JP6311761B2 - Photocurable inkjet ink set and inkjet recording method using the same - Google Patents

Description

  The present invention relates to a photocurable ink jet ink set and an ink jet 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 these, the ink jet system is an inexpensive apparatus, and ink is ejected only to a required image portion to form an image directly on a recording medium. Therefore, the ink can be used efficiently, and the running cost is low. Furthermore, the inkjet method is excellent as a recording method because of its low noise.

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

  On the other hand, a recording method in which various photocurable inks are successively overcoated has been widely used. For example, a color image can be formed by overlaying inks of different colors such as cyan (C), magenta (M), yellow (Y), black (K), and white (W), In some cases, a colorless and transparent clear ink is 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 discloses a clear coating for energy beam curable ink jet recording after a printed coating film is formed on a recording member using the energy ink curable color ink composition (Ia). 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 of overcoating 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). In the description of 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. The problem is that the wettability of the ink is poor.

  Accordingly, an object of the present invention is to provide a photocurable inkjet ink set having excellent wettability of ink.

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

  Therefore, the inventors of the present application have further studied, and as a result, a lower-layer photocurable inkjet ink (hereinafter, also simply referred to as “lower-layer ink”) containing a predetermined vinyl ether group-containing (meth) acrylic acid ester. A photocurable inkjet ink set for the upper layer (hereinafter also referred to simply as “upper layer ink”), and the surface tension of the lower layer ink is larger than the surface tension of the upper layer ink. (Hereinafter, also simply referred to as “ink set”), the inventors have found that the above problems can be solved, and completed the present invention.

That is, the present invention is as follows.
[1]
The following general formula (I):
_{^{CH 2 = CR 1 -COOR 2 -O}} -CH = CH-R 3 ··· (I)
Wherein R ¹ is a hydrogen atom or a methyl group, R ² is a divalent organic residue having 2 to 20 carbon atoms, and R ³ is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms. Group.)
A photocurable inkjet ink set comprising: a lower layer photocurable inkjet ink containing a vinyl ether group-containing (meth) acrylic acid ester represented by: and an upper layer photocurable inkjet ink; A photocurable inkjet ink set, wherein the surface tension of the photocurable inkjet ink is larger than the surface tension of the upper layer photocurable inkjet ink.
[2]
The photocurable inkjet ink set according to [1], wherein a difference between a surface tension of the lower layer photocurable inkjet ink and a surface tension of the upper layer photocurable inkjet ink 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% by mass.
[4]
The photocurable inkjet ink for the upper layer contains a vinyl ether group-containing (meth) acrylic acid ester represented by the general formula (I), and the content of the vinyl ether group-containing (meth) acrylic acid ester is The photocurable inkjet ink set according to any one of [1] to [3], wherein the lower layer photocurable inkjet ink is larger than the upper layer photocurable inkjet ink.
[5]
The photocurable inkjet ink set according to any one of [1] to [4], wherein the lower layer photocurable inkjet ink contains an acrylic surfactant.
[6]
The photo-curable inkjet ink set according to any one of [1] to [5], wherein the upper-layer photocurable inkjet ink contains a silicone-based surfactant.
[7]
A coating film of the photocurable inkjet ink for a lower layer according to any one of [1] to [6] is formed on a recording medium, and the 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 apparatus that can be used in an embodiment of the present invention.

  Hereinafter, embodiments for carrying out the present invention will be described in detail. In addition, this invention is not restrict | limited to the following embodiment, A various deformation | transformation can be implemented within the range of the summary.

  In the present specification, “wetting and spreading properties of ink” refers to a property in which the line width becomes relatively wide when the upper layer ink is linearly applied on the lower layer ink coating. “Curing” means that when an ink containing a polymerizable compound is irradiated with radiation, the polymerizable compound is polymerized and the ink is solidified. “Curable” refers to the property of curing in response to light. “Filling property” is also referred to as filling property, and refers to a property in which a recording medium as a base cannot be seen when a recorded material is viewed from a side on which a cured product (image) is formed. “Ejection stability” refers to the property of ejecting stable ink droplets from a nozzle without clogging the nozzle. “Storage stability” refers to the property that the viscosity before and after storage is difficult to change when the ink is stored.

  In this specification, “lower layer” and “upper layer” merely represent relative positional relationships in an image obtained by overcoating ink, and may be rephrased as “undercoating” and “overcoating”, respectively. it can.

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

In this specification, “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, a cured product means a cured substance, and an ink coating film means a film formed by applying and curing ink, that is, a cured coating film. Curing of the light-curable inkjet ink for the lower layer when a coating film for the light-curable inkjet ink for the lower layer described below is formed and the film of the photo-curable inkjet ink for the upper layer is formed on the coating film. Means a state judged to have been cured by a curing test described later. The coating film may be a solid pattern image or a design pattern image such as a picture, character, or pattern.
Note that the “solid pattern image” in the present specification means an image in which dots are recorded for all of 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 includes a lower layer photocurable inkjet ink containing a vinyl ether group-containing (meth) acrylic acid ester represented by the following general formula (I), and an upper layer: And a photocurable inkjet ink. In addition, 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 include an ink other than the lower layer photocurable inkjet ink and the upper layer photocurable inkjet ink.

  First, the surface tension will be described. The ink set of the present embodiment needs to satisfy the above-mentioned relationship. On that basis, 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, and more preferably 10 or more. When the difference is within the above range, the ink wettability is further improved.

  It can be said that each surface tension of the lower layer ink and the upper layer ink is preferable when it is within the following range on the premise that the above relationship is satisfied. That is, it is preferable that the surface tension 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 More preferably, it is 25-40 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 ink jet head is excellent.

  Here, the surface tension of the ink is determined by its composition (type and content of each component), and the main factor is the type and content of the surfactant contained in the ink. The surfactant will be described later. In addition, the value obtained by the measuring method performed in the below-mentioned Example column is employ | adopted for the surface tension in this specification.

  Hereinafter, additives (components) that are or can be included in the ink (composition) in the present embodiment will be described. Hereinafter, 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 radiation irradiation by the action of a photopolymerization initiator described later, and the printed ink can be cured.

(1. Vinyl ether group-containing (meth) acrylic acid esters)
Among the inks in the present embodiment, at least the lower layer ink is a polymerizable compound represented by the following general formula (I):
_{^{CH 2 = CR 1 -COOR 2 -O}} -CH = CH-R 3 ··· (I)
(In the above formula (I), R ¹ is a hydrogen atom or a methyl group, R ² is a divalent organic residue having 2 to 20 carbon atoms, and R ³ is a hydrogen atom or 1 to 11 carbon atoms. Valent organic residue.)
The vinyl ether group containing (meth) acrylic acid ester represented by these is contained.

  When the ink contains the vinyl ether group-containing (meth) acrylic acid esters, the curability of the ink can be improved, and the ink can be reduced in viscosity. In addition, it is better to use a compound having both a vinyl ether group and a (meth) acryl group in one molecule than to use a compound having a vinyl ether group and a compound having a (meth) acryl group separately. It is preferable for improving curability.

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

  On the other hand, the upper layer ink may or may not contain the vinyl ether group-containing (meth) acrylic acid ester, but it is preferable to contain it because the following effects are obtained. When the upper layer ink also contains a vinyl ether group-containing (meth) acrylic acid ester, the lower layer ink and the upper layer ink contain the same compound as the vinyl ether group-containing (meth) acrylic acid ester. It may also 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 having 2 to 20 carbon atoms. May be an alkylene group, an optionally substituted alkylene group having an oxygen atom by an ether bond and / or an ester bond in the structure, and an optionally substituted divalent alkylene group having 6 to 11 carbon atoms. Aromatic groups are preferred. Among these, C2-C6 alkylene groups such as ethylene group, n-propylene group, isopropylene group, and butylene group, oxyethylene group, oxy n-propylene group, oxyisopropylene group, and oxybutylene group An alkylene group having 2 to 9 carbon atoms having an oxygen atom due to an ether bond in the structure is preferably used.

In the general formula (I), the monovalent organic residue having 1 to 11 carbon atoms represented by R ³ is a linear, branched or cyclic substituted group having 1 to 10 carbon atoms. Suitable alkyl groups and optionally substituted aromatic groups having 6 to 11 carbon atoms are preferred. Among these, C6-C2 aromatic groups, such as a C1-C2 alkyl group which is a methyl group or an ethyl group, a phenyl group, and a benzyl group, are used suitably.

  When each of the organic residues is an optionally substituted group, the substituent is divided into a group containing a carbon atom and a group not containing a carbon atom. First, when the 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 not containing a carbon atom include, but are not limited to, a hydroxyl group and a halo group.

  Examples of the vinyl ether group-containing (meth) acrylic acid esters include, but are not limited to, for example, 2-vinyloxyethyl (meth) acrylate, 3-vinyloxypropyl (meth) acrylate, and 1-methyl (meth) acrylate. 2-vinyloxyethyl, 2-vinyloxypropyl (meth) acrylate, 4-vinyloxybutyl (meth) acrylate, 1-methyl-3-vinyloxypropyl (meth) acrylate, 1-vinyloxymethyl (meth) acrylate Propyl, 2-methyl-3-vinyloxypropyl (meth) acrylate, 1,1-dimethyl-2-vinyloxyethyl (meth) acrylate, 3-vinyloxybutyl (meth) acrylate, 1-methyl- (meth) acrylate 2-vinyloxypropyl, 2-vinyloxybutyl (meth) acrylate, (meth) a 4-vinyloxycyclohexyl silylate, 6-vinyloxyhexyl (meth) acrylate, 4-vinyloxymethylcyclohexylmethyl (meth) acrylate, 3-vinyloxymethylcyclohexylmethyl (meth) acrylate, (meth) acrylic acid 2-vinyloxymethylcyclohexylmethyl, p-vinyloxymethylphenylmethyl (meth) acrylate, m-vinyloxymethylphenylmethyl (meth) acrylate, o-vinyloxymethylphenylmethyl (meth) acrylate, (meth) 2- (vinyloxyethoxy) ethyl acrylate, 2- (vinyloxyisopropoxy) ethyl (meth) acrylate, 2- (vinyloxyethoxy) propyl (meth) acrylate, 2- (vinyloxy) (meth) acrylate Ethoxy) isopropyl, (meth) acrylic acid 2- Vinyloxyisopropoxy) propyl, 2- (vinyloxyisopropoxy) isopropyl (meth) acrylate, 2- (vinyloxyethoxyethoxy) ethyl (meth) acrylate, 2- (vinyloxyethoxyisopropoxy) (meth) acrylate ) Ethyl, 2- (vinyloxyisopropoxyethoxy) ethyl (meth) acrylate, 2- (vinyloxyisopropoxyisopropoxy) ethyl (meth) acrylate, 2- (vinyloxyethoxyethoxy) propyl (meth) acrylate 2- (vinyloxyethoxyisopropoxy) propyl (meth) acrylate, 2- (vinyloxyisopropoxyethoxy) propyl (meth) acrylate, 2- (vinyloxyisopropoxyisopropoxy) propyl (meth) acrylate, (Meth) acrylic acid 2- (vinylo) Xylethoxyethoxy) isopropyl, (meth) acrylic acid 2- (vinyloxyethoxyisopropoxy) isopropyl, (meth) acrylic acid 2- (vinyloxyisopropoxyethoxy) isopropyl, (meth) acrylic acid 2- (vinyloxyisopropoxy) Isopropoxy) isopropyl, (meth) acrylic acid 2- (vinyloxyethoxyethoxyethoxyethoxy) ethyl, (meth) acrylic acid 2- (vinyloxyethoxyethoxyethoxyethoxy) ethyl, (meth) acrylic acid 2- (isopropenoxyethoxy) ) Ethyl, (meth) acrylic acid 2- (isopropenoxyethoxyethoxyethoxy) ethyl, (meth) acrylic acid 2- (isopropenoxyethoxyethoxyethoxy) ethyl, (meth) acrylic acid 2- (isopropenoxyethoxyethoxyethoxy) D ) Ethyl, and (meth) Polyethylene glycol monovinyl ether acrylate, and (meth) acrylic acid polypropylene glycol monovinyl ether.

  Among these, since the viscosity of the ink can be further reduced, the flash point is high, and the curability of the ink is excellent, 2- (vinyloxyethoxy) ethyl (meth) acrylate, that is, 2- (vinyloxy) acrylate At least one of ethoxy) ethyl and 2- (vinyloxyethoxy) ethyl methacrylate is preferable, and 2- (vinyloxyethoxy) ethyl acrylate is more preferable. In particular, since 2- (vinyloxyethoxy) ethyl acrylate and 2- (vinyloxyethoxy) ethyl methacrylate have a simple structure and a small molecular weight, the viscosity of the ink can be significantly reduced. 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. Incidentally, 2- (vinyloxyethoxy) ethyl acrylate is superior in terms of curability compared to 2- (vinyloxyethoxy) ethyl methacrylate.

  A vinyl ether group containing (meth) acrylic acid ester may be used individually by 1 type, and may be used in combination of 2 or more type.

  The content of the above-mentioned 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 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 lowered, the curability of the lower layer ink can be improved, and the wettability of the upper layer ink can be further improved. It 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 lower in the lower layer ink. The upper layer ink is preferably larger than the upper layer ink, and the lower layer ink is more preferably 10% by mass or more than the upper layer ink. As a result, it is possible to further improve the wetting and spreading property of the upper layer ink and to obtain an ink set having various performances.
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 this 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 application of the lower layer ink is performed. The reason why the upper layer ink wettability on the film is excellent is that the coating film derived from the vinyl ether group of the (meth) acrylic acid ester containing vinyl ether group has some influence on the upper layer ink wettability. It is guessed. 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 of esterifying (meth) acrylic acid and a hydroxyl group-containing vinyl ether (Production Method B), (meth) acrylic acid halide. And esterification of (meth) acrylic anhydride and hydroxyl group-containing vinyl ether (production method D), esterification of (meth) acrylic acid ester and hydroxyl group-containing vinyl ether Method of exchange (Production method E), Method of esterifying (meth) acrylic acid and halogen-containing vinyl ether (Method of production F), Method of esterifying alkali (earth) metal salt of (meth) acrylic acid and halogen-containing vinyl ether (Production G), hydroxyl group-containing (meth) acrylic acid ester and carboxylic acid vinyl ester How to vinyl exchange and Le (Procedure H), hydroxyl group-containing (meth) How to ether exchange and acrylic acid ester and an alkyl vinyl ether (Process I).

  Among these, since the desired effect can be further exhibited in this embodiment, the production method E is preferable.

(2. Polymerizable compounds other than the above)
As the polymerizable compound other than the above (hereinafter, referred to as “other polymerizable compound”), conventionally known various monomers and oligomers such as monofunctional, bifunctional, and trifunctional or more 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 anhydride thereof, acrylonitrile, styrene, various types 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 are mentioned.

  Moreover, an N-vinyl compound may be included 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, monofunctional (meth) acrylates include, for example, isoamyl (meth) acrylate, stearyl (meth) acrylate, lauryl (meth) acrylate, octyl (meth) acrylate, decyl (meth) acrylate, iso Myristyl (meth) acrylate, isostearyl (meth) acrylate, 2-ethylhexyl-diglycol (meth) acrylate, 2-hydroxybutyl (meth) acrylate, butoxyethyl (meth) acrylate, ethoxydiethylene glycol (meth) acrylate, methoxydiethylene glycol ( (Meth) acrylate, methoxypolyethylene glycol (meth) acrylate, methoxypropylene glycol (meth) acrylate, phenoxyethyl (meth) acrylate , Tetrahydrofurfuryl (meth) acrylate, isobornyl (meth) acrylate, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 2-hydroxy-3-phenoxypropyl (meth) acrylate, lactone-modifiable Examples include flexible (meth) acrylate, t-butylcyclohexyl (meth) acrylate, dicyclopentanyl (meth) acrylate, and dicyclopentenyloxyethyl (meth) acrylate. Among these, phenoxyethyl (meth) acrylate is preferable.

  Among the (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, 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 Di (meth) acrylate, neopentyl glycol di (meth) acrylate, dimethylol-tricyclodecane di (meth) acrylate, EO (ethylene oxide) adduct di (meth) acrylate of bisphenol A DOO, PO bisphenol A (propylene oxide) adduct 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, trifunctional or more polyfunctional (meth) acrylates include, for example, trimethylolpropane tri (meth) acrylate, EO-modified trimethylolpropane tri (meth) acrylate, and pentaerythritol tri (meth) acrylate. , Pentaerythritol tetra (meth) acrylate, dipentaerythritol hexa (meth) acrylate, ditrimethylolpropane tetra (meth) acrylate, glycerin propoxytri (meth) acrylate, cowprolactone modified trimethylolpropane tri (meth) acrylate, pentaerythritol Examples 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 ink has a low viscosity, the solubility of the photopolymerization initiator and other additives is excellent, and good ejection stability at the time of ink jet recording is easily obtained. Furthermore, since the toughness, heat resistance, and chemical resistance of the coating film are increased, it is more preferable to use a monofunctional (meth) acrylate and a bifunctional (meth) acrylate, and among them, phenoxyethyl (meth) acrylate and dipropylene glycol. It is more preferable to use di (meth) acrylate in combination.

  The said other polymeric compound may be used individually by 1 type, and may use 2 or more types together.

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

[Surfactant]
The ink in the present 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. Furthermore, not only the upper layer ink but also the lower layer ink contains the surfactant, so that the embedding property of the lower layer ink on the recording medium becomes excellent.

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

  Examples of the silicone surfactant include, but are not limited to, modified silicones such as polyester-modified silicone and polyether-modified silicone. Among these, the polyether-modified silicone is preferable because the surface tension can be greatly reduced, and the wettability of the ink on the recording medium is 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.

  As a commercial item of said silicone type surfactant, BYK-347, BYK-348, BYK-UV3500, BYK-UV3510, BYK-UV3530, BYK-UV3570 (above, a product name by the BYK company) etc. are mentioned, for example. It is done. Among these, BYK-UV3500 is preferable because the wetting and spreading property of the upper layer ink is further improved.

  Moreover, as said acrylic surfactant, although it is not limited to the following, For example, a (meth) acrylic acid type copolymer is mentioned preferably.

  Examples of commercially available acrylic surfactants include 1970, 230, LF-1980, LF-1982 (-50), LF-1983 (-50), LF-1984 (-50), and LHP-95. , LHP-96, UVX-35, UVX-36, UVX-270, UVX-271, UVX-272, AQ-7120, AQ-7130 (above, trade names manufactured by Enomoto Kasei Co., Ltd.), BYK-350, BYK-352 , BYK-354, BYK-355, BYK-358, BYK-380, BYK-381, BYK-392 (above, trade names manufactured by BYK), Polyflow No. 57, 95 (above, trade name manufactured by Kyoeisha Yushi Chemical Co., Ltd.) and the like. Among these, when the lower layer ink contains an acrylic surfactant, BYK-350 is preferable because the discharge stability of the lower layer ink is excellent.

  In the case of the following first to third cases, the combination of surfactants that may or may be included in the upper layer ink and the lower layer ink can satisfy the above-described surface tension relationship more reliably. Any of these is preferable, and the following third case is more preferable.

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

  Second, both the upper layer ink and the lower layer ink contain a modified silicone surfactant (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. The size is preferably larger than the lower layer ink. The content difference is preferably 0.2% by mass or more, and more preferably 0.3% by mass or more.

  As the modified silicone surfactant in the second case, the upper layer ink and the lower layer ink are preferably independent of each other, and at least one of polyether-modified silicone and polyester-modified silicone is preferable. More preferred. In addition, it is preferable to use the same modified silicone 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 second case, the embedding property of the lower layer ink on the recording medium is excellent.

  Third, 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 wetting and spreading property of the upper layer ink is further improved. Examples of the modified silicone surfactants include, but are not limited to, polyether-modified silicones and polyester-modified silicones. Among them, they are easily available, and the embedding property of the lower layer ink on the recording medium is high. A polyether-modified silicone is preferred 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 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 included in the ink in the present embodiment is used to form a print by curing the ink present on the surface of the recording medium by photopolymerization by irradiation of radiation. 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 the energy of light and initiates the polymerization of the polymerizable compound, but a photoradical polymerization initiator or a photocationic polymerization initiator should be used. Among them, it is preferable to use a photo radical polymerization initiator.

  Examples of the photo radical polymerization initiator include aromatic ketones, acyl phosphine oxide compounds, aromatic onium salt compounds, organic peroxides, thio compounds (thioxanthone compounds, thiophenyl group-containing compounds, etc.), hexaarylbiphenyls, and the like. Examples include imidazole compounds, ketoxime ester compounds, borate compounds, azinium compounds, metallocene compounds, active ester compounds, compounds having a carbon halogen bond, and alkylamine compounds.

  Among these, it is preferable to use at least an acyl phosphine 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 photo radical 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, benzoin propyl ether, benzoin ethyl ether, benzyldimethyl ketal, 1- (4-isopropylphenyl) ) -2-hydroxy-2-methylpropan-1-one, 2-hydroxy-2-methyl-1-phenylpropan-1-one, thioxanthone, diethylthio Xanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone, 2-methyl-1- [4- (methylthio) phenyl] -2-morpholino-propan-1-one, bis (2,4,6-trimethylbenzoyl) -phenyl Phosphine 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 photo radical 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-morphol Linopropan-1-one), IRGACURE 369 (2-benzyl-2-dimethylamino-1- (4-morpholinophenyl) -butanone-1), IRGACURE 379 (2- (dimethylamino) -2-[(4- Methylphenyl) methyl] -1- [4- (4-morpholinyl) phenyl] -1-butanone), DAROCUR TPO (2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide), IRGACURE 819 (bis (2, 4,6-trimethylbenzoyl) -phenylphosphine oxide), IRGACURE 784 (bis (η5-2,4-cyclopentadien-1-yl) -bis (2,6-difluoro-3- (1H-pyrrole-1-) Yl) -phenyl) titanium), IRGACURE OXE 01 (1.2-octanedione, 1- [4- (phenylthio)-, 2- (O-benzoyloxime)]), IRGACURE OXE 02 (ethanone, 1- [9-ethyl-6- (2-methylbenzoyl) -9H-carbazol-3-yl]-, 1- (O-acetyloxime)), IRGACURE 754 (oxyphenylacetic acid, 2- [2-oxo-2-phenylacetoxyethoxy] ethyl ester and oxyphenylacetic acid, 2- (2-hydroxyethoxy) ethyl ester mixture) (above, manufactured by BASF), Speedcure TPO, Speedcure DETX (2,4-diethylthioxanthone), Speedcure ITX (2-isopropylthioxanthone) (above, manufactured by Lambson), KAYACURE DETX-S (2 , 4-diethylthioxanthone) (manufactured by Nippon Kayaku Co., Ltd.), Lucirin TPO, LR8883, LR8970 (above, manufactured by BASF), and Ubekrill P36 (manufactured by UCB).

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

  The content of the photopolymerization initiator can improve the ultraviolet curing rate and have excellent curability, and in order to avoid undissolved photopolymerization initiator and coloring derived from the photopolymerization initiator, It is preferable that it is 3-20 mass% with respect to the total mass (100 mass%) of an ink.

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

  Examples of the polymerization inhibitor include, but are not limited to, p-methoxyphenol, cresol, t-butylcatechol, 3,5-di-t-butyl-4-hydroxytoluene, 2,2′-methylenebis ( 4-methyl-6-tert-butylphenol), 2,2′-methylenebis (4-ethyl-6-butylphenol), and 4,4′-thiobis (3-methyl-6-tert-butylphenol) Inhibitors, and hindered amines, hydroquinone monomethyl ether (MEHQ), and hydroquinone are included.

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

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

(Pigment)
In this embodiment, the light resistance of the ink can be improved by using a pigment as the color material. As the pigment, both inorganic pigments and organic pigments 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.

  Organic pigments include insoluble azo pigments, condensed azo pigments, azo lakes and chelate azo pigments, phthalocyanine pigments, perylene and perinone pigments, anthraquinone pigments, quinacridone pigments, dioxane pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, etc. Polycyclic pigments, dye chelates (eg basic dye chelates, acid dye chelates, etc.), dye rakes (basic dye rakes, acid dye rakes), nitro pigments, nitroso pigments, aniline black, daylight A fluorescent pigment is mentioned.

  More specifically, as carbon black used as black ink, No. 2300, no. 900, MCF88, No. 33, no. 40, no. 45, no. 52, MA7, MA8, MA100, no. 2200B and the like (trade names manufactured by Mitsubishi Chemical Corporation), Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven 1255, Raven 700, and the like (above, Columbia Carbon Co., Carbon Carbon Co., Carbon 1) 400R, Regal 1 330R, Regal 1 660R, Mogul L, Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400, etc. Name), Color Black FW1, C lor Black FW2, Color Black FW2V, Color Black FW18, Color Black FW200, Color B1ack S150, Color Black S160, Color Black S170, Printex 35, Printex U, Printex V, Printex 140U, Special Black 6, Special Black 5, Special Black 4A, Special Black 4, etc. (above, trade name of Degussa).

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

  Examples of 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. Bat Blue 4, 60.

  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 said pigment may be used individually by 1 type, and may use 2 or more types together.

  When using said pigment, the average particle diameter has preferable 300 nm or less, and 50-200 nm is more preferable. When the average particle diameter is in the above range, the ink can be more excellent in reliability such as ejection stability and dispersion stability, 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 the present embodiment, a dye can be used as the color material. The dye is not particularly limited, and acid dyes, direct dyes, reactive dyes, and basic dyes 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. Directed Black 19, 38, 51, 71, 154, 168, 171, 195, C.I. I. Reactive Red 14, 32, 55, 79, 249, C.I. I. Reactive black 3, 4, and 35 are mentioned.
The said dye may be used individually by 1 type, and may use 2 or more types together.

  The content of the color material is preferably 0.5 to 20% by mass with respect to the total mass (100% by mass) of the ink because excellent concealability and color reproducibility can be obtained.

[Other additives]
The ink in the present 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 enhancers, wetting agents (humectants), and other additives. Examples of the 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 lower layer ink film is not narrowed, the photocurable inkjet ink set having excellent wettability of the upper layer ink can be obtained. Can be provided. Furthermore, according to the present embodiment, a photocurable inkjet ink set is provided that is excellent in both the curability and ejection stability of both the upper layer ink and the lower layer ink and the embedding property of the lower layer ink on the recording medium. can do. Thus, in the photocurable inkjet ink set, the lower layer ink and the upper layer ink have a relationship in which they affect each other in terms of composition and physical properties. In other words, when determining the composition and physical properties of the 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 ink jet ink set of the present embodiment provides a recorded matter by being ejected onto a recording medium using an ink jet recording method described later. Examples of the recording medium include an absorptive or non-absorbable recording medium. The ink jet recording method of the embodiment described later can be widely applied to a recording medium having various absorption performances from a non-absorbing recording medium in which ink penetration is difficult to an absorbent recording medium in which ink penetration is easy. .

  The absorbent recording medium is not particularly limited. For example, plain paper such as electrophotographic paper having high ink permeability, ink jet paper (an ink absorbing layer composed of silica particles or alumina particles, or polyvinyl alcohol ( Art paper, coated paper, etc. used for general offset printing with relatively low ink permeability, such as PVA) and polyvinyl pyrrolidone (PVP). Examples include cast paper.

  The non-absorbable recording medium is not particularly limited. For example, films, sheets and plates of plastics such as polyvinyl chloride (PVC), polyethylene, polypropylene, polyethylene terephthalate (PET), iron, silver, copper, Examples thereof include a metal plate such as aluminum, a metal plate produced by vapor deposition of these various metals, a plastic film, a plate made of an alloy such as stainless steel or 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 for the ink jet recording method of the present embodiment.

[Inkjet recording apparatus for carrying out inkjet recording method]
An example of an ink jet recording apparatus (hereinafter, also simply referred to as “recording apparatus”) for carrying out the above ink jet 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 with the lower layer ink in the first recording mode and the second recording mode.
Hereinafter, an example of the recording apparatus 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-like recording medium S is transported from the upstream side in the transport direction to the downstream side as the upstream roller 12A, the transport drum 12, and the downstream roller 12B rotate. The head unit includes a first white head unit 30A that discharges white ink W (on the paper, the left side in FIG. 1), a yellow head unit 30B that discharges yellow ink Y, and a magenta head unit that discharges magenta ink M. 30C, a cyan head unit 30D that discharges cyan ink C, a black head unit 30E that discharges black ink K, and a second white head unit 30A that discharges white ink W (on the paper surface, on the right side of FIG. 1) Is provided. The irradiation unit 40 includes irradiation units 42a, 41a, 41b, 41c, 41d, and 42b provided on the downstream side in the transport direction for each head unit, and an irradiation unit 43 provided at the end in the transport direction. Yes. Such a recording apparatus can be configured, for example, as shown in FIG. 2 in Japanese Patent Application Laid-Open No. 2010-208218.

  Here, when the first recording mode is performed, first, in the order in which the recording medium S is transported with respect to the recording medium S transported to a position facing each head unit on the transport drum, 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 of the lower layer ink. Next, the upper layer ink is discharged from at least one of the yellow head unit 30B, the magenta head unit 30C, the cyan head unit 30D, and the black head unit 30E 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, in the case where the second recording mode is performed, first, in the order in which the recording medium S is transported with respect to the recording medium S transported to a position facing each head unit on the transport drum, first, yellow 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 an irradiation unit provided on the downstream side of the head. An ink film is formed. Next, the lower layer ink is discharged from the second white head unit 30A onto the coating film, and irradiation is performed from the irradiation 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 recording control unit, when the lower layer ink coating is a solid pattern, the second recording mode preferably has a lower ink amount per dot of the lower layer ink than the first recording mode. It may be adjusted to be larger by 10% or more, more preferably 10 to 50%. Thereby, the wetting spread property of the lower layer ink equivalent to that in the first recording mode is ensured even in the second recording mode. Note that the one dot means an ink droplet to be attached per pixel of a pixel which is the minimum recording unit defined by the recording resolution.

  Here, the second recording mode is not necessarily provided, and in this case, the second white head unit 30A and the irradiation unit 42b are not necessarily provided. Also, the ink that forms the coating film later among the inks, that is, the upper layer ink in the first recording mode and the lower layer ink in the second recording mode, are applied to the irradiation section (41a) provided on the downstream side of the head unit. ~ 41d, 42b) and at least one of irradiation by the irradiation unit 43 may be cured, and when cured by the irradiation unit provided on the downstream side of the head unit, the irradiation unit 43 may be omitted. Good.

[Ink for upper layer and ink for lower layer]
In the present embodiment, the lower layer ink is an ink that forms a coating film on the recording medium prior to the upper layer ink in the first recording mode, and the upper layer ink is the recording medium in the first recording mode. It is an ink which forms a coating film on the coating film by the ink for lower layers. For example, one of the upper layer ink and the lower layer ink may be the main ink and the other may be the auxiliary ink. The main ink may be ink that is used by visually recognizing a coating film such as color ink. The auxiliary ink is, for example, a special color ink such as a white ink, a clear ink, or a metallic ink, or a functional ink that improves performance such as adhesion, wet spread, and abrasion resistance of the main ink. That's fine.
Here, both the upper layer ink and the lower layer ink may be the main ink, or both may be the auxiliary ink, but using one as the main ink and the other as the auxiliary ink broadens the application of the recorded matter. Moreover, it is preferable because the performance of the main ink is improved. 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 used as the main ink may be determined in consideration of the properties of the main ink and the auxiliary ink described above. Therefore, in FIG. 1 described above, the head that discharges 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.

  As described above, the amount of ink differs between the first recording mode and the second recording mode. In the second recording mode, the line width of the lower layer ink on the coating film of the upper layer ink becomes narrower. This is because the filling property of the solid pattern image of the lower layer ink deteriorates. Therefore, by increasing the amount of ink as described above, the embedding property of the lower layer ink can be improved.

[Inkjet recording method using inkjet recording apparatus]
The ink jet recording method of this embodiment includes a first recording step of forming a coating film of the lower layer ink in the above embodiment on a recording medium, and an overcoat ink in the above embodiment on 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 process and the second recording process includes an ejection stage for ejecting ink onto a recording medium, and a curing stage for irradiating the ejected 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 steps will be described.

(Discharge stage)
In the ejection stage, 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 ink viscosity at the time of ejection is preferably 15 mPa · s or less, and more preferably 12 mPa · s or less. If the ink viscosity is as described above with the ink temperature set to room temperature or the ink not heated, the ink may be discharged at room temperature or without heating the ink. In that case, it is preferable that the temperature of the ink at the time of discharge is 20-30 degreeC. On the other hand, the ink may be discharged with a preferable viscosity by heating the ink to a predetermined temperature. In this way, good discharge stability is realized.

  The lower layer ink and the upper layer ink in the present embodiment are both photocurable inks and have a higher viscosity than ordinary water-based inks. Therefore, the viscosity variation due to temperature variation during ejection is large. 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, and can cause image quality degradation. Therefore, it is preferable to keep the temperature of the ink during ejection as constant as possible.

[Curing stage]
Next, in the curing stage, the ink ejected and attached (landed) toward the recording medium 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 rays) to generate starting species such as radicals, acids, and bases, and the polymerization reaction of the polymerizable compound This is because it is promoted by function. Or it is because the polymerization reaction of a polymeric compound starts by irradiation of an ultraviolet-ray. 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 contacting with the photopolymerization initiator. And a more sensitive curing reaction can be achieved.

  As an ultraviolet ray source, a mercury lamp, a gas / solid laser or the like is mainly used, and as a light source used for curing a photocurable inkjet ink, a mercury lamp and a metal halide lamp are widely known. On the other hand, there is a strong demand for mercury-free from the viewpoint of environmental protection, and replacement with a GaN-based semiconductor ultraviolet light-emitting device is very useful industrially and environmentally. Furthermore, ultraviolet light-emitting diodes (UV-LEDs) and ultraviolet laser diodes (UV-LDs) are small, have a long life, have high efficiency, and are low in cost, and are expected as light sources for photo-curable ink jets. 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 200 mJ / cm ² or less, preferably using a UV-LED having an emission peak wavelength of preferably 350 to 420 nm. It is preferable to use this set for the ink jet recording method. In this case, low cost printing and high printing speed can be realized. Such an ink can be obtained by including at least one of a photopolymerization initiator that decomposes upon irradiation with ultraviolet rays in the wavelength range and a polymerizable compound that starts polymerization upon irradiation with ultraviolet rays 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 embedding property of the lower layer ink on the recording medium are all selected. In addition, an excellent ink jet recording method using a photocurable ink jet ink set can be provided.

  Hereinafter, the embodiments of the present invention will be described more specifically by way of examples. However, the embodiments are 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, hereinafter abbreviated as “PB15: 3”)
・ C. I. Pigment White 6 (titanium oxide, manufactured by Teica, 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., hereinafter abbreviated as “VEEA”)
(2. Other polymerizable compounds)
Biscoat # 192 (phenoxyethyl acrylate, trade name of OSAKA ORGANIC CHEMICAL INDUSTRY LTD., Hereinafter abbreviated as “PEA”)
NK ester APG-100 (trade name, dipropylene glycol diacrylate, manufactured by SHIN-NAKAMURA CHEMICAL CO., LTD., Hereinafter abbreviated as “DPGDA”)
(Polymerization inhibitor)
・ MEHQ (hydroquinone monomethyl ether, manufactured by Kanto Chemical Co., Inc.) [Surfactant]
BYK-UV3500 (Brand name, manufactured by BYK, silicone type, hereinafter abbreviated as “UV3500”)
BYK-UV3570 (trade name, manufactured by BYK, silicone type, abbreviated as “UV3570” below)
BYK-350 (trade name of BYK, acrylic, abbreviated as “BYK350” below)
BYK-381 (trade name of BYK, acrylic, abbreviated as “BYK381” below)
(Photopolymerization initiator)
IRGACURE 819 (trade name, manufactured by BASF, solid content: 100%, hereinafter abbreviated as “819”)
DAROCUR TPO (trade name, manufactured by BASF, solid content: 100%, hereinafter abbreviated as “TPO”)
KAYACURE DETX-S (trade name, manufactured by Nippon Kayaku Co., Ltd., solid content: 100%, hereinafter abbreviated as “DETX”)

[Preparation of upper layer ink and lower layer ink]
Each material was added so that it might become a composition (unit: mass%) of the following Table 1 and Table 2, and this was stirred with a high-speed water-cooled stirrer, and the cyan upper layer ink and the white lower layer An ink was prepared.
Table 1 shows the composition of each upper layer ink, and 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. The lower layer inks 1, 2, 3, 4, 5, 6, and 7 are hereinafter referred to as lower layers 1, 2, 3, 4, 5, 6 respectively. , And 7.

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

[Evaluation item]
About the ink set produced in each Example and each comparative example, wetting spreadability, embedding property, ejection stability, and sclerosis | hardenability were evaluated with the following method.

(1. Wetting and spreading properties)
To the recording medium (PET50A, manufactured by Lintec Corporation), the lower layer ink was ejected under the conditions of 720 × 720 dpi and 15 ng / dot, and then cured to form a cured product. Thereafter, 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.

Each ink was ejected after the temperature of the ink was adjusted so that the viscosity was 10 mPa · s. 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. Burial property)
To the recording medium (PET50A), the lower layer ink was ejected and cured under the conditions of 720 × 720 dpi and 10 ng / dot.

Each ink was discharged after its temperature was adjusted so that the viscosity was 10 mPa · s. About this obtained hardened | cured material, this evaluation was performed by observing the embedding property on the lower layer ink visually.
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 stripe was not visible).
B: Underground stripes were 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 ejected continuously from a head having 180 nozzles for 60 minutes. In this way, the ejection stability in 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 missing occurred during continuous discharge.

(4. Curability)
The curability of each of the upper layer ink and the lower layer ink was evaluated. Using an ink jet recording apparatus provided with a piezo ink jet nozzle, the upper layer ink and the lower layer ink in each Example and each Comparative Example were filled in each nozzle row. Each of the upper layer ink and the lower layer ink is ejected toward the recording medium (PET50A) at normal temperature and normal pressure under the conditions of 720 × 720 dpi and 10 ng / dot, and the film thickness of the printed material (recorded material) is 7 to 8 μm. A solid pattern image was printed so that

After that, from the UV-LED in the ultraviolet irradiation device mounted on the side of the carriage, the irradiation energy is gradually irradiated to the maximum of 200 mJ / cm ² per pass with the irradiation intensity of 1 W / cm ² and the peak wavelength of 395 nm. The amount was increased and irradiated toward the solid pattern image. In this way, a recorded matter was produced. During the irradiation, when there was no tackiness on the surface of the coated film on the recorded matter, it was assumed to be cured at that time.
Whether or not the tackiness disappeared was determined under the following conditions. That is, it is determined whether ink adheres to the cotton swab or whether the solid pattern image on the recording medium is scratched. The ink does not adhere to the cotton swab and the solid pattern image on the recording medium The case where no scratch was found was considered that the feeling of tack was lost. The cotton swab used here was a Johnson swab manufactured by Johnson & Johnson. The number of rubbing was 10 reciprocations and the rubbing strength was 100 g load.
The “number of passes” means the number of times the head moves relative to the recorded matter and the coating film is irradiated with ultraviolet rays from the ultraviolet irradiation device mounted on the head. The irradiation energy [mJ / cm ² ] was obtained from the product of the irradiation intensity [mW / cm ² ] on the irradiated surface irradiated from the light source 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 an irradiation energy of 200 mJ / cm ² or less.
B: Not cured with irradiation energy of 200 mJ / cm ² .

  From the above results, the lower layer ink containing the 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) that satisfies a relationship larger than the surface tension of the upper layer ink is superior in the wet property of the upper layer ink to the ink set (each comparative example) that does not, and further, the upper layer ink. It was found that both the curability and ejection stability of both the ink for ink and the ink for the lower layer and the embedding property of the ink for the lower layer on the recording medium are excellent.

  Moreover, although not described in the said table | surface, except for the point which added 0.05 mass% of UV3500 as surfactant after making DPGDA content 26.75 mass% among the inks 3 for lower layers, it is lower layer Evaluation was performed in the same manner as in the above example using the ink prepared in the same manner as the ink 3 for the upper layer, using the upper layer ink 1 and the upper layer ink 4 as the upper layer ink, and using the lower layer ink as the lower layer ink. . As a result, the embedding 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 ink is the lower layer ink γ. -The upper layer ink γ was 5 [mN / m], and the wetting spreadability of the upper layer ink was B.

  DESCRIPTION OF SYMBOLS 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 ... irradiation unit, S ... recording medium, W ... white ink, Y ... yellow ink, M ... magenta ink, C ... cyan ink, K ... black ink.

Claims (13)

The following general formula (I):
_{^{CH 2 = CR 1 -COOR 2 -O}} -CH = CH-R 3 ··· (I)
Wherein R ¹ is a hydrogen atom or a methyl group, R ² is a divalent organic residue having 2 to 20 carbon atoms, and R ³ is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms. Group.)
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,
The surface tension of the lower layer photocurable inkjet ink is larger than the surface tension of the upper layer photocurable inkjet ink,
In the lower layer photocurable ink-jet ink, the content of the vinyl ether group-containing (meth) acrylic acid esters is more than the content of the vinyl ether group-containing (meth) acrylic acid esters in the upper layer photocurable ink-jet ink. 10 mass% or more and 70 mass% or less with respect to the total mass of the photocurable inkjet ink for the lower layer, and other than the vinyl ether group-containing (meth) acrylic acid esters represented by the general formula (I) A photocurable inkjet ink set comprising a functional (meth) acrylate in an amount of 15 to 90% by mass with respect to the total mass of the lower layer photocurable inkjet ink.   The photocurable inkjet ink set according to claim 1, wherein the difference between the surface tension of the lower layer photocurable inkjet ink and the surface tension of the upper layer photocurable inkjet ink is 5 mN / m or more at room temperature. .   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 or 2, wherein the content is 20 to 70% by mass.   The photocurable inkjet according to any one of claims 1 to 3, wherein the upper layer photocurable inkjet ink contains a vinyl ether group-containing (meth) acrylic acid ester represented by the general formula (I). Ink set.   The photocurable inkjet ink set according to any one of claims 1 to 4, wherein the lower layer photocurable inkjet ink 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 photocurable inkjet ink set according to any one of claims 1 to 6, wherein each of the lower layer photocurable inkjet ink and the upper layer photocurable inkjet ink contains a color material.   The content of the vinyl ether group-containing (meth) acrylic acid esters in the lower layer photocurable inkjet ink is more than the content of the vinyl ether group-containing (meth) acrylic acid esters in the upper layer photocurable inkjet ink. The photocurable inkjet ink set according to any one of claims 1 to 7, which is 10 to 50% by mass larger.   The lower layer photocurable ink-jet ink has a polymerizable compound other than the vinyl ether group-containing (meth) acrylic acid ester represented by the general formula (I) in the total mass of the lower layer photocurable ink-jet ink. The photocurable inkjet ink set according to any one of claims 1 to 8, which is contained in an amount of 30 to 90% by mass.   The photocurable inkjet ink for lower layer contains 30% by mass or less of the vinyl ether group-containing (meth) acrylic acid ester represented by the general formula (I) with respect to the total mass of the photocurable inkjet ink for lower layer. The photocurable inkjet ink set according to any one of claims 1 to 9.   The photocurable inkjet ink set according to any one of claims 1 to 10, wherein the lower layer photocurable inkjet ink contains an acylphosphine oxide compound as a photopolymerization initiator.   The photocurable inkjet ink set according to any one of claims 1 to 11, wherein the ink attached to the recording medium is used after being cured by irradiating with ultraviolet rays from an ultraviolet ray source which is an ultraviolet light emitting diode.   A coating film of the photocurable inkjet ink for a lower layer according to any one of claims 1 to 12 is formed on a recording medium, and the coating film according to any one of claims 1 to 12 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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