Flexographic printing ink composition

Abstract

To enhance storage stability of a water-based flexographic printing ink composition, while improving physical properties of a coating film thereof, including durability such as water resistance.SOLUTION: A flexographic printing ink composition containing a pigment, an acrylic resin, a styrene-maleic acid resin, and an aqueous medium, wherein a content of the styrene-maleic acid resin is 2 mass% or more in the ink composition. The flexographic printing ink composition preferably further contains a urea-based compound, and a content of the urea-based compound is 15 mass% or less in the ink composition.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a flexographic printing ink composition. [Background technology]

[0002] Flexographic printing is a type of letterpress printing method that generally uses rubber or synthetic resin as the plate (flexographic plate) material. An anilox roll (made of metal) is often used to supply ink to the plate, and it is widely used for printing on cardboard, packaging, cloth, etc. In recent years, flexographic printing techniques using low-coat aqueous inks have also been proposed. Flexographic printing, particularly flexographic printing using low-coat aqueous inks, requires the contradictory properties of suppressing the drying of the ink composition on the plate while accelerating the drying of the ink composition on the printed material, such as paper. If the ink composition dries on the plate, it becomes difficult to transfer the ink to the substrate and the plate becomes difficult to clean. Furthermore, if the ink composition does not dry easily on the printed material, problems such as color transfer of the coating film on the printed material are more likely to occur.

[0003] It is known that aqueous acrylic resins and urea are blended together to produce aqueous flexographic printing inks (see Patent Documents 1 and 2). Patent Document 1 discloses an aqueous flexographic printing ink blended with an alkali-soluble resin, glycols, and urea, which is said to improve printability and durability of the coating film. Patent Document 2 discloses an aqueous flexographic printing ink blended with a water-soluble acrylic resin and an aqueous acrylic resin emulsion, and urea, which is said to improve the suitability of the printed layer, such as flexographic printability and bending resistance of the coating film. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 55-164265 [Patent Document 2] Japanese Patent Application Publication No. 2023-110818 Summary of the Invention [Problem to be solved by the invention]

[0005] As described above, it is known that various physical properties (including the physical properties of the coating film) can be improved by blending urea with various resins into an aqueous flexographic printing ink composition. However, conventional aqueous flexographic printing ink compositions have sometimes been insufficient in terms of storage stability as an ink, particularly viscosity stability.

[0006] Therefore, an object of the present invention is to enhance the storage stability of an aqueous flexographic printing ink composition and to improve the physical properties of the coating film (including durability such as water resistance), and further to preferably inhibit drying of the ink composition on the plate while promoting drying of the ink composition (coating film) on a printed material such as paper. [Means for solving the problem]

[0007] That is, the present invention relates to the following flexographic printing ink composition. [1] A flexographic printing ink composition containing a pigment, an acrylic resin, a styrene-maleic acid resin, and an aqueous medium, A flexographic printing ink composition, wherein the content of the styrene-maleic acid resin in the ink composition is 2% by mass or more.

[0008] [2] The flexographic printing ink composition according to [1] above, further comprising a urea compound, wherein the content of the urea compound in the ink composition is 0.1 to 15 mass % or less. [3] The flexographic printing ink composition according to [1] or [2] above, wherein the content of the styrene-maleic acid resin in the ink composition is 3 to 12 mass %. [4] The flexographic printing ink composition according to any one of [1] to [3] above, wherein the urea compound is urea. [5] The flexographic printing ink composition according to any one of [1] to [4], wherein the styrene-maleic acid resin is a styrene-maleic acid half ester resin. [6] The flexographic printing ink composition according to any one of [1] to [5] above, wherein the styrene-maleic acid resin has an acid value of 100 to 300 mgKOH / g. [7] The weight average molecular weight of the styrene-maleic acid resin is 4 × 10 3 ~2×10 5 The flexographic printing ink composition according to any one of [1] to [6] above, [8] The flexographic printing ink composition according to any one of [1] to [7] above, wherein the acrylic resin is an alkali-soluble resin. [9] The flexographic printing ink composition according to any one of [1] to [8], wherein the mass ratio of the acrylic resin to the styrene-maleic acid resin (acrylic resin / styrene-maleic acid resin) is 1 / 1 to 1 / 5.

[10] The flexographic printing ink composition according to any one of [1] to [9], which is substantially free of additives that have a boiling point exceeding 130°C and are liquid at room temperature. [Effects of the Invention]

[0009] The flexographic printing ink composition of the present invention has high storage stability and can be applied to various flexographic printing applications. Preferably, the flexographic printing ink composition of the present invention can simultaneously achieve the contradictory properties of inhibiting drying of the ink composition on the plate while accelerating drying of the ink composition (coating film) on a printed material such as paper. DETAILED DESCRIPTION OF THE INVENTION

[0010] [1. Flexographic printing ink composition] The flexographic printing ink composition of the present invention contains a pigment, an acrylic resin, a styrene-maleic acid resin, and an aqueous medium, and preferably further contains a urea compound, and may further contain other optional additives.

[0011] [1-1. Pigments] The flexographic printing ink composition of the present invention contains a pigment. The pigment is a component added to the ink composition to impart coloring power, hiding power, etc., and examples thereof include color pigments, white pigments, metal powders, etc. Examples of such pigments include, without particular limitation, the following organic and / or inorganic pigments that have conventionally been used in ink compositions.

[0012] Examples of pigments include dye lake pigments, various organic pigments such as azo, benzimidazolone, phthalocyanine, quinacridone, anthraquinone, dioxazine, indigo, thioindigo, perylene, perinone, diketopyrrolopyrrole, isoindolinone, nitro, nitroso, flavanthrone, quinophthalone, pyranthrone, and indanthrone, as well as various inorganic pigments.

[0013] Examples of yellow pigments include CI Pigment Yellow 1, 2, 3, 12, 13, 14, 16, 17, 42, 73, 74, 75, 81, 83, 87, 93, 95, 97, 98, 108, 109, 114, 120, 128, 129, 138, 139, 150, 151, 155, 166, 180, 184, 185, and 213.

[0014] Examples of magenta pigments include CI Pigment Red 5, 7, 12, 22, 38, 48:1, 48:2, 48:4, 49:1, 53:1, 57, 57:1, 63:1, 101, 102, 112, 122, 123, 144, 146, 149, 168, 177, 178, 179, 180, 184, 185, 190, 202, 209, 224, 242, 254, 255, 270, and CI Pigment Violet 19.

[0015] Examples of cyan pigments include CI Pigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 18, 22, 27, 29, and 60.

[0016] Examples of black pigments include carbon black (CI Pigment Black 7).

[0017] Examples of white pigments include titanium oxide and aluminum oxide, which may be surface-treated with various materials such as alumina and silica.

[0018] The content of the pigment in the flexographic printing ink composition varies depending on the type of pigment and the desired degree of coloring or hiding, and is not particularly limited, but may be, for example, 5.0 to 35.0 mass%, and preferably about 8.0 to 30.0 mass%. When preparing a colored ink composition, it is possible to use a pigment or dye of another color in combination as a complementary color, or to add an ink composition of another color.

[0019] [1-2. Acrylic resin] The flexographic printing ink composition of the present invention contains an acrylic resin. The acrylic resin may be a water-soluble acrylic resin (dissolved in the ink composition) or an acrylic resin emulsion (dispersed in the ink composition). In the flexographic printing ink composition, the acrylic resin can also function as a pigment dispersant. The acrylic resin is preferably an alkali-soluble resin.

[0020] The acrylic resin is preferably a copolymer resin containing styrene, acrylic acid, and an acrylic acid ester as monomer units. The weight-average molecular weight of the acrylic resin is preferably 1,500 to 150,000, more preferably 5,000 to 100,000, and even more preferably 10,000 to 50,000. The acid value of the acrylic resin is preferably 100 mgKOH / g or more, more preferably 150 mgKOH / g or more, and even more preferably 200 mgKOH / g or more; on the other hand, it is preferably 300 mgKOH / g or less, and more preferably 250 mgKOH / g or less.

[0021] The solid content of the acrylic resin in the flexographic printing ink composition may be set to any amount that allows the pigment to be dispersed appropriately, and is preferably 1 to 10 mass %, more preferably 2 to 5 mass %.

[0022] [1-3. Styrene-maleic acid resin] The flexographic printing ink composition of the present invention contains a styrene-maleic acid resin. The styrene-maleic acid resin is preferably dissolved in the ink composition and can be formulated as a varnish. The styrene-maleic acid resin may be a copolymer of a styrene-based monomer and maleic acid, a copolymer of a styrene-based monomer and maleic anhydride, a copolymer of a styrene-based monomer and a maleic acid half ester, or a copolymer of a styrene-based monomer and a maleic acid diester; however, a copolymer of a styrene-based monomer and a maleic acid half ester is preferred.

[0023] The copolymer of a styrene monomer and a maleic acid half ester may be an esterification reaction product of a copolymer of a styrene monomer and maleic anhydride with an alcohol, or a copolymer of a styrene monomer with an esterification reaction product of maleic anhydride and an alcohol. In this copolymer, some or all of the rings consisting of the acid anhydride groups contained therein are ring-opened, and only one of the two carboxyl groups generated by ring-opening of the acid anhydride groups is esterified (half-esterified) (see the formula below). [ka]

[0024] The styrene-based monomer constituting the styrene-maleic acid-based resin (preferably, styrene-maleic acid half ester resin) may be styrene, α-methylstyrene, vinyltoluene, or a styrene derivative thereof.

[0025] The alcohol used to esterify the acid anhydride group of maleic acid is preferably a monohydric alcohol, including aliphatic alcohols having 1 to 8 carbon atoms, cyclic (e.g., alicyclic) alcohols having 1 to 8 carbon atoms, and aromatic alcohols having 6 to 10 carbon atoms; cyclic alcohols or aliphatic alcohols having 1 to 8 carbon atoms are preferred. Among the above alcohols, 1-butanol, ethyl carbitol, 2-butoxyethanol, cyclohexanol, n-propanol, and isopropanol are particularly preferred. These alcohols may be used alone or in combination of two or more.

[0026] The monomer components constituting the styrene-maleic acid resin (preferably, a styrene-maleic acid half ester resin) may contain other monomer components in addition to the styrene-based monomer and maleic acid (preferably, a maleic acid half ester). The other monomer components are preferably (meth)acrylic acid or its esters; examples of (meth)acrylic acid esters include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, and 2-hydroxystearyl (meth)acrylate. When the styrene-maleic acid resin contains acrylic acid or its esters as a monomer unit, the styrene-maleic acid resin is distinguished as a substance different from the acrylic resin of the present invention.

[0027] The styrene-maleic acid resin (preferably, styrene-maleic acid half ester resin) has an acid value of 100 mgKOH / g or more, preferably 150 mgKOH / g or more; and 300 mgKOH / g or less, preferably 280 mgKOH / g or less, more preferably 200 mgKOH / g or less.

[0028] The weight average molecular weight of the styrene-maleic acid resin (preferably, styrene-maleic acid half ester resin) is not particularly limited, but is, for example, 4×10 3 It is preferable that the value is 6×10 or more. 3 More preferably, it is 2 x 10 or more. 5 It is preferable that:

[0029] The styrene-maleic acid resin (preferably, styrene-maleic acid half ester resin) preferably has a glass transition temperature of 30°C or higher, more preferably 50°C or higher; and preferably 120°C or lower, more preferably 100°C or lower.

[0030] The content of the styrene-maleic acid resin in the ink composition may be 2% by mass or more, preferably 3% by mass or more, and more preferably 5% by mass or more; on the other hand, it is preferably 12% by mass or less, and more preferably 10% by mass or less.

[0031] The mass ratio of the acrylic resin to the styrene-maleic acid resin in the ink composition (acrylic resin / styrene-maleic acid resin) is preferably in the range of 1 / 1 to 1 / 5, more preferably in the range of 1 / 2 to 1 / 5, and even more preferably in the range of 1 / 1 to 1 / 4.

[0032] In the ink composition of the present invention, the styrene-maleic acid resin can improve the storage stability of the ink composition. For example, it can suppress an increase in viscosity when the ink composition is left in a storage environment. The stabilization mechanism is not particularly limited, but it is thought that because styrene-maleic acid resins are often formed by alternating copolymerization of styrene-based monomer units and maleic acid-based monomer units, the styrene skeleton increases the affinity and adsorption of the styrene-maleic acid resin to pigments, and the maleic acid skeleton increases the dispersibility of the styrene-maleic acid resin in water, resulting in a resin that is excellent in both properties.

[0033] [1-4. Urea compounds] The flexographic printing ink composition of the present invention preferably contains a urea compound. The urea compound may be any compound having a urea skeleton, and preferably has a structure represented by general formula (I). [ka]

[0034] R in general formula (I) 1 represents an oxygen atom or a sulfur atom, preferably an oxygen atom.

[0035] R in general formula (I) 2 and R 3 each independently represents a hydrogen atom, an optionally substituted alkyl group having 1 to 4 carbon atoms, or an optionally substituted aryl group having 6 to 10 carbon atoms; or R 2 and R 3 may bond with each other to form an alkylene group having 1 to 3 carbon atoms. The alkyl group having 1 to 4 carbon atoms may be a methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group, sec-butyl group, tert-butyl group, etc., with methyl group and ethyl group being preferred. The aryl group having 6 to 10 carbon atoms may be a phenyl group, naphthyl group, etc., with phenyl group being preferred. The substituent that the alkyl group or aryl group may have may be a hydroxy group, amino group, carboxyl group, halogen atom, cyano group, thio group, silyl group, nitro group, etc., with hydroxy group being preferred. The aryl group may have an alkyl group having 1 to 3 carbon atoms as a substituent.

[0036] Specific examples of the urea compound include urea, 2-hydroxyethylurea, thiourea, 1,3-dimethylurea, urea derivatives such as ethyleneurea synthesized by reacting ammonium carbamate with a titanium catalyst, and thiourea derivatives such as N-phenylthiourea; preferably, urea, 2-hydroxyethylurea, and 1,3-dimethylurea; more preferably, urea and 1,3-dimethylurea; and even more preferably, urea. The urea compound may be used alone or in combination of two or more.

[0037] It is sometimes preferable that the urea compound has a certain level of solubility in water, preferably 20% by mass or more, and more preferably 30% by mass or more in water at 25°C.

[0038] The content of the urea compound in the flexographic printing ink composition is 15% by mass or less, preferably 12% by mass or less, more preferably 10% by mass or less, and even more preferably 8% by mass or less; while it may be 0% by mass (no urea compound), it is preferably 0.1% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more. Furthermore, the mass ratio of the urea compound to the resin (total of acrylic resin and styrene-maleic acid resin) in the flexographic printing ink composition (urea compound / resin) is preferably 1 / 0.1 or less, more preferably 1 / 400 to 1 / 0.5, even more preferably 1 / 200 to 1 / 1, and even more preferably 1 / 50 to 1 / 1.1. The urea compound may further improve the storage stability of the flexographic printing ink composition.

[0039] Furthermore, urea compounds may be able to reduce the drying speed of flexographic printing ink compositions on plates. In flexographic printing, a flexographic printing ink composition is supplied to an anilox roll, transferred to a flexographic plate, and then transferred to a substrate; however, it is required that the ink composition does not dry before transfer. This is because if the ink composition dries, it becomes difficult to transfer or transfer. By reducing the drying speed of the flexographic printing ink composition, urea compounds can prevent the ink composition from drying on the anilox roll or flexographic plate.

[0040] Furthermore, urea compounds do not inhibit the drying properties of the coating film on the printed matter of the flexographic printing ink composition. Generally, when attempting to reduce the drying properties of the ink on the flexographic plate, the drying properties of the coating film on the printed matter are also inhibited (the coating becomes difficult to dry), which can result in a decrease in the quality of the printed matter. In contrast, urea compounds can reduce the drying properties of the ink on the plate without inhibiting the drying properties of the coating film.

[0041] [1-5.Aqueous medium] The flexographic printing ink composition of the present invention contains an aqueous medium (aqueous liquid medium). The aqueous medium is preferably a solvent containing water as a main component, and may contain any water-soluble organic solvent. The water-soluble organic solvent may be any liquid that is miscible with water, but in order to improve the drying properties of the coating film, it is preferable that the boiling point be 200°C or less, and more preferably 150°C or less. The proportion of water in the aqueous medium is preferably 60 to 98% by mass, and more preferably 70 to 95% by mass.

[0042] The water-soluble organic solvent may be an alcohol-based solvent, a glycol-based solvent, etc. Examples of the alcohol-based solvent include methanol, ethanol, propanol, isopropanol, isobutanol, normal butanol, tertiary butanol, hexanol, octanol, decanol, etc. Examples of glycol-based solvents include ethylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, ethylene glycol monooctyl ether, diethylene glycol, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, triethylene glycol, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monopropyl ether, triethylene glycol monobutyl ether, propylene glycol, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, dipropylene glycol, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol monopropyl ether, dipropylene glycol monobutyl ether, tripropylene glycol, tripropylene glycol monomethyl ether, tripropylene glycol monoethyl ether, tripropylene glycol monopropyl ether, tripropylene glycol monobutyl ether, dibutyl glycol, etc. The water-soluble organic solvents may be used alone or in combination of two or more.

[0043] The content of the aqueous medium in the flexographic printing ink composition is preferably 80% by mass or less, more preferably 70% by mass or less; on the other hand, it is preferably 30% by mass or more, more preferably 40% by mass or more.

[0044] [1-6. Other additives] The aqueous flexo ink composition of the present invention may contain any other additives as needed. Examples of other additives include dispersants, waxes, extender pigments, leveling agents, antifoaming agents, and coalescents. However, the aqueous flexo ink composition of the present invention is substantially free of high-boiling liquid additives (additives that are liquid at room temperature); specifically, the content of high-boiling liquid additives is less than 0.5% by mass, preferably less than 0.1% by mass, or is completely free of high-boiling liquid additives. High-boiling liquid additives are liquid additives with a boiling point of 200°C or higher, or liquid additives with a boiling point of 300°C or higher. Examples of high-boiling liquid additives include high-boiling liquid amines and high-boiling glycol-based solvents.

[0045] Specifically, various additives may be added, such as a dispersion of wax resin particles (e.g., polyethylene wax) to improve abrasion resistance; extender pigments (e.g., calcium carbonate, kaolin, barium sulfate, aluminum hydroxide, clay, and talc) to improve drying properties and coating film hiding power; inorganic particles and adhesive resins (e.g., acrylic resins and vinyl acetate resins) to impart slip resistance; leveling agents to improve leveling; antifoaming agents to impart defoaming properties; basic compounds (e.g., sodium hydroxide and potassium hydroxide) to impart resolubility; and film-forming aids. However, it may be preferable for the aqueous flexo ink composition of the present invention to be free of additives that are high-boiling liquid components. This is because the addition of high-boiling liquid components may excessively reduce the drying properties of the aqueous flexo ink composition. A high-boiling liquid component is a liquid component with a boiling point at atmospheric pressure of 200°C or higher, preferably above 130°C. Examples of high-boiling liquid components include polyhydric alcohols (e.g., diethylene glycol) and organic amines (e.g., diethanolamine).

[0046] [1-7. Viscosity of Flexographic Printing Ink Composition] The flexographic printing ink composition of the present invention may have a viscosity sufficient for flexographic printing; for example, the viscosity is preferably 1 to 500 mPa·s, and more preferably 5 to 300 mPa·s. The flexographic printing ink composition of the present invention also has high storage stability, meaning that the viscosity is less likely to change (thicken) even when left in a certain storage environment.

[0047] [2. Method for producing flexographic printing ink composition] The flexographic printing ink composition of the present invention can be produced by mixing the various components; however, it is preferable to prepare a base ink containing a pigment and then mix the base ink with other components. The base ink can be obtained by mixing the pigment, some or all of the acrylic resin, water, and optionally some of the styrene-maleic acid resin. In other words, it is preferable that at least some of the styrene-maleic acid resin is not used to prepare the base ink, but is instead mixed with the base ink.

[0048] The components can be mixed using a dispersing machine such as a wet circulation mill, a bead mill, a ball mill, a sand mill, an attritor, a roll mill, a DCP mill, an agitator, a Henschel mixer, a colloid mill, an ultrasonic homogenizer, a high-pressure homogenizer (Microfluidizer, Nanomizer, Ultimizer, Genus PY, DeBEE2000, etc.), or a pearl mill.

[0049] [3. Flexographic printing] The flexographic printing ink composition of the present invention can be printed on a substrate by flexographic printing. Flexographic printing methods include two-roll, doctor, and doctor chamber methods; in all of these methods, the flexographic printing ink composition is supplied to an anilox roll, and excess aqueous flexographic ink is scraped off the surface of the anilox roll with a doctor blade. The ink is then passed through a flexographic printing plate made of resin or the like, and finally printed onto a substrate (object to be printed).

[0050] Examples of printing substrates (printed materials) to be printed with the flexographic printing ink composition of the present invention include plastic films such as polyethylene, polypropylene, polyethylene terephthalate, and nylon, cellophane, paper, and aluminum foil, as well as films and sheets made of composite materials thereof. In the present invention, paper substrates are particularly preferred. Paper substrates are preferably selected from uncoated paper, one-side gloss kraft paper treated on one side, bleached kraft paper, unbleached kraft paper, and the like. [Example]

[0051] A. Preparation of Flexographic Printing Ink Composition The materials used to prepare the flexographic printing ink compositions of the Examples and Comparative Examples shown in Tables 1 and 2 are listed below.

[0052] <Pigments> PR48:3 (CI, Pigment Red 48:3) <Styrene-maleic acid resin and styrene-maleic acid resin varnish> The styrene-maleic acid resin used was a copolymer resin (glass transition temperature 71°C, acid value 170 mgKOH / g, weight average molecular weight 18000) consisting of 30% by mass of a ring-opening reaction product of maleic anhydride and 1-butanol, 30% by mass of a ring-opening reaction product of maleic anhydride and ethyl carbitol, and 40% by mass of styrene. 30 parts by mass of the styrene-maleic acid resin, 8 parts by mass of 18% aqueous ammonia, and 62 parts by mass of water were heated and stirred to obtain a styrene-maleic acid resin varnish (solid content 30% by mass). <Urea compounds> urea <Aqueous medium> Methoxypropanol water

[0053] <Styrene acrylic resin 1 and styrene acrylic resin varnish 1> A copolymer resin (glass transition temperature 107°C, acid value 210 mgKOH / g, weight average molecular weight 10,000) consisting of 25% by mass of acrylic acid, 10% by mass of lauryl acrylate, and 65% by mass of styrene was used as the styrene acrylic resin 1. 25 parts by mass of the styrene acrylic resin 1, 5 parts by mass of 18% aqueous ammonia, and 70 parts by mass of water were heated and stirred to form a varnish, to obtain styrene acrylic resin varnish 1 (solid content 25% by mass). <Styrene-acrylic acid resin 2 and styrene-acrylic acid resin varnish 2> A styrene acrylic resin (weight average molecular weight 15,000 to 17,000, acid value 210 to 225 mgKOH / g, glass transition temperature 130°C) with the trade name "Indurez SR-200 (INDULOR)" was used as the styrene acrylic resin 2. 25 parts by mass of the styrene acrylic resin 2, 7.4 parts by mass of 18% aqueous ammonia, and 67.6 parts by mass of water were heated and stirred to form a varnish, thereby obtaining a styrene acrylic resin varnish 2 (solid content 25% by mass).

[0054] Preparation of base ink: A mixture of 45 parts by mass of pigment (PR48:3), 20 parts by mass of styrene acrylic resin varnish 1, 10 parts by mass of styrene maleic acid resin varnish, and 25 parts by mass of water was kneaded and dispersed in a bead mill to obtain a base ink.

[0055] Flexographic printing ink compositions of the examples and comparative examples were obtained according to the formulations shown in Tables 1 and 2.

[0056] [B. Evaluation of Flexographic Printing Ink Compositions and Printed Matter] The flexographic printing ink compositions obtained in each of the Examples and Comparative Examples were evaluated for the following points. The results are shown in Tables 1 and 2.

[0057] [B-1. Storage stability of ink composition (thickening rate)] The viscosity of the ink compositions of each example and comparative example at 25°C was measured using a Zahn Cup No. 4 manufactured by Rigaud. A Zahn cup is a container with an orifice at the bottom, and the time it takes for a sample (here, the ink composition) to flow down through the orifice at the bottom was measured, and the flow time (number of seconds) was measured. The viscosity (number of seconds) of the ink composition at 25°C immediately after production was taken as the initial viscosity. Each ink composition was then placed in a glass bottle and stored at an ambient temperature of 25°C for 3 days. The viscosity (number of seconds) of each ink composition after storage was measured in the same manner as the initial viscosity (viscosity after 3 days). The rate of viscosity change (thickness increase rate) from the initial viscosity to the viscosity after 3 days was calculated and used as an evaluation index for storage stability.

[0058] [B-2. Water resistance of printed matter] The inks of the examples and comparative examples were printed on a paper sheet with a basis weight of 170 g / m 2 The color was applied to a C5 white liner using a 200-line hand proofer. The resulting applied color was conditioned for 24 hours at 23°C and 50% humidity. The applied color was placed as a test piece in a Gakushin-type rub fastness tester, with a white cloth placed on the test piece. Five drops of water were placed on the white cloth and rubbed back and forth once with a 200g load. The color transfer to the white cloth was evaluated using the JIS L 0805 staining gray scale. 5: Grayscale rating of 4 or higher 4: Grayscale rating of 3 to 3-4 3: Grayscale rating of 2 to 2-3 2: Grayscale rating is 1-2 level 1: Grayscale evaluation is equivalent to level 1

[0059] [Table 1]

[0060] [Table 2]

[0061] As shown in Comparative Examples 1 to 4, the ink compositions that did not contain a predetermined amount of styrene-maleic acid resin showed a significant increase in viscosity, and the storage stability was poorly evaluated.

[0062] In contrast, ink compositions containing a predetermined amount of styrene-maleic acid resin exhibit high storage stability without increasing in viscosity during storage, as shown in Examples 1 to 5. Furthermore, a comparison between Example 1 and Examples 2 to 5 reveals that the addition of urea further improves storage stability. [Industrial Applicability]

[0063] The flexographic printing ink composition of the present invention can be applied to the production of various flexographic printed items, for example, printing on various paper substrates such as cardboard and paper bags.

Claims

1. A flexographic printing ink composition containing a pigment, an acrylic resin, a styrene-maleic acid resin, and an aqueous medium, A flexographic printing ink composition, wherein the content of the styrene-maleic acid resin in the ink composition is 2 mass% or more.

2. 2. The flexographic printing ink composition according to claim 1, further comprising a urea compound, and a content of the urea compound in the ink composition is 0.1 to 15% by mass.

3. 3. The flexographic printing ink composition according to claim 1, wherein the content of the styrene-maleic acid resin in the ink composition is 3 to 12% by mass.

4. 3. The flexographic printing ink composition according to claim 1, wherein the urea compound is urea.

5. 3. The flexographic printing ink composition according to claim 1, wherein the styrene-maleic acid resin is a styrene-maleic acid half ester resin.

6. 3. The flexographic printing ink composition according to claim 1, wherein the styrene-maleic acid resin has an acid value of 100 to 300 mgKOH / g.

7. The weight average molecular weight of the styrene-maleic acid resin is 4×10 ３ ~2 x 10 ５ 3. The flexographic printing ink composition according to claim 1 or 2,

8. 3. The flexographic printing ink composition according to claim 1, wherein the acrylic resin is an alkali-soluble resin.

9. 3. The flexographic printing ink composition according to claim 1, wherein a mass ratio of the acrylic resin to the styrene-maleic acid resin (acrylic resin / styrene-maleic acid resin) is 1 / 1 to 1 / 5.

10. 3. The flexographic printing ink composition according to claim 1, wherein the flexographic printing ink composition is substantially free of additives that are liquid at room temperature and have a boiling point exceeding 130°C.

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