Ultraviolet curable inkjet ink and inkjet recording method

Abstract

Provided is an ultraviolet-curable inkjet ink containing a polymerizable compound and a photopolymerization initiator, the polymerizable compound containing a monofunctional (meth) acrylate, the monofunctional (meth) acrylate comprising an amino group-containing monofunctional (meth) acrylate (A), and the photopolymerization initiator comprising an ethoxyphenyl (2, 4, 6, 7, 8, 8, 9, 10, 11, 12, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, the content of the monofunctional (meth) acrylate is 0.1-15% by mass on the basis of the total mass of the ultraviolet-curable inkjet ink, and the content of the monofunctional (meth) acrylate is 0.1-15% by mass on the basis of the total mass of the ultraviolet-curable inkjet ink. An inkjet recording method includes: discharging the ultraviolet curable inkjet ink from an inkjet head onto a recording medium; and curing the ultraviolet curable inkjet ink on the recording medium by irradiating ultraviolet light from an ultraviolet light emitting diode.

Description

Technical Field

[0001] Embodiments of the present invention relate to an ultraviolet-curable inkjet ink and an inkjet recording method using the ultraviolet-curable inkjet ink. Background Art

[0002] Conventionally, in the existing printing market, offset printing methods and the like have been mainly used. However, with the improvement of the performance of inkjet heads in recent years, the inkjet printing method is expected to be developed in the existing printing market. When the inkjet printing method is adopted in the above-mentioned existing printing market, high productivity is required. Therefore, for example, a single-pass printing method capable of high-speed printing needs to be addressed.

[0003] In addition, in the above-mentioned existing printing market, various recording media such as coated paper and various films are used as substrates. Among the recording media, the surface states and the wetting and spreading properties of the ink are different. Therefore, in order to adopt the above-mentioned inkjet printing method in the existing printing market, it is important to be able to ensure a certain level or more of printing quality for the above-mentioned various recording media using the same ink.

[0004] From the viewpoint of simultaneously achieving such productivity and versatility on various printing media, an active energy ray-curable inkjet ink can be most suitably used.

[0005] In addition, recently, the requirements regarding environmental concerns from the market have increased. From this viewpoint, as a method for curing an active energy ray-curable inkjet ink, the UV-LED method is mostly adopted. The UV-LED method is a method of irradiating ultraviolet rays (UV), which is a kind of active energy rays, from a light-emitting diode (LED), and has a characteristic of a narrow wavelength range of the radiated ultraviolet rays. In contrast, in the conventionally used high-pressure mercury lamp and metal halide lamp, ultraviolet rays of various wavelengths from short-wavelength ultraviolet rays to long-wavelength ultraviolet rays are radiated. Among ultraviolet rays, short-wavelength ultraviolet rays are effective for curing the surface of the ink film. In addition, long-wavelength ultraviolet rays are effective for curing the inside of the ink film.

[0006] However, as described above, since the wavelength region of the ultraviolet rays emitted from the UV-LED is narrow, there is a possibility that the curability of the surface and / or inside of the ink film is insufficient. In addition, if a part of the photoinitiator is in an unreacted state, there is a possibility that the unreacted photoinitiator decomposes and / or cracks over time, resulting in the discoloration of the printed matter over time. Furthermore, the emission maximum wavelength of the commonly used UV-LED is 360 to 410 nm, and photoinitiators having absorption in this wavelength region are mostly colored yellow due to their absorption characteristics. Therefore, when a vivid hue is expressed in the printed matter, the use of such photoinitiators becomes a drawback.

[0007] In the case of using UV-LED, improving curability is particularly important in the above problems. On the premise of using UV-LED, various studies have been carried out to improve the curability of ultraviolet curable inkjet inks.

[0008] For example, in order to allow the polymerization reaction to proceed sufficiently, there is a method of using a polymerizable compound having a large number of functional groups. However, in order to impart sufficient ejection stability to the inkjet ink, the ink must have a low viscosity. Generally, polymerizable compounds having a large number of functional groups have a high viscosity. Therefore, the viscosity of the inkjet ink containing the polymerizable compound becomes high, and ejection stability may be problematic.

[0009] Patent Document 1 discloses an ink for UV-LED photo-curable inkjet printing, which contains predetermined amounts of 2-(2-vinyloxyethoxy)ethyl acrylate, trifunctional and / or tetrafunctional (meth)acrylate monomers, and acylphosphine oxide-based photoinitiators, respectively. According to Patent Document 1, it is described that the above ink has excellent curability, ejection stability, and substrate adhesion when using UV-LED. However, for the ink having the constitution specifically disclosed in Patent Document 1, the present inventors actually evaluated it, and as a result, it was found that the printed matter discolored over time and the viscosity stability of the ink was poor.

[0010] On the other hand, Patent Document 2 discloses a photo-curable inkjet recording ink having excellent curability, adhesion to a polyvinyl chloride sheet substrate, ejection stability, storage stability (viscosity stability over time and presence or absence of precipitates), etc. in the case of using UV-LED, in the case of normal film thickness and thin film. The photo-curable inkjet recording ink is disclosed to have the following constitution: containing 65 to 85% by mass of a monofunctional monomer having a cyclic structure in the polymerizable compound, and further containing a monofunctional monomer having an ether group, a polyfunctional monomer having an amino group, and a monomer having a trimethylolpropane skeleton, etc.

[0011] In addition, Patent Document 3 discloses an ink for photo-curable inkjet printing, which has excellent ejection stability and good curability even when a UV-LED having a small cumulative light amount is used as a light source. The ink for photo-curable inkjet printing contains 60 to 99.5% by mass of a monofunctional monomer in the polymerizable compound, and further contains an acrylamide-based monomer, vinylcaprolactam, and / or an amine-modified acrylate compound, etc.

[0012] However, since the content of the monofunctional monomer in the photo-curable inkjet printing inks disclosed in Patent Documents 2 and 3 is large, curability may be insufficient when used under high-speed printing conditions and when the amount of the ink applied to the recording medium is large.

[0013] Prior Art Documents

[0014] Patent Document

[0015] Patent Document 1: Japanese Patent Application Laid-Open No. 2014-125557

[0016] Patent Document 2: Japanese Patent Application Laid-Open No. 2016-160274

[0017] Patent Document 3: Japanese Patent Application Laid-Open No. 2017-149825 Summary of the Invention

[0018] Problems to be Solved by the Invention

[0019] The present invention has been completed to solve the above problems, and an object thereof is to provide an ultraviolet-curable inkjet ink (hereinafter also simply referred to as "ink") that can obtain a printed matter having excellent curability and hue stability (not discoloring over time) when using a UV-LED regardless of the amount applied to the recording medium, and also has excellent ejection stability and viscosity stability over time. Another object of the present invention is to provide an inkjet recording method that can obtain a printed matter having excellent quality (image quality) by using the above ultraviolet-curable inkjet ink.

[0020] Means for Solving the Problems

[0021] To solve the above problems, the present inventors conducted in-depth research and as a result, found an ink having the following constitution, and completed the present invention.

[0022] That is, the embodiments of the present invention relate to the following [1] to [6]. However, the embodiments of the present invention are not limited to the following and include various embodiments.

[0023] [1] An ultraviolet-curable inkjet ink containing a polymerizable compound and a photoinitiator, wherein the polymerizable compound contains a monofunctional (meth)acrylate, the monofunctional (meth)acrylate includes a monofunctional (meth)acrylate (A) containing an amino group, the photoinitiator contains ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, and based on the total mass of the ultraviolet-curable inkjet ink, the content of the monofunctional (meth)acrylate is 0.1 to 15% by mass.

[0024] [2] The ultraviolet-curable inkjet ink according to the above [1], wherein the amine value of the monofunctional (meth)acrylate (A) containing an amino group is 150 to 250 mgKOH / g.

[0025] [3] The ultraviolet-curable inkjet ink as described in [1] or [2] above, wherein the polymerizable compound further contains at least one polymerizable compound selected from the group consisting of the polymerizable compound represented by the following general formula (B) and the polymerizable compound represented by the following general formula (C).

[0026] General formula (B): CH2=CH-CO-(O-CH2CH2) n -O-R 1

[0027] [In general formula (B), R 1 represents an acryloyl group or a vinyl group, and n represents an integer of 2 to 10.]

[0028] General formula (C): CH2=CH-CO-O-R 2 -O-CO-CH=CH2

[0029] [In general formula (C), R 2 represents an alkylene group having 2 to 10 carbon atoms which may have a branched structure.]

[0030] [4] The ultraviolet-curable inkjet ink as described in [3] above, wherein the content ratio of the amino group-containing monofunctional (meth)acrylate (A) to the total content of the polymerizable compound represented by the general formula (B) and the polymerizable compound represented by the general formula (C) is 0.02 to 0.2.

[0031] [5] The ultraviolet-curable inkjet ink as described in any one of [1] to [4] above, wherein the content ratio of the amino group-containing monofunctional (meth)acrylate (A) to the content of the ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide is 0.02 to 1.

[0032] [6] An inkjet recording method, which includes: ejecting the ultraviolet-curable inkjet ink as described in any one of [1] to [5] above from an inkjet head onto a recording medium; and curing the ultraviolet-curable inkjet ink ejected onto the recording medium by irradiating ultraviolet rays from an ultraviolet light-emitting diode.

[0033] Advantages of the Invention

[0034] According to the present invention, it is possible to provide an ultraviolet-curable inkjet ink which is not affected by the amount applied to the recording medium, and a printed matter having excellent curability and hue stability when using a UV-LED can be obtained, and the ejection stability and the viscosity stability over time are also excellent. In addition, it is possible to provide an inkjet recording method which can obtain a printed matter having excellent quality (image quality) by using the above ultraviolet-curable inkjet ink. Detailed Embodiments

[0035] Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the embodiments described below.

[0036] It should be noted that in this specification, unless otherwise specified, "parts" and "%" represent "parts by mass" and "mass %". In addition, in this specification, notations such as "(meth)acrylate", "(meth)acryloyl", and "(meth)acrylic acid" respectively refer to "acrylate and / or methacrylate", "acryloyl and / or methacryloyl", and "acrylic acid and / or methacrylic acid".

[0037] <1>UV-curable inkjet ink

[0038] One embodiment of the present invention relates to a UV-curable inkjet ink containing a polymerizable compound and an acylphosphine oxide-based photoinitiator. Generally, acylphosphine oxide-based photoinitiators can absorb ultraviolet light with a wide wavelength range, which is effective not only in improving the curability of the ink film surface but also in improving the curability inside the ink film. In addition, it is known that an ink containing this acylphosphine oxide-based photoinitiator has excellent curability compared to an ink containing other types of photoinitiators.

[0039] On the other hand, ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (hereinafter, also referred to as "EO-TPO" in this specification), which belongs to acylphosphine oxide-based photoinitiators, has a tendency of poor improvement effect on curability compared to other acylphosphine oxide-based photoinitiators. For example, as acylphosphine oxide-based photoinitiators other than EO-TPO, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (hereinafter, also referred to as "BTPO" in this specification) and 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (hereinafter, also referred to as "TPO" in this specification) can be cited, and the effect of EO-TPO is lower than theirs. It is considered that this is because the aromatic ring structure in the photoinitiator molecule participates in the absorption of ultraviolet light, so EO-TPO, which has a smaller number of aromatic ring structures compared to BTPO and TPO, has poor ultraviolet absorption ability.

[0040] In particular, as described above, when an LED is used as a light source for ultraviolet light, the wavelength range of the ultraviolet light emitted from the LED is narrow. Therefore, it is difficult to exhibit the advantages of acylphosphine oxide-based photoinitiators that can absorb a wide range of ultraviolet wavelengths. Therefore, the curability of an ink using EO-TPO is likely to be a problem when using a UV-LED.

[0041] When EO-TPO is used in an inkjet ink, it has been necessary to compensate for the curability by some method in the past. For example, a method of increasing the amount of a photoinitiator in the ink may be considered. However, an increase in the amount of the photoinitiator may cause deterioration of ejection stability and viscosity stability over time.

[0042] Therefore, the present inventors conducted intensive studies and found that when EO-TPO is used in combination with a mono-functional (meth)acrylate (A) containing an amino group and further the content of the mono-functional (meth)acrylate is restricted, even if EO-TPO is not used excessively, the curability when using a UV-LED is significantly improved, and thus the present invention was completed.

[0043] Therefore, one embodiment of the present invention relates to an ultraviolet curable inkjet ink (hereinafter, also referred to as the ink of the present embodiment), which contains a polymerizable compound and a photoinitiator. The polymerizable compound contains a mono-functional (meth)acrylate, the mono-functional (meth)acrylate includes a mono-functional (meth)acrylate (A) containing an amino group, the photoinitiator contains ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, and based on the total mass of the ultraviolet curable inkjet ink, the content of the mono-functional (meth)acrylate is 0.1 to 15% by mass.

[0044] The detailed mechanism is not yet clear, but the present inventors et al. presume that the mono-functional (meth)acrylate (A) containing an amino group functions as a photosensitizer. That is, first, it is considered that the mono-functional (meth)acrylate (A) containing an amino group absorbs ultraviolet rays from the UV-LED and becomes an excited state, and then this energy moves to EO-TPO, whereby EO-TPO becomes an excited state and starts a polymerization reaction.

[0045] It is considered that when the above energy moves, in the ink, the (meth)acrylate containing an amino group exists close to EO-TPO. Therefore, when the (meth)acrylate containing an amino group is a poly-functional compound, EO-TPO and the (meth)acrylate containing an amino group will preferentially react, and it may not be possible to improve the curability of the whole ink. However, in the ink of the present embodiment, among the (meth)acrylates containing an amino group, a mono-functional compound (that is, a mono-functional (meth)acrylate containing an amino group) is used. It is considered that the mono-functional (meth)acrylate containing an amino group has a lower reactivity compared to other polymerizable compounds (especially poly-functional compounds), and as a result, other polymerizable compounds present in the ink can also participate in the above polymerization reaction, and the curability of the whole ink is improved.

[0046] In addition, as described above, EO-TPO is an acylphosphine oxide-based photoinitiator, which is originally a material effective for improving the curability inside the ink film. On the other hand, the monofunctional (meth)acrylate (A) containing an amino group has an amino group, and thus has an effect of suppressing the polymerization reaction hindered by oxygen on the surface of the printed matter. As a result, the polymerization reaction proceeds evenly inside and on the surface of the ink, and there is no difference in the curing rate between the inside and the surface of the ink, and polymerization and curing proceed rapidly. Therefore, it is considered that excellent curability can be achieved even when using a UV-LED through the above combination.

[0047] On the other hand, if the content of the monofunctional (meth)acrylate present in the ink is too high, the curability when using a UV-LED as a light source may be insufficient even when the reactivity of EO-TPO is increased. Therefore, in the ink of the present embodiment, based on the total mass of the ink, it is preferable to limit the content of the monofunctional (meth)acrylate to 0.1 to 15% by mass. When the content of the monofunctional (meth)acrylate is adjusted to the above range, the above-mentioned effect of improving curability can be easily obtained.

[0048] Here, the above-mentioned "monofunctional (meth)acrylate" naturally also includes the above-mentioned monofunctional (meth)acrylate (A) containing an amino group.

[0049] Generally speaking, as sensitizers and initiators having an amino group, there are also non-polymerizable compounds (for example, aminobenzoate compounds, alkylaminophenone compounds, etc.). In the case of an ink containing these non-polymerizable compounds, the curability is also excellent, similar to the case of using an amino group-containing (meth)acrylate. However, generally speaking, the yellow tone of the above-mentioned non-polymerizable compounds is strong. In addition, the tone of a printed matter produced using an ink containing these non-polymerizable compounds may change over time. The reason is considered to be that a part of the above-mentioned non-polymerizable compounds remaining in the printed matter in an unreacted state decomposes over time.

[0050] In contrast, the amino group-containing (meth)acrylate used in the ink of the present embodiment can participate in the polymerization reaction. Therefore, it does not remain in the printed matter in an unreacted state like a non-polymerizable compound, and it is also not easily decomposed over time. As a result, a printed matter using the ink of the present embodiment is also likely to have excellent hue stability.

[0051] Even in the absence of an excess amount of EO-TPO, the ink of the present embodiment can achieve sufficient curability and hue stability by using a mono-functional (meth)acrylate (A) containing an amino group and restricting the content of the mono-functional (meth)acrylate, and can achieve excellent ejection stability and viscosity stability over time. Although the details are not clear, it is also considered that: through the interaction between EO-TPO and the amino group present in the mono-functional (meth)acrylate (A) containing an amino group, the micro viscoelasticity of the ink is optimized, and the formation of an intermolecular chain network occurs inside the cured ink, further improving the ejection stability.

[0052] By configuring the ink as described above, a printed matter excellent in curability and hue stability when using a UV-LED, which is not affected by the amount applied to the recording medium, can be obtained, and an ink excellent in ejection stability and viscosity stability over time can be achieved.

[0053] Next, each component constituting the ink of the present embodiment will be described in detail below.

[0054] <Polymerizable compound>

[0055] In the present specification, a polymerizable compound refers to a compound that undergoes a polymerization reaction or a crosslinking reaction by an initiating species such as a free radical generated by a photopolymerization initiator described later, and has a function of curing a composition containing the polymerizable compound.

[0056] The polymerizable compound may be any compound having the above characteristics. There is no limitation on the compound that can be used as the polymerizable compound, and monomers, oligomers, polymers, etc. can be used. In the ink of the present embodiment, a radically polymerizable polymerizable compound (radically polymerizable compound) is particularly preferably used. The above-mentioned "oligomer" and "polymer" are both polymers formed by bonding multiple monomers, and are classified according to the degree of polymerization. In the present specification, the above-mentioned degree of polymerization of 2 to 5 is referred to as an "oligomer", and that of 6 or more is referred to as a "polymer".

[0057] Examples of the polymerizable group possessed by the above-mentioned radically polymerizable compound include (meth)acryloyl group, vinyl ether group, allyl group, vinyl group (excluding vinyl ether group and allyl group), unsaturated carboxylic acid group, etc. Among the above-mentioned polymerizable groups, from the viewpoint of excellent curability, (meth)acryloyl group and vinyl ether group are preferred. From such a viewpoint, in one embodiment, the radically polymerizable compound contained in the ink of the present embodiment preferably contains (meth)acryloyl group and / or vinyl ether group as the polymerizable group, and more preferably contains only (meth)acryloyl group and / or vinyl ether group.

[0058] In the ink of the present embodiment, as long as the above requirements are satisfied, a monofunctional compound or a polyfunctional compound having two or more functional groups can be used as the radical polymerizable compound. Further, only one kind of polymerizable compound can be used, or a plurality of polymerizable compounds can be used in combination. When a plurality of polymerizable compounds are used in combination, the ratio of the monofunctional compound to the polyfunctional compound can be arbitrarily adjusted within the range of satisfying the above-mentioned content requirement of the monofunctional (meth)acrylate.

[0059] When using a plurality of polymerizable compounds, the ratio of the monofunctional compound to the polyfunctional compound can be determined, for example, in consideration of the following viewpoints. That is, when the blending ratio of the monofunctional compound is large, the viscosity of the ink is lowered and the ejection stability is improved. However, the printed matter tends to become soft, and the curability of the ink is also poor. On the other hand, when the ratio of the polyfunctional compound is large, the curability of the ink is good. However, there is a tendency that the viscosity becomes high and the ejection stability is poor.

[0060] In this specification, "monofunctional" means a compound having only one polymerizable group in one molecule. In addition, "bifunctional" and "trifunctional" mean a compound having two polymerizable groups in one molecule and a compound having three polymerizable groups in one molecule, respectively. Moreover, those having two or more functional groups are collectively referred to as "polyfunctional". Hereinafter, the monofunctional compound is also referred to as a monofunctional monomer, and the polyfunctional compound is also referred to as a polyfunctional monomer.

[0061] (Amino-containing monofunctional (meth)acrylate (A))

[0062] As described above, the ink of the present embodiment contains an amino-containing monofunctional (meth)acrylate (A) as a polymerizable compound.

[0063] Specific examples of the amino-containing monofunctional (meth)acrylate (A) include: monofunctional monomers such as 2-(diethylamino)ethyl (meth)acrylate, 2-(diisopropylamino)ethyl (meth)acrylate, tert-butylaminoethyl (meth)acrylate, morpholinoethyl (meth)acrylate, 2-(diethylamino)ethyl (meth)acrylate; and commercially available products of monofunctional oligomers such as "CN374", "CN383", "CN384", "CN386", "CN3715" manufactured by Arkema, "GENOMER 5142", "GENOMER 5161" manufactured by Rahn AG, "Agisyn 002", "Agisyn 007" manufactured by DSM, "Photomer 4967", "Photomer5006" manufactured by IGM RESINS, "EBECRYL 81" manufactured by Allnex.

[0064] In addition, as the amino group-containing monofunctional (meth)acrylate (A), a compound obtained by leaving only one polymerizable group present in a polyfunctional monomer (polyfunctional (meth)acrylate) described later and adding (Michael addition) a primary organic amine or a secondary organic amine to the remaining polymerizable groups can be used. When using such a compound, as the above polyfunctional monomer, a trifunctional monomer or a tetrafunctional monomer is preferably used, and a trifunctional monomer is particularly preferably used. When a trifunctional monomer or a tetrafunctional monomer is used for preparing the above compound, it is easy to suppress the viscosity of the ink to a low level, and the ejection stability can be easily improved. In addition, there is a tendency that the curability of the ink is also excellent.

[0065] The amino group-containing monofunctional (meth)acrylate (A) preferably has an epoxyethyl group in addition to the amino group. Since EO-TPO has an ethoxy group, the affinity with the amino group-containing monofunctional (meth)acrylate (A) having an epoxyethyl group is improved. Therefore, it is considered that EO-TPO and the amino group-containing monofunctional (meth)acrylate (A) are likely to exist in close proximity to each other. Furthermore, it is considered that the above-mentioned energy transfer is likely to occur due to the proximity of the two, and as a result, the curing property of the ink when using a UV-LED is extremely improved.

[0066] From the above viewpoints, in the ink of the present embodiment, for example, the amino group-containing monofunctional (meth)acrylate (A) represented by the following general formula (A-1) can be suitably used.

[0067] [Chemical formula 1]

[0068] General formula (A-1):

[0069]

[0070] In the above general formula (A-1), x1 and x2 represent integers from 1 to 10, and x3 represents an integer from 1 to 5. R 3 is a hydrogen atom or a methyl group, R 4 is a hydrocarbon group having 1 to 12 carbon atoms which may have a branched structure, R 5 and R 6 are each independently a hydrogen atom or an alkyl group having 1 to 6 carbon atoms which may have a branched structure.

[0071] From the viewpoint of ejection stability, in the above general formula (A-1), x1 and x2 are each independently preferably an integer from 1 to 6, and each independently particularly preferably an integer from 1 to 4. In addition, from the above viewpoints, x3 is preferably 1 or 2. From the viewpoint of being able to suppress the viscosity of the ink and having excellent ejection stability, R 4 is preferably a hydrocarbon group having 1 to 6 carbon atoms which may have a branched structure. From the viewpoint of curability, the above R 5 and R 6Each is independently preferably a hydrogen atom or an alkyl group having 1 to 2 carbon atoms.

[0072] In the ink of the present embodiment, the amine value of the amino group-containing monofunctional (meth)acrylate (A) can be 150 to 250 mgKOH / g. The amino group-containing monofunctional (meth)acrylate (A) having the above amine value can appropriately absorb ultraviolet rays from the UV-LED. Therefore, the curability of the ink containing the amino group-containing monofunctional (meth)acrylate (A) becomes good. In addition, since an appropriate amount of amino groups are present in the ink, the hue stability and ejection stability of the printed matter also become excellent. It should be noted that from the viewpoint of more appropriately exhibiting the above effects, the above amine value is particularly preferably 180 to 240 mgKOH / g.

[0073] The above "amine value" is the number of milligrams of potassium hydroxide equivalent to the acid required to neutralize 1 g of the sample. An example of the method for measuring the amine value is as follows.

[0074] First, a certain amount of the amino group-containing monofunctional (meth)acrylate (A) is added to a mixed solution of ethanol or tetrahydrofuran and acetic acid, and they are thoroughly mixed. Then, for the mixed solution, titration is performed using a perchloric acid-acetic acid solution according to the potentiometric titration method. Then, the titration amount read from the titration curve obtained by titration is converted into the number of milligrams of potassium hydroxide.

[0075] As another method, when the structural formula of the amino group-containing monofunctional (meth)acrylate (A) as the object is known, the amine value can also be calculated.

[0076] Based on the total mass of the ink, the content of the amino group-containing monofunctional (meth)acrylate (A) (in the ink) is preferably 0.1 to 15% by mass, more preferably 1 to 10% by mass, and further preferably 2 to 8% by mass. When the above content is adjusted within the above range, the curability of the above ink can be greatly improved while maintaining the viscosity range that can be ejected as an inkjet ink.

[0077] In addition, by making the ratio of the blending amount of the amino group-containing monofunctional (meth)acrylate (A) to the blending amount of EO-TPO within an appropriate range, energy transfer occurs appropriately with respect to EO-TPO, and the curability of the ink containing the amino group-containing monofunctional (meth)acrylate (A) is particularly improved. From this viewpoint, in one embodiment, it is appropriate to set the ratio ((ii) / (i)) of the content (ii) of the amino group-containing monofunctional (meth)acrylate (A) to the content (i) of EO-TPO to 0.02 to 1. The above content ratio is more appropriately set to 0.05 to 0.8, and particularly appropriately set to 0.1 to 0.5.

[0078] (The polymerizable compound represented by the general formula (B) and the polymerizable compound represented by the general formula (C))

[0079] In one embodiment, it is suitable that the ink contains at least one polymerizable compound selected from the group consisting of the polymerizable compound represented by the following general formula (B) and the polymerizable compound represented by the following general formula (C).

[0080] General formula (B): CH2=CH-CO-(O-CH2CH2) n -O-R 1

[0081] In the above general formula (B), R 1 represents an acryloyl group or a vinyl group, and n represents an integer of 2 to 10.

[0082] General formula (C): CH2=CH-CO-O-R 2 -O-CO-CH=CH2

[0083] In the above general formula (C), R 2 represents an alkylene group having 2 to 10 carbon atoms which may have a branched structure.

[0084] The detailed situation is not clear, but these polymerizable compounds can effectively receive free radicals from EO-TPO, and the curability of the ink is greatly improved. On the other hand, since the viscosity of the above polymerizable compound is relatively small, the ejection stability of the ink also becomes excellent.

[0085] Examples of the compound represented by the general formula (B) include diethylene glycol diacrylate, triethylene glycol diacrylate, polyethylene glycol 200 diacrylate, polyethylene glycol 300 diacrylate, polyethylene glycol 400 diacrylate, 2-(2-vinyloxyethoxy)ethyl acrylate, 2-[2-(2-vinyloxyethoxy)ethoxy]ethyl acrylate, and the like.

[0086] Among them, in one embodiment, the ink preferably contains at least one of 2-(2-vinyloxyethoxy)ethyl acrylate and polyethylene glycol 200 diacrylate, and more preferably contains 2-(2-vinyloxyethoxy)ethyl acrylate.

[0087] Examples of the compound represented by the general formula (C) include alkylene glycol diacrylates having no branched structure such as 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, 1,9-nonanediol diacrylate, and 1,10-decanediol diacrylate; and alkylene glycol diacrylates having a branched structure such as neopentyl glycol diacrylate, 2,4-dimethyl-1,5-pentanediol diacrylate, 3-methyl-1,5-pentanediol diacrylate, 2-ethyl-2-butylbutanediol diacrylate, and 2-ethyl-2-butylpropanediol diacrylate.

[0088] Among them, in one embodiment, the ink preferably contains at least one of 1,6-hexanediol diacrylate and 3-methyl-1,5-pentanediol diacrylate.

[0089] In one embodiment, when the ink contains at least one polymerizable compound selected from the group consisting of the polymerizable compounds represented by the above general formula (B) and the polymerizable compounds represented by the above general formula (C), its content is preferably 20 to 70% by mass, more preferably 25 to 65% by mass based on the total mass of the above ink.

[0090] Further, from the viewpoint of obtaining an ink excellent in both curability and viscosity stability over time, the ratio (a / bc) of the content (a) of the amino group-containing monofunctional (meth)acrylate (A) to the total content (bc) of the polymerizable compounds represented by the above general formula (B) and the polymerizable compounds represented by the above general formula (C) is preferably 0.02 to 0.2, particularly preferably 0.05 to 0.16.

[0091] (Other Polymerizable Compounds)

[0092] The ink of the present embodiment may further contain a polymerizable compound other than the above compounds (hereinafter referred to as "other polymerizable compounds").

[0093] As a monofunctional monomer that can be used as other polymerizable compounds, for example, compounds having one (meth)acryloyl group can be mentioned. Therefore, in one embodiment, the above-mentioned amino group-containing monofunctional (meth)acrylate (A) and other monofunctional (meth)acrylates can be used in combination. Specific examples of the compounds (monofunctional (meth)acrylates) that can be used in combination with the above-mentioned amino group-containing monofunctional (meth)acrylate (A) include benzyl (meth)acrylate, 2-phenoxyethyl (meth)acrylate, (ethoxy(or propoxy)ylated)2-phenoxyethyl (meth)acrylate, dicyclopentenyl(oxyethyl) (meth)acrylate, phenoxydiethylene glycol (meth)acrylate, 2-methoxyethyl (meth)acrylate, methoxytriethylene glycol (meth)acrylate, 2-ethoxyethyl (meth)acrylate, 2-(2-ethoxyethoxy)ethyl (meth)acrylate, methoxydipropylene glycol (meth)acrylate, dipropylene glycol (meth)acrylate, nonylphenol EO-modified acrylate, nonylphenol PO-modified acrylate, o-phenylphenol EO-modified acrylate, 2-ethylhexyl EO-modified acrylate, β-carboxyethyl (meth)acrylate, trimethylolpropane formal (meth)acrylate, isopentyl (meth)acrylate, cyclohexyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, norbornyl (meth)acrylate, dicyclopentyl (meth)acrylate, isononyl (meth)acrylate, stearyl (meth)acrylate, n-octyl acrylate, isooctyl (meth)acrylate, lauryl (meth)acrylate, isodecyl (meth)acrylate, tridecyl (meth)acrylate, caprolactone (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 1,4-cyclohexanedimethanol (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, acryloylmorpholine, N-acryloyloxyethylhexahydrophthalimide, and the like. In the present specification, "EO" means "ethylene oxide" and "PO" means "propylene oxide".

[0094] In addition, as another example of a monofunctional monomer that can be used as other polymerizable compounds, compounds having one vinyl group can be mentioned. Specific examples include N-vinylcaprolactam, N-vinylpyrrolidone, 5-methyl-3-vinyloxazolidin-2-one, and the like.

[0095] Among these exemplified monofunctional monomers, from the viewpoint of easily improving curability, it is preferable to use nonylphenol EO-modified acrylate, lauryl acrylate, trimethylolpropane formal acrylate, tetrahydrofurfuryl acrylate, N-vinylcaprolactam, 5-methyl-3-vinyloxazolidin-2-one, and the like.

[0096] As a difunctional monomer that can be used as other polymeric compounds, for example, compounds having two (meth)acryloyl groups can be cited. Specific examples of such compounds include 1,4-butanediol dimethacrylate, 1,6-hexanediol dimethacrylate, ethoxylated (or propoxylated) 1,6-hexanediol di(meth)acrylate, 1,10-decanediol dimethacrylate, neopentyl glycol dimethacrylate, 2,4-dimethyl-1,5-pentanediol dimethacrylate, 3-methyl-1,5-pentanediol dimethacrylate, 2-ethyl-2-butylbutanediol dimethacrylate, 2-ethyl-2-butylpropanediol dimethacrylate, 2-ethyl-2-butylpropanediol dimethacrylate, ethoxylated cyclohexanemethanol di(meth)acrylate, ethylene glycol di(meth)acrylate, neopentyl glycol hydroxypivalate di(meth)acrylate, bisphenol A di(meth)acrylate, EO-modified bisphenol A di(meth)acrylate, PO-modified bisphenol A di(meth)acrylate, EO-modified or PO-modified bisphenol A di(meth)acrylate, bisphenol F di(meth)acrylate, EO-modified bisphenol F di(meth)acrylate, PO-modified bisphenol F di(meth)acrylate, dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, oligopropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, EO-modified isocyanuric acid diacrylate, tricyclodecane di(meth)acrylate, EO-modified neopentyl glycol di(meth)acrylate, PO-modified neopentyl glycol di(meth)acrylate, dimethylol tricyclodecane di(meth)acrylate, cyclohexanedimethanol di(meth)acrylate, trimethylolpropane di(meth)acrylate, neopentyl glycol-modified trimethylolpropane di(meth)acrylate, dicyclopentyl di(meth)acrylate, etc.

[0097] As a trifunctional monomer that can be used as other polymerizable compounds, for example, compounds having 3 (meth)acryloyl groups can be cited. Specific examples of such compounds include trimethylolpropane tri(meth)acrylate, trimethylolethane tri(meth)acrylate, alkylene oxide-modified tri(meth)acrylates of trimethylolpropane (e.g., trimethylolpropane EO-modified triacrylate, trimethylolpropane PO-modified triacrylate, etc.), tetramethylolmethane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, trimethylolpropane tris((meth)acryloyloxypropyl) ether, alkylene oxide-modified tri(meth)acrylates of isocyanuric acid, dipentaerythritol propionate tri(meth)acrylate, tris((meth)acryloyloxyethyl) isocyanurate, hydroxypivalaldehyde-modified dimethylolpropane tri(meth)acrylate, sorbitol tri(meth)acrylate, propoxylated trimethylolpropane tri(meth)acrylate, ethoxylated glycerol tri(meth)acrylate, propoxylated glycerol tri(meth)acrylate, etc. Among the exemplified compounds, from the viewpoint of the balance between curability and ejection stability, trimethylolpropane triacrylate and trimethylolpropane EO-modified triacrylate can be preferably used.

[0098] As a tetrafunctional monomer that can be used as other polymerizable compounds, for example, compounds having 4 (meth)acryloyl groups can be cited. Specific examples of such compounds include pentaerythritol tetra(meth)acrylate, sorbitol tetra(meth)acrylate, bis(trimethylolpropane) tetra(meth)acrylate, dipentaerythritol propionate tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, tetramethylolmethane tetra(meth)acrylate, etc. Among the exemplified compounds, pentaerythritol tetraacrylate is preferred.

[0099] As a pentafunctional monomer that can be used as other polymerizable compounds, for example, compounds having 5 (meth)acryloyl groups can be cited. Specific examples of such compounds include sorbitol penta(meth)acrylate, dipentaerythritol penta(meth)acrylate, etc.

[0100] As a hexafunctional monomer that can be used as other polymerizable compounds, for example, compounds having 6 (meth)acryloyl groups can be cited. Specific examples of such compounds include dipentaerythritol hexa(meth)acrylate, sorbitol hexa(meth)acrylate, alkylene oxide-modified hexa(meth)acrylates of phosphazene, ε-caprolactone-modified dipentaerythritol hexa(meth)acrylate, etc. Among the exemplified compounds, dipentaerythritol hexaacrylate can be preferably used.

[0101] In one embodiment, the polymerizable compound in the ink may include (A) a mono-functional (meth)acrylate containing an amino group, the polymerizable compound (B) represented by the above general formula (B) and / or the polymerizable compound (C) represented by the above general formula (C), and other polymerizable compounds other than the above (A), (B), and (C). In the above embodiment, the polymerizable compound (B) preferably includes at least one of 2-(2-vinyloxyethoxy)ethyl acrylate and polyethylene glycol 200 diacrylate. The polymerizable compound (C) preferably includes at least one of 1,6-hexanediol diacrylate and 3-methyl-1,5-pentanediol diacrylate.

[0102] In one embodiment, the polymerizable compound in the ink is composed of a mono-functional compound containing (A) a mono-functional (meth)acrylate containing an amino group and a multi-functional compound containing the above (B) and / or (C), and the other polymerizable compound may be either a mono-functional compound or a multi-functional compound.

[0103] In one embodiment, the other polymerizable compound may include at least one selected from the group consisting of lauryl (meth)acrylate, isobornyl (meth)acrylate, dipropylene glycol diacrylate, and trimethylolpropane triacrylate. In other embodiments, the other polymerizable compound preferably includes a difunctional monomer containing dipropylene glycol diacrylate.

[0104] Furthermore, in other embodiments, as the other polymerizable compound, a radically polymerizable oligomer may also be used. In this case, a compound having a (meth)acryloyl group as a polymerizable group is preferably used. From the viewpoint of the balance of curability, hue stability, and viscosity stability over time, the number of polymerizable groups contained in the oligomer is preferably 1 to 15 per molecule. The number of the polymerizable groups is more preferably 1 to 6, further preferably 1 to 4, and particularly preferably 1 to 2. In addition, the weight average molecular weight of the oligomer is preferably 400 to 10,000, and more preferably 500 to 5,000.

[0105] Examples of the oligomer having a (meth)acryloyl group include urethane (meth)acrylate oligomers such as aliphatic urethane (meth)acrylate oligomers and aromatic urethane (meth)acrylate oligomers, acrylic (meth)acrylate oligomers, polyester (meth)acrylate oligomers, polyether (meth)acrylate oligomers, and epoxy (meth)acrylate oligomers.

[0106] In one embodiment, the total amount of the polymerizable compounds contained in the above ink (including the content of the amino group-containing monofunctional (meth)acrylate (A), the polymerizable compounds (B) and (C), etc.) is preferably 50 to 95% by mass based on the total mass of the ink. The total amount of the polymerizable compounds is more preferably 60 to 95% by mass, and further preferably 70 to 95% by mass. When the total amount of the polymerizable compounds is adjusted within the above range, excellent properties in terms of curability, ejection stability, and viscosity stability over time are easily obtained.

[0107] Based on the mass of all the polymerizable compounds contained in the ink, the total content of the difunctional monomers is preferably 20 to 99% by mass, more preferably 35 to 97% by mass, and particularly preferably 50 to 95% by mass. Here, the polymerizable compounds represented by the above general formulas (B) and (C) are also included in the difunctional monomers. When the total content of the difunctional monomers is adjusted within the above range, it becomes easy to improve the ejection stability due to the low viscosity of the ink, optimize the curability and viscosity stability over time of the ink, and reduce the residual amount of unreacted components after curing.

[0108] In addition, when the above trifunctional or higher monomers are included, based on the total mass of the ink, it is preferably 10% by mass or less, more preferably 7% by mass or less, further preferably 5% by mass or less, and particularly preferably 2% by mass or less. If within the above range, it is easy to balance curability and ejection stability.

[0109] On the other hand, in the ink of the present embodiment, as described above, it is necessary to adjust the content of monofunctional monomers such as monofunctional (meth)acrylate present in the ink. Specifically, based on the total mass of the ink, the content of the monofunctional monomers is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and further preferably 1% by mass or more. The above content is preferably 15% by mass or less, more preferably 12% by mass or less, and further preferably 8% by mass or less. In one embodiment, the above content can be 5% by mass or less. In one embodiment, the monofunctional monomers present in the ink are preferably only monofunctional (meth)acrylate.

[0110] In one embodiment, from the viewpoint of easily improving curability, it is preferred to use only the amino group-containing monofunctional (meth)acrylate as the monofunctional (meth)acrylate. From such a viewpoint, based on the total mass of the ink, the content of the monofunctional (meth)acrylate is preferably 0.1 to 15% by mass, more preferably 0.5 to 12% by mass, and further preferably 1 to 8% by mass.

[0111] <Photoinitiator>

[0112] The ink of this embodiment contains ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (also known as ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate) having the structure shown by the following formula as a photopolymerization initiator.

[0113] [Chemical formula 2]

[0114]

[0115] As described above, by using ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide ("also referred to as EO-TPO in this specification") in combination with an amino group-containing monofunctional (meth)acrylate (A), the curability when using a UV-LED is improved.

[0116] It should be noted that the "ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide" described in this specification also includes a polymer of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (hereinafter, also referred to as "EO-TPO polymer") in this specification.

[0117] Commercially available products can be used for EO-TPO. Specifically, "Omnirad TPO-L", "OMNIPOL TP" manufactured by IGM RESINS, "Speedcure TPO-L" manufactured by Lambson, "GR-TPO-L" manufactured by Hubei Gurun Technology, etc. can be cited. It should be noted that OMNIPOL TP is an EO-TPO polymer.

[0118] EO-TPO can be used alone in the ink. In addition, in order to further improve the curability and film flexibility, EO-TPO and a photopolymerization initiator other than this EO-TPO (hereinafter, also referred to as "other photopolymerization initiator") can also be used in combination.

[0119] As a specific example of other photopolymerization initiators, for example, acylphosphine oxide-based photopolymerization initiators other than EO-TPO (hereinafter, also referred to as "other acylphosphine oxide-based photopolymerization initiators"), benzophenone-based photopolymerization initiators, indane-based photopolymerization initiators, thioxanthone-based photopolymerization initiators, hydroxyacetophenone-based photopolymerization initiators, alkylaminobenzophenone-based photopolymerization initiators, oxime ester-based photopolymerization initiators, etc. can be cited.

[0120] In this specification, the "polymerization initiator" also includes materials that are generally called sensitizers and promote the generation of free radicals of other photopolymerization initiators. As such materials, for example, aminobenzoate-based compounds and anthracene-based compounds can be cited.

[0121] The above-mentioned other acylphosphine oxide-based photoinitiators can be commercially available products. For example, "Omnirad TPO" (a commercially available product of the above-mentioned TPO), "Omnirad 819" (a commercially available product of the above-mentioned BTPO), etc. manufactured by IGM RESINS can be cited. In addition, acylphosphine oxide-based photoinitiators described in, for example, International Publication No. 2017 / 086224 and International Publication No. 2020 / 049378, and lithium phenyl(2,4,6-trimethylbenzoyl)phosphinate can also be used.

[0122] Benzophenone-based photoinitiators can be commercially available products. For example, "Omnirad BP", "Omnirad BMS", "Omnirad 4PBZ", "OMNIRAD EMK", "Esacure 1001M", etc. manufactured by IGM RESINS can be cited.

[0123] Indan-based photoinitiators can be commercially available products. For example, "SpeedCureXFs01" manufactured by LAMBSON can be cited.

[0124] The above-mentioned thioxanthone-based photoinitiators can be commercially available products. For example, "Omnirad ITX", "Omnirad DETX", etc. manufactured by IGM RESINS can be cited.

[0125] The above-mentioned hydroxyacetophenone-based photoinitiators can be commercially available products. For example, "Omnirad 127", "Omnirad 184", "Omnirad 1173", "Omnirad2959", "Esacure KIP150", etc. manufactured by IGM RESINS can be cited.

[0126] The above-mentioned alkylaminobenzophenone-based photoinitiators can be commercially available products. For example, "Omnirad 907", "Omnirad 369", "Omnirad 379", etc. manufactured by IGM RESINS can be cited.

[0127] The above-mentioned oxime ester-based photoinitiators can be commercially available products. For example, "IRGACUREOXE01", "IRGACURE OXE02", "IRGACURE OXE04", etc. manufactured by BASF can be cited.

[0128] The above-mentioned benzoate compounds can be commercially available products. For example, "Esacure A198", "Omnipol ASA", "Omnirad EDB", "Omnirad EHA" manufactured by IGM RESINS, "GENOPOL AB-1", "GENOPOL AB-2" manufactured by Rahn AG, etc. can be cited.

[0129] In addition, the above-mentioned anthracene compounds can be commercially available products. For example, "ANTHRACURE UVS-581" manufactured by Kawasaki Chemical Industry Co., Ltd. etc. can be cited.

[0130] In addition to those listed above, as other photoinitiators, for example, "Omnirad 651", "Omnirad MBF" manufactured by IGM RESINS, etc. can be used.

[0131] Among the substances listed above, the ink of the present embodiment preferably contains other acylphosphine oxide photoinitiators in addition to EO-TPO. As described above, other acylphosphine oxide photoinitiators not only have an excellent effect of improving curability compared to EO-TPO, but also can absorb ultraviolet rays with a wide wavelength range. Therefore, even ultraviolet rays from a UV-LED can be used to generate free radicals. As a result, it is considered that an ink with particularly excellent surface and internal curability can be achieved. In one embodiment, from the viewpoint of particularly improving curability, as other acylphosphine oxide photoinitiators, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BTPO) is particularly suitably used.

[0132] When EO-TPO and other acylphosphine oxide photoinitiators are used in combination, from the viewpoint of balancing curability and ejection stability, it is preferable to set the mixing ratio thereof to 1:1 to 3:1.

[0133] <Inhibitor>

[0134] In one embodiment, in order to improve the viscosity stability of the ink over time and suppress the hue stability and curing wrinkles of the printed matter, an inhibitor can be used. As specific examples of the inhibitor, hindered phenol compounds, phenol compounds, hydroquinone compounds, phenothiazine compounds, phosphorus compounds, and nitrosophenylhydroxylamine compounds can be cited, and they can be suitably used.

[0135] More specifically, 4-methoxyphenol, tert-butylhydroquinone, 2,6-di-tert-butyl-4-methylphenol, pentaerythritol tetra[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], hydroquinone, methylhydroquinone, phenothiazine, di-cumylphenothiazine, triphenylphosphine, aluminum salt of N-nitrosophenylhydroxylamine, etc. can be cited.

[0136] Based on the total mass of the ink, the content of the polymerization inhibitor is preferably 0.01 to 2% by mass, more preferably 0.05 to 1% by mass, and particularly preferably 0.1 to 0.5% by mass. When the above content is adjusted to the above range, it is easy to improve the viscosity stability over time while maintaining the curability, and improve the hue stability of the printed matter, and suppress curing wrinkles and the like.

[0137] <Colorant>

[0138] In one embodiment, the ink may contain a colorant. As the above colorant, conventionally known dyes and pigments can be used. In one embodiment, it is appropriate to use pigments from the viewpoints of improving the concentration of the printed matter and the storage stability and ejection stability of the ink.

[0139] The above pigments are not particularly limited. For example, organic pigments or inorganic pigments represented by the following pigment index numbers can be used.

[0140] For example, as red pigments, C.I. Pigment Red 5, 7, 12, 17, 48 (Ca), 48 (Mn), 49:2, 57 (Ca), 57:1, 112, 122, 123, 147, 149, 150, 166, 168, 176, 177, 178, 184, 188, 202, 209, 242, 254, 255, 264, 266, 269, 282, etc. can be cited;

[0141] As purple pigments, C.I. Pigment Violet 19, 23, etc. can be cited;

[0142] As orange pigments, C.I. Pigment Orange 5, 13, 34, 38, 43, 61, 62, 64, etc. can be cited;

[0143] As blue pigments, C.I. Pigment Blue 1, 2, 3, 15:3, 15:4, 15:6, 16, 22, 60, C.I. Vat Blue 4, 60, etc. can be cited;

[0144] As green pigments, C.I. Pigment Green 7, 26, 36, 50, 58, etc. can be cited;

[0145] As yellow pigments, C.I. Pigment Yellow 1, 2, 3, 12, 14, 16, 17, 73, 74, 75, 83, 93, 95, 97, 98, 109, 110, 114, 128, 129, 138, 139, 147, 150, 151, 154, 155, 180, 185, 213, etc. can be cited;

[0146] As black pigments, C.I. Pigment Black 1, 7, etc. can be cited; and

[0147] C.I. Pigment White 6, 18, 21, etc. as white pigments. These pigments can be arbitrarily used according to the desired color reproducibility and color rendering property. Two or more of the above-listed pigments can also be used in combination.

[0148] When measured by the dynamic light scattering method, the average particle diameter (D50) of the above pigments is preferably 50 to 500 nm, more preferably 100 to 400 nm. When D50 is within this range, the color rendering property (hiding power in the case of white ink), storage stability of the ink, and ejection stability are excellent. Furthermore, deterioration of curability can also be suppressed. The above D50 represents the median particle diameter based on volume. The median particle diameter is a value that can be measured using a dynamic light scattering type particle size distribution measuring device (for example, "Nanotrac UPA-EX150" manufactured by Microtrac BEL Corporation) after diluting the ink 200 to 1000 times with ethyl acetate or the like.

[0149] The content of the pigment in the ink can be appropriately selected according to the color and use purpose of the ink. For example, based on the total mass of the ink, it is preferably 0.1 to 30% by mass. In addition, from the viewpoints of color rendering property, storage stability of the ink, and ejection stability, except in the case of white ink, the content of the above pigment is more preferably 0.5 to 15% by mass, and further preferably 1 to 10% by mass. On the other hand, in the case of white ink, the content of the above pigment is more preferably 5 to 30% by mass, and further preferably 13 to 25% by mass.

[0150] According to the required use and image quality, the ink of this embodiment can also be configured as a light-colored ink with a small pigment content. Specific examples of the light-colored ink include light yellow, light magenta, light cyan, and light black. When configuring the light-colored ink, the above-listed pigments can be used as colorants for the light-colored ink.

[0151] <Pigment dispersion resin>

[0152] When the ink of this embodiment contains a pigment, a pigment dispersion resin can be used for the initial dispersibility, ejection stability, and viscosity stability over time of the pigment. The above pigment dispersion resin can be a commercially available product or a pigment dispersion resin synthesized by a conventionally known method.

[0153] As specific examples of commercially available products, “Ajisper-PB-821” and “Ajisper-PB-822” manufactured by Ajinomoto Fine-Techno Co., “BYKJET-9150”, “BYKJET-9151” and “BYKJET-9152” manufactured by BYK-Chemie Co., “Solsperse32000”, “Solsperse39000”, “SolsperseJ180” and “SolsperseJ200” manufactured by Lubrizol Corporation, etc. can be cited. In addition, as the pigment dispersion resin, for example, a copolymer of a polymerizable compound containing an acid group such as acrylic acid and methacrylic acid, a polymerizable compound containing an amino group such as acrylamide, dimethylaminoethyl methacrylate and diethylaminoethyl methacrylate, and other polymerizable compounds as required can be used.

[0154] The weight average molecular weight (hereinafter referred to as Mw) of the above-mentioned pigment dispersion resin is preferably 5,000 to 100,000, more preferably 10,000 to 50,000, and further preferably 15,000 to 30,000. If it is within the above range, the compatibility of the pigment dispersion resin with the polymerizable compound becomes good, and the storage stability of the ink is improved. In addition, the decrease in the glossiness of the printed matter caused by the pigment dispersion resin is suppressed, and the color rendering property is improved.

[0155] The above Mw can be determined by gel permeation chromatography (hereinafter referred to as GPC). Specifically, it is a value obtained as the polystyrene equivalent molecular weight measured using a TSKgel column (manufactured by Tosoh Corporation) in GPC equipped with an RI detector (for example, “HLC-8320GPC” manufactured by Tosoh Corporation) using DMF as an eluent.

[0156] The amine value of the pigment dispersion resin is preferably 5 to 50 mgKOH / g, more preferably 10 to 50 mgKOH / g, and further preferably 20 to 45 mgKOH / g. The acid value of the pigment dispersion resin is preferably 2 to 25 mgKOH / g, more preferably 5 to 20 mgKOH / g.

[0157] When the acid value and amine value of the pigment dispersion resin are within the above range, the compatibility with the mono-functional (meth)acrylate (A) containing an amino group is improved, and the curability, ejection stability and viscosity stability over time become good. In addition, during printing, inks of different colors do not mix with each other, and a printed matter with excellent color reproducibility can be obtained.

[0158] It should be noted that the method for measuring the amine value of the pigment dispersion resin is the same as that of the above-mentioned mono-functional (meth)acrylate (A) containing an amino group.

[0159] In addition, the above "acid value" is the number of milligrams of potassium hydroxide required to neutralize 1 g of the solid component of the resin dispersion resin, and can be determined by potentiometric titration in accordance with JIS K 0070. As an example of a specific measurement method, after dissolving the resin to be measured in a solvent prepared by mixing diethyl ether and ethanol at a mass ratio of 1:1, a 0.1 mol / L potassium hydroxide-ethanol solution is used for titration by potentiometric titration. Then, the acid value can be calculated using the titration amount read from the obtained titration curve.

[0160] In a preferred embodiment, from the viewpoint of balancing storage stability and ejection stability, as the pigment dispersion resin, a pigment dispersion resin having an Mw of 10,000 to 50,000 and an amine value of 5 to 50 mgKOH / g is particularly preferably used.

[0161] Except in the case of white ink, the addition amount of the pigment dispersion resin is preferably 20 to 120% by mass, more preferably 30 to 80% by mass, based on the total amount of the pigments. In the case of white ink, the addition amount of the pigment dispersion resin is preferably 1 to 100% by mass, more preferably 3 to 50% by mass, based on the total amount of the pigments. By using the pigment dispersion resin within the above-mentioned blending amounts, a pigment dispersion having excellent initial dispersibility of the pigment and viscosity stability over time can be easily obtained.

[0162] <Other components>

[0163] In addition to the above components, the ink of the present embodiment may also be used in combination with a surface conditioner, an organic solvent, water, an inert resin, and other additives.

[0164] (Surface conditioner)

[0165] In order to improve the wetting spreadability on the recording medium, improve the adhesion, and prevent depressions, the ink of the present embodiment preferably contains a surface conditioner. As the surface conditioner, for example, a silicone-based surface conditioner, a fluorine-based surface conditioner, an acrylic-based surface conditioner, an alkynediol-based surface conditioner, etc. can be used. Among them, from the viewpoints of surface tension reduction ability, adhesion improvement, and compatibility with polymerizable compounds, a silicone-based surface conditioner is preferably used.

[0166] As the silicone-based surface conditioner, for example, a compound having a dimethylsiloxane structure and / or its modified product can be used. Among them, a polyether-modified silicone-based surface conditioner is particularly preferably used. By using a polyether-modified silicone-based surface conditioner, the ink deposited on the recording medium (substrate) can be sufficiently wetted and spread, and an interaction is generated between the polyether group and EO-TPO, enabling both the hardness and flexibility of the coating film and the curability to be balanced.

[0167] As specific examples of the above polyether group, polyethylene oxide and polypropylene oxide can be cited. They may contain only either one of them as the polyether group in the molecule, or may contain both. From the viewpoint of the interaction with the compound represented by the general formula (A), it is preferably at least provided with a polyethylene oxide structure.

[0168] As commercially available products of the polyether-modified silicone-based surface conditioner, for example, BYK (registered trademark)-378, 348, 349, 3420, 3760, BYK-UV3500, UV3510 manufactured by BYK-Chemie; TEGO (registered trademark) Glide 450, 440, 435, 432, 410, 406, 130, 110, 100 manufactured by EVONIK, etc. can be preferably used. Among them, from the viewpoints of improving the adhesion and the image quality of the printed matter, BYK-378, 348, UV3510; TEGO Glide 450, 440, 432, 410, etc. are particularly preferably used.

[0169] When using the silicone-based surface conditioner, its content is preferably 0.1 to 5.0% by mass based on the total mass of the ink. When the above content is adjusted to 0.1% by mass or more, the wetting spreadability to the recording medium can be easily improved, and the adhesion is also improved. On the other hand, when the above content is adjusted to 5.0% by mass or less, the viscosity stability and ejection stability of the ink over time are easily ensured.

[0170] (Organic solvent, water)

[0171] In the ink of the present embodiment, in order to improve the low viscosity, wetting spreadability to the recording medium, and adhesion of the ink, an organic solvent and / or water can be used. When an organic solvent and / or water is contained, based on the total mass of the ink, its content is preferably 0.01 to 30% by mass, more preferably 0.05 to 20% by mass, and further preferably 0.1 to 10% by mass. In addition, from the viewpoints of drying property, wetting spreadability to the recording medium, and adhesion, when using an organic solvent, an organic solvent having a boiling point of 140 to 300°C is preferably used.

[0172] As the organic solvent, for example, alkylene glycol monoalkyl ether acetates, alkylene glycol diacetates, alkylene glycol monoalkyl ethers, alkylene glycol dialkyl ethers, alkanediols, lactams, lactones, other nitrogen-containing solvents, and other oxygen-containing solvents can be used.

[0173] Among them, it preferably contains at least one selected from the group consisting of alkylene glycol monoalkyl ethers, alkylene glycol dialkyl ethers, and alkylene glycol monoalkyl ether acetates. Tripropylene glycol monomethyl ether, dipropylene glycol monomethyl ether, propylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, tetraethylene glycol dialkyl ether, ethylene glycol monobutyl ether acetate, and diethylene glycol diethyl ether are particularly preferred. In one embodiment, the organic solvent preferably contains at least one selected from the group consisting of tetraethylene glycol dialkyl ether, ethylene glycol monobutyl ether acetate, and diethylene glycol diethyl ether.

[0174] (Inert resin)

[0175] In order to impart adhesion to various recording media and adjust the viscoelasticity of the ink to improve ejection stability, the ink of the present embodiment may contain an inert resin. As the inert resin, (meth)acrylic resins, polyurethane resins, vinyl chloride-vinyl acetate copolymer resins, ketone resins, etc. can be used. Among them, from the viewpoint of optimizing both adhesion and ejection stability, the inert resin preferably contains (meth)acrylic resins and / or ketone resins.

[0176] When the ink of the present embodiment contains an inert resin, based on the total mass of the ink, 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. When the above content is adjusted within the above range, adhesion and ejection stability can be easily improved without deteriorating curability.

[0177] In this specification, "inert resin" refers to a resin that does not participate in the polymerization reaction and contributes to the adhesion to the recording medium, and is a resin that is soluble in the ink.

[0178] (Other additives)

[0179] The ink of the present embodiment may contain additives such as ultraviolet absorbers and anti-fading agents as needed in addition to the above components. These components can be arbitrarily used with conventionally known compounds.

[0180] (Physical properties of the ink)

[0181] The viscosity of the ink of the present embodiment at 25 °C is preferably 5 to 25 mPa·s, more preferably 8 to 20 mPa·s. When the viscosity is 5 mPa·s or more, it can be ejected well. When it is 25 mPa·s or less, the ejection accuracy will not decrease and the reduction of the image quality is small. Furthermore, from the viewpoint of imparting high-frequency adaptability for high-speed printing, the above viscosity is preferably 8 to 14 mPa·s. The viscosity can be measured by reading the viscosity at 50 rpm at 25 °C using a TVE25L type viscometer manufactured by Toki Sangyo Co., Ltd.

[0182] From the viewpoint of the balance between ejection stability and the reliability of ink dot formation after landing, the surface tension of the ink at 25°C is preferably 20 mN / m or more and 50 mN / m or less, more preferably 25 mN / m or more and 40 mN / m or less. The surface tension can be measured by using an automatic surface tensiometer CBVP-Z manufactured by Kyowa Interface Science Co., Ltd. and confirming the surface tension when the platinum plate is wetted with the ink in a 25°C environment.

[0183] <Method for manufacturing ink>

[0184] The ink of this embodiment can be manufactured by a conventionally known method. For example, it can be manufactured as follows, but the method for manufacturing the ink is not limited to the method described below.

[0185] First, a colorant, a polymerizable compound, and, if necessary, a pigment dispersion resin, a surface conditioner, a polymerization inhibitor, an organic solvent, and / or water are mixed, and then a dispersion treatment is performed to prepare a colorant dispersion. The dispersion treatment can use a paint stirrer, a sand mill, a roll mill, a media-free disperser, etc.

[0186] Next, for the obtained colorant dispersion, the remaining part of the polymerizable compound, a photopolymerization initiator containing EO-TPO, and, if necessary, a surface conditioner, a polymerization inhibitor, an organic solvent, and / or water are added in such a manner as to achieve the desired ink properties, and they are thoroughly mixed. Then, it is filtered with a filter or the like to remove coarse particles, and an ink is prepared.

[0187] In the case of manufacturing the colorant dispersion, the colorant dispersion preferably contains 3 to 50% by mass of the colorant, more preferably 5 to 40% by mass, and particularly preferably 10 to 30% by mass.

[0188] As the polymerizable compound used when manufacturing the colorant dispersion, a difunctional monomer is preferably used. In particular, from the viewpoints of the storage stability, viscosity, and ejection stability of the colorant dispersion and the ink manufactured using the colorant dispersion, the difunctional monomer is preferably at least one selected from the group consisting of the polymerizable compound represented by the above general formula (B), the polymerizable compound represented by the above general formula (C), and a difunctional monomer having a main skeleton of an EO chain or a PO chain (wherein, the polymerizable compound represented by the above general formula (B) is not included). Among them, from the viewpoint of storage stability, it preferably contains at least one selected from the group consisting of dipropylene glycol diacrylate and 2-(2-vinyloxyethoxy)ethyl acrylate, and more preferably contains dipropylene glycol diacrylate.

[0189] It should be noted that the mono-functional (meth)acrylate (A) containing an amino group can be added during the production of the colorant dispersion, or can be added together with a photopolymerization initiator such as EO-TPO after the production of the colorant dispersion.

[0190] <2> Inkjet recording method

[0191] One embodiment of the present invention relates to an inkjet recording method using the ink of the above embodiment (hereinafter also referred to as "the inkjet recording method of the present embodiment"). This recording method includes: ejecting the ink of the above embodiment from an inkjet head onto a recording medium (step I); and irradiating the inkjet ink ejected onto the recording medium with ultraviolet rays to cure the inkjet ink (step II).

[0192] The inkjet recording method of the present embodiment can adopt a method of ejecting and applying the same ink from the same inkjet head to the same part on the recording medium multiple times, that is, a method of performing the above step I on the same part on the recording medium multiple times (multi-pass printing method). However, in the case of the recording method of the present embodiment, from the viewpoint of fully exerting the effects of the ink of the above embodiment and obtaining a printed matter with excellent quality (image quality), it is preferable to adopt a method of ejecting and applying the same ink from the same inkjet head to the same part on the recording medium only once. That is, it is preferable to adopt a method of performing the above step I on the same part on the recording medium only once (single-pass printing method).

[0193] The single-pass printing method can be implemented, for example, by using a line printer. From the viewpoint of obtaining a printed matter with productivity and good quality, the printing speed (conveying speed of the recording medium) at this time is preferably 35 to 150 m / min, more preferably 50 to 125 m / min, and further preferably 75 to 100 m / min.

[0194] The ink of the present embodiment is an ink for inkjet recording. Therefore, as described above, in the ink ejection mechanism in step I, an inkjet head is used.

[0195] As a method of ejecting ink using an inkjet head, examples include: an electrostatic attraction method of ejecting ink using electrostatic force; a drop on demand method (pressure pulse method) using the vibration pressure of a piezoelectric element; an acoustic inkjet method of changing an electric signal into a sound beam and irradiating the ink, and ejecting the ink using the radiation pressure at this time; a thermal inkjet method of heating the ink to form bubbles and ejecting the ink using the generated pressure, etc. Among them, in one embodiment, from the viewpoint of ejection stability, it is preferable to adopt a drop on demand method (pressure pulse method) using the vibration pressure of a piezoelectric element.

[0196] From the viewpoints of the image quality and ejection stability of the printed matter, the droplet volume of the ink droplets ejected from the inkjet nozzles is preferably 1 to 100 pL (picoliters), more preferably 2 to 50 pL, and still more preferably 3 to 20 pL. In addition, the design resolution of the inkjet head is preferably 9,000 dot / inch 2 (150×600 dpi) or more, and more preferably 180,000 dot / inch 2 (300×600 dpi) or more, and still more preferably 360,000 dot / inch 2 (600×600 dpi) or more. It should be noted that dpi represents the number of dots per 2.54 cm (per inch).

[0197] Examples of the inkjet head that satisfy the above conditions include KJ4A-AA, KJ4A-TA, KJ4A-RH manufactured by Kyocera Corporation, Samba G3L manufactured by Fujifilm Corporation, S3200, S1600, S800, I3200, I1600 manufactured by Seiko Epson Corporation, KM1024i, KM1024 manufactured by Konica Minolta Corporation, MH5320, MH5340, MH5240, MH5440 manufactured by Ricoh Company, etc., and all of them can be suitably used.

[0198] In one embodiment, the ink can be heated while being ejected by a heating device such as a heater provided in the inkjet head so as to make the ink have an appropriate viscosity. From the viewpoint of continuously and stably ejecting the ink, it is preferably heated so that the viscosity of the ink at the time of ejection becomes 20 mPa·s or less, and more preferably becomes 15 mPa·s or less.

[0199] In the above step II, the ink ejected onto the recording medium is cured by irradiating ultraviolet rays to form a cured film.

[0200] As a light source for ultraviolet rays, for example, a high-pressure mercury lamp, a low-pressure mercury lamp, an ultra-high-pressure mercury lamp, a metal halide lamp, an ultraviolet laser, a UV-LED, sunlight, etc. are known. On the other hand, as described above, in the case of the present embodiment, a UV-LED is preferably used. The maximum emission wavelength thereof is preferably 260 to 450 nm, more preferably 280 to 420 nm, and particularly preferably 320 to 410 nm.

[0201] Since the shape of the UV-LED is small, a plurality of them can be arranged and provided. Therefore, a plurality of LEDs can be arranged to increase the irradiation intensity on the recording medium and used. It should be noted that at this time, a plurality of UV-LEDs having different peak wavelengths can also be arranged and used. In addition, in step II, a light source other than the UV-LED such as the above-listed metal halide lamp and the UV-LED can also be used in combination.

[0202] When using a UV-LED in Process II, from the viewpoint of being able to fully exhibit the above effects and improve the quality of the printed matter, the maximum illuminance of ultraviolet rays on the recording medium is preferably 1,000 mW / cm 2 or more. The above maximum illuminance is more preferably 2,000 mW / cm 2 or more, and particularly preferably 3,000 mW / cm 2 or more. In addition, the cumulative light amount when irradiated onto the recording medium varies depending on the type and content of the polymerizable compound and photoinitiator contained in the ink. For example, it is preferably 50 mJ / cm 2 or more. The above cumulative light amount is more preferably 100 mJ / cm 2 or more, and particularly preferably 150 mJ / cm 2 or more.

[0203] After the ink adheres to the recording medium through the ejection in the above Process I (after the end of the above Process I), the irradiation of ultraviolet rays starts (the start of the above Process II). The time from the end of Process I to the start of Process II is preferably adjusted to 0.03 to 3 seconds. The above time is more preferably 0.04 to 2.5 seconds, and further preferably 0.6 to 2 seconds. By adjusting the above time to the above range, the dot formation property of the ink becomes good, the inks do not mix with each other, and a printed matter with good quality can be obtained.

[0204] <Ink set>

[0205] As an embodiment of the present invention, it is also possible to prepare a plurality of inks with different colors to form an ink set. It is preferred that all the inks constituting the ink set satisfy the ink requirements of the present embodiment.

[0206] When manufacturing a printed matter using an ink set, as a method thereof, for example, after performing Process I according to the number of inks constituting the ink set, Process II can be performed (only the formal curing described later is performed). As another example, it is possible to first continuously perform Process I on a part of the inks constituting the ink set and then perform Process II, further continuously perform Process I on the remaining (unsprayed) inks, and finally perform Process II again. That is, it is also possible to be a method of performing only the pre-curing described later on a part of the inks constituting the ink set, spraying all the inks, and finally performing the formal curing.

[0207] However, in the case of the ink of the present embodiment, from the viewpoint of being able to improve the quality of the printed matter, a method of performing Process II after each performance of Process I is preferred. That is, it is preferred to manufacture a printed matter by a method of performing pre-curing on all the inks constituting the ink set and further performing formal curing after spraying all the inks.

[0208] <Pre-curing>

[0209] "Pre-curing" refers to Process II that is carried out before the ejection of all the inks that make up the ink set is completed. Specifically, it means that after the ink lands on the recording medium, before the next ink lands, ultraviolet light is irradiated from a light source to cure the ink portion on the recording medium. When pre-curing is carried out in this embodiment, the light source used in this pre-curing is also preferably a UV-LED. In addition, from the viewpoint of being able to particularly improve the quality of the printed matter, the maximum illuminance of the ultraviolet light on the recording medium during pre-curing is preferably 2 to 20 mW / cm 2 , more preferably 5 to 15 mW / cm 2 .

[0210] <Full curing>

[0211] On the other hand, for the above-mentioned pre-curing, after all the inks that make up the ink set are ejected onto the recording medium, Process II carried out to completely cure the ink on the recording medium is generally referred to as "full curing". That is, when Process II is carried out multiple times in the manufacture of a printed matter using an ink set, the last Process II is not pre-curing but full curing. When full curing is carried out using a UV-LED, it is appropriate to use the above-mentioned maximum illuminance of 1,000 mW / cm 2 or more and a cumulative light amount of 50 mJ / cm 2 or more.

[0212] On the other hand, as the light source of the ultraviolet light for full curing, ultraviolet lamps such as high-pressure mercury lamps, low-pressure mercury lamps, ultra-high-pressure mercury lamps, and metal halide lamps can also be used. At this time, the maximum illuminance of the ultraviolet light is preferably 80 mW / cm 2 or more, more preferably 120 mW / cm 2 or more. In addition, the cumulative light amount is preferably 100 mJ / cm 2 or more, more preferably 150 mJ / cm 2 or more, and further preferably 200 mJ / cm 2 or more.

[0213] <Recording medium>

[0214] As a recording medium used in a printing method using the ink of the present embodiment, a resin film substrate or a paper substrate is preferred. The above resin film substrate preferably has a thickness of 10 to 90 μm. In addition, as the above resin film substrate, a 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 above paper substrate, coated paper, art paper, laminated paper, etc. are preferably selected. In one embodiment, the ink of the present embodiment can be suitably used for printing on a package formed of the above-listed recording media. In the printing of this package, it can be particularly suitably used for printing food packaging.

[0215] It should be noted that the above "substrate containing a material selected from the group consisting of polypropylene, polyethylene, polyethylene terephthalate, and nylon" is not limited to a single-layer structure and may also have a multilayer structure. That is, the above substrate may be a resin film substrate having one layer made of a material selected from the group consisting of the above polyethylene terephthalate, polyethylene, polypropylene, and nylon, or a resin film substrate having two or more of the above layers (laminated film substrate). In addition, for the purpose of improving the strength of the package, oxygen barrier, etc., a layer made of AL (aluminum foil) and VM (vacuum evaporation) film (aluminum evaporation film, transparent evaporation film), etc. may be present in the layers constituting the above laminated film substrate.

[0216] Examples

[0217] Hereinafter, the present invention will be described in more detail, but the following examples do not limit the scope of the rights of the present invention in any way. In addition, unless otherwise specified, "parts" means parts by mass and "%" means mass%.

[0218] (Examples 1 to 20, Comparative Examples 1 to 4)

[0219] <1> Preparation of Ink

[0220] Before preparing the ink, a black pigment dispersion is prepared. 20 parts of a pigment ("Special Black 350" from Orion Engineered Carbons), 10 parts of a pigment dispersion resin ("Solsperse 32000" from Lubrizol), and 70 parts of dipropylene glycol diacrylate (DPGDA, "SR508" from Arkema) are put into a high-speed mixer and stirred until homogeneous. Then, a dispersion treatment is carried out using a horizontal sand mill for about 1 hour to prepare a black pigment dispersion A.

[0221] In addition, except for using 2-(2-vinyloxyethoxy)ethyl acrylate (VEEA, manufactured by Nippon Shokubai Co., Ltd.) instead of dipropylene glycol diacrylate, a black pigment dispersion B is prepared by the same method as the above black pigment dispersion A.

[0222] Next, while stirring the above-prepared black pigment dispersion A or black pigment dispersion B according to the formulation described in Table 1, each raw material such as a polymerizable compound, a photoinitiator, a polymerization inhibitor, and a surface modifier is added in sequence. Then, it is gently mixed until the solid photoinitiator is dissolved. For the obtained mixture, it is filtered using a membrane filter with a pore size of 1 μm to remove coarse particles, thereby obtaining a black ink. It should be noted that the addition order of the above raw materials can be different.

[0223] [Table 1-1]

[0224]

[0225] [Table 1-2]

[0226]

[0227] The details of the materials described in Table 1 are as follows.

[0228] [Polymerizable compound]

[0229] (Mono-functional (meth)acrylate (A) containing an amino group)

[0230] ·Genomer 5161: Mono-functional acrylate containing an amino group (manufactured by Rahn AG, amine value 230 mg KOH / g)

[0231] ·CN383: Mono-functional acrylate containing an amino group (manufactured by Arkema, amine value 150 mg KOH / g)

[0232] ·CN374: Mono-functional acrylate containing an amino group (manufactured by Arkema, amine value 250 mg KOH / g)

[0233] ·EBECRYL 81: Mono-functional acrylate containing an amino group (manufactured by Allnex, amine value 56 mg KOH / g)

[0234] (Polymerizable compound represented by the general formula (B))

[0235] ·VEEA: 2-(2-Vinyloxyethoxy)ethyl acrylate (manufactured by Nippon Shokubai Co., Ltd.)

[0236] ·Photomer 4050: Polyethylene glycol 200 diacrylate (manufactured by IGM RESINS)

[0237] (Polymerizable compound represented by the general formula (C))

[0238] ·Photomer 4017: 1,6 - hexanediol diacrylate (manufactured by IGM RESINS)

[0239] ·SR341: 3 - methyl - 1,5 - pentanediol diacrylate (manufactured by ARKEMA)

[0240] ·SR595: 1,10 - decanediol diacrylate (manufactured by ARKEMA)

[0241] (Other polymerizable compounds)

[0242] ·SR335: Lauryl acrylate (manufactured by ARKEMA)

[0243] ·SR506: Isobornyl acrylate (manufactured by ARKEMA)

[0244] ·SR508: Dipropylene glycol diacrylate (manufactured by ARKEMA)

[0245] ·SR351: Trimethylolpropane triacrylate (manufactured by ARKEMA)

[0246] (Photoinitiator)

[0247] (EO - TPO)

[0248] ·Omnirad TPO - L: Ethoxy phenyl (2,4,6 - trimethylbenzoyl) phosphine oxide (manufactured by IGM RESINS)

[0249] ·Omnipol TP: Polymer of ethoxy phenyl (2,4,6 - trimethylbenzoyl) phosphine oxide (manufactured by IGM RESINS)

[0250] (Other polymerization initiators)

[0251] ·Omnirad 819: Bis(2,4,6 - trimethylbenzoyl) - phenyl phosphine oxide (manufactured by IGM RESINS)

[0252] ·Omnirad TPO: 2,4,6 - Trimethylbenzoyl - phenyl phosphine oxide (manufactured by IGM RESINS)

[0253] ·ESACURE 1001M: 1 - [4 - (4 - benzoylphenylthio)phenyl] - 2 - methyl - 2 - (4 - methylphenylsulfonyl) propane - 1 - one (manufactured by IGM RESINS)

[0254] ·Omnirad DETX: 2,4 - Diethylthioxanthone - 9 - one (manufactured by IGM RESINS)

[0255] ·Omnirad ITX: 2-Isopropylthioxanthone (manufactured by IGM RESINS)

[0256] <Polymerization inhibitor>

[0257] ·BHT: 2,6-Di-tert-butyl-4-methylphenol (“BHT SWANOX” manufactured by Seiko Chemical Co., Ltd.)

[0258] <Surface conditioner>

[0259] ·BYK3760: Silicone-based surface conditioner (manufactured by BYK Chemie)

[0260] ·TEGO GLIDE 450: Silicone-based surface conditioner (manufactured by EVONIK)

[0261] <Inert resin>

[0262] ·JONCRYL586: (Meth)acrylic resin (manufactured by BASF, weight average molecular weight 4,300)

[0263] <2>Production of printed matter

[0264] Using the inks of the above-prepared examples and comparative examples, printed matter was manufactured as follows.

[0265] An inkjet head (resolution 600 dpi × 600 dpi) manufactured by Kyocera Corporation was provided above a conveyor capable of conveying a substrate. Furthermore, an inkjet ejection device (“OnePassJET” manufactured by Tritek Corporation) equipped with a UV-LED for formal curing on the downstream side with respect to the substrate conveyance direction was prepared.

[0266] The ink manufactured previously was filled in the above inkjet head. Next, after fixing the PET substrate “K2411” manufactured by Lintec Corporation to the conveyor, the conveyor was driven at a speed of 50 m / min, and printing was performed when the above PET substrate passed through the installation part of the inkjet head. Specifically, under the printing conditions of a droplet ejection amount of 14 pl, a solid image with a coverage rate of 100% and a halftone solid image with a coverage rate of 30% were printed. Then, after ejecting the ink, the conveyor was also continuously driven at the same speed, and ultraviolet rays were irradiated when the PET substrate passed through the installation part of the UV-LED for formal curing to produce printed matter.

[0267] As the above UV-LED for formal curing, FirePower FP300 manufactured by Phoseon (maximum emission wavelength 395 nm, maximum illuminance 16,000 mW / cm 2)。In addition, the output of the above UV-LED is pre-adjusted so that the illuminance during irradiation of the ejected ink is 7,000 mW / cm 2 and the cumulative light quantity is 500 mJ / cm 2 .

[0268] <3> Various evaluations

[0269] Using the inks and printed matters of the examples and comparative examples prepared by the above method, various evaluations were carried out according to the method shown below. The evaluation results are shown in Table 1.

[0270] <Evaluation of thick film (internal) curability>

[0271] The surface of the printed matter of the solid image with a coverage rate of 100% prepared by the above method was wiped with a cotton swab, and it was confirmed whether the ink in the uncured state was attached to the cotton swab. When the ink was attached to the cotton swab, the printed matter was fixed to the conveyor of the above inkjet printing device, and without printing the ink, only the UV-LED for formal curing was irradiated. Thereafter, it was again confirmed whether the ink was attached when wiped with a cotton swab. This operation was repeated, and the number of passes required until the ink in the uncured state was no longer attached to the cotton swab was confirmed.

[0272] The evaluation criteria for thick film curability are as described below, and when the evaluation is "3" or more, it is considered practical.

[0273] (Evaluation criteria)

[0274] 5: At the moment of passing a total of once (no additional ultraviolet irradiation is required), the ink in the uncured state is not attached to the cotton swab

[0275] 4: At the moment of passing a total of twice (one additional ultraviolet irradiation was performed), the ink in the uncured state is no longer attached to the cotton swab

[0276] 3: At the moment of passing a total of 3 times (two additional ultraviolet irradiations were performed), the ink in the uncured state is no longer attached to the cotton swab

[0277] 2: At the moment of passing a total of 4 times (three additional ultraviolet irradiations were performed), the ink in the uncured state is no longer attached to the cotton swab

[0278] 1: Until the ink in the uncured state is no longer attached to the cotton swab, it is necessary to perform a total of more than 5 UV-LED irradiations.

[0279] <Evaluation of thin film (surface) curability>

[0280] As a printed matter, a printed matter using a halftone solid image with a coverage rate of 30% was used. In addition, the evaluation of the thin film curability was carried out in the same method and evaluation criteria as the evaluation of the above-mentioned thick film curability.

[0281] <Evaluation of color change over time>

[0282] A printed matter with a solid image having a coverage rate of 100% was produced by the above method. Then, the printed matter was fixed to the conveyor of the above inkjet printing apparatus, and irradiation with a UV-LED for formal curing (i.e., additional ultraviolet irradiation) was carried out 4 times without printing ink. Then, using the X-Rite 500 series manufactured by X-Rite Inc., the hue (L value, a value, b value) of the solid image printed matter immediately after the additional ultraviolet irradiation was measured. The measurement conditions were a viewing angle of 2° and a light source D65, and the L value, a value, and b value obtained by the measurement were set as L1, a1, and b1, respectively.

[0283] Next, after leaving the printed matter after the above hue measurement at room temperature for 24 hours, the hue of the printed matter was measured again. The measurement conditions were the same as above, and the L value, a value, and b value obtained by the measurement were set as L2, a2, and b2, respectively.

[0284] Next, using the values obtained by the above measurement, the color difference was calculated according to the following formula 1, and the evaluation of the color change over time was carried out.

[0285] Formula 1: ΔE = {(L2 - L1) 2 + (a2 - a1) 2 + (b2 - b1) 2} 1 / 2

[0286] The evaluation criteria for the color change over time are as follows. When the evaluation is 3 or more, it is considered practical.

[0287] (Evaluation criteria)

[0288] 5: ΔE < 1

[0289] 4: 1 ≤ ΔE < 1.5

[0290] 3: 1.5 ≤ ΔE < 2

[0291] 2: 2 ≤ ΔE < 3

[0292] 1: ΔE ≥ 3

[0293] <Evaluation of ejection stability>

[0294] In a jig equipped with an inkjet head capable of temperature adjustment (Toshiba TEC Corporation's "CA4", number of nozzles: 318), the above-prepared inks were filled respectively. Next, the temperature of the print head was controlled so that the viscosity of the ink at the time of ejection became 8 to 9 mPa·s. After confirming that there were no nozzles that did not eject ink, the ink was continuously ejected at a driving frequency of 6 kHz. Then, the number of nozzles that became unable to eject ink before and after 30 minutes of continuous ejection (number of nozzle losses) was confirmed. The evaluation criteria are as described below, and cases evaluated as "3" or higher are considered practical.

[0295] (Evaluation Criteria)

[0296] 5: The number of nozzle losses is 0 or 1

[0297] 4: The number of nozzle losses is 2 or 3

[0298] 3: The number of nozzle losses is 4 or 5

[0299] 2: The number of nozzle losses is 6 to 10

[0300] 1: The number of nozzle losses is 11 or more

[0301] <Viscosity Stability Evaluation>

[0302] After each ink was manufactured, its viscosity was immediately measured. Then, each ink was filled into a glass container with a capacity of 20 mL at a filling rate of 90% of the container capacity. Then, the glass container filled with the ink was left standing in a sealed and light-shielded state at 70°C for 1 week. Then, the viscosity of the ink in the glass container was measured again after 1 week. Based on the measured values, the change rate of viscosity (viscosity increase rate, %) was calculated according to the following formula 2, and the viscosity stability was evaluated.

[0303] Formula 2: (Viscosity increase rate) = (V2 - V1) / V1 × 100

[0304] In Formula 2, V1 represents the viscosity of the ink just after manufacture, and V2 represents the viscosity of the ink after being left standing at 70°C for 1 week.

[0305] The evaluation criteria for viscosity stability are as described below, and cases evaluated as "3" or higher are considered practical.

[0306] (Evaluation Criteria)

[0307] 5: Viscosity increase rate < 3 (%)

[0308] 4: 3 (%) ≤ viscosity increase rate < 5 (%)

[0309] 3: 5 (%) ≤ viscosity increase rate < 8 (%)

[0310] 2: 8 (%) ≤ viscosity increase rate < 10 (%)

[0311] 1: Tackifying rate ≥ 10 (%)

[0312] As shown in Table 1, the inks of Examples 1 to 19 all have the composition of the present invention and obtained results of "3" or more in all evaluation items, which are practical. On the other hand, Comparative Example 1 is a system that does not contain the amino group-containing monofunctional (meth)acrylate (A). In the evaluation results of the ink of Comparative Example 1, both the thick film curability and the thin film curability are insufficient. Comparative Example 2 is a system that does not contain EO-TPO. In order to supplement the curability instead of this EO-TPO, other photoinitiators were used. In the evaluation results of the ink of Comparative Example 2, although the curability reaches a practical level, the color change over time and the viscosity stability are greatly deteriorated.

[0313] In Comparative Example 3, the content of the monofunctional (meth)acrylate is 18% by mass, which is more than the content specified in the present invention. In the evaluation results of the ink of Comparative Example 3, both the thick film curability and the thin film curability are poor. In addition, a color change over time also occurred. It is considered that this color change is caused by a large amount of unreacted photoinitiator remaining in the printed matter.

[0314] Comparative Example 4 reproduced the ink composition described in Example 1 of the above Patent Document 1 (substitutes were used for the materials that could not be obtained). In the evaluation results of the ink of Comparative Example 4, the thick film curability and the ejection stability are at a practical level. However, deterioration of the thin film curability and the viscosity stability was confirmed, and in addition, a color change of the printed matter over time also occurred.

Claims

1. A UV-curable inkjet ink containing a polymerizable compound and a photoinitiator, wherein the polymerizable compound contains a monofunctional (meth)acrylate, and the monofunctional (meth)acrylate includes an amino-containing monofunctional (meth)acrylate (A), the photoinitiator contains ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, based on the total mass of the UV-curable inkjet ink, the content of the monofunctional (meth)acrylate is 0.1 to 15% by mass.

2. The ultraviolet curable inkjet ink according to claim 1, wherein, The amine value of the amino-containing monofunctional (meth)acrylate (A) is 150 to 250 mgKOH / g.

3. The ultraviolet curable inkjet ink according to claim 1 or 2, wherein, The polymerizable compound further includes at least one polymerizable compound selected from the group consisting of the polymerizable compound represented by the following general formula (B) and the polymerizable compound represented by the following general formula (C), General formula (B): CH2=CH-CO-(O-CH2CH2) n -O-R 1 In general formula (B), R 1 represents an acryloyl group or a vinyl group, and n represents an integer of 2 to 10. General formula (C): CH2=CH-CO-O-R 2 -O-CO-CH=CH2 In general formula (C), R 2 represents an alkylene group having 2 to 10 carbon atoms which may have a branched structure.

4. The ultraviolet curable inkjet ink according to claim 3, wherein, the ratio of the content of the amino-containing monofunctional (meth)acrylate (A) to the total content of the polymerizable compound represented by the general formula (B) and the polymerizable compound represented by the general formula (C) is 0.02 to 0.

2.

5. The ultraviolet curable inkjet ink according to any one of claims 1 to 4, wherein, The ratio of the content of the amino-containing monofunctional (meth)acrylate (A) to the content of ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide is 0.02 to 1.

6. An inkjet recording method, comprising: ejecting the UV-curable inkjet ink according to any one of claims 1 to 5 from an inkjet head onto a recording medium, and curing the UV-curable inkjet ink ejected onto the recording medium by irradiating ultraviolet rays from a UV light-emitting diode.

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