Patent Publication Number: US-6713241-B2

Title: Thermally developable emulsions and imaging materials containing binder mixture

Description:
FIELD OF THE INVENTION 
     This invention relates to thermally developable imaging emulsions and materials. In particular, it relates to such emulsions and thermographic and photothermographic materials containing the emulsions that comprise a mixture of specific binders in at least one imaging layer. The invention also relates to methods for imaging these materials. 
     BACKGROUND OF THE INVENTION 
     Silver-containing photothermographic imaging materials that are developed with heat and without liquid development have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, visible, ultraviolet, or infrared radiation) and developed by the use of thermal energy. These materials, also known as “dry silver” materials, generally comprise a support having coated thereon: (a) a photosensitive catalyst (such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a hydrophilic or hydrophobic binder. The latent image is then developed by application of thermal energy. 
     In such materials, the photosensitive catalyst is generally a photo-graphic type photosensitive silver halide that is considered to be in catalytic proximity to the non-photosensitive source of reducible silver ions. Catalytic proximity requires intimate physical association of these two components either prior to or during the thermal image development process so that when silver atoms, (Ag 0 ) 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 [Kiosterboer,  Imaging Processes and Materials  ( Neblette&#39;s Eighth Edition ), Sturge, Walworth &amp; Shepp (Eds.), Van Nostrand-Reinhold, New York, Chapter 9, pp. 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)]. 
     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 co-precipitated [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, Sep. 7-11, 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. 
     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. 
     In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag 0 ) 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. 
     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). 
     The various distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials ( Neblette&#39;s Eighth Edition ), noted above,  Unconventional Imaging Processes , E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74-75, in Zou et al.,  J. Imaging Sci. Technol . 1996, 40, 94-103, and in M. R. V. Sahyun,  J. Imaging Sci. Technol . 1998, 42, 23. 
     Problem to be Solved 
     Most common photothermographic materials are prepared using organic solvents for layer formulation and coating, and therefore often identified as “solvent-based” or “non-aqueous” materials. The various chemical components and binders required for such materials are generally soluble in the organic solvents and insoluble in water. 
     However, photothermographic materials that can be formulated and coated out of water (“aqueous-based” materials) would have a number of manufacturing, environmental, and cost advantages. Use of the same chemical components that are present in solvent-based materials is not always possible in aqueous environments without the use of expensive or tedious solubilizing or dispersing techniques. The water-insoluble chemical components tend to precipitate and cause variability in photosensitive response and coating defects when used in aqueous formulations even with adequate dispersion. 
     Aqueous-based photothermographic materials require specific components to provide optimal coating, imaging, and development. In addition, those components need to be formulated in specific ways to achieve uniform dispersion throughout the imaging layers. Gelatin is the most common binder for aqueous-based imaging formulations and photothermographic materials but mixtures of various hydrophilic binders or water-soluble or water-dispersible polymers or latexes are also used for this purpose, as described for example in U.S. Pat. No. 6,143,481 (Uytterhoeven et al.) and U.S. Pat. No. 6,071,687 (Yamashita). 
     However, it has been observed that aqueous formulations using only a gelatin binder exhibit certain problems when certain silver carboxylates, such as silver behenate, are present as the source of reducible silver ions. It has been observed that some of the behenic acid crystallizes when gelatin is used as the predominant binder. Such crystallization causes spot defects in the developed (imaged) areas. 
     Hence, there is a need for improved aqueous-based (hydrophilic) thermally developable formulations and materials that include conventional silver ion reducing agents, silver carboxylates such as silver behenate, and hydrophilic binders that do not exhibit crystallization in the imaging layers. 
     SUMMARY OF THE INVENTION 
     The present invention provides an aqueous-based thermally sensitive emulsion comprising: 
     a) a non-photosensitive source of reducible silver ions that includes an organic silver carboxylate formed from a fatty acid that is insoluble in gelatin, 
     b) a reducing agent composition for the reducible silver ions, and 
     c) a binder mixture comprising a hydrophilic binder in an amount of from about 70 to about 99 weight % of total binder weight and a hydrophobic binder or water-dispersible polymer or latex comprising the remainder of the total binder weight. 
     Still other embodiments of this invention includes aqueous-based thermally sensitive emulsions comprising: 
     a) a non-photosensitive source of reducible silver ions that includes an organic silver carboxylate formed from a fatty acid that is insoluble in gelatin, 
     b) a reducing agent composition for the reducible silver ions, and 
     c) a binder mixture comprising a hydrophilic binder in an amount of from about 70 to about 99 weight % of total binder weight and a water-soluble polyvinyl butyral or water-soluble cellulose ester comprising the remainder of the total binder weight. 
     Aqueous-based photothermographic emulsions of this invention include components a) through c) noted above as well as a photosensitive silver halide. 
     In addition, this invention provides an aqueous-based thermally developable material comprising a support having thereon at least one aqueous-based imaging layer comprising a mixture of binders, and having in reactive association: 
     a) a non-photosensitive source of reducible silver ions that includes an organic silver carboxylate formed from a fatty acid that is insoluble in gelatin, and 
     b) a reducing agent composition for the reducible silver ions, 
     wherein the mixture of binders comprises a bydrophilic binder in an amount of from 70 to about 99 weight % of total binder weight and a hydrophobic binder or water-dispersible polymer or latex comprising the remainder of the total binder weight. 
     Still again, this invention provides an aqueous-based thermally developable material comprising a support having thereon at least one aqueous-based imaging layer comprising a mixture of binders, and having in reactive association: 
     a) a non-photosensitive source of reducible silver ions that includes an organic silver carboxylate formed from a fatty acid that is insoluble in gelatin, and 
     b) a reducing agent composition for the reducible silver ions, 
     wherein the mixture of binders comprises a hydrophilic binder in an amount of from 70 to about 99 weight % of total binder weight and a water-soluble polyvinyl butyral or water-soluble cellulose ester comprising the remainder of the total binder weight. 
     This invention also provides an aqueous-based photothermographic material comprising a support having thereon at least one aqueous-based imaging layer comprising a binder mixture, and having in reactive association: 
     a) a photosensitive silver halide, 
     b) a non-photosensitive source of reducible silver ions that includes an organic silver salt that is formed from a fatty acid that is insoluble in gelatin, and 
     c) a reducing agent composition for the reducible silver ions, 
     wherein the binder mixture comprises a hydrophilic binder in an amount of from 70 to about 99 weight % of total binder weight and a hydrophobic binder or water-dispersible polymer or latex comprising the remainder of the total binder weight. 
     In preferred embodiments, the aqueous-based photothermographic material comprises a transparent support having thereon an aqueous-based imaging layer comprising a binder mixture, and having in reactive association: 
     a) photosensitive grains of silver bromide, silver iodobromide, or both, 
     b) a non-photosensitive source of reducible silver ions that comprises one or more silver carboxylates provided as an aqueous nanoparticulate dispersion, at least one of which silver carboxylates is silver behenate, 
     c) a reducing agent composition for the reducible silver ions that includes one or more bisphenols, with or without a high contrast co-developer, and 
     d) one or more antifoggants, toners, or spectral sensitizing dyes, 
     wherein the binder mixture comprises gelatin or a gelatin derivative in an amount of from about 90 to about 95 weight % based on total binder weight, and a hydrophobic binder or water-dispersible polymer or latex that is poly(styrene-co-butadiene), a core-shell latex of poly(styrene-co-butyl acrylate-co-2-acrylamido-2-methylpropane-1-sulfonic acid, sodium salt) as the core and poly(styrene-co-2-hydroxyethyl methacrylate-co-2-acrylamido-2-methylpropane-1-sulfonic acid, sodium salt) as the shell, a polyurethane dispersion, or a polyester dispersion comprising the remainder of the total binder weight, 
     the photothermographic material further comprising an aqueous-based protective layer disposed over said imaging layer, and an aqueous-based antihalation layer disposed on the backside of the support. 
     A method of forming a visible image of this invention comprises: 
     A) imagewise exposing the aqueous-based photothermographic material of this invention to electromagnetic radiation to form a latent image, 
     B) simultaneously or sequentially, heating the exposed photothermographic material to develop the latent image into a visible image. 
     This image-forming method can further comprise: 
     C) positioning the exposed and heat-developed photothermographic material having the visible image therein between a source of imaging radiation and an imageable material that is sensitive to the imaging radiation, and 
     D) thereafter exposing the imageable material to the imaging radiation through the visible image in the exposed and heat-developed photothermographic material to provide a visible image in the imageable material. 
     This invention also includes a method of forming a visible image comprising imagewise heating the aqueous-based thermally developable material (thermographic material) of this invention to provide a visible image. 
     We have discovered that the aqueous-based thermally developable emulsions and materials of the present invention can be prepared with a specific mixture of binders in one or more imaging layers. This mixture of binders contains predominantly hydrophilic binders, that is from 70 to about 99 weight % of the total binder weight, and the remainder comprises one or more hydrophobic binders or water-dispersible binders such as water-dispersible polymers or latexes. This critical mixture of binders enables the preparation of aqueous-based materials that avoid the disadvantages of the solvent-based imaging materials while providing other advantages. 
     By using this unique combination of binders in the imaging layer(s), we discovered that crystallization of behenic acid and other fatty acids that are insoluble in gelatin and are used to form the organic silver salts, is reduced or eliminated. In addition, we discovered unexpectedly that the photographic speed of the photothermographic emulsions and materials was increased. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The thermally sensitive emulsions and thermally developable materials of this invention can be used, for example, in conventional black-and-white or color thermography or photothermography, in electronically generated black-and-white or color hardcopy recording. They can be used in microfilm applications, in radiographic imaging (for example digital medical imaging), and industrial radiography. They can also be used in the graphic arts area (for example, image setting and phototypesetting), in the manufacture of printing plates, in duplicating, and in proofing. The thermographic and photothermographic materials of the present invention are particularly useful for medical radiography to obtain black-and-white images. 
     In the thermally developable materials of this invention, the components for imaging can be in one or more layers. The layer(s) that contain a photosensitive silver halide (in the case of photothermographic materials) and non-photosensitive source of reducible silver ions, or both, are referred to herein as emulsion layer(s). The photosensitive silver halide and the non-photosensitive source of reducible silver ions are in catalytic proximity and preferably in the same emulsion layer. In addition, the non-photosensitive source of reducible silver ions and the reducing agent composition are in catalytic proximity and preferably in the same emulsion layer. 
     Various layers can be disposed on the “backside” (non-emulsion or non-imaging side) of the materials, including antihalation layer(s), protective layers, antistatic layers, conducting layers, and transport enabling layers. 
     Various layers are also usually disposed on the “frontside” imaging 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. 
     In thermography, imaging is carried out by imagewise heating the thermographic materials of the present invention using any suitable source of imaging thermal energy (such as a laser or thermal print head). 
     For photothermography, the process for the formation of a visible image (usually a black-and-white image) comprises first exposing to suitable electromagnetic radiation and thereafter heating the photothermographic material. 
     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 photo-sensitive printing plate) through the exposed and heat-developed photothermo-graphic material using steps C) and D) noted above. 
     When the photothermographic materials of this invention are heat-developed as described below in a substantially water-free condition after, or simultaneously with, imagewise exposure, a silver image (preferably a black-and-white silver image) is obtained. The photothermographic material may be exposed in step A using ultraviolet, visible, infrared or laser radiation using an infrared laser, a laser diode, an infrared laser diode, a light-emitting screen, a CRT tube, a light-emitting diode, or other light or radiation source readily apparent to one skilled in the art. 
     Definitions 
     As used herein: 
     In the descriptions of the thermally developable materials of the present invention, “a” or “an” component refers to “at least one” of that component. 
     Heating in a substantially water-free condition as used herein, means heating at a temperature of from about 50° C. 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. 
     “Photothermographic material(s)” means a construction comprising at least one photothermographic emulsion or imaging layer or a photothermo-graphic set of layers (wherein the photosensitive silver halide and the non-photo-sensitive source of reducible silver ions are in one layer and the other components or 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. 
     “Thermographic material(s)” are similarly defined except they do not include a photosensitive silver halide (or other photosensitive component) for imaging. 
     “Thermally sensitive” emulsions refer to both “photothermographic emulsions” as well as “thermographic emulsions” (that are useful for thermography and therefore lack the photosensitive component). 
     “Photothermographic emulsion” refers to a dispersion that comprises as essential components: at least one photosensitive silver halide and at least one non-photosensitive source of reducible silver ions. As is well known in the art, the emulsion can include many other components (including the non-crystalline reducing agent compositions described below) and other addenda that are described in more detail below. These layers are usually on what is known as the “frontside” of the support. 
     “Non-photosensitive” means not intentionally light sensitive. 
     The sensitometric terms “photospeed” or “photographic speed” (also known as “sensitivity”), “contrast”, D min , and D max , have conventional definitions known in the imaging arts. 
     “Transparent” means capable of transmitting visible light or imaging radiation without appreciable scattering or absorption. 
     “Hydrophilic binder” refers to a material that is soluble in an aqueous solvent that includes at least 50 volume % water. 
     “Hydrophobic binder” refers to a material that is antagonistic to water or incapable of dissolving in water (see Hawley&#39;s Condensed Chemical Dictionary, 14 th  ed., R. J. Lewis, ed., J. Wiley and Sons). 
     “Water-dispersible polymer or latex” refers to water insoluble or slightly soluble polymers that are dispersed in an aqueous medium or polymers that are prepared as latexes or emulsions such as described in  Principles of Polymerization , Odian, Wiley-Interscience. 
     As is well understood in this art, for the various essential compounds described herein, 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. 
     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 CH 3 —CH 2 —CH 2 —O—CH 2 — or CH 3 —CH 2 —CH 2 —S—CH 2 —), 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. 
       Research Disclosure  is 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). 
     Other aspects, advantages, and benefits of the present invention are apparent from the detailed description, examples, and claims provided in this application. 
     The Photocatalyst 
     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. Typical techniques for preparing and precipitating silver halide grains are described in  Research Disclosure , 1978, Item 17643. 
     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, rhombic, dodecahedral, orthorhombic, tetrahedral, other polyhedral, laminar, twinned, platelet, or tabular morphologies and may have epitaxial growth of crystals thereon. If desired, a mixture of these crystals can be employed. Silver halide grains having cubic and tabular morphology are preferred. 
     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.) incorporated herein by reference. 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. 
     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. 
     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.” 
     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 from 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 wavelengths to which the grains are spectrally sensitized. Such a lower limit, for example, is typically from about 0.01 to about 0.005 μm. 
     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. 
     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. 
     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.)]. 
     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). 
     In some instances, it may be helpful to prepare the photosensitive silver halide grains in the presence of a hydroxytetraazindene (such as 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene) or an N-heterocyclic compound comprising at least one mercapto compound (such as 1-phenyl-5-mercaptotetrazole) to provide increased photospeed. 
     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.). 
     Preferably, the photosensitive silver halide used in the practice of this invention is provided as a hydrophilic photosensitive silver halide emulsion comprising the unique binder mixture described herein. Most likely, the photosensitive silver halide emulsion includes one or more conventional peptizers that are well known to one skilled in the art, including but not limited to, gelatino peptizers such as phthalated gelatin, non-phthalated gelatin, and acid or base hydrolyzed gelatins. The amount of peptizer in this emulsion will depend upon such factors as the particular photosensitive silver halide, the desired image, the particular components of the photothermographic emulsion, and coating conditions. In some embodiments, the peptizer(s) is present in an amount of from about 5 to about 40 grams per mole of silver from the silver halide. Useful procedures for preparing such photosensitive silver halide emulsions are described for example in Product Licensing Index, Vol., 92, Item 9232, December 1971 (now know as  Research Disclosure ). 
     The one or more light-sensitive silver halides used in the photothermographic materials of the present invention are preferably present in an emulsion (imaging) layer 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. 
     Chemical and Spectral Sensitizers 
     The photosensitive silver halides used in the invention may be employed without modification. However, they are 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. 
     Thus, the photothermographic material of this invention 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), U.S. Pat. No. 5,049,485 (Deaton), U.S. Pat. No. 5,252,455 (Deaton), U.S. Pat. No. 5,391,727 (Deaton), U.S. Pat. No. 5,912,111 (Lok et al.), U.S. Pat. No. 5,759,761 (Lushington et al.), and EP 0 915 371A1 (Lok et al.). 
     One method of chemical sensitization is by oxidative decomposition of a spectral sensitizing dye in the presence of a photothermo-graphic emulsion, as described in U.S. Pat. No. 5,891,615 (Winslow et al.), incorporated herein by reference. 
     Sulfur-containing chemical sensitizers useful in the present invention are well known in the art and described for example, in Sheppard et al.,  J. Franklin Inst ., 1923, 196, pp. 653 and 673, C. E. K. Mees and T. H. James,  The Theory of the Photographic Process , 4 th  Edition, 1977, pp. 152-3, Tani, T.,  Photographic Sensitivity: Theory and Mechanisms , Oxford University Press, NY, 1995, pp. 167-176, U.S. Pat. No. 5,891,615 (Winslow et al.), Zavlin et al., IS&amp;T&#39;s 48 th  Annual Conference Papers, May 7-11 1995 Washington D.C., pp. 156-6), U.S. Pat. No. 4,810,626 (Burgmaier et al.), U.S. Pat. No. 4,036,650 (Kobayashi et al.), U.S. Pat. No. 4,213,784 (Ikenoue et al.), and U.S. Pat. No. 4,207,108 (Hiller). 
     Particularly useful sulfur-containing chemical sensitizers are tetrasubstituted thiourea compounds, preferably such thiourea compounds that are substituted with the same or different aliphatic substituents, and more preferably such thiourea compounds that are substituted with the same aliphatic substituent. Such useful thioureas are described for example in U.S. Pat. No. 5,843,632 (Eshelman et al.) and U.S. Pat. No. 6,368,779 (Lynch et al.), both incorporated herein by reference. 
     Particularly, useful tellurium-containing chemical sensitizing compounds are described in copending and commonly assigned U.S. Ser. No. 09/923,039 (filed Aug. 6, 2001 by Gysling, Dickinson, Lelental, and Boettcher), incorporated herein by reference. 
     Useful combinations of sulfur- or tellurium-containing chemical sensitizers with gold(III)-containing chemical sensitizers are described in copending and commonly assigned U.S. Pat. No. 6,413,710 (Shor et al.), incorporated herein by reference. 
     The total amount of chemical sensitizers that may be used during formulation of the photographic imaging composition will generally vary depending upon the average size of silver halide grains. The total amount is generally at least 10 −10  mole per mole of total silver, and preferably from 10 −8  to about 10 −2  mole 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. 
     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.), 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. All of the patents above are incorporated herein by reference. 
     An appropriate amount of spectral sensitizing dye added is generally about 10 −10  to 10 −1  mole, and preferably, about 10 −7  to 10 −2  mole per mole of silver halide. 
     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 as “supersensitizers”. 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 0 559 228A1 (Philip Jr. et al.) as supersensitizers. 
     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). 
     Heteroaromatic mercapto compounds are most preferred. Examples of preferred heteroaromatic mercapto compounds are 2-mercaptobenz-imidazole, 2-mercapto-5-methylbenzimidazole, 2-mercaptobenzothiazole and 2-mercaptobenzoxazole, and mixtures thereof. 
     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. 
     Non-Photosensitive Reducible Silver Source Material 
     The non-photosensitive source of reducible silver ions used in thermographic and photothermographic materials of the present invention can be one or more materials that contains reducible silver ions, provided that at least one organic silver carboxylate is present that is formed from a fatty acid that is insoluble in gelatin. By “insoluble in gelatin”, we mean that the mixture of fatty acid and gelatin can separate as two phases or the fatty acid will have the tendency to crystallize when in the presence of gelatin. The organic silver salts are comparatively stable to light and form a silver image when heated to 80° C. or higher in the presence of an exposed photosensitive silver halide and/or a reducing agent composition. 
     Silver salts of organic acids, particularly silver salts of long-chain carboxylic (fatty) acids are used as the predominant organic silver salt. By “predominant”, we mean that at least 50 mol % of the total silver present in the organic silver salts is provided by the organic silver carboxylates. The chains of the carboxylic acids typically contain 10 to 30, and preferably 15 to 28, carbon atoms. 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. It is particularly useful to have at least silver behenate present as the sole organic silver salt or predominant (at least 50 mol %) in a mixture of organic silver salts. 
     Additional organic silver salts can also be present in “minor” (less than 50 mol %) amounts. Preferred examples of silver salts of aromatic carboxylic acid and other carboxylic acid group-containing compounds include, but are not limited to, silver benzoates, a silver substituted-benzoate, 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 gallate, 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 comprising 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 providing coatings with less light scattering can also be used. Such silver carboxylates are described in U.S. Pat. No. 5,491,059 (noted above). Mixtures of any of the silver salts described herein can also be used if desired. 
     Silver salts of sulfonates can also be present. 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 EPO 227 141A1 (Leenders et al.). 
     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-amino-thiadiazole, a silver salt of  2 -(2-ethylglycolamido)benzothiazole, 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)]. 
     Furthermore, a silver salt of a compound containing an imino group can be used, particularly in admixture with silver behenate. 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.). 
     It is also convenient to use silver half soaps. A preferred example of a silver half soap is an equimolar blend of an organic silver carboxylate and the corresponding 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 carboxylic acid and analyzing for about 22% silver, can be used. 
     The methods used for making silver soap dispersions 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. 
     It is particularly preferred that the non-photosensitive source of reducible silver ions be provided in the form of an aqueous nanoparticulate dispersion of silver salt particles (such as silver carboxylate particles). The silver salt particles in such dispersions generally have a weight average particle size of less than 1000 nm when measured by any useful technique such as sedimentation field flow fractionation, photon correlation spectroscopy, or disk centrifugation. Obtaining such small silver salt particles can be achieved using a variety of techniques that are described in the copending applications identified in the following paragraphs, but generally they are achieved using high speed milling using a device such as those manufactured by Morehouse-Cowles and Hochmeyer. The details for such milling are well known in the art. 
     Such dispersions also advantageously include a surface modifier so the silver salt can more readily be incorporated into aqueous-based photothermographic formulations. Useful surface modifiers include, but are not limited to, vinyl polymers having an amino moiety, such as polymers prepared from acrylamide, methacrylamide, or derivatives thereof, as described in U.S. Pat. No. 6,391,537 (Lelental et al.), incorporated herein by reference. A particularly useful surface modifier is dodecylthiopolyacrylamide that can be prepared as described in the noted copending application using the teaching provided by Pavia et al.,  Makromoleculare Chemie , 193(9), 1992, pp. 2505-17. 
     Other useful surface modifiers are phosphoric acid esters, such as mixtures of mono- and diesters of orthophosphoric acid and hydroxy-terminated, oxyethylated long-chain alcohols or oxyethylated alkyl phenols as described for example in U.S. Pat. No. 6,387,611 (Lelental et al.), incorporated herein by reference. Particularly useful phosphoric acid esters are commercially available from several manufacturers under the trademarks or tradenames EMPHOS™ (Witco Corp.), RHODAFAC (Rhone-Poulenc), T-MULZ® (Hacros Organics), and TRYFAC (Henkel Corp./Emery Group). 
     Such dispersions contain smaller particles and narrower particle size distributions than dispersions that lack such surface modifiers. Particularly useful nanoparticulate dispersions are those comprising silver carboxylates such as silver salts of long chain fatty acids having from 8 to 30 carbon atoms, including, but not limited to, silver behenate, silver caprate, silver hydroxystearate, silver myristate, silver palmitate, and mixtures thereof. Silver behenate nanoparticulate dispersions are most preferred. These nanoparticulate dispersions can be used in combination with the conventional silver salts described above, including but not limited to, silver benzotriazole, silver imidazole, and silver benzoate. 
     The one or more non-photosensitive sources of reducible silver ions are preferably present 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 layer. 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/m 2  of the dry photothermographic material, and preferably from about 0.01 to about 0.05 mol/m 2  of that material. 
     The total amount of silver (from all silver sources) in the photothermographic materials of this invention is generally at least 0.002 mol/m 2  and preferably from about 0.01 to about 0.05 mol/m 2 . For thermographic materials of this invention, the total amount of silver (from the organic silver salts) is at least 0.002 mol/m 2  and preferably from about 0.01 to about 0.05 mol/m 2 . 
     Reducing Agents 
     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, catechol, 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.). 
     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 of reducing agents described below. 
     Hindered phenol reducing agents are preferred (alone or in combination with one or more high-contrast co-developing agents and co-developer contrast enhancing 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 (that is bisphenols), hindered phenols, and hindered naphthols, each of which may be variously substituted. 
     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. 
     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′-dichloro-biphenyl, 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). 
     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). 
     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), 1,1′-bis(3,5-di-t-butyl-4-hydroxyphenyl)methane, 2,2′-bis(4-hydroxy-3-methylphenyl)propane, 4,4′-etbylidene-bis(2-t-butyl-6-methylphenol), 2,2′-isobutylidene-bis(4,6-dimethylphenol) (LOWINOX 221B46), and 2,2′-bis(3,5-dimethyl-4-hydroxyphenyl)propane. For additional compounds see U.S. Pat. No. 5,262,295 (noted above). 
     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. 
     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). 
     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 polybydroxybenzene 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-aminohexose reductone and anhydrodihydro-piperidone-hexose reductone), sulfonamidophenol reducing agents (such as 2,6-dichloro-4-benzenesulfonamido-phenol, and p-benzenesulfonamidophenol), indane-1,3-diones (such as 2-phenylindane-1,3-dione), 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), ascorbic acid derivatives (such as 1-ascorbylpalmitate, ascorbylstearate and unsaturated aldehydes and ketones), and 3-pyrazolidones. 
     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.). All of these patents are incorporated herein by reference. 
     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 carboxaldehydes, 5-(hydroxymethylene)-2,2-dimethyl-1,3-dioxane-4,6-diones, 5-(hydroxymethylene)-1,3-dialkylbarbituric acids, and 2-(ethoxymethylene)-1H-indene-1,3(2H)-diones. 
     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.), 2-substituted malondialdehyde compounds as described in U.S. Pat. No. 5,654,130 (Murray), and 4-substituted isoxazole compounds as described in U.S. Pat. No. 5,705,324 (Murray). Additional developers are described in U.S. Pat. No. 6,100,022 (Inoue et al.). All of the patents above are incorporated herein by reference. 
     Yet another class of co-developers includes substituted acrylonitrile compounds that are described in U.S. Pat. No. 5,635,339 (Murray) and U.S. Pat. No. 5,545,515 (Murray et al.), both incorporated herein by reference. Examples of such compounds include, but are not limited to, the compounds identified as HET-01 and HET-02 in U.S. Pat. No. 5,635,339 (noted above) and CN-01 through CN-13 in U.S. Pat. No. 5,545,515 (noted above). Particularly useful compounds of this type are (hydroxymethylene)cyanoacetates and their metal salts. 
     In some embodiments, one or more non-crystalline or amorphous hindered phenols can be used. Most hindered phenols used as reducing agents in thermally developable materials are naturally crystalline materials, and when incorporated as solid-particle dispersions, they retain their crystalline nature. Such hindered phenols can be rendered non-crystalline or amorphous. By “non-crystalline”, we mean that the reducing agent composition exhibits no birefringence when examined by optical microscopy using polarized light. 
     The hindered phenol reducing agents 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), bisphenols (that is dihydroxybiphenyls), bis(hydroxynaphthyl)methanes, bis(hydroxyphenyl)methanes, and hindered naphthols each of which may be variously substituted, many of which are described in U.S. Pat. No. 3,094,417 (Workman) and U.S. Pat. No. 5,262,295 (Tanaka et al.), both incorporated herein by reference. 
     Particularly useful mixtures of hindered phenols are mixtures of bisphenols. One particularly useful mixture includes the non-crystalline forms of both 2,2′-(2-methylpropylidene)bis(4,6-dimethylphenol) and 2,2′-(3,5,5-trimethylhexylidene)bis(4,6-dimethylphenol). 
     While the non-crystalline form of hindered phenols can be obtained in any conventional manner, in preferred embodiments, they are provided in what are known as “evaporated dispersions” that have reduced the likelihood of crystallization during and after coating. Such dispersions are prepared by dissolving two or more crystalline hindered phenol silver ion reducing agents in one or more “low boiling” organic solvents to provide a solvent solution. By “low boiling” organic solvents is meant solvents that have a boiling point less than 150° C. under atmospheric pressure. Examples of such solvents include, but are not limited to, lower alkyl acetates (such as methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, and butyl acetates), lower alkyl propionates (such as methyl propionate and ethyl propionate), chlorinated hydrocarbons (such as carbon tetrachloride, sym-dichloroethylene, trichloroethylene, 1,2-dichloropropane, and chloroform), amyl chloride, diethyl carbonate, ketones (such as diethyl ketone, methyl ethyl ketone, methyl n-propylketone, and diethyl ketone), diisopropyl ether, cyclohexane, methylcyclohexane, ligroin, benzene, toluene, xylene, nitromethane, and other water-immiscible organic solvents that would be readily apparent to one skilled in the art. 
     Low boiling water-miscible organic solvents can also be used. These include, but are not limited to, alcohols (such as methanol, ethanol, and isopropanol), dimethylsulfoxide, tetrahydrofuran, N-methyl-pyrrolidone, dioxane, acetone, butyrolactone, ethylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, glycerol, acetonitrile, formamide, N,N-dimethylformamide, tetrahydrothiophene dioxide, and dimethoxyethane. Other useful solvents are described in U.S. Pat. No. 4,430,421 (Van de Sande et al.) and references cited therein. Ethyl acetate is the most preferred low boiling organic solvent. Generally, up to 50 weight % of the crystalline hindered phenols is dissolved in the one or more low boiling solvents at the beginning of this process. 
     The hindered phenols described herein can be dissolved within the one or more low boiling organic solvents at any suitable temperature from room temperature up to the boiling point of the low boiling organic solvents. 
     The non-crystalline reducing agent composition may also include one or more “permanent” high boiling organic solvents as long as they comprise less than 50 volume % of the total composition solvent volume. Preferably, the compositions of this invention comprise less than 10 volume % of such “permanent” high boiling organic solvents and more preferably, they include no “permanent” high boiling organic solvents. Such solvents generally have a boiling point greater than 150° C. and are also known in the art as “oil-formers” as described for example in U.S. Pat. No. 4,430,421 (noted above). This patent is incorporated herein by reference for its listing (Col. 9) of representative “oil-formers” or “permanent” organic solvents. 
     In some instances, the reducing agent composition also includes a contrast enhancing agent. Examples of useful contrast enhancing agents include, but are not limited to, hydroxylamines (including hydroxylamine and alkyl- and aryl-substituted derivatives thereof), 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.). All of the above patents are incorporated herein by reference. 
     In other embodiments, ascorbic acid reducing agents are preferred. An “ascorbic acid” reducing agent (also referred to as a developer or developing agent) means ascorbic acid, complexes thereof, and derivatives thereof. Ascorbic acid developing agents are described in a considerable number of publications in photographic processes, including U.S. Pat. No. 5,236,816 (Purol et al.) and references cited therein. 
     Useful ascorbic acid developing agents include ascorbic acid and the analogues, isomers, complexes, and derivatives thereof. Such compounds include, but are not limited to, D- or L-ascorbic acid, 2,3-dihydroxy-2-cyclohexen-1-one, 3,4-dihydroxy-5-phenyl-2(5H)-furanone, sugar-type derivatives thereof (such as sorboascorbic acid, γ-lactoascorbic acid, 6-desoxy-L-ascorbic acid, L-rhamnoascorbic acid, imino-6-desoxy-L-ascorbic acid, glucoascorbic acid, fucoascorbic acid, glucoheptoascorbic acid, maltoascorbic acid, L-arabosascorbic acid), sodium ascorbate, niacinamide ascorbate, potassium ascorbate, isoascorbic acid (or L-erythroascorbic acid), and salts thereof (such as alkali metal, ammonium or others known in the art), endiol type ascorbic acid, an enaminol type ascorbic acid, a thioenol type ascorbic acid, and an enamin-thiol type ascorbic acid, as described for example in U.S. Pat. No. 5,498,511 (Yamashita et al.), EP 0 585,792A1 (Passarella et al.), EP 0 573 700A1 (Lingier et al.), EP 0 588 408A1 (Hieronymus et al.), U.S. Pat. No. 5,089,819 (Knapp), U.S. Pat. No. 5,278,035 (Knapp), U.S. Pat. No. 5,384,232 (Bishop et al.), U.S. Pat. No. 5,376,510 (Parker et al.), Japanese Kokai 7-56286 (Toyoda), U.S. Pat. No. 2,688,549 (James et al.), and  Research Disclosure , publication 37152, March 1995. D-, L-, or D,L-ascorbic acid (and alkali metal salts thereof) or isoascorbic acid (or alkali metal salts thereof) are preferred. Sodium ascorbate and sodium isoascorbate are most preferred. Mixtures of these developing agents can be used if desired. 
     The reducing agent composition of this invention is generally present in an amount of from about 5 to about 30% (dry weight) of an emulsion layer. In multilayer constructions, if the reducing agents are added to a layer other than an emulsion layer, slightly higher amounts may be used. Any contrast enhancing agents are present in conventional amounts. 
     Other Addenda 
     The photothermographic materials of this invention preferably include one or more water-soluble or water-dispersible antifoggants that have a pKa of 8 or less. In addition, they are represented by the following Structure I: 
     
       
         R 1 —SO 2 —C(R 2 )R 3 —(CO) m —(L) n —SG  I 
       
     
     wherein R 1  is a substituted or unsubstituted aliphatic or cyclic group of any size as long as the antifoggant remains soluble or readily dispersible in water. Substituted or unsubstituted aliphatic groups for R 1  include monovalent groups having 1 to 20 carbon, nitrogen, sulfur, and oxygen atoms in the chain including, but not limited to, chains that include one or more substituted or unsubstituted alkyl groups (having 1 to 10 carbon atoms), substituted or unsubstituted alkenylene groups (having 2 to 20 carbon atoms), substituted or unsubstituted alkylenearylene groups (having 7 to 20 carbon atoms in the chain), and combinations of any of these groups, as well as combinations of these groups that are connected with one or more amino, amido, carbonyl, sulfonyl, carbonamido, sulfonamido, thio, oxy, oxycarbonyl, oxysulfonyl, and other connecting groups that would be readily apparent to one skilled in the art. The various types of useful aliphatic groups would be readily apparent to one skilled in the art. 
     Preferred aliphatic groups for R 1  include substituted or unsubstituted t-butyl and trifluoromethyl groups. 
     R 1  can also be substituted or unsubstituted cyclic groups including substituted or unsubstituted carbocyclic aryl groups (having 6 to 14 carbon atoms to form the cyclic ring), substituted or unsubstituted cycloalkylene groups (having 5 to 10 carbon atoms to form the cyclic ring) and heterocyclic groups (having 5 to 10 carbon, nitrogen, sulfur, or oxygen atoms to form the cyclic ring), both aromatic and nonaromatic. The various types of cyclic groups would be readily apparent to one skilled in the art. 
     Preferred cyclic groups for R 1  include substituted or unsubstituted aryl groups having 6 to 10 carbon atoms to form the cyclic ring. Substituted or unsubstituted phenyl groups are most preferred. Methyl groups are preferred substituents on the phenyl group. 
     More preferably, R 1  is 4-methylphenyl, phenyl, trifluoromethyl, adamantyl, or tertiary butyl. 
     In Structure I, R 2  and R 3  are independently hydrogen or bromine as long as one of them is bromine. Preferably, both R 2  and R 3  are bromine. 
     In addition, L is a substituted or unsubstituted aliphatic divalent linking group that can have the same definition as R 1  except that L is divalent. Thus, one skilled in the art would be able to determine suitable L groups that would serve the desired purpose while maintaining compound water solubility or dispersibility. Preferably, L is an —NH-alkylene group wherein “alkylene” is substituted or unsubstituted and has 1 to 10 carbon atoms (more preferably 1 to 3 carbon atoms). 
     When m and n are each 1, L is preferably an —N(CH 3 )-alkylene- or -NH-alkylene- group. 
     Substituents on R 1  and L can be any chemical moiety that would not adversely affect the desired function of the antifoggant and can include, but are not limited to, alkyl, aryl, heterocyclic, cycloalkyl, amino, carboxy, hydroxy, phospho, sulfonamido, sulfo, and other groups that would be readily apparent to one skilled in the art. The number of substituents is limited only by the number of available valences (available hydrogen atoms). Alkyl groups are preferred substituents for cyclic R 1  groups. However, as would be apparent, the antifoggants can have multiple sulfo, carboxy, phospho, and sulfonamido groups that impart water solubility to the molecule. 
     Further, in Structure I, m and n are independently 0 or 1, and preferably, both are 1. 
     SG can be any solubilizing group having a pKa of 8 or less that does not interfere with its antifogging activity. SG may be in the free acid form or it may be a salt, particularly a suitable metal salt (for example, an alkali metal salt) or ammonium ion salt. Preferably, SG is a salt. When SG is in its free acid form, the salt can be generated in situ by neutralization with any basic material commonly used by one skilled in the art. Preferably, SG is a carboxy, phospho, sulfo, or sulfonamido group. When SG is a sulfonamido group, it may be —SO 2 N − COR 4 M + , or —NSO 2 R 4 M +  wherein R 4  is a substituted or unsubstituted aliphatic or cyclic group as defined from R 1 . R 1  and R 4  can be the same or different group. More preferably, SG is a carboxy or sulfo group (or salts thereof), particularly when both m and n are 1. 
     M +  is a suitable cation such as hydrogen or a metal cation (preferably an alkali metal cation) or an ammonium ion. When M +  is a hydrogen atom, the resulting free acid can be easily solubilized by neutralization with a suitable base such as for example, potassium hydroxide or sodium bicarbonate. 
     In preferred embodiments, when m and n are both 0, SG is carboxy (or a salt thereof), sulfo (or a salt thereof), phospho (or a salt thereof), —SO 2 N 31  COR 4 M + , or —NSO 2 R 4 M +  wherein M +  is as defined above. 
     Additionally, when m and n are both 1, SG is carboxy (or a salt thereof), sulfo (or a salt thereof), phospho (or a salt thereof), or —SO 2 N 31  COR 4 M +  wherein M +  is as defined above. 
     Moreover, when m is 1 and n is 0, SG is carboxy (or a salt thereof), sulfo (or a salt thereof), phospho (or a salt thereof), or —N 31  SO 2 R 4 M +  wherein M +  is as defined above. 
     These preferred antifoggants can be prepared by techniques described in U.S. Pat. No. 6,514,678 (Burgmaier et al). For example, where m is I (and n is 0 or 1), the compounds can be prepared by reacting a salt of a sulfinic acid with a 2-bromomethylcarbonyl derivative, followed by bromination of the resulting sulfone using molecular bromine or another suitable brominating agent. 
     The antifoggants can be used individually or in combination in the photothermographic materials of this invention. Generally, they are present in an amount of at least 0.0001 mol/mol of total silver. Preferably, they are present in an amount of from about 0.001 to about 0.1 mol/mol of total silver. 
     Preferably, the antifoggants are included in the one or more photothermographic emulsion layers, but during manufacture, they can also be incorporated into interlayers, underlayers, and protective topcoat layers on the frontside of the support. If they are placed in a non-emulsion layer, they tend to migrate into the emulsion layer(s) where they become effective in reducing D min . 
     The thermographic and photothermographic materials of the present invention can also contain other additives, where appropriate, such as shelf-life stabilizers, toners, additional antifoggants besides those described above, 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. 
     The materials can be further protected against the production of fog and can be stabilized against loss of sensitivity during storage. It may be advantageous to add mercury (II) salts to the emulsion layer(s) as an antifoggant. Preferred mercury (II) 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). 
     Other suitable optional 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.). 
     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 U.S. Pat. No. 6,171,767 (Kong et al.). 
     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. 
     Other optional 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 —SO 2 CBr 3  groups 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 U.S. Pat. No. 6,143,487 (Philip, Jr. et al.), diisocyanate compounds as described in EP 0 600 586A1 (Philip, Jr. et al.), and tribromomethylketones as described, for example, in EP 0 600 587A1 (Oliff et al.). 
     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 thermographic or photothermo graphic 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-Gevaert). 
     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 hexaaminecobalt (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 bexachlororhodate (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-aminopyrimidine 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]. 
     Phthalazine and phthalazine derivatives [such as those described in U.S. Pat. No. 6,146,822 (noted above), incorporated herein by reference] and cyclic imides such as succinimide are particularly useful toners. 
     Binders 
     The photosensitive silver halide (if present), the non-photosensitive source of reducible silver ions, the non-crystalline reducing agent composition, and any other additives used in the present invention are incorporated into a mixture of two or more binders that are predominantly hydrophilic in nature. By “predominantly” is meant that at least 70% by weight of the total binders are hydrophilic in nature. The amount of hydrophilic binders no greater than 99% by weight (based on total binder weight). The remaining of the binder weight is comprised of one or more binders that are hydrophobic in nature or water-dispersible polymer or latex. Preferably, the amount of hydrophilic binders is from about 75 to about 95%, and more preferably from about 90 to about 95%, based on total binder weight. As described in more detail below, the formulations for the emulsion layers are intended to be prepared and coated out of aqueous coating solvents (meaning water and mixtures of water and water-miscible solvents where water is the predominant solvent). 
     Useful hydrophilic binders in the various layers (especially emulsion layers) include, but are not limited to, proteins and protein derivatives, “gelatins” such as gelatin and gelatin-like derivatives (hardened or unhardened, including alkali- and acid-treated gelatins, acetylated gelatin, oxidized gelatin, phthalated gelatin, and deionized gelatin), poly(silicic acid), acrylamide/methacrylamide polymers, acrylic/methacrylic polymers, polyvinyl pyrrolidones, poly(vinyl acetates), poly(vinyl alcohols), poly(vinyl lactams), polymers of sulfoalkyl acrylate and methacrylates, hydrolyzed poly(vinyl acetates), and polysaccharides (such as dextrans and starch ethers) and other synthetic or naturally-occurring vehicles that are hydrophilic in nature (as defined above) (see for example,  Research Disclosure , Item 38957). Gelatin and gelatin derivatives and poly(vinyl alcohols) are most preferred binders while gelatin and gelatin derivatives are most preferred. 
     Examples of typical hydrophobic binders include, but are not limited to, poly(vinyl acetals), poly(vinyl chloride), poly(vinyl acetate), cellulose acetate, cellulose acetate butyrate, cellulose esters and derivatives such as hydroxypropyl methyl cellulose or hydroxyethylcellulose, polystyrenes, polyacrylonitrile, polycarbonates, maleic anhydride ester copolymers, and other materials that are hydrophobic in nature (as defined above). Copolymers (including terpolymers) are also included in the definition of polymers. The poly(vinyl acetals) [such as poly(vinyl butyral) and poly(vinyl formal)] and vinyl copolymers [such as poly(vinyl acetate) and poly(vinyl chloride)] are particularly preferred. Particularly suitable binders are poly(vinyl butyral) resins that are available as BUTVAR® B79 (Solutia, Inc.), Pioloform BS-18, or Pioloform BL-16 (Wacker Chemical Company) and KSW (Sekesui) and cellulose esters. These materials may be added to the system as water based dispersions. 
     The binder mixture can also include up to 30 weight % of a water-dispersible polymer or latex that is prepared using conventional dispersion or emulsion polymerization techniques in which the appropriate ethylenically unsaturated polymerizable monomers are dispersed with polymerization catalyst in water. Generally, the resulting polymer latex particles have a mean particle size of from about 1 to 50,000 nm and the polymers can have a molecular weight (Mn) generally from about 5,000 to 1,000,000. Polymers comprising the polymer latex include, but are not limited to, acrylic resins, methacrylic resins, polyester resins, vinyl acetate resins, rubbery resins (for example, styrene-butadiene resins), polyurethanes resins, vinyl chloride resins, vinylidene chloride resins, polyolefin resins. Examples of such polymer latexes are described in U.S. Pat. No. 6,071,687 (Yamashita), U.S. Pat. No. 6,132,949 (Fujita et al.), EP 0 883 022A1 (Tsuzaki et al.), all incorporated herein by reference. 
     Particularly useful polymers latexes include poly(styrene-co-butadiene) and a core-shell latex of poly(styrene-co-butyl acrylate-co-2-acrylamido-2-methylpropane-1-sulfonic acid, sodium salt) as the core and poly(styrene-co-2-hydroxyethyl methacrylate-co-2-acrylamido-2-methylpropane-1-sulfonic acid, sodium salt) as the shell. 
     Particularly useful polymer dispersions include dispersions of polyurethanes, polyesters, and poly(vinyl butyral). 
     Hardeners for various hydrophilic binders may be present if desired. Useful hardeners are well known and include diisocyanate compounds as described for example in EP 0 600 586B1, vinyl sulfone compounds as described in U.S. Pat. No. 6,143,487 (Philip, Jr. et al), and aldehydes and various other hardeners as described in U.S. Pat. No. 6,190,822 (Dickerson et al.). 
     Where the proportions and activities of the photothermographic materials require a particular developing time and temperature, the binder mixture should be able to withstand those conditions. Generally, it is preferred that the binders be resistant to decomposition or loss of structural integrity at 120° C. for 60 seconds. It is more preferred that they not be decomposed or lose their structural integrity at 177° C. for 60 seconds. 
     The binder mixture 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, the binder mixture 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. 
     Particularly useful binder mixtures include a mixture of gelatin or a gelatin derivative with poly(styrene-co-butadiene) and a core-shell latex of poly(styrene-co-butyl acrylate-co-2-acrylamido-2-methylpropane-1-sulfonic acid, sodium salt) as the core and poly(styrene-co-2-hydroxyethyl methacrylate-co-2-acrylamido-2-methylpropane-1-sulfonic acid, sodium salt) as the shell. 
     In other embodiments, mixtures of such hydrophilic binders as gelatin or a gelatin derivative with a water-soluble polyvinyl butyral or water-soluble cellulose ester (such as a hydroxypropyl methyl cellulose) can be used. 
     Support Materials 
     The thermographic and photothermographic materials of this invention can be prepared using 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 poly(ethylene terephthalate) and poly(ethylene naphthalate)], cellulose acetate and other cellulose esters, poly(vinyl 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. Poly(ethylene terephthalate) film is a particularly useful support. Various support materials are described, for example, in  Research Disclosure , August 1979, item 18431. A method of making dimensionally stable polyester films is described in  Research Disclosure , September, 1999, item 42536. 
     Opaque supports can also be used such as dyed polymeric films and resin-coated papers that are stable to high temperatures. 
     Support materials can contain various colorants, pigments, antihalation or acutance dyes if desired. Blue-tinting pigments or dyes are particularly useful. 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. 
     Formulations and Construction 
     The formulations for the emulsion layer(s) can be prepared by dissolving and dispersing the binder mixture, the emulsion components, the reducing agent composition, and optional addenda in an aqueous solvent that includes water and possibly minor amounts (less than 50 volume %) of a water-miscible solvent (such as acetone or a lower alcohol) to provide aqueous-based coating formulations. Preferably, at least 95 % of the solvent volume is composed of water. 
     The non-imaging layers of the thermally developable materials (both frontside and backside) of this invention can also be coated out of various solvents. Preferably, they are formulated and coated out of aqueous solvents also. 
     The 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). 
     EP 0 792 476A1 (Geisler et al.) describes various means of modifying 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. 
     The thermographic and photothermographic materials of this invention 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.), fluorinated compounds such as those described in EP 0 663 612A1 (Gomez et al.) and electrically-conductive metal-containing particles dispersed in a polymeric binder such as those described in EP 0 678 776A1 (Melpolder et al.). Other antistatic agents are well known in the art. 
     The thermographic and photothermographic materials can be constructed of one or more layers on a support. Single layer materials should contain the photosensitive silver halide, the non-photosensitive source of reducible silver ions, the non-crystalline reducing agent composition, the hydrophilic binder, as well as optional materials such as toners, acutance dyes, coating aids, and other adjuvants. 
     Two-layer constructions comprising a single imaging layer coating containing all the ingredients and a protective topcoat are generally found in the thermographic or photothermographic materials of this invention. However, two-layer constructions containing photosensitive silver halide and non-photosensitive source of reducible silver ions in an emulsion layer (usually the layer adjacent to the support) and the reducing agent composition and other ingredients in a different layer or distributed between both layers are also envisioned. Generally, the multiple layers are coated out of an aqueous solvent as described above. Thus, where the thermally developable materials comprise protective overcoat and/or antihalation layers, they are generally coated as aqueous formulations. 
     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.). 
     Protective overcoats or topcoats can also be present over the one or more emulsion layers. The overcoats are generally transparent are composed of one or more film-forming hydrophilic binders such as poly(vinyl alcohol), gelatin (and gelatin derivatives), and poly(silicic acid). A combination of poly(vinyl alcohol) and poly(silicic acid) is particularly useful. Such layers can further comprise matte particles, plasticizers, and other additives readily apparent to one skilled in the art. The protective layer can also be a backing layer (such as an antihalation layer) that is on the backside of the support. 
     Preferred photothermographic materials of this invention comprise a protective overcoat on the imaging side, an antihalation layer on the backside, or both. 
     The emulsions and other 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), all incorporated herein by reference. 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. 
     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 (Yonkoski et al.) or by using particular drying techniques as described, for example, in U.S. Pat. No. 5,621,983 (Ludemann et al.). 
     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. 
     While the first and second layers can be coated on one side of the film support, the manufacturing 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. 
     To promote image sharpness, photothermographic materials of this 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 antibalation 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 contain an antihalation coating on the support opposite to the side on which the emulsion and topcoat layers are coated. 
     Dyes particularly useful as antihalation and acutance dyes include dihydroperimidine squaraine dyes. Details of such dyes 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 dihydropyrimidine squaraine dye is cyclobutenediylium, 1,3-bis[2,3-dihydro-2,2-bisf[1-oxohexyl)oxy]methyl]-1H-pyrimidin-4-yl]-2,4-dihydroxy-, bis(inner salt). 
     Dyes particularly useful as antihalation dyes in a backside layer of the photothermographic material also include indolenine cyanine dyes as described in EP 0 342 810A1 (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. 
     Heat-bleachable compositions can be used in backside layers as antihalation compositions. Under practical conditions of use, such compositions are heated to provide bleaching at a temperature of at least 90° C. for at least 0.5 seconds. Preferably, bleaching is carried out at a temperature of from about 100° C. to about 200° C. for from about 5 to about 20 seconds. Most preferred bleaching is carried out within 20 seconds at a temperature of from about 110° C. to about 130° C. 
     Useful heat-bleachable antihalation compositions can include an infrared radiation absorbing compound such as an oxonol dyes and various other compounds used in combination with a hexaarylbiimidazole (also known as a “HABI”), or mixtures thereof. Such HABI compounds are well known in the art, such as U.S. Pat. No. 4,196,002 (Levinson et al.), U.S. Pat. No. 5,652,091 (Perry et al.), and U.S. Pat. No. 5,672,562 (Perry et al.), all incorporated herein by reference. Examples of such heat-bleachable compositions are described for example in commonly assigned U.S. Pat. Nos. 6,558,880 (Goswami et al.) and 6,514,677 (Ramsden et al.). 
     Other antihalation compositions (such as dyes) that decolorize with heat during processing are described for example in 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.). 
     Imaging/Development 
     While the thermographic and photothermographic materials of this 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 therrnographic 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 190 to about 850 nm (preferably from about 400 to about 850 nm). 
     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. Generally, imaging is carried out at a wavelength of from about 600 to about 1150 nm to which the photothermographic material is sensitive. 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.). 
     For using the photothermographic materials, 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. 
     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. 
     When a thermographic material is imaged, the image may be created (and developed) merely by imagewise heating at the above noted temperatures using a thermal stylus, laser, or thermal print head, or by heating while in contact with a heat absorbing material. 
     Thermographic materials of this invention may also include a dye to facilitate direct imaging and development by exposure to laser radiation. Preferably, the dye is an infrared absorbing dye and the laser is a diode laser emitting in the infrared region of the electromagnetic spectrum. Upon exposure to radiation, the dye converts radiation to heat to develop an image. 
     Methods and Materials for the Examples 
     All materials used in the following examples are readily available from standard commercial sources or prepared using known procedures and starting materials unless otherwise specified. All percentages are by weight unless otherwise indicated. 
     Infrared Spectral Sensitizing IR Dye 1 is                    
     Dye deaggregant (Deag-1) is 2,2′-(1,2-ethenediyl)bis(5-((4-chloro-6-((2-chlorophenyl)amino)-1,3,5-triazin-2-yl)amino)benzenesulphonic acid, disodium salt.                    
     Antifoggant AF-1 is 2,2′-dibromo-(4-methylphenyl)sulfonyl-N-(2-sulfoethyl)acetamide, potassium salt, and has the following structure:                    
     Antifoggant AF-1 can be prepared as follows: 
     To a 5-liter flask equipped with a mechanical stirrer and reflux condenser was added p-toluenesulfinic acid, lithium salt (308.57 g), N-(2-sulfoethyl)-2-bromoacetamide, lithium salt (527.39 g), water (180 ml), and ethyl alcohol (3380 ml). The resulting suspension was heated to reflux. After about an hour of reflux, nearly all of the reactants had dissolved. Reflux was continued another four hours, and the solution was filtered hot through a Celite pad to remove some haziness. The solution was cooled overnight to room temperature. The solid that formed was collected and washed with 1 liter of 95% ethyl alcohol/water. The white solid was air dried and then dried at high vacuum, providing 553.88 g (89% yield) of 2-(4-methylphenyl)sulfonyl-N-(2-sulfoethyl)acetamide, lithium salt (Intermediate 1). HPLC analysis showed no detectable impurities. Ion chromatography indicated 0.035 weight % bromide and 1.8 weight % lithium. The material exhibited an acceptable proton spectrum. 
     To glacial acetic acid (660 ml) was added Intermediate 1 (98.19 g), and 1,3-dibromo-5,5-dimethylhydantoin (42.89 g). The resulting suspension was heated to reflux where solution occurred. After about 3-5 minutes at reflux, the slight bromine color was discharged, and reflux was continued to another 15 minutes. Analysis of the reaction mixture by HPLC indicated conversion to one main product. After cooling to near room temperature, most of the acetic acid was removed on the rotary film evaporator using a water aspirator (water bath temperature at 40° C.). The residue was diluted with 2500 ml of ethyl alcohol. Complete solution occurred after stirring the suspension for one hour at room temperature. To this stirring solution at room temperature was added drop-wise a solution of potassium acetate (58.88 g) dissolved in ethyl alcohol (500 ml). A white solid formed immediately. Upon complete addition of the potassium acetate solution, the suspension was stirred at room temperature for 90 minutes, and the desired antifoggant AF-1, 2,2-dibromo-2-(4-methylphenyl)sulfonyl-N-(2-sulfoethyl)acetamide, potassium salt, was collected by filtration and washed with ethyl alcohol. The solid was then dried under high vacuum at 40° C. The yield of crude antifoggant AF-1, which had a slight odor of acetic acid, was 145.22 g (94%). 
     Two separate synthetic batches of AF-1 were made, combined, and recrystallized by dissolving 182.33 g of product in a mixture of water (85 ml) and ethyl alcohol (600 ml) while boiled, filtered hot, and adding about 7 ml water upon cooling to prevent oiling. After letting the solution stand overnight at room temperature, the desired antifoggant product was collected and washed with about 300 ml (10:1 v/v) ethyl alcohol/water mixture. The product was then air-dried and then dried under high vacuum at 40° C., providing 159.87 g of desired product. HPLC analysis indicated an assay of 99.2% of the desired component. The product exhibited the expected proton NMR spectrum and mass spectrum consistent with the AF-1 structure shown above. 
     Antifoggant AF-2 is 2-bromo-2-(4-methylphenylsulfonyl)acetamide, can be obtained using the teaching provided in U.S. Pat. No. 3,955,982 (Van Allan), and has the following structure:                    
     Reducing agent (developer) DEV-1 is 2,2′-(3,5,5-trimethylhexylidene)bis(4,6-dimethyl-phenol) and has the following structure:                    
     The following examples are representative of the present invention and its practice and are not meant to be limiting in any manner. 
    
    
     EXAMPLE 1 
     A) Preparation of Nanoparticulate Silver Behenate: 
     A reactor was initially charged with demineralized water, a 10% solution of dodecylthiopolyacrylamide surfactant (72 g), and behenic acid [46.6 g, nominally 90% behenic acid (Unichema) recrystallized from isopropanol]. The reactor contents were stirred at 150 rpm and heated to 70° C. at which time a 10.85% w/w KOH solution (65.1 g) were added to the reactor. The reactor contents were then heated to 80° C. and held for 30 minutes until a hazy solution was achieved. The reaction mixture was then cooled to 70° C. and a silver nitrate solution consisting of silver nitrate (166.7 g of 12.77% solution) was added to the reactor at a controlled rate during 30 min. The reactor contents were then held at the reaction temperature for 30 minutes, cooled to room temperature, and decanted. A nanoparticulate silver behenate dispersion (NPSBD) with a median particle size of 140 nm was obtained (3% solids). 
     B) Purifying and Concentrating NPSBD: 
     The 3% solids nanoparticulate silver behenate dispersion (12 kg) was loaded into a diafiltration/ultrafiltration apparatus (with an Osmonics model 21-HZ20-S8J permeator membrane cartridge having an effective surface area of 0.34 m 2  and a nominal molecular weight cutoff of 50,000). The apparatus was operated so that the pressure going into the perneator was 50 lb/in 2  (3.5 kg/cm 2 ) and the pressure downstream from the permeator was 20 lb/in 2  (1.4 kg/cm 2 ). The permeate was replaced with deionized water until 24 kg of permeate were removed from the dispersion. At this point the replacement water was turned off and the apparatus was run until the dispersion reached a concentration of 28% solids to provide a nanoparticulate silver behenate dispersion (NPSB). 
     C) Dyed Silver Bromoiodide Imaging Emulsion: 
     A silver bromoiodide emulsion was prepared using conventional precipitation techniques. The resulting AgBrI emulsion comprised 3 mol % iodide (based on total silver in the silver halide) cubic grains having a mean edge length of 57 nm, and gelatin (20 g/mol silver in the silver halide). 
     To prepare the dyed emulsion, 2.04 g of the AgBrI emulsion was mixed with 0.56g of a 10% solution of Olin 10G, surfactant. To this was added 1.3 g of a 0.3% dispersion of Deag-1 in water and 0.1 7g of a 0.7% solution of IR-1 in methanol. 
     D) Preparation of Comparative Sample A Photothermographic Material: 
     An imaging composition to yield 0.1 kg of liquid mixture was prepared by mixing at 40° C. an aqueous solution of deionized bone gelatin (15.7 g of 35%), water (32.2 g), and the NPSBD (36.7 g) and adjusting to pH 6.5 under PAN lighting. To this were added Antifoggant AF-1 (0.8 g of 2.5% aqueous solution), Antifoggant AF-2 (0.27 g of 20.3% by weight solid-particle dispersion prepared using conventional milling techniques), succinimide (0.6 g), an aqueous solution (1.13 g) of sodium iodide (50 g/l) and a solid-particle dispersion of reducing agent DEV-1 (9.85 g of 20.1 % by weight) that had been prepared using conventional milling techniques. After stirring the mixture for 60 minutes 4.1 g of the dyed AgBrI emulsion was added. After stirring at 40° C. for 60 min. 1.11 of 4-methylphthalic acid (0.9 g of 10% aqueous solution) was added. This final mixture was stirred at 40° C. until coating. 
     This formulation was coated onto a clear, gelatin-subbed, 0.178 mm thick poly(ethylene terephthalate) support to give a wet coverage of 99 g/m 2  to provide a Control photothermographic material outside the scope of the present invention. 
     Preparation of Invention samples B-M: 
     The process used to prepare the photothermographic material of Comparative Example 1 was repeated here except the gelatin was partially replaced with a hydrophobic binder as indicated in TABLE I. 
     This formulation was coated onto a clear, gelatin-subbed, 0.178 mm thick poly(ethylene terephthalate) support to give a wet coverage of 99 g/m 2  to provide a photothermographic material of the present invention. 
     Both Control and Inventive photothermographic materials identified above were exposed with a xenon light source for 0.001 second using a conventional Mark VI EG&amp;G sensitometer and heat processed at 122° C. for 15 seconds on a moving drum to provide a silver image in each material. The transmission density of the developed silver was measured with a conventional X-rite model 310 photographic densitometer. For unexposed areas, low values are desirable. In exposed areas, particularly mid-scale (density 1.0), no observation of crystals is also desirable. 
     The results of the following TABLE I demonstrate that the incorporation of a hydrophobic binder material in the emulsion to provide a binder mixture reduces or eliminates the formation of crystals. The percentage of hydrophobic binder refers to the % of total weight of all binders. Latex 1 was a core-shell polymer latex prepared by reacting the following monomer mixture (by weight): 47% of styrene, 47% of butyl acrylate, 2% of 2-acrylamido-2-methylpropane-1-sulfonic acid, sodium salt, and 4% of divinyl benzene to form the core and 12% of styrene, 85% of 2-hydroxyethyl-methacrylate, and 3% of 2-acrylamido-2-methylpropane-1-sulfonic acid, sodium salt to form the shell. This core-shell polymer latex was prepared using conventional techniques and starting materials. ΔDmin 30NAK-Fresh corresponds to the difference in measured density between processed unexposed areas of the film 30 days after coating and fresh-coated film. ΔDmin 60NAK-Fresh corresponds similarly to 60 days keeping differences. The photographic speed of the photothermographic emulsions and materials was also increased. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE I 
               
               
                   
               
               
                 Sample 
                   
                 Hydrophobic 
                 Crystal specks 
                 ΔDmin 
                 ΔDmin 
                 Speed at 1.0 
               
               
                 Number 
                 Hydrophobic Binder 
                 Binder (%) 
                 present 
                 30NAK-Fresh 
                 60NAK-Fresh 
                 Density 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 A 
                 None (Control) 
                 0 
                 Yes 
                 0.02 
                 0.02 
                 49 
               
               
                 (comparative) 
               
               
                 B 
                 Latex 1 
                 1 
                 No 
                 0.01 
                 0.21 
                 55 
               
               
                 C 
                 ″ 
                 3 
                 No 
                 0.01 
                 0.03 
                 55 
               
               
                 D 
                 ″ 
                 5 
                 No 
                 0.00 
                 0.03 
                 55 
               
               
                 E 
                 ″ 
                 10 
                 No 
                 0.00 
                 0.05 
                 56 
               
               
                 F 
                 Poly(styrene-co-butadiene latex), 
                 1 
                 No 
                 0.01 
                 0.12 
                 53 
               
               
                   
                 DL-242NA (Dow Chemical) 
               
               
                 G 
                 Poly(styrene-co-butadiene latex), 
                 3 
                 No 
                 0.01 
                 0.03 
                 54 
               
               
                   
                 DL-242NA (Dow Chemical) 
               
               
                 H 
                 Poly(styrene-co-butadiene latex), 
                 5 
                 No 
                 0.00 
                 0.10 
                 55 
               
               
                   
                 DL-242NA (Dow Chemical) 
               
               
                 I 
                 Poly(styrene-co-butadiene latex), 
                 10 
                 No 
                 0.01 
                 0.22 
                 54 
               
               
                   
                 DL-242NA (Dow Chemical) 
               
               
                 J 
                 Poly(styrene-co-butadiene latex), 
                 20 
                 No 
                 0.04 
                 0.63 
                 56 
               
               
                   
                 DL-242NA (Dow Chemical) 
               
               
                 K 
                 Poly(styrene-co-butadiene latex), 
                 30 
                 No 
                 0.09 
                 1.11 
                 57 
               
               
                   
                 DL-242NA (Dow Chemical) 
               
               
                 L 
                 Poly(styrene-co-butadiene latex), 
                 40 
                 No 
                 0.45 
                 2.08 
                 56 
               
               
                   
                 DL-242NA (Dow Chemical) 
               
               
                 M 
                 Sancure Polyurethane 898 
                 10 
                 No 
                 0.00 
                 0.06 
                 54 
               
               
                   
                 dispersion (B. F. Goodrich) 
               
               
                   
               
            
           
         
       
     
     This example shows that the incorporation of a small amount of polymer latex or dispersion prevents the formation of crystal defects in the developed coating and improves the photographic speed with no impact on keeping. 
     EXAMPLE 2 
     Invention samples N through Q were prepared as the Invention samples in Example 1 except a higher percent of the total binder of Latex 1 was included as indicated in TABLE II. Comparative Sample A2 was prepared as described in Example 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE II 
               
               
                   
               
               
                   
                   
                 Hydrophobic 
                 Crystal specks 
                 ΔDmin 
                 ΔDmin 
                 Speed at 1.0 
               
               
                 Sample 
                 Hydrophobic Binder 
                 Binder (%) 
                 present 
                 30NAK-Fresh 
                 60NAK-Fresh 
                 Density 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 A2 
                 None (Control) 
                 0 
                 Yes 
                 0.02 
                 0.02 
                 33 
               
               
                 (comparative) 
               
               
                 N 
                 Latex 1 
                 5 
                 No 
                 −0.01 
                 −0.01 
                 38 
               
               
                 O 
                 ″ 
                 10 
                 No 
                 0.00 
                 −0.01 
                 38 
               
               
                 P 
                 ″ 
                 15 
                 No 
                 −0.01 
                 −0.02 
                 36 
               
               
                 Q 
                 ″ 
                 20 
                 No 
                 0.00 
                 −0.01 
                 37 
               
               
                   
               
            
           
         
       
     
     This example shows that the incorporation of polymer latex prevents the formation of crystal defects in the developed coating and improves the photographic speed with no impact on keeping. 
     EXAMPLE 3 
     This example was prepared in a similar manner as described in Example 1 except the gelatin was partially replaced with a further variety of hydrophobic binders as indicated in TABLE III. The polymer latexes were prepared using conventional techniques and starting materials. 
     Latex 2 was a polymer latex prepared by reacting the following monomer mixture (by weight): 90% of butyl acrylate, 6% of 2-acrylamido-2-methylpropane-1-sulfonic acid, sodium salt, and 4% of 2-acetoacetoxyethyl methacrylate. 
     Latex 3 was a polymer latex prepared by reacting the following monomer mixture (by weight): 85% of glycidyl methacrylate and 15% of butyl acrylate. 
     Latex 4 was a polymer latex prepared by reacting the following monomer mixture (by weight): 98% of methyl methacrylate and 2% of 2-acrylamido-2-methylpropane-1-sulfonic acid, sodium salt. 
     Latex 5 was a polymer latex prepared by reacting the following monomer mixture (by weight): 95% of ethyl methacrylate and 2% of 2-acrylamido-2-methylpropane-1 sulfonic acid, sodium salt. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE III 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Crystal 
               
               
                   
                   
                   
                 Hydrophobic 
                 specks 
               
               
                   
                 Sample 
                 Hydrophobic Binder 
                 Binder (%) 
                 present 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 R 
                 AQ55 Polyester ionomer 
                 10 
                 No 
               
               
                   
                   
                 dispersion (Eastman 
               
               
                   
                   
                 Chemical) 
               
               
                   
                 S 
                 AQ55 Polyester ionomer 
                 40 
                 No 
               
               
                   
                   
                 dispersion (Eastman 
               
               
                   
                   
                 Chemical) 
               
               
                   
                 T 
                 E3 Water-soluble 
                 100 
                 No 
               
               
                   
                   
                 hydroxypropyl methyl 
               
               
                   
                   
                 cellulose (Dow 
               
               
                   
                   
                 Chemical) 
               
               
                   
                 U 
                 KW3, Water soluble 
                 10 
                 slight 
               
               
                   
                   
                 poly(vinyl butyral) 
               
               
                   
                   
                 (Sekesui) 
               
               
                   
                 V 
                 Latex 2 
                 50 
                 No 
               
               
                   
                 W 
                 Latex 3 
                 10 
                 No 
               
               
                   
                 X 
                 Latex 4 
                 50 
                 No 
               
               
                   
                 Y 
                 Latex 5 
                 50 
                 No 
               
               
                   
                 Z 
                 Latex 6 
                 50 
                 No 
               
               
                   
                 AA 
                 Poly(styrene-co- 
                 10 
                 No 
               
               
                   
                   
                 butadiene) latex, DL- 
               
               
                   
                   
                 242NA (Dow Chemical) 
               
               
                   
                 BB 
                 Poly(styrene-co- 
                 30 
                 No 
               
               
                   
                   
                 butadiene) latex, DL- 
               
               
                   
                   
                 242NA (Dow Chemical) 
               
               
                   
                 CC 
                 Poly(styrene-co- 
                 50 
                 No 
               
               
                   
                   
                 butadiene) latex, DL- 
               
               
                   
                   
                 242NA (Dow Chemical) 
               
               
                   
                 DD 
                 Poly(styrene-co- 
                 70 
                 No 
               
               
                   
                   
                 butadiene) latex, DL- 
               
               
                   
                   
                 233NA (Dow Chemical) 
               
               
                   
                 EE 
                 Poly(styrene-co- 
                 50 
                 No 
               
               
                   
                   
                 butadiene) latex, DL- 
               
               
                   
                   
                 233NA (Dow Chemical) 
               
               
                   
                 FF 
                 Sancure Polyurethane 
                 40 
                 No 
               
               
                   
                   
                 898 dispersion (B. F. 
               
               
                   
                   
                 Goodrich) 
               
               
                   
                   
               
            
           
         
       
     
     This example shows that the incorporation of a hydrophobic polymer prevents the formation of crystal defects in the developed coating. 
     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.