JP2025051583A - Ink composition for seamless printed cans, printed and coated seamless cans, and method for producing same - Google Patents

Abstract

To provide an ink composition for a seamless printed can which has a bluish color tone that is difficult to be expressed in conventional seamless can printing, is excellent in jet-blackness and pitting corrosion resistance, and is also excellent in stability on a machine and high speed printability, and a seamless printed coated can using the same.SOLUTION: An ink composition for a seamless printed can contains a carbon black, a resin and a solvent, wherein the carbon black contains a carbon black (CBI) having an average primary particle diameter of 15 nm or more and less than 35 nm and a carbon black (CBII) having an average primary particle diameter of 35 nm or more and 60 nm or less, and a mass ratio of the CBI to the CBII is 40/60 to 95/5.SELECTED DRAWING: None

Description

The present invention relates to a black ink composition for seamless printed cans and seamless cans printed and painted using the same. In particular, the present invention relates to an ink composition for seamless printed cans that imparts excellent blueness and jet blackness to the printed and painted surface, and seamless cans printed and painted using the same.

Metal containers are used in a wide range of applications, including beverage containers, general-purpose cans, and confectionery cans. Metal beverage containers in particular offer superior storage stability for the contents compared to plastic beverage containers, and are therefore used for alcoholic beverages, carbonated beverages, soft drinks, coffee beverages, etc.

Metal beverage containers are mainly manufactured using two methods. One is to print and paint a sheet metal plate, then cut, weld, and other processes are used to create the desired can body. The other is to punch out a metal plate, iron, and other processes to form a cylindrical shape with a bottom, and then print and paint the body of the can. The latter method is specifically called a seamless can.

The most commonly used seamless cans are steel cans, which are primarily made of iron, and aluminum cans, which are primarily made of aluminum. To display images or content on these seamless cans, there are several methods, such as printing directly onto the container or attaching a printed film to the container.

Patent Document 1 describes an ink composition for metal printing that contains carbon black. Among the inks for seamless printing cans used when printing directly on containers, black inks used on beverage containers such as those for black coffee require a high level of jet blackness. Furthermore, in recent years, there has been an increasing demand for black inks with a bluish hue rather than black inks with a reddish or brownish hue.

In addition, black inks used on aluminum cans must be resistant to pitting corrosion. Pitting corrosion of aluminum cans is a corrosion phenomenon in which the surface of the printed coating becomes rough or holes appear in the printed and painted aluminum can due to the action of foreign matter adhering to the surface of the aluminum can.

Patent No. 7284896

The present invention aims to provide an ink for seamless printing on cans that has a bluish hue that was previously difficult to achieve, has excellent jet blackness and pitting corrosion resistance, and is also excellent in on-press stability and suitability for high-speed printing.

In order to solve the above problems, the present invention has developed a black ink for seamless printing cans that has excellent jet black color and a bluish tint by combining carbon blacks with different particle sizes. In addition, it has been discovered that by including a specific ratio of acidic carbon black, which has acidic functional groups introduced on the surface, in the total carbon black, the ink also has excellent pitting corrosion resistance on aluminum cans, which led to the completion of the present invention.

That is, the present invention relates to an ink composition for seamless printed cans that contains carbon black, a resin, and a solvent, and is characterized in that the carbon black contains carbon black (CBI) having an average primary particle diameter of 15 nm or more and less than 35 nm, and carbon black (CBII) having an average primary particle diameter of 35 nm or more and 60 nm or less, and the mass ratio of CBI to CBII is 40/60 to 95/5.

Furthermore, the present invention relates to the ink composition for seamless printing cans, in which the content of acidic carbon black in the carbon black is 80 mass% or more.

Furthermore, the present invention relates to the ink composition for seamless printing cans, in which the difference in average primary particle size between CBI and CBII is 5 nm or more.

Furthermore, the present invention relates to the ink composition for seamless printing cans, in which the carbon black content in the ink composition is 20 to 45 mass %.

Furthermore, the present invention relates to a printed and coated seamless can having a printing layer made of the ink composition for seamless printed cans provided on a metal printing medium, and an overprint layer made of an overprint varnish provided on the printing layer.

Furthermore, the present invention relates to a method for producing printed and coated seamless cans, which comprises dry offset printing or offset printing the ink composition for seamless printed cans onto a metal printing medium to form a printed layer, and applying an overprint varnish onto the printed layer to form an overprint layer.

According to the present invention, it is possible to provide an ink composition for seamless printing cans that has a bluish tint and is excellent in jet blackness, on-press stability, and suitability for high-speed printing, and a printed item obtained by printing the ink composition, and it is also possible to provide a printed item that exhibits good pitting corrosion resistance on seamless cans made of aluminum material.

The carbon black used in the present invention can be various types of carbon black, such as furnace black, channel black, thermal black, and acetylene black, which are produced using various manufacturing equipment.

The carbon black in the ink composition of the present invention is a combination of carbon black (CBI) having an average primary particle diameter of 15 nm or more and less than 35 nm and carbon black (CBII) having an average primary particle diameter of 35 nm or more and 60 nm or less. Carbon black having an average primary particle diameter of 15 nm or more and less than 35 nm is easy to obtain an ink with high jet blackness, and carbon black having an average primary particle diameter of 20 nm or more and 30 nm or less is more preferable. Carbon black having an average primary particle diameter of 35 nm or more and 60 nm or less is easy to obtain an ink with a bluish hue, and carbon black having an average primary particle diameter of 40 nm or more and 50 nm or less is more preferable. By using these in combination, an ink composition for seamless printing cans that is excellent in jet blackness, bluishness, and suitability for high-speed printing can be obtained.

Here, the average primary particle size refers to a value measured by an electron microscope. For example, the maximum particle sizes of approximately 2,000 to 3,000 randomly selected pigment particles are measured using a field emission scanning electron microscope (JSM-7000F manufactured by JEOL Ltd.) and the average value thereof is regarded as the average primary particle size.
When the average primary particle size of carbon black is expressed as a frequency distribution, commercially available carbon black generally exhibits a symmetrical mountain shape with the average primary particle size as a peak. The distribution curve expressing the average primary particle size of carbon black in the ink composition of the present invention as a frequency distribution exhibits an asymmetrical mountain shape or a mountain shape with two or more peaks.

The difference in average primary particle size between CBI and CBII is preferably 3 nm or more, more preferably 5 nm or more, and even more preferably 10 nm or more. This is because it provides better jet blackness, blueness, suitability for high-speed printing, etc.

The nitrogen adsorption specific surface area of CBI is preferably 75 to 350 ^{m 2} /g, more preferably 90 to 180 m ² /g. The nitrogen adsorption specific surface area of CBII is preferably 30 to 70 ^{m 2} /g, more preferably 35 to 60 m ² /g. By using carbon black in these ranges in combination, a composition for seamless printed cans that is superior in jet blackness, bluish color, and high-speed printability can be obtained. The nitrogen adsorption specific surface areas are all defined in accordance with JIS K 6217.

CBI includes MA77 (23 nm), MA7 (24 nm), MA8 (24 nm), MA11 (29 nm) manufactured by Mitsubishi Chemical Corporation, SpecialBlack5 (20 nm), SpecialBlack4 (25 nm), SpecialBlack4A (25 nm) manufactured by Orion Engineered Carbons Co., Ltd., and can be used alone or in combination of two or more types. CBII includes MOGUL-E (48 nm) manufactured by CABOT Corporation, MA14 (40 nm), MA285 (40 nm), MA220 (55 nm) manufactured by Mitsubishi Chemical Corporation, and can be used alone or in combination of two or more types.

The ratio of the carbon blacks used together is a mass ratio of CBI to CBII in the range of 40/60 to 95/5, more preferably in the range of 50/50 to 93/7, and even more preferably in the range of 60/40 to 90/10. Within the above range, it is easy to obtain an ink composition for seamless printing cans that has excellent blueness, jet blackness, and high-speed printability.

The carbon black in the ink composition of the present invention preferably contains 80% by mass or more of acidic carbon black having an acidic functional group introduced therein. More preferably, 85% or more, and even more preferably, 90% or more. The pH of the acidic carbon black is in the range of 2.0 to 6.0, and preferably in the range of 2.5 to 5.0. The pH here is a value measured by a glass electrode pH meter for a mixture of carbon black and distilled water. By using acidic carbon black within the above range, the ink composition exhibits good pitting corrosion resistance even when the substrate is an aluminum container that has not been subjected to size coating or coating coating. It is believed that the oxygen on the surface of the carbon black is less likely to receive electrons released from the aluminum, and as a result, corrosion due to battery action is less likely to progress. However, the above is based on scientific considerations, and the present invention is not limited to this action alone.
Examples of the acidic functional group include a carboxyl group and a phenolic hydroxyl group.

The carbon black content in the ink composition of the present invention is in the range of 20 to 45% by mass, and more preferably 25 to 40% by mass. By having the carbon black content within the above range, the ink composition for seamless printing cans has excellent dispersion stability, and in addition to the coating properties, it also has excellent jet blackness and pitting corrosion resistance.

The solvents used in the ink composition for seamless printed cans of the present invention may be high-boiling petroleum-based solvents, aliphatic hydrocarbon solvents, higher alcohol-based solvents, and other solvents suitable for printing inks, and it is particularly preferable to use high-boiling petroleum-based solvents. The above solvents may be used alone or in combination of two or more types.

High-boiling petroleum solvents are primarily composed of naphthenic or isoparaffinic solvents, and non-aromatic solvents that do not contain aromatic compounds are preferable in consideration of the environmental impact. Non-aromatic solvents among high-boiling petroleum solvents include AF Solvent Nos. 4 to 7 manufactured by Nippon Oil Corporation and Exxon Mobil Corp.'s Exxol D110 and D130. Some higher alcohol solvents may also be used in combination.

In addition to petroleum-based solvents, fatty acid esters can also be used.

The content of the solvent is not particularly limited. As an example, the content of the solvent in the ink composition for seamless printing cans is preferably 10% by mass or more, and more preferably 15% by mass or more. The content of the solvent in the ink composition for seamless printing cans is preferably 60% by mass or less, and more preferably 50% by mass or less. By having the solvent content within the above range, the ink composition for seamless printing cans can be easily adjusted to an ink tack value that shows good printability on seamless cans that are printed in large quantities at high speed.

The resin used in the ink composition of the present invention is preferably a fatty acid-modified alkyd resin. Fatty acid-modified alkyd resin has a high affinity for the particle surface of carbon black, making it easy to obtain an ink for seamless printing cans that has a bluish hue, excellent jet blackness, and excellent pitting corrosion resistance. Fatty acid-modified alkyd resin is obtained by condensation of a polybasic acid, a polyhydric alcohol, and a fatty acid component.

The fatty acid component is not particularly limited as long as it is a fatty acid or oil with an iodine value of 100 or less, and examples include coconut oil fatty acid, palm oil fatty acid, palm kernel oil fatty acid, olive oil, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, etc. From the viewpoint of jet blackness, it is preferable to include a fatty acid or oil having a hydroxyl group as the fatty acid component. Examples include 12-hydroxystearic acid, ricinoleic acid, castor oil fatty acid, hydrogenated castor oil fatty acid, castor oil, hydrogenated castor oil, etc. These are classified as non-drying oils, and are less likely to produce the characteristic odor due to components derived from fatty acids, and the flavor of the contents of the seamless can is less likely to be impaired.

Examples of polybasic acids that can be used to form the fatty acid modified alkyd resin include dibasic acids such as phthalic acid, phthalic anhydride, isophthalic acid, terephthalic acid, succinic anhydride, adipic acid, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, and maleic anhydride, and tribasic acids such as trimellitic anhydride and methylcyclohexene tricarboxylic anhydride. Polybasic acids may be used in combination.

Examples of polyhydric alcohols that can be used to compose the fatty acid modified alkyd resin include dihydric alcohols such as ethylene glycol, diethylene glycol, triethylene glycol, 1,6-hexanediol, bisphenol A, and hydrogenated bisphenol A; trihydric alcohols such as glycerin, trimethylolethane, trimethylolpropane, and tris(2-hydroxyethyl)isocyanurate; and tetrahydric or higher alcohols such as pentaerythritol and dipentaerythritol. Polyhydric alcohols may be used in combination.

The method for producing the fatty acid-modified alkyd resin is not particularly limited. For example, it can be produced by known methods such as the ester exchange method using oil as a raw material, or the fatty acid method using fatty acid as a raw material. For example, the fatty acid, polybasic acid, and polyhydric alcohol described above are charged together with xylene into a reaction vessel equipped with a stirrer, reflux condenser, and thermometer, and the temperature is raised to 240°C while stirring in a nitrogen atmosphere to carry out the ester reaction. The reaction is then terminated after confirming that the desired acid value is reached, thereby obtaining a fatty acid-modified alkyd resin, which is a condensation polymer of the polybasic acid, polyhydric alcohol, and fatty acid component.

The styrene-equivalent weight average molecular weight of the fatty acid-modified alkyd resin is preferably 3,000 or more and 30,000 or less. When the weight average molecular weight of the fatty acid-modified alkyd resin is within the above range, the ink composition for seamless printing cans maintains the coating properties and suppresses misting in terms of printability, which is preferable. In this embodiment, the weight average molecular weight is a value measured by gel permeation chromatography (GPC).

The resin of this embodiment may be a fatty acid-modified alkyd resin, or a conventionally used resin for ink. In other words, depending on the required performance such as printability and coating film properties, the ink composition for seamless printing cans may use a known resin that is compatible with the fatty acid-modified alkyd resin. Examples include the fatty acid-modified alkyd resins described above, as well as oil-free polyester resins, petroleum resins, epoxy resins, ketone resins, rosin-modified phenolic resins, rosin-modified maleic acid resins, melamine resins, benzoguanamine resins, etc.

The resin content is not particularly limited as long as it can be adjusted to a predetermined ink tack value. As an example, the resin content in the ink composition for seamless printing cans is preferably 20% by mass or more, and more preferably 30% by mass or more. The resin content in the ink composition for seamless printing cans is preferably 60% by mass or less, and more preferably 50% by mass or less. By having the resin content in the above range, the ink composition for seamless printing cans maintains its printability performance, such as transferability and misting, and also has excellent coating film properties.

In addition to the above components, the ink composition for seamless printed cans of this embodiment may contain optional additives that are typically included in ink compositions for seamless printed cans. Optional components include pigment dispersants, driers, lubricants, viscosity modifiers, storage stabilizers, etc. Inorganic pigments and organic pigments may also be used as necessary.

The method for preparing the ink composition for seamless printed cans of this embodiment is not particularly limited. As an example, the ink composition for seamless printed cans can be prepared by a conventional method using a three-roll mill, a ball mill, a bead mill, or the like.

The viscosity of the ink composition for seamless printed cans of this embodiment is not particularly limited, but is from 5 Pa·s to 30 Pa·s, and preferably from 7 Pa·s to 20 Pa·s.
The viscosity here is a value measured using a viscoelasticity measuring device (Discovery HR-2, manufactured by TA Instruments Japan) at a temperature of 30° C. and a shear rate of 100 sec ⁻¹ .

The printing method for the ink composition for seamless printed cans of the present invention is not particularly limited, and can be appropriately selected from methods such as the dry offset method using a resin relief plate used for printing on printed and coated seamless cans, and the offset method using a waterless flat plate.

The printed and coated seamless can of this embodiment has a printed layer made of an ink composition for seamless printed cans provided on a metal printing medium, and an overprint layer made of an overprint varnish provided on the printed layer.

The metal printing medium on which the ink composition for seamless printed cans of the present invention is printed includes, but is not limited to, aluminum plates, steel plates, and coated plates laminated with polyester films or the like. These substrates may be subjected to chemical conversion treatment, plating, size coating, white coating, silver coating, etc.

The step of providing the printing layer is preferably one of the following methods: a dry offset method using a resin letterpress plate, and an offset method using a waterless flat plate. By using these printing plates, clear letters and images can be formed on the curved surface of a printed and painted seamless can printed with the ink for seamless printed cans of the present invention, even at high speeds of over one thousand and several hundred cans per minute.

The film thickness of the printed layer is not particularly limited. As an example, the film thickness of the printed layer is preferably 0.3 μm or more, and more preferably 0.4 μm or more. The film thickness of the printed layer is preferably 6.0 μm or less, and more preferably 4.0 μm or less. By having the film thickness of the printed layer within the above range, the method for manufacturing seamless printed cans can prevent problems from occurring during the can manufacturing process.

The process for providing the overprint layer is not particularly limited, but it is preferable to apply the overprint layer using a wet-on-wet method in which the overprint layer is formed without curing the print layer, and then the coating film is cured.

The overprint varnish is not particularly limited, and any conventionally known varnish can be used. Examples include thermosetting overprint varnishes such as polyester-melamine, polyester-epoxy-melamine, and polyester-acrylic-melamine varnishes that are typically used for seamless printed cans. In addition, the varnish may be either water-based or solvent-based.

The thickness of the coating film including the print layer and the overprint layer is not particularly limited. As an example, the coating film thickness is preferably 3.0 μm or more, and more preferably 6.0 μm or more. The coating film thickness is preferably 30.0 μm or less, and more preferably 20.0 μm or less. When the coating film thickness is within the above range, the resulting printed and coated seamless can has excellent appearance such as gloss, and it is easy to obtain a coating film with excellent coating film properties aimed at protecting the ink layer.

The baking conditions in the process of baking and hardening the coating film are not particularly limited. As an example, the baking conditions are a first bake at a temperature of 180°C to 300°C for about 3 to 90 seconds, and a second bake at a temperature of 180°C to 300°C for about 30 to 150 seconds for heat hardening. However, depending on the type of can, baking may be done once. This produces a printed and painted seamless can made of a multi-layer coating film.

The printed and coated seamless cans and test panels of the present invention are evaluated based on the values obtained by measuring L ^* and b ^* in the L ^* a ^* b ^* color system (CIE 1976). In the L ^* a ^* b ^* color system, lightness is represented by L ^* , and hue is represented by a ^* and b ^* .

The larger the L ^* value, the brighter the color, and the smaller the value, the darker the color. In the case of black ink, the smaller the L ^* value, the better the jet blackness.
The black ink composition is transferred to a metal printing medium in a film thickness of 2 μm, then an overprint varnish is applied, and the L ^* value of the print which is baked and cured is preferably 15.0 or less, more preferably 10.0 or less.

The a ^* and b ^* values indicate the color direction, with a ^* (+) representing a red hue and a ^* (-) representing a green hue, b ^* (+) representing a yellow hue and b ^* (-) representing a blue hue. In black ink, the smaller the b ^* value, the more bluish the ink is.
The black ink composition is transferred to a metal printing medium in a film thickness of 2 μm, then an overprint varnish is applied, and the print is baked and cured, and the b ^* value of the print is preferably 2.0 or less, more preferably 1.5 or less.

The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to these. The numbers in the tables below are based on mass.

An ink composition for seamless printing cans was prepared according to the formula shown below.

<Synthesis of fatty acid modified alkyd resin>
10.5 parts of 12-hydroxystearic acid, 24.5 parts of coconut oil fatty acid, 35.6 parts of phthalic anhydride, 18.2 parts of pentaerythritol, and 18.2 parts of trimethylolpropane were esterified in a conventional manner to obtain a fatty acid-modified alkyd resin having a fatty acid content of 35%, an acid value of 4.3 mgKOH/g, and a weight average molecular weight of 7,800. The amount of dehydration was 7.0%.
In Examples 1 to 30 and Comparative Examples 1 to 5, the above fatty acid-modified alkyd resin was used.

<Preparation of printing ink composition>
Using the obtained fatty acid-modified alkyd resin, inks for seamless printing cans of Examples and Comparative Examples were prepared in the formulations shown in Tables 1 and 2. The carbon black used was as follows, and the solvent was Linear Alkyl Benzene manufactured by Nippon Oil Corporation, and the additive was Solsperse 20000 manufactured by Lubrizol Japan, Ltd.

CB(A): MA77 manufactured by Mitsubishi Chemical Corporation (average primary particle size 23 nm, specific surface area 130 ^{m 2} /g, pH 2.5)
CB(B): MOGUL-E manufactured by CABOT Corporation (average primary particle size 48 nm, specific surface area 57 m ² /g, pH 2.5)
CB(C): #47 manufactured by Mitsubishi Chemical Corporation (average primary particle size 23 nm, specific surface area 132 ^{m 2} /g, pH 7.5)
CB(D): REGAL350R manufactured by CABOT Corporation (average primary particle size 48 nm, specific surface area 58 ^{m 2} /g, pH 7.0)
CB(E): MA11 manufactured by Mitsubishi Chemical Corporation (average primary particle size 29 nm, specific surface area 92 m ² /g, pH 2.5)
CB(F): MA14 manufactured by Mitsubishi Chemical Corporation (average primary particle size 40 nm, specific surface area 56 m ² /g, pH 3.0)
CB(A), CB(B), CB(E), and CB(F) are acidic carbon blacks whose surfaces have been treated with an acid, and CB(C) and CB(D) are neutral carbon blacks whose surfaces have not been treated with an acid.

The T.V. in the table indicates the tack value of the ink, and is the value measured using a digital incometer (manufactured by Toyo Seiki Seisakusho Co., Ltd.) with 1.31 cc of ink, at a room temperature of 25°C, a roller temperature of 30°C, and a rotation speed of 400 rpm for 1 minute. The F.V. indicates the flow value of the ink, and is the diameter of the spread (unit: mm) after 60 seconds at a room temperature of 25°C using a parallel plate viscometer (spread meter) (manufactured by Yasuda Seiki Seisakusho Co., Ltd.).

For the printing inks prepared in Examples 1 to 30 and Comparative Examples 1 to 5, test panels were prepared as described below, and the jet blackness, hue, and pitting corrosion resistance were evaluated using the test panels. In-press stability was also evaluated.

＜Jet black＞
The printing inks prepared in Examples 1 to 30 and Comparative Examples 1 to 5 were uniformly transferred to a test rubber roll using a high-speed printability tester (PM904PT manufactured by SMT Co., Ltd.) so that the thickness of the ink film was 2 μm on the substrate, and then transferred to an aluminum plate at a printing speed of 8 m/s. Immediately thereafter, a thermosetting overprint varnish was applied onto the printed ink layer at a speed of 2 m/s so that the film thickness was 13 μm. Thereafter, the printed coating film was baked at 200° C. for 3 minutes to prepare a printed coating film. The printed coating film was measured using a spectrophotometer (manufactured by X-rite Co., Ltd.) and evaluated by the numerical value of the L ^* value in the L ^* a ^* b ^* color system.
(Evaluation Criteria)
A: The color measurement result of the L ^* value is 10 or less, and the jet blackness is good. B: The color measurement result of the L ^* value is more than 10 and less than 15, and there is no problem in terms of quality. C: The color measurement result of the L ^* value is more than 15, and the jet blackness is significantly poor and the product cannot be used as a product. Furthermore, A and B are ratings for practical use.

<Hue>
The printing inks prepared in Examples 1 to 30 and Comparative Examples 1 to 5 were uniformly transferred to a test rubber roll using a high-speed printability tester (PM904PT manufactured by SMT Co., Ltd.) so that the thickness of the ink film was 2 μm on the substrate, and then transferred to an aluminum plate at a printing speed of 8 m/s. Immediately thereafter, a thermosetting overprint varnish was applied onto the printed ink layer at a speed of 2 m/s so that the film thickness was 13 μm. Thereafter, the printed coating film was baked at 200° C. for 3 minutes to prepare a printed coating film. The printed coating film was measured using a spectrophotometer (manufactured by X-rite Co., Ltd.) and evaluated by the numerical value of the b ^* value in the L ^* a ^* b ^* color system.
(Evaluation Criteria)
A: The color measurement result of the b ^* value is 1.5 or less, and the color has a strong bluish tinge. B: The color measurement result of the b ^* value is greater than 1.5 and less than 2.0, and the color has a slightly strong bluish tinge. C: The color measurement result of the b ^* value is greater than 2.0, and the color has a weak bluish tinge. Furthermore, A and B are ratings that indicate practical use.

<Pitting corrosion resistance>
The printing inks prepared in Examples 1 to 30 and Comparative Examples 1 to 5 were uniformly transferred to a test rubber roll using an RI tester (manufactured by Kokubo Seimitsu Co., Ltd.) so that the thickness of the ink film on the substrate was 2 μm, and then transferred to an aluminum plate. Then, a thermosetting overprint varnish was applied onto the printed ink layer so that the film thickness was 13 μm. Then, the printed coating film was produced by baking at 200° C. for 3 minutes. The printed coating plate was immersed in a 1% aqueous solution of sodium chloride at 40° C. for 168 hours. Then, the coated plate was taken out, and cellophane adhesive tape was applied to the printed part, and the tape was then peeled off. It was visually judged whether the ink on the printed part had peeled off (corrosion state).
(Evaluation Criteria)
A: No paint peeling occurred. B: Very slight paint peeling was observed, but it did not affect the quality. C: Slight paint peeling was observed, but it did not affect the quality. D: A little paint peeling was observed, and the product could not be used as a product. E: Clear paint peeling was observed, and the product could not be used as a product. Furthermore, A, B, and C are rated as usable products.

<On-board stability>
For the printing inks prepared in Examples 1 to 30 and Comparative Examples 1 to 5, 1.31 cc of ink was evenly placed on the rubber roll of a digital inkometer (manufactured by Toyo Seiki Seisakusho, Ltd.), and the time until the tack value reached a maximum was measured under conditions of 40°C and 1200 rpm to evaluate the on-press stability. Note that the longer the time it takes to reach the maximum value, the more suppressed the release of the solvent from the ink, and the less change in the fluidity and viscosity of the ink on the ink roller or blanket, which can be said to be excellent on-press stability.
(Evaluation Criteria)
A: The maximum value is reached after 20 minutes. B: The maximum value is reached in 10 to 20 minutes. C: The maximum value is reached in less than 10 minutes. Note that A and B are evaluations of practical use.

<High-speed printing suitability (inking property)>
For the printing inks prepared in Examples 1 to 30 and Comparative Examples 1 to 5, 0.2 cc of ink was supplied to a leveling roll using a high-speed printability tester (PM904PT manufactured by SMT Co., Ltd.), and after homogenization, it was transferred to a test rubber roll, and then transferred to an aluminum plate at a printing speed of 8 m/s, and immediately thereafter, a thermosetting overprint varnish was applied onto the printing ink layer at a speed of 2 m/s to a film thickness of 13 μm. Then, it was baked at 200° C. for 3 minutes to prepare a printing coating film.
(Evaluation Criteria)
A: The substrate is sufficiently concealed, and ink adhesion is good.
B: Slight loss of ink was observed.
C: Somewhat noticeable ink loss is observed.
D: There was ink void or the aluminum substrate was not sufficiently concealed, and ink adhesion was poor.
The practical evaluations are A, B and C.

The formulations of the examples and comparative examples are as follows.
(I) Table 1 (Examples 1 to 30)
The ink composition contains a fatty acid-modified alkyd resin, carbon black, and a solvent, and the carbon black component contained in the ink composition includes: (1) carbon black having an average primary particle diameter of 15 nm or more and less than 35 nm, and carbon black having an average primary particle diameter of 35 nm or more and less than 60 nm; and (2) a mass ratio of carbon black having an average primary particle diameter of 15 nm or more and less than 35 nm to carbon black having an average primary particle diameter of 35 nm or more and less than 60 nm is 40/60 to 95/5.
(II) Table 2 (Comparative Examples 1 to 4)
The difference from the compositions of the examples is that it does not contain carbon black having an average primary particle size of 15 nm or more and less than 35 nm, or carbon black having an average primary particle size of 35 nm or more and less than 60 nm.
(III) Table 2 (Comparative Example 5)
The difference between the compositions of the examples is that the mass ratio of carbon black having an average primary particle size of 15 nm or more and less than 35 nm to carbon black having an average primary particle size of 35 nm or more and less than 60 nm is not in the range of 40/60 to 95/5.

As shown in Tables 1 and 2, when the ink compositions for seamless printed cans described in the Examples of the present invention were used, ink compositions for seamless printed cans having high jet blackness, expressing a bluish color, excellent pitting corrosion resistance, and practical level on-press stability and excellent suitability for high-speed printing could be obtained, compared to the case where the ink compositions for seamless printed cans of the Comparative Examples were used.

Claims (6)

An ink composition for seamless printing cans, comprising carbon black, a resin, and a solvent, characterized in that the carbon black contains carbon black (CBI) having an average primary particle diameter of 15 nm or more and less than 35 nm, and carbon black (CBII) having an average primary particle diameter of 35 nm or more and 60 nm or less, and the mass ratio of CBI to CBII is 40/60 to 95/5. The ink composition for seamless printing cans according to claim 1, wherein the content of acidic carbon black in the carbon black is 80% by mass or more. The ink composition for seamless printing cans according to claim 1, wherein the difference in average primary particle size between CBI and CBII is 5 nm or more. The ink composition for seamless printing cans according to claim 1, wherein the carbon black content in the ink composition is 20 to 45% by mass. A printed and coated seamless can having a printed layer made of the ink composition for seamless printed cans described in any one of claims 1 to 4 provided on a metal printing medium, and an overprint layer made of an overprint varnish provided on the printed layer. A method for producing printed and coated seamless cans, comprising: forming a printed layer on a metal printing medium by dry offset printing or offset printing the ink composition for seamless printed cans according to any one of claims 1 to 4; and applying an overprint varnish onto the printed layer to form an overprint layer.