UV-curing inkjet inks and printed materials

Abstract

The present invention provides an ultraviolet-curable inkjet ink that exhibits excellent surface and internal curing properties, as well as discharge stability, and furthermore, produces printed materials with excellent color development and free from image defects such as white spots and pinholes. [Solution] An ultraviolet-curable inkjet ink comprising a photopolymerizable compound and a pigment, wherein the photopolymerizable compound comprises 5-methyl-3-vinyloxazolidine-2-one and a radically polymerizable difunctional monomer having a specific structure, the content of each of the above-mentioned components is defined, the content of monofunctional polymerizable compounds excluding 5-methyl-3-vinyloxazolidine-2-one (C) is 2.5 times or less the content of 5-methyl-3-vinyloxazolidine-2-one (A), and the total amount of the radically polymerizable difunctional monomer content (B) and the above content (C) is 7 to 65% by mass of the total amount of the ultraviolet-curable inkjet ink.

Description

[Technical Field]

[0001] Embodiments of the present invention relate to an ultraviolet-curable inkjet ink and a printed material obtained using the ultraviolet-curable inkjet ink. [Background technology]

[0002] In recent years, digital printing has rapidly become widespread in the printing industry. Because digital printing does not require printing plates, it is possible to miniaturize printing equipment compared to traditional plate printing, which requires plates. Furthermore, digital printing is superior to other printing methods in various aspects, such as lower running costs and ease of full-color printing, and its use is progressing particularly in the industrial printing industry.

[0003] One type of digital printing method is inkjet printing. Inkjet printing is a method in which ink droplets of inkjet ink are ejected from nozzles (discharge ports) on an inkjet head, and these droplets are deposited onto the printing substrate to perform printing. Inkjet inks used in inkjet printing include water-based, oil-based, solvent-based, and UV-curing types, but due to their characteristics such as being applicable to non-absorbent printing substrates such as plastics and glass, having a fast drying (curing) time, and producing high-strength printed materials, the demand for UV-curing inkjet inks is particularly high in the industrial printing industry.

[0004] In printed materials produced using UV-curing inkjet inks, the curing properties tend to vary depending on the thickness of the film (ink film) formed by curing the inkjet ink. This is due to factors such as the influence of oxygen in the air that the surface of the ink film is in contact with (oxygen inhibition), and insufficient UV energy in the deeper parts of the ink film. In particular, improving the responsiveness of the inkjet ink to UV light is necessary to achieve printing speeds equivalent to those of the plate printing method described above.

[0005] Furthermore, in the industrial printing industry, when using UV-curable inkjet inks for commercial printing, labels, etc., it is important to be able to stably produce printed materials with high color reproduction and no image defects. In other words, for the above-mentioned applications, it is important that the UV-curable inkjet ink has good ejection stability, and that it produces an ink film that is highly concentrated and free from image defects such as white spots (where the inkjet ink does not adhere to areas where it should be applied, leaving the printing substrate exposed) and pinholes (tiny dot-like white spots).

[0006] One example of a method to improve the reactivity of inkjet inks to ultraviolet light is to reduce the amount of pigment contained in the inkjet ink. Generally, pigments have the property of absorbing, reflecting, or scattering ultraviolet light, and even if ultraviolet light is irradiated onto an inkjet ink containing pigment, there is a risk that the ultraviolet light will not penetrate sufficiently into the interior. However, if the amount of pigment, which is a coloring agent, is reduced, it will naturally be impossible to obtain a high-concentration ink film. Another method is to use a polyfunctional polymerizable compound (details will be described later) as the photopolymerizable compound contained in the inkjet ink. However, in that case, the viscosity of the inkjet ink will increase, and there is a high risk that the ejection stability will deteriorate. In addition, there is a risk that the inkjet ink that lands on the printing substrate will not wet and spread sufficiently, resulting in problems such as solid areas not being filled.

[0007] In this disclosure, the condition in which there are no defects such as white gaps or pinholes in areas printed at 100% coverage will also be described as "solid color filling" or "good solid color filling."

[0008] In recent years, in order to meet market demands for environmental considerations, light-emitting diodes (UV-LEDs) capable of emitting ultraviolet light have increasingly been adopted as a means of ultraviolet irradiation. However, UV-LEDs have the characteristic of emitting a narrow range of ultraviolet wavelengths, and improving curing performance is a particular challenge when used in combination with UV-curing inkjet inks.

[0009] Studies to improve the curing properties of UV-curable inkjet inks when used in combination with UV-LEDs have been conducted for some time (Patent Documents 1-4). [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] International Publication No. 2014 / 014017 [Patent Document 2] Japanese Patent Publication No. 2022-149919 [Patent Document 3] Japanese Patent Publication No. 2021-042321 [Patent Document 4] Japanese Patent Publication No. 2023-163470 [Overview of the project] [Problems that the invention aims to solve]

[0011] For example, Patent Document 1 discloses a photocurable inkjet printing ink composition containing 4 to 40% by mass of vinyloxyethoxyethyl acrylate and 10 to 65% by mass of benzyl acrylate, and further containing 50% or more by mass of monofunctional monomers. Patent Document 2 discloses an active energy ray curable inkjet ink composition containing CI pigment violet 19 and CI pigment orange 71 in a certain ratio, and further containing a polyfunctional amine-modified oligomer, as well as multiple types of monofunctional monomers with different glass transition temperatures and nitrogen-containing monofunctional monomers. Patent Document 2 states that the ink composition having the above configuration has excellent color development, can be cured with UV-LED, and further produces printed materials with excellent hardness, abrasion resistance, and water resistance. However, the examples in Patent Documents 1 and 2 do not include actual printing evaluations using an inkjet printer (printing device), and do not address whether highly colored, clear, and defect-free printed materials can be stably printed under high-frequency conditions. Furthermore, considering the adoption in the printing market as described above, it is thought that the ink compositions specifically disclosed in Patent Documents 1 and 2 require further improvement in curability. It should be noted that Patent Document 2 specifies the type of pigment to be used, and does not examine the color development when using arbitrary pigments such as carbon black.

[0012] Furthermore, Patent Document 3 discloses a radiation-curable inkjet composition containing vinylmethyl oxazolidinone and 2-(2-vinyloxyethoxy)ethyl (meth)acrylate as polymerizable compounds, and Patent Document 4 discloses a radiation-curable inkjet composition containing a predetermined amount of glycerin diacrylate and / or glycerin triacrylate. The inkjet compositions disclosed in Patent Documents 3 and 4 are said to produce printed materials with low viscosity but excellent abrasion resistance, and the examples show that the above inkjet compositions are cured using an LED light source. However, Patent Documents 3 and 4 do not include evaluations using inkjet printers, and depending on the printing conditions, there is a risk that ejection stability and solid filling may be insufficient. In addition, Patent Documents 3 and 4 do not include any studies on the color development of the printed materials.

[0013] As described above, conventionally, there were no UV-curable inkjet inks that could maintain good ejection stability even when printing at high speeds, prevent image defects such as white spots and pinholes, produce an ink film with sufficient color development, and furthermore, exhibit excellent surface and internal curing properties even when cured using UV-LEDs.

[0014] The present invention has been made to solve the above-mentioned problems, and the objective of one embodiment is to provide an ultraviolet-curable inkjet ink that has excellent surface and internal curability and ejection stability, as well as excellent color development, and that produces printed materials free from image defects such as white spots and pinholes. The objective of another embodiment is to provide printed materials that have excellent color development and are free from image defects such as white spots and pinholes. [Means for solving the problem]

[0015] To solve the above problems, as a result of intensive studies by the present inventors, it has been found that by using a specific photopolymerizable compound in combination, and specifying the content and ratio of the specific photopolymerizable compound, as well as the pigment concentration (content of the pigment), the above problems can be preferably solved by an ultraviolet-curable inkjet ink, and the present invention has been completed.

[0016] That is, the embodiments of the present invention include the following. However, the present invention is not limited to the following embodiments and includes various embodiments.

[0017] One embodiment is an ultraviolet-curable inkjet ink containing a photopolymerizable compound and a pigment, where the photopolymerizable compound includes 5-methyl-3-vinyloxazolidin-2-one and a radical-polymerizable difunctional monomer represented by the general formula (1), the content (A) of 5-methyl-3-vinyloxazolidin-2-one with respect to the total amount of the ultraviolet-curable inkjet ink is 3 to 25% by mass, the content (B) of the radical-polymerizable difunctional monomer represented by the general formula (1) with respect to the total amount of the ultraviolet-curable inkjet ink is 7 to 65% by mass, the content (D) of the pigment with respect to the total amount of the ultraviolet-curable inkjet ink is 2.2 to 7% by mass, the ratio (C / A) of the content (C) of the monofunctional polymerizable compound (excluding 5-methyl-3-vinyloxazolidin-2-one) to the content (A) of 5-methyl-3-vinyloxazolidin-2-one is 0 to 2.5, and the total amount (B + C) of the content (B) of the radical-polymerizable difunctional monomer represented by the general formula (1) and the content (C) of the monofunctional polymerizable compound (excluding 5-methyl-3-vinyloxazolidin-2-one) is from 7 to 65% by mass with respect to the total amount of the ultraviolet-curable inkjet ink. It relates to an ultraviolet-curable inkjet ink. General formula (1): CH2=CH-CO-O-R 1 -O-CO-CH=CH2 [In general formula (1), R 1 This represents an alkylene group having 3 to 10 carbon atoms, which may have a branched structure.

[0018] Another embodiment relates to a printed material in which the ultraviolet-curable inkjet ink is printed on a printing substrate. [Effects of the Invention]

[0019] One embodiment of the present invention provides an ultraviolet-curable inkjet ink that exhibits excellent surface and internal curability and discharge stability, as well as excellent color development, and produces printed materials free from image defects such as white spots and pinholes. Another embodiment of the present invention provides printed materials that exhibit excellent color development and are free from image defects such as white spots and pinholes. [Modes for carrying out the invention]

[0020] The present invention will be described in detail below. However, the present invention is not limited to the following embodiments and can be implemented with various modifications without departing from the spirit of the invention.

[0021] The ultraviolet-curable inkjet ink of the present invention (hereinafter also simply referred to as "the inkjet ink of the present invention" or "the ink of the present invention") contains a pigment in an amount of 2.2 to 7% by mass of the total amount of inkjet ink. As described above, when an inkjet ink containing a pigment and a photopolymerization initiator is irradiated with ultraviolet light, there is a risk that a sufficient amount of ultraviolet light will not penetrate into the interior of the inkjet ink due to the ultraviolet absorption properties of the pigment. In such cases, the photopolymerization initiator contained in the inkjet ink does not function properly, and there is a problem that the curability of the inkjet ink, especially the internal curability of thick films, deteriorates. In particular, when an ultraviolet light-emitting diode (UV-LED) is used as the ultraviolet light source, the wavelength range of ultraviolet light emitted from the UV-LED is narrow, so the tendency for curability to deteriorate is stronger.

[0022] In contrast, the UV-curable inkjet ink of the present invention contains 3 to 25% by mass of 5-methyl-3-vinyloxazolidine-2-one as a photopolymerizable compound in the total amount of inkjet ink. Generally, cyclic N-vinyl compounds are known to have a higher reaction rate with radicals derived from photopolymerization initiators compared to acrylate compounds used as photopolymerizable compounds. Furthermore, the reaction rate of 5-methyl-3-vinyloxazolidine-2-one is particularly high compared to the reaction rates of other cyclic N-vinyl compounds. Compared to N-vinylcaprolactam and N-vinylpyrrolidone, which are generally known as cyclic N-vinyl compounds, 5-methyl-3-vinyloxazolidine-2-one has an oxygen atom in its ring structure. It is thought that electrons in the ring structure are attracted to this oxygen atom, making the vinyl group more receptive to radicals derived from photopolymerization initiators, thus increasing the reaction rate of 5-methyl-3-vinyloxazolidine-2-one.

[0023] Furthermore, the UV-curable inkjet ink of the present invention contains 7 to 65% by mass of a radical polymerizable difunctional monomer represented by general formula (1) as a polyfunctional polymerizable compound, based on the total amount of the inkjet ink. The radical polymerizable difunctional monomer represented by general formula (1) has low surface tension and improves wettability and spreadability on printing substrates such as polyolefin substrates and paper substrates, thereby enabling the formation of a uniform and smooth ink film. In addition, the radical polymerizable difunctional monomer represented by general formula (1) has low viscosity and a relatively small molecular weight, and is highly reactive among acrylate compounds that can be used in inkjet inks. Therefore, when used in combination with 5-methyl-3-vinyloxazolidine-2-one, the overall curability of the inkjet ink can be significantly improved. As a result, even in inkjet inks containing a certain amount or more of pigment, improved surface and internal curability can be achieved.

[0024] As mentioned above, 5-methyl-3-vinyloxazolidine-2-one is highly polar. Therefore, inkjet inks containing a certain amount of 5-methyl-3-vinyloxazolidine-2-one may experience poor wetting and spreading properties on the printing substrate, potentially leading to white spots and / or pinholes during high-speed printing. In contrast, the radical polymerizable difunctional monomer represented by general formula (1) has low surface tension. By using both together, it is possible to maintain curability while improving wetting and spreading properties on the printing substrate, suppressing the occurrence of white spots and pinholes even during high-speed printing. Furthermore, when 5-methyl-3-vinyloxazolidine-2-one is used, the dispersion stability of the pigment may deteriorate, potentially leading to pigment aggregation and a deterioration in the color development of the ink film. However, by using the radical polymerizable difunctional monomer represented by general formula (1) in combination, the deterioration of dispersion stability can be suppressed, making it easier to maintain good color development of the ink film. Furthermore, 5-methyl-3-vinyloxazolidine-2-one and the radically polymerizable difunctional monomer represented by general formula (1) have low viscosity among photopolymerizable compounds, and their viscoelasticity is optimized by intermolecular interactions (for example, hydrogen bonds formed between oxygen and / or nitrogen atoms via hydrogen atoms), thus improving ejection stability even in inkjet inks containing a certain amount or more of pigment.

[0025] Furthermore, in the UV-curable inkjet ink of the present invention, when the content of monofunctional polymerizable compounds (excluding 5-methyl-3-vinyloxazolidine-2-one) is C and the content of 5-methyl-3-vinyloxazolidine-2-one is A, the value expressed as C / A is 0 to 2.5. Monofunctional polymerizable compounds other than 5-methyl-3-vinyloxazolidine-2-one have low viscosity and are effective materials for improving, for example, the ejection stability of inkjet inks. However, compared to other acrylate compounds used in inkjet inks, many of them have a low reaction rate and tend to remain unreacted in the ink film, especially inside the ink film where ultraviolet light does not easily reach. Therefore, by limiting the content of monofunctional polymerizable compounds other than 5-methyl-3-vinyloxazolidine-2-one, the amount of monofunctional polymerizable compounds remaining inside the ink film can be reduced, and the internal curing performance can be dramatically improved.

[0026] Furthermore, in the UV-curable inkjet ink of the present invention, when the content of the radical polymerizable difunctional monomer represented by the general formula (1) is B, the total amount of B and C (B+C) is 7 to 65% by mass of the total amount of the inkjet ink. Both the radical polymerizable difunctional monomer represented by the general formula (1) and the monofunctional polymerizable compounds, excluding 5-methyl-3-vinyloxazolidine-2-one, have low viscosity. As mentioned above, the radical polymerizable difunctional monomer represented by the general formula (1) also has low surface tension. As a result, inkjet inks containing a certain amount of these components have improved wettability on the printing substrate, and it is easier to prevent the occurrence of white spots and pinholes in the ink film. Although the detailed mechanism is unknown, the viscoelasticity of the inkjet ink is also optimized, and the ejection stability is improved.

[0027] As described above, the above configuration is essential to obtain an ultraviolet-curable inkjet ink that exhibits excellent surface and internal curing properties, as well as discharge stability, and furthermore, has excellent color development and produces printed materials free from image defects such as white spots and pinholes.

[0028] Furthermore, while Patent Document 1 discloses specific examples using a radically polymerizable difunctional monomer (1,6-hexanediol diacrylate, HDDA) represented by general formula (1), it does not disclose any specific examples using 5-methyl-3-vinyloxazolidine-2-one. In addition, although N-vinylcaprolactam is described in the specification of Patent Document 1, 5-methyl-3-vinyloxazolidine-2-one is not mentioned at all. Furthermore, in the specific example of Patent Document 2, neither the radical polymerizable difunctional monomer represented by general formula (1) nor 5-methyl-3-vinyloxazolidine-2-one is used. While paragraph 0031 of Patent Document 2 lists compounds included in the radical polymerizable difunctional monomer represented by general formula (1), the same paragraph also contains a statement that "if included in large quantities, adhesion to the substrate will be poor," thus discouraging the active use of these compounds. While Patent Documents 3 and 4 disclose specific examples using 5-methyl-3-vinyloxazolidine-2-one, they do not describe its use in combination with the radically polymerizable difunctional monomer represented by general formula (1). Furthermore, although the specifications of Patent Documents 3 and 4 list compounds included in the radically polymerizable difunctional monomer represented by general formula (1) along with other polyfunctional photopolymerizable compounds, there is no description of which of these compounds are preferably used. Furthermore, regarding the composition of the inkjet ink of the present invention, which involves using 5-methyl-3-vinyloxazolidine-2-one, which has a high reaction rate, in combination with a radically polymerizable difunctional monomer represented by general formula (1), and adjusting the content of each component, and the effects of this composition, there is neither description nor suggestion in the above-mentioned Patent Documents 1 to 4.

[0029] Next, each of the components constituting the inkjet ink of the present invention will be described in detail below.

[0030] <Photopolymerizable compound> The ink of the present invention contains a photopolymerizable compound. The photopolymerizable compound also contains 5-methyl-3-vinyloxazolidine-2-one and a radical-polymerizable difunctional monomer represented by the above general formula (1). In this disclosure, "photopolymerizable compound" refers to a compound that reacts with radicals generated from a photopolymerization initiator, etc., to cause polymerization and / or crosslinking reactions. Furthermore, "monofunctional polymerizable compound" refers to a photopolymerizable compound having one photopolymerizable group, and "polyfunctional polymerizable compound" refers to a photopolymerizable compound having two or more photopolymerizable groups. Here, examples of the above photopolymerizable groups include (meth)acryloyl groups and vinyl groups (excluding (meth)acryloyl groups).

[0031] In this disclosure, "(meth)acryloyl" means "acryloyl" and / or "methacryloyl," and "(meth)acrylate" means "acrylate" and / or "methacrylate." Furthermore, in this disclosure, "monomer" refers to a compound consisting of molecules that can be the smallest unit in a polymer obtained by polymerization and / or crosslinking reactions. In addition, "photopolymerizable monomer" refers to a monomer among photopolymerizable compounds that has one or more photopolymerizable groups.

[0032] ≪5-methyl-3-vinyloxazolidine-2-one≫ As described above, 5-methyl-3-vinyloxazolidine-2-one exhibits particularly superior curing properties compared to other N-vinyl compounds. Furthermore, 5-methyl-3-vinyloxazolidine-2-one is also superior in terms of safety and odor, making it an essential material in the inkjet ink of the present invention.

[0033] The content (A) of 5-methyl-3-vinyloxazolidine-2-one relative to the total amount of the inkjet ink of the present invention is 3 to 25% by mass. Furthermore, it is more preferable that A be 5 to 20% by mass, and particularly preferable that be 5 to 15% by mass. By setting the content of 5-methyl-3-vinyloxazolidine-2-one within the above range, it is possible to take advantage of the fast curing speed of 5-methyl-3-vinyloxazolidine-2-one while maintaining the wetting spread of the inkjet ink on the printing substrate, thereby obtaining an inkjet ink that suppresses the occurrence of white spots and / or pinholes in the ink film, while also having excellent curability of the ink film surface. In addition, it is possible to suppress the deterioration of the dispersion stability of the pigment due to 5-methyl-3-vinyloxazolidine-2-one, resulting in good color development of the ink film and good ejection stability of the inkjet ink.

[0034] <<Other monofunctional polymerizable compounds>> The inkjet ink of the present invention may contain monofunctional polymerizable compounds other than the 5-methyl-3-vinyloxazolidine-2-one described above. These monofunctional polymerizable compounds may include, for example, photopolymerizable monomers having one (meth)acryloyl group. Specific examples of the compound include 2-phenoxyethyl (meth)acrylate, benzyl (meth)acrylate, (ethoxylated (or propoxylated)) 2-phenoxyethyl (meth)acrylate, dicyclopentenyl (oxyethyl) (meth)acrylate, phenoxydiethylene glycol (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 o-phenylphenol acrylate, ethylene oxide-modified 2-ethylhexyl acrylate, β-carboxymethyl Examples include methyl(meth)acrylate, cyclic trimethylolpropaneformal(meth)acrylate, cyclohexyl(meth)acrylate, tetrahydrofurfuryl(meth)acrylate, isobornyl(meth)acrylate, norbornyl(meth)acrylate, dicyclopentanyl(meth)acrylate, stearyl(meth)acrylate, tridecyl(meth)acrylate, caprolactone(meth)acrylate, 2-hydroxy-3-phenoxypropyl(meth)acrylate, 1,4-cyclohexanedimethanol(meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, 4-hydroxybutylacrylate, acryloylmorpholine, and N-acryloyloxyethylhexahydrophthalimide.

[0035] Furthermore, the above monofunctional polymerizable compounds may also contain a photopolymerizable monomer having one vinyl group (excluding the (meth)acryloyl group) (excluding 5-methyl-3-vinyloxazolidine-2-one). Specific examples of such compounds include N-vinylpyrrolidone, N-vinylvalerolactam, N-vinylcaprolactam, N-vinylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide, and N-vinyloxazolidinone.

[0036] Furthermore, the above monofunctional polymerizable compounds may contain a photopolymerizable oligomer having one (meth)acryloyl group. Specific examples of such compounds include KRM9276 from Daicel Ornex, and CN131NS, CN131BNS, CN146NS, CN153NS, CN3108NS, CN7002NS, CN8004NS, and CN9003NS from Arkema.

[0037] In the ink of the present invention, the ratio of the content of monofunctional polymerizable compounds other than 5-methyl-3-vinyloxazolidine-2-one (C) relative to the total amount of inkjet ink, to the content of 5-methyl-3-vinyloxazolidine-2-one (A) relative to the total amount of inkjet ink, i.e., the value expressed as C / A, is 0 to 2.5, and may be 0.0 to 2.5. By keeping the value expressed as C / A within the above range, even in an ink film containing a certain amount of pigment, the amount of monofunctional polymerizable compounds remaining inside the ink film is reduced, resulting in good internal curing properties of the ink film and good ejection stability. From this viewpoint, the value expressed as C / A is preferably 0.0 to 1.0, and particularly preferably 0.0 to 0.5.

[0038] Generally, when a photopolymerizable compound contains both monofunctional polymerizable compounds and polyfunctional polymerizable compounds, the higher the proportion of the monofunctional polymerizable compound, the more flexible the ink film tends to be. On the other hand, a higher proportion of the polyfunctional polymerizable compound improves both surface curability and internal curability. From this viewpoint, in the ink of the present invention, the total amount of monofunctional polymerizable compounds, that is, the total amount (A+C) of the content of 5-methyl-3-vinyloxazolidine-2-one (A) and the content of monofunctional polymerizable compounds other than 5-methyl-3-vinyloxazolidine-2-one (C) relative to the total amount of inkjet ink, is preferably 2 to 65% by mass, more preferably 4 to 55% by mass, and particularly preferably 7 to 45% by mass.

[0039] ≪Radical polymerizable difunctional monomer represented by general formula (1)≫ The ink of the present invention contains a radically polymerizable difunctional monomer represented by the following general formula (1) as a polyfunctional polymerizable compound. General formula (1): CH2=CH-CO-OR 1 -O-CO-CH=CH2 [In general formula (1), R 1 This represents an alkylene group having 3 to 10 carbon atoms, which may have a branched structure. Furthermore, the content (B) of the radical polymerizable difunctional monomer represented by the general formula (1) above, relative to the total amount of inkjet ink, is 7 to 65% by mass. Because the radical polymerizable difunctional monomer represented by the general formula (1) has low surface tension, inkjet inks containing the radical polymerizable difunctional monomer represented by the general formula (1) exhibit excellent wetting and spreading properties on the printing substrate, improving solid coverage in the ink film. In addition, among photopolymerizable compounds that can be used in inkjet inks, the radical polymerizable difunctional monomer represented by the general formula (1) has relatively low viscosity and molecular weight, and its viscoelasticity is optimized due to intermolecular interactions, thus improving the curability and ejection stability of the inkjet ink. Furthermore, the deterioration of the dispersion stability of pigments in the inkjet ink can be suppressed, improving the color development of the ink film. Thus, by keeping the content of the radical polymerizable difunctional monomer represented by the general formula (1) above within the above range, improvements in solid coverage and color development of printed materials, as well as improvements in the ejection stability and curability of the inkjet ink, can be achieved.

[0040] In the ink of the present invention, from the viewpoint of improving the solid filling of the ink film and improving the ink discharge stability and curability, the content (B) of the radical polymerizable difunctional monomer represented by the above general formula (1) relative to the total amount of inkjet ink is particularly preferably 20 to 65% by mass.

[0041] Examples of radically polymerizable difunctional monomers represented by general formula (1) include 1,3-propanediol diacrylate, 1,3-butylenediol diacrylate, 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, 1,8-octanediol diacrylate, 1,9-nonanediol diacrylate, 1,10-decanediol diacrylate, neopentyl glycol diacrylate, 3-methyl-1,5-pentanediol diacrylate, 2,4-dimethyl-1,5-pentanediol diacrylate, 2-ethyl-2-butyl-1,3-propanediol diacrylate, and 2-ethyl-2-butyl-1,3-butanediol diacrylate. From the viewpoint of improving curability, all polymerizable groups contained in the radically polymerizable difunctional monomers represented by general formula (1) are acryloyl groups.

[0042] In the ink of the present invention, it is preferable to include one or more compounds selected from the group consisting of 1,4-butanediol diacrylate, 3-methyl-1,5-pentanediol diacrylate, and 1,6-hexanediol diacrylate. These compounds have an excellent balance between viscosity and surface tension, and exhibit excellent effects in improving ejection stability, while also improving the wettability and spreadability of the inkjet ink on the printing substrate and improving the solid filling of the ink film. Furthermore, the compounds listed above are also effective in improving the curability inside the ink film.

[0043] In particular, the ink of the present invention preferably contains a radically polymerizable difunctional monomer represented by general formula (1) that comprises 3-methyl-1,5-pentanediol diacrylate and 1,4-butanediol diacrylate and / or 1,6-hexanediol diacrylate. Although the detailed mechanism is unknown, by using in combination an alkanediol diacrylate consisting of a branched alkylene group and an alkanediol diacrylate consisting of a non-branched alkylene group, it becomes possible to obtain an ink with excellent discharge stability and curing properties, as well as printed materials with excellent solid filling and good print quality. In one example, the radically polymerizable difunctional monomer represented by general formula (1) comprises 3-methyl-1,5-pentanediol diacrylate and 1,4-butanediol diacrylate, but does not contain 1,6-hexanediol diacrylate. In another example, the radical polymerizable difunctional monomer represented by general formula (1) includes 3-methyl-1,5-pentanediol diacrylate and 1,6-hexanediol diacrylate, but does not include 1,4-butanediol diacrylate. In another example, the radical polymerizable difunctional monomer represented by general formula (1) includes 3-methyl-1,5-pentanediol diacrylate, 1,4-butanediol diacrylate, and 1,6-hexanediol diacrylate. From the viewpoint of obtaining the above-mentioned good effects, the content of radical polymerizable difunctional monomers other than 3-methyl-1,5-pentanediol diacrylate, 1,4-butanediol diacrylate, and 1,6-hexanediol diacrylate in the radical polymerizable difunctional monomer represented by general formula (1) is preferably 0 to 10% by mass, more preferably 0 to 5% by mass, and particularly preferably 0 to 1% by mass, relative to the total amount of the radical polymerizable difunctional monomer represented by general formula (1).

[0044] When using both an alkanediol diacrylate consisting of a branched alkylene group and an alkanediol diacrylate consisting of a non-branched alkylene group as radical polymerizable difunctional monomers represented by general formula (1), the WB / WN ratio is preferably 1.0 to 18.0, and particularly preferably 1.5 to 9.0, when WB / WN is the content of the alkanediol diacrylate consisting of a branched alkylene group relative to the total amount of inkjet ink, and WN is the content of the alkanediol diacrylate consisting of a non-branched alkylene group relative to the total amount of inkjet ink. By keeping the WB / WN ratio within the above range, it becomes easy to obtain an inkjet ink with excellent ejection stability, curability, and print quality of printed materials.

[0045] Furthermore, when 3-methyl-1,5-pentanediol diacrylate and 1,4-butanediol diacrylate and / or 1,6-hexanediol diacrylate are used in combination as radical polymerizable difunctional monomers represented by general formula (1), the WMP (mass%) is the content of 3-methyl-1,5-pentanediol diacrylate relative to the total amount of inkjet ink, and the WBH (mass%) is the total content of 1,4-butanediol diacrylate and 1,6-hexanediol diacrylate (either of which may be 0% by mass). In this case, the WMP / WBH value is preferably 1.0 to 18.0, and particularly preferably 1.5 to 9.0. By keeping the WMP / WBH value within the above range, it becomes easy to obtain an inkjet ink with excellent ejection stability, curability, and print quality of printed materials.

[0046] As described above, in the ink of the present invention, when B is the content of the radical polymerizable difunctional monomer represented by general formula (1) and C is the content of monofunctional polymerizable compounds other than 5-methyl-3-vinyloxazolidine-2-one relative to the total amount of inkjet ink, the total amount (B+C) is 7 to 65% by mass of the total amount of inkjet ink.

[0047] Furthermore, in the ink of the present invention, when the content of 5-methyl-3-vinyloxazolidine-2-one relative to the total amount of inkjet ink is A, and the content of the radical polymerizable difunctional monomer represented by the general formula (1) is B, the value of the ratio expressed as B / A is preferably 1.8 to 15.0, and particularly preferably 8.0 to 13.0. By keeping the value expressed as B / A within the above range, the unreacted portion of the photopolymerizable compound can be reduced both on the surface and inside the ink film, resulting in good surface and internal curing properties of the inkjet ink, even when printing at high speeds.

[0048] <<Other polyfunctional polymerizable compounds>> The ink of the present invention may contain polyfunctional polymerizable compounds other than the radically polymerizable difunctional monomer represented by general formula (1) (also referred to in this disclosure as "other polyfunctional polymerizable compounds"), and two or more compounds may be used in combination as such other polyfunctional polymerizable compounds. The other polyfunctional polymerizable compounds may include, for example, a photopolymerizable monomer having two (meth)acryloyl groups. Specific examples of the compound include 1,3-propanediol dimethacrylate, 1,3-butylenediol dimethacrylate, 1,4-butanediol dimethacrylate, 1,6-hexanediol dimethacrylate, ethylene oxide-modified 1,6-hexanediol di(meth)acrylate, propylene oxide-modified 1,6-hexanediol di(meth)acrylate, 1,8-octanediol dimethacrylate, 1,9-nonanediol dimethacrylate, neopentyl glycol dimethacrylate, ethylene oxide-modified neopentyl glycol di(meth)acrylate, propylene oxide-modified neopentyl glycol di(meth)acrylate, 3-methyl-1,5-pentanediol dimethacrylate, 2,4-dimethyl-1,5-Pentanediol dimethacrylate, 2-ethyl-2-butylpropanediol dimethacrylate, 2-ethyl-2-butylbutanediol dimethacrylate, 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, hydroxypivalate neopentyl glycol di(meth)acrylate, bisphenol A di(meth)acrylate, ethylene oxide-modified bisphenol A di(meth)acrylate, propylene oxide-modified bisphenol Examples include bisphenol 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, dimethylol tricyclodecane di(meth)acrylate, cyclohexanedimethanol di(meth)acrylate, trimethylolpropane di(meth)acrylate, neopentyl glycol-modified trimethylolpropane di(meth)acrylate, and dicyclopentanyl di(meth)acrylate.

[0049] In particular, from the viewpoint of achieving both curability on the surface of the ink film and curability inside the ink film, as well as improving discharge stability, it is preferable that the other polyfunctional polymerizable compound contains one or more compounds selected from the group consisting of dipropylene glycol di(meth)acrylate and tripropylene glycol di(meth)acrylate. In that case, from the viewpoint of the above-mentioned viewpoint, namely the improvement of curability and discharge stability, the total amount of dipropylene glycol di(meth)acrylate and tripropylene glycol di(meth)acrylate (either one may be 0% by mass) is preferably 1 to 55% by mass, and particularly preferably 10 to 30% by mass, of the total amount of the photopolymerizable compound.

[0050] On the other hand, other polyfunctional polymerizable compounds may also contain photopolymerizable monomers having three (meth)acryloyl groups. Specific examples of such photopolymerizable monomers 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 Examples include isocyanuric acid tri(meth)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, hydroxypivalaldehyde-modified dimethylolpropane tri(meth)acrylate, sorbitol tri(meth)acrylate, and the like.

[0051] Of the compounds listed above, from the viewpoint of maintaining discharge stability while also improving internal curability, it is preferable that the other polyfunctional polymerizable compound contains glycerin triacrylate and / or ethylene oxide-modified trimethylolpropane triacrylate (with 3 or fewer ethylene oxide groups). In that case, from the viewpoint of the above-mentioned viewpoint, namely internal curability and discharge stability, the total amount of glycerin triacrylate and ethylene oxide-modified trimethylolpropane triacrylate (with 3 or fewer ethylene oxide groups) (either one may be 0% by mass) is preferably 1 to 15% by mass, and particularly preferably 2 to 12% by mass, of the total amount of the photopolymerizable compound.

[0052] Furthermore, other polyfunctional polymerizable compounds may also contain photopolymerizable monomers having four (meth)acryloyl groups. Specific examples of such photopolymerizable monomers 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.

[0053] Of the compounds listed above, from the viewpoint of improving internal curability and discharge stability, it is preferable that the other polyfunctional polymerizable compound contains ditrimethylolpropanetetraacrylate. In that case, from the viewpoint of the above-mentioned viewpoint, namely internal curability and discharge stability, the content of ditrimethylolpropanetetraacrylate is preferably 1 to 12% by mass, and particularly preferably 1 to 10% by mass, of the total amount in the photopolymerizable compound.

[0054] Furthermore, when using a photopolymerizable monomer having five (meth)acryloyl groups as another polyfunctional polymerizable compound, specific examples of such photopolymerizable monomers include sorbitol penta(meth)acrylate and dipentaerythritol penta(meth)acrylate.

[0055] Furthermore, when using a photopolymerizable monomer having six (meth)acryloyl groups as another polyfunctional polymerizable compound, specific examples of such photopolymerizable monomers include dipentaerythritol hexa(meth)acrylate, sorbitol hexa(meth)acrylate, alkylene oxide-modified phosphazene hexa(meth)acrylate, and ε-captolactone-modified dipentaerythritol hexa(meth)acrylate.

[0056] Furthermore, the photopolymerizable monomers included as other polyfunctional polymerizable compounds may be modified versions of the radical polymerizable difunctional monomer represented by the above general formula (1), and the other polyfunctional polymerizable compounds listed above. Examples of such modification include sulfonic acid modification, phosphoric acid modification, amine modification, mercapto modification, etc. For example, a compound obtained by reacting (Michael addition reaction) an amine compound with one or more compounds selected from the group consisting of 1,6-hexanediol diacrylate, ethylene oxide-modified 1,6-hexanediol diacrylate, propylene oxide-modified 1,6-hexanediol diacrylate, trimethylolpropane triacrylate, ethylene oxide-modified trimethylolpropane triacrylate, propylene oxide-modified trimethylolpropane triacrylate, glycerin triacrylate, ethylene oxide-modified glycerin triacrylate, and propylene oxide-modified glycerin triacrylate (provided that it has one or more acryloyl groups) can be used as other polyfunctional polymerizable compounds. These compounds can simultaneously improve discharge stability, the curability of the ink film surface, and the curability of the ink film interior, and are therefore suitable for use in the ink of the present invention.

[0057] Furthermore, other polyfunctional polymerizable compounds may also contain photopolymerizable oligomers. In this case, the photopolymerizable oligomer may preferably contain a compound having a (meth)acryloyl group as a polymerizable group. The number of polymerizable groups contained in the polymerizable oligomer is preferably 2 to 6 per molecule from the viewpoint of balancing curability, discharge stability, and dispersion stability. The number of polymerizable groups is more preferably 2 to 4, and particularly preferably 2. The mass-average molecular weight of the polymerizable oligomer is preferably 400 to 12,000, and more preferably 500 to 10,000.

[0058] The mass-average molecular weight in this disclosure is a value calculated using a calibration curve for standard polystyrene, measured by gel permeation chromatography (GPC). The HLC-8320GPC from Tosoh Corporation can be used as the gel permeation chromatography analyzer, the TSKgel® SuperMultiporeHZ-N from Tosoh Corporation can be used as the analytical column, and tetrahydrofuran can be used as the developing solvent.

[0059] In this disclosure, "oligomer" refers to a compound obtained by polymerizing a small number of monomers in a finite number (e.g., 2 to 20). "Photopolymerizable oligomer" refers to an oligomer among photopolymerizable compounds that has one or more photopolymerizable groups. However, monomers used to synthesize a photopolymerizable oligomer may include monomers that are not photopolymerizable monomers (except monomers used to impart photopolymerizable groups). For example, a photopolymerizable urethane oligomer obtained by reacting a urethane oligomer having an isocyanate group at its terminus, which is a reaction product of 1,6-hexanediol and isophorone diisocyanate, with 2-hydroxyethyl acrylate contains 1,6-hexanediol and isophorone diisocyanate as monomers.

[0060] Examples of photopolymerizable oligomers having the above-mentioned (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; epoxy (meth)acrylate oligomers, etc. The above-mentioned oligomers may be modified. Examples of such modification include sulfonic acid modification, phosphoric acid modification, amine modification, mercapto modification, etc.

[0061] If the ink of the present invention contains other polyfunctional polymerizable compounds, the content thereof is preferably 1 to 50% by mass of the total ink amount, and more preferably 3 to 35% by mass. By setting the content within the above range, the surface curability, internal curability, and discharge stability can all be easily improved.

[0062] <Pigments> The ink of the present invention contains a pigment in order to improve the color development of printed materials (opacity in the case of white ink, and gloss in the case of metallic ink). As described above, pigments absorb, reflect, and scatter ultraviolet light, which reduces the efficiency of radical generation from the photopolymerization initiator. In particular, when a UV-LED is used as an ultraviolet light source, the wavelength range of the emitted ultraviolet light is narrow, which may further exacerbate the decrease in the radical generation efficiency described above. Therefore, in the embodiments of the present invention, by using the above-mentioned photopolymerizable monomers in a specific content and content ratio, and by specifying the pigment content (D) in the total amount of inkjet ink to be 2.2 to 7% by mass, it is possible to maintain good ejection stability and the color development of the ink film without worsening the curability inside the ink film. The pigment content (D) may be 2.2 to 7.0% by mass, 2.4 to 5.0% by mass, or 2.5 to 4.0% by mass.

[0063] The pigments contained in the ink of the present invention are not particularly limited, but include, for example, organic pigments and inorganic pigments represented by the following color index names. For example, as red pigments, there are CI 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, 255, 264, 266, 269, 282, etc.; as violet pigments, there is CI Pigment Violet 19, etc.; as orange pigments, there are CI Pigment Orange 5, 13, 34, 38, 43, 61, 62, 64, etc.; as blue pigments, there are CI Pigment Blue 1, 2, 3, 15:3, 15:4, 15:34, 16, 22, 60, CI Bat Blue -4, 60 etc; as green pigments, CI Pigment Green 7, 26, 36, 50, 58 etc; as yellow pigments, CI 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; as black pigments, CI Pigment Black 1, 7 etc; and as white pigments, CI Pigment White 6, 18, 21 etc; and as metallic pigments, CI Pigment Metal 1, 2 etc. may be used as desired, depending on the color reproducibility and color development. Two or more of the above-listed pigments may be used in combination.

[0064] The average particle size (D50) of the above pigment is preferably 50 to 500 nm, and more preferably 100 to 400 nm. Ink containing pigments with a D50 within this range exhibits excellent color development, opacity, or glossiness, as well as superior storage stability and discharge stability. Furthermore, it becomes easier to suppress deterioration of surface hardening and internal hardening properties. Note that D50 represents the median diameter on a volume basis and can be measured using a dynamic light scattering particle size distribution analyzer (for example, Microtrac-Bell's "Nanotrac UPA-EX150") after diluting the ink 200 to 1000 times with ethyl acetate or the like.

[0065] The pigment content (D) relative to the total amount of inkjet ink is preferably 2.2 to 5.5% by mass. By keeping the content within this range, the storage stability and ejection stability of the ink are further improved, making it possible to stably produce high-quality printed materials even when printing at high speeds. In particular, by setting the pigment content (D) to 2.5 to 5.5% by mass, it becomes easier to make the color development of the printed material better than that of printed materials produced by conventional printing methods such as offset printing and gravure printing.

[0066] Furthermore, depending on the required application and image quality, the ink of the present invention can also be made into a light-colored ink with a low pigment content (for example, light yellow, light magenta, light cyan, or light black). In this case, the pigments listed above can be included as colorants in the light-colored ink.

[0067] Furthermore, in the present invention, it is preferable that the ratio (D / A) of the pigment content (D) to the 5-methyl-3-vinyloxazolidine-2-one content (A) relative to the total amount of inkjet ink is 0.10 to 1.50. By keeping the value expressed as D / A within the above range, the ink ejection stability is improved, and the color development of the ink film becomes sufficiently practical for printed materials. Moreover, the internal curing properties are improved as the curing progresses sufficiently to the interior of the ink film.

[0068] <Pigment-dispersed resin> In the present invention, it is preferable that the ink contains a pigment dispersion resin, as this improves the dispersion stability of the pigment, as well as the storage stability and ejection stability of the ink, and furthermore, it improves the color development of the printed material by making the pigment uniform within the ink film. The pigment dispersion resin can be a commercially available product or one synthesized by a conventionally known method. Specific examples of commercially available products include: Ajinomoto Fine Techno's "Ajisper PB-821", "Ajisper PB-822", "Ajisper PB-824", "Ajisper PB-881"; Bic Chemie's "DISPERBYK-162", "DISPERBYK-163", "DISPERBYK-168", "DISPERBYK-182", "DISPERBYK-184", "DISPERBYK-185", "DISPERBYK-2013", "DISPERBYK-2155", "BYKJET-9150", "BYKJET-9151", "BYKJET-9152"; Lubrizol's "Solspers 24000", "Solspers 32000", "Solspers 33000", "Solspers 39000", "Solspers J180", "Solspers J200"; and BASF's "EFKA Examples include "PX4701," "EFKA PX4703," and "EFKA PX4733." Furthermore, resins obtained by polymerizing acid group-containing polymerizable compounds such as acrylic acid and methacrylic acid, amino group-containing polymerizable compounds such as acrylamide, dimethylaminoethyl methacrylate, and diethylaminoethyl methacrylate, and other polymerizable compounds such as styrene, α-methylstyrene, methyl methacrylate, butyl methacrylate, and lauryl methacrylate may be used as pigment dispersion resins.

[0069] The mass-average molecular weight of the above-mentioned pigment dispersion resin is preferably 5,000 to 100,000, more preferably 10,000 to 50,000, and even more preferably 15,000 to 30,000. Within this range, the compatibility of the pigment dispersion resin with the radically polymerizable difunctional monomer represented by general formula (1) is good, improving the storage stability and discharge stability of the ink. Furthermore, it becomes easier to homogenize the above-mentioned pigment within the ink film, improving the abrasion resistance and color development of the printed material.

[0070] The amine value of the pigment dispersion resin is preferably 11 to 50 mg KOH / g, more preferably 15 to 45 mg KOH / g, and particularly preferably 18 to 40 mg KOH / g. A pigment dispersion resin having an amine value within the above range has a sufficient number of adsorption sites and can strongly adsorb to the pigment. As a result, the dispersion stability of the pigment is improved, and the ejection stability of the inkjet ink is also improved. Furthermore, since the adsorption sites (basic groups) do not inhibit the polymerization reaction of the photopolymerizable compound, the surface curability and internal curability of the inkjet ink are also improved.

[0071] The "amine value" of a pigment-dispersing resin is the amount of potassium hydroxide (in mg) equivalent to the amount of acid required to neutralize 1 g of the resin. As an example of a method for measuring the amine value, the resin in question is dissolved in a solvent mixture of ethanol or tetrahydrofuran and acetic acid, and then titrated using a 0.1 mol / L perchloric acid-acetic acid solution by potentiometric titration. The amine value can then be calculated by converting the titration volume read from the resulting titration curve to the amount of potassium hydroxide (in mg).

[0072] From the viewpoint of improving the ejection stability of the inkjet ink of the present invention, it is preferable that the acid value of the pigment dispersion resin is 30 mg KOH / g or less (it may also be 0 mg KOH / g). Furthermore, it is particularly preferable that the acid value of the pigment dispersion resin is 20 mg KOH / g or less (it may also be 0 mg KOH / g) from the viewpoint that the pigment dispersion resin can be stably present in the inkjet ink, further improving the ejection stability, and also improving the internal curing properties of the inkjet ink.

[0073] The "acid value" of a pigment dispersion resin is the number of milligrams of potassium hydroxide required to neutralize 1 gram of the resin. This value can be determined by potentiometric titration, in accordance with JIS K 0070:1992. As a specific example of the measurement method, the target resin is dissolved in a solvent consisting of diethyl ether and ethanol in a 1:1 mass ratio. Then, the solution is titrated using a 0.1 mol / L potassium hydroxide-ethanol solution by potentiometric titration. The acid value can then be calculated using the titration volume read from the resulting titration curve.

[0074] Except for white inks, the amount of pigment dispersion resin added is preferably 20 to 120% by mass, and more preferably 30 to 80% by mass, relative to the total amount of pigment. In the case of white inks, the amount of pigment dispersion resin added is preferably 1 to 100% by mass, and more preferably 3 to 50% by mass, relative to the total amount of pigment. By using the above content ranges, a pigment dispersion with excellent dispersion stability and storage stability can be obtained.

[0075] <Photopolymerization initiator> The inkjet ink of the present invention preferably contains a photopolymerization initiator. A photopolymerization initiator is a compound that serves as the starting point for polymerization and crosslinking reactions of a photopolymerizable compound. That is, the photopolymerization initiator absorbs energy from active energy rays, undergoes intramolecular cleavage, or is excited and further extracts hydrogen atoms from a hydrogen donor, thereby generating radicals. In this disclosure, "polymerizable initiator" also includes compounds that can function as the above-mentioned hydrogen donor, which are generally referred to as sensitizers.

[0076] As the photopolymerization initiator, one or more conventionally known compounds can be arbitrarily used. For example, compounds that can absorb ultraviolet energy and generate radicals can be used. Specifically, acylphosphine oxide compounds, benzophenone compounds, indan compounds, thioxanthone compounds, hydroxyacetophenone compounds, alkylaminoacetophenone compounds, oxime ester compounds, aminobenzoate compounds, ketocoumarin compounds, anthracene compounds, etc., can be used. Of these, aminobenzoate compounds, ketocoumarin compounds, and anthracene compounds are generally classified as sensitizers.

[0077] Among these photopolymerization initiators, it is preferable that the photopolymerization initiator contains a thioxanthone compound, as this allows for improved surface and internal curability while maintaining optimal discharge stability.

[0078] ≪Thioxanthone compounds≫ Specific examples of the thioxanthone compounds mentioned above 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. Furthermore, examples of commercially available thioxanthone compounds include "Omnirad ITX," "Omnirad DETX," "OMNIPOL TX," "OMNIPOL 3TX," and "OMNIPOL BL 728" from IGM RESINS, "SPEEDCURE ITX," "SPEEDCURE 2-ITX," "SPEEDCURE DETX," "SPEEDCURE LTX," "SPEEDCURE CPTX," and "SPEEDCURE 7010" from Lambson, and "Genopol TX-2" from RAHN. Of these commercially available products, "OMNIPOL TX," "SPEEDCURE 7010," and "Genopol TX-2" are the above-mentioned polymers. The inkjet ink of the present invention may contain only one of the thioxanthone compounds listed above, or it may contain two or more of them.

[0079] When the inkjet ink of the present invention contains a thioxanthone compound, from the viewpoint of obtaining an inkjet ink that is excellent in all aspects of curability, ejection stability, and pigment dispersion stability, the content of the thioxanthone compound (E1) relative to the total amount of inkjet ink is preferably 0.6 to 3% by mass, and particularly preferably 0.8 to 2.5% by mass. By keeping the content of the thioxanthone compound within the above range, it becomes easier to achieve both curability on the surface of the ink film and curability inside the ink film.

[0080] ≪Acylphosphine oxide compounds≫ In the inkjet ink of the present invention, it is particularly preferable that the photopolymerization initiator contains an acylphosphine oxide compound in addition to the thioxanthone compound, since both ejection stability and surface and internal curability are at desirable levels. Specific examples of the acylphosphine oxide initiator 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. Examples of commercially available acylphosphine oxide compounds include "Omnirad TPO," "Omnirad TPO-L," "Omnirad TPO-H," "Omnirad 819," and "OMNIPOL TP" from IGM RESINS, and "Speedcure TPO," "Speedcure TPO-L," and "Speedcure BPO" from Lambson. For example, acylphosphine oxide compounds described in International Publication No. 2017 / 086224 and International Publication No. 2020 / 049378, as well as lithium phenyl(2,4,6-trimethylbenzoyl)phosphinate, can also be used. The inkjet ink of the present invention may contain only one of the above-listed acylphosphine oxide compounds, or two or more.

[0081] Among these compounds, the present invention prefers to contain ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide. Ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide is a liquid at room temperature and is an effective material for improving the curability of the ink film surface while maintaining discharge stability. Although the detailed mechanism is unknown, it is thought that when ink droplets land on a printing substrate, ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide quickly orients on the surface of the ink droplets. Therefore, ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide is considered particularly effective in improving the curability of the ink film surface. Furthermore, acylphosphine oxide-based photopolymerization initiators generally exhibit a photobleaching effect. That is, after generating radicals and decomposing, the acylphosphine oxide-based photopolymerization initiator loses its ultraviolet absorption ability, and the ultraviolet transmittance into the ink film improves. As mentioned above, ethoxyphenyl (2,4,6-trimethylbenzoyl)phosphine oxide is thought to easily orient itself on the droplet surface, and as the polymerization reaction progresses, a photobleaching effect is expected to occur, making it easier for ultraviolet light to penetrate into the ink film.

[0082] Furthermore, if the above-mentioned photopolymerization initiator contains ethoxyphenyl (2,4,6-trimethylbenzoyl)phosphine oxide, its content is preferably 2 to 20% by mass, and most preferably 5 to 15% by mass, in the UV-curable inkjet ink. By setting it within this range, it is possible to achieve both ink ejection stability and curability of the ink film surface.

[0083] Furthermore, when the content of the acylphosphine oxide initiator relative to the total amount of the inkjet ink of the present invention is E2 and the content of 5-methyl-3-vinyloxazolidine-2-one is A, it is preferable that the product of the two, i.e., the value expressed as A (mass%) × E2 (mass%), is between 57.5 and 300. The acylphosphine oxide initiator has a high affinity for 5-methyl-3-vinyloxazolidine-2-one, and by setting the value expressed as A × E2 within the above range, it becomes easier to achieve both curability on the surface of the ink film and curability inside the ink film.

[0084] In one embodiment, when the ink of the present invention contains a monoacylphosphine oxide initiator, its content is preferably 45 to 85% by mass, and particularly preferably 60 to 75% by mass, of the total amount of photopolymerization initiator contained in the ink. The monoacylphosphine oxide initiator has high solubility not only for 5-methyl-3-vinyloxazolidine-2-one but also for the radically polymerizable difunctional monomer represented by the general formula (1) described above. On the other hand, the monoacylphosphine oxide initiator also has excellent radical generation efficiency. Therefore, by setting the content of the monoacylphosphine oxide initiator within the above range, it becomes easy to obtain an ink that is excellent in all aspects: curability on the surface of the ink film, curability inside the ink film, and discharge stability.

[0085] Examples of the above-mentioned monoacylphosphine oxide initiators include ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide.

[0086] As described above, the inkjet ink of the present invention preferably contains an acylphosphine oxide compound and a thioxanthone compound. In this case, when the content of the thioxanthone compound relative to the total amount of inkjet ink is E1 and the content of the acylphosphine oxide initiator is E2, the value expressed as E2 / E1 is preferably 1 to 25, more preferably 2 to 20, particularly preferably 2 to 12, and particularly preferably 2.5 to 12. Furthermore, from the viewpoint of improving not only curability and discharge stability but also solid filling, when the content of 5-methyl-3-vinyloxazolidine-2-one relative to the total amount of inkjet ink is A, the value expressed as E2 / (A+E1) is preferably 0.20 to 2.00, and particularly preferably 0.40 to 1.70.

[0087] From the viewpoint of maintaining favorable ejection stability even under high-frequency ejection conditions without excessively increasing the viscosity of the ink, and also improving the curability of the surface and interior of the ink film, it is preferable that the ink contains 5 to 15% by mass of a photopolymerization initiator in the ultraviolet-curable inkjet ink.

[0088] <Other ingredients> In addition to the components described above, the ink of the present invention may optionally contain a surface tension modifier, a polymerization inhibitor, an organic solvent, water, and other additives.

[0089] Surface tension modifier The ink of the present invention preferably contains a surface tension modifier, from the viewpoint of improving the wetting and spreading properties of inkjet ink droplets on a printing substrate and enabling the production of printed materials with good solid coverage. In the ink of the present invention, silicone-based surface tension modifiers, fluorine-based surface tension modifiers, acetylene glycol-based surface tension modifiers, acetylene monool-based surface modifiers, etc., can be used as the surface tension modifier. Among these, it is preferable to contain a silicone-based surface tension modifier, and particularly preferable to contain a polyether-modified silicone-based surface tension modifier, because it has excellent surface tension reduction ability, easily prevents white spots and pinholes, has good compatibility with radical polymerizable difunctional monomers represented by general formula (1), and can improve the wetting and spreading of the ink in a short time without impairing the ink discharge stability. Furthermore, the surface tension modifier is particularly preferable to contain a polyether-modified silicone surface tension modifier having a (meth)acryloyl group, given that the curability of the ink and the abrasion resistance of the printed material are significantly improved, as well as the compatibility with photopolymerizable compounds containing radical polymerizable difunctional monomers represented by general formula (1) is improved and the discharge stability is also improved. The above-mentioned polyether-modified silicone surface tension modifier having a (meth)acryloyl group may be one synthesized by conventionally known methods or a commercially available product. Examples of commercially available products include BYK-UV3500, BYK-UV3505, BYK-UV3530, BYK-UV3570, BYK-UV3575, BYK-UV3576 (all manufactured by Bic Chemie Co., Ltd.), KF-2012, X-22-164, X-22-164AE, X-22-164A, X-22-164B, X-22-164C, X-22-164E, X-22-174ASX, X-22-174BX, X-22-2426, X-22-2404 (all manufactured by Shin-Etsu Silicone Co., Ltd.), TEGO Rad 2100, TEGO Rad 2200N, TEGO Rad 2250, TEGO Rad 2300, TEGO Rad 2330, TEGO Rad Examples include the 2500, TEGO Rad 2550, TEGO Rad 2650, TEGO Rad 2700, and TEGO Rad 2800 (all manufactured by Evonik).

[0090] When the ink of the present invention contains a polyether-modified silicone surface tension modifier having a (meth)acryloyl group, its content (F) relative to the total amount of inkjet ink is preferably 0.05 to 5% by mass, more preferably 0.05 to 3% by mass, and particularly preferably 0.8 to 3% by mass. The content (F) may be 0.05 to 5.0% by mass, 0.05 to 3.0% by mass, or 0.8 to 3.0% by mass. Adding 0.05% by mass or more easily improves the wetting spread of ink droplets on the printing substrate, resulting in an ink film without white spots or pinholes even during high-speed printing, as well as improving ejection stability and internal curability of the ink. Furthermore, by limiting the amount added to 5% by mass or less, deterioration of ejection stability can be suppressed, and surface curability can be easily ensured without inhibiting the curing of 5-methyl-3-vinyloxazolidine-2-one and the radical polymerizable difunctional monomer represented by general formula (1) on the ink surface.

[0091] Furthermore, when the ink of the present invention contains a polyether-modified silicone surface tension modifier having a (meth)acryloyl group, compatibility with the radically polymerizable difunctional monomer represented by general formula (1) is improved, both curability and ejection stability are improved, and the color development of printed materials is improved, reducing white spots and pinholes. Therefore, when the content of the polyether-modified silicone surface tension modifier having a (meth)acryloyl group relative to the total amount of inkjet ink is F, and the content of the radically polymerizable difunctional monomer represented by general formula (1) is B, the ratio of the two, expressed as B / F, is preferably 5.0 to 200.0, and particularly preferably 10.0 to 150.0.

[0092] Polymerization inhibitors Polymerization inhibitors can be added to the ink of the present invention because they improve discharge stability and balance surface hardening and internal hardening, thereby improving overall hardening performance. Specifically, examples include hindered phenol compounds, phenol compounds (excluding hindered phenol compounds), hydroquinone compounds, phenothiazine compounds, phosphorus compounds, and nitrosophenylhydroxylamine compounds, and these compounds can be suitably used as polymerization inhibitors. More specifically, examples of compounds that can be used as polymerization inhibitors include 4-methoxyphenol, t-butylhydroquinone, 2,6-di-t-butyl-4-methylphenol, pentaerythritol tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], hydroquinone, methylhydroquinone, phenothiazine, dicumylphenothiazine, triphenylphosphine, and aluminum salts of N-nitrosophenylhydroxylamine. In order to allow the effects of the polymerization inhibitor to be suitably expressed without hindering the effects of the present invention as described above, when the ink of the present invention contains a polymerization inhibitor, the content thereof is preferably 0.01 to 2% by mass, and more preferably 0.1 to 1% by mass, relative to the total amount of the ink.

[0093] Organic solvents, water The ink of the present invention may contain an organic solvent and / or water, as this improves discharge stability through optimization of the ink's viscosity and surface tension, and improves the wetting spread of the ink droplets onto the printing substrate, enabling the production of printed materials with good solid coverage. When the ink of the present invention contains an organic solvent and / or water, the content is preferably 0.01 to 30% by mass, more preferably 0.05 to 20% by mass, and particularly preferably 0.1 to 10% by mass, based on the total amount of ink. Furthermore, when the ink of the present invention contains an organic solvent, it is preferable to use an organic solvent with a boiling point of 140 to 300°C at 1 atmosphere, and more preferably an organic solvent with a boiling point of 150 to 235°C at 1 atmosphere, as this improves both discharge stability and wetting spread on the printing substrate.

[0094] Furthermore, from the viewpoint of suitably improving all of the above-mentioned effects, namely ejection stability, wetting spread on the printing substrate, and reduction of image defects, it is preferable that the organic solvent having a boiling point of 140 to 300°C at 1 atmosphere contains at least one selected from the group consisting of alkylene glycol monoalkyl ethers, alkylene glycol dialkyl ethers, and alkylene glycol monoalkyl ether acetates. Among these, at least one selected from the group consisting of dipropylene glycol monomethyl ether, dipropylene glycol monopropyl ether, dipropylene glycol monobutyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, ethylene glycol monobutyl ether acetate, diethylene glycol monoethyl ether acetate, diethylene glycol methyl ethyl ether, diethylene glycol diethyl ether, and triethylene glycol dimethyl ether is preferably included, and at least one selected from the group consisting of diethylene glycol monoethyl ether, ethylene glycol monobutyl ether acetate, diethylene glycol methyl ethyl ether, and diethylene glycol diethyl ether is particularly preferably included.

[0095] Inert Resin The inkjet ink of the present invention can provide adhesion to various printing substrates and may contain an inert resin to adjust the viscoelasticity of the ink and improve ejection stability. The inert resin can contain (meth)acrylic resin, urethane resin, vinyl chloride-vinyl acetate copolymer resin, ketone resin, etc. Among these, it is preferable to include a (meth)acrylic resin and / or a ketone resin as the inert resin, from the viewpoint of improving both adhesion and ejection stability.

[0096] When the inkjet ink of the present invention contains an inert resin, its content is preferably 0.1 to 10% by mass, more preferably 0.5 to 5% by mass, and particularly preferably 1 to 3% by mass, based on the total mass of the inkjet ink. By adjusting the above content within the above range, adhesion and discharge stability can be easily improved without worsening curability.

[0097] In this disclosure, "inert resin" refers to a resin that does not participate in polymerization reactions, contributes to adhesion to a printing substrate, and has solubility in inkjet inks.

[0098] <<Other additives>> In addition to the components described above, the ink of the present invention may optionally contain ultraviolet absorbers, fade inhibitors, other polymer compounds, etc. These components can be any conventionally known components.

[0099] ≪Physical Properties of Inkjet Inks≫ The inkjet ink of the present invention preferably has a viscosity of 5 to 20 mPa·s at 25°C, and more preferably 8 to 20 mPa·s, from the viewpoint of improving ejection stability, improving the wetting and spreading properties of the inkjet ink droplets on the printing substrate, and improving the adhesion of the printed material. If the viscosity is 5 mPa·s or higher, the inkjet ink can be ejected well from the inkjet head. If the viscosity is 25 mPa·s or lower, it is possible to continue ejecting stably without a decrease in ejection accuracy, and the ink spreads suitably on the printing substrate, improving solid coverage. Furthermore, from the viewpoint of enabling stable ejection even at high-speed printing, the viscosity is particularly preferably 8 to 15 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 with a cone angle of 1°34' (diameter 48 mm), under conditions of a 25°C environment and a rotation speed of 20 rpm.

[0100] Furthermore, from the viewpoint of improving ejection stability and the curability of printed materials, 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 above static surface tension includes values ​​measured by the plate method (Wilhelmi method). Specifically, for example, it includes an automatic surface tension meter "CBVP-Z" manufactured by Kyowa Interface Science Co., Ltd. and a platinum plate, and can be measured in a 25°C environment.

[0101] ≪Inkjet Ink Manufacturing Method≫ The ink of the present invention can be manufactured by conventionally known methods. For example, it can be manufactured by the method shown below, but the method of manufacturing the ink is not limited to the method shown below.

[0102] First, the pigment, a portion of the photopolymerizable compound, and, if necessary, pigment dispersion resin, surface tension modifier, polymerization inhibitor, organic solvent, and / or water are thoroughly mixed. After mixing for a certain period of time, the mixture is dispersed using a paint shaker, sand mill, roll mill, medialess disperser, etc., to produce a pigment dispersion (pigment dispersion process).

[0103] Next, to the obtained pigment dispersion, the remaining photopolymerizable compounds not used in the pigment dispersion step, such as 5-methyl-3-vinyloxazolidine-2-one and a radically polymerizable difunctional monomer represented by general formula (1), as well as a photopolymerization initiator, surface tension modifier, polymerization inhibitor, organic solvent and / or water as needed, are added and thoroughly mixed. After that, the mixture is filtered using a filter or the like to remove coarse particles and obtain ink (ink formation step).

[0104] The pigment content in the above-mentioned pigment dispersion is preferably 10 to 70% by mass, and particularly preferably 15 to 60% by mass.

[0105] Furthermore, as the photopolymerizable compound used in the pigment dispersion process, it is preferable to use a radical polymerizable difunctional monomer represented by general formula (1) and / or other polyfunctional polymerizable compounds having two (meth)acryloyl groups. In addition, from the viewpoint of storage stability and discharge stability, and as mentioned above, from the viewpoint of ink curability, etc., when using a radical polymerizable difunctional monomer represented by general formula (1) as the above photopolymerizable compound, it is preferable to use one or more compounds selected from the group consisting of 1,4-butanediol diacrylate, 3-methyl-1,5-pentanediol diacrylate, and 1,6-hexanediol diacrylate. Furthermore, when using other polyfunctional polymerizable compounds having two (meth)acryloyl groups as the above photopolymerizable compound, from the viewpoint of improving the storage stability and discharge stability of the pigment dispersion and the ink produced using the pigment dispersion, it is preferable to use a monomer having an ethylene oxide structure and / or a propylene oxide structure as its main skeleton, and it is particularly preferable to use dipropylene glycol diacrylate and / or tripropylene glycol diacrylate.

[0106] <Printed material> The printed material of the present invention is obtained by printing the ink of the present invention described above onto a printing substrate, as described later; that is, it is a printing substrate on which images and / or characters are recorded. Therefore, the "printed material" in the present invention includes images and / or characters, which are made of a film (ink film) formed by curing the ink of the present invention, and a printing substrate. The "image" mentioned above includes solid images (images printed at 100% density so as to completely cover the surface of the printing substrate) and seamless images such as checkerboard patterns.

[0107] <Manufacturing methods for printed materials> As a method for manufacturing the above-mentioned printed material, for example, a method is provided that includes, in this order, a step of ejecting the ink of the present invention onto a printing substrate (step 1), and a step of irradiating the substrate having the ejected ink with ultraviolet light (step 2). Any step may be included between step 1 and step 2, or it may not be included.

[0108] In the present invention, a method of ejecting and applying the same inkjet ink from the same inkjet head to the same location on the printing substrate multiple times (multi-pass printing method) may be employed. However, in the case of the present invention, it is preferable to employ a method of ejecting and applying the same inkjet ink from the same inkjet head to the same location on the printing substrate only once (one-pass printing method) in order to fully realize the effects of the present invention described above.

[0109] The above one-pass printing method can be implemented, for example, by using a line printer. The printing speed in this case is preferably 35 to 150 m / min, and particularly preferably 75 to 100 m / min, from the viewpoint of productivity and obtaining printed materials with good quality.

[0110] ≪Process 1 (discharge process)≫ The ink of the present invention can be suitably used in inkjet printing. Therefore, in step 1, it is preferable to eject the ink from the inkjet head.

[0111] When the ink of the present invention is ejected from an inkjet head, the ejection volume (drop volume) is preferably 1 to 50 pL, and more preferably 3 to 20 pL. Furthermore, the design resolution of the inkjet head is preferably 600 dpi or higher. Examples of inkjet heads that satisfy the above conditions include Kyocera's KJ4A-AA, KJ4A-TA, KJ4A-RH; Fujifilm's SambaG3L; Seiko Epson's S3200, S1600, S800, I3200, I1600; Konica Minolta's KM1024i, KM1024; and Ricoh's MH5320, MH5340, MH5240, MH5440, etc., all of which can be suitably used.

[0112] Further, in order for the ink of the present invention to have an appropriate viscosity, it can be ejected while heating the ink with a heating device such as a heater provided in the inkjet head. From the viewpoint of stable and continuous ejection of the ink, it is preferable to heat the ink so that the viscosity of the ink during ejection is 20 mPa·s or less, and more preferably 15 mPa·s or less.

[0113] ≪Step 2 (Curing Step)≫ The ink of the present invention is ejected onto a printing substrate and then cured by irradiation with ultraviolet rays to form a printed matter.

[0114] As the ultraviolet irradiation means used in the above step 2, for example, a high-pressure mercury lamp, an ultra-high-pressure mercury lamp, a metal halide lamp, an excimer laser lamp, a xenon lamp, a UV-LED, etc. can be used.

[0115] Among the above ultraviolet irradiation means, the UV-LED has characteristics such as a narrow wavelength width of the irradiated ultraviolet rays and easy miniaturization. Therefore, the irradiation wavelength and usage method of the UV-LED can be adjusted arbitrarily to some extent according to the characteristics of pigments, photopolymerizable compounds, photoinitiators, etc. contained in the ink. The peak wavelength of the UV-LED used in step 2 is preferably 280 to 420 nm, more preferably 320 to 410 nm, and particularly preferably 340 to 400 nm.

[0116] In step 2, when using a UV-LED, from the viewpoint of sufficiently exerting the above-described effects and obtaining a printed matter excellent in curability and printing image quality, the maximum illuminance of ultraviolet rays on the printing substrate is preferably 1 W / cm 2 or more, more preferably 2 W / cm 2 or more, and particularly preferably 3 W / cm 2 or more. Also, the integrated light amount when irradiating the substrate varies depending on the types and contents of the photopolymerizable compounds and photoinitiators contained in the ink, but it is preferably 100 mJ / cm 2 or more, and preferably 200 mJ / cm 2It is even more preferable to do the above.

[0117] Furthermore, due to their small size, multiple UV-LEDs can be installed side by side. Therefore, multiple LEDs can be arranged to increase the irradiation intensity to the substrate. In this case, multiple UV-LEDs with different peak wavelengths may be used side by side. In step 2, UV-LEDs may also be used in combination with other ultraviolet irradiation means such as high-pressure mercury lamps or metal halide lamps. In this case, metal halide lamps are preferred because they effectively irradiate with ultraviolet light in the UV-A region and allow ultraviolet light to sufficiently reach the interior of the ink film.

[0118] Furthermore, the time between the completion of step 1 and the start of step 2 (the time from when the ink adheres to the printing substrate until the start of ultraviolet irradiation) is preferably 0.03 to 3 seconds, more preferably 0.04 to 2.5 seconds, and even more preferably 0.06 to 2 seconds. This makes it possible to obtain printed materials with excellent adhesion and abrasion resistance, and also prevents ink droplets from coalescing, making it easy to obtain printed materials with good print quality.

[0119] In the above inkjet printing method, step 2 can be repeated multiple times. For example, immediately after applying the inkjet ink to the substrate, ultraviolet light can be irradiated to partially cure the inkjet ink, and then ultraviolet light can be irradiated again to completely cure the inkjet ink. In this way, it becomes easy to obtain printed materials with particularly excellent print quality. In this disclosure, the step of partially curing the inkjet ink described above is referred to as "preliminary curing," and the step of completely curing it is referred to as "final curing."

[0120] When performing the above-mentioned pre-curing, it is preferable that the ultraviolet irradiation means used for the pre-curing be a UV-LED, and the maximum irradiance of ultraviolet light on the substrate should be 2 to 20 W / cm². 2 Preferably, it is 5-15 W / cm². 2 It is preferable that it be so.

[0121] On the other hand, in this curing process, 2 W / cm 2 The above maximum illuminance, and 100 mJ / cm² 2 This can be done with the above accumulated light intensity.

[0122] <Printing base material> As printing substrates to which the ink of the present invention can be applied, resin film substrates and paper substrates can be preferably used. Furthermore, as the resin film substrate, a thickness of 10 to 90 μm and containing a material selected from the group consisting of polypropylene, polyethylene, polyethylene terephthalate, and nylon is preferably selected. On the other hand, as the paper substrate, coated paper, art paper, laminated paper, etc., are preferably selected. The ink of the present invention can be suitably used, for example, for printing on packages manufactured using the above-listed printing substrates.

[0123] The above-mentioned "materials selected from the group consisting of polypropylene, polyethylene, polyethylene terephthalate, and nylon" also includes resin film substrates (laminated film substrates) having a multilayer structure and having at least one layer made of a material selected from the group consisting of polyethylene terephthalate, polyethylene, polypropylene, and nylon. Furthermore, for the purpose of improving the strength of the package, blocking oxygen, etc., the layers constituting the laminated film may include layers made of AL (aluminum foil), VM (vacuum deposition) film (aluminum deposition film, transparent deposition film, etc.).

[0124] Embodiments of the present invention include the ultraviolet-curable inkjet inks shown in [1] to [5] below, and printed materials produced using the ultraviolet-curable inkjet inks shown in [6]. However, the present invention is not limited to the embodiments shown below and includes various embodiments.

[0125] [1] An ultraviolet-curable inkjet ink comprising a photopolymerizable compound and a pigment, The photopolymerizable compound comprises 5-methyl-3-vinyloxazolidine-2-one and a radical polymerizable difunctional monomer represented by general formula (1), The content (A) of 5-methyl-3-vinyloxazolidine-2-one relative to the total amount of the UV-curable inkjet ink is 3 to 25% by mass. The content (B) of the radical polymerizable difunctional monomer represented by the general formula (1) relative to the total amount of the UV-curable inkjet ink is 7 to 65% by mass. The pigment content (D) relative to the total amount of the UV-curable inkjet ink is 2.2 to 7% by mass. The ratio (C / A) of the content (C) of monofunctional polymerizable compounds (excluding the aforementioned 5-methyl-3-vinyloxazolidine-2-one) to the content (A) of the aforementioned 5-methyl-3-vinyloxazolidine-2-one is 0 to 2.5. A UV-curable inkjet ink in which the total amount (B+C) of the radical polymerizable difunctional monomer represented by the general formula (1) (B) and the monofunctional polymerizable compound (excluding the 5-methyl-3-vinyloxazolidine-2-one) (C) is 7 to 65% by mass of the total amount of the UV-curable inkjet ink. General formula (1): CH2=CH-CO-OR 1 -O-CO-CH=CH2 [In general formula (1), R 1 This represents an alkylene group having 3 to 10 carbon atoms, which may have a branched structure. [2] The ultraviolet-curable inkjet ink according to [1], wherein the ratio (D / A) of the content of the pigment (D) to the content of the 5-methyl-3-vinyloxazolidine-2-one (A) is 0.1 to 1.5. [3] The ultraviolet-curable inkjet ink according to [1] or [2], wherein the ratio (B / A) of the content (B) of the radical polymerizable difunctional monomer represented by the general formula (1) to the content (A) of the 5-methyl-3-vinyloxazolidine-2-one is 1.8 to 15.0. [4] Furthermore, containing a photopolymerization initiator, The photopolymerization initiator comprises a thioxanthone compound, The ultraviolet-curable inkjet ink according to any one of [1] to [3], wherein the content (E1) of the thioxanthone compound relative to the total amount of the ultraviolet-curable inkjet ink is 0.6 to 3.0% by mass. [5] Furthermore, it contains a surface tension modifier, The surface tension modifier includes a polyether-modified silicone surface tension modifier having a (meth)acryloyl group. The ultraviolet-curable inkjet ink according to any one of [1] to [4], wherein the content (F) of the polyether-modified silicone surface tension modifier having a (meth)acryloyl group, relative to the total amount of the ultraviolet-curable inkjet ink, is 0.05 to 3% by mass. A printed material obtained by printing an ultraviolet-curing inkjet ink described in any of [6][1] to [5] onto a printing substrate.

[0126] The disclosures of this application relate to the subject matter described in Japanese Patent Application No. 2024-184463, filed on 18 October 2024, all of which are incorporated herein by reference. [Examples]

[0127] The present invention will be described in more detail below with reference to examples. However, the following examples do not limit the scope of the present invention in any way. Unless otherwise specified, "parts" refers to parts by mass, and "%" refers to percentage by mass.

[0128] <Manufacturing of pigment dispersions> First, pigment dispersions 1 to 8 were prepared using the materials listed in each column of Table 1 below. Specifically, the pigment dispersion resin and photopolymerizable compound shown in Table 1 were first added to a mixing container (8 L volume) equipped with a stirrer and stirred for 1 hour (premixing) to produce a pigment dispersion resin varnish. Next, while stirring the pigment dispersion resin varnish, the pigments listed in Table 1 were added little by little, and after the addition was complete, stirring was continued for another hour (pre-dispersion). After that, the pigment dispersion was produced by circulating and dispersing the mixture for 4 hours using a "DinoMill" (0.6 L volume) manufactured by Synmaru Enterprises, filled with zirconia beads with a diameter of 0.8 mm to a packing rate of 70%.

[0129] [Table 1]

[0130] The details of the abbreviations used in Table 1 are as follows: • CB: Carbon Black (Special Black 350, manufactured by Orion Engineered Carbons) ·PY155:Pigment Yellow 155(5515C LYSOPAC JAUNE, manufactured by Ferro) • SP32000: Solspers 32000 (Acid value 16 mg KOH / g, Amine value 35 mg KOH / g, manufactured by Lubrizol) • SP33000: Solspers 33000 (Acid value 26 mg KOH / g, Amine value 0 mg KOH / g, manufactured by Lubrizol) • PX4701: EFKA PX4701 (Acid value 0 mg KOH / g, Amine value 40 mg KOH / g, manufactured by BASF) • BJ9151: BYKJET-9151 (Acid value 8 mg KOH / g, Amine value 18 mg KOH / g, manufactured by Bic Chemie) • PB821: Ajisper PB-821 (Acid value 17 mg KOH / g, Amine value 10 mg KOH / g, manufactured by Ajinomoto Fine Techno Co., Ltd.) • DPGDA: Dipropylene glycol diacrylate (Miramer M222, manufactured by Miwon) • HDDA: 1,6-Hexanediol diacrylate (Viscote #240, manufactured by Osaka Organic Chemical Industry Co., Ltd.)

[0131] <Manufacturing of inkjet ink compositions> The pigment dispersion prepared above, photopolymerizable compound, photopolymerization initiator, polymerization inhibitor, surface tension modifier, and organic solvent were added to a mixing container equipped with a stirrer in the order shown in the columns of Tables 2-1 to 2-9, respectively, to achieve the content levels indicated in each column. The contents of the mixing container were stirred while the stirrer was operated as each material was added. For components containing two or more materials, the order of addition within that component was arbitrary. After adding all the materials, gentle stirring and mixing were continued until the photopolymerization initiator dissolved. Then, the mixture was filtered through a 1 μm pore size membrane filter to remove coarse particles, thereby producing the inkjet ink.

[0132] [Table 2-1]

[0133] [Table 2-2]

[0134] [Table 2-3]

[0135] [Table 2-4]

[0136] [Table 2-5]

[0137] [Table 2-6]

[0138] [Table 2-7]

[0139] [Table 2-8]

[0140] [Table 2-9]

[0141] Details of the raw material names listed in Tables 2-1 to 2-9 are as follows. <Photopolymerizable compound> • VMOX: 5-methyl-3-vinyloxazolidine-2-one (manufactured by BASF) • PDDA: 1,3-propanediol diacrylate (manufactured by Tokyo Chemical Industry Co., Ltd.) • BDDA: 1,4-butanediol diacrylate (Viscote #195, manufactured by Osaka Organic Chemical Industry Co., Ltd.) • MPDDA: 3-methyl-1,5-pentanediol diacrylate (EBECRYL MPDDA, manufactured by Daicel Ornex) • HDDA: 1,6-Hexanediol diacrylate (Viscote #240, manufactured by Osaka Organic Chemical Industry Co., Ltd.) • NDDA: 1,9-nonanediol diacrylate (Viscote #260, manufactured by Osaka Organic Chemical Industry Co., Ltd.) • DDDA: 1,10-decadiolic acid diacrylate (NK ester A-DOD-N, manufactured by Shin-Nakamura Chemical Industry Co., Ltd.) • IBXA: Isobornyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd.) • ACMO: Acryloylmorpholine (ACMO, manufactured by KJ Chemicals) • BzA: Benzyl acrylate (Viscote #160, manufactured by Osaka Organic Chemical Industry Co., Ltd.) • CBA: Ethyl carbitol (Viscote #190, manufactured by Osaka Organic Chemical Industry Co., Ltd.) • PEA: 2-Phenoxyethyl acrylate (Viscote #192, manufactured by Osaka Organic Chemical Industry Co., Ltd.) • DPGDA: Dipropylene glycol diacrylate (Miramer M222, manufactured by Miwon) • TMP(EO)3TA: Ethylene oxide-modified trimethylolpropane triacrylate (3 ethylene oxide groups) (MIRAMER M3130, manufactured by Miwon) • DiTMPTA: Ditrimethylolpropanetetraacrylate (Ebecryl 1142, Daicel Ornex Co., Ltd.) <Photopolymerization initiator> • DETX: 2,4-Diethylthioxanthone (Omnirad DETX, manufactured by IGM Resins) • OmnTX: Diester (multimer) of carboxymethoxythiooxane (Omnipol TX, manufactured by IGM Resins) • TPO-L: Ethoxyphenyl (2,4,6-trimethylbenzoyl)phosphine oxide (Omnirad TPO-L, manufactured by IGM RESINS) • Omn380: Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Omnirad380, manufactured by IMG Resins) Omn379: 1-(4-morpholinophenyl)-2-(dimethylamino)-2-(4-methylbenzyl)-1-butanone (manufactured by IMG Resins) <Polymerization inhibitors> • Phenothiazine: Contains a mixture with Seiko Chemicals' phenol (mass ratio: 1 / 1), manufactured by Bic Chemie. <Surface tension modifier> TR2100: TEDO Rad 2100, a polyether-modified silicone surface tension modifier containing (meth)acryloyl groups (manufactured by Evonik). • UV3500: Polyether-modified silicone surface tension modifier containing (meth)acryloyl groups (BYK-UV3500, manufactured by BYK Chemie). • BYK-315N: Polyester-modified polymethylalkyl silicone-based surface tension modifier (25% solids by mass, containing methoxypropyl acetate and phenoxyethanol as solvent components in a 1:1 mass ratio) <Organic solvents> • DEDG: Diethylene glycol diethyl ether

[0142] <Creating printed materials> Using the ink prepared above, printed materials were manufactured as follows. First, one Kyocera inkjet head (designed resolution 600 dpi) is installed above the conveyor that can transport the printing substrate. Furthermore, above the conveyor that is downstream of the transport direction of the printing substrate, a UV-LED for curing (Phoseon FirePower FP300, maximum emission wavelength 395 nm, maximum illuminance 16 W / cm²) is installed. 2 An inkjet printing device (Trytech's "OnePassJET") equipped with a [specific type of inkjet head] was prepared. Next, the inkjet inks manufactured above were filled into the inkjet heads, and the temperature of the inkjet heads was adjusted so that the viscosity of the inkjet ink at the time of ejection was 6-7 mPa·s. Furthermore, Avery Dennison's semi-gloss coated paper "Fasson 60# Semi-Gloss Elite FSC" was fixed on a conveyor. Then, the conveyor was driven at a speed of 50 m / min, and when the semi-gloss coated paper passed below the installation area of ​​the inkjet head, the image described later was printed with printing conditions of an ejection droplet volume of 11 pL and a print resolution of 600 dpi × 600 dpi. After the inkjet ink was ejected, the conveyor was driven at the same speed, and when the semi-gloss coated paper passed below the installation area of ​​the UV-LED for curing, ultraviolet light was irradiated to produce the printed material. Furthermore, the irradiance of ultraviolet light irradiated onto the inkjet ink on semi-gloss coated paper is 6,000 mW / cm². 2 Furthermore, the cumulative light intensity is 200 mJ / cm². 2The output of the UV-LED used for curing was adjusted in advance to achieve the desired result before the printing described above was performed. In addition, two types of images were prepared as described above: a solid image with 100% print coverage (10 x 25 cm) and a light tone solid image with 25% print coverage (10 x 25 cm). Two types of printed materials were produced for each inkjet ink.

[0143] [Examples 1-91, Comparative Examples 1-9] The inks and printed materials prepared as described above were used for the evaluations shown below. The evaluation results are shown in Tables 2-1 to 2-9.

[0144] <Evaluation 1: Evaluation of dispensing stability> The inkjet inks manufactured as described above were filled into jigs equipped with Kyocera inkjet heads (KJ4A, design resolution 600 dpi) that allow for temperature control. Next, the temperature of the inkjet heads was adjusted so that the viscosity of the inkjet ink at ejection was 6-7 mPa·s. After confirming that there were no nozzles that were not ejecting inkjet ink, continuous ejection of inkjet ink was performed from all nozzles at a drive frequency of 20 kHz. After continuous ejection for 5 minutes, a nozzle check pattern was printed, and the number of nozzles that were not ejecting inkjet ink (number of nozzle losses) was counted to evaluate the ejection stability. The evaluation criteria for the above ejection stability were as follows, and the evaluations of ○, ○△, and △ were considered practical.

[0145] ≪Evaluation Criteria for Discharge Stability≫ ○: Nozzle loss was 2 or less. ○△: Nozzle loss was 3 to 5. △: Nozzle loss was 6 to 9. ×: The number of nozzle losses was 10 or more.

[0146] <Evaluation 2: Evaluation of concentration> The density of solid images with 100% print coverage, created using the method described above, was measured and evaluated. A spectrophotometer X-RITE528 was used for density measurement, with the following conditions: light source D50, viewing angle 2°, CIE color system, and status E. The measured density of the solid images was then compared to the densities defined by the standard printing color characteristics of Japan Color 2011 for sheet-fed printing (yellow ink: 1.32, black ink: 1.70). The density evaluation criteria are as follows, with ○, ○△, and △ ratings being considered practical.

[0147] ≪Concentration Evaluation Criteria≫ ○: The density was 0.1 or more higher than that of Japan Color 2011. ○△: The concentration was the same as or higher than that of Japan Color 2011, with a difference of less than 0.1. △: The density was lower than that of Japan Color 2011, but the difference was less than 0.1. ×: The density was lower than that of Japan Color 2011, and the difference was 0.1 or more.

[0148] <Rating 3: Evaluation of complete occupancy> Using the method described above, ten solid images with 100% print coverage were produced consecutively. Each of the resulting solid images was then observed visually and with a magnifying glass to evaluate the degree of solid coverage. The evaluation criteria for solid coverage were as follows, and the ratings ○, ○△, and △ were made practical.

[0149] ≪Evaluation Criteria for Complete Filling≫ ○: Visual inspection revealed no white spots in any of the 10 images. Furthermore, when examined with a magnifying glass, white spots were observed in 2 or fewer images. ○△: Visually, no white spots were observed in any of the 10 images. However, when examined with a magnifying glass, white spots were observed in 3 to 4 images. △: Visual inspection revealed no white spots in any of the 10 images. However, when examined with a magnifying glass, white spots were observed in 5 or more images. ×: White areas were visible to the naked eye in one or more printed pages.

[0150] <Evaluation 4: Evaluation of surface hardening properties> The surface of a printed material with a light tone solid image at 25% print density, prepared using the method described above, was rubbed with a cotton swab, and it was checked whether any marks were left in the rubbed area. If inkjet ink adhered to the cotton swab, the printed material was fixed to the conveyor of the inkjet printing apparatus, and without printing with inkjet ink, only irradiation with the UV-LED for curing was performed. After that, the surface of the printed material was rubbed again with a cotton swab, and it was checked for the presence or absence of marks. This procedure was repeated, and the number of times the UV-LED was irradiated until no marks were left in the rubbed area was investigated to evaluate the surface hardening properties. The evaluation criteria for surface hardening properties were as follows, and the evaluations of ○, ○△, and △ were made practical.

[0151] ≪Evaluation Criteria for Surface Hardening≫ ○: After a total of 1-2 UV irradiations (either no additional UV irradiation or one additional UV irradiation), the area rubbed with the cotton swab no longer showed any marks. ○△: After a total of 3 irradiations (including 2 additional UV irradiations), the area rubbed with a cotton swab no longer showed any marks. △: After a total of 4 irradiations (including 3 additional UV irradiations), the area rubbed with a cotton swab no longer showed any marks. ×: It was necessary to perform UV-LED irradiation a total of 5 or more times (an additional 4 or more UV irradiations) until the area rubbed with a cotton swab no longer left a mark.

[0152] <Evaluation 5: Evaluation of internal hardening properties> Using the method described above, a solid image print with 100% print coverage was prepared. Cuts were made in the print using a cutter, and Nichiban cellophane tape (18mm wide) was firmly attached to intersect the cuts. The cellophane tape was then held at one end and peeled off instantly while maintaining a 60-degree angle. The adhesive surface of the cellophane tape was visually inspected after peeling. If ink adhered to the adhesive surface of the cellophane tape, the print was fixed to the conveyor of the inkjet printing machine, and without printing with inkjet ink, only irradiation with the UV-LED for curing was performed. Then, the presence or absence of ink adhesion to the cellophane tape was checked again using the method described above. This procedure was repeated, and the number of times the UV-LED was irradiated until no more ink adhered to the cellophane tape was counted. The evaluation criteria were as follows, and ○, ○△, and △ ratings were considered practical.

[0153] Criteria for evaluating internal hardening properties ○: After passing the tape through a total of 1-2 times (either without additional UV irradiation or with one additional UV irradiation), the ink stopped adhering to the cellophane tape. ○△: After passing it through a total of 3 times (with 2 additional UV irradiations), the ink stopped adhering to the cellophane tape. △: After passing through the device a total of four times (with three additional UV irradiations), the ink stopped adhering to the cellophane tape. ×: It was necessary to pass the cellophane tape through the UV lamp a total of five or more times (additional UV irradiation four or more times) until no more ink adhered to it.

[0154] As shown in Tables 2-1 to 2-9 above, the inkjet inks of Examples 1 to 91 having the configuration of the present invention were excellent in all aspects: ejection stability, density, solid coverage, surface curability, and internal curability.

[0155] On the other hand, as shown in Comparative Examples 1 and 2, when the pigment content (D) relative to the total amount of inkjet ink was less than 2.2% by mass or more than 7% by mass, one or more of the balances of ejection stability, concentration, and internal curing properties did not reach a practical level. Also, as shown in Comparative Examples 3 and 4, when the content (A) of 5-methyl-3-vinyloxazolidine-2-one relative to the total amount of inkjet ink was less than 3% by mass, ejection stability, surface curing properties, and internal curing properties did not reach a practical level, and conversely, when the content (A) exceeded 25% by mass, the internal curing properties were also inferior. Similarly, as shown in Comparative Examples 8 and 9, when the content of the radical polymerizable difunctional monomer represented by general formula (1) relative to the total amount of inkjet ink was less than 7% by mass or more than 65% by mass, in addition to internal curing properties, ejection stability, solid filling, surface curing properties, etc., did not reach a practical level.

[0156] Furthermore, as shown in Comparative Examples 5 to 7, in inks where the ratio (C / A) of the content of monofunctional polymerizable compounds other than 5-methyl-3-vinyloxazolidine-2-one relative to the total amount of inkjet ink to the above content (A) was greater than 2.5, deterioration of the internal curing properties of the printed material was observed, which is thought to be due to a large amount of photopolymerizable compounds remaining inside the coating film.

Claims

1. A UV-curable inkjet ink comprising a photopolymerizable compound and a pigment, The photopolymerizable compound comprises 5-methyl-3-vinyloxazolidine-2-one and a radically polymerizable difunctional monomer represented by general formula (1), The content (A) of 5-methyl-3-vinyloxazolidine-2-one relative to the total amount of the UV-curable inkjet ink is 3 to 25% by mass. The content (B) of the radical polymerizable difunctional monomer represented by the general formula (1) relative to the total amount of the ultraviolet-curable inkjet ink is 7 to 65% by mass. The pigment content (D) relative to the total amount of the UV-curable inkjet ink is 2.2 to 7% by mass. The ratio (C / A) of the content (C) of monofunctional polymerizable compounds (excluding the aforementioned 5-methyl-3-vinyloxazolidine-2-one) to the content (A) of the aforementioned 5-methyl-3-vinyloxazolidine-2-one is between 0 and 2.

5. A UV-curable inkjet ink in which the total amount (B + C) of the radical polymerizable difunctional monomer represented by the general formula (1) (B) and the monofunctional polymerizable compound (excluding the 5-methyl-3-vinyloxazolidine-2-one) (C) is 7 to 65% by mass of the total amount of the UV-curable inkjet ink. General formula (1): CH 2 =CH-CO-O-R 1 -O-CO-CH=CH 2 [In general formula (1), R 1 This represents an alkylene group having 3 to 10 carbon atoms, which may have a branched structure.

2. The ultraviolet-curable inkjet ink according to claim 1, wherein the ratio (D / A) of the content of the pigment (D) to the content of the 5-methyl-3-vinyloxazolidine-2-one (A) is 0.1 to 1.

5.

3. The ultraviolet-curable inkjet ink according to claim 1 or 2, wherein the ratio (B / A) of the content (B) of the radical polymerizable difunctional monomer represented by the general formula (1) to the content (A) of the 5-methyl-3-vinyloxazolidine-2-one is 1.8 to 15.

0.

4. Furthermore, it contains a photopolymerization initiator, The photopolymerization initiator comprises a thioxanthone compound, The ultraviolet-curable inkjet ink according to claim 1 or 2, wherein the content (E1) of the thioxanthone compound relative to the total amount of the ultraviolet-curable inkjet ink is 0.6 to 3.0% by mass.

5. Furthermore, it contains a surface tension modifier, The surface tension modifier includes a polyether-modified silicone surface tension modifier having a (meth)acryloyl group. The ultraviolet-curable inkjet ink according to claim 1 or 2, wherein the content (F) of the polyether-modified silicone surface tension modifier having a (meth)acryloyl group, relative to the total amount of the ultraviolet-curable inkjet ink, is 0.05 to 3% by mass.

6. A printed article comprising an ultraviolet-curable inkjet ink according to claim 1 or 2 printed on a printing substrate.

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