Abstract:
This invention relates to negative photosensitized sheet constructions which, upon exposure to an actinic radiation source through a screened image, can accurately reproduce said image. The construction is useful as a color proofing film which can be employed to predict the image quality from a lithographic printing process.

Description:
BACKGROUND OF THE INVENTION 
     In the graphic arts, it is desirable to produce a color proof to assist a printer in correcting a set of photomasks which will be used in exposing printing plates. The proof should reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplicate of the desired half tone or line image, and should neither gain nor lose color. Visual examination of a color proof should reveal the following characteristics: 
     1. Any defects on the photomask. 
     2. The best color rendition to be expected from press printing of the material. 
     3. The correct gradation of all colors and whether grays are neutral. 
     4. The need, if any, for subduing any of the colors and/or giving directions for altering the film photomask before making the printing plates. 
     Color proofing sheets for multi-colored printing have heretofore been made by using a printing press proof which requires taking all the steps necessary for actual multicolor printing. Such a conventional method of color proofing has been costly and time consuming. Alternate color proofing methods have therefore been developed to simulate the quality of press proofs. There are two known types of photographic color proofing methods, namely, the surprint type and the overlay type. 
     In the overlay type of color proofing, an independent transparent plastic support is used for producing an image of each color separation film. A number of such supports carrying colored images are then superimposed upon each other and placed on a white sheet to produce a color proof. The overlay type of color proofing method has the disadvantage that the superimposed plastic supports tend to darken the color proofing sheet, and, as a result, the impression of the color proofing sheet thus prepared becomes vastly different from copies actually obtained by a conventional printing press proof. Its primary advantage is that it is quick and can serve as a progressive proof by combining any two or more colors in register. 
     In the surprint type of color proofing method, a color proofing sheet is prepared by successively producing images of different colors from different color separation films onto a single receptor sheet. This is done by utilizing a single opaque support and by applying toners, photosensitive solutions or coatings of photosensitive materials of corresponding colors on the opaque support in succession. An example of this approach is described in U.S. Pat. No. 3,671,236. An advantage of the surprint type of color proof is that the color saturation is not influenced by superimposed plastic supports. This method more closely resembles the actual printing and eliminates the color distortion inherent in the overlay system. 
     Various processes for producing copies of an image embodying photopolymerization and thermal transfer techniques are known as shown in U.S. Pat. Nos. 3,060,023; 3,060,024; 3,060,025 3,481,736; and 3,607,264. In these processes, a photopolymerizable layer coated on a suitable support is imagewise exposed to a photographic transparency. The surface of the exposed layer is then pressed into contact with the image receptive surface of a separate element and at least one of the elements is heated to a temperature above the transfer temperature of the unexposed portions of the layer. The two elements are then separated, whereby the thermally transferrable, unexposed, image areas of the composite transfer to the image receptive element. If the element is not precolored, the tacky unexposed image may now be selectively colored with a desired toner. The colored matter preferentially adheres to the clear unpolymerized material. U.S. Pat. No. 3,574,049 provides a transfer process for printing a design on a final support which comprises (a) printing a design onto a temporary support, (b) superimposing the temporary support and the final support, (c) applying heat and/or pressure to the superimposed structure formed in (b), and (d) separating the temporary support from the final support which retains the printed design. The affinity of the design for the temporary support is lower than its affinity for the final support. 
     In U.S. Pat. No. 3,721,557 a method of transferring colored images is claimed which provides a stripping layer coated between the photosensitive element and the support. When the photosensitive layer is exposed to actinic light and developed, the more soluble portions are selectively removed to produce a visible image. The image-carrying support is pressed against a suitable adhesive coated receptor member and, subsequently, the carrier support sheet is stripped to accomplish the transfer of the image. A fresh layer of adhesive is applied to the receptor for each subsequent transfer. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved method for forming a colored image which comprises: 
     (A) providing a photosensitive element which comprises, in order: 
     (i) a substrate having a release surface; and 
     (ii) a photosensitive layer on said release surface, which photosensitive layer comprises a light sensitive, negative working, polymeric diazonium compound; a resinous binder composition, which composition contains at least 20% of a resin having the general formula 
     
         --A--B--C-- 
    
      wherein a plurality of each of components A, B, and C occur in ordered or random sequence in the resin and wherein A is present in said resin at about 5% to about 20% by weight and comprises groups of the formula ##STR1##  B is present in said resin at about 4% to about 30% by weight and comprises groups of the formula ##STR2##  and C is present in said resin at about 50% to about 91% by weight; the weight amount of components A, B and C being based on the total weight of the resin; and comprises acetal groups consisting of groups of the formulae ##STR3##  R is lower alkyl or hydrogen, and wherein said group I. is present in component C from about 75% to about 85%; group II. is present in component C from about 3% to about 5%; and group III. is present in component C from about 10% to about 22% the percent basis for the acetal groups I, II and III being on a mol basis of acetal groups I, II, and III of component C; and at least one colorant; and 
     (iii) an adhesive layer in direct contact with said photosensitive layer, which adhesive layer comprises a thermoplastic polymer and which adhesive layer is nontacky at room temperature, thermally activated and can be transferred at temperatures between 60° C. and 90° C.; and 
     (B.) either 
     (i) laminating said element with heat and pressure via said adhesive layer to a developer resistant receiver sheet; and removing said substrate by the application of peeling forces; and imagewise exposing said photosensitive layer to actinic radiation; or 
     (ii) imagewise exposing said photosensitive layer to actinic radiation; and laminating said element with heat and pressure via said adhesive layer to a developer resistant receiver sheet; and removing said substrate by the application of peeling forces; or 
     (iii) laminating said element with heat and pressure via said adhesive layer to a developer resistant receiver sheet; and imagewise exposing said photosensitive layer to actinic radiation; and removing said substrate by the application of peeling forces; and 
     (C.) removing the nonexposed areas of said photosensitive layer with a suitable liquid developer, which removing is conducted at a temperature at which said adhesive layer is substantially nontacky; and preferably 
     (D.) repeating steps A through C at least once whereby another photosensitive element having at least one different colorant is laminated onto said receptor sheet over the non-removed portions of the previously laminated photosensitive layer or layers. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In carrying out the method of the invention, one employs a photographic element which broadly comprises a substrate having a release surface, a colored photosensitive layer on the release surface and an adhesive layer on the photosensitive layer. Optional additional layers containing antihalation materials, adhesion promoters or release agents may also be used. 
     In the preferred embodiment, the substrate is composed of a dimensionally and chemically stable base material which does not significantly change its size, shape or chemical properties as the result of the heating coating or other treatments which it must undergo. One preferred material is polyethylene terephthalate. In the usual case it has a thickness of from about 1 to about 10 mils, a more preferred thickness is from about 2-5 mils and most preferably from about 2-3 mils. Suitable films include Hostaphan 3000, available from American Hoechst Corporation, Mylar D, available from DuPont and Melinex grades 0; 052; 442; 516 and S, available from ICI. The surface of the substrate may be smooth or may be provided with a matte texture by various methods known in the art. 
     Matte films include Melinex 377 and 470 from ICI. These materials have the unique property of giving the final image a desired matte finish without any extra steps. One can control the gloss of the final image by properly selecting the matte finish of the temporary support. This effect works because the top layer of the final image is originally in contact with this matte surface. This does not occur with a separate release layer between the temporary support and photosensitive layer. An additional advantage of coating on a matte surface is that subsequent transferred layers generally adhere better to a rough surface than to a smooth surface. 
     A similar matte finish of the final image can be obtained by embossing the shiny, top surface of the image with a matte material, such as described above. This is done by laminating together the final image and matte material under pressure and temperature. The matte material is then generally removed after lamination. The advantage of this method is that the finish of the final proof can be varied. Furthermore, the matting material can be used repeatedly. 
     A third method for producing a matte finish uses a heat transferable layer, such as Butvar 90, available from Monsanto, coated onto a film with a rough surface, such as Melinex 329, available from ICI. The transferable layer is laminated to the final image under pressure and temperature. Then the film with the rough surface is peeled off. The rough surface of the transferred layer imparts a matte finish to the final image. The advantage is that all layers appear matte and that the extra transferred layer protects the image. U.S. Pat. Nos. 4,294,909 and 4,376,159, also suggests various methods for making a matte surface. 
     In either case, the substrate must have a release surface, that is, it must be capable of releasably holding the photosensitive layer thereto. This may be accomplished either by the substrate surface being inherently releasable, being rendered releasable by a suitable treatment or being provided with a release layer over the substrate surface. Such a release layer may comprise polyvinyl alcohol. 
     Releasably bonded to the release surface is the photosensitive layer. The photosensitive layer broadly comprises a photosensitizers, colorants, binding resins, and other optional ingredients such as plasticizers, acid stabilizers, surfactants, antistatic compositions, uv absorbers and residual coating solvents. 
     The preferred photosensitizer is a light sensitive, polymeric diazonium salt. The most preferred photosensitizer is the polycondensation product of 3-methoxy-4-diazo-diphenyl amine sulfate and 4,4&#39;-bis-methoxy methyl-diphenyl ether, precipitated as mesitylene sufonate as taught in U.S. Pat. No. 3,849,392. Other suitable photosensitizers are taught in U.S. Pat. No. 4,436,804. The diazo compounds of choice are preferably soluble in organic solvents. 
     Suitable binding resins have the general formula 
     
         --A--B--C 
    
     wherein a plurality of each of components A, B, and C occur in ordered or random sequence in the resin and wherein A is present in said resin at about 5% to about 20% by weight and comprises groups of the formula ##STR4## B is present in said resin at about 4% to about 30% by weight and comprises groups of the formula ##STR5## and C is present in said resin at about 50% to about 91% by weight; the weight amount of components A, B and C being based on the total weight of the resin and comprises acetal groups consisting of groups of the formulae ##STR6## wherein R is lower alkyl or hydrogen, and wherein said group I. is present in component C from about 75% to about 85%; group II. is present in component C from about 3% to about 5%; and group III. is present in component C from about 10% to about 22% the percent basis for the acetal groups I, II and III being on a mol basis of component C. An important resin selection criterion is that it must be a good film former. 
     These resins are more fully described in U.S. patent application Ser. No. 762,089 filed Aug. 2, 1985 which is incorporated herein by reference. 
     The colorants useful for the present invention include various classes of dyes and pigments. In the most preferred embodiment, pigments having an average particle size diameter of about 1 micrometer or less are used. 
     Optional plasticizers which may be incorporated into the photosensitive layer include those of the phthalate and phosphate types. Preferred plasticizers include dibutyl phthalate and dimethyl phthalate. Polymeric plasticizers include acrylic resins such as Carboset 525 available from BF Goodrich. Acid stabilizers include phosphoric acid and p-toluene sulfonic acid. Developing aids include polymers with acid groups such as 
     Carboset XL27 available from BF Goodrich, Scripset 540 available from Monsanto and Elvacite 2028 available from DuPont. 
     Suitable acid stabilizers include phosphoric, citric, benzoic, m-nitro benzoic, p-toluene sulfonic acid, and mixtures thereof. Preferably, the acid stabilizer is phosphoric acid. 
     These ingredients may be blended with such compatible solvents as gamma butyrolactone, diacetone alcohol, propylene glycol monomethyl ether, ethanol, methyl cellosolve and methyl ethyl ketone, coated on the release surface, and dried. In the preferred embodiment, the photosensitive layer has a coating weight between approximately 0.1 and 5.0 g/m 2 . The most preferred weight is from about 0.4 to 2.0 g/m 2 . 
     In the preferred embodiment, the photosensitizer is present in the photosensitive layer in an amount of from about 5 to about 40 percent by weight; or more preferably from about 10 to about 35 percent by weight. 
     In the preferred embodiment, the colorant is present in the photosensitive layer in an amount of from about 10 to about 40 percent by weight; or more preferably from about 13 to about 34 percent by weight. 
     In the preferred embodiment, the binding resin is present in the photosensitive layer in an amount of from about 10 to about 80 parts by weight; or more preferably from about 32 to about 75 parts by weight. 
     In the preferred embodiment, the plasticizer, when one is used, is present in the photosensitive layer in an amount of up to about 20 parts by weight; or more preferably up to about 15 parts by weight and most preferably from about 12 to about 15 parts by weight. 
     In the preferred embodiment, the acid stabilizer, when one is used, is present in the photosensitive layer in an amount of up to about 10 parts by weight. 
     Typical formulations for the photosensitive layer include: 
     
         ______________________________________I.              Yellow  Magenta  Cyan  Black______________________________________propylene glycol monomethyl           39.81   39.66    43.94 39.54ethermethyl ethyl ketone           33.78   33.00    23.07 31.15gamma-butryolactone           6.01    6.35     8.52  8.41diacetone alcohol           16.89   16.50    17.79 15.58polyvinyl acetal/           0.74    2.08     1.06  1.03alcohol/acetateresin in example #1Scripset 540    0.68    0.66     --    --Elvacite 2028   --      --       2.11  --Carboset XL27   --      --       1.05  1.56Above diazo from U.S. Pat.           1.35    0.99     1.23  1.87No. 3,849,392phthalo blue pigment           --      0.02     1.14  --yellow pigment  0.74    0.05     --    --magenta pigment --      0.69     0.09  --black pigment   --      --       --    0.86______________________________________ 
    
     The adhesive layer is substantially nontacky at ordinary temperatures. The adhesive layer comprises a thermoplastic polymer or mixture of thermoplastic polymers. The polymers can be selected from thermoplastic polymers which have a glass transition temperature (Tg) preferably between about 20° and 60° C., more preferably between about 25° and 55° C. The thermoplastic polymer should form a flexible film. They should preferably be transparent and colorless. 
     Examples of suitable adhesive materials nonexclusively include polyvinyl acetates such as Mowilith DM-6, 20, DM-22, 25, and 30, available from Hoechst AG; polyvinyl acetals such as Butvar B79, available from Monsanto; polyvinyl acrylates such as Elvacite 2044 available from DuPont; polyvinyl chloride copolymers such as UCAR VYNC available from Union Carbide. Novolaks and epoxy resins can also be used. These are usually dispersed in water, or dissolved in methyl isobutyl ketone or n-butyl acetate or other solvent compositions for coating on the photosensitive layer. It is then dried to a coating weight of from about 5 to about 30 g/m 2 , more preferably from about 10 to about 20 g/m 2 . The layer may optionally contain a uv absorber such as Uvinul D-50 from G.A.F. It may also contain a polymeric plasticizer such as Resoflex R-296, available from Cambridge Industries or Carboset 525 available from BF Goodrich. It may also contain antistats, such as Gafac and Gafstat from G.A.F. It may also contain other resins, such as Nitrocellulose RS 1/2, available from Hercules. The adhesive layer should not betacky to the touch, during storage or during development of the photosensitive element. The layer should have a softening point in the range of from about 60° C. to about 180° C., preferably 60° C. to 120°  C., more preferably 60° C. to 100° C. In the preferred embodiment, the polyvinyl acetate is present in the adhesive layer in an amount of greater than about 50 percent by weight. The plasticizer may be present in an amount of up to about 30 percent by weight, the uv absorber up to about 20 percent by weight, and other resins up to about 50 percent by weight. 
     Typical adhesive formulations include. 
     
         ______________________________________I.Water            50.00Mowilith DM-22   50.00II.n-butyl acetate  78.00Resoflex R-296   1.00Mowilith 30      21.00III.n-butyl acetate  68.70Uvinul D-50      1.30Mowilith 20      30.00IV.n-butyl acetate  50.00UCAR VYNC in     50.00isopropylacetate______________________________________ 
    
     In operation, the photosensitive element is laminated to a receptor sheet via the adhesive layer. The receiver sheet should be resistant to any adverse effects which may be caused by the developer of choice. For example, the receiver sheet should be water resistant if aqueous developers are used. Plastic or plastic coated receiver sheets are useful for this purpose. 
     Useful receiver sheets include Melinex 329; 339; 994 and 3020 from ICI. Other white and nonwhite receiver sheets may also be used. Rough textured and/or adhesion promoted surfaces are preferred for the receiver, which must be able to withstand the laminating and development processes. 
     Lamination may be conducted by putting the receiver sheet in contact with the adhesive side of the colored composite and then introducing the two materials into the nip of a pair of heated laminating rollers under suitable pressure. Suitable laminating temperatures usually range from about 60° C. to about 90° C., preferably about 75° C. to about 85° C. After lamination, the substrate is peeled away, usually merely employing manual peeling forces. The adhesive and photosensitive layers thus remain on the receiver sheet. 
     The photosensitive layer is imagewise exposed by means well known in the art either before or after lamination. Such exposure may be conducted by exposure to a uv light source through a photomask under vacuum frame conditions. Exposure may be performed with actinic light through a conventional negative flat. Exposures after lamination and peel apart are preferred for emulsion-toemulsion contact. Mercury vapor discharge lamps are preferred over metal halide lamps. Filters may be used to reduce light scattering in the material. 
     After lamination, peel apart, and exposure, the photosensitive layer is developed by dissolving the nonexposed area in a suitable developer and dried. A suitable developer useful for developing a lithographic printing plate made with the resin of the present invention includes an aqueous solution containing one or more of the following groups: 
     (a) a sodium, potassium or lithium salt of octyl, decyl, dodecyl, or tetradecyl monosulfate; 
     (b) a sodium, lithium, potassium or ammonium metasilicate salt; 
     (c) a lithium, potassium, sodium or ammonium borate salt; 
     (d) an aliphatic dicarboxylic acid, or sodium, potassium or ammonium salt thereof having from 2 to 6 carbon atoms; and 
     (e) mono, di-, or tri-sodium or potassium phosphate. 
     Other suitable developers include water, benzoic acid or sodium, lithium and potassium benzoates and hydroxy substituted analogs thereof as well as those developers described in U.S. Pat. No. 4,436,807. The adhesive layer is not substantially removed by this development. Specific examples of suitable developers non-exclusively include: 
     
         ______________________________________I.water              95.0sodium decyl sulphate              3.0disodium phosphate 1.5sodium metasilicate              0.5II.water              89.264monosodium phosphate              0.269trisodium phosphate              2.230sodium tetradecyl sulfate              8.237______________________________________ 
    
     Any developer solution which satisfactorily removes the nonexposed areas of the photosensitive layer after exposure while retaining the image areas may be used. The selection of developer is well within the ability of the skilled artisan. 
     The process can then be repeated whereby another photosensitive element having a different color is laminated to the same receiver sheet over the previously formed image. In the usual case, four colored layers are employed to produce a full color reproduction of a desired image. These are cyan, magenta, yellow and black. 
     The following non-limiting example serves to illustrate the invention. 
     EXAMPLE I 
     The resin is made from a copolymer of polyvinyl alcohol/polyvinyl acetate, Vinol 523. 75.0 g of Vinol 523 which has from about 75% to 80% hydroxyl groups by weight and an average molecular weight of about 70,000, is dissolved in a solution comprising 225.0 g of water and 200.0 g of ethanol for 16 hours at 70° C. after which 10.13 g of hydrochloric acid (37%) is added and the temperature adjusted to 60° C. while mixing vigorously. 37.66 g of propionaldehyde is slowly titrated into the reaction mixture. Simultaneously, 250.0 g of ethanol is likewise titrated into the reaction mixture. The mixture is then neutralized to a pH of 7.0 with a sodium carbonate/sodium hydroxide (50/50) mixture. The product is isolated in granular form by precipitation with water. It is then dried so as to have a moisture residue of not greater than 1.0%. A yield of 107 g or about 96% is obtained. The average molecular weight is about 90,000. 
     Using standard analytical techniques, the product is found to consist of 13.6% acetate, 9.8% hydroxyl, and 76.6% acetal groups. Of the acetal groups, 80% are found to be six-membered cyclic acetal, 4% are five-membered cyclic acetal, and 16% are intermolecular acetals. 
     Four photosensitive solutions of cyan, yellow, magenta, and black are produced according to the photosensitive formulations described above. The pigment is introduced as a dispersion of the above polyvinyl acetal/alcohol/acetate resin and the appropriate pigment in a 1:1 solvent mixture of gamma butryolactone and propylene glycol monomethyl ether. The pigment loading in the dispersion is 4.7%. The resin loading in the dispersion is 5.6%. The solutions are coated and dried separately to the required optical density onto 3 mil Melinex 516 polyester films as temporary support. The surface densities are roughly 0.8 g/m 2  for cyan, 0.9 g/m 2  for yellow, 0.7 g/m 2  for magenta, and 1.6 g/m 2  for black. The adhesive solution, in particular adhesive formulation number II from above, is coated on top of the photosensitive layers and dried to an surface density of 12 g/m 2 . The yellow composite is then laminated at 80° C. with the adhesive side onto either side of a 7 mil Melinex 3020 polyester receiver sheet. The 516 temporary support is peeled away after lamination, leaving the adhesive and photosensitive layers on the receiver sheet. The yellow photosensitive layer is then exposed to actinic light through a photographic flat for the yellow color. The receiver sheet with the exposed yellow layer is then immersed for 15 sec in developer II from above at 27° C. with gentle pad rubbing on the photosensitive side. The nonexposed, yellow areas are thereby washed off and the exposed areas remain during development. The adhesive layer is not effected by the developer. After this treatment, the imaged material is rinsed and then dried. The magenta composite is then laminated as before onto the imaged, yellow side of the receptor sheet. The temporary support is removed as before. The magenta layer is then exposed through the magenta flat. It is then processed as with the yellow. The magenta is followed in a like manner by cyan and then by black to give a four color image which is an accurate representation of the original from which separations are prepared. 
     EXAMPLE II 
     The pigment can be introduced as a dispersion of the appropriate pigment and a binder other than the polyvinyl acetal/alcohol/acetate resin mentioned in Example I, in an appropriate solvent. For example, the following formulation for cyan has the pigments dispersed with Scripset 540 in methyl ethyl ketone. 
     
         ______________________________________methyl cellosolve    48.65methyl ethyl ketone  34.85gamma butryolacetone 10.46Scripset 540         2.71resin Example I      1.04diazo from US 3,849,392                1.21phthalo blue pigment 1.01quinacrid one magenta pigment                0.07______________________________________ 
    
     The material is coated and treated like in Example I.