JP7056343B2 - Active ray-curable ink and image formation method using it - Google Patents

Description

The present invention relates to an active photocurable ink and an image forming method using the same.

In recent years, the inkjet recording method has been used in various printing fields such as photography, various printing, marking, and special printing such as color filters from the viewpoint of being able to easily and inexpensively create an image.

The inkjet inks used in such an inkjet recording method include water-based inkjet inks containing water as the main solvent, oil-based inkjet inks mainly containing non-volatile solvents that do not volatilize at room temperature, and oil-based ink inks that do not substantially contain water and volatilize at room temperature. Non-aqueous inkjet inks that mainly contain solvents and do not contain substantially water, hot melt inks that heat and melt solid inks at room temperature for printing, and active light-curing inkjet inks that cure with active rays such as ultraviolet rays after printing. There are various types of inkjet inks, such as, and they are used properly according to the application. Among them, for example, the active light-curing inkjet ink described in Patent Document 1 has a fast-curing property and can print on a wide variety of recording media, so that the drying load is large and the recording medium is limited. It is attracting attention as a next-generation inkjet ink that replaces water-based inkjet inks, oil-based inkjet inks, and non-aqueous inkjet inks, and is expected to expand its applications.

Japanese Unexamined Patent Publication No. 2009-191183

However, the image formed from the active ray-curable inkjet ink described in Patent Document 1 has a problem that the pigment is dropped due to uneven density or friction. Therefore, there has been a demand for an active light-curable ink capable of forming an image having less uneven density (excellent in image quality) and less likely to cause pigments to fall off due to friction (excellent in rubbing resistance).

Therefore, an object of the present invention is to provide an active ray-curable ink capable of forming an image having excellent image quality and rubbing resistance.

As a result of diligent research, the present inventor is an active photocurable ink containing a photopolymerizable compound, a photopolymerization initiator, a pigment and a pigment dispersant, and the photopolymerization initiator and the pigment dispersant are ( We have found that the above-mentioned problems can be solved by an active photocurable ink which is a compound having a meta) acryloyl group, and have completed the present invention.

According to the active light curable ink according to the present invention, it is possible to form an image having excellent image quality and rubbing resistance.

As used herein, "(meth) acryloyl" refers to methacryloyl and / or acryloyl. Similarly, "(meth) acrylate" refers to methacrylate and / or acrylate.

In the present specification, "XY" indicating a range means "X or more and Y or less". Further, in the present specification, unless otherwise specified, the operation and the measurement of physical properties are performed under the conditions of room temperature (20 to 25 ° C.) / relative humidity of 40 to 50% RH.

<Active ray curable ink>
One embodiment of the present invention is an active photocurable ink containing a photopolymerizable compound, a photopolymerization initiator, a pigment and a pigment dispersant, wherein the photopolymerization initiator and the pigment dispersant are (meth) acryloyl groups. It is an active light curable ink which is a compound having. According to the active ray-curable ink according to the present invention, it is possible to form an image in which density unevenness is small (excellent in image quality) and the pigment is not easily removed by friction (excellent in rubbing resistance). Hereinafter, the active light curable ink of the present invention is also simply referred to as "ink".

The present inventor has made a pigment, a photopolymerization initiator, and a pigment in the cured product as the reason why the pigment is dropped due to uneven density or friction in the image (cured product) formed from the ink described in Patent Document 1. It is presumed that this is because the dispersant and the like exist in a free state, and unevenness occurs due to the aggregation of each component, especially the aggregation of the pigment. Therefore, the design of the active ray-curable ink has been diligently studied, and the configuration of the active ray-curable ink according to the present invention has been found.

Since the photopolymerization initiator and pigment dispersant constituting the ink of the present invention have a (meth) acryloyl group, they can be irradiated with active light (ultraviolet rays, visible light, X-rays, etc.) to form a cured product together with the photopolymerizable compound. Form. Therefore, the pigment dispersed in the ink due to the action of the pigment dispersant is also incorporated into the cured product. As a result, in the cured product formed from the ink of the present invention, no free material is present, or even if it is present, the amount is very small. Therefore, it is considered that the aggregation of the pigment is less likely to occur and the density unevenness of the formed image is improved (that is, the image quality is improved). Further, it is considered that the pigment is incorporated into the cured product as described above, so that the pigment is less likely to come off due to friction (that is, the rubbing resistance is improved).

The above mechanism is speculative, and the present invention is not limited to the above mechanism.

Hereinafter, the constituent components of the active light curable ink of the present invention will be described.

[Photopolymerizable compound]
The photopolymerizable compound used in the ink of the present invention is a compound that is cured by irradiation with active light (ultraviolet rays, visible light, electron beam, etc.), and can be used in any of the photopolymerization initiators and pigment dispersants described later. It means something that does not apply.

The weight average molecular weight of the photopolymerizable compound is preferably less than 1,500. The weight average molecular weight is a value measured by gel permeation chromatography (GPC) using polystyrene as a standard material.

The photopolymerizable compound is preferably an unsaturated carboxylic acid ester compound, more preferably a (meth) acrylate.

Examples of the (meth) acrylate include triethylene glycol di (meth) acrylate, tetraethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, tripropylene glycol di (meth) acrylate, and polypropylene glycol di (meth). Acrylate, 1,4-butanediol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate, 1,9-nonanediol di (meth) acrylate, 1,10-decanediol di (meth) acrylate, Neopentyl glycol di (meth) acrylate, dimethylol-tricyclodecanedi (meth) acrylate, hydroxypivalate neopentyl glycol di (meth) acrylate, polytetramethylene glycol di (meth) acrylate, polyethylene glycol diacrylate, tripropylene glycol Bifunctional (meth) acrylates such as diacrylates; trimethylol propanetri (meth) acrylates, pentaerythritol tri (meth) acrylates, pentaerythritol tetra (meth) acrylates, dipentaerythritol hexa (meth) acrylates, ditrimethylol propanetetra (ditrimethylol propanetetra (meth) acrylates. Examples thereof include trifunctional or higher polyfunctional (meth) acrylates such as meth) acrylates, glycerin propoxytri (meth) acrylates, and pentaerythritol ethoxytetra (meth) acrylates.

The (meth) acrylate may be a modified product, for example, an ethylene oxide-modified (meth) acrylate such as an ethylene oxide-modified trimethylolpropane tri (meth) acrylate or an ethylene oxide-modified pentaerythritol tetraacrylate, or a propylene oxide-modified trimethylol. Polypropylene oxide-modified (meth) acrylates such as propanetriacrylates alkylene oxide-modified (meth) acrylates such as caprolactone-modified trimethylolpropanetri (meth) acrylates and other caprolactone-modified (meth) acrylates; and caprolactam-modified dipentaerythritol hexa (meth). ) Caprolactam-modified (meth) acrylate such as acrylate can be mentioned. Of these, alkylene oxide-modified (meth) acrylate is preferable.

As the photopolymerizable compound, a commercially available product or a synthetic product may be used. Commercially available products include CD561, SR454, SR499, SR494 manufactured by Sartmer, NK Ester A-400, NK Ester A-600, NK Ester 9G, NK Ester 14G, and EM2381 manufactured by Choko Chemical Industry Co., Ltd. , V # 335HP manufactured by Osaka Organic Chemical Industry Co., Ltd., Photomer (registered trademark) 4072 manufactured by Cognis, Miramer M360 manufactured by MIWON, and the like can be used.

The photopolymerizable compound may be used alone or in combination of two or more. Above all, from the viewpoint of curing speed, curing shrinkage, and degree of curing, it is preferable to use a non-modified (meth) acrylate and an alkylene oxide-modified (meth) acrylate in combination. At this time, the content of the non-modified (meth) acrylate in the photopolymerizable compound is preferably 35 to 65% by mass, and the content of the alkylene oxide-modified (meth) acrylate in the photopolymerizable compound is preferably. It is 35 to 65% by mass.

[Photopolymerization initiator]
The photopolymerization initiator used in the ink of the present invention is a compound having a (meth) acryloyl group and generating an active species by irradiation with active light (ultraviolet rays, visible light, electron beam, etc.).

The photopolymerization initiator of the present invention preferably has an absorption wavelength region of 340 to 440 nm. When the energy is 340 nm or more, the energy is low, so that the components contained in the ink are not easily decomposed and the generation of free substances is suppressed. Further, when it is 440 nm or less, it does not overlap with the absorption wavelength region of the pigment, so that the photopolymerization initiator functions well. Above all, from the viewpoint of further improving the rubbing resistance, it is more preferable that the photopolymerization initiator has an absorption wavelength region at 380 to 400 nm.

In the present invention, the photopolymerization initiator is preferably a compound represented by the following general formula 1.

In the general formula 1, X ₁ , X ₂ , X ₃ and Y are each independently a hydrogen atom, an alkyl group or-(CH ₂ ) _n -R, and at least one of X ₁ to X ₃ is. -(CH ₂ ) _n -R, where n is an integer of 1 to 6 and R is a (meth) acryloyloxy group. In the general formula 1, the two X _1s may be the same or different, but are preferably the same. The same applies to two _X2s and _two X3s.

The compound represented by the general formula 1 has a high reaction rate when irradiated with light having a wavelength of 340 to 440 nm (particularly 380 to 400 nm). Therefore, when the ink is cured using such a compound as a photopolymerization initiator, the image quality and / and the rubbing resistance of the formed image may be further improved.

Here, examples of the alkyl group include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group and an isopentyl group. It refers to a neopentyl group, an n-hexyl group, an n-heptyl group, an n-octyl group, an n-nonyl group, an n-decyl group and the like.

Above all, from the viewpoint of further improving the effect of the present invention, the photopolymerization initiator is preferably a compound represented by the following general formula 1'.

In the above general formula 1', R is a (meth) acryloyloxy group. Further, in the above general formula 1', n is an integer of 1 to 6 independently, and is preferably an integer of 1 to 4 independently from the viewpoint of further improving the image quality, and is independent of each other. It is more preferably an integer of 1 to 3, and even more preferably 1 or 2 independently of each other, and particularly preferably 2.

The photopolymerization initiator may be used alone or in combination of two or more. Further, when the photopolymerization initiator represented by the general formula 1 is used, it may be used in combination with a conventionally known photopolymerization initiator.

As the photopolymerization initiator, a commercially available product or a synthetic product may be used. When synthesizing the photopolymerization initiator represented by the above general formula 1', for example, as described in Examples described later, after halogenating the alkyl group added to the phosphine oxide, a halogen-lithium exchange reaction is carried out. It can be obtained by producing an organic lithium compound and reacting it with a (meth) acrylic acid halogenated alkyl ester.

In the active photocurable ink of the present invention, the photopolymerization initiator is preferably contained in an amount of 0.3 to 10% by mass based on the photopolymerizable compound. If it is 0.3% by mass or more, the curability is sufficient. Further, when it is 10% by mass or less, the brittleness of the image is low and the rubbing resistance is good. Above all, from the viewpoint of further improving the image quality and the rubbing resistance, it is preferably 1 to 8% by mass, and more preferably 2 to 6% by mass.

[Pigment]
In the present invention, either an inorganic pigment or an organic pigment can be used as the pigment.

Examples of the inorganic pigment include carbon black, titanium oxide, calcium carbonate and the like.

Organic pigments include insoluble azo pigments such as toluidine red, toluidine maroon, hanza yellow, benzidine yellow, and pyrazolone red, and soluble azo pigments such as litol red, heliobordeaux, pigment scarlet, and permanent red 2B; alizarin, indantron, and thio. Derivatives from building dyes such as indigo maroon; phthalocyanine organic pigments such as phthalocyanine blue and phthalocyanine green; quinacridone organic pigments such as quinacridone red and quinacridone magenta; perylene organic pigments such as perylene red and perylene carlet; isoindolinone Isoindolinone-based organic pigments such as yellow and isoindolinone orange; pyranthron-based organic pigments such as pyranthron red and pyranthron orange; thioindigo-based organic pigments, condensed azo-based organic pigments, benzimidazolone-based organic pigments, quinophthalone yellow, etc. Kinophthalone-based organic pigments; Isoindrin-based organic pigments such as Isoindrin Yellow; Other pigments include flavanthron yellow, acylamide yellow, nickel azo yellow, copper azomethin yellow, perinone orange, anthron orange, and dianthraquinonyl red. , Dioxazine violet and the like.

Organic pigments are exemplified below by Color Index (CI) number:
C. I. Pigment Yellow 12, 13, 14, 17, 20, 24, 74, 83, 86, 93, 109, 110, 117, 120, 125, 128, 129, 137, 138, 139, 147, 148, 150, 151, 153, 154, 155, 166, 168, 180, 185;
C. I. Pigment Orange 16, 36, 43, 51, 55, 59, 61;
C. I. Pigment Red 9, 48, 49, 52, 53, 57, 97, 122, 123, 149, 168, 177, 180, 192, 202, 206, 215, 216, 217, 220, 223, 224, 226, 227, 228, 238, 240;
C. I. Pigment Violet 19, 23, 29, 30, 37, 40, 50;
C. I. Pigment Blue 15, 15: 1, 15: 3, 15: 4, 15: 6, 22, 60, 64;
C. I. Pigment Green 7, 36;
C. I. Pigment Brown 23, 25, 26.

Among the above pigments, quinacridone-based, phthalocyanine-based, benzimidazolone-based, isoindolinone-based, condensed azo-based, quinophthalone-based, and isoindoline-based organic pigments are preferable because they have excellent light resistance.

The pigment may be used alone or in combination of two or more. Further, as the pigment, a commercially available product or a synthetic product may be used.

The organic pigment is preferably fine particles having an average particle size (volume-based median diameter) of 10 to 150 nm in the ink as measured by laser scattering. When the diameter is 10 nm or more, it is possible to suppress a decrease in light resistance due to a small particle size. Further, when the size is 150 nm or less, the dispersion can be stably maintained, the precipitation of the pigment can be suppressed, the discharge stability can be improved, and the generation of minute mist called satellite can be suppressed.

The pigment may be surface-treated by a known technique such as an acid treatment, a basic treatment, a sinargist, and a coupling treatment in order to promote the adsorption of the pigment dispersant on the surface thereof.

In the active photocurable ink of the present invention, the pigment is preferably contained in an amount of 0.5 to 15% by mass, more preferably 1 to 6% by mass, based on the photopolymerizable compound.

[Pigment dispersant]
The pigment dispersant used in the ink of the present invention is a compound having a (meth) acryloyl group and having a function of dispersing the pigment in the ink. In general, a pigment dispersant has a site that adsorbs to a pigment and a site that has a high affinity for a photopolymerizable compound or a solvent, and the (meth) acryloyl group corresponds to the latter. According to the study by the present inventor, when a pigment dispersant having a (meth) acrylamide group was used instead of the (meth) acryloyl group, it was inferior in terms of both image quality and rubbing resistance (comparison described later). See Example 7). The reason is not clear, but it is speculated that the affinity with the pigment is weaker with the amide than with the ester, and it may be difficult to stabilize the dispersed state of the pigment.

The weight average molecular weight of the pigment dispersant is preferably 1,500 or more, more preferably 3,000 or more, from the viewpoint of the dispersion stability of the pigment. The weight average molecular weight of the pigment dispersant is preferably 200,000 or less, more preferably 100,000 or less, from the viewpoint of suppressing an increase in the viscosity of the system and the resulting influence on the dispersion time. The weight average molecular weight is a value measured by gel permeation chromatography (GPC) using polystyrene as a standard material.

In the present invention, when a pigment having an aromatic ring such as an azo pigment or a quinacridone pigment is used, the pigment dispersant preferably has a structural unit represented by the following formula 1. By having the main chain of the styrene skeleton as shown in the following formula 1, the adsorptivity to the pigment becomes good, and the pigment can be stably dispersed in the ink. Therefore, it is considered that the density unevenness of the image formed from the ink can be improved.

In the above formula 1, _Ra1 is a (meth) acryloyloxy group, and above all, from the viewpoint of further improving image quality and / or rubbing resistance, _Ra1 is more preferably a methacryloyloxy group. In Equation 1, _Ra1 is preferably located at the para position (4th position) of the benzene ring.

Among all the structural units constituting the pigment dispersant, the structural unit represented by the formula 1 is preferably 10 to 100 mol%, more preferably 20 to 40 mol%. The numerical value can be controlled within a desired range by adjusting, for example, the amount of the raw material monomer charged and the amount of (meth) acrylic acid or a derivative thereof added in the production method described later.

When a π-conjugated system, particularly a pigment having a cyclic π-conjugated group, is used, the pigment dispersant is represented by the following formula 2 in addition to the structural unit represented by the formula 1 from the viewpoint of compatibility with the substituent. It is preferable to have a structural unit.

In the above formula 2, _Ra2 is a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and is preferably a hydrogen atom (that is, in the formula 2, the benzene ring is preferably unsubstituted).

Among all the structural units constituting the pigment dispersant, the structural unit represented by the formula 2 is preferably 0 to 60 mol%, more preferably 20 to 40 mol%. The numerical value can be controlled within a desired range by adjusting the amount of the raw material monomer charged, for example, in the production method described later.

Above all, from the viewpoint of further improving the image quality, it is preferable that the pigment dispersant has a structural unit represented by the following formula 3 in addition to the structural unit represented by the formula 1 and the structural unit represented by the formula 2. ..

In the above formula 3, _Ra3 is a hydrogen atom or a methyl group, preferably a hydrogen atom. Further, _Ra4 is an alkyl group having 1 to 8 carbon atoms, preferably an alkyl group having 1 to 4 carbon atoms, and more preferably an ethyl group.

Among all the structural units constituting the pigment dispersant, the structural unit represented by the formula 3 is preferably 0 to 40 mol%, more preferably 20 to 40 mol%. The numerical value can be controlled within a desired range by adjusting the amount of the raw material monomer charged, for example, in the production method described later.

The pigment dispersant may be used alone or in combination of two or more. Further, as the pigment dispersant, a commercially available product or a synthetic product may be used.

In the active photocurable ink of the present invention, the pigment dispersant is preferably contained in an amount of 10 to 80% by mass with respect to the pigment. When it is 10% by mass or more, the dispersion stability of the pigment is sufficient. Further, when it is 80% by mass or less, the excess pigment dispersant in the ink is small, and the increase in the viscosity of the ink can be suppressed. Above all, from the viewpoint of further improving the image quality and the rubbing resistance, it is preferably 30 to 60% by mass.

(Manufacturing method of pigment dispersant)
The pigment dispersant can be produced by using a conventionally known polymerization method such as a solution polymerization method, a bulk polymerization method, an emulsion polymerization method, a suspension polymerization method, a reverse phase suspension polymerization method, a thin film polymerization method, and a spray polymerization method. can. Of these, the solution polymerization method is preferable because the molecular weight can be easily adjusted.

In the case of the solution polymerization method, it is preferable to use a thermal polymerization initiator. The amount of the thermal polymerization initiator added may be appropriately adjusted, and is, for example, 0.005 to 0.05 molar equivalent with respect to the total molar equivalent of the monomers to be polymerized.

Examples of thermal polymerization initiators are 2,2-azobisisobutyronitrile (AIBN), 2,2'-azobis (2-methylbutyronitrile) (AMBN), azobiscyanovaleric acid, 2,2'-. Azo compounds such as azobis (4-methoxy-2.4-dimethylvaleronitrile), 2,2'-azobis (2.4-dimethylvaleronitrile); tert-butylperoxypivalate, tert-butylperoxybenzoate, Organic peroxides such as tert-butylperoxy-2-ethylhexanoate, di-tert-butyl peroxide, cumene hydroperoxide, benzoyl peroxide, tert-butyl hydroperoxide; hydrogen peroxide, ammonium persulfate, Examples thereof include inorganic peroxides such as potassium persulfate and sodium persulfate. These may be used alone or in combination of two or more.

In synthesizing the pigment dispersant, a chain transfer agent may be used to control the molecular weight. The amount of the chain transfer agent added can be adjusted as appropriate, and is, for example, 0.005 to 0.5 molar equivalent with respect to the total molar equivalent of the monomers to be polymerized.

Examples of chain transfer agents are methyl mercaptan, tert-butyl mercaptan, decyl mercaptan, benzyl mercaptan, lauryl mercaptan, stearyl mercaptan, n-dodecyl mercaptan, tert-dodecyl mercaptan, mercaptoacetic acid, mercaptopropionic acid and its esters, 2- Mercaptans such as ethylhexylthioglycol and octyl thioglycolate; trithiocarbonate compounds such as S-dodecyl-S'-(α, α'-dimethyl-α''-acetic acid) trithiocarbonate; methyl-4- Cyclohexene-1,2-dicarboxylic acid anhydrides, α-methylstyrene, α-methylstyrene dimer and the like can be mentioned. These may be used alone or in combination of two or more.

The polymerization conditions are not particularly limited and may be appropriately adjusted depending on the type of the thermal polymerization initiator used, etc., but the polymerization temperature is preferably 60 to 120 ° C. under an inert gas (preferably nitrogen) atmosphere, and the polymerization time. Is preferably 6 to 48 hours.

The polymerization solvent is not particularly limited as long as it can dissolve each monomer to be polymerized, the polymer precursor to be produced, and if necessary, a thermal polymerization initiator and other additives, and methanol, ethanol, isopropanol, etc. Tetrahydrofuran, cyclohexanone, methyl ethyl ketone, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, 2-methoxyethyl acetate, diethylene glycol dimethyl ether, 1-methoxy-2-propanol, 1-methoxy-2-propyl acetate, N, N-dimethylformamide, N, N-dimethylacetamide, toluene, ethyl acetate, ethyl lactate, methyl lactate, dimethyl sulfoxide, water and the like can be used. These may be used alone or in combination of two or more.

The product after polymerization can be purified by a general purification method such as a reprecipitation method, a dialysis method, an ultrafiltration method, or an extraction method.

<< Manufacturing method of pigment dispersant having a structural unit represented by the formula 1 >>
The method for producing the pigment dispersant having the structural unit represented by the formula 1 is not particularly limited, but for example, a pigment dispersant precursor having a hydroxyl group in the side chain by polymerizing a styrene-based monomer having a hydroxyl group. (Step 1), (meth) acrylic acid or a derivative thereof is added to the hydroxyl group by an esterification reaction, and a (meth) acryloyl group is introduced into the side chain of the pigment dispersant precursor (step 1). 2) A method having the above can be mentioned.

[Step 1: Production of Pigment Dispersant Precursor]
Since the preferable polymerization conditions in step 1 are the same as described above, the description thereof will be omitted.

Examples of the styrene-based monomer having a hydroxyl group used in step 1 include 2-ethenylphenol, 3-ethenylphenol, 4-ethenylphenol and the like.

In step 1, the raw material monomer of the structural unit represented by the formula 2 and / and the raw material monomer of the structural unit represented by the formula 3 are added and copolymerized with the styrene-based monomer having a hydroxyl group. You may let me.

Examples of the raw material monomer of the structural unit represented by the formula 2 include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, α-methylstyrene, p-phenylstyrene, and p-ethylstyrene. 2,4-dimethylstyrene, p-tert-butylstyrene, pn-hexylstyrene, pn-octylstyrene, pn-nonylstyrene, pn-decylstyrene, pn-dodecylstyrene, etc. Examples include styrene-based monomers.

Examples of the raw material monomer of the structural unit represented by the formula 3 include methyl (meth) acrylate, ethyl (meth) acrylate, isopropyl (meth) acrylate, n-butyl (meth) acrylate, and (meth). ) T-butyl acrylate, isobutyl (meth) acrylate, n-octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate and other (meth) acrylic acid alkyl esters having an alkyl group of 1 to 8 carbon atoms. Can be mentioned.

The content (mol%) of the raw material monomer of the constituent unit represented by the formula 2 in all the monomers used in the step 1 is represented by the formula 2 in all the constituent units constituting the pigment dispersant. It is substantially equal to the content (mol%) of the constituent units. Further, the content (mol%) of the raw material monomer of the structural unit represented by the formula 3 in all the monomers used in the step 1 is determined by the formula 3 in all the structural units constituting the pigment dispersant. Substantially equal to the content (mol%) of the represented building blocks.

[Step 2: Introduction of (meth) acryloyl group into the pigment dispersant precursor]
In step 2, it is preferable to add 1.2 molar equivalents or more of (meth) acrylic acid or a derivative thereof with respect to 1 molar equivalent of the hydroxyl group contained in the pigment dispersant precursor (upper limit value: for example, 4 molar equivalents). Less than). Within the above range, theoretically, (meth) acryloyl groups are introduced for all hydroxyl groups in the pigment dispersant precursor. That is, substantially all the structural units derived from the styrene-based monomer having a hydroxyl group are converted into the structural units represented by the formula 1. Therefore, the content (mol%) of the styrene-based monomer having a hydroxyl group in all the monomers used in step 1 is represented by the formula 1 in all the constituent units constituting the pigment dispersant. It is substantially equal to the content of the unit (mol%).

Examples of the (meth) acrylic acid or a derivative thereof used in step 2 include (meth) acryloyl chloride and the like.

When the esterification reaction is carried out, the reaction temperature is not particularly limited, but is, for example, -18 to 40 ° C. The reaction time is also not particularly limited, but is, for example, 1 to 6 hours.

In the esterification reaction, it is preferable to add a base such as triethylamine or pyridine in order to neutralize by-products (for example, hydrochloric acid).

The product after the esterification reaction can be appropriately adjusted in pH and purified by a general purification method such as a reprecipitation method, a dialysis method, an ultrafiltration method, or an extraction method.

[Other ingredients]
In addition to the above components, the ink of the present invention contains various known additives such as a solvent, a photopolymerization inhibitor, a surface tension adjuster, a surfactant, a lubricant, a filler, and an antifoaming agent, if necessary. A gelling agent, a thickener, a specific resistance adjusting agent, a film forming agent, an ultraviolet absorber, an antioxidant, a fading inhibitor, an antibacterial agent, an anticorrosive agent and the like can be appropriately selected and used.

(Photopolymerization inhibitor)
From the viewpoint of storage stability, it is preferable to add a photopolymerization inhibitor to the active photocurable ink of the present invention.

Examples of the photopolymerization inhibitor include methquinone (hydroquinone monomethyl ether), hydroquinone, 4-methoxy-1-naphthol, hindered amine-based antioxidants, nitrogen-containing heterocyclic mercapto-based compounds, thioether-based antioxidants, and hindered phenol-based agents. Antioxidants, ascorbic acids, zinc sulfate, thiocyanates, thiourea derivatives, various saccharides, phosphoric acid-based antioxidants, nitrites, sulfites, thiosulfates, hydroxylamine derivatives, nitroxyl radicals, dicyandiamide and poly Examples thereof include polycondensates of alkylene polyamines.

In the active photocurable ink of the present invention, the amount of the photopolymerization inhibitor added is preferably 0.05 to 2% by mass with respect to the total mass of the photopolymerizable compound, the photopolymerization initiator and the pigment dispersant. .. When it is 0.05% by mass or more, the storage stability is good. Further, when it is 2% by mass or less, high curability can be maintained without impairing the activity of generating active species of the photopolymerization initiator.

These radical polymerization inhibitors may be used alone or in combination of two or more.

(Surface tension adjuster)
As the surface tension adjusting agent, a silicone compound is preferable, and as a commercially available product, KF-65 manufactured by Shin-Etsu Chemical Co., Ltd. and the like can be mentioned.

The amount of the surface tension adjusting agent added is preferably 0.05 to 2% by mass with respect to the total mass of the ink.

[Preparation method of active ray curable ink]
The active photocurable ink of the present invention uses the above-mentioned photopolymerizable compound, photopolymerization initiator, pigment, pigment dispersant, and if necessary, other components using a normal disperser such as a sand mill or a paint shaker. Manufactured by dispersion. When dispersing, zirconia beads or the like may be used.

As a method for preparing an ink, for example, a pigment, a pigment dispersant, and a part of a photopolymerizable compound are mixed in advance to prepare a pigment dispersion, and then the remaining photopolymerization compound and a photopolymerization initiator are added. Ink may be prepared. With such a method, sufficient dispersion is possible even in the dispersion by a normal disperser, and therefore, excessive dispersion energy is not applied and a large dispersion time is not required, so that when the ink components are dispersed, the ink components are dispersed. It is possible to prepare an ink with excellent stability that does not easily cause deterioration. At this time, the ink preparation conditions (temperature and time) are not particularly limited, but the ink is prepared, for example, at 60 to 120 ° C. for 30 to 120 minutes.

The prepared ink is preferably filtered through a filter having a pore size of 3 μm or less.

[Physical characteristics of active ray curable ink]
When the active ray-curable ink of the present invention is used for inkjet printing, it is preferable that the physical properties have the same physical characteristics as those of ordinary inkjet inks. That is, the viscosity is 2 to 50 mPa · s at 25 ° C., the share rate dependence is as small as possible, the surface tension is in the range of 15 to 35 mN / m at 25 ° C., and when a pigment is used as a colorant, the pigment is used. It is preferable that there is no gel-like substance having an average particle size of more than 1.0 μm other than the particles, the electric conductivity is 10 μS / cm or less, and the ink is free from electrical corrosion inside the head. .. In the continuous type, it is necessary to adjust the electric conductivity by an electrolyte, and in this case, it is necessary to adjust the electric conductivity to 0.5 mS / cm or more.

That is, the active light curable ink according to the embodiment of the present invention is used for inkjet printing.

<Image formation method>
The present invention comprises a step of landing a droplet of the active light-curing ink on a recording medium (step 1) and a cured film constituting an image by irradiating the droplet landed on the recording medium with an active ray. Also provided is an image forming method including a step of forming (step 2).

[Step 1: Ink droplet landing process]
In step 1, droplets of the active ray-curable ink of the present invention are ejected from an inkjet head or the like and landed on a recording medium at a position corresponding to an image to be formed.

The inkjet head may be an on-demand method or a continuous method. The discharge method includes an electric-mechanical conversion method (for example, a single cavity type, a double cavity type, a bender type, a piston type, a shared mode type, a shared wall type, a piezo type, etc.), and an electric-heat conversion method (for example). For example, any ejection method such as thermal inkjet type, bubble jet (registered trademark) type, etc. may be used.

Ink droplets can be ejected from the inkjet head in a heated state to improve ejection stability. From the viewpoint of improving ejection stability, the temperature of the ink at the time of ejection (the set temperature of the inkjet head) is preferably 35 to 100 ° C, more preferably 35 to 80 ° C.

The method of heating the ink is not particularly limited. For example, at least one of the ink tank, the supply pipe, the ink supply system such as the front chamber ink tank immediately before the head, the pipe with a filter, the piezo head, etc. constituting the head carriage is heated by a panel heater, a ribbon heater, heat insulating water, or the like. be able to.

The amount of ink droplets to be ejected is preferably 2 to 20 pL from the viewpoint of recording speed and image quality.

The recording medium is not particularly limited, and in addition to ordinary uncoated paper, coated paper, etc., synthetic paper YUPO, various plastics used for flexible packaging, and films thereof can be used. Examples of various plastic films include PP film, PET film, OPS film, OPP film, ONy film, PVC film, PE film, and TAC film. As other plastics, polycarbonate, acrylic resin, ABS, polyacetal, PVA, rubber and the like can be used. It can also be applied to metals and glasses. The recording medium for food packaging is appropriately selected according to the intended use, and may be polypropylene or the like, for example.

From the viewpoint of dot formation including wet spread when the ink droplets land, the temperature of the recording medium (the set temperature of the transport table of the recording medium) when the ink droplets land is preferably 20 to 40 ° C. ..

[Step 2: Hardened film (image) forming step]
In step 2, the ink landed on the recording medium in step 1 is irradiated with active light rays to form a cured film (image) constituting an image.

Examples of the active ray include ultraviolet rays, visible rays, electron beams, α rays, γ rays, X-rays and the like, and ultraviolet rays are preferable. Irradiation of ultraviolet rays can be carried out under the condition of a wavelength of 395 nm by using, for example, a water-cooled LED manufactured by Phoseon Technology. By using the LED as a light source, it is possible to suppress the occurrence of poor curing of the ink due to the melting of the ink by the radiant heat of the light source. Further, when the wavelength is set to the above, it does not overlap with the absorption wavelength region of the pigment, so that curing failure does not easily occur, and since the energy is low, the ink component does not easily decompose.

The integrated light amount of the active light is not particularly limited, but is preferably 30 to 500 mJ / cm ² .

Irradiation with active light rays is preferably performed within 0.001 to 2.0 seconds after the ink lands, and is performed between 0.001 and 1.0 seconds in order to form a high-definition image. Is more preferable.

Irradiation with active light may be performed in two stages. First, the ink may be temporarily cured by irradiating the ink with an active ray within 0.001 to 2.0 seconds after the ink has landed, and after all printing is completed, the ink may be further irradiated with an active ray to cure the ink. .. By dividing the irradiation of the active light into two stages, the shrinkage of the recording material that occurs during ink curing is less likely to occur.

In the image forming method of the present invention, the total ink film thickness after the ink lands on the recording medium and is cured by irradiating with active light rays is 2 from the viewpoint of curl, wrinkles, texture change of the recording medium, and the like. It is preferably about 20 μm. The "total ink film thickness" here means the maximum value of the ink film thickness drawn on the recording medium, and even if it is a single color, other two-color overlay (secondary color), three-color overlay, and so on. The meaning of the total ink film thickness is the same even when recording is performed by the four-color overlapping (white ink-based) inkjet recording method.

Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto. Although the display of "parts" or "%" is used in the examples, it represents "parts by mass" or "% by mass" unless otherwise specified.

<Manufacturing of pigment dispersant>
[Manufacturing Example 1-1]
To a nitrogen-substituted eggplant flask, add a DMF solution containing 1 mol equivalent of styrene, 1 mol equivalent of 4-ethenylphenol, and 1 mol equivalent of ethyl acrylate, and further add S-dodecyl-S'-(α, α'-dimethyl-. A reaction solution containing 0.05 molar equivalent of α''-acetic acid) trithiocarbonate and 0.05 molar equivalent of AIBN was added and stirred at 100 ° C. for 24 hours under a nitrogen atmosphere to prepare a reaction solution containing the pigment dispersant precursor 1.

After adding 1N aqueous sodium hydroxide solution to the reaction solution containing the pigment dispersant precursor 1 and stirring, 5 molar equivalents of triethylamine and 1.5 molar equivalents of methacryloyl chloride were added, and the mixture was stirred at 0 ° C. for 3 hours. Then, 1N hydrochloric acid was added until the pH reached 2, chloroform and water were added to separate and recover the organic layer, washed with water three times, and the organic layer was concentrated to dryness with a rotary evaporator to obtain a pigment dispersant 1. ..

The weight average molecular weight of the obtained pigment dispersant 1 was 5,000. The weight average molecular weight is a value measured by gel permeation chromatography (GPC) using polystyrene as a standard material.

[Manufacturing Example 1-2]
The pigment dispersant 2 was produced in the same manner as in Production Example 1-1 except that the methacryloyl chloride was changed to acryloyl chloride. The weight average molecular weight of the pigment dispersant 2 was 5,000.

[Manufacturing Example 1-3]
Change 1 molar equivalent of styrene, 1 molar equivalent of 4-ethenylphenol, and 1 molar equivalent of ethyl acrylate to 1.5 molar equivalent of styrene and 1.5 molar equivalent of 4-ethenylphenol, and add methacryloyl chloride. The pigment dispersant 3 was produced in the same manner as in Production Example 1-1 except that the equivalent was changed to 2.25 molar equivalents. The weight average molecular weight of the pigment dispersant 3 was 5,000.

[Manufacturing Example 1-4]
1 molar equivalent of styrene, 1 molar equivalent of 4-ethenylphenol, 1 molar equivalent of ethyl acrylate was changed to 3 molar equivalent of 4-ethenylphenol, and the amount of methacryloyl chloride added was changed to 4.5 molar equivalent. The pigment dispersant 4 was produced in the same manner as in Production Example 1-1 except for the above. The weight average molecular weight of the pigment dispersant 4 was 5,000.

[Manufacturing Example 1-5]
To the reaction solution containing the pigment dispersant precursor 1 obtained in Production Example 1-1, 1N hydrochloric acid was added until the pH reached 2, chloroform and water were added to separate and recover the organic layer, and the mixture was washed 3 times with water. The organic layer was concentrated to dryness with a rotary evaporator to obtain a pigment dispersant 5. The weight average molecular weight of the pigment dispersant 5 was 5,000.

[Manufacturing Example 1-6]
To the nitrogen-substituted eggplant flask, add a DMF solution containing 3 mol equivalents of styrene, and further add 0.05 mol equivalents of S-dodecyl-S'-(α, α'-dimethyl-α''-acetic acid) trithiocarbonate. 0.05 mol equivalent of AIBN was added, and the mixture was stirred at 100 ° C. for 24 hours under a nitrogen atmosphere to obtain a reaction solution. Chloroform and water were added to the obtained reaction solution to separate and recover the organic layer, washed with water three times, and the organic layer was concentrated to dryness with a rotary evaporator to obtain a pigment dispersant 6. The weight average molecular weight of the pigment dispersant 6 was 5,000.

[Manufacturing Example 1-7]
Change 1 molar equivalent of styrene, 1 molar equivalent of 4-ethenylphenol, and 1 molar equivalent of ethyl acrylate to 1.5 molar equivalent of styrene and 1.5 molar equivalent of 4-aminostyrene, and add 2 methacryloyl chloride. The pigment dispersant 7 was produced in the same manner as in Production Example 1-1 except that the equivalent was changed to .25 molar equivalents. The weight average molecular weight of the pigment dispersant 7 was 5,000.

The structures of the pigment dispersants 1 to 7 produced above are shown in Table 1.

<Manufacturing of photopolymerization initiator>
In preparing the active light-curing inkjet ink described later, the photopolymerization initiators 1 to 6 shown in Table 2 were used. Of these, the photopolymerization initiators 2 to 5 were produced by the following methods. Further, as the photopolymerization initiator 6, IRGACURE (registered trademark) 819 manufactured by BASF was used. The absorption wavelength region (nm) shown in Table 2 is based on the absorption spectrum obtained by measurement by the spectrophotometric method in the wavelength range of 250 to 600 nm. Therefore, the actual lower limit absorption wavelength of the photopolymerization initiators 1 to 3, 5 and 6 may be 250 nm or less.

[Manufacturing Example 2-1]
A reference amount of bis (2,4,6-trimethylbenzoyl) -phenylphosphinoxide (IRGACURE® 819, manufactured by BASF), 2.5 mol equivalent of N-bromosuccinimide, chlorobenzene in a nitrogen-substituted eggplant flask. Was added, and the mixture was stirred at 80 ° C. for 1 hour. Then, 1.3 mol equivalents of azoisobutyronitrile were added, and the mixture was stirred at 80 ° C. for 6 hours. The reaction mixture was separated by ethyl acetate and water, the organic layer was concentrated to dryness, and purified by column chromatography to obtain Intermediate A. The dichloromethane solution of Intermediate A was cooled to −78 ° C., 2.2 molar equivalents of n-butyllithium was added, and the mixture was stirred for 30 minutes. Then, a dichloromethane solution having an equivalent amount of 2 mol of bromomethylmethacrylate was added dropwise, and the mixture was stirred at −20 ° C. for 2 hours and then at room temperature (25 ° C.) for 4 hours. The reaction solution was separated by dichloromethane and water, the organic layer was concentrated to dryness, and purified by column chromatography to obtain a photopolymerization initiator 1.

[Manufacturing Example 2-2]
Photopolymerization initiator 2 was produced in the same manner as in Production Example 2-1 except that bromomethylmethacrylate was changed to bromomethylacrylate.

[Manufacturing Example 2-3]
The photopolymerization initiator 3 was produced in the same manner as in Production Example 2-1 except that bromomethylmethacrylate was changed to 5-bromopentylmethacrylate.

[Manufacturing Example 2-4]
A reference amount of 2-phenyl-1-propanol and 1 mol equivalent of thiosalicylic acid were suspended in sulfuric acid and stirred at 80 ° C. for 18 hours. The reaction mixture was cooled to room temperature, separated by dichloromethane and water, the organic layer was concentrated to dryness, and purified by column chromatography to obtain Intermediate B. The dichloromethane solution of Intermediate B was cooled to 0 ° C., 1.2 mol equivalents of methacryloyl chloride and 1.5 mol equivalents of triethylamine were added, and the mixture was stirred for 1 hour. The reaction solution was separated by dichloromethane and water, the organic layer was concentrated to dryness, and purified by column chromatography to obtain a photopolymerization initiator 4.

[Manufacturing Example 2-5]
A reference amount of a solution of 4-bromomethylbenzyl in dichloromethane was cooled to −78 ° C., 1.1 mol equivalent of n-butyllithium was added, and the mixture was stirred for 30 minutes. Then, 1.1 mol equivalent of dichloromethane solution of bromomethyl acrylate was added dropwise, and the mixture was stirred at −20 ° C. for 2 hours and then at room temperature (25 ° C.) for 4 hours. The reaction mixture was separated by dichloromethane and water, the organic layer was concentrated to dryness, and purified by column chromatography to obtain Intermediate C. To the acetic acid solution of the obtained intermediate C, 1.1 mol equivalents of p-chlorobenzaldehyde and 3 mol equivalents of ammonium acetate were added, and the mixture was stirred at 80 ° C. for 8 hours. The reaction mixture was separated by dichloromethane and water, the organic layer was concentrated to dryness, and purified by column chromatography to obtain Intermediate D. 2 molar equivalents of potassium ferricyanide and 2 molar equivalents of potassium hydroxide were added to a suspension solution of dichloromethane and water of intermediate D, and the mixture was stirred at room temperature for 5 hours under shading. Under shading, the reaction solution was separated by dichloromethane and water, the organic layer was concentrated to dryness, and purified by column chromatography to obtain a photopolymerization initiator 5.

<Manufacturing of active ray curable ink>
[Example 1]
Magenta pigment Cinquasia Magenta D4550J (BASF, quinacridone red (CI pigment red 122)) 8 g, pigment dispersant 1 3.6 g, photopolymerizable compound polyethylene glycol diacrylate (NK ester A-400, new) 34.5 g of Nakamura Kagaku Kogyo Co., Ltd., 0.3 g of photopolymerization inhibitor (Irgastab (registered trademark) UV10, manufactured by BASF), 100 ml of zirconia beads (YTZ ball 0.3 mm, manufactured by Nikkato Co., Ltd.) The mixture was measured in a plastic container and dispersed in a paint shaker at room temperature (25 ° C.) for 3 hours to obtain a pigment dispersion liquid 1.

5.6 g of pigment dispersion 1, 9.8 g of polyethylene glycol diacrylate (NK ester A-400, weight average molecular weight 508, manufactured by Shin-Nakamura Chemical Industry Co., Ltd.) as a photopolymerizable compound, 4EO-modified pentaerythritol tetraacrylate ( SR494, weight average molecular weight less than 1,500, manufactured by Sartmer) 8 g, 3PO-modified trimethylolpropane triacrylate (EM2381, weight average molecular weight less than 1,500, manufactured by Choko Chemical Industry Co., Ltd.) 5 g, photopolymerization initiator 1 1.2 g, Irgastab (registered trademark) UV10 (hindered amine antioxidant, manufactured by BASF) as a photopolymerization inhibitor, 0.04 g, and KF-65 (manufactured by Shinetsu Chemical Industry Co., Ltd.) as a surface tension adjuster, 0. After adding .36 g and stirring at 105 ° C. for 45 minutes, filtration was performed with an ADVANTEC Teflon (registered trademark) 3 μm polymer filter to obtain an active photocurable inkjet ink 1 (hereinafter, also referred to as ink 1).

[Examples 2 to 15, 24, Comparative Examples 1 to 7]
Active photocurable inkjet inks 2 to 15 and 24 to 31 were prepared in the same manner as in Example 1 except that the types of the photopolymerization initiator and / and the pigment dispersant were changed as shown in Table 3. ..

[Examples 16 to 19]
The amount of the pigment dispersant 1 added in the preparation of the pigment dispersion is 0.8 g (10% by mass), 2.4 g (30% by mass), 4.8 g (60% by mass) or 6.4 g (80% by mass). Active photocurable inkjet inks 16 to 19 were prepared in the same manner as in Example 1 except that the photopolymerization initiator 1 was changed to the photopolymerization initiator 2. The numerical value in parentheses represents the mass% with respect to the pigment.

[Examples 20 to 23]
The photopolymerization initiator 1 is changed to the photopolymerization initiator 2, and the addition amounts thereof are 0.27 g (1.0% by mass), 0.67 g (2.5% by mass), and 1.62 g (6.0% by mass). %) Or 2.16 g (8.0% by mass), active photocurable inkjet inks 20 to 23 were prepared in the same manner as in Example 1. The numerical value in parentheses represents the mass% with respect to the photopolymerizable compound.

<Image formation>
Each of the above-prepared active ray-curable inkjet inks was loaded into a line-type inkjet recording apparatus having an inkjet recording head equipped with a piezo-type inkjet nozzle. The ink supply system includes an ink tank, an ink flow path, a sub ink tank immediately before the inkjet recording head, a pipe with a filter, and a piezo head, and the temperature of the inkjet head is set to 80 ° C. The transport table of the recording medium was set to 40 ° C. After printing a solid image on a recording medium and forming an image, an LED lamp (Phoseon Technology, peak emission wavelength 395 nm, water-cooled LED) placed downstream of the recording device is used to print an ultraviolet ray (integrated light amount 350 mJ / cm) on the image. ² ) was irradiated to cure the ink, and a cured film (image) having an ink thickness of 5 μm was formed.

<Image evaluation>
[image quality]
Ink is ejected from the inkjet head, and the recording medium is uncoated high-quality paper (J paper, 68 g / m ² , manufactured by Konica Minolta Co., Ltd.) and the printing coated paper A (OK top coat, 128 g / m ² , Oji Paper). A solid image of 5 cm × 5 cm printed on (manufactured by Co., Ltd.) was visually evaluated, and the image quality (density unevenness) was evaluated according to the following evaluation criteria (passed with ○ or more). Normally, non-coated paper tends to cause uneven penetration of ink, so that uneven density tends to occur as compared with coated paper.

⊚: No density unevenness is observed in the image when observed from a position 5 cm away ○: Density unevenness is observed in a part of the image when observed from a position 5 cm away, but the image is observed when observed from a position 15 cm away. No density unevenness is observed in Δ: When observed from a position 15 cm away, density unevenness is observed in a part of the image, but when observed from a position 30 cm away, no density unevenness is observed ×: From a position 30 cm away Observing, uneven density is observed in the image.

[Rubbing resistance]
Ink was ejected from the inkjet head to create a solid image of 5 cm × 5 cm printed on a PET film as a recording medium, and the weight was measured. After installing the PET film on which the image is printed on the rubbing resistance evaluation machine, the # 15000 ultra-precision polishing film is pressed against the image with a load of 9.81 kPa (0.1 kgf / cm ² ) and rubbed for 30 minutes to evaluate the following. The rubbing resistance (difficulty of the pigment coming off due to rubbing) was evaluated according to the standard (passed with ○ or higher).

⊚: Pigment coloring is not observed on the ultra-precision polishing film even when using an optical microscope ○: Pigment coloring is not observed on the ultra-precision polishing film visually, but coloring is observed by the optical microscope △: Ultra-precision Pigment coloring is observed visually from a position 10 cm away from the polishing film, but pigment coloring is not observed at a position 30 cm away. ×: Pigment coloring is observed visually from a position 30 cm away from the ultra-precision polishing film. Is recognized.

As shown in Table 3, the active ray-curable inkjet ink according to the examples was superior to the active ray-curable inkjet ink according to the comparative example in terms of both image quality and rubbing resistance.

Further, comparing Examples 4 to 6 and 24, when the pigment dispersant has a structural unit represented by the formula 1, a structural unit represented by the formula 2, and a structural unit represented by the formula 3, the image quality is high. It has improved further. Above all, when _Ra1 in the formula 1 was a methacryloyl group, the rubbing resistance was further improved.

Further, comparing Examples 1, 4, 7, 10 and 13, the photopolymerization initiators 1 and 2, that is, the photopolymerization initiator represented by the general formula 1'(in this case, n is 2). Was particularly excellent in terms of both image quality and rubbing resistance.

Claims (10)

An active photocurable ink containing a photopolymerizable compound, a photopolymerization initiator, a pigment and a pigment dispersant.
The photopolymerizable compound is (meth) acrylate and is
The pigment is a pigment having an aromatic ring.
The photopolymerization initiator contains at least one selected from the group consisting of the compound represented by the following general formula 1', the following compound 4 and the following compound 5.

(In the above general formula 1', R is a (meth) acryloyloxy group, and n is an independently integer of 1 to 6).

The pigment dispersant has a weight average molecular weight of 1,500 or more and has a structural unit represented by the following formula 1.

(In the above formula 1, _Ra1 is a (meth) acryloyloxy group). The active photocurable ink according to claim 1, wherein the pigment dispersant further has a structural unit represented by the following formula 2.

(In the above formula 2, R _a2 Is a hydrogen atom or an alkyl group having 1 to 4 carbon atoms). The active photocurable ink according to claim 1 or 2 , wherein the pigment dispersant has a weight average molecular weight of 200,000 or less . The active photocurable ink according to any one of claims 1 to 3 , wherein the pigment dispersant is contained in an amount of 10 to 80% by mass with respect to the pigment. The active photocurable ink according to any one of claims 1 to 4 , wherein the photopolymerization initiator has an absorption wavelength region of 340 to 440 nm. The active photocurable ink according to any one of claims 1 to 5, wherein the pigment is a quinacridone-based pigment. The active photocurable ink according to any one of claims 1 to 6 , wherein the photopolymerization initiator is contained in an amount of 0.3 to 10% by mass with respect to the photopolymerizable compound. The active photocurable ink according to any one of claims 1 to 7 , further containing a gelling agent. The active light curable ink according to any one of claims 1 to 8 , which is used for inkjet printing. The step of landing a droplet of the active photocurable ink according to any one of claims 1 to 9 on a recording medium, and irradiating the droplet landed on the recording medium with an active ray to form an image. A method for forming an image, which comprises a step of forming a cured film.