JP7056149B2 - Active energy ray curing inkjet ink set - Google Patents

Description

The present invention relates to an inkjet ink set containing at least one pair of dark and light inks having the same hue.

Conventionally, various methods have been used as a recording method for forming an image on a recording medium such as paper based on an image data signal. Of these, the inkjet method, which forms images and characters by flying and landing minute droplets of ink, is an inexpensive device that ejects ink only to the required image area and prints the image directly on the recording medium. Because it is formed, it is efficient and the running cost is low. Further, the inkjet method has low noise and is excellent as a recording method.

In recent years, in an inkjet recording method, various recording media such as plastic and glass, which have higher coating resistance than water-based inkjet inks and solvent inkjet inks, have excellent drying properties, and are less likely to cause image bleeding. As an ink that can be recorded on a non-absorbable recording medium, an active energy ray-curable inkjet ink that cures by irradiation with ultraviolet rays or the like is attracting attention.

Further, in the inkjet method, a method of using an ink set having the same hue consisting of a dark color ink and a light color ink is known in order to obtain smooth gradation with improved graininess.

However, in an active energy ray-curable inkjet ink composed of a polymerizable compound having one or more ethylenically unsaturated bonds, a photopolymerization initiator and the like, the hue of the cured film changes depending on the photopolymerization initiator used. Furthermore, since it is cured by irradiating it with strong active energy rays, the hue of the cured film changes even when the photopolymerization initiator decomposes. In particular, it is remarkable in light-colored inks, and there is a problem that the desired high-quality image quality cannot be obtained even if an ink set having the same hue consisting of dark-colored inks and light-colored inks is used.

For example, in Patent Document 1, the content of the colorant is 2% by mass or less of the entire ink composition, the curing sensitivity using a specific sensitizer is excellent, and the color reproducibility of the image obtained by curing is excellent. Light color ink compositions are disclosed.

Further, in Patent Document 2, at least one pair of dark ink and light color ink having the same hue is contained, and the mass ratio (X) of the monofunctional monomer / polyfunctional monomer in the dark color ink and the monofunctional ink in the light color ink are contained. By setting the mass ratio (Y) of the monomer / polyfunctional monomer to X> Y, an image having excellent productivity, excellent stretchability of the obtained cured film, excellent hue, and reduced granularity can be obtained. The obtained ink set for inkjet recording is disclosed.

Further, in Patent Document 3, at least one pair of a dark color ink composition and a light color ink composition having the same hue is contained, and the mass ratio of the pigment and the thioxanthone-based polymerization initiator in the dark color ink composition is 4: 1. By setting the mass ratio of the pigment to the thioxanthone-based polymerization initiator in the light-color ink composition to 1: 1 to 1: 1, the curing sensitivity is excellent, and the obtained cured film can be used as a recording medium. Disclosed is an ink set for inkjet recording that can obtain an image having excellent adhesion, excellent hue, and reduced graininess.

Japanese Unexamined Patent Publication No. 2010-018703 Japanese Unexamined Patent Publication No. 2012-140490 JP2012-140492A

However, the light-colored ink of Patent Document 1 has a problem that the light-colored ink is yellowed by a specific sensitizer and the color reproducibility is inferior. Further, while the curing sensitivity is increased, there is a problem that the stretchability of the coating film is deteriorated when the recorded material is exhibited outdoors. In the inkjet ink set of Patent Document 2, since the light color contains thioxanthone, the light color ink turns yellow, and there is a problem that the color reproducibility is inferior. Further, it is not possible to obtain sufficient stretchability, and there is a problem that the stretchability of the coating film deteriorates, especially when the recorded material is exhibited outdoors. In the inkjet ink set of Patent Document 3, since the light color contains thioxanthone, the light color ink turns yellow, and there is a problem that the color reproducibility is inferior.

The present invention has been made to solve the above problems, and has excellent curability, excellent stretchability of the obtained cured film, particularly excellent stretchability at low temperature, excellent hue, and excellent ejection stability. It is to provide an inkjet ink set that can obtain high image quality.

As a result of diligent studies to solve the above problems, the present inventors have found that the above problems can be solved by the inkjet ink shown below, and have completed the present invention.

That is, the present invention is an inkjet ink set containing at least one pair of dark ink and light ink having the same hue, and the dark ink and the light ink are both the pigment (A) and the photopolymerization initiator (B). ), The polymerizable compound (C),
When the content of the photopolymerization initiator (B) in the light-colored ink is (α) and the content of the photopolymerization initiator (B) in the dark-colored ink is (β), 0. 1 <α-β <10 ,
The hue angle H ° (X) of the dark ink and the hue angle H ° (Y) of the light ink at the hue angle H ° defined in the CIELAB color space on the recording medium are
｜ XY ｜ <30 °
The present invention relates to an inkjet ink set, which is characterized by satisfying the above-mentioned relationship.

The present invention also relates to the inkjet ink set, wherein the dark ink and the light ink contain an acylphosphine oxide compound (B-1) as the photopolymerization initiator (B).

Further, in the present invention, the dark color ink further contains a thioxanthone-based compound (B-2) and / or an α-aminoalkylphenone-based compound (B-3) as the photopolymerization initiator (B). The present invention relates to the above-mentioned inkjet ink set.

The present invention also relates to the inkjet ink set, which is characterized in that the light-colored ink does not substantially contain the thioxanthone-based compound (B-2) as the photopolymerization initiator (B).

Further, the present invention is characterized in that the light-colored ink does not substantially contain the α-aminoalkylphenone compound (B-3) as the photopolymerization initiator (B). Regarding.

Further, in the present invention, the dark color ink and the light color ink contain a monofunctional monomer (C-1) and a polyfunctional monomer (C-2) as the polymerizable compound (C).
The mass ratio (α) of the polyfunctional monomer (C-2) / polymerizable compound (C) in the dark color ink and the polyfunctional monomer (C-2) / polymerizable compound (C) in the light color ink. ) Satisfy the relationship of α <β, the mass ratio (β) of the above-mentioned inkjet ink set.

The present invention also relates to the inkjet ink set, wherein the monofunctional monomer (C-1) contains a monofunctional monomer (C-1-1) having a cyclic structure in the molecule.

Further, the present invention has a monofunctional monomer having an acryloyl group and / or an acryloyl group as the monofunctional monomer (C-1-1) having a cyclic structure in the molecule. The present invention relates to the above-mentioned inkjet ink set, which comprises a monofunctional monomer having an acryloyl structure.

Further, in the present invention, the monofunctional monomer (C-1-1) having a cyclic structure in the molecule contains N-vinylcaprolactam.
The present invention relates to an inkjet ink set characterized in that the content of the N-vinylcaprolactam is 5 to 35% by mass with respect to the total mass of the inkjet ink.

The present invention also relates to the inkjet ink set, wherein the polyfunctional monomer (C-2) is a bifunctional monomer having two acryloyl groups and having a cyclic structure in the molecule.

Further, the present invention includes cyan ink and magenta ink as the dark color ink.
The present invention relates to the inkjet ink set, which comprises light cyan ink and light magenta ink as the light color ink.

INDUSTRIAL APPLICABILITY According to the present invention, there is provided an inkjet ink set which has excellent curability, excellent stretchability of the obtained cured film, particularly excellent stretchability at low temperature, excellent hue, and excellent ejection stability and high image quality. I was able to.

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

<Inkjet ink set>
The inkjet ink set of the present invention is an inkjet ink set containing at least one pair of dark and light inks having the same hue at a hue angle H ° defined in the CIELAB color space on the recording medium. It is characterized in that the hue angle H ° (X) of the dark color ink and the hue angle H ° (Y) of the light color ink satisfy the relationship of | XY | <30 °.
Hereinafter, the inkjet ink set of the present invention will be described in detail.

The inks constituting the inkjet ink set of the present invention include at least one pair of dark inks and light inks having the same hue. Here, "a pair of dark inks and light inks having the same hue" means that the hues of the ink inks used for multicolor inkjet recording are substantially the same, and only the shades of colors are used. It is a pair of at least two kinds of inks having different contents, and means a pair of inks having different content of coloring material. Further, three or more kinds of inks having different densities may be contained.

In the present invention, the light color ink is represented by adding "light" to the name of the dark color ink having the same hue. For example, when the dark ink is cyan ink, the light ink having the same hue is light cyan ink.

The hue of the pair of inks composed of the dark ink and the light ink is not particularly limited, but the dark ink is selected from the group consisting of, for example, cyan ink, magenta ink, yellow ink, and black ink. At least one can be used. As the light color ink, for example, at least one selected from the group consisting of light cyan ink, light magenta ink, light yellow ink and gray ink can be used. Above all, when forming an image, it is preferable to select from cyan ink, magenta ink and black ink which are dark color inks which are frequently used, and to be selected from the group consisting of cyan ink and magenta ink which are more frequently used. Is more preferable. Further, cyan ink is particularly preferable.

In the present invention, for example, when the light color ink is a light cyan ink, the entire ink set contains cyan ink as a dark color ink, and further, a dark color selected from the group consisting of yellow, magenta, black, and white. It preferably contains ink. Further, as the dark color ink, orange ink, green ink, and violet ink can be included.

[Hue angle H °]
The inkjet ink set of the present invention is an inkjet ink set comprising at least one pair of dark and light inks having the same hue at a hue angle H ° defined in the CIELAB color space on the recording medium. , The hue angle H ° (X) of the dark color ink and the hue angle H ° (Y) of the light color ink preferably satisfy the relationship of | XY | <30 °, | XY | < 20 ° is more preferable. Within the above range, graininess is suppressed and image quality with smooth gradation can be obtained.
As used herein, the term "hue angle H °" means a correlation amount calculated in accordance with JIS Z 8781-4: 2013. The hue angle H ° is H ° = tan ^-1 (b ^* / a ^* ) + 180 (a ^* <0) or H ° = tan ^-1 (b ^* / a ^* ) + 360 (a ^* > 0). In the case of).
“A ^* ” and “b ^* ” are values in the CIE1976La ^* b ^* color space when an image of each color of ink is used to form an image having a halftone dot area ratio of 100%. Further, as the measuring device, SPM100-II manufactured by Gretag is used.
The halftone dot area ratio means the ratio of the area of halftone dots to the unit area expressed as a percentage.

The recording medium is preferably soft vinyl chloride. Examples of the soft vinyl chloride include MD5 manufactured by METAMARK.

As the dark color ink, the hue angle H ° of cyan ink is in the range of 200 to 260 °, the hue angle H ° of magenta ink is in the range of 330 to 360 °, and the hue angle H ° of yellow ink is in the range of 80 to 110 °. preferable.

As the light color ink, the hue angle H ° of the light cyan ink is in the range of 200 to 260 °, the hue angle H ° of the light magenta ink is in the range of 330 to 360 °, and the hue angle H ° of the yellow ink is in the range of 80 to 110 °. preferable.

Hereinafter, the components contained in or may be contained in the dark color ink and the light color ink (hereinafter, also referred to as the inkjet ink of the present invention) constituting the inkjet ink of the present invention will be described.

<Inkjet ink>
[Pigment (A)]
The inkjet ink of the present invention contains a pigment. The pigment that can be used in the present invention is not particularly limited, and known pigments can be used. As the pigment, either an inorganic pigment or an organic pigment can be used. It may be a pigment generally used for printing applications and paint applications, and an appropriate pigment can be selected from these pigments according to the required applications such as color development and light resistance.

Examples of the inorganic pigment include carbon blacks such as furnace black, lamp black, acetylene black and channel black, iron oxide and titanium oxide.

Examples of the organic pigment include soluble azo pigments such as β-naphthol type, β-oxynaphthoic acid type, β-oxynaphthoic acid type anilide type, acetoacetate anilide type and pyrazolone type; β-naphthol type and β-oxynaphthoic acid. Insoluble azo pigments such as anilide-based, acetoacetate anilide-based monoazo, acetoacetate anilide-based disazo, pyrazolone-based; copper phthalocyanine blue, halogenated (eg, chlorinated or brominated) copper phthalocyanine blue, sulfonated copper phthalocyanine blue, metal Phthalocyanine pigments such as free phthalocyanines; quinacridone-based, dioxazine-based, slene-based (pirantron, antoanthron, indantron, anthrapyrimidine, flavantron, thioindigo-based, anthraquinone-based, perinone-based, perylene-based, etc.), isoindolinone-based, Examples thereof include polycyclic pigments such as metal complex type, quinophthalone type and diketopyrrolopyrrole type, and heterocyclic pigments.

More specifically, C.I. I. In terms of the color index, the pigment exhibiting the cyan color is C.I. I. Pigment Blue 1, 2, 14, 15, 15: 1, 15: 2, 15: 3, 15: 4, 60, 62 and the like.

Examples of the pigment exhibiting the magenta color include C.I. I. Pigment RED 1, 3, 5, 19, 21, 22, 31, 38, 42, 43, 48: 1, 48: 2, 48: 3, 48: 4, 48: 5, 49: 1, 50, 52, 53: 1, 57: 1, 57: 2, 58: 4, 63: 1, 81, 81: 1, 81: 2, 81: 3, 81: 4, 83, 90, 104, 108, 112, 114, 122, 144, 146, 148, 149, 150, 166, 168, 169, 170, 172, 173, 176, 177, 178, 184, 185, 187, 193, 202, 209, 214, 242, 254, 255, 264, 266, 269, C.I. I. Pigment Violet 19 and the like.

Examples of the pigment exhibiting a yellow color include C.I. I. Pigment Yellow 1, 2, 3, 12, 13, 14, 16, 17, 18, 24, 73, 74, 75, 83, 93, 95, 97, 98, 100, 108, 109, 110, 114, 120, 128, 129, 138, 139, 174, 150, 151, 154, 155, 167, 180, 185, 213 and the like.

Examples of the pigment exhibiting a black color include C.I. I. Pigment Black 1, 6, 7, 9, 10, 11, 28, 26, 31 and the like.

Examples of the pigment exhibiting a white color include C.I. I. Pigment White 5, 6, 7, 12, 28 and the like.

Examples of the pigment exhibiting a green color include C.I. I. Pigment Green 1, 2, 3, 4, 7, 8, 10, 15, 17, 26, 36, 45, 50 and the like.

Examples of the pigment exhibiting the violet color include C.I. I. Pigment Violet 1, 2, 3, 4, 5: 1, 12, 13, 15, 16, 17, 19, 23, 25, 29, 31, 32, 36, 37, 39, 42 and the like.
Examples of the orange-colored pigment include C.I. I. Pigment Orange 13, 16, 20, 34, 36, 38, 39, 43, 51, 61, 63, 64, 74 and the like.

Among the above pigments, the pigment exhibiting a cyan color is C.I. I. Pigment Blue 15: 3 and 15: 4 are more preferable, and the pigment exhibiting a magenta color is C.I. I. Pigment RED 122, 202, C.I. I. Pigment Violet 19 is more preferable, and the pigment exhibiting a yellow color is C.I. I. Pigment Yellow 74, 120, 150, 180, 185, 213 are more preferable, and the pigment exhibiting a black color is C.I. I. Pigment Black 7 is more preferable.

In the present invention, the pigment may be used alone or in combination of two or more.

In the present invention, the pigment (A) of the dark color ink and the light color ink is the same C.I. I. It is more preferable to use a pigment that is a color index. For example, cyan ink is used as a dark ink, and C.I. I. When Pigment Blue 15: 3 is used, C.I. I. Pigment Blue 15: 3 is used.

In the present invention, the content of the pigment (A) in the dark color ink is preferably 0.5 to 20% by mass, more preferably 1 to 10% by mass, based on the total mass of the dark color ink. 2 to 5% by mass is more preferable. By setting the content in the above range, it becomes easy to obtain good results in terms of color density and light resistance of the printed matter. In particular, in the case of cyan ink, 2 to 4% by mass is preferable, and in the case of magenta ink, 4 to 6% by mass is preferable.

The content of the pigment (A) in the light-colored ink is preferably 0.05 to 2.0% by mass, more preferably 0.1 to 1.5% by mass, based on the total mass of the light-colored ink. , 0.2 to 0.1.2% by mass is more preferable. In particular, in the case of light cyan ink, 0.2 to 0.8% by mass is preferable, and in the case of light magenta ink, 0.7 to 1.2% by mass is preferable.

The mass ratio of the content of the pigment (A) of the dark ink and the light ink having the same hue is preferably 30: 1 to 4: 1, more preferably 20: 1 to 5: 1, and 10: 1 to 6: 1. 1 is more preferable. When the mass ratio of the content is within the above range, smooth image quality with gradation can be obtained.

[Photopolymerization Initiator (B)]
The inkjet ink of the present invention contains a photopolymerization initiator (B). The photopolymerization initiator that can be used in the present invention is not particularly limited, and a known photopolymerization initiator can be used.
Further, the photopolymerization initiator may be used alone or in combination of two or more.
The photopolymerization initiator is not only a compound that absorbs external energy such as active radiation to generate a polymerization initiator species, but also a compound that absorbs a specific active energy ray and promotes the decomposition of the polymerization initiator (so-called increase). Sensitive agent) is also included.

In the present invention, the photopolymerization initiator (B) contained in the dark-colored ink and the light-colored ink preferably contains an acylphosphine oxide-based compound (B-1).

(Acylphosphine oxide compound (B-1))
Specific examples of the acylphosphine oxide compound (B-1) include diphenylacylphenylphosphine oxide, ethoxy (2,4,6-trimethylbenzoyl) phenylphosphinoxide, and 2,4,6-trimethylbenzoyl-. Examples thereof include diphenylphosphine oxide and bis (2,4,6-trimethylbenzoyl) -phenylphosphine oxide. Of these, 2,4,6-trimethylbenzoyl-diphenylphosphine oxide is preferable from the viewpoint of curability, hue, and solubility of the polymerizable compound.

In the present invention, the content of the acylphosphine oxide compound (B-1) in the dark color ink is preferably 1 to 20% by mass, preferably 5 to 15% by mass, based on the total mass of the dark color ink. % Is more preferable, and 7 to 12% by mass is further preferable. When the content is within the above range, the dark color ink has excellent curability.

The content of the acylphosphine oxide compound (B-1) in the light-colored ink is preferably 3 to 20% by mass, more preferably 6 to 15% by mass, based on the total mass of the light-colored ink. , 9-12% by mass is more preferable. When the content is within the above range, the light-colored ink has less hue change in the cured film and is excellent in curability.

Further, the content of the acylphosphine oxide-based compound (B-1) in the light-colored ink is preferably higher than the content of the acylphosphine oxide-based compound (B-1) in the dark-colored ink.

The mass ratio of the content of the acylphosphine oxide compound (B-1) in the light-colored ink to the content of the acylphosphine oxide-based compound (B-1) in the dark-colored ink is 3: 1 to 1. 1 is preferable, and 2: 1 to 1.01: 1 is more preferable.

In the present invention, when a light emitting diode (hereinafter, "UV-LED") that generates ultraviolet rays having an emission peak wavelength in the range of 380 to 420 nm is used as a light source, the dark color ink is a thioxanthone-based compound from the viewpoint of curability. It is preferable to contain (B-2) and / or an α-aminoalkylphenone compound (B-3).

(Thioxanthone compound (B-2))
Specific examples of the thioxanthone compound (B-2) include thioxanthone, 2-isopropylthioxanthone, 4-isopropylthioxanthone, 2-chlorothioxanthone, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone and the like. Of these, 2-isopropylthioxanthone is preferable from the viewpoint of curability.

In the present invention, the content of the thioxanthone compound (B-2) in the dark color ink is preferably 0.1 to 10% by mass, and 0.2 to 7% by mass, based on the total mass of the dark color ink. The mass% is more preferable, and 0.2 to 5% by mass is further preferable.

The content of the thioxanthone compound (B-2) in the light-colored ink is preferably 0 to 5% by mass, more preferably 0 to 2% by mass, and further preferably substantially not contained. It should be noted that substantially not contained is less than 0.05% by mass with respect to the total mass of the light-colored ink.

(Α-Aminoalkylphenone compound (B-3))
Specific examples of the α-aminoalkylphenone compound (B-3) include 2-methyl-1- [4- (methoxythio) -phenyl] -2-morpholinopropane-1-one and 2-benzyl-2. -Dimethylamino-1- (4-morpholinophenyl) -butanone-1,2- (dimethylamino) -2-[(4-methylphenyl) methyl] -1- [4- (4-morpholinyl) phenyl-1 -Butanon etc. can be mentioned. Above all, when UV-LED is used as a light source, 2-benzyl-2-dimethylamino-1- (4-morpholinophenyl) -butanone-1,2- (dimethylamino) -2- is used from the viewpoint of curability. [(4-Methylphenyl) methyl] -1- [4- (4-morpholinyl) phenyl-1-butanone is preferred.

In the present invention, the content of the α-aminoalkylphenone compound (B-3) in the dark color ink is preferably 0.1 to 6% by mass with respect to the total mass of the dark color ink, and is 0. .2 to 5% by mass is more preferable.

The content of the α-aminoalkylphenone compound (B-3) in the light-colored ink is preferably 0 to 5% by mass, more preferably 0 to 2% by mass, and even more preferably substantially not contained. It should be noted that substantially not contained is less than 0.05% by mass with respect to the total mass of the light-colored ink.

In the present invention, the thioxanthone-based compound (B-2) and / or the α-aminoalkylphenone-based compound (B-3) is preferably used for an inkjet ink containing 1.0% by mass or more of the pigment (A). 5.5% by mass or more is more preferable, and 2% by mass or more is further preferable.

As described above, the hue of the cured film changes depending on the photopolymerization initiator or its decomposition products, and becomes particularly remarkable in light-colored inks. For example, when a UV-LED is used as a light source, an α-aminoalkylphenone compound (B-3) which is a photopolymerization initiator having high absorption at 380 to 420 nm and a thioxanthone compound which accelerates the decomposition of the polymerization initiator. (B-2) is used. When such a compound is used, the curability is excellent, but the hue of the cured film shifts to yellowish and the color reproducibility is impaired. Therefore, particularly when the mass ratio of the content of the pigment (A) of the dark color ink and the light color ink is 6: 1 or more, the light color ink does not substantially contain the thioxanthone compound (B-2). Is preferable, and in the case of 8: 1 to 10: 1, it is more preferable that the thioxanthone compound (B-2) is not substantially contained. Further, it is particularly preferable that the α-aminoalkylphenone compound (B-3) is not substantially contained.
It should be noted that the term “substantially” means less than 0.05% by mass with respect to the total mass of the inkjet ink.

Other photopolymerization initiators other than the photopolymerization initiators (B-1) to (B-3) may be contained as long as the effect of the present invention is not reduced. Specific examples thereof include benzophenone compounds, amine compounds, α-hydroxyalkylphenone compounds, and dialkoxyacetophenone compounds. Of these, benzophenone-based compounds and amine-based compounds are preferable from the viewpoint of surface curability.

Examples of the benzophenone compound include benzophenone, 4-phenylbenzophenone, isophthalphenone, 4-benzoyl-4'-methyl-diphenylsulfide and the like. Of these, 4-benzoyl-4'-methyl-diphenylsulfide is preferable from the viewpoint of surface curability. When the benzophenone compound is contained, it is preferably 0.1 to 5% by mass, more preferably 0.2 to 3% by mass, based on the total mass of the inkjet ink.

Examples of the amine compound include trimethylamine, methyldimethanolamine, triethanolamine, p-diethylaminoacetophenone, ethyl p-dimethylaminobenzoate, isoamyl p-dimethylaminobenzoate, N, N-dimethylbenzylamine and ethyl. Examples thereof include tertiary amine compounds such as -4-dimethylaminobenzoate, 4,4'-bis (diethylamino) benzophenone, and ethylhexyl-4-dimethylaminobenzoate. Of these, ethyl-4-dimethylaminobenzoate and 4,4'-bis (diethylamino) benzophenone are preferable from the viewpoint of curability. When the amine compound is contained, it is preferably 0.1 to 5% by mass, more preferably 0.2 to 3% by mass, based on the total weight of the inkjet ink.

The total content of the photopolymerization initiator (B) is preferably 2 to 25% by mass, more preferably 5 to 20% by mass, based on the total mass of the polymerizable compound (C) described later. When the total content of the photopolymerization initiator (B) is 5% by mass or more, it is easy to obtain good curability. On the other hand, when the total content of the photopolymerization initiator (B) is 20% by mass or less, the undissolved component of the photopolymerization initiator is not generated even at a low temperature, and good ejection stability can be obtained.

In the present invention, when the total content of the photopolymerizable initiator (B) in the light-colored ink is (α) and the total content of the photopolymerizable initiator (B) in the dark-colored ink is (β). It is preferable that α> β. More preferably, 0.1 <α-β <10, and even more preferably 0.2 <α-β <5. Within the above range, the curability of the dark ink and the light ink is the same, bleeding between the dark ink and the light ink is suppressed, and smooth image quality without graininess can be obtained.

[Polymerizable compound (C)]
Here, in the present specification, the descriptions such as "(meth) acryloyl", "(meth) acrylic acid", "(meth) acrylate", and "(meth) acryloyloxy" are referred to as "(meth) acryloyloxy", respectively, unless otherwise specified. It means "acryloyl and / or methacrylic acid", "acrylic acid and / or methacrylic acid", "acrylate and / or methacrylate", and "acryloyloxy and / or methacryloyloxy". Further, "PO" represents "propylene oxide" and "EO" represents "ethylene oxide".

The inkjet ink of the present invention contains the polymerizable compound (C). As the polymerizable compound that can be used in the present invention, known polymerizable or crosslinkable materials such as radical polymerization reaction, cationic polymerization reaction, and dimerization reaction can be used. Above all, from the viewpoint of coating film resistance, adhesion and curability, a radically polymerizable compound is preferable, and an addition polymerizable compound having at least one ethylenically unsaturated double bond is more preferable. Specifically, a compound having a (meth) acryloyl group, an allyl group, a vinyl group, a vinyl ether group, and an internal double bond group (maleic acid or the like) in the molecule is preferable. Of these, compounds having a (meth) acryloyl group and a vinyl group are more preferable in terms of curability.
Further, the polymerizable compound that can be used in the present invention may be used alone or in combination of two or more.

The inkjet ink of the present invention contains a monofunctional monomer (C-1) and a polyfunctional monomer (C-2) as the polymerizable compound (C), and polymerizes with the polyfunctional monomer (C-2) in the dark ink. The mass ratio of the sex compound (B) ((C-2) / (B)), γ, and the mass ratio of the polyfunctional monomer (C-2) and the polymerizable compound (C) in the light-colored ink ((C-). It is preferable that δ of 2) / (C)) satisfies the relationship of γ <δ. More preferably, 1.05 <δ / γ <4, and even more preferably 1.1 <δ / γ <2. Within the above range, the curability of the dark ink and the light ink is the same, bleeding between the dark ink and the light ink is suppressed, and smooth image quality without graininess can be obtained.

(Monofunctional monomer (C-1))
The monofunctional monomer (C-1) is a compound having one ethylenically unsaturated bond in the molecule. Specifically, a compound having a (meth) acryloyl group, an allyl group, a vinyl group, a vinyl ether group, and an internal double bond group (maleic acid or the like) is preferable. Of these, compounds having a (meth) acryloyl group and a vinyl group are more preferable in terms of curability. The monofunctional monomer (C-1) may be used alone or in combination of two or more.

Specific examples of the monofunctional monomer having a (meth) acryloyl group include 2-ethylhexyl (meth) acrylate, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, and β-carboxyethyl (meth). Acrylate, 4-tert-butylcyclohexinol (meth) acrylate, tetrahydrofurfuryl acrylate, alkoxylated tetrahydrofurfuryl acrylate, caprolactone (meth) acrylate, lauryl (meth) acrylate, stearyl (meth) acrylate, isoamyl (meth) acrylate, 2-Phenoxyethyl (meth) acrylate, isodecyl (meth) acrylate, 3,3,5-trimethylcyclohexanol (meth) acrylate, cyclohexyl (meth) acrylate, isobornyl (meth) acrylate, norbornyl (meth) acrylate, dicyclopenta Nyl (meth) acrylate, dicyclopentenyl (oxyethyl) (meth) acrylate, 1,4-cyclohexanedimethanol (meth) acrylate, cyclic trimethylolpropaneformal (meth) acrylate, benzyl (meth) acrylate, EO-modified nonylphenol acrylate , 2-Methyl-2-ethyl-1,3-dioxolan-4-yl) methyl acrylate, acryloylmorpholine and the like.

Specific examples of the monofunctional monomer having a vinyl group include N-vinylcarbazole, 1-vinylimidazole, N-vinyl-2-pyrrolidone, N-vinylcaprolactam, and N-vinylformamide.

In the present invention, the monofunctional monomer (C-1) preferably contains a monofunctional monomer (C-1-1) having a cyclic structure. The monofunctional monomer having a cyclic structure includes a monofunctional monomer having a heterocyclic structure, a monofunctional monomer having an alicyclic structure, and a monofunctional monomer having an aromatic ring structure.

In the present invention, the monofunctional monomer (C-1-1) having a cyclic structure preferably contains a monofunctional monomer having a (meth) acryloyl group and having a heterocyclic structure.

Specific examples of the monofunctional monomer having a (meth) acryloyl group and having a heterocyclic structure include (2-methyl-2-ethyl-1,3-dioxolan-4-yl) methyl acrylate and tetrahydro. Examples include furfuryl acrylate, alkoxylated tetrahydrofurfuryl acrylate, cyclic trimethylolpropaneformal (meth) acrylate, 2-methyl-2-ethyl-1,3-dioxolan-4-yl) methyl acrylate, acryloylmorpholin and the like. .. Among them, from the viewpoint of the stretchability of the cured film, particularly the stretchability of the cured film after outdoor exposure, (2-methyl-2-ethyl-1,3-dioxolan-4-yl) methyl acrylate, tetrahydrofurfuryl acrylate, Alkoxylated tetrahydrofurfuryl acrylate, cyclic trimethylolpropaneformal (meth) acrylate, 2-methyl-2-ethyl-1,3-dioxolan-4-yl) methyl acrylate is preferably contained, preferably tetrahydro. It is more preferable to contain any one of furfuryl acrylate and cyclic trimethylolpropane formal (meth) acrylate.

In the present invention, the content of the monofunctional monomer having the (meth) acryloyl group and having a heterocyclic structure is preferably 5 to 20% by mass with respect to the total mass of the inkjet ink. More preferably, 5 to 15% by mass. When the content is within the above range, the stretchability of the cured film after outdoor exposure is excellent.

In the present invention, it is preferable that the monofunctional monomer (C-1-1) having a cyclic structure further contains a monofunctional monomer having a vinyl group and having a heterocyclic structure. Above all, from the viewpoint of curability, it is preferable to contain N-vinylcaprolactam.

In the present invention, the content of N-vinylcaprolactam is preferably 5 to 35% by mass, more preferably 10 to 30% by mass, still more preferably 10 to 25% by mass, based on the total mass of the inkjet ink. .. When the content is within the above range, excellent curability can be obtained without impairing the discharge stability.

Specific examples of the monofunctional monomer having an alicyclic structure include 4-tert-butylcyclohexanol (meth) acrylate, 3,3,5-trimethylcyclohexanol (meth) acrylate, cyclohexyl (meth) acrylate, and isobornyl (). Examples thereof include meta) acrylate, norbornyl (meth) acrylate, dicyclopentanyl (meth) acrylate, dicyclopentenyl (oxyethyl) (meth) acrylate, and 1,4-cyclohexanedimethanol (meth) acrylate. Above all, from the viewpoint of adhesion to the substrate, it is preferable to contain any one of 4-tert-butylcyclohexinol (meth) acrylate, cyclohexyl (meth) acrylate, and isobornyl (meth) acrylate.

In the present invention, the content of the monofunctional monomer having an alicyclic structure is preferably 1 to 20% by mass, more preferably 3 to 10% by mass, based on the total mass of the inkjet ink. When the content is within the above range, excellent adhesion can be obtained without impairing the discharge stability.

Specific examples of the monofunctional monomer having an aromatic ring structure include benzyl (meth) acrylate, 2-phenoxyethyl methacrylate, phenoxydiethylene glycol (meth) crylate, phenoxybenzyl (meth) acrylate, and phenoxypropylene glycol (meth) acrylate. Phenoxydipropylene glycol (meth) acrylate, phenoxypolyethylene glycol (meth) acrylate, phenol EO modified (meth) acrylate, nonylphenol EO modified (meth) acrylate, nonylphenol PO modified (meth) acrylate, o-phenylphenol EO modified (meth) Examples thereof include acrylates and 2-hydroxy-3-phenoxypropyl (meth) acrylates. Among them, from the viewpoint of stretchability of the cured film, any one of benzyl (meth) acrylate, 2-phenoxyethyl methacrylate, phenoxydiethylene glycol (meth) crylate, phenoxybenzyl (meth) acrylate, and nonylphenol EO-modified (meth) acrylate is used. It is preferable to include it.

In the present invention, the content of the monofunctional monomer having an aromatic ring structure is preferably 30 to 80% by mass with respect to the total mass of the inkjet ink.

The glass transition temperature of the homopolymer of the monofunctional monomer (C-1) is preferably −50 to 120 ° C., more preferably −20 to 100 ° C., and even more preferably −10 to 80 ° C. Flexibility is improved at −50 ° C. or higher, and curability is improved at 120 ° C. or lower.
Here, the glass transition temperature of the homopolymer was measured by the differential scanning calorimetry (DSC) method when there was no catalog value. As the DSC device, Seiko Instruments DSC120U was used, and the measurement was performed at a measurement temperature of 30 ° C. to 300 ° C. and a temperature rise rate of 2.5 ° C. per minute.

In the present invention, the total content of the monofunctional monomer (C) is preferably 60 to 90% by mass, more preferably 65 to 80% by mass, based on the total mass of the inkjet ink from the viewpoint of curability. .. Further, from the viewpoint of stretchability, the content of the monofunctional monomer (B-1-1) having a cyclic structure is 80 to 100% by mass with respect to the total mass of the monofunctional monomer (B-1). Is preferable, and 90 to 100% by mass is more preferable.

(Polyfunctional monomer (C-2))
The polyfunctional monomer (C-2) is a compound having two or more ethylenically unsaturated bonds in the molecule. Specifically, a compound having a (meth) acryloyl group, an allyl group, a vinyl group, a vinyl ether group, and an internal double bond group (maleic acid or the like) is preferable. Among them, a compound having a (meth) acryloyl group is more preferable in terms of curability. The polyfunctional monomer (C-2) may be used alone or in combination of two or more.

Specific examples of the polyfunctional monomer having a (meth) acryloyl group include (poly) ethylene glycol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate, 1,9-nonanediol acrylate, and 1, 10-decanediol diacrylate, neopentyl glycol di (meth) acrylate, hydroxypivalate neopentyl glycol di (meth) acrylate, dipropylene glycol di (meth) acrylate, (neopentyl glycol modified) trimethyl propandi (meth) Acrylate, Tripropylene glycol di (meth) acrylate, Dimethyloltricyclodecanedi (meth) acrylate, EO adduct di (meth) acrylate of bisphenol A, EO adduct di (meth) acrylate of bisphenol F, PO of bisphenol A Bifunctional monomers such as adduct di (meth) acrylate, cyclohexanedimethanol di (meth) acrylate, dimethylol-tricyclodecanedi (meth) acrylate, dicyclopentanyldi (meth) acrylate,
Trimethylolpropane tri (meth) acrylate, ε-caprolactone-modified tris- (2-acryloxyethyl) isocyanurate, ethoxylated isocyanuric acid tri (meth) acrylate, tris (2-hydroxyethyl) isocyanurate tri (meth) acrylate, Trifunctional monomers such as pentaerythritol tri (meth) acrylate and trimethylolpropane tri (meth) acrylate,
Tetra-functional monomers such as pentaerythritol tetra (meth) acrylate and ditrimethylolpropane tetra (meth) acrylate,
Dipentaerythritol Penta (meth) acrylate pentafunctional monomer,
Examples thereof include a hexafunctional monomer of dipentaerythritol hexa (meth) acrylate.

In the present invention, the molecular weight of the polyfunctional monomer (C-2) is preferably 200 to 2,000, more preferably 300 to 1,500, and even more preferably 500 to 1,000. When the molecular weight is 200 or more, the stretchability is good, and when the molecular weight is 2,000 or less, the curability is good.

In the present invention, the glass transition temperature of the homopolymer of the polyfunctional monomer (C-2) is preferably −30 to 120 ° C., more preferably −20 to 100 ° C., and even more preferably −10 to 80 ° C. Flexibility is improved at −30 ° C. or higher, and curability is improved at 120 ° C. or lower.

In the present invention, the polyfunctional monomer (C-2) preferably contains a bifunctional monomer having two ethylenically unsaturated bonds in the molecule. From the viewpoint of the stretchability of the cured film, the content of the trifunctional or higher functional monomer is preferably 5% by mass or less, more preferably 3% by mass or less, and substantially the total mass of the inkjet ink. It is more preferable that it is not contained in. It should be noted that substantially no inclusion is less than 0.1% mass.

In the present invention, the polyfunctional monomer (C-2) preferably contains a polyfunctional monomer having a cyclic structure in the molecule. The polyfunctional monomer having a cyclic structure includes a polyfunctional monomer having a heterocyclic structure, a polyfunctional monomer having an alicyclic structure, and a polyfunctional monomer having an aromatic ring structure.
Specific examples of the polyfunctional monomer having a cyclic structure in the molecule include dimethyloltricyclodecanedi (meth) acrylate, EO adduct di (meth) acrylate of bisphenol A, and EO adduct di (meth) of bisphenol F. Acrylate, PO adduct di (meth) acrylate of bisphenol A, cyclohexanedimethanol di (meth) acrylate, dimethylol-tricyclodecanedi (meth) acrylate, dicyclopentanyldi (meth) acrylate, cyclohexanedimethanol divinyl ether, etc. Bifunctional monomer,
Examples thereof include trifunctional monomers such as ε-caprolactone-modified tris- (2-acryloxyethyl) isocyanurate, ethoxylated isocyanurate tri (meth) acrylate, and tris (2-hydroxyethyl) isocyanurate tri (meth) acrylate.
Above all, from the viewpoint of the stretchability of the cured film, particularly the stretchability of the cured film after outdoor exposure, it is preferable to contain a polyfunctional monomer having an aromatic ring structure, for example, the EO adduct di (meth) of bisphenol A. Examples thereof include acrylate, EO adduct di (meth) acrylate of bisphenol F, and PO adduct di (meth) acrylate of bisphenol A. From the viewpoint of curability and stretchability, EO4 molar adduct di (meth) acrylate of bisphenol A, EO10 mol additive di (meth) acrylate of bisphenol A and EO4 mol additive di (meth) acrylate of bisphenol F are preferable.

In the present invention, the content of the polyfunctional monomer having an aromatic ring structure is preferably 1 to 20% by mass, more preferably 2 to 15% by mass, and 5 to 15% by mass with respect to the total mass of the inkjet ink. % Is more preferable. When the content is in the above range, excellent curability can be obtained without impairing the stretchability of the cured film after outdoor exposure.

In the present invention, the content of the polyfunctional monomer (C-2) contained in the inkjet ink is preferably 1 to 25% by mass, more preferably 2 to 20% by mass, based on the total mass of the inkjet ink. 5 to 15% by mass is more preferable. When the content is in the above range, excellent curability can be obtained without impairing the stretchability of the cured film.

[Polymerizable oligomer]
The polymerizable oligomer (C) may contain a polymerizable oligomer. The polymerizable oligomer preferably has a (meth) acryloyl group as an ethylenically unsaturated bond in the molecule. The number of tylene unsaturated bonds contained in the oligomer is preferably 1 to 15 per molecule, more preferably 2 to 6, further preferably 2 to 4, and particularly preferably 2 from the viewpoint of the balance between flexibility and curability. .. The weight average molecular weight of the oligomer is preferably 400 to 10,000, more preferably 500 to 5,000. Here, the "weight average molecular weight" can be determined as a styrene-equivalent molecular weight by general gel permission chromatography (hereinafter referred to as GPC).
In the present invention, it is a polystyrene-equivalent molecular weight when a TSKgel column (manufactured by Tosoh Corporation) is used and a GPC (manufactured by Tosoh Co., Ltd., HLC-8320GPC) equipped with an RI detector and DMF is used as a developing solvent.

Examples of the polymerizable oligomer include urethane acrylate oligomers such as aliphatic urethane acrylate oligomers and aromatic urethane acrylate oligomers, acrylic ester oligomers, polyester acrylate oligomers, and epoxy acrylate oligomers. From the viewpoint of stretchability, aliphatic urethane acrylate oligomers and aromatic urethane acrylate oligomers are preferable. Further, the polymerizable oligomer may be used alone or in combination of two or more.

From the viewpoint of achieving both curability and stretchability, the content of the polymerizable oligomer is preferably 0.1 to 15% by mass, more preferably 0.5 to 10% by mass, based on the total mass of the inkjet ink. It is preferable, and 1 to 8% by mass is more preferable.

In the present invention, the content of the polymerizable compound having a cyclic structure in the molecule is preferably 90% by mass or more, more preferably 100% by mass, based on the total mass of the polymerizable compound (C). preferable.

[Pigment dispersant]
In the present invention, it is preferable to use a pigment dispersant in order to improve the dispersibility of the pigment and the storage stability of the inkjet ink.
The pigment dispersant that can be used in the present invention is not particularly limited, and known pigment dispersants can be used. Among them, a resin-type pigment dispersant having a basic functional group is preferable, and examples of the basic functional group include a primary, secondary, or tertiary amino group, a nitrogen-containing heterocycle such as pyridine, pyrimidine, and pyrazine. can.
Further, as the skeleton constituting the resin-type pigment dispersant, a fatty acid amine skeleton and / or a urethane skeleton is more preferable because a pigment dispersion having good storage stability can be easily obtained.

The pigment dispersant preferably has a weight average molecular weight of 5,000 to 50,000, an acid value (mgKOH / g) of 5 to 20, and an amine value (mgKOH / g) of 20 to 50.
The "acid value" represents the acid value per 1 g of the solid content of the dispersant, and can be determined by the potentiometric titration method according to JIS K 0070.
The "amine value" represents an amine value per 1 g of a solid dispersant, and is a value obtained by a potentiometric titration method using a 0.1 N hydrochloric acid aqueous solution and then converted into an equivalent amount of potassium hydroxide.

Specific examples of the pigment dispersant include Solspurs 32000, 76400, 76500, J100, and J180; and Disperbyk-161, 162, 163, 164, 165, 166, 167, and 168 manufactured by Lubrizol. Be done.

The content of the pigment dispersant is arbitrarily selected in order to ensure the desired stability. For example, the flow characteristics of the inkjet ink are excellent when the pigment dispersant is 25 to 150 parts by mass with respect to 100 parts by mass of the pigment. When a pigment dispersant is used within the above range, the dispersion stability of the inkjet ink is good, and the quality tends to be the same as that of the initial stage even after a long period of time, which is preferable. Further, when the pigment dispersant is 40 to 100 parts by mass with respect to 100 parts by mass of the pigment, it is preferable from the viewpoint of both the dispersibility and the ejection property of the inkjet ink.
In particular, in the light cyan ink, the content of the pigment dispersant is preferably 25 to 200 parts by mass, more preferably 40 to 150 parts by mass, still more preferably 50 to 120 parts by mass with respect to 100 parts by mass of the pigment. .. In the light magenta ink, the content of the pigment dispersant is preferably 25 to 150 parts by mass, more preferably 40 to 100 parts by mass, still more preferably 50 to 100 parts by mass with respect to 100 parts by mass of the pigment.

[Other ingredients]
The inkjet ink of the present invention may contain, if necessary, an inert resin, a surface conditioner, a polymerization inhibitor, an organic solvent, an antifoaming agent, an antioxidant and the like, in addition to the above components.

(Inactive resin)
In one embodiment, an inert resin may be contained for the purpose of imparting arbitrary physical properties to the printed matter. The "inactive resin" does not contain a pigment dispersant.

As used herein, the term "inactive resin" means a compound containing no reactive group and having a weight average molecular weight of 1,000 or more.

The inert resin used in one embodiment is, for example, an acrylic resin such as a polyacrylic acid resin, a styrene acrylic acid copolymer resin, and a butadiene-acrylic nitrile copolymer resin; a styrene maleic acid copolymer resin, a polyvinyl chloride resin, and chloride. Vinyl chloride-based resins such as vinyl vinyl acetate copolymer resin and vinyl chloride vinyl acetate maleic acid copolymer resin; polyvinyl-based resins such as polyvinyl acetal resin, polyvinyl butyral resin, and polyvinylpyrrolidone resin; polyurethane resin; polyketone resin; polyester resin; Polypropylene resin; polylactic acid resin; cellulose resin; cellulose acetate resin; butyral resin and the like can be mentioned. Among the above, a polyacrylic acid resin and / or a styrene acrylic acid copolymer resin is used because it has excellent compatibility with the polymerizable compound contained in the ink and can exhibit excellent ejection stability and adhesion. Is more preferable.

The polyacrylic acid resin is, for example, a homopolymer of (meth) acrylic acid or (meth) acrylic acid alkyl ester, or a copolymer of (meth) acrylic acid and (meth) acrylic acid alkyl ester. The styrene acrylic acid copolymer resin is, for example, a copolymer of (meth) acrylic acid and / or (meth) acrylic acid alkyl ester and a styrene-based monomer.

The polyacrylic acid resin and / or the styrene acrylic acid copolymer preferably used in one embodiment is commercially available in the form of flakes, an aqueous solution, or an aqueous dispersion (emulsion). Specific examples of commercially available products include, but are limited to, the Joncryl (registered trademark) series manufactured by BASF, the ARUFON (registered trademark) series manufactured by Toagosei, and the SMA series manufactured by Cray Valley USA. is not.

In one embodiment, the acid value of the inert resin is preferably 5 mgKOH / g or more, more preferably 20 mgKOH / g or more, still more preferably 50 mgKOH / g or more. Further, 400 mg / KOH or less is preferable, 300 mg / KOH or less is more preferable, and 250 mg / KOH / g or less is further preferable. Within the above range, not only the adhesion is excellent, but also the storage stability of the ink can be suitably maintained.

The weight average molecular weight of the inert resin is preferably 1,000 or more and 50,000 or less, more preferably 1,500 or more and 45,000 or less, and 1,500 or more and 15,000 or less. It is more preferable to have. By keeping it within the above range, not only the ejection stability during high-speed and continuous printing is improved, but also the wet spreadability, print image quality, and glossiness of the inkjet ink during printing are improved, and the adhesion to the recording medium is improved. The sex is also excellent.

(Surface conditioner)
As used herein, the term "surface conditioner" means a substance that reduces the surface tension of an ink when added. Examples of the surface adjusting agent include a silicone-based surface adjusting agent, a fluorine-based surface adjusting agent, an acrylic-based surface adjusting agent, an acetylene glycol-based surface adjusting agent, and the like. From the viewpoint of the ability to reduce the surface tension and the compatibility with the polymerizable monomer, it is preferable to use a silicone-based surface preparation agent.

Specific examples of the silicone-based surface conditioner include modified products having a dimethylsiloxane skeleton. Of these, a polyether-modified siloxane-based surface conditioner is preferable.
The polyether may be, for example, polyethylene oxide and / or polypropylene oxide. In one embodiment of the present invention, a commercially available polyether-modified silicone-based surfactant can be used.
Representative products that can be preferably used include polyether-modified siloxanes such as BYK®-378, 348, and 349 from Big Chemie; and polyether-modified polydimethylsiloxanes such as BYK-UV3500 and UV3510. Be done. Also mentioned are polyether-modified siloxane copolymers such as TEGO® GLIDE 450, 440, 435, 432, 410, 406, 130, 110, and 100 from Evonik Degussa. Among these, a polyether-modified silicone-based surface conditioner such as BYK-331, 333, 378, 348, UV3510, TEGO GLIDE 450, 440, 432, and 410 is preferable from the viewpoint of improving print image quality.

The content of the silicone-based surface conditioner is preferably 0.05 to 5% by mass with respect to the total mass of the inkjet ink. By setting the content to 0.05% by mass or more, the wettability of the inkjet ink to the recording medium can be improved. On the other hand, by setting the content to 5% by mass or less, it becomes easy to secure the storage stability of the inkjet ink.

(Polymerization inhibitor)
A polymerization inhibitor can be used to enhance the viscosity stability of the inkjet ink over time, the ejection stability after aging, and the viscosity stability in the inkjet recording apparatus. As the polymerization inhibitor, a hindered phenol-based compound, a phenothiazine-based compound, a hindered amine-based compound, and a phosphorus-based compound are particularly preferably used.
Specific examples thereof include 4-methoxyphenol, hydroquinone, methylhydroquinone, t-butylhydroquinone, 2,6-di-t-butyl-4-methylphenol, phenothiazine, and aluminum salts of N-nitrosophenylhydroxylamine. ..
Among them, it is preferable to contain a hindered phenol-based compound and / or a phenothiazine-based compound, and it is more preferable to contain 2,6-di-t-butyl-4-methylphenol and phenothiazine.

From the viewpoint of improving the stability over time while maintaining the curability, the content of the polymerization inhibitor is preferably 0.01 to 2% by mass with respect to the total mass of the inkjet ink.

(Organic solvent)
In one embodiment, an organic solvent may be contained in order to reduce the viscosity of the inkjet ink and improve the wettability to the recording medium. The boiling point of the organic solvent is preferably 120 ° C. to 300 ° C., more preferably 140 to 270 ° C., and even more preferably 160 to 260 ° C.

Examples of the organic solvent include glycol compounds such as monoacetates, diacetates, diols, monoalkyl ethers, dialkyl ethers, and lactic acid esters.
Of these, monoacetates, monoalkyl ethers, and dialkyl ethers, which are glycol compounds, are preferable.
Of these, tetraethylene glycol dialkyl ether, ethylene glycol monobutyl ether acetate, and diethylene glycol diethyl ether are preferable.

The content of the organic solvent is preferably 0.1 to 10% by mass with respect to the total mass of the inkjet ink. By setting the content within the above range, it becomes easy to obtain good results for each property of curability, discharge stability and adhesion. From the viewpoint of curability, the content of the organic solvent is more preferably 0.2 to 5% by mass with respect to the total mass of the inkjet ink. Further, from the viewpoint of discharge stability, 0.5 to 4% by mass is preferable.

<Ink physical characteristics>
[viscosity]
The inkjet ink of the present invention preferably has a viscosity of 40 mPa · s or less at 25 ° C. in consideration of ejection properties. It is more preferably 5 to 40 mPa · s, and even more preferably 7 to 30 mPa · s.
Further, the viscosity at the discharge temperature (preferably 25 to 80 ° C., more preferably 25 to 50 ° C.) is preferably 3 to 15 mPa · s, and more preferably 3 to 13 mPa · s.
In the inkjet ink of the present invention, it is preferable to appropriately adjust the composition ratio so that the viscosity is within the above range.

[surface tension]
The surface tension of the inkjet ink of the present invention at 25 ° C. is preferably 20 to 40 mN / m. More preferably, it is 22 to 39 mN / m.
When recording on various recording media such as soft vinyl chloride, acrylic, polyolefin, PET, coated paper, and uncoated paper, 20 mN / m or more is preferable from the viewpoint of bleeding and penetration, and 40 mN / m or less in terms of wettability. Is preferable.

<Inkjet recording method>
The inkjet ink set of the present invention is used for inkjet recording.
The inkjet ink set of the present invention includes a step of ejecting the inkjet ink of the present invention onto a recording medium and a step of irradiating the ejected inkjet ink with active energy rays to cure the ink jet ink. It is characterized by that.

[Recorded medium]
In the present invention, the recording medium is specifically polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), polycarbonate, hard vinyl chloride, soft vinyl chloride, polystyrene, foamed styrol, acrylic (PMMA, etc.) and the like. Examples thereof include a plastic base material made of the above materials; a paper base material such as art-coated paper, semi-gloss-coated paper, cast-coated paper, and high-quality paper; a glass base material; a metal base material such as stainless steel and aluminum vapor-deposited paper.

The surface of the recording medium may be smooth, may have an uneven shape, and may be transparent, translucent, or opaque. Further, the recording medium may be a medium in which two or more of the above-mentioned various base materials are bonded to each other. Further, the recording medium may have a functional layer such as a peeling adhesive layer on the opposite side of the printing surface. In another embodiment of the recording medium, a functional layer such as an adhesive layer may be provided on the printing surface after printing.

In particular, plastic substrates such as hard vinyl chloride, soft vinyl chloride, acrylic (PMMA and the like), and polycarbonate are preferable.
The thickness of the plastic base material is preferably 2 to 100 μm, more preferably 6 to 50 μm.

It is preferable to apply a surface treatment such as a corona discharge treatment or an easy-adhesion treatment to the surface of the plastic base material or the coated paper. For example, the corona discharge treatment breaks the molecular bonds on the surface of the substrate, and further reacts with the oxygen radicals and ozone in the atmosphere generated by the corona discharge to introduce polar functional groups such as hydroxyl groups, carbonyl groups, and carboxyl groups. Will be done. Good adhesion and gloss can be exhibited by the interaction between this polar group and the resin having a diallyl phthalate structure or a diallyl phthalate structure contained in the ink.

The recording device used for recording the inkjet ink set of the present invention is not particularly limited, and a known inkjet recording device provided with an inkjet head for achieving a desired resolution can be used. That is, any known inkjet recording device including a commercially available product can eject the inkjet ink of the present invention onto a recording medium.
[Discharge process]
The resolution (nozzle density) of the inkjet head is preferably 90,000 dots / inch2 (150 × 600 dpi) or more, more preferably 180,000 dots / inch2 (300 × 600 dpi) or more, and 360,000 dots / inch2 (600 × 600 dpi) or more. Is even more preferable.

[Curing process]
The light source in the curing step is not particularly limited, and a known light source can be used.
Specific examples thereof include mercury lamps, xenon lamps, metal hydride lamps, UV-LEDs, LEDs (light emitting diodes) such as ultraviolet laser diodes (UV-LD), and gas / solid-state lasers. Above all, it is preferable to use UV-LED.
From the viewpoint of curability, 380 to 420 nm is preferable, and 380 to 410 nm is more preferable. When it is 380 nm or more, the safety is excellent. Further, when it is 420 nm or less, it is preferable because it has excellent curability.

Since the inkjet ink of the present invention has sufficient sensitivity, it can be sufficiently cured even with low-power active radiation. Specifically, the maximum illuminance on the surface of the recording medium is sufficiently cured within the range of 10 to 2,000 mW / cm ² .
When the maximum illuminance on the surface of the recording medium is 10 mW / cm ² or more, the curability is excellent, and the image is not sticky or the image quality is not deteriorated.
Further, when the maximum illuminance on the surface of the recording medium is 2,000 mW / cm ² or less, the cured ink composition does not proceed excessively quickly, and the image quality is caused by the formation of irregularities on the image surface. The decrease in ink is suppressed.
The maximum illuminance on the surface of the recording medium is preferably 100 to 1,800 mW / cm ² and more preferably 200 to 1,500 mW / cm ² from the viewpoint of image quality and productivity.

It is appropriate that the inkjet ink of the present invention is irradiated with such ultraviolet rays for preferably 0.01 to 2 seconds, more preferably 0.1 to 1.5 seconds, and even more preferably 0.3 to 1 second. be.

Specifically, the method of irradiating the active energy rays is performed by providing light sources on both sides of the inkjet head unit including the inkjet recording device and scanning the head unit and the light source by a so-called shuttle method.
As described above, by using a small and lightweight UV-LED as a light source provided in the moving portion, it is possible to reduce the size and energy saving of the inkjet recording device, and it is possible to form an image with high productivity. Further, since the UV-LED has excellent variability of exposure conditions, suitable exposure conditions can be set according to the inkjet ink, and an image can be formed with high productivity.

In the curing step, the energy applied by the UV-LED, that is, the energy (integrated light amount) applied to the inkjet ink on the recording medium by irradiation with ultraviolet rays is preferably 100 to 1,000 mJ / cm ² , preferably 150 to 800 mJ / cm. ² is more preferable, and 200 to 700 mJ / cm ² is even more preferable. Within the above range, productivity and curability can be achieved at the same time, which is preferable.

The inkjet ink set of the present invention is used as the inkjet recording method. The order of the inkjet inks to be ejected is not particularly limited, but it is preferable to apply the inkjet inks having the lowest brightness to the recording medium.
For example, when using as an inkjet ink set containing at least 7 colors of light color ink such as light cyan and light magenta and dark color ink of cyan, magenta, black, yellow and white, white → light cyan → light magenta → yellow. It is preferable to apply magenta, cyan, and black on the recording medium in this order. In this way, an image can be formed on the surface of the recording medium by curing with high sensitivity by irradiation with ultraviolet rays.

Examples and comparative examples are shown below, and the present invention will be described in more detail. However, the present invention is not limited thereto. In the examples and comparative examples, "parts" and "%" represent "parts by mass" and "% by mass", respectively.

<Manufacturing method of inkjet ink>
[Creation of pigment dispersion]
A cyan pigment dispersion was prepared with the following composition. Hereinafter, the dispersion was prepared by adding a pigment and a dispersant to the polymerizable compound, stirring the mixture with a high-speed mixer or the like until uniform, and then dispersing the obtained mill base with a horizontal sand mill for about 1 hour.
・ LIONOL BLUE FG-7400G (Toyo Color Phthalocyanine Pigment CI Pigment Blue 15: 4) 30 parts ・ Sol Spurs 32000 (Lubrisole Pigment Dispersant) 9 parts ・ Phenoxyethyl acrylate 61 parts

A magenta pigment dispersion was prepared with the following composition. The dispersion was prepared in the same manner as the cyan dispersion.
・ Hostaperm Red E5B02 (Clariant quinacridone pigment CI Pigment Violet 19) 20 parts ・ Sol Spurs 24000 (Lubrisole pigment dispersant) 6 parts ・ Phenoxyethyl acrylate 74 parts

[Creating inkjet ink]
The raw materials shown in Table 1 were mixed, and after stirring for 2 hours, it was confirmed that there was no undissolved residue, and filtration was performed with a membrane filter to remove coarse particles causing head clogging, and an inkjet was prepared. It should be noted that the numerical values in Table 1 represent "parts by mass" unless otherwise specified, and blanks indicate that they are not blended.

Each component in Table 1 is as follows.
-VCAP: N-vinylcaprolactam (BASF's "NVC")
THA: Tetrahydrofurfuryl acrylate ("SR285" manufactured by Arkema Co., Ltd.)
-CTFA: Cyclic trimethylolpropane formal acrylate ("SR531" manufactured by Arkema Co., Ltd.)
TBCH: 4-tert-butylcyclohexylacrylate ("SR217" manufactured by Arkema Co., Ltd.)
-IBXA: Isobornyl acrylate ("SR506" manufactured by Arkema Co., Ltd.)
-PEA: Phenoxyethyl acrylate ("SR339" manufactured by Arkema Co., Ltd.)
-LA: Lauryl acrylate ("SR335" manufactured by Arkema Co., Ltd.)
BisA-EO4A: Ethoxylation (4) Bisphenol A diacrylate ("SR601" manufactured by Arkema Co., Ltd.)
BisA-EO10A: Ethoxylation (10) Bisphenol A diacrylate ("SR602" manufactured by Arkema Co., Ltd.)
BisA-EO30A: Ethoxylated (30) bisphenol A diacrylate (Miwon Specialty Chemical Co., Ltd. "Miramer M2300")
-DPGDA: Dipropylene glycol diacrylate (SR508 manufactured by Arkema Co., Ltd.)
-TMPTA: Trimethylolpropane triacrylate ("SR351" manufactured by Arkema Co., Ltd.)
-TPO: 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (BASF's "DAROCUR TPO")
-Ir819: Bis (2,4,6-trimethylbenzoyl) -Phenylphosphine oxide (BASF's "Irgacure 819")
-Ir369: 2-Benzyl-2-dimethylamino-1- (4-morpholinophenyl) -butanone-1 ("Irgacure 369" manufactured by BASF)
-ITX: 2-Isopropylthioxanthone ("SpeedCure2-ITX" manufactured by LAMBSON)
Ir907: 2-Methyl-1- (4-Methylthiophenyl) -2-morpholinopropan-1-one (BASF's "Irgacure 907")
-BHT: Dibutylhydroxytoluene ("H-BHT" manufactured by Honshu Chemical Industry Co., Ltd.)
・ Phenothiazine: "Phenothiazine" manufactured by Seiko Kagaku Co., Ltd.
BYK-UV3510: Polyether-modified polydimethylsiloxane (manufactured by BYK-Chemie)

<How to make a test sample>
Using an inkjet recording device, a 12 μm cured film was prepared on soft vinyl chloride as a recording medium by the following printing method, and a test sample was prepared.
The inkjet head was driven so that it could eject at a resolution of 360 x 2160 dpi with 10 to 30 pl multi-size dots.
The exposure conditions, main scanning speed, and ejection frequency were adjusted so that the inkjet ink landed on the recording medium and irradiation started 0.1 to 0.3 seconds later, and the ultraviolet rays of 200 mW / cm ² were irradiated 6 times. ..
As the ultraviolet lamp, a UV-LED manufactured by Phoseon Technology, which emits ultraviolet rays having a wavelength of 395 nm, was used.

[Hue angle]
Using the test sample prepared by the above method, the hue angle was measured with SPM100-II manufactured by Gretag. If it is 2 or more, it is evaluated that there is no problem in practical use.
5: The difference in hue angle between the cured film of dark color ink and the cured film of light color ink is 0 ° or more and less than 8 ° 4: The difference in hue angle between the cured film of dark color ink and the cured film of light color ink is 8 ° or more 15 Less than ° 3: The difference in hue angle between the cured film of dark color ink and the cured film of light color ink is 15 ° or more and less than 23 ° 2: The difference in hue angle between the cured film of dark color ink and the cured film of light color ink is 23 ° Less than 30 ° 1: The difference in hue angle between the cured film of dark color ink and the cured film of light color ink is 30 ° or more.

[Graininess evaluation]
Using the above printing method, a 16-gradation halftone image was created with each dark ink and light ink, and the graininess of the highlight portion (10 to 30%) was evaluated. If it is 3 or more, it is evaluated that there is no problem in practical use.
5: There is no graininess in the image.
4: There is a slight graininess in the image.
3: There is a slight graininess in the image.
2: There is a feeling of roughness in the image.
1: The image is quite grainy.

[Curability evaluation]
One minute after the test sample prepared by the above method was prepared, the curability was evaluated by rubbing the surface of the cured film with a nail and confirming the cured state. If it is 2 or more, it is evaluated that there is no problem in practical use.
5: The cured film does not come off even if it is rubbed strongly with a nail. There is no feeling of tack on the surface.
4: If you rub it strongly with your nails, part of the cured film will come off. There is no feeling of tack on the surface.
3: Part of the cured film peels off when rubbed with a nail. There is no feeling of tack on the surface.
2: Part of the cured film peels off when rubbed with a nail. There is a slight tack on the surface.
1: The cured film can be easily peeled off by touching your nails. There is a slight tack on the surface.

[Evaluation of stretchability]
After bending the test sample prepared by the above method to 180 °, the bent portion was visually observed and observed with a loupe. If it is 3 or more, it is evaluated that there is no problem in practical use.
5: No cracks were observed when observed visually or with a loupe.
4: No cracks were visually observed, but slight cracks were observed on the loupe.
3: A slight crack was visually observed.
2: A clear crack was visually observed.
1: The coating film peeled off.

[Evaluation of stretchability after outdoor exposure]
The test sample prepared by the above method was left outdoors for 2 weeks, and then evaluated by the same method as the above-mentioned stretchability evaluation.

[Discharge stability evaluation]
A nozzle check pattern was printed on coated paper (Raflacoat manufactured by UPM Rafratac) with the inkjet device, and it was confirmed that there was no nozzle omission. Next, after 100,000 droplets were ejected from each nozzle, the nozzle check pattern was printed again on the coated paper, and the ejection stability was evaluated by counting the number of nozzles missing. If it is 2 or more, it is evaluated that there is no problem in practical use.
5: Nozzle missing after 100,000 printing 4: Nozzle missing after 100,000 printing 3: Nozzle missing after 100,000 printing 3-5 nozzle missing after 100,000 printing 6: Nozzle missing after 100,000 printing 1 : 10 or more nozzles missing after printing 100,000 shots

According to the embodiment of the present invention, in an inkjet ink set containing at least one pair of dark ink and light ink having the same hue, the curability is excellent, and the stretchability of the obtained cured film, particularly the stretchability at a low temperature, is excellent. It was found that it is possible to obtain an inkjet ink set that is excellent in color, excellent in hue, and excellent in ejection stability and can obtain high image quality.

Claims (11)

An inkjet ink set containing at least one pair of dark and light inks having the same hue.
Both the dark ink and the light ink contain a pigment (A), a photopolymerization initiator (B), and a polymerizable compound (C).
When the content of the photopolymerization initiator (B) in the light-colored ink is (α) and the content of the photopolymerization initiator (B) in the dark-colored ink is (β), 0. 1 <α-β <10 ,
The hue angle H ° (X) of the dark ink and the hue angle H ° (Y) of the light ink at the hue angle H ° defined in the CIELAB color space on the recording medium are
｜ XY ｜ <30 °
Inkjet ink set characterized by satisfying the relationship of. The inkjet ink set according to claim 1, wherein the dark-colored ink and the light-colored ink contain an acylphosphine oxide-based compound (B-1) as the photopolymerization initiator (B). The dark ink is characterized in that the photopolymerization initiator (B) further contains a thioxanthone-based compound (B-2) and / or an α-aminoalkylphenone-based compound (B-3). The inkjet ink set according to 1 or 2. The inkjet according to any one of claims 1 to 3, wherein the light-colored ink does not substantially contain the thioxanthone-based compound (B-2) as the photopolymerization initiator (B). Ink set. One of claims 1 to 4, wherein the light-colored ink does not substantially contain the α-aminoalkylphenone compound (B-3) as the photopolymerization initiator (B). Inkjet ink set described in. The dark-colored ink and the light-colored ink contain a monofunctional monomer (C-1) and a polyfunctional monomer (C-2) as the polymerizable compound (C).
The mass ratio (γ) of the polyfunctional monomer (C-2) / polymerizable compound (C) in the dark-colored ink and the polyfunctional monomer (C-2) / polymerizable compound (C) in the light-colored ink. The inkjet ink set according to any one of claims 1 to 5, wherein the mass ratio (δ) of) satisfies the relationship of γ <δ. The inkjet ink set according to claim 6, wherein the monofunctional monomer (C-1) contains a monofunctional monomer (C-1-1) having a cyclic structure in the molecule. The monofunctional monomer (C-1-1) having a cyclic structure in the molecule has an acryloyl group and / or a monofunctional monomer having a heterocyclic structure and / or an acryloyl group and has an acryloyl structure. The inkjet ink set according to claim 7, further comprising a monofunctional monomer having the above. The monofunctional monomer (C-1-1) having a cyclic structure in the molecule contains N-vinylcaprolactam, and the content of the N-vinylcaprolactam is 5 to 35 mass with respect to the total mass of the inkjet ink. The inkjet ink set according to claim 7 or 8, characterized in that it is%. The invention according to any one of claims 6 to 9, wherein the polyfunctional monomer (C-2) contains a bifunctional monomer having two acryloyl groups and having a cyclic structure in the molecule. Inkjet ink set. The dark ink contains cyan ink and magenta ink.
The inkjet ink set according to any one of claims 1 to 10, wherein the light-colored ink contains a light cyan ink and a light magenta ink.