Active energy ray-curable inkjet black ink and printed matter

Abstract

This active energy ray-curable inkjet black ink comprises carbon black, a basic pigment dispersion resin having an acid value of at most 30 mg KOH / g, and 5-methyl-3-vinyloxazolidin-2-one, wherein: when the DBP oil absorption amount of the carbon black is AC (mL / 100 g) and the specific surface area of the carbon black is SC (m2 / g), the value represented by AC×SC is 2,000-12,000; and the content of a monofunctional polymerizable compound (excluding the 5-methyl-3-vinyloxazolidin-2-one) is at most 15 mass% with respect to the total mass of said active energy ray-curable inkjet black ink.

Description

Active energy ray curable inkjet black ink and printed matter

[0001] An embodiment of the present invention relates to an actinic ray-curable inkjet black ink and a printed matter obtained by printing the actinic ray-curable inkjet black ink on a printing substrate.

[0002] In recent years, digital printing has been increasingly adopted in printing markets that emphasize productivity, such as the commercial printing market, the office printing market, and the specialty printing market. The reasons for this include, first, that digital printing does not require plate-making, so printed materials can be produced at low cost and in a short time. Second, the printing devices are smaller and less expensive than those used in plate-based printing. Finally, uniform printed materials can be easily produced regardless of the skill level of the printing personnel.

[0003] Among these, inkjet printing, which is one type of digital printing method, has many advantages over other digital printing methods, such as low running costs during printing, ease of full color printing, and the fact that print quality is not dependent on the installation environment of the printing device, etc. For this reason, there is a particularly high demand for the adoption of inkjet printing in the printing market.

[0004] The inks used in the inkjet printing method range widely, including water-based, oil-based, solvent-based, active energy ray-curable, etc. Among these, there has been an increasing demand in recent years for active energy ray-curable inks because of their ability to produce high-quality prints even on non-permeable printing substrates such as resin films and glass, their rapid drying (curing) time, and their strength in printed matter.

[0005] In the production of printed materials, black is an extremely important color because it can improve the visibility and readability of printed characters and barcodes, the clarity of printed photographs, and other images. However, it is known that it is difficult to improve the curability of black in actinic radiation-curable inks used in inkjet printing (hereinafter simply referred to as "actinic radiation-curable inkjet inks"). This is because carbon black, which is commonly used as a black colorant, absorbs actinic radiation. Insufficient curability leads to a decrease in printing speed. Therefore, improving the curability of black actinic radiation-curable inkjet inks is a very important issue when considering expansion into the above-mentioned printing market, which places importance on productivity.

[0006] In the past, active energy ray-curable inkjet inks have been investigated that reproduce black by mixing multiple colorants without using carbon black (see, for example, Patent Document 1). However, it is difficult to reproduce a vivid black color with such a method, and further improvements are needed from the viewpoints of improving the visibility, readability, and sharpness described above.

[0007] Furthermore, there has been a growing demand in the market for environmental considerations in recent years. From this perspective, LED lamps have been increasingly adopted as a method for curing actinic radiation-curable inkjet inks. However, LED lamps have the characteristic of emitting actinic radiation with a narrow wavelength range. Therefore, when used in combination with actinic radiation-curable inkjet inks, improving curability becomes an issue.

[0008] Studies have been conducted to improve the curability of actinic radiation-curable inkjet inks when used in combination with LED lamps. For example, Patent Document 2 discloses a photocurable inkjet printing ink composition containing 4 to 40% by mass of vinyloxyethoxyethyl acrylate, 10 to 65% by mass of benzyl acrylate, and 50% by mass or more of a monofunctional monomer. However, the examples in Patent Document 2 do not include evaluation of printing using an actual inkjet printer (printing device), and there is no mention of whether printed matter with excellent visibility, readability, etc. can be stably printed. Furthermore, considering adoption in the printing market described above, it is considered necessary to further improve the curability of the photocurable inkjet printing ink composition disclosed in Patent Document 2.

[0009] Furthermore, Patent Document 3 discloses an ultraviolet-curable ink composition for inkjet printing, which contains an α-aminoalkylphenone initiator, a thioxanthone initiator, and a tertiary amine (photopolymerization initiator aid). However, in the examples of Patent Document 3, curability is evaluated using an inkjet printer employing a multi-pass printing method in which actinic radiation is irradiated to the same location multiple times. Furthermore, in the examples, only a yellow ink composition is evaluated. As described above, it is more difficult to improve the curability of a black actinic radiation-curable inkjet ink than other colors. Therefore, considering adoption in the printing market, simply applying the configuration of Patent Document 3 to a black actinic radiation-curable inkjet ink is likely to result in the desired curability being insufficient.

[0010] When the configuration disclosed in Patent Document 3 is applied to a black actinic radiation-curable inkjet ink, it is possible to increase the amount of the α-aminoalkylphenone initiator, the thioxanthone initiator, and the tertiary amine to enhance sensitivity to actinic radiation and improve curability. However, many of the above-mentioned components are solid at room temperature. Therefore, incorporating large amounts of these components can cause problems such as deterioration in ejection stability from the inkjet head and the dispersion state of the colorant (carbon black, etc.). For this reason, simply increasing the amount of the above-mentioned components is not necessarily a good solution.

[0011] As described above, it has been extremely difficult to achieve both dispersion stability and ejection stability of a colorant (carbon black, etc.) and curability and fineness of printed matter in a black actinic ray-curable inkjet ink (also referred to as an "actinic ray-curable inkjet black ink" in this specification), particularly when used in combination with an LED lamp.

[0012] In this specification, the term "definition of printed matter" refers to the excellent visibility and readability even for printed matter containing minute characters and barcodes.

[0013] JP 2008-266548 A International Publication No. 2014 / 014017 JP 2014-136795 A

[0014] The present invention has been made to solve the above-mentioned problems, and an object of one embodiment of the present invention is to provide an actinic energy ray-curable inkjet black ink that is excellent in all of the dispersion stability of carbon black, ejection stability, curability, and fineness of printed matter.

[0015] As a result of extensive research, the present inventors have found that all of the above-mentioned problems can be solved simultaneously and to a high degree by an active energy ray-curable inkjet black ink having the following composition.

[0016] That is, one embodiment of the present invention relates to an actinic ray-curable inkjet black ink. Another embodiment of the present invention relates to a printed matter obtained using the actinic ray-curable inkjet black ink. Specific embodiments of the present invention will be listed below, but the present invention is not limited to the following and includes various embodiments. [1] An actinic ray-curable inkjet black ink containing carbon black, a basic pigment dispersion resin, and a polymerizable compound, wherein the DBP oil absorption of the carbon black is AC (mL / 100 g), the specific surface area of ​​the carbon black is SC (m ２

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[0099] [ ２ =CR １ -CO-O-R ２ -O-CO-CR １ =CH ２ (In general formula (A), R １ represents a hydrogen atom or a methyl group, R ２represents an alkylene group having 3 to 6 carbon atoms, which may have a branched structure.) [3] The active energy ray-curable inkjet black ink according to [1] or [2], wherein the polymerizable compound contains a radical polymerizable trifunctional monomer represented by the following general formula (B), and the content of the radical polymerizable trifunctional monomer represented by the general formula (B) is 2 to 15 mass % based on the total mass of the active energy ray-curable inkjet black ink. General formula (B): (In general formula (B), R ３ represents a hydrogen atom or a methyl group, R ４is an ethylene group or a propylene group. Furthermore, l, m, and n each represent an integer from 0 to 9, and l + m + n is an integer from 0 to 9.) [4] The active energy ray-curable inkjet black ink according to any one of [1] to [3], further comprising a photopolymerization initiator, wherein the photopolymerization initiator comprises an acylphosphine oxide-based compound. [5] The active energy ray-curable inkjet black ink according to [4], further comprising a thioxanthone-based compound. [6] The actinic ray-curable inkjet black ink according to [4] or [5], wherein the acylphosphine oxide compound comprises at least one compound selected from the group consisting of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and a polymer of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, and the total content of the ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and the polymer of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide is 45 to 80 mass% based on the total mass of the photopolymerization initiator. [7] The actinic ray-curable inkjet black ink according to any one of [1] to [6], wherein the pH of the carbon black is 2 to 5. [8] A printed matter obtained by printing the actinic ray-curable inkjet black ink according to any one of [1] to [7] on a printing substrate. The disclosure of this application is related to the subject matter described in Japanese Patent Application No. 2024-022367, filed February 16, 2024, the entire disclosure of which is incorporated herein by reference.

[0017] The active energy ray-curable inkjet black ink according to one embodiment of the present invention exhibits the effects of being excellent in all of the dispersion stability of the carbon black, the ejection stability, the curing properties, and the definition of the printed matter.

[0018] An active energy ray-curable inkjet black ink (hereinafter simply referred to as "inkjet ink of this embodiment") according to one embodiment of the present invention will be described in detail below. The present invention is not limited to the following embodiment, and includes modifications that are implemented within the scope of the present invention. Furthermore, in this specification, the terms "(meth)acrylate," "(meth)acryloyl," and "(meth)acrylic acid" mean "acrylate and / or methacrylate," "acryloyl and / or methacryloyl," and "acrylic acid and / or methacrylic acid," respectively.

[0019] First, the mechanism by which the inkjet ink of the present embodiment achieves the above-mentioned effects will be explained. However, the mechanism described below is based on the inference of the inventors and does not limit the present invention in any way.

[0020] First, the inkjet ink of this embodiment contains 5-methyl-3-vinyloxazolidin-2-one as a polymerizable compound. As described above, cyclic N-vinyl compounds are generally known to have a higher reaction rate with radicals derived from photopolymerization initiators than acrylate compounds. Furthermore, the reaction rate of 5-methyl-3-vinyloxazolidin-2-one is particularly high compared to other compounds. The reason for this is thought to be that, compared to N-vinylcaprolactam and N-vinylpyrrolidone, which are commonly known as cyclic N-vinyl compounds, 5-methyl-3-vinyloxazolidin-2-one differs in that it contains an oxygen atom in its ring structure. That is, electrons in the ring structure are attracted to this oxygen atom, which is thought to facilitate the reaction of the vinyl group with radicals derived from the photopolymerization initiator. Furthermore, as a result, bleeding of fine characters and thin lines can be suppressed, making it easier to obtain high-definition printed materials.

[0021] Furthermore, by setting the content of the monofunctional polymerizable compound (excluding 5-methyl-3-vinyloxazolidin-2-one) to 15 mass % or less based on the total mass of the inkjet ink of this embodiment, the reaction between 5-methyl-3-vinyloxazolidin-2-one and radicals is not inhibited, and it is possible to further improve the curability and the definition of the printed matter.

[0022] On the other hand, cyclic N-vinyl compounds such as 5-methyl-3-vinyloxazolidin-2-one have the problem of easily deteriorating the dispersion stability of pigments and the ejection stability of inkjet inks. Possible reasons for this include the fact that cyclic N-vinyl compounds inhibit stable adsorption of pigment dispersants to the pigment surface, and that vinyl groups in the cyclic N-vinyl compounds react with acid groups present in pigment dispersants, etc. Furthermore, improving the curability of active energy ray-curable inkjet black inks containing carbon black is particularly difficult. Reasons for this include the fact that active energy rays are absorbed by the carbon black, making it difficult for them to penetrate the ink, and that quinone groups present on the surface of the carbon black trap radicals. Therefore, simply blending 5-methyl-3-vinyloxazolidin-2-one into an active energy ray-curable inkjet black ink containing carbon black may result in insufficient curability. Furthermore, there is a risk that the dispersion stability of the carbon black and the ejection stability of the inkjet black ink may be impaired. Furthermore, deterioration in dispersion stability may also adversely affect the definition of printed matter.

[0023] In contrast, in the inkjet ink of this embodiment, the DBP oil absorption is AC (mL / 100 g) and the specific surface area is SC (m ２The carbon black used is one that has a value of 2,000 to 12,000 expressed as AC x SC when the total surface area is 100% (AC x SC / g). Carbon black that satisfies the above requirements has large primary particles and relatively small aggregates, which are agglomerations of the primary particles. When such carbon black is used, it is thought that the amount of active energy rays absorbed is reduced, and the small external surface area can reduce the amount of radicals trapped on the surface, although the details are unknown.

[0024] Furthermore, in the inkjet ink of this embodiment, a basic pigment dispersion resin having an acid value of 30 mgKOH / g or less is used to disperse the carbon black. As will be described in detail below, the basic pigment dispersion resin has basic groups that serve as adsorption sites for the pigment (carbon black). It is believed that these basic groups strongly interact with functional groups, such as carboxyl groups, hydroxyl groups, and quinone groups, present on the surface of the carbon black, thereby ensuring the dispersion stability of the carbon black and the ejection stability of the inkjet ink, even in the presence of 5-methyl-3-vinyloxazolidin-2-one. Furthermore, the acid value of the basic pigment dispersion resin is set to 30 mgKOH / g or less (or 0 mgKOH / g). By using such a specific basic pigment dispersion resin, the amount of acid groups that can react with the vinyl groups in 5-methyl-3-vinyloxazolidin-2-one can be limited, thereby further improving the ejection stability of the inkjet ink.

[0025] As described above, when an active energy ray-curable inkjet black ink having the above-described configuration is prepared, it is possible to improve the dispersion stability of the carbon black, the ejection stability, the curability, and the definition of the printed matter.

[0026] Next, each of the components constituting the inkjet ink of this embodiment will be described in detail below.

[0027] The inkjet ink of this embodiment is black in color. The "black" inkjet ink includes not only inkjet ink that can be used to produce black printed matter, but also inkjet ink that can be used to produce light black (gray) printed matter. However, it goes without saying that in both the former and latter cases, the inkjet ink must have the above-described configuration.

[0028] <Carbon Black> The inkjet ink of this embodiment contains carbon black. The DBP oil absorption of the carbon black is AC (mL / 100 g), and the specific surface area is SC (m ２ / g), the value expressed by AC×SC is 2,000 to 12,000.

[0029] (DBP Oil Absorption) DBP oil absorption is a value that represents the amount of DBP (dibutyl phthalate) absorbed into the particle voids present within aggregates. Generally, the more aggregates developed in a carbon black, the higher the DBP oil absorption value. The DBP oil absorption of carbon black that can be used in the inkjet ink of this embodiment is preferably 40 to 100 mL / 100 g, more preferably 40 to 80 mL / 100 g, and particularly preferably 40 to 65 mL / 100 g. Inkjet inks using carbon black with such DBP oil absorption values ​​exhibit excellent curing properties. Furthermore, the strong adsorption of the basic pigment dispersion resin improves the dispersion stability of the carbon black. The DBP oil absorption can be measured using a Brabender "Absorbtometer Type C" or the like in accordance with JIS K 6217-4.

[0030] (Specific Surface Area) Generally, carbon black with a large specific surface area has small primary particles and / or many pores. The specific surface area of ​​carbon black that can be used in the inkjet ink of this embodiment is 45 to 120 m ２ / g, and 50 to 95m ２ / g is particularly preferred. Carbon black having such a specific surface area is less likely to absorb active energy rays that have penetrated into the inkjet ink, and is less likely to trap radicals on the surface of the carbon black, resulting in improved curability. Furthermore, the amount of carbon black with an excessively large primary particle size is reduced, resulting in good discharge stability. In this specification, the value measured by the nitrogen BET method (nitrogen BET specific surface area) is used as the "specific surface area." The method for measuring the specific surface area of ​​carbon black by the nitrogen BET method is specified in JIS K 6217-2, and can be applied mutatis mutandis in this specification. As a specific example of the measurement method, nitrogen is adsorbed onto degassed carbon black at liquid nitrogen temperature, and the specific surface area (m) is calculated from the amount of nitrogen adsorption when equilibrium is reached. ２ / g).

[0031] (Multiplication of DBP Oil Absorption and Specific Surface Area) As described above, for the carbon black contained in the inkjet ink of this embodiment, the DBP oil absorption is AC (mL / 100 g) and the specific surface area is SC (m ２ / g), the value expressed by AC×SC is 2,000 to 12,000. In some embodiments, the value is preferably 2,000 to 7,500, and particularly preferably 2,000 to 6,000. According to the above embodiment, not only is curability improved, but the definition of printed matter can also be easily improved. Furthermore, by firmly adsorbing to the basic pigment dispersion resin, the dispersion stability of the carbon black and the ejection stability of the inkjet ink can also be easily improved.

[0032] (pH) In addition to the above specifications, the carbon black contained in the inkjet ink of this embodiment preferably has a pH of 2 to 5, and particularly preferably 2 to 4. Carbon black with such a pH value strongly adsorbs the basic pigment dispersion resin, significantly improving the dispersion stability of the carbon black and the ejection stability of the inkjet ink. The pH of carbon black can be measured by conventional methods. For example, 5 g of carbon black and 50 mL of ion-exchanged water are mixed in a glass container such as a beaker, and the opening of the beaker is covered with a watch glass or the like, and the mixture is heated for 15 minutes. A small amount (approximately 0.1 mL) of ethanol or acetone may be added to facilitate wetting of the carbon black. After boiling, the mixture is cooled to 25°C, and the supernatant liquid is removed to obtain a sludge. A pH electrode is inserted into the sludge to measure the pH. For measuring pH, for example, a tabletop pH meter "F-71" (manufactured by Horiba Ltd.) equipped with a pH electrode "9681S-10D" (manufactured by Horiba Ltd.) can be used.

[0033] (Primary Particle Diameter) For the same reasons as in the case of the specific surface area described above, i.e., because curability and ejection stability are improved, the primary particle diameter of carbon black is preferably 20 to 50 nm, more preferably 25 to 50 nm, and particularly preferably 30 to 50 nm. The primary particle diameter of carbon black can be measured by a conventional method. For example, the primary particle diameter can be obtained by observing carbon black with a transmission electron microscope (TEM), measuring the size (diameter) of 100 primary particles of carbon black, and then calculating the average value.

[0034] The content of carbon black contained in the inkjet ink of this embodiment that satisfies the above-mentioned requirements is preferably 0.5 to 3.5 mass %, and particularly preferably 1.0 to 3.0 mass %, based on the total mass of the inkjet ink. When the content of carbon black that satisfies the above-mentioned requirements is within the above range, the amount of carbon black present in the inkjet ink is optimal, resulting in an inkjet ink that is excellent in curability, ejection stability, and fineness of printed matter.

[0035] Furthermore, the particle size (secondary particle size) of the carbon black contained in the inkjet ink of this embodiment that satisfies the above-mentioned requirements is preferably 80 to 250 nm, and particularly preferably 100 to 200 nm. If the secondary particle size of the carbon black that satisfies the above-mentioned requirements is within the above range, it can be said that the carbon black is in a favorable dispersion state. This makes it possible to easily obtain an inkjet ink that is excellent in all of curability, ejection stability, and carbon black dispersion stability.

[0036] For the same reason, that is, because the carbon black is in a favorable dispersed state, the curing property and ejection stability of the inkjet ink, as well as the dispersion stability of the carbon black, are all improved, the secondary particle diameter (unit: nm) is calculated as the specific surface area (unit: m) of the carbon black that satisfies the above-mentioned requirements. ２ / g) is preferably 1.0 to 4.0, and particularly preferably 1.5 to 3.5.

[0037] The secondary particle size refers to a median size measured on a volume basis. The secondary particle size can be measured using a dynamic light scattering particle size distribution analyzer (e.g., Nanotrac UPA-EX150 manufactured by Microtrac-Bell) and an inkjet ink diluted with ethyl acetate to a concentration that allows measurement of the secondary particle size using the particle size distribution analyzer.

[0038] The inkjet ink of this embodiment may contain carbon black other than the carbon black that satisfies the above-mentioned requirements (also referred to herein as "other carbon black"). However, when the inkjet ink of this embodiment contains other carbon black, it is preferable that the amount of other carbon black is an amount that does not inhibit the effects of the carbon black that satisfies the above-mentioned requirements. Specifically, the content of other carbon black contained in the inkjet ink of this embodiment is preferably in the range of 0 to 35% by mass, based on the total mass of the carbon black contained in the inkjet ink. In other words, the content is preferably 35% by mass or less, more preferably 20% by mass or less, and particularly preferably 10% by mass, and may even be 0% by mass.

[0039] In this specification, the expression "amount (content) of 0 mass %" means that the target component is not contained.

[0040] <Other Pigments> The inkjet ink of this embodiment may also contain pigments other than carbon black (also referred to herein as "other pigments") in order to improve curing properties and increase the definition and blackness of printed matter. When the inkjet ink of this embodiment contains other pigments, for the same reasons as in the case of the other carbon black described above, namely, because the effects of carbon black that satisfies the above-mentioned requirements are effectively exerted and the curing properties, ejection stability, and dispersion stability of the carbon black are improved, the content of the other pigments contained in the inkjet ink of this embodiment is preferably in the range of 0 to 50% by mass relative to the content of carbon black that satisfies the above-mentioned requirements. That is, the content is preferably 50% by mass or less, more preferably 35% by mass or less, and particularly preferably 20% by mass or less, and may even be 0% by mass.

[0041] The other pigments are not particularly limited, but may be, for example, organic or inorganic pigments represented by the following color index names. For example, red pigments include C.I. Pigment Red 5, 7, 12, 17, 48(Ca), 48(Mn), 49:2, 57(Ca), 57:1, 112, 122, 123, 147, 149, 150, 166, 168, 176, 177, 178, 184, 188, 202, 209, 242, 254, 255, 264, 266, 269, 282, etc.; violet pigments include C.I. Pigment Violet 19, 23, etc.; orange pigments include C.I. Pigment Orange 5, 13, 34, 38, 43, 61, 62, 64, etc.; and blue pigments include C.I. Examples of suitable pigments include C.I. Pigment Blue 1, 2, 3, 15:3, 15:4, 15:6, 16, 22, 60, C.I. Vat Blue 4, 60, etc.; green pigments include C.I. Pigment Green 7, 26, 36, 50, 58, etc.; and yellow pigments include C.I. Pigment Yellow 1, 2, 3, 12, 14, 16, 17, 73, 74, 75, 83, 93, 95, 97, 98, 109, 110, 114, 128, 129, 138, 139, 147, 150, 151, 154, 155, 180, 185, 213, etc. These pigments can be used as desired depending on the curing properties and the definition and blackness of the printed matter. Two or more of the pigments listed above may be used in combination.

[0042] <Basic pigment dispersion resin> The inkjet ink of this embodiment contains a basic pigment dispersion resin. The basic pigment dispersion resin has an acid value of 30 mgKOH / g or less. The acid value may be 0 mgKOH / g, i.e., the basic pigment dispersion resin may have substantially no acid groups.

[0043] In this specification, the term "basic pigment dispersing resin" refers to a dispersing resin in which basic groups serve as adsorption points on the pigment (carbon black) surface. Examples of the basic groups include primary amino groups, secondary amino groups, tertiary amino groups, quaternary ammonium groups, and imino groups. The organic groups bonded to the nitrogen atoms in the tertiary amino groups and the quaternary ammonium groups may also be bonded to each other to form a ring structure (a heterocyclic ring containing the nitrogen atom). Examples of such rings include pyridine, pyrrolidine, pyrrolidone, imidazoline, and caprolactam.

[0044] In addition, even if a pigment dispersion resin has an acid group in addition to a basic group, the basic group functions as the adsorption site, so long as the basic group functions as the adsorption site, it is considered to be included in the "basic pigment dispersion resin" in this specification. However, as described above, it goes without saying that in this embodiment, it is necessary to use at least a basic pigment dispersion resin having an acid value of 30 mgKOH / g or less (it may be 0 mgKOH / g).

[0045] Examples of basic pigment dispersing resins include the above-mentioned acrylic resins having a basic group, the above-mentioned maleic anhydride resins having a basic group, polyethyleneimine, polyallylamine, polydiallylamine, polyvinylimidazoline, and polyvinylpyrrolidone, as well as graft resins having these resins as the main chain. In addition, examples of commercially available basic pigment dispersing resins include the following: "ADISPER-PB-821," "ADISPER-PB-822," "ADISPER-PB-824," and "ADISPER-PB-881" manufactured by Ajinomoto Fine-Techno Co., Inc. "DISPERBYK-162," "DISPERBYK-163," "DISPERBYK-168," "DISPERBYK-182," "DISPERBYK-184," "DISPERBYK-185," "DISPERBYK-2013," "DISPERBYK-2155," "BYKJET-9150," "BYKJET-9151," and "BYKJET-9152" manufactured by BYK-Chemie Co., Ltd. Lubrizol's Solsperse 24000, Solsperse 32000, Solsperse 33000, Solsperse 35000, Solsperse 39000, Solsperse 86000, Solsperse J200, and Solsperse X300; BASF's EFKA PX4701, EFKA PX4703, and EFKA PX4733.

[0046] The term "acrylic resin" refers to a resin using one or more monomers selected from the group consisting of acrylic acid, methacrylic acid, acrylic acid esters, and methacrylic acid esters as the monomers constituting the resin. A styrene-based monomer may also be used as a monomer constituting the resin. However, resins using maleic acid (anhydride) (maleic anhydride and / or maleic acid) as the monomer are not included in the term "acrylic resin." The term "maleic acid (anhydride) resin" refers to a resin using at least maleic acid (anhydride) as the monomer constituting the resin. The maleic acid (anhydride) resin may also use one or more monomers selected from the group consisting of α-olefins, acrylic acid, methacrylic acid, acrylic acid esters, methacrylic acid esters, styrene, and styrene derivatives as the monomers.

[0047]

[0043] As described above, from the viewpoint of improving the ejection stability and curability of the inkjet ink of this embodiment, the acid value of the basic pigment dispersion resin is 30 mgKOH / g or less (may be 0 mgKOH / g). Furthermore, from the viewpoint of enabling the basic pigment dispersion resin to be stably present in the inkjet ink, further improving the ejection stability, and further improving the curability of the inkjet ink, it is particularly preferable that the acid value of the basic pigment dispersion resin is 20 mgKOH / g or less (may be 0 mgKOH / g).

[0048] The "acid value" of a basic pigment dispersion resin is the number of milligrams of potassium hydroxide required to neutralize 1 g of resin, and can be determined by potentiometric titration in accordance with JIS K 0070. As a specific example of a measurement method, the target resin is dissolved in a solvent obtained by mixing diethyl ether and ethanol in a 1:1 mass ratio, and then titrated by potentiometric titration using a 0.1 mol / L potassium hydroxide-ethanol solution. The acid value can then be calculated using the titration amount read from the obtained titration curve.

[0049] The amine value of the basic pigment dispersion resin is preferably 10 to 50 mgKOH / g, and particularly preferably 15 to 40 mgKOH / g. A basic pigment dispersion resin having an amine value within the above range has a sufficient number of adsorption sites, allowing it to firmly adsorb to carbon black that satisfies the above-mentioned requirements. As a result, the dispersion stability of the carbon black and the ejection stability of the inkjet ink are improved. Furthermore, the adsorption sites (basic groups) do not inhibit the polymerization reaction of the polymerizable compound, thereby improving the curability of the inkjet ink.

[0050] The "amine value" of a basic pigment dispersion resin is the number of milligrams of potassium hydroxide, which is the equivalent amount of acid required to neutralize 1 g of resin. As an example of a method for actually measuring the amine value, the target resin is dissolved in a solvent mixture of ethanol or tetrahydrofuran and acetic acid, and then titrated by potentiometric titration using a 0.1 mol / L perchloric acid-acetic acid solution. The titration amount is then read from the obtained titration curve, and converted into the number of milligrams of potassium hydroxide, allowing the amine value to be calculated.

[0051] The mass average molecular weight of the basic pigment dispersion resin is preferably 5,000 to 60,000, more preferably 8,000 to 55,000, and particularly preferably 10,000 to 50,000. When the mass average molecular weight of the basic pigment dispersion resin is within the above range, the compatibility of the basic pigment dispersion resin with the polymerizable compound is improved, and the ejection stability is improved. Furthermore, since the basic pigment dispersion resin can be firmly adsorbed to carbon black that satisfies the above-mentioned requirements, the dispersion stability of the carbon black is improved.

[0052] The mass average molecular weight can be measured by gel permeation chromatography (GPC). Specifically, it is a value obtained as a polystyrene-equivalent molecular weight measured using a TSKgel column (manufactured by Tosoh Corporation) and a GPC (for example, "HLC-8320GPC" manufactured by Tosoh Corporation) equipped with an RI detector, using DMF as a developing solvent.

[0053] The blending amount of the basic pigment dispersion resin is preferably 10 to 100% by mass, more preferably 15 to 70% by mass, and particularly preferably 20 to 50% by mass, relative to the total amount of pigment components contained in the inkjet ink of this embodiment. The total amount of pigments refers to the total amount of carbon black that satisfies the above-mentioned requirements and any other carbon black and / or other pigments that are optionally used. Using the basic pigment dispersion resin in the blending amount range improves the dispersion stability of the pigment components, including the carbon black that satisfies the above-mentioned requirements, and also improves the ejection stability of the inkjet ink of this embodiment.

[0054] <Polymerizable Compound> In this embodiment, the polymerizable compound refers to a compound that undergoes a polymerization reaction or a crosslinking reaction due to radicals generated from a photopolymerization initiator or the like, which will be described later, and has the function of curing a composition containing the polymerizable compound.

[0055] As described above, the inkjet ink of this embodiment contains 5-methyl-3-vinyloxazolidin-2-one as a polymerizable compound. The content of monofunctional polymerizable compounds excluding 5-methyl-3-vinyloxazolidin-2-one is 15% by mass or less based on the total mass of the inkjet ink.

[0056] (5-methyl-3-vinyloxazolidin-2-one) As described above, 5-methyl-3-vinyloxazolidin-2-one has particularly excellent curing properties compared to N-vinyl compounds other than 5-methyl-3-vinyloxazolidin-2-one. Furthermore, 5-methyl-3-vinyloxazolidin-2-one is also excellent in terms of safety and odor. Therefore, the inkjet ink of this embodiment containing 5-methyl-3-vinyloxazolidin-2-one has excellent properties as described above.

[0057] The content of 5-methyl-3-vinyloxazolidin-2-one is preferably 5 to 40% by mass, more preferably 10 to 35% by mass, and particularly preferably 15 to 30% by mass, based on the total mass of the inkjet ink of this embodiment. By ensuring that the content of 5-methyl-3-vinyloxazolidin-2-one falls within the above range, an inkjet ink with excellent curability can be obtained. As a result, the resolution of printed matter is also improved. Furthermore, it is possible to suppress the inhibition of stable adsorption of the basic pigment dispersion resin to the surface of carbon black that satisfies the above-mentioned requirements, and to suppress reaction with acid groups present in the basic pigment dispersion resin. These factors also improve the dispersion stability of the carbon black and the ejection stability of the inkjet ink.

[0058] (Other Polymerizable Compounds) The inkjet ink of this embodiment may contain a radically polymerizable compound (also referred to herein as "other polymerizable compounds") other than the above-mentioned 5-methyl-3-vinyloxazolidin-2-one. There are no limitations on the compounds that can be used as the other polymerizable compounds, and any monomer, oligomer, polymer, etc. having one or more polymerizable groups can be used. Note that the above-mentioned "oligomer" and "polymer" both refer to polymers in which multiple monomers are bonded together, and are classified according to their degree of polymerization. In this specification, those with a degree of polymerization of 2 to 10 are called "oligomers," and those with a degree of polymerization of 11 or more are called "polymers."

[0059] Examples of the polymerizable group possessed by the radically polymerizable polymerizable compound (radical polymerizable compound) include a (meth)acryloyl group, a vinyl ether group, an allyl group, a vinyl group (excluding a vinyl ether group and an allyl group), etc. Among the above polymerizable groups, it is preferable to use a radical polymerizable compound having one or more polymerizable groups selected from the group consisting of an acryloyl group, a vinyl ether group, and a vinyl group (excluding a vinyl ether group and an allyl group) in terms of excellent curability.

[0060] Furthermore, as the other polymerizable compound, a monofunctional polymerizable compound (monofunctional polymerizable compound) or a bifunctional or higher (polyfunctional) polymerizable compound (polyfunctional polymerizable compound) may be used. Furthermore, only one type of other polymerizable compound may be used, or a mixture of multiple polymerizable compounds may be used. Note that the term "monofunctional" refers to a compound having only one polymerizable group in one molecule, while "bifunctional" and "trifunctional" refer to a compound having two polymerizable groups in one molecule and a compound having three polymerizable groups in one molecule, respectively. Bifunctional or higher functional compounds are collectively referred to as "polyfunctional."

[0061] As described above, the inkjet ink of this embodiment may optionally contain other polymerizable compounds. However, in the inkjet ink of this embodiment, the content of monofunctional polymerizable compounds (excluding 5-methyl-3-vinyloxazolidin-2-one) is limited to 15% by mass or less, based on the total mass of the inkjet ink of this embodiment. This is because limiting the content of monofunctional polymerizable compounds excluding 5-methyl-3-vinyloxazolidin-2-one further improves the curability and the resolution of printed matter. From this perspective, the lower the content of monofunctional polymerizable compounds (excluding 5-methyl-3-vinyloxazolidin-2-one), the more preferable. Specifically, the content of monofunctional polymerizable compounds (excluding 5-methyl-3-vinyloxazolidin-2-one) is preferably 10% by mass or less, more preferably 6% by mass or less, and particularly preferably 3% by mass or less, based on the total mass of the inkjet ink of this embodiment.

[0062] Other examples of monofunctional radically polymerizable compounds (radical polymerizable monofunctional monomers) that can be used as polymerizable compounds include compounds having one (meth)acryloyl group. Specific examples of the compound include benzyl (meth)acrylate, 2-phenoxyethyl (meth)acrylate, ethylene oxide-modified 2-phenoxyethyl (meth)acrylate, propylene oxide-modified 2-phenoxyethyl (meth)acrylate, dicyclopentenyl (oxyethyl) (meth)acrylate, 2-methoxyethyl (meth)acrylate, methoxytriethylene glycol (meth)acrylate, 2-ethoxyethyl (meth)acrylate, 2-(2-ethoxyethoxy)ethyl (meth)acrylate, methoxydipropylene glycol (meth)acrylate, dipropylene glycol (meth)acrylate, ethylene oxide-modified nonylphenol acrylate, propylene oxide-modified nonylphenol acrylate, ethylene oxide-modified o-phenylphenol acrylate, and ethylene oxide-modified 2-ethyl Hexyl acrylate, β-carboxylethyl (meth)acrylate, trimethylolpropane formal (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, cyclohexyl (meth)acrylate, 3,3,5-trimethylcyclohexyl acrylate, 4-tert-butylcyclohexyl acrylate, isobornyl (meth)acrylate, norbornyl (meth)acrylate, dicyclopentanyl (meth)acrylate, isoamyl (meth)acrylate, isononyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, lauryl (meth)acrylate, isodecyl (meth)acrylate, tridecyl (meth)acrylate, stearyl (meth)acrylate, caprolactone (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 1,Examples of such acrylates include 4-cyclohexanedimethanol (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, (meth)acryloylmorpholine, 2-(diethylamino)ethyl (meth)acrylate, 2-(diisopropylamino)ethyl (meth)acrylate, tert-butylaminoethyl (meth)acrylate, morpholinoethyl (meth)acrylate, 2-(diethylamino)ethyl (meth)acrylate, and N-(meth)acryloyloxyethyl hexahydrophthalimide.

[0063] Further, other examples of radically polymerizable monofunctional monomers that can be used as other polymerizable compounds include compounds in which only one polymerizable group is left in a polyfunctional radically polymerizable compound (specific examples will be described later) and a primary or secondary organic amine is added (Michael addition) to the remaining polymerizable group.

[0064] Further, other examples of radically polymerizable monofunctional monomers that can be used as other polymerizable compounds include compounds having one vinyl group (excluding 5-methyl-3-vinyloxazolidin-2-one), such as N-vinylcaprolactam and N-vinylpyrrolidone.

[0065] Other examples of the difunctional radically polymerizable compound (radical polymerizable difunctional monomer) that can be used as the polymerizable compound include compounds having two (meth)acryloyl groups. Specific examples of the compound include 1,3-propanediol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,3-butylenediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, ethylene oxide-modified 1,6-hexanediol di(meth)acrylate, propylene oxide-modified 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, ethylene oxide-modified neopentyl glycol di(meth)acrylate, propylene oxide-modified neopentyl glycol di(meth)acrylate, 3-methyl-1,5-pentanediol di(meth)acrylate, 2,4-dimethyl-1,5-Pentanediol di(meth)acrylate, 2-ethyl-2-butylpropanediol di(meth)acrylate, 2-ethyl-2-butylbutanediol di(meth)acrylate, ethylene oxide modified cyclohexanemethanol di(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, polyethylene glycol 200 di(meth)acrylate, polyethylene glycol 300 di(meth)acrylate, polyethylene glycol 400 di(meth)acrylate, neopentyl glycol hydroxypivalate di(meth)acrylate, bisphenol A di(meth)acrylate, ethylene oxide modified bisphenol A di(meth)acrylate, propylene oxide modified bis Examples of the di(meth)acrylate include phenol A di(meth)acrylate, bisphenol F di(meth)acrylate, ethylene oxide-modified bisphenol F di(meth)acrylate, propylene oxide-modified bisphenol F di(meth)acrylate, dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, ethylene oxide-modified isocyanuric acid di(meth)acrylate, tricyclodecane di(meth)acrylate, dimethyloltricyclodecane di(meth)acrylate, cyclohexanedimethanol di(meth)acrylate, trimethylolpropane di(meth)acrylate, neopentyl glycol-modified trimethylolpropane di(meth)acrylate, and dicyclopentanyl di(meth)acrylate.

[0066] Further, other examples of radically polymerizable monofunctional monomers that can be used as other polymerizable compounds include compounds in which two polymerizable groups are left in a polyfunctional radically polymerizable compound (specific examples will be described later) having three or more polymerizable groups, and a primary or secondary organic amine is added to the remaining polymerizable group (Michael addition).

[0067] Further, other examples of radically polymerizable bifunctional monomers that can be used as other polymerizable compounds include compounds having one (meth)acryloyl group and one vinyl ether group, such as 2-(2-vinyloxyethoxy)ethyl (meth)acrylate and 2-[2-(2-vinyloxyethoxy)ethoxy]ethyl (meth)acrylate.

[0068] Other examples of trifunctional radically polymerizable compounds (radical polymerizable trifunctional monomers) that can be used as polymerizable compounds include compounds having three (meth)acryloyl groups. Specific examples of the compounds include trimethylolpropane tri(meth)acrylate, trimethylolethane tri(meth)acrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, propylene oxide-modified trimethylolpropane tri(meth)acrylate, glycerin tri(meth)acrylate, ethylene oxide-modified glycerin tri(meth)acrylate, propylene oxide-modified glycerin tri(meth)acrylate, and ethylene oxide-modified isocyanuric acid tri(meth)acrylate. acrylate, propylene oxide-modified isocyanuric acid tri(meth)acrylate, propylene oxide-modified dipentaerythritol tri(meth)acrylate tetramethylolmethane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, tri((meth)acryloyloxyethyl)isocyanurate, hydroxypivalaldehyde-modified dimethylolpropane tri(meth)acrylate, and sorbitol tri(meth)acrylate.

[0069] Other examples of tetrafunctional radically polymerizable compounds (radical polymerizable tetrafunctional monomers) that can be used as polymerizable compounds include compounds having four (meth)acryloyl groups. Specific examples of such compounds include pentaerythritol tetra(meth)acrylate, sorbitol tetra(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, ethylene oxide-modified pentaerythritol tetra(meth)acrylate, propylene oxide-modified pentaerythritol tetra(meth)acrylate, and tetramethylolmethane tetra(meth)acrylate.

[0070] Other examples of the pentafunctional radically polymerizable compound (radical polymerizable pentafunctional monomer) that can be used as the polymerizable compound include compounds having five (meth)acryloyl groups. Specific examples of such compounds include sorbitol penta(meth)acrylate and dipentaerythritol penta(meth)acrylate.

[0071] Other examples of hexafunctional radically polymerizable compounds (radical polymerizable hexafunctional monomers) that can be used as polymerizable compounds include compounds having six (meth)acryloyl groups. Specific examples of such compounds include dipentaerythritol hexa(meth)acrylate, sorbitol hexa(meth)acrylate, alkylene oxide-modified hexa(meth)acrylate of phosphazene, and ε-caprolactone-modified dipentaerythritol hexa(meth)acrylate.

[0072] Furthermore, as the other polymerizable compound, a radically polymerizable compound (radical polymerizable oligomer) that is an oligomer can also be used. In this case, a compound having a (meth)acryloyl group as the polymerizable group is preferably used. The number of polymerizable groups contained in the radically polymerizable oligomer is preferably 1 to 6 per molecule from the viewpoint of the balance between curability, discharge stability, and dispersion stability. The number of polymerizable groups is more preferably 1 to 4, and particularly preferably 1 to 2. The mass average molecular weight of the radically polymerizable oligomer is also preferably 400 to 6,000, and more preferably 500 to 4,500.

[0073] Examples of the radically polymerizable oligomer having a (meth)acryloyl group include urethane (meth)acrylate oligomers such as aliphatic urethane (meth)acrylate oligomers and aromatic urethane (meth)acrylate oligomers; acrylic (meth)acrylate oligomers; polyester (meth)acrylate oligomers; polyether (meth)acrylate oligomers; and epoxy (meth)acrylate oligomers. The oligomers may be modified. Examples of the modified oligomers include sulfonic acid-modified, phosphoric acid-modified, amino-modified, and mercapto-modified.

[0074] When the inkjet ink of this embodiment contains other polymerizable compounds, it is preferable that the inkjet ink contains a radically polymerizable bifunctional monomer and / or a radically polymerizable trifunctional monomer, from the viewpoint of improving all of curability, ejection stability, and resolution of printed matter. In particular, in the inkjet ink of this embodiment, it is preferable to use, as the other polymerizable compounds, a radically polymerizable bifunctional monomer represented by the following general formula (A) and / or a radically polymerizable trifunctional monomer represented by the following general formula (B):

[0075] General formula (A): CH ２ =CR １ -CO-O-R ２ -O-CO-CR １ =CH ２

[0076] In the above general formula (A), R １ represents a hydrogen atom or a methyl group, R ２ represents an alkylene group having 3 to 6 carbon atoms, which may have a branched structure.

[0077] General formula (B):

[0078] In the above general formula (B), R ３ represents a hydrogen atom or a methyl group, R ４ is an ethylene group or a propylene group. Furthermore, l, m, and n each represent an integer of 0 to 9, and l+m+n is an integer of 0 to 9.

[0079] The radically polymerizable bifunctional monomer represented by the general formula (A) has low viscosity and relatively low surface tension. Therefore, inkjet inks containing the radically polymerizable bifunctional monomer represented by the general formula (A) exhibit excellent wetting and spreading properties on printing substrates, resulting in improved curability. Furthermore, the radically polymerizable bifunctional monomer represented by the general formula (A) is also highly compatible with 5-methyl-3-vinyloxazolidin-2-one, allowing the polymerization reaction to proceed rapidly, further improving curability. Furthermore, deterioration of the dispersion stability of carbon black due to 5-methyl-3-vinyloxazolidin-2-one is suppressed, thereby improving the dispersion stability of the carbon black.

[0080] Specific examples of the radically polymerizable bifunctional monomer represented by the general formula (A) include 1,3-propanediol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,3-butylene diol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, and 3-methyl-1,5-pentanediol di(meth)acrylate.

[0081] When the inkjet ink of this embodiment contains the radical polymerizable bifunctional monomer represented by general formula (A), the blending amount thereof is preferably 15 to 55 mass %, and particularly preferably 25 to 45 mass %, based on the total mass of the inkjet ink. When the blending amount is within the above range, both the curability and the dispersion stability of the carbon black are improved, and further the definition of the printed matter is also improved.

[0082] On the other hand, the radical polymerizable trifunctional monomer represented by the general formula (B) has a low viscosity among radical polymerizable trifunctional monomers, and therefore it is easy to achieve both ejection stability and curability of the inkjet ink. Furthermore, since it does not inhibit the effects of 5-methyl-3-vinyloxazolidin-2-one, further improvement in curability can be achieved.

[0083] Specific examples of the radical polymerizable trifunctional monomer represented by the general formula (B) include glycerin tri(meth)acrylate, ethylene oxide-modified glycerin tri(meth)acrylate (number of ethylene oxide groups: 3), ethylene oxide-modified glycerin tri(meth)acrylate (number of ethylene oxide groups: 9), and propylene oxide-modified glycerin tri(meth)acrylate (number of propylene oxide groups: 3).

[0084] From the viewpoint of improving both the curability and ejection stability and also improving the definition of printed matter, when the inkjet ink of this embodiment contains the radical polymerizable trifunctional monomer represented by general formula (B), the blending amount thereof is preferably 2 to 15 mass %, and particularly preferably 5 to 12 mass %, based on the total mass of the inkjet ink.

[0085] In one embodiment, it is preferable that the inkjet ink contains both the radical polymerizable bifunctional monomer represented by the above general formula (A) and the radical polymerizable trifunctional monomer represented by the above general formula (B), which makes it easy to obtain an inkjet ink that is excellent in all of curability, ejection stability, carbon black dispersion stability, and print resolution.

[0086] When the inkjet ink of the present embodiment contains the radical polymerizable bifunctional monomer represented by the general formula (A) above and the radical polymerizable trifunctional monomer represented by the general formula (B) above, the mass content of the radical polymerizable bifunctional monomer represented by the general formula (A) above contained in the inkjet ink is preferably 2 to 20, and particularly preferably 2.5 to 15, relative to the mass content of the radical polymerizable trifunctional monomer represented by the general formula (B) above contained in the inkjet ink, which is taken as 1.

[0087] From the viewpoint of improving both the curability and the ejection stability, when the inkjet ink of this embodiment contains a radically polymerizable bifunctional monomer and / or a radically polymerizable trifunctional monomer as the other polymerizable compound, the total content of 5-methyl-3-vinyloxazolidin-2-one and the content of the radically polymerizable bifunctional monomer and / or the radically polymerizable trifunctional monomer is preferably 75 to 100 mass %, more preferably 90 to 100 mass %, and particularly preferably 95 to 100 mass %, based on the total mass of the polymerizable compounds contained in the inkjet ink.

[0088] In some embodiments, when the mass content of 5-methyl-3-vinyloxazolidin-2-one is taken as 1, the mass content of the other polymerizable compound is preferably 1 to 15, and particularly preferably 1.2 to 5. According to the above embodiment, the dispersion stability of carbon black and the ejection stability of the inkjet ink can be easily improved without inhibiting the effects of 5-methyl-3-vinyloxazolidin-2-one described above.

[0089] As described above, in the inkjet ink of this embodiment, the content of monofunctional polymerizable compounds other than 5-methyl-3-vinyloxazolidin-2-one is limited. Therefore, in some embodiments, it is preferable not to use a radically polymerizable monofunctional monomer as the other polymerizable compound. In some embodiments, it is preferable to set the amount of radically polymerizable monofunctional monomer used to 15% by mass or less, preferably 10% by mass or less, more preferably 6% by mass or less, and particularly preferably 3% by mass or less.

[0090] On the other hand, when a radically polymerizable monofunctional monomer is used as the other polymerizable compound, a (meth)acrylate having an alicyclic structure is preferably used because it improves the wetting and spreading properties of the inkjet ink on the printing substrate, resulting in improved curing properties, and it can prevent deterioration of the dispersion stability of carbon black. Specific examples of the (meth)acrylate having an alicyclic structure include cyclohexyl (meth)acrylate, 3,3,5-trimethylcyclohexyl acrylate, 4-tert-butylcyclohexyl acrylate, isobornyl (meth)acrylate, norbornyl (meth)acrylate, and dicyclopentanyl (meth)acrylate.

[0091] <Photopolymerization initiator> The inkjet ink of this embodiment preferably contains a photopolymerization initiator that serves as a radical generation source. As the photopolymerization initiator, one or more conventionally known compounds can be used as desired. Specifically, acylphosphine oxide compounds, benzophenone compounds, indan compounds, thioxanthone compounds, hydroxyacetophenone compounds, alkylaminoacetophenone compounds, oxime ester compounds, etc. can be used.

[0092] The term "polymerization initiator" as used herein also includes materials that promote radical generation in other photopolymerization initiators, commonly referred to as sensitizers. Examples of such materials include aminobenzoate compounds, ketocoumarin compounds, and anthracene compounds.

[0093] Among these photopolymerization initiators, it is preferable to use one or more photopolymerization initiators selected from the group consisting of acylphosphine oxide compounds and thioxanthone compounds, and it is particularly preferable to use at least an acylphosphine oxide compound, because this allows the curing property to be significantly improved while maintaining the dispersion stability of carbon black in a suitable state.

[0094] Furthermore, from the viewpoint that an inkjet ink having particularly excellent curability and fineness of printed matter can be obtained, and furthermore, the inkjet ink has good ejection stability and carbon black dispersion stability, it is extremely preferable to use an acylphosphine oxide-based compound and a thioxanthone-based compound in combination as the photopolymerization initiator.

[0095] In one embodiment, from the viewpoint of significantly improving curability, it is preferable to use one or more photopolymerization initiators selected from the group consisting of acylphosphine oxide compounds and thioxanthone compounds in combination with one or more photopolymerization initiators selected from the group consisting of aminobenzoate compounds and ketocoumarin compounds.

[0096] (Acylphosphine oxide compounds) Specific examples of the acylphosphine oxide compounds include diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, as well as polymers of these compounds. Commercially available examples of acylphosphine oxide compounds include "Omnirad TPO," "Omnirad TPO-L," "Omnirad TPO-H," "Omnirad 819," and "OMNIPOL TP," all manufactured by IGM RESINS; and "Speedcure TPO," "Speedcure TPO-L," and "Speedcure BPO," all manufactured by Lambson. Furthermore, for example, the acylphosphine oxide compounds and lithium phenyl(2,4,6-trimethylbenzoyl)phosphinate described in WO 2017 / 086224 and WO 2020 / 049378 can also be used. In the inkjet ink of this embodiment, the above-listed acylphosphine oxide compounds may be used alone or in combination of two or more.

[0097] Among these compounds, ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and / or a polymer of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide can be preferably used because they have high affinity with 5-methyl-3-vinyloxazolidin-2-one and improve curability and ejection stability.

[0098] From the viewpoint of improving the curability and the resolution of printed matter without deteriorating the discharge stability and the dispersion stability of the carbon black, the content (total amount) of at least one compound selected from the group consisting of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and polymers of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide is preferably 45 to 80 mass%, and particularly preferably 50 to 70 mass%, based on the total mass of the photopolymerization initiator contained in the inkjet ink of this embodiment. Note that when the inkjet ink of this embodiment contains only one compound selected from the group consisting of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and polymers of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, the expression "content (total amount)" refers to the content of that compound. When the inkjet ink of this embodiment contains two or more compounds, the expression refers to the total content of those compounds.

[0099] When the inkjet ink of this embodiment contains an acylphosphine oxide compound, the blending amount thereof is preferably 4 to 15 mass %, and particularly preferably 5 to 12 mass %, based on the total mass of the inkjet ink. When the blending amount is adjusted within the above range, an inkjet ink that is excellent in all of curability, ejection stability, and carbon black dispersion stability can be easily obtained. Furthermore, from the viewpoint of easily obtaining an inkjet ink that is excellent in curability and ejection stability, the blending amount of the acylphosphine oxide compound is preferably 0.15 to 2, and particularly preferably 0.2 to 0.8, when the blending amount of 5-methyl-3-vinyloxazolidin-2-one is taken as 1.

[0100] (Thioxanthone-Based Compounds) Specific examples of the thioxanthone-based compounds include 2-isopropylthioxanthone, 4-isopropylthioxanthone, 2,4-diethylthioxanthone, 3-methoxythioxanthone, 2-carboxymethoxythioxanthone, 3-ethoxycarbonylmethoxythioxanthone, 3-butoxycarboxymethoxythioxanthone, 1,3-dimethyl-2-(2-ethylhexyloxy)thioxanthone, 2-[2,2-bis(ethoxycarbonyl)]ethylthioxanthone, 1-chloro-4-propoxythioxanthone, and polymers of these compounds. Examples of commercially available thioxanthone compounds include "Omnirad ITX," "Omnirad DETX," and "OMNIPOL TX" manufactured by IGM RESINS; "SPEEDCURE ITX," "SPEEDCURE 2-ITX," "SPEEDCURE DETX," "SPEEDCURE LTX," "SPEEDCURE CPTX," and "SPEEDCURE 7010" manufactured by Lambson; and "Genopol TX-2" manufactured by RAHN. Of these commercially available products, "OMNIPOL TX," "SPEEDCURE 7010," and "Genopol TX-2" are the above-mentioned multimers. In the inkjet ink of this embodiment, the above-listed thioxanthone compounds may be used alone or in combination of two or more.

[0101] When the inkjet ink of this embodiment contains a thioxanthone compound, the blending amount thereof is preferably 0.5 to 8 mass %, and particularly preferably 1 to 5 mass %, based on the total mass of the inkjet ink. When the blending amount of the thioxanthone compound is adjusted to fall within the above range, an inkjet ink that is excellent in all of curability, discharge stability, and carbon black dispersion stability can be easily obtained.

[0102] As described above, it is preferable that the inkjet ink of this embodiment uses an acylphosphine oxide compound and a thioxanthone compound in combination. In embodiments in which the above compounds are used in combination, the mass of the acylphosphine oxide compound is preferably 1 to 25, and particularly preferably 2 to 12, relative to the mass of the thioxanthone compound, which is taken as 1. When the above contents are adjusted within the above ranges, an inkjet ink that exhibits excellent curability and print definition can be easily produced. Furthermore, when the sum of the mass of the thioxanthone compound and the mass of 5-methyl-3-vinyloxazolidin-2-one is taken as 1, the mass of the acylphosphine oxide compound is preferably 0.1 to 1.0, and particularly preferably 0.2 to 0.85. When the above contents are adjusted within the above ranges, ejection stability is improved in addition to curability and print definition.

[0103] (Aminobenzoate Compounds) Specific examples of the aminobenzoate compounds include methyl 2-(dimethylamino)benzoate, ethyl 4-(dimethylamino)benzoate, ethyl 4-(diethylamino)benzoate, 2-ethylhexyl 2-(dimethylamino)benzoate, 2-butoxyethyl 2-(dimethylamino)benzoate, bis-[(4-dimethylaminobenzoyl)oxyethylene-1-yl]-methylamine, and polymers of these compounds (for example, polyethylene glycol-bis(methyl 4-dimethylaminobenzoate)). Examples of commercially available aminobenzoate compounds include "Omnirad EDB," "Omnirad EHA," "Esacure A198," and "Omnipol ASA" manufactured by IGM RESINS; "SPEEDCURE EDB," "SPEEDCURE EHA," "SPEEDCURE BEDB," and "SPEEDCURE 7040" manufactured by Lambson; and "GENOPOL AB-1" and "GENOPOL AB-2" manufactured by Rahn AG. In the inkjet ink of this embodiment, the above-listed aminobenzoate compounds may be used alone or in combination of two or more.

[0104] When the inkjet ink of this embodiment contains an aminobenzoate compound, the blending amount thereof is preferably 0.2 to 8 mass %, and particularly preferably 0.5 to 5 mass %, based on the total mass of the inkjet ink.

[0105] (Ketocoumarin Compounds) Specific examples of the ketocoumarin compounds include 3-benzoyl-7-methoxycoumarin, 3-benzoyl-5,7-dimethoxycoumarin, 3-(4-tert-butylbenzoyl)-5,7-dimethoxycoumarin, 3-(4-hexylbenzoyl)-5,7-dimethoxycoumarin, 3-[4-(2-ethylhexyl)benzoyl]-5,7-dimethoxycoumarin, 5,7-dimethoxy-3-[4-(3,5,5-trimethylhexyl)benzoyl]coumarin, 7-methoxy-3-(4-methylbenzoyl)coumarin, 7-methoxy-3-(4-tert-butylbenzoyl)coumarin, 7-methoxy-3-(4-hexylbenzoyl)coumarin, and 7-methoxy-3-[4-(2-ethylhexyl)benzoyl]coumarin.

[0106] From the viewpoint of significantly improving curability, it is preferable to use the above-listed ketocoumarin compounds in combination with the above-mentioned acylphosphine oxide compounds. In particular, it is preferable to use, as the acylphosphine oxide compound, at least one selected from the group consisting of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and polymers of this ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide.

[0107] When the inkjet ink of this embodiment contains a ketocoumarin compound, the blending amount thereof is preferably 1 to 10% by mass, and particularly preferably 2 to 8% by mass, based on the total mass of the inkjet ink.

[0108] (Other Photopolymerization Initiators) Examples of commercially available photopolymerization initiators (also referred to as "other photopolymerization initiators" in this specification) other than acylphosphine oxide compounds, thioxanthone compounds, and ketocoumarin compounds are shown below.

[0109] Examples of commercially available benzophenone compounds include "Omnirad BP", "Omnirad BMS", "Omnirad 4PBZ", "OMNIRAD EMK" and "Esacure 1001M" manufactured by IGM RESINS.

[0110] An example of a commercially available indane compound is "SpeedCure XFs01" manufactured by LAMBSON.

[0111] Examples of commercially available hydroxyacetophenone compounds include "Omnirad 127", "Omnirad 184", "Omnirad 1173", "Omnirad 2959" and "Esacure KIP150" manufactured by IGM RESINS.

[0112] Examples of commercially available alkylaminoacetophenone compounds include "Omnirad 907," "Omnirad 369," and "Omnirad 379" manufactured by IGM RESINS.

[0113] Examples of commercially available oxime ester compounds include "IRGACURE OXE01," "IRGACURE OXE02," and "IRGACURE OXE04," all manufactured by BASF.

[0114] An example of a commercially available product of the anthracene-based compound is "Anthracure UVS-581" manufactured by Kawasaki Kasei Chemical Industries, Ltd.

[0115] In addition to the above-listed photopolymerization initiators, for example, "Omnirad 651" and "Omnirad MBF" manufactured by IGM RESINS can be used.

[0116] When the inkjet ink of this embodiment contains a photopolymerization initiator, the total amount of the photopolymerization initiator blended is preferably 3 to 20 mass %, more preferably 4 to 17 mass %, and particularly preferably 5 to 15 mass %, based on the total mass of the inkjet ink. By keeping the total amount of the photopolymerization initiator blended within the above range, it is possible to achieve both curability and ejection stability.

[0117] <Other Components> In addition to the components described above, the inkjet ink of this embodiment may contain a surface conditioner, a polymerization inhibitor, an organic solvent, water, an inert resin, and other additives, if necessary.

[0118] (Surface Conditioner) The inkjet ink of this embodiment preferably contains a surface conditioner for the purpose of improving the wetting and spreading properties on the printing substrate, the print image quality including the definition of the printed matter, the substrate adhesion, and the ejection stability. Examples of surface conditioners that can be used include siloxane-based surface conditioners, fluorine-based surface conditioners, acetylene glycol-based surface conditioners, and acetylene monool-based surface conditioners. Among these, it is preferable to use a siloxane-based surface conditioner, from the viewpoint of improving the wetting and spreading properties on the printing substrate, the print image quality including the definition of the printed matter, the substrate adhesion, and the ejection stability without deteriorating the dispersion stability of the carbon black.

[0119] Examples of the siloxane-based surface conditioner include compounds having a dimethylsiloxane structure and / or modified products thereof. Among these, polyether-modified siloxane-based surface conditioners are particularly preferred. The use of a polyether-modified siloxane-based surface conditioner allows the ink that has landed on the printing substrate to be sufficiently wetted and spread without impairing the effects of 5-methyl-3-vinyloxazolidin-2-one. This allows for a balance of curability, print image quality including the resolution of the printed matter, and substrate adhesion, while maintaining favorable ejection stability and carbon black dispersion stability. Specific examples of the polyether group include a polyethylene oxide group and a polypropylene oxide group. These polyether groups may contain either one or both in the molecule.

[0120] Commercially available polyether-modified siloxane-based surface conditioners that can be preferably used include, for example, BYK (registered trademark)-378, 348, 349, 3420, 3760, BYK-UV3500, and UV3510 manufactured by BYK-Chemie; and TEGO (registered trademark) Glide 450, 440, 435, 432, 410, 406, 130, 110, and 100 manufactured by EVONIK.

[0121] When a siloxane-based surface conditioner is used, its content is preferably 0.1 to 5.0% by mass based on the total mass of the inkjet ink. By adjusting the content to 0.1% by mass or more, wetting and spreading properties on the printing substrate are improved, and print quality, including the definition of the printed matter, and adhesion are also improved. On the other hand, by adjusting the content to 5.0% by mass or less, curability, carbon black dispersion stability, and ejection stability can be easily ensured.

[0122]

[0043] (Polymerization Inhibitor) In order to improve the ejection stability of the inkjet ink, and further to improve the hue stability of the printed matter and suppress hardening wrinkles, a polymerization inhibitor may be blended into the inkjet ink of this embodiment. Specific examples of the polymerization inhibitor include hindered phenol compounds, phenol compounds, hydroquinone compounds, phenothiazine compounds, phosphorus compounds, and nitrosophenylhydroxylamine compounds, and these can be suitably used.

[0123] More specific examples of polymerization inhibitors that can be used in the inkjet ink of this embodiment include 4-methoxyphenol, tert-butylhydroquinone, 2,6-di-tert-butyl-4-methylphenol, pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], hydroquinone, methylhydroquinone, phenothiazine, dicumylphenothiazine, triphenylphosphine, and aluminum salts of N-nitrosophenylhydroxylamine.

[0124] The content of the polymerization inhibitor is preferably 0.01 to 2 mass %, more preferably 0.05 to 1 mass %, and particularly preferably 0.1 to 0.8 mass %, based on the total mass of the inkjet ink. When the content of the polymerization inhibitor is adjusted to the above range, it becomes easy to improve the ejection stability of the inkjet ink while maintaining the curability.

[0125] (Organic Solvent, Water) The inkjet ink of this embodiment may contain an organic solvent and / or water in order to reduce the viscosity of the inkjet ink, improve its wetting and spreading properties and adhesion to the printing substrate, and ensure ejection stability. When an organic solvent and / or water is contained, the content thereof is preferably 0.01 to 20% by mass, more preferably 0.05 to 10% by mass, and particularly preferably 0.1 to 5% by mass, based on the total mass of the inkjet ink. Furthermore, from the viewpoints of ejection stability and its wetting and spreading properties and adhesion to the printing substrate, when an organic solvent is used, it is preferable to use an organic solvent having a boiling point of 140 to 300°C.

[0126] Examples of the organic solvent that can be used include alkylene glycol monoalkyl ether acetates, alkylene glycol diacetates, alkylene glycol monoalkyl ethers, alkylene glycol dialkyl ethers, alkanediols, lactams, lactones, other nitrogen-containing solvents, and other oxygen-containing solvents.

[0127] Among these, it is preferable to include at least one selected from the group consisting of alkylene glycol monoalkyl ethers, alkylene glycol dialkyl ethers, and alkylene glycol monoalkyl ether acetates. In particular, tripropylene glycol monomethyl ether, dipropylene glycol monomethyl ether, propylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, tetraethylene glycol dialkyl ether, ethylene glycol monobutyl ether acetate, and diethylene glycol diethyl ether are preferred. In one embodiment, the organic solvent preferably includes at least one selected from the group consisting of tetraethylene glycol dialkyl ether, ethylene glycol monobutyl ether acetate, and diethylene glycol diethyl ether.

[0128] (Inert Resin) The inkjet ink of this embodiment may contain an inert resin for the purposes of imparting adhesion to various printing substrates and adjusting the viscoelasticity of the ink to improve ejection stability. Examples of inert resins that can be used include (meth)acrylic resins, urethane resins, vinyl chloride-vinyl acetate copolymer resins, and ketone resins. Of these, from the viewpoint of improving both adhesion and ejection stability, it is preferable that the inert resin contain a (meth)acrylic resin and / or a ketone resin.

[0129] When the inkjet ink of this embodiment contains an inert resin, the content thereof is preferably 0.1 to 10 mass %, more preferably 0.5 to 5 mass %, and particularly preferably 1 to 3 mass %, based on the total mass of the inkjet ink. When the content is adjusted to within the above range, adhesion and ejection stability can be easily improved without deteriorating curability.

[0130] In this specification, the term "inert resin" refers to a resin that does not participate in the polymerization reaction, contributes to adhesion to the printing substrate, and has solubility in inkjet ink.

[0131] (Other Additives) In addition to the components described above, the inkjet ink of this embodiment may further contain additives such as an ultraviolet absorber, an anti-fading agent, etc. Any conventionally known compounds can be used as these components.

[0132] <Physical Properties of Inkjet Ink> From the viewpoint of improving ejection stability, the inkjet ink of this embodiment preferably has a viscosity at 25°C of 5 to 25 mPa·s, and more preferably 8 to 20 mPa·s. A viscosity of 5 mPa·s or higher allows the inkjet ink to be ejected satisfactorily from the inkjet head. A viscosity of 25 mPa·s or lower allows for continuous, stable ejection without a decrease in ejection accuracy. Furthermore, from the viewpoint of imparting high frequency suitability and enabling stable ejection even in high-speed printing, the viscosity is particularly preferably 8 to 14 mPa·s. The viscosity can be measured using 1.1 mL of inkjet ink and an E-type viscometer (for example, "TVE25L" manufactured by Toki Sangyo Co., Ltd.) equipped with a cone (diameter 48 mm) with a cone angle of 1°34' at 25°C and a rotation speed of 20 rpm.

[0133] Furthermore, from the viewpoint of improving all of the ejection stability, the definition of printed matter, and the curing properties, the static surface tension of the inkjet ink at 25°C is preferably 20 to 45 mN / m, and particularly preferably 22 to 40 mN / m. The static surface tension is measured by the plate method (Wilhelmy method). Specifically, for example, the static surface tension can be measured in a 25°C environment using an automatic surface tensiometer "CBVP-Z" manufactured by Kyowa Interface Science Co., Ltd. and a platinum plate.

[0134] <Method for Producing Inkjet Ink> The inkjet ink of this embodiment can be produced by a conventionally known method, for example, as follows, but the method for producing the inkjet ink of this embodiment is not limited to the method described below.

[0135] First, a basic pigment dispersion resin is dissolved in a polymerizable compound to produce a basic pigment dispersion resin varnish. Next, carbon black satisfying the above-mentioned requirements is gradually added to the stirred basic pigment dispersion resin varnish and mixed. After mixing for a certain period of time, a dispersion treatment is carried out using a dispersing machine such as a paint shaker, sand mill, roll mill, or media-less disperser to produce a carbon black dispersion. In order to improve the dispersion stability of the carbon black, it is preferable to use a radically polymerizable bifunctional monomer and / or a radically polymerizable trifunctional monomer as the polymerizable compound used to produce the basic pigment dispersion resin varnish. It is particularly preferable to use a radically polymerizable bifunctional monomer represented by the above general formula (A) and / or a radically polymerizable trifunctional monomer represented by the above general formula (B).

[0136] Thereafter, 5-methyl-3-vinyloxazolidin-2-one, and, if necessary, a photopolymerization initiator, a surface conditioner, a polymerization inhibitor, an organic solvent, water, an inert resin, and other additives are added to the carbon black dispersion and thoroughly mixed. The mixture is then filtered using a filter or the like to remove coarse particles, thereby obtaining the inkjet ink of this embodiment.

[0137] The amount of carbon black present in the carbon black dispersion is preferably 10 to 50% by mass, more preferably 12 to 40% by mass, and particularly preferably 15 to 30% by mass.

[0138] <Inkjet printing method> The inkjet ink of the present embodiment described above is suitably used in an inkjet printing method. The inkjet printing method preferably includes, in this order, a step (Step I) of ejecting the inkjet ink of the present embodiment from an inkjet head onto a printing substrate, and a step (Step II) of irradiating the inkjet ink ejected onto the printing substrate with actinic energy rays to cure the inkjet ink.

[0139] The inkjet printing method may employ a method in which the same inkjet ink is ejected and applied multiple times from the same inkjet head to the same location on the printing substrate, i.e., a method in which step I is performed multiple times to the same location on the printing substrate (multi-pass printing method). However, in the case of the inkjet printing method using the inkjet ink of this embodiment, from the viewpoint of fully utilizing the effects of the inkjet ink, such as excellent curability and ejection stability, and of obtaining printed matter quickly and stably, a method in which the same inkjet ink is ejected and applied only once from the same inkjet head to the same location on the printing substrate is preferred. In other words, it is preferred to employ a method in which step I is performed only once to the same location on the printing substrate (one-pass printing method).

[0140] The one-pass printing method can be carried out, for example, by using a line printer, etc. In this case, the printing speed (the conveying speed of the printing substrate) is preferably 35 to 150 m / min, more preferably 50 to 125 m / min, and even more preferably 75 to 100 m / min, from the viewpoints of productivity and obtaining printed matter with good quality.

[0141] The inkjet ink of this embodiment is an ink for inkjet printing, and therefore, as described above, an inkjet head is used as the inkjet ink ejection means in step I.

[0142] Methods of ejecting ink using an inkjet head include an electrostatic attraction method that uses electrostatic force to eject ink, a drop-on-demand method (pressure pulse method) that uses the vibration pressure of a piezoelectric element, an acoustic inkjet method that converts an electric signal into an acoustic beam and irradiates the ink with the radiation pressure generated at that time to eject the ink, and a thermal inkjet method that heats the ink to form bubbles and ejects the ink using the generated pressure, etc. Among these, in one embodiment, from the viewpoint of ejection stability, the drop-on-demand method (pressure pulse method) that uses the vibration pressure of a piezoelectric element can be preferably used.

[0143] From the viewpoint of print resolution and ejection stability, the drop volume of inkjet ink droplets ejected from the inkjet nozzle is preferably 1 to 50 pL (picoliters), more preferably 2 to 30 pL, and even more preferably 3 to 20 pL. The design resolution of the inkjet head is preferably 300 dpi or higher, more preferably 480 dpi or higher, and even more preferably 600 dpi or higher. dpi refers to the number of dots per 2.54 cm (1 inch).

[0144] Examples of inkjet heads that satisfy the above conditions include KJ4A-AA, KJ4A-TA, and KJ4A-RH manufactured by Kyocera Corporation, Samba G3L manufactured by Fujifilm Corporation, S3200, S1600, S800, I3200, and I1600 manufactured by Seiko Epson Corporation, KM1024i and KM1024 manufactured by Konica Minolta, Inc., and MH5320, MH5340, MH5240, and MH5440 manufactured by Ricoh Co., Ltd., and any of these can be suitably used.

[0145] In one embodiment, the inkjet ink can be heated by a heater or other heating device provided in the inkjet head so that the inkjet ink has an appropriate viscosity during ejection. From the viewpoint of continuously and stably ejecting the inkjet ink, the inkjet ink is preferably heated so that the viscosity of the inkjet ink during ejection is 15 mPa s or less, and more preferably 12 mPa s or less.

[0146] On the other hand, in the above-mentioned step II, the inkjet ink ejected onto the printing substrate is cured by irradiation with active energy rays, and a cured film (printed matter) is formed. That is, a printed matter having a cured film of the inkjet ink is formed on the printing substrate.

[0147] In this specification, "active energy rays" refers to energy rays that can provide the energy necessary to generate active species such as radicals, cations, anions, etc. in the irradiated object (inkjet ink). Specific examples of active energy rays include ultraviolet rays, electron beams, visible light, and infrared rays. However, it is preferable to select ultraviolet rays because they can easily improve the curability of the inkjet ink and provide a high degree of freedom in designing the inkjet ink and the printing device.

[0148] Furthermore, examples of the ultraviolet light source include high-pressure mercury lamps, low-pressure mercury lamps, ultra-high-pressure mercury lamps, metal halide lamps, ultraviolet lasers, and LED lamps. Only one of these may be used, or two or more may be used in combination. For example, because the LED lamps are small, it is easy to arrange a plurality of them side by side, or to use them in combination with high-pressure mercury lamps or metal halide lamps, which can easily achieve further improvement in curing properties. Furthermore, when arranging a plurality of LED lamps side by side, it is also possible to use a plurality of types of LED lamps with different peak emission wavelengths in combination.

[0149] Generally, the ultraviolet light emitted from an LED lamp has a narrow wavelength range and is highly directional (i.e., poorly diffusible), which makes it difficult to cure active energy ray-curable inkjet inks. In particular, in the case of inkjet inks containing carbon black, curing with an LED lamp is difficult for the reasons mentioned above, namely, because the ultraviolet light is absorbed by the carbon black and because the quinone groups on the surface of the carbon black may trap radicals. However, the inkjet ink of this embodiment has particularly excellent curability and can therefore be suitably combined with an LED lamp.

[0150] In the inkjet printing method using the inkjet ink of this embodiment, ultraviolet light is preferably selected as the actinic energy ray. Furthermore, when an LED lamp is used as the light source of the ultraviolet light, the peak wavelength of the emitted ultraviolet light is preferably 260 to 450 nm, more preferably 280 to 420 nm, and particularly preferably 320 to 410 nm.

[0151] When an LED lamp that emits ultraviolet light is used in step II, the above-mentioned effects are fully exerted, and from the viewpoint of obtaining a printed matter that is excellent in print quality, including curability and print definition, the maximum illuminance of the ultraviolet light on the printing substrate is set to 1,000 mW / cm. ２ The maximum illuminance is preferably 2,000 mW / cm or more. ２ More preferably, it is 3,000 mW / cm or more. ２ The cumulative light amount when irradiating the printing substrate can be adjusted by the type and content of the polymerizable compound and photopolymerization initiator contained in the inkjet ink. For example, the cumulative light amount is 50 mJ / cm or more. ２ The integrated light amount is preferably 100 mJ / cm or more. ２ More preferably, it is 150 mJ / cm or more. ２ It is particularly preferable that the above amount is set to 100%.

[0152] In the above step I, after the inkjet ink droplets have adhered to the printing substrate (i.e., after the completion of the above step I), irradiation with active energy rays is initiated (i.e., the above step II is initiated). The time from the completion of step I to the start of step II is preferably adjusted to 0.03 to 3 seconds. The above time is more preferably 0.04 to 2.5 seconds, and even more preferably 0.6 to 2 seconds. By adjusting the above time within the above range, the dot formability of the inkjet ink is improved, and printed matter with excellent definition can be obtained.

[0153] In the inkjet printing method, step II can be repeated multiple times. For example, immediately after applying the inkjet ink to the printing substrate, the inkjet ink can be partially cured by irradiating it with active energy rays, and then the inkjet ink can be completely cured by irradiating it with active energy rays again. This makes it easy to obtain printed matter with exceptionally excellent print quality, including the fineness of the printed matter. In this specification, the above-mentioned step of partially curing the inkjet ink is referred to as "pre-curing." Furthermore, the step of completely curing the inkjet ink is referred to as "main curing."

[0154] That is, in one embodiment, an inkjet printing method using the inkjet ink of the present embodiment may include, in this order, a step of ejecting the inkjet ink from an inkjet head onto a printing substrate (step I), a step of irradiating the inkjet ink ejected onto the printing substrate with active energy rays to provisionally cure the inkjet ink (step II-A), and a step of fully curing the inkjet ink (step II-B).

[0155] When carrying out the step II-A, that is, when provisionally curing the inkjet ink ejected onto the printing substrate, it is preferable to use an LED lamp that emits ultraviolet light. In this case, from the viewpoint of significantly improving the print quality, including the definition of the printed matter, the maximum illuminance of the ultraviolet light on the printing substrate during the provisional curing is 2 to 20 mW / cm. ２ is preferably 5 to 15 mW / cm ２ It is more preferable that:

[0156] Furthermore, when carrying out the above-mentioned step II-B, that is, when the inkjet ink discharged onto the printing substrate is completely cured, an LED lamp that emits ultraviolet rays can also be used. In this case, the above-mentioned 1,000 mW / cm ２ The maximum illuminance is 50 mJ / cm or more. ２ It is preferable to use the above integrated light amount.

[0157] On the other hand, in step II-B, ultraviolet light can be irradiated using a high-pressure mercury lamp or a metal halide lamp. In this case, the maximum irradiance of the ultraviolet light is 80 mW / cm. ２ It is preferable that the power is 120 mW / cm or more. ２ It is more preferable that the integrated light amount is 100 mJ / cm or more. ２ It is preferable to set the dose to 150 mJ / cm or more. ２ More preferably, it is 200 mJ / cm or more. ２ More preferably, it is set to the above.

[0158] <Printing Substrate> The printing substrate used in the printing method using the inkjet ink of this embodiment is preferably a resin film substrate or a paper substrate. The resin film substrate may preferably have a thickness of 10 to 90 μm. Furthermore, a substrate containing a material selected from the group consisting of polypropylene, polyethylene, polyethylene terephthalate, and nylon is preferably selected as the resin film substrate. Meanwhile, coated paper, art paper, laminated paper, etc. are preferably selected as the paper substrate. In one embodiment, the inkjet ink of this embodiment is preferably used for printing on packages formed from the printing substrates listed above. Among the printing on such packages, it is particularly preferably used for printing on food packaging.

[0159] The "substrate containing a material selected from the group consisting of polypropylene, polyethylene, polyethylene terephthalate, and nylon" is not limited to a single-layer structure and may have a multilayer structure. That is, the substrate may be, for example, a resin film substrate having one layer made of at least one material selected from the group consisting of polyethylene terephthalate, polyethylene, polypropylene, and nylon. As another example, the substrate may be a resin film substrate (laminated film substrate) having two or more of the above layers. Furthermore, for the purpose of improving the strength of the package, blocking oxygen, etc., layers made of AL (aluminum foil), VM (vacuum vapor deposition) film (aluminum vapor deposition film, transparent vapor deposition film), etc. may be provided among the layers constituting the laminated film substrate.

[0160] <Printed Material> The inkjet ink of this embodiment can be used to produce a printed material (a printing substrate on which print information is recorded). The above-mentioned inkjet printing method can be used as a method for producing the printed material.

[0161] The present invention will be described in more detail below, but the following examples are not intended to limit the scope of the present invention. Unless otherwise specified, "parts" represents parts by mass, and "%" represents % by mass.

[0162] <Raw Materials Used> In the examples shown below, the carbon blacks used were those shown in Table 1. In Table 3 described later, the carbon blacks used are listed by their abbreviations.

[0163]

[0164] In the examples shown below, the pigment dispersing resins shown in Table 2 were used. In Table 3 described later, the pigment dispersing resins used are listed by abbreviation. Of the pigment dispersing resins shown in Table 2 below, the pigment dispersing resins except for Solsperse 41000 are basic pigment dispersing resins, and Solsperse 41000 is an acidic pigment dispersing resin (a pigment dispersing resin whose adsorption point with the pigment is an acid group).

[0165]

[0166] <Preparation of Carbon Black Dispersions> Carbon black dispersions 1 to 27 were prepared using the raw materials listed in each column of Table 3 below. Specifically, a pigment dispersion resin and a polymerizable compound were first placed in a mixing vessel (volume 8 L) equipped with a stirrer and stirred for one hour (premixing) to prepare a pigment dispersion resin varnish. Next, carbon black was gradually added to the stirred pigment dispersion resin varnish, and after the addition was completed, stirring was continued for another hour (predispersion). The mixture was then circulated and dispersed for two hours using a Shinmaru Enterprises "Dynomill" (volume 0.6 L) filled with zirconia beads with a diameter of 0.8 mm to achieve a filling rate of 70%, thereby preparing a carbon black dispersion.

[0167]

[0168]

[0169]

[0170] Table 3 also lists the secondary particle size and the pigment dispersion resin content relative to the carbon black content for carbon black dispersions 1 to 27 produced by the above-mentioned method. Details of the abbreviations used in Table 3 are as follows. Abbreviations not listed below are as explained above in Tables 1 and 2. HDDA: 1,6-hexanediol diacrylate DPGDA: dipropylene glycol diacrylate NDDA: 1,9-nonanediol diacrylate

[0171] <Production of Inkjet Inks> The materials listed in each column of Table 4 below were added to a mixing vessel equipped with a stirrer. After all the materials had been added, the mixture was heated while stirring until the temperature of the mixture reached 40°C. After reaching 40°C, stirring was continued for an additional hour while maintaining the temperature. The mixture was then filtered through a membrane filter with a pore size of 0.8 μm to produce inkjet inks 1 to 96.

[0172] In producing the inkjet ink, each material was added while stirring the mixture in the mixing vessel. The materials were added in the following order: carbon black dispersion, polymerizable compound other than 5-methyl-3-vinyloxazolidin-2-one, 5-methyl-3-vinyloxazolidin-2-one, photopolymerization initiator, polymerization initiator, and surface conditioner. However, when producing an inkjet ink that did not contain one or more of these components, that component was not added, and the next component was added in the specified order. For components containing two or more materials, the order of addition was determined by the amount of the component that was added first.

[0173]

[0174]

[0175]

[0176]

[0177]

[0178]

[0179]

[0180]

[0181]

[0182]

[0183] Details of the abbreviations used in Table 4 are as follows: Abbreviations not listed below are as explained above in Tables 1 to 3. PDDA: 1,3-propanediol diacrylate BDDA: 1,4-butanediol diacrylate MPDDA: 3-methyl-1,5-pentanediol diacrylate GlyTA: glycerin triacrylate Gly(EO)3TA: ethylene oxide-modified glycerin triacrylate (ethylene oxide group number: 3) Gly(EO)9TA: ethylene oxide-modified glycerin triacrylate (ethylene oxide group number: 9) CHA: cyclohexyl acrylate IBXA: isobornyl acrylate BzA: benzyl acrylate VCL: N-vinyl caprolactam PEG400DA: polyethylene glycol 400 diacrylate TMP(EO)3TA: ethylene oxide-modified trimethylolpropane triacrylate (ethylene oxide group number: 3) TMP(PO)2TA: propylene oxide modified trimethylolpropane tri(meth)acrylate (number of propylene oxide groups: 2) MVOZ: 5-methyl-3-vinyloxazolidin-2-one TPO-L: ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (manufactured by IGM RESINS, "Omnirad TPO-L") OmnTP: polymer of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (manufactured by IGM RESINS, "OMNIPOL TP") Omn819: phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (manufactured by IGM RESINS, "Omnirad 819") DETX: 2,4-Diethylthioxanthone ("Omnirad DETX" manufactured by IGM RESINS) OmnTX: 3-ethoxycarbonylmethoxythioxanthone polymer ("OMNIPOL TX" manufactured by IGM RESINS) Omn379: 2-(dimethylamino)-2-(4-methylbenzyl)-1-(4-morpholinophenyl)butan-1-one ("Omnirad 379" manufactured by IGM RESINS) ASA: polyethylene glycol-bis(p-dimethylaminobenzoate) ("Omnipol ASA" manufactured by IGM RESINS) BHT: 2,6-di-tert-butyl-4-methylphenol UV3510: Polyether-modified siloxane surface conditioner (BYK-Chemie "BYK-UV3510")

[0184] <Production of Printed Material> A printed material was produced using the inkjet ink prepared above by the method described below. A temperature-controllable inkjet head (KJ4A, design resolution 600 dpi) manufactured by Kyocera Corporation was installed above a conveyor capable of transporting the printing substrate. In addition, a main curing LED lamp (FirePower FP300 manufactured by Phoseon Corporation (maximum emission wavelength 395 nm, maximum illuminance 16,000 mW / cm) was installed downstream with respect to the transport direction of the printing substrate. ２)) was installed. The inkjet head was filled with the inkjet ink prepared above. Next, the temperature of the inkjet head was adjusted so that the viscosity of the inkjet ink during ejection was 6 to 7 mPa·s. After that, a PET substrate "K2411" manufactured by Lintec Corporation was fixed on a conveyor, and the conveyor was driven at a speed of 50 m / min. When the PET substrate passed the area where the inkjet head was installed, inkjet ink droplets were ejected from the inkjet head to perform printing. Specifically, a solid image with a 100% print ratio and a character image were printed under printing conditions of an ejected droplet volume of 11 pL and a printing resolution of 600 dpi x 600 dpi. The character images were images in which random hiragana characters in MS Mincho font were printed at 4-point, 6-point, and 8-point sizes, with 20 of each. Next, after printing the inkjet ink, the conveyor was continued to be driven at the same speed, and when the PET substrate passed through the installation area of ​​the main curing LED lamp, ultraviolet light was irradiated to produce a printed product. Note that the illuminance of the ultraviolet light irradiated on the inkjet ink after printing was 6,000 mW / cm. ２ , and the cumulative light amount is 150 mJ / cm ２ The output of the LED lamp was adjusted in advance so that the above printing was performed.

[0185] [Examples 1 to 87, Comparative Examples 1 to 9] Using the inkjet inks and printed materials produced by the methods described above, various evaluations were carried out according to the methods described below. The evaluation results are shown in Table 4.

[0186] <Evaluation 1: Evaluation of Dispersion Stability> For each of the above inkjet inks, the secondary particle diameter was measured immediately after production using the measuring device and method described above. Next, each inkjet ink was filled into a 20 mL glass container so that the filling rate was 80% of the container volume. The glass container filled with the inkjet ink was then left to stand for two weeks in a sealed, light-shielded environment at 60°C, and the secondary particle diameter of the inkjet ink after standing was measured again. The dispersion stability was evaluated by calculating the rate of increase in secondary particle diameter before and after standing. The evaluation criteria for the dispersion stability were as follows: A rating of "2" or higher was considered practical, and a rating of "3" or higher was considered practically suitable.

[0187] (Evaluation criteria for dispersion stability) 4: The increase rate of secondary particle diameter was less than 10%. 3: The increase rate of secondary particle diameter was 10% or more and less than 20%. 2: The increase rate of secondary particle diameter was 20% or more and less than 30%. 1: The increase rate of secondary particle diameter was 30% or more.

[0188] <Evaluation 2: Evaluation of Discharge Stability> An inkjet ink was filled into a jig equipped with a temperature-adjustable inkjet head (KJ4A, design resolution 600 dpi) manufactured by Kyocera Corporation. The temperature of the inkjet head was then adjusted so that the inkjet ink viscosity during discharge was 6 to 7 mPa·s. After confirming that there were no nozzles from which the inkjet ink was not being discharged, the inkjet ink was continuously discharged from all nozzles at a drive frequency of 20 kHz. After five minutes of continuous discharge, the number of nozzles from which the inkjet ink was not being discharged (nozzle loss number) was counted to evaluate discharge stability. The evaluation criteria for the discharge stability were as follows, and a rating of "2" or higher was deemed practical. However, for inkjet inks that received a rating of "1" in the dispersion stability evaluation described above, the inkjet ink was not evaluated for discharge stability due to a high risk of damaging the inkjet head.

[0189] (Evaluation criteria for ejection stability) 4: The number of nozzle losses was 0 to 2. 3: The number of nozzle losses was 3 to 5. 2: The number of nozzle losses was 6 to 9. 1: The number of nozzle losses was 10 or more.

[0190] <Evaluation 3: Evaluation of Curability> The surface of a printed material with a 100% solid image coverage produced by the above method was rubbed with a cotton swab to check whether uncured ink adhered to the cotton swab. If inkjet ink adhered to the cotton swab, the printed material was fixed to the conveyor of the inkjet printing device and irradiated with the main curing LED lamp without printing the inkjet ink. After that, the presence or absence of inkjet ink adhesion when rubbed with the cotton swab was checked again. This procedure was repeated, and the number of passes required until the uncured inkjet ink no longer adhered to the cotton swab was counted to evaluate curability. The evaluation criteria for the curability were as follows: A rating of "2" or higher was considered practical, and a rating of "3" or higher was considered practically suitable.

[0191] (Curability evaluation criteria) 4: After one total pass (no additional UV irradiation required), uncured inkjet ink no longer adhered to the cotton swab. 3: After two total passes (one additional UV irradiation performed), uncured inkjet ink no longer adhered to the cotton swab. 2: After three total passes (two additional UV irradiation performed), uncured inkjet ink no longer adhered to the cotton swab. 1: It was necessary to irradiate the cotton swab with the LED lamp four or more times in total until uncured inkjet ink no longer adhered to the cotton swab.

[0192] <Evaluation 4: Evaluation of the precision (character visibility) of the printed matter> The precision (character visibility) of the printed matter was evaluated by visually checking whether the hiragana characters in the character images prepared by the above method could be read. The evaluation criteria for the precision were as follows, with a rating of "2" or higher being considered practically usable and a rating of "3" or higher being considered practically suitable.

[0193] (Evaluation criteria for print resolution (character legibility)) 4: All 20 hiragana characters could be distinguished at all 4-point, 6-point, and 8-point sizes. 3: All 20 hiragana characters could be distinguished at all 6-point and 8-point sizes, but some hiragana characters printed at 4-point were indistinguishable. 2: All 20 hiragana characters could be distinguished at all 8-point sizes, but some hiragana characters printed at 6-point were indistinguishable. 1: Some hiragana characters printed at 8-point were indistinguishable.

[0194] As shown in Table 4, the inkjet inks of Examples 1 to 87 having the above-described configuration were excellent in all of the dispersion stability of carbon black, ejection stability, curing properties, and definition of printed matter.

[0195] On the other hand, the inkjet inks of Comparative Examples 1 to 4, which used carbon black with a product (AC x SC) of DBP oil absorption (AC) and specific surface area (SC) greater than 12,000, did not achieve practically acceptable dispersion stability or ejection stability. Furthermore, the inkjet inks of Comparative Examples 3 and 4, which had particularly large values ​​for this product, achieved practically acceptable levels of curability and print resolution, but did not achieve levels suitable for practical use. In contrast, the inkjet inks of Examples 1 to 7, which used carbon black with a product value of 2,000 to 12,000, achieved practically acceptable dispersion stability and ejection stability, and also achieved practically acceptable levels of curability and print resolution. These results confirm that controlling the value of this product is important for achieving the effects described above.

[0196] Furthermore, the dispersion stability and ejection stability did not reach practical levels in Comparative Example 5, which used Solsperse 41000, an acidic pigment dispersion resin, and Comparative Example 6, which used a basic pigment dispersion resin (DISPERBYK-180) with an acid value of more than 30 mgKOH / g. In these examples, the acid value of the pigment dispersion resin used was too high, causing the acid groups in the pigment dispersion resin to react with the vinyl groups in 5-methyl-3-vinyloxazolidin-2-one, presumably resulting in a deterioration in the dispersion stability and ejection stability of the carbon black.

[0197] The inkjet ink of Comparative Example 7 was a system in which the content of monofunctional polymerizable compounds excluding 5-methyl-3-vinyloxazolidin-2-one was greater than 15% by mass, and evaluation confirmed that the curability did not reach a practically acceptable level. Furthermore, the definition of the printed matter was at a practically acceptable level, but not at a level suitable for practical use.

[0198] Furthermore, the inkjet ink of Comparative Example 8, in which N-vinylcaprolactam, a typical example of an N-vinyl compound, was used instead of 5-methyl-3-vinyloxazolidin-2-one, did not achieve a practically acceptable level of dispersion stability, and while the curability and definition of the printed matter were at a practically acceptable level, they were not at a level suitable for practical use. Compared to the inkjet ink of Example 2, in which 5-methyl-3-vinyloxazolidin-2-one was used instead of N-vinylcaprolactam, the inkjet ink of Comparative Example 8 was significantly inferior in quality. This confirms that 5-methyl-3-vinyloxazolidin-2-one is an essential ingredient in the inkjet ink of this embodiment.

[0199] Furthermore, the importance of 5-methyl-3-vinyloxazolidin-2-one in the inkjet ink of this embodiment was also confirmed by the fact that the inkjet ink of Comparative Example 9, which did not contain 5-methyl-3-vinyloxazolidin-2-one, did not achieve practically acceptable levels in curability and definition of printed matter.

Claims

1. An active energy ray curable inkjet black ink containing carbon black, a basic pigment dispersion resin, and a polymerizable compound, wherein the DBP oil absorption of the carbon black is AC (mL / 100g), the specific surface area of ​​the carbon black is SC (m ２ / g), the value expressed by AC×SC is 2,000 to 12,000, the basic pigment dispersion resin has an acid value of 30 mgKOH / g or less, the polymerizable compound contains 5-methyl-3-vinyloxazolidin-2-one, and the content of monofunctional polymerizable compounds (excluding 5-methyl-3-vinyloxazolidin-2-one) is 15 mass % or less based on the total mass of the active energy ray-curable inkjet black ink.

2. The actinic ray-curable inkjet black ink according to claim 1, wherein the polymerizable compound comprises a radically polymerizable bifunctional monomer represented by general formula (A), and the content of the radically polymerizable bifunctional monomer represented by the following general formula (A) is 15 to 55 mass % based on the total mass of the actinic ray-curable inkjet black ink. General formula (A): CH ２ =CR １ -CO-O-R ２ -O-CO-CR １ =CH ２ (In general formula (A), R １ represents a hydrogen atom or a methyl group, R ２ represents an alkylene group having 3 to 6 carbon atoms, which may have a branched structure.

3. The actinic energy ray-curable inkjet black ink according to claim 1 or 2, wherein the polymerizable compound comprises a radical polymerizable trifunctional monomer represented by the following general formula (B), and the content of the radical polymerizable trifunctional monomer represented by general formula (B) is 2 to 15 mass % based on the total mass of the actinic energy ray-curable inkjet black ink. General formula (B): (In general formula (B), R ３ represents a hydrogen atom or a methyl group, R ４ is an ethylene group or a propylene group. Furthermore, l, m, and n each represent an integer of 0 to 9, and l+m+n is an integer of 0 to 9.

4. The actinic ray-curable inkjet black ink according to claim 1, further comprising a photopolymerization initiator, wherein the photopolymerization initiator comprises an acylphosphine oxide compound.

5. The actinic ray-curable inkjet black ink according to claim 4, wherein the photopolymerization initiator further contains a thioxanthone compound.

6. The active energy ray-curable inkjet black ink according to claim 4 or 5, wherein the acylphosphine oxide compound comprises at least one selected from the group consisting of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and a polymer of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, and the total content of the ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and the polymer of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide is 45 to 80 mass % based on the total mass of the photopolymerization initiator.

7. The actinic ray-curable inkjet black ink according to claim 1, 2, 4 or 5, wherein the pH of the carbon black is 2 to 5.

8. A printed matter obtained by printing the actinic radiation curable inkjet black ink according to claim 1, 2, 4 or 5 onto a printing substrate.

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