Negative working diazo color proofing method
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 single 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 a major amount of at least one resin selected from the group consisting of a polyvinyl butyral polymer and a styrene-maleic anhydride copolymer half ester; and at least one colorant; and 
     (iii) an adhesive layer directly adhered to said colored photosensitive layer, which adhesive layer comprises a polyvinyl acetate polymer and which adhesive layer has a softening point in the range of from about 60° C. to about 180° C.; and thereafter 
     (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 non-exposed areas of said photosensitive layer with a suitable liquid developer, which removing is conducted at a temperature at which said adhesive layer is substantially non-tacky; 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 EMBODIMENT 
     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 anti-halation 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 adhesive layer is laminated to the final image under pressure and temperature. Then the film with the rough surface is peeled off. The rough surface imparts a matte finish to the final image. The advantage is that all layers appear matte and that the extra adhesive 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 photosensitizer, a colorant, a binding resin, and other optional ingredients such as plasticizers, acid stabilizers, surfactants, anti-static compositions, uv absorbers and residual coating solvents. 
     The photosensitizer is preferably 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 sulfonate 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 include polyvinyl butyral and styrene maleic anhydride copolymer half esters and mixtures thereof. Such resins include Butvar B72, B76 and B90 and Scripset 540 and 550, respectfully, all available from Monsanto. An important resin selection criterion is that it must be a good film former. 
     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 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. Acid stabilizers include phosphoric acid and p-toluene sulfonic acid. 
     These ingredients may be blended with such compatible solvents as 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 0.5 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 25 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: 
     
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I.                                                                        
                  Cyan     Yellow  Black                                  
______________________________________                                    
methyl cellosolve 27.37    62.16   58.35                                  
methyl ethyl ketone                                                       
                  40.30    33.00   35.65                                  
ethanol           27.20    --      --                                     
phosphoric acid (conc.)                                                   
                  0.40     --      0.50                                   
p-toluene sulfonic acid                                                   
                  --       0.25    --                                     
dibutyl phthalate --               0.75                                   
Butvar 72         1.08     1.62     1.125                                 
Butvar 90         0.60     0.80    1.50                                   
Above diazo From US 3,849,392                                             
                  1.70     0.55    1.00                                   
phthalo blue pigment                                                      
                  1.35     --      --                                     
yellow pigment    --       1.62    --                                     
black pigment     --       --       1.125                                 
optical density   1.2      0.9     1.3                                    
______________________________________                                    
II.                                                                       
              Cyan    Yellow   Magenta Black                              
______________________________________                                    
methyl cellosolve                                                         
              41.00   41.00    46.50   41.00                              
methyl ethyl ketone                                                       
              41.00   40.99    46.48   41.00                              
gamma-butryolactone                                                       
              10.00   10.00    --      10.00                              
dimethyl phthalate                                                        
              0.75    0.75     0.88    0.75                               
dibutyl phthalate                                                         
              0.25    0.25     --      0.25                               
p-toluene sulfonic acid                                                   
              --      --       0.18    0.35                               
Scripset 540  3.33    2.60     3.15    3.71                               
Scripset 550  1.17    --       --      --                                 
hydrolyzed Scripset 540                                                   
              --      --       0.67    --                                 
SMA 2625      --      2.02     --      --                                 
Above diazo from US                                                       
              1.33    1.35     0.70    2.00                               
3,849,392                                                                 
phthalo blue pigment                                                      
              1.17    --       --      --                                 
yellow pigment                                                            
              --      1.04     --      --                                 
magenta pigment                                                           
              --      --       1.44    --                                 
black pigment --      --       --      0.94                               
optical density                                                           
              1.1     0.9      1.2     1.5                                
______________________________________                                    
 Scripset resins are from Monsanto. SMA resins are from Arco.             
 
    
     The adhesive layer comprises polyvinyl acetate and may optionally contain such other desired components as uv absorbers, anti-static compositions and plasticizers. Useful polyvinyl acetates non-exclusively include Mowilith DM-6, 20, DM-22, 25, 30 and mixtures thereof, available from Hoechst AG. 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 plasticizer such as Resoflex R-296, available from Cambridge Industries. 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 be tacky 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            50.00                                                    
Mowilith DM-22   50.00                                                    
II.                                                                       
n-butyl acetate  78.00                                                    
Resoflex R-296   1.00                                                     
Mowilith 25      21.00                                                    
III.                                                                      
n-butyl acetate  68.70                                                    
Uvinul D-50      1.30                                                     
Mowilith 20      30.00                                                    
______________________________________                                    
 
    
     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 non-white 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-to-emulsion 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 non-exposed area in a suitable developer and dried. The adhesive layer is not removed by this development. Suitable developers non-exclusively include: 
     
         ______________________________________                                    
I.                                                                        
water              95.0                                                   
sodium decyl sulphate                                                     
                   3.0                                                    
disodium phosphate 1.5                                                    
sodium metasilicate                                                       
                   0.5                                                    
II.                                                                       
water              89.264                                                 
monosodium phosphate                                                      
                   0.269                                                  
trisodium phosphate                                                       
                   2.230                                                  
sodium tetradecyl sulfate                                                 
                   8.237                                                  
______________________________________                                    
 
    
     Any developer solution which satisfactorily removes the non-exposed 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 
     Four photosensitive solutions of cyan, yellow, magenta, and black are produced according to photosensitive formulations II described above. The pigment is introduced as a dispersion of methyl ethyl ketone, Scripset 540, and the appropriate pigment. 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 1.3 g/m 2  for cyan, 0.9 g/m 2  for yellow, 1.8 g/m 2  for magenta, and 1.2 g/m 2  for black. The adhesive solution, in particular adhesive formulation number II 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 above at 27° C. with gentle pad rubbing on the photosensitive side. The non-exposed, 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.