Patent Publication Number: US-2007111145-A1

Title: Thermally developable materials with backside conductive layer

Description:
FIELD OF THE INVENTION  
      This invention relates to thermally developable materials having certain backside conductive layers. In particular, this invention relates to thermographic and photothermographic materials having conductive backside layers with improved “conductive efficiency.” This invention also relates to methods of imaging using these thermally developable materials.  
     BACKGROUND OF THE INVENTION  
      Silver-containing thermographic and photothermographic imaging materials (that is, thermally developable imaging materials) that are imaged and/or developed using heat and without liquid processing have been known in the art for many years.  
      Silver-containing thermographic imaging materials are non-photo-sensitive materials that are used in a recording process wherein images are generated by the use of thermal energy. These materials generally comprise a support having disposed thereon (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing composition (usually including a developer) for the reducible silver ions, and (c) a suitable hydrophilic or hydrophobic binder.  
      In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. At elevated temperatures, the silver of the silver carboxylate is reduced by a reducing agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechols, pyrogallol, ascorbic acid, and ascorbic acid derivatives, whereby an image of elemental silver is formed. Some thermographic constructions are imaged by contacting them with the thermal head of a thermographic recording apparatus such as a thermal printer or thermal facsimile. In such constructions, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated to an elevated temperature, typically in the range of from about 60 to about 225° C., resulting in the formation of an image.  
      Silver-containing photothermographic imaging materials (that is, photosensitive thermally developable imaging materials) that are imaged with actinic radiation and then developed using heat and without liquid processing 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, X-radiation, or ultraviolet, visible, 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 photocatalyst (that is, a photosensitive compound 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 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. 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 photothermographic imaging layer(s).  
      Differences Between Photothermography and Photography  
      The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.  
      In photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer incorporated within the material. Heating at 50° C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30° C. to 50° C.) to provide a visible image.  
      In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example, a silver carboxylate or a silver benzotriazole) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself at least partially converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.  
      In photothermographic materials, all of the “chemistry” for imaging is incorporated within the material itself. For example, such materials include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of “fog” or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems.  
      Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is in the aqueous fixing step).  
      Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the underlying chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.  
      These and other distinctions between photothermographic and photographic materials are described in  Unconventional Imaging Processes , E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74-75, in D. H. Klosterboer,  Imaging Processes and Materials , ( Neblette&#39;s Eighth Edition ), J. Sturge, V. Walworth, and A. Shepp, Eds., Van Nostrand-Reinhold, New York, 1989, Chapter 9, pp. 279-291, in Zou et al.,  J. Imaging Sci. Technol.  1996, 40, pp. 94-103, and in M. R. V. Sahyun,  J. Imaging Sci. Technol.  1998, 42, 23.  
      Problem to be Solved  
      Many of the chemicals used to make supports or supported layers in thermally developable materials have electrically insulating properties, and electrostatic charges frequently build up on the materials during manufacture, packaging, and use. The accumulated charges can cause various problems. For example, in photothermographic materials containing photosensitive silver halides, accumulated electrostatic charge can generate light to which the silver halides are sensitive. This may result in imaging defects that are a particular problem where the images are used for medical diagnosis.  
      Build-up of electrostatic charge can also cause sheets of thermally processable materials to stick together causing misfeeds and jamming within processing equipment. Additionally, accumulated electrostatic charge can attract dust or other particulate matter to the materials, thereby requiring more cleaning to insure rapid transport through the processing equipment and quality imaging.  
      Build-up of electrostatic charge also makes handling of developed sheets of imaged material more difficult. For example, radiologists desire a static free sheet for viewing on light boxes. This problem can be particularly severe when reviewing an imaged film that has been stored for a long period of time because many antistatic materials loose their effectiveness over time.  
      In general, electrostatic charge is related to surface resistivity (measured in ohm/sq) and charge level. While electrostatic charge control agents (or antistatic agents) can be included in any layer of an imaging material, the accumulation of electrostatic charge can be prevented by reducing the surface resistivity or by lowering the charge level. These results can usually be achieved by including charge control agents in surface layers such as protective overcoats. In thermally processable materials, charge control agents may be used in backing layers that are on the opposite side of the support as the imaging layers. Another approach taken to reduce surface resistivity is to include a “buried” conductive layer incorporating conductive particles.  
      A wide variety of charge control agents, both inorganic and organic, have been devised and used for electrostatic charge control and numerous publications describe such agents. Metal oxides are described in conductive layers in U.S. Pat. No. 5,340,676 (Anderson et al.), U.S. Pat. No. 6,464,413 (Oyamada), U.S. Pat. No. 5,368,995 (Christian et al.), and U.S. Pat. No. 5,457,013 (Christian et al.).  
      U.S. Pat. No. 5,731,119 (Eichorst et al.) describes the use of acicular metal oxides in aqueous-coated conductive layers for use in antistatic compositions. An aqueous-coated sample containing granular zinc antimonate served as a comparison.  
      U.S. Pat. No. 6,355,405 (Ludemann et al.) describes thermally developable materials that include very thin adhesion-promoting layers on either side of the support. These adhesion-promoting layers include specific mixtures of polymers and other compounds to promote adhesion, and are also known as “carrier” layers.  
      U.S. Pat. No. 6,689,546 (LaBelle et al.) describes thermally developable materials that contain a backside conductive layer comprising non-acicular metal antimonate particles in the amount of from about 40 to about 55% (based on total dry weight).  
      Despite these advances, little attention has been paid to the effect of large-scale processing conditions on the “conductive efficiency” of the formulations described above. For example, when production quantities of backside conductive materials are prepared, the speed required for efficient mixing often results in shear conditions that are different from those involved for the preparation of laboratory quantities. As a result, the conductive backside materials so produced can have properties different from those of laboratory-prepared samples. Often these different properties result in materials having poorer conductive efficiency. It would be useful to be able to prepare large quantities of conductive backside coating formulations whose preparation are independent of mixing conditions such as blade size, mixing speed, and preparation vessel size.  
      There is, therefore, a continuing need in the industry to find more efficient and less costly ways to reduce electrostatic charge, particularly in “buried” layers on the backside of thermally developable imaging materials, without loss of adhesion of the conductive layer to underlying substrates or decreased conductive efficiency. There is also a need to develop constructions with improved conductive efficiency, so that the same performance can be achieved at lower coating weights.  
     SUMMARY OF THE INVENTION  
      The present invention provides a thermally developable material that comprises a support having on one side thereof, one or more thermally developable imaging layers comprising a binder and in reactive association, a non-photosensitive source of reducible silver ions, and a reducing agent composition for the non-photosensitive source reducible silver ions, and  
      having disposed on the backside of the support a non-imaging backside conductive layer comprising non-acicular metal antimonate particles in a one or more binder polymers,  
      wherein the non-acicular metal antimonate particles comprise greater than 55 and up to 85% by dry weight of the backside conductive layer, are present at a coverage of from about 0.06 to about 0.5 g/m 2 , and the ratio of total binder polymers in the backside conductive layer to the non-acicular metal antimonate particles is less than 0.75:1, based on dry weights.  
      The present invention also provides a photothermographic material that comprises a support having on one side thereof, one or more thermally developable imaging layers comprising a binder and in reactive association, a photosensitive silver halide, a non-photosensitive source of reducible silver ions, and a reducing agent composition for the non-photosensitive source reducible silver ions, and  
      having disposed on the backside of the support:  
      a) a first layer comprising a film-forming polymer, and  
      b) interposed between the support and the first layer and directly adhering the first layer to the support, a non-imaging backside conductive layer comprising non-acicular metal antimonate particles in a mixture of two or more polymers that include a first polymer serving to promote adhesion of the backside conductive layer directly to the support, and a second polymer that is different than and forms a single phase mixture with the first polymer,  
      wherein the non-acicular metal antimonate particles comprise greater than 60 and up to 76% by dry weight of the backside conductive layer, are present at a coverage of from about 0.06 to about 0.2 g/m 2 , and the ratio of total binder polymers in the backside layer to the non-acicular metal antimonate particles is less than 0.75:1, based on dry weights, and  
      wherein the film-forming polymer of the first layer and the second polymer of the backside conductive layer are the same or different polyvinyl acetal resins, polyester resins, cellulosic polymers, maleic anhydride-ester copolymers, or vinyl polymers.  
      Preferred embodiments include a black-and-white photothermographic material that comprises a transparent polymeric support having on one side thereof one or more thermally developable imaging layers comprising predominantly one or more hydrophobic binders, and in reactive association, preformed photosensitive silver bromide or silver iodobromide present as tabular and/or cubic grains, a non-photosensitive source of reducible silver ions that includes silver behenate, a reducing agent composition for the non-photosensitive source reducible silver ions comprising a hindered phenol, and a protective layer disposed over the one or more thermally developable imaging layers, and  
      having disposed on the backside of the support:  
      a) a backside protective layer comprising a film-forming polymer that is cellulose acetate butyrate and an antihalation composition, and  
      b) interposed between the support and the backside protective layer and directly adhering the backside protective layer to the support, a non-imaging backside conductive layer comprising non-acicular metal antimonate particles in a mixture of two or more polymers that include a first polymer serving to promote adhesion of the conductive layer directly to the support, and a second polymer that is different than and forms a single phase mixture with the first polymer,  
      wherein the first polymer of the backside conductive layer is a polyester and the second polymer of the backside conductive layer is cellulose acetate butyrate, and  
      wherein the non-acicular metal antimonate particles are composed of zinc antimonate (ZnSb 2 O 6 ) and comprise from about 70 to about 76% by dry weight of the backside conductive layer, are present at a coverage of from about 0.06 to about 0.2 g/m 2 , and the dry thickness of the backside conductive layer is from about 0.09 to about 0.15 μm.  
      Preferred embodiments also include a black-and-white thermographic material that comprises a transparent polymeric support having on one side thereof one or more thermally developable imaging layers comprising predominantly one or more hydrophobic binders, and in reactive association, a non-photosensitive source of reducible silver ions that includes silver behenate, a reducing agent composition for the non-photosensitive source reducible silver ions comprising an aromatic di- and tri-hydroxy compound having at least two hydroxy groups in ortho- or para-relationship on the same aromatic nucleus or mixture thereof, and a protective layer disposed over the one or more thermally developable imaging layers, and  
      having disposed on the backside of the support:  
      a) a backside protective layer comprising a film-forming polymer, and,  
      b) interposed between the support and the backside protective layer and directly adhering the backside protective layer to the support, a non-imaging backside conductive layer comprising non-acicular metal antimonate particles in a mixture of two or more polymers that include a first polymer serving to promote adhesion of the backside conductive layer directly to the support, and a second polymer that is different than and forms a single phase mixture with the first polymer,  
      wherein the non-acicular metal antimonate particles comprise greater than 70 and up to 76% by dry weight of the backside conductive layer, are present at a coverage of from about 0.06 to about 0.2 g/m 2 , and the ratio of total binder polymers in the backside layer to the non-acicular metal antimonate particles is less than 0.75:1, based on dry weights, and  
      wherein the film-forming polymer of the first layer and the second polymer of the backside conductive layer are the same or different polyvinyl acetal resins, polyester resins, cellulosic polymers, maleic anhydride-ester copolymers, or vinyl polymers.  
      This invention further provides a method of forming a visible image in thermographic materials that comprises:  
      (A) thermal imaging of the thermographic material.  
      This invention also provides a method for forming a visible image in photothermographic materials comprising:  
      (A) imagewise exposing any of the photothermographic materials of the present invention to 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.  
      These imaging forming methods are particularly useful for providing a medical diagnosis of a human or animal subject.  
      The present invention provides a means for providing exceptional conductivity on a backside conductive layer with the use of a unique amount of conductive metal antimonate in the backside conductive layer to provide improved conductive efficiency. It was surprising that a lesser amount of conductive metal antimonate particles could be used to provide the same or improved conductivity especially in “buried” conductive layers. The overall dry thickness of such layers can also be reduced because it has been discovered that a lesser amount of binder polymer(s) is needed to achieve the desired layer integrity and adhesion to other layers or to the support. In some embodiments, the improved conductive layer is the only backside layer in the thermally developable materials but in preferred embodiments, the conductive layer is “buried” and the binders of that layer and the overlying conductive layer are designed for maximum adhesion and coating advantages. It has also been found that formulations containing high amounts of non-acicular zinc antimonate (that is, a low binder to non-acicular zinc antimonate ratio) provide highly conductive materials having decreased loss of resistivity when scaled-up under high shear conditions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The thermally developable materials described herein are both thermographic and photothermographic materials. While the following discussion will often be directed primarily to the preferred photothermographic embodiments, it would be readily understood by one skilled in the art that thermographic materials can be similarly constructed and used to provide black-and-white or color images using appropriate imaging chemistry and particularly non-photosensitive organic silver salts, reducing agents, toners, binders, and other components known to a skilled artisan. In both thermographic and photothermographic materials, the non-acicular metal antimonate particles described herein are preferably incorporated into a separate buried conductive (“antistatic”) layer on at least the backside and optionally on both sides of the support.  
      The thermally developable materials of this invention can be used in black-and-white or color thermography and photothermography and 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), X-ray radiography, and in industrial radiography. Furthermore, the absorbance of these photothermographic materials between 350 and 450 nm is desirably low (less than 0.5), to permit their use in the graphic arts area (for example, imagesetting and phototypesetting), in the manufacture of printing plates, in contact printing, in duplicating (“duping”), and in proofing.  
      The thermally developable materials are particularly useful for imaging of human or animal subjects in response to visible, X-radiation, or infrared radiation for use in a medical diagnosis. Such applications include, but are not limited to, thoracic imaging, mammography, dental imaging, orthopedic imaging, general medical radiography, therapeutic radiography, veterinary radiography, and autoradiography. When used with X-radiation, the photothermographic materials may be used in combination with one or more phosphor intensifying screens, with phosphors incorporated within the photothermographic emulsion, or with combinations thereof. Such materials are particularly useful for dental radiography when they are directly imaged by X-radiation. The materials are also useful for non-medical uses of X-radiation such as X-ray lithography and industrial radiography.  
      The photothermographic materials can be made sensitive to radiation of any suitable wavelength. Thus, in some embodiments, the materials are sensitive at ultraviolet, visible, infrared, or near infrared wavelengths, of the electromagnetic spectrum. In preferred embodiments, the materials are sensitive to radiation greater than 700 nm (and generally up to 1150 nm). Increased sensitivity to a particular region of the spectrum is imparted through the use of various spectral sensitizing dyes.  
      In the photothermographic materials, the components needed for imaging can be in one or more photothermographic imaging layers on one side (“frontside”) of the support. The layer(s) that contain the photosensitive photocatalyst (such as a photosensitive silver halide) or non-photosensitive source of reducible silver ions, or both, are referred to herein as photothermographic emulsion layer(s). The photocatalyst and the non-photosensitive source of reducible silver ions are in catalytic proximity and preferably are in the same emulsion layer.  
      Similarly, in the thermographic materials of this invention, the components needed for imaging can be in one or more layers. The layer(s) that contain the non-photosensitive source of reducible silver ions are referred to herein as thermographic emulsion layer(s).  
      Where the materials contain imaging layers on one side of the support only, various non-imaging layers are usually disposed on the “backside” (non-emulsion or non-imaging side) of the materials, including at least one buried conductive layer described herein, and optionally antihalation layer(s), protective layers, and transport enabling layers.  
      Various non-imaging layers can also be disposed on the “frontside” or 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 other layers readily apparent to one skilled in the art.  
      For some embodiments, it may be useful that the thermally developable materials be “double-sided” or “duplitized” and have the same or different thermally developable coatings (or imaging layers) on both sides of the support. In such constructions each side can also include one or more protective topcoat layers, primer layers, interlayers, acutance layers, auxiliary layers, anti-crossover layers, and other layers readily apparent to one skilled in the art, as well as the required conductive layer(s).  
      When the thermally developable materials 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.  
      Definitions  
      As used herein:  
      In the descriptions of the thermally developable materials, “a” or “an” component refers to “at least one” of that component (for example, the specific conductive metal oxide described herein).  
      Unless otherwise indicated, when the terms “thermally developable materials,” “photothermographic materials,” and “thermographic materials” are used herein, the terms refer to materials of the present invention.  
      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, Eastman Kodak Company, Rochester, N.Y., 1977, p. 374.  
      “Photothermographic material(s)” means a construction comprising a support and at least one photothermographic emulsion layer or a photothermographic set of emulsion layers, wherein the photosensitive silver halide and the source of reducible silver ions are in one layer and the other necessary components or additives are distributed, as desired, in the same layer or in an adjacent coated layer. These materials also include multilayer constructions in which one or more imaging components are in different layers, but are in “reactive association.” For example, one layer can include the non-photosensitive source of reducible silver ions and another layer can include the reducing composition, but the two reactive components are in reactive association with each other.  
      “Thermographic materials” are similarly defined except that no photosensitive silver halide catalyst is purposely added or created.  
      When used in photothermography, the term, “imagewise exposing” or “imagewise exposure” means that the material is imaged using any exposure means that provides a latent image using electromagnetic radiation. This includes, for example, by analog exposure where an image is formed by projection onto the photosensitive material as well as by digital exposure where the image is formed one pixel at a time such as by modulation of scanning laser radiation.  
      When used in thermography, the term, “imagewise exposing” or “imagewise exposure” means that the material is imaged using any means that provides an image using heat. This includes, for example, by analog exposure where an image is formed by differential contact heating through a mask using a thermal blanket or infrared heat source, as well as by digital exposure where the image is formed one pixel at a time such as by modulation of thermal print-heads or by thermal heating using scanning laser radiation.  
      “Catalytic proximity” or “reactive association” means that the reactive components are in the same layer or in adjacent layers so that they readily come into contact with each other during imaging and thermal development.  
      “Emulsion layer,” “imaging layer,” “thermographic emulsion layer,” or “photothermographic emulsion layer” means a layer of a thermographic or photothermographic material that contains the photosensitive silver halide (when used) and/or non-photosensitive source of reducible silver ions, or a reducing composition. Such layers can also contain additional components or desirable additives. These layers are usually on what is known as the “frontside” of the support, but they can also be on both sides of the support.  
      “Photocatalyst” means a photosensitive compound such as silver halide that, upon exposure to radiation, provides a compound that is capable of acting as a catalyst for the subsequent development of the image-forming material.  
      Many of the chemical components used herein are provided as a solution. The term “active ingredient” means the amount or the percentage of the desired chemical component contained in a sample. All amounts listed herein are the amount of active ingredient added unless otherwise specified.  
      “Ultraviolet region of the spectrum” refers to that region of the spectrum less than or equal to 410 nm (preferably from about 100 nm to about 410 nm) although parts of these ranges may be visible to the naked human eye.  
      “Visible region of the spectrum” refers to that region of the spectrum of from about 400 nm to about 700 nm.  
      “Short wavelength visible region of the spectrum” refers to that region of the spectrum of from about 400 nm to about 450 nm.  
      “Red region of the spectrum” refers to that region of the spectrum of from about 600 nm to about 700 nm.  
      “Infrared region of the spectrum” refers to that region of the spectrum of from about 700 nm to about 1400 nm.  
      “Non-photosensitive” means not intentionally light sensitive.  
      The sensitometric terms “photospeed,” “speed,” or “photographic speed” (also known as sensitivity), absorbance, and contrast have conventional definitions known in the imaging arts.  
      In photothermographic materials, the term D min  (lower case) is considered herein as image density achieved when the photothermographic material is thermally developed without prior exposure to radiation. The term D max  (lower case) is the maximum image density achieved in the imaged area of a particular sample after imaging and development. In thermographic materials, D min  is considered herein as the image density in the areas with the minimum application of heat by the thermal printhead. In thermographic materials, the term D max  is the maximum image density achieved when the thermographic material is thermally imaged with a given amount of thermal energy.  
      In both photothermographic and thermographic materials, the term D MIN  (upper case) is the density of the unimaged material. In photothermographic materials, the term D MAX  (upper case) is the maximum image density achievable when the photothermographic material is exposed and then thermally developed. In thermographic materials, the term D MAX  is the maximum image density achievable when the thermographic material is thermally developed. D MAX  is also known as “Saturation Density.” 
      The sensitometric term absorbance is another term for optical density (OD).  
      “Transparent” means capable of transmitting visible light or imaging radiation without appreciable scattering or absorption.  
      As used herein, the phrase “silver organic coordinating ligand” refers to an organic molecule capable of forming a bond with a silver atom. Although the compounds so formed are technically silver coordination compounds they are also often referred to as silver salts.  
      The term “buried layer” means that there is at least one other layer disposed over the layer (such as a “buried” backside conductive layer).  
      The terms “coating weight,” “coat weight,” and “coverage” are synonymous, and are usually expressed in weight per unit area such as g/m 2 .  
      “Conductive efficiency” refers to the amount of conductive particles necessary to achieve a given conductivity. Samples with a high conductive efficiency require fewer conductive particles to achieve a given conductivity than those of a comparative sample. Alternatively, conductive efficiency can also refer to samples having a higher conductivity with the same number of particles (that is, the same coating weight).  
      As is well understood in this art, for the chemical compounds herein described, substitution is not only tolerated, but is often advisable and various substituents are anticipated on the compounds used in the present invention unless otherwise stated. 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.  
      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 “alkyl group” is intended to include not only pure hydrocarbon alkyl chains, such as methyl, ethyl, n-propyl, t-butyl, cyclohexyl, iso-octyl, and octadecyl, 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, and carboxy. For example, alkyl group includes ether and thioether groups (for example CH 3 —CH 2 —CH 2 —O—CH 2 — and CH 3 —CH 2 —CH 2 —S—CH 2 —), haloalkyl, nitroalkyl, alkylcarboxy, carboxyalkyl, carboxamido, 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 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. It is also available from Emsworth Design Inc., 147 West 24th Street, New York, N.Y. 10011.  
      Other aspects, advantages, and benefits of the present invention are apparent from the detailed description, examples, and claims provided in this application.  
      The Photocatalyst  
      As noted above, photothermographic materials include one or more photocatalysts in the photothermographic emulsion layer(s). Useful photocatalysts are typically photosensitive 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 generally having up to 10 mol % silver iodide.  
      In some embodiments of aqueous-based photothermographic materials, higher amounts of iodide may be present in homogeneous photo-sensitive silver halide grains, and particularly from about 20 mol % up to the saturation limit of iodide as described, for example, U.S. Published application 2004-0053173 (Maskasky et al.).  
      The silver halide grains may have any crystalline habit or morphology including, but not limited to, cubic, octahedral, tetrahedral, orthorhombic, rhombic, dodecahedral, other polyhedral, tabular, laminar, twinned, or platelet morphologies and may have epitaxial growth of crystals thereon. If desired, a mixture of grains with different morphologies can be employed. Silver halide grains having cubic and tabular morphology (or both) are preferred.  
      The silver halide grains may have a uniform ratio of halide throughout. They may also 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 or more silver halides, and a discrete shell of one or more different silver halides. Core-shell silver halide grains useful in photothermographic materials and methods of preparing these materials are described 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), both incorporated herein by reference.  
      In some instances, it may be helpful to prepare the photosensitive silver halide grains in the presence of a hydroxytetrazaindene (such as 4-hydroxy-6-methyl-1,3,3a,7-tetrazaindene) or an N-heterocyclic compound comprising at least one mercapto group (such as 1-phenyl-5-mercaptotetrazole) as described in U.S. Pat. No. 6,413,710 (Shor et al.) that is 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. With this technique, one has the possibility of more precisely controlling the grain size, grain size distribution, dopant levels, and composition of the silver halide, so that one can impart more specific properties to both the silver halide grains and the resulting photothermographic material.  
      In some constructions, it is preferable to form the non-photo-sensitive 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 source of reducible silver ions in the presence of silver halide provides a more intimate mixture of the two materials [see U.S. Pat. No. 3,839,049 (Simons)] to provide a material often referred to as a “preformed soap.” 
      In some constructions, it is preferred that preformed silver halide grains be added to and “physically mixed” with the non-photosensitive source of reducible silver ions.  
      Preformed silver halide emulsions can be prepared by aqueous or organic processes and can be unwashed or washed to remove soluble salts. Soluble salts can be removed by any desired procedure for example as 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- or a halogen-containing compound is added to an organic silver salt to partially convert the silver of the organic silver salt to silver halide. Inorganic halides (such as zinc bromide, zinc iodide, calcium bromide, lithium bromide, lithium iodide, or mixtures thereof) or an organic halogen-containing compound (such as N-bromo-succinimide or pyridinium hydrobromide perbromide) can be used. The details of such in-situ generation of silver halide are well known and described in U.S. Pat. No. 3,457,075 (Morgan et al.).  
      It is particularly effective to use a mixture of both preformed and in-situ generated silver halide. The preformed silver halide is preferably present in a preformed soap.  
      Additional methods of preparing silver halides and organic silver salts and blending them are described in  Research Disclosure , June 1978, item 17029, U.S. Pat. No. 3,700,458 (Lindholm), U.S. Pat. No. 4,076,539 (Ikenoue et al.), JP Kokai 49-013224 (Fuji), JP Kokai 50-017216 (Fuji), and JP Kokai 51-042529 (Fuji).  
      The silver halide grains used in the imaging formulations can vary in average diameter of up to several micrometers (μm) depending on the 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.  
      The average size of the photosensitive 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. Representative grain sizing methods are described 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, Macmillan, New York, 1966, Chapter 2. 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.  
      The one or more light-sensitive silver halides are preferably present in an amount of from about 0.005 to about 0.5 mole, more preferably from about 0.01 to about 0.25 mole, and most preferably from about 0.03 to about 0.15 mole, per mole of non-photosensitive source of reducible silver ions.  
      Chemical Sensitization  
      The photosensitive silver halides can be chemically sensitized using any useful compound that contains 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. Pat. No. 5,759,761 (Lushington et al.), and EP 0 915 371A1 (Lok et al.), all of which are incorporated herein by reference.  
      Mercaptotetrazoles and tetraazindenes as described in U.S. Pat. No. 5,691,127 (Daubendiek et al.), incorporated herein by reference, can also be used as suitable addenda for tabular silver halide grains.  
      Certain substituted and unsubstituted thiourea compounds can be used as chemical sensitizers including those described in U.S. Pat. No. 6,368,779 (Lynch et al.) that is incorporated herein by reference.  
      Still other additional chemical sensitizers include certain tellurium-containing compounds that are described in U.S. Pat. No. 6,699,647 (Lynch et al.), and certain selenium-containing compounds that are described in U.S. Pat. No. 6,620,577 (Lynch et al.), that are both incorporated herein by reference.  
      Combinations of gold (3+)-containing compounds and either sulfur-, tellurium-, or selenium-containing compounds are also useful as chemical sensitizers as described in U.S. Pat. No. 6,423,481 (Simpson et al.) that is also incorporated herein by reference.  
      In addition, sulfur-containing compounds can be decomposed on silver halide grains in an oxidizing environment according to the teaching in U.S. Pat. No. 5,891,615 (Winslow et al.). Examples of sulfur-containing compounds that can be used in this fashion include sulfur-containing spectral sensitizing dyes.  
      Other useful sulfur-containing chemical sensitizing compounds that can be decomposed in an oxidized environment are the diphenylphosphine sulfide compounds represented by Structure (PS) described in copending and commonly assigned U.S. Ser. No. 10/731,251 (filed Dec. 9, 2003 by Simpson, Burleva, and Sakizadeh) which application is incorporated herein by reference.  
      The chemical sensitizers can be present 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 for silver halide grains having an average size of from about 0.01 to about 2 μm.  
      Spectral Sensitization  
      The photosensitive silver halides may be spectrally sensitized with one or more spectral sensitizing dyes that are known to enhance silver halide sensitivity to ultraviolet, visible, and/or infrared radiation. Non-limiting examples of spectral 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. They may be added at any stage in chemical finishing of the photothermographic emulsion, but are generally added after chemical sensitization is achieved.  
      Suitable spectral 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 Kokai 2000-063690 (Tanaka et al.), JP Kokai 2000-112054 (Fukusaka et al.), JP Kokai 2000-273329 (Tanaka et al.), JP Kokai 2001-005145 (Arai), JP Kokai 2001-064527 (Oshiyama et al.), and JP Kokai 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. Useful spectral sensitizing dyes are also described in  Research Disclosure , December 1989, item 308119, Section IV and  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. patent, 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 and patents are incorporated herein by reference.  
      Also useful are spectral sensitizing dyes that decolorize by the action of light or heat as 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 Kokai 2001-183770 (Hanyu et al.), all incorporated herein by reference.  
      Dyes may be selected for the purpose of supersensitization to attain much higher sensitivity than the sum of sensitivities that can be achieved by using each dye alone.  
      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 Source of Reducible Silver Ions  
      The non-photosensitive source of reducible silver ions in the thermally developable materials is a silver-organic compound that contains reducible silver (1+) ions. Such compounds are generally silver salts of silver organic coordinating ligands that are comparatively stable to light and form a silver image when heated to 50° C. or higher in the presence of an exposed photocatalyst (such as silver halide, when used in a photothermographic material) and a reducing agent composition.  
      The primary organic silver salt is often a silver salt of an aliphatic carboxylate (described below). Mixtures of silver salts of aliphatic carboxylates are particularly useful where the mixture includes at least silver behenate.  
      Useful silver carboxylates include silver salts of long-chain aliphatic carboxylic acids. The aliphatic carboxylic acids generally have aliphatic chains that contain 10 to 30, and preferably 15 to 28, carbon atoms. Examples of such preferred silver salts 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. Most preferably, at least silver behenate is used alone or in mixtures with other silver carboxylates.  
      Silver salts other than the silver carboxylates described above can be used also. Such silver salts include 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 as described in U.S. Pat. No. 5,491,059 (Whitcomb), silver salts of dicarboxylic acids, silver salts of sulfonates as described in U.S. Pat. No. 4,504,575 (Lee), silver salts of sulfosuccinates as described in EP 0 227 141A1 (Leenders et al.), silver salts of aromatic carboxylic acids (such as silver benzoate), silver salts of acetylenes 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.), and silver salts of heterocyclic compounds containing mercapto or thione groups and derivatives as described in U.S. Pat. No. 4,123,274 (Knight et al.) and U.S. Pat. No. 3,785,830 (Sullivan et al.).  
      It is also convenient to use silver half soaps such as an equimolar blend of silver carboxylate and carboxylic acid that analyzes for about 14.5% by weight solids of silver in the blend and that is prepared by precipitation from an aqueous solution of an ammonium or an alkali metal salt of a commercially available fatty carboxylic acid, or by addition of the free fatty acid to the silver soap.  
      The methods used for making silver soap emulsions are well known in the art and are disclosed in  Research Disclosure , April 1983, item 22812 , Research Disclosure , October 1983, item 23419, U.S. Pat. No. 3,985,565 (Gabrielsen et al.) and the references cited above.  
      Sources of non-photosensitive reducible silver ions can also be core-shell silver salts as described in U.S. Pat. No. 6,355,408 (Whitcomb et al.) that is incorporated herein by reference, wherein a core has one or more silver salts and a shell has one or more different silver salts, as long as one of the silver salts is a silver carboxylate.  
      Other useful sources of non-photosensitive reducible silver ions are the silver dimer compounds that comprise two different silver salts as described in U.S. Pat. No. 6,472,131 (Whitcomb) that is incorporated herein by reference.  
      Still other useful sources of non-photosensitive reducible silver ions are the silver core-shell compounds comprising a primary core comprising one or more photosensitive silver halides, or one or more non-photosensitive inorganic metal salts or non-silver containing organic salts, and a shell at least partially covering the primary core, wherein the shell comprises one or more non-photosensitive silver salts, each of which silver salts comprises a organic silver coordinating ligand. Such compounds are described in U.S. Published application 2004-0023164 (Bokhonov et al.) that is incorporated herein by reference.  
      Organic silver salts that are particularly useful in organic solvent-based thermographic and photothermographic materials include silver carboxylates (both aliphatic and aromatic carboxylates), silver triazolates, silver sulfonates, silver sulfosuccinates, and silver acetylides. Silver salts of long-chain aliphatic carboxylic acids containing 15 to 28 carbon atoms and silver salts are particularly preferred.  
      Organic silver salts that are particularly useful in aqueous based thermographic and photothermographic materials include silver salts of compounds containing an imino group. 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-chloro-benzotriazole), silver salts of 1,2,4-triazoles or 1-H-tetrazoles such as phenyl-mercaptotetrazole 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.). Particularly useful silver salts of this type are the silver salts of benzotriazole and substituted derivatives thereof. A silver salt of a benzotriazole is particularly preferred in aqueous-based thermographic and photothermographic formulations.  
      The one or more non-photosensitive sources of reducible silver ions are preferably present in an amount of from about 5% to about 70%, and more preferably from about 10% to about 50%, based on the total dry weight of the emulsion layers. Alternatively stated, the amount of the sources of reducible silver ions is generally from about 0.001 to about 0.2 mol/m 2  of the dry photothermographic material (preferably from about 0.01 to about 0.05 mol/m 2 ).  
      The total amount of silver (from all silver sources) in the thermographic and photothermographic materials is generally 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 (1+) ion to metallic silver. The “reducing agent” is sometimes called a “developer” or “developing agent.” 
      Conventional photographic developers can be used as reducing agents, including aromatic di- and tri-hydroxy compounds (such as hydroquinones, gallatic acid and gallic acid derivatives, catechols, and pyrogallols), aminophenols (for example, N-methylaminophenol), p-phenylenediamines, alkoxynaphthols (for example, 4-methoxy-1-naphthol), pyrazolidin-3-one type reducing agents (for example PHENIDONE®), pyrazolin-5-ones, polyhydroxy spiro-bis-indanes, indan-1,3-dione derivatives, hydroxytetrone acids, hydroxy-tetronimides, hydroxylamine derivatives such as for example those described in U.S. Pat. No. 4,082,901 (Laridon et al.), hydrazine derivatives, hindered phenols, amidoximes, azines, reductones (for example, ascorbic acid and ascorbic acid derivatives), leuco dyes, and other materials readily apparent to one skilled in the art.  
      When a silver benzotriazole silver source is used, ascorbic acid reducing agents are preferred. An “ascorbic acid” reducing agent (also referred to as a developer or developing agent) means ascorbic acid, complexes, and derivatives thereof. An “ascorbic acid” reducing agent means ascorbic acid, complexes, and derivatives thereof. Ascorbic acid reducing agents are described in a considerable number of publications 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 and derivatives thereof. Such compounds include, but are not limited to, D- or L-ascorbic acid, sugar-type derivatives thereof (such as sorboascorbic acid, γ-lacto ascorbic acid, 6-desoxy-L-ascorbic acid, L-rhamnoascorbic acid, imino-6-desoxy-L-ascorbic acid, glucoascorbic acid, fucoascorbic acid, glucohepto ascorbic acid, malto ascorbic acid, L-arabosascorbic acid), sodium 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 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 , item 37152, March 1995. Mixtures of these developing agents can be used if desired.  
      When a silver carboxylate silver source is used in a photothermographic material, one or more hindered phenol reducing agents are preferred. 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 co-developers and 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 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 reducing agents may contain more than one hydroxy group as long as each hydroxy group is located on different phenyl rings. Hindered phenol reducing agents include, for example, binaphthols (that is dihydroxybinaphthyls), biphenols (that is dihydroxy-biphenyls), bis(hydroxynaphthyl)methanes, bis(hydroxyphenyl)methanes (that is bisphenols), hindered phenols, and hindered naphthols, each of which may be variously substituted.  
      Preferred hindered phenol reducing agents are bis(hydroxy-phenyl)methanes such as, 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), and 2,2′-isobutylidene-bis(4,6-dimethyl-phenol) (LOWINOX® 221B46). Mixtures of hindered phenol reducing agents can be used if desired.  
      An additional class of reducing agents that can be used includes 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 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.), U.S. Pat. No. 3,887,417 (Klein et al.), and U.S. Pat. No. 5,981,151 (Leenders et al.). All of these patents are incorporated herein by reference.  
      Additional reducing agents that may be used include amidoximes, azines, a combination of aliphatic carboxylic acid aryl hydrazides and ascorbic acid, a reductone and/or a hydrazine, piperidinohexose reductone or formyl-4-methylphenylhydrazine, hydroxamic acids, a combination of azines and sulfonamidophenols, α-cyanophenylacetic acid derivatives, reductones, indane-1,3-diones, chromans, 1,4-dihydropyridines, and 3-pyrazolidones.  
      Useful co-developer reducing agents can also be used as described in U.S. Pat. No. 6,387,605 (Lynch et al.) that is incorporated herein by reference. 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 such as the compounds identified as HET-01 and HET-02 in U.S. Pat. No. 5,635,339 (Murray) and CN-01 through CN-13 in U.S. Pat. No. 5,545,515 (Murray et al.).  
      Various contrast enhancing agents can be used in some photothermographic materials with specific co-developers. Examples of useful contrast enhancing agents include, but are not limited to, hydroxylamines, alkanolamines and ammonium phthalamate compounds as described 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 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 patents above are incorporated herein by reference.  
      When used with a silver carboxylate silver source in a thermographic material, preferred reducing agents are aromatic di- and tri-hydroxy compounds having at least two hydroxy groups in ortho- or para-relationship on the same aromatic nucleus. Examples are hydroquinone and substituted hydroquinones, catechols, pyrogallol, gallic acid and gallic acid esters (for example, methyl gallate, ethyl gallate, propyl gallate), and tannic acid.  
      Particularly preferred are reducing catechol-type reducing agents having no more than two hydroxy groups in an ortho-relationship. Preferred catechol-type reducing agents include, for example, catechol, 3-(3,4-dihydroxy-phenyl)-propionic acid, 2,3-dihydroxy-benzoic acid, 2,3-dihydroxy-benzoic acid esters, 3,4-dihydroxy-benzoic acid, and 3,4-dihydroxy-benzoic acid esters.  
      One particularly preferred class of catechol-type reducing agents are benzene compounds in which the benzene nucleus is substituted by no more than two hydroxy groups which are present in 2,3-position on the nucleus and have in the 1-position of the nucleus a substituent linked to the nucleus by means of a carbonyl group. Compounds of this type include 2,3-dihydroxy-benzoic acid, methyl 2,3-dihydroxy-benzoate, and ethyl 2,3-dihydroxy-benzoate.  
      Another particularly preferred class of catechol-type reducing agents are benzene compounds in which the benzene nucleus is substituted by no more than two hydroxy groups which are present in 3,4-position on the nucleus and have in the 1-position of the nucleus a substituent linked to the nucleus by means of a carbonyl group. Compounds of this type include, for example, 3,4-dihydroxy-benzoic acid, methyl 3,4-dihydroxy-benzoate, ethyl 3,4-dihydroxy-benzoate, 3,4-dihydroxy-benzaldehyde, and phenyl-(3,4-dihydroxyphenyl)ketone. Such compounds are described, for example, in U.S. Pat. No. 5,582,953 (Uyttendaele et al.).  
      Still another useful class of reducing agents includes polyhydroxy spiro-bis-indane compounds described as photographic tanning agents in U.S. Pat. No. 3,440,049 (Moede).  
      Aromatic di- and tri-hydroxy reducing agents can also be used in combination with hindered phenol reducing agents and further in combination with one or more high contrast co-developing agents and co-developer contrast-enhancing agents).  
      The reducing agent (or mixture thereof) described herein is generally present as 1 to 10% (dry weight) of the emulsion layer. In multilayer constructions, if the reducing agent is added to a layer other than an emulsion layer, slightly higher proportions, of from about 2 to 15 weight % may be more desirable. Co-developers may be present generally in an amount of from about 0.001% to about 1.5% (dry weight) of the emulsion layer coating.  
      Other Addenda  
      The thermally developable materials can also contain other additives such as shelf-life stabilizers, antifoggants, contrast enhancers, development accelerators, acutance dyes, post-processing stabilizers or stabilizer precursors, thermal solvents (also known as melt formers), and other image-modifying agents as would be readily apparent to one skilled in the art.  
      To further control the properties of photothermographic materials, (for example, contrast, D min , speed, or fog), it may be preferable to add one or more heteroaromatic mercapto compounds or heteroaromatic disulfide compounds of the formulae Ar—S-M 1  and Ar—S—S—Ar, wherein M 1  represents a hydrogen atom or an alkali metal atom and Ar represents a heteroaromatic ring or fused heteroaromatic ring containing one or more of nitrogen, sulfur, oxygen, selenium, or tellurium atoms. Preferably, the heteroaromatic ring comprises benzimidazole, naphthimidazole, benzothiazole, naphthothiazole, benzoxazole, naphthoxazole, benzoselenazole, benzotellurazole, imidazole, oxazole, pyrazole, triazole, thiazole, thiadiazole, tetrazole, triazine, pyrimidine, pyridazine, pyrazine, pyridine, purine, quinoline, or quinazolinone. Useful heteroaromatic mercapto compounds are described as supersensitizers for infrared photothermographic materials in EP 0 559 228B1 (Philip Jr. et al.).  
      Heteroaromatic mercapto compounds are most preferred. Examples of preferred heteroaromatic mercapto compounds are 2-mercaptobenz-imidazole, 2-mercapto-5-methylbenzimidazole, 2-mercaptobenzothiazole and 2-mercaptobenzoxazole, and mixtures thereof.  
      A heteroaromatic mercapto compound is generally present in an emulsion layer in an amount of at least 0.0001 mole (preferably from about 0.001 to about 1.0 mole) per mole of total silver in the emulsion layer.  
      The photothermographic materials can be further protected against the production of fog and can be stabilized against loss of sensitivity during storage. Suitable antifoggants and stabilizers that can be used alone or in combination include thiazolium salts as described in U.S. Pat. No. 2,131,038 (Brooker) 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), urazoles as 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), compounds having —SO 2 CBr 3  groups as described in U.S. Pat. No. 5,594,143 (Kirk et al.) and U.S. Pat. No. 5,374,514 (Kirk et al.), 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 as described in 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 benzo-triazoles (for example alkylsulfonylbenzotriazoles and arylsulfonylbenzotriazoles) may be useful as described in U.S. Pat. No. 6,171,767 (Kong et al.).  
      Other useful antifoggants/stabilizers are described in U.S. Pat. No. 6,083,681 (Lynch et al.). Still other antifoggants are hydrobromic acid salts of heterocyclic compounds (such as pyridinium hydrobromide perbromide) as described in U.S. Pat. No. 5,028,523 (Skoug), benzoyl acid compounds as described in U.S. Pat. No. 4,784,939 (Pham), substituted propenenitrile compounds as described in U.S. Pat. No. 5,686,228 (Murray et al.), silyl blocked compounds as described in U.S. Pat. No. 5,358,843 (Sakizadeh et al.), vinyl sulfones as described 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 in EP 0 600 587A1 (Oliff et al.).  
      Preferably, the photothermographic materials include one or more polyhalo antifoggants that include one or more polyhalo substituents including but not limited to, dichloro, dibromo, trichloro, and tribromo groups. The antifoggants can be aliphatic, alicyclic or aromatic compounds, including aromatic heterocyclic and carbocyclic compounds. Particularly useful antifoggants are polyhalo antifoggants, such as those having a —SO 2 C(X′) 3  group wherein X′ represents the same or different halogen atoms.  
      The photothermographic materials may also include one or more thermal solvents (or melt formers) such as disclosed in U.S. Pat. No. 3,438,776 (Yudelson), U.S. Pat. No. 5,250,386 (Aono et al.), U.S. Pat. No. 5,368,979 (Freedman et al.), U.S. Pat. No. 5,716,772 (Taguchi et al.), and U.S. Pat. No. 6,013,420 (Windender).  
      It is often advantageous to include a base-release agent or base precursor in photothermographic materials. Representative base-release agents or base precursors include guanidinium compounds and other compounds that are known to release a base but do not adversely affect photographic silver halide materials (such as phenylsulfonyl acetates) as described in U.S. Pat. No. 4,123,274 (Knight et al.).  
      “Toners” or derivatives thereof that improve the image are highly desirable components of the thermally developable materials. Toners (also known as “toning agents”) are compounds that when added to the imaging layer(s) shift the color of the developed silver image from yellowish-orange to brown-black or blue-black. Generally, one or more toners described herein are present in an amount of about 0.01% by weight to about 10%, and more preferably about 0.1% by weight to about 10% by weight, based on the total dry weight of the layer in which it is included. Toners may be incorporated in the photothermographic emulsion layer(s) or in an adjacent non-imaging layer.  
      Compounds useful as toners are described 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).  
      Phthalazine and phthalazine derivatives [such as those described in U.S. Pat. No. 6,146,822 (Asanuma et al.), incorporated herein by reference], phthalazinone, and phthalazinone derivatives are particularly useful toners.  
      Additional useful toners are substituted and unsubstituted mercaptotriazoles as described in U.S. Pat. No. 3,832,186 (Masuda et al.), U.S. Pat. No. 6,165,704 (Miyake et al.), U.S. Pat. No. 5,149,620 (Simpson et al.), U.S. Pat. No. 6,713,240 (Lynch et al.), and U.S. Published application 2004-0013984 (Lynch et al.), all of which are incorporated herein by reference.  
      Also useful are the phthalazine compounds described in U.S. Pat. No. 6,605,418 (Ramsden et al.), the triazine thione compounds described in U.S. Pat. No. 6,703,191 (Lynch et al.), and the heterocyclic disulfide compounds described in U.S. Pat. No. 6,737,227 (Lynch et al.), all of which are incorporated herein by reference.  
      The photothermographic materials can also include one or more image stabilizing compounds that are usually incorporated in a “backside” layer. Such compounds can include phthalazinone and its derivatives, pyridazine and its derivatives, benzoxazine and benzoxazine derivatives, benzothiazine dione and its derivatives, and quinazoline dione and its derivatives, particularly as described in U.S. Pat. No. 6,599,685 (Kong). Other useful backside image stabilizers include anthracene compounds, coumarin compounds, benzophenone compounds, benzotriazole compounds, naphthalic acid imide compounds, pyrazoline compounds, or compounds described in U.S. Pat. No. 6,465,162 (Kong et al), and GB 1,565,043 (Fuji Photo). All of these patents and patent applications are incorporated herein by reference.  
      Phosphors are materials that emit infrared, visible, or ultraviolet radiation upon excitation and can be incorporated into the photothermographic materials. Particularly useful phosphors are sensitive to X-radiation and emit radiation primarily in the ultraviolet, near-ultraviolet, or visible regions of the spectrum (that is, from about 100 to about 700 nm). An intrinsic phosphor is a material that is naturally (that is, intrinsically) phosphorescent. An “activated” phosphor is one composed of a basic material that may or may not be an intrinsic phosphor, to which one or more dopant(s) has been intentionally added. These dopants or activators “activate” the phosphor and cause it to emit ultraviolet or visible radiation. Multiple dopants may be used and thus the phosphor would include both “activators” and “co-activators.” 
      Any conventional or useful phosphor can be used, singly or in mixtures. For example, useful phosphors are described in numerous references relating to fluorescent intensifying screens as well as U.S. Pat. No. 6,440,649 (Simpson et al.) and U.S. Pat. No. 6,573,033 (Simpson et al.) that are directed to photothermographic materials, both of which references are incorporated herein.  
      Some particularly useful phosphors are primarily “activated” phosphors known as phosphate phosphors and borate phosphors. Examples of these phosphors are rare earth phosphates, yttrium phosphates, strontium phosphates, or strontium fluoroborates (including cerium activated rare earth or yttrium phosphates, or europium activated strontium fluoroborates) as described in U.S. Ser. No. 10/826,500 (filed Apr. 16, 2004 by Simpson, Sieber, and Hansen).  
      The one or more phosphors can be present in the photothermographic materials in an amount of at least 0.1 mole per mole, and preferably from about 0.5 to about 20 mole, per mole of total silver in the photothermographic material. As noted above, generally, the amount of total silver is at least 0.002 mol/m 2 . While the phosphors can be incorporated into any imaging layer on one or both sides of the support, it is preferred that they be in the same layer(s) as the photosensitive silver halide(s) on one or both sides of the support.  
      Binders  
      The photosensitive silver halide (if present), the non-photosensitive source of reducible silver ions, the reducing agent composition, and any other imaging layer additives are generally combined with one or more binders that are generally hydrophobic or hydrophilic in nature. Thus, either aqueous or organic solvent-based formulations can be used to prepare the thermally developable materials of this invention. Mixtures of either or both types of binders can also be used. It is preferred that the binder be selected from predominantly hydrophobic polymeric materials (at least 50 dry weight % of total binders).  
      Examples of typical hydrophobic binders include polyvinyl acetals, polyvinyl chloride, polyvinyl acetate, cellulose acetate, cellulose acetate butyrate, polyolefins, polyesters, polystyrenes, polyacrylonitrile, polycarbonates, methacrylate copolymers, maleic anhydride ester copolymers, butadiene-styrene copolymers, and other materials readily apparent to one skilled in the art. Copolymers (including terpolymers) are also included in the definition of polymers. The polyvinyl acetals (such as polyvinyl butyral and polyvinyl formal) and vinyl copolymers (such as polyvinyl acetate and polyvinyl chloride) are particularly preferred. Particularly suitable binders are polyvinyl butyral resins that are available under the names BUTVAR® (Solutia, Inc., St. Louis, Mo.) and PIOLOFORM® (Wacker Chemical Company, Adrian, Mich.).  
      Hydrophilic binders or water-dispersible polymeric latex polymers can also be present in the formulations. Examples of useful hydrophilic binders include, but are not limited to, proteins and protein derivatives, gelatin and gelatin-like derivatives (hardened or unhardened), cellulosic materials such as hydroxymethyl cellulose and cellulosic esters, acrylamide/methacrylamide polymers, acrylic/methacrylic polymers polyvinyl pyrrolidones, polyvinyl alcohols, poly(vinyl lactams), polymers of sulfoalkyl acrylate or methacrylates, hydrolyzed polyvinyl acetates, polyacrylamides, polysaccharides and other synthetic or naturally occurring vehicles commonly known for use in aqueous-based photographic emulsions (see for example,  Research Disclosure , item 38957, noted above). Cationic starches can also be used as a peptizer for tabular silver halide grains as described in U.S. Pat. No. 5,620,840 (Maskasky) and U.S. Pat. No. 5,667,955 (Maskasky).  
      Hardeners for various binders may be present if desired. Useful hardeners are well known and include diisocyanate compounds as described in EP 0 600 586 B1 (Philip, Jr. et al.), vinyl sulfone compounds as described in U.S. Pat. No. 6,143,487 (Philip, Jr. et al.) and EP 0 640 589 A1 (Gathmann et al.), aldehydes and various other hardeners as described in U.S. Pat. No. 6,190,822 (Dickerson et al.). The hydrophilic binders used in the photothermographic materials are generally partially or fully hardened using any conventional hardener. Useful hardeners are well known and are described, for example, in T. H. James,  The Theory of the Photographic Process , Fourth Edition, Eastman Kodak Company, Rochester, N.Y., 1977, Chapter 2, pp. 77-8.  
      Where the proportions and activities of the thermally developable materials require a particular developing time and temperature, the binder(s) should be able to withstand those conditions. When a hydrophobic binder is used, it is preferred that the binder (or mixture thereof) does not decompose or lose its structural integrity at 120° C. for 60 seconds. When a hydrophilic binder is used, it is preferred that the binder does not decompose or lose its structural integrity at 150° C. for 60 seconds. It is more preferred that it does not decompose or lose its structural integrity at 177° C. for 60 seconds.  
      The polymer binder(s) is used in an amount sufficient to carry the components dispersed therein. Preferably, a binder is used at a level of from about 10% to about 90% by weight (more preferably at a level of from about 20% to about 70% by weight) based on the total dry weight of the layer. It is particularly useful that the thermally developable materials include at least 50 weight % hydrophobic binders in both imaging and non-imaging layers on both sides of the support (and particularly the imaging side of the support).  
      Support Materials  
      The thermally developable materials comprise a polymeric support that is preferably a flexible, transparent film that has any desired thickness and is composed of one or more polymeric materials. 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 polyesters [such as poly(ethylene terephthalate) and poly(ethylene naphthalate)], cellulose acetate and other cellulose esters, polyvinyl acetal, polyolefins, polycarbonates, and polystyrenes. Preferred supports are composed of polymers having good heat stability, such as polyesters and polycarbonates. Support materials may also be treated or annealed to reduce shrinkage and promote dimensional stability.  
      It is also useful to use supports comprising dichroic mirror layers as described in U.S. Pat. No. 5,795,708 (Boutet), incorporated herein by reference. Also useful are transparent, multilayer, polymeric supports comprising numerous alternating layers of at least two different polymeric materials as described in U.S. Pat. No. 6,630,283 (Simpson et al.), incorporated herein by reference.  
      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. For example, the support can include one or more dyes that provide a blue color in the resulting imaged film. 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.  
      Thermographic and Photothermographic Formulations and Constructions.  
      An organic solvent-based coating formulation for the thermographic and photothermographic emulsion layer(s) can be prepared by mixing the various components with one or more binders in a suitable organic solvent system that usually includes one or more solvents such as toluene, 2-butanone (methyl ethyl ketone), acetone, or tetrahydrofuran, or mixtures thereof.  
      Alternatively, the desired 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.  
      Thermally developable materials can contain plasticizers and lubricants such as poly(alcohols) and diols as described in U.S. Pat. No. 2,960,404 (Milton et al.), fatty acids or esters as described in U.S. Pat. No. 2,588,765 (Robijns) and U.S. Pat. No. 3,121,060 (Duane), and silicone resins as described in GB 955,061 (DuPont). The materials can also contain inorganic and organic matting agents as 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 as described in U.S. Pat. No. 5,468,603 (Kub).  
      U.S. Pat. No. 6,436,616 (Geisler et al.), incorporated herein by reference, describes various means of modifying photothermographic materials to reduce what is known as the “woodgrain” effect, or uneven optical density.  
      Layers to promote adhesion of one layer to another are also known, as described 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 in U.S. Pat. No. 5,928,857 (Geisler et al.).  
      Layers to reduce emissions from the material may also be present, including the polymeric barrier layers described in U.S. Pat. No. 6,352,819 (Kenney et al.), U.S. Pat. No. 6,352,820 (Bauer et al.), U.S. Pat. No. 6,420,102 (Bauer et al.), and U.S. Pat. No. 6,667,148 (Rao et al.), and in U.S. Pat. No. 6,746,831 (Hunt), all incorporated herein by reference.  
      Mottle and other surface anomalies can be reduced by incorporation of a fluorinated polymer as described 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.).  
      The thermally developable materials can also include one or more antistatic or conductive layers on the frontside of the support. Such layers may contain metal antimonates as described above, or other conventional antistatic agents known in the art for this purpose such as 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 above and in U.S. Pat. No. 5,368,995 (Christian et al.), electrically-conductive metal-containing particles dispersed in a polymeric binder such as those described in EP 0 678 776A1 (Melpolder et al.), and fluoro-chemicals that are described in numerous publications.  
      The photothermographic and thermographic materials may also usefully include a magnetic recording material as described in  Research Disclosure , Item 34390, November 1992, or a transparent magnetic recording layer such as a layer containing magnetic particles on the underside of a transparent support as described in U.S. Pat. No. 4,302,523 (Audran et al.).  
      To promote image sharpness, photothermographic materials 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 compositions may be incorporated into one or more antihalation backing layers, underlayers, or overcoats. Additionally, one or more acutance dyes may be incorporated into one or more frontside layers.  
      Dyes useful as antihalation and acutance dyes include squaraine dyes as described in U.S. Pat. No. 5,380,635 (Gomez et al.), U.S. Pat. No. 6,063,560 (Suzuki et al.), and EP 1 083 459A1 (Kimura), indolenine dyes as described in EP 0 342 810A1 (Leichter), and cyanine dyes as described in U.S. Pat. No. 6,689,547 (Hunt et al.), all incorporated herein by reference.  
      It is also useful to employ compositions including acutance or antihalation dyes that will decolorize or bleach with heat during processing, as described 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.), U.S. Pat. No. 6,306,566, (Sakurada et al.), JP Kokai 2001-142175 (Hanyu et al.), and JP Kokai 2001-183770 (Hanye et al.). Useful bleaching compositions are also described in JP Kokai 11-302550 (Fujiwara), JP Kokai 2001-109101 (Adachi), JP Kokai 2001-51371 (Yabuki et al.), and JP Kokai 2000-029168 (Noro). All of the noted publications are incorporated herein by reference.  
      Other useful heat-bleachable antihalation compositions can include an infrared radiation absorbing compound such as an oxonol dye or various other compounds used in combination with a hexaarylbiimidazole (also known as a “HABI”), or mixtures thereof. HABI compounds are described in 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 in U.S. Pat. No. 6,455,210 (Irving et al.), U.S. Pat. No. 6,514,677 (Ramsden et al.), and U.S. Pat. No. 6,558,880 (Goswami et al.), all incorporated herein by reference.  
      Under practical conditions of use, these compositions are heated to provide bleaching at a temperature of at least 90° C. for at least 0.5 seconds (preferably, at a temperature of from about 100° C. to about 200° C. for from about 5 to about 20 seconds).  
      In some embodiments, the thermally developable materials include a surface protective layer over one or more imaging layers on one or both sides of the support. In other embodiments, the materials include a surface protective layer on the same side of the support as the one or more emulsion layers and a layer on the backside that includes the required conductive antistatic composition (with or without an antihalation composition or layer). A separate non-conductive, backside surface protective layer can also be included in these embodiments.  
      The thermally developable formulations can be coated by various coating procedures including wire wound rod coating, dip coating, air knife coating, curtain coating, slide coating, or extrusion coating using hoppers of the type described in U.S. Pat. No. 2,681,294 (Beguin). Layers can be coated one at a time, or two or more layers can be coated simultaneously by the procedures described in U.S. Pat. No. 2,761,791 (Russell), U.S. Pat. No. 4,001,024 (Dittman et al.), U.S. Pat. No. 4,569,863 (Keopke et al.), U.S. Pat. No. 5,340,613 (Hanzalik et al.), U.S. Pat. No. 5,405,740 (LaBelle), U.S. Pat. No. 5,415,993 (Hanzalik et al.), U.S. Pat. No. 5,525,376 (Leonard), U.S. Pat. No. 5,733,608 (Kessel et al.), U.S. Pat. No. 5,849,363 (Yapel et al.), U.S. Pat. No. 5,843,530 (Jerry et al.), U.S. Pat. No. 5,861,195 (Bhave et al.), and GB 837,095 (Ilford). A typical coating gap for the emulsion layer can be from about 10 to about 750 μm, and the layer can be dried in forced air at a temperature of from about 20° C. to about 100° C. It is preferred that the thickness of the layer be selected to provide maximum image densities greater than about 0.2, and more preferably, from about 0.5 to 5.0 or more, as measured by an X-rite Model 361 V Densitometer equipped with 301 Visual Optics, available from X-rite Corporation, (Granville, Mich.).  
      Subsequently to or simultaneously with application of the emulsion formulation to the support, a protective overcoat formulation can be applied over the emulsion formulation.  
      Preferably, two or more layer formulations are applied simultaneously to a support using slide coating, the first layer being coated on top of the second layer while the second layer is still wet. The first and second fluids used to coat these layers can be the same or different solvents.  
      In other embodiments, a “carrier” layer formulation comprising a single-phase mixture of the two or more polymers described above may be applied directly onto the support and thereby located underneath the emulsion layer(s) as described in U.S. Pat. No. 6,355,405 (Ludemann et al.), incorporated herein by reference. The carrier layer formulation can be applied simultaneously with application of the emulsion layer formulation.  
      While the thermographic and photothermographic layers can be coated on one side of the film support, manufacturing methods can also include forming on the opposing or backside of the polymeric support, one or more additional layers, including the required conductive layer, and optionally an antihalation layer, a layer containing a matting agent (such as silica), or a combination of such layers. Alternatively, one backside layer can perform several or all of the desired functions.  
      Backside Conductive Compositions/Layers  
      The thermally developable materials have at least one conductive layer on the backside (non-imaging side) of the support that includes non-acicular metal antimonate particles. This conductive layer can be the sole backside layer, but preferably, there are multiple backside layers and the non-imaging conductive layer is a “buried” conductive layer and a protective overcoat layer is disposed over it. More preferably the conductive layer is a “buried” carrier layer.  
      The non-acicular metal antimonate particles generally have a composition represented by the following Structure I or II: 
 
M +2 Sb +5   2 O 6   (I) 
 
 wherein M is zinc, nickel, magnesium, iron, copper, manganese, or cobalt, 
 
M a   +3 Sb +5 O 4   (II) 
 
 wherein M a  is indium, aluminum, scandium, chromium, iron, or gallium. 
 
      Thus, these particles are generally metal oxides that are doped with antimony.  
      Preferably, the non-acicular metal antimonate particles are composed of zinc antimonate (ZnSb 2 O 6 ). Several conductive metal antimonates are commercially available from Nissan Chemical America Corporation including the preferred non-acicular zinc antimonate (ZnSb 2 O 6 ) particles that are available as a 60% (solids) organosol dispersion in methanol under the tradename CELNAX® CX-Z641M.  
      Alternatively, the metal antimonate particles can be prepared using methods described for example in U.S. Pat. No. 5,457,013 (noted above) and references cited therein.  
      The metal antimonate particles in the backside conductive layer are predominately in the form of non-acicular particles as opposed to “acicular” particles. By “non-acicular” particles is meant not needlelike, that is, not acicular. Thus, the shape of the metal antimonate particles can be granular, spherical, ovoid, cubic, rhombic, tabular, tetrahedral, octahedral, icosahedral, truncated cubic, truncated rhombic, or any other non-needle like shape.  
      Generally, these metal particles have an average diameter of from about 15 to about 20 nm as measured across the largest particle dimension using the BET method.  
      The non-acicular metal antimonate particles are generally present in an amount sufficient to provide a backside water electrode resistivity (WER) of 1×10 12  ohms/sq or less and preferably 1×10 11  ohms/sq or less at 70° F. (21.1° C.) and 50% relative humidity.  
      The non-acicular metal antimonate particles generally comprise from about more than 55 and up to 85% (preferably from 60 to about 76% and more preferably from about 70 to about 76%) by weight of the dry backside conductive layer. Another way of defining the amount of particles is that they are generally present in the backside conductive layer in an amount of from about 0.06 to about 0.5 g/m 2  (preferably from about 0.06 to about 0.4 g/m 2  and more preferably from about 0.06 to about 0.2 g/m 2 ) of the dry layer coverage. Mixtures of different types of non-acicular metal antimonate particles can be used if desired.  
      The backside conductive layer includes one or more binders (described in detail below) in an amount to provide a total binder to conductive particle ratio of less than 0.75:1 and preferably of from about 0.4:1 to about 0.3:1, based on dry weights. The optimum ratio of total binder to conductive particles can vary depending upon the specific binders used, the conductive particle size, the coverage of conductive particles, and the dry thickness of the conductive layer. One skilled in the art would be able to determine the optimum parameters to achieve the desired conductivity and adhesion to adjacent layers and/or support.  
      Preferably, the conductive metal antimonate particles are present in one or more backside conductive layers that are “buried” on the backside of the support. In such embodiments, the relationship of the backside conductive layer(s), and the layer or layers immediately adjacent is important because the types of polymers and binders in these layers are designed to provide excellent adhesion to one another as well as acceptably dispersing the conductive metal antimonate particles and/or or layer components, and are readily coated simultaneously or separately.  
      The backside conductive layer may also be relatively thin, especially if it is a “buried” conductive layer. For example, it can have a dry thickness of from about 0.05 to about 0.55 μm (preferably from 0.09 to about 0.3 μm, and most preferably of from about 0.09 to about 0.15 μm). The thin “buried” backside conductive layers are useful as “carrier” layers. The term “carrier layer” is often used when multiple layers are coated using slide coating and the buried backside conductive layer is a thin layer adjacent to the support.  
      In one preferred embodiment, the buried backside conductive layer is a carrier layer and is directly disposed on the support without the use of primer or subbing layers, or other adhesion-promoting means such as support surface treatments. Thus, the support can be used in an “untreated” and “uncoated” form when a buried backside conductive layer is used. The carrier layer formulation is applied simultaneously with application of these other backside layer formulations and is thereby located underneath these other backside layers. In a preferred construction, the backside conductive carrier layer formulation comprises a single-phase mixture of the two or more polymers described above and non-acicular metal antimonate particles.  
      The layer directly disposed over the conductive layers is known herein as a “first” layer and can be known as a “protective” layer that can be the outermost topcoat layer or have further layer(s) disposed thereon. This first layer comprises a film-forming polymer. The backside conductive layer immediately underneath comprises the non-acicular metal antimonate particles in a mixture of two or more polymers that includes a “first” polymer serving to promote adhesion of the backside conductive layer directly to the polymeric support, and a “second” polymer that is different than and forms a single-phase mixture with the first polymer.  
      It is preferred that film-forming polymer of the first layer and the second polymer of the backside conductive layer are the same or different polyvinyl acetal resins, polyester resins, cellulosic polymers, maleic anhydride-ester copolymers, or vinyl polymers. It is more preferred that the film-forming polymer of the first layer and the second polymer of the backside conductive layer is a polyvinyl acetal such as polyvinyl butyral or cellulose ester such as cellulose acetate butyrate. It is preferred that the “first” polymer of the backside conductive layer is a polyester resin. It is most preferred that the backside conductive layer is a single phase mixture of a polyester resin as a “first” polymer and cellulose acetate butyrate as a “second” polymer.” 
      It is preferred to use a mixture of polymers, that is, a first polymer that promotes adhesion to the support and a second polymer that promotes adhesion to the first layer. For example, when the support is a polyester film, and the backside conductive layer contains a polyvinyl acetal or a cellulose ester, then a preferred mixture of polymers in that conductive layer is a single-phase mixture of a polyester resin and a polyvinyl acetal such as polyvinyl butyral or cellulose ester such as cellulose acetate butyrate.  
      In another embodiment, the buried backside conductive layer is disposed between a “first” layer and a “second” layer directly adhering the support. In this embodiment, the “first” layer is directly above the backside conductive layer and is known herein as a “first” layer, a “protective” layer, or a “protective topcoat” layer. It can be the outermost topcoat layer or have further layer(s) disposed thereon. This first layer comprises a film-forming polymer. The conductive layer immediately beneath the first layer comprises the non-acicular metal antimonate particles in a polymer that serves to promote adhesion of the backside conductive layer to the first layer as well as to a “second” layer immediately beneath it. This second layer is directly adhered to the polymeric support. The second layer directly adhered to the support comprises a mixture of two or more polymers. The first polymer serves to promote adhesion of the second layer directly to the polymeric support. The second polymer serves to promote adhesion of the second layer to the backside conductive layer.  
      It is preferred that the film-forming polymer of the first layer, the polymer of the backside conductive layer, and the second polymer of the second layer are the same or different polyvinyl acetal resins, polyester resins, cellulosic ester polymers, maleic anhydride-ester copolymers, or vinyl polymers. A preferred polymer is cellulose acetate butyrate.  
      It is preferred that the second, adhesion-promoting, layer use a single phase mixture of a polyester resin as a “first” polymer and a polyvinyl acetal such as polyvinyl butyral or cellulose ester such as cellulose acetate butyrate as a “second” polymer.” 
      In another embodiment, the buried backside conductive layer is disposed between a “first” layer and a “second” layer directly adhering to the support. In this embodiment, the first layer is directly above the backside conductive layer is known herein as a “first” layer, a “protective” layer, or a “protective topcoat” layer. It can be the outermost topcoat layer or have further layer(s) disposed thereon. This first layer comprises a film-forming polymer. The conductive layer immediately beneath the first layer comprises the non-acicular metal antimonate particles in a mixture of two or more polymers, a “first” polymer that serves to promote adhesion of the conductive layer to the second layer, and a “second” polymer that serves to promote adhesion of the conductive layer to the first layer.  
      It is preferred that the film-forming polymer of the first layer, and the “second” polymer of the backside conductive layer are the same or different polyvinyl acetal resins, polyester resins, cellulosic ester polymers, maleic anhydride-ester copolymers, or vinyl polymers. A preferred polymer is cellulose acetate butyrate.  
      It is also preferred that the polymer of the second, adhesion promoting, layer and the “first” polymer of the backside conductive layer are the same or different polyester resins.  
      Representative “first” polymers can be chosen from one or more of the following classes: polyvinyl acetals (such as polyvinyl butyral, polyvinyl acetal, and polyvinyl formal), cellulosic ester polymers (such as cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose acetate propionate, hydroxy-methyl cellulose, cellulose nitrate, and cellulose acetate butyrate), polyesters, polycarbonates, epoxies, rosin polymers, polyketone resin, vinyl polymers (such as polyvinyl chloride, polyvinyl acetate, polystyrene, polyacrylonitrile, and butadiene-styrene copolymers), acrylate and methacrylate polymers, and maleic anhydride ester copolymers. The polyvinyl acetals, polyesters, cellulosic ester polymers, and vinyl polymers such as polyvinyl acetate and polyvinyl chloride are particularly preferred, and the polyvinyl acetals, polyesters, and cellulosic ester polymers are more preferred. Polyester resins are most preferred. Thus, the adhesion-promoting polymers are generally hydrophobic in nature.  
      Representative “second” polymers include polyvinyl acetals, cellulosic polymers, vinyl polymers (as defined above for the “first” polymer), acrylate and methacrylate polymers, and maleic anhydride-ester copolymers. The most preferred “second” polymers are polyvinyl acetals and cellulosic ester polymers (such as cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose acetate propionate, hydroxymethyl cellulose, cellulose nitrate, and cellulose acetate butyrate). Cellulose acetate butyrate is a particularly preferred second polymer. Of course, mixtures of these second polymers can be used in the backside conductive layer. These second polymers are also soluble or dispersible in the organic solvents described above.  
      It is preferred that the “first” and “second” polymers are compatible with each other or are of the same polymer class. One skilled in the art would readily understand from the teaching herein which polymers are “compatible with” or “of the same class” as those film-forming polymers. For example, it is most preferred to use a single phase mixture of a polyester resin as a “first” polymer and a cellulose ester such as cellulose acetate butyrate as a “second” polymer.” Many of the film-forming polymers useful in the first layer are described in other places herein (for example, binders used in imaging layers and or other conventional backside layers).  
      The backside conductive layers are generally coated out of one or more miscible organic solvents including, but not limited to, methyl ethyl ketone (2-butanone, MEK), acetone, toluene, tetrahydrofuran, ethyl acetate, ethanol, methanol, or any mixture of any two or more of these solvents.  
      The backside conductive layers can be coated using various coating procedures such as those described above for the thermographic and photothermographic imaging layers.  
      The weight ratio of “first” polymer to “second” polymer in the backside conductive layer is generally from about 10:90 to about 40:60, and preferably from about 10:90 to about 30:70. A most preferred polymer combination is of polyester and cellulose acetate butyrate having a weight ratio of about 20:80.  
      The backside conductive layer can also include still other polymers that are not defined herein as first or second polymers. These additional polymers can be either hydrophobic or hydrophilic. Some hydrophilic polymers that may be present include, but are not limited to, proteins or polypeptides such as gelatin and gelatin derivatives, polysaccharides, gum arabic, dextrans, polyacrylamides (including polymethacrylamides), polyvinyl pyrrolidones and others that would be readily apparent to one skilled in the art.  
      Other components of the backside conductive layer include materials that may improve coatability or adhesion, crosslinking agents (such as diisocyanates), surfactants and shelf-aging promoters.  
      The backside conductive layer may also include other addenda commonly added to such formulations including, but not limited to, shelf life extenders, antihalation dyes, colorants to control tint and tone, magnetic recording materials to record data, UV absorbing materials to improve light-box stability, and coating aids such as surfactants to achieve high quality coatings, all in conventional amounts. It is also useful to add inorganic matting agents such as the polysilicic acid particles as described in U.S. Pat. No. 4,828,971 (Przezdziecki), poly(methyl methacrylate) beads as described in U.S. Pat. No. 5,310,640 (Markin et al.), or polymeric cores surrounded by a layer of colloidal inorganic particles as described in U.S. Pat. No. 5,750,328 (Melpolder et al.). Alternatively, such materials can also be present in the “first” backside layer.  
      In one preferred embodiment, the “first” backside layer (usually referred to as a protective or topcoat layer) includes an antihalation composition, such as those antihalation compositions described above.  
      When production quantities of materials are prepared, the speed required for efficient mixing often results in shear conditions that are different from those involved for the preparation of laboratory quantities. As a result, the materials so produced can have properties different from those of laboratory-prepared samples. Backside coating formulations containing high amounts of non-acicular zinc antimonate (that is, a low binder to non-acicular zinc antimonate ratio) show less sensitivity to change in shear conditions when compared to similarly prepared materials having a high binder to non-acicular zinc antimonate ratio. This permits the preparation of large quantities of conductive backside coating formulations whose properties are independent of mixing conditions such as blade size, mixing speed, and preparation vessel size. Moreover, coatings prepared using these backside coating formulations containing high amounts of non-acicular zinc antimonate (that is, a low binder to non-acicular zinc antimonate ratio) demonstrate improved conductive efficiency. This permits lower coating weight of the backside conductive layer to achieve a given resistivity.  
      Still other conductive compositions include fluorochemicals that are reaction products of R f -CH 2 CH 2 —SO 3 H with amines wherein R f  comprises 4 or more fully fluorinated carbon atoms as described in U.S. Pat. No. 6,699,648 (Sakizadeh et al.) that is incorporated herein by reference.  
      Additional conductive compositions include one or more fluoro-chemicals described in U.S. Pat. No. 6,762,013 (Sakizadeh et al.) that is incorporated herein by reference.  
      Imaging/Development  
      The thermally developable materials 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 a source of thermal energy for thermographic materials). In some embodiments, the materials are sensitive to radiation in the range of from about at least 300 nm to about 1400 nm, and preferably from about 300 nm to about 850 nm. In other embodiments, the materials are sensitive to radiation at 700 nm or greater (such as from about 750 to about 950 nm).  
      Imaging can be achieved by exposing the photothermographic materials to a suitable source of radiation to which they are sensitive, including X-radiation, ultraviolet radiation, visible light, near infrared radiation and infrared radiation to provide a latent image. Suitable exposure means are well known and include sources of radiation, including: incandescent or fluorescent lamps, xenon flash lamps, lasers, laser diodes, light emitting diodes, infrared lasers, infrared laser diodes, infrared light-emitting diodes, infrared lamps, or any other ultraviolet, visible, or infrared radiation source readily apparent to one skilled in the art, and others described in the art, such as in  Research Disclosure , September, 1996, item 38957. Particularly useful infrared exposure means include laser diodes, including laser diodes that are modulated to increase imaging efficiency using what is known as multi-longitudinal 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.).  
      Thermal development conditions will vary, depending on the construction used but will typically involve heating the imagewise exposed material at a suitably elevated temperature, 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.  
      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 imaging thermographic materials, the image may be “written” simultaneously with development at a suitable temperature using a thermal stylus, a thermal print head or a laser, or by heating while in contact with a heat-absorbing material. The thermographic materials may include a dye (such as an IR-absorbing dye) to facilitate direct development by exposure to laser radiation.  
      Use as a Photomask  
      The thermographic and photothermographic materials can be sufficiently transmissive in the range of from about 350 to about 450 nm in non-imaged areas to allow their use in a method where there is a subsequent exposure of an ultraviolet or short wavelength visible radiation sensitive imageable medium. The heat-developed materials absorb ultraviolet or short wavelength visible radiation in the areas where there is a visible image and transmit ultraviolet or short wavelength visible radiation where there is no visible image. The heat-developed materials may then be used as a mask and positioned between a source of imaging radiation (such as an ultraviolet or short wavelength visible radiation energy source) and an imageable material that is sensitive to such imaging radiation, such as a photopolymer, diazo material, photoresist, or photosensitive printing plate. Exposing the imageable material to the imaging radiation through the visible image in the exposed and heat-developed thermographic or photothermographic material provides an image in the imageable material. This method is particularly useful where the imageable medium comprises a printing plate and the photothermographic material serves as an imagesetting film.  
      Thus, in some other embodiments wherein the thermographic or photothermographic material comprises a transparent support, the image-forming method further comprises, after step (A) or steps (A) and (B) noted above:  
      (C) positioning the imaged, heat-developed photothermographic or thermographic material 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 an image in the imageable material.  
      The following examples are provided to illustrate the practice of the present invention and the invention is not meant to be limited thereby.  
      Materials and Methods for the Experiments and Examples:  
      All materials used in the following examples are readily available from standard commercial sources, such as Aldrich Chemical Co. (Milwaukee Wis.) unless otherwise specified. All percentages are by weight unless otherwise indicated. The following additional methods and materials were used.  
      PARALOID® A-21 is an acrylic copolymer available from Rohm and Haas (Philadelphia, Pa.).  
      CAB 171-15S and CAB 381-20 are cellulose acetate butyrate resins available from Eastman Chemical Co. (Kingsport, Tenn.).  
      CELNAX® CX-Z641M is an organosol dispersion containing 60% of non-acicular zinc antimonate particles in methanol. It was obtained from Nissan Chemical America Corporation (Houston, Tex.).  
      MEK is methyl ethyl ketone (or 2-butanone).  
      SYLOID® 74×6000 is a synthetic amorphous silica that is available from Grace-Davison (Columbia, Md.).  
      VITEL® PE-2700B LMW is a polyester resin available from Bostik, Inc. (Middleton, Mass.).  
      Backcoat Dye BC-1 is cyclobutenediylium, 1,3-bis[2,3-dihydro-2,2-bis[[1-oxohexyl)oxy]methyl]-1H-perimidin-4-yl]-2,4-dihydroxy-, bis(inner salt). It is believed to have the structure shown below.  
                 
 
      Ethyl-2-cyano-3-oxobutanoate is described in U.S. Pat. No. 5,686,228 and has the structure shown below.  
                 
 
      Vinyl Sulfone-1 (VS-1) is described in U.S. Pat. No. 6,143,487 and has the following structure:  
                 
 
 Resistivity Measurements: 
 
      The charge control performance of antistatic backside conductive layers can be reported in terms of their surface electrode resistivity (SER) as described in U.S. Pat. No. 6,689,546 (noted above). For buried conductive layers, which are a preferred embodiment of this invention, a better measuring technique is their water electrode resistivity (WER). For materials where the backside conductive layer is the surface layer, the WER and SER are essentially the same. For buried conductive layers however, WER measurement removes the influence of any protective overcoats on the measured resistivity. Although the advantages of the present invention are described in terms of WER, it would be apparent to one skilled in the art that the same advantages maybe demonstrated in terms of SER measurements.  
      Resistivity of antistatic coatings was measured using the “water electrode resistivity” (WER) test. In this test, antistatic performance is evaluated by measuring the internal resistivity of the overcoated electrically conductive antistatic layer using a salt bridge water electrode resistivity measurement technique. This technique is described in R. A. Elder “Resistivity Measurements on Buried Conductive Layers,” EOS/ESD Symposium Proceedings, Lake Buena Vista, Fla., 1990, pp. 251-254, incorporated herein by reference. [EOS/ESD stands for Electrical Overstress/Electrostatic Discharge]. Typically, antistatic layers with WER values greater than about 1×10 12  ohm/square are considered to be ineffective at providing static protection for photographic imaging elements. We have also found WER measurements to be predictive of how an antistatic material will perform when used in a commercial product. Resistivity was measured in a room maintained at 70° F. (21.1° C.) and 50% relative humidity (RH) after samples had been acclimated for 18 to 24 hours.  
      Adhesion Test:  
      Samples were evaluated using a “cross-hatch” adhesion test performed according to ASTM D3359-92A. A coated film was cut with a razor blade in a cross-hatched pattern, a 1 inch (2.54 cm) wide piece of commercially available 3M Type 610 semi-transparent pressure-sensitive tape was placed on the pattern and then quickly lifted off. The amount of coating left on the film is the measure of adhesion. The adhesion test ratings are from 0 to 5 where 0 refers to complete removal of the coating and 5 refers none or very little coating removed. A rating of “3” or greater is considered to be acceptable. 3M Type 610 semi-transparent pressure-sensitive tape was obtained from 3M Company (Maplewood, Minn.).  
     EXAMPLE 1  
     Photothermographic Materials with Improved Conductive Efficiency  
      Photothermographic materials were prepared containing buried backside antistatic layers. The ratio of non-acicular zinc antimonate [CELNAX® CX-Z 641M (ZnSb 2 O 6 )] to binder was varied. A control sample was also prepared containing a ratio of binder to non-acicular zinc antimonate as described in U.S. Pat. No. 6,689,546 (noted above).  
      Buried Backside Conductive Layer Formulation:  
      A buried backside conductive layer formulation was prepared by mixing the following materials:  
                               Solution A:                                                VITEL ® PE-2700B LMW   (see TABLE I below)           CAB 381-20   (see TABLE I below)           MEK   1,269 g                      
 
     
       
         
           
               
             
               
                   
               
               
                   
               
               
                 Solution B: 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 CELNAX ® CX-Z641M 
                 (see TABLE I below) 
               
               
                 (containing 60% non-acicular zinc antimonate 
               
               
                 solids in methanol) 
               
               
                 MEK 
                 120 g 
               
               
                   
               
            
           
         
       
     
      Solution A: VITEL® PE-2700B LMW and CAB 381-20 were dissolved in 1.269 Kg of MEK.  
      Solution B: CELNAX® CX-Z641M non-acicular zinc antimonate was placed in a second reaction vessel, stirring was begun and 120 g of MEK was added.  
      Buried Backside Layer Solution: Solution A was added to Solution B with stirring.  
      Backside Topcoat Formulation:  
      A backside topcoat formulation was prepared by mixing the following materials:  
                                                      MEK   88.9 weight %           CAB 381-20   10.7 weight %           SYLOID ® 74X6000   0.28 weight %           Antihalation Dye BC-1   0.14 weight %                      
 
      The buried backside conductive layer formulation and backside topcoat formulations were simultaneously coated onto one side of a 7 mil (178 μm) blue tinted poly(ethylene terephthalate) support. A precision multilayer coater equipped with an in-line dryer was used. The dry coating weight of the backside topcoat layer was 0.2 g/ft 2  (2.2 g/m 2 ). TABLE I shows the amount of non-acicular zinc antimonate and binders in each sample as well as the non-acicular zinc antimonate binder ratio.  
      Photothermographic Emulsion Formulation:  
      An infrared-sensitive photothermographic emulsion coating formulation was prepared using a silver salt homogenate prepared substantially as described in Col. 25 of U.S. Pat. No. 5,434,043 (noted above), incorporated herein by reference. The photothermographic emulsion formulation was then prepared substantially as described in Cols. 19-24 of U.S. Pat. No. 5,541,054 (Miller et al.) that is also incorporated herein by reference.  
      Photothermographic Emulsion Topcoat Formulation:  
      A topcoat formulation was prepared for application over the photothermographic emulsion formulation with the following components:  
                                                      MEK   86.10 weight %            VS-1   0.35 weight %           Benzotriazole   0.27 weight %           Silica   0.21 weight %           PARALOID ® A-21   0.47 weight %           CAB 171-15S   12.25 weight %            Antihalation dye BC-1   0.21 weight %           Ethyl-2-cyano-3-oxobutanoate   0.23 weight %                      
 
      Photothermographic Emulsion Carrier Layer Formulation:  
      A “carrier” layer formulation for the photothermographic emulsion and topcoat layers was prepared as described in U.S. Pat. No. 6,355,405 (Ludemann et al.).  
      Preparation of Photothermographic Materials:  
      The photothermographic emulsion, topcoat, and carrier layer formulations were coated onto the side of the support opposite to that containing the antistatic coating using a precision multilayer coater equipped with an in-line dryer.  
      Evaluation of Samples:  
      The coating weight of CELNAX® non-acicular zinc antimonate and the layer thickness necessary to achieve a WER of 1×10 9  ohm/sq was determined. The results, shown below in TABLE II, demonstrate that samples with a lower binder to CELNAX® non-acicular zinc antimonate ratio have improved conductive efficiency.  
      The resistivity of samples with a constant coating weight was also determined. The results, also shown below in TABLE II, demonstrate that samples having a thin coating with a low binder to CELNAX® non-acicular zinc antimonate ratio provide highly conductive materials.  
      Samples were also evaluated for adhesion of the backside conductive layer and backside topcoat layer to the support. The results, shown below in TABLE II demonstrate that excellent adhesion is maintained even at a Binder to CELNAX® non-acicular zinc antimonate ratio of 0.37:1.  
                               TABLE I                           CELNAX ®   VITEL ®       Binder:CELNAX ®       Sample   CX-Z641M   PE-2700B LMW   CAB 381-20   Ratio                  Control-1   48.0 weight %   10.4 weight %    41.8 weight %   1.09:1       1-1   57.99 weight %    8.4 weight %   33.7 weight %   0.73:1       1-2   73.1 weight %   5.4 weight %   21.5 weight %   0.37:1                  
 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE II 
               
             
            
               
                   
                   
               
               
                   
                   
               
               
                   
                 Thickness of 
                   
                   
                   
               
               
                   
                 Conductive Layer 
               
               
                   
                 at a WER of 
                 CELNAX ® coverage 
               
               
                   
                 1 × 10 9  ohm/sq 
                 at a WER of 
                 WER at a CELNAX ® 
                 Cross Hatch 
               
            
           
           
               
               
               
               
               
               
            
               
                 Sample 
                 Wet 
                 Dry 
                 1 × 10 9  ohm/sq 
                 coverage of 0.24 g/m 2   
                 Adhesion 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 1-Control 
                 10.7 (μm) 
                 0.70 (μm) 
                 0.31 g/m 2   
                 6.3 × 10 11   
                 ohm/sq 
                 4 
               
               
                 1-1 
                 8.48 (μm) 
                 0.55 (μm) 
                 0.26 g/m 2   
                 2.0 × 10 10   
                 ohm/sq 
                 4 
               
               
                 1-2 
                 3.53 (μm) 
                 0.23 (μm) 
                 0.22 g/m 2   
                 6.3 × 10 8   
                 ohm/sq 
                 4 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 2  
     Effect Of Binder to CELNAX® Non-Acicular Zinc Antimonate Ratio on Adhesion to the Support  
      The following Example demonstrates the effect of the binder to CELNAX® non-acicular zinc antimonate ratio on adhesion of the backside conductive layer and backside topcoat layer to the support.  
      Buried Backside Conductive Layer Formulation:  
      A buried backside conductive layer formulation was prepared by mixing the following materials:  
                                      CELNAX ® CX-Z641M   (see TABLE III below)       (containing 60% non-acicular zinc antimonate       solids in methanol)       MEK   (see TABLE III below)       VITEL ® PE-2700B LMW   (see TABLE III below)       CAB 381-20   (see TABLE III below)                  
 
      Backside Topcoat Formulation:  
      A backside topcoat formulation was prepared by mixing the following materials:  
                                                      MEK   88.9 weight %           CAB 381-20   10.7 weight %           SYLOID ® 74X6000   0.28 weight %           Antihalation Dye BC-1   0.14 weight %                      
 
      The buried backside conductive layer formulation and backside topcoat formulations were simultaneously coated onto one side of a 7 mil (178 μm) blue tinted poly(ethylene terephthalate) support. A precision multilayer coater equipped with an in-line dryer was used. The dry coating weight of the backside topcoat layer was 0.2 g/ft 2  (2.2 g/m 2 ). The coating weight of the backside conductive layer was 0.03 g/ft 2  (0.323 g/m 2 ). No photothermographic coating was on the opposite side.  
      Evaluation of Samples:  
      Samples were prepared to evaluate the binder to CELNAX® non-acicular zinc antimonate ratio on adhesion of the backside conductive layer and backside topcoat layer to the support. The results, shown below in TABLE III demonstrate that good adhesion is maintained at a binder to CELNAX® non-acicular zinc antimonate ratio of 0.37:1 (73% CELNAX® non-acicular zinc antimonate) and adhesion became poor at a binder to CELNAX® non-acicular zinc antimonate ratio of 0.28:1 (78% CELNAX® non-acicular zinc antimonate). It is to be understood that different polymer systems will affect the maximum CELNAX® non-acicular zinc antimonate loading and its effect on adhesion.  
                                   TABLE III                           CELNAX ®                           CX-Z641M   VITEL ®       Binder:CELNAX ®   Cross Hatch       Sample   (Dry wt.)   PE-2700B LMW   CAB 381-20   Ratio   Adhesion                                                        2-1   47.6 weight %   10.5 weight %    41.9 weight %    1.1:1   5       2-2   57.6 weight %   8.5 weight %   33.9 weight %   0.73:1   5       2-3   64.5 weight %   7.1 weight %   28.4 weight %   0.55:1   5       3-4   67.0 weight %   6.6 weight %   26.4 weight %   0.49:1   5       2-5   73.1 weight %   5.4 weight %   21.5 weight %   0.37:1   4.5       2-6   75.7 weight %   4.9 weight %   19.4 weight %   0.33:1   5       2-7   78.4 weight %   4.3 weight %   17.3 weight %   0.28:1   0                  
 
     EXAMPLE 3  
     Effect of Mixing Shear on Resistivity  
      When production quantities of materials are prepared, the speed required for efficient mixing often results in shear conditions that are different from those involved for the preparation of laboratory quantities. As a result, the materials so produced can have properties different from those of laboratory-prepared samples. The following example demonstrates the effect of high shear mixing on the resistivity of the resulting backside layer.  
      Conductive backside coating formulations were prepared as described in Example 2 containing various ratios of binder to non-acicular zinc antimonate [CELNAX®, (ZnSb 2 O 6 )].  
      Formulations were coated before and after being subjected to high shear homogenization. Homogenization was carried for one pass at 8,000 psi using a Model 15 MR Laboratory Homogenizer manufactured by APV Gaulin, Inc. (Everett, Mass.). A control sample was also prepared containing a ratio of binder to non-acicular zinc antimonate as described in U.S. Pat. No. 6,689,546 (noted above). All solutions were coated on a laboratory knife spread coater without a backside topcoat.  
      Samples were evaluated using the WER test described above. The results, shown below in TABLE IV, demonstrate that backside coating formulations containing high amounts CELNAX® non-acicular zinc antimonate (that is, a low binder to CELNAX® non-acicular zinc antimonate ratio) provide highly conductive materials having decreased of resistivity when prepared under high shear conditions.  
                                   TABLE IV                           CELNAX ®                           CX-Z641M   Binder:CELNAX ®   CELNAX ®       Sample   (Dry wt. %)   Ratio   Coating Weight   Homogenized   WER                                                            3-Control   47.6    1.1:1   0.32 g/m 2     No   2.0 × 10 9     ohm/sq       3-Control   47.6    1.1:1   0.31 g/m 2     Yes   4.0 × 10 13     ohm/sq       3-1-1   57.6   0.75:1   0.35 g/m 2     No   1 × 10 9     ohm/sq       3-1-2   57.6   0.75:1   0.34 g/m 2     Yes   4.0 × 10 12     ohm/sq       3-2-1   64.5   0.55:1   0.34 g/m 2     No   6.3 × 10 8     ohm/sq       3-2-2   64.5   0.55:1   0.32 g/m 2     Yes   1 × 10 11     ohm/sq       3-3-1   73.1   0.37:1   0.33 g/m 2     No   5 × 10 8     ohm/sq       3-3-2   73.1   0.37:1   0.29 g/m 2     Yes   2 × 10 9     ohm/sq                  
 
     EXAMPLE 4  
     Low Coating Weights of Non-Acicular Zinc Antimonate  
      A conductive backside coating formulation and backside topcoat formulation were prepared, simultaneously coated, and dried as described in Example 2. The dry coating weight of the backside topcoat layer was 0.2 g/ft 2  (2.2 g/m 2 )  
      TABLE V shows the amount of non-acicular zinc antimonate, the non-acicular zinc antimonate binder ratio, the wet and dry coating thickness, the WER, and the cross-hatch adhesion. The results demonstrate that even at a non-acicular zinc antimonate coating weight of 0.131 g/m 2 , resistivity is acceptable. These results show the improved conductive efficiency at a lower binder:CELNAX (non-acicular zinc antimonate) ratio.  
                                       TABLE V                                      Thickness of                           Conductive Layer   Binder:CELNAX ®   CELNAX ®       Cross Hatch                                         Sample   Wet   Dry   Ratio   Coating Weight   WER   Adhesion               4-Control   6.9 (μm)   0.45 (μm)    1.1:1   0.22 g/m 2     1.5 × 10 12  ohm/sq   5       4-1   1.9 (μm)   0.12 (μm)   0.37:1   0.13 g/m 2     1.5 × 10 10  ohm/sq   4                  
 
      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.