US2311058A - Photographic silver halide emulsion - Google Patents

Description

Patented Feb. 16, 1943 PHOTOGRAPHIO SILVER HALIDE EMULSION Wesley G. Lowe, Rochester, N. Y., aslignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application October 29, 1941, Serizi 2 Claims.

*This invention relates to photographic silver halide emulsion in which polyvinyl alcohol is employed as the protective colloid for the silver halide therein. This invention also relates to photographic silver halide emulsions in which a polyvinyl alcohol, gelled by means of a-naphthol, is employed as the protective colloid.

This is a continuation-in-part application of my U. S. application Serial No. 318,559, filed February 12, 1940, now Patent No. 2,286,215.

Gelatin has ordinarily been employed as the protective colloid for the light-sensitive silver halides in photographic emulsions due to its ability to hold these salts in suspension, its good water permeability, its insolubility in water and in photographic developing solutions at ordinary tempertaures and its ability to form a thin coating of the light-sensitive silver halide emulsion upon the film base. i

Various other materials have been suggested for use as the protective colloids in photographic emulsions but in most instances the suggested materials have evidenced inferiority in one or more of the properties enumerated, compared with gelatin. For instance, cellulose esters, such as cellulose nitrate, have been suggested as carriers for silver salts in photographic emulsions. The emulsion layer formed, however, when dry, is none too permeable to developing solutions and, therefore, cellulose nitrate has not been successful with theconventional developing and fixing baths. Another disadvantage of cellulose nitrate is its insolubility in solvent mixtures containing appreciable amounts of water. This prevents the use of the water-soluble salts which are now ordinarily employed in forming lightsensitive photographic emulsions.

Various other colloidal materials have been mentioned in this connection. However, most of those that have been suggested heretofore are deficient for some reason or other, such as lack of good water permeability, inability to hold silver halide in suspension or inability to form a hard emulsion layer when coated upon a support, such as photographic film base or paper, and dried.

Gelatin as a protective colloid for photographic silver halide emulsions leaves much to be desired. Gelatin is susceptible to depredations by organisms, both animal and vegetable. For instance, insects attack gelatin, particularly in the tropics. Gelatin is susceptible to the action of molds and of bacteria, particularly under conditions of high humidity and a. compatible temperature. Gelatin, because of its natural source often varies in properties from time to time.

In Great Britain February 1 Also, to obtain gelatin in a highly refined con dition (as needed for photographic purposes) requires considerable care and it can be procured in limited amounts only from animal products by a slow and expensive process.

One object of my invention is to provide carriers or protective colloids for light-sensitive silver halides, which can be employed instead of gelatin forvarious photographic silver halide emulsions. Another object of my invention is to provide carriers orprotective colloids forlightsensitive silver halides in photographic emulsions which possess many of the advantages but which lack some of the disadvantages evidenced by gelatin. Other objects will appear herein. 1 have found that polyvinyl alcohol is. eminently suitable as a carrierior the silver halide in photographic emulsions, the physical properties of the -material being easily controlled in production so as to get a material of constant properties. Polyvinyl alcohol has the ability to hold silver halide in suspension, is easily. coated out upon a support, such as paper, readily sets and dries to a hard, scratch-resistant emulsion layer, which may be treated with developing and fixing baths, and it may .be washed thoroughly with water after processing without dissolving. I have found that a polyvinyl alcohol emulsionlayer swells but does not dissolve when developed, fixed and washed in the usual way. After drying, it exhibits good physical properties. I have found that photographic silver halide emulsions having polyvinyl alcohol as the protective colloid, in which an amount of a-naphthol has been incorporated therewith to cause gelling of the polyvinyl alcohol, have good photographic properties. The treatment of an aqueous solution of polyvinyl alcohol with u-naphthol to convert the polyvinyl alcohol to a rigid gel, which is thermalreversible, is described and claimed in U. S. Patent No. 2,249,536 of McDowell and Kenyon. This is distinct from the hardening action of some materials on polyvinyl alcohol.

A photographic silver halide emulsion in accordance with my invention may, for instance, be prepared after a silver halide has been dispersed in aqueous polyvinyl alcohol, such as 540% concentration, by treating the mass. with an amount of a-naphthol at a slightly elevated temperature (preferably 40-90 C.) which, when cooled, forms a rigid thermal-reversible gel. After the cooling has taken place, the emulsion gel is formed and may be washed or otherwise treated as desired. This phenomena is valuable in that it makes possible the coating of the emulsion upon a support while the mass is liquid at the elevated temperature and then gel-setting of the emulsion. The emulsion can then be dried and forms a photo-sensitive layer on the support. The gelling agent should only be added after the formation of the emulsion to satisfactorily form a gelled product.

"Polyvinyl alcohols, which term as used herein includes also water-soluble hydrolyzed polyvinyl esters having a polyvinyl ester content of approximately 5% or sometimes less-the remainder being polyvinyl alcohol-are particularly adapted to the preparation of unwashed emulsions and when coated on a suitable support such as paper in the form of a photosensitive silver halide emulsion will record images accurately and sharply, forming permanent prints when processed in the ordinary manner.

Polyvinyl alcohols are eminently suitable for the preparation of washed photographic emulsions in accordance with my invention. These emulsions are particularly suitable for coating onto a cellulose ester support and when so coated in the form of a photosensitive silver halide emulsion will, upon proper exposure, developing and printing, give a sharp image.

The photographic emulsions of my invention may be prepared by a process involving first dispersing silver halide in polyvinyl alcohol, such as by running aqueous solutions of silver nitrate and alkali metal halide into an aqueous solution of polyvinyl alcohol. Preferably after adding a gelling amount of a-naphthol and warming to cause better dispersion and increase the speed, as is well known in the art, followed by cooling and, ifv desired, sensitizing with a small amount of a sensitizing dye, theemulsion is ready to coat out onto a support, such a paper. The emulsion is coated out in a warm condition so as to render it fiowable. After the emulsion layer is dried, such as by a current of warm air, it may be exposed, processed, washed and dried in the usual manner and prints of good appearance result. Where the a-naphthol gelling agent is used, the emulsion, after coating out onto a support, can be set by cooling.

It is preferable to incorporate the gelling agent in the emulsion composition after the formation of the silver halide because otherwise there may sometimes be a tendency towards fog. The gelling agent may be added after the addition of the polyvinyl alcohol protective colloid, or it may be incorporated in polyvinyl alcohol which is mixed with the silver halide. In the case of the washed emulsions it is often desirable to add some m-naphthol thereto after the washing operation to replace any gelling agent which was lost in the washing operation, prior to coating of the emulsion on to the support.

a-naphthol or other like gelling agents convert polyvinyl alcohol to a thermal-reversible gel as described in McDowell and Kenyon Patent No.

1 2,249,536. This gelled polyvinyl alcohol forms a graphic emulsions in accordance with my invention need be only a gelling amount. For instance the usual emulsions using p lyvinyl alcohol as the protective colloid as finally prepared, contain about 3-5% of polyvinyl alcohol. For 50 gms. of emulsion usually approximately V, or a gm. of a-naphthol will be satisfactory altho the amount of a-naphthol within the limits given is a matter of choice. ,In preparing a washed emulsion some of thegelling agent is usually washed out and therefore may be employed in a somewhat greater amount. However, after the washing operation it is desirable that the amount of a-naphthol be not so great as to result in the formation of crystals in the emulsion or that will render the handling of the emulsion more difllcult than necessary because of excessive toughness or a higher melting point or both.

a-naphthol is distinctive as a gelling agent for polyvinyl alcohol photographic emulsions in that the emulsions which have been gelled with a-naphthol exhibit an adherence between the emulsion layer and a cellulose nitrate subbing layer, which is superior, as a rule, than is the case with other emulsions in which a gelled polyvinyl alcohol is the protective colloid.

My invention is concerned with the preparation of photographic silver halide emulsions adapted to be coated upon paper or upon a transparent base, such as cellulose derivative film base or glass plates, to form what is normally known in the art as photographic paper, photographic film or photographic plates.

The photographic silver halide emulsions of my invention may be sensitized in the same manner as are silver halide emulsions generally. This is ordinarily accomplished with my emulsions by thoroughly incorporating a small amount of a sensitizing dye after the emulsion prepared. If staining of the paper on which the emulsion is coated is objectionable, an amount of sensitizing dye, which is not great enough to cause staining, may be employed. If a more sensitive emulsion is desired and staining is not objectionable, or 'a support which will-not absorb the dye is used, a larger proportion of sensitizing dye may be incorporated. The emulsions, containing the smaller amounts of sensitizing dye, are ordinarily adapted for making contact prints or enlargements by projection.

The sensitizers, which have been previously employed to optically sensitize silver halide emulsions, may be incorporated in the emulsions of my invention to increase their sensitivity. Examples of sensitizers, which may be employed in the emulsions-described herein, are the cyanines and the sensitizing dyes disclosed in the following patents:

' U. S. No, 2,186,608, Keyes, January 9, 1940; U. S.

' No. 2,165,339, Brooker, July 11, 1939.

The following examples illustrate my invention:

Example I cc. of a 10% solution of polyvinyl alcohol I was diluted with cc. of water. Into this the following solutions were run with vigorous stirring, the precipitation time being five minutes. Solution A: 40 cc. of a 50% solution of silver nitrate, Solution B: 12.8 grams of potassium bromide, 3 grams of potassium chloride, 0.5 gram of potassium iodide and 44 cc. of water. 80 cc. of a 10% solution of polyvinyl alcohol was then added :and the emulsion was stirred for 20 minutes at 50. C. 120 cc. of a solution of polyvinyl a1- cohol was added and the emulsion was cooled. 50grams of this emulsion was heated to 40 C. and Amie. gram of a-naphthol was added. One milligram of 2,3-diethyl-4'-methyl-oxathiazolocarbocyanine dissolved ina little methyl alcohol was added-tosensitize the emulsion. The emulsion was coated on a glass plate and found to gel in 2-3 minutes at room temperature and more rapidly when chilled. The plate was exposed wet and processed. It was found to develop and fix readily. Y

A plate coated as before was washed by placing in cold water for 30 minutes and was then dried.

On exposing and processing, the plate was found to develop and fix readily giving a clear, clean result. The emulsion was alsocoated on paper. It was dried by subjecting to a current of warm air. Upon exposing and processingdt' was found to give a clear,'clean result.

Example II bromide and 0.25 gram 'of potassium iodide made lowered temperature.

up to 21cc. with water. grams of 10% polyvinyl alcohol solution was then added followed by the addition of 10 cc. of glacial acetic acid. The emulsion was then heated to 145 F. and 20 cc. of a 6% solution of a-naphthol in ethanol was added. The emulsion was boiled for 16 hours. It set to a flrm gel which was cut into strips and thoroughly washed with water. The washed emulsion was melted by heating to F. and

10 cc. of a 6% solution of a-naphthol was added thereto. The emulsion was coated on glass plates. It set to a gel and dried rapidly. 'These plates were exposed and processed. They were found to be of medium speed and to give a clear. clean result. 1

1. A light-sensitive photographic emulsion comprising a silver halide dispersed in a thermalreversible gel composed of polyvinyl alcohol mixed with a gelling amount of a-naphthol.

2. A method of preparing a light-sensitive photographic emulsion which comprises preparing a silver halide in an aqueous'solution of a polyvinyl alcohol and subsequently adding thereto at a temperature of 40-90" C. a sufflcient amount of a-naphthol to form a rigid gel at a WESLEY G. LOWE.