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(11) | EP 0 896 245 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Silver halide emulsion with large tabular grains with a high bromide |
| (57) The present invention concerns a novel silver halide emulsion containing fine tabular
grains having a high aspect ratio as well as a method of preparing this emulsion. It also concerns a novel photographic product containing such an emulsion. The emulsion of the present invention is particularly advantageous for high-sensitivity photographic products. |
[0001] The present invention concerns a novel silver halide emulsion containing fine tabular grains having a high aspect ratio. It also concerns a novel photographic product containing such an emulsion as well as a method of preparing this emulsion.
[0002] Tabular grain emulsions are generally characterised by a grain population where at least 50% of the total projected surface area of the grains of the emulsion is represented by tabular grains. The grains are said to be "tabular" since they comprise two parallel main faces which are appreciably larger than all the other faces.
[0003] Tabular grains are characterised by their aspect ratio (R) defined as the ratio ECD/e > 5, where ECD is the mean equivalent circular diameter of the grains constituting the emulsion and e the mean thickness of the grains, ECD and e being expressed in the same units.
[0004] In the prior art many advantages have been identified resulting from the use of tabular grains with a high aspect ratio. Non-limitatively, these advantages consist of a better relationship between speed and granularity, improved sharpness, a possibility of faster processing, increased covering power, reduced loss of covering power for greater pretanning, better separation of speeds in the blue and minus blue region, etc.
[0005] Many patents describe tabular grains of silver bromide or silver bromoiodide with a high aspect ratio, for example US Patents 4 433 048, 4 434 226 and 4 439 520. These emulsions are prepared by means of a method which consists of growing the silver halide grains by introducing the halide ions and silver ions necessary for the growth of the grains in the form of grains preformed in a dedicated nucleation reactor. The drawback of this technique lies in the complexity of the equipment which must be used.
[0006] Another technique for preparing tabular grains with a high aspect ratio consists of using grain growth modifier. These compounds, being adsorbed selectively on certain faces of the grains, promote the growth of tabular grains with a high aspect ratio. The drawback of this technique lies in the fact that these compounds must be eliminated from the surface of the grains before sensitisation. Eliminating these compounds is difficult to implement and often causes a thickening of the grains.
[0007] In order to obtain highly sensitive photographic products, it is important to use large silver halide grains. These grains have the drawback of giving granularity. One means of obtaining a high sensitivity whilst limiting the granularity of the photographic products is the use of tabular grains. A means for reducing the granularity further is to use thin tabular grains. It is therefore important to be able to have tabular grains having a size which is as broad as possible and where the thickness of the grains is kept as low as possible.
[0008] It is known to a person skilled in the art that increasing the equivalent diameter of the tabular grains is to the detriment of the thickness. This is because, when the growth step of the tabular grains is prolonged, the speed increase of the thickness increases with the size. As for a certain size of grain, it therefore becomes of little advantage to continue the growth steps as it contributes mainly to increasing the thickness of the grains.
[0009] It is therefore particularly advantageous to provide a novel silver halide photographic emulsion containing fine tabular grains with a high equivalent circular diameter. These photographic products have a high photographic sensitivity whilst keeping low granularity.
[0010] The present invention concerns a silver halide photographic emulsion in which at least 95% of the grains constituting the emulsion are high tabular grains with a high bromide content, characterised in that the emulsion comprises tabular grains having a size greater than or equal to 5µm and a thickness below 0.1 µm.
[0011] In the context of the present invention, the size of the grain is measured by the diameter of a circle having the same projected surface area as this silver halide grain.
[0012] The present invention also concerns a method of preparing a silver halide emulsion in which at least 95% of the grains of the emulsion are tabular grains with a high bromide content and comprising grains having a size greater than 5 µm and a thickness less than or equal to 0.07 µm, the method comprising: (a) a nucleating step providing a mole number x of nucleated silver halides, (b) a ripening step and (c) a growing step providing a total mole number of silver halides y, the ratio y/x being greater than or equal to 500.
[0013] Figure 1 is a schematic depiction of the increase in the thickness of a tabular grain according to the size of this grain with the conventional methods of the prior art (curve 1a) and with the method of the present invention (curve 1b).
[0014] Figure 2 is a graph which shows the relationship between the individual thicknesses and sizes of the tabular grains obtained with the method of the Comparative Example 1 and the method of the present invention. The size and thickness of each of the grains depicted is measured by atomic force microscopy (AFM).
[0015] With the method of the present invention, a rate of grain thickening is obtained which is less than that observed with the methods of the prior art.
[0016] Apart from the size of the grains obtained, the emulsions of the invention have a better covering power. It is therefore possible, with such emulsions, to reduce the silver content of the photographic products.
[0017] Because of the high size of the silver halide tabular grains of the present invention, the presence of such grains greatly increases the speed of the emulsion. In addition, because of this size, a gain in speed is obtained without substantially impairing the granularity, by increasing the grain number per surface unit of coated emulsion.
[0018] Compared with the teachings of the prior art, it is surprising to obtain tabular grains whose size exceeds 4 µm whilst keeping such low thicknesses.
[0019] According to a particular embodiment, the emulsion of the present invention comprises at least 20% tabular grains having a diameter greater than or equal to 5 µm and a thickness below 0.1 µm.
[0020] According to another embodiment of the invention, the emulsion comprises tabular grains having a diameter greater than or equal to 5 µm and a thickness below 0.07 µm. According to another embodiment, the emulsion comprises tabular grains having a diameter greater than or equal to 8 µm and a thickness below 0.07 µm.
[0021] In the context of the invention, the tabular grains with a high silver bromide content are silver halide grains which contain at least 50% mol bromide, preferably at least 70% mol bromide, preferably at least 90% mol based on silver, and the remaining halides can be chloride, iodide or a mixture. In the context of the present invention, the emulsions can be pure bromide emulsions, or chlorobromide, iodochlorobromide, chloroiodobromide or iodobromide emulsions. When reference is made to silver halide grains or silver halide emulsions containing at least two silver halides, the halides are cited in increasing order of concentration.
[0022] Preferably, the silver halide tabular grains contained in the emulsion of the present invention contain the quantity of iodide below 3% mol. Very small quantities of iodide are generally sufficient to increase the sensitivity of the emulsion. The iodide is preferably introduced evenly during at least part of the making of the silver halide grains.
[0023] According to a particular embodiment of the invention, the emulsion is a silver iodobromide emulsion containing less than 3% mol iodide based on the total number of silver halides.
[0024] The emulsions of the present invention are obtained by means of the double jet technique. The nucleating, ripening and growing steps which are implemented in the method of the present invention are known conventional steps for obtaining silver halide tabular grains. Information for producing tabular grain emulsions are given in Research Disclosure, January 1983, No. 22534, and August 1983, No. 23212.
[0025] In the context of the invention, the method can be implemented in several steps. For example, after the nucleating step, it can be envisaged dividing the emulsion containing the nucleated silver halide moles and implementing the growing step on some of this emulsion. This embodiment makes it possible to use, for the growing step, growing reactors having a conventional volume.
[0026] According to a particular embodiment of the present invention, the ratio of the total mole number of precipitated silver halides to the nucleated silver halide mole number is greater than or equal to 800.
[0027] The emulsions of the present invention can be used in any radiation-sensitive photographic product.
Conventionally, these products comprise a support covered on at least one of its faces with a silver halide emulsion.
EXAMPLE 1 (comparative)
[0029] In an 18 litre precipitation reactor, 6 l of oxidised gelatin solution in water (1.15 g/l) was introduced. The pH was adjusted to 1.8 by adding HNO3, and the pAg to 9.16 by adding NaBr (0.6 g/l). This solution was stirred at 3500 rpm and maintained at 45°C.
[0030] A solution of AgNO3 (1.7 M) and a solution containing KI (0.025 M) and NaBr (1.645 M) were added simultaneously over 4 seconds at a rate of 110 ml/min. During these additions, the pAg was maintained at 9.16.
[0031] The temperature was then increased to 60°C (temperature rise 9 min). The mixture was maintained at this temperature for 9 in with stirring at 4000 rpm. The pAg was then around 8.54.
[0032] 100 g of oxidised and deionized gelatin was introduced into the reactor. The pH was adjusted to 5.85 by adding NaOH.
[0033] 8.78 g of NaCl and then NaBr (98 ml of a 1 M solution) were then introduced. The pAg was 8.47.
[0034] A solution of AgNO3 (0.4 M) and a solution of NaBr (2.5 mol/l) were introduced simultaneously at a rate of 14 ml/min over 30 seconds, and the pAg of the solution was 9.01. The flow rate of AgNO3 was then adjusted to 50 ml/min and that of NaBr to 9 ml/min. These flow rates were maintained for 40 min, and the pAg was then 9.06. This was adjusted to 9.29 by adding NaBr (2.5 M).
[0035] The growth of the silver halide grains was continued by introducing, into the reaction medium, the solutions of AgNO3 and NaBr described above over 60 min at accelerated rates (31.3 ml/min to 106 ml/min for AgNO3 and 5.9 ml/min to 19.5 ml/min for NaBr).
[0036] In this way 4 moles of silver were precipitated. The ratio of the total number of precipitated silver halide moles to the number of nucleated silver halide moles was around 300.
[0037] The emulsion obtained had a mean ECD of 1.65 µm and a mean thickness of 39 nm. The mean aspect ratio of such an emulsion was around 40. These measurements were made on the entire population.
[0038] Figure 2a is an individual analysis of a representative sample of the population of tabular grains constituting the emulsion. This Figure 2a shows that the emulsion thus obtained comprises no tabular grains with a size above 4 µm.
EXAMPLE 2
[0039] The operating method of Example 1 was reproduced, but in which, after the first growing step, an additional growing step was conducted in a 32 litre reactor. This step consisted of maintaining a flow rate of AgNO3 (0.4 M) at 220 ml/min and of NaBr (2.5 M) at 40 ml/min for 75 min and increasing the stirring to 5000 rpm.
[0040] In this way 6.6 additional moles of silver was precipitated. The ratio of the total number of precipitated silver halide moles to the number of nucleated silver halide moles was greater than 800.
[0041] The emulsion thus obtained had a mean ECD of 3.74 µm and a mean thickness of 48 nm. The mean aspect ratio of such an emulsion was around 80. These measurements were made on the entire population.
[0042] Figure 2b depicts an analysis of this emulsion by AFM, limited to the largest grains contained in the emulsion.
[0043] This figure shows the existence of grains having simultaneously an equivalent circular diameter to about or greater than 5 µm and a thickness less than or equal to 0.07 µm.
1. Silver halide photographic emulsion in which at least 95% of grains constituting the
emulsion are tabular grains with a high bromide content, characterised in that the
emulsion comprises tabular grains having a diameter greater than or equal to 5 µm
and a thickness below 0.1 µm.
2. Emulsion according to Claim 1 comprising tabular grains having a diameter greater
than or equal to 5 µm and a thickness below 0.07 µm.
3. Emulsion according to Claim 2 comprising tabular grains having a diameter greater
than or equal to 8 µm and a thickness below 0.07 µm.
4. Emulsion according to Claim 1 in which at least 20% of the tabular grains of the emulsion
have a diameter greater than or equal to 5 µm and a thickness below 0.1 µm.
5. Emulsion according to Claim 1 in which the silver halide grains are silver bromide
grains.
6. Emulsion according to Claim 1 in which the silver halide grains are silver iodobromide
grains comprising a quantity of iodide of less than 3% mol based on the total number
of moles of silver halide.
7. Emulsion according to any one of the preceding claims, comprising tabular grains having
individually an aspect ratio greater than or equal to 100.
8. Silver halide photographic product comprising a support and at least one light-sensitive
layer comprising a tabular-grain silver halide emulsion according to any one of Claims
1 to 7.
9. Method of preparing a silver halide emulsion in which at least 95% of the grains of
the emulsion are tabular grains with a high bromide content and comprising grains
having a size greater than 5 µm and a thickness less than or equal to 0.07 µm, the
method comprising:
(a) a nucleating step providing a number of nucleated silver halide moles x,
(b) a ripening step, and
(c) a growing step providing a total number of silver halide moles y,
the ratio y/x being greater than or equal to 500.
10. Method according to Claim 8 in which the ratio y/x is greater than or equal to 800.