WO1993005442A1

WO1993005442A1 - Improved photographic element which employs a bromide shell and supersensitization and stabilization thereof

Info

Publication number: WO1993005442A1
Application number: PCT/US1992/007396
Authority: WO
Inventors: Lloyd George Sidwell
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1991-09-11
Filing date: 1992-09-09
Publication date: 1993-03-18

Abstract

A silver halide emulsion prepared from core-shell grains wherein the core is mainly chloride and the shell bromoiodide, is described. These unique and novel grains can be further stabilized by thiotosylate salts thereby providing a silver halide element with excellent speed and stability and good processability, thus fully utilizing the advantages of chloride grains.

Description

TITLE

IMPROVED PHOTOGRAPHIC ELEMENT WHICH EMPLOYS A BROMIDE SHELL AND SUPERSENSITIZATION ftNP STABILIZATION THEREOF

BACKGROUND OF THE INVENTION

1. Field of the Invention:

This invention relates to silver halide emulsions usef l in the preparation of photosensitive elements. Still more particularly, this invention relates to a particular silver halide grain elements in which a bromide shell is present. Even more particularly, this invention relates to silver chloride elements that can be successfully used in X-ray elements.

2. Discussion of the Prior Art:

It is well known that silver halide elements prepared mainly from silver chloride have a greater processability because of the increased solubility of the silver chloride grain as compared to bromide or iodide. Thus, these elements can be processed in a shorter time period with better results, e.g., cleaner image with good image color and good image density. However, elements which employ a grain that has a considerable amount of chloride are much slower than those which contain bromide or bromoiodide grains. Thus, the challenge to prepare silver halide elements having a high chloride content and good image quality with good speed and sensitometry is an on-going goal. Silver halide elements employing a core-shell grain are also well known as exemplified in Ohashi et al., U.S. Patent 4,835,095. These elements can be made with various halides that comprise the core and the shell of the grain and have some advantages over regular grains not employing a core/shell. Some of these advantages include sensitivity and grain size distribution, for example and these grains can be made into any of the conventional grain shapes, e.g., cubic, octahedral, tabular, etc. Once again, however, when the grain is comprised mainly of chloride, sensitometric results are lowered over mostly bromide or bromoiodide grains.

Tabular silver chloride grains are also well known as exemplified in Takada, et al., U.S. Patent 4,783,398 and Maskasky, U.S. Patent 4,713,323. However, these grains are not generally stable and when chemically sensitized, tend to revert back to a cubic or spherical shape. Thus, the advantages which can be achieved with a tabular shape are lost. There have been processes to stabilize these tabular grains and some of these are very successful. For example, Tufano and Chan in the Journal of Imaging Science, Vol. 34, No. 2, March/April 1990 teach the use of certain aminopyrimidine crystal growth modifiers to stabilize chloride-rich grain shapes.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a photographic element which employs a grain comprised mainly of chloride and having a bromide shell associated therewith. It is yet another object of this invention to provide a core-shell emulsion useful as an X-ray element and having good image color, improved sensitometry and image density as well as excellent processability. These and yet other objects are achieved in a gelatino, silver halide element having improved stability and sensitometry comprising a silver chlorobromoiodide grain emulsion, said grain comprising a core and a shell wherein said grain is predominantly silver chloride having 70-98 mol % chloride, 1-29 mol % bromide and 0.001 to 7 mol % iodide with the proviso that no more than 49% by weight of said bromide is present in said core with the remainder in a shell around the core of said grain and that all of the iodide is present in said shell. It is a further object of this invention to provide the emulsion with a stabilizing amount of

wherein

R = H or straight chain or branched aliphatic hydrocarbon with not greater than 6 carbon atoms, and A+ = Na+ or K+.

DETAILS OF THE INVENTION

The use of high chloride emulsions has been limited by the relative slow speed of these emulsions to photosensitive systems such as papers and the like that do not require high speed. However, as previously mentioned, chloride emulsions have some very desirable properties such as the ability to "clear" quickly from the processing fluids. Additionally, these products have good density and excellent image color. All of these characteristics would be useful in high speed photographic systems such as medical X-ray, for example. By making core—shell emulsions in which a majority of the halide is chloride and where less than half the bromide is present in the core and more than half is present in the shell, following the preparation steps described herein, it is possible to manufacture photographic emulsions which can be sensitized to high speed with both chemical and spectral sensitizers and which also exhibit many of the desirable characteristics of chloride emulsions as described above. Preferably, these emulsions will contain 70-98 mol % chloride, 1-29 mol % bromide and 0.001-7 mol % iodide. Most preferably these emulsions will contain 70-92 mol % chloride, 7-29 mol % bromide and 1-3 mol % iodide. These emulsions can be made by preparing a requisite "heel" or starting solution which will contain chloride and bromide salts in the desired ratio. Additionally, this heel will contain some water and a requisite amount of gelatin. If it is desired to make a tabular grain emulsion, a grain growth modifier such as 4- aminopyrazolo(3,4d)pyrimidine, as described in the above reference Tufano et al. paper, the substance of which is incorporated herein by reference, is also added.

Silver halide seeds are prepared within this heel after the heel is adjusted to the desired pH and temperature. This step is accomplished by adding a solution of silver nitrate at a predetermined rate. By controlling the temperature of the emulsion and the rate at which the silver salts are added at this stage it is possible to generate cubic or tabular shaped grains. Generally, these temperatures range from about 40°C to about 70°C.

Then, a solution containing the desired ratio of halide salts (chloride and bromide) and a solution of silver nitrate is added by balanced-double-jet addition, a procedure that is well-known in the prior art, to this emulsion during the so-called "growth" period. During this time, the rate of addition of the two solutions is also controlled to produce the desired grain shape. The entire emulsion is, of course, stirred during this process and heat applied to obtain the desired temperature. An antifoamant may also be added if desired.

Then, a solution containing the desired halides to place within the shell of the above grain, is added at predetermined rate. Usually, this solution will contain both bromide and iodide since elements prepared with a bromoiodide shell will have the best sensitometry. After all of this solution has been added, the emulsion is digested for a period of time at 46-60°C with some stirring and the silver halide grains are coagulated to form a crude, curd-type emulsion and washed thoroughly with water. By decanting the wash water, the excess salts are removed. Coagulation and washing could be replaced with an ultrafiltration process as known in the art.

Then, this emulsion is suitable for the preparation of a silver halide element. First, the curds are redispersed in a bulking amount of gelatin and water. Chemical sensitization and stabilization is well known in the art for increasing photographic speed and decreasing minimum density. Sensitization with gold salts have proven to be very effective for the formation of high speed photographic emulsions. The use of prior art techniques for chemical sensitization of the novel grains described herein are unacceptable due to the high minimal density which is observed at useful photographic speeds. The photographic elements comprising core shell silver chlorobromoiodide grains as described above can be suitably prepared by the addition of stabilizing amounts of a thiotosylate, i.e.:

wherein R = H or straight chain or branched aliphatic hydrocarbon with 6 carbons or less, and A+ = Na⁺ or K⁺.

Thiotosylates are typically used as an aqueous solution wherein a stabilizing amount of a similarly substituted sulfinic acid is added.

Additionally, spectral sensitizing dyes may also be added to raise the spectral sensitivity of this emulsion. Various wetting agents, complimentary antifoggants or stabilizers, coating aides, hardeners, etc. may also be added before coating on a suitable support. Exemplary examples of sensitization chemicals and additional additives are included herein by reference to Research Disclosures, Vol. 308, December 1989, Item 308119. This support may be any of those well-known supports such as films and papers and the likes. I prefer to use dimensionally stable, polyethylene terephthalate which has been subbed with a resin sub on both sides followed by a thin, hardened gelatin layer to improve the adhesion of the silver halide emulsion layer. The support may also be tinted to impart color thereto. For example, film supports used as supports for medical X-ray elements usually have a blue tint imparted therein. Antistatic agents may be added to the emulsion or incorporated in a thin, hardened gelatin antiabrasion layer that is normally coated supra to the emulsion layer or layers. In fact. when medical X-ray elements are prepared, it is conventional to coat an emulsion layer on each side of the film support or an emulsion on one side of a film support and an auxiliary layer on the opposite side of the support.

Auxiliary layers typically may contain dyes for printthrough reduction as well as adjuvants for antistatic protection, developer and fixer enhancers and other functions as known in the art. Photographic elements made according to the teachings of this invention can be used for any photographic, recording process. However, they are eminently useful as medical X-ray recording elements. Various grain shapes can be used within these structures. For example, tabular grains have some advantages in that they can be coated at thinner coating weights thus saving silver. Additionally, tabular grain elements can be forehardened without significant loss of covering powers. On the other hand, cubic grains can be made with a very narrow grain size distribution and this has its advantages. Thus, the high chloride grains of this invention with the novel bromoiodide shell, which produces a distorted surface on the grain, can be used extremely well in the aforementioned systems. There seems to be an increase in the ability of the grain to absorb sensitizing dyes. This is possibly due to distortions in the grain surface. Thus, elements made herein exhibit improved response to the addition of spectral sensitizers. Since tabular grains must be spectrally sensitized to improve their efficiency, and since most tabular grain emulsions exhibit dye stain and the like, this fact is an important attribute of the grains of this invention. The grains themselves are stable and will process with improved clearing rate as previously mentioned. This invention will now be illustrated by the following examples. All parts and percentages are molar unless otherwise indicated.

£2_i_i__EI___l

A cubic grain, core-shell, silver chlorobromide emulsion with the bromide contained mainly in the shell, was prepared. This emulsion, which was made by the procedure described below, contained a core of ca. 80% chloride and ca. 20% bromide with the shell being all bromide. This emulsion represents a core- shell emulsion which is outside the limits of this invention. Also for control purposes, a standard, splash mix cubic bromide emulsion with a 1.5 mol % iodide content was also made by conventional methods.

Representing the core-shell, chlorobromoiodide grains of this invention, a procedure similar to that used to prepare the prior art core-shell grains but with iodide added in the shell of this grain, was also prepared. Changes to the procedure are indicated:

Solutions to Prepare a Core-Shell, Cubic Grain, Chlorobromoiodide Emulsion:

(A) Heel Preparation:

Ingredient Amt/1 Mol Silver (gms) Preferred Gelatin 12 - 30 20

Distilled Water 200-300 267

NH₄CI 5.0 - 7.1 5.7

NH₄Br 0.0 - 3.9 2.6 The salts were dissolved in the gelatin and water at 40°C for about 15 minutes and then held at 45-60°C. The pH was adjusted to 4.0-4.1.

(B) Silver Salt Solution:

3 molar gN03 made up at 509.6 gms of silver nitrate per liter of distilled water.

(C) Growth Salt Solution:

Ingredient Amt/1 mol Ag (gms) Preferred Distilled Water 200-400 400

NH₄C1 44-65 51.4 NH₄Br 0.00-36 23.5

(D) Shell Salt Solution:

0.5 M KBr (59.5 g/1) 0-60 20 0.5 M KI (83.0 g/1) 0-60 40

The iodide-containing mixture was used when the grains of this invention were prepared.

Silver Seeding Step:

While the heel was held at 46°C, with stirring at 500-1000 rpm, some silver salt solution was added at an appropriate rate to maintain a pAg of +205 mv. An optional antifoamant, tributylphosphate was used. After completion of this step, the mv was allowed to level off which took about 10-15 seconds. This is referred to as the "stabilization" step.

Growth Period: Using a balanced-double-jet addition process, silver salt and the halide salt solution (C) was added at a rate suitable for maintaining a pAg of +205 mV. The growth step was complete when all of the silver salt solution had been added.

Shell Procedure:

The bromide and iodide shell salt solutions (D) , were added at a predetermined rate over a period of 10 minutes at an agitation rate of 1000 rpm. At 6°C the mv reading decreased from +235 to +128 and the shell procedure was over when all of the bromide salt had been added. This mixture was then allowed to digest at 1000 rpm and 46°C for 10 minutes. The silver halide and gelatin was coagulated using:

and the pH adjusted to 2.8-3.0. The so-called "curds" were allowed to settle in the kettle for 20 minutes with no agitation and the supernate then drained off the top of these curds. Distilled water was then added to cover the curds and agitation started again. More of the above mentioned coagulating.agent was added along with 1.5 N sulfuric acid (pH 2.7-3.2). After stirring for 10 minutes, settling for 20 minutes, the supernate water was removed again and the curds were ready for use. Separate portions of the curds employed in formulations are set forth in parts A and B.

PART A A sample of these curds were redispersed in gelatin, digested with sulfur and gold as known in the art and spectrally sensitized using:

i-

After sensitization, the usual antifoggants, coating aids and hardeners were added and the emulsion coated on a subbed film support. The coating weight was 60 mg/dm^. A thin, gelatin, antiabrasion layer was coated supra to this emulsion layer. A sample of this film, after drying, was then given an exposure to a HeNe laser through a 2 step wedge and then developed, fixed, washed and dried in the conventional manner. In a like manner, the control (cubic, splash prepared bromoiodide grains) and grains containing the iodide in the shell, were also redispersed, sensitized, coated, dried, the samples were exposed, developed, fixed, washed and dried. The sensitometry of each sample was determined with the following results: Relative

Sample Speed Gradient Dmax

Control 100 1.28 2.90

Of This Invention

1 mol % Iodide

2 mol % Iodide

3 mol % Iodide

Thus it can be seen, when core-shell grains-of mainly chloride in the core and mainly bromide and iodide in the shell are prepared as described above, the speed of the emulsion is increased over prior art core-shell grains. Without the iodide in the shell, the chloride grains are much too slow for commercial use.

Part B A sample of these curds were redispersed in gelatin, digested with sulfur sodium aurousthiosulfate. Sodium thiotosylate was added in the amounts indicated in Table 1. Spectral sensitization was accomplished as known in the art using:

After sensitization, the usual antifoggants, coating aids and hardeners were added and the emulsion coated on a subbed film support. The coating weight was 60 mg/dm². A thin, gelatin, antiabrasion layer was coated supra to this emulsion layer. A sample of this film, after drying, was then given an exposure to a HeNe laser through a 2 step wedge and then developed, fixed, washed and dried in the conventional manner. The sensitometry of each sample was determined and the results contained in the attached table. As indicated in the table the addition of stabilizing amounts of thiotosylate salts to core shell chlorobromoiodide grains provide a lower minimum density (Dmin) at comparable speeds and similar gradients and maximum density (Dmax) .

TABLE 1: Sensitometric Performance

Rela¬ tive __B Pm n Speed Gradient Dmax

Comparative Example 23.3 0 .53 100 1.96 3.38

Inventive

Example 11.6 9.24 .20 106 1.83 3.02 Inventive

Example 23.3 9.24 .18 71 2.02 3.29

Inventive

Example 35.0 9.24 .15 52 1.98 3.44

A: mg of sodium aurousthiosulfate per mole of Ag B: mg of sodium thiotosylate per mole of Ag

Claims

What is Claimed is:

1. A gelatino, silver halide element having improved stability and sensitometry comprising a silver chlorobromoiodide grain emulsion, said grains comprising a core and a shell wherein said grains are predominantly silver chloride having 70-98 mole % chloride, 1-29 mole % bromide and 0.001 to 7 mole % iodide with the proviso that no more than 49% by weight of said bromide is present in said core with the remainder in a shell around the core of said grain and that all of the iodide is present in said shell

2. The element of claim 1 wherein said emulsion further comprises a stabilizing amount of

wherein R = H or straight chain or branched aliphatic hydrocarbon with 6 carbons or less, and A+ = Na⁺ or K+.