Ink set and inkjet method

Abstract

To provide an ink set and an inkjet method capable of improving a glossiness while suppressing an odor generation, in a coating film of a clear ink.SOLUTION: An ink set is for a radiation-curable inkjet ink containing a polymerizable compound. The ink set includes a color ink and a clear ink. The polymerizable compound included in the clear ink contains: a monomer A having a hydroxy group; and a monomer B having a cubic volume of 0.26 cubic nm or more defined by a Van-der-Waals radius and an area of 0.25 sq. nm or more in a height direction with respect to a long side. The total content of the monomer A and the monomer B is 80 mass% or more with respect to a total amount of the polymerizable compound included the clear ink.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an ink set and an ink-jet method. [Background technology]

[0002] Conventionally, ink sets containing color inks and clear inks, which are radiation-curable inkjet inks, have been known. Such ink sets contain polymerizable compounds that are cured by radiation such as ultraviolet light. For example, Patent Document 1 discloses an ink set containing a clear ink that uses phenoxyethyl acrylate (PEA) or isobornyl acrylate (IBXA) as a polymerizable compound. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2013-67770 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, the ink set of Patent Document 1 has a problem that it is difficult to improve the glossiness of the coating film formed by the clear ink. In detail, when a coating film of a clear ink containing PEA or the like is formed on a coating film of a color ink, the glossiness may decrease. If the glossiness decreases, there is a risk that sufficient appearance quality cannot be obtained when the ink set is used for printed matter such as signage.

[0005] In addition, there is also a problem that the coating film formed by the clear ink tends to have a strong odor. In detail, when a polymerizable compound having a relatively bulky molecular structure such as IBXA is used, the above-mentioned decrease in glossiness is unlikely to occur, but a strong odor may be generated from the coating film formed. In other words, there has been a demand for an ink set for clear ink that improves the glossiness of the coating film while suppressing the generation of an odor from the coating film. [Means for solving the problem]

[0006] The ink set is an ink set of radiation-curable inkjet inks containing a polymerizable compound, the ink set including color inks and a clear ink, the polymerizable compound contained in the clear ink includes monomer A having a hydroxyl group and monomer B having a volume defined by the Van der Waals radius of 0.26 cubic nm or more and an area in the height direction relative to the long side of 0.25 square nm or more, and the total content of monomer A and monomer B is 80 mass% or more of the total amount of polymerizable compounds contained in the clear ink.

[0007] In the above ink set, the content of monomer A is preferably 5% by mass or more and 15% by mass or less relative to the total amount of polymerizable compounds contained in the clear ink.

[0008] In the above ink set, the content of monomer B is preferably 70 mass % or more relative to the total amount of polymerizable compounds contained in the clear ink.

[0009] In the above ink set, the polymerizable compound contained in the clear ink preferably contains a monofunctional monomer, and the content of the monofunctional monomer relative to the total amount of the polymerizable compound contained in the clear ink is preferably 85 mass % or more.

[0010] In the above ink set, the polymerizable compound contained in the clear ink preferably contains a polyfunctional monomer, and the content of the polyfunctional monomer relative to the total amount of the clear ink is preferably 0.01% by mass or more and 15.00% by mass or less.

[0011] In the ink set, the weighted average of the glass transition temperatures of the homopolymers in the polymerizable compounds, where the weights are the mass ratios of the contents of the polymerizable compounds contained in the clear ink, is preferably 48° C. or higher.

[0012] In the ink set, the polymerizable compound contained in the color inks preferably contains a monofunctional monomer, and the content of the monofunctional monomer relative to the total amount of the polymerizable compounds contained in the color inks is preferably 85 mass % or more.

[0013] The inkjet method is an inkjet method using the above-mentioned ink set, and includes a first ejection step of ejecting color inks from an inkjet head and adhering them to a recording medium, a first curing step of irradiating the color inks adhered to the recording medium with radiation, a second ejection step of ejecting clear ink from the inkjet head and adhering it to an area of ​​the recording medium containing the color inks irradiated with radiation, and a second curing step of irradiating the clear ink adhered to the recording medium with radiation.

[0014] The above inkjet method preferably includes a leveling step of leaving the recording medium to which the clear ink has been applied still between the second ejection step and the second curing step. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic perspective view illustrating a configuration of an inkjet printer according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Embodiment 1. Ink set The ink set according to the present embodiment includes color inks and clear inks, which are radiation-curable inkjet inks containing a polymerizable compound. The ink set may include a plurality of color inks having different color materials. The inkjet method according to the present embodiment is used to form images, colors, characters, patterns, and the like on a recording medium by the inkjet method using the ink set. By including a plurality of color inks exhibiting different color tones in the ink set, it becomes easier to obtain a desired color tone in the image to be formed. The use of the ink set according to the present embodiment is not limited to the above, and may be, for example, for 3D modeling.

[0017] Here, the radiation-curable inkjet ink is an inkjet ink that is cured by being irradiated with radiation. Examples of radiation include ultraviolet light, electron beams, infrared light, visible light, X-rays, etc. Among these, it is preferable to use ultraviolet light as the radiation, because radiation sources are easily available and widely used, and materials suitable for curing with respect to the peak wavelength of the radiation are easily available and widely used.

[0018] 1.1.Clear ink The clear ink according to the present embodiment contains a photopolymerization initiator and a polymerizable compound, which is a monomer A and a monomer B, which will be described later. The clear ink may contain other polymerizable compounds that do not fall under the category of monomer A and monomer B. The clear ink is not used to color a recording medium, but is used to protect a coating film formed from color inks and to adjust the gloss of printed matter. Therefore, the clear ink preferably contains 0.2 mass % or less of coloring material relative to the total amount of the clear ink, and more preferably does not contain coloring material. Hereinafter, the components contained in the clear ink according to the present embodiment will be described. In the following description, the coating film formed from the clear ink is also referred to as a clear ink coating film, and the coating film formed from the color ink is also referred to as a color ink coating film.

[0019] When a conventional clear ink coating is formed on a color ink coating containing PEA (phenoxyethyl acrylate), the gloss of the coating may not be ensured. The inventors have therefore conducted research and found that the gloss of the clear ink coating can be improved by using a monomer B having a bulky molecular structure in the clear ink. Here, the monomer B having a bulky molecular structure refers to a polymerizable compound having a volume defined by the Van-der-Waals radius of 0.26 cubic nm or more and an area in the height direction relative to the long side of 0.25 square nm or more.

[0020] In addition, when a polymerizable compound having a relatively bulky molecular structure is used, the coating film of the conventional clear ink may have a strong odor. Therefore, the inventors conducted further research and found that the odor generated from the coating film of the clear ink can be suppressed by using a monomer A having a hydroxyl group in combination.

[0021] In the clear ink of this embodiment, the total content of monomer A and monomer B is 80% by mass or more, preferably 82% by mass or more, and more preferably 84% by mass or more, based on the total amount of polymerizable compounds contained in the clear ink. This means that when a polymerizable compound other than monomer A and monomer B, such as PEA, is contained in the clear ink, the content of the polymerizable compound other than monomer A and monomer B is inevitably reduced, improving the gloss of the coating film. The upper limit of the total content of monomer A and monomer B contained in the clear ink is not particularly limited, but is preferably 99% by mass or less, more preferably 97% by mass or less, and even more preferably 95% by mass or less. This means that the curability of the clear ink can be improved.

[0022] Here, the polymerizable compounds used in the clear ink may include polymerizable compounds corresponding to both monomer A and monomer B. In this case, when calculating the total content of monomer A and monomer B, a monomer corresponding to both monomer A and monomer B is not counted as both monomer A and monomer B. In other words, the total content of monomer A and monomer B refers to the total content of monomer A that does not correspond to monomer B, monomer B that does not correspond to monomer A, and monomer that corresponds to both monomer A and monomer B. Details of monomer A and monomer B will be described later. The components contained in the clear ink of this embodiment will be described below.

[0023] 1.1.1. Polymerizable compounds The polymerizable compound includes a monofunctional monomer having one polymerizable functional group, a polyfunctional monomer having multiple polymerizable functional groups, and an oligomer having one or more polymerizable functional groups. These polymerizable compounds may be used alone or in combination of two or more.

[0024] The polymerizable functional group of the polymerizable compound of the present embodiment is not particularly limited as long as it can undergo a polymerization reaction by radiation, and any known polymerizable functional group can be used. In particular, from the viewpoint of polymerization reactivity, the polymerizable functional group is preferably a polymerizable functional group having an unsaturated double bond between carbon atoms, more preferably a methacryloyl group, and even more preferably an acryloyl group.

[0025] In this specification, (meth)acryloyl means at least one of acryloyl and the corresponding methacryloyl, (meth)acrylate means at least one of acrylate and the corresponding methacrylate, and (meth)acrylic means at least one of acrylic and the corresponding methacrylic.

[0026] The weighted average of the glass transition temperatures of the homopolymers in the polymerizable compound, weighted by the mass ratio of each content of the polymerizable compounds contained in the clear ink, is 48° C. or higher. The weighted average of the glass transition temperatures is preferably 70° C. or higher, more preferably 85° C. or higher. By having the weighted average of the glass transition temperatures be 48° C. or higher, the scratch resistance of the coating film of the clear ink in a room temperature environment of about 25° C. can be improved. The upper limit of the weighted average of the glass transition temperatures is not particularly limited, but is preferably 140° C. or lower, more preferably 120° C. or lower, and even more preferably 110° C. or lower.

[0027] The method for calculating the weighted average of the above glass transition temperatures will be described. The weighted average of the above glass transition temperatures is called Tg All (℃), and the glass transition temperature of each homopolymer of the polymerizable compound is Tg N (° C.), and the mass ratio of the content of the polymerizable compound corresponding to each homopolymer, in other words, the content ratio of each polymerizable compound to the total amount of the polymerizable compounds, is X N (% by mass). N is a number starting from 1 depending on the type of polymerizable compound contained in the clear ink. Specifically, when three types of polymerizable compounds are used, Tg 1 , Tg 2 , Tg 3 is set.

[0028] Weighted average glass transition temperature Tg All is the glass transition temperature Tg of the homopolymer corresponding to each polymerizable compound. N and the content ratio X of each polymerizable compound N Therefore, the following formula (1) holds true. Tg All =ΣTg N X N (1)

[0029] Weighted average Tg of the above glass transition temperatures Allcan be adjusted by the glass transition temperature and mass ratio of each polymerizable compound used in the clear ink. The glass transition temperature of each homopolymer of the polymerizable compound can be obtained from the safety data sheet (SDS) or catalog information of the corresponding polymerizable compound.

[0030] 1.1.1.1. Monomer A Monomer A has a hydroxyl group in its molecular structure. The number of hydroxyl groups that Monomer A has in its molecular structure is 1 or 2 or more. From the viewpoint of making it difficult for the viscosity of the clear ink to increase, the number of the hydroxyl group is preferably 1.

[0031] Specific examples of monomer A are not particularly limited, but include, for example, 4-hydroxybutyl (meth)acrylate, hydroxypropyl (meth)acrylate, etc., as monofunctional monomers having one hydroxyl group. In addition, as monomer A, a commercially available product such as 2-hydroxy-3-phenoxypropyl acrylate may be used. Note that 2-hydroxy-3-phenoxypropyl acrylate is a monomer that corresponds to both monomer A and monomer B.

[0032] The content of monomer A in the clear ink is 5% by mass or more and 15% by mass or less, preferably 7% by mass or more and 13% by mass or less, and more preferably 10% by mass or more and 12% by mass or less, based on the total amount of polymerizable compounds in the clear ink. This further reduces odors emitted from the clear ink coating, and prevents excessive increases in the viscosity of the clear ink. The content of monomer A in the clear ink includes the content of monomers that fall under both monomer A and monomer B.

[0033] 1.1.1.2.Monomer B Monomer B has the bulkiness of its molecular structure as described below. The volume of monomer B defined by the Van-der-Waals radius is 0.26 cubic nm or more, preferably 0.27 cubic nm or more, and more preferably 0.28 cubic nm or more. The upper limit of the volume of monomer B defined by the Van-der-Waals radius is not particularly limited, but is preferably 0.60 cubic nm or less, more preferably 0.55 cubic nm or less, and even more preferably 0.50 cubic nm or less.

[0034] The area of ​​monomer B in the height direction relative to the long side defined by the Van der Waals radius is 0.25 nm2 or more, preferably 0.27 nm2 or more, and more preferably 0.29 nm2 or more. The upper limit of the area of ​​monomer B in the height direction relative to the long side defined by the Van der Waals radius is not particularly limited, but is preferably 0.50 nm2 or less, more preferably 0.45 nm2 or less, and even more preferably 0.40 nm2 or less. The molecular structure of monomer B has the bulkiness characteristic described above, and thus the gloss of the coating film of the clear ink can be improved.

[0035] Here, the volume and the area in the height direction relative to the long side defined by the Van der Waals radius are calculated as the volume and the area in the height direction relative to the long side in the molecular structure with the lowest energy among the structural isomers of the molecule. For example, known software such as thermodynamic property prediction software can be used to specify the three-dimensional shape defined by the Van der Waals radius of the molecule and to calculate the volume and the area in the height direction relative to the long side based on the specified three-dimensional shape.

[0036] The volume defined by the Van-der-Waals radius refers to the volume of the three-dimensional shape of a molecule, i.e., the cavity, formed by the Van-der-Waals radius of each atom constituting the molecule, when the molecule is suspended in a vacuum approximated by a chemical formula. The long side defined by the Van-der-Waals radius is the longest side in the three-dimensional shape. The long side is obtained by modeling the structure in which the molecule is most stable, and calculating the distance between the farthest terminal skeletal atoms, such as carbon, oxygen, and nitrogen. Furthermore, the area in the height direction relative to the long side is the value obtained by dividing the volume by the long side, and is an index of the area in a plane perpendicular to the long side.

[0037] Monomer B may be a monofunctional monomer or a polyfunctional monomer. In addition, as described above, monomer B may have a feature of having a hydroxyl group in its molecular structure, which is a requirement corresponding to monomer A.

[0038] Examples of monomer B include, but are not limited to, monofunctional monomers such as dicyclopentenyl acrylate (DCPA), isobornyl acrylate (IBXA), isobornyl methacrylate, 3,3,5-trimethylcyclohexyl acrylate (TMCHA), tert-butylcyclohexanol acrylate (TBCHA), isononyl acrylate (INAA), and lauryl acrylate (LA); polyfunctional monomers such as dipropylene glycol diacrylate (DPGDA); and monomers having characteristics of monomer A and monomer B such as 2-hydroxy-3-phenoxypropyl acrylate.

[0039] The content of monomer B contained in the clear ink is 70% by mass or more, preferably 73% by mass or more, and more preferably 78% by mass or more, based on the total amount of polymerizable compounds contained in the clear ink. This can further improve the gloss of the clear ink coating. There is no particular limit to the upper limit of the content of monomer B, but it is preferably 90% by mass or less, more preferably 87% by mass or less, and even more preferably 85% by mass or less, based on the total amount of polymerizable compounds contained in the clear ink. This can improve the curability of the clear ink while suppressing the odor of the cured clear ink coating.

[0040] 1.1.1.3. Monofunctional monomers The polymerizable compound contained in the clear ink includes a monofunctional monomer. The monofunctional monomer includes those that fall under Monomer A and Monomer B, as well as other monofunctional monomers that do not fall under these categories.

[0041] The monofunctional monomer is not particularly limited, but for example, a nitrogen-containing monofunctional monomer, a monofunctional (meth)acrylate having a crosslinked condensed ring structure, an aromatic group-containing monofunctional monomer, and a saturated aliphatic group-containing monofunctional monomer can be mentioned. If necessary, other monofunctional monomers may be used in place of or in addition to the above-mentioned monofunctional monomer. The polymerizable functional group of the monofunctional monomer is not particularly limited, but a known polymerizable functional group, particularly a polymerizable functional group having an unsaturated double bond between carbon atoms, can be applied.

[0042] The content of the monofunctional monomer contained in the clear ink is 85% by mass or more, preferably 90% by mass or more, and more preferably 95% by mass or more, based on the total amount of the polymerizable compounds contained in the clear ink. This makes the coating film of the clear ink flexible and improves the stretchability. There is no particular limit to the upper limit of the content of the monofunctional monomer, but it is preferably 99% by mass or less, more preferably 98% by mass or less, and even more preferably 97% by mass or less, based on the total amount of the polymerizable compounds contained in the clear ink. This can improve the curability of the clear ink and the abrasion resistance of the coating film.

[0043] The content of the monofunctional monomer contained in the clear ink is preferably 70% by mass or more, more preferably 75% by mass or more, and even more preferably 80% by mass or more, based on the total amount of the clear ink. This further improves the extensibility of the coating film. The upper limit of the content of the monofunctional monomer is preferably 95% by mass or less, more preferably 92% by mass or less, and even more preferably 90% by mass or less, based on the total amount of the clear ink. This further improves the curability of the clear ink and the abrasion resistance of the coating film.

[0044] Specific examples of the monofunctional monomer are shown below, but the monofunctional monomer in the present embodiment is not limited to the following.

[0045] 1.1.1.3.1. Nitrogen-containing monofunctional monomers The nitrogen-containing monofunctional monomer is not particularly limited, but examples thereof include nitrogen-containing monofunctional vinyl monomers such as N-vinylcaprolactam, N-vinylformamide, N-vinylcarbazole, N-vinylacetamide, and N-vinylpyrrolidone; nitrogen-containing monofunctional acrylate monomers such as acryloylmorpholine; and nitrogen-containing monofunctional acrylamide monomers such as (meth)acrylamide, N-hydroxymethyl(meth)acrylamide, diacetone acrylamide, N,N-dimethyl(meth)acrylamide, and dimethylaminoethyl acrylate benzyl chloride quaternary salt.

[0046] Among these, either a nitrogen-containing monofunctional vinyl monomer or a nitrogen-containing monofunctional acrylate monomer is preferred, and a monomer having a nitrogen-containing heterocyclic structure such as N-vinylcaprolactam, N-vinylcarbazole, N-vinylpyrrolidone, or acryloylmorpholine is more preferred, and one containing acryloylmorpholine is even more preferred.

[0047] By using such nitrogen-containing monofunctional monomers, the scratch resistance of the clear ink coating film is improved. Furthermore, nitrogen-containing monofunctional acrylate monomers having a nitrogen-containing heterocyclic structure such as acryloylmorpholine improve the stretchability and adhesion of the clear ink coating film to the recording medium and the coating film of the color ink. In the following description, the adhesion to the recording medium and the coating film of other inks may be simply referred to as adhesion.

[0048] The content of the nitrogen-containing monofunctional monomer in the clear ink is preferably 2% by mass or more and 15% by mass or less, more preferably 3% by mass or more and 13% by mass or less, and even more preferably 4% by mass or more and 12% by mass or less, based on the total amount of the polymerizable compounds. This improves the abrasion resistance and adhesion of the clear ink coating.

[0049] The content of the nitrogen-containing monofunctional monomer contained in the clear ink is preferably 2% by mass or more and 15% by mass or less, more preferably 3% by mass or more and 12% by mass or less, and even more preferably 4% by mass or more and 11% by mass or less, relative to the total amount of the clear ink, which improves the abrasion resistance and adhesion of the clear ink coating.

[0050] 1.1.1.3.2. Monofunctional (meth)acrylates having a bridged condensed ring structure The bridged condensed ring structure in the monofunctional (meth)acrylate having a bridged condensed ring structure refers to a structure in which two or more ring structures share a side in a one-to-one relationship, and two or more atoms of the same ring structure or different ring structures that are not adjacent to each other are linked. Examples of the monofunctional (meth)acrylate having a bridged condensed ring structure include dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, and dicyclopentanyl (meth)acrylate. In addition, other bridged condensed ring structures include structures of the following general formulas (a) and (b).

[0051] [ka]

[0052] [ka]

[0053] Among these, it is more preferable to include dicyclopentenyl acrylate (DCPA), which corresponds to monomer B. By using such a monofunctional (meth)acrylate having a crosslinked condensed ring structure, the scratch resistance, stretchability, and adhesion of the clear ink coating film are further improved. Furthermore, the gloss of the clear ink coating film is further improved.

[0054] The content of the monofunctional (meth)acrylate having a crosslinked condensed ring structure in the clear ink is preferably 10% by mass or more and 65% by mass or less, more preferably 15% by mass or more and 63% by mass or less, and even more preferably 20% by mass or more and 60% by mass or less, based on the total amount of the polymerizable compounds. This improves the abrasion resistance and adhesion of the clear ink coating.

[0055] The content of the monofunctional (meth)acrylate having a crosslinked condensed ring structure in the clear ink is preferably 10% by mass or more and 60% by mass or less, more preferably 15% by mass or more and 57% by mass or less, and even more preferably 20% by mass or more and 55% by mass or less, based on the total amount of the clear ink. This improves the abrasion resistance and adhesion of the clear ink coating.

[0056] 1.1.1.3.3. Aromatic group-containing monofunctional monomers Examples of aromatic group-containing monofunctional monomers include, but are not limited to, phenoxyethyl (meth)acrylate, benzyl (meth)acrylate, alkoxylated 2-phenoxyethyl (meth)acrylate, ethoxylated nonylphenyl (meth)acrylate, alkoxylated nonylphenyl (meth)acrylate, p-cumylphenol EO-modified (meth)acrylate, and 2-hydroxy-3-phenoxypropyl (meth)acrylate.

[0057] Among these, phenoxyethyl (meth)acrylate and benzyl (meth)acrylate are preferred, phenoxyethyl (meth)acrylate is more preferred, and phenoxyethyl acrylate (PEA) is even more preferred. By using such aromatic group-containing monofunctional monomers, the solubility of the photopolymerization initiator is further improved, and the curing property of the clear ink is improved. In particular, it is suitable when an acylphosphine oxide-based photopolymerization initiator or a thioxanthone-based photopolymerization initiator is used as the photopolymerization initiator described later.

[0058] The content of aromatic group-containing monofunctional monomer in the clear ink is preferably 2% by mass or more and 15% by mass or less, more preferably 3% by mass or more and 13% by mass or less, and even more preferably 4% by mass or more and 12% by mass or less, based on the total amount of polymerizable compounds. This improves the adhesion and abrasion resistance of the clear ink coating.

[0059] The content of aromatic group-containing monofunctional monomer contained in the clear ink is preferably 2% by mass or more and 15% by mass or less, more preferably 3% by mass or more and 12% by mass or less, and even more preferably 4% by mass or more and 11% by mass or less, based on the total amount of the clear ink. This improves the adhesion and abrasion resistance of the clear ink coating.

[0060] 1.1.1.3.4. Monofunctional monomers containing saturated aliphatic groups The saturated aliphatic group-containing monofunctional monomer is not particularly limited, and examples thereof include alicyclic group-containing (meth)acrylates such as isobornyl (meth)acrylate, tert-butylcyclohexanol acrylate (TBCHA), and 2-(meth)acrylic acid-1,4-dioxaspiro[4,5]dec-2-ylmethyl; isoamyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, octyl (meth)acrylate, and decyl (meth)acrylate. Examples of the monomer include linear or branched aliphatic group-containing (meth)acrylates and lactone-modified flexible (meth)acrylates, such as isodecyl (meth)acrylate, isomyristyl (meth)acrylate, isostearyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, butoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, and 2-hydroxypropyl (meth)acrylate. In this specification, the saturated aliphatic group-containing monofunctional monomer is not a compound having a crosslinked condensed ring structure.

[0061] Among these, isobornyl (meth)acrylate and tert-butylcyclohexanol acrylate (TBCHA) are preferred, which correspond to monomer B. By using such a saturated aliphatic group-containing monofunctional monomer, the curability of the clear ink and the scratch resistance of the coating film are improved, and the gloss of the coating film of the clear ink is also improved.

[0062] The content of the saturated aliphatic group-containing monofunctional monomer in the clear ink is preferably 15% by mass or more and 85% by mass or less, more preferably 20% by mass or more and 80% by mass or less, and even more preferably 25% by mass or more and 75% by mass or less, based on the total amount of the polymerizable compounds. This improves the curability of the clear ink and the scratch resistance of the coating film.

[0063] The content of the saturated aliphatic group-containing monofunctional monomer contained in the clear ink is preferably 15% by mass or more and 80% by mass or less, more preferably 17% by mass or more and 75% by mass or less, and even more preferably 20% by mass or more and 70% by mass or less, based on the total amount of the clear ink. This improves the curability of the rear ink and the scratch resistance of the coating film.

[0064] 1.1.1.3.5. Other monofunctional monomers Examples of other monofunctional monomers other than those mentioned above include unsaturated carboxylic acids such as (meth)acrylic acid, itaconic acid, crotonic acid, isocrotonic acid, and maleic acid, salts of the unsaturated carboxylic acids, esters, urethanes, amides, and anhydrides of the unsaturated carboxylic acids, acrylonitrile, styrene, various unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes.

[0065] 1.1.1.4. Multifunctional monomers The polymerizable compound contained in the clear ink may contain a polyfunctional monomer. Examples of the polyfunctional monomer include vinyl ether group-containing (meth)acrylate and polyfunctional (meth)acrylate. In addition, the polyfunctional monomer may be one corresponding to Monomer B, such as dipropylene glycol diacrylate (DPGDA).

[0066] The content of the polyfunctional monomer contained in the clear ink is preferably 0.01% by mass or more and 20.00% by mass or less, more preferably 0.01% by mass or more and 15.00% by mass or less, and even more preferably 1.00% by mass or more and 15.00% by mass or less, based on the total amount of the polymerizable compounds. When the content of the polyfunctional monomer is 0.01% by mass or more, the curability of the clear ink and the scratch resistance of the coating film are improved. In addition, when the content of the polyfunctional monomer is 20.00% by mass or less, the stretchability and adhesion of the coating film of the clear ink are improved.

[0067] The content of the polyfunctional monomer contained in the clear ink is preferably 0.01% by mass or more and 15.00% by mass or less, more preferably 0.50% by mass or more and 15.00% by mass or less, and even more preferably 1.00% by mass or more and 15.00% by mass or less, based on the total amount of the clear ink. When the content of the polyfunctional monomer is 0.01% by mass or more, the curability and the scratch resistance of the coating film are improved. In addition, when the content of the polyfunctional monomer is 15.00% by mass or less, the stretchability and adhesion of the coating film tend to be further improved.

[0068] Specific examples of the polyfunctional monomer are shown below, but the polyfunctional monomer in the present embodiment is not limited to the following.

[0069] 1.1.1.4.1. (Meth)acrylates containing vinyl ether groups The vinyl ether group-containing (meth)acrylate is not particularly limited, but may be, for example, a compound represented by the following general formula (c). By including such a vinyl ether group-containing (meth)acrylate, the viscosity of the clear ink is reduced, and the ejection stability is improved. In addition, the curing property of the clear ink is improved, and the improved curing property allows the recording speed on the recording medium to be increased. CH 2 =CR 1 -COOR 2 -O-CH=CH-R 3 (c) (In the formula, R 1is a hydrogen atom or a methyl group, and R 2 is a divalent organic residue having 2 to 20 carbon atoms, and R 3 is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms.

[0070] In the above general formula (c), R 2 Examples of the divalent organic residue having 2 to 20 carbon atoms and represented by the formula (I) include linear, branched or cyclic alkylene groups having 2 to 20 carbon atoms which may be substituted, alkylene groups having 2 to 20 carbon atoms which have an oxygen atom in the structure via an ether bond and / or an ester bond and which may be substituted, and divalent aromatic groups having 6 to 11 carbon atoms which may be substituted.

[0071] Among these, alkylene groups having 2 to 6 carbon atoms, such as an ethylene group, an n-propylene group, an isopropylene group, and a butylene group, and alkylene groups having 2 to 9 carbon atoms and an oxygen atom due to an ether bond in the structure, such as an oxyethylene group, an oxy n-propylene group, an oxyisopropylene group, and an oxybutylene group, are preferred. Furthermore, from the viewpoint of reducing the viscosity of the clear ink and improving the curing property of the clear ink, R 2 More preferred are those having a glycol ether chain, which is an alkylene group having 2 to 9 carbon atoms and having an oxygen atom by an ether bond in the structure, such as an oxyethylene group, an oxy-n-propylene group, an oxyisopropylene group, and an oxybutylene group.

[0072] In the general formula (c), R 3 Preferred examples of the monovalent organic residue having 1 to 11 carbon atoms and represented by the formula (I) are linear, branched or cyclic alkyl groups having 1 to 10 carbon atoms which may be substituted, and aromatic groups having 6 to 11 carbon atoms which may be substituted. Among these, alkyl groups having 1 to 2 carbon atoms, such as a methyl group or an ethyl group, and aromatic groups having 6 to 8 carbon atoms, such as a phenyl group and a benzyl group, are preferably used.

[0073] When each of the organic residues is a group that may be substituted, the substituent is divided into a group that contains carbon atoms and a group that does not contain carbon atoms.First, when the substituent is a group that contains carbon atoms, the carbon atom is counted in the number of carbon atoms of the organic residue.The group that contains carbon atoms includes, but is not limited to, a carboxyl group and an alkoxy group.The group that does not contain carbon atoms includes, but is not limited to, a hydroxyl group and a halo group.

[0074] Specific examples of the compound of general formula (c) include, but are not limited to, 2-vinyloxyethyl (meth)acrylate, 3-vinyloxypropyl (meth)acrylate, 1-methyl-2-vinyloxyethyl (meth)acrylate, 2-vinyloxypropyl (meth)acrylate, 4-vinyloxybutyl (meth)acrylate, 1-methyl-3-vinyloxypropyl (meth)acrylate, 1-vinyloxymethylpropyl (meth)acrylate, 2-methyl-3-vinyloxypropyl (meth)acrylate, 1,1-dimethyl-2-vinyloxyethyl (meth)acrylate, 3-vinyloxybutyl (meth)acrylate, 1-methyl-2-vinyloxypropyl (meth)acrylate, 2-vinyloxybutyl (meth)acrylate, 4-vinyloxycyclohexyl (meth)acrylate, 6-vinyloxyhexyl (meth)acrylate, 4-vinyloxymethylcyclohexylmethyl (meth)acrylate, 3-vinyloxymethylcyclohexylmethyl (meth)acrylate, 2-vinyloxymethylcyclohexylmethyl (meth)acrylate, p-vinyloxymethylphenylmethyl (meth)acrylate, m-vinyloxymethylphenyl (meth)acrylate ethyl methyl, o-vinyloxymethylphenylmethyl (meth)acrylate, o-vinyloxymethylphenylmethyl (meth)acrylate, 2-(2-vinyloxyethoxy)ethyl (meth)acrylate, 2-(2-vinyloxyethoxy)ethyl acrylate, 2-(vinyloxyisopropoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxy)propyl (meth)acrylate, 2-(vinyloxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxy)propyl (meth)acrylate, 2-(vinyloxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyisopropoxy)ethyl (meth)acrylate, 2-(vinyloxyisopropoxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyisopropoxyisopropoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyethoxy)propyl (meth)acrylate, 2-(vinyloxyethoxyisopropoxy)propyl (meth)acrylate, 2-(vinyloxyisopropoxyethoxy)propyl (meth)acrylate, 2-(vinyloxyisopropoxyisopropoxy)propyl (meth)acrylate,2-(vinyloxyethoxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxy)ethyl (meth)acrylate acrylate, 2-(isopropenoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, polyethylene glycol monovinyl ether (meth)acrylate, and polypropylene glycol monovinyl ether (meth)acrylate. Of these specific examples, 2-(2-vinyloxyethoxy)ethyl acrylate is particularly preferred because it is easy to balance the curability and viscosity of the clear ink.

[0075] The content of the vinyl ether group-containing (meth)acrylate in the clear ink is preferably from 1% to 10% by mass, more preferably from 2% to 8% by mass, and even more preferably from 2% to 6% by mass, relative to the total amount of the polymerizable compounds. This reduces the viscosity of the clear ink, improving the ejection stability of the clear ink from the inkjet head.

[0076] The content of the vinyl ether group-containing (meth)acrylate in the clear ink is preferably 1% by mass or more and 10% by mass or less, more preferably 1% by mass or more and 8% by mass or less, and even more preferably 2% by mass or more and 6% by mass or less, based on the total amount of the clear ink. This reduces the viscosity of the clear ink and improves the ejection stability from the inkjet head.

[0077] 1.1.1.4.2. Multifunctional (meth)acrylates The polyfunctional (meth)acrylate is not particularly limited, and examples thereof include the above-mentioned dipropylene glycol diacrylate (DPGDA), diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, dipropylene glycol dimethacrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, dimethylol-tricyclodecane di(meth)acrylate, EO (ethylene oxide) adduct di(meth)acrylate of bisphenol A, PO(propylene glycol) di(meth)acrylate of bisphenol A, and the like. Examples of the polyfunctional (meth)acrylate include bifunctional (meth)acrylates such as hydroxypivalic acid neopentyl glycol di(meth)acrylate, and polytetramethylene glycol di(meth)acrylate, and polyfunctional (meth)acrylates having three or more functional groups such as trimethylolpropane tri(meth)acrylate, EO-modified trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, glycerin propoxy tri(meth)acrylate, caprolactone-modified trimethylolpropane tri(meth)acrylate, pentaerythritol ethoxy tetra(meth)acrylate, and caprolactam-modified dipentaerythritol hexa(meth)acrylate.

[0078] The content of the polyfunctional (meth)acrylate contained in the clear ink is preferably 1% by mass or more and 20% by mass or less, more preferably 1% by mass or more and 17% by mass or less, and even more preferably 2% by mass or more and 15% by mass or less, based on the total amount of the polymerizable compounds. When the content of the polyfunctional (meth)acrylate is 1% by mass or more, the scratch resistance of the clear ink coating is improved. When the content of the polyfunctional (meth)acrylate is 20% by mass or less, the stretchability and adhesion of the clear ink coating are improved.

[0079] The content of the polyfunctional (meth)acrylate contained in the clear ink is preferably 1% by mass or more and 20% by mass or less, more preferably 1% by mass or more and 17% by mass or less, and even more preferably 2% by mass or more and 15% by mass or less, based on the total amount of the clear ink. When the content of the polyfunctional (meth)acrylate is 1% by mass or more, based on the total amount of the clear ink, the scratch resistance of the coating film of the clear ink is improved. When the content of the polyfunctional (meth)acrylate is 20% by mass or less, based on the total amount of the clear ink, the stretchability and adhesion of the coating film of the clear ink are improved.

[0080] 1.1.2. Photopolymerization initiator The photopolymerization initiator generates active species by irradiation with radiation, and has the function of promoting the polymerization reaction of the polymerizable compound by the active species. Specifically, the active species generated from the photopolymerization initiator are radicals, acids, bases, etc. The photopolymerization initiator is not particularly limited as long as it has the above-mentioned function, and examples thereof include known photopolymerization initiators such as acylphosphine oxide-based photopolymerization initiators, alkylphenone-based photopolymerization initiators, titanocene-based photopolymerization initiators, and thioxanthone-based photopolymerization initiators. Among these, acylphosphine oxide-based photopolymerization initiators are preferred. By using such a photopolymerization initiator, the curing property of the clear ink, particularly the curing property in the curing process by the light of UV-LED (ultraviolet light-emitting diode), is improved. The photopolymerization initiator may be used alone or in combination of two or more types.

[0081] The acylphosphine oxide photopolymerization initiator is not particularly limited, but examples thereof include 2,4,6-trimethylbenzoyldiphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, and bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.

[0082] Examples of commercially available products of such acylphosphine oxide photopolymerization initiators include IRGACURE (registered trademark) 819 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), IRGACURE 1800 (a mixture of bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide and 1-hydroxy-cyclohexyl-phenyl ketone in a mass ratio of 25:75), and IRGACURE TPO (2,4,6-trimethylbenzoyldiphenylphosphine oxide), all of which are manufactured by BASF.

[0083] The content of the photopolymerization initiator in the clear ink is preferably 3% by mass or more and 12% by mass or less, more preferably 5% by mass or more and 10% by mass or less, and even more preferably 7% by mass or more and 9% by mass or less, relative to the total amount of the clear ink. This improves the curing property of the clear ink and ensures the solubility of the photopolymerization initiator.

[0084] 1.1.3. Other additives The clear ink may further contain other additives, such as a polymerization inhibitor, a slip agent, and a photosensitizer, as necessary.

[0085] 1.1.3.1. Polymerization inhibitors The polymerization inhibitor has a function of suppressing the progress of unintended polymerization reactions of the polymerizable compound during storage, etc., and improving the storage stability of the clear ink. One type of polymerization inhibitor may be used alone, or two or more types may be used in combination.

[0086] The polymerization inhibitor is not particularly limited, but examples thereof include 4-methoxyphenol (MEHQ), 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl, hydroquinone, cresol, t-butylcatechol, 3,5-di-t-butyl-4-hydroxytoluene, 2,2'-methylenebis(4-methyl-6-t-butylphenol), 2,2'-methylenebis(4-ethyl-6-butylphenol), and 4,4'-thiobis(3-methyl-6-t-butylphenol), and hindered amine compounds.

[0087] The content of the polymerization inhibitor in the clear ink is preferably 0.05% by mass or more and 1.00% by mass or less, and more preferably 0.05% by mass or more and 0.50% by mass or less, relative to the total amount of the clear ink.

[0088] 1.1.3.2. Slip agents The slip agent has a function of improving the scratch resistance of the coating film of the clear ink. The slip agent may be used alone or in combination of two or more kinds.

[0089] The slip agent is preferably a silicone surfactant, more preferably a polyester-modified silicone or a polyether-modified silicone. Commercially available products can be used as the slip agent, such as polyester-modified silicones such as BYK (registered trademark)-347, -348, BYK-UV3500, -3510, and -3530, and polyether-modified silicones such as BYK-3570, all of which are manufactured by BYK Additives & Instruments.

[0090] The content of the slip agent in the clear ink is preferably 0.01% by mass or more and 2.00% by mass or less, and more preferably 0.05% by mass or more and 1.00% by mass or less, relative to the total amount of the clear ink.

[0091] Photosensitizers The photosensitizer absorbs radiation to become excited and has the function of promoting the generation of active species from the photopolymerization initiator. The photosensitizer may be used alone or in combination of two or more kinds.

[0092] Examples of photosensitizers include aliphatic amines, amines containing aromatic groups, piperidine, reaction products of epoxy resins and amines, and amine compounds such as triethanolamine triacrylate, urea compounds such as allylthiourea and o-tolylthiourea, sulfur compounds such as sodium diethyldithiophosphate and soluble salts of aromatic sulfinic acids, nitrile compounds such as N,N-diethyl-p-aminobenzonitrile, phosphorus compounds such as tri-n-butylphosphine and sodium diethyldithiophosphide, nitrogen compounds such as Michler's ketone, N-nitrisohydroxylamine derivatives, oxazolidine compounds, tetrahydro-1,3-oxazine compounds, and condensates of formaldehyde or acetaldehyde with diamines, and chlorine compounds such as carbon tetrachloride and hexachloroethane.

[0093] When a photosensitizer is used in the clear ink, the content thereof is preferably 0.5% by mass or more and 3.0% by mass or less relative to the total amount of the clear ink.

[0094] 1.2.Color ink The color ink according to the present embodiment contains a polymerizable compound, a photopolymerization initiator, and a coloring material. The color ink is used for coloring a recording medium or forming an image or the like on a recording medium. Components contained in the color ink according to the present embodiment will be described below.

[0095] 1.2.1. Polymerizable compounds The polymerizable compound includes a monofunctional monomer having one polymerizable functional group, a polyfunctional monomer having multiple polymerizable functional groups, and an oligomer having one or more polymerizable functional groups. Each of these polymerizable compounds may be used alone or in combination of two or more.

[0096] The weighted average of the glass transition temperatures of the homopolymers in the polymerizable compounds, weighted by the mass ratio of each content of the polymerizable compounds contained in the color ink, is 48° C. or higher. The weighted average of the glass transition temperatures is preferably 70° C. or higher, more preferably 85° C. or higher. By having the weighted average of the glass transition temperatures be 48° C. or higher, the scratch resistance of the coating film in a room temperature environment can be improved. The upper limit of the weighted average of the glass transition temperatures is not particularly limited, but is preferably 140° C. or lower, more preferably 120° C. or lower, and even more preferably 110° C. or lower.

[0097] The method for calculating the weighted average of the glass transition temperatures is the same as that described for the clear ink. The weighted average of the glass transition temperatures can be adjusted by the glass transition temperatures and the mass ratios of the polymerizable compounds used in the color inks.

[0098] 1.2.1.1. Monofunctional monomers The polymerizable compound contained in the color ink includes a monofunctional monomer. The monofunctional monomer is not particularly limited, but examples thereof include a nitrogen-containing monofunctional monomer, a monofunctional (meth)acrylate having a crosslinked condensed ring structure, an aromatic group-containing monofunctional monomer, and a saturated aliphatic group-containing monofunctional monomer. If necessary, other monofunctional monomers may be used in place of or in addition to the above monofunctional monomers. The polymerizable functional group of the monofunctional monomer is not particularly limited, but a known polymerizable functional group, particularly a polymerizable functional group having a carbon-carbon unsaturated double bond, may be used.

[0099] The content of the monofunctional monomer contained in the color ink is 85% by mass or more, preferably 90% by mass or more, based on the total amount of the polymerizable compounds contained in the color ink. This makes the coating film of the color ink flexible and improves the stretchability. In addition, when the content of the monofunctional monomer contained in the color ink is 85% by mass or more, the gloss of conventional clear inks tends to decrease, so the present invention is particularly effective.

[0100] The upper limit of the content of the monofunctional monomer is not particularly limited, but is preferably 99% by mass or less, more preferably 98% by mass or less, and even more preferably 97% by mass or less, based on the total amount of the polymerizable compounds contained in the color ink. This can improve the scratch resistance of the coating film of the color ink.

[0101] The content of the monofunctional monomer contained in the color ink is preferably 65% ​​by mass or more, more preferably 70% by mass or more, and even more preferably 75% by mass or more, based on the total amount of the color ink. This improves the extensibility of the coating film of the color ink. The upper limit of the content of the monofunctional monomer is preferably 97% by mass or less, more preferably 95% by mass or less, and even more preferably 90% by mass or less, based on the total amount of the color ink. This improves the curability of the color ink.

[0102] The nitrogen-containing monofunctional monomer, the monofunctional (meth)acrylate having a crosslinked condensed ring structure, the aromatic group-containing monofunctional monomer, the saturated aliphatic group-containing monofunctional monomer, and other monofunctional monomers contained in the color inks may be the same as those in the clear ink. The monofunctional monomers contained in the color inks may include those corresponding to the above-mentioned monomer A or monomer B.

[0103] The content of the nitrogen-containing monofunctional monomer in the color ink is preferably 3% by mass or more and 17% by mass or less, more preferably 5% by mass or more and 15% by mass or less, and even more preferably 8% by mass or more and 12% by mass or less, based on the total amount of the polymerizable compounds. This improves the abrasion resistance and adhesion of the coating film of the color ink.

[0104] The content of the nitrogen-containing monofunctional monomer contained in the color ink is preferably 3% by mass or more and 17% by mass or less, more preferably 5% by mass or more and 15% by mass or less, and even more preferably 8% by mass or more and 12% by mass or less, based on the total amount of the color ink. This improves the abrasion resistance and adhesion of the coating film of the color ink.

[0105] The content of the monofunctional (meth)acrylate having a crosslinked condensed ring structure contained in the color ink is preferably 20% by mass or more and 55% by mass or less, more preferably 25% by mass or more and 50% by mass or less, and even more preferably 30% by mass or more and 45% by mass or less, based on the total amount of the polymerizable compounds. This improves the adhesion and abrasion resistance of the coating film of the color ink.

[0106] The content of the monofunctional (meth)acrylate having a crosslinked condensed ring structure contained in the color ink is preferably 15% by mass or more and 50% by mass or less, more preferably 20% by mass or more and 45% by mass or less, and even more preferably 25% by mass or more and 40% by mass or less, based on the total amount of the color ink. This improves the adhesion and abrasion resistance of the coating film of the color ink.

[0107] The content of the aromatic group-containing monofunctional monomer in the color ink is preferably 2% by mass or more and 12% by mass or less, more preferably 3% by mass or more and 10% by mass or less, and even more preferably 4% by mass or more and 7% by mass or less, based on the total amount of the polymerizable compounds. This tends to further improve the adhesion and abrasion resistance of the coating film of the color ink.

[0108] The content of the aromatic group-containing monofunctional monomer contained in the color ink is preferably 2% by mass or more and 12% by mass or less, more preferably 3% by mass or more and 10% by mass or less, and even more preferably 4% by mass or more and 7% by mass, based on the total amount of the color ink. This improves the adhesion and abrasion resistance of the coating film of the color ink.

[0109] The content of the saturated aliphatic group-containing monofunctional monomer in the color ink is preferably 15% by mass or more and 45% by mass or less, more preferably 20% by mass or more and 40% by mass or less, and even more preferably 25% by mass or more and 35% by mass or less, based on the total amount of the polymerizable compounds. This improves the curability of the color ink.

[0110] The content of the saturated aliphatic group-containing monofunctional monomer contained in the color ink is preferably 15% by mass or more and 40% by mass or less, more preferably 20% by mass or more and 35% by mass or less, and even more preferably 25% by mass or more and 30% by mass or less, based on the total amount of the color ink. This improves the curability of the color ink.

[0111] 1.2.1.2. Multifunctional monomers The polymerizable compound contained in the color ink preferably contains a polyfunctional monomer. Examples of the polyfunctional monomer include vinyl ether group-containing (meth)acrylate and polyfunctional (meth)acrylate. The polyfunctional monomer is not limited to the above. The polyfunctional monomer may be the same as that of the clear ink. The polyfunctional monomer contained in the color ink may include the monomer A or monomer B described above.

[0112] The content of the polyfunctional monomer contained in the color ink is preferably 0.01% by mass or more and 20.00% by mass or less, more preferably 0.01% by mass or more and 15.00% by mass or less, and even more preferably 1.00% by mass or more and 15.00% by mass or less, based on the total amount of the polymerizable compound. When the content of the polyfunctional monomer is 0.01% by mass or more, the abrasion resistance of the coating film of the color ink is improved. In addition, when the content of the polyfunctional monomer is 20.00% by mass or less, the stretchability and adhesion of the coating film tend to be further improved.

[0113] The content of the polyfunctional monomer contained in the color ink is preferably 0.01% by mass or more and 15.00% by mass or less, more preferably 0.50% by mass or more and 15.00% by mass or less, and even more preferably 1.00% by mass or more and 15.00% by mass or less, based on the total amount of the color ink. When the content of the polyfunctional monomer is 0.01% by mass or more, the abrasion resistance of the coating film of the color ink is improved. In addition, when the content of the polyfunctional monomer is 15.00% by mass or less, the stretchability and adhesion of the coating film of the color ink are improved.

[0114] The content of the vinyl ether group-containing (meth)acrylate in the color ink is preferably 1% by mass or more and 10% by mass or less, more preferably 2% by mass or more and 8% by mass or less, and even more preferably 2% by mass or more and 6% by mass or less, based on the total amount of the polymerizable compounds. This reduces the viscosity of the color ink, improving the ejection stability of the color ink from the inkjet head.

[0115] The content of the vinyl ether group-containing (meth)acrylate contained in the color ink is preferably 1% by mass or more and 10% by mass or less, more preferably 1% by mass or more and 8% by mass or less, and even more preferably 2% by mass or more and 6% by mass or less, based on the total amount of the color ink. This reduces the viscosity of the color ink, improving the ejection stability of the color ink from the inkjet head.

[0116] The content of the polyfunctional (meth)acrylate contained in the color ink is preferably 1% by mass or more and 20% by mass or less, more preferably 1% by mass or more and 17% by mass or less, and even more preferably 2% by mass or more and 15% by mass or less, based on the total amount of the polymerizable compound. When the content of the polyfunctional (meth)acrylate is 1% by mass or more, the abrasion resistance of the coating film of the color ink is improved. In addition, when the content of the polyfunctional (meth)acrylate is 20% by mass or less, the stretchability and adhesion of the coating film of the color ink are improved.

[0117] The content of the polyfunctional (meth)acrylate contained in the color ink is preferably 1% by mass or more and 20% by mass or less, more preferably 1% by mass or more and 17% by mass or less, and even more preferably 2% by mass or more and 15% by mass or less, based on the total amount of the color ink. When the content of the polyfunctional (meth)acrylate is 1% by mass or more, the abrasion resistance of the coating film of the color ink is improved. In addition, when the content of the polyfunctional (meth)acrylate is 20% by mass or less, the stretchability and adhesion of the coating film of the color ink are improved.

[0118] 1.2.2. Photoinitiator The photopolymerization initiator contained in the color ink may be the same as that in the clear ink. The content of the photopolymerization initiator contained in the color ink is preferably 3% by mass or more and 12% by mass or less, more preferably 5% by mass or more and 10% by mass or less, and even more preferably 7% by mass or more and 9% by mass or less, based on the total amount of the color ink. This improves the curability of the color ink and ensures the solubility of the photopolymerization initiator.

[0119] 1.2.3.Colorants The coloring material has a function of coloring the coating film formed by the color ink. By coloring the coating film of the color ink, it becomes possible to color a recording medium or form a color image on the recording medium. Examples of the coloring material contained in the color ink include pigments and dyes.

[0120] 1.2.3.1. Pigments By using a pigment as the coloring material, the light resistance of the coloring material in the coating film of the color ink can be improved. Either an inorganic pigment or an organic pigment can be used as the pigment. One type of pigment may be used alone, or two or more types may be used in combination.

[0121] As the pigment, any of known organic pigments and inorganic pigments can be used. As the organic pigment, for example, azo pigments such as azo lake pigments, insoluble azo pigments, condensed azo pigments, and chelate azo pigments, phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, isoindoline pigments, quinophthalone pigments, and polycyclic pigments such as diketopyrrolopyrrole pigments, dye lake pigments such as basic dye lakes and acid dye lakes, nitro pigments, nitroso pigments, aniline black, and daylight fluorescent pigments can be used. As the inorganic pigment, for example, metal oxide pigments such as titanium dioxide, zinc oxide, and chromium oxide, and carbon black can be used. In addition, luster pigments such as pearl pigments and metallic pigments can be used.

[0122] Examples of black pigments include CI (Colour Index Generic Name) Pigment Black 1, 7, and 11. Among these, it is preferable to use carbon black, which is CI Pigment Black 7.

[0123] Specific examples of carbon black include Mitsubishi Chemical's No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52, MA7, MA8, MA100, and No. 2200B; Columbia Carbon's Raven (registered trademark) 5750, 5250, 5000, 3500, 1255, and 700; CABOT's Rega1 (registered trademark) 400R, 330R, and 660R, Mogul (registered trademark) L, and Monarch (registered trademark) 700, 800, 880, 900, 1000, 1100, 1300, and 1400; and Degussa's Color Black FW1, FW2, FW2V, FW18, FW200, S150, S160, and S170, Printex (registered trademark) 35, U, V, and 140U, and Special Black. 6, 5, 4A, and 4 are examples.

[0124] Examples of white pigments include CI Pigment White 6, 18, and 21.

[0125] Examples of yellow pigments include CI Pigment Yellow 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 16, 17, 24, 34, 35, 37, 53, 55, 65, 73, 74, 75, 81, 83, 93, 94, 95, 97, 98, 99, 108, 109, 110, 113, 114, 117, 120, 124, 128, 129, 133, 138, 139, 147, 151, 153, 154, 155, 167, 172, and 180.

[0126] Examples of magenta pigments include CI Pigment Red 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 19, 21, 22, 23, 30, 31, 32, 37, 38, 40, 41, 42, 48(Ca), 48(Mn), 57(Ca), 57:1, 88, 112, 114, 122, 123, 144, 146, 149, 150, 166, 168, 170, 171, 175, 176, 177, 178, 179, 184, 185, 187, 202, 209, 219, 224, 245, and CI Pigment Violet. 19, 23, 32, 33, 36, 38, 43, and 50 are listed.

[0127] Examples of cyan pigments include CI Pigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:34, 15:4, 16, 18, 22, 25, 60, 65, and 66, and CI Vat Blue 4 and 60.

[0128] Examples of pigments for colors other than those mentioned above include CI Pigment Green 7 and 10, CI Pigment Brown 3, 5, 25 and 26, and CI Pigment Orange 1, 2, 5, 7, 13, 14, 15, 16, 24, 34, 36, 38, 40, 43 and 63.

[0129] The above pigments may be used alone or in combination of two or more kinds. When a pigment is used, a dispersant, which will be described later, may be used in combination. The average particle diameter of the pigment is preferably 300 nm or less, and more preferably 50 nm or more and 200 nm or less. When the average particle diameter of the pigment is within the above range, the discharge stability and dispersion stability of the radiation-curable inkjet ink as a color ink are improved, and the image formed on the recording medium can have excellent quality. The average particle diameter here refers to the volume-based particle size distribution (50%) measured by dynamic light scattering.

[0130] The content of the pigment in the color ink is, relative to the total amount of the color ink, more than 0.2% by mass and not more than 20% by mass, preferably 1% by mass or more and not more than 20% by mass, more preferably 1% by mass or more and not more than 15% by mass, and even more preferably 1% by mass or more and not more than 10% by mass.

[0131] 1.2.3.2.Dye A dye may be used as the coloring material. The dye is not particularly limited, but acid dyes, direct dyes, reactive dyes, and basic dyes can be used. One type of dye may be used alone, or two or more types may be used in combination.

[0132] The dye is not particularly limited, and examples thereof include CI Acid Yellow 17, 23, 42, 44, 79, 142, CI Acid Red 52, 80, 82, 249, 254, 289, CI Acid Blue 9, 45, 249, CI Acid Black 1, 2, 24, 94, CI Food Black 1, 2, CI Direct Yellow 1, 12, 24, 33, 50, 55, 58, 86, 132, 142, 144, 173, CI Direct Red 1, 4, 9, 80, 81, 225, 227, CI Direct Blue 1, 2, 15, 71, 86, 87, 98, 165, 199, 202, CI Direct Black 19, 38, 51, 71, 154, 168, 171, 195, and CI Reactive Red. 14, 32, 55, 79, 249, and CI Reactive Black 3, 4, and 35.

[0133] When a dye is used in the color ink, the content of the dye relative to the total amount of the color ink is more than 0.2 mass % and not more than 20.0 mass %, preferably 1.0 mass % or more and 20.0 mass % or less, more preferably 1.0 mass % or more and 15.0 mass % or less, and even more preferably 1.0 mass % or more and 10.0 mass % or less.

[0134] 1.2.4. Other additives The clear ink may further contain other additives, such as a dispersant, a polymerization inhibitor, a slip agent, and a photosensitizer, as necessary.

[0135] Dispersants Dispersants have the function of imparting dispersibility to color materials such as pigments in color inks. By using a dispersant, pigments and the like are stably dispersed in the color inks, and sedimentation resistance during storage and ejection stability from an inkjet head are improved.

[0136] The dispersant is not particularly limited, but may be a known dispersant commonly used for preparing a pigment dispersion, such as a polymer dispersant.Specific examples of the dispersant include polyoxyalkylene polyalkylene polyamine, vinyl polymer and copolymer, acrylic polymer and copolymer, polyester, polyamide, polyimide, polyurethane, amino polymer, silicon-containing polymer, sulfur-containing polymer, fluorine-containing polymer, and epoxy resin.The dispersant may be used alone or in combination of two or more.

[0137] As the polymer dispersant, commercially available products may be used, and examples thereof include the Ajisper (registered trademark) series from Ajinomoto Fine-Techno Co., Ltd., the Solsperse (registered trademark) series such as Solsperse 36000 from Lubrizol Corporation, the Disperbyk series from BYK Additives & Instruments, and the Disparlon (registered trademark) series from Kusumoto Chemicals Co., Ltd.

[0138] 1.2.4.2. Polymerization inhibitors The polymerization inhibitor may be the same as that used in the clear ink. The polymerization inhibitor may be used alone or in combination of two or more. The content of the polymerization inhibitor contained in the color ink is preferably 0.05% by mass or more and 1.00% by mass or less, more preferably 0.05% by mass or more and 0.50% by mass or less, based on the total amount of the color ink.

[0139] 1.2.4.3. Slip agents The slip agent may be the same as that used in the clear ink. The slip agent may be used alone or in combination of two or more. The content of the slip agent in the color ink is preferably 0.01% by mass or more and 2.00% by mass or less, more preferably 0.05% by mass or more and 1.00% by mass or less, based on the total amount of the color ink.

[0140] Photosensitizers The photosensitizer may be the same as that used in the clear ink. The photosensitizer may be used alone or in combination of two or more. When the photosensitizer is used in the color ink, the content is preferably 0.01% by mass or more and 5.00% by mass or less with respect to the total amount of the color ink.

[0141] 1.3.How to adjust the clear ink and color ink In preparing the clear ink and the color ink, the above-mentioned components are mixed and thoroughly stirred so that the components are uniformly mixed. In this embodiment, in the preparation process, at least one of ultrasonic treatment and heating treatment is applied to a mixture of a photopolymerization initiator and at least a part of a polymerizable compound. This reduces dissolved oxygen in the prepared color ink and clear ink, improving ejection stability and storage stability. Here, in the following description, the color ink and the clear ink are sometimes collectively referred to as simply ink.

[0142] The mixture may contain at least a photopolymerization initiator and a portion of the polymerizable compound, and may further contain other components, or may contain all of the components of the ink.

[0143] 1.4.Ink properties The viscosity of the ink at 20°C is preferably 22 mPa·s (millipascal seconds) or more and 28 mPa·s or less, more preferably 22 mPa·s or more and 26 mPa·s or less, and even more preferably 22 mPa·s or more and 24 mPa·s or less. This allows an appropriate amount of ink to be ejected from the inkjet head, and prevents the ink droplets from deflecting or scattering. The ink viscosity can be measured using a viscoelasticity tester MCR-300 made by Pysica at a temperature of 20°C by increasing the shear rate from 10 to 1000 and reading the viscosity at a shear rate of 200.

[0144] The surface tension of the ink at 20°C is preferably 20 mN / m or more and 40 mN / m or less. This makes it difficult for the ink to wet the nozzle surface of the inkjet head that has been treated to be liquid repellent. This allows the ink to be ejected normally and in an appropriate amount from the inkjet head, preventing the ink droplets from deflecting or scattering. The surface tension of the ink can be measured by checking the surface tension when a platinum plate is wetted with the ink in an environment of 20°C using an automatic surface tensiometer CBVP-Z manufactured by Kyowa Interface Science Co., Ltd.

[0145] 2.1. Inkjet recording device Next, a serial type inkjet printer will be exemplified as an inkjet recording device used in the inkjet method of this embodiment described later. Fig. 1 is a schematic perspective view showing the configuration of an inkjet printer according to an embodiment. Note that in Fig. 1, the scale of each component is different from the actual scale so that each component is large enough to be recognizable.

[0146] The inkjet printer 1 of this embodiment is what is known as a serial type printer. A serial printer is a printer in which an inkjet head is mounted on a carriage that moves in a specific direction, and printing is performed while the inkjet head moves in conjunction with the movement of the carriage. Hereinafter, the inkjet printer 1 will also be referred to simply as the printer 1.

[0147] 1, the printer 1 has an inkjet head 3, a carriage 4, a main scanning mechanism 5, a platen roller 6, a light source 11, and a control unit (not shown) that controls the overall operation of the printer 1. The carriage 4 mounts the inkjet head 3 and the light source 11, and is capable of detachably mounting ink cartridges 7a, 7b, 7c, 7d, and 7e as containers that contain ink to be supplied to the inkjet head 3.

[0148] The main scanning mechanism 5 has a timing belt 8 connected to the carriage 4, a motor 9 that drives the timing belt 8, and a guide shaft 10. The guide shaft 10, as a support member for the carriage 4, is installed in the main scanning direction, which is the scanning direction of the carriage 4. The carriage 4 is driven by the motor 9 via the timing belt 8, and is capable of reciprocating along the guide shaft 10. In this way, the main scanning mechanism 5 moves the carriage 4 back and forth in the main scanning direction.

[0149] The platen roller 6 transports the recording medium 2, to which the ink is to be applied, in a sub-scanning direction perpendicular to the main scanning direction. Therefore, the recording medium 2 is transported in the sub-scanning direction, which is approximately the same as the length direction of the recording medium 2. The carriage 4 is capable of reciprocating in the main scanning direction, which is approximately the same as the width direction of the recording medium 2. Therefore, the inkjet head 3 and the light source 11 are capable of scanning relatively to the recording medium 2 in the main scanning direction and the sub-scanning direction.

[0150] The ink cartridges 7a, 7b, 7c, 7d, and 7e are five independent ink cartridges. The ink cartridges 7a, 7b, 7c, 7d, and 7e contain the above-mentioned color inks and clear ink. For example, the ink cartridges 7a, 7b, 7c, 7d, and 7e can contain color inks of colors such as black, cyan, magenta, and yellow, and clear ink separately to be used as an ink set. In FIG. 1, the number of ink cartridges is five, but is not limited to this. The bottoms of the ink cartridges 7a, 7b, 7c, 7d, and 7e are provided with supply ports (not shown) for supplying the contained ink to the inkjet head 3.

[0151] The components of the ink cartridges 7a to 7e are made of materials that are resistant to evaporation and deterioration of the ink components, deterioration of the components due to the ink components, etc. In this embodiment, an ink cartridge is exemplified as the ink container, but the ink container is not limited to this, and other than the ink cartridge, an ink pack, an ink bottle, etc. may be used as the ink container.

[0152] An inkjet head 3 and a light source 11 are disposed on the side of the carriage 4 facing the recording medium 2. The inkjet head 3 has a nozzle surface (not shown) on the surface facing the recording medium 2. A polymer film containing a fluorine compound and a silicone compound, or a co-deposition plating film containing nickel and a fluorine compound may be formed as a liquid-repellent film on the nozzle surface. Although not shown, a nozzle row consisting of a plurality of ejection nozzles is disposed on the nozzle surface individually corresponding to each ink. Each ink is supplied to the inkjet head 3 from the ink cartridges 7a, 7b, 7c, 7d, and 7e, respectively, and ejected as droplets from the ejection nozzles by an actuator (not shown) in the inkjet head 3. The ejected ink droplets land on and adhere to the recording medium 2.

[0153] In this embodiment, the printer 1 is provided with one inkjet head 3 that ejects color inks and clear ink, but the printer 1 is not limited to this. The printer 1 may also be provided with separate inkjet heads that eject color inks and inkjet heads that eject clear ink.

[0154] In the inkjet head 3, a piezoelectric element is used as an actuator, which is a driving means, but is not limited to this. For example, an electromechanical conversion element that displaces a vibration plate as an actuator by electrostatic adsorption, or an electrothermal conversion element that ejects ink as droplets by bubbles generated by heating may be used as the driving means.

[0155] The light source 11 is arranged alongside the inkjet head 3 in the main scanning direction. The light source 11 is a radiation emitting device, and includes a light emitting element (not shown), such as a UV-LED (ultraviolet light emitting diode). The radiation emitted from the light source 11 is not limited to ultraviolet light, and may be infrared light, electron beams, visible light, X-rays, or the like. Here, a lamp or the like may be used as the light source 11 instead of a light emitting element such as an LED (light emitting diode) or an LD (semiconductor laser), but it is preferable to use a light emitting element from the viewpoint of miniaturization, cost reduction, and the like.

[0156] When the ink droplets attached to the recording medium 2 are irradiated with radiation from the light source 11, a photopolymerization reaction of the polymerizable compound in the ink proceeds, causing the ink droplets to harden and form an ink coating. This results in coloring of the recording medium 2, formation of images, colors, characters, patterns, etc. on the recording medium 2, and formation of a clear ink coating.

[0157] Here, the light source 11 is not limited to being arranged next to the inkjet head 3 in the main scanning direction, and the light source 11 may be offset from the inkjet head 3 in the sub-scanning direction opposite to the direction in which the recording medium 2 is transported. The light source 11 is not limited to being arranged on the carriage 4, and may be separate from the carriage 4.

[0158] In this embodiment, an on-carriage type printer 1 is exemplified as an inkjet recording device, but the invention is not limited to this. The inkjet recording device may be, for example, an off-carriage type such as a large format printer in which an ink container is not mounted on a carriage. In addition, the inkjet recording device used in this embodiment is not limited to a serial printer, and may be a line head printer in which the inkjet head is formed to be as wide as or wider than the width of the recording medium 2 and printing is performed without moving the inkjet head.

[0159] 2.2. Recording medium The recording medium 2 is appropriately selected depending on the purpose of the printed matter produced from the ink and the recording medium. The material for forming the recording medium 2 is not particularly limited, but examples thereof include plastics such as polyvinyl chloride (PVC), polyethylene terephthalate, polypropylene, polyethylene, polycarbonate, cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate butyrate, cellulose nitrate, polyethylene terephthalate, polyethylene, polystyrene, polypropylene, polycarbonate, and polyvinyl acetal, as well as those whose surfaces have been treated, glass, paper, metal, and wood.

[0160] The form of the recording medium 2 is not particularly limited, and examples thereof include a film, a sheet, a board, and a cloth. The ink set of this embodiment has improved stretchability and abrasion resistance, so that when the printed matter is post-processed for signage use, the ink coating is less likely to crack or peel off due to cutting or bending. In addition, the gloss of the clear ink coating is improved, so the ink set of this embodiment can be suitably used for signage use.

[0161] 3. Inkjet method The inkjet method according to this embodiment uses the ink set described above and includes a first ejection step of ejecting color inks from the inkjet head 3 to adhere to the recording medium 2, a first curing step of irradiating the color inks adhered to the recording medium 2 with radiation, a second ejection step of ejecting clear ink from the inkjet head 3 to adhere to an area of ​​the recording medium 2 containing the color ink irradiated with radiation, a leveling step of leaving the recording medium 2 with the clear ink adhered thereto still, and a second curing step of irradiating the clear ink adhered to the recording medium 2 with radiation. Each step included in the inkjet method according to this embodiment will be described below with reference to FIG. 1.

[0162] 3.1.First discharge process In the first ejection step, the color ink is ejected from the inkjet head 3 and adhered to the recording medium 2. More specifically, the piezoelectric element is driven to eject the color ink filled in the pressure generating chamber of the inkjet head 3 from the ejection nozzle. This ejection method is also called an inkjet method. At this time, a multi-pass may be used in which the carriage 4 repeats reciprocating movement in the main scanning direction multiple times relative to a predetermined position on the recording medium 2. That is, the conveyance of the recording medium 2 in the sub-scanning direction is stopped for a certain period of time to eject the color ink in multiple passes, and then the recording medium 2 is conveyed a short distance before performing the multi-pass again.

[0163] By depositing color ink onto the recording medium 2 while moving the inkjet head 3 relatively to the recording medium 2, a liquid layer of color ink is formed on the recording medium 2, which serves as a base for a desired image or the like.

[0164] 3.2. First curing process In the first curing step, radiation is irradiated from the light source 11 to the liquid layer of the color ink attached to the recording medium 2 to cure the color ink on the recording medium 2 and form a coating film of the color ink. More specifically, radiation is irradiated from the light source 11 toward the recording medium 2 while the carriage 4 is moved in the main scanning direction relative to a predetermined position on the recording medium 2. The movement of the carriage 4 in the main scanning direction may be performed once or may be repeated. In addition, the radiation irradiated area of ​​the recording medium 2 may include an area where the color ink is attached. The radiation irradiates the liquid layer of the color ink on the recording medium 2, curing it to form a coating film of the color ink. The first ejection step and the first curing step may or may not be repeated for a specific position on the recording medium 2.

[0165] 3.3.Second discharge process In the second ejection step, the clear ink is ejected from the inkjet head 3 and adhered to the recording medium 2. At this time, the area to which the clear ink is adhered may include the area on the recording medium 2 where the color ink coating is formed. In other words, the clear ink does not have to be adhered to the area where the color ink coating is not formed, or may be adhered in a solid manner to substantially the entire surface of the recording medium 2, including the area where the color ink coating is not formed. Note that, similar to the color ink in the first ejection step, the second ejection step may be a multi-pass ejection step, or a single-pass ejection step without multiple passes. As a result, a liquid layer of the clear ink is formed at least on the color ink coating.

[0166] 3.4.Leveling process The inkjet method includes a leveling step between the second ejection step and the second curing step described below, in which the recording medium 2 with the clear ink attached thereto is left standing for a predetermined period of time. By providing the leveling step, the liquid layer of the clear ink is leveled on the coating film of the color inks. Therefore, after the liquid layer of the clear ink is leveled, a coating film of the clear ink is formed from the liquid layer in the second curing step described below. This increases the smoothness of the coating film of the clear ink, further improving the gloss of the coating film of the clear ink.

[0167] The term "standing still" used here means that the recording medium 2 to which the clear ink has been applied in the second ejection step is not irradiated with radiation in the second curing step. Therefore, in the leveling step, the recording medium 2 may be transported in the sub-scanning direction.

[0168] The standing time in the leveling step is preferably from 1 to 120 seconds, more preferably from 2 to 100 seconds, and even more preferably from 4 to 80 seconds, whereby the liquid layer of the clear ink is sufficiently leveled on the coating film of the color ink, further improving the gloss of the coating film of the clear ink.

[0169] In addition, the inkjet method of the present embodiment is configured to include a leveling step, but is not limited to this. The leveling step can be omitted if the glossiness of the clear ink coating is ensured. From the viewpoint of further improving the glossiness, it is preferable to provide the leveling step.

[0170] 3.5.Second curing process In the second curing step, the liquid layer of the clear ink attached to the recording medium 2 is irradiated with radiation from the light source 11 to cure the clear ink and form a coating of clear ink. More specifically, radiation is irradiated from the light source 11 toward the recording medium 2 at a predetermined position on the recording medium 2, including on the coating of the color ink, while the carriage 4 is moved in the main scanning direction. The carriage 4 may be moved in the main scanning direction once or repeatedly. Moreover, the radiation irradiated area on the recording medium 2 may include the area where the clear ink is attached. The radiation irradiates the liquid layer of the clear ink on the recording medium 2, curing it to form a coating of clear ink. The second ejection step to the second curing step may be repeated for a specific position on the recording medium 2, or may not be repeated.

[0171] According to this embodiment, the following effects can be obtained.

[0172] In the clear ink, the gloss of the coating film is improved and the generation of odor from the coating film can be suppressed. More specifically, monomer B has a bulky molecular structure. By using monomer B, the gloss of the coating film formed from the clear ink can be improved. Since monomer A has a hydroxyl group in its molecular structure, it is less likely to generate odor from the coating film compared to polymerizable compounds with bulky molecular structures. Therefore, by using monomer A in combination with monomer B, the odor generated from the coating film can be suppressed. Furthermore, monomer A has little effect on the gloss of the coating film and does not reduce the gloss of the coating film.

[0173] Furthermore, because the total content of monomer A and monomer B is 80 mass % or more, when a polymerizable compound other than monomer A and monomer B, such as PEA, is contained, the content of the polymerizable compound other than monomer A and monomer B will necessarily be small. As a result, it is possible to provide an ink set that improves the gloss of the coating film in the clear ink and suppresses the emission of odor from the coating film.

[0174] Since the content of monomer A is 5% by mass or more relative to the total amount of polymerizable compounds contained in the clear ink, odors generated from the coating film can be further suppressed. Furthermore, since the content of monomer A is 15% by mass or less, excessive increase in viscosity of the clear ink can be suppressed. Since the content of monomer B is 70% by mass or more relative to the total amount of polymerizable compounds contained in the clear ink, the gloss of the coating film of the clear ink can be further improved.

[0175] The content of monofunctional monomers is 85% by mass or more relative to the total amount of polymerizable compounds contained in the clear ink, so the extensibility of the coating film of the clear ink is improved. Therefore, even if the coating film is formed on a flexible sheet for signage applications, cracking and peeling are unlikely to occur. In other words, it is possible to provide an ink set suitable for signage applications.

[0176] Since the content of the polyfunctional monomer is 0.01% by mass or more and 15.00% by mass or less with respect to the total amount of the clear ink, a crosslinked structure is formed in the coating film by the polyfunctional monomer, and the curability and abrasion resistance of the coating film can be improved compared to a case where the polyfunctional monomer is not used. Also, since the content of the polyfunctional monomer is 15.00% by mass or less, the extensibility of the coating film can be ensured.

[0177] The weighted average of the glass transition temperatures of the homopolymers in the polymerizable compounds, where the weights are the mass ratios of the contents of the polymerizable compounds contained in the clear ink, is 48°C or higher, which can improve the scratch resistance of the clear ink coating.

[0178] Since the content of the monofunctional monomer is 85% by mass or more relative to the total amount of the polymerizable compounds contained in the color ink, the above-mentioned clear ink can be suitably used for the color ink. More specifically, in the past, when a clear ink coating film was formed on a color ink coating film, if the clear ink contained a large amount of monofunctional monomer, the glossiness of the clear ink coating film was likely to decrease. In contrast, the clear ink of the present invention can improve the glossiness of the clear ink coating film.

[0179] By using the inkjet method, a clear ink film is formed on a color ink film. Therefore, the clear ink film protects the color ink film and improves gloss. This makes it possible to produce a printed matter with improved abrasion resistance and gloss of the color ink.

[0180] In the leveling step, the liquid layer of the clear ink attached to the recording medium 2 is made more flat. As a result, a printed matter with improved gloss can be produced.

[0181] 4. Examples and Comparative Examples The effects of the present invention will be described more specifically below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0182] 4.1. Preparation of clear ink and color ink Tables 1 and 2 show the composition of each clear ink and each color ink, the type of polymerizable compound used, the compound name, whether it is monomer A or monomer B, the glass transition temperature (Tg) of the homopolymer of each polymerizable compound, a parameter defined by the Van-der-Waals radius, and the content of the type of polymerizable compound. In the composition columns of Tables 1 and 2, the units of values ​​are mass%, and columns with no value or - sign mean that the component is not contained. In addition, abbreviations are used for the names of each component in Tables 1 and 2. The abbreviations will be described later.

[0183] According to the compositions of clear ink 1 to clear ink 16 shown in Tables 1 and 2, the polymerizable compound, photopolymerization initiator, polymerization inhibitor, and slip agent were weighed and placed in a stainless steel mixing tank. Next, the mixture was stirred for 1 hour in an environment of about 20°C using a mechanical stirrer, and solids such as the photopolymerization initiator were completely dissolved in the polymerizable compound. After that, filtration was performed using a membrane filter with a pore size of 5 μm, and clear inks 1 to 16 were prepared.

[0184] The pigment (coloring material), dispersant, and a portion of the polymerizable compound were weighed and placed in a tank for dispersion using a bead mill according to the compositions of Color Ink 1 and Color Ink 2 shown in Table 2. Ceramic beads having a diameter of 1 mm were then placed in the tank and dispersed using a bead mill to prepare a pigment dispersion in which the pigment was dispersed in the polymerizable compound.

[0185] Separately from the pigment dispersion, the remaining polymerizable compound, photopolymerization initiator, polymerization inhibitor, and slip agent other than the components blended in the pigment dispersion were weighed and added to a stainless steel mixing tank. Then, the mixture was stirred using a mechanical stirrer to completely dissolve the solids such as the photopolymerizable compound in the polymerizable compound. Next, the pigment dispersion was weighed and stirred for another hour in an environment of about 20°C. After that, filtration was performed using a membrane filter with a pore size of 5 μm to prepare color ink 1 and color ink 2, respectively.

[0186] [Table 1]

[0187] [Table 2]

[0188] Details of the abbreviations used in Tables 1 and 2 are as follows:

[0189] Monofunctional Monomers 4HBA: 4-Hydroxybutyl acrylate. Osaka Organic Chemical Industry Co., Ltd. ·DA-141: 2-Hydroxy-3-phenoxypropyl acrylate. Nagase ChemteX Corporation HPA: Hydroxypropyl acrylate. Osaka Organic Chemical Industry Co., Ltd. IBXA: Isobornyl acrylate. Osaka Organic Chemical Industry Co., Ltd. DCPA: Dicyclopentenyl acrylate. Hitachi Chemical TBCHA: tert-butyl cyclohexanol acrylate. Product name SR217, Sartomer Corporation PEA: Phenoxyethyl acrylate. Product name: Viscoat #192, Osaka Organic Chemical Industry Co., Ltd. ACMO: Acroylmorpholine. KJ Chemicals Among the above monofunctional monomers, those corresponding to Monomer A are 4HBA and HPA, those corresponding to Monomer B are IBXA, DCPA and TBCHA, and those corresponding to both Monomer A and Monomer B are DA-141.

[0190] Multifunctional Monomers DPGDA: Dipropylene glycol diacrylate. Sartomer Corporation DPGDA corresponds to monomer B.

[0191] Photoinitiator IRGACURE 819: Trade name. Bis(2,4,6-trimethylbenzoyl)-phenylphosphonoxide. BASF IRGACURE TPO: Trade name. 2,4,6-trimethylbenzoyldiphenylphosphonoxide. BASF

[0192] Polymerization Inhibitor MEHQ: 4-Methoxyphenol. Kanto Chemical Co., Ltd.

[0193] Slip Agent BYK-UV3500: Polyether-modified polydimethylsiloxane with acroyl groups. BYK Additives&Instruments.

[0194] Pigments PB 15:3:CI Pigment Blue 15:3

[0195] Dispersants Solsperse 36000: Trade name. Polymeric dispersant. Lubrizol

[0196] In Tables 1 and 2, the volume value of the parameters defined by the Van-der-Waals radius is the volume of the cavity of the molecule formed by the Van-der-Waals radius of each atom constituting the molecule when the molecule is floating in a vacuum, calculated using the software COSMOtherm by MOLSIS. Among the above parameters, the value of the length of the long side is the longest side of the cavity used to calculate the volume. Among the above parameters, the area in the height direction is the value obtained by dividing the volume by the length of the long side.

[0197] 4.2.Ink set Table 3 shows the details of the ink sets of Example 1 to Example 14 and Comparative Example 1 to Comparative Example 5, and the evaluation results of each ink set. Note that hereinafter, Example 1 to Example 14 may be collectively referred to simply as Examples, and Comparative Example 1 to Comparative Example 5 may be collectively referred to simply as Comparative Examples.

[0198] Here, the ink set of Comparative Example 1 uses clear ink 5. In clear ink 5, the content of monomer A relative to the total amount of polymerizable compounds is 0 mass %, which is less than 5 mass %.

[0199] The ink sets of Comparative Examples 2 and 4 use clear ink 12. In clear ink 12, the combined content of monomer A and monomer B relative to the total amount of polymerizable compounds is 74 mass %, which is less than 80 mass %.

[0200] The ink sets of Comparative Examples 3 and 5 use clear ink 13. In clear ink 13, the combined content of monomer A and monomer B relative to the total amount of polymerizable compounds is 79 mass %, which is less than 80 mass %.

[0201] [Table 3]

[0202] Evaluation The ink sets of the examples and comparative examples were evaluated as described below, and the evaluation results are shown in Table 3.

[0203] 4.3.1. Clear ink viscosity The viscosity of the clear ink at 20° C. was measured by the above-mentioned method for measuring the viscosity of ink at 20° C., and evaluated according to the following evaluation criteria. Evaluation criteria AA: 22 mPa s or more and 24 mPa s or less. A: More than 24 mPa·s and 26 mPa·s or less. B: More than 26 mPa·s and 28 mPa·s or less. C: More than 28 mPa·s.

[0204] 4.3.2. Odor of clear ink coating Using a bar coater, the clear ink corresponding to each ink set was applied to a recording medium to a thickness of 10 μm. The recording medium used was a vinyl chloride film (trade name JT5829 by MACtac). Next, a UV-LED was used to apply 400 mJ / cm 2 A clear ink coating film was formed by irradiating radiation with an energy of 100 Hz. A sensory evaluation was performed in an environment of about 25° C. on the odor generated from the clear ink coating film, and the odor was evaluated according to the following evaluation criteria. Evaluation criteria A: Either odorless, barely detectable odor, or a weak odor that is easy to identify. B: The odor is easily detectable. C: Strong or overpowering odor.

[0205] 4.3.3. Gloss of clear ink coating Using a bar coater, the color ink was applied to a recording medium to a thickness of 10 μm. For the recording medium, Mitsubishi Gas Chemical Company's Iupilon (registered trademark) NF2000 was used as polycarbonate. Next, a UV-LED was used to apply 400 mJ / cm 2The color ink coating was formed by irradiating radiation with an energy of 400 mJ / cm. Then, clear ink was applied to the color ink coating with a bar coater to a thickness of 10 μm. After applying the clear ink, the coating was left to stand for 1 minute as a leveling step. Next, the above-mentioned UV-LED was used to apply radiation with an energy of 400 mJ / cm. 2 The clear ink coating was formed by irradiating radiation with an energy of 1000 nm. The clear ink coating was visually observed under a fluorescent lamp, and the distance between the eye and the clear ink coating at which the reflection of the fluorescent lamp could be confirmed was measured. The glossiness was evaluated based on the above distance according to the following evaluation criteria. Evaluation criteria A: The reflection of the fluorescent light can be seen even at a distance of 50 cm or more. B: The reflection of the fluorescent light can be seen at a distance of 30 cm or more but less than 50 cm. C: The reflection of the fluorescent light can be seen at a distance of 10 cm or more but less than 30 cm. D: The reflection of the fluorescent light can be seen at a distance of less than 10 cm, or the reflection cannot be seen.

[0206] 4.3.4. Stretchability of clear ink coating Using a bar coater, the clear ink corresponding to each ink set was applied to a recording medium to a thickness of 10 μm. The recording medium used was a vinyl chloride film (trade name JT5829, manufactured by MACtac). Next, a metal halide lamp manufactured by iGraphics was used to apply 400 mJ / cm 2 A coating of clear ink was formed by irradiating the substrate with radiation at an energy of 100 .mu.m.

[0207] Next, the release paper of the PVC film was peeled off from the clear ink coating, and the clear ink coating together with the PVC film was cut into a strip of 1 cm width and 8 cm length to prepare a test piece. The test piece was measured for elongation, which is an index of extensibility, in an environment of about 25°C using a tensile tester TENSILON (registered trademark) manufactured by ORIENTEC. The test piece was stretched at a speed of 5 mm per minute, and the value at the time when cracks occurred in the clear ink coating was adopted as the elongation. Specifically, the elongation was calculated from the distance between the chucks of the tensile tester on which the test piece was set, using the following formula: Elongation = {(distance between the chucks when cracks occurred - initial distance between the chucks before stretching) / initial distance between the chucks before stretching} x 100. The extensibility was evaluated according to the following evaluation criteria. Evaluation criteria A: The elongation rate is 300% or more. B: The elongation rate is 200% or more and less than 300%. C: The elongation rate is less than 200%.

[0208] 4.3.5. Scratch resistance of clear ink and color ink coatings Using a bar coater, the color ink was applied to the recording medium to a thickness of 10 μm. The recording medium used was a vinyl chloride film, trade name JT5829, manufactured by MACtac. Next, a metal halide lamp manufactured by iGraphics was used to apply 400 mJ / cm 2 The color ink film was then formed by irradiating radiation with an energy of 400 mJ / cm. Then, clear ink was applied to the color ink film with a bar coater to a thickness of 10 μm. Next, the above metal halide lamp was used to apply radiation with an energy of 400 mJ / cm. 2 A coating of clear ink was formed by irradiating the substrate with radiation at an energy of 100 .mu.m.

[0209] The coatings of the color ink and clear ink were subjected to a micro-scratch test according to JIS R3255 as an index of abrasion resistance. Specifically, the coatings were scratched by varying the load applied to the stylus using Nanotech's ultra-thin scratch tester CSR-5000, and the load was measured when the stylus penetrated the coating and reached the PVC film, which is a recording medium. Here, the load applied to the stylus when the stylus reached the PVC film is also called the load resistance. The measurement conditions were a stylus diameter of 15 μm, an amplitude of 100 μm, and a scratching speed of 10 μm per second. The measured load resistance was evaluated for abrasion resistance according to the following evaluation criteria. The higher the load resistance, the better the abrasion resistance. Evaluation criteria A: Load capacity is 25mN / cm 2 That's all. B: Load capacity is 20mN / cm 2 More than 25mN / cm 2 is less than. C: Load capacity is 20mN / cm 2 is less than.

[0210] 4.4. Summary of evaluation results As shown in Table 3, in the examples other than Example 4 and Example 7, the odor and gloss of the clear ink coating film were both rated A, which corresponds to excellent. Furthermore, in Example 4, the odor of the coating film was rated B, which corresponds to fair, and the gloss of the coating film was rated A. In Example 7, the odor of the coating film was rated A, and the gloss of the coating film was rated B, which corresponds to fair. This shows that the ink sets of the examples improve the gloss of the clear ink coating film and suppress the generation of odor from the coating film.

[0211] Furthermore, the viscosity of the clear ink was rated A or higher, which corresponds to excellent, in all examples other than Example 3 and Example 10. The stretchability of the clear ink coating film was rated A, which corresponds to excellent, in all examples other than Example 9 and Example 10. The abrasion resistance of the clear ink and color ink coating films was rated B or higher, which corresponds to fair, in all examples other than Example 13, and in particular, it was rated A, which corresponds to excellent, in all examples other than Example 5 and Example 6. This demonstrated that the ink sets of the examples are likely to improve the viscosity of the clear ink, and the stretchability and abrasion resistance of the coating film.

[0212] On the other hand, in Comparative Examples 2 to 5, the glossiness of the clear ink coating film was rated C or lower, which corresponds to "unacceptable." In addition, in Comparative Example 1, the odor of the clear ink coating film was rated C, which corresponds to "unacceptable." In other words, it was found that it is difficult to improve both the odor and glossiness of the clear ink coating film with the ink sets of the comparative examples. In particular, Comparative Examples 2 and 3, which used color ink 1, are inferior in glossiness to Comparative Examples 4 and 5, which used color ink 2. In contrast, the ink sets of the examples, even the ink sets using color ink 1, were rated B or higher. In other words, it was shown that the clear ink of the examples is suitable for color ink 1.

[0213] The following describes what can be derived from the embodiments.

[0214] The ink set is an ink set of radiation-curable inkjet inks containing a polymerizable compound, the ink set including color inks and a clear ink, the polymerizable compound contained in the clear ink includes monomer A having a hydroxyl group and monomer B having a volume defined by the Van der Waals radius of 0.26 cubic nm or more and an area in the height direction relative to the long side of 0.25 square nm or more, and the total content of monomer A and monomer B is 80 mass% or more of the total amount of polymerizable compounds contained in the clear ink.

[0215] According to this configuration, in the clear ink, the gloss of the coating film is improved and the generation of odor from the coating film can be suppressed. More specifically, monomer B has a bulky molecular structure. By using monomer B, the gloss of the coating film formed from the clear ink can be improved. Since monomer A has a hydroxyl group in its molecular structure, it is less likely to generate odor from the coating film compared to polymerizable compounds with bulky molecular structures. Therefore, by using monomer A in combination with monomer B, the odor generated from the coating film can be suppressed. Furthermore, monomer A has little effect on the gloss of the coating film and does not reduce the gloss of the coating film.

[0216] Furthermore, because the total content of monomer A and monomer B is 80 mass % or more, when a polymerizable compound other than monomer A and monomer B, such as PEA, is contained, the content of the polymerizable compound other than monomer A and monomer B will necessarily be small. As a result, it is possible to provide an ink set that improves the gloss of the coating film in the clear ink and suppresses the emission of odor from the coating film.

[0217] In the above ink set, the content of monomer A is preferably 5% by mass or more and 15% by mass or less relative to the total amount of polymerizable compounds contained in the clear ink.

[0218] According to this configuration, since the content of monomer A is 5% by mass or more, it is possible to further suppress odors generated from the coating film. Also, since the content of monomer A is 15% by mass or less, it is possible to suppress an excessive increase in viscosity of the clear ink.

[0219] In the above ink set, the content of monomer B is preferably 70 mass % or more relative to the total amount of polymerizable compounds contained in the clear ink.

[0220] According to this configuration, the gloss of the coating film formed from the clear ink can be further improved.

[0221] In the above ink set, the polymerizable compound contained in the clear ink preferably contains a monofunctional monomer, and the content of the monofunctional monomer relative to the total amount of the polymerizable compound contained in the clear ink is preferably 85 mass % or more.

[0222] According to this configuration, the extensibility of the coating film formed from the clear ink is improved. Therefore, even if the coating film is formed on a flexible sheet for signage applications, cracks and peeling are unlikely to occur. In other words, it is possible to provide an ink set suitable for signage applications.

[0223] In the above ink set, the polymerizable compound contained in the clear ink preferably contains a polyfunctional monomer, and the content of the polyfunctional monomer relative to the total amount of the clear ink is preferably 0.01% by mass or more and 15.00% by mass or less.

[0224] According to this configuration, a crosslinked structure is formed in the coating film by the polyfunctional monomer. Therefore, the curability and abrasion resistance of the coating film can be improved compared to a case where the polyfunctional monomer is not used. In addition, since the content of the polyfunctional monomer is 15.00 mass% or less, the extensibility of the coating film can be ensured.

[0225] In the ink set, the weighted average of the glass transition temperatures of the homopolymers in the polymerizable compounds, where the weights are the mass ratios of the contents of the polymerizable compounds contained in the clear ink, is preferably 48° C. or higher.

[0226] According to this configuration, it is possible to improve the scratch resistance of the coating film formed from the clear ink.

[0227] In the ink set, the polymerizable compound contained in the color inks preferably contains a monofunctional monomer, and the content of the monofunctional monomer relative to the total amount of the polymerizable compounds contained in the color inks is preferably 85 mass % or more.

[0228] According to this configuration, the above-mentioned clear ink can be suitably used for the color ink. More specifically, in the past, when a clear ink coating film was formed on a color ink coating film, if the color ink contained a large amount of monofunctional monomers, the glossiness of the clear ink coating film was likely to decrease. In contrast, the clear ink of the present invention can improve the glossiness of the clear ink coating film.

[0229] The inkjet method is an inkjet method using the above-mentioned ink set, and includes a first ejection step of ejecting color inks from an inkjet head and adhering them to a recording medium, a first curing step of irradiating the color inks adhered to the recording medium with radiation, a second ejection step of ejecting clear ink from the inkjet head and adhering it to an area of ​​the recording medium containing the color inks irradiated with radiation, and a second curing step of irradiating the clear ink adhered to the recording medium with radiation.

[0230] According to this configuration, a coating film of the clear ink is formed on a coating film of the color ink. Therefore, the coating film of the clear ink protects the coating film of the color ink and improves the glossiness. This makes it possible to produce a printed matter in which the abrasion resistance and glossiness of the color ink are improved.

[0231] The above inkjet method preferably includes a leveling step of leaving the recording medium to which the clear ink has been applied still between the second ejection step and the second curing step.

[0232] According to this configuration, the liquid layer of the clear ink adhered to the recording medium is further flattened in the leveling process, making it possible to produce a printed matter with further improved gloss. [Explanation of symbols]

[0233] 1...inkjet printer, 2...recording medium, 3...inkjet head.

Claims

1. An ink set of radiation curable inkjet inks containing a polymerizable compound, the ink set includes a color ink and a clear ink, The polymerizable compound contained in the clear ink is a monomer A having a hydroxyl group and The volume defined by the n-der-Waals radius is 0.26 cubic nm or more, and the Monomer B having a height area of ​​0.25 nm2 or more; A multifunctional monomer, and a nitrogen-containing monofunctional acrylate monomer having a nitrogen-containing heterocyclic structure, The total content of the monomer A and the monomer B in the clear ink is The amount of the polymerizable compound is 80% by mass or more based on the total amount of the polymerizable compound, The content of the monomer A relative to the total amount of the polymerizable compound contained in the clear ink The amount is 3% by mass or more and 17% by mass or less, the content of the nitrogen-containing monofunctional acrylate monomer having a nitrogen-containing heterocyclic structure is 5% by mass or more relative to the total amount of the clear ink, The content of the polyfunctional monomer is 0.01% by mass or more relative to the total amount of the clear ink. There is an ink set.

2. The content of the monomer A relative to the total amount of the polymerizable compound contained in the clear ink The ink set according to claim 1 , wherein the amount of the pigment is from 5% by weight to 15% by weight.

3. The content of the monomer B relative to the total amount of the polymerizable compound contained in the clear ink The ink set according to claim 1 or 2, wherein the amount of the pigment is 70% by mass or more.

4. the polymerizable compound contained in the clear ink contains a monofunctional monomer, The amount of the monofunctional monomer relative to the total amount of the polymerizable compound contained in the clear ink The inkjet printer according to any one of claims 1 to 3, wherein the content is 85 mass % or more. Yat.

5. a weight is a mass ratio of the content of each of the polymerizable compounds contained in the clear ink; The weighted average of the glass transition temperatures of the homopolymers in the polymerizable compound is 48° C. or higher. The ink set according to claim 1 .

6. the polymerizable compound contained in the color ink contains a monofunctional monomer, The amount of the monofunctional monomer relative to the total amount of the polymerizable compound contained in the color ink The inkjet printer according to any one of claims 1 to 5, wherein the content is 85 mass% or more. Yat.

7. The content of the aromatic group-containing monofunctional monomer with respect to the total amount of the clear ink is 7% by mass.

7. The ink set according to claim 1, wherein:

8. An ink-jet method using the ink set according to any one of claims 1 to 7. It is a law, A first ejection step of ejecting the color ink from an inkjet head and depositing the color ink on a recording medium. The output process and a first curing step of irradiating the color ink applied to the recording medium with radiation; and, The clear ink is ejected from an inkjet head, a second ejection step of applying radiation to the area containing the color ink; A second curing step of irradiating the clear ink attached to the recording medium with radiation. and an ink jet method comprising:

9. Between the second ejection step and the second curing step, The ink-jet method according to claim 8 , further comprising a leveling step of leaving the recording medium stationary.

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