WO2025197697A1 - Ink composition for metal printing - Google Patents

Abstract

[Problem] To provide a white ink composition for metal printing, said ink composition enabling pre-mixing to be finished in a shorter time and having sufficient coating film physical properties such as impact resistance. [Solution] An ink composition for metal printing that may be used comprises a coloring pigment, a resin, a solvent, silica and calcium carbonate, and is characterized in that the coloring pigment contains titanium oxide, the content of the silica is 5 mass% or less relative to the entire composition, and the total content of the silica and the calcium carbonate is 8 mass% or less relative to the entire composition.

Description

Metal printing ink composition

The present invention relates to an ink composition for metal printing.

For printing on the exterior surfaces of metal materials, such as zinc- or tin-plated iron sheets, aluminum sheets, or metal cans made from these metal materials, metal printing ink compositions are used, the main vehicle components of which are binder resins such as alkyd resins, polyester resins, and epoxy resins, and organic solvents such as mineral oils or higher alcohols.

In addition, these printed surfaces are typically coated with an overprint varnish to improve the ink film's adhesion, bending resistance, impact resistance, abrasion resistance, etc. These overprint varnishes are commonly solvent-based, consisting of binder resins such as alkyd resin, polyester resin, acrylic resin, and epoxy resin, hardeners such as melamine resin and benzoguanamine resin, and organic solvents such as mineral oil and cellosolve-based solvents.

When printing on the exterior metal surface, ink is printed using an offset printing machine, dry offset printing machine, etc., and then an overprint varnish is applied wet-on-wet onto the ink coating using a coater, etc., and then baked at 150-280°C.

Metal printing is almost always performed using a white ink composition in addition to the four process colors (yellow, magenta, cyan, and black). This is because the printing target for metal printing is metallic, not white like paper, and therefore a white ink composition must be used to express the white color. Furthermore, to enhance the color development of full-color printing using process colors, the entire printing area may be printed with a white ink composition prior to the process color printing. This white ink composition is required to have high hiding power to hide the color of the base, and therefore contains a high concentration of titanium oxide, a white pigment. In fact, the white ink compositions described in Patent Documents 1 to 3 contain high concentrations of titanium oxide, and the invention described in Patent Document 3 is said to use small-particle silica in addition to titanium oxide to further enhance hiding power. Thus, white ink compositions tend to contain a high content of solids, such as color pigments and extender pigments, to ensure high hiding power.

Japanese Patent Application Publication No. 6-279722 JP 2011-26404 A Japanese Patent Application Laid-Open No. 2021-91806

As mentioned above, white ink compositions tend to have a high solids content, which can be a factor in deteriorating the physical properties of the coating. A typical solution to this problem is to extend the time of the dispersion process (a milling process using a roll mill, etc.) to improve the dispersibility of the solids in the ink composition. However, extending the time of the dispersion process increases production lead times and reduces productivity. Furthermore, as a step prior to the dispersion process, premixing is performed, in which solids such as pigments are added to the varnish and mixed, thoroughly blending the solids into the varnish until no powder remains. However, if the solids content is high, this premixing process also takes a long time, which also reduces productivity.

The present invention was made in light of the above circumstances, and aims to provide a white ink composition for metal printing that can be premixed in a shorter time and has sufficient coating properties such as impact resistance.

As a result of extensive research into resolving the above-mentioned problems, the inventors discovered that the above-mentioned problems could be solved by adding calcium carbonate to silica as an extender pigment in a white ink composition for metal printing, and further limiting the silica content to 5% by mass or less, and limiting the total content of silica and calcium carbonate to 8% by mass or less, thereby completing the present invention. Specifically, the present invention provides the following:

(1) The present invention is an ink composition for metal printing comprising a color pigment, a resin, a solvent, silica, and calcium carbonate, wherein the color pigment contains titanium oxide, the content of the silica is 5% by mass or less relative to the total composition, and the total content of the silica and the calcium carbonate is 8% by mass or less relative to the total composition.

(2) The present invention also provides an ink composition for metal printing according to item (1), characterized in that the solvent has a solubility parameter (sp value) of less than 10.00 (cal/cm ³ ) ^1/2 and contains at least one compound selected from the group consisting of compounds represented by the following general formula (1):
(In the above general formula (1), each A is independently an alkylene group having 2 to 4 carbon atoms which may have a branch; R is an alkyl group having 1 to 13 carbon atoms which may have a branched and/or cyclic structure; and n is an integer of 2 to 8.)

(3) The present invention also relates to an ink composition for metal printing according to item (2), in which the divalent group represented by AO in general formula (1) is an oxypropylene group.

(4) The present invention also relates to an ink composition for metal printing according to any one of items (1) to (3), in which the silica is surface-hydrophobically treated silica.

(5) The present invention also relates to an ink composition for metal printing according to any one of items (1) to (4), in which the resin contains an alkyd resin.

(6) The present invention also relates to an ink composition for metal printing according to item (5), in which the alkyd resin has a mass average molecular weight of less than 10,000.

(7) The present invention also relates to an ink composition for metal printing according to item (5) or (6), in which the alkyd resin has a pentaerythritol skeleton.

(8) The present invention also relates to an ink composition for metal printing according to any one of items (5) to (7), further comprising a rosin-modified resin as the resin.

(9) The present invention also relates to an ink composition for metal printing according to any one of items (1) to (8), further comprising an alkanolamine.

The present invention provides a white ink composition for metal printing that can be premixed in a shorter time and has sufficient coating properties such as impact resistance.

The following describes one embodiment of the ink composition for metal printing of the present invention. Note that the present invention is not limited to the following embodiment, and can be practiced with appropriate modifications within the scope of the present invention.

The ink composition for metal printing of the present invention (hereinafter referred to as "the ink composition of the present invention") is for metal printing and is preferably applied to printing using the so-called dry offset printing method, which uses a relief plate as the printing plate, or the offset printing method, which uses a lithographic plate as the printing plate, but is also applicable to all printing methods commonly used in metal printing. Furthermore, the printed film formed by baking after printing with the ink composition of the present invention has good impact resistance not only in its original state but also after retort treatment. Therefore, the ink composition of the present invention is not only applicable to general metal printing, but is also preferably applied to printing on metals that require retort treatment, such as two-piece and three-piece cans used for beverages such as coffee and tea, and canned goods.

The ink composition of the present invention is a metal printing ink composition containing a color pigment, a resin, a solvent, silica, and calcium carbonate, and is characterized in that the color pigment contains titanium oxide, the content of the silica is 5% by mass or less based on the total composition, and the total content of the silica and the calcium carbonate is 8% by mass or less based on the total composition. Each component is described below.

[Coloring pigments]
The ink composition of the present invention contains titanium oxide as a color pigment. Examples of titanium oxide that have been used in ink compositions to date are not particularly limited. Examples of such titanium oxide include anatase-type titanium oxide and rutile-type titanium oxide, with rutile-type titanium oxide being preferred from the viewpoint of achieving higher hiding power. The titanium oxide may be surface-treated with a metal oxide such as alumina, silica, or zirconia, and the average particle size thereof may preferably be, for example, about 0.1 μm to 0.5 μm.

The content of titanium oxide in the ink composition is preferably about 10 to 60% by mass, more preferably about 20 to 50% by mass, and even more preferably about 30 to 45% by mass, based on the total composition.

[resin]
The resin used in the ink composition of the present invention may be any resin that has been used in ink compositions for metal printing, without any particular limitation. Among these resins, alkyd resins and rosin-modified resins are preferably used in the ink composition of the present invention. These resins will now be described.

Alkyd resins are condensation polymers of polyhydric alcohols and polybasic acids, and are a type of polyester. However, they can also be prepared by condensation polymerization with animal and vegetable oils and/or their fatty acids. In this process, the animal and vegetable oils undergo transesterification with the polyhydric alcohol to form fatty acids, which are then incorporated into the alkyd resin structure. The proportion of the alkyd resin derived from fatty acids of animal and vegetable oils is called the oil length, and the oil length of the alkyd resin used in the present invention is preferably 20 to 50% by mass. Oil-free alkyd resins that do not contain fatty acid components from animal and vegetable oils may also be used.

Preferably, the alkyd resin used in the present invention has a pentaerythritol skeleton in the molecule. Using an alkyd resin with such a skeleton is preferable because it can improve the impact resistance of the printed film after retort treatment. Such an alkyd resin is prepared using pentaerythritol as the polyhydric alcohol. The alkyd resin used in the present invention may also be prepared using other polyhydric alcohols in addition to pentaerythritol.

An alkyd resin with a pentaerythritol skeleton in its molecule can be obtained, for example, as a condensation polymer of an acid component consisting of a fatty acid and a polybasic acid with a polyhydric alcohol containing at least pentaerythritol. Next, a method for preparing such an alkyd resin will be described.

Fatty acids are obtained by hydrolyzing natural fats and oils such as vegetable oils and animal oils, and because they have one carboxy group, they can form esters with polyhydric alcohols, which will be described later. Introducing such fatty acids into alkyd resins can improve the transferability of ink compositions that use them and increase the proportion of biomass-derived components. From this perspective, it is preferable to use fatty acids in an amount such that the oil length, which is the ratio (mass %) of the mass of the fatty acid portion to the mass of the entire resin, is approximately 20 to 50 mass %. A preferred example of such a fatty acid is coconut oil fatty acid. Various types of fatty acids can be used, and these can be used alone or in combination of two or more.

Polybasic acids are compounds containing multiple carboxyl groups and are components that undergo condensation polymerization with polyhydric alcohols (described below) to achieve high molecular weight. Examples of such polybasic acids include phthalic acid, phthalic anhydride, isophthalic acid, terephthalic acid, adipic acid, trimellitic acid, 1,2-cyclohexanedicarboxylic acid, 1,3-cyclohexenedicarboxylic acid, 1,4-cyclohexenedicarboxylic acid, hexahydrophthalic anhydride, 5-sodiosulfoisophthalic acid, fumaric acid, benzoic acid, tert-butylbenzoic acid, tetrahydrophthalic anhydride, maleic anhydride, succinic acid, succinic anhydride, fumaric acid, sebacic acid, azelaic acid, tetrabromophthalic anhydride, methylhimic anhydride, tetrachlorophthalic anhydride, hexahydrophthalic anhydride, pyromellitic anhydride, trimellitic anhydride, and methylcyclohexenedicarboxylic anhydride. Among these, phthalic acid and phthalic anhydride are preferred. These polybasic acids can be used alone or in combination of two or more.

The polyhydric alcohol forms an ester with the above-mentioned acid component, increasing the molecular weight of these components. Any polyhydric alcohol that has been used in the synthesis of alkyd resins can be used without restriction, including compounds with two or more hydroxyl groups.

Such compounds include, in addition to the above-mentioned pentaerythritol, ethylene glycol, 1,3-propanediol, 1,2-propanediol, 1,4-butanediol, diethylene glycol, triethylene glycol, polyethylene glycol, dipropylene glycol, 1,3-butanediol, neopentyl glycol, spiroglycol, dioxane glycol, adamantanediol, 3-methyl-1,5-pentanediol, methyloctanediol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, 2-methylpropanediol, 1,3-methylpentanediol, 1,3-methylpentanediol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, 2-methylpropanediol, 1,3-methylpentanediol, 1,3-methylpentanediol, 1,4-methylpentanediol, 1,5-methylpentanediol, 1,6-hexanediol, 1,4-cyclohexanedimethanol ...5-methylpentanediol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, 1,3-methylpentanediol, 1,3-methylpentanediol, 1,5-methylpentanediol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, 1,3-methylpentanedi , 5, hexamethylene glycol, octylene glycol, 9-nonanediol, 2,4-diethyl-1,5-pentanediol, ethylene oxide-modified compounds of bifunctional phenols such as bisphenol A, propylene oxide-modified compounds of bifunctional phenols such as bisphenol A, ethylene oxide and propylene oxide copolymer-modified compounds of bisphenol A, ethylene oxide and propylene oxide copolymer-based polyether polyols, polycarbonate diols, adamantane diols, polyether diols, polyester diols, polycaprolactone diols, etc. These can be used alone or in combination of two or more.

To prepare alkyd resins, a reaction vessel containing the acid components and polyhydric alcohol is charged with an inert gas such as nitrogen, and a small amount of a solvent such as xylene is added. The mixture is then heated, and condensation polymerization is carried out while the condensation water is removed by azeotropy. Alternatively, an alkyd resin with a high degree of crosslinking and a tough cured coating can be obtained by using the condensation polymerization reaction of the acid components and polyhydric alcohol as the first step and condensation polymerization with a trifunctional or higher polybasic acid such as trimellitic acid as the second step. The reaction temperature can be approximately 170-250°C, and the reaction time can be approximately 5-25 hours, but is not limited thereto. The completion of the reaction can be determined by monitoring the acid value of the reaction mixture over time. That is, the reaction is considered complete when the decrease in the acid value of the reaction mixture accompanying the condensation polymerization stops. The condensation polymerization reaction can be carried out more quickly by distilling the water produced by the condensation polymerization out of the system or by using a reaction catalyst. Examples of reaction catalysts include tetrabutyl zirconate, monobutyltin oxide, zirconium naphthate, and tetrabutyl titanate.

The mass average molecular weight of the alkyd resin is preferably less than 10,000, more preferably 8,500 or less, and even more preferably 7,000 or less.

The mass average molecular weight of the resin in this invention can be measured by gel permeation chromatography (GPC). As an example, chromatography can be performed using a Waters Acquity APC (Waters) GPC apparatus and columns ACQUITY APC XT 45 1.7 μm 4.6 x 150 mm, ACQUITY APC XT 200 2.5 μm 4.6 x 75 mm, or ACQUITY APC XT 900 2.5 μm 4.6 x 75 mm (Waters), under the following conditions: tetrahydrofuran as the mobile phase, a column temperature of 40°C, a flow rate of 0.8 milliliters/minute, an RI detector, a sample injection concentration of 10 milligrams/5 milliliters, and an injection volume of 10 microliters. The polystyrene-equivalent mass average molecular weight can be calculated.

The content of alkyd resin in the ink composition is preferably 10 to 40% by mass of the entire composition, and more preferably 20 to 40% by mass of the entire composition.

Rosin-modified resins are resins prepared using rosin as one of the raw materials. Rosin contains a mixture of resin acids such as abietic acid, palustric acid, isopimaric acid, and levopimaric acid. These resin acids contain hydrophilic, chemically active carboxyl groups, and some even contain conjugated double bonds. Therefore, a variety of rosin-modified resins have been prepared by combining polyhydric alcohols and polybasic acids and performing condensation polymerization; adding resol, a phenol condensate, to the benzene rings contained in the rosin skeleton; or performing a Diels-Alder reaction with dienophiles such as maleic anhydride or maleic acid to add a maleic acid or maleic anhydride skeleton. Various types of rosin-modified resins are commercially available, and they can also be purchased and used.

Examples of rosin-modified resins include rosin ester resin, maleated rosin, fumarated rosin resin, rosin-modified maleic acid resin, rosin-modified fumaric acid resin, rosin-modified phenolic resin, rosin-modified alkyd resin, and rosin-modified polyester resin. While any of these rosin-modified resins may be used in the present invention, rosin ester resins are preferred.

The rosin-modified resin used in the present invention preferably has a hydroxyl value of 10 mgKOH/g or more. By including a rosin-modified resin with such a high hydroxyl value in the ink composition of the present invention, the transferability of the ink composition during printing can be further improved. Furthermore, as the polarity of the composition itself increases, affinity with water-based OP varnishes, which also have high polarity, is enhanced, resulting in the effect of suppressing repelling even when the composition is applied wet-on-wet. The hydroxyl value of the rosin-modified resin is more preferably 15 mgKOH/g or more, and even more preferably 20 mgKOH/g or more. Furthermore, the upper limit of the hydroxyl value of the rosin-modified resin is not particularly limited, but an example would be approximately 200 mgKOH/g, preferably approximately 150 mgKOH/g, and more preferably approximately 100 mgKOH/g.

Furthermore, although not particularly limited, the acid value of the rosin-modified resin is preferably 100 mgKOH/g or less. Having an acid value of 100 mgKOH/g or less is preferable because it can suppress cissing when the aqueous OP varnish is applied wet-on-wet, while also achieving printability such as suppressing misting and blobbing. The acid value of the rosin-modified resin is more preferably 80 mgKOH/g or less, and even more preferably 50 mgKOH/g or less.

The rosin-modified resin is used in the form of a varnish obtained by heating with a solvent, as described below, to dissolve or disperse it. The rosin-modified resin may be used as a dissolved varnish in which it remains dissolved or dispersed in a solvent, or it may be used in the form of a gelled varnish obtained by dissolving the resin in the varnish and adding a divalent or higher metal alkoxy compound as a gelling agent to the dissolved varnish obtained by preparing the varnish. Of these, preparing a dissolved varnish from the rosin-modified resin and using this to prepare the ink composition is preferred, as it improves the transferability of the ink composition during printing. Furthermore, preparing a gelled varnish from the rosin-modified resin and using this to prepare the ink composition imparts appropriate viscoelasticity to the ink composition, thereby improving flowability and reducing misting, and also forming a tougher cured coating.

The content of the rosin-modified resin in the ink composition is preferably 1 to 20% by mass of the entire composition, more preferably 1 to 10% by mass of the entire composition, and even more preferably 2 to 5% by mass of the entire composition.

In addition to the alkyd resins and rosin-modified resins described above, the ink composition of the present invention can also contain resins that are conventionally used in preparing ink compositions for metal printing. That is, depending on the required performance, such as printability and coating properties, known resins that are compatible with the alkyd resins and rosin-modified resins described above can be used alone or in combination. Examples of such resins include polyester resins, petroleum resins, epoxy resins, ketone resins, amino resins, and benzoguanamine resins.

[solvent]
The solvent used in the ink composition of the present invention can be any solvent that has been used in the field of ink compositions for metal printing, without any particular limitation. Examples of such solvents include aliphatic hydrocarbons, alicyclic hydrocarbons, alkylbenzenes, polyalkylene glycols, and the like, each having a boiling point range of approximately 230 to 400°C. Among these, the ink composition of the present invention preferably uses at least one solvent having a solubility parameter (sp value) of less than 10.00 (cal/cm ³ ) ^1/2 and selected from the group consisting of compounds represented by the following general formula (1). Hereinafter, at least one solvent having a solubility parameter (sp value) of less than 10.00 (cal/cm ³ ) ^1/2 and selected from the group consisting of compounds represented by the following general formula (1) will also be referred to as a specific solvent. The sp value of the specific solvent is preferably 9.80 (cal/cm ³ ) ^1/2 or less. The lower limit of the sp value of the specific solvent is preferably about 8.50 (cal/cm ³ ) ^1/2 , and more preferably about 9.00 (cal/cm ³ ) ^1/2 . A specific solvent having such an sp value can also be said to be a polyalkylene glycol monoalkyl ether having hydrophobic properties.

In the general formula (1), each A is independently an alkylene group having 2 to 4 carbon atoms, which may have a branch. Examples of such alkylene groups include an ethylene group [-(CH ₂ ) ₂ -], a propylene group [-CH ₂ (CH ₃ )-CH ₂ - or -CH ₂ CH ₂ (CH ₃ )-], a trimethylene group [-(CH ₂ ) ₃ -], and an isopropylidene group [-C(CH ₃ ) ₂ -]. Among these, a propylene group is preferred. In this case, the divalent group represented by AO in general formula (1) is an oxypropylene group.

In the above general formula (1), R is an alkyl group having 1 to 13 carbon atoms, which may have a branched and/or cyclic structure. This alkyl group may be an aliphatic group or an alicyclic group. Examples of such alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, hexyl, 2-ethylhexyl, octyl, decyl, and cyclohexyl groups.

In the above general formula (1), n is an integer from 2 to 8. An n of 2 or greater is preferred because it ensures a sufficient boiling point for the specific solvent to provide stability to the ink composition on the printing press, while an n of 8 or less ensures a viscosity suitable for use as a solvent for the ink composition.

Examples of compounds represented by the above general formula (1) include dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, dipropylene glycol monooctyl ether, dipropylene glycol tridecyl ether, tripropylene glycol monobutyl ether, tripropylene glycol monodecyl ether, tetrapropylene glycol monohexyl ether, pentapropylene glycol monobutyl ether, and hexapropylene glycol monomethyl ether.

In addition, the sp value used in this invention is calculated using the Fedros method (see R.F. Fedros, Polym. Eng. Sci., 14(2)147(1974)).

The content of solvent in the ink composition of the present invention is preferably 10 to 50% by mass of the entire composition, and more preferably 20 to 45% by mass of the entire composition. Among the solvents, the content of the specific solvent is preferably 15 to 40% by mass of the entire composition, and it is preferable that all of the solvents in the composition are specific solvents. By including such specific solvents in the ink composition of the present invention, misting during printing can be reduced, which is preferable.

[silica]
The ink composition of the present invention contains silica. Silica contributes to improving the hiding power of the ink composition of the present invention. The silica used in the present invention is preferably in the form of extremely fine particles with an average primary particle diameter of 100 nm or less, and may be hydrophilic silica in which the particle surface is covered with silanol groups, or surface-hydrophobized silica in which the silanol groups present on the particle surface are modified with an alkyl group or the like. Among these, fumed silica with an average primary particle diameter of 10 to 30 nm and surface-hydrophobized can be preferably used.

The silica content in the ink composition of the present invention is 5% by mass or less. By keeping the silica content at 5% by mass or less, the premixing time in the ink composition manufacturing stage can be shortened. Furthermore, the lower limit of the silica content in the ink composition of the present invention is preferably about 0.1% by mass, more preferably about 1% by mass, and even more preferably about 2% by mass.

[Calcium carbonate]
The ink composition of the present invention contains calcium carbonate. Calcium carbonate contributes to improving the hiding power of the ink composition of the present invention. Such calcium carbonate may be in the form of powder particles, and its average primary particle diameter is preferably 70 nm or less. A primary particle diameter of 70 nm or less of calcium carbonate is preferred because it can ensure sufficient storage stability of the ink composition. Furthermore, the lower limit of the primary particle diameter of calcium carbonate is preferably about 10 nm, more preferably about 20 nm, and even more preferably about 30 nm.

The calcium carbonate used in the present invention is preferably one whose particle surface has been modified by chemical modification. In this case, preferred examples of the treatment agent used for modification include fatty acids and rosin acids. Various types of such modified calcium carbonate are commercially available, such as the Hakuenka series manufactured by Shiraishi Calcium.

The content of calcium carbonate in the ink composition of the present invention is preferably approximately 2 to 6% by mass, and more preferably approximately 3 to 6% by mass.

In the present invention, it is necessary that the total content of the silica and calcium carbonate be 8% by mass or less relative to the total composition. By keeping the total content of silica and calcium carbonate 8% by mass or less relative to the total composition, the impact resistance of the coating film obtained using the ink composition of the present invention can be ensured. The total content of silica and calcium carbonate is preferably 7.5% by mass or less.

[Other ingredients]
The ink composition of the present invention preferably contains an alkanolamine in addition to the above components. By containing an alkanolamine in the ink composition of the present invention, misting during printing can be reduced.

Alkanolamines include monoethanolamine, diethanolamine, triethanolamine, ethylmonoethanolamine, n-butylmonoethanolamine, dimethylethanolamine, diethylethanolamine, ethyldiethanolamine, n-butyldiethanolamine, di-n-butylethanolamine, triisopropanolamine, etc. Among these, triethanolamine is preferred.

The content of alkanolamine in the ink composition of the present invention is preferably 0.1 to 3 mass % of the total composition, and more preferably 0.1 to 1 mass part.

Other components that can be added to the ink composition of the present invention, as needed, include known hardeners; pigment dispersants; waxes; extender pigments such as silica particles, benton clay, kaolin, and talc; stabilizers, etc.

As a curing agent, for example, an amino resin such as melamine resin or benzoguanamine resin can be used.

To prepare the ink composition of the present invention, first, the above-mentioned components are mixed and stirred until the powder components such as titanium oxide, silica, and calcium carbonate are no longer powdery. As already explained, this process is called premixing, and is a process for thoroughly blending the solvent and varnish with the powder components. The mixture obtained from premixing is then milled using a roll mill, ball mill, bead mill, or the like to obtain the ink composition of the present invention. The viscosity of the ink composition is, for example, 10 to 70 Pa·s at 25°C as measured using a Raleigh viscometer, but is not particularly limited.

Metals for use in metal printing with the ink composition of the present invention are not particularly limited, but examples include zinc-plated or tin-plated iron sheets, aluminum sheets, and metal cans made from these metal materials.

The ink composition of the present invention will be explained in more detail below by showing examples, but the present invention is not limited to the following examples in any way.

[Preparation of alkyd resin varnish]
First-stage esterification was carried out by reacting 5.98 parts by weight of neopentyl glycol, 8.53 parts by weight of pentaerythritol, 10.10 parts by weight of coconut oil fatty acid, 11.95 parts by weight of isophthalic acid, and 2.48 parts by weight of terephthalic acid under a nitrogen atmosphere at 220°C until the acid value of the mixture reached 7 mgKOH/g. Then, 0.70 parts by weight of trimellitic anhydride was added, and the mixture was heated at 165°C for 30 minutes under a nitrogen atmosphere to carry out second-stage esterification. These esterification reactions were carried out according to conventional methods to obtain an alkyd resin with a mass average molecular weight of 6,049 and a number average molecular weight of 2,591. To this alkyd resin, 21.8 parts by weight of tripropylene glycol monobutyl ether was added to prepare an alkyd resin varnish. The sp value of this tripropylene glycol monobutyl ether was 9.73 (cal/ ^cm3 ) ^1/2 , which corresponds to the specific solvent of the present invention.

[Preparation of rosin-modified resin varnish]
A rosin-modified resin varnish was obtained by heating 63.2 parts by mass of a rosin ester resin (hydroxyl value 20-30 mgKOH/g, acid value <10 mgKOH/g, mass average molecular weight 632, number average molecular weight 565) and 35.9 parts by mass of tripropylene glycol monobutyl ether at 130°C for 1 hour to dissolve them. The sp value of this tripropylene glycol monobutyl ether was 9.73 (cal/cm ³ ) ^1/2 , and it corresponds to the specified solvent of the present invention.

[Examples 1 to 6, Comparative Examples 1 to 8]
First, the components were mixed according to the formula of "Component 1" shown in Tables 1 and 2, and premixed. Premixing was performed by heating the components other than the powder to about 60°C while stirring with a stirring blade, and then gradually adding the powder while continuing stirring and heating, and finishing the process when the powder had disappeared. The mixture obtained by premixing was kneaded in a three-roll mill, and then "Component 2" shown in Tables 1 and 2 was added and mixed well, thereby preparing the ink compositions of Examples 1 to 6 and Comparative Examples 1 to 8. In Tables 1 and 2, "specific solvent" is tripropylene glycol monobutyl ether (sp value 9.73 (cal/cm ³ ) ^1/2 ), "titanium oxide" is titanium oxide with an average primary particle diameter of 250 nm and DBP oil absorption of 18 g/100 g, "silica" is Aerosil R972 (surface-hydrophobic treated silica, average primary particle diameter 0.016 μm, specific surface area 110±20 m ² /g) manufactured by Nippon Aerosil Co., Ltd., and "clay" is CLAYTONE (product name) manufactured by BYK Japan K.K. APA (Benton clay), "Calcium Carbonate 1" is manufactured by Shiraishi Calcium Co., Ltd. under the product name Hakuenka O (rosin acid-treated calcium carbonate, average primary particle diameter 30 nm), "Calcium Carbonate 2" is manufactured by Shiraishi Calcium Co., Ltd. under the product name Hakuenka CC (fatty acid-treated calcium carbonate, average primary particle diameter 50 nm), "Calcium Carbonate 3" is manufactured by Shiraishi Calcium Co., Ltd. under the product name Hakuenka T-DD (rosin acid-treated calcium carbonate, average primary particle diameter 80 nm), "Calcium Carbonate 4" is manufactured by Shiraishi Calcium Co., Ltd. under the product name Hakuenka DD (rosin acid-treated calcium carbonate, average primary particle diameter 50 nm), and "Calcium Carbonate 5" is manufactured by Shiraishi Calcium Co., Ltd. under the product name Viscoexcel 30 (fatty acid-treated calcium carbonate, average primary particle diameter 30 nm).

[Premixing suitability evaluation]
For each of the Examples and Comparative Examples, when the ink compositions were prepared according to the above procedure, the time required for premixing was measured, and the premixing suitability was evaluated based on the measurement results according to the following criteria. The evaluation results are shown in the "Premixing Suitability" column in Tables 1 and 2.
○: The time required for premixing was less than 10 minutes. △: The time required for premixing was 10 minutes or more but less than 15 minutes. ×: The time required for premixing was 15 minutes or more.

[Impact resistance evaluation after retort processing]
For each of the Examples and Comparative Examples, 0.2 cc of the ink composition was applied to a 50 μm-thick aluminum substrate using an RI paint coater to obtain a painted product. Then, a retort overprint varnish (manufactured by AkzoNobel) was applied to the painted surface of the painted product using a No. 0.4 bar coater (Meyer bar), and the product was held in an oven at 240°C for 2 minutes to obtain a retort sample. The retort sample was then retorted at 125°C for 30 minutes using an autoclave (high-pressure steam sterilizer, HG-50 manufactured by Hirayama Seisakusho Co., Ltd.) to obtain a retort-treated product. The retort-treated product was subjected to impacts at four locations from the opposite side of the painted surface using a DuPont impact tester (Toyo Seiki Seisaku-sho, product name H-50) with a core diameter of 1/4Φ (6.35 mm), a weight of 300 g, and a weight drop height of 50 mm. The degree of peeling of the coating film on the painted surface was evaluated according to the following criteria. The results are shown in the "Impact Resistance" column in Tables 1 and 2.
○: In all four locations, the coating film did not peel off at the location that came into contact with the impact core. ×: In one or more of the four locations, at least a portion of the coating film in the location that came into contact with the impact core peeled off, exposing the aluminum base material.

As shown in Tables 1 and 2, while Examples 1 to 6, which are ink compositions of the present invention, had all evaluation items within practical ranges, Comparative Examples 1 to 8 were found to have problems in terms of manufacturing and impact resistance. From the above, it was demonstrated that the ink composition of the present invention allows premixing to be completed in a shorter time, and provides a coating film with good impact resistance after printing.

Claims (9)

An ink composition for metal printing comprising a color pigment, a resin, a solvent, silica, and calcium carbonate,
The color pigment contains titanium oxide,
The content of the silica is 5% by mass or less based on the total amount of the composition,
An ink composition for metal printing, characterized in that the total content of the silica and the calcium carbonate is 8 mass % or less based on the total composition. The ink composition for metal printing according to claim 1, characterized in that the solvent has a solubility parameter (sp value) of less than 10.00 (cal/cm ³ ) ^1/2 and contains at least one compound selected from the group consisting of compounds represented by the following general formula (1):
(In the above general formula (1), each A is independently an alkylene group having 2 to 4 carbon atoms which may have a branch; R is an alkyl group having 1 to 13 carbon atoms which may have a branched and/or cyclic structure; and n is an integer of 2 to 8.) The ink composition for metal printing according to claim 2, wherein the divalent group represented by AO in general formula (1) is an oxypropylene group. The ink composition for metal printing according to any one of claims 1 to 3, wherein the silica is surface-hydrophobically treated silica. The ink composition for metal printing according to any one of claims 1 to 4, wherein the resin contains an alkyd resin. The ink composition for metal printing according to claim 5, wherein the alkyd resin has a mass average molecular weight of less than 10,000. The ink composition for metal printing according to claim 5, wherein the alkyd resin has a pentaerythritol skeleton. The ink composition for metal printing according to claim 5, further comprising a rosin-modified resin as the resin. The ink composition for metal printing according to any one of claims 1 to 8, further comprising an alkanolamine.