Presensitized Planographic Printing Plate

Abstract

A presensitized printing plate comprising a support, a hardened gelatin photographic silver halide emulsion layer, a developer-containing layer and a reflecting layer between the emulsion and the support which contains a metal oxide and has certain Reflectance characteristics. The emulsion can be a direct-positive one and can optionally contain a halogen-accepting compound and a sulfonated compound to increase sensitivity.

Description

Background of the Invention

1. Field of the Invention

This invention relates to novel photographic materials, their preparation and use. In one of its aspects, this invention relates to novel presensitized printing plates and their use in lithography. In another of its aspects, this invention relates to novel presensitized printing plates, particularly positive-working plates, which contain a metal oxide in a layer between the support and the photographic silver halide emulsion layer.

2. Description of the Prior Art

Lithographic printing plates are well known and have been prepared, for example, from metal, cellulose ester or paper supports carrying light-sensitive layers. An outstanding lithographic printing plate which does not depend upon the printing character being substantially above or below the hydrophilic nonprinting surface, as opposed to other plates such as relief or intaglio, is described in U.S. Pat. No. 3,146,104 of Yackel et al. issued Aug. 25, 1964. In this lithographic printing plate, a gelatin photographic silver halide emulsion layer either contains or is coated effectively adjacent to a layer containing a developing agent which oxidizes upon development to render gelatin oleophilic in developed areas. Upon thinking the exposed and developed plate and printing on a lithographic press, the desired lithographic reproduction, which can be positive or negative with respect to the original subject is obtained. To avoid halation effects and achieved good resolution with the lithographic printing plates described in the Yackel et al. patent, antihalation materials such as carbon black have been employed in a layer between the photographic silver halide layer and the support. The use of such materials as carbon black for this purpose has resulted in a significant loss in photographic speed. However, omitting the carbon black results in poor exposure latitude and resolution. It would be very desirable to have a printing plate which combines good photographic speed, exposure latitude and resolution.

Summary of the Invention

According to one embodiment of this invention, there is provided a presensitized printing plate comprising a support, a hardened gelatin photographic silver halide emulsion layer, a layer no farther from said support than said emulsion layer containing a polyhydroxybenzene developing agent which oxidizes upon development to render gelatin oleophilic in developed areas, and between said emulsion layer and said support, a reflecting layer containing a metal oxide and having the Reflectance characteristics described hereinafter.

According to another embodiment of this invention, there is provided a process employing a presensitized printing plate comprising a hardened gelatin photographic silver halide emulsion layer and a reflecting layer containing a metal oxide, 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 layer to render gelatin oleophilic in developed areas.

The reflecting layer present in the presensitized plate of this invention requires a certain opacity, as defined by its average Reflectance described herein, and must contain a metal oxide. Pigments which have been used in the past to impart opacity to reflecting layers but which are not metal oxides do not give the desired results in the practice of this invention. Thus, as shown in example 2 which follows, pigments such as barium sulfate impart the desired Reflectance characteristics to the reflecting layer but the presensitized plates have poor resolution. Furthermore, it could not have been expected that a carbon black containing layer which absorbs light could be replaced by a metal oxide reflecting layer in a printing plate to obtain good exposure latitude without losing photographic speed or resolution.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The gelatin photographic silver halide emulsion layer used in the presensitized printing plate described herein can be any negative or direct-positive silver halide emulsion. These emulsions can contain silver halide grains which form latent images predominantly on the surface of the grains or those which form latent images predominantly inside the silver halide crystal, as exemplified by those described in Davey and Knott U.S. Pat No. 2,592,250 issued Apr. 8, 1952, direct-positive emulsions such as those described in Kendall and Hill U.S. Pat No. 2,541,472 issued Feb. 13, 1951, or Fallesen U.S. Pat. No. 2,497,875 issued Feb. 21, 1950. If desired, a positive-working plate can be obtained by employing a photographic emulsion layer which comprises separate layers of fogged photographic silver halide coated over unfogged silver halide as described in Yackel and Abbott U.S. Pat. No. 3,146,104 issued Aug. 25, 1964. A negative-working plate is obtained when the unfogged silver halide emulsion is used alone, as described in U.S. Pat. No. 3,146,104.

In practicing this invention, it has been found that particularly good results are obtained with positive-working plates in which the photographic silver halide emulsion layers are layers of photographic reversal or direct-positive emulsions containing grains comprising a central core of a silver halide containing centers which promote the deposition of photolytic silver and an outer shell or covering for said core of a fogged or spontaneously developable silver halide. Such emulsions can be called "covered grain emulsions." The fogged shell of such grains develops to silver without exposure. In preparing this type of reversal emulsion, before a shell of silver halide is added to the silver halide core, the core emulsion is first chemically or physically treated by methods previously described in the prior art to produce centers which promote deposition of photolytic silver, i.e., latent image nucleating centers. Such centers can be obtained by various techniques such as chemical sensitization, particularly good results being obtained with techniques of the type described by Antoine Hautot and Henri Saubenier in Science et Industries Photographiques, Volume XXVIII, Jan. 1957, pages 57 to 65. Chemical sensitization techniques which can be employed include 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. The shell of the grains comprising the reversal emulsions is prepared by precipitating over the core grains a light-sensitive silver halide salt that can be fogged and which fog is removable by bleaching. The shell is of sufficient thickness to prevent access to the core by the developer used in processing. The silver halide 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 chemical sensitization to fog using 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, high pH and low pAg silver halide precipitating conditions and the like can be suitably utilized.

In one embodiment of the invention, the direct-positive 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 -1.0. These emulsions have an unusually high sensitivity or photographic speed. If the halogen-accepting compound does not have the polarographic halfwave 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.

The halogen-accepting compounds employed in practicing the one embodiment of this invention referred to above are absorbed 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 b 1 .times.10.sup.-4 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 wave being designated E.sub.(C). Anodic measurements can be made with 1 .times.10 .sup.-4 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 voltametric half peak potential for the most negative anodic response being designated E.sub.(a). In each measurement, the reference electrode can be aqueous silver -silver chloride (saturated potassium chloride) electrode at 20.degree. 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 Kolthoff 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 acceptors in the practice of this invention include organic or inorganic compounds having an anodic polarographic halfwave potential E.sub.(a) less than 0.85 and a cathodic polarographic potential E.sub.(c) 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 cathode 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: ##SPC1## where A represents the atoms necessary to complete an acid heterocyclic nucleus, e.g., rhodanine, 2-thiohydantoin and the like, B represents the atoms necessary to complete a basic nitrogen-containing heterocyclic nucleus, e.g., benzothiazole, naphthothiazole, benzoxazole and the like, each L represents a methine linkage, e.g., and n is an integer from 0 to 2 , i.e., 0. 1or 2. Specific examples of merocyanine dyes falling within the above formula include:

3-carboxymethyl-5-[(3-ethyl-2-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-o xazolidinedione;

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

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

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

For a further description of suitable halogen acceptors see Wise U.S. application Ser. No. 615,360 filed Feb. 13, 1967. Suitable procedures for preparing merocyanine dyes are described in Brooker et al. U.S. Pat. Nos. 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 milligrams 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:

R-so.sub.3 m 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:

Calcofluor White-MR. This is the trade name for a bis(s-triazin-2-ylamino)stilbene-2,2'-disulfonic acid, sodium salt.

Leucophor B. This is the trade name for a bis(s-triazin-2-ylamino)stilbene-2,2'-disulfonic acid, sodium salt.

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

3,4-Bis(4-methoxy-3-sulfobenzamido)-dibenzothiophene dioxide, sodium salt.

4',4"-Bis(2,4-dimethoxy-5-sulfobenzamido)-p-terphenyl, disodium salt.

Chyrsene-6-sulfonic acid, sodium salt.

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

These sulfonated derivatives may be used in any concentration effective for the intended purpose. Good results 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.

The silver halide employed in the preparation of the photographic emulsion layers of the printing plates described herein include any of the photographic silver halides, as exemplified by silver bromide, silver iodide, silver chloride, silver chlorobromide, silver bromiodide, silver chlorobromiodide and the like, These photographic silver halides can be coated at silver coverages of about 10 to about 400, preferably about 20 to about 100 milligrams per square foot.

The developing agents employed in the practice of this invention include the developing agents which are capable of oxidation in the presence of the hardened gelatin 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 developed 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 two and preferably from two to six carbon atoms have the property of making gelatin oleophilic in developed areas. The 1,2-dihydroxybenzene developing agents substituted by halogen, monocyclic aryl of the benzene series and alkyl groups of at least two carbon atoms and preferably two to six carbon atoms are particularly useful in the process. Developing agents possessing the necessary properties thus include certain polyhydroxybenzene developing agents such as pyrogallol and substituted polyhydroxybenzene developing agents, particularly dihydroxybenzene substituted with, for example, halogen, alkyl groups of at least two and preferably from two to six carbon atoms and a monocycylic 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, 4-n-butylpyrogallol, nordihydroguiauretic acid, 4,5-dibromocatechol, 3,3,6-tribromo-4 -phenylcatechol and 1-phenyl 3-(N-n-hexylcarboxamide) -4[p-(.beta.-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 incorporated 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 pyrogallol, an auxiliary developing agent such as monoethyl-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 yield oleophilic images. The developing agents are generally employed in the photographic elements at coverages of about 5 to about 200, preferably about 10 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 0.5 to about 10 grams of dry formaldehyde per pound of gelatin, i.e., it should have a melting point in water greater than 150.degree. F. and preferably greater than 200.degree. F.

The reflecting layers included in the presensitized printing plates of this invention provide good antihalation qualities without the significant photographic speed loss generally associated with the use of carbon black in antihalation layers. These good antihalation qualities are shown by resolution characteristics which are comparable to those of plates containing carbon black containing antihalation layers.

The reflecting layers, which are between the photographic silver halide emulsion layer and the support in the presensitized printing plates of this invention, contain a metal oxide, generally in a gelatin binder, and have an average Reflectance of at least 60percent, often at least 80 percent, over wavelengths in the visible region of the spectrum in the range of about 420 to about 500 millimicrons. Reflectance is that fraction of light which is reflected from the surface of the metal oxide containing layer coated on the support which Reflectance is measured at the defined wavelengths of the spectrum. This Reflectance can be determined using any method suitable for this purpose, including for example, the use of a standard recording spectrophotometer which will measure Reflectance at different wavelengths over the range of abut 420 to about 500 millimicrons. When averaged, the Reflectance at the wavelengths over this range must be at least 60 percent, generally in the range of about 60 to about 95 percent, and preferably in the range of about 80 to about 95 percent. The metal oxides employed in these layers can be any photographically inert metal oxide which will impart the desired average Reflectance such as aluminum or magnesium oxide, although oxides of metals from Group IIB and IVB of the Periodic Table, such as zinc oxide and titanium dioxide, are preferred. The metal oxides employed have an average particle size up to about 10 microns, although the particle size is generally less than about 5 microns and can be 0.1 micron or less. The metal oxide is generally coated at a coverage in the range of about 0.1 to about 10, preferably about 0.5 to about 5 grams per square foot of support. The metal oxides are in a layer between the emulsion layer and the support which layer can also contain the polyhydroxybenzene developing agent when developing agent is coated in the layer beneath the photographic emulsion layer. Where the support employed is paper or some other support coated with an alphaolefin polymer, it is convenient to incorporate the metal oxide into the resin coating on the photographic emulsion side surface of the support.

As already indicated, the presensitized printing plates of this invention give prints having good resolution even without the usual antihalation pigments such as carbon black or antihalation dyes. However, the metal oxide containing layers in the presensitized plates of this invention can contain antihalation dyes, particularly merocyanine dyes, if desired. These layers can also contain other addenda, for example, organic or inorganic pigments and brightening agents, as long as the metal oxide is present and the layer has the required Reflectance characteristics.

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 percent, by weight, of the binder in the photographic emulsion layer. Other binding agents can be used with the gelatin in the emulsion layer or in other layers alone or with 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 generally 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 are the flexible supports which are generally employed in printing plates, as exemplified by supports of 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 two to 10 carbon atoms, as exemplified by polyethylene, polypropylene, ethylene-butene copolymers and the like, give good results.

In practicing this invention, the ink-receptive areas of the printing plates on a background of hydrophilic material are obtained by alkaline activation of a silver halide developing agent in the presence of a gelatin photographic silver halide 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 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 toluthydroquinone, 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 halide emulsion layer do not form oleophilic images, whereas other closely related silver halide developing agents such as chlorohydroquinone, are quite useful in the process of the invention.

The photographic silver halide emulsions employed in the printing plates of this invention can be chemically sensitized by any method suitable for this purpose. For example, the emulsions can be digested with naturally active gelatin or they can contain such addenda as chemical sensitizers, for example, sulfur sensitizers (for example, allyl thiocarbamide, thiourea, allyl isothiocyanate, cystine, etc.), they can be treated during or after the formation of the silver salts with salts of polyvalent metals such as bismuth, the noble metals and/or the metals of Group VIIIB of the Periodic Table such as ruthenium, rhodium, palladium, iridium, osmium, platinum and the like, and can contain various gold compounds (e.g., potassium chloroaurate, auric trichloride, etc.) (see Baldsiefen U.S. Pat. No. 2,540,085 issued Feb. 6, 1951; Damschroder U.S. Pat. No. 2,597,856 issued May 27, 1952; and yutzy and leermakers u.s. Pat. No. 2,597,915 issued may 27, 1952), various palladium compounds such as potassium chloropalladate (Stauffer and Smith U.S. Pat. No. 2,598,079 issued May 27, 1952) etc., reduction sensitizers such as stannous salts (Carrol U.S. Pat. No. 2,478,850 issued Nov. 15, 1949), polyamines, such as diethyl triamine (Lowe and Jones U.S. Pat. No. 2,518,698 issued Aug. 15, 1950), polyamines, such as spermine (Lowe and Allen U.S. Pat. No. 2,521,925 issued Sept. 12, 1950), or bis(.beta.-aminoethyl)sulfide and its water-soluble salts (Lowe and Jones U.S. Pat. No. 2,521,926 issued Sept. 12, 1950) or mixtures of such sensitizers, antifoggants, such as ammonium chloroplatinate (Trivelli and Smith U.S. Pat. No. 2,566,245 issued Aug. 28, 1951), benzotriazole, nitrobenzimidazole, 5-nitroindazole, benzidine, mercaptans, etc. (see Mees, "The Theory of the Photographic Process," Mac Millian Publishing Company, 1942, page 460) or mixtures thereof, hardeners such as aldehyde hardeners, aziridine hardeners, hardeners which are derivatives of dioxane, oxypolysaccharides such as oxystarch or oxy plant gums and other types of hardeners for gelatin and hydrophilic colloids. These photographic emulsions 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. U.S. Pat. Nos. 2,493,747 and 2,493,748 issued Jan. 10, 1950.

The invention can be further illustrated by reference to the accompanying drawing in which:

FIGS. 1, 2 and 3 are each diagrammatical cross-sectional views of printing plates representing separate embodiments of this invention.

FIG. 4 is a set of Reflectance curves for three supports coated with a gelatin layer containing titanium dioxide, barium sulfate or zinc oxide.

In FIG. 1 there is shown a cross-sectional view of a presensitized printing plate of the type which can be advantageously employed in the practice of this invention. The plate comprises a support 1 such as paper, layer 2 is a hydrophilic organic colloid layer, for example, gelatin containing polyhydroxybenzene developing agent and metal oxide, as described herein and layer 3 is a hardened gelatine photographic silver halide emulsion layer (negative or positive). The element illustrated, if desired, can also contain additional layers (not shown) such as gelatin layers, subbing layers and the like.

FIGS. 2 and 3 are variations of FIG. 1 in which, in FIG. 2, the polyhydroxybenzene developing agent is in the photographic silver halide emulsion layer 4 and, in FIG. 3, the emulsion layer 5 comprises a fogged gelatin photographic silver halide layer coated over an unfogged gelatin photographic silver halide layer to give a positive-working plate.

FIG. 4 is gives the Reflectance curves for three paper supports coated with gelatin layers containing (A) titanium dioxide having an average particle size of 2 microns, (B) barium sulfate having an average particle size of 2 microns and (C) zinc oxide having an average particle size of 2 microns, all as in example 2.

This 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 1

A positive-working presensitized plate having good exposure latitude is obtained when the covered grain photographic emulsions described herein are coated over metal oxide containing layers which may also contain a dye such as merocyanine dyes, e.g., thiazoline-rhodanine merocyanines. To illustrate, the following gelatin dispersions are coated over a polyethylene coated paper support: ##SPC2##

Coatings 2 and 3 have an average Reflectance over the 420-500 millimicron of the spectrum over 60 percent.

Each of the resulting coatings 1-3 is overcoated with a fogged covered grain photographic emulsion prepared as follows:

A gelatin silver chloride photographic emulsion is prepared by simultaneously adding at 70.degree. 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-quarter of the resulting gelatin silver chloride emulsion (containing 1.0 mole percent silver chloride) is melted at 40.degree. C., 100 milligrams of potassium chloroiridite (dissolved in water) is added and the emulsion heated to 70.degree. 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 4 molar silver nitrate aqueous solution and 500 milliliters of 4 molar sodium chloride aqueous solution simultaneously over a period of 20 minutes. 160 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. Sufficient 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 of silver chloride are added to the emulsion at 40.degree. C. The emulsion is fogged by heating it to 55.degree. C. and holding it for 40 minutes at this temperature. It is cooled immediately to 40.degree. C. The following additional additives are incorporated into one mole of the fogged emulsion: i240 grams gelatin, 200 milligrams of a thiazoline-rhodanine merocyanine green sensitizer of the type described in Brooker et al. U.S. Pat. No. 2,493,748 issued Jan. 10, 1950, 1 gram of a sulfonated triazinyl-stilbene of the type described in Formula II of McFall U.S. Pat. No. 2,933,390 issued Apr. 19, 1960, and 25 milliliters of a 10 percent formaldehyde solution.

The emulsion is coated at a coverage of 72 milligrams silver per square foot and 215 milligrams of gelatin per square foot. The resulting coatings are deliberately overexposed by exposing for 8 seconds to a 40-watt bulb at 27 inches through a parallel test line object covered with a step tablet, processed for 20 seconds at 85.degree. F. in an 8 percent potassium phosphate solution containing 0.1 gram potassium bromide per liter and stopped for 20 seconds in a phosphoric acid bath.

The plate is then placed in a lithographic printing press, inked in the conventional manner and printed to give a positive lithographic print.

Plates with coatings 2 and 3 exhibit very little or no loss in image quality with overexposure as compared to those containing coating 1. This illustrated the good exposure latitude of the former in comparison to the latter. Furthermore, sensitometric tests show that the use of the metal oxide interlayer of coating 2 does not decrease the photographic speed, although with the dye of coating 3 there is a slight speed loss of 0.15 log E.

Similar results are obtained when the 4-phenyl catechol developing agent is replaced in the above procedure with 4-t-butyl catechol, pyrogallol or 1-phenyl-3-(N-n-hexylcarboxamide)-4-[p-(.beta.-hydroquinolylethyl)-phenyla zo]-5-pyrazolone.

EXAMPLE 2

As already indicated, replacing a carbon black containing antihalation layer of the presensitized plates described herein with the metal oxide reflecting layer results in improved photographic speed while maintaining good resolution. Furthermore, such pigments as barium sulfate cannot be substituted for the metal oxide disclosed herein. To illustrate these features, a silver chloride direct-positive emulsion is prepared as described in example 1. The emulsion is coated on a series of gelatin layers as described in the following table, which gelatin layers are coated on polyethylene coated paper supports. The emulsion is coated at a coverage of 72 milligrams of silver and 215 milligrams of gelatin per square foot of polyethylene coated support. In each case, the gelatin emulsion and the gelatin developer layers are hardened using about 1 gram of formaldehyde per 100 grams of gelatin. The emulsion is green sensitized by the addition of a merocyanine dye. ##SPC3##

Each of the above elements is exposed to a revolving power chart, processed for 20 seconds at 85.degree. F. in an 8 percent potassium phosphate solution containing 0.1 gram potassium bromide per liter (pH 12), and stopped for 20 seconds in a 2 percent phosphoric acid bath. The plate is put on a conventional lithographic printing press, inked and several lithographic prints are made. The following characteristics are noted.

Coating Relative Resolution No. Speed (Lines/mm.) Addenda __________________________________________________________________________ 1 33 7 Carbon 2 100 5 Zinc oxide 3 100 3 Barium sulfate 4 100 7 Titanium dioxide 5 100 3 None

In the above table, as well as in the following tables, the relative speed indicated is a function of the exposure necessary to give a density of 0.2 above background fog.

Resolution, as reported in the above and following tables is measured in lines per millimeter (mm.) printed at the optimum exposure level.

It is obvious from an inspection of the above table that the presensitized printing plates of this invention exhibit excellent resolution without the substantial loss in photographic speed which results from the use of carbon black. Furthermore, the use of pigments such as barium sulfate gives a plate having good speed but very poor resolution.

EXAMPLE 3

As previously pointed out, the polyhydroxybenzene developing agents can be included in the photographic emulsion layer of the printing plates described herein and the metal oxide can be dispersed in polyethylene coated over a support such as paper. To illustrate, a direct-positive emulsion is prepared as described in example 1. A dispersion of 4-phenyl catechol in tri-o-cresyl phosphate is added directly to the emulsion which is coated on a paper fiber sheet coated with polyethylene containing titanium dioxide having an average particle size of about 2 microns. The metal oxide layer has an average Reflectance at 420-500 millimicrons of about 85 percent, and is coated at a coverage of about 3.4 grams of polyethylene and 0.34 gram of titanium dioxide per square foot of support. Two paper supports have the polyethylene surfaces electron bombarded, as described in British Pat. No. 975,108 to improve adhesion, and the reversal emulsion is coated on the supports at the coverages indicated in the following table. About 1 gram of formaldehyde per 100 grams of gelatin is added to the emulsion as a hardener just prior to coating and the emulsion is green sensitized by the addition of a merocyanine dye. A sample of each coating is processed and used as a lithographic plate as described in example 2 to give the results indicated below. ##SPC4##

EXAMPLE 4

A positive-working presensitized plate using a photographic silver halide emulsion layer which comprises a fogged emulsion coated over an unfogged emulsion, as described in Yackel et al. U.S. Pat. No. 3,146,104 issued Aug. 25, 1964, together with the metal oxide layers described herein has good photographic speed and resolution. To illustrate, a direct-positive printing plate is prepared as described in example 3 of the Yackel et al. patent. The layers are coated on a support comprising a paper fiber sheet having a polyethylene coated surface. The top layer comprises fogged gelatino-silver chloride emulsion. The middle layer comprises an unfogged gelatino-silver chloride emulsion green sensitized with a merocyanine dye. The bottom layer comprises a gelatin layer containing 4-phenyl catechol dispersed in tri-o-cresyl phosphate, and titanium dioxide. In each case the gelatin layers are hardened by the addition of approximately 1 gram of formaldehyde per 100 grams of gelatin. The layers are coated at the coverages indicated in the following table. ##SPC5##

The elements are exposed, processed, inked and printed on a lithographic press, as described in example 2, to give an image resolution of 7 lines per millimeter. In contrast, similar coatings in which the titanium dioxide is omitted have a resolution of only 3 lines per millimeter.

EXAMPLE 5

A negative-working presensitized plate which comprises a hardened silver halide gelatin emulsion layer, as described in the Yackel et al. patent together with the metal oxide layers described herein has good photographic speed and resolution. To illustrate, a negative-working printing plate is prepared as described in example 2 of the Yackel et al. patent. The layers are coated on a support comprising a paper fiber sheet having a polyethylene coated surface. The top layer comprises a gelatin silver chloride emulsion green sensitized with a merocyanine dye. The bottom layer comprises a gelatin layer containing 4-phenyl catechol dispersed in tri-o-cresyl phosphate, and titanium dioxide. In each case the gelatin layers are hardened by the addition of approximately 1 gram of formaldehyde per 100 grams of gelatin. The layers are coated at the coverages indicated in the following table. ##SPC6##

The elements are exposed, processed, inked and printed in a lithographic press, as described in example 2, to give an image resolution of 7 lines per millimeter. In contrast, similar coatings in which the titanium dioxide is omitted have a resolution of only 3 lines per millimeter.

Image resolution and exposure latitude of the presensitized printing plates of this invention are improved by the elimination of excess developing agent. This is accomplished by developing the printing plate in face-to-face contact with a web such as a hardened gelatin pad, a fogged silver halide emulsion layer or polyethylene-coated paper. The following example will illustrate this feature.

EXAMPLE 6

A. A photographic element of the type described in example 4 above is exposed in a process camera to a line and halftone original, processed in an 8 percent K.sub.3 PO.sub.4 activator at 85.degree. F. with agitation for 20 seconds followed by a 20-second immersion in a 2 percent phosphoric acid stop bath and then squeegeed.

B. A photographic element of the type described in example 4 above is exposed in a process camera to a line and halftone original, processed in an 8 percent K.sub.3 PO.sub.4 activator at 85.degree. F. for 5 seconds and then brought into face-to-face contact with a hardened gelatin layer coated on a paper support. The two sheets are held into contact for 30 seconds and then separated. The processed element is then immersed for 20 seconds in a 2 percent phosphoric acid stop bath and squeegeed.

C. A photographic element of the type described in example 4 above is processed in the same manner as in B. with the exception of bringing the element into face-to-face contact with a hardened fogged silver halide gelatin emulsion layer coated on a paper support.

The above plates are inked and printed on a lithographic press. The prints made from plates B and C produce a much better reproduction of the halftone picture and the fine line subject matter than the prints made from plate A.

When the photographic elements of example 6 are exposed in a process camera, the image formed is wrong-reading, which necessitates the use of a prism or mirror system in order to give right-reading prints. Such prisms or mirror systems, however, affect the optical properties of the camera and lower the image quality. A way to achieve right-reading lithographic prints using a process camera without an image-reversing device is to use the gelatin-coated support from part B. above as a lithographic plate. When the photographic element of part B. above is brought into face-to-face contact with the hardened gelatin layer coated on a paper support, oxidized developing agent is transferred imagewise to the gelatin-coated layer and forms a hydrophobic image which is a mirror image of the photographic element. When the gelatin-coated sheet is immersed in a phosphoric acid stop bath, squeegeed, inked and printed on a lithographic press, right-reading prints are obtained which are similar in quality to the prints obtained from the photographic element.

Thus, by the practice of this invention there is provided a means for obtaining a presensitized printing plate having good exposure latitude, photographic speed and resolution characteristics. Furthermore, the use of metal oxides rather than carbon black in antihalation layers of the presensitized printing plates of this invention gives comparable exposure latitude and resolution without the speed loss encountered when carbon black is used. In addition, the use of a metal oxide such as titanium dioxide or zinc oxide in the antihalation layer rather than carbon black gives a presensitized plate having a white background which makes it easy to inspect the dark image after processing. In contrast, the use of carbon black in an antihalation layer gives a dark background which makes it difficult to inspect the dark image.

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.

Claims

I claim:

1. In a presensitized printing plate comprising a support, a hardened gelatin photographic silver halide emulsion layer, a layer no farther from said support than said emulsion layer and containing a polyhydroxybenzene developing agent which is a halogen substituted, phenyl substituted, or two-six carbon atoms containing alkyl substituted polyhydroxybenzene developing agent and which oxidizes upon development to render gelatin oleophilic in developed areas; the improvement comprising a layer between said emulsion layer and said support containing an oxide of titanium or zinc and having an average Reflectance of at least 60 percent over wavelengths in the visible region of the spectrum in the range of about 420 to about 500 millimicrons.

2. The presensitized printing plate of claim 1 in which the layer containing a metal oxide has an average Reflectance of at least 80 percent over wavelengths in the visible region of the spectrum in the range of about 420 to about 500 millimicrons.

3. The presensitized printing plate of claim 1 in which said oxide is titanium dioxide.

4. The presensitized printing plate of claim 1 in which said developing agent is 4-phenyl catechol and said oxide is zinc oxide.

5. The presensitized printing plate of claim 4 in which said silver halide is silver chloride and said 4-phenyl catechol is in said emulsion layer.

6. The presensitized printing plate of claim 1 in which said silver halide is silver chloride and said developing agent is 4-phenyl catechol which is contained in said layer between said emulsion layer and said support.

7. The presensitized printing plate of claim 1 in which said layer between said emulsion layer and said support contains an antihalation dye.

8. The presensitized printing plate of claim 1 in which said emulsion is a direct-positive emulsion.

9. The presensitized printing plate of claim 8 in which said emulsion layer contains 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.

10. The presensitized printing plate of claim 9 in which said developing agent is 4-phenyl catechol and said oxide is titanium dioxide.

11. The presensitized printing plate of claim 9 in which said developing agent is 4-phenyl catechol and said oxide is zinc oxide.

12. The presensitized printing plate of claim 10 in which said emulsion layer is a hardened gelatin photographic silver chloride emulsion layer containing grains comprising a central core of silver chloride containing centers which promote deposition of photolytic silver and an outer shell covering said core comprising a fogged silver chloride that develops to silver without exposure and said 4-phenyl catechol is in said emulsion layer.

13. The presensitized printing plate of claim 9 in which 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 -1.0.

14. The presensitized printing plate of claim 13 in which said halogen-accepting compound is a merocyanine dye.

15. The presensitized printing plate of claim 13 in which said emulsion also contains a sulfonated compound having the formula:

R-so.sub.3 m wherein R represents a polynuclear aromatic group and M represents a hydrogen atom or a water-soluble cation salt group.

16. A process which comprises (a) exposing to a subject a presensitized printing plate comprising a support, a hardened gelatin photographic silver halide emulsion layer, and between said emulsion layer and said support, a layer containing an oxide of titanium or zinc and having an average Reflectance of at least 60 percent over wavelengths in the visible region of the spectrum in the range of about 420 to about 500 millimicrons and (b) developing the emulsion layer with a polyhydroxybenzene developing agent which is a halogen substituted, phenyl substituted or two-six carbon atoms containing alkyl substituted polyhydroxybenzene developing agent and which renders gelatin oleophilic in developed areas.

17. The process of claim 16 wherein said silver halide is silver chloride, said developing agent is 4-phenyl catechol which is contained in said layer between said emulsion layer and said support, and said developing step (b) is accomplished by contacting said exposed emulsion layer with an alkaline solution.

18. The process of claim 16 wherein said printing plate is a positive-working presensitized printing plate comprising a support, a hardened gelatin photographic emulsion layer containing grains comprising a central core of photographic silver halide containing centers which promote deposition of photolytic silver and an outer shell covering said core comprising a fogged photographic silver halide that develops to silver without exposure.

19. The process of claim 18 which includes step (c) inking the developed areas with greasy printing ink and printing therefrom in a lithographic printing press.

20. The process of claim 18 wherein said emulsion has adsorbed to the fogged grains a halogen-accepting compound having an anodic polarographic halfwave potential which is more negative than -1.0.

21. The process of claim 20 wherein said halogen-accepting compound is a merocyanine dye.

22. The process of claim 20 wherein said emulsion also contains a sulfonated compound having the formula:

R-so.sub.3 m wherein R represents a polynuclear aromatic group and M represents a hydrogen atom or a water-soluble cation salt group.