Ultraviolet-curable inkjet ink and printed matter

Abstract

To provide an ultraviolet curable inkjet ink excellent in curability and discharge stability, and further excellent in adhesion and scratch resistance of a printed matter.SOLUTION: An ultraviolet-curable inkjet ink containing titanium oxide, a photopolymerizable compound, a photopolymerization initiator, and a fluorescent brightening agent, wherein the photopolymerizable compound contains a polyfunctional polymerizable compound in an amount of at least 50% by mass relative to the total mass of the photopolymerizable compound, the polyfunctional polymerizable compound contains a specific alkanediol diacrylate in an amount of at least 50% by mass relative to the total mass of the polyfunctional polymerizable compound, and the fluorescent brightening agent contains a thiophene benzoxazoyl-based compound and / or a naphthalene benzoxazoyl-based compound.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to an ultraviolet-curable inkjet ink and a printed matter obtained using the ultraviolet-curable inkjet ink. [Background technology]

[0002] In recent years, digital printing methods, which do not require plates, have rapidly become popular in the printing industry. Because digital printing methods do not require plate making, printing equipment can be made smaller than plate-based printing methods, which require plates. Digital printing methods also have many advantages over other printing methods, such as reduced running costs during printing and ease of full-color printing, and are therefore increasingly used in the industrial printing industry in particular.

[0003] Inks used in inkjet printing methods include water-based, oil-based, solvent-based, and UV-curable inks, but demand for UV-curable inkjet inks is increasing due to their ability to be used on non-absorbent printing substrates such as plastic and glass, their fast drying (curing) time, and the high strength of printed materials.

[0004] Printed materials produced using UV-curable inkjet printing methods tend to have differences in curing and scratch resistance depending on the thickness of the cured ink film. This is due to factors such as the influence of oxygen in the air that comes into contact with the ink film surface (oxygen inhibition). In particular, to achieve printing speeds equivalent to those of plate-based printing methods, it is necessary to improve the ink's reactivity to UV light.

[0005] One method for improving the reactivity is to improve the efficiency of radical generation upon ultraviolet irradiation. To achieve this, a conventional method has been to use a sensitizer such as a thioxanthone compound in combination with a photopolymerization initiator. For example, Patent Document 1 discloses an ink composition containing a white pigment, an acylphosphine compound, and a thioxanthone compound, with the contents of the acylphosphine compound and the thioxanthone compound specified.

[0006] Patent Document 2 discloses a white ink composition for inkjet recording, which contains a difunctional (meth)acrylate, a trifunctional (meth)acrylate, and a tetrafunctional (meth)acrylate in a total amount of 60 mass % or more, an acylphosphine oxide compound having a polymerizable group, and a white pigment.

[0007] Furthermore, Patent Document 3 discloses an ink composition containing a white pigment, an aromatic monofunctional ethylenically unsaturated compound, and an N-vinyl lactam and / or a monofunctional ethylenically unsaturated compound having an aliphatic cyclic structure.

[0008] In addition, Patent Document 4 discloses a white inkjet ink composition containing a white pigment, an acylphosphine oxide-based photopolymerization initiator, and a non-acylphosphine oxide-based photopolymerization initiator (for example, oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone)). [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-140491 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-196425 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-191118 [Patent Document 4] Japanese Patent Application Laid-Open No. 2011-207960 Summary of the Invention [Problem to be solved by the invention]

[0010] However, when a thioxanthone compound is used in Patent Document 1, there are problems such as coloration of the ink or printed matter, and an initial hue that is inferior to that of a product without the compound. Furthermore, there is also the problem that the ink present near the nozzle of the inkjet head is easily cured by ultraviolet light leaking from the ultraviolet irradiation means within the device or light incident from outside the device, which tends to deteriorate the ejection stability. In fact, in the examples of Patent Document 1, specific examples of ink compositions containing an acylphosphine compound and a thioxanthone compound are disclosed, but the ejection stability is not evaluated, and further, the inventors have found that the ejection stability may be deteriorated depending on the printing conditions, etc.

[0011] Furthermore, the white ink disclosed in Patent Document 2 may not exhibit good ejection stability depending on the printing device and printing conditions used.

[0012] Furthermore, the ink composition specifically disclosed in Patent Document 3 may have poor curability, as well as poor adhesion and abrasion resistance of the printed matter, depending on the printing substrate (for example, when a substrate that allows the ink composition to penetrate is used) and printing conditions.

[0013] Additionally, in the case of the white inkjet ink composition disclosed in Patent Document 4, the curability and abrasion resistance may be deteriorated depending on the ultraviolet irradiation means used (for example, when a UV-LED, which will be described later, is used) and the type of printing substrate.

[0014] As described above, until now, there has been no white ultraviolet-curable inkjet ink that is excellent in all of curability, adhesion, abrasion resistance, and ejection stability.

[0015] The present disclosure has been made to solve the above-mentioned problems, and an object of the present disclosure is to provide an ultraviolet-curable inkjet ink that has excellent curability and ejection stability, as well as excellent adhesion and abrasion resistance of printed matter. [Means for solving the problem]

[0016] As a result of intensive research conducted by the present inventors to solve the above-mentioned problems, they found that the above-mentioned problems can be suitably solved by the ultraviolet-curable inkjet ink of the present disclosure, which uses a specific polyfunctional polymerizable compound and a specific fluorescent brightening agent together with titanium oxide, and thus completed the present invention.

[0017] That is, the embodiments of the present disclosure relate to the following [1] to [4]. However, the present disclosure is not limited to the following embodiments and includes various embodiments.

[0018] [1] An ultraviolet-curable inkjet ink containing titanium oxide, a photopolymerizable compound, a photopolymerization initiator, and a fluorescent brightening agent, the photopolymerizable compound includes a polyfunctional polymerizable compound, the content of the polyfunctional polymerizable compound is 50% by mass or more of the total amount of the photopolymerizable compound; The polyfunctional polymerizable compound contains a compound represented by general formula (1), the content of the compound represented by the general formula (1) is 50 mass % or more of the total amount of the polyfunctional polymerizable compound, The ultraviolet-curable inkjet ink, wherein the fluorescent brightener comprises a thiophene benzoxazoyl compound and / or a naphthalene benzoxazoyl compound. General formula (1) CH2=CH-CO-OR 1 -O-CO-CH=CH2 [In general formula (1), R 1 represents an alkylene group having 4 to 10 carbon atoms, which may have a branched structure.] [2] The ultraviolet-curable inkjet ink according to [1], wherein the ratio of the content of the photopolymerization initiator to the content of the fluorescent brightening agent (photopolymerization initiator / fluorescent brightening agent) is 11 to 1100 by mass. [3] The ultraviolet-curable inkjet ink according to [1] or [2], wherein the photopolymerization initiator contains ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide. [4] A printed matter obtained by printing the ultraviolet-curable inkjet ink according to any one of [1] to [3] on a printing substrate. [Effects of the Invention]

[0019] The present disclosure makes it possible to provide an ultraviolet-curable inkjet ink that is excellent in curability and ejection stability, and further in adhesion and abrasion resistance of printed matter. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present disclosure will be described in detail below. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made without departing from the spirit and scope of the present disclosure.

[0021] The ultraviolet-curable inkjet ink of the present disclosure (hereinafter simply referred to as "the ink of the present disclosure") uses a polyfunctional polymerizable compound containing a specific compound and a specific fluorescent brightening agent together with titanium oxide, and this configuration solves the above-mentioned problems.

[0022] Specifically, the ultraviolet-curable inkjet ink of the present disclosure contains a polyfunctional polymerizable compound in an amount of 50% by mass or more of the total amount of photopolymerizable compounds. The polyfunctional polymerizable compound includes a compound represented by general formula (1). By using a certain amount or more of a compound having a large number of polymerizable groups that serve as the starting point for the polymerization reaction, the curability of the ink can be improved. Furthermore, the crosslink density of the ink film is increased, thereby increasing the strength of the ink film regardless of its thickness. General formula (1) CH2=CH-CO-OR 1 -O-CO-CH=CH2 [In general formula (1), R 1 represents an alkylene group having 4 to 10 carbon atoms, which may have a branched structure.]

[0023] On the other hand, the compound represented by the general formula (1) has an alkylene group. Since an alkylene group is generally considered to form a soft segment, the use of the compound represented by the general formula (1) can impart flexibility to printed matter. Furthermore, the use of a photopolymerizable compound containing a medium- to long-chain alkylene group in its structure, such as the compound represented by the general formula (1), enhances affinity with printing substrates such as polyolefin substrates and paper substrates. As a result, adhesion between the printed matter and the printing substrate is improved. Furthermore, since the compound represented by the general formula (1) has a relatively low viscosity among photopolymerizable compounds, the ink jetting stability can also be improved. From these perspectives, i.e., from the perspective of improving the ink curability and jetting stability, as well as the flexibility and adhesion of printed matter, the ultraviolet-curable inkjet ink of the present disclosure contains the compound represented by the general formula (1) in an amount of 50% by mass or more of the total amount of polyfunctional polymerizable compounds in the inkjet ink.

[0024] Generally, white pigments, including titanium oxide, have the property of reflecting and scattering ultraviolet light. Therefore, when ultraviolet light is irradiated onto an ultraviolet-curable inkjet ink containing such a white pigment, there is a risk that a sufficient amount of ultraviolet light may not penetrate the inkjet ink. In such cases, the photopolymerization initiator may not function properly, resulting in problems such as poor curability, adhesion, and abrasion resistance of the inkjet ink. In particular, titanium oxide has a higher hardness than other white pigments, and also a high ability to reflect and scatter ultraviolet light. Therefore, while it is easy to obtain printed matter with excellent abrasion resistance, it is prone to the aforementioned poor curability and poor adhesion and abrasion resistance of the printed matter. In particular, when an ultraviolet light-emitting diode (UV-LED) is used as a UV light source, the narrow wavelength range of the ultraviolet light emitted from the UV-LED tends to result in poor curability, adhesion, and abrasion resistance.

[0025] Therefore, in this disclosure, a thiophene benzoxazolyl-based compound and / or a naphthalene benzoxazolyl-based compound is used as a fluorescent brightening agent to enhance the curing properties of the inkjet ink. While the detailed mechanism is unknown, it is believed that these fluorescent brightening agents are excited by ultraviolet light, and then the photopolymerization initiator in the inkjet ink reacts with the emitted fluorescence, or alternatively, the excitation energy is transferred to the photopolymerization initiator. As a result, even in inkjet inks containing titanium oxide, the reaction of the photopolymerizable compound proceeds sufficiently, improving the curing properties of the ink film interior as well as adhesion to the printing substrate and abrasion resistance.

[0026] Furthermore, unlike the above-mentioned thioxanthone compounds, fluorescent brightening agents are not themselves directly involved in the polymerization reaction, and therefore are less likely to cause a deterioration in ejection stability, and although the detailed mechanism is unknown, it is thought that the viscoelasticity of the ink is optimized and the ejection stability is improved by the interaction via the thiophene benzoxazoyl compound and / or the naphthalene benzoxazoyl compound.Furthermore, because fluorescent brightening agents have the property of emitting purple to blue fluorescence, they also have the effect of making the yellowish color tone of the ink film appear paler.

[0027] As described above, in order to simultaneously improve all of the curability and ejection stability, as well as the adhesion and abrasion resistance of printed matter, the ultraviolet-curable inkjet ink of the present disclosure having the above-described configuration is essential.

[0028] In the UV-curable inkjet ink of the present disclosure, an acylphosphine oxide initiator is preferably used as the photopolymerization initiator used in the polymerization reaction. Compared to other photopolymerization initiators, acylphosphine oxide initiators can absorb UV light over a wide wavelength range, including long-wavelength UV light. As described above, the UV-curable inkjet ink of the present disclosure contains a fluorescent brightening agent. Generally, the fluorescence emitted from the fluorescent brightening agent has lower energy than the energy of the absorbed UV light. That is, the wavelength of the fluorescence is longer than the wavelength of the absorbed UV light. Therefore, in the UV-curable inkjet ink of the present disclosure, by using a photopolymerization initiator that absorbs in the long-wavelength range, i.e., an acylphosphine oxide initiator, in combination with the ink, the fluorescence emitted from the fluorescent brightening agent can be effectively utilized, resulting in significantly improved curability. Furthermore, as described above, the acylphosphine oxide initiator can absorb UV light over a wide wavelength range, thereby improving the curability of the ink film not only on the surface but also inside the ink film.

[0029] In particular, in the present disclosure, it is preferable to use ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide as the acylphosphine oxide initiator. Because ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide is a liquid at room temperature, it is possible to increase the amount of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide blended in the ink while suppressing an increase in the viscosity of the inkjet ink, compared to other acylphosphine oxide initiators that are solid at room temperature, and this is effective in improving the ejection stability and curability.

[0030] Next, each of the components constituting the ink of the present disclosure will be described in detail below.

[0031] ≪Photopolymerizable compound≫ The ink of the present disclosure contains a photopolymerizable compound. The photopolymerizable compound also contains a polyfunctional polymerizable compound. In this disclosure, the term "photopolymerizable compound" refers to a compound that reacts with radicals generated from a photopolymerization initiator or the like to cause a polymerization reaction or a crosslinking reaction. In addition, the term "polyfunctional polymerizable compound" refers to a photopolymerizable compound having two or more photopolymerizable groups. Examples of the photopolymerizable group include a (meth)acryloyl group and a vinyl group (excluding a (meth)acryloyl group).

[0032] Generally, when a photopolymerizable compound contains a monofunctional polymerizable compound (a photopolymerizable compound having one photopolymerizable group) and a polyfunctional polymerizable compound, the greater the content of the monofunctional polymerizable compound, the more flexible the ink film tends to be, and the greater the content of the polyfunctional polymerizable compound, the more the curability of thin and thick films and the strength of the ink film tend to improve. As mentioned above, in the ink of the present disclosure, the content of the polyfunctional polymerizable compound is 50% by mass or more of the total amount of photopolymerizable compounds, preferably 70% by mass or more, and particularly preferably 85% by mass or more.

[0033] The ink of the present disclosure also contains a compound represented by general formula (1) as the polyfunctional polymerizable compound. The compound represented by general formula (1) is present in an amount of 50% by mass or more, preferably 65% ​​by mass to 85% by mass, of the total amount of the polyfunctional polymerizable compound. As described above, the medium- to long-chain alkylene group present in the compound represented by general formula (1) imparts flexibility to the ink film while enhancing affinity with printing substrates such as polyolefin substrates and paper substrates, thereby enhancing adhesion to the printing substrate. Furthermore, the compound represented by general formula (1) has a relatively low viscosity among photopolymerizable compounds, thereby improving the ink ejection stability. Furthermore, the compound represented by general formula (1) itself is a polyfunctional polymerizable compound. Therefore, it goes without saying that the compound represented by general formula (1) is also effective in improving the curability of the ink and the abrasion resistance of printed matter. In this way, by setting the blending amount of the polyfunctional polymerizable compound to 50 mass % or more of the total amount of photopolymerizable compounds, and further setting the blending amount of the compound represented by the general formula (1) above within the above range, the adhesion and abrasion resistance of the printed matter, as well as the ejection stability and curing properties of the ink, are all at good levels.

[0034] In one embodiment, it is also suitable that the amount of the compound represented by general formula (1) is 90 to 100 mass % of the total amount of the polyfunctional polymerizable compounds. When most or all of the polyfunctional polymerizable compounds contained in the ink of the present disclosure are compounds represented by general formula (1), it becomes easier to obtain printed matter with exceptionally excellent adhesion.

[0035] In general formula (1), R 1 The number of carbon atoms in R may be 4 to 10, 4 to 8, or 4 to 6. 1 may be a branched or linear alkylene group.

[0036] Examples of the compound represented by general formula (1) include 1,3-butylenediol diacrylate, 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, 1,8-octanediol diacrylate, 1,9-nonanediol diacrylate, neopentyl glycol diacrylate, 3-methyl-1,5-pentanediol diacrylate, 2,4-dimethyl-1,5-pentanediol diacrylate, 2-ethyl-2-butylpropanediol diacrylate, 2-ethyl-2-butylbutanediol diacrylate, etc. In order to improve curability, all of the polymerizable groups contained in the compound represented by general formula (1) are acryloyl groups.

[0037] Among these compounds, the ink of the present disclosure preferably uses 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 are effective in improving ejection stability, while also improving the wetting and spreading properties of the ink on the printing substrate, and improving the curing properties of the ink, as well as the adhesion and abrasion resistance of the printed matter.

[0038] In one embodiment, it is preferable to use 1,6-hexanediol diacrylate and / or 1,4-butanediol diacrylate in combination with 3-methyl-1,5-pentanediol diacrylate. By using these compounds in combination, the cure shrinkage of the ink film is suppressed, and the inclusion of a certain amount of 3-methyl-1,5-pentanediol diacrylate, which contains an alkylene group with a branched structure, significantly improves adhesion. Furthermore, while the detailed mechanism is unclear, optimizing the viscoelasticity of the ink also significantly improves ejection stability.

[0039] When 1,6-hexanediol diacrylate and / or 1,4-butanediol diacrylate are used in combination with 3-methyl-1,5-pentanediol diacrylate as the compound represented by general formula (1), the content of 3-methyl-1,5-pentanediol diacrylate relative to the total content of 1,6-hexanediol diacrylate and 1,4-butanediol diacrylate is preferably 100 to 900% by mass, more preferably 150 to 900% by mass, and particularly preferably 230 to 900% by mass. By keeping the content of 3-methyl-1,5-pentanediol diacrylate within the above range, adhesion and ejection stability are improved.

[0040] From the viewpoint of improving the adhesion and abrasion resistance of the printed matter, as well as the ejection stability and curing properties of the ink, the amount of the compound represented by the general formula (1) is preferably 45 mass % or more, and particularly preferably 60 to 85 mass %, of the total amount of the photopolymerizable compounds.

[0041] In one embodiment, the content of the compound represented by general formula (1) in the photopolymerizable compound is preferably 90 to 100% by mass. In this case, the medium- to long-chain alkylene group present in the compound represented by general formula (1) is thought to increase the flexibility of the ink film. As a result, the curability is maintained in a suitable state, while the adhesion to the printing substrate is also improved.

[0042] The ink of the present disclosure may contain a polyfunctional polymerizable compound other than the compound represented by general formula (1) (also referred to as "other polyfunctional polymerizable compounds" in the present disclosure), and two or more of these may be used in combination. As the other polyfunctional polymerizable compound, for example, a compound having two (meth)acryloyl groups among photopolymerizable compounds (photopolymerizable monomers) that are monomers can be used. Specific examples of the compound include 1,3-propanediol di(meth)acrylate, 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, 1,10-decanediol 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, hydroxypivalic acid neopentyl glycol di(meth)acrylate, bisphenol A di(meth)acrylate, ethylene oxide modified bisphenol A di(meth)acrylate, propylene oxide modified bisphenol Examples of the di(meth)acrylate 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, dimethyloltricyclodecane di(meth)acrylate, cyclohexanedimethanol di(meth)acrylate, trimethylolpropane di(meth)acrylate, neopentyl glycol-modified trimethylolpropane di(meth)acrylate, and dicyclopentanyl di(meth)acrylate.

[0043] Among these, it is preferable to use one or more compounds selected from the group consisting of dipropylene glycol di(meth)acrylate and tripropylene glycol di(meth)acrylate, because the compatibility of the curability of the ink film surface and the curability of the ink film is significantly improved, and also from the viewpoint of improving the discharge stability and the abrasion resistance of the ink film. In this case, from the viewpoints mentioned above, i.e., the curability and abrasion resistance, the total content of dipropylene glycol di(meth)acrylate and tripropylene glycol di(meth)acrylate is preferably 1 to 45 mass%, and particularly preferably 5 to 35 mass%, of the total amount of the photopolymerizable compounds.

[0044] As mentioned above, in one embodiment, it is also suitable that the content of the compound represented by general formula (1) in the photopolymerizable compound is 90 to 100 mass %. In this case, it is preferable to use substantially no dipropylene glycol di(meth)acrylate and no tripropylene glycol di(meth)acrylate, and to use 1,6-hexanediol diacrylate and / or 1,4-butanediol diacrylate, and 3-methyl-1,5-pentanediol diacrylate in combination, from the viewpoint of improving adhesion and curability.

[0045] In the present disclosure, "(meth)acryloyl" refers to "acryloyl" and / or "methacryloyl", and "(meth)acrylate" refers to "acrylate" and / or "methacrylate". In addition, the term "monomer" in this disclosure refers to the smallest unit in a polymer obtained by a polymerization reaction and / or a crosslinking reaction. Furthermore, the terms "photopolymerizable compound as a monomer" and "photopolymerizable monomer" refer to a monomer among photopolymerizable compounds that has one or more photopolymerizable groups.

[0046] Additionally, "substantially not used" means that the target component is not added intentionally, and does not prevent its inclusion as, for example, an impurity, by-product, etc. Specifically, when the amount of impurities, by-products, etc. mixed in is 0.1% by mass or less (preferably 0.05% by mass or less) of the total amount of ink, this is considered to be included in the state of "substantially not used."

[0047] On the other hand, as other polyfunctional polymerizable compounds, photopolymerizable monomers having three (meth)acryloyl groups can also be used. Specific examples of the 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 ... Examples of the monomer include hydroxypropyl 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, and sorbitol tri(meth)acrylate.

[0048] Among the compounds listed above, it is preferable to use glycerin triacrylate from the viewpoint of improving curability, discharge stability, and adhesion to the printing substrate. In this case, from the viewpoints mentioned above, i.e., curability, discharge stability, and coating film resistance, the content of glycerin triacrylate is preferably 1 to 15 mass %, and particularly preferably 2 to 12 mass %, of the total amount of the photopolymerizable compound.

[0049] Furthermore, as other polyfunctional polymerizable compounds, photopolymerizable monomers having four (meth)acryloyl groups can also be used. 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.

[0050] Among the compounds listed above, ditrimethylolpropane tetraacrylate is preferably used from the viewpoint of improving curability, adhesion to the printing substrate, and abrasion resistance of the printed matter. In this case, from the viewpoints mentioned above, i.e., curability, adhesion, and abrasion resistance, the total content of ditrimethylolpropane tetraacrylate is preferably 1 to 12 mass%, and particularly preferably 2 to 10 mass%, of the total amount of the photopolymerizable compound.

[0051] Furthermore, when a photopolymerizable monomer having five (meth)acryloyl groups is used as another polyfunctional polymerizable compound, specific examples of the photopolymerizable monomer include sorbitol penta(meth)acrylate and dipentaerythritol penta(meth)acrylate.

[0052] Furthermore, when a photopolymerizable monomer having six (meth)acryloyl groups is used as another polyfunctional polymerizable compound, specific examples of the photopolymerizable monomer include dipentaerythritol hexa(meth)acrylate, sorbitol hexa(meth)acrylate, alkylene oxide-modified hexa(meth)acrylate of phosphazene, and ε-caprolactone-modified dipentaerythritol hexa(meth)acrylate.

[0053] The photopolymerizable monomer used as the other polyfunctional polymerizable compound may be a modified version of the compound represented by the general formula (1) or the other polyfunctional polymerizable compound listed above. Examples of such modifications include sulfonic acid modification, phosphoric acid modification, amine modification, and mercapto modification. For example, a compound (having one or more acryloyl groups) obtained by reacting (Michael addition reaction) 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 with an amine compound can be used as the other polyfunctional polymerizable compound. These compounds can be suitably used in the ink of the present disclosure because they can simultaneously improve all of the ejection stability, curing properties, and adhesion properties.

[0054] Furthermore, as other polyfunctional polymerizable compounds, photopolymerizable compounds (photopolymerizable oligomers) that are oligomers can also be used. In this case, compounds having a (meth)acryloyl group as the polymerizable group are preferably used. The number of polymerizable groups contained in the polymerizable oligomer is preferably 2 to 6 per molecule from the viewpoint of the balance between curability, discharge stability, and dispersion stability. The number of polymerizable groups is more preferably 2 to 4, and particularly preferably 2. The weight average molecular weight of the polymerizable oligomer is preferably 400 to 12,000, and more preferably 500 to 10,000.

[0055] In this disclosure, the term "oligomer" refers to a compound formed by polymerizing a small number and finite number of monomers (e.g., 2 to 20). The terms "photopolymerizable compound that is an oligomer" and "photopolymerizable oligomer" refer to a photopolymerizable compound having one or more photopolymerizable groups. However, the monomers constituting the photopolymerizable oligomer may contain a monomer that is not a photopolymerizable monomer (excluding monomers used to impart photopolymerizable groups). For example, a photopolymerizable urethane oligomer obtained by reacting 2-hydroxyethyl acrylate with a urethane oligomer having an isocyanate group at its terminal, which is a reaction product of 1,6-hexanediol and isophorone diisocyanate, contains 1,6-hexanediol and isophorone diisocyanate as monomers.

[0056] Examples of the polymerizable oligomer having a (meth)acryloyl group include urethane (meth)acrylate oligomers such as aliphatic urethane (meth)acrylate oligomers and aromatic urethane (meth)acrylate oligomers; acrylic (meth)acrylate oligomers; polyester (meth)acrylate oligomers; polyether (meth)acrylate oligomers; and epoxy (meth)acrylate oligomers. The oligomers may be modified. Examples of such modifications include sulfonic acid modification, phosphoric acid modification, amine modification, and mercapto modification.

[0057] When the ink of the present disclosure contains another polyfunctional polymerizable compound, the content thereof is preferably 1 to 50 mass % of the total amount of the ink, and more preferably 5 to 35 mass %. By setting the blending amount within the above range, it is possible to easily improve all of the curing property and discharge stability of the ink, as well as the adhesion and abrasion resistance of the printed matter.

[0058] On the other hand, a monofunctional polymerizable compound can also be used as the other polymerizable compound. For example, a compound having one (meth)acryloyl group among photopolymerizable monomers can be used as the monofunctional polymerizable compound. 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, o-phenylphenol EO-modified acrylate, 2-ethylhexyl EO-modified acrylate, β-carboxylethyl (meth)acrylate, cyclic trimethylolpropane formal ( (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-hydroxybutyl acrylate, N-vinylcaprolactam, N-vinylpyrrolidone, 5-methyl-3-vinyloxazolidin-2-one, acryloylmorpholine, N-acryloyloxyethyl hexahydrophthalimide, and the like.

[0059] Furthermore, as the monofunctional polymerizable compound, a compound having one vinyl group (excluding a (meth)acryloyl group) among photopolymerizable monomers can also be used. Specific examples of such compounds include N-vinylpyrrolidone, N-vinylvalerolactam, N-vinylcaprolactam, N-vinylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide, N-vinyloxazolidinone, and N-vinyl-5-methyloxazolidinone.

[0060] The monofunctional polymerizable compound may be a photopolymerizable oligomer having one (meth)acryloyl group. Specific examples of such compounds include KRM9276 manufactured by Daicel-Allnex Co., Ltd. and CN131NS, CN131BNS, CN146NS, CN153NS, CN3108NS, CN7002NS, CN8004NS, and CN9003NS manufactured by Arkema.

[0061] Of the monofunctional polymerizable compounds listed above, it is preferable to use N-vinyl-5-methyloxazolidinone in the ink of the present disclosure. N-vinyl-5-methyloxazolidinone is highly reactive, which enhances the curing properties of the ink and also improves adhesion to the printing substrate. Furthermore, even when N-vinyl-5-methyloxazolidinone is used, the viscosity of the ink is less likely to increase, which prevents deterioration of the ejection stability.

[0062] When the ink of the present disclosure contains a monofunctional polymerizable compound, the content thereof is preferably 1 to 20 mass% of the total amount of the ink, more preferably 1 to 10 mass%, and particularly preferably 1 to 8 mass%. By adjusting the blending amount within the above range, flexibility is added to the ink film, improving the adhesion of the printed matter. Furthermore, it becomes easier to maintain good curability and abrasion resistance of the ink.

[0063] Furthermore, when the ink of the present disclosure contains a monofunctional polymerizable compound, the compound having one vinyl group (excluding a (meth)acryloyl group) is preferably used in an amount of 50 mass % or more, more preferably 70 mass % or more, and particularly preferably 90 mass % or more of the total amount of the monofunctional polymerizable compound. By keeping the amount of the compound having one vinyl group (excluding a (meth)acryloyl group) used within the above range, it is easy to significantly improve the adhesion of the printed matter and the curing properties of the ink while maintaining favorable discharge stability.

[0064] Furthermore, when the ink of the present disclosure contains a compound having one vinyl group (excluding a (meth)acryloyl group) as a monofunctional polymerizable compound, the content of the compound having one vinyl group (excluding a (meth)acryloyl group) relative to the content of the compound represented by general formula (1) above is preferably 1 to 20 mass%, and particularly preferably 1.5 to 15 mass%. By keeping the content of the compound having one vinyl group within the above range, the adhesion of the printed matter, as well as the curing properties and ejection stability of the ink, are all favorable.

[0065] The content of the photopolymerizable compound is preferably 60 to 80 mass % of the total amount of the ink, more preferably 65 to 80 mass %, and particularly preferably 70 to 80 mass %.

[0066] <Photopolymerization initiator> The ink of the present disclosure contains a photopolymerization initiator. One or more conventionally known compounds can be used as the photopolymerization initiator. For example, a compound capable of absorbing the energy of actinic rays and generating radicals can be used. Specific examples of usable compounds include acylphosphine oxide compounds, benzophenone compounds, indan compounds, thioxanthone compounds, hydroxyacetophenone compounds, alkylaminoacetophenone compounds, and oxime ester compounds.

[0067] Among these photopolymerizable compounds, it is preferable to use an acylphosphine oxide initiator in the ink of the present disclosure, from the viewpoint that, as described above, it is possible to absorb the energy of light in a wide wavelength range, including the fluorescence emitted from the fluorescent brightener, and to improve not only the curability of the ink film surface but also the curability of the interior of the ink film.

[0068] 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. Commercially available acylphosphine oxide compounds include "Omnirad TPO," "Omnirad TPO-L," "Omnirad TPO-H," "Omnirad 819," and "OMNIPOL TP" manufactured by IGM Resins, and "Speedcure TPO," "Speedcure TPO-L," and "Speedcure BPO" manufactured by Lambson. In addition, for example, acylphosphine oxide compounds and lithium phenyl(2,4,6-trimethylbenzoyl)phosphinate described in WO 2017 / 086224 and WO 2020 / 049378 can also be used. In the ink of the present disclosure, the above-listed acylphosphine oxide compounds may be used alone or in combination of two or more.

[0069] Among these compounds, the present disclosure preferably uses ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide. As described above, ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide is a liquid at room temperature and is an effective material for improving curability while maintaining ejection stability. Although the detailed mechanism is unknown, when ink droplets land on a printing substrate, ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide tends to orient on the ink droplet surface. Radicals are generated when irradiated with ultraviolet light in this state, so ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide is thought to be particularly effective in improving the curability of the ink film surface. Furthermore, acylphosphine oxide photopolymerization initiators generally exhibit a photobleaching effect. That is, after generating radicals and decomposing, the acylphosphine oxide photopolymerization initiator loses its ultraviolet absorption ability, improving the ultraviolet transmittance into the ink film interior. As mentioned above, ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide is thought to be easily oriented on the droplet surface, and as the polymerization reaction progresses, a photobleaching effect is likely to occur, making it easier for ultraviolet light to penetrate into the ink film.

[0070] When the ink of the present disclosure contains ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, it is preferable that it further contains another photopolymerization initiator. In particular, as described above, it is preferable to use an acylphosphine oxide initiator other than ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide as the other photopolymerization initiator, from the viewpoint that it can absorb light energy over a wide wavelength range, including fluorescence emitted from fluorescent brighteners, and thereby significantly improve curing properties. For example, it is believed that the reactivity can be further improved by blending phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide with ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide in the ink. Although this is speculation, the mechanism is thought to be that phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide can be uniformly present in the ink droplets, and after ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide exhibits a photobleaching effect, the phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide reacts with ultraviolet light that penetrates into the ink film, improving the internal curing of the ink film. Furthermore, when an acylphosphine oxide initiator other than ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (e.g., phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide) is used as the other photopolymerization initiator, both the surface curing and internal curing of the ink are improved, which also improves the abrasion resistance of the ink film.

[0071] From the viewpoint of improving the curing properties while maintaining the ejection stability without excessively increasing the viscosity of the ink, it is preferable to incorporate the photopolymerization initiator in an amount of 5 to 15% by mass in the ultraviolet-curable inkjet ink.

[0072] Furthermore, when the photopolymerization initiator contains ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, the content thereof in the ultraviolet-curable inkjet ink is preferably 3 to 10% by mass.

[0073] Furthermore, when the photopolymerization initiator contains phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, the content thereof in the ultraviolet-curable inkjet ink is preferably 2 to 5% by mass.

[0074] In one embodiment, when the ink of the present disclosure contains a monoacylphosphine oxide initiator, its content is preferably 45 to 85 mass% of the total amount of photopolymerization initiators contained in the ink, and particularly preferably 60 to 75 mass%. Monoacylphosphine oxide initiators have high solubility in compounds represented by general formula (1) and also have high radical generation efficiency. Therefore, by setting the content of the monoacylphosphine oxide initiator within the above range, it is easy to obtain an ink that is excellent in all of curability, ejection stability, and even abrasion resistance of printed matter.

[0075] Examples of the monoacylphosphine oxide initiator include ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide.

[0076] Furthermore, from the viewpoint that an ink with excellent curability can be obtained by reacting effectively with the fluorescence emitted from the fluorescent brightening agent, when the photopolymerization initiator contains ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, the content thereof is preferably 5 to 150, and particularly preferably 10 to 75, relative to the mass content of the fluorescent brightening agent in the ink, taken as 1.

[0077] <Fluorescent whitening agents> The ink of the present disclosure contains a fluorescent brightening agent. A fluorescent brightening agent is a compound that absorbs light in a wavelength range of approximately 200 to 400 nm and emits fluorescence with a wavelength longer than the absorption wavelength. Specific examples of compounds used as fluorescent brightening agents include naphthalene benzoxazoyl compounds, thiophene benzoxazoyl compounds, stilbene benzoxazoyl compounds, stilbene biphenyl compounds, distyrylbenzene compounds, coumarin compounds, carbostyryl compounds, pyrazolone compounds, naphthalimide compounds, pyrene compounds, and pyridotriazole compounds.

[0078] Among these compounds, the present disclosure uses thiophene benzoxazoyl compounds and / or naphthalene benzoxazoyl compounds as fluorescent brightening agents. Compared to other fluorescent brightening agents, these fluorescent brightening agents have absorption and fluorescence wavelengths that are suitable for improving curability, and even adding a small amount makes it easy to significantly improve the curability of inkjet inks. Furthermore, thiophene benzoxazoyl compounds and naphthalene benzoxazoyl compounds are relatively less colored, so they can prevent the ink itself from being colored. Furthermore, they have the property of emitting purple to blue fluorescence, which has the effect of making the ink film appear paler.

[0079] Furthermore, in the ink of the present disclosure, it is particularly preferable to use a thiophene benzoxazolyl-based compound as a fluorescent brightening agent, because the high absorbance significantly improves the curing properties of the ink, and the amount used can be reduced, making it easy to improve the discharge stability.

[0080] Specific examples of the thiophene benzoxazolyl compounds and naphthalene benzoxazolyl compounds include "Tinopal OB" and "Tinopal OB CO" manufactured by BASF, "SpeedBlock OB-184" manufactured by Arkema, "NF-TH01" and "NF-NA01" manufactured by Nippon Chemical Industry Co., Ltd., and "FLUORESCENT BRIGHTNER KCB," "FLUORESCENT BRIGHTNER OB," and "FLUORESCENT BRIGHTNER PB" manufactured by Xcolor.

[0081] From the viewpoint of obtaining an ink that is excellent in all of curability, ejection stability, and abrasion resistance of printed matter, the content of the fluorescent brightener in the ultraviolet-curable inkjet ink is preferably 0.01 to 1.5 mass%, more preferably 0.05 to 1 mass%, and particularly preferably 0.1 to 0.7 mass%.

[0082] On the other hand, in the present disclosure, it is preferable to control the amount of fluorescent brightening agent to improve both the ink curability and the adhesion of printed matter. Specifically, the ratio of the total photopolymerization initiator content to the fluorescent brightening agent content (photopolymerization initiator / fluorescent brightening agent) is preferably 11 to 1100 by mass, and particularly preferably 25 to 250. By keeping the fluorescent brightening agent content within the above range, the fluorescent brightening agent dissolves uniformly in the photopolymerizable compound in the ink, allowing the agent to fully function as a fluorescent brightening agent, improving the ink curability and the abrasion resistance of printed matter. Furthermore, clogging of the inkjet head is prevented, and ejection stability is improved.

[0083] <Titanium oxide> In the present disclosure, titanium dioxide is used as a white pigment. As mentioned above, white pigments reflect and scatter light, reducing the efficiency of radical generation from the photopolymerization initiator. In particular, when a UV-LED is used as the ultraviolet light source, the narrow wavelength range of the emitted ultraviolet light may further accelerate the reduction in the efficiency of radical generation. Therefore, in the present disclosure, by using a thiophene benzoxazolyl-based compound and / or a naphthalene benzoxazolyl-based compound as a fluorescent brightening agent, it is possible to improve the curability of a system containing titanium dioxide while reducing the amount of photopolymerization initiator added.

[0084] The titanium oxide used in the ink of the present disclosure preferably has a secondary particle diameter of 180 to 250 nm. By using titanium oxide with a secondary particle diameter of 180 nm or more, printed matter with sufficient hiding power can be obtained, and the scratch resistance of the printed matter can also be improved. Furthermore, if the secondary particle diameter is 250 nm or less, excessive settling of titanium oxide can be suppressed, making it easier to obtain an ink film with good curing properties while suppressing deterioration of discharge stability. The secondary particle diameter is preferably 200 to 250 nm, and more preferably 220 to 250 nm.

[0085] The "secondary particle size" refers to the particle size of titanium oxide dispersed in the ink, and represents the median size measured on a volume basis. The secondary particle size can be measured using a dynamic light scattering particle size analyzer (e.g., Microtrac-Bell's Nanotrac UPA-EX150) and inkjet ink diluted with ethyl acetate to a concentration that allows measurement of the secondary particle size using the particle size analyzer.

[0086] The primary particle diameter of the titanium oxide is not particularly limited, but is preferably equal to or smaller than the secondary particle diameter. This is because, in order to keep the secondary particle diameter within the preferred range, it is not necessary to destroy the titanium oxide or intentionally remove the surface treatment agent, resulting in good dispersion stability and ejection stability. Specifically, it is preferable to use titanium oxide having a primary particle diameter of 150 to 240 nm, more preferably titanium oxide having a primary particle diameter of 180 to 230 nm, and most preferably titanium oxide having a primary particle diameter of 200 to 230 nm.

[0087] The primary particle diameter of titanium oxide can be calculated, for example, by taking multiple images using a transmission electron microscope (TEM), measuring and calculating the diameters of circles having the same area as the areas of a total of 100 titanium oxide particles present in the multiple images (equivalent circle diameters), and then averaging these values.

[0088] Commercially available titanium oxide products having a primary particle size of 150 to 240 nm include Typepaque CR-60, CR-60-2, CR-63, CR-67, CF-80, PF-690, PF-691, A-100, and A-220 manufactured by Ishihara Sangyo Kaisha, Ltd., and 2064, 2190, and 2310 manufactured by Cronos Co., Ltd. Among these, CR-60-2, CR-63, PF-690, PF-691, PF-726, and PF-728 manufactured by Ishihara Sangyo Kaisha, Ltd., and 2064, 2190, and 2310 manufactured by Cronos Co., Ltd., in which the surface of titanium oxide has been organically treated, are particularly preferably used.

[0089] The titanium oxide used in the present disclosure can be either anatase or rutile, but it is preferable to use rutile, as it can easily improve the hiding power of printed matter. Titanium oxide produced by either the chlorine method or the sulfuric acid method can be used, but it is preferable to use titanium oxide produced by the chlorine method, as it can produce printed matter with high whiteness.

[0090] The titanium oxide may be surface-treated with an inorganic and / or organic substance. Surface treatment can be carried out as needed to reduce the catalytic activity of the titanium oxide surface, control hydrophilicity, and improve weather resistance and dispersion stability. Examples of inorganic substances used for surface treatment include alumina, silica, zirconia, titania, tin oxide, and hydrates of these compounds (e.g., hydrous alumina, hydrous zirconia, etc.). Examples of organic substances include polyhydric alcohols, alkanolamines, organosilicon compounds, organophosphate compounds, and higher fatty acids.

[0091] Among these, titanium oxide treated with (hydrated) alumina and (hydrated) silica, as well as with an organic substance, is preferred because it provides an ink with high dispersion stability and ejection stability. Titanium oxide surface-treated with silica has hydroxyl groups on its surface, which improves its adsorption to the pigment dispersion resin described below due to intermolecular interactions such as hydrogen bonding. For the same reason, it is particularly preferred to use one or more organic substances selected from the group consisting of polyhydric alcohols, alkanolamines, and organosilicon compounds as the surface treatment.

[0092] The above-mentioned "(hydrous) alumina" refers to alumina and / or hydrous alumina, "(hydrous) silica" refers to silica and / or hydrous silica, and "(hydrous) zirconia" refers to zirconia and / or hydrous zirconia.

[0093] The amount of titanium oxide in the ink is preferably 10 to 25% by mass. By keeping the amount within this range, high hiding power can be achieved even with a thin film thickness of the printed matter, and the storage stability and discharge stability of the ink, as well as the abrasion resistance and adhesion of the printed matter, can be easily improved. From this perspective, the amount is more preferably 13 to 23% by mass.

[0094] Incidentally, light (including ultraviolet light) incident on titanium oxide is scattered on its surface. The wavelength distribution of the scattered light changes depending on the secondary particle size of the white pigment. In the case of the ink of the present disclosure, it is thought that curing due to the scattered light progresses depending on the absorption wavelength of the fluorescent whitening agent contained in the ink.

[0095] That is, when the maximum absorption wavelength of the thiophene benzoxazoyl compound and / or naphthalene benzoxazoyl compound contained in the ink of the present disclosure is A W (nm), the amount of titanium oxide having a particle size (unit: nm) measured on a volume basis of A W x 1 / 4 to A W x 2 / 3 is preferably 10 to 50 mass%, and particularly preferably 20 to 50 mass%, of the total mass of titanium oxide contained in the ink. Inks containing a predetermined amount of titanium oxide within the above particle size range not only have excellent hiding power of printed matter, but also improve the curing properties of the ink and the scratch resistance of printed matter.

[0096] For the same reason, i.e., from the viewpoint of significantly improving the curability of the ink and the abrasion resistance and hiding power of the printed matter, the amount of titanium oxide having a particle size (unit: nm) measured on a volume basis of AW×2 / 5 to AW×5 / 9 is preferably 8 to 45 mass%, and particularly preferably 15 to 45 mass%, of the total mass of titanium oxide contained in the ink.

[0097] Furthermore, from the viewpoint of improving the curability of the ink and the abrasion resistance and hiding power of the printed matter regardless of the types of thiophene benzoxazoyl compound and naphthalene benzoxazoyl compound used, the amount of titanium oxide having a particle size measured on a volume basis of 150 to 200 nm is preferably 8 to 45 mass%, and particularly preferably 15 to 45 mass%, of the total mass of titanium oxide contained in the ink.

[0098] The maximum absorption wavelength can be confirmed by measuring the absorption spectrum of a dimethylformamide solution of the target compound using a visible-ultraviolet spectrophotometer (e.g., "UV-3600" manufactured by Shimadzu Corporation). The particle size measured on a volume basis is measured in the same manner as in the case of the secondary particle size.

[0099] <Pigment dispersion resin> In the present disclosure, the use of a pigment dispersion resin is preferred from the viewpoints of improving the initial dispersibility of titanium oxide, as well as the storage stability and ejection stability of the ink, and furthermore, of improving the scratch resistance of printed matter by enabling the titanium oxide to be uniformly dispersed within the ink film. The pigment dispersion resin may be commercially available or may be synthesized by a conventional method. Specific examples of commercially available products include "Ajisper PB-821," "Ajisper PB-822," "Ajisper PB-824," and "Ajisper PB-881" manufactured by Ajinomoto Fine-Techno Co., Ltd.; "BYKJET-9150," "BYKJET-9151," and "BYKJET-9152" manufactured by BYK-Chemie; and "Solsperse 24000," "Solsperse 32000," "Solsperse 33000," "Solsperse 39000," "Solsperse J180," and "Solsperse J200" manufactured by The Lubrizol Corporation. 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 another polymerizable compound such as styrene, α-methylstyrene, methyl methacrylate, butyl methacrylate, or lauryl methacrylate may also be used as the pigment dispersing resin.

[0100] The weight-average molecular weight of the 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 compound represented by general formula (1) is improved, resulting in improved ink storage stability and ejection stability. Furthermore, it becomes easier to homogenize the titanium oxide within the ink film, improving the abrasion resistance and hiding power of the printed material.

[0101] The amount of pigment dispersing resin added is preferably 1 to 100% by mass, more preferably 3 to 50% by mass, and even more preferably 5 to 25% by mass, based on the total amount of titanium oxide. By using the pigment dispersing resin within this range, the initial dispersibility of titanium oxide, as well as the storage stability and discharge stability of the ink, are improved, and the scratch resistance of the printed matter is also improved.

[0102] <Other ingredients> In addition to the components described above, the ink of the present disclosure may contain a surface tension adjuster, a polymerization inhibitor, an organic solvent, water, and other additives.

[0103] <Surface tension adjuster> The ink of the present disclosure preferably contains a surface tension modifier, from the viewpoints of improving the wetting and spreading of inkjet ink droplets on a printing substrate, improving abrasion resistance and adhesion, and improving the ejection stability of the inkjet ink. In the ink of the present disclosure, the surface tension modifier may be a silicone-based surface tension modifier, a fluorine-based surface tension modifier, an acetylene glycol-based surface tension modifier, or the like. Among these, it is preferable to use a silicone-based surface tension modifier, and in particular a polyether-modified silicone-based surface tension modifier, because they have excellent surface tension-reducing properties, making it easy to improve abrasion resistance and adhesion, and because they have good compatibility with the compound represented by general formula (1), improving the storage stability and ejection stability of the ink. Furthermore, it is particularly preferable to use a polyether-modified silicone-based surface tension modifier having a (meth)acryloyl group, because this significantly improves the curability of the ink and the scratch resistance of the printed matter, and because it has excellent compatibility with the photopolymerizable compound containing the compound represented by general formula (1), thereby improving discharge stability. The polyether-modified silicone-based surface tension modifier having a (meth)acryloyl group may be synthesized by a conventionally known method, or a commercially available product may be used. Examples of commercially available products include BYK-UV3500, BYK-UV3505, BYK-UV3530, BYK-UV3570, BYK-UV3575, and BYK-UV3576 (all manufactured by BYK-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, and X-22-2404 (all manufactured by Shin-Etsu Silicones Co., Ltd.), TEGO Rad 2100, TEGO Rad 2200N, TEGO Rad 2250, TEGO Rad 2300, TEGO Rad 2330, and TEGO Rad 2500, TEGO Rad 2550, TEGO Rad 2650, TEGO Rad 2700, TEGO Rad 2800 (all manufactured by Evonik).

[0104] When the ink of the present disclosure contains a silicone-based surface tension modifier, its content in the ink is preferably 0.1 to 5.0% by mass. Addition of 0.1% by mass or more easily improves the wetting and spreading of ink droplets onto the printing substrate, improving abrasion resistance and adhesion. Furthermore, adding an amount of 5.0% by mass or less makes it easy to ensure the storage stability and ejection stability of the ink.

[0105] Furthermore, when the ink of the present disclosure contains a polyether-modified silicone-based surface tension modifier having a (meth)acryloyl group, the content of the compound represented by general formula (1) is preferably 60 to 150, and particularly preferably 80 to 130, when the content of the polyether-modified silicone-based surface tension modifier having a (meth)acryloyl group is taken as 1, in order to improve compatibility with the compound represented by general formula (1), thereby improving both curability and discharge stability, and improving the scratch resistance of printed matter.

[0106] <Polymerization inhibitor> A polymerization inhibitor can be added to the ink of the present disclosure to improve ejection stability, balance the surface and internal curing properties to improve overall curing, and simultaneously improve the adhesion of printed matter and prevent curing wrinkles. Specific examples of polymerization inhibitors include hindered phenol compounds, phenol compounds (excluding hindered phenol compounds), hydroquinone compounds, phenothiazine compounds, phosphorus compounds, and nitrosophenylhydroxylamine compounds. These compounds are suitable for use as polymerization inhibitors. More specific 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 salt of N-nitrosophenylhydroxylamine. In order to optimally exert the effects of the polymerization inhibitor described above, when the ink of the present disclosure contains a polymerization inhibitor, the content thereof is preferably 0.01 to 2 mass % and more preferably 0.1 to 1 mass % relative to the total amount of the ink.

[0107] <Organic solvents, water> The ink of the present disclosure may contain an organic solvent and / or water to improve the jetting stability by optimizing the viscosity and surface tension of the ink and to improve the adhesion of the printed matter by improving the wetting and spreading properties of the ink droplets onto the printing substrate. When the ink of the present disclosure contains an organic solvent and / or water, the content thereof 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 the ink. Furthermore, when the ink of the present disclosure contains an organic solvent, in order to improve both the jetting stability and the wetting and spreading properties and adhesion properties of the ink to the printing substrate, it is preferable to use an organic solvent having a boiling point of 140 to 300°C at 1 atmosphere, and it is more preferable to use an organic solvent having a boiling point of 150 to 235°C at 1 atmosphere.

[0108] Furthermore, from the viewpoint of suitably improving all of the above-mentioned effects, i.e., ejection stability, wetting and spreading on the printing substrate, and adhesion of the printed matter, 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 used, and at least one selected from the group consisting of diethylene glycol monoethyl ether, ethylene glycol monobutyl ether acetate, diethylene glycol methyl ethyl ether, and diethylene glycol diethyl ether is particularly preferably used, in terms of excellent compatibility with photopolymerizable compounds containing compounds represented by general formula (1), particularly improving discharge stability, wetting and spreading properties on printing substrates, and adhesion of printed matter.

[0109] <Other ingredients> In addition to the components described above, the ink of the present disclosure may contain, as needed, ultraviolet absorbers, anti-fading agents, other polymer compounds, etc. Any conventionally known components may be used as these components.

[0110] <Examples of suitable configurations of the ink of the present disclosure> Below, examples of the configuration of ultraviolet-curable inkjet inks that can suitably solve the problems of the present disclosure are shown. (A) The ultraviolet-curable inkjet ink, wherein, based on the total amount of the ultraviolet-curable inkjet ink, the content of titanium oxide is 10 to 25% by mass, the total content of photopolymerizable compounds is 60 to 80% by mass, the content of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide is 3 to 10% by mass, the content of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide is 2 to 5% by mass, and the total content of thiophene benzoxazoyl compounds and naphthalene benzoxazoyl compounds is 0.05 to 1.0% by mass, the photopolymerizable compounds contain a polyfunctional polymerizable compound in an amount of 70% by mass or more based on the total amount of the photopolymerizable compounds, the content of the compound represented by general formula (1) is 60 to 85% by mass based on the total amount of the photopolymerizable compounds, and the total content of dipropylene glycol di(meth)acrylate and tripropylene glycol di(meth)acrylate is 1 to 45% by mass based on the total amount of the photopolymerizable compounds.

[0111] (B) In the total amount of the ultraviolet-curable inkjet ink, the content of titanium oxide is 10 to 25% by mass, the total content of photopolymerizable compounds is 60 to 80% by mass, the content of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide is 3 to 10% by mass, the content of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide is 2 to 5% by mass, and the total content of thiophenebenzoxazoyl-based compounds and naphthalenebenzoxazoyl-based compounds is 0.05 to 1.0% by mass, and the photopolymerizable compounds contain a polyfunctional polymerizable compound in an amount of 85% by mass or more of the total amount of the photopolymerizable compounds, and the content of the compound represented by the general formula (1) is 90% by mass or more of the total amount of the photopolymerizable compound, the compound represented by general formula (1) contains 3-methyl-1,5-pentanediol diacrylate, and 1,6-hexanediol diacrylate and / or 1,4-butanediol diacrylate, but does not contain dipropylene glycol di(meth)acrylate or tripropylene glycol di(meth)acrylate, and the content of 3-methyl-1,5-pentanediol diacrylate is 100 to 900% by mass of the total content of 1,6-hexanediol diacrylate and 1,4-butanediol diacrylate.

[0112] <Physical properties of inkjet ink> The ink of the present disclosure preferably has a viscosity at 25°C of 5 to 20 mPa·s, more preferably 8 to 20 mPa·s, from the viewpoints of improving ejection stability, improving the wetting and spreading of ink droplets on a printing substrate, and improving the adhesion of printed matter. A viscosity of 5 mPa·s or higher allows the ink to be ejected smoothly from the inkjet head. A viscosity of 25 mPa·s or lower allows for stable, continuous ejection without a decrease in ejection accuracy, and also allows the ink to be smoothly wetted and spread on a printing substrate, improving adhesion to the printing substrate. Furthermore, from the viewpoints of enabling stable ejection even during high-speed printing and improving the adhesion of printed matter, the viscosity is particularly preferably 8 to 15 mPa·s. The viscosity can be measured using 1.1 mL of inkjet ink and an E-type viscometer (e.g., "TVE25L" manufactured by Toki Sangyo Co., Ltd.) equipped with a cone (diameter: 48 mm) with a cone angle of 1°34' at 25°C and a rotation speed of 20 rpm.

[0113] Furthermore, from the viewpoint of improving the ejection stability and the curing properties of printed matter, the static surface tension of the inkjet ink at 25°C is preferably 20 to 45 mN / m, and particularly preferably 22 to 40 mN / m. The static surface tension is measured by the plate method (Wilhelmy method). Specifically, for example, the static surface tension can be measured in a 25°C environment using an automatic surface tensiometer "CBVP-Z" manufactured by Kyowa Interface Science Co., Ltd. and a platinum plate.

[0114] <Inkjet ink manufacturing method> The ink of the present disclosure can be produced by a conventionally known method, for example, as follows, but the method for producing the ink is not limited to the following.

[0115] First, titanium oxide, a portion of the photopolymerizable compound (the remainder is used when preparing the ink, as described below), and, if necessary, a pigment dispersing resin, a surface tension modifier, a polymerization inhibitor, an organic solvent and / or water, etc. are thoroughly mixed. After mixing for a certain period of time, a dispersion treatment is carried out using a paint shaker, sand mill, roll mill, medialess disperser, etc. to produce a titanium oxide dispersion.

[0116] Next, the remainder of the photopolymerizable compound, a photopolymerization initiator, and, if necessary, a surface tension modifier, a polymerization inhibitor, an organic solvent and / or water, etc. are added to the obtained titanium oxide dispersion so as to obtain the desired ink characteristics, and after thorough mixing, the mixture is filtered through a filter or the like to remove coarse particles, thereby obtaining an ink.

[0117] The content of titanium oxide present in the titanium oxide dispersion is preferably 30 to 70% by mass, and particularly preferably 40 to 60% by mass, in the titanium oxide dispersion.

[0118] In addition, the photopolymerizable compound used in producing a titanium oxide dispersion is preferably a compound represented by general formula (1) and / or other polyfunctional polymerizable compounds that are photopolymerizable monomers having two (meth)acryloyl groups. Furthermore, from the viewpoints of storage stability, ejection stability, and ink curing properties, when a compound represented by general formula (1) is used 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. When a photopolymerizable monomer having two (meth)acryloyl groups is used as the photopolymerizable compound, it is preferable to use a monomer represented by general formula (1) having an EO chain or a PO chain as the main skeleton, and dipropylene glycol diacrylate and / or tripropylene glycol diacrylate are particularly preferable to use in order to improve the storage stability and ejection stability of the titanium oxide dispersion and the ink produced using the titanium oxide dispersion.

[0119] ≪Printed matter≫ The printed matter of the present disclosure is formed by printing the ink of the present disclosure onto a printing substrate described below, i.e., a printing substrate on which an image and / or characters are recorded. Therefore, the "printed matter" of the present disclosure includes the printing substrate and the image and / or characters formed as a film (ink film) formed by curing the ink of the present disclosure. The "image" also includes solid images (images printed at 100% coverage so as to completely cover the surface of the printing substrate) and seamless images such as checkerboard patterns.

[0120] <<Manufacturing method for printed matter>> An example of a method for producing the printed matter is a method that includes, in this order, a step (step 1) of ejecting the ink of the present disclosure onto a printing substrate, and a step (step 2) of irradiating the substrate with the ejected ink with ultraviolet light.

[0121] In the present disclosure, a method of ejecting and applying the same inkjet ink from the same inkjet head multiple times to the same location on the printing substrate (multi-pass printing method) may be employed. However, in the case of the present disclosure, in order to fully exhibit the effects of the present disclosure described above, it is preferable to employ a method of ejecting and applying the same inkjet ink from the same inkjet head only once to the same location on the printing substrate (one-pass printing method).

[0122] The one-pass printing method can be carried out, for example, by using a line printer, and the printing speed is preferably 20 to 150 m / min, and particularly preferably 30 to 100 m / min, from the viewpoints of productivity and obtaining printed matter of good quality.

[0123] ≪Process 1 (discharge process)≫ The ink of the present disclosure can be suitably used in inkjet printing, and therefore, in step 1, the ink is preferably ejected from an inkjet head.

[0124] When the ink of the present disclosure is ejected from an inkjet head, the ejection amount (drop volume) is preferably 2 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, and KJ4A-RH; Fujifilm Samba G3L; Seiko Epson S3200, S1600, S800, I3200, and I1600; Konica Minolta KM1024i and KM1024; and Ricoh MH5320, MH5340, MH5240, and MH5440, all of which can be suitably used.

[0125] Furthermore, to ensure that the ink of the present disclosure has an appropriate viscosity, the ink can be heated while being ejected using a heating device such as a heater provided in the inkjet head. From the viewpoint of enabling 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 more preferably 15 mPa s or less.

[0126] ≪Process 2 (hardening process)≫ The ink of the present disclosure is ejected onto a printing substrate and then cured by irradiation with ultraviolet light to form a printed matter.

[0127] Examples of the ultraviolet irradiating means used in step 2 include a high-pressure mercury lamp, an ultra-high-pressure mercury lamp, a metal halide lamp, an excimer laser lamp, a xenon lamp, and a UV-LED.

[0128] Among the above ultraviolet irradiation means, UV-LEDs have the advantage of emitting ultraviolet light with a narrow wavelength range and being easily miniaturized. Therefore, the irradiation wavelength and usage method of the UV-LED can be adjusted to some extent to suit the properties of the photopolymerization initiator, fluorescent brightener, and titanium oxide 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.

[0129] When UV-LED is used in step 2, the above-mentioned effects are fully exerted, and from the viewpoint of obtaining a printed matter with excellent curing properties and print quality, the maximum irradiance of ultraviolet light on the printing substrate is 1 W / cm. 2 It is preferable that the power is 2 W / cm or more. 2 More preferably, it is 3 W / cm or more. 2 The integrated amount of light when irradiating the substrate varies depending on the type and content of the polymerizable compound and photopolymerization initiator contained in the ink, but is preferably 100 mJ / cm or more. 2 It is preferable to set the dose to 200 mJ / cm or more. 2 More preferably, it is set to the above.

[0130] Furthermore, due to their small size, multiple UV-LEDs can be installed side by side. Therefore, multiple LEDs can be lined up to increase the irradiation intensity on the substrate. In this case, multiple UV-LEDs with different peak wavelengths may be lined up. Furthermore, in step 2, UV-LEDs may be used in combination with other ultraviolet irradiation means, such as high-pressure mercury lamps and metal halide lamps. For example, metal halide lamps are preferably used, as they effectively irradiate ultraviolet rays in the UV-A region and allow the ultraviolet rays to reach the interior of the ink film.

[0131] Furthermore, the time from the end of step 1 to the start of step 2 (the time from when the ink adheres to the printing substrate to when ultraviolet light irradiation begins) 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 not only makes it possible to obtain a printed product with excellent adhesion and abrasion resistance, but also prevents the ink droplets from coalescing, resulting in a printed product with good print quality.

[0132] In the inkjet printing method, step 2 can be repeated multiple times. For example, immediately after applying the inkjet ink to the substrate, the inkjet ink can be partially cured by irradiating it with ultraviolet light, and then the inkjet ink can be completely cured by irradiating it with ultraviolet light again. This makes it easy to obtain printed matter with exceptionally excellent print quality. In the present disclosure, the step of partially curing the inkjet ink described above is referred to as "pre-curing," and the step of completely curing the inkjet ink is referred to as "main curing."

[0133] When the temporary curing is carried out, the ultraviolet ray irradiation means used for the temporary curing is preferably a UV-LED, and the maximum illuminance of the ultraviolet ray on the substrate is 2 to 20 W / cm. 2 is preferably 5 to 15 W / cm 2 It is more preferable that:

[0134] On the other hand, in the present disclosure, the curing is 2 W / cm 2 The maximum illuminance is 100mJ / cm 2 This can be done with the above integrated light amount.

[0135] ≪Printing base material≫ Resin film substrates and paper substrates are preferably used as printing substrates to which the ink of the present disclosure can be applied. The resin film substrate is preferably selected from those having a thickness of 10 to 90 μm and containing a material selected from the group consisting of polypropylene, polyethylene, polyethylene terephthalate, and nylon. On the other hand, coated paper, art paper, laminated paper, etc. are preferably selected as the paper substrate. The ink of the present disclosure can be suitably used, for example, for printing on packages manufactured using the printing substrates listed above.

[0136] The above "material selected from the group consisting of polypropylene, polyethylene, polyethylene terephthalate, and nylon" also includes a resin film substrate (laminated film substrate) 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 a layer made of AL (aluminum foil), VM (vacuum vapor deposition) film (aluminum vapor deposition film, transparent vapor deposition film, etc.), etc. [Example]

[0137] The present disclosure will be described in more detail below with reference to examples. However, the following examples are not intended to limit the scope of the present disclosure. Unless otherwise specified, "parts" represents parts by mass, and "%" represents % by mass.

[0138] First, prior to preparing the ink, a titanium oxide dispersion was prepared by the following method.

[0139] <Production of Titanium Oxide Dispersion A> 2,500 g of titanium dioxide pigment (2310, primary particle size 200 nm, manufactured by Cronos) surface-treated with (hydrous) alumina, (hydrous) silica, (hydrous) zirconia, and organic matter, 150 g of pigment dispersing resin (Solsperse 32000, manufactured by Lubrizol), and 2,350 g of dipropylene glycol diacrylate were placed in a mixing vessel equipped with a stirrer and pre-dispersed by stirring with the stirrer. The mixture was then dispersed for 2 hours using a 0.6 L sand mill (Dynomill, manufactured by Shinmaru Enterprises) filled with 1,800 g of 1 mm diameter zirconia beads to obtain titanium dioxide dispersion A.

[0140] <Production of Titanium Oxide Dispersion B> Titanium oxide dispersion B was produced using the same materials and method as in the case of titanium oxide dispersion A, except that 1,6-hexanediol diacrylate was used instead of dipropylene glycol diacrylate.

[0141] <Production of Titanium Oxide Dispersion C> Titanium oxide dispersion C was produced using the same materials and method as in the case of titanium oxide dispersion A, except that (hydrated) alumina, (hydrated) silica, and Typepaque PF-690 (primary particle diameter 210 nm) manufactured by Ishihara Sangyo Kaisha, Ltd., whose surface was treated with an organic substance, was used as the titanium oxide pigment.

[0142] <Production of Titanium Oxide Dispersion D> Titanium oxide dispersion D was produced using the same materials and method as in the case of titanium oxide dispersion A, except that Typepaque PF-726 (primary particle diameter: 210 nm) manufactured by Ishihara Sangyo Kaisha, Ltd., whose surface was treated with (hydrated) alumina and (hydrated) silica, was used as the titanium oxide pigment.

[0143] <Production of Titanium Oxide Dispersion E> Titanium oxide dispersion E was produced using the same materials and method as in the case of titanium oxide dispersion A, except that Typepaque CR60-2 (primary particle diameter 210 nm) manufactured by Ishihara Sangyo Kaisha, Ltd., whose surface was treated with (hydrous) alumina and an organic substance, was used as the titanium oxide pigment.

[0144] <Production of Titanium Oxide Dispersion F> Titanium oxide dispersion F was produced using the same materials and method as in the case of titanium oxide dispersion A, except that 3-methyl-1,5-pentanediol diacrylate was used instead of dipropylene glycol diacrylate.

[0145] <Production of Inkjet Ink Composition> Next, the titanium oxide dispersion, photopolymerizable compound, photopolymerization initiator, fluorescent brightening agent, and surface tension modifier prepared above were added in this order to a mixing vessel equipped with a stirrer to obtain the formulation shown in each column of Tables 1-1 to 1-8. Note that each material was added while the stirrer was running to stir the contents of the mixing vessel. Furthermore, for components containing two or more types of materials, the order of addition within the component was arbitrary. After all the materials were added, gentle stirring and mixing was continued until the photopolymerization initiator was dissolved, and then the mixture was filtered through a membrane filter with a pore size of 1 μm to remove coarse particles, thereby obtaining a white inkjet ink.

[0146] [Table 1-1]

[0147] [Table 1-2]

[0148] [Table 1-3]

[0149] [Table 1-4]

[0150] [Table 1-5]

[0151] [Table 1-6]

[0152] [Table 1-7]

[0153] [Table 1-8]

[0154] Details of the raw material names listed in Tables 1-1 to 1-8 are as follows: In the "Specifications" section of Tables 1-1 to 1-8, the "Photopolymerizable compound content (mass%)" is calculated and rounded to one decimal place, and the other items are calculated and rounded to one decimal place. <Polymerizable compound> HDDA: 1,6-hexanediol diacrylate (Viscoat #240, manufactured by Osaka Organic Chemical Industry Co., Ltd.) MPDDA: 3-methyl-1,5-pentanediol diacrylate (EBECRYL MPDDA, manufactured by Daicel-Allnex Co., Ltd.) BDDA: 1,4-butanediol diacrylate (Viscoat #195, manufactured by Osaka Organic Chemical Industry Co., Ltd.) NPGDA: Neopentyl glycol diacrylate (Light Acrylate NP-A, manufactured by Kyoeisha Chemical Co., Ltd.) DPGDA: Dipropylene glycol diacrylate (Miramer M222, manufactured by Miwon) GlyTA: Glycerin triacrylate (Aronix M-930, manufactured by Toagosei Co., Ltd.) DiTMPTA: Ditrimethylolpropanetetraacrylate (Ebecryl 1142, manufactured by Daicel-Allnex Co., Ltd.) PEA: 2-phenoxyethyl acrylate (Viscoat #192, manufactured by Osaka Organic Chemical Industry Co., Ltd.) BzA: Benzyl acrylate (Viscoat #160, manufactured by Osaka Organic Chemical Industry Co., Ltd.) IBXA: Isobornyl acrylate (Osaka Organic Chemical Industry Co., Ltd.) VMOX: N-vinyl-5-methyloxazolidinone (BASF) <Photopolymerization initiator> TPO-L: Ethoxyphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (Omnirad TPO-L, manufactured by IGM Resins) Omn380: Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Omnirad380, manufactured by IGM Resins) TPO: Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (Omnirad TPO, manufactured by IGM Resins) Omn369: 2-benzyl-2-(dimethylamino)-4'-morpholinobutyrophenone (Omnirad 369, IGM Resins) <Fluorescent whitening agents> OB184: 2,5-bis(5-tert-butyl-2-benzoxazol-2-yl)thiophene (SpeedBlock OB-184, Sartomer) KCB: 1,4-bis(benzoxazol-2-yl)naphthalene (FLOURESCENT BRIGHTNER KCB, manufactured by Xcolor Pigments) DP74: 7-diethylamino-4-methylcoumarin (SpeedBlock DP74, Sartomer) <Surface tension adjuster> TEDO Rad 2100: A polyether-modified silicone surface tension modifier with a (meth)acryloyl group (manufactured by Evonik) BYK-UV3510: A polyether-modified silicone surface tension modifier without a (meth)acryloyl group (manufactured by BYK-Chemie)

[0155] <Creating printed materials> Using the ink prepared above, printed matter was produced as follows. First, one Kyocera inkjet head (design resolution 600 dpi) was installed above the conveyor that could transport the printing substrate. Furthermore, a UV-LED for main curing (Phoseon FirePower FP300 (maximum emission wavelength 395 nm, maximum illuminance 16 W / cm)) was installed downstream of the transport direction of the printing substrate. 2 An inkjet printing device (Tritec Corporation's "OnePassJET") equipped with the inkjet head was prepared. Next, the inkjet inks prepared above were filled into the inkjet heads, and the temperature of the inkjet heads was adjusted so that the inkjet inks had a viscosity of 6 to 7 mPa·s during ejection. Furthermore, a PET substrate (Lintec Corporation's "PET50 (K2411)") was fixed onto the conveyor. The conveyor was then driven at a speed of 50 m / min, and when the PET substrate passed the area where the inkjet heads were installed, a solid image was printed with a 100% coverage under printing conditions of an ejected droplet volume of 14 pL and a printing resolution of 600 dpi x 600 dpi. After the inkjet ink was ejected, the conveyor was continued to be driven at the same speed. When the PET substrate passed the area where the main curing UV-LED was installed, ultraviolet light was irradiated, producing a printed product. The irradiance of the ultraviolet light irradiating the inkjet ink on the PET substrate was 6,000 mW / cm. 2 , and the cumulative light intensity is 200mJ / cm 2 The output of the UV-LED for main curing was adjusted in advance so that the print was as described above.

[0156] [Examples 1 to 92, Comparative Examples 1 to 5] The inks and printed matter prepared above were used to carry out the following evaluations, and the evaluation results are shown in Tables 1-1 to 1-8.

[0157] <Evaluation 1: Evaluation of curability> The surface of a printed material with a 100% solid image coverage produced by the above method was rubbed with a cotton swab, and it was checked whether or not a mark was left at the location rubbed with the cotton swab. If inkjet ink adhered to the cotton swab, the printed material was fixed to the conveyor of the inkjet printing device, and without printing the inkjet ink, only the UV-LED for main curing was irradiated, and then the presence or absence of a mark when rubbed with the cotton swab was checked again. This procedure was repeated, and the number of passes required until the area rubbed with the cotton swab no longer left a mark was counted to evaluate the curability. The evaluation criteria for the curability were as follows: A rating of "2" or higher was considered practical, and a rating of "3" or higher was considered practically suitable.

[0158] <Curability evaluation criteria> 4: After one total pass (without the need for additional UV exposure), the area rubbed with the cotton swab no longer left any marks. 3: After passing the cloth through the cloth twice (and irradiating it with UV rays once more), the area rubbed with the cotton swab no longer left any marks. 2: After passing the cloth through the cloth three times (and irradiating it with UV rays two more times), the area rubbed with the cotton swab no longer left any marks. 1: It took more than four UV-LED irradiations in total to remove any marks from the area rubbed with the cotton swab.

[0159] <Evaluation 2: Adhesion evaluation> Each of the solid image prints with a printing rate of 100% produced by the above method was subjected to two additional rounds of UV irradiation in the same manner as in the evaluation of curing properties. That is, the total cumulative amount of UV light irradiated onto the prints was 600 mJ / cm. 2 This becomes: Next, after further UV irradiation, the solid image print was cut perpendicular to the print substrate with a cutter blade, creating an approximately 1.5 cm long incision. Eleven incisions were made vertically and horizontally, spaced approximately 1 mm apart, to create a 10 x 10 grid. A piece of transparent cellophane tape approximately 75 mm long was then applied to the grid. Within 5 minutes of application, the tape was pulled at an angle of nearly 60 degrees, peeled off from the print, and the condition of the print was visually inspected. The evaluation criteria were as follows: A rating of "3" or higher was deemed usable, and a rating of "4" was deemed practically suitable. For inks that received a rating of "1" in the above evaluation of curability, the evaluation of adhesion was not carried out.

[0160] <Adhesion evaluation criteria> 4: No peeling of the ink film was observed. 3: Peeling of the ink film was observed in an area of ​​25% or less of the area where the cellophane tape was attached. 2: Peeling of the ink film was observed in an area exceeding 25% and not exceeding 50% of the area where the cellophane tape was applied. 1: Peeling of the ink film was observed in more than 50% of the area where the cellophane tape was attached.

[0161] For inks that received a rating of "4" in the above adhesion evaluation, an additional evaluation of adhesion was conducted. The evaluation method was as follows: the number of additional UV irradiations was counted as one, i.e., the total cumulative amount of UV light irradiated was 400 mJ / cm2. 2 The same procedure was followed as above, except that the printed matter prepared as in Example 1 was used. If no peeling of the ink film was observed on the printed matter after peeling off the cellophane tape, the ink was rated as "5." However, for inks that were rated "1" or "2" in the curing evaluation above, no additional evaluation of adhesion was performed, even if they were rated "4" in the adhesion evaluation above.

[0162] <Evaluation 3: Evaluation of abrasion resistance> Each of the solid image prints with a printing rate of 100% produced by the above method was subjected to two additional rounds of UV irradiation in the same manner as in the evaluation of curing properties. That is, the total cumulative amount of UV light irradiated onto the prints was 600 mJ / cm. 2 This becomes: The surface of the solid image print after additional UV irradiation was then scratched with a cupronickel coin and the degree of damage to the print was visually inspected. The evaluation criteria were as follows, with a rating of "2" or higher being considered usable. For inks that received a rating of "1" in the above evaluation of curability, the evaluation of scratch resistance was not carried out.

[0163] <Evaluation criteria for abrasion resistance> 4: No scratches were found on the coin. 3: Scratches caused by coin abrasion were observed, but the extent of the scratches was smaller than that of the print produced using the ink of Example 27. 2: Scratches caused by coin abrasion were observed, but the extent of the scratches was similar to that of the print produced using the ink of Example 27. 1: Scratches caused by coin abrasion were observed, and the extent of the scratches was greater than that of the print produced using the ink of Example 27.

[0164] <Evaluation 4: Discharge stability evaluation> Each of the inkjet inks prepared above was filled into a jig equipped with a temperature-adjustable Kyocera inkjet head (design resolution 600 dpi). Next, the temperature of the head was controlled so that the ink viscosity during ejection was 6-7 mPa·s. After confirming that there were no nozzles from which ink was not being ejected, the inkjet ink was continuously ejected from all nozzles at a drive frequency of 20 kHz. After 20 minutes of continuous ejection, the number of nozzles from which ink was not being ejected (nozzle loss number) was counted to evaluate ejection stability. The evaluation criteria were as follows, with a rating of "2" or higher being considered practical.

[0165] <Evaluation criteria for ejection stability> 4: The number of nozzle losses was 2 or less. 3: The number of nozzle losses was 3 to 5. 2: The number of nozzle losses was 6 to 10. 1: The number of nozzle losses was 11 or more.

[0166] As shown in Tables 1-1 to 1-8 above, the inkjet inks containing titanium oxide of Examples 1 to 92 were excellent in all of curability, adhesion, and abrasion resistance, and also in ejection stability as inkjet inks.

[0167] On the other hand, as shown in Comparative Example 3, when the content of the polyfunctional polymerizable compound was less than 50 mass% of the total amount of photopolymerizable compounds, the curability did not reach a practical level. Furthermore, as shown in Comparative Examples 1 and 2, when the content of the compound represented by general formula (1) was less than 50 mass% of the total amount of the polyfunctional polymerizable compounds, the adhesion did not reach a practical level, and the ejection stability was not necessarily good. As described above, it was confirmed that in order to improve all of the curability and ejection stability of the ink, as well as the abrasion resistance and adhesion of the printed matter, it is necessary to use a certain amount of polyfunctional polymerizable compound, and for 50 mass% or more of the polyfunctional polymerizable compound to be the compound represented by general formula (1).

[0168] Furthermore, in the case of the ink shown in Comparative Example 4, which does not contain a fluorescent brightening agent, and in the case of the ink shown in Comparative Example 5, which uses a fluorescent brightening agent other than a thiophene benzoxazoyl compound or a naphthalene benzoxazoyl compound, it is thought that the light-reflecting and scattering properties of titanium oxide deteriorated the efficiency of photopolymerization initiator generation, and as a result, the adhesion and abrasion resistance of the printed matter did not reach a practical level.

[0169] Although the present invention has been described with reference to the above-mentioned several embodiments, the present invention is not limited to these several embodiments. Various modifications can be made to the configuration and details of the present invention within the scope of the present invention.

[0170] This disclosure is related to the subject matter described in Japanese Patent Application No. 2024-109845, filed on July 8, 2024, the entire disclosure of which is incorporated herein by reference.

Claims

1. An ultraviolet-curable inkjet ink comprising titanium oxide, a photopolymerizable compound, a photopolymerization initiator, and a fluorescent brightening agent, the photopolymerizable compound includes a polyfunctional polymerizable compound, the content of the polyfunctional polymerizable compound is 50% by mass or more of the total amount of the photopolymerizable compound; The polyfunctional polymerizable compound contains a compound represented by general formula (1), the content of the compound represented by the general formula (1) is 50 mass% or more of the total amount of the polyfunctional polymerizable compound, The ultraviolet-curable inkjet ink, wherein the fluorescent brightener comprises a thiophene benzoxazoyl compound and / or a naphthalene benzoxazoyl compound. General formula (1) CH 2 =CH-CO-O-R 1 -O-CO-CH=CH 2 [In general formula (1), R 1 represents an alkylene group having 4 to 10 carbon atoms, which may have a branched structure.

2. 2. The ultraviolet-curable inkjet ink according to claim 1, wherein a ratio of the content of the photopolymerization initiator to the content of the fluorescent brightening agent (photopolymerization initiator / fluorescent brightening agent) is 11 to 1,100 in mass ratio.

3. 3. The ultraviolet-curable inkjet ink according to claim 1, wherein the photopolymerization initiator comprises ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide.

4. 3. A printed matter obtained by printing the ultraviolet-curable inkjet ink according to claim 1 or 2 on a printing substrate.

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