Inkjet recording method and inkjet recording apparatus

Abstract

To provide an ink jet recording method and an ink jet recording apparatus which can cure a yellow ink effectively and can produce a highly reliable recorded object efficiently.SOLUTION: An ink jet recording method of the present invention comprises: an ejection step of ejecting a yellow ink, which is a radiation curable ink, from an inkjet head onto a recording medium; and a curing step of irradiating the yellow ink ejected onto the recording medium with a radiation having a peak wavelength of less than 350 nm from an LED irradiator and curing the yellow ink.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an inkjet recording method and an inkjet recording apparatus. [Background technology]

[0002] Conventionally, various methods have been used to form images on recording media such as paper based on image data signals. Among these, the inkjet method uses inexpensive equipment to eject ink only onto the required image area to form an image directly on the recording media, thereby enabling efficient use of ink and low running costs. Furthermore, the inkjet method is excellent as a recording method due to its low noise level.

[0003] 2. Description of the Related Art In recent years, radiation-curable inks that cure when irradiated with radiation have come to be used in inkjet recording methods in order to improve water resistance, solvent resistance, abrasion resistance, and the like.

[0004] In inkjet recording methods using radiation-curable inks, radiation with a wavelength of 365 nm is generally used due to the reactivity of the polymerization reaction of polymerizable compounds and ease of availability (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-117621 Summary of the Invention [Problem to be solved by the invention]

[0006] However, for example, when an ink contains a colorant, the radiation absorption spectrum varies depending on the colorant. In particular, with a yellow radiation-curable ink, the colorant has a high absorption rate of radiation around 365 nm, which is conventionally used in polymerization reactions. This means that the irradiated radiation is not effectively utilized in the polymerization reaction, resulting in problems such as poor curing. Such poor curing can lead to problems such as tackiness in the recorded area formed using the radiation-curable ink, reduced abrasion resistance, and odor from the recorded material. While extending the radiation irradiation time is considered to sufficiently promote the curing reaction, this approach cannot achieve high-speed printing or improved productivity of the recorded material and can also result in undesired degradation of the components of the recording medium and the recorded area. It is also undesirable from the perspective of energy conservation. [Means for solving the problem]

[0007] The present invention has been made to solve the above-mentioned problems, and can be realized as the following application examples.

[0008] The inkjet recording method according to an application example of the present invention includes: a discharge step of discharging a yellow ink, which is a radiation curable ink, from an inkjet head onto a recording medium; and a curing step of irradiating the yellow ink ejected onto the recording medium with radiation having a peak wavelength of less than 350 nm from an LED irradiator to cure the yellow ink.

[0009] The inkjet recording apparatus according to an application example of the present invention includes: an inkjet head that ejects yellow ink, which is a radiation curable ink, onto a recording medium; and an LED illuminator that emits radiation having a peak wavelength of less than 350 nm. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a perspective view of an inkjet recording apparatus according to a preferred embodiment. [Figure 2]FIG. 2 is a front view of the radiation irradiation apparatus shown in FIG. [Figure 3] FIG. 3 is a view taken along the line AA in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0011] Preferred embodiments of the present invention will be described in detail below. The embodiments described below are examples of the present invention. The present invention is not limited to the following embodiments, and includes various modifications that are implemented within the scope of the present invention. Note that not all of the configurations described below are necessarily essential configurations of the present invention.

[0012] [1] Inkjet recording method First, the ink jet recording method of the present invention will be described.

[0013] The inkjet recording method of the present invention includes a discharge step of discharging a yellow ink, which is a radiation-curable ink, from an inkjet head onto a recording medium, and a curing step of irradiating the yellow ink discharged onto the recording medium with radiation having a peak wavelength of less than 350 nm from an LED irradiator, thereby curing the yellow ink.

[0014] This allows for effective curing of the yellow ink, thereby providing an inkjet recording method capable of efficiently producing highly reliable recorded materials. More specifically, by irradiating the yellow ink applied to a recording medium with radiation having a peak wavelength of less than 350 nm, the rate of radiation absorption by the colorant contained in the yellow ink can be reduced. The irradiated radiation can be efficiently utilized in the curing reaction of the radiation-curable ink, effectively preventing problems caused by insufficient curing reaction. Furthermore, by using radiation emitted from an LED irradiator as the radiation, thermal damage to the recording medium, etc., can be suppressed. As a result, the reliability of the recorded materials produced can be improved. Furthermore, the efficient curing reaction can be achieved, thereby improving the productivity of the recorded materials. Furthermore, the short-time curing process can eliminate stickiness and tackiness of the recorded area. Therefore, even when the printed surface comes into contact with other recorded materials or other components, for example, when the recorded materials are stacked or wound, image distortion can be effectively prevented. Therefore, recorded materials can be produced efficiently even in a small space. Furthermore, the use of an LED irradiator allows the radiation source to be made smaller, thereby enabling the overall size of the equipment used to produce the recorded material to be reduced. This is more advantageous from the viewpoint of energy conservation than using a metal halide lamp, and it also makes it possible to more effectively prevent odors from being generated during the production of the recorded material and from the produced recorded material.

[0015] On the other hand, if the above conditions are not met, the above excellent effects cannot be obtained.

[0016] For example, if radiation with a peak wavelength of 350 nm or longer is used in the curing process, the irradiated radiation is absorbed by the coloring material contained in the yellow ink, and the irradiated radiation cannot be efficiently utilized in the curing reaction of the radiation-curable ink, preventing the curing reaction from proceeding sufficiently. As a result, the recorded area is likely to become sticky and tacky, the image is likely to be distorted, and the reliability of the recorded product is reduced.

[0017] Furthermore, when radiation is irradiated from a radiation source other than an LED irradiator, for example, when radiation is irradiated from a metal halide lamp, thermal damage to the recording medium and the like is likely to occur, reducing the reliability of the resulting recorded matter. Furthermore, a relatively strong odor may be generated during or from the recording matter produced. Another problem is that the device for producing the recorded matter is likely to become large.

[0018] [1-1] Discharge process In the ejection step, yellow ink, which is a radiation curable ink, is ejected from an inkjet head onto a recording medium.

[0019] The yellow ink will be described in detail later. In this step, at least one other ink may be used together with the yellow ink. In other words, the inkjet recording method of the present invention may use an ink set including a yellow ink and at least one other ink than the yellow ink.

[0020] When inks other than yellow ink are used, it is preferable to use at least cyan ink and magenta ink.

[0021] In addition to the yellow ink, black ink may also be used. Inks other than yellow ink will also be described later.

[0022] The volume of each droplet of ink ejected in this step is preferably 1 pL or more and 20 pL or less.

[0023] This makes it possible to obtain higher quality images while improving the ink ejection stability.

[0024] As an inkjet method for ejecting ink from an inkjet head, for example, a piezo method or a method in which ink is ejected by bubbles generated by heating the composition to be ejected can be used. However, the piezo method is preferred from the viewpoint of being less susceptible to unintended deterioration of the ink components.

[0025] Any recording medium may be used, including, for example, ordinary paper, paper specifically for inkjet printing, plastic materials, metals, ceramics, wood, natural and synthetic fibers such as cotton, polyester, and wool, and nonwoven fabrics.

[0026] In particular, in the present invention, it is more preferable that the recording medium is a non-absorbent recording medium. Non-absorbent recording media generally tend to shrink due to heat. Therefore, when a metal halide lamp or the like is used as a radiation source, problems due to undesired shrinkage of the recording medium are likely to occur. In contrast, the present invention uses an LED irradiator in the curing process, which can reduce damage to the recording medium. In other words, the effects of the present invention are more pronounced when the recording medium is a non-absorbent recording medium.

[0027] In this specification, the term "non-absorbent recording medium" refers to a recording medium that does not absorb ink at all or absorbs very little ink. Quantitatively, a non-absorbent recording medium is one that "absorbs ink within 30 msec from the start of contact in the Bristow method." 1 / 2 Water absorption up to 10mL / m 2This refers to a recording medium that is non-absorbent. The Bristow method is the most widely used method for measuring the amount of liquid absorbed in a short period of time, and is also adopted by the Japan Pulp and Paper Technology Association (JAPAN TAPPI). Details of the test method are described in Standard No. 51 "Paper and Paperboard - Liquid Absorbency Test Method - Bristow Method" of the "JAPAN TAPPI Paper and Pulp Test Methods 2000 Edition." Examples of recording media that have non-absorbent properties include recording media that do not have an ink-absorbing ink-receiving layer on the recording surface, and recording media that have a non-ink-absorbent coating layer on the recording surface.

[0028] Examples of non-absorbent recording media include plastic films that do not have an ink-absorbing layer, substrates such as paper coated with plastic, and substrates with a plastic film adhered thereto. Examples of plastics include, but are not limited to, polyvinyl chloride, polyethylene terephthalate, polycarbonate, polystyrene, polyurethane, polyethylene, polypropylene, and polyvinyl acetal.

[0029] The recording medium may be colorless and transparent, translucent, colored and transparent, chromatic and opaque, achromatic and opaque, etc. The recording medium may itself be colored, translucent, or transparent. The shape of the recording medium is not particularly limited, and may be any shape, such as a sheet.

[0030] The inkjet recording apparatus equipped with an inkjet head for ejecting ink will be described in detail later.

[0031] [1-2]Curing process In the curing step, the yellow ink ejected onto the recording medium in the ejection step is irradiated with radiation having a peak wavelength of less than 350 nm from an LED irradiator, thereby curing the yellow ink.

[0032] As described above, the peak wavelength of the radiation emitted from the LED irradiator may be less than 350 nm, but is preferably 200 nm or more and 330 nm or less, more preferably 250 nm or more and 320 nm or less, and even more preferably 280 nm or more and 310 nm or less. This makes the above-mentioned effects more pronounced.

[0033] The radiation energy emitted from the LED irradiator is 10 mW / cm 2 More than 1000mW / cm 2 Preferably, it is 20 mW / cm or less. 2 More than 500mW / cm 2 More preferably, it is 30 mW / cm or less. 2 More than 200mW / cm 2 More preferably, it is:

[0034] This makes it possible to more effectively prevent damage to the recording medium and the like while allowing the curing reaction to proceed more favorably, thereby improving the reliability of the recorded matter produced. This is also advantageous from the viewpoint of further improving the productivity of the recorded matter, saving energy, etc.

[0035] The cumulative amount of radiation irradiated from the LED irradiator per unit area of ​​the area where yellow ink is applied in this process is 100 mJ / cm 2 More than 600mJ / cm 2 Preferably it is 150 mJ / cm or less. 2 More than 450mJ / cm 2 More preferably, it is 200 mJ / cm or less. 2 More than 300mJ / cm 2 More preferably, it is:

[0036] This makes it possible to more effectively prevent damage to the recording medium and the like while allowing the curing reaction to proceed more favorably, thereby improving the reliability of the recorded matter produced. This is also advantageous from the viewpoint of further improving the productivity of the recorded matter, saving energy, etc.

[0037] In the ejection step, when inks other than the yellow ink are used together with the yellow ink, these inks may be cured in this step.

[0038] In such a case, the inks other than the yellow ink may be cured under the same conditions as those described above as the curing conditions for the yellow ink, but may also be cured under conditions different from those described above.

[0039] In particular, when curing magenta ink, cyan ink, or black ink, the wavelength of the irradiated radiation is preferably 350 nm or more, more preferably 355 nm or more and 420 nm or less, and even more preferably 360 nm or more and 410 nm or less.

[0040] This allows these inks to be cured more suitably, and also makes it easier to match conditions such as curing speed with those of the yellow ink on the printed matter.

[0041] [1-3] Radiation-curable yellow ink Next, the yellow ink used in the ink jet recording method of the present invention will be described.

[0042] The yellow ink is not particularly limited as long as it is an ink that can form a yellow recording section when a recording section is formed using only the yellow ink, and the ink undergoes a polymerization reaction and is cured by irradiation with the radiation.

[0043] In this specification, the term "yellow" when referring to yellow ink is a concept that includes colors that are visible as yellow.

[0044] [1-3-1] Coloring materials Yellow ink typically contains a yellow colorant.

[0045] The coloring material may be, for example, at least one of a yellow pigment and a yellow dye. The yellow ink may contain a coloring material other than yellow, as long as it can form a yellow recording portion.

[0046] Examples of yellow pigments include CI Pigment Yellow 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 16, 17, 24, 34, 35, 37, 53, 55, 65, 73, 74, 75, 81, 83, 93, 94, 95, 97, 98, 99, 108, 109, 110, 113, 114, 117, 120, 124, 128, 129, 133, 138, 139, 147, 151, 153, 154, 155, 167, 172, and 180.

[0047] Examples of the dye include disperse dyes, acid dyes, basic dyes, direct dyes, and reactive dyes.

[0048] Examples of yellow disperse dyes include CI Disperse Yellow 3, 4, 5, 7, 9, 13, 23, 24, 30, 33, 34, 42, 44, 49, 50, 51, 54, 56, 58, 60, 63, 64, 66, 68, 71, 74, 76, 79, 82, 83, 85, 86, 88, 90, 91, 93, 98, 99, 100, 104, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 168, 169, 170, 171, 172, 173, 174, 175, 176 14, 116, 118, 119, 122, 124, 126, 135, 140, 141, 149, 160, 162, 163, 164, 165, 179, 180, 182, 183, 184, 186, 192, 198, 199, 202, 204, 210, 211, 215, 216, 218, 224, 227, 231, 232, etc.

[0049] Examples of yellow acid dyes include CI Acid Yellow 1, 3, 6, 11, 17, 18, 19, 23, 25, 36, 38, 40, 40:1, 42, 44, 49, 59, 59:1, 61, 65, 67, 72, 73, 79, 99, 104, 142, 159, 169, 176, 184, 193, 200, 204, 207, 215, 219, 219:1, 220, 230, 232, 235, 241, 242, and 246.

[0050] Examples of yellow basic dyes include CI Basic Yellow 1, 2, 13, 19, 21, 25, 32, 36, 40, and 51.

[0051] Examples of yellow direct dyes include CI Direct Yellow 1, 8, 9, 10, 11, 12, 22, 24, 27, 28, 33, 39, 44, 50, 55, 58, 86, 87, 98, 105, 106, 130, 132, 137, 142, 144, 147, 153, and 173.

[0052] Examples of yellow reactive dyes include CI Reactive Yellow 2, 3, 7, 15, 17, 18, 22, 23, 24, 25, 27, 37, 39, 42, 57, 69, 76, 81, 84, 85, 86, 87, 92, 95, 102, 105, 111, 125, 135, 136, 137, 142, 143, 145, 151, 160, 161, 165, 167, 168, 175, and 176.

[0053] In particular, it is preferable that the yellow ink contains CI Pigment Yellow 155 as a coloring material.

[0054] CI Pigment Yellow 155 has a significant difference in radiation absorption between wavelengths of 350 nm or more and less than 350 nm. Therefore, when the yellow ink contains CI Pigment Yellow 155, the effects of the present invention are more pronounced.

[0055] When the above pigments are used, the average particle size thereof is preferably 500 nm or less, and more preferably 50 nm or more and 300 nm or less.

[0056] This makes it possible to improve the reliability of the yellow ink, such as ejection stability when using an inkjet method and storage stability, and also to form images of higher quality.

[0057] In this specification, the average particle size refers to the average particle size on a volume basis unless otherwise specified. The average particle size can be determined, for example, by measurement using Nanotrac UPA (manufactured by Nikkiso Co., Ltd.).

[0058] The content of the coloring material in the yellow ink is preferably 0.5% by mass to 10.0% by mass, more preferably 1.0% by mass to 7.0% by mass, and even more preferably 3.0% by mass to 6.0% by mass.

[0059] This makes it possible to, for example, ensure that the color density of the image formed on the recording medium is sufficiently high, while also improving the ejection stability of the yellow ink when ink-jet printing, storage stability, and the like.

[0060] [1-3-2] Polymerizable compound The yellow ink usually contains a polymerizable compound.

[0061] The polymerizable compound can be polymerized by itself or in the presence of a polymerization initiator when irradiated with radiation, thereby curing the ink on the recording medium.

[0062] The polymerizable compound is not particularly limited, but specifically, conventionally known monofunctional, bifunctional, and trifunctional or higher polyfunctional monomers and oligomers can be used. The polymerizable compound may be used alone or in combination of two or more.

[0063] The polymerizable compound preferably contains a radical polymerizable compound from the viewpoint of further enhancing the curability of the ink and from the viewpoint of obtaining high versatility and high ease of use.

[0064] Furthermore, from the viewpoints of improving the curability, further reducing the viscosity of the ink, and increasing the solubility of a polymerization initiator when one is used, it is preferable that the polymerizable compound contains a polymerizable compound having a vinyl ether group and a (meth)acrylate group.

[0065] The polymerizable compound having a vinyl ether group and a (meth)acrylate group is preferably a radical polymerizable compound having a vinyl ether group and a (meth)acrylate group.

[0066] Examples of such polymerizable compounds include (meth)acrylates having a monofunctional or polyfunctional vinyl ether group. Hereinafter, (meth)acrylates having a vinyl ether group will also be referred to as "vinyl group-containing (meth)acrylates."

[0067] The vinyl group-containing (meth)acrylate is not particularly limited, but from the viewpoints of further reducing the viscosity of the composition, increasing the flash point, and further enhancing the curability of the composition, it is preferable that the vinyl group-containing (meth)acrylate contains a compound represented by the following general formula (I):

[0068] H2C=CR 1 -CO-OR 2 -O-CH=CH-R 3 (I) (In formula (I), R 1 is a hydrogen atom or a methyl group, and R 2 is a divalent organic residue having 2 to 20 carbon atoms, and R 3 is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms.

[0069] Hereinafter, the vinyl group-containing (meth)acrylate represented by general formula (I) may be simply referred to as the "compound of formula (I)."

[0070] By including a compound of formula (I) in the yellow ink, the curability of the yellow ink tends to be excellent. Furthermore, by including a compound of formula (I), the viscosity of the yellow ink tends to be easily reduced. Furthermore, in terms of improving the curability of the yellow ink, it is preferable to use a compound having both a vinyl ether group and a (meth)acrylic group in one molecule, rather than using a compound having a vinyl ether group and a compound having a (meth)acrylic group separately.

[0071] In the above general formula (I), R 2 Preferred examples of the divalent organic residue having 2 to 20 carbon atoms and represented by the formula (I) include linear, branched, or cyclic alkylene groups having 2 to 20 carbon atoms, which may be substituted; alkylene groups having 2 to 20 carbon atoms and containing an oxygen atom via at least one of an ether bond and 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, n-propylene, isopropylene, and butylene, and alkylene groups having 2 to 9 carbon atoms and containing an oxygen atom via an ether bond in the structure, such as oxyethylene, oxy-n-propylene, oxyisopropylene, and oxybutylene, are preferably used. Furthermore, from the viewpoint of further reducing the viscosity of the radiation-curable yellow ink and further improving its curability, R 2 However, more preferred are compounds having a glycol ether chain, which is an alkylene group having 2 to 9 carbon atoms and an oxygen atom due to an ether bond in the structure, such as an oxyethylene group, an oxy-n-propylene group, an oxyisopropylene group, or an oxybutylene group.

[0072] In the above general formula (I), R 3The monovalent organic residue having 1 to 11 carbon atoms, represented by the formula (I), 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 or 2 carbon atoms, such as a methyl group or an ethyl group, or an aromatic group having 6 to 8 carbon atoms, such as a phenyl group or a benzyl group, is preferably used.

[0073] When each of the above organic residues is a group that may be substituted, the substituent is divided into a group containing carbon atoms and a group that does not contain carbon atoms. First, when the above substituent is a group that contains carbon atoms, the carbon atom is counted in the number of carbon atoms of the organic residue. Examples of the group that contains carbon atoms include a carboxyl group, an alkoxy group, etc. Next, examples of the group that does not contain carbon atoms include a hydroxyl group, a halo group, etc.

[0074] The content of the compound of formula (I) in the yellow ink is preferably 10.0% by mass or more and 70.0% by mass or less, and more preferably 20.0% by mass or more and 50.0% by mass or less.

[0075] This makes it possible to more effectively reduce the viscosity of the yellow ink, thereby improving the ejection stability of the yellow ink when used in an inkjet method, improving the curing properties of the yellow ink, and improving the storage stability of the yellow ink.

[0076] Specific examples of the compound of formula (I) include 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, 1-methyl-2-vinyloxypropyl (meth)acrylate, 2-vinyloxybutyl (meth)acrylate, 4-vinyloxycyclohexyl (meth)acrylate, 6-vinyloxyhexyl (meth)acrylate, 4-vinyloxymethylcyclohexylmethyl (meth)acrylate, 3-vinyloxymethylcyclohexylmethyl (meth)acrylate, 2-vinyloxymethylcyclohexylmethyl (meth)acrylate, p-vinyloxymethylphenylmethyl (meth)acrylate, m-vinyloxymethylphenylmethyl (meth)acrylate, (meth) ) o-vinyloxymethylphenylmethyl acrylate, 2-(2-vinyloxyethoxy)ethyl methacrylate, 2-(2-vinyloxyethoxy)ethyl acrylate (VEEA), 2-(vinyloxyisopropoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxy)propyl (meth)acrylate, 2-(vinyloxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxy)propyl (meth)acrylate, 2-(vinyloxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxy)ethyl (meth)acrylate (meth)acrylate 2-(vinyloxyethoxyisopropoxy)ethyl, (meth)acrylate 2-(vinyloxyisopropoxyethoxy)ethyl, (meth)acrylate 2-(vinyloxyisopropoxyisopropoxy)ethyl, (meth)acrylate 2-(vinyloxyethoxyethoxy)propyl, (meth)acrylate 2-(vinyloxyethoxyisopropoxy)propyl, (meth)acrylate 2-(vinyloxyisopropoxyethoxy)propyl, (meth)acrylate 2-(vinyloxyisopropoxyisopropoxy)propyl, (meth)acrylate 2-(vinyloxyisopropoxyisopropoxy)propyl,2-(vinyloxyethoxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxyethoxy)ethyl (meth)acrylate ethyl, 2-(isopropenoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, polyethylene glycol monovinyl ether (meth)acrylate, polypropylene glycol monovinyl ether (meth)acrylate, etc.

[0077] Among these, from the viewpoints of achieving a high flash point, a more suitable low viscosity for the yellow ink, and more suitable curing properties, 2-(vinyloxyethoxy)ethyl (meth)acrylate, that is, at least one selected from the group consisting of 2-(vinyloxyethoxy)ethyl acrylate and 2-(vinyloxyethoxy)ethyl methacrylate, is preferred, and 2-(vinyloxyethoxy)ethyl acrylate is more preferred.

[0078] Both 2-(vinyloxyethoxy)ethyl acrylate and 2-(vinyloxyethoxy)ethyl methacrylate have a simple structure and a small molecular weight, and therefore can more suitably reduce the viscosity of the yellow ink.

[0079] Examples of 2-(vinyloxyethoxy)ethyl (meth)acrylate include 2-(2-vinyloxyethoxy)ethyl (meth)acrylate and 2-(1-vinyloxyethoxy)ethyl (meth)acrylate, and examples of 2-(vinyloxyethoxy)ethyl acrylate include 2-(2-vinyloxyethoxy)ethyl acrylate and 2-(1-vinyloxyethoxy)ethyl acrylate. Note that 2-(vinyloxyethoxy)ethyl acrylate is superior to 2-(vinyloxyethoxy)ethyl methacrylate in terms of curability.

[0080] The content of the vinyl ether group-containing (meth)acrylic acid esters, in particular 2-(2-vinyloxyethoxy)ethyl (meth)acrylate, in the yellow ink is preferably 5.0% by mass or more and 60.0% by mass or less, and more preferably 10.0% by mass or more and 50.0% by mass or less.

[0081] This makes it possible to more effectively reduce the viscosity of the yellow ink, improve the ejection stability of the yellow ink when ink-jet printing, improve the curing properties of the yellow ink, improve the storage stability of the yellow ink, and improve the surface gloss of the recorded material.

[0082] The yellow ink may contain at least one selected from the group consisting of monofunctional, difunctional, and trifunctional or higher polyfunctional monomers other than those mentioned above.

[0083] Examples of such monomers include, but are not limited to, 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 acid anhydrides of unsaturated carboxylic acids; acrylonitrile, styrene, various unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes.

[0084] Furthermore, the copolymer may contain an N-vinyl compound as another monofunctional or polyfunctional monomer.

[0085] The N-vinyl compound is not particularly limited, but examples thereof include N-vinylformamide, N-vinylcarbazole, N-vinylacetamide, N-vinylpyrrolidone, N-vinylcaprolactam, acryloylmorpholine, and derivatives thereof.

[0086] The yellow ink may contain a monofunctional (meth)acrylate as the monofunctional monomer.

[0087] This makes it possible to more suitably reduce the viscosity of the yellow ink, improve the solubility of the polymerization initiator and other additives, and improve the ejection stability of the yellow ink when used in an inkjet method.

[0088] The monofunctional (meth)acrylate is not particularly limited, and examples thereof include isoamyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, isomyristyl (meth)acrylate, isostearyl (meth)acrylate, 2-ethylhexyl-diglycol (meth)acrylate, 2-hydroxybutyl (meth)acrylate, butoxyethyl (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, methoxydiethylene glycol (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, methoxypropylene glycol (meth)acrylate, phenoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl methyl ... hydroxypropyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 4-hydroxybutyl acrylate, lactone-modified flexible (meth)acrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentanyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, 2-(isopropenoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, polyethylene glycol monovinyl ether (meth)acrylate, polypropylene glycol monovinyl ether (meth)acrylate, cyclic trimethylolpropane formal (meth)acrylate, and the like. Among these, at least one selected from the group consisting of phenoxyethyl (meth)acrylate, isobornyl (meth)acrylate, and cyclic trimethylolpropane formal (meth)acrylate is preferred.

[0089] This makes it possible to improve the curability of the yellow ink, and also to suitably reduce the content of the polymerization initiator, which can cause coloring.

[0090] The content of the monofunctional monomer in the yellow ink is preferably 10.0% by mass or more and 80.0% by mass or less, and more preferably 20.0% by mass or more and 60.0% by mass or less.

[0091] This can improve the curing properties, storage stability, and ejection stability of the yellow ink, and can improve the solubility of the polymerization initiator in the yellow ink.

[0092] The yellow ink may contain a polyfunctional (meth)acrylate as the polyfunctional monomer.

[0093] Among these, the bifunctional (meth)acrylate is not particularly limited, but examples thereof include triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonyl 2-methylpropional di(meth)acrylate, 1,6 ... Examples of such an alkyl acrylate include nanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, dimethylol-tricyclodecane di(meth)acrylate, EO (ethylene oxide) adduct di(meth)acrylate of bisphenol A, PO (propylene oxide) adduct di(meth)acrylate of bisphenol A, hydroxypivalic acid neopentyl glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, and diethylene glycol di(meth)acrylate.

[0094] Furthermore, examples of trifunctional or higher functional (meth)acrylates include 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, cowprolactone-modified trimethylolpropane tri(meth)acrylate, pentaerythritol ethoxy tetra(meth)acrylate, and caprolactam-modified dipentaerythritol hexa(meth)acrylate.

[0095] Among these, dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol hexa(meth)acrylate are preferred, and dipropylene glycol di(meth)acrylate and dipentaerythritol hexa(meth)acrylate are more preferred.

[0096] The content of the polyfunctional monomer in the yellow ink is preferably 60.0% by mass or less, more preferably 50.0% by mass or less, and even more preferably 40.0% by mass or less.

[0097] This makes it possible to improve the curing properties, storage stability, and ejection stability of the yellow ink, and also to improve the surface gloss of the produced recording material.

[0098] Furthermore, in order to improve the toughness, heat resistance, and chemical resistance of the recording portion formed using the yellow ink, it is preferable to use the monofunctional (meth)acrylate in combination with the polyfunctional (meth)acrylate, and it is particularly preferable to use phenoxyethyl (meth)acrylate, isobornyl (meth)acrylate, dipropylene glycol di(meth)acrylate, and dipentaerythritol hexa(meth)acrylate in combination.

[0099] Furthermore, in order to improve the adhesion and toughness of the recording portion formed using the yellow ink to the recording medium, the ink may contain a monofunctional or polyfunctional oligomer having two or more functionalities in addition to the above-mentioned monomers.

[0100] The type of oligomer is not particularly limited, but examples thereof include various oligomers such as acrylic oligomers formed from acrylic monomers, styrene-acrylic oligomers formed from styrene and acrylic monomers, aliphatic, alicyclic, aromatic, etc. urethane acrylate oligomers, epoxy acrylate oligomers, and polyester acrylate oligomers.

[0101] The content of the polymerizable compound in the yellow ink is preferably 35.0% by mass or more and 95.0% by mass or less, and more preferably 45.0% by mass or more and 90.0% by mass or less.

[0102] This makes it possible to make the viscosity of the yellow ink more suitable, reduce the odor of the yellow ink and of recorded matter produced using the same, and improve the solubility of the polymerization initiator in the yellow ink, the reactivity of the yellow ink, and the surface gloss of recorded matter produced using the yellow ink.

[0103] [1-3-3] Polymerization initiator The yellow ink may contain a polymerization initiator.

[0104] This allows the polymerization reaction to be suitably initiated and progressed when irradiated with the above-mentioned radiation.

[0105] As the polymerization initiator, for example, various compounds that generate active species such as radicals or cations when irradiated with actinic radiation and initiate the polymerization reaction of the above-mentioned polymerizable compound can be used. More specifically, as the polymerization initiator, a photoradical polymerization initiator or a photocationic polymerization initiator can be used, but it is preferable to use a photoradical polymerization initiator.

[0106] In particular, the yellow ink preferably contains an α-hydroxyketone-based polymerization initiator.

[0107] This allows the yellow ink to have better curability with radiation having a peak wavelength of less than 350 nm.

[0108] An α-hydroxyketone initiator refers to a polymerization initiator that has an α-hydroxyketone structure in the molecule.

[0109] Examples of the α-hydroxyketone initiator include 2-hydroxy-1-(4-(4-(2-hydroxy-2-methyl-propionyl)-benzyl)-phenyl)-2-methyl-propane, 1-(4-(2-hydroxyethoxyl)-phenyl)-2-hydroxy-methylpropanone, oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone), 2-hydroxy-1-[4-{4-(2-hydroxy-2-methylpropanoyl)phenoxy}phenyl]-2-methylpropan-1-one, 1-hydroxycyclohexyl phenyl ketone, and 2-hydroxy-2-methoxypropiophenone.

[0110] Among these, α-hydroxyketone initiators having a molecular weight of 210 or more are preferred.

[0111] This makes it possible to more effectively suppress the generation of odor from the recording section and the migration of substances from the recording section.

[0112] Examples of α-hydroxyketone initiators having a molecular weight of 210 or more include 2-hydroxy-1-(4-(4-(2-hydroxy-2-methyl-propionyl)-benzyl)-phenyl)-2-methyl-propane (molecular weight: 340), 1-(4-(2-hydroxyethoxyl)-phenyl)-2-hydroxy-methylpropanone (molecular weight: 224), oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone) (molecular weight: 483), 2-hydroxy-1-[4-{4-(2-hydroxy-2-methylpropanoyl)phenoxy}phenyl]-2-methylpropan-1-one (molecular weight: 342), and the like.

[0113] As mentioned above, the molecular weight of the α-hydroxyketone initiator is preferably 210 or more, more preferably 300 or more, even more preferably 400 or more, and most preferably 450 or more. This makes the above-mentioned effects more pronounced.

[0114] Commercially available α-hydroxyketone initiators include, for example, Omnirad 127 (trade name, manufactured by IGM Resins BV, 2-hydroxy-1-(4-(4-(2-hydroxy-2-methyl-propionyl)-benzyl)-phenyl)-2-methyl-propane), Omnirad 2959 (trade name, manufactured by IGM Resins BV, 1-(4-(2-hydroxyethoxyl)-phenyl)-2-hydroxy-methylpropanone), ESACURE KIP 150 (trade name, manufactured by Lamberti, oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone)), ESACURE KIP 160 (trade name, manufactured by Lamberti, 2-hydroxy-1-[4-{4-(2-hydroxy-2-methylpropanoyl)phenoxy}phenyl]-2-methylpropan-1-one), Omnirad 184 (trade name, manufactured by IGM Resins BV, 1-hydroxycyclohexyl phenyl ketone), Omnirad 1173 (trade name, manufactured by IGM Resins BV, 2-hydroxy-2-methoxypropiophenone), and the like.

[0115] The content of the α-hydroxyketone initiator in the yellow ink is preferably 4.0% by mass or more and 20.0% by mass or less, more preferably 6.0% by mass or more and 15.0% by mass or less, and even more preferably 7.5% by mass or more and 10.0% by mass or less.

[0116] This makes it possible to increase the curing speed, effectively prevent the α-hydroxyketone initiator from remaining undissolved in the yellow ink, and more effectively prevent the generation of odor.

[0117] The yellow ink may also contain a polymerization initiator other than an α-hydroxyketone-based initiator. Examples of such polymerization initiators include photoradical polymerization initiators such as aromatic ketones other than α-hydroxyketone-based compounds, acylphosphine oxide compounds, aromatic onium salt compounds, organic peroxides, thio compounds (such as thioxanthone compounds and thiophenyl group-containing compounds), hexaarylbiimidazole compounds, ketoxime ester compounds, borate compounds, azinium compounds, metallocene compounds, active ester compounds, compounds having a carbon-halogen bond, and alkylamine compounds.

[0118] Among these, at least one selected from acylphosphine oxide compounds and thioxanthone compounds is preferred. Furthermore, acylphosphine oxide compounds and thioxanthone compounds may be used in combination.

[0119] Among the acylphosphine oxide compounds, 2,4,6-trimethylbenzoyldiphenylphosphine oxide and bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide are particularly preferred.

[0120] 2,4,6-trimethylbenzoyldiphenylphosphine oxide is preferred because it has excellent compatibility with the above-mentioned polymerizable compound.

[0121] Commercially available products of 2,4,6-trimethylbenzoyldiphenylphosphine oxide include, for example, "SpeedCure TPO" (trade name, manufactured by BASF Japan Ltd.).

[0122] Bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide is also preferred because it has a wide absorption range.

[0123] Commercially available bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide products include, for example, "IRGACURE 819" (trade name, BASF Japan Ltd.).

[0124] Specific examples of the photoradical polymerization initiator include acetophenone, acetophenone benzyl ketal, xanthone, fluorenone, benzaldehyde, fluorene, anthraquinone, triphenylamine, carbazole, 3-methylacetophenone, 4-chlorobenzophenone, 4,4'-dimethoxybenzophenone, 4,4'-diaminobenzophenone, Michler's ketone, benzoin propyl ether, benzoin ethyl ether, benzil dimethyl ketal, thioxanthone, and diene. Examples of suitable thioxanthone include thiolthioxanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propan-1-one, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, 2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide, 2,4-diethylthioxanthone, and bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.

[0125] Commercially available products of the photoradical polymerization initiator include, for example, IRGACURE 907 (2-methyl-1-(4-methylthiophenyl)-2-morpholinopropan-1-one), IRGACURE 369 (2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1), Omnirad 379 (2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]-1-butanone), DAROCUR TPO (2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide), IRGACURE 819 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), IRGACURE 820 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), and IRGACURE 830 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide). 784 (bis(η5-2,4-cyclopentadien-1-yl)-bis(2,6-difluoro-3-(1H-pyrrol-1-yl)-phenyl)titanium), IRGACURE OXE 01 (1,2-octanedione, 1-[4-(phenylthio)-, 2-(O-benzoyloxime)]), IRGACURE OXE 02 (ethanone, 1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl]-, 1-(O-acetyloxime)), IRGACURE 754 (mixture of oxyphenylacetic acid, 2-[2-oxo-2-phenylacetoxyethoxy]ethyl ester and oxyphenylacetic acid, 2-(2-hydroxyethoxy)ethyl ester), Lucirin TPO, LR8893, LR8970 (all manufactured by BASF Japan), KAYACURE Examples include DETX-S (2,4-diethylthioxanthone) (manufactured by Nippon Kayaku Co., Ltd.), Ubecryl P36 (manufactured by UCB), Speedcure TPO (diphenyl-2,4,6-trimethylbenzoylphosphine oxide), and Speedcure TPO (diphenyl-(2,4,6-trimethylbenzoyl)phosphine oxide) (all manufactured by Lambson).

[0126] The content of the polymerization initiator in the yellow ink is preferably 4.0% by mass or more and 25.0% by mass or less, more preferably 6.0% by mass or more and 18.0% by mass or less, and even more preferably 7.5% by mass or more and 11.0% by mass or less.

[0127] [1-3-4] Polymerization inhibitor The yellow ink may contain a polymerization inhibitor.

[0128] This makes it possible to more effectively prevent undesired polymerization reactions from proceeding during storage of the yellow ink.

[0129] As the polymerization inhibitor, for example, various known polymerization inhibitors can be used, but hindered amine compounds are preferred.

[0130] The hindered amine compound is not particularly limited, and examples thereof include 4-acetamido-2,2,6,6-tetramethylpiperidinyl-1-oxyl, 4-amino-2,2,6,6-tetramethylpiperidinyl-1-oxyl, 4-carboxy-2,2,6,6-tetramethylpiperidinyl-1-oxyl, 4-(2-chloroacetamido)-2,2,6,6-tetramethylpiperidinyl-1-oxyl, 4-cyano-2,2,6,6-tetramethylpiperidinyl-1-oxyl, 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxyl, 4-hydroxybenzoate-2,2 ,6,6-tetramethylpiperidinyl-1-oxyl, 4-(2-iodoacetamido)-2,2,6,6-tetramethylpiperidinyl-1-oxyl, 4-isothiocyanate-2,2,6,6-tetramethylpiperidinyl-1-oxyl, 4-methacryloyloxy-2,2,6,6-tetramethylpiperidinyl-1-oxyl, 4-methoxy-2,2,6,6-tetramethylpiperidinyl-1-oxyl, 4-oxo-2,2,6,6-tetramethylpiperidinyl-1-oxyl, 4-(2-propynyloxy)-2,2,6,6-tetramethylpiperidinyl-1-oxyl, and the like.

[0131] Examples of polymerization inhibitors other than hindered amine compounds include p-methoxyphenol, hydroquinone monomethyl ether (MEHQ), 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).

[0132] The content of the polymerization inhibitor in the yellow ink is preferably 0.05% by mass or more and 0.6% by mass or less, and more preferably 0.1% by mass or more and 0.5% by mass or less.

[0133] [1-3-5] Surfactants The yellow ink may contain a surfactant.

[0134] As the surfactant, a silicone surfactant is preferred, and polyester-modified silicone or polyether-modified silicone is more preferred.

[0135] Examples of polyester-modified silicones include BYK-347, 348, BYK-UV3500, 3510, and 3530 (all manufactured by BYK Additives & Instruments), and examples of polyether-modified silicones include BYK-3570 (manufactured by BYK Additives & Instruments).

[0136] The content of the surfactant in the yellow ink is preferably 0.05% by mass or more and 0.6% by mass or less, and more preferably 0.1% by mass or more and 0.5% by mass or less.

[0137] [1-3-6] Dispersants The yellow ink may contain a dispersant.

[0138] This makes it possible to improve the dispersion stability of the pigment in the yellow ink and the storage stability of the yellow ink, for example, when the yellow ink contains a pigment as a coloring material.

[0139] The dispersant is not particularly limited, and examples thereof include dispersants commonly used in preparing pigment dispersions, such as polymer dispersants, etc. 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.

[0140] Examples of 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 Disperbic series (BYK 180, etc.) manufactured by BYK Additives & Instruments, and the Disparlon series manufactured by Kusumoto Chemicals Co., Ltd.

[0141] [1-3-7] Other ingredients The yellow ink may contain components other than those described above. Hereinafter, such components will also be referred to as "other components" in this section.

[0142] Examples of other components include fluorescent brighteners, dispersing aids, coloring materials such as pigments and dyes, fixing agents, antifungal agents, preservatives, antioxidants, ultraviolet absorbers, chelating agents, and pH adjusters.

[0143] The content of other components in the yellow ink is preferably 5.0% by mass or less, and more preferably 1.0% by mass or less.

[0144] [1-3-8] Other conditions The surface tension of the yellow ink at 25°C is not particularly limited, but is preferably 20 mN / m or more and 60 mN / m or less, more preferably 25 mN / m or more and 50 mN / m or less, and even more preferably 30 mN / m or more and 40 mN / m or less.

[0145] This makes it more difficult for nozzle clogging of the inkjet head to occur, further improving the ejection stability of the yellow ink. Furthermore, even if nozzle clogging does occur, the recovery from the problem can be improved by capping the nozzle.

[0146] The surface tension can be measured by the Wilhelmy method or the Ring method using a surface tensiometer (e.g., DY-300, DY-500, DY-700, etc., manufactured by Kyowa Interface Science Co., Ltd.).

[0147] The viscosity of the yellow ink at 25° C. is preferably 2 mPa·s or more and 10 mPa·s or less, and more preferably 3 mPa·s or more and 8 mPa·s or less.

[0148] This improves the ejection stability of the yellow ink when it is ejected by the inkjet method.

[0149] The viscosity can be determined by measurement using a vibration viscometer, a rotational viscometer, a capillary viscometer, or a falling ball viscometer. For example, when using a vibration viscometer, the viscosity can be determined by measurement in accordance with JIS Z8809.

[0150] [1-4] Inks other than yellow ink Next, inks other than the yellow ink that may be used in the ink jet recording method of the present invention will be described.

[0151] Hereinafter, such inks will also be referred to as "other inks." The other inks are the same as the yellow ink described above, except for the color tone.

[0152] To achieve a color tone different from that of the yellow ink, the composition of the coloring material is usually made different. The coloring material contained in the other ink may be, for example, at least one of a pigment other than yellow and a dye other than yellow. Furthermore, the other ink may contain a yellow coloring material in addition to such a coloring material other than yellow.

[0153] Examples of pigments of colors other than yellow include black pigments such as carbon black; white pigments such as CI Pigment White 6, 18, and 21, white inorganic pigments such as titanium oxide, zinc oxide, zinc sulfide, antimony oxide, magnesium oxide, and zirconium oxide, and white organic pigments such as white hollow resin particles and polymer particles; and CI Pigment Red 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 19, 21, 22, 23, 30, 31, 32, 37, 38, 40, 41, 42, 48(Ca), 48(Mn), 57(Ca), 57:1, 88, 112, 114, 122, 123, 144, 146, 149, and 150. , 166, 168, 170, 171, 175, 176, 177, 178, 179, 184, 185, 187, 202, 209, 219, 224, 245; CI Pigment Violet 19, 23, 32, 33, 36, 38, 43, 50; CI Pigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:34, 15:4, 16, 18, 22, 25, 60, 65, 66; CI Bat Blue 4, 60; CI Pigment Green 7, 10; CI Pigment Brown 3, 5, 25, 26; CI Pigment Orange 1, 2, 5, 7, 13, 14, 15, 16, 24, 34, 36, 38, 40, 43, 63, etc. In addition, examples of pigments that may be used include pearl pigments having a pearlescent or interference luster, such as titanium dioxide-coated mica, fish scale foil, and bismuth oxychloride; and metallic pigments, such as particles composed of a single element or an alloy of aluminum, silver, gold, platinum, nickel, chromium, tin, zinc, indium, titanium, copper, and the like.

[0154] Examples of the dye include disperse dyes, acid dyes, basic dyes, direct dyes, and reactive dyes.

[0155] Examples of disperse dyes in colors other than yellow include CI Disperse Orange 1, 3, 5, 7, 11, 13, 17, 20, 21, 25, 29, 30, 31, 32, 33, 37, 38, 42, 43, 44, 45, 46, 47, 48, 49, 50, 53, 54, 55, 56, 57, 58, 59, 61, 66, 71, 73, 76, 78, 80, 89, 90, 91, 93, 96, 97, 119, 127, 130, 139, 142;CI Disperse Red 1, 4, 5, 7, 11, 12, 13, 15, 17, 27, 43, 44, 50, 52, 53, 54, 55, 56, 58, 59, 60, 65, 72, 73, 74, 75, 76, 78, 81, 82, 86, 88, 90, 91, 92, 93, 96, 103, 105, 106, 107, 108, 110, 111, 113, 117, 118, 121, 122, 126, 127, 128, 1 31, 132, 134, 135, 137, 143, 145, 146, 151, 152, 153, 154, 157, 159, 164, 167, 169, 177, 179, 181, 183, 184, 185, 188, 189, 190, 191, 192, 200, 201, 202, 203, 205, 206, 207, 210, 221, 224, 225, 227, 229, 239, 240, 257, 258, 277, 278, 279, 281, 288, 298, 302, 303, 310, 311, 312, 320, 324, 328; CI Disperse Violet 1, 4, 8, 23, 26, 27, 28, 31, 33, 35, 36, 38, 40, 43, 46, 48, 50, 51, 52, 56, 57, 59, 61, 63, 69, 77; CI Disperse Green 9; CI Disperse Brown 1, 2, 4, 9, 13, 19; CIDisperse Blue 3, 7, 9, 14, 16, 19, 20, 26, 27, 35, 43, 44, 54, 55, 56, 58, 60, 62, 64, 71, 72, 73, 75, 79, 81, 82, 83, 87, 91, 93, 94, 95, 96, 102, 106, 108, 112, 113, 115, 118, 120, 122, 125, 128, 130, 139, 141, 142, 143, 146, 148, 149, 153, 154, 158 , 165, 167, 171, 173, 174, 176, 181, 183, 185, 186, 187, 189, 197, 198, 200, 201, 205, 207, 211, 214, 224, 225, 257, 259, 267, 268, 270, 284, 285, 287, 288, 291, 293, 295, 297, 301, 315, 330, 333, 359, 360; CI Disperse Black 1, 3, 10, 24.

[0156] Examples of acid dyes with colors other than yellow include CI Acid Orange 3, 7, 8, 10, 19, 22, 24, 33, 51, 51S, 56, 67, 74, 80, 86, 87, 88, 89, 94, 95, 107, 108, 116, 122, 127, 140, 142, 144, 149, 152, 156, 162, 166, and 168; CI Acid Red 1, 6, 8, 9, 13, 18, 27, 35, 37, 52, 54, 57, 60, 73, 82, 88, 97, and 97:1, 106, 111, 114, 118, 119, 127, 131, 138, 143, and 145. 151, 183, 195, 198, 211, 215, 217, 225, 226, 249, 251, 254, 256, 257, 260, 261, 265, 266, 274, 276, 277, 289, 296, 299, 315, 318, 336, 337, 357, 359, 361, 362, 364, 366, 399, 407, 415; CI Acid Violet 17, 19, 21, 42, 43, 47, 48, 49, 54, 66, 78, 90, 97, 102, 109, 126; CI Acid Blue 1, 7, 9, 15, 23, 25, 40, 61:1, 62, 7 2, 74, 80, 83, 90, 92, 103, 104, 112, 113, 114, 120, 127, 127:1, 128, 129, 138, 140, 142, 156, 158, 171, 182, 185, 193, 199, 201, 203, 204, 205, 207, 209, 220, 221, 224, 225, 229, 230, 239, 258, 260, 264, 277:1, 278, 279, 280, 284, 290, 296, 298, 300, 317, 324, 333, 335, 338, 342, 350; CI Acid Green 9, 12, 16, 19, 20, 25, 27, 28, 40, 43, 56, 73, 81, 84, 104, 108, 109; CI Acid Brown 2, 4, 13, 14, 19, 28, 44, 123, 224, 226, 227, 248, 282, 283, 289, 294, 297, 298, 301, 355, 357, 413; CI Acid Black 1, 2, 3, 24, 24:1, 26, 31, 50, 52, 52:1, 58, 60, 63, 63S, 107, 109, 112, 119, 132, 140, 155, 172, 187, 188, 194, 207, 222, etc.

[0157] Examples of basic dyes of colors other than yellow include CI Basic Red 1, 5, 12, 19, 22, 29, 37, 39, and 92; CI Basic Blue 1, 3, 9, 11, 16, 17, 24, 28, 41, 45, 54, 65, and 66; and CI Basic Black 2 and 8.

[0158] Examples of direct dyes other than yellow include CI Direct Orange 6, 26, 27, 34, 39, 40, 46, 102, 105, 107, and 118; CI Direct Red 2, 4, 9, 23, 24, 31, 54, 62, 69, 79, 80, 81, 83, 84, 89, 95, 212, 224, 225, 226, 227, 239, 242, 243, and 254; CI Direct Violet 9, 35, 51, 66, 94, and 95; and CI Direct Blue 1, 15, 71, 76, 77, 78, 80, 86, 87, 90, 98, 106, 108, and 16. 0, 168, 189, 192, 193, 199, 200, 201, 202, 203, 218, 225, 229, 237, 244, 248, 251, 270, 273, 274, 290, 291; CI Direct Green 26, 28, 59, 80, 85; CI Direct Brown 44, 44:1, 106, 115, 195, 209, 210, 212:1, 222, 223; CI Direct Black 17, 19, 22, 32, 51, 62, 108, 112, 113, 117, 118, 132, 146, 154, 159, 169, etc.

[0159] Examples of reactive dyes with colors other than yellow include CI Reactive Orange 1, 4, 5, 7, 11, 12, 13, 15, 16, 20, 30, 35, 56, 64, 67, 69, 70, 72, 74, 82, 84, 86, 87, 91, 92, 93, 95, 99, and 107; CI Reactive Red 2, 3, 3:1, 5, 8, 11, 21, 22, 23, 24, 28, 29, 31, 33, 35, 43, 45, 49, and 55. 56, 58, 65, 66, 78, 83, 84, 106, 111, 112, 113, 114, 116, 120, 123, 124, 128, 130, 136, 141, 147, 158, 159, 171, 174, 180, 183, 184, 187, 190, 193, 194, 195, 198, 218, 220, 222, 223, 226, 228, 235, 245; CI Reactive Violet 1, 2, 4, 5, 6 , 22, 23, 33, 36, 38; CI Reactive Blue 2, 3, 4, 7, 13, 14, 15, 19, 21, 25, 27, 28, 29, 38, 39, 41, 49, 50, 52, 63, 69, 71, 72, 77, 79, 89, 104, 109, 112, 113, 114, 116, 119, 120, 122, 137, 140, 143, 147, 160, 161, 162, 163, 168, 171, 176, 182 , 184, 191, 194, 195, 198, 203, 204, 207, 209, 211, 214, 220, 221, 222, 231, 235, 236; CI Reactive Green 8, 12, 15, 19, 21; CI Reactive Brown 2, 7, 9, 10, 11, 17, 18, 19, 21, 23, 31, 37, 43, 46; CI Reactive Black 5, 8, 13, 14, 31, 34, 39, etc.

[0160] Furthermore, if the other ink is a clear ink, the other ink does not need to contain a coloring material.

[0161] As for the constituent components of the other inks other than the coloring material, the same components as those explained as the constituent components of the yellow ink can be used.

[0162] These components preferably satisfy the same conditions as those explained in the section on yellow ink.

[0163] Furthermore, it is preferable that the surface tension and viscosity of the other inks satisfy the same conditions as those explained in the section on yellow ink.

[0164] [2] Inkjet recording device Next, the ink jet recording apparatus of the present invention will be described.

[0165] The inkjet recording apparatus of the present invention includes an inkjet head that ejects yellow ink, which is a radiation-curable ink, onto a recording medium, and an LED irradiator that irradiates radiation having a peak wavelength of less than 350 nm.

[0166] This makes it possible to provide an inkjet recording apparatus that can effectively cure the yellow ink and efficiently produce highly reliable recorded material.

[0167] Hereinafter, preferred embodiments of the inkjet recording apparatus of the present invention will be described in detail with reference to the drawings.

[0168] Fig. 1 is a perspective view of an inkjet recording apparatus according to a preferred embodiment, Fig. 2 is a front view of the radiation irradiation apparatus shown in Fig. 1, and Fig. 3 is a view taken along the line AA in Fig. 2.

[0169] The inkjet recording device 20 executes the inkjet recording method of the present invention described above, and is equipped with a motor 30 that feeds the recording medium P in the sub-scanning direction SS, a platen 40, an inkjet head 52 that sprays yellow ink, which is a radiation-curable ink, in fine droplets from head nozzles onto the recording medium P, a carriage 50 that carries the inkjet head 52, a carriage motor 60 that moves the carriage 50 in the main scanning direction MS, and a pair of radiation irradiation devices 90A, 90B that irradiate the yellow ink deposited on the recording medium P with radiation.

[0170] The carriage 50 is pulled by a traction belt 62 driven by a carriage motor 60 and moves along a guide rail 64 .

[0171] The inkjet head 52 is mounted on a carriage 50 and moves in accordance with the movement of the carriage 50 in the main scanning direction MS.

[0172] The inkjet head 52 is also capable of ejecting the aforementioned yellow ink, which is a radiation curable ink. The inkjet head 52 is, for example, a serial type head for full-color printing that ejects four types of ink, and is provided with a large number of head nozzles for ejecting not only the yellow ink but also other radiation curable inks, such as cyan ink, magenta ink, and black ink, for each color.

[0173] The carriage 50 on which the inkjet head 52 is mounted also carries, in addition to the inkjet head 52, a black ink cartridge 54 serving as a black ink container containing black ink to be supplied to the inkjet head 52, and a color ink cartridge 56 serving as color ink containing each radiation-curable color ink, i.e., yellow ink, cyan ink, and magenta ink, separately supplied to the inkjet head 52.

[0174] A capping device 80 is provided at the home position of the carriage 50, i.e., the position on the right side in Figure 1, for sealing the nozzle surface 52a of the inkjet head 52 when the carriage 50 is stopped. When the print job is completed and the carriage 50 reaches above this capping device 80, the capping device 80 automatically rises by a mechanism (not shown) and seals the nozzle surface 52a of the inkjet head 52. This capping prevents the ink inside the nozzles from drying out or changing quality.

[0175] A wiping unit (not shown) for wiping the nozzle surface 52a of the inkjet head 52 when the carriage 50 is stopped may be provided at the home position of the carriage 50. The wiping unit is provided at a position that does not interfere with the capping device 80, and can remove droplets, mist, and hardened products thereof adhering to the nozzle surface 52a by scrubbing the nozzle surface 52a, on which the nozzles of the inkjet head 52 are formed, with a squeegee-like wiper blade.

[0176] As shown in FIGS. 1 to 3, the radiation irradiation devices 90A and 90B are attached to both ends of the carriage 50 along the direction of movement thereof.

[0177] 2, the radiation irradiation device 90A attached to the left side of the inkjet head 52 irradiates droplets ejected onto the recording medium P with radiation when the carriage 50 moves rightward, i.e., in the direction of arrow B in Fig. 2, during a rightward scan. On the other hand, the radiation irradiation device 90B attached to the right side of the inkjet head 52 irradiates droplets ejected onto the recording medium P with radiation when the carriage 50 moves leftward, i.e., in the direction of arrow C in Fig. 2, during a leftward scan.

[0178] Each of the radiation irradiation devices 90A and 90B is attached to the carriage 50 and includes a housing 94 that supports an LED irradiator 92, and a light source control circuit (not shown) that controls the LED irradiator 92 to turn on and off.

[0179] Each housing 94 is provided with an LED irradiator 92 capable of irradiating radiation having a peak wavelength of less than 350 nm. This allows the yellow ink applied to the recording medium P to be cured appropriately.

[0180] In addition to the LED illuminator 92 capable of emitting radiation having a peak wavelength of less than 350 nm, the housing 94 may further include an LED illuminator that irradiates radiation that does not have a peak wavelength of less than 350 nm.

[0181] This allows, for example, yellow ink and other inks to be irradiated with different radiation.

[0182] As shown in FIG. 2, with the radiation irradiation devices 90A and 90B, droplets deposited on the recording medium P by ejection from the inkjet head 52 are irradiated with radiation 92a by an LED irradiator 92 that irradiates the recording medium P near the inkjet head 52, thereby hardening at least the surface of the droplets and forming an image on the recording medium.

[0183] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these.

[0184] For example, the ink jet recording method of the present invention may include other steps than those described above, if necessary.

[0185] For example, the method may include a step of heating the recording medium before or after applying ink, or a step of cooling the recording medium. .

[0186] The ink jet recording method of the present invention may be any method as long as it includes the above-mentioned ejection step and curing step, and may be carried out without using the ink jet recording apparatus described above.

[0187] In the above-described embodiment, the inkjet recording method of the present invention has been mainly described as being performed using a single apparatus, but the inkjet recording method of the present invention may also be performed using a combination of a plurality of different apparatuses. For example, the apparatus performing the ejection step and the apparatus performing the curing step may be different. [Example]

[0188] Next, specific examples of the present invention will be described. [3] Preparation of yellow ink

[0189] (Preparation examples Y1 to Y7) First, the pigment, dispersant, and base monomer phenoxyethyl acrylate were weighed out and placed in a tank for pigment dispersion so as to obtain the compositions shown in Table 1. A ceramic bead mill with a diameter of 1 mm was then placed in the tank and stirred to obtain each dispersion in which the pigment was dispersed in the base monomer.

[0190] Next, a polymerizable compound, a polymerization initiator, a polymerization inhibitor, and a surfactant were placed in a stainless steel container, a mixing tank, so as to obtain the composition shown in Table 1, and the mixture was mixed and stirred to completely dissolve them. Then, the dispersion obtained above was added, and the mixture was further mixed and stirred at 25°C for 1 hour, and then filtered through a 5 μm membrane filter, thereby obtaining a yellow ink for each Preparation Example.

[0191] The compositions of the yellow inks prepared in the above Preparation Examples are summarized in Table 1. CI Pigment Yellow 155 is "PY155", CI Pigment Yellow 150 is "PY150", Solsperse 36000 (manufactured by Lubrizol) as a polymer dispersant is "S36000", phenoxyethyl acrylate (Viscoat #192, manufactured by Osaka Organic Chemical Industry Co., Ltd.) as a polymerizable compound is "PEA", 2-(2-vinyloxyethoxy)ethyl acrylate (manufactured by Nippon Shokubai Co., Ltd.) as a polymerizable compound is "VEEA", isobornyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd.) as a polymerizable compound is "IBXA", dipropylene glycol diacrylate (APG-100, manufactured by Shin-Nakamura Chemical Co., Ltd.) as a polymerizable compound is "DPGDA", dipentaerythritol hexaacrylate (manufactured by Shin-Nakamura Kogyo Co., Ltd.) as a polymerizable compound is "A-DPH", Omnirad 127 (trade name, IGM 2-hydroxy-1-(4-(4-(2-hydroxy-2-methyl-propionyl)-benzyl)-phenyl)-2-methyl-propane) manufactured by IGM Resins BV is designated as "O127", Omnirad 2959 (trade name, manufactured by IGM Resins BV, 1-(4-(2-hydroxyethoxyl)-phenyl)-2-hydroxy-methylpropanone) is designated as "O2959" as an α-hydroxyketone polymerization initiator, ESACURE KIP 150 (trade name, manufactured by Lamberti, oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone)) is designated as "EK150" as an α-hydroxyketone polymerization initiator, and ESACURE KIP 160 (trade name, manufactured by Lamberti, 2-hydroxy-1-[4-{4-(2-hydroxy-2-methylpropanoyl)phenoxy}phenyl]-2-methylpropan-1-one) was used as "EK160," and Omnirad 184 (trade name, manufactured by IGM Resins BV) was used as an α-hydroxyketone polymerization initiator.1-hydroxycyclohexyl phenyl ketone (manufactured by IGM Resins) as "O184", Speedcure TPO (trade name, manufactured by Lambson) as an acylphosphine polymerization initiator as "SCTPO", Omnirad 819 (trade name, manufactured by IGM Resins) as an acylphosphine polymerization initiator as "SCTPO", Bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (manufactured by BV) is indicated as "O819," BKY-UV3500 (trade name, polyether-modified polydimethylsiloxane, manufactured by BYK Japan) as a surfactant is indicated as "UV3500," LA-7RD (trade name, 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl, manufactured by ADEKA Corporation) as a polymerization inhibitor is indicated as "LA-7RD," and MEHQ (trade name, hydroquinone monomethyl ether, manufactured by Kanto Chemical Co., Inc.) as a polymerization inhibitor is indicated as "MEHQ." Furthermore, the surface tension of the yellow inks of each of the above preparation examples at 25°C, measured by the Wilhelmy method, was in the range of 20 mN / m to 25 mN / m. Furthermore, the viscosity of each of the yellow inks in the above preparation examples at 25°C, measured using a vibration viscometer in accordance with JIS Z8809, was in the range of 15 mPa·s to 20 mPa·s.

[0192] [Table 1]

[0193] [4] Production of recorded materials Example 1 First, a modified inkjet printer "PX-G5000" (manufactured by Seiko Epson Corporation) was prepared as an inkjet recording device as shown in Figures 1 to 3. This inkjet recording device was equipped with a radiation source in a radiation irradiation device with a peak wavelength of 280 nm and irradiation energy of 100 mW / cm. 2 It is equipped with a "Deep Ultraviolet LED Irradiator" (product name, manufactured by Eye Graphics Co., Ltd.).

[0194] The inkjet recording device was filled with the yellow ink prepared in Preparation Example Y1, and a solid pattern image was printed on a polyethylene terephthalate film (PET50A, manufactured by Lintec Corporation) as a recording medium at 25°C and 1 atmosphere, with the ink dots having a medium diameter and a film thickness of 10 μm. The accumulated light intensity per unit area of ​​the solid pattern image was 260 mJ / cm. 2 Radiation was irradiated from the LED irradiator so as to obtain a recorded matter.

[0195] It should be noted that a "solid pattern image" is an image in which dots are recorded for all pixels, which are the minimum recording unit area defined by the recording resolution.

[0196] Examples 2 to 7 A recorded material was produced in the same manner as in Example 1, except that the yellow ink shown in Table 2 was used, the radiation source provided in the inkjet recording apparatus was an LED irradiator shown in Table 2, and the radiation irradiation conditions for the solid pattern image were set as shown in Table 2.

[0197] (Comparative Examples 1 to 3) A recorded material was produced in the same manner as in Example 1, except that the yellow ink shown in Table 2 was used, the radiation source provided in the inkjet recording apparatus was an LED irradiator shown in Table 2, and the radiation irradiation conditions for the solid pattern image were set as shown in Table 2.

[0198] Comparative Example 4 A recorded material was produced in the same manner as in Example 1, except that the radiation source provided in the inkjet recording apparatus was a metal halide lamp (manufactured by Eye Graphics Co., Ltd.).

[0199] The conditions for producing the recorded matter in each of the examples and comparative examples are summarized in Table 2. In the table, the peak wavelength is 280 nm, and the irradiation energy is 100 mW / cm 2 "Deep UV LED Irradiator" (product name, manufactured by Eye Graphics Co., Ltd.) was "LED-1", with a peak wavelength of 310 nm and irradiation energy of 50 mW / cm 2"Evaluation surface irradiation 7 × 7" (product name, manufactured by Nikkiso Giken Co., Ltd.) is "LED-2", peak wavelength 365 nm, irradiation energy 500 mW / cm 2 "FireJet ONE" (product name, manufactured by Phoseon Technology) "LED-3", peak wavelength 395nm, irradiation energy 500mW / cm 2 "FireJet ONE" (product name, manufactured by Phoseon Technology) with "LED-4" and irradiation energy of 1000mW / cm 2 The metal halide lamp (manufactured by Eye Graphics) was referred to as "Metahara."

[0200] [Table 2]

[0201] [5] Evaluation [5-1] Curing property The surface of the recorded portion of the recorded matter obtained in each of the Examples and Comparative Examples was rubbed with a cotton swab and evaluated according to the following criteria. The cotton swab used was a Johnson cotton swab manufactured by Johnson & Johnson (registered trademark). The swabs were rubbed back and forth 10 times, with a rubbing force of 100 g load. A grade of B or higher was considered to be good.

[0202] A: There is no mark at all. B: Slight scratches. C: Scratch marks are clearly visible.

[0203] [5-2] Recorded material odor The recorded materials obtained in each of the examples and comparative examples were smelled and evaluated according to the following criteria, with B or better being considered as a good level.

[0204] A: There is no odor. B: There is a slight odor. C: There is an odor. D: Has a strong odor. These results are summarized in Table 3.

[0205] [Table 3]

[0206] As is clear from Table 3, excellent results were obtained in the present invention. In contrast, satisfactory results were not obtained in the comparative examples. [Explanation of symbols]

[0207] 20...inkjet recording device, 30...motor, 40...platen, 50...carriage, 52...inkjet head, 52a...nozzle surface, 54...black ink cartridge, 56...color ink cartridge, 60...carriage motor, 62...traction belt, 64...guide rail, 80...capping device, 90A...radiation irradiation device, 90B...radiation irradiation device, 92...LED irradiator, 92a...radiation, 94...casing, MS...main scanning direction, P...recording medium, SS...sub-scanning direction

Claims

1. Yellow ink, which is a radiation curable ink, is ejected onto a recording medium from an inkjet head. a discharge step of discharging the The yellow ink ejected onto the recording medium has a peak wavelength of less than 350 nm. and a curing step of irradiating radiation from an LED irradiator to cure the yellow ink. 、 the yellow ink contains an α-hydroxyketone initiator as a polymerization initiator, The molecular weight of the α-hydroxyketone initiator is 210 or more, The inkjet recording method includes the α-hydroxyketone initiator containing at least one of 2-hydroxy-1-(4-(4-(2-hydroxy-2-methyl-propionyl)-benzyl)-phenyl)-2-methyl-propane, oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone), and 2-hydroxy-1-[4-{4-(2-hydroxy-2-methylpropanoyl)phenoxy}phenyl]-2-methylpropan-1-one).

2. 2. The inkjet recording method according to claim 1, wherein the recording medium is a non-absorbent recording medium.

3. The yellow ink contains C.I. Pigment Yellow 155 as a colorant.

3. The ink jet recording method according to any one of items 1 and 2.

4. The surface tension of the yellow ink at 25°C is 20 mN / m or more and 60 mN / m or less, 4. The inkjet recording method according to claim 1, wherein the ejecting step ejects the yellow ink from a head nozzle.

5. An inkjet printer that ejects yellow radiation-curable ink onto a recording medium. Head and an LED illuminator that irradiates radiation having a peak wavelength of less than 350 nm; the yellow ink contains an α-hydroxyketone initiator as a polymerization initiator, The molecular weight of the α-hydroxyketone initiator is 210 or more, The α-hydroxyketone initiator includes at least one of 2-hydroxy-1-(4-(4-(2-hydroxy-2-methyl-propionyl)-benzyl)-phenyl)-2-methyl-propane, oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone), and 2-hydroxy-1-[4-{4-(2-hydroxy-2-methylpropanoyl)phenoxy}phenyl]-2-methylpropan-1-one).

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