CS270388B1 - A method of preparing photo halogen! - Google Patents

Abstract

Nucleation is carried out in a protective gelatin colloid environment made from fish bones or skin, in the range of pBr 0.8 to 2.7, pH 1 to 11, at temperatures of 1 to 50 °C. Lowering the temperature during nucleation allows narrowing the distribution function of tabular crystal sizes and preparing hexagonal tabular silver halide crystals.

Description

The invention relates to a method for producing photographic silver halide emulsions which consist of a protective colloid and tabular silver halide crystals.

Photographic emulsions with tabular crystals exhibit a number of advantageous properties that can be used to improve the properties of a photographic image, increase light sensitivity, and save silver.

Tabular crystals of silver halides are defined as follows:

1. they are silver halide crystals bounded by parallel crystal faces (111) or (100), with the diameter of the crystal considerably exceeding its thickness,

2. the maximum thickness of tabular crystals is h< 0.5 /um and the diameter d>0.6 /um. The diameter of a tabular crystal is understood as the diameter of a circle that has the same area as the projection surface of the tabular crystal. The thickness of the crystal is understood as the distance between two parallel surfaces. The term mean aspect ratio is introduced, which characterizes tabular crystals from the point of view of their geometry:

n»1

For a set of tabular crystals, a mean aspect ratio of at least f>5:1 is required.

3. for a set of tabular crystals ▼ emulsion it is required that of the total projection area of all halogen-silver crystals present in the emulsion at least 50% of the projection area falls on tabular crystals.

In the two-stream precipitation or Ostwald growth mode, it is possible to prepare sets of tabular crystals with a mean aspect ratio f > 5 : 1 to f < 200 : 1. For practical use, emulsions with tabular crystals are suitable, the mean aspect ratio of which lies in the interval f > 5 : 1 to f < 20 : 1. For practical use, it is advantageous when tabular crystals account for more than 90 % of the total projection area of all crystals in the emulsion.

Due to the many advantageous properties of emulsions with tabular silver halide crystals, considerable attention has been paid to the preparation of these photographic emulsions. An overview of the state of knowledge of the preparation of tabular silver halide crystals can be obtained, for example, in the literature reviews:

Stávek J., Šípek M.: Methods of building the volume of silver halide crystals, Final report 82226/03/85, VŠOHT Prague 1985

Breslav J. A., Pejsachov V. V., Kaplun I. J.: Preparation and properties of tabular microcrystals of silver halides, Usp. Kauč. Pot., Moscow, Kauka, 1986.

A major difficulty in preparing tabular crystals of silver halides is solving the following problems:

1. relatively wide distribution of tabular crystal sizes,

2. formation and growth of non-tabular crystals,

3. formation and growth of tabular crystals with different numbers of twinning planes.

These three undesirable properties then lead to the disadvantages of photographic tabular silver halide emulsions:

1. the gradation of the characteristic curve cannot be increased,

2. a wide distribution of crystal sizes will not allow finding optimal chemical sensitization conditions for both small and large tabular crystals, since the optimal conditions for chemical ripening of large and small crystals differ significantly.

During the formation and growth of tabular crystals of silver halides, several twinning planes can form in the crystal. The influence of the number of twinning planes on the resulting shape of tabular crystals was studied by J. Maskasky in J. Imag. Sci. 21» '» »tr. ^5-26, 1987· If there are two such twinning planes in the crystal, then the resulting growth shape is hexagonal tabular

CS 270 388 B1 crystal. If the number of twins is three or five, then a tabular crystal of triangular shape is formed.

Because the optimal conditions for chemical ripening of hexagonal and triangular tabular crystals differ, it is not possible to find an optimum for chemical ripening for both types of tabular crystals.

If the dimensions of triangular and hexagonal tabular crystals are compared with the same projection surfaces of both types of crystals, then it follows that the size of triangular tabular crystals is 1.23 times larger than hexagonal ones. Therefore, to improve the graininess of the image, it will be more advantageous to use only hexagonal tabular crystals of silver halides.

The aim of the invention is to prepare negative and direct positive silver halide emulsions with silver halide crystals with the same growth form and narrow particle size distribution. Such emulsions then exhibit increased light sensitivity, gradation, less graininess, better image sharpness, higher hiding power, better overall image quality and resistance to pressure desensitization.

The subject of the invention is to find the conditions for the preparation of silver halide crystals in a protective colloid with the following properties:

1. at least 90% of the total projection area is made up of tabular silver halide crystals, 2. at least 70% of the total projection area is made up of tabular hexagonal silver halide crystals, «

3. a hexagonal crystal is required if the ratio of the longest edge to the shortest edge is less than 2:1 (see Fig. 1),

4. the distribution of tabular crystal sizes is less than C _T <20%, with the coefficient of variation defined as:

C . standard deviation ₁₀₀ ^T mean crystal size *

5. sets of tabular silver halide crystals exhibit a mean focal length greater than f>5 : 1.

In the preparation of tabular crystals of silver halides, the critical step is the nucleation stage, where it is necessary to create a twinning defect in the crystals (most preferably two twinning planes for the growth of hexagonal tabular crystals), while the size distribution of crystals with a twinning defect should be as narrow as possible.

The influence of pBr and pi on the formation and growth of tabular crystals is well known from the literature, see e.g. DOS 36 44 159. The following have also been studied: the influence of pH, type and concentration of gelatin on the formation of a twinning defect, see e.g. Eur. Pat. Appl. 0 228 256.

Several works are devoted to the study of the mechanism of the formation and growth of tabular crystals. One of the possible mechanisms that can explain the formation of the twinning defect and the growth of tabular crystals is the coagulation of microcrystals, see e.g. V. K. Walworth, W. D. Slafer, A. B. Holland: J. Imag. Sci. 3j, 3, 108 (1987) and Kaplun I». J., Sergeeva I. T., Breslav J. A., Andrejanov V. V. in Sborníku naučných trudov, pp. 5-15, GOSWII1ÍFOTOPROJEKT, Moscow 1987.

In the work of Breslavs et al. the dependence of the formation of twinning defects and the growth of tabular crystals on the concentration of silver halide microcrystals with a size of 20 to 200 nm clearly shows how significantly the formation of twinning defects can be assisted by increasing the number of silver halide microcrystal particles per unit volume of emulsion. In the next step, Ostwald ripening is carried out, when particles without twinning defects dissolve, which favorably affects the percentage of tabular crystals in the emulsion.

Silver halide emulsions with high particle concentration can be prepared in several ways. For the preparation of tabular crystals, it has been shown to be most advantageous to control the particle concentration per unit volume by decreasing the temperature of the system. It has been shown that by decreasing the temperature

OS 270388 Bl 3 system, during nucleation, hexagonal tabular crystals of silver halides begin to predominate in the system.

Low temperature nucleation offers the following advantages:

1. the proportion of tabular crystals increases, these crystals are small, less than 0.1 /um, 2. the nuclei formed and the twinning defect are stable, because Ostwald ripening is suppressed at low temperature. The growth rate of the formed nuclei is small at low temperature, therefore the formed nuclei have a narrow particle size distribution function, 3· at low temperature, nucleation proceeds in favor of the formation of tabular crystals with the suppression of the formation of other growth forms.

Gelatin is used as a protective colloid for the preparation of photographic silver halide emulsions. The commonly used gelatin, produced from animal bones or skins, has a sol-gel transition in the range of 31 to 33 °C. If precipitation is required at a lower temperature, it is necessary to reduce the concentration of gelatin in the solution. For example, in a 0.8% by weight solution, the sol-gel transition drops to about 25 to 28 °C. However, the concentration of gelatin cannot be reduced arbitrarily, because less concentrated gelatin solutions lose their protective colloidal effects.

Another option to reduce the sol-gel transition is to use gelatin with a low molecular weight in the range of approximately 2,000 to 100,000. Such gelatins are prepared by cleavage using, for example, enzymes. However, this process is difficult to control.

The novelty of the invention lies in the fact that at least for the nucleation stage of tabular crystals of silver halides, a new type of gelatin was used, which was made from fish bones. Gelatin prepared from this raw material differs from the generally used gelatins from bones and animal skins in the composition and representation of amino acids. This different chemical composition of gelatin made from fish bones also changes some properties of this gelatin, such as, for example, aqueous solutions of this gelatin have an aol - gel transition of 8 to 10 °C. More dilute solutions (1% by weight) of this gelatin can be cooled down to 3 °C.

The entire process of formation and growth of tabular crystals can be carried out according to the generally known procedure for the preparation of tabular crystals. The current state of knowledge of the preparation of tabular crystals of silver halides is explained in DOS 37 07 135. Under suitable conditions, nucleation occurs, followed by Ostwald ripening and finally the growth of tabular crystals.

Nucleation is carried out under the following conditions: the pBr value can be in the range of 0.8 to 2.5j the pi value in the range of 4.5 to 1.5 (see DOS 36 44 159) · Gelatin concentration 0.05 to 2% by weight. In the examples in DOS 37 07 135 the gelatin concentration is 0.8% by weight · and the temperature at which nucleation was carried out is 30 °C. The gelatin used by us allows the nucleation temperature to be reduced to 1 °C. The hydrogen ion concentration - pH - ranges from pH - 1 to 10.

After nucleation is complete, Ostwald ripening (dissolution of non-tabular crystals) takes place. Ostwald ripening is carried out as follows:

1. by increasing the reaction temperature to 40 to 80 °C,

2. adjusting the gelatin concentration to 1 to 10% by weight, whereby the added gelatin can be made from fish as well as from animal bones or skins,

3. Set a higher pBr in the range of 1.5 to 3.0 by adding silver nitrate. Neutralization of bromide ions reduces the solubility of the system, because Ostwald ripening at low pBr leads to a broadening of the particle size distribution function.

After the Ostwald ripening is complete, the growth of tabular crystals can begin. Two or more streams of silver and halide ions flow into the crystallizer. To accelerate the growth, a solvent such as thioether, ammonia, rhodanide, etc. can be added to the crystallizer. The rate of flow into the crystallizer must be regulated tsk so that the so-called critical flow rate is not exceeded, at which new stable nuclei are formed. The volume of tabular crystals can be varied over a wide range: the halide composition, for example, the iodide to bromide ratio can be increased up to half the solubility of iodide in the crystal lattice of silver bromide. Likewise

CS 270 388 Bl, various dopants can be added during the growth of tabular crystals, such as metal ions of cadmium, lead, thallium, erbium, bismuth, iridium, silver, platinum, etc.

By choosing the volume construction of tabular silver crystals, photographic emulsions can be prepared for both negative emulsions and direct positive emulsions.

After the growth of tabular crystals has ended and soluble salts have been removed, further steps in the production of photographic material are carried out, such as chemical and optical desensitization, addition of other necessary components - stabilizers, color-forming components, DIB components, etc. After coating, the obtained recording material is evaluated according to generally known procedures in photographic chemistry.

The essence of the invention will be explained by the following examples.

Example 1

The following conditions were set in the crystallizer: volume set to 1000 ml pBr « 1.3 pi « 3.3 pH · 6.0 t « 30 °C 8 g gelatin made from fish bones.

20 ml of 2U AgBTOj and 20 ml of 2M KBr were fed into the crystallizer at a rate of 15 ml/«4 a.

Then the temperature of the system was raised to 70°C and 200 ml of 10% gelatin (phthaloyl derivative) and 150 mg of 3,6-dithiooctane-1,8-diol were added. Then the pBr ³ value was adjusted to 2.5 by adding 2M AgNO 4 at a rate of 1 ml/min.

Then, 25% AgKO and 2M KBr solutions were fed into the system at a rate of 500 ml/min over 80 minutes, with the volumetric flow rate of the reactants being increased linearly from an initial value of 1 ml/min to a final value of 15 ml/min.

During the growth phase of the tabular crystals, the value was maintained at pBr * 2.5.

After the precipitation was completed, a sample was taken and the characteristics of the obtained photographic emulsion were evaluated using electron microscopy. The result is shown in Table I.

Example 2

The procedure in this example is identical to that in Example 1, with the difference that the temperature during nucleation was 3°C. The results of the properties of the prepared photographic emulsion are given in Table I (see also Fig. 2).

Table I

Geometric characteristics of prepared photographic emulsions with tabular crystals of silver halides

Ho mean size mean thickness % crystal crystal _M M mm *MMM mmmmmMmMmmmmmmmwmmmmmm mmmmmmmmmmMmmmmmmmmmmmm*mmmM mmmmmm« 1 1.25 0.27 projected area of T-crystals 90 « i — ΐ o í ® r 2 1.38 0.18 94% 12 The effect of the present invention is that by lowering the temperature, the coefficient of variation of growing tabular halide crystals increases. silver nucleation phase. ee ani-

Claims (1)

CS 270 388 Bl P S BD Μ £ T OF THE INVENTION A method for producing photographic silver halide emulsions consisting of a protective colloidal medium and tabular crystals of silver halides, the size of which is greater than 0.5 /u and the thickness is not more than 0.35 /U· and with an average aspect ratio f>5:1, wherein the projection area of the tabular crystals represents at least 50% of the total projection area of all crystals present in the emulsion, characterized in that nucleation takes place in a protective gelatin colloid made from fish bones 61 skins in the temperature range of 1 to 50 °C at a gelatin concentration of 0.05 to 10% by weight and at a pBr of 0.8 to 2.7, a concentration of iodide ions of maximum 0.1 mol/l and a pH of 1 to 11.