US 2935405 A
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May 3, 1960 J. SOTTYSIAK STABILITY OF PHOTOGRAPHIC PAPER EMULS ION COATINGS Filed Aug. 25, 1955 DENSITY LOG EXPOSURE DENSETY V F l LOG EXPOSURE INVENTOR JOSEPH SOTTYSIA K United States Patent Q ce "STABILITY OF PHOTOGRAPHIC PAPER EMULSION COATINGS Joseph So ttysiak, B in g hamton, assignor to General Film gorporation, New York, N.Y., a corporation of Delaware Application August 25, 1955, Serial No. 530,615 6 Claims. (c1. 96- 110) Tli'e present invention relates to photographic paper coatings and, specifically, to the improvement of gradation stability of photographic paper coatings prepared Q frorn clilo'ro bromide type photographic paper emulsions,
characteristiccurves are plotted for the coated emulsion according to the well-known method of Hurter and Driffield (so-called H and D curve).
Photographic emulsions are generally coated on -a paper base in various grades of contrast, i.e., soft, 'mehard "and extra hard. The spacing between the gradations is so de'signatedthatphotographic prints can be made by the'usual methods of, exposure'to light from neg'atives'of 'varyin'g contrast. The photographic paper manufacturer attempts to stabilize'thcse contrast spacings so they will maintain themselves throughout the life of "thepaper. In most'ca'ses, the commercial quality of a photographic [paper is expected to continue for a period of 'upfto, two years, but considerable change occurs throughout the life of the paper. Various methods are used to obtain and maintain the spacing between con- 'trasts. A common one is to coat the photographic emulsion 'at the highest limit of its contrast specification so "that, upon aging and when ready for use, it will have leveled ofiat tlie designated gradation. This practice 'hasmany obvious deficiencies.
The other alternative is for the consumer to adjust "processing conditions so as to compensate for shifts in paper quality. This is not satisfactory since he may have fvari qus contrasts or different coating ages. By my'invention intend to, prepare a photographic paper emulsion which 'is considerably more stable toward change with aging by use of strontiumcliloride in the preparation 'of chloride-bromide type emulsions and Whilecm- ,ployin g a critical quantityof strontiurnchloride during silver halide precipitation. The preparation of such emulsions and the emulsion so prepared constitutetlie purposes and objects of the present invention.
The "literatur pertaining to photographic emulsion making technique is quite meager. The use of certain ehemicalsgduripg the precipitation of the silver halides in the carrier colloid gives, photographic results totally unpredictable from a normal knowledge of their chem "ical properties. s "Reference to'theu'se of strontium chlorideis-giv'en in Bakers Photographic Emulsion T echnique7"where strontium and lithium chloride are used as a source of chloride ion in forming silver chloride in a collodion type chloride printing out paper. Reference in this same book is made to the fact that halides of barium, cadmium and other metals give warmer toned emulsions than do those of 2,935,405 Patented May'3, 1960 2 potassium, sodium and ammonium. No other reference was found in the literature for the specific use of strontium chloride. As stated, I do not propose to use strontium chloride the known relationship nor as a means of obtaining warmer tones, but'as a gradation stabilizer.
I procure stabilization of gradation through the use of strontium "chloride in combination with sodium chloride in the presence of other halides during the precipitation of silver halide in a carrier colloid such as gelatin to form a photographic paper emulsion. In the preparation of unwashed chloro bromide emulsions, it is normal practice to use halide salts such as potassium bromide and sodium chloride and to form, by chemical interaction with silver nitrate in the presence of the carrier colloid such as gelatin, a silver bromide-silver chloride crystal complex. A small quantity, usually less than 10% of the bromide, of potassium iodide is added to give added sensitivity to the emulsion. Sodium chloride and potassium bromide are normally used as sources of halide ions because of their abundance, availability and favorablecost. Other metal halides can be used as sources of chloride ions. Strontium chloride in this invention is not; used merely as a source of chloride ions, but as an adduct to the sodium chloride-potassium bromide combination. It is possible without strontium chloride to manufacture a suitable chloro-bromide emulsion but, surprisingly with the addition of critical amounts of strontium chloride, a' markedly superior product is obtained.
A chloro broinide'emiilsion is prepared by interacting in the customary way in the presence of gelatin an aqueous solution or potassium bromide, sodium chloride and anarnoiiritofpotassium iodide, not 'to exceed 10% of'tl'le potassium'bremide, 'with silver nitrate so as not to completely precipitate all of the halide ions, but to leave a'slight excess, not more than 15% of the total.. This excess of halide in an unwashed emulsion is beneficial in stabilizing the emulsion against fog. At the proper temperature (SO- C.) and with the appropriate silver nitrate addition, a photographic emulsion is obtained which is subsequently coated on a paper base. This emulsion has the characteristics of Figure 1 of the accompanying self-explanatory drawing which illustrates the invention by sensitometric'curves prepared in the manner of Hurter and Drifiield.
It is-foundthat When the chloride salt concentration is changed so as to include the chlorides of strontium and sodium in a definite ratio of mol percentages, an
emulsion is obtained which, when coated on a paper base, gives a -product with the characteristics of Figure 2. F
in each figure refers to the freshly coated paper exposed in a sensitometer togive a characteristic H and D curve.
0, on the other hand, refers to the same coating exposed in a sensitometer after a four-day accelerated oven aging test effected at F. (60 C.) and a relative, humidity of 20-25% This'type of test treatment corresponds substantially to a normal natural aging of approximately one year. It will be observed that in Example l, there is considerable loss in gradation after aging, whereas in Example II, "there is relativel little less in gradation with aging;
It 'is necessary to operate within certain limits of halide salts to obtain the desired improvement. This range extends from a ratio of mor eicem of (a) 12.0 of sodium chloride "to one ofstrontium chloride to ('b) 2.7 of sodium 'chloride 'to one or strontium chloride. Concentrations lower than the 12/ 1 ratio have 'littleor no effect in improving gradational stability. Concentrations in a ratio higher than 2.7/l give an emulsion with poor photographic properties. Thus, the emulsions are diflicult to coat on a paper base and after aging, exhibit peculiar properties in normal photographic developer solutions.
The particular reasons for the improvement obtained have not been determined and are not easily explainable. Two theories present themselves but whether either is applicable is unknown. It is impossible to determine which of the halides present reacts with the silver nitrate. It is barely possible that the strontium chloride does not react or reacts only in part. This would mean that free strontium chloride is present in the emulsion, it being emphasized that the emulsion being a paper emulsion is unwashed.
It is noted, on the other hand, that in addition to the strontium chloride there is present potassium bromide, sodium chloride and potassium iodide; The possibility, therefore, exists that, upon reaction of the halides with silver nitrate, a peculiar heretofore unattainable crystal lattice is produced. However, regardless of the theory, the fact remains that by utilization of the various halides including strontium chloride and the particular ratio pointed out, new and unexpected results are obtained from the standpoint of gradational stability.
The following examples will serve to illustrate the invention but it is to be understood that the invention is not restricted thereto.
Example I The following basic formula defines a composition with the lowest concentration of strontium chloride. The strontium chloride used is the normally available crystalline hexahydrate', i.e., SrCl .6H O. The mol percent is calculated on the actual SrCl present in the crystal.
I. Potassium bromide --g..- 14.40 Sodium chloride --g-.. 8.40 Strontium chloride .g 3.20 Potassium iodide ..g 0.96
II. Gelatin g 98.20 Water 690.00
III. Silver nitrate ....g 36.00 Water c..- 360.00
The same basic formula was used as in Example I, excepting that the ratio of salts was as follows:
I. Potassium bromide 9.95 Sodium chloride 7.80 Strontium chloride 12.70 Potassium iodide 0.96
Example Ill The same basic formula was used as in Example I, excepting that the ratio of salts was as follows:
1. Potassium bromide 9.95 Sodium chloride 7.85 Strontium chloride 6.25 Potassium iodide 0.96
Example IV An ordinary emulsion was prepared for use in comparison in those of Examples I, H and III with the following ratio of salts;
G. I. Potassium bromide 9.95 Sodium chloride 12.50 Potassium iodide 0.96
The following table depicts the differences that exist on a mol percent basis with the aforestated ratio of sodium chloride to strontium chloride:
M01 Percent Example Ratio, NaOl/SrOI, N aOl SrOlz Gamma Example Fresh Aged Change -o. 11 -0. 10 O. 12 -o. 25
An examination of the table shows that by my invention the changes which occur upon aging have been greatly minimized. Thus, the emulsions of Examples I, II and III have considerably better gradation stability than emulsions prepared according to usual practice.
1. The process of producing an unwashed silver halide paper emulsion having improved gradation stability which comprises precipitating in a colloidal carrier therefor, a mixed silver halide by reacting silver nitrate with an aqueous solution of potassium bromide, sodium chloride, potassium iodide and strontium chloride in the presence of said colloidal carrier, the molar ratio of sodium chloride to strontium chloride being between 2.7 and 12 of the former to 1 of the latter.
2. The silver halide emulsion made by the process of claim 1. I
3. The process as defined in claim 1 wherein the colloidal carrier is gelatin.
References Cited in the file of this patent UNITED STATES PATENTS Crouse et al May 28, 1946 OTHER REFERENCES Baker: Photographic Emulsion Technique (2nd edition), 1948, American Phot. Publishing Company, Boston, page 251. Copy in Division 67. p