CH643371A5 - Process for producing photographic silver halide emulsions which contain silver halide crystals of the twinned type - Google Patents

Description

The present invention relates to a process for the preparation of photographic silver halide emulsions which contain silver halide crystals of the twin type and represents an improvement or modification of the invention according to patent 681,555.

Silver halide emulsions are composed of silver halide crystals dispersed in a colloid medium, which often consists of gelatin. The properties of photographic emulsions depend very considerably on the various stages carried out to prepare the photographic emulsion and in each case on the ratio and sequence of one or more such stages.

Thus, a common method for producing such an emulsion is the initial precipitation (nucleation) of microscopic silver halide crystals, usually by mixing a silver salt solution with a solution of a water-soluble halide salt, growth of these crystals by further addition of reagent solutions, washing the emulsion to remove water-soluble ones , as a by-product of the double reaction formed in the previous precipitation stages and sensitization, in order to increase the intrinsic sensitivity of the finished emulsion by treatment with chemical sensitizers such as sulfur and gold salts, and in many cases by adding spectral sensitizing dyes.

The stages of such an emulsion making process can be tailored precisely to achieve the various desirable goals, so that emulsions 45 having the required photographic properties can be obtained. The precipitation stages in the process can be adjusted in such a way that the average size of the silver halide crystals (which generally determines the sensitivity of the photographic emulsion), the size distribution of such crystals (which influences the photographic contrast), the shape and the habit of the crystals ( including the outer lattice surfaces and the degree of twin formation) and the halide composition of the crystals. It is particularly advantageous to control the uniformity of the halide distribution within the crystal population and the halide composition profile within each crystal. The shape and internal structure of the crystals also have an important influence on the photographic behavior of the emulsion. In particular, twinned silver halide crystals are favorable in many applications because of their high photographic sensitivity and opacity (ratio of the developed blackening to the weight of the developed silver).

65 This invention relates in particular to silver halide crystals which have a special shape and a special habit. In order to achieve this choice in shape and habitus, however, there has to be a certain one

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Restriction of the halide composition also proved to be necessary. This invention specifically relates to an improved process for the production of twin-type emulsions from silver iodide bromide, silver iodide chloride or silver iodide chloride bromide by the targeted incorporation of silver iodide into the silver halide crystals during their growth.

Improved photographic properties are often observed when a mixture of water-soluble halides is used in the precipitation of the silver halide. As by Gutoff in the U.S. No. 3,773,516, there are two generally accepted methods of controlling silver halide precipitation, namely the single nozzle and the double nozzle emulsification method. In the single nozzle process, aqueous silver nitrate solution is added to a solution containing a small amount of gelatin and a mixture of soluble halides. The crystal size distribution can be checked after partial or complete introduction of the silver nitrate by an Ostwald ripening stage, the emulsion being kept at elevated temperatures in the presence of a silver halide solvent. During this stage, the least soluble large crystals grow by diffusion and incorporation of silver halide extracted from the more soluble small crystals.

The ripening step leads to an increase in the average crystal size of the emulsion (and often an increase in the photographic sensitivity of the finished emulsion) and to a broadening of the crystal size distribution. Because of the large excess of halide ions present during the precipitation and ripening stages, the photographic emulsions produced by the single-nozzle process often have predominantly the crystal habit of twinned octahedron crystals.

This applies in particular to iodide bromide precipitation. "An Introduction to Crystallography" by F.C. Phillips, 3rd edition, Longmans (1966), pp. 162-165 and "The Crystalline State" by P. Gay, Oliver and Boyd (1972), pp. 328-338, give a description of crystal twins . The disadvantage of such a single nozzle process is that the twinned crystals produced inevitably have a relatively wide size distribution. On the other hand, in the double nozzle process, aqueous solutions of silver nitrate and soluble halide salts are simultaneously added to a gelatin solution while stirring. The flow rates of the reagent solutions can be regulated in such a way that the amount of excess halide is kept constant and predominantly un-twinned crystals are formed.

Flat or tabular silver halide crystals show exceptional opacity when developing, and this leads to very good silver utilization in comparison to silver halide crystals of other shapes, for example cubic crystals. Many twinned octahedron crystals belong to this type, especially if the crystals contain more than one twin plane and the twin planes are parallel. It is a particular object of the present invention to increase the proportion of twin crystals with parallel twin levels in an emulsion.

With improvements in commercial production of photographic emulsions, another object is to increase the contrast of the finished material, which is a desirable property for the graphic arts and radiography products. This can be achieved in part by narrowing the size distribution, as described, for example, in British Patent No. 1,469,480, and in part by making it larger

Ensure similarity of the iodide content and the iodide distribution in different types of silver halide crystals in the emulsion. As is known, the point where soluble iodide salt is added in various emulsification processes influences the sensitivity and size distribution of the emulsion (Research Disclosure No. 13 452 (1975).

The use of photographic emulsions containing twin crystals in materials with increased sensitivity and increased contrast has therefore been hampered by the deficiencies in conventional emulsification techniques, particularly by the uncontrolled incorporation of iodide into the crystals. An improved process for the preparation of photographic emulsions from silver iodide bromide, silver iodide chloride or silver iodide chloride bromide was described in patent 631 555, in which iodide ions were added to the growing crystals by dissolving a uniform dispersion of silver iodide crystals. Furthermore, the conditions have been described under which this process promotes the desired tabular twinned habit and largely uniform photographic emulsion crystals of this type can be produced.

Thus, in the patent 631 555 a process for the production of a twin type silver halide emulsion is described, whereby stepwise (a) silver halide crystals containing at least 90 mol% iodide are formed in a colloidal dispersion medium, (b) an aqueous silver salt solution and an aqueous alkali or Ammonium bromide or chloride solution or mixtures thereof are mixed into the dispersing medium containing these silver halide crystals, so that twinned silver halide crystals are formed which contain iodide and the added halide (s), (c) the dispersing medium with a silver halide solvent and thus leads to growth of the twinned crystals by Ostwald ripening, optionally (d) by adding further aqueous silver salt solution and further alkali metal or ammonium halide to the colloidal dispersion to increase the size of the twinned crystals and finally if necessary afterwards (e) the educated removes water-soluble salts and chemically sensitizes the emulsion.

It has now been found that the process just described can be modified by chemically sensitizing the silver halide crystals containing at least 90 mol% of iodide in step (a).

The present invention accordingly relates to a process for producing a twin-type silver halide emulsion, which is characterized in that silver halide crystals containing at least 90 mol% of iodide are formed stepwise (a) in a colloidal dispersion medium, these crystals predominantly having the hexagonal lattice structure, and then these silver halide crystals containing at least 90 mol% of iodide are chemically sensitized, (b) an aqueous silver salt solution and an aqueous alkali metal or ammonium bromide or chloride solution or mixtures thereof are mixed into the dispersing medium containing these silver halide crystals, so that twinned silver halide crystals form, which form iodide and contain the added halide (s) and (c) the dispersion medium is mixed with a silver halide solvent and thus brings about growth of the twinned crystals by Ostwald ripening.

The present invention also relates to special embodiments of the process according to the invention, which contain the additional process measures (d) and / or (e), according to (d) further aqueous silver salt solution and further alkali metal or ammonium halide for the colloidal dispersion of the twin crystals

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Stalle is added to achieve further growth of these crystals, and according to (e) the water-soluble salts formed are removed and the silver halide crystals are chemically sensitized. The present invention furthermore relates to the emulsions prepared by the process according to the invention and to the photographic material containing these emulsions.

In the process according to the invention, the silver halide crystals with a high iodide content first formed in stage (a) serve as nuclei for the twinned crystals formed in stage (b). Silver halide crystals of high iodide content, i.e. about 90 to 100 mol% iodide, predominantly have the hexagonal lattice structure. Methods for producing silver iodide with a predominantly hexagonal lattice structure are well known and are described, for example, by B.L. Byerley and H. Hirsch, J. Phot. Sci., Vol. 18, p. 53 (1970). Such crystals have the shape of hexagonal pyramids or bipyramids. The base surfaces of these pyramids represent the lattice planes {0001}. Silver iodide crystals of a hexagonal lattice structure are shown in FIG.

Silver halide crystals, which contain minor amounts of iodide (i.e. up to approximately 45 mol% iodide), however, have the face-centered lattice structure and an octahedral crystal shape (the outer shapes represent {111} lattice planes). It is known that these crystal shapes can be modified by twin formation, so that twin crystals face-centered cubic lattice structure can have the shape of triangular or hexagonal plates, the shape depending on the number and geometry of the twin planes formed.

In step (b) of the process according to the invention, aqueous solutions of a silver salt and an alkali metal or ammonium bromide or chloride (or mixtures thereof) are added to the dispersion medium containing the silver iodide crystals, which has a predominantly hexagonal lattice structure, so that silver bromide (or chloride or chloride bromide) precipitates becomes. However, since no total growth of the silver iodide crystals of this structure can take place, nuclei of the added silver halide form as very small crystals of the face-centered cubic lattice type, since the iodide portion in these crystals is very small. During this stage, however, the silver iodide crystals initially formed dissolve, and silver iodide is incorporated into the growing crystals with face-centered cubic lattice. Electron micrographs have shown that in stage (b), while there is no overall growth of the silver iodide crystals, the crystals of the halide added in stage (b) with face-centered cubic lattice type form and grow oriented on the base surfaces of the silver iodide crystals formed in stage (a) . Oriented growth between {0001} -AgJ surfaces and {11 l} -AgBr or -AgCl surfaces is possible because both are hexagonally packed, homoionic lattice planes. It was observed in electron microscopy that the oriented growing crystals, as long as they are bound to the silver iodide basic crystal, show a high degree of twin formation (recognizable by the parallel striations characteristic of several twin planes intersecting the surface). It is believed that this twin formation is promoted by the continuous supply of iodide ions to the growing (face-centered cubic, fzk) phase, either by mass diffusion through the dispersing medium or by anionic diffusion through the crystal interface. Generally, a twinned (fzk) crystal forms on the single base surface of a hexagonal pyramidal silver iodide crystal, while twinned crystals form on the two base surfaces of the hexagonal bipyramidal silver iodide crystal.

1 illustrates schematically and with the aid of electron micrographs the recrystallization of the silver iodide crystals with a hexagonal lattice in order to form silver iodide bromide crystals with a cubic lattice. It is shown how a hexagonal bipyramidal silver iodide crystal undergoes recrystallization according to the oriented growth mechanism. In general, the twinned silver iodide bromide crystals each form on the base surface of the (truncated) hexagonal-bipyramidal silver iodide crystal (a). As stage {b) progresses and silver halide continues to precipitate, the iodide content of the total halide suspended in the dispersion medium falls below 30-40 mol% of silver iodide. The dissolution of the originally formed silver iodide crystals becomes predominant, and the "dumbbell" -shaped crystals shown in Fig. 1 (b) are observed. During this stage, the twinned crystals increase in size and the silver iodide crystals decrease. Finally, the silver iodide bond between the two twinned crystals is broken, and the two twinned crystals are released (le). The rest of the silver iodide can remain on the twinned crystals with a face-centered cubic lattice, or it can finally be dissolved away. The lower part of FIG. 1 shows electron micrographs (magnification 30,000 ×), which illustrate the progress of the formation of dumbbell-shaped crystals and the eventual release of the hexagonal or triangular platelet shapes characteristic of twinned silver halide crystals. After the Ostwald maturation stage (c) has been carried out, the smaller, un-twinned crystals, which are also produced and visible in the last electron micrograph, will preferably dissolve.

A similar process takes place in the recrystallization of silver iodide to form silver iodide chloride or silver iodide chloride bromide crystals.

In the process according to the invention, chemically sensitized silver halide crystals with a high iodide content are produced in step (a). It was found that during step (b) such crystals dissolve in accordance with the formation of twinned crystals and the chemical sensitization products are transferred to the twinned crystals with the incorporation of the iodide, which gives the finished crystals an increased photographic sensitivity.

There are four known skin-sensitive methods for chemical sensitization of silver halide emulsions. These consist of gold and other precious metal sensitization, heavy metal sensitization with e.g. bismuth and lead salts, sulfur and other medium chalcogen sensitization and reductive sensitization. Any of these methods can be used in the method according to the invention. A combination of these methods can also be carried out simultaneously, e.g. B. Gold and sulfur sensitization. This sensitization method is explained in the following example using the preparation of emulsion A.

A particularly suitable method for sensitizing silver halide crystals consists in using noble metals from group VIII. Metal compounds suitable for such sensitization are, for example, polyvalent cations, including divalent ions (e.g. lead), trivalent ions (e.g. antimony, bismuth) , Arsenic, gold, iridium and rhodium) as well as tetravalent ions (e.g. osmium, iridium and platinum).

Among the noble metal compounds typically used are those such as ammonium and potassium chloropalladate, ammonium, sodium and potassium chloroplatinate, ammonium, potassium and sodium bromoplatinate,

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Ammonium chlororhodate, ammonium chlororuthenate, ammonium chloroiridate, ammonium. Potassium and sodium chloroplatinite, and ammonium, potassium and sodium chloropalladite, etc. Gold sensitizers include, for example, potassium auricyanide, potassium aurorhodanide, gold sulfide, gold selenide, gold iodide, potassium chloroaurate, ethylenediamine bis-gold chloride, and various organic gold compounds found in the U.S. Patent Specification 3,753,721 structure.

Hydrazine, thiourea and tin-II salts are mentioned among the compounds used for reductive sensitization.

Both sulfur compounds such as thiosulfate and selenium compounds such as selenosulfate are also used for chemical sensitization.

In the method according to the invention it is understood that

that the twinned crystals formed at the end of stage (b) can be relatively small, so that they can often only be used as seed crystals. These crystals can be grown to a useful size during step (d). However, as previously indicated, it is possible to extend step (b) so that useful crystals are obtained at the end. Nevertheless, steps (b) and (c) can pass into step (d) in the process according to the invention without in any way interrupting the addition of the aqueous solutions.

However, the twinned crystals formed at the end of step (b) are generally seed crystals, so that the silver iodide detached from the silver iodide crystals formed in step (a) will be present in the seed crystal and therefore after the growth step (d) in the core of the crystal if iodide is not added in step (b). After transfer of the products of chemical sensitization from the chemically sensitized silver iodide formed in stage (a), the size of the chemically sensitized twinned crystals formed in stage (b) is therefore increased in stage (c), the Ostwald maturation stage, and in stage ( d) increase, which is preferably carried out in the process according to the invention, and these twinned crystals form together with the core of the finished crystals formed in step (d). For example, if the silver iodide crystals formed in step (a) are formed by inclusion of a noble metal, e.g. Rhodium, chemically sensitized during stage (a), therefore the rhodium during stage (b) is then enclosed in the growing twin crystals. After stages (c) and (d), the rhodium is then present as part of the core of the finished silver halide crystals. The method according to the invention thus represents a novel method for chemical sensitization of the core of core / shell silver halide emulsions.

Chemical sensitization using this method has several advantages; e.g. For example, only the twinned crystals in step (b) are sensitized and not the other un-twinned crystals formed in step (b), which would occur if the chemical sensitization took place in step (b). It is also possible to chemically sensitize the silver iodide under chemical conditions which would be inappropriate for other silver halides if the chemical sensitization was carried out during step (b).

The chemical sensitization of the silver iodide in step (a) can take place at any time in step (a), e.g. B. during the formation of the silver iodide crystals or as an end section in stage (a) after the production of the silver iodide crystals has ended in this stage.

The chemical sensitization is carried out by adding the various chemical sensitizers mentioned above to the emulsion reagents.

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These compounds can be added either continuously during part or the entire duration of the crystallization process in stage (a), for example by introducing them into the raw material solutions; or you can also stop the crystallization process, treat the partially grown crystals with the appropriate reagent and start growing again.

A few mol% of other halides, for example silver bromide or silver chloride, can be added to the silver iodide crystals by precipitating them with the silver iodide. This can improve the effectiveness of chemical sensitization or the viability of the silver iodide emulsions, as described by James and Van-selow, Photographic Science and Engineering 5.1 (1961) in attempts to track the ripening process.

A large number of the agents normally used during chemical sensitization can be used; for example, stabilizers can be added to effectively stop the sensitization (chemical ripening), or the sensitization can take place in the presence of retarders such as nucleic acids or cysteine. The chemical conditions must be selected so that the optimal properties are achieved. For example, it is preferable to use sulfur and gold salts at a fixed pAg, e.g. 8-9, and pH, e.g. B. 5-7, and elevated temperature (50-60 'C) and in the case of heavy metal sensitization, using, for example, bismuth or lead salts, at a relatively low pH, e.g. 3, to raise awareness.

The chemically sensitized silver emulsions prepared in step (a) can be flocculated and washed by any known method to remove water soluble salts or excess sensitizers prior to performing step (b).

Pure silver iodide crystals are preferably formed in step (a), but up to 10 mol% of other halides (chloride or bromide) can be present in the silver iodide crystals as long as they still predominantly retain their hexagonal lattice shape. It is therefore understood that the term silver iodide crystals also includes crystals with up to 10 mol% of other halides. It goes without saying that a small proportion of the crystals formed in stage (a) (ie up to 10%, in terms of weight or number, of the crystals) can be predominantly silver chloride or bromide and can belong to the face-centered cubic lattice type without clearly relating to the process according to the invention impact. Preferably, no additional iodide is added to the halide solution in step (b), but the possibility of adding small amounts is not excluded (i.e. up to 10 mol% of the halide added in this step can be iodide). '

The average linear size of the silver iodide crystals formed in step (a) should preferably be in the range 0.05-5.0 microns, particularly preferably in the range 0.1-1 micron. The silver iodide content in the dispersing medium at the beginning of stage (b) should preferably be in the range 0.05-2.0 mol / liter, particularly preferably in the range 0.10-1.0 mol / liter.

It is a special feature of the present invention that, in order to produce a crystal population of the highest uniformity in stage (b), which can be used to produce monodisperse emulsions, the addition rates of the silver halide solutions used in stage (b) are predetermined by tests. The optimal flow rates in this regard depend on the type of halide and decrease with the number of silver iodide crystals in the aqueous dispersion medium.

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the crystal size of the silver iodide crystals, the pAg in the range described above and the temperature.

For example, higher rates of addition are required in the preparation of silver iodide chloride or silver iodide chloride bromide emulsions than for their iodide bromide equivalents.

In the recrystallization stage (b) it is preferred to use such volumes of silver nitrate and ammonium or alkali halide solutions that the silver iodide accounts for 0.01-20 mol% of the total silver halide in the finished emulsion. As a guideline for an appropriate flow rate, the rate should be adjusted until the dissolution of the silver iodide is almost complete at the time when the amount of silver nitrate added is one to three times equivalent to the silver iodide. The optimal speed can thus be derived from electron micrographs, which are taken at different times during recrystallization, since the silver, iodide crystals with their characteristic crystal habit can be distinguished from the silver halide crystals of the usual face-centered cubic lattice.

From the above discussion of the mechanism of the method according to the invention, it can be seen that electron micrographs of emulsion samples taken during experimental preparations, in which the rate of addition is varied in the course of step (b), can be used as a further indication of the optimal flow rates.

Preferably, a constant flow rate is used in step (b) and electron micrographs of the finished ripened emulsion at the end of step (c) can be used to select the optimal rate of addition during step (b) which is a population of twinned crystals of the greatest uniformity and shape would generate. The optimum flow rate during stage (b) that best corresponds to the conditions selected for the maturation stage (c) can thus be determined by preliminary tests. The addition rate should preferably also be selected so that Ostwald ripening does not take place among the existing population of twinned crystals. The experimental trials necessary to ensure that the range of flow velocities to be used is optimal are similar to those described in British Patent No. 1,469,480. A too slow addition rate in step (b) would lead to incomplete recrystallization of the silver iodide crystals formed in step (a) and, due to Ostwald ripening, to an excessive broadening of the size distribution of the twinned crystals formed. Too high a rate of addition in step (b) would lead to considerable new formation of nuclei of unbranched crystals, which are easily detectable due to their characteristic regular cubic or octahedral shape. In this case, the dissolution of the untwinned crystals during Ostwald's maturation stage (c) may be incomplete, which leads to a broad distribution of the iodide content and to a bimodal size distribution of the finished emulsion. Both of these effects would result in a loss of photographic contrast in the finished emulsion. Selection of an appropriate intermediate rate of addition during step (b) results in a population of twin crystals of more uniform size and shape.

It has been shown that the silver halide emulsions produced by the process of patent 631 555 have a high internal sensitivity to unconfined silver halide emulsions, and this is shown in the example. However, according to the method according to the invention it is possible to considerably increase the internal sensitivity of the finished silver halide emulsions, and this is explained in the example.

The sensitivity of internally sensitized emulsions can be increased by adding one or more reagents commonly used in negative emulsions. In particular, these emulsions can be spectrally sensitized with dyes of the type usually used for surface-sensitive negative emulsions. It is advantageous to use such a large amount of dye that it would cause desensitization in a surface-sensitive emulsion of the same size, since the image on the inside is not subject to dye-reduced desensitization.

Internally sensitive emulsions exposed imagewise can be developed into an internal image by any method known in the art. It is primarily a standard type developer with the addition of certain amounts of either free iodide or a silver halide solvent such as alkali thiosulfate. If necessary, the surface can be bleached with an oxidizing agent to remove a surface image before development (Sutherns, J. Phot. Sci. 9, 217 (1961)).

If the shell silver halide layer is thin (about 15 lattice levels), the crystal can be developed in a surface developer, this technique provides an emulsion which gives a conventional surface appearance but also avoids the desensitization resulting from large additions of dyes to surface-sensitive emulsions.

When using a surface developer that contains certain fogging agents (or nucleating agents), you can create a direct positive image with the internally sensitive emulsions described above. Commonly used nucleating or fogging agents contain a hydrazine, hydrazone or a nitrogen-containing heterocyclic ring or combinations thereof. Suitable hydrazine compounds are given in British patents 702 162, 712 355, 1 269 640 and 1 385 039 and hydrazone compounds in British patent 1,371,366 (the latter can also act as sensitizing dyes). Examples of heterocyclic N-containing systems can be found in British Patents 1,363,772, 1,362,859 and 1,283,835 as well as in U.S. Pat. Patent 3,850,638. In addition to organic foggants, inorganic compounds, typically stannous complex chelates or boron compounds, are sometimes used, for example in French Patent 1,579,422 and U.S. Pat. Patents 3,617,282 and 3,246,987. A veil can also be achieved by means of a prebath containing a fogging agent, or a fogging agent can be incorporated into the emulsion layer or an adjacent separate layer. As in the U.S. Patent specification 3,796,577, a veil can also be created by uniform exposure before or during processing in a surface developer.

The method according to the invention can be used for the production of direct-positive emulsions, where an otherwise known technique is used, such as, for example, in British patent specification 723 019 and in the article by Vanassche et al., J. Phot. Sci. 22, 121 (1974). The silver halide emulsion produced by the process according to the invention is obtained using a combination of a reducing agent (thiourea dioxide, hydrazine, tin salts and several others are known for this) and a compound of a more electropositive metal than silver (gold and or palladium are preferred)

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provided with a veil. An electron trapping compound, which is preferably also a spectral sensitizer for the direct positive method, is added. After pouring, imagewise exposure and treatment with a surface developer, the emulsion will give a direct positive image. If necessary, the usual additives can be applied to the direct-positive emulsion, for example soluble halides to increase the sensitivity, sensitizing or desensitizing dyes to expand the spectral range and compounds which increase the sensitivity in the blue. You can also protect the surface veil against air oxidation by covering it with a thin layer of silver halide so that it is still accessible to conventional surface developers.

In order to change the properties of the finished silver halide crystals, there is the possibility of changing the halides added during stage (b) or of completely changing the halides or halide ratios used during the transition from stage (b) to stage (d). As a result, layers with a specific halide ratio in the finished crystals can be obtained by ensuring that a specific halide or halide mixture is used in successive sections in stage (b) or in stage (d) in the process according to the invention.

If the emulsions produced by the process according to the invention are to be used for negative-working photographic material, it is expedient to

after the recrystallization stage (b) and the maturation stage (c), the halides in stage (d) are added in such a way that up to 10 mol% of iodide is precipitated in a “shell” surrounding the “core” from twinned crystals formed in stage (b) and that up to 10 mol% of chloride is precipitated in the outermost shell of the crystals. Thus, silver iodide chloride bromide emulsions with “internal” iodide (in addition to the crystals originating from the original silver iodide crystals) and “surface” chloride layers containing crystals can be produced according to the invention.

If the emulsions produced by the process according to the invention are to be used for directly positive materials or for other purposes where internally sensitive crystals are desired, it is expedient that the halide precipitated during the first section or the entire recrystallization step (b) is predominantly chloride and the halide precipitated during the entire growth stage (d) or its last section is predominantly bromide. Thus, according to the invention, silver iodide chloride-bromide emulsions can be produced with “inside” chloride and crystals containing “surface” bromide layers.

Such "core / shell" emulsions are well known and are also described in British Patent 1,027,146.

The process according to the invention is particularly suitable for the production of monodisperse emulsions.

In order to produce monodisperse emulsions using the method according to the invention, the silver iodide emulsion produced in stage (a) preferably belongs to the monodisperse type. Such emulsions can be prepared by mixing aqueous silver salt and alkali or ammonium iodide solutions at a fixed temperature and pAg with stirring in a solution of a protective colloid. The final crystal size of the silver iodide emulsion is preferably in the range 0.05-5.0 microns. Preferably, the halide solution contains ammonium or potassium iodide alone, but up to about 10 mole percent ammonium chloride or bromide can optionally be used. To suitably high addition speed643 371

To be able to use the manufacturing temperature, the production temperature is preferably at least 60 ° C., and the pAg of the solution is kept at a controlled value in the range 3-5 or in the range 11-13. The pAg is particularly preferably kept at a value of approximately 11.8 + 0.3. The pAg value can be very suitably maintained by means of a suitable electrode system and automatic adjustment of the flow rate of one of the solutions.

As just stated, the preferred size range of the silver iodide crystals produced in step (a) is between 0.05 and 5.0 microns. The silver iodide crystals produced in step (a) can also be monodisperse. It was found that the average size of the silver iodide crystals formed in step (a) influences the size of the twinned crystals formed in step (b). In general, the larger the silver iodide crystals formed in step (a), the larger the twinned crystals formed in step (b).

One method for increasing the size of the silver iodide crystals formed in step (a) is to carry out step (a) in the presence of a silver iodide solvent. The solubility of the silver iodide can also be expediently regulated by changing the temperature, the amount of excess iodide and the proportion of silver iodide solvent in the dispersing medium.

Furthermore, it is obvious that the crystal size distribution of the finished twinned emulsion also depends on the crystal size distribution of the silver iodide formed in step (a). Thus, for high-contrast applications such as X-ray films, the silver iodide crystals in stage (a) should preferably be monodisperse, but for low-contrast applications such as camera films it may be preferable to prepare a relatively polydisperse twinned silver halide emulsion in accordance with the present process in such a way that in the stages (a ) formed silver iodide crystals produces a relatively wide size distribution. Such a broad size distribution can also be achieved by mixing monodisperse silver iodide emulsions of various sizes before the start of step (b). Mastering the size and size distribution of the twinned silver halide crystals produced in stages (b) and (c) or (d) can thus be achieved by selecting the size and size distribution of the silver iodide crystals formed in stage (a).

The water-soluble salts formed in the process of the invention can be removed by any well known method. Such methods often involve coagulating the silver halide and the colloid dispersant, removing this coagulate from the now aqueous medium, washing it and redispersing it in water.

As just stated, emulsions produced according to the invention may have a tendency to form latent images inside the crystals, such emulsions being particularly suitable for use with solvent developers with the formation of a negative image as shown in FIG. 2 with reference to the examples, or with fog developers under formation of a direct positive image as shown in Fig. 3 are suitable. However, by appropriate chemical surface sensitization in the optional step (e) of the present process, using any of the agents described above, the emulsions can be given a much greater tendency to form latent surface images upon exposure, which then appears with a standard type negative working developer would kick. Depending on the respective type or extent of the chemical used in stage (a) and stage (e)

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See sensitization process, a precisely targeted balance between the formation of latent images inside and on the outside of the crystal is possible. For example, the chemical sensitization carried out in step (a) could be achieved by adding rhodium or iridium salts in an appropriate amount, and the chemical sensitization, step (e), on the surface of the crystals could be achieved by prolonged ripening with sulfur and gold salts. Such a method would be particularly suitable for those emulsions produced according to the invention which predominantly contain silver chloride, which gives emulsions which form a negative image in a negative working standard developer, but which are compared to a similar emulsion in which the sensitization in step (a) is omitted has an increased contrast. Numerous other combinations of such internal and surface sensitization processes are readily apparent to those skilled in the art.

It is also readily apparent that the photographic contrast could be regulated by adding two or more emulsions produced by the present process, which, however, are chemically sensitized to different degrees or according to different methods in either step (a) or (e), mixed.

The emulsions produced by the process according to the invention can be optically sensitized to the emulsions by adding optical sensitizers, for example carbocyanine and merocyanine dyes.

The emulsions optionally contain any of the additives commonly used in photographic emulsions, for example wetting agents such as polyalkylene oxides, stabilizers such as tetraazaindenes, metal chelating agents and agents used for silver halide, such as adenine, which are used for growth or crystal habit modifying.

The dispersing medium is preferably gelatin or a mixture of gelatin and a water-soluble latec, e.g. a latex polymer containing vinyl acrylate. If such a latex is present in the finished emulsion, it is particularly preferably added after the crystal growth has completely ended. However, other water-soluble colloids, for example casein, polyvinylpyrrolidone or polyvinyl alcohol, can be used on their own or together with gelatin.

To the extent desired, the silver halide emulsions prepared by the process according to the invention have high opacity and high development speed, as is shown in the example below.

The silver halide emulsions produced by the process according to the invention can therefore be used in many types of photographic materials, such as in X-ray films, both in black and white and color films for cameras, in paper products and in direct positive materials.

The invention thus comprises silver halide emulsions produced by the method according to the invention and cast photographic silver halide material which contains at least one such emulsion.

example

The following emulsions are made: Emulsion A

This example explains the production of a monodisperse twinned octahedral emulsion according to the invention, in which internally sensitive twinned crystals are produced by recrystallization of silver iodide crystals which are chemically sensitized by heat treatment with sulfur and gold salts, and to produce a direct positive image by suitable processing is used.

Production of monodisperse silver iodide emulsion (stage a)

1 liter of 5% aqueous solution of an inert gelatin is stirred at 65 ° C. and 200 rpm with 2 ml of n-octanol as an anti-foaming agent. Every hour, 3000 ml of aqueous 4.7m silver nitrate and potassium iodide solutions are injected into the gelatin solution with stirring, until an addition of 150 ml of silver nitrate solution. During the precipitation the pAg is kept at a value of 11.8.

At this stage, the growth of the silver iodide crystals is stopped and the chemical sensitization is carried out by the emulsion at 57 ° C for 100 minutes at pH 6.3 and pAg 8.8 in the presence of 14 mg sodium thiosulfate and 2.4 mg sodium tetrachloroaurate-di hydrate per mole of silver halide ripens.

The precipitation is then continued with the addition of further volumes of 4.7 m silver nitrate and potassium iodide solutions at 2100 ml per hour until an addition of 525 ml of silver nitrate solution. The pAg is again kept at a value of 11.8. The crystals of this silver iodide emulsion have an average size of 0.18 microns.

Recrystallization and maturation (stages b and c)

230 g of the silver iodide emulsion prepared in stage (a) and 1 liter of 5% aqueous solution of an inert gelatin are added with stirring at 200 rpm at 65 ° C. with 2 ml of n-octanol and 30 ml of 5N sulfuric acid. Aqueous 4.7m solutions of silver nitrate and ammonium bromide are sprayed into the silver iodide emulsion at a rate of 3000 ml per hour until 500 ml of silver nitrate are added. The pAg is kept at 7.7 ± 0.3 throughout.

Ostwald ripening is carried out in the presence of 100 ml of 1 l, 8 m ammonia solution, which is added with the halide solution, so that the ammonia concentration increases with the onset of recrystallization. At the end of the addition of the silver nitrate and ammonium halide solutions, the dissolution of the non-twinned crystals by Ostwald's ripening is essentially complete, and mostly twinned crystals of silver iodide bromide remain.

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Further growth (level d)

The pH of the emulsion prepared in stage (c) is adjusted to 5.0 to neutralize the ammonia present with 5N sulfuric acid. At a flow rate of 3000 ml per hour, while maintaining the temperature at 65 ° C. and the pAg at 9.5, further 4.7 m solutions of silver nitrate and ammonium bromide are added with stirring at 200 rpm until an addition of 750 ml of silver nitrate solution . The finished emulsion has an average crystal size of 0.76 micron with a scattering coefficient of 16%. The emulsion is flocculated using conventional methods, washed and redispersed with a total of 210 g of limed bone glue gelatin. In this case there is no further chemical sensitization, i.e. the optional step (s) is omitted. The emulsion is adjusted to pH 6.3 and pAg 8.8 and poured onto a photo carrier with a casting weight of 50 mg Ag / dm2.

Emulsion B

This example explains the production of a monodisperse twinned silver iodide chloride

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bromide emulsion, which contains twinned crystals with a “core” of silver iodide chloride and an outer “shell” of silver bromide, which is produced by recrystallization of silver iodide crystals that are chemically sensitized by the addition of a rhodium salt during the precipitation.

Preparation of monodisperse silver iodide emulsion (stage a)

A monodisperse silver iodide emulsion is prepared exactly according to the method given in step (a) of Example 1, but omitting the ripening with the sulfur and the gold salts.

Instead, after the growth of the silver iodide crystals to an average size of 0.18 micron, 35 ml of an aqueous solution containing 35 • 10 ~ 4 g of sodium hexachlororhodite dissolved in 10n lithium chloride are added to the silver iodide emulsion in order to incorporate rhodium into the surface layers of the silver iodide crystals, most likely in the form of an adsorbed rhodium complex ion genus. The emulsion is stirred at 65 ° C for 15 minutes to allow adsorption and chemical sensitization to take place.

Recrystallization and maturation (stages b and c)

230 g of the silver iodide emulsion prepared in step (a) are added to 1 liter of 5% aqueous solution of an inert gelatin with stirring at 200 rpm at 65 CC with 2 ml of n-oc-tanol and 30 ml of 5N sulfuric acid. Aqueous 4.7m solutions of silver nitrate and ammonium chloride are injected into the silver iodide emulsion with stirring at a rate of 3500 ml per hour until 75 ml of silver nitrate solution are added. The pAg is kept at 7.3 + 0.2 throughout. Up to the addition of a further 425 ml of silver nitrate solution, 4.7m aqueous solutions of silver nitrate and ammonium bromide are injected into the emulsion with stirring at a rate of 3000 ml per hour. During this period of precipitation, the pAg is kept at 7.7 ± 0.2.

Ostwald's ripening is carried out in the presence of 100 ml of 1 l, 8 m ammonia solution, which is added during this stage with the halide solutions, so that the ammonia concentration increases with the recrystallization of the chemically sensitized silver iodide crystals. At the end of the addition of the silver nitrate and ammonium halide solutions, the dissolution of the non-twinned crystals by Ostwald's ripening is essentially complete, and mostly twinned crystals of silver iodide chloride bromide remain. The size and the habit of the crystals are similar to those in Emulsion A.

Further growth (level d)

This emulsion is subjected to further growth in the same way as described for stage (d) of the preparation of emulsion A and is prepared for casting on photo supports with a casting weight of 50 mg Ag / dm2 as described above.

Emulsion C

For comparison, an emulsion is produced by a process according to Belgian patent 855 519, where the high internal sensitivity of the emulsion, as produced, is used to produce a direct positive image for comparison with that from emulsion A.

A monodisperse twinned octahedral emulsion is prepared exactly as described for Emulsion A, but omitting the chemical sensitization of the silver iodide emulsion with sulfur and gold salts described in step (a) of Example A.

Emulsion D

As a reference standard, an unincorporated monodisperse cubic silver bromide emulsion with an average edge length of 0.39 microns is produced. The cubic monodisperse emulsion is prepared by the pAg-controlled technique described in British Patent 1,335,925, but with chemical sensitization inside and on the surface to obtain a satisfactory reversal characteristic. The preparation of this emulsion comprises the following stages:

(i) Growth of crystals to 0.12 micron average edge length.

(ii) Ripening at pH 6.3 and pAg 7.8 for 40 minutes at 70 C with 14 mg sodium thiosulfate and 0.6 mg sodium tetrachloroaurate dihydrate per mole of silver halide.

(iii) Further growth of the cubic crystals to 0.39 micron average edge length.

(iv) Further ripening at pH 6.3 and pAg 8.8 for 40 minutes at 57 ° C with 12.5 mg sodium thiosulfate and 1.8 mg sodium tetrachloroaurate dihydrate per mole of silver halide.

The reference emulsion is then poured onto the photocarrier at a casting weight of 30 mg Ag / dm2. If the maturation in stage (ii) is omitted, a constant density (D max) is obtained without reversal. If the surface sensitization in step (iv) is omitted, a very low maximum density (D max) is observed.

Photographic Results The cast strips are then imagewise exposed on an intensity scale sensitometer for 0.2 seconds under a tungsten source with an intensity of 1000 lux.

To assess the internal sensitivity of the emulsions, the cast samples are 4 minutes at 20 ° C in an internal developer (solvent developer) of the following composition:

Developer I

Methyl p-aminophenol sulfate 0.7 g

Hydroquinone 2.7 g

Sodium sulfite (anhydrous) 25 g

Sodium carbonate 12.5g

Sodium thiosulfate 10 g

Potassium bromide 0.7 g of water processed to 1 liter, then fixed, watered and dried in the usual way. The photographic results obtained are shown in Fig. 2.

All emulsions were anhydro- [3- (3-sulfo-propyl) -2-benzothiazole] - (3-ethyl-2-benzothiazole) -ß-methyl-trimethyne cyanine hydroxide per mole by adding 0.2 mg of the cyanine dye before pouring Silver halide has been spectrally sensitized.

To obtain a direct positive image, the cast samples are placed in a fog developer of the following composition for 8 minutes at 20 ° C:

Developer II

l-phenyl-3-pyrazolidone 0.25 g

Hydroquinone 10 g

Sodium sulfite (anhydrous) 25 g

Sodium carbonate 12.5 g

Sodium hydroxide 10 g

Potassium bromide 0.7 g

5-methylbenzotriazole 50 mg

N-Formyl-N'-p-tolylhydrazine 50 mg water per 1 liter

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processed, then fixed, watered and dried in the usual way. The photographic results obtained are shown in Figure 3.

Discussion of the photographic results

The photographic results shown in FIGS. 2 and 3 demonstrate the improved photographic behavior of the emulsions produced according to the invention.

The results obtained for the internal development of the cast strips are shown in the usual form as development density / log exposure curves in FIG. 2. These demonstrate the increased internal sensitivity which results from the chemical sensitization carried out in stage (a) of the new process according to the invention, in the case of emulsions A and B compared to emulsion C which was prepared by a process according to the patent 631 555. Similar results are obtained by pretreating the cast strips with a ferricyanide bleach to remove any surface image, as described by Sutherns, J. Phot. Sci. 9, 217 (1961).

Furthermore, it is evident that the twinned emulsions produced by both processes have considerably greater internal sensitivity than the reference emulsion D,

an unincorporated cubic silver bromide emulsion. Fig. 3 shows the photographic results obtained by fogging the cast strips. These demonstrate that the higher internal sensitivity of Emulsion A and B leads to a corresponding improvement in the direct reversal characteristic compared to Emulsions io C and D. The photographic sensitivities measured on the cast strips after processing in fog developer II (a direct positive image is obtained) have values which are similar to the internal sensitivities measured after processing in solvent developer I. It can be seen from FIG. 3 that an excellent reversing characteristic can be obtained for the twinned emulsions according to the invention without the chemical surface sensitization treatment which is necessary for the untwisted reference emulsion D.

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Claims (15)

643 371 1. A process for the preparation of silver halide photographic emulsions containing twin-type silver halide crystals, characterized in that stepwise (a) silver halide crystals containing at least 90 mol% of iodide are formed in a colloidal dispersion medium, these crystals predominantly having the hexagonal lattice structure, and then these silver halide crystals containing at least 90 mol% of iodide are chemically sensitized, (b) an aqueous silver salt solution and an aqueous alkali metal or ammonium bromide or chloride solution or mixtures thereof are mixed into the dispersing medium containing these silver halide crystals, so that twisted silver halide crystals are formed which contain iodide and the halide (s) added and (c) a silver halide solvent is added to the dispersing medium and thus the growth of the twinned crystals is brought about by Ostwald ripening. 2. The method according to claim 1, characterized in that in an additional step (d) further aqueous silver salt solution and further alkali or ammonium halide are added to the colloidal dispersion in order to achieve growth of the twinned crystals. 2nd PATENT CLAIMS 3. The method according to claim 1 or 2, characterized in that in an additional step (e) the water-soluble salts formed are removed and the surface of the silver halide crystals of the emulsion is chemically sensitized. 4. The method according to claim I, characterized in that the chemical sensitization in step (a) consists of gold or other noble metal sensitization. 5. The method according to claim 1, characterized in that the chemical sensitization in step (a) takes place by means of a heavy metal compound. 6. The method according to claim 1, characterized in that the chemical sensitization in step (a) consists in a sulfur or selenium sensitization. 7. The method according to claim 1, characterized in s that chemical sensitization in stage (a) takes place in a ductive awareness. 8. The method according to any one of claims 2 to 7, characterized in that stage (d) is carried out and the silver halide crystals formed in stage (b) represent the core and io the halide added in stage (d) the shell of the finished silver halide crystals. 9. The method according to any one of claims 2 to 7, characterized in that in step (b) the halide precipitated at least in the initial stage of the step is predominantly chloride 15 and in step (d) the halide precipitated at least in the final stage of the step is predominantly bromide. 10. The method according to any one of claims 3 to 9, characterized in that the surface of the silver halide crystals in step (e) is chemically sensitized. 20th 11. The method according to any one of claims 3 to 9, characterized in that the surface of the silver halide crystals is veiled in step (e). 12. The method according to claim 11, characterized in that the concealment using a reduc- 25 agent together with a compound of a more electro-positive metal than silver. 13. The method according to claim 12, characterized in that an electron-trapping compound is adsorbed on the surface of the silver halide crystals. 30th 14. Silver halide emulsions prepared according to the process according to one of claims 1 to 13. 15. Photographic material, characterized in that it has at least one layer containing a silver halide emulsion according to claim 14.