BACKGROUND OF THE INVENTION Silver-containing photothermographic imaging materials (that is, photosensitive thermally developable imaging materials) that are imaged with actinic radiation and then developed using heat and without liquid processing have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermo- graphic material to specific electromagnetic radiation (for example, X-radiation, or ultraviolet, visible, or infrared radiation) and developed by the use of thermal energy. These materials, also known as"dry silver"materials, generally comprise a support having coated thereon: (a) a photocatalyst (that is, a photosensitive compound such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a hydrophilic or hydrophobic binder. The latent image is then developed by application of thermal energy.

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

and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced.

In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a"developer, "may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. At elevated temperatures, the reducible silver ions are reduced by the reducing agent.

In photothermographic materials, upon heating, this reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the imaging layer (s).

Differences Between Photothermography and Photography The imaging arts have long recognized that the field of photo- thermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with 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 or a silver benzotriazole) is used to generate the visible image using thermal development. Thus, the imaged photosensitive

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

In photothermographic materials, all of the"chemistry"for imaging is incorporated within the material itself. For example, such materials include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of"fog"or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems.

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

Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the underlying chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and

chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.

These and other distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials (Neblette's Eightla Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al. (Eds. ), The Focal Press, London and New York, 1978, pp. 74-75, in Zou et al., J. Imaging Sci. Technol. 1996, 40, pp. 94-103, and in M. R. V. Sahyun, J. Imaging Sci. Technol. 1998, 42, 23.

Problem to be Solved One of the challenges in the use of photothermographic materials is attaining sufficient photothermographic speed in such materials that are also compatible with available imaging sources.

Each of the pure photographic silver halides (silver chloride, silver bromide and silver iodide) and mixed silver halides (such as silver bromochloro- iodide) has its own natural response to radiation, in both wavelength (spectral sensitivity) and efficiency (speed), within the UV, near UV and blue regions of the electromagnetic spectrum. Thus, silver halide grains, when composed of only silver and halogen atoms, have defined levels of sensitivity depending upon the levels of specific halogen, crystal morphology (shape and structure of the crystals or grains), crystal defects, stresses, and dislocations, and dopants incorporated within or on the crystal lattice of the silver halide.

Chemical sensitization (generally sulfur-sensitization) is a process, during or after silver halide crystal formation, in which sensitization centers [for example, silver sulfide clusters such as (Ag2S) n] are introduced onto the individual silver halide grains. For example, silver sulfide specks can be introduced by direct reaction of sulfur-contributing compounds with the silver halide during various

stages or after completion of silver halide grain growth. These specks usually function as shallow electron traps for the preferential formation of a latent image center. Other chalcogens (Se and Te) can function similarly. The presence of these specks increases the speed or sensitivity of the resulting silver halide grains to radiation. Sulfur-contributing compounds useful for this purpose include thiosulfates (such as sodium thiosulfate) and various thioureas (such as allyl thiourea, thiourea, triethyl thiourea and 1, 1'-diphenyl-2-thiourea) as described for example, by Sheppard et al., J. Franklin Inst., 1923, pp. 196,653, and 673, C. E.

K. Mees and T. H. James, The Theory of the Photographic Process, 4th Edition, 1977, pp. 152-3, and Tani, T., Photographic Sensitivity : Theory and Mechanisms, Oxford University Press, NY, 1995, pp. 167-176).

In photothennographic emulsions, the photosensitive silver halide must be in catalytic proximity to (or in reactive association with) the non-photo- sensitive source of reducible silver ions. Because of the different emulsion making procedures and chemical environments of photothermographic emulsions, the effects achieved by compounds (such as chemical sensitizers) in conventional photographic emulsions are not necessarily possible in photothermographic emulsions.

For example, in photothermographic emulsions, two types of chemical sensitization have been used to increase speed: (a) chemical sensitization of preformed silver halide grains that are then mixed into the solution containing reducible silver ions in some manner, and (b) chemical sensitization of preformed silver halide grains that are already in intimate contact with the reducible silver ions.

In the first approach (a), many of the traditional methods (used for photographic emulsions) can be used, but for the second approach (b), quite specific methods and unique compounds are often needed. Regardless of which approach is used, there is considerable difficulty in attaining additional speed while maintaining low fog (Dmin).

Another method of chemical sensitization is achieved by oxidative decomposition of a sulfur-containing spectral sensitizing dye in a photothermo

graphic emulsion as described in U. S. Patent 5,891, 615 (Winslow et al. ). In general, this method involves providing the sulfur-containing dye (such as a sulfur-containing merocyanine dye) on or around preformed silver halide grains in an emulsion containing both the silver halide grains and a non-photosensitive source of silver ions. The sulfur-containing compound on or around the silver halide grains is then decomposed by addition of a strong oxidizing agent such as pyridinium hydrobromide perbromide (PHP) to the emulsion. Subsequent to this decomposition, a portion of the non-photosensitive silver salt is converted in-situ to silver halide by addition of an inorganic halide compound.

Photothermographic materials are constantly being redesigned to meet ever-increasing performance, storage, and manufacturing demands raised by customers, regulators, and manufacturers. One of these demands is increased photospeed without a significant increase in fog (Dmin) or a loss in Dmax. Thus, while the current method of chemical sensitization described in U. S. Patent 5,891, 615 (noted above) has provided desired speed for photothermographic emulsions, there is a continued need for an improved method that provides even greater photospeed for such emulsions.

SUMMARY OF THE INVENTION This invention provides a method of preparing a photothermo- graphic emulsion comprising: (A) providing a photothermographic dispersion of a preformed photosensitive silver halide and a non-photosensitive source of reducible silver ions, and performing the following steps in order, (B) providing one or more diphenylphosphine sulfide compounds, in association with the preformed silver halide grains and the non-photosensitive source of reducible silver ions, the one or more diphenylphosphine sulfide compounds being represented by the following Structure PS:

(PS) wherein Ph, and Ph2 are the same or different phenyl groups, Rl and R2 are each independently hydrogen or an alkyl or phenyl group, L is a direct bond or a divalent linking group, m is 1 or 2, and when m is 1, R3 is a monovalent group and when m is 2, R3 is a divalent aliphatic linking group having 1 to 20 carbon, nitrogen, oxygen, or sulfur atoms in the chain, (C) chemically sensitizing the preformed silver halide grains by decomposing the one or more diphenylphosphine sulfide compounds represented above by Structure (PS) on or around the silver halide grains in an oxidizing environment to provide a photothermographic emulsion comprising chemically sensitized photosensitive silver halide grains in reactive association with the non-photosensitive source of reducible silver ions, and (D) converting some of the reducible silver ions in the non-photo- sensitive source of reducible silver ions into photosensitive silver halide grains.

In preferred embodiments of this invention, a method of preparing a black-and-white photothermographic emulsion comprises: (A) providing a photothermographic dispersion of a preformed photosensitive silver halide and a non-photosensitive source of reducible silver ions, and performing the following steps in order: (B) providing one or more diphenylphosphine sulfide compounds in association with the preformed silver halide grains and the non-photosensitive source of reducible silver ions, the one or more diphenylphosphine sulfide compounds selected from at least one of the compounds PS-1 to PS-19 described below, (C) chemically sensitizing the preformed silver halide grains by decompsing the one or more diphenylphosphine sulfide compounds on or around the silver halide grains by the addition, in one or more stages, of pyridinium

hydrobromide perbromide to the silver halide grains at from 20°C to 30°C for up to 60 minutes, to provide a photothermographic emulsion comprising chemically sensitized photosensitive silver bromide grains in reactive association with the non-photosensitive source of reducible silver ions comprising silver behenate, and (D) converting from 0.1 to 10 mol % of the reducible silver ions in the non-photosensitive source of reducible silver ions into photosensitive silver bromide grains by addition of a bromide salt.

This invention provides a method of preparing a photothermo- graphic material comprising: (A) providing a photothermographic dispersion of a preformed photosensitive silver halide and a non-photosensitive source of reducible silver ions, and performing the following steps in order, (B) providing one or more diphenylphosphine sulfide compounds, in association with the preformed silver halide grains and the non-photosensitive source of reducible silver ions, the one or more diphenylphosphine sulfide compounds being represented by the Structure (PS) noted above, (C) chemically sensitizing the preformed silver halide grains by decomposing the one or more diphenylphosphine sulfide compounds on or around the silver halide grains in an oxidizing environment to provide a photothermo- graphic emulsion comprising chemically sensitized photosensitive silver halide grains in reactive association with the non-photosensitive source of reducible silver ions, and (D) converting some of the reducible silver ions in the non-photo- sensitive source of reducible silver ions into photosensitive silver halide grains.

(E) simultaneously with any of steps (A) through (D), or subsequently to step (D), adding a binder to form a photothermographic emulsion formulation, and (F) after step (E), coating and drying the photothermographic emulsion formulation on a support to provide a photothermographic material.

The present invention provides photothermographic emulsions and materials having increased photospeed ("speed"), improved silver coverage, and better reproducibility without significant loss in Dmin (fog) or Dmax.

During the ex-situ preparation of the preformed silver halide as well as during the formation of the silver carboxylate soap dispersion, impurities are formed. These impurities can act as fog centers and, upon aging and processing, cause an increase in Dmin. These impurities are conventionally removed by addition of an oxidizing agent. One impurity is believed to be silver atoms, clusters, or particles of silver (0). Addition of an oxidizing agent is believed to remove these fog centers by converting silver (0) species to silver (I). Bromine- containing oxidizing agents are often used to convert these silver (0) species to silver bromide.

U. S. Patent 5,891, 615 (Winslow et al. , noted above) describes a method for chemically sensitizing photothermographic emulsions by oxidative decomposition of a sulfur-containing spectral sensitizing dye. This method is believed to also simultaneously remove some fog centers. Subsequent to the decomposition of the sulfur-containing spectral sensitizing dye, some of the silver ions in the non-photosensitive source of reducible silver ions are converted to silver halide.

We have found that the use of diphenylphosphine sulfide compounds provides an improved method for chemically sensitizing photothermo- graphic emulsions when compared to the use of sulfur-containing spectral sensitizing dyes known in the art.

DETAILED DESCRIPTION OF THE INVENTION The photothermographic materials prepared by this invention can be used in black-and-white or color photothermography and in electronically generated black-and-white or color hardcopy recording. They can be used in microfilm applications, in radiographic imaging (for example digital medical imaging), X-ray radiography, and in industrial radiography. Furthermore, the absorbance of these photothermographic materials between 350 and 450 nm is

desirably low (less than 0.5), to permit their use in the graphic arts area (for example, imagesetting and phototypesetting), in the manufacture of printing plates, in contact printing, in duplicating ("duping"), and in proofing.

The photothermographic materials prepared by this invention are particularly useful for medical imaging of human or animal subjects in response to visible or X-radiation for use in diagnosis. Such applications include, but are not limited to, thoracic imaging, mammography, dental imaging, orthopedic imaging, general medical radiography, therapeutic radiography, veterinary radiography, and auto-radiography. When used with X-radiation, the photothermographic materials of this invention may be used in combination with one or more phosphor intensifying screens, with phosphors incorporated within the photothermographic emulsion, or with a combination thereof. Such materials are particularly useful for dental radiography.

The photothennographic materials prepared by the methods of this invention can be made sensitive to radiation of any suitable wavelength. Thus, in some embodiments, the materials are sensitive at ultraviolet, visible, infrared, or near infrared wavelengths, of the electromagnetic spectrum. In preferred embodiments, the materials are sensitive to radiation greater than 350 nm (such as sensitivity to from 250 to 1100 nm). Increased sensitivity to a particular region of the spectrum is imparted through the use of various sensitizing dyes. In other embodiments they are sensitive to X-radiation directly. Increased sensitivity to X-radiation is imparted through the use of phosphors.

The photothermographic materials prepared by the methods of this invention are also useful for non-medical uses of visible or X-radiation (such as X-ray lithography and industrial radiography). In such imaging applications, it is often desirable that the photothermographic materials be"double-sided." In the photothermographic materials prepared by this invention, the components needed for imaging can be in one or more photothermographic imaging layers on one side ("frontside") of the support. The layer (s) that contain the photosensitive photocatalyst (such as a photosensitive silver halide) or non-photosensitive source of reducible silver ions, or both, are referred to herein

as photothermographic emulsion layer (s). The photocatalyst and the non-photo- sensitive source of reducible silver ions are in catalytic proximity (that is, in reactive association with each other) and preferably are in the same emulsion layer.

Where the materials contain imaging layers on one side of the support only, various non-imaging layers are usually disposed on the"backside" (non-emulsion or non-imaging side) of the materials, including conductive layers, antihalation layers, protective layers, and transport enabling layers.

In such instances, various non-imaging layers can also be disposed on the"frontside"or imaging or emulsion side of the support, including protective topcoat layers, primer layers, interlayers, opacifying layers, antistatic layers, antihalation layers, acutance layers, auxiliary layers, and other layers readily apparent to one skilled in the art.

For some applications it may be useful that the photothermographic materials be"double-sided"and have the same or different photothennographic coatings (or imaging layers) on both sides of the support. In such constructions each side can also include one or more protective topcoat layers, primer layers, interlayers, antistatic layers, acutance layers, auxiliary layers, anti-crossover layers, and other layers readily apparent to one skilled in the art.

When the photothermographic materials prepared by this invention are heat-developed as described below in a substantially water-free condition after, or simultaneously with, imagewise exposure, a silver image (preferably a black- and-white silver image) is obtained.

Definitions As used herein: In the descriptions of the photothermographic materials prepared by the present invention,"a"or"an"component refers to"at least one"of that component (for example, the specific diphenylphosphine sulfide compounds used for chemical sensitization).

Heating in a substantially water-free condition as used herein, means heating at a temperature of from 50°C 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, Eastman Kodak Company, Rochester, NY, 1977, p. 374.

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

When used in photothermography, the term, "imagewise exposing" or"imagewise exposure"means that the material is imaged using any exposure means that provides a latent image using electromagnetic radiation. This includes, for example, by analog exposure where an image is formed by projection onto the photosensitive material as well as by digital exposure where the image is formed one pixel at a time such as by modulation of scanning laser radiation.

"Catalytic proximity"or"reactive association"means that the materials are in the same layer or in adjacent layers so that they readily come into contact with each other during thermal imaging and development.

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

"Photocatalyst"means a photosensitive compound such as silver halide that, upon exposure to radiation, provides a compound that is capable of acting as a catalyst for the subsequent development of the image-forming material.

Many of the materials used herein are provided as a solution. The term"active ingredient"means the amount or the percentage of the desired material contained in a sample. All amounts listed herein are the amount of active ingredient added.

"Ultraviolet region of the spectrum"refers to that region of the spectrum less than or equal to 410 nm, and 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 to 405 nm.

"Visible region of the spectrum"refers to that region of the spectrum of from 400 nm to 700 nm.

"Short wavelength visible region of the spectrum"refers to that region of the spectrum of from 400 nm to 450 nm.

"Red region of the spectrum"refers to that region of the spectrum of from 600 nm to 700 nm.

"Infrared region of the spectrum"refers to that region of the spectrum of from 700 nm to 1400 nm.

"Non-photosensitive"means not intentionally light sensitive.

The sensitometric terms"photospeed", "speed", or"photographic speed" (also known as sensitivity), absorbance, contrast, Dmin, and Dmax have conventional definitions known in the imaging arts.

In photothermographic materials, Dmin is considered herein as image density achieved when the photothermographic material is thermally developed without prior exposure to radiation. It is the average of eight lowest density values on the exposed side of the fiducial mark.

Dmax is the maximum density of film in the imaged area.

The sensitometric term absorbance is another term for optical density (OD).

"SP-2" (Speed-2) is Logl/E + 4 corresponding to the density value of 1.00 above Dmin where E is the exposure in ergs/cm2.

"SP-3" (Speed-3) is Logl/E + 4 corresponding to the density value of 2.9 above Dmin where E is the exposure in ergs/cm2.

"AC-1" (Average Contrast-1) is the absolute value of the slope of the line joining the density points of 0.60 and 2.00 above Dmin.

"AC-2" (Average Contrast-2) is the absolute value of the slope of the line joining the density points of 1.00 and 2.40 above Dmin.

Dmax/Ag coat weight is the maximum density divided by the silver coating weight in g/m2. It represents the efficiency of development.

"Transparent"means capable of transmitting visible light or imaging radiation without appreciable scattering or absorption.

As used herein, the phrase"organic silver coordinating ligand" refers to an organic molecule capable of forming a bond with a silver atom.

Although the compounds so formed are technically silver coordination compounds they are also often referred to as silver salts.

The terms"double-sided, "and"double-faced coating"are used to define photothermographic materials having one or more of the same or different thermally developable emulsion layers disposed on both sides (front and back) of the support. Another term for double-sided is"duplitized."

In the compounds described herein, no particular double bond geometry (for example, cis or trans) is intended by the structures drawn.

Similarly, in compounds having alternating single and double bonds and localized charges are drawn as a formalism. In reality, both electron and charge delocalization exists throughout the conjugated chain.

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

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

Research Disclosure is a publication of Kenneth Mason Publications Ltd. , Dudley House, 12 North Street, Emsworth, Hampshire PO10 7DQ England. It is also available from Emsworth Design Inc., 147 West 24th Street, New York, N. Y. 10011.

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

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

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, rhombic, dodecahedral, other polyhedral, tabular, laminar, twined, or platelet morphologies and may have epitaxial growth of crystals thereon. If desired, a mixture of these crystals can be employed. Silver halide grains having cubic and tabular morphology are preferred, and mixtures of both cubic and tabular grains can be used in the present invention.

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 or more silver halides, and a discrete shell of one or more different silver halides. Core-shell silver halide grains useful in photothermographic materials and methods of preparing these materials are

described for example in U. S. Patent 5,382, 504 (Shor et al. ). Iridium and/or copper doped core-shell and non-core-shell grains are described in U. S. Patent 5,434, 043 (Zou et al. ) and U. S. Patent 5,939, 249 (Zou).

In some instances, it may be helpful to prepare the photosensitive silver halide grains in the presence of a hydroxytetrazaindene (such as 4-hydroxy- 6-methyl-1, 3,3a, 7-tetrazaindene or an N-heterocyclic compound comprising at least one mercapto group (such as 1-phenyl-5-mercaptotetrazole) to provide increased photospeed. Details of this procedure are provided in U. S. Patent 6,413, 710 (Shor et al.).

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

It is preferred that the silver halides be preformed and prepared by an ex-situ process. The silver halide grains prepared ex-situ may then be added to and physically mixed with the non-photosensitive source of reducible silver ions.

It is more preferable to form the non-photosensitive source of reducible silver ions in the presence of ex-situ-prepared silver halide. In this process, the source of reducible silver ions, such as a long chain fatty acid silver carboxylate (commonly referred to as a silver"soap"), is formed in the presence of the preformed silver halide grains. Co-precipitation of the source of reducible silver ions in the presence of silver halide provides a more intimate mixture of the two materials [see, for example U. S. Patent 3,839, 049 (Simons) ]. Materials of this type are often referred to as"preformed soaps".

Preformed silver halide emulsions used in the material of this invention can be prepared by aqueous or organic processes and can be unwashed or washed to remove soluble salts. In the latter case, the soluble salts can be removed by ultrafiltration, by chill setting and leaching, or by washing the coagulum [for example, by the procedures described in U. S. Patent 2,618, 556 (Hewitson et al. ), U. S. Patent 2,614, 928 (Yutzy et al. ), U. S. Patent 2,565, 418 (Yackel), U. S. Patent 3,241, 969 (Hart et al. ), and U. S. Patent 2,489, 341 (Waller et al.)].

It is also effective to use an in-site process in which a halide-or a halogen-containing compound is added to an organic silver salt to partially convert the silver of the organic silver salt to silver halide. The compound can be one or more inorganic halides (such as zinc bromide, zinc iodide, calcium bromide, lithium bromide, lithium iodide, or mixtures thereof) or an organic halogen- containing compound (such as N-bromosuccinimide or pyridinium hydrobromide perbromide). As noted above, a portion of the silver halide grains used in the present invention are prepared using an in-situ process. The details of such in-situ generation of silver halide are well known and described for example in U. S.

Patent 3,457, 075 (Morgan et al. ). Further details of this procedure are provided below. Zinc bromide is preferably added in the practice of this invention.

It is particularly effective to use a mixture of both preformed and in-situ generated silver halide.

Additional methods of preparing silver halide and organic silver salts and manners of blending them are described in Research Disclosure, June 1978, item 17029, U. S. Patent 3,700, 458 (Lindholm), U. S. Patent 4,076, 539 (Ikenoue et al. ), JP Kokai 49-013224, (Fuji), JP Kokai 50-017216 (Fuji), and JP Kokai 51-042529 (Fuji).

The silver halide grains used in the imaging formulations can vary in average diameter of up to several micrometers (pm) depending on the desired use. Preferred silver halide grains are those having an average particle size of from 0.01 to 1.5 Mm, more preferred are those having an average particle size of from 0.03 to 1. 0 um, and most preferred are those having an average particle size of from 0.05 to 0.8 pm. Those of ordinary skill in the art understand that there is a finite lower practical limit for silver halide grains that is partially dependent upon the wavelengths to which the grains are spectrally sensitized. Such a lower limit, for example, is typically from 0.01 to 0.005 Fm.

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

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

The one or more light-sensitive silver halides provided 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, and most preferably from 0.03 to 0.15 mole, per mole of non-photosensitive source of reducible silver ions.

Chemical Sensitization The photothermographic emulsions useful in the present invention can be prepared by: (A) providing a photothermographic dispersion of a preformed photosensitive silver halide and a non-photosensitive source of reducible silver ions, and performing the following steps in order, (B) providing one or more diphenylphosphine sulfide compound, in association with the preformed silver halide grains and the non-photosensitive source of reducible silver ions (for example, by incorporating the organic sulfur- containing compound into the photothermographic dispersion), the diphenyl- phosphine sulfide compound being represented by the following Structure (PS):

wherein Phl and Ph2 are the same or different phenyl groups, Rl and R2 are each independently hydrogen or an alkyl or phenyl group, L is a direct bond or a divalent linking group, m is 1 or 2 and when m is 1, R3 is a monovalent group and when m is 2, R3 is a divalent aliphatic linking group having 1 to 20 carbon, nitrogen, oxygen, or sulfur atoms in the chain, and (C) chemically sensitizing the preformed silver halide grains by decomposing the diphenylphosphine sulfide compound on or around the silver halide grains in an oxidizing environment to provide a photothermographic emulsion comprising chemically sensitized photosensitive silver halide grains in reactive association with the non-photosensitive source of reducible silver ions, and (D) converting some of the reducible silver ions in the non-photo- sensitive source of reducible silver ions into photosensitive silver halide grains.

This invention provides a method of preparing a photothermo- graphic material comprising: (A) providing a photothermographic dispersion of a preformed photosensitive silver halide and a non-photosensitive source of reducible silver ions, and performing the following steps in order, (B) providing one or more diphenylphosphine sulfide compound, in association with the preformed silver halide grains and the non-photosensitive source of reducible silver ions, the diphenylphosphine sulfide compound being represented by the Structure (PS) defined herein, (C) chemically sensitizing the preformed silver halide grains by decomposing the diphenylphosphine sulfide compound on or around the silver halide grains in an oxidizing environment to provide a photothermographic emulsion comprising chemically sensitized photosensitive silver halide grains in reactive association with the non-photosensitive source of reducible silver ions, (D) converting some of the reducible silver ions in the non-photo- sensitive source of reducible silver ions into photosensitive silver halide grains, (E) simultaneously with any of steps (A) through (D), or subsequently to step (D), adding a binder to form an emulsion formulation, and

(F) after step (E), coating and drying the emulsion formulation on a support to provide a photothermographic material.

A photothermographic dispersion of the photosensitive silver halide and the non-photosensitive source of reducible silver ions can be provided in a conventional fashion. Representative examples of such dispersions and methods for preparing them are described in detail in U. S. Patent 5,434, 043 (Zou et al. ) and U. S. Patent 5,939, 249 (Zou), and in the examples provided below.

Generally such dispersions comprise photosensitive silver halides and non-photosensitive organic silver salts in suitable solvents such as acetone, methyl ethyl ketone (MEK, 2-butanone), methyl isobutyl ketone (MIBK), toluene, methanol, ethanol, isopropanol, and mixtures thereof.

In one embodiment, one or more diphenylphosphine sulfide compounds are then added and suitably mixed with the photothermographic dispersion. We believe that in this step, the diphenylphosphine sulfide compound becomes located on or around the surface of the silver halide grains.

The silver halide grains are then chemically sensitized by decomposing the diphenylphosphine sulfide compound on or around the silver halide grains in an oxidizing environment to provide a photothermographic emulsion comprising chemically sensitized photosensitive silver halide grains in reactive association with the non-photosensitive source of reducible silver ions.

After the silver halide grains are chemically sensitized by oxidative decomposition of the diphenylphosphine sulfide compound, some of the silver ions in the source of reducible silver are converted in-situ into photosensitive silver halide grains. This is generally achieved by adding one or more halide- containing compounds to the photothermographic dispersion. Such halide- containing compounds include, but are not limited to zinc bromide, calcium bromide, lithium bromide, lithium iodide or mixtures thereof. The conversion of the reducible silver ions can be carried out by one addition of a halide-containing compound or by multiple additions at various times in the preparation of the photothermographic emulsion. For example, a portion of the halide-containing compound can be added before the organic sulfur-containing compound and a

second portion can be added after the addition of the organic sulfur-containing compound. Different halide-containing compounds can be used in these multiple additions if desired.

Useful sulfur-containing chemical sensitizing compounds are diphenylphosphine sulfides that can be defined using the following general Structure (PS): (ps) wherein Phl and Ph2 are the same or different substituted or unsubstituted phenyl groups. Substituents on the phenyl groups can include but are not limited to, halogen, alkyl, alkoxy, cyano, and nitro.

Also in Structure (PS), Rl and R2 are each independently hydrogen, a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms (such as methyl, ethyl, iso-propyl, or cyclohexyl), or a substituted or unsubstituted phenyl group (such as phenyl, 4-methylphenyl, and 3-chlorophenyl). Preferably, Ri and R2 are both hydrogen or at least one of R, and R2 is hydrogen. More preferably, Ri and R2 are both hydrogen.

Further, m is 1 or 2 and preferably m is 1.

L represents a direct bond, or an organic linking group having 1 to 3 atoms in the chain. Preferred linking groups are sulfonyl [-S02-], carbonyl [-(C=O)-], and sulfoxide [-SO-]. Most preferably the linking group is carbonyl [-(C=O)-] When m is 1, R3 is monovalent group such as a substituted or unsubstituted alkyl group having 1 to 16 carbon atoms, preferably 1 to 7 carbon atoms (such as methyl, benzyl, and'methylcarbophenyl groups), a substituted or unsubstituted aryl group (such as phenyl, naphthyl, and furanyl groups), a disubstituted amino group (such as methylamino, dimethylamino, diethylamino,

morpholino, and piperdino groups). When m is 2, R3 is a substituted or unsubstituted divalent aliphatic linking group having 1 to 20 carbon, nitrogen, oxygen, or sulfur atoms in the chain (such as methylene, ethylene, propylene, polyether, and polythioether groups). Preferably, m is 1 and R3 is a diethylamino or benzyl group.

Representative compounds of Structure (PS) include the following compounds PS-1 to PS-19: (PS-1) (PS-2) (PS-3) (PS-4)

(PS-5) (PS-6) (PS-7) (PS-8)

(PS-9) (PS-10) (PS-11) (PS-12)

(PS-13) (PS-14) (PS-15)

(PS-19).

Mixtures of such sulfur-containing compounds can be used if desired. Compounds PS-1 and PS-2 are most preferred.

The diphenylphosphine sulfides useful in the practice of this invention can be prepared generally by alkylation of diphenylphosphine sulfide in methylene chloride at a temperature of from 0°C to room temperature for from 30 minutes to about 24 hours in the presence of powdered potassium hydroxide.

They can also be prepared using the teaching described in copending and commonly assigned U. S. S. N. 10/10/731,251 (filed December 9,2003, by Simpson, Burleva, and Sakizadeh).

The following SCHEME I depicts the preparation of the diphenyl- phosphine sulfide compounds of this invention were L is a carbonyl group.

SCHEME I The diphenylphosphine sulfide compound is used in the practice of the present invention in an amount of from 1.5 x 10-6 to 4 x 10-3 mol per mol of total silver from the non-photosensitive source of reducible silver ions in the photothermographic dispersion. A preferred amount is from 4 x 104 to 1 x 10-3 mol/mol of total silver from the non-photosensitive source of reducible silver ions in the photothermographic dispersion.

As noted above, once the diphenylphosphine sulfide compound has been added and the diphenylphosphine sulfide compound is then decomposed on or around the silver halide grains in an oxidative environment, the in-situ silver halide grains are generated. Decomposition is generally carried out using one or more oxidizing agents, and preferably a"strong"oxidizing agent, that is capable

of fonning species on the grains that act as the chemical sensitizer at a temperature from 10°C up to 30°C for up to 60 minutes. Preferably, the reaction is carried out from ambient temperature (generally 20°C) up to 30°C.

The efficiency of the decomposition is influenced by the function and efficiency of the oxidizing agent (s), the diphenylphosphine sulfide compound that is decomposed, the length of decomposition time, and the decomposition temperature. More reactive oxidizing agents can be used at lower temperature and/or shorter times, and the converse is true for less reactive oxidizing agents.

Decomposition can be carried out in a single reaction or in stages where the reaction is interrupted or completed before addition of the same or different oxidizing agent. Thus, in some embodiments, a single oxidizing agent can be provided in a"portioned"addition where the total amount is divided into portions and added in stages.

Preferred oxidizing agents that may be added to remove any impurities in the photothennographic soap dispersion include hydrobromic acid salts of nitrogen-containing heterocyclic ring compounds that are further associated with a pair of bromine atoms. These compounds are also known as quaternary nitrogen-containing 5-, 6-, or 7-membered monocyclic or polycyclic rings that are associated with hydrobromic acid perbromide. Examples of such compounds are described as antifoggants in U. S. Patent 5,028, 523 (Skoug) that is cited herein, and include compounds with substituted or unsubstituted pyridine, pyrrolidone, pyrrolidinone, pyrrolidine, phthalazinone, and phthalazine rings. The compounds with a pyridine ring are more preferred and a particularly useful oxidizing agent is pyridinium hydrobromide perbromide (PHP).

In a preferred embodiment, PHP is used as the oxidizing agent at a temperature of from 20°C to 30°C for up to 60 minutes.

The conversion of some of the silver ions of the reducible source of silver into photosensitive silver halide grains is generally achieved by adding one or more halogen-containing compounds to the photothermographic dispersion.

Such compounds can be inorganic halides (such as zinc bromide, zinc iodide, calcium bromide, lithium bromide, lithium iodide, and mixtures thereof) or

organic halogen-containing compounds (such as N-bromosuccinimide). The details of such in-situ generation of silver halide are well known and described for example in U. S. Patent 3,457, 075 (Morgan et al. ). Zinc bromide is preferably added in the practice of this invention.

It is particularly effective to use a mixture of both preformed and in-situ generated silver halide.

The halogen-containing compound (s) is added in an amount sufficient to convert from 0.1 to 10 mol % of the reducible silver ions to photosensitive silver halide. Preferably from 0.5 to 5 mol % of the reducible silver ions are converted to photosensitive silver halide. More preferably from 1 to 3 mol % of the reducible silver ions are converted. The halogen-containing compound (s) is added in an amount of from 10-4 to 10-1 mole halogen atom per mole of non-photosensitive source of reducible silver ions.

Generally, conversion of the reducible silver ions occurs within 30 minutes at an appropriate temperature. In some embodiments, however, the halogen-containing compound (s) can be added in stages to control silver halide formation and composition. For example, a bromide salt can be added with an iodide salt, and then a bromide salt can be added alone. If mixtures of halides are added, they are added in a proportion to provide desired halide composition in the resulting silver halide grains.

Following the in-situ conversion of some of the silver ions in the reducible source of silver into photosensitive silver halide grains silver halide, the resulting photothermographic emulsion can be further modified by the addition of additional chemical sensitizers that do not require oxidization, binders, toners, antifoggants, spectral sensitizing dyes, matting agents, phosphors, high-contrast agents, and other addenda commonly included within such emulsions. Further details of these compounds are provided below as well as in considerable published literature.

Useful additional chemical sensitizers may be used in the preparation of the photosensitive silver halides. Such compounds may contain sulfur, tellurium, or selenium, or may comprise a compound containing gold,

platinum, palladium, ruthenium, rhodium, iridium, or combinations thereof, a reducing agent such as a tin halide or a combination of any of these. The details of these materials are provided for example, in T. H. James, The Tlaeory of the Photographic Process, Fourth Edition, Eastman Kodak Company, Rochester, NY, 1977, Chapter 5, pp. 149-169. Suitable conventional chemical sensitization procedures are also described in U. S. Patent 1,623, 499 (Sheppard et al. ), U. S.

Patent 2,399, 083 (Waller et al. ), U. S. Patent 3,297, 447 (McVeigh), U. S. Patent 3,297, 446 (Dunn), U. S. Patent 5,049, 485 (Deaton), U. S. Patent 5,252, 455 (Deaton), U. S. Patent 5,391, 727 (Deaton), U. S. Patent 5,912, 111 (Lok et al. ), U. S.

Patent 5,759, 761 (Lushington et al. ), and EP 0 915 371A1 (Lok et al.).

Certain substituted and unsubstituted thiourea compounds can be used as chemical sensitizers. Particularly useful tetra-substituted thioureas are described in U. S. Patent 6,368, 779 (Lynch et al.).

Still other additional chemical sensitizers include certain tellurium- containing compounds that are described in U. S. Published Application 2002-0164549 (Lynch et al. ), and certain selenium-containing compounds that are described in U. S. Patent 6,620, 577 (Lynch et al.).

Combinations of gold (3+) -containing compounds and either sulfur-or tellurium-containing compounds are also useful as chemical sensitizers as described in U. S. Patent 6,423, 481 (Simpson et al.).

The additional chemical sensitizers can be present in conventional amounts that generally depend upon the average size of the silver halide grains.

Generally, the total amount is at least 10-1° mole per mole of total silver, and preferably from 10-8 to 10-2 mole per mole of total silver for silver halide grains having an average size of from 0.01 to 2 um. The upper limit can vary depending upon the compound (s) used, the level of silver halide and the average grain size, and would be readily determinable by one of ordinary skill in the art.

Spectral Sensitizers The photosensitive silver halides used in the photothermographic features of the invention may be spectrally sensitized with various spectral

sensitizing dyes that are known to enhance silver halide sensitivity to ultraviolet, visible, and/or infrared radiation. Non-limiting examples of sensitizing dyes that can be employed include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, holopolar cyanine dyes, hemicyanine dyes, styryl dyes, and hemioxanol dyes. Cyanine dyes, merocyanine dyes and complex merocyanine dyes are particularly useful. Spectral sensitizing dyes are chosen for optimum photosensitivity, stability, and ease of synthesis. They may be added at any stage in chemical finishing of the photothermographic emulsion. Spectral sensitization is generally carried out by adding one or more spectral sensitizing dyes to the photothennographic emulsion after chemical sensitization is achieved.

They may be added at any stage in chemical finishing of the photothermographic emulsion. Spectral sensitization is generally carried out by adding one or more spectral sensitizing dyes to the photothermographic emulsion after chemical sensitization is achieved. It is particularly desired to use one or more spectral sensitizing dyes to provide spectral sensitization at from 600 to 1100 nm.

Suitable sensitizing dyes such as those described in U. S. Patent 3,719, 495 (Lea), U. S. Patent 4,396, 712 (Kinoshita et al. ), U. S. Patent 4,439, 520 (Kofron et al. ), U. S. Patent 4,690, 883 (Kubodera et al. ), U. S. Patent 4,840, 882 (Iwagaki et al. ), U. S. Patent 5,064, 753 (Kohno et al. ), U. S. Patent 5,281, 515 (Delprato et al. ), U. S. Patent 5,393, 654 (Burrows et al. ), U. S. Patent 5,441, 866 (Miller et al. ), U. S. Patent 5,508, 162 (Dankosh), U. S. Patent 5,510, 236 (Dankosh), U. S. Patent 5,541, 054 (Miller et al. ), JP Kokai 2000-063690 (Tanaka et al.), JP Kokai 2000-112054 (Fukusaka et al. ), JP Kokai 2000-273329 (Tanaka et al.), JP Kokai 2001-005145 (Arai), JP Kokai 2001-064527 (Oshiyama et al. ), and JP Kokai 2001-154305 (Kita et al. ), can be used in the practice of the invention. A summary of generally useful spectral sensitizing dyes is contained in Research Disclosure, December 1989, item 308119, Section IV. Additional classes of dyes useful for spectral sensitization, including sensitization at other wavelengths are described in Research Disclosure, 1994, item 36544, section V Teachings relating to specific combinations of spectral sensitizing dyes also include U. S. Patent 4,581, 329 (Sugimoto et al. ), U. S. Patent 4,582, 786

(Ikeda et al. ), U. S. Patent, U. S. Patent 4,609, 621 (Sugimoto et al. ), U. S. Patent 4,675, 279 (Shuto et al. ), U. S. Patent 4,678, 741 (Yamada et al. ), U. S. Patent 4,720, 451 (Shuto et al. ), U. S. Patent 4,818, 675 (Miyasaka et al.), U. S. Patent 4,945, 036 (Arai et al. ), and U. S. Patent 4,952, 491 (Nishikawa et al.).

Also useful are spectral sensitizing dyes that decolorize by the action of light or heat. Such dyes are described in U. S. Patent 4,524, 128 (Edwards et al. ), JP Kokai 2001-109101 (Adachi), JP Kokai 2001-154305 (Kita et al. ), and JP Kokai 2001-183770 (Hanyu et al.).

Spectral sensitizing dyes may be used singly or in combination.

The dyes are selected for the purpose of adjusting the wavelength distribution of the spectral sensitivity, and for the purpose of supersensitization. When using a combination of dyes having a supersensitizing effect, it is possible to attain much higher sensitivity than the sum of sensitivities that can be achieved by using each dye alone. It is also possible to attain such supersensitizing action by the use of a dye having no spectral sensitizing action by itself, or a compound that does not substantially absorb visible light. Diaminostilbene compounds are often used as supersensitizers.

An appropriate amount of spectral sensitizing dye added is generally 10-1° to 10-1 mole, and preferably, 10-7 to 10-2 mole per mole of silver halide.

Non-Photosensitive Source of Reducible Silver Ions The non-photosensitive source of reducible silver ions used in the photothermographic materials prepared by this invention can be any metal-organic compound that contains reducible silver (1+) ions. Such compounds are generally silver salts of silver coordinating ligands. Preferably, it is an organic 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, when used in a photothermographic material) and a reducing composition.

Silver salts of organic acids including silver salts of long-chain carboxylic acids are preferred. The chains typically contain 10 to 30, and

preferably 15 to 28, carbon atoms. Suitable organic silver salts include silver salts of organic compounds having a carboxylic acid group. Examples thereof include a silver salt of an aliphatic carboxylic acid or a silver salt of an aromatic carboxylic acid. Preferred examples of the silver salts of aliphatic carboxylic acids include silver behenate, silver arachidate, silver stearate, silver oleate, silver laurate, silver caprate, silver myristate, silver palmitate, silver maleate, silver fumarate, silver tartarate, silver furoate, silver linoleate, silver butyrate, silver camphorate, and mixtures thereof. Preferably, at least silver behenate is used alone or in mixtures with other silver carboxylates.

Representative silver salts of aromatic carboxylic acid and other carboxylic acid group-containing compounds include, but are not limited to, silver benzoate, silver substituted-benzoates (such as silver 3,5-dihydroxy-benzoate, silver o-methylbenzoate, silver m-methylbenzoate, silverp-methylbenzoate, silver 2,4-dichlorobenzoate, silver acetamidobenzoate, silverp-phenylbenzoate), silver tannate, silver phthalate, silver terephthalate, silver salicylate, silver phenylacetate, and silver pyromellitate.

Silver salts of aliphatic carboxylic acids containing a thioether group as described in U. S. Patent 3,330, 663 (Weyde et al. ) are also useful.

Soluble silver carboxylates comprising hydrocarbon chains incorporating ether or thioether linkages, or sterically hindered substitution in the oc- (on a hydrocarbon group) or ortho- (on an aromatic group) position, and displaying increased solubility in coating solvents and affording coatings with less light scattering can also be used. Such silver carboxylates are described in U. S. Patent 5,491, 059 (Whitcomb). Mixtures of any of the silver salts described herein can also be used if desired.

Silver salts of dicarboxylic acids are also useful. Such acids may be aliphatic, aromatic, or heterocyclic. Examples of such acids include, for example, phthalic acid, glutamic acid, or homo-phthalic acid.

Silver salts of sulfonates are also useful in the practice of this invention. Such materials are described for example in U. S. Patent 4,504, 575

(Lee). Silver salts of sulfosuccinates are also useful as described for example in EP 0 227 141A1 (Leenders et al.).

Silver salts of compounds containing mercapto or thione groups and derivatives thereof can also be used. Preferred examples of these compounds include, but are not limited to, a heterocyclic nucleus containing 5 or 6 atoms in the ring, at least one of which is a nitrogen atom, and other atoms being carbon, oxygen, or sulfur atoms. Such heterocyclic nuclei include, but are not limited to, triazoles, oxazoles, thiazoles, thiazolines, imidazoles, diazoles, pyridines, and triazines. Representative examples of these silver salts include, but are not limited to, a silver salt of 3-mercapto-4-phenyl-1, 2,4-triazole, a silver salt of 5-carboxylic- 1-methyl-2-phenyl-4-thiopyridine, a silver salt of mercaptotriazine, a silver salt of 2-mercaptobenzoxazole, silver salts as described in U. S. Patent 4,123, 274 (Knight et al. ) (for example, a silver salt of a 1,2, 4-mercaptothiazole derivative, such as a silver salt of 3-amino-5-benzylthio-1, 2,4-thiazole), and a silver salt of thione compounds [such as a silver salt of 3- (2-carboxyethyl)-4-methyl-4-thiazoline- 2-thione as described in U. S. Patent 3, 785, 830 (Sullivan et al.)].

Examples of other useful silver salts of mercapto or thione substituted compounds that do not contain a heterocyclic nucleus include but are not limited to, a silver salt of thioglycolic acids such as a silver salt of an S-alkyl- thioglycolic acid (wherein the alkyl group has from 12 to 22 carbon atoms), a silver salt of a dithiocarboxylic acid such as a silver salt of a dithioacetic acid, and a silver salt of a thioamide.

In some embodiments, a silver salt of a compound containing an imino group is preferred, especially in aqueous-based imaging formulations.

Preferred examples of these compounds include, but are not limited to, silver salts of benzotriazole and substituted derivatives thereof (for example, silver methyl- benzotriazole and silver 5-chlorobenzotriazole), silver salts of 1,2, 4-triazoles or 1-H-tetrazoles such as phenylmercaptotetrazole as described in U. S. Patent 4,220, 709 (deMauriac), and silver salts of imidazoles and imidazole derivatives as described in U. S. Patent 4,260, 677 (Winslow et al. ). Particularly useful silver salts of this type are the silver salts of benzotriazole and substituted derivatives

thereof. A silver salt of benzotriazole is preferred in aqueous-based photothermo- graphic formulations.

Moreover, silver salts of acetylenes can also be used as described, for example in U. S. Patent 4,761, 361 (Ozaki et al. ) and U. S. Patent 4,775, 613 (Hirai et al.).

Organic silver salts that are particularly useful in organic solvent- based photothermographic materials include silver carboxylates (both aliphatic and aromatic carboxylates), silver triazolates, silver sulfonates, silver sulfo- succinates, and silver acetylides. Silver salts of long-chain aliphatic carboxylic acids containing 15 to 28, carbon atoms (and including silver behenate) are particularly preferred.

It is also convenient to use silver half soaps. A preferred example of a silver half soap is an equimolar blend of silver carboxylate and carboxylic acid, which analyzes for 14.5% by weight solids of silver in the blend and which is prepared by precipitation from an aqueous solution of an ammonium or an alkali metal salt of a commercially available fatty carboxylic acid, or by addition of the free fatty acid to the silver soap. For transparent films a silver carboxylate full soap, containing not more than 15% of free fatty carboxylic acid and analyzing for 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, U. S. Patent 3, 985,565 (Gabrielsen et al. ) and the references cited above.

Sources of non-photosensitive reducible silver ions can also be provided as core-shell silver salts such as those described in U. S. Patent 6,355, 408 (Whitcomb et al. ). These silver salts include a core comprised of one or more silver salts and a shell having one or more different silver salts.

Another useful source of non-photosensitive reducible silver ions in the practice of this invention are the silver dimer compounds that comprise two different silver salts as described in U. S. Patent 6,472, 131 (Whitcomb). Such

non-photosensitive silver dimer compounds comprise two different silver salts, provided that when the two different silver salts comprise straight-chain, saturated hydrocarbon groups as the silver coordinating ligands, those ligands differ by at least 6 carbon atoms.

Still other useful sources of non-photosensitive reducible silver ions in the practice of this invention are the silver core-shell compounds comprising a primary core comprising one or more photosensitive silver halides, or one or more non-photosensitive inorganic metal salts or non-silver containing organic salts, and a shell at least partially covering the primary core, wherein the shell comprises one or more non-photosensitive silver salts, each of which silver salts comprises a organic silver coordinating ligand. Such compounds are described in copending and commonly assigned U. S. Serial No. 10/208, 603 (filed July 30,2002 by Bokhonov, Burleva, Whitcomb, Howlader, and Leichter).

As one skilled in the art would understand, the non-photosensitive source of reducible silver ions can include various mixtures of the various silver salt compounds described herein, in any desirable proportions.

The silver halide and the non-photosensitive source of reducible silver ions must be in catalytic proximity (that is, reactive association). It is preferred that these reactive components be present in the same emulsion layer.

The one or more non-photosensitive sources of reducible silver ions are 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 another way, the amount of the sources of reducible silver ions is generally present in an amount of from 0.001 to 0.2 mol/m2 of the dry photothermographic material, and preferably from 0.01 to 0.05 mol/m2 of that material.

The total amount of silver (from all silver sources) in the photo- thermographic materials is generally at least 0.002 mol/m2 and preferably from 0.01 to 0.05 mol/m2.

Reducing Agents The reducing agent (or reducing agent composition comprising two or more components) for the source of reducible silver ions can be any material, preferably an organic material, that can reduce silver (1+) ion to metallic silver.

Conventional photographic developers can be used as reducing agents, including aromatic di-and tri-hydroxy compounds (such as hydro- quinones, gallatic acid and gallic acid derivatives, catechols, and pyrogallols), aminophenols (for example, N-methylaminophenol), p-phenylenediamines, alkoxynaphthols (for example, 4-methoxy-1-naphthol), pyrazolidin-3-one type reducing agents (for example PHENIDONE@), pyrazolin-5-ones, polyhydroxy spiro-bis-indanes, indan-1, 3-dione derivatives, hydroxytetrone acids, hydroxy- tetronimides, hydroxylamine derivatives such as for example those described in U. S. Patent 4,082, 901 (Laridon et al. ), hydrazine derivatives, hindered phenols, amidoximes, azines, reductones (for example, ascorbic acid and ascorbic acid derivatives), leuco dyes, and other materials readily apparent to one skilled in the art.

When a silver benzotriazole silver source is used, ascorbic acid reducing agents are preferred. An"ascorbic acid"reducing agent (also referred to as a developer or developing agent) means ascorbic acid, complexes, and derivatives thereof. Ascorbic acid developing agents are described in a considerable number of publications in photographic processes, including U. S.

Patent 5,236, 816 (Purol et al. ) and references cited therein. Useful ascorbic acid developing agents include ascorbic acid and the analogues, isomers and derivatives thereof. Such compounds include, but are not limited to, D-or L- ascorbic acid, sugar-type derivatives thereof (such as sorboascorbic acid, y-lactoascorbic acid, 6-desoxy-L-ascorbic acid, L-rhamnoascorbic acid, imino- 6-desoxy-L-ascorbic acid, glucoascorbic acid, fucoascorbic acid, glucohepto- ascorbic acid, maltoascorbic acid, L-arabosascorbic acid), sodium ascorbate, potassium ascorbate, isoascorbic acid (or L-erythroascorbic acid), and salts thereof (such as alkali metal, ammonium or others known in the art), endiol type ascorbic acid, an enaminol type ascorbic acid, a thioenol type ascorbic acid, and

an enamin-thiol type ascorbic acid, as described for example in U. S. Patent 5,498, 511 (Yamashita et al. ), EP 0 585 792A1 (Passarella et al. ), EP 0 573 700A1 (Lingier et al. ), EP 0 588 408A1 (Hieronymus et al. ), U. S. Patent 5,089, 819 (Knapp), U. S. Patent 5,278, 035 (Knapp), U. S. Patent 5,384, 232 (Bishop et al. ), U. S. Patent 5,376, 510 (Parker et al. ), Japanese Kokai 7-56286 (Toyoda), U. S. Patent 2,688, 549 (James et al. ), and Research Disclosure, item 37152, March 1995. D-, L-, or D, L-ascorbic acid (and alkali metal salts thereof) or isoascorbic acid (or alkali metal salts thereof) are preferred. Mixtures of these developing agents can be used if desired.

When a silver carboxylate silver source is used, hindered phenol reducing agents are preferred. In some instances, the reducing agent composition comprises two or more components such as a hindered phenol developer and a co-developer that can be chosen from the various classes of co-developers and reducing agents described below. Ternary developer mixtures involving the further addition of contrast enhancing agents are also useful. Such contrast enhancing agents can be chosen from the various classes of reducing agents described below. Hindered phenol reducing agents are preferred (alone or in combination with one or more high-contrast co-developing agents and co-developer contrast enhancing agents).

"Hindered phenol reducing agents"are compounds that contain only one hydroxy group on a given phenyl ring and have at least one additional substituent located ortho to the hydroxy group. Hindered phenol reducing agents may contain more than one hydroxy group as long as each hydroxy group is located on different phenyl rings. Hindered phenol reducing agents include, for example, binaphthols (that is dihydroxybinaphthyls), biphenols (that is dihydroxy- biphenyls), bis (hydroxynaphthyl) methanes, bis (hydroxyphenyl) methanes (that is bisphenols), hindered phenols, and hindered naphthols, each of which may be variously substituted.

Representative binaphthols include, but are not limited, to 1, 1'-bi-2-naphthol, 1, 1'-bi-4-methyl-2-naphthol and 6, 6'-dibromo-bi-2-naphthol.

For additional compounds see U. S. Patent 3,094, 417 (Workman) and U. S. Patent 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'-dichloro- biphenyl, 2- (2-hydroxy-3-t-butyl-5-methylphenyl)-4-methyl-6-n-hexylpheno l, 4,4'-dihydroxy-3, 3', 5, 5'-tetra-t-butylbiphenyl and 4, 4'-dihydroxy-3,3', 5,5'-tetra- methylbiphenyl. For additional compounds see U. S. Patent 5,262, 295 (noted above).

Representative bis (hydroxynaphthyl) methanes include, but are not limited to, 4,4'-methylenebis (2-methyl-1-naphthol). For additional compounds see U. S. Patent 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 (NONOXt or PERMANAX WSO), 1, 1'-bis (3, 5-di-t-butyl-4-hydroxyphenyl) methane, 2,2'-bis (4-hydroxy-3-methylphenyl) propane, 4,4'-ethylidene-bis (2-t-butyl- 6-methylphenol), 2, 2'-isobutylidene-bis (4,6-dimethylphenol) (LOWINOX 221B46), and 2,2'-bis (3,5-dimethyl-4-hydroxyphenyl) propane. For additional compounds see U. S. Patent 5,262, 295 (noted above).

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

Representative hindered naphthols include, but are not limited to, 1-naphthol, 4-methyl-1-naphthol, 4-methoxy-1-naphthol, 4-chloro-1-naphthol and 2-methyl-1-naphthol. For additional compounds see U. S. Patent 5,262, 295 (noted above).

Mixtures of hindered phenol reducing agents can be used if desired.

Still another useful class of reducing agents are polyhydroxy spiro-bis-indane compounds described as photographic tanning agents in U. S.

Patent 3,440, 049 (Moede). Examples include 3,3, 3', 3'-tetramethyl-5,6, 5', 6'-tetra

hydroxy-1, 1'-spiro-bis-indane (called indane I) and 3,3, 3', 3'-tetramethyl- 4,6, 7,4', 6', 7'-hexahydroxy-1, 1'-spiro-bis-indane (called indane II).

An additional class of reducing agents that can be used as developers are substituted hydrazines including the sulfonyl hydrazides described in U. S. Patent 5,464, 738 (Lynch et al. ). Still other useful reducing agents are described, for example, in U. S. Patent 3,074, 809 (Owen), U. S. Patent 3,094, 417 (Workman), U. S. Patent 3,080, 254 (Grant, Jr. ), and U. S. Patent 3,887, 417 (Klein et al. ). Auxiliary reducing agents may be useful as described in U. S. Patent 5, 981, 151 (Leenders et al.).

More specific alternative reducing agents that have been disclosed in dry silver systems including amidoximes such as phenylamidoxime, 2-thienyl- amidoxime andp-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-ß-phenyl hydrazide in combination with ascorbic acid], a combination of polyhydroxy- benzene and hydroxylamine, a reductone and/or a hydrazine [for example, a combination of hydroquinone and bis (ethoxyethyl) hydroxylamine], piperidino- hexose reductone or formyl-4-methylphenylhydrazine, hydroxamic acids (such as phenylhydroxamic acid, p-hydroxyphenylhydroxamic acid, and o-alanine- hydroxamic acid), a combination of azines and sulfonamidophenols (for example, phenothiazine and 2, 6-dichloro-4-benzenesulfonamidophenol), a-cyanophenyl- acetic acid derivatives (such as ethyl oc-cyano-2-methylphenylacetate and ethyl o-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, andp-benzenesulfon- amidophenol), indane-1, 3-diones (such as 2-phenylindane-1, 3-dione), chromans

(such as 2, 2-dimethyl-7-t-butyl-6-hydroxychroman), 1,4-dihydropyridines (such as 2,6-dimethoxy-3, 5-dicarbethoxy-1, 4-dihydropyridine), ascorbic acid derivatives (such as 1-ascorbylpalmitate, ascorbylstearate and unsaturated aldehydes and ketones), and 3-pyrazolidones.

Useful co-developer reducing agents can also be used as described for example, in U. S. Patent 6, 387, 605 (Lynch et al. ). Examples of these compounds include, but are not limited to, 2,5-dioxo-cyclopentane carbox- aldehydes, 5- (hydroxymethylene)-2, 2-dimethyl-1,3-dioxane-4, 6-diones, 5-(hydroxymethylene)-1, 3-dialkylbarbituric acids, and 2- (ethoxymethylene)- 1 H-indene-1, 3 (2H)-diones.

Additional classes of reducing agents that can be used as co-developers are trityl hydrazides and formyl phenyl hydrazides as described in U. S. Patent 5,496, 695 (Simpson et al. ), 2-substituted malondialdehyde compounds as described in U. S. Patent 5,654, 130 (Murray), and 4-substituted isoxazole compounds as described in U. S. Patent 5,705, 324 (Murray). Additional developers are described in U. S. Patent 6,100, 022 (Inoue et al.).

Yet another class of co-developers includes substituted acrylonitrile compounds that are described in U. S. Patent 5,635, 339 (Murray) and U. S. Patent 5,545, 515 (Murray et al. ). Examples of such compounds include, but are not limited to, the compounds identified as HET-01 and HET-02 in U. S. Patent 5, 635, 339 (noted above) and CN-01 through CN-13 in U. S. Patent 5,545, 515 (noted above). Particularly useful compounds of this type are (hydroxymethylene) cyanoacetates and their metal salts.

Various contrast enhancing agents can be used in some photo- thermographic materials with specific co-developers. Examples of useful contrast enhancing agents include, but are not limited to, hydroxylamines (including hydroxylamine and alkyl-and aryl-substituted derivatives thereof), alkanolamines and ammonium phthalamate compounds as described for example, in U. S. Patent 5,545, 505 (Simpson), hydroxamic acid compounds as described for example, in U. S. Patent 5,545, 507 (Simpson et al. ), N-acylhydrazine compounds as described

for example, in U. S. Patent 5,558, 983 (Simpson et al. ), and hydrogen atom donor compounds as described in U. S. Patent 5,637, 449 (Harring et al.).

Aromatic di-and tri-hydroxy reducing agents can also be used in combination with hindered phenol reducing agents either together or in or in combination with one or more high contrast co-developing agents and co-developer contrast-enhancing agents).

The reducing agent (or mixture thereof) described herein is generally present as 1 to 10% (dry weight) of the emulsion layer. In multilayer constructions, if the reducing agent is added to a layer other than an emulsion layer, slightly higher proportions, of from 2 to 15 weight % may be more desirable. Co-developers may be present generally in an amount of from 0. 001 % to 1.5% (dry weight) of the emulsion layer coating.

For color imaging materials (for example, monochrome, dichrome, or full color images), one or more reducing agents can be used that can be oxidized directly or indirectly to form or release one or more dyes.

The dye-forming or releasing compound may be any colored, colorless, or lightly colored compound that can be oxidized to a colored form, or to release a preformed dye when heated, preferably to a temperature of from 80°C to 250°C for a duration of at least 1 second. When used with a dye-or image- receiving layer, the dye can diffuse through the imaging layers and interlayers into the image-receiving layer of the photothermographic material.

Leuco dyes or"blocked"leuco dyes are one class of dye-forming compounds (or"blocked"dye-forming compounds) that form and release a dye upon oxidation by silver ion to form a visible color image in the practice of the present invention. Leuco dyes are the reduced form of dyes that are generally colorless or very lightly colored in the visible region (optical density of less than 0.2). Thus, oxidation provides a color change that is from colorless to colored, an optical density increase of at least 0.2 units, or a substantial change in hue.

Representative classes of useful leuco dyes include, but are not limited to, chromogenic leuco dyes (such as indoaniline, indophenol, or azomethine dyes), imidazole leuco dyes such as 2- (3, 5-di-t-butyl-4-hydroxy

phenyl) -4,5-diphenylimidazole as described for example in U. S. Patent 3,985, 565 (Gabrielson et al.), dyes having an azine, diazine, oxazine, or thiazine nucleus such as those described for example in U. S. Patent 4,563, 415 (Brown et al. ), U. S.

Patent 4,622, 395 (Bellus et al. ), U. S. Patent 4,710, 570 (Thien), and U. S. Patent 4,782, 010 (Mader et al. ), and benzylidene leuco compounds as described for example in U. S. Patent 4,932, 792 (Grieve et al. ). Further details about the chromogenic leuco dyes noted above can be obtained from U. S. Patent 5,491, 059 (noted above, Column 13) and references noted therein.

Another useful class of leuco dyes includes what are known as "aldazine"and"ketazine"leuco dyes that are described for example in U. S. Patent 4,587, 211 (Ishida et al. ) and U. S. Patent 4,795, 697 (Vogel et al.).

Still another useful class of dye-releasing compounds includes those that release diffusible dyes upon oxidation. These are known as preformed dye release (PDR) or redox dye release (RDR) compounds. In such compounds, the reducing agents release a mobile preformed dye upon oxidation. Examples of such compounds are described in U. S. Patent 4,981, 775 (Swain).

Still further, the reducing agent can be a compound that releases a conventional photographic dye forming color coupler or developer upon oxidation as is known in the photographic art.

The dyes that are formed or released can be the same in the same or different imaging layers. A difference of at least 60 nm in reflective maximum absorbance is preferred. More preferably, this difference is from 80 to 100 nm.

Further details about the various dye absorbance are provided in U. S. Patent 5,491, 059 (noted above, Col. 14).

The total amount of one or more dye-forming or releasing compound that can be incorporated into the photothermographic materials of this invention is generally from 0.5 to 25 weight % of the total weight of each imaging layer in which they are located. Preferably, the amount in each imaging layer is from 1 to 10 weight %, based on the total dry layer weight. The useful relative proportions of the leuco dyes would be readily known to a skilled worker in the art.

Other Addenda The photothermographic materials prepared by this invention can also contain other additives such as shelf-life stabilizers, antifoggants, contrast enhancers, development accelerators, acutance dyes, post-processing stabilizers or stabilizer precursors, thermal solvents (also known as melt formers), and other image-modifying agents as would be readily apparent to one skilled in the art.

To further control the properties of photothermographic materials, (for example, contrast, Dmin, speed, or fog), it may be preferable to add one or more heteroaromatic mercapto compounds or heteroaromatic disulfide compounds of the formulae Ar-S-MI and Ar-S-S-Ar, wherein M, represents a hydrogen atom or an alkali metal atom and Ar represents a heteroaromatic ring or fused hetero- aromatic 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, tetrazol, triazine, pyrimidine, pyridazine, pyrazine, pyridine, purine, quinoline, or quinazolinone. Compounds having other heteroaromatic rings and compounds providing enhanced sensitization at other wavelengths are also envisioned to be suitable. For example, heteroaromatic mercapto compounds are described as supersensitizers for infrared photothermographic materials in EP 0 559 228B1 (Philip Jr. et al.).

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

Heteroaromatic mercapto compounds are most preferred.

Examples of preferred heteroaromatic mercapto compounds are 2-mercaptobenz- imidazole, 2-mercapto-5-methylbenzimidazole, 2-mercaptobenzothiazole and 2-mercaptobenzoxazole, and mixtures thereof.

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

The photothermographic materials can be further protected against the production of fog and can be stabilized against loss of sensitivity during storage. While not necessary for the practice of the invention, it may be advantageous to add mercury (2+) salts to the emulsion layer (s) as an antifoggant.

Preferred mercury (2+) salts for this purpose are mercuric acetate and mercuric bromide. Other useful mercury salts include those described in U. S. Patent 2,728, 663 (Allen).

Other suitable antifoggants and stabilizers that can be used alone or in combination include thiazolium salts as described in U. S. Patent 2,131, 038 (Brooker) and U. S. Patent 2,694, 716 (Allen), azaindenes as described in U. S.

Patent 2,886, 437 (Piper), triazaindolizines as described in U. S. Patent 2,444, 605 (Heimbach), the urazoles described in U. S. Patent 3,287, 135 (Anderson), sulfocatechols as described in U. S. Patent 3,235, 652 (Kennard), the oximes described in GB 623,448 (Carrol et al. ), polyvalent metal salts as described in U. S.

Patent 2,839, 405 (Jones), thiuronium salts as described in U. S. Patent 3,220, 839 (Herz), palladium, platinum, and gold salts as described in U. S. Patent 2,566, 263 (Trirelli) and U. S. Patent 2,597, 915 (Damshroder), compounds having-S02CBr3 groups as described for example in U. S. Patent 5,594, 143 (Kirk et al. ) and U. S.

Patent 5,374, 514 (Kirk et al. ), and 2- (tribromomethylsulfonyl) quinoline compounds as described in U. S. Patent 5,460, 938 (Kirk et al.).

Stabilizer precursor compounds capable of releasing stabilizers upon application of heat during development can also be used. Such precursor compounds are described in for example, U. S. Patent 5,158, 866 (Simpson et al.), U. S. Patent 5,175, 081 (Krepski et al. ), U. S. Patent 5,298, 390 (Sakizadeh et al.), and U. S. Patent 5,300, 420 (Kenney et al.).

In addition, certain substituted-sulfonyl derivatives of benzo- triazoles (for example alkylsulfonylbenzotriazoles and arylsulfonylbenzotriazoles) have been found to be useful stabilizing compounds (such as for post-processing print stabilizing), as described in U. S. Patent 6,171, 767 (Kong et al.).

Furthermore, other specific useful antifoggants/stabilizers are described in more detail in U. S. Patent 6,083, 681 (Lynch et al.).

Other antifoggants are hydrobromic acid salts of heterocyclic compounds (such as pyridinium hydrobromide perbromide) as described, for example, in U. S. Patent 5,028, 523 (Skoug), benzoyl acid compounds as described, for example, in U. S. Patent 4,784, 939 (Pham), substituted propenenitrile compounds as described, for example, in U. S. Patent 5,686, 228 (Murray et al.), silyl blocked compounds as described, for example, in U. S. Patent 5,358, 843 (Sakizadeh et al. ), vinyl sulfones as described, for example, in U. S. Patent 6,143, 487 (Philip, Jr. et al. ), diisocyanate compounds as described, for example, in EP 0 600 586A1 (Philip, Jr. et al. ), and tribromomethylketones as described, for example, in EP 0 600 587A1 (Oliff et al. ).

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

Particularly useful antifoggants are polyhalo antifoggants, such as those having a-S02C (X') 3 group wherein X'represents the same or different halogen atoms.

Advantageously, the photothermographic materials prepared according to this invention also include one or more thermal solvents (or melt formers). Representative examples of such compounds include, but are not limited to, salicylanilide, phthalimide, N-hydroxyphthalimide, N-potassium- phthalimide, succinimide, N-hydroxy-1, 8-naphthalimide, phthalazine, 1- (2H)-phthalazinone, 2-acetylphthalazinone, benzanilide, dimethylurea, D-sorbitol, and benzenesulfonamide. Combinations of these compounds can also be used

including a combination of succinimide and dimethylurea. Known thermal solvents are disclosed, for example, in U. S. Patent 3, 438, 776 (Yudelson), U. S.

Patent 5,250, 386 (Aono et al. ), U. S. Patent 5,368, 979 (Freedman et al. ), U. S.

Patent 5,716, 772 (Taguchi et al. ), and U. S. Patent 6,013, 420 (Windender).

It is often advantageous to include a base-release agent or base precursor in the photothermographic materials prepared by the method of this invention to provide improved and more effective image development. A base- release agent or base precursor as employed herein is intended to include compounds which upon heating in the photothermographic material provide a more effective reaction between the described photosensitive silver halide, and the image-forming combination comprising a silver salt and the silver halide developing agent. Representative base-release agents or base precursors include guanidinium compounds, such as guanidinium trichloroacetate, and other compounds that are known to release a base but do not adversely affect photographic silver halide materials, such as phenylsulfonyl acetates. Further details are provided in U. S. Patent 4,123, 274 (Knight et al.).

A range of concentrations of the base-release agent or base precursor is useful in the described photothermographic materials. The optimum concentration of base-release agent or base precursor will depend upon such factors as the desired image, particular components in the photothermographic material, and processing conditions.

The use of"toners"or derivatives thereof that improve the image are highly desirable components of the photothermographic materials. Toners are compounds that when added to the photothermographic imaging layer (s) shift the color of the developed silver image from yellowish-orange to brown-black or blue-black. Generally, one or more toners described herein are present in an amount of 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 may be incorporated in the photothermographic emulsion layer (s) or in an adjacent layer.

Compounds useful as toners are described, for example, in U. S.

Patent 3,080, 254 (Grant, Jr. ), U. S. Patent 3,847, 612 (Winslow), U. S. Patent 4,123, 282 (Winslow), U. S. Patent 4,082, 901 (Laridon et al. ), U. S. Patent 3,074, 809 (Owen), U. S. Patent 3,446, 648 (Workman), U. S. Patent 3,844, 797 (Willems et al. ), U. S. Patent 3,951, 660 (Hagemann et al.), U. S. Patent 5,599, 647 (Defieuw et al. ) and GB 1, 439,478 (AGFA).

Phthalazine and phthalazine derivatives [such as those described in U. S. Patent 6,146, 822 (Asanuma et al. )], phthalazinone, and phthalazinone derivatives are particularly useful toners.

Additional useful toners are substituted and unsubstituted mercaptotriazoles as described for example in U. S. Patent 3, 832, 186 (Masuda et al. ), U. S. Patent 6,165, 704 (Miyake et al. ), U. S. Patent 5,149, 620 (Simpson et al.), and copending and commonly assigned U. S. Serial No. 10/193,443 (filed July 11, 2002 by Lynch, Zou, and Ulrich) and U. S. Serial No. 10/192,944 (filed July 11, 2002 by Lynch, Ulrich, and Zou).

Also useful are the phthalazine compounds described in commonly assigned U. S. Patent 6,605, 418 (Ramsden et al. ), the triazine thione compounds described in U. S. Serial No. 10/341,754 (filed January 14,2003 by Lynch, Ulrich, and Skoug), and the heterocyclic disulfide compounds described in U. S. Serial No.

10/384, 244 (filed March 7,2003 by Lynch and Ulrich).

Examples of toners include, but are not limited to, phthalimide and N-hydroxyphthalimide, cyclic imides (such as succinimide), pyrazoline-5-ones, quinazolinone, 1-phenylurazole, 3-phenyl-2-pyrazoline-5-one, and 2,4-thiazolidinedione, naphthalimides (such as N-hydroxy-1, 8-naphthalimide), cobalt complexes [such as hexaaminecobalt (3+) trifluoroacetate], mercaptans (such as 3-mercapto-1, 2,4-triazole, 2,4-dimercaptopyrimidine, 3-mercapto- 4, 5-diphenyl-1, 2,4-triazole and 2, 5-dimercapto-1, 3, 4-thiadiazole), N (amino- methyl) aryldicarboximides (such as (N, N-dimethylaminomethyl) phthalimide), and N- (dimethylaminomethyl) naphthalene-2,3-dicarboximide, a combination of blocked pyrazoles, isothiuronium derivatives, merocyanine dyes {such as 3-ethyl- 5- [ (3-ethyl-2-benzothiazolinylidene)-l-methyl-ethylidene]-2-thi o-2, 4-o-azolidine

dione}, 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 hexa- chlororhodate (3+), rhodium bromide, rhodium nitrate, and potassium hexachloro- rhodate (3+) ], benzoxazine-2,4-diones and naphthoxazine diones (such as 1, 3-benzoxazine-2,4-dione, 8-methyl-1, 3-benzoxazine-2,4-dione, 3,4-dihydro- 2,4-dioxo-1, 3,2H-benzoxazine, 3,4-dihydro-2, 4-dioxo-1,3, 7- ethylcarbonatobenzoxazine, and 6-nitro-1, 3-benzoxazine-2,4-dione) as described in U. S. Patent 5,817, 598 (Defieuw et al. ), 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- IH, 4H-2, 3a, 5, 6a-tetraazapentalene and 1, 4-di- (o-chlorophenyl)-3, 6-dimercapto- lH, 4H-2, 3a, 5,6a-tetraazapentalene].

The photothermographic materials prepared by the methods of this invention can also include one or more image stabilizing compounds that are usually incorporated in a"backside"layer. Such compounds can include, but are not limited to, phthalazinone and its derivatives, pyridazine and its derivatives, benzoxazine and benzoxazine derivatives, benzothiazine dione and its derivatives, and quinazoline dione and its derivatives, particularly as described in copending and commonly assigned U. S. Patent 6,599, 685 (Kong). Other useful backside image stabilizers include, but are not limited to, anthracene compounds, coumarin compounds, benzophenone compounds, benzotriazole compounds, naphthalic acid imide compounds, pyrazoline compounds, or compounds described for example, in U. S. Patent 6,465, 162 (Kong et al), and GB 1,565, 043 (Fuji Photo).

Phosphors In some embodiments, it is also effective to incorporate X-radiation-sensitive phosphors in the chemically sensitized photothermographic emulsions and materials prepared as described herein. Organic solvent-based emulsions and materials are described in U. S. Patent 6,440, 649 (Simpson et al.) and aqueous-based emulsions and materials are described in U. S. Patent 6,573, 033 (Simpson et al.).

Any conventional or useful phosphor can be used, singly or in mixtures, in the practice of this invention. More specific details of useful phosphors are provided as follows.

Phosphors are materials that emit infrared, visible, or ultraviolet radiation upon excitation. An intrinsic phosphor is a material that is naturally (that is, intrinsically) phosphorescent. An"activated"phosphor is one composed of a basic material that may or may not be an intrinsic phosphor, to which one or more dopant (s) has been intentionally added. These dopants"activate"the phosphor and cause it to emit infrared, visible, or ultraviolet radiation. For example, in Gd202S: Tb, the Tb atoms (the dopant/activator) give rise to the optical emission of the phosphor.

Some phosphors, such as BaFBr, are known as storage phosphors.

In these materials, the dopants are involved in the storage as well as the emission of radiation. When storage phosphors are incorporated within the photothermo- graphic materials, the initial exposure to X-radiation is"stored"within the phosphor particles. When the material is then later exposed a second time to stimulating electromagnetic radiation (usually to visible light or infrared radiation), the"stored"energy is then released as an emission of visible or infrared radiation. BaFBr described herein is such a storage phosphor.

For example, useful phosphors are described in numerous references relating to fluorescent intensifying screens, including but not limited to, Research Disclosure, Vol. 184, August 1979, Item 18431, Section IX, X-ray Screens/Phosphors, and U. S. Patent 2,303, 942 (Wynd et al. ), U. S. Patent 3,778, 615 (Luckey), U. S. Patent 4,032, 471 (Luckey), U. S. Patent 4,225, 653

(Brixner et al. ), U. S. Patent 3, 418, 246 (Royce), U. S. Patent 3, 428, 247 (Yocon), U. S. Patent 3,725, 704 (Buchanan et al. ), U. S. Patent 2,725, 704 (Swindells), U. S.

Patent 3,617, 743 (Rabatin), U. S. Patent 3,974, 389 (Ferri et al. ), U. S. Patent 3,591, 516 (Rabatin), U. S. Patent 3,607, 770 (Rabatin), U. S. Patent 3,666, 676 (Rabatin), U. S. Patent 3,795, 814 (Rabatin), U. S. Patent 4,405, 691 (Yale), U. S.

Patent 4,311, 487 (Luckey et al. ), U. S. Patent 4,387, 141 (Patten), U. S. Patent 5,021, 327 (Bunch et al. ), U. S. Patent 4,865, 944 (Roberts et al. ), U. S. Patent 4,994, 355 (Dickerson et al. ), U. S. Patent 4,997, 750 (Dickerson et al. ), U. S. Patent 5,064, 729 (Zegarski), U. S. Patent 5,108, 881 (Dickerson et al. ), U. S. Patent 5,250, 366 (Nakajima et al. ), U. S. Patent 5, 871, 892 (Dickerson et al.), EP 0 491 116A1 (Benzo et al. ), the disclosures of all of which are cited herein with respect to the phosphors.

Useful classes of phosphors include, but are not limited to, calcium tungstate (CaW04), activated or unactivated lithium stannates, niobium and/or rare earth activated or unactivated yttrium, lutetium, or gadolinium tantalates, rare earth (such as terbium, lanthanum, gadolinium, cerium, and lutetium) -activated or unactivated middle chalcogen phosphors such as rare earth oxychalcogenides and oxyhalides, and terbium-activated or unactivated lanthanum and lutetium middle chalcogen phosphors.

Still other useful phosphors are those containing hafnium as described for example in U. S. Patent 4,988, 880 (Bryan et al. ), U. S. Patent 4,988, 881 (Bryan et al. ), U. S. Patent 4,994, 205 (Bryan et al. ), U. S. Patent 5,095, 218 (Bryan et al. ), U. S. Patent 5,112, 700 (Lambert et al. ), U. S. Patent 5,124, 072 (Dole et al. ), and U. S. Patent 5,336, 893 (Smith et al. ). These include rare earth-activated lanthanum oxybromides, and terbium-activated or thulium- activated gadolinium oxides such as Gd202S : Tb.

Other suitable phosphors are described in U. S. Patent 4,835, 397 (Arakawa et al. ) and U. S. Patent 5, 381, 015 (Dooms), both cited herein, and including for example divalent europium and other rare earth activated alkaline earth metal halide phosphors and rare earth element activated rare earth oxyhalide phosphors. Of these types of phosphors, the more preferred phosphors include

alkaline earth metal fluorohalide prompt emitting and/or storage phosphors [particularly those containing iodide such as alkaline earth metal fluorobromo- iodide storage phosphors as described in U. S. Patent 5,464, 568 (Bringley et al.)].

Another useful class of phosphors includes rare earth hosts that are rare earth activated mixed alkaline earth metal sulfates such as europium-activated barium strontium sulfate.

Particularly useful phosphors are those containing doped or undoped tantalum such as YTa04, YTa04 : Nb, Y (Sr) Ta04, and Y (Sr) Ta04 : Nb.

These phosphors are described in U. S. Patent 4,226, 653 (Brixner), U. S. Patent 5,064, 729 (Zegarski), U. S. Patent 5,250, 366 (Nakajima et al.), and U. S. Patent 5,626, 957 (Benso et al. ). Other useful phosphors are alkaline earth metal phosphors.

Storage phosphors can also be used in the practice of this invention.

Various storage phosphors are described for example, in U. S. Patent 5,464, 568 (noted above). Such phosphors include divalent alkaline earth metal fluorohalide phosphors that may optionally contain iodide. Some embodiments of these phosphors are described in more detail in U. S. Patent 5,464, 568 (noted above).

Still other storage phosphors are described in U. S. Patent 4,368, 390 (Takahashi et al. ), cited herein, and include divalent europium and other rare earth activated alkaline earth metal halides and rare earth element activated rare earth oxyhalides, as described in more detail above.

Examples of useful phosphors include: SrS: Ce, SM, SrS: Eu, Sm, ThO2 : Er, La202S: Eu, Sm, ZnS: Cu, Pb, and others described in U. S. Patent 5,227, 253 (Takasu et al.).

The one or more phosphors used in the practice of this invention are present in the photothermographic materials in an amount of at least 0.1 mole per mole, and preferably from 0.5 to 20 mole, per mole of total silver in the photo- thermographic material. Generally, the amount of total silver is at least 0.002 mol/m2.

Because of the size of the phosphors used in the invention, generally the layers in which they are incorporated (usually one or more emulsion

layers), have a dry coating weight of at least 5 g/m2, and preferably from 5 g/m2, to 200 g/m2. Most preferably, the one or more phosphors and the photosensitive silver halide are incorporated within the same imaging layer that has a dry coating weight within the noted preferred range.

Binders The chemically sensitized photosensitive silver halide, the non-photosensitive source of reducible silver ions, the reducing agent composition described above, and any other imaging layer additives used in the present invention are generally combined with one or more binders that are either hydrophilic or hydrophobic. Thus, either aqueous or organic solvent-based formulations can be used to prepare the thermally developable materials of this invention. Mixtures of either or both types of binders can also be used. It is preferred that the binder be selected from 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 under the names BUTVARX (Solutia, Inc.) and PIOLOFORMX (Wacker Chemical Company).

Aqueous dispersions (or latexes) of hydrophobic binders may also be used either alone as binders or in combination with other binders.

Examples of useful hydrophilic binders include, but are not limited to, proteins and protein derivatives, gelatin and gelatin-like derivatives (hardened

or unhardened, including alkali-and acid-treated gelatins, acetylated gelatin, oxidized gelatin, phthalated gelatin, and deionized gelatin), cellulosic materials such as hydroxymethyl cellulose and cellulosic esters, acrylamide/methacrylamide polymers, acrylic/methacrylic polymers polyvinyl pyrrolidones, polyvinyl alcohols, poly (vinyl lactams), polymers of sulfoalkyl acrylate or methacrylates, hydrolyzed polyvinyl acetates, polyacrylamides, polysaccharides (such as dextrans and starch ethers), and other synthetic or naturally occurring vehicles commonly known for use in aqueous-based photographic emulsions (see for example, Research Disclosure, item 38957, noted above). Cationic starches can be used as a peptizer for tabular silver halide grains as described in U. S. Patent 5,620, 840 (Maskasky) and U. S. Patent 5,667, 955 (Maskasky).

Hardeners for various binders may be present if desired. Useful hardeners are well known and include diisocyanate compounds as described for example, in EP 0 600 586 Bl (Philip, Jr. et al. ), vinyl sulfone compounds as described in U. S. Patent 6,143, 487 (Philip, Jr. et al. ), and EP 0 640 589 Al (Gathmann et al. ), aldehydes and various other hardeners as described in U. S.

Patent 6,190, 822 (Dickerson et al. ). The hydrophilic binders used in the photo- thermographic materials are generally partially or fully hardened using any conventional hardener. Useful hardeners are well known and are described, for example, in T. H. James, The Theory of the Photographic Process, Fourth Edition, Eastman Kodak Company, Rochester, NY, 1977, Chapter 2, pp. 77-78.

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. When a hydrophobic binder is used, it is preferred that the binder does not decompose or lose its structural integrity at 120°C for 60 seconds. When a hydrophilic binder is used, it is preferred that the binder does not decompose or lose its structural integrity at 150°C for 60 seconds.

It is more preferred that it does not decompose or lose its structural integrity at 177°C for 60 seconds.

The polymer binder (s) is used in an amount sufficient to carry the components dispersed therein. The effective range of amount of polymer 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 it is included. The amount of binders in double-sided photothermographic materials may be the same or different.

It is particularly useful in the photothermographic materials to use predominantly (more than 50% by weight of total binder weight) hydrophobic binders in both imaging and non-imaging layers on both sides of the support.

Thus, the hydrophobic binder is mixed into the photothermographic emulsion prepared according to this invention to form a photothermographic emulsion formulation for coating onto a support.

Support Materials The photothermographic materials prepared by this invention comprise a polymeric support that is preferably a flexible, transparent film that has any desired thickness and is composed of one or more polymeric materials, depending upon their use. The supports are generally transparent (especially if the material is used as a photomask) or at least translucent, but in some instances, opaque supports may be useful. They are required to exhibit dimensional stability during thermal development and to have suitable adhesive properties with overlying layers. Useful polymeric materials for making such supports include, but are not limited to, polyesters (such as polyethylene terephthalate and polyethylene naphthalate), cellulose acetate and other cellulose esters, polyvinyl acetal, polyolefins (such as polyethylene and polypropylene), polycarbonates, and polystyrenes (and polymers of styrene derivatives). Preferred supports are composed of polymers having good heat stability, such as polyesters and polycarbonates. Polyethylene terephthalate film is a particularly preferred support.

Various support materials are described, for example, in Research Disclosure, August 1979, item 18431. A method of making dimensionally stable polyester films is described in Research Disclosure, September 1999, item 42536. Support

materials may also be treated or annealed to reduce shrinkage and promote dimensional stability.

It is also useful to use supports comprising dichroic mirror layers wherein the dichroic mirror layer reflects radiation at least having the predetermined range of wavelengths to the emulsion layer and transmits radiation having wavelengths outside the predetermined range of wavelengths. Such dichroic supports are described in U. S. Patent 5,795, 708 (Boutet).

It is further useful to use transparent, multilayer, polymeric supports comprising numerous alternating layers of at least two different polymeric materials. Such multilayer polymeric supports preferably reflect at least 50% of actinic radiation in the range of wavelengths to which the photothermo- graphic material is sensitive, and provide photothennographic materials having increased speed. Such transparent, multilayer, polymeric supports are described in U. S. Patent 6,630, 283 (Simpson et al.).

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

Support materials can contain various colorants, pigments, antihalation or acutance dyes if desired. For example, the support can include one or more dyes that provide a blue color in the resulting imaged film. Support materials may be treated using conventional procedures (such as corona discharge) to improve adhesion of overlying layers, or subbing or other adhesion-promoting layers can be used. Useful subbing layer formulations include those conventionally used for photographic materials such as vinylidene halide polymers.

Photothermographic Formulations An organic solvent-based coating formulation for the photothenno- graphic emulsion layer (s) can be prepared by mixing the photothermographic emulsion prepared according to the present invention with one or more binders, the reducing composition, toner (s), and optional addenda in a suitable solvent

system that usually includes an organic solvent, such as toluene, 2-butanone (methyl ethyl ketone), acetone, or tetrahydrofuran.

Alternatively, the photothermographic emulsion formulation can be composed with a hydrophilic binder (such as gelatin, a gelatin-derivative, or a latex) in water or water-organic solvent mixtures to provide aqueous-based coating formulations.

Photothermographic materials prepared by this invention can contain plasticizers and lubricants such as poly (alcohols) and diols of the type described in U. S. Patent 2,960, 404 (Milton et al. ), fatty acids or esters such as those described in U. S. Patent 2,588, 765 (Robijns) and U. S. Patent 3,121, 060 (Duane), and silicone resins such as those described in GB 955,061 (DuPont).

The materials can also contain matting agents such as starch, titanium dioxide, zinc oxide, silica, and polymeric beads including beads of the type described in U. S. Patent 2,992, 101 (Jelley et al. ) and U. S. Patent 2,701, 245 (Lynn). Polymeric fluorinated surfactants may also be useful in one or more layers of the imaging materials for various purposes, such as improving coatability and optical density uniformity as described in U. S. Patent 5,468, 603 (Kub).

U. S. Patent 6, 436, 616 (Geisler et al. ) describes various means of modifying photothermographic materials to reduce what is known as the "woodgrain"effect, or uneven optical density. This effect can be reduced or eliminated by several means, including treatment of the support, adding matting agents to the topcoat, using acutance dyes in certain layers or other procedures described in the noted publication.

The photothermographic materials prepared according to this invention can include one or more antistatic agents in any of the layers including the photothermographic emulsion layer, or in separate conductive layers, on either or both sides of the support. Thus, conductive components include, but are not limited to, soluble salts (for example, chlorides or nitrates), evaporated metal layers, or ionic polymers such as those described in U. S. Patent 2,861, 056 (Minsk) and U. S. Patent 3,206, 312 (Sterman et al. ), or insoluble inorganic salts such as those described in U. S. Patent 3, 428, 451 (Trevoy), electroconductive underlayers

such as those described in U. S. Patent 5,310, 640 (Markin et al. ), electronically- conductive metal antimonate particles such as those described in U. S. Patent 5,368, 995 (Christian et al. ), and electrically-conductive metal-containing particles dispersed in a polymeric binder such as those described in EP 0 678 776 Al (Melpolder et al. ). Particularly useful conductive particles are the non-acicular metal antimonate particles described in copending and commonly assigned U. S.

Serial No. 10/304, 224 (filed on November 27,2002 by LaBelle, Sakizadeh, Ludemann, Bhave, and Pham). Other antistatic agents are well known in the art.

Still other conductive compositions include one or more fluoro- chemicals each of which is a reaction product of Rf-CH2CH2-SO3H with an amine wherein Rf comprises 4 or more fully fluorinated carbon atoms. These antistatic compositions are described in more detail in copending and commonly assigned U. S. Published Application 2003-0198901 (Sakizadeh et al.).

Additional conductive compositions include one or more fluoro- chemicals having the structure Rf-R-N (R'l) (R'2) (R'3) + X'wherein Rf is a straight or branched chain perfluoroalkyl group having 4 to 18 carbon atoms, R is a divalent linking group comprising at least 4 carbon atoms and a sulfide group in the chain, R'l, R'2, R'3 are independently hydrogen or alkyl groups or any two of R'l, R'2, and R'3 taken together can represent the carbon and nitrogen atoms necessary to provide a 5-to 7-membered heterocyclic ring with the cationic nitrogen atom, and X is a monovalent anion. These antistatic compositions are described in more detail in copending and commonly assigned U. S. Serial No.

10/265,058 (filed October 4,2002 by Sakizadeh, LaBelle, and Bhave).

The photothermographic materials prepared according to this invention can be constructed of one or more layers on the imaging side of the support. Single layer materials should contain the chemically sensitized silver halide, the non-photosensitive source of reducible silver ions, the reducing agent composition, the binder, as well as optional materials such as toners, acutance dyes, coating aids, and other adjuvants.

Two-layer constructions comprising a single imaging layer coating containing all the ingredients and a surface protective topcoat are generally found

on the frontside of the photothermographic materials prepared by this invention.

However, two-layer constructions containing chemically sensitized silver halide and non-photosensitive source of reducible silver ions in one imaging layer (usually the layer closer to the support) and the reducing agent composition and other ingredients in the second imaging layer or distributed between both layers are also envisioned.

Layers to promote adhesion of one layer to another in photothermo- graphic materials are also known, as described for example in U. S. Patent 5,891, 610 (Bauer et al. ), U. S. Patent 5,804, 365 (Bauer et al. ), and U. S. Patent 4,741, 992 (Przezdziecki). Adhesion can also be promoted using specific polymeric adhesive materials as described for example in U. S. Patent 5,928, 857 (Geisler et al.).

Layers to reduce emissions from the film may also be present, including the polymeric barrier layers described in U. S. Patent 6,352, 819 (Kenney et al.), U. S. Patent 6,352, 820 (Bauer et al. ), and U. S. Patent 6,420, 102 (Bauer et al. ), and copending and commonly assigned U. S. Serial No. 10/341,747 (filed January 14,2003 by Rao, Hammerschmidt, Bauer, Kress, and Miller), and U. S.

Serial No. 10/351,814 (filed January 27,2003 by Hunt).

The photothermographic formulations described herein (including the photothermographic emulsion formulation) can be coated by various coating procedures including wire wound rod coating, dip coating, air knife coating, curtain coating, slide coating, or extrusion coating using hoppers of the type described in U. S. Patent 2, 681, 294 (Beguin). Layers can be coated one at a time, or two or more layers can be coated simultaneously by the procedures described in U. S. Patent 2,761, 791 (Russell), U. S. Patent 4,001, 024 (Dittman et al. ), U. S.

Patent 4,569, 863 (Keopke et al. ), U. S. Patent 5,340, 613 (Hanzalik et al. ), U. S.

Patent 5,405, 740 (LaBelle), U. S. Patent 5,415, 993 (Hanzalik et al. ), U. S. Patent 5,525, 376 (Leonard), U. S. Patent 5,733, 608 (Kessel et al. ), U. S. Patent 5,849, 363 (Yapel et al. ), U. S. Patent 5,843, 530 (Jerry et al. ), U. S. Patent 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 lem, and the layer can be dried in forced air at a temperature of

from 20°C to 100°C. It is preferred that the thickness of the layer be selected to provide maximum image densities greater than 0.2, and more preferably, from 0.5 to 5.0 or more, as measured by a MacBeth Color Densitometer Model TD 504.

For example, after or simultaneously with application of the photo- thermographic emulsion formulation to the support, a protective overcoat formulation can be applied over the emulsion formulation. Preferably, the two formulations are applied simultaneously.

In other embodiments, a"carrier"layer formulation comprising a single-phase mixture of the two or more polymers described above may be applied directly onto the support and thereby located underneath the emulsion layer (s).

Such formulations are described in U. S. Patent 6,355, 405 (Ludemann et al.).

Preferably, the carrier layer formulation is applied to the support simultaneously with application of the photothermographic emulsion layer formulation.

Mottle and other surface anomalies can be reduced in the photo- thermographic materials prepared by this invention by incorporation of a fluorinated polymer as described for example in U. S. Patent 5,532, 121 (Yonkoski et al. ) or by using particular drying techniques as described, for example in U. S.

Patent 5,621, 983 (Ludemann et al.).

Preferably, two or more layer formulations 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 solvents (or solvent mixtures).

While the first and second layers can be coated on one side of the film support, manufacturing methods can also include forming on the opposing or backside of the polymeric support, one or more additional layers, including a conductive layer, antihalation layer, or a layer containing a matting agent (such as silica), or a combination of such layers. Alternatively, one backside layer can perform all of the desired functions.

It is also contemplated that the photothermographic materials prepared according to this invention can include photothermographic emulsion layers on both sides of the support and at least one infrared radiation absorbing

heat-bleachable compositions as an antihalation underlayer beneath at least one emulsion layer.

Photothermographic materials having thermally developable layers disposed on both sides of the support often suffer from"crossover."Crossover results when radiation used to image one side of the photothermographic material is transmitted through the support and images the photothermographic layers on the opposite side of the support. Such radiation causes a lowering of image quality (especially sharpness). As crossover is reduced, the sharper becomes the image. Various methods are available for reducing crossover. Such "anti-crossover"materials can be materials specifically included for reducing crossover or they can be acutance or antihalation dyes. In either situation, when imaged with visible radiation, it is often necessary that they be rendered colorless during processing.

To promote image sharpness, photothermographic materials prepared by the methods of the present invention can contain one or more layers containing acutance and/or antihalation dyes. These dyes are chosen to have absorption close to the exposure wavelength and are designed to absorb scattered light. One or more antihalation compositions may be incorporated into one or more antihalation layers according to known techniques, as an antihalation backing layer, as an antihalation underlayer, or as an antihalation overcoat.

Additionally, one or more acutance dyes may be incorporated into one or more frontside layers such as the photothermographic emulsion layer, primer layer, underlayer, or topcoat layer according to known techniques. It is preferred that the photothermographic materials contain an antihalation composition on the backside of the support, and more preferably in a backside conductive layer.

Dyes useful as antihalation and acutance dyes include squaraine dyes described in U. S. Patent 5,380, 635 (Gomez et al. ), U. S. Patent 6,063, 560 (Suzuki et al. ), and EP 1 083 459A1 (Kimura), the indolenine dyes described in EP 0 342 810A1 (Leichter), and the cyanine dyes described in U. S. Published Application 2003-0162134 (Hunt et al.).

It is also useful to employ compositions including acutance or antihalation dyes that will decolorize or bleach with heat during processing. Dyes and constructions employing these types of dyes are described in, for example, U. S. Patent 5,135, 842 (Kitchin et al. ), U. S. Patent 5,266, 452 (Kitchin et al. ), U. S.

Patent 5,314, 795 (Helland et al. ), U. S. Patent 6,306, 566, (Sakurada et al. ), JP Kokai 2001-142175 (Hanyu et al. ), and JP Kokai 2001-183770 (Hanye et al.).

Also useful are bleaching compositions described in JP Kokai 11-302550 (Fujiwara), JP Kokai 2001-109101 (Adachi), JP Kokai 2001-51371 (Yabuki et al.), and JP Kokai 2000-029168 (Noro).

Particularly useful heat-bleachable backside antihalation compositions can include an infrared radiation absorbing compound such as an oxonol dyes and various other compounds used in combination with a hexaarylbiimidazole (also known as a"HABI"), or mixtures thereof. Such HABI compounds are well known in the art, such as U. S. Patent 4,196, 002 (Levinson et al. ), U. S. Patent 5,652, 091 (Perry et al. ), and U. S. Patent 5,672, 562 (Perry et al.).

Examples of such heat-bleachable compositions are described for example in U. S.

Patent 6,455, 210 (Irving et al. ), U. S. Patent 6,514, 677 (Ramsden et al. ), and U. S.

Patent 6,558, 880 (Goswami et al.).

Under practical conditions of use, these compositions are heated to provide bleaching at a temperature of at least 90°C for at least 0.5 seconds.

Preferably, bleaching is carried out at a temperature of from 100°C to 200°C for from 5 to 20 seconds. Most preferred bleaching is carried out within 20 seconds at a temperature of from 110°C to 130°C.

In some preferred embodiments, the photothermographic materials prepared by this invention include a surface protective layer over one or more imaging layers one both sides of the support.

In other preferred embodiments, the photothermographic materials include a surface protective layer on the same side of the support as the one or more photothermographic emulsion layers and a layer on the backside that includes an antihalation composition and/or conductive antistatic components. A

separate backside surface protective layer can also be included in these embodiments.

Imaging/Development The photothermographic materials prepared according to 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). In some embodiments, the materials are sensitive to radiation in the range of from at least 300 nm to 1400 nm, and preferably from 300 nm to 850 nm.

Imaging can be achieved by exposing the photothermographic materials prepared by this invention to a suitable source of radiation to which they are sensitive, including ultraviolet radiation, visible light, near infrared radiation and infrared radiation to provide a latent image. Suitable exposure means are well known and include sources of radiation, including: incandescent or fluorescent lamps, xenon flash lamps, lasers, laser diodes, light emitting diodes, infrared lasers, infrared laser diodes, infrared light-emitting diodes, infrared lamps, or any other ultraviolet, visible, or infrared radiation source readily apparent to one skilled in the art, and others described in the art, such as in Research Disclosure, September, 1996, item 38957. Particularly useful infrared exposure means include laser diodes, including laser diodes that are modulated to increase imaging efficiency using what is known as multi-longitudinal exposure techniques as described in U. S. Patent 5,780, 207 (Mohapatra et al.). Other exposure techniques are described in U. S. Patent 5,493, 327 (McCallum et al.).

In some embodiments, the photothermographic materials can be imaged using any suitable X-radiation imaging source to provide a latent image.

Suitable exposure means are well known and include medical, mammography, dental, and industrial X-ray units.

When storage phosphors are incorporated within the photothermo- graphic materials, the initial exposure to X-radiation is"stored"within the phosphor particles. When the material is then later exposed a second time to

stimulating electromagnetic radiation (usually to visible light or infrared radiation), the"stored"energy is then released as an emission of visible or infrared radiation. The photothermographic materials may then be developed by heating.

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°C to 250°C (preferably from 80°C to 200°C and more preferably from 100°C 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. A preferred heat- development procedure includes heating at from 110°C to 135°C for from 3 to 25 seconds.

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.

In another two-step development method, thermal development can take place using a preheating step (for example at 110°C for up to 10 seconds), immediately followed by a final development step (for example at 125°C for up to 20 seconds).

Use as a Photomask The photothennographic materials described herein can be sufficiently transmissive in the range of from 350 to about 450 nm in non-imaged areas to allow their use in a method where there is a subsequent exposure of an ultraviolet or short wavelength visible radiation sensitive imageable medium. For example, imaging the materials and subsequent development affords a visible image. The heat-developed photothermographic materials absorb ultraviolet or short wavelength visible radiation in the areas where there is a visible image and transmit ultraviolet or short wavelength visible radiation where there is no visible

image. The heat-developed materials may then be used as a mask and positioned between a source of imaging radiation (such as an ultraviolet or short wavelength visible radiation energy source) and an imageable material that is sensitive to such imaging radiation, such as a photopolymer, diazo material, photoresist, or photosensitive printing plate. Exposing the imageable material to the imaging radiation through the visible image in the exposed and heat-developed photo- thermographic material provides an image in the imageable material. This method is particularly useful where the imageable medium comprises a printing plate and the photothermographic material serves as an imagesetting film.

Thus, a method for the formation of a visible image (usually a black-and-white image) comprises: (A) imagewise exposing the photothermographic material to electro- magnetic radiation to which the chemically sensitized photosensitive silver halide is sensitive, to form a latent image, and (B) simultaneously or sequentially, heating the exposed material to develop the latent image into a visible image.

The photothermographic material may be exposed in step A using any source of radiation, to which it is sensitive, including: ultraviolet radiation, visible light, infrared radiation or any other infrared radiation source readily apparent to one skilled in the art.

This visible image prepared from the photothermographic material can then be used as a mask for exposure of other photosensitive imageable materials, such as graphic arts films, proofing films, printing plates and circuit board films, that are sensitive to suitable imaging radiation (for example, UV radiation). This can be done by imaging an imageable material (such as a photopolymer, a diazo material, a photoresist, or a photosensitive printing plate) through the heat-developed photothermographic material. Thus, in some other embodiments wherein the photothermographic material comprises a transparent support, the image-forming 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.

Imaging Assemblies The photothermographic materials described herein are also useful in an imaging assembly comprising one or more phosphor intensifying screens adjacent the front and/or back of the photothermographic material. Such screens are well known in the art [for example, U. S. Patent 4,865, 944 (Roberts et al. ) and U. S. Patent 5,021, 327 (Bunch et al. )]. An assembly (often known as a cassette), can be prepared by arranging the photothermographic material, and the one or more screens in a suitable holder and appropriately packaging them for transport and imaging uses.

In use, the phosphor intensifying screen can be positioned in "front"of the photothermographic material to absorb X-radiation and to emit electromagnetic radiation having a wavelength greater than 300 nm and to which the photothermographic material has been sensitized.

Double-coated X-radiation sensitive photothermographic materials (that is, materials having one or more thermally developable imaging layers on both sides of the support) are preferably used in combination with two intensifying screens, one screen in the"front"and one screen in the"back"of the material.

The front and back screens can be appropriately chosen depending upon the type of emissions desired, the desired photicity, emulsion speeds, and percent crossover. A metal (such as copper or lead) screen can also be included if desired.

Other arrangements of screens and photothermographic materials in imaging assemblies or cassettes would be readily apparent to a skilled artisan.

Constructions and imaging assemblies useful in industrial radiography include, for

example, U. S. Patent 4, 480, 024 (Lyons et al), U. S. Patent 5,900, 357 (Feumi- Jantou et al. ), and EP 1 350 883 Al (Pesce et al.).

The following examples are provided to illustrate the practice of the present invention and the invention is not meant to be limited thereby.

Materials and Methods for the Experiments and Examples: All materials used in the following examples are readily available from standard commercial sources, such as Aldrich Chemical Co. (Milwaukee WI) unless otherwise specified. All percentages are by weight unless otherwise indicated. The following additional terms and materials were used.

ACRYLOIDe A-21 is an acrylic copolymer available from Rohm and Haas (Philadelphia, PA).

BUTVAR B-79 is a polyvinyl butyral resin available from Solutia, Inc. (St. Louis, MO).

CAB 171-15S and CAB 381-20 are cellulose acetate butyrate resins available from Eastman Chemical Co,. (Kingsport, TN).

DESMODURTM N3300 is an aliphatic hexamethylene diisocyanate that is available from Bayer Chemicals (Pittsburgh, PA).

The Fischer X-Ray machine was a Model 36600G and was obtained from Fischer Imaging Corporation (Denver, CO).

LOWINOX 221B446 is 2,2'-isobutylidene-bis (4,6-dimethylphenol) available from Great Lakes Chemical (West Lafayette, IN).

Diphenylphosphine sulfide (DPPS) was obtained from Organometallics, Inc (East Hampstead, NH) PERMANAXO WSO (or NONOXC) is 1, 1-bis (2-hydroxy- 3,5-dimethylphenyl)-3, 5,5-trimethylhexane [CAS RN=7292-14-0] and is available from St-Jean PhotoChemicals, Inc. (Quebec, Canada).

MEK is methyl ethyl ketone (or 2-butanone).

"PHP"is pyridinium hydrobromide perbromide and is available from Great Western Inorganics, Inc, Arvada, CO

PIOLOFORM BL-16 and PIOLOFORMX BN-18 are polyvinyl butyral resins available from Wacker Polymer Systems (Adrian, MI).

The X-Rite# Model 301 densitometer was obtained from X-Rite Inc. (Grandville, MI).

Vinyl Sulfone-1 (VS-1) is described in U. S. Patent 6,143, 487 and has the structure shown below.

(VS-1) 2- (Tribromomethylsulfonyl) pyridine (Antifoggant-A) has the following structure: (AF-A) Ethyl-2-cyano-3-oxobutanoate (Antifoggant-B) is described in U. S.

Patent 5,686, 228 and is believed to have the structure shown below.

Sensitizing Dye A has the structure shown below.

(SD-A) Compound Au-2 is the gold (III) terpyridine trichloride and has the structure shown below.

(Au-2) Backcoat Dye BC-1 is cyclobutenediylium, 1, 3-bis [2,3-dihydro- 2,2-bis [ [l-oxohexyl) oxy] methyl]-1H-perimidin-4-yl]-2, 4-dihydroxy-, bis (inner salt). It is believed to have the structure shown below.

(BC-1) Acutance Dye AD-1 has the following structure: (AD-1) Tinting Dye TD-1 has the following structure : (TD-1) Organic Sulfur Dye compound OSD-1 has the following structure :

(OSD-1) Example 1: This example compares the compounds of chemical sensitization of the present invention with those described in U. S. Patent 5,891, 615 (Winslow et al. ). A control example containing no chemical sensitizer was also prepared. It is labeled Control Example 1-1 Preparation of Control Example 1-1 : A photothermographic emulsion of silver behenate full soap containing preformed silver halide was prepared as described in U. S. Patent 5,939, 249 (noted above). The emulsion was homogenized to 27.2 % solids in MEK containing 2% PIOLOFORMX BM-18 polyvinyl butyral binder. Mixing for 15 minutes at 20°C was followed by addition of 1.6 parts of a 15% solution of pyridinium hydrobromide perbromide in methanol with continued stirring. After 60 minutes of mixing, 2.1 parts of an 11% zinc bromide solution in methanol was added. Stirring was continued and after 30 minutes, a solution of 0.15 parts of 2-mercapto-5-methylbenzimidazole, 0.007 parts Sensitizing Dye A, 1.7 parts of

2- (4-chlorobenzoyl) benzoic acid, 10.8 parts of methanol, and 3.8 parts of MEK was added.

After stirring for another 75 minutes, 26 parts of PIOLOFORM BM-18 polyvinyl butyral and 20 parts of PIOLOFORMs BL-16 were added, the temperature was lowered to 10°C, and mixing was continued for another 30 minutes.

Photothermographic Coating Formulation: Solution A: Antifoggant-A : 1.2 parts Tetrachlorophthalic acid 0.37 parts 4-Methylphthalic acid 0.60 parts MEK 16 parts Methanol 0.28 parts LOWINOXX 221B446 9.5 parts Solution B: DESMODURTM N3300 0.66 parts MEK 0.33 parts Solution C: Phthalazine 1.3 parts MEK 6.3 parts

The photothermographic coating formulation was completed by adding Solution A, LOWINOX 221B446, Solution B, and Solution C. These materials were added 5 minutes apart. Mixing was maintained.

Protective Topcoat Formulation : A stock solution formulation for the protective topcoat for the photothermographic emulsion layer was prepared as follows: ACRYLOID A-21 2.9 parts CAB 171-15S 32 parts MEK 459 parts Vinyl sulfone (VS-1) 1.6 parts Benzotriazole 0.9 parts Antifoggant-B 0. 8 parts Acutance dye (AD-1) 0.5 parts Tinting dye (TD-1) 0.02 parts Preparation of Comparative Example 1-2: Comparative Example 1-2 was prepared in the same way as the Control Example 1-1 except: - Three parts of a 0.66% solution of Organic Sulfur Dye compound (OSD-1) in a mixture of MEK/methanol (1: 1) was added to 196 parts of the photothermographic emulsion prior to the addition of PHP.

Preparation of Inventive Examples 1-3 and 1-4: Inventive Examples 1-3 and 1-4 were prepared in the same way as Comparative Example 1-1, except that: - Inventive Example 1-3 used 8 parts of a 0.5% solution of diphenylphosphine sulfide compound PS-1 in MEK/Methanol (1: 1) was used instead of Organic Sulfur Dye compound OSC-1. Compound PS-1 was added to the emulsion before the addition of PHP.

- Inventive Example 1-4 used 2 parts of a 1 % solution of diphenylphosphine sulfide compound PS-15 in MEK.

Compound PS-15 was added to the emulsion before the addition of PHP.

The photothermographic and topcoat formulations were simultaneously dual knife coated onto a 178 jim polyethylene terephthalate support to provide photothermographic materials with the topcoat being farthest from the support. The web (support and applied layers) was conveyed at a rate of 5 m/min during coating and drying. Immediately after coating, the samples were dried in an oven at about 85°C for 5 minutes. The imaging layer formulation was coated to provide about 2 g/m2 of silver dry coating weight. The topcoat formulation was coated to provide about 2.6 g/m2 dry coating weight.

The coated and dried photothermographic materials prepared above were cut into 1.5 inch x 10 inch strips (3.6 cm x 25.4 cm) and exposed through a 10 cm continuous wedge with a scanning laser sensitometer incorporating an 811 nm laser diode. The total scan time for the sample was 6 seconds. The samples were developed using a heated roll processor for 15 seconds at 252°F (122. 2°C).

Densitometry measurements were made on a custom built computer-scanning densitometer and meeting ISO Standards 5-2 and 5-3. They are believed to be comparable to measurements from commercially available densitometers. Density of the wedges was then measured using a filter appropriate

to the sensitivity of the photothermographic material to obtain graphs of density versus log exposure (that is, D log E curves).

The results, shown below in TABLE I, demonstrate that diphenylphosphine sulfide compounds provide significant improved SP-2 over the organic sulfur dye compound. A slight increase in Dmin was found in Inventive Example 1-3.

TABLE I<BR> Example Chemical Sensitizer Dmin SP-2 SP-3<BR> Compound<BR> Control Example 1-1 None 0.226 1.70 1.26<BR> Comparative Example 1-2 OSD-1 0.231 1.77 1.34<BR> Inventive Example 1-3 PS-1 0.317 2.05 1.50<BR> Inventive Example 1-4 PS-15 0.226 1.87 1.17

Example 2.

This example shows the effect on Dmin, Speed-2 and Speed-3 of temperature at which the compounds of chemical sensitization are added. Control, Comparative and Inventive examples were made as described in Example 1 except that the initial temperature during preparation of the emulsion formulation was 23°C instead of 20°C.

TABLE II<BR> Example Chemical Sensitizer Dmin SP-2 SP-3<BR> Compound<BR> Control Example 2-1 None 0.218 1.67 0 1.19<BR> Comparative Example 2-2 OSD-1 0.207 1.79 1.26<BR> Inventive Example 2-3 PS-1 0.215 2.01 1.36<BR> Inventive Example 2-4 PS-15 0.479 2.19 1.49

Example 3: The preparation of a photothermographic formulation was carried out as follows: A preformed silver bromide, silver carboxylate"soap"was prepared as described in U. S. Patent 6,413, 710 (Shor et al. ). The average grain size was 0. 15 jim.

Photothermographic Emulsion Formulation Chemically sensitized photothermographic emulsions were prepared according to procedures described in U. S. Patent 6,423, 481 (Simpson et al. ) but incorporating the diphenylphosphine sulfide compounds described herein and using the materials and amounts as described below. The materials were added 10 to 60 minutes apart and the temperature during addition ranged from 50°F to 70°F (10°C to 21°C).

To 163.0 g of this silver soap dispersion at 28.4% solids was added in order: Chemical Sensitizer (PS-1) 8.1 ml of a 1. 53xl04 mol solution in 8.64 g of MEOH PHP 0.20 g in 1.58 g of MeOH ZnBr2 0.169 g in 1.19 g of MeOH Au-2 4.8 ml of a solution of 0. 0052 g in 50 g of MeOH Chlorobenzoyl benzoic acid 1.42 g BUTVAR B-79 20 g Antifoggant-A 1.71 g in 19.4 g of MEK DESMODURTM N3300 0.63 g in 1.5 g of MEK Phthalazine 1.0 g in 5 g of MEK Tetrachlorophthalic acid 0.35 g in 2 g of MEK 4-Methylphthalic acid 0.45 g in 4 g of MEK

PERMANAX WSO 10.6 g Protective topcoat Formulation A protective topcoat for the photothermographic emulsion layer was prepared as follows: ACRYLOID A-21 0.58 g CAB 171-15S 14.9 g MEK 200 g VS-1 0. 3 g Benzotriazole 1. 6 g Antifoggant-A 0.24 g Antifoggant-B 0.12 g The photothermographic emulsion and topcoat formulations were coated under safelight conditions using a dual knife coating machine onto a 7 mil (178 im) blue-tinted polyethylene terephthalate support provided with a backside antihalation layer containing Dye BC-1 in CAB 171-15S resin binder. Samples were dried for 7 minutes at 87°C. The silver coating weights were approximately 2.2 to 2.3 g/m2.

Samples of the photothermographic materials were imagewise exposed for 10-3 seconds using an EG&G Flash sensitometer equipped with both a P-16 filter and a 0.7 neutral density filter to provide continuous tone"wedges." Following exposure, the films were developed using a heated roll processor for 15 seconds at 122. 2°C to 122. 8°C.

Densitometry measurements were made on a custom built computer-scanning densitometer as described in Example 1 above. The sensitometric results, shown below in TABLE III demonstrate that the addition of ZnBr2 after the addition of diphenylphosphine sulfide compound (PS-1) and oxidizing compound (PHP) provides photothermographic materials with good Dmin, speed, and contrast.

TABLE III Example Dmin SP-2 AC-1 AC-2 Invention 3-1 0.29 3.93 2.95 1.96 Example 4-Use in Phosphor-Containing Photothermographic Material: To 25 g of each of the photothermographic emulsion formulations prepared above in Example 3, was added 18.2 g of YSrTa04 phosphor having an average size of 4. 0 im. The materials were mixed for 5 minutes to prepare the final photothermographic coating formulations. Photothermographic materials were coated and dried as described in Example 4. The approximate phosphor coating weights were from 76 to 77 g/m2.

The photothermographic materials were imaged, developed, and evaluated as described above in Example 4. The sensitometric results, shown below in TABLE IV, demonstrate the effects on Dmin, speed and contrast by the addition of ZnBr2 after the diphenylphosphine sulfide compound (PS-1) and the oxidizing compound (PHP).

TABLE IV Example Dmin SP-2 AC-1 AC-2 Invention 4-1 0.82 4.31 3.39 3.95 The X-ray sensitometric response of these photothermographic materials was determined by exposing the samples using a Fischer X-ray unit operating at 200 mA and 76 KeV and filtered with a 3.0 mm sheet of aluminum.

The samples were placed on a table set 85.5 cm from the X-ray source. A series of X-ray exposures of constant intensity and exposure times from 0.1 sec to 1.5 sec were made. Exposed samples were developed in a manner similar to that described in Example 1.

The density of these samples were measured with an X-Rite 310 densitometer using the Status A filters and measured with the visible filter. The sensitometric results, shown below in TABLE V, demonstrate that the addition of ZnBr2 after the addition of compound PS-1 and PHP oxidizing compound provides photothermographic materials with good differentiation between developed density and Dmin. In addition, D log E curves showed low Dmin, and good speed and contrast.

TABLE V<BR> Example (Developed Density - Dmin) (Developed Density - Dmin)<BR> at 0.8 sec at 1.5 sec<BR> Invention 4-1 2.54 3.40