Radiation imagery chemistry: process – composition – or product th – Radiation sensitive product – Silver compound sensitizer containing
Reexamination Certificate
2001-11-09
2003-10-28
Letscher, Geraldine (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Radiation sensitive product
Silver compound sensitizer containing
C430S567000
Reexamination Certificate
active
06638703
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the preparation of photographic emulsions containing silver halide tabular grains.
BACKGROUND OF THE INVENTION
The preparation of silver halide grains generally includes a nucleation step and at least one crystal growth step.
In this patent application, the term “nuclei” designates grains of small size (less than 0.1 micrometer, for example) obtained in the nucleation step. The term “seeds” designates the grains obtained after the nuclei have been submitted to a first growth step. These seeds, generally smaller than 0.4 micrometer, are then subjected to a second growth step to obtain the final silver halide grains.
There are various conventional processes for achieving the nucleation step of silver halide grains. In single-jet processes, an aqueous solution of a silver salt is introduced into a stirred reactor containing a colloid, generally gelatin, and an aqueous solution of halides. In double-jet processes, the silver salt and halide solutions are introduced either simultaneously or alternately from two separate sources in a stirred reactor containing the colloid. In both cases, the growth step typically follows immediately and is achieved by double-jet precipitation.
In these conventional processes it can be difficult to correlate the number of nuclei formed during the nucleation with the final number of grains, in particular because of Ostwald ripening, which causes the less soluble larger grains to grow at the expense of the more soluble small ones. For a given number of nuclei initially formed, the number of grains remaining after the growth step will thus generally be lower than the number of nuclei.
There exists a third type of process that involves carrying out a nucleation step in a first reactor by simultaneously introducing solutions of silver salts, halides and colloid, and then a growth step in a second reactor containing the nuclei formed in the first reactor and where a solution of a silver salt and one or more halide solutions are introduced.
U.S. Pat No. 5,254,454 describes a process for preparing silver halide grains for photographic emulsions in which the nucleation step is carried out in a vigorously stirred mixer (10,000 revolutions per minute) into which a solution of silver salt, a solution of halides, and a solution of colloid are introduced. According to U.S. Pat. No. 5,254,454, a first emulsion is thereby formed containing fine silver halide grains (size less than or equal to 0.01 micrometer). This first emulsion is then transferred to a reactor in which the pAg is modified. The modified emulsion is then transferred to a second reactor containing a second emulsion made up of small sized silver halide crystals. The crystals that are present in the second reactor, after dissolution, allow the growth of the fine grains in the first emulsion.
Another process involves separating the nucleation and growth operations temporally and spatially. In a first reactor a seed solution, which is stored temporarily, is generated by precipitation of silver salts and halide salts in the presence of a colloidal agent. A part of this seed solution is subsequently used to seed a second reactor initially containing a colloidal agent and halide salts. The final growth of these seeds is then achieved by a conventional double-jet method. In some cases this process can afford silver halide emulsions that display special properties, such as reduced pressure sensitivity. This process has been described for tabular grains. However, none of the publications that describe the use of this process for tabular grains provides for any improvement of the characteristics of the industrial production of the grains over any of the conventional processes. The processes using tabular grains do not, according to the descriptions that are provided, afford any reduction in the variability observed in the operations performed for the industrial production of tabular crystals.
U.S. Pat. No. 5,712,083 uses the general idea of tabular seeds and describes an intermediate washing step to remove the growth modifying agent used in the operation that served to generate the seeds. It does not, however, describe any intermediate adjustment and therefore does not aim to improve the overall reproducibility of the precipitation processes.
U.S. Pat. No. 5,378,600 also describes the tabular seed approach, similarly with no intermediate adjustment. The size of the seed crystals is relatively small (0.3 micrometer), but their thickness is high (at best 0.1 micrometer), which is relatively easy to achieve, whereas it is difficult to produce small thin crystals in sufficient amounts. In addition, these seeds are used for the final growth operation at concentrations greater than 0.5% of the volume of the initial solution present in the growth reactor. This value is high, and so does not necessarily favor obtaining high industrial yields. With smaller seeds such as those generated in this invention, smaller quantities can be added, in all cases less than 0.5% by volume.
In view of the wide range of silver halide photographic emulsions used in photographic products, it is most desirable to have a method for the preparation of emulsions that are either identical or different as regards the size of their silver halide grains or the size range of their grains, from one single nucleation step, irrespective of whether the precipitations are carried out at the laboratory, pilot or production scale. It is known that nucleation is a precipitation step that induces wide variability in the final crystal size. An identical but well controlled nucleation for all crystals could therefore reduce that variability, while making it easier to make emulsions that are identical at all scales.
Furthermore, in tabular grain emulsions there is often an appreciable proportion of grains that are unwanted because they have not the required shape, diameter or thickness specifications. In addition, the dispersity of the grain characteristics is an important parameter for ensuring that the grains respond to light excitation and to image forming development in as even a manner as possible. To overcome these problems seed emulsions can be prepared from which the final tabular grains, as indicated above, can be obtained by growing. However, if the population of these grains is not sufficiently monodisperse and homogeneous, or not sufficiently stable, it is unlikely that the final emulsion will display the characteristics that are wanted. Because of this difficulty it is not possible at will to obtain emulsions with a high morphological purity after growth from an initial population of nuclei. By high morphological purity is meant an emulsion in which the tabular grains account for at least 50% and advantageously more than 80% and even more than 90% of the total surface area of the grains. By “tabular grain” is meant grains whose aspect ratio (equivalent circular diameter: thickness) is at least equal to 2, preferably greater than 3 and is advantageously greater than 8.
SUMMARY OF THE INVENTION
This invention solves the problems stated above and relates to a method to produce a quantity of thin tabular grains, substantially monodisperse, and of high morphological purity, from a controlled stable seed emulsion. A further object of this invention is a method for preparing different emulsions or several batches of silver halide tabular grains emulsion from a single controlled stabilized seed emulsion.
The method of this invention, for preparing a silver halide tabular grain emulsion, comprises the following steps:
(a) A batch of nuclei emulsion is prepared in a nucleation reactor in the presence of a peptizing agent of the hydrophilic colloid type, and these nuclei are then submitted to a physical or Ostwald ripening.
(b) The nuclei obtained in step (a) are grown into stable tabular seeds, keeping the ratio of initial volume of nucleation medium to final volume of medium after growth of nuclei in the nucleation reactor between 0.4 and 0.95.
(c) A portion of the batch of seed emulsion obtained in step (
Barillon Bruno C.
Jezequel Pierre-Henri
Schmuckle Christian S.
Anderson Andrew J.
Eastman Kodak Company
Letscher Geraldine
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