JP7463894B2 - Inkjet recording device - Google Patents

Description

The present invention relates to an inkjet recording device.

Conventionally, as described in, for example, Patent Document 1, it is known that by providing each color ink composition containing a radical polymerizable compound including an N-vinyl compound and a radical polymerization initiator, and a clear ink composition containing a radical polymerizable compound, an acylphosphine oxide radical polymerization initiator, and a surfactant in a specified ratio, it is possible to obtain an ink set for forming a multilayer that has excellent image quality and gloss, provides a good surface condition, has excellent blocking resistance, and is capable of forming an image without a tint of the base.

JP 2013-67770 A

Patent Document 1 discloses an inkjet recording device that ejects clear ink from inkjet heads corresponding to each color and hardens the ink using a light source provided at an adjacent position in the transport direction of the recording medium. However, it has been found that the use of such a recording device can result in a loss of gloss after the clear ink is applied.

The present invention provides an inkjet recording device including a first inkjet head that ejects a radiation-curable clear ink, and a radiation source that irradiates the clear ink with radiation to form a coating of the clear ink, wherein the clear ink contains a polymerizable compound, the polymerizable compound containing a polymerizable compound A having a volume, defined by the van der Waals radius, of 0.26 ^nm3 or more and an area in a height direction relative to a long side of 0.25 ^nm2 or more, the content of the polymerizable compound A being 80 mass% or more with respect to the total amount of the polymerizable compound, and the first inkjet head and the radiation source are configured to satisfy the following formula (1):
0.5≦X×B×T/L≦60 (1)
X (mm): distance from the nozzle end of the inkjet head to the radiation source in the sub-scanning direction L (mm): length of the nozzle row of the inkjet head in the sub-scanning direction B: number of passes during printing T (sec): time required for one pass

FIG. 1 is a perspective view showing one aspect of an ink jet device according to an embodiment of the present invention. FIG. 2 is a plan view showing one embodiment of a recording section. FIG. 11 is a plan view showing another embodiment of the recording section.

Below, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail with reference to the drawings as necessary, but the present invention is not limited to this, and various modifications are possible without departing from the gist of the invention. In the drawings, the same elements are given the same reference numerals, and duplicated explanations will be omitted. Furthermore, unless otherwise specified, positional relationships such as up, down, left, and right will be based on the positional relationships shown in the drawings. Furthermore, the dimensional ratios of the drawings are not limited to those shown in the drawings.

1. Inkjet recording device The inkjet recording device of this embodiment includes a first inkjet head that ejects radiation-curable clear ink, and a radiation source (hereinafter also referred to as a "first radiation source") that irradiates the clear ink with radiation to form a coating of clear ink, wherein the clear ink contains a polymerizable compound, and the polymerizable compound contains polymerizable compound A having a volume, defined by the van der Waals radius, of 0.26 ^nm3 or more and an area in the height direction relative to the long side of 0.25 ^nm2 or more, the content of polymerizable compound A is 80 mass% or more with respect to the total amount of the polymerizable compound, and the first inkjet head and the radiation source are configured to satisfy the following formula (1):
0.5≦X×B×T/L≦60 (1)
X (mm): distance from the nozzle end of the inkjet head to the radiation source in the sub-scanning direction L (mm): length of the nozzle row of the inkjet head in the sub-scanning direction B: number of passes during printing T (sec): time required for one pass

To improve image quality, color inks are cured by a light source relatively soon after application. Conventionally, recording devices have been used that cure clear ink by a light source relatively soon after application, just like color inks. However, it has become clear that the use of such recording devices creates the problem of reduced gloss in the resulting recorded matter.

After investigating this issue, it was found that this phenomenon occurs when a coating is formed using a clear ink that contains a relatively large amount of monomers with low bulkiness, and that even when relatively bulky and large monomers are used, a sufficient gloss may not be obtained if the recording device has a recording unit that is configured to shorten the time from application of the clear ink to curing.

Therefore, in this embodiment, a predetermined amount of bulky polymerizable compound A is used in the clear ink, and the first inkjet head that ejects the clear ink and the radiation source that hardens the clear ink have a predetermined configuration, so that the liquid layer of the clear ink is sufficiently smoothed (leveled) on the recording medium before hardening, thereby suppressing the decrease in gloss.

In particular, by using a clear ink containing a predetermined amount of polymerizable compound A having a predetermined bulkiness expressed using the volume and area defined by the van der Waals radius as indicators, the glossiness of a color ink coating formed on a base is improved, and the image formed by the color ink coating can be suitably protected by the clear ink coating.

1.1 Apparatus Configuration As an example of an inkjet recording apparatus, a serial printer 200 is shown in a perspective view in Fig. 1. As shown in Fig. 1, the serial printer 200 includes a conveying unit 220 and a recording unit 230.

The conveying unit 220 conveys the recording medium F fed to the serial printer to the recording unit 230, and ejects the recording medium after recording outside the serial printer. Specifically, the conveying unit 220 has feed rollers and conveys the fed recording medium F in the sub-scanning direction (from T1 to T2).

The recording unit 230 includes a first inkjet head 231 that ejects radiation-curable clear ink onto the recording medium F sent from the transport unit 220, a second radiation source 232 that irradiates the applied clear ink with radiation, a carriage 235 that carries these, and a carriage movement mechanism 236 that moves the carriage 235 in the main scanning directions S1 and S2 of the recording medium F.

In the case of a serial printer, the inkjet head 231 is provided with a length that is shorter than the width of the recording medium, and the head moves to perform printing in multiple passes (multi-pass). In addition, in a serial printer, the first inkjet head 231 and the first radiation source 232 are mounted on a carriage 235 that moves in a predetermined direction, and clear ink is ejected onto the recording medium as the carriage 235 moves. In this way, printing is performed in two or more passes (multi-pass). Note that a pass is also called a main scan.

In this embodiment, the first inkjet head and the first radiation source are configured to satisfy the above formula (1), so that the clear ink adhered to the recording medium is irradiated with radiation by the first radiation source 232 when the recording medium F is moved by the conveying unit 220 in the sub-scanning direction by a distance of X or more. In other words, in this embodiment, the first inkjet head and the first radiation source are configured so that the application of the clear ink to the recording medium and the irradiation of the adhered clear ink with radiation are not performed in the same pass, and the radiation is irradiated when a predetermined time has elapsed since the clear ink was applied. This causes the clear ink to be leveled on the color ink layer, further improving the gloss of the resulting recorded matter.

To further explain the configuration of the recording unit 230, Figs. 2 and 3 show one embodiment of a plan view of the recording unit 230, which includes a first inkjet head 231 and a first radiation source 232. Figs. 2 and 3 show the surface of the recording unit 230 facing the recording medium in Fig. 1, as viewed from the recording medium side.

In this embodiment, the distance X (mm) from the nozzle end of the first inkjet head 231 to the first radiation source 232 in the sub-scanning direction, the length L (mm) of the nozzle row of the first inkjet head 231 in the sub-scanning direction, the number of passes B during printing, and the time T (seconds) required for one pass are configured to satisfy the above formula (1). X x B x T/L expressed in formula (1) is an index showing the time from when the clear ink lands to when it is cured by the radiation source and a coating of the clear ink is formed.

X×B×T/L represented by formula (1) is 0.5 to 60, preferably 1.0 to 50, more preferably 1.0 to 40, even more preferably 1.0 to 30, even more preferably 1.0 to 20, and even more preferably 1.5 to 15. When X×B×T/L is 0.5 or more, the clear ink deposited on the recording medium is leveled, and the gloss of the resulting recording material tends to be improved. Furthermore, when X×B×T/L is 60 or less, the printing speed can be increased.

The first inkjet head 231 shown in Figures 2 and 3 has multiple nozzle rows N in which nozzle holes are arranged. Also, as shown in Figures 2 and 3, the nozzle rows N may be arranged with a slight shift in the sub-scanning direction. In either case, the length L (mm) in this embodiment refers to the distance from the nozzle hole closest to the sub-scanning direction T2 to the nozzle hole closest to the sub-scanning direction T1.

The length L (mm) is preferably 10 to 40 mm, more preferably 15 to 35 mm, and even more preferably 20 to 30 mm. By having the length L (mm) within the above range, the recording device can be made even more compact.

The first inkjet head 231 and the first radiation source 232 may be arranged on the same surface of the recording unit 320 as shown in FIG. 2, or may be arranged in separate recording units as shown in FIG. 3. Furthermore, the first radiation source 232 may have various shapes other than a rectangle. In either case, the distance X (mm) in this embodiment refers to the distance from the nozzle hole closest to the sub-scanning direction T2 to the end of the first radiation source 232 closest to the sub-scanning direction T1.

Furthermore, the distance X (mm) is preferably 5.0 to 250 mm, more preferably 10 to 250 mm, even more preferably 10 to 200 mm, even more preferably 10 to 100 mm, and even more preferably 10 to 50 mm. By making the distance X (mm) 5.0 mm or more, the clear ink adhered to the recording medium is leveled, and even when the printing speed is increased, the glossiness of the resulting recorded matter tends to be improved. Furthermore, by making the distance X (mm) 250 mm or less, the recording device can be made smaller.

Furthermore, X/L is preferably 0.4 to 10, more preferably 0.4 to 8.0, even more preferably 0.4 to 6.0, and even more preferably 0.4 to 4.0. By making X/L 0.4 or more, the clear ink deposited on the recording medium is leveled, and even when the printing speed is increased, the gloss of the resulting recorded matter tends to be improved. Furthermore, by making X/L 10 or less, the recording device can be made smaller.

As mentioned above, in the serial method, a sub-scan is performed between passes to transport the recording medium. In other words, main and sub-scans are performed alternately. For example, if the length of the head in the sub-scanning direction is 20 mm, one main scan (one pass) is performed and the recording medium is moved 2.0 mm in the sub-scanning direction. This process repeats, and 10 main scans (10 passes) are required for the recording medium to move 20 mm in the sub-scanning direction the length of the head. When viewed from the recording medium side, ink is applied to any position by 10 main scans (10 passes). Here, the number of passes B during printing in this embodiment refers to the number of main scans required until the recording medium is transported a distance L in the sub-scanning direction.

The number of passes B is preferably 4 to 24, more preferably 4 to 20, and even more preferably 4 to 16. By having the number of passes B be 4 or more, it is possible to apply a larger amount of ink, and the clear ink deposited on the recording medium is leveled, while the thickness of the clear ink coating is improved, and the gloss of the resulting recording material tends to be improved. In addition, by having the number of passes B be 22 or less, the printing speed can be increased.

The recording unit 230 repeats movement in the main scanning direction S1 and movement in the main scanning direction S2 by the carriage movement mechanism 236. At this time, when switching the direction from the movement in the main scanning direction S1 to the movement in the main scanning direction S2, the recording unit 230 may move continuously without stopping the movement, or may stop temporarily. In either case, the time T (seconds) in this embodiment refers to half the time from the point when the recording unit 230 at the end (start position) of the main scanning direction S1 starts moving in the main scanning direction S2 to the point before the recording unit 230 returns to the end (start position) of the main scanning direction S1 and starts moving in the main scanning direction S2 again. In other words, the time T (seconds) refers to half the round trip time of the recording unit 230 from the point when the recording unit 230 starts moving from the start position to the point when the recording unit 230 starts moving from the start position again.

The time T (seconds) required for one pass depends on the size of the recording medium, but is preferably 0.10 to 2.5 seconds, more preferably 0.15 to 1.5 seconds, and even more preferably 0.20 to 1.0 seconds. By making the time T (seconds) 0.10 seconds or more, more ink can be applied, and the clear ink adhered to the color ink coating is leveled, which tends to improve the gloss of the resulting recording. Also, by making the time T (seconds) 2.5 seconds or less, the printing speed can be increased.

As shown in Figures 2 and 3, the recording unit 230 may further include a second inkjet head 130 that ejects radiation-curable color ink, and a second radiation source 140 that irradiates the color ink with radiation to form a coating of the color ink. The second inkjet head is disposed upstream of the first inkjet head in the order of ink deposition. This allows clear ink to be deposited on the recording medium to which the color ink has been deposited.

Note that while FIG. 2 shows an embodiment in which the first radiation source 232 is mounted on the same carriage (recording unit 230) as the first inkjet head, as shown in FIG. 3, the recording device of this embodiment may have a first radiation source 232 that is not mounted on a carriage.

In addition, the inkjet device of this embodiment is not limited to the serial type printer, but may be the line type printer described above.

The inkjet recording device of this embodiment preferably performs recording so that the thickness of the clear ink coating is 20 μm or more. The thickness of the clear ink coating is more preferably 25 μm or more, and even more preferably 30 μm or more. The upper limit of the film thickness is preferably 80 μm or less. By making the thickness of the clear ink coating 20 μm or more, the gloss of the resulting recorded matter tends to be further improved.

1.2. Clear ink The clear ink is a radiation curable inkjet composition containing a specific polymerizable compound. Here, the term "clear ink" refers not to an ink used for coloring a recording medium, but to an ink used for adjusting the glossiness of a recording medium. The purpose of using the clear ink includes the purpose of improving the characteristics of a recording medium, such as the abrasion resistance, and the purpose of improving the fixation and color development of a color ink. Specifically, the clear ink is preferably an ink composition containing 0.2% by mass or less of a coloring material, and more preferably does not contain a coloring material. On the other hand, the color ink is an ink used for coloring a recording medium, and preferably contains more than 0.2% by mass of a coloring material.

The radiation curable inkjet composition in this embodiment is a composition that is ejected from an inkjet head by an inkjet method. Below, a radiation curable ink composition is described as one embodiment of the radiation curable inkjet composition, but the composition according to this embodiment may be a composition other than an ink composition, for example, a composition used for 3D modeling.

The radiation-curable inkjet composition is cured by irradiating it with radiation. Examples of radiation include ultraviolet light, electron beams, infrared light, visible light, and X-rays. As the radiation, ultraviolet light is preferred because radiation sources are readily available and widely used, and materials suitable for curing by ultraviolet radiation are readily available and widely used.

Below, we will explain the components that may be contained in the clear ink of this embodiment.

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

1.2.1.1. Polymerizable compound A
The polymerizable compound A of the present embodiment is a polymerizable compound having a volume of 0.26 ^nm3 or more and an area in the height direction relative to the long side of 0.25 ^nm2 or more, as defined by the van der Waals radius. The polymerizable compound A may be a monomer or an oligomer. The polymerizable compound A is a monofunctional polymerizable compound A1 having one polymerizable functional group (hereinafter, simply referred to as "polymerizable compound A1"). ), a multifunctional polymerizable compound A2 having a plurality of polymerizable functional groups (hereinafter, also simply referred to as "polymerizable compound A2"); an oligomer polymerizable compound A3 (hereinafter, also simply referred to as "polymerizable compound A3"); The polymerizable compound A may be a compound satisfying the above bulkiness requirement, and may be used alone or in combination of two or more kinds.

The volume of polymerizable compound A defined by the van der Waals radius is 0.26 ^nm3 or more, preferably 0.27 ^nm3 or more, and more preferably 0.28 ^nm3 or more. The upper limit of the volume of polymerizable compound A defined by the van der Waals radius is not particularly limited, but is preferably 0.60 ^nm3 or less, more preferably 0.55 ^nm3 or less, and even more preferably 0.50 ^nm3 or less. When the bulkiness of polymerizable compound A is within the above defined range, the gloss of the coating film of the clear ink is further improved.

The area of the polymerizable compound A in the height direction relative to the long side defined by the van der Waals radius is 0.25 ^nm2 or more, preferably 0.27 ^nm2 or more, and more preferably 0.29 ^nm2 or more. The upper limit of the area of the polymerizable compound A in the height direction relative to the long side defined by the van der Waals radius is not particularly limited, but is preferably 0.50 ^nm2 or less, more preferably 0.45 ^nm2 or less, and even more preferably 0.40 ^nm2 or less. When the bulkiness of the polymerizable compound A is in the above-defined range, the gloss of the coating film of the clear ink is further improved.

The volume and height area relative to the long side defined by the van der Waals radius are calculated as the volume and height area relative to the long side of the molecular structure with the lowest energy among the structural isomers of the molecule. Known software, such as thermodynamic property prediction software, can be used to identify the three-dimensional shape defined by the van der Waals radius of the molecule and to calculate the volume and height area relative to the long side based on the identification.

"Volume" is the volume of the cavity created by approximating the molecular state in a vacuum from the chemical formula and using the van der Waals radius. The "long side" is the longest side in the three-dimensional shape defined by the van der Waals radius, and is found by calculating the distance between the farthest terminal skeletal atoms (C, O, N, etc.) when modeling the structure in which the molecule is most stable. "Area in the height direction relative to the long side" is the value obtained by dividing the volume by the long side, and is an index of the area on the plane perpendicular to the long side.

The above-mentioned polymerizable compound A is not particularly limited, but examples thereof include monofunctional polymerizable compounds A1 having one polymerizable functional group such as dicyclopentenyl acrylate (DCPA), isobornyl acrylate (IBXA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA), tert-butylcyclohexanol acrylate (TBCHA), isononyl acrylate (INAA), and lauryl acrylate (LA); polyfunctional polymerizable compounds A2 having multiple polymerizable functional groups such as dipropylene glycol diacrylate (DPGDA), and oligomer polymerizable compounds A3.

The content of the polymerizable compound A is 80% by mass or more, preferably 82% by mass or more, and more preferably 84% by mass or more, based on the total amount of the polymerizable compounds. When the content of the polymerizable compound A is 80% by mass or more, the gloss of the coating film of the clear ink is further improved. In addition, the upper limit of the content of the polymerizable compound A is not particularly limited, but is preferably 99% by mass or less, more preferably 97% by mass or less, and even more preferably 95% by mass or less, based on the total amount of the polymerizable compounds. When the content of the polymerizable compound A is 99% by mass or less, the curability and abrasion resistance tend to be further improved. The content of the polymerizable compound A means the sum of the content of A1, the content of A2, and the content of A3.

Furthermore, assuming that the content of the polymerizable compound A is 80% by mass, the content of the polymerizable compound A1 is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and even more preferably 85% by mass or more, based on the total amount of the polymerizable compounds. When the content of the polymerizable compound A1 is 60% by mass or more, the glossiness of the coating film of the clear ink tends to be further improved. Furthermore, when the content of the polymerizable compound A1 is within the above range, the wetting and spreading property of the clear ink on the color ink layer is further improved, and an image with even higher glossiness can be obtained. Furthermore, the content of the polymerizable compound A1 is preferably 95% by mass or less, more preferably 92% by mass or less, and even more preferably 90% by mass or less, based on the total amount of the polymerizable compounds. When the content of the polymerizable compound A1 is 95% by mass or less, the curing property and abrasion resistance of the coating film obtained tend to be further improved.

Furthermore, assuming that the content of polymerizable compound A is 80% by mass, the content of polymerizable compound A2 is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, based on the total amount of polymerizable compounds. When the content of polymerizable compound A2 is 1% by mass or more, the abrasion resistance of the resulting coating film tends to be further improved. In addition, the content of polymerizable compound A2 is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less, based on the total amount of polymerizable compounds.

1.2.1.2. Monofunctional Monomer In this embodiment, the monofunctional monomer is a concept including the above-mentioned polymerizable compound A1 and other monofunctional monomers. The monofunctional monomer is not particularly limited, but for example, a nitrogen-containing monofunctional monomer, a monofunctional (meth)acrylate having a crosslinked condensed ring structure, an aromatic group-containing monofunctional monomer, and a saturated aliphatic group-containing monofunctional monomer can be mentioned. In addition, instead of or in addition to these, other monofunctional monomers may be included as necessary. In addition, the monofunctional monomer is not particularly limited, but a conventionally known monofunctional monomer having a polymerizable functional group, particularly a polymerizable functional group having an unsaturated double bond between carbons, can be used.

In this embodiment, for example, when simply referring to a saturated aliphatic group-containing monofunctional monomer, this includes a saturated aliphatic group-containing monofunctional monomer that is polymerizable compound A1 and a saturated aliphatic group-containing monofunctional monomer that is not polymerizable compound A1. The same applies to other nitrogen-containing monofunctional monomers, monofunctional (meth)acrylates having a crosslinked condensed ring structure, aromatic group-containing monofunctional monomers, etc.

The content of the monofunctional monomer contained in the clear ink is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, and even more preferably 95% by mass or more, based on the total amount of the polymerizable compounds. When the content of the monofunctional monomer is 80% by mass or more based on the total amount of the polymerizable compounds, the flexibility of the coating film is further improved. In addition, the upper limit of the content of the monofunctional monomer is not particularly limited, but is preferably 99% by mass or less, more preferably 98% by mass or less, and even more preferably 97% by mass or less, based on the total amount of the polymerizable compounds. When the content of the monofunctional monomer is 99% by mass or less based on the total amount of the polymerizable compounds, the curing property and abrasion resistance of the coating film tend to be further improved.

The content of the monofunctional monomer contained in the clear ink is preferably 70% by mass or more, more preferably 75% by mass or more, and even more preferably 80% by mass or more, based on the total amount of the clear ink. When the content of the monofunctional monomer is 70% by mass or more, based on the total amount of the clear ink, the flexibility of the coating film tends to be further improved. Furthermore, the upper limit of the content of the monofunctional monomer is preferably 95% by mass or less, more preferably 92% by mass or less, and even more preferably 90% by mass or less, based on the total amount of the clear ink. When the content of the monofunctional monomer is 95% by mass or less, based on the total amount of the clear ink, the curing property and abrasion resistance of the coating film tend to be further improved.

Below, examples of monofunctional monomers are given, but the monofunctional monomers in this embodiment are not limited to the following.

Nitrogen-containing monofunctional monomers include, but are not limited to, nitrogen-containing monofunctional vinyl monomers such as N-vinylcaprolactam, N-vinylformamide, N-vinylcarbazole, N-vinylacetamide, vinylmethyloxazolidinone, and N-vinylpyrrolidone; nitrogen-containing monofunctional acrylate monomers such as acryloylmorpholine (ACMO); and nitrogen-containing monofunctional acrylamide monomers such as (meth)acrylamide, N-hydroxymethyl(meth)acrylamide, diacetone acrylamide, N,N-dimethyl(meth)acrylamide, and dimethylaminoethyl acrylate benzyl chloride quaternary salt.

Among these, it is preferable to include either a nitrogen-containing monofunctional vinyl monomer or a nitrogen-containing monofunctional acrylate monomer, and more preferable to include a monomer having a nitrogen-containing heterocyclic structure such as N-vinylcaprolactam, N-vinylcarbazole, N-vinylpyrrolidone, vinylmethyloxazolidinone, or acryloylmorpholine, and even more preferable to include acryloylmorpholine.

The use of such nitrogen-containing monofunctional monomers tends to further improve the scratch resistance of the coating film. Furthermore, nitrogen-containing monofunctional acrylate monomers having a nitrogen-containing heterocyclic structure such as acryloylmorpholine tend to further improve the flexibility and adhesion of the coating film.

The content of the nitrogen-containing monofunctional monomer contained in the clear ink is preferably 2 to 15 mass % relative to the total amount of polymerizable compounds, more preferably 3 to 13 mass %, and even more preferably 4 to 12 mass %. When the content of the nitrogen-containing monofunctional monomer relative to the total amount of polymerizable compounds is within the above range, the abrasion resistance and adhesion of the coating film tend to be further improved.

The content of the nitrogen-containing monofunctional monomer contained in the clear ink is preferably 2 to 15 mass % relative to the total amount of the clear ink, more preferably 3 to 12 mass %, and even more preferably 4 to 11 mass %. By having the content of the nitrogen-containing monofunctional monomer relative to the total amount of the clear ink within the above range, the abrasion resistance and adhesion of the coating film tend to be further improved.

1.2.1.2.2. Monofunctional (meth)acrylate having a bridged condensed ring structure As one of the other monofunctional monomers, there can be mentioned a monofunctional (meth)acrylate having a bridged condensed ring structure. In the present invention, the bridged condensed ring structure means a structure in which two or more ring structures share a side in a one-to-one relationship and two or more non-adjacent elements of the same ring structure or different ring structures are linked. Examples of the monofunctional (meth)acrylate having a bridged condensed ring structure include dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, and dicyclopentanyl (meth)acrylate. In addition to the above, the following can be mentioned as examples of the bridged condensed ring structure.

Among these, it is more preferable to include dicyclopentenyl acrylate (DCPA), which corresponds to the above-mentioned polymerizable compound A. By using such a monofunctional (meth)acrylate having a cross-linked condensed ring structure, the abrasion resistance, flexibility, and adhesion of the coating film tend to be further improved, and the gloss of the coating film of the clear ink also tends to be further improved.

The content of the monofunctional (meth)acrylate having a cross-linked condensed ring structure contained in the clear ink is preferably 10 to 65 mass % relative to the total amount of polymerizable compounds, more preferably 15 to 63 mass %, and even more preferably 20 to 60 mass %. When the content of the monofunctional (meth)acrylate having a cross-linked condensed ring structure relative to the total amount of polymerizable compounds is within the above range, the adhesion and abrasion resistance of the coating film tend to be further improved.

The content of the monofunctional (meth)acrylate having a cross-linked condensed ring structure contained in the clear ink is preferably 10 to 60 mass % relative to the total amount of the clear ink, more preferably 15 to 57 mass %, and even more preferably 20 to 55 mass %. When the content of the monofunctional (meth)acrylate having a cross-linked condensed ring structure relative to the total amount of the clear ink is within the above range, the adhesion and abrasion resistance of the coating film tend to be further improved.

1.2.1.2.3. Aromatic Group-Containing Monofunctional Monomer The aromatic group-containing monofunctional monomer is not particularly limited, but examples thereof include phenoxyethyl (meth)acrylate, benzyl (meth)acrylate, alkoxylated 2-phenoxyethyl (meth)acrylate, ethoxylated nonylphenyl (meth)acrylate, alkoxylated nonylphenyl (meth)acrylate, p-cumylphenol EO-modified (meth)acrylate, and 2-hydroxy-3-phenoxypropyl (meth)acrylate.

Among these, phenoxyethyl (meth)acrylate and benzyl (meth)acrylate are preferred, phenoxyethyl (meth)acrylate is more preferred, and phenoxyethyl acrylate (PEA) is even more preferred. By using such aromatic group-containing monofunctional monomers, the solubility of the photopolymerization initiator tends to be improved, and the curing properties of the clear ink tend to be improved. In particular, when an acylphosphine oxide-based photopolymerization initiator or a thioxanthone-based photopolymerization initiator is used, the solubility tends to be good.

The content of the aromatic group-containing monofunctional monomer contained in the clear ink is preferably 2 to 15 mass % relative to the total amount of polymerizable compounds, more preferably 3 to 13 mass %, and even more preferably 4 to 12 mass %. By having the content of the aromatic group-containing monofunctional monomer relative to the total amount of polymerizable compounds within the above range, the adhesion and abrasion resistance of the coating film tend to be further improved.

The content of the aromatic group-containing monofunctional monomer contained in the clear ink is preferably 2 to 15 mass % relative to the total amount of the clear ink, more preferably 3 to 12 mass %, and even more preferably 4 to 11 mass %. By having the content of the aromatic group-containing monofunctional monomer relative to the total amount of the clear ink within the above range, the adhesion and abrasion resistance of the coating film tend to be further improved.

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

Among these, isobornyl (meth)acrylate (IBXA) and tert-butylcyclohexanol acrylate (TBCHA), which correspond to the above-mentioned polymerizable compound A, are preferred. By using such a monofunctional monomer containing a saturated aliphatic group, the curing property and abrasion resistance of the clear ink tend to be further improved, and the gloss of the coating film of the clear ink also tends to be further improved.

The content of the saturated aliphatic group-containing monofunctional monomer contained in the clear ink is preferably 15 to 85% by mass, more preferably 20 to 80% by mass, and even more preferably 25 to 75% by mass, relative to the total amount of polymerizable compounds. When the content of the saturated aliphatic group-containing monofunctional monomer relative to the total amount of polymerizable compounds is within the above range, the curing properties and abrasion resistance of the clear ink tend to be further improved.

The content of the saturated aliphatic group-containing monofunctional monomer contained in the clear ink is preferably 15 to 80% by mass, more preferably 17 to 75% by mass, and even more preferably 20 to 70% by mass, relative to the total amount of the clear ink. By having the content of the saturated aliphatic group-containing monofunctional monomer relative to the total amount of the clear ink within the above range, the curing properties and abrasion resistance of the clear ink tend to be further improved.

1.2.1.2.5. Others As other monofunctional monomers, in addition to the above, for example, unsaturated carboxylic acids such as (meth)acrylic acid, itaconic acid, crotonic acid, isocrotonic acid, and maleic acid; salts of the unsaturated carboxylic acids; esters, urethanes, amides, and anhydrides of unsaturated carboxylic acids; acrylonitrile, styrene, various unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes may be used.

1.2.1.3. Multifunctional Monomer In this embodiment, the multifunctional monomer is a concept including the above-mentioned polymerizable compound A2 and other multifunctional monomers. Examples of the multifunctional monomer in this embodiment include vinyl ether group-containing (meth)acrylate and multifunctional (meth)acrylate. Note that the multifunctional monomer is not limited to the above.

The content of the polyfunctional monomer contained in the clear ink is preferably 0.01 to 20% by mass, more preferably 0.01 to 15% by mass, and even more preferably 1 to 15% by mass, relative to the total amount of polymerizable compounds. When the content of the polyfunctional monomer relative to the total amount of polymerizable compounds is 0.01% by mass or more, the ink tends to have excellent curability and improved abrasion resistance. Furthermore, when the content of the polyfunctional monomer relative to the total amount of polymerizable compounds is 20% by mass or less, the ink tends to have improved flexibility and adhesion.

The content of the polyfunctional monomer contained in the clear ink is preferably 0.01 to 18% by mass, more preferably 0.01 to 15% by mass, and even more preferably 1 to 15% by mass, relative to the total amount of the clear ink. When the content of the polyfunctional monomer relative to the total amount of the clear ink is 0.01% by mass or more, the ink tends to have excellent curing properties and improved abrasion resistance. When the content of the polyfunctional monomer relative to the total amount of the clear ink is 20% by mass or less, the ink tends to have improved flexibility and adhesion.

Below, examples of polyfunctional monomers are given, but the polyfunctional monomers in this embodiment are not limited to the following.

1.2.1.3.1 Vinyl ether group-containing (meth)acrylates Examples of vinyl ether group-containing (meth)acrylates include, but are not limited to, compounds represented by the following formula (2). By including such vinyl ether group-containing (meth)acrylates, the viscosity of the clear ink tends to decrease and the ejection stability tends to improve. In addition, the curability of the clear ink is improved, and the improved curability allows the recording speed to be increased.
CH ₂ ═CR ¹ -COOR ² -O-CH═CH-R ³ ... (2)
(In the formula, R ¹ is a hydrogen atom or a methyl group, R ² is a divalent organic residue having 2 to 20 carbon atoms, and R ³ is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms.)

In the above formula (2), examples of the divalent organic residue having 2 to 20 carbon atoms represented by R ² include linear, branched or cyclic alkylene groups having 2 to 20 carbon atoms which may be substituted, alkylene groups having 2 to 20 carbon atoms which have an oxygen atom due to an ether bond and/or an ester bond in the structure which may be substituted, and divalent aromatic groups having 6 to 11 carbon atoms which may be substituted. Among these, alkylene groups having 2 to 6 carbon atoms such as ethylene groups, n-propylene groups, isopropylene groups, and butylene groups, and alkylene groups having 2 to 9 carbon atoms which have an oxygen atom due to an ether bond in the structure such as oxyethylene groups, oxy n-propylene groups, oxyisopropylene groups, and oxybutylene groups are preferred. Furthermore, from the viewpoint of being able to further reduce the viscosity of the clear ink and further improving the curing properties of the clear ink, a compound having a glycol ether chain in which ^R2 is an alkylene group having 2 to 9 carbon atoms and having an oxygen atom via an ether bond in the structure, such as an oxyethylene group, an oxy-n-propylene group, an oxyisopropylene group, or an oxybutylene group, is more preferable.

In the above formula (2), the monovalent organic residue having 1 to 11 carbon atoms represented by ^R3 is preferably a linear, branched or cyclic alkyl group having 1 to 10 carbon atoms which may be substituted, or an aromatic group having 6 to 11 carbon atoms which may be substituted. Among these, an alkyl group having 1 to 2 carbon atoms, such as a methyl group or an ethyl group, and an aromatic group having 6 to 8 carbon atoms, such as a phenyl group or a benzyl group, are preferably used.

When each of the above organic residues is a group that may be substituted, the substituent is divided into a group that contains a carbon atom and a group that does not contain a carbon atom. First, when the above substituent is a group that contains a carbon atom, the carbon atom is counted in the number of carbon atoms of the organic residue. Examples of groups that contain a carbon atom include, but are not limited to, a carboxyl group and an alkoxy group. Next, examples of groups that do not contain a carbon atom include, but are not limited to, a hydroxyl group and a halo group.

Specific examples of the compound of formula (2) include, but are not limited to, 2-vinyloxyethyl (meth)acrylate, 3-vinyloxypropyl (meth)acrylate, 1-methyl-2-vinyloxyethyl (meth)acrylate, 2-vinyloxypropyl (meth)acrylate, 4-vinyloxybutyl (meth)acrylate, 1-methyl-3-vinyloxypropyl (meth)acrylate, 1-vinyloxymethylpropyl (meth)acrylate, 2-methyl-3-vinyloxypropyl (meth)acrylate, 1,1-dimethyl-2-vinyloxyethyl (meth)acrylate, 3-vinyloxybutyl (meth)acrylate, 1-methyl-2-vinyloxypropyl (meth)acrylate, 2-vinyloxybutyl (meth)acrylate, 4-vinyloxycyclohexyl (meth)acrylate, and 6-vinyloxyhexyl (meth)acrylate. acrylate, 4-vinyloxymethylcyclohexylmethyl (meth)acrylate, 3-vinyloxymethylcyclohexylmethyl (meth)acrylate, 2-vinyloxymethylcyclohexylmethyl (meth)acrylate, p-vinyloxymethylphenylmethyl (meth)acrylate, m-vinyloxymethylphenylmethyl (meth)acrylate, o-vinyloxymethylphenylmethyl (meth)acrylate, 2-(2-vinyloxyethoxy)ethyl (meth)acrylate, 2-(2-vinyloxyethoxy)ethyl acrylate, 2-(vinyloxyisopropoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxy)propyl (meth)acrylate, 2-(vinyloxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxy)propyl (meth)acrylate, 2-(vinyloxyisopropoxy)propyl (meth)acrylate, (meth)acrylic acid 2-(vinyloxyisopropoxy)isopropyl, (meth)acrylic acid 2-(vinyloxyethoxyethoxy)ethyl, (meth)acrylic acid 2-(vinyloxyethoxyisopropoxy)ethyl, (meth)acrylic acid 2-(vinyloxyisopropoxyethoxy)ethyl, (meth)acrylic acid 2-(vinyloxyisopropoxyisopropoxy)ethyl, (meth)acrylic acid 2-(vinyloxyethoxyethoxy)propyl, (meth)acrylic acid 2-(vinyloxyethoxyisopropoxy)propyl, (meth)acrylic acid 2-(vinyloxyisopropoxyethoxy)propyl, (meth)acrylic acid 2-(vinyloxyisopropoxyisopropoxy)propyl, (meth)acrylic acid 2-(vinyloxyisopropoxyethoxy)propyl, (meth)acrylic acid 2-(vinyloxyisopropoxyisopropoxy)propyl, (meth)acrylic acid 2-(vinyloxyethoxyethoxy)isopropyl, (meth)acrylic acid 2-(vinyloxyethoxyisopropoxy)isopropyl, (meth)acrylic acid 2-(vinyloxyethoxyethoxy)isopropyl, (meth)acrylic acid 2-(vinyloxyethoxyisopropoxy)isopropyl, [0113] Examples of the acrylate include 2-(vinyloxyisopropoxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, polyethylene glycol monovinyl ether (meth)acrylate, and polypropylene glycol monovinyl ether (meth)acrylate. Among these specific examples, 2-(2-vinyloxyethoxy)ethyl acrylate is particularly preferred because it is easy to balance the curing properties and viscosity of the clear ink. In this embodiment, 2-(2-vinyloxyethoxy)ethyl acrylate is sometimes referred to as VEEA.

The content of the vinyl ether group-containing (meth)acrylate contained in the clear ink is preferably 1 to 10% by mass or more, more preferably 2 to 8% by mass, and even more preferably 2 to 6% by mass, relative to the total amount of polymerizable compounds. When the content of the vinyl ether group-containing (meth)acrylate relative to the total amount of polymerizable compounds is within the above range, the viscosity of the clear ink tends to decrease and the ejection stability tends to be improved.

The content of vinyl ether group-containing (meth)acrylate in the clear ink is preferably 1 to 10% by mass or more, more preferably 1 to 8% by mass, and even more preferably 2 to 6% by mass, relative to the total amount of the clear ink. When the content of vinyl ether group-containing (meth)acrylate relative to the total amount of the clear ink is within the above range, the viscosity of the clear ink tends to decrease and the ejection stability tends to be improved.

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

Among these, dipropylene glycol diacrylate (DPGDA), which corresponds to the above-mentioned polymerizable compound A, is preferred. By using such a multifunctional (meth)acrylate, the curing property and abrasion resistance of the clear ink tend to be further improved, and the gloss of the coating film of the clear ink also tends to be further improved.

The content of the polyfunctional (meth)acrylate contained in the clear ink is preferably 1 to 20% by mass or more, more preferably 1 to 17% by mass, and even more preferably 2 to 15% by mass, relative to the total amount of polymerizable compounds. When the content of the polyfunctional (meth)acrylate relative to the total amount of polymerizable compounds is 1% by mass or more, the abrasion resistance tends to be further improved. Furthermore, when the content of the polyfunctional (meth)acrylate relative to the total amount of polymerizable compounds is 20% by mass or less, the flexibility and adhesion of the coating film tends to be further improved.

The content of the polyfunctional (meth)acrylate contained in the clear ink is preferably 1 to 20% by mass or more, more preferably 1 to 17% by mass, and even more preferably 2 to 15% by mass, relative to the total amount of the clear ink. When the content of the polyfunctional (meth)acrylate relative to the total amount of the clear ink is 1% by mass or more, the abrasion resistance tends to be further improved. When the content of the polyfunctional (meth)acrylate relative to the total amount of the clear ink is 20% by mass or less, the flexibility and adhesion of the coating film tends to be further improved.

1.2.2. Photopolymerization initiator The photopolymerization initiator is not particularly limited as long as it generates active species by irradiation with radiation, and examples thereof include known photopolymerization initiators such as acylphosphine oxide-based photopolymerization initiators, alkylphenone-based polymerization initiators, titanocene-based polymerization initiators, and thioxanthone-based photopolymerization initiators. Among these, acylphosphine oxide-based photopolymerization initiators are preferred. By using such a photopolymerization initiator, the curing property of the clear ink is further improved, and in particular, there is a tendency for the curing property by the curing process using UV-LED light to be further improved. The photopolymerization initiator may be used alone or in combination of two or more types.

Acylphosphine oxide photopolymerization initiators are not particularly limited, but examples include 2,4,6-trimethylbenzoyldiphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, etc.

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

The content of the photopolymerization initiator contained in the clear ink is preferably 3 to 12% by mass, more preferably 5 to 10% by mass, and even more preferably 7 to 9% by mass, relative to the total amount of the clear ink. By having the content of the photopolymerization initiator within the above range, the curability of the clear ink and the solubility of the photopolymerization initiator tend to be further improved.

1.1.4 Other Additives The clear ink according to this embodiment may further contain additives such as a dispersant, a polymerization inhibitor, and a slip agent, as necessary.

The clear ink according to this embodiment may further contain a polymerization inhibitor. The polymerization inhibitor may be used alone or in combination of two or more kinds.

Polymerization inhibitors include, but are not limited to, p-methoxyphenol, hydroquinone monomethyl ether (MEHQ), 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl, hydroquinone, cresol, t-butylcatechol, 3,5-di-t-butyl-4-hydroxytoluene, 2,2'-methylenebis(4-methyl-6-t-butylphenol), 2,2'-methylenebis(4-ethyl-6-butylphenol), and 4,4'-thiobis(3-methyl-6-t-butylphenol), hindered amine compounds, etc.

The content of the polymerization inhibitor is preferably 0.05 to 1 mass % relative to the total amount of the clear ink, and more preferably 0.05 to 0.5 mass %.

The clear ink according to this embodiment may further contain a slip agent. The slip agent may be used alone or in combination of two or more types.

As the slip agent, a silicone-based surfactant is preferred, and polyester-modified silicone or polyether-modified silicone is more preferred. Examples of polyester-modified silicone include BYK-347, 348, BYK-UV3500, 3510, and 3530 (all manufactured by BYK Additives & Instruments), and examples of polyether-modified silicone include BYK-3570 (manufactured by BYK Additives & Instruments).

The amount of slip agent contained is preferably 0.01 to 2% by mass, and more preferably 0.05 to 1% by mass, relative to the total amount of clear ink.

1.3. Color ink The inkjet device of this embodiment may further include color ink. The color ink is a radiation curable inkjet composition containing a polymerizable compound. Here, the "color ink" refers to an ink used to color a recording medium. Components that may be contained in the color ink of this embodiment will be described below.

1.3.1 Polymerizable Compound The polymerizable compound includes a monofunctional monomer having one polymerizable functional group, a polyfunctional monomer having multiple polymerizable functional groups, and an oligomer having one or multiple polymerizable functional groups. Each polymerizable compound may be used alone or in combination of two or more kinds.

1.3.1.1. Monofunctional Monomer In this embodiment, the monofunctional monomer is not particularly limited, but examples thereof include nitrogen-containing monofunctional monomers, monofunctional (meth)acrylates having a crosslinked condensed ring structure, aromatic group-containing monofunctional monomers, and saturated aliphatic group-containing monofunctional monomers. In addition, other monofunctional monomers may be included instead of or in addition to these as necessary. The other monofunctional monomers are not particularly limited, but conventionally known monofunctional monomers having polymerizable functional groups, particularly polymerizable functional groups having carbon-carbon unsaturated double bonds, can be used.

The content of the monofunctional monomer contained in the color ink is preferably 80% by mass or more, preferably 85% by mass or more, and more preferably 90% by mass or more, based on the total amount of the polymerizable compounds. When the content of the monofunctional monomer is 80% by mass or more based on the total amount of the polymerizable compounds, the flexibility of the coating film is further improved. In addition, when the content of the monofunctional monomer contained in the color ink is within the above range, gloss defects are likely to occur, so the present invention is particularly useful. Furthermore, the upper limit of the content of the monofunctional monomer is not particularly limited, but is preferably 99% by mass or less, more preferably 98% by mass or less, and even more preferably 97% by mass or less, based on the total amount of the polymerizable compounds. When the content of the monofunctional monomer is 99% by mass or less based on the total amount of the polymerizable compounds, the abrasion resistance of the coating film tends to be further improved.

The content of the monofunctional monomer contained in the color ink is preferably 65% by mass or more, more preferably 70% by mass or more, and even more preferably 75% by mass or more, based on the total amount of the color ink. When the content of the monofunctional monomer is 75% by mass or more, based on the total amount of the color ink, the flexibility of the coating film tends to be further improved. Furthermore, the upper limit of the content of the monofunctional monomer is preferably 97% by mass or less, more preferably 95% by mass or less, and even more preferably 90% by mass or less, based on the total amount of the color ink. When the content of the monofunctional monomer is 97% by mass or less, based on the total amount of the clear ink, the curability of the coating film tends to be further improved.

The nitrogen-containing monofunctional monomers, monofunctional (meth)acrylates having a crosslinked condensed ring structure, aromatic group-containing monofunctional monomers, saturated aliphatic group-containing monofunctional monomers, and other monofunctional monomers contained in the color inks can be similar to those exemplified for the clear ink.

The content of the nitrogen-containing monofunctional monomer contained in the color ink is preferably 3 to 17% by mass, more preferably 5 to 15% by mass, and even more preferably 8 to 12% by mass, relative to the total amount of polymerizable compounds. When the content of the nitrogen-containing monofunctional monomer relative to the total amount of polymerizable compounds is within the above range, the abrasion resistance and adhesion of the coating film tend to be further improved.

The content of the nitrogen-containing monofunctional monomer contained in the color ink is preferably 3 to 17% by mass, more preferably 5 to 15% by mass, and even more preferably 8 to 12% by mass, relative to the total amount of the color ink. When the content of the nitrogen-containing monofunctional monomer relative to the total amount of the color ink is within the above range, the abrasion resistance and adhesion of the coating film tend to be further improved.

The content of the monofunctional (meth)acrylate having a cross-linked condensed ring structure contained in the color ink is preferably 20 to 60% by mass, more preferably 25 to 55% by mass, and even more preferably 30 to 55% by mass, relative to the total amount of polymerizable compounds. When the content of the monofunctional (meth)acrylate having a cross-linked condensed ring structure relative to the total amount of polymerizable compounds is within the above range, the adhesion and abrasion resistance of the coating film tend to be further improved.

The content of the monofunctional (meth)acrylate having a cross-linked condensed ring structure contained in the color ink is preferably 20 to 60% by mass, more preferably 25 to 55% by mass, and even more preferably 30 to 55% by mass, relative to the total amount of the color ink. When the content of the monofunctional (meth)acrylate having a cross-linked condensed ring structure relative to the total amount of the color ink is within the above range, the adhesion and abrasion resistance of the coating film tend to be further improved.

The content of the aromatic group-containing monofunctional monomer contained in the color ink is preferably 2 to 12 mass % relative to the total amount of the polymerizable compounds, more preferably 3 to 10 mass %, and even more preferably 4 to 7 mass %. When the content of the aromatic group-containing monofunctional monomer relative to the total amount of the polymerizable compounds is within the above range, the adhesion and abrasion resistance of the coating film tend to be further improved.

The content of the aromatic group-containing monofunctional monomer contained in the color ink is preferably 2 to 12 mass % relative to the total amount of the color ink, more preferably 3 to 10 mass %, and even more preferably 4 to 7 mass %. When the content of the aromatic group-containing monofunctional monomer relative to the total amount of the color ink is within the above range, the adhesion and abrasion resistance of the coating film tend to be further improved.

The content of the saturated aliphatic group-containing monofunctional monomer contained in the color ink is preferably 10 to 45% by mass, more preferably 15 to 40% by mass, and even more preferably 15 to 35% by mass, based on the total amount of polymerizable compounds. By having the content of the saturated aliphatic group-containing monofunctional monomer in the above range, the curing properties of the color ink tend to be further improved.

The content of the saturated aliphatic group-containing monofunctional monomer contained in the color ink is preferably 10 to 45% by mass, more preferably 15 to 40% by mass, and even more preferably 15 to 35% by mass, based on the total amount of the color ink. By having the content of the saturated aliphatic group-containing monofunctional monomer in the above range, the curing properties of the color ink tend to be further improved.

1.3.1.2. Multifunctional Monomer Examples of the multifunctional monomer of this embodiment include vinyl ether group-containing (meth)acrylate and multifunctional (meth)acrylate. Note that the multifunctional monomer is not limited to the above. Examples of the multifunctional monomer include the same ones as those exemplified for the clear ink.

The content of the polyfunctional monomer contained in the color ink is preferably 1 to 30% by mass, more preferably 3 to 25% by mass, and even more preferably 3 to 20% by mass, relative to the total amount of polymerizable compounds. When the content of the polyfunctional monomer relative to the total amount of polymerizable compounds is 1% by mass or more, the abrasion resistance tends to be further improved. Also, when the content of the polyfunctional monomer relative to the total amount of polymerizable compounds is 30% by mass or less, the flexibility and adhesion of the coating film tends to be further improved.

The content of the polyfunctional monomer contained in the color ink is preferably 1 to 30% by mass, more preferably 3 to 25% by mass, and even more preferably 3 to 20% by mass, relative to the total amount of the color ink. When the content of the polyfunctional monomer relative to the total amount of the color is 1% by mass or more, the abrasion resistance tends to be further improved. When the content of the polyfunctional monomer relative to the total amount of the color is 30% by mass or less, the flexibility and adhesion of the coating film tends to be further improved.

The content of the vinyl ether group-containing (meth)acrylate contained in the color ink is preferably 1 to 10% by mass or more, more preferably 1 to 7% by mass, and even more preferably 2 to 5% by mass, relative to the total amount of polymerizable compounds. When the content of the vinyl ether group-containing (meth)acrylate relative to the total amount of polymerizable compounds is within the above range, the viscosity of the color ink tends to decrease and the ejection stability tends to be improved.

The content of vinyl ether group-containing (meth)acrylate contained in the color ink is preferably 1 to 10% by mass or more, more preferably 1 to 7% by mass, and even more preferably 2 to 5% by mass, relative to the total amount of the color ink. When the content of vinyl ether group-containing (meth)acrylate relative to the total amount of the color ink is within the above range, the viscosity of the color ink tends to decrease and the ejection stability tends to be improved.

The content of the polyfunctional (meth)acrylate contained in the color ink is preferably 1 to 25% by mass or more, more preferably 3 to 20% by mass, and even more preferably 3 to 17% by mass, relative to the total amount of polymerizable compounds. When the content of the polyfunctional (meth)acrylate relative to the total amount of polymerizable compounds is 1% by mass or more, the abrasion resistance tends to be further improved. Furthermore, when the content of the polyfunctional (meth)acrylate relative to the total amount of polymerizable compounds is 25% by mass or less, the flexibility and adhesion of the coating film tends to be further improved.

The content of the polyfunctional (meth)acrylate contained in the color ink is preferably 1 to 25% by mass or more, more preferably 3 to 20% by mass, and even more preferably 3 to 17% by mass, relative to the total amount of the color ink. When the content of the polyfunctional (meth)acrylate relative to the total amount of the color ink is 1% by mass or more, the abrasion resistance tends to be further improved. Furthermore, when the content of the polyfunctional (meth)acrylate relative to the total amount of the color ink is 20% by mass or less, the flexibility and adhesion of the coating film tends to be further improved.

1.3.2. Photopolymerization initiator Examples of photopolymerization initiators contained in the color ink include those exemplified for the clear ink. The content of the photopolymerization initiator is preferably 3 to 12 mass % relative to the total amount of the color ink, more preferably 5 to 10 mass %, and even more preferably 7 to 9 mass %. By having the content of the photopolymerization initiator within the above range, the curability of the color ink and the solubility of the photopolymerization initiator tend to be further improved.

1.3.3. Coloring Material The coloring material contained in the color ink can be at least one of a pigment and a dye.

The total content of coloring materials is preferably 0.2 to 20% by mass, more preferably 0.5 to 15% by mass, and even more preferably 1 to 10% by mass, relative to the total amount of color ink.

1.3.3.1. Pigment By using a pigment as a coloring material, the light resistance of the color ink can be improved. Either an inorganic pigment or an organic pigment can be used as the pigment. The pigment may be used alone or in combination of two or more kinds.

As inorganic pigments, carbon blacks such as furnace black, lamp black, acetylene black, and channel black (C.I. (Colour Index Generic Name) Pigment Black 7), iron oxide, and titanium oxide can be used.

Organic pigments include azo pigments such as insoluble azo pigments, condensed azo pigments, azo lakes, and chelate azo pigments, polycyclic pigments such as phthalocyanine pigments, perylene and perinone pigments, anthraquinone pigments, quinacridone pigments, dioxane pigments, thioindigo pigments, isoindolinone pigments, and quinophthalone pigments, dye chelates (e.g., basic dye chelates, acid dye chelates, etc.), dye lakes (basic dye lakes, acid dye lakes), nitro pigments, nitroso pigments, aniline black, and daylight fluorescent pigments.

More specifically, the carbon blacks used for black include No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52, MA7, MA8, MA100, and No. 2200B, etc. (all manufactured by Mitsubishi Chemical Corporation), Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven 1255, Raven 700, etc. (all manufactured by Carbon Columbia), Rega1 400R, Rega1 330R, Rega1 660R, Mogul L, Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400 (manufactured by Cabot Corporation (CABOT JAPAN K.K.)), Color Black FW1, Color Black FW2, Color Black FW2V, Color Black FW18, Color Black FW200, Color Black S150, Color Black S160, Color Black S170, Printex 35, Printex U, Printex V, Printex 140U, Special Black 6, Special Black 5, Special Black 4A, Special Black 4 (all manufactured by Degussa).

Pigments used for white include C.I. Pigment White 6, 18, and 21.

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

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

Pigments used for cyan include C.I. Pigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:34, 15:4, 16, 18, 22, 25, 60, 65, 66, C.I. Vat Blue 4, 60.

In addition, examples of pigments other than magenta, cyan, and yellow include C.I. Pigment Green 7, 10, C.I. Pigment Brown 3, 5, 25, 26, and C.I. Pigment Orange 1, 2, 5, 7, 13, 14, 15, 16, 24, 34, 36, 38, 40, 43, and 63.

The content of the pigment in the color ink is preferably 1 to 20% by mass, more preferably 1 to 15% by mass, and even more preferably 1 to 10% by mass, based on the total amount of the color ink.

1.3.3.1. Dyes Dyes can be used as coloring materials. The dyes are not particularly limited, and acid dyes, direct dyes, reactive dyes, and basic dyes can be used. The dyes may be used alone or in combination of two or more.

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

1.3.4. Other Additives The color ink may further contain additives such as a dispersant, a polymerization inhibitor, and a slip agent, as necessary.

1.3.4.1. Dispersant The dispersant is not particularly limited, but examples thereof include dispersants commonly used for preparing pigment dispersions, such as polymer dispersants. Specific examples thereof include those containing one or more of polyoxyalkylene polyalkylene polyamines, vinyl polymers and copolymers, acrylic polymers and copolymers, polyesters, polyamides, polyimides, polyurethanes, amino polymers, silicon-containing polymers, sulfur-containing polymers, fluorine-containing polymers, and epoxy resins as the main component. The dispersant may be used alone or in combination of two or more.

Commercially available polymer dispersants include the Ajisper series manufactured by Ajinomoto Fine-Techno Co., Ltd., the Solsperse series (Solsperse 36000, etc.) available from Avecia and Noveon, the Disperbyk series manufactured by BYK Additives & Instruments, and the Disparlon series manufactured by Kusumoto Chemicals Co., Ltd.

The content of the dispersant in the color ink is preferably 0.1 to 2% by mass, more preferably 0.1 to 1% by mass, and even more preferably 0.1 to 0.5% by mass, based on the total amount of the color ink.

The color ink may further contain a polymerization inhibitor. The polymerization inhibitor may be used alone or in combination of two or more. Examples of the polymerization inhibitor include the same ones as those exemplified for the clear ink.

The content of the polymerization inhibitor in the color ink is preferably 0.05 to 1 mass % relative to the total amount of the color ink, and more preferably 0.05 to 0.5 mass %.

The color ink may further contain a slip agent. The slip agent may be used alone or in combination of two or more. Examples of the slip agent include the same ones as those exemplified for the clear ink.

The content of the slip agent in the color ink is preferably 0.01 to 2% by mass, and more preferably 0.05 to 1% by mass, based on the total amount of the color ink.

1.4. Method for manufacturing clear ink and color ink The clear ink and color ink are manufactured (prepared) by mixing the components contained in the ink and stirring the components to mix them sufficiently uniformly. In the present embodiment, the preparation of the clear ink and color ink preferably includes a step of subjecting a mixture of a polymerization initiator and at least a part of a monomer to at least one of ultrasonic treatment and heating treatment during the preparation process. This can reduce the amount of dissolved oxygen in the ink after preparation, and can provide clear ink and color ink with excellent ejection stability and storage stability. The mixture may contain at least the above components, and may further contain other components contained in the clear ink and color ink, or may contain all the components contained in the clear ink and color ink. The monomer contained in the mixture may be at least a part of the monomer contained in the clear ink and color ink.

2. Inkjet Method The inkjet method according to this embodiment is an inkjet method that uses the inkjet recording apparatus described above, and includes the steps of: ejecting color inks from the second inkjet head to cause the color inks to adhere to the recording medium (first ejection step); irradiating the color inks adhered to the recording medium with radiation from a second radiation source (first irradiation step); ejecting clear ink from the first inkjet head to cause the clear ink to adhere to at least a portion of the region of the recording medium to which the color inks have adhered (second ejection step); and irradiating the clear ink adhered to the recording medium with radiation from the first radiation source (second irradiation step).

In the inkjet method according to this embodiment, the use of the above-mentioned clear ink and color inks makes it possible to obtain an image with excellent gloss. In addition, in the inkjet method according to this embodiment, the use of the above-mentioned clear ink and color inks makes it possible to obtain an image with excellent flexibility and abrasion resistance. Each step will be described in detail below.

In the first ejection process, the color ink is ejected from the second inkjet head and adhered to the recording medium. More specifically, the pressure generating means is driven to eject the color ink filled in the pressure generating chamber of the second inkjet head from the nozzle. This ejection method is also called the inkjet method.

2.2. First irradiation step In the first irradiation step, radiation is irradiated to the color ink attached to the recording medium. When radiation is irradiated, the polymerization reaction of the monomer starts, and the color ink hardens to form a coating film. If a polymerization initiator is present at this time, it generates active species (initiation species) such as radicals, acids, and bases, and the polymerization reaction of the monomer is promoted by the function of the initiation species. If a photosensitizer is present, it absorbs radiation and becomes excited, and by coming into contact with the polymerization initiator, it promotes the decomposition of the polymerization initiator, thereby achieving a more effective curing reaction.

Here, examples of the radiation include ultraviolet light, infrared light, visible light, and X-rays. The radiation source irradiates the ink with the second radiation source provided downstream of the second inkjet head. The radiation source is not particularly limited, but an example thereof is an ultraviolet light-emitting diode. By using such a radiation source, it is possible to reduce the size and cost of the device. The ultraviolet light-emitting diode as the ultraviolet light source is small, so it can be installed inside the inkjet device.

For example, the ultraviolet light-emitting diodes can be attached to the carriage (both ends along the medium width direction and/or the medium transport direction side) on which the second inkjet head is mounted. Furthermore, due to the composition of the radiation-curable inkjet composition described above, curing can be achieved at low energy and high speed. The irradiation energy is calculated by multiplying the irradiation time by the irradiation intensity. Therefore, the irradiation time can be shortened and the printing speed can be increased. On the other hand, the irradiation intensity can also be reduced. This allows the coating film to cure slowly, resulting in a cured coating film with excellent color development.

In the second ejection step, the clear ink is ejected from the first inkjet head to deposit the clear ink on at least a portion of the area of the recording medium to which the color inks have been deposited. The ejection method and the head used can be the same as those in the first ejection step.

In the second irradiation step, the clear ink adhered to the recording medium is irradiated with radiation from the first radiation source. In the inkjet method of the present embodiment, by using the ink set described above, the gloss of the coating of the clear ink is further improved.

3. Recorded Matter The recorded matter of this embodiment is recorded by the inkjet recording device, and is formed by adhering and curing the radiation curable inkjet composition onto a recording medium. By using an ink set of the radiation curable inkjet composition, the image recorded with the color inks is protected by the clear ink, and thus has good glossiness. Furthermore, since the radiation curable inkjet composition has good flexibility and adhesion, cracks and chipping of the coating film can be suppressed when post-processing such as cutting and bending is performed. Therefore, the recorded matter of this embodiment can be suitably used for sign applications, etc.

Materials for the recording medium are not particularly limited, but examples include plastics such as polyvinyl chloride, polyethylene terephthalate, polypropylene, polyethylene, polycarbonate, cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate butyrate, cellulose nitrate, polyethylene terephthalate, polyethylene, polystyrene, polypropylene, polycarbonate, polyvinyl acetal, and the like, as well as plastics whose surfaces have been treated, glass, paper, metal, wood, and the like.

The form of the recording medium is also not particularly limited. Examples include film, board, cloth, etc.

The present invention will be described in more detail below using examples and comparative examples. The present invention is not limited in any way by the following examples.

1. Preparation of clear ink and color ink First, the colorant, dispersant, and a portion of each monomer were weighed and placed in a pigment dispersion tank, and a ceramic bead mill with a diameter of 1 mm was placed in the tank and stirred to obtain a pigment dispersion in which the colorant was dispersed in the monomer. Next, the remaining monomers, polymerization initiator, and polymerization inhibitor were placed in a stainless steel container for mixing, and mixed and stirred to completely dissolve, so as to obtain the composition shown in Table 1. The pigment dispersion obtained above was then added, and further mixed and stirred at room temperature for 1 hour, and further filtered through a 5 μm membrane filter to obtain a radiation-curable inkjet composition of each example. The numerical values of each component shown in each example in the table represent mass % unless otherwise specified.

The abbreviations and product ingredients used in Table 1 are as follows:

<Monofunctional Monomer>
DCPA (dicyclopentenyl acrylate, manufactured by Hitachi Chemical Co., Ltd.)
IBXA (Osaka Organic Chemical Industry Ltd., isobornyl acrylate)
PEA (product name "Viscoat #192", manufactured by Osaka Organic Chemical Industry Co., Ltd., phenoxyethyl acrylate)
- ACMO (Acryloylmorpholine, manufactured by KJ Chemicals Co., Ltd.)
<Polyfunctional Monomer>
VEEA (2-(2-vinyloxyethoxy)ethyl acrylate, manufactured by Nippon Shokubai Co., Ltd.)
DPGDA (product name "SR508", manufactured by Sartomer Corporation, dipropylene glycol diacrylate)
HDDA (product name "Viscoat #230", manufactured by Osaka Organic Chemical Industry Ltd., 1,6-hexanediol di(meth)acrylate)
<Polymerization initiator>
Irg. 819 (product name "IRGACURE 819" manufactured by BASF, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide)
TPO (product name "IRGACURE TPO", manufactured by BASF, 2,4,6-trimethylbenzoyldiphenylphosphine oxide)
<Polymerization inhibitor>
MEHQ (product name "p-methoxyphenol", manufactured by Kanto Chemical Co., Ltd., hydroquinone monomethyl ether)
<Slip agent>
・BYK-UV3500 (BYK Additives & Instruments, polyether-modified polydimethylsiloxane having an acryloyl group)
<Coloring material (pigment)>
PB15:3 (C.I. Pigment Blue 15:3)
<Dispersant>
Solsperse 36000 (Lubrizol, polymer dispersant)

In Table 1, the "monofunctional monomer ratio" in the physical properties column indicates the content of monofunctional monomer relative to the total amount of polymerizable compounds.

In Table 1, the "Proportion of polymerizable compound A" in the physical properties column indicates the content of polymerizable compound A relative to the total amount of polymerizable compounds.

In Table 1, the "Proportion of polymerizable compound A relative to the total amount of monofunctional monomers" in the physical properties column indicates the content of polymerizable compound A relative to the total amount of monofunctional monomers.

In Table 1, "Volume" represents the volume of the cavity created by the van der Waals radius when the molecule is suspended in a vacuum, calculated using the software "COSMOtherm" (MOLSIS).

The "Length" in Table 1 represents the longest side when calculating the "Volume," calculated using the software "COSMOtherm" (MOLSIS).

"Volume/Length" in Table 1 represents the value obtained by dividing the "Volume" above by the "Length" above. This value represents the area in the height direction relative to the long side.

2. Evaluation Method A serial printer as shown in Fig. 1 was prepared as a recording device. The recording unit 320 of the prepared printer had the configuration as shown in Fig. 2, and recording was performed by the recording unit configured as shown in Table 2 below. Specifically, color inks were applied to the recording medium to a thickness of 10 μm and cured, and clear ink was applied to the color ink coating and cured to a thickness as shown in Table 3 to obtain a coating. In Table 2, L/B represents the feed amount (mm) of the recording medium in the sub-scanning direction per pass.

For the evaluation of flexibility, a vinyl chloride film (JT5829R, manufactured by MACtac) was used as the recording medium, and for the evaluation of gloss, a polycarbonate film (Iupilon NF2000, manufactured by Mitsubishi Gas Chemical Company, Inc.) was used as the recording medium.

2.1. Evaluation of flexibility The release paper was peeled off from the PVC film on which the coating film was formed, and the film was cut into a strip of 1 cm width and 8 cm length to prepare a test piece. The elongation percentage as flexibility was measured for each test piece using a tensile tester (TENSILON, manufactured by ORIENTEC). The elongation percentage was the value at the time when a crack occurred when pulled at 5 mm/min. The value was calculated by {(length at crack - length before stretching) / length before stretching x 100}.
The evaluation criteria are as follows:
(Evaluation criteria)
A: 300% or more B: 200% or more but less than 300% C: Less than 200%

The coating film obtained as described above was visually observed from a certain distance to confirm the reflection of fluorescent light from the coating film. The evaluation criteria are as follows.
(Evaluation criteria)
A: The reflection of the fluorescent light can be seen even if the distance is more than 50 cm. B: The reflection of the fluorescent light can be seen if the distance is between 30 cm and 50 cm. C: The reflection of the fluorescent light can be seen if the distance is between 10 cm and 30 cm. D: The reflection of the fluorescent light can be seen if the distance is less than 10 cm, or the reflection cannot be seen.

3. Evaluation Results The composition of the ink set used in each example and the evaluation results are shown in Table 3. It is clear from Table 3 that when the inkjet recording devices of Examples 1 to 10, which contain a predetermined amount of polymerizable compound A and have a predetermined recording unit configuration, are used, images having excellent gloss can be obtained.

200... serial printer, 220... transport unit, 230... recording unit, 231... first inkjet head, 232... first radiation source, 233... first inkjet head, 234... first radiation source, 235... carriage, 236... carriage movement mechanism, F... recording medium, S1, S2... main scanning direction, T1... sub-scanning direction, N... nozzle, L... length of nozzle row of first inkjet head in the sub-scanning direction, X... distance from nozzle end of first inkjet head to first radiation source in the sub-scanning direction

Claims (6)

a first inkjet head that ejects radiation curable clear ink;
a radiation source that irradiates the clear ink with radiation to form a coating of the clear ink,
the clear ink contains a polymerizable compound,
the polymerizable compound includes a polymerizable compound A having a volume defined by the van der Waals radius of 0.26 ^nm3 or more and an area in a height direction relative to a long side of 0.25 ^nm2 or more;
The content of the polymerizable compound A is 80% by mass or more based on the total amount of the polymerizable compounds,
The inkjet recording apparatus, wherein the first inkjet head and the radiation source are configured to satisfy the following formula (1):
0.5≦X×B×T/L≦60 (1)
X (mm): distance from the nozzle end of the first inkjet head to the radiation source in the sub-scanning direction L (mm): length of the nozzle row of the first inkjet head in the sub-scanning direction B: number of passes during printing T (sec): time required for one pass a second inkjet head that ejects radiation curable color ink;
the second inkjet head is disposed upstream of the first inkjet head in terms of ink deposition order;
The inkjet recording apparatus according to claim 1 . the color ink contains a polymerizable compound,
a content of the monofunctional monomer in the color ink is 90% by mass or more based on a total amount of the polymerizable compound;
The inkjet recording apparatus according to claim 2 . The X is 10 mm or more and 250 mm or less.
The inkjet recording apparatus according to any one of claims 1 to 3. X/L is 0.4 or more and 10 or less;
The inkjet recording apparatus according to any one of claims 1 to 4. Recording is performed so that the coating thickness of the clear ink is 20 μm or more.
The inkjet recording apparatus according to any one of claims 1 to 5.

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