JP2020114928A - 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. Among them, the ink jet method is an inexpensive apparatus and ejects ink only to a necessary image portion to directly form an image on a recording medium, so that the ink can be efficiently used and the running cost is low. Further, the ink jet method is excellent as a recording method because of low noise.

In recent years, in order to form a print having excellent water resistance, solvent resistance, scratch resistance, and the like 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. It is used.

On the other hand, a recording method in which various photo-curable inks are successively overlaid 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, and image protection and A color image may be overcoated with a colorless transparent clear ink for the purpose of imparting gloss. In such a case, a technique for obtaining an image with a desired image quality is required.

For example, Patent Document 1 discloses that after forming a printed coating film on a recording member using the energy ray curable inkjet recording color ink composition (Ia), an energy ray curable inkjet recording clear film is formed on the coating film. 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 lower layer to form a clear coating film using the ink composition (Ib). Discloses an overcoating method of an energy ray-curable ink composition satisfying the relationships of 31.5≦Sa<35.0 or 27.0≦Sa<31.5 and Sb<Sa (Reference 1). Paragraph 0009).

JP 2006-181801 A

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 the clear ink is very small. However, there is a problem in that the wettability and spreadability of the ink is poor.

Therefore, one of the objects of the present invention is to provide a photo-curable inkjet ink set that is excellent in ink wetting and spreading properties.

The inventors of the present application have made extensive studies to solve the above problems. As a result, they have 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 photo-curable inkjet ink and the upper layer photo-curable inkjet ink have the relationship as disclosed in Patent Document 1, the lower layer photo-curable inkjet ink has a predetermined value. It was found that the upper layer photocurable inkjet ink has excellent spreadability by containing the vinyl ether group-containing (meth)acrylic acid ester.

Then, the inventors of the present application have further studied, and as a result, a photocurable inkjet ink for a lower layer (hereinafter, also simply referred to as “ink for a lower layer”) containing a predetermined vinyl ether group-containing (meth)acrylic acid ester. And a photocurable inkjet ink for the upper layer (hereinafter, also simply referred to as “upper layer ink”), and 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 for a lower layer containing a vinyl ether group-containing (meth)acrylic acid ester represented by the formula (1), and a photocurable inkjet ink for an upper layer. And 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 ester represented by the general formula (I) contained in the lower layer photocurable inkjet ink is based on the total mass of the lower layer photocurable inkjet ink. The photocurable inkjet ink set according to [1] or [2], which is 20 to 80 mass %.
[4]
The photocurable inkjet ink for the upper layer 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 for the lower layer is the photocurable inkjet ink set according to any one of [1] to [4], which contains an acrylic surfactant.
[6]
The photocurable inkjet ink 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 lower layer 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 an upper layer.

FIG. 3 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 an embodiment of the present invention.

Hereinafter, modes 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.

In the present specification, the “wetting and spreading property of ink” refers to the property that the line width becomes relatively wide when the upper layer ink is linearly overlaid on the coating film of the lower layer ink. “Curing” means that when the ink containing a polymerizable compound is irradiated with radiation, the polymerizable compound is polymerized to solidify the ink. "Curable" refers to the property of curing in response to light. The “filling property” is also referred to as filling property, and refers to the property that the recording medium as the base cannot be seen when the recorded product is viewed from the side where the cured product (image) is formed. The “ejection stability” refers to the property of always ejecting stable ink droplets from the nozzle without clogging of the nozzle. “Storage stability” refers to the property that the viscosity of an ink does not easily change before and after the storage.

In the present specification, the “lower layer” and the “upper layer” merely represent the relative positional relationship in the image obtained by applying the ink repeatedly, and may be rephrased as “undercoat” and “topcoat”, respectively. it can.

In the present specification, "(meth)acrylate" means at least one of acrylate and corresponding methacrylate, "(meth)acryloyl" means at least one of acryloyl and corresponding methacryloyl, and " “(Meth)acrylic” means at least one of acrylic and methacrylic corresponding thereto.

In the present specification, the “recorded material” refers to a material in which ink is recorded on a recording medium to form a cured material. Further, in the present specification, a cured product means a cured substance, and a coating film of ink means a film obtained by coating and curing ink, that is, a cured coating film. Curing in the coating film of the photocurable inkjet ink for the lower layer when forming the coating film of the photocurable inkjet ink for the lower layer described below and forming the coating film of the photocurable inkjet ink for the upper layer on the coating film Indicates a state in which it is determined that the resin has been cured in the curability test described later, among curing. 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 on all the pixels that are the minimum recording unit area defined by the recording resolution.

[Photo-curable 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 an upper layer. And a photo-curable inkjet ink. In addition to this, the ink set satisfies the relationship that the surface tension of the lower layer ink is higher than that of the upper layer ink.
The ink set may further include an ink other than the photocurable inkjet ink for the lower layer and the photocurable inkjet ink for the upper layer.

First, the 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 lower layer ink”−“upper layer The surface tension of the ink for use") is preferably 5 or more, more preferably 10 or more. When the difference is within the above range, the wettability and spreadability of the ink becomes further 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, provided that the above relationships are satisfied. That is, it is preferable that both the surface tension of the upper layer ink and the lower layer ink be 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. 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, respectively, 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 this specification, a value obtained by the measuring method performed in the below-mentioned Example section is adopted.

Hereinafter, additives (components) contained in the ink (composition) according to the present embodiment or that may be contained therein 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 ink for the upper layer and the ink for the lower layer in the present embodiment can be polymerized at the time of irradiation with radiation by the action of the photopolymerization initiator described later to cure the printed ink.

(1. Vinyl ether group-containing (meth)acrylic acid esters)
Among the inks in this embodiment, at least the lower layer ink has the following general formula (I) as a polymerizable compound:
_{^{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 to 1 to 11 carbon atoms. A vinyl ether group-containing (meth)acrylic acid ester represented by the formula (1).

By containing the vinyl ether group-containing (meth)acrylic acid ester in the ink, the curability of the ink can be made excellent and the viscosity of the ink can be lowered. Furthermore, rather than using a compound having a vinyl ether group and a compound having a (meth)acrylic group separately, it is better to use a compound having both a vinyl ether group and a (meth)acrylic group in one molecule. It is preferable for improving the curability.

In particular, since the lower layer ink contains the vinyl ether group-containing (meth)acrylic acid ester, the upper layer ink has excellent wettability and spreadability, and further, the lower layer ink curability and the lower layer ink on the recording medium are excellent. 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 ester, but it is preferable to contain the following effects 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. Or may contain different compounds.

In the above general formula (I), the divalent organic residue having 2 to 20 carbon atoms represented by R ² may be linear, branched or cyclic substituted having 2 to 20 carbon atoms. An alkylene group which may be substituted, an alkylene group having 2 to 20 carbon atoms which may have an oxygen atom by an ether bond and/or an ester bond in the structure, and an optionally substituted divalent C 6 to 11 carbon atom Aromatic groups are preferred. Among these, alkylene groups having 2 to 6 carbon atoms such as ethylene group, n-propylene group, isopropylene group, and butylene group, oxyethylene group, oxyn-propylene group, oxyisopropylene group, and oxybutylene group. An alkylene group having 2 to 9 carbon atoms having an oxygen atom by 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 may be linear, branched or cyclic substituted having 1 to 10 carbon atoms. A good alkyl group and an optionally substituted aromatic group having 6 to 11 carbon atoms are preferable. 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 a group which may be substituted, the substituent is divided into a group containing a carbon atom and a group containing no carbon atom. First, when the substituent is a group containing a carbon atom, the carbon atom is counted in the carbon number 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.

Examples of the vinyl ether group-containing (meth)acrylic acid esters include, but are not limited to, 2-vinyloxyethyl (meth)acrylate, 3-vinyloxypropyl (meth)acrylate, 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-methacrylate 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) 2-(vinyloxyisopropoxyethoxy)propyl acrylate, 2-(vinyloxyisopropoxyisopropoxy)propyl (meth)acrylate, 2-(vinylo)methacrylate Xyethoxyethoxy)isopropyl, (meth)acrylic acid 2-(vinyloxyethoxyisopropoxy)isopropyl, (meth)acrylic acid 2-(vinyloxyisopropoxyethoxy)isopropyl, (meth)acrylic acid 2-(vinyloxyisopropoxy) Isopropoxy)isopropyl, 2-(vinyloxyethoxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxy)methacrylate ) Ethyl, 2-(isopropenoxyethoxyethoxy)ethoxy (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy) (meth)acrylate Examples include 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), because the ink can have a lower viscosity, 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 each have a simple structure and a small molecular weight, so that the viscosity of the ink can be remarkably lowered. 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. Note that 2-(vinyloxyethoxy)ethyl acrylate is superior in curability to 2-(vinyloxyethoxy)ethyl methacrylate.

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

The content of the vinyl ether group-containing (meth)acrylic acid esters contained in the lower layer ink is preferably 20 to 80% by mass, and 30 to 80% by mass with respect to 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 excellent, 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 ester, the content of the vinyl ether group-containing (meth)acrylic acid ester is lower in the lower layer ink. The ink for the upper layer is preferably larger than the ink for the upper layer, and more preferably 10% by mass or more for the ink for the upper layer. This makes it possible to further improve the wettability and spreadability of the upper layer ink and to provide an ink set having various excellent performances.
In addition, the content of the upper layer ink in the above case is not particularly limited because it is preferably determined in consideration of performances 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 ester, the coating of the lower layer ink is performed. The reason why the wettability of the upper layer ink on the film is excellent is that the coating film derived from the vinyl ether group of the vinyl ether group-containing (meth)acrylic acid ester has some influence on the wettability of the upper layer ink. Presumed to be. 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 for esterifying (meth)acrylic acid and a hydroxyl group-containing vinyl ether (production method B), (meth)acrylic acid halide. With a hydroxyl group-containing vinyl ether (Production Method C), with a (meth)acrylic acid anhydride with a hydroxyl group-containing vinyl ether (Production Method D), with a (meth)acrylic acid ester and a hydroxyl group-containing vinyl ether Exchange method (Production method E), Method of esterifying (meth)acrylic acid with halogen-containing vinyl ether (Production method F), Method of esterifying alkali (earth) metal salt of (meth)acrylic acid with halogen-containing vinyl ether (Production method G), a method of vinyl-exchange a hydroxyl group-containing (meth)acrylic acid ester with vinyl carboxylate (production method H), and a method of ether-exchange a hydroxyl group-containing (meth)acrylic acid ester with an alkyl vinyl ether (production method I). Can be mentioned.

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

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

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

Among the (meth)acrylates, examples of monofunctional (meth)acrylates include 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 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, 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, neopentyl glycol di(meth)acrylate, dimethylol-tricyclodecane di(meth)acrylate, EO (ethylene oxide) adduct of bisphenol A di(meth)acrylate, PO of bisphenol A (propylene oxide) ) Adducts di(meth)acrylate, hydroxypivalic acid neopentyl glycol di(meth)acrylate, and polytetramethylene glycol di(meth)acrylate. Among these, dipropylene glycol di(meth)acrylate is preferable.

Among the above (meth)acrylates, examples of the trifunctional or higher polyfunctional (meth)acrylates include trimethylolpropane tri(meth)acrylate, EO-modified trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate. , Pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, glycerin propoxytri(meth)acrylate, cowprolactone modified trimethylolpropane tri(meth)acrylate, pentaerythritol Ethoxy tetra (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 ink has a low viscosity, the solubility of the photopolymerization initiator and other additives is excellent, and good ejection stability during inkjet recording is 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 together, and among them, phenoxyethyl (meth)acrylate and dipropylene glycol. 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, and even 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 reduced, and the solubility and reactivity of the photopolymerization initiator can be made excellent.

[Surfactant]
The ink according to this embodiment preferably contains a surfactant. In particular, when the upper layer ink contains a surfactant, it is possible to more surely satisfy the relationship that the surface tension of the lower layer ink is larger than the surface tension of the upper layer ink. Furthermore, not only the ink for the upper layer but also the ink for the lower layer contains a surfactant, so that the burying property of the ink for the lower layer on the recording medium becomes excellent.

Examples of the surfactant include, but are not limited to, silicone-based surfactants, acrylic surfactants, cationic surfactants, anionic surfactants, nonionic surfactants, amphoteric surfactants, And fluorine-based surfactants. Among these, the surface tension can be significantly reduced, and damage to the members in contact with the ink such as the head and the ink supply device can be prevented, so that the silicone-based surfactant and the acrylic-based surfactant can be used. At least one of the agents is preferred.

The above-mentioned silicone-based surfactant is not limited to the following, but for example, modified silicone such as polyester-modified silicone and polyether-modified silicone can be used. Among these, the polyether-modified silicone is preferable because the surface tension can be greatly reduced and the wettability and spreadability of the ink on the recording medium can be further improved.

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 products of the above silicone-based surfactants include BYK-347, BYK-348, BYK-UV3500, BYK-UV3510, BYK-UV3530, BYK-UV3570 (these are trade names of BYK). To be Among these, BYK-UV3500 is preferable because the wettability and spreadability of the upper layer ink is further improved.

The above-mentioned acrylic surfactant is not limited to the following, but (meth)acrylic acid-based copolymers are preferable.

Examples of commercially available products of the above-mentioned 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, Kusumoto Kasei Co., Ltd. trade name), BYK-350, BYK-352. , BYK-354, BYK-355, BYK-358, BYK-380, BYK-381, BYK-392 (above, trade name by BYK), Polyflow No. 57, 95 (above, trade name of Kyoeisha Oil and Fat Chemical Co., Ltd.) and the like. 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.

In the following first to third cases, the combination of the surfactants contained in each of the upper layer ink and the lower layer ink, or contained therein, can more surely satisfy the above-mentioned surface tension relationship. Any of the above is preferable, and the following third case is more preferable.

First, it is preferable that the upper layer ink and the lower layer ink contain the same surfactant, and that the content of the surfactant in the upper layer ink is larger than that in the lower layer ink. 0.2 mass% or more is preferable and, as for the difference of the said content, 0.3 mass% or more is more preferable.
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 wettability and spreadability 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-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 it is larger than the lower layer ink. 0.2 mass% or more is preferable and, as for the difference of the said content, 0.3 mass% or more is more preferable.

As the modified silicone surfactant in the second case, the upper layer ink and the lower layer ink are independently of each other preferably at least one of a polyether modified silicone and a polyester modified silicone, and a polyether modified silicone is More preferable. Further, it is preferable to use the same modified silicone type 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 in the lower layer ink is more preferably 0.1 to 0.3% by mass.
In the case of the second case, the filling property of the lower layer ink on the recording medium is excellent.

Thirdly, the upper layer ink preferably contains a modified silicone surfactant. On the other hand, the lower layer 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 surfactant, the upper layer ink is more excellent in the wettability and spreadability. Examples of the modified silicone-based surfactant include, but are not limited to, polyether-modified silicone and polyester-modified silicone, and among them, they are easily available and have a lower layer ink burying property on a recording medium. Polyether-modified silicone is preferable because it is 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 in the lower layer ink is more preferably 0.1 to 0.3% by mass.
In the third case, the wettability and spreadability of the ink is further improved, which is advantageous in that a 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 for curing the ink existing on the surface of the recording medium by photopolymerization by irradiation of radiation to form a print. By using ultraviolet rays (UV) among radiation (light), 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 polymerization of the polymerizable compound, but a photoradical polymerization initiator or a photocationic polymerization initiator should be used. In particular, it is preferable to use a photoradical polymerization initiator.

Examples of the above-mentioned radical photopolymerization initiator include aromatic ketones, acylphosphine oxide compounds, aromatic onium salt compounds, organic peroxides, thio compounds (thioxanthone compounds, thiophenyl group-containing compounds, etc.), and hexaarylbi compounds. Examples thereof include an imidazole compound, a ketoxime ester compound, a borate compound, an azinium compound, a metallocene compound, an active ester compound, a compound having a carbon-halogen bond, and an alkylamine compound.

Among these, at least an acylphosphine oxide compound is preferably used 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-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy-2-phenylacetophenone, xanthone, fluorenone, benzaldehyde, fluorene, anthraquinone, triphenylamine. , Carbazole, 3-methylacetophenone, 4-chlorobenzophenone, 4,4'-dimethoxybenzophenone, 4,4'-diaminobenzophenone, Michler's ketone, benzoinpropyl ether, benzoinethyl ether, benzyldimethylketal, 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- Examples include phosphine oxide, 2,4-diethylthioxanthone, and bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.

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), 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-cyclopentadiene-1-yl) -Bis(2,6-difluoro-3-(1H-pyrrol-1-yl)-phenyl)titanium), IRGACURE OXE 01 (1.2-octanedione, 1-[4-(phenylthio)-,2-( O-benzoyl oxime)]), IRGACURE OXE 02 (ethanone, 1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl]-, 1-(O-acetyloxime)), IRGACURE 754 (a mixture of oxyphenylacetic acid, 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, Lambson), KAYACURE DETX-S (2,4-diethylthioxanthone) (Nippon Kayaku Co., Ltd.), Lucirin TPO, LR8893, LR8970 (above, BASF), and Ubecryl. P36 (manufactured by UCB) and the like can be mentioned.

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

The content of the photopolymerization initiator can improve the ultraviolet curing rate and make the curability excellent, and to avoid undissolved photopolymerization initiator or 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 the present embodiment may further contain a polymerization inhibitor. When the ink contains the polymerization inhibitor, the polymerization reaction of the polymerizable compound before curing can be prevented, and the storage stability of the ink can be made excellent.

The above-mentioned polymerization inhibitor is not limited to the following. 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), 4,4'-thiobis(3-methyl-6-t-butylphenol) and other phenolic polymerizations Inhibitors, as well as hindered amines, hydroquinone monomethyl ether (MEHQ), and hydroquinone.

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

[Coloring material]
The ink in this embodiment may contain a coloring material. At least one of a pigment and a dye can be used as the coloring material.

(Pigment)
In this embodiment, the light resistance of the ink can be improved by using the pigment as the coloring material. As the pigment, both an inorganic pigment and 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, azo lakes, chelate azo pigments, phthalocyanine pigments, perylene and perinone pigments, anthraquinone pigments, quinacridone pigments, dioxane pigments, thioindigo pigments, isoindolinone pigments, and quinophthalone pigments. Polycyclic pigments, dye chelates (for example, basic dye chelates, acid dye chelates, etc.), dye lakes (basic dye lakes, acid dye lakes), nitro pigments, nitroso pigments, aniline black, daylight Examples include fluorescent pigments.

More specifically, as carbon black used as a black ink, No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52, MA7, MA8, MA100, No. 52. 2200B etc. (above, trade name manufactured by Mitsubishi Chemical Corporation), Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven 1255, Raven 700 etc. (above, made by Columbia Carbon (Carbona Carbon Colomba)). 400R, Regal1 330R, Regal1 660R, Mogul L, Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400, etc. Name), Color Black FW1, Color Black FW2, Color Black FW2V, Color Black FW18, Color Black FW200, Color B1ack ri Puri Spl, Color Black SFW, Color Black FW2, Color Black FW2, Color Black FW2, Color Black FW200 , Special Black 5, Special Black 4A, Special Black 4 and the like (these are trade names manufactured by Degussa).

Examples of pigments used in white ink include C.I. I. Pigment White 6, 18, 21.

Examples of 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.

Examples of 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 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. Butt Blue 4, 60 may be mentioned.

Examples of pigments other than magenta, cyan, and yellow include 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 alone or in combination of two or more.

When the above pigment is used, its average particle size is preferably 300 nm or less, more preferably 50 to 200 nm. When the average particle diameter is within the above range, reliability such as ejection stability and dispersion stability of the ink is further excellent, and an image with excellent image quality can be formed. Here, the average particle diameter 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 an acid dye, a direct dye, a reactive dye, and a basic dye can be used. 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. Dilectd 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 properties and color reproducibility are obtained.

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

As described above, according to the present embodiment, since the line width of the upper layer ink that is overcoated on the coating film of the lower layer ink does not become narrow, a photocurable inkjet ink set having excellent wettability and spreadability of the upper layer ink is provided. Can be provided. Furthermore, according to the present embodiment, a photocurable inkjet ink set is provided which is excellent in curability and ejection stability of both the upper layer ink and the lower layer ink, and the burying property of the lower layer ink on the recording medium. can do. As described above, in the photo-curable inkjet ink set, the lower layer ink and the upper layer ink are in a relationship of affecting 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 can be used to obtain a recorded matter by being ejected onto a recording medium using an inkjet recording method described later. Examples of this recording medium include an absorptive or non-absorptive recording medium. The inkjet recording method of the embodiment described later can be widely applied to recording media having various absorption performances, from non-absorbent recording media in which it is difficult for ink to penetrate to absorptive recording media in which ink can penetrate easily. ..

The absorptive recording medium is not particularly limited, but for example, plain paper such as electrophotographic paper having high ink permeability, inkjet paper (ink absorbing layer composed of silica particles or alumina particles, or polyvinyl alcohol ( (PVA) or polyvinylpyrrolidone (PVP), an ink jet dedicated paper provided with an ink absorbing layer composed of a hydrophilic polymer) to an art paper, a coated paper, which is used for general offset printing with relatively low ink permeability. Examples include cast paper.

The non-absorbent recording medium is not particularly limited, and examples thereof include films, sheets, and plates of plastics such as polyvinyl chloride (PVC), polyethylene, polypropylene, polyethylene terephthalate (PET), iron, silver, copper, Examples thereof include plates of metals such as aluminum, metal plates produced by vapor deposition of these various metals, films made of plastic, plates of alloys such as stainless steel and brass.

[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 in the inkjet recording method of the present embodiment.

[Inkjet recording apparatus for implementing 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 unit.

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

The recording control unit controls the ink amount 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 apparatus that can be used in this embodiment. The continuous sheet-shaped recording medium S is transported from the upstream side to the downstream side in the transport direction along with the rotation of the upstream roller 12A, the transport drum 12, and the downstream roller 12B. The head unit is a first white head unit 30A that ejects the white ink W (on the left side of FIG. 1 on the paper surface), a yellow head unit 30B that ejects the yellow ink Y, and a magenta head unit that ejects the 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 (on the paper surface, the one on the right side in FIG. 1). Equipped with. The irradiation unit 40 includes, for each head unit, irradiation units 42a, 41a, 41b, 41c, 41d, 42b provided on the downstream side in the carrying direction, and an irradiation unit 43 provided last in the carrying direction. There is. Such a recording device can be configured as shown in FIG. 2 of JP 2010-208218 A, for example.

Here, in the case where the first recording mode is performed, first, in the order in which the recording medium S is conveyed with respect to the recording medium S conveyed to a position facing each head unit on the conveying drum, The lower layer ink is ejected from the first white head unit 30A and is irradiated from the irradiation section 42a to form a coating film of the lower layer ink. Next, the upper layer ink is ejected onto the coating film from at least one of the yellow head unit 30B, the magenta head unit 30C, the cyan head unit 30D, and the black head unit 30E, and is provided on the downstream side of the head. Irradiation is performed from the irradiated unit. 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, in the order in which the recording medium S is conveyed with respect to the recording medium S conveyed to a position facing each head unit on the conveying drum, the yellow recording medium S is first transferred. Ink for the upper layer 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 an irradiation unit provided on the downstream side of the head to perform the upper layer ink. A coating film 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 section 42a are not used.

When the coating layer of the lower layer ink has a solid pattern, the recording control unit preferably has a lower ink amount per dot of the lower layer ink in the second recording mode than in the first recording mode. It may be adjusted to be 10% or more, more preferably 10 to 50% larger. As a result, even in the second recording mode, the same wetting and spreading property of the lower layer ink as in the first recording mode is ensured. The one 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 do not have to be provided. Further, among the inks, the ink that is to be subsequently formed into a coating film, that is, the upper layer ink in the first recording mode and the lower layer ink in the second recording mode, is the irradiation unit (41a) provided on the downstream side of the head unit. ˜41d, 42b) and/or irradiation by the irradiation unit 43, the curing may be performed by at least one of the irradiation by the irradiation unit provided on the downstream side of the head unit. 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. It is an ink that forms a coating film on the coating film by the lower layer ink. One of the upper layer ink and the lower layer ink 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. 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 performance such as adhesion, wet spreadability, and abrasion resistance of the main ink. Good.
Here, the upper layer ink and the lower layer ink may both be main inks, or both may be auxiliary inks, but when one is used as the main ink and the other is used as the auxiliary ink, the use of recorded matter is widened, In addition, the performance of the main ink is improved, which is preferable. When one is the main ink and the other is the auxiliary ink, which of the upper layer ink and the lower layer ink is to be used as the main ink may be determined in consideration of the properties of each of the above-described main ink and auxiliary ink. Therefore, in FIG. 1 described above, 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 ink amount is made different between the first recording mode and the second recording mode because the line width of the lower layer ink on the coating film of the upper layer ink becomes narrow in the second recording mode. Therefore, the filling property of the solid pattern image of the lower layer ink is deteriorated. Therefore, by increasing the ink amount as described above, the filling property of the lower layer ink can be improved.

[Inkjet recording method using inkjet recording apparatus]
The ink jet 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 an upper layer ink of the above embodiment is applied over the coating film. And 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 discharging step of discharging the ink onto the recording medium and a curing step of irradiating the discharged ink with ultraviolet rays to cure the ink. It is a waste. In this way, an image, that is, a cured ink coating film is formed by the ink cured on the recording medium. Hereinafter, these stages will be described.

[Discharging 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 at the time of ejection is preferably 15 mPa·s or less, more preferably 12 mPa·s or less. If the viscosity of the ink is as described above when the temperature of the ink is room temperature or when the ink is not heated, the temperature of the ink may be room temperature or the ink may be ejected without being heated. At that time, the temperature of the ink during ejection is preferably 20 to 30°C. 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.

Since the lower layer ink and the upper layer ink in this embodiment are both photo-curable inks and have higher viscosities than ordinary water-based inks, the viscosity fluctuations due to temperature fluctuations during ejection are large. Such fluctuations in the viscosity of the ink have a great influence on the change in the droplet size and the change in the droplet ejection speed, which can lead to deterioration in image quality. Therefore, it is preferable to keep the temperature of the ink during ejection as constant as possible.

[Curing stage]
Next, in the curing step, the ink ejected and adhered (landed) to the recording medium is cured by irradiation of ultraviolet rays. This is because the photopolymerization initiator that can be contained in the ink is decomposed by irradiation of light (ultraviolet rays) to generate a starting species such as a radical, an acid, and a base, and the polymerization reaction of the polymerizable compound is This is because it is promoted by the function. Alternatively, it is because the polymerization reaction of the polymerizable compound is started by the irradiation of ultraviolet rays. At this time, if the 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 promotes decomposition of the photopolymerization initiator by contact with the photopolymerization initiator. And a curing reaction with higher sensitivity can be achieved.

A mercury lamp, a gas/solid laser, etc. are mainly used as an ultraviolet ray source, and a mercury lamp and a metal halide lamp are widely known as a light source used for curing a photocurable inkjet ink. On the other hand, from the viewpoint of environmental protection, there is a strong demand for mercury-free materials at present, and the replacement with a GaN-based semiconductor ultraviolet light-emitting device is industrially and environmentally very useful. Further, the ultraviolet light emitting diode (UV-LED) and the ultraviolet laser diode (UV-LD) have small size, long life, high efficiency, and low cost, and are expected as a light source for photocurable inkjet. Among these, UV-LED is preferable.

Here, since it is easy to increase the output of the LED, a photo-curable inkjet ink that can be cured with irradiation energy of preferably 200 mJ/cm ² or less by using a UV-LED having an emission peak wavelength of preferably 350 to 420 nm. It is preferable to use the above set for the ink jet recording method. In this case, low cost printing and high printing speed can be realized. Such an ink is obtained by including at least one of a photopolymerization initiator that decomposes by irradiation with ultraviolet rays in the above wavelength range and a polymerizable compound that initiates polymerization by irradiation with ultraviolet rays in the above wavelength range.

As described above, according to the present embodiment, the wettability and spreadability of the upper layer ink, the curability and ejection stability of both the upper layer ink and the lower layer ink, and the burying property of the lower layer ink on the recording medium are improved. It is also possible to provide an excellent inkjet recording method using a photo-curable inkjet ink set.

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

[Materials used]
The materials used in the following examples and comparative examples are as follows.
[Pigment]
C. I. Pigment Blue 15:3 (Phthalocyanine Blue, manufactured by DIC, abbreviated as "PB15:3" below.)
C. I. Pigment White 6 (titanium oxide, manufactured by Teika, abbreviated as "PW6" below).
[Polymerizable compound]
(1. Vinyl ether group-containing (meth)acrylic acid esters)
-VEEA (2-(2-vinyloxyethoxy)ethyl acrylate, trade name of Nippon Shokubai Co., Ltd., abbreviated as "VEEA" below.)
(2. Other polymerizable compounds)
-Biscoat #192 (phenoxyethyl acrylate, trade name of OSAKA ORGANIC CHEMICAL INDUSTRY LTD., abbreviated as "PEA" below)
-NK Ester APG-100 (trade name of SHIN-NAKAMURA CHEMICAL CO., LTD., dipropylene glycol diacrylate, abbreviated as "DPGDA" below).
[Polymerization inhibitor]
・MEHQ (hydroquinone monomethyl ether, manufactured by Kanto Chemical Co., Inc.)
[Surfactant]
BYK-UV3500 (trade name by BYK, silicone type, abbreviated as "UV3500" below)
BYK-UV3570 (trade name by BYK, silicone type, abbreviated as "UV3570" below).
BYK-350 (trade name by BYK, acrylic, abbreviated as "BYK350" below)
BYK-381 (trade name by BYK, acrylic, abbreviated as "BYK381" below)
[Photopolymerization initiator]
-IRGACURE 819 (trade name, manufactured by BASF, solid content 100%, abbreviated as "819" below).
DAROCUR TPO (trade name by BASF, solid content 100%, abbreviated as "TPO" below)
KAYACURE DETX-S (trade name by Nippon Kayaku Co., solid content 100%, abbreviated as "DETX" below).

[Preparation of upper layer ink and lower layer ink]
Each material was added so as to have the composition (unit: mass%) described in Tables 1 and 2 below, and the resulting mixture was stirred with a high-speed water-cooled stirrer to give a cyan upper layer ink and a white lower layer. An 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. Further, Table 2 shows the composition of each lower layer ink, and the lower layer inks 1, 2, 3, 4, 5, 6, and 7 will be described below as the lower layers 1, 2, 3, 4, 5, 6 respectively. , And 7 are abbreviated.

[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 described in the upper part of each of Tables 3 and 4 below. The surface tension γ (mN/m) of the ink for the upper layer and the ink for the lower layer in each of Examples and Comparative Examples was measured by a Wilhelmy method using a surface tensiometer CBVP-Z (trade name, manufactured by Kyowa Interface Science Co., Ltd.). The values are measured at room temperature under normal pressure and are shown in Tables 3 and 4 below.

[Evaluation item]
With respect to the ink sets prepared in each of the examples and each of the comparative examples, the wetting and spreading property, the filling property, the ejection stability, and the curability were evaluated by the following methods.

(1. wettability)
The lower layer ink was ejected toward a 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 (one dot row) was formed on the cured product under the conditions of 720 dpi and 10 ng/dot using the upper layer ink.

It should be noted that the temperature of each ink was adjusted so that each ink had a viscosity of 10 mPa·s before being ejected. 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 (PET 50A) under the conditions of 720×720 dpi and 10 ng/dot to cure.

The temperature of each ink was adjusted so that the viscosity was 10 mPa·s, and then the ink was ejected. This evaluation was carried out by visually observing the burying property of the obtained cured product on the lower layer ink.
The evaluation criteria are as follows. The evaluation results are shown in Tables 3 and 4 below.
A: Visually observed from a height of 30 cm from the recording medium, it was buried (no streaks in the background were visible).
B: The streak of the base was visually observed 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. Each of the upper layer ink and the lower layer ink was continuously ejected for 60 minutes from a head having 180 nozzles. In this way, the ejection stability of the head when each of the upper layer ink and the lower layer ink was ejected was evaluated.
The evaluation criteria are as follows. The evaluation results are shown in Tables 3 and 4 below.
A: No nozzle omission occurred during continuous discharge.
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. An ink jet recording apparatus equipped with a piezo type ink jet nozzle was used to fill each nozzle row with the ink for the upper layer and the ink for the lower layer in each of Examples and Comparative Examples. 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 thickness of the printed matter (recorded matter) is 7 to 8 μm The solid pattern image was printed so that

After that, from the UV-LED in the ultraviolet irradiation device mounted beside the carriage, the irradiation energy of the irradiation intensity of 1 W/cm ² and the peak wavelength of 395 nm is gradually increased up to 200 mJ/cm ² per pass. The amount was increased and irradiation was performed toward the solid pattern image. A recorded matter was produced in this manner. During the irradiation, when the tackiness of the coating film surface on the recorded material disappeared, it was determined to be cured at that time.
Whether or not the tackiness disappeared was judged under the following conditions. That is, it is determined whether or not the ink adheres to the swab, or whether or not the solid pattern image on the recording medium is scratched, the ink does not adhere to the swab, and the solid pattern image on the recording medium is When there was no scratch, the tackiness was eliminated. At that time, the cotton swab used was a Johnson swab manufactured by Johnson &amp; Johnson. The number of times of rubbing was 10 reciprocations, and the rubbing strength was 100 g load.
Further, the "pass number" means the number of times that the head moves with respect to the recorded matter and ultraviolet rays are irradiated toward the coating film 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 surface to be irradiated by the light source, and calculating the product of this and 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: Cured with irradiation energy of 200 mJ/cm ² or less.
B: It was not cured by 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, and the surface tension of the lower layer ink is The ink set (each example) satisfying the relationship larger than the surface tension of the upper layer ink has excellent wettability and spreadability of the upper layer ink as compared with the ink set not having the same (each comparative example). It was found that the curability and ejection stability of both the ink for the lower layer and the ink for the lower layer and the burying property of the ink for the lower layer on the recording medium are excellent.

Further, although not shown in the above table, the lower layer ink 3 except that the DPGDA content was 26.75% by mass and UV3500 as a surfactant was added at 0.05% by mass. Using the ink prepared in the same manner as the ink for ink 3, the ink for the upper layer 1 and the ink for the upper layer 4 were used as the ink for the upper layer, and the lower layer ink was used 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 both of the upper layer inks are lower layer ink γ. -The upper layer ink γ was 5 [mN/m], and the upper layer ink had a wettability and spreadability of 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):
_{^{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 for a lower layer containing a vinyl ether group-containing (meth)acrylic acid ester represented by the formula (1), and a photocurable inkjet ink for an upper layer. hand,
A photocurable inkjet ink set, wherein the surface tension of the lower layer photocurable inkjet ink is larger than the surface tension of the upper layer photocurable inkjet ink. 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 lower layer photocurable inkjet ink is based on the total mass of the lower layer photocurable inkjet ink. The photocurable inkjet ink set according to claim 1, which is 20 to 80 mass %. The photocurable inkjet ink for the upper layer 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 greater than that of the photocurable inkjet ink for the lower layer than that of 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 the 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-based surfactant. The coating film of the photocurable inkjet ink for lower layer 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 an upper layer.

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