Flexible printing plate

Abstract

The present application provides a flexible printing plate which reduces dot enlargement of a fine dot in a fade-out section and improves printability. A flexible printing plate characterized by being obtained from a flexible printing master plate in which a support (A), a photosensitive resin layer (B), and a heat-sensitive mask layer (C) are at least sequentially stacked, wherein when a dot having a diameter of 16 μm is formed on the printing plate by a 175 line, the dot does not show bending until a load of 0.2 N. The flexible printing plate is preferably such that when a dot having a diameter of 16 μm is formed on the printing plate by a 175 line, the shoulder angle of the dot is 105 to 125°, and the dot depth is 70 to 85 μm.

Description

Technical Field

[0001] This invention relates to a flexible printing plate with excellent printability of tiny dots in fade-out areas. Background Technology

[0002] Flexographic printing is a printing method that transfers ink from the plate to the substrate by pressing the plate against the substrate. Because the plates used in flexographic printing are flexible and can conform to various shapes, it can print on a wide range of substrates. Examples of substrates include packaging films, labels, beverage cartons, paper containers, envelopes, and cardboard. Flexographic printing is particularly suitable for substrates with rough surfaces. Furthermore, flexographic printing can use water-based and alcohol-based inks with low VOC emissions, making it a highly environmentally friendly printing method. These advantages, such as substrate adaptability and environmental friendliness, are driving the transition from gravure and offset printing to flexographic printing.

[0003] However, since flexographic printing involves pressing a relatively flexible printing plate onto a substrate, the plate deforms during the pressing process, resulting in the printed dots appearing coarser compared to the plate's dots (dot gain). This dot gain is particularly prone to occur at the transition points between dot gradation and zero (fade-out parts). Furthermore, the coarser printing in these areas is very noticeable due to the contrast with zero, significantly reducing print quality.

[0004] Previously, as a method to reduce dot gain, methods to increase the dynamic hardness of the dots have been proposed (see Patent Document 1). In Patent Document 1, the dynamic hardness of the dots is increased by mixing multifunctional crosslinking monomers into the photosensitive resin layer to increase the crosslinking density. However, while this method can reduce dot gain in the center of the image, it cannot suppress the thickening of the fade-out areas.

[0005] To suppress printing coarsening in the fade-out areas, a method was proposed to pre-set the height of the dot tops at the ends to be lower than that at the center (Patent Document 2). However, this method can only be implemented through laser engraving and cannot be used in the conventional exposure and development process. Laser engraving suffers from problems such as expensive equipment and low production speed.

[0006] On the other hand, Patent Document 3 is cited as a general example of a flexographic printing plate. The photosensitive resin layer of the flexographic printing plate in Patent Document 3 is characterized by being composed of (A) a hydrophobic polymer obtained from an aqueous dispersion latex, (B) a hydrophilic polymer, (C) a photopolymerizable unsaturated compound, and (D) a photopolymerization initiator. In (C) the photopolymerizable unsaturated compound, (C-1) a hydroxyl-free photopolymerizable oligomer, (C-2) a hydroxyl-containing photopolymerizable monomer, and (C-3) a hydroxyl-free photopolymerizable monomer are used. While the flexographic printing plate of the embodiment in Patent Document 3 shows improvements in printability and printability, the printability of fade-out areas is poor.

[0007] Therefore, there is a strong demand for flexible printing plates that can reduce dot gain in the fade-out areas. Existing technical documents Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 9-80743 Patent Document 2: Japanese Patent No. 6395920 Patent Document 3: Japanese Patent No. 6079625 Summary of the Invention The problem that the invention aims to solve

[0009] The present invention was created in view of the current state of the prior art, and its purpose is to provide a flexographic printing plate that reduces dot gain of tiny dots in fade-out areas and improves printability. The means to solve the problem

[0010] The inventors conducted in-depth research to achieve the above-mentioned objectives and discovered that conventional printing plates produce softer dots with small shoulder angles and deeper dot depths. This results in dots that are prone to collapsing (bending) even under low loads, leading to large dots and poor image reproducibility during printing. Furthermore, it was found that by controlling the shape and hardness of the tiny dots when forming them on the printing plate, the dots do not bend even under specific high loads. This reduces dot gain not only in the center of the image but also in fade-out areas. Specifically, it was found that by controlling the shape of the tiny dots, including their shoulder angle and dot depth, within a specific range, and further controlling the hardness of the tiny dots within a specific range, the effect of reducing dot gain can be improved.

[0011] Furthermore, it was discovered that controlling the shape and hardness of the dots is effective, thus completing the present invention. These methods include: mixing a larger quantity of photopolymerization initiator in the photosensitive resin composition constituting the photosensitive resin layer than previously implemented; furthermore, including a certain amount of low-molecular-weight olefin unsaturated compound in the photosensitive resin composition, and achieving a higher mass ratio of photopolymerization initiator to low-molecular-weight olefin unsaturated compound than before; furthermore, mixing high-molecular-weight olefin unsaturated compound in addition to the low-molecular-weight olefin unsaturated compound in the photosensitive resin composition; furthermore, using two specific photopolymerization initiators; and furthermore, providing an oxygen barrier layer between the photosensitive resin layer and the mask layer.

[0012] That is, the present invention has the following configurations (1) to (10). (1) A flexible printing plate, characterized in that it is a flexible printing plate obtained by forming a flexible printing master plate consisting of at least a support (A), a photosensitive resin layer (B) and a thermal mask layer (C) in sequence, wherein when the printing plate forms halftone dots with a diameter of 16 μm by 175 lines, the halftone dots do not show bending until a load of 0.2 N is applied. (2) The flexible printing plate according to (1) is characterized in that when the halftone dots with a diameter of 16 μm are formed by 175 lines on the printing plate, the shoulder angle of the halftone dots is 105 to 125°. (3) The flexible printing plate according to (1) or (2) is characterized in that when the dots with a diameter of 16 μm are formed by 175 lines on the printing plate, the dot depth is 70 to 85 μm. (4) The flexible printing plate according to any one of (1) to (3), characterized in that the photosensitive resin composition forming the photosensitive resin layer (B) contains (a) a polymer formed by polymerizing a conjugated diene, (b) an olefinic unsaturated compound and (c) a photopolymerization initiator, wherein the content of (c) the photopolymerization initiator in the photosensitive resin composition is 2 to 9% by mass. (5) The flexible printing plate according to (4) is characterized in that (b) the olefinic unsaturated compound contains a (meth)acrylate compound (i) with a number average molecular weight of 100 or more and 600 or less, the content of the (meth)acrylate compound (i) with a number average molecular weight of 100 or more and 600 or less in the photosensitive resin composition is 5 to 16% by mass, and the ratio of the mass of (c) the photopolymerization initiator to the mass of the (meth)acrylate compound (i) with a number average molecular weight of 100 or more and 600 or less in the photosensitive resin composition is in the range of 0.20 to 0.55. (6) The flexible printing plate according to (5), characterized in that (b) the olefinic unsaturated compound further contains a (meth)acrylate compound (ii) with a number average molecular weight greater than 600 and less than 20,000, and the content of the (meth)acrylate compound (ii) with a number average molecular weight greater than 600 and less than 20,000 in the photosensitive resin composition is 5 to 25% by mass. (7) The flexible printing plate according to (5) or (6) is characterized in that (c) the photopolymerization initiator contains two compounds: a benzoyl alkyl ketal compound and a benzophenone compound, wherein the mass ratio of the benzoyl alkyl ketal compound to the benzophenone compound is in the range of 99:1 to 80:20. (8) The flexible printing plate according to any one of (1) to (7), characterized in that the flexible printing plate has an oxygen barrier layer (D) between the photosensitive resin layer (B) and the thermal mask layer (C). (9) The flexographic printing plate according to any one of (1) to (8), characterized in that it is obtained by developing the flexographic printing plate with an aqueous developer. (10) A flexible printing method, characterized in that a flexible printing plate as described in any one of (1) to (9) is used. Invention Effects

[0013] The flexible printing plate of the present invention is configured such that when specific micro dots are formed on the printing plate, the dots do not show bending until a specific high load is applied. Therefore, not only in the center of the image, but also in the fade-out area, the dot gain of the micro dots can be reduced, and as a result, the printing quality of the micro dots in the fade-out area can be improved. Attached Figure Description

[0014] Figure 1 This is a schematic longitudinal section diagram illustrating the shoulder angle of the halftone dots. Figure 2 This is a graph used to calculate the bending loads of Example 2 and Comparative Example 1. Detailed Implementation

[0015] The flexible printing plate of the present invention is a flexible printing plate obtained by sequentially stacking a support (A), a photosensitive resin layer (B), and a thermal mask layer (C) on a flexible printing master. Specifically, it is a printing plate obtained by exposing and developing such a flexible printing master. Its characteristic feature is that when specific micro-dots are formed on the printing plate, these dots do not exhibit bending up to a specific high load. By configuring the micro-dots formed on the printing plate as described above so that they are not easily bent even under a specific high load, dot gain caused by the pressure during printing is effectively prevented not only in the center of the image but also in the fade-out areas.

[0016] The support (A) used in the flexographic printing master is preferably made of a material that is flexible yet has excellent dimensional stability. For example, it can be made of metals such as steel, aluminum, copper, or nickel, or thermoplastic resins such as polyethylene terephthalate (PET), polyethylene naphthalate (PET), polybutylene terephthalate (PET), or polycarbonate. Among these, PET film, which has excellent dimensional stability and sufficiently high viscoelasticity, is particularly preferred. From the viewpoints of mechanical properties, shape stability, and operability during printing plate making, the thickness of the support is ideally 50–350 μm, and preferably 100–250 μm. Furthermore, if necessary, an adhesive can be provided between the support (A) and the photosensitive resin layer (B) to improve the adhesion between them.

[0017] The photosensitive resin composition forming the photosensitive resin layer (B) used in flexographic printing master plates is a composition containing (a) a polymer obtained by polymerizing a conjugated diene, (b) an olefinically unsaturated compound, and (c) a photopolymerization initiator, and, if necessary, further containing additives such as plasticizers, hydrophilic compounds, ultraviolet absorbers, surface tension modifiers, thermal polymerization inhibitors, dyes, pigments, fragrances, or antioxidants. In particular, the present invention is characterized by designing the composition of (b) the olefinically unsaturated compound in the photosensitive resin composition forming the photosensitive resin layer (B), and using (c) the photopolymerization initiator in a greater mass ratio than previously used.

[0018] As for the polymer obtained by (a) polymerizing conjugated dienes, conventionally known synthetic polymers used in printing plates can be used. Specifically, polymers obtained by polymerizing conjugated diene hydrocarbons, or copolymers obtained by copolymerizing conjugated diene hydrocarbons and monoolefin unsaturated compounds, can be listed. Examples include butadiene polymers, isoprene polymers, chloroprene polymers, styrene-butadiene copolymers, styrene-butadiene-styrene copolymers, styrene-isoprene copolymers, styrene-isoprene copolymers, styrene-chloroprene copolymers, acrylonitrile-butadiene copolymers, acrylonitrile-isoprene copolymers, methyl methacrylate-butadiene copolymers, methyl methacrylate-isoprene copolymers, acrylonitrile-butadiene-styrene copolymers, acrylonitrile-isoprene-styrene copolymers, etc. Among these, from the viewpoint of the characteristics of a flexographic printing plate, namely the resilience of the plate surface, the elongation property, the hardness of the resin plate, and the morphological stability or availability when unexposed, butadiene polymers are preferred. These polymers can be used alone or in combination of two or more. The proportion of component (a) in the photosensitive resin composition forming the photosensitive resin layer (B) is preferably in the range of 40 to 70% by mass.

[0019] As for (b) the olefinic unsaturated compound, conventionally known compounds used in printing originals can be used, preferably containing (meth)acrylate compounds with a number average molecular weight of 100 or more and 600 or less (hereinafter also referred to as low molecular weight (meth)acrylate compounds) (i), and more preferably containing (meth)acrylate compounds with a number average molecular weight greater than 600 and 20,000 or less (hereinafter also referred to as high molecular weight (meth)acrylate compounds) (ii). Here, low molecular weight (meth)acrylate compounds are compounds that form a dense cross-linked network through cross-linking and curing by a photopolymerization initiator, while high molecular weight (meth)acrylate compounds are compounds that form a loose cross-linked network through cross-linking and curing by a photopolymerization initiator. The former is further preferably with a number average molecular weight of 200 or more and 500 or less, and the latter is further preferably with a number average molecular weight of 2000 or more and 10,000 or less. By mixing not only low-molecular-weight (meth)acrylate compounds as described above, but also high-molecular-weight (meth)acrylate compounds, even when mixing a larger amount of (c) photopolymerization initiator than before, the reproducibility of individual points and the durability during printing are not compromised. This is believed to be due to the increased toughness of the printing plate by containing a certain proportion of high-molecular-weight (meth)acrylate compounds. The proportion of component (b) in the photosensitive resin composition forming the photosensitive resin layer (B) is preferably in the range of 10 to 50% by mass.

[0020] The content of the high molecular weight (meth)acrylate compound in the photosensitive resin composition is preferably 5-25% by mass, more preferably 8-20% by mass. If the content is below the above range, the durability is easily reduced when a large amount of (c) photopolymerization initiator is mixed. If the content exceeds the above range, the complex elastic modulus of the solid portion is easily increased, and the ink spread in the solid portion is insufficient, which may result in poor print quality. Furthermore, the content of the low molecular weight (meth)acrylate compound in the photosensitive resin composition is preferably 5-16% by mass, more preferably 7-13% by mass. When the content of the low molecular weight (meth)acrylate compound is less than the above range, the effect of increasing the complex elastic modulus of the dots is insufficient, and small dots may easily show bending under low load. If the content exceeds the above range, the complex elastic modulus of the solid portion becomes high, and the ink spread in the solid portion is insufficient, which may result in poor print quality.

[0021] As low molecular weight (meth)acrylate compounds, there are no particular restrictions as long as the number average molecular weight is between 100 and 600. Examples include (meth)hexyl acrylate, (meth)nonyl acrylate, (meth)lauryl acrylate, (meth)stearyl acrylate, 2-ethylpropanediol (meth)acrylate, 2-butylpropanediol (meth)acrylate, (meth)hydroxyethyl acrylate, 2-(meth)acryloyloxyethyl hexahydrophthalic acid ester, 2-(meth)acryloyloxyethyl phthalic acid ester, (meth)acrylate dimer, ECH modified allyl acrylate, benzyl acrylate, (meth)caprolactone, (meth)acrylate dicyclopentenyl acrylate, (meth)acrylate isobornyl acrylate, (meth)acrylate cyclohexyl acrylate, etc., which are straight-chain, branched, or cyclic monofunctional monomers. In addition, examples include hexanediol di(meth)acrylate, nonanediol di(meth)acrylate, 2-butyl-2-ethylpropane di(meth)acrylate, neopentyl glycol di(meth)acrylate, neopentyl glycol hydroxypentanoate di(meth)acrylate, ECH-modified phthalic acid di(meth)acrylate, dicyclopentadiene di(meth)acrylate, tricyclodecanediethanol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, ECH-modified glycerol tri(meth)acrylate, trimethylolpropane benzoate (meth)acrylate, EO(PO)-modified trimethylolpropane tri(meth)acrylate, and dipentaerythritol hexa(meth)acrylate, as well as other straight-chain, branched, and cyclic polyfunctional monomers. These compounds can be used individually, or in combination of two or more to achieve the desired resin properties.

[0022] As for high molecular weight (meth)acrylate compounds, there are no particular restrictions as long as the number average molecular weight is in the range of more than 600 and less than 20,000. Examples include substances that impart (meth)acrylate groups to butadiene oligomers and isobutylene oligomers; and urethane (meth)acrylates. These compounds can be used alone or in combination to achieve the desired resin properties.

[0023] Preferably, the ratio of the mass of the photopolymerization initiator to the mass of the low molecular weight (meth)acrylate compound (i) [(mass of photopolymerization initiator) / (mass of low molecular weight (meth)acrylate compound)] is 0.20 to 0.55. More preferably, it is 0.22 to 0.50, and even more preferably, it is 0.25 to 0.45. By setting such a ratio, the shoulder angle, dot depth, and complex elastic modulus of the dots can be adequately satisfied. If the ratio is less than the above range, there is too little photopolymerization initiator, and the crosslinking reaction will not occur under a small amount of light (scattered light in the photosensitive resin layer). As a result, the dots become unstable with small shoulder angles and deep dot depths, and the complex elastic modulus of the dots also decreases. Ultimately, the bending load of the dots on the printing plate may become lower. If the ratio exceeds the above range, there is too much photopolymerization initiator relative to the (meth)acrylate, and the effect caused by increasing the amount of photopolymerization initiator is not observed.

[0024] Examples of photopolymerization initiators (c) include benzophenones, benzoin derivatives, acetophenones, benzoyl derivatives, benzoin alkyl ethers, benzoyl alkyl ketals, anthraquinones, and thioxanthones. Specific examples include benzophenone, chlorobenzophenone, benzoin, acetophenone, benzoyl, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutyl ether, benzoyl dimethyl ketal, benzoyl diethyl ketal, benzoyl diisopropyl ketal, anthraquinone, 2-ethylanthraquinone, 2-methylanthraquinone, 2-allylanthraquinone, 2-chloroanthraquinone, thioxanthone, and 2-chlorothioxanthone.

[0025] (c) The photopolymerization initiator preferably contains compounds composed of benzoyl alkyl ketals and benzophenone compounds. Furthermore, the mass ratio of the benzoyl alkyl ketal compound to the benzophenone compound is preferably 99:1 to 80:20, and more preferably in the range of 97:3 to 85:15. Since the photopolymerization initiator has an inherent light absorption spectrum, by using both photopolymerization initiators, the light energy from the subsequent post-exposure and exposure using a germicidal lamp can be used without waste. In particular, by using both benzoyl alkyl ketals and benzophenone compounds as photopolymerization initiators, the complex elastic modulus of the dots can be further improved, resulting in increased flexural load on the micro-dots of the printing plate. Benzoyl dimethyl ketal is preferred as the benzoyl alkyl ketal compound, and benzophenone is preferred as the benzophenone compound. In particular, benzophenone does not absorb light from the lamp used for main exposure (peak wavelength 370nm), but it does absorb light from the lamp used for germicidal exposure (peak wavelength 250nm). Therefore, the complex elastic modulus of the dots can be increased by exposure using a germicidal lamp alone, without causing excessive cross-linking reaction by the main exposure. As a result, the bending load of the tiny dots on the printing plate can be increased.

[0026] In this invention, the content of (c) photopolymerization initiator in the photosensitive resin composition is preferably 2-9% by mass, more preferably 2.5-7.5% by mass, and even more preferably 3-7% by mass. By mixing the photopolymerization initiator in such a larger amount than conventionally used, the crosslinking reaction occurs even under a small amount of light (scattered light in the photosensitive resin layer), thus forming micro-dots with large shoulder angles, shallow dot depth, and stable shapes, and the dot complex modulus also increases. As a result, the micro-dots do not show bending even under certain high loads. Conventionally, if the content of photopolymerization initiator is increased, the light absorption of the photopolymerization initiator after the reaction will block the light reaching in the thickness direction of the plate, resulting in problems such as reduced reproducibility of independent points important for crosslinking in the thickness direction and reduced durability during printing. Therefore, the content of photopolymerization initiator in the photosensitive resin composition is usually only about 1% by mass in practical use. In contrast, in this invention, by containing a high molecular weight (meth)acrylate compound in a certain proportion in the photosensitive resin composition, the toughness of the plate can be improved, overcoming the aforementioned conventional disadvantages (reduced reproducibility of individual dots and reduced durability during printing) when a large amount of photopolymerization initiator is mixed. Furthermore, the advantages resulting from the increased content of the photopolymerization initiator can be enjoyed (increased bending load of micro-dots due to the formation of stable-shaped micro-dots and the improvement of dot composite elasticity, and improved printability due to the reduction of dot gain of micro-dots in the fade-out areas as a result).

[0027] The photosensitive resin composition used to form the photosensitive resin layer (B) in the flexographic printing master is a composition containing (a) a polymer obtained by polymerizing conjugated diene, (b) an olefinic unsaturated compound and (c) a photopolymerization initiator, and may also contain, as needed, additives such as plasticizers, hydrophilic compounds, ultraviolet absorbers, surface tension modifiers, thermal polymerization inhibitors, dyes, pigments, fragrances or antioxidants.

[0028] Plasticizers are substances that impart flexibility to the photosensitive resin layer (B). Examples of plasticizers include liquid rubber, oils, polyesters, and phosphoric acid compounds. Examples of liquid rubbers include liquid polybutadiene, liquid polyisoprene, or rubbers formed by imparting hydroxyl and carboxyl groups to them. Examples of oils include paraffin wax, naphthenic acids, and fragrances. Examples of polyesters include adipic acid-based polyesters. Examples of phosphoric acid compounds include phosphate esters. From the viewpoint of compatibility with polymers obtained by polymerizing conjugated dienes, liquid polybutadiene and liquid polybutadiene with hydroxyl and carboxyl groups are preferred. When developing with an aqueous developer, liquid polybutadiene with hydroxyl and carboxyl groups is particularly preferred. The content of plasticizer in the photosensitive resin composition is preferably 5 to 15% by mass.

[0029] The hydrophilic compound is a substance that improves the developability of the photosensitive resin layer (B) in an aqueous developer. Examples of hydrophilic compounds include acrylic polymers, urethane polymers, polyamide polymers, and polyester polymers having hydrophilic groups such as carboxylic acid, carboxylates, sulfonic acids, sulfonates, hydroxyl groups, amino groups, phosphate groups, ethylene oxide, and propylene oxide. Furthermore, known surfactants can also be used. From the viewpoint of developability in aqueous developers, urethane polymers having carboxylates are preferred. The content of the hydrophilic compound in the photosensitive resin composition is preferably 1 to 15% by mass.

[0030] Surface tension modifiers are substances that adjust the surface tension of printing plates. By adjusting the surface tension of the printing plate, the transferability of ink and ink clogging of the printing plate can be controlled. Examples of surface tension modifiers include paraffin oil, long-chain alkyl compounds, surfactants, fatty acid amides, silicone oil, modified silicone oil, fluorinated compounds, and modified fluorinated compounds. The content of the surface tension modifier in the photosensitive resin composition is preferably 0.1% to 2% by mass.

[0031] Ultraviolet (UV) absorbers are substances that improve the exposure tolerance of the photosensitive resin layer (B). Examples of UV absorbers include benzophenone-based, salicylate-based, benzotriazole-based, acrylonitrile-based, metal complex salt-based, hindered amine-based, anthraquinone-based, azo-based, coumarin-based, and furan-based compounds. Among these, benzotriazole-based compounds are preferred from the viewpoint of availability and exposure tolerance. The content of the UV absorber in the photosensitive resin composition is preferably 0.005–0.1% by mass.

[0032] Typically, when manufacturing flexographic printing plates from flexographic master plates, four types of exposure are performed: back exposure, main exposure, post-exposure, and exposure using a germicidal lamp. Back exposure is used to illuminate the entire surface from the support side to form the base of the printing plate. Main exposure involves illuminating the flexographic master plate in a patterned manner using a mask, causing the unsaturated compounds in the photosensitive resin layer of the illuminated areas to cross-link and solidify, forming the image portion. Post-exposure is performed after the main exposure, illuminating the entire surface of the plate after the formation of halftone dots and solid areas. It supplements the cross-linking and solidification in the main exposure and cross-links and solidifies the sides of the halftone dots. Exposure using a germicidal lamp is performed to remove surface adhesion of the plate, but like post-exposure, it also partially cross-links and solidifies the sides of the halftone dots. Furthermore, main and post-exposure are typically performed using UVA, while exposure using a germicidal lamp is performed using UVC.

[0033] The curing of the dot sides caused by post-exposure and exposure using a germicidal lamp has occurred to some extent even in conventional exposures. However, since post-exposure and exposure using a germicidal lamp are usually performed in the atmosphere, polymerization inhibition caused by atmospheric oxygen occurs. Therefore, in the past, the degree of curing of the dot sides caused by post-exposure and exposure using a germicidal lamp was not significant. In contrast, in this invention, by mixing in a larger amount of photopolymerization initiator than in the past, the effect of polymerization inhibition caused by oxygen can be sufficiently suppressed, and the dot sides can be fully cured, thereby significantly increasing the complex elastic modulus of the dot and improving the bending load of small dot surfaces. As for the effect of fully curing the dot sides, it is presumably because the deformation of the dot surface is suppressed due to the full curing of the sides.

[0034] When the content of the photopolymerization initiator is below the preferred range described above, the hardness of the dots during post-exposure and exposure using a germicidal lamp becomes insufficient. Furthermore, the shape of the microdots cannot achieve the shoulder angle and depth range specified in this invention. It is easy to observe that the microdots bend under loads lower than those specified in this invention, and dot gain may occur in the fade-out areas. On the other hand, if the content of the photopolymerization initiator exceeds the preferred range described above, the reproducibility of individual dots and the durability of the printing plate may decrease.

[0035] The thermal mask layer (C) used in flexographic printing master plates can be any thermal mask layer used in printing master plates, but preferably, for example, a substance composed of carbon black, a material that has the function of absorbing infrared laser light and converting it into heat and blocking ultraviolet light, its dispersing binder, and a film-forming adhesive polymer. The dispersing binder and the film-forming adhesive polymer can also be used in combination. In addition, as any other component, without impairing the effects of the present invention, pigment dispersants, fillers, surfactants, or coating aids may be included.

[0036] The thermal mask layer (C) used in the flexographic printing master of the present invention is preferably water-developable. Examples of specific thermal mask layers (C) include a thermal mask layer obtained by combining a polyamide containing polar groups and a butyral resin (Japanese Patent No. 4200510), a thermal mask layer containing a polymer with the same structure as the polymer in the photosensitive resin layer and an acrylic resin (Japanese Patent No. 5710961), and a thermal mask layer containing an anionic polymer and a polymer having ester bonds in its side chains and a saponification degree of 0% or more and 90% or less (Japanese Patent No. 5525074), etc.

[0037] In the flexographic printing master of the present invention, an oxygen barrier layer (D) is preferably provided between the photosensitive resin layer (B) and the thermal mask layer (C). By providing the oxygen barrier layer (D), the inhibition of oxygen polymerization during the main exposure is suppressed, resulting in a sufficient curing reaction. As a result, tiny dots with large shoulder angles, shallow dot depth, and stable shapes are formed, and these tiny dots do not show bending up to a certain high load. Examples of adhesive polymers in the barrier layer include polyvinyl alcohol, partially saponified vinyl acetate, alkyl cellulose, cellulose-based polymers, and polyamides. These polymers are not limited to using one, and two or more polymers may be used in combination. Preferred adhesive polymers in terms of oxygen barrier properties are polyvinyl alcohol, partially saponified vinyl acetate, and polyamides. By selecting an adhesive polymer with oxygen barrier properties within a preferred range, image reproducibility can be appropriately controlled.

[0038] The thickness of the barrier layer is preferably 0.2 μm to 3.0 μm, more preferably 0.2 μm to 1.5 μm. If the layer thickness is less than the above range, the oxygen barrier effect becomes insufficient, and roughness may occur on the relief surface. If the thickness exceeds the above range, poor reproduction of fine lines may occur.

[0039] The method for manufacturing the flexible printing original of the present invention is not particularly limited, and is generally as follows. First, the binder and other components of the thermal mask layer (C) are dissolved in a suitable solvent to disperse the carbon black, thus creating a dispersion. Next, this dispersion is coated onto a thermal mask layer support (e.g., a polyethylene terephthalate film), and the solvent is evaporated. Then, the oxygen barrier layer (D) is applied externally to prepare a laminate. Furthermore, another laminate is prepared by coating a photosensitive resin layer (B) onto the support (A). The two laminates thus obtained are then laminated under pressure and / or heat, such that the photosensitive resin layer (B) is adjacent to the oxygen barrier layer (D). Additionally, the thermal mask layer support functions as a protective film on the surface of the printed master.

[0040] As a method for manufacturing a flexographic printing plate from a flexographic printing master as described above, the protective film is first removed from the flexographic printing master when it is present. Then, a thermal mask layer (C) is patterned by irradiating it with an IR laser, forming a mask on the photosensitive resin layer (B). Suitable IR lasers include ND / YAG lasers (1064 nm) or diode lasers (e.g., 830 nm). Laser systems suitable for computer plate-making technology are commercially available; for example, CDI (Esco Graphics) can be used. The laser system includes a rotating cylindrical roller holding the printing master, an IR laser irradiation device, and a planning output computer from which image information is directly transmitted to the laser device.

[0041] After image information is written onto the thermal mask layer (C), the entire surface of the flexographic printing plate is irradiated with active light (main exposure) through a patterned mask. This can also be done with the plate mounted in the laser tube, but to handle non-standard plate sizes, it is advantageous to remove the plate from the laser device and irradiate it with a conventional irradiation unit with a flat plate shape, which is the usual method. As the active light, ultraviolet light with a emission peak at wavelengths of 330–380 nm can be used. As the light source, low-pressure mercury lamps, high-pressure mercury lamps, ultra-high-pressure mercury lamps, metal halide lamps, xenon lamps, zirconium lamps, carbon arc lamps, and ultraviolet fluorescent lamps can be used. The irradiated plate is then developed, post-exposed, and then exposed to a germicidal lamp to obtain the flexographic printing plate. The developing process can be performed using a conventional developing unit.

[0042] The flexographic printing plate obtained as described above can have the following characteristics: when forming tiny dots with a diameter of 16 μm at 175 lines on the printing plate, the dots do not show bending under loads up to 0.2 N, further up to 0.3 N, further up to 0.5 N, and further up to 1.0 N. Because the flexographic printing plate of the present invention has such characteristics, the tiny dots on the printing plate do not bend during actual printing, resulting in no thickening of the print in the fade-out areas. When the tiny dots bend, ink is transferred not only from the dot protrusions but also from the sides of the dots to the printed material, potentially resulting in very large dot gain.

[0043] In this invention, the determination of the load under which the dots do not show bending is performed by finding the minimum load (bending load) at which bending occurs. Specifically, the following method is used. The TMAQ400, manufactured by TA Instruments Japan Co., Ltd., was used as the instrument for measuring bending. A probe with a contact diameter of 0.89 mm was inserted into the halftone dot at a load speed of 0.5 N / min to measure the degree of compression (deformation) of the halftone dot. The inflection point of the halftone dot deformation was taken as the bending load. The preload was set to 0.01 N, the holding time before measurement was set to 1 minute, and the maximum load was set to 1 N. The halftone dots used for measurement were 175 lines, 16 μm in diameter, and cut into 4 mm square dots. The cut printing plates were stored at 25°C and 65% RH for 24 hours before measurement. Five measurements were performed, and the average value was calculated.

[0044] Furthermore, the flexographic printing plate obtained as described above can have the following characteristics: when forming tiny dots with a diameter of 16 μm at 175 lines on the printing plate, the dot shoulder angle is 105–125°, and more specifically 110–120°. If the shoulder angle is smaller than the above range, the tiny dots are prone to bending, and the printing of the tiny dots tends to become coarser in the fade-out areas. Furthermore, if the shoulder angle exceeds the above range, dot filling tends to occur in shadow parts and the like.

[0045] In this invention, a color 3D laser microscope VK-9700 (manufactured by Keyence Co., Ltd.) is used as the instrument for measuring the shoulder angle of the halftone dots. A 50x objective lens is used, and the measurement interval is set to 0.5 μm. Let the longitudinal section representing the halftone dots be... Figure 1 The area in the middle is set as the dot shoulder angle. Dots with a diameter of 16μm are measured using 175 lines.

[0046] Furthermore, the flexographic printing plate obtained as described above can have the following characteristics: when forming minute dots with a diameter of 16 μm at 175 lines on the printing plate, the dot depth is 70–85 μm, and more specifically 72–83 μm. If the dot depth is less than the above range, the minute dots are prone to bending, and the printing of the minute dots tends to become coarser in fade-out areas. Furthermore, if the dot depth exceeds the above range, dot filling tends to occur in shadow areas, etc.

[0047] In this invention, a color 3D laser microscope VK-9700 (manufactured by Keyence Co., Ltd.) is used as the instrument for measuring dot depth. A 50x objective lens is used, and the measurement interval is set to 0.5 μm. The distance between the top and bottom of the dot is defined as the dot depth. Dots with a diameter of 16 μm are measured at 175 lines.

[0048] The printing plates used to measure the bending load, dot shoulder angle, and dot depth were prepared under the following conditions. Regarding back exposure, main exposure, and post-exposure, a Philips TL-K 40W / 10R lamp (with a peak wavelength of 370nm and a peak wavelength of 350nm, and an illuminance of 10mW / cm²) was used. 2 The germicidal lamp used is the Panasonic GL-40 (peak wavelength 250nm, illuminance of 4.5mW / cm²) manufactured by Panasonic Corporation. 2The thermal mask layer image was created using a CDI 4850 manufactured by ESCO Graphics at a resolution of 4000 dpi. Evaluation images were set to have at least 175 lines, halftone dots ranging from 0% to 10% with a 0.3% amplitude, and a halftone gradient (fade-out) transitioning to zero. The back exposure was adjusted to the time it took for the relief depth to reach 0.6 mm, and the main exposure was adjusted to the time it took to reproduce 1% halftone dots (16 μm in diameter) on the printing plate. After development and drying, a 7-minute post-exposure was performed, followed by 5 minutes of UV sterilization to obtain the printing plate amplitude back exposure.

[0049] Furthermore, the complex elastic modulus of the dots in the flexographic printing plate obtained as described above can be 30–80 MPa, more specifically 35–75 MPa, and even more specifically 40–70 MPa. With this complex elastic modulus, the flexographic printing plate of the present invention can reduce dot gain in the fade-out areas, particularly for small dots.

[0050] The complex elastic modulus in this invention was measured using a Bruker-manufactured nanoindentation apparatus, the Hysitron TIPremier, under the following conditions. Calibration of the apparatus was performed using quartz, and the measurement was conducted at a pressure of 69.6 ± 5% GPa. Indenter: Berkovich-type diamond indenter with a tip opening angle of 142.3°. Maximum indentation depth: 4500nm Loading speed, unloading speed: 4500nm / sec Hold time: 0 seconds Feedback mode: Displ.Control Furthermore, the minimum test force of the dynamic hardness tester DUH-201 used in Patent Document 1 is 100 μN, while the minimum test force of TI Premier is 75 nN. TI Premier is capable of high-precision measurements. Due to this difference in stress-sensitive precision, TI Premier can perform measurements with higher accuracy than the tester used in Patent Document 1.

[0051] The printing plate used for determining the complex modulus of elasticity was prepared under the following conditions. Regarding back exposure, main exposure, and post-exposure, a Philips TL-K 40W / 10R lamp (peak wavelength 370nm, illuminance 10mW / cm²) was used. 2 The germicidal lamp used is the Panasonic GL-40 (peak wavelength 250nm, illuminance of 4.5mW / cm²) manufactured by Panasonic Corporation. 2The thermal mask layer image was created using a CDI 4850 manufactured by ESCO Graphics at a resolution of 4000 dpi. Evaluation images were set to have at least 175 lines, dots ranging from 0% to 10% with a 0.3% amplitude, and a dot gradient (fade-out) transitioning to zero. The back exposure was adjusted to the time it took for the texture depth to reach 0.6 mm, and the main exposure was adjusted to the time required to reproduce 1% dots (16 μm in diameter) on the printing plate. After development and drying, a 7-minute post-exposure was performed, followed by 5 minutes of UV sterilization to obtain the printing plate.

[0052] A portion with a dot diameter of 25 μm was cut from the printing plate prepared under the above-described plate-making conditions, and the complex elastic modulus of the dot was measured. The cut printing plate was stored at 25°C and 65% RH for 24 hours, and then the complex elastic modulus was measured. First, the position was adjusted so that the indenter descended to the center of the dot, and the indenter was pressed into the sample, and the load-displacement curve was measured. Based on the obtained load-displacement curve, the complex elastic modulus of the dot was calculated using the following formula. Here, Er is the complex elastic modulus, S is the contact stiffness, and Ac is the contact area. The lower fit of the fitted curve was set to 20%, and the upper fit was set to 95%. The measurement was performed 50 times, and the average value was calculated. The complex elastic modulus of the solid portion was measured by cutting a solid portion from the printing plate prepared under the above-described plate-making conditions, under the same measurement conditions as the complex elastic modulus of the dot. Complex elastic modulus

[0053] As can be seen from the above formula, the complex elastic modulus is a parameter that takes into account elastic deformation. In contrast, the dynamic hardness used as a parameter for evaluating hardness in the prior art is a parameter that does not consider elastic deformation.

[0054] Furthermore, in terms of measurement accuracy, the complex elastic modulus, as mentioned above, has the advantage of being able to be measured with high precision compared to dynamic hardness. This is because the stress sensing accuracy of the devices used to measure their respective parameters differs. Example

[0055] The effects of the printing plate of the present invention are illustrated by the following embodiments, but the present invention is not limited to these embodiments. Furthermore, "parts" in the embodiments means parts by mass, and the numerical values ​​representing the composition ratios in the table also refer to parts by mass.

[0056] The evaluation in the examples was conducted using the following methods. (1) Bending load of mesh The measuring instrument used was a TMAQ400 manufactured by TA Instruments Japan Co., Ltd. A probe with a contact diameter of 0.89 mm was inserted into the halftone dot at a load speed of 0.5 N / min to measure the degree of compression (deformation) of the halftone dot. The inflection point of the halftone dot deformation was taken as the bending load. The preload was set to 0.01 N, the holding time before measurement was set to 1 minute, and the maximum load was set to 1 N. The halftone dots used for measurement were 175 lines, 16 μm in diameter, and cut into 4 mm square dots.

[0057] (2) Shoulder angle of the dot The measuring instrument used was a VK-9700 color 3D laser microscope (manufactured by Keyence Co., Ltd.). A 50x objective lens was used, and the measurement interval was set to 0.5 μm. The longitudinal section representing the dots was... Figure 1 The area in the middle is designated as the dot shoulder angle. Dots with a diameter of 16 μm are measured using 175 line strength. The measurement is performed 5 times, and the average value is calculated.

[0058] (3) Network Depth The measuring instrument used was a VK-9700 color 3D laser microscope (manufactured by Keyence Co., Ltd.). A 50x objective lens was used, and the measurement interval was 0.5 μm. The distance between the top and bottom of the halftone dot was defined as the halftone depth. Halftone dots with a diameter of 16 μm were measured at 175 lines. The measurement was performed 5 times, and the average value was calculated.

[0059] (4) Complex elastic modulus of the network For the flexographic printing plate produced in Example 1, the complex elastic modulus of the dots was calculated using the Hysitron TIPremier manufactured by Bruker Company using the method described above.

[0060] (5) Printability of fade-out sections Regarding the flexographic printing plate prepared in Example 1, dot gain in the fade-out region was evaluated using a flexographic printing press FPR302 (manufactured by MCK Corporation) and a 900 LPI anilox roller. UV ink (trade name: FLEXOCURE CYAN (manufactured by Flint Corporation)) was used. Coated paper (trade name: Pearl Coat, manufactured by Oji Paper) was used. The printing speed was 50 m / min. The point of contact between the plate and the printed material was set as 0, and the indentation distance of 90 μm from that point was set as the indentation amount during printing. The size of the printed dots at the very end of the fade-out region in the printed material was measured. The area of ​​the printed dots was less than 700 μm. 2 At that time, it was determined to be ◎, 700μm 2 Above and below 1200μm 2 When the value is ○, 1200μm 2Above and below 1800μm 2 At that time, it was determined to be △, 1800μm 2 If the value is above the specified value, it is judged as ×. The lower the value of the printing dot area, the better the printability of the fade-out area.

[0061] (6) Reproducibility of independent points The reproducibility of the smallest independent point on the printed plate is determined. If the smallest independent point is less than 50 μm, it is marked with ◎; if it is greater than 50 μm but less than 100 μm, it is marked with ○; if it is greater than 100 μm but less than 200 μm, it is marked with △; and if it is greater than 200 μm, it is marked with ×. A smaller smallest independent point indicates better reproducibility.

[0062] (7) Durability of printing plates For the flexographic printing plate prepared in Example 1, the durability of the printing plate was evaluated using a flexographic printing press FPR302 (manufactured by MCK Corporation) and a 900 LPI anilox roller. UV ink (trade name: FLEXOCURE CYAN (manufactured by Flint Corporation)) was used. Coated paper (trade name: Pearl Coat, manufactured by Oji Paper) was used. The printing speed was 50 m / min. The point of contact between the plate and the printed material was set as 0, and the indentation amount at a distance of 150 μm from that point was set as the indentation amount during printing. 5,000 m of printing was performed using this method. The dots with a diameter of 16 μm were observed under a microscope after printing. No change before and after printing was marked as ◎, slight wear only at the ends was marked as ○, localized defects or wear were marked as △, and overall defects or wear were marked as ×.

[0063] Example 1 Preparation of photosensitive resin composition In a container, 86 parts by weight of butadiene latex (Nipol LX111NF, 55% non-volatile component, manufactured by Zeon Corporation, Japan), 24 parts by weight of acrylonitrile-butadiene latex (Nipol SX1503, 42% non-volatile component, manufactured by Zeon Corporation, Japan), 15 parts by weight of polybutadiene-terminated acrylate (BAC45, manufactured by Osaka Organic Chemicals Co., Ltd.), a number-average molecular weight compound with an olefinic unsaturated compound, and trimethylolpropane trimethacrylate (LIGHT ESTER), a number-average molecular weight compound with an olefinic unsaturated compound, were mixed. The dopant was prepared by mixing 10 parts by weight of TMP (manufactured by Kyoeisha Chemical Co., Ltd.), 3 parts by weight of benzoyl dimethyl ketal as a photopolymerization initiator, 20 parts by weight of hydrophilic polymer (PFT-4, 25% non-volatile component, manufactured by Kyoeisha Chemical Co., Ltd.), 9.9 parts by weight of butadiene oligomer (B2000 manufactured by Nippon Soda Co., Ltd.), 0.1 parts by weight of heat stabilizer (4-methoxyphenol), and 0.01 parts by weight of ultraviolet absorber (Tinuvin 326). The dopant was then placed in a pressure kneader and the solvent was removed under reduced pressure at 80°C to obtain a photosensitive resin composition.

[0064] Production of flexographic printing originals A thermal mask coating solution was prepared by mixing carbon black dispersion (manufactured by ORIENT Chemical Industry Co., Ltd., AMBK-8), copolyamide (PA223, manufactured by Toyobo Co., Ltd.), propylene glycol, and methanol in a mass ratio of 45 / 5 / 5 / 45. After demolding treatment on both sides of a PET film (Toyobo Co., Ltd., E5000, 100 μm thickness), the thermal mask coating solution was applied using a doctor blade coater until the dried film thickness reached 2 μm. After drying at 120°C for 5 minutes, a film laminate (I) was obtained. The optical density was 2.3. The optical density was measured using a DM-520 black and white transmission densitometer (manufactured by Dai Nippon Screen Co., Ltd.). An oxygen barrier coating solution was prepared by mixing polyvinyl acetate (KH20, manufactured by Nippon Seizo Co., Ltd.) with a saponification degree of 80% and a plasticizer (glycerin) in a mass ratio of 70 / 30. On the film laminate (I), an oxygen barrier coating liquid is applied using a doctor blade coater until the dried coating thickness is 2.0 μm. After drying at 120°C for 5 minutes, a film laminate (II) is obtained. The above-mentioned photosensitive resin composition is prepared on a PET film support (Toyobo Co., Ltd., E5000, thickness 125 μm) coated with a copolyester adhesive, and the film laminate (II) is stacked on it. Lamination is performed using a hot press at 100°C to obtain a flexible printing master consisting of a PET support, an adhesive layer, a photosensitive resin layer, an oxygen barrier layer, a thermal mask layer, and a cover film. The total thickness of the master is 1.14 mm.

[0065] Printing plates are made from flexographic printing originals. A 10-second back exposure was performed on the PET support of the printing master. Next, the cover film was peeled off. The plate was then rolled onto a CDI 4530 manufactured by EscoGraphics and ablated at a resolution of 4000 dpi using a test image with halftone dots ranging from 0.3% to 175 lines, individual dots ranging from 50 μm to 0 to 300 μm, and a dot gradient (fade-out area) transitioning to zero. After ablation, the plate was removed and restored to a flat surface, and a 7-minute main exposure was performed. Afterward, development was performed for 8 minutes using a developing machine manufactured by A&V Corporation (Stuck System, 1% soapy water solution, 40°C), and water droplets on the plate surface were removed with a draining stick. Then, it was dried in a dryer at 60°C for 10 minutes. Next, a 7-minute back exposure was performed, and finally, it was irradiated with a germicidal lamp for 5 minutes to obtain the flexographic printing plate. Back exposure, main exposure, and post-exposure all used Philips TL-K 40W / 10R lamps (peak wavelength 370nm, illuminance 10mW / cm²) manufactured by Philips Corporation. 2 The sterilization process was carried out using Panasonic GL-40 germicidal lamps (peak wavelength 250nm, illuminance of 4.5mW / cm²). 2 The process was performed. The resulting printing plate had a relief depth of 0.6 mm, confirming the reproduction of dots with a diameter of 16 μm on the printing plate.

[0066] Examples 2-15, Comparative Examples 1-3 Except for changing the mixing ratio of each component in the photosensitive resin composition constituting the photosensitive resin layer as shown in Tables 1 and 2, a flexographic printing master was prepared using the same method as in Example 1, from which a printing plate was obtained. Furthermore, the back exposure time was adjusted to make the embossing depth 0.6 mm, and the main exposure time was set to the time required for a 16 μm diameter dot to be reproduced on the printing plate.

[0067] The evaluation results of Examples 1-15 and Comparative Examples 1-3 are shown in Tables 1 and 2.

[0068]

[0069] [Table 2]

[0070] In addition, details of the olefinic unsaturated compounds in the table above are as follows. Light Ester TMP: Trimethylolpropane trimethacrylate, number average molecular weight 338, manufactured by Kyoei Chemical Co., Ltd. Light Ester 1,6HX: 1,6-hexanediol dimethacrylate, number average molecular weight 254, manufactured by Kyoei Chemical Co., Ltd. Light Ester 19ND: 1,9-nonanediol dimethacrylate, number average molecular weight 298, manufactured by Kyoei Chemical Co., Ltd. BAC45: Polybutadiene-terminated acrylate, number average molecular weight 10,000, manufactured by Osaka Organic Chemical Industry Co., Ltd. TE2000: Polybutadiene with methacrylic acid introduced at the end, urethane-bonded, number average molecular weight 3,000, manufactured by Nippon Soda Co., Ltd.

[0071] As can be seen from the evaluation results in the table above, in Examples 1-15, where the bending load, shoulder angle, and depth of the microdots are within the range of the present invention, the printability of the fade-out portion is excellent. As a method for maintaining the bending load, shoulder angle, and depth of the microdots within the range of the present invention, it is important to mix a larger amount of photopolymerization initiator than before, and to mix a certain amount of low molecular weight (meth)acrylate compound. Furthermore, by mixing a high molecular weight (meth)acrylate compound, even with a large amount of photopolymerization initiator mixed, the reproducibility of individual dots and the durability of the printing plate are not impaired (comparison of Examples 1-8, 11-13, 15 with Examples 9, 10, 14). In addition, by using benzophenone in combination with the photopolymerization initiator, the complex elastic modulus of the dots can be increased (comparison of Examples 1-6, 9-15 with Examples 7, 8). Furthermore, by having an oxygen barrier layer, the printability of the fade-out portion of the microdots can be further improved (comparison of Examples 1-14 with Example 15).

[0072] In contrast, in Comparative Example 1, because the amount of photopolymerization initiator mixed was small, similar to that in the past, the bending load, shoulder angle, and depth of the micro-dots were outside the scope of the present invention, resulting in poor printability of the fade-out areas. In Comparative Example 2, because the amount of photopolymerization initiator mixed was small, even if the mixing ratio of the photopolymerization initiator to the low molecular weight (meth)acrylate compound was appropriate, the bending load, shoulder angle, and depth of the micro-dots were still outside the scope of the present invention, resulting in poor printability of the fade-out areas. In Comparative Example 3, because the amount of the low molecular weight (meth)acrylate compound mixed was small, the bending load, shoulder angle, and depth of the micro-dots were still outside the scope of the present invention, resulting in poor printability of the fade-out areas.

[0073] Additionally, for reference, the bending load measurement results of Example 2 and Comparative Example 1 are shown below. Figure 2 .from Figure 2 As can be seen from the graphs, in Example 2, the bending of the micro-dots occurred at 0.32N, while in Comparative Example 1, the bending of the micro-dots occurred at 0.14N. Industrial utilization potential

[0074] In the flexographic printing plate of the present invention, since the bending load of the micro-dots is increased to a specific value, dot gain of the micro-dots can be reduced not only in the center of the image but also in the fade-out areas. As a result, the printing quality of the micro-dots in the fade-out areas can be improved. Therefore, the flexographic printing plate of the present invention is extremely useful in the art.

Claims

1. A flexible printing plate, characterized in that, A flexible printing plate is obtained by sequentially stacking a support (A), a photosensitive resin layer (B), and a thermal mask layer (C) on a flexible printing master. When halftone dots with a diameter of 16 μm are formed on the printing plate using 175 lines, the halftone dots do not show bending up to a load of 0.2 N. The photosensitive resin composition forming the photosensitive resin layer (B) contains (a) a polymer formed by polymerizing a conjugated diene, (b) an olefinic unsaturated compound, and (c) a photopolymerization initiator, wherein the content of (c) the photopolymerization initiator in the photosensitive resin composition is 2.5% to 9% by mass. (b) The olefinic unsaturated compound further contains a (meth)acrylate compound (ii) with a number average molecular weight greater than 600 and less than 20,000, wherein the content of the (meth)acrylate compound (ii) with a number average molecular weight greater than 600 and less than 20,000 in the photosensitive resin composition is 5% to 20% by mass. (b) The olefinic unsaturated compound contains a (meth)acrylate compound (i) with a number average molecular weight of 100 or more and 600 or less, and the content of the (meth)acrylate compound (i) with a number average molecular weight of 100 or more and 600 or less in the photosensitive resin composition is 5 to 16% by mass, and the ratio of the mass of (c) the photopolymerization initiator to the mass of the (meth)acrylate compound (i) with a number average molecular weight of 100 or more and 600 or less in the photosensitive resin composition is in the range of 0.20 to 0.

45.

2. The flexible printing plate according to claim 1, characterized in that, When a halftone dot with a diameter of 16 μm is formed from 175 lines on a printing plate, the shoulder angle of the halftone dot is 105–125°.

3. The flexible printing plate according to claim 1 or 2, characterized in that, When a dot with a diameter of 16 μm is formed by 175 lines on a printing plate, the dot depth is 70–85 μm.

4. The flexible printing plate according to claim 1, characterized in that, (c) The photopolymerization initiator contains two compounds: benzoyl alkyl ketal compounds and benzophenone compounds, with the mass ratio of benzoyl alkyl ketal compounds to benzophenone compounds ranging from 99:1 to 80:

20.

5. The flexible printing plate according to claim 1 or 2, characterized in that, The flexographic printing master has an oxygen barrier layer (D) between the photosensitive resin layer (B) and the thermal mask layer (C).

6. The flexible printing plate according to claim 1 or 2, characterized in that, It is obtained by developing the flexographic printing original using an aqueous developer.

7. A flexographic printing method, characterized in that, The flexible printing plate according to any one of claims 1 to 6 was used.

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