Radiation imagery chemistry: process – composition – or product th – Thermographic process – Heat applied after imaging
Reexamination Certificate
2000-08-15
2002-10-15
Chea, Thorl (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Thermographic process
Heat applied after imaging
C430S512000, C430S523000, C430S607000, C430S613000, C430S614000
Reexamination Certificate
active
06465162
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to thermally-developable imaging materials such as photothermographic materials. More particularly, it relates to photothermographic imaging materials that provide images that are more stable to light and heat over time particularly under light box conditions. The invention also relates to methods of imaging using these materials. This invention is directed to the photothermographic imaging industry.
BACKGROUND OF THE INVENTION
Heat-developable thermographic and photothermographic imaging materials (that is, heat-developable photographic materials) have been known in the art for many years.
Thermography or thermal imaging is a recording process wherein images are generated by the use of thermal energy. In direct thermography, a visible image is formed by imagewise heating a recording material containing components that changes color or optical density upon heating. Thermographic materials generally comprise a support having coated thereon: (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing system (usually including a developer) for the reducible silver ions, and (c) a hydrophilic or hydrophobic binder.
Thermal recording materials become photothermographic materials upon incorporating a photosensitive catalyst such as silver halide. Upon imagewise exposure to irradiation energy (ultraviolet, visible or IR radiation) the exposed silver halide grains form a latent image. Application of thermal energy causes the latent image of exposed silver halide grains to act as a catalyst for the development of the non-photosensitive source of reducible silver to form a visible image. These photothermographic materials are also known as “dry silver” materials.
In such materials, the photosensitive compound is generally a photographic type photosensitive silver halide that is considered to be in catalytic proximity to the non-photosensitive source of reducible silver ions. Catalytic proximity requires intimate physical association of these two components either prior to or during the thermal image development process so that when silver atoms (Ag
o
, also known as silver specks, clusters or nuclei) are generated by irradiation or light exposure of the photosensitive silver halide, those silver atoms are able to catalyze the reduction of the reducible silver ions within a catalytic sphere of influence around the silver atoms [Klosterboer, Neblette's Eighth Edition:
Imaging Processes and Materials
, Sturge, Walworth & Shepp (Eds.), Van Nostrand-Reinhold, New York, Chapter 9, pages 279-291, 1989]. It has long been understood that silver atoms act as a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive reducible silver ions in a number of different fashions (see, for example,
Research Disclosure
, June 1978, Item No. 17029). Other photosensitive catalysts such as titanium dioxide and zinc oxide can be used in place of silver halide.
The photosensitive silver halide may be made “in situ,” for example by mixing a halogen-containing source (either organic or inorganic halogen source) with the source of reducible silver ions to achieve partial methasis and thus causing the in-situ formation of silver halide (AgX) grains throughout the reducible silver ion source [see, for example, U.S. Pat. No. 3,457,075 (Morgan et al.), by coprecipitation of the silver halide and the reducible silver ion source [see for example U.S. Pat. No. 3,839,049 (Simons)], or by complete conversion of a portion of the silver ions to the silver halide and adding that portion back to the reducible silver ion source.
The silver halide may also be pre-formed and prepared by an ex situ process whereby the silver halide (AgX) grains are prepared and grown separately in an aqueous or an organic solvent before mixing and/or coating with the source of reducible silver ions. With this technique, one has the possibility of controlling the grain size, grain size distribution, dopant levels, and composition much more precisely, so that one can impart more specific properties to the photothermographic material.
The non-photosensitive source of reducible silver ions is a material that contains silver ions. Typically, the preferred non-photosensitive source of reducible silver ions is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, or mixtures of such salts. Salts of other organic acids or other organic compounds, such as silver imidazolates, silver benzotriazoles, silver benzotetrazoles, silver benzothiazoles and silver acetylides have also been proposed. U.S. Pat. No. 4,260,677 (Winslow et al.) discloses the use of complexes of various inorganic or organic silver salts.
In photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag
o
) in what is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photosensitive emulsion must be further developed to produce a visible image by the reduction of silver ions that are in catalytic proximity to the silver halide grains bearing the clusters of silver atoms. This produces a black-and-white image. The non-photosensitive silver source is reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced depending upon the reducing agent in the materials.
In both thermographic and photothermographic materials, the reducing agent for the reducible silver ion of the light-insensitive silver salt, often referred to as a “developer,” may be any compound that can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for both thermographic and photothermographic materials. At clevated temperatures the reducible silver ions arc reduced by the reducing agent. In thermographic materials, simply heating above the development temperature is sufficient to cause the reduction reaction. In photothermographic materials, upon heating, this reaction occurs preferentially in the regions surrounding the latent image. In both thermographic and photothermographic materials, this reaction produces an image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the imaging emulsion.
Differences Between Photothermography and Photography
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing using aqueous processing solutions.
In photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer incorporated within the material. Heating at 50° C. or more is essential for this dry development. In contrast, conventional wet-processed photographic imaging materials require processing in aqueous processing baths to provide a visible image at more moderate temperatures (from 30° C. to 50° C.).
In photothermographic materials, only a small amount of silver halide is needed to capture light and a different form of silver (for example a silver carboxylate) is used to generate the image using thermal development. Thus, the silver halide serves as a catalyst for the physical development of the non-photosensitive reducible silver ions. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver that, upon chemical development, is itself converted into the silver image, or that upon physical development requires addition of an external silver source. Thus, photothermographic materials require an amou
Kong Steven H.
Sakizadeh Kumars
Chea Thorl
Eastman Kodak Company
Tucker J. Lanny
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