HK1139747B - A resin relief plate for relief printing and a method for producing the organic el element obtained by using the same - Google Patents

Description

Resin relief plate for relief printing and method for producing organic EL element using the same

Technical Field

The present invention relates to a resin relief printing plate for relief printing for printing an organic light emitting layer of an organic electroluminescence (hereinafter, referred to as "organic EL") device, and a method for manufacturing an organic EL device using the relief printing plate.

Background

In recent years, organic EL elements having features such as thinness, low power consumption, and light weight have been attracting attention as display elements in mobile phones and the like. The organic EL element has a structure in which an organic light emitting layer including an organic EL light emitting body is formed between 2 opposed electrodes, and the organic light emitting layer emits light when a current flows through the organic light emitting layer.

The organic EL light-emitting body includes a low-molecular material and a high-molecular material. In general, an organic light-emitting layer is formed on a substrate by vacuum deposition or the like using a low-molecular material, but in this case, patterning is performed using a mask having a fine pattern, and therefore, if the substrate is increased in size, the vacuum deposition apparatus is increased in size, and there is a problem that the material use efficiency, the throughput, and the yield are significantly reduced. Therefore, recently, in order to cope with mass production even when the substrate is large in size, the following methods have been attempted: a polymer material is used as an organic EL light-emitting body, and the polymer material is dissolved and dispersed in a solvent to form an ink, and then an organic light-emitting layer is formed on a substrate by a printing method (for example, see patent document 1). However, since all of the conjugated polymer compounds known as polymer materials have low solubility, it is necessary to use an aromatic system such as xylene or toluene as a solvent in the ink-forming process. Since the aromatic solvent portion has high swellability with respect to the polymer, there are problems in that, for example, when an organic light-emitting layer is formed by an offset printing method: swell in a solvent, and cause a reduction in dimensional accuracy due to repeated printing processes.

On the other hand, various printing methods such as the above-mentioned offset printing method are available, but since the above-mentioned organic EL element often uses a glass substrate as a substrate, an offset printing method using a rubber blanket having elasticity, and a relief printing method using a rubber blanket or a resin plate having elasticity are preferable. However, in the offset printing method, since there is a risk that the dimensional accuracy of the rubber blanket is lowered as described above, it is more preferable to use the relief printing method. Further, even in this relief printing method, since there is the above-mentioned risk in the case of a rubber plate, a resin plate, among which a commercially available solid plate (for example, a product manufactured by the east beauty company) is often used, can be used.

Patent document 1: japanese patent laid-open publication No. 2006 and 252787

Disclosure of Invention

Problems to be solved by the invention

However, the solid plate is a resin relief plate obtained by evaporating and drying a soluble emulsion composition of a photosensitive resin and a water-soluble polymer such as water-soluble nylon or polyvinyl alcohol, and therefore has a problem of high hardness and large damage to a substrate during printing.

The present invention has been made in view of these circumstances, and an object thereof is to provide a resin relief plate for relief printing which is stable in dimensional accuracy even when printing processing is repeated and causes little damage to a substrate during printing, and a method for manufacturing an organic EL element using the relief plate.

Means for solving the problems

In order to achieve the above object, the 1 st gist of the present invention is a resin relief plate for relief printing formed by using, as a plate material, a photosensitive resin composition obtained by blending a prepolymer obtained by addition polymerization of an acrylate ester with a polyester polyol and a diisocyanate and a monofunctional or polyfunctional monomer, and the 2 nd gist of the present invention is a method for producing an organic EL element, comprising the steps of: the organic EL light-emitting layer is formed by printing a printing ink, which is obtained by dissolving an organic EL light-emitting body in a solvent, on a substrate using the resin relief for relief printing, and then evaporating and vaporizing the solvent.

Effects of the invention

That is, the present inventors have conducted intensive studies to obtain a resin relief plate for relief printing (hereinafter, simply referred to as "resin relief plate") which is stable in dimensional accuracy even when a printing process is repeatedly performed and which causes little damage to a substrate at the time of printing, and as a result, have found that the initial object can be achieved when a photosensitive resin composition is used as a plate material, the photosensitive resin composition being obtained by blending a prepolymer formed by addition of an acrylate ester with a polyaddition product of a polyester polyol and a diisocyanate and a monofunctional or polyfunctional monomer, and have completed the present invention. Since the plate material of the resin relief plate of the present invention is a photosensitive resin composition having a polyester skeleton, it does not swell excessively in an aromatic solvent as compared with a plate material having an aliphatic skeleton, and therefore, even if a printing process is repeated, the dimensional accuracy of the resin relief plate is not lowered and stable dimensional accuracy can be maintained for a long period of time. Further, the resin relief plate has a soft hardness and causes little damage to the substrate during printing.

In addition, since the method of manufacturing an organic EL device of the present invention forms an organic light-emitting layer on a substrate using the resin relief plate, dimensional accuracy of the organic light-emitting layer of the obtained organic EL device is stable even when a plurality of organic EL devices are manufactured using 1 resin relief plate. Further, the damage to the substrate during the production of the organic EL element is small, and the substrate is not damaged.

Drawings

Fig. 1 is a side view showing an embodiment of the resin relief plate of the present invention.

Fig. 2 is an enlarged plan view of an important part of the above-described resin relief plate.

Fig. 3 is a sectional view a-a of fig. 2.

Fig. 4 is an enlarged plan view showing an important part of the resin relief plate modification example.

Fig. 5 is a B-B sectional view of fig. 4.

Fig. 6 is an explanatory view showing the above-described resin relief printing method.

Fig. 7 is an explanatory view showing an embodiment of the organic EL element.

Fig. 8 is an explanatory view showing a method for manufacturing the organic EL element.

Detailed Description

Next, the best mode for carrying out the present invention will be described. However, the present invention is not limited to these embodiments.

Fig. 1 shows an embodiment of the resin relief plate of the present invention. In this embodiment, the resin relief plate is used for printing an organic light emitting layer 23 (see fig. 7) on an organic EL element constituting an organic EL color display or the like (not shown) described later, and is composed of a substrate 1, and a plurality of printing projections 2 formed in a stripe shape on a pattern printing surface of the substrate 1 and projecting in parallel at a predetermined interval (pitch), as shown in fig. 2 and fig. 3 which is a cross-sectional view a-a of the substrate 2. A plurality of minute protrusions (minute convex portions) 3 are distributed and formed on the top surface of each printing convex portion 2, and a groove portion 4 is formed between each minute protrusion 3 so as to be connected in a row, and printing ink (not shown) is held in the groove portion 4.

The resin relief plate has a JIS rubber hardness in the range of 60 to 90 DEG Shore A (shore A), and preferably has flexibility in the range of 65 to 80 DEG Shore A. If the hardness of the JIs rubber is higher than 90 ° shore a, the hardness is too high, and there is a risk of damaging the substrate, and if the hardness is lower than 60 ° shore a, the hardness is too soft, and there is a risk of causing dripping of the printing ink during printing, and a sharp image cannot be obtained.

The resin relief plate may be developed with an aqueous detergent, and the uncured portion may be washed with an aqueous detergent such as washing water in the production of the resin relief plate described later.

The resin relief plate has a swelling ratio in the range of 0.2 to 15% in the weight change ratio before and after immersion in an aromatic solvent such as anisole, cyclohexylbenzene, or tetralin, or a mixed solvent thereof, which dissolves or disperses an organic EL light-emitting body, when the resin relief plate is immersed continuously for 24 hours at room temperature in the range of 20 to 25 ℃. Thus, the resin relief plate does not swell with an aromatic solvent. Therefore, when a printing ink obtained by dissolving and dispersing the organic EL light-emitting body in an aromatic solvent is used, the resin relief plate does not swell, and therefore, even if the printing process is repeated, the dimensional accuracy of the resin relief plate is not lowered, and stable dimensional accuracy can be maintained for a long period of time.

The weight change rate is preferably set within the range of 1 to 10%. If the weight change rate exceeds 10%, there is a risk that the dimensional accuracy of the resin relief plate is lowered by repeating the printing process. Further, the lower the weight change rate is, the more preferable the weight change rate is, and if the weight change rate is about 1%, the reduction in the dimensional accuracy is hardly observed, but in consideration of the number of times of ordinary use, the reduction in the dimensional accuracy is not likely to occur if the weight change rate is within a range of 5 to 7%. When a mixed solvent of anisole, cyclohexylbenzene, or tetralin is used as the aromatic solvent, the mixing ratio can be arbitrarily set. In addition, when the weight change rate is measured, a resin relief plate with a thickness of 0.13-0.30 cm is cut into a size of 2cm × 3 cm.

The resin relief plate preferably has resin characteristics such that the elongation at cutting is 15 to 200%, the tensile strength is 2 to 100MPa, and the maximum tensile force is 30 to 200N. If the rubber hardness exceeds this range, the rubber hardness increases, and the substrate is damaged as described above, while if the rubber hardness falls below this range, the rubber hardness decreases, and there is the problem of the occurrence of dripping during printing.

For example, in the case of line printing, the shape of the fine protrusions 3 is preferably a truncated cone shape or a cylindrical shape (truncated cone shape in fig. 2 and 3), and the height H of the fine protrusions 3 is more favorable from the viewpoint of better transferability₁Preferably in the range of 1 to 500 μm, and the diameter D of the top surface₁Preferably in the range of 5 to 500 μm, the spacing W between adjacent minute protrusions 3₁Preferably in the range of 5 to 500. mu.m.

Instead of the fine protrusions 3, as shown in fig. 4 and fig. 5 which is a cross-sectional view B-B of fig. 4, a stripe-shaped uneven portion may be formed on the top surface of the printing convex portion 2, the stripe-shaped uneven portion being composed of a plurality of stripe-shaped ridges 5 arranged in parallel at predetermined intervals and stripe-shaped grooves (stripe-shaped concave portions) 6 formed between the adjacent 2 stripe-shaped ridges 5. The cross-sectional shape of the linear ridges 5 is preferably a mesa shape or a rectangular shape (mesa shape in fig. 4 and 5), and the height H of the linear ridges 5 is more favorable from the viewpoint of better transferability₂Preferably in the range of 1 to 500 μm, and the width D of the top surface₂Preferably, the groove width (recess width) W of the linear groove 6 is in the range of 5 to 500 μm₂Preferably in the range of 5 to 500. mu.m. When the organic light-emitting layer 23 of the organic EL element is applied to illumination applications other than display applications, a printing plate having a full pattern (BETA pattern) of a single color over the entire surface without performing the above-described line-shaped patterning may be used.

Such a resin relief plate is produced with a photosensitive resin composition obtained by compounding a prepolymer obtained by addition of an acrylate with a polyaddition product of a polyester polyol and a diisocyanate and a monofunctional or polyfunctional monomer as a plate material.

Examples of the polyester polyol include polyester copolymers obtained by polycondensation of a dicarboxylic acid composed of a saturated fatty acid, an unsaturated fatty acid, an aromatic acid, and the like, and a polyhydric alcohol.

Here, the polyhydric alcohol means an alcohol having 2 or more hydroxyl groups in 1 molecule, and specifically, ethylene glycol, propylene glycol, diethylene glycol, and the like are mentioned, and they may be used alone or in combination.

Further, as the diisocyanate, for example, aliphatic, alicyclic or aromatic diisocyanates can be used, examples thereof include 2, 4-tolylene diisocyanate, 2, 6-tolylene diisocyanate, 4 '-diphenylmethane diisocyanate, 4-dicyclohexylmethane diisocyanate, dimethoxyaniline isocyanate (dianisidineisocyanate), 3' -xylene-4, 4 '-diisocyanate, p-xylene diisocyanate, 1, 3-cyclohexanedimethylene isocyanate, 1, 5-naphthalene diisocyanate, trans-vinylidene diisocyanate, 2, 6-diisocyanate methylhexanate, diphenyl ether-4, 4' -diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, and the like. As such a method for addition polymerization of a polyester polyol and a diisocyanate, conventionally known methods can be used.

Examples of the acrylic acid ester include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, and polyethylene glycol mono (meth) acrylate.

The addition of the acrylate to the prepolymer can be carried out by, for example, a conventionally known graft polymerization method. Here, the addition ratio of the acrylate to the prepolymer is usually set in the range of 0.5 to 5.0 mol of acrylate to 1 mol of prepolymer. If the addition ratio of the acrylic ester exceeds the above range, there is a problem that the rubber hardness of the resin relief plate increases, and if it is less than the above range, the resin relief plate cannot be formed.

Examples of the monofunctional or polyfunctional monomer include various monomers mainly comprising 1, 3-butanediol diacrylate, 1, 4-butanediol diacrylate, 1, 6-hexanediol diacrylate, diethylene glycol diacrylate, neopentyl glycol diacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, dipentaerythritol hexaacrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and 2-hydroxy-3-phenoxypropyl acrylate.

The ratio of the prepolymer obtained by adding the above addition-polymerized product to the acrylate and the monofunctional or polyfunctional monomer can be arbitrarily determined in accordance with the purpose of achieving a predetermined hardness, flexibility and swelling ratio. In addition, a photopolymerization initiator is added to the prepolymer and monomer mixture in a range of 0.001 to 10 wt% relative to the mixture.

The photosensitive resin composition may be in a liquid state, a solid state, or a powder state, and when it is in a solid state or a powder state, it is in a liquid state at the time of use.

Using such a plate material, the above-described resin relief can be manufactured, for example, as described below. That is, first, the negative film 11 shown in fig. 6 is prepared. The negative film 11 has a portion corresponding to the minute projection 3 of the relief plate formed in a circular hole 11a in a region corresponding to the above-described resin relief plate, and the inside of the circular hole 11a is transparent, and the other portion is black. Next, the negative film 11 is laminated on the surface of the lower glass substrate 12, and then a liquid photosensitive resin composition having a polyester skeleton is applied to the surface of the negative film 11 to have a predetermined thickness. In fig. 6, the coating layer is denoted by reference numeral 13, and in the coating layer 13, an uncured portion to be described later is denoted by reference numeral 13a, and a cured portion is denoted by a hatched portion S. Then, a transparent base film (not shown) is laminated on the surface of the above coating layer 13, and the upper glass substrate 14 is laminated on the surface of the base film.

Next, as shown in fig. 6, the lamp 15 is used to irradiate light such as ultraviolet rays through the upper glass substrate 14 and the base film, and to irradiate light such as ultraviolet rays through the lower glass substrate 12 and the negative film 11. Thereby, the portion (hatched portion S in fig. 6) where the light incident from the entire upper surface of the layer formed of the photosensitive resin composition 13 and the light incident from the circular hole 11a of the negative film 11 reach is cured. At this time, the depth to which the light reaches is adjusted by the intensity of the irradiated light. Next, the upper and lower glass plates 12 and 14 and the negative film 11 are removed, and the negative film 11 is washed with a developer to remove a black portion of the negative film 11 and a portion (uncured portion) that is not yet irradiated with light and thus is uncured. Subsequently, after the cured portion is dried, light such as ultraviolet light is irradiated on the formation side of the fine protrusions 3 (post exposure), thereby accurately curing the fine wires and the like. This makes it possible to produce the resin relief shown in fig. 1.

In the above embodiment, since the plate material is a photosensitive resin composition having a polyester skeleton with high polarity, the resin relief plate produced using the plate material hardly swells in an aromatic solvent. Therefore, even if the printing process is repeated, the dimensional accuracy of the resin relief plate does not decrease, and stable dimensional accuracy can be maintained for a long period of time. In addition, the resin relief plate has a relatively soft hardness and causes almost no damage to a substrate of a printing object during printing.

Fig. 7 shows an embodiment of an organic EL element manufactured using the resin relief plate. In this embodiment, the organic EL element is formed by forming a transparent or translucent anode 22, an organic light-emitting layer 23, and a cathode 24 on the surface of a glass substrate 21. This element emits light by passing a current through the organic light-emitting layer 23 formed between the two electrodes 22 and 24 by a voltage supplied from the power supply 25 connected between the two electrodes 22 and 24. Then, the EL light is displayed through the glass substrate 21. The organic light-emitting layers 23 are of 3 types, red (R), green (G), and blue (B), and the organic light-emitting layers 23 of each color are formed in a stripe shape, are arranged in parallel at predetermined intervals, and are arranged by repeating the order of red, green, and blue. And at the time of printing of the organic light emitting layer 23 of 1 color, 1 sheet of resin relief is used, and resin relief corresponding to the number of the organic light emitting layers 23 may be used.

The organic EL device is manufactured, for example, as follows. That is, first, the anode 22 is formed on the surface of the glass substrate 21. In this step, a transparent anode 22 is formed by coating a transparent conductive material such as Indium Tin Oxide (ITO) or zinc aluminum oxide on the surface of the glass substrate 21 by a vacuum deposition method, a sputtering method, or the like, or a translucent anode 22 is formed by depositing thin gold or platinum.

Next, using the above resin relief, a positive hole transport layer and an organic light emitting layer 23 are sequentially formed on the above anode 22 by a conventional relief printing method. In the step of forming the organic light-emitting layer 23, first, 3 kinds of printing inks of red, green, and blue colors, which are obtained by dissolving and dispersing the organic EL light-emitting body in an aromatic solvent, are prepared. A printer shown in fig. 8 including a printing roller 31, an anilox roller 32, a printing table 33, an ink supply device 34, and a doctor blade 35 for scraping off excess printing ink on the anilox roller 32 was also prepared. Next, the resin relief plate is attached to the printing roller 31, and the glass substrate 21 on which the anode 22 (not shown) is formed is placed on the printing table 33. Next, printing ink having a certain color (for example, red) is supplied from the printing supply device 34 to the anilox roller 32, and the printing roller 31 and the anilox roller 32 are rotated. At this time, the printing ink is held in the grooves 4 on the top surfaces of the printing projections 2 of the resin relief plate. Next, the printing table 33 is moved in synchronization with the rotation of the printing roller 31, printing ink having a certain color is printed on the anode 22 on the surface of the glass substrate 21, and then the organic light emitting layer 23 is formed by heating to evaporate and vaporize the aromatic solvent in the printing ink. This printing is performed similarly for the other 2 colors, and 3 organic light emitting layers 23 are formed at predetermined positions.

Next, a cathode 24 is formed on the organic light-emitting layer 23. The cathode 24 is composed of elementary metal elements such as Li, Na, Mg, La, Ce, Ca, and the like. In the step of forming the cathode 24, the metal element monomers are co-deposited on the organic light-emitting layer 23 by the respective deposition sources by using a resistance heating method under a predetermined vacuum degree while monitoring with a crystal resonator thickness meter. Then, the two electrodes 22 and 24 are connected to a power supply 25. The organic EL element is thus manufactured.

According to this manufacturing method, even if a plurality of organic EL elements are manufactured using 1 sheet of resin relief plate, the dimensional accuracy of the organic light emitting layer 23 of the obtained organic EL element is stable. In addition, the damage to the substrate during the manufacture of the organic EL element is small, and the glass substrate 21 is not damaged.

The organic EL light-emitting body has a viscosity of 2 to 100 mPas, and a low-molecular material or a high-molecular material can be used. Examples of the low-molecular material include triphenylbutadiene, coumarin, nile red (nile red), and oxadiazole derivatives. Examples of the polymer material include poly (2-decyloxy-1, 4-phenylene) (DO-PPP), poly [2- (2' -ethylhexyloxy) -5-methoxy-1, 4-phenylacetylene ] (MEH-PPV), poly [ 5-methoxy- (sulfonylated 2-propoxy) -1, 4-phenylacetylene ] (MP S-PPV), poly [2, 5-bis (hexyloxy-1, 4-phenylene) - (1-cyanovinylene) ] (CN-PPV), poly [2- (2-ethylhexyloxy) -5-methoxy-1, 4-phenylene- (1-cyanovinylene) ] (MEH-CN-PPV), Poly (dioctylfluorene), and the like. Examples of the solvent used herein include cyclohexylbenzene, trichlorobenzene, anisole, xylene, ethylbenzoate, cyclohexylpyrrolidone, butylcellosolve, dichlorobenzene, and toluene, and these solvents may be used alone or in a mixture of 2 or more. The mixing ratio at the time of mixing is determined according to the organic light-emitting layer 23.

Examples

Next, examples will be described.

A resin relief plate having the structure shown in fig. 1 to 3 was prepared. The resin relief plate had a JIS rubber hardness of Shore A85, and a rate of change in the resin relief plate weight was 5% when the plate was continuously immersed in anisole at room temperature of 23 ℃ for 24 hours. This resin relief plate was produced by the same method as the above-mentioned resin relief plate production method using the plate materials shown in table 1 below, and had a total plate thickness of 1.3mm and a substrate thickness of 0.7 mm.

TABLE 1

	Total thickness of plate (mm)	Rubber hardness (°)	Rate of change in weight (%)	Tensile Strength (MPa)	Elongation at Break (%)
						Material A	1.30	88	1.5	2.4	20
Material B	1.30	84	3.1	3.6	30
						Material C	1.30	90	4.2	2.4	25

(Note 1) rubber hardness was measured in accordance with JIS K6253 "hardness test method for rubber", using dumbbell shape No. 3.

(Note 2) tensile strength and elongation at cut were measured in accordance with JIS K6251 "tensile test method for rubber".

(Note 3) the weight change rate was measured from the weight of a test piece of 1.3 mm. times.20 mm. times.30 mm before and after being continuously immersed for 24 hours in a room temperature environment of 23 to 25 ℃.

As a printing machine, a machine shown in fig. 8 (MTTEC co., flexible printing machine manufactured by ltd., FC33S) was prepared. The printing conditions are as follows: the width of a gap between the resin relief printing plate and the anilox roller is adjusted to be 3.5-6.5 mm, the width of the gap between the resin relief printing plate and the glass substrate is adjusted to be 7.5-12.5 mm, and the printing speed is adjusted to be 20 m/min.

As the pseudo ink, a mixture of anisole and tetralin in a ratio of 1: 1 was used, and 0.2 to 1.5 wt% of polyvinylcarbazole (molecular weight 90000, manufactured by Kanto chemical Co., Ltd.) was dissolved therein. The pseudo ink is transported by 0.01 to 0.1MPa of dry air, and is adjusted to be supplied to the anilox roller by only 0.5 to 5 ml for 40 seconds.

The resin relief plate is mounted on a cylinder of a printing machine, the printing machine is driven under the printing conditions and the printing speed, a pseudo ink is printed on a glass substrate for constituting an organic EL element, and then anisole and tetralin are evaporated and vaporized on a hot plate heated to 60 to 90 ℃ to be removed, and only effective components are precipitated to form an organic light emitting layer.

The organic light-emitting layer is measured by using a light interference microscope, and the result is detected to be 400-700Film thickness of (2). Further, the ultraviolet lamp was brought close to the film surface, and the suspected ink was confirmed to emit light, and the organic EL light-emitting body was applied.

Claims (5)

1. A method for manufacturing an organic EL element, comprising the steps of:

a step of preparing a resin relief plate for relief printing, which is obtained by adding a product of addition polymerization of an acrylate, a polyester polyol and a diisocyanate, and a photosensitive resin composition obtained by blending a prepolymer and a monofunctional or polyfunctional acrylate monomer as a plate material; and the combination of (a) and (b),

and a step of forming an organic light-emitting layer by printing a printing ink obtained by dissolving an organic EL light-emitting body in an aromatic solvent on a substrate having an anode formed on the surface thereof, and evaporating and vaporizing the aromatic solvent, while holding the printing ink in a groove portion on the top surface of a printing projection of the resin relief plate for relief printing.

2. The method for producing an organic EL element according to claim 1, wherein the resin relief plate for relief printing has a hardness in the range of Shore A60 to 90 ° in terms of JIS rubber hardness, and the plate material is developable with an aqueous detergent.

3. The method for producing an organic EL element according to claim 1 or 2, wherein the relief printing resin relief plate has a swelling ratio in a range of 0.2 to 15% in a weight change ratio before and after immersion in an aromatic solvent such as anisole, cyclohexylbenzene, or tetralin, or a mixed solvent thereof, which dissolves or disperses the organic EL light-emitting body, when the relief printing resin relief plate is immersed continuously in an environment at room temperature of 20 to 25 ℃ for 24 hours.

4. The method for producing an organic EL element according to claim 1 or 2, wherein the resin relief plate for relief printing has resin characteristics such that the elongation at cutting is 15 to 200%, the tensile strength is 2 to 100MPa, and the maximum tensile force is 30 to 200N.

5. The method for producing an organic EL element according to claim 1 or 2, wherein the resin relief plate for relief printing has a printing convex portion on a pattern printing surface, and a stripe-shaped concave portion having a concave width of 10 to 500 μm or a minute convex portion having a convex diameter of 10 to 500 μm for holding printing ink is formed on a surface thereof.