CH645468A5 - Process for the preparation of 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 containing twin-type silver halide crystals and is an improvement or modification of the invention described and claimed in main patent No. 132,454.

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 significantly on the various stages involved in the preparation of the photographic emulsion, and in each case on the mutual relationships and the 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 than By-product of the double reaction formed in the previous precipitation stages and sensitization 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 preparation process can be tailored precisely to achieve the various desirable goals, so that emulsions with the required photographic properties can be obtained. The precipitation stages in the process can be adjusted so that the average size of the silver halide crystals (which is generally the Emp2

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sensitivity of the photographic emulsion), the size distribution of such crystals (which influences the photographic contrast), the shape and the shape of the crystals (including the outer lattice surface and the degree of twinning) 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 particularly suitable in many applications because of their high photographic sensitivity and opacity (ratio of the developed density to the weight of developed silver).

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 habit, however, a certain restriction of the halide composition has also been found 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, for example in the single-nozzle process in which aqueous silver nitrate is added to a solution containing a small amount of gelatin and a mixture of soluble halides. One advantage of the single-nozzle mode of operation over the alternative double-nozzle process, in which aqueous solutions of silver nitrate and soluble halide salts are added to a gelatin solution while stirring, is that the emulsion crystals produced are predominantly twinned octahedral crystals, since their seeds are formed in an excess of halide ions. This applies in particular to the case of 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 an individual nozzle process is that the twinned crystals produced in this way inevitably have a relatively broad size distribution.

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. Figure 1 shows silver halide crystals of the tabular twin type. 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, for example in British Patent No.

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1,469,480, and partly by ensuring a greater 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 application of photographic emulsions containing twin crystals to products requiring increased sensitivity and contrast has therefore been hampered by the deficiencies in conventional emulsifying techniques, particularly by the uncontrolled incorporation of iodide into the crystals. An improved method for producing photographic emulsions from silver iodide bromide, silver iodide chloride or silver iodide chloride bromide has been described in Swiss Patent No. 631,555, in which iodide ions are 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.

For example, a method for producing a twin-type silver halide emulsion is described in Swiss Patent No. 631,555, wherein (a) silver halide crystals containing at least 90 mol% of iodide are gradually 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 is added and thus the growth of the twinned crystals is brought about by Ostwald ripening and optionally (d) then by adding further aqueous silver salt solution and further alkali * or ammonium halide to the colloidal dispersion, the twinned crystals grow in size and finally give them If necessary, (e) the water-soluble salts formed are removed and the emulsion is chemically sensitized.

It has now been found that the process just described can be modified by controlling step (b) and leaving step (c) out, but still obtaining the required twinned crystals.

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 (i) in a colloidal dispersion medium, these crystals predominantly having the hexagonal lattice structure and (ii ) 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 to form silver halide crystals which contain iodide and the added halide (s), wherein regulates the rate of addition of the silver salt and alkali or ammonium halide so that the silver halide crystals formed in this stage are predominantly twinned.

This process can be followed by a further process step (iii), in which the twinned crystals grow in size by adding further aqueous silver salt solution and further alkali metal or ammonium halide to the colloidal dispersion.

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Furthermore, in a process step (iv), the water-soluble salts formed can be removed and the emulsion can be chemically sensitized.

In the process according to the invention, silver halide crystals with a high iodide content are first formed. Silver halide crystals of high iodide content, i.e. about 90-100 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.PhotSci. Volume 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 with a hexagonal lattice structure are shown in FIG.

However, silver halide crystals containing minor amounts of iodide (i.e. up to about 45 mol% iodide) 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 twinning, 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 (ii) 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 the overall growth of the silver iodide crystals of this structure is difficult, nuclei of the added silver halide form as very small face-centered cubic lattice type crystals because the iodide content 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 step (ii), while there is no overall growth of the silver iodide crystals, the crystals of the halide added in step (ii) with face-centered cubic lattice type are formed and oriented on the base surfaces of those formed in step (i) Silver iodide crystals grow up. Oriented growth between {0001} -Agj surfaces and {111} -AgBr or AgCl surfaces is possible because both are hexagonally packed, homoionic lattice planes. In electron microscopy it was observed that the oriented growing crystals, as long as they are bound to the silver iodide basic crystal, have a high degree of twinning (recognizable by the parallel striations characteristic of several twin planes intersecting the surface). This twinning is believed to be facilitated by the continuous supply of iodide ions to the growing face centered cubic phase, either by mass diffusion through the dispersing medium or by anionic diffusion through the crystal interface. Generally, a twinned (face centered cubic) 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.

FIG. 3 shows a hexagonal-pyramidal silver iodide crystal (3a) and a hexagonal-bipyramidal crystal (3b). As the further precipitation of silver halide progresses and the total iodide content in the total halide suspended in the dispersing medium falls below 30-40 mol% silver iodide, the dissolution of the originally formed silver iodide crystal becomes predominant and the "dumbbell-shaped" crystals in FIG. 4 are observed.

FIG. 4 shows a twin-face-centered cubic crystal formed on a hexagonal-pyramidal silver iodide crystal (4a) and two twin-face-centered cubic crystals, each of which forms on the base surface of a (truncated) hexagonal-bipyramidal silver iodine crystal (4b). During step (ii), the twin face-centered cubic crystals increase in size and the iodide crystals decrease. This state is shown in Figure 5. Finally, the syllable qodide connection between the two twinned crystals (5b) is interrupted and the two twinned crystals are formed.

The rest of the syllable iodide can remain on the twin face-centered cubic crystals, or it can finally be dissolved away.

The electron micrograph in Figure 6 shows the dumbbell crystals of Figure 4 (b) during the recrystallization process.

The twinned crystals in FIG. 5 are similar to the twinned crystals shown in FIG. 7 of Swiss Patent No. 631 555, which formed after step c) had ended. The purpose of step c) was to dissolve the more soluble, smaller, non-twinned crystals formed in step b) which were formed from the halide added in step b). However, by carefully controlling the rate of addition of the aqueous solutions of silver salt and alkali or ammonium bromide or chloride (or their mixtures), it is possible to form predominantly twinned mixed halide crystals as shown in FIG. 5, and step c) is therefore not in the process according to the invention required.

For example, in the process according to Swiss Patent No. 631 555, a silver halide solvent is used in step c) in order to dissolve the undesired, unavoidable, unavoidable silver halide crystals formed in step b). In the process according to the invention, however, stage (ii) is controlled in such a way that very few or no undesirable, unincorporated crisis traps form. With this method, it is unnecessary to carry out stage c) of Swiss Patent No. 631 555. The silver halide crystals formed after step c) are similar to those formed according to the invention after step (ii), i.e. the crystals are mainly twinned crystals of the face-centered cubic lattice type. In the process according to Swiss Patent No. 631 555, stages (b) and (c) can be combined by adding a solvent for Ostwald ripening before the end of stage (b).

In this case, crystal growth takes place either by mass transfer from small crystals to the large crystals due to the presence of the solvent for Ostwald ripening or by direct incorporation of the silver halide precipitated during step (b). However, the growth of silver halide crystals by double injection alone (as in step (ii)) is conceptually very similar to the growth by Ostwald ripening in that small crystals can dissolve in both growth processes. In the double nozzle process, the incomplete mixing of concentrated reagent solutions generally used in silver halide precipitation inevitably leads to local areas in the emulsifying vessel where the absorption capacity of the existing crystals for new material is greatly exceeded. Since silver halide has a low solubility in water, the excess silver halide therefore tends to precipitate very small

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Germs. Because of the high surface energy of small crystals, such nuclei will generally only be temporarily stable and will dissolve as the mixing progresses. The method according to Swiss Patent No. 631 555 and the method according to the invention are in fact very similar in principle, because the growth of the twinned crystals taking place in step (ii) of the present invention presumably takes place in part by such a ripening process, although the rates of addition the solution are regulated so that no significant formation of new crystals is observed in step (ii). This applies in particular to a preferred method according to the invention, this growth taking place in the presence of a fixed concentration of excess halide, which is itself a silver halide solvent, or during the preparation of silver halide emulsions which contain a substantial proportion of chloride, which is the silver halide solubility and thus the Tendency to Ostwald ripening increased.

In the process according to the invention, iodide ions are added in stage (ii), hereinafter referred to as the recrystallization stage, by further dissolution of silver joist particles by the relationship

[Ag +] [J-] = k where [Ag +] and [J—] are the activities (the concentrations in dilute solution) of the silver and iodide ions and k is a constant (k is the well-known solubility product) to maintain given equilibrium concentration .

As indicated above, the incorporation of iodide into the crystals growing in step (ii) promotes the formation of octahedral surfaces, and in particular the formation of stacking errors known as twin planes. According to a further object of the present invention, the formation of crystals with parallel twin planes is particularly favored. This leads to a change in the crystal shape, so that many crystals formed belong to the tabular twinned type shown in FIG. 1. It is known that the formation of twin planes is not possible if the outer surfaces of the crystals are the cubic (100) lattice planes (Berry and Skillman, Photographic Science and Engineering 6, p. 159 (1962)), but can only take place if the Outer surfaces at least partially include the octahedral (111) lattice planes. The incorporation of iodide in the recrystallization stage (ii) therefore has a twin-promoting effect, even under conditions where cubic outer surfaces normally appear in crystals that do not contain iodide.

To the extent that iodide ions are removed from the solution by precipitation in step (ii), they are rapidly replaced by further dissolution of silver iodide crystals, so that, depending on the rates of addition of the silver and halide solutions, the silver iodide crystals at the end of the precipitation or recrystallization stage (ii) are largely dissolved. However, it is possible that at the end of step (ii), after the oriented crystals have been deposited, a small amount of undissolved silver iodide is present on some of the twinned mixed halide crystals.

The silver halide crystals of the photographic emulsion produced by the process according to the invention can furthermore predominantly belong to the desired tabular twin type if the growth stage (ii) is carried out under conditions which favor the octahedral habit, and under such conditions usually more than 50% by weight or number of the present silver halide crystals of this type.

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The recrystallization stage (ii), in which nuclei for the twin crystals are formed, is preferably carried out by adding aqueous 3m-5m silver nitrate and ammonium bromide or chloride solutions or mixtures thereof with stirring to give a silver iodide dispersion in gelatin solution at a fixed temperature and while maintaining a pAg value in the range from 5.0 to 11.0, particularly preferably in the range from 6.0 to 10.0. The set temperature can be within a wide range, e.g. B. 35 ° to 90 ° C can be set. Most advantageously, the flow rate of the silver nitrate solution is initially maintained constant during this stage, adjusting the rate of addition of the halide solution as required. However, as indicated, the rate of addition of the aqueous solutions in stage (ii) must be regulated so that at the end of this stage the silver halide crystals formed are predominantly twisted.

It goes without saying that stages (i) and (ii) need not follow one another directly. For example, the silver iodide colloid dispersion can be prepared before it is needed and then stored. If rather small silver halide crystals are required, step (iii) can be unnecessary. However, step (iii) is particularly useful in the preparation of monodispersed twin silver halide emulsions as described below.

Pure silver iodide crystals are preferably formed in step (i), but up to 10 mol% of other halides (chloride or bromide) can be present in the silver iodide crystals as long as they still 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 (i) (i.e. up to 10%, by weight or in number, of the crystals) can belong to the face-centered cubic lattice type without having a significant effect on the process according to the invention. Preferably, no additional iodide is added to the halide solution in step (ii), but the possibility of adding small amounts is not excluded (i.e. up to 10 mol% of the halide added in this step may be iodide).

The average linear size of the silver iodide crystals formed in stage (i) should preferably be in the range from 0.05 to 5.0 microns, and particularly preferably in the range from 0.1 to 0.4 microns.

The silver iodide content in the dispersing medium at the beginning of stage (ii) should preferably be in the range from 0.05 to 2.0 mol / liter, and particularly preferably in the range from 0.10 to 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 (ii), which can be used to produce monodisperse emulsions, the rates of addition of the silver halide solutions introduced in stage (ii) are predetermined by tests. The optimal flow rates in this regard depend on the type of halide and increase with the number of silver iodide crystals in the aqueous dispersion medium, the decreasing 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 manufacture of silver iodide chloride or silver iodide chloride bromide emulsions than for their iodide bromide equivalents.

In the recrystallization step (ii) it is preferred that silver nitrate and ammonium or alkali halides are added in such volumes that the silver iodide accounts for 0.01 to 20 mol% of the total silver halide in the finished emulsion. As a guideline for a practical

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Flow rate should be adjusted until the dissolution of the silver iodide is almost complete by the time the amount of silver nitrate added is one to three times equivalent to the amount of 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 (ii), can be used as a further indication of the optimal flow rates. A special feature of the present invention is that in step (ii) the rate of addition of the reagent solutions is regulated in such a way that the silver halide crystals formed in this step are predominantly of the twinned type and that there is no substantial formation of new untwinned crystals. The addition rates 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 stage (ii) leads to incomplete recrystallization of the silver iodide crystals formed in stage (i) and, due to Ostwald ripening, to an excessive broadening of the size distribution of the twinned crystals formed. An excessively high addition rate in stage (ii) would lead to considerable new formation of nuclei of unincorporated crystals, which are easily detectable due to their characteristic regular cubic or octahedral shape. In this case, the finished crystals would have formed only partially under the direct influence of the silver iodide, 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. The selection of an appropriate intermediate rate of addition during step (ii) results in a population of twin crystals of more uniform size and shape.

In order to increase the rate of addition of the aqueous solutions in stage (ii) and still ensure that the crystals obtained at the end of stage (ii) are predominantly of the twinned type, it is preferable to have small proportions in stages (i) and (ii) To use alkali halides which have cation radii, which differ significantly from the commonly used sodium, potassium or ammonium salts. Thus, the optimal rate of addition used during step (ii) can be increased by using a small proportion of an alkali halide, such as lithium, with a smaller cation radius than silver during the preparation of the silver iodide crystals in step (i) or during the recrystallization step (ii) uses a small proportion of an alkali halide, such as rubidium, with a larger cation radius than silver. A table of cation sizes is available from R. A. Robinson and R.H. Stokes in “Electrolyte Solutions” p. 461.2. Edition, Butterworths (1959). It is believed that small amounts of these ions form inclusions in the respective silver halide lattices during precipitation and increase the transition rate of the hexagonal lattice type crystals formed in step (i). Further methods for increasing the rate of oriented growth (or the rate of dissolution of the silver iodide crystals) during step (ii) are to carry out step (ii) in the presence of a wetting agent such as a polyalkylene oxide condensate or a silver iodide solvent. Polyalkylene oxides are believed to accelerate the conversion of silver iodide to silver iodide bromide or iodide chloride by complexing with iodide ions or by displacing gelatin from the surface of the crystals subject to recrystallization, while incorporating a portion of silver iodide solvent into the dispersing medium during step (ii) the rate of conversion direct influence on solubility affect karm.

A high concentration of ammonia promotes the formation of the cubic habit in silver iodide bromide crystals, and for this reason it is preferred to carry out the recrystallization step (ii) for silver iodide bromide emulsions at a low ammonia concentration. Conversely, an ammonia concentration in silver iodide chloride or silver chloride crystals promotes the formation of the octahedral habit (Berg et al. The Basics of Photographic Processes with Silver Halides, Volume 2, p. 640), and the recrystallization stage (ii) for producing twinned silver iodide chloride emulsions according to the invention should therefore be carried out continuously in a preferred range from 0.5 to Im lying ammonia concentration. This is conveniently achieved by adding concentrated ammonia solution to the alkali or ammonium chloride solution. However, twin cubic silver iodide chloride emulsions can be prepared in the absence of ammonia.

The process according to the invention is particularly suitable for the production of twinned silver halide emulsions of the monodisperse type. In this embodiment of the invention, step (iü) is included, and during this step further silver and halide solutions are added by the double nozzle method and with controlled pAg. It is preferable to add so much additional halide in this stage that the iodide content of the finished crystals is about 1 to 10 mol%, which amount of iodide has proven to be highly advantageous since it provides highly sensitive emulsions which are capable of rapid development.

A process for producing monodisperse twinned octahedral silver halide crystals is described in British Patent No. 1,649,480, and the silver halide crystals produced in the process of this invention after step (ii) can be increased in size according to the method described therein.

A particular subject of the present invention is a process for the production of monodisperse silver halide emulsions of the twin type, which is characterized in that it comprises the steps (i) to (iii) or (i) to (iv) set out above and in the step ( iii) further aqueous silver nitrate and ammonium or alkali halide solutions are added to the twin crystals at such a rate that crystal nucleation no longer takes place, the pAg being at a specified value in the range 5 to 12 at a specified temperature in the range 35 ° to 90 ° C holds.

The choice of the pAg depends on the required crystal habit, for example FIG. 6 shows the twinned cubic crystals obtained which form when the emulsion is prepared with a growth step (iii) carried out at a pAg of 6.0. The Monodi6

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This twinned cubic emulsion, as shown in FIG. 7, is particularly high. FIG. 8 shows an example of the corresponding emulsion produced with a growth step (iii) carried out at pAg 10 and illustrates the production of a monodisperse twinned octahedral emulsion.

To obtain twin monodisperse octahedral emulsions, step (iii) is preferably carried out at a fixed pAg between 9 and 11. In order to obtain twin monodisperse cubic emulsions, stage (iii) is preferably carried out at a fixed pAg between 6 and 9 and at an ammonia concentration in the range between 0 and 0.5 molar. It is also possible to produce twinned silver halide crystals of the intermediate tetradecahedral habit by selecting the appropriate solution conditions.

In order to produce monodisperse emulsions using the method according to the invention, the silver iodide emulsion produced in stage (i) 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 to 5.0 microns. Preferably, the halide solution contains ammonium iodide alone, but up to about 10 mole percent ammonium chloride or bromide can optionally be used. In order to be able to use expediently high addition rates, 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 to 5 or in the range 11 to 13. The pAg is particularly preferably kept at a value of approximately 11.8 + 0.3. Figure 2 shows a silver iodide emulsion of this type. 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 (i) is between 0.05 and 5.0 microns. The silver iodide crystals produced in step (i) can also be monodisperse. It was found that the average size of the silver iodide crystals formed in step (i) influences the size of the twinned crystals formed in step (ii). In general, the larger the silver iodide crystals formed in step (i), the larger the twinned crystals formed in step (ii).

One method of increasing the size of the silver iodide crystals formed in step (i) is to perform step (i) 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 (i). Thus, for high-contrast applications such as X-ray films, the silver iodide crystals in stage (i) 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 according to the present process in such a way that in the stage (i ) formed silver iodide crystals produces a relatively wide size distribution. A broad size distribution can also be achieved by using different monodisperse silver iodide emulsions

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Size mixed before the start of stage (ii). Control of the size and size distribution of the twinned silver halide crystals produced in stages (ii) or (iii) can thus be achieved by selecting the size and size distribution of the silver iodide crystals formed in stage (i).

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.

The silver halide crystals can be chemically sensitized in any growth step in any well known manner.

For certain applications, however, it has proven useful to chemically sensitize the silver iodide crystals in step (i).

A preferred embodiment of the present invention therefore relates to a process for the preparation of a twin-type silver halide emulsion which is characterized in that it comprises steps (i), (ii), (iii) and (iv) as set out above and in step (i) the silver halide crystals containing at least 90 mol% of iodide are chemically sensitized.

In this preferred method according to the invention, chemically sensitized silver halide crystals with a high iodide content are thus produced in stage (i). It has been found that during step (ii), where these crystals are dissolved and twinned crystals are formed, the chemical sensitization products are transferred to the twinned crystals with the incorporation of the iodide, giving the finished crystals increased photographic sensitivity.

There are four main known 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 are applicable to this method. A combination of these methods can also be carried out simultaneously, for example gold and sulfur sensitization. This sensitization method is explained below using the preparation of emulsion A.

A particularly useful method for sensitizing silver halide crystals is the use of 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) and tetravalent ions (e.g. osmium, iridium and platinum).

The noble metal compounds typically used include those such as ammonium and potassium chloropalladate, ammonium, sodium and potassium chloroplatinate, ammonium, potassium and sodium bromoplatinate, ammonium chlororhodate, ammonium chlororuthenate, ammonium chloroiridate, ammonium, potassium and sodium triumchloroplatinite and ammonium, potassium and sodium chloropalladite. 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 of the type described in U.S. Pat. Patent Specification 3,753,721 structure.

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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 (ii) 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 (iii). However, as previously indicated, it is possible to extend step (ii) so that useful crystals are produced at the end. Nevertheless, stage (ii) can proceed to stage (iii) 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 (ii) are generally seed crystals, so that the silver iodide detached from the silver iodide crystals formed in step (i) will be present in the seed crystal and therefore after the growth step (iii) in the core of the crystal. After transfer of the products of chemical sensitization from the chemically sensitized syllable iodide formed in step (i), the size of the chemically sensitized twinned crystals formed in step (ii) will therefore increase in step (iii), which is preferably carried out in the process according to the invention, and these twinned crystals then form the core of the finished crystals formed after step (iii).

For example, if the silver iodide crystals formed in step (i) are included by inclusion of a noble metal e.g. Rhodium chemically sensitized during stage (i), the rhodium during stage (ii) is then included in the growing twin crystals. After stage (iii) the rhodium is then present as part of the core of the finished silver halide crystals. This 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; for example, the twinned crystals can be sensitized without extensive fog, which could occur if the chemical sensitization took place in step (ii). 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 (ii).

The chemical iodide in step (i) can be sensitized at any time in step (i), e.g. during the formation of the silver iodide crystals or as an end portion in step (i) after the production of the silver iodide crystals has been completed in this step.

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

These compounds can be added either continuously during part or the entire duration of the crystallization process in stage (i), for example by adding them to the starting solutions; or you can also stop the crystallization process, treat the partially grown crystals with the appropriate reagent and resume growth.

As mentioned previously, a few mol% of other halides, for example silver bromide or silver chloride, can be mixed into the silver iodide crystals by precipitating them with the silver iodide. This can improve the effectiveness of chemical sensitization or the viability of the syllable iodide emulsions, as described by James and Vanselow, Photographic Science and Engineering 5.1 (1961) in attempts to monitor the progress of 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. 5-7, and elevated temperature (50-60 ° C) and in the case of heavy metal sensitization, using e.g. bismuth or lead salts, at a relatively low pH, e.g. 3, to raise awareness.

The chemically sensitized silver iodide emulsions prepared in step (i) can be flocculated and washed to remove water soluble salts or excess sensitizers by any known method before step (ii) is carried out.

It has been shown that the unsensitized silver halide emulsions produced by the process according to the invention have a high internal sensitivity to unconfined silver halide emulsions. In this embodiment of the method according to the invention, however, it is possible to increase the internal sensitivity of the finished silver halide emulsions and thus to increase the sensitivity to reversal; this is explained in Example 2.

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-sensitized emulsion of the same size, since the image on the inside is less subject to the dye-induced desensitization.

Emulsions sensitive to images exposed inside can be developed into an inside image by any method known in the art. This 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 cladding silver halide layer is thin (about 15 lattice planes), 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 dye additions to surface sensitive emulsions.

When using a surface developer that contains certain fogging agents (or nucleating agents), a directly positive image can be created 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 hydra

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Tin 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 tin-II 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 fog can also be obtained by means of a fog agent in a pretreatment or a fog agent can be incorporated into the emulsion layer or a separate layer adjacent to it. 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 provided with a veil using the 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) . 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 to 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.

To change the properties of the finished silver halide crystals; there is a possibility that during stage

(ii) change added halides or completely change the halides or halide ratios used in the transition from step (ii) to step (iii). As a result, layers with a certain halide ratio in the finished crystals can be obtained by ensuring that a certain halide or halide mixture is used in successive sections in stage (ii) or in stage (iii) 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 step (ii) the halides in step

(iii) to be added in such a way that up to 10 mol% of iodide is precipitated in an “envelope” surrounding the “core” from twinned crystals formed in stage (ii) and that up to 10 mol% of chloride is precipitated in the outermost envelope of the crystals . This means that silver iodide chloride bromide emulsions can be produced with “inside” iodide (in addition to the crystals that come from the original silver iodide crystals) and “surface” chloride layers.

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 stage (ii) is predominantly chloride and the halide precipitated during the entire growth stage (iii) or its last section is predominantly bro-mid. 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.

In step (iv), the silver halide crystals can be chemically sensitized using any of the well-known chemical sensitizers mentioned above. The water-soluble salts formed in the process of the invention can be removed by any well known method.

Such methods frequently involve coagulating the silver halide and the colloid dispersant, removing this coagulate from the aqueous medium, washing it and dispersing it again in water.

As just stated, emulsions produced according to the invention, and in particular with regard to those emulsions in which the chemical sensitization is carried out in step (i), 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 or with fog developers with the formation of a direct positive image, as shown in FIG. 11, are suitable. However, by appropriate chemical surface sensitization in optional step (iv) of the present method, 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 type or extent of the chemical sensitization methods used in stage (i) and stage (iv), a precisely controlled 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 stage (i) could be achieved by adding rhodium or iridium salts in an appropriate amount, and the chemical sensitization, stage (iv), on the surface of the crystals could be achieved by prolonged ripening with sulfur and gold salts achieve.

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 (i) 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 prepared by the present process, but to different extents or using different methods in either step (i)

or (iv) chemically sensitized, mixed.

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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.

If appropriate, the emulsions contain any of the additives commonly used in photographic emulsions, for example wetting agents such as polyethylene oxide adducts, stabilizers such as tetrazaindenes, metal chelating agents and agents used for silver halide, such as adenine, which are used for growth or crystal habit modification.

The dispersing medium is preferably gelatin or a mixture of gelatin and a water-soluble latex, for example 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 ceased. 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 contrast during development, as is shown in the examples 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 photographic papers 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.

The following examples serve to explain the invention.

example 1

In this example, the preparation of a monodisperse twinned octahedral emulsion is explained, wherein surface-sensitive twinned crystals are produced by chemical sensitization with sulfur and gold salts and are used to produce a negative image.

Production of twin octahedral silver iodide bromide emulsion Production of monodisperse silver iodide emulsion (stage ©)

1 liter of 5% 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 μm silver nitrate and potassium iodide solutions are injected into the gelatin with stirring, until an addition of 150 ml of silver nitrate solution. Then, every hour, 2100 ml of further volumes of these solutions are added until an addition of 525 ml of silver nitrate solution. The pAg of the emulsion is kept at a value of 11.8 + 0.2 throughout. The silver iodide emulsion has an average crystal size of 0.18 microns.

Recrystallization (stage (ii))

812 g of the silver iodide emulsion prepared in stage (i) are added to 500 ml of distilled water which contains 120 g of inert limed bone gelatin and is stirred with 2 ml of n-octanol at 65 ° C. and 200 rpm. Aqueous 4.7m solutions of silver nitrate and ammonium bromide are injected into the silver iodide emulsion with stirring after the pAg has been adjusted to 8.6 as follows.

Flow velocity added

Solution volumes

600 ml / hour 200 ml

960 ml / hour 400 ml

1500 ml / hour 800 ml

Then 120 ml of distilled water containing 80 g of inert gelatin are added and the double injection of 1500 ml / hour is continued until an additional 400 ml of silver nitrate solution is added. The pAg is continuously at 8.6 + 0.2. held.

A sample of the silver halide emulsion is then examined under the microscope and relatively few undeveloped silver halide crystals are observed. Stage (ii) was therefore carried out with such a rate of addition of silver salt and ammonium bromide that the silver halide crystals formed at the end of this stage are predominantly twinned.

Further growth (stage (iii))

The pAg is set to 9.8. At a flow rate of 2400 ml / hour, while maintaining the temperature at 65 ° C. and the pAg at 9.8, further 4.7 m solutions of silver nitrate and ammonium bromide are added to the emulsion with stirring at 200 rpm until a further addition of 2000 ml of silver nitrate solution . The finished emulsion has an average crystal size of 0.54 microns with a scattering coefficient of 16%. Figure 9 shows the crystals of this finished emulsion. In the electron micrograph, it can be seen that the crystals of the emulsion produced are predominantly of the twin type.

Sensitization of the emulsion (step (iv)) The emulsion is flocculated using conventional methods, washed and redispersed with a total of 210 g of calcified bone glue gelatin and then at 57 ° C. for various times at pH 6.3 and pAg 8.8 in the presence of 14 mg sodium thiosulfate and 2.6 mg sodium tetrachloroaurate dihydrate per mole of silver halide. The emulsion is stabilized with 7-hydroxy-5-methyl-2-methylthio-l, 3,4-triazaindoline. Then the sensitized emulsion is poured onto a photocarrier with a casting weight of 24 mg Ag / dm2.

Photographic Results The cast strips are then imagewise exposed for 0.02 seconds and developed in a metol / hydroquinone based developer at 20 ° C for 4 minutes.

FIG. 10 shows the photographic results obtained in comparison with a low-sensitivity camera emulsion used as a reference standard:

Emulsion silver DMAX gamma casting weight

Emulsion from this 24mg / dm2 2.6 1.43

example

(as manufactured)

Low sensitivity 22 1.9 0.84

Camera emulsion

The low-sensitivity camera film used in this example is a commercial formulation prepared as follows: 1.5 mol of 3 m silver nitrate solution are injected into 3 liters of 3.3% gelatin solution containing 2.25 mol of 3 m ammonium

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contains nium bromide, 0.11 mol l, 2m potassium iodide and 2.8 mol 11.8m ammonia solution. After ripening, the mean linear size of this emulsion is 0.50 microns.

Discussion of the photographic results The photographic results with the emulsion prepared in this example show that with increased opacity and higher contrast, a sensitivity comparable to that of the reference emulsion is obtained.

Example 2

The following emulsions are made:

EMULSION A This example illustrates the preparation 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 have been chemically sensitized by heat treatment with sulfur and gold salts, and which are used to produce a direct positive image by suitable ones Processing is used.

Preparation of monodisperse silver iodide emulsion (stage (i))

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 chemical sensitization is carried out by running 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 dihydrate 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 syllable iodide emulsion have an average size of 0.18 microns.

Recrystallization (Step (ii)) The recrystallization step is carried out in exactly the same way as described for the emulsion of Example 1, but using the chemically sensitized silver iodide emulsion prepared as just described.

Further growth (stage (iii))

The further growth step (iii) is carried out exactly as described for the emulsion of Example 1.

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 (iv) 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 30 mg aeg / dm2.

EMULSION B This example explains the production of a monodisperse twinned silver iodide chloride bromide emulsion according to the invention, which twinned crystals with a “core” made of silver iodide chloride and an outer “shell”

contains from silver bromide and which is produced by recrystallization of silver iodide crystals which are chemically sensitized by the addition of a rhodium salt during the precipitation.

Production of monodisperse silver iodide emulsion

(Level (i))

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

Instead, after the growth of the silver iodide crystals has ended, 50 ml of an aqueous solution containing 5 • 10 "6 g of sodium hexachlororhodite dissolved in ion lithium chloride are added to the silver iodide emulsion in order to incorporate rhodium into the surface layers of the silver iodide crystals to a mean size of 0.18 microns. Most likely in the form of an adsorbed rhodium complex ion, the emulsion is stirred for 15 minutes at 65 ° C to allow adsorption and chemical sensitization to occur.

Recrystallization (stage (ii))

812 g of the silver iodide emulsion prepared in step (1) are added to 500 g of distilled water which contains 120 g of inert limed bone gelatin and is stirred with 2 ml of n-octanol at 65 ° C. and 200 rpm. Aqueous 4.7m solutions of silver nitrate and ammonium chloride are injected into the silver iodide emulsion with stirring at a rate of 600 ml per hour until an addition of 600 ml of silver nitrate solution. The pAg is kept at 7.3 ± 0.2 throughout. Up to the addition of a further 800 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 1500 ml per hour. During this precipitation phase, the pAg is kept at 7.7 + 0.2.

Then 250 ml of distilled water containing 200 g of inert gelatin are added and the double injection at 2000 ml per hour is continued until an additional 400 ml of silver nitrate solution is added. The pAg is kept at 8.6 + 0.2.

Further growth (stage (iii))

This emulsion is subjected to further growth in the same way as described for stage (iii) of emulsion A and is prepared for pouring onto photo supports with a casting weight of 30 mg Ag / dm2 as described above.

EMULSION C

This example illustrates the preparation of a monodisperse twinned octahedral silver-beriodide bromide emulsion according to the invention, the internally sensitive twinned crystals produced by recrystallization of the silver iodide crystals without intentional chemical sensitization in stages (i) or (iv) being used to produce a direct positive image by suitable processing.

A monodisperse twinned octahedral emulsion is prepared exactly as described for the emulsion of Example 1, but omitting the chemical sensitization with sulfur and gold salts described in step (iv) of Example 1. After flocculation and washing, the emulsion is adjusted to pH 6.3 and pAg 8.8 and poured onto photocarriers with a casting weight of 30 mg Ag / dm2.

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12

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

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

(ii) Maturation at pH 6.3 and pAg 7.8 for 40 minutes

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 tetrachioroaurate dihydrate per mole of silver halide.

The reference emulsion is then poured onto photo support with a casting weight of 30 mg Ag / dm2. If the ripening in stage (ii) is omitted, a constant density (D max) is obtained without reversal. If surface sensitization is omitted in stage (iv), 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 obtain a direct positive image, the cast samples are placed in a fog developer of the following composition for 8 minutes at 20 ° C:

Veil developer

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-methylbenztriazole 50 mg

N-Formyl-N'-p-tolylhydrazine processed 50 mg water per liter, then fixed, watered and dried in the usual way. The photographic results obtained are shown in Figure 11.

Discussion of the Photographic Results The photographic results obtained by fogging the cast strips and shown in Figure 11 demonstrate the improved photographic behavior of the emulsions prepared according to the invention.

Figure 11 shows the photographic results obtained by fogging the cast strips. These demonstrate that the higher internal sensitivity of emulsions A and B leads to a corresponding improvement in the direct reversal characteristic compared to emulsions C and in particular D. It can be seen from FIG.

that one can obtain an excellent reversal characteristic for the twinned emulsions according to the invention without the chemical surface sensitization treatment which is necessary for the untwisted reference emulsion D.

The increased internal sensitivity compared to emulsion C, which is responsible for the high sensitivity to reversal, is a result of the chemical sensitization carried out in the case of emulsions A and B in stage (i) of the process according to the invention and can be demonstrated by using a solvent developer.

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Claims (19)

645468 PATENT CLAIMS 1. A process for the production of silver halide emulsions of the twin type, characterized in that stepwise (i) silver halide crystals containing at least 90 mol% iodide are formed in a colloidal dispersion medium, these crystals predominantly having the hexagonal lattice structure, and (ii) 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 to form silver halide crystals which contain iodide and the added halide (s), whereby the addition rate of the silver salt and alkali or ammonium halide so regulated that the silver halide crystals formed in this stage are predominantly twinned. 2. The method according to claim 1, characterized in that in a further process step (iii) by adding further aqueous silver salt solution and further alkali metal or ammonium halide to the colloidal dispersion the twinned crystals are allowed to grow in size. 3. The method according to any one of claims 1 or 2, characterized in that in a further process step (iv) the water-soluble salts formed are removed and the emulsion is chemically sensitized. 4. The method according to any one of claims I to 3, characterized in that the silver iodide content in the dispersing medium at the beginning of step (ii) is in the range of 0.05 to 2.0 mol / liter. 5. The method according to any one of claims 1 to 4, characterized in that in stage (ii) aqueous 3-5m silver nitrate and ammonium bromide or chloride solutions at a specified temperature and while maintaining a pAg value in the range of 5 , 0 to 11.0 with stirring to a silver-iodide dispersion in gelatin solution. 6. The method according to any one of claims 1 to 5, characterized in that in step (ii) the rate of addition of the silver salt is kept constant. 7. The method according to any one of claims 1 to 6, characterized in that in step (ii) such volumes of silver salt and ammonium or alkali halides are added that the silver iodide 0.01 to 20 mol .-% of the total silver halide in the finished Makes up emulsion. 8. The method according to any one of claims 1 to 7, characterized in that step (ii) is carried out in the presence of a wetting agent. 9. A process for the preparation of silver halide emulsions of uniform particle size of the twin type according to claim I, characterized in that it comprises the steps (i), (ii), (iii) and (iv) according to claims 1 to 3 and as in one of the Modified claims 2 to 7, wherein in step (iii) further aqueous silver nitrate and ammonium or alkali halide solutions are added to the twin crystals at such a rate that crystal nucleation no longer takes place, and wherein in the solution the pAg value is at a fixed value im Range 5 to 12 and the temperature in the range 35 to 90 ° C constant. 10. The method according to claim 9, characterized in that a twinned monodisperse octahedral emulsion is prepared by performing step (iii) at a fixed pAg between 9 and 11. 11. The method according to claim 9, characterized in that a twinned monodisperse cubic emulsion is prepared by performing step (iii) at a fixed pAg between 6 and 9 and an ammonia concentration in the range from 0 to 0.5 molar . 12. The method according to any one of claims 1 to 11, characterized in that it comprises the steps (i), (ii), (iii) and (iv) according to claims 1 to 3 and in step (i) the at least 90 mol% iodide containing silver halide crystals are chemically sensitized. 13. The method according to claim 12, characterized in that one carries out in step (i) two types of chemical sensitization. 14. The method according to any one of claims 12 or 13, characterized in that step (iii) is carried out and the silver halide crystals formed in step (ii) form the core and the halide added in step (iii) the shell of the finished silver halide crystals. 15. The method according to any one of claims 12 or 13, characterized in that in stage (ii) the halide precipitated at least in the initial stage of the stage is predominantly chloride and in stage (iii) the halide precipitated at least in the final stage of the stage is predominantly bromide. 16. The method according to any one of claims 1 to 15, characterized in that the surface of the silver halide crystals in step (iv) is chemically sensitized. 17. The method according to any one of claims 1 to 15, characterized in that the surface of the silver halide crystals is veiled in step (iv). 18. Silver halide emulsion prepared according to the process according to one of claims 1 to 17. 19. Photographic material, characterized in that it has at least one layer containing a silver halide emulsion according to claim 18.