Process of sensitizing converted-type silver halide emulsions with noble-metal salts

United States Patent Office

3,703,584

Patented Nov. 21, 1972

3,703,584

PROCESS OF SENSITIZING CONVERTED-TYPE SILVER HALIDE EMULSIONS WITH NOBLEMETAL SALTS

Robert Franklin Motter, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y.

No Drawing. Filed Aug. 20, 1970, Ser. No. 65,696

Int. Cl. G03c 1/28

U.S. Cl. 96—108 IO· Claims

ABSTRACT OF THE DISCLOSURE

Conversion-type silver halide emulsions are made by adding a noble-metal salt to the emulsion before or during the conversion step. In one embodiment, gold salts, which are generally known as silver halide sensitizers, are added after initial formation of the grain in the conversion process and before 80% of the conversion step is complete. Improved internal-radiation sensitivity is observed in converted-type emulsions made by this process.

This invention relates to methods for sensitizing conversion-type silver halide emulsions and to the products produced thereby. In one aspect, this invention relates to a process wherein a noble-metal salt, and preferably a gold salt, is added to the silver halide emulsion before or during the initial stages of the conversion step. In another aspect, this invention relates to converted-type silver halide emulsions with improved light sensitivity (i.e., photographic speed).

It is known in the art to make converted-type silver halide emulsions such as disclosed in Davey and Knott, U.S. Pat. 2,592,250. Generally, emulsions of this type are made by converting a more soluble silver halide into a less soluble silver halide, e.g., a silver chloride emulsion is converted in the presence of a water-soluble bromide, the amounts of which are dependent on the desired final composition, such as silver chlorobromide or silver bromide including conversion only on the surface of the more soluble grains. While converted-type emulsions have been found to be very useful in some applications, it has been difficult to improve the photographic speed of converted-type emulsions as they cannot be chemically sensitized as the ordinary surface latent image emulsion, as mentioned in Japanese Pat. 29,405/68. Improved converted-type silver halide emulsions having better photographic response characteristics would be desirable.

I have now found that when noble-metal salts, and preferably gold salts, are added to the silver halide emulsion before 80% of the conversion step has been carried out, it will provide improved total sensitivity of the silver halide emulsion. This result is quite unexpected, since some of the other chemical sensitizers do not provide the same increase in photographic speed when added during the conversion step.

In one embodiment of this invention, the noble-metal salt is added before 50% of the conversion has taken place.

In a preferred embodiment of this invention, the noblemetal salt is a gold salt.

In another preferred embodiment, the gold salt is added in combination with an antifoggant.

In another highly preferred embodiment, this invention relates to a method of improving the internal sensitivity of a converted-type silver halide emulsion.

The converted-halide silver halide grains employed in the practice of this invention are preferably unfogged and have a halide content of at least 50 mole percent, and preferably at least 80 mole percent, bromide and contain up to 10 mole percent, and preferably less than 5 mole percent, iodide, any remaining halide being chloride. Especially good results are obtained with silver halide grains containing about 90 mole percent bromide and about 10 mole percent chloride.

The term “converted-halide silver halide grains” is employed herein as a word of art and denotes silver halide grains prepared iby forming an emulsion of silver salt grains consisting at least partly of a silver salt more soluble in water than silver bromide and “converting” at least 10 a portion of such grains to silver bromide or silver bromoiodide salts.

More specifically, such silver halide grains can be prepared by forming a silver salt which is (1) more soluble in water than silver bromide and (2) has a solubility of 15 less than about .02 gram, and preferably less than about .00002 gram, per 100 milliliters of water at 20° C. Useful silver salts more soluble than silver bromide include silver chloride, silver thiocyanate and silver citrate. Such salts are conveniently formed by adding separate solutions of 20 silver nitrate in water and a suitable aqueous alkali metal salt solution, such as potassium chloride, potassium citrate or potassium thiocyanate, to an aqueous gelatin solution containing a small amount of the alkali metal salt. The first two solutions are preferably added slowly 25 to the aqueous gelatin solution. The temperature of all the solutions is preferably maintained between about 20° and 80° C. After the formation of the silver salts more soluble than silver bromide, a suitable bromide salt can be added to convert at least part of the more soluble silver salt to 30 silver bromide. If desired, a suitable iodide salt can also be added to form silver bromoiodide. Potassium bromide and potassium iodide are especially useful for this purpose. When silver chloride is used as the silver salt more soluble than silver bromide, it is not essential to convert 35 all the chloride to bromide or bromoiodide, nor it is essential that the silver halide grains contains iodide. The useful silver halide grains thus include silver bromide, silver bromoiodide, silver chlorobromide and silver chlorobromoiodide. A method for preparing emulsions of the ⁴type employed in this invention is described by Davey and Knott, U.S. Pat. 2,592,250 issued Apr. 8, 1952.

The converted-halide silver halide emulsions described herein have also been referred to by those skilled in the _r art as “halide-conversion emulsions” and “halide-con⁴⁵ verted photographic emulsion.”

The silver halide emulsions of this invention are treated with a noble-metal salt before 80% of the conversion is complete, and preferably before 60% of the conversion has been completed. Typical noble-metal salts which can ⁵⁽¹ be used are the salts of such metals as ruthenium, rhodium, pallidium, iridium, osmium, platinum and gold. Representative compounds are ammonium chloropalladate, potassium chloroplatinate, sodium chloropalladite, aurous sulfide, ammonium hexachloroiridate (IV), ³³ sodium dithiosulfatoaurate (I), potassium tetrachloroiridate (II), ammonium hexachloroosmate (IV), rhodium trichloride, diaminopalladium (II) chloride, sodium hexachloroplatinate (IV), and the like. Preferably, those noble-metal salts are used which are known to be ⁶⁰ useful in the art as silver halide sensitizers.

Generally, the noble-metal salts are added to the emulsion in concentrations of about 3X10^-7 mole percent to about 3 X10^-5 mole percent based on silver, and preferably from about 2.92χ 10^-7 mole percent to about 1X 10~^{5 63} mole percent.

In a highly preferred embodiment of this invention, gold salts are used during the conversion step to provide highly improved photographic properties. Typical useful gold compounds include those described in U.S. Pats. ⁷⁰ 2,399,083 by Waller et al. issued Apr. 23, 1946, 2,642,361 by Damschroder et al. issued June 16, 1953, and the

3,703,584 like. Typical representative compounds are potassium chloroaurite, potassium aurithiocyanate, potassium chloroaurate, auric trichloride, 2-aurosulfobenzothiazole methochloride, and especially those gold compounds or complexes containing both sulfur atoms and gold atoms such as aurous sulfide, the alkali metal and ammonium aurous thiosulfates and the like.

After completion of the conversion step, the silver halide can be further treated, sensitized, etc., as known in the art to provide further light sensitivity, resistance to fogging, and the like.

The noble-metal salts used during the conversion step are preferably added to the emulsion in combination with an antifoggant.

In one highly preferred embodiment, the noble-metal salts are added to the emulsion in combination with a mercury salt to reduce the fog level of the emulsion. Generally, any mercury salt can be used during the formation of the silver halide. The term “mercury salt” is intended to include compounds consisting of a molecular compound of a mercury salt with an organic compound containing a basic nitrogen atom, an organic compound of mercury in which the mercury atom is attached by a non-ionic bond to an organic nucleus and by a non-ionic bond to an anion, or simple mercury salts of organic or inorganic acids. Typical useful mercuric salts include mercuric sulfate, mercuric acetate, mercuric nitrate, mercuric halides such as mercuric chloride, mercuric iodide, and organic mercuric salts such as mercuric salts with amines; useful mercury salts of this class are generally described in Allen et al., U.S. Pat. 2,728,663 issued Dec. 27, 1955, and British Pat. 742,219 and 742,223, the disclosures of which are incorporated herein by reference.

The mercury salts are utilized in the emulsion at concentrations of about 6χ10~⁶ moles per mole of silver to about 6χ10^-8 moles per mole of silver, and preferably from about 3 X 10~⁷ moles per mole of silver to about 1.2χ10~⁸ moles per mole of silver. The mercury salt is generally added to the emulsion at any stage of the formation of the silver halide grain and is preferably added prior to the conversion step in making the converted-type silver halide.

After formation of the converted-type emulsion grains, they can, of course, be washed, modified, coated and the like as other silver halide emulsions. Additional chemical and spectral sensitizers can be added, antifoggants and stabilizers can be used, developing agents can be incorporated in elements containing the emulsions of this invention, and various natural and synthetic vehicles can be used in the formation of the grains, as well as for binders, which in turn can be hardened by techniques used in the photographic art.

The emulsions of this invention can generally be utilized in practical applications where internal-image emulsions are useful including negative-forming systems, reversal systems, direct-positive systems, dry physical development systems, direct-print systems, image-transfer systems, color systems, lithographic systems and the like.

The invention can be further illustrated by the following examples of preferred embodiments thereof.

EXAMPLE 1

Emulsion A.—An internal-image AgCl/Br/I emulsion is prepared in a manner similar to that of Example II in U.S. Pat. 2,592,250 with the exception that all of the iodide is added in a second stage of the conversion with the resulting concentration at 2.5 mole percent iodide.

Emulsion B.—A similar emulsion is prepared to which 0.9 mg./mole of silver of potassium chloroaurate is added when 10% of the first conversion is completed. This addition results in increased speed but increased fog.

Emulsion C.—An emulsion similar to Emulsion B is prepared which contains a small amount of mercuric iodide in the silver nitrate solution and results in increased internal speed at the same fog as Emulsion A.

These emulsions are coated on cellulose acetate film support at 540 mg. Ag/ft.^{* 1 2 * 4} and 1225 mg. gelatin/ft.², exposed 1/25 Neg. on an Eastman IB sensitometer through a light-intensity step wedge, and processed for 5' 5 in a surface developer (Kodak D-19 developer) and also an internal-type developer (Kodak D-19 developer-]-10 g. sodium thiosulfate/liter). The following results show a great increase in internal speed by the addition of potassium chloroaurate without any increase in surface sen10 sitivity.

Internal developer

Emul- Surface Relative sion Feature developer speed 7 Fog

A_________ Control.................No image. 100 .73.05

B_________Plus gold salt..............do______ 324 .74.32

C_....... Plus gold salt and ...do______ 324 . 82. 08 antifoggant.

EXAMPLE 2 ²⁰ The following example shows that the point of addition of the gold salt is important for optimum sensitization. As the gold salt is added progressively later during the conversion, the speed decreases, as the following table shows.

²⁵ Emulsion D.—An emulsion is prepared as in Emulsion A above except that the concentration of iodide is 1 mole percent.

Emulsion E.—An emulsion is prepared as in Emulsion C above except that the concentration of the iodide is 1 ³⁰ mole percent and .0009 g. of potassium chloroaurate is added per mole of silver when 10% of the conversion is completed.

Emulsion F.—An emulsion is prepared as in Emulsion E above except that the potassium chloroaurate is added ³⁵ when 50% of the first conversion is completed.

Emulsion G.—An emulsion is prepared as in Emulsion E above except that the potassium chloroaurate is added when 75% of the first conversion is completed.

The emulsions are coated, exposed and developed as ⁴⁰ in Example 1.

Internal developer

Emulsion Feature

Surface Relative developer speed 7 Fog

E________

F_________

G________

-------Control_________________No image.

Gold salt after 10% ...do______ conversion.

Gold salt after 50% ...do.......

conversion.

Gold salt after 75% ...do.......

conversion.

<td> 100</td><td> .61</td><td> .06</td>

<td> 363</td><td> .75</td><td> .10</td>

<td> 302</td><td> .71</td><td> .09</td>

<td> 182</td><td> .79</td><td> .04</td>

Although the invention has been described in considerable detail with particular reference to certain preferred embodiments thereof, variations and modifications can be effected within the spirit and scope of the invention.