Ultraviolet curable ink, cured product forming method, cured product, and cured product forming device

Abstract

The present invention relates to an ultraviolet curable ink capable of suppressing yellowing of a cured product formed, suppressing decrease in brightness, and suppressing decrease in curability of the cured product. The ultraviolet-curable ink contains a photopolymerizable compound, a light-storing pigment that emits light having a wavelength of 405 nm, and a photopolymerization initiator that has a light absorption coefficient of 100 mL / g.cm or more with respect to the light having a wavelength of 405 nm, and the content of the photopolymerization initiator is 0.1-1.5 mass% with respect to the total mass of the photopolymerizable compound.

Description

Technical Field

[0001] The present invention relates to an ultraviolet curing ink, a method for forming a cured product, a cured product and a device for forming a cured product. Background Art

[0002] As ink used in the field of printing, UV curable ink containing photopolymerizable compounds is known. When UV curable ink is irradiated with ultraviolet rays, it can be cured by polymerization of the photopolymerizable compound to form a printed matter. Printed matters formed by UV curable ink are also used for outdoor billboards, etc., because of their excellent firmness.

[0003] However, when printed materials formed by the above-mentioned inks are used for outdoor billboards, etc., the printed parts are difficult to see in the dark at night, and there is a need to improve the brightness of the printed parts.

[0004] As inks capable of improving the brightness of printed parts, for example, inks containing fluorescent whitening agents are known, as in Patent Document 1. Fluorescent whitening agents absorb light of a specific wavelength and emit light of a wavelength different from the absorbed wavelength. However, fluorescent whitening agents stop emitting light immediately when the light irradiation stops, and thus can only improve the brightness during the period when the light source irradiates the ink.

[0005] Therefore, there is a growing expectation that the visibility of the printed portion can be improved even during a period when the ink is not irradiated with light, and the development of ink that can achieve this expectation is being promoted.

[0006] As such ink, there is known an ultraviolet curing ink containing a light-storing pigment and a polymerizable compound (for example, patent documents 2 and 3). The light-storing pigment can absorb light of a specific wavelength and store light energy. Moreover, the light energy stored in the light-storing pigment can be released in the form of light with a wavelength different from the absorbed light for a long time after the absorption of light stops. Thus, for example, when a printed material formed using an ink containing a light-storing pigment is used outdoors, the light-storing pigment absorbs light such as sunlight and stores light energy, and can emit light even at night. Therefore, the printed part can be seen even in the dark.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Publication No. 2011-174076

[0010] Patent Document 2: Japanese Patent Application Publication No. 2014-028484

[0011] Patent Document 3: Japanese Patent Application Publication No. 2005-205258 Summary of the invention

[0012] UV curable inks containing luminescent pigments are known, such as Patent Documents 2 and 3. However, according to the present inventors, when a cured product is formed using the UV curable inks containing luminescent pigments described in Patent Documents 2 and 3, yellowing occurs, and the brightness of the cured product decreases.

[0013] The present inventors have conducted studies and found that yellowing is more likely to occur when the amount of photopolymerization initiator added to the ink increases.

[0014] However, if the amount of the photopolymerization initiator added is reduced in order to suppress yellowing, there is a problem that the polymerization of the polymerizable compound contained in the ink does not proceed sufficiently, and the curability of the cured product is reduced.

[0015] The present invention is made in view of the above situation, and its purpose is to provide an ultraviolet curing ink containing a light-storing pigment and capable of suppressing yellowing and brightness reduction of the formed cured product, and capable of suppressing reduction in curability of the cured product, a method for forming a cured product using the ink, a cured product, and a cured product forming device.

[0016] One embodiment of the present invention for solving the above-mentioned problems relates to the ultraviolet curable inks of the following [1] to

[12] .

[0017] [1] An ultraviolet curable ink comprising a photopolymerizable compound, a light-storing pigment emitting light at a wavelength of 405 nm, and a photopolymerization initiator having an absorption coefficient of light at a wavelength of 405 nm of 100 mL / g·cm or more, wherein the content of the photopolymerization initiator is 0.1% by mass to 1.5% by mass relative to the total mass of the photopolymerizable compound.

[0018] [2] The ultraviolet curable ink according to [1], which is an inkjet ink.

[0019] [3] The ultraviolet curable ink according to [1] or [2], further comprising a polymerization inhibitor.

[0020] [4] The ultraviolet curable ink according to any one of [1] to [3], wherein the particle size (d 50 ) is 1μm~10μm.

[0021] [5] The ultraviolet curable ink according to any one of [1] to [4], wherein the content of the light-storing pigment is less than 50% by mass relative to the total mass of the ultraviolet curable ink.

[0022] [6] The ultraviolet curable ink according to any one of [1] to [5], wherein the light-storing pigment is a light-storing pigment obtained by activating strontium aluminate with an activator.

[0023] [7] The ultraviolet curable ink according to any one of [1] to [6], wherein the photopolymerization initiator is an intramolecular cleavage type photopolymerization initiator.

[0024] [8] The ultraviolet curable ink according to any one of [1] to [7], wherein the extinction coefficient of the photopolymerization initiator is 500 mL / g·cm or more.

[0025] In addition, one embodiment of the present invention for solving the above-mentioned problems relates to the method for forming a cured product according to the following [9] to

[11] .

[0026] [9] A method for forming a cured product, comprising: applying the ultraviolet curable ink described in any one of [1] to [8] to a substrate; and irradiating the applied ultraviolet curable ink with ultraviolet rays.

[0027]

[10] The method for forming a cured product according to [9], further comprising: a step of further imparting the above-mentioned ultraviolet curing ink to the ink irradiated with the above-mentioned ultraviolet rays; and a step of irradiating the above-mentioned ultraviolet curing ink imparted to the above-mentioned ink with ultraviolet rays.

[0028]

[11] A method for forming a solidified product according to [9] or

[10] , wherein the step of imparting the ultraviolet curing ink to a substrate comprises ejecting the ultraviolet curing ink from a nozzle of an inkjet head and imparting the ultraviolet curing ink to the substrate, and the method further comprises: a step of discharging the ultraviolet curing ink that has not been ejected from the nozzle of the inkjet head from the inkjet head, and a step of resupplying the discharged ultraviolet curing ink to the inkjet head.

[0029] In addition, one embodiment of the present invention for solving the above-mentioned problems relates to the cured products of the following

[12] and

[13] .

[0030]

[12] A cured product obtained by curing the ultraviolet curable ink according to any one of [1] to [8].

[0031]

[13] The cured product according to

[12] , wherein the cured product has a thickness of 0.3 mm or more.

[0032] In addition, one embodiment of the present invention for solving the above-mentioned problems relates to a solidified material forming apparatus according to the following

[14] and

[15] .

[0033]

[14] A cured product forming device comprising: an ink applying unit for applying the ultraviolet curing ink described in any one of [1] to [8] to a substrate; and an LED light source for irradiating ultraviolet light to the ultraviolet curing ink.

[0034]

[15] The solidified material forming device according to

[14] , wherein the device has an inkjet head for ejecting the ultraviolet curing ink, discharges the ultraviolet curing ink that has not been ejected from the inkjet head from the inkjet head, and supplies the ejected ultraviolet curing ink to the inkjet head again.

[0035] According to the present invention, there can be provided an ultraviolet curable ink containing a light-storing pigment and capable of further suppressing yellowing of the formed cured product and reduction in brightness, and capable of suppressing reduction in curability of the cured product, a method for forming a cured product using the ink, a cured product, and a cured product forming apparatus. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic diagram showing the structure of a solidified material forming apparatus according to one embodiment of the present invention. DETAILED DESCRIPTION

[0037] Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

[0038] 1. UV curable ink

[0039] The ultraviolet curable ink (hereinafter also simply referred to as "ink") of the present embodiment contains an ultraviolet polymerizable compound, a light-storing pigment, and a photopolymerization initiator. The ink is preferably an inkjet ink.

[0040] As described above, when the active ray curing ink containing the luminescent pigment described in Patent Documents 2 and 3 is used, the cured product turns yellow. If the cured product turns yellow, not only the appearance is poor, but also the brightness of the cured product decreases. It is believed that this is because the luminescence of the luminescent pigment is partially masked by the yellowing.

[0041] The inventors of the present invention have conducted research and found that when the amount of photopolymerization initiator added to the ink containing the light-storing pigment is large, yellowing of the cured product is likely to occur. This is believed to be because the residue of the photopolymerization initiator present in the cured product has a molecular structure with a covalent bond such as benzene as a parent nucleus capable of absorbing ultraviolet rays. In other words, it is believed that since the parent nucleus absorbs ultraviolet rays, a lot of yellow corresponding to the wavelength that is not absorbed is seen, resulting in yellowing. On the other hand, if the amount of photopolymerization initiator added is reduced in order to suppress the yellowing of the cured product caused by the residue of the photopolymerization initiator, the polymerization of the polymerizable compound in the ink cannot be fully carried out, and the curability of the cured product is reduced.

[0042] However, the solidified material formed by the ink containing non-photoluminescent pigment (such as ink for image formation) is not easy to see the yellowing caused in the solidified material due to the coloring of the pigment. On the other hand, in the ink containing the photoluminescent pigment, in order to improve the transparency of the solidified material and further improve the brightness, the non-photoluminescent pigment is not added, or even if it is added, the amount is small. Therefore, the yellowing of the solidified material is easy to be conspicuous. In addition, due to the improved transparency, if the solidified material yellows, it is easy to see not only the yellowing near the surface of the solidified material, but also the yellowing inside the solidified material closer to the substrate, which is easy to cause poor appearance. Moreover, if yellowing occurs, the luminescence of the photoluminescent pigment is partially covered by the yellowing, and the brightness is reduced. Therefore, the problem caused by the yellowing of the solidified material occurs significantly in the ink containing the photoluminescent pigment.

[0043] The inventors of the present invention have repeatedly studied these problems and found that by using an ultraviolet curable ink containing a light-storing pigment that emits light at a wavelength of 405 nm and a photopolymerization initiator with an absorption coefficient of 100 mL / g·cm or more at a wavelength of 405 nm, it is possible to reduce the amount of photopolymerization initiator added, suppress yellowing and reduction in brightness, and suppress reduction in curability.

[0044] If such an ultraviolet curing ink is irradiated with ultraviolet rays, the photopolymerization initiator in the ink absorbs part of the ultraviolet rays and initiates polymerization of the polymerizable compound, thereby curing the ink. On the other hand, part of the ultraviolet rays irradiated to the ink is absorbed by the light-storing pigment. The light-storing pigment that absorbs the ultraviolet rays releases light of a wavelength of 405 nm in the cured product for a long time. At this time, the photopolymerization initiator with an absorption coefficient of 100 mL / g·cm or more at a wavelength of 405 nm remaining in the cured product easily absorbs the light released from the light-storing pigment. Therefore, it is believed that the polymerization reaction of the unreacted polymerizable compound can be further initiated.

[0045] In this way, in addition to the ink curing caused by ultraviolet irradiation, the ink curing caused by the light emission of the light-storing pigment also occurs, thereby improving the curability of the cured product. In addition, the light emission of the light-storing pigment occurs inside the cured product, so the ink can be cured more effectively. It is believed that this is because the polymerization reaction is not easily affected by oxygen hindrance. Based on these reasons, it is believed that even if the amount of photopolymerization initiator added is reduced, the curability can be suppressed from decreasing.

[0046] Furthermore, the present inventors have found that yellowing of the cured product and reduction in brightness can be suppressed by setting the amount of the photopolymerization initiator added to 0.1% by mass to 1.5% by mass.

[0047] 1-1. Photopolymerizable compounds

[0048] The photopolymerizable compound contained in the ink of the present embodiment is a compound that is polymerized and cross-linked by irradiation with ultraviolet rays.

[0049] Examples of photopolymerizable compounds include free radical polymerizable compounds and cationic polymerizable compounds. Among these, the photopolymerizable compound is preferably a free radical polymerizable compound. The photopolymerizable compound can be any one of a monomer, a polymerizable oligomer, and a mixture thereof. It should be noted that the photopolymerizable compound can be included in the ink only one type, or two or more types can be included in combination.

[0050] The photopolymerizable compound may be monofunctional or polyfunctional. From the viewpoint of further improving the curability of the cured product, the photopolymerizable compound preferably includes a polyfunctional compound.

[0051] The radical polymerizable compound is a monofunctional or polyfunctional compound having an ethylenically unsaturated bond that can be free radically polymerized in the molecule. Examples of compounds having an ethylenically unsaturated bond that can be free radically polymerized include unsaturated carboxylic acids and salts thereof, unsaturated carboxylic acid ester compounds, unsaturated carboxylic acid carbamate compounds, unsaturated carboxylic acid amide compounds and anhydrides thereof, acrylonitrile, styrene, unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, unsaturated carbamates, and the like. Examples of unsaturated carboxylic acids include (meth) acrylic acid, itaconic acid, crotonic acid, isocrotonic acid, maleic acid, and the like.

[0052] The radical polymerizable compound is preferably an unsaturated carboxylic acid ester and a (meth)acrylate, and more preferably a (meth)acrylate.

[0053] Examples of monofunctional (meth)acrylates include isoamyl (meth)acrylate, stearyl (meth)acrylate, isobornyl (meth)acrylate, lauryl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, isomyristyl (meth)acrylate, isostearyl (meth)acrylate, 2-phenoxyethyl (meth)acrylate, 2-ethylhexyl-diglycol (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 2-(meth)acryloyloxyethyl hexahydrophthalate, butoxyethyl (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, methoxy Diethylene 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 (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 2-(meth)acryloyloxyethyl succinate, 2-(meth)acryloyloxyethyl phthalate, 2-(meth)acryloyloxyethyl-2-hydroxyethyl-phthalate and tert-butylcyclohexyl (meth)acrylate, etc.

[0054] Examples of the multifunctional (meth)acrylate include triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene 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-nonanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, tricyclodecane dimethanol diacrylate, dihydroxymethyl-tricyclodecane di(meth)acrylate, bisphenol A di(meth)acrylate, hydroxy neopentyl glycol di(meth)acrylate, The invention also includes bifunctional (meth)acrylates such as acrylate, polytetramethylene glycol di(meth)acrylate, polyethylene glycol diacrylate, dipropylene glycol diacrylate and tripropylene glycol diacrylate, trifunctional or higher (meth)acrylates such as trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, glycerol propoxy tri(meth)acrylate and pentaerythritol ethoxy tetra(meth)acrylate, oligomers having a (meth)acryloyl group including polyester acrylate oligomers, and the like.

[0055] Examples of the cationically polymerizable compound include epoxy compounds, vinyl ether compounds, and oxetane compounds.

[0056] Examples of the epoxy compounds include: 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexanecarboxylate, bis(3,4-epoxycyclohexylmethyl)adipate, vinylcyclohexene monoepoxide, ε-caprolactone-modified 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexanecarboxylate, 1-methyl-4-(2-methyloxiranyl)-7-oxabicyclo[4,1,0]heptane, 2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexanone-m-dicyclohexane Aliphatic epoxy compounds such as alicyclic epoxy resins such as diglycidyl ether of 1,4-butanediol and bis(2,3-epoxycyclopentyl) ether, diglycidyl ether of 1,4-butanediol, diglycidyl ether of 1,6-hexanediol, triglycidyl ether of glycerol, triglycidyl ether of trimethylolpropane, diglycidyl ether of polyethylene glycol, diglycidyl ether of propylene glycol, polyglycidyl ether of polyether polyols obtained by adding one or two or more alkylene oxides (such as ethylene oxide and propylene oxide) to aliphatic polyols such as ethylene glycol, propylene glycol and glycerol, and aromatic epoxy compounds such as di- or polyglycidyl ethers of bisphenol A or its alkylene oxide adducts, di- or polyglycidyl ethers of hydrogenated bisphenol A or its alkylene oxide adducts and novolac type epoxy resins.

[0057] Examples of the above-mentioned vinyl ether compounds include monovinyl ether compounds including ethyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, octadecyl vinyl ether, cyclohexyl vinyl ether, hydroxybutyl vinyl ether, 2-ethylhexyl vinyl ether, cyclohexanedimethanol monovinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, isopropenyl ether-o-propylene carbonate, dodecyl vinyl ether, diethylene glycol monovinyl ether and octadecyl vinyl ether, and di- or trivinyl ether compounds including ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, propylene glycol divinyl ether, dipropylene glycol divinyl ether, butanediol divinyl ether, hexanediol divinyl ether, cyclohexanedimethanol divinyl ether and trimethylolpropane trivinyl ether.

[0058] Examples of the oxetane compound include 3-hydroxymethyl-3-methyloxetane, 3-hydroxymethyl-3-ethyloxetane, 3-hydroxymethyl-3-propyloxetane, 3-hydroxymethyl-3-n-butyloxetane, 3-hydroxymethyl-3-phenyloxetane, 3-hydroxymethyl-3-benzyloxetane, 3-hydroxyethyl-3-methyloxetane, 3-hydroxyethyl-3-ethyloxetane, 3-hydroxyethyl ... Ethyl-3-phenyloxetane, 3-hydroxypropyl-3-methyloxetane, 3-hydroxypropyl-3-ethyloxetane, 3-hydroxypropyl-3-propyloxetane, 3-hydroxypropyl-3-phenyloxetane, 3-hydroxybutyl-3-methyloxetane, 1,4-bis{[(3-ethyl-3-oxetane)methoxy]methyl}benzene, 3-ethyl-3-(2-ethylhexyloxymethyl)oxetane and di[1-ethyl(3-oxetane)]methyl ether, etc.

[0059] The molecular weight of the photopolymerizable compound is preferably 1000 or less, more preferably 600 or less. When the molecular weight is 1000 or less, the viscosity of the ink can be further reduced, and when the ink is used as an inkjet ink, the ejection stability can be further improved. The lower limit of the molecular weight is not particularly limited, but is preferably 200 or more.

[0060] Among the photopolymerizable compounds having a molecular weight of 1000 or less, preferred are photopolymerizable compounds having an alkylene oxide structure or an aromatic ring, such as dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, 2-phenoxyethyl (meth)acrylate, and bisphenol A di(meth)acrylate. According to the knowledge of the present inventors, these compounds can easily improve the transparency of the cured product formed from the ink.

[0061] The content of the photopolymerizable compound may be, for example, 1% by mass to 95% by mass, preferably 30% by mass to 95% by mass, and more preferably 50% by mass to 95% by mass, relative to the total mass of the ink.

[0062] 1-2. Luminescent pigments

[0063] In this embodiment, the ink contains a light-storing pigment.

[0064] Phosphorescent pigments can absorb light of a predetermined wavelength, store light energy, and release the stored light energy in the form of light of a wavelength different from the absorbed light for a long period of time after the absorption of light stops. In other words, phosphorescent pigments can emit phosphorescence.

[0065] In this embodiment, the luminescent pigment emits light at a wavelength of 405 nm. In this specification, "emitting light at a wavelength of 405 nm" means that the luminescent pigment has a luminescence spectrum in which the luminescence intensity at a wavelength of 405 nm is 0.1% or more of the luminescence intensity at the maximum luminescence wavelength.

[0066] From the viewpoint of further absorbing the light from the light-storing pigment by the photopolymerization initiator described later and further promoting the polymerization reaction, the light emission intensity at a wavelength of 405 nm is preferably 0.15% or more of the light emission intensity at the maximum light emission wavelength, and more preferably 0.2% or more. When the light emission intensity is within the above range, the curability of the ink can be further improved. The upper limit of the light emission intensity is not particularly limited, and is, for example, 80% or less of the light emission intensity at the maximum light emission wavelength.

[0067] The emission intensity of the luminescent pigment can be measured, for example, using a spectrofluorophotometer (F-7000, manufactured by Hitachi High-Tech Sciences, Ltd.) In the above measurement, the wavelength of the excitation light of the luminescent pigment is appropriately selected depending on the type of the luminescent pigment.

[0068] The type of luminescent pigment is not particularly limited as long as it has the above-mentioned characteristics. Examples of luminescent pigments include luminescent pigments obtained by activating a mother crystal, which is a metal compound, with an activator.

[0069] Examples of the above-mentioned mother crystal include sulfides such as zinc sulfide, calcium sulfide, germanium sulfide, strontium sulfide, and yttrium sulfide, metal oxides such as calcium oxide, strontium oxide, barium oxide, aluminum oxide, and cerium oxide, aluminates such as calcium aluminate, strontium aluminate, and barium aluminate, strontium aluminum silicate, etc. Among these, strontium aluminate and strontium aluminum silicate are preferred.

[0070] Examples of the activator for activating the mother crystal include europium, terbium, yttrium, zirconium, dysprosium, neodymium, manganese, magnesium, cobalt, titanium, copper, and barium, etc. Among these, europium, dysprosium, and neodymium are preferred.

[0071] The ink may contain only one type of the luminescent pigment or a combination of two or more types.

[0072] The maximum absorption wavelength in the absorption spectrum of the luminescent pigment is preferably 280 nm to 380 nm, more preferably 300 nm to 360 nm. If the peak wavelength of the absorption spectrum of the luminescent pigment is within the above range, the emission intensity of the luminescent pigment at a wavelength of 405 nm is easily within the above preferred range.

[0073] The maximum absorption wavelength of the luminescent pigment can be measured, for example, using a spectrofluorophotometer (F-7000, manufactured by Hitachi High-Tech Sciences, Ltd.).

[0074] The peak wavelength (maximum emission wavelength) in the emission spectrum of the luminescent pigment is preferably 400 nm to 700 nm, more preferably 400 nm to 550 nm. When the peak wavelength of the emission spectrum of the luminescent pigment is within the above range, the emission intensity of the luminescent pigment at a wavelength of 405 nm is likely to be within the above preferred range.

[0075] The peak wavelength of the light emission spectrum of the luminescent pigment can be measured, for example, using a spectrofluorophotometer (F-7000, manufactured by Hitachi High-Tech Sciences, Ltd.).

[0076] The particle size (d 50 ) is preferably 1 μm to 10 μm, more preferably 1 μm to 5 μm. If the particle size is 1 μm or more, it is not easily affected by Mie scattering, and the scattering of the light of the light-storage pigment in the cured product can be further suppressed, thereby further improving the brightness of the cured product. In addition, by further suppressing the scattering of the light of the light-storage pigment, the photopolymerization initiator can more fully absorb the light and further improve the curability of the cured product. If the particle size is 10 μm or less, when the ink is used as an inkjet ink, the reduction in ejection stability can be further suppressed.

[0077] The particle size of the luminescent pigment can be measured using a particle size distribution analyzer using the Stokes sedimentation method (for example, LUMiSizer, manufactured by LUM Japan Co., Ltd.).

[0078] The content of the light-storage pigment is preferably less than 50% by mass, preferably 20% by mass or more and less than 50% by mass, relative to the total mass of the ink. If the above content is less than 50% by mass, the proportion of the photopolymerizable compound in the ink can be increased to further improve the curability and adhesion of the cured product to the substrate. In addition, by being less than 50% by mass, when the ink is used as an inkjet ink, the reduction in ejection stability can be suppressed. If the above content is 20% by mass or more, the brightness of the cured product can be further improved.

[0079] 1-3. Photopolymerization initiator

[0080] In the present embodiment, the ink contains a photopolymerization initiator having an extinction coefficient of light of a wavelength of 405 nm of 100 mL / g·cm or more.

[0081] As described above, by including the above-mentioned photopolymerization initiator in the ink, the curing reaction can be further promoted inside the ink by the light emission of the light-storing pigment, so the amount of the photopolymerization initiator added can be reduced. As a result, the yellowing of the cured product from the photopolymerization initiator can be suppressed, and the brightness of the cured product can be further improved.

[0082] From the viewpoint of further improving the curability of the cured product, the extinction coefficient of the photopolymerization initiator is preferably 100 mL / g·cm to 10,000 mL / g·cm, and more preferably 500 mL / g·cm to 1,000 mL / g·cm.

[0083] The extinction coefficient of the photopolymerization initiator can be measured, for example, using a spectrophotometer (V650, manufactured by JASCO Corporation) using a solution obtained by diluting the photopolymerization initiator with methanol so as to have a concentration in the range of 0.1% by mass to 0.5% by mass.

[0084] The photopolymerization initiator may be a radical initiator when the ink contains a radical polymerizable compound, and may be a cationic initiator (photoacid generator) when the ink contains a cation polymerizable compound.

[0085] Examples of the radical polymerization initiator include intramolecular cleavage-type photopolymerization initiators, hydrogen abstraction-type polymerization initiators, etc. The radical polymerization initiator may be contained in the ink alone or in combination of two or more.

[0086] Examples of intramolecular cleavage type photopolymerization initiators include Irgacure 819 (absorption coefficient: 9.0×10 2 mL / g·cm), Irgacure 784 (absorption coefficient: 1.2×10 3 mL / g·cm), Irgacure 379 (absorption coefficient: 2.8×10 2 mL / g·cm), Irgacure 369 (absorption coefficient: 2.8×10 2 mL / g·cm) (both manufactured by BASF), Omnirad TPO (absorption coefficient: 1.6×10 2 mL / g·cm, manufactured by IGM Resins Co., Ltd.) etc. (“absorption coefficient” means the absorption coefficient of light with a wavelength of 405 nm).

[0087] Examples of hydrogen abstraction type photopolymerization initiators include Omnirad ITX (absorption coefficient: 1.7×10 3 mL / g·cm, manufactured by IGM Resins Co., Ltd.) etc.

[0088] In the present embodiment, the photopolymerization initiator is preferably an intramolecular cleavage type photopolymerization initiator. Compared with the hydrogen abstraction type photopolymerization initiator, the number of parent nuclei capable of absorbing ultraviolet rays contained in one molecule of the residue of the intramolecular cleavage type photopolymerization initiator is smaller. Therefore, it is believed that the yellowing of the cured product is less likely to occur. In addition, the intramolecular cleavage type photopolymerization initiator can further improve the curability of the cured product.

[0089] Examples of cationic photopolymerization initiators include photoacid generators. Examples of photoacid generators include: Ammonium, iodine Matte and Aromatic Compound B(C6F5)4 － PF6 － 、AsF6 － 、SbF6 － CF3SO3 － Sulfonates of sulfonic acids such as salts, halides of photogenerated hydrogen halides, and iron arene complexes.

[0090] The maximum absorption wavelength in the absorption spectrum of the photopolymerization initiator is preferably 250 nm to 420 nm, more preferably 280 nm to 400 nm. When the peak wavelength of the photopolymerization initiator is within the above range, the light emission of the luminescent pigment can be more easily absorbed, and the curability of the ink can be further improved.

[0091] The maximum absorption wavelength of the absorption spectrum of the photopolymerization initiator can be measured, for example, using a spectrophotometer (V650, manufactured by JASCO Corporation) using a solution obtained by diluting the photopolymerization initiator with methanol or acetonitrile to a concentration of 0.1% by mass to 0.5% by mass.

[0092] In this embodiment, the content of the photopolymerization initiator is 0.5% to 1.5% by mass, more preferably 0.7% to 1.3% by mass, relative to the total mass of the photopolymerizable compound. When the content is 0.5% by mass or more, the curability of the cured product can be improved, and when the content is 1.5% by mass or less, yellowing of the cured product can be suppressed and the brightness can be improved.

[0093] In the present embodiment, the ink may contain the photopolymerization initiator having an absorption coefficient of less than 100 mL / g·cm within a range that exhibits the effects of the present invention.

[0094] Examples of the photopolymerization initiator having an extinction coefficient of less than 100 mL / g·cm include Irgacure 127 (extinction coefficient: 7.0 mL / g·cm, manufactured by BASF) and Omnirad MBF (extinction coefficient: less than 0.1 mL / g·cm, manufactured by IGM Resins).

[0095] The content of the photopolymerization initiator having an extinction coefficient of less than 100 mL / g·cm may be, for example, 0.3 to 1.3% by mass relative to the total mass of the photopolymerizable compound. From the viewpoint of brightness improvement and yellowing, the content is preferably less than 1.3% by mass, and more preferably less than 1% by mass.

[0096] 1-4. Inhibitor

[0097] In this embodiment, the ink may contain a polymerization inhibitor.

[0098] The inclusion of a polymerization inhibitor in the ink can further suppress a decrease in ejection stability when the ink is used as an inkjet ink because the polymerization inhibitor can suppress ultraviolet rays that cure the ink from reaching a small amount of ink remaining in the inkjet head nozzle and curing the ink around the nozzle.

[0099] However, according to the research of the present inventors, when a polymerization inhibitor is included in the ink together with a photopolymerization initiator, yellowing may occur further depending on the amount of the photopolymerization initiator added. The reason for this phenomenon is not clear, but it is believed that the yellowing occurs further due to the reaction product of the photopolymerization initiator residue and the polymerization inhibitor present in the cured product.

[0100] In contrast, in the present invention, by using the above-mentioned photopolymerization initiator, the curability of the cured product can be improved and the amount thereof added can be reduced, thereby suppressing the generation of the above-mentioned reaction products generated by the combined use of the polymerization inhibitor and the photopolymerization initiator, and suppressing the generation of yellowing.

[0101] Examples of the polymerization inhibitor include N-oxyl radical polymerization inhibitors, phenol polymerization inhibitors, quinone polymerization inhibitors, amine polymerization inhibitors, copper dithiocarbamate polymerization inhibitors, etc. The polymerization inhibitor may be contained in the ink alone or in combination of two or more.

[0102] Examples of N-oxyl radical-based polymerization inhibitors include 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO), 4-hydroxy-2,2,6,6-tetramethyl-piperidinyl-N-oxyl, 4-oxo-2,2,6,6-tetramethyl-piperidinyl-N-oxyl, 4-methoxy-2,2,6,6-tetramethyl-piperidinyl-N-oxyl, 4-acetoxy-2,2,6,6-tetramethyl-piperidinyl-N-oxyl, etc. Examples of commercial products of N-oxyl radical-based polymerization inhibitors include Irgastab UV10 (manufactured by BASF ("Irgastab" is a registered trademark of the company)).

[0103] Examples of the phenolic polymerization inhibitor include 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2-tert-butyl-4,6-dimethylphenol, 2,6-di-tert-butyl-4-methylphenol, 2,4,6-tri-tert-butylphenol, 2,6-di-tert-butyl-p-cresol (butylated hydroxytoluene: BHT), 4-methoxyphenol, and 2-methoxy-4-methylphenol.

[0104] Examples of the quinone-based polymerization inhibitor include hydroquinone, methoxyhydroquinone, benzoquinone, 1,4-naphthoquinone, and p-tert-butylcatechol.

[0105] Examples of the amine-based polymerization inhibitor include alkylated diphenylamine, N,N′-diphenyl-p-phenylenediamine, and phenothiazine.

[0106] Examples of the copper dithiocarbamate-based polymerization inhibitor include copper dimethyl dithiocarbamate, copper diethyl dithiocarbamate, and copper dibutyl dithiocarbamate.

[0107] The content of the polymerization inhibitor is preferably 0.005% by mass to 1% by mass, more preferably 0.05% by mass to 0.1% by mass.

[0108] 1-5. Others

[0109] The ink of the present embodiment may further contain other components such as a non-photoluminescent pigment, a pigment dispersant, a surfactant, and a polymerization inhibitor within a range in which the effects of the present invention are exhibited.

[0110] Examples of non-photoluminescent pigments include red pigments, yellow pigments, blue pigments, white pigments, etc. that can be used in image forming inks. These pigments can be known pigments.

[0111] Examples of the pigment dispersant include polymer dispersants, wetting dispersants, and the like.

[0112] The content of the pigment dispersant is preferably 0.1% by mass to 30% by mass based on the total mass of the luminescent pigment.

[0113] Examples of surfactants include anionic surfactants such as dialkyl sulfosuccinates, alkylnaphthalene sulfonates and fatty acid salts, nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl allyl ethers, acetylene glycols and polyoxyethylene-polyoxypropylene block copolymers, cationic surfactants such as alkylamine salts and quaternary ammonium salts, and silicone-based and fluorine-based surfactants.

[0114] The content of the surfactant relative to the total mass of the ink is not particularly limited within a range that exhibits the effects of the present invention, and may be, for example, 0.001 mass % or more and less than 1.0 mass %.

[0115] 1-6.Physical properties

[0116] The viscosity of the ink of this embodiment is preferably 1 mPa·s or more at 10° C., and preferably 15 mPa·s or less at 90° C. When the viscosity is within the above range, when the ink is used as an inkjet ink, the temperature of the head can be adjusted to control the viscosity within a range suitable for inkjet ejection, thereby improving ejection stability.

[0117] The viscosity of the ink at 10°C and 90°C can be determined by measuring the temperature change of the dynamic viscoelasticity of the ink using a rheometer. Specifically, the ink is heated to 100°C, cooled to 25°C under the conditions of a shear rate of 11.7 (1 / s) and a cooling rate of 0.1°C / s, and the temperature change curve of the viscosity at this time is obtained. Then, the viscosity at 10°C and 90°C is read from the temperature change curve of the viscosity, and the viscosity can be determined.

[0118] The rheometer may be a stress-controlled rheometer PhysicaMCR series manufactured by Anton Paar. The cone plate may have a diameter of 75 mm and a cone angle of 1.0°.

[0119] 1-7. Method for producing ultraviolet curing ink

[0120] The ink of this embodiment can be prepared by mixing the above-mentioned components. When the ink contains a pigment dispersant, a pigment dispersion containing a light-storing pigment and a pigment dispersant can be prepared in advance, and the remaining components can be added thereto and mixed. At this time, in order to improve the solubility of the pigment dispersant, etc., it is preferred to prepare the pigment dispersion by mixing the pigment and the dispersant while heating.

[0121] 2. Method for forming a solidified product

[0122] The method for forming a cured product according to the present embodiment includes a step of applying an ultraviolet curing ink to a substrate, and a step of irradiating the applied ultraviolet curing ink with ultraviolet rays.

[0123] 2-1. Process of applying ink

[0124] In this step, the above-mentioned ultraviolet curable ink is applied to the substrate.

[0125] Examples of methods for imparting ink to a substrate include methods of imparting ink using a roll coater, a spin coater, etc., methods of imparting ink using a screen printing method, methods of imparting ink using an inkjet method, etc. Among these, the inkjet method is preferred from the viewpoint of forming a cured product at a desired position on the substrate more finely.

[0126] When ink is applied to a substrate by an inkjet method, ink is ejected from a nozzle of an inkjet head to apply ink to the substrate. The ejection method from the inkjet head can be any of an on-demand method and a continuous method. The on-demand method inkjet head can be any of an electro-mechanical conversion method such as a single-chamber type, a dual-chamber type, a curved type, a piston type, a shared pattern type, and a shared wall type, and an electro-thermal conversion method such as a thermal inkjet type and a bubble jet ("Bubble Jet" is a registered trademark of Canon Inc.) type.

[0127] The type of substrate is not particularly limited, and examples thereof include plastic plates made of acrylic, ABS resin, polycarbonate, PMMA, etc., films made of nylon, polypropylene, polystyrene, etc., metal plates made of copper, stainless steel, iron, aluminum, etc., rags made of nylon, polyester, etc., glass plates, ceramic tiles made of alumina titanium dioxide, etc., etc., and the like.

[0128] 2-2. Ultraviolet irradiation process

[0129] In this step, the ink applied to the substrate is irradiated with ultraviolet rays to cure the ink. The wavelength of the ultraviolet rays to be irradiated can be appropriately set according to the maximum absorption wavelength of the photopolymerization initiator.

[0130] Ultraviolet rays are preferably irradiated from an LED light source. LEDs radiate less heat than conventional light sources (such as metal halide lamps), so by using LEDs, ink is less likely to melt when irradiated with ultraviolet rays, and uneven gloss is less likely to occur.

[0131] In this process, the wavelength of the ultraviolet rays irradiated is preferably 300nm or more. If the wavelength of the ultraviolet rays is 300nm or more, the curing of the ink is promoted, and the damage to the substrate is also less. In particular, from the viewpoint of reducing damage to the substrate and improving the operability of the light source, the wavelength of the ultraviolet rays irradiated is preferably 380nm or more. It should be noted that if ultraviolet rays of this wavelength are irradiated, the light-storing pigment can fully emit light and improve the curing property of the ink.

[0132] 2-3. Step of discharging ink from inkjet head

[0133] The method for forming a cured product according to the present embodiment may further include a step of discharging, from the inkjet head, the ultraviolet curable ink that has not been discharged from the nozzles of the inkjet head when the step of applying the ink to the substrate is performed using the inkjet head.

[0134] In this embodiment, this step is described later. Figure 1 In the illustrated cured product forming apparatus, the ink that is not discharged from the inkjet head 150 in the ultraviolet irradiation step is discharged from the ink discharge port of the inkjet head 150 through the ink discharge flow path 170 to the ink storage tank 110 .

[0135] 2-4. Process of resupplying ink to the inkjet head

[0136] The method for forming a cured product according to the present embodiment may further include a step of resupplying the ink discharged from the inkjet head in the step of discharging the ink to the inkjet head.

[0137] In this embodiment, this step is described later. Figure 1 In the illustrated solidified material forming apparatus, the ink discharged from the inkjet head 150 and returned to the ink storage tank 110 is supplied to the inkjet head 150 again through the ink supply flow path 120 by the liquid feeding pump 130 .

[0138] By repeating the ink discharge step and the ink resupply step (performing ink circulation), it is possible to suppress an increase in ink viscosity near the nozzle due to precipitation or aggregation of the light-storing pigment in the ink, thereby easily improving ejection stability.

[0139] The above-mentioned process of discharging ink and the above-mentioned process of re-supplying ink can be performed after the above-mentioned process of applying ink, and can also be performed in parallel with the above-mentioned process of irradiating ultraviolet rays, the process of further applying ink (described later), and the process of irradiating ultraviolet rays on the ink applied to the ink (described later).

[0140] 2-5. Further ink application step

[0141] The method for forming a cured product according to the present embodiment may include a step of further applying ink to the ink irradiated with ultraviolet rays.

[0142] The method of further applying the ink may be the same method as the method of applying the ink to the substrate used in the above-mentioned step of applying the ink, or may be a different method.

[0143] 2-6. Step of irradiating the ink applied on the ink with ultraviolet light

[0144] The method for forming a cured product according to the present embodiment may include a step of irradiating the ink further provided with ultraviolet rays.

[0145] In this step, the ink further applied in the step of further applying the ink is irradiated with ultraviolet rays to cure the ink.

[0146] When ultraviolet rays are used, the wavelength may be the same as in the above-mentioned step of irradiating ultraviolet rays.

[0147] In the method for forming a solidified product of the present embodiment, by repeating the above-mentioned step of further imparting ink and the above-mentioned step of irradiating the ink imparted with ultraviolet light, the thickness of the solidified product formed can be further increased. If the thickness of the solidified product becomes larger, more light-storing pigments can be contained in the solidified product. Since the surface area of ​​the solidified product can be increased, the area of ​​the light-emitting surface in the solidified product can be increased. For these reasons, the brightness of the entire solidified product can be more fully improved.

[0148] In the past, when using ultraviolet curing ink containing light-storage pigment as inkjet ink, if it is desired to further suppress the decrease in ink ejection stability, it is not possible to further increase the concentration of the light-storage pigment, and it is difficult to further increase the brightness of the cured product. In contrast, by performing the above-mentioned step of further imparting ink and the above-mentioned step of irradiating the ink imparted with ultraviolet light, even if the concentration of the light-storage pigment in the ink is reduced, the thickness of the cured product can be increased to further increase the brightness. Therefore, performing these steps is particularly useful from the perspective of suppressing the decrease in ejection stability when the ink is used as an inkjet ink and further increasing the brightness of the cured product.

[0149] The method of further applying the ink may be the same method as the method of applying the ink to the substrate used in the above-mentioned step of applying the ink to the substrate, or may be a different method.

[0150] 3. Cured product

[0151] The cured product of the present embodiment is obtained by curing the above-mentioned ultraviolet curable ink.

[0152] The above-mentioned cured product can be formed by the above-mentioned cured product forming method.

[0153] The thickness of the cured product is preferably 0.3 mm or more.

[0154] For a solidified product formed of ink containing a light-storing pigment, the thickness is sometimes made greater than the thickness of a solidified product formed of ink for image formation from the viewpoint of further including a light-storing pigment in the solidified product to further improve the brightness. However, according to the view of the inventors, if the thickness of the solidified product is large, the yellowed part inside the solidified product is difficult to oxidatively decompose and disappear. Therefore, the problem of yellowing of the solidified product is particularly likely to occur when the thickness of the solidified product formed of ink containing a light-storing pigment is large.

[0155] In contrast, the ultraviolet curable ink of the present embodiment can suppress yellowing of the cured product, and thus can increase the thickness of the formed cured product, improve the brightness of the cured product, and suppress yellowing.

[0156] The thickness of the cured product is preferably 0.3mm to 2.0mm, more preferably 0.5mm to 1.5mm, and further preferably 0.7mm to 1.5mm. If the above-mentioned thickness is less than 2.0mm, the yellowing of the cured product can be further suppressed. It is believed that this is because the yellowing produced in the cured product is easily eliminated due to oxidative decomposition. Furthermore, by being less than 2.0mm, it is also easier to see the luminescence of the light-storage pigment present near the substrate, which can further improve the brightness of the entire cured product. It should be noted that from the viewpoint of cost reduction and the processability of the cured product, it is also preferred that the above-mentioned thickness is less than 2.0mm. In addition, if the thickness is more than 0.3mm, it is not easily affected by oxygen hindrance, and the curability of the cured product can also be further improved.

[0157] 4. Solidification forming device

[0158] The cured product forming apparatus in the present embodiment is an apparatus for forming a cured product using the above-mentioned ultraviolet curing ink. Figure 1 It is a schematic diagram showing the structure of the solidified material forming apparatus 100 according to the present embodiment.

[0159] The solidified material forming device 100 includes an ink storage tank 110, an ink supply flow path 120, a liquid feeding pump 130, an inlet side tank 140, an ink applying unit 150, an ink discharge flow path 170, an outlet side tank 170, a conveying device 180, and an ultraviolet irradiation unit 190. It should be noted that in the present embodiment, the ink applying unit 150 is an inkjet head that ejects ink, but is not limited thereto. In this specification, the ink applying unit 150 is hereinafter referred to as the inkjet head 150.

[0160] In the solidified material forming apparatus 100, the ink stored in the ink storage tank 110 is supplied to the inlet side tank 140 via the ink supply flow path 120 by the liquid feeding pump 130. Then, the ink is supplied from the inlet side tank 140 to the inkjet head 150. Then, the ink that is not ejected from the nozzle of the inkjet head 150 is discharged to the outlet side tank 160, and then is discharged to the ink discharge flow path 170 by the liquid feeding pump 130, and the discharged ink is returned to the ink storage tank 110. Thus, the ink that is not ejected from the inkjet head 150 can be circulated from the ink discharge flow path 170 to the ink supply flow path 120, and then supplied to the inkjet head 150, so that the ink is circulated.

[0161] By circulating the ink in the solidified material forming apparatus 100 in this manner, it is possible to suppress an increase in the viscosity of the ink near the nozzle due to sedimentation or aggregation of the light-storing pigment in the ink, and to easily improve discharge stability.

[0162] It should be noted that, in this embodiment, the solidified material forming device 100 may not have Figure 1 Specifically, the ink discharge flow path 170 is not provided, and the ink stored in the ink storage tank 110 is supplied to the inkjet head 150 via the ink supply flow path 120 by the liquid delivery pump 130, and the supplied ink is ejected from the nozzle of the inkjet head 150.

[0163] The ink storage tank 110 is a tank for storing ink to be supplied to the inkjet head 150 and ink discharged without being ejected from the inkjet head 150 .

[0164] The ink supply flow path 120 is a flow path for supplying ink from the ink storage tank 110 to the ink supply port of the inkjet head 150 .

[0165] The liquid feeding pump 130 is respectively arranged in the ink supply flow path 120 and the ink discharge flow path 170. As described above, the liquid feeding pump 130 feeds the ink in the first tank 110 to the inlet side tank 140, and feeds the ink discharged from the inkjet head 150 to the outlet side tank 160. In this embodiment, the liquid feeding pump 130 can receive signals from the inlet side tank 140 and the outlet side tank 160 described later.

[0166] The inlet side tank 140 is a tank having an internal pressure adjustment mechanism and an ink amount detection mechanism. The pressure adjustment mechanism can increase the internal pressure of the inlet side tank, and the ink inside can be supplied to the inkjet head 150. In addition, the detection mechanism can send a signal to the liquid delivery pump 130 when the amount of ink inside decreases, so that ink can be supplied from the ink storage tank 110.

[0167] The outlet tank 160 is a tank having an internal pressure adjustment mechanism and an ink amount detection mechanism. The pressure adjustment mechanism can reduce the internal pressure of the outlet tank, so that the ink in the inkjet head 150 can be discharged to the outlet tank 160 by using the pressure difference with the pressure in the inkjet head 150. In addition, the detection mechanism can send a signal to the liquid delivery pump 130 to discharge the ink to the ink storage tank 110 if the amount of ink inside increases.

[0168] The inlet-side tank 140 and the outlet-side tank 160 can appropriately adjust the flow of ink in the inkjet head 150 , and can further suppress the entrainment of bubbles in the nozzles.

[0169] The pressure setting values ​​of the inlet tank 140 and the outlet tank 160 differ depending on the size and type of the inkjet head 150. For example, when KM1024aLHG-RC (manufactured by Konica Minolta, Inc.) is used as the inkjet head, the internal pressure of the inlet tank 140 can be set to 5 kPa, and the internal pressure of the outlet tank 160 can be set to -20 kPa.

[0170] The inkjet head 150 has a plurality of nozzles for ejecting ink droplets onto a recording medium M such as paper, which is a printed object.

[0171] The ink discharge flow path 170 is a flow path for discharging ink that is not ejected from the inkjet head 150 from the ink discharge ports of the inkjet head 150 .

[0172] The transport device 180 is a device for transporting the substrate M. The transport device 180 includes, for example, a conveyor belt 180 a and a rotatable feed roller 180 b .

[0173] The ultraviolet irradiation unit 190 is disposed downstream of the inkjet head 150 in the conveyance direction of the substrate M, and irradiates ultraviolet rays to the surface of the substrate M to which the ultraviolet curable ink is applied. Thus, the ultraviolet irradiation unit 190 cures the ultraviolet curable ink.

[0174] The ultraviolet irradiation unit 190 is preferably an LED light source that emits ultraviolet light having a wavelength between the peak wavelength of the absorption spectrum of the luminescent pigment contained in the ultraviolet curable ink and the peak wavelength of the absorption spectrum of the photopolymerization initiator contained in the ultraviolet curable ink.

[0175] When further applying ultraviolet curing ink to the cured ink, the feed roller 180b of the conveying device 180 is rotated in the direction opposite to the conveying direction of the substrate M to return the substrate M to a predetermined position. Then, the feed roller 180b is rotated in the conveying direction again to convey the substrate M, and ink is applied from the inkjet head 150 to the cured ink.

[0176] Example

[0177] Hereinafter, the present invention will be specifically described with reference to Examples, but the scope of the present invention is not limited to the description of the Examples.

[0178] 1. Preparation of inks 1 to 19

[0179] (Preparation of Pigment Dispersion)

[0180] 99.9 parts by mass of a photopolymerizable compound (dipropylene glycol diacrylate, molecular weight: 242) and 0.01 parts by mass of a polymerization inhibitor (Irgastab UV-10, manufactured by BASF) were mixed and stirred for 1 hour while heating at 70° C. to prepare an ink solvent.

[0181] 69 parts by mass of the ink solvent prepared above, 30 parts by mass of luminescent pigment (G-300FF, manufactured by Nemoto Chemical Co., Ltd.) and 1 part by mass of pigment dispersant (EFKAPX4701, manufactured by BASF) were placed in a polypropylene container together with 50 parts by mass of zirconium oxide beads having an average particle size of 0.3 mm, and a paint shaker was used to disperse the mixture for 30 minutes, and the zirconium oxide beads were removed to prepare a pigment dispersion.

[0182] The prepared pigment dispersion was diluted with dipropylene glycol diacrylate, and the particle size d of the pigment was measured using a particle size distribution analyzer (LUMiSizer, manufactured by LUM Japan Co., Ltd.). 50 The measured particle size of the pigment is d 50 It is 3.4μm.

[0183] The ratio of the luminescence intensity at a wavelength of 405 nm to the luminescence intensity at the maximum absorption wavelength of the luminescent pigment used was 0.2%, the maximum absorption wavelength of the absorption spectrum was 325 nm, and the peak wavelength of the luminescence spectrum was 520 nm.

[0184] The luminescence intensity at a wavelength of 405 nm and the peak wavelength of the luminescence spectrum of the luminescent pigment were measured using a spectrofluorophotometer (F-7000, manufactured by Hitachi High-Tech Sciences, Ltd., excitation light wavelength 360 nm). In addition, the peak wavelength of the absorption spectrum of the luminescent pigment was measured using a spectrofluorophotometer (F-7000, manufactured by Hitachi High-Tech Sciences, Ltd.).

[0185] (Preparation of inks 1 to 13)

[0186] A solution prepared by dissolving 1 part by mass of photopolymerization initiator A (Irgacure 819, manufactured by BASF) in 19 parts by mass of an ink solvent was added to 100 parts by mass of the prepared pigment dispersion and stirred, and then filtered five times using a 40 μm polypropylene pleated filter to obtain ink 1. At this time, the concentration of the luminescent pigment in ink 1 was 25% by mass, and the content of the photopolymerization initiator was 1.1% by mass relative to the mass of the photopolymerizable compound contained in ink 1.

[0187] In the above preparation, inks 2 to 16 were obtained in the same manner as ink 1 except that the composition of the solution added to 100 parts by mass of the pigment dispersion and the presence or absence of the addition of a polymerization inhibitor were changed as shown in Table 1.

[0188] (Preparation of inks 17 and 18)

[0189] Inks 17 and 18 were obtained in the same manner as Ink 1 except that the luminescent pigment was changed to a white pigment (CR-52, manufactured by Ishihara Industries, Ltd.) and the composition of the solution added to 100 parts by mass of the pigment dispersion was changed as shown in Table 1.

[0190] It should be noted that the photopolymerization initiators A to H in Tables 1 and 2 used the following substances. It should be noted that the "absorption coefficient (405)" shown below refers to the absorption coefficient of light of a wavelength of 405 nm of the photopolymerization initiator, and the "absorption coefficient (365)" refers to the absorption coefficient of light of a wavelength of 365 nm of the photopolymerization initiator. In addition, the "absorption wavelength" refers to the maximum absorption wavelength in the absorption spectrum of the photopolymerization initiator.

[0191] Photopolymerization initiator A: Irgacure 819 (manufactured by BASF, intramolecular cleavage type, absorption coefficient (405): 9.0×10 2 mL / g·cm, Absorption coefficient (365): 2.3×10 3 mL / g·cm, absorption wavelength: 300nm)

[0192] Photopolymerization initiator B: Omnirad ITX (manufactured by IGM Resins, hydrogen abstraction type, absorption coefficient (405): 1.7×10 3 mL / g·cm, absorption wavelength: 320nm)

[0193] Photopolymerization initiator C: Omnirad TPO (manufactured by IGM Resins, intramolecular cleavage type, absorption coefficient (405): 1.6×10 2 mL / g·cm, Absorption coefficient (365): 4.7×10 3 mL / g·cm, absorption wavelength: 300nm)

[0194] Photopolymerization initiator D: Irgacure 784 (manufactured by BASF, intramolecular cleavage type, absorption coefficient (405): 1.2×10 3 mL / g·cm, absorption wavelength: 400nm)

[0195] Photopolymerization initiator E: Irgacure 379 (manufactured by BASF, intramolecular cleavage type, absorption coefficient (405): 2.8×10 2 mL / g·cm, absorption wavelength: 325nm)

[0196] Photopolymerization initiator F: Irgacure 369 (manufactured by BASF, intramolecular cleavage type, absorption coefficient (405): 2.8×10 2 mL / g·cm, absorption wavelength: 325nm)

[0197] Photopolymerization initiator G: Irgacure 127 (manufactured by BASF, intramolecular cleavage type, absorption coefficient (405): 7.0 mL / g·cm, absorption coefficient (365): 1.1×10 3 mL / g·cm, absorption wavelength: 260nm)

[0198] Photopolymerization initiator H: Omnirad MBF (manufactured by IGM Resins, hydrogen abstraction type, absorption coefficient (405): less than 0.1 mL / g·cm, absorption wavelength: 260 nm)

[0199] Photopolymerization initiator I: Irgacure 907 (manufactured by BASF, intramolecular cleavage type, extinction coefficient (405): less than 0.1 mL / g·cm, extinction coefficient (365): 4.7×10 2 mL / g·cm, absorption wavelength 300nm)

[0200] The extinction coefficient (405) and extinction coefficient (365) of the photopolymerization initiator used in each ink were determined by diluting the photopolymerization initiator with methanol to a concentration of 0.1 mass % and measuring the resulting diluted solution using a spectrophotometer (V650, manufactured by JASCO Corporation).

[0201] [Table 1]

[0202]

[0203] [Table 2]

[0204]

[0205] 2. Formation of solidified material

[0206] The prepared ink 1 was filled in a circulation type inkjet head (KM1024aLHG-RC, manufactured by Konica Minolta Co., Ltd.), and the ink 1 was applied to a white acrylic plate under the conditions of a resolution of 720 dpi, a droplet ejection amount of 140 pL (20 pL×7 shots), and a conveying speed of 176 mm / s. Then, a UV light source (H-LSC03, manufactured by HOYA Co., Ltd., wavelength 405 nm) was used at an illumination of 1 W / cm when the plate passed under the lamp at a conveying speed of 176 mm / s. 2 The ink was cured for 0.14 seconds by adjusting ultraviolet rays using an illuminometer. The intensity of the ultraviolet rays was measured using an illuminometer (C9536-02, manufactured by Hamamatsu Photonics KK).

[0207] Ink 1 was further applied to the cured ink under the same conditions, and the applied ink 1 was irradiated with ultraviolet rays under the same conditions to cure the ink. This operation was repeated to form a cured product 1 having a thickness of 0.3 mm.

[0208] The types of ink used, the wavelength of ultraviolet rays, and the thickness of the cured product were changed as shown in Tables 1 and 2, and the cured products 2 to 27 were obtained in the same manner as the cured product 1. It should be noted that the thickness of the cured product was adjusted by changing the number of repetitions of the operation of applying the ink and curing the ink. In addition, the wavelength of ultraviolet rays shown in Tables 1 and 2 was adjusted by changing the type of UV light source to the light source shown below.

[0209] Wavelength 365nm: UV light source (FireJet FJ100 225*20AC 365-6W, manufactured by PHOSEON)

[0210] Wavelength 395nm: UV light source (FireJet FJ100 225*20AC 395-12W, manufactured by PHOSEON)

[0211] 3. Evaluation

[0212] (brightness)

[0213] The cured products 1 to 27 were irradiated with white light from a fluorescent lamp at 200 lx for 20 minutes, and the brightness at that time was measured using a brightness meter (LS-110, manufactured by Konica Minolta Japan Co., Ltd.) 10 minutes after the irradiation. The brightness was evaluated according to the following criteria based on the measured brightness values. It should be noted that for the cured products with a curability evaluation of ×× described later, UV was further irradiated to cure them to the extent that the brightness could be measured.

[0214] ◎:0.20cd / m 2 above

[0215] 0: 0.15cd / m 2 Above and less than 0.20cd / m 2

[0216] △: 0.10cd / m 2 Above and less than 0.15cd / m 2

[0217] ×: 0.10cd / m 2 the following

[0218] (Yellowing)

[0219] The appearances of the cured products 1 to 27 were visually observed and evaluated according to the following criteria.

[0220] ◎: No yellowing was observed

[0221] 0:Slight yellowing was observed

[0222] △: The whole color is confirmed to be light yellow

[0223] ×: Confirmed to be dark yellow in color

[0224] ××: The overall coloring was confirmed to be a darker yellow

[0225] (Curing property)

[0226] For the cured products 1 to 27, after being left to stand for 3 minutes after formation, the cured products were palpated and the curability was evaluated according to the following criteria.

[0227] ◎: The ink is fully cured and no stickiness (tackiness) is observed.

[0228] ○: The cured product is slightly sticky (tackiness), and there are some areas where the curing is not sufficient.

[0229] △: The cured product is sticky (tacky) and there are many areas where the curing is insufficient.

[0230] ×: The cured product peels off from the acrylic plate when rubbed with fingers

[0231] ××: The ink is not cured as a whole

[0232] The evaluation results are shown in Tables 3 to 5.

[0233] [Table 3]

[0234]

[0235] [Table 4]

[0236]

[0237] [Table 5]

[0238]

[0239] In the cured products 1 to 17, since a light-storing pigment having a luminescent spectrum with a wavelength of 405 nm and a photopolymerization initiator having an absorption coefficient of light of a wavelength of 405 nm of 100 mL / g·cm or more are used, yellowing of the cured product, reduction in brightness, and reduction in curability can be suppressed. It is believed that this is because the photopolymerization initiator absorbs ultraviolet rays irradiated to the ink to cure the ink, and the light-storing pigment absorbs the ultraviolet rays and emits light, and the remaining photopolymerization initiator absorbs the light emission, which can further cure the ink. Therefore, even if the amount of photopolymerization initiator added is small, the reduction in curability can be suppressed. In addition, it is believed that by reducing the amount of photopolymerization initiator added, yellowing and reduction in brightness can be suppressed.

[0240] In the cured product 2 having a thickness greater than that of the cured product 1, the brightness and curability of the cured product were evaluated well. It is believed that this is because the thickness becomes larger, and more light-storing pigments can be included in the cured product, so the brightness is further improved. In addition, it is believed that the thickness becomes larger, and the curability is further improved because it is not easily affected by oxygen hindrance.

[0241] In addition, the cured product 2 had better curability than the cured product 11, and had better yellowing and brightness evaluations than the cured product 12. It is believed that if the amount of the photopolymerization initiator is 0.7% by mass or more, the polymerization reaction can be sufficiently initiated to further improve the curability, and if it is 1.3% by mass or less, the yellowing from the photopolymerization initiator can be further suppressed, and the decrease in brightness can also be further suppressed.

[0242] On the other hand, when the content of the photopolymerization initiator relative to the mass of the photopolymerizable compound is more than 1.5 mass %, yellowing and brightness are evaluated low in the cured products 18 to 23. In addition, in the cured products 7 to 9, 24, and 25 in which the above-mentioned extinction coefficient of the photopolymerization initiator is less than 100 mL / g·cm, the brightness is improved, but the curability is reduced. It is believed that this is because the residue of the photopolymerization initiator having the above-mentioned extinction coefficient less than 100 mL / g·cm easily absorbs wavelengths shorter than the wavelength of ultraviolet rays, and yellow, which is a complementary color, is not easily seen.

[0243] In the cured products 26 and 27 using white pigment instead of the light-storing pigment, yellowing is hardly observed even when the content of the photopolymerization initiator exceeds 1.5 mass %. This is probably because the coloring by the white pigment can mask the yellowing of the cured product.

[0244] This application claims the priority based on Japanese Patent Application No. 2022-087969 filed on May 30, 2022. The contents described in the specification and drawings of that application are all cited in the specification of this application.

[0245] Industrial Applicability

[0246] The present invention can suppress yellowing and brightness reduction of a cured product formed of an ink containing a light-storing pigment, and suppress reduction in curability. Therefore, the present invention is useful in, for example, manufacturing outdoor billboards.

[0247] Explanation of symbols

[0248] 100 Solidification forming device

[0249] 110 ink storage tank

[0250] 120 ink supply flow path

[0251] 130 Liquid delivery pump

[0252] 140 inkjet head

[0253] 150 ink discharge flow path

[0254] 160 conveying device

[0255] 170 Ultraviolet irradiation unit

Claims

1. A UV curable ink comprising: Photopolymerizable compounds, Luminescent pigment that emits light at a wavelength of 405nm, and A photopolymerization initiator having an absorption coefficient of light at a wavelength of 405 nm of 100 mL / g·cm or more; The content of the photopolymerization initiator is 0.1% by mass to 1.5% by mass based on the total mass of the photopolymerizable compound. 2 . The ultraviolet curable ink according to claim 1 , which is an inkjet ink.

3. The ultraviolet curable ink according to claim 1 or 2, wherein: Further comprising a polymerization inhibitor.

4. The ultraviolet curable ink according to claim 1 or 2, wherein: The particle size distribution of the light-storing pigment based on volume is 50% of the cumulative value, i.e., the particle size d 50 1μm~10μm.

5. The ultraviolet curable ink according to claim 1 or 2, wherein: The content of the light-storing pigment is less than 50% by mass based on the total mass of the ultraviolet curable ink.

6. The ultraviolet curable ink according to claim 1 or 2, wherein: The light-storage pigment is a light-storage pigment obtained by activating strontium aluminate with an activator.

7. The ultraviolet curable ink according to claim 1 or 2, wherein: The photopolymerization initiator is an intramolecular cleavage type photopolymerization initiator.

8. The ultraviolet curable ink according to claim 1 or 2, wherein: The extinction coefficient of the photopolymerization initiator is 500 mL / g·cm or more.

9. A method for forming a solidified product, comprising: A step of applying the ultraviolet curable ink according to claim 1 or 2 to a substrate, and A step of irradiating the applied ultraviolet curing ink with ultraviolet rays.

10. The method for forming a solidified product according to claim 9, wherein: Further having: a step of further applying the ultraviolet curing ink to the ink irradiated with the ultraviolet rays, and A step of irradiating the ultraviolet curing ink applied onto the ink with ultraviolet rays.

11. The method for forming a solidified product according to claim 9, wherein: The step of applying the ultraviolet curing ink to a substrate comprises ejecting the ultraviolet curing ink from a nozzle of an inkjet head to apply the ultraviolet curing ink to a substrate. The method for forming a solidified product further comprises: a step of discharging the ultraviolet curable ink which has not been discharged from the nozzles of the inkjet head from the inkjet head, and A step of supplying the discharged ultraviolet curing ink to the inkjet head again.

12. A cured product obtained by curing the ultraviolet curable ink according to claim 1 or 2.

13. The cured product according to claim 12, wherein The thickness of the solidified material is greater than 0.3 mm.

14. A solidified material forming device, comprising: An ink applying unit for applying the ultraviolet curable ink according to claim 1 or 2 to a substrate, and The LED light source irradiates the ultraviolet curing ink with ultraviolet rays.

15. The solidified material forming device according to claim 14, wherein: The ink applying unit is an inkjet head that ejects the ultraviolet curing ink. The ultraviolet curing ink which has not been discharged from the inkjet head is discharged from the inkjet head, and the discharged ultraviolet curing ink is re-supplied to the inkjet head.

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