Core-shell silver salts and imaging compositions, materials and methods using same

[0002] This application claim benefit to provisional No. 60/201,858 filed May 4, 2000.

[0003] This invention relates to novel non-photosensitive core-shell silver salts and their use in imaging compositions, materials and methods. In particular, it relates to core-shell silver salts comprising one or more silver salts in the core, and one or more different silver salts in the shell. These salts are useful in thermally-developable imaging materials such as thermographic and photothermographic imaging materials.

[0004] Silver-containing thermographic and photothermographic imaging materials (that is, heat-developable photographic materials) that are developed with heat and without liquid development have been known in the art for many years.

[0005] 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 matter 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 composition (usually including a developer) for the reducible silver ions, and (c) a hydrophilic or hydrophobic binder.

[0006] In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. At elevated temperatures, silver behenate is reduced by a reducing agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechols, pyrogallol, ascorbic acid, ascorbic acid derivatives, and the like, whereby an image of elemental silver is formed. Some thermographic constructions are imaged by contacting them with the thermal head of a thermographic recording apparatus, such as a thermal printer, thermal facsimile, and the like. In such: an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated to an elevated temperature, typically in the range of from about 60 to about 225° C., resulting in the formation of an image.

[0007] Thermal recording materials become photothermographic upon incorporating a photosensitive catalyst (such as a silver halide) that upon exposure to irradiation energy (ultraviolet, visible or IR radiation) is capable of providing a latent image. This latent image can be developed by application of thermal energy. Photothermographic materials are also known as “dry silver” materials.

[0008] In such materials, the photosensitive catalyst 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, (AgO)_n, also known as silver specks, clusters, nuclei, or latent image, 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 in catalytic proximity with the non-photosensitive source of reducible silver ions in a number of different ways (see, for example, Research Disclosure, June 1978, item 17029). Other photosensitive materials, such as titanium dioxide, cadmium sulfide, and zinc oxide, have also been reported to be useful in place of silver halide as the photocatalyst in photothermographic materials [see for example, Shepard, J. Appl. Photog. Eng. 1982, 8(5), 210-212, Shigeo et al., Nippon Kagaku Kaishi, 1994, 11, 992-997, and FR 2,254,047 (Robillard)].

[0009] The photosensitive silver halide may be made “in situ,” for example, by mixing an organic or inorganic halide-containing source with a source of reducible silver ions to achieve partial metathesis and thus causing the in-situ formation of silver halide (AgX) grains throughout the silver source [see, for example, U.S. Pat. No. 3,457,075 (Morgan et al.)]. In addition, photosensitive silver halides and sources of reducible silver ions can be coprecipitated [see Usanov et al., J. Imag. Sci. Tech.40, 104 (1996)]. Alternatively, a portion of the reducible silver ions can be completely converted to silver halide, and that portion can be added back to the source of reducible silver ions (see Usanov et al., International Conference on Imaging Science, 7-11 September 1998) The silver halide may also be “preformed” and prepared by an “ex situ” process whereby the silver halide (AgX) grains are prepared and grown separately. 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 both the silver halide grains and the photothermographic material. The preformed silver halide grains may be introduced prior to, and be present during, the formation of the source of reducible silver ions. Co-precipitation of the silver halide and the source of reducible silver ions provides a more intimate mixture of the two materials [see for example, U.S. Pat. No. 3,839,049 (Simons)]. Alternatively, the preformed silver halide grains may be added to and physically mixed with the source of reducible silver ions.

[0010] The non-photosensitive source of reducible silver ions is a material that contains reducible 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. Such acids are also known as “fatty acids” or “fatty carboxylic acids”. Silver salts of other organic acids or other organic compounds, such as silver imidazoles, silver tetrazoles, 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.

[0011] In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (AgO)_n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver containing-clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically 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.

[0012] In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a “developer,” may be any compound that, in the presence of the latent image, 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 photothermographic materials. At elevated temperatures, the reducible silver ions are reduced by the reducing agent for silver ion. In photothermographic materials, upon heating, this reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative 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 layer(s).

[0013] 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 with aqueous processing solutions.

[0014] As noted above, 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 photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30° C. to 50° C.) to provide a visible image.

[0015] In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example, a silver carboxylate) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.

[0016] In photothermographic materials, all of the “chemistry” for imaging is incorporated within the material itself. For example, they include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. Even in so-called “instant photography”, the developer chemistry is physically separated from the photosensitive silver halide until development is desired. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of “fog” or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems during the preparation of the photothermographic emulsion as well as during coating, use, storage, and post-processing handling.

[0017] Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is, in the aqueous fixing step).

[0018] In photothermographic materials, the binder is capable of wide variation and a number of binders (both hydrophilic and hydrophobic) are useful. In contrast, conventional photographic materials are limited almost exclusively to hydrophilic colloidal binders such as gelatin.

[0019] Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the underlying chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.

[0020] These and other distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials(Neblette's Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp 74-75, and in Zou et al., J. Imaging Sci. Technol.1996, 40, 94-103.

[0021] While a number of useful thermographic and photothermographic products are available in the market for medical and graphic arts uses, there is a continuing need for improving the reactivity of the compounds used to provide reducible silver ions. In particular, there is a need for imaging materials that have improved image stability and that can be imaged and/or developed at lower temperatures, while providing high D_max, and maintaining good image tone and quality.

[0022] The present invention provides a core-shell non-photosensitive silver salt comprising:

[0023] a core comprising a non-photosensitive first silver salt comprising a first silver organic coordinating ligand, and

[0024] at least one shell at least partially covering the core, the shell comprising a non-photosensitive second silver salt comprising a second silver organic coordinating ligand,

[0025] wherein the first and second silver organic coordinating ligands are different, and the molar ratio of the first salt to the second salt in the is from about 0.01:1 to about 100:1.

[0026] This invention also provides a composition comprising:

[0027] a) a core-shell non-photosensitive silver salt comprising:

[0028] a core comprising a non-photosensitive first silver salt comprising a first silver organic coordinating ligand, and

[0029] at least one shell at least partially covering the core, the shell comprising a non-photosensitive second silver salt comprising a second silver organic coordinating ligand,

[0030] wherein the first and second silver organic coordinating ligands are different, and the molar ratio of the first salt to the second salt is from about 0.01:1 to about 100:1, and

[0031] b) a non-photosensitive non-core-shell silver salt.

[0032] Still again, this invention provides a composition comprising:

[0033] a) a core-shell non-photosensitive silver salt comprising:

[0034] a core comprising a non-photosensitive first silver salt comprising a first silver organic coordinating ligand, and

[0035] at least one shell at least partially covering the core, the shell comprising a non-photosensitive second silver salt comprising a second silver organic coordinating ligand,

[0036] wherein the first and second silver organic coordinating ligands are different, and the molar ratio of the first salt to the second salt is from about 0.01:1 to about 100:1, and

[0037] b) a binder.

[0038] A thermally-sensitive emulsion of this invention comprises:

[0039] a) a source of non-photosensitive silver ions comprising a core-shell non-photosensitive silver salt comprising:

[0040] a core comprising a non-photosensitive first silver salt comprising a first silver organic coordinating ligand, and

[0041] at least one shell at least partially covering the core, the shell comprising a non-photosensitive second silver salt comprising a second silver organic coordinating ligand,

[0042] wherein the first and second silver organic coordinating ligands are different, and the molar ratio of the first salt to the second salt is from about 0.01:1 to about 100:1,

[0043] b) a reducing composition for the non-photosensitive silver ions, and

[0044] c) a binder.

[0045] Still again, a thermally-sensitive imaging material of this invention comprises a support having thereon a one or more layers comprising:

[0046] a) a source of non-photosensitive silver ions comprising a core-shell non-photosensitive silver salt comprising:

[0047] a core comprising a non-photosensitive first silver salt comprising a first silver organic coordinating ligand, and

[0048] at least one shell at least partially covering the core, the shell comprising a non-photosensitive second silver salt comprising a second silver organic coordinating ligand,

[0049] wherein the first and second silver organic coordinating ligands are different, and the molar ratio of the first salt to the second salt is from about 0.01:1 to about 100:1,

[0050] b) a reducing composition for the non-photosensitive silver ions, and

[0051] c) a binder.

[0052] Moreover, a photothermographic composition of this invention comprises:

[0053] a) a source of non-photosensitive silver ions comprising a core-shell non-photosensitive silver salt comprising:

[0054] a core comprising a non-photosensitive first silver salt comprising a first silver organic coordinating ligand, and

[0055] at least one shell at least partially covering the core, the shell comprising a non-photosensitive second silver salt comprising a second silver organic coordinating ligand,

[0056] wherein the first and second silver organic coordinating ligands are different, and the molar ratio of the first salt to the second salt is from about 0.01:1 to about 100:1,

[0057] b) a reducing composition for the non-photosensitive silver ions, and

[0058] c) a photocatalyst, and

[0059] d) a binder.

[0060] A preferred embodiment of this invention is a photothermographic material comprising a support having thereon one or more layers comprising:

[0061] a) a source of non-photosensitive silver ions comprising a core-shell non-photosensitive silver salt comprising:

[0062] a core comprising a non-photosensitive first silver salt comprising a first silver organic coordinating ligand, and

[0063] at least one shell at least partially covering the core, the shell comprising a non-photosensitive second silver salt comprising a second silver organic coordinating ligand,

[0064] wherein the first and second silver organic coordinating ligands are different, and the molar ratio of the first salt to the second salt is from about 0.01:1 to about 100:1,

[0065] b) a reducing composition for the non-photosensitive silver ions,

[0066] c) a photocatalyst, and

[0067] d) a binder.

[0068] This invention also comprises a method of making the core-shell non-photosensitive silver salts described above, which method comprises:

[0069] A) preparing a dispersion of a first non-photosensitive silver salt from silver ions and a first silver organic coordinating ligand, and

[0070] B) preparing a second non-photosensitive silver salt as a shell on the first non-photosensitive silver salt by adding silver ions and a second silver organic coordinating ligand to the dispersion of the first non-photosensitive silver salt, the first and second silver organic coordinating ligands being different.

[0071] Further a method of making the core-shell non-photosensitive silver salt described above comprises:

[0072] A) preparing a dispersion of a first non-photosensitive silver salt from silver ions and a first silver organic coordinating ligand, and

[0073] B) adding to the dispersion, a second silver organic coordinating ligand that is different from the first silver organic coordinating ligand.

[0074] Another preferred embodiments of this invention is a method of making a photosensitive imaging composition comprising:

[0075] A) preparing a dispersion of photosensitive silver halide grains,

[0076] B) adding to the dispersion of photosensitive silver halide grains, silver ions and a first silver organic coordinating ligand to form a first non-photosensitive silver salt on the photosensitive silver halide grains, and

[0077] C) preparing a second non-photosensitive silver salt as a shell on the first non-photosensitive silver salt by adding silver ions and a second silver organic coordinating ligand to the dispersion, the first and second organic coordinating ligands being different.

[0078] Thermographic and photothermographic materials incorporating the novel core-shell silver salts described herein as the non-photosensitive silver salt can provide images with improved image stability that can be developed at lower temperatures, while providing high quality images with high D_maxand good image tone.

[0079] The thermographic and photothermographic materials of this invention can be used, for example, in conventional black-and-white thermography and photothermography, in electronically generated black-and-white hardcopy recording, in the graphic arts area (for example, imagesetting and photo-typesetting), in the manufacture of printing plates, in proofing, in microfilm applications, and in radiographic imaging. Furthermore, the absorbance of these photothermographic materials between 350 and 450 nm is sufficiently low (less than 0.5) to permit their use in graphic arts applications such as contact printing, proofing, and duplicating (“duping”).

[0080] In the thermographic and photothermographic materials of this invention, the components of the imaging layer can be in one or more layers. The layer(s) that contain a photosensitive photocatalyst (for photothermographic materials) and non-photosensitive source of reducible silver ions, or both, are referred to herein as emulsion layer(s). The photosensitive photocatalyst and the non-photosensitive source of reducible silver ions are in catalytic proximity and preferably in the same emulsion layer.

[0081] Various layers are usually disposed on the “backside” (non-emulsion side) of the materials, including antihalation layer(s), protective layers, antistatic layers, conducting layers, and transport enabling layers.

[0082] Various layers are also usually disposed on the “frontside” or emulsion side of the support, including protective topcoat layers, primer layers, interlayers, opacifying layers, antistatic layers, antihalation layers, acutance layers, auxiliary layers, and others readily apparent to one skilled in the art.

[0083] For the inventive thermographic materials, an image (usually a black-and-white image) is provided by exposing the materials to heat from a suitable source in an imagewise fashion. Thermal development of the image occurs at essentially the same time.

[0084] The present invention also provides a process for the formation of a visible image (usually a black-and-white image) by first exposing to electromagnetic radiation and thereafter heating the inventive photothermographic material. In one embodiment, the present invention provides a process comprising:

[0085] (A) exposing the photothermographic material of this invention to electromagnetic radiation to which the photosensitive catalyst of the material is sensitive, to generate a latent image, and

[0086] (B) simultaneously or sequentially, heating the exposed material to develop the latent image into a visible image.

[0087] In some methods of practicing this invention, the imaging method includes the further steps of:

[0088] (C) positioning the exposed material with a visible image thereon between a source of imaging radiation and an imageable material that is sensitive to the imaging radiation, and

[0089] (D) thereafter exposing the imageable material to the imaging radiation through the visible image in the exposed and developed photothermographic material to provide a visible image in the imageable material.

[0090] This visible image can also be used as a mask for exposure of other photosensitive imageable materials, such as graphic arts films, proofing films, printing plates and circuit board films, that are sensitive to suitable imaging radiation (for example, UV radiation). This can be done by imaging an imageable material (such as a photopolymer, a diazo material, a photoresist, or a photosensitive printing plate) through the exposed and heat-developed photothermographic material of this invention using steps C and D noted above.

[0091] When the photothermographic materials used in this invention are heat developed as described below in a substantially water-free condition after, or simultaneously with, imagewise exposure, a silver image (preferably black-and-white silver image) is obtained. The photothermographic element may be exposed in step (a) with ultraviolet, visible, infrared radiation using an infrared laser, a laser diode, an infrared laser diode, a light-emitting screen, CRT tube, a light-emitting diode, or other light or radiation source readily apparent to one skilled in the art.

[0092] As used herein:

[0093] In the descriptions of the thermographic and photothermographic materials of the present invention, “a” or “an” component refers to “at least one” of that component. For example, the core-shell silver salts described herein for chemical sensitization can be used individually or in mixtures.

[0094] Heating in a substantially water-free condition as used herein, means heating at a temperature of from about 50° to about 250° C. with little more than ambient water vapor present. The term “substantially water-free condition” means that the reaction system is approximately in equilibrium with water in the air and water for inducing or promoting the reaction is not particularly or positively supplied from the exterior to the material. Such a condition is described in T. H. James, The Theory of the Photographic Process, Fourth Edition, Macmillan 1977, p 374.

[0095] “Photothermographic material(s)” means a construction comprising at least one photothermographic emulsion layer or a photothermographic set of layers (wherein the silver halide and the source of reducible silver ions are in one layer and the other essential components or desirable additives are distributed, as desired, in an adjacent coating layer) and any supports, topcoat layers, image-receiving layers, blocking layers, antihalation layers, subbing or priming layers. These materials also include multilayer constructions in which one or more imaging components are in different layers, but are in “reactive association” so that they readily come into contact with each other during imaging and/or development. For example, one layer can include the non-photosensitive source of reducible silver ions and another layer can include the reducing composition, but the two reactive components are in reactive association with each other.

[0096] “Thermographic material(s)” are similarly defined except that no photosensitive photocatalyst is present.

[0097] “Emulsion layer,” “imaging layer,” “thermographic emulsion layer,” or “photothermographic emulsion layer,” means a layer of a thermographic or photothermographic material that contains the photosensitive silver halide (when used) and/or non-photosensitive source of reducible silver ions for photothermographic materials). It can also mean a layer of the photothermographic material that contains, in addition to the photosensitive silver halide (when used) and/or non-photosensitive source of reducible ions, additional essential components and/or desirable additives. These layers are usually on what is known as the “frontside” of the support.

[0098] “Ultraviolet region of the spectrum” refers to that region of the spectrum less than or equal to 410 nm, and preferably from about 100 nm to about 410 nm, although parts of these ranges may be visible to the naked human eye. More preferably, the ultraviolet region of the spectrum is the region of from about 190 to about 405 nm.

[0099] “Visible region of the spectrum” refers to that region of the spectrum of from about 400 nm to about 750 nm.

[0100] “Short wavelength visible region of the spectrum” refers to that region of the spectrum from about 400 nm to about 450 nm.

[0101] “Red region of the spectrum” refers to that region of the spectrum of from about 600 nm to about 750 nm.

[0102] “Infrared region of the spectrum” refers to that region of the spectrum of from about 750 nm to about 1400 nm.

[0103] “Non-photosensitive” means not intentionally light sensitive.

[0104] “Transparent” means capable of transmitting visible light or imaging radiation without appreciable scattering or absorption.

[0105] As is well understood in this area, for the various compounds defined herein (including core-shell silver salts), substitution is not only tolerated, but is often advisable and various substituents are anticipated on the compounds used in the present invention. Thus, when a compound is referred to as “having the structure” of a given formula, any substitution that does not alter the bond structure of the formula or the shown atoms within that structure is included within the formula, unless such substitution is specifically excluded by language (such as “free of carboxy-substituted alkyl”). For example, where a benzene ring structure is shown (including fused ring structures), substituent groups may be placed on the benzene ring structure, but the atoms making up the benzene ring structure may not be replaced.

[0106] As a means of simplifying the discussion and recitation of certain substituent groups, the term “group” refers to chemical species that may be substituted as well as those that are not so substituted. Thus, the term “group,” such as “alkyl group” is intended to include not only pure hydrocarbon alkyl chains, such as methyl, ethyl, propyl, t-butyl, cyclohexyl, iso-octyl, octadecyl and the like, but also alkyl chains bearing substituents known in the art, such as hydroxyl, alkoxy, phenyl, halogen atoms (F, Cl, Br, and I), cyano, nitro, amino, carboxy and the like. For example, alkyl group includes ether and thioether groups (for example, CH3—CH2—CH2—O—CH2—), haloalkyl, nitroalkyl, carboxyalkyl, hydroxyalkyl, sulfoalkyl, and other groups readily apparent to one skilled in the art. Substituents that adversely react with other active ingredients, such as very strongly electrophilic or oxidizing substituents, would, of course, be excluded by the ordinarily skilled artisan as not being inert or harmless.

[0107] Research Disclosureis a publication of Kenneth Mason Publications Ltd., Dudley House, 12 North Street, Emsworth, Hampshire PO10 7DQ England (also available from Emsworth Design Inc., 147 West 24th Street, New York, N.Y. 10011).

[0108] Other aspects, advantages, and benefits of the present invention are apparent from the detailed description, examples, and claims provided in this application.

[0109] As noted above, the photothermographic materials of the present invention include one or more photocatalysts in the photothermographic emulsion layer(s). Useful photocatalysts are typically silver halides such as silver bromide, silver iodide, silver chloride, silver bromoiodide, silver chlorobromoiodide, silver chlorobromide, and others readily apparent to one skilled in the art. Mixtures of silver halides can also be used in any suitable proportion. Silver bromide and silver bromoiodide are more preferred, with the latter silver halide having up to 10 mol % silver iodide.

[0110] The shape of the photosensitive silver halide grains used in the present invention is in no way limited. The silver halide grains may have any crystalline habit including, but not limited to, cubic, octahedral, tetrahedral, dodecahedral, other polyhedral, rhombic, orthorhombic, tabular, laminar, twinned, or platelet morphologies and may have epitaxial growth of crystals thereon. If desired, a mixture of these crystals may be employed. Silver halide grains having cubic and tabular morphology are preferred.

[0111] The silver halide grains may have a uniform ratio of halide throughout. They may have a graded halide content, with a continuously varying ratio of, for example, silver bromide and silver iodide or they may be of the core-shell type, having a discrete core of one halide ratio, and a discrete shell of another halide ratio. Core-shell silver halide grains useful in photothermographic materials and methods of preparing these materials are described for example, in U.S. Pat. No. 5,382,504 (Shor et al.). Iridium and/or copper doped core-shell and non-core-shell grains are described in U.S. Pat. No. 5,434,043 (Zou et al.), and U.S. Pat. No. 5,939,249 (Zou), incorporated herein by reference.

[0112] The photosensitive silver halide can be added to (or formed within) the emulsion layer(s) in any fashion as long as it is placed in catalytic proximity to the non-photosensitive source of reducible silver ions.

[0113] It is preferred that the silver halides be preformed and prepared by an ex-situ process. The silver halide grains prepared ex-situ may then be added to and physically mixed with the non-photosensitive source of reducible silver ions. It is more preferable to form the source of reducible silver ions in the presence of ex-situ-prepared silver halide. In this process, the source of reducible silver ions, such as a long chain fatty acid silver carboxylate (commonly referred to as a silver “soap”), is formed in the presence of the preformed silver halide grains. Co-precipitation of the reducible source of silver ions in the presence of silver halide provides a more intimate mixture of the two materials [see, for example, U.S. Pat. No. 3,839,049 (Simons)]. Materials of this type are often referred to as “preformed soaps.”

[0114] The silver halide grains used in the imaging formulations can vary in average diameter of up to several micrometers (μm) depending on their desired use. Preferred silver halide grains are those having an average particle size of from about 0.01 to about 1.5 μm, more preferred are those having an average particle size of from about 0.03 to about 1.0 μm, and most preferred are those having an average particle size of form about 0.05 to about 0.8 μm. Those of ordinary skill in the art understand that there is a finite lower practical limit for silver halide grains that is partially dependent upon the wavelength to which the grains are spectrally sensitized. Such a lower limit, for example, is typically about 0.01 to 0.005 μm.

[0115] The average size of the photosensitive doped silver halide grains is expressed by the average diameter if the grains are spherical, and by the average of the diameters of equivalent circles for the projected images if the grains are cubic or in other non-spherical shapes.

[0116] Grain size may be determined by any of the methods commonly employed in the art for particle size measurement. Representative methods are described by in “Particle Size Analysis,” ASTM Symposium on Light Microscopy, R. P. Loveland, 1955, pp. 94-122, and in C. E. K. Mees and T. H. James, The Theory of the Photographic Process, Third Edition, Chapter 2, Macmillan Company, 1966. Particle size measurements may be expressed in terms of the projected areas of grains or approximations of their diameters. These will provide reasonably accurate results if the grains of interest are substantially uniform in shape.

[0117] Preformed silver halide emulsions used in the material of this invention can be prepared by aqueous or organic processes and can be unwashed or washed to remove soluble salts. In the latter case, the soluble salts can be removed by ultrafiltration, by chill setting and leaching, or by washing the coagulum [for example, by the procedures described in U.S. Pat. No. 2,618,556 (Hewitson et al.), U.S. Pat. No. 2,614,928 (Yutzy et al.), U.S. Pat. No. 2,565,418 (Yackel), U.S. Pat. No. 3,241,969 (Hart et al.) and U.S. Pat. No. 2,489,341 (Waller et al.)].

[0118] It is also effective to use an in situ process in which a halide-containing compound is added to an organic silver salt to partially convert the silver of the organic silver salt to silver halide. The halogen-containing compound can be inorganic (such as zinc bromide or lithium bromide) or organic (such as N-bromosuccinimide).

[0119] Additional methods of preparing these silver halide and organic silver salts and manners of blending them are described in Research Disclosure, June 1978, item 17029, U.S. Pat. No. 3,700,458 (Lindholm) and U.S. Pat. No. 4,076,539 (Ikenoue et al.), and JP Applications 13224/74, 42529/76, and 17216/75.

[0120] The one or more light-sensitive silver halides used in the photothermographic materials of the present invention are preferably present in an amount of from about 0.005 to about 0.5 mole, more preferably from about 0.01 to about 0.25 mole per mole, and most preferably from about 0.03 to about 0.15 mole, per mole of non-photosensitive source of reducible silver ions.

[0121] The silver halide used in the present invention may be employed without modification. However, it is preferably chemically and/or spectrally sensitized in a manner similar to that used to sensitize conventional wet-processed silver halide photographic materials or state-of-the-art heat-developable photothermographic materials.

[0122] For example, the photothermographic material may be chemically sensitized with one or more chemical sensitizing agents, such as a compound containing sulfur, selenium, or tellurium, or with a compound containing gold, platinum, palladium, ruthenium, rhodium, iridium, or combinations thereof, a reducing agent such as a tin halide or a combination of any of these. The details of these procedures are described in T. H. James, The Theory of the Photographic Process, Fourth Edition, Chapter 5, pp. 149-169. Suitable chemical sensitization procedures are also disclosed in U.S. Pat. No. 1,623,499 (Sheppard et al.), U.S. Pat. No. 2,399,083 (Waller et al.), U.S. Pat. No. 3,297,447 (McVeigh) and U.S. Pat. No. 3,297,446 (Dunn). One preferred method of chemical sensitization is by oxidative decomposition of a spectral sensitizing dye in the presence of a photothermographic emulsion, as described in U.S. Pat. No. 5,891,615 (Winslow et al.) incorporated herein by reference.

[0123] Useful sulfur-containing chemical sensitizing compounds are described in copending and commonly assigned U.S. Ser. No. 09/667,748 (filed Sep. 21, 2000 by Lynch, Simpson, Shor, Willett, and Zou, incorporated herein by reference.

[0124] Useful tellurium-containing chemical sensitizing compounds are described in copending and commonly assigned U.S. Ser. No. 09/746,400 (filed Dec. 21, 2000 by Lynch, Opatz, Shor, Simpson, Willett, and Gysling) and entitled “High Speed Photothermographic Materials Containing Tellurium Compounds and Methods of Using Same” incorporated herein by reference.

[0125] The total amount of chemical sensitizers that may be used during formulation of the imaging composition will generally vary depending upon the average size of silver halide grains. The total amount is generally at least 10⁻⁷mole per mole of total silver, and preferably from 10⁻⁵to about 10⁻²mole per mole of total silver for silver halide grains having an average size of from about 0.01 to about 2 μm. The upper limit can vary depending upon the compound used, the level of silver halide and the average grain size, and it would be readily determinable by one of ordinary would be readily determinable by one of ordinary skill in the art.

[0126] In general, it may also be desirable to add spectral sensitizing dyes to enhance silver halide sensitivity to ultraviolet, visible and infrared light. Thus, the photosensitive silver halides may be spectrally sensitized with various dyes that are known to spectrally sensitize silver halide. Non-limiting examples of sensitizing dyes that can be employed include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, holopolar cyanine dyes, hemicyanine dyes, styryl dyes, and hemioxanol dyes. The cyanine dyes, merocyanine dyes and complex merocyanine dyes are particularly useful. Suitable sensitizing dyes such as those described in U.S. Pat. No. 3,719,495 (Lea), U.S. Pat. No. 5,393,654 (Burrows et al.), U.S. Pat. No. 5,441,866 (Miller et al.) and U.S. Pat. No. 5,541,054 (Miller et al.), U.S. Pat. No. 5,281,515 (Delprato et al.) and U.S. Pat. No. 5,314,795 (Helland et al.) are effective in the practice of the invention.

[0127] An appropriate amount of sensitizing dye added is generally about 10⁻¹⁰to 10⁻¹mole, and preferably, about 10⁻⁷to 10⁻²mole per mole of silver halide.

[0128] To further control the properties of photothermographic materials, (for example, contrast, D_min, speed, or fog), it may be preferable to add one or more heteroaromatic mercapto compounds or heteroaromatic disulfide compounds. Examples include compounds of the formulae: Ar—S—M and Ar—S—S—Ar, wherein M represents a hydrogen atom or an alkali metal atom and Ar represents a heteroaromatic ring or fused heteroaromatic ring containing one or more of nitrogen, sulfur, oxygen, selenium, or tellurium atoms. Preferably, the heteroaromatic ring comprises benzimidazole, naphthimidazole, benzothiazole, naphthothiazole, benzoxazole, naphthoxazole, benzoselenazole, benzotellurazole, imidazole, oxazole, pyrazole, triazole, thiazole, thiadiazole, tetrazole, triazine, pyrimidine, pyridazine, pyrazine, pyridine, purine, quinoline, or quinazolinone. Compounds having other heteroaromatic rings and compounds providing enhanced sensitization at other wavelengths are also envisioned to be suitable. Many of the above compounds are described in EP-A-0 559 228 (Philip Jr. et al.) as supersensitizers for infrared photothermographic materials.

[0129] The heteroaromatic ring may also carry substituents. Examples of preferred substituents are halo groups (such as bromo and chloro), hydroxy, amino, carboxy, alkyl groups (for example, of 1 or more carbon atoms and preferably 1 to 4 carbon atoms), and alkoxy groups (for example, of 1 or more carbon atoms and preferably of 1 to 4 carbon atoms).

[0130] Heteroaromatic mercapto compounds are most preferred. Examples of preferred heteroaromatic mercapto compounds are 2-mercaptobenzimidazole, 2-mercapto-5-methylbenzimidazole, 2-mercaptobenzothiazole and 2-mercaptobenzoxazole, and mixtures thereof.

[0131] If used, a heteroaromatic mercapto compound is generally present in an emulsion layer in an amount of at least about 0.0001 mole per mole of total silver in the emulsion layer. More preferably, the heteroaromatic mercapto compound is present within a range of about 0.001 mole to about 1.0 mole, and most preferably, about 0.005 mole to about 0.2 mole, per mole of total silver.

[0132] The primary source of reducible, non-photosensitive silver in the practice of this invention are the core-shell silver salts described herein that comprise one or more silver salts in the core, and one or more silver salts in the shell, but at least one of the silver salts in the core is different from at least one of the silver salts in the shell.

[0133] There is no particular limitation on the structure of each of the non-photosensitive silver salts used to prepare the core and shell of the non-photosensitive silver salt. In some embodiments, the core can comprise a mixture of two or more different silver salts, or said shell can comprise a mixture of two or more different silver salts, or both the core and shell can comprise mixtures of two or more different silver salts, as long as at least one silver salt in the core is different from at least one silver salt in the shell.

[0134] In still other embodiments, the core can be comprised of one or more silver salts, an “inner” shell can be comprised of one or more different silver salts, and an “outer” shell can be comprised of one or more of silver salts that are the same or different as those in the core. Further still, the “inner” and “outer” shells can be composed of the same mixture of silver salt(s), but have different molar ratios of the salts in those mixtures. Additionally, the transition between the surface layer (shell) and internal phase (core) of the non-photosensitive core-shell silver salt may be abrupt, so as to provide a distinct boundary, or diffuse so as to create a gradual transition from one non-photosensitive silver salt to another.

[0135] Within the core-shell silver salts of the invention, the molar ratio of one or more core (first) silver salts to the one or more shell (second) silver salts is from about 0.01:1 to about 100:1, and preferably from about 0.1:1 to about 10:1.

[0136] The silver salts used to make the core-shell salts are comprised of silver salts of silver organic coordinating ligands. Many examples of such organic coordinating ligands are described below in this section of the disclosure. Preferably, either or both of the first (core) and second (shell) silver organic coordinating ligands are carboxylates that are also defined below. More preferably, the first (core) and second (shell) silver organic coordinating ligands are carboxylates having different chain lengths, such as those differing in chain length by at least 2 carbon atoms.

[0137] While most useful core-shell silver salts include only one silver salt in the core and a single different silver salt in the shell, other core-shell structures of the present invention comprises a mixture of two or more different silver salts in the core, a mixture of two or more different silver salts in the shell, or mixtures of two or more different silver salts in each of the core and shell (as long as at least one silver organic coordinating ligand in the core is different from at least one silver organic coordinating ligand in the shell).

[0138] Other compositions useful in this invention can include one or more core-shell silver salts as described above and one more conventional silver salts as described below (that is, non-core-shell silver salts or mixtures thereof).

[0139] The non-photosensitive source of reducible silver ions (that is, silver salts) used in the core or shell can be any compound that contains reducible silver (1+) ions. Preferably, it is a silver salt that is comparatively stable to light and forms a silver image when heated to 50° C. or higher in the presence of an exposed photocatalyst (such as silver halide) and a reducing composition.

[0140] Silver salts of organic acids, particularly silver salts of long-chain carboxylic acids are preferred. The chains typically contain 10 to 30, and preferably 15 to 28, carbon atoms. Suitable organic silver salts include silver salts of organic compounds having a carboxylic acid group. Examples thereof include a silver salt of an aliphatic carboxylic acid or a silver salt of an aromatic carboxylic acid. Preferred examples of the silver salts of aliphatic carboxylic acids include silver behenate, silver arachidate, silver stearate, silver oleate, silver laurate, silver caprate, silver myristate, silver palmitate, silver maleate, silver fumarate, silver tartarate, silver furoate, silver linoleate, silver butyrate, silver camphorate, and mixtures thereof. Preferred examples of the silver salts of aromatic carboxylic acid and other carboxylic acid group-containing compounds include, but are not limited to, silver benzoate, silver-substituted benzoates, such as silver 3,5-dihydroxy-benzoate, silver o-methylbenzoate, silver m-methylbenzoate, silver p-methylbenzoate, silver 2,4-dichlorobenzoate, silver acetamidobenzoate, silver p-phenylbenzoate, silver tannate, silver phthalate, silver terephthalate, silver salicylate, silver phenylacetate, silver pyromellitate, a silver salt of 3-carboxymethyl-4-methyl-4-thiazoline-2-thione or others as described in U.S. Pat. No. 3,785,830 (Sullivan et al.), and silver salts of aliphatic carboxylic acids containing a thioether group as described in U.S. Pat. No. 3,330,663 (Weyde et al.). Soluble silver carboxylates having hydrocarbon chains incorporating ether or thioether linkages, or sterically hindered substitution in the α- (on a hydrocarbon group) or ortho- (on an aromatic group) position, and displaying increased solubility in coating solvents and affording coatings with less light scattering can also be used. Such silver carboxylates are described in U.S. Pat. No. 5,491,059 (Whitcomb). Mixtures of any of the silver salts described herein can also be used if desired.

[0141] Silver salts of sulfonates are also useful in the practice of this invention. Such materials are described, for example, in U.S. Pat. No. 4,504,575 (Lee). Silver salts of sulfosuccinates are also useful as described for example, in EP-A-0 227 141 (Leenders et al.).

[0142] Silver salts of compounds containing mercapto or thione groups and derivatives thereof can also be used. Preferred examples of these compounds include, but are not limited to, a silver salt of 3-mercapto-4-phenyl-1,2,4-triazole, a silver salt of 2-mercaptobenzimidazole, a silver salt of 2-mercapto-5-aminothiadiazole, a silver salt of 2-(2-ethylglycolamido)benzothiazole, silver salts of thioglycolic acids (such as a silver salt of a S-alkylthioglycolic acid, wherein the alkyl group has from 12 to 22 carbon atoms), silver salts of dithiocarboxylic acids (such as a silver salt of dithioacetic acid), a silver salt of thioamide, a silver salt of 5-carboxylic-1-methyl-2-phenyl-4-thiopyridine, a silver salt of mercaptotriazine, a silver salt of 2-mercaptobenzoxazole, silver salts as described in U.S. Pat. No. 4,123,274 (Knight et al.) (for example, a silver salt of a 1,2,4-mercaptothiazole derivative, such as a silver salt of 3-amino-5-benzylthio-1,2,4-thiazole), and a silver salt of thione compounds such as a silver salt of 3-(2-carboxyethyl)4-methyl-4-thiazoline-2-thione [as described in U.S. Pat. No. 3,201,678 (Meixell)].

[0143] Furthermore, a silver salt of a compound containing an imino group can be used. Preferred examples of these compounds include, but are not limited to, silver salts of benzotriazole and substituted derivatives thereof (for example, silver methylbenzotriazole and silver 5-chlorobenzotriazole), silver salts of 1,2,4-triazoles or 1-H-tetrazoles such as phenylmercaptotetrazole as described in U.S. Pat. No. 4,220,709 (deMauriac), and silver salts of imidazoles and imidazole derivatives as described in U.S. Pat. No. 4,260,677 (Winslow et al.). Moreover, silver salts of acetylenes can also be used as described, for example, in U.S. Pat. No. 4,761,361 (Ozaki et al.) and U.S. Pat. No. 4,775,613 (Hirai et al.).

[0144] It is also convenient to use silver half soaps. A preferred example of a silver half soap is an equimolar blend of silver carboxylate and carboxylic acid, which analyzes for about 14.5% by weight solids of silver in the blend and which is prepared by precipitation from an aqueous solution of the sodium salt of a commercial fatty carboxylic acid, or by addition of the free fatty acid to the silver soap. For transparent films a silver carboxylate full soap, containing not more than about 15% of free fatty carboxylic acid and analyzing for about 22% silver, can be used. For opaque photothermographic materials, different amounts can be used.

[0145] The methods used for making silver soap emulsions are well known in the art and are disclosed in Research Disclosure, April 1983, item 22812, Research Disclosure, October 1983, item 23419, U.S. Pat. No. 3,985,565 (Gabrielsen et al.) and the references cited above.

[0146] However, there are unique methods for preparing the core-shell silver salts of the present invention as well as for preparing photosensitive dispersions containing them.

[0147] For example, in one embodiment a method of making the core-shell non-photosensitive silver salt comprises:

[0148] A) preparing a dispersion of a first non-photosensitive silver salt from silver ions and a first silver organic coordinating ligand, and

[0149] B) preparing a second non-photosensitive silver salt as a shell on the first non-photosensitive silver salt by adding silver ions and a second silver organic coordinating ligand to the dispersion of the first non-photosensitive silver salt, the first and second organic coordinating ligands being different.

[0150] In another embodiment, a method of making the core-shell non-photosensitive silver salt comprises:

[0151] A) preparing a dispersion of a first non-photosensitive silver salt from silver ions and a first silver organic coordinating ligand, and

[0152] B) adding to the dispersion, a second silver organic coordinating ligand that is different from the first silver organic coordinating ligand.

[0153] The details for using these methods are illustrated in Examples 1-5 below.

[0154] In addition, a method of making a photosensitive imaging composition comprises:

[0155] A) preparing a dispersion of photosensitive silver halide grains,

[0156] B) adding to the dispersion of photosensitive silver halide grains, silver ions and a first silver organic coordinating ligand to form a first non-photosensitive silver salt on the photosensitive silver halide grains, and

[0157] C) preparing a second non-photosensitive silver salt as a shell on the first non-photosensitive silver salt by adding silver ions and a second silver organic coordinating ligand to the dispersion, the first and second organic coordinating ligands being different.

[0158] In this method, the photosensitive silver halide grains can already be chemically and/or spectrally sensitized as described herein. Thus, the silver halide dispersion can further comprise one or more spectral sensitizing dyes.

[0159] Alternatively, the silver halide grains are chemically sensitized after step A, for example between steps A and B, between steps B and C, or after step C.

[0160] When used in photothermographic materials emulsions, the non-photosensitive core-shell silver salts can be prepared at any stage of preparation of the of the photothermographic emulsion. Non-limiting examples of other methods of preparation of non-photosensitive core-shell silver salts are:

[0161] The non-photosensitive core-shell silver salts can be prepared after the addition of and in the presence of preformed silver halide grains.

[0162] The non-photosensitive core-shell silver salts can be prepared before the addition of preformed silver halide grains.

[0163] The core of the non-photosensitive core-shell silver salts can be prepared before the addition of the preformed silver halide grains. The shell can then be grown around these previously prepared cores and in the presence of preformed silver halide grains.

[0164] The non-photosensitive core-shell silver salts can be prepared and the silver halide can be prepared in situ, that is, in the presence of the non-photosensitive core-shell silver salts.

[0165] The core of the non-photosensitive core-shell silver salts can be prepared and the silver halide can be prepared in situ, that is, in the presence of the non-photosensitive core-shell silver salts. The shell can then be grown around these previously prepared cores.

[0166] The core of the non-photosensitive core-shell silver salts can be prepared and the silver halide can be prepared in situ, that is, in the presence of the non-photosensitive core-shell silver salts. The shell can then be grown around these previously prepared cores and in the presence of preformed silver halide grains.

[0167] In all of the above preparations, the boundary between the core and shell of the non-photosensitive silver salts need not be discrete but may be continuous and the ratio of said first and second silver organic coordinating ligands may continuously decrease as the distance from the center of the core increases.

[0168] The photocatalyst and the non-photosensitive source of reducible silver ions must be in catalytic proximity (that is, reactive association). “Catalytic proximity” or “reactive association” means that they should be in the same layer, or in adjacent layers. It is preferred that these reactive components be present in the same emulsion layer.

[0169] The one or more non-photosensitive sources of reducible silver ions are preferably present in both thermographic and photothermographic materials in an amount of about 5% by weight to about 70% by weight, and more preferably, about 10% to about 50% by weight, based on the total dry weight of the emulsion layers. Stated another way, the amount of the sources of reducible silver ions is generally present in an amount of from about 0.001 to about 0.2 mol/m²of dried thermographic or photothermographic material, and preferably from about 0.01 to about 0.05 mol/m²of that material.

[0170] The reducing agent (or reducing agent composition comprising two or more components) for the source of reducible silver ions can be any material, preferably an organic material, that can reduce silver (I) ion to metallic silver. Conventional photographic developers such as methyl gallate, hydroquinone, substituted hydroquinones, hindered phenols, amidoximes, azines, catechols, pyrogallol, ascorbic acid (and derivatives thereof), leuco dyes and other materials readily apparent to one skilled in the art can be used in this manner as described for example, in U.S. Pat. No. 6,020,117 (Bauer et al.).

[0171] In some instances, the reducing agent composition comprises two or more components such as a hindered phenol developer and a co-developer that can be chosen from the various classes of reducing agents described below. Ternary developer mixtures involving the further addition of contrast enhancing agents are also useful. Such contrast enhancing agents can be chosen from the various classes described below.

[0172] Hindered phenol reducing agents are preferred (alone or in combination with one or more co-developers and contrast-reducing agents). These are compounds that contain only one hydroxy group on a given phenyl ring and have at least one additional substituent located ortho to the hydroxy group. Hindered phenol developers may contain more than one hydroxy group as long as each hydroxy group is located on different phenyl rings. Hindered phenol developers include, for example, binaphthols (that is, dihydroxybinaphthyls), biphenols (that is, dihydroxybiphenyls), bis(hydroxynaphthyl)methanes, bis(hydroxyphenyl)methanes, and hindered naphthols, each of which may be variously substituted.

[0173] Representative binaphthols include, but are not limited to, 1,1′-bi-2-naphthol, 1,1′-bi-4-methyl-2-naphthol, and 6,6′-dibromo-bi-2-naphthol. For additional compounds see U.S. Pat. No. 3,094,417 (Workman) and U.S. Pat. No. 5,262,295 (Tanaka et al.), both incorporated herein by reference.

[0174] Representative biphenols include, but are not limited to, 2,2′-dihydroxy-3,3′-di-t-butyl-5,5-dimethylbiphenyl, 2,2′-dihydroxy-3,3′,5,5′-tetra-t-butylbiphenyl, 2,2′-dihydroxy-3,3′-di-t-butyl-5,5′-dichlorobiphenyl, 2-(2-hydroxy-3-t-butyl-5-methylphenyl)-4-methyl-6-n-hexylphenol, 4,4′-dihydroxy-3,3′,5,5′-tetra-t-butylbiphenyl, and 4,4′-dihydroxy-3,3′,5,5′-tetramethylbiphenyl. For additional compounds see U.S. Pat. No. 5,262,295 (noted above).

[0175] Representative bis(hydroxynaphthyl)methanes include, but are not limited to, 4,4′-methylenebis(2-methyl-1-naphthol). For additional compounds see U.S. Pat. No. 5,262,295 (noted above).

[0176] Representative bis(hydroxyphenyl)methanes include, but are not limited to, bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane (CAO-5), 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane (NONOX or PERMANAX WSO, [CAS RN=7292-14-0]), 1,1-bis(3,5-di-t-butyl-4-hydroxyphenyl)methane, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 4,4-ethylidene-bis(2-t-butyl-6-methylphenol), and 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane. For additional compounds see U.S. Pat. No. 5,262,295 (noted above).

[0177] Representative hindered phenols include, but are not limited to, 2,6-di-t-butylphenol, 2,6-di-t-butyl-4-methylphenol, 2,4-di-t-butylphenol, 2,6-dichlorophenol, 2,6-dimethylphenol, and 2-t-butyl-6-methylphenol.

[0178] Representative hindered naphthols include, but are not limited to, 1-naphthol, 4-methyl-1-naphthol, 4-methoxy-1-naphthol, 4-chloro-1-naphthol, and 2-methyl-1-naphthol. For additional compounds see U.S. Pat. No. 5,262,295 (noted above).

[0179] More specific alternative reducing agents that have been disclosed in dry silver systems including amidoximes such as phenylamidoxime, 2-thienylamidoxime and p-phenoxyphenylamidoxime, azines (for example, 4-hydroxy-3,5-dimethoxybenzaldehydrazine), a combination of aliphatic carboxylic acid aryl hydrazides and ascorbic acid, such as 2,2′-bis(hydroxymethyl)-propionyl-β-phenyl hydrazide in combination with ascorbic acid, a combination of polyhydroxybenzene and hydroxylamine, a reductone and/or a hydrazine [for example, a combination of hydroquinone and bis(ethoxyethyl)hydroxylamine], piperidinohexose reductone or formyl-4-methylphenylhydrazine, hydroxamic acids (such as phenylhydroxamic acid, p-hydroxyphenylhydroxamic acid, and o-alaninehydroxamic acid), a combination of azines and sulfonamidophenols (for example, phenothiazine and 2,6-dichloro-4-benzenesulfonamidophenol), α-cyanophenylacetic acid derivatives (such as ethyl α-cyano-2-methylphenylacetate and ethyl-α-cyanophenylacetate), bis-o-naphthols [such as 2,2′-dihydroxyl-1-binaphthyl, 6,6′-dibromo-2,2′-dihydroxy-1,1′-binaphthyl, and bis(2-hydroxy-1-naphthyl)methane], a combination of bis-o-naphthol and a 1,3-dihydroxybenzene derivative (for example, 2,4-dihydroxybenzophenone or 2,4-dihydroxyacetophenone), 5-pyrazolones such as 3-methyl-1-phenyl-5-pyrazolone, reductones (such as dimethylaminohexose reductone, anhydrodihydro-amino hexose reductone and anhydrodihydro-piperidone-hexose reductone), sulfonamidophenol reducing agents (such as 2,6-dichloro-4-benzenesulfonamidophenol, and p-benzenesulfonamidophenol), 2-phenylindane-1,3-dione and similar compounds, chromans (such as 2,2-dimethyl-7-t-butyl-6-hydroxychroman), 1,4-dihydropyridines (such as 2,6-dimethoxy-3,5-dicarbethoxy-1 4-dihydropyridine), bisphenols [such as bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 4,4-ethylidene-bis(2-t-butyl-6-methylphenol) and 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane], ascorbic acid derivatives (such as 1-ascorbylpalmitate, ascorbylstearate and unsaturated aldehydes and ketones), 3-pyrazolidones, and certain indane-1,3-diones.

[0180] An additional class of reducing agents that can be used as developers are substituted hydrazines including the sulfonyl hydrazides described in U.S. Pat. No. 5,464,738 (Lynch et al.). Still other useful reducing agents are described for example, in U.S. Pat. No. 3,074,809 (Owen), U.S. Pat. No. 3,094,417 (Workman), U.S. Pat. No. 3,080,254 (Grant, Jr.) and U.S. Pat. No. 3,887,417 (Klein et al.). Auxiliary reducing agents may be useful as described in U.S. Pat. No. 5,981,151 (Leenders et al.).

[0181] Useful co-developer reducing agents can also be used as described for example, in copending U.S. Ser. No. 09/239,182 (filed Jan. 28, 1999 by Lynch and Skoog), incorporated herein by reference. Examples of these compounds include, but are not limited to, 2,5-dioxo-cyclopentane carboxaldehyde, 5-(hydroxymethylene)-2,2-dimethyl-1,3-dioxane-4,6-dione, 5-(hydroxymethylene)-1,3-dialkylbarbituric acids, 2-(ethoxymethylene)-1H-indene-1,3(2H)-dione.

[0182] Additional classes of reducing agents that can be used as co-developers are trityl hydrazides, and formyl phenyl hydrazides as described in U.S. Pat. No. 5,496,695 (Simpson et al.), 3-heteroaromatic-substituted acrylonitrile compounds as described in U.S. Pat. No. 5,635,339 (Murray), 2-substituted malondialdehyde compounds as described in U.S. Pat. No. 5,654,130 (Murray), substituted propenitriles as described in U.S. Pat. No. 5,686,228 (Murray et al.), and 4-substituted isoxazole compounds as described in U.S. Pat. No. 5,705,324 (Murray), 2,5-dioxo-cyclopentane carboxaldehydes, 5-(hydroxymethylene)-1,3-dialkylbarbituric acids, and 2-(ethoxymethylene)-1H-indene-1,3(2H)-diones. Additional developers are described in U.S. Pat. No. 6,100,022 (Inoue et al.).

[0183] Various contrast enhancers can be used in some photothermographic materials with specific co-developers. Examples of useful contrast enhancers include, but are not limited to, hydroxylamine, alkanolamines and ammonium phthalamate compounds as described for example, in U.S. Pat. No. 5,545,505 (Simpson), hydroxamic acid compounds as described for example, in U.S. Pat. No. 5,545,507 (Simpson et al.), N-acylhydrazine compounds as described for example, in U.S. Pat. No. 5,558,983 (Simpson et al.), and hydrogen atom donor compounds as described in U.S. Pat. No. 5,637,449 (Harring et al.).

[0184] The reducing agent (or mixture thereof) described herein is generally present as 1 to 10% (dry weight) of the emulsion layer. In multilayer constructions, if the reducing agent is added to a layer other than an emulsion layer, slightly higher proportions, of from about 2 to 15 weight % may be more desirable. Any co-developers may be present generally in an amount of from about 0.001% to about 1.5% (dry weight) of the emulsion layer coating.

[0185] The thermographic and photothermographic materials of the invention can also contain other additives such as shelf-life stabilizers, toners, antifoggants, contrast enhancers, development accelerators, acutance dyes, post-processing stabilizers or stabilizer precursors, and other image-modifying agents as would be readily apparent to one skilled in the art.

[0186] The materials of the present invention can be further protected against the production of fog and can be stabilized against loss of sensitivity during storage. While not necessary for the practice of the invention, it may be advantageous to add mercury (II) salts to the emulsion layer(s) as an antifoggant. Preferred mercury (HI) salts for this purpose are mercuric acetate and mercuric bromide. Other useful mercury salts include those described in U.S. Pat. No. 2,728,663 (Allen).

[0187] Other suitable antifoggants and stabilizers that can be used alone or in combination include thiazolium salts as described in U.S. Pat. No. 2,131,038 (Staud) and U.S. Pat. No. 2,694,716 (Allen), azaindenes as described in U.S. Pat. No. 2,886,437 (Piper), triazaindolizines as described in U.S. Pat. No. 2,444,605 (Heimbach), the urazoles described in U.S. Pat. No. 3,287,135 (Anderson), sulfocatechols as described in U.S. Pat. No. 3,235,652 (Kennard), the oximes described in GB 623,448 (Carrol et al.), polyvalent metal salts as described in U.S. Pat. No. 2,839,405 (Jones), thiuronium salts as described in U.S. Pat. No. 3,220,839 (Herz), palladium, platinum and gold salts as described in U.S. Pat. No. 2,566,263 (Trirelli) and U.S. Pat. No. 2,597,915 (Damshroder), and 2-(tribromomethylsulfonyl)quinoline compounds as described in U.S. Pat. No. 5,460,938 (Kirk et al.). Stabilizer precursor compounds capable of releasing stabilizers upon application of heat during development can also be used. Such precursor compounds are described in for example, U.S. Pat. No. 5,158,866 (Simpson et al.), U.S. Pat. No. 5,175,081 (Krepski et al.), U.S. Pat. No. 5,298,390 (Sakizadeh et al.) and U.S. Pat. No. 5,300,420 (Kenney et al.).

[0188] In addition, certain substituted-sulfonyl derivatives of benzotriazoles (for example, alkylsulfonylbenzotriazoles and arylsulfonylbenzotriazoles) have been found to be useful stabilizing compounds (such as for post-processing print stabilizing), as described in copending U.S. Ser. No. 09/301,652 (filed Apr. 28, 1999 by Kong, Sakizadeh, LaBelle, Spahl, and Skoug).

[0189] Furthermore, other specific useful antifoggants/stabilizers are described in more detail in U.S. Pat. No. 6,083,681 (Lynch et al.), incorporated herein by reference.

[0190] Other antifoggants are hydrobromic acid salts of heterocyclic compounds (such as pyridinium hydrobromide perbromide) as described, for example, in U.S. Pat. No. 5,028,523 (Skoug), compounds having —SO₂CBr₃groups as described, for example, in U.S. Pat. No. 5,594,143 (Kirk et al.) and U.S. Pat. No. 5,374,514 (Kirk et al.), benzoyl acid compounds as described, for example, in U.S. Pat. No. 4,784,939 (Pham), substituted propenenitrile compounds as described, for example, in U.S. Pat. No. 5,686,228 (Murray et al.), silyl blocked compounds as described, for example, in U.S. Pat. No. 5,358,843 (Sakizadeh et al.), vinyl sulfones as described, for example, in EP-A-0 600,589 (Philip, Jr. et al.) and EP-A-0 600,586 (Philip, Jr. et al.), and tribromomethylketones as described, for example, in EP-A-0 600,587 (Oliff et al.).

[0191] Preferably, the materials of this invention include one or more polyhalo antifoggants that include one or more polyhalo substituents including but not limited to, dichloro, dibromo, trichloro and tribromo groups. The antifoggants can be aliphatic, alicyclic or aromatic compounds, including aromatic heterocyclic and carbocyclic compounds.

[0192] The use of “toners” or derivatives thereof that improve the image is highly desirable. Preferably, if used, a toner can be present in an amount of about 0.01% by weight to about 10%, and more preferably about 0.1% by weight to about 10% by weight, based on the total dry weight of the layer in which it is included. Toners may be incorporated in the photothermographic emulsion layer or in an adjacent layer. Toners are well known materials in the photothermographic art, as shown in U.S. Pat. No. 3,080,254 (Grant, Jr.), U.S. Pat. No. 3,847,612 (Winslow), U.S. Pat. No. 4,123,282 (Winslow), U.S. Pat. No. 4,082,901 (Laridon et al.), U.S. Pat. No. 3,074,809 (Owen), U.S. Pat. No. 3,446,648 (Workman), U.S. Pat. No. 3,844,797 (Willems et al.), U.S. Pat. No. 3,951,660 (Hagemann et al.), U.S. Pat. No. 5,599,647 (Defieuw et al.) and GB 1,439,478 (AGFA).

[0193] Examples of toners include, but are not limited, to phthalimide and N-hydroxyphthalimide, cyclic imides (such as succinimide), pyrazoline-5-ones, quinazolinone, 1-phenylurazole, 3-phenyl-2-pyrazoline-5-one, and 2,4-thiazolidinedione, naphthalimides (such as N-hydroxy-1,8-naphthalimide), cobalt complexes [such as hexaamminecobalt(3+) trifluoroacetate], mercaptans (such as 3-mercapto-1,2,4-triazole, 2,4-dimercaptopyrimidine, 3-mercapto-4,5-diphenyl-1,2,4-triazole and 2,5-dimercapto-1,3,4-thiadiazole), N-(aminomethyl)aryldicarboximides [such as (N,N-dimethylaminomethyl)phthalimide, and N-(dimethylaminomethyl)naphthalene-2,3-dicarboximide, a combination of blocked pyrazoles, isothiuronium derivatives, and certain photobleach agents [such as a combination of N,N′-hexamethylene-bis(1-carbamoyl-3,5-dimethylpyrazole), 1,8-(3,6-diazaoctane)bis(isothiuronium)trifluoroacetate, and 2-(tribromomethylsulfonyl benzothiazole)], merocyanine dyes (such as 3-ethyl-5-[(3-ethyl-2-benzothiazolinylidene)-1-methyl-ethylidene]-2-thio-2,4-o-azolidinedione), phthalazine and derivatives thereof [such as those described in U.S. Pat. No. 6,146,822 (Asanuma et al.)], phthalazinone and phthalazinone derivatives, or metal salts or these derivatives (such as 4-(1-naphthyl)phthalazinone, 6-chlorophthalazinone, 5,7-dimethoxyphthalazinone, and 2,3-dihydro-1,4-phthalazinedione), a combination of phthalazine (or derivative thereof) plus one or more phthalic acid derivatives (such as phthalic acid, 4-methylphthalic acid, 4-nitrophthalic acid, and tetrachlorophthalic anhydride), quinazolinediones, benzoxazine or naphthoxazine derivatives, rhodium complexes functioning not only as tone modifiers but also as sources of halide ion for silver halide formation in situ [such as ammonium hexachlororhodate (III), rhodium bromide, rhodium nitrate, and potassium hexachlororhodate (III)], inorganic peroxides and persulfates (such as ammonium peroxydisulfate and hydrogen peroxide), benzoxazine-2,4-diones (such as 1,3-benzoxazine-2,4-dione, 8-methyl-1,3-benzoxazine-2,4-dione and 6-nitro-1,3-benzoxazine-2,4-dione), pyrimidines and asym-triazines (such as 2,4-dihydroxypyrimidine, 2-hydroxy-4-amino-pyrimidine and azauracil) and tetraazapentalene derivatives (such as 3,6-dimercapto-1,4-diphenyl-1H,4H-2,3a,5,6a-tetraazapentalene and 1,4-di-(o-chlorophenyl)-3,6-dimercapto-1H,4H-2,3a,5,6a-tetraazapentalene).

[0194] Phthalazine and various phthalazine derivatives [such as those described in U.S. Pat. No. 6,146,822 (Asanuma et al.), incorporated herein by reference] are particularly useful toners.

[0195] The photocatalyst (such as photosensitive silver halide), the non-photosensitive source of reducible silver ions, the reducing agent composition, and any other additives used in the present invention are generally added to one or more binders that are either hydrophilic or hydrophobic. Thus, either aqueous- or solvent-based formulations can be used to prepare materials of this invention. Mixtures of either or both types of binders can also be used. It is preferred that the binder be selected from hydrophobic polymeric materials, such as, for example, natural and synthetic resins that are sufficiently polar to hold the other ingredients in solution or suspension.

[0196] Examples of typical hydrophobic binders include, but are not limited to, polyvinyl acetals, polyvinyl chloride, polyvinyl acetate, cellulose acetate, cellulose acetate butyrate, polyolefins, polyesters, polystyrenes, polyacrylonitrile, polycarbonates, methacrylate copolymers, maleic anhydride ester copolymers, butadiene-styrene copolymers and other materials readily apparent to one skilled in the art. Copolymers (including terpolymers) are also included in the definition of polymers. The polyvinyl acetals (such as polyvinyl butyral and polyvinyl formal) and vinyl copolymers (such as polyvinyl acetate and polyvinyl chloride) are particularly preferred. Particularly suitable binders are polyvinyl butyral resins that are available as BUTVAR® B79 (Solutia, Inc.) and Pioloforrn BS-18 or Pioloform BL16 (Wacker Chemical Company).

[0197] Examples of useful hydrophilic binders include, but are not limited to, gelatin and gelatin-like derivatives (hardened or unhardened), cellulosic materials such as cellulose acetate, cellulose acetate butyrate, hydroxymethyl cellulose, acrylamide/methacrylamide polymers, acrylic/methacrylic polymers polyvinyl pyrrolidones, polyvinyl acetates, polyvinyl alcohols and polysaccharides (such as dextrans and starch ethers).

[0198] Hardeners for various binders may be present if desired. Useful hardeners are well known and include diisocyanate compounds as described for example, in EP-0 600 586B 1 and vinyl sulfone compounds as described in EP-0 600 589B1.

[0199] Where the proportions and activities of the thermographic and photothermographic materials require a particular developing time and temperature, the binder(s) should be able to withstand those conditions. Generally, it is preferred that the binder not decompose or lose its structural integrity at 120° C. for 60 seconds. It is more preferred that it not decompose or lose its structural integrity at 177° C. for 60 seconds.

[0200] The polymer binder(s) is used in an amount sufficient to carry the components dispersed therein. The effective range can be appropriately determined by one skilled in the art. Preferably, a binder is used at a level of about 10% by weight to about 90% by weight, and more preferably at a level of about 20% by weight to about 70% by weight, based on the total dry weight of the layer in which it is included.

[0201] The thermographic and photothermographic materials of this invention comprise a polymeric support that is preferably a flexible, transparent film that has any desired thickness and is composed of one or more polymeric materials, depending upon their use. The supports are generally transparent (especially if the material is used as a photomask) or at least translucent, but in some instances, opaque supports may be useful. They are required to exhibit dimensional stability during thermal development and to have suitable adhesive properties with overlying layers. Useful polymeric materials for making such supports include, but are not limited to, polyesters (such as polyethylene terephthalate and polyethylene naphthalate), cellulose acetate and other cellulose esters, polyvinyl acetal,.polyolefins (such as polyethylene and polypropylene), polycarbonates, and polystyrenes (and polymers of styrene derivatives). Preferred supports are composed of polymers having good heat stability, such as polyesters and polycarbonates. Polyethylene terephthalate film is the most preferred support. Various support materials are described, for example, in gust 1979, item 18431. A method of making dimensionally stable polyester films is described in Research Disclosure, September, 1999, item 42536.

[0202] Opaque supports can also be used, such as dyed polymeric films and resin-coated papers that are stable to high temperatures.

[0203] Support materials can contain various colorants, pigments, antihalation or acutance dyes if desired. Support materials may be treated using conventional procedures (such as corona discharge) to improve adhesion of overlying layers, or subbing or other adhesion-promoting layers can be used. Useful subbing layer formulations include those conventionally used for photographic materials such as vinylidene halide polymers.

[0204] The formulation for the emulsion layer(s) can be prepared by dissolving and dispersing a hydrophobic binder, the photocatalyst (for photothermographic materials), the non-photosensitive source of reducible silver ions, the reducing composition, and optional addenda in an organic solvent, such as toluene, 2-butanone, acetone or tetrahydrofuran.

[0205] Alternatively, these components can be formulated with a hydrophilic binder in water or water-organic solvent mixtures to provide aqueous-based coating formulations.

[0206] Thermographic and photothermographic materials of this invention can also contain plasticizers and lubricants such as polyalcohols and diols of the type described in U.S. Pat. No. 2,960,404 (Milton et al.), fatty acids or esters such as those described in U.S. Pat. No. 2,588,765 (Robijns) and U.S. Pat. No. 3,121,060 (Duane), and silicone resins such as those described in GB 955,061 (DuPont). The materials can also contain matting agents such as starch, titanium dioxide, zinc oxide, silica, and polymeric beads, including beads of the type described in U.S. Pat. No. 2,992,101 (Jelley et al.) and U.S. Pat. No. 2,701,245 (Lynn). Polymeric fluorinated surfactants may also be useful in one or more layers of the imaging materials for various purposes, such as improving coatability and optical density uniformity as described in U.S. Pat. No. 5,468,603 (Kub).

[0207] EP-A-0 792 476 (Geisler et al.) describes various means of modifying the photothermographic materials to reduce what is known as the “woodgrain” effect, or uneven optical density. This effect can be reduced or eliminated by several means, including treatment of the support, adding matting agents to the topcoat, using acutance dyes in certain layers, or other procedures described in the noted publication.

[0208] The thermographic and photothermographic materials can include antistatic or conducting layers. Such layers may contain soluble salts (for example, chlorides or nitrates), evaporated metal layers, or ionic polymers such as those described in U.S. Pat. No. 2,861,056 (Minsk) and U.S. Pat. No. 3,206,312 (Sterman et al.), or insoluble inorganic salts such as those described in U.S. Pat. No. 3,428,451 (Trevoy), electroconductive underlayers such as those described in U.S. Pat. No. 5,310,640 (Markin et al.), electronically-conductive metal antimonate particles such as those described in U.S. Pat. No. 5,368,995 (Christian et al.), and electrically-conductive metal-containing particles dispersed in a polymeric binder such as those described in EP-A-0 678 776 (Melpolder et al.). Other antistatic agents are well known in the art.

[0209] The thermographic and photothermographic materials can be constructed of one or more layers on a support. Single layer materials should contain the photocatalyst (for photothermographic materials), the non-photo-sensitive source of reducible silver ions, the reducing composition, the binder, as well as optional materials such as toners, acutance dyes, coating aids and other adjuvants.

[0210] Two-layer constructions comprising a single imaging layer coating containing all the ingredients and a protective topcoat are generally found in the materials of this invention. However, two-layer constructions containing photocatalyst and non-photosensitive source of reducible silver ions in one imaging layer (usually the layer adjacent to the support) and the reducing composition and other ingredients in the second imaging layer or distributed between both layers are also envisioned.

[0211] Layers to promote adhesion of one layer to another are also known, as described for example, in U.S. Pat. No. 5,891,610 (Bauer et al.), U.S. Pat. No. 5,804,365 (Bauer et al.) and U.S. Pat. No. 4,741,992 (Przezdziecki). Adhesion can also be promoted using specific polymeric adhesive materials as described for example, in U.S. Pat. No. 5,928,857 (Geisler et al.).

[0212] Thermographic and photothermographic formulations described herein can be coated by various coating procedures including wire wound rod coating, dip coating, air knife coating, curtain coating, slide coating, or extrusion coating using hoppers of the type described in U.S. Pat. No. 2,681,294 (Beguin). Layers can be coated one at a time, or two or more layers can be coated simultaneously by the procedures described in U.S. Pat. No. 2,761,791 (Russell), U.S. Pat. No. 4,001,024 (Dittman et al.), U.S. Pat. No. 4,569,863 (Keopke et al.), U.S. Pat. No. 5,340,613 (Hanzalik et al.), U.S. Pat. No. 5,405,740 (LaBelle), U.S. Pat. No. 5,415,993 (Hanzalik et al.), U.S. Pat. No. 5,525,376 (Leonard), U.S. Pat. No. 5,733,608 (Kessel et al.), U.S. Pat. No. 5,849,363 (Yapel et al.), U.S. Pat. No. 5,843,530 (Jerry et al.), U.S. Pat. No. 5,861,195 (Bhave et al.) and GB 837,095 (Ilford). A typical coating gap for the emulsion layer can be from about 10 to about 750 μm, and the layer can be dried in forced air at a temperature of from about 20° C. to about 100° C. It is preferred that the thickness of the layer be selected to provide maximum image densities greater than about 0.2, and more preferably, from about 0.5 to 5.0 or more, as measured by a MacBeth Color Densitometer Model TD 504.

[0213] When the layers are coated simultaneously using various coating techniques, a “carrier” layer formulation comprising a single-phase mixture of the two or more polymers, described above, may be used. Such formulations are described in copending and commonly assigned U.S. Ser. No. 09/510,648 filed Feb. 23, 2000 by Ludemann et al. that is based on Provisional Application No. 60/121,794, filed Feb. 26, 1999.

[0214] Mottle and other surface anomalies can be reduced in the materials of this invention by incorporation of a fluorinated polymer as described for example, in U.S. Pat. No. 5,532,121 (Yonkonski et al.) or by using particular drying techniques as described, for example, in U.S. Pat. No. 5,621,983 (Ludemann et al.).

[0215] Preferably, two or more layers are applied to a film support using slide coating. The first layer can be coated on top of the second layer while the second layer is still wet. The first and second fluids used to coat these layers can be the same or different organic solvents (or organic solvent mixtures).

[0216] While the first and second layers can be coated on one side of the film support, the method can also include forming on the opposing or backside of said polymeric support, one or more additional layers, including an antihalation layer, an antistatic layer, or a layer containing a matting agent (such as silica), or a combination of such layers. It is also contemplated that the thermographic and photothermographic materials of this invention can include emulsion layers on both sides of the support.

[0217] To promote image sharpness, photothermographic materials according to the present invention can contain one or more layers containing acutance and/or antihalation dyes. These dyes are chosen to have absorption close to the exposure wavelength and are designed to absorb scattered light. One or more antihalation dyes may be incorporated into one or more antihalation layers according to known techniques, as an antihalation backing layer, as an antihalation underlayer, or as an antihalation overcoat. Additionally, one or more acutance dyes may be incorporated into one or more frontside layers such as the photothermographic emulsion layer, primer layer, underlayer, or topcoat layer according to known techniques. It is preferred that the photothermographic materials of this invention contain an antihalation coating on the support opposite to the side on which the emulsion and topcoat layers are coated.

[0218] Dyes particularly useful as antihalation and acutance dyes include dihydroperimidine squaraine dyes having the nucleus represented by the following general structure:

[0219] Details of such dyes having the dihydroperimidine squaraine nucleus and methods of their preparation can be found in U.S. Pat. No. 6,063,560 (Suzuki et al.) and U.S. Pat. No. 5,380,635 (Gomez et al.), both incorporated herein by reference. These dyes can also be used as acutance dyes in frontside layers of the materials of this invention. One particularly useful dihydroperimidine squaraine dye is cyclobutenediylium, 1,3-bis[2,3-dihydro-2,2-bis[[1-oxohexyl)oxy]methyl]-1H-perimidin-4-yl]-2,4-dihydroxy-, bis(inner salt).

[0220] Dyes particularly useful as antihalation dyes in a backside layer of the photothermographic material also include indolenine cyanine dyes having the nucleus represented by the following general structure:

[0221] Details of such antihalation dyes having the indolenine cyanine nucleus and methods of their preparation can be found in EP-A-0 342 810 (Leichter), incorporated herein by reference. One particularly useful cyanine dye, compound (6) described therein, is 3H-Indolium, 2-[2-[2-chloro-3-[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)ethylidene]-5-methyl-1-cyclohexen-1-yl]ethenyl]-1,3,3-trimethyl-, perchlorate.

[0222] It is also useful in the present invention to employ acutance or antihalation dyes that will decolorize with heat during processing. Dyes and constructions employing these types of dyes are described in, for example, U.S. Pat. No. 5,135,842 (Kitchin et al.), U.S. Pat. No. 5,266,452 (Kitchin et al.), U.S. Pat. No. 5,314,795 (Helland et al.), and EP-A-0 911 693 (Sakurada et al.).

[0223] While the imaging materials of the present invention can be imaged in any suitable manner consistent with the type of material using any suitable imaging source (typically some type of radiation or electronic signal for photothermographic materials and some type of thermal source for thermographic materials), the following discussion will be directed to the preferred imaging means for photothermographic materials. Generally, the materials are sensitive to radiation in the range of from about 300 to about 850 nm.

[0224] Imaging can be achieved by exposing the photothermographic materials to a suitable source of radiation to which they are sensitive, including ultraviolet light, visible light, near infrared radiation and infrared radiation to provide a latent image. Suitable exposure means are well known and include laser diodes that emit radiation in the desired region, photodiodes and others described in the art, including Research Disclosure, September 1996, item 38957, (such as sunlight, xenon lamps and fluorescent lamps). Particularly useful exposure means uses laser diodes, including laser diodes that are modulated to increase imaging efficiency using what is known as multilongitudinal exposure techniques as described in U.S. Pat. No. 5,780,207 (Mohapatra et al.). Other exposure techniques are described in U.S. Pat. No. 5,493,327 (McCallum et al.).

[0225] For using the materials of this invention, development conditions will vary, depending on the construction used but will typically involve heating the imagewise exposed material at a suitably elevated temperature. Thus, the latent image can be developed by heating the exposed material at a moderately elevated temperature of, for example, from about 50° C. to about 250° C. (preferably from about 80° C. to about 200° C. and more preferably from about 100° C. to about 200° C.) for a sufficient period of time, generally from about 1 to about 120 seconds. Heating can be accomplished using any suitable heating means such as a hot plate, a steam iron, a hot roller or a heating bath.

[0226] In some methods, the development is carried out in two steps. Thermal development takes place at a higher temperature for a shorter time (for example, at about 150° C. for up to 10 seconds), followed by thermal diffusion at a lower temperature (for example, at about 80° C.) in the presence of a transfer solvent.

[0227] The thermographic and photothermographic materials of the present invention are sufficiently transmissive in the range of from about 350 to about 450 nm in non-imaged areas to allow their use in a process where there is a subsequent exposure of an ultraviolet or short wavelength visible radiation sensitive imageable medium. For example, imaging the materials and subsequent development affords a visible image. The heat-developed photothermographic materials absorb ultraviolet or short wavelength visible radiation in the areas where there is a visible image and transmits ultraviolet or short wavelength visible radiation where there is no visible image. The heat-developed materials may then be used as a mask and positioned between a source of imaging radiation (such as an ultraviolet or short wavelength visible radiation energy source) and an imageable material that is sensitive to such imaging radiation, such as a photopolymer, diazo material, photoresist, or photosensitive printing plate. Exposing the imageable material to the imaging radiation through the visible image in the exposed and heat-developed thermographic or photothermographic material provides an image in the imageable material. This process is particularly useful where the imageable medium comprises a printing plate and the thermographic or photothermographic material serves as an imagesetting film.

[0228] The following examples are provided to illustrate the practice of this invention, and are not intended to be limiting in any manner.

[0229] The preparation of the core-shell non-photosensitive silver salts of the present invention must be carried out in a specifically defined manner. For example, simply mixing two different silver salts of fatty carboxylic acids in the desired ratio will not produce the core-shell structure. Similarly, forming the silver salts from a mixture of two fatty acids as described in EP-0 964 300 (Loccufier et al.) will not produce the silver carboxylate salts having a core-shell structure. In general, the preparation of the core-shell silver carboxylate compounds of the present invention begins with the preparation of the one or more silver salts used as the “core”, followed by preparation of the one or more silver salts used for the “shell”.

[0230] Specifically, in one embodiment, the preparation of a “core” was carried out by dissolving sodium hydroxide (5 mmol) in water (250 ml) at about room temperature followed by addition of dodecanoic acid (5 mmol). The resulting solution was stirred for 5 minutes. Silver nitrate (10 mmol in 15 ml of water) was added to form a dispersion of the silver dodecanoate to be used as the “core” silver salt.

[0231] Meanwhile, behenic acid (5 mmol) in 250 ml of water was dissolved in water (250 ml) at 80° C. containing sodium hydroxide (5 mmol). After 5 minutes of stirring, the resulting sodium behenate solution was allowed to cool to about room temperature and then added to the silver dodecanoate “core” dispersion. The timing was such that the silver dodecanoate contained excess silver nitrate for about 1 minute prior to the addition of the sodium behenate. The resulting mixture was stirred another 10 minutes, and filtered. The resulting solid core-shell silver salt was re-dispersed in an equal volume of water, stirred, filtered, and air-dried.

[0232] The reverse procedure was used to prepare a core-shell silver salt having a silver behenate core and a silver dodecanoate shell.

[0233] The imaging properties of these core shell dispersions were evaluated by homogenizing (10 minutes) a 3% dispersion in polyvinyl butyral (Pioloform BL-16, Wacker Chemical Company), 10% in acetone, and coating to 100 μm wet thickness on a 4 mil (102 μm) transparent polyester support. The resulting films were air-dried and coated with developers (reducing composition), as shown below, to provide thermographic materials of this invention. The imaging results of the various invention thermographic materials (Examples 1-5) are shown in TABLE I below.

[0234] A silver salt was also prepared using the ratios described above, but with the fatty acids simply physically mixed together prior to the addition of AgNO₃. A thermographic material (Control A) outside of the present invention was similarly prepared using this mixed silver salt. Films identified as Controls B and C were prepared using homogeneous (not core-shell) silver salts.

[0235] It should be noted that the silver salt in Example 3 had a multilayer core-shell construction. Example 4 had a used a silver salt of a non-carboxylic acid in the shell. Example 5 used a silver salt of an α-substituted carboxylic acid in the core.

[0236] Photothermographic materials of this invention were made by including suitable photocatalysts (such as a silver halide) with core-shell silver salt as the non-photosensitive sources of silver ion, and the binder and reducing composition (for example, developer) were provided either in the same layer or a separate layer.

[0237] The core-shell silver salt of Example 2, 3, or 4 (0.6 g) was dispersed in acetone (10 ml) containing the polyvinyl butyral noted above (10 mg) and homogenized 15 minutes. Addition of calcium bromide (60 mg) in ethanol (2 ml) produced ˜20 mole % in situ photosensitive silver bromide grains. After 15 minutes, polyvinyl butyral (0.5 g) was added, and the dispersion was coated at 100 μm (wet) onto a 4 mil (102 μm) transparent polyester support and air-dried to provide an imaging layer. A topcoat formulation comprising polyvinyl butyral (0.3 g), phthalazine (0.2 g), 4-methylphthalic acid (0.2), and NONOX developer (0.2 g) in ethanol (10 ml) was applied at ˜50 μm (wet) on the imaging layer and air-dried.

[0238] Samples were evaluated by exposing half (lengthwise) of a strip of the film at 364 nm using a Spectraline ENF-24 ultraviolet lamp followed by thermal development on a Hotbench™ (Cambridge Instruments, Buffalo, N.Y.) thermal gradient bar. In these negative-acting systems, the onset temperatures of the light activated, thermally developed area, T_exposed, and unexposed, T_unexposed, define the imageability of the construction. The difference between them, ΔT, is a measure of the thermal process latitude. The results are shown below in Table II.

[0239] A photothermographic silver soap dispersion was prepared as described in U.S. Pat. No. 5,434,043. A second ligand, tetrachlorophthalic acid, capable of coordination with silver was then added and allowed to exchange with the dispersed silver salt to form a shell of silver tetrachlorophthalate on the original core. Photothermographic films were then constructed also as described in U.S. Pat. No. 5,434,043.

[0240] As can be seen in the following TABLE III, tetrachlorophthalic acid can be added to the imaging layer formulation at certain levels to construct core-shell silver salts in situ and to provide improved image stability, that is reduced change in D_minover time. As one skilled in the art would understand from the data in TABLE III, the amount of tetrachlorophthalic acid can be optimized to provide the desired image stability while retaining desired D_maxand photospeed. Similar results were obtained with 2-chloro-4-nitrobenzoic acid, 2,4-dichlorobenzoic acid, and p-bromophenyl acetic acid.

[0241] Tetrachlorophthalic acid has the following structure:

[0242] Two photothermographic materials of the present invention were prepared in the following manner. Red safelights were used.

[0243] Preformed core-shell silver bromide grains (1 g) in gelatin (0.055 μm cubes, 1.32 mmol/g, bromide containing copper and 2% iodide) was added to a sodium stearate dispersion (prepared from 1.3 g of stearic acid and 0.18 g of sodium hydroxide in 140 ml of water at 70° C.) and cooled to 48° C. After 15 minutes, silver nitrate (0.75 g) in water (10 ml) was added. After stirring for 20 minutes with ambient cooling, silver nitrate (0.41 g) in water (5 ml) was added, followed immediately by addition of a sodium decanoate dispersion (prepared from 0.41 g of decanoic acid and 0.088 g of sodium hydroxide in 20 ml of water). After 15 minutes, the resulting dispersion was filtered and washed. At this point, the silver soap dispersion was divided into two portions for making two different photothermographic films.

[0244] The film of Example 11 was prepared by dispersing the silver soap dispersion (2 g) described above, while wet, in water (14 g) containing gelatin (1 g of 35% solution) at 45° C. Phthalazine (0.16 g) was added and the resulting dispersion was homogenized using a conventional mixer for 15 minutes. This formulation was then coated at a wet thickness of 100 μm on a 4 mil (102 μm) transparent polyester support and air-dried to provide an imaging layer. A topcoat formulation containing polyvinyl butyral (Pioloform BL-16, 0.3 g), 4-methylphthalic acid (0.2 g), and NONOX developer (0.2 g) in ethanol (10 ml) was applied to the imaging layer at 30 μm (wet) and air-dried. The results of imaging and heat-development are provided in the following TABLE IV.

[0245] The film of Example 12 was prepared by dispersing the silver soap dispersion (0.6 g) described above, after being air-dried, in acetone (10 g) containing polyvinyl butyral (12% solution) at room temperature. Phthalazine (0.2 g) was added and the resulting dispersion was homogenized using a conventional mixer for 15 minutes. This formulation was then coated at a wet thickness of 100 μm on a (102 μm) transparent polyester support and air-dried to provide an imaging layer. A topcoat formulation containing polyvinyl butyral (Pioloform BL-16, 0.3 g), 4-methylphthalic acid (0.2 g), and NONOX developer (0.2 g) in ethanol (10 ml) was applied to the imaging layer at 30 μm (wet) and air-dried. The results of imaging and heat-development are provided in the following TABLE IV.

[0246] The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.