US 3099209 A
Description (OCR text may contain errors)
United States Patent 3,099,209 PROCESS OF TREATING RESIDUAL POSITIVE SIL- VER I-IALIDE IMAGES WITH ORGANIC SULFUR T0 RENDER SAID IMAGES OLEOPHILIC Rudolph E. Damschroder and Edward C. Yackel, Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Feb. 26, 1960, Ser. No. 11,158 7 Claims. (Cl. 101-1492) This invention relates to a lithographic printing process and more particularly to a photographic method for the preparation of lithographic printing plates.
In lithography a greasy ink is employed and the printing plate comprises a support coated with a colloid carrier which when wet is hydrophilic and carries a line or dot image which is oleophilic.
The oleophilic properties of the image and hydrophilic properties of the surrounding support surface are relative in nature to some extent. When using an image bearing plate for printing, both ink and water are applied and the colloid coated on the support is sufiiciently hydrophilic to accept water but not the greasy ink, while the image accepts the ink but not the water. Hence, to obtain good prints it is necessary that the difference in hydrophilic and oleophilic properties of the image and the back ground surface is sufficiently great that when water and ink are applied, the image will accept suflicient ink without the background accepting any ink at all. Hence, lithographic printing plates are supports the nature of whose surface is such that when wetted with water will not accept greasy ink.
The practice in making a photolithographic printing plate is to produce an oleophilic image on the hydrophilic support which can be put into an inking machine whereby large numbers of copies can usually be made very rapidly from the one plate.
' Where only a few hundreds of copies are required, or even up to about a thousand copies, it is quite common to use a paper type printing plate, but where many thousands of copies are required it is necessary to use a more substantial type of printing plate and the ones commonly used are made of aluminum sheet or zinc sheet. From these metal sheets it is a common practice in the art to take as many as twenty thousand copies. Aluminum sheet employed in the art is provided with an inert hydrophilic surface. This is often done by treating the aluminum to produce thereon an aluminum compound which is hydrophilic for instance, aluminum oxide as in the case of anodised aluminum, aluminum silicate and chromati zed aluminum.
When making such printing plates, usually an ordinary negative is first made and then this is used for making a positive oleophilic image on the printing plate. A large number of processes have been used and suggested for doing this.
A printing process employing greasy ink on metal plates was suggested many years ago by K. W. C. Webb in which an image formed of silver halide or silver and mercury was brought into contact with a smooth metal surface such as a polished copper plate in the presence of certain deposition agents. The plates used in that suggested process, for instance the polished copper plate, were not as such suitable as lithographic printing plates and it was necessary to etch the bare copper parts after the image had been formed. One object of the present invention is to provide a process which entirely avoids etching after formation of the image by making the image areas very oleophilic compared with the background areas.
In view of the wide variety of commercial uses of 3,099,209 Patented July 30, 1 963 "ice photolithographic copying, it is very desirable to have a process which is both simple and efiective.
'An object of the present invention is to provide a process which satisfies these requirements in a very surprisingly simple way by merely forming a negative silver image in a silver halide emulsion layer and treating the residual positive silver halide image in the presence of the silver negative image with solutions of certain organic sulfur compounds which react with the silver halide positive image in the emulsion layer to render it oleophilic whereas the hydrophilic property of the silver negative image in the emulsion layer is not appreciably affected. This treatment of the silver'halide positive image areas of the emulsion with the solutions of the sulfur compounds can very easily be done by swabbing the emulsion layer with the solution, bathing the emulsion layer in the solution for a short time or otherwise applying the solution to the emulsion layer in any convenient manner to render the residual silver halide oleophilic and the emulsion surface thus ink-receptive in that region.
The organic sulfur compounds which can be used as described alone or in combination are the following thiols Lmercaptobenzothiazole The above thiol compounds are further characterized by forming with the silver halide, an oleophilic silver compound more insoluble in water than the silver halide used, thus the thiols will not wash out of the emulsion layer if a washing step is used in the process or during the normal treatment of the oleophilic image with water and greasy ink. v
Adducts and unsaturated derivatives, e.g. cyanamid, chalcone or disulfide derivatives of the above thiol compounds which hydrolyze in alkaline solution to yield the above thiol compounds and no other photographically active species act as donors, in situ, of the above thiols. These thiol donors can be present either directly in the emulsion layer or in a layer contiguous to it. The thiol donors can also be present in the emulsion together with an alkaline developer composition so that when the element is exposed and then subjected to moisture or heat, or both, in the presence of the alkali, the developer composition develops the latent silver negative image to a visible image while the thiol donor simultaneously splits to release a thiol of our invention which reacts with the residual silver halide positive image to render it oleophilic and capable ofacting as alithographic plate.
It is interesting to note that the compound S-nitroindazole can be used in a similar manner to obtain an elecphilic image in the region of a silver halide positive image. Many organic sulfur compounds are inactive or do not react at the surface of emulsion layers in the region of the silver halide positive image in the presence of a silver negative image and may react only in the region of the silver image or render the entire emulsion surface more or less oleophilic, e.g. the following are representative of a large number which have been found to be inactive in our invention in that they did not render silver halide oleophilic:
a,o-Dimercapto adipic acid 2,5 -dimercapto 1,3 ,4-thiadiazole B-Mercapto ethyl amine HCl Di ethylaminoethanethiol HCl B-Carboxymethylmercaptobenzalacetophenone The following organic sulfur compounds were found to be unacceptable in that while they imparted some degree of oleophilictivity to the emulsion surface, they failed to differentiate between the silver negative image and the silver halide positive image:
Thiosalicycl-ic acid l-phenyl-S-mercapto tetrazole Z-mercapto-6-nitrobenzothiazole N-Methyl-2-rnercapto benzamide Biphenyl-4,4'-dithiol t-Dodecylmercaptan benzamide Accordingly, our invention contemplates the preparation and use of a lithographic printing plate prepared by treating a developed but unfixed silver halide emulsion layer, such as a gelatin-silver halide emulsion layer containing a silver negative image and residual silver halide positive image, with the mentioned compounds alone or in combination so as to render the surface of the emulsion layer oleophilic only in the region of the silver halide positive image with the result that upon subsequent acidification of the emulsion layer and inking, lithographic prints can be made in the usual manner.
The fiollowing example will serve to illustrate the process of our invention:
A photographic paper having a contact speed gelatinsil'ver chloride emulsion was exposed to a graphic subject (line or halftone subject), then developed for 30 seconds at 70-75 F. in a conventional Elon-hydroquinone photographic paper developer solution after which it was removed from the developing bath and squeegeed. The surface was then treated for 10 to 20 seconds with a percent solution of p-toluenethiol dissolved in 5 percent sodium hydroxide solution. The surface thus treated was rinsed, acidified with 5 percent acetic acid solution and placed on a Multilith printing press. The undeveloped areas of the emulsion were found to be oleophilic and printed a positive image, the p-toluenethiol presumably being reactive with the undeveloped silver halide to form an oleophiliic surface.
When alkaline solutions of the other effective thiol compounds mentioned were used in a similar manner for treating a developed silver halide emulsion layer, the surface became oleophilic in the region of the undeveloped silver halide and printed readily in a lithographic printing press.
In the process of the invention illustrated above, after development of the emulsion in the conventional manner, the emulsion may then be acidified with dilute acid solution to stop development and after a brief wash of about 2 minutes in running water, treated with the alkaline solution of the thiol compound. The subsequent acidification step may be carried out in a convenient manner, for example, by application of dilute acetic acid solution or other acid fountain solution on the printing press.
The hydrophilic organic colloid of the silver halide emulsion layers used in the invention is not especially critical and may include gelatin, colloidal albumen, zein, partially hydrolyzed cellulose esters, polyvinyl alcohol, partially hydrolyzed polyvinyl ester, po lyacrylamide, and other hydrophilic naturally occurring and synthetic organic colloids. Similarly, the silver halide emulsions may be selected from well-known emulsions containing silver chloride, silver bromide and silver iodide or mixtures thereof as well as containing optical and chemical sensitizing agents, fog-stabilizing compounds, emulsion hardeners, pl asticizing compounds and wetting agents.
. l. A lithographic printing process which comprises reacting the residual silver halide remaining after formation of a developed silver image in the emulsion layer with an alkaline solution of a thiol selected from the group consisting of p-toluenethiol, potassium n-butyl xanthate, Z-mercapto-S-phenyl-1,3,4-oxadiazole, Z-mercaptobenzothiazole, Z-mercaptobenzimidazole and 2-ethyl-hexanethiol until the surface of the moist emulsion layer in only the region of the undeveloped silver halide has become receptive to greasy inks, acidifying the emu sion layer, inking the moist emulsion layer and printing a lithographic print therefrom.
2. The process of claim 1 wherein the emulsion layer is a gelatin-silver halide emulsion layer.
3. The process of claim 1 wherein the thiol used is ptoluenethiol.
4. The process of claim 1 wherein the thiol used is potassium n-butyl xanthate.
5. The process of claim 1 wherein the thiol used is Z-mercapto-S-phenyi-1,3,4-oxadiazole.
6. The process of claim 1 wherein the thiol used is Z-mercaptobenzothiazole.
7. The process of claim 1 wherein the thiol used is Z-mercaptobenzimidazole.
References Cited in the file of this patent UNITED STATES PATENTS 2,453,346 Russell Nov. 9, 1948 2,843,491 Allen et a1. July 15, 1958 FOREIGN PATENTS 1,058,844 Germany June 4, 1959 1,064,343 Germany Aug. 27, 1959 1,064,807 Germany Sept. 3, 1959 758,759 Great Britain Oct. 10, 1956