JP2005200560A - Inkjet ink - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a UV-curable inkjet ink ejectable with stable and precise landing, excellent in storability and curability. <P>SOLUTION: This UV-curable inkjet ink comprises at least a pigment, a dispersant having an acid value larger than its amine value, a polymerizable compound and a photopolymerization initiator, where the total amount of cationic impurities, metallic impurities and strongly acidic substances is ≤500 ppm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an inkjet ink that is cured by ultraviolet rays, and more particularly to an ultraviolet curable inkjet ink that can be ejected stably and with high landing accuracy, has excellent storage stability, and is excellent in curability.

  A method of recording by irradiating ultraviolet rays and ejecting ink curable by ultraviolet rays and irradiating with ultraviolet rays has been developed in recent years, and its practical application is progressing. As inks to be used, WO99 / 29787, WO99 / 29788, WO97 / 31071, JP-A-5-214280, JP-A-2002-1888025 and the like are known.

  WO99 / 29787 states that the ink viscosity at 30 ° C. is preferably 35 mPa · s or less. This indicates a preferable condition for ejection in ink jet recording. In the above-mentioned other documents relating to inks, formulation examples almost corresponding to this condition are shown.

  The UV curable ink jet recording system has the advantage that it can be recorded on a substrate that does not absorb ink, but on the other hand, the reactive diluent (polymerizable monomer, oligomer, etc.) evaporates or shrinks. However, since it remains as it is, there is a drawback that the image becomes embossed and the texture is impaired.

  The present inventor has studied to obtain a high image quality by discharging ink droplets of a finer size and appropriately leveling the droplets in an ultraviolet curable ink jet recording system.

  In general, ultraviolet curable ink has a higher viscosity than water-based ink, and it has been found that it is very difficult to perform high-speed, stable and accurate ejection. In particular, in order to eject small droplets of less than 10 pl with a gray scale type head with variable droplet size, the conventional inks can be used for stable ejection or landing. It was difficult to improve accuracy.

  Such a phenomenon also varies depending on the type of pigment, and becomes a problem particularly in yellow and magenta where it is difficult to obtain good dispersibility.

  Patent Document 1 proposes a driving method corresponding to a weak thixotropic behavior in water-based ink as a driving device and method for an ink jet printer head, but a high-viscosity liquid such as UV ink is not sufficient. there were.

  As means for improving pigment dispersibility in UV inks, Patent Document 2 proposes using several dispersants, and Patent Document 3 uses surface-treated pigments. It was difficult to achieve both sensitivity and storage stability while maintaining the properties. In order to cure small droplets or thin films, cationic polymerization that is not subjected to oxygen inhibition is more advantageous than radical polymerization, but it is difficult to stabilize pigment dispersion particularly in a cationic polymerization type ink.

  In order to obtain good pigment dispersibility using a polymerizable monomer with low polarity as a medium, a polymer dispersant is adsorbed on the pigment surface by polarity or acid / base, and the dispersion is stabilized by steric hindrance of the polymer dispersant Is a conventional means. However, even if the initial ink dispersibility is good only by using a method such as making one of the dispersant and the pigment acidic, making one of the basics polar, or one of them polar, the long-term storage stability is sufficient. Was not obtained.

In particular, cationically polymerizable UV inks are susceptible to sensitivity deterioration due to excess basic components and storage stability deterioration due to excess acidic components, and it is difficult to balance sensitivity and storage stability. In addition, since photopolymerization initiators such as iodonium salts and sulfonium salts have a strong polarity, the adsorption equilibrium between the pigment and the dispersant is broken, and the pigments flocculate with each other. Problems are likely to occur. Also in radical polymerization, the same dispersibility degradation occurs by adding a relatively polar material such as an amine compound or a surfactant.
JP 2000-203020 A JP 2001-288387 A JP 2003-253155 A

  An object of the present invention is to provide an ultraviolet curable inkjet ink that can be discharged stably and with high landing accuracy, has excellent storage stability, and has excellent curability.

  The above object of the present invention is achieved by the following configurations.

(Claim 1)
An ultraviolet curable inkjet ink containing at least a pigment, a dispersant having a larger acid value than the amine value, a polymerizable compound, and a photopolymerization initiator, and the total amount of cationic impurities, metal impurities, and strongly acidic substances is 500 ppm or less An ink-jet ink characterized by

(Claim 2)
The cation type impurity and the metal impurity are at least one element selected from B, F, Na, Mg, Al, P, S, Cl, K, Ca, Cu, Br, Ag, Sn, Sb, As and W. The inkjet ink according to claim 1, wherein the ink is an impurity.

(Claim 3)
The inkjet ink according to claim 1, which is cationically polymerizable and contains an amine compound.

(Claim 4)
The inkjet ink according to any one of claims 1 to 3, wherein the water content is 1 to 3% by mass.

  According to the present invention, it is possible to provide an ultraviolet curable inkjet ink that can be discharged stably and with high landing accuracy, has excellent storability, and excellent curability.

  As a result of diligent research, the inventor of the present invention is an ultraviolet curable inkjet ink containing at least a pigment, a dispersant having an acid value larger than the amine value, a polymerizable compound, and a photopolymerization initiator. And the total amount of strongly acidic substances was found to be 500 ppm or less, and it was found that a highly sensitive UV curable inkjet ink that was stable and could be ejected with high accuracy and had excellent storage stability was obtained. .

  The present invention will be described in detail below.

[Pigment]
The ink-jet ink of the present invention (hereinafter also simply referred to as ink) contains a pigment. In the present invention, the amine value of the pigment is larger than the acid value, and the difference is preferably 1 mg / g KOH or more and less than 10 mg / g. If it is less than 1 mg / g KOH, the effect is not obtained, and if it is 10 mg / g or more, it is necessary to carry out the basic treatment excessively.

  The acid value and amine value of the pigment can be determined by potentiometric titration. For example, it can be measured by the method described in Color Material Association Journal 61, [12], 692-698 (1988).

As the pigment that can be used in the present invention, various pigments can be used as long as the amine value is larger than the acid value. As specific examples of the pigment, those listed below which have an amine value larger than the acid value can be used, but the present invention is not limited thereto.
C. I. Pigment Yellow;
1, 2, 3, 12, 13, 14, 16, 17, 73, 74, 75, 81, 83, 87, 93, 95, 97, 98, 109, 114, 120, 128, 129, 138, 151, 154, 180
C. I. Pigment Red;
5, 7, 12, 22, 38, 48: 1, 48: 2, 48: 4, 49: 1, 53: 1, 57: 1, 63: 1, 101, 112, 122, 123, 144, 146, 168, 184, 185, 202
C. I. Pigment Violet;
19, 23
C. I. Pigment Blue;
1, 2, 3, 15: 1, 15: 2, 15: 3, 15: 4, 18, 22, 27, 29, 60
C. I. Pigment Green;
7, 36
C. I. Pigment Black;
7
C. I. Pigment White;
6, 18, 21
For example, a process color ink set having preferable pigment types is prepared by appropriately combining four types of the following pigments.
Y: C.I. I. Pigment Yellow; 120, 128, 74, 83, 138, 151, 180
M: C.I. I. Pigment Violet; 19,122
C :. I. Pigment Blue; 15; 4
K: C.I. I. Pigment Black; 7
W: C.I. I. Pigment White; 6
The pigment is preferably subjected to any surface treatment of basic, acidic, or polar treatment on the surface. As the surface treatment method, it is preferable to use a synergist having a structure similar to that of the pigment and having any of basic, acidic and polar treatments.

  In the present invention, the synergist is a direct or via a joint with an organic substance having a dye or organic pigment mother nucleus having a structure similar to that of a pigment and modified with a polar group such as an acidic group or a basic group. A group to which a group is bonded. The dispersibility of the pigment is improved by adsorbing it on the pigment surface and bonding it to the dispersant. Most of these synergists are inferior in light resistance, and excessive treatment cannot exhibit the light resistance of the original pigment.

  Examples of the polar group include a sulfonic acid group, a carboxylic acid group, a phosphoric acid group, a boric acid group, and a hydroxyl group, preferably a sulfonic acid group and a carboxylic acid group, and more preferably a sulfonic acid group.

  Examples of a method for obtaining pigment particles having a polar group on the surface include WO97 / 48769, JP-A-10-110129, JP-A-11-246807, JP-A-11-57458, JP-A-11-189739, JP-A-11-189739. By treating the surface of the pigment particles described in JP-A Nos. 11-323232 and 2000-265094 with an appropriate oxidizing agent, polar groups such as sulfonic acid groups or salts thereof are formed on at least a part of the pigment surface. The method to introduce is mentioned. Specifically, it is prepared by oxidizing carbon black with concentrated nitric acid, or in the case of color pigments by oxidizing with sulfamic acid, sulfonated pyridine salt, amide sulfuric acid, etc. in sulfolane or N-methyl-2-pyrrolidone. can do. A pigment dispersion can be obtained by removing and purifying a substance that has been excessively oxidized and becomes water-soluble by these reactions. Moreover, when a sulfonic acid group is introduced onto the surface by oxidation, the acidic group may be neutralized with a basic compound as necessary.

  Other methods include a method of adsorbing the pigment derivatives described in JP-A-11-49974, JP-A-2000-273383, JP-A-2000-303014, etc. on the surface of the pigment particles by a treatment such as milling. Examples include a method of dissolving the pigment described in each publication such as 2002-179777, 2002-20141, and the like together with a pigment derivative, followed by crystallization in a poor solvent. Pigment particles having a polar group on the surface can be obtained.

  In the present invention, the polar group may be free or in a salt state, or may have a counter salt. Examples of the counter salt include inorganic salts (lithium, sodium, potassium, magnesium, calcium, aluminum, nickel, ammonium) and organic salts (triethylammonium, diethylammonium, pyridinium, triethanolammonium, etc.), preferably 1 A counter salt having a valence.

  As the synergist, those modified with acid such as sulfonic acid modification, carboxyl group modification and the like having an amine value larger than the acid value are preferable.

  These synergists are preferably added in an amount of 1 to 50 parts by mass with respect to 100 parts by mass of the pigment.

  For pigment dispersion, the selection of the pigment, dispersant, and dispersion medium, dispersion conditions, and filtration conditions are appropriately set so that the average particle diameter of the pigment particles is 80 to 150 nm. The average particle diameter can be measured by laser scattering or the like. By controlling the particle size, clogging of the head nozzle can be suppressed, and ink storage stability, ink transparency, and curing sensitivity can be maintained.

  In the ink of the present invention, the pigment concentration is preferably 1 to 10% by mass of the whole ink.

[Dispersant]
In the present invention, the dispersion stability is improved by using a dispersant having an acid value larger than the amine value. The acid value and amine value of the dispersant can be measured by the potentiometric titration.

  As the dispersant, any of low molecular weight and high molecular weight can be used, but a polymer type dispersant is preferable. The polymer type dispersant preferably has a base or an acid as an adsorption site with the pigment, and good results can be obtained with either a base or an acid. Even with a basic polymer dispersant, the amine value is not necessarily higher than the acid value when the amine value and acid value are measured. The present invention pays attention to this point, and has discovered that, as a whole, the dispersant has a higher acid value than the amine value for the discharge stability, rather than the type of anchor. The weighted average value can be used for the amine value and acid value when a plurality of dispersants are used.

  The acid value of the dispersant is larger than the amine value, and the difference is preferably 1 mg / g KOH or more and less than 30 mg / g KOH. If it is less than 1 mg / g KOH, the effect is not obtained, and if it is 30 mg / g KOH or more, there is a concern that it is cured by a thermal reaction.

  Preferable specific examples of the dispersant include Ajimoto PB822 and Ajisper PB821 manufactured by Ajinomoto Fine Techno Co., but the present invention is not limited thereto.

  The amount of the dispersant used is 10 to 100% by mass, preferably 15 to 50% by mass, based on the pigment.

  For the dispersion of the pigment, a ball mill, a sand mill, an attritor, a roll mill, an agitator, a Henschel mixer, a colloid mill, an ultrasonic homogenizer, a pearl mill, a wet jet mill, a paint shaker, or the like can be used.

[Polymerizable compound]
Among the polymerizable compounds that can be used in the present invention, examples of the radical polymerizable compound include, for example, JP-A-7-159983, JP-B-7-31399, JP-A-8-224982, and JP-A-10-863. The compounds described in each publication can be mentioned, and various known cationic polymerizable compounds can be used as the cationic polymerizable compound. For example, in JP-A-6-9714, JP-A-2001-31892, 2001-40068, 2001-55507, 2001-310938, 2001-310937, 2001-220526, etc. Examples include epoxy compounds, vinyl ether compounds, oxetane compounds and the like.

  The radical polymerizable compound is a compound having an ethylenically unsaturated bond capable of radical polymerization, and may be any compound as long as it has at least one ethylenically unsaturated bond capable of radical polymerization in the molecule. , Oligomers, polymers and the like having chemical forms. Only one kind of radically polymerizable compound may be used, or two or more kinds thereof may be used in combination at an arbitrary ratio in order to improve desired properties.

  The addition amount of the radical polymerizable compound is preferably 1 to 97% by mass and more preferably 30 to 95% by mass with respect to the ink composition.

  In the case of a cationically polymerizable ink, it preferably contains at least one oxetane compound and at least one compound selected from an epoxy compound and a vinyl ether compound. In particular, the oxetane compound and the following alicyclic epoxy compound are included. It is preferable to use together. As the epoxy compound, an alicyclic epoxide is particularly preferable. As the alicyclic epoxide, a compound having at least one cycloalkane ring such as cyclohexene or cyclopentene ring is converted to an appropriate oxidation such as hydrogen peroxide or peracid. A cyclohexene oxide or cyclopentene oxide-containing compound obtained by epoxidation with an agent is preferred.

(Photopolymerization initiator)
In the present invention, in order to perform the curing reaction more efficiently, a photopolymerization initiator is added and cured. The photopolymerization initiator is a radical generator when a radically polymerizable compound is used as the polymerizable compound, and a photoacid generator when a cationically polymerizable compound is used as the polymerizable compound.

  Radical generators used for radical polymerization can be broadly classified into two types: intramolecular bond cleavage type and intramolecular hydrogen abstraction type. Examples of the intramolecular bond cleavage type radical generator include diethoxyacetophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, benzyldimethyl ketal, 1- (4-isopropylphenyl) -2- Hydroxy-2-methylpropan-1-one, 4- (2-hydroxyethoxy) phenyl- (2-hydroxy-2-propyl) ketone, 1-hydroxycyclohexyl-phenylketone, 2-methyl-2-morpholino (4- Acetophenone series such as thiomethylphenyl) propan-1-one; α-aminoketone series such as 2-benzyl-2-dimethylamino-1- (4-morpholinophenyl) -butanone; benzoin, benzoin methyl ether, benzoin isopropyl ether Benzoins such as bis (2,4,6 Such as trimethyl benzoyl) phenyl phosphine oxide bisacylphosphine oxide-based; benzyl - and methyl phenylglyoxylate esters.

  On the other hand, examples of the intramolecular hydrogen abstraction type radical generator include, for example, benzophenone, methyl 4-phenylbenzophenone, o-benzoylbenzoate, 4,4'-dichlorobenzophenone, hydroxybenzophenone, 4-benzoyl-4'-methyl- Benzophenone series such as diphenyl sulfide, acrylated benzophenone, 3,3 ', 4,4'-tetra (t-butylperoxycarbonyl) benzophenone, 3,3'-dimethyl-4-methoxybenzophenone; 2-isopropylthioxanthone, 2, Thioxanthone series such as 4-dimethylthioxanthone, 2,4-diethylthioxanthone, 2,4-dichlorothioxanthone; Amibenzotone, aminobenzophenone series such as 4,4'-diethylaminobenzophenone; 10-butyl 2-chloro-acridone, 2-ethyl anthraquinone, 9,10-phenanthrenequinone, camphorquinone, and the like. When the radical generator is used, the blending amount is preferably 0.1 to 15.0% by mass of the ultraviolet curable ink. It is also effective to use an amine compound in combination.

  Of these photopolymerization initiators, bisacylphosphine oxide having photobleaching property and α-aminoketone which is not easily affected by oxygen inhibition are particularly preferable because of high sensitivity-up effect in small droplets. As the bisacylphosphine oxide and α-aminoketone, for example, Irgacure series of Ciba Specialty Chemicals can be used.

  Examples of photoacid generators used for cationic polymerization include chemically amplified photoresists and compounds used for photocationic polymerization (edited by Organic Electronics Materials Study Group, “Organic Materials for Imaging”, Bunshin Publishing ( 1993), pages 187-192). Examples of compounds suitable for the present invention are listed below.

First, B (C ₆ F ₅ ) ₄ ⁻ , PF ₆ ⁻ , AsF ₆ ⁻ , SbF ₆ ⁻ , CF ₃ SO ₃ ⁻ salt of aromatic onium compounds such as diazonium, ammonium, iodonium, sulfonium, phosphonium, etc. be able to.

  Specific examples of onium compounds that can be used in the present invention are shown below.

  Secondly, sulfonated substances that generate sulfonic acid can be mentioned, and specific compounds thereof are exemplified below.

  Of these, triarylsulfonium salts are preferred.

  Thirdly, halides that generate hydrogen halide can also be used, and specific compounds thereof are exemplified below.

  Fourthly, an iron allene complex can be mentioned.

  A triarylsulfonium salt is preferable from the viewpoint of storage stability. Moreover, since a thing with low polarity does not destroy a dispersibility, it is preferable. The sulfonium salt is preferably a monocation type rather than a dication type.

〔impurities〕
The present invention is an ultraviolet curable inkjet ink containing at least a pigment, a dispersant having a larger acid value than an amine value, a polymerizable compound, and a photopolymerization initiator, and comprising a cationic impurity, a metal impurity, and a strongly acidic substance. The total amount is 500 ppm or less.

  By keeping the total amount of cationic impurities, metal impurities and strongly acidic substances to 500 ppm or less, storage stability over a long period of time can be obtained. This is probably because these impurities break the adsorption equilibrium between the pigment and the dispersant. Many polymerizable monomers such as acrylates, oxetanes, and epoxies are esters. In particular, if there are a few strongly acidic impurities, they react with water in the ink to cause hydrolysis, and the decomposed carboxylic acid or alcohol breaks the dispersion. It is possible. Particularly good dispersion stability can be obtained particularly in a cationic polymerization system, in particular, a cationic polymerization system using oxetane.

  In the present invention, the total amount of cationic impurities, metal impurities, and strongly acidic substances contained in the ink needs to be 500 ppm or less, preferably 100 ppm or less including 0 ppm.

  These impurities include impurities during pigment synthesis and surface treatment, impurities during synthesis of dispersants and surfactants, impurities eluted from ink packaging materials, polymerizable compounds, photopolymerization initiators or other Impurities brought in from additives. When these impurities exceed 500 ppm, storage stability is impaired.

  The cation type impurity or metal impurity is at least one selected from B, F, Na, Mg, Al, P, S, Cl, K, Ca, Cu, Br, Ag, Sn, Sb, As and W. An impurity containing an element can be given.

Examples of strongly acidic substances include sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, p-toluenesulfonic acid, benzenesulfonic acid, naphthalenesulfonic acid, methanesulfonic acid, ethanesulfonic acid, trifluoromethanesulfonic acid, AlCl ₃ , AlBr. ₃ , FeCl ₃ , BCl ₃ , BBr ₃ , BF ₃ .OEt ₂ , BF ₃ , SbF ₅ , PF ₅ , ZnCl ₂ , TiCl _{4 and the} like.

  In the present invention, a cationic impurity, a metal impurity, or a strongly acidic substance as an impurity is any one of a polymerizable compound, a photopolymerization initiator, and an ink, an adsorption treatment with a basic adsorbent, column chromatography (silica gel). The ink of the present invention having a total content of 500 ppm or less can be obtained by subjecting it to separation / purification means such as column chromatography), activated carbon treatment, crystallization or recrystallization.

  Particularly for strongly acidic substances, it is preferable to perform an adsorption treatment with a basic adsorbent and a separation treatment with column chromatography among the above purification means. As the basic adsorbent, for example, a basic inorganic adsorbent (eg, hydrotalcite) such as a trade name “KYOWARD” can be used. Although the usage-amount of a basic adsorbent can be suitably selected with the kind and content of a strongly acidic substance, it is about 1-200 mass parts normally with respect to 100 mass parts of compounds used for a process. The adsorption treatment with the basic adsorbent is performed by dissolving a solvent capable of dissolving the compound to be treated. Processing temperature is about 10-100 degreeC, for example. The treatment can be performed by a conventional method such as a batch method, a continuous method, a fluidized bed method, or a packed tower method.

  The content of these impurities can be determined by known analytical means such as high performance liquid chromatography, gas chromatography, elemental analysis, atomic absorption analysis, infrared absorption spectroscopy, NMR spectroscopy, mass spectrometry, and titration analysis. it can.

(Polymerization inhibitor)
In the present invention, a polymerization inhibitor that suppresses polymerization by heat or actinic rays can be added to the ink. As the polymerization inhibitor, various compounds are known, and those widely blended in general polymerizable compositions can be used as they are.

  As a polymerization inhibitor, it is possible to use phenolic antioxidants, amine compounds, phosphorus antioxidants, hydroquinone monomethyl ether widely used for (meth) acrylic monomers, hydroquinone, t-butylcatechol, pyrogallol, water, etc. Is possible. These are detailed in “Development Technology of Polymer Additives” (issued by CMC). Since phenolic compounds with double bonds derived from acrylic acid in the molecule can capture R · from their reaction mechanism, they can suppress polymerization even when heated in a system without sealed oxygen. Is preferable. Specifically, Sumitizer Chemical's Sumilizer GA-80, Sumilizer GM, Sumilizer GS, etc. can be mentioned.

  These polymerization inhibitors are preferably added at the time of preparing the pigment dispersion, but excessive addition of the polymerization inhibitor causes a decrease in sensitivity as an ink, while maintaining the storage stability as an ink, It is desirable that the amount capable of preventing polymerization at the time of pigment dispersion is appropriately set and blended. Or it is preferable to mix | blend excessively and to extract after dispersion | distribution and to reduce the quantity of a polymerization inhibitor.

  When the photopolymerization is cationic polymerizable, the polymerization inhibitor is preferably an amine compound. The amine compound can synergistically maintain a balance between storage stability and sensitivity in combination with a dispersant having an acid value higher than the amine value. Among amine compounds, a quaternary ammonium salt is particularly preferable for a cationic polymerizable ink because it can suppress a dark reaction during storage without suppressing a reaction during exposure. When the dispersibility is unstable, a tertiary amine having a small polarity is a particularly preferred polymerization inhibitor. The addition amount of the amine compound is preferably 0.01 to 1% by mass of the total ink. If it is less than 0.01% by mass, storage stability cannot be obtained. When it is larger than 1% by mass, the curing sensitivity is lowered.

〔moisture〕
The water content in the ink varies depending on the type and content ratio of the polymerizable compound, the type and content ratio of the photopolymerization initiator, the storage conditions, the curing conditions, etc., but the water content in the ink is 1 to 3 masses. Storage stability improves by setting it as%. In particular, the storage stability of the cationically polymerizable ink is greatly improved. The sensitivity to the storage period can be improved as compared with the case where only the amine compound is added. If it is 1% by mass or less, this effect is poor, and if it is 3% by mass or more, problems such as precipitation during low-temperature storage occur.

  The water content can be measured by the Karl Fischer method (automatic moisture measuring device, AQV-2000, manufactured by Hiranuma Sangyo Co., Ltd.).

  The surface tension of the ink is 22 to 38 mN / m, more preferably 24 to 35 mN / m.

[Other additives]
Moreover, although the ink of the present invention is cured by irradiation with ultraviolet rays, a photosensitizer can be used in combination in order to perform the curing reaction more efficiently. Examples of such a photosensitizer include triethanolamine, methyldiethanolamine, triisopropanolamine, methyl 4-dimethylaminobenzoate, ethyl 4-dimethylaminobenzoate, isoamyl 4-dimethylaminobenzoate, benzoic acid ( 2-dimethylamino) ethyl, 4-dimethylaminobenzoic acid (n-butoxy) ethyl, amines such as 4-dimethylaminobenzoic acid 2-ethylhexyl, cyanine, phthalocyanine, merocyanine, porphyrin, spiro compound, ferrocene, fluorene, fulgide , Imidazole, perylene, phenazine, phenothiazine, polyene, azo compound, diphenylmethane, triphenylmethane, polymethine acridine, coumarin, ketocoumarin, quinacridone, indigo, styryl, pyriri Compounds, pyromethene compounds, pyrazolotriazole compounds, benzothiazole compounds, barbituric acid derivatives, thiobarbituric acid derivatives and the like, and further, European Patent No. 568,993, US Patent No. 4,508,811 No. 5,227,227, JP-A-2001-125255, JP-A-11-271969, and the like can also be used.

  As the cationic photosensitizer, known ones are used, and specific examples include anthracene, pyrene, perylene, acridine orange, thioxanthone, 2-chlorothioxanthone, benzoflavin, eosin, rose bengal, erythrosine, methylene blue, and the like. is there.

  The amount of the photosensitizer used is preferably in the range of 0.01 to 10.00% by mass in the ink composition.

  The ink of the present invention preferably contains a surfactant. As the surfactant, any of cationic, anionic, amphoteric and nonionic can be used, and a nonionic surfactant is particularly preferable.

  Nonionic surfactants include polyoxyethylene alkyl ether, polyoxyethylene secondary alcohol ether, polyoxyethylene alkylphenyl ether (eg, Emulgen 911), polyoxyethylene sterol ether, polyoxyethylene lanolin derivative, polyoxyethylene Polyoxypropylene alkyl ether (for example, Newpol PE-62), polyoxyethylene glycerin fatty acid ester, polyoxyethylene castor oil, hydrogenated castor oil, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene sorbitol fatty acid ester, polyethylene glycol fatty acid ester, Fatty acid monoglyceride, polyglycerin fatty acid ester, sorbitan fatty acid ester, propylene glycol fatty acid ester, ® sugar fatty acid esters, fatty acid alkanolamides, polyoxyethylene fatty acid amides, polyoxyethylene alkyl amines, alkyl amine oxides, acetylene glycol, acetylene alcohol, and the like, especially those fluorosurfactants preferred.

  In the present invention, the nonionic surfactant is particularly preferably a fluorosurfactant having a perfluoroalkyl group in the molecule. Examples of the fluorosurfactant having a perfluoroalkyl group in the molecule include perfluoroalkylethylene oxide adducts, perfluoroalkylamine oxides, perfluoroalkyl-containing oligomers, specifically, for example, “SURFLON S-141. ”,“ Surflon S-145 ”,“ Surflon S-381 ”,“ Surflon S-383 ”,“ Surflon S-393 ”,“ Surflon SC-101 ”,“ Surflon SC-105 ”,“ Surflon KH-40 ” , "Surflon SA-100" (product of Seimi Chemical Co., Ltd.), "Megafuck F-171", "Megafuck F-172", "Megafuck F-173", "Megafuck F-177" , “Megafuck F-178A”, “Megafuck F-178K”, "Megafuck F-179", "Megafuck F-183", "Megafuck F-184", "Megafuck F-815", "Megafuck F-470", "Megafuck F-471" Product of Nippon Ink Chemical Co., Ltd.).

  When these surfactants are used, they can be used singly or as a mixture of two or more, and by adding them in the range of 0.001 to 1.0% by mass with respect to the total amount of the ink, the surface of the ink The tension can be adjusted arbitrarily. When implementing this invention, it is not limited to these. In order to maintain the long-term storage stability of the ink, an antiseptic and an antifungal agent may be added to the ink.

(Inkjet recording method)
Next, an ink jet recording method using the ink of the present invention will be described.

  In the present invention, ink is ejected and drawn on a substrate with a small droplet size by an ink jet recording method, and then the ink is cured by irradiation with ultraviolet rays. By irradiating the landed ink with ultraviolet rays, the ink is cured and an image is completed.

  Since the ink continues to be leveled on the base material after landing and the surface area per unit volume increases, it is more strongly affected by polymerization inhibition. Therefore, the dot diameter is small, for example, 80 μm or less. By irradiating ultraviolet rays at the timing, bleeding and residual monomers can be reduced.

  Moreover, it is preferable to irradiate ultraviolet rays after the ink has landed on the substrate and before the landed ink penetrates into the substrate, for example, before it penetrates 30% by mass or more. When an ink-absorbing base material is used, the ink continues to permeate the base material after the landing until the ultraviolet irradiation, and the ink that permeates the inside of the base material makes it difficult for the ultraviolet light to reach, resulting in poor curing.

  In the present invention, the ink film thickness after the ink has landed on the substrate and cured by irradiation with ultraviolet rays is preferably 2 to 80 μm. In the field of flexible packaging printing, where the base material is often a thin plastic material, not only the problem of curl and wrinkles of the base material, but also the problem that the overall print material's strain and texture will change. Ink ejection is not preferred.

  In addition, "ink film thickness" means the maximum value of the film thickness of the ink drawn on the base material, and it is a single color or other two color overlap (secondary color), three color overlap, four color overlap Even when recording is performed by the (white ink base) inkjet recording method, the meaning of the ink film thickness is the same.

  As the ink discharge conditions, it is preferable from the viewpoint of discharge stability that the ink jet recording head and the ink are heated to 35 to 100 ° C. and discharged. The UV curable ink has a large viscosity fluctuation range due to temperature fluctuations, and the viscosity fluctuations directly affect the droplet size and droplet ejection speed, causing image quality degradation. It is necessary to keep. The control range of the ink temperature is set temperature ± 5 ° C., preferably set temperature ± 2 ° C., more preferably set temperature ± 1 ° C.

  A basic method for irradiating ultraviolet rays is disclosed in Japanese Patent Application Laid-Open No. 60-132767. According to this, the light source is provided on both sides of the head unit, and the head and the light source are scanned by the shuttle method. Irradiation is performed after a certain period of time after ink landing. Further, the curing is completed by another light source that is not driven. In US Pat. No. 6,145,979, as an irradiation method, a method using an optical fiber or a method of applying a collimated light source to a mirror surface provided on the side of the head unit and irradiating the recording unit with UV light is disclosed. Yes. Any of these irradiation methods can be used in the image forming method according to the present invention.

  Next, an ink jet recording apparatus (hereinafter also simply referred to as a recording apparatus) that can be used in the present invention will be described.

  Hereinafter, a recording apparatus that can be used in the present invention will be described with reference to the drawings as appropriate. Note that the recording apparatus shown in the drawings is merely an embodiment of the recording apparatus that can be preferably used in the present invention, and the present invention is not limited to the drawing of the recording apparatus exemplified here.

  FIG. 1 is a front view showing an example of a configuration of a main part used in a serial printing method in an ink jet recording apparatus that can be used in the present invention. The recording apparatus 1 includes a head carriage 2, a recording head 3, an irradiation unit 4, a platen unit 5, and the like. In the recording apparatus 1, the platen unit 5 is installed under the base material P. The platen unit 5 has a function of absorbing ultraviolet rays, and absorbs excess ultraviolet rays that have passed through the substrate P. As a result, a high-definition image can be stably reproduced.

  The base material P is guided by the guide member 6 and moves from the near side to the far side in FIG. 1 by the operation of the conveying means (not shown). A head scanning unit (not shown) scans the recording head 3 held by the head carriage 2 by reciprocating the head carriage 2 in the Y direction in FIG.

  The head carriage 2 is installed on the upper side of the substrate P, and stores a plurality of recording heads 3 to be described later according to the number of colors used for image printing on the substrate P, with the discharge ports arranged on the lower side. The head carriage 2 is installed with respect to the main body of the recording apparatus 1 in such a manner that it can reciprocate in the Y direction in FIG. 1, and reciprocates in the Y direction in FIG. 1 by driving the head scanning means.

  In FIG. 1, the head carriage 2 is illustrated as accommodating the recording heads 3 of yellow (Y), magenta (M), cyan (C), and black (K). The number of colors of the recording head 3 housed in the head carriage 2 can be determined as appropriate.

  The recording head 3 is directed from the discharge port toward the substrate P by the operation of a plurality of discharge means (not shown) provided therein with ultraviolet curable ink supplied by an ink supply means (not shown). To discharge. The ink ejected by the recording head 3 is composed of a pigment, a dispersant, a polymerizable compound, a photopolymerization initiator, etc., and the photopolymerization initiator acts as a catalyst when irradiated with ultraviolet rays. It has the property of being cured by crosslinking and polymerization reaction of the accompanying polymerizable compound.

  The recording head 3 has a certain region (landing possible region) on the substrate P during scanning of moving from one end of the substrate P to the other end of the substrate P in the Y direction in FIG. Ink is ejected as ink droplets, and the ink droplets are landed on the landable area. It is preferable that the image is completed by repeating the process of irradiating light for each ejection multiple times by ejecting ink.

  The above-described scanning is performed as many times as necessary, and after ink is ejected toward one landable area, the substrate P is appropriately moved from the front to the back in FIG. 1 by the conveying means, and scanning by the head scanning means is performed again. In the meantime, the recording head 3 discharges ink to the next landable area adjacent to the landable area in FIG.

  By repeating the above operation and ejecting ink from the recording head 3 in conjunction with the head scanning means and the conveying means, an image made up of an aggregate of ink droplets is formed on the substrate P.

  The irradiation unit 4 includes an ultraviolet lamp that emits ultraviolet light in a specific wavelength region with stable exposure energy and a filter that transmits ultraviolet light of a specific wavelength. Here, as the ultraviolet lamp, a mercury lamp, a metal halide lamp, an excimer laser, an ultraviolet laser, a hot cathode tube, a cold cathode tube, a black light, an LED (light emitting diode), etc. can be applied. Cathode tubes, mercury lamps or black lights are preferred.

  The irradiating means 4 has the same shape as the largest one that can be set by the recording apparatus (inkjet printer) 1 among the landable areas where the recording head 3 ejects ink by one scan driven by the head scanning means. It has a shape larger than the possible area.

  The irradiating means 4 is fixed on both sides of the head carriage 2 so as to be substantially parallel to the substrate P.

  As a means for adjusting the illuminance of the ink discharge portion, not only the entire recording head 3 is shielded, but also the distance h1 between the irradiation means 4 and the substrate P, the ink discharge portion 31 of the recording head 3 and the base. It is effective to reduce both the distance h2 from the material P (2 mm or less, preferably 1.5 mm or less) or to increase the distance d between the recording head 3 and the irradiation means 4 (d is increased). Further, it is more preferable that a bellows structure 7 is provided between the recording head 3 and the irradiation means 4.

  Here, the wavelength of the ultraviolet rays irradiated by the irradiation unit 4 can be changed as appropriate by exchanging the ultraviolet lamp or the filter provided in the irradiation unit 4.

  When a cationically polymerizable ink is used in the recording method of the present invention, it is preferable to heat-treat the ink jet ink that has landed on the substrate during light irradiation or after light irradiation.

  As the heating means, a method using a heat plate that guides the conveyance of the substrate and generates heat is preferable. Heat is transmitted to the substrate from the heat plate that conveys and guides the substrate, and the ink jet ink landed by this heat is heated. Is done.

  It is also preferable that the heating means is a hot air blowing means for blowing hot air onto the ink jet ink landed on the substrate.

  The heating temperature of the ink jet ink landed on the substrate is preferably 30 to 60 ° C. If the heating temperature is less than 30 ° C., the landed ink is not cured depending on the environmental humidity, and the image quality is impaired. If the temperature exceeds 60 ° C., the film base material shrinks and wrinkles, which is not preferable.

  EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto.

[Synthesis of Photopolymerization Initiator 1]
20 ml of tin tetrachloride was added to a 500 ml three-necked flask, and 48.1 g of anisole was added with stirring. This solution was ice-cooled and 6.75 g of thionyl chloride was added dropwise while maintaining a temperature range of 10 to 15 ° C. At this time, the reaction became dark brown. Stirring was continued for 4 hours at room temperature, and 70 ml of water was added to the reaction solution. The oil layer was then removed and washed 3 times with diethyl ether. To this was added 18.4 g of KPF ₆ dissolved in 100 g of water, and the mixture was allowed to stand overnight. The oil layer was taken out again, washed with diethyl ether and dried under reduced pressure to obtain 32.3 g (yield 59.6%) of white powder of tris (4-methoxyphenyl) sulfonium hexafluorophosphate. In addition, this photoinitiator 1 identified the structure by the infrared absorption spectrum, the elemental analysis, and NMR.

[Synthesis of Photoinitiator 2]
The photopolymerization initiator 1 was dissolved in methylene chloride, passed through an alumina column, and mixed and stirred with an aqueous sodium carbonate solution. Next, the methylene chloride layer was taken out, dehydrated with magnesium sulfate, methylene chloride was removed and dried, recrystallized with methanol, and a white powder of photopolymerization initiator 2 obtained by subjecting photopolymerization initiator 1 to base treatment was obtained. It was.

[Preparation of inks 1 to 6]
Inks 1 to 6 having the compositions shown in Table 1 below were prepared by a conventional method. After dispersing the pigment, the dispersant and the polymerizable compound for 3 hours with a bead mill, a photopolymerization initiator, a surfactant and an amine compound were added, mixed and filtered to obtain an ink. All of these inks are cationically polymerizable.

  The compounds used are shown below.

PR122: Pigment Red 122 (a pigment obtained by acid-treating the surface of a quinacridone pigment, an acid value of 4.7, an amine value of 6.1)
PB821: Ajimoto PB821 (dispersant) manufactured by Ajinomoto Fine Techno Co., Ltd.
ED-251: Enomoto Kasei Co., Ltd. ED-251 (dispersant)
S32000: Solsperse 32000 (dispersant) manufactured by Avecia
OXT221: Aron Oxetane OXT221 (polymerizable compound) manufactured by Toa Gosei Co., Ltd.
C2021P: Daicel Corporation Celoxide 2021P (polymerizable compound)
TAS1: Photopolymerization initiator 1
TAS2: Photoinitiator 2
F1405: Dai Nippon Ink Co., Ltd. MegaFuck F1405 (surfactant)
TIPA: Triisopropanolamine (amine compound)
As a result of measuring the acid value and amine value of the three kinds of dispersants, it was as follows.

Type of dispersant Acid value Amine value PB821 30.4 10.2
ED-251 19.0 13.4
S32000 4.8 27.1

[Measurement and evaluation of ink characteristic values]
Ink characteristic values were measured and evaluated by the following methods. The results are shown in Table 2.

(Measurement of moisture content)
Immediately after the ink was prepared, the water content was measured by the Karl Fischer method (automatic moisture measuring device AQV-2000 manufactured by Hiranuma Sangyo Co., Ltd.).

(Measurement of total amount of impurities)
The total amount of impurities (ppm) of the ink prepared above was measured using atomic absorption analysis, mass spectrometry, and titration analysis. As impurities, Sn, Cl, S, K, P, Mg, P, B, Na, F, HCl, HPF ₆ , phosphoric acid, and methanesulfonic acid were detected.

(Evaluation of storage stability)
The prepared ink is sealed in a glass bottle, subjected to accelerated storage for 1 week at 70 ° C. in a light-shielding container, and combined with the sample before storage, the average particle size, viscosity, and satellite generation rate by the following methods. Further, the rate of decrease in intermittent discharge rate, continuous discharge property and curing rate were evaluated.

<Average particle size>
Measured with Malvern Photon Correlation Zetasizer 1000HS immediately after the ink was diluted to about 0.5% by mass with a pigment concentration and then subjected to ultrasonic treatment with an ultrasonic cleaner under conditions of 100 W and 28 kHz for 5 minutes. ZAve. Is the value of

<viscosity>
The viscosity was measured using a rheometer MCR300 (manufactured by Physica) under the conditions of 55 ° C. and Shear Rate = 1000 (1 / s).

<Satellite generation speed>
Using a piezo-type head that can express 6 gradations by continuously ejecting 5 drops of ink droplets 4 pl heated to 55 ° C. per cycle at a drive frequency of 4 kHz, while adjusting the drive voltage and changing the droplet speed The minimum droplet velocity (m / s) at which satellites began to occur following the main drop was measured.

<Intermittent discharge speed reduction rate>
Using a piezo-type head capable of expressing 6 gradations by continuously ejecting 5 drops of ink droplets 4 pl heated to 55 ° C per cycle at a driving frequency of 4 kHz, it is driven so that the droplet velocity is 6 m / s. The voltage was adjusted, and the first drop speed reduction rate (%) immediately after stopping for 5 seconds after continuous discharge was measured.

<Continuous discharge>
Using a piezo-type head capable of expressing 6 gradations by continuously ejecting 5 drops of ink droplets 4 pl heated to 55 ° C per cycle at a driving frequency of 4 kHz, it is driven so that the droplet velocity is 6 m / s. The voltage was adjusted and discharged continuously for 30 minutes, the discharge state from the nozzles of each recording head was visually observed, and the continuous discharge property was evaluated according to the following criteria.

○: No nozzle missing even after 30 minutes of continuous discharge △: No nozzle missing due to continuous 30 minute discharge, but slightly satellite generated ×: No nozzle missing at several nozzles over 30 minutes of continuous ejection <Curing speed>
Under an environment of 30 ° C. and 60% RH, the piezo head speed is changed in units of 50 mm / s from 50 to 800 mm / s, and each ink droplet 4 pl heated to 55 ° C. is 5 dots (drop). After discharging with the H bulb (1.5 kW light source) and curing with each finger, the surface of each dot is rubbed with a finger to determine the maximum piezo head speed at which the cured ink droplets will not peel off the substrate. (Mm / s).

  As is clear from the results in Table 2, the ink of the present invention in which the total content of cationic impurities, metal impurities, and strongly acidic substances is 500 ppm or less is higher than the comparative example in terms of storage stability, curability and ejection stability. It turns out that it is excellent in.

It is a front view which shows the structure of the principal part of the recording device used for this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Recording device 2 Head carriage 3 Recording head 4 Irradiation means 5 Platen part 6 Guide member 7 Bellows structure P Recording medium

Claims (4)

An ultraviolet curable inkjet ink containing at least a pigment, a dispersant having a larger acid value than the amine value, a polymerizable compound, and a photopolymerization initiator, and the total amount of cationic impurities, metal impurities, and strongly acidic substances is 500 ppm or less An ink-jet ink characterized by The cation type impurity and the metal impurity are at least one element selected from B, F, Na, Mg, Al, P, S, Cl, K, Ca, Cu, Br, Ag, Sn, Sb, As and W. The inkjet ink according to claim 1, wherein the inkjet ink contains impurities. The inkjet ink according to claim 1, which is cationically polymerizable and contains an amine compound. The inkjet ink according to any one of claims 1 to 3, wherein the water content is 1 to 3% by mass.

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