US3072496A

US3072496A - Process for preparing photographic negatives

Info

Publication number: US3072496A
Application number: US9444A
Authority: US
Inventors: Halpern Bernard Robert
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1960-02-18
Filing date: 1960-02-18
Publication date: 1963-01-08
Anticipated expiration: 1980-01-08

Description

.which represent the printing areas of the plate.

United States Patent Q r 3,072,496 PROCESS FOR PREPARING PHOTOGRAPHIC NEGATIVES Bernard Robert Halpern, Swarthmore, Pa., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Feb. 18, 1960, Ser. No. 9,444 7 Claims. (Cl. 117-5.5)

This invention relate to the graphic arts field and more particularly it relates to photomechanical processes in the graphic arts field. Still more particularly it relates to improvements in processes for producing photographic printing negatives for use in making photomechanical printing plates.

It is old in the art to make duplicate printing plates for any method of printing by photographing a reproduction proof pulled from a type form, or by photographing a prepared form directly so as to obtain a negative that can be used to make a photomechanical plate. These negatives, commonly referred to as line negatives, are placed in contact with a plate coated with a light-sensitive emulsion. Exposure of the plate to a light source through the negative insolubilizes the exposed areas to provide a resist for etching or for chemical differentiation, that after appropriate processing, results in a printing plate that is suitable for a printing process such as letterpress, gravure, or offset lithography. The above methods require exacting preparation of the printing forms for pulling a satisfactory proof, and precise process cameras for transmitting all of the character detail to thenegative. Since the bulk of all text composition is currently cast on type composing machines, it is necessary to convert this composition into a suitable negative or positive before the text can be used on a photomechanical plate. This transparency can then be used to prepare any number of duplicate plates as may be needed.

A simplified and more direct method for producing a negative from an existing printing form for use in photomechanical platemaking, is to pull an impression directly on a film coated with a photographic gelatino-silver halide emulsion. When this is done, using any conventional hydrophobic printing ink, a resist is formed which prevents the action of processing solutions in those areas In the case of gelatino-silver halide emulsion layers, photographic developers will develop silver in the non-printing areas, assuming,.of course that the operation is done under normal room lighting, which, in effect, flash exposes the silver halide layer. By dissolving, wiping away, or otherwise (i.e., surface tension reduction with wetting agents to permit penetration) removing the inked impression and then fixing out the film with a conventional photographic fixer, the silver halide which was protected by the ink during development is removed to give clear areas, and the film becomes, in effect, an accurate photographic negative of the printing plate. The inked impressions may be made in any conventional proof press, in the manner known to those skilled in the art. No camera or dark room equipment is necessary with this latter method and the films are normally handled under ordinary room lighting conditions. In the case of lightsensitive gelatino-silve-r halide emulsion coated film, the

latter must be exposed to light so that black silver will be developed except where the printed impression covered 3,072,496 Patented Jan. 8, 1963 by the ink prevents the developer from acting to reduce the silver halide to silver. The ink, after development, is removed by a suitable solvent and the film is then immersed in a fixing composition which dissolves the silver halide, which has not been acted on by the developer and forms clear areas which gives a negative of the printed impression. It will be evident that because light is not involved in the actual making of the image, inks or other pigmented compounds need not necessarily be used. Any suitable composition which will resist the action of the processing compositions may be used. It is also evident that a positive can be produced by making a print resist and then fixing, removing the ink resist and then developing. In this case the printed impression will be developed to black silver and the unprinted areas will be clear thus forming a positive of the printed material. It is with this method of inking directly on a gelatino-silver halide layer that the present invention is concerned.

Although the above method ofinking directly on a gelatino-silver halide layer eliminates the necessity of using cumbersome proces cameras and processing the exposed film under photographic dark-room conditions, the system has some serious drawbacks which become particularly evident when used in conjunction with the newer printing plates now commercially available. In place of the usual metal plates sensitized with bichromated gelatin, the new plates are composed of photopolymerizable layers such as those described and claimed in Plambeck US. Patent 2,791,504 and Martin et al., US. Serial No. 596,766, filed July 7, 1956, now US. Patent 2,927,022, issued March 1, 1960. Printing plates composed of the said photopolymerizable layers are easy to make and more faithfully reproduce the subject matter to be printed from the negative to which the plate is photographically exposed than the plates of the prior art. The outstanding difficulties encountered in making negatives by inking directly'on the gelatino-silver halide layers are two-fold. Ink squeezeout is one of the difli culties which results in loss of fidelity and grayed impression areas during the printing of the proof and also the tendency of characters to be slightly bolder with mottled centers than in the original. Wherever large quantities of paper are used, as is the case in commercial printing plants, the atmosphere nearly always contains substantial amounts of lint and dust. This causes discontinuous inked images because the particles in the air will mix with the ink on the form rollers of the press and also settle on surfaces and prevent satisfactory ink transfer. When films containing such images are processed,

a poor negative results. An exposed printing plate from high-quality negatives from relief printing forms. A further object is to provide such a process which overcomes the deficiencies of prior art procedures due to lint and dust particles. A still further object is to provide such a process which eliminates the undesirable effects of ink squeezeout. Still further advantages will appear hereinafter.

These and other objects are realized by the instant invention which consists in a process for preparing highquality negatives from a relief printing form, e.g., line, halftone or combined line and halftone plate or type form, which comprises lightly coating the relief surface of said form with a hydrophobic developer resist, pulling a first pro-of on the emulsion surface of a flexible, transparent film coated with a light-sensitive photographic emulsion layer, cleaning said relief surface of said form to remove any remaining developer resist, again lightly coating said relief surface with said developer resist and pulling a second proof on said film in exact register with said first proof, developing said emulsion layer in areas not coated with said developer resist and subsequently removing said developer resist and the photographic emulsion layer thereunder from said film.

High-quality negatives can be produced by the above process to take full advantage of the new polymeric printing plates referred to above. The invention resides in the novel process of inking which will be hereinafter more fully described. It comprises first lightly inking the plate or type form to be duplicated by the usual proof press and then pulling a proof on a photographic emulsion coated film by making an impression on the film thin enough so that the tendency for ink squeezeout is minimized. The printing plate is then wiped or washed free of its remaining ink coating. The plate is then lightly re-inked and a second impression is made on the film in exact register with the first impression. The intermediate step of washing the plate and second inking serve to shift the location of defects caused by dust and lint specks on the printing form so that in two or more inkings a continuous film of ink duplicating the printing surface is produced as a developer resist on the gelatino-silver halide layer of the film. By this method large reverses and solids as well as text and half-tones are converted with good quality. Any type of gelatino-silver halide emulsion may be used, including high-contrast negative materials and direct positive films. The latter films have been found to be particularly suitable as they are less sensitive to prolonged handling in the proof press room. Chemical blackeners, such as sodium sulfide, can be used instead of developers thus eliminating the need for the fogging exposure to light. Normal room lighting i satisfactory although high intensity fluorescent lights and mercury lamps should not be in use during the proofing operation, particularly in the case of high speed and direct positive emulsions because of the formation of photolytic sliver which is independent of the developing process.

The type of ink used is generally only a matter of personal choice, since any hydrophobic ink which will transfer satisfactorily from the printing form to the gelatinosilver halide coated film to form a developer resist will be suitable. Many suitable lithographic inks are commercially available. A suitable ink may also be prepared by melting together parts of petroleum jelly, /2 part of amorphous silica and A part of carbon black. Inks need not necessarily be used as a processing solution resist and can incorporate photographic desensitizers which will act directly on the emulsion to prevent development of the image areas. Resists can also be prepared without inclusion of a pigment or desensitizer such as by melting together one part of beeswax and one part petroleum jelly.

The processing solutions, namely alkaline photographic developers and acid photographic fixers may be of the conventional type and are so chosen as to be the most elficacious for the particular gelatino-silver halide emulsion used.

The invention will now be illustrated in and by the following examples:

4 Example I A printing form of good quality was locked in a proof press capable of printing exact dot on dot register. After the normal make-ready and test proofs, a photographic direct positive clear film was secured to the proof press cylinder. A thin ink application is then printed on the emulsion side of the film. The printing form was then wiped clean with a solvent for the ink applied sparingly to a lint-free rag and the form then re-inked. A second thin ink application was printed in exact register over the first impression. The film element was then placed, emulsion side up, in a tray containing a developer having the following composition:

Water liter 1 Sodium sulfite (anhy.) ..grams 120.0 Paraformaldehyde do 30.0 Potassium metabisulfite do 150.0 Boric acid, crystals do 30.0 Hydroquinone do 90.0 Potassium bromide do 6.0 Water to make 2 liters.

The tray was rocked for about 45 seconds to uniformly develop the film and provide a maximum density of at least 3.5. The film was then transferred to a tray containing an acid short stop bath composed of 5% acetic acid and agitated for about five seconds. After this treatment, a cotton wad which was dampened with 50-50 ethyl alcohol and hypo fixing bath solution was used to wipe off the ink. Care was taken to remove all of the ink by turning the cotton and rewiping. The film was then transferred to a tray containing a fixing bath having the following composition:

Water (15 C.) ml 600.0 Sodium thiosulfate (anhy.) ..grams 15.0 Sodium sulfite (anhy.) do 18.0 Potassium alum do 20.0 Acetic acid (glacial) ml 12.0

Water to make 1.0 liter.

The film was agitated in the above bath for about one minute or until the area which had been covered by the ink was clear. The film was then washed and dried by conventional methods. The negative obtained was an excellent reproduction of the original printing form. The negative was placed over a photopolymerizable printing element prepared as described in Example IV of Martin et al., U.S. Serial No. 596,766, filed July 9, 1956, now U.S. Patent 2,927,022; and Example 3 of Burg, U.S. Serial No. 750,868, filed July 25, 1958. The element and negative were placed in a vacuum frame and exposed to 1.75 watts of actinic radiation per square inch for 12 seconds from an l800-watt, high-pressure, mercury-arc lamp. After exposure, the unexposed areas of the element were removed by spray washing for 8 minutes using an 0.08 N aqueous solution of sodium hydroxide. A sharp, polymerized relief printing plate was obtained, the relief areas corresponding to the clear areas and relief or printing areas of the original plate. The resulting plate, when used in a conventional printing press, gave reproductions of good quality comparable to those obtained from the original printing form. There were no pinhole or other defects discernible in the text, half-tone or solid areas of the image as is so often the case where a single inking process is used. Naturally, any gross surface defects such as pits, batters, or gouges in the printing surface will be reproduced. During the making of the above negative, a control negative was made under the same conditions except that only a single normally inked impression was made on the proof press. All other conditions were the same. A photopolymerization printing plate exposed to the resulting processed negative printed larger and more Example II Example I was repeated using in place of the direct positive film, a high contrast gelatino-silver chlorobromide film. After inking in the manner described in Example I the film element was placed emulsion side up, in a tray containing a developer having the following composition:

Water ml 500 Methyl-p-aminophenol grams 3.0 Sodium sulfite (anhy.) do 45.0 Hydroquinone do 12.0 Sodium carbonate (anhy.) do 67.5 Potassium bromide do 1.9

Water to make 2.0 liters.

The tray was rocked for about 20 seconds to uniformly develop the film and provide a maximum density of at least 3.5. The processing of the film was then continued as described in Example I including the removal of the ink, fixing, washing, drying and exposure steps. The polymerized relief printing plate had characteristics similar to those of a plate exposed through the negative formed using the direct positive clear film.

An important aspect of the invention is that any film having a coating of a gelatino-silver halide emulsion may be used in the process. For example, high-contrast films which are suitable for graphic arts work may be used. As indicated above, where high speed and/or very sensitive films are used, subdued room lighting is desirable to reduce the possibility of producing photolytic silver which would be formed in the layer in the areas which would be eventually covered by the ink when an impression is pulled. During the fixing operation after the ink is removed, such spontaneously formed silver would remain and could not be removed by the fixing solution. This would, of course, prevent printing a clear image unmarred by defects of the negative onto the photopolymerizable printing plate. The emulsion layer to receive the ink impression need not necessarily have a transparent film support. A translucent support may be used and may have auxiliary layers in addition to the emulsion layer to provide such translucency. The emulsion layer itself may contain delustering agents which provides a matted surface which can aid in preventing ink squeezeout. It also aids in allowing trapped air to escape to provide rapid contact between the negative and the plate when printing down in vacuum printing frames.

Any relief, intaglio, or planographic printing surface may be used as the printing form for taking the impression. For example, slug composition (Linotype and Intertype), individual type (Monotype and foundry type), photoengravings and duplicate plate (electrotypes) and lithographic printing plates may be used. The printing can be made from printing plates composed of zinc, copper, aluminum, magnesium,,steel, nickel, chromium, brass, type metal, plastic, etc.

It will be appreciated that many variations can be taken advantage of using the negatives of the novel process of the invention. For example, either right-reading or wrong-reading printing plates may be made by either exposing the photopolymer plate through the emulsion side or through the non-emulsion side of the processed negative film.

When it is desired to make gravure plates, for example, the inked film may be processed by first fixing and then, after washing, and removing the ink, developing the emulsion which was covered by the ink. In this case the area covered by the ink will become the image and a r 6 transparent positive will result. Upon printing a suitable plate from this film one can obtain a gravure plate wherein the printing areas will be depressed and non-printing areas will be raised.

The film support for the emulsion layers used in the novel process may be any suitable transparent plastic. For example, the cellulosic supports, e.g., cellulose acetate, cellulose triacetate, cellulose mixed esters, etc. may be used. Polymerized vinyl compounds, e.g., copolymerized vinyl acetate and vinyl chloride, polystyrene, and polymerized acrylates may also be mentioned. An important aspect which governs the selection of the proper support is dimensional stability because of necessity of exact register both in the inking operation and the printing operation. The film formed from the polyesterification product of a dicarboxylic acid and a dihydric alcohol made according to the teachings of Alles, US. Patent 2,779,684 and the patents referred to in the specification of that patent. Other suitable supports are the polyethylene terephthalate/isophthalates of British Patent 766,290 and Canadian Patent 562,672 and those obtainable by condensing terephthalic acid and dimethyl terephthalate with propylene glycol, diethylene glycol, tetramethylene glycol or cyclohexane 1,4-dimethanol (hexahydro-p-xylene alcohol). The above polyester films are particularly suitable because of their dimensional stability.

It will be evident that the double inking process can also be applied to non-photographic, but chemicallysensitized films which, by being brought into contact with a suitable reagent, form image transparencie which may be used to expose the above-described printing plates.

The main advantage of the novel process of the invention is that it overcomes the loss of fine detail and the mottling due to incomplete inking which is the inherent deficiency of the prior art processes.

What i claimed is: V

1. A process for preparing high-quality photographic negatives from a relief printing form which comprises lightly coating the relief surface of said form with a thin layer of a hydrophobic developer resist, pulling a first proof on the emulsion surface of a flexible, transparent film coated with a light-sensitive photographic emulsion layer, cleaning said relief surface of said form to remove any remaining developer resist, again lightly coating said relief surface with a thin layer of said developer resist and pulling a second proof on said film in exact register with said first proof, developing said emulsion layer in areas not coated with said developer resist and subsequently removing said developer resist and the photographic emulsion layer thereunder from said film.

2. Process according to claim 1 wherein said developer resist is a hydrophobic ink.

3. Process according to claim 1 wherein said photographic film is a high-contrast gelatino-silver chlorobromide film.

4. Process according to claim 1 wherein said light- 'sensitive photographic emulsion is a direct positive type emulsion.

5. Process according to claim 4 wherein said developer resist is a hydrophobic ink.

6. Process according to claim 4 wherein said photographic film is a high-contrast gelatino-silver chlorobromide film.

7. A process for preparing high-quality photographic right-reading negatives from a relief printing form which comprises lightly coating the relief surface of said form with a thin layer of a hydrophobic developer resist, pulling a first proof on the emulsion surface of a flexible, transparent film coated with a light-sensitive photographic emulsion layer, cleaning said relief surface of said form to remove any remaining developer resist, again lightly coating said relief surface with a thin layer of said developer resist and pulling a second proof on said film in exact register with said first proof, removing said emulsion from said film in areas not covered by said developer resist, removing only said developer resist from said film, and developing said photographic emulsion on said film previously coated by said developer resist.

References Cited in the file of this patent UNITED STATES PATENTS 117,376 Brandon July 25, 1871 8 Eader May 12, 1936 Boni Feb. 7, 1956 M011 et al Feb. 14, 1956 OTHER REFERENCES

Claims

1. A PROCESS FOR PREPARING HIGH-QUALITY PHOTOGRAPHIC NEGATIVES FROM A RELIEF PRINTING FORM WHICH COMPRISES LIGHTLY COATING THE RELIEF SURFACE OF SAID FORM WITH A THIN LAYER OF A HYDROPHOBIC DEVELOPER RESIST, PULLING A FIRST PROOF ON THE EMULSION SURRFACE OF A FLEXIBLE, TRANSPARENT FILM COATED WITH A LIGHT-SENSITIVE PHOTOGRAPHIC EMULSION LAYER, CLEANING SAID RELIEF SURFACE OF SAID FORM TO REMOVE ANY REMAINING DEVELOPER RESIST, AGAIN LIGHTLY COATING SAID RELIEF SURFACE WITH A THIN LAYER OF SAID DEVELOPER RESIST AND PULLING A SECOND PROOF ON SAID FILM IN EXACT REGISTER WITH SAID FIRST PROOF, DEVELOPING SAID EMULSION LAYER IN AREAS NOT COATED WITH SAID DEVELOPER RESIST AND SUBSEQUENTLY REMOVING SAID DEVELOPER RESIST AND THE PHOTOGRAPHIC EMULSION LAYER THEREUNDER FROM SAID FILM.