US3597201A - Positive-working presensitized planographic printing plate - Google Patents

Abstract

POSITIVE-WORKING PRESENSITIZED PRINTING PLATES COMPRISING A SUPPORT, A HARDENED GELATIN PHOTOGRAPHIC DIRECTPOSITIVE COVERED GRAIN EMULSION AND APHOLYHYDROXYBENZENE DEVELOPING AGENT. OPTIONALLY, A HALOGEN-ACCEPTING COMPOUND AND A SULFONATED COMPOUND MAY BE ADDED TO THE EMULSION TO INCREASE SENSITIVITY.

Description

Ilnited States Patent Ofice 3,597,201 Patented Aug. 3, 1971 ABSTRACT OF THE DISCLOSURE Positive-working presensitized printing plates comprising a support, a hardened gelatin photographic directpositive covered grain emulsion and a polyhydroxybenzene developing agent. Optionally, a halogen-accepting compound and a sulfonated compound may be added to the emulsion to increase sensitivity.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to novel photographic elements, their preparation and use. In one of its aspects, this invention relates to novel positive-acting presensitized printing plates. In another of its aspects, the invention relates to using such plates in lithography without etching or washing off the hardened areas.

Description of the prior art The art of lithographic printing depends upon the immiscibility of grease and water and involves the use of a plate, the printing surface of which is differentially inkreceptive when moistened with water. In the past, the use of direct-positive emulsions to obtain positive working plates has resulted in plates exhibiting very low photographic speed. Furthermore, many prior art positive working plates generally have a complex structure involving at least two photographic emulsion layers and require complex processing techniques or unstable processing solutions for development. It is evident, therefore, that the art will be substantially enhanced by a direct positive photographic printing plate which will not be subject to the aforementioned deficiencies. Likewise, the provision of a printing process employing an improved positiveworking printing plate which is free from the aforementioned deficiencies will represent an advance in the art.

SUMMARY OF THE INVENTION According to one embodiment of this invention, there is provided a positive-working presensitized printing plate comprising a support and a hardened gelatin photographic emulsion layer containing a particular type of reversal or direct-positive emulsion, as hereinafter described, in combination with a polyhydroxybenzene developing agent which oxidizes in the development reaction and renders gelatin oleophilic in unexposed areas, i.e., it forms an oleophilic image (ink-receptive image).

According to another embodiment of this invention, there is provided a process employing a positive-working presensitized printing plate comprising a hardened gelatin direct-positive photographic emulsion layer, as described hereinafter, and processing this plate after exposure with a polyhydroxybenzene developing agent which oxidizes in the presence of the gelatin in the silver halide emulsion to render gelatin in the unexposed areas oleophilic, i.e., it forms an oleophilic image (ink-receptive image).

A significant feature of this invention resides in the fact that only certain combinations of photographic directpositive emulsions and polyhydroxylbenzene developing agents can be employed in its practice. Thus, as shown in Examples 2 and 3 which follow, positive-working presensitized printing plates employing photographic directpositive emulsions of a different type from that employed in the practice of this invention have a very low photographic speed and require very long exposures. Furthermore, a polyhydroxybenzene developing agent which does not oxidize upon development to render the gelatin oleophilic in an unexposed area cannot be employed in the practice of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The silver halide emulsions used in the practice of this invention are photographic reversal or direct-positive emulsions containing grains comprising a central core of a water insoluble silver salt containing centers which promote the deposition of photolytic silver and an outer shell or covering for such core of a fogged or spontaneously developable water-insoluble silver salt. The fogged shell of such grains develops to silver without exposure.

In one embodiment of the invention, the direct-positive emulsion has adsorbed to the fogged grains a halogenaccepting compound having an anodic polarographic halfwave potential less than 0.85 and a cathodic polarographic halfwave potential which is more negative than -l.0. These emulsions have an unusually high sensitivity or photographic speed. If the halogen-accepting compound does not have the polarographic halfware potential set forth above, the emulsions containing the compound will not have an unusually high sensitivity.

In another embodiment of the invention, certain high molecular weight organic compounds, particularly sulfonated compounds as described hereinafter, can be used in combination with halogen-accepting compounds to effect an even greater increase in photographic speed or sensitivity.

Before the shell of water-insoluble salt is added to the silver salt core, the core emulsion is first chemically or physically treated by methods previously described in the prior art to produce centers which promote the deposition of photolytic silver, i.e., latent image nucleating centers. Such centers can be obtained by various technuiques as described herein. Chemical sensitization techniques of the type described by Antoine Hautot and Henri Saubenier in Science et Industries Photographiques, Volume XXVIII, January 1957, pages 57-65, are particularly useful. Such chemical sensitization includes three major classes, namely, gold or noble metal sensitization, sulfur sensitization, such as by a labile sulfur compound, and reduction sensitization, i.e., treatment of the silver halide with a strong reducing agent which introduces small specks of metallic silver into the silver salt crystal or grain.

When the core emulsion is chemically sensitized, it is preferably sensitized so that when examined according to normal photographic testing techniques by coating a test portion of the emulsion on a transparent support, exposing to a light intensity scale for a fixed time between 0.01 and 1 second and development for 6 minutes at 68 F. in Developer A, as hereinafter defined, it has sensitivity greater than the sensitivity of an identical test portion of the same emulsion (measured at a density of 0.1 above fog), which has been exposed in the same way, bleached 5 minutes in an aqueous 0.3 percent potassium ferricyanide solution at 65 F., and developed for 5 minutes at 65 F., in Developer B, as hereinafter defined. Developer A is the usual type of surface image developer and Developer B is an internal developer having high silver halide solvent activity.

3 DEVELOPER A Grams N-methyl-p-aminophenolsulfate 2.5 Ascorbic acid 10.

Potassium metaborate 35.0 Potassium bromide 1.0 Water to 1 liter. pH of 9.6.

DEVELOPER B Grams N-methyl-p-aminophenolsulfate 2.0 Sodium sulfite, desiccated 90. Hydroquinone 8.0 Sodium carbonate, monohydrate 52.5 Potassium bromide 5.0 Sodium thiosulfate 10.0

Water to 1 liter.

The core emulsions can be chemically sensitized by any method suitable for this purpose. For example, the core emulsions can be digested with naturally active gelatin, or sulfur compounds can be added, such as those described in Sheppard US. Patent 1,574,944 issued Mar. 2, 1926; Sheppard et al. U.S. Patent 1,623,499 issued Apr.

5, 1927; and Sheppard et al. US. Patent 2,410,689 issued Nov. 5, 1946.

The core emulsions can also be chemically sensitized with gold salts as described in Waller et al. US. Pat. 2,399,083 issued Apr. 23, 1946, and Damschroder et al. US. Pat. 2,642,361 issued June 16, 1953. Suitable com pounds are potassium chloroaurite, potassium aurithiocyanate, potassium chloroaurate, auric trichloride and 2- aurosulfobenzothiazole methochloride.

The core emulsions can also be chemically sensitized with reducing agents, such as stannous salts (Carroll US. Pat. 2,487,850 issued Nov. 15, 1949), polyamines, such as diethylene triarnine (Lowe and Jones US. Pat. 2,518,- 698 issued Aug. 15, 1950), polyamines, such as spermine (Lowe and Allen US. Pat. 2,521,925, issued Sept. 12, 1950), or bis(betaaminoethyl)sulfide and its water-soluble salts (Lowe and Jones US. Pat. 2,521,926 issued Sept. 12, 1950).

The core emulsions can also be treated during or after the formation of the silver salt With salts of polyvalent metals such as bismuth, the noble metals and/or the metals of Group VIII of the Periodic Table, such as ruthenium, rhodium, palladium, iridium, osmium, platinum and the like. Representative compounds are ammonium chloropalladate, potassium chloroplatinate, sodium chloropalladite and the like.

The core emulsions can also be subjected to fogging by exposure to light either to low or high intensity light, to produce centers which promote the deposition of photolytic silver prior to forming the shell thereon.

The emulsions employed in practicing this invention can also contain spectral sensitizers such as the cyanines, merocyanines, complex (trinuclear) cyanines, complex (trinuclear) merocyanines, styryls and hemicyanines. Particularly good spectral sensitizers which can be used are the merocyanines disclosed in Brooker et al. US. Pats. 2,493,747 and 2,493,748 issued Jan. 10, 1950.

The shell of the grains comprising the emulsions used in practicing this invention is prepared by precipitating over the core grains a light-sensitive water-insoluble silver salt that can be fogged and which fog is removable by bleaching. The shell is of sufficient thickness to prevent access of the developer used in processing the emulsions of the invention to the core. The silver salt shell is surface fogged to make it developable to metallic silver with conventional surface image developing compositions. Substantially all of the silver halide grains in an emulsion are fogged prior to exposure and/or processing, i.e., such emulsions are uniformly fogged. Such fogging can be effected by chemically sensitizing to fog with the sensitizing agents described for chemically sensitizing the core emulsion, high intensity light and like fogging means well known to those skilled in the art. While the core need not be sensitized to fog, the shell is fogged, for example, reduction fogged with a reducing agent such as stannous chloride. Fogging by means of a reduction sensitizer, a noble metal salt such as gold salt plus a reduction sensitizer, high pH and low pAg silver halide precipitating conditions, and the like can be suitably utilized.

In one embodiment of the invention, the core of the grains in the emulsion layer is a coarse grained silver salt and silver halide from a finer grained silver salt is deposited thereon by Ostwald ripening to form the shell. Also, coarse grained silver salts can be used to form a shell over a finer grained core when the shell-forming silver salt is more water-soluble than the core silver salt. In another embodiment of the invention, the silver halide shell is formed immediately after formation of the core without interrupting the precipitation.

Generally, about 2 to 8 molar equivalents of shell silver salt per molar equivalent of core silver salt are used in the grains comprising the emulsions of the invention. The emulsions used in the presensitized plates of this invention, which can be termed covered grain emulsions, contain a population of grains which are substantially uniform in grain-size distribution, as contrasted with emulsion blends with contain at least two types of grains, which are separate and distinct in their physical, and frequently, photographic properties. The grain size of these covered grain emulsions widely varies, typical emulsions having an average grain size of about 0.05 to 10 microns in diameter. Such emulsions are generally coated at silver coverages in the range of about 10 to about 400 milligrams silver per square foot, preferably about 20 to 100 milligrams silver per square foot, and when exposed to an image and thereafter developed in a conventional surface image developer having low silver salt solvent action, form a reversal or direct-positive silver image. The unexposed grains develop without substantial reduction of the imagewise exposed grains.

The silver halide grains employed in the practice of this invention are fogged sufiiciently to give a density of at least 0.5 when developed without exposure for five minutes in Developer A, described hereinbefore, when a directpositive emulsion layer containing such grains is coated at a coverage of about 50 to about 500 milligrams of silver per square foot of support.

The halogen-accepting compounds employed in practicing the one embodiment of this invention referred to above are adsorbed to the fogged silver halide grains. The halogen acceptors which give particularly good results in the practice of this invention can be characterized in terms of their polarographic halfwave potentials, i.e., their oxidation reduction potentials determined by polarography. Cathodic measurements can be made with a 1X 1O molar solution of the halogen acceptor in a solvent, for example, methanol which is 0.05 molar in lithium chloride using a dropping mercury electrode with the polarographic halfwave potential of the most positive cathodic wa ve being designated E Anodic measurements can be made with 1X10 molar aqueous solvent solution, for example, methanolic solutions of the halogen acceptor which are 0.05 molar in sodium acetate and 0.005 molar in acetic acid using a carbon paste or pyrolytic graphite electrode, with the voltommetric half peak potential for the most negative anodic response being designated E In each measurement, the reference electrode can be an aqueous silver-silver chloride (saturated potassium chloride) electrode at 20 C. Electrochemical measurements of this type are known in the art and are described in New Instrumental Methods in Electrochemistry, by Delahay, Interscience Publishers, New York, N.Y., 1954; Polarography, by Kolthofi and Lingane, 2nd Edition, Interscience Publishers, New York, N.Y., 1952; Analytical Chemistry, 36, 2426 (1964) by Elving; and Analytical Chemistry, 30, 1576 (1958) by Adams.

Compounds which can be employed as halogen accep tors in the practice of this invention include organic or inorganic compounds having an anodic polarographic halfwave potential E less than 0.85 and a cathodic polarographic potential E which is more negative than 1.0. A preferred class of halogen-accepting compounds is characterized by an anodic halfwave potential which is less than 0.62 and a cathodic halfwave potential which is more negative than -1.3. A preferred class of halogen acceptors that can be used in the practice of this invention comprises the spectral sensitizing merocyanine dyes having the formula:

1'; b= L L)..- where A represents the atoms necessary to complete an acid heterocyclic nucleus, e.g., rhodanine, 2-thiohydant0in and the like, B represents the atoms necessary to com.- plete a basic nitrogen containing heterocyclic nucleus, e.g., benzothiazole, naphthothiazole, benzoxazole and the like, each L represents a methine linkage, e.g.,

CH=, C= or (|J= CH: 0 m

and n is an integer from 0 to 2, i.e., 0, 1 or 2. Specific examples of merocyanine dyes falling within the above formula include:

3 -carboxymethyl--[ (3-ethyl-Z-benzothiazolinylidene ethylidene] rhodanine 3 -ethyl-5-[1-(4-sulfobutyl)-4(1H)-pyridylidene]- rhodanine, sodium salt;

3-carboxymethyl-5-[ (3-ethyl-2-benzoxazolinylidene) ethylidene] -2-thio-2,4-oxazolidinedione;

1-carboxymethyl-5-[ (3ethyl-2benzothiazolinylidene)- ethylidene] 3-phenyl-3 thiohydantoin;

4-[ 1-ethylnaphtho[ 1,2-d] thiazolin-Z-ylidene)1-methylethylidene] 3-methyll- (4-sulfophenyl) 2-pyrazolin- 5 one;

4-[ (3 ethyl-6-nitro-2-benzothiazolinylidene) ethylidene] 3phenyl-2-isoxazolin-5-one; etc.

For a further description of suitable halogen acceptors, see Wise US. application Ser. No. 615,360 filed Feb. 13, 1967. Suitable procedures for preparing merocyanine dyes are described in Brooker et al. U.S. Patents 2,493,747 and 2,493,748 issued Jan. 10, 1950.

The halogen-accepting compounds employed in practicing the one embodiment of this invention referred to above can be used in widely varying concentrations. However, the halogen-accepting compounds are generally employed at concentrations in the range of about 100 rmilligrams to about 1.0 gram, preferably about 150 to about 600 milligrams per mole of silver halide.

As already indicated, the halogen accepting compounds described herein can be employed in combination with certain types of high molecular weight organic compounds to achieve an even greater increase in the photographic speed of direct-positive emulsions. These compounds are sulfonated and comprise polynuclear aromatic compounds containing at least one sulfo group. The term polynuclear aromatic as used herein is intended to mean 2 or more benzene rings fused together (for example, as in naphthalene, pyrene, etc.), or at least 2 benzene rings or aromatic rings directly joined together (for example, as in diphenyl, terphenyl, quaterphenyl, etc.), or through an aliphatic linkage. Such sulfonated derivatives can conveniently be represented by the following general formula:

wherein R represents a polynuclear aromatic group as defined above and M represents a cation such as a hydrogen atom or a water-soluble cation salt group (e.g., sodium, potassium, ammonium, triethylammonium, triethanolammonium, pyridinium, etc.).

Included among the sulfonated derivatives of the above formula are the following typical examples:

Calcofiuor White-MRThis is the trade name for a his (s-triazin-2-y1amino)stilbene-2,2-disulfonic acid, sodium salt.

Leucophor BThis is the trede name for a bis(s-triazin- 2-ylamino)stilbene-2,2-disulfonic acid, sodium salt.

Sodium 6-(4-methoxy-3-sulfo-w-phenylacryloyl)pyrene.

3,4-bis(4-methoxy-3asulfobenzamido)dibenzothiophene dioxide, sodium salt.

4',4-bis(2,4-dimethoxy-5sulfobenzamido)p-terphenyl,

disodium salt.

Chyrsene-6-sulfonic acid, sodium salt.

4,4'-bis[2-phenoxy-4- (Z-hydroxyethylamino) 1,3 ,5- triazin-6-ylamino]stilbene-2,2'-disulfonic acid, disoduim salt.

These sulfonated derivatives may be used in any concentration effective for the intended purpose. Good re sults are generally obtained by employing the compounds in concentrations in the range of about 0.02 to about 10 grams per mole of silver halide.

Silver halides are preferably utilized to prepare the core as well as the shell of the grains comprising the covered grain emulsions used in the practice of the invention. Suitable silver halides include silver chloride, silver bromide, silver iodide, silver chlorobrornide, silver bromoiodide and siver chlorobromoiodide. Other waterinsoluble silver salts for the core and/or shell include silver thiocyanate, silver phosphate, silver cyanide, silver carbonate and the like.

In the practice of this invention, the ink-receptive areas of the printing plates on a background of hydrophilic material are obtained by alkaline activation of a silver salt, preferably silver halide, developing agent in the presence of a gelatin, silver salt emulsion layer. The result is to form a silver image and oxidized developing agent in the region of development. The element can then be inked in the developed areas with greasy printing ink and a positive print made therefrom in a lithographic press. The gelatin in the emulsion layer can undergo additional hardening at the same time as development. However, the hardening of the gelatin layer in the region of development is incidental to successful operation of the invention since some developing agents, such as catechol, hydroquinone and toluhydroquinone, which are known to be strong gelatin-tanning silver halide developing agents, are not useful in the sensitive elements of this invention because the oxidation products formed in the presence of the gelatin, silver salt emulsion layer do not form oleophilic images, Whereas other closely related silver salt developing agents such as chlorohydroquinone, are quite useful in the process of the invention.

The developing agents employed in the practice of this invention include any developing agents which are capable of oxidation in the presence of the hardened gela tin present in the photographic emulsion layer to produce an image receptive to greasy printing ink, i.e., upon development, they render the gelatin in the unexposed areas oleophilic. They can be incorporated into one or more layers of the photographic element of this invention or they can be supplied from outside of the element, e.g., from solution or from a layer on a separate support. The polyhydroxybenzene developing agents substituted with halogen, monocyclic aryl groups of the benzene series and alkyl groups of at least 2 and preferably from 2 to 6 carbon atoms have this property. The 1,2-dihydroxybenzene developing agents substituted by halogen, monocyclic aryl of the benzene series and alkyl groups of at least 2 carbon atoms and preferably 2 to 6 carbon atoms are particularly useful in the process. Developing agents possessing the necessary properties thus include certain polyhyroxybenzene developing agents such as pyrogallol and substituted polyhydroxybenzene developing agents, particularly dihydroxybenzenes substituted with, for example, halogen, alkyl groups of at least 2 and preferably from 2 to 6 carbon atoms and a monocyclic aryl group of the benzene series, e.g., o-chlorohydroquinone, o-bromohydroquinone, 4-phenyl catechol, 4-phenethyl catechol, 4-phenpropyl catechol, 4-t-butyl catechol, pyrogallol, 4- n-butylpyrogallol, nordihydroguiauretic acid, 4,.5-dibromocatecho, 3,3,6-tribromo-4-phenylcatechol and 1 phenyl 3 (N nhexylcarboxarnide)-4-[p-(,8-hydroquinolylethyl)-phenylazo]-5-pyrazolone. Esters of such developing agents, e.g., formates and acetates of pyrogallol hydrolyze in alkaline solutions and can be used in the processes of the invention. Such esters are intended to be included in the specification and claims where reference is made to polyhydroxybenzene developing agents. In certain cases it has been found to be advantageous to include with the polyhydroxybenzene developing agent such as pyrogallol, an auxiliary developing agent such as monomethyl-p-aminophenol or a 3-pyrazolidone, which latter developing agents by themselves do not yield oleophilic images in the processes described, but which do appear to act synergistically with the polyhydroxybenzenes to t yield oleophilic images. The developing agents are generally employed in the photographic elements at coverages of about 5 to about 200, preferably about to about 50 milligrams per square foot of support and are incorporated in a layer which is no farther from the support than the photographic emulsion layer, i.e., they are incorporated into the photographic emulsion layer or a layer between the emulsion layer and the support, e.g., an adjacent or contiguous layer. However, these developing agents can be incorporated in a layer, e.g., a gelatin layer, on a separate support which is wetted with activator and brought into contact with the photographic emulsion layer during processing.

In the photosensitive elements of this invention, the silver salt emulsion layer should be substantially hardened in order to prevent the processed plate from adhering to printing blankets, papers, etc. For this purpose, the emulsion should be as hard as a gelatin layer containing at least about 0.2 gram and preferably about 2 to grams of dry formaldehyde per pound of gelatin, i.e., it should have a melting point in water greater than 150 F. and preferably greater than 200 F.

Although gelatin can be the sole binding agent, various colloids can be used with gelatin as vehicles or binding agents in the photographic emulsion layers employed in the practice of this invention. However, gelatin or a gelatin derivative which is primarily gelatin, is used for at least a part, for example, at least 10%, by weight, of the binder in the photographic element. Other binding agents that can be used with the gelatin include any of the hydrophilic colloids generally employed in the photographic field such as colloidal albumin, polysaccharides, cellulose derivatives, synthetic resins such as polyvinyl compounds, including polyvinyl alcohol derivatives, acrylamide polymers, and the like. In addition to the hydrophilic colloids,

the vehicle or binding agent can contain dispersed polymerized vinyl compounds, particularly those which increase the dimensional stability of photographic materials. Suitable compounds of this type include Water-insoluble polymers of alkyl acrylates or methacrylates, acrylic acid, sulfoalkyl acrylates or methacrylates, and the like. The binding agent is generaly coated at a coverage in the range of about 50 to about 2000, preferably about 100 milligrams to about 1000 milligrams per square foot of support.

The photographic layers described herein can be superimposed upon a wide variety of supports. Typical flexible supports include those generally employed in printing plates, as exemplified by metals such as aluminum, paper, cellulose nitrate film, cellulose acetate film, polyvinyl acetal film, polystyrene film, polyethylene terephthalate film, and related films or resinous materials and other related materials. Supports such as paper which are partially acetylated or coated with an alpha-olefin polymer, particularly a polymer of an alpha-olefin containing 210 carbon atoms, as exemplified by polyethylene, polypropylene, ethylene-butene copolymers and the like, give good results.

The photographic elements of printing plates of this invention can include antihalation layers which are generally coated between a photographic emulsion layer and the support. Suitable layers of this type include gelatin layers containing organic and inorganic pigments such as Napthol Red type pigments, colloidal carbon or silver, zinc oxide, titanium dioxide, iron oxide and the like, which can be dyed, if desired, with such dyes as the merocyanines and the like. Where paper coated with an alphaolefin polymer is employed as the support, it is also convenient to incorporate a dye and/or a pigment into the resin coating on the photographic emulsion side surface of the support which serves as an antihalatio-n layer. In addition, the dyes and/or pigments can be in the same layer with the polyhydroxybenzene developing agent when it is coated in a layer beneath the photographic emulsion layer.

The invention can be further illustrated by the following examples of preferred embodiments thereof although it will be understood that these examples are included merely for purposes of illustration and are not intended to limit the scope of the invention unless otherwise specifically indicated.

Example l.A superior positive-working presensitized lithographic printing plate can be prepared using the covered grain emulsions described herein in combination with polyhydroxybenzene developing agents located in an adjacent layer beneath the photographic emulsion layer. To illustrate, a gelatin silver chloride photographic emulsion is prepared simultaneously adding at C., over a period of about 20 minutes, 1000 milliliters of a 4 molar silver nitrate aqueous solution and 1000 milliliters of a 4 molar sodium chloride aqueous solution, to a Well-stirred aqueous solution of 1000 milliliters of 0.01 molar sodium chloride containing 40 grams of gelatin. 5000 milliliters of water containing 280 grams of gelatin is added and the emulsion is cooled. One-eighth of the resulting gelatin silver chloride emulsion (containing 0.5 mole percent silver chloride) is melted at 40 C., milligrams of potassium chloroiridite (dissolved in Water) is added and the emulsion heated to 70 C. This prepared emulsion constitutes the silver chloride core containing physical discontinuities that trap electrons over which is formed a shell of silver chloride.

The shell of silver chloride is formed by adding to the core emulsion 500 milliliters of 40 molar silver nitrate aqueous solution and 500 milliliters of 4 molar silver chloride aqueous solution simultaneously over a period of 20 minutes. grams of gelatin, previously soaked in 340 milliliters of water, is stirred in and the emulsion cooled. During both additions of the silver nitrate and sodium chloride (i.e., to form both the core and the shell), the two solutions are added at approximately constant rates. Sufiicient silver chloride is formed in the shell to give a ratio of 4 moles of shell silver chloride to 1 mole of core silver chloride. The resulting covered grain emulsion is melted, the gelatin content increased to 160 grams per mole of silver chloride and water added to 4000 grams per mole of silver chloride.

Two milligrams of thiourea dioxide per mole are added to the emulsion at 40 C. The emulsion is fogged by heating it to 55 C. and holding it for 40 minutes at this temperature. It is cooled immediately to 40 C. The following additional additives are incorporated into one mole of the fogged emulsion: 100 grams gelatin, 200 milligrams of a thiazoline-rhodanine merocyanine green sensitizer, of the type described in Brooker et al. US. Patent 2,493,748 issued Jan. 10, 1950; 1 gram of a sulfonated triazinyl-stilbene of the type described in Formula II of McFall et al. US. Patent 2,933,390 issued Apr.

9 9, 1960; and 25 milliliters of a 10 percent formaldehyde solution.

The emulsion is coated at a coverage of 72 milligrams of silver per square foot over a gelatin layer containing the developing agent 4-phenyl catechol. The gelatin layer is prepared from a gelatin solution containing a dispersion of 4-phenyl catechol in tricresyl phosphate and is coated at 30 milligrams per square foot phenyl catechol and 0.58 gram per square foot gelatin on a polyethylene coated paper support.

The direct-positive photosensitive element obtained is exposed by projection to a line original in a conventional process camera. After seconds activation at 75 F. in an 8 percent potassium phosphate solution, the plate is stopped in a phoshoric acid bath. The plate is then printed on a lithographic press giving a positive lithographic print.

Similar results are obtained when silver chloride in the above procedure is replaced by another silver salt such as silver chlorobromide or silver bromoiodide.

Example 2.The direct-positive photographic printing plates prepared according to this invention have much higher photographic speed than direct-positive plates previously described in the art. For example, the photographic direct-positive elements prepared according to this invention are considerably more sensitive than direct positive plates of the type described in Yackel et al. US. Pat. 3,146,104 issued Aug. 25, 1964. To illustrate, a direct-positive photographic printing plate is prepared as described in Example 11 of U.S. Pat. 3,146,104 and exposed as described in Example 1 above. The photographic element of Example 1 has more than ten times the sensitivity of the element prepared according to Example 11 of US. Pat. 3,146,104. In fact, this latter element does not have enough sensitivity to be exposed on conventional projection copying equipment and must be exposed on a contact printer.

Similar improvements in photographic speed are shown for the photographic elements prepared according to this invention when they are compared with elements using solarizing emulsions of the type described in Leermakers US. Pat. 2,184,013 issued Dec. 19,1939, and referred to in Example 11 of Yackel et al. U.S. Pat. 3,146,104 issued Aug. 25, 1964.

Example 3.-The printing plates disclosed herein exhibit several significant advantages over positive-working presensitized printing plates disclosed in the prior art. To illustrate, a direct-positive element is prepared as described in Example 3 of Yackel et al. US. Pat. 3,146,104 issued Aug. 25, 1964. This element is exposed and printed as described in Example 1 above, This photographic element requires more than three times the 20 seconds used to process the element described in Example 1. Also, the photographic plates of this invention are of simple construction in comparison to that described in Example 3 of U8. Pat. 3,146,104 since they eliminate at least one layer.

When the above procedure is repeated with plates employing photographic reversal emulsion layers of the type described in Fallesen US. Patent 2,497,875 issued Feb. 21, 1950, they yield lower contrast than do the plates of Example 1 above. Furthermore, such prior art printing plates require longer processing times and the use of processing solutions having poor stability.

Example 4.An emulsion is prepared as described in Example 1 except that the core emulsion is reduction sensitized into fog by adding an excess of silver nitrate and holding for 20 minutes at 70 C. at a pAg of 2.5 and a pH of 75. Following this treatment, the pH is adjusted to 5.5 and the pAg to 6.8. After the reduction sensitization, aqueous solutions of silver nitrate and potassium chloride are added simultaneously to the core emulsion over a period of 20 minutes at 70 C. to form a silver chloride shell over the reduction sensitized silver chloride core.

10 Suflicient silver chloride is formed in this shell to give a ratio of 4 moles of shell silver chloride to 1 mole of core silver chloride. The emulsion is reduction sensitized to fog by adding 2 milligrams of thiourea dioxide per mole of slver halide and holding the resulting emulsion for 15 minutes at 55 C.

The additional additives described in Example 1 are added and the emulsion is coated over a gelatin layer containing a polyhydroxybenzene developing agent as described in Example 1. When processed and printed on a lithographic printing press, as described in Example 1, the photographic element provides a direct-positive print of good quality.

Example 5 .As previously pointed out, the developing agent can be incorporated into the photographic emulsion layer which gives a plate of very simple structure. Furthermore, the plate can contain an antihalation layer under the photographic emulsion layer. To illustrate these features, the procedure of Example 1 is repeated except that the photographic emulsion layer is coated at coverages per square foot of 72 milligrams silver, 15 milligrams of 4-phenyl catechol and 100 milligrams gelatin. The photographic emulsion layer is coated on a polyethylene coated paper support containing titanium dioxide and red pigments dispersed in the polyethylene for antihalation purposes.

After exposure and processing, as in Example 1, the plate is printed on a conventional lithographic press to give a positive print.

Example 6.A gelatin layer containing a dispersion of 4-phenyl catechol dissolved in tri-o-cresyl phosphate is coated on conventional cellulose acetate film support at coverages of 30 milligrams of 4-phenyl catechol per square foot, 300 milligrams of tri-o-cresyl phosphate per square foot and 580 milligrams of gelatin per square foot. The gelatin layer is overcoated with a fogged, directpositive silver chloride emulsion prepared using the procedure of Example 1 at coverages of 48 milligrams of silver per square foot and 96 milligrams of gelatin per square foot. A gelatin antihalation layer containing 25 milligrams of gelatin per square foot and 15 milligrams of carbon per square foot is coated over the emulsion layer.

The direct-positive photosensitive element obtained is exposed through the base in a conventional process camera to a positive original. After 20 seconds activation at F. in an 8% potassium phosphate solution containing 0.1 gram of potassium bromide per liter, the plate is stopped in a phosphoric acid bath. The plate is then printed on a lithographic press to give a right reading positive lithographic print.

Example 7.A gelatin layer containing carbon black pigment in a dispersion of 4-phenyl catechol dissolved in tri-o-cresyl phosphate is coated on conventional polyethylene terephthalate film support (.004 inch thick). The layer is coated at coverages of 10 milligrams of carbon black, 30 milligrams of 4-phenyl catechol, 300 milligrams of tri-o-cresyl phosphate and 580 milligrams of gelatin per square foot of support. This layer is overcoated with a fogged, direct-positive silver chloride emulsion prepared according to the procedure of Example 1 and coated at coverages of 48 milligrams of silver and 96 milligrams of gelatin per square foot of support.

The direct-positive photosensitive element obtained is exposed, processed and printed as in Example 6 to give a right reading positive lithographic print.

Example 8.A gelatin layer containing carbon black pigment in a dispersion of 4-phenyl catechol dissolved in tri-o-cresyl phosphate is coated on conventional polyethylene terephthalate film support (.004 inch thick) having an antihalation backing layer with a density of 0.3. The gelatin layer is overcoated with a fogged, direct-positive silver chloride emulsion prepared according to the procedure of Example 1 and coated at 48 milligrams of silver and 96 milligrams of gelatin per square foot.

The direct-positive photosensitive element obtained is exposed, processed and printed in the manner described in Example 6 to give a right reading positive lithographic print.

Thus, by the practice of this invention there is provided a means for obtaining direct-positive prints in a lithographic press using a high speed plate. The plate offers many significant advantages over many other positive working plates heretofore available. For example, the plate has a relatively simple structure, does not require complex processing and can be processed quickly in stable processing solutions prior to printing.

Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, it will be understood that variations and modifications can be effected without departing from the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

We claim:

1. A positive-working presensitized printing plate comprising a support, a hardened gelatin photographic emulsion layer containing grains comprising a central core of water-insoluble silver salt containing centers which promote deposition of photolytic silver and an outer shell covering said core comprising a fogged water-insoluble silver salt that develops to silver without exposure, and a polyhydroxybenzene developing agent which is a halogen-substituted, phenyl-substituted or 26 carbon atomcontaining alkyl-substituted polyhydroxy benzene devel oping agent and which oxidizes upon development of the emulsion layer to render gelatin oleophilic in an unexposed area.

2. The printing plate of claim 1 wherein said developing agent is contained in a layer no farther from said support than said emulsion layer.

3. The printing plate of claim 2 wherein said emulsion layer is a silver halide emulsion layer containing grains comprising a central core of silver halide containing centers which promote deposition of photolytic silver and an outer shell covering said core comprising a fogged silver halide that develops to silver without exposure.

4. The printing plate of claim 3 wherein said. centers which promote deposition of photolytic silver are obtained by treatment of said central core with an iridium salt.

5. The printing plate of claim 3 wherein said central core of silver halide is chemically sensitized.

6. The printing plate of claim 3 wherein said fogged silver halide is reduction fogged silver halide.

7. The printing plate of claim 3 wherein said polyhydroxybenzene developing agent is in a layer between said emulsion layer and said support.

8. The printing plate of claim 3 wherein said silver halide is silver chloride and said polyhydroxybenzene developing agent is 4-phenyl catechol.

9. The printing plate of claim 3 wherein said emulsion has adsorbed to the fogged grains a halogen-accepting compound having an anodic polarographic halfwave potential less than 0.85 and a cathodic polarographic halfwave potential which is more negative than l.().

10. The printing plate of claim 9 wherein said halogenaccepting compound is a merocyanine dye.

12 11. The printing plate of claim 9 wherein said emulsion also contains a sulfonated compound having the formula:

RSO M wherein R represents a polynuclear aromatic group and M represents a hydrogen atom or a water-soluble cation salt group.

12. A process which comprises (a) exposing to a subject a positive-working presensitized printing plate comprising a support, a hardened gelatin photographic silver halide emulsion layer containing grains comprising a central core of silver halide containing centers which promote deposition of photolytic silver and an outer shell covering said core comprising a fogged silver halide that develops to silver without exposure and (b) developing the emulsion layer with a polyhydroxybenzene developing agent which is a halogen substittued, phenylsubstituted or 26 carbon atom-containing alkyl-substituted polyhydroxybenzene developing agent and which renders gelatin oleophilic in an unexposed area.

13. The process of claim 12 wherein said printing plate contains a polyhydroxybenzene developing agent which is a halogen-substituted, phenyl-substituted or 2-6 carbon atom-containing aklyl-substituted polyhydroxybenzene developing agent and which oxidizes upon development to render gelatin oleophilic in an unexposed area and said step (b) is accomplished by contacting said exposed emulsion layer with an alkaline solution to produce a printing surface which is positive with respect to the subject.

14. The process of claim 12 wherein said silver halide is silver chloride and said polyhydroxybenzene developing agent is 4-phenyl catechol.

15. The process of claim 12 which includes step (c) inking the developed area with greasy printing ink and printing therefrom in a lithographic printing press.

16. The process of claim 12 wherein said emulsion has adsorbed to the fogged grains a halogen-accepting compound having an anodic polarographic halfwave potential less than 0.85 and a cathodic polarographic halfwave potential which is more negative than l.0.

17. The process of claim 16 wherein said halogenaccepting compound is a merocyanine dye.

18. The process of claim 16 wherein said emulsion also contains a sulfonated compound having the formula RSO M wherein R represents a polynuclear aromatic group and M represents a hydrogen atom or a water-soluble cation salt group.

References Cited UNITED STATES PATENTS 3,146,104 8/1964 Yackel 9633 3,367,778 2/1968 Berriman 96-64 3,501,306 3/1970 Illingsworth 96107 3,501,310 3/1970 Illingsworth et al. 96-101 NORMAN G. TORCHIN, Primary Examiner I. WINKELMAN, Assistant Examiner US. Cl. X.R. 96-64, 76

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,597, 0 Dated August 3, 1971 Inventor(s) Dorothy J. Beavers and William B. Kendall It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In column 6, line 6 (page 10, line 19 of the application), "trede" should read ---trade-.

In column 7, lines 5-6 (page 12, line 21 of the application) l-,5-dibromocatecho" should read h.,5- dibromocatech0l--.

In column 8, line n9 (page 15, line 32 of the application) "L O molar" should read --L molar-.

. In column 10, line 5 (page 18, line 30 of the application) "slver" should read --silver--.

In column 12, line 17 (page 23, Claim 13 of the application) "halogen substittued" should read ---halogen substituted-.

S gned and sealed this l h day of May 1072.

(SEAL) guess:

IUDI-JAIYD ILFLWPJHEH ,JI-i. ROBERT GOTTSGIIALK A i,l ;c,-sti.n;" Officer Commissioner of Pa tents