[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.
[0028] The present invention is described in more detail in the following examples.
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 HNO
3, 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 AgNO
3 (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 AgNO
3 (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 AgNO
3 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 AgNO
3 and NaBr described above over 60 min at accelerated rates (31.3 ml/min to 106 ml/min
for AgNO
3 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 AgNO
3 (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.
[0044] A measurement of the sample representing the population of tabular grains in this
emulsion by AFM shows that the grains of the emulsion represent at least 20% of this
population.
[0045] A comparison with Figures 2a and 2b shows the difference in the size and thickness
of the grains in the emulsion of the present invention and in the comparative emulsion
of example 1.
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.