Ultraviolet curable inkjet ink and printed matter

Abstract

One embodiment of the present invention pertains to an ultraviolet curable inkjet ink which contains a photopolymerizable compound and a pigment. The photopolymerizable compound contains 5-methyl-3-vinyloxazolidin-2-one and a radically polymerizable bifunctional monomer having a specific structure. The contained amounts of abovementioned components are specified as: the contained amount (C) of a monofunctional polymerizable compound excluding the 5-methyl-3-vinyloxazolidin-2-one is 2.5 times or less of the contained amount (A) of the 5-methyl-3-vinyloxazolidin-2-one; and the sum of the contained amount (C) and the contained amount (B) of the radically polymerizable bifunctional monomer is 7-65 mass%.

Description

UV-curing inkjet inks and printed materials

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

[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 manufactured using UV-curing inkjet inks, the curing properties tend to vary depending on the thickness of the ink film formed by curing the 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 reactivity 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-curing 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-curing 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 is also 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).

[0010] International Publication No. 2014 / 014017, Japanese Patent Publication No. 2022-149919, Japanese Patent Publication No. 2021-042321, Japanese Patent Publication No. 2023-163470

[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 comprising 50% by mass or more of monofunctional monomers. Patent Document 2 discloses an active energy ray curable inkjet ink composition containing C.I. pigment violet 19 and C.I. pigment orange 71 in a certain ratio, and further comprising 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 yields 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 in the examples, the 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 printed materials.

[0013] As described above, conventionally, there was no UV-curable inkjet ink 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.

[0015] In order to solve the above-mentioned problems, the inventors diligently conducted research and found that the above-mentioned problems can be suitably solved by using a UV-curable inkjet ink that incorporates a specific photopolymerizable compound in combination, and specifies the content and ratio of the specific photopolymerizable compound, as well as the pigment concentration (pigment content). As a result, the present invention was completed.

[0016] In other words, embodiments of the present invention include the following. However, the present invention is not limited to the following embodiments, but includes various embodiments.

[0017] One embodiment is 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 radical-polymerizable difunctional monomer represented by general formula (1), the content of 5-methyl-3-vinyloxazolidine-2-one (A) relative to the total amount of the ultraviolet-curable inkjet ink is 3 to 25% by mass, the content of the radical-polymerizable difunctional monomer represented by general formula (1) (B) relative to the total amount of the ultraviolet-curable inkjet ink is 7 to 65% by mass, the content of the pigment (D) relative to the total amount of the ultraviolet-curable inkjet ink is 2.2 to 7% by mass, and the ratio (C / A) of the content of monofunctional polymerizable compounds (excluding 5-methyl-3-vinyloxazolidine-2-one) (C) to the content of 5-methyl-3-vinyloxazolidine-2-one (A) is 0 to 2.5. This invention relates to an ultraviolet-curable inkjet ink in which 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 the 5-methyl-3-vinyloxazolidine-2-one) is 7 to 65% by mass. General formula (1): CH ２ =CH-CO-OR 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. Another embodiment relates to a printed material obtained by printing the ultraviolet-curable inkjet ink on a printing substrate.

[0018] 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.

[0019] 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.

[0020] 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 sufficiently, 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.

[0021] 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 the 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.

[0022] 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.

[0023] 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 radical 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.

[0024] Furthermore, in the UV-curable inkjet ink of the present invention, when C is the content of monofunctional polymerizable compounds (excluding 5-methyl-3-vinyloxazolidine-2-one) and A is the content of 5-methyl-3-vinyloxazolidine-2-one, 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.

[0025] 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 other than 5-methyl-3-vinyloxazolidine-2-one have low viscosity. Also, as mentioned above, the radical polymerizable difunctional monomer represented by the general formula (1) has low surface tension. As a result, in an inkjet ink containing a certain amount of these components, the wettability on the printing substrate is improved, and it becomes easier to prevent the occurrence of white spots and pinholes in the ink film. In addition, although the detailed mechanism is unknown, the viscoelasticity of the inkjet ink is optimized, and the ejection stability is also improved.

[0026] 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.

[0027] Furthermore, while Patent Document 1 discloses specific examples using a radically polymerizable difunctional monomer represented by general formula (1) (1,6-hexanediol diacrylate, HDDA), it does not disclose any specific examples using 5-methyl-3-vinyloxazolidine-2-one. Also, although N-vinylcaprolactam is described in the specification of Patent Document 1, 5-methyl-3-vinyloxazolidine-2-one is not mentioned at all. In addition, neither the radically polymerizable difunctional monomer represented by general formula (1) nor 5-methyl-3-vinyloxazolidine-2-one is used in the specific examples of Patent Document 2. Furthermore, while paragraph 0031 of Patent Document 2 lists compounds included in the radically 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," which discourages the active use of the above-mentioned 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 compounds included in the radically polymerizable difunctional monomer represented by general formula (1) are listed together with other polyfunctional photopolymerizable compounds in the specifications of Patent Documents 3 and 4, there is no description of which of these compounds is suitably used. Moreover, Patent Documents 1 to 4 do not describe or suggest the configuration 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 the radically polymerizable difunctional monomer represented by general formula (1), and adjusting the content of each component, nor do they describe or suggest the effects of this configuration.

[0028] Next, each of the components constituting the inkjet ink of the present invention will be described in detail below.

[0029] <Photopolymerizable Compounds> The ink of the present invention contains photopolymerizable compounds. The photopolymerizable compounds include 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 photopolymerization initiators, 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).

[0030] In this disclosure, "(meth)acryloyl" means "acryloyl" and / or "methacryloyl," and "(meth)acrylate" means "acrylate" and / or "methacrylate." In this disclosure, "monomer" means a compound consisting of molecules that can be the smallest unit in a polymer obtained by polymerization and / or crosslinking reactions. Furthermore, "photopolymerizable monomer" means a monomer among photopolymerizable compounds that has one or more photopolymerizable groups.

[0031] ≪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.

[0032] The content (A) of 5-methyl-3-vinyloxazolidin-2-one with respect to the total amount of the inkjet ink of the present invention is 3 to 25% by mass. Further, it is more preferable that the A is 5 to 20% by mass, and particularly preferably 5 to 15% by mass. By setting the content of 5-methyl-3-vinyloxazolidin-2-one within the above range, while maintaining the wet spreading of the inkjet ink on the printing substrate, the fast curing speed of the above 5-methyl-3-vinyloxazolidin-2-one can be utilized, and an inkjet ink excellent in the curability of the ink film surface can be obtained while suppressing the occurrence of white spots and / or pinholes in the ink film. In addition, the deterioration of the dispersion stability of the pigment due to 5-methyl-3-vinyloxazolidin-2-one can be suppressed, and the color development property of the ink film and the ejection stability of the inkjet ink are also good.

[0033] <<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, a photopolymerizable monomer 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, N-acryloyloxyethylhexahydrophthalimide, and the like.

[0034] Further, the monofunctional polymerizable compound may contain, among the photopolymerizable monomers, a compound having one vinyl group (excluding the (meth)acryloyl group) (excluding 5-methyl-3-vinyloxazolidin-2-one). Specific examples of such a compound include N-vinylpyrrolidone, N-vinylvalerolactam, N-vinylcaprolactam, N-vinylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide, N-vinyloxazolidinone, and the like.

[0035] Further, the monofunctional polymerizable compound may contain, among the photopolymerizable oligomers, a compound having one (meth)acryloyl group. Specific examples of such a compound include KRM9276 manufactured by Daicel Ornex Co., Ltd.; CN131NS, CN131BNS, CN146NS, CN153NS, CN3108NS, CN7002NS, CN8004NS, CN9003NS, etc. manufactured by Arkema Co., Ltd.

[0036] In the ink of the present invention, the content (C) of the monofunctional polymerizable compound other than 5-methyl-3-vinyloxazolidin-2-one with respect to the total amount of the inkjet ink, and the content (A) of 5-methyl-3-vinyloxazolidin-2-one with respect to the total amount of the inkjet ink, that is, the value represented by C / A is 0 to 2.5, and may be 0.0 to 2.5. By setting the value represented by C / A within the above range, even in an ink film containing a certain amount of pigment, the amount of the monofunctional polymerizable compound remaining inside the ink film is reduced, the internal curability of the ink film is improved, and the ejection stability is also in a good state. From this viewpoint, the value represented by C / A is preferably 0.0 to 1.0, and particularly preferably 0.0 to 0.5.

[0037] 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, of the total amount of photopolymerizable compounds.

[0038] <<Radical polymerizable difunctional monomer represented by general formula (1)>> The ink of the present invention contains a radical polymerizable difunctional monomer represented by the following general formula (1) as a polyfunctional polymerizable compound. General formula (1): CH ２ =CH-CO-OR １ -O-CO-CH=CH ２ [In general formula (1), R １represents 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 above general formula (1) 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) have excellent wetting spread on the printing substrate and improved 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 by intermolecular interactions, thus improving the curability and ejection stability of the inkjet ink. Furthermore, because deterioration of the dispersion stability of pigments in the inkjet ink can be suppressed, the color development of the ink film is improved. In this way, by keeping the content of the radical polymerizable difunctional monomer represented by the above general formula (1) within the above range, it is possible to improve the solid filling and color development of printed materials, as well as improve the ejection stability and curing properties of inkjet inks.

[0039] 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.

[0040] 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.

[0041] In the ink of the present invention, it is preferable to contain 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.

[0042] 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, based on the total amount of the radical polymerizable difunctional monomer represented by general formula (1).

[0043] When using a combination of an alkanediol diacrylate consisting of a branched alkylene group and an alkanediol diacrylate consisting of a non-branched alkylene group as a radical polymerizable difunctional monomer 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 (mass%) is the content of the alkanediol diacrylate consisting of a branched alkylene group relative to the total amount of inkjet ink, and WN (mass%) is the content of the alkanediol diacrylate consisting of a non-branched alkylene group. 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.

[0044] 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 setting 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.

[0045] 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.

[0046] 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 radically 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 curing and internal curing properties of the inkjet ink, even when printing at high speeds.

[0047] <<Other Polyfunctional Polymerizable Compounds>> The ink of the present invention may contain polyfunctional polymerizable compounds other than the radical polymerizable difunctional monomer represented by general formula (1) (also referred to as "other polyfunctional polymerizable compounds" in this disclosure), 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.

[0048] In particular, from the viewpoint of achieving both curability of the ink film surface and curability of the inside of 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, that is, from the viewpoint of improving 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.

[0049] 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.

[0050] Of the compounds listed above, from the viewpoint of maintaining discharge stability while also improving internal curability, the other polyfunctional polymerizable compound preferably contains glycerin triacrylate and / or ethylene oxide-modified trimethylolpropane triacrylate (having 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 (having 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] Furthermore, the photopolymerizable monomers included as other polyfunctional polymerizable compounds may be modified versions of the radical polymerizable difunctional monomer represented by the general formula (1) above, 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.

[0056] Furthermore, other polyfunctional polymerizable compounds may also contain photopolymerizable oligomers. In this case, the photopolymerizable oligomer preferably contains 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.

[0057] 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.

[0058] 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 monomers 1,6-hexanediol and isophorone diisocyanate.

[0059] 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.

[0060] 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.

[0061] <Pigments> The ink of the present invention contains pigments from the viewpoint of improving the color development of printed materials (opacity in the case of white inks, and gloss in the case of metallic inks). 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 reduce the above-mentioned reduction in radical generation efficiency. 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 as 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.

[0062] The pigments contained in the ink of the present invention are not particularly limited, but include, for example, organic and inorganic pigments represented by the following color index names. For example, as red pigments, C.I. Pigment Red 5, 7, 12, 17, 48 (Ca), 48 (Mn), 49:2, 57 (Ca), 57:1, 112, 122, 123, 147, 149, 150, 166, 168, 176, 177, 178, 184, 188, 202, 209, 242, 255, 264, 266, 269, 282, etc.; as violet pigments, C.I. Pigment Violet 19, etc.; as orange pigments, C.I. Pigment Orange 5, 13, 34, 38, 43, 61, 62, 64, etc.; as blue pigments, C.I. Pigment Blue 1, 2, 3, 15:3, 15:4, 15:34, 16, 22, 60, C.I. Bat Blue 4, 60, etc.; as green pigments, C.I. Pigment Green 7, 26, 36, 50, 58, etc.; as yellow pigments, C.I. Pigment Yellow 1, 2, 3, 12, 14, 16, 17, 73, 74, 75, 83, 93, 95, 97, 98, 109, 110, 114, 128, 129, 138, 139, 147, 150, 151, 154, 155, 180, 185, 213, etc.; as black pigments, C.I. Pigment Black 1, 7, etc.; and as white pigments, C.I. Pigment whites 6, 18, 21, etc.; as metallic pigments, C.I. Pigment Metals 1, 2, etc., can be used as desired, depending on the desired color reproduction and color development. Two or more of the pigments listed above may be used in combination.

[0063] 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 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.

[0064] 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.

[0065] 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.

[0066] 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 a sufficiently practical density for printed materials. Moreover, the internal curing properties are improved as the curing progresses sufficiently to the interior of the ink film.

[0067] <Pigment Dispersion Resin> In the present invention, it is preferable for the ink to contain a pigment dispersion resin, as this improves the dispersion stability of the pigment, as well as the storage stability and discharge stability of the ink, and furthermore, it is preferable to improve 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 "Azisper PB-821", "Azisper PB-822", "Azisper PB-824", and "Azisper PB-881" from Ajinomoto Fine Techno Co., Ltd., and "Disperby K-162", "Disperby K-163", "Disperby K-168", "Disperby K-182", "Disperby K-184", and "Disperby K-1" from Big Chemie Co., Ltd. Examples include "85", "DISPERBYK-2013", "DISPERBYK-2155", "BYKJET-9150", "BYKJET-9151", "BYKJET-9152", Lubrizol's "Solspers 24000", "Solspers 32000", "Solspers 33000", "Solspers 39000", "Solspers J180", "Solspers J200", BASF's "EFKA PX4701", "EFKA PX4703", "EFKA PX4733", etc. Furthermore, a resin obtained by polymerizing an acid group-containing polymerizable compound such as acrylic acid or methacrylic acid, an amino group-containing polymerizable compound such as acrylamide, dimethylaminoethyl methacrylate, or diethylaminoethyl methacrylate, and other polymerizable compounds such as styrene, α-methylstyrene, methyl methacrylate, butyl methacrylate, or lauryl methacrylate may be used as a pigment dispersion resin.

[0068] 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.

[0069] 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.

[0070] 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).

[0071] 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.

[0072] The "acid value" of a pigment-dispersing 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, a 0.1 mol / L potassium hydroxide-ethanol solution is used for titration by potentiometric titration. The acid value can then be calculated using the titration volume read from the resulting titration curve.

[0073] 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 within the above content ranges, a pigment dispersion with excellent dispersion stability and storage stability can be obtained.

[0074] <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 photopolymerizable compounds. That is, a 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.

[0075] 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.

[0076] Among these photopolymerization initiators, it is preferable that the photopolymerization initiator contains a thioxanthone compound, as this can improve both surface curability and internal curability while maintaining optimal discharge stability.

[0077] ≪Thioxanthone Compounds≫ Specific examples of the above thioxanthone compounds include 2-isopropylthioxanthone, 4-isopropylthioxanthone, 2,4-diethylthioxanthone, 3-methoxythioxanthone, 2-carboxymethoxythioxanthone, 3-ethoxycarbonylmethoxythioxanthone, 3-butoxycarboxymethoxythioxanthone, 1,3-dimethyl-2-(2-ethylhexyloxy)thioxanthone, 2-[2,2-bis(ethoxycarbonyl)]ethylthioxanthone, 1-chloro-4-propoxythioxanthone, and polymers of these compounds. 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 aforementioned 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.

[0078] When the inkjet ink of the present invention contains a thioxanthone compound, from the viewpoint of obtaining an inkjet ink that is excellent in 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.

[0079] <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. Furthermore, 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. Additionally, 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.

[0080] 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.

[0081] 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.

[0082] 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, the product of the two, i.e., the value expressed as A (mass%) × E2 (mass%), is preferably 57.5 to 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.

[0083] 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.

[0084] Examples of the above-mentioned monoacylphosphine oxide initiators include ethoxyphenyl (2,4,6-trimethylbenzoyl)phosphine oxide and diphenyl (2,4,6-trimethylbenzoyl)phosphine oxide.

[0085] 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 curability, ejection stability, and 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.

[0086] 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.

[0087] <Other Components> In addition to the components described above, the ink of the present invention may contain, as necessary, a surface tension modifier, a polymerization inhibitor, an organic solvent, water, and other additives.

[0088] <Surface Tension Adjusting Agent> The ink of the present invention preferably contains a surface tension adjusting agent, 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 adjusting agents, fluorine-based surface tension adjusting agents, acetylene glycol-based surface tension adjusting agents, acetylene monool-based surface adjusting agents, etc., can be used as the surface tension adjusting agent. Among these, it is preferable to contain a silicone-based surface tension adjusting agent, and it is particularly preferable to contain a polyether-modified silicone-based surface tension adjusting agent, 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), 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 Examples include the TEGO Rad 2250, TEGO Rad 2300, TEGO Rad 2330, TEGO Rad 2500, TEGO Rad 2550, TEGO Rad 2650, TEGO Rad 2700, and TEGO Rad 2800 (all manufactured by Evonik).

[0089] 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.

[0090] 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 the printed material 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.

[0091] <<Polymerization Inhibitors>> Polymerization inhibitors can be added to the ink of the present invention because they improve discharge stability and balance surface curability and internal curability, thereby improving overall curability. 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.

[0092] <<Organic Solvents, Water>> The ink of the present invention may contain organic solvents and / or water, as this improves discharge stability due to the optimization of the viscosity and surface tension of the ink, and improves the wetting spread of the ink droplets onto the printing substrate, thereby enabling the production of printed materials with good solid coverage. When the ink of the present invention contains organic solvents 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 organic solvents, it is preferable to use organic solvents with a boiling point of 140 to 300°C at 1 atmosphere, and more preferably organic solvents 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.

[0093] Furthermore, from the viewpoint of suitably improving all of the above-mentioned effects, namely discharge 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, as it exhibits excellent compatibility with photopolymerizable compounds containing radical polymerizable bifunctional monomers represented by general formula (1), and particularly improves discharge stability, wettability on printing substrates, and adhesion of printed materials.

[0094] <Inert Resin> The inkjet ink of the present invention may contain an inert resin in order to provide adhesion to various printing substrates and to adjust the viscoelasticity of the ink and improve ejection stability. The inert resin may 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.

[0095] 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.

[0096] 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.

[0097] <<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 of the conventionally known ones.

[0098] ≪Physical Properties of Inkjet Ink≫ 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 printed materials. 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, ejection can be continued 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 speeds, the viscosity is particularly preferably 8 to 15 mPa·s. The viscosity described above 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.

[0099] 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.

[0100] ≪Method for Manufacturing Inkjet Ink≫ 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 for manufacturing the ink is not limited to the following.

[0101] 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).

[0102] 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).

[0103] The pigment content in the above-mentioned pigment dispersion is preferably 10 to 70% by mass, and particularly preferably 15 to 60% by mass.

[0104] 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. Moreover, 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 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 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.

[0105] <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 also 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.

[0106] <Method for Manufacturing Printed Materials> As a method for manufacturing the printed materials described above, for example, a method is given which includes, in this order, a step of dispensing the ink of the present invention onto a printing substrate (step 1), and a step of irradiating the substrate having the dispensed ink with ultraviolet light (step 2). Any step may be included between step 1 and step 2, or it may not be included.

[0107] 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.

[0108] 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.

[0109] <Step 1 (Ejection Step)> The ink of the present invention can be suitably used in an inkjet printing method. Therefore, in Step 1, it is preferable to eject the ink from the inkjet head.

[0110] 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. Specific examples of inkjet heads that satisfy the above conditions include Kyocera KJ4A-AA, KJ4A-TA, KJ4A-RH; Fujifilm Samba G3L; Seiko Epson S3200, S1600, S800, I3200, I1600; Konica Minolta KM1024i, KM1024; and Ricoh MH5320, MH5340, MH5240, MH5440, etc., all of which can be suitably used.

[0111] Furthermore, the ink of the present invention can be ejected while being heated by a heating device such as a heater provided in the inkjet head so that the ink has an appropriate viscosity. 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 at the time of ejection is 20 mPa·s or less, and it is even more preferable to heat it so that it is 15 mPa·s or less.

[0112] <<Step 2 (Hardening Step)>> After the ink of the present invention is discharged onto a printing substrate, it is irradiated with ultraviolet rays and hardened to form a printed matter.

[0113] 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.

[0114] Among the above ultraviolet irradiation means, the UV-LED has characteristics such as a narrow ultraviolet wavelength range to be irradiated and easy miniaturization. Therefore, the irradiation wavelength and usage method of the UV-LED can be adjusted arbitrarily to some extent according to the properties of the pigment, photopolymerizable compound, photoinitiator, 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.

[0115] 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 compound and photoinitiator contained in the ink, but it is preferably 100 mJ / cm ２ or more, and more preferably 200 mJ / cm ２ or more.

[0116] 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.

[0117] Furthermore, the time between the completion of step 1 and the start of step 2 (the time between the ink adhering to the printing substrate and 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 makes it easy to obtain printed materials with good print quality without ink droplets coalescing together.

[0118] 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."

[0119] 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 to 15 W / cm². ２ It is preferable that it be so.

[0120] 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.

[0121] <Printing Substrates> 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.

[0122] 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.).

[0123] 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.

[0124] [1] 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 radical-polymerizable difunctional monomer represented by general formula (1), the content of 5-methyl-3-vinyloxazolidine-2-one (A) relative to the total amount of the ultraviolet-curable inkjet ink is 3 to 25% by mass, the content of the radical-polymerizable difunctional monomer represented by general formula (1) (B) relative to the total amount of the ultraviolet-curable inkjet ink is 7 to 65% by mass, the content of the pigment (D) relative to the total amount of the ultraviolet-curable inkjet ink is 2.2 to 7% by mass, and the ratio (C / A) of the content of monofunctional polymerizable compounds (excluding 5-methyl-3-vinyloxazolidine-2-one) (C) to the content of 5-methyl-3-vinyloxazolidine-2-one (A) 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. General formula (1): CH ２ =CH-CO-OR １ -O-CO-CH=CH ２ [In general formula (1), R １is a C3 to C10 alkylene group 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 of the radical polymerizable difunctional monomer represented by the general formula (1) (B) to the content of the 5-methyl-3-vinyloxazolidine-2-one (A) is 1.8 to 15.0. [4] The ultraviolet-curable inkjet ink according to any one of [1] to [3], further comprising a photopolymerization initiator, wherein the photopolymerization initiator comprises a thioxanthone compound, and 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] The ultraviolet-curable inkjet ink according to any one of [1] to [4], further comprising a surface tension modifier, wherein the surface tension modifier comprises a polyether-modified silicone surface tension modifier having a (meth)acryloyl group, and 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 obtained by printing the ultraviolet-curable inkjet ink according to any one of [1] to [5] on a printing substrate.

[0125] The disclosures of this application are related 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.

[0126] 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 mass percent.

[0127] <Manufacturing of Pigment Dispersions> First, pigment dispersions 1 to 8 were manufactured 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 above 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 manufactured by circulating and dispersing the mixture for 4 hours using a "DinoMill" (0.6 L volume) manufactured by Synmaru Enterprises, which was filled with zirconia beads with a diameter of 0.8 mm to a packing rate of 70%.

[0128]

[0129] 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: Adisper 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 Co., Ltd.) • HDDA: 1,6-Hexanediol diacrylate (Viscote #240, manufactured by Osaka Organic Chemical Industry Co., Ltd.)

[0130] <Manufacturing of Inkjet Ink Composition> 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 Table 2-1 to 2-10, so that the contents matched. Each material was added while the contents of the mixing container were stirred by driving the stirrer. For components containing two or more materials, the order of addition within that component was arbitrary. After all materials had been added, gentle stirring and mixing were continued until the photopolymerization initiator dissolved. Then, the mixture was filtered through a membrane filter with a pore size of 1 μm to remove coarse particles, thereby manufacturing the inkjet ink.

[0131]

[0132]

[0133]

[0134]

[0135]

[0136]

[0137]

[0138]

[0139]

[0140]

[0141] Details of the raw material names listed in Tables 2-1 to 2-10 are as follows. <Photopolymerizable Compounds> ・VMOX: 5-methyl-3-vinyloxazolidine-2-one (BASF) ・PDDA: 1,3-propanediol diacrylate (Tokyo Chemical Industries, Ltd.) ・BDDA: 1,4-butanediol diacrylate (Viscote #195, Osaka Organic Chemical Industry Co., Ltd.) ・MPDDA: 3-methyl-1,5-pentanediol diacrylate (EBECRYL MPDDA, Daicel Ornex Co., Ltd.) ・HDDA: 1,6-hexanediol diacrylate (Viscote #240, Osaka Organic Chemical Industry Co., Ltd.) ・NDDA: 1,9-nonanediol diacrylate (Viscote #260, Osaka Organic Chemical Industry Co., Ltd.) ・DDDA: 1,10-decadiolic acid diacrylate (NK Ester A-DOD-N, Shin Nakamura Chemical Industry Co., Ltd.) • IBXA: Isobornyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd.) • ACMO: Acryloyl morpholine (ACMO, manufactured by KJ Chemicals Co., Ltd.) • 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 Co., Ltd.) • TMP(EO)3TA: Ethylene oxide modified trimethylolpropane triacrylate (ethylene oxide group count: 3) (MIRAMER M3130, manufactured by Miwon Co., Ltd.)・DiTMPTA: Ditrimethylolpropanetetraacrylate (Ebecryl 1142, Daicel Ornex Co., Ltd.) <Photopolymerization initiator> ・DETX: 2,4-Diethylthioxanthone (Omnirad DETX, IGM RESINS) ・OmnTX: Carboxymethoxythiooxane diester (multimer) (Omnipol TX, IGM RESINS) ・TPO-L: Ethoxyphenyl (2,4,6-trimethylbenzoyl)phosphine oxide (Omnirad TPO-L, IGM RESINS) ・Omn380: Phenylbis(2,4,6-Trimethylbenzoyl)phosphine oxide (Omnirad 380, manufactured by IMG Resins) ・Omn 379: 1-(4-morpholinophenyl)-2-(dimethylamino)-2-(4-methylbenzyl)-1-butanone (manufactured by IMG Resins) <Polymerization inhibitor> ・Phenothiazine: Contains a mixture with Seiko Chemical's Nol (mass ratio: 1 / 1), manufactured by Bic Chemie <Surface tension modifier> ・TR2100: TEDO Rad 2100, polyether-modified silicone surface tension modifier having (meth)acryloyl groups (manufactured by Evonik) ・UV3500: Polyether-modified silicone surface tension modifier having (meth)acryloyl groups (BYK-UV3500, manufactured by Bic Chemie)・BYK-315N: Polyester-modified polymethylalkyl silicone surface tension modifier (25% by mass solids, containing methoxypropyl acetate and phenoxyethanol as solvent components in a 1:1 mass ratio) <Organic solvent> ・DEDG: Diethylene glycol diethyl ether,

[0142] <Production of Printed Materials> Using the ink produced above, printed materials were manufactured as follows. First, one Kyocera inkjet head (design resolution 600 dpi) was 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, the UV-LED for curing (Phoseon FirePower FP300, maximum emission wavelength 395 nm, maximum illuminance 16 W / cm) was installed. ２An inkjet printing device (Trytech's "OnePassJET") equipped with the above-mentioned 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 to 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 the ejection droplet amount of 11 pL and the print resolution of 600 dpi × 600 dpi. Then, even 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 section of the UV-LED for curing, ultraviolet light was irradiated to produce the printed material. The irradiance of the ultraviolet light irradiated onto the inkjet ink on the semi-gloss coated paper was 6,000 mW / cm². ２ Furthermore, the cumulative light intensity is 200 mJ / cm² ２ The 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 above were used for the evaluations described below. The evaluation results are shown in Tables 2-1 to 2-10.

[0144] <Evaluation 1: Evaluation of Discharge Stability> The inkjet inks manufactured above were filled into jigs equipped with Kyocera inkjet heads (KJ4A, design resolution 600 dpi) that allow temperature adjustment. Next, the temperature of the inkjet heads was adjusted so that the viscosity of the inkjet ink at the time of discharge was 6 to 7 mPa·s. After confirming that there were no nozzles that were not discharging inkjet ink, continuous discharging of inkjet ink was performed from all nozzles at a drive frequency of 20 kHz. After continuous discharging for 5 minutes, a nozzle check pattern was printed, and the number of nozzles that were not discharging inkjet ink (number of nozzle losses) was counted to evaluate the discharge stability. The evaluation criteria for the above discharge stability were as follows, and the evaluations of ○, ○△, and △ were considered to be 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 ×: Nozzle loss was 10 or more

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

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

[0148] <Evaluation 3: Evaluation of Solid Color Filling> Ten solid color images with 100% print coverage were created consecutively using the method described above. Each of the obtained solid color images was then observed visually and with a magnifying glass to evaluate the solid color filling. The evaluation criteria for solid color filling were as follows, and the ○, ○△, and △ ratings were made practical.

[0149] ≪Evaluation Criteria for Solid Fill≫ ○: No white spots were observed in any of the 10 sheets upon visual inspection. Furthermore, when observed with a magnifying glass, the number of sheets with white spots was 2 or less. ○△: No white spots were observed in any of the 10 sheets upon visual inspection. Furthermore, when observed with a magnifying glass, the number of sheets with white spots was 3 to 4. △: No white spots were observed in any of the 10 sheets upon visual inspection. Furthermore, when observed with a magnifying glass, the number of sheets with white spots was 5 or more. ×: White spots were observed in one or more printed materials, even upon visual inspection.

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

[0151] ≪Surface Hardening Evaluation Criteria≫ ○: After a total of 1-2 irradiations (no additional UV irradiation, or 1 additional UV irradiation), no marks were left on the area rubbed with a cotton swab. ○△: After a total of 3 irradiations (2 additional UV irradiations), no marks were left on the area rubbed with a cotton swab. △: After a total of 4 irradiations (3 additional UV irradiations), no marks were left on the area rubbed with a cotton swab. ×: It was necessary to irradiate with UV-LED a total of 5 or more times (4 or more additional UV irradiations) until no marks were left on the area rubbed with a cotton swab.

[0152] <Evaluation 5: Evaluation of Internal Curing> Cuts were made with a cutter on a solid image print with 100% print coverage, prepared using the method described above, and Nichiban cellophane tape (18 mm wide) was firmly attached to the cuts so as to intersect them. Then, the end of the cellophane tape was held and peeled off instantaneously while maintaining a 60-degree angle, and 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, and 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 curing≫ ○: After passing through a total of 1-2 times (no additional UV irradiation, or 1 additional UV irradiation), the ink no longer adheres to the cellophane tape. ○△: After passing through a total of 3 times (2 additional UV irradiations), the ink no longer adheres to the cellophane tape. △: After passing through a total of 4 times (3 additional UV irradiations), the ink no longer adheres to the cellophane tape. ×: It was necessary to pass the cellophane tape through the UV lamp a total of 5 or more times (4 or more additional UV irradiations) until the ink no longer adhered to the cellophane tape.

[0154] As shown in Tables 2-1 to 2-10 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., also 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 amount of monofunctional polymerizable compounds other than 5-methyl-3-vinyloxazolidine-2-one relative to the total amount of inkjet ink to the above amount (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. 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 radical-polymerizable difunctional monomer represented by general formula (1), the content of 5-methyl-3-vinyloxazolidine-2-one (A) relative to the total amount of the ultraviolet-curable inkjet ink is 3 to 25% by mass, the content of the radical-polymerizable difunctional monomer represented by general formula (1) (B) relative to the total amount of the ultraviolet-curable inkjet ink is 7 to 65% by mass, the content of the pigment (D) relative to the total amount of the ultraviolet-curable inkjet ink is 2.2 to 7% by mass, and the ratio (C / A) of the content of monofunctional polymerizable compounds (excluding 5-methyl-3-vinyloxazolidine-2-one) (C) to the content of 5-methyl-3-vinyloxazolidine-2-one (A) 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. General formula (1): CH ２ =CH-CO-OR 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. The ultraviolet-curable inkjet ink according to any one of claims 1 to 3, further comprising a photopolymerization initiator, wherein the photopolymerization initiator comprises a thioxanthone compound, and 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. The ultraviolet-curable inkjet ink according to any one of claims 1 to 4, further comprising a surface tension modifier, wherein the surface tension modifier comprises a polyether-modified silicone surface tension modifier having a (meth)acryloyl group, and 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 obtained by printing an ultraviolet-curable inkjet ink according to any one of claims 1 to 5 onto a printing substrate.

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