Radiation imagery chemistry: process – composition – or product th – Radiation sensitive product – Silver compound sensitizer containing
Reexamination Certificate
2000-10-20
2003-10-07
Chea, Thorl (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Radiation sensitive product
Silver compound sensitizer containing
C430S631000
Reexamination Certificate
active
06630293
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for producing a silver salt of an organic acid and a photothermographic material utilizing the silver salt.
BACKGROUND OF THE INVENTION
In recent years, reduction of amount of waste processing solutions is strongly desired in the medical field from standpoints of environmental protection and space savings. Techniques relating to photosensitive thermographic materials for use in the medical field and photographic-art processes are required which enables efficient exposure by a laser image setter or a laser imager and formation of a clear black image having high resolution and sharpness. The photosensitive thermographic materials can provide users with a more simple and non-polluting heat development processing system that eliminates the use of solution-type processing chemicals.
The same need applies to the field of ordinary image-forming materials. However, photo-images for medical use require high quality excellent in sharpness and graininess as they need very fine images. In addition, for easy diagnosis, cold monochromatic images are preferred. At present, various types of hard copy systems using pigments and dyes, for example, ink jet printers and electrophotographic systems are available as ordinary image forming systems. However, no satisfactory system is available for medical use.
Methods utilizing a silver salt of an organic acid for forming an image by heat development are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 and Klostervoer, “Thermally Processed Silver Systems”, Imaging Processes and Materials, Neblette, 8th ed., compiled by J. Sturge, V. Walworth and A. Shepp, Chapter 9, p.279, (1989). Generally, the phototlhermographic material, in particular, comprises a image-forming layer (photosensitive layer) containing a photocatalyst (e.g., silver halide) in a catalytically active amount, a reducing agent, a reducible silver salt (e.g., silver salt of an organic acid), and optionally a toning agent for controlling tone of silver, which are usually dispersed in a binder matrix. When the photothermographic material is heated at a high temperature (e.g., 80° C. or higher) after light exposure, a monochromatic black silver image is produced through an oxidation-reduction reaction between the silver halide or the reducible silver salt (which functions as an oxidizing agent) and the reducing agent. The oxidation-reduction reaction is accelerated by catalytic action of a latent image of silver halide generated upon exposure. Therefore, the monochromatic silver images are formed in exposed areas of the materials. This technique is disclosed in many references including U.S. Pat. No. 2,910,377 and Japanese Patent Publication (Kokoku, hereinafter referred to as JP-B) 43-4924. The photothermographic systems using a silver salt of an organic acid can achieve image quality and tones that satisfy the needs in the medical filed.
The silver source used in these systems is generally a silver salt of a fatty acid, and various methods for producing the same are known. Examples of the methods include a method of preparing a silver salt of an organic acid under coexistence of water and a hardly water-soluble solvent as disclosed in Japanese Patent Laid-open Publication (Kokai, hereinafter referred to as JP-A) 49-93310, JP-A-49-94619 and JP-A-53-68702, a method of preparing a silver salt of an organic acid in an aqueous solution as disclosed in JP-A-53-31611, JP-A-54-4117 and JP-A-54-46709, a method of preparing a silver salt of an organic acid in an organic solvent as disclosed in JP-A-57-186745, JP-A-47-9432 and U.S. Pat. No. 3,700,458 and so forth. Basically, the preparation is carried out by heating a fatty acid to a temperature higher than melting point thereof to dissolve the acid in water, adding sodium hydroxide or an alkali metal salt with vigorous stirring, and then adding silver nitrate to convert the alkali soap into silver soap.
Such alkali soap forms micelles in an aqueous solution, and gives a solution of whitely turbid appearance. The reaction from such a micelle state to the silver soap often suffers from problems concerning production stability. Therefore, as a method for obtaining the alkali soap as a uniform solution, a method of using a mixed solution of water and alcohol as the solvent is disclosed in JP-A-55-40607.
Further, alkali soap presents alkalinity as indicated by its name. Therefore, the silver soap will be prepared under a high pH condition in the above method. However, addition of a solution containing silver ions into an alkaline solution not only produces silver oxide as a byproduct, but generates unintended silver nuclei produced by a trace amount of reducing contaminants, which are unavoidable in a production process and exhibit high reducing property due to the high pH. Such byproducts are highly disadvantageous since they degrade performance of photothermographic materials, in particular, cause undesired fog. The problem of fog is not solved even by the method disclosed in JP-A-55-40607, which aims at obtaining a uniform solution to suppress generation of the byproducts.
In addition, JP-A-9-127643 discloses a method for producing a silver salt by simultaneous addition of measured amounts of an alkali metal salt solution and a silver nitrate solution, and the reference describes simultaneous addition of a solution of sodium behenate in a mixture of water and isopropyl alcohol and a solution of silver nitrate. This method can at least lower the high pH of the reaction to a neutral region, and thus is preferred to reduce the generation amount of silver oxide. However, isopropyl alcohol has weak reducing property, which makes the method insufficient to completely solve the problem of fog.
Moreover, the silver behenate grains formed by this method are two-dimensionally and anisotropically grown acicular grains having a size of 0.04 &mgr;m to 0.05 &mgr;m, and the reference contains no description concerning control of the grain size or grain shape.
In order to obtain a uniform dispersion practically usable as a coating solution containing a silver salt of an organic acid, it is necessary to achieve a state in which the silver salt of an organic acid is finely dispersed in a solvent without aggregation. For this reason, it is necessary to develop a method for dispersing the silver salt of an organic acid as fine grains. An ordinarily used method includes a method comprising the steps of separating the formed hydrophobic grains of silver salt of an organic acid as solid by filtration, mixing a dispersing agent with the solid and re-dispersing the mixture as described by Kloosterboer (Imaging Processes and Materials, Noblette, 8th ed., compiled by J. Sturge, V. Walworth and A. Shepp, Chapter 9, p.279, (1989)).
As the method for dispersing a silver salt of an organic acid as fine grains, a method of mechanically dispersing the salt in the presence of a dispersing aid by a known pulverization means (e.g., high-speed mixer, homogenizer, high-speed impact mill, Banbary mixer, homomixer, kneader, ball mill, vibrating ball mill, planetary ball mill, attriter, sand mill, bead mill, colloid mill, jet mill, roller mill, trone mill and high-speed stone mill). However, these methods only produce a coating solution containing a lot of aggregated particles, i.e., a coating solution that gives bad coated surface quality, as well as they cause a, problem that, because the methods possibly grind primary grains of a silver salt of an organic salt originally crystallized as a hardly wafer-soluble salt without any selectivity, silver nuclei are formed at crystal cleavage surfaces and causes increase of fog.
Several methods have been proposed, wherein the primary grains obtained during the reaction of a solution of alkali metal salt and a solution containing silver ions are utilized as they are, not separating the silver salt of an organic acid as solid and finely dispersing the solid.
For example, JP-A-8-234358 discloses a method of adding silver nitrate to an aqueous dispersion in which fine
Kawanishi Naoyuki
Kuno Koichi
Nagai Yoichi
Chea Thorl
Fuji Photo Film Co. , Ltd.
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