Thermally developable imaging materials containing surface barrier layer

Abstract: Thermographic and photothermographic materials comprise a surface barrier layer to provide physical protection and to prevent migration of diffusible imaging components and by-products resulting from high temperature development. The barrier layer comprises a film-forming acrylate or methacrylate polymer(s) that has a molecular weight of at least 8000 g/mole and comprises epoxy functionality and is capable of retarding diffusion of mobile chemicals such as fatty acids. This polymer is preferably present in admixture with at least one other film-forming polymer to provide a clear and scratch-resistance surface film.

Description: This invention relates to thermally developable imaging materials such as thermographic and photothermographic materials. More particularly, it relates to thermographic and photothermographic imaging materials having improved physical protection by the presence of a unique surface barrier layer. The invention also relates to methods of imaging using these materials. This invention is directed to the photothermographic and thermographic imaging industries. Silver containing thermographic and photothermographic imaging materials that are developed with heat and without liquid development have been known in the art for many years. Thermography or thermal imaging is a recording process wherein images are generated by the use of thermal energy. In direct thermography, a visible image is formed by imagewise heating a recording material containing matter that changes color or optical density upon heating. Thermographic materials generally comprise a support having coated thereon: (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing composition (usually including a developer) for the reducible silver ions, and (c) a hydrophilic or hydrophobic binder. Thermal recording materials become photothermographic upon incorporating a photosensitive catalyst such as silver halide. Upon imagewise exposure to irradiation energy (ultraviolet, visible or IR radiation) the exposed silver halide grains form a latent image. Application of thermal energy causes the latent image of exposed silver halide grains to act as a catalyst for the development of the non-photosensitive source of reducible silver to form a visible image. These photothermographic materials are also known as "dry silver" materials. In such materials, the photosensitive catalyst is generally a photographic type photosensitive silver halide that is considered to be in catalytic proximity to the non-photosensitive source of reducible silver ions. Catalytic proximity requires an intimate physical association of these two components either prior to or during the thermal image development process so that when silver atoms [Ag(0)], also known as silver specks, clusters or nuclei are generated by irradiation or light exposure of the photosensitive silver halide, those silver atoms are able to catalyze the reduction of the reducible silver ions within a catalytic sphere of influence around the silver atoms [Klosterboer, Neblette's Eighth Edition: Imaging Processes and Materials, Sturge, Walworth & Shepp (Eds.), Van Nostrand-Reinhold, New York, Chapter 9, pages 279-291, 1989]. It has long been understood that silver atoms act as a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive source of reducible silver ions in a number of different ways (see, for example, Research Disclosure, June 1978, Item No. 17029). Other photosensitive materials, such as titanium dioxide, zinc oxide, and cadmium sulfide have been reported as useful in place of silver halide as the photocatalyst in photothermographic materials [see, for example, Shepard, J. Appl. Photog. Eng. 1982, 8(5), 210-212, Shigeo et al., Nippon Kagaku Kaishi, 1994, 11 , 992-997, and FR 2,254,047 (Robillard)]. The photosensitive silver halide may be made " in situ, " for example by mixing an organic or inorganic halide-containing source with a source of reducible silver ions to achieve partial metathesis and thus causing the in-situ formation of silver halide (AgX) grains on the surface of the silver source [see for example, US-A-3,457,075 (Morgan et al.)]. The silver halide may also be "preformed" and prepared by an " ex situ " process whereby the silver halide (AgX) grains are prepared and grown separately. With this technique, one has the possibility of controlling the grain size, grain size distribution, dopant levels, and composition much more precisely, so that one can impart more specific properties to both the silver halide grains and photothermographic material. The preformed silver halide grains may be introduced prior to and be present during the formation of the silver soap. Co-precipitation of the silver halide and source of reducible silver ions provides a more intimate mixture of the two materials [see for example, US-A-3,839,049 (Simons)]. Alternatively, the preformed silver halide grains may be added to and physically mixed with the source of reducible silver ions. The non-photosensitive source of reducible silver ions is a material that contains reducible silver ions. Typically, the preferred non-photosensitive source of reducible silver ions is a silver salt of a long chain aliphatic carboxylic acid (such as a silver fatty acid carboxylate) having from 10 to 30 carbon atoms, or mixtures of such salts. Such acids are also known as "fatty acids". Salts of other organic acids or other organic compounds, such as silver imidazolates, silver benzotriazoles, silver tetrazoles, silver benzotetrazoles, silver benzothiazoles and silver acetylides have been proposed. US-A-4,260,677 (Winslow et al.) discloses the use of complexes of various non-photosensitive inorganic or organic silver salts. In photothermographic emulsions, exposure of the photosensitive silver halide to light produces small clusters of 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 emulsion 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 clusters of silver atoms (that is, the latent image). This produces a black-and-white image. The non-photosensitive silver source is reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced. In photothermographic materials, the reducing agent for the non-photosensitive reducible silver ions, often referred to as a "developer," may be any compound that in the presence of the latent image, can reduce silver ions 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. In photothermographic materials, upon heating, this reduction occurs preferentially in the regions surrounding the latent image. In photothermographic materials, this reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the imaging layer(s). Differences Between Photothermography and Photography The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing using aqueous processing solutions. As noted above, in photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer, incorporated within the material. Heating at 50°C or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30°C to 50°C) to provide a visible image. In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example a silver carboxylate) is used to generate the visible image using thermal development. Thus, the photosensitive silver halide serves as a catalyst for the physical development of the non-photosensitive source of reducible silver ions. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver that, upon chemical development, is itself converted into the silver image, or that upon physical development requires addition of an external silver source. Thus, photothermographic materials require an 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, they include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. Even in so-called instant photography, the developer chemistry is physically separated from the photosensitive silver halide until development is desired. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of "fog" or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems during the preparation of the photothermographic emulsion as well as during coating, storage, and post-processing handling. 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, the silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is in the aqueous fixing step). In photothermographic materials, the binder is capable of wide variation and a number of binders (both hydrophilic and hydrophobic) are useful. In contrast, conventional photographic materials are limited almost exclusively to hydrophilic colloidal binders such as gelatin. Because photothermographic materials require dry thermal processing, they pose different considerations and present distinctly different problems in manufacture and use, compared to conventional, wet-processed silver halide materials. These and other distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials ( Neblette's Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al (Eds.), The Focal Press, London and New York, 1978, pages 74-75, and in Zou, Sahyun, Levy and Serpone, J. Imaging Sci. Technol. 1996, 40 , pages 94-103. As noted above, thermographic and photothermographic materials generally include a source of reducible silver ions for thermal development. The most common sources of reducible silver ions are the silver fatty acid carboxylates described above. Other components in such materials include a reducing agent system that usually includes a reducing agent, and optionally a toning agent in photothermographic materials (common ones being phthalazine and derivatives thereof) in one or more binders (usually hydrophobic binders). These components are generally formulated for coating using polar organic solvents. We have found that by-products, including various fatty carboxylic acids (such as behenic acid), are formed in the materials during thermal development. These fatty acid by-products as well as the reducing agent and any toner that is present can readily diffuse out of the materials during thermal development and cause debris build-up on the thermal processing equipment (such as processor drums). This may result in the processed materials sticking to the processing equipment and causing a jam in the machine, as well as scratching of the outer surface of the developed materials. It is known from US-A-5,422,234 (Bauer et al.) to use a surface overcoat layer in photothermographic materials to minimize the problems noted above. This overcoat layer comprises gelatin, poly(vinyl alcohol), poly(silicic acid) or combinations of such hydrophilic materials. While these overcoat layer materials provide suitable barriers to diffusion of reagents from the photothermographic materials, they are typically coated from water. Coating a separate hydrophilic layer from water when the imaging layer(s) are generally coated from polar organic solvents is not desirable for a number of reasons. While organic solvent-soluble polymers (such as polyacrylates and cellulosic materials) can also be used as barrier layer materials to provide physical protection, they do not adequately prohibit diffusion of all by-products of thermal development out of the thermographic and photothermographic materials. There remains a need for thermally developable materials that have suitable barrier layers that provide physical protection while inhibiting the diffusion of various chemicals out of the materials during thermal development. It would be particularly desirable to have improved thermographic and photothermographic materials that include a layer that acts as a barrier to the diffusion of fatty acids from materials during thermal development. The problems noted above are solved with a thermally developable material comprising a support having thereon: a) a thermally developable, imaging layer(s) comprising a binder and in reactive association, a non-photosensitive source of reducible silver ions and a reducing composition for the non-photosensitive source of reducible silver ions, the material characterized as further comprising: b) a surface barrier layer that is on the same side of but farther from the support than the imaging layer(s), the barrier layer comprising a film-forming acrylate or methacrylate polymer having a molecular weight of at least 8000 g/mole and epoxy functionality. This invention also provides a black-and-white photothermographic material comprising a support having thereon: a) a thermally developable imaging layer(s) comprising a binder and in reactive association, a photocatalyst, a non-photosensitive source of reducible silver ions, and a reducing composition for the non-photosensitive source of reducible silver ions, the photothermographic material characterized as further comprising: b) a surface barrier layer that is on the same side of but farther from the support than the imaging layer(s), the barrier layer comprising a film-forming acrylate or methacrylate polymer having a molecular weight of at least 8000 g/mole and epoxy functionality. Further, a method of this invention for forming a visible image comprises: A) imagewise exposing the black-and-white photothermographic material described above to electromagnetic radiation to form a latent image, and B) simultaneously or sequentially, heating the exposed photothermographic material to develop the latent image into a visible image. This method can further include: C) positioning the exposed and heat-developed photothermographic material between a source of imaging radiation and an imageable material that is sensitive to the imaging radiation, and D) 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 thermographic materials of this invention can also be used to provide a desired black-and-white image by imagewise heating and development. It has been found that the particular surface barrier layer used in the present invention effectively inhibits the diffusion of fatty acids and other chemicals (such as developers and toners) from thermally developable imaging materials. Thus, the surface barrier layer reduces the buildup of debris on the processing equipment and improves imaging efficiencies and quality. These advantages are achieved by using certain film-forming acrylate and methacrylate polymers having epoxy functionality (that is, epoxy groups) in the surface barrier layer. These polymers are preferably used in admixture with other film-forming polymers, and the combined formulation is believed to provide an excellent chemical and/or physical barrier to the fatty acids and other mobile chemicals. The epoxy groups are believed to improve the compatibility of the polymer mixtures, thereby providing improved clarity and reduced haze. The thermographic and photothermographic materials of this invention can be used, for example, in conventional black-and-white thermography and photothermography, in electronically generated black-and-white hardcopy recording, in the graphic arts area (for example imagesetting, and phototypesetting), in the manufacture of printing plates, in proofing, in microfilm applications and in radiographic imaging. The remaining disclosure will be directed to the preferred photothermographic materials, but it would be readily apparent that such materials can be readily modified to act as thermographic materials and used under thermographic imaging conditions known in the art. In the photothermographic materials of this invention, the components needed for imaging can be in one or more layers. The layer(s) that contain the photosensitive photocatalyst (such as photosensitive silver halide), non-photosensitive source of reducible silver ions, or both, are referred to herein as imaging layer(s) or photothermographic emulsion layer(s). The photocatalyst and the non-photosensitive source of reducible silver ions are in catalytic proximity (or reactive association) and preferably are in the same layer. The materials are generally sensitive to radiation of from 300 to 850 nm. Various layers are usually disposed on the "backside" (non-emulsion side) of the materials, including antihalation layer(s), protective layers, conducting layers, transport enabling layers, primer or subbing layers, and antistatic layers. Various layers are also disposed on the "frontside" or emulsion side of the support including the surface barrier layer described herein, interlayers, opacifying layers, protective layers, antistatic layers, acutance layers, conducting layers, subbing or primer layers, auxiliary layers and other layers readily apparent to one skilled in the art. The present invention also provides a process for the formation of a visible image (usually a black-and-white image) by first exposing to suitable electromagnetic radiation and thereafter heating the inventive photothermographic material. Thus, in one embodiment, the present invention provides a process comprising: A) imagewise exposing the photothermographic material of this invention to electromagnetic radiation to which the photocatalyst (for example a photosensitive silver halide) of the material is sensitive, to generate a latent image, and B) simultaneously or sequentially, heating the exposed material to develop the latent image into a visible black-and-white image. This visible image can also be used as a mask for exposure of other photosensitive imageable materials, such as graphic arts films, proofing films, printing plates and circuit board films, that are sensitive to suitable imaging radiation (for example UV radiation). This is done by imaging an imageable material (such as a photopolymer, a diazo material, a photoresist, or a photosensitive printing plate through the exposed and heat-developed photothermographic material of this invention using steps C and D noted above. For thermographic imaging, imaging is carried out entirely with thermal energy from a suitable thermal imaging source. When the photothermographic materials of this invention are heat developed as described below in a substantially water-free condition after, or simultaneously with, imagewise exposure, a silver image (preferably black-and-white silver image) is obtained. The photothermographic material may be exposed using ultraviolet, visible, infrared, or laser radiation such as from an infrared laser, a laser diode, an infrared laser diode, a light emitting diode, a light emitting screen, a CRT tube, or any other radiation source readily apparent to one skilled in the art. Definitions In the descriptions of the photothermographic materials of the present invention, "a" or "an" component refers to "at least one" of that component. For example, the chemical materials (including polymers) described herein for the barrier layer can be used individually or in mixtures. Heating in a substantially water-free condition as used herein, means heating at a temperature of from 50° to 250°C with little more than ambient water vapor present. The term "substantially water-free condition" means that the reaction system is approximately in equilibrium with water in the air and water for inducing or promoting the reaction is not particularly or positively supplied from the exterior to the material. Such a condition is described in T. H. James, The Theory of the Photographic Process, Fourth Edition, Macmillan 1977, page 374. "Photothermographic material(s)" means a construction comprising at least one photothermographic emulsion layer or a "two trip" photothermographic set of layers (the "two-trip coating where the silver halide and the source of reducible silver ions are in one layer and the other essential components or desirable additives are distributed as desired in an adjacent coating layer) and any supports, protective layers, surface barrier layers, image-receiving layers, blocking layers, antihalation layers, subbing or priming layers. These materials also include multilayer constructions in which one or more imaging components are in different layers, but are in "reactive association" so that they readily come into contact with each other during imaging and/or development. For example, one layer can include the non-photosensitive source of reducible silver ions and another layer can include the reducing composition, but the two reactive components are in reactive association with each other. "Emulsion layer," "imaging layer," or "photothermographic emulsion layer" means a layer of a photothermographic material that contains the photosensitive silver halide and/or non-photosensitive source of reducible silver ions. These layers are usually on what is known as the "frontside" of the support. "Ultraviolet region of the spectrum" means that region of the spectrum less than or equal to 410 nm, preferably from 100 nm to 410 nm although parts of these ranges may be visible to the naked human eye. More preferably, the ultraviolet region of the spectrum is the region of from 190 nm to 405 nm. "Visible region of the spectrum" refers to that region of the spectrum of from 400 nm to 750 nm. "Short wavelength visible region of the spectrum" refers to that region of the spectrum from 400 nm to 450 nm. "Red region of the spectrum" refers to that region of the spectrum of from 600 nm to 750 nm. Preferably the red region of the spectrum is from 620 nm to 700 nm. "Infrared region of the spectrum" refers to that region of the spectrum of from 750 nm to 1400 nm. "Non-photosensitive" means not intentionally light sensitive. "Transparent" means capable of transmitting visible light or imaging radiation without appreciable scattering or absorption. As is well understood in this area, substitution is not only tolerated, but is often advisable and substitution is anticipated on the compounds used in the present invention. For compounds disclosed herein, when a compound is referred to as "having the structure" of a given formula, any substitution that does not alter the bond structure of the formula or the shown atoms within that structure is included within the formula, unless such substitution is specifically excluded by language (such as "free of carboxy-substituted alkyl"). For example, where there is a benzene ring structure shown substituent groups may be placed on the benzene ring structure, but the atoms making up the benzene ring structure may not be replaced. As a means of simplifying the discussion and recitation of certain substituent groups, the term "group" refers to chemical species that may be substituted as well as those that are not so substituted. For example, the term "alkyl group" is intended to include not only pure hydrocarbon alkyl chains (such as methyl, ethyl, propyl, t -butyl, cyclohexyl, iso -octyl, and octadecyl) but also alkyl chains bearing substituents known in the art, such as hydroxyl, alkoxy, thioalkyl, phenyl, halogen atoms (F, Cl, Br, and I), cyano, nitro, amino, and carboxy. Further, alkyl group includes ether groups (for example CH)-CH 2 -CH 2 -O-CH 2 -), thioether group, haloalkyls, nitroalkyls, carboxyalkyls, hydroxyalkyls, sulfoalkyls, and others readily apparent to one skilled in the art. Substituents that adversely react with other active ingredients, such as very strongly electrophilic or oxidizing substituents, would of course be excluded by the ordinarily skilled artisan as not being inert or harmless. Other aspects, advantages, and benefits of the present invention are apparent from the detailed description, examples, and claims provided in this application. Surface Barrier Layer The advantages of the present invention are achieved by using certain film-forming acrylate and methacrylate polymers in a surface barrier layer. The surface barrier layer is the outermost layer on the "frontside" of the thermographic and photothermographic materials of this invention. A single homogeneous (that is, uniform throughout) surface barrier layer is preferred. However, as used herein, "surface barrier layer" also includes the use of multiple layers containing the same or different polymer composition can be disposed over emulsion and other layers to provide a surface barrier "structure" having multiple strata that serve as "barriers" to the diffusion of the various chemical components present in the material or produced during thermal development. The surface barrier layer can also act as a protective overcoat, but in some embodiments, a protective layer is interposed between it and underlying emulsion layers. The surface barrier layer is generally transparent and colorless. If it is not transparent and colorless, it must be at least transparent to the wavelength of radiation used to provide and view the resulting image. The surface barrier layer does not significantly adversely affect the imaging properties of the photothermographic materials of this invention, such as the sensitometric properties including minimum density, maximum density and photospeed. That is, haze is desirably as low as possible. The optimum surface barrier layer dry thickness depends upon various factors including type of imaging material, thermal processing means, desired image and various imaging components. Generally, the surface barrier layer has a dry thickness of at least 0.2 µm, and preferably a dry thickness of from 1.5 to 3 µm. The upper limit to the dry thickness is dependent only upon what is practical for meeting imaging needs. The surface barrier layer useful in this invention comprises one or more film-forming acrylate or methacrylate polymers having epoxy functionality that are preferably mixed with one or more additional film-forming polymers that lack such functionality. The various film-forming polymers used in this layer must be compatible with each other so that a clear, non-hazy film is provided in a given layer. Mixtures of the various types of film-forming polymers can also be used. By "film-forming" is meant that the polymers provide such a smooth film at temperatures below 250°C. Polymers having epoxy functionality that are useful in the practice of this invention can vary widely in structure and composition. They can include homopolymers or epoxy group-containing monomers and copolymers formed from two or more acrylate or methacrylate monomers at least one that provides the epoxy functionality (that is epoxy group). The epoxy functionality can be present in the monomers prior to polymerization, or the monomers can include chemically reactive groups (such as amine, halogen, hydroxy or carboxylic acid groups) that can be converted to epoxy functionality after polymerization. The film-forming polymers containing epoxy functionality are vinyl polymers prepared by polymerization of one or more ethylenically unsaturated polymerization monomers using conventional procedures and starting materials that would be readily apparent to one skilled in the polymer chemistry art. The molecular weight of the useful film-forming polymers is at least 8000 g/mole, and preferably the molecular weight is at least 25,000 g/mole. It is preferred at least 25 mol % of the recurring units in the film-forming epoxy-containing polymer(s) in the surface barrier layer comprise a pendant oxirane ring. Preferably, from 25 to 100 mol % (and more preferably from 50 to 100 mol %) of the recurring units comprise a pendant oxirane ring. More particularly, the epoxy-containing film-forming polymers useful in this invention are represented by the following Formula I: -(A) m -(B) n - wherein A represents recurring units derived from one or more ethylenically unsaturated polymerizable acrylate or methacrylate monomers comprising a pendant oxirane ring, B represents recurring units derived from one or more ethylenically unsaturated polymerizable acrylate or methacrylate monomers other than those represented by A, m is from 25 to 100 mol %, and n is from 0 to 75 mol %. More preferably, in Formula I, m is from 50 to 100 mol % and n is from 0 to 50 mol %. The "A" recurring units shown in Formula I can be derived from one or more ethylenically unsaturated polymerizable acrylate or methacrylate monomers such as glycidyl methacrylate, glycidyl acrylate, allyl glycidyl ether, 2,3-epoxybutyl methacrylate, 3,4-epoxybutyl methacrylate, 2,3-epoxycyclohexyl methacrylate, and others that would be readily apparent to one skilled in the art. Glycidyl methacrylate is preferred. Most of these monomers can be obtained from a number of commercial sources including Aldrich Chemical Company and Scientific Polymer Products. Other monomers can be prepared using known starting materials and procedures. The "B" recurring units shown in Formula I can be derived from one or more ethylenically unsaturated polymerizable acrylate or methacrylate monomers such as methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, ethyl acrylate, n -butyl acrylate, cyclohexyl methacrylate, cyclohexyl acrylate, lauryl methacrylate, allyl methacrylate, and others that would be readily apparent to one skilled in the art. Most of these compounds are readily available from a number of commercial sources including the commercial sources noted above. Other monomers can be prepared using known starting materials and procedures. Representative film-forming polymers having epoxy functionality that are useful in the practice of this invention include, but are not limited to the following materials: poly(glycidyl methacrylate), poly(glycidyl methacrylate-co-ethyl methacrylate), poly(glycidyl methacrylate-co-methyl methacrylate), poly(glycidyl methacrylate-co-ethyl methacrylate-co-methyl methacrylate), poly(glycidyl acrylate-co-ethyl methacrylate), poly(glycidyl methacrylate-co-isopropyl methacrylate), poly(allyl glycidyl ether-co- n -butyl acrylate), poly(glycidyl methacrylate-co-glycidyl acrylate-co-methyl methacrylate), and poly(glycidyl acrylate-co-allyl glycidyl ether-co-styrene). The most preferred polymers are poly(glycidyl methacrylate) and poly(glycidyl methacrylate-co-ethyl methacrylate). If desired, the polymers can be crosslinked or contain crosslinkable moieties using polymer chemistry known to one skilled in the art. The "additional" film-forming polymers that are preferably present in the surface barrier layer can be of any structure or composition as long as they are film-forming (as defined above), compatible with the epoxy-containing polymers, provide scratch-resistant films, and are stable as thermal development temperatures and conditions. They do not contain epoxy functionality. Such polymers can be cellulosic materials, polyacrylates (including copolymers), polymethacrylates (including copolymers), polyesters, polyurethanes that do not have epoxy functionality. Such materials can be obtained from a number of commercial sources including Eastman Chemical Company and DuPont or they can be prepared using known starting materials and procedures. The polyacrylates and polymethacrylates, for example, can be prepared from the various acrylate and methacrylate monomers described above in the definition of the "B" recurring units, with or without other ethylenically unsaturated polymerizable monomers that are not acrylates or methacrylates. Mixtures of these "additional" polymers can be used if desired. The cellulosic materials are preferred in the practice of this invention. Such materials include but are not limited to, cellulose acetate, cellulose acetate butyrate, hydroxymethyl cellulose, cellulose acetate propionate, and cellulose derivatives as described in E. Doelker, Advances in Polymer Science, Vol. 107, pp. 199-265. Mixtures of cellulose polymers can be used if desired. Cellulose acetate butyrate is preferred. In the surface barrier layer used in this invention, the film-forming polymers comprising epoxy functionality generally comprise from 5 to 100 weight %, and preferably from 25 to 50 weight %, based on total dry layer weight. The additional film-forming polymers (not having epoxy functionality) generally comprise from 0 to 95 weight %, and preferably from 50 to 75 weight %, based on total dry layer weight. The surface barrier layer(s) can also include various addenda such as surfactants, lubricants, matting agents, crosslinking agents, photothermographic toners, acutance dyes and other chemicals that would be readily apparent to one skilled in the art. These components can be present in conventional amounts. The surface barrier layer(s) can be applied to other layers in the thermographic or photothermographic materials using any suitable technique (see coating described below). Generally, the components of the layers are formulated and coated out of predominantly one or more suitable polar organic solvents such as methyl ethyl ketone, acetone, and methanol at from 2 to 35% solids, coated in a suitable fashion, and dried. Alternatively, the surface barrier layer(s) can be formulated in and coated as an aqueous formulation wherein water comprises at least 50 weight % of the total amount of solvents. Components of the layer(s) can be dissolved or dispersed within such coating formulations using known procedures. The Photocatalyst As noted above, the photothermographic materials of the present invention include one or more photocatalysts in the photothermographic emulsion layer(s). Useful photocatalysts include, but are not limited to, silver halides, titanium oxide, cupric salts [such as copper (II) salts)], zinc oxide, cadmium sulfide and other photocatalysts that would be readily apparent to one skilled in the art. Preferred photocatalysts are 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 various types of silver halides can also be used in any suitable proportion. Silver bromide and silver bromoiodide are more preferred, the latter silver halide including up to 10 mol % silver iodide. The shape of the photosensitive silver halide grains used in the present invention is in no way limited. The silver halide grains may have any crystalline habit including, but not limited to, cubic, octahedral, tetrahedral, orthorhombic, tabular, laminar, twinned, and platelet morphologies. If desired, a mixture of these crystals may be employed. Silver halide grains having cubic or tabular morphology are preferred. The silver halide grains may have a uniform ratio of halide throughout. They may have a graded halide content, with a continuously varying ratio of, for example, silver bromide and silver iodide or they may be of the core-shell-type, having a discrete core of one halide ratio, and a discrete shell of another halide ratio. Core-shell silver halide grains useful in photothermographic materials and methods of preparing these materials are described for example, in US-A-5,382,504 (Shor et al.). Iridium and/or copper doped core-shell grains of this type are described in US-A-5,434,043 (Zou et al.), US-A-5,939,249 (Zou), and EP-A-0 627 660 (Shor et al.). The photocatalyst 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. For the preferred photocatalysts, it is preferred that the silver halide be preformed and prepared by an ex-situ process. The silver halide grains prepared ex-situ may then be added to and physically mixed with the non-photosensitive source of reducible silver ions. It is more preferable to form the source of reducible silver ions in the presence of ex-situ prepared silver halide. In this process, the source of reducible silver ions, such as a long chain fatty acid silver carboxylate (commonly referred to as a silver "soap") is formed in the presence of the preformed silver halide grains. Co-precipitation of the reducible source of silver ions in the presence of silver halide provides a more intimate mixture of the two materials [see, for example, US-A- 3,839,049 (Simons)]. Materials of this type are often referred to as "preformed soaps." The silver halide grains used in the imaging formulations can vary in average diameter of up to several micrometers (µm) depending on their desired use. Preferred silver halide grains are those having an average particle size of from 0.01 to 1.5 µm, more preferred are those having an average particle size of from 0.03 to 1.0 µm, and most preferred are those having an average particle size of from 0.05 to 0.8 µm. Those of ordinary skill in the art understand that there is a finite lower practical limit for silver halide grains that is partially dependent upon the wavelengths to which the grains are spectrally sensitized, such lower limit, for example being 0.01 or 0.005 µm. The average size of the photosensitive doped silver halide grains is expressed by the average diameter if the grains are spherical and by the average of the diameters of equivalent circles for the projected images if the grains are cubic or in other non-spherical shapes. Grain size may be determined by any of the methods commonly employed in the art for particle size measurement. Representative methods are described by in "Particle Size Analysis," ASTM Symposium on Light Microscopy, R. P. Loveland, 1955, pp. 94-122, and in The Theory of the Photographic Process, C. E. Kenneth Mees and T. H. James, Third Edition, Chapter 2, Macmillan Company, 1966. Particle size measurements may be expressed in terms of the projected areas of grains or approximations of their diameters. These will provide reasonably accurate results if the grains of interest are substantially uniform in shape. Preformed silver halide emulsions used in the material of this invention can be prepared by aqueous or organic processes and can be unwashed or washed to remove soluble salts. In the latter case, the soluble salts can be removed by chill setting and leaching or the emulsion can be coagulation washed [for example by the procedures described in US-A-2,618,556 (Hewitson et al.), US-A-2,614,928 (Yutzy et al.), US-A-2,565,418 (Yackel), US-A-3,241,969 (Hart et al.) and US-A-2,489,341 (Waller et al.) and by ultrafiltration to remove soluble salts. It is also effective to use an in situ process in which an organic or inorganic halide-containing compound is added to an organic silver salt to partially convert the silver of the organic silver salt to silver halide. The halide-containing compound can be inorganic (such as zinc bromide or lithium bromide) or organic (such as N-bromosuccinimide). Additional methods of preparing these silver halide and organic silver salts and manners of blending them are described in Research Disclosure, June 1978, item 17029, US-A-3,700,458 (Lindholm) and US-A-4,076,539 (Ikenoue et al.), and JP Applications 13224/74, 42529/76 and 17216/75. Research Disclosure is a publication of Kenneth Mason Publications Ltd., Dudley House, 12 North Street, Emsworth, Hampshire PO10 7DQ England (also available from Emsworth Design Inc., 147 West 24 th Street, New York, N.Y. 10011). The one or more light-sensitive silver halides used in the photothermographic materials of the present invention are preferably present in an amount of from 0.005 to 0.5 mole, more preferably from 0.01 to 0.25 mole per mole, and most preferably from 0.03 to 0.15 mole, per mole of non-photosensitive source of reducible silver ions. The silver halide used in the present invention may be employed without modification. However, it is preferably chemically and/or spectrally sensitized in a manner similar to that used to sensitize conventional wet-processed silver halide photographic materials or state-of-the-art heat-developable photothermographic materials. For example, the photothermographic material may be chemically sensitized with one or more chemical sensitizing agents, such as a compound containing sulfur, selenium, or tellurium, or with a compound containing gold, platinum, palladium, ruthenium, rhodium, iridium, or combinations thereof, a reducing agent such as a tin halide or a combination of any of these. The details of these procedures are described in James, The Theory of the Photographic Process, Fourth Edition, Chapter 5, pages 149 to 169. Suitable chemical sensitization procedures are also disclosed in US-A-1,623,499 (Sheppard et al.), US-A-2,399,083 (Waller et al.), US-A-3,297,447 (McVeigh) and US-A-3,297,446 (Dunn). One preferred method of chemical sensitization is by oxidative decomposition of a spectral sensitizing dye in the presence of a photothermographic emulsion, as described in US-A-5,891,615 (Winslow et al.). Other useful chemical sensitizers include tetrasubstituted thiourea compounds that are described in copending and commonly assigned U.S. Serial No. 09/667,748 (filed on September 21, 2000 by Lynch, Simpson, Shor, Willett, and Zou). These compounds are broadly defined as thioureas in which the nitrogen atoms directed attached to the one or more sulfur atoms are fully substituted with monovalent or divalent groups. The addition of sensitizing dyes to the photosensitive silver halides provides high sensitivity to ultraviolet, visible and infrared light by spectral sensitization. Thus, the photosensitive silver halides may be spectrally sensitized with various known dyes that spectrally sensitize silver halide. Non-limiting examples of sensitizing dyes that can be employed include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, holopolar cyanine dyes, hemicyanine dyes, styryl dyes, and hemioxanol dyes. The cyanine dyes, merocyanine dyes and complex merocyanine dyes are particularly useful. Suitable sensitizing dyes such as those described in US-A-3,719,495 (Lea), US-A-5,393,654 (Burrows et al.), US-A-5,441,866 (Miller et al.) and US-A-5,541,054 (Miller et al.), US-A-5,281,515 (Delprato et al.) and US-A-5,314,795 (Helland et al.) are effective in the practice of the invention. An appropriate amount of sensitizing dye added is generally 10 -10 to 1 mole, and preferably, 10 -6 to 10 -1 moles per mole of silver halide. To enhance the speed and sensitivity of the photothermographic materials, it is often desirable to use one or more supersensitizers that increase the sensitivity to light. For example, preferred infrared supersensitizers are described in US-A-5,922,529 (Tsuzuki et al.) and EP-A-0 559 228 (Philip Jr. et al.) and include heteroaromatic mercapto compounds or heteroaromatic disulfide compounds of the formulae: Ar-S-M and Ar-S-S-Ar, wherein M represents a hydrogen atom or an alkali metal atom. 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. However, compounds having other heteroaromatic rings are envisioned to be suitable supersensitizers. The heteroaromatic ring may also carry substituents. Examples of preferred substituents are halogens (such as bromine and chlorine), hydroxy, amino, carboxy, alkyl groups (for example of 1 or more carbon atoms and preferably 1 to 4 carbon atoms) and alkoxy groups (for example of 1 or more carbon atoms and preferably of 1 to 4 carbon atoms). Most preferred supersensitizers are 2-mercaptobenzimidazole, 2-mercapto-5-methylbenzimidazole, 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, and mixtures thereof. If used, a supersensitizer is generally present in an emulsion layer in an amount of at least 0.0001 mole per mole of silver in the emulsion layer. More preferably, a supersensitizer is present within a range of 0.0001 mole to 1.0 mole, and most preferably, 0.005 mole to 0.2 mole, per mole of silver halide. Non-Photosensitive Source of Reducible Silver Ions The non-photosensitive source of reducible silver ions used in photothermographic materials of this invention can be any material that contains reducible silver ions. Preferably, it is a silver salt that is comparatively stable to light and forms a silver image when heated to 50°C or higher in the presence of an exposed photocatalyst (such as silver halide) and a reducing composition. Silver salts of organic acids, particularly silver salts of long-chain fatty carboxylic acids are preferred. The chains typically contain 10 to 30, and preferably 15 to 28, carbon atoms. Suitable organic silver salts include silver salts of organic compounds having a carboxylic acid group. Examples thereof include silver salts of aliphatic carboxylic acids and silver salts of aromatic carboxylic acids. Preferred examples of the silver salts of aliphatic carboxylic acids include silver behenate, silver arachidate, silver stearate, silver oleate, silver laurate, silver caprate, silver myristate, silver palmitate, silver maleate, silver fumarate, silver tartarate, silver furoate, silver linoleate, silver butyrate, silver camphorate, and mixtures thereof, hydrocarbon chains having ether or thioether linkages, or sterically hindered substitution in the α- (on a hydrocarbon group) or ortho- (on an aromatic group) position. Preferred examples of the silver salts of aromatic carboxylic acid and other carboxylic acid group-containing compounds include, but are not limited to, silver benzoate, a silver-substituted benzoate, such as silver 3,5-dihydroxy-benzoate, silver o -methylbenzoate, silver m -methylbenzoate, silver p -methylbenzoate, silver 2,4-dichlorobenzoate, silver acetamidobenzoate, silver p -phenylbenzoate, silver gallate, silver tannate, silver phthalate, silver terephthalate, silver salicylate, silver phenylacetate, silver pyromellitate, a silver salt of 3-carboxymethyl-4-methyl-4-thiazoline-2-thione or others as described in US-A-3,785,830 (Sullivan et al.), and silver salts of aliphatic carboxylic acids containing a thioether group as described in US-A-3,330,663 (Weyde et al.). Soluble silver carboxylates having increased solubility in coating solvents and affording coatings with less light scattering can also be used. Such silver carboxylates are described in US-A-5,491,059 (Whitcomb). Mixtures of any of the silver salts described herein can also be used if desired. Silver salts of sulfonates are also useful in the practice of this invention. Such materials are described for example in US-A-4,504,575 (Lee). Silver salts of sulfosuccinates are also useful as described for example in EP-A-0 227 141 (Leenders et al.). Silver salts of compounds containing mercapto or thione groups and derivatives thereof can also be used. Preferred examples of these compounds include, but are not limited to, a silver salt of 3-mercapto-4-phenyl-1,2,4-triazole, a silver salt of 2-mercaptobenzimidazole, a silver salt of 2-mercapto-5-aminothiadiazole, a silver salt of 2-(2-ethylglycolamido)benzothiazole, silver salts of thioglycolic acids (such as a silver salt of a S-alkylthioglycolic acid, wherein the alkyl group has from 12 to 22 carbon atoms), silver salts of dithiocarboxylic acids (such as a silver salt of dithioacetic acid), a silver salt of thioamide, a silver salt of 5-carboxylic-1-methyl-2-phenyl-4-thiopyridine, a silver salt of mercaptotriazine, a silver salt of 2-mercaptobenzoxazole, silver salts as described in US-A-4,123,274 (Knight et al.) (for example, a silver salt of a 1,2,4-mercaptothiazole derivative, such as a silver salt of 3-amino-5-benzylthio-1,2,4-thiazole), and a silver salt of thione compounds [such as a silver salt of 3 -(2-carboxyethyl)-4-methyl-4-thiazoline-2-thione as described in US-A-3,201,678]. Furthermore, a silver salt of a compound containing an imino group can be used. Preferred examples of these compounds include but are not limited to, silver salts of benzotriazole and substituted derivatives thereof (for example, silver methylbenzotriazole and silver 5-chlorobenzotriazole), silver salts of 1,2,4-triazoles or 1- H -tetrazoles such as phenylmercaptotetrazole as described in US-A-4,220,709 (deMauriac), and silver salts of imidazoles and imidazole derivatives as described in US-A-4,260,677 (Winslow et al.). Moreover, silver salts of acetylenes can also be used as described for example in US-A-4,761,361 (Ozaki et al.) and US-A-4,775,613 (Hirai et al.). It may also be convenient to use silver half soaps. A preferred example of a silver half soap is an equimolar blend of silver carboxylate and carboxylic acid, which analyzes for 14.5% by weight solids of silver in the blend and which is prepared by precipitation from an aqueous solution of the sodium salt of a commercial fatty carboxylic acid, or by addition of the free fatty acid to the silver soap. For transparent films a silver carboxylate full soap, containing not more than 15% of free fatty carboxylic acid and analyzing 22% silver, can be used. For opaque photothermographic materials, different amounts can be used. 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, US-A-3,985,565 (Gabrielsen et al.) and the references cited above. The photocatalyst and the non-photosensitive source of reducible silver ions must be in catalytic proximity (that is reactive association). "Catalytic proximity" or "reactive association" means that they should be in the same emulsion layer or in adjacent layers. It is preferred that these reactive components be present in the same emulsion layer. The source of non-photosensitive reducible silver ions is preferably present in an amount of 5% by weight to 70% by weight, and more preferably, 10% to 50% by weight, based on the total dry weight of the emulsion layers. Stated in another way, the amount of the source of reducible silver ions is generally present in an amount of from 0.001 to 0.5 mol/m 2 of material, and preferably from 0.01 to 0.05 mol/m 2 of material. As noted above, mixtures of reducible silver ion sources can be used. The photocatalyst, the total amount of silver (from all silver sources) in the photothermographic materials is generally at least 0.002 mol/m 2 , and preferably from 0.01 to 0.05 mol/m 2 . Reducing Agents The reducing agent (or reducing agent composition comprising two or more components) for the source of reducible silver ions can be any material, preferably an organic material, that can reduce silver (I) ion to metallic silver. Conventional photographic developers such as methyl gallate, hydroquinone, substituted hydroquinones, hindered phenols, amidoximes, azines, catechol, pyrogallol, ascorbic acid (and derivatives thereof), leuco dyes and other materials readily apparent to one skilled in the art can be used in this manner as described for example in US-A-6,020,117 (Bauer et al.). In some instances, the reducing agent composition comprises two or more components such as a hindered phenol developer and a co-developer that can be chosen from the various classes of reducing agents described below. For example, hindered phenols can be used in combination with hydrazine, sulfonyl hydrazide, trityl hydrazide, formyl phenyl hydrazide, 3-heteroaromatic-substituted acrylonitrile, and 2-substituted malondialdehyde co-developer compounds described below. Ternary developer mixtures involving the further addition of contrast enhancing agents such as hydrogen atom donor, hydroxylamine, alkanolamine, ammonium phthalamate, hydroxamic acid, and N-acylhydrazine compounds are also useful. Hindered phenol developers are preferred (individually or mixtures). These are compounds that contain only one hydroxy group on a given phenyl ring and have at least one additional substituent located ortho to the hydroxy group. Hindered phenol developers may contain more than one hydroxy group as long as each hydroxy group is located on different phenyl rings. Hindered phenol developers include, for example, binaphthols (that is dihydroxybinaphthyls), biphenols (that is dihydroxybiphenyls), bis(hydroxynaphthyl)methanes, bis(hydroxyphenyl)methanes, hindered phenols, and hindered naphthols each of which may be variously substituted. Representative binaphthols include but are not limited to 1,1'-bi-2-naphthol, 1,1'-bi-4-methyl-2-naphthol and 6,6'-dibromo-bi-2-naphthol. For additional compounds see US-A-3,094,417 (Workman) and US-A-5,262,295 (Tanaka et al.). Representative biphenols include but are not limited to 2,2'-dihydroxy-3,3'-di- t -butyl-5,5-dimethylbiphenyl, 2,2'-dihydroxy-3,3',5,5'-tetra- t -butylbiphenyl, 2,2'-dihydroxy-3,3'-di- t -butyl-5,5'-dichlorobiphenyl, 2-(2-hydroxy-3- t -butyl-5-methylphenyl)-4-methyl-6- n -hexylphenol, 4,4'-dihydroxy-3,3',5,5'-tetra- t -butylbiphenyl and 4,4'-dihydroxy-3,3',5,5'-tetramethylbiphenyl. For additional compounds see US-A-5,262,295 (noted above). Representative bis(hydroxynaphthyl)methanes include but are not limited to 4,4'-methylenebis(2-methyl-1-naphthol). For additional compounds see US-A-5,262,295 (noted above). Representative bis(hydroxyphenyl)methanes include but are not limited to bis(2-hydroxy-3- t -butyl-5-methylphenyl)methane (CAO-5), 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane (NONOX or PERMANAX WSO), 1,1-bis(3,5-di- t -butyl-4-hydroxyphenyl)methane, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 4,4-ethylidene-bis(2- t -butyl-6-methylphenol) and 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane. For additional compounds see US-A-5,262,295 (noted above). Representative hindered phenols include but are not limited to 2,6-di- t -butylphenol, 2,6-di- t -butyl-4-methylphenol, 2,4-di- t -butylphenol, 2,6-dichlorophenol, 2,6-dimethylphenol and 2- t -butyl-6-methylphenol. Representative hindered naphthols include but are not limited to 1-naphthol, 4-methyl-1-naphthol, 4-methoxy-1-naphthol, 4-chloro-1-naphthol and 2-methyl-1-naphthol. For additional compounds see US-A-5,262,295 (noted above). More specific alternative reducing agents that have been disclosed in dry silver systems including amidoximes such as phenylamidoxime, 2-thienylamidoxime and p -phenoxyphenylamidoxime, azines (for example 4-hydroxy-3,5-dimethoxybenzaldehydrazine), a combination of aliphatic carboxylic acid aryl hydrazides and ascorbic acid, such as 2,2'-bis(hydroxymethyl)propionyl-betaphenyl hydrazide in combination with ascorbic acid, a combination of polyhydroxybenzene and hydroxylamine, a reductone and/or a hydrazine [for example, a combination of hydroquinone and bis(ethoxyethyl)hydroxylamine], piperidinohexose reductone or formyl-4-methylphenylhydrazine, hydroxamic acids (such as phenylhydroxamic acid, p -hydroxyphenylhydroxamic acid, and o -alaninehydroxamic acid), a combination of azines and sulfonamidophenols (for example phenothiazine and 2,6-dichloro-4-benzenesulfonamidophenol), α-cyanophenylacetic acid derivatives (such as ethyl α-cyano-2-methylphenylacetate and ethyl α-cyanophenylacetate), bis- o -naphthols [such as 2,2'-dihydroxyl-1-binaphthyl, 6,6'-dibromo-2,2'-dihydroxy-1,1'-binaphthyl, and bis(2-hydroxy-1-naphthyl)methane], a combination of bis- o -naphthol and a 1,3-dihydroxybenzene derivative (for example 2,4-dihydroxybenzophenone or 2,4-dihydroxyacetophenone), 5-pyrazolones such as 3-methyl-1-phenyl-5-pyrazolone, reductones (such as dimethylaminohexose reductone, anhydrodihydro-aminohexose reductone and anhydrodihydro-piperidone-hexose reductone), sulfonamidophenol reducing agents (such as 2,6-dichloro-4-benzenesulfonamido-phenol, and p -benzenesulfonamidophenol), 2-phenylindane-1,3 -dione and similar compounds, chromans (such as 2,2-dimethyl-7-t-butyl-6-hydroxychroman), 1,4-dihydropyridines (such as 2,6-dimethoxy-3,5-dicarbethoxy-1 4-dihydropyridine), bisphenols [such as bis(2-hydroxy-3- t -butyl-5-methylphenyl)methane, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 4,4-ethylidene-bis(2- t -butyl-6-methylphenol) and 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane], ascorbic acid derivatives (such as 1-ascorbylpalmitate, ascorbylstearate and unsaturated aldehydes and ketones), 3-pyrazolidones, and certain indane-1,3-diones. Still other useful reducing agents are described for example in US-A-3,074,809 (Owen), US-A-3,094,417 (Workman), US-A-3,080,254 (Grant, Jr.) and US-A-3,887,417 (Klein et al.). Auxiliary reducing agents may be useful as described in US-A-5,981,151 (Leenders et al.). The reducing agent (or mixture thereof) described herein is generally present as 1 to 20% (dry weight) of the emulsion layer. In multilayer constructions, if the reducing agent is added to a layer other than an imaging layer, slightly higher proportions, of from 2 to 25 weight % may be more desirable. Any co-developers may be present generally in an amount of from 0.01% to 1.5% (dry weight) of the imaging layer coating. High Contrast Agents The thermographic and photothermographic materials of this invention may include one or more high contrast agents. Such materials are sometimes identified as "co-developers" or "auxiliary developers", but their main function is to increase the contrast of the material by reducing most or all of the reducible silver ions in the non-photosensitive source of reducible silver ions in the radiation-exposed areas (that is in the latent image). High contrast agents that are particularly useful in the materials of this invention include, but are not limited to, acrylonitrile co-developers, hydrazide co-developers and isoxazole co-developers. For example, useful acrylonitrile co-developers can be represented by Formula II as follows: H(R')C=C(R)CN wherein R is a substituted or unsubstituted aryl group of 6 to 14 carbon atoms in the single or fused ring structure (such as phenyl, naphthyl, p -methylphenyl, p -chlorophenyl, 4-pyridinyl and o -nitrophenyl groups) or an electron withdrawing group (such as a halo atom, cyano group, carboxy group, ester group and phenylsulfonyl group). R' is a halo atom (such as fluoro, chloro and bromo), hydroxy or metal salt thereof, a thiohydrocarbyl group, an oxyhydroxycarbyl group, or a substituted or unsubstituted 5- or 6-membered aromatic heterocyclic group having only carbon atoms and 1 to 4 nitrogen atoms in the central ring (with or without fused rings attached), and being attached through a non-quaternary ring nitrogen atom (such as pyridyl, furyl, diazolyl, triazolyl, pyrrolyl, tetrazolyl, benzotriazolyl, benzopyrrolyl and quinolinyl groups). Further details of these compounds and their preparation can be found in US-A-5,635,339 (Murray) and US-A-5,654,130 (Murray). Examples of such compounds include, but are not limited to, the compounds identified as HET-01 and HET-02 in US-A-5,635,339 (noted above) and MA-01 through MA-07 in US-A-5,654,130 (noted above) Other useful high contrast agents are hydrazide co-developers having the following Formula III: R 1 (CO)-NHNH 2 wherein R 1 is a substituted or unsubstituted aliphatic group having up to 20 carbon atoms. Useful aliphatic groups include, but are not limited to, alkyl group of 1 to 20 carbon atoms (linear or branched, and preferably from 1 to 10 carbon atoms, and more preferably from 1 to 5 carbon atoms including methyl, ethyl, isopropyl, t -butyl and n -pentyl groups), a substituted or unsubstituted alkenyl group of 2 to 20 carbon atoms (linear or branched, and preferably from 2 to 10 carbon atoms, and more preferably from 2 to 5 carbon atoms such as 1-ethenyl, 2-propenyl, isopropenyl and 2- n -pentenyl groups), and a substituted or unsubstituted alkoxy or thioalkoxy group of 1 to 20 carbon atoms (linear or branched, and preferably 1 to 10 carbon atoms and more preferably from 1 to 5 carbon atoms). R 1 can also be a carbocyclic or heterocyclic group, each of which can be substituted. Useful carbocyclic groups are substituted or unsubstituted aryl, aryalkyl or alkaryl groups having 6 to 14 carbon atoms in the ring structure (such as phenyl, naphthyl, p -methylphenyl and benzyl groups), a substituted or unsubstituted aryloxy or thioaryloxy group of 6 to 14 carbon atoms in the ring structure (such as phenoxy and naphthoxy groups), and useful heterocyclic groups include substituted or unsubstituted aromatic or non-aromatic heterocyclic groups having up to 10 carbon, nitrogen, sulfur and oxygen atoms in the single or fused ring structure, a substituted or unsubstituted carbocyclyl group of 5 to 14 carbon atoms in the nonaromatic ring structure, an amido group having up to 20 carbon atoms, a substituted or unsubstituted anilino group having up to 20 carbon atoms, and R 3 is a trityl group. Further details of such compounds, including methods of making them, are provided in US-A-5,558,983 (Simpson et al.). Useful compounds within Formula III include, but are not limited to those identified as CA-1 through CA-6 in US-A-5,558,983 (noted above). Still other useful hydrazide co-developer high contrast agents have the following Formula IV: R 2 -(C=O)-NHNH-R 3 wherein R 2 is hydrogen and R 3 is a substituted or unsubstituted aryl group of 6 to 14 carbon atoms in the ring structure (such as phenyl, naphthyl, anthryl, p -methylphenyl, o -chlorophenyl groups). Alternatively, R 2 is hydrogen, a substituted or unsubstituted alkyl group of 1 to 20 carbon atoms (linear or branched, and preferably from 1 to 10 carbon atoms, and more preferably from 1 to 5 carbon atoms including methyl, ethyl, isopropyl, t -butyl and n -pentyl groups), a substituted or unsubstituted alkenyl group of 2 to 20 carbon atoms (linear or branched, and preferably from 2 to 10 carbon atoms, and more preferably from 2 to 5 carbon atoms such as 1-ethenyl, 2-propenyl, isopropenyl and 2- n -pentenyl groups), a substituted or unsubstituted alkoxy or thioalkoxy group of 1 to 20 carbon atoms (linear or branched, and preferably 1 to 10 carbon atoms and more preferably from 1 to 5 carbon atoms), a substituted or unsubstituted aryl, aryalkyl or alkaryl group having 6 to 14 carbon atoms in the ring structure (such as phenyl, naphthyl, p -methylphenyl and benzyl groups), a substituted or unsubstituted aryloxy or thioaryloxy group of 6 to 14 carbon atoms in the ring structure (such as phenoxy and naphthoxy groups), a substituted or unsubstituted aromatic or non-aromatic heterocyclyl group having up to 10 carbon, nitrogen, sulfur and oxygen atoms in the single or fused ring structure, a substituted or unsubstituted carbocyclyl group of 5 to 14 carbon atoms in the nonaromatic ring structure, an amido group having up to 20 carbon atoms, a substituted or unsubstituted anilino group having up to 20 carbon atoms, and R 3 is a trityl group. Further details of such compounds, including methods of making them, are provided in US-A-5,496,695 (Simpson et al.) and US-A-5,545,505 (Simpson et al.). Representative compounds of Formula III include, but are not limited to, the compounds identified as H-1 through H-28 in US-A-5,496,695 and the compounds identified as H-1 through H-29 in US-A-5,545,505. Still another class of useful high contrast agents includes hydrazide co-developers having the following Formula V: R 4 -CO-NHNH-SO 2 R 5 wherein R 4 and R 5 are independently a substituted or unsubstituted alkyl group of 1 to 20 carbon atoms (linear or branched, and preferably from 1 to 10 carbon atoms, and more preferably from 1 to 5 carbon atoms including methyl, ethyl, isopropyl, t -butyl and n -pentyl groups), a substituted or unsubstituted alkenyl group of 2 to 20 carbon atoms (linear or branched, and preferably from 2 to 10 carbon atoms, and more preferably from 2 to 5 carbon atoms such as 1-ethenyl, 2-propenyl, isopropenyl and 2- n -pentenyl groups), a substituted or unsubstituted alkoxy group of 1 to 20 carbon atoms (linear or branched, and preferably 1 to 10 carbon atoms and more preferably from 1 to 5 carbon atoms), a substituted or unsubstituted aryl group having 6 to 14 carbon atoms in the ring structure (such as phenyl, naphthyl, p -methylphenyl and o -chlorophenyl groups), a substituted or unsubstituted aryloxy group of 6 to 14 carbon atoms in the ring structure (such as phenoxy and naphthoxy groups), a substituted or unsubstituted aromatic or non-aromatic heterocyclyl group having up to 10 carbon, nitrogen, sulfur and oxygen atoms in the single or fused ring structure, or a substituted or unsubstituted carbocyclyl group of 5 to 14 carbon atoms in the nonaromatic ring structure. Additional details of these compounds, including their preparation and representative cyclic groups useful as R 4 or R 5 , are provided in US-A-5,464,738 (Lynch et al.). Representative compounds within Formula V include, but are not limited to, the compounds identified as Sulfonyl Hydrazide Developers 1-12 of US-A-5,464,738 (noted above). Still other useful co-developer reducing agents are described for example in JP 2000-221632 by Lynch and Skoog. These compounds are generally defined as having the following formula: wherein Y is H, a metal cation (such as zinc ion, ammonium ion, alkali metals, alkaline earth metals but preferably, sodium or potassium), or an alkyl group (preferably, an alkyl group having from 1 to 4 carbon atoms, and more preferably, a methyl or ethyl group), and the solid curved line represents the atoms and bonds necessary to complete a 5- to 6-membered carbocyclic or heterocyclic main ring structure that may include heteroatoms (for example nitrogen, oxygen and sulfur). The main ring structure can include one or more additional rings, including pendant and fused rings. Of all of the possible high contrast agents that can be used in the materials of this invention, the most preferred compounds are formyl phenyl hydrazine, trityl hydrazide and various alkali metal salts of alkyl(hydroxymethylene)cyanoacetates. The most preferred high contrast agent is a potassium salt of ethyl(hydroxymethylene)cyanoacetate. Mixtures of the same or different type of high contrast agents can be used in the photothermographic materials of this invention. The one or more high contrast agents are preferably present in the photothermographic materials of this invention in an amount of at least 0.001 g/m 2 , and preferably in an amount of at least 0.01 g/m 2 . The upper limit is generally determined by practical considerations of cost, amount of activity desired, structure and activity and is generally 1 g/m 2 . Other Addenda The photothermographic materials of the invention can also contain other additives such as shelf-life stabilizers, toners, antifoggants, contrast enhancers, development accelerators, acutance dyes, post-processing stabilizers or stabilizer precursors, and other image-modifying agents as would be readily apparent to one skilled in the art. The photothermographic materials of the present invention can be further protected against the production of fog and can be stabilized against loss of sensitivity during storage. While not necessary for the practice of the invention, it may be advantageous to add a mercury (II) salt to the imaging layer(s) as an antifoggant. Preferred mercury (II) salts for this purpose are mercuric acetate and mercuric bromide. Other suitable antifoggants and stabilizers that can be used alone or in combination include thiazolium salts as described in US-A-2,131,038 (Staud) and US-A-2,694,716 (Allen), azaindenes as described in US-A-2,886,437 (Piper), triazaindolizines as described in US-A-2,444,605 (Heimbach), mercury salts as described in US-A-2,728,663 (Allen), the urazoles described in US-A-3,287,135 (Anderson), sulfocatechols as described in US-A-3,235,652 (Kennard), the oximes described in GB 623,448 (Carrol et al.), polyvalent metal salts as described in US-A-2,839,405 (Jones), thiuronium salts as described in US-A-3,220,839 (Herz), palladium, platinum and gold salts as described in US-A-2,566,263 (Trirelli) and US-A-2,597,915 (Damshroder), and 2-(tribromomethyl-sulfonyl)quinoline compounds as described in US-A-5,460,938 (Kirk et al.). Stabilizer precursor compounds capable of releasing stabilizers upon application of heat during development can also be used. Such precursor compounds are described in for example, US-A-5,158,866 (Simpson et al.), US-A-5,175,081 (Krepski et al.), US-A-5,298,390 (Sakizadeh et al.) and US-A-5,300,420 (Kenney et al.). In addition, certain sulfonyl-substituted derivatives of benzotriazoles (for example alkylsulfonylbenzotriazoles and arylsulfonylbenzotriazoles) have been found to be useful stabilizing compounds (such as for post-processing print stabilizing), as described in copending and commonly assigned U.S. Patent 6,171,767 by Kong, Sakizadeh, LaBelle, Spahl, and Skoug. Still other antifoggants are hydrobromic acid salts of heterocyclic compounds (such as pyridinium hydrobromide perbromide) and substituted propenenitrile compounds as described for example in US-A-5,594,143 (Kirk et al.), US-A-5,028,523 (Skoug), US-A-4,784,939 (Pham), US-A-5,374,514 (Kirk et al.), US-A-5,496,696 (Patel et al.), US-A-5,686,228 (Murray et al.), US-A-5,358,843 (Sakizadeh et al.) EP-A-0 600,589 (Philip, Jr. et al.), EP-A-0 600,586 (Philip, Jr. et al.), US-A-6,083,861 (Lynch et al.), and EP-A-0 600,587 (Oliff et al.). Preferably, the photothermographic materials of this invention include one or more polyhalo antifoggants that include one or more polyhalo substituents including but not limited to, dichloro, dibromo, trichloro and tribromo groups. The antifoggants can be aliphatic, alicyclic or aromatic compounds, including aromatic heterocyclic and carbocyclic compounds. The use of "toners" or derivatives thereof that improve the image is highly desirable. Preferably, if used, a toner can be present in an amount of 0.01% by weight to 10%, and more preferably 0.1% by weight to 10% by weight, based on the total dry weight of the layer in which it is included. Toners are usually incorporated in the photothermographic emulsion layer or in adjacent layers. Toners are well known materials in the photothermographic art, as shown in US-A-3,080,254 (Grant, Jr.), US-A-3,847,612 (Winslow), US-A-4,123,282 (Winslow), US-A-4,082,901 (Laridon et al.), US-A-3,074,809 (Owen), US-A-3,446,648 (Workman), US-A-3,844,797 (Willems et al.), US-A-3,951,660 (Hagemann et al.), US-A-5,599,647 (Defieuw et al.) and GB 1,439,478 (AGFA). Examples of toners include but are not limited to phthalimide and N -hydroxyphthalimide, cyclic imides (such as succinimide), pyrazoline-5-ones, quinazolinone, 1-phenylurazole, 3-phenyl-2-pyrazoline-5-one, and . 2,4-thiazolidinedione, naphthalimides (such as N -hydroxy-1,8-naphthalimide), cobalt complexes (such as cobaltic hexamine trifluoroacetate), mercaptans (such as 3-mercapto-1,2,4-triazole, 2,4-dimercaptopyrimidine, 3-mercapto-4,5-diphenyl-1,2,4-triazole and 2,5-dimercapto-1,3,4-thiadiazole), N -(aminomethyl)aryldicarboximides [such as (N,N-dimethylaminomethyl)phthalimide, and N-(dimethylaminomethyl)naphthalene-2,3-dicarboximide, a combination of blocked pyrazoles, isothiuronium derivatives, and certain photobleach agents [such as a combination of N,N'-hexamethylene-bis(1-carbamoyl-3,5-dimethylpyrazole), 1,8-(3,6-diazaoctane)bis(isothiuronium)trifluoroacetate, and 2-(tribromomethylsulfonyl benzothiazole)], merocyanine dyes {such as 3-ethyl-5-[(3-ethyl-2-benzothiazolinylidene)-1-methyl-ethylidene]-2-thio-2,4- o -azolidinedione}, phthalazine and derivatives thereof, phthalazinone and phthalazinone derivatives, or metal salts or these derivatives [such as 4-(1-naphthyl)-phthalazinone, 6-chlorophthalazinone, 5,7-dimethoxyphthalazinone, and 2,3-dihydro-1,4-phthalazinedione], a combination of phthalazine (or derivative thereof) plus one or more phthalic acid derivatives (such as phthalic acid, 4-methylphthalic acid, 4-nitrophthalic acid, and tetrachlorophthalic anhydride), quinazolinediones, benzoxazine or naphthoxazine derivatives, rhodium complexes functioning not only as tone modifiers but also as sources of halide ion for silver halide formation in situ [such as ammonium hexachlororhodate (III), rhodium bromide, rhodium nitrate, and potassium hexachlororhodate (III)], inorganic peroxides and persulfates (such as ammonium peroxydisulfate and hydrogen peroxide), benzoxazine-2,4-diones (such as 1,3-benzoxazine-2,4-dione, 8-methyl-1,3-benzoxazine-2,4-dione and 6-nitro-1,3-benzoxazine-2,4-dione), pyrimidines and asym-triazines (such as 2,4-dihydroxypyrimidine, 2-hydroxy-4-aminopyrimidine and azauracil) and tetraazapentalene derivatives [such as 3,6-dimercapto-1,4-diphenyl- 1H , 4H- 2,3a,5,6a-tetraazapentalene and 1,4-di-( o -chlorophenyl)-3,6-dimercapto- 1H,4H -2,3a,5,6a-tetraazapentalene]. Phthalazine and various phthalazine derivatives [such as described in US-A-6,146,822 (Asanuma et al)] are particularly useful toners. Binders The photocatalyst (such as photosensitive silver halide for photothermographic materials), the non-photosensitive source of reducible silver ions, the reducing agent composition, and any other additives used in the present invention are generally present in one or more layers admixed within at least one binder that is either hydrophilic or hydrophobic. Thus, either aqueous or solvent-based formulations can be used to prepare materials of this invention. Mixtures of either or both types of binders can also be used. It is preferred that the binder be selected from hydrophobic polymeric materials, such as, for example, natural and synthetic resins that are sufficiently polar to hold the other ingredients in solution or suspension. Examples of typical hydrophobic binders include, but are not limited to, polyvinyl acetals, polyvinyl chloride, polyvinyl acetate, cellulose acetate, cellulose acetate butyrate, polyolefins, polyesters, polystyrenes, polyacrylonitrile, polycarbonates, methacrylate copolymers, maleic anhydride ester copolymers, butadiene-styrene copolymers and other materials readily apparent to one skilled in the art. Copolymers (including terpolymers) are also included in the definition of polymers. The polyvinyl acetals (such as polyvinyl butyral and polyvinyl formal) and vinyl copolymers (such as polyvinyl acetate and polyvinyl chloride) are particularly preferred. Particularly suitable binders are polyvinyl butyral resins that are available as BUTVAR® B79 (Solutia, Inc.) and Pioloform BS-18 or Pioloform BL-16 (Wacker Chemical Company). Examples of useful hydrophilic binders include, but are not limited to, gelatin and gelatin-like derivatives (hardened or unhardened), cellulosic materials such as cellulose acetate, cellulose acetate butyrate, hydroxymethyl cellulose, acrylamide/methacrylamide polymers, acrylic/methacrylic polymers polyvinyl pyrrolidones, polyvinyl acetates, polyvinyl alcohols and polysaccharides (such as dextrans and starch ethers). Hardeners for various binders (especially hydrophilic binders) may be present if desired. Useful hardeners are well known and include diisocyanate compounds as described for example in EP-0 600 586 B1 and vinyl sulfone compounds as described in EP-0 600 589 B1. Where the proportions and activities of the photothermographic materials require a particular developing time and temperature, the binder(s) should be able to withstand those conditions. Generally, it is preferred that the binder not decompose or lose its structural integrity at 120°C for 60 seconds, and more preferred that it 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 that is within the effective range of the action as the binder. The effective range can be appropriately determined by one skilled in the art. Preferably, a binder is used at a level of 10% by weight to 90% by weight, and more preferably at a level of 20% by weight to 70% by weight, based on the total dry weight of the layer in which they are included. Support Materials The photothermographic materials of this invention comprise a polymeric support that is preferably a flexible, transparent film that has any desired thickness and is composed of one or more polymeric materials depending upon their use. The supports are generally transparent or at least translucent, but in some instances, opaque supports may be useful. They are required to exhibit dimensional stability during development and to have suitable adhesive properties with overlying layers. Useful polymeric materials for making such supports include, but are not limited to, polyesters (such as polyethylene terephthalate and polyethylene naphthalate), cellulose acetate and other cellulose esters, polyvinyl acetal, polyolefins (such as polyethylene and polypropylene), polycarbonate, and polystyrenes (including polymers of styrene derivatives). Preferred supports are composed of polymers having good heat stability, such as polyesters and polycarbonate. Polyethylene terephthalate film is the most preferred support. Various support materials are described, for example, in Research Disclosure August 1979, publication 18431. A method of making dimensionally stable polyester films is described in Research Disclosure, September, 1999, publication 42536. Opaque supports can also be used including 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. Support materials may be treated using conventional procedures (such as corona discharge) to improve adhesion of overlying layers, or subbing or other adhesion-promoting layers can be used. Useful subbing layer formulations include those conventionally used for photographic materials including vinylidene halide polymers. Formulations For solvent-based formulations, thermographic or photothermographic emulsion layer(s) can be prepared by dissolving and dispersing the binder, the photocatalyst, the non-photosensitive source of reducible silver ions, the reducing composition, and optional addenda in an organic solvent, such as toluene, 2-butanone, acetone or tetrahydrofuran. Methods of making such formulations are described for example in US-A-5,275,927 (Pham et al.), US-A-5,422,234 (noted above), and US-A-5,928,857 (Geisler et al.). For aqueous-based formulations, the components of the emulsion layer(s) are dissolved or dispersed within water or mixtures of water and various water-miscible polar organic solvents such as alcohols. Methods of making such formulations are described for example in US-A-5,891,616 (Gilliams et al.), US-A-6,030,765 (Leenders et al.), EP-A-0 803,764 (Katoh et al.). Thus, this invention may also be useful in a method of preparing a thermally developable material comprising a support having thereon a thermally developable imaging layer(s) comprising a binder and in reactive association, a non-photosensitive source of reducible silver ions and a reducing composition for the non-photosensitive source reducible silver ions. This method comprises forming a surface barrier layer that is on the same side of but farther from the support than the imaging layer(s), by applying a formulation comprising a film-forming acrylate or methacrylate polymer having a molecular weight of at least 8000 g/mole and epoxy functionality that is in admixture with one or more additional film-forming polymers, and drying. Photothermographic materials can contain plasticizers and lubricants such as polyalcohols and diols of the type described in US-A-2,960,404 (Milton et al.), fatty acids or esters such as those described in US-A-2,588,765 (Robijns) and US-A-3,121,060 (Duane), and silicone resins such as those described in GB 955,061 (DuPont). The materials can also contain matting agents such as starch, titanium dioxide, zinc oxide, silica, and polymeric beads including beads of the type described in US-A-2,992,101 (Jelley et al.) and US-A-2,701,245 (Lynn) in various layers for conventional purposes. Polymeric fluorinated surfactants may also be useful in one or more layers of the imaging materials for various purposes, such as improving coatability and optical density uniformity as described in US-A-5,468,603 (Kub). EP-A-0 792 476 (Geisler et al.) describes various means of modifying photothermographic materials to reduce what is known as the "woodgrain" effect, or uneven optical density. This effect can be reduced or eliminated by treating the support, adding matting agents to the topcoat to provide a certain amount of haze, using acutance dyes in certain layers, or other procedures described in the noted publication. The imaging materials can include antistatic or conducting layers. Such layers may contain soluble salts (for example chlorides or nitrates), evaporated metal layers, or ionic polymers such as those described in US-A-2,861,056 (Minsk) and US-A-3,206,312 (Sterman et al.), or insoluble inorganic salts such as those described in US-A-3,428,451 (Trevoy), electroconductive underlayers such as those described in US-A-5,310,640 (Markin et al.) electronically-conductive metal antimonate particles such as those described in US-A-5,368,995 (Christian et al.), and electrically-conductive metal-containing particles dispersed in a polymeric binder such as those described in EP-A-0 678 776 (Melpolder et al.). Other antistatic agents are well known in the art. The imaging materials may also contain electroconductive underlayers to reduce static electricity effects and improve transport through processing equipment. Such layers are described in US-A-5,310,640 (Markin et al.). Imaging materials can be constructed of one or more layers on a support. Single layer materials contain the photocatalyst, the non-photosensitive source of reducible silver ions, the reducing composition, the binder, as well as optional materials such as toners, acutance dyes, coating aids and other adjuvants. The imaging formulations can be provided as two or more layers. For example, two-layer constructions (having two distinct layers on the frontside of the support) can contain photocatalyst and non-photosensitive source of reducible silver ions in one emulsion layer (usually the layer adjacent to the support) and the reducing composition and other ingredients in a second layer or distributed between both layers. If desired, the developer and co-developer may be in separate layers. Layers to promote adhesion of one layer to another in photothermographic materials are also known, as described for example in US-A-5,891,610 (Bauer et al.), US-A-5,804,365 (Bauer et al.) and US-A-4,741,992 (Przezdziecki). Adhesion can also be promoted using specific polymeric adhesive materials is adhered layers as described for example in US-A-5,928,857 (noted above). Photothermographic formulations described 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 US-A-2,681,294 (Beguin). Layers can be coated one at a time or simultaneously. It is preferred that two or more layers can be coated simultaneously by the procedures described in US-A-2,761,791 (Russell), US-A-4,001,024 (Dittman et al.), US-A-4,569,863 (Keopke et al.), US-A-5,340,613 (Hanzalik et al.), US-A-5,405,740 (LaBelle), US-A-5,415,993 (Hanzalik et al.), US-A-5,733,608 (Kessel et al.), US-A-5,849,363 (Yapel et al.), US-A-5,843,530 (Jerry et al.), US-A-5,861,195 (Bhave et al.) and GB 837,095 (Ilford). A typical coating gap for the emulsion layer can be from 10 to 750 µm, and the layer can be dried in forced air at a temperature of from 20°C to 150°C. It is preferred that the thickness of the layer be selected to provide maximum image densities greater than 0.2, more preferably greater than 3.0 and most preferably greater than 5.0, as measured by a commercially available X-Rite Model 361T Densitometer. When the layers are coated simultaneously using various coating techniques, a "carrier" layer formulation comprising a single-phase mixture of the two or more polymers described above may be used. Such formulations are described in copending and commonly assigned U.S. Serial No. 09/510,648 (filed February 23, 2000 by Ludemann, LaBelle, Geisler, Warren, Crump, and Bhave) that is based on Provisional Application 60/121,794, filed February 26, 1999. Mottle and other surface anomalies can be reduced in the materials of this invention by incorporation of a fluorinated polymer as described for example in US-A-5,532,121 (Yonkonski et al.) or by using particularly drying techniques as described for example in US-A-5,621,983 (Ludemann et al.). Preferably, two or more layers are applied to a film support using slide coating. The first layer can be coated on top of the second layer while the second layer is still wet. The first and second fluids used to coat these layers can be the same or different organic solvents (or organic solvent mixtures). While the first and second layers can be coated on one side of the film support, the method can also include forming on the opposing or backside of said polymeric support, one or more additional layers, including an antihalation layer, an antistatic layer, or a layer containing a matting agent (such as silica), or a combination of such layers. Imaging materials having emulsion layers on both sides of the support are also contemplated in this invention. Photothermographic materials of this present invention can comprise one or more layers containing one or more acutance dyes and/or antihalation dyes. These dyes are chosen to have absorption close to the exposure wavelength and are designed to absorb scattered light. One or more antihalation dyes may be incorporated into one or more antihalation layers according to known techniques as an antihalation backing layer, an antihalation underlayer or as an overcoat. It is preferred that the photothermographic materials of this invention contain an antihalation coating on the support opposite to the side on which the emulsion and topcoat layers are coated. To promote image sharpness, one or more acutance dyes may be usually incorporated into one or more frontside layers such as the photothermographic emulsion layer or topcoat layers according to known techniques. Dyes particularly useful as antihalation and acutance dyes include dihydroperimidine squaraine dyes having a nucleus represented by the following structure: Details of such dyes and methods of their preparation can be found in US-A-6,063,560 (Suzuki et al.) and US-A-5,380,635 (Gomez et al). These dyes can also be used as acutance dyes in frontside layers of the materials of this invention. One particularly useful dihydroperimidine squaraine dye is cyclobutenediylium, 1,3-bis[2,3-dihydro-2,2-bis[[1-oxohexyl)oxy]methyl]-1H-perimidin-4-yl]-2,4-dihydroxy-, bis(inner salt). Dyes particularly useful as antihalation dyes on the backside layer of the photothermographic materials also include indolenine cyanine dyes having the nucleus represented by the following structure: Details of such antihalation dyes having the indolenine cyanine nucleus and methods of their preparation can be found in EP-A-0 342 810 (Leichter). One particularly useful cyanine dye, compound (6) described therein, is 3H-Indolium, 2-[2-[2-chloro-3-[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)ethylidene]-5-methyl-1-cyclohexen-1-yl]ethenyl]-1,3,3-trimethyl-, perchlorate. It is also useful in the present invention to employ acutance or antihalation dyes that will decolorize with heat during processing. Dyes and constructions employing these types of dyes are described in, for example, US-A-5,135,842 (Kitchin et al.), US-A-5,266,452 (Kitchin et al.), US-A-5,314,795 (Helland et al.), and EP-0 911 693A1 (Sakurada et al.). Imaging/Development While the imaging materials of the present invention can be imaged in any suitable manner consistent with the type of material using any suitable imaging source (typically some type of radiation or electronic signal), the following discussion will be directed to the preferred imaging means. Generally, the materials are sensitive to radiation in the range of from 300 to 850 nm. Imaging of photothermographic materials can be achieved by exposing the materials to a suitable source of radiation to which they are sensitive, including ultraviolet light, visible light, near infrared radiation and infrared radiation to provide a latent image. Suitable exposure means are well known and include laser diodes that emit radiation in the desired region, photodiodes and others described in the art, including Research Disclosure, Vol. 389, Publication 38957, September 1996 (such as sunlight, xenon lamps and fluorescent lamps). Particularly useful exposure means are laser diodes that are modulated to increase imaging efficiency using what is known as multilongitudinal exposure techniques as described in US-A-5,780,207 (Mohapatra et al.). Other exposure techniques are described in US-A-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. Thus, the latent image can be developed by heating the exposed material at a moderately elevated temperature of, for example, from 50 to 250°C (preferably from 80 to 200°C, and more preferably from 100 to 200°C) for a sufficient period of time, generally from 1 to 120 seconds. Heating can be accomplished using any suitable heating means such as a hot plate, a steam iron, a hot roller or a heating bath. In some methods, the development is carried out in two steps. Thermal development takes place at a higher temperature for a shorter time (for example at 150°C for up to 10 seconds), followed by thermal diffusion at a lower temperature (for example at 80°C) in the presence of a transfer solvent. The second heating step prevents further development. Use as a Photomask The photothermographic materials of the present invention are sufficiently transmissive in the range of from 350 to 450 nm in non-imaged areas to allow their use in a process where there is a subsequent exposure of an ultraviolet or short wavelength visible radiation sensitive imageable medium. For example, imaging the photothermographic material and subsequent heat development affords a visible image. The heat-developed photothermographic material absorbs ultraviolet or short wavelength visible radiation in the areas where there is a visible image and transmits ultraviolet or short wavelength visible radiation where there is no visible image. The heat-developed material 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, for example, 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 photothermographic material provides an image in the imageable material. This process is particularly useful where the imageable medium comprises a printing plate and the photothermographic material serves as an imagesetting film. The following examples are provided to illustrate the practice of this invention, and are not intended to be limiting in any manner. The examples provide exemplary synthetic procedures and preparatory procedures using the surface barrier layers described herein. Unless otherwise indicated, all materials are commercially available from one or more sources. Examples 1-6: Photothermographic materials were prepared using the following layer formulations and procedures. Photothermographic Formulation: This formulation was prepared similarly to that described in US-A-5,939,249 of Zou. The following TABLE I shows the components of this formulation, their formulation concentrations (% weight based on total formulation weight in methyl ethyl ketone), and dry coating coverage (g/m 2 ). Component Formulation Concentration (% weight) Coating Coverage (g/m 2 ) Pioloform BS-18 polyvinyl butyral (Wacker Chemical) 2.85 1.54 AgBr preformed grains 0.34 0.184 Behenic acid 0.52 0.281 Arachidic acid 0.37 0.201 Stearic acid 0.26 0.139 Ag behenate 7.44 4.03 Ag arachidate 5.10 2.77 Ag stearate 0.82 0.443 Pyridinium hydrobromide perbromide 0.08 0.043 Zinc bromide 0.08 0.042 2-Mercapto-5-methylbenzimidazole 0.05 0.027 2-(4-chlorobenzoyl)-benzoic acid 0.55 0.298 Benzothiazolium, 3-ethyl-2-[[7-[[3-ethyl-5-(methylthio)-2(3H)-benzothiazolylidene]-methyl]-4,4a,5,6-tetrahydro-2(3H)-naphthalenylidene]methyl]-5-(methylthio)-, iodide 0.002 0.001 VITEL PE2200 polyester resin (Bostik, Inc.) 0.08 0.045 Pioloform BL-16 polyvinyl butyral (Wacker Chemical) 13.6 7.40 2-Tribromomethyl-sulfonylquinoline 0.43 0.233 DESMODUR N3300 hardener (Bayer Plastic & Coatings) 0.22 0.119 2,2-Isobutylidene-bis(4,6-dimethylphenol) 3.15 1.71 Tetrachlorophthalic acid 0.12 0.065 Phthalazine 0.44 0.239 4-Methylphthalic acid 0.20 0.108 Carrier Layer Formulation: A formulation that was coated underneath the photothermographic formulation comprised the components and amounts shown in TABLE II below formulated in methyl ethyl ketone solvent. Component Formulation Concentration (% weight) Coating Coverage (g/m 2 ) VITEL 2200 polyester (Bostik, Inc.) 0.274 0.012 Pioloform BL-16 polyvinyl butyral (Wacker Chemical) 6.57 0.296 The surface barrier layer formulation contained the components and amounts shown in TABLE III formulated in methyl ethyl ketone solvent. Component Formulation Concentration (% weight) Coating Coverage (g/m 2 ) 1,3-Bis(vinylsulfonyl)-2-propanol 0.091 0.056 Benzotriazole 0.068 0.042 Sylysia 310 amorphous silica (Fuji Silysia) 0.054 0.033 Acryloid 21 (Rohm & Haas) 0.172 0.106 Binder polymers, see TABLE V below 4.464 2.75 Cyclobutenediylium, 1,3-bis[2,3-dihydro-2,2-bis[[1-oxohexyl)oxy]methyl]-1H-perimidin-4-yl]-2,4-dihydroxy-, bis(inner salt) 0.054 0.033 Ethyl 2-cyano-3-oxobutanoate 0.060 0.037 An antihalation backing layer formulation was prepared in methyl ethyl ketone to have the components and concentrations shown in TABLE IV below formulated in methyl ethyl ketone solvent. Component Formulation Concentration (% weight) Coating Coverage (g/m 2 ) VITEL 2200 polyester (Bostik, Inc.) 0.173 0.057 Cellulose acetate butyrate 12.4 4.13 Cyclobutenediylium, 1,3-bis[2,3-dihydro-2,2-bis[[1-oxohexyl)oxy]methyl]-1H-perimidin-4-yl]-2,4-dihydroxy-, bis(inner salt) 0.088 0.029 Syloid 74x6000 silica (Grace-Davison) 0.161 0.054 4-Methylphthalic acid 0.218 0.073 α-(2-aminoethyl)-ω-(2-aminoethoxy)-poly(oxy-1,2-ethanediyl)-1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8 -heptadecafluoro- 1-octane sulfonate antistatic agent 0.986 0.328 The carrier layer and photothermographic formulations were coated onto a 7 mil (0.018 cm) thick transparent poly(ethylene terephthalate) film using conventional coating techniques and equipment to give a dry emulsion layer coverage of 20 g/m 2 . Once dried, the resulting photothermographic emulsion layer was overcoated with a surface topcoat formulation. A Control A material was prepared by coating a surface topcoat formulation comprising solely cellulose acetate butyrate (CAB) as the binder material in methyl ethyl ketone (MEK) to provide a dry coverage of 2.75 g/m 2 . This material was considered a "Control" film because the surface topcoat layer is not a surface barrier layer within the scope of the present invention. Control B comprised a surface topcoat layer comprised of a 50:50 weight mixture of cellulose acetate butyrate and a liquid epoxy resin derived from bisphenol A and epichlorohydrin, EPON 828 (available from Shell Chemical Co.). This epoxy resin is not an acrylate or methacrylate resin. Photothermographic materials of the present invention were prepared similarly except that over the dried emulsion layer was coated a solution of poly(glycidyl methacrylate) and CAB in methyl ethyl ketone (MEK). The dry coverage (thickness) of the resulting surface barrier layers is shown in TABLE V below as well as the various weight ratios of CAB to the "epoxy polymer" (containing the epoxy functionality). The coating coverage was varied by changing the % solids of the mixture of film-forming polymers. In Examples 1-6, the "epoxy polymer" was poly(glycidyl methacrylate) and in Example 7, it was poly(glycidyl methacrylate-co-ethyl methacrylate) (75:25 molar ratio). The effectiveness of the various surface barrier layers to inhibit the diffusion of chemical components (such as fatty acids like behenic acid) from the emulsion layer was evaluated as follows. A sample of the photothermographic material was placed between clean conventional glass microscope slides. About 1110 g of weight was evenly applied to the resulting laminate while it was heated at 120°C for 30 minutes. The glass slide in contact with the photothermographic material topcoat was then analyzed for the relative amount of fatty acid transferred to it using Attenuated Total Reflectance Fourier Transform InfraRed Spectroscopy (ATR FTIR) and a conventional Bio-Rad FTS60 FTIR spectrometer fitted with a diamond ATR stage. At least two spectra of the glass slide from each photothermographic material sample were collected. The CH 2 stretching bands (2920 and 2850 cm -1 ) and the CH 3 stretching band (2955 cm -1 ) of the fatty acid were divided by the SiO 2 band (910 cm -1 ) of the glass to provide a ratio after baseline correction. The relative amount of fatty acid transferred is directly related to the value of the ratio. That is, lower ratios mean lower fatty acid transfer and that the surface layer acts as a better surface barrier layer. The FTIR ratios are also shown in TABLE V below. Material CAB/Epoxy Polymer Ratio Dry Coverage (g/m 2 ) FTIR Ratio Control A 100:0 2.70 0.016 - 0.021 Control B 50:50 2.70 0.039 Example 1 85:15 2.70 0.007 Example 2 75:25 2.70 0.008 Example 3 50:50 2.70 0.006 Example 4 50:50 2.20 0.010 Example 5 50:50 3.30 0 Example 6 0:100 2.70 0.008 Example 7 50:50 2.75 0.008

Claims: A thermally developable material comprising a support having thereon: a) a thermally developable imaging layer(s) comprising a binder and in reactive association, a non-photosensitive source of reducible silver ions and a reducing composition for the non-photosensitive source reducible silver ions, the material characterized as further comprising: b) a surface barrier layer that is on the same side of but farther from the support than the imaging layer(s), the barrier layer comprising a film-forming acrylate or methacrylate polymer having a molecular weight of at least 8000 g/mole and epoxy functionality. The thermally developable material of claim 1 that further comprises a protective layer that is disposed between the barrier layer and the imaging layer(s). The thermally developable material of claim 1 or 2 wherein the reducing composition comprises at least one hindered phenol and the imaging layer(s) further comprises a high contrast agent that is an acrylonitrile co-developer, an isoxazole co-developer or a hydrazide co-developer. The thermally developable material of any of claims 1 to 3 that is a photothermographic material further comprising a photocatalyst. The thermally developable material of any of claims 1 to 4 wherein the one or more film-forming acrylate or methacrylate polymers having epoxy functionality are comprised of recurring units, 25 mol % or more of which recurring units comprise a pendant oxirane ring. The thermally developable material of any of claims 1 to 5 wherein the one or more film-forming acrylate or methacrylate polymers having epoxy functionality are vinyl polymers represented by Formula I: -(A) m -(B) n - wherein A represents recurring units derived from one or more ethylenically unsaturated polymerizable acrylate or methacrylate monomers comprising a pendant oxirane ring, B represents recurring units derived from one or more ethylenically unsaturated polymerizable acrylate or methacrylate monomers other than those represented by A, m is from 25 to 100 mol %, and n is from 0 to 75 mol %. The thermally developable material of claim 6 wherein A represents recurring units derived from one or more of glycidyl methacrylate, 2,3-epoxybutyl methacrylate, 3,4-epoxybutyl methacrylate, 2,3-epoxycyclohexyl methacrylate, glycidyl acrylate, or allyl glycidyl ether. The thermally developable material of any of claims 1 to 7 further comprising one or more additional film-forming polymers that do not contain epoxy functionality. The thermally developable material of claim 8 wherein the one or more additional film-forming polymers are cellulosic materials, polyacrylates, polymethacrylates, polyesters or polyurethanes. The thermally developable material of any of claims 1 to 9 wherein the surface barrier layer is capable of retarding the diffusion of or reacting with fatty carboxylic acids. The thermally developable material of claim 10 wherein the surface barrier layer is capable of retarding the diffusion of or is reactive with behenic acid. The photothermographic material of claim 4 comprising a support having thereon: a) a thermally developable imaging layer(s) comprising a binder and in reactive association, a photocatalyst, a non-photosensitive source of reducible silver ions, and a reducing composition for the non-photosensitive source reducible silver ions, and b) a surface barrier layer that is on the same side of but farther from the support than the imaging layer(s), the barrier layer comprising a film-forming acrylate or methacrylate polymer having a molecular weight of at least 8000 g/mole and epoxy functionality. A method of forming a visible image comprising: A) imagewise exposing the photothermographic material of claim 12 to electromagnetic radiation to form a latent image, B) simultaneously or sequentially, heating the exposed photothermographic material to develop the latent image into a visible image. The method of claim 13 wherein the photothermographic material has a transparent support and the method further comprises: C) positioning the exposed and heat-developed photothermographic material between a source of imaging radiation and an imageable material that is sensitive to the imaging radiation, and D) 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.

Inventors: BAUER CHARLES LEO, HORCH MICHELLE L, MILLER ANNE M, TEEGARDEN DAVID MORRISON, BAUER, CHARLES LEO, HORCH, MICHELLE L., MILLER, ANNE M., TEEGARDEN, DAVID MORRISON, BAUER, CHARLES LEO, C/O EASTMAN KODAK COMPANY, Horch, Michelle L., c/o Eastman Kodak Company, Miller, Anne M., c/o Eastman Kodak COmpany, Teegarden, David Morrison, Eastman Kodak Company
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 2001
Main classification: G03C1
IPC codes: G03C1