Patent Publication Number: US-6667148-B1

Title: Thermally developable materials having barrier layer with inorganic filler particles

Description:
FIELD OF THE INVENTION 
     This invention relates to thermally developable materials. In particular, it relates to thermographic and photothermographic materials having improved properties form the presence of a unique barrier layer containing inorganic filler particles. 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 photographic type photosensitive silver halide that is considered to be in catalytic proximity to the non-photosensitive source of reducible silver ions. Catalytic proximity requires intimate physical association of these two components either prior to or during the thermal image development process so that when silver atoms, (Ag 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 [Klosterboer,  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 
     As noted above, thermographic and photothermographic materials generally include a source of reducible silver ions for thermal development. The most common sources of reducible silver ions are the silver fatty acid carboxylates described above. Other components in such materials include a reducing agent system that usually includes a reducing agent, and optionally a toning agent in photothermographic materials (common ones being phthalazine and derivatives thereof) in one or more binders. These components are generally formulated for coating using the appropriate solvents. 
     We have found that by-products including various fatty carboxylic acids (such as behenic acid) are formed in the materials during thermal development. These fatty acid by-products as well as the reducing agent and any toner that is present can readily diffuse out of the materials during thermal development and cause debris build-up on the thermal processing equipment (such as processor drums). This may result in the processed materials sticking to the processing equipment and causing a jam in the machine, as well as scratching of the outer surface of the developed materials. 
     Various overcoats or barrier layers have been developed to address the problems noted above. For example, it is known from U.S. Pat. No. 5,422,234 (Bauer et al.) and U.S. Pat. No. 5,989,796 (Moon) to use a surface overcoat layer in photothermographic materials comprising gelatin, poly(vinyl alcohol), poly(silicic acid), or combinations of such hydrophilic materials. Water-soluble barrier layer polymers including water-soluble polyesters, as described in U.S. Pat. No. 6,352,819 (Kenney et al.). Useful film-forming barrier layer polymers having epoxy functionality are described in U.S. Pat. No. 6,250,561 (Miller et al.). Polyesters useful as barrier layers are described in U.S. Pat. No. 6,352,820 (Bauer et al.). Hydroxy-containing polymeric barrier layers are described in U.S. Pat. No. 6,420,102 (Bauer et al.). 
     Cellulose acetate polymers and their ester derivatives have been widely described as useful in overcoats of photothermographic materials. To achieve less surface deformation during thermal development, a polycarbonate copolymers has been tried as described in U.S. Pat. No. 5,536,696 (Uyttendaele et al.). Polyvinyl alcohols have been reinforced with nanoparticulate silica sols as described in U.S. Pat. No. 4,741,992 (Przezdziecki et al.). A crosslinked polyvinyl alcohol silica network is also described as an overcoat material in U.S. Pat. No. 5,759,752 (Uyttendaele et al.). 
     There remains a need for thermally developable materials that have improved suitable barrier layers that provide physical protection while inhibiting the diffusion of various chemicals out of the materials during thermal development. It would be particularly desirable to have improved thermographic and photothermographic materials that include a layer that acts as a barrier to the diffusion of fatty acids from the materials during thermal development. 
     SUMMARY OF THE INVENTION 
     The present invention provides a thermally developable material comprising a support and having thereon one or more imaging layers comprising a binder and, in reactive association, 
     a) a non-photosensitive source of reducible silver ions that includes an organic silver salt, and 
     b) a reducing agent composition for the reducible silver ions, and 
     disposed over the one or more imaging layers, a barrier layer comprising inorganic filler particles having a length to thickness ratio of from about 10 to about 1000, the inorganic filler particles being intercalated or exfoliated with a hydrophilic or water-dispersible polymer. 
     These thermally developable materials can be thermographic or photothermographic materials. 
     Thus, the present invention also provides an aqueous-based photothermographic material comprising a support and having thereon one or more imaging layers comprising a hydrophilic binder and, in reactive association, 
     a) a photosensitive silver halide, 
     b) a non-photosensitive source of reducible silver ions that includes an organic silver salt, and 
     c) a reducing agent composition for the reducible silver ions, and 
     disposed over the one of more imaging layers, a barrier layer comprising inorganic filler particles having a length to thickness ratio of from about 10 to about 1000, the inorganic filler being intercalated or exfoliated with a hydrophilic or water-dispersible polymer. 
     In preferred embodiments, an aqueous-based photothermographic material comprises a transparent support having thereon an aqueous-based photothermographic imaging layer comprising a hydrophilic binder, 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, 
     the hydrophilic binder comprising one or more of a poly(vinyl alcohol), gelatin or a gelatin derivative, or poly(vinyl pyrrolidone), 
     the photothermographic material further comprising an outermost barrier layer disposed over the imaging layer comprising inorganic filler comprising platelets of a phyllosilicate, the platelets having a length to thickness ratio of from about 20 to about 200 and being intercalated with one or more of a poly(vinyl alcohol), gelatin or a gelatin derivative, poly(ethylene oxide), or poly(vinyl pyrrolidone), the weight ratio of the platelets to the hydrophilic or water-dispersible polymer being from about 0.01:1 to about 0.1:1. 
     Further, this invention provides a method of forming a visible image comprising: 
     A) imagewise exposing the photothermographic material of this invention to electromagnetic radiation to form a latent image, and 
     B) simultaneously or sequentially, heating the exposed photothermographic material to develop the latent image into a visible image. 
     In some embodiments, the photothermographic material comprises a transparent support, and the image-forming method further comprising: 
     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. 
     In addition, another method of this invention for forming a visible image comprises imagewise heating the aqueous-based thermally developable material of this invention that is a thermographic material to provide a visible image. 
     It has been found that the particular barrier layer used in the present invention effectively inhibits (or retards) the diffusion of or reacts with fatty carboxylic acids (such as behenic acid) and other chemicals (such as developers and toners) from thermally developable imaging materials. Thus, the barrier layer reduces the buildup of debris on the processing equipment and improves imaging efficiencies and quality. The barrier layer can be the outermost layer and therefore also serve as a protective overcoat layer for the thermographic and photothermographic materials. Alternatively, the barrier layer can be interposed between the imaging layer(s) and a protective overcoat layer. Still again, a protective layer can be disposed between the barrier layer and the imaging layer(s). 
     These advantages are achieved by using an inorganic phase comprised of inorganic filler particles that are intercalated or exfoliated with a hydrophilic or water-dispersible polymer. The inorganic filler particles are preferably a clay nanoparticulate. This inorganic phase is preferably coated out of aqueous solvents but are not limited to aqueous-based coating formulations. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The 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, imagesetting 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 photosensitive printing plate) through the exposed and heat-developed photothermographic 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 photothermographic set of layers (wherein the photosensitive silver halide and the non-photosensitive 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. 
     “Platelets” refers to particles with two comparable dimensions significantly greater than the third dimension. For example, platelets are generally particles useful in this invention that have a length to thickness ratio greater than 10 (that is 10:1), and preferably greater than 20 (that is 20:1). 
     “Layered material” refers to an inorganic material (or filler), such as a smectite clay, that is in the form of a plurality of adjacent bound layers. 
     “Intercalation” refers to the insertion of one or more foreign molecules or parts of foreign molecules (such as an organic polymer) between platelets of the layered material, usually detected by X-ray diffraction technique, as illustrated in U.S. Pat. No. 5,554,670 (Giannelis et al.). 
     “Intercalant” refers to the foreign molecule inserted between platelets of the layered material. 
     “Exfoliation” refers to the separation of individual platelets into a disordered structure without any stacking order. 
     “Polymer” is meant to include oligomers, copolymers, terpolymers, and interpolymers. 
     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. 
     Barrier Layer 
     The barrier layer used in the thermally developable materials of this invention is located further from the support than the one or more thermally developable layers. Preferably, the barrier layer is the outermost layer on one or both sides of the thermally developable materials. The “barrier layer” can be a single homogeneous (uniform throughout) layer or it can be composed of multiple layers containing the same or different compositions disposed over the one or more thermally developable layers to provide a barrier “structure” or composite (having multiple strata) that serves as the physical and/or chemical barrier to the diffusion of the various chemical components (such as developers, toners, and fatty carboxylic acids as described herein) during thermal imaging and/or development. 
     The barrier layer can also act as the outermost surface protective overcoat, but in some embodiments, an additional protective overcoat layer is disposed over the barrier layer and the underlying imaging layer(s). For example, protective overcoat layer comprise common overcoat materials such as poly(vinyl butyral), cellulose acetate butyrate, poly(vinyl alcohols), poly(vinyl pyrrolidones), gelatin (and derivatives thereof), and other film-forming polymers (including copolymers) can be disposed over the barrier layer. 
     In other embodiments, a protective overcoat composed of poly(vinyl butyral), cellulose acetate butyrate, poly(vinyl alcohols), poly(vinyl pyrrolidones), gelatin (and derivatives thereof), or other film-forming polymers can be interposed between the barrier layer and the one or more underlying imaging layers. For example, the protective layer can comprise a poly(vinyl alcohol) or a mixture of a poly(vinyl alcohol) and a poly(vinyl pyrrolidone). 
     The barrier layer is generally transparent and colorless although the inorganic filler particles described below can add some opacity and/or color to the layer but the amount is insufficient to influence the imaging and sensitometric characteristics of the thermally developable materials. Thus, the barrier layer does not significantly adversely affect the imaging properties of the thermographic and photothermographic materials including such sensitometric properties as D min , D max , contrast, and photospeed. That is, haze is desirably as low as possible. 
     The optimum barrier layer dry thickness depends upon various factors including the type of imaging material, thermal and/or development imaging means, desired image, barrier layer binders, and various imaging components. Generally, the dry thickness is at least 0.5 μm, and preferably the dry thickness is from about 0.5 to about 5 μm, and more preferably from about 1.5 to about 3 μm. The upper limit to the dry thickness is dependent only upon what is practical for meeting imaging needs. In some instances, thicker barrier layers will provide desirable properties without the presence of the inorganic filler particles, but there is a desired in the industry to reduce barrier layer thickness as much as possible and that is where the present invention provides the greater benefit. 
     The barrier layer used in the thermally developable materials of the present invention comprise an inorganic phase that comprises inorganic filler particles (such as platelets of a layered material) having a length to thickness ratio (that is, aspect ratio) of from about 10 to about 1000. These particles can also be referred to as “platelets.” A preferred aspect ratio is from about 20 to about 200. The inorganic filler particles are intercalated or exfoliated with a hydrophilic or water-dispersible polymer (defined below). In addition, the longest dimension of the inorganic filler particles is no greater than 1 μm. 
     The preferred clay particles should have a lateral dimension of from about 0.01 μm to about 1 μm, and preferably from about 0.01 μm to about 0.05 μm. The thickness or the vertical dimension of the clay particles can vary but it is preferably from about 0.5 nm to about 10 nm, and more preferably from about 1 nm to about 5 nm. 
     Moreover, the weight ratio of the inorganic filler particles to the hydrophilic or water-dispersible polymer is at least 0.005:1, and preferably, the ratio is from about 0.01:1 to about 0.1:1. 
     Any useful inorganic filler particles can be used in the practice of this invention as long as they have the properties described above. Generally, the inorganic filler particles are composed of one or more “clays”. Useful clays include illites, mixed layered illite/smectite minerals, such as ledikite and admixtures of illites. Such clays are described in detail in relevant literature, such as “Clay Colloid Chemistry” by H. van Olphen, 2 nd  Ed., Publishers:John Wiley &amp; Sons, (1977). Other useful layered inorganic filler particles, particularly useful with anionic polymers, are the layered hydrotalcites or double hydroxides, such as Mg 6 Al 3.4 (OH) 18.8 (CO 3 ) 1.7 H 2 O, that have positively charged layers and exchangeable anions in the in the interlayer spaces. Yet other useful layered inorganic filler particles include chlorides such as FeCl 3 , FeOCl, chalcogenides, such as TiS 2 , MoS 2 , and MoS 3 , cyanides such as Ni(CN) 2  and oxides such as H 2 Si 2 O 5 , V 6 O 13 , HTiNbO 5 , Cr 0.5 V 0.5 S 2 , V 2 O 5 , Ag doped V 2 O 5 , W 0.2 V 2.8 O7, Cr 3 O 8 , MoO 3 (OH) 2 , VOPO 4 -2H 2 O, Zr(HPO 4 ) 2 -2H 2 O, CaPO 4 CH 3 -H 2 O, MnHAsO 4 -H 2 O, and Ag 6 Mo 10 O 33 . 
     Preferred inorganic filler particles include smectite clays such as montmorillonite, nontronite, beidellite, volkonskoite, hectorite, saponite, sauconite, sobockite, stevensite, svinfordite, halloysite, magadiite, kenyaite, and vermiculite as well as hydrotalcites, chalcogenites and oxides. 
     The useful smectite clay can be natural or synthetic. This distinction can influence the particle size and/or the level of associated impurities. Typically, synthetic clays are smaller in lateral dimension and therefore possess a smaller aspect ratio. However, synthetic clays are purer and are of narrower size distribution, compared to natural clays and may not require any further purification or separation. 
     More preferably, the inorganic filler particles useful in this invention comprise platelets of a phyllosilicate. Phyllosilicates such as those described in U.S. Pat. No. 4,739,007 (Okada et al.), U.S. Pat. No. 4,810,734 (Kawasumi et al.), U.S. Pat. No. 4,889,885 (Usuki et al.), U.S. Pat. No. 4,894,411 (Okada et al.), U.S. Pat. No. 5,102,948 (Dsguchi et al.), U.S. Pat. No. 5,164,440 (Deguchi et al.), U.S. Pat. No. 5,164,460 (Yano et al.), U.S. Pat. No. 5,248,720 (Deguchi et al.), U.S. Pat. No. 5,973,053 (Usuki et al.), U.S. Pat. No. 5,578,672 (Beall et al.), all incorporated herein by reference, are preferred inorganic filler particles useful in the practice of this invention. Phyllosilicates suitable for the present invention include smectite clay, for example, montmorillonite, particularly sodium montmorillonite, magnesium montmorillonite, and/or calcium montmorillonite, nontronite, beidellite, volkonskoite, hectorite, saponite, sauconite, sobockite, stevensite, svinfordite, vermiculite, magadiite, kenyaite, talc, mica, kaolinite, and mixtures thereof. 
     The inorganic filler particles useful in the barrier are intercalated or exfoliated with one or more hydrophilic polymers. There are many of such polymers known in the art, for example as described in U.S. Pat. No. 5,683,862 (Majumdar et al.), U.S. Pat. No. 5,891,611 (Majumdar et al.), and U.S. Pat. No. 6,060,230 (Christian et al.). The water soluble polymers can comprise polyalkylene oxides such as polyethylene oxide, poly 6, (2-ethyloxazolines), poly(ethyleneimine), poly(vinyl pyrrolidone), poly(vinyl alcohols), poly(vinyl acetate), poly(styrene sulfonate), poly(acrylamides), poly(methacrylamides), poly(N,N-dimethacrylamide), poly(N-isopropylacrylamide), polysaccharides, dextrans, and cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, and others known in the art. 
     The hydrophilic polymers can also include gelatin or gelatin grafted polymers, including alkali-treated gelatin (cattle bone or hide gelatin), acid-treated gelatin (pigskin or bone gelatin), and gelatin derivatives such as partially phthalated gelatin, acetylated gelatin, and deionized gelatins. 
     Particularly useful intercalating hydrophilic or water-dispersible polymers comprise one or more poly(vinyl alcohols), gelatin and gelatin derivatives, poly(ethylene oxide), poly(vinyl pyrrolidones), poly(carboxylic acids), a poly(sulfonic acid), poly(acrylamides), and quatemized polymers. Mixtures of these materials also can be used if desired. 
     The smectite clay of the invention can also be further treated with a compatibilizing agent. The purpose of the compatibilizing agent is to render the inorganic clay phase compatible with the binder polymer in which the clay is preferably dispersed. Typically, the compatibilizing agent comprises a component that bonds with the clay surface and another component that interacts favorably with the binder polymer. Effective compatibilization leads to a homogenous dispersion of the clay in the binder polymer. 
     The inorganic filler particles used in the present invention can be dispersed in one or more hydrophilic or hydrophobic binders. The binders can be a wide variety of as described in the “Binder” section provided below. The binder can e the same or different material as the hydrophilic or water-dispersible polymer described above. Particularly useful binders are poly(vinyl alcohols), poly(vinyl pyrrolidones), gelatin and its various derivatives, and mixtures of these materials. In preferred embodiments, the same material used for intercalation or exfoliation is used as the binder. 
     The amount of intercalated or exfoliated inorganic filler particles in the barrier layer comprises at least 0.5 weight %, and preferably from about 3 to about 10 weight % of the total dry barrier layer weight. 
     The barrier layer can also include various addenda such as surfactants, lubricants, matting agents, crosslinking agents, hardeners, photothermographic toners, acutance dyes, and other chemicals that would be readily apparent to one skilled in the art, depending upon whether the barrier layer is the outermost layer or underneath another layer. These components can be used in conventional amounts. 
     The barrier layers can be applied to other layers using any suitable technique (see coating techniques described below). The coating solvents used for coating the barrier layer can be aqueous- or organic solvent-based depending upon the type of binder materials being used. Coating solvents are also described below. However, preferred organic solvent-based barrier layer formulations include methyl ether ketone, and preferred aqueous-based barrier layer formulations include water as at least 50 volume % of the solvents. 
     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 photothermographic features of the invention may be employed without modification. However, one or more conventional chemical sensitizers may be used in the preparation of the photosensitive silver halides to increase photospeed. Such compounds may contain sulfur, tellurium, or selenium, or may comprise 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 materials are provided for example, in T. H. James,  The Theory of the Photographic Process , Fourth Edition, Eastman Kodak Company, Rochester, N.Y., 1977, Chapter 5, pp. 149-169. Suitable conventional chemical sensitization procedures are also described 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), 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. Patent 5,759,761 (Lushington et al.), U.S. Pat. No. 6,296,998 (Eikenberry et al), and EP 0 915 371 A1 (Lok et al.), all of which are incorporated herein by reference. 
     In addition, mercaptotetrazoles and tetraazindenes as described in U.S. Pat. No. 5,691,127 (Daubendiek et al.), incorporated herein by reference, can be used as suitable addenda for tabular silver halide grains. 
     When used, sulfur sensitization is usually performed by adding a sulfur sensitizer and stirring the emulsion at an appropriate temperature for a predetermined time. Various sulfur compounds can be used. Some examples of sulfur sensitizers include thiosulfates, thioureas, thioamides, thiazoles, rhodanines, phosphine sulfides, thiohydantoins, 4-oxo-oxazolidine-2-thiones, dipolysulfides, mercapto compounds, polythionates, and elemental sulfur. 
     Certain tetrasubstituted thiourea compounds are also useful in the present invention. Such compounds are described, for example in U.S. Pat. No. 6,296,998 (Eikenberry et al.), U.S. Pat. No. 6,322,961 (Lam et al.), and U.S. Pat. No. 6,368,779 (Lynch et al.). Also useful are the tetrasubstituted middle chalcogen (that is, sulfur, selenium, and tellurium) thiourea compounds disclosed in U.S. Pat. No. 4,810,626 (Burgmaier et al.). All of the above patents are incorporated herein by reference. 
     The amount of the sulfur sensitizer to be added varies depending upon various conditions such as pH, temperature and grain size of silver halide at the time of chemical ripening, it is preferably from 10 −7  to 10 −2  mole per mole of silver halide, and more preferably from 10 −6  to 10 −4  mole. 
     In one preferred embodiment, chemical sensitization is achieved by oxidative decomposition of a sulfur-containing spectral sensitizing dye in the presence of a photothermographic emulsion. Such sensitization is described in U.S. Pat. No. 5,891,615 (Winslow et al.), incorporated herein by reference. 
     Still other useful chemical sensitizers include certain selenium-containing compounds. When used, selenium sensitization is usually performed by adding a selenium sensitizer and stirring the emulsion at an appropriate temperature for a predetermined time. Some specific examples of useful selenium compounds can be found in U.S. Pat. No. 5,158,892 (Sasaki et al.), U.S. Pat. No. 5,238,807 (Sasaki et al.), U.S. Pat. No. 5,942,384 (Arai et al.) and in co-pending and commonly assigned U.S. Ser. No. 10/082,516 (filed Feb. 25, 2002 by Lynch, Opatz, Gysling, and Simpson). All of the above documents are incorporated herein by reference. 
     Still other useful chemical sensitizers include certain tellurium—containing compounds. When used, tellurium sensitization is usually performed by adding a tellurium sensitizer and stirring the emulsion at an appropriate temperature for a predetermined time. Tellurium compounds for use as chemical sensitizers can be selected from those described in  J. Chem. Soc,. Chem. Commun . 1980, 635, ibid., 1979, 1102, ibid., 1979, 645 , J. Chem. Soc. Perkin. Trans , 1980, 1, 2191 , The Chemistry of Organic Selenium and Tellurium Compounds , S. Patai and Z. Rappoport, Eds., Vol. 1 (1986), and Vol. 2 (1987), U.S. Pat. No. 1,623,499 (Sheppard et al.), U.S. Pat. No. 3,320,069 (Illingsworth), U.S. Pat. No. 3,772,031 (Berry et al.), U.S. Pat. No. 5,215,880 (Kojima et al.), U.S. Pat. No. 5,273,874 (Kojima et al.), U.S. Pat. No. 5,342,750 (Sasaki et al.), U.S. Pat. No. 5,677,120 (Lushington et al.), British Patent 235,211 (Sheppard), British Patent 1,121,496 (Halwig), British Patent 1,295,462 (Hilson et al.), British Patent 1,396,696 (Simons), JP-04-271341 A (Morio et al.), in co-pending and commonly assigned U.S. Published Application 2002-0164549 (Lynch et al.), and in co-pending and commonly assigned U.S. Ser. No. 09/923,039 (filed Aug. 6, 2001 by Gysling, Dickinson, Lelental, and Boettcher). All of the above documents are incorporated herein by reference. 
     The amount of the selenium or tellurium sensitizer used in the present invention varies depending on silver halide grains used or chemical ripening conditions. However, it is generally from 10 −8  to 10 −2  mole per mole of silver halide, preferably on the order of from 10 −7  to 10 −3  mole. 
     Noble metal sensitizers for use in the present invention include gold, platinum, palladium and iridium. Gold sensitization is particularly preferred. 
     When used, the gold sensitizer used for the gold sensitization of the silver halide emulsion used in the present invention may have an oxidation number of 1 or 3, and may be a gold compound commonly used as a gold sensitizer. U.S. Pat. No. 5,858,637 (Eshelman et al.) describes various Au (I) compounds that can be used as chemical sensitizers. Other useful gold compounds can be found in U.S. Pat. No. 5,759,761 (Lushington et al.). Useful combinations of gold (I) complexes and rapid sulfiding agents are described in U.S. Pat. No. 6,322,961 (Lam et al.). Combinations of gold (III) compounds and either sulfur- or tellurium-containing compounds are useful as chemical sensitizers and are described in U.S. Pat. No. 6,423,481 (Simpson et al.). All of the above references are incorporated herein by reference. 
     Reduction sensitization may also be used. Specific examples of compounds useful in reduction sensitization include, but are not limited to, stannous chloride, hydrazine ethanolamine, and thioureaoxide. Reduction sensitization may be performed by ripening the grains while keeping the emulsion at pH 7 or above, or at pAg 8.3 or less. 
     The chemical sensitizers can be used in making the silver halide emulsions in conventional amounts that generally depend upon the average size of the silver halide grains. Generally, the total amount is at least 10 −10  mole per mole of total silver, and preferably from about 10 −8  to about 10 −2  mole per mole of total silver. The upper limit can vary depending upon the compound(s) used, the level of silver halide, and the average grain size and grain morphology, and would be readily determinable by one of ordinary skill in the art. 
     Spectral Sensitizers 
     The photosensitive silver halides used in the photothermographic features of the invention may be spectrally sensitized with various spectral sensitizing dyes that are known to enhance silver halide sensitivity to ultraviolet, visible, and/or infrared radiation. 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. Cyanine dyes, merocyanine dyes and complex merocyanine dyes are particularly useful. Spectral sensitizing dyes are chosen for optimum photosensitivity, stability, and synthetic ease. They may be added at any stage in chemical finishing of the photothermographic emulsion. 
     Suitable sensitizing dyes such as those described in U.S. Pat. No. 3,719,495 (Lea), U.S. Pat. No. 4,396,712 (Kinoshita et al.), U.S. Pat. No. 4,439,520 (Kofron et al.), U.S. Pat. No. 4,690,883 (Kubodera et al.), U.S. Pat. No. 4,840,882 (Iwagaki et al.), U.S. Pat. No. 5,064,753 (Kohno et al.), U.S. Pat. No. 5,281,515 (Delprato et al.), U.S. Pat. No. 5,393,654 (Burrows et al), U.S. Pat. No. 5,441,866 (Miller et al.), U.S. Pat. No. 5,508,162 (Dankosh), U.S. Pat. No. 5,510,236 (Dankosh), U.S. Pat. No. 5,541,054 (Miller et al.), JP 2000-063690 (Tanaka et al.), JP 2000-112054 (Fukusaka et al.), JP 2000-273329 (Tanaka et al.), JP 2001-005145 (Arai), JP 2001-064527 (Oshiyama et al.), and JP 2001-154305 (Kita et al.), can be used in the practice of the invention. All of the publications noted above are incorporated herein by reference. A summary of generally useful spectral sensitizing dyes is contained in  Research Disclosure , item 308119, Section IV, December, 1989. Additional classes of dyes useful for spectral sensitization, including sensitization at other wavelengths are described in  Research Disclosure , 1994, item 36544, section V. 
     Teachings relating to specific combinations of spectral sensitizing dyes also include U.S. Pat. No. 4,581,329 (Sugimoto et al.), U.S. Pat. No. 4,582,786 (Ikeda et al.), U.S. Pat. No. 4,609,621 (Sugimoto et al.), U.S. Pat. No. 4,675,279 (Shuto et al.), U.S. Pat. No. 4,678,741 (Yamada et al.), U.S. Pat. No. 4,720,451 (Shuto et al.), U.S. Pat. No. 4,818,675 (Miyasaka et al.), U.S. Pat. No. 4,945,036 (Arai et al.), and U.S. Pat. No. 4,952,491 (Nishikawa et al.). All of the above publications are incorporated herein by reference. 
     Specific examples of useful spectral sensitizing dyes for the photothermographic materials of this invention include, for example, 2-[[5-chloro-3-(3-sulfopropyl)-2(3H)-benzothiazolylidene]methyl]-1-(3-sulfopropyl)-naphtho[1,2-d]thiazolium, inner salt, N,N-diethylethanamine salt (1:1), 2-[[5,6-dichloro-1-ethyl-1,3-dihydro-3-(3-sulfopropyl)-2H-benzimidazol-2-ylidene]methyl]-5-phenyl-3-(3-sulfopropyl)-benzoxazolium, inner salt, potassium salt, 5-chloro-2-[[5-chloro-3-(3-sulfopropyl)-2(3H)-benzothiazolylidene]methyl]-3-(3-sulfopropyl)-benzothiazolium, inner salt, N,N-diethylethanamine salt (1:1), and 5-phenyl-2-((5-phenyl-3-(3-sulfopropyl)-2(3H)-benzoxazolylidene)methyl)-3-(3-sulfopropyl)-benzothiazolium, inner salt, N,N-diethylethanamine salt(1:1). 
     Also useful are spectral sensitizing dyes that decolorize by the action of light or heat. Such dyes are described in U.S. Pat. No. 4,524,128 (Edwards et al.), JP Kokai 2001-109101 (Adachi), JP Kokai 2001-154305 (Kita et al.), and JP 2001-183770 (Hanyu et al.). 
     Spectral sensitizing dyes may be used singly or in combination. The dyes are selected for the purpose of adjusting the wavelength distribution of the spectral sensitivity, and for the purpose of supersensitization. When using a combination of dyes having a supersensitizing effect, it is possible to attain much higher sensitivity than the sum of sensitivities that can be achieved by using each dye alone. It is also possible to attain such supersensitizing action by the use of a dye having no spectral sensitizing action by itself, or a compound that does not substantially absorb visible light. Diaminostilbene compounds are often used as supersensitizers. 
     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. 
     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. 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 (fatty acid) 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 227 141 A1 (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 patents 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′-dichlorobiphenyl, 2-(2-hydroxy-3-t-butyl-5-methylphenyl)-4-methyl-6-n-hexylphenol, 4,4′-dihydroxy-3,3′,5,5′-tetra-t-butylbiphenyl and 4,4′-dihydroxy-3,3′,5,5′-tetramethylbiphenyl. For additional compounds see U.S. Pat. No. 5,262,295 (noted above). 
     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-ethylidene-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-thienyl-amidoxime and p-phenoxyphenylamidoxime, azines (for example, 4-hydroxy-3,5-dimethoxybenzaldehydrazine), a combination of aliphatic carboxylic acid aryl hydrazides and ascorbic acid [such as 2,2′-bis(hydroxymethyl)-propionyl-β-phenyl hydrazide in combination with ascorbic acid], a combination of polyhydroxybenzene and hydroxylamine, a reductone and/or a hydrazine [for example, a combination of hydroquinone and bis(ethoxyethyl)hydroxylamine], piperidinohexose reductone or formyl-4-methylphenylhydrazine, hydroxamic acids (such as phenylhydroxamic acid, p-hydroxyphenylhydroxamic acid, and o-alanine-hydroxamic acid), a combination of azines and sulfonamidophenols (for example, phenothiazine and 2,6-dichloro-4-benzenesulfonamidophenol), α-cyanophenylacetic acid derivatives (such as ethyl a-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. Pat. No. 6,387,605 (Lynch et al.), 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 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-dihydroky-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 can 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 − 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 − 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 − SO 2 R 4 M +  wherein M +  is as defined above. 
     These antifoggants can be prepared by using known techniques. For example, where m is 1 (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 photothermographic emulsion layer or in an adjacent layer. Toners are well known materials in the photothermographic art, as shown in U.S. Pat. No. 3,080,254 (Grant, Jr.), U.S. Pat. No. 3,847,612 (Winslow), U.S. Pat. No. 4,123,282 (Winslow), U.S. Pat. No. 4,082,901 (Laridon et al.), U.S. Pat. No. 3,074,809 (Owen), U.S. Pat. No. 3,446,648 (Workman), U.S. Pat. No. 3,844,797 (Willems et al.), U.S. Pat. No. 3,951,660 (Hagemann et al.), U.S. Pat. No. 5,599,647 (Defieuw et al.), and GB 1,439,478 (Agfa-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-dimethyl-pyrazole), 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-azolidine-dione}, phthalazine and derivatives thereof [such as those described in U.S. Pat. No. 6,146,822 (Asanuma et al.)], phthalazinone and phthalazinone derivatives, or metal salts or these derivatives [such as 4-(1-naphthyl)phthalazinone, 6-chlorophthalazinone, 5,7-dimethoxyphthalazinone, and 2,3-dihydro-1,4-phthalazinedione], a combination of phthalazine (or derivative thereof) plus one or more phthalic acid derivatives (such as phthalic acid, 4-methylphthalic acid, 4-nitrophthalic acid, and tetrachlorophthalic anhydride), quinazolinediones, benzoxazine or naphthoxazine derivatives, rhodium complexes functioning not only as tone modifiers but also as sources of halide ion for silver halide formation in situ [such as ammonium hexachlororhodate (III), rhodium bromide, rhodium nitrate, and potassium hexachlororhodate (III)], inorganic peroxides and persulfates (such as ammonium peroxydisulfate and hydrogen peroxide), benzoxazine-2,4-diones (such as 1,3-benzoxazine-2,4-dione, 8-methyl-1,3-benzoxazine-2,4-dione and 6-nitro-1,3-benzoxazine-2,4-dione), pyrimidines and asym-triazines (such as 2,4-dihydroxypyrimidine, 2-hydroxy-4-amino-pyrimidine and azauracil) and tetraazapentalene derivatives [such as 3,6-dimercapto-1,4-diphenyl-1H,4H-2,3a,5,6a-tetraazapentalene and 1,4-di-(o-chlorophenyl)-3,6-dimercapto-]H,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 one 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. Dyes in the support can be the same or different than any dyes (such as acutance dyes) provided in layers on either side of the support. 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 
     An organic-based formulation for the thermographic and photothermographic emulsion layer(s) can be prepared by dissolving and dispersing the binder, the photocatalyst (when used), the source of non-photosensitive silver ions, the reducing composition, toner(s), and optional addenda in an organic solvent, such as toluene, 2-butanone (methyl ethyl ketone), acetone, or tetrahydrofuran. 
     Alternatively and preferably, the imaging components can be formulated with a hydrophilic binder (such as gelatin, a gelatin-derivative, or a latex) in water or water-organic solvent mixtures to provide aqueous-based coating formulations. These formulations may contain 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 two or more layers on a support. Two layer materials should contain the photosensitive silver halide, the non-photosensitive source of reducible silver ions, the non-crystalline reducing agent composition, and a binder in one layer. The barrier layer and/or imaging layer can include optional materials such as toners, acutance dyes, coating aids, and other adjuvants. 
     Three-layer constructions comprising a two imaging layers coating containing all the ingredients and a barrier layer are generally found in the thermographic or photothermographic materials of this invention. However, three-layer constructions containing photosensitive silver halide and non-photo-sensitive 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 barrier layer, protective overcoat and/or antihalation layers, they are preferably 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 and barrier layer. 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 antihalation backing layer, as an antihalation underlayer, or as an antihalation overcoat. Additionally, one or more acutance dyes may be incorporated into one or more frontside layers such as the photothermographic emulsion layer, primer layer, underlayer, or topcoat layer according to known techniques. It is preferred that the photothermographic materials 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-bis[[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 0 342 810 A1 (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 copending and commonly assigned U.S. Ser. No. 09/875,772 (filed Jun. 6, 2001 by Goswami, Ramsden, Zielinski, Baird, Weinstein, Helber, and Lynch) and U.S. Ser. No. 09/944,573 (filed Aug. 31, 2001 by Ramsden and Baird), incorporated herein by reference. 
     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 0 911 693A1 (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 thermographic materials), the following discussion will be directed to the preferred imaging means for photothermographic materials. Generally, the materials are sensitive to radiation in the range of from about 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, and has the following structure:                    
     Antifoggant AF-1 is 2,2′-dibromo-(4-methylphenyl)sulfonyl-N-(2-sulfoethyl)acetamide, potassium salt. 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. 
     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). 
     Reducing agent (developer) DEV-1 is N-(4-hydroxyphenyl)-benzenesulfonamide. 
     Reducing agent (developer) DEV-2 is 2,2′-(3,5,5-trimethylhexylidene)bis(4,6-dimethyl-phenol). 
     Gelatin crosslinking agent HAR-1 is bis(vinylsulfonyl)methane. 
     The following examples are representative of the present invention and its practice and are not meant to be limiting in any manner. 
     Control A Photothermographic Material 
     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 permeator was 50 lb/in 2  (3.5 kg/cm2) 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 AgBrl emulsion was mixed with 0.56 g 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 0.17 g of a 0.7% solution of IR Dye 1 in methanol. 
     D) Preparation of Photothermogranhic Material: 
     An imaging composition to yield 0.1 kg of liquid mixture was prepared by mixing at 40° C. an aqueous solution of polyvinyl alcohol (Dupont Elvanol 52-22) (42.1 g of 7.1%), water (7.33 g), methanol (0.16 g) and the NPSBD (33.0 g) and adjusting to pH 6.5 under PAN lighting. To this were added Antifoggant AF-1 (1.47 g of 2.5% aqueous solution), Antifoggant AF-2 (2.79 g of 20.3% by weight solid-particle dispersion prepared using conventional milling techniques), succinimide (0.8 g), an aqueous solution (1.79 g) of sodium iodide (50 g/l) and a solid-particle dispersion of reducing agent DEV-1 (6.61 g of 20.1% by weight) that had been prepared using conventional milling techniques. After stirring the mixture for 60 minutes 3.1 g of the dyed AgBrI emulsion 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. 
    
    
     COMPARATIVE EXAMPLES 1-4 AND INVENTION EXAMPLES 1-12 
     The imaging layer of the Control photothermographic material was overcoated with various barrier layer formulations to provide Comparative and Invention photothermographic materials. Comparative Examples 1-4 were prepared by coating a overcoat barrier formulation comprising solely of polyvinyl alcohol (PVA, Nippon Gohsei Gohsenol GH23) as the binder material in 100% water. The coating solution was coated to yield a dry coverage as shown in TABLE II below. These materials were considered Comparative films because the overcoat barrier layer was not within the scope of the present invention. 
     Photothermographic materials of the present invention were prepared similarly except that over the dried imaging layer of the Control material was coated, from water, a solution containing an inorganic platelet like barrier material. Coatings with variable weight percent inorganic particles, relative to PVA coverage, cover a range up to 10% as shown in TABLE II. The barrier layer formulation used for the present invention is shown in the following TABLE 1. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE I 
               
               
                   
               
               
                 Inorganic 
                   
                   
                 Aspect 
                 Diameter b   
               
               
                 Particles ID 
                 Name 
                 Supplier 
                 Ratio a   
                 (Å) 
               
               
                   
               
             
            
               
                 L1 
                 Cloisite ® 
                 Southern 
                 200 
                 200 
               
               
                   
                 Na+ 
                 Clay 
               
               
                   
                   
                 Products 
               
               
                   
               
               
                   a Southern Clay Products Product Manual  
               
               
                   b measured by TEM, average size  
               
            
           
         
       
     
     Transmission electron microscopy (TEM) uses a microscope operating at 200 kV accelerating voltage (JEM-2000FX from JEOL, Inc.). 
     The solution of inorganic particle barrier formulation was prepared as follows: 
     An aqueous mixture of 3% solid concentration of platelets was made in a 50° C. water bath using a high shear device. Water was used to disperse the platelets. Poly(vinyl pyrrolidone) (PVP, weight average of about 10,000) was introduced to the platelet dispersion at a weight ratio of 30/70 (PVP to platelets) and the polyvinyl alcohol binder polymer were brought together under high sheer to form a total 3% solids dispersion at different platelet to binder ratios. The resulting dispersions were allowed to stir for an additional three hours to ensure total mixing. 
     To determine the dry overcoat layer thickness, the film was cross sectioned. The cross section of the film was then examined using an Olympus BH-2 optical microscope. The measurement was done with 1000 times magnification and an eyepiece with standard scale of 0.1 μm resolution. 
     The effectiveness of the various barrier layers to inhibit the diffusion of chemical components (such as fatty acids like behenic acid) from the imaging layer was evaluated as follows. A sample of the photothermographic material was placed between clean conventional (7.62 cm×2.54 cm) glass microscope slides. About 110 g of weight was evenly applied to the resulting laminate while it was heated at 120° C. for 30 minutes. The glass slide in contact with the photothermographic material barrier layer was then analyzed for the relative amount of fatty acid transferred to it using Attenuated Total Reflectance Fourier Transform InfraRed Spectroscopy (ATR FTIR) and a conventional Bio-Rad FTS60 FTIR spectrometer fitted with a diamond ATR stage. At least two spectra of the glass slide from each photothermographic material sample were collected. The CH 2  stretching bands (2920 and 2850 cm −1 ) and the CH 3  stretching band (2955 cm −1 ) of the fatty acid were divided by the SiO 2  band (910 cm −1 ) of the glass to provide a ratio after baseline correction. The relative amount of fatty acid transferred was directly related to the value of the ratio. That is, lower ratios mean lower fatty acid transfer and that the barrier layer served as a more effective barrier to fatty acid movement. 
     The effectiveness of the Invention barrier layers compared to the various Comparative barrier layers is reported as % Reduction in Fatty Acid Transfer in TABLE II below. A larger % reduction indicates greater effectiveness as a barrier layer. It is apparent from the data that most of the films of the present invention containing clay particles in the barrier layer exhibited reduced fatty acid transfer compared to the respective Comparative films. The films of Examples 10-12 provided no improvement over Comparative film  4  because the barrier layer thickness (binder) alone provided a sufficient barrier to fatty acid transfer. However, the invention films may provide an improvement over other films having barrier layers of the same thickness but composed of different materials. As noted above, it is preferred that the dry thickness of the barrier layer be less than 5 μm. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE II 
               
               
                   
               
               
                   
                   
                   
                 Overcoat 
                   
                   
                   
                   
               
               
                   
                   
                   
                 Barrier 
                 Weight % 
               
               
                   
                   
                   
                 Layer Dry 
                 Inorganic 
                   
                 % Reduction 
                 Aspect Ratio 
               
               
                   
                 Imaging Layer 
                 Overcoat Barrier 
                 Thickness 
                 Material in 
                 Barrier Layer Clay 
                 in Fatty 
                 of Inorganic 
               
               
                 Example 
                 Binder 
                 Layer Binder 
                 (μm) 
                 Barrier Layer 
                 Type 
                 Acid Transfer 
                 Particles 
               
               
                   
               
             
            
               
                 Comparative 1 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 1.5 
                 0 
                 None 
                 — 
                 — 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Invention 1 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 1.5 
                 3 
                 Southern Clay 
                 45 
                 200 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
                   
                   
                 Products Cloisite Na +   
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Invention 2 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 1.5 
                 5 
                 Southern Clay 
                 55 
                 200 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
                   
                   
                 Products Cloisite Na +   
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Invention 3 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 1.5 
                 10 
                 Southern Clay 
                 59 
                 200 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
                   
                   
                 Products Cloisite Na +   
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Comparative 2 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 2.0 
                 0 
                 None 
                 — 
                 — 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Invention 4 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 2.0 
                 3 
                 Southern Clay 
                 61 
                 200 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
                   
                   
                 Products Cloisite Na +   
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Invention 5 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 2.0 
                 5 
                 Southern Clay 
                 70 
                 200 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
                   
                   
                 Products Cloisite Na +   
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Invention 6 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 2.0 
                 10 
                 Southern Clay 
                 67 
                 200 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
                   
                   
                 Products Cloisite Na +   
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Comparative 3 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 3.2 
                 0 
                 None 
                 — 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Invention 7 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 3.2 
                 3 
                 Southern Clay 
                 74 
                 200 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
                   
                   
                 Products Cloisite Na +   
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Invention 8 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 3.2 
                 5 
                 Southern Clay 
                 78 
                 200 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
                   
                   
                 Products Cloisite Na +   
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Invention 9 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 3.2 
                 10 
                 Southern Clay 
                 70 
                 200 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
                   
                   
                 Products Cloisite Na +   
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Comparative 4 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 5.1 
                 0 
                 None 
                 — 
                 — 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Invention 10 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 5.1 
                 3 
                 Southern Clay 
                 0 
                 200 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
                   
                   
                 Products Cloisite Na +   
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Invention 11 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 5.1 
                 5 
                 Southern Clay 
                 0 
                 200 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
                   
                   
                 Products Cloisite Na +   
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                 Invention 12 
                 Polyvinyl alcohol 
                 Polyvinyl alcohol 
                 5.1 
                 10 
                 Southern Clay 
                 0 
                 200 
               
               
                   
                 (Dupont Elvanol 
                 (Nippon Gohsei Gohsenol 
                   
                   
                 Products Cloisite Na +   
               
               
                   
                 52-22) 
                 GH 23) 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 13 AND COMPARATIVE EXAMPLE 5 
     The film of the example was prepared like Example 8 except deionized bone gelatin with 1.8% HAR-1 (weight % relative to gelatin) was used in place of the binder polyvinyl alcohol (PVA, Nippon Gohsei Gohsenol GH23) as indicated in TABLE III. Comparative Example 5 was prepared with a barrier layer containing no inorganic particles present. The film of the present invention comprised a highly effective barrier layer since fatty acid transfer was reduced more than 95% from that exhibited by the Comparative 5 film. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE III 
               
               
                   
               
               
                   
                   
                   
                 Overcoat 
                   
                   
                   
                   
               
               
                   
                   
                   
                 Barrier 
                 Weight % 
               
               
                   
                   
                   
                 Layer Dry 
                 Inorganic 
                   
                 % Reduction 
                 Aspect Ratio 
               
               
                   
                 Imaging Layer 
                 Overcoat Barrier 
                 Thickness 
                 Material in 
                 Barrier Inorganic Clay 
                 in Fatty 
                 of Inorganic 
               
               
                 Example 
                 Binder 
                 Layer Binder 
                 (μm) 
                 Barrier Layer 
                 Type 
                 Acid Transfer 
                 Particles 
               
               
                   
               
             
            
               
                 Comparative 5 
                 Polyvinyl alcohol 
                 Gelatin 
                 3.2 
                 0 
                 None 
                 — 
                 — 
               
               
                   
                 (Dupont Elvanol 
               
               
                   
                 52-22) 
               
               
                 Invention 13 
                 Polyvinyl alcohol 
                 Gelatin 
                 3.2 
                 5 
                 Southern Clay 
                 &gt;95 
                 200 
               
               
                   
                 (Dupont Elvanol 
                   
                   
                   
                 Products Cloisite Na +   
               
               
                   
                 52-22) 
               
               
                   
               
            
           
         
       
     
     Control B 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 (31.2 g), and the NPSBD (37.0 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 (0.8 g), an aqueous solution (1.13 g) of sodium iodide (50 g/l), and a solid-particle dispersion of reducing agent DEV-2 (9.49 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 were added. After stirring at 40° C. for 60 minutes, 1.11 g of 4-methylphthalic acid (0.9 g of 10% aqueous solution) were then 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 to provide a Control B photothermographic material that is outside the scope of the present invention. 
     EXAMPLES 14-15 AND COMPARATIVE EXAMPLE 6 
     Once dried, the Control B imaging layer was overcoated with various barrier layer formulations. The Comparative Example 6 film was prepared by coating a overcoat formulation comprising solely polyvinyl alcohol (PVA, Nippon Gohsei Gohsenol GH23) as the binder material in 100% water. The coating solution was coated to yield a dry coverage as shown in TABLE V below. 
     Photothermographic materials of the present invention were prepared similarly except that over the dried imaging layer was coated, from water, formulations containing various weight % inorganic particles to form barrier layers. 
     The inorganic particles used in this manner are identified in the following TABLE IV. The data in TABLE V shows that the photothermographic materials of the present invention comprised effective barrier layers because of the considerable reduction in fatty acid transfer compared to the Comparative 6 film. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE IV 
               
               
                   
               
               
                 Inorganic 
                   
                   
                 Aspect 
                 Diameter b   
               
               
                 Particles ID 
                 Name 
                 Supplier 
                 Ratio a   
                 (Å) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 L1 
                 NaCloisite ® 
                 Southern 
                 200 
                 200 
               
               
                   
                   
                 Clay 
               
               
                   
                   
                 Products 
               
               
                 L2 
                 Laponite ® 
                 Southern 
                 20-30 
                 25 
               
               
                   
                 RDS 
                 Clay 
               
               
                   
                   
                 Products 
               
               
                   
               
               
                   a Southern Clay Products Product Manual  
               
               
                   b measured by TEM, average size  
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE V 
               
               
                   
               
               
                   
                   
                   
                 Overcoat 
                   
                   
                   
                   
               
               
                   
                   
                   
                 Barrier 
                 Weight % 
               
               
                   
                   
                   
                 Layer Dry 
                 Inorganic 
                   
                 % Reduction 
                 Aspect Ratio 
               
               
                   
                 Imaging Layer 
                 Overcoat Barrier 
                 Thickness 
                 Material in 
                 Inorganic Barrier Clay 
                 in Fatty 
                 of Inorganic 
               
               
                 Example 
                 Binder 
                 Layer Binder 
                 (μm) 
                 Barrier Layer 
                 Type 
                 Acid Transfer 
                 Particles 
               
               
                   
               
             
            
               
                 Comparative 6 
                 Gelatin 
                 Polyvinyl alcohol 
                 3.2 
                 0 
                 None 
                 — 
                 — 
               
               
                   
                   
                 (Nippon Gohsei 
               
               
                   
                   
                 Gohsenol GH 23) 
               
               
                 Invention 14 
                 Gelatin 
                 Polyvinyl alcohol 
                 3.2 
                 5 
                 Southern Clay 
                 38 
                 200 
               
               
                   
                   
                 (Nippon Gohsei 
                   
                   
                 Products Cloisite Na +   
               
               
                   
                   
                 Gohsenol GH 23) 
               
               
                 Invention 15 
                 Gelatin 
                 Polyvinyl alcohol 
                 3.2 
                 10 
                 Southern Clay 
                 &gt;95 
                 20-30 
               
               
                   
                   
                 (Nippon Gohsei 
                   
                   
                 Products Laponite 
               
               
                   
                   
                 Gohsenol GH 23) 
                   
                   
                 RDS 
               
               
                   
               
            
           
         
       
     
     EXAMPLES 16-17 AND COMPARATIVE EXAMPLE 7 
     Invention materials 16 and 17 were prepared like Example 14 except deionized bone gelatin was used in place of the binder polyvinyl alcohol (PVA, Nippon Gohsei Gohsenol GH23) and the dry barrier layer thickness was 2 μm as indicated in TABLE VI below. Comparative Example 7 was prepared similarly without inorganic particles in the barrier layer. The data in TABLE VI shows that the photothermographic materials of the present invention comprised effective barrier layers because of the considerable reduction in fatty acid transfer compared to the Comparative 7 film. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE VI 
               
               
                   
               
               
                   
                   
                   
                 Overcoat 
                   
                   
                   
                   
               
               
                   
                   
                   
                 Barrier 
                 Weight % 
               
               
                   
                   
                   
                 Layer 
                 Inorganic 
                   
                 % Reduction 
                 Aspect Ratio 
               
               
                   
                 Imaging Layer 
                 Overcoat Barrier 
                 Thickness 
                 Material in 
                 Inorganic Barrier Clay 
                 in Fatty 
                 of Inorganic 
               
               
                 Example 
                 Binder 
                 Layer Binder 
                 (μm) 
                 Barrier Layer 
                 Type 
                 Acid Transfer 
                 Particles 
               
               
                   
               
             
            
               
                 Comparative 7 
                 Gelatin 
                 Gelatin 
                 2.0 
                 0 
                 None 
                 — 
                 — 
               
               
                 Invention 16 
                 Gelatin 
                 Gelatin 
                 2.0 
                 3 
                 Southern Clay 
                 15 
                 200 
               
               
                   
                   
                   
                   
                   
                 Products Cloisite Na +   
               
               
                 Invention 17 
                 Gelatin 
                 Gelatin 
                 2.0 
                 5 
                 Southern Clay 
                 58 
                 200 
               
               
                   
                   
                   
                   
                   
                 Products Cloisite Na +   
               
               
                   
               
            
           
         
       
     
     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.