Title:
Method of making photographic silver halide emulsions and products thereof
United States Patent 3917485
Abstract:
A method is disclosed of making an internally sensitive photographic silver halide emulsion. In this method further silver halide is laid down on the grains of a surface sensitive or surface fogged emulsion so that excesses of silver and halide ions are alternately produced. In one preferred form a desensitizer is added to the completed internally sensitive emulsion, which desensitizer has a cathodic polarographic half-wave potential less negative that -1.0 volt. In another preferred form a sensitizing dye can be added to the internally sensitive emulsion. The invention is also directed to the silver halide emulsions formed by these methods.
US Patent References:
Direct positive photographs from hydrazine-containing developers
Ives - August 1951 - 2563785
Photographic silver bromide emulsion containing some silver iodide
Davey et al. - April 1952 - 2592250
Chemically sensitized emulsions having low surface sensitivity and high internal sensitivity
Porter et al. - September 1965 - 3206313
Silver salt direct positive emulsion
Berriman - February 1968 - 3367778
/3687676.html
Spence et al. - August 1972 - 3687676
Direct positive photographs from hydrazine-containing developers
Ives - August 1951 - 2563785
Photographic silver bromide emulsion containing some silver iodide
Davey et al. - April 1952 - 2592250
Chemically sensitized emulsions having low surface sensitivity and high internal sensitivity
Porter et al. - September 1965 - 3206313
Silver salt direct positive emulsion
Berriman - February 1968 - 3367778
/3687676.html
Spence et al. - August 1972 - 3687676
Application Number:
05/502695
Publication Date:
11/04/1975
Filing Date:
09/03/1974
View Patent Images:
Export Citation:
Assignee:
Eastman Kodak Company (Rochester, NY)
Primary Class:
Other Classes:
430/576
International Classes:
G03C1/015; G03C1/12; G03C1/36; G03C1/485; G03C1/36; G03C1/02; G03C1/08
Field of Search:
96/101,64,94R,120,125,108
Primary Examiner:
Klein, David
Assistant Examiner:
Suro Pico, Aalfonso T.
Attorney, Agent or Firm:
Thomas C. O.
Parent Case Data:
CROSS-REFERENCE TO RELATED APPLICATION
This is a continuation-in-part of my earlier filed, copending U.S. Pat. application Ser. No. 431,453, filed Jan. 7, 1974 now abandoned.
Claims:
I claim
1. A method of making an internally sensitive photographic silver halide emulsion in which additional silver halide is precipitated onto silver halide grains of a surface sensitive or surface fogged silver halide emulsion characterized in that alternate excesses of silver ions and halide ions are caused to be present during precipitation.
2. A method according to claim 1 characterized in that the alternate excesses comprise a pAg adjustment cycle in which sufficient silver salt is caused to be present to adjust the pAg of the emulsion so that an excess of silver ions are present as compared to halide ions and sufficient alkali metal halide salt is then caused to be present to adjust the pAg so that an excess of halide ions are present as compared to silver ions.
3. A method according to claim 2 in which the silver salt is silver nitrate and the alkali metal salt is potassium halide.
4. A method according to claim 3 in which the potassium halide is at least one of potassium bromide and potassium iodide.
5. A method according to claim 2 in which the alternate excesses take place at a temperature of not more than 40°C and for times of from 15 to 30 seconds for each part of the pAg adjustment cycle.
6. A method according to claim 5 in which the pAg is at least 4 when an excess of silver ions are present and up to 10 when as excess of halide ions are present.
7. A method according to claim 5 in which the pAg is in the range of from 4 to 6.5 when an excess of silver ions are present and in the range of from 8 to 10 when an excess of halide ions are present.
8. A method according to claim 2 in which at least 3 pAg cycles are employed.
9. A method according to claim 5 in which at least 7 pAg cycles are employed.
10. A method according to claim 1 in which the surface sensitive or surface fogged silver halide emulsion is a silver bromoiodide emulsion.
11. A method according to claim 1 in which a spectral sensitizing dye or a desensitizer is caused to be present in the internally sensitive photographic silver halide emulsion.
12. A method according to claim 11 in which the sensitizing dye or desensitizer is added to the internally sensitive silver halide photographic emulsion after the precipitation of additional silver halide onto the grains.
13. A method according to claim 11 in which the sensitizing dye or desensitizer is added to the internally sensitive silver halide emulsion in a concentration that would at least partially desensitize the surface sensitive emulsion.
14. A method according to claim 13 in which the sensitizing dye or desensitizer is added to the internally sensitive silver halide emulsion in a concentration that would produce a loss in blue speed sensitivity in a sulfur and gold surface sensitized silver bromoiodide emulsion (6 mole percent iodide) of similar grain size of at least 0.3 log E when developed in surface developer Kodak D-19.
15. A method according to claim 11 in which a desensitizer is caused to be present having a cathodic polarographic half-wave potential less negative than -1.0 volt.
16. A method according to claim 1 in which the surface sensitive emulsion has been chemically sensitized on its surface.
17. A method according to claim 16 in which the surface sensitive emulsion has been sulfur sensitized
18. A method according to claim 1 including the additional step of fogging the internally sensitive photographic silver halide emulsion to form an emulsion useful in forming direct positive images.
1. A method of making an internally sensitive photographic silver halide emulsion in which additional silver halide is precipitated onto silver halide grains of a surface sensitive or surface fogged silver halide emulsion characterized in that alternate excesses of silver ions and halide ions are caused to be present during precipitation.
2. A method according to claim 1 characterized in that the alternate excesses comprise a pAg adjustment cycle in which sufficient silver salt is caused to be present to adjust the pAg of the emulsion so that an excess of silver ions are present as compared to halide ions and sufficient alkali metal halide salt is then caused to be present to adjust the pAg so that an excess of halide ions are present as compared to silver ions.
3. A method according to claim 2 in which the silver salt is silver nitrate and the alkali metal salt is potassium halide.
4. A method according to claim 3 in which the potassium halide is at least one of potassium bromide and potassium iodide.
5. A method according to claim 2 in which the alternate excesses take place at a temperature of not more than 40°C and for times of from 15 to 30 seconds for each part of the pAg adjustment cycle.
6. A method according to claim 5 in which the pAg is at least 4 when an excess of silver ions are present and up to 10 when as excess of halide ions are present.
7. A method according to claim 5 in which the pAg is in the range of from 4 to 6.5 when an excess of silver ions are present and in the range of from 8 to 10 when an excess of halide ions are present.
8. A method according to claim 2 in which at least 3 pAg cycles are employed.
9. A method according to claim 5 in which at least 7 pAg cycles are employed.
10. A method according to claim 1 in which the surface sensitive or surface fogged silver halide emulsion is a silver bromoiodide emulsion.
11. A method according to claim 1 in which a spectral sensitizing dye or a desensitizer is caused to be present in the internally sensitive photographic silver halide emulsion.
12. A method according to claim 11 in which the sensitizing dye or desensitizer is added to the internally sensitive silver halide photographic emulsion after the precipitation of additional silver halide onto the grains.
13. A method according to claim 11 in which the sensitizing dye or desensitizer is added to the internally sensitive silver halide emulsion in a concentration that would at least partially desensitize the surface sensitive emulsion.
14. A method according to claim 13 in which the sensitizing dye or desensitizer is added to the internally sensitive silver halide emulsion in a concentration that would produce a loss in blue speed sensitivity in a sulfur and gold surface sensitized silver bromoiodide emulsion (6 mole percent iodide) of similar grain size of at least 0.3 log E when developed in surface developer Kodak D-19.
15. A method according to claim 11 in which a desensitizer is caused to be present having a cathodic polarographic half-wave potential less negative than -1.0 volt.
16. A method according to claim 1 in which the surface sensitive emulsion has been chemically sensitized on its surface.
17. A method according to claim 16 in which the surface sensitive emulsion has been sulfur sensitized
18. A method according to claim 1 including the additional step of fogging the internally sensitive photographic silver halide emulsion to form an emulsion useful in forming direct positive images.
Description:
This invention relates to a method of making photographic silver halide emulsions and to silver halide emulsions made thereby.
Photographic silver halide emulsions which form latent image predominantly in the interior of the silver halide grain are often called internal latent image emulsions. A number of ways of preparing them have been described, for example, in British Specification Nos. 581,792, 581,789, 635,841, 1,027,146 and 1,011,062.
A typical method for preparing such emulsions is described in British Specification No. 1,011,062 (U.S. Pat. No. 3,206,313). This involves blending a very fine grain silver halide emulsion, such as a Lippmann emulsion, with a surface sensitized emulsion having considerably larger average grain size. This blend is then held under conditions and for a period of time sufficient to cause the silver halide of the fine grain emulsion to dissolve and recrystallize on the grains of the surface sensitized emulsion. This results in silver halide grains having a core derived from the surface sensitized grains and a shell of silver halide derived from the fine grain emulsion. Since the silver halide shell covers the sensitivity sites previously present on the surface of the silver halide grains, these sensitivity sites are now buried in the silver halide grains.
This method of preparing internally sensitive photographic silver halide emulsions can be relatively complicated. It requires careful control of the conditions under which the blend of fine grain and coarse grain emulsions is ripened if a desirable product is to be obtained. Furthermore, not all emulsions are useful as the "coarse grain" core emulsions. This is particularly true of emulsions whose grains vary considerably in size, such as can occur in polydispersed emulsions.
The present invention provides a relatively simple procedure for making internally sensitive photographic silver halide emulsions, which procedure is applicable to convert a variety of surface sensitive or surface fogged silver halide emulsions (including polydispersed emulsions of varied grain size) to internally sensitive photographic silver halide emulsions.
According to the present invention there is provided a method of making an internally sensitive photographic silver halide emulsion in which further silver halide is laid down on grains of a surface sensitive or surface fogged silver halide emulsion by producing alternate excesses of silver ions and halide ions in solution during precipitation of additional silver halide onto the silver halide grains.
The present invention also provides an internally sensitive photographic silver halide emulsion comprising initially surface sensitive or surface fogged grains and, on each of said grains, a surface layer of silver halide precipitated from a solution containing alternate excesses of silver and halide ions during precipitation.
The present invention can be applied to any conventional surface sensitive or surface fogged silver halide emulsion. The surface sensitive or surface fogged silver halide emulsion can be that which has been employed alone, in combination with other emulsions or as a core emulsion in forming an internally sensitive emulsion. Such emulsions and their formation are well known to those skilled in the art and are disclosed, for example, in U.S. Pat. Nos. 2,222,264; 3,320,069; 3,271,175; 2,592,250; 3,206,313; 3,367,778; 3,447,927; 3,447,927; 2,184,013; 2,541,472; 3,501,307; 2,563,785; 2,456,953 and 2,861,885, the disclosures of which are here incorporated by reference.
Preferred surface sensitive emulsions are those that have been chemically sensitized--e.g. gold or noble metal sensitized, sulfur or selenium sensitized and/or reduction sensitized. The silver halide grains in one preferred form can also be treated with salts of noble metals, such as ruthenium, palladium, platinum, iridium, rhodium, and iron. When the emulsion is surface fogged this can be achieved using conventional nucleating agents or by fogging through uniform exposure.
My invention is particularly applicable to surface sensitive or surface fogged emulsions having a wide distribution of grain sizes, as can be formed either by blending or by the method of precipitation chosen--e.g., using a single jet precipitation technique. My invention is also fully compatible with the use of monodispersed emulsions or those having a relatively restricted range of grain sizes, as have been heretofore conventionally employed in forming internally sensitized emulsions. It is a specific advantage of my invention that it can be applied to emulsions generally regardless of grain size or grain size distribution. Thus, my invention is applicable to Lippman emulsions, relatively coarse grain radiographic emulsions, monodispersed emulsions and polydispersed emulsions generally.
To form silver halide layers on the surface sensitive or surface fogged silver halide grains in order to convert these grains to internally sensitive silver halide grains the surface sensitive emulsion is brought into contact with a conventional silver halide precipitation solution. Silver halide layers can then be formed on the surface sensitive or fogged grains using generally the same materials and techniques useful in forming silver halide grains (i.e., the procedures disclosed in the patents cited above), except as specifically modified to achieve the advantages of my invention.
Typically the precipitation solution includes a silver salt and an alkali metal halide which interact in a double decomposition reaction to form silver halide and an alkali metal salt by-product which remains in solution. To avoid clumping of the silver halide grains during additional silver halide precipitation, small amounts of a peptizer, such as gelatin or some other conventional hydrophilic colloid, is present during precipitation. Suitable peptizers are disclosed, for example in Product Licensing Index, Volume 92, December 1971, publication 9232, paragraph VIII. The peptizer can be that used to form the surface sensitive or surface fogged silver halide emulsion or, if desired, additional peptizer can be added during formation of the silver halide layers.
Any conventional technique for bringing together the silver and the halide containing salts in the presence of the surface sensitive or surface fogged silver halide emulsion can be employed, provided an excess of silver or halide ions can be produced and the excess reversed at will. According to one approach, to the surface sensitive or fogged emulsion is added both the silver and bromide salts, but with one being present in excess. Subsequently additional quantities of the silver and the halide salts are alternately added so that the excess of ions above the stoichiometric required amount is successively shifted between the silver halide ions present. In one form then, the formation of silver halide layers can be viewed as an "alternate jet" mode of precipitation. Instead of actually interrupting the flow of silver salt and halide salt periodically, it is also possible to sequentially restrict the flow of the separate jets and/or sequentially increase the flow of the separate jets. The silver and the halide salts can be added directly as salts, but are preferably added in aqueous solution.
As an example, sufficient silver nitrate solution is added to the precipitation solution to adjust the pAg of the emulsion to the silver side, e.g., to a pAg of 5.0 (from 4 to 6.5, most preferably generally), then sufficient potassium halide solution is added to adjust the pAg of the emulsion to the halide side, e.g., to a pAg of 8.0 (from 7.5 to 10, most preferable generally), and this pAg adjustment cycle may be repeated several times. After each successive cycle the "surface" speed and contrast is reduced, as judged by development in a normal commercial developer of low silver halide solvent content. Development in an "internal" developer, i.e., one containing an appreciable content of silver halide solvent, will however yield an image with speed and contrast similar to the original surface emulsion.
Good results can be obtained using pAg figures other than those given above. It is desirable that the silver excess should not be too great--typically a pAg no less than 4 is desirable, and the temperature and time that the emulsion is held at the low pAg should be as low as conveniently possible, otherwise there is a tendency for fog to develop. In general, temperatures of not more than 40°C and times of 15 to 30 seconds for each part of the cycle yield the best results.
If precipitation is conducted above 40°C, the shift in the neutral pAg of the precipitation solution must be considered in chosing the pAg levels in cycling. For example, whereas a neutral pAg is about 7.0 at room temperature, at 80°C the neutral pAg is about 5.5. Generally cycling of at least one pAg unit and, preferably, two pAg units on either side of pAg neutrality is contemplated. As expressed in this specification, pAg values and ranges are related to temperatures in the range of from room temperature to 40°C.
Although the added silver halide may be of any constitution, the best results are obtained when the silver halide is silver bromoiodide. In adjusting the pAg it is preferred to employ bromoiodide solutions rich in iodide. Thus, for example, good results are obtained with 75 percent bromide, 25 percent iodide (mole percentages).
Generally between 7 and 20 cycles are sufficient to suppress the normal surface sensitivity completely, although a beneficial degree of internal sensitivity can be achieved with a few as 3 cycles or less. The number of cycles for any specific application will vary with the grain size of the emulsion, halide make-up of the added layers, and the crystal habit of the original grains, but can be readily ascertained by one skilled in the art.
The emulsions prepared by the present process need not be fully internal latent image emulsions, but can retain significant surface sensitivity if only a few pAg cycles have been used. Thus, emulsions having any desired balance of internal and surface sensitivities can be produced using my process.
It is not certain whether all the additional silver halide is laid down on the surface of the grains or whether some is precipitated separately in the gelatin phase and subsequently transferred to the grains by Ostwald ripening, or whether some remains permanently as a separate fine grain precipitate. Nevertheless it is thought that by this technique thin layers, of perhaps 10 to 100 ion pairs of additional silver halide each, are laid down. It is not necessary to the practice of the invention to accept this particular theory, but it is supported by the fact that the number of cycles necessary to suppress the surface sensitivity is less in the case of octahedral grains than those of cubic habit, since in the former case it might be expected that a whole layer of ions would more easily be absorbed for each pAg change.
It is not even necessary to complete the pAg cycles before exposure; conversion of an existing latent image from `surface` to `internal` can be achieved by the pAg cycling technique in the same way as conversion of the sensitivity. Thus if a liquid emulsion is fogged by exposure to light, and subjected to an appropriate number of cycles, a `clean` emulsion with internal fog results.
An advantage of this invention is the suppression of the desensitizing action of spectral sensitizing dyes. All sensitizing dyes tend to desensitize the natural sensitivity of a photographic emulsion in the blue region, and is practice this sets a limit to the amount of dye it is possible to add to an emulsion. Moreover there are specific interactions between emulsions of well defined crystal habit and certain classes of sensitizing dye (e.g. octahedral emulsions and carbocyanines) that results in excessive desensitization, as noted by Markocki (J. Photo. Sci. 1965, 13, 85).
This desensitization can be substantially reduced or even completely overcome by using the present cycling technique to interpolate silver halide between the dye at the surface and the sensitivity below. Moreover, this reduction in desensitization is often not accompanied by any adverse action on the efficiency of sensitization, i.e. the relation between the speed in the dye band and the basic blue sensitivity of the emulsion is unchanged.
Thus, in a preferred embodiment, the initially sensitive silver halide emulsions of this invention are particularly suitable for use with sensitizing dyes or desensitizers which are added to the emulsion at a concentration which would desensitize the emulsion if it had not been subjected to the present cycling technique. Typical sensitizing dyes having a primary absorption peak less than 700 millimicrons are described in Belgian Pat. No. 770,293 while typical sensitizing dyes having a primary absorption peak greater than 700 millimicrons are described in U.S. Pat. No. 3,690,891. The desensitizers or electron acceptors useful in the emulsion of this invention are described in U.S. Pat. No. 3,687,676. The concentrations of the sensitizing dyes and desensitizers can be described as being above that which would produce a loss in blue sensitivity in a sulfur and gold surface sensitized silver bromoiodide emulsion (6 mole percent iodide) of similar grain size of at least 0.3 log E when developed in a surface developer such as Kodak D-19.
It is, of course, recognized that lower, conventional concentrations of sensitizers and desensitizers can be employed, as is taught in the art in connection with core-shell emulsions. Preferred desensitizers are those conventional desensitizers characterized by having a cathodic polarographic half-wave potential (Ec) less negative than -1.0 volt.
Cathodic measurements are made with a 1 × 10 - 4 molar solution of the electron acceptor in a solvent, for example, methanol which is 0.05 molar in lithium chloride using a dropping mercury electrode with the polarographic half-wave potential for the most positive cathodic wave being designated E c . Anodic measurements are made with 1 × 10 - 4 molor aqueous solvent solution, for example, aqueous methanolic solutions of the electron acceptor which are 0.05 molar in sodium acetate and 0.005 molar in acetic acid using a carbon paste or pyrolytic graphite electrode, with the voltametric half peak potential for the most negative anodic response being designated E a . In each measurement, the reference electrode is an aqueous silver- silver chloride (saturated potassium chloride) electrode at 20°C. Electrochemical measurements of this type are known in the art and are described in New Instrumental Methods in Electrochemistry, by Delahay, Interscience Publishers, New York, New York, 1954; Polarography, by Kolthoff and Lingane, 2nd Edition, Interscience Publishers, New York, New York, 1952, Analytical Chemistry, 36, 2426 (1964) by Elving; and Analytical Chemistry, 30, 1576 (1958) by Adams.
The present emulsions may be made by any of the techniques employed in the art and may contain emulsion addenda including coating aids, antifoggants and speed-increasing compounds. The emulsions may be coated on a variety of supports. A review of emulsion chemistry and applications of photographic emulsions appear in "Product Licensing Index" December 1971 pages 107-110, Industrial Opportunities Ltd., Havant, Hampshire.
The present emulsions may be used to obtain directpositive images preferably by exposing them imagewise unfogged and then developing in the presence of fogging or nucleating agents in known manner.
It is to be appreciated that the emulsions formed according to my invention can be further modified, as by the incorporation of addenda, coated, exposed and processed by procedures known to those skilled in the art. Such teachings can be summarized at least in part, for example, in Product Licensing Index, Volume 92, published December 1971, publication 9232, pages 107 through 110.
The following Examples illustrate the invention. The developers referred to as D19 and D19b have compositions corresponding to Kodak Developers D19 and D19b respectively given in the Kodak Limited Data Book. The work "Kodak" is a registered trade mark.
EXAMPLE 1
An emulsion was prepared by running a solution of 100 grams silver nitrate in 500 cc of distilled water at 55°C. into a solution of 15 grams gelatin, 75 grams potassium bromide and 4.5 grams potassium iodide in 750 cc of distilled water at 60°C. for over 10 minutes. After heating for 30 minutes at 60°C., 125 grams of active gelatin dissolved in 500 cc of water at 60°C. was added, the suspension was set to gel, shredded and washed 40 minutes in running water. It was then re-melted and heated for a further period of 40 minutes at 55°C., the final volume being adjusted to 1750 cc with water.
This method of preparation gives crystals which form latent image preferentially at the surface, as shown below:
A 200 cc sample of the suspension was exposed for one minute at a distance of 6 feet from a 100 w lamp in the liquid state at 35°C. with efficient stirring, held at this temperature for 5 minutes, then coated on glass plates and allowed to dry. Two of these plates were then developed, one for 15 minutes in D19b developer and the other for 5 minutes in D19b containing 20 g/liter of hypo, after first bleaching for 5 minutes in 3 percent potassium ferricyanide and rinsing in water for 5 minutes. The first plate had a density of 3.6, while the second had a density of 0.92, showing that the latent image was predominantly on the surface of the grains.
A second sample was exposed in exactly the same manner and subjected to 10 `pAg cycles` in the following manner. 1.0 N silver nitrate solution was added to give a silver ion excess (pAg 5.0), the suspension held for 15 seconds, then 0.95 N potassium bromide, 0.05 N potassium iodide solution (the same bromide-iodide ratio as the silver halide crystals) was added to restore the halide excess (pAg 8.5) followed by another 15 seconds holding. The quantities of the solutions added for each part of the cycle had been pre-determined by electrometric titration of a separate sample. At the conclusion of the ten cycles, the suspension was coated on glass and dried as before. In this case, however, a plate developed for 5 minutes in D19b had a density of 0.31, while one developed in the D19b containing hypo, without preliminary bleach, had a density of 2.40.
It would thus appear that the passage through successive baths of silver and halide ions had in fact laid down a shell of fresh silver halide on the crystals covering the latent image. Development then only occurs when solvent is present to remove this shell. Further runs showed that for most dyes the major degree of desensitization is suppressed after six or seven cycles and after ten cycles little further improvement can be observed.
EXAMPLE 2
A fully chemically sensitized fast negative bromoiodide emulsion of 8 mole percent iodide content with mainly triangular grains of average edge length 1.55 μ and thickness 0.3 μ was subjected to 10 pAg cycles as in Example 1 but using a 0.75 N potassium bromide, 0.25 N potassium iodide solution to restore the pAg to the higher level. On coating plates, exposing and developing for 5 minutes in D19b at 20°C, only a ghost image resulted. Addition of 20 g/liter of sodium thiosulphate to the developer gave an image of speed and contrast only slightly lower than that of the uncycled emulsion.
EXAMPLE 3
Addition of 0.44 grams of the sensitizing dye 3-3-dimethyl-9-ethyl-4,5,4', 5'-dibenzothiacarbocyanine iodide (E c ==1.12v and E a =+0.58v) per mole of silver halide to the pAg cycled emulsion of Example 2 yielded an emulsion that gave an internal image of normal dye sensitivity, judged by a wedge spectrogram, but again virtually no image when developed in a surface developer. A similar result was obtained even when the dye was added to the emulsion before the pAg cycles.
EXAMPLE 4
A pure bromide emulsion with monodispersed grains of octahedral habit and edge-length 0.51 μ was prepared, and sulphur sensitized to optimum speed by digestion for 30 minutes at 65°C with 3.5 mg sodium thiosulphate per mole Ag. One portion was then subjected to 10 pAg cycles as in Example 2, and film coatings with 225 mg Ag per square foot were prepared of
1. the untreated emulsion
2. the same as 1, with the addition of 400 mg of the sensitizating dye anhydro-bis-(5,6-dichloro-1-ethyl-3,3'-sulfobutyl-2-benzimid azole) trimethinecyanine hydroxide (E c ==1.60v and E a =+.53v)
3. the pAg cycled emulsion
4. the same as 3, with the addition of the sensitizing dye.
Strips of the film were exposed on an Eastman IB sensitometer through blue (Wratten 47B) and minus blue (Wratten 12 + 58) filters, and developed in D19 developer for 3 minutes at 20°C. A further set was exposed in the same way and developed in D19 developer to which 10 g of sodium thiosulphate has been added per liter. The results are shown below, the speeds quoted being the relative log speeds for a density of 0.1 above fog.
______________________________________ Developer Compositions D19 D19b ______________________________________ p-methylamlnophenol sulphate 2.0 g 2.2 g sodium sulphite (anhydrous) 90.0 g 72.0 g hydroquinone 8.0 g 8.8 g sodium carbonate 45.0 g 48.0 g potassium bromide 5.0 g 4.0 g water to 1 liter 1 liter ______________________________________
______________________________________ Sample Blue Exposure Minus Blue Exposure Speed Υ Speed Υ ______________________________________ D19 Development 1. Control 2.27 1.90 -- -- 2. Control + dye 0.30 1.82 0.69 1.54 3. 10 pAg cycles 0 0 -- -- (ghost) 4. 10 pAg cycles 0 0 0 0 + dye (ghost) D19 + Sodium Thiosulphate Development 1. Control 2.19 0.81 -- -- 2. Control + dye 0.38 0.68 0.74 0.83 3. 10 pAg cycles 1.77 0.90 -- -- 4. 10 pAg cycles 1.38 0.52 1.73 0.46 + dye ______________________________________
EXAMPLE 5
The gradual covering of the sensivitity is shown in this example. The same emulsion, cycling technique, sensitizing dye and development conditions were used as in Example 4, but this time film coatings were made after different numbers of pAg cycles. In the following results, the speeds quoted are relative log speeds for a density of 0.3 above fog after 3 minutes D19 development at 20°C.
______________________________________ Number of Without Dye With Dye Minus pAg cycles Blue Blue Blue Speed Υ Speed Υ Speed ______________________________________ 0 2.16 2.66 0.60 1.60 1.04 1 2.22 2.93 1.66 2.01 2.15 2 2.14 2.06 1.38 1.33 1.80 5 1.92 1.14 0.32 0.38 0.88 10 0 0 0 0 0 ______________________________________
This illustrates that the emulsion was entirely converted to an internally sensitized emulsion lacking surface sensitivity in 10 pAg cycles and that in 5 cycles the major portion of surface sensitivity had been internalized.
EXAMPLE 6
The same cycling conditions and solutions were used as in Example 5, but a fully digested cubic-grained AgBr emulsion of the same grain size was employed, while the sensitizing dye was a merocyanine, 3-carboxymethyl-5-[1'-(3-methylthiazolidin-2-ylidene)-prop-2 '-ylidene]-2-t hiothiazolid-4-one (E c =-1.47v and E a =0.53v). The results show that more cycles are necessary to suppress the surface sensitivity of a cubic emulsion, but again the desensitizing action of the dye is reduced.
______________________________________ D19 Development D19 + 10 g/liter Sodium Thiosulphate Number Sensi- Minus Minus of pAg tizing Blue Blue Blue Blue cycles Dye Speed Υ Speed Speed Υ Speed ______________________________________ 0 0 2.55 1.68 -- 2.37 0.88 -- 0 400 mg 1.10 1.70 1.53 1.03 1.25 1.43 mole Ag 10 0 1.95 0.34 -- 2.47 0.85 -- 10 400 mg/ 2.02 0.50 2.32 2.17 0.77 2.57 mole Ag 20 0 -- 0.20 -- 2.24 0.66 -- ______________________________________
Examples 5 and 6 show further that the reduction of desensitization by dyes may be obtained when insufficient silver halide has been added to reduce the surface sensitivity substantially, so that normal commercial developers with no added solvent may still develop the latent image effectively.
EXAMPLE 7
Internal latent image emulsions may be used for preparing direct positive images by using suitable treatment and processing as revealed e.g. in British Pat. Specification No. 581,773 and U.S. Pat. Nos. 2,568,786 and 2,497,875. The emulsions prepared by pAg cycling technique of the present invention may be used in the same manner. Thus a pAg cycled emulsion coating prepared and coated as in Example 4 was exposed and processed with a viscous processing solution. The solution contained 20 g sodium hydroxide, 40 g sodium sulphite, 1 g of potassium bromide, 0.52 g 1-phenyl-3-pyrazolidone, 12 g hydroquinone, 0.60 g N-formyl-N'-p-tolylhydrazine and 25 g hydroxyethyl cellulose in one litre of water. This viscous developer was spread on the emulsion coating at a thickness of approximately 0.1mm, after 45 secs. development at room temperature, was stopped by washing the solution with dilute acetic acid, and the image fixed with sodium thiosulphate solution in the normal way. The image thus obtained was a direct positive image of approximately the same speed as the negative images obtained by internal image development.
EXAMPLE 8
Portions of the bromide emulsion of Example 4 were sensitized with a variety of dyes, while other portions of this emulsion were subjected to one pAg cycle using the solutions of Example 2. The sensitizers used were:
A. (3,3'-diethylbenzothiazole) heptamethinecyanine iodide, E c = -0.72 E a =0.26.
B. (3,3'-dicthylbenzoselenazole)pentamethinecyanine bromide, E c = -0.84 E a = 0.46.
C. (5,5'-dichloro-3,3'-diethylbenzothiazole) trimethincyanine iodide E c = -0.86 E a = 0.84.
D. (1,3-diethyl-2-imidazo-[4,5-6]quinoxaline) (1-methyl-2-phenyl -3-indole) dimethinecyanine iodide, E c -0.63 E a = >1.0.
E. (1,3-diallyl-2-imidazo-[4,5,-6]quinoxaline) (3,5,-dimethyl-1-phenyl-4-pyrazole) dimethinecyanine iodide, E c = -0.52 E a = >1.0 calc. 2.14
Strips of the film were exposed as in Example 4 and developed using a normal commercial surface developer for 5 minutes at 21°C. The results are shown below.
______________________________________ Blue Speed Minus Blue Speed No + pAg No + pAg Cycle cycle Cycle cycle ______________________________________ Control for dye A 2.04 2.03 -- -- " + 25mg dye A/mol 1.17 1.58 0.35 1.16 Control for dye B 1.85 1.90 -- -- " + 150mg Dye B/mol 0.58 1.67 <0.40 1.55 Control for dye C 2.11 1.74 -- -- " + 400mg dye C/mol 0.97 1.16 0.91 1.27 Control for dye D 2.05 2.27 -- -- " + 25mg dye D/mol 1.37 2.16 0.98 2.26 Control for dye E 2.22 1.98 -- -- " + 5mg dye E/mol 1.05 1.70 -- -- (Dye E does not extend the spectral sensitivity far enough to give minus blue speed figures). ______________________________________
EXAMPLE 9
A polydispersed, high speed sulfur and gold sensitized bromoidide emulsion was pAg cycled as follows:
A 1.0 N solution of silver nitrate was added to the chemically sensitized emulsion at 40°C until a pAg of 4.0 was obtained. The emulsion was then held for seconds. A 1.0 N solution of halide salts (a mixture of potassium bromide and potassium iodide, 11.2 mole percent iodide) was then added to bring the pAg up to 8.0. The above procedure was repeated five times, and the emulsion was then adjusted to a pAg of 8.1.
To portions of the pAg cycled emulsion and the parent emulsion were added the infrared sensitizing dye 3,3'-diethylselenadicarbocyanine ethylsulfate at 0, 100, 200, 400 and 800 mg dye per silver mole. The emulsions were then coated on a film support, exposed and developed for 6 minutes in Kodak Developer D-19 to which a silver halide solvent had been added. The following results were observed:
At 100 mg of dye per silver mole and at all higher levels of dye the parent emulsion was severely desensitized in the blue region. At 200 mg of dye per mole of silver the parent emulsion produced no response in the infrared region of the spectrum. By contrast the pAg cycled emulsion showed improved infrared spectral sensitization with maximum infrared speed being achieved at 200 mg of dye per mole of silver.
The invention has been described 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.
Photographic silver halide emulsions which form latent image predominantly in the interior of the silver halide grain are often called internal latent image emulsions. A number of ways of preparing them have been described, for example, in British Specification Nos. 581,792, 581,789, 635,841, 1,027,146 and 1,011,062.
A typical method for preparing such emulsions is described in British Specification No. 1,011,062 (U.S. Pat. No. 3,206,313). This involves blending a very fine grain silver halide emulsion, such as a Lippmann emulsion, with a surface sensitized emulsion having considerably larger average grain size. This blend is then held under conditions and for a period of time sufficient to cause the silver halide of the fine grain emulsion to dissolve and recrystallize on the grains of the surface sensitized emulsion. This results in silver halide grains having a core derived from the surface sensitized grains and a shell of silver halide derived from the fine grain emulsion. Since the silver halide shell covers the sensitivity sites previously present on the surface of the silver halide grains, these sensitivity sites are now buried in the silver halide grains.
This method of preparing internally sensitive photographic silver halide emulsions can be relatively complicated. It requires careful control of the conditions under which the blend of fine grain and coarse grain emulsions is ripened if a desirable product is to be obtained. Furthermore, not all emulsions are useful as the "coarse grain" core emulsions. This is particularly true of emulsions whose grains vary considerably in size, such as can occur in polydispersed emulsions.
The present invention provides a relatively simple procedure for making internally sensitive photographic silver halide emulsions, which procedure is applicable to convert a variety of surface sensitive or surface fogged silver halide emulsions (including polydispersed emulsions of varied grain size) to internally sensitive photographic silver halide emulsions.
According to the present invention there is provided a method of making an internally sensitive photographic silver halide emulsion in which further silver halide is laid down on grains of a surface sensitive or surface fogged silver halide emulsion by producing alternate excesses of silver ions and halide ions in solution during precipitation of additional silver halide onto the silver halide grains.
The present invention also provides an internally sensitive photographic silver halide emulsion comprising initially surface sensitive or surface fogged grains and, on each of said grains, a surface layer of silver halide precipitated from a solution containing alternate excesses of silver and halide ions during precipitation.
The present invention can be applied to any conventional surface sensitive or surface fogged silver halide emulsion. The surface sensitive or surface fogged silver halide emulsion can be that which has been employed alone, in combination with other emulsions or as a core emulsion in forming an internally sensitive emulsion. Such emulsions and their formation are well known to those skilled in the art and are disclosed, for example, in U.S. Pat. Nos. 2,222,264; 3,320,069; 3,271,175; 2,592,250; 3,206,313; 3,367,778; 3,447,927; 3,447,927; 2,184,013; 2,541,472; 3,501,307; 2,563,785; 2,456,953 and 2,861,885, the disclosures of which are here incorporated by reference.
Preferred surface sensitive emulsions are those that have been chemically sensitized--e.g. gold or noble metal sensitized, sulfur or selenium sensitized and/or reduction sensitized. The silver halide grains in one preferred form can also be treated with salts of noble metals, such as ruthenium, palladium, platinum, iridium, rhodium, and iron. When the emulsion is surface fogged this can be achieved using conventional nucleating agents or by fogging through uniform exposure.
My invention is particularly applicable to surface sensitive or surface fogged emulsions having a wide distribution of grain sizes, as can be formed either by blending or by the method of precipitation chosen--e.g., using a single jet precipitation technique. My invention is also fully compatible with the use of monodispersed emulsions or those having a relatively restricted range of grain sizes, as have been heretofore conventionally employed in forming internally sensitized emulsions. It is a specific advantage of my invention that it can be applied to emulsions generally regardless of grain size or grain size distribution. Thus, my invention is applicable to Lippman emulsions, relatively coarse grain radiographic emulsions, monodispersed emulsions and polydispersed emulsions generally.
To form silver halide layers on the surface sensitive or surface fogged silver halide grains in order to convert these grains to internally sensitive silver halide grains the surface sensitive emulsion is brought into contact with a conventional silver halide precipitation solution. Silver halide layers can then be formed on the surface sensitive or fogged grains using generally the same materials and techniques useful in forming silver halide grains (i.e., the procedures disclosed in the patents cited above), except as specifically modified to achieve the advantages of my invention.
Typically the precipitation solution includes a silver salt and an alkali metal halide which interact in a double decomposition reaction to form silver halide and an alkali metal salt by-product which remains in solution. To avoid clumping of the silver halide grains during additional silver halide precipitation, small amounts of a peptizer, such as gelatin or some other conventional hydrophilic colloid, is present during precipitation. Suitable peptizers are disclosed, for example in Product Licensing Index, Volume 92, December 1971, publication 9232, paragraph VIII. The peptizer can be that used to form the surface sensitive or surface fogged silver halide emulsion or, if desired, additional peptizer can be added during formation of the silver halide layers.
Any conventional technique for bringing together the silver and the halide containing salts in the presence of the surface sensitive or surface fogged silver halide emulsion can be employed, provided an excess of silver or halide ions can be produced and the excess reversed at will. According to one approach, to the surface sensitive or fogged emulsion is added both the silver and bromide salts, but with one being present in excess. Subsequently additional quantities of the silver and the halide salts are alternately added so that the excess of ions above the stoichiometric required amount is successively shifted between the silver halide ions present. In one form then, the formation of silver halide layers can be viewed as an "alternate jet" mode of precipitation. Instead of actually interrupting the flow of silver salt and halide salt periodically, it is also possible to sequentially restrict the flow of the separate jets and/or sequentially increase the flow of the separate jets. The silver and the halide salts can be added directly as salts, but are preferably added in aqueous solution.
As an example, sufficient silver nitrate solution is added to the precipitation solution to adjust the pAg of the emulsion to the silver side, e.g., to a pAg of 5.0 (from 4 to 6.5, most preferably generally), then sufficient potassium halide solution is added to adjust the pAg of the emulsion to the halide side, e.g., to a pAg of 8.0 (from 7.5 to 10, most preferable generally), and this pAg adjustment cycle may be repeated several times. After each successive cycle the "surface" speed and contrast is reduced, as judged by development in a normal commercial developer of low silver halide solvent content. Development in an "internal" developer, i.e., one containing an appreciable content of silver halide solvent, will however yield an image with speed and contrast similar to the original surface emulsion.
Good results can be obtained using pAg figures other than those given above. It is desirable that the silver excess should not be too great--typically a pAg no less than 4 is desirable, and the temperature and time that the emulsion is held at the low pAg should be as low as conveniently possible, otherwise there is a tendency for fog to develop. In general, temperatures of not more than 40°C and times of 15 to 30 seconds for each part of the cycle yield the best results.
If precipitation is conducted above 40°C, the shift in the neutral pAg of the precipitation solution must be considered in chosing the pAg levels in cycling. For example, whereas a neutral pAg is about 7.0 at room temperature, at 80°C the neutral pAg is about 5.5. Generally cycling of at least one pAg unit and, preferably, two pAg units on either side of pAg neutrality is contemplated. As expressed in this specification, pAg values and ranges are related to temperatures in the range of from room temperature to 40°C.
Although the added silver halide may be of any constitution, the best results are obtained when the silver halide is silver bromoiodide. In adjusting the pAg it is preferred to employ bromoiodide solutions rich in iodide. Thus, for example, good results are obtained with 75 percent bromide, 25 percent iodide (mole percentages).
Generally between 7 and 20 cycles are sufficient to suppress the normal surface sensitivity completely, although a beneficial degree of internal sensitivity can be achieved with a few as 3 cycles or less. The number of cycles for any specific application will vary with the grain size of the emulsion, halide make-up of the added layers, and the crystal habit of the original grains, but can be readily ascertained by one skilled in the art.
The emulsions prepared by the present process need not be fully internal latent image emulsions, but can retain significant surface sensitivity if only a few pAg cycles have been used. Thus, emulsions having any desired balance of internal and surface sensitivities can be produced using my process.
It is not certain whether all the additional silver halide is laid down on the surface of the grains or whether some is precipitated separately in the gelatin phase and subsequently transferred to the grains by Ostwald ripening, or whether some remains permanently as a separate fine grain precipitate. Nevertheless it is thought that by this technique thin layers, of perhaps 10 to 100 ion pairs of additional silver halide each, are laid down. It is not necessary to the practice of the invention to accept this particular theory, but it is supported by the fact that the number of cycles necessary to suppress the surface sensitivity is less in the case of octahedral grains than those of cubic habit, since in the former case it might be expected that a whole layer of ions would more easily be absorbed for each pAg change.
It is not even necessary to complete the pAg cycles before exposure; conversion of an existing latent image from `surface` to `internal` can be achieved by the pAg cycling technique in the same way as conversion of the sensitivity. Thus if a liquid emulsion is fogged by exposure to light, and subjected to an appropriate number of cycles, a `clean` emulsion with internal fog results.
An advantage of this invention is the suppression of the desensitizing action of spectral sensitizing dyes. All sensitizing dyes tend to desensitize the natural sensitivity of a photographic emulsion in the blue region, and is practice this sets a limit to the amount of dye it is possible to add to an emulsion. Moreover there are specific interactions between emulsions of well defined crystal habit and certain classes of sensitizing dye (e.g. octahedral emulsions and carbocyanines) that results in excessive desensitization, as noted by Markocki (J. Photo. Sci. 1965, 13, 85).
This desensitization can be substantially reduced or even completely overcome by using the present cycling technique to interpolate silver halide between the dye at the surface and the sensitivity below. Moreover, this reduction in desensitization is often not accompanied by any adverse action on the efficiency of sensitization, i.e. the relation between the speed in the dye band and the basic blue sensitivity of the emulsion is unchanged.
Thus, in a preferred embodiment, the initially sensitive silver halide emulsions of this invention are particularly suitable for use with sensitizing dyes or desensitizers which are added to the emulsion at a concentration which would desensitize the emulsion if it had not been subjected to the present cycling technique. Typical sensitizing dyes having a primary absorption peak less than 700 millimicrons are described in Belgian Pat. No. 770,293 while typical sensitizing dyes having a primary absorption peak greater than 700 millimicrons are described in U.S. Pat. No. 3,690,891. The desensitizers or electron acceptors useful in the emulsion of this invention are described in U.S. Pat. No. 3,687,676. The concentrations of the sensitizing dyes and desensitizers can be described as being above that which would produce a loss in blue sensitivity in a sulfur and gold surface sensitized silver bromoiodide emulsion (6 mole percent iodide) of similar grain size of at least 0.3 log E when developed in a surface developer such as Kodak D-19.
It is, of course, recognized that lower, conventional concentrations of sensitizers and desensitizers can be employed, as is taught in the art in connection with core-shell emulsions. Preferred desensitizers are those conventional desensitizers characterized by having a cathodic polarographic half-wave potential (Ec) less negative than -1.0 volt.
Cathodic measurements are made with a 1 × 10 - 4 molar solution of the electron acceptor in a solvent, for example, methanol which is 0.05 molar in lithium chloride using a dropping mercury electrode with the polarographic half-wave potential for the most positive cathodic wave being designated E c . Anodic measurements are made with 1 × 10 - 4 molor aqueous solvent solution, for example, aqueous methanolic solutions of the electron acceptor which are 0.05 molar in sodium acetate and 0.005 molar in acetic acid using a carbon paste or pyrolytic graphite electrode, with the voltametric half peak potential for the most negative anodic response being designated E a . In each measurement, the reference electrode is an aqueous silver- silver chloride (saturated potassium chloride) electrode at 20°C. Electrochemical measurements of this type are known in the art and are described in New Instrumental Methods in Electrochemistry, by Delahay, Interscience Publishers, New York, New York, 1954; Polarography, by Kolthoff and Lingane, 2nd Edition, Interscience Publishers, New York, New York, 1952, Analytical Chemistry, 36, 2426 (1964) by Elving; and Analytical Chemistry, 30, 1576 (1958) by Adams.
The present emulsions may be made by any of the techniques employed in the art and may contain emulsion addenda including coating aids, antifoggants and speed-increasing compounds. The emulsions may be coated on a variety of supports. A review of emulsion chemistry and applications of photographic emulsions appear in "Product Licensing Index" December 1971 pages 107-110, Industrial Opportunities Ltd., Havant, Hampshire.
The present emulsions may be used to obtain directpositive images preferably by exposing them imagewise unfogged and then developing in the presence of fogging or nucleating agents in known manner.
It is to be appreciated that the emulsions formed according to my invention can be further modified, as by the incorporation of addenda, coated, exposed and processed by procedures known to those skilled in the art. Such teachings can be summarized at least in part, for example, in Product Licensing Index, Volume 92, published December 1971, publication 9232, pages 107 through 110.
The following Examples illustrate the invention. The developers referred to as D19 and D19b have compositions corresponding to Kodak Developers D19 and D19b respectively given in the Kodak Limited Data Book. The work "Kodak" is a registered trade mark.
EXAMPLE 1
An emulsion was prepared by running a solution of 100 grams silver nitrate in 500 cc of distilled water at 55°C. into a solution of 15 grams gelatin, 75 grams potassium bromide and 4.5 grams potassium iodide in 750 cc of distilled water at 60°C. for over 10 minutes. After heating for 30 minutes at 60°C., 125 grams of active gelatin dissolved in 500 cc of water at 60°C. was added, the suspension was set to gel, shredded and washed 40 minutes in running water. It was then re-melted and heated for a further period of 40 minutes at 55°C., the final volume being adjusted to 1750 cc with water.
This method of preparation gives crystals which form latent image preferentially at the surface, as shown below:
A 200 cc sample of the suspension was exposed for one minute at a distance of 6 feet from a 100 w lamp in the liquid state at 35°C. with efficient stirring, held at this temperature for 5 minutes, then coated on glass plates and allowed to dry. Two of these plates were then developed, one for 15 minutes in D19b developer and the other for 5 minutes in D19b containing 20 g/liter of hypo, after first bleaching for 5 minutes in 3 percent potassium ferricyanide and rinsing in water for 5 minutes. The first plate had a density of 3.6, while the second had a density of 0.92, showing that the latent image was predominantly on the surface of the grains.
A second sample was exposed in exactly the same manner and subjected to 10 `pAg cycles` in the following manner. 1.0 N silver nitrate solution was added to give a silver ion excess (pAg 5.0), the suspension held for 15 seconds, then 0.95 N potassium bromide, 0.05 N potassium iodide solution (the same bromide-iodide ratio as the silver halide crystals) was added to restore the halide excess (pAg 8.5) followed by another 15 seconds holding. The quantities of the solutions added for each part of the cycle had been pre-determined by electrometric titration of a separate sample. At the conclusion of the ten cycles, the suspension was coated on glass and dried as before. In this case, however, a plate developed for 5 minutes in D19b had a density of 0.31, while one developed in the D19b containing hypo, without preliminary bleach, had a density of 2.40.
It would thus appear that the passage through successive baths of silver and halide ions had in fact laid down a shell of fresh silver halide on the crystals covering the latent image. Development then only occurs when solvent is present to remove this shell. Further runs showed that for most dyes the major degree of desensitization is suppressed after six or seven cycles and after ten cycles little further improvement can be observed.
EXAMPLE 2
A fully chemically sensitized fast negative bromoiodide emulsion of 8 mole percent iodide content with mainly triangular grains of average edge length 1.55 μ and thickness 0.3 μ was subjected to 10 pAg cycles as in Example 1 but using a 0.75 N potassium bromide, 0.25 N potassium iodide solution to restore the pAg to the higher level. On coating plates, exposing and developing for 5 minutes in D19b at 20°C, only a ghost image resulted. Addition of 20 g/liter of sodium thiosulphate to the developer gave an image of speed and contrast only slightly lower than that of the uncycled emulsion.
EXAMPLE 3
Addition of 0.44 grams of the sensitizing dye 3-3-dimethyl-9-ethyl-4,5,4', 5'-dibenzothiacarbocyanine iodide (E c ==1.12v and E a =+0.58v) per mole of silver halide to the pAg cycled emulsion of Example 2 yielded an emulsion that gave an internal image of normal dye sensitivity, judged by a wedge spectrogram, but again virtually no image when developed in a surface developer. A similar result was obtained even when the dye was added to the emulsion before the pAg cycles.
EXAMPLE 4
A pure bromide emulsion with monodispersed grains of octahedral habit and edge-length 0.51 μ was prepared, and sulphur sensitized to optimum speed by digestion for 30 minutes at 65°C with 3.5 mg sodium thiosulphate per mole Ag. One portion was then subjected to 10 pAg cycles as in Example 2, and film coatings with 225 mg Ag per square foot were prepared of
1. the untreated emulsion
2. the same as 1, with the addition of 400 mg of the sensitizating dye anhydro-bis-(5,6-dichloro-1-ethyl-3,3'-sulfobutyl-2-benzimid azole) trimethinecyanine hydroxide (E c ==1.60v and E a =+.53v)
3. the pAg cycled emulsion
4. the same as 3, with the addition of the sensitizing dye.
Strips of the film were exposed on an Eastman IB sensitometer through blue (Wratten 47B) and minus blue (Wratten 12 + 58) filters, and developed in D19 developer for 3 minutes at 20°C. A further set was exposed in the same way and developed in D19 developer to which 10 g of sodium thiosulphate has been added per liter. The results are shown below, the speeds quoted being the relative log speeds for a density of 0.1 above fog.
______________________________________ Developer Compositions D19 D19b ______________________________________ p-methylamlnophenol sulphate 2.0 g 2.2 g sodium sulphite (anhydrous) 90.0 g 72.0 g hydroquinone 8.0 g 8.8 g sodium carbonate 45.0 g 48.0 g potassium bromide 5.0 g 4.0 g water to 1 liter 1 liter ______________________________________
______________________________________ Sample Blue Exposure Minus Blue Exposure Speed Υ Speed Υ ______________________________________ D19 Development 1. Control 2.27 1.90 -- -- 2. Control + dye 0.30 1.82 0.69 1.54 3. 10 pAg cycles 0 0 -- -- (ghost) 4. 10 pAg cycles 0 0 0 0 + dye (ghost) D19 + Sodium Thiosulphate Development 1. Control 2.19 0.81 -- -- 2. Control + dye 0.38 0.68 0.74 0.83 3. 10 pAg cycles 1.77 0.90 -- -- 4. 10 pAg cycles 1.38 0.52 1.73 0.46 + dye ______________________________________
EXAMPLE 5
The gradual covering of the sensivitity is shown in this example. The same emulsion, cycling technique, sensitizing dye and development conditions were used as in Example 4, but this time film coatings were made after different numbers of pAg cycles. In the following results, the speeds quoted are relative log speeds for a density of 0.3 above fog after 3 minutes D19 development at 20°C.
______________________________________ Number of Without Dye With Dye Minus pAg cycles Blue Blue Blue Speed Υ Speed Υ Speed ______________________________________ 0 2.16 2.66 0.60 1.60 1.04 1 2.22 2.93 1.66 2.01 2.15 2 2.14 2.06 1.38 1.33 1.80 5 1.92 1.14 0.32 0.38 0.88 10 0 0 0 0 0 ______________________________________
This illustrates that the emulsion was entirely converted to an internally sensitized emulsion lacking surface sensitivity in 10 pAg cycles and that in 5 cycles the major portion of surface sensitivity had been internalized.
EXAMPLE 6
The same cycling conditions and solutions were used as in Example 5, but a fully digested cubic-grained AgBr emulsion of the same grain size was employed, while the sensitizing dye was a merocyanine, 3-carboxymethyl-5-[1'-(3-methylthiazolidin-2-ylidene)-prop-2 '-ylidene]-2-t hiothiazolid-4-one (E c =-1.47v and E a =0.53v). The results show that more cycles are necessary to suppress the surface sensitivity of a cubic emulsion, but again the desensitizing action of the dye is reduced.
______________________________________ D19 Development D19 + 10 g/liter Sodium Thiosulphate Number Sensi- Minus Minus of pAg tizing Blue Blue Blue Blue cycles Dye Speed Υ Speed Speed Υ Speed ______________________________________ 0 0 2.55 1.68 -- 2.37 0.88 -- 0 400 mg 1.10 1.70 1.53 1.03 1.25 1.43 mole Ag 10 0 1.95 0.34 -- 2.47 0.85 -- 10 400 mg/ 2.02 0.50 2.32 2.17 0.77 2.57 mole Ag 20 0 -- 0.20 -- 2.24 0.66 -- ______________________________________
Examples 5 and 6 show further that the reduction of desensitization by dyes may be obtained when insufficient silver halide has been added to reduce the surface sensitivity substantially, so that normal commercial developers with no added solvent may still develop the latent image effectively.
EXAMPLE 7
Internal latent image emulsions may be used for preparing direct positive images by using suitable treatment and processing as revealed e.g. in British Pat. Specification No. 581,773 and U.S. Pat. Nos. 2,568,786 and 2,497,875. The emulsions prepared by pAg cycling technique of the present invention may be used in the same manner. Thus a pAg cycled emulsion coating prepared and coated as in Example 4 was exposed and processed with a viscous processing solution. The solution contained 20 g sodium hydroxide, 40 g sodium sulphite, 1 g of potassium bromide, 0.52 g 1-phenyl-3-pyrazolidone, 12 g hydroquinone, 0.60 g N-formyl-N'-p-tolylhydrazine and 25 g hydroxyethyl cellulose in one litre of water. This viscous developer was spread on the emulsion coating at a thickness of approximately 0.1mm, after 45 secs. development at room temperature, was stopped by washing the solution with dilute acetic acid, and the image fixed with sodium thiosulphate solution in the normal way. The image thus obtained was a direct positive image of approximately the same speed as the negative images obtained by internal image development.
EXAMPLE 8
Portions of the bromide emulsion of Example 4 were sensitized with a variety of dyes, while other portions of this emulsion were subjected to one pAg cycle using the solutions of Example 2. The sensitizers used were:
A. (3,3'-diethylbenzothiazole) heptamethinecyanine iodide, E c = -0.72 E a =0.26.
B. (3,3'-dicthylbenzoselenazole)pentamethinecyanine bromide, E c = -0.84 E a = 0.46.
C. (5,5'-dichloro-3,3'-diethylbenzothiazole) trimethincyanine iodide E c = -0.86 E a = 0.84.
D. (1,3-diethyl-2-imidazo-[4,5-6]quinoxaline) (1-methyl-2-phenyl -3-indole) dimethinecyanine iodide, E c -0.63 E a = >1.0.
E. (1,3-diallyl-2-imidazo-[4,5,-6]quinoxaline) (3,5,-dimethyl-1-phenyl-4-pyrazole) dimethinecyanine iodide, E c = -0.52 E a = >1.0 calc. 2.14
Strips of the film were exposed as in Example 4 and developed using a normal commercial surface developer for 5 minutes at 21°C. The results are shown below.
______________________________________ Blue Speed Minus Blue Speed No + pAg No + pAg Cycle cycle Cycle cycle ______________________________________ Control for dye A 2.04 2.03 -- -- " + 25mg dye A/mol 1.17 1.58 0.35 1.16 Control for dye B 1.85 1.90 -- -- " + 150mg Dye B/mol 0.58 1.67 <0.40 1.55 Control for dye C 2.11 1.74 -- -- " + 400mg dye C/mol 0.97 1.16 0.91 1.27 Control for dye D 2.05 2.27 -- -- " + 25mg dye D/mol 1.37 2.16 0.98 2.26 Control for dye E 2.22 1.98 -- -- " + 5mg dye E/mol 1.05 1.70 -- -- (Dye E does not extend the spectral sensitivity far enough to give minus blue speed figures). ______________________________________
EXAMPLE 9
A polydispersed, high speed sulfur and gold sensitized bromoidide emulsion was pAg cycled as follows:
A 1.0 N solution of silver nitrate was added to the chemically sensitized emulsion at 40°C until a pAg of 4.0 was obtained. The emulsion was then held for seconds. A 1.0 N solution of halide salts (a mixture of potassium bromide and potassium iodide, 11.2 mole percent iodide) was then added to bring the pAg up to 8.0. The above procedure was repeated five times, and the emulsion was then adjusted to a pAg of 8.1.
To portions of the pAg cycled emulsion and the parent emulsion were added the infrared sensitizing dye 3,3'-diethylselenadicarbocyanine ethylsulfate at 0, 100, 200, 400 and 800 mg dye per silver mole. The emulsions were then coated on a film support, exposed and developed for 6 minutes in Kodak Developer D-19 to which a silver halide solvent had been added. The following results were observed:
At 100 mg of dye per silver mole and at all higher levels of dye the parent emulsion was severely desensitized in the blue region. At 200 mg of dye per mole of silver the parent emulsion produced no response in the infrared region of the spectrum. By contrast the pAg cycled emulsion showed improved infrared spectral sensitization with maximum infrared speed being achieved at 200 mg of dye per mole of silver.
The invention has been described 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.