WO2024176659A1

WO2024176659A1 - Photocurable primer ink composition for ink-jet recording, ink set, and recording method

Info

Publication number: WO2024176659A1
Application number: PCT/JP2024/001007
Authority: WO
Inventors: 智久西本
Original assignee: マクセル株式会社
Priority date: 2023-02-21
Filing date: 2024-01-16
Publication date: 2024-08-29
Also published as: JP2024118878A; JP7456032B1

Abstract

The present invention relates to a photocurable primer ink composition for ink-jet recording comprising a polymerizable compound and a photopolymerization initiator, the polymerizable compound including (A) a monofunctional monomer containing an acidic group in a molecule, (B) a monofunctional monomer containing an aromatic hydrocarbon cyclic structure in a molecule, and (C) a monofunctional monomer containing an aliphatic hydrocarbon cyclic structure in a molecule, wherein the amount of (C) the monofunctional monomer is 60 mass % or more based on the mass of the whole composition.

Description

Photocurable inkjet recording primer ink composition, ink set, and recording method

The present invention relates to a photocurable primer ink composition for inkjet recording, an ink set, and a recording method.

Inkjet printing, which ejects ink droplets from an ink nozzle head, is used in many printers because it is small, inexpensive, and capable of forming images without contacting the surface of a recording medium. In recent years, inkjet printers have been used for commercial and industrial printing for advertising, posters, decorative purposes, and the like. As a result, there is a strong demand for the ability to print on recording media made of various materials, including non-permeable recording media surfaces such as plastics, and for the realization of printing surfaces with higher image quality.

In order to meet such demands, various studies have been conducted on ink compositions used in inkjet printers. For example, Patent Document 1 proposes an ink composition that contains an acrylate monomer that imparts adhesion to metal or glass, and at least one selected from the group consisting of a multifunctional acrylate monomer and a multifunctional acrylate oligomer that imparts rigidity.

Patent Document 2 also proposes a primer ink composition that contains a carboxyl group-containing mono(meth)acrylate monomer and a hydroxyl group-containing (meth)acrylate, and the ratio of the amounts of the carboxyl group-containing mono(meth)acrylate monomer and the hydroxyl group-containing (meth)acrylate blended is within a certain range.

JP 2017-019899 A JP 2021-195467 A

However, when printing is performed using the ink composition described above, the printed matter may have low strength, may be tacky (sticky), or may have poor adhesion to the recording medium, and the properties of the printed matter may not be sufficient. In addition, when printing is performed under higher speed and higher resolution conditions, the ink composition may not be ejected stably.

The object of the present invention is therefore to provide a photocurable primer ink composition for inkjet recording that has excellent print strength and adhesion, low tackiness, and excellent ejection stability even during high-speed printing.

The present invention provides the following preferred embodiments.
[1] A photocurable primer ink composition for inkjet recording, comprising a polymerizable compound and a photopolymerization initiator, wherein the polymerizable compound comprises a monofunctional monomer (A) containing an acidic group in its molecule, a monofunctional monomer (B) containing an aromatic hydrocarbon-based cyclic structure in its molecule, and a monofunctional monomer (C) containing an aliphatic hydrocarbon-based cyclic structure in its molecule, and the content of the monofunctional monomer (C) is 60 mass% or more based on the mass of the entire composition.
[2] The composition according to [1], wherein the monofunctional monomer (C) includes a monofunctional monomer (C') having an SP value of 9.4 (cal/cm ³ ) ^1/2 or more and 12.0 (cal/cm ³ ) ^1/2 or less.
[3] The composition according to [1] or [2], wherein the ratio of the total content of the monofunctional monomers (A) and (B) to the content of the monofunctional monomer (C) in the composition is 1:1 to 1:7.
[4] The composition according to any one of [1] to [3], wherein the content ratio of the monofunctional monomers (A) and (B) in the composition is 2.1:1 to 10:1.
[5] The composition according to any one of [1] to [4], wherein the monofunctional monomer (A) includes a monomer containing a carboxy group.
[6] The composition according to any one of [1] to [5], wherein the monofunctional monomer (B) includes a monomer containing a hydroxy group.
[7] The composition according to any one of [1] to [6], wherein the monofunctional monomer (C) includes cyclic trimethylolpropane formal acrylate.
[8] The composition according to any one of [1] to [7], wherein the content of the photopolymerization initiator is 5.1% by mass or more and 20% by mass or less based on the mass of the entire composition.
[9] The composition according to any one of [1] to [8], wherein the polymerizable compound does not contain a polyfunctional monomer.
[10] The composition according to any one of [1] to [9], having a surface tension of 20 mN/m or more and 35 mN/m or less.
[11] The composition according to any one of [1] to [10], having a viscosity at 25°C of 50 mPa·s or less.
[12] An ink set comprising the photocurable primer ink composition for inkjet recording according to any one of [1] to [11], and a color ink composition and/or a clear ink composition containing a colorant.
[13] An inkjet recording method for forming an image on a recording medium made of a non-absorbent substrate using the ink set according to [12], the method comprising: a step of ejecting the primer ink composition from an inkjet head onto the recording medium and photo-curing the primer ink composition; and a step of ejecting the color ink composition and/or the clear ink composition from an inkjet head onto the photo-cured primer ink composition and photo-curing the primer ink composition.
[14] The method according to [13], wherein the non-absorbent substrate is subjected to a surface modification treatment.
[15] The method according to [13] or [14], wherein the inkjet head has a design resolution of 600 dpi or more.

The present invention provides a photocurable inkjet recording primer ink composition that has excellent print strength and adhesion, low tackiness, and excellent ejection stability even during high-speed printing.

The following is a detailed explanation of the embodiments of the present invention. Note that the scope of the present invention is not limited to the embodiments described here, and various modifications can be made without departing from the spirit of the present invention.

<Photocurable primer ink composition for inkjet recording>
The photocurable primer ink composition for inkjet recording of the present invention (hereinafter may be referred to as "primer ink composition") contains a polymerizable compound and a photopolymerization initiator, the polymerizable compound contains a monofunctional monomer (A) containing an acidic group in its molecule, a monofunctional monomer (B) containing an aromatic hydrocarbon-based cyclic structure in its molecule, and a monofunctional monomer (C) containing an aliphatic hydrocarbon-based cyclic structure in its molecule, and the content of the monofunctional monomer (C) is 60 mass% or more based on the mass of the entire composition.

[Polymerizable compound]
The primer ink composition of the present invention contains a polymerizable compound. The polymerizable compound in the present invention is a compound that undergoes a polymerization reaction by irradiation with light and has the function of curing a composition containing the polymerizable compound. Specifically, a compound having at least one ethylenic double bond selected from the group consisting of an acryloyl group, a methacryloyl group, a vinyl group, an allyl group, and a vinyl ether group is preferred, and a compound having at least one ethylenic double bond selected from the group consisting of an acryloyl group, a methacryloyl group, and a vinyl ether group is more preferred.
In this specification, "(meth)acrylate" is a general term for "acrylate" and "methacrylate." The terms "(meth)acrylamide,""(meth)acryloyloxy," and the like also have the same meaning.

The primer ink composition of the present invention contains, as polymerizable compounds, a monofunctional monomer (A) containing an acidic group in the molecule, a monofunctional monomer (B) containing an aromatic hydrocarbon-based cyclic structure in the molecule, and a monofunctional monomer (C) containing an aliphatic hydrocarbon-based cyclic structure in the molecule.

In the present invention, the monofunctional monomer (A) containing an acidic group in the molecule (hereinafter, may be simply referred to as "monomer (A)") may refer to a compound that has one ethylenic double bond in the molecule and contains one or more functional groups such as a carboxy group, a sulfo group, or a phosphate group in the molecule. When the primer ink composition of the present invention contains monomer (A), it can exhibit excellent adhesion, particularly to glass and metal.

Such monomers (A) include non-cyclic carboxy group-containing monofunctional (meth)acrylates such as ω-carboxy-polycaprolactone mono(meth)acrylate, β-carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, 2-(meth)acryloyloxyethyl succinate, and 2-acryloyloxyethyl succinic acid; 2-(meth)acryloyloxyethyl hexahydrophthalic acid, 2-(meth)acryloyloxyethyl phthalic acid, and 2-(meth)acryloyloxyethyl phthalic acid. p) Carboxy group-containing monofunctional (meth)acrylates having a cyclic structure such as acryloyloxypropyl phthalate and 2-(meth)acryloyloxyethyl-2-hydroxyethyl phthalate; non-cyclic sulfo group-containing monofunctional (meth)acrylates such as 2-(meth)acrylamido-2-methylpropanesulfonic acid and 2-sulfoethyl (meth)acrylate; and non-cyclic phosphoric acid group-containing monofunctional (meth)acrylates such as 2-(meth)acryloyloxyethyl acid phosphate. These can be used alone or in combination of two or more.

The glass transition temperature (Tg) of the monomer (A) as a homopolymer is preferably -70°C or higher, more preferably -60°C or higher, even more preferably -50°C or higher, and preferably 100°C or lower, more preferably 90°C or lower, even more preferably 80°C or lower, and even more preferably 70°C or lower. That is, the glass transition temperature (Tg) of the monomer (A) is preferably -70 to 100°C, more preferably -60 to 90°C, even more preferably -50 to 80°C, and even more preferably -50 to 70°C. When the glass transition temperature (Tg) of the monomer (A) is within the above range, the adhesion and tackiness to the substrate can be improved. The glass transition temperature (Tg) can be measured, for example, by differential scanning calorimetry (DSC) or dynamic mechanical analysis (DMA). The glass transition temperature of a homopolymer may depend on the degree of polymerization, but if a homopolymer with a weight average molecular weight of 20,000 or more is produced and measured, the effect of the degree of polymerization can be ignored.

The surface tension of the monomer (A) is preferably 5 mN/m or more, more preferably 10 mN/m or more, even more preferably 15 mN/m or more, and is preferably 60 mN/m or less, more preferably 55 mN/m or less, even more preferably 50 mN/m or less. That is, the surface tension of the monomer (A) is preferably 5 to 60 mN/m, more preferably 10 to 55 mN/m, even more preferably 15 to 50 mN/m. If the surface tension of the monomer (A) is within the above range, the surface tension of the primer ink composition can be appropriate. The surface tension of the monomer (A) can be measured, for example, by the ring method (Dunouil method) or the platinum plate method (Wilhelmy method).

The viscosity of the monomer (A) is preferably 1.0 mPa·s or more, more preferably 2.0 mPa·s or more, and even more preferably 3.0 mPa·s or more, and is preferably 200 mPa·s or less, more preferably 190 mPa·s or less, and even more preferably 180 mPa·s or less. That is, the viscosity of the monomer (A) is preferably 1.0 to 200 mPa·s, more preferably 2.0 to 190 mPa·s, and even more preferably 3.0 to 180 mPa·s. When the viscosity of the monomer (A) is within the above range, the viscosity of the primer ink composition can be appropriate. The viscosity of the monomer can be measured, for example, by a rotational viscometer.

The monomer (A) preferably contains a monomer containing a carboxy group. When the monomer (A) contains a monomer containing a carboxy group, the adhesion of the primer ink composition, particularly to glass and metal, is further improved. Among the monomers containing a carboxy group, it is more preferable that the monomer (A) contains at least one selected from the group consisting of ω-carboxy-polycaprolactone mono(meth)acrylate, β-carboxyethyl(meth)acrylate, and carboxypentyl(meth)acrylate, and it is even more preferable that the monomer (A) contains ω-carboxy-polycaprolactone monoacrylate. The content of the monomer containing a carboxy group in the monomer (A) is preferably 40 to 100% by mass, more preferably 50 to 100% by mass. When the content of the monomer containing a carboxy group is within the above range, the adhesion of the primer ink composition, particularly to glass and metal, is further improved.

The content of the monomer (A) is preferably 5% by mass or more, more preferably 8% by mass or more, even more preferably 10% by mass or more, even more preferably 11% by mass or more, particularly preferably 12% by mass or more, or 13% by mass or more, based on the mass of the entire primer ink composition. The content of the monomer (A) is preferably less than 30% by mass, more preferably 29% by mass or less, even more preferably 25% by mass or less, and even more preferably 20% by mass or less. That is, the content of the monomer (A) is preferably 5% by mass or more and less than 30% by mass, more preferably 8 to 29% by mass, even more preferably 10 to 25% by mass, even more preferably 11 to 25% by mass, particularly preferably 12 to 20% by mass, or 13 to 20% by mass. When the content of the monomer (A) is within the above range, excellent adhesion can be exhibited, especially to glass and metal.

The monomer (A) preferably contains a monofunctional monomer (A') having a solubility parameter SP value of 7.0 (cal/cm ³ ) ^1/2 or more and 14.0 (cal/cm ³ ) ^1/2 or less. When the monomer (A) contains a monofunctional monomer (A') having a solubility parameter SP value of 7.0 (cal/cm ³ ) ^1/2 or more and 14.0 (cal/cm ³ ) ^1/2 or less, the compatibility between the monomers is improved and the monomers can be dissolved well. Therefore, the printed strength and adhesion of the primer ink composition are improved, tackiness is low, and excellent ejection stability can be exhibited even in high-speed printing. The SP value of the monomer (A') is more preferably 7.5 (cal/ ^cm3 ) ^1/2 or more, even more preferably 8.0 (cal/ ^cm3 ) ^1/2 or more, still more preferably 8.5 (cal/ ^cm3 ) ^1/2 or more, and is preferably, more preferably 13.5 (cal/ ^cm3 ) ^1/2 or less, and even more preferably 12.0 (cal/ ^cm3 ) ^1/2 or less. That is, the SP value of the monomer (A') is preferably 7.0 to 14.0 (cal/ ^cm3 ) ^1/2 , more preferably 7.5 to 13.5 (cal/ ^cm3 ) ^1/2 , even more preferably 8.0 to 12.0 (cal/ ^cm3 ) ^1/2 , and even more preferably 8.5 to 12.0 (cal/ ^cm3 ) ^1/2 . The SP value can be determined by the method described later.

The monomer (A') is preferably contained in the monomer (A) at 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of the content of the monomer (A') is 100% by mass. That is, the monomer (A') is preferably contained in the monomer (A) at 50 to 100% by mass, more preferably 60 to 100% by mass, and even more preferably 70 to 100% by mass. When the content of the monomer (A') is within the above range, the compatibility between the monomers is further improved, and the monomer can be dissolved better. Therefore, the printed matter strength, adhesion, tackiness, and ejection stability in high-speed printing of the primer ink composition can be improved.

In the present invention, the monofunctional monomer (B) containing an aromatic hydrocarbon-based cyclic structure in the molecule (hereinafter, may be simply referred to as "monomer (B)") may refer to a compound having one ethylenic double bond in the molecule and containing one or more aromatic hydrocarbon-based cyclic structures in the molecule. Note that monomer (B) preferably does not contain a functional group that exhibits acidity. In addition to the aromatic hydrocarbon-based cyclic structure, it may also have an aliphatic hydrocarbon-based cyclic structure. When the primer ink composition of the present invention contains monomer (B), it can exhibit excellent adhesion, particularly to resins such as acrylic and polyacetal.

Examples of such monomers (B) include (meth)acrylates having one or more aromatic hydrocarbon-based ring structures in the molecule, such as 2-hydroxy-3-phenoxypropyl (meth)acrylate, phenoxymethyl (meth)acrylate, phenoxyethyl (meth)acrylate, phenoxydiethylene glycol (meth)acrylate, phenoxypolyethylene glycol (meth)acrylate, alkoxy-modified phenol (meth)acrylate, EO-modified nonylphenol (meth)acrylate, PO-modified nonylphenol (meth)acrylate, alkoxy-modified nonylphenol (meth)acrylate, benzyl (meth)acrylate, 4-butylphenyl (meth)acrylate, phenyl (meth)acrylate, 2,4,5-tetramethylphenyl (meth)acrylate, 4-chlorophenyl (meth)acrylate, and EO-modified cresol (meth)acrylate. In addition, aromatic vinyls such as styrene, methylstyrene, dimethylstyrene, trimethylstyrene, ethylstyrene, isopropylstyrene, chloromethylstyrene, methoxystyrene, acetoxystyrene, chlorostyrene, dichlorostyrene, bromostyrene, vinyl benzoic acid methyl ester, 3-methylstyrene, 4-methylstyrene, 3-ethylstyrene, 4-ethylstyrene, 3-propylstyrene, 4-propylstyrene, 3-butylstyrene, 4-butylstyrene, 3-hexylstyrene, 4-hexylstyrene, 3-octylstyrene, 4-octylstyrene, 3-(2-ethylhexyl)styrene, 4-(2-ethylhexyl)styrene, allylstyrene, isopropenylstyrene, butenylstyrene, octenylstyrene, 4-t-butoxycarbonylstyrene, 4-methoxystyrene, and 4-t-butoxystyrene, and vinyl ethers having one or more aromatic hydrocarbon-based ring structures in the molecule such as benzyl vinyl ether, phenylethyl vinyl ether, and phenoxy polyethylene glycol vinyl ether can also be used. These can be used alone or in combination of two or more.

The glass transition temperature (Tg) of the monomer (B) as a homopolymer is preferably -30°C or higher, more preferably -25°C or higher, even more preferably -20°C or higher, even more preferably 0°C or higher, particularly preferably 5°C or higher, and is preferably 60°C or lower, more preferably 50°C or lower, even more preferably 40°C or lower, and even more preferably 30°C or lower. That is, the glass transition temperature (Tg) of the monomer (B) is preferably -30 to 60°C, more preferably -25 to 50°C, even more preferably -20 to 40°C, even more preferably 0 to 30°C, or 5 to 30°C. When the glass transition temperature (Tg) of the monomer (B) is within the above range, the adhesion and tackiness to the substrate can be good.

The surface tension of the monomer (B) is preferably 5 mN/m or more, more preferably 10 mN/m or more, even more preferably 15 mN/m or more, and is preferably 60 mN/m or less, more preferably 55 mN/m or less, even more preferably 50 mN/m or less. In other words, the surface tension of the monomer (B) is preferably 5 to 60 mN/m, more preferably 10 to 55 mN/m, even more preferably 15 to 50 mN/m. When the surface tension of the monomer (B) is within the above range, the surface tension of the primer ink composition can be appropriate.

The viscosity of the monomer (B) is preferably 1.0 mPa·s or more, more preferably 2.0 mPa·s or more, even more preferably 3.0 mPa·s or more, even more preferably 9.0 mPa·s or more, and particularly preferably 12.0 mPa·s or more, and is preferably 200 mPa·s or less, more preferably 195 mPa·s or less, and even more preferably 190 mPa·s or less. That is, the viscosity of the monomer (B) is preferably 1.0 to 200 mPa·s, more preferably 2.0 to 195 mPa·s, even more preferably 3.0 to 190 mPa·s, and even more preferably 9.0 to 190 mPa·s. When the viscosity of the monomer (B) is within the above range, the viscosity of the primer ink composition can be appropriate.

The monomer (B) preferably contains a monomer containing a hydroxy group. When the monomer (B) contains a monomer containing a hydroxy group, the adhesion of the primer ink composition to resins, particularly acrylic and polyacetal, is further improved. Among the monomers (B) containing a hydroxy group, it is more preferable that the monomer (B) contains at least one selected from the group consisting of 2-hydroxy-3-phenoxypropyl (meth)acrylate, phenoxymethyl (meth)acrylate, phenoxyethyl (meth)acrylate, and phenoxydiethylene glycol (meth)acrylate, and it is even more preferable that the monomer (B) contains 2-hydroxy-3-phenoxypropyl acrylate (HPPA). The content of the monomer containing a hydroxy group in the monomer (B) is preferably 40 to 100% by mass, more preferably 50 to 100% by mass. When the content of the monomer containing a hydroxy group is within the above range, the adhesion of the primer ink composition to resins, particularly acrylic and polyacetal, is further improved.

The content of the monomer (B) is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 3% by mass or more, even more preferably 4% by mass or more, and particularly preferably 5% by mass or more, based on the mass of the entire primer ink composition. The content of the monomer (B) is preferably 15% by mass or less, more preferably 12% by mass or less, and even more preferably 10% by mass or less. That is, the content of the monomer (B) is preferably 1 to 15% by mass, more preferably 2 to 15% by mass, even more preferably 3 to 12% by mass, even more preferably 4 to 10% by mass, and 5 to 10% by mass. When the content of the monomer (B) is within the above upper and lower limit ranges, excellent adhesion can be exhibited, especially to resins such as acrylic and polyacetal.

The monomer (B) preferably contains a monofunctional monomer (B') having a solubility parameter SP value of 9.0 (cal/cm ³ ) ^1/2 or more and 13.0 (cal/cm ³ ) ^1/2 or less. When the monomer (B) contains a monofunctional monomer (B') having a solubility parameter SP value of 9.0 (cal/cm ³ ) ^1/2 or more and 13.0 (cal/cm ³ ) ^1/2 or less, the compatibility between the monomers is improved and the monomers can be dissolved well. Therefore, the printed strength and adhesion of the primer ink composition are improved, tackiness is low, and excellent ejection stability can be exhibited even in high-speed printing. The SP value of the monomer (B') is more preferably 9.3 (cal/ ^cm3 ) ^1/2 or more, even more preferably 9.7 (cal/ ^cm3 ) ^1/2 or more, still more preferably 10.0 (cal/ ^cm3 ) ^1/2 or more, particularly preferably 10.1 (cal/ ^cm3 ) ^1/2 or more, and is preferably, more preferably 12.7 (cal/ ^cm3 ) ^1/2 or less, even more preferably 12.5 (cal/ ^cm3 ) ^1/2 or less, even more preferably 12.3 (cal/ ^cm3 ) ^1/2 or less, particularly preferably 12.0 (cal/ ^cm3 ) ^1/2 or less. That is, the solubility parameter SP value of monomer (B') is preferably 9.0 to 13.0 (cal/cm ³ ) ^1/2 , more preferably 9.3 to 12.7 (cal/cm ³ ) ^1/2 , even more preferably 9.7 to 12.7 (cal/cm ³ ) ^1/2 , still more preferably 10.0 to 12.5 (cal/cm ³ ) ^1/2 , particularly preferably 10.1 to 12.3 (cal/cm ³ ) ^1/2 or 10.1 to 12.0 (cal/cm ³ ) ^1/2 . The SP value can be determined by the method described later.

The monomer (B') is preferably contained in the monomer (B) at 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of the content of the monomer (B') is 100% by mass. That is, the monomer (B') is preferably contained in the monomer (B) at 50 to 100% by mass, more preferably 60 to 100% by mass, and even more preferably 70 to 100% by mass. When the content of the monomer (B') is within the above range, the compatibility between the monomers is further improved, and the monomer can be dissolved better. Therefore, the printed matter strength, adhesion, tackiness, and ejection stability in high-speed printing of the primer ink composition can be improved.

The content (mass) ratio of the monomers (A) and (B) in the primer ink composition of the present invention (monomer (A):monomer (B)) is preferably 2.1:1 to 10:1, more preferably 2.2:1 to 10:1, even more preferably 2.3:1 to 10:1, even more preferably 2.4:1 to 9.5:1, particularly preferably 3:1 to 9.5:1, and extremely preferably 4:1 to 9:1. When the content ratio of monomers (A) and (B) is within the above range, the adhesion of the primer ink composition to non-absorbent substrates is improved, and excellent adhesion can be exhibited, in particular, to both inorganic substrates such as glass and metal, and organic substrates such as acrylic and polyacetal.

In the present invention, a monofunctional monomer (C) containing an aliphatic hydrocarbon-based cyclic structure in the molecule (hereinafter, sometimes simply referred to as "monomer (C)") may refer to a compound having one ethylenic double bond in the molecule and containing one or more cyclic hydrocarbon groups in the molecule. Monomer (C) preferably does not contain a functional group exhibiting acidity and an aromatic hydrocarbon-based cyclic structure.

In the present invention, it was found that the monomer (C) has excellent compatibility with the monomer (A) and the monomer (B). Therefore, when the primer ink composition of the present invention contains the monomer (C) in addition to the monomer (A) and the monomer (B), the monomer (A) and the monomer (B) which are thought to be the cause of poor ejection under high-speed printing conditions, which are not completely dissolved in the conventional composition and may precipitate slightly, are thought to be sufficiently dissolved in the primer ink composition of the present invention without precipitating. Therefore, the primer ink composition can be ejected stably even under high-speed printing conditions where even a small amount of precipitates is easily affected. In addition, if a distortion due to the precipitates (discontinuity or non-uniformity of the reaction between the monomer molecules) occurs between the ejected primer layer and the recording medium, the adhesion to the recording medium and the strength of the printed matter may decrease. Therefore, it was found that the primer ink composition of the present invention in which the monomer (A) and the monomer (B) are sufficiently dissolved has improved adhesion to the recording medium and the strength of the printed matter compared to the conventional composition. Furthermore, it was found that the addition of the monomer (C) can improve compatibility and curing properties, thereby reducing tackiness (stickiness). That is, it is believed that monomer (C) has the effect of reducing the localization of monomers of the same kind in the monomer distribution of the entire coating film, such as monomer (A) and monomer (B), which are tacky when used alone, and homogenizing them in the monomer distribution on the coating film surface, inner surface of the coating film, and substrate interface. In addition, since monomer (C) itself has good curing properties, it is believed to have the effect of reducing the ratio of the total remaining monomers in the coating film. However, even if the actual situation differs from the above assumption, it is still within the scope of the present invention.

Examples of the monomer (C) include monomers having an alicyclic hydrocarbon structure in the molecule, such as cyclohexyl (meth)acrylate, 4-n-butylcyclohexyl (meth)acrylate, bornyl (meth)acrylate, isobornyl (meth)acrylate, 3,5,5-trimethylcyclohexyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, and dicyclopentanyl (meth)acrylate; cyclic trimethylolpropane formal ( Examples of monomers having a heterocyclic structure in the molecule include methyl methacrylate, acryloylmorpholine, tetrahydrofurfuryl (meth)acrylate, glycidyl (meth)acrylate, glycidyloxybutyl (meth)acrylate, glycidyloxyethyl (meth)acrylate, glycidyloxypropyl (meth)acrylate, 3-ethyl-3-oxetanylmethyl (meth)acrylate, (2-methyl-2-ethyl-1,3-dioxolan-4-yl)methyl acrylate, and N-vinyl-2-caprolactam. In addition, vinyl ethers having an alicyclic hydrocarbon structure in the molecule, such as cyclohexyl vinyl ether, cyclohexyl methyl vinyl ether, 4-methylcyclohexyl methyl vinyl ether, 4-hydroxymethylcyclohexyl methyl vinyl ether, and tetrahydrofurfuryl vinyl ether, can also be used. These can be used alone or in combination of two or more.

The monomer (C) preferably contains a monofunctional monomer (C') having a solubility parameter SP value of 9.4 (cal/cm ³ ) ^1/2 or more and 12.0 (cal/cm ³ ) ^1/2 or less. When the monomer (C) contains a monofunctional monomer (C') having a solubility parameter SP value of 9.4 (cal/cm ³ ) ^1/2 or more and 12.0 (cal/cm ³ ) ^1/2 or less, the compatibility between the monomers is improved and the monomers can be dissolved well. Therefore, the printed strength and adhesion of the primer ink composition are improved, tackiness is low, and excellent ejection stability can be exhibited even in high-speed printing. The SP value of the monofunctional monomer is preferably 9.4 (cal/cm ³ ) ^1/2 or more, more preferably 9.5 (cal/cm ³ ) ^1/2 or more, even more preferably 9.6 (cal/cm ³ ) ^1/2 or more, still more preferably 9.8 (cal/cm ³ ) ^1/2 or more, and is preferably 12.0 (cal/cm ³ ) ^1/2 or less, more preferably 11.8 (cal/cm ³ ) ^1/2 or less, even more preferably 11.5 (cal/cm ³ ) ^1/2 or less, and still more preferably 11.1 (cal/cm ³ ) ^1/2 or less. That is, the SP value of the monomer (C') is preferably 9.4 to 12.0 (cal/ ^cm3 ) ^1/2 , more preferably 9.5 to 11.8 (cal/ ^cm3 ) ^1/2 , even more preferably 9.6 to 11.5 (cal/ ^cm3 ) ^1/2 , and even more preferably 9.8 to 11.1 (cal/ ^cm3 ) ^1/2 . The monomer (C') is contained in the monomer (C) at preferably 50 mass% or more, more preferably 55 mass% or more, even more preferably 60 mass% or more, and even more preferably 65 mass% or more. The upper limit of the content of the monomer (C') is 100 mass%. That is, the monomer (C') is contained in the monomer (C) at preferably 50 to 100 mass%, more preferably 60 to 100 mass%, and even more preferably 70 to 100 mass%.

The monomer (C) may contain a monomer (C'') having a solubility parameter SP value of less than 9.4 (cal/cm ³ ) ^1/2 and/or a monomer (C''') having a solubility parameter SP value of more than 12.0 (cal/cm ³ ) ^1/2 , within a range that does not impair compatibility with the monomer (A) and the monomer (B). The content of monomer (C") having a solubility parameter SP value less than 9.4 (cal/ ^cm3 ) ^1/2 and/or monomer (C'") having a solubility parameter SP value greater than 12.0 (cal/ ^cm3 ) ^1/2 is preferably less than the content of monomer (C') having a solubility parameter SP value of 9.4 (cal/ ^cm3 ) ^1/2 or more and 12.0 (cal/ ^cm3 ) ^1/2 or less, and the total amount in monomer (C) is preferably less than 50% by mass, more preferably 45% by mass or less, even more preferably 40% by mass or less, and even more preferably 35% by mass or less. The lower limit of the content of monomer (C") and/or (C'") is 0% by mass. That is, the monomers (C'') and/or (C''') are contained in the monomer (C) in a total amount of preferably 0 to less than 50 mass%, more preferably 0 to 45 mass%, even more preferably 0 to 40 mass%, and still more preferably 0 to 35 mass%.

Here, it is known that the SP value of each monomer can be calculated from the molecular structure. The solubility parameter in this specification is the value at 25°C obtained by the Fedors method (Yuji Harazaki, "Basic Science of Coatings", Chapter 3, p. 35, 1977, Maki Shoten Publishing Co., Ltd.)

The glass transition temperature (Tg) of the monomer (C) as a homopolymer is preferably -20°C or higher, more preferably -10°C or higher, even more preferably -4°C or higher, even more preferably 10°C or higher, and particularly preferably 20°C or higher, and is preferably 160°C or lower, more preferably 140°C or lower, even more preferably 120°C or lower, and even more preferably 95°C or lower. In other words, the glass transition temperature (Tg) of the monomer (C) is preferably -20 to 160°C, more preferably -10 to 140°C, even more preferably -4 to 120°C, and even more preferably 10 to 95°C. When the glass transition temperature (Tg) of the monomer (C) is within the above range, the adhesion and tackiness to the substrate can be good.

The surface tension of the monomer (C) is preferably 5 mN/m or more, more preferably 10 mN/m or more, even more preferably 15 mN/m or more, and is preferably 60 mN/m or less, more preferably 55 mN/m or less, even more preferably 50 mN/m or less. That is, the surface tension of the monomer (C) is preferably 5 to 60 mN/m, more preferably 10 to 55 mN/m, even more preferably 15 to 50 mN/m. When the surface tension of the monomer (C) is within the above range, the surface tension of the primer ink composition can be appropriate.

The viscosity of the monomer (C) is preferably 1.0 mPa·s or more, more preferably 2.0 mPa·s or more, even more preferably 3.0 mPa·s or more, even more preferably 5.0 mPa·s or more, and is preferably 50 mPa·s or less, more preferably 30 mPa·s or less, even more preferably 20 mPa·s or less. That is, the viscosity of the monomer (B) is preferably 1.0 to 50 mPa·s, more preferably 2.0 to 30 mPa·s, even more preferably 3.0 to 20 mPa·s, even more preferably 5.0 to 20 mPa·s. When the viscosity of the monomer (C) is within the above range, the viscosity of the primer ink composition can be appropriate.

The monomer (C) preferably includes a monomer containing a heterocyclic structure in the molecule. Examples of heteroatoms include oxygen atoms, nitrogen atoms, and sulfur atoms. It is more preferable that the monomer (C) includes a monomer containing a heterocyclic structure containing an oxygen atom or a nitrogen atom, and it is even more preferable that the monomer (C) includes at least one selected from the group consisting of cyclic trimethylolpropane formal acrylate, 3-ethyl-3-oxetanylmethyl (meth)acrylate, (2-methyl-2-ethyl-1,3-dioxolan-4-yl)methyl acrylate, N-vinyl-2-caprolactam, and acryloylmorpholine. When the monomer (C) includes a monomer containing a heterocyclic structure in the molecule, particularly cyclic trimethylolpropane formal acrylate, the printed matter strength and adhesion of the primer ink composition are further improved, tackiness is reduced, and better ejection stability can be exhibited even in high-speed printing.

As an embodiment of the present invention, when using the monofunctional monomer (A) containing an acidic group in the molecule and the monofunctional monomer (B) containing a hydroxyl group among the monofunctional monomers (B) containing an aromatic hydrocarbon-based ring structure in the molecule, it is preferable that the monofunctional monomer (C) contains a monomer having a heterocyclic structure containing an oxygen atom and/or a nitrogen atom. In these combinations, it is considered that the monofunctional monomer (C) contains a monomer having a heterocyclic structure containing an oxygen atom, and the polarity of the molecules becomes closer to each other due to the oxygen atom contained in the acidic group of the monofunctional monomer (A) and the hydroxyl group of the monofunctional monomer (B) and the oxygen atom of the monofunctional monomer (C), and the compatibility is improved. In addition, it is considered that the compatibility is improved by chemical interaction when the monofunctional monomer (C) contains a monomer having a heterocyclic structure containing a nitrogen atom.
Furthermore, since the monofunctional monomer (B) has an aromatic hydrocarbon-based cyclic structure, when the monofunctional monomer (C) has a monomer having a heterocyclic structure, the compatibility between the two monomers having similar chemical structures can be improved.

The content of monomer (C) is 60% by mass or more based on the total mass of the primer ink composition. When the content of monomer (C) is 60% by mass or more, the compatibility of the monomers can be improved, so that the print strength and adhesion (particularly adhesion when boiled) of the primer ink composition can be improved, tackiness can be reduced, and excellent ejection stability can be exhibited even in high-speed printing. The content of monomer (C) is preferably 66% by mass or more, more preferably 67% by mass or more, even more preferably 68% by mass or more, even more preferably 69% by mass or more, particularly preferably 70% by mass or more, and extremely preferably 71% by mass or more. When the content of monomer (C) is equal to or more than the lower limit, the print strength and adhesion (particularly adhesion when boiled) of the primer ink composition can be improved, tackiness can be reduced, and excellent ejection stability can be exhibited even in high-speed printing. In addition, the content of monomer (C) is preferably 90% by mass or less, more preferably 85% by mass or less.

The ratio (mass ratio) of the content of monomer (A) to the content of monomer (C) (monomer (A):monomer (C)) is preferably 1:2 to 1:15, more preferably 1:2.5 to 1:12, even more preferably 1:3 to 1:10, even more preferably 1:4 to 1:9.5, and particularly preferably 1:5.5 to 1:9. When the ratio of the content of monomer (A) to the content of monomer (C) is equal to or greater than the lower limit and equal to or less than the upper limit, monomer (C) can dissolve monomer (A) well, improving the printed matter strength and adhesion of the primer ink composition, reducing tackiness, and exhibiting excellent ejection stability even in high-speed printing.

The ratio of the content of monomer (B) to the content of monomer (C) is preferably 1:5 to 1:25, more preferably 1:8 to 1:20, even more preferably 1:10 to 1:18, and even more preferably 1:12 to 1:18. When the ratio of the content of monomer (B) to the content of monomer (C) is equal to or greater than the lower limit and equal to or less than the upper limit, monomer (C) can dissolve monomer (B) well, improving the print strength and adhesion of the primer ink composition, reducing tackiness, and showing excellent ejection stability even in high-speed printing.

The ratio of the total content of monomers (A) and (B) to the content (mass) of monomer (C) (monomer (A) + monomer (B): monomer (C)) is preferably 1:1 to 1:7, more preferably 1:2 to 1:7, even more preferably 1:2.7 to 1:7, even more preferably 1:3.6 to 1:7, and particularly preferably 1:4.5 to 1:6.5. When the ratio of the total content of monomers (A) and (B) to the content of monomer (C) is equal to or greater than the lower limit and equal to or less than the upper limit, monomer (C) can dissolve monomers (A) and (B) well, so that the printed matter strength and adhesion of the primer ink composition are improved, tackiness is reduced, and excellent ejection stability can be exhibited even in high-speed printing.

The primer ink of the present invention may contain, in addition to the above-mentioned monofunctional monomers (A), (B) and (C), a monofunctional monomer having a structure different from the above-mentioned three types of monofunctional monomers as a polymerizable compound.

Specific examples include 2-ethylhexyl diglycol (meth)acrylate, 2-ethylhexyl (meth)acrylate, butoxyethyl (meth)acrylate, 2-chloroethyl (meth)acrylate, 4-bromobutyl (meth)acrylate, cyanoethyl (meth)acrylate, butoxymethyl (meth)acrylate, 3-methoxybutyl (meth)acrylate, alkoxymethyl (meth)acrylate, alkoxyethyl (meth)acrylate, 2-(2-methoxyethoxy)ethyl (meth)acrylate, 2-(2-butoxyethoxy)ethyl (meth)acrylate, 2,2,2-trifluoroethyl (meth)acrylate, 1H,1H,2H,2H-perfluorodecyl (meth)acrylate, hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, acrylate, diethylaminoethyl (meth)acrylate, dimethylaminopropyl (meth)acrylate, diethylaminopropyl (meth)acrylate, trimethoxysilylpropyl (meth)acrylate, trimethylsilylpropyl (meth)acrylate, polyethylene oxide monomethyl ether (meth)acrylate, oligoethylene oxide monomethyl ether (meth)acrylate, polyethylene oxide (meth)acrylate, oligoethylene oxide (meth)acrylate, oligoethylene oxide monoalkyl ether (meth)acrylate, polyethylene oxide monoalkyl ether (meth)acrylate, dipropylene glycol (meth)acrylate, polypropylene oxide monoalkyl ether (meth)acrylate, oligopropylene oxide monoalkyl ether (meth)acrylate, 2-methacryloyloxyethyl succinate, butoxydiethylene glycol (meth)acrylate, trifluoroethyl (meth)acrylate and perfluorooctylethyl (meth)acrylate. In addition, (meth)acrylamides such as (meth)acrylamide, N-butoxymethyl(meth)acrylamide, N-methylol(meth)acrylamide, and (meth)acryloylmorpholine, and vinyl ethers such as methoxyethyl vinyl ether, ethoxyethyl vinyl ether, butoxyethyl vinyl ether, methoxyethoxyethyl vinyl ether, ethoxyethoxyethyl vinyl ether, methoxypolyethylene glycol vinyl ether, 2-hydroxyethyl vinyl ether, 2-hydroxypropyl vinyl ether, 4-hydroxybutyl vinyl ether, diethylene glycol monovinyl ether, polyethylene glycol vinyl ether, chloroethyl vinyl ether, chlorobutyl vinyl ether, and chloroethoxyethyl vinyl ether can be used.
In addition to the above, vinyl esters (vinyl acetate, vinyl propionate, vinyl versatate, etc.), allyl esters (allyl acetate, etc.), halogen-containing monomers (vinylidene chloride, vinyl chloride, etc.), vinyl cyanides ((meth)acrylonitrile, etc.), and olefins (ethylene, propylene, etc.) can be used.
These may be used alone or in combination of two or more.

The primer ink composition of the present invention may contain a polyfunctional monomer in addition to the monofunctional monomer described above. However, if a polyfunctional monomer is contained, the distortion between the printed matter and the recording medium may increase due to cure shrinkage, and the adhesion of the primer ink composition (particularly the adhesion when boiled) may decrease. In addition, the viscosity of the primer ink composition may increase, which may deteriorate the ejection stability. Therefore, when the primer ink composition of the present invention contains a bifunctional or higher polyfunctional monomer, it is preferably 10% by mass or less based on the mass of the entire primer ink composition, more preferably 8% by mass or less, even more preferably 5% by mass or less, even more preferably 3% by mass or less, and it is particularly preferable that it does not contain any. In other words, it is preferable that all the polymerizable compounds contained in the primer ink composition of the present invention are monofunctional polymerizable compounds.

The content of the polymerizable compound in the primer ink composition of the present invention is preferably 80 to 98% by mass, and more preferably 85 to 95% by mass, based on the total mass of the primer ink composition.

[Photopolymerization initiator]
The primer ink composition of the present invention contains a photopolymerization initiator. The photopolymerization initiator is not particularly limited as long as it is a compound that can initiate polymerization of the primer ink composition by irradiation with light.

Examples of photopolymerization initiators include benzoin compounds having 14 to 18 carbon atoms (e.g., benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin propyl ether, benzoin isobutyl ether, etc.), acetophenone compounds having 8 to 18 carbon atoms (e.g., acetophenone, 2,2-diethoxy-2-phenylacetophenone, 2,2-diethoxy-2-phenylacetophenone, 1,1-dichloroacetophenone, 2-hydroxy-2-methyl-phenylpropan-1-one, diethoxyacetophenone, 1-hydroxycyclohexyl phenyl ketone, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one, etc.), anthraquinone compounds having 14 to 19 carbon atoms (e.g., 2-ethylanthraquinone, 2-t-butylanthraquinone, 2-chloroanthraquinone, 2-amylanthraquinone, etc.), non, etc.), thioxanthone compounds having 13 to 17 carbon atoms [e.g., 2,4-diethylthioxanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone, etc.], ketal compounds having 16 to 17 carbon atoms [e.g., acetophenone dimethyl ketal, benzyl dimethyl ketal, etc.], benzophenone compounds having 13 to 21 carbon atoms [e.g., benzophenone, 4-benzoyl-4'-methyldiphenyl sulfide, 4,4'-bismethylaminobenzophenone, etc.], acylphosphine oxide compounds having 22 to 28 carbon atoms [e.g., 2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide, bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide], mixtures of these compounds, etc. These may be used alone or in combination of two or more. Among these, from the viewpoint of curability, it is preferable to include at least one selected from acetophenone compounds and acylphosphine oxide compounds, and 1-hydroxycyclohexyl phenyl ketone, 2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one, etc. are preferable. In addition, as the photopolymerization initiator, commercially available products may be used, such as DAROCURE TPO, IRGACURE 819, IRGACURE 184, IRGACURE 907, etc. manufactured by BASF Corporation.

The content of the photopolymerization initiator in the primer ink composition of the present invention is preferably 5.1% by mass or more, more preferably 6% by mass or more, even more preferably 7% by mass or more, even more preferably 8% by mass or more, particularly preferably 9% by mass or more, and preferably 20% by mass or less, more preferably 15% by mass or less, based on the mass of the entire primer ink composition. When the content of the photopolymerization initiator is equal to or greater than the lower limit, the amount of unreacted monomer components is reduced and the monomer components are less likely to precipitate, so that the strength and adhesion of the printed matter are improved and tackiness is reduced. On the other hand, when the content of the photopolymerization initiator is equal to or less than the upper limit, the amount of photopolymerization initiator remaining undissolved in the primer ink composition is reduced and the undissolved photopolymerization initiator is less likely to precipitate, so that ejection stability during high-speed printing is good.

The primer ink composition of the present invention may contain other additives as necessary, provided that the effects of the present invention are not impaired. Examples of other additives include surface conditioners, polymerization inhibitors, sensitizers, co-sensitizers, storage stabilizers, preservation stabilizers, antioxidants, colorants, UV absorbers, light stabilizers, chain transfer agents, conductive salts, fillers, organic solvents, diluting solvents, thickeners, etc.

The surface conditioner is used to improve the wettability of the recording medium and to prevent cissing.
Specific examples of surface conditioners that can be used in the present invention include anionic surface conditioners such as dialkyl sulfosuccinates, alkyl naphthalene sulfonates, and fatty acid salts; nonionic surface conditioners such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl allyl ethers, acetylene glycols, and polyoxyethylene-polyoxypropylene block copolymers; cationic surface conditioners such as alkylamine salts and quaternary ammonium salts; silicone-based surface conditioners; and fluorine-based surface conditioners.

The amount of surface conditioner added is selected appropriately depending on the intended use, but is generally preferably 0.0001 to 1% by mass relative to the total mass of the primer ink composition. By adjusting the amount of surface conditioner added as necessary within the above range, the surface tension of the primer ink composition can be adjusted.

The polymerization inhibitor is used to suppress excessive polymerization at the temperature at which the primer ink composition is ejected, and improves the storage stability of the primer ink composition and the ejection stability from an inkjet head.
Specific examples of the polymerization inhibitor that can be used in the present invention include nitroso-based polymerization inhibitors, hydroquinone, methoxyhydroquinone, benzoquinone, p-methoxyphenol, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), 4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl [TEMPOL (HO-TEMPO)], cupferron Al, and hindered amines.

The content of the polymerization inhibitor is generally preferably 0.001 to 1.5% by mass relative to the total mass of the primer ink composition. When the content of the polymerization inhibitor is within this range, the storage stability of the primer ink composition is further improved, and the ejection stability from the inkjet head is further improved.

The primer ink composition according to this embodiment is preferably transparent so as not to affect the image quality of the image formed on the primer layer. In other words, it is preferable that it does not contain a colorant. Furthermore, if the primer ink composition contains a colorant, the content of non-curable components increases, which may reduce adhesion to the substrate. For these reasons, it is also preferable that the primer ink composition does not contain a colorant.

The primer ink composition of the present invention is applied onto a recording medium by an inkjet system, and therefore the physical properties of the primer ink composition must be suitable for the inkjet system.
Specifically, the surface tension of the primer ink composition of the present invention is preferably 20 mN/m or more, more preferably 25 mN/m or more, and preferably 35 mN/m or less, more preferably 32 mN/m or less. When the surface tension of the primer ink composition is equal to or more than the lower limit and equal to or less than the upper limit, the droplets ejected from the nozzle can be normally formed even during high-speed ejection, and an image can be properly drawn. The surface tension can be adjusted within the above range by appropriately adjusting, for example, the type and/or amount of the monofunctional monomer; the type and/or amount of the surface conditioner, etc. The surface tension can be measured, for example, using a surface tensiometer, and can be measured by the method described in the examples.

The viscosity of the primer ink composition of the present invention at 25°C is preferably 50 mPa·s or less, more preferably 40 mPa·s or less, and even more preferably 30 mPa·s or less. When the viscosity is equal to or less than the upper limit, the ink can be appropriately ejected from the ink head even at high speed. There is no particular limit to the lower limit of the viscosity of the primer ink composition, but it is usually about 3.0 mPa·s. The viscosity can be adjusted to within the above range by appropriately adjusting, for example, the type and/or amount of the monofunctional monomer; the type and/or amount of the photopolymerization initiator, etc. The viscosity can be measured, for example, using a viscometer, and can be measured by the method described in the examples.

The method for producing the primer ink composition of the present invention is not particularly limited, and it can be produced, for example, by uniformly mixing the components that make up the primer ink composition using a mixing and stirring device or the like.

<Ink set>
In general, in inkjet recording methods, adjacent ink droplets applied with overlapping portions to obtain high image density remain on the recording medium and contact each other before drying, so that adjacent ink droplets merge with each other, causing image bleeding and unevenness in the line width of thin lines, which is likely to impair sharp image formation. In the present invention, by applying a primer layer onto a recording medium using the primer ink composition of the present invention, it is possible to suppress the coalescence of adjacent ink droplets due to the interaction between the primer ink and the ink droplets forming the image. This effectively prevents image bleeding, uneven line widths of thin lines in an image, and color unevenness on a colored surface.

Here, adjacent ink droplets refer to droplets ejected from an ink ejection port using ink of a single color and having overlapping portions, or droplets ejected from an ink ejection port using ink of different colors and having overlapping portions. Adjacent ink droplets may be droplets that are ejected simultaneously, or may be a preceding droplet and a succeeding droplet that are ejected in a preceding and succeeding droplet relationship.

Therefore, the primer ink composition of the present invention is preferably used together with at least one type of image-forming ink composition as a liquid for forming an image.
In the present invention, there is no particular restriction on the image-forming ink composition used together with the primer ink composition of the present invention, and any known ink composition conventionally used in inkjet systems can be used, but a color ink composition containing a photocurable polymerizable compound and a colorant, and/or a clear ink composition containing a photocurable polymerizable compound but no colorant, is preferred. That is, in the present invention, the image-forming ink composition includes a color ink composition and a clear ink composition.
Therefore, the present invention encompasses an ink set comprising the primer ink composition of the present invention and a color ink composition and/or a clear ink composition containing a colorant.

Below, we will explain an image-forming ink composition that is particularly suitable for use with the primer ink composition of the present invention. In this specification, the term "image-forming ink composition" means each ink composition that constitutes the image-forming ink used to form an image (normally, an image-forming ink is composed of multiple image-forming ink compositions).

The image-forming ink composition is configured to be a composition for forming at least an image. Specifically, for example, it contains at least one polymerizable compound, and, if necessary, contains a colorant, a polymerization initiator, a polymerization inhibitor, a surface conditioner, and other additives.

As the polymerizable compound used in the image-forming ink composition, from the viewpoint of curing speed, (meth)acrylates and (meth)acrylamides are preferred. In particular, from the viewpoints of print strength and low tackiness, it is preferable to mainly use polyfunctional polymerizable compounds, and it is preferable to contain a tetrafunctional or higher functional (meth)acrylate. Furthermore, from the viewpoints of reducing the viscosity and improving the adhesive strength of the image-forming ink composition, it is preferable to use a trifunctional or higher functional (meth)acrylate in combination with a monofunctional (meth)acrylate or a bifunctional (meth)acrylate or (meth)acrylamide.

The mixing ratio of the monofunctional polymerizable compound to the polyfunctional polymerizable compound in the image-forming ink composition is preferably 9:1 to 1:9, more preferably 8:2 to 2:8, and even more preferably 4:6 to 2:8, in terms of mass ratio (monofunctional polymerizable compound:polyfunctional polymerizable compound). When the mixing ratio of the monofunctional polymerizable compound to the polyfunctional polymerizable compound is within the above-mentioned range, the curing speed and viscosity of the image-forming ink composition can be appropriate.
The content of the polymerizable compound is preferably from 50 to 99.6% by mass, and more preferably from 60 to 99.0% by mass, based on the solid content (mass) of the image-forming ink composition.

As the polymerization initiator, for example, the photopolymerization initiators exemplified as those usable in the primer ink composition of the present invention can be appropriately selected and used. The polymerization initiators can be used alone or in combination of two or more kinds. They can also be used in combination with a known sensitizer for the purpose of improving sensitivity.

It is preferable to appropriately select and include a polymerization initiator within a range that allows the storage stability of the image-forming ink composition to be maintained to a desired degree. The content is, for example, preferably 0.1 to 20 mass % relative to the solid content of the image-forming ink composition, and more preferably 0.5 to 15 mass %. Furthermore, the content ratio of the polymerization initiator to the polymerizable compound is preferably 0.5:100 to 30:100, and more preferably 1:100 to 20:100, in mass ratio (polymerization initiator:polymerizable compound).

The image forming ink may be a multi-color ink set consisting of a plurality of color ink compositions. The image forming ink may basically contain at least one colored ink composition selected from the group consisting of a cyan ink composition (cyan ink composition), a magenta ink composition (magenta ink composition), a yellow ink composition (yellow ink composition), a black ink composition (black ink composition), and a white ink composition (white ink composition). In some cases, an image with excellent color reproducibility can be formed by using special colors such as violet, blue, green, orange, and red.

Therefore, it is preferable that the color ink composition contains at least one type of colorant. Each color ink composition contains at least one type of colorant that exhibits the corresponding color. The colorant used in the color ink composition is not particularly limited and can be appropriately selected from known water-soluble dyes, oil-soluble dyes, pigments, etc., but since the primer ink composition of the present invention is non-aqueous, it is preferable that the colorant of the color ink composition used together with the primer ink composition of the present invention is an oil-soluble dye or pigment that is easily dispersed and dissolved uniformly in a non-aqueous medium, and it is particularly preferable to use a pigment.

As the pigment, either an organic pigment or an inorganic pigment can be used.
Preferred examples of black pigments include carbon black pigments, etc. Generally, pigments of the three primary colors of black, cyan, magenta, and yellow are used, but other hues, such as metallic luster pigments of gold, silver, etc., and colorless or pale-colored extender pigments, can also be used depending on the purpose.

Examples of pigments that exhibit a magenta color include monoazo pigments such as C.I. Pigment Red 3 (Toluidine Red, etc.), disazo pigments such as C.I. Pigment Red 38 (Pyrazolone Red B, etc.), azo lake pigments such as C.I. Pigment Red 53:1 (Lake Red C, etc.) and C.I. Pigment Red 57:1 (Brilliant Carmine 6B), condensed azo pigments such as C.I. Pigment Red 144 (Condensed Azo Red BR, etc.), perinone pigments such as C.I. Pigment Red 194 (Perinone Red, etc.), perylene pigments such as C.I. Pigment Red 149 (Perylene Scarlet, etc.), and C.I. Examples of the pigments include quinacridone pigments such as C.I. Pigment Violet 19 (unsubstituted quinacridone, CINQUASIA Magenta RT-355T; manufactured by Ciba Specialty Chemicals), C.I. Pigment Red 122 (dimethylquinacridone), and C.I. Pigment Red 202 (dichloroquinacridone), isoindolinone pigments such as C.I. Pigment Red 180 (isoindolinone red 2BLT, etc.), and alizarin lake pigments such as C.I. Pigment Red 83 (madder lake, etc.).

Cyan pigments include disazo pigments such as C.I. Pigment Blue 25 (dianisidine blue, etc.), phthalocyanine pigments such as C.I. Pigment Blue 15, C.I. Pigment Blue 15:3 (IRGALITE BLUE GLO; manufactured by Chiba Specialty Chemicals), and C.I. Pigment Blue 15:4, acid dye lake pigments such as C.I. Pigment Blue 24 (peacock blue lake, etc.), and alkali blue pigments such as C.I. Pigment Blue 18 (alkali blue V-5:1), etc.

Pigments that exhibit a yellow color include monoazo pigments such as C.I. Pigment Yellow 1 (Fast Yellow G, etc.) and C.I. Pigment Yellow 74, disazo pigments such as C.I. Pigment Yellow 12 (Disazo Yellow AAA, etc.) and C.I. Pigment Yellow 17, non-benzidine azo pigments such as C.I. Pigment Yellow 180 and C.I. Pigment Yellow 200 (Novoperm Yellow 2HG), azo lake pigments such as C.I. Pigment Yellow 100 (Tartrazine Yellow Lake, etc.), condensed azo pigments such as C.I. Pigment Yellow 95 (Condensed Azo Yellow GR, etc.), acid dye lake pigments such as C.I. Pigment Yellow 115 (Quinoline Yellow Lake, etc.), and the like. Examples of the pigment include basic dye lake pigments such as C.I. Pigment Yellow 18 (thioflavin lake, etc.), anthraquinone pigments such as Flavanthrone Yellow (Y-24), isoindolinone pigments such as Isoindolinone Yellow 3RLT (Y-110), quinophthalone pigments such as Quinophthalone Yellow (Y-138), isoindoline pigments such as Isoindoline Yellow (Y-139), nitroso pigments such as C.I. Pigment Yellow 153 (nickel nitroso yellow, etc.), metal complex azo pigments such as C.I. Pigment Yellow 150 (nickel complex salt), and metal complex azomethine pigments such as C.I. Pigment Yellow 117 (copper azomethine yellow, etc.).

Black pigments include carbon black, titanium black, and aniline black. An example of carbon black is SPECIAL BLACK 250 (manufactured by Degussa).

Examples of white pigments include C.I. Pigment White 6, 18, and 21. Specific examples of white pigments include basic lead carbonate (2PbCO ₃ Pb(OH) ₂ , also known as silver white), zinc oxide (ZnO, also known as zinc white), titanium oxide (TiO ₂ , also known as titanium white), and strontium titanate (SrTiO ₃ , also known as titanium strontium white). Titanium oxide has a smaller specific gravity than other white pigments, a larger refractive index, and is chemically and physically stable, so it has a large hiding power and coloring power as a pigment, and is also excellent in durability against acids, alkalis, and other environments. Therefore, it is preferable to use titanium oxide as the white pigment. Other white pigments (which may be other than the white pigments listed) may be used as necessary.

Among these known pigments, the pigment that exhibits a cyan color is preferably Pigment Blue 15:3 or Pigment Blue 15:4, the pigment that exhibits a magenta color is preferably Pigment Red 122, Pigment Red 202 or Pigment Violet 19, and the pigment that exhibits a yellow color is more preferably Pigment Yellow 150, Pigment Yellow 180 or Pigment Yellow 155. In addition, the pigment that exhibits a white color is preferably titanium oxide.

In order to form an image having excellent color reproducibility, the following pigments may be used as characteristic colors such as violet, blue, green, orange and red, if necessary.
Pigments that exhibit a violet color include C.I. Pigment Violet 1 (Rhodamine B), C.I. Pigment Violet 2 (Rhodamine 3B), C.I. Pigment Violet 3 (Methyl Violet Lake), C.I. Pigment Violet 3:1 (Methyl Violet Lake), C.I. Pigment Violet 3:3 (Methyl Violet Lake), C.I. Pigment Violet 5:1 (Alizarin Maroon), C.I. Pigment Violet 13 (Ultramarine Pink), C.I. Pigment Violet 17 (Naphthol AS), C.I. Pigment Violet 23 (Dioxazine Violet), C.I. Pigment Violet 25 (Naphthol AS), C.I. Pigment Violet 29 (Perylene Violet), C.I. C.I. Pigment Violet 31 (Violanthrone Violet), C.I. Pigment Violet 32 (Benzimidazolone Bordeaux HF3R), C.I. Pigment Violet 36 (Thioindigo), C.I. Pigment Violet 37 (Dioxazine Violet), C.I. Pigment Violet 42 (Quinacridone Maroon B), C.I. Pigment Violet 50 (Naphthol AS) and the like are commercially available.

Commercially available blue pigments include C.I. Pigment Blue 1, C.I. Pigment Blue 2, C.I. Pigment Blue 16, C.I. Pigment Blue 22, C.I. Pigment Blue 60, and C.I. Pigment Blue 66.

Commercially available green pigments include C.I. Pigment Green 1 (Brilliant Green Lake), C.I. Pigment Green 4 (Malachite Green Lake), C.I. Pigment Green 7 (Phthalocyanine Green), C.I. Pigment Green 8 (Pigment Green B), C.I. Pigment Green 10 (Nickel Azo Yellow), and C.I. Pigment Green 36 (Brominated Phthalocyanine Green).

Orange pigments include C.I. Pigment Orange 1 (Hansa Yellow 3R), C.I. Pigment Orange 2 (Orthonitro Orange), C.I. Pigment Orange 3 (β-naphthol), C.I. Pigment Orange 4 (Naphthol AS), C.I. Pigment Orange 5 (β-naphthol), C.I. Pigment Orange 13 (Pyrazolone Orange G), C.I. Pigment Orange 15 (Disazo Orange), C.I. Pigment Orange 16 (Anisidine Orange), C.I. Pigment Orange 17 (Persian Orange Lake), C.I. Pigment Orange 19 (Naphthalene Yellow Lake), C.I. Pigment Orange 24 (Naphthol Orange Y), C.I. Pigment Orange 31 (Condensed Azo Orange), C.I. C.I. Pigment Orange 34 (Disazopyrazolone Orange), C.I. Pigment Orange 36 (Benzimidazolone Orange HL), C.I. Pigment Orange 38 (Naphthol Orange), C.I. Pigment Orange 40 (Pyranthrone Orange), C.I. Pigment Orange 43 (Perinone Orange), C.I. Pigment Orange 46 (Ethyl Red Lake C), C.I. Pigment Orange 48 (Quinacridone Gold), C.I. Pigment Orange 49 (Quinacridone Gold), C.I. Pigment Orange 51 (Pyranthrone Orange), C.I. Pigment Orange 60 (Imidazolone Orange HGL), C.I. Pigment Orange 61 (Isoindolinone Orange), C.I. Pigment Orange 62 (Benzimidazolone Orange H5G), C.I. C.I. Pigment Orange 64 (benzimidazolone), C.I. Pigment Orange 65 (azomethine orange), C.I. Pigment Orange 66 (isoindolinone orange), C.I. Pigment Orange 67 (pyrazoloquinazolone orange), C.I. Pigment Orange 68 (azomethine orange), C.I. Pigment Orange 69 (isoindolinone orange), C.I. Pigment Orange 71 (diketopyrrolopyrrole orange), C.I. Pigment Orange 72 (imidazolone orange H4GL), C.I. Pigment Orange 73 (diketopyrrolopyrrole orange), C.I. Pigment Orange 74 (naphthol orange 2RLD), C.I. Pigment Orange 81 (diketopyrrolopyrrole orange), etc. are commercially available.

Commercially available red pigments include C.I. Pigment Red 171, C.I. Pigment Red 175, C.I. Pigment Red 176, C.I. Pigment Red 177, C.I. Pigment Red 209, C.I. Pigment Red 220, C.I. Pigment Red 224, C.I. Pigment Red 242, C.I. Pigment Red 254, C.I. Pigment Red 255, C.I. Pigment Red 264, and C.I. Pigment Red 270.

Among these, from the viewpoints of color reproducibility, light resistance, and pigment dispersion stability, C.I. Pigment Violet 23 is preferred as a pigment that exhibits a violet color, C.I. Pigment Orange 36 and C.I. Pigment Orange 71 are preferred as pigments that exhibit an orange color, and C.I. Pigment Green 7 and C.I. Pigment Green 36 are preferred as pigments that exhibit a green color.

In the present invention, it is preferable to use a color ink composition containing the above-mentioned characteristic pigment, and a color ink composition containing a cyan, magenta, yellow, black, or white pigment and/or dye.

To disperse the colorant, a dispersing device such as a bead mill, ball mill, sand mill, attritor, roll mill, jet mill, homogenizer, paint shaker, kneader, agitator, Henschel mixer, colloid mill, ultrasonic homogenizer, pearl mill, wet jet mill, etc. can be used, and a mixer such as a line mixer can also be used. Furthermore, after dispersing the colorant, classification processing can be performed using a centrifuge, filter, cross flow, etc., in order to remove coarse particles of the colorant. When dispersing the colorant, a dispersing agent can be added. When adding the colorant, a synergist corresponding to each colorant can also be used as a dispersing aid, if necessary. A solvent can be added as a dispersing medium for various components such as the colorant, and a polymerizable compound that is a solvent-free low molecular weight component can also be used as a dispersing medium. Since the primer ink composition of the present invention is preferably a photocurable liquid, the image forming ink composition used therewith is also preferably a photocurable liquid, and since the primer ink composition and the image forming ink composition are cured after application onto a recording medium, they are preferably solvent-free. If solvent remains in the formed image, the solvent resistance of the image may decrease, or problems may occur with the VOC (Volatile Organic Compound) of the remaining solvent. From this perspective, it is preferable to use a polymerizable compound as the dispersion medium, and among them, to select a polymerizable compound with a low viscosity from the viewpoint of improving the dispersion suitability and the handleability of the ink composition.

Since the finer the average particle size of the colorant used, the better the color development, the average particle size of the colorant is preferably 0.01 μm or more and 0.4 μm or less, and more preferably 0.02 μm or more and 0.2 μm or less. It is preferable to select the colorant, dispersant, and dispersion medium, and set the dispersion conditions, classification conditions, and filtration conditions so that the maximum particle size is preferably 3 μm or less, more preferably 1 μm or less. By controlling the particle size in this way, clogging of the head nozzle can be suppressed, and the storage stability, transparency, and curing sensitivity of the ink composition can be maintained.
The particle size of the colorant can be measured by a known method, such as a centrifugal sedimentation light transmission method, an X-ray transmission method, a laser diffraction/scattering method, or a dynamic light scattering method.

The content of the colorant in the color ink composition is appropriately selected depending on the color and the purpose of use. From the viewpoints of image density and storage stability, the content of the colorant is preferably 0.1 to 30 mass % and more preferably 0.5 to 20 mass % relative to the total mass of the image-forming ink composition.
The colorants may be used alone or in combination of two or more. Also, two or more organic pigments or solid solutions of organic pigments may be used in combination. Furthermore, different colorants may be used for each of the droplets and liquids to be ejected, or the same colorant may be used.

It is preferable to add a dispersant when dispersing the colorant. The type of dispersant is not particularly limited, but it is preferable to use a known polymer dispersant. Examples of polymer dispersants include DisperBYK-101, DisperBYK-102, DisperBYK-103, DisperBYK-106, DisperBYK-111, DisperBYK-161, DisperBYK-162, DisperBYK-163, DisperBYK-164, DisperBYK-166, DisperBYK-167, DisperBYK-16 8. DisperBYK-170, DisperBYK-171, DisperBYK-174, DisperBYK-182 (manufactured by BYK Chemie), EFKA4010, EFKA4046, EFKA4080, EFKA5010, EFKA5207, EFKA5244 , EFKA6745, EFKA6750, EFKA7414, EFKA7462, EFKA7500, EFKA7570, EFK Polymer dispersants such as EFKA A7575, EFKA A7580 (all manufactured by EFKA Additive Co., Ltd.), Disperse Aid 6, Disperse Aid 8, Disperse Aid 15, and Disperse Aid 9100 (manufactured by San Nopco); Solsperse 3000, 5000, 9000, 12000, 13240, 13940, 17000, 24000, 26000, 28000, 32000, 33000, 3 6000, 39000, 41000, 71000, and various Solsperse dispersants (manufactured by Avecia); Adeka Pluronic L31, F38, L42, L44, L61, L64, F68, L72, P95, F77, P84, F87, P94, L101, P103, F108, L121, P-123 (manufactured by ADEKA Corporation) and Isonet S-20 (manufactured by Sanyo Chemical Industries, Ltd.), and Kusumoto Chemical Industries, Ltd.'s "Disparlon KS-860, 873SN, 874 (polymer dispersant), #2150 (aliphatic polycarboxylic acid), #7004 (polyether ester type)".
Pigment derivatives such as phthalocyanine derivatives (product name: EFKA-745 (manufactured by EFKA Corporation)), Solsperse 5000, 12000, and Solsperse 22000 (manufactured by Avecia Corporation) can also be used. By using the dispersants with excellent dispersibility and stability, a uniform and stable dispersion can be obtained even when a fine particle colorant is used.

The amount of dispersant contained is selected appropriately depending on the intended use, but is preferably 0.01 to 5% by mass relative to the total mass of the color ink composition.

The clear ink composition in the ink set of the present invention is a highly transparent ink that does not contain a pigment or contains only a small amount of a pigment and/or dye such as a bluing agent. The pigment content in the clear ink composition is usually 0.1 mass % or less, and more preferably 0.05 mass % or less, based on the total amount of the clear ink composition. The lower limit of the pigment content in the clear ink composition is 0 mass %.

The manufacturing method of the color ink composition and the clear ink composition is not particularly limited, and for example, they can be manufactured by uniformly mixing the above-mentioned components using a mixing and stirring device or the like.

The image-forming ink composition used together with the primer ink of the present invention can contain, in addition to the above-mentioned components, sensitizers, co-sensitizers, storage stabilizers, preservation stabilizers, antioxidants, UV absorbers, light stabilizers, chain transfer agents, conductive salts, fillers, organic solvents, diluting solvents, thickeners and other additives depending on the purpose.

<Inkjet recording method>
In the inkjet recording method using the ink set of the present invention, an image is formed on a recording medium made of a non-absorbent substrate using the ink set described above. More specifically, the primer ink composition described above is ejected from an inkjet head onto a recording medium and photocured, and then the color ink composition and/or the clear ink composition described above is ejected from an inkjet head onto the photocured primer ink composition and photocured.

In the present invention, it is preferable to apply the primer ink to the same area as the image formed by discharging the image-forming ink composition onto the recording medium or to an area larger than the image. That is, one specific embodiment of the inkjet recording method using the primer ink composition of the present invention includes a step of discharging the primer ink composition of the present invention onto the same area as the image formed by droplets of the image-forming ink composition of multiple colors deposited on the recording medium or to an area larger than the image, a step of applying light to the primer ink composition discharged onto the recording medium, and a step of discharging droplets of the image-forming ink composition of multiple colors onto the photocured primer ink composition. In particular, when discharging the primer ink composition onto an area larger than the image formed by droplets of the image-forming ink composition, it is important that the printed matter of the primer ink composition has high strength and low tackiness from the viewpoint of the case where the printed matter is stored in layers and the processability in the subsequent process.

The photocuring of the primer ink composition and the color ink composition and/or the clear ink composition may be performed simultaneously with the ejection or immediately after the ejection. The reactivity (monomer double bond reaction rate) of the polymerizable compound in the primer ink composition may be adjusted according to the process or application. For example, when the primer ink composition is semi-cured (monomer double bond reaction rate is about 50%), the monomers of the primer ink composition may copolymerize at the interface with the color ink composition, and stronger adhesion may be exhibited. Ultimately, from the viewpoint of the strength of the printed matter and low tackiness, it is preferable that both the primer ink composition and the color ink composition are fully cured (monomer double bond reaction rate is 90 to 100%). The reaction rate can be determined, for example, by measuring the infrared absorption spectrum (IR) of the coating film. Examples of the light for photocuring include far infrared rays, infrared rays, visible light, near ultraviolet rays, ultraviolet rays, and the like. Among these, near ultraviolet rays or ultraviolet rays are preferable from the viewpoint of ease and efficiency of the curing work.

The amount of energy required for the photocuring reaction varies depending on the type and content of the polymerization initiator, but in general, it is preferable that the integrated light amount be about 100 mJ/cm ² or more and 10,000 mJ/cm ² or less.

In the recording method of the present invention, the drop volume of the primer ink composition ejected from the inkjet head is preferably 2 pl or more and 30 pl or less, and more preferably 3 pl or more and 20 pl or less. The design resolution of the inkjet head is preferably 600 dpi or more. The primer ink composition of the present invention contains monomer (C), which provides excellent compatibility between monomers (A) and (B), and therefore has excellent print strength and adhesion, low tackiness, and excellent ejection stability even under the high-speed, high-definition conditions described above.

In the present invention, the droplets of the image forming ink ejected onto the primer ink are preferably ejected (preferably by an inkjet nozzle) with a droplet size of 1 pL to 25 pL, and more preferably with a droplet size of 1 pL to 15 pL. Droplet sizes within the above range are effective in that they can depict images with high sharpness and density.

The primer ink composition and the color ink composition are preferably ejected from the inkjet head so that the coating thickness on the non-absorbent substrate is 1 to 20 μm.

The non-absorbent substrate may be, for example, a plastic film, a substrate such as paper coated with plastic, or a substrate with a plastic film adhered thereto. Examples of plastic include polyvinyl chloride, polyethylene terephthalate, polycarbonate, polystyrene, polyurethane, polyethylene, polypropylene, acrylic resin, polyacetal, etc. Other non-absorbent substrates that may be used include metals such as aluminum, iron, gold, silver, copper, nickel, titanium, chromium, molybdenum, silicon, lead, zinc, and stainless steel, and glass.

The non-absorbent substrate is preferably subjected to a surface modification treatment. When the surface of the non-absorbent substrate is modified, the strength and adhesion of the printed matter of the primer ink composition can be further improved. Examples of such modification treatments include corona treatment, plasma treatment, and frame treatment. The surface modification treatment can be carried out by a method known in the art.

The present invention will be explained in more detail below with reference to examples. In the examples, "%" and "parts" are by mass % and parts by mass unless otherwise specified.

The components of the ink compositions used in each example and comparative example are shown in Table 1 below.

(Preparation of photocurable inkjet primer ink composition)
A photopolymerizable compound, a photopolymerization initiator, a polymerization inhibitor, and a surface conditioner were mixed uniformly using a mixer/stirrer according to the formulation shown in Table 2. The mixture was then suction filtered using a glass filter (manufactured by Kiriyama Seisakusho Co., Ltd.) to prepare the photocurable inkjet primer ink compositions of Examples 1 to 10 and Comparative Examples 1 to 9.

(SP value)
The SP value of a monomer is a value at 25°C obtained by the Fedors method (Harasaki Yuji, "Basic Science of Coating", Chapter 3, page 35, published by Maki Shoten in 1977), and specifically, is a value calculated according to the following method.
Fedors believes that both the cohesive energy density and the molar volume depend on the type and number of substituents, and has proposed the following formula and constants corresponding to each substituent.

In the formula, δ is the SP value (cal/cm ³ ) ^1/2 , ΔE is the cohesive energy density, V is the molar volume, Δei is the evaporation energy of each atom or atomic group (cal/mol), and Δvi is the molar volume of each atom or atomic group (cm ³ /mol).
In addition, for compounds having a glass transition temperature Tg of 25° C. or higher, the following value was added to the molar volume: when the number of main chain skeleton atoms in a repeating unit in the compound is n, 4n was added to Δvi when n<3, and 2n was added to Δvi when n≧3.

(Glass Transition Temperature)
The glass transition temperature (Tg) of the monomer when made into a homopolymer was measured by differential scanning calorimetry (DSC).

(Measurement of Surface Tension of Monomer)
The surface tension of the monomer was measured at 25° C. after 20 seconds from the start of measurement using a fully automatic balanced electrosurface tensiometer ESB-V (manufactured by Kyowa Interface Science Co., Ltd.).

(Measurement of Monomer Viscosity)
The viscosity of the monomer was measured using an R100 type viscometer (manufactured by Toki Sangyo Co., Ltd.) under conditions of 25° C. and a cone rotation speed of 5 rpm.

(Measurement of Viscosity of Composition)
The viscosity of the primer ink compositions of the examples and comparative examples was measured using an R100 type viscometer (manufactured by Toki Sangyo Co., Ltd.) under conditions of 25° C. and a cone rotation speed of 5 rpm.

(Measurement of surface tension)
The surface tension of the primer ink compositions of the Examples and Comparative Examples was measured at 25° C., 20 seconds after the start of measurement, using a fully automatic balanced electrosurface tensiometer ESB-V (manufactured by Kyowa Interface Science Co., Ltd.).

(Preparation of Photocurable Black Ink Composition for Inkjet)
First, a primary dispersion of colorant (pigment) was prepared as follows: Polymerizable compound HDDA, colorant MA8, dispersing aid S5000, and dispersing agent S32000 were weighed out into a plastic bottle in the amounts (unit: mass %) shown in Table 3, to which 100 parts by mass of zirconia beads having a diameter of 0.3 mm were added, and the mixture was dispersed for 1 hour using a paint conditioner.
Next, a pigment ink was prepared using the primary dispersion as follows: The remaining components were added to the primary dispersion in the amounts (unit: mass %) shown in Table 3, and the mixture was stirred for 30 minutes with a magnetic stirrer. After stirring, the mixture was suction filtered using glass fiber filter paper GFP (manufactured by Kiriyama Seisakusho, capture particle size 0.8 μm) to prepare a black ink composition.

(Preparation of print samples for evaluation)
The primer ink compositions of the examples and comparative examples were applied to each substrate (glass, SUS, POM (polyacetal) (corona treated)) using a bar coater to form a 15 μm thick solid ink coating film. This coating film was irradiated with ultraviolet light using a high pressure mercury lamp (peak illuminance: 300 mW/ ^cm2 ) as an irradiation means so that the total irradiation light amount was 500 mJ/ ^cm2 , and cured to obtain a solid print.
The black ink composition was then applied onto the primer ink solid print using a bar coater to form a 15 μm thick ink solid coating film. The coating film was irradiated with ultraviolet light using a high pressure mercury lamp (peak illuminance: 300 mW/cm ² ) as the irradiation means, with the total irradiation amount being 500 mJ/cm ² for glass and SUS, and 1500 mJ/cm ² for POM, to harden the coating film and obtain a solid print.
The corona treatment of the POM was carried out using a corona surface modification evaluation device (TEC-4AX, manufactured by Kasuga Electric Co., Ltd.) under conditions of an output of 100 W and an electrode moving speed of 1.5 m/min.

(Evaluation of print strength)
The primer ink compositions and the black ink composition of the Examples and Comparative Examples were used to visually inspect the solid prints formed on glass substrates by the above procedure, and the print strength was evaluated according to the following criteria: In other words, if the print strength of the primer layer is weak, it means that the primer layer is physically scraped off with a bar coater when the black layer is formed, and the unevenness of the coating film becomes apparent, and a flat film is not obtained.
◎: Both the primer layer and the black layer have a flat coating. ◯: Both the primer layer and the black layer have slight coating disturbances (horizontal stripes). △: The primer layer has been scraped off, and along with the black layer, coating disturbances (horizontal stripes) have occurred. ×: The primer layer has been scraped off significantly, and along with the black layer, a coating has not been formed.

(Evaluation of tackiness)
The primer ink compositions and the black ink composition of the Examples and Comparative Examples were used to form solid prints on glass substrates according to the above procedure, and the tackiness was evaluated by touching the prints with a finger according to the following criteria.
◎: Not sticky, ink does not stick to fingers. ◯: Slightly sticky, but ink does not stick to fingers. △: Sticky, ink sticks slightly to fingers. ×: Very sticky, ink sticks to fingers.

(Evaluation of Adhesion)
The solid prints formed by the above procedure using the primer ink compositions and the black ink composition of the Examples and Comparative Examples were cross-cut, and an adhesive sheet [Cellotape (registered trademark) (manufactured by Nichiban Co., Ltd.)] was attached to the adhesive layer side and then peeled off.
The adhesion was evaluated according to the following criteria.
◎: No peeling at all ◯: Slight peeling △: Partial peeling ×: Peeling occurred over the entire area where the adhesive sheet was attached

(Evaluation of boiling resistance)
A solid print formed on a POM substrate using the primer ink compositions and the black ink composition of the Examples and Comparative Examples in the above-mentioned procedure was immersed in boiling water and boiled for 20 minutes, the moisture on the surface was dried and removed, and then the print was cross-cut and an adhesive sheet [Cellotape (registered trademark) (manufactured by Nichiban Co., Ltd.)] was attached to the adhesive layer side and then peeled off.
The boiling resistance was evaluated according to the following criteria.
◎: No peeling at all ◯: Slight peeling △: Partial peeling ×: Peeling occurred over the entire area where the adhesive sheet was attached

(Filtration test)
The primer ink compositions of the Examples and Comparative Examples were filled into glass bottles, which were then subjected to one cycle of heat-cold testing in an environmental testing machine, in which the bottles were stored at 60° C. for 14 days and then at −10° C. for 14 days. Thereafter, the primer ink composition was suction-filtered using a SUS mesh (pore size: 5 μm) to check for the occurrence of precipitates in the primer ink composition, and the state of the residue on the mesh was observed using an optical microscope, and the storage stability of the primer ink composition was evaluated according to the following criteria.
◎: No residue 〇: Slight residue △: Residue ×: Large amount of residue

(Evaluation of ejection properties)
The primer ink compositions of the Examples and Comparative Examples and the above-mentioned black ink composition were filled into a commercially available LED-UV curing inkjet printer (droplet size 7 pl, resolution 720 x 600 dpi, LED wavelength 385 nm, peak illuminance 1200 mW/cm ² , cumulative light amount 2600 mJ/cm ² , UV lamp installed next to the head), ejected from the inkjet head onto a glass substrate, and each time an ink droplet landed on the substrate, it was simultaneously irradiated with UV light. Ten text prints were produced in succession, with the order from the printer stage being substrate/primer layer/color layer.
The ejection property was evaluated according to the following criteria.
◎: No missing ink droplets, no ink droplet misalignment, and the primer droplets and black droplets overlap. ◯: No missing ink droplets, slight ink droplet misalignment, but no practical problem. △: Missing ink droplets, ink droplet misalignment, and the primer effect cannot be achieved. ×: Many missing ink droplets, and characters cannot be formed.

The evaluation results are shown in Table 4.

Claims

A photocurable inkjet recording primer ink composition comprising a polymerizable compound and a photopolymerization initiator, the polymerizable compound comprising: a monofunctional monomer (A) having an acidic group in the molecule;
A monofunctional monomer (B) containing an aromatic hydrocarbon-based cyclic structure in the molecule, and a monofunctional monomer (C) containing an aliphatic hydrocarbon-based cyclic structure in the molecule.
Including,
The content of the monofunctional monomer (C) is 60 mass% or more based on the mass of the entire composition.
The composition according to claim 1, wherein the monofunctional monomer (C) comprises a monofunctional monomer (C') having an SP value of 9.4 (cal/cm 3 ) 1/2 or more and 12.0 (cal/cm 3 ) 1/2 or less.
The composition according to claim 1 or 2, wherein the ratio of the total content of the monofunctional monomers (A) and (B) to the content of the monofunctional monomer (C) in the composition is 1:1 to 1:7.
The composition according to claim 1 or 2, wherein the content ratio of monofunctional monomers (A) and (B) in the composition is 2.1:1 to 10:1.
The composition according to claim 1 or 2, wherein the monofunctional monomer (A) includes a monomer containing a carboxy group.
The composition according to claim 1 or 2, wherein the monofunctional monomer (B) includes a monomer containing a hydroxy group.
The composition according to claim 1 or 2, wherein the monofunctional monomer (C) includes cyclic trimethylolpropane formal acrylate.
The composition according to claim 1 or 2, wherein the content of the photopolymerization initiator is 5.1% by mass or more and 20% by mass or less based on the mass of the entire composition.
The composition according to claim 1 or 2, wherein the polymerizable compound does not contain a polyfunctional monomer.
The composition according to claim 1 or 2, having a surface tension of 20 mN/m or more and 35 mN/m or less.
The composition according to claim 1 or 2, having a viscosity of 50 mPa·s or less at 25°C.
3. An ink set comprising: the photocurable primer ink composition for inkjet recording according to claim 1 or 2; and a color ink composition and/or a clear ink composition containing a colorant.
An inkjet recording method for forming an image on a recording medium made of a non-absorbent substrate by using the ink set according to claim 12,
ejecting the primer ink composition from an inkjet head onto a recording medium and photo-curing the primer ink composition; and ejecting the color ink composition and/or the clear ink composition from an inkjet head onto the photo-cured primer ink composition and photo-curing the color ink composition and/or the clear ink composition.
The method of claim 13, wherein the non-absorbent substrate is surface-modified.
The method according to claim 13, wherein the design resolution of the inkjet head is 600 dpi or more.