Inkjet ink and image forming method

Abstract

To provide an inkjet ink which is cured by an active ray, can form an image which is less likely to peel off from a recording medium while preventing the occurrence of cracks of an image when a recording medium is folded and enhances injection stability, and to provide an image forming method using the same.SOLUTION: There is provided an inkjet ink containing a polymerizable compound which polymerizes by an active ray, which is cured by an active ray, wherein the content of a polymerizable compound having a melting point of 25°C or more is less than 3 mass% based on the total mass of the polymerizable compound and when the weighted average value (on a mass basis) of ethylene oxide groups per molecule in the polymerizable compound is defined as X and the weighted average value (on a mass basis) of propylene oxide groups per molecule in the polymerizable compound is defined as Y, the following expression (1) is satisfied. 3.5≤X+Y≤9 (1).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to ink-jet inks and imaging methods. [Background technology]

[0002] Inkjet recording methods are used in various printing fields because they can form images easily and inexpensively. Known inks for use in inkjet recording methods include inkjet inks that are cured by actinic radiation (hereinafter referred to as actinic radiation-curable inkjet inks), which contain a polymerizable compound that is polymerized by actinic radiation (hereinafter referred to as actinic radiation-polymerizable compound) as a liquid component. When actinic radiation-curable inkjet inks are irradiated with actinic radiation, the actinic radiation-polymerizable compound is polymerized and the coloring material is firmly attached to the recording medium. It is known that a desired image can be formed by the formation of this cured film.

[0003] For example, Patent Document 1 discloses an active energy ray-curable composition containing a monofunctional polymerizable compound and a polyfunctional polymerizable compound. According to Patent Document 1, a coating film (average thickness 10 μm) formed using the above composition is irradiated with an integrated light dose of 1500 mJ / cm. 2 The laminate (3 layers) of the cured product obtained by irradiating the active energy rays is said to have excellent stretchability and strength. In addition, according to Patent Document 1, the composition can be used as an inkjet ink. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2016-172841 A Summary of the Invention [Problem to be solved by the invention]

[0005] As disclosed in Patent Document 1, actinic ray curable inkjet inks are known.

[0006] Incidentally, a recording medium on which an image is formed using an actinic ray curable inkjet ink may be folded when being carried or stored. In this case, cracks may occur in the formed image. Therefore, it is desirable that the actinic ray curable inkjet ink can form an image that is less likely to crack (fold crack) when the recording medium is folded.

[0007] In addition, in order to prevent the formed image from peeling off from the recording medium, actinic radiation curable inkjet inks are required to form images with improved abrasion resistance.

[0008] Additionally, a need exists for actinic radiation curable ink-jet inks that have enhanced jetting stability.

[0009] The present invention has been made in consideration of the above circumstances, and has an object to provide an ink-jet ink that is cured by actinic radiation, that is capable of forming an image that is highly resistant to abrasion while making the image less susceptible to fold cracks, and that has improved ejection stability, and an image forming method using the ink-jet ink. [Means for solving the problem]

[0010] In order to solve the above problems, one aspect of the present invention relates to an ink-jet ink as set forth in the following items [1] to

[10] . [1] An ink-jet ink containing a polymerizable compound that is polymerized by actinic radiation, the content of the polymerizable compound having a melting point of 25° C. or higher is less than 3% by mass relative to the total mass of the polymerizable compound; When the weighted average value (mass basis) of the number of ethylene oxide groups per molecule in the polymerizable compound is X and the weighted average value (mass basis) of the number of propylene oxide groups per molecule in the polymerizable compound is Y, formula (1) is satisfied. Inkjet ink that cures with actinic radiation. 3.5≦X+Y≦9 (1) [2] The ink-jet ink according to [1], wherein in formula (1), X is 1.5 or more and 7 or less. [3] The ink-jet ink according to [1] or [2], wherein the weighted average (mass basis) number of repeating ethylene oxide groups and propylene oxide groups per molecule in the polymerizable compound is 2 or more and 10 or less. [4] The ink-jet ink according to any one of [1] to [3], wherein the ink-jet ink has a viscosity at 60° C. of 10 mPa·s or more and 25 mPa·s or less. [5] The ink-jet ink according to any one of [1] to [4], wherein the weighted average equivalent weight (mass basis) of the polymerizable group in the polymerizable compound is 160 g / eq or more and 300 g / eq or less. [6] The polymerizable compound includes a monofunctional polymerizable compound, the content of the monofunctional polymerizable compound is more than 5% by mass and not more than 40% by mass with respect to the total mass of the inkjet ink; The inkjet ink according to any one of [1] to [5]. [7] The ink-jet ink according to [6], wherein the monofunctional polymerizable compound includes a compound having a benzene ring in the molecule. [8] The ink-jet ink according to [7], wherein the weighted average (mass basis) number of ethylene oxide groups per molecule in the monofunctional polymerizable compound having a benzene ring is 2 or more. [9] The ink-jet ink according to [8], wherein the weighted average (mass basis) number of ethylene oxide groups per molecule in the monofunctional polymerizable compound having a benzene ring is 4 or more.

[10] further comprising a pigment; When the pigment is a yellow pigment, the weighted average equivalent weight (mass basis) of the polymerizable group in the polymerizable compound is 160 g / eq or more and 200 g / eq or less; when the pigment is a black pigment, the weighted average equivalent weight (by mass) of the polymerizable group in the polymerizable compound is 160 g / eq or more and 210 g / eq or less; When the pigment is a red pigment, the weighted average equivalent weight (mass basis) of the polymerizable group in the polymerizable compound is 200 g / eq or more and 250 g / eq or less. or when the pigment is a blue pigment, the weighted average equivalent weight (mass basis) of the polymerizable group in the polymerizable compound is 200 g / eq or more and 260 g / eq or less; The inkjet ink according to any one of [1] to [9].

[0011] In order to solve the above problems, another aspect of the present invention relates to an image forming method according to the following item

[11] .

[11] A step of ejecting the inkjet ink according to any one of [1] to

[10] from an inkjet head and applying it to a surface of a recording medium; irradiating the applied inkjet ink with actinic radiation to cure the inkjet ink; The image forming method according to claim 1, Effect of the Invention

[0012] According to the present invention, it is possible to provide an inkjet ink that is cured by actinic radiation, which is capable of forming an image that is highly resistant to abrasion while reducing the likelihood of cracking of the image when folded, and has improved ejection stability, and an image forming method using the inkjet ink. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a flowchart showing an image forming method according to the present embodiment. [Diagram 2] FIG. 2 is a schematic diagram showing the configuration of an image forming apparatus capable of carrying out the image forming method according to this embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present invention will be described in detail. However, the present invention is not limited to the following embodiment.

[0015] 1. Inkjet ink The inkjet ink cured by actinic radiation (hereinafter referred to as actinic radiation-curable inkjet ink) in this embodiment contains a polymerizable compound polymerized by actinic radiation (hereinafter referred to as actinic radiation polymerizable compound), and satisfies formula (1) when the weighted average (by mass) of the number of ethylene oxide groups per molecule in the actinic radiation polymerizable compound is X and the weighted average (by mass) of the number of propylene oxide groups per molecule in the polymerizable compound is Y. Furthermore, the content of polymerizable compounds having a melting point of 25° C. or higher relative to the total mass of the actinic radiation polymerizable compound is less than 3 mass%. 3.5≦X+Y≦9 (1)

[0016] The present inventors have discovered that, when the actinic radiation-polymerizable compound contained in the ink satisfies the above formula (1), it is possible to make it possible to reduce the occurrence of cracks in the image when the recording medium is folded (hereinafter referred to as "fold cracks"), and to form an image with high abrasion resistance.

[0017] When X+Y in formula (1) is 3.5 or more, the amount of ethylene oxide groups (hereinafter referred to as EO groups) and propylene oxide groups (hereinafter referred to as PO groups) contained in the polymer chain of the actinic radiation polymerizable compound increases. Since EO groups and PO groups increase molecular mobility, when X+Y is 3.5 or more, the flexibility of the cured film (image) is sufficiently increased, and the image easily follows the recording medium when the recording medium is folded. This makes it possible to prevent the image from cracking when folded.

[0018] On the other hand, if X+Y is too high and the flexibility of the image is excessively increased, the hardness of the cured film decreases, and the scratch resistance of the image decreases. Therefore, by setting X+Y to 9 or less, the flexibility of the cured film is not increased too much, and the hardness of the cured film is not easily decreased. This makes it possible to increase the scratch resistance of the image.

[0019] Furthermore, by making X+Y 9 or less, it is possible to suppress an excessive increase in the viscosity of the ink, which occurs when excessive hydrogen bonds are formed between the molecules of the actinic radiation-polymerizable compound, reducing the mobility of the molecules, thereby improving the ejection stability.

[0020] Furthermore, by making the content of polymerizable compounds having a melting point of 25°C or higher less than 3% by mass relative to the total mass of the actinic radiation polymerizable compounds, ejection stability can also be improved. Polymerizable compounds having a melting point of 25°C or higher tend to increase the viscosity of the ink at room temperature or at a temperature to which the ink is heated during ink ejection. In contrast, by making the content less than 3% by mass, ejection stability can be improved.

[0021] In this specification, the weighted average value X of the number of EO groups per molecule refers to the total value of the values ​​calculated by multiplying the number of EO groups per molecule of the polymerizable compound contained in the actinic ray polymerizable compound by the mass ratio of the polymerizable compound in the actinic ray polymerizable compound for each polymerizable compound. Similarly, the weighted average value Y of the number of PO groups per molecule refers to the total value of the values ​​calculated by multiplying the number of PO groups per molecule of the polymerizable compound contained in the actinic ray polymerizable compound by the mass ratio of the polymerizable compound in the actinic ray polymerizable compound for each polymerizable compound.

[0022] The actinic ray curable inkjet ink of the present invention based on the above findings will be described in more detail below.

[0023] 1-1.Active radiation polymerizable compounds The actinic radiation polymerizable compound is a compound that polymerizes and crosslinks when irradiated with actinic radiation. Examples of actinic radiation include electron beams, ultraviolet rays, α rays, γ rays, and X-rays. Among these, ultraviolet rays and electron beams are preferred, and ultraviolet rays are more preferred.

[0024] As described above, when the weighted average (mass basis) of the number of EO groups per molecule in the actinic ray-polymerizable compound is X and the weighted average (mass basis) of the number of PO groups per molecule in the polymerizable compound is Y, the formula (1) is satisfied. 3.5≦X+Y≦9 (1)

[0025] In the above formula (1), X+Y is preferably 3.5 or more and 8 or less, more preferably 3.5 or more and 7 or less. When X+Y is 3.5 or more, the flexibility of the cured film can be increased, making it more difficult for the image to crack when folded. Furthermore, when X+Y is 8 or less, the hardness of the cured film can be more difficult to decrease, making it possible to improve the abrasion resistance of the image. Furthermore, when X+Y is 8 or less, the ink viscosity is more difficult to increase.

[0026] In the above formula (1), X is preferably 1.5 to 7, more preferably 1.5 to 6. Since the EO group has higher molecular mobility than the PO group, the flexibility of the cured film can be increased by increasing the number of EO groups. Therefore, when X is 1.5 or more, the image is less likely to be cracked. In addition, when X is 7 or less, the hardness of the cured film is less likely to decrease, and the abrasion resistance of the image is increased.

[0027] In the actinic ray polymerizable compound, the weighted average (by mass) of the number of repetitions of EO groups and PO groups per molecule is preferably 2 or more and 10 or less, and more preferably 2 or more and 8 or less.

[0028] In this specification, the repeat number of EO groups and PO groups per molecule refers to the number of consecutive EO groups or PO groups in a molecular chain. When a molecule contains multiple units of consecutive EO groups or PO groups, the repeat number is a value obtained by dividing the total number of consecutive EO groups or PO groups by the number of the units. The weighted average repeat number refers to a value calculated by multiplying the repeat number per polymerizable compound molecule contained in the actinic radiation polymerizable compound by the mass ratio of the polymerizable compound in the actinic radiation polymerizable compound, and then summing the values ​​for each polymerizable compound.

[0029] When the weighted average value of the repeat number is 2 or more, the EO group or PO group is continuous in the polymer chain, so that the part with high molecular mobility is lengthened, and the flexibility of the cured product can be further increased. This makes it possible to further suppress the image from being broken. In addition, when the weighted average value of the repeat number is 10 or less, the hardness of the cured film is less likely to decrease, and the abrasion resistance of the image can be further increased.

[0030] In the present embodiment, the actinic ray polymerizable compound includes an actinic ray polymerizable compound having an EO group or an actinic ray polymerizable compound having a PO group. Examples of the actinic ray polymerizable compound having an EO group and the actinic ray polymerizable compound having a PO group include a radical polymerizable compound and a cationic polymerizable compound. Among these, the radical polymerizable compound is preferable. The radical polymerizable compound is a compound having an ethylenically unsaturated bond capable of radical polymerization. The ink may contain only one type of radical polymerizable compound, or may contain two or more types in combination.

[0031] Examples of the radical polymerizable compound include unsaturated carboxylic acid esters and (meth)acrylates. Among these, (meth)acrylates are preferred. In this specification, "(meth)acrylate" means acrylate or methacrylate, and "(meth)acrylic" means acrylic or methacrylic.

[0032] Examples of (meth)acrylates having an EO group include polyethylene glycol diacrylate, EO-modified 1,6-hexanediol di(meth)acrylate, EO-modified pentaerythritol tetraacrylate, EO-modified dipentaerythritol pentaacrylate, EO-modified dipentaerythritol hexaacrylate, EO-modified trimethylolpropane tri(meth)acrylate, EO-modified phenol acrylate, EO-modified nonylphenol (meth)acrylate, EO-modified cresol (meth)acrylate, and EO-modified bisphenol A diacrylate.

[0033] Examples of the (meth)acrylate having a PO group include tripropylene glycol diacrylate, a PO-modified neopentyl glycol diacrylate, a PO-modified trimethylolpropane triacrylate, a PO-modified glyceryl triacrylate, a PO-modified bisphenol A diacrylate, and a PO-modified nonylphenol acrylate.

[0034] The actinic radiation polymerizable compound may include a cationically polymerizable compound having an EO group and a cationically polymerizable compound having a PO group.

[0035] The content of the actinic radiation polymerizable compound having an EO group and the actinic radiation polymerizable compound having a PO group is preferably 65% ​​by mass or more based on the total mass of the actinic radiation polymerizable compounds. The upper limit of the content is not particularly limited, and is, for example, 100% by mass.

[0036] In this embodiment, the actinic ray polymerizable compound may include a polymerizable compound that does not contain an EO group or a PO group, so long as formula (1) is satisfied.

[0037] Examples of the polymerizable compound not containing an EO group or a PO group include radically polymerizable compounds and cationic polymerizable compounds. Of these, the radically polymerizable compounds are preferred.

[0038] Examples of the radical polymerizable compound include unsaturated carboxylic acid esters and (meth)acrylates. Among these, (meth)acrylates are preferred. The (meth)acrylates may be monofunctional or polyfunctional.

[0039] Examples of monofunctional (meth)acrylates include isoamyl acrylate, stearyl acrylate, lauryl acrylate, octyl acrylate, decyl acrylate, isomyristyl acrylate, isostearyl acrylate, 2-ethylhexyl-diglycol acrylate, butoxyethyl acrylate, phenoxyethyl acrylate, cumylphenoxylethyl acrylate, tetrahydrofurfuryl acrylate, isobornyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxybutyl acrylate, and t-butylcyclohexyl acrylate.

[0040] Examples of the difunctional (meth)acrylate include 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, hydroxypivalic acid neopentyl glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, and the like.

[0041] Examples of the tri- or higher functional (meth)acrylate include trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and the like.

[0042] Examples of the cationically polymerizable compound include epoxy compounds and vinyl ether compounds.

[0043] Examples of the epoxy compounds include 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexanecarboxylate, bis(3,4-epoxycyclohexylmethyl)adipate, ε-caprolactone-modified 3,4-epoxycyclohexylmethyl 3',4'-epoxycyclohexanecarboxylate, 1-methyl-4-(2-methyloxiranyl)-7-oxabicyclo[4,1,0]heptane, 2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexanone-meta-dioxane, and bis(2,3-epoxycyclopentyl)ether, diglycidyl ether of 1,4-butanediol, diglycidyl ether of 1,6-hexanediol, triglycidyl ether of glycerin, and the like.

[0044] Examples of the vinyl ether compound include monovinyl ether compounds including ethyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, octadecyl vinyl ether, cyclohexyl vinyl ether, hydroxybutyl vinyl ether, 2-ethylhexyl vinyl ether, cyclohexanedimethanol monovinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, isopropenyl ether-o-propylene carbonate, dodecyl vinyl ether, diethylene glycol monovinyl ether, and octadecyl vinyl ether, as well as di- or trivinyl ether compounds including ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, propylene glycol divinyl ether, dipropylene glycol divinyl ether, butanediol divinyl ether, hexanediol divinyl ether, cyclohexanedimethanol divinyl ether, and trimethylolpropane trivinyl ether.

[0045] The weighted average value (mass basis) of the equivalent weight of the polymerizable group in the actinic radiation polymerizable compound is preferably 160 g / eq or more and 300 g / eq or less, more preferably 180 g / eq or more and 300 g / eq or less, and even more preferably 200 g / eq or more and 300 g / eq or less. By being 160 g / eq or more, the crosslinking degree of the polymer of the actinic radiation polymerizable compound can be appropriately lowered, and the flexibility of the cured film can be further increased. This can further suppress the folding crack of the image. By being 300 g / eq or less, the crosslinking degree of the polymer is not lowered too much, and the hardening degree of the cured film can be further increased, and peeling of the image can be further suppressed.

[0046] The weighted average value (mass basis) of the equivalent weight of the polymerizable group is preferably 160 g / eq or more and 200 g / eq or less when the actinic ray curable inkjet ink contains a yellow pigment as a colorant. The weighted average value (mass basis) of the equivalent weight of the polymerizable group is preferably 160 g / eq or more and 210 g / eq or less when the actinic ray curable inkjet ink contains a black pigment as a colorant. The weighted average value (mass basis) of the equivalent weight of the polymerizable group is preferably 200 g / eq or more and 250 g / eq or less when the actinic ray curable inkjet ink contains a red pigment as a colorant. The weighted average value (mass basis) of the equivalent weight of the polymerizable group is preferably 200 g / eq or more and 260 g / eq or less when the actinic ray curable inkjet ink contains a blue pigment as a colorant. Yellow pigments and black pigments are easy to absorb actinic rays. Therefore, when a yellow pigment or a black pigment is used, in order to facilitate sufficient curing of the actinic radiation-curable inkjet ink, it is preferable to set the weighted average value lower than when other pigments (e.g., red pigments and blue pigments) are used.

[0047] The equivalent weight of the polymerizable group refers to the acrylic equivalent weight when the actinic ray polymerizable compound is, for example, an acrylate, and the weighted average value of the equivalent weight of the polymerizable group refers to the sum of values ​​obtained by multiplying the equivalent weight of the polymerizable group of each polymerizable compound by the mass ratio of each polymerizable compound in the actinic ray polymerizable compound.

[0048] The actinic radiation polymerizable compound includes a monofunctional polymerizable compound, and the content of the monofunctional polymerizable compound is preferably more than 5% by mass and 40% by mass or less with respect to the total mass of the actinic radiation curable inkjet ink. By including more than 5% by mass of the monofunctional polymerizable compound, the degree of crosslinking of the polymer of the actinic radiation polymerizable compound can be appropriately reduced, and the flexibility of the cured film can be further increased. This can further suppress fold cracks in the image. In addition, by including the monofunctional polymerizable compound in an amount of 40% by mass or less, the degree of crosslinking is not too low, and a decrease in the abrasion resistance of the image can be suppressed.

[0049] In this case, it is preferable that the actinic radiation polymerizable compound further contains a polyfunctional polymerizable compound. This can further suppress the deterioration of the scratch resistance of the image. The content mass ratio of the monofunctional polymerizable compound to the polyfunctional polymerizable compound (monofunctional / polyfunctional weight ratio) is preferably 0 to 0.6, more preferably 0.1 to 0.4.

[0050] The monofunctional polymerizable compound preferably contains a compound having a benzene ring in the molecule. It is considered that the presence of a benzene ring in the molecule causes the interaction between the benzene rings (π-π interaction) to form a stack structure of the benzene rings, which further increases the hardness of the cured film. Therefore, the monofunctional polymerizable compound having a benzene ring in the molecule can further suppress folding cracks of the image while further increasing the abrasion resistance of the image.

[0051] The monofunctional polymerizable compound having a benzene ring in the molecule preferably contains 1 to 2 benzene rings in the molecule. Such a polymerizable compound can increase the strength of the cured film by stacking the benzene rings.

[0052] The monofunctional polymerizable compound having a benzene ring in the molecule may or may not contain an EO group or a PO group, but preferably contains an EO group or a PO group, and more preferably contains an EO group.

[0053] When the monofunctional polymerizable compound having a benzene ring contains an EO group, the weighted average number (mass basis) of the number of EO groups per molecule in the monofunctional polymerizable compound having a benzene ring is preferably 2 or more, more preferably 4 or more. The EO group and the PO group can increase the flexibility of the cured film. When the monofunctional polymerizable compound has both a benzene ring, an EO group, and a PO group, it is possible to achieve both an improvement in the strength of the cured film due to the stack of the benzene rings and an improvement in the flexibility of the cured film due to the EO group or the PO group. Furthermore, when the cured film having the flexibility increased by the EO group or the PO group is stretched, the stack of the benzene rings temporarily separates to make the cured film easier to stretch, and makes the folded cured film less likely to break (fracture resistance is further increased). On the other hand, since the temporarily separated benzene rings are stacked again when the stretching is released, it is considered that the strength of the cured film due to stretching is also less likely to decrease. In this way, the polymerizable compound is considered to further increase the flexibility of the cured film without decreasing the strength of the cured film, while further suppressing the folding cracks of the image. The upper limit of the weighted average number of EO groups is preferably 8. The weighted average number of EO groups per molecule refers to a value calculated by multiplying the number of EO groups contained in one molecule of the monofunctional polymerizable compound having a benzene ring by the mass ratio of the monofunctional polymerizable compound in the actinic ray-polymerizable compound, and the total value for each monofunctional polymerizable compound (containing a benzene ring).

[0054] In this case, the weighted average value (mass basis) of the number of repetitions of EO groups per molecule in the monofunctional polymerizable compound having a benzene ring is preferably not less than 2, and more preferably not less than 4. From the viewpoint of facilitating stacking of the benzene rings, the upper limit of the weighted average value (mass basis) of the number of repetitions is preferably 8.

[0055] Examples of the monofunctional polymerizable compound having a benzene ring in the molecule include radical polymerizable compounds and cationic polymerizable compounds. Among these, the radical polymerizable compound is preferred. The radical polymerizable compound is preferably a (meth)acrylate.

[0056] Examples of monofunctional (meth)acrylates having a benzene ring in the molecule include phenoxyethyl acrylate, 2-hydroxy-3-phenoxypropyl acrylate, cumylphenoxylethyl acrylate, phenol acrylate, nonylphenol (meth)acrylate, cresol (meth)acrylate, and EO or PO modified versions of these.

[0057] The content of the monofunctional polymerizable compound having a benzene ring in the molecule is preferably 0% by mass or more and 40% by mass or less, and more preferably 5% by mass or more and 25% by mass or less, based on the total mass of the actinic radiation polymerizable compound. By being 5% by mass, the degree of curing of the cured film can be further increased, and peeling of the image can be further suppressed. By being 25% by mass or less, a decrease in the flexibility of the cured film can be further suppressed, and folding cracks of the image can be further suppressed.

[0058] As described above, in the actinic ray polymerizable compound in this embodiment, the content of the polymerizable compound having a melting point of 25° C. or more is less than 3 mass % relative to the total mass of the actinic ray polymerizable compound. From the viewpoint of further improving the injection stability, the content of the polymerizable compound having a melting point of 25° C. or more is preferably less than 1 mass %, more preferably less than 0.1 mass %, and even more preferably 0 mass %.

[0059] Examples of the polymerizable compound having a melting point of 25° C. or higher include octadecyl acrylate, tris(2-acryloyloxyethyl)isocyanurate, behenyl acrylate, and the like.

[0060] Whether the melting point is 25° C. or higher can be determined based on whether the polymerizable compound is liquid (has fluidity) or solid (has no fluidity) at 25° C. Alternatively, it may be measured by a thermal analysis method (DSC, DTA) or the like.

[0061] The weight average molecular weight (Mw) of the actinic radiation polymerizable compound is preferably from 160 to 300, and more preferably from 200 to 300. The weight average molecular weight can be measured by using gel permeation chromatography (GPC).

[0062] The content of the actinic ray-polymerizable compound can be from 1% by mass to 97% by mass, preferably from 30% by mass to 95% by mass, more preferably from 50% by mass to 95% by mass, and even more preferably from 70% by mass to 95% by mass, based on the total mass of the actinic ray-curable inkjet ink.

[0063] 1-2.Other The actinic ray curable inkjet ink composition according to this embodiment may further contain other components such as a gelling agent, a colorant, a surfactant, a polymerization inhibitor, and a moisturizing agent, as long as the effects of the present invention are achieved.

[0064] (Gelling agent) In this embodiment, the actinic ray curable inkjet ink may contain a gelling agent. When the actinic ray curable inkjet ink contains a gelling agent, the ink undergoes a sol-gel phase transition (before irradiation with actinic rays). Specifically, the gelling agent is a compound that dissolves in the actinic ray polymerizable compound contained in the ink when the ink containing the gelling agent is heated (e.g., to 80° C.), and crystallizes in the ink at around room temperature (e.g., 35° C.), gelling the ink.

[0065] It is preferable that the gelling agent crystallizes in the ink, and forms a structure in which the actinic radiation-polymerizable compound is encapsulated in a three-dimensional space formed by the gelling agent crystallized into plate-like shapes (such a structure will be referred to hereinafter as a "house of cards structure"). When a house of cards structure is formed, the liquid actinic radiation-polymerizable compound is held within the space, further enhancing the gelling properties of the ink. This makes it more difficult for the ink to wet and spread when formed on the recording medium, improving the pinning properties of the ink.

[0066] Examples of gelling agents that can facilitate the formation of a house-of-cards structure through crystallization include aliphatic ketones, aliphatic esters, glycerol-based compounds, pentaerythritol-based compounds, petroleum-based waxes, vegetable-based waxes, animal-based waxes, mineral-based waxes, hydrogenated castor oil, modified waxes, higher fatty acids, higher alcohols, hydroxystearic acid, fatty acid amides including N-substituted fatty acid amides and special fatty acid amides, higher amines, esters of sucrose fatty acids, synthetic waxes, dibenzylidene sorbitol, dimer acids and dimer diols.

[0067] Among these, from the viewpoint of increasing the polarity and further increasing the solubility in the actinic radiation polymerizable compound, the gelling agent is preferably an aliphatic ketone, an aliphatic ester, a higher fatty acid, or a higher alcohol, and more preferably an aliphatic ketone or an aliphatic ester. Note that only one type of gelling agent may be contained, or two or more types may be contained in combination.

[0068] Examples of the aliphatic ketones include dibehenyl ketone, distearyl ketone, dieicosyl ketone, dipalmityl ketone, dilauryl ketone, dimyristyl ketone, myristyl palmityl ketone, and palmityl stearyl ketone.

[0069] Examples of fatty esters include fatty acid esters of monoalcohols such as behenyl behenate, icosanoic acid ethyl ester, stearyl stearate, palmityl stearate, myristyl myristate, cetyl myristate, and oleyl palmitate, and fatty acid esters of polyhydric alcohols such as glycerin fatty acid esters, sorbitan fatty acid esters, propylene glycol fatty acid esters, ethylene glycol fatty acid esters, and polyoxyethylene fatty acid esters.

[0070] Examples of higher fatty acids include behenic acid, arachidic acid, stearic acid, palmitic acid, myristic acid, lauric acid, oleic acid, and erucic acid.

[0071] Examples of higher alcohols include stearyl alcohol and behenyl alcohol.

[0072] Among the aliphatic ketones and aliphatic esters, from the viewpoint of further enhancing the gelling property, an aliphatic ketone represented by the following general formula (G1) or an aliphatic ester represented by the following general formula (G2) is more preferable.

[0073] General formula (G1):R a -CO-R b General formula (G2):R c -COO-R d

[0074] In general formula (G1), R a and R b each independently represents a linear hydrocarbon group having 12 to 22 carbon atoms, and in general formula (G2), R c and R d each independently represents a linear hydrocarbon group having 12 to 22 carbon atoms.

[0075] In general formulae (G1) and (G2), R a ~R d When the number of carbon atoms is 12 or more, the crystallinity of the gelling agents represented by general formula (G1) and general formula (G2) is increased, and more sufficient spaces are generated in the house-of-cards structure. Therefore, the actinic radiation polymerizable compound (A) is easily and sufficiently encapsulated in the spaces, and the gelling property and pinning property of the ink are improved.

[0076] Also, R a ~R d When the number of carbon atoms is 22 or less, the melting point of the gelling agent represented by general formula (G1) and general formula (G2) does not become too high, and the solubility of the gelling agent is increased.

[0077] Examples of aliphatic ketones represented by general formula (G1) include dibehenyl ketone (carbon number: 21-22), distearyl ketone (carbon number: 17-18), dieicosyl ketone (carbon number: 19-20), dipalmityl ketone (carbon number: 15-16), dimyristyl ketone (carbon number: 13-14), dilauryl ketone (carbon number: 11-12), lauryl myristyl ketone (carbon number: 13-14), and ketones represented by general formula (G2). Prime numbers: 11-14), lauryl palmityl ketone (11-16), myristyl palmityl ketone (13-16), myristyl stearyl ketone (13-18), myristyl behenyl ketone (13-22), palmityl stearyl ketone (15-18), palmityl behenyl ketone (15-22) and stearyl behenyl ketone (17-22). The number of carbon atoms in the parentheses above indicates the number of carbon atoms in each of the two hydrocarbon groups separated by the carbonyl group.

[0078] Examples of the aliphatic ester represented by the general formula (G2) include behenyl behenate (carbon number: 21-22), icosanoic acid icosyl (carbon number: 19-20), stearyl stearate (carbon number: 17-18), palmityl stearate (carbon number: 17-16), lauryl stearate (carbon number: 17-12), cetyl palmitate (carbon number: 15-16), stearyl palmitate (carbon number: 15-18), myristyl Myristyl phosphate (carbon number: 13-14), cetyl myristate (carbon number: 13-16), octyldodecyl myristate (carbon number: 13-20), stearyl oleate (carbon number: 17-18), stearyl erucate (carbon number: 21-18), stearyl linoleate (carbon number: 17-18), behenyl oleate (carbon number: 18-22), and arachidyl linoleate (carbon number: 17-20). The number of carbons in the parentheses above indicates the number of carbons in each of the two hydrocarbon groups separated by the ester group.

[0079] The content of the gelling agent is preferably 0.5% by mass or more and 10% by mass or less, and more preferably 1.5% by mass or more and 8% by mass or less, based on the total mass of the actinic radiation curable inkjet ink. When the content is 1.5% by mass or more, the gelling property and pinning property of the ink can be further improved. Also, when the content is 8% by mass or less, the solubility of the gelling agent in the actinic radiation polymerizable compound can be further improved.

[0080] (Coloring agent) The colorant may be a dye or a pigment, with pigments being preferred since they have good dispersibility in the ink components and are weather resistant. The pigment may be selected from, for example, yellow pigments, red pigments, blue pigments, black pigments, and white pigments depending on the color of the image to be formed.

[0081] Examples of yellow pigments include Pigment Yellow (PY) 1, 3, 12, 13, 14, 17, 34, 35, 37, 55, 74, 81, 83, 93, 94, 95, 97, 108, 109, 110, 137, 138, 139, 153, 154, 155, 157, 166, 167, 168, 180, 185, and 193. Of these, from the viewpoints of further reducing the amount of impurities in the ink composition and further increasing the electrical resistance value of the insulating film, PY185 and PY150 are preferred.

[0082] Examples of red pigments include Pigment Red (PR) 3, 5, 19, 22, 31, 38, 43, 48:1, 48:2, 48:3, 48:4, 48:5, 49:1, 53:1, 57:1, 57:2, 58:4, 63:1, 81, 81:1, 81:2, 81:3, 81:4, 88, 104, 108, 112, 122, 123, 144, 146, 149, 166, 168, 169, 170, 177, 178, 179, 184, 185, 202, 208, 216, 226, 257, Pigment Violet (PV) 3, 19, 23, 29, 30, 37, 50, 88, Pigment Orange (PO) 13, 16, 20, 36, etc. Among these, PR122 and PV19 are preferred from the viewpoint of further reducing the amount of impurities in the ink composition and further increasing the electrical resistance value of the insulating film.

[0083] Examples of blue pigments include Pigment Blue (PB) 1, 15, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 17:1, 22, 27, 28, 29, 36, and 60. Of these, PB15:3 and PB15:4 are preferred from the viewpoints of further reducing the amount of impurities in the ink composition and further increasing the electrical resistance value of the insulating film.

[0084] Examples of black pigments include CI Pigment Black (hereinafter also simply referred to as "PBk") 7, PBk26, and PBk28.

[0085] The white pigment may be any pigment that imparts a white color to the cured film formed by curing the white ink. Examples of the white pigment include inorganic pigments such as titanium oxide, zinc oxide, calcium carbonate, barium sulfate, and aluminum hydroxide. Among these, titanium oxide is preferred.

[0086] The crystal form of the titanium oxide may be any of rutile, anatase, and brookite types. From the viewpoint of making it easier to reduce the particle size of the white pigment, the anatase type, which has a small specific gravity, is preferred, and from the viewpoint of further improving the hiding power of the image to be formed, the rutile type, which has a large refractive index in the visible light region, is preferred.

[0087] The content of the colorant is preferably from 0.1% to 10% by mass, and more preferably from 1% to 5% by mass, based on the total mass of the ink composition. The content of the white pigment is preferably from 3% to 8% by mass.

[0088] (Pigment dispersant) The actinic radiation curable ink may contain a pigment dispersant for dispersing the pigment. Examples of the pigment dispersant include hydroxyl group-containing carboxylate ester, salt of long chain polyaminoamide and high molecular weight acid ester, salt of high molecular weight polycarboxylate, salt of long chain polyaminoamide and polar acid ester, high molecular weight unsaturated acid ester, polymer copolymer, modified polyurethane, modified polyacrylate, polyether ester type anionic surfactant, naphthalene sulfonate formalin condensate salt, aromatic sulfonate formalin condensate salt, polyoxyethylene alkyl phosphate ester, polyoxyethylene nonylphenyl ether, and stearylamine acetate.

[0089] The content of the pigment dispersant is preferably 10% by mass or more and 200% by mass or less, and more preferably 20% by mass or more and 100% by mass or less, based on the total mass of the pigment. When the content of the dispersant is 10% by mass or more based on the total mass of the pigment, the dispersion stability of the pigment is improved, and when the content of the dispersant is 200% by mass or less based on the total mass of the pigment, the ejection of the ink from the inkjet head becomes stable.

[0090] (Activin Radiation Polymerization Initiator) In the present embodiment, the actinic ray curable inkjet ink may contain an actinic ray polymerization initiator (hereinafter, simply referred to as a "polymerization initiator"). The polymerization initiator may be any one that can initiate polymerization of the actinic ray polymerizable compound by irradiation with actinic rays. For example, when the actinic ray curable inkjet ink contains a radical polymerizable compound, the polymerization initiator may be a radical polymerization initiator, and when the actinic ray curable ink contains a cationic polymerizable compound, the polymerization initiator may be a cationic polymerization initiator (photoacid generator). Note that when the actinic ray curable inkjet ink can be sufficiently cured without a polymerization initiator, such as when the actinic ray curable inkjet ink is cured by irradiation with electron beams, the polymerization initiator is not necessary.

[0091] The radical polymerization initiator includes an intramolecular bond cleavage type radical polymerization initiator and an intramolecular hydrogen abstraction type radical polymerization initiator.

[0092] Examples of the intramolecular bond cleavage type radical polymerization initiator include acetophenone-based initiators including diethoxyacetophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, benzyl dimethyl ketal, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one, 4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone, 1-hydroxycyclohexyl-phenyl ketone, 2-methyl-2-morpholino(4-methylthiophenyl)propan-1-one, and 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone, benzoins including benzoin methyl ether, and benzoin isopropyl ether, acylphosphine oxide-based initiators including 2,4,6-trimethylbenzoin diphenylphosphine oxide, and benzyl and methylphenyl glyoxyesters.

[0093] Examples of the intramolecular hydrogen abstraction type radical polymerization initiator include benzophenone-based initiators including benzophenone, o-benzoylmethylbenzoate, 4-phenylbenzophenone, 4,4'-dichlorobenzophenone, hydroxybenzophenone, 4-benzoyl-4'-methyl-diphenyl sulfide, acrylated benzophenone, 3,3',4,4'-tetra(t-butylperoxycarbonyl)benzophenone, and 3,3'-dimethyl-4-methoxybenzophenone; thioxanthone-based initiators including 2-isopropylthioxanthone, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, and 2,4-dichlorothioxanthone; aminobenzophenone-based initiators including Michler's ketone and 4,4'-diethylaminobenzophenone; 10-butyl-2-chloroacridone, 2-ethylanthraquinone, 9,10-phenanthrenequinone, and camphorquinone.

[0094] Examples of cationic polymerization initiators include photoacid generators. Examples of photoacid generators include aromatic onium compounds such as diazonium, ammonium, iodonium, sulfonium, and phosphonium, such as B(C6F5)4. - , PF6 - , AsF6 - , SbF6 - , CF3SO3 - These include salts, sulfonates which generate sulfonic acids, halides which photogenerate hydrogen halides, and iron-allene complexes.

[0095] The content of the polymerization initiator is not particularly limited as long as the actinic ray curable inkjet ink is sufficiently cured by irradiation with actinic rays (e.g., ultraviolet rays) and does not deteriorate the coatability on the surface of a recording medium. For example, the content of the polymerization initiator is preferably 0.1% by mass or more and 20% by mass or less, and more preferably 1% by mass or more and 10% by mass or less, based on the total mass of the actinic ray curable inkjet ink.

[0096] (Polymerization inhibitor) In this embodiment, the actinic ray curable inkjet ink may contain a polymerization inhibitor.

[0097] Examples of polymerization inhibitors include (alkyl)phenols, hydroquinone, catechol, resorcinol, p-methoxyphenol, t-butylcatechol, t-butylhydroquinone, pyrogallol, 1,1-picrylhydrazyl, phenothiazine, p-benzoquinone, nitrosobenzene, 2,5-di-t-butyl-p-benzoquinone, dithiobenzoyl disulfide, picric acid, cupferron, aluminum N-nitrosophenylhydroxylamine, tri-p-nitrophenylmethyl, N-(3-oxyanilino-1,3-dimethylbutylidene)aniline oxide, dibutylcresol, cyclohexanone oxime cresol, guaiacol, o-isopropylphenol, butylaldoxime, methyl ethyl ketoxime, cyclohexanone oxime, and the like.

[0098] The content of the polymerization inhibitor is not particularly limited, but is preferably from 0.05% by mass to 10.00% by mass with respect to the total mass of the actinic ray-curable inkjet ink.

[0099] (Surfactant) In this embodiment, the actinic ray curable inkjet ink may contain a surfactant for adjusting the surface tension.

[0100] Examples of surfactants include anionic surfactants including dialkyl sulfosuccinates, alkyl naphthalene sulfonates, and fatty acid salts; nonionic surfactants including polyoxyethylene alkyl ethers, polyoxyethylene alkyl allyl ethers, acetylene glycols, and polyoxyethylene-polyoxypropylene block copolymers; cationic surfactants including alkylamine salts and quaternary ammonium salts, silicone surfactants, and fluorine-based surfactants.

[0101] The content of the surfactant is not particularly limited, but is preferably from 0.001% to 10% by mass, and more preferably from 0.001% to 1.0% by mass, based on the total mass of the actinic ray-curable inkjet ink.

[0102] In this embodiment, in addition to the above-mentioned components, the actinic ray curable inkjet ink may contain, as necessary, a fixing resin, a viscosity adjuster, a resistivity adjuster, a film forming agent, an ultraviolet absorbing agent, an antioxidant, a color fading inhibitor, an antifungal agent, an antirust agent, and the like.

[0103] 1-4. Physical properties of actinic radiation curable inkjet ink The viscosity of the actinic ray curable ink at 60°C is preferably 10 mPa·s or more and 25 mPa·s or less, and more preferably 10 mPa·s or more and 15 mPa·s or less. This can improve the ejection properties when the ink is heated and ejected from the inkjet head. In this embodiment, the viscosity can be adjusted to fall within the above range by satisfying formula (1). Furthermore, when the ink contains a gelling agent, the viscosity can also be adjusted to fall within the above range by adjusting the content of the gelling agent.

[0104] When the actinic ray curable ink contains a gelling agent, the viscosity of the actinic ray curable ink at 80° C. is preferably 6 mPa·s or more and 25 mPa·s or less, and more preferably 7 mPa·s or more and 15 mPa·s or less, which can improve the ejection property when the ink is heated and ejected from the inkjet head.

[0105] The viscosity can be measured by a rheometer. For example, the precoat agent is heated to 100°C, and while measuring the viscosity by a stress-controlled rheometer (Physica MCR301 (diameter of cone plate: 75 mm, cone angle: 1.0°), the ink is cooled to 20°C under conditions of a shear rate of 11.7 (1 / s) and a temperature drop rate of 0.1°C / s to obtain a temperature change curve of the viscosity. The viscosity can be obtained by reading the viscosity at 80°C from the obtained temperature change curve.

[0106] 1-5. Method for preparing actinic radiation curable inkjet ink The actinic radiation curable inkjet ink can be prepared by mixing the actinic radiation polymerizable compound, the gelling agent, and any other components under heating. At this time, it is preferable to filter the obtained mixture through a predetermined filter. When preparing an ink containing a pigment, it is preferable to prepare a pigment dispersion containing the pigment and the actinic radiation polymerizable compound, and then mix the pigment dispersion with other components. The pigment dispersion may further contain a dispersant.

[0107] The pigment dispersion can be prepared by dispersing a pigment in an actinic radiation polymerizable compound. The pigment can be dispersed using, for example, a ball mill, a sand mill, an attritor, a roll mill, an agitator, a Henschel mixer, a colloid mill, an ultrasonic homogenizer, a pearl mill, a wet jet mill, a paint shaker, etc. At this time, a dispersant may be added.

[0108] In addition, when a plurality of types of actinic radiation-polymerizable compounds are used, the actinic radiation-polymerizable compounds may be mixed first to prepare an actinic radiation-polymerizable composition, and then the actinic radiation-polymerizable composition, the gelling agent, and any other components may be mixed under heating.

[0109] 2. Image forming method 1 is a flowchart showing an image forming method according to this embodiment. The image forming method according to this embodiment includes a step (step S10) of ejecting droplets of the actinic ray curable inkjet ink from an inkjet head and applying the droplets to the surface of a recording medium, and a step (step S20) of irradiating the applied droplets of the actinic ray curable inkjet ink with actinic rays to cure the droplets.

[0110] 2-1. Step of applying actinic radiation curable inkjet ink to a recording medium (step S10) In this process, the actinic radiation curable inkjet ink described above is ejected from an inkjet head and applied to the surface of a recording medium (at a position corresponding to the image to be formed).

[0111] The ejection method from the inkjet head may be either an on-demand method or a continuous method. The on-demand inkjet head may be either an electro-mechanical conversion type such as a single cavity type, a double cavity type, a bender type, a piston type, a share mode type, or a shared wall type, or an electro-thermal conversion type such as a thermal inkjet type or a bubble jet type ("bubble jet" is a registered trademark of Canon Inc.). The inkjet head may be either a scan type or a line type.

[0112] When the ink contains a gelling agent, droplets of the actinic ray curable ink are ejected from the inkjet head in a heated, solvated state, and therefore it is preferable to set the temperature of the ink when filled into the inkjet head to the gelling temperature of the ink +10°C or higher and the gelling temperature +30°C or lower. If the temperature of the actinic ray curable ink in the inkjet head is the gelling temperature +10°C or higher, the ink is less likely to gel inside the inkjet head or on the nozzle surface, resulting in a decrease in ejection performance. On the other hand, if the temperature of the ink in the inkjet head is the gelling temperature +30°C or lower, deterioration of the components due to high temperatures is less likely to occur.

[0113] The method for heating the actinic radiation-curable inkjet ink is not particularly limited. For example, at least one of the ink supply system such as the ink tank constituting the head carriage, the supply pipe, and the anterior ink tank immediately before the head, the piping with a filter, and the piezo head can be heated by a panel heater, a ribbon heater, or warm water.

[0114] The amount of droplets of the actinic ray curable inkjet ink ejected is preferably 2 pL or more and 20 pL or less, from the viewpoint of further increasing the recording speed and image quality.

[0115] The recording medium is not particularly limited, and in addition to ordinary uncoated paper and coated paper, synthetic paper Yupo ("Yupo" is a registered trademark of Yupo Corporation), various plastics used in soft packaging, and their films 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. Other plastics that can be used include polycarbonate, (meth)acrylic resin, ABS, polyacetal, PVA, and rubbers.

[0116] The actinic ray curable inkjet ink may be applied to a recording medium by directly impacting the ejected ink on the recording medium, or by impacting the ejected ink on an intermediate transfer body to form an intermediate image, and then transferring the intermediate image from the intermediate transfer body to the recording medium.

[0117] 2-2. Step of curing the actinic ray curable inkjet ink (step S20) In this process, the droplets of the actinic ray-curable inkjet ink applied to the recording medium in step S10 are irradiated with actinic rays to cure the droplets, thereby forming an image made of a cured film of the actinic ray-curable inkjet ink.

[0118] The actinic radiation can be selected from, for example, electron beams, ultraviolet rays, α rays, γ rays, and X-rays, but is preferably ultraviolet rays or electron beams. The ultraviolet rays are preferably light having a peak wavelength of 360 nm or more and 410 nm or less. The ultraviolet rays are preferably irradiated from an LED light source. LEDs emit less radiant heat than conventional light sources (e.g., metal halide lamps, etc.). Therefore, when exposed to actinic radiation, LEDs are less likely to melt the ink and less likely to cause uneven gloss.

[0119] 3. Image forming equipment An image forming apparatus 100 capable of carrying out the above-mentioned image forming method will now be described.

[0120] Fig. 2 is a schematic diagram showing the configuration of an image forming apparatus 100 according to this embodiment. As shown in Fig. 2, the image forming apparatus 100 has an inkjet head 110, a conveying section 120, and an irradiating section 130. In Fig. 2, the arrow indicates the conveying direction of the recording medium.

[0121] The inkjet head 110 has a nozzle surface 113 on which the nozzles 111 are provided, on a surface facing the transport unit 120 when forming an image, and ejects actinic ray curable inkjet ink onto the recording medium 200 transported by the transport unit 120. From the viewpoint of improving the ejection properties of the actinic ray curable inkjet ink, the inkjet head 110 may have a temperature adjustment means for adjusting the temperature of the ink to adjust the ink to a low viscosity. Examples of the temperature adjustment means include a panel heater, a ribbon heater, and a heating means that uses warm water.

[0122] The inkjet head 110 may be a scanning type inkjet head whose width in a direction perpendicular to the transport direction of the recording medium is smaller than that of the recording medium 200, or may be a line type inkjet head whose width in a direction perpendicular to the transport direction of the recording medium is larger than that of the recording medium 200.

[0123] The nozzles 111 have ejection openings on a nozzle surface 113. The number of nozzles 111 may be equal to or greater than the number of inks used in image formation (for example, four).

[0124] When forming an image, the conveying unit 120 conveys the recording medium 200 so that the recording medium 200 facing the inkjet head 110 moves directly below the inkjet head 110 in the vertical direction. For example, the conveying unit 120 has a driving roller 121, a driven roller 122, and a conveying belt 123.

[0125] The irradiation unit 130 irradiates the upper surface of the transport unit 120 with active rays. This allows the droplets of the active ray-curable inkjet ink that has landed on the transported recording medium 200 to be irradiated with active rays, thereby curing the droplets. The irradiation unit 130 can be disposed immediately above the transport unit 120, downstream of the inkjet head 110.

[0126] In addition to the above configuration, the image forming apparatus 100 may have an ink tank (not shown) for storing actinic ray curable ink before ejection, an ink flow path (not shown) that connects the ink tank and the inkjet head 110 so that the ink can flow therethrough, and a control unit (not shown) that controls the operation of the inkjet head 110, the transport unit 120, and the irradiation unit 130.

[0127] The image forming apparatus 100 may also have an intermediate transfer body and a transfer unit (both not shown). At this time, the inkjet head 110 ejects actinic ray curable inkjet ink onto the intermediate transfer body, causing it to land on the surface of the intermediate transfer body, and forms an intermediate image on the surface of the intermediate transfer body, which is made up of droplets of the actinic ray curable inkjet ink. Thereafter, the transfer unit transfers the intermediate image from the surface of the intermediate transfer body to the surface of the recording medium. Then, the irradiation unit 130 irradiates the intermediate image transferred to the surface of the recording medium with actinic rays, thereby curing the droplets of the actinic ray curable inkjet ink. EXAMPLES

[0128] The present invention will now be described with reference to examples, which should not be construed as limiting the scope of the present invention.

[0129] 1. Preparation / synthesis of materials The materials used in the preparation of the actinic radiation curable inkjet ink are shown below.

[0130] 1-1. Pigment dispersion 9 parts by mass of a pigment dispersant (EFKA-7701, manufactured by BASF) and 71 parts by mass of tripropylene glycol diacrylate were placed in a stainless steel beaker, and heated and stirred for 1 hour while being heated to 65°C on a hot plate. Then, the mixture was cooled to room temperature, and 20 parts by mass of any of the pigments listed below were added to the stainless steel beaker after stirring, and then the mixture was placed in a glass bottle together with 200 g of zirconia beads (diameter 0.3 mm, manufactured by Nikkato Corporation) and sealed. This pigment-containing liquid was dispersed using a paint shaker, and the zirconia beads were removed to obtain a pigment dispersion. The dispersion time was 4 hours when a cyan pigment and a black pigment were used, and 6 hours when a magenta pigment and a yellow pigment were used. Cyan pigment: Pigment Blue 15:4 (Chromofine Blue 6332JC, manufactured by Dainichi Seika Color & Chemicals Co., Ltd.) Magenta pigment: Mixture of Pigment Violet 19 and Red 202 (CINQUASIA MAGENTA RT-355D, manufactured by BASF) Yellow pigment: Pigment Yellow 185 (D1155, manufactured by BASF) Black pigment: Pigment Black 7 (#52, manufactured by Mitsubishi Chemical Corporation)

[0131] 1-2.Active radiation polymerizable compound (Monofunctional actinic radiation polymerizable compound) m1: 4EO modified nonylphenol acrylate (Number of EO groups: 4, Number of repeating EO groups (average): 4, Molecular weight: 450, Acrylic equivalent: 450g / eq, Melting point: less than 25°C) m2: Lauryl acrylate (Number of EO or PO groups: 0, molecular weight: 240, acrylic equivalent: 240g / eq, melting point: less than 25℃) m3:4EO modified phenol acrylate (Number of EO groups: 4, Number of repeating EO groups (average): 4, Molecular weight: 324, Acrylic equivalent: 324g / eq, Melting point: less than 25°C) m4: octadecyl acrylate (Number of EO or PO groups: 0, molecular weight: 324, acrylic equivalent: 324g / eq, melting point: 30℃) m5:8EO modified nonylphenol acrylate (Number of EO groups: 8, Number of repeating EO groups (average): 8, Molecular weight: 626, Acrylic equivalent: 624g / eq, Melting point: less than 25°C) m6: 2.5PO modified nonylphenol acrylate (Number of PO groups: 2.5, Number of repeating PO groups (average): 2.5, Molecular weight: 419, Acrylic equivalent: 419g / eq, Melting point: less than 25°C) m7: (5-ethyl-1,3-dioxane-5-yl)methyl acrylate (Number of EO or PO groups: 0, molecular weight: 200, acrylic equivalent: 200g / eq, melting point: less than 25°C) m8: isobornyl acrylate (Number of EO or PO groups: 0, molecular weight: 208, acrylic equivalent: 208g / eq, melting point: less than 25°C) m9: EO modified phenol acrylate (Number of EO groups: 1, Number of repeating EO groups (average): 1, Molecular weight: 192, Acrylic equivalent: 192g / eq, Melting point: less than 25°C) m10: 2EO modified phenol acrylate (Number of EO groups: 2, Number of repeating EO groups (average): 2, Molecular weight: 236, Acrylic equivalent: 236g / eq, Melting point: less than 25°C)

[0132] (Polyfunctional actinic radiation polymerizable compound) M1: Polyester acrylate (8 functionalities, number of EO groups: 8, number of repeating EO groups (average): 1, molecular weight: 561, acrylic equivalent: 70g / eq, melting point: less than 25°C) M2: Tripropylene glycol diacrylate (2 functionalities, number of PO groups: 3, number of repeating PO groups (average): 3, molecular weight: 300, acrylic equivalent: 150g / eq, melting point: less than 25°C) M3: PO-modified neopentyl glycol diacrylate (2 functionalities, number of PO groups: 5, number of repeating PO groups (average): 2.5, molecular weight: 503, acrylic equivalent: 251 g / eq, melting point: less than 25°C) M4: 3EO modified trimethylolpropane triacrylate (Trifunctional, number of EO groups: 9, number of repeating EO groups (average): 3, molecular weight: 692, acrylic equivalent: 231g / eq, melting point: less than 25°C) M5: 14EO modified polyethylene glycol diacrylate #600 (2 functionalities, number of EO groups: 14, number of repeating EO groups (average): 14, molecular weight: 708, acrylic equivalent: 354g / eq, melting point: less than 25°C) M6: 3PO modified trimethylolpropane triacrylate (Trifunctional, number of PO groups: 3, number of repeating PO groups (average): 1, molecular weight: 470, acrylic equivalent: 157g / eq, melting point: less than 25°C) M7: 3PO modified glyceryl triacrylate (Trifunctional, number of PO groups: 3, number of repeating PO groups (average): 1, molecular weight: 429, acrylic equivalent: 143g / eq, melting point: less than 25°C) M8: Decanediol diacrylate (Difunctional, number of EO or PO groups: 0, molecular weight: 282, acrylic equivalent: 141g / eq, melting point: less than 25°C) M9: Tricyclodecane dimethanol diacrylate (Difunctional, number of EO or PO groups: 0, molecular weight: 304, acrylic equivalent: 152 g / eq, melting point: less than 25°C) M10: 9EO modified trimethylolpropane triacrylate (Trifunctional, number of EO groups: 9, number of repeating EO groups (average): 3, molecular weight: 693, acrylic equivalent: 231g / eq, melting point: less than 25°C) M11: Ditrimethylolpropane tetraacrylate (4 functionalities, number of EO or PO groups: 0, molecular weight: 466, acrylic equivalent: 117g / eq, melting point: less than 25°C) M12: Tris(2-acryloyloxyethyl) isocyanurate (Trifunctional, number of EO or PO groups: 0, molecular weight: 423, acrylic equivalent: 141g / eq, melting point: 53℃)

[0133] Of the above actinic ray polymerizable compounds, the melting points of m4 and M12 were determined using catalog values, and the melting points of the other actinic ray polymerizable compounds were determined to be less than 25°C by confirming that they were liquid at room temperature (25°C).

[0134] 1-3. Gelling agents G1: Behenyl behenate (WEP-3, NOF Corporation) G2: Stearyl stearate (Excepearl SS, manufactured by Kao Corporation)

[0135] 1-4. Actinic polymerization initiator Initiator 1 (Omnirad 819, IGM Resins BV) Initiator 2 (Speedcure 2-ITX, Arkema)

[0136] 1-5. Polymerization inhibitors Irgasutab UV-10 (BASF)

[0137] 1-6.Surfactants KF-352A (Shin-Etsu Chemical Co., Ltd.)

[0138] 2. Preparation of actinic radiation curable inkjet ink The pigment dispersion, actinic radiation polymerizable compound, gelling agent, polymerization initiator, polymerization inhibitor, and surfactant were placed in a stainless steel beaker so as to obtain the compositions shown in Tables 1 to 7, and the mixture was stirred at 105° C. for 45 minutes. The mixture was then filtered through a 3 μm Teflon (registered trademark) membrane filter manufactured by ADVANTEC Corporation to obtain actinic radiation curable inkjet inks 1 to 34. The notation "benzene ring" in Tables 1 to 7 indicates whether or not an actinic radiation polymerizable compound containing a benzene ring is contained in the molecule. The notation "number of monofunctional EO groups" in Tables 1 to 7 indicates the weighted average value (by mass) of the number of ethylene oxide groups per molecule in a monofunctional actinic radiation polymerizable compound containing a benzene ring.

[0139] [Table 1]

[0140] [Table 2]

[0141] [Table 3]

[0142] [Table 4]

[0143] [Table 5]

[0144] [Table 6]

[0145] [Table 7]

[0146] 3. Image Formation A monochromatic image was formed using any one of the actinic radiation curable inkjet inks 1 to 34 using a line type inkjet recording device. The temperature of the inkjet head of the inkjet recording device was set to 80°C. A solid image of 5 cm x 5 cm was printed on a recording medium (OK topcoat + 127 g, manufactured by Oji Paper Co., Ltd.). After the image was formed, the image was irradiated with ultraviolet light by an LED lamp (395 nm, water-cooled LED manufactured by Phoseon Technology Co., Ltd.) arranged downstream of the recording device to cure the ink. A piezo head was used as the ejection recording head. The ejection conditions were that the amount of liquid per drop was 9.0 pl, and the ink was ejected at a liquid speed of about 6 m / s, and the image was recorded at a resolution of 1200 dpi x 1200 dpi. The recording speed was 500 mm / s. The image was formed in an environment of 23°C and 55% RH. dpi refers to the number of dots per inch (2.54 cm).

[0147] 4. Evaluation (Bending cracks) Using a paper folder (AFV-564FKT, manufactured by Horizon Co., Ltd.), the image formed on the recording medium was folded in half. The state of the image was then observed, and the ratio of the area of ​​the portion of the recording medium that was not exposed due to cracks (non-exposed portion) was determined by image analysis. The fold crack resistance was evaluated according to the following criteria. 5: No cracks were observed in the image 4: Cracks were visible in the image, and only a small amount of white was exposed. 3: The image was cracked and less than half of the area was exposed white. 2: Cracks were visible in the image, and more than half of the area was exposed white. 1: Cracks were visible in the image, and white was exposed over the entire area.

[0148] (Cutting and peeling (image abrasion resistance)) The image-formed portion of the recording medium was cut using a tabletop cutter (PC-P430, manufactured by Horizon Co., Ltd.) The cut portion was rubbed with a finger, and the degree of peeling of the coating film (image) was evaluated according to the following criteria. 5: The coating did not peel off. 4: Peeling was present but not visible to the naked eye. 3: Peeling was present but difficult to see with the naked eye 2: Peeling was visible to the naked eye 1: Large peeling was clearly visible

[0149] (viscosity) The viscosity at 60°C of the obtained actinic ray curable inkjet inks 1 to 34 was measured. The above viscosity was determined from the complex viscosity at the temperature of 60°C in a viscosity change curve measured while changing the temperature from 90°C to 30°C in vibration mode using a modular compact rheometer (MCR302e, manufactured by Anton Paar). Based on the measured viscosity, evaluation was performed according to the following criteria. 2: Viscosity was 10-25 mPa·s 1: Viscosity was outside the range of 10 to 25 mPa·s

[0150] (Injection properties) <Evaluation criteria> The image output of each sample was visually checked for white streaks (white streaks due to poor ejection) in the 100% printed area when the 10th and 100th sheets were printed. 2: No white spots occurred, or there were one or two white spots, but they were not problematic for practical use. 1: There were many white spots, which caused problems in practical use.

[0151] The evaluation results are summarized in Tables 8 to 14.

[0152] [Table 8]

[0153] [Table 9]

[0154] [Table 10]

[0155] [Table 11]

[0156] [Table 12]

[0157] [Table 13]

[0158] [Table 14]

[0159] From the results of inks 5 to 34, it was found that when the actinic ray-polymerizable compound satisfies formula (1) and the desired amount of the actinic ray-polymerizable compound having a melting point of 25°C or higher is less than 3 mass%, it is possible to suppress image cracking, suppress image peeling, and improve ejection stability. [Industrial Applicability]

[0160] The ink-jet ink of the present invention can form an image that is not easily peeled off from a recording medium while preventing cracks from occurring when the recording medium is folded, and can also improve ejection stability, so that the present invention is useful in the field of image formation. [Explanation of symbols]

[0161] 100 Image forming device 110 Inkjet head 120 Conveyor 130 Irradiation unit

Claims

1. An ink-jet ink containing a polymerizable compound that is polymerized by actinic radiation, the content of the polymerizable compound having a melting point of 25° C. or higher is less than 3% by mass relative to the total mass of the polymerizable compounds; When a weighted average value (based on mass) of the number of ethylene oxide groups per molecule in the polymerizable compound is defined as X and a weighted average value (based on mass) of the number of propylene oxide groups per molecule in the polymerizable compound is defined as Y, Formula (1) is satisfied. Inkjet ink that cures with actinic radiation. 3.5≦X+Y≦9 (1)

2. The ink-jet ink according to claim 1 , wherein, in formula (1), X is 1.5 or more and 7 or less.

3. The ink-jet ink according to claim 1 , wherein the weighted average (by mass) of the number of repetitions of ethylene oxide groups and propylene oxide groups per molecule of the polymerizable compound is 2 or more and 10 or less.

4. The ink-jet ink according to claim 1 , wherein the ink-jet ink has a viscosity at 60° C. of 10 mPa·s or more and 25 mPa·s or less.

5. The ink-jet ink according to claim 1, wherein the weighted average equivalent weight (by mass) of the polymerizable group in the polymerizable compound is 160 g / eq or more and 300 g / eq or less.

6. The polymerizable compound includes a monofunctional polymerizable compound, the content of the monofunctional polymerizable compound is more than 5% by mass and not more than 40% by mass with respect to the total mass of the inkjet ink; The ink-jet ink of claim 1.

7. The ink-jet ink according to claim 6 , wherein the monofunctional polymerizable compound includes a compound having a benzene ring in the molecule.

8. The ink-jet ink according to claim 7, wherein the weighted average number (by mass) of ethylene oxide groups per molecule in the monofunctional polymerizable compound having a benzene ring is 2 or more.

9. The ink-jet ink according to claim 8, wherein the weighted average number (by mass) of ethylene oxide groups per molecule in the monofunctional polymerizable compound having a benzene ring is 4 or more.

10. Further comprising a pigment, when the pigment is a yellow pigment, a weighted average equivalent weight (mass basis) of the polymerizable group in the polymerizable compound is 160 g / eq or more and 200 g / eq or less; when the pigment is a black pigment, the weighted average equivalent weight (by mass) of the polymerizable group in the polymerizable compound is 160 g / eq or more and 210 g / eq or less; When the pigment is a red pigment, the weighted average equivalent weight (mass basis) of the polymerizable group in the polymerizable compound is 200 g / eq or more and 250 g / eq or less. or when the pigment is a blue pigment, the weighted average equivalent weight (mass basis) of the polymerizable group in the polymerizable compound is 200 g / eq or more and 260 g / eq or less; The ink-jet ink of claim 1.

11. A step of applying the ink-jet ink according to any one of claims 1 to 10 from an ink-jet head onto a surface of a recording medium; irradiating the applied inkjet ink with actinic radiation to cure the inkjet ink; The image forming method according to claim 1,

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