Process For Preparing Silver Halide Emulsions

Abstract

Poly-disperse photographic silver halide emulsions comprising silver halide grains of uniform habit are produced by utilizing a continuous precipitation technique.

Parent Case Text

The present invention is a continuation-in-part of my copending U.S. Pat. application Ser. No. 7817, filed Feb. 2, 1970 now abandoned.

Description

The present invention is concerned with the preparation of photographic silver halide emulsions and, more particularly, with the preparation of emulsions having predictable properties and comprising grains of uniform crystal habit.

In a common method of preparing photographic silver halide emulsions, a water-soluble silver salt, such as silver nitrate, is reacted with at least one water-soluble halide such as potassium or ammonium bromide, preferably together with potassium or sodium iodide in an aqueous solution of a colloidal peptizing agent, such as gelatin. The dispersion of silver halide thus formed contains water-soluble salts as a by-product of the double decomposition reaction in addition to an unreacted excess of either of the initial salts. Such water-soluble salts must be removed from the emulsion before chemical ripening (afterripening) to enhance the emulsion's sensitivity to light, which will be discussed more fully hereinbelow. Such techniques are well known to those skilled in emulsion-making technology.

There are generally two recognized techniques for controlling the precipitation of silver halide in a gelatinuous protective colloid environment. Such techniques comprise, respectively, the single jet and double jet precipitation systems. In the former system, a peptizing agent is combined with one of the reactants, normally the halide, and the other reactant, e.g., silver nitrate, is introduced therein with agitation. The first portion of silver nitrate reacts with the halide solution to form silver halide nuclei; the latter portions form additional nuclei and also add onto other existing nuclei to form large crystals. The rate of addition thus effects the crystal size distribution. The crystal size distribution is also normally adjusted by holding the reaction mixture at elevated temperatures either after all the silver nitrate has been introduced or after only a portion has been introduced, or both. The smaller particles having a higher solubility than the larger particles dissolve to subsequently recrystallize out on the larger ones. This process of physical or Ostwald ripening broadens the particle size distribution and produces a coarse grain. The crystal habit of photographic emulsions made by a single jet technique normally consists of highly twinned crystals due to the large excess of halide ions present during the precipitation and physical ripening stages. Double jet precipitation, on the other hand, involves the introduction of a stream of an aqueous solution of a silver salt, as for example, silver nitrate; and the concomitant introduction of a stream of an aqueous solution of at least one alkali metal halide with or without a peptizing agent, into a reaction vessel generally containing a peptizing agent such as, preferably, gelatin. As the flow rates of both the silver nitrate and the halide solutions can be carefully regulated, the excess halide present in the reaction vessel can be maintained constant and untwinned crystals may be formed. When the flow rates of the solutions are low and the solutions are highly concentrated (about 3 N), nucleation takes place rapidly, usually in less than a minute, and then only growth of the nuclei occurs. Crystals over 1 micron in size can be grown with a very narrow particle size distribution. With continued additions the distance between particles increases until the time it takes for new material to reach the surface of the particles is larger than the nucleation time, fresh nucleation occurs, and a dimodal distribution results.

According to the present invention, streams of reactants are continuously fed at a controlled rate in a reaction vessel equipped with an efficient agitator, with the reaction product being removed at a continuous rate. At steady state, nuclei are continuously being formed and grown. Since the reaction mixture is well mixed the effluent contains grains of all sizes. In the presence of ammonia, crystals of cubic or octahedral habit can be rapidly grown to a micron size in an average residence time of only a few minutes compared to the many hours required by conventional double jet precipitation techniques.

The art has taught that silver halide emulsions can be made continuously. See U.S. Pat. No. 3,519,426 which defines a continuous technique for producing fine grain emulsions. Monodisperse emulsions comprising crystals about 1 micron in size would take about at least 16 minutes since typical growth rates are up to about 0.06 micron per minute under ammoniacal conditions, 0.2 N NH.sub.4 OH, and less at neutral conditions (Berry & Skillman, J. Photo. Chem. 68, 1138 [1964]). If growth conditions are altered to enhance crystal growth, one would expect twinned crystals to result because of the inability of stacking faults to diffuse out at such high growth rates, so twin planes form and continue to grow (Berry & Skillman, J. Photo. Sci. 16, 137 [1968]).

Contrary to what is dictated by the art, the process of the present invention provides polydisperse emulsions with average grain size between 0.5 and 1.1 microns, the grains being of uniform octahedral or cubic habit, in average residual times of less than 8 minutes, and preferably between 0.8 to 5.0 minutes.

It is, accordingly, a primary object of the present invention to provide a novel method for the preparation of photographic silver halide emulsions with highly predictable photographic characteristics.

It is another object of the present invention to provide photographic silver halide emulsions comprising silver halide grains of uniform habit and wide particle size distribution.

It is an additional object of the present invention to provide a method for the preparation of poly-disperse silver halide photographic emulsions comprising silver halide grains of uniform habit which comprises a system for continuously precipitating silver halide.

Other objects of the present invention will, in part, be obvious and will, in part, appear hereinafter.

The invention, accordingly, comprises the several steps and the relation and order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure and the scope of the application which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

FIG. 1 denotes a typical flow diagram definitive of the present invention;

FIGS. 2 through 5 are graphical representations depicting the particle size distribution relationship of emulsions of the present invention.

It will be recognized that numerous advantages may be achieved within photographic systems if the characteristics of silver halide emulsions utilized therein can be predicted with reasonable certainty; characteristics such as speed, sensitivity, distribution, and so forth, being considered the most desirable parameters which would hopefully be made subject to accurate prediction and control according to the present invention.

Although crystals of pure silver halide such as the chloride, or bromide, may be and have been used in photographic emulsions, highest sensitivity is generally produced with silver bromide, which additonally contains a small percentage of iodide ions. Microcrystals of pure or mixed silver halide of uniform crystal habit which are suitable for utilization in photographic emulsions may be precipitated according to the present invention by feeding into a reaction vessel at a continuous rate streams comprising a silver salt, such as silver nitrate; halides of at least 80 percent bromide constituency with up to 20 percent iodide and/or chloride, such as potassium or ammonium bromide, etc; a protective colloid such as gelatin; and, preferably, ammonia. Upon mixing in the reaction vessel, microcrystals of silver halides are formed and grow to a predetermined particle size distribution and average particle size. This growth mechanism takes place in the presence of continuously incoming streams of reactants. Physical ripening, if desired, can take place in an additional vessel or vessels and depends upon the assumption that very small crystals are more soluble than larger ones and so dissolve, thereby forming a super-saturated solution with reference to the larger crystals. Thus, larger crystals, by this physical or Ostwald ripening mechanism, grow at the expense of the smaller ones. In the absence of ammonia, physical ripening takes place at a very slow rate due to the low solubility of silver halides with the small excess bromide ion levels required to produce cubic or octahedral grains.

The ultimate particle size distribution of crystals emanating from the precipitation vessel can be accurately regulated by controlling the average residence time of the crystals within the reaction vessel, the concentration of reactants, the temperature and the degree of agitation. It will be appreciated that under these conditions, certain particles will remain within the reaction vessel longer than others, thereby attaining larger sizes than the particles removed from the system prior to the average residence time. Simple experimentation is all that is required to determine the necessary conditions (agitation, concentration, temperature, etc.) to produce a silver halide emulsion with a particle size distribution in the predetermined desired range. By judiciously choosing such a range, speed latitude of a photographic product produced with the emulsion fabricated according to the present procedure may be accurately predicted. The particle size distribution may be further regulated by controlling the degree of physical ripening subsequent to the precipitation stage.

A second parameter for which the instant invention provides accurate predictability is crystal habit. According to the present invention, emulsions comprising, substantially, a uniform crystal habit, either cubic or octahedral, may be prepared; the predominance of each being a function mainly of the silver ion concentration (bromide ion concentration) and of the ammonia concentration in the reaction mixture. It has been found that cubic crystals are generally produced at low halide ion concentrations and octahedral crystals are produced at higher concentrations. At intermediate values of halide ion concentration, as more fully herein below described, emulsions may be produced which comprise both octahedral and cubic crystals. If the halide ion concentration becomes too high, twinned crystals result. By raising the ammonia concentration, the halide ion concentration at which the transformation occurs from cubes to octahedra and from octahedra to twinned crystals is increased and also the crystal growth rates are increased.

As aforenoted, iodide ion content in the silver halide emulsion aids in increasing the speed of the emulsion. Iodide also increases the nucleation rate of silver halide.

It will be recognized that the halide ion concentration and the silver ion concentration are related by means of the solubility product, and the silver ion may, in fact, be the primary parameter for controlling crystal habit. However, since emulsions of at least 80 percent bromide constituency are contemplated by the present invention, and further since the examples define control parameters in terms of bromide ion concentration, for convenience sake, bromide ion concentration is referred to as a primary control throughout the specification. The negative common logarithm of bromide ion concentration, or pBr, is commonly used as a measure of bromide ion concentration. At zero pBr, or one normal bromide, only twinned crystals, such as flat platelets, truncated pyramides, etc., are formed in a system utilized according to the present invention. At a temperature of about 60.degree. C. and an ammonia concentration of 0.66 N, with a pBr of 1.0 to 1.4, octahedra are formed; and at pBr values above 1.6, cubic grains are formed. Mixed grains consisting of grains with cubic and octahedral habits are generally formed at pBr values of about 1.5. In the absence of ammonia, uniform crystal habits may be achieved only at higher pBr values, which are difficult to control.

It is necessary to utilize an ammoniacal precipitation gelatinous medium in the present invention in order to control the bromide ion concentration to provide the requisite value in the production of the habit and particle size distribution desired, and to obtain high growth rates. It is theorized that the ammoniacal condition, that is, the presence of ammonium hydroxide in the precipitating system, allows stacking faults to anneal out rapidly enough to repress twin grain formation and results in untwinned silver halide grains at high concentrations of bromide ion.

The precipitating system is generally operated at normal emulsion making temperatures, e.g., 60.degree. C. with ultimate flocculation and washing being generally accomplished below the gelation temperature of the emulsion being formed. Such temperatures, washing means, etc., are within the expertise of one of ordinary skill in the art to determine and are well known to such artisans. Further in the context of the present invention, it has been recognized that the precipitation vessel should preferably be maintained in a constant temperature environment, e.g., water jacket, etc. The same holds true, for example, for any ripening system which is utilized in conjunction with the precipitation mechanism in a continuous emulsion-making procedure. Agitation of the precipitating medium and/or agitation of the medium in the physical ripening environment (that is, ripening) under conditions where no additional reactants are being added to the system) is also suitably determined by the operator and is in the range of good engineering practice. Adequate mixing is needed in the precipitation stage to prevent agglomeration, etc.

The instant invention may be more fully appreciated with reference to FIG. 1 of the drawings which is a flow diagram of the preferred embodiment of the present invention. In that figure, a stream of silver nitrate solution and a stream of a solution comprising ammonium bromide, potassium iodide, gelatin, and ammonium hydroxide are continuously fed into a jacketed, baffled reaction vessel which is kept at a predetermined constant temperature. The volume of the vessel, and the respective concentrations and flows of the incoming streams are adjusted to provide predetermined residence time and reactant concentrations under constant agitation. The resultant emulsion is then continuously withdrawn from the precipitation vessel and, if desired, fed into a second jacketed, agitated vessel where physical ripening takes place during a second predetermined average residence time after which the material is fed to flocculating, washing, afterripening, and storage stages, in that order, such flocculation, washing, etc. being accomplished either continuously or according to conventional batch procedures. It will be appreciated that the essence of the present invention is the continuous precipitation and withdrawal of silver halide emulsion while maintaining the bromide ion concentration in the system at a predetermined constant level. Sensors to measure the bromide ion concentration may be placed, for example, in the precipitation vessel, in the physical ripening stage, in the flocculation stage, or in the lines from one stage to another. They are preferably placed just after the precipitating stage. Likewise suitable temperature sensors may be placed strategically throughout the system to accurately measure temperature. These sensors may be mated with control devices to provide automatic variable control.

The present invention will be more clearly understood with reference to the following examples:

EXAMPLE I

Into a 144 ml. reaction vessel containing four vertical baffles and a four-bladed turbine rotating at 1725 rpm is fed a stream comprising 1.0 N silver nitrate, at a metered rate of 24.8 mls. per minute, and a second stream comprising a mixture of 16.6 mls, per minute of 3.30 N ammonium hydroxide, and 41.4 mls. per minute of 0.589 N ammonium bromide, 0.03 N potassium iodide, and 1.293 percent trimellitic anhydride derivatized gelatin for a total flow of 82.8 mls. per minute into the vessel. The reaction vessel is maintained at 60.degree. C. by means of a water jacket and by heating the reactant streams. It will be appreciated that the average residence time of the silver halide in the vessel is 144 mls./82.4 mils./min. or 1.74 minutes. The gelatin is, in particular, acid pigskin gelatin derivatized with 5.5 percent trimellitic anhydride. In one case, the silver halide emulsion is continuously removed from the vessel and fed into a 144 ml. physical ripening vessel for an average residence time of 1.74 minutes. In another case, the ripening vessel is bypassed. In both cases, the emulsions are continuously fed into a flocculating stage maintained at 20.degree. C. and 2 N sulfuric acid is continuously added to maintain a pH of 2 to 3. From the flocculation stage, the flocculated emulsion is collected, washed, afterripened, sensitized with gold and sulfur, and stored. During this experiment bromide ion concentration was measured by a silver bromide coated silver electrode and a standard reference electrode using a standard pH meter and the pH was measured by a glass electrode and the same reference electrode, all electrodes being in the flocculation stage. The pBr measured in this experiment was 1.8 to 1.9. The uniform cubic structure and poly-disperse condition of the finished emulsion are immediately evident. FIG. 2 graphically depicts the particle size distribution in the finished emulsion. Likewise, FIG. 3 shows a graphical depiction of the particle size distribution of the emulsion which has been through the physical ripening stage. Both emulsions were found suitable for use in photographic negatives.

EXAMPLE II

The procedure of Example I was repeated, all parameters being the same except that the ammonium bromide concentration was 0.697 N resulting in a measured pBr of 1.2 to 1.26. The particle size distribution of the finished emulsion with no physical ripening is graphically depicted in FIG. 4. FIG. 5 is a graphical depiction of the particle size distribution of the same emulsion which has been physically ripened.

It will be appreciated that by the present invention, photographic silver halide emulsions may be produced with a high degree of predictability particularly as to speed and sensitiivity distribution, which, it is recognized, is responsible for the gray scale in a photographic silver halide environment.

Heretofore, if one desired to produce emulsions with silver halide grains of cubic or octahedral habit comprising the wide particle size distributions evident in the instant invention, it was necessary to provide separate monodisperse emulsions of uniform habit and blend such emulsions. It will be evident that such has been obviated by the instant invention.

The emulsions of the present invention may be chemically sensitized by any of the accepted procedures. For example, the emulsion may be digested with naturally active gelatin, or sulfur compounds can be added such as those described in U. S. Pat. Nos. 1,574,944; 1,623,499; and 2,410,689.

The emulsions may also be treated with salts of the noble metals such as ruthenium rhodium, palladium, iridium, and platinum, all of which belong to Group VIII of the Periodic Table of Elements and have an atomic weight greater than 100. The salts may be used for sensitizing in amounts below that which produce any substantial fog inhibition, as described in U. S. Pat. No. 2,448,060 and as antifoggants in higher amounts, as described in U. S. Pat. Nos. 2,566,245 and 2,566,263.

The emulsions may also be chemically sensitized with gold salts as described in U. S. Pat. No. 2,399,083 or stabilized with gold salts as described in U. S. Pat. Nos. 2,597,856 and 2,597,915.

The emulsions may also be chemically sensitized with reducing agents such as stannous chloride as described in U. S. Pat. No. 2,487,850; amines such as diethylenetriamine as described in U. S. Pat. No. 2,518,698; polyamides such as spermine as described in U. S. Pat. No. 2,521,925; or bis-(.beta. -aminoethyl)-sulfide and its water-soluble salts as described in U. S. Pat. No. 2,521,926.

The emulsions may also be stabilized with the mercury compounds of U. S. Pat. Nos. 2,728,663; 2,728,664; and 2,728,665.

The emulsions may also be optically sensitized with cyanine and merocyanine dyes as described in U. S. Pat. Nos. 1,846,301; 1,846,302; 1,942,854; 1,990,507; 2,112,140; 2,165,338; 2,493,747; 2,493,748; 2,503,776; 2,519,001; 2,666,761; 2,734,900; 2,739,149; and 2,739,964.

The emulsions may also contain speed-increasing compounds of the quaternary ammonium type as described in U. S. Pat. Nos. 2,271,623; 2,288,226; and 2,334,864; and of the polyethylene glycol type as described in U. S. Pat. No. 2,708,162.

Where desired, suitable antifoggants, restrainers, accelerators, preservatives, coating aids, and/or stabilizers may be included in the composition of the emulsions.

Hardening agents such as inorganic agents providing polyvalent metallic atoms, especially polyvalent aluminum or chromium ions, for example, potash alum [K.sub.2 Al.sub.2 (SO.sub.4).sub. 4.sup. . 24H.sub.2 O] and chrome alum [K.sub.2 Cr.sub.2 (SO.sub.4).sub. 4.sup. . 24H.sub.2 0] and organic agents of the aldehyde type such as formaldehyde, glyoxal, mucochloric acid, etc.; the ketone type such as diacetal; and the quinone type may be incorporated in the emulsions according to procedures well known in the art.

The term "photosensitive" and other terms of similar import are herein employed in the generic sense to describe materials possessing physical and chemical properties which enable them to form usable images when exposed to actinic radiation.

Throughout the specification, the term "poly-disperse" has been utilized. Such term is intended to define emulsions possessing wide particle size distributions.

Since certain changes may be made in the above process and product without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

Claims

What is claimed is:

1. A process for preparing a photosensitive poly-disperse silver halide emulsion comprising octahedric silver halide grains of at least 80 percent silver bromide constituency which comprises the continuous precipitation in a reaction vessel of said silver halide grains in the presence of a protective colloid, with constant agitation, under ammoniacal conditions which promote formation of an octahedric grain habit, and at a pBr between 1.0 and 1.4, and with the continuous removal of the resultant silver halide emulsion in order to provide a predetermined average grain residence time in said vessel of less than 8 minutes.

2. The invention of claim 1 wherein said protective colloid is gelatin.

3. A process for preparing a photosensitive poly-disperse silver halide emulsion comprising cubic silver halide grains of at least 80 percent silver bromide constituency which comprises the continuous precipitation in a reaction vessel of said silver halide grains in the presence of a protective colloid, with constant agitation, under ammoniacal conditions which promote formation of a cubic grain habit, at a pBr above 1.6, and with the continuous removal of the resultant silver halide emulsion in order to provide a predetermined average grain residence time in said vessel of less than 8 minutes.

4. The invention of claim 3 wherein said protective colloid is gelatin.