JP2021155742A - Photocurable inkjet ink set and inkjet recording method using the same - Google Patents

Abstract

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

Description

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

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. Of these, the inkjet method is an inexpensive device that ejects ink only to the required image portion to form an image directly on the recording medium, so that the ink can be used efficiently and the running cost is low. Further, since the inkjet method has low noise, it is excellent as a recording method.

In recent years, in order to form prints having excellent water resistance, solvent resistance, and scratch resistance on the surface of a recording medium, a photocurable ink that cures when irradiated with light (ultraviolet rays) is used in an inkjet recording method. in use.

On the other hand, a recording method in which various photocurable inks are sequentially overcoated is widely used. For example, inks of different colors such as cyan (C), magenta (M), yellow (Y), black (K), and white (W) are overcoated to form a color image, or image protection is performed. A clear ink that is colorless and transparent may be overcoated on a color image for the purpose of imparting gloss. In such a case, a technique for obtaining an image having a desired image quality is required.

For example, Patent Document 1 uses an energy ray-curable inkjet recording color ink composition (Ia) to form a print coating film on a member to be recorded, and then clears the energy ray-curable inkjet recording on the coating film. A clear coating film is formed using the ink composition (Ib), and 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. Discloses a method for recoating an energy ray-curable ink composition in which the relationship of 31.5 ≦ Sa <35.0 or 27.0 ≦ Sa <31.5 and Sb <Sa is established (Reference 1). Specified paragraph 0009).

Japanese Unexamined Patent Publication No. 2006-181801

However, according to the recoating 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 the clear ink is very small. , There arises a problem that the wettability of the ink is inferior.

Therefore, one of the objects of the present invention is to provide a photocurable inkjet ink set having excellent wettability and spreadability of ink.

The inventors of the present application have diligently studied 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 relationship of the surface tension between the ink and the photocurable inkjet ink for the upper layer are correlated. More specifically, even when the surface tensions of the lower layer photocurable inkjet ink and the upper layer photocurable inkjet ink are related as disclosed in Patent Document 1, the lower layer photocurable inkjet ink is predetermined. It was found that by containing the vinyl ether group-containing (meth) acrylic acid esters of the above, the spreadability of the photocurable inkjet ink for the upper layer is excellent.

Therefore, as a result of further studies by the inventors of the present application, a photocurable inkjet ink for a lower layer containing a predetermined vinyl ether group-containing (meth) acrylic acid ester (hereinafter, also simply referred to as "ink for a lower layer"). , A photocurable inkjet ink set for an upper layer (hereinafter, also simply referred to as "upper layer ink"), and a photocurable inkjet ink set in which the surface tension of the lower layer ink is larger than the surface tension of the upper layer ink. The present invention has been completed by finding that the above problems can be solved by (hereinafter, also simply referred to as “ink set”).

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)
(In the formula, R ¹ is a hydrogen atom or a methyl group, R ² is a divalent organic residue having 2 to 20 carbon atoms, and R ³ is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms. A photocurable inkjet ink set containing a photocurable inkjet ink for a lower layer containing a vinyl ether group-containing (meth) acrylic acid ester represented by () and a photocurable inkjet ink for an upper layer. A photocurable inkjet ink set in which the surface tension of the photocurable inkjet ink for the lower layer is larger 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 esters represented by the general formula (I) contained in the photocurable inkjet ink for the lower layer is based on the total mass of the photocurable inkjet ink for the lower layer. The photocurable inkjet ink set according to [1] or [2], which is 20 to 80% by mass.
[4]
The photocurable inkjet ink for the upper layer contains vinyl ether group-containing (meth) acrylic acid esters represented by the general formula (I), and the content of the vinyl ether group-containing (meth) acrylic acid esters 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 photocurable inkjet ink for the lower layer contains an acrylic surfactant.
[6]
The photocurable inkjet ink set for the upper layer is the photocurable inkjet ink set according to any one of [1] to [5], which contains a silicone-based surfactant.
[7]
A coating film of the photocurable inkjet ink for the lower layer according to any one of [1] to [6] is formed on the 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 an upper layer.

It is a conceptual diagram for demonstrating an example of the head unit and the transfer unit of the recording apparatus which can be used in one Embodiment of this invention.

Hereinafter, embodiments for carrying out the present invention will be described in detail. The present invention is not limited to the following embodiments, and can be variously modified and implemented within the scope of the gist thereof.

In the present specification, the “wetting and spreading property of the ink” refers to the property that the line width becomes relatively wide when the upper layer ink is linearly overcoated on 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 polymerizes and the ink solidifies. "Curing property" refers to the property of being cured by being sensitive to light. The “filling property” is also referred to as a filling property, and refers to a property in which the recording medium, which is the base, cannot be seen when the recorded material is viewed from the side where the cured product (image) is formed. “Ejection stability” refers to the property of ejecting stable ink droplets from a nozzle without clogging of the nozzle. “Storage stability” refers to the property that the viscosity of ink does not change easily before and after storage when the ink is stored.

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

As used herein, "(meth) acrylate" means at least one of acrylate and its corresponding methacrylate, and "(meth) acryloyl" means at least one of acryloyl and its corresponding methacryloyl. "(Meta) acrylic" means at least one of acrylic and its corresponding methacryl.

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

[Photo-curable inkjet ink set]
The photocurable inkjet ink set according to the embodiment of the present invention includes a photocurable inkjet ink for a lower layer containing vinyl ether group-containing (meth) acrylic acid esters represented by the general formula (I) described later, and an upper layer. Includes photo-curable inkjet inks for use. In addition to this, the ink set satisfies the relationship that the surface tension of the lower layer ink is larger than the surface tension of the upper layer ink.
The ink set may further contain 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 described. The ink set of the present embodiment needs to satisfy the above relationship, and on top of that, the difference between the surface tension of the lower layer ink and the surface tension of the upper layer ink (“surface tension of the lower layer ink”-“upper layer). The surface tension of the ink for use ”) is preferably 5 or more, and more preferably 10 or more. When the difference is within the above range, the wettability of the ink becomes more excellent.

It can be said that the surface tensions of the lower layer ink and the upper layer ink are suitable when they are within the following ranges on the premise that the above relationship is satisfied. That is, it is preferable that the surface tensions of the upper layer ink and the lower layer ink are both in the range of 20 to 40 mN / m, the surface tension of the upper layer ink is 20 to 30 mN / m, and the surface tension of the lower layer ink is set to 20 to 30 mN / m. It is more preferably 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 becomes excellent.

Here, the surface tension of the ink is determined by its composition (type and content of each component), and the main factor thereof is the type and content of the surfactant contained in the ink. The surfactant will be described later. As the surface tension in the present specification, a value obtained by the measurement method performed in the Example column described later is adopted.

Hereinafter, additives (components) that are or may be contained in the ink (composition) of the present embodiment will be described. In the following, 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 at the time of irradiation by the action of the photopolymerization initiator described later, and the printed ink can be cured.

(1. Vinyl ether group-containing (meth) acrylic acid esters)
Of the inks in this embodiment, at least the lower layer ink is a polymerizable compound having 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 1 of 1 to 11 carbon atoms. It contains vinyl ether group-containing (meth) acrylic acid esters represented by (valent organic residues).

When the ink contains the vinyl ether group-containing (meth) acrylic acid esters, it is possible to improve the curability of the ink and further reduce the viscosity of the ink. Furthermore, it is better to use a compound having both a vinyl ether group and a (meth) acrylic group in one molecule than to use a compound having a vinyl ether group and a compound having a (meth) acrylic group separately. It is preferable for improving the curability.

In particular, since the lower layer ink contains the vinyl ether group-containing (meth) acrylic acid esters, the upper layer ink has excellent wettability and spreadability, and the lower layer ink has curability and is used for the lower layer on the recording medium. The ink filling property is also excellent.

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

In the above general formula (I), the ^{divalent organic residue having 2 to 20 carbon atoms represented by R 2} is a linear, branched or cyclic substituted organic residue having 2 to 20 carbon atoms. May be an alkylene group, an optionally substituted alkylene group having an oxygen atom due to an ether bond and / or an ester bond in the structure, an optionally substituted divalent 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 due to an ether bond in the structure of is preferably used.

In the above general formula (I), the ^{monovalent organic residue having 1 to 11 carbon atoms represented by R 3} is substituted linearly, branched or cyclically having 1 to 10 carbon atoms. Alkyl groups may be suitable, and aromatic groups having 6 to 11 carbon atoms which may be substituted are suitable. Among these, an aromatic group having 6 to 8 carbon atoms such as an alkyl group having 1 to 2 carbon atoms, a phenyl group and a benzyl group, which is a methyl group or an ethyl group, is preferably used.

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

The vinyl ether group-containing (meth) acrylic acid esters are not limited to the following, but are, for example, 2-vinyloxyethyl (meth) acrylic acid, 3-vinyloxypropyl (meth) acrylic acid, and 1-methyl (meth) acrylic acid. -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-methyl (meth) acrylate- 2-vinyloxypropyl, 2-vinyloxybutyl (meth) acrylate, 4-vinyloxycyclohexyl (meth) acrylate, 6-vinyloxyhexyl (meth) acrylate, 4-vinyloxymethylcyclohexyl (meth) acrylate, (Meta) 3-vinyloxymethylcyclohexylmethyl acrylate, 2-vinyloxymethylcyclohexylmethyl (meth) acrylate, p-vinyloxymethylphenylmethyl (meth) acrylate, m-vinyloxymethylphenyl (meth) acrylate Methyl, (meth) acrylic acid o-vinyloxymethylphenylmethyl, (meth) acrylic acid 2- (vinyloxyethoxy) ethyl, (meth) acrylic acid 2- (vinyloxyisopropoxy) ethyl, (meth) acrylic acid 2 -(Vinyloxyethoxy) propyl, 2- (meth) acrylate) isopropyl, (meth) acrylate 2- (vinyloxyisopropoxy) propyl, (meth) acrylate 2- (vinyloxyisopropoxy) 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) 2- (Vinyloxyisopropoxyethoxy) propyl acrylate, 2- (vinyloxyisopropoxyisopropoxy) propyl (meth) acrylate, 2- (vinylo) acrylate Xiethoxyethoxy) isopropyl, 2- (meth) acrylate 2- (vinyloxyethoxyisopropoxy) isopropyl, 2- (meth) acrylate 2- (vinyloxyisopropoxyethoxy) isopropyl, 2- (meth) acrylate 2- (vinyloxyisopropoxy) Isopropoxy) isopropyl, 2- (meth) acrylate 2- (vinyloxyethoxyethoxyethoxy) ethyl, (meth) acrylate 2- (vinyloxyethoxyethoxyethoxy) ethyl, (meth) acrylate 2- (isopropenoxyethoxy) ) Ethyl, 2- (isopropenoxyethoxyethoxy) ethyl (meth) acrylate, 2- (isopropenoxyethoxyethoxyethoxy) ethyl (meth) acrylate, 2- (isopropenoxyethoxyethoxyethoxy) (meth) acrylate Examples thereof include ethoxy) ethyl, polyethylene glycol monovinyl ether (meth) acrylate, and polypropylene glycol monovinyl ether (meth) acrylate.

Among these, 2- (meth) acrylate 2- (vinyloxyethoxy) ethyl (meth) acrylate, that is, 2- (vinyloxy) acrylate, is used because the viscosity of the ink can be lowered, the flash point is high, and the curability of the ink is excellent. At least one of ethoxy) ethyl and 2- (vinyloxyethoxy) ethyl methacrylate is preferable, and 2- (vinyloxyethoxy) ethyl acrylate is more preferable. In particular, 2- (vinyloxyethoxy) ethyl acrylate and 2- (vinyloxyethoxy) ethyl methacrylate both have a simple structure and a small molecular weight, so that the ink can be remarkably reduced in viscosity. Examples of 2- (vinyloxyethoxy) ethyl (meth) acrylate include 2- (2-vinyloxyethoxy) ethyl (meth) acrylate and 2- (1-vinyloxyethoxy) ethyl (meth) acrylate. Examples of 2- (vinyloxyethoxy) ethyl acrylate include 2- (2-vinyloxyethoxy) ethyl acrylate and 2- (1-vinyloxyethoxy) ethyl acrylate. In addition, 2- (vinyloxyethoxy) ethyl acrylate is superior to 2- (vinyloxyethoxy) ethyl methacrylate in terms of curability.

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 lower layer ink is preferably 20 to 80% by mass, preferably 30 to 80% by mass, based on the total mass (100% by mass) of the lower layer ink. Is more preferable. 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 wettability and 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 made excellent.

In particular, when not only the lower layer ink but also the upper layer ink contains the vinyl ether group-containing (meth) acrylic acid esters, the content of the vinyl ether group-containing (meth) acrylic acid esters is higher in the lower layer ink. It is preferably larger than the upper layer ink, and more preferably 10% by mass or more larger than the upper layer ink. As a result, the wettability and spreadability of the upper layer ink can be further improved, and an ink set having excellent various performances can be obtained.
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 larger than the surface tension of the lower layer ink and the lower layer ink contains the vinyl ether group-containing (meth) acrylic acid esters, the lower layer ink is applied. The reason why the upper layer ink has excellent wet spreadability on the film is that the coating film derived from the vinyl ether group of the vinyl ether group-containing (meth) acrylic acid esters has some influence on the wet spreadability of the upper layer ink. It is presumed. However, the cause is not limited to this.

The method for producing the vinyl ether group-containing (meth) acrylic acid esters is not limited to the following, but is a method for esterifying (meth) acrylic acid and a hydroxyl group-containing vinyl ether (Production Method B), (meth) acrylic acid halide. And a method for esterifying a hydroxyl group-containing vinyl ether (manufacturing method C), a method for esterifying a (meth) acrylic acid anhydride and a hydroxyl group-containing vinyl ether (manufacturing method D), an ester of a (meth) acrylic acid ester and a hydroxyl group-containing vinyl ether. Exchange method (manufacturing method E), (meth) acrylic acid and halogen-containing vinyl ether esterifying method (manufacturing method F), (meth) acrylic acid alkali (earth) metal salt and halogen-containing vinyl ether esterifying method (Production method G), a method of exchanging vinyl between a hydroxyl group-containing (meth) acrylic acid ester and vinyl carboxylate (Production method H), and a method of ether exchange of a hydroxyl group-containing (meth) acrylic acid ester with an alkyl vinyl ether (Production method I). Can be mentioned.

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

(2. Polymerizable compounds other than the above)
As the polymerizable compound other than the above (hereinafter, referred to as “other polymerizable compound”), various monomers and oligomers known conventionally such as monofunctional, bifunctional, and trifunctional or higher polyfunctional can be used. .. Examples of the monomer include unsaturated carboxylic acids such as (meth) acrylic acid, itaconic acid, crotonic acid, isocrotonic acid and maleic acid, salts or esters thereof, urethane, amide and its anhydride, acrylonitrile and styrene. Examples thereof include unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes. Examples of the oligomer include oligomers formed from the above monomers such as linear acrylic oligomers, epoxy (meth) acrylate, oxetane (meth) acrylate, aliphatic urethane (meth) acrylate, and aromatic urethane (meth). Acrylate and polyester (meth) acrylate can be mentioned.

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

Among other polymerizable compounds, an ester of (meth) acrylic acid, that is, (meth) acrylate is preferable.

Among the above (meth) acrylates, examples of the monofunctional (meth) acrylate include isoamyl (meth) acrylate, stearyl (meth) acrylate, lauryl (meth) acrylate, octyl (meth) acrylate, decyl (meth) acrylate, and 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 ( Meta) 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 preferable.

Among the above (meth) acrylates, examples of the bifunctional (meth) acrylate include triethylene glycol di (meth) acrylate, tetraethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, and dipropylene glycol di (meth) acrylate. Meta) 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 Didioldi (meth) acrylate, neopentyl glycol di (meth) acrylate, dimethylol-tricyclodecanedi (meth) acrylate, EO (ethylene oxide) adduct di (meth) acrylate of bisphenol A, PO (propylene oxide) of bisphenol A ) Additives Di (meth) acrylate, neopentyl glycol di (meth) acrylate of hydroxypivalate, and polytetramethylene glycol di (meth) acrylate can be mentioned. Among these, dipropylene glycol di (meth) acrylate is preferable.

Among the above (meth) acrylates, as the trifunctional or higher functional (meth) acrylate, for example, trimethyl propantri (meth) acrylate, EO-modified trimethylol propanthry (meth) acrylate, pentaerythritol tri (meth) acrylate , Pentaerythritol tetra (meth) acrylate, dipentaerythritol hexa (meth) acrylate, ditrimethylol propanetetra (meth) acrylate, glycerin propoxytri (meth) acrylate, cow prolactone modified trimethylol propanetri (meth) acrylate, pentaerythritol Examples thereof include 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 inkjet recording can be easily obtained. Further, 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 bifunctional (meth) acrylate in combination, among which phenoxyethyl (meth) acrylate and dipropylene glycol are used. It is more preferable to use di (meth) acrylate in combination.

The above-mentioned other polymerizable compounds may be used alone 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, still more preferably 60 to 90% by mass, based on the total mass (100% by mass) of the ink. When the content is within the above range, the viscosity and odor can be lowered, 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, the relationship that the surface tension of the lower layer ink is larger than the surface tension of the upper layer ink can be more reliably satisfied. Further, since not only the upper layer ink but also the lower layer ink contains the surfactant, the filling property of the lower layer ink on the recording medium is also excellent.

The surfactant is not limited to the following, but for example, a silicone-based surfactant, an acrylic-based surfactant, a cationic-based surfactant, an anionic surfactant, a nonionic surfactant, an amphoteric surfactant, and the like. And fluorine-based surfactants. Among these, silicone-based surfactants and acrylic-based 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.

The silicone-based surfactant is not limited to the following, and for example, modified silicones such as polyester-modified silicones and polyether-modified silicones can be used. Among these, a polyether-modified silicone is preferable because the surface tension can be significantly reduced, which makes the ink more excellent in wettability and spreadability on the recording medium.

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

Examples of commercially available products of the above-mentioned silicone-based surfactants include BYK-347, BYK-348, BYK-UV3500, BYK-UV3510, BYK-UV3530, BYK-UV3570 (hereinafter, trade names manufactured by BYK) and the like. Be done. Among these, BYK-UV3500 is preferable because the wett-spreading property of the upper layer ink becomes more excellent.

Further, the above-mentioned acrylic surfactant is not limited to the following, and for example, a (meth) acrylic acid-based copolymer is preferably mentioned.

Commercially available products of the above acrylic surfactant include, for example, 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 (above, trade name manufactured by Kusumoto Kasei Co., Ltd.), BYK-350, BYK-352 , BYK-354, BYK-355, BYK-358, BYK-380, BYK-381, BYK-392 (above, BYK trade name), Polyflow No. 57, 95 (above, product name manufactured by Kyoei Co., Ltd. Oil and Fat Chemical Industry Co., Ltd.) and the like can be mentioned. Among these, when the lower layer ink contains an acrylic surfactant, BYK-350 is preferable because the ejection stability of the lower layer ink is excellent.

As a combination of surfactants that can be contained in or can be contained in the upper layer ink and the lower layer ink, respectively, the above-mentioned relationship of surface tension can be more reliably satisfied. Is preferable, and the third case below is more preferable.

First, it is preferable that the upper layer ink and the lower layer ink contain the same surfactant, and the content of the surfactant is larger in the upper layer ink than in the lower layer ink. The difference in the contents 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 of the lower layer ink is more preferably 0.1 to 0.3% by mass.
In the first case, the wettability and spreadability of the upper layer ink becomes more excellent, 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-based surfactant (however, the compounds may be the same or different from each other), and the content of the surfactant is the same as that of the upper layer ink. It is preferable that the size is larger than that of the lower layer ink. The difference in the contents 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, at least one of the polyether-modified silicone and the polyester-modified silicone is preferable for the upper layer ink and the lower layer ink independently of each other, and the polyether-modified silicone is preferable. More preferred. Further, it is preferable to use the same modified silicone-based surfactant because the cost of the ink material can be reduced due to the 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 of the lower layer ink is more preferably 0.1 to 0.3% by mass.
In the second case, the ink for the lower layer is excellently filled on the recording medium.

Third, the upper layer ink preferably contains a modified silicone-based surfactant. On the other hand, the lower layer ink may or may not contain a surfactant. However, when the ink for the lower layer contains a surfactant, the surfactant is preferably an acrylic surfactant. When the upper layer ink contains a modified silicone-based surfactant, the wettability and spreadability of the upper layer ink becomes further excellent. Examples of the modified silicone-based surfactant include, but are not limited to, polyether-modified silicone and polyester-modified silicone. Among them, the modified silicone-based surfactant is easily available and has the ability to fill the ink for the lower layer on the recording medium. Polyester-modified silicones are preferred because they are excellent.

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 of the lower layer ink is more preferably 0.1 to 0.3% by mass.
In the third case, the wettability of the ink becomes more excellent, which is advantageous in that the difference in surface tension between the upper layer ink and the lower layer ink is stably secured.

[Photopolymerization initiator]
The photopolymerization initiator that can be contained in the ink in the present embodiment is used to cure the ink existing on the surface of the recording medium to form a print by photopolymerization by irradiation with radiation. By using ultraviolet rays (UV) among the radiation (light), safety is excellent and the cost of the light source lamp can be suppressed. As long as an active species such as a radical or a cation is generated by the energy of light and the polymerization of the above-mentioned polymerizable compound is started, there is no limitation, but a photoradical polymerization initiator or a photocationic polymerization initiator should be used. Of these, 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.), and hexaarylbi. Examples thereof include imidazole compounds, ketooxime 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 because the curability of the ink can be further improved. It is also preferable to use a thioxanthone compound in addition to the acylphosphine oxide compound.

Specific examples of the photoradical polymerization initiator 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 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 can be mentioned.

Examples of commercially available photoradical polymerization initiators include IRGACURE 651 (2,2-dimethoxy-1,2-diphenylethane-1-one), IRGACURE 184 (1-hydroxy-cyclohexyl-phenyl-ketone), and DAROCUR 1173. (2-Hydroxy-2-methyl-1-phenyl-propane-1-one), IRGACURE 2959 (1- [4- (2-hydroxyethoxy) -phenyl] -2-hydroxy-2-methyl-1-propane- 1-on), 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) -phenylphosphin oxide), IRGACURE 784 (bis (η5-2,4-cyclopentadiene-1-yl)) -Bis (2,6-difluoro-3- (1H-pyrrole-1-yl) -phenyl) titanium), IRGACURE OXE 01 (1.2-octanedione, 1- [4- (phenylthio)-, 2- ( O-benzoyloxime)]), IRGACURE OXE 02 (Etanone, 1- [9-ethyl-6- (2-methylbenzoyl) -9H-carbazole-3-yl]-, 1- (O-acetyloxime)), IRGACURE 754 (oxyphenylacetic acid, a mixture of 2- [2-oxo-2-phenylacetoxyethoxy] ethyl ester and oxyphenylacetic acid, 2- (2-hydroxyethoxy) ethyl ester) (above, manufactured by BASF), Speedcure TPO , Speedcure DETX (2,4-diethylthioxanthone), Speedcure ITX (2-isopropylthioxanthone) (above, Rambson), KAYACURE DETX-S (2,4-diethylthioxanthone) (Nippon Kayaku Co., Ltd.), Lucirin TPO, LR8883, LR8970 (BASF), and Yubekrill P36 (manufactured by UCB) and the like can be mentioned.

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

The content of the photopolymerization initiator is such that the ultraviolet curing rate can be improved to improve the curability, and the undissolved residue of the photopolymerization initiator and the coloring derived from the photopolymerization initiator are avoided. 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 above-mentioned polymerizable compound before curing can be prevented, and the storage stability of the ink can be improved.

The above-mentioned polymerization inhibitor is not limited to the following, but for example, 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) and other phenolic polymers. Prohibition agents and include hindered amines, hydroquinone monomethyl ethers (MEHQ), and hydroquinone.

The polymerization inhibitor may be used alone or in combination of two or more. The content of the polymerization inhibitor is not particularly limited because it is determined by the relationship with the content of other components.

[Color material]
The ink in this embodiment may contain a coloring material. As the coloring material, at least one of a pigment and a dye can be used.

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

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

Examples of organic pigments include azo pigments such as insoluble azo pigments, condensed azo pigments, azolakes 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 chelate (for example, basic dye type chelate, acidic dye type chelate, etc.), dyeing rake (basic dye type rake, acidic dye type rake), nitro pigment, nitroso pigment, aniline black, daylight Fluorescent pigments can be mentioned.

More specifically, the carbon black used as the black ink is No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52, MA7, MA8, MA100, No. 2200B, etc. (above, trade name manufactured by Mitsubishi Chemical Corporation (Mitsubishi Chemical Corporation)), Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven 1255, Raven 700, etc. 400R, Rega1 330R, Rega1 660R, Mogul L, Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400, etc. ), Color Black FW1, Color Black FW2, Color Black FW2V, Color Black FW18, Color Black FW200, Color B1ack S150, Color Black , Special Black 5, Special Black 4A, Special Black 4 and the like (these are trade names manufactured by Degussa).

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

Examples of the pigment used in the 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 can be mentioned.

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.

Examples of the pigment used for 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 can be mentioned.

Examples of pigments other than magenta, cyan, and yellow include C.I. I. Pigment Greens 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 pigment may be used alone or in combination of two or more.

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

(dye)
In this 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. As the dye, for example, 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. Dilekdo 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 alone 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 (100% by mass) of the ink because excellent hiding property and color reproducibility can be obtained.

[Other additives]
The ink in 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 accelerators, penetration accelerators, wetting agents (moisturizers), and other additives. Examples of the above-mentioned other additives include conventionally known fixing agents, fungicides, preservatives, antioxidants, ultraviolet absorbers, chelating agents, and thickeners.

From the above, according to the present embodiment, since the line width of the upper layer ink overlaid on the coating film of the lower layer ink is not narrowed, a photocurable inkjet ink set having excellent wettability and spreadability of the upper layer ink can be obtained. Can be provided. Further, according to the present embodiment, a photocurable inkjet ink set having excellent curability and ejection stability of both the upper layer ink and the lower layer ink and the filling property of the lower layer ink on the recording medium is provided. can do. As described above, in the photocurable inkjet ink set, the lower layer ink and the upper layer ink have a relationship of influencing 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.

[Recorded medium]
In the photocurable inkjet ink set of the present embodiment, a recorded material can be obtained by ejecting the ink onto a recording medium by using an inkjet recording method described later. Examples of the recording medium include an absorbent or non-absorbable recording medium. The inkjet recording method of the embodiment described later can be widely applied to a recording medium having various absorption performances, from a non-absorbable recording medium in which ink is difficult to penetrate to an absorbent recording medium in which ink can easily penetrate. ..

The absorbent recording medium is not particularly limited, but is, for example, 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 (polyvinyl alcohol). From ink-absorbing layers made of hydrophilic polymers such as PVA) and polyvinylpyrrolidone (PVP) to art papers and coated papers used for general offset printing with relatively low ink permeability. Cast paper and the like can be mentioned.

The non-absorbable recording medium is not particularly limited, but for example, plastic films such as polyvinyl chloride (PVC), polyethylene, polypropylene, polyethylene terephthalate (PET), sheets, and plates, iron, silver, copper, and the like. Examples thereof include metal plates such as aluminum, metal plates manufactured by vapor deposition of various metals, plastic films, alloy plates such as stainless steel and true cast, and the like.

[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 this embodiment.

[Inkjet recording device for implementing the inkjet recording method]
An example of an inkjet recording device (hereinafter, also simply referred to as “recording device”) for carrying out the inkjet recording method using the ink set of the above embodiment will be described. The recording device includes a first recording mode, a second recording mode, and a recording control unit.

In the first recording mode, the upper layer ink is overcoated on the coating film formed of the lower layer ink to form the coating film.

In the second recording mode, the lower layer ink is overcoated on the coating film formed of the upper layer ink to form the coating film.

The recording control unit controls the amount of ink per dot of the solid pattern image formed by the lower layer ink in the first recording mode and the second recording mode.
Hereinafter, an example of the above recording device will 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 the present embodiment. The continuous sheet-shaped recording medium S is transported from the upstream side to the downstream side in the transport direction as the upstream roller 12A, the transport drum 12, and the downstream roller 12B rotate. The head units are the first white head unit 30A (on the paper, the one on the left side of FIG. 1) that ejects white ink W, the yellow head unit 30B that ejects yellow ink Y, and the magenta head unit that ejects magenta ink M. 30C, cyan head unit 30D for discharging cyan ink C, black head unit 30E for discharging black ink K, and second white head unit 30A for discharging white ink W (on the paper, the one on the right side of FIG. 1). To be equipped with. The irradiation unit 40 includes irradiation units 42a, 41a, 41b, 41c, 41d, 42b provided downstream of each head unit in the transport direction, and an irradiation unit 43 provided at the end of the transport direction. There is. Such a recording device can be configured as shown in FIG. 2, for example, in Japanese Patent Application Laid-Open No. 2010-208218.

Here, when the first recording mode is performed, first, the recording medium S is conveyed to the recording medium S at a position facing each head unit on the transfer drum in the order in which the recording medium S is conveyed. The lower layer ink is ejected from the first white head unit 30A, and irradiation is performed from the irradiation unit 42a to form a coating film with the lower layer ink. Next, 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 onto the coating film, and is provided on the downstream side of the head. Irradiation is performed from the irradiated part. Finally, irradiation is performed from the irradiation unit 43 to form a coating film. The second white head unit 30A and the irradiation unit 42b are not used.

On the other hand, when the second recording mode is performed, first, yellow is used in the order in which the recording medium S is conveyed to the recording medium S that has been conveyed to the position facing each head unit on the transfer drum. Upper layer ink is ejected from at least one of the 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 on the downstream side of the head for the upper layer. A coating of ink is formed. 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. The first white head unit 30A and the irradiation unit 42a are not used.

In the above recording control unit, when the coating film of the lower layer ink is a solid pattern, the amount of ink per dot of the lower layer ink is preferably higher in the second recording mode than in the first recording mode. It may be adjusted to be 10% or more larger, more preferably 10 to 50% larger. As a result, even in the second recording mode, the same wettability and spreadability of the lower layer ink as in the first recording mode is ensured. The 1 dot means an ink droplet attached to each pixel, which is the minimum 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 may not be provided. Further, among the inks, the ink on which the coating film is formed later, that is, the upper layer ink in the first recording mode and the lower layer ink in the second recording mode, is an irradiation unit (41a) provided on the downstream side of the head unit. It may be cured by at least one of the irradiation by (-41d, 42b) and the irradiation by the irradiation unit 43, and when it is cured by the irradiation unit provided on the downstream side of the head unit, the irradiation unit 43 may be omitted. good.

[Upper layer ink and lower layer ink]
In the present embodiment, the lower layer ink is an 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 is the ink of the recording medium in the first recording mode. This is an ink that forms a coating film on the coating film made of the lower layer ink. For the upper layer ink and the lower layer ink, for example, one may be the main ink and the other may be the auxiliary ink. Then, the main ink may be an ink that is used by visually recognizing a coating film such as a color ink. Further, the auxiliary ink may be, for example, a special color ink such as white ink, clear ink, or metallic ink, or a functional ink that improves performance such as adhesion, wettability, and abrasion resistance of the main ink. You just have to.
Here, both the upper layer ink and the lower layer ink may be used as the main ink, or both may be used as the auxiliary ink, but using one as the main ink and the other as the auxiliary ink expands the use of the recorded material. Moreover, it is preferable because the performance of the main ink is improved. When one is used as the main ink and the other is used as the auxiliary ink, which of the upper layer ink and the lower layer ink is used may be determined in consideration of the respective properties of the main ink and the auxiliary ink described above. Therefore, in FIG. 1, the head that ejects the lower layer ink in the first recording mode is the first white head unit 30A, but the lower layer ink is not limited to the white ink.

In this way, the difference in the amount of ink 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. Therefore, the filling property of the solid pattern image of the lower layer ink is deteriorated. Therefore, by increasing the amount of ink as described above, the filling property of the lower layer ink can be improved.

[Inkjet recording method using an inkjet recording device]
In the inkjet recording method of the present embodiment, the first recording step of forming a coating film of the lower layer ink of the above embodiment on the recording medium and the upper layer ink of the above embodiment are overcoated on the coating film. This includes a second recording step of forming an image by forming a coating film of the ink.

Each of the first recording step and the second recording step includes a 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 waste. In this way, the ink cured on the recording medium forms an image, that is, a cured ink coating film. Hereinafter, these stages will be described.

[Discharge stage]
In the ejection stage, 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 at the time of 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 temperature of the ink is set to room temperature or the ink is not heated, the temperature of the ink may be set to room temperature or the ink may be ejected without being heated. At that time, the temperature of the ink at the time of ejection is preferably 20 to 30 ° C. On the other hand, the ink may be discharged to a preferable viscosity by heating the ink to a predetermined temperature. In this way, good discharge stability is achieved.

Both the lower layer ink and the upper layer ink in the present embodiment are photocurable inks and have higher viscosities than ordinary water-based inks, so that the viscosity fluctuates greatly due to temperature fluctuations during ejection. Such a variation in the viscosity of the ink has a great influence on the change in the droplet size and the change in the droplet ejection speed, which may cause deterioration in image quality. Therefore, it is preferable to keep the temperature of the ink at the time of ejection as constant as possible.

[Curing stage]
Next, in the curing step, the ink ejected toward the recording medium and adhered (landed) is cured by irradiation with ultraviolet rays. This is because the photopolymerization initiator that can be contained in the ink is decomposed by irradiation with light (ultraviolet light) to generate starting species such as radicals, acids, and bases, and the polymerization reaction of the polymerizable compound is the starting species of the starting species. This is because it is promoted by the function. Alternatively, the polymerization reaction of the polymerizable compound is started by irradiation with ultraviolet rays. At this time, if a sensitizing dye is present together with the photopolymerization initiator in the ink, the sensitizing dye in the system absorbs ultraviolet rays to be in an excited state, and contacts with the photopolymerization initiator to promote decomposition of the photopolymerization initiator. It is possible to achieve a more sensitive curing reaction.

Mercury lamps, gas / solid-state lasers, and the like are mainly used as ultraviolet sources, and mercury lamps and metal halide lamps are widely known as light sources used for curing photocurable inkjet inks. On the other hand, mercury-free is currently strongly desired from the viewpoint of environmental protection, and replacement with a GaN-based semiconductor ultraviolet light emitting device is extremely useful industrially and environmentally. Further, an ultraviolet light emitting diode (UV-LED) and an ultraviolet laser diode (UV-LD) are small in size, have a long life, have high efficiency, and are low in cost, and are expected as light sources for photocurable inkjet. 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 by using a UV-LED having an emission peak wavelength preferably in the range of 350 to 420 nm. It is preferable to use the set of the above for the inkjet recording method. In this case, low-cost printing and high printing speed can be realized. Such an ink is obtained by containing at least one of a photopolymerization initiator that decomposes by ultraviolet irradiation in the wavelength range and a polymerizable compound that initiates polymerization by ultraviolet irradiation in the 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 filling property of the lower layer ink on the recording medium are all satisfied. It is also possible to provide an inkjet recording method using a photocurable inkjet ink set, which is also excellent.

Hereinafter, embodiments of the present invention will be described in more detail by way of examples, but the present embodiment is not limited to these examples.

[Material used]
The materials used in the following examples and comparative examples are as follows.
[Pigment]
・ C. I. Pigment Blue 15: 3 (Parthalocyanine Blue, manufactured by DIC Corporation, 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 acid esters)
-VEEA (2- (2-vinyloxyethoxy) ethyl acrylate, trade name manufactured by Nippon Shokubai Co., Ltd., abbreviated as "VEEA" below)
(2. Other polymerizable compounds)
-Viscoat # 192 (Phenoxyethyl acrylate, product name manufactured by OSAKA ORGANIC CHEMICAL INDUSTRY LTD., Abbreviated as "PEA" below)
-NK Ester APG-100 (Product name manufactured by Shin Nakamura Chemical Industry Co., Ltd. (SHIN-NAKAMURA CHEMICAL CO., LTD.), Dipropylene glycol diacrylate, hereinafter abbreviated as "DPGDA")
[Polymerization inhibitor]
・ MEHQ (hydroquinone monomethyl ether, manufactured by Kanto Chemical Co., Inc.)
[Surfactant]
-BYK-UV3500 (Product name manufactured by BYK, silicone type, hereinafter abbreviated as "UV3500")
-BYK-UV3570 (Product name manufactured by BYK, silicone type, hereinafter abbreviated as "UV3570")
-BYK-350 (Product name manufactured by BYK, acrylic type, hereinafter abbreviated as "BYK350")
-BYK-381 (Product name manufactured by BYK, acrylic type, hereinafter abbreviated as "BYK381")
[Photopolymerization initiator]
-IRGACURE 819 (BASF product name, solid content 100%, hereinafter abbreviated as "819")
-DAROCUR TPO (BASF product name, 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 upper layer ink and lower layer ink]
Each material is added so as to have the composition (unit: mass%) shown in Tables 1 and 2 below, and this is stirred with a high-speed water-cooled stirrer to obtain a cyan-colored upper layer ink and a white-colored lower layer. Ink for use was prepared.
Table 1 shows the composition of each upper layer ink, and the upper layer inks 1, 2, 3, and 4 are abbreviated as upper layers 1, 2, 3, and 4, respectively. Table 2 shows the composition of each lower layer ink, and the lower layer inks 1, 2, 3, 4, 5, 6, and 7 are shown below, respectively. , And 7 are abbreviated.

[Examples 1 to 7, Comparative Examples 1 to 5]
An ink set was prepared by combining the upper layer ink and the lower layer ink described in the upper rows of Tables 3 and 4 below. The surface tension γ (mN / m) of the upper layer ink and the lower layer ink in each Example and each Comparative Example was determined by the Millinewton method using a tensiometer CBVP-Z (trade name manufactured by Kyowa Interface Science Co., Ltd.). The values were measured under normal temperature and pressure, and are shown in Tables 3 and 4 below.

[Evaluation item]
The ink sets produced in each Example and each Comparative Example were evaluated for wettability, filling property, ejection stability, and curability by the following methods.

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

The temperature of each ink was adjusted so that the viscosity was 10 mPa · s, and then the ink was discharged. 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).
The 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. Fillability)
The lower layer ink was ejected toward the recording medium (PET50A) under the conditions of 720 × 720 dpi and 10 ng / dot and cured.

The temperature of each ink was adjusted so that the viscosity was 10 mPa · s, and then the ink was discharged. This evaluation was carried out by visually observing the filling property of the obtained cured product on the ink for the lower layer.
The evaluation criteria are as follows. The evaluation results are shown in Tables 3 and 4 below.
A: It was buried visually from a height of 30 cm from the recording medium (the underlying streaks could not be seen).
B: Underground streaks were visible visually from a height of 30 cm from the recording medium.

(3. Discharge stability)
The ejection stability of each of the upper layer ink and the lower layer ink was evaluated. From the head having 180 nozzles, each of the upper layer ink and the lower layer ink was continuously ejected for 60 minutes. In this way, the ejection stability of the head when ejecting each of the upper layer ink and the lower layer ink was evaluated.
The evaluation criteria are as follows. The evaluation results are shown in Tables 3 and 4 below.
A: Nozzle omission did not occur during continuous ejection.
B: Nozzle omission occurred during continuous discharge.

(4. Curability)
The curability of each of the upper layer ink and the lower layer ink was evaluated. Using an inkjet recording device equipped with a piezo-type inkjet nozzle, the upper layer ink and the lower layer ink in each Example and each Comparative Example were filled into each nozzle row. Each of the upper layer ink and the lower layer ink is ejected toward the recording medium (PET50A) at room temperature and normal pressure under the conditions of 720 × 720 dpi and 10 ng / dot, and the film thickness of the printed matter (recorded matter) is 7 to 8 μm. The solid pattern image was printed so as to be.

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.} The amount was increased and the solid pattern image was irradiated. A record was prepared in this way. When the tackiness on the surface of the coating film on the recorded material disappeared during the irradiation, it was assumed that the coating film was cured at that time.
Whether or not the feeling of tackiness disappeared was judged under the following conditions. That is, it is judged whether or not the ink adheres to the cotton swab or whether or not the solid pattern image on the recording medium is scratched, and the ink does not adhere to the cotton swab and the solid pattern image on the recording medium is scratched. When there is no scratch, the feeling of tack is eliminated. At that time, the cotton swab used was a Johnson swab manufactured by Johnson &amp; Johnson. The number of rubbing was 10 times for reciprocation, and the rubbing strength was 100 g load.
Further, the “number of passes” means the number of times that the head moves with respect to the recorded object and irradiates the coating film with ultraviolet rays from the ultraviolet irradiation device mounted on the head. 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 this by the irradiation duration [s]. The irradiation intensity was measured using an ultraviolet 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: It was cured with an irradiation energy of 200 mJ / cm ^{2 or less.}
B: It did not cure with an irradiation energy of 200 mJ / cm ^2.

From the above results, the lower layer ink containing the vinyl ether group-containing (meth) acrylic acid esters represented by the general formula (I) and the upper layer ink are contained, and the surface tension of the lower layer ink is high. An ink set (each example) that satisfies a relationship larger than the surface tension of the upper layer ink is superior in wet spreadability of the upper layer ink as compared with an ink set (each comparative example) that does not, and further, an upper layer. It was found that both the curability and ejection stability of both the ink for the lower layer and the ink for the lower layer were excellent, and the filling property of the ink for the lower layer on the recording medium was also excellent.

In addition, although not described in the above table, the lower layer of the lower layer ink 3 except that the content of DPGDA was set to 26.75% by mass and UV3500 was added as a surfactant in an amount of 0.05% by mass. An evaluation was carried out in the same manner as in the above embodiment, using the ink adjusted in the same manner as the ink 3 for the upper layer, the ink 1 for the upper layer and the ink 4 for the upper layer as the ink for the upper layer, and the ink for the lower layer as the ink for the lower layer. .. As a result, the filling property of the lower layer ink on the recording medium is B, the ejection stability of the lower layer ink is A, and the curability of the lower layer ink is A, while the upper layer inks are both the lower layer ink γ. -The upper layer ink γ was 5 [mN / m], and the wet spreadability 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 Unit, 41a, 41b, 41c, 41d, 42a, 42b, 43 ... Irradiating part, 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)
(In the formula, R ¹ is a hydrogen atom or a methyl group, R ² is a divalent organic residue having 2 to 20 carbon atoms, and R ³ is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms. It is a photocurable inkjet ink set containing a photocurable inkjet ink for a lower layer containing a vinyl ether group-containing (meth) acrylic acid ester represented by () and a photocurable inkjet ink for an upper layer. hand,
A photocurable inkjet ink set in which the surface tension of the photocurable inkjet ink for the lower layer is larger than the surface tension of the photocurable inkjet ink for the upper layer. 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 esters represented by the general formula (I) contained in the photocurable inkjet ink for the lower layer is based on the total mass of the photocurable inkjet ink for the lower layer. The photocurable inkjet ink set according to claim 1 or 2, which is 20 to 80% by mass. The photocurable inkjet ink for the upper layer contains vinyl ether group-containing (meth) acrylic acid esters represented by the general formula (I), and
The content of the vinyl ether group-containing (meth) acrylic acid ester is larger than that of the photocurable inkjet ink for the upper layer, according to any one of claims 1 to 3. 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 the lower layer contains an acrylic surfactant. The photocurable inkjet ink set according to any one of claims 1 to 5, wherein the photocurable inkjet ink for the upper layer contains a silicone-based surfactant. The coating film of the photocurable inkjet ink for the lower layer according to any one of claims 1 to 6 is formed on the 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 an upper layer.

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