US3531288A - Direct positive silver halide emulsions containing excess iodide - Google Patents

Description

United States Patent O1 3,531,288 Patented Sept. 29, 1970 3,531,288 DIRECT POSITIVE SILVER HALIDE EMULSIONS CONTAINING EXCESS IODIDE Evan T. Jones, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New York No Drawing. Filed Feb. 24, 1967, Ser. No. 618,320

Int. Cl. G03c 1/08 US. Cl. 96-108 24 Claims ABSTRACT OF THE DISCLOSURE Direct positive photographic silver halide emulsion, the halide of which is at least 50 mole percent bromide, contains an electron acceptor and has iodide on the surface thereof.

This invention relates to photographic emulsions and methods for their preparation. In one aspect, it relates to improved direct positive photographic emulsions. In another aspect, it relates to improved processes for preparing direct positive photographic emulsions.

Fogged direct positive photographic silver halide emulsions described in the literature have slow speeds. It would be highly desirable to provide novel direct positive photographic emulsions which exhibit increased speed, and processes for preparing such emulsions.

Direct positive photographic emulsions comprising silver halide grains in which at least 50 percent of the silver halide is bromide, are especially useful. These direct positive emulsions have high contrast and low background, i.e., low minimum density in exposed areas.

One object of this invention is to provide novel direct positive photographic silver halide emulsions.

Another object of this invention is to provide a process for preparing novel photographic silver halide direct positive emulsions.

A further object of this invention is to provide novel direct positive photographic silver halide emulsions which demonstrate increased speed, and processes for preparing such emulsions.

Still another object of this invention is to provide novel photographic elements.

Other objects of this invention will be apparent from the disclosure herein and the appended claims.

In accordance with one embodiment of this invention, a substantially uniformly fogged direct positive photographic emulsion is provided which comprises silver halide grains, the halide of said silver halide being at least 50 mole percent bromide, said silver halide grains including an electron acceptor and, on the surface thereof, a suflicient quantity of iodide to increase the speed of said silver halide grains. The quantity of iodide employed is in addition to any iodide present in said silver halide emulsion as mixed silver halide.

In another embodiment of this invention, novel photographic elements are provided comprising a support having coated thereon the novel emulsions described herein.

In another embodiment of this invention, a process is provided for improving the speed of a substantially uniformly fogged direct positive photographic emulsion comprising silver halide grains, the halide of said silver halide being at least 50 mole percent bromide, which comprises providing said grains with an electron acceptor and contacting the surface of the silver halide grains with a sufficient quantity of water soluble iodide salt to effectively increase the speed of the silver halide grains.

Any suitable water soluble iodide salt can be used in the practice of this invention. Typical useful iodide salts include the ammonium, potassium, lithium, sodium, cadmium and strontium iodide salts. In carrying out the processes of this invention, the water soluble iodide salt is added to the emulsion at a concentration sufiicient to effectively increase the speed of the emulsion. The addition of about .002 to about .03 mole iodide salt (e.g., about .5 to 5 grams potassium iodide) per mole of silver produces useful increases in speed. Preferred concentration ranges for most emulsions are from about .003 to about .012 mole water soluble iodide salt per mole of silver in the emulsion. Stated another way, the surface of the silver halide grains of the invention carry from about .002 to about .03 mole iodide per mole of silver, and preferably from about .003 to about .012 mole iodide permole of silver. This concentration of iodide is, of course, in addition to any iodide uniformly present throughout the grains as mixed silver halide (such as that iodide present in silver bromoiodide or silver chlorobromoiodide emulsions).

This invention is applicable to direct positive silver halide emulsions which contain an electron acceptor on the surface of the grain, and to direct positive silver halide emulsions which contain, within the emulsion grains, centers or sites which accept (or trap) electrons. When the latter type emulsion is employed, additional electron acceptors can be adsorbed to the surface of the silver halide grains, if desired.

The electron 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. The electron acceptors useful herein have an anodic polarographic potential and a cathodic polarographic potential which, when added together, give a positive sum. Cathodic measurements can be made with a 1 10 molar solution of the electron 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 for the most positive cathodic wave being designated E Anodic measurements can be made with 1 10 molar aqueous solvent solutions, for example methanolic solutions of the electron acceptor which are 0.05 molar in sodium acetate and 0.005 molar in acetic acid using a carbon paste of pyrolytic graphite electrode, with the volumetric half peak potential for the most negative anodic response being designated E In each measurement, the reference electrode can be an aqueous silversilver 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, New York, 1954; Polarography, by Kolthoff and Lingane, 2nd edition, Interscience Publishers, New York, New York, 1952; Analytical Chemistry, 36, 2426 (1964) by Elving; and Analytical Chemistry 30, 1576 (1958) by Adams. Signs are according to IUPAC, Stockholm Convention 1953.

Advantageously, the electron acceptors used herein also provide spectral sensitization such that the ratio of minus blue relative speed to blue relative speed of the emulsion is greater than 7, and preferably greater than 10, when exposed to a tungsten light source through Wratten No. 16 and No. 35 plus 38A filters respectively. Such electron acceptors can be termed spectrally sensitizing electron acceptors. However electron acceptors can be used which do not spectrally sensitize the emulsion.

An especially useful class of electron acceptors which can be used in the direct-positive photographic silver halide emulsions and processes of this invention are cyanine dyes, such as the imidazo[4,5-b]quinoxaline dyes. Dyes of this class are described in Brooker and Van Lare Belgian Pat. 660,253, issued Mar. 15, 1965. In these dyes, the imidazo[4,5-b1quinoxaline nucleus is attached,

through the Z-carbon atom thereof to the methine chain.

Very good results are obtained with Z-aromatically substituted indole dyes, e.g., cyanine dyes containing an indole nucleus aromatically substituted in the 2-position, i.e., a cyanine dye containing a 2-aromatically substituted indole nucleus. Advantageously, such dyes also include a densenitizing nucleus in addition to the indole nucleus. desensitizing nucleus is one which, when converted to a symmetrical carbocyanine dye and added to a silver chlorobromide emulsion containing mole percent chloride and mole percent bromide, at a concentration in the range of about 0.01 to about 0.2 g. of dye per mole of silver, causes, by electron trapping, at least an loss in speed to blue radiation, and preferably more than a or loss in blue speed. One useful class of spectral sensitizing electron acceptors suitable for use in this invention has the following general formula:

where L represents a methine chain containing from 2 to 3 carbon atoms; A represents a 2-aromatically substituted indole nucleus attached to the methine chain through the 3-carbon atom of the indole nucleus; and B represents an organic heterocyclic nucleus, said nucleus being, where L represents a methine chain of 2 carbon atoms, a desensitizing nucleus to provide an unsymmetrical dimethine cyanine dye, and, where L represents a methine chain of 3 carbon atoms, B represents a 2-aromatically substituted indole nucleus attached to the methine chain through the 3-carbon atom of the indole nucleus. An especially useful desensitizing nucleus, where L is a methine chain containing 2 carbon atoms, is an imidazo[4,5-b1quinoxaline nucleus attached through the 2-carbon atom itself to the methine chain. Spectral sensitizing electron acceptors of this type are dyes and can be prepared using any of the methods generally used for preparing such dyes. One convenient method involves refluxing, in a suitable solvent, a carboxaldehyde derivative of a 2-aromatically substituted indole with an alkyl substituted quaternary salt of a compound containing the desired desensitizing nucleus. For example, a 2-aromatically substituted indole-3-carboxaldehyde can be refluxed in a solvent such as acetic anhydride with a 2- alkylimidazo[4,5-b1quinoxalinium salt or a 2-alkylene pyrrolo[2,3-b]pyridine compound to provide the desired dye.

A preferred group of spectral sensitizing electron acceptors employed herein has the following general formula:

FormulaI 1.11 N -L= C (117) R3 i Rn A N i r t R, R1 5 2& wherein L represents a methine linkage, e.g., -CH C(CH C(C H etc.; A represents an aromatic nucleus, such as a phenyl nucleus which can contain various groups, such as alkyl (e.g., methyl, ethyl, propyl, butyl, etc.), alkoxy (e.g., methoxy, ethoXy, propoxy, butoxy, etc.), halogen groups such as Br, C1 or F, aryl such as phenyl, or A can be a heterocyclic aromatic nucleus, preferably containing from 5 to 6 carbon atoms, and the hetero atom is preferably nitrogen, sulfur or oxygen; R and R each represents a hydrogen atom, a halogen atom such as Cl, Br or F, an alkyl or alkoxy substituent such as methyl, ethyl, propyl, butyl, methoxy, propoxy, hydroxy ethyl, etc.; or, R and R taken together, represent the atoms necessary to complete a fused aromatic ring having 6 carbon atoms; R represents an alkyl substituent (including substituted alkyl) preferably containing from 1 to 8 carbon atoms, including methyl,

ethyl, propyl, butyl, octyl, sulfoalkyl such as sulfopropyl or sulfobutyl, sulfatoalkyl such as sulfatopropyl or sulfatobutyl, carboxyalkyl such as carboxyethyl or carboxybutyl and the like; R and R each represents an alkyl substituent (including substituted alkyl), preferably containing from 1 to 18 carbon atoms, including methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, hexyl, dodecyl, octadecyl, benzyl, beta-phenylethyl, etc., sulfoalkyl such as sulfatobutyl; carboxyalkyl such as carboxyethyl and carboxybutyl; hydroxyalkyl such as hydroxyethyl, hydroxypropyl and hydroxybutyl; allyl, alkenyl such as propenyl and butenyl; alkynyl such as propargyl; cycloalkyl such as cyclobutyl and cyclohexyl; dialkylaminoalkyl such as dimethylaminoethyl and aryl such as phenyl, p-tolyl, otolyl, 3,4-dichlorophenyl, etc.; R has the same meaning as R or, taken together with R represents an alkylene group such as trimethine or dimethine; R represents halogen, CN or N0 n is an integer from O to 3, and, X represents an anion, preferably an acid anion such as chloride, bromide, iodide, p-toluenesulfonate, thiocyanate, sulfonate, methyl sulfate, ethyl sulfate, perchlorate, etc.

A related class of highly useful spectral sensitizing electron acceptors are pyrrolo[2,3-b]pyrido dyes, e.g., those having the following formula:

Formula. IIA R H iz X wherein R R and R each represents an alkyl group, such as methyl, ethyl, propyl or butyl, or an aryl group such as phenyl; L and X have the meaning given above; and Q represents a substituent selected from the group consisting of (l) L=Q wherein Q represents the atoms necessary to complete a desensitizing nucleus to form a trimethine cyanine dye, such as a 6-nitrobenzthiazole nucleus, a S-nitroindolenine nucleus, an imidazo[4,5- b]quinoxaline nucleus or a pyrrolo[2,3-b]pyrido nucleus, e.g.,

-o A l H iii: W

wherein R R R and R have the same meanings given above.

Another useful group of spectral sensitizing electron acceptors have the following general formula:

Formula II E ]L=LCY=O R3 R10 R R 9 A A1 69 Yr Ha lit R3 ltn where X, L, A, R R R and R have the meanings given above; A R R R and R have the same values, respectively, as A, R R R and R and, Y represents a hydrogen atom, an aryl group, such as phenyl, an alkyl (e.g., methyl, ethyl, propyl or butyl) or alkoxy (e.g., methoxy, ethoxy or propoxy) substituted phenyl, or a heterocyclic aromatic group, such as a thiophene radical. Dyes of this type can be prepared using the method described in Coenen et al. US Pat. 2,930,694, issued Mar. 29, 1960.

Symmetrical imidazo[4,5-blquinoxaline trimethine cyanine dyes, wherein each nucleus is attached through the 2-carbon atom thereof to the methine chain, are useful electron acceptors in the practice of this invention. Typical of such dyes are those having the following general formula:

and merocyanine dyes in which as least one nucleus, and preferably two nuclei, contain desensitizing substituents such as N Some other specific electron acceptors which give outstanding results in the practice of this invention are the reaction product of a cyanine dye with a halogenating agent. Preferred electron acceptors of this type are those which have a hydrogen atom of at least one methine group of the cyanine dye replaced with a halogen atom having an atomic Weight in the range of about 35 to about 127( i.e., chlorine, bromine or iodine atoms. In these compounds, one carbon atom linking the two nuclei thereof can carry two halogen atoms. Suitable halogen containing compounds can be represented by one of the fol- 15 lowing formulas:

wherein X, L, R, and R have the meanings given above, and R and R have the same values given for R and R each X is halogen such as Br, C1 or F and each n is an integer from O to 3. Dyes of this type can be prepared by the method described in Belgian Pat. 660,253, published Mar. 15, 1965.

Still another group of electron acceptors or pyrazolyl dyes, such as those having the following general formula:

wherein R n, R R L and X each have the meanings given in Formula I above, R and R each represents a substituent selected from the group consisting of hydrogen atom, an alkyl substituent, preferably containing 1 to 18 carbon atoms, as exemplified by methyl, butyl, octyl, dodecyl, octadecyl, an aryl substituent such as phenyl, ptolyl, 3,4-dichlorophenyl, etc., and R has the same value as R,,. Dyes of this type can be conveniently prepared by conventional techniques suitable for preparing such materials. For example, a suitable method involves refluxing in a suitable solvent such as acetic anhydride, a Z-alkylimidazo[4,5-blquinoxalinium salt with a pyrazole 4-carboxaldehyde. A typical dye of this type is l,3-diallyl2- l2 (3,5 dimethyl-1-phenyl-4-pyrazolyl)vinyl]imidazo [4,5-b1quinoxalinium iodide which has the formula:

CH2=CHOH2 Me wherein Z and Z each represents the non-metallic atoms necessary to complete a heterocyclic nucleus of the type used in cyanine dyes, such as a nucleus of the benzothiazole series (e.g., benzothiazole, 4-chlorobenzothiazole, 5-chlorobenzothiazole, 7-chlorobenzothiazole, 4-methylbenzothiazole, S-methylbenzothiazole, S-bromobenzothiazole, 4-phenylbenzothiazole, S-phenylbenzothiazole, 6- phenylbenzothiazole, 4-methoxybenzothiazole, S-methoxybenzothiazole, 5-iodobenzothiazole, 4 ethoxybenzothiazole, 5-ethoxybenzothiazole, 5 hydroxybenzothiazole, etc.); the naphthothiazole series (e.g., a-naphthothiazole, l3-naphthothiazole, 5-rnethoxy-fi-naphthothiazole, S-ethylfl-naphthothiazole, 8-methoxy-a-naphthothiazole, 7 methoxy-a-naphthothiazole, etc.); those of the benzoxazole series (e.g., benzoxazole, 5-chlorobenzoxazole, S-methylbenzoxazole, 5 phenylbenzoxazole, 5 methoxybenzoxazole, 5-ethoxybenzoxazole, S-hydroxybenzoxazole, etc.); those of the naphthoxazole series (e.g., a-naphthoxazole, fl-naphthoxazole, etc.); those of the benzoselenazole series (e.g., benzoselenazole, 5-chlorobenzoselenazole, S-methylbenzoselenazole, S-hydroxybenzoselenazole, etc.); those of the naphthoselenazole series (e.g., ot-naphthoselenazole, fl-naphthoselenazole, etc.); those of the quinoline series including the Z-quinolines (e.g., quinoline, 3-methylquinoline, S-methylquinoline, 7-methylquinoline, S-methyL quinoline, 6 chloroquinoline, 8-chloroquinoline, 6-mothoxyquinoline, 6-hydroxyquinoline, 8-hydroxyquinoline, etc.); the 4-quinolines (e.g., quinoline, 6-methoxyquinoline, 7 methoxyquinoline, S-methoxyquinoline, etc.); those of the isoquinoline series (e.g., the l-isoquinolines, the 3-isoquinolines, etc.); each L represents a methine linkage as described above; X and X each represents an atom selected from the group consisting of hydrogen, chlorine, bromine and iodine, at least one of X and X being chlorine, bromine or iodine; X represents a chlorine, bromine or iodine atom; R and R each represents alkyl, e.g., lower alkyl such as methyl, ethyl, propyl, isopropyl, butyl, secondary butyl, tertiary butyl, etc., a sulfoalkyl group in which the alkyl group has from 1 to 4 carbon atoms, such as sulfomethyl, sulfoethyl, sulfopropyl, sulfobutyl, etc., and a carboxyalkyl group in which the alkyl group has from 1 to 4 carbon atoms such as carboxymethyl, carboxyethyl, carboxypropyl, carboxybutyl, etc.; A represents an acid anion such as chloride, bromide, iodide, p-toluenesulfonate, thiocyanate, methyl sulfate, ethyl sulfate, perchlorate, and the like; y represents an integer of from 1 to 3 and d, m, n and p each represents a positive integer of from 1 to 2.

The halogen containing compounds described above 75 can be prepared by halogenating a cyanine dye with chlorine, bromine or iodine. Any suitable halogenating agent may be used, such as aqueous alcoholic (e.g., methanol or ethanol) solutions of the halogen, N-chlorosuccinimide, N-bromosuccinimide, N-iodosuccinimide, or a commercially available halogen-pyrrolidone complex, such as a bromo-pyrrolidone complex sold by General Aniline and Film Corp. Using such halogenating agents causes replacement by halogen of a hydrogen atom in the methine chain. In carbocyanines, or dicarbocyanines, it is believed that halogen substitution occurs on a terminal carbon atom of the methine chain. As noted above, one linking carbon atom can carry two halogen atoms.

The compounds which accept electrons in the directpositive photographic silver halide emulsions and processes of this invention can be employed in widely varying concentrations. However, such compounds are preferably employed at concentrations in the range of about 100 milligrams to about 2 grams of electron acceptor per mole of silver halide. Best results are obtained using from 300 to 600 milligrams electron acceptor per mol of silver halide. Specific examples of suitable electron acceptors include;

1,1-dimethyl-2,2-diphenyl-3,3-indoloearbocyanine bromide;

2,2-di-p-methoxyphenyl-1,l'-dimethyl-3,3-indolocarbocyanine bromide;

1,1'-dimethyl-2,2-8-triphenyl-3,3'-indolocarbocyanine perchlorate;

1,3-diallyl-2- 2- 9-methyl-3 -carbazolyl vinyl] imidazo [4,5-blquinoxalinium p-toluenesulfonate;

l,3-diethyl-1-methyl-2-phenyl imidazo[4,5-b]quinoxalino-3-indolocarbocyanine iodide;

1 1 ,3,3'-tetraethylimidazo [4,5 -b] quinoxalinocarbocyanine chloride;

6-chloro-1'-methyl-1,2,3-triphenylimidazo[4,5-b]quinoxalino-3-indolocarbocyanine p-toluenesulfonate;

6,6'-dichloro-1, 1-3,3'-tetraphenylimidazo[4,5-b1quinoxalinocarbocyanine p-toluenesulfonate;

1,1',3,3 -tetramethyl-2-phenyl-3-indolopyrrolo [2,3-b]

pyridocarbocyanine iodide;

1,1',3,3,3,3'-hexamethylpyrrolo[2,3-b1pyridocarbocyanine perchlorate;

1,1',3,S-tetramethyl-S-nitro-2'-phenylindo-3-indolocarbocyanine iodide;

1,1,3,3,3,3'-hexamethyl-5,5'-dinitroindocarbocyanine p-toluenesulfonate;

3'-ethyl-1-methyl-2-phenyl-6'-nitro-3-indolothiacarbocyanine iodide;

5'-chloro-1,3'-dimethyl-2-phenyl-6'-nitro-3-indolothiacarbocyanine p-toluenesulfonate;

5,5'-dichloro-3,3'-diethyl-6,6-clinitrothiacarbocyanine iodide;

pinacrytol yellow, S-m-nitrobenzylidenerhodanine;

S-m-nitrobenzylidene-3-phenylrhodanine;

1,3-diallyl-2-[2-(3,S-dimethyl-1-phenyl-4-pyrazolyl) vinyl]imidazo[4,5-b]quinoxalinium iodide;

3-ethyl-5-m-nitrobenzylidenerhodanine;

3-ethyl-5-( 2,4-dinitrobenzylidene) rhodanine;

5-o-nitrobenzyilidene-3-phenylrhodanine;

1',3-diethyl-6-nitr0thia-2'-cyanine iodide;

6-chloro-4-nitrobenzotriazole;

6-amino-1-methyl-2- l-methyl-(6'-quinolinium) vinyl] quinolinium dichloride;

4- (p-n-amyloxyphenyl -2,6-di-p-ethylphenyl) thiapyrylium perchlorate and the like.

If desired, the emulsions of the invention can be provided with a combination of electron acceptor and halOgen acceptor.

In carrying out the processes of this invention, the electron acceptor and the water soluble iodide salt can be added to the emulsion in any order or simultaneously. The preferred order, however, is the addition of electron acceptor followed by addition of water soluble iodide salt. The preferred sequence gives the greatest increase in speed. No holding period is required between the addition of electron acceptor and iodide salt in this preferred sequence of addition. Thus, the electron acceptor can be added to a stirred emulsion followed directly, but with continued stirring, by the addition of iodide salt. However, a holding period can be used if desired.

The direct positive silver halide emulsions useful herein can be uniformly fogged in any suitable manner, such as by light or with chemical fogging agents. Chemical fogging agents are preferred. Typical useful chemical fogging agents include reducing agents such as stannous chloride, formaldehyde, thiourea dioxide and the like. In preferred embodiments of this invention, the emulsion is fogged by the addition thereto of a reducing agent, such as thiourea dioxide, and a compound of a metal more electropositive than silver, such as a gold salt (e.g., potassium chloroaurate) as described in British Pat. 723,019 (1955).

Useful concentrations of reducing agent and metal compound (e.g., metal salt) can be varied over a considerable range. As a general guideline, good results are obtained using about .05 to 40 mg. reducing agent per mole of silver halide, and 0.5 to 15.0 mg. metal compound per mole of silver halide. Best results are obtained at lower concentration levels of both reducing agent and metal compound.

As used herein, and in the appended claims, fogged refers to emulsions containing silver halide grains which produce a density of at least 0.5 when developed, without exposure, for 5 minutes at 68 F. in developer Kodak DK-SO having the composition set forth below, when the emulsion is coated at a silver coverage of mg. to 500 Potassium bromide Water to make 1 liter.

This invention can be practiced with direct positive emulsions of the type in which a silver halide grain has a water-insoluble silver salt center and an outer shell composed of a fogged water-insoluble silver salt that develops to silver without exposure. These emulsions can be prepared in various ways, such as those described in.

Berriman US. patent application Ser. No. 448,467, filed Apr. 15, 1965 now US. Pat. 3,367,778 issued Feb. 6, 1968. For example, the shell of the grains in such emulsions may be 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. The silver salt of the shell is sufficiently fogged to produce a density of at least about 0.5 when developed for 6 minutes at 68 F. in developer A below when the emulsion is coated at a silver coverage of mg. per square foot. 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 the like fogging means well known to those skilled in the art. While the core need not be sensitized to fog, the shell is fogged. Fogging by means of a reduction sensitizer, a noble metal salt such as gold salt plus a reduction sensitizer, a sulfur sensitizer, high pH and low pAg silver halide precipitating conditions,

9 and the like can be suitably utilized. The shell portion of the subject grains can also be coated prior to fogging.

DEVELOPER A G. N-methyl-p-aminophenol sulfate 2.5 Ascorbic acid 10.0 Potassium metaborate 35.0 Potassium bromide 1.0

Water to make 1 liter. pH of 9.6.

Before the shell of water-insoluble silver 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 techniques as described in the Berriman application referred to above. Silver salt cores containing centers attributable to a metal of Group VIII of the Periodic Table, e.g., palladium, iridium or platinum and the like, are especially useful since these centers also appear to function as electron acceptors. Chemical sensitization techniques of the type described by Antoine Hautot and Henri Saubeneir in Science et Industries Photographiques, vol. XXVIII, January 1957, pages 1 to 23 and January 1957, pages 57 to 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, e.g., 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 practice of this invention is particularly suitable for high speed direct positive emulsions comprising fogged silver halide grains and a compound which accepts electrons, as described and claimed in Illingsworth US. patent application Ser. No. 609,794, filed Jan. 17, 1967 now abandoned and titled Photographic Reversal Materials III. The fogged silver halide grains of such emulsions are such that a test portion thereof, when coated as a photographic silver halide emulsion on a support to give a maximum density of at least about one upon processing for six minutes at about 68 F. in Kodak DK-50 developer, has a maximum density which is at least about 30% greater than the maximum density of an identical coated test portion which is processed for six minutes at about 68 F. in Kodak DK-50 developer after being bleached for about 10 minutes at about 68 F. in a bleach composition of:

Potassium cyanide50 mg. Acetic acid (glacial)3.47 cc. Sodium acetate-11.49 g. Potassium bromide--l19 mg.

Water to make 1 liter.

The grains of such emulsions will lose at least about 25% and generally at least about 40% of their fog when bleached for ten minutes at 68 F. in a potassium cyanide bleach composition as described herein. This fog loss can be illustrated by coating the silver halide grains as a photographic silver halide emulsion on a support to give a maximum density of at least 1.0 upon processing for six minutes at about 68 F. in Kodak DK-SO developer and comparing the density of such a coating with an identical coating which is processed for six minutes at 68 F. in Kodak DK- developer after being bleached for about 10 minutes at 68 F. in the potassium cyanide bleach composition. As already indicated, the maximum density of the unbleached coating will be at least 30% greater, generally at least greater, than the maxi mum density of the bleached coating.

The silver halides employed in the preparation of the photographic emulsions useful in this invention include any of the photographic silver halides which contain at least 50 mole percent bromide as exemplified by silver bromide, silver bromoiodide, silver chlorobromide, and silver chlorobromoiodide and the like. Emulsion blends, e.g., blends of silver bromide and silver bromoiodide can be used. Also, the core of the silver halide grain can be composed of silver halide of different composition than that in the outer shell of the grain. In any case, the total bromide present as silver bromide or silver bromohalide should be at least 50 mole percent of the total silver halide in the emulsion grains.

Silver halide grains having an average grain size less than about one micron, preferably less than about 0.5 micron, give particularly good results. The silver halide grains can be regular and can be any suitable shape such as cubic or octahedral, as described and claimed in Illingsworth U.S. patent application Ser. No. 609,778, filed Jan. 17, 1967, now abandoned and titled Direct Positive Photographic Emulsions I. Such grains advantageously have a rather uniform diameter frequency distribution, as described and claimed in Illingsworth US. patent application Ser. No. 609,790, filed Ian. 17, 1967 now abandoned and titled Photographic Reversal Emulsions II. For example, at least by weight, of the photographic silver halide grains can have a diameter which is within about 40%, preferably within about 30% of the mean grain diameter. Mean grain diameter, i.e., average grain size, can be determined using conventional methods, e.g., as shown in an article by Trivelli and Smith entitled Empirical Relations Between Sensitometric and Size-Frequency Characteristics in Photographic Emulsion Series in The Photographic Journal, vol. LXXIX, 1949, pages 330-338. The fogged silver halide grains in these direct-positive photographic emulsions of this invention produce a density of at least 0.5 when developed without exposure for five minutes at 68 F. in Kodak DK50 developer when such an emulsion is coated at a coverage of 50 to about 500' mg. of silver per square foot of support. The photographic silver halides can be coated at silver coverages in the range of about 50 to about 500 milligrams of silver per square foot of support.

In the preparation of the above photographic emulsions, the electron acceptor, and iodide salt are advantageously incorporated in the washed, finished silver halide emulsion and should, of course, be uniformly distributed throughout the emulsion. The methods of incorporating such addenda in emulsions are relatively simple and well known to those skilled in the art of emulsion making. For example, it is convenient to add them from solutions in appropriate solvents, in which case the solvent selected should be completely free from any deleterious effect on the ultimate light-sensitive materials. Methanol, isopropanol, pyridine, Water, etc., alone or in admixtures, have proven satisfactory as solvents for the electron acceptors and halogen acceptors. The type of silver halide emulsions that can be sensitized with these dyes include any of those prepared with hydrophilic colloids that are known to be satisfactory for dispersing silver halides, for example, emulsions comprising natural materials such as gelatin, albumin, agar-agar, gum arabic, alginic acid, etc. and hydrophilic synthetic resins such as polyvinyl alcohol, polyvinyl pyrrolidone, cellulose ethers, partially hydrolyzed cellulose acetate, and the like. The binding agents for the emulsion layer of the photographic element can also contain dispersed polymerized vinyl compounds. Such compounds are disclosed, for example, in US Pats. 3,142,568; 3,193,386; 3,062,674 and 3,220,844 and include the water insoluble polymers of alkyl acrylates and methacrylates, acrylic acid, sulfoalkyl acrylates or methacrylates and the like.

The novel emulsions of this invention may be coated on any suitable photographic support, such as glass, film base such as cellulose acetate, cellulose acetate butyrate, polyesters such as polyethylene terephthalate, paper, baryta coated paper, polyolefin coated paper, e.g., polyethylene or polypropylene coated paper, which may be electron bombarded to promote emulsion adhesion, to produce the novel photographic elements of the invention.

This invention will be further illustrated by the following examples. Example 1 illustrates the practice of the invention with a direct positive fogged silver halide emulsion of the type having, within the silver halide grains, centers which accept electrons. As Example 1 shows, the mere addition of potassium iodide salt to the surface of such grains results in a substantial and unexpected increase in speed. The example further illustrates the substantial increase in speed when the combination of electron acceptor is added to the surface of the silver halide grains along with the iodide salt.

EXAMPLE 1 A gelatin silver bromoiodide (2.5 mole percent of the halide being iodide) emulsion containing iridium centers in the core is prepared by simultaneously adding at 70 C., over a period of about 35 minutes in a controlled pAg of 8.9, (a) 1200 milliliters of a 3.81 molar aqueous solution of potassium bromide and a 0.1 molar aqueous solution of potassium iodide and (b) 1275 milliliters of a 3.69 molar aqueous solution of silver nitrate, to 4000 milliliters of a gelatin aqueous solution. Fifty milligrams of potassium chloroiridate i.e., K. IrCl is added five minutes after the run is started. At the end of the run the emulsion is cooled to 40 C., gelatin is added to a total of 159 grams per mole of silver and the emulsion is chilled for about hours, noodled and Washed to remove soluble salts. The emulsion is then melted at 40 C., adjusted to a final weight of 12.4 kilograms, a pH of 6.5 and a pAg of 9.2. Thiourea dioxide, in aqueous solution, is added to the melted emulsion at a concentration of 0.002 gram per mole of silver. The melt is then digested for one hour at 55 C. While holding at 55 C., 40 milligrams per mole of silver of potassium chloroaurate are added to the melt from aqueous solution. The melt is then digested for minutes at 65 C. and then cooled to 40 C. The grains of this emulsion contain centers, attributable to iridium, which accept electrons. This emulsion is divided into several portions and potassium iodide alone, and together with the electron acceptor 1,3 diethyl 1' methyl 2 phenylimidazo [4,5-b]quinoxalino-3'-indolocarbocyanine iodide, as indicated in Table I. The electron acceptor, when employed, is added prior to the addition of potassium iodide with no holding (but with stirring) between the addition of the two addenda. The emulsions obtained are coated on a cellulose acetate film support at a coating rate of 400 mg. silver per square foot, and are chill set and dried.

The coatings are exposed on an intensity scale sensitometer, developed for 6 minutes in Kodak developer Dl9, fixed, washed and dried. Speeds are read at 0.3 below maximum density in each instance. The results are shown Example 2 illustrates the practice of the invention with a fogged direct positive silver halide emulsion which does not contain electron acceptors within the silver halide grains. As shown in Example 2, the addition of electron acceptor and a water-soluble iodide salt to the surface of such silver halide grains imparts a substantial and unexpected increase in the speed of such emulsion.

EXAMPLE 2 A gelatin silver bromoiodide emulsion (2.5 mole percent of the halide being iodide) and having an average grain size of about 0.5 micron is prepared by adding an 12 aqueous solution of potassium bromide and potassium iodide, and an aqueous solution of silver nitrate, simultaneously to a rapidly agitated aqueous gelatin solution at a temperature of C., over a period of about 35 minutes. The emulsion is chill-set, shredded and washed by leaching with cold water in the conventional manner. The emulsion is reduction-gold fogged by first adding 0.2 mg. of thiourea dioxide per mole of silver and heating for 60 minutes at 65 C. and then adding 4.0 mg. of potassium chloroaurate per mole of silver and heating for 60 minutes at 65 C. This emulsion is split into several portions, and electron acceptor and potassium iodide are added as indicated in Table II. The emulsions are coated and processed as described in Example 1, with the results shown in Table II below.

TABLE II Electron Dmnx. In Dmin. in Acceptor, KI, g./mole Relative Unexposed Exposed g./rnole Ag Ag Clear Speed Area Area.

(11 1.0 (l) I (0. 5) 162 1.77 0.02 I (0. 5) 1.0 214 1.82 0.02

1 No reversal.

' ethyl-2,2-cyanine chloride.

Results similar to those in Example 2 are obtained when the emulsion employed is a light fogged or a reduction fogged (e.g., with stannous chloride) silver halide emulsion, rather a reduction and gold fogged silver halide emulsion.

The invention has been described in detail with particular reference to preferred embodiments thereof, but, it will be understood that variations and modifications can be effected within the spirit and scope of the invention described hereinabove and in the appended claims.

I claim:

1. In a direct positive photographic emulsion comprising silver halide grains, the halide of said silver halide being at least 50 mol percent bromide, at least the outer shell of said grains being substantially uniformly fogged, and, said grains including an electron acceptor; the improvement which comprises a quantity of iodide, in addition to any iodide present in said grains as mixed silver halide, on the surface of said grains, said quantity of iodide being sufiicient to effectively increase the speed of said silver halide grains.

2. A direct positive photographic emulsion as defined in claim 1 wherein said quantity of iodide is from about .002 to about .03 mole per mole of silver.

3. A direct positive photographic emulsion as defined in claim 2 wherein said silver halide grains are chemically fogged.

4. A direct positive photographic emulsion as defined in claim 3 wherein said silver halide grains are fogged with a combination of a reducing agent and a compound of a metal more electropositive than silver; the halide of said silver halide grains being at least mole percent bromide; and, said grains having adsorbed to the surface thereof an electron acceptor which has an anodic polarographic potential and a cathodic polarographic potential which, when added together, give a positive sum.

5. A direct positive photographic emulsion as defined in claim 4 wherein said compound of a metal more electropositive than silver is a gold compound; and, said electron acceptor is selected from the group consisting of a 2-aromatically substituted indole dye, an imidazo[4,5- b]quinoxaline dye, a pyrrolo[2,3-b]pyrido dye, a nitrosubstituted dye and the reaction product of a cyanine dye with a halogenating agent.

6. A direct positive, photographic emulsion in accordance with claim 2 which comprises fogged silver halide grains, said grains being such that a test portion thereof, when coated as a photographic silver halide emulsion on a support to give a maximum density of at least about 1 upon processing for 6 minutes at about 68 F. in Kodak DK-SO developer, has a maximum density which is at least about 30% greater than the maximum density of an identical coated test portion which is processed for 6 minutes at about 68 F. in Kodak DK-SO developer after being bleached for about minutes at about 68 F. in a bleach composition of:

Potassium cyanide50 mg. Acetic acid (glacial)--3.47 cc. Sodium acetatel1.49 g. Potassium bromide-l 19 mg. Water to make 1 liter.

7. A direct positive, photographic emulsion in accordance with claim 2 which comprises fogged silver halide grains, at least 95 by weight, of said grains having a size which is within about 40% of the average grain size.

8. A direct positive photographic emulsion as defined in claim 2 in which said fogged silver halide grains comprise a central core of a water insoluble silver salt containing centers which promote the deposition of photolytic silver, and an outer shell covering said core comprising said fogged silver halide grains.

9. A direct positive photographic emulsion as defined in claim 8 wherein said central core contains centers attributable to Group VIII metal ions, which centers acicept electrons and promote deposition of photolytic s1 ver.

10. A direct positive photographic emulsion as defined in claim 9 wherein said outer shell has adsorbed to the surface thereof an electron acceptor which has an anodic polarographic potential and a cathodic polarographic potential which, when added together, give a positive sum.

11. A direct positive photographic silver halide emulsion comprising silver halide grains, the halide of said silver halide being at least 80 mole percent bromide, said grains comprising a central core of a water insoluble silver salt containing centers attributable to iridium ions, which centers accept electrons and promote the deposition of photolytic silver, and an outer shell covering said core comprising silver halide grains being substantially uni formly fogged with a combination of thiourea dioxide and potassium chloroaurate, said outer shell having on the surface thereof, (1) from about .003 to about .012 mole iodide per mole of silver, said iodide being in addition to any iodide present in said silver halide grains as mixed silver halide, and (2) l,3-diethyl-1-methyl-2- phenylimidazo [4,5 b]quinoxalino-3'-indolocarbocyanine iodide as electron acceptor.

12. A direct positive silver halide emulsion comprising silver halide grains, the halide of said silver halide being at least 80 mole percent bromide, said grains being substantially uniformly fogged with a combination of thiourea dioxide and potassium chloroaurate, and having on the surface thereof (1) from about .003 to about .012 mole iodide per mole of silver, said iodide being in addition to any iodide present in said grains as mixed silver halide, and (2) 1,3-diethyl 1 methyl-2'-phenylimidazo [4,5-b]quinoxalino-3'-indolocarbocyanine iodide as electron acceptor.

13. In the process for improving the speed of a direct positive photographic emulsion comprising silver halide grains, the halide of said silver halide being at least 50 mol percent bromide, at least the outer shell of said grains being substantially uniformly fogged, which process includes providing said grains with an electron acceptor: the improvement which comprises contacting the surface of said grains with sufiicient quantity of water-soluble iodide salt to effectively increase the speed of said silver halide grains, said quantity of iodide being in addition to any iodide present in said grains as mixed silver halide.

14. The process as defined in claim 13 wherein said quantity of iodide salt is from about .002 to about .03 mole per mole of silver.

15. The process as defined in claim 14 wherein said silver halide grains are chemically fogged, and said electron acceptor is adsorbed to the surface of said silver halide grains prior to contacting said grains with said iodide salt.

16. The process as defined in claim 15 wherein said silver halide grains are fogged with the combination of a reducing agent and a compound of a metal more electropositive than silver; the halide of said silver halide is at least mole percent bromide; and, said electron acceptor is adsorbed to the surface of said grains, said electron acceptor having an anodic polarographic potential and a cathodic polarographic potential which, when added together, give a positive sum.

17. The process as defined in claim 16 wherein said compound of a metal more electropositive than silver is a gold compound and said electron acceptor is selected from the group consisting of a Z-aromatically substituted indole dye, an imidazo[4,5-b]quinoxaline dye, a pyrrolo [2,3-b]pyrido dye, a nitro-substituted dye and the reaction product of a cyanine dye with a halogenating dye.

18. The process as defined in claim 15 wherein said silver halide grains comprise a central core of a water insoluble silver salt containing centers which promote the deposition of photolytic silver, and an outer shell covering said core comprising said fogged silver halide grains.

19. The process defined in claim 13 wherein said silver halide grains are such that a test portion thereof, when coated as a photographic silver halide emulsion on a support to give a maximum density of at least about 1 upon processing for 6 minutes at about 68 F. in Kodak DK50 developer, has a maximum density which is at least about 30% greater than the maximum density of an identical coated test portion which is processed for 6 minutes at about 68 F. in Kodak DK-SO developer after being bleached for about 10 minutes at about 68 F. in a bleach composition of Potassium cyanide-50 mg. Acetic acid (glacial)3.47 cc. Sodium acetate11.49 g. Potassium bromide119 mg. Water to make 1 liter.

20. The process defined in claim 13 wherein at least by weight, of said silver halide grains have a size which is within about 40% of the average grain size.

21. A method for increasing the speed of a direct positive photographic emulsion which comprises silver halide grains, the halide of said silver halide being at least 80 mole percent bromide, said grains comprising a central core of a water insoluble silver salt containing centers attributable to iridium ions, which centers accept electrons and promote the deposition of photolytic silver, and an outer shell covering said core comprising silver halide grains substantially uniformly fogged with a combination of thiourea dioxide and potassium chloroaurate, which comprises contacting said grains with about .003 to about .012 mole potassium iodide per mole of silver and 1,3-diethyl-1 -methyl-2'-phenylimidazo [4,5 -b quinoxalino-3'-indolocarbocyanine iodide as electron acceptor.

22. A method for increasing the speed of a direct positive silver halide emulsion comprising silver halide grains, a halide of said silver halide being at least 80 mole percent bromide, said grains being substantially uniformly fogged with thiourea dioxide and potassium chloroaurate, comprising contacting said grains with about .003 to about .012 mole potassium iodide per mole of silver and 1,3- diethyl 1' methyl-2'-phenylimidazo[4,5-b]quinoxalino- 3'-indolocarbocyanine iodide as electron acceptor.

23. A photographic element having coated thereon a direct positive emulsion as defined in claim 1.

24. A photographic element having coated thereon a direct positive emulsion as defined in claim 5.

References Cited UNITED STATES PATENTS 4/1952 Davey et a1. 9694 2/1962 Dersch et a1 96101 2/1968 Berriman 9664 NORMAN G. TORCHIN, Primary Examiner R. E. FICHTER, Assistant Examiner US. Cl. X.R.