UV-curing inkjet inks and printed materials

Abstract

To provide an ultraviolet-curable inkjet ink that offers excellent storage stability, good adhesion and curing properties, and produces high-gloss printed materials. [Solution] An ultraviolet-curable inkjet ink comprising a colorant, 5-methyl-3-vinyloxazolidine-2-one, a hydrogen abstraction type photopolymerization initiator, and a polymerization inhibitor, wherein the content of 5-methyl-3-vinyloxazolidine-2-one is 1 to 25% by mass of the total amount of the inkjet ink, the polymerization inhibitor comprises an N-nitrosophenylhydroxylaluminum salt, and the ratio of the content of the hydrogen abstraction type photopolymerization initiator to the content of the N-nitrosophenylhydroxylaluminum salt is 8:1 to 80:1 by mass.

Description

[Technical Field]

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

[0002] In recent years, digital printing methods have rapidly become widespread in the printing industry. Because digital printing methods do not require platemaking, printing equipment can be made smaller compared to plate printing methods that require plates. Furthermore, digital printing methods offer several advantages over other printing methods, such as lower running costs and easy full-color printing, and their use is particularly advancing in the industrial printing industry.

[0003] Inkjet printing inks come in various types, including water-based, oil-based, solvent-based, and UV-curing. Among these, UV-curing inkjet inks are gaining increasing demand due to their suitability for non-absorbent printing substrates such as plastics and glass, their fast drying (curing) time, and the high strength of the printed materials they produce.

[0004] In this disclosure, inks used in inkjet printing methods will also be simply referred to as "inkjet inks."

[0005] Furthermore, there has been a growing market demand for environmentally conscious products. Therefore, UV-LEDs (ultraviolet light-emitting diodes) are increasingly being used as a means of curing ultraviolet-curing inkjet inks. However, UV-LEDs have a narrow wavelength range of ultraviolet light they emit, making improving curing performance a particular challenge when used in combination with ultraviolet-curing inkjet inks.

[0006] Thus, in the design of UV-curable inkjet inks in recent years, it is essential to select materials that achieve both printability and curability. For example, it is preferable to use a polymerizable compound, which is the main component of UV-curable inkjet ink, that is a low-viscosity liquid at room temperature and has excellent reactivity.

[0007] 5-methyl-3-vinyloxazolidine-2-one is one polymerizable compound with such properties, and there are documents disclosing inkjet ink compositions containing it. For example, Patent Documents 1 and 2 show specific examples of photocurable inkjet printing ink compositions containing the above-mentioned 5-methyl-3-vinyloxazolidine-2-one. Patent Document 3 also discloses a photocurable inkjet printing ink composition containing 5 to 50% by mass of 5-methyl-3-vinyloxazolidine-2-one and 10 to 50% by mass of a monofunctional monomer with a low glass transition temperature. However, the present inventors have found that, depending on the printing conditions and the selection of the printing substrate, the curability and adhesion were sometimes insufficient. On the other hand, Patent Document 4 describes that a radiation-curable inkjet ink composition using 5-methyl-3-vinyloxazolidine-2-one and a monomer having two vinyl ether groups can ensure good adhesion of the ink film while having low viscosity. Furthermore, in the examples of Patent Document 4, in addition to the adhesion described above, curability using UV-LEDs and storage stability are also evaluated. However, it has been found that, depending on the peak wavelength and intensity of the UV-LEDs used in combination, and the printing substrate used, good curability and adhesion may not always be achieved.

[0008] Generally speaking, UV-curable inkjet inks, which have excellent curability, often face challenges in terms of storage stability. This is thought to be because, even when inkjet inks are not exposed to UV light during storage, trace amounts of radicals are generated due to the cleavage of polymerizable groups in the polymerizable compound, or the reaction of the polymerizable compound with cationic or anionic components present in the composition.

[0009] Furthermore, in the case of highly curable UV-curable inkjet inks, there is a strong tendency for the inkjet ink to harden before it can be smoothed out on the printing substrate, or for volume shrinkage (curing shrinkage) to occur due to the polymerization reaction. As a result, the surface of the printed material may become uneven, and the gloss (glossiness) may decrease.

[0010] Thus, conventionally, there were no UV-curable inkjet inks that offered excellent storage stability, and superior adhesion, curing properties, and gloss of printed materials, even when using UV-LEDs. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] International Publication No. 2021 / 261208 [Patent Document 2] International Publication No. 2022 / 030261 [Patent Document 3] International Publication No. 2021 / 199760 [Patent Document 4] Japanese Patent Publication No. 2022-006388 [Overview of the project] [Problems that the invention aims to solve]

[0012] Therefore, in one embodiment of the present invention, the objective is to provide an ultraviolet-curable inkjet ink that has excellent storage stability, good adhesion and curing properties, and can produce printed materials with high gloss. [Means for solving the problem]

[0013] As a result of diligent research by the inventors, we have discovered that all of the above-mentioned problems can be solved simultaneously and at a high level by an ultraviolet-curable inkjet ink having the following configuration, and have completed the present invention.

[0014] That is, one embodiment of the present invention relates to an ultraviolet-curable inkjet ink shown in the following [1] to [5], and a printed matter formed using the ultraviolet-curable inkjet ink shown in the following [6]. [1] An ultraviolet-curable inkjet ink comprising a colorant, a polymerizable compound, a photopolymerization initiator, and a polymerization inhibitor, wherein the polymerizable compound comprises 5-methyl-3-vinyloxazolidin-2-one, the content of said 5-methyl-3-vinyloxazolidin-2-one is 1 to 25% by mass based on the total mass of said inkjet ink, said photopolymerization initiator comprises a hydrogen abstraction-type photopolymerization initiator, said polymerization inhibitor comprises an N-nitrosophenylhydroxyl aluminum salt, wherein the mass ratio of the content of said hydrogen abstraction-type photopolymerization initiator to the content of said N-nitrosophenylhydroxyl aluminum salt (hydrogen abstraction-type photopolymerization initiator : N-nitrosophenylhydroxyl aluminum salt) is 8:1 to 80:1. [2] The ultraviolet-curable inkjet ink according to [1], wherein said hydrogen abstraction-type photopolymerization initiator comprises a thioxanthone compound. [3] The ultraviolet-curable inkjet ink according to [1] or [2], wherein the mass ratio of the content of said hydrogen abstraction-type photopolymerization initiator to the content of said 5-methyl-3-vinyloxazolidin-2-one (hydrogen abstraction-type photopolymerization initiator : 5-methyl-3-vinyloxazolidin-2-one) is 0.1:1 to 2.0:1. [4] The ultraviolet-curable inkjet ink according to any one of [1] to [3], wherein said photopolymerization initiator further comprises an acylphosphine oxide-based compound, and the mass ratio of the content of said acylphosphine oxide-based compound to the content of said hydrogen abstraction-type photopolymerization initiator (acylphosphine oxide-based compound : hydrogen abstraction-type photopolymerization initiator) is 2:1 to 20:1. [5] Furthermore, an ultraviolet-curable inkjet ink according to any one of [1] to [4], comprising a polyether-modified polysiloxane. A printed material obtained by printing with any of the ultraviolet-curing inkjet inks described in [6][1] to [5]. [Effects of the Invention]

[0015] The present invention provides an ultraviolet-curable inkjet ink that exhibits excellent storage stability, good adhesion and curing properties, and yields printed materials with high gloss. [Modes for carrying out the invention]

[0016] The following describes in detail an ultraviolet-curable inkjet ink (hereinafter also simply referred to as "the inkjet ink of this embodiment") which is one embodiment of the present invention. It should be noted that the present invention is not limited to the following embodiments, and includes modifications that do not alter the essence of the present invention.

[0017] The inkjet ink of this embodiment contains 5-methyl-3-vinyloxazolidine-2-one as a polymerizable compound. Its content is 1 to 25% by mass of the total amount of the inkjet ink. Generally, cyclic N-vinyl compounds are known to have a higher reaction rate with radicals derived from photopolymerization initiators than (meth)acrylate compounds. The reaction rate of 5-methyl-3-vinyloxazolidine-2-one is particularly high compared to other cyclic N-vinyl compounds. This is thought to be because, unlike N-vinylcaprolactam and N-vinylpyrrolidone, which are generally known as cyclic N-vinyl compounds, 5-methyl-3-vinyloxazolidine-2-one has an oxygen atom in its ring structure. That is, it is thought that electrons in the ring structure are attracted to this oxygen atom, making the vinyl group more likely to react with radicals derived from photopolymerization initiators. As a result, the inkjet ink of this embodiment containing a certain amount of 5-methyl-3-vinyloxazolidine-2-one exhibits excellent curability. It is known that the polymerization reaction rate between vinyl groups and (meth)acryloyl groups is faster than the polymerization reaction rate between (meth)acrylic groups, and in particular, the reaction rate between vinyl groups and (meth)acryloyl groups present in 5-methyl-3-vinyloxazolidine-2-one is considered to be faster than the reaction rate between vinyl groups and (meth)acryloyl groups present in other compounds. As a result, in the inkjet ink of this embodiment, the amount of polymerizable compound containing (meth)acryloyl groups with unreacted (meth)acryloyl groups can be suitably reduced, and the curability is easily improved.

[0018] However, as mentioned above, generally speaking, compositions that are highly reactive and have excellent curability tend to have storage stability issues. Furthermore, a well-known method for ensuring the above-mentioned storage stability is to use polymerization inhibitors that can capture radicals. In the inkjet ink of this embodiment, it is preferable that N-nitrosophenylhydroxylaluminum salt is included as the polymerization inhibitor.

[0019] N-nitrosophenylhydroxylaluminum salt contains a nitrosamine structure. This nitrosamine structure captures trace amounts of radicals generated during the storage of inkjet ink, forming a stable intermediate. As a result, the storage stability of the inkjet ink is expected to improve.

[0020] However, since N-nitrosophenylhydroxylaluminum salt can capture radicals generated from photopolymerization initiators during inkjet ink printing, i.e., under ultraviolet irradiation, the addition of the above-mentioned N-nitrosophenylhydroxylaluminum salt may be a factor that reduces the curability of inkjet ink.

[0021] Therefore, in this embodiment of inkjet ink, a hydrogen abstraction type photopolymerization initiator is used to suppress the aforementioned decrease in curability and to achieve both good curability and storage stability.

[0022] Examples of hydrogen abstraction type photopolymerization initiators include thioxanthone compounds, benzophenone compounds, and ketocoumarin compounds. These hydrogen abstraction type photopolymerization initiators become excited by light energy and then abstract hydrogen atoms from amines and other hydrogen atom donors, generating radicals. The radicals generated at this time contain oxygen atoms, aromatic rings, etc., and are therefore considered to be highly anionic and sterically bulky. On the other hand, N-nitrosophenylhydroxylaluminum salt also contains nitrogen atoms, oxygen atoms, and aromatic rings. Therefore, due to charge repulsion and steric repulsion, N-nitrosophenylhydroxylaluminum salt is considered to be less reactive with radicals generated from the above-mentioned hydrogen abstraction type photopolymerization initiators, and these radicals are thought to readily react with polymerizable compounds such as 5-methyl-3-vinyloxazolidine-2-one before being captured by the N-nitrosophenylhydroxylaluminum salt. Furthermore, when irradiated with ultraviolet light, a large number of the above-mentioned radicals are generated, and combined with the high reactivity of 5-methyl-3-vinyloxazolidine-2-one mentioned above, it is thought that the polymerization reaction proceeds at a rate that exceeds the rate at which the N-nitrosophenylhydroxylaluminum salt captures radicals during the polymerization reaction.

[0023] On the other hand, as mentioned above, hydrogen abstraction type photopolymerization initiators require a hydrogen atom donor to generate radicals. In particular, compounds having a carbon atom to which an amino group and a hydrogen atom are bonded function effectively as the above-mentioned hydrogen atom donor. The 5-methyl-3-vinyloxazolidine-2-one contained in the inkjet ink of this embodiment has such a carbon atom. From the above, it is considered that in the inkjet ink of this embodiment, 5-methyl-3-vinyloxazolidine-2-one also functions as a hydrogen atom donor. Furthermore, from this perspective, hydrogen abstraction by the hydrogen abstraction type photopolymerization initiator occurs around the 5-methyl-3-vinyloxazolidine-2-one molecule. As a result, the above-mentioned radicals can participate in the polymerization reaction before being captured by the N-nitrosophenylhydroxylaluminum salt, and it is considered that further improvement in curability can be achieved.

[0024] In the inkjet ink of this embodiment, the ratio of the content of the hydrogen abstraction type photopolymerization initiator to the content of N-nitrosophenylhydroxylaluminum salt is specified in order to suitably exhibit the above-mentioned effects and simultaneously improve curability and storage stability. Specifically, the ratio of the content of the hydrogen abstraction type photopolymerization initiator to the content of N-nitrosophenylhydroxylaluminum salt (hydrogen abstraction type photopolymerization initiator: N-nitrosophenylhydroxylaluminum salt) is preferably 8:1 to 80:1. When the content of the hydrogen abstraction type photopolymerization initiator is 8 times or more the content of the N-nitrosophenylhydroxylaluminum salt, the amount of radicals generated in relation to the N-nitrosophenylhydroxylaluminum salt becomes good, improving the curing speed and curability. Furthermore, when the content is 80 times or less, the hydrogen abstraction type photopolymerization initiator does not excessively absorb ultraviolet light, resulting in a suitable amount of radical generation and improved curability. Furthermore, during storage of inkjet inks, radical scavenging by N-nitrosophenylhydroxylaluminum salt is effectively carried out, thus improving storage stability.

[0025] There are other effects of adjusting the ratio of the amount of hydrogen abstraction type photopolymerization initiator to the amount of N-nitrosophenylhydroxylaluminum salt. The N-nitrosophenylhydroxylaluminum salt contained in the inkjet ink of this embodiment is less affected by oxygen present inside and around the inkjet ink when capturing radicals. In other words, compared to other polymerization inhibitors that can be affected by oxygen, the N-nitrosophenylhydroxylaluminum salt can exhibit the same level of function inside the ink film as on the surface of the ink film, even in the interior of the ink film that is not in contact with oxygen (air). As a result, differences in curability are less likely to occur between the surface and the interior of the ink film, and the amount of (meth)acryloyl group-containing polymerizable compound having unreacted (meth)acryloyl groups is uniformly reduced both on the surface and inside. As a result, the generation of internal stress is mitigated, and the wetting spread of the inkjet ink on the surface of the printing substrate is improved, making it easier to obtain a smooth ink film, thus easily obtaining printed materials with high adhesion and gloss.

[0026] As described above, the configuration of the inkjet ink of this embodiment is essential to solving the aforementioned problems. However, the mechanism for solving the problems is based on the inventors' reasoning and does not limit the present invention in any way.

[0027] Furthermore, Patent Documents 1 to 4 disclose inkjet inks containing 5-methyl-3-vinyloxazolidine-2-one as examples. However, Patent Documents 1 to 4 do not explicitly state that these inkjet inks contain N-nitrosophenylhydroxylaluminum salt as a polymerization inhibitor. Specifically, the polymerization inhibitor used in the examples of Patent Documents 1 to 3 is dioctyl maleate (paragraph 0042 of Patent Document 1, paragraph 0041 of Patent Document 2, and paragraph 0031 of Patent Document 3), and the polymerization inhibitor used in the example of Patent Document 4 is 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxyl and 4-methoxyphenol (paragraph 0126 of Patent Document 3). In addition, while Patent Documents 1 to 3 state that inkjet inks may contain polymerization inhibitors, they do not specify which compounds are suitable for use. On the other hand, while Patent Document 4 lists specific examples of polymerization inhibitors that can be suitably used in paragraphs 0081 to 0082, it does not mention N-nitrosophenylhydroxylaluminum salt. Furthermore, Patent Documents 1 to 4 do not describe or suggest the effects achieved by combining N-nitrosophenylhydroxylaluminum salt, hydrogen abstraction type photopolymerization initiator, and 5-methyl-3-vinyloxazolidine-2-one as described above.

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

[0029] <Polymerizable compound> In the inkjet ink of this embodiment, a polymerizable compound refers to a compound that undergoes a polymerization reaction and / or crosslinking reaction by radicals generated from a photopolymerization initiator, etc., as described later, and has the function of curing a composition containing the polymerizable compound. Any compound having the above-described properties can be used without particular limitation as the polymerizable compound. Specifically, monomers, oligomers, polymers, etc. can be used as polymerizable compounds. Here, "monomer" refers to the smallest unit in a polymer obtained by a polymerization reaction and / or crosslinking reaction, and represents a compound having one or more polymerizable groups. On the other hand, both "oligomer" and "polymer" are polymers having polymerizable groups formed by the bonding of multiple monomers. The two are classified by their degree of polymerization; specifically, those with a degree of polymerization of 2 to 10 are called "oligomers," and those with a degree of polymerization of 11 or more are called "polymers." However, monomers used to impart polymerizable groups to oligomers and polymers are not included in the monomers constituting the oligomers and polymers. Furthermore, the monomers constituting the oligomers and polymers may include compounds other than the polymerizable compounds mentioned above. For example, an oligomer obtained by reacting a pre-oligomer having an isocyanate group at its terminal end, which is a reaction product of 1,6-hexanediol and isophorone diisocyanate, with 2-hydroxyethyl acrylate has 1,6-hexanediol and isophorone diisocyanate as monomers.

[0030] A "polymerizable group" is a functional group that can react with radicals, and examples include (meth)acryloyl groups, vinyl ether groups, allyl groups, and vinyl groups (excluding vinyl ether and allyl groups). Among the polymerizable groups mentioned above, one or more selected from the group consisting of acryloyl groups, vinyl ether groups, and vinyl groups (excluding vinyl ether and allyl groups) are preferred due to their excellent curability. Furthermore, "monofunctional" refers to having only one polymerizable group in one molecule, while "difunctional" and "trifunctional" refer to having two polymerizable groups and three polymerizable groups in one molecule, respectively. Compounds with two or more functionalities are collectively referred to as "polyfunctional."

[0031] As described above, the inkjet ink of this embodiment contains 5-methyl-3-vinyloxazolidine-2-one as a polymerizable compound. Preferably, the amount of 5-methyl-3-vinyloxazolidine-2-one is 1 to 25% by mass of the total amount of ink. Furthermore, it is more preferable that the amount of 5-methyl-3-vinyloxazolidine-2-one is 1 to 15% by mass of the total amount of inkjet ink, and particularly preferable that it is 2 to 10% by mass. In an inkjet ink containing 1% by mass or more of 5-methyl-3-vinyloxazolidine-2-one, the reaction rate between the vinyl groups present in the 5-methyl-3-vinyloxazolidine-2-one and the (meth)acryloyl groups contained in the inkjet ink is fast, and the amount of unreacted (meth)acryloyl group-containing polymerizable compound can be effectively reduced, thereby improving curability and adhesion. Furthermore, by limiting the content of 5-methyl-3-vinyloxazolidine-2-one in the inkjet ink to 25% by mass or less, it becomes easier to improve storage stability and the gloss of printed materials.

[0032] The inkjet ink of this embodiment may contain radical polymerizable compounds other than 5-methyl-3-vinyloxazolidine-2-one (also referred to as "other polymerizable compounds" in this disclosure). These other polymerizable compounds may be monofunctional polymerizable compounds (monofunctional polymerizable compounds) or bifunctional or multifunctional polymerizable compounds (polyfunctional polymerizable compounds). Furthermore, only one type of other polymerizable compound may be used, or a mixture of multiple polymerizable compounds may be used.

[0033] As described above, the inkjet ink of this embodiment can optionally use other polymerizable compounds. Examples of monofunctional monomers that can be used as other polymerizable compounds include compounds having one (meth)acryloyl group. Specific examples of such compounds include benzyl (meth)acrylate, 2-phenoxyethyl (meth)acrylate, ethylene oxide-modified 2-phenoxyethyl (meth)acrylate, propylene oxide-modified 2-phenoxyethyl (meth)acrylate, dicyclopentenyl (oxyethyl) (meth)acrylate, 2-methoxyethyl (meth)acrylate, methoxytriethylene glycol (meth)acrylate, and 2-ethoxyethyl (meth)acrylate, 2-(2-ethoxyethoxy)ethyl (meth)acrylate, methoxydipropylene glycol (meth)acrylate, dipropylene glycol (meth)acrylate, ethylene oxide-modified nonylphenol (meth)acrylate, propylene oxide-modified nonylphenol (meth)acrylate, ethylene oxide-modified o-phenylphenol (meth)acrylate, ethylene oxide-modified 2-ethyl Hexyl (meth)acrylate, β-carboxylethyl (meth)acrylate, trimethylolpropaneformal (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, cyclohexyl (meth)acrylate, 3,3,5-trimethylcyclohexyl (meth)acrylate, 4-tert-butylcyclohexyl (meth)acrylate, isobornyl (meth)acrylate, norbornyl (meth)acrylate, disyl Clopentanyl (meth)acrylate, isoamyl (meth)acrylate, isononyl (meth)acrylate, N-octyl (meth)acrylate, isooctyl (meth)acrylate, lauryl (meth)acrylate, isodecyl (meth)acrylate, tridecyl (meth)acrylate, stearyl (meth)acrylate, caprolactone (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 1,Examples include 4-cyclohexanedimethanol(meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, 4-hydroxybutyl(meth)acrylate, (meth)acryloylmorpholine, 2-(diethylamino)ethyl(meth)acrylate, 2-(diisopropylamino)ethyl(meth)acrylate, tert-butylaminoethyl(meth)acrylate, morpholinoethyl(meth)acrylate, 2-(diethylamino)ethyl(meth)acrylate, and N-(meth)acryloyloxyethylhexahydrophthalimide.

[0034] Other examples of monofunctional monomers that can be used as polymerizable compounds include compounds having one vinyl group (excluding 5-methyl-3-vinyloxazolidine-2-one). Specific examples include N-vinylcaprolactam and N-vinylpyrrolidone.

[0035] Furthermore, if the inkjet ink of this embodiment contains monofunctional monomers as other polymerizable compounds, it is preferable that the content is 15% by mass or less (it does not have to contain monofunctional monomers) of the total amount of the inkjet ink, and particularly preferable that it is 10% by mass or less (it does not have to contain monofunctional monomers). When the content of monofunctional monomers included as other polymerizable compounds is 15% by mass or less, the wetting spread of the inkjet ink on the printing substrate is improved, and the curability and adhesion are improved. On the other hand, from the viewpoint of particularly improving the curability of the inkjet ink, it is preferable that the inkjet ink of this embodiment substantially does not contain monofunctional monomers as other polymerizable compounds.

[0036] In this disclosure, "substantially absent" means that the component in question is not intentionally added, and does not prevent contamination as an impurity, by-product, etc. Specifically, if the amount of contamination as an impurity, by-product, etc. is 0.1% by mass or less (preferably 0.05% by mass or less) of the total amount of inkjet ink, it shall be included in the state of "substantially absent".

[0037] Furthermore, other examples of monofunctional polymerizable compounds that can be used as other polymerizable compounds include polyfunctional radical polymerizable compounds (specific examples will be described later) in which only one polymerizable group is left, and a primary or secondary organic amine is added to the remaining polymerizable group (Michael addition).

[0038] Other bifunctional monomers that can be used as polymerizable compounds include, for example, compounds having two (meth)acryloyl groups. Specific examples of such compounds include 1,3-propanediol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,3-butylenediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, ethylene oxide-modified 1,6-hexanediol di(meth)acrylate, propylene oxide-modified 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, neopentyl glycoside di(meth)acrylate, ethylene oxide modified neopentyl glycoside di(meth)acrylate, propylene oxide modified neopentyl glycoside di(meth)acrylate, 3-methyl-1,5-pentanediol di(meth)acrylate, 2,4-dimethyl-1,5-Pentanedioldi(meth)acrylate, 2-ethyl-2-butylpropanedioldi(meth)acrylate, 2-ethyl-2-butylbutanedioldi(meth)acrylate, ethylene oxide-modified cyclohexanemethanoldi(meth)acrylate, ethylene glycoldi(meth)acrylate, diethylene glycoldi(meth)acrylate, triethylene glycoldi(meth)acrylate Polyethylene glycol 200 di(meth)acrylate, polyethylene glycol 300 di(meth)acrylate, polyethylene glycol 400 di(meth)acrylate, hydroxypivalate neopentyl glycol di(meth)acrylate, bisphenol A di(meth)acrylate, ethylene oxide modified bisphenol A di(meth)acrylate, propylene oxide modified bis Phenol A di(meth)acrylate, bisphenol F di(meth)acrylate, ethylene oxide modified bisphenol F di(meth)acrylate, propylene oxide modified bisphenol F di(meth)acrylate, dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, ethylene oxide Examples include modified isocyanuric acid di(meth)acrylate, tricyclodecane di(meth)acrylate, methylol tricyclodecane di(meth)acrylate, cyclohexanedimethylol di(meth)acrylate, trimethylolpropane di(meth)acrylate, neopentyl glycol modified trimethylolpropane di(meth)acrylate, and dicyclopentanyl di(meth)acrylate.

[0039] Furthermore, other examples of bifunctional monomers that can be used as polymerizable compounds include compounds having one (meth)acryloyl group and one vinyl ether group. Specific examples include 2-(2-vinyloxyethoxy)ethyl (meth)acrylate and 2-[2-(2-vinyloxyethoxy)ethoxy]ethyl (meth)acrylate.

[0040] Furthermore, other examples of bifunctional polymerizable compounds that can be used as other polymerizable compounds include polyfunctional radical polymerizable compounds having three or more polymerizable groups, in which two polymerizable groups are left intact and primary or secondary organic amines are added to the remaining polymerizable groups (Michael addition).

[0041] Other trifunctional monomers that can be used as polymerizable compounds include, for example, compounds having three (meth)acryloyl groups. Specific examples of such compounds include trimethylolpropane tri(meth)acrylate, trimethylolethane tri(meth)acrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, propylene oxide-modified trimethylolpropane tri(meth)acrylate, glycerin tri(meth)acrylate, ethylene oxide-modified glycerin tri(meth)acrylate, propylene oxide-modified glycerin tri(meth)acrylate, and ethylene oxide-modified isocyanuric acid tri(meth)acrylate. Examples include phosphates, propylene oxide-modified isocyanurate tri(meth)acrylate, propylene oxide-modified dipentaerythritol tri(meth)acrylate, tetramethylolmethane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, tri((meth)acryloyloxyethyl)isocyanurate, hydroxypivalaldehyde-modified dimethylolpropane tri(meth)acrylate, sorbitol tri(meth)acrylate, etc.

[0042] Other tetrafunctional monomers that can be used as polymerizable compounds include, for example, compounds having four (meth)acryloyl groups. Specific examples of such compounds include pentaerythritol tetra(meth)acrylate, sorbitol tetra(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, ethylene oxide-modified pentaerythritol tetra(meth)acrylate, propylene oxide-modified pentaerythritol tetra(meth)acrylate, and tetramethylolmethane tetra(meth)acrylate.

[0043] In one embodiment, in an inkjet ink containing a low amount of 5-vinyl-3-vinyloxazolidine-2-one, for example, 1 to 10% by mass of the total amount of inkjet ink, the curability and adhesion are further improved by using a trifunctional monomer and / or a tetrafunctional monomer in combination. In this case, the amount of trifunctional monomer added is preferably 3 to 12% by mass of the total amount of inkjet ink, and the amount of tetrafunctional monomer added is preferably 2 to 8% by mass of the total amount of inkjet ink.

[0044] Other polymerizable compounds that can be used include pentafunctional monomers (pentafunctional monomers) having five (meth)acryloyl groups, for example. Specific examples of such compounds include sorbitolpenta(meth)acrylate and dipentaerythritolpenta(meth)acrylate.

[0045] Other hexafunctional monomers that can be used as polymerizable compounds include, for example, compounds having six (meth)acryloyl groups. Specific examples of such compounds include dipentaerythritol hexa(meth)acrylate, sorbitol hexa(meth)acrylate, alkylene oxide-modified phosphazene hexa(meth)acrylate, and ε-captolactone-modified dipentaerythritol hexa(meth)acrylate.

[0046] Furthermore, oligomers can also be used as the other polymerizable compounds mentioned above. In this case, compounds having a (meth)acryloyl group as the polymerizable group are preferably used. The number of polymerizable groups contained in the oligomer is preferably 1 to 6 per molecule, from the viewpoint of balancing curability, discharge stability, and dispersion stability. The number of polymerizable groups is more preferably 1 to 4, and particularly preferably 1 to 2. The mass-average molecular weight of the oligomer is preferably 400 to 6,000, and more preferably 500 to 4,500.

[0047] Examples of oligomers having the (meth)acryloyl group include aliphatic urethane (meth)acrylate oligomers, aromatic urethane (meth)acrylate oligomers, acrylic (meth)acrylate oligomers, polyester (meth)acrylate oligomers, polyester (meth)acrylate oligomers, epoxy (meth)acrylate oligomers, etc. Furthermore, the above-mentioned oligomers may be modified. Examples of such modification include sulfonic acid modification, phosphoric acid modification, amine modification, mercapto modification, etc.

[0048] If the inkjet ink of this embodiment contains other polymerizable compounds, it is preferable to include a bifunctional monomer and / or a trifunctional monomer, as this improves curability, ejection stability, and gloss of the printed material.

[0049] In particular, compounds containing alkylene groups are preferably used as other polymerizable compounds because they have relatively low viscosity and improve the wettability and spreadability of inkjet inks on printing substrates, thereby improving adhesion, curability, and gloss of printed materials. The alkylene group, in particular, is thought to act as a soft segment within the ink film, making it easy to impart flexibility to the printed material. Furthermore, it increases affinity with printing substrates such as polyolefin substrates and paper substrates, thereby improving the adhesion of inkjet inks. Specifically, it is preferable to use polymerizable compounds having an alkylene group, such as 1,3-butylenediol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,8-octanediool di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, neopentylglycol di(meth)acrylate, 3-methyl-1,5-pentanediol di(meth)acrylate, 2,4-dimethyl-1,5-pentanediol di(meth)acrylate, 2-ethyl-2-butylpropanediol di(meth)acrylate, and 2-ethyl-2-butylbutanediol di(meth)acrylate.

[0050] Among these compounds, it is particularly preferable to use one or more compounds selected from the group consisting of 1,4-butanedioldiaacrylate, 3-methyl-1,5-pentanedioldiaacrylate, and 1,6-hexanedioldiaacrylate as polymerizable compounds having an alkylene group. These compounds have an excellent balance between viscosity and surface tension and exhibit a great effect in improving discharge stability. On the other hand, the wettability and spreadability of inkjet inks on the printing substrate are improved, curability and adhesion of printed materials are good, and furthermore, printed materials with high gloss can be obtained.

[0051] When the inkjet ink of this embodiment contains other polymerizable compounds, the content of these other polymerizable compounds relative to the total amount of polymerizable compounds is preferably 65 to 99% by mass, and more preferably 75 to 95% by mass. When the content of other polymerizable compounds falls within this range, the curability of the inkjet ink is improved, and the gloss of the resulting printed material is also improved. Furthermore, the adhesion and durability of the ink film are at or above the level practically required for printed materials.

[0052] <Photopolymerization initiator> The inkjet ink of this embodiment contains a photopolymerization initiator that acts as a radical source, and as described above, it is preferable to use a hydrogen abstraction type photopolymerization initiator. Examples of hydrogen abstraction type photopolymerization initiators include thioxanthone compounds, benzophenone compounds, ketocoumarin compounds, and the like. These compounds can be used individually or in combination of two or more.

[0053] Specific examples of thioxanthone compounds include 2-isopropylthioxanthone, 4-isopropylthioxanthone, 2,4-diethylthioxanthone, 3-methoxythioxanthone, 2-carboxymethoxythioxanthone, 3-ethoxycarbonylmethoxythioxanthone, 3-butoxycarboxymethoxythioxanthone, 1,3-dimethyl-2-(2-ethylhexyloxy)thioxanthone, 2-[2,2-bis(ethoxycarbonyl)]ethylthioxanthone, 1-chloro-4-propoxythioxanthone, and polymers of these compounds. Furthermore, examples of commercially available thioxanthone compounds include "Omnirad ITX," "Omnirad DETX," and "OMNIPOL TX" from IGM RESINS, "SPEEDCURE ITX," "SPEEDCURE 2-ITX," "SPEEDCURE DETX," "SPEEDCURE LTX," "SPEEDCURE CPTX," and "SPEEDCURE 7010" from Lambson, and "Genopol TX-2" from RAHN. In addition, polymerizable compounds disclosed in International Publication No. 2010 / 29017, International Publication No. 2022 / 157274, etc., can also be used as thioxanthone compounds. Of the commercially available products listed above, "OMNIPOL TX," "SPEEDCURE 7010," and "Genopol TX-2" are the aforementioned polymers.

[0054] Specific examples of benzophenone compounds include benzophenone, 2,4,6-trimethylbenzophenone, 4-phenylbenzophenone, 2-ethylanthraquinone, 4,4′-bis(dimethylamino)benzophenone, 4,4′-bis(diethylamino)benzophenone, p-methylbenzophenone, and methyl 2-benzoylbenzoate. Examples of commercially available products include "Omnirad BP," "Omnirad BMS," "Omnirad4PBZ," "Omnirad EMK," and "Esacure 1001M" from IGM RESINS.

[0055] Specific examples of ketocoumarin compounds include 3-benzoyl-7-methoxycoumarin, 3-benzoyl-5,7-dimethoxycoumarin, 3-(4-tert-butylbenzoyl)-5,7-dimethoxycoumarin, 3-(4-hexylbenzoyl)-5,7-dimethoxycoumarin, 3-[4-(2-ethylhexyl)benzoyl]-5,7-dimethoxycoumarin, 5,7-dimethoxy-3-[4-(3,5,5-trimethylhexyl)benzoyl]coumarin, 7-methoxy-3-(4-methylbenzoyl)coumarin, 7-methoxy-3-(4-tert-butylbenzoyl)coumarin, 7-methoxy-3-(4-hexylbenzoyl)coumarin, and 7-methoxy-3-[4-(2-ethylhexyl)benzoyl]coumarin. In addition, commercially available ketocoumarin compounds such as "Esacure 3644" from IGM RESINS, as well as compounds disclosed in International Publication Nos. 2014 / 63997, 2017 / 216699, 2019 / 30631, and 2019 / 116176, can also be used.

[0056] In the inkjet ink of this embodiment, it is particularly preferable to use a thioxanthone compound as the hydrogen abstraction type photopolymerization initiator. The inkjet ink of this embodiment, which contains a thioxanthone compound, has excellent storage stability, adhesion, and curing properties.

[0057] When a thioxanthone compound is used as the hydrogen abstraction type photopolymerization initiator described above, its content is preferably 0.5 to 4% by mass, more preferably 1 to 3.5% by mass, and particularly preferably 1.5 to 2.5% by mass of the total amount of inkjet ink. When the content of the thioxanthone compound is 0.5% by mass or more, the polymerization reaction proceeds without excessive radical capture by the N-nitrosophenylhydroxylaluminum salt contained in the inkjet ink of this embodiment, and curability is improved. Furthermore, when the content of the thioxanthone compound is 4% by mass or less, the thioxanthone compound does not excessively absorb ultraviolet light, and light can reach the inside of the inkjet ink, making it easier to prevent a decrease in curing speed, poor curing inside the film, and deterioration of adhesion.

[0058] As described above, from the viewpoint of simultaneously improving curability and storage stability, and furthermore, from the viewpoint of reducing differences in curability between the surface and interior of the ink film, and easily obtaining printed materials with high adhesion and gloss, the ratio of the content mass of the hydrogen abstraction type photopolymerization initiator to the content mass of N-nitrosophenylhydroxylaluminum salt (hydrogen abstraction type photopolymerization initiator: N-nitrosophenylhydroxylaluminum salt) is preferably 8:1 to 80:1. In particular, the above ratio is preferably 20:1 to 60:1, as this allows the above effects to be suitably exhibited and improves curability, storage stability, adhesion, and gloss of the printed material all at the same time.

[0059] On the other hand, in the inkjet ink of this embodiment, the ratio of the amount of hydrogen abstraction type photopolymerization initiator to the amount of 5-methyl-3-vinyloxazolidine-2-one (hydrogen abstraction type initiator: 5-methyl-3-vinyloxazolidine-2-one) is preferably 0.080:1 to 2.0:1. By setting the amount of hydrogen abstraction type photopolymerization initiator to 0.080 to 2.0 times the amount of 5-methyl-3-vinyloxazolidine-2-one, 5-methyl-3-vinyloxazolidine-2-one functions effectively as a hydrogen source donor, which is thought to significantly improve curability. Furthermore, the polymerization reaction proceeds uniformly on the surface and inside the inkjet ink, improving adhesion and gloss. These effects are more favorably expressed, and curability, adhesion, and gloss of the printed material are improved. Therefore, the ratio of the mass content of the hydrogen abstraction type photopolymerization initiator to the mass content of 5-methyl-3-vinyloxazolidine-2-one is preferably 0.10:1 to 2.0:1.

[0060] In the inkjet ink of this embodiment, in addition to the hydrogen abstraction type photopolymerization initiator described above, one or more conventionally known photopolymerization initiators can be optionally used. Specifically, acylphosphine oxide compounds, indan compounds, hydroxyacetophenone compounds, alkylaminoacetophenone compounds, oxime ester compounds, aminobenzoate compounds, and anthracene compounds can be used as such photopolymerization initiators.

[0061] Among these compounds, in the inkjet ink of this embodiment, acylphosphine oxide compounds are particularly preferred as photopolymerization initiators other than hydrogen abstraction type photopolymerization initiators, because they can easily achieve desirable levels of ejection stability and curability of the surface and interior of the ink film. Generally, acylphosphine oxide compounds exhibit a photobleaching effect. The photobleaching effect is a phenomenon in which a compound decomposes due to irradiation with ultraviolet light, etc., and the compound fades. In the case of acylphosphine oxide compounds, after radical generation, they fade and lose their ultraviolet absorption ability, thereby improving ultraviolet transmittance into the ink film. As a result, both internal and surface curability of the ink film can be achieved, and the gloss of the ink film surface can be easily improved due to the uniform progress of the reaction. From this point of view as well, acylphosphine oxide compounds can be suitably used.

[0062] When the inkjet ink of this embodiment contains an acylphosphine oxide compound as a photopolymerization initiator, the content of the acylphosphine oxide compound relative to the total amount of the photopolymerization initiator is preferably 70 to 96% by mass, and particularly preferably 75 to 92% by mass. If the content of the acylphosphine oxide compound is within the above range, it becomes easy to improve curability, adhesion, and gloss of the printed material in a balanced manner.

[0063] Specific examples of acylphosphine oxide compounds include diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, as well as polymers of these compounds. Examples of commercially available acylphosphine oxide compounds include "Omnirad TPO," "Omnirad TPO-L," "Omnirad TPO-H," "Omnirad 819," and "OMNIPOL TP" from IGM RESINS, and "Speedcure TPO," "Speedcure TPO-L," and "Speedcure BPO" from Lambson. For example, acylphosphine oxide compounds described in International Publication No. 2017 / 086224 and International Publication No. 2020 / 049378, as well as lithium phenyl(2,4,6-trimethylbenzoyl)phosphinate, can also be used. In the inkjet ink of this embodiment, only one of the above-listed acylphosphine oxide compounds may be used, or two or more may be used in combination.

[0064] Among these acylphosphine oxide compounds, the inkjet ink of this embodiment preferably uses ethoxyphenyl (2,4,6-trimethylbenzoyl)phosphine oxide and / or ethoxyphenyl (2,4,6-trimethylbenzoyl)phosphine oxide polymers. Ethoxyphenyl (2,4,6-trimethylbenzoyl)phosphine oxide is a liquid at room temperature and is a useful material because it easily improves the curability and adhesion of the ink film surface while maintaining discharge stability.

[0065] From the standpoint of improving discharge stability, curability, and gloss of printed materials, the ratio of the mass content of the acylphosphine compound to the mass content of the hydrogen abstraction type photopolymerization initiator (acylphosphine compound: hydrogen abstraction type initiator) is preferably 2.0:1 to 20:1. More preferably 2.5:1 to 20:1, and particularly preferably 3.0:1 to 10:1.

[0066] <Polymerization inhibitor> As already mentioned above, in the inkjet ink of this embodiment, N-nitrosophenylhydroxylaluminum salt can be used as a polymerization inhibitor. Adding the above polymerization inhibitor makes it easier to improve the storage stability and ejection stability of the inkjet ink, improve the hue stability of printed materials, and prevent curing wrinkles.

[0067] The N-nitrosophenylhydroxylaluminum salt is preferably present in the inkjet ink of this embodiment in an amount of 0.05 to 0.5% by mass, and particularly preferably in an amount of 0.05 to 0.1% by mass. By incorporating it within this range, storage stability can be ensured while maintaining curability. As a result, wrinkle formation during curing is suppressed, a uniform cured film is obtained, and the adhesion and gloss of the printed material are also improved.

[0068] <Coloring agent> When the inkjet ink of this embodiment contains a colorant, conventionally known dyes and pigments can be used as the colorant. Among these, the use of pigments is preferable from the viewpoint of improving the color development of the printed material (opacity in the case of white ink, and gloss in the case of metallic ink), as well as storage stability and ejection stability. Specific examples of organic and inorganic pigments that can be used as the above-mentioned pigments include: Red pigments include CI Pigment Red 5, 7, 12, 17, 48(Ca), 48(Mn), 49:2, 57(Ca), 57:1, 112, 122, 123, 147, 149, 150, 166, 168, 176, 177, 178, 179, 184, 188, 202, 209, 242, 254, 255, 264, 266, 269, and 282; Orange pigments: CI Pigment Orange 5, 13, 34, 38, 43, 61, 62, 64; Yellow pigments, CI Pigment Yellow-1, 2, 3, 12, 14, 16, 17, 73, 74, 75, 83, 93, 95, 97, 98, 109, 110, 114, 120, 128, 129, 138, 139, 147, 150, 151, 154, 155, 180, 185, 213; Green pigments: CI Pigment Green 7, 26, 36, 50, 58; Blue pigments, CI Pigment Blue-1, 2, 3, 15, 15:3, 15:4, 15:6, 16, 22, 60; Violet pigments, CI Pigment Violet 19, 23, 31, 37; As black pigments, CI Pigment Black 1, 6, 7, 32; As white pigments, CI Pigment White 4, 5, 6, 12, 18, 21, 25; and, Examples of metallic pigments include CI Pigment Metal 1 and 2. Furthermore, two or more of the pigments listed above may be used in combination.

[0069] When the inkjet ink of this embodiment contains a pigment, its content can be adjusted according to the color and intended use of the inkjet ink. For example, from the viewpoint of achieving both storage stability, ejection stability, and curability of the inkjet ink, the pigment content is preferably 0.5 to 15% by mass, and particularly preferably 1 to 10% by mass, except in the case of white ink and metallic ink. On the other hand, when the inkjet ink of this embodiment is a white ink, from the viewpoint of achieving both storage stability, ejection stability, curability, and opacity of the ink film, the pigment content is more preferably 5 to 30% by mass, and even more preferably 15 to 25% by mass. Furthermore, when the inkjet ink is a metallic ink, from the viewpoint of achieving both storage stability, ejection stability, and glossiness of the ink film, the pigment content is more preferably 0.5 to 10% by mass, and even more preferably 1 to 5% by mass.

[0070] <Pigment derivatives> When the inkjet ink of this embodiment contains a pigment, a pigment derivative can be used to ensure and improve the dispersion stability of the pigment, as well as the storage stability and ejection stability of the inkjet ink.

[0071] As the above-mentioned pigment derivatives, compounds in which substituents are introduced to the basic skeleton of the pigment can be used. In particular, it is preferable to use compounds in which substituents are introduced to the same basic skeleton as the pigment contained in the inkjet ink. For example, when using CI Pigment Blue-15:3 as the pigment, a compound in which substituents are introduced to the copper phthalocyanine skeleton can be preferably used, and when using CI Pigment Red 122 as the agent, a compound in which substituents are introduced to the quinacridone skeleton can be preferably used.

[0072] On the other hand, the substituents include a carboxylate group having a counterion (COO - ), and sulfonate groups having counterions (SO3 -) are examples of counterions. In addition, hydrogen ions (H) are examples of counterions. + ), sodium ions, potassium ions, magnesium ions, calcium ions, primary amine cations, secondary amine cations, tertiary amine cations, quaternary ammonium cations, etc. can be used.

[0073] When the inkjet ink of this embodiment contains a pigment derivative, it is preferable that the substituent is a sulfonate group having a counterion selected from the group consisting of a hydrogen ion, a primary amine cation, a secondary amine cation, and a quaternary ammonium cation. By using a pigment derivative having such substituents, it becomes easy to obtain an inkjet ink with excellent storage stability and ejection stability.

[0074] When the inkjet ink of this embodiment contains a pigment and a pigment derivative, the amount of the pigment derivative varies depending on the type of pigment used in combination, but is preferably 0.5 to 10% by mass relative to the pigment content, and particularly preferably 1 to 8% by mass.

[0075] <Pigment-dispersed resin> When the inkjet ink of this embodiment contains a pigment, a pigment dispersion resin can be used to ensure and improve the dispersion stability of the pigment, as well as the storage stability and ejection stability of the inkjet ink.

[0076] The mass-average molecular weight of the pigment dispersion resin is preferably 3,500 to 60,000, more preferably 5,000 to 50,000, and even more preferably 10,000 to 45,000. Within this range, the compatibility of the pigment dispersion resin with the polymerizable monomer is good, improving the storage stability and discharge stability of the ink. Furthermore, since the pigment is uniformly dispersed by the pigment dispersion resin, the adhesion and gloss of the printed material are also improved.

[0077] The above mass-average molecular weight can be determined by GPC (Gel Permission Chromatography). Specifically, it is the value obtained as the polystyrene-equivalent molecular weight measured using a TSKgel column (manufactured by Toso Corporation) and a GPC equipped with an RI detector (for example, Toso Corporation's "HLC-8320GPC"), with DMF as the developing solvent.

[0078] From the viewpoint of suitably improving the dispersion stability of the pigment, the storage stability and ejection stability of the inkjet ink, and the adhesion of the printed material, the above-mentioned pigment dispersion resin is preferably a basic pigment dispersion resin.

[0079] In this embodiment, the "basic pigment dispersion resin" refers to a dispersion resin in which basic groups serve as adsorption sites on the pigment surface. Examples of the basic groups include primary amino groups, secondary amino groups, tertiary amino groups, quaternary ammonium groups, and imino groups. Furthermore, organic groups bonded to the nitrogen atoms in the tertiary amino groups and quaternary ammonium groups may bond to each other to form a ring structure (a heterocyclic ring containing the nitrogen atoms). Examples of such rings include pyridine, pyrrolidine, pyrrolidone, imidazoline, and caprolactam.

[0080] Examples of the basic pigment dispersion resins mentioned above include acrylic resins having the basic group, (anhydride) maleic acid resins having the basic group, polyethyleneimine, polyallylamine, polydiallylamine, polyvinylimidazoline, and polyvinylpyrrolidone, as well as graft resins with these resins as the main chain. Examples of commercially available basic pigment dispersion resins include "Ajispa--PB-821", "Ajispa--PB-822", "Ajispa--PB-824", and "Ajispa--PB-881" from Ajinomoto Fine Techno Co., Ltd., and "DISPERBYK-162", "DISPERBYK-163", "DISPERBYK-168", "DISPERBYK-182", "DISPERBYK-184", "DISPERBYK-185", and "DISPERBYK-185" from Big Chemi Co., Ltd. Examples include "SPERBYK-2013", "DISPERBYK-2155", "BYKJET-9150", "BYKJET-9151", "BYKJET-9152", Lubrizol's "Solspas 24000", "Solspas 32000", "Solspas 33000", "Solspas 35000", "Solspas 39000", "Solspas 86000", "Solspas J200", and "Solspas X300", and BASF's "EFKA PX4701", "EFKA PX4703", and "EFKA PX4733".

[0081] In this disclosure, "acrylic resin" refers to a resin that uses one or more monomers selected from the group consisting of acrylic acid, methacrylic acid, acrylic acid esters, and methacrylic acid esters as constituent monomers (styrene monomers may also be used). However, resins using (anhydride) maleic acid (maleic anhydride and / or maleic acid) as the monomer are not included in the above "acrylic resin". Furthermore, in this disclosure, "(anhydride) maleic acid resin" refers to a resin that uses at least (anhydride) maleic acid as a monomer constituting the resin. In addition, the (anhydride) maleic acid resin may also use one or more selected from the group consisting of α-olefins, acrylic acid, methacrylic acid, acrylic acid esters, methacrylic acid esters, styrene, and styrene derivatives as the above monomer.

[0082] When the inkjet ink of this embodiment contains a basic pigment dispersion resin, its amine value is preferably 10 to 50 mgKOH / g, and particularly preferably 15 to 40 mgKOH / g. A basic pigment dispersion resin having an amine value within the above range has a sufficient number of adsorption sites and can therefore be strongly adsorbed to the pigment. As a result, the dispersion stability of the pigment, as well as the storage stability and ejection stability of the inkjet ink, are improved. Furthermore, since the adsorption sites (basic groups) do not inhibit the polymerization reaction of polymerizable compounds, the curability and adhesion of the inkjet ink are also improved.

[0083] The "amine value" mentioned above refers to the amount of potassium hydroxide (in mg) equivalent to the acid required to neutralize 1 g of resin. As an example of a method for measuring the amine value, the target resin is dissolved in a solvent mixture of ethanol or tetrahydrofuran and acetic acid, and then titrated using a 0.1 mol / L perchloric acid-acetic acid solution by potentiometric titration. The amine value can then be calculated by converting the titration volume read from the resulting titration curve to the amount of potassium hydroxide (in mg).

[0084] Furthermore, if the inkjet ink of this embodiment contains a basic pigment dispersion resin, its acid value is preferably 2 to 30 mg KOH / g, and particularly preferably 5 to 20 mg KOH / g. If the acid value of the basic pigment dispersion resin is within the above range, the adsorption of basic groups to the pigment surface is less likely to be inhibited, while the pigment dispersed by the basic dispersion resin is uniformly and stably dispersed within the inkjet ink, thereby improving the dispersion stability of the pigment, the storage stability of the inkjet ink, the ejection stability, and the adhesion.

[0085] The "acid value" mentioned above refers to the number of milligrams of potassium hydroxide required to neutralize 1 gram of resin, and can be determined by potentiometric titration in accordance with JIS K 0070. As an example of a specific measurement method, the target resin is dissolved in a solvent in which diethyl ether and ethanol are mixed in a 1:1 mass ratio, and then titrated using a 0.1 mol / L potassium hydroxide-ethanol solution by potentiometric titration. The acid value can then be calculated using the titration volume read from the resulting titration curve.

[0086] When the inkjet ink of this embodiment contains a basic pigment dispersion resin, its content is preferably 15 to 120% by mass, and more preferably 20 to 80% by mass, relative to the total amount of pigment, except when the inkjet ink is a white ink. In the case of a white ink, the content is preferably 2 to 60% by mass, and more preferably 5 to 30% by mass, relative to the total amount of pigment. Using the ink within the above blending range improves the initial dispersibility of the pigment, the storage stability of the inkjet ink, and the adhesion.

[0087] <Other ingredients> In addition to the components described above, the inkjet ink of this embodiment may also contain surface modifiers, organic solvents, water, inert resins, fluorescent whitening agents, and other additives.

[0088] <Surface modifier> The inkjet ink of this embodiment preferably contains a surface modifier for the purpose of improving wetting spread on the printing substrate, print quality including the fineness of the printed material, adhesion, and ejection stability. Examples of surface modifiers that can be used include siloxane-based surface modifiers, fluorine-based surface modifiers, acetylene glycol-based surface modifiers, and acetylene monool-based surface modifiers. Among these, a siloxane-based surface modifier is preferred because it can improve wetting spread on the printing substrate, print quality including the fineness of the printed material, adhesion, and ejection stability without worsening the dispersion stability of the pigment.

[0089] As the siloxane-based surface modifier mentioned above, for example, compounds having a dimethylsiloxane structure and / or modified versions thereof can be used. Among these, polyethylene-modified siloxane-based surface modifiers are particularly preferred. By using polyethylene-modified siloxane-based surface modifiers, the effect of 5-methyl-3-vinyloxazolidine-2-one is not inhibited, and the ink that lands on the printing substrate can be sufficiently wetted and spread. This makes it possible to achieve both curability, print quality including the fineness of the printed material, and adhesion while maintaining favorable discharge stability and pigment dispersion stability. Furthermore, even when printing on a printing substrate with pores, it becomes less susceptible to oxygen inhibition by air contained in the pores, making it easier to improve curability and adhesion to the aforementioned printing substrate with pores.

[0090] Specific examples of the above-mentioned polyethylene group include polyethylene oxide group and polypropylene oxide group. These may contain only one of these polyethylene groups in the molecule, or both. Furthermore, from the viewpoint of improving curability and adhesion, it is also preferable to use a siloxane-based surface modifier having a (meth)acryloyl group in the molecule ((meth)acrylic-modified siloxane-based surface modifier). Particularly preferable is a siloxane-based surface modifier having both a polyethylene group and a (meth)acryloyl group in the molecule ((meth)acrylic-polyethylene-modified siloxane-based surface modifier).

[0091] As commercially available polyethylene-modified siloxane-based surface modifiers, for example, BYK(registered trademark)-378, 348, 349, 3420, and 3760 from BIC Chemi; and TEGO(registered trademark) Glide 450, 440, 435, 432, 410, 406, 130, 110, and 100 from EVONIK are preferably used. In addition, as siloxane-based surface modifiers having polyethylene groups and (meth)acryloyl groups, TEGO Rad 2100, 2200N, 2250, 2300, 2330, 2500, 2550, BYK-UV3500, and UV3530 are preferably used.

[0092] When using a siloxane-based surface modifier, its content is preferably 0.1 to 5% by mass, based on the total mass of the inkjet ink. Adjusting the content to 0.1% by mass or more improves the wettability and spreadability on the printing substrate, as well as the print quality, including the fineness of the printed material, and the adhesion. On the other hand, adjusting the content to 5% by mass or less makes it easier to ensure curability, pigment dispersion stability, and ejection stability.

[0093] <Organic solvents, water> In the inkjet ink of this embodiment, an organic solvent and / or water may be used to reduce the viscosity of the inkjet ink, improve its wetting and spreading properties and adhesion to the printing substrate, and ensure ejection stability. When an organic solvent and / or water is included, the total amount contained therein is preferably 0.01 to 10% by mass, more preferably 0.05 to 5% by mass, and particularly preferably 0.1 to 3% by mass, based on the total mass of the inkjet ink. Furthermore, from the viewpoint of ejection stability and wetting and spreading properties and adhesion to the printing substrate, when an organic solvent is used, it is preferable to use an organic solvent with a boiling point of 140 to 300°C.

[0094] Examples of organic solvents that can be used include alkylene glycosulfones, alkylene glycosulfones, alkylene glycosulfones, alkylene glycosulfones, alkylene glycosulfones, alkanediols, lactams, lactones, other nitrogen-containing solvents, and other oxygen-containing solvents.

[0095] In particular, it is preferable to include at least one selected from the group consisting of alkylene glycosulfone monoalkyl ethers, alkylene glycosulfone dialkyl ethers, and alkylene glycosulfone monoalkyl ether acetates. Tripropylene glycosulfone monomethyl ether, dipropylene glycosulfone monomethyl ether, propylene glycosulfone monomethyl ether, ethylene glycosulfone monobutyl ether, diethylene glycosulfone monobutyl ether, triethylene glycosulfone monobutyl ether, ethylene glycosulfone monobutyl ether acetate, diethylene glycosulfone methylethyl ether, and diethylene glycosulfone diethyl ether are preferred. In one embodiment, the organic solvent preferably includes at least one selected from the group consisting of dipropylene glycol monomethyl ether, diethylene glycol monobutyl ether, ethylene glycol monobutyl ether acetate, diethylene glycol methyl ethyl ether, and diethylene glycol diethyl ether. By using these organic solvents, it is easy to improve adhesion and print quality while maintaining the storage stability and ejection stability of the inkjet ink in a suitable state.

[0096] <Inert resin> The inkjet ink of this embodiment can improve adhesion to various printing substrates, and the viscoelasticity of the inkjet ink is adjusted, improving ejection stability. Therefore, it may contain an inert resin. As the inert resin, (meth)acrylic resin, urethane resin, vinyl chloride-vinyl acetate copolymer resin, ketone resin, etc. can be used. In particular, from the viewpoint of improving both adhesion and ejection stability, it is preferable that the inert resin contains (meth)acrylic resin and / or ketone resin.

[0097] When the inkjet ink of this embodiment contains an inert resin, its content is preferably 0.1 to 10% by mass, more preferably 0.5 to 5% by mass, and particularly preferably 1 to 3% by mass, based on the total mass of the inkjet ink. By adjusting the above content within the above range, adhesion and discharge stability can be easily improved without worsening curability.

[0098] In this disclosure, "inert resin" refers to a resin that does not participate in polymerization reactions, contributes to adhesion to a printing substrate, and has solubility in inkjet inks.

[0099] <Fluorescent whitening agent> The inkjet ink of this embodiment may contain a fluorescent whitening agent. A fluorescent whitening agent is a compound that absorbs light in the 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 whitening agents include naphthalenebenzoxazoyl compounds, thiophenebenzoxazoyl compounds, stilbenebenzoxazoyl compounds, stilbenebiphenyl compounds, distyrylbenzene compounds, coumarin compounds, carbostyryl compounds, pyrazolone compounds, naphthalimide compounds, pyrene compounds, pyridotriazole compounds, and the like.

[0100] In the inkjet ink of this embodiment, it is preferable to use a thiophenebenzoxazoyl compound and / or a naphthalenebenzoxazoyl compound as a fluorescent whitening agent. Compared to other fluorescent whitening agents, these fluorescent whitening agents have absorption and fluorescence wavelengths that are suitable for improving curability, and even with the addition of a small amount, it is easy to significantly improve the curability of the inkjet ink. Furthermore, they have high affinity with the materials contained in the inkjet ink of this embodiment, and deterioration of ejection stability and adhesion can be suppressed. Moreover, since thiophenebenzoxazoyl compounds and naphthalenebenzoxazoyl compounds have little coloration, the original color of the inkjet ink does not change significantly.

[0101] Specific examples of the above-mentioned thiophene-benzoxazoyl compounds and naphthalene-benzoxazoyl compounds include "Tinopal OB" and "Tinopal OB CO" from BASF, "SpeedBlock OB-184" from Arkema, "NF-TH01" and "NF-NA01" from Nippon Chemical Industries, Ltd., and "FLUORESCENT BRIGHTNER KCB," "FLUORESCENT BRIGHTNER OB," and "FLUORESCENT BRIGHTNER PB" from Xcolor.

[0102] From the viewpoint of obtaining an inkjet ink that is excellent in terms of curability, discharge stability, and adhesion, the content of the fluorescent whitening agent is preferably 0.01 to 1.5% by mass, more preferably 0.05 to 1% by mass, and particularly preferably 0.1 to 0.7% by mass, of the total amount of the above inkjet ink.

[0103] Furthermore, when using a UV-LED as a light source, it is particularly preferable to use a thioxanthone compound as the hydrogen abstraction type photopolymerization initiator and a thiophenebenzoxazoyl compound as the fluorescent whitening agent. Both thioxanthone compounds and thiophenebenzoxazoyl compounds have absorption wavelengths suitable for UV-LED light sources, and effective radical generation is facilitated, resulting in a particularly improved curability. This is presumed to be because both have heterocyclic and conjugated systems in their structures, which makes the transfer of electrons and energy more efficient and facilitates the formation of stable radicals. Also, as mentioned above, since the molecular structures of the two are relatively similar, they have high affinity, and deterioration of discharge stability and adhesion can also be easily suppressed.

[0104] In the inkjet ink of this embodiment, from the viewpoint of improving both curability and adhesion of printed materials, the ratio of the hydrogen abstraction type photopolymerization initiator content to the fluorescent whitening agent content (hydrogen abstraction type photopolymerization initiator / fluorescent whitening agent) is preferably 0.2 to 100 by mass ratio, and particularly preferably 0.5 to 40. By setting the fluorescent whitening agent content within the above range, the function of the fluorescent whitening agent is fully expressed without inhibiting the functional expression of the hydrogen abstraction type photopolymerization initiator, thereby improving curability and adhesion of printed materials.

[0105] <Other additives> The inkjet ink of this embodiment may, if necessary, further contain additives such as ultraviolet absorbers and fade inhibitors in addition to the components described above. These components can be any conventionally known compounds.

[0106] <Physical properties of inkjet inks> In this embodiment, the inkjet ink preferably has a viscosity of 5 to 25 mPa·s at 25°C, and more preferably 8 to 20 mPa·s, from the viewpoint of improving ejection stability. If the viscosity is 5 mPa·s or higher, the inkjet ink can be ejected well and stably from the inkjet head. If it is 25 mPa·s or lower, the ejection accuracy will not decrease, and stable ejection can be continued. Furthermore, from the viewpoint of providing high-frequency suitability and enabling stable ejection even at high-speed printing, the viscosity is particularly preferably 8 to 14 mPa·s. The viscosity can be measured using 1.1 mL of inkjet ink and an E-type viscometer (for example, "TVE25L" manufactured by Toki Sangyo Co., Ltd.) equipped with a cone with a cone angle of 1°34' (diameter 48 mm), under conditions of a 25°C environment and a rotation speed of 20 rpm.

[0107] Furthermore, from the viewpoint of improving ejection stability, print resolution, and curability, 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 used is the value measured by the plate method (Wilhelmi method). Specifically, for example, it can be measured at 25°C using an automatic surface tension meter "CBVP-Z" manufactured by Kyowa Interface Science Co., Ltd. and a platinum plate.

[0108] <Method of manufacturing inkjet ink> The inkjet ink of this embodiment can be manufactured by conventionally known methods. For example, it can be manufactured as follows, but the method for manufacturing the inkjet ink of this embodiment is not limited to the method described below.

[0109] First, a pigment dispersion resin is dissolved in a polymerizable compound to produce a pigment dispersion resin varnish. Next, the pigment is gradually added to the pigment dispersion resin varnish while stirring and mixed. After mixing for a certain period of time (premixing), a pigment dispersion liquid is produced by performing a dispersion treatment using a dispersion machine such as a paint shaker, sand mill, roll mill, or medialess disperser. It is preferable to use the above-mentioned bifunctional or higher polymerizable compound as the polymerizable compound used in the production of the pigment dispersion resin varnish, as this facilitates improvements in the dispersion stability of the pigment and the storage stability and ejection stability of the inkjet ink.

[0110] Subsequently, 5-methyl-3-vinyloxazolidine-2-one and N-nitrosophenylhydroxylaluminum salt, along with other polymerizable compounds, other photopolymerization initiators, surface modifiers, organic solvents, water, inert resin, fluorescent whitening agents, and other additives as needed, are added to the pigment dispersion and thoroughly mixed. Then, the mixture is filtered using a filter or the like to remove coarse particles, and the inkjet ink of this embodiment can be obtained.

[0111] The amount of pigment present in the above-mentioned pigment dispersion is preferably 20 to 70% by mass, and particularly preferably 25 to 60% by mass, when using a white pigment. When using a pigment other than a white pigment, the amount of pigment is preferably 10 to 40% by mass, and particularly preferably 15 to 30% by mass, in the pigment dispersion.

[0112] <Printed material> An embodiment of the present invention is a printed material obtained by printing the inkjet ink of this embodiment onto a printing substrate, as described later; that is, a printing substrate on which images and / or characters are recorded. Therefore, "printed material" in this disclosure includes images and / or characters, which are made of a film (ink film) formed by curing the inkjet ink of this embodiment, and a printing substrate. The above-mentioned "image" also includes solid images and seamless images such as checkerboard patterns. Furthermore, the inkjet printing method shown below can be used as a method for manufacturing the above-mentioned printed material.

[0113] <Inkjet Printing Method> The inkjet ink of this embodiment described above is preferably used in an inkjet printing method. Furthermore, the inkjet printing method preferably includes, in this order, the steps of ejecting the inkjet ink of this embodiment from an inkjet head onto a printing substrate (Step I), and curing the inkjet ink ejected onto the printing substrate by irradiating it with ultraviolet light (Step II).

[0114] In the above inkjet printing method, a method may be employed in which the same inkjet ink is ejected and applied multiple times from the same inkjet head to the same location on the printing substrate, that is, a method in which step I is performed multiple times on the same location on the printing substrate (multi-pass printing method). However, in the case of the inkjet printing method using the inkjet ink of this embodiment, from the viewpoint of fully demonstrating the effects of the inkjet ink, such as its excellent curability and ejection stability, and obtaining printed materials quickly and stably, a method in which the same inkjet ink is ejected and applied only once from the same inkjet head to the same location on the printing substrate is preferred. In other words, it is preferable to employ a method in which step I is performed only once on the same location on the printing substrate (one-pass printing method).

[0115] One-pass printing can be implemented, for example, by using a line printer. The printing speed (transport speed of the printing substrate) in this case is preferably 35 to 150 m / min, more preferably 50 to 125 m / min, and even more preferably 75 to 100 m / min, from the viewpoint of productivity and obtaining printed materials of good quality.

[0116] The inkjet ink in this embodiment is an inkjet printing ink. Therefore, as described above, an inkjet head is used as the inkjet ink ejection means in step I.

[0117] Methods for ejecting ink using an inkjet head include electrostatic induction, which ejects ink using electrostatic force; drop-on-demand (pressure pulse) method, which utilizes the vibration pressure of a piezoelectric element; acoustic inkjet method, which converts an electrical signal into an acoustic beam and irradiates the ink, using the resulting radiation pressure to eject the ink; and thermal inkjet method, which heats the ink to form bubbles and uses the resulting pressure to eject the ink. Among these, in one embodiment, from the viewpoint of ejection stability, the drop-on-demand (pressure pulse) method, which utilizes the vibration pressure of a piezoelectric element, is preferably used.

[0118] From the viewpoint of print quality and ejection stability, the drop volume of inkjet ink droplets ejected from the inkjet nozzle is preferably 1 to 50 pL (picoliters), more preferably 2 to 30 pL, and even more preferably 3 to 20 pL. Furthermore, the design resolution of the inkjet head is preferably 300 dpi or higher, more preferably 480 dpi or higher, and even more preferably 600 dpi or higher. Note that dpi represents the number of dots per 2.54 cm (1 inch).

[0119] Examples of inkjet heads that meet the above conditions include Kyocera's KJ4A-AA, KJ4A-TA, KJ4A-RH; Fujifilm's Samba G3L; Seiko Epson's S3200, S1600, S800, I3200, I1600; Konica Minolta's KM1024i, KM1024; and Ricoh's MH5320, MH5340, MH5240, MH5440, etc., all of which can be used suitably.

[0120] In one embodiment, the inkjet ink can be ejected while being heated by a heating device such as a heater provided in the inkjet head so that the inkjet ink has an appropriate viscosity. From the viewpoint of continuously and stably ejecting the inkjet ink, it is preferable to heat the inkjet ink so that the viscosity of the inkjet ink at the time of ejection is 15 mPa·s or less, and it is even more preferable to heat it so that it is 12 mPa·s or less.

[0121] On the other hand, in step II described above, the inkjet ink ejected onto the printing substrate hardens when irradiated with ultraviolet light, forming an ink film.

[0122] Examples of ultraviolet light sources include high-pressure mercury lamps, low-pressure mercury lamps, ultra-high-pressure mercury lamps, metal halide lamps, ultraviolet lasers, and LED light sources. Only one type may be used, or two or more types may be used in combination. For example, because the LED light source is small, it is easy to install multiple units side by side, or to use them in combination with high-pressure mercury lamps or metal halide lamps, which makes it easy to further improve curing performance. When installing multiple LED light sources side by side, multiple types of LED light sources with different emission peak wavelengths may be used in combination.

[0123] Generally, ultraviolet light emitted from LED light sources has a narrow wavelength range and exhibits excellent directional propagation (i.e., poor diffusion), making it difficult to cure inkjet inks. However, the inkjet ink of this embodiment has particularly excellent curability, making it suitable for combination with LED light sources.

[0124] In the inkjet printing method using the inkjet ink of this embodiment, when an LED light source is used as the ultraviolet light source, the peak wavelength of its emission is preferably 260 to 450 nm, more preferably 280 to 420 nm, and particularly preferably 320 to 410 nm.

[0125] In step II, when using an LED light source that emits ultraviolet light, the above-mentioned effects are fully realized, and from the viewpoint of obtaining printed materials with excellent print quality, including curability and fineness of the printed material, the irradiance of ultraviolet light on the printing substrate is set to 1,000 mW / cm². 2 It is preferable that the illuminance is 2,000 mW / cm². 2 It is more preferable that the level be greater than or equal to 3,000 mW / cm². 2 The above is particularly preferable. Furthermore, the cumulative light intensity when irradiating the printing substrate varies depending on the type and content of polymerizable compounds and photopolymerization initiators contained in the inkjet ink, but for example, 50 mJ / cm² is preferable. 2 It is preferable to have the above. The above integrated light quantity is 100 mJ / cm 2is more preferably at least 150 mJ / cm 2 is particularly preferably at least.

[0126] In the above step I, after droplets of the inkjet ink adhere to the printing substrate (that is, after the completion of the above step I), ultraviolet irradiation is started (that is, the above step II is started). The time from the completion of step I to the start of step II is preferably adjusted to 0.03 to 3 seconds. The above time is more preferably 0.04 to 2.5 seconds, and still more preferably 0.06 to 2 seconds. By adjusting the time within the above range, the dot formability of the inkjet ink is improved, and a printed matter excellent in print image quality can be obtained.

[0127] In the above inkjet printing method, the above step II can be repeated a plurality of times. For example, immediately after the inkjet ink is applied onto a printing substrate, after the inkjet ink is partially cured by ultraviolet irradiation, the inkjet ink can be completely cured by ultraviolet irradiation again. By doing this, it becomes easy to obtain a printed matter that is particularly excellent in print image quality including the definition of the printed matter. In the present disclosure, the above-described step of partially curing the inkjet ink is referred to as "pre-curing", and the step of completely curing the above inkjet ink is referred to as "main curing".

[0128] That is, in one embodiment, the inkjet printing method using the inkjet ink of the present embodiment may include, in this order: a step (step I) of discharging the inkjet ink from an inkjet head onto a printing substrate; a step (step II-A) of irradiating the inkjet ink discharged onto the printing substrate with ultraviolet rays to pre-cure the inkjet ink; and a step (step II-B) of main-curing the inkjet ink.

[0129] When performing step II-A described above, that is, when pre-curing the inkjet ink ejected onto the printing substrate, it is preferable to use an LED light source that emits ultraviolet light. In this case, from the viewpoint of particularly improving the print quality of the printed material, the irradiance of ultraviolet light on the printing substrate during the pre-curing process should be 2 to 20 mW / cm². 2 Preferably, it is 5-15 mW / cm². 2 It is preferable that it be so.

[0130] Generally, when performing the above process II-A, the time from when the ejected inkjet ink lands on the printing substrate until it partially hardens is an extremely short time, about 0.03 to 0.5 seconds. However, the inkjet ink of this embodiment has particularly excellent wetting and spreading properties, so it can be suitably combined with partial hardening.

[0131] Furthermore, when performing step II-B described above, that is, when curing the inkjet ink ejected onto the printing substrate, an LED light source emitting ultraviolet light can also be used. In this case, the aforementioned 1,000 mW / cm² can be used. 2 The above illuminance and 50 mJ / cm² 2 It is preferable to perform the procedure with the above-mentioned cumulative light intensity.

[0132] Alternatively, in process II-B, ultraviolet light can be irradiated using a high-pressure mercury lamp or a metal halide lamp. In this case, the irradiance of the ultraviolet light is 80 mW / cm². 2 Preferably, it should be 120 mW / cm² or higher. 2 It is more preferable to set it to the above. Also, the integrated light intensity should be 100 mJ / cm 2 Preferably, it should be 150 mJ / cm² or higher. 2 It is more preferable to set it to 200 mJ / cm² or higher. 2 It is even more preferable to keep it as above.

[0133] <Printing base material> In the printing method using the inkjet ink of this embodiment, the printing substrate used is preferably a resin film substrate or a paper substrate. The resin film substrate may preferably have a thickness of 10 to 90 μm. Furthermore, as the resin film substrate, a printing substrate containing a material selected from the group consisting of polypropylene, polyethylene, polyethylene terephthalate, and nylon is preferably selected. On the other hand, as the paper substrate, coated paper, art paper, laminated paper, etc., are preferably selected. As described above, the inkjet ink of this embodiment can produce printed materials with excellent curability and adhesion even on printing substrates having pores such as coated paper.

[0134] In one embodiment, the inkjet ink of this embodiment is suitably used for printing on packages formed from the printing substrates listed above. Among the printing of such packages, it is particularly suitable for printing on food packaging.

[0135] Furthermore, the "printing substrate containing a material selected from the group consisting of polypropylene, polyethylene, polyethylene terephthalate, and nylon" is not limited to a single-layer structure, but may also have a multilayer structure. That is, the printing substrate may be a resin film substrate having one layer made of a material selected from the group consisting of polyethylene terephthalate, polyethylene, polypropylene, and nylon, or a resin film substrate (laminated film substrate) having two or more of the above layers. In addition, for the purpose of improving the strength of the package, blocking oxygen, etc., the layers constituting the laminated film substrate may include layers made of AL (aluminum foil) and VM (vacuum deposition) film (aluminum deposition film, transparent deposition film), etc. [Examples]

[0136] The present invention will be described in more detail below, but the following examples do not limit the scope of the present invention in any way. Unless otherwise specified, "parts" refers to parts by mass, and "%" refers to percentage by mass.

[0137] <Manufacturing of Carbon Black Dispersion> Solspas 32000 varnish A was produced by first filling a mixing container (volume 10L) equipped with a stirrer with 150g of "Solspas 32000" from Lubrizol, a basic pigment dispersion resin, and 3,250g of dipropylene glycol diacrylate ("Miramer M222" from Miwon), and thoroughly stirring and mixing the contents until they were uniform. Next, while stirring Solspace 32000 Varnish A in the mixing container, 600g of Carbon Black (Special Black 350, manufactured by Orion Engineered Carbons Co., Ltd.) was gradually added to the mixing container. After all of the Carbon Black had been added, stirring was continued for another hour (premixing). Subsequently, the mixture was circulated and dispersed using a Synmaru Enterprises "Dino-Mill" (volume 0.6 L) filled with 1,800 g of 0.5 mm diameter zirconia beads. The average particle size of the mixture was then measured at regular intervals (e.g., every hour) using the apparatus and method described later. The circulation and dispersion was terminated when the average particle size fell below 180 nm, thereby producing a carbon black dispersion (pigment concentration 15% by mass). In this disclosure, "average particle size" refers to the volume-based median diameter (D50) measured using a dynamic light scattering particle size distribution analyzer (Microtrac-Bell "Nanotrac UPA-EX150"), and, if necessary, using inkjet ink diluted with ethyl acetate to a measurable concentration.

[0138] <Manufacturing of Titanium Dioxide Dispersion> Solspas 32000 varnish B was produced by first adding 180g of "Solspas 32000" (manufactured by Lubrizol), a basic pigment dispersion resin, and 2,820g of dipropylene glycol diacrylate ("Miramer M222" (manufactured by Miwon)) to a mixing container (volume 10L) equipped with a stirrer, and thoroughly stirring and mixing until the contents were uniform. Next, while stirring the Solspas 32000 varnish B in the mixing container, 3,000g of titanium dioxide (KRONOS 2310, manufactured by KRONOS) was gradually added to the container. After all the titanium dioxide had been added, stirring was continued for another hour (premixing). Subsequently, the mixture was circulated and dispersed using a Synmaru Enterprises "Dino-Mill" (volume 0.6 L) filled with 1,800 g of 1 mm diameter zirconia beads. The average particle size of the mixture was then measured at regular intervals (e.g., every hour) using the apparatus and method described above. The circulation and dispersion was terminated when the average particle size fell below 270 nm, thereby producing a titanium dioxide dispersion (pigment concentration 50% by mass).

[0139] <Manufacturing of cyanide dispersions, magenta dispersions, and yellow dispersions> In the method for producing the carbon black dispersion described above, the same materials and procedures were used to produce cyanide dispersions, magenta dispersions, and yellow dispersions, respectively, except that the carbon black was changed to "LIONOL BLUE FG-7400" (manufactured by Toyo Color Co., Ltd., CIPigment Blue FG-15:4), "Fastogen Super Magent RG" (manufactured by DIC Corporation, CIPigment Red 122), and "BAYSCRIPT YELLOW 4FG" (manufactured by Lanxess Corporation, CIPigment Yellow 150).

[0140] <Manufacturing of inkjet inks> The materials listed in columns 1-1 to 1-6 of the table below were placed in a mixing vessel equipped with a stirrer. After adding all the materials, the mixture was heated while stirring until its temperature reached 40°C. Once 40°C was reached, stirring was continued for another hour while maintaining the temperature. The mixture was then filtered through a membrane filter with a pore size of 0.8 μm to produce inkjet ink.

[0141] In the manufacture of the above inkjet ink, the pigment dispersion, other polymerizable compounds, 5-methyl-3-vinyloxazolidine-2-one, polymerization inhibitor, photopolymerization initiator, surface modifier, and organic solvent were added in the following order while stirring the mixture in the mixing container. Furthermore, for components containing two or more materials, the order of addition within that component was determined by the amount of each material added first.

[0142] [Table 1-1]

[0143] [Table 1-2]

[0144] [Table 1-3]

[0145] [Table 1-4]

[0146] [Table 1-5]

[0147] [Table 1-6]

[0148] The details of the raw material names listed in Tables 1-1 to 1-6 are as follows. <Polymerizable compound> • VMOX: 5-methyl-3-vinyloxazolidine-2-one (manufactured by BASF) • DPGDA: Dipropylene glycol diacrylate (Miramer M222, manufactured by Miwon) • HDDA: 1,6-Hexanediol diacrylate (Viscoat #240, manufactured by Osaka Organic Chemical Industry Co., Ltd.) • BDDA: 1,4-butanediol diacrylate (Viscoat #195, manufactured by Osaka Organic Chemical Industry Co., Ltd.) • TMP(PO)3TA: Trimethylol propanepropoxylate triacrylate (Miramer M360, manufactured by Miwon) Gly-TA: Glycerin triacrylate (Arronix M-930, manufactured by Toagosei Co., Ltd.) • Di-TMPTA: Ditrimethylolpropanetetraacrylate (Ebecryl 1142, Daicel Ornex Co., Ltd.) • PETTA: Pentaerythritol tetraacrylate (SR295NS, manufactured by Sartmar) • BzA: Benzyl acrylate (Viscoat #160, manufactured by Osaka Organic Chemical Industry Co., Ltd.) • ACMO: Acryloylmorpholine (ACMO, manufactured by KJ Chemicals) • IBXA: Isobornyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd.) • PEA: 2-Phenoxyethyl acrylate (Viscoat #192, manufactured by Osaka Organic Chemical Industry Co., Ltd.) <Photopolymerization initiator> • DETX: 2,4-Diethylthioxanthone (Omnirad DETX, manufactured by IGM Resins) • ITX: 2-Isopropylthioxanthone (Speedcure ITX, manufactured by Arkema) • Omnipol TX: Diester (multimer) of carboxymethoxythiooxane (Omnipol TX, manufactured by IGM Resins) • EMK: 4,4'-bis(diethylamino)benzophenone (Speedcure EMK, manufactured by Arkema) • TPO: 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (Omipol TPO, manufactured by IGM RESINS) • Omnipol TP: (2,4,6-trimethylbenzoyl)phosphine oxide polymer (manufactured by IGM RESINS) • TPO-L: Ethoxyphenyl (2,4,6-trimethylbenzoyl)phosphine oxide (Omnirad TPO-L, manufactured by IGM RESINS) • BAPO: Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Omnirad 819, manufactured by IGM Resins) <Organic solvents> ·DEDG: Diethyleneglycol-diethyl ether <Polymerization inhibitor> • Q1301: N-nitrosophenylhydroxylaluminum salt (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) • DOM: Dioctyl maleate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) • PTZ: Phenothiazine (manufactured by Seiko Chemical Co., Ltd.) <Surface modifier> • TEDO Rad 2100: Polyether-modified silicone surface tension modifier containing (meth)acryloyl groups (manufactured by Evonik). • BYK-315N: Polyester-modified polymethylalkyl silicane-based surface tension modifier (25% solids by mass, containing methoxypropyl acetate and phenoxyethanol in a 1:1 mass ratio as diluent solvent components)

[0149] <Manufacturing of printed materials> First, a Kyocera inkjet head (KJ4A, design resolution 600dpi) with temperature control was installed above the conveyor that transports the printing substrate. Additionally, a temporary curing LED light source (Phoseon FireEdge FE300, peak wavelength 395nm, design maximum illuminance 3,000mW / cm²) was installed above the conveyor on the downstream side of the transport direction of the printing substrate. 2 Furthermore, above the conveyor downstream from the installation position of the temporary curing LED light source, the main curing LED light source (Phoseon FirePower FP300, radiant peak wavelength 395nm, designed maximum illuminance 16,000mW / cm²) is installed. 2The following were installed: The distance between the inkjet head and the temporary curing LED light source was 15 cm, and the distance between the temporary curing LED light source and the main curing LED light source was 35 cm. The inkjet inks manufactured as described above were filled into the respective inkjet heads. Next, the temperature of the inkjet heads was adjusted so that the viscosity of the inkjet ink at ejection was 6-7 mPa·s. After that, the paper substrate, as described later, was fixed onto a conveyor, and the conveyor was driven at a speed of 50 m / min. When the paper substrate passed the inkjet head mounting area, droplets of inkjet ink were ejected from the inkjet head to perform printing. Specifically, under printing conditions of ejection droplet volume of 11 pL and print resolution of 600 dpi x 600 dpi, a composite image consisting of a solid image with a size of 10 cm square and 100% print density, and a character image (an image with 20 randomly printed hiragana characters each in MS Mincho font at sizes of 4 points, 6 points, and 8 points) was printed. Then, even after printing with inkjet ink, the conveyor was driven at the same speed, and ultraviolet light was irradiated onto the paper substrate as it passed through the installation sections of the preliminary curing LED light source and the final curing LED light source, thereby manufacturing the printed material.

[0150] Furthermore, the illuminance and cumulative light amount of ultraviolet light irradiated onto the inkjet ink after printing are 900 mW / cm² using an LED light source for temporary curing. 2 , integrated light intensity 25 mJ / cm 2 This curing LED light source provides an illuminance of 6,000 mW / cm². 2 Total luminous intensity 150 mJ / cm 2 To achieve this, the output of the two types of LED light sources was adjusted in advance before performing the printing described above. Furthermore, for inkjet inks containing titanium dioxide, Fuji Kyowa Paper Co., Ltd.'s black cast-coated paper substrate "Fantas" was used, and for inkjet inks containing pigments other than titanium dioxide, UPM RAFLATAC's coated paper substrate "Raflacoat" was used as the paper substrate.

[0151] [Examples 1-48, Comparative Examples 1-13] Using the inkjet inks and printed materials manufactured by the method described above, various evaluations were conducted according to the methods shown below. The evaluation results are shown in Tables 1-1 to 1-6.

[0152] <Evaluation 1: Evaluation of curing properties> The surface of the solid image area with 100% print coverage of the printed material produced by the above method was rubbed with a cotton swab to check whether uncured ink adhered to the cotton swab. If inkjet ink adhered to the cotton swab, the printed material was fixed to the conveyor of the inkjet printing apparatus, and without printing inkjet ink, only the curing LED light source was irradiated. Then, the presence or absence of inkjet ink adhered when rubbed with a cotton swab was checked again. This procedure was repeated, and the number of passes required until uncured inkjet ink no longer adhered to the cotton swab was investigated to evaluate the curability. The evaluation criteria for the curability were as follows, with an evaluation of "3" or higher being considered practically usable, and an evaluation of "4" or higher being considered practically suitable.

[0153] ≪Evaluation Criteria for Curing Properties≫ 5. After passing the swab through the material once in total (without requiring additional UV irradiation), no uncured inkjet ink adhered to the cotton swab. 4. After passing the swab through it a total of two times (with one additional UV irradiation), no uncured inkjet ink adhered to the cotton swab. 3. After passing the swab through a total of three times (with two additional UV irradiations), no uncured inkjet ink adhered to the cotton swab. 2. After passing the swab through a total of four times (with three additional UV irradiations), no uncured inkjet ink adhered to the cotton swab. 1. It was necessary to irradiate the cotton swab with ultraviolet light a total of five or more times until no uncured inkjet ink adhered to it.

[0154] <Evaluation 2: Evaluation of adhesion> In the printed material produced using the method described above (without additional UV irradiation as performed in Evaluation 1), six vertical and six horizontal cuts were made at 2.5 mm intervals in the solid image area with 100% print coverage. Next, cellophane tape was attached over the cuts and rubbed from above with an eraser to ensure the cellophane tape adhered firmly to the solid image area. Then, the cellophane tape was peeled off while maintaining a 90° angle between the printed surface of the solid image area and the cellophane tape, and the adhesion was evaluated by calculating the ratio of the area of ​​the solid printed material peeled off with the cellophane tape to the area where the cellophane tape was adhered. The evaluation criteria for the adhesion were as follows, with an evaluation of "3" or higher considered practically usable, and an evaluation of "4" or higher considered practically suitable.

[0155] ≪Criteria for evaluating adhesion≫ 5: Area of ​​peeled solid print is less than 5% 4: Area of ​​peeled solid print is 5% or more but less than 15% 3: Area of ​​peeled solid print is 15% or more but less than 25% 2: Area of ​​peeled solid print is 25% or more but less than 50% 1: More than 50% of the solid print area has been peeled off.

[0156] <Evaluation 3: Evaluation of storage stability> The viscosity of each inkjet ink manufactured as described above was measured at 25°C. Then, each inkjet ink was filled into a 20mL airtight glass container to 90% of its capacity, and stored in a light-shielded environment at 60°C for two weeks. After storage, the viscosity at 25°C was measured again, and the viscosity increase rate was determined. Anton Pahl's "Lovis 2000 ME" was used for viscosity measurement. The evaluation criteria for storage stability were as follows, with a rating of "3" or higher considered usable, and a rating of "4" or higher considered practically suitable.

[0157] ≪Evaluation Criteria for Storage Stability≫ 5: Thickening rate less than 1% 4: Thickening rate of 1% or more but less than 3% 3: Thickening rate of 3% or more but less than 5% 2: Thickening rate of 5% or more but less than 15% 1: Thickening rate of 15% or more

[0158] <Rating 4: Gross rating> The 60° gloss value of the solid image area with 100% print coverage of the printed material produced by the above method (without additional UV irradiation as performed in Evaluation 1) was measured using a surface gloss meter (BYK's "Microtrigloss"). The gloss evaluation criteria were as follows, with a score of "3" or higher considered usable, and a score of "4" considered practically suitable.

[0159] ≪Gross Evaluation Criteria≫ 5: Glossiness level of 90 or higher 4: Gloss level between 85 and 90 3: Gloss level between 75 and 85 2: Gloss level between 65 and 75 1: Gloss level is less than 65

[0160] As shown in Tables 1-1 to 1-5 above, the inkjet inks of Examples 1 to 48 having the above-described configurations yielded printed materials with excellent curability, adhesion, and storage stability, as well as high gloss.

[0161] On the other hand, in Comparative Example 1, an inkjet ink that contained no 5-methyl-3-vinyloxazolidine-2-one, the curability and adhesion did not reach a practical level, and the gloss of the printed material was also low. Conversely, in inkjet inks with a 5-methyl-3-vinyloxazolidine-2-one content greater than 25% by mass, as in Comparative Examples 2 and 13, the storage stability, adhesion, and gloss of the printed material were inferior. Furthermore, in Comparative Examples 3-5 and 9-11, where the ratio of the hydrogen abstraction type photopolymerization initiator to the N-nitrosophenylhydroxylaluminum salt content was not within the preferred range described above, deterioration in adhesion and a decrease in the gloss of the printed material were observed. In addition, in Comparative Examples 6-8 and 12, which used polymerization inhibitors other than N-nitrosophenylhydroxylaluminum salt, the storage stability did not reach a practical level, and the curability, adhesion, and gloss of the cured film were also inferior. These results confirm that, in order to simultaneously solve all of the aforementioned problems, it is essential to keep the ratio of the 5-methyl-3-vinyloxazolidine-2-one content to the hydrogen abstraction initiator content and the N-nitrosophenylhydroxylaluminum salt content within a predetermined range.

Claims

1. A UV-curable inkjet ink containing a colorant, a polymerizable compound, a photopolymerization initiator, and a polymerization inhibitor, The polymerizable compound comprises 5-methyl-3-vinyloxazolidine-2-one, The content of 5-methyl-3-vinyloxazolidine-2-one is 1 to 25% by mass of the total amount of the inkjet ink. The photopolymerization initiator includes a hydrogen abstraction type photopolymerization initiator. The polymerization inhibitor comprises an N-nitrosophenylhydroxylaluminum salt, A UV-curable inkjet ink in which the ratio of the content of the hydrogen abstraction type photopolymerization initiator to the content of the N-nitrosophenylhydroxylaluminum salt (hydrogen abstraction type photopolymerization initiator: N-nitrosophenylhydroxylaluminum salt) is 8:1 to 80:1 by mass.

2. The ultraviolet-curable inkjet ink according to claim 1, wherein the hydrogen abstraction type photopolymerization initiator comprises a thioxanthone compound.

3. The ultraviolet-curable inkjet ink according to claim 1 or 2, wherein the ratio of the content of the hydrogen abstraction type photopolymerization initiator to the content of the 5-methyl-3-vinyloxazolidine-2-one (hydrogen abstraction type photopolymerization initiator: 5-methyl-3-vinyloxazolidine-2-one) is 0.1:1 to 2.0:1 by mass.

4. The ultraviolet-curable inkjet ink according to claim 1 or 2, wherein the photopolymerization initiator further comprises an acylphosphine oxide compound, and the ratio of the content of the acylphosphine oxide compound to the content of the hydrogen abstraction type photopolymerization initiator (acylphosphine oxide compound: hydrogen abstraction type photopolymerization initiator) is 2:1 to 20:1 by mass.

5. Furthermore, the UV-curable inkjet ink according to claim 1 or 2 further contains a polyether-modified polysiloxane.

6. A printed article obtained by printing with the ultraviolet-curable inkjet ink described in claim 1 or 2.

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