US 2921914 A
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States PREPARATION OF BLUE COLLOIDAL SILVER DISPERSIONS FOR ANTIHALO LAYER No Drawing. Application February 13, 1957 Serial No. 639,863
4 Claims. (Cl. 252-313) The present invention relates to colloidal dispersions of silver which are blue in color, and as such, particularly well adapted for use as antihalation layers for color film.
The formation of colloidal dispersions of silverin aqueous gelatin solutions by reduction of silver salts is a well known process. Such colloidal dispersions have been used in photography as antihalation layers or as filter layers for color film, particularly when located between the upper blue sensitive and the intermediate green sensitive layers of multilayer color film.
Typically, in US. Patent 2,220,187, yellow colloidal silver filter layers are recommended for absorbing blue light to prevent exposure thereby of the layers sensitized to the longer wave lengths but which also possess an inherent blue sensitivity. The colloidal silver is produced, according to this patent, by the reduction of silver nitrate in an alkaline medium in the presence of a peptizing agent by means of such organic reducing agents as dextrin, ferrous citrate or hydroquinone followedby dispersion in gelatin. The quantity of silver used is roughly 30 grams of silver nitrate to 1 kilo of emulsion.
The procedure of this patent leads to a dispersion of very fine particles of colloidal silver which, as a consequence, is typified by a yellow color. Thus it has been observed that the color of colloidal dispersions of silver in aqueous gelatin, depends on the size of the colloidal particles, the finest being yellow. The color then shifts to red, violet, blue and finally green as the particle size increases.
The blue colored dispersions are particularly desirable for forming antihalation layers on color film for the reason that they are more highly efficient in absorbing the longer wave lengths of light to which the layers adjacent the base are sensitive and by which they would be exposed through light reflected from the surface of the base. However, it has not been a simple matter to produce such dispersions without creating particles so coarse that they settle out from the dispersion.
The importance of the blue colloidal dispersions for use an antihalation layers and a method for their preparation are described in US. Patent 2,688,601. According to this patent, silver nitrate in gelatin is reduced with hydroquinone in the presence of an alkali metal sulfite and a calcium salt which may be separately added or may be present in the gelatin used in the reduction. It is assumed in this patent, that silver sulfite is first precipitated as a very fine dispersion and that this is subsequently reduced to the metal. Operators are cautioned that the silver sulfite is unstable and should undergo reduction before other and unwanted changes take place. In any case, the critical feature of the patent is the employment, in the reduction, of a calcium salt such as calcium acetate or the like, to which is attributed the ability to obtain large particles without the particles precipitating from the gelatin. To this end, it is emphasized also that the calcium salt must be present in a quantity equal to at least about .0017 mole per liter or reaction mixture.
I have now discovered that colloidal silver-gelatin dis- 'nitrate and strontium nitrate .several minutes, say
2 persions of an exceptionally dense blue color can be obtained providing that the reduction of the silver salt is effected by means of tamiic acid as the reducing agent in the presence of a water soluble strontium salt. The preparation of such dispersions 'by this procedure, and such dispersions themselves, constitute the purposes and ob jects of the present invention.
It will be appreciated that, according to my method,
I replace the calcium salt of US. Patent 2,688,601 by a strontium salt and, for this purpose, I may use any Water soluble strontium salt, the anion of which does not form an insoluble silver salt such as strontium nitrate, strontium acetate, and the like. It is to be emphasized, how
ever, that, whereas the strontium salt leads tov a'very marked improvement over the use of the calcium salt, much larger quantities of thestrontium salt must be employed. For example, whereas the patentee uses a minimum of .0017 mole ofa soluble calcium salt perliter, I use aminimum of roughly about three to four times this quantity of the strontium salt, -i;e;,-about .005 mole per liter.
Tannic acid operates to much greater advantage than the conventional reducing ag'ents such as dextr in, hydroquinone, ferrous citrate and the like, particularly in the presence of the strontium salt. In fact, my results are directly dependent upon the conjoint-use of tannic acid and strontium salts.
The procedure contemplated herein involves dissolving gelatin in water and adding suflicient alkali to give a pH of above 7.5, preferably between 8 and 9 (Solution 1). Separate solutions are then made up from water, silver (Solution 1) and from water, sodium sulfite and tannic acid (Solution III).
Solution II is added to Solution I and after a pause of 2 to 5,:Solution III is dumped inwith vigorous agitation, preferably while using several stirrers.
The emulsion is digested at 40 C., for about 20 minutes, chilled, shredded and Washed. Alternatively, the dispersion may be coagulated, washed, remelted and chilled.
The quantities of the various essential components may vary so long as there is present, the aforesaid minimum of strontium nitrate and an adequate quantity of silver salt and reducing agent to yield the dense blue colored dispersion. Typically, for 1 kilo of gelatin, I may employ .2 to .4 kilo of silver nitrate, .015 to .10 kilo of strontium nitrate, .1 to .4 kilo of sodium sulfite and .2 to .4 kilo of tannic acid. Conversely, excepting for the strontium salt, the proportions of the ingredients used in the above patent US. 2,688,601 are adequate.
My procedure has several outstanding advantages directly attributable to the use of the strontium nitrate and tannlc acid. First of all, a much better blue color is obtamed, a color which, in fact, is exceptionally dense. This leads to superior antihal-ation properties. Secondly, the procedure envisaged in U.S. Patent 2,688,601 involving the conjoint use of a calcium salt and hydroquinone yields a product which has poorer keeping qualities than the product hereof.
The great advantage arising from the use of a strontium salt with tannic acid is evident if one considers the results which ensue if a calcium salt be used with tannic acid as the reducing agent, or a strontium salt with hydroquinone as the reducing agent. In these cases, the color is quite inferior being much less dense and the antihalo effect is much less efficient. Conversely, if in my procedure I were to use tannic acid and an amount of strontium salt equivalent to the minimum.- amount of calcium salt stated by the patentee to be critical, a red colored rather than a dense blue colored product would be obtained.
following my procedure andin using my antihalo layer in lieu of the present gray colloidal silver antihalo layers'. "Thus; my antihalo layers give much better'protection against foggingjof, theZ'first frames of. film in 35 cartridges: by the light coming through thepartridge lip. This featureis particularly important-in connection with the. higher speed colorfilms of present day manufacture. The invention is illustratedby the following example, the parts being by weight unless otherwise stated. It is to be'understood, however, that the invention is not restricted to this example.
.EXAMPLE I Part'l Water liters' 63 Gelatin kgs 10 GN'NaOH to pH 8.8 x
7 Part 11- 7 Water liters; 12.5 AgNO kgs 3.2 Sr(NO kg 0.5
I Part III p v Water 1iters 27 Na SO kgs Tann ic. acid a kg 0.37 Add Hm I. Dump. Pause 5'. p Add Dump rapidly-use two stirrers;
Temp. 40, C. V
Digest--20 minutes. 7 Chill, shred, wash to conductivity of 1400; Yield-130435 kgs.
The resulting dispersion is coated on a film support in the usual manner and dried to produce a very effective bluev colored antihalation layer- When coated at a concentration of 3.2 milligrams of silver per 100 square centimeters, the optical density is 0.15 at 450 my and 2.35 at 650 ma. 7
Modifications of the invention will occur to persons skilled in the art and I, therefore, do not intend to be limited in the patent granted except as necessitated by the appended claims.
I claim; 7 -1. A method ofpreparing a blue color dispersion of silver in gelatin comprising admixing an aqueous alkaline gelatin solution with a water soluble silver salt and at least about .005 mol'of a water soluble strontium salt per liter of the reaction mixture, and eflecting reduction of the silver salt by adding an amount of an alkali metal sulfite which is at least one half of the weight of the silver salt and an excess of tannic acid. I
2.' The 'method as defined in :claim 1' wherein for each kilo of gelatin there aroused .2 to .4 kilosof 'a water soluble silver salt, .1 to .4 kilos of an alkali metal sulfite and .2 to .4 kilos of tannic acid.
'3. The process as defined in claim 1 wherein the silver salt is silvernitrate and the strontium salt is strontium nit-rate.
4. A stable dispersion of silver in gelatin, said dispersion having a dense 'blue color and being particularly adapted for use as an anti-halation layer for a multilayer color film, said dispersion being prepared according to the procedure of, claim 1.
References Cited in the file of this patent V UNITED'STATES PATENTS' 'Herz Sept. 7, 1954 OTHER REFERENCES Uses and Applications of Chemicals and'Related Materials--Gregory. Reinhold Pub Co. N.Y. vol. I (1939), p. 585.