JP2002038065A - Resin composition for intaglio ink, ink composition using the same, and printed matter thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resin composition for an intaglio ink, which is capable of removing excess ink with an aqueous wiping liquid, has excellent ink surface drying properties by ultraviolet rays, and has high ink film flexibility. Provided are an ink composition used and a printed material thereof. SOLUTION: At least one of an acrylic ester obtained by dehydrating and condensing polyglycerol having an average repeating unit number of 4 or more and 20 or less with acrylic acid, and an epoxy acrylate obtained by adding acrylic acid to glycidyl ether of polyglycerin. UV curable material containing
Various pigments are kneaded with a resin composition for an intaglio ink containing, as essential components, an oxidatively polymerizable material, a photopolymerization initiator and an oxidative polymerization catalyst to obtain an ink composition.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resin composition for an intaglio ink, which can remove excess ink with an aqueous wiping liquid, has excellent ink surface drying properties by ultraviolet rays, and has a high flexibility of an ink film, and an ink composition using the same. And printed matter.

[0002]

2. Description of the Related Art Intaglio printing is characterized by its unique touch and fine and sharp image lines, which cannot be realized unless a special printing machine is used. Is widely used for printing of banknotes, government bonds, stock certificates, gift certificates such as gift certificates.

[0003] On the other hand, the ink dried by irradiation with ultraviolet rays has a problem in that the printing speed is increased and the weather resistance of the printed matter is increased.
It is becoming widely used. However, in the case of an ink in which a pigment that absorbs ultraviolet light is dispersed, the thickness of a curable ink film is limited to a few μm. Although possible in principle and widely generalized, UV curing systems are not available in the field of intaglio printing. We have for these reasons:
We applied an ink that combines UV curing and oxidative polymerization functions to prevent set-off by curing the surface with UV irradiation and gradually cure the uncured portion inside the ink by oxidative polymerization, and a printed material using it. It was announced in Heisei 8-892.

[0004] In this ink, the unreacted ultraviolet curing component is polymerized and cured by radicals generated from the oxidized polymerization component within the ink to which ultraviolet light has not reached. This reaction is not restricted by the type of pigment,
It is just described in Japanese Patent Publication No. 8-892 that various inks can be used by appropriately combining an ultraviolet curable material, an oxidatively polymerizable material, a photopolymerization initiator, and an oxidative polymerization catalyst.

In intaglio printing, there is a process of wiping off excess intaglio ink on the printing plate surface. The wiping process includes a paper wiping method and a roll wiping method. In this case, a roll wiping method is mainly used. The roll wiping method enables continuous wiping by dispersing or dissolving the wiped ink in an oil solution or an aqueous solution containing a surfactant. The case where an oil solution is used is referred to as oil wiping, and the case where an aqueous solution is used is referred to as aqueous wiping. However, the water wiping method is mainly used because the load on the working environment is small.

Intaglio inks having both ultraviolet curing and oxidative polymerization functions are required to have an ink film surface having a high curing speed applicable to water-based wiping and high-speed printing. A flexible film that is more resistant to bending than a printing method is required. However, among the ink component vehicles, the UV-curable component that greatly affects the surface curability and the flexibility of the film is dipentaerythritol hexaacrylate, which has excellent curability used as the main component of the vehicle of the UV-curable offset ink. And the like cannot be used because the cured film becomes hard and brittle and does not dissolve or disperse in an aqueous alkaline solution.

As an ultraviolet curing component soluble in an aqueous alkali solution or water, there is an acrylate ester having a carboxyl group in a molecule so as to be dissolved in an aqueous alkaline solution. For example, in Japanese Patent Publication Nos. Hei 7-21015 and Hei 7-21016, a copolymer of maleic anhydride and a vinyl compound,
A compound obtained by reacting a hydroxyl group-containing acrylate is disclosed. JP-A-6-298851 discloses a compound having an acid anhydride group (specifically, a polymer obtained by polymerizing an unsaturated acid anhydride and a radical polymerizable monomer copolymerizable therewith, a conjugated double bond, A compound having maleic anhydride added thereto, a compound having two or more acid anhydride groups and a polyhydroxy compound reacted so that the acid anhydride groups become excessive), and reacting with hydroxyethyl acrylate to form a carboxyl group. Discloses a water-soluble active energy ray-curable resin that neutralizes carboxylic acid with a tertiary amine. JP-A-5-140251 and JP-A-6-16751 disclose resins which are made water-soluble by neutralizing a carboxyl group-containing acrylate obtained by reacting an epoxy acrylate with a polybasic acid anhydride. Further, there is known an active energy ray-curable resin in which acrylic acid is reacted with a novolak type epoxy resin, and then a generated hydroxyl group is reacted with an acid anhydride to generate a carboxyl group. Japanese Patent Application Laid-Open No. Hei 6-93082 discloses a development in which a part of the hydroxyl group is increased in molecular weight through a urethane bond. Although different from novolak type epoxy, a water-soluble active energy ray-curable resin produced so as to contain an anionic hydrophilic group in a polyurethane resin and neutralized with a tertiary amine is disclosed in JP-A-7-102037. . Furthermore, JP-A-6-93065, JP-A-6-1576991,
JP-A-6-57695 discloses a urethane acrylate in which a polyol compound having a quaternary ammonium halide and a hydroxyl group-containing acrylate are bonded by a polyisocyanate compound. JP-A-10-195361 discloses an acrylate in which a (meth) acrylate having an epoxy ring structure is partially added to a polymer having a carboxyl group and a number average molecular weight of 1,000 to 15,000. These acrylates can be selected from various materials in order to adjust the desired viscosity and mechanical properties, but they are still insufficient from the viewpoint of rapid curability.

[0008] None of the above alkali-soluble or water-soluble acrylates have high curability, cannot be applied to printing speeds of 50 m / min or more in intaglio printing, and considerable blocking occurs and set-off occurs. In some cases.

As an alkali-soluble acrylate having a very high curability, an acrylate obtained by adding an acid anhydride to a hydroxyl group of pentaerythritol triacrylate, which has a curability close to that of dipentaerythritol hexaacrylate, can be considered. Because the crosslink density is high for a small molecular weight, the cured film becomes hard and brittle,
It is difficult to use as an intaglio ink vehicle.

[0010]

SUMMARY OF THE INVENTION In view of the above, the present invention has been made to solve the above-mentioned problems, and has an excellent surface curability by ultraviolet rays, and a set-off caused by high-speed printing. , Without blocking, and
When wiping excess ink, it is possible to dissolve or disperse it in water or an aqueous alkali solution without using an oil-based wiping method due to problems in the work environment. An object of the present invention is to provide a resin composition for ink, an ink composition using the same, and a printed material thereof.

[0011]

Means for Solving the Problems As a result of intensive studies in view of the above situation, the present inventors have found that acrylic esters produced by condensation of polyglycerol having a large number of hydroxyl groups in one molecule with acrylic acid, and Epoxy acrylate obtained by adding acrylic acid to glycidyl ether of polyglycerin is water-soluble, has a high ultraviolet curability, and when it is made into a composition diluted with other water-soluble acrylate, it is a modified acid anhydride of novolak epoxy acrylate. It has been found that the same degree of cured product flexibility can be obtained, and the present invention has been completed.

The present invention relates to an acrylic ester obtained by dehydrating and condensing acrylic acid with polyglycerin having an average number of repeating units of 4 or more and 20 or less, and an epoxy acrylate obtained by adding acrylic acid to glycidyl ether of polyglycerin. A resin composition for an intaglio ink comprising, as essential components, at least one UV-curable material, an oxidatively polymerizable material, a photopolymerization initiator, and an oxidative polymerization catalyst.

Further, the present invention provides an acrylic ester obtained by dehydrating and condensing polyglycerol having an average number of repeating units of 4 or more and 20 or less with acrylic acid, wherein the number of acrylate groups per molecule is 4 or more. , 22 or less acrylic ester.

Further, the present invention is characterized in that the epoxy acrylate obtained by adding acrylic acid to the glycidyl ether of polyglycerin described above has an acrylic ester group number of 4 or more and 22 or less per molecule. It is a resin composition for intaglio ink.

Further, the present invention provides the resin composition for an intaglio ink according to the invention, wherein the oxidatively polymerizable material is a drying oil or a semi-dry oil, or a material modified from the drying oil or the semi-dry oil. It is.

In the present invention, the composition ratio of the ultraviolet curable material to the oxidatively polymerizable material is 95: 5 to 7 in the description.
5:25 (% by weight:% by weight).

The present invention is also an ink composition obtained by kneading various pigments with the resin composition for intaglio ink described above.

Further, the present invention is an ink composition characterized in that at least one of hydroquinone and methylhydroquinone is blended with the ink composition described above.

The present invention also provides a printed material characterized by being intaglio-printed with the ink composition described above.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The ink of the present invention has a surface cured with ultraviolet rays to prevent set-off, and the inside of the ink film which has not passed ultraviolet rays is coated with a dry oil-modified or semi-dry oil-modified alkyd resin or a dry oil-modified alkyd resin. Radicals generated by the oxidative polymerization of oils and semi-drying oils themselves cause the curing of the acrylate ester to cure the entire ink.Among the constituent materials, the condensation reaction of polyglycerin and acrylic acid with the ultraviolet curing component The ink is characterized by comprising, as an essential component, an acrylic ester of polyglycerin or an epoxy acrylate obtained by adding acrylic acid to glycidyl ether of polyglycerin.

The acrylic ester of polyglycerin obtained by condensing polyglycerin and acrylic acid is synthesized by heating polyglycerin and acrylic acid to 80 to 110 ° C. with an acid catalyst in the same manner as in the synthesis of ordinary acrylic esters. Is done. Polyglycerin is glycerin condensed,
A compound having 6 or more hydroxyl groups in one molecule, and polyglycerin having 6 to 12 hydroxyl groups is commercially available. The number of repeating units of the polyglycerol in the polyglycerol acrylate used in the present invention is 4 or more and 20 or less, preferably 6 or more and 20 or less. When the number of repeating units of polyglycerin is 3 or less, water solubility is not high,
If the number of repeating units of polyglycerin exceeds 20, the viscosity becomes too high, the amount of diluted acrylate necessary for viscosity adjustment increases, and the curability deteriorates, which is not preferable.
Further, when polyglycerin having a repeating unit number of 4 or more and 20 or less and acrylic acid are condensed, a sufficient curability can be obtained when the esterification rate is 60% to 100%. An acrylate of polyglycerin having a range of 22 acryloyl groups is obtained.

Epoxy acrylate obtained by adding acrylic acid to glycidyl ether of polyglycerin is synthesized by performing an addition reaction of epoxidized polyglycerin and acrylic acid in the presence of a catalyst without using a solvent. As glycidyl ether of polyglycerin, Denasel EX-512 and EX-521 of Nagase Kasei Kogyo Co., Ltd. are available from the market. Epoxy acrylates obtained by adding acrylic acid to glycidyl ether of polyglycerin are also available on the market. Polyglycerin serving as the skeleton is similar to polyglycerin acrylate,
Those having an average number of repeating units of 4 or more and 20 or less are preferable, and those having 4 or more and 10 or less are particularly preferable from the viewpoint of viscosity and curability. The number of acryloyl functional groups per molecule is preferably in the range of 4 to 22, particularly preferably in the range of 4 to 12.

In order to further adjust the viscosity and physical properties of the cured film, the ultraviolet curable component may be any of various water-soluble compounds other than epoxy acrylate obtained by adding acrylic acid to polyglycerin acrylate or polyglycerin glycidyl ether. Alternatively, an alkali-soluble polymerizable monomer or oligomer can be added. Other water-soluble or alkali-soluble polymerizable monomers include polyethylene glycol acrylate, polypropylene glycol acrylate, carboxyl group-containing acrylate, and the like. Examples of the oligomers include the aforementioned carboxyl group-containing urethane acrylate and carboxyl group-containing epoxy acrylate.

The other components are as described in JP-B 8-892, but again, the oxidation-polymerizable material must be modified with dry oil or semi-cured, since radicals must be generated by oxidation. Dry oil-modified alkyd resin, or
Dry or semi-dry oils per se, as well as materials of various structures derived therefrom, can be used, with the requirement that unsaturated fatty acids be present in the molecule.

The composition ratio of the ultraviolet curable material and the oxidatively polymerizable material of the present invention is 95: 5 to 75:25 (% by weight:% by weight).
Use within the range. If the amount of the ultraviolet curable material is less than this, the ultraviolet curability of the surface of the ink film after printing is reduced, causing set-off and blocking. When the amount of the oxidative polymerization component is smaller than this, the oxidative polymerization is insufficient, and poor curing of the ink may occur.

As the photopolymerization initiator of the present invention, various commercially available photopolymerization initiators can be used. These can be used alone or 2
It is possible to use a combination of more than one species. The amount used depends on the type of polymerization initiator. Also, 4-
Photopolymerization initiation aids such as ethyl dimethylaminobenzoate and isoamyl 4-dimethylaminobenzoate can also be added.

The oxidative polymerization catalyst of the present invention includes metal compounds such as cobalt, manganese, lead and iron, cobalt borate, cobalt octylate, cobalt naphthenate, lead monoxide and the like.

The resin composition for an intaglio ink of the present invention comprises 5 to 25 parts by weight of an oxidatively polymerizable material and 2 to 10 parts by weight of a photopolymerization initiator per 100 parts by weight of a mixed material of an ultraviolet curable material and an oxidatively polymerizable material. The mixing range of the oxidation polymerization catalyst is preferably 0.5 to 3 parts by weight.

An organic pigment is added to the resin composition for intaglio ink.
The ink composition of the present invention is obtained by adding and kneading additives such as an inorganic pigment and a wax. The kneading method is not particularly limited.

In order to improve the storage stability of the ink, it is usually necessary to add a polymerization inhibitor (inhibitor). As the polymerization inhibiting (suppressing) agent of the present invention, known polymerization inhibiting (suppressing) agents such as p-methoxyphenol and hydroquinone can be used, and hydroquinone and methylhydroquinone are particularly preferable. The amount added varies depending on the type of polymerization inhibitor (inhibitor).

[0031]

The present invention will be described in more detail with reference to the following examples, but the content of the present invention is not limited to the scope of these examples. In this example, an ultraviolet curable material was prepared by the following synthesis example, and the gel fraction and the flexibility of the film were evaluated. FIG. 1 shows the evaluation results of the gel fraction and the flexibility of the film. Further, an intaglio ink was prepared at the compounding ratio shown in FIG. 2 for each of the prepared ultraviolet curable materials, and the blocking was evaluated. The results of the blocking evaluation are shown in FIG.

(Synthesis Example 1) 150 g of polyglycerin # 750 (manufactured by Sakamoto Yakuhin Kogyo), 225 ml of acrylic acid (manufactured by Osaka Organic Chemical Industry) were placed in a glass separable flask equipped with an air inlet tube, a thermometer and a stirrer. 0.38 g of methylhydroquinone, 5 g of p-toluenesulfonic acid, and 200 ml of benzene were charged. Next, stirring was performed while blowing air into the mixed solution,
Heated. The water generated by the dehydration reaction flows out together with the benzene, and the benzene that has been received by the water receiver and overflowed is returned to the reaction system. After the reaction for 16 hours, the pressure was reduced, and benzene and acrylic acid were distilled out of the reaction system. After cooling, the acid value was measured, and an aqueous alkali solution in which sodium hydroxide required for neutralization was dissolved in 300 ml of water was gradually added dropwise. After separating the organic layer, the acid value of the organic layer is 3 mgK
It was washed with an aqueous solution of sodium hydroxide until OH / g or less. Further, it was washed twice with an aqueous solution of sodium sulfate. Water was removed under reduced pressure from the obtained acrylate containing a large amount of water, and 230 g of acrylate was obtained. The esterification rate of the acrylate of polyglycerin obtained in this manner is determined from the measurement of the hydroxyl value of the raw material and the product.
85%. 53g of this material and polyethylene glycol 40
47 g of 0 diacrylate was mixed to obtain a water-soluble active energy ray-curable resin. 3 parts by weight of Irgacure 907 (manufactured by Ciba Specialty Chemicals) was mixed with 100 parts by weight of the obtained water-soluble active energy ray-curable resin to prepare an ultraviolet curable material of Example 1.

(Synthesis Example 2) In a glass separable flask equipped with an air inlet tube, a thermometer and a stirrer, 150 g of polyglycerin PG06 (manufactured by Daicel Chemical Industries), 266 ml of acrylic acid (manufactured by Osaka Organic Chemical Industry), methyl Hydroquinone 0.43g, p
5 g of toluenesulfonic acid and 200 ml of benzene were charged.
The following was performed in the same manner as in Example 1, and 270 g of an acrylate ester was obtained. The esterification rate of the polyglycerol acrylate thus obtained was 79% from the measurement of the hydroxyl value of the raw material and the product. 48 g of this material and 52 g of polyethylene glycol 400 diacrylate were mixed to obtain a water-soluble active energy ray-curable resin. 3 parts by weight of Irgacure 907 (manufactured by Ciba Specialty Chemical) was mixed with 100 parts by weight of the obtained water-soluble active energy ray-curable resin to prepare an ultraviolet curable material of Example 2.

(Water-soluble active energy ray-curable resin of Example 3) A mixture of 34 g of NK oligo EA-5323 manufactured by Shin-Nakamura Chemical Co., Ltd. and 66 g of polyethylene glycol 400 diacrylate was mixed with a water-soluble active energy ray-curable resin. Got. 3 parts by weight of Irgacure 907 (manufactured by Ciba Specialty Chemical) was mixed with 100 parts by weight of the obtained water-soluble active energy ray-curable resin to prepare an ultraviolet-curable material of Example 3.

(Synthesis Example 3) 187 g of phenol novolak epoxy resin EPPN-201 (Nippon Kayaku, epoxy equivalent 187)
To 74 g of acrylic acid, 600 g of polyethylene glycol 400 diacrylate and 0.1 g of methylhydroquinone were added, 0.6 g of dimethylbenzylamine was added, and reacted at 110 ° C. for 13 hours.After confirming that the acid value was 3.0 or less, 30 ° C. And add 150 g of tetrahydrophthalic anhydride,
The mixture was reacted at a temperature of 3 ° C. for 3 hours to obtain a mixture. The resulting mixture 10
3 parts by weight of Irgacure 907 (manufactured by Ciba Specialty Chemical) was mixed with 0 parts by weight to prepare an ultraviolet curable material of Comparative Example 1.

(Synthesis Example 4) 101 g of a styrene maleic anhydride copolymer SMA1000 (manufactured by elf atochem) was added to 100 ml of methyl ethyl ketone, heated to 80 ° C. and dissolved, and 65 g of hydroxyethyl acrylate, 0.5 g of pyridine and 0.4 g of p-methoxyphenol were added. The mixture was stirred at 90 ° C. for 4 hours. After cooling, it was poured into water to precipitate a polymer. The polymer was collected by filtration and dried. 28 g of the obtained polymer was dissolved in 72 g of polyethylene glycol 400 diacrylate to obtain a mixture.
3 parts by weight of Irgacure 907 (manufactured by Ciba Specialty Chemicals) was mixed with 100 parts by weight of the obtained mixture to prepare an ultraviolet-curable material of Comparative Example 2.

(Synthesis Example 5) 100 g of isobutylene maleic anhydride copolymer Isovan 600 (manufactured by Kuraray) was added to 200 ml of dimethylformamide, heated to 100 ° C. and dissolved, and then 84 g of hydroxyethyl acrylate, 0.7 g of pyridine and 0.5 g of p-methoxyphenol were added. g was added and stirred at 90 ° C. for 4 hours. After cooling, it was poured into water to precipitate a polymer. The polymer was collected by filtration and dried. 24 g of the obtained polymer was dissolved in 76 g of polyethylene glycol 400 diacrylate to obtain a mixture. Irgacure 9 based on 100 parts by weight of the obtained mixture
07 (Ciba Specialty Chemical) 3 parts by weight
The ultraviolet curable material of Comparative Example 3 was produced.

(Synthesis Example 6) 100 g of succinic anhydride was added to 295 g of pentaerythritol triacrylate (M-305 manufactured by Toa Gosei).
Was added, and 0.2 g of methylhydroquinone and 1 g of dimethylbenzylamine were added, followed by stirring at 120 ° C. for 3 hours. 6.1 g of polyethylene glycol 400 diacrylate was added thereto to obtain a mixture. The obtained water-soluble active energy ray-curable resin 10
3 parts by weight of Irgacure 907 (manufactured by Ciba Specialty Chemical) was mixed with 0 parts by weight to prepare an ultraviolet curable material of Comparative Example 4.

(Examples 1-3, Comparative Examples 1-4) Example 1
The UV curable materials of Comparative Examples 1 to 3 and Comparative Examples 1 to 4 were evaluated for gel fraction and film flexibility by the following methods (1) and (2). FIG. 1 shows the results of these evaluations.

(1) Gel Fraction A coating having a thickness of about 100 μm was applied on a glass plate, and irradiated with various amounts of ultraviolet light to form a cured film. The proportion (gel fraction) of the portion of the cured film that did not dissolve in methyl ethyl ketone was determined.

(2) Flexibility of coating A coating having a thickness of 100 μm was applied on a glass plate, and irradiated with ultraviolet rays at an irradiation amount of 380 mJ / cm ² to form a cured film of 15 mm × 100 mm. This film is used for tensile tester Tensilon UTM-3-500
(Toyo Baldwin) tensile test (tensile speed 5mm / min,
Span 5 cm), and the elongation and tensile strength were determined.

Next, an intaglio ink was prepared for each of the UV-curable materials of Examples 1 to 3 and Comparative Examples 1 to 4 by a three-roll mill using the composition shown in FIG. Was evaluated. FIG. 3 shows the evaluation results.

(3) Blocking Property According to the composition shown in FIG. 2, an intaglio ink was prepared, and a 65 mm × 160 mm basis weight 86 g / m ^{2 was} obtained on an intaglio plate having an image depth of 100 μm.
Intaglio printing on paper, and conveyor speed: 50 immediately after printing
m / min, UV lamps: ultraviolet under the conditions of air-cooled 120 W / cm metal halide lamp 1 lamp irradiation, superimposed sheets of the same size on the printing surface side, across the glass plate, applying a load of 215 kg / m ² Was. After being left for one day, the magnitude of the force (peeling strength) required to peel the overlaid paper from the printed paper was measured.

Comparative Examples 1 to 3 are insufficient in terms of quick-curing properties compared to Examples 1 to 3, and Comparative Example 4 is more practical than Examples 1 to 3 as a material for an intaglio ink in terms of flexibility. Absent.

[0045]

The resin composition for intaglio ink of the present invention and the ink composition using the same have excellent ink surface curability due to ultraviolet rays, so that set-off and blocking in high-speed printing can be prevented and excess ink can be prevented. When wiping the ink, it dissolves or disperses in water or an aqueous alkaline solution, so the excess ink can be washed by aqueous wiping. Excellent wire durability.

[Brief description of the drawings]

FIG. 1 is a diagram showing the results of evaluation of the gel fraction and the flexibility of a film in an ultraviolet curable material.

FIG. 2 is a diagram showing a mixing ratio of an intaglio ink.

FIG. 3 is a view showing the evaluation results of blocking of an intaglio ink.

Claims (8)

[Claims] 1. An acrylic ester obtained by dehydrating and condensing polyglycerin having an average repeating unit number of 4 or more and 20 or less with acrylic acid, and an epoxy acrylate obtained by adding acrylic acid to glycidyl ether of polyglycerin. UV curable material containing one
A resin composition for an intaglio ink comprising, as essential components, an oxidatively polymerizable material, a photopolymerization initiator and an oxidative polymerization catalyst. 2. The average number of repeating units according to claim 1 is 4
As described above, the acrylic ester obtained by dehydrating and condensing acrylic acid with polyglycerin having 20 or less is an acrylic ester having 4 to 22 acrylate groups per molecule. Resin composition. 3. The intaglio plate of the epoxy acrylate obtained by adding acrylic acid to the glycidyl ether of polyglycerin according to claim 1, wherein the number of acrylate groups per molecule is 4 or more and 22 or less. Resin composition for ink. 4. The resin composition for intaglio ink according to claim 1, wherein the oxidatively polymerizable material is a drying oil or a semi-dry oil, or a material modified from the drying oil or the semi-dry oil. 5. The composition according to claim 1, wherein the composition ratio of the ultraviolet curable material to the oxidatively polymerizable material is 95: 5 to 75:25.
(% By weight:% by weight). 6. An ink composition obtained by kneading various pigments with the resin composition for intaglio ink according to claim 1. 7. An ink composition comprising the ink composition according to claim 6 and at least one of hydroquinone and methylhydroquinone. 8. A printed matter obtained by intaglio printing with the ink composition according to claim 6.