Fogged Direct Positive Silver Halide Element Containing A Selenium Compound Sensitizer

Abstract

Direct-positive photographic elements are described which comprise uniformly fogged radiation-sensitive silver halide and a selenium compound of the formula : R -- Se -- Q wherein R is alkyl, aralkyl, alkaryl, aryl or a heterocyclic ring system and Q represents --Se--R' or --S--R' wherein R' has one of the significances given for R, halogen, cyano, or wherein each of R.sub.1, R.sub.2 and R.sub.3 is alkyl, aralkyl or aryl and X.sup.- is an anion but does not exist when R itself contains an anionic group. The selenium compounds increase the speed of the direct-positive silver halide elements.

Description

The present invention relates to radiation sensitive direct-positive silver halide emulsions, more particularly to direct-positive emulsions having an improved radiation-sensitivity.

It is known that positive photographic images can be obtained without previously forming a negative silver image by the use of silver halide emulsions that have been pre-fogged by overall exposure to actinic radiation or by overall chemically fogging with reducing substances. Upon image-wise exposure of the pre-fogged emulsions the development centers formed by said fogging are destroyed at the exposed areas and remain at the unexposed areas. By subsequent development by means of silver halide developers a direct-positive image is obtained.

The image-wise exposure of the pre-fogged silver halide emulsion, which results in an image-wise destruction of the development specks, can be accomplished for instance by utilizing the solarization effect or the Herschel-effect. Solarization is a reversal phenomenon which is produced by over-exposure of a silver halide emulsion wherein a uniform latent image corresponding to the critical exposure has been formed by pre-fogging, said over-exposure resulting in a destruction of the development centers.

According to the Herschel-effect a latent image can be reversed by a second exposure to red or infra-red light. The sensitive layer is first fogged uniformly to its maximum density with blue-violet light or by chemical means for example reducing agents and is then image-wise exposed with light of long wavelength, preferably at low intensity and for a long time. The Herschel-effect may, however, also be obtained with light of shorter wavelength such as yellow light if the photographic material has been treated with so-called "desensitizing dyes."

From the foregoing it is clear that the production of direct-positive images can proceed according to two different techniques. In both processes an increase in sensitivity is pursued.

It has now been found that the speed of direct-positive elements can be increased by incorporating into said elements a selenium compound corresponding to the following general formula:

R -- Se -- Q

wherein:

R stands for an alkyl, aralkyl, alkaryl, aryl or heterocyclic group which groups may carry substituents for example halogen, e.g. chlorine and bromine, carboxyl, sulpho, nitro, amino, substituted amino, acylamino e.g. acetamide, carbamoyl, etc.,

Or form part of a heterocyclic ring system, and

Q stands for an electron-withdrawing group or atom with strong leaving properties for example the group --Se--R' or S--R' wherein R' has the same significance as R, halogen,

wherein each of R.sub.1, R.sub.2 and R.sub.3 stands for alkyl, aralkyl or aryl and X.sup.- stands for an anion but does not exist when R itself contains an anionic group.

Therefore, in accordance with the present invention a direct-positive photographic silver halide element is provided comprising radiation-sensitive silver halide, which has been uniformly fogged, and a selenium compound as defined above.

The present invention further provides a method of producing direct-positive images which comprises image-wise exposing a direct-positive element having prefogged silver halide and comprising a selenium compound as defined above, and developing said element in a silver halide developing solution.

The selenium compounds corresponding to the above general formula are especially suitable for increasing the sensitivity of direct-positive emulsions designed to exhibit the solarization technique. However, it was found that they also promote reversal by the Herschel effect.

The direct-positive emulsions can be prepared according to known methods. The silver halide composition is not critical and may consist of silver chloride, silver bromide, silver iodide or mixtures thereof.

As is known, fogging of the emulsions may occur by means of an overall uniform exposure to actinic radiation or by means of chemically fogging agents e.g. by means of reducing agents such as hydrazine, hydroxylamine, formaldehyde, tin(II)chloride, thiourea dioxide also called formamidine sulphinic acid, etc.

In addition to the light-sensitive silver halide, the direct-positive emulsions may comprise all kinds of other known emulsion ingredients for instance compounds of metals that are more electro-positive than silver such as compounds of gold, platinum, palladium and iridium for example gold(III) chloride, potassium chloroaurate and (NH.sub.4).sub. 2 PdCl.sub.6, wetting agents, development accelerators, optical brightening agents, hardeners, stabilizers, electron-accepting compounds for example the known desensitizing dyes for direct-positive emulsions, etc.

Representative examples of selenium compounds which correspond to the above general formula and which have been found particularly suitable for increasing the reversal speed of direct-positive radiation-sensitive silver halide elements are those listed below. They can be prepared according to the methods described in the literature referred or as described hereinafter. ##SPC1##

Compound 3 was prepared by dissolving 158 g of compound 5, prepared as described below, in 400 ml of acetic acid with heating and then pouring the reaction mixture into a mixture of 1,800 ml of concentrated hydrochloric acid and 900 ml of water. The mixture was stirred and refluxed for 12 hours. Upon cooling, the benzoic acid formed was filtered off by suction and the filtrate concentrated by evaporation. The yellow residue was recrystallized from 90 percent methanol. Melting point: 182.degree.C.

Compound 5 was prepared by dissolving 0.5 mole of bromoethyl-benzamide in 300 ml of methanol and treating the solution while stirring with a solution of 0.55 mole of KSeCN in 300 ml of methanol. The mixture was refluxed for 30 minutes and cooled, whereupon the potassium bromide was filtered off by suction and the filtrate treated with 91 g of potassium hydroxide. Then, 300 ml of ice-water was added and the yellow precipitate collected after washing with methanol/water (1:1) and ether. Melting point : 158.degree.C.

Compound 6 was prepared by treating 1 mole of freshly prepared Na.sub.2 Se.sub.2 in aqueous solution with 1 mole of benzyl chloride dissolved in alcohol at 80.degree.C. After extraction with ether, drying the either extract and concentrating the extract by evaporation compound 6 having a melting point of 91.degree.C was obtained.

Compound 11 was prepared as follows:

To 0.5 mole of meta-chlorobromobenzene in 500 ml of ether, 12 g of magnesium and 0.5 mole of selenium were added. The mixture was stirred for a few hours whereupon it was poured into ice-water in the presence of 75 ml of hydrochloric acid. The ether layer was separated and dried whereupon an air-current was bubbled through the ether layer. After evaporation of the ether, the residual oil was distilled. Boiling point: 208.degree.-211.degree.C/2.5 mm.

Compound 19 was prepared as follows: 28.8 g (0.2 mole) of potassium selenocyanate were dissolved in 75 ml of water whereupon a solution was added, with stirring, of 24.4 g (0.2 mole) of propane sultone in 75 ml of alcohol. The mixture was refluxed for 15 min., filtered and concentrated by evaporation. The residue was recrystallized from ethanol/water (1/1 ). Melting point: 258.degree.C.

Compound 27 was prepared as follows :

Twenty-two g of 2-methyl-6-amino-benzothiazole in 80 ml of 5N hydrochloric acid and 400 ml of water were diazotized at 0.degree.C with a solution of 10 g of sodium nitrite in 20 ml of water whereupon the solution was buffered to pH 4 by means of 75 g of sodium acetate. A solution of 28.8 g of potassium selenocyanate in 50 ml of water was added and the precipitate formed was filtered by suction. The product was recrystallized from ethylene glycol monomethyl ether. Melting point: 137.degree.C.

Compound 28 was prepared as follows: 0.05 mole of tributylphosphine selenide and 0.1 mole of propane sultone were heated for 30 minutes on an oil bath of 100.degree.C. The oil obtained was treated with ether and then with acetone. The precipitate formed was dried over phosphorus trioxide.

The tributylphosphine selenide used was prepared by allowing to react equimolecular amounts of tributylphosphine and selenium in toluene, first at room temperature and then by reflux. The solution was filtered and the toluene removed by evaporation. The remaining liquid was distilled under reduced pressure. Boiling point: 177.degree.C/0.75 mm.

Compound 29 was prepared as follows: 0.03 mole of triphenyl-phosphine selenide, prepared as described in Inorg.Synth., 10, 157 (1967) in 15 g of dimethysulphate was heated for 30 min. at 100.degree. C. The solution was treated, after having been cooled, with ether whereupon the white precipitate formed was washed with ether and dried. The triphenylphosphine selenide used in the preparation of compound 29 was found itself to have a speed-increasing effect in direct-positive emulsions as described in the present invention.

The selenium compounds of use according to the invention may be used in amounts varying between very wide limits. The optimum amounts can be readily determined by trial. Generally they are used in amounts between 1 and 100 mg per mole of silver halide. They are added to the direct-positive silver halide emulsion preferably just before coating on a suitable support.

The selenium compounds as defined above are particularly suitable for use in direct-positive emulsions containing in the interior of the silver halide grains, centers promoting the deposition of photolytic silver.

Photographic emulsions comprising in the interior of the silver halide grains, centers promoting the deposition of photolytic silver are known, for instance, from United Kingdom pat. specification Nos. 1,011,062, 1,027,146 and 1,151,781.

According to United Kingdom Pat. specification No. 1,011,062 emulsions having ripening nuclei in the interior of the grains are prepared by admixture of a coarse-grained silver halide emulsion, which has been chemically ripened and comprises ripening nuclei at the surface of the grains, with a fine-grain, silver halide emulsion followed by physical ripening of the mixture whereby the large grains grow at the cost of the small grains so that the unripened fine-grain emulsion deposits around the ripened coarse-grain emulsion.

In United Kingdom Pat. specification No. 1,027,146 a process has been described and claimed according to which silver halide emulsions are prepared having a narrow grain-size distribution and containing composite silver halide granules of which the internal structure is different from the surface structure. According to one embodiment of this process a fine-grained silver halide emulsion with narrow-grain-size distribution is prepared first whereupon the precipitation of the silver halide is interrupted and the surface of the silver halide nuclei is chemically or physically modified e.g. by treatment with reducing agents such as hydrazine and derivatives, ascorbic acid and formaldehyde, by treatment with noble metal salts such as gold salts or by exposure to actinic radiation, and finally precipitation of the same or another silver halide is continued.

By fogging the emulsions formed according to these United Kingdom Pat. specifications Nos. 1,011,062 and 1,027,146 before or after coating on a support, a photographic direct-positive silver halide emulsion can be obtained.

According to United Kingdom Pat. specification No. 1,151,781 there is provided a method of making a photographic direct-positive material which comprises forming grains of a first light-sensitive silver salt, treating these grains so as to produce in them centers which promote the deposition of photolytic silver, covering the treated grains with a layer of a second light-sensitive silver salt and fogging the resulting emulsion before or after coating on a support.

Further details as regards the preparation of Direct-positive silver halide emulsions having in the interior of the silver halide grains centers promoting the deposition of photolytic silver can be found in the above United Kingdom Patent specifications.

In accordance with the present invention, direct-positive silver halide emulsions containing in the interior of the silver halide grains centers promoting the deposition of photolytic silver are preferably obtained by the following steps :

1. the formation of silver halide grains by mixing in an aqueous gelatin solution a dissolved water-soluble silver salt and a dissolved water-soluble alkali metal halide salt,

2. the interruption of the precipitation,

3. treatment of the silver halide grains (cores) so as to produce centers which promote the deposition of photolytic silver,

4. the further mixing of silver salt and halide salt solutions thus forming a shell of silver halide around the cores, and

5. the fogging of the resulting silver halide emulsion at the surface of the silver halide grains to such an extent that after a sufficient image-wise exposure of the emulsion layer to radiation to which the grains are sensitive, a direct-positive silver image can be obtained by treatment of the exposed emulsion in a silver halide developer.

In preparing suchlike direct-positive emulsions, the cores may be treated according to any of the known procedures for producing centers which promote the deposition of photolytic silver. Thus the cores may be fogged by exposure to radiation or they may be chemically sensitized. The cores are preferably chemically sensitized and any of the usual procedures may be used therefor. Thus the cores may be digested with naturally active gelatin or with a labile sulphur compound. They are preferably chemically sensitized with a gold or other noble metal sensitizer or with reduction sensitizers or with a mixture of both. Gold sensitization preferably occurs by means of a mixture of water-soluble gold salts such as gold(III)chloride, and thiocyanates forming complexes with gold and having a solvent action on the silver halide grains e.g. alkali metal and ammonium thiocyanates. The cores may also contain speed increasing addenda such as quaternary ammonium compounds and compounds of the polyethylene glycol type.

The precipitation of the silver halide is preferably carried out by simultaneous addition of the halide salt solution and the silver salt solution to the gelatin solution and keeping the pAg value as constant as possible during the precipitation.

The composite silver halide granules formed may contain silver chloride, silver bromide or silver iodide or mixtures thereof. For example, a core of silver bromide can be coated with a layer of silver chloride or a mixture of silver bromide and silver iodide, or a core of silver chloride can have deposited thereon a layer of silver bromide. However, the composite silver halide granules preferably have an interior core of silver bromide around which silver bromide has been deposited.

Gelatin is preferably used as vehicle for the composite silver halide granules but like in other silver halide emulsions the gelatin may be wholly or partly replaced by other hydrophilic colloids, for example colloidal albumin, zein, agar-agar, polyvinyl alcohols, polyvinyl acetals, hydrolyzed cellulose esters or ethers, etc.

In addition to the selenium compounds the direct-positive silver halide emulsions comprising in the interior of the silver halide grains centers promoting the deposition of photolytic silver may also comprise all kinds of known emulsion ingredients for direct-positive emulsions for example wetting agents, development accelerators, optical brightening agents, hardeners, stabilizers, electron-accepting compounds such as the known desensitizing dyes for direct-positive emulsions, blue speed increasing merocyanine dyes as described in United Kingdom Patent specification No. 1,186,718, carbocyanine dyes, etc.

The emulsions containing in the interior of the silver halide grains centers promoting the deposition of photolytic silver are uniformly fogged at the surface of the grains. Fogging may occur by exposure of the emulsion, before or after coating, to actinic radiation, but preferably chemical fogging is applied. The emulsion may be chemically fogged for instance, by sensitizing to fog using one of the chemical sensitization methods outlined above. For this purpose reducing agents, e.g. hydrazine, hydroxylamine, tin(II) salts, such as stanno chloride, ascorbic acid, formaldehyde, thioureadioxide, etc. can be successfully used.

According to a special embodiment of the present invention, the centers promoting the deposition of photolytic silver in the interior of the silver halide grains are produced by chemical sensitization with a gold compound together with reduction sensitizer for example thiourea dioxide also called formamidine sulphinic acid and derivatives thereof whereas the external fogging of the composite silver halide granules occurs with the same reduction sensitizer. The use of thiourea dioxide or formamidine sulphinic acid and derivatives thereof is known from United Kingdom Patent specification No. 789,823 and from U.S. Pat. Specs. Nos. 2,983,609 and 2,983,610.

The emulsions of the invention can be coated on any of a wide variety of supports in accordance with usual practice to provide sensitive materials of the invention. When these materials are exposed to an image and thereafter developed in a conventional developer for example a p-monomethylaminophenol/hydroquinone developer a direct-positive silver image is formed. This image can thereafter be fixed or stabilized by conventional techniques.

Direct-positive silver halide materials comprising selenium compounds of the above general formula and having centers promoting the deposition of photolytic silver in the interior of the silver halide grains prepared according to the processes described above by interrupted precipitation can also be developed by means of so-called lith-developers, as described in co-pending application No. 17, 162/69 in order to obtain direct-positive images with very steep gradation curve. By a "lith"-developer there is understood a developing composition for light-exposed silver halide containing a bisulphite-addition compound of an aliphatic aldehyde or ketone e.g. formaldehyde bisulphite, and hydroquinone as the sole developing agent.

The following examples illustrate the present invention.

EXAMPLE 1

A washed silver chloroiodide (99.65 mole percent of chloride and 0.35 mole percent of iodide) emulsion comprising per kg an amount of silver halide equivalent to 75 g of silver nitrate was melted by heating to 50.degree.C and the pH was adjusted to 7 by means of sodium hydroxide. Per kg of emulsion were added 20 ml of a 4 percent aqueous formaldehyde solution and 100 ml of a 0.1 percent aqueous solution of sodium hydroxide. The emulsion was kept at 50.degree.C for 50 min. Thereupon the pH was reduced to 6 by means of an aqueous solution of hydrogen chloride, whereupon 200 mg of the desensitizing compound 1-(2,4-dinitroanilino)-1,4-dihydro-4,4,6-trimethyl-2-pyrimidinethiol the preparation of which is described in example 2 of the United Kingdom Patent specification No. 796,873 were incorporated into the emulsion.

After addition of the necessary coating aids, the emulsion was divided into two aliquot portions A and B. Per kg of emulsion portion B 16 mg of the selenium compound 23 were added whereas to emulsion portion A no selenium compound was added. Both emulsions were then coated in a similar way on a polyethylene terephthalate support and dried.

Both light-sensitive materials were exposed reflectographically through a grey test wedge, a yellow filter layer being applied to the back of the polyester base. During the exposure the emulsion layers stood in contact with an original containing white and black areas. The materials were then developed for 120 sec. at 20.degree.C in a developing bath of the following composition:

water (40.degree.C) 800 ml p-monomethylaminophenol sulphate 1.5 g anhydrous sodium sulphite 25 g hydroquinone 6 g anhydrous sodium carbonate 40 g potassium bromide 1 g water to make 1000 ml

The sensitometric results obtained are listed in the table below.

The speed (S) is expressed on a percentage basis with respect to the reference emulsion A, the speed of which is arbitrarily given the value 100.

The density in the exposed areas (minimum density) is a measure of the brightness of the whites and is denoted in the table by D.sub.min whereas the density in the unexposed areas (maximum density) is denoted in the table by D.sub.max.

TABLE

Material D.sub.min D.sub.max S __________________________________________________________________________ A 0.01 4.20 100 B 0.01 4.20 118 __________________________________________________________________________

EXAMPLE 2

Example 1 was repeated with the only difference that development took place using a "lith"-developer having the following composition:

hydroquinone 13 g formaldehyde bisulphite 50 g anhydrous sodium carbonate 60 g potassium bromide 1.5 g the oxyethylene units containing polycondensation product prepared as described in preparation 2 of the United Kingdom Patent Specification 920,637 0.5 g water to make 1000 ml

The sensitometric results obtained are listed in the table below.

Material D.sub.min D.sub.max S __________________________________________________________________________ A 0.01 4.30 100 B 0.01 4.20 135 __________________________________________________________________________

EXAMPLE 3

75 ml of a 3N aqueous solution of potassium bromide and 75 ml of a 3N aqueous solution of silver nitrate were simultaneously introduced at a rate of 10 ml per minute into a precipitation flask containing 650 ml of a 4 percent by weight aqueous gelatin solution to which at 50.degree.C an aqueous solution of ammonia was added in order to obtain a pH-value of 9.3. During precipitation of the silver bromide the pAg-value was adjusted to and maintained at a value corresponding to an E.M.F. of +20 mV (Ag/saturated reference calomel electrode).

Subsequently, the internal ripening was effected. For that purpose, 3.5 ml of a 10.sup..sup.-2 molar solution of formamidine sulphinic acid were added and the emulsion digested for 30 minutes at 50.degree.C. Then the pH was lowered to 6.5 and 1.5 ml of an aqueous 0.08 percent by weight solution of gold(III) chloride and 1.5 ml of aqueous 2 percent by weight solution of ammonium thiocyanate were added. After said addition the digestion was continued for 10 minutes at 50.degree.C.

The precipitation was then continued (second precipitation) as follows : an aqueous solution of ammonia was added to obtain a pH value of 9.3 whereupon 665 ml of an aqueous 3N potassium bromide solution and 665 ml of an aqueous 3N silver nitrate solution were introduced at a rate of 19 ml per minute. During precipitation the pAg value was maintained at a value corresponding to an E.M.F. of + 20 mV (Ag/saturated calomel electrode). The emulsion was solidified after the addition of 234 g of gelatin was washed in the usual way.

After washing, the pAg was again adjusted to a value corresponding to an E.M.F. of + 20 mV (Ag/saturated calomel electrode) and the pH of the emulsion was adjusted to 6.5.

The external fogging of the composite silver bromide granules formed was then carried out for 80 minutes at 55.degree.C with 9 ml of a 10.sup..sup.-2 molar aqueous solution of formamidine sulphinic acid per kg emulsion, containing 110 g of silver bromide and 80 g of gelatin.

After the fogging treatment, the emulsion was divided into several aliquot portions. To each portion, were then added per kg 1 g of saponin as coating aid and one of the selenium compounds listed in the table below in the amount given. Just before coating 500 mg of formaldehyde were added per kg for hardening purposes. The various emulsion samples were then coated on a polyethylene terephthalate resin support and dried.

The materials formed were exposed to incandescent bulb light through a grey test wedge and developed for 120 sec. at 20.degree.C in a developing bath having the composition given in example 1.

The sensitometric results obtained with freshly prepared materials and with materials stored for 36 hours at 57.degree.C and 34 percent of relative humidity are listed in the table hereinafter.

In this table, the speed (S) is expressed on a percentage basis with respect to freshly prepared reference emulsions, which comprise no selenium compounds and for which arbitrarily a total speed of 100 is given.

The density in the exposed areas (minimum density) is a measure of the brightness of the whites and is denoted in the table by D.sub.min.

In the table are also given the maximum density obtained with freshly prepared material (D.sub.max) as well as the silver bromide content per sq.m of the materials examined. ##SPC2##

EXAMPLE 4

Example 3 was repeated with the only difference that development took place using the "lith"-developer of example 2.

The sensitometric results obtained are listed in the table below. ##SPC3##

Claims

We claim:

1. A direct-positive photographic element comprising uniformly fogged radiation-sensitive silver halide and a selenium compound corresponding to the following formula:

R -- Se -- Q

wherein:

R stands for an alkyl group, an aralkyl group, an alkaryl group, an aryl group or a heterocycle,

Q stands for --Se--R' or --S--R' wherein R' stands for an alkyl group, an aralkyl group, an alkaryl group, an aryl group or a heterocycle; halogen; --CN or

wherein each of R.sub.1, R.sub.2 and R.sub.3 stands for alkyl, aralkyl or aryl and X.sup.- stands for an anion but does not exist when R itself contains an anionic group, said selenium compound being present in an amount sufficient to increase the speed of said direct-positive element.

2. A direct-positive photographic element according to claim 1, wherein the said selenium compound is present in an amount of from about 1 mg - 100 mg per mole of silver halide.

3. A direct-positive photographic element according to claim 1, wherein the silver halide has been uniformly fogged by chemical means.

4. A direct-positive photographic element according to claim 3, wherein the silver halide has been uniformly fogged by means of reducing agents.

5. A direct-positive photographic element according to claim 1, wherein the fogged silver halide emulsion comprises silver halide grains having an interior core of silver halide comprising centers promoting the deposition of photolytic silver and an outer shell of silver halide.

6. A direct-positive photographic element according to claim 5, wherein the said centers are formed by chemical sensitization of the interior core of silver halide.

7. A direct-positive photographic element according to claim 6, wherein said centers are formed by treatment of the interior core of silver halide with a reduction sensitizer and/or noble metal compound.

8. A direct-positive photographic element according to claim 7, wherein said centers are formed by treatment with a reduction sensitizer and a gold sensitizer.

9. A direct-positive photographic element according to claim 8, wherein fogging of the silver halide emulsion comprising silver halide grains having an interior core of chemically sensitized silver halide and an outer shell of silver halide, is effected by treatment with a reduction sensitizer.

10. A direct-positive element according to claim 9, wherein the reduction sensitizer used in the chemical sensitization of the silver halide core is the same as the reduction sensitizer used for fogging the silver halide emulsion.

11. A direct-positive element according to claim 10, wherein said reduction sensitizer is thiourea dioxide.

12. A direct-positive element according to claim 5, wherein the silver halide of the interior core as well as the silver halide of the outer shell is silver bromide.

13. A direct-positive photographic element according to claim 2, wherein said selenium compound is

14. A direct-positive photographic element according to claim 2, wherein said selenium compound is

15. A direct-positive photographic element according to claim 2, wherein said selenium compound is

16. A direct-positive photographic element according to claim 2, wherein said selenium compound is

17. A direct-positive photographic element according to claim 2, wherein said selenium compound is