Flexible printing plate

Abstract

The present application provides a flexographic printing plate which reduces the ink adhesion problem of ink direct connection of adjoining dot protrusions. A flexographic printing plate characterized by being a flexographic printing plate obtained from a flexographic 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, and in which dot protrusions, dot valley portions formed between the dot protrusions, and a bottom portion are provided on a surface of the flexographic printing plate, and in which the side surface portions and the bottom portion of the dot valley portions have received the same post-exposure treatment and exposure treatment using a germicidal lamp, the adhesion of the bottom portion of the flexographic printing plate being 2.0 N / mm 2 The following.

Description

Technical Field

[0001] This invention relates to a flexible printing plate for reducing ink entanglement of ink connections between adjacent dot protrusions. Background Technology

[0002] Flexographic printing is a printing method that transfers ink from the plate to the substrate by pressing the plate against the raised parts of the printing plate. Because the printing plates used in flexographic printing are flexible and can conform to various shapes, they can be used 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] On the other hand, a problem with flexographic printing is ink bridging, where ink from adjacent dot protrusions connects and transfers to the printed material. When ink accumulates between the dots and dot valleys on the printing plate, and a certain amount accumulates, the ink from adjacent dot protrusions directly connects and transfers to the printed material, resulting in ink bridging. Ink bridging significantly reduces print quality. Therefore, when ink bridging is visible on the printed material, the printing press is usually stopped and the printing plate cleaned, but stopping the printing press results in a significant loss of paper. Ink bridging occurs more significantly when using water-based inks than when using UV inks. This is because water-based inks generally have a lower viscosity than UV inks, making it easier for ink to move from the dot protrusions to the dot valleys.

[0004] One proposed method for suppressing ink adhesion involves incorporating silicone into the photosensitive resin layer or spraying modified silicone onto the printing plate to reduce the surface tension of the printing plate and thus suppress ink movement from the dot protrusions to the dot valleys (see Patent Documents 1-3). While these methods are effective at suppressing UV ink adhesion, they are not satisfactory for water-based inks. Furthermore, because these methods reduce the surface tension of the printing plate, the ink transferability in solid areas may be reduced.

[0005] Therefore, regardless of ink viscosity, there is a strong demand for flexible printing plates that are less likely to cause ink adhesion problems, such as ink bonding between adjacent dot protrusions.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: WO07 / 116941 Publication

[0009] Patent Document 2: Japanese Patent No. 5751492

[0010] Patent Document 3: Japanese Patent No. 5601606 Summary of the Invention

[0011] The problem that the invention aims to solve

[0012] The present invention was created in view of the current state of the prior art, and its purpose is to provide a flexible printing plate that reduces the problem of ink adhesion between adjacent dot protrusions.

[0013] The means to solve the problem

[0014] To achieve the aforementioned objectives, the inventors meticulously studied the causes and countermeasures of ink adhesion. They discovered that ink adhesion is caused by ink accumulating between the dot valleys of adjacent dot protrusions. To prevent this adhesion, it is effective to reduce the adhesion force on the side surfaces of the dot valleys of the printing plate, thus preventing ink from moving into these valleys. Furthermore, since it is difficult to accurately measure the adhesion force on the side surfaces of the dot valleys of the printing plate at a specific location, this invention uses the adhesion force at the bottom of the printing plate, which exhibits substantially the same adhesion force, as an indicator.

[0015] Furthermore, the following method was found to be suitable, and the present invention was thus completed. This method involves mixing a larger amount of photopolymerization initiator in the photosensitive resin composition constituting the photosensitive resin layer than previously implemented, in order to reduce the adhesion force of the side portions between the dot valleys. Furthermore, as an olefin unsaturated compound in the photosensitive resin composition, a certain amount of low molecular weight olefin unsaturated compound is contained, and the mass ratio of photopolymerization initiator to low molecular weight olefin unsaturated compound is higher than previously implemented. Furthermore, as an olefin unsaturated compound in the photosensitive resin composition, in addition to the low molecular weight olefin unsaturated compound, a high molecular weight olefin unsaturated compound is also mixed. Moreover, two specific photopolymerization initiators are included and used.

[0016] That is, the present invention has the following configurations (1) to (7).

[0017] (1) A flexible printing plate, characterized in that it is a flexible printing plate obtained by sequentially stacking a support (A), a photosensitive resin layer (B), and a thermal mask layer (C) on the surface of the printing plate, wherein dot protrusions, dot valleys formed between the dot protrusions, and a bottom are provided on the surface of the printing plate, and the side portions and bottom portions of the dot valleys are subjected to the same post-exposure treatment and exposure treatment using a germicidal lamp, wherein the adhesion strength of the bottom of the printing plate is 2.0 N / mm. 2 the following.

[0018] (2) The flexible printing plate according to (1) is characterized in that the photosensitive resin composition forming the photosensitive resin layer (B) contains (a) a polymer obtained 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, and the (b) olefinic unsaturated compound contains a (meth)acrylate compound (i) with a number average molecular weight of 100 or more and 600 or less, wherein the content of (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 (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.

[0019] (3) The flexible printing plate according to (2), characterized in that (b) the olefinic unsaturated compound further contains (meth)acrylate compound (ii) with a number average molecular weight greater than 600 and less than 20,000, and the content of (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.

[0020] (4) The flexible printing plate according to (2) or (3) 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.

[0021] (5) The flexible printing plate according to any one of (1) to (4), 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).

[0022] (6) The flexographic printing plate according to any one of (1) to (5), characterized in that it is obtained by developing the flexographic printing plate with an aqueous developer.

[0023] (7) A flexible printing method, characterized in that it uses any one of (1) to (6) flexible printing plates.

[0024] Invention Effects

[0025] The flexible printing plate of the present invention is configured to reduce the adhesion force at the bottom (adhesion force on the side surface of the halftone dots). As a result, the adhesion force on the side surface of the halftone valleys formed between adjacent halftone dot protrusions of the printing plate is reduced compared to the past. Therefore, the ink will not move to the halftone valleys, and ink adhesion between adjacent halftone dot protrusions can be effectively prevented. Attached Figure Description

[0026] Figure 1 It is a schematic diagram illustrating the halftone dots, dot protrusions, dot valleys, dot sides, and bottom using a cross-section of the printing plate. Detailed Implementation

[0027] The flexible printing plate of the present invention is a flexible printing plate obtained by sequentially stacking a flexible printing master plate consisting of at least a support (A), a photosensitive resin layer (B) and a thermal mask layer (C). Specifically, it is a printing plate obtained by exposing and developing the flexible printing master plate involved.

[0028] Here, the halftone dots, halftone dot protrusions, halftone dot valleys, halftone dot side surfaces, and bottom of the printing plate mentioned in this invention are described through... Figure 1 The cross-section of a printing plate is shown. Generally, the surface of a printing plate is roughly divided into the dot portion and the bottom portion. The dots are the parts that form the image; by placing ink on the raised parts of the dots and pressing it onto the substrate, the ink is transferred to the substrate to form an image. On the other hand, the bottom portion acts as a buffer, improving print quality. The bottom is a cured layer formed during the back exposure process of manufacturing a flexographic printing plate from a flexographic master, where active light is irradiated across the entire back side of the printing master through a support; it is essentially flat. Since the appropriate thickness of the bottom varies depending on the substrate being printed on, the thickness of the bottom needs to be adjusted according to the substrate. The thickness of the bottom can be adjusted by changing the back exposure conditions. Specifically, increasing the amount of back exposure results in a thicker bottom, while decreasing the amount of back exposure results in a thinner bottom. Furthermore, back exposure is used to control the thickness of the bottom and has no substantial impact on the adhesion of the bottom. As shown in Figure 1, the dot portion is composed of multiple raised dots. Between adjacent halftone dot protrusions, halftone valleys with left and right dot side faces are formed. As shown in Figure 1, the depth of the halftone valley (the difference between the height of the apex of the halftone dot protrusion and the height of the lowest point of the halftone valley) is shallower than the depth of the bottom (the difference between the height of the apex of the halftone dot protrusion and the height of the bottom). As mentioned above, ink placed on the halftone dot protrusions easily moves and accumulates in the halftone valleys, easily causing ink sticking problems.

[0029] In this invention, the characteristic feature is that when forming halftone dots on a printing plate, the adhesive force of the side surfaces between the halftone valleys formed between adjacent halftone dot protrusions (for convenience, the adhesive force at the bottom is used instead as an indicator) is reduced to 2.0 N / mm. 2 The following is an explanation of how this configuration effectively prevents ink from adjacent dot protrusions from moving to and accumulating in the dot valleys between them. Consequently, it effectively prevents ink adhesion problems caused by direct contact between the inks of adjacent dot protrusions. Furthermore, the reason why the adhesion strength of the bottom of the printing plate is specified instead of the adhesion strength of the dot side portions is that it is practically difficult to measure the adhesion strength of the dot side portions in a given location. Therefore, even if the adhesion strength of the bottom portion, which undergoes the same post-exposure treatment and exposure treatment using a germicidal lamp as the dot side portions, is used as a substitute, it can represent an adhesion strength that is substantially the same as that of the dot side portions. Therefore, in the following explanation, the adhesion strength (adhesion) of the "bottom" should be understood as equivalent to the adhesion strength (adhesion) of the "dot side portions".

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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 referred to as low molecular weight (meth)acrylate compounds) (i), and even 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, the low molecular weight (meth)acrylate compounds are cross-linked and cured by a photopolymerization initiator to form a dense cross-linked network, while the high molecular weight (meth)acrylate compounds are cross-linked and cured by a photopolymerization initiator to form a loose cross-linked network. 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 plate strength caused 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.

[0034] 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 of the printing plate 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. In addition, 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. By containing a low molecular weight (meth)acrylate compound within the above range, the adhesion of the halftone dots on the sides and bottom can be effectively reduced. However, when the content is below the above range, the reduction in adhesion of the halftone dots on the sides and bottom is insufficient. When 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.

[0035] 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 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.

[0036] 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.

[0037] (c) 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 preferably 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 adhesion of the dot sides and bottom can be adequately satisfied. If the ratio is less than the above range, there is too little photopolymerization initiator, which cannot reduce the adhesion of the dot sides and bottom. If it exceeds the above range, there is too much photopolymerization initiator relative to (meth)acrylate, and the amount of (meth)acrylate for crosslinking may become insufficient. In this case, it is also easy to fail to reduce the adhesion of the dot sides and bottom.

[0038] 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.

[0039] (c) The photopolymerization initiator preferably contains a compound composed of both a benzoyl alkyl ketal and a benzophenone compound. 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 two photopolymerization initiators together, the light energy from the subsequent post-exposure and the exposure using a germicidal lamp can be used without waste. In particular, by using both benzoyl alkyl ketal and benzophenone photopolymerization initiators together, the adhesion of the dots to the sides and bottom can be reduced more effectively. As a benzoyl alkyl ketal, benzoyl dimethyl ketal is preferred, and as a benzophenone compound, benzophenone is preferred. In particular, benzophenone does not absorb light from the lamp used for the main exposure (peak wavelength 370nm), but it does absorb light from the germicidal lamp (peak wavelength 250nm). Therefore, exposure using the germicidal lamp can reduce the adhesion of the halftone dots on the sides and bottom, instead of using the main exposure to increase the polyelastic modulus of the solid areas.

[0040] In this invention, the content of (c) photopolymerization initiator in the photosensitive resin composition is preferably 2 to 9% by mass, more preferably 2.5 to 7.5% by mass, and even more preferably 3 to 7% by mass. As described above, by mixing the photopolymerization initiator in a larger amount than conventionally used, the adhesion of the dot sides and bottom can be reduced. It has been conventionally believed that if the content of the photopolymerization initiator is increased, the light absorption of the photopolymerization initiator after the reaction will block the light reaching the plate in the thickness direction, resulting in a decrease in the reproducibility of independent points important for crosslinking in the thickness direction and a decrease in printing durability. Therefore, conventionally, the content of the photopolymerization initiator in the photosensitive resin composition is usually only about 1% by mass at most 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, and the advantages (reduced adhesion of the dot sides and bottom) can be enjoyed due to the increased content of photopolymerization initiator.

[0041] In this invention, the photosensitive resin composition forming the photosensitive resin layer (B) used in the flexographic printing master may further contain an organosilicon compound. By mixing the organosilicon compound, the surface tension of the printing plate surface is reduced, thus preventing ink bridging more effectively. The organosilicon compound is preferably an amino-modified organosilicon compound having an amino group within its molecule. When an amino-modified organosilicon compound is used, the ink bridging prevention effect obtained by the organosilicon compound can be sustained. This is believed to be because the amino group within the molecule undergoes Michael addition to the double bond in the photopolymerizable unsaturated compound, fixing the organosilicon compound in the photosensitive resin layer, thereby preventing the movement of the organosilicon compound during printing. The position of the amino group within the molecule is not limited; for example, terminal positions, side chain positions, etc., are examples. In addition, the number of amino groups is only one or more, and multiple amino groups are also allowed. Furthermore, functional groups other than amino groups may also be included. The amino equivalent of the amino-modified organosilicon compound is preferably in the range of 500 g / mol to 10,000 g / mol, and more preferably in the range of 1,000 g / mol to 5,000 g / mol. When the amino equivalent is below the lower limit mentioned above, the initial effect of preventing ink sticking may be poor; conversely, when it exceeds the upper limit, the effect may be less sustainable. The kinematic viscosity of the amino-modified silicone compound is not particularly limited, but is preferably 20–5000 mm⁻¹. 2 The range of / s is further preferably 40–1000 mm. 2 / s. When the kinematic viscosity is less than the lower limit mentioned above, the effect of preventing ink sticking may be poor; on the other hand, when it exceeds the upper limit mentioned above, the solubility stability may be poor. Suitable organosilicon compounds used in this invention include, for example, X221660B-3 and X229409 manufactured by Shin-Etsu Chemical Co., Ltd., which are available commercially. Alternatively, for products not available commercially, synthesis methods disclosed in, for example, Japanese Patent Application Publication Nos. 3-197486, 8-34855, 2006-213856, and 2017-52737 can be appropriately referred to. The number-average molecular weight of the organosilicon compound is preferably 500 to 50,000, more preferably 1,000 to 10,000. When the number-average molecular weight is less than the lower limit mentioned above, the effect of preventing ink sticking may be insufficient; on the other hand, when it exceeds the upper limit mentioned above, the compatibility with the photosensitive resin may decrease. The content of organosilicon compound in the photosensitive resin layer is preferably 0.1 to 10% by mass, more preferably 0.2 to 5% by mass. If the content of organosilicon compound is less than the above lower limit, it may not be able to fully exert the effect of preventing ink sticking. On the other hand, if it exceeds the above upper limit, the mechanical strength of the printing plate may decrease.

[0042] The photosensitive resin composition used to form the photosensitive resin layer (B) in the flexographic printing master is a composition comprising (a) a polymer obtained by polymerizing conjugated diene, (b) an olefinic unsaturated compound and (c) a photopolymerization initiator, and, as needed, additives such as plasticizers, hydrophilic compounds, ultraviolet absorbers, surface tension modifiers, thermal polymerization inhibitors, dyes, pigments, fragrances or antioxidants, in addition to the aforementioned organosilicon compounds.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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 bottom 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 areas that become the image (dots). Post-exposure involves developing the plate after the main exposure, illuminating the entire surface of the plate with the formed dot areas, and supplementing the cross-linking and solidification in the main exposure while also cross-linking and solidifying the surfaces of the dot projections (dot side portions) and the bottom of the dot portions. 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 surfaces of the dot projections and the bottom of the dot portions. Furthermore, main and post exposures are typically performed using UVA, while exposure using a germicidal lamp is performed using UVC.

[0047] Curing of the sides and bottom surfaces of the dot protrusions, achieved through post-exposure and exposure with a germicidal lamp, occurs to some extent even in conventional exposures. However, since post-exposure and exposure with a germicidal lamp are typically performed in the atmosphere, polymerization inhibition caused by atmospheric oxygen occurs. Therefore, conventionally, the degree of curing of the sides and bottom surfaces of the dot protrusions resulting from post-exposure and exposure with a germicidal lamp is not very high. In contrast, in this invention, by mixing in a greater amount of photopolymerization initiator than in conventional methods, the effect of polymerization inhibition caused by oxygen can be sufficiently suppressed, allowing for thorough curing of the sides and bottom surfaces of the dot protrusions and significantly reducing the adhesion of the sides and bottom surfaces of the dot protrusions.

[0048] When the content of the photopolymerization initiator is below the above-mentioned preferred range, the adhesion of the surfaces of the halftone dots on the sides and bottom becomes insufficient during post-exposure and exposure using a germicidal lamp, and ink adhesion may occur. On the other hand, when the content of the photopolymerization initiator exceeds the above-mentioned preferred range, curing occurs during the main exposure, the complex elastic modulus of the solid areas becomes too high, and there is a tendency for poor ink spreading in the solid areas.

[0049] 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.

[0050] 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).

[0051] 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, and sufficient curing reaction occurs in both the dot and solid areas. As a result, a flat top suitable for printing is formed in the dot area. By forming a flat top, printing can be performed stably. Examples of adhesive polymers in the barrier layer include polyvinyl alcohol, partially saponified vinyl acetate, alkyl cellulose, cellulose polymers, and polyamides. These polymers are not limited to using one type, and two or more polymers can 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.

[0052] 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.

[0053] The method for manufacturing the flexible printing original of the present invention is not particularly limited, and is generally as follows.

[0054] 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.

[0055] 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.

[0056] After image information is written onto a 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 a 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 commonly used flat-shaped irradiation unit, 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 carried out using a conventional developing unit.

[0057] The flexographic printing plate obtained as described above can have the following characteristics: the adhesion force at the bottom (dot side) of the printing plate is 2.0 N / mm. 2 The following is further 1.5 N / mm 2 The following is further 1.0 N / mm 2 The flexographic printing plate of the present invention, by having the features described above, makes it virtually impossible for ink adhesion problems to occur in actual printing, where inks from adjacent halftone dot protrusions directly connect. When ink adhesion occurs, the inks from adjacent halftone dot protrusions connect, which can lead to a significant decrease in print quality. Furthermore, the flexographic printing plate of the present invention reduces the adhesion of dust during printing by reducing the adhesion force at the bottom, thus reducing printing defects caused by this. The reason why reducing the adhesion force at the bottom (the adhesion force on the side surface of the halftone dot) can reduce ink adhesion is presumably because the curing of the bottom surface (the curing of the surface of the side surface of the halftone dot) becomes sufficient, thereby reducing the affinity with the ink.

[0058] In this invention, the adhesive strength at the bottom is specifically measured according to the following method.

[0059] The adhesive strength at the bottom was measured using an ORIENTEC RTC1210A universal testing machine. Flexible printed plates for measuring adhesive strength were fixed at both the top and bottom, respectively, with an application rate of 20 N / mm². 2 The bottoms of the upper and lower flexographic printing plates were brought into close contact for 60 seconds, and the maximum load when peeled at a peeling speed of 1 mm / min was taken as the adhesive force. The test was conducted 10 times at 25°C and 50% RH, and the average value was taken. Furthermore, a 30 mm × 30 mm square flexographic printing plate was used for the lower part, and a circular flexographic printing plate with a perimeter of 10 mm was used for the upper part.

[0060] The printing plates used in the above-mentioned bottom adhesion strength measurement were 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. 2 The 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 with dots ranging from 0% to 10% dot coverage at 0.3% amplitude, dots ranging from 10% to 100% dot coverage at 5% amplitude, bottom dots, and independent dots ranging from 0 to 300 μm at 50 μm amplitude. Back exposure was adjusted to the time required for a texture depth of 0.6 mm, and 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 sterilization to obtain the printing plate.

[0061] Example

[0062] 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.

[0063] The evaluation in the examples was conducted using the following methods.

[0064] (1) Adhesion at the bottom

[0065] The adhesive strength at the bottom was measured using an ORIENTEC RTC1210A universal testing machine. Flexible printed plates for measuring adhesive strength were fixed at both the top and bottom, respectively, with an application rate of 20 N / mm². 2The bottoms of the upper and lower flexographic printing plates were brought into close contact for 60 seconds, and the maximum load when peeled at a peeling speed of 1 mm / min was taken as the adhesive force. The test was conducted 10 times at 25°C and 50% RH, and the average value was taken. Furthermore, a 30 mm × 30 mm square flexographic printing plate was used for the lower part, and a circular flexographic printing plate with a perimeter of 10 mm was used for the upper part.

[0066] (2) Ink adhesion

[0067] Regarding the flexographic printing plate, an FPR302 flexographic printing press (manufactured by MCK Co., Ltd.) was used, and ink adhesion was evaluated using a 900 LPI anilox roller. Water-based ink (trade name: Marine Flex LM (manufactured by DIC Co., Ltd.) was used. PP film (trade name: P4256, manufactured by Toyobo) was used as the substrate. The printing speed was 50 m / min. The point of contact between the plate and the substrate was set as 0, and the indentation depth 90 μm from that point was set as the indentation amount during printing. Ink adhesion was evaluated according to the following criteria.

[0068] 5 points: Even after printing 10,000m, there was no ink sticking together.

[0069] 4 points: After printing 10,000m, there is slight ink adhesion in some halftone dots.

[0070] 3 points: After printing 10,000m, ink sticks together.

[0071] 2 points: After printing 5000m, ink sticks together.

[0072] 1 point: After printing 1000m, ink sticks together.

[0073] (3) Reproducibility of independent points

[0074] 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 Δ; and if it is greater than 200 μm, it is marked with ×. A smaller smallest independent point indicates better reproducibility.

[0075] (4) Durability of the printing plate

[0076] For the flexographic printing plate manufactured 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 contact point between the plate and the printed surface was set as 0, and the indentation depth 150 μm from that point was set as the indentation amount during printing. 5,000 m of printing was performed using this method. The 16 μm diameter dots on the printed plate were observed under a microscope. 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 Δ, and overall defects or wear were marked as ×.

[0077] Example 1

[0078] Preparation of photosensitive resin composition

[0079] The container contains 86 parts by weight of butadiene latex (Nipol LX111NF, 55% non-volatile component, manufactured by Zeon Corporation, Japan) as a polymer obtained by polymerizing conjugated dienes, 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 Chemical Industry Co., Ltd.) as an olefinic unsaturated compound with a number average molecular weight of 10,000, and trimethylolpropane trimethacrylate (LIGHT ESTER) with a number average molecular weight of 338. 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.) as other components, 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.

[0080] Production of flexographic printing originals

[0081] 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 to achieve a dried film thickness of 2 μm. The film was then dried at 120°C for 5 minutes to obtain a film laminate (I). The optical density was 2.3. The optical density was measured using a black and white transmission densitometer DM-520 (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 to achieve a dried coating thickness of 2.0 μm, and then dried at 120°C for 5 minutes to obtain the film laminate (II). The aforementioned photosensitive resin composition is prepared on a PET film support (Toyobo Co., Ltd., E5000, 125 μm thickness) coated with a copolyester adhesive, and the film laminate (II) is then overlapped thereon. Lamination is performed using a hot press at 100°C to obtain a flexible printing master consisting of a PET support, adhesive layer, photosensitive resin layer, oxygen barrier layer, thermal mask layer, and cover film. The total thickness of the master is 1.14 mm.

[0082] Printing plates are made from flexographic printing originals.

[0083] A 10-second back exposure is performed on the PET support side of the printing master. Next, the cover film is peeled off. The plate is rolled onto a CDI4530 manufactured by Esco Graphics and ablated at a resolution of 4000 dpi with an image having 175 lines of dots at 0.3% amplitude in the 0%–10% dot range, dots at 5% amplitude in the 10%–100% dot range, a bottom dot, and individual dots at 50 μm amplitude in the 0–300 μm range. After ablation, the plate is removed, restored to a flat surface, and subjected to a 7-minute main exposure. Then, it is developed for 8 minutes using a developer manufactured by A&V Corporation (Stuck System, 1% soapy water solution, 40°C), and water droplets on the plate surface are removed with a draining stick. It is then dried in a 60°C dryer for 10 minutes, subjected to a 7-minute back exposure, and finally 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). 2 The sterilization process was carried out using a Panasonic GL-40 sterilization lamp (peak wavelength 250nm, illuminance of 4.5mW / cm²) manufactured by Panasonic Corporation. 2The process was performed. The resulting printing plate had a relief depth of 0.6 mm, confirming that dots with a diameter of 16 μm were reproduced on the printing plate.

[0084] Examples 2-14, Comparative Examples 1-3

[0085] 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 flexible 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 until the embossing depth was 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.

[0086] Example 15

[0087] X22 1660B-3 (an organosilicon compound containing amino and phenyl groups within the molecule, with an amino equivalent of 2200 g / mol, a phenyl equivalent of 210 g / mol, and a number-average molecular weight of 4400) manufactured by Shin-Etsu Chemical Industry Co., Ltd., was further mixed as an organosilicon compound in the composition of the photosensitive resin. Except for changing the mixing ratios of each component as shown in Table 1, a flexographic printing master was prepared using the same method as in Example 1, and a printing plate was obtained from this master. Furthermore, the back exposure time was adjusted until the embossing depth was 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.

[0088] The evaluation results of Examples 1-15 and Comparative Examples 1-3 are shown in Tables 1 and 2.

[0089]

[0090] [Table 2]

[0091]

[0092] In addition, details of the olefinic unsaturated compounds in the table above are as follows.

[0093] Light Ester TMP: Trimethylolpropane trimethacrylate, number average molecular weight 338, manufactured by Kyoei Chemical Co., Ltd.

[0094] Light Ester 1,6HX: 1,6-hexanediol dimethacrylate, number average molecular weight 254, manufactured by Kyoeisha Chemical Co., Ltd.

[0095] Light Ester 19ND: 1,9-nonanediol dimethacrylate, number average molecular weight 298, manufactured by Kyoeisha Chemical Co., Ltd.

[0096] BAC45: Polybutadiene-terminated acrylate, number average molecular weight 10,000, manufactured by Osaka Organic Chemical Industry Co., Ltd.

[0097] TE2000: Polybutadiene with terminal methacrylic acid groups, urethane-bonded, number average molecular weight 3,000, manufactured by Nippon Soda Co., Ltd.

[0098] As can be seen from the evaluation results in the table above, in Examples 1-15, where the adhesion at the bottom (adhesion at the side of the halftone dot) is within the scope of this invention, ink adhesion is minimal. As a method for setting the adhesion at the bottom (adhesion at the side of the halftone dot) within the scope of this invention as described above, it is important to mix a larger quantity of photopolymerization initiator than before, mix a certain amount of low molecular weight (meth)acrylate compound, and control the mixing ratio of the photopolymerization initiator to the low molecular weight (meth)acrylate compound within a certain range. 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 adhesion at the bottom (adhesion at the side of the halftone dot) can be reduced (comparison of Examples 1-6, 9-15 with Examples 7, 8). Furthermore, by mixing with organosilicon compounds, ink sticking can be prevented even more effectively (comparison between Examples 1-14 and Example 15).

[0099] In contrast, in Comparative Example 1, because the amount of photopolymerization initiator mixed was small, the same as in the past, the adhesion at the bottom (adhesion at the side of the dot) was high, and ink adhesion could not be suppressed. In Comparative Example 2, because the amount of photopolymerization initiator mixed was small, even if the mixing ratio of photopolymerization initiator to low molecular weight (meth)acrylate compound was appropriate, the adhesion at the bottom was high, and ink adhesion could not be suppressed. In Comparative Example 3, because the amount of low molecular weight (meth)acrylate compound mixed was small, the adhesion at the bottom (adhesion at the side of the dot) was high, and ink adhesion could not be suppressed.

[0100] Industrial utilization potential

[0101] The flexographic printing plate of the present invention reduces the adhesive force at the bottom (adhesive force on the side surface of the halftone dots) to a specific value, thereby reducing ink adhesion at the ink connection of adjacent halftone dot protrusions and preventing the resulting reduction in print quality. Therefore, the flexographic printing plate of the present invention is extremely useful in the art.

Claims

1. A flexible printing plate, characterized in that, The flexographic printing plate is a flexible printing master plate formed by at least sequentially stacking a support (A), a photosensitive resin layer (B), and a thermal mask layer (C). The printing plate has dot protrusions, dot valleys formed between the dot protrusions, and a bottom portion on its surface. The side portions and bottom portion of the dot valleys undergo the same post-exposure treatment and exposure treatment using a germicidal lamp. The adhesion strength at the bottom of the printing plate is 2.0 N / mm. 2 the following, The photosensitive resin composition forming the photosensitive resin layer (B) contains (a) a polymer obtained 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 2-9% by mass. (b) The olefinic unsaturated compound further contains a (meth)acrylate compound (ii) with a number average molecular weight of 3,000 or more and 20,000 or less, wherein the content of the (meth)acrylate compound (ii) with a number average molecular weight of 3,000 or more and 20,000 or less in the photosensitive resin composition is 5 to 20% by mass.

2. The flexible printing plate according to claim 1, 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, 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.

55.

3. The flexible printing plate according to claim 2, 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.

4. The flexographic printing plate according to any one of claims 1 to 3, 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).

5. The flexographic printing plate according to any one of claims 1 to 3, characterized in that, It is obtained by developing the flexographic printing original using an aqueous developer.

6. A flexographic printing method, characterized in that, The flexible printing plate according to any one of claims 1 to 5 was used.

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