Radiation imagery chemistry: process – composition – or product th – Radiation sensitive product – Silver compound sensitizer containing
Reexamination Certificate
2002-07-11
2004-01-06
Chea, Thorl (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Radiation sensitive product
Silver compound sensitizer containing
C430S567000
Reexamination Certificate
active
06673529
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an improved method for preparing tabular grain silver halide emulsions, and to photographic elements which contain one or more of such improved emulsions.
BACKGROUND OF THE INVENTION
The photographic advantages of high bromide tabular grain emulsions were first demonstrated by Wilgus et al U.S. Pat. No. 4, 434,226, Kofron et al U.S. Pat. No. 4,439,520 and Solberg et al. U.S. Pat. No. 4,433,048, in which patents it was recognized that the chemically and spectrally sensitized emulsions disclosed in one or more of their various forms would be useful in color photography and in black-and-white photography (including indirect radiography). The high bromide tabular grain emulsions were prepared by creating tabular grain nuclei under a specified set of conditions and then growing these nuclei by the concurrent addition of silver and halide ions. Kofron et al, e.g., disclosed and demonstrated striking photographic advantages for chemically and spectrally sensitized tabular grain emulsions in which tabular grains having an average aspect ratio of greater than 8 account for greater than 50 percent of total grain projected area of the emulsion grains. Solberg et al. demonstrated advantages for high bromide tabular silver iodobromide grain emulsions having a lower proportion of iodide in a central region than in a laterally displaced region.
The advantages of tabular grain emulsions have further been demonstrated in many subsequent references, especially in regard to their increased surface area to volume ratio compared to three dimensional grains of equal mass, such as cubes or octahedra. As a consequence of this larger surface area to volume ratio, tabular emulsions offer a number of photographic advantages, including a higher spectral to intrinsic speed ratio, improved granularity at a given spectral speed, higher covering power, and decreased turbidity. Aspect ratio, which is the ratio of tabular grain thickness to diameter (usually the diameter of a circle of equivalent area), is a figure of merit that correlates directly with the magnitude of potential advantage that a tabular grain emulsion offers. Thus if two tabular grain emulsions have the same mean grain diameter, but different aspect ratios, they have potential to be of approximately equivalent spectral speed when optimally sensitized. However, the one that has higher aspect ratio (i.e., the emulsion comprising thinner grains where such emulsions have the same mean grain diameter) has potential to form images of lower granularity, for example, in a color negative film.
U.S. Pat. No. 4,914,014 describes a process for making tabular grain emulsions wherein nucleation is carried out at high pBr, which is useful for precipitation of thin tabular emulsions. U.S. Pat. No. 5,250,403 demonstrates improved specularity (decreased turbidity) of very thin tabular emulsions vs. that of thicker tabular grain emulsions, and the importance of morphological purity in thin tabular emulsions.
The use of thiocyanate ions during preparation of photographic elements has been suggested in the art, primarily for increasing photographic sensitivity. U.S. Pat. No. 2,222,264, e.g., discloses use of thiocyanate in precipitation of AgBrI emulsions. The inventors indicate that an “amount of thiocyanate equal to from about 2 to about 15% by moles of the silver halide in the emulsion” may be employed. They further state that “larger amounts of thiocyanate can be employed, particularly if the thiocyanate is added before washing”, and they claim an emulsion containing about 0.05 to 2 mole-% (after washing). These inventors make no mention of grain morphology, and in particular, they do not teach levels that are appropriate for precipitation of tabular grain emulsions.
U.S. Pat. No. 3,320,069 discloses use of a water-soluble thiocyanate compound at a level of “5-50 grams per mole of silver halide.” Although the inventors do not specify a particular water-soluble thiocyanate compound, if it pertains to the sodium salt as used in some of their examples, this 5-50 gram level corresponds to 6.16 to 61.6 mole-% based on silver halide. The inventors do not describe the morphology of their emulsions, and in particular offer no teaching regarding use of thiocyanate in precipitation of tabular grain emulsions. U.S. Pat. No. 4,853,323 discloses use of thiocyanate at a concentration of 0.5 to 60 mole % (preferably 2 to 40 mole %) of Ag in precipitation of emulsions. This patent, and closely related patent U.S. Pat. No. 4,921,784, also specify a spectral characteristic in which the resulting emulsion shows a particular intensity of IR absorption of thiocyanate ion at 2052 cm
−1
. While a statement is included that tabular grain emulsions having an aspect ratio of 5 or more can be used in the invention, there is no teaching as to the effect thiocyanate addition would actually have on the preparation of such emulsions.
U.S. Pat. No. 4,433,048 mentions possible use of thiocyanate in tabular emulsions, but gives no guidance regarding the specific time in the emulsion preparation process at which it should be added, does not indicate preferred amounts, and does not describe expected effects.
Because aspect ratio is a direct correlate to potential advantage of tabular emulsions, methods to increase this figure of merit during tabular grain preparations are sought. Also, because of the continuous desire for higher photographic speed, larger grain emulsions are required. Thus, precipitation methods for producing tabular grain emulsions which maintain or increase aspect ratios would be especially valued.
SUMMARY OF THE INVENTION
In one aspect this invention is directed towards a process of preparing a high bromide tabular grain emulsion comprising a dispersion medium and silver halide grains including tabular grains having {111 } major faces and an aspect ratio of at least 2, which contain greater than 50 mole percent bromide, based on silver, and which account for greater than 50 percent of total grain projected area, where such tabular grains have an average aspect ratio of at least 5, the process comprising
(i) in a grain nucleation step creating in a dispersing medium tabular silver halide grain nuclei containing parallel twin planes and
(ii) in a grain growth step subsequently growing the tabular grain nuclei into tabular grains in a silver halide reaction vessel by adding a silver ion source and a halide ion source to the reaction vessel and precipitating silver halide onto the tabular grain nuclei, wherein thiocyanate ion is introduced into the silver halide reaction vessel prior to the addition of at least the final 10 mole percent of the total silver added to the reaction vessel, and further wherein the introduced thiocyanate ion concentration is at most 0.4 mole %, based on the total silver added to the reaction vessel.
In a further aspect, this invention is directed towards a photographic element comprised of a support, and a silver halide emulsion layer coated on the support comprised of an emulsion prepared in accordance with the process of the invention.
We have found that while use of prior art suggested levels of thiocyanate in tabular grain emulsion precipitation typically results in decreased aspect ratio and degraded morphological purity, use of levels of thiocyanate lower than taught in prior art during grain precipitation surprisingly result in increased aspect ratio without degrading morphological purity.
DETAILED DESCRIPTION OF THE INVENTION
The invention is directed to an improved process for the preparation of tabular grain photographic emulsions. The emulsions are specifically contemplated for incorporation in camera speed color or black and white photographic films, as well as in radiographic film elements, including photothermographic elements.
As used herein, the term “tabular” grain refers to silver halide grains having an aspect ratio of at least 2, where aspect ratio is defined as the equivalent circular diameter (ECD) of the major face of the grain divided by the grain thickness. Tabu
Daubendiek Richard L.
Eikenberry Jon N.
Fisher Barbara J.
Gersey Timothy R.
Lighthouse Joseph G.
Anderson Andrew J.
Chea Thorl
Eastman Kodak Company
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