Preparation Of A Silver Salt Of A Fatty Acid

Abstract

A process for preparing a silver salt of a fatty acid in finely divided form comprising (a) adding an aqueous solution of a water-soluble noble metal salt to a colloidal dispersion comprising the ammonium or alkali metal salt of the fatty acid, (b) optionally adding an acidic solution to the resulting reaction mixture to convert unreacted ammonium or alkali metal salt to the free fatty acid, and (c) recovering the noble metal salt of the fatty acid, provides noble metal salts of fatty acids which can be used in photographic materials.

Description

The present invention relates to the preparation of silver salts of fatty acids in finely divided form. According to the present invention there is provided a process for the preparation of a silver salt of a fatty acid in finely divided form, which comprises the steps of adding an aqueous solution of a water-soluble silver salt to a colloidal dispersion comprising the ammonium or an alkali metal salt of the fatty acid, optionally adding an acidic solution to the reaction mixture to convert any remaining ammonium or alkali metal salt to the free fatty acid, and separating the precipitate containing the silver salt from the reaction mixture. The process of the present invention provides finely divided silver salts of fatty acids which are particularly useful for photographic applications. Further, the process has the advantage that it is technically simple and uses readily available materials.

Preferably, the colloidal dispersion contains free fatty acid in admixture with the ammonium or alkali metal salt of the acid.

In a preferred embodiment of the invention, the fatty acid is behenic acid. In this embodiment of the invention, at least 30 mole per cent of the behenic acid in the colloidal dispersion should be present as the ammonium or alkali metal salt. Preferably, from 35 to 60 mole per cent of the behenic acid in the colloidal dispersion is present as the ammonium or alkali metal salt since such compositions form colloidal dispersions which are stable at room temperature. It has been found that extremely good results are obtained when the composition contains about 50 mole per cent of the ammonium or alkali metal behenate and about 50 mole per cent of the free behenic acid. Also, it has been found that colloids containing less than 40 mole per cent of behenic acid tend to be relatively unstable at room temperature and therefore may have to be used at elevated temperatures. The colloidal dispersion is preferably formed by heating the fatty acid and the aqueous medium to a temperature above the melting point of the acid but below the boiling point of the medium and adding a solution of alkali metal hydroxide or ammonium hydroxide, preferably sodium hydroxide, so as to at least partially convert the acid to the corresponding salt. Preferably, the concentration of the fatty acid used is between 50 and 250 grams per liter of aqueous medium although concentrations outside of this range can be used if desired. Preferably, the reaction mixture is agitated such as by stirring or shaking to facilitate the reaction. Where the fatty acid is behenic acid then the behenic acid and aqueous medium should be heated to at least 80.degree.C. Any water-soluble silver salt may be used to form the silver salt of the fatty acid but silver nitrate is preferred. If it is desired to change the relative proportions of fatty acid and ammonium or alkali metal salt in the colloidal dispersion where it contains free fatty acid, then the water-soluble silver salt solution may contain a base, such as ammonia, or the base may be added simultaneously with the silver salt so as to further neutralize the acid. This enables a greater amount of the silver salt of the fatty acid to be formed since it is the ammonium or alkali metal salt thereof which reacts with the water-soluble silver salt rather than the free fatty acid. It is possible to reduce the particle size of the starting colloidal dispersion, by, for example, subjecting the dispersion to ultrasonic vibration. This gives a starting material for use in the process of smaller particle size which reacts more readily with the water-soluble silver salt.

Where the starting colloid contains free fatty acid then the precipitate containing the silver salt which is separated from the reaction mixture will also contain free fatty acid. The product can be used in this form for photographic applications but if desired the fatty acid may be removed by treating the product with a solvent, such as diethyl ether, in which the silver salt of the fatty acid is less soluble.

Further, the product may be washed such as with water to remove adventitious impurities therefrom.

The present invention also provides finely divided silver salts of fatty acids whenever prepared by the process of the present invention.

In a modification of the process of the present invention, light-sensitive silver halide grains are added to the colloidal dispersion of the ammonium or alkali metal salt of the fatty acid prior to the addition of the water-soluble silver salt so as to provide a source of catalytic centers for physical development.

A composition prepared in this way can be used in photographic applications as is described more fully hereinafter with favorable results. The use of preformed silver halide grains permits variation and control of the sensitometric properties of the compositions of the invention, in accordance with the properties of the grain themselves. Generally, a hydrophilic colloid photographic emulsion containing the silver halide grains is used and this can be prepared by well known emulsion techniques. Preferably, a low-gelatin photographic emulsion is used, i.e., one containing less than about 30 grams of gelatin per mole of silver halide.

The silver halide grains may be treated, prior to use, in various ways to modify their properties as is well known in the photographic art. For example, they may be chemically sensitized. The silver halide emulsion may contain conventional addenda such as surfactants to improve dispersion thereof.

The silver halide is preferably added in a concentration of from 3 to 50 mole per cent based on the fatty acid silver salt which is to be formed.

As mentioned hereinabove, the product of the process of the present invention is particularly useful in photographic applications. In particular, it is useful in a radiation sensitive material of the type comprising a support and containing in a layer or layers thereof a radiation sensitive compound capable of forming catalytic centers for physical development on exposure to actinic radiation such as light, electron radiation, X-rays or gamma-rays, the silver salt of the fatty acid produced by the process of the invention to provide a source of silver ions for physical development when heated to a suitable temperature and a reducing agent. In use, these materials are given an imagewise radiation exposure to form a latent image therein and are then processed by heating. Preferably, they also contain a toning agent to provide a more neutral image tone. Such photosensitive materials are described, for example, in U.S. Pat. No. 3,672,904 of DeMauriac, issued June 27, 1972; Belgian Pat. No. 772,371 issued Oct. 15, 1971 and U.S. Pat. No. 3,152,904 of Sorensen et al, issued Oct. 13, 1964.

An especially useful method of the invention is a method of preparing a photothermographic composition comprising (a) mixing an aqueous solution of a water soluble noble metal salt with a colloidal dispersion comprising a fatty acid reactant comprising less than 70 mole percent free fatty acid and at least 30 mole percent ammonium or alkali metal salt of said fatty acid to provide a reaction mixture, (b) optionally adding an acidic solution to the reaction mixture to convert any unreacted ammonium or alkali metal salt to the free fatty acid, (c) recovering the noble metal salt of the fatty acid and (d) mixing the resulting noble metal salt of the fatty acid with (i) a photosensitive component, (ii) an organic reducing agent and (iii) a binder.

The following examples are included for a further understanding of the invention.

EXAMPLE 1

Behenic acid (10g) was heated with water (100ml) to 85.degree.C. 4M sodium hydroxide solution (3.7ml) was added slowly to convert 50 mole per cent of the behenic acid to the sodium salt. The mixture was vigorously stirred. The resulting opalescent suspension was cooled to 25.degree.C., cold water (60ml) added, and 5M AgNO.sub.3 (2.90ml) added dropwise with stirring to convert the sodium behenate to silver behenate. The resulting flocculent white precipitate was filtered at the pump, washed three times with water, sucked reasonably dry and dried in vacuo over potassium hydroxide. The product (10.9g) was a loose, free-flowing white powder, with an average particle size of less than 1.mu.m.

EXAMPLE 2

Behenic acid (10g) was heated with water (50ml) to 85.degree.C. 4M sodium hydroxide solution (3.7ml) was added slowly with vigorous stirring to convert 50 mole per cent of the acid to the sodium salt. The resulting viscous liquid was cooled to room temperature, and cold water (120ml) added. To this colloidal suspension was added 5M silver nitrate solution (5.8ml) to which had been added 25 percent ammonia solution (6ml). The ammonia converted a further 40 mole per cent of the original behenic acid to the ammonium salt. The mixture was stirred gently for four hours, with the addition of two 1ml aliquots of 25 percent ammonia solution, to replace ammonia losses. The mixture at this point resembled a thick cream, and 3M nitric acid (32ml) was added to neutralize excess ammonia. This caused the mixture to flocculate rendering it easily filterable. The product was filtered and washed three times with water (250ml portions) at the pump, and dried. Examination under an optical microscope showed the particle size to be less than 0.3.mu.m, with the exception of a few larger particles or aggregates. The silver ion in the filtrate was estimated gravimetrically by precipitation with chloride ion, and showed 11 percent of the silver added had not been converted, corresponding to an 88 percent conversion of behenic acid to silver behenate.

Examples 3 and 4 illustrate the photographic application of the products of Examples 1 and 2 respectively.

EXAMPLE 3

In this Example, the silver behenate behenic acid powder prepared in Example 1 was used in a coating on paper.

The following composition was ball-milled for 90 minutes:

Silver behenate-behenic acid powder prepared in Example 1 3.0 g polyvinyl butyral (binder) 0.4 g phthalimide (toning agent) 0.34 g acetone 10 ml toluene 10 ml sodium bromide 0.010 g

Then 0.45g 1,1'-bis-2-naphthol reducing agent dissolved in acetone, 14ml, plus toluene, 5ml was added. The mixture was mixed thoroughly, coated on paper at a wet thickness of 0.10mm and dried.

The sodium bromide was added to form a light sensitive silver bromide by reaction with some of the silver behenate.

Samples of the dried coating were exposed to an Edgerton, Germeshausen and Grier Mark VII sensitometer for 10.sup..sup.-3 seconds, with no filters, through a bar pattern or a photographic `step-wedge`. They were processed by contact with a curved aluminum block heated to 138.degree.C. for about ten seconds. The bar-pattern-exposed sample showed a good image of black bars on a white background, with a reflection density difference of 1.2 units between image and background. The step-wedge exposure showed eleven 0.15 density increment steps. The coatings were smooth and of good quality.

EXAMPLE 4

This example illustrates the use of the product of Example 2 in the preparation of film coatings. The following composition was ball-milled for one hour:

silver behenate/behenic acid composition 1.75 g From Example 2 30% polyvinyl butyral (binder) in 1:1 acetone/toluene 20 ml phthalimide (toning agent) 0.17 g sodium bromide (to provide light sensitive silver bromide) 0.05 g

Then the following solution was added:

1,1'-bis-2-naphthol (reducing agent) 0.22 g polyvinyl butyral (binder) 1.00 g acetone 14 ml toluene 5 ml and the mixture ball-milled a further five minutes. It was coated on a polyethylene terephthalate support at a wet thickness of 0.1 mm, allowed to dry and overcoated with a 6.6 percent solution of polyvinyl butyral in 2:1 acetone/toluene, at a wet thickness of 0.1 mm which was also allowed to dry.

A sample was exposed through a negative to a small electronic flash-gun nine inches distant for about 10.sup..sup.-3 seconds. The exposed material was heat-processed for 10 seconds at 138.degree.C. as in Example 3. A brown-black image, optical density 0.90 with a background density of 0.05 was obtained.

EXAMPLE 5

This example illustrates the preparation of lightsensitive silver behenate/silver bromide/behenic acid compositions in accordance with the modification of the process of the invention described above. The silver bromide grains used were in the form of a coagulum cubic silver bromide grains of edge-length 0.31 microns and were prepared by conventional double-jet techniques in the presence of 25g of phthalated gelatin per mole of silver bromide, and the pH of the resultant emulsion then lowered so as to precipitate the silver bromide and gelatin together as a coagulum. The aqueous phase was then substantially removed.

Behenic acid (10g) was heated with water (50ml) to 82.degree.C. and 4M sodium hydroxide solution (3.7ml) was then added slowly with vigorous stirring. To the resulting milky-opalescent suspension was added cold water (150ml). This diluted suspension was strained through muslin and brought to 40.degree.C.

Under safelight conditions, some coagulum as described above, containing about 0.15g of silver bromide, was dispersed in water (50ml), to which a drop or two of sodium hydroxide solution had been added to bring the pH to about 8, at 40.degree.C. This dispersion was added to the behenic acid/sodium behenate suspension, and the combined suspension stirred vigorously for 5 minutes. 5M silver nitrate solution (2.9ml), diluted with 10ml of water, was then run in slowly with vigorous stirring, and the mixture stirred thoroughly for 10 minutes at 40.degree.C. It was cooled to room temperature and stood in the dark for ninety minutes. Then the mixture was filtered, the precipitate washed three times with water and dried in vacuo over potassium hydroxide. The result was a fine white powder, yield 10g. Example 6 illustrates the photographic application of the product of Example 5.

EXAMPLE 6

The following composition was ball-milled in the dark for seventeen hours, using 5mm diameter glass balls:

silver behenate/silver bromide/ behenic acid powder prepared in Example 5 1.5 g phthalimide (toning agent) 0.16 g polyvinyl butyral, (binder) 3% w/v solution in 50-50 acetone-toluene 10 ml

Then the following mixture was added and the combined mixture ball-milled a further ten minutes:

1,1'-bis-2-naphthol (reducing agent) 0.20 g polyvinyl butyral (binder) 0.50 g acetone 8 ml toluene 3 ml

The dispersion was coated on paper at a wet thickness of 0.10 mm (0.004 inches), and the coating dried.

The resulting material was divided into two portions. One portion was given a saturation exposure through a bar-pattern to an electronic flash gun 12 inches distant and the other portion was exposed through a step tablet having 0.15 density increment steps to an Edgerton, Germeshausen and Grier Mark VII Sensitometer for 10.sup..sup.-3 seconds with no filters. The exposed portions were processed by heating on an aluminum block at 128.degree.C. until background density started to form. Densities were read by a diffuse reflection densitometer and the results are given in Table I below. Examples 7 and 8 illustrate the photographic applications of compositions prepared as in Example 1.

EXAMPLE 7

A coating was prepared as in Example 6, except that a silver behenate/behenic acid powder prepared as in Example 1 was used in place of the powder prepared in Example 5. Further, 1 percent lithium bromide solution in acetone (0.5 ml) was added to the composition before being milled for seventeen hours. The resulting material was tested in the same way as the material of Example 6. The results are shown in Table I below.

EXAMPLE 8

A coating was prepared as in Example 7, except that some coagulum as described in Example 5 (0.02 g AgBr) was washed thoroughly with acetone and added in place of the lithium bromide solution. The resulting material was tested in the same way as the material of Example 6. The results are shown in Table I below.

Table I ______________________________________ Saturation Speed-Step No. at Stated Exposure Density above D.sub.min Example D.sub.max D.sub.min 0.05 0.1 0.2 0.5 ______________________________________ 6 1.57 0.17 21 201/2 20-11 9 7 1.50 0.17 9 81/2 71/2 61/2 8 1.10 0.34 7 5 3 -- ______________________________________

The speeds are quoted as the step-number (densest step in the table 21) which give the stated density above fog. The material of Example 7 was of high contrast, while that of Example 6 was of moderate contrast but having a very long `toe` of low, uniform density. The material of Example 8 showed lower density, contrast and speed. It will be noticed that the material of Example 6 shows comparable image density and substantially greater photographic speed compared with the material of Example 7.

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.

Claims

1. A method of preparing a composition comprising photosensitive silver halide and silver behenate comprising (1) mixing (i) a preformed photosensitive silver halide emulsion comprising less than about 30 grams of gelatin per mole of silver halide with (ii) a fatty acid reactant comprising less than 65 mole percent behenic acid and 35 to 60 mole percent alkali salt of behenic acid selected form the group consisting of ammonium and alkali metal salts of behenic acid to provide a photosensitive silver halide composition, (2) mixing an aqueous solution of a water soluble silver salt with the photosensitive silver halide composition to provide a reaction mixture and (3) adding an acidic solution to the reaction mixture to convert any unreacted ammonium or

2. A method as in claim 1 also comprising (4) recovering the resulting

3. A method of preparing a photothermographic composition comprising (a) mixing an aqueous solution of a water soluble noble metal salt with a colloidal dispersion comprising a fatty acid reactant comprising less than 70 mole percent free fatty acid and at least 30 mole percent ammonium or alkali metal salt of said fatty acid to provide a reaction mixture, (b) adding an acidic solution to the reaction mixture to convert any unreacted ammonium or alkali metal salt to the free fatty acid, (c) recovering the noble metal salt of the fatty acid and (d) mixing the resulting noble metal salt of the fatty acid with (i) a preformed photosensitive

4. A method of preparing a photothermographic composition comprising (a) mixing an aqueous solution of silver nitrate with a colloidal dispersion comprising a fatty acid reactant comprising less than 70 mole percent free fatty acid and at least 30 mole percent ammonium or alkali metal salt of behenic acid to provide a reaction mixture, (b) adding an acidic solution to the reaction mixture to convert unreacted ammonium or alkali metal salt to the free acid, (c) recovering the noble metal salt of the fatty acid as a finely divided solid, and (d) mixing said finely divided solid with preformed photosensitive silver halide, a phenolic reducing agent and a

5. A method of preparing a finely divided noble metal salt of a fatty acid comprising (a) mixing (i) an aqueous solution of a water-soluble noble metal salt with (ii) a colloidal dispersion comprising photosensitive silver halide and a fatty acid reactant comprising less than 70 mole percent free fatty acid and at least 30 mole percent ammonium or alkali metal salt of said fatty acid to provide a photosensitive reaction mixture, (b) adding sufficient acid solution to the reaction mixture at least partially to convert unreacted ammonium or alkali metal salt to the free fatty acid and (c) recovering the resulting product as finely divided

6. A method of preparing a finely divided noble metal salt of a fatty acid comprising (a) mixing (i) an aqueous solution of a water-soluble noble metal salt with (ii) a colloidal dispersion consisting essentially of (1) a photosensitive silver halide gelatino emulsion containing less than about 30 grams of gelatin per mole of silver halide in said emulsion, and (2) a fatty acid reactant comprising less than 70 mole percent free fatty acid and at least 30 mole percent ammonium or alkali metal salt of said fatty acid to provide a reaction mixture, (b) adding sufficient acidic solution to the reaction mixture at least partially to convert unreacted ammonium or alkali metal salt to the free fatty acid and (c) recovering the resulting product as finely divided noble metal salt of fatty acid and photosensitive silver halide.