Patent Publication Number: US-2005142480-A1

Title: Photosensitive element for use as flexographic printing plate

Description:
FIELD OF THE INVENTION  
      This invention pertains to a photosensitive element for use as a flexographic printing plate. Furthermore, the invention pertains to a process for preparing the photosensitive element, a process for preparing a flexographic printing plate from the photosensitive element, and the flexographic printing plate made by that process.  
     BACKGROUND OF THE INVENTION  
      Flexographic printing plates are well known for use in relief printing on a variety of substrates such as paper, corrugated board, films, foils and laminates. Flexographic printing plates can be prepared from photosensitive elements comprising a photopolymerizable layer containing an elastomeric binder, a monomer, and a photoinitiator, interposed between a support and a cover sheet or multilayer cover element. A preferred process of making such photosensitive elements is described in U.S. Pat. No. 4,460,675 where a previously extruded photopolymerizable composition is fed into the nip of a calender and is calendered between a support and a multilayer cover element to form a photopolymerizable layer. Upon imagewise exposure of the photosensitive element with actinic radiation through a photomask, the exposed areas of the photopolymerizable layer are insolubilized. Treatment with a suitable solvent removes the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing. Such materials are described in U.S. Pat. No. 4,323,637; U.S. Pat. No. 4,427,759; and U.S. Pat. No. 4,894,315.  
      A common technique for bringing a photosensitive element and a photomask into close contact with one another is to juxtapose the elements and draw a vacuum from between them usually by use of a vacuum frame. When smooth-surfaced elements are brought into such vacuum contact, however, the time required to evacuate air from between the elements and obtain a substantially uniform and complete contact between them becomes exceedingly high. Moreover, even after long periods of time, uniform and complete contact might not be achieved and the photomask may stick so strongly to the photosensitive element that it is damaged when stripped off.  
      Sometimes, at least one of the photosensitive element or the photomask has a rough outermost layer to avoid these disadvantages. Use of photomasks having rough surfaces are disclosed in U.S. Pat. No. 4,997,735 and U.S. Pat. No. 5,124,227. It is also known to use photosensitive elements with s rough outermost layers in combination with photomasks having a smooth surface. Several methods are known to provide such rough layers. Cover sheets having a rough layer are stripped off from the photosensitive element, before or after contacting the photomask with the photosensitive element thereby transferring their roughness to the surface of the photosensitive element. Such processes are described in U.S. Pat. No. 5,294,474; U.S. Pat. No. 4,994,344; U.S. Pat. No. 4,957,845; U.S. Pat. No. 4,567,128; U.S. Pat. No. 4,559,292; U.S. Pat. No. 3,891,443; EP-A 0 549 946; DE-C 26 31 837. Furthermore, it is known to incorporate a small amount of particles into the photosensitive layer itself as disclosed by U.S. Pat. No. 4,599,299; U.S. Pat. No. 4,560,636; U.S. Pat. No. 4,298,678; U.S. Pat. No. 3,891,443; EP-A 0 549 946; EP-A 0 260 943. Such particles can also be incorporated into a temporary protective layer, the so called “release layer”, which is a flexible and transparent polymeric film on top of the photosensitive layer and which is removed during development of the imagewise exposed photosensitive elements, as disclosed by U.S. Pat. No. 6,040,116; U.S. Pat. No. 5,593,811; U.S. Pat. No. 5,254,437; U.S. Pat. No. 4,238,560; U.S. Pat. No. 4,072,527; EP-A 0 617 331; DE-A 41 17 127; DE-A 25 12 043; DE-C 21 27 767. It is also known that in addition to the better vacuum contact, a rough outermost layer can also improve the print quality of the final printing plate.  
      But incorporation of particles into photosensitive layers or release layers often leads to difficulties during development of the imagewise exposed photosensitive elements, for example, sludge deposits in the development processor. Therefore, special materials and processes are used, such as, special polymeric particles having the same composition as the photosensitive layer, and incorporating particles by special process steps, such as spray coating or embossing the surface of the photosensitive element after it has been produced. These techniques are disclosed in U.S. Pat. No. 5,795,647; U.S. Pat. No. 5,576,137; U.S. Pat. No. 5,028,512; U.S. Pat. No. 4,842,982; U.S. Pat. No. 4,557,994; U.S. Pat. No. 4,288,526; EP-A 1 014 194; EP-A 0 649 063; EP-A 0617 331; EP-A 0152 653; DE-C 30 09 928. But these techniques are elaborate because they are restricted to special materials and always require additional process steps.  
      Therefore, it is an objective of the present invention to provide in an easy and economical way for photosensitive elements to be used with photomasks having a smooth surface. It is another object of the present invention to provide photosensitive elements for preparing flexographic printing plates which show improved printing quality.  
     SUMMARY OF THE INVENTION  
      This objective is solved by a photosensitive element for use as a flexographic printing plate comprising a) a support, b) at least one elastomeric photopolymerizable layer on the support containing at least lo one elastomeric binder, at least one ethylenically unsaturated compound photopolymerizable by actinic radiation, and at least one photoinitiator or photoinitiator system, and c) a matted layer disposed above a surface of the photopolymerizable layer opposite the support comprising a polymeric binder and at least one matting agent, the at least one matting agent is capable of being anchored in the surface of the photopolymerizable layer, and selected from the group consisting of i) matting agents having a pore volume of ≧0.9 ml/g, ii) matting agents having a BET surface of ≧150 m 2 /g, iii) matting agents having an oil number of ≧150 g/100 g, iv) matting agents having at least one crosslinkable group; and v) combinations thereof.  
      In another embodiment, the invention is directed to a process for preparing the photosensitive element wherein the matted layer is contacted, either by laminating or extrusion followed by calendering, with one surface of the elastomeric photopolymerizable layer.  
      In a further embodiment, the invention is directed to a process for preparing a flexographic printing plate comprising (A) exposing a photosensitive element to actinic radiation through a photomask forming polymerized areas and unpolymerized areas in the photopolymerizable layer, (B) removing the photomask, and (C) treating the exposed photosensitive element to remove unpolymerized areas and form a relief surface suitable for printing; and a flexographic printing plate made by such a process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)  
      The present invention provides an easy and economical way to prepare a photosensitive element used to make a flexographic printing plate in which the photosensitive element can be used with photomasks having a smooth surface. The present invention also provides for a flexographic printing plate prepared from the photosensitive element showing good ink transfer and printing quality and having enhanced print life. The present invention can be integrated in usual production processes with no additional, special production steps being necessary. It can be adopted for all sorts of flexographic printing plates and various types of flexographic composition. The photosensitive element provides better vacuum contact with the photomask during imagewise exposure than conventional photosensitive elements. Even photomasks having smooth surfaces can be used. Developability of the present photosensitive element is comparable to conventional elements. But the present photosensitive element can have the advantage that no sludge is formed in the processor during development.  
      A main advantage of the present photosensitive element is that during printing the flexographic printing plate made from this photosensitive element picks up less deposits of paper fibers and dried ink which would fill in reverses areas of the plate. Therefore, high printing quality can be maintained much longer than with former flexographic printing plates and the time between plate cleanings during the printing press run can be extended, resulting in higher productivity.  
      Matted Layer  
      The matted layer of the photosensitive element comprises a polymeric binder and at least one matting agent which is capable of being anchored in an outermost surface of the photopolymerizable layer opposite the support. The term “anchored” means that the matting agent remains attached on and/or partially in the outermost surface of imagewise photopolymerized areas of the photopolymerizable layer during imagewise exposure and development as described below, whereas the polymeric binder of the matted layer is removed from the imagewise photopolymerized areas during development. The matting agent is selected from the group consisting of i) matting agents having a pore volume of ≧0.9 ml/g, ii) matting agents having a BET surface of ≧150 m 2 /g, iii) matting agents having an oil number of ≧150 g/100 g, iv) matting agents having at least one crosslinkable group; and v) combinations thereof.  
      The pore volume of the matting agent under i) is determined by titration with water. The test sample is activated by heating for 2 hours at 200° C. 10-40 g of the test sample are weighed into a screw top jar. Sufficient water is added from a burette to fill about 80% of the expected pore volume. The jar is then vigorously shaken to distribute the water evenly throughout the sample. The sample is cooled to room temperature under cold water and further small additions of water are made until a saturated gel has formed, which adheres to the base of the jar when inverted. At this point the water has filled all of the pores and begins to cover the external surface of the sample. After each addition the jar is shaken vigorously. The pore volume is calculated by: pore volume (ml/g)=quantity of water added (ml)/weight of sample (g). Preferably, a matting agent with a pore volume of 1.0-2.5 ml/g is used. Especially matting agents with a pore volume of 1.5-2.5 ml/g are suitable.  
      BET surface means the specific surface determined according to the method of Brunauer, Emmett, and Teller (see also DIN 66131). By this method, the volume of nitrogen gas is measured, which is adsorbed on the surface of the adsorbing material at −196° C. dependent upon the applied pressure. This method is well known to those skilled in the art. Preferably, a matting agent with a BET surface of ≧200 m 2 /g is used. Especially matting agents with a BET surface of ≧280 m 2 /g are suitable.  
      The oil number is measured according to DIN ISO 787-5 (ASTM D 281). The oil number represents the amount of oil in g necessary to process 100 g of a pigment (here the matting agent) to form an adhering cementlike mass. Preferably, a matting agent with an, oil number of ≧200 g/100 g is used. Especially matting agents with an oil number of ≧250 g/100 g are suitable.  
      The term “crosslinkable group” for matting agents having at least one crosslinkable group of iv) is intended to encompass photopolymerizable and photocrosslinkable groups as well as such groups which are thermally crosslinkable. Preferred crosslinkable groups are ethylenically unsaturated groups, such as, e.g., vinyl groups, acryloyl groups or methacryloyl groups, and epoxy groups. Especially preferred are ethylenically unsaturated groups. Matting agents comprising such crosslinkable groups are prepared by conventional surface modification of silicic acids or silica matting agents with silanes comprising the crosslinkable group and an alkoxy group or halogen atom. Such surface modifications are described in DE-A 33 25 064 and DE-B 23 04 602.  
      Preferably, the matting agent has a mean particle size of ≧3 μm. The mean particle size is the particle size determined according to ASTM-D 4438-85 (Coulter Counter method). Especially suitable are matting agents having a mean particle size of 3-15 μm. Preferably, ≧60% by volume of the total particles of the matting agent are between 3 to 15 μm; especially preferred are ≧80% by volume. The matting agent may include ≦20% by volume of particles with a particle size of ≧15 μm. Such particles are useful to provide roughness to the surface of the photosensitive element after peel-off of the cover sheet resulting in reduced vacuum draw-down time and easier lift-off of the photomask after exposure. Furthermore, the matting agent may comprise ≧10%, preferably 10-30%, by volume of particles with a particle size of ≦3 μm, and preferably 5-15% of ≦2 μm. Particles with a particle size of ≦3 μm may advantageously enhance the surface coverage of the flexographic printing plate with matting agent, whereby the print quality of such flexographic printing plate can be further improved due to the enhanced hydrophilicity of the printing plate surface. Such printing plates are especially useful for polar printing inks containing water, alcohols, and/or other polar compounds. Especially, these printing plates can provide the further advantage of forming a barrier layer on top of the surface resulting in less penetration of ink solvent components thereby lowering the swelling of the printing plates and providing higher surface hardness for longer print runs.  
      It is advantageous to use a matting agent having a combination of the particle characteristics described for i), ii), iii), and/or iv). A matting agent having a mean particle size of ≧3 μm, a pore volume of ≧0.9 ml/g, and an oil number of ≧150 g/100 g is suitable, and further including a BET surface of ≧150 m 2 /g is particularly suitable. Most suitable are those matting agents having a mean particle size of ≧3 μm, a pore volume of 1.0-2.5 ml/g, an oil number of ≧200 g/100 g, and a BET surface of ≧200 m 2 /g.  
      Preferably, the matting agent has a refractive index similar to that of the other components of the photosensitive element.  
      The matting agent may be any inorganic or organic matting agent having one or more of the above described properties. Preferably, the matting agent is selected from the group consisting of silicas, silicic acids, silicates, like clays, kaolinites, zeolithes, then aluminas and/or aluminates, and mixtures thereof.  
      It is also possible to use a matting agent that is filled and/or loaded with at least one ethylenically unsaturated compound photopolymerizable by actinic radiation. Suitable monomers are those which are used in the photopolymerizable layer and which are disclosed below. Preferably, polar monomers with good adsorption to the surface of the matting particles, especially such with low viscosity and high diffusion capability in the photopolymerizable layer, can be used, for example, hexamethylene glycol diacrylate and/or hexamethylene glycol dimethacrylate. By the use of matting agents filled and/or loaded with monomers it is possible to reduce the ageing time of the photosensitive elements after their production which is necessary to equilibrate the photopolymerizable layer to achieve suitable sensitometric characteristics and to equilibrate the photopolymerizable layer and the matted layer for purpose of monomer migration from the photopolymerizable layer into the matted layer, especially into the pores and/or onto the surface of the matting agent. This migration of monomers seems to provide anchoring of the matting agent during imagewise exposure of the photosensitive element. Another possibility to reduce the ageing time is to apply heat to the photosensitive element instead of ageing at ambient temperature. Longer ageing time and/or ageing at higher temperatures may provide better anchoring of the matting agent. But not in all embodiments of the invention, it seems to be necessary to have a good anchoring of the matting agent for improved printing quality.  
      Preferably, the matting agent is present in an amount of ≧10% by weight of the matted layer, preferably ≧15% by weight, and most preferably 15-20% by weight. The matting agent may be used in an amount up to 60% by weight of the matted layer. Furthermore, the matting agent may include ≦20% by weight of other particulate materials not capable of being anchored in the surface of the photopolymerizable layer, such as, for example, polymer beads, etc.  
      It is preferred that the matted layer and the photopolymerized printing plate have special surface roughness. Surface roughness, as described herein, is measured with a Hommeltester by profile measurement. The surface roughness Rz according to DIN 4768 of the matted layer alone, i.e., before association with the photopolymerizable layer, is preferably ≧3 μm, especially ≧5 μm. The surface roughness Rz of the photosensitive element after removal of a cover sheet is preferably ≧2 μm, especially ≧2.5 μm. The roughness Rz of the printing surface of the flexographic printing plate after treating, and optionally after-treatment, is preferably ≧2.5 μm, especially ≧3 μm.  
      In addition to the matting agent, the matted layer comprises a polymeric binder. Suitable polymers are soluble or strippable or removable during treating of the photosensitive element. Such are, for example, polyamides as nylon and nylon copolymers, polyvinyl alcohols, polyurethanes, polyvinyl pyrrolidones, polyethylene oxides of Mw ≧100,000, copolymers of ethylene and vinyl acetate, cellulose ester, cellulose ether. It is especially preferred to use polyamides. Usually, the polymeric binder is present in an amount of ≦85% by weight of the matted layer. Preferably, the matted layer comprises 40-90% by weight of the polymeric binder.  
      Optionally, the matted layer may comprise colorants, e.g., dyes and/or pigments as well as photochromic additives, i.e., for identification or for better contrast between imaged and non-imaged areas of the photosensitive elements directly after imagewise exposure or after imagewise exposure and development. In one embodiment of the invention, a dye can provide enhanced exposure latitude to the photosensitive element. Exposure latitude means the fill-in of reverse depths due to light scattering. This is especially important for print quality and good reproduction of fine elements. These colorants must not interfere with the imagewise exposure of the photopolymerizable layer. Suitable colorants are, e.g., soluble acridine dyes, anthraquinone dyes, phenazine dyes, and phenoxazine dyes, such as, for example, methylene violet (C.I. Basic Violet 5), methylene blue B (C.I. 52015), Solvent Black 3 (C.I. 26150), Rhodamin 3 GO (C.1. Basic Red 4), Solvent Blue 11 (C.I. 61525), Victoria Pure Blue BO (C.I. Basic Blue 7 or C.I. 42595), anthraquinone Blue 2 GA (C.I. Acid Blue 58), Safranin T (C.I. 50240), etc. Usually, the colorant is present in an amount of 0.0001-2% by weight of the matted layer. Preferably, the matted layer comprises 0.001-1% by weight of the colorant.  
      The matted layer may optionally form an integrated masking layer for the photosensitive element. As such, the matted layer becomes an infrared (IR) sensitive layer, which means that the matted layer can be imaged with an infrared laser radiation. Therefore, the matted layer can contain material having high infrared absorption in the wavelength range between 750 and 20,000 nm, such as, for example, polysubstituted phthalocyanine compounds, cyanine dyes, merocyanine dyes, etc., inorganic pigments, such as, for example, carbon black, graphite, chromium dioxide, etc., or metals, such as aluminum, copper, etc. The quantity of infrared absorbing material is usually 0.1-40% by weight, relative to the total weight of the layer. Furthermore, the matted layer in this embodiment is opaque to ultraviolet or visible light, that is, has an optical density ≧2.5. To achieve this optical density, the matted layer contains a material that prevents the transmission of actinic radiation.  
      This actinic radiation blocking material can be the same or different than the infrared absorbing material, and can be for example, dyes or pigments, and in particular the aforesaid inorganic pigments. The quantity of the radiation blocking material is usually 1-70% by weight relative to the total weight of the layer. When the matted layer is IR-sensitive, it may include as the polymeric binder, for example, nitrocellulose, homopolymers or copolymers of acrylates, methacrylates and styrenes, polyamides, polyvinyl alcohols, etc. All these compounds are described in detail, for example in WO 94/03838 and WO 94/3839 which disclose IR-sensitive layers as integrated photomasks for flexographic printing plates. Other auxiliary agents, such as plasticizers, coating aids, waxes, surfactants, etc. can be included in the matted layer. Preferably, waxes with a softening temperature ≧70° C., especially polyethylene waxes having a softening temperature ≧90° C., can be used.  
      Conventional methods like slot coating, roll coating, gravure coating, or spray coating are used to prepare the matted layer from a solution or dispersion of the components in suitable solvents. The matted layer is applied on the cover sheet, and subsequently dried. The thickness of the matted layer is usually 0.02-40 μm, preferably 0.05-20 μm, especially 0.5-10 μm with a usual dry coating weight of 0.01-10 g/m 2 , preferably 0.1-5 g/m 2 . The dry coating weight of the matting agent usually is 0.001-5 g/m 2 , preferably 0.01-2 g/m 2 .  
      Photopolymerizable Layer  
      The photopolymerizable layer of the photosensitive element for use as flexographic printing plate consist of known photopolymerizable materials. As used herein, the term “photopolymerizable” is intended to encompass systems which are photopolymerizable, photocrosslinkable, or both. All photopolymerizable materials of the state of the art can be used. Especially preferred are the materials disclosed in U.S. Pat. No. 4,323,637; U.S. Pat. No. 4,427,759; and U.S. Pat. No. 4,894,315. They usually comprise at least one elastomeric binder, at least one photopolymerizable, ethylenically unsaturated monomer, and at least one photoinitiator or photoinitiator system, wherein the photoinitiator is sensitive to actinic radiation, which usually includes ultraviolet radiation and/or visible radiation.  
      Examples of elastomeric binders are polyalkadienes, alkadiene/acrylonitrile copolymers; ethylene/propylene/alkadiene copolymers; ethylene/(meth)acrylic acid((meth)acrylate copolymers; and thermoplastic, elastomeric block copolymers of styrene, butadiene, and/or isoprene. Linear and radial thermoplastic, elastomeric block copolymers of styrene and butadiene and/or isoprene are preferred. Preferably, the binder is present in an amount of ≧65% by weight of the photopolymerizable material.  
      Monomers that can be used in the photopolymerizable layer are well known in the art and include ethylenically unsaturated, copolymerizable, organic compounds, such as, for example, acrylates and methacrylates of monovalent or polyvalent alcohols; (meth)acrylamides; vinyl ethers and vinyl esters; etc., in particular acrylic and/or methacrylic of butanediol, hexanediol, diethylene glykol, trimethylol propane, pentaerythritol, etc.; and mixtures of such compounds. Preferably, the monomer is present in an amount of ≧5% by weight of the photopolymerizable material.  
      Suitable photoinitiators are individual photoinitiators or photoinitiator systems, such as, for example, benzoin derivatives, benzil acetals, diarylphosphine oxides, etc., also mixed with triphenyl phosphine, tertiary amines, etc. Preferably, the photoinitiator is present in an amount of 0.001-10.0% by weight of the photopolymerizable material  
      In addition to the main components described in the foregoing, the photopolymerizable compositions may comprise conventional additives like, for example, UV absorbers, thermal stabilizers, plasticizers, colorants, antioxidants, fillers, etc.  
      The thickness of the photopolymerizable layer can vary over a wide range depending upon the type of flexographic printing plate desired. For so called “thin plates” the photopolymerizable layer can be from about 0.05-0.17 cm in thickness. Thicker plates will have a photopolymerizable layer up to 0.25-0.64 cm in thickness or greater.  
      Support  
      The support can be any flexible material which is conventionally used with photosensitive elements for use as flexographic printing plates. Examples for suitable support materials include polymeric films such those formed by addition polymers and linear condensation polymers, transparent foams and fabrics, and metals such as aluminum. A preferred support is a polyester film; particularly preferred is polyethylene terephthalate. The support typically has a thickness from 0.001-0.030 inch.  
      Cover Sheet  
      The photosensitive element optionally comprises a cover sheet as outermost protective layer on top of the matted layer or if present on top of the IR-sensitive layer. Useful cover sheets consist of flexible polymeric films, e.g., polyethylene terephthalate, which preferably is unsubbed but optionally may be subcoated with a thin silicone layer, polystyrene, polyethylene, polypropylene, or other strippable polymeric films. Preferably, polyethylene terephthalate is used.  
      Additional Layers  
      Additional layers may be present on top of the photopolymerizable layer. Suitable layers are those disclosed as elastomeric layers in the multilayer cover element described in U.S. Pat. No. 4,427,759 and U.S. Pat. No. 4,460,675. Such elastomeric layers comprise layers, which are insensitive to actinic radiation themselves but become photosensitive when contacted with the photopolymerizable layer as well as such layers which are photosensitive themselves. These photosensitive elastomeric layers comprise preferably an elastomeric binder, a monomer, and a photoinitiator, and optionally fillers or other additives. Elastomeric layers which become photosensitive when contacted with the photopolymerizable layer do not comprise any monomer. Binder, monomer, and other compounds can be the same or similar to those compounds comprised in the photopolymerizable layer. These elastomeric layers are disposed between the photopolymerizable layer and the matted layer.  
      The photosensitive element can optionally include a wax layer as disclosed in DE-C 199 09 152 between the matted layer and the photopolymerizable layer, or between the matted layer and the elastomeric layer if present. Suitable waxes are all natural and synthetic waxes, such as polyolefin waxes, paraffin waxes, carnauba waxes, stearin waxes, and steramide waxes. Preferred are waxes with a softening temperature ≧70° C., especially polyethylene waxes having a softening temperature ≧90° C. Conventional methods like casting, printing, or spray coating are used to prepare the wax layers from dispersions of the waxes in suitable solvents. The wax layer is usually 0.02-1.0 μm thick, preferably 0.05-0.5 μm.  
      Furthermore, the photosensitive element may optionally comprise an IR-sensitive layer on top of the matted layer. The IR-sensitive layer can form an integrated masking layer for the photosensitive element. The preferred IR-sensitive layer is removable during treating, i.e., soluble or dispersible in a developer solution or removable during thermal development; opaque to actinic radiation, i.e., ultraviolet or visible light, that is, has an optical density ≧2.5; and can be imaged with an infrared laser. The IR sensitive layer contains material having high infrared absorption in the wavelength range between 750 and 20,000 nm, such as, for example, polysubstituted phthalocyanine compounds, cyanine dyes, merocyanine dyes, etc., inorganic pigments, such as, for example, carbon black, graphite, chromium dioxide, etc., or metals, such as aluminum, copper, etc. The quantity of infrared absorbing material is usually 0.1-40% by weight, relative to the total weight of the layer. To achieve the optical density of ≧2.5 to block actinic radiation, the infrared-sensitive layer contains a material that prevents the transmission of actinic radiation. This actinic radiation blocking material can be the same or different than the infrared absorbing material, and can be, for example, dyes or pigments, and in particular the aforesaid inorganic pigments. The quantity of this material is usually 1-70% by weight relative to the total weight of the layer. The infrared-sensitive layer optionally includes a polymeric binder, such as, for example, nitrocellulose, homopolymers or copolymers of acrylates, methacrylates and styrenes, polyamides, polyvinyl alcohols, etc. Other auxiliary agents, such as plasticizers, coating aids, etc. are possible. The infrared-sensitive layer is usually prepared by coating or printing a solution or dispersion of the aforesaid components on the cover sheet, and subsequently drying it before the matted layer is applied onto the cover sheet. The thickness of the infrared-sensitive layer is usually 2 nm to 50 μm, preferably 4 nm to 40 μm. These infrared-sensitive layers and their preparation are described in detail, for example in WO 94/03838 and WO 9413839.  
      The photosensitive element can optionally include an adhesive layer between the support and the photopolymerizable layer. Such adhesive materials are disclosed in U.S. Pat. No. 3,036,913 or U.S. Pat. No. 2,760,863. Alternatively, the support can have an adhesion promoting surface by flame-treatment or electron-treatment or the adhesion of the photopolymerizable layer to the support can be enhanced by exposure to actinic radiation through the support.  
      Furthermore, the photosensitive element can optionally include an antihalation layer between the support and the photopolymerizable layer. Such antihalation layer can be made by dispersing a finely divided dye or pigment which substantially absorbs actinic radiation in a solution or aqueous dispersion of a resin or polymer which is adherent to both the support and the photopolymerizable layer and coating it on the support and drying. Suitable antihalation pigments and dyes include carbon black, manganese dioxide, Acid Blue Black (CI 20470), and Acid Magenta O (CI 42685). Suitable polymeric or resin carriers include polyvinyl compounds, e.g., polyvinyl chloride homo- and copolymers, copolymers of acrylic and methacrylic acid, etc.  
      Process for Preparing Photosensitive Elements  
      The present photosensitive element can be prepared by contacting the matted layer with one surface of a photopolymerizable layer. In one instance, the matted layer, is disposed on a cover sheet, and is then laminated onto the outermost surface of the element opposite the support, which is typically the photopolymerizable layer, with a conventional laminator. The photopolymerizable layer itself may be prepared in many ways by admixing the binder, monomer, initiator, and other ingredients and forming it into a sheet layer. It is preferred that the application of the matted layer onto the photopolymerizable layer is integrated within the usual production process of photosensitive elements for use as flexographic printing plates. Generally, the photopolymerizable mixture is formed into a hot melt and then calendered to the desired thickness. An extruder can be used to perform the function of melting, mixing, deaerating and filtering the composition. The extruded mixture is then calendered between the support and a cover element. Concerning the present invention, this cover element comprises a cover sheet, optionally an IR-sensitive layer, the matted layer, and optionally a wax layer and/or an elastomeric layer. Alternatively, the photopolymerizable material can be placed between the support and the cover element in a mold. The layers of material are then pressed flat by the application of heat and/or pressure. The combination of extrusion/calendering process is particularly preferred. The better the contact between the matted layer and the photopolymerizable layer, the better seems to be the anchoring of matting agent to the surface of the photopolymerized layer later on. After the photosensitive element is prepared, it is cooled, e.g., with blown air, and is passed under a bank of fluorescent lamps, e.g., black light tubes, placed traverse to the path of movement. The photosensitive element is continually exposed through the support to partially polymerize a predetermined thickness of the photopolymer layer adjacent the support.  
      Process for Preparing Flexographic Printing Plates  
      The photosensitive element produced as described above is then imagewise exposed by common processes through a photomask having areas transparent to actinic radiation and areas substantially opaque to actinic radiation. Actinic radiation means ultraviolet and visible radiation. The photomask can be a separate film, i.e., an image-bearing transparency or phototool, such as a silver halide film; or can be the photomask integrated with the photosensitive element as described above. In the case in which the photomask is a separate film, the optional cover sheet is usually stripped before imagewise exposure leaving the matted layer on the photosensitive element. The photomask is brought into close contact with the matted layer of the photosensitive element by the usual vacuum processes, e.g., by use of a common vacuum frame. Thus a substantially uniform and complete contact between the matted layer and the photomask can be achieved in acceptable time. By applying vacuum, the atmospheric oxygen is removed and the effect of oxygen inhibiting polymerization is minimized. The absence of atmospheric oxygen may furthermore lead to better anchoring of the matting agent.  
      In the case in which there is a IR-sensitive layer on the matted layer or the matted layer also functions as the IR-sensitive layer, the IR-sensitive layer is imagewise exposed to IR laser radiation to form the photomask on the photosensitive element. The infrared laser exposure can be carried out using various types of infrared lasers, which emit in the range 750 to 20,000 nm. Infrared lasers including, diode lasers emitting in the range 780 to 2,000 nm and Nd:YAG lasers emitting at 1064 nm are preferred. The radiation opaque layer is exposed imagewise to infrared laser radiation to form the image on or disposed above the photopolymerizable layer, i.e., the in-situ mask. The infrared laser radiation can selectively remove, e.g., ablate or vaporize, the infrared sensitive layer (i.e., radiation opaque layer) from the photopolymerizable layer, as disclosed by Fan in U.S. Pat. Nos. 5,262,275 and 5,719,009; and Fan in EP 0 741 330 B1. The integrated photomask remains on the photosensitive element for subsequent steps of overall exposure to actinic radiation and treating. For photosensitive elements having the integrated photomask, the imagewise exposure may be conducted in the presence or absence of atmospheric oxygen. However, it is preferred that the imagewise exposure is conducted in the absence of atmospheric oxygen, for example, under vacuum, to minimize the effect of oxygen inhibiting polymerization whereby also better anchoring of the matting agent may be achieved.  
      Upon imagewise exposure, the radiation-exposed areas of the photopolymerizable layer are converted to the insoluble state with no significant polymerization or crosslinking taking place in the unexposed areas of the layer. Any conventional source of actinic radiation can be used for this exposure. Examples of suitable radiation sources include xenon lamps, mercury vapor lamps, carbon arcs, argon glow lamps, fluorescent lamps with fluorescent materials emitting UV radiation and electron flash units, and photographic flood lamps. The most suitable sources of UV radiation are the mercury vapor lamps, particularly the sun lamps. The exposure time may vary from a few seconds to minutes, depending upon the intensity and spectral energy distribution of the radiation, its distance from the photosensitive element, and the nature and amount of the photopolymerizable material. A long exposure time may improve the anchoring of the matting agent on the surface of the flexographic printing plate and thereby enhance the surface coverage by the matting agent. An overall back exposure may be conducted before or after the imagewise exposure to polymerize a predetermined thickness of the photopolymer layer adjacent the support. This polymerized portion of the photopolymer layer is designated a floor. The floor thickness varies with the time of exposure, exposure source, etc. This exposure may be done diffuse or directed. All radiation sources suitable for imagewise exposure may be used. The exposure is generally for 1-30 minutes.  
      Following overall exposure to UV radiation through the mask, the photosensitive printing element is treated to remove unpolymerized areas in the photopolymerizable layer and thereby form a relief image. The treating step removes at least the photopolymerizable layer in the areas which were not exposed to actinic radiation, i.e., the unexposed areas or uncured areas, of the photopolymerizable layer. The matted layer disposed above the non-exposed areas of the photopolymerizable layer and the polymeric binder of the matted layer which are disposed above the exposed areas of the photopolymerizable layer are removed with the treating step.  
      After the photopolymerizable layer has been polymerized by exposure and the relief formed by treatment, the surface opposite the support ultimately is the printing surface of the printing element. The printing surface is the raised area/s of the relief which carry ink. This printing surface of the photopolymerized printing plate may be covered with particles of the matting agent from the matted layer. Surface coverage, as herein described is measured under a microscope in transmission or by SEM/EDX techniques as described below. In one embodiment of the invention, ≧0.5% of the printing surface of the flexographic printing plate is covered with the matting agent, preferably, 0.5-1% of the printing surface. In another embodiment of the invention, ≧1% of the printing surface of the flexographic printing plate is covered with the matting agent, preferably, ≧2% of the printing surface. In a further embodiment of the invention, ≧10% of the printing surface of the flexographic printing plate is covered with the matting agent, preferably, ≧20% of the printing surface. In a further embodiment of the invention, 20-30% of the printing surface can be covered with the matting agent. For special applications, it may be especially advantageous when ≧30% of the surface of the flexographic printing plate are covered with the matting agent. Flexographic printing plates showing a high covering of the printing surface with matting agent can advantageously be used in off-set lacquering applications.  
      Except for the elastomeric capping layer, typically the additional layers that may be present on the photopolymerizable layer are removed or substantially removed from the polymerized areas of the photopolymerizable layer. For photosensitive elements including a separate IR-sensitive layer for digital formation of the mask, the treating step also removes the mask image (which had been exposed to actinic radiation).  
      Treatment of the photosensitive printing element includes (1) “wet” development wherein the photopolymerizable layer is contacted with a suitable developer solution to washout unpolymerized areas and (2) “dry” development wherein the photosensitive element is heated to a development temperature which causes the unpolymerized areas of the photopolymerizable layer to melt or soften or flow and is wicked away by contact with an absorbent material. Dry development may also be called thermal development.  
      Wet development is usually carried out at about room temperature. The developers can be organic solvents, aqueous or semi-aqueous solutions, or water. The choice of the developer will depend primarily on the chemical nature of the photopolymerizable material to be removed. Suitable organic solvent developers include aromatic or aliphatic hydrocarbon, and aliphatic or aromatic halohydrocarbon solvents, for example, n-hexane, petrol ether, hydrated petrol oils, limonene or other terpenes or toluene, isopropyl benzene, etc., ketones such as methyl ethyl ketone, halogenated hydrocarbons such as chloroform, trichloroethane, or tetrachloroethylene, esters such as acetic acid or acetoacetic acid esters, or mixtures of such solvents with suitable alcohols. Other organic solvent developers have been disclosed in published German Application 38 28 551. Suitable semi-aqueous developers usually contain water and a water miscible organic solvent and an alkaline material. Suitable aqueous developers usually contain water and an alkaline material. Other suitable aqueous developer combinations are described in U.S. Pat. No. 3,796,602. Additives such as surfactants or alcohols may be used.  
      Development time can vary, but it is preferably in the range of about 2 to about 25 minutes. Developer can be applied in any convenient manner, including immersion, spraying and brush or roller application. Brushing aids can be used to remove the unpolymerized portions of the element. Washout can be carried out in an automatic processing unit which uses developer and mechanical brushing action to remove the unexposed portions of the plate, leaving a relief constituting the exposed image and the floor.  
      Following treatment by developing in solution, the relief printing plates are generally blotted or wiped dry, and then more fully dried in a forced air or infrared oven. Drying times and temperatures may vary, however, typically the plate is dried for 60 to 200 minutes at 60° C. High temperatures are not recommended because the support can shrink and this can cause registration problems.  
      Treating the element thermally includes heating the photosensitive element having at least one photopolymerizable layer (and the additional layer/s) to a temperature sufficient to cause the uncured portions of the photopolymerizable layer to soften or melt or flow, and contacting an outermost surface of the element to an absorbent surface to absorb or wick away the melt or flow portions. The polymerized areas of the photopolymerizable layer have a higher melting temperature than the unpolymerized areas and therefore do not melt, soften, or flow at the thermal development temperatures. Thermal development of photosensitive elements to form flexographic printing plates is described by Martens in U.S. Pat. Nos. 5,015,556; 5,175,072; 5,215,859; and by Wang et al. in WO 98/13730.  
      The term “melt” is used to describe the behavior of the unirradiated portions of the photopolymerizable elastomeric layer subjected to an elevated temperature that softens and reduces the viscosity to permit flow and absorption by the absorbent material. The material of the meltable portion of the photopolymerizable layer is usually a viscoelastic material which does not have a sharp transition between a solid and a liquid, so the process functions to absorb the heated composition layer at any temperature above some threshold for absorption in the absorbent material. A wide temperature range may be utilized to “melt” the composition layer for the purposes of this invention. Absorption may be slower at lower temperatures and faster at higher temperatures during successful operation of the process.  
      The thermal treating steps of heating the photosensitive element and contacting an outermost surface of the element with an absorbent material can be done at the same time, or in sequence provided that the uncured portions of the photopolymerizable layer are still soft or in a melt state when contacted with the absorbent material. The at least one photopolymerizable layer (and the additional layer/s) are heated by conduction, convection, radiation, or other heating methods to a temperature sufficient to effect melting of the uncured portions but not so high as to effect distortion of the cured portions of the layer. The one or more additional layers disposed above the photopolymerizable layer may soften or melt or flow and be absorbed as well by the absorbent material. The photosensitive element is heated to a surface temperature above about 40° C., preferably from about 40° C. to about 230° C. (104-446° F.) in order to effect melting or flowing of the uncured portions of the photopolymerizable layer. By maintaining more or less intimate contact of the absorbent material with the photopolymerizable layer that is molten in the uncured regions, a transfer of the uncured photosensitive material from the photopolymerizable layer to the absorbent material takes place. While still in the heated condition, the absorbent material is separated from the cured photopolymerizable layer in contact with the support layer to reveal the relief structure. A cycle of the steps of heating the photopolymerizable layer and contacting the molten (portions) layer with an absorbent material can be repeated as many times as necessary to adequately remove the uncured material and create sufficient relief depth. However, it is desirable to minimize the number of cycles for suitable system performance, and typically the photopolymerizable element is thermally treated for 5 to 15 cycles. Intimate contact of the absorbent material to the photopolymerizable layer (while in the uncured portions are melt) may be maintained by the pressing the layer and the absorbent material together.  
      A preferred apparatus to thermally develop the photosensitive element is disclosed by Peterson et al. in U.S. Pat. No. 5,279,697, and also by Johnson et al. in Patent Cooperation Treaty Application No. PCT/US00/24400 filed Sep. 6, 2000 (IM-1289 PCT). The photosensitive element may be placed on a drum or a planar surface in order for thermal treatment to be carried out.  
      The absorbent material is selected having a melt temperature exceeding the melt temperature of the uncured portions of the photopolymerizable layer and having good tear resistance at the same operating temperatures. Preferably, the selected material withstands the temperatures required to process the photosensitive element during heating. The absorbent material is selected from non-woven materials, paper stocks, fibrous woven material, open-celled foam materials, porous materials that contain more or less a substantial fraction of their included volume as void volume. The absorbent material can be in web or sheet form. The absorbent materials should also possess a high absorbency for the molten elastomeric composition as measured by the grams of elastomer that can be absorbed per square millimeter of the absorbent material. Preferred is a non-woven nylon web.  
      It is also contemplated that the photosensitive element may undergo one or more treating steps to sufficiently remove the uncured portions to form the relief. The photosensitive element may undergo both wet development and dry development, in any order, to form the relief. A pre-development treating step may be necessary to remove one or more of the additional layers disposed above the photopolymerizable layer if such additional layers are not removable by the washout solution and/or by heating.  
      The flexographic printing plate may be post exposed and/or chemically or physically after-treated in any sequence to detackify the surface of the flexographic printing plate.  
     EXAMPLES  
      The following examples illustrate the invention, but do not limit it. Unless otherwise indicated, the parts and percentages indicated refer to the weight.  
      In the following examples, CYREL® flexographic printing plates (including plates identified herein as DuPont types, HOS, NOW, HiQ), CYREL® exposure unit, CYREL® processor, Cyrel® mounting tape, and CYREL® Flexosol® washout solvent are products sold by E. I. du Pont de Nemours and Company, Wilmington, Del.  
      SEM/EDX Evaluation of Flexoplate Surfaces  
      For confirming silica particles anchored on the plate surface Scanning Electron Microscope (SEM) techniques were used in combination with Energy Dispersive X-ray (EDX) spectrometry. The surfaces of the samples were vacuum-sputtered with a thin gold film of some few Angstroem to ensure a sufficient surface conductivity, which is needed fir the SEM/EDX evaluation. For the surface analysis a SEM Type Zeiss DSM 962 (from Carl Zeiss Company, now Leo Elektronenmikroskopie GmbH, Carl-Zeiss Str. 56, D-7344 Oberkochen/Germany) equipped with Secondary-Electrons (SE)- and Back-Scattered Electrons (BSE)-detectors and with an EDX spectrometer Type Noran Vantage (from THERMO NORAN, 2551 W. Beltline Hwy., Middleton, Wis. 53562, USA) for element analysis and element mapping option was used. The images were taken at magnification of 50×, 100×, 200× for studying surface areas in the range of about 0.5-5 mm 2 . By moving the sample in XY-direction by microdrives, larger areas of about 5 cm×5 cm (areas of about 5 cm 2  up to 25 cm 2 ) were evaluated and inspected for structure uniformity.  
      Higher magnifications were used to evaluate details of the surface pattern and of particles anchored on the sample surface. At magnifications of 500× and 1000× the surface was evaluated for finer details and finer surface structures. The SEM images and the correlating EDX mapping images were taken at 200× and 500×. In addition, the BSE mode of the microscope was used for higher element contrasting, so that particles anchored at the surface could be easier recognized.  
      By scanning sections of the surface area and simultaneous recording of the X-ray signals an element spectrum was obtained. Single particles were analyzed separately to confirm Si from the silica particles dispersed into the release layer and bonded to the plate surface. Element distribution was recorded for Si (silica) and the images were further analyzed to determine the silica surface coverage on the different plate samples. Only particles containing Silicium were found. On all plates tested, Si was found, however, at different Si levels corresponding with different levels of silica coverage, depending on photopolymer type and plate preparation or manufacturing conditions.  
      The BSE mode provided a good contrast for the silica particles which can be seen as bright areas due to the larger amount of back-scattered electrons from Si versus the surrounding matrix of organic photopolymer material. High contrast images were also obtained with EDX mapping; mapping run typically for about 8 hours and allowed to detect small silica particles of 1-2 μm diameter difficult to recognize with other methods.  
      High contrast images from BSE and EDX mapping were further analyzed using the image analysis software analySIS® Pro Version 3.00 from Soft Imaging Systems GmbH, Johann-Krane-Weg 39, D48149 Müinster, Germany. The images were filtered to remove the noise level, and contrasted; from the EDX images a binary element distribution image was obtained from which the Si surface coverage was determined. Beside Si only the elements C, O, and Au from the gold sputtering were found: consequently, the Si detected correlated directly with silica particles anchored on the plate surface.  
     Example 1  
      7.00% of a polyamide (Macromelt® 6900, from Henkel KGaA, D-40191, Düsseldorf, Germany) were dissolved in a solvent blend of n-propanol, toluene, and cyclohexane (17.50%/27.50%/45.00%). A matting agent/polymeric binder ratio of 30170 was adjusted by adding 3.00% of a highly porous, highly polar and hydrophilic silica gel (Syloid® ED-5 from GRACE GmbH &amp; Co. KG, D-67545 Worms, Germany, having a pore volume of 1.8 ml/g; oil number of 320 g/100 g; a BET surface of 380 m 2 /g; mean particle size of 5 μm (D50=9.44 μm, measured with Mastersizer 2000 from Malvern Instruments/UK), particle size distribution of &gt;80% by volume within 2.5-15 μm). The matting agent was dispersed with glass beads having a diameter of 2 mm for 5 min on a disperser (BAS-20 from Lau GmbH, D-58675, Germany) with a vibration frequency of 620 min −1  according to DIN 53238-13 (draft ISO 8780). The dispersion was coated with a 30 μm doctor-box on a polyester cover sheet to form a matted layer. After air drying at ambient room conditions (22° C., 45% relative humidity) the resulting dry matted layer had a measured total coating weight of 27.50 mg/dm 2  and a corresponding calculated coating weight of the matting agent of 8.25 mg/dm 2 .  
      The so produced cover element was hot laminated to a Cyrel® 100HOS flexoplate to produce a photosensitive element with the matted layer contacting the photopolymerizable layer of the Cyrel® plate. The original cover sheet and release layer of the Cyrel® plate had been peeled-off before lamination. Lamination was carried out by a hot stamping press at 120° C. and with a lamination holding time of 10 sec at a pressure of 1 bar. After accelerated aging for 2 hours at 70° C. in a hot oven, the photosensitive element was back exposed in a Cyrel® Exposure Unit 1001 E for 25 sec to form a floor of 0.7 mm. The cover sheet was peeled off whereby the matted layer remained on the photopolymerizable layer. The matted layer of the photosensitive element was covered with a glossy phototool and the photosensitive element was imagewise-exposed under vacuum for 10 min in a Cyrel® Exposure Unit 1001 E. The vacuum draw-down time was 3 min. After imagewise-exposure, the glossy phototool could be easily taken off and the element was developed in a Cyrel® Processor 1002 P with Flexosol® at 32° C. for 400 sec. The resulting printing plate was dried at 60° C. for 3 hours in a Cyrel® Dryer 1002 D and simultaneously finished by UV-A and UV-C exposure in a Cyrel® Light Finisher 1002 LF for 6 min.  
      The surface roughness Rmax, Rz and Ra of the dry matted layer before lamination to the Cyrel® plate was measured with a surface profile tester T20-DC/RP50 (from Hommelwerke GmbH, D-78056 Villingen-Schwenningen, Germany) according to DIN 4768 (Rmax), DIN 4768 (Rz) and DIN 4762/DIN 4768/ISO 4287/1 (Ra). Filtering (R profil), selection of the cut-off length (0.8 mm) and selection of the total measuring length (4 mm) was adjusted according to DIN 4768/ISO 4288. The surface roughness Rmax was 9.2 μm, Rz was 8.4 μm and Ra was 1.5 μm measured on top of the dry matted layer before lamination to the Cyrel® plate. In a similar way the surface roughness of the photosensitive element after cover sheet peel-off and before exposure was measured. The following data were obtained: for Rmax 3.25 μm, for Rz 2.63 μm and for Ra 0.35 μm. Then the photosensitive element was back and front exposed, washed-out, dried and finished. After processing the press-ready printing plate was measured again and the following surface roughness data were measured: for Rmax 4.85 μm, for Rz 4.10 μm and for Ra 0.40 μm. Furthermore, the processed plate was evaluated under the microscope in transmission and it was found that more than 20% of the surface was covered with matting agent particles.  
     Example 2  
      As described in Example 1, a cover element with a matted layer was produced wherein the matted layer was prepared from a coating dispersion containing 5.00% of Macromelt® 6900 and 5.00% of Syloid® ED-5. The resulting dry matted layer had a measured total coating weight of 31.5 mg/dm 2  and a calculated matting agent/polymeric binder ratio of 50/50. A photosensitive element comprising this cover element was prepared and processed as described in Example 1. After processing more than 30% of the printing plate surface were covered with matting agent.  
     Example 3  
      Example 1 was repeated with a coating dispersion comprising 6.93% Macromelts® 6900, 2.97% Syloid® ED5, 19.64% n-propanol, 27.89% of toluene, and 41.25% cyclohexane. Additionally, 1.32% hexamethylene glycol diacrylate were added to the dispersion to fill and/or load the matting agent with monomer. The resulting dry matted layer had a total coating weight of 30.8 mg/dm 2 . A photosensitive element comprising this cover element was prepared and processed as described in Example 1. After processing more than 20% of the printing plate surface were covered with matting agent.  
     Example 4  
      A 100 μm thick polyester film was coated on one side with a matted layer consisting of 70% by weight of a polyamide (Macromelt® 6900, from Henkel Corp.) and 30% by weight of a porous silica (Syloid® ED-5 from Grace &amp; Co., having a pore volume of 1.8 ml/g; oil number of 320 g/100 g; a BET surface of 380 m 2 /g; mean particle size of 5 μm (D50=9.44 μm, measured with Mastersizer 2000 from Malvern Instruments/UK), particle size distribution of &gt;80% by volume within 2.5-15 μm), the matted layer having a dry coating weight of 3.4 g/m 2 . This matted layer was overcoated with a polyethylene wax layer. In the standard manufacturing process of DuPont Cyrel® 45HiQ and 67HiQ photopolymerizable printing plates, this cover element according to the present invention was used as cover element instead of the conventional cover element. As comparative examples DuPont Cyrel® 45HiQ and 67HiQ photopolymerizable printing plates with standard cover element (polyester cover sheet with release layer comprising Macromelt® 6900 and an amphoteric interpolymer) were produced, too. In the manufacturing process, an extruder was used to perform the function of melting, mixing, deaerating and filtering the conventional photopolymerizable mixture for Cyrel® HiQ photopolymer printing plates. This mixture was extruded through a flat die between a polyester base film and the cover element with the matted layer in contact with the photopolymerizable layer. The material was calendered to the required Cyrel® photopolymer plate thickness, cooled down to room temperature and cut into plates.  
      According to the standard procedures for the production of flexographic printing plates from photosensitive elements, as described in Example 1, the inventive and standard photopolymerizable plates were imagewise exposed through a photographic film containing a test target for flexographic printing plate performance evaluation and processed in a standard DuPont Cyrel® processing unit using DuPont Flexosol® as the processing solution. Then the plates were dried for 3 hours at 60° C. in an dryer for flexographic printing plates and finished by exposing with UV-light in a standard UV-finishing unit. Surprisingly the surface of the inventive plates was not tacky after drying for 20 minutes in a dryer at 60° C., while the standard plates exhibited the usual tackiness. Evaluations of the photosensitivity of standard and inventive plates showed no differences in the maximum allowable exposure time.  
      Inventive and conventional printing plates were printed on a Lemo Masterflex® flexographic printing machine using a 500 er anilox with a ink volume of 3.5 cm 3 /m 2  at a print speed of 120 meters per minute. For the print trials, paper (Sappi Algro Finess; 70 g/m 2 ) and film (Trespaphan® SHD 50.0) were used in combination with solvent based ink for film (LM magenta from Siegwerk Druckfarben/Siegburg) and water based ink for paper (Hartmann Magenta). In both cases the inventive printing plates improved the quality of the print compared to the standard printing plates. Especially, an improved reproduction of the middle tone to shadow areas was achieved.  
     Example 5  
      A photosensitive element based on Cyrel® 67NOW was prepared according to Example 4, wherein the standard release layer was replaced by a release layer comprising Syloid® ED5 and Macromelt® 6900 as described in Example 4 and having a matting agent/polymeric binder ratio of 17/83. The photosensitive element was processed to a flexographic printing plate as described in Example 4. The final printing plate was evaluated with the previously described SEM/EDX techniques. A silica surface coverage of 2-5% was determined. The surface appeared matte. The roughness was measured as described in Example 1 and the following data were found: Rmax=4.90 μm, Rz=3.85 μm, and Ra=0.60 μm. As a comparative example a commercial Cyrel® 67NOW plate was processed in the same way. The surface of the press-ready plate was highly glossy. The roughness was measured as follows: Rmax=2.80 μm, Rz=2.35 μm and Ra=0.35 μm.  
     Example 6  
      Two photosensitive elements based on Cyrel® 67HiQ were prepared according to Example 4, wherein the standard release layer was replaced by a release layer comprising Syloid® ED5 and Macromelt® 6900 as described in Example 4 and having a matting agent/polymeric binder ratio of 17/83. Before the photosensitive elements were processed to flexographic printing plates as described in Example 4, one element was stored at ambient temperature for 1 month (Plate A), whereas the other element was stored at 70° C. in a hot oven for 3 days (Plate B). The final printing plates were evaluated with the previously described SEM/EDX techniques. Plate A showed a silica surface coverage of 0.5-1%, and Plate B showed a silica surface coverage of 1-3%.  
     Example 7  
      A photosensitive element based on Cyrel® 67HiQ was prepared according to the process described in Example 1. The matted layer comprised Syloid® ED5 and Macromelt® 6900 as described in Example 1 and had a matting agent/polymeric binder ratio of 25/75 and a silica coating weight of 5.2 mg/dm 2 . Before the photosensitive element was processed to a flexographic printing plate as described in Example 4 (but without a finishing step), the element was stored at 100° C. in a hot oven for 16 h. The final printing plate was evaluated with the previously described SEM/EDX techniques. A silica surface coverage of 26% was determined.  
     Example 8  
      A printing test was performed with 2 sets of flexographic printing plates for printing on PE-film and paper and for comparing the print performance of the following plate types: 
          Sample 1 Cyrel® 67HiQ—with standard release layer;     Sample 2 HiQ-NS—a Cyrel® 67HiQ plate with a matted layer instead of the standard release layer;     Sample 3 BASF 67ACE—with standard release layer.        

      The photopolymerizable plate of Sample 2 was prepared according to Example 4, except that the matting agent/polymeric binder ratio of the matted layer was adjusted to 17/83.  
      Two sets of printing plates were prepared by standard procedure as described in Example 1 and Example 4. Exposure conditions for backflash were 65 sec and 70 sec, respectively, and for main exposure 18 min and 15 min, respectively. The plates were exposed in a Cyrel® Exposure Unit 1000 EC/LF and washed-out in a Cyrel® Processor Smart 1000 P with Cyrel® Flexosol as wash-out solvent. The plates were dried in a Cyrel® Dryer 1002 D for 3 h at 60° C. Light finishing and post exposure was done in a Cyrel® Lightfinisher 1002 LF for 3 min with UV-A and UV-C radiation simultaneously.  
      After the processing was completed Sample 2 showed a matte surface different from Sample 1 which had a glossy surface. The surface roughness was measured according to Example 1. The following roughness data were measured: for Sample 1 Rmax=3.00 μm, Rz=2.55 μm, and Ra=0.43 μm; for Sample 2 Rmax=6.10 μm, Rz=4.93 μm, and Ra=0.78 μm, for Sample 3 Rmax=1.93 μm, Rz=2.33 μm, and Ra=0.35 μm. All plates were similar in hardness, between 68 and 70 Shore A.  
      These printing plates were tested in separate print runs. The first set of plates was printed on PE-film with a solvent based ink (Type ZPEA 2 HOKO RA from Siegwerk Druckfarben, Siegburg). The second set was printed on paper with a water based ink (Type TP RW from Hartmann).  
      Inventive and conventional printing plates were printed on a Lemo Masterflex® flexographic printing machine at a speed of 120 m/min. The printing impression on film was adjusted to 120 μm and for printing on paper to 180 μm. The anilox rolls were Type Praxair with 320 L/cm at 60° with a cell volume of 4 cm 3 /m 2 . The anilox impression was set to 60 μm. The substructure of the plate was a GFK sleeve (no. 40) with mounting tape Cyrel® MONT 2038: The trials were printed on coated paper (SAPPI; 5302 ALGRO-FINESS/70 g/m 2 ) and on PE-film (Type PE 55 μm; white opaque coloured; pre-treated with a 1350 W adjustment). The ambient temperature and humidity conditions were measured as 25° C. and 43-52% relative humidity during the print run.  
      The print results on PE-film are listed in Table 1, and the print results on paper are listed in Table 2. The test prints were evaluated by using common techniques familiar to those skilled in the art, i.e. measurement of optical densites and dot gain on a Gretag D 19C reflection densitometer (GratagMacbeth, Little Windsor N.Y.). The evaluation is based on a screen ruling of 54 L/cm. Overall best results were achieved for Sample 2. Sample 2 preformed with lowest dot gain and highest solid densities on both PE-film and paper. Sample 3 gave poor solid densities on paper. Sample 2 was best in ink uniformity and ink lay-down in the solid areas compared to Sample 1 and Sample 3.  
                                   TABLE 1                                   tonal value film                       [%]   Sample 1   Sample 2   Sample 3                                                            100   1.60   1.71   1.57                         Tonal value on print [%]                                     90   101   100   101           80   100   99   100           70   98   97   99           60   96   94   97           50   94   92   94           40   91   88   92           30   85   79   84           20   73   67   72           10   60   53   58           5   44   36   39           4   39   33   38           3   34   31   31           2   28   28   24           1   24 . . . 27   21 . . . 24   22 . . . 25                      
 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                 tonal value film 
                   
                   
                   
               
               
                   
                 [%] 
                 Sample 1 
                 Sample 2 
                 Sample 3 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 100 
                 1.24 
                 1.30 
                 1.20 
               
            
           
           
               
               
            
               
                   
                 Tonal value on print [%] 
               
            
           
           
               
               
               
               
               
            
               
                   
                 90 
                 99 
                 99 
                 100 
               
               
                   
                 80 
                 98 
                 95 
                 99 
               
               
                   
                 70 
                 93 
                 89 
                 95 
               
               
                   
                 60 
                 90 
                 84 
                 91 
               
               
                   
                 50 
                 84 
                 77 
                 85 
               
               
                   
                 40 
                 75 
                 71 
                 81 
               
               
                   
                 30 
                 68 
                 61 
                 70 
               
               
                   
                 20 
                 52 
                 47 
                 56 
               
               
                   
                 10 
                 37 
                 32 
                 39 
               
               
                   
                 5 
                 24 
                 21 
                 26 
               
               
                   
                 4 
                 22 
                 18 
                 24 
               
               
                   
                 3 
                 20 
                 15 
                 23 
               
               
                   
                 2 
                 17 
                 13 
                 19 
               
               
                   
                 1 
                 15 
                 11 
                 16 
               
               
                   
                   
               
            
           
         
       
     
     Example 9  
      Two photosensitive elements based on Cyrel® 67HiQ were prepared and processed as described in Example 4. In one element (Sample 1), the standard release layer was replaced by a matted layer comprising 17.25% of wieght of Syloid® ED5 as described in Example 4, 76.5% of weight of the Macromelt® 6900, and 6.25% of weight of a blue dye (Orasol Blue GL from Ciba Specialty Chemicals Inc., CH-4002 Basel, Switzerland), the total coating weight was 32 mg/dm 2 , the silica coating weight was 5.5 mg/dm 2 . In the second element (Sample 2), the standard release layer was replaced by a matted layer comprising Syloid® ED5 and Macromelt® 6900 as described in Example 4 and having a matting agent/polymeric binder ratio of 17/83.  
      The surface roughnesses Rz of the matted layers were 10.03 (Sample 1) and 9.48 (Sample 2). The surface roughnesses Rz of the flexographic printing plates were 3.99 (Sample 1) and 3.98 (Sample 2).  
      The element with the blue dyed marred layer (Sample 1) had a significant higher exposure latitude than the element with the non-dyed matted layer. That means, even on longer exposure times, the fill-in after 5 min additional exposure time was only 4% loss for Sample 1 versus 2× more (9%) for Sample 2. The dye-free element had a much faster fill-in, especially when exposure time was further increased. The loss of reverse depth after increased exposure to 35 min of exposure time was only 11% for Sample 1 versus 32% compared to Sample 2, which was 3× better then Sample 2. In summary, Sample 1 showed only minor loss of reverse depth at longer times, which is important for printing of fine elements to maintain a high resolution. These results were confirmed by printing test.  
               TABLE 3                          Reverse Depth [μm] and % loss of 0.8 mm line versus exposure [min]                             Sample 1   Sample 2                                         [min]   [μm]   %   % loss   [μm   %   % loss                                                 15   441   100       306   100           20   424   96   4   277   91   9       25   413   94   6   254   83   17       30   424   96   4   233   76   24       35   391   89   11   208   68   32                  
 
     Example 10  
      Three flexographic printing plates were prepared according to the process described in Example 4, 
          a) HiQ-NS-A was a flexo plate based on a Cyrel® 67HiQ plate wherein the standard release layer was replaced by a matted layer comprising Syloid® ED5 and Macromelt® 6900 and having a matting agent/polymeric binder ratio of 17/83 and a silica coating weight of 5.5 mg/dm 2 ;     b) HiQ-NS-B was a flexo plated based on a Cyrel® 67HiQ plate wherein the standard release layer was replaced by a matted layer comprising 17.25% of weight of Syloid® ED5, 76.5% of weight of the Macromelt® 6900, and 6.25of weight of a blue dye (Orasol Blue GL from Ciba Specialty Chemicals Inc., CH-4002 Basel, Switzerland), the total coating weight was 32 mg/dm 2,  the silica coating weight was 5.5 mg/dm 2 ;     c) HiQ-NS-C was a flexo plate based on a Cyrel® 67HiQ plate wherein the standard release layer was replaced by a matted layer comprising Syloid® ED5 and Macromelt® 6900, and having a matting agent/polymeric binder ratio of 30/70 and a silica coating weight of 9.6 mg/dm 2 .        

      These printing plates were compared with a commercially available Cyrel® 67HiQ printing plate comprising a standard release layer (polyester cover sheet with release layer comprising Macromelt® 6900 and an amphoteric interpolymer), (HiQ in Table 4).  
      The surface roughness Rz of the flexographic printing plates was measured as described in Example 1.  
      Also ink spreading and ink wettability performance of these flexographic printing plates were determined by evaluation with test inks of different surface tension. A set of test inks from Arcotec GmbH, Rotweg 24, D-71297 Moensheim/Germany was used. The lower number of surface tension represents the test ink which wetted the plate surface; the higher number of surface tension represents the test ink which showed de-wetting on plate surface. The higher the surface tensions the better the wetting of the flexographic printing plate with polar printing inks.  
                                   TABLE 4                                           Plate Surface   Surface Tension           Plate Type   Release Layer   Rz [μm]   [mN/m]                          HiQ   Standard   1.06   32-35           HiQ-NS-A   ED5; 17/83   3.98   35-38           HiQ-NS-B   NS-A + Dye   3.99   35-38           HiQ-NS-C   ED5; 30/70   4.63   44-48                      
 
     Example 11  
      Macromelt® 6900 (3.90 weight %) was dissolved in a solvent blend of i-propanol (47.00 weight %) and toluene (47 weight %). Then Aerosil® 200 (Degussa A G, Weissfrauenstr. 9, D-602287 Frankfurt/Main, Germany) (0.90%) was added and dispersed for 30 min in a disperser as described in Example 1 to achieve a good dispersion, then Syloid® ED5 (1.20 weight %) was added and dispersed for additional 5 min. A dispersion of Aerosil® 200 and Syloid® ED5 was obtained and coated on a Mylar®) 100 A polyester film and treated as described in Example 1. This cover element was laminated to a Cyrel® 67HiQ printing plate instead of the standard cover element under the conditions described in Example 1. After storage for 16 h at 100° C., UV exposure, wash-out and drying as described in Example 1, the final printing plate was evaluated with the previously described SEM/EDX techniques. A silica surface coverage of 15.9% was determined.  
      By the addition of Aerosil® 200 the coating viscosity of the coating dispersion could be increased. This allowed to coat a much lower dry coating weight than without Aerosil® addition. The dry coating weight of this example could be reduced by reducing the solids from 8% to 6% without jeopardizing the coatability. The coatability was maintained and very good coating uniformity was observed. No drying structures of the coating even with longer drying times were seen. The coating weight could be reduced from 32 mg/dm 2  to 24 mg/ dm 2 . The flexographic printing plate resulting from this cover element (sample plate) gave equivalent physical properties compared to a Cyrel® 67HiQ printing plate with a matted layer without Aerosil® 200 (Sample 2 of Example 9, here: reference plate). The surface roughness of the sample plate was found to be slightly lower (Rz=3.25 μm), but still close and equivalent to the reference plate (Rz=3.98 μm).