Photothermographic element

Details Photothermographic element Photothermographic element

1999-04-07

2002-08-13

Chea, Thorl (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Radiation sensitive product

Silver compound sensitizer containing

C430S531000, C430S567000, C430S569000, C430S568000, C430S570000, C430S604000, C430S607000, C430S613000, C430S944000

Reexamination Certificate

active

06432627

ABSTRACT:

This invention relates to a method of preparing a photothermographic element and more particularly, to a method of preparing a photothermographic element having a high sensitivity and experiencing, after processing, a of fog during storage in daylight.
BACKGROUND OF THE INVENTION
From the contemporary standpoints of environmental protection and space saving, it is strongly desired in the medical diagnostic field to reduce the quantity of spent solution. Needed in this regard is a technology relating to thermographic photosensitive materials for use in the medical diagnostic and photographic fields which can be effectively exposed by means of laser image setters or laser imagers and produce clear black images of high resolution and sharpness. These thermographic photosensitive materials eliminate a need for wet processing chemicals and offer a simple, environmentally friendly, thermographic system to the customer.
However, these photothermographic elements do not entail developing agents (reducing agents) and the fixation of undeveloped silver as opposed to wet processing, and give rise to the problem that the print-out of silver halide after processing can alter the images. This problem can be avoided by reducing the size of silver halide grains or by reducing the silver coverage thereof, but at the expense of sensitivity or image density.
It is well known in the art that if the size of silver halide grains is significantly reduced to the range below 50 nm, which is within the scope of the present invention, various inefficiencies during exposure considerably accumulate to induce a substantial drop of sensitivity. On the other hand, increasing sensitivity often invites an increase of fog at the same time. This makes it difficult to find a compromise between sensitivity and fog for photothermographic elements susceptible to fog.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a photothermographic element which has a high sensitivity and low fog, an ability to minimize a drop of image quality by a fog increase due to the print-out silver during storage in daylight after processing, and age stability before processing.
Another object of the present invention is to provide a photothermographic element in which age stability is endowed to a silver halide emulsion so that the variation among different coating lots is minimized.
According to the invention, there is provided a photothermographic element comprising at least one photosensitive layer on a support. The element contains an emulsion of silver halide fine grains having a mean equivalent spherical diameter of 10 to 50 nm, and further contains a non-photosensitive silver salt of organic aliphatic acid (referred to as organic silver salt, hereinafter), and a reducing agent for silver ion. The silver halide emulsion has been preformed independent of the organic silver salt and subsequently mixed with the organic silver salt before coating. That is, the silver halide emulsion is a preformed one. Differently stated, the silver halide emulsion is not formed by partial halogen conversion of an organic silver salt. Additionally, the silver halide fine grains have been chemically sensitized in the presence of a spectral sensitizing dye.
Several preferred embodiments of the invention are given below.
(1) The emulsion has been subjected to selenium sensitization or tellurium sensitization or both.
(2) The emulsion contains 10
−5
to 10
−3
mol of a polyvalent metal ion per mol of the silver halide. More preferably, the polyvalent metal ion is an iridium, ruthenium or iron ion and coordinates with a Cl, Br or CN ion.
(3) Of the faces of the silver halide fine grains of the emulsion, {100} face accounts for 50 to 100%.
(4) The silver halide fine grains of the emulsion have been formed in the presence of an oxidizing agent for silver.
(5) The silver halide fine grains have a mean equivalent spherical diameter of 10 to 30 nm.
(6) The spectral sensitizing dye is a merocyanine dye having an absorption peak at a wavelength in the range of 600 to 1,000 nm.
(7) The silver halide fine grains have a silver iodide content of 0 to 5 mol %.
(8) The emulsion further contains a triazine compound.
(9) A water-soluble polymer or water-dispersible polymer or both is used as a binder in the element.
DETAILED DESCRIPTION OF THE INVENTION
Silver Halide
The halogen composition of photosensitive silver halide is not critical and may be any of silver chloride, silver chlorobromide, silver bromide, silver iodobromide, and silver iodochlorobromide. The silver halide preferably has a silver iodide content of up to 5 mol %. The halogen composition in silver halide grains may have a uniform distribution or a non-uniform distribution wherein the halogen concentration changes in a stepped or continuous manner. Silver halide grains of the core/shell structure are also useful. Such core/shell grains preferably have a multilayer structure of 2 to 5 layers, more preferably 2 to 4 layers. Silver chloride or silver chlorobromide grains having silver bromide localized on surfaces thereof are also useful.
A method for forming the photosensitive silver halide is well known in the art. Any of the methods disclosed in
Research Disclosure
No. 17029 (June 1978) and U.S. Pat. No. 3,700,458, for example, may be used. One illustrative method which can be used herein is a method of adding a silver-providing compound and a halogen-providing compound to a solution of gelatin or another polymer to preform photosensitive silver halide grains and mixing the grains with an organic silver salt.
The photosensitive silver halide should preferably have a smaller grain size for the purposes of minimizing white turbidity, fog after image formation and a fog increase by print-out silver. Specifically, the grain size is 10 to 50 nm, preferably 10 to 45 nm, more preferably 10 to 40 nm, most preferably 10 to 30 nm. The term grain size designates the diameter of an equivalent sphere having the same volume as a grain.
The shape of silver halide grains may be cubic, octahedral, tabular, spherical, rod-like and potato-like, with cubic grains being preferred in the practice of the invention. Silver halide grains having rounded corners are also preferably used. No particular limit is imposed on the face indices (Miller indices) of an outer surface of photosensitive silver halide grains. Preferably silver halide grains have a high proportion of {100} face featuring high spectral sensitization efficiency upon adsorption of a spectral sensitizing dye. The proportion of {100} face is preferably 50% to 100%, more preferably at least 65%, most preferably at least 80% of the entire faces. Note that the proportion of Miller index {100} face can be determined by the method described in T. Tani, J. Imaging Sci., 29, 165 (1985), utilizing the adsorption dependency of {111} face and {100} face upon adsorption of a sensitizing dye.
Polyvalent Metal Ion
The silver halide grains used herein preferably contain in their crystal lattices a metal belonging to Groups 3 to 14 of Periods 4, 5 and 6 in the Periodic Table (new IUPAC), in the form of a coordination metal complex or metal ion. These metals may take the form of metal salts such as ammonium salts, acetate salts, nitrate salts, sulfate salts, phosphate salts, hydroxide salts, and be used as metal ions. The use of metals in the form of mononuclear coordination metal complex salts (such as six-coordinate complexes and four-coordinate complexes), binuclear metal complex salts or polynuclear metal complex salts can take advantage of the function of a ligand or complex structure.
When metals are used in the form of coordination metal complexes, the preferred ligands include halo (X), aquo (H
2
O), azido (N
3
), cyano (CN), cyanate (OCN), thiocyanate (SCN), selenocyanate (SeCN), tellurocyanate (TeCN), nitrosil (NO), thionitrosil (NS), oxo (O) or carbonyl (CO). The metal complex may have an organic ligand containing at least one carbon-carbon, carbon-hydrogen or carbon-nitrog

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