COVER FILM FOR PRINTING PLATE

Abstract: Provided is a cover film for a printing plate which cover film comprises a light-transmissible smooth base film and a light-transmissible resinous coating layer formed on one side of the base film, the resinous coating layer having 50 to 10,000 convex projections per square centimeter, with a half-value width, b, of each such convex projection being 5 to 300 times the height, a, of the vertex of the convex projec­ tion which height, a, is in the range of 1 to 10 µ. This cover film is superior in contact with an image-bearing film and little badly affects the image reproducibility of the printing plate. It is useful for a printing plate of the type which is exposed to light while being covered with a cover film, for example, a dry planographic printing plate having an ink-­ repellent layer formed by a silicone rubber layer.

Description: COVER FILM FOR PRINTING PLATE BACKGROUND OF THE INVENTION The present invention relates to a cover film for a printing plate and particularly to a cover film for a printing plate superior in contact with an imagebearing film and little badly affecting the image reproducibility. Heretofore, in order to improve the contact between a printing plate and an image-bearing film, there has been proposed and known a technique in which a mat layer is provided on the printing plate which is brought into contact with the image-bearing film, such as the methods disclosed in U.S, Patent Nos.4,216,289 and 4,288,526. In all of these methods, a mat layer is provided directly on a photosensitive layer. On the other hand, in the case of a printing plate which requires a cover film, it is necessary to provide a mat layer on the cover film in order to improvetthe contact with an imagebearing film. In this connection, reference is made below to the effect of a mat layer in the presence of a cover film and also in the absence of such film. (1) As to the contact with an image-bearing film, the mat layer is effective almost equally in both cases. (2) As to the influence of scattering of light in the mat layer upon the image reproducibility of the printing plate, there is a great difference between both cases. More particularly, in the absence of a cover film, scattering of light in the mat layer will little badly affect the image reproducibility because the photosensitive layer is adjacent to the mat layer. On the other hand, in the presence of a cover film, scattering of light in the mat layer will cause a bad influence on the image reproducibility because the scattered light passes through the cover film before reaching the photosensitive layer. Thus, the effect of a mat layer differs between the case where a cover film is present and the case where it is omitted. Therefore, in case the technique of providing a mat layer directly on a photosensitive layer is applied as it is to a printing plate which requires a cover film, it is difficult to attain both a superior contact with an image-bearing film and a superior image reproducibility at a time. Reference is here made to Japanese Patent Laid Open Publication No.55343/80 which shows a technique for improving the contact with an image-bearing film by providing a mat layer on the surface of a cover film. This technique involves the following two methods according to a broad classification: 1) Method in which the film surface is matted by processing. 2) Method in which the film surface is matted by the provision of a coating layer which contains inorganic particles. In the above method 1), all of the embodiments described in the Japanese Patent Laid Open Publication in question are difficult to attain both a superior contact and a superior image reproducibility at a time. Since this method requires processing of the film itself, it involves associated problems, for example, an increase in scale of the manufacturing equipment is required, and thus the above method 1) is not suitable for a small volume production. In the above method 2), surface unevenness is formed by the inorganic particles. In this case, since the organic particles are generally spherical, scattering of light by the surface unevenness is increased, which is liable to deteriorate the image reproducibility. Therefore, with the provision of a coating layer containing inorganic particles on the cover film surface, it is difficult to attain both a superior contact with an image-bearing film and a superior image reproducibility at a time SUMMARY OF THE INVENTION It is an object of the present invention to provide a cover film for a printing plate superior in contact with an image-bearing film and little badly affecting the image reproducibility. It is another object of the present invention to provide a printing plate superior in contact with an image-bearing film and also superior in image reproducibility. It is a further object of the present invention to provide a method of producing a cover film for a printing plate superior in contact with an image-bearing film and little badly affecting the image reproducibility. Other and further objects and effects of the present invention will becomes apparent from the following detailed description According to the present invention, there is provided a cover film for a printing plate, which cover film comprises a light-transmissible smooth base film and a light-transmissible-resinous coating layer formed on one side of the base film, the resinous coating layer having 50 to 10,000 convex projections per square centimeter, with a half-value width,b, of each such convex projection being 5 to 300 times the height,a, of the vertex of the convex projection which height,a, is in the range of 1 to 10 p The cover film of the present invention is superior in contact with an image-bearing film and diminishes scattering of light induced by surface unevenness. Consequently, in a printing plate of the type which is exposed to light without removing a cover film, for example, in a dry planographic printing plate having an ink-repellent portion formed by a silicone rubber layer, it is possible to attain both a superior contact with an image-bearing film and a superior image reproducibility at a time. BRIEF DESCRIPTION OF THE DRAWINGS Fig.l is a sectional view of the cover film of the present invention; and Figs.2 and 3 are plan views (a) and sectional views (b) of cells of gravure rolls used in the production of the cover film of the present invention DESCRIPTION OF PREFERRED EMBODIMENTS In a printing plate which must be covered with a cover film until just before development because the plate surface is easily scratched or for some other reason, for example, in a dry planographic printing plate having an ink-repellent portion formed by a silicone rubber layer, such as those disclosed in U.S. Patent Nos. 3,894,873, 4,342,820, 4,347,303 and 4,358,522, the cover film exists between an image-bearing film and a photosensitive layer at the time of exposure. As a result, the light scattered between the image-bearing film and the cover film is diffused while passing through the cover film, and for this reason the reproduction of very small dots of the image-bearing film tends to be difficult as compared with a printing plate not requiring a cover film. Consideration will now be given about the scattering of light occurring between an image-bearing film and a cover film. The smoother the cover film surface, the smaller the amount of light scattered and the easier the reproduction of very small dots. On the other hand, as to the vacuum contact between an image bearing film and a cover film, the higher the degree of matting of the cover film surface, the better the vacuum contact. Thus, the degree of matting of the cover film surface should be decided while considering both image reproducibility and vacuum contact. But, it has been difficult for the conventional techniques to attain both a satisfactory image reproducibility and a satisfactory vacuum contact at a time. It has now been found that a superior reproducibility is attainable while retaining a good vacuum contact by using a cover film which has a specific shape of convex projections formed on its surface. More particularly, it has now been found that the contact with an image-bearing film is improved and a bad influence upon image reproducibility is minimized by using a cover film which comprises a light-transmissible smooth base film and a light-transmissible resinous coating layer formed on one side of the base film, the resinous coating layer having 50 to 10,000, preferably 100 to 2,000, convex projections per square centimeter, with a half-value width,b, of each such convex projection being 5 to 300, preferably 10 to 150, times the height,a, of the vertex of the convex projection which height,a, is in the range of 1 to 10 , preferably 2 to 5 It is preferable that the area of the coating layer portion having a thickness not larger than one fifth of the height,a, of the vertex of the convex projection be in the range of 10 to 90% of the surface area of the base film, and that an inter-vertex distance,c, between adjacent convex projections be in the range of 100 to 1,500 p. Values outside the ranges defined above would result in impairment of either the image reproducibility or the contact with an image-bearing film. The "light-transmissible" as referred to herein means a light transmissibility not less than 50% for an actinic light (wavelength: 300-500nm), for example. The "smooth" as referred to herein means a smoothness having a value of Ra not larger than 0.4 wherein Ra represents the center line average roughness as defined in paragraph 5 of page 4 of JIS B-0601. The "vertex of the convex projection" as referred to herein indicates the point of maximum thickness of the coating layer. The l'height,a, of the vertex of the convex projection" as referred to herein indicates a height obtained by substracting the minimum thickness from the maximum thickness of the coating layer 2 on the base film 1, as shown in Fig.l. The "half-value width,b, of the convex projection" as referred to herein indicates the width of the convex projection at a half point of the height,a, of the vertex of the convex projection, as shown in Fig.1. The base film used in the present invention is not specially limited provided it is lighttransmittable as previously defined. Examples of such base film include plastic films such as those formed of polyethylene, polypropylene, polyester, polyamide, polyvinylidene chloride, polystyrene, polyvinyl chloride, polybutadiene and polyvinyl alcohol, as well as composite films consisting of two or more of the plastic films just exemplified. It is also preferable that these films be subjected in advance to a surface treatment such as a corona discharge treatment or the like and/or a metal deposition treatment in a range not impairing the light transmissibility of the film. As the material of the base film, it is desirable from the standpoint of its strength and versatility that a polyolefin such as polyethylene or polypropylene, a polyester and/or a polyamide be contained in a propor tin not less than 90%. Particularly preferred is a polyester film. The base film has a thickness preferably in the range of 1 to 20 p Its thickness larger than 20u is likely to deteriorate the image reproducibility, and its thickness smaller than lji causes problems associated with the strength, etc. of the base film itself. The printing plate used in the present invention is not specially limited provided it is of the type which forms image upon irradiation while being covered with a cover film. A particularly outstanding effect of the present invention is developed in the case of using as the printing plate a dry planographic printing plate comprising a base substrate, a photosensitive layer overlying the base substrate and a silicone rubber layer overlying the photosensitive layer, especially a dry planographic printing plate of such structure wherein the photosensitive layer contains a photo-radicalpolymerizable monomer or oligomer. In the latter dry planographic printing plate, it is preferable that the oxygen permeability of the base film be not more than 2,000 cc/m2 24hr atm/sheet. As examples of the printing plate described above, mention may be made of the dry planographic printing plates disclosed in U.S. Patent Nos. 3,894,873, 4,342,820, 4,347,303 and 4,358,522. The material of the resinous coating layer used in the present invention is not specially limited provided it is light-transmissible as previously defined and can retain its shape at normal temperatures (-20 C to 60 C). Eamples of such resin include polyolefin resins, chlorinated polyolefin resins, polyester resins, butyral resins, vinylidene chloride resins, vinyl chloride resins, alkyd resins, acrylic resins, epoxy resins, poly amide resins, polyurethane resins, polyvinyl alcohol resins, phenolic resins, and natural resins. These resins may be used alone or in combination of two or more. Further, since the resinous coating layer is formed by means of coating, it is desirable to use a thermoplastic resin as the resin of the coating layer from the standpoint of stability of the coating solution and easiness of its application. Preferred examples of such thermoplastic resin are polyester resins and butyral resins The method of preparing the resinous coating layer is not specially limited, but preferably a solution of a resin or a precursor thereof having a viscosity in the range of 0.5 to 30 poise is applied onto the surface of the base film by means of a gravure roll having an area of cell portion in the range of 10 to 80% of the surface area of the gravure roll, and then the coating is hardened to form a coating layer having convex projections. The shape of the cells of the gravure roll is not specially limited, for example, the cells may be in the form of a geometrical pattern such as a lattice, oblique lines, pyramids or the like, or even a pattern not having a specific regularity, provided the area of cell portion of the roll is in the range of 10 to 80%, preferably 10 to 60%, of the roll surface area. If the area of cell portion exceeds 80% of the roll surface areat the concaves and convexes formed on the film will assume a shape with overlapped projections, so it is impossible to obtain a film having the desired concaves and cOnvexes. If the area of cell portion is below 10% of the roll surface area, it is difficult to obtain a convex projection of a relatively large height, and a film having the desired concaves and convexes is not obtained. It is necessary that the cells of the gravure roll be disposed substantially uniformly on the roll surface. The cell as referred to herein is as shown, for example, in Figs.2 and 3 in each of which (a) is a plan view and (b) is a sectional view, both illustrated schematically. The area of cell portion as referred to herein indicates the area of the opening portion of the cell, namely, i x i. The ratio of the area of cell portion to the surface area of the gravure roll is equal to (i x i) . j(i + j) x (i + j)3 in the case where cells of such a shape as shown in Fig.2 or Fig.3 are existent uniformly on the roll surface. The coating solution used herein should have a viscosity in the range of 0.5 to 30 poise, preferably 1 to 15 poise, when applied to-the base film. If its viscosity is below 0.5 poise, the concaves and convexes formed just after coating will immediately be levelled and not be recognized after drying. If its viscosity exceeds 30 poise, application per se of the coating solution will be difficult, so it is impossible to obtain a film having stable characteristics. Further, the coating solution used herein should form the foregoing coating layer after drying or curing. The solvent of the coating solution is not specially limited. For example, there may be used petroleum solvents; aromatic hydrocarbons, e.g. toluene; ketones, e.g. methyl ethyl ketone; alcohols, e.g. etharLol ethers, e.g. tetrahydrofuran; and inorganic polar solJen'cs r e.g. water. These solvents may be used alone or in combination of two or more. For imparting thixotropy to the coating solution and for improving the slipperiness of the uneven surface layer, there may be added a small amount of an inorganic substance such as silicon dioxide, titanium dioxide or calcium carbonate, or an organic substance incompatible with the coating resin. Further, in order to improve the dispersion stability of such inorganic or organic substance and to impart an antistatic property, there may be added a surface active agent. The coating solution thus prepared is applied onto the film surface by means of the foregoing gravure and then cured to obtain a light-transmissible film having concaves and convexes on the surface thereof. The curing of the coating solution is performed by a conventional method such as drying, crosslinking, or the like. The contour of such a convex projection as shown in Fig.l may be measured by the following methods: 1) Method which utilizes a photomicrograph of a sectional portion of the cover film. 2) Method which utilizes a tracer meter (JIS B-0651). In the case of adopting the above method 2), it is necessary that the load applied to the stylus tip of the tracer meter be kept low The value of the oxygen permeability as referred to herein is a value determined by a measuring method using gas chromatography at 20 C, 0% RH and under a pressure of 1.5 kg/cm2. The following working and comparative examples are given to further illustrate the present invention. Example 1 A coating solution of the following composition was applied to a polypropylene film (8 thick) by means of a gravure roll having such a fine pattern as shown in Fig.2 (pyramid type, i/j = 2, 55 lines/inch, the area of cell portion being 44% of the roll surface area), and then dried. The viscosity of the coating solution was 2.5 poises and its drying was performed with hot-air at 700C for 40 seconds. (a) Chlorinated polypropylene ("Superchlon 803H" a product of Sanyo-Kokusaku Pulp Co.) 20.0 wt.% (b) Silicon dioxide (average particle size: 3P) 0.6 wt.% (c) Toluene 55.4 wt.% (d) Methyl ethyl ketone 24.Q wt.% The film thus obtained proved to have the following characteristic values: 1) Height of convex projection, a = 8¯10 2) Half-value width of convex projection, b = 180¯ 220 3) b/a = 18¯28 4) Number of convex projections: : 470 pcs./cmê¯ 500 pcs./cmê 5) Distance between adjacent convex projections, c = 460 p 6) Area of the coating layer portion having a thickness not larger than one fifth of the height,a, of the vertex of convex projection: 40##60% of the surface area of the base film. Using this film, there was produced a dry planographic printing plate of a structure as will be discribed hereinafter. A solution having the following composition (a 10% solution in ethyl cellosolve) was applied onto an aluminum base substrate and dried in hot air at 800c to form a photopolymerizable layer having a thickness of (a) Polyurethane resin "Impranil ELN" (a product of Bayer) 56 parts by weight (b) Adduct of 4 moles glycidyl methacrylate and 1 mole xylylenediamine 40 parts by weight (c) Michler's ketone 4 parts by weight (d) Crystal Violet 0.4 part by weight Then, a 10% diluted solution in n-hexane of a silicone having the following composition was applied onto the photopolymerizable layer and dried in hot air at 500C to form a silicone rubber layer having a thickness of (a) Polydimethylsiloxane {molecular weight: : about 40,000) 100 parts by weight (b) Methyltriacetoxysilane 5 parts by weight (c) Dibutyltin acetate 0.2 part by weight Onto the surface of the thus-formed silicone rubber layer was laminated the previously-formed film so that the smooth side of the film (the side not having the concaves and convexes) was in contact with the silicone rubber layer. Thus, using a vacuum printing frame, a positive film was brought into close contact with the printing plate having the above-described structure. One minute was required for attaining a close contact. As to the image reproducibility of this printing plate, 2 ¯98% of 200 //inch dots was reproduced. Comparative Example 1 A coating solution of the following composition was applied to a polypropylene film (8 thick) by means of a gravure roll having such a fine pattern as shown in Fig.3 (lattice type, i/j = 19, 180 lines/inch, the area of cell portion being 90% of the roll surface area) and then dried. The viscosity of the coating. solution was 0.5 poise and its drying was performed with hot-air at 700C for 40 seconds. (a) Chlorinated polypropylene ('tSuperchlon 803H", a product of Sanyo-Kokusaku Pulp Co.) 15.0 wt.% (b) Silicon dioxide (average particle size: 5 ) 1.2 wt.% (c) Toluene 58.3 wt.% (d) Methyl ethyl ketone 25.5 wt.% The film thus obtained proved to have the following characteristic values: 1) Height of convex projection, a = 5 ¯6 ji 2) Half-value width of convex projection, b = 10#'20 3) b/a = 2#4 4) Number of convex projections: 1 x 105 pcs./cmê 5) Distance between adjacent convex projections, c = 0#vS0 6) There was no coating layer portion having a thickness not larger than one fifth of the height,a, of the vertex of convex projection. Using this film, there was produced a dry planographic printing plate in the same way as in Example 1. Then, using a vacuum printing frame, a positive film was brought into close contact with the printing plate. One minute was required for attaining a close contact. As to the image reproducibility of this printing plate, however, only 5¯95% of 200 //inch dots was reproduced. Example 2 and Comparative Example 2 Coating solutions having the compositions shown in Table 1 were each applied to a polyester film (6 thick) by using the similar gravure roll to that used in Example 1 (pyramid type, i/j = 1, 55 lines/inch, the area of cell portion being 25% of the roll surface area) , and then dried with hot-air at 700c for 40 seconds. Table 1 EMI17.1 <tb> <SEP> Concentration <SEP> of <SEP> Viscosity <SEP> of <tb> <SEP> saturated <SEP> poly- <SEP> Methyl <SEP> ethyl <SEP> coating <SEP> solu <tb> <SEP> ester <SEP> resin <SEP> * <SEP> Toluene <SEP> ketone <SEP> tion <tb> <SEP> (wt.%) <SEP> (wt.%) <SEP> <SEP> (wt.%) <SEP> (poise) <tb> No.1 <SEP> 40 <SEP> 48 <SEP> 12 <SEP> 5.0 <tb> No.2 <SEP> 25 <SEP> 60 <SEP> 15 <SEP> 0.7 <tb> No.3 <SEP> 15 <SEP> 68 <SEP> 17 <SEP> 0.2 <tb> * "Vylon 200" (a product of Toyobo Co.) Table 2 below shows characteristic values of the films thus obtained. Table 2 EMI17.2 <tb> <SEP> Height <SEP> of <SEP> convex <SEP> Half-value <SEP> Area <SEP> of <SEP> portion <tb> <SEP> projection <SEP> ( ) <SEP> width <SEP> ( ) <SEP> <SEP> b/a <SEP> with <SEP> thickness <tb> <SEP> a <SEP> b <SEP> not <SEP> larger <SEP> than <tb> <SEP> 1/5 <SEP> of <SEP> a <SEP> (%) <SEP> <tb> No.1 <SEP> 10¯12 <SEP> 150¯180 <SEP> 12¯18 <SEP> 60¯80 <SEP> <tb> No.2 <SEP> 4¯5 <SEP> 170¯200 <SEP> 34¯50 <SEP> 50¯70 <tb> No.3 <SEP> 0.4¯ <SEP> 0.5 <SEP> 200¯220 <SEP> 400¯550 <SEP> 10¯20 <SEP> <tb> The number of convex projections was 470 pcs./cm2 500 pcs./cm2, and the distance between adjacent convex projections, c = 460 . Using these films, there were produced dry planographic printing plate mn the same way as in Eample 1. Table 3 shows performances of the printing plates obtained. Table 3 EMI18.1 <tb> <SEP> Time <SEP> required <SEP> for <SEP> close <SEP> Image <tb> Film <SEP> contact <SEP> withpositive <SEP> film <SEP> reproducibility <tb> No.1 <SEP> 0.2 <SEP> 5¯95 <tb> No.2 <SEP> 1.0 <SEP> <SEP> 2¯98 <tb> No.3 <SEP> 6.0 <SEP> 2 <SEP> ¯ <SEP> 98 <tb> From Table 3 it is seen that the printing plate using film No.1 is unsatisfactory in point of image reproducibility and the printing plate using film No.3 unsatisfactory in point of contact with positive film.

Claims: WHAT IS CLAIMED IS: 1. A cover film for a printing plate, said cover film comprising a light-transmissible smooth base film and a lighttransmissible resinous coating layer formed on one side of said base film, said resinous coatning layer having 50 to 10,000 convex projections per square centimeter, with a half-value width,b,.of each said convex projection being in the range of 5 to 300 times the height,a, of the vertex of said convex projection which height,a, is in the range of 1 to 10 microns. 2. The cover film for a printing plate as defined in claim 1, wherein said printing plate is a dry planograhic printing plate comprising a base substrate, a photosensitive layer overlying said base substrate and a silicone rubber layer overlying said photosensitive layer. 3. The cover film for a printing plate as defined in claim 2, wherein said photosensitive layer contains a photoradical-polymerizable monomer or oligomer. 4. The cover film for a printing plate as defined in claim 1, wherein said base film has an oxygen permeability not more than 2,000 cc/m2 24hr-atm/sheet. 5. The cover film for a printing plate as defined in claim 1, wherein said base film has a thickness in the range of 1 to 20 microns. 6. The cover film for a printing plate as defined in claim 1, wherein the area of the coating layer portion having a thickness not larger than one fifth of the height,a, of the vertex of said convex projection is in the range of 10 to 90 % of the surface area of said base film. 7. The cover film for a printing plate as defined in claim 1, wherein an inter-vertex distance,c, between adjacent said convex projections is in the range of 100 to 1,500 microns. 8. The cover film for a printing plate as defined in claim 1, wherein the half-value width,b, of each said convex pr!ojection is in the range of 10 to 150 times the height,a, of the vertex of said convex projection. 9. The cover film for a printing plate as defined in claim 1, wherein the height,a, of the vertex of each said convex projection is in the range of 2 to 5 microns. 10. The cover film for a printing plate as defined in claim 1, wherein said resinous coating layer having said convex projections has been formed by applying a solution of a resin or a precursor thereof onto the surface of said base film by means of a gravure roll and then curing the thusapplied solution, said solution having a viscosity in the range of 0.5 to 30 poises,and said gravure roll having an area of cell portion in the range of 10 to 80 % of the surface area of the gravure roll. 11. A dry planographic printing plate comprising a base substrate, a photosensitive layer overlying said base substrate, a silicone rubber layer overlying said photosensitive layer and a cover film overlying said silicone rubber layer, said cover film comprising a light-transmissible smooth base film and a light-transmissible resinous coating layer formed on one side of said base film, said resinous coating layer having 50 to 10,000 convex projections per square centimeter, with a half-value width,b, of each said convex projection being in the range of 5 to 300 times the height,a, of the vertex of said convex projection which height,a, is in the range of 1 to 10 microns, said cover film being laminated onto said silicone rubber layer so that the other smooth side of said base film contacts the upper surface of said silicone rubber layer.

Inventors: FUJIKAWA, JUNICHI, KASIO, SHIGETORA, NAGASE, KIMIKAZU, SHIMOKAWA, YOICHI
Applicants: TORAY INDUSTRIES, INC.
Application year: 1983
Main classification: G03F7
IPC codes: G03F7