FIELD OF THE INVENTION This invention relates to thermally developable imaging materials comprising certain boron compounds. In particular the invention relates to photo- thermographic and thermographic materials containing these boron compounds and to methods of imaging these materials.

BACKGROUND OF THE INVENTION

Silver-containing thermographic and photothermographic imaging materials (that is, thermally developable imaging materials) that are imaged and/or developed using heat and without liquid processing have been known in the art for many years. Silver-containing thermographic imaging materials are non-photosensitive materials that are used in a recording process wherein images are generated by the use of thermal energy. These materials generally comprise a support having disposed thereon (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing composition (usually including a developer) for the reducible silver ions, and (c) a suitable hydrophilic or hydrophobic binder.

In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. At elevated temperatures, the silver of the silver carboxylate is reduced by a reducing agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechols, pyrogallol, ascorbic acid, and ascorbic acid derivatives, whereby an image of elemental silver is formed. Some thermographic constructions are imaged by contacting them with the thermal head of a thermo

graphic recording apparatus such as a thermal printer or thermal facsimile. In such constructions, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated to an elevated temperature, typically in the range of from 60 to 225°C, resulting in the formation of an image.

Silver-containing photothermographic imaging materials (that is, thermally developable photosensitive imaging materials) that are imaged with actinic radiation and then developed using heat and without liquid processing have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation 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 photo- sensitive 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 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 most instances, the source of reducible silver

ions is an organic silver salt in which silver ions are complexed with organic silver coordinating ligands.

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.

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. Even in so-called "instant

photography," the developer chemistry is physically separated from the photosensitive silver halide until development is desired. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of "fog" or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems.

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 farther 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 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 (Neblettes ' Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74-75, in Zou et al., J. Imaging ScL Technol. 1996, 40, pp. 94-103, and in M. R. V. Sahyun, J. Imaging Sd. Technol. 1998, 42, 23.

Problem to be Solved

A challenge in photothermographic materials is the need to improve their stability at ambient temperature and relative humidity during storage prior to imaging. This stability is referred to as "Natural Age Keeping" (NAK), "Raw Stock Keeping" (RSK) or Shelf-Life Stability. It is desirable that photothermographic materials be capable of maintaining imaging properties, including photospeed and Dmax, while minimizing any increase in Dmin during storage periods. Natural Age Keeping is a problem especially for photothermographic films compared to conventional silver lialide photographic films because, as noted above, all the components needed for development and image formation in photothermographic systems are incorporated into the imaging element, in intimate proximity, prior to development. Thus, there are a greater number of potentially reactive components that can prematurely react during storage.

Another challenge in photothermographic materials is the need to improve the "Dark Stability" of the imaged and processed photothermographic film upon storage. It is desirable that the Dmin not increase, and that the Dmax, tint, and tone of the image not change.

A further challenge in photothermographic materials is the need to improve their stability to light exposure after imaging and processing. Referred to as "Desktop Print Stability," the formation of additional image or "print-out" is usually most evident as an increase in Dmin. This effect tends to be especially problematic under high humidity conditions.

Boron compounds, such as boric acid, borates, and boron alkoxides, have been used in photothermographic materials to crosslink film- forming and binder components (especially polyvinyl acetals and polyvinyl alcohols) as described in U.S. Patents 4,558,003 (Sagawa), 5,004,667 (Arahara et al.), 5,804,365 (Bauer et al.), 6,071,688 (Moose et al.), and 6,203,972 (Katoh et al.). Boric acid has also been used to crosslink poly( vinyl alcohol) to provide a viscosity modifier in coating compositions as described in U.S. Patents 6,419,987 (Bauer et al.) and 6,551 ,770 (Hirabyashi) and particularly in backcoat compositions.

There remains a need to effectively incorporate compounds into photothermographic imaging formulations and materials so that sensitometric properties are not changed during Natural Age Keeping, and so that Dark Stability and Desktop Print Stability are improved, all without sacrifice of desired photospeed and other sensitometric properties.

SUMMARY OF THE INVENTION

This invention provides a black-and-white photothermographic material comprising a support having on a frontside thereof, a) one or more frontside photothermographic imaging layers comprising a polymer binder, and in reactive association, a photosensitive silver halide, a non-photosensitive source of reducible silver ions, and a reducing agent for the non-photosensitive source reducible silver ions, b) the material comprising on the backside of the support, one or more backside photothermographic imaging layers having the same or different composition as the frontside photothermographic imaging layers, and c) optionally, an overcoat layer disposed over the one or more photothermographic imaging layers on either or both sides of the support, wherein the material comprises one or more of the same or different boron compounds on one or both sides of the support, each of the boron compounds being present in the photothermographic imaging layers or in the overcoat, if present, or both.

This invention also provides a black-and-white thermally developable material comprising a support and having on a frontside thereon at least one thermally developable imaging layer, and optionally an overcoat layer disposed over the thermally developable imaging layer, each of the thermally developable layer and overcoat layer, if present, comprising the same or different polymer binder, the material further comprising, in reactive association: a. a non-photosensitive source of reducible silver ions, b. a reducing agent for the reducible silver ions, and

c. one or more of the same or different boron compounds, each of the boron compounds being present in said thermally developable imaging layers or in said overcoat, if present, or both, the boron compounds being represented by Structure (II):

X-B(OLO-Z ¹ (II)

wherein X' is hydroxy, alkoxy groups having 5 or more carbon atoms, alkyl groups having 5 or more carbon atoms, acyloxy groups, aryl groups, or heteroaryl groups,

Z' is alkyl groups having 5 to 18 carbon atoms, acyloxy groups, aryl groups, or heteroaryl groups.

L' is hydrogen, alkyl, acyl, aryl, or heteroaryl, or

X' and Z' together represent carbon or heteroatoms sufficient to provide a heterocyclic ring with the boron atom, or still again, X', L', and Z' together represent carbon or heteroatoms sufficient to provide heterocyclic rings with the boron atom, provided that when the polymer binder in the thermally developable imaging layer is a polyvinyl acetal, X' is not alkoxy.

This invention also provides a method of forming a visible image comprising:

(A') thermal imaging of the thermally developable material of this invention that is a thermographic material.

In alternative methods of this invention, a method of forming a visible image comprises: (A) imagewise exposing a photothermographic material of this invention to form a latent image,

(B) simultaneously or sequentially, heating the exposed photothermographic material to develop the latent image into a visible image.

An imaging assembly of this invention comprises a photothermo- graphic material of this invention that is arranged in association with one or more phosphor intensifying screens.

We have found that certain boron compounds (boric acid, boronic acid, borinic acid and their derivatives) are useful stabilizers and improve Dark Stability, Natural Age Keeping, and Desktop Print Stability of thermally developable materials including duplitized photothermographic materials.

DETAILED DESCRIPTION OF THE INVENTION The thermally developable materials described herein are both thermographic and photothermographic materials. While the following discussion will often be directed primarily to the preferred photothermographic embodiments, it would be readily understood by one skilled in the art that thermographic materials can be similarly constructed and used to provide black-and-white or color images using appropriate imaging chemistry and particularly non-photosensitive organic silver salts, reducing agents, toners, binders, and other components known to a skilled artisan.

The thermally developable materials can be used in black-and- white or color thermography and photothermography and in electronically generated black-and-white or color hardcopy recording. They can be used in microfilm applications, in radiographic imaging (for example digital medical imaging), X-ray radiography, and in industrial radiography. Furthermore, the absorbance of these photothermographic materials between 350 and 450 nm is desirably low (less than 0.5), to permit their use in the graphic arts area (for example, imagesetting and phototypesetting), in the manufacture of printing plates, in contact printing, in duplicating ("duping"), and in proofing.

The thermally developable materials are particularly useful for medical imaging of human or animal subjects in response to visible, X-radiation, or infrared radiation for use in a medical diagnosis. Such applications include, but are not limited to, thoracic imaging, mammography, dental imaging, orthopedic imaging, general medical radiography, therapeutic radiography, veterinary

radiography, and autoradiography. When used with X-radiation, the photothermo- graphic materials may be used in combination with one or more phosphor intensifying screens, with phosphors incorporated within the phototheraiographic emulsion, or with combinations thereof. Such materials are particularly useful for dental radiography when they are directly imaged by X-radiation. The materials are also useful for non-medical uses of X-radiation such as X-ray lithography and industrial radiography.

The photothermographic materials can be made sensitive to radiation of any suitable wavelength. Thus, in some embodiments, the materials are sensitive at ultraviolet, visible, near infrared, or infrared wavelengths, of the electromagnetic spectrum, hi these embodiments, the materials are preferably sensitive to radiation greater than 300 nm (such as sensitivity to, from 300 nm to 750 nm, preferably from 300 to 600 nm, and more preferably from 300 to 450 nm). In other embodiments they are sensitive to X-radiation. Increased sensitivity to X-radiation can be imparted through the use of phosphors.

The photothermographic materials are also useful for non-medical uses of visible or X-radiation (such as X-ray lithography and industrial radiography), hi these and other imaging applications, it is particularly desirable that the photothermographic materials be "double-sided." In some embodiments of the photothermographic materials, the components needed for imaging can be in one or more imaging or emulsion layers on one side ("frontside") of the support. The layer(s) that contain the photosensitive photocatalyst (such as a photosensitive silver halide) for photothermographic materials or the non-photosensitive source of reducible silver ions, or both, are referred to herein as the emulsion layer(s). hi photothermographic materials, the photocatalyst and non-photosensitive source of reducible silver ions are in catalytic proximity and preferably are in the same emulsion layer.

Similarly, in the thermographic materials of this invention, the components needed for imaging can be in one or more layers. The layer(s) that contain the non-photosensitive source of reducible silver ions are referred to herein as thermographic emulsion layer(s).

Where the photothermographic materials contain imaging layers on one side of the support only, various non-imaging layers can also be disposed on the "backside" (non-emulsion or non-imaging side) of the materials, including, conductive layers, antihalation layer(s), protective layers, antistatic 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 overcoat 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 preferred embodiments, the photothermographic materials are "double-sided" or "duplitized" and have the same or different emulsion coatings (or photothermographic imaging layers) on both sides of the support. Such constructions can also include one or more protective overcoat layers, primer layers, interlayers, antistatic layers, acutance layers, antihalation layers, auxiliary layers, conductive layers, and other layers readily apparent to one skilled in the art on either or both sides of support. Preferably, such photothermographic materials have essentially the same layers on each side of the support.

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

Definitions

As used herein: In the descriptions of the photothermographic materials, "a" or

"an" component refers to "at least one" of that component (for example, the boron compounds described herein).

Unless otherwise indicated, the terms "photothermographic materials" and "imaging assemblies" are used herein in reference to embodiments of the present invention.

Heating in a substantially water-free condition as used herein, means heating at a temperature of from 50 ⁰ C to 25O ⁰ 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 ₃ 1977, p. 374.

"Photothermographic material(s)" means a construction comprising at least one photothermographic emulsion layer or a pliotothermographic 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 coated layer). These materials also include multilayer constructions in which one or more imaging components are in different layers, but are in "reactive association." For example, one layer can include the non-photosensitive source of reducible silver ions and another layer can include the reducing agent and/or photosensitive silver halide.

"Thermographic materials" are similarly defined except that no photosensitive silver halide catalyst is purposely added or created.

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

When used in thermography, the term, "imagewise exposing" or "imagewise exposure" means that the material is imaged using any means that provides an image using heat. This includes, for example, by analog exposure where an image is formed by differential contact heating through a mask using a thermal blanket or infrared heat source, as well as by digital exposure where the

image is formed one pixel at a time such as by modulation of thermal print-heads or by thermal heating using scanning laser radiation.

"Catalytic proximity" or "reactive association" means that the 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," "thermographic emulsion layer," or "photothermographic emulsion layer" means a layer of a thermographic or photothermographic material that contains the photosensitive silver halide (when used) and/or non-photosensitive source of reducible silver ions, and a reducing composition. Such layers can also contain additional components or desirable additives. These layers are on what is known as the "frontside" of the support.

The term "double sided" and duplitized are used to define photothermographic materials having one or more of the same or different photothermo- graphic emulsion layers disposed on both sides (front and back) of the support. In double-sided materials the emulsion layers can be of the same or different chemical composition, thickness, or sensitometric properties. In such double- sided materials, the terms frontside and backside are used a convention to refer to opposite sides of the photothermographic material. hi addition, "frontside" also generally means the side of a photothermographic material that is first exposed to imaging radiation, and "backside" generally refers to the opposite side of the photothermographic material.

"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 photothermographic 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 ran, although parts of these ranges may be visible to the naked human eye. More preferably, the ultraviolet region of the spectrum is the region of from 190 nm to 405 nm. The near ultraviolet region of the spectrum refers to that region of from 300 to 400 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.

"Blue region of the spectrum" refers to that region of the spectrum of from 400 nm to 500 nm.

"Green region of the spectrum" refers to that region of the spectrum of from 500 nm to 600 nm. "Red region of the spectrum" refers to that region of the spectrum of from 600 am 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. "Transparent" means capable of transmitting visible light or imaging radiation without appreciable scattering or absorption.

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.

In photothermographic materials, the term Dmax is the maximum image density achieved when the photothermographic material is exposed to a

particular radiation source and a given amount of radiation energy and then thermally developed.

The terms "density," "optical density (OD)," and "image density" refer to the sensitometric term absorbance. Speed-2 is Logl/E + 4 corresponding to the density value of 1.0 above Dmin where E is the exposure in ergs/cm .

"Desktop Print Stability" is the stability of the imaged and processed film when stored for a period of time under given conditions of temperature, relative humidity, and light exposure. It is one type of post-processing stability.

"Natural Age Keeping" (NAK), also known as "Raw Stock Keeping" (RSK) or Shelf-Life Stability, is the stability of the non-imaged film when stored in the dark for a period of time under a given set of temperature and relative humidity conditions. "Dark Stability," also known as "Archival Stability," is the stability of the imaged and processed film when stored in the dark for a period of time under given conditions of temperature and relative humidity. It is one type of post-processing stability.

"Aspect ratio" refers to the ratio of particle or grain "ECD" to particle or grain thickness wherein ECD (equivalent circular diameter) refers to the diameter of a circle having the same projected area as the particle or grain.

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 complexes or silver compounds they are also often referred to as silver salts.

In the compounds described herein, no particular double bond geometry (for example, cis or trans) is intended by the structures drawn unless otherwise specified. Similarly, in compounds having alternating single and double bonds and localized charges their structures are drawn as a formalism. In reality, both electron and charge derealization 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, or as "a derivative" of, a given formula, any substitution that does not alter the bond structure of the formula or the shown atoms within that structure is included within the formula, unless such substitution is specifically excluded by language.

As a means of simplifying the discussion and recitation of certain substituent groups, the term "group" refers to chemical species that may be substituted as well as those that are not so substituted. Thus, the term "alkyl group" is intended to include not only pure hydrocarbon alkyl chains, such as methyl, ethyl, n-propyl, t-butyl, cyclohexyl, ώo-octyl, and octadβcyl, but also alkyl chains bearing substituents known in the art, such as hydroxy, alkoxy, phenyl, halogen atoms (F, Cl, Br, and I), cyano, nitro, amino, and carboxy. For example, alkyl group includes ether and thioether groups (for example CH ₃ -CH ₂ -CH ₂ -O-CH ₂ - and CH ₃ -CH ₂ -CH ₂ -S-CH ₂ -), hydroxyalkyl (such as 1,2-dihydroxyethyl), haloalkyl, nitroalkyl, alkylcarboxy, carboxyalkyl, carboxamido, sulfoalkyl, and other groups readily apparent to one skilled in the art. Substituents that adversely react with other active ingredients, such as very strongly electrophilic or oxidizing substituents, would, of course, be excluded by the ordinarily skilled artisan as not being inert or harmless.

Research Disclosure (http://www.researchdisclosure.com) is a publication of Kenneth Mason Publications Ltd., The Book Barn, Westbourne, Hampshire POlO 8RS, UK. It is also available from Emsworth Design Inc., 200 Park Avenue South, Room 1101, New York, NY 10003.

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

The Piiotocataϊyst

The photothermographic materials include one or more photocatalysts in the photothermographic emulsion layer(s). Useful photocatalysts are typically photosensitive silver halides such as silver bromide, silver iodide, silver chloride, silver bromoiodide, silver chlorobromoiodide, silver chloro- bromide, 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 silver halides, with the latter silver halide having up to nearly 100 mol% silver iodide (more preferably up to 40 mol % silver iodide) based on total silver halide, and up to the saturation limit of iodide as described in U.S. Patent Application Publication 2004/0053173 (Maskasky et al). The shape (morphology) of the photosensitive silver halide grains used in the present need not be limited. The silver halide grains may have any crystalline habit including cubic, octahedral, tetrahedral, orthorhombic, rhombic, dodecahedral, other polyhedral, tabular, laminar, twinned, or platelet morphologies and may have epitaxial growth of crystals thereon. If desired, a mixture of these crystals can be employed. Silver halide grains having cubic and tabular morphology (or both) are preferred. More preferably, the silver halide grains are predominantly (at least 50% based on total silver halide) present as tabular grains. 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 of 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.). Indium and/or copper doped core-shell and non-core-shell grains are described in U.S. Patents 5,434,043 (Zou et al.) and 5,939,249 (Zou).

In some instances, it may be helpful to prepare the photosensitive silver halide grains in the presence of a hydroxytetraazaindene or an

N-heterocyclic compound comprising at least one mercapto group as described 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 halide grains be preformed and prepared by an ex-situ process, chemically and spectrally sensitized, and then be added to and physically mixed with the non-photosensitive source of reducible silver ions. It is also possible to form the source of reducible silver ions in the presence of ex-szYw-prepared silver halide grains. In this process, the source of reducible silver ions is formed in the presence of the preformed silver halide grains. Precipitation of the reducible source of silver ions in the presence of silver halide provides a more intimate mixture of the two materials [see, for example U.S. Patent 3,839,049 (Simons)] to provide a "preformed emulsion." This method is useful when non-tabular silver halide grains are used.

In general, the non-tabular silver halide grains used in this invention can vary in average diameter of up to several micrometers (μm) and they usually have an average particle size of from 0.01 to 1.5 μm (preferably from 0.03 to 1.0 μm, and more preferably from 0.05 to 0.8 μm). The average size of the photosensitive silver halide grains is expressed by the average diameter if the grains are spherical, and by the average of the diameters of equivalent circles for the projected images if the grains are cubic, tabular, or other non-spherical shapes. Representative grain sizing 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.

In preferred embodiments of this invention, the silver halide grains are provided predominantly (based on at least 50 mol % silver) as tabular silver halide grains that are considered "ultrathin" and have an average thickness of at least 0.02 μm and up to and including 0.10 μm (preferably an average thickness of

at least 0.03 μm and more preferably of at least 0.04 μm, and up to and including 0.08 μm and more preferably up to and including 0.07 μm).

In addition, these ultrathin tabular grains have an equivalent circular diameter (ECD) of at least 0.5 μm (preferably at least 0.75 μm, and more preferably at least 1 μm). The ECD can be up to and including 8 μm (preferably up to and including 6 μm, and more preferably up to and including 4 μm).

The aspect ratio of the useful tabular grains is at least 5:1 (preferably at least 10:1, and more preferably at least 15:1) and generally up to 50: 1. The grain size of ultrathin tabular grains may be determined by any of the methods commonly employed in the art for particle size measurement, such as those described above. Ultrathin tabular grains and their method of preparation and use in photothermographic materials are described in U.S. Patents 6,576,410 (Zou et al.) and 6,673,529 (Daubendiek et al.).

The ultrathin tabular silver halide grains can also be doped using one or more of the conventional metal dopants known for this puipose including those described in Research Disclosure, item 38957, September, 1996 and U.S. Patent 5,503,970 (Olm et al.). Preferred dopants include indium (III or IV) and ruthenium (II or III) salts. Particularly preferred silver halide grains are ultrathin tabular grains containing iridium-doped azole ligands. Such tabular grains and their method of preparation are described in copending and commonly assigned U.S. Serial No. 10/826,708 (filed on April 16, 2004 by Olm, McDugle, Hansen, Pawlik, Lewis, Mydlarz, Wilson, and Bell).

It is also possible to form some in-situ silver halide, by a process in which an inorganic halide- or an organic halogen-containing compound is added to an organic silver salt to partially convert the silver of the organic silver salt to silver halide as described in U.S. Patent 3,457,075 (Morgan et al.).

The one or more light-sensitive silver halides used in the photothermographic materials 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 Sensitizers

If desired, the photosensitive silver halides used in the photo- thermographic materials can be chemically sensitized using any useful compound that contains sulfur, tellurium, or selenium, or may comprise a compound containing gold, platinum, palladium, ruthenium, rhodium, iridium, or combinations thereof, a reducing agent such as a tin halide or a combination of any of these. The details of these materials are provided for example, in T. H. James, The Theory of the Photographic Process, Fourth Edition, Eastman Kodak Company, Rochester, NY ₅ 1977, Chapter 5, pp. 149-169. Suitable conventional chemical sensitization procedures and compounds are also described in U.S. Patents 1,623,499 (Sheppard et al), 2,399,083 (Waller et al.), 3,297,447 (McVeigh), 3,297,446 (Dunn), 5,049,485 (Deaton), 5,252,455 (Deaton), 5,391,727 (Deaton), 5,912,111 (Lok et al.), 5,759,761 (Lushington et al.), 6,296,998 (Eikenberry et al), and 5,691,127 (Daubendiek et al.), and EP 0 915 371 Al (Lok et al.).

Certain substituted or and unsubstituted thioureas can be used as chemical sensitizers including those described in U.S. Patents 6,296,998 (Eikenberry et al.), 6,322,961 (Lam et al.), 4,810,626 (Burgmaier et al.), and 6,368,779 (Lynch et al.). Still other useful chemical sensitizers include tellurium- and selenium-containing compounds that are described in and U.S. Patents 5,158,892 (Sasaki et al.), 5,238,807 (Sasaki et al.), 5,942,384 (Arai et al.), 6,620,577 (Lynch et al.), and 6,699,647 (Lynch et al.).

Noble metal sensitizers for use in the present invention include gold, platinum, palladium and iridium. GoId(I or III) sensitization is particularly preferred, and described in U.S. Patents 5,858,637 (Eshelman et al.) and 5,759,761 (Lushington et al.). Combinations of gold(III) compounds and either sulfur- or tellurium-containing compounds are useful as chemical sensitizers and are described in U.S. Patent 6,423,481 (Simpson et al.). In addition, sulfur-containing compounds can be decomposed on silver halide grains in an oxidizing environment according to the teaching in U.S.

Patent 5,891,615 (Winslow et al.). Examples of sulflir-containing compounds that can be used in this fashion include sulfur-containing spectral sensitizing dyes.

Other useful sulfur-containing chemical sensitizing compounds that can be decomposed in an oxidized environment are the diphenylphosphine sulfide compounds described in copending and commonly assigned U.S. Serial No. 10/731,251 (filed December 9, 2003 by Simpson, Burleva, and Sakizadeh).

The chemical sensitizers can be used in making the silver halide emulsions in conventional amounts that generally depend upon the average size of silver halide grains. Generally, the total amount is at least 10 ^"10 mole per mole of total silver, and preferably from 10 ^"8 to 10 ^"2 mole per mole of total silver. The upper limit can vary depending upon the compound(s) used, the level of silver halide, and the average grain size and grain morphology.

Spectral Sensitizers

The photosensitive silver halides used in the photothermographic materials may be spectrally sensitized with one or more spectral sensitizing dyes that are known to enhance silver halide sensitivity to ultraviolet, visible, and/or infrared radiation of interest. 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. They may be added at any stage in chemical finishing of the photothermographic emulsion, but are generally added after chemical sensitization. It is particularly useful that the photosensitive silver halides be spectrally sensitized to a wavelength of from 300 to 750 nm, preferably from 300 to 600 nm, more preferably to a wavelength of from 300 to 450 nm, even more preferably from a wavelength of from 360 to 420 nm, and most preferably from a wavelength of from 380 to 420 nm. In other embodiments, the photosensitive silver halides are spectrally sensitized to a wavelength of from 650 to 1150 nm. A worker skilled in the ait would know which dyes would provide the desired spectral sensitivity. Suitable sensitizing dyes such as those described in U.S. Patents

3,719,495 (Lea), 4,396,712 (Kinoshita et al.), 4,439,520 (Kofron et al.), 4,690,883

(Kubodera et al.), 4,840,882 (Iwagaki et al.), 5,064,753 (Kohno et al.), 5,281,515

(Delprato et al.), 5,393,654 (Burrows et al), 5,441,866 (Miller et al.), 5,508,162

(Dankosh), 5,510,236 (Dankosh), and 5,541,054 (Miller et al.), and Japanese

Kokai 2000-063690 (Tanaka et al.), 2000-112054 (Fukusaka et al.), 2000-273329 (Tanaka et al.), 2001-005145 (Arai), 2001-064527 (Oshiyama et al.), and

2001-154305 (Kita et al.), and Research Disclosure, item 308119, Section IV,

December, 1989.

Teachings relating to specific combinations of spectral sensitizing dyes also provided in U.S. Patents 4,581,329 (Sugimoto et al.), 4,582,786 (Ikeda et al.), 4,609,621 (Sugimoto et al.), 4,675,279 (Shuto et al.), 4,678,741 (Yamada et al.), 4,720,451 (Shuto et al.), 4,818,675 (Miyasaka et al.), 4,945,036 (Arai et al.), and 4,952,491 (Nishikawa et al.).

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

2001-183770 (Hanyu et al.).

Dyes may be selected for the purpose of supersensitization to attain much higher sensitivity than the sum of sensitivities that can be achieved by using each dye alone. An appropriate amount of spectral sensitizing dye added is generally 10 ^"10 to 10 ^"1 mole, and preferably, from 10 ^"7 to 10 ^"2 mole per mole of silver halide.

Non-Photosensitive Source of Reducible Silver Ions

The non-photosensitive source of reducible silver ions in the photo- thermographic materials is a silver-organic compound that contains reducible silver(I) ions. Such compounds are generally silver salts of silver organic coordinating ligands that are comparatively stable to light and form a silver image when heated to 5O ⁰ C or higher in the presence of an exposed photocatalyst (such as silver halide, when used in a photothermographic material) and a reducing agent composition.

Organic silver salts that are particularly useful in photothermo- graphic materials include silver salts of compounds containing an imino group. Such salts include, but are not limited to, silver salts of benzotriazole and substituted derivatives thereof (for example, silver methylbenzotriazole and silver 5-chlorobenzotriazole), silver salts of nitrogen acids selected from the group consisting of imidazole, pyrazole, urazole, 1,2,4-triazole and lH-tetrazole, nitrogen acids or combinations thereof, as described in U.S. Patent 4,220,709 (deMauriac). Also included are the silver salts of imidazole and imidazole derivatives as described in U.S. Patent 4,260,677 (Winslow et al.). A nitrogen acid as described herein is intended to include those compounds that have the moiety -NH- in the heterocyclic nucleus. Particularly useful silver salts are the silver salts of benzotriazole, substituted derivatives thereof, or mixtures of two or more of these salts. A silver salt of benzotriazole is most preferred.

Useful nitrogen-containing organic silver salts and methods of preparing them are also described in copending and commonly assigned U.S.

Serial No 10/826,417 (filed April 16, 2004 by Zou and Hasberg). Such silver salts (particularly the silver benzotriazoles) are rod-like in shape and have an average aspect ratio of at least 3:1 and a width index for particle diameter of 1.25 or less. Silver salt particle length is generally less than 1 μm. Also useful are the silver salt-toner co-precipitated nano-crystals comprising a silver salt of a nitrogen- containing heterocyclic compound containing an imino group, and a silver salt comprising a silver salt of a mercaptotriazole as described in copending and commonly assigned U.S. Serial No. 10/935,384 (filed September 7, 2004 by Hasberg, Lynch, Chen-Ho, and Zou). Other organic silver salts that are useful in photothermographic and thermographic materials are silver carboxylates (both aliphatic and aromatic carboxylates) The aliphatic carboxylic acids generally have aliphatic chains that contain 10 to 30. Silver salts of long-chain aliphatic carboxylic acids having 15 to 28 carbon atoms are particularly preferred. Examples of such preferred silver salts include silver behenate, silver arachidate, silver stearate, silver oleate, silver laurate, silver caprate, silver myristate, silver palmitate, silver maleate, silver

fumarate, silver tartarate, silver furoate, silver linoleate, silver butyrate, silver camphorate, and mixtures thereof. Most preferably, at least silver behenate is used alone or in mixtures with other silver carboxylates. Silver carboxylates are particularly useful in organic solvent-based and aqueous latex-based photothermo- graphic materials.

It is also convenient to use silver half soaps such as an equimolar blend of silver carboxylate and carboxylic acid that analyzes for 14.5% by weight solids of silver in the blend and that is prepared by precipitation from an aqueous solution of an ammonium or an alkali metal salt of a commercially available fatty carboxylic acid, or by addition of the free fatty acid to the silver soap.

The methods used for making silver soap emulsions are well known in the art and are disclosed in Research Disclosure, April 1983, item 22812, Research Disclosure, October 1983, item 23419, U.S. Patent 3,985,565 (Gabrielsen et al.) and the references cited above. While the noted organic silver salts are the predominant silver salts in the materials, secondary organic silver salts can be used if present in "minor" amounts (less than 40 mol % based on the total moles of organic silver salts).

Such secondary organic silver salts include silver salts of heterocyclic compounds containing mercapto or thione groups and derivatives thereof such as silver triazoles, oxazoles, thiazoles, thiazolines, imidazoles, diazoles, pyridines, and triazines as described in U.S. Patent 4,123,274 (Knight et al.) and U.S. Patent 3,785,830 (Sullivan et al.). Also included are silver salts of aliphatic carboxylic acids containing a thioether group as described in U.S. Patent 3,330,663 (Weyde et al.), soluble silver carboxylates comprising hydrocarbon chains incorporating ether or thioether linkages or sterically hindered substitution in the α- (on a hydrocarbon group) or ortho- (on an aromatic group) position as described in U.S. Patent 5,491,059 (Whitcomb), silver salts of dicarboxylic acids, silver salts of sulfonates as described in U.S. Patent 4,504,575 (Lee), silver salts of sulfosuccinates as described in EP 0 227 141 Al (Leenders et al.), silver salts of aromatic carboxylic acids (such as silver benzoate), silver salts of acetylenes as described, for example in U.S. Patents 4,761,361 (Ozaki et al.) and 4,775,613

(Hirai et al.). Examples of other useful silver salts of mercapto or thione substituted compounds that do not contain a heterocyclic nucleus include silver salts of thioglycolic acids, dithiocarboxylic acids, and thioamides

Sources of non-photosensitive reducible silver ions can also be in the form of core-shell silver salts as described in U.S. Patent 6,355,408

(Wbitcomb et al.) or the silver dimer compounds that comprise two different silver salts as described in U.S. Patent 6,472,131 (Whitcomb).

Still other useful sources of non-photosensitive reducible silver ions are the silver core-shell compounds comprising a primary core comprising one or more photosensitive silver halides, or one or more non-photosensitive inorganic metal salts or non-silver containing organic salts, and a shell at least partially covering the primary core, wherein the shell comprises one or more non-photosensitive silver salts, each of which silver salts comprises a organic silver coordinating ligand. Such compounds are described in U.S. Patent 6,802,177 (Bokhonov et al.).

The one or more non-photosensitive sources of reducible silver ions (both primary and secondary organic silver salts) are preferably present in a total 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. Alternatively, the total amount of reducible silver ions is generally present in an amount of from

0.001 to 0.2 mol/m ² of the dry photothermographic material (preferably from 0.01 to 0.05 mol/m ² ).

The total amount of silver (from all silver sources) in the photothermographic materials is generally at least 0.002 mol/m ² and preferably from 0.01 to 0.05 mol/m ² for single-sided materials. For double-sided coated materials, total amount of silver from all sources would be doubled.

Reducing Agents

The reducing agent (or reducing agent composition comprising two or more components) for the source of reducible silver ions can be any material (preferably an organic material) that can reduce silver(I) ion to metallic silver. The "reducing agent" is sometimes called a "developer" or "developing agent."

When a silver benzotriazole silver source is used, ascorbic acid and reductone reducing agents are preferred. An "ascorbic acid" reducing agent means ascorbic acid, complexes, and derivatives thereof. Ascorbic acid reducing agents are described in a considerable number of publications including U.S. 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, γ-lactoascorbic acid. 6-desoxy-L-ascorbic acid, L-rharnno- ascorbic acid, imino-6-desoxy-L-ascorbic acid, glucoascorbic acid, fucoascorbic acid, glucoheptoascorbic 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 in EP 0 585 792Al

(Passarella et al.), EP 0 573 700Al (Lingier εt al.), EP 0 588 408Al (ffieronymus et al.), U.S. Patents 5,089,819 (Knapp), 2,688,549 (James et al.), 5,278,035 (Knapp), 5,384,232 (Bishop et al.), 5,376,510 (Parker et al.), and 5,498,511 (Yamashita et al.), Japanese Kokai 7-56286 (Toyoda), and Research Disclosure, item 37152, March 1995. Mixtures of these developing agents can be used if desired.

Particularly useful reducing agents are ascorbic acid mono- or di-fatty acid esters such as the monolaurate, monomyristate, monopalmitate, monostearate, monobehenate, diluarate, distearate, dipalmitate, dibehenate, and dimyristate derivatives of ascorbic acid as described in U.S. Patents 3,832,186 (Masuda et al.) and 6,309,814 (Ito). Preferred ascorbic acid reducing agents and their methods of preparation are those described in copending and commonly assigned U.S. Serial No. 10/764,704 (filed on January 26, 2004 by Ramsden, Lynch, Skoug, and Philip) and those described in copending and commonly assigned U.S. Serial No. 10/935,645 (filed on September 7, 2004 by Brick,

Ramsden, and Lynch). A preferred reducing agent is L-ascorbic acid ό-O-palmitate.

A "reductone" reducing agent means a class of unsaturated, di- or poly-enolic organic compounds which, by virtue of the arrangement of the enolic hydroxy groups with respect to the unsaturated linkages, possess characteristic strong reducing power. The parent compound, "reductone" is 3-hydroxy-2-oxo- propionaldehyde (enol form) and has the structure HOCH=CH(OH)-CHO. Examples of reductone reducing agents can be found in U.S. Patents 2,691,589 (Henn et al), 3,615,440 (Bloom), 3,664,835 (Youngquist et al.), 3,672,896 (Gabrielson et al.), 3,690,872 (Gabrielson et al.), 3,816,137 (Gabrielson et al.), 4,371,603 (Bartels-Keith et al.), 5,712,081 (Andriesen et al.), and 5,427,905 (Freedman et al.).

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

"Hindered phenol reducing agents" are compounds that contain only one hydroxy group on a given phenyl ring and have at least one additional substituent located ortho to the hydroxy group.

One type of hindered phenol reducing agent includes hindered phenols and hindered naphthols.

Another type of hindered phenol reducing agent are hindered bis-phenols. These compounds contain more than one hydroxy group each of which is located on a different phenyl ring. This type of hindered phenol includes, for example, binaphthols (that is dihydroxybinaphthyls), biphenols (that is dihydroxybiphenyls), bis(hydroxynaphthyl)methanes, bis(hydroxyphenyl)

methanes bis(hydroxyphenyl)ethers, bis(hydroxyphenyl)sulfones, and bis(hydroxyphenyl)thioethers, each of which may have additional substituents.

Preferred hindered phenol reducing agents are bis(hydroxy- phenyl)methanes such as, bis(2-hydroxy-3-?-butyl-5-methylphenyl)methane (CAO-5), l,r-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane

(NONOX ^® or PERMANAX WSO), and l,l'-bis(2-hydroxy-3.5-dimethylphenyl)- isobutane (LOWINOX ^® 22IB46). Mixtures of hindered phenol reducing agents can be used if desired.

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

Additional reducing agents that may be used include amidoximes, azines, a combination of aliphatic carboxylic acid aryl hydrazides and ascorbic acid, a reductone and/or a hydrazine, piperidinohexose reductone or formyl- 4-methylphenylhydrazine, hydroxamic acids, a combination of azines and sulfonamidophenols, α-cyanophenylacetic acid derivatives, reductones, indane- 1,3-diones, chromans, 1,4-dihydropyridines, and 3-pyrazolidones. Useful co-developer reducing agents can also be used as described in U.S. Patent 6,387,605 (Lynch et al.). 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. Patents 6,100,022 (Inoue et al.), 5,635,339 (Murray), and 5,545,515 (Murray et al.).

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, hydroxamic acid compounds, N- acylhydrazine compounds, hydrogen donor compounds, hydroxylamines,

alkanolamines and ammonium phthalamate compounds as described in U.S. Patents 5,545,505 (Simpson), 5,545,507 (Simpson et al.), 5,558,983 (Simpson et al.), and 5,637,449 (Harring et al.).

When used with a silver carboxylate silver source in a thermo- graphic material, preferred reducing agents are aromatic di- and tri-hydroxy compounds having at least two hydroxy groups in ortho- or pαrα-rβlationship on the same aromatic nucleus. Examples are hydroquinone and substituted hydroquinones, catechols, pyrogallol, gallic acid and gallic acid esters (for example, methyl gallate, ethyl gallate, propyl gallate), and tannic acid. Particularly preferred are catechol-type reducing agents having no more than two hydroxy groups in an ort/zo-relationship.

One particularly preferred class of catechol-type reducing agents are benzene compounds in which the benzene nucleus is substituted by no more than two hydroxy groups which are present in 2,3-position on the nucleus and have in the 1 -position of the nucleus a substituent linked to the nucleus by means of a carbonyl group. Compounds of this type include 2,3-dihydroxy-benzoic acid, and 2,3-dihydroxy-benzoic acid esters (such as methyl 2,3-dihydroxy-benzoate, and ethyl 2,3-dihydroxy-benzoate).

Another particularly preferred class of catechol-type reducing agents are benzene compounds in which the benzene nucleus is substituted by no more than two hydroxy groups which are present in 3.4-position on the nucleus and have in the 1 -position of the nucleus a substituent linked to the nucleus by means of a carbonyl group. Compounds of this type include, for example, 3,4-dihydroxy-benzoic acid, 3-(3,4-dihydroxy-phenyl)-propionic acid, 3,4-dihydroxy-benzoic acid esters (such as methyl 3,4-dihydroxy-benzoate, and ethyl 3,4-dihydroxy-benzoate), 3,4-dihydroxy-benzaldehyde, 3,4-dihydroxy- benzonitrile, and phenyl-(3,4-dihydroxyphenyl)ketone. Such compounds are described, for example, in U.S. Patent 5,582,953 (Uyttendaele et al.).

Still another useful class of reducing agents includes polyhydroxy spiro-bis-indane compounds described as photographic tanning agents in U.S. Patent 3,440,049 (Moede).

Aromatic di- and tri-hydroxy reducing agents can also be used in combination with hindered phenol reducing agents and further in combination with one or more high contrast co-developing agents and co-developer contrast- enhancing agents). The reducing agent (or mixture thereof) described herein is generally present as 1 to 10% (dry weight) of the emulsion layer. In multilayer constructions, if the reducing agent is added to a layer other than an emulsion layer, slightly higher proportions, of from 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.

Boron Compounds

In general, the boron compounds useful in this invention have at least one >B-O- group. Preferably, they have two or more of such groups or comprise ring structures in which two or three bonds to the boron atom are attached to oxy groups (that is, they contain B-O- bonds).

More particularly, boron compounds useful in the present invention can be represented by Structure (I):

X-B(OL)-Z (I)

wherein, X and Z are independently hydroxy, alkoxy (preferably having 1 to 18 carbon atoms and more preferably having 1 to 10 carbon atoms), alkyl groups (preferably having 1 to 18 carbon atoms and more preferably having 1 to 10 carbon atoms), acyloxy groups (preferably having 2 or more carbon atoms, more preferably having 2 to 18 carbon atoms, and most preferably having 2 to 10 carbon atoms), aryl groups (preferably having 6 to 14 atoms in the aromatic ring), or heteroaryl groups (preferably having 5 to 14 atoms in the aromatic ring),

L is hydrogen, an acyl group having 2 to 18 carbon atoms (preferably having 2 to 10 carbon atoms), an alkyl group having 1 to 18 carbon atoms (preferably having 1 to 10 carbon atoms), an aryl group (preferably having ό to 14 atoms in the aromatic ring), or an heteroaryl group (preferably having 5 to 14 atoms in the aromatic ring),

or X and Z together represent carbon or heteroatoms sufficient to provide a heterocyclic ring with the boron atom, or still again, X ₅ L, and Z together represent carbon or heteroatoms sufficient to provide heterocyclic rings with the boron atom. Still more particularly, compounds useful in the present invention can be represented by Structure (II).

X'-B(OL')~Z' (II)

wherein X ¹ is a hydroxy group, an alkoxy group having 5 or more carbon atoms (preferably having 5 to 18 carbon atoms and more preferably having 5 to 10 carbon atoms), an alkyl group having 5 or more carbon atoms (preferably having 5 to 18 carbon atoms and more preferably having 5 to 10 carbon atoms), an acyloxy group (preferably having 2 or more carbon atoms, more preferably having 2 to 18 carbon atoms, and most preferably having 5 to 10 carbon atoms), an aryl group (preferably having 6 to 14 atoms in the aromatic ring), or an heteroaryl group (preferably having 5 to 14 atoms in the aromatic ring),

Z' is an alkyl group having 5 or more carbon atoms (preferably having 5 to 18 carbon atoms and more preferably having 5 to 10 carbon atoms), an acyloxy group (preferably having 2 or more carbon atoms, more preferably having 2 to 18 carbon atoms, and most preferably having 2 to 10 carbon atoms), an aryl group, (preferably having 6 to 14 atoms in the aromatic ring), or an heteroaryl group (preferably having 5 to 14 atoms in the aromatic ring),

L ¹ is hydrogen, an acyl groups having from 2 to 18 carbon atoms (preferably from 2 to 10 carbon atoms), an alkyl group having 5 or more carbon atoms (preferably having 5 to 18 carbon atoms and preferably having 5 to 10 carbon atoms), an aryl group (preferably having 6 to 14 atoms in the aromatic ring), or an heteroaryl group (preferably having 5 to 14 atoms in the aromatic

X ¹ and Z' together represent carbon or heteroatoms sufficient to provide a heterocyclic ring with the boron atom, or still again, X', L', and Z' together

represent carbon or heteroatoms sufficient to provide heterocyclic rings with the boron atom.

The boron compounds described herein can be prepared by synthetic methods known in the art and as further described below in the section entitled "Synthesis of Boron Compounds." Many of the compounds can be purchased from commercial sources such as Sigma- Aldrich Chemical Co.

The following compounds (B-I) to (B-29) are representative of the boron compounds useful in the present invention:

(B-3) (B-4)

(B-7)

(B-8)

(B-9)

(B-IO)

(B-13) (B-U)

(B-15) (B-16)

(B-17) (B-18)

(B-19) (B-20)

(B-21) (B-22)

(B-23)

(B-24)

(B-25)

(B-26) (B-27)

(B-28) (B-29).

The boron compounds described herein that contain a nitrogen atom can alternatively be depicted with an additional coordination bond between the boron atom(s) and the nitrogen atom(s). For example, compound B-19 above, can be depicted with Structure B- 19a.

(B-19a)

For the purposes of this invention, the two depictions are not meant to limit the description to nitrogen coordinated or un-coordinated to boron. The two depictions should be taken as identical.

The preferred boron compounds include B-I , B-5, B-17, and B-29. B-I, and B-17 are most preferred.

The noted boron compounds can be present in the thermally developable material in an amount of at least 0.001 g/m and preferably from 0.010 to 0.50 g/m ² , and more preferably from 0.016 to 0.40 g/m ² .

Other Addenda

The photothermographic materials can also contain other additives where appropriate, such as toners, additional shelf-life stabilizers, antifoggants, contrast enhancing agents, development accelerators, speed-enhancing agents, acutance dyes, additional post-processing stabilizers or stabilizer precursors, thermal solvents (also known as melt formers), humectants, and other image- modifying agents as would be readily apparent to one skilled in the art.

Toners are compounds that when added to the imaging layer shift the color of the developed silver image from yellowish-orange to brown-black or blue-black, and/or act as development accelerators to speed up thermal development. "Toners" or derivatives thereof that improve the black-and-white image are highly desirable components of the photothermographic materials.

Thus, compounds that either act as toners or react with a reducing agent to provide toners can be present in an amount of 0.01% by weight to 10% (preferably from 0.1% to 10% by weight) based on the total dry weight of the layer in which they are included. The amount can also be defined as being within the range of from 1 x 10 ^"5 to 1.0 mol per mole of non-photosensitive source of reducible silver in the photothermographic material. The toner compounds may be incorporated in one or more of the photothermographic layers as well as in adjacent layers such as the outermost protective layer or underlying "carrier" layer. Toners can be located on both sides of the support if photothermographic layers are present on both sides of the support.

Compounds useful as toners are described in U.S. Patents 3,074,809 (Owen), 3,080,254 (Grant, Jr.), 3,446,648 (Workman), 3,844,797 (Willems et al.), 3,847,612 (Winslow), 3,951,660 (Hagemann et al.), 4,082,901 (Laridon et al.), 4,123,282 (Winslow), 5,599,647 (Defieuw et al.), and 3,832,186 (Masuda et al.), and GB 1 ,439,478 (AGFA).

Particularly useful toners are mercaptotriazoles as described in U.S. Patent 6,713,240 (Lynch et al.), the heterocyclic disulfide compounds described in U.S. Patent 6,737,227 (Lynch et al.), the triazine-thione compounds described in U.S. Patent 6,703,191 (Lynch et al.), and the silver salt-toner co-precipitated nano-crystals described in copending and commonly assigned U.S. Serial No. 10/935,384 (noted above).

Also useful as toners are phthalazine and phthalazine derivatives [such as those described in U.S. Patent 6,146,822 (Asanuma et al.)], phthalazinone, and phthalazinone derivatives as well as phthalazinium compounds [such as those described in U.S. Patent 6,605,418 (Ramsden et al.)].

To further control the propeities of photothermographic materials, (for example, super-sensitization, 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-M ¹ and Ar-S-S-Ar, wherein M ¹ represents a hydrogen atom or an alkali metal atom and Ar represents a heteroaromatic ring or fused heteroaromatic ring containing one or more of

nitrogen, sulfur, oxygen, selenium, or tellurium atoms. Useful heteroaromatic rnercapto compounds are described as supersensitizers in EP 0 559 228 Bl (Philip Jr. et al.).

The photothermographic materials can be further protected against the production of fog and can be stabilized against loss of sensitivity during storage. Suitable antifoggants and stabilizers that can be used alone or in combination include thiazolium salts as described in U.S. Patents 2,131,038 (Brooker et al.) and 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), urazoles as described in U.S. Patent 3,287,135 (Anderson), sulfocatechols as described in U.S. Patent 3,235,652 (Kennard), oximes as described in GB 623,448 (Carrol et al.), polyvalent metal salts as described in U.S. Patent 2,839,405 (Jones), and thiuronium salts as described in U.S. Patent 3,220,839 (Herz). The photothermographic materials may also 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 of this type are polyhalo antifoggants, such as those having a -SO ₂ C(X') ₃ group wherein X' represents the same or different halogen atoms. Compounds having -SO ₂ CBr ₃ groups are particularly preferred. Such compounds are described, for example, in U.S. Patents 5,460,938 (Kirk et al.), 5,594,143 (Kirk et al.), and 5,374,514 (Kirk et al.). Another class of useful antifoggants includes those compounds described in U.S. Patent 6,514,678 (Burgmaier et al.).

Advantageously, the photothermographic materials also include one or more thermal solvents (also called "heat solvents," "thermosolvents," "melt formers," "melt modifiers," "eutectic formers," "development modifiers," "waxes," or "plasticizers").

By the term "thermal solvent" is meant an organic material that becomes a plasticizer or liquid solvent for at least one of the imaging layers upon heating at a temperature above 6O ⁰ C. Representative examples of such compounds include polyethylene glycols having a mean molecular weight in the range of 1 ,500 to 20,000, ethylene carbonate, niacinamide, hydantoin,

5,5-dimethylhydantoin, salicyϊanilide, succinimide, phthalimide, N-potassium- phthalimide, N-hydroxyphthalimide, N-hydroxy-l,8-naphthalimide, phthalazine, l-(2H)-phthalazinone, 2-acetylphthalazinone, benzanilide, urea, 1,3-dimethylurea, 1,3-diethylurea, 1,3-diallylurea, meso-erythήtol, D-sorbitol, tetrahydro- 2-pyrimidone, glycouril, 2-imidazolidone, 2-imidazolidone-4-carboxylic acid, methyl sulfonamide, and benzenesulfonamide. Combinations of these compounds can also be used including, for example, a combination of succinimide and 1,3-dimethylurea. Known thermal solvents are disclosed, for example, in U.S. Patents 3,347,675 (Henn et al.), 3,438,776 (Yudelson), 5,250,386 (Aono et al), 5,368,979 (Freedman et al.), 5,716,772 (Taguchi et al.), and 6,013,420 (Windender), and in Research Disclosure, December 1976, item 15027.

It may be advantageous to include a base-release agent or base precursor in the photothermographic materials. 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 as described in U.S. Patent 4,123,274 (Knight et al.).

Phosphors

In some embodiments, it is also effective to incorporate X-radiation-sensitive phosphors in the photothermographic materials as described in U.S. Patents 6,573,033 (Simpson et al.) and 6,440,649 (Simpson et al.). Other useful phosphors are primarily "activated" phosphors known as phosphate phosphors and borate phosphors. Examples of these phosphors are rare earth phosphates, yttrium phosphates, strontium phosphates, or strontium fluoroborates (including cerium activated rare earth or yttrium phosphates, or europium

activated strontium fluoroborates) as described in U.S. Serial No. 10/826,500 (filed April 16, 2004 by Simpson, Sieber, and Hansen).

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 of total silver in the photothermographic material.

Binders

The photosensitive silver halide (if present), the non-photosensitive source of reducible silver ions, the reducing agent, antifoggant(s), and any other additives used in the present invention are added to and coated in one or more binders using a suitable aqueous or non-aqueous solvent. Thus, aqueous-based formulations can be used to prepare the photothermographic materials. Mixtures of different types of hydrophilic and/or hydrophobic binders can also be used. Preferably, hydrophilic polymer binders and water-dispersible polymeric latexes are used to provide aqueous-based formulations and photothermographic materials. Alternatively, hydrophobic binders can be used to provide non-aqueous based formulations and photothermographic materials.

Examples of useful hydrophilic polymer binders include, but are not limited to, proteins and protein derivatives, gelatin and gelatin derivatives (hardened or unhardened), cellulosic materials, acrylamide/methacrylamide polymers, acrylic/methacrylic polymers, polyvinyl pyrrolidones, polyvinyl alcohols, polyvinyl lactams), polymers of sulfoalkyl acrylate or methacrylates, hydrolyzed polyvinyl acetates, polyamides, polysaccharides, and other naturally occurring or synthetic vehicles commonly known for use in aqueous-based photographic emulsions (see for example Research Disclosure, item 38957, noted above). Minor amounts (less than 50 weight % based on total binder weight) of hydrophobic binders may also be used.

Particularly useful hydrophilic polymer binders are gelatin, gelatin derivatives, polyvinyl alcohols, and cellulosic materials. Gelatin and its derivatives are most preferred, and comprise at least 75 weight % of total binders when a mixture of binders is used.

Aqueous dispersions of water-dispersible polymeric latexes may also be used, alone or with hydrophilic or hydrophobic binders described herein. Such dispersions are described in, for example, U.S. Patents 4,504,575 (Lee), 6,083,680 (Ito et al), 6,100,022 (Inoue et al), 6,132,949 (Fujita et al), 6,132,950 (Ishigaki et al.), 6,140,038 (Ishizuka et al.), 6,150,084 (Ito et al.), 6,312,885 (Fujita et al.), and 6,423,487 (Naoi).

Examples of typical hydrophobic binders include polyvinyl acetals, polyvinyl chloride, polyvinyl acetate, cellulose acetate, cellulose acetate butyrate, polyolefϊns, polyesters, polystyrenes, polyacrylonitrile, polycarbonates, methacrylate copolymers, maleic anhydride ester copolymers, butadiene-styrene copolymers, and other materials readily apparent to one skilled in the art. The polyvinyl acetals (such as polyvinyl butyral and polyvinyl formal), cellulose ester polymers, and vinyl copolymers (such as polyvinyl acetate and polyvinyl chloride) are preferred. Particularly suitable binders are polyvinyl butyral resins that are available under the name BUTVAR ^® from Solutia, Inc. (St. Louis, MO) and

PIOLOFORM ^® from Wacker Chemical Company (Adrian, MI) and cellulose ester polymers.

Hardeners for various binders may be present if desired. Useful hardeners are well known and include diisocyanates as described for example, in EP 0 600 586Bl (Philip, Jr. et al.) and vinyl sulfone compounds as described in U.S. Patent 6,143,487 (Philip, Jr. et al.), and EP 0 640 589Al (Gathmann et al.), aldehydes and various other hardeners as described in U.S. Patent 6,190,822 (Dickerson et al.).

Where the proportions and activities of the photothermo graphic materials require a particular developing time and temperature, the binder(s) should be able to withstand those conditions. Generally, it is preferred that the binder does not decompose or lose its structural integrity at 12O ⁰ 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 binder(s) is used in an amount sufficient to carry the components dispersed therein. 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 on opposing sides of the support in double-sided materials may be the same or different. Support Materials

The photothermographic materials comprise a polymeric support that is preferably a flexible, transparent film that has any desired thickness and is composed of one or more polymeric materials. They are required to exhibit dimensional stability during thermal development and to have suitable adhesive properties with overlying layers. Useful polymeric materials for making such supports include, but are not limited to, polyesters, cellulose acetate and other cellulose esters, polyvinyl acetal, polyolefms, polycarbonates, and polystyrenes. Preferred supports are composed of polymers having good heat stability, such as polyesters and polycarbonates. Polyethylene terephthalate film is a particularly preferred support. Support materials may also be treated or annealed to reduce shrinkage and promote dimensional stability.

It is also useful to use supports comprising dichroic mirror layers as described in U.S. Patent 5,795,708 (Boutet).

Also useful are transparent, multilayer, polymeric supports comprising numerous alternating layers of at least two different polymeric materials as described in U.S. Patent 6,630,283 (Simpson et al.).

Support materials can contain various colorants, pigments, antihalation or acutance dyes if desired. For example, blue-tinted supports are particularly useful for providing images useful for medical diagnosis. 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.

Photothermographic Formulations and Constructions

The imaging components are prepared in a formulation containing a polymer binder (such as gelatin, a gelatin-derivative, a cellulosic material, or

polyvinyl butyral) or a water-dispersible polymer in latex form in an aqueous or non-aqueous solvent (or mixtures thereof) to provide coating formulations. Thus, the photothermographic imaging layers on one or both sides of the support are prepared and coated out of such formulations. The photothermographic materials can contain plasticizers and lubricants such as poly(alcohols) and diols as described in U.S. Patent 2,960,404 (Milton et al.), fatty acids or esters as described in U.S. Patents 2,588,765 (Robijns) and 3,121,060 (Duane), and silicone resins as described in GB 955,061 (DuPont). The materials can also contain inorganic or organic matting agents as described in U.S. Patents 2,992,101 (Jelley et al.) and 2,701,245 (Lynn).

Polymeric fluorinated surfactants may also be useful in one or more layers 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.

The photothermographic materials can include one or more antistatic agents in any of the layers on either or both sides of the support. Conductive components include soluble salts, evaporated metal layers, or ionic polymers as described in U.S. Patents 2,861,056 (Minsk) and 3,206,312 (Sterman et al.), insoluble inorganic salts as described in U.S. Patent 3,428,451 (Trevoy), polythiophenes as described in U.S. Patent 5,747,412 (Leenders et al.) ₃ electro- conductive underlayers as described in U.S. Patent 5,310,640 (Markin et al.), electronically-conductive metal antimonate particles as described in U.S. Patent 5,368,995 (Christian et al.), and electrically-conductive metal-containing particles dispersed in a polymeric binder as described in EP 0 678 776 Al (Melpolder et al.). Particularly useful conductive particles are the non-acicular metal antimonate particles described in U.S. Patent 6,689,546 (LaBeIIe et al.) ₅ and in copending and commonly assigned U.S. Serial Numbers 10/930,428 (filed August 31, 2004 by Ludemann, LaBeIIe, Koestner, Hefley, Bhave, Geisler, and Philip), 10/930,438 (filed August 31, 2004 by Ludemann, LaBeIIe, Philip, Koestener, and Bhave), and 10/978,205 (filed October 29, 2004 by Ludemann, LaBeIIe, Koestner, and Chen).

In addition, fluorochemicals such as Fhiorad ^® FC- 135 (3M Corporation), ZONYL ^® FSN (E. I. DuPont de Nemours & Co.). ^{as wel} l as those described in U.S. Patents 5,674,671 (Brandon et al.), 6,287,754 (Melpolder et al.), 4,975,363 (Cavallo et al.), 6,171,707 (Gomez et al.), 6,699,648 (Sakizadeh et al.), and 6,762,013 (Sakizadeh et al.) can be used.

The photothermographic materials can have a protective overcoat layer (or outermost overcoat layer) disposed over the one or more imaging layers on one or both sides of the support. The binders for such overcoat layers can be any of the binders described in the Binders Section. Preferably, the protective layers include gelatin or a gelatin derivative as the predominant binder(s) especially when the one or more imaging layers also include gelatin or a gelatin derivative as the predominant binder(s), or the protective layers include a cellulosic ester (such as cellulose acetate butyrate) when the predominant binder is polyvinylbutyral . For double-sided photothermographic materials, each side of the support can include one or more of the same or different imaging layers, interlayers, and protective overcoat layers. In such materials preferably a overcoat is present as the outermost layer on both sides of the support. The photoihermo- graphic layers on opposite sides can have the same or different construction and can be overcoated with the same or different protective layers.

Layers to promote adhesion of one layer to another are also known, as described in U.S. Patents 5,891,610 (Bauer et al.), 5,804,365 (Bauer et al.), and 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. Patents 6,352,819 (Kenney et al.), 6,352,820 (Bauer et al.), 6,420,102 (Bauer et al.), 6,667,148 (Rao et al.), and U.S. Patent 6,746,831 (Hunt). The formulations described herein (including the thermographic and photothermographic formulations) can be coated by various coating

procedures including wire wound rod coating, dip coating, air knife coating, curtain coating, slide coating, or extrusion coating using hoppers of the type described in U.S. 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. Patents 2,761,791 (Russell), 4,001,024 (Dittman et al.), 4,569,863 (Keopke et al.), 5,340,613 (Hanzalik et al.), 5,405,740 (LaBeIIe), 5,415,993 (Hanzalik et al.), 5,525,376 (Leonard), 5,733,608 (Kessel et al.), 5,849,363 (Yapel et al.), 5,843,530 (Jerry et al.), and 5,861,195 (Bhave et al.), and GB 837,095 (Ilford). Atypical coating gap for the emulsion layer can be from 10 to 750 μm, and the layer can be dried in forced air at a temperature of from 20°C to 100 ⁰ 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.

Simultaneously with or subsequently to application of an emulsion formulation to the support, a protective overcoat formulation can be applied over the emulsion formulation.

Preferably, two or more layer formulations are applied simultaneously to a film support using slide coating techniques, the overcoat layer being coated on top of the photothermographic emulsion layer while the photo- thermographic emulsion layer is still wet.

In other embodiments, a "carrier" layer formulation comprising a single-phase mixture of the two or more polymers may be applied directly onto the support and thereby located underneath the emulsion layer(s) as described in U.S. Patent 6,355,405 (Ludemann et al.). The carrier layer formulation can be applied simultaneously with application of the photothermographic emulsion layer formulation and any overcoat formulations.

Mottle and other surface anomalies can be reduced in the materials by incorporation of a fluorinated polymer as described in U.S. Patent 5,532,121 (Yonkoski et al.) or by using particular drying techniques as described in U.S. Patent 5,621 ,983 (Ludemann et al.).

While the overcoat and photothermographic layers can be coated on one side of the film support, manufacturing methods can also include forming on the opposing or backside of the polymeric support, one or more additional layers, including 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.

To promote image sharpness, photothermographic materials can contain one or more layers containing acutance and/or antihalation dyes that are chosen to have absorption close to the exposure wavelength and are designed to absorb scattered light. One or more antihalation compositions may be incorporated into one or more antihalation backing layers, antihalation underlayers, or as antihalation overcoats.

Dyes useful as antihalation and acutance dyes include squaraine dyes described in U.S. Patents 5,380,635 (Gomez et al.) and 6,063,560 (Suzuki et al.), and EP 1 083 459Al (Kimura), indolenine dyes described in EP 0 342 810Al (Leichter), and cyanine dyes described in U.S. Patent 6,689,547 (Hunt et al.).

It may also be useful to employ compositions including acutance or antihalation dyes that will decolorize or bleach with heat during processing, as described in U.S. Patents 5,135,842 (Kitchin et al.), 5,266,452 (Kitchin et al.), 5,314,795 (Helland et al.) _s and 6,306,566, (Sakurada et al.), and Japanese Kokai 2001-142175 (Hanyu et al.) and 2001-183770 (Hanye et al.). Useful bleaching compositions are also described in Japanese Kokai 11-302550 (Fujiwara), 2001-109101 (Adachi), 2001-51371 (Yabuki et al.), and 2000-029168 (Noro). Other useful heat-bleachable backside antihalation compositions can include an infrared radiation absorbing compound such as an oxonol dye or other compounds used in combination with a hexaarylbiimidazole (also known as a "HABI"), or mixtures thereof. HABI compounds are described in U.S. Patents 4,196,002 (Levinson et al.), 5,652,091 (Perry et al.), and 5,672,562 (Perry et al.). Examples of such heat-bleachable compositions are described for example in U.S. Patents 6,455,210 (Irving et al.), 6,514,677 (Ramsden et al.), and 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, at a temperature of from 100°C to 200°C for from 5 to 20 seconds).

Imaging/Development The photothermographic materials can be imaged in any suitable manner consistent with the type of material, using any suitable imaging source (typically some type of radiation or electronic signal for photothermographic materials and a source of thermal energy for thermographic materials). In some embodiments, the materials are sensitive to radiation in the range of from at least 100 run to 1400 nm, and normally from 300 nm to 750 nm (preferably from 300 to 600 nm, more preferably from 300 to 450 nm, even more preferably from a wavelength of from 360 to 420 nm, and most preferably from 380 to 420 nm), using appropriate spectral sensitizing dyes.

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

In preferred embodiments, the photothermographic materials can be indirectly imaged using an X-radiation imaging source and one or more prompt-emitting or storage X-ray sensitive phosphor screens adjacent to the photothermographic material. The phosphors emit suitable radiation to expose the

photothermographic material. Preferred X-ray screens are those having phosphors emitting in the near ultraviolet region of the spectrum (from 300 to 400 run), in the blue region of the spectrum (from 400 to 500 nm), and in the green region of the spectrum (from 500 to 600 nm). In other embodiments, the photothermographic materials can be imaged directly using an X-radiation imaging source to provide a latent image.

Thermal development conditions will vary, depending on the construction used but will typically involve heating the imagewise exposed photothermographic material at a suitably elevated temperature, for example, from 50°C to 250°C (preferably from 80°C to 2OQ ⁰ C and more preferably from 100 ⁰ C to 200 ⁰ C) for a sufficient period of time, generally from 1 to 120 seconds. Heating can be accomplished using any suitable heating means. A preferred heat development procedure for photothermographic materials includes heating at from 130 ⁰ C to 170 ⁰ C for from 3 to 25 seconds. A particularly preferred development procedure is heating at 150 ⁰ C for 15 to 25 seconds.

When imaging thermographic materials, the image may be "written" simultaneously with development at a suitable temperature using a thermal stylus, a thermal print-head or a laser, or by heating while in contact with a heat-absorbing material. The thermographic materials may include a dye (such as an ΪR-absorbing dye) to facilitate direct development by exposure to laser radiation.

Use as a Photomask

In some embodiments, the photothermographic materials are sufficiently transmissive in the range of from 350 to 450 nm in non-imaged areas to allow their use in a method where there is a subsequent exposure of an ultraviolet or short wavelength visible radiation sensitive imageable medium. The heat-developed materials absorb ultraviolet or short wavelength visible radiation in the areas where there is a visible image and transmit ultraviolet or short wavelength visible radiation where there is no visible image. The 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

iniageable material that is sensitive to such imaging radiation, such as a photopolymer, diazo material, photoresist, or photosensitive printing plate.

Thus, in some other embodiments wherein the thermographic or photothermographic material comprises a transparent support, the image-forming method further comprises, after step (A') or steps (A) and (B) noted above:

These embodiments of the imaging method of this invention are carried out using step (A') or steps (A) and (B) noted above with the following Steps (C) and (D):

(C) positioning the exposed and photothermographic material with the visible image therein 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 photothermographic material to provide an image in the imageable material. Imaging Assemblies

In some embodiments, the photothermographic materials are used or arranged in association with one or more phosphor intensifying screens and/or metal screens in what is known as "imaging assemblies." Double-sided visible light sensitive photothermographic materials are preferably used in combination with two adjacent 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, and emulsion speeds. The imaging assemblies can be prepared by arranging the photothermographic material and one or more phosphor intensifying screens in a suitable holder (often known as a cassette), and appropriately packaging them for transport and imaging uses.

There are a wide variety of phosphors known in the art that can be formulated into phosphor intensifying screens as described in hundreds of publications. U.S. Patent 6,573,033 (noted above) describes phosphors that can be used in this manner. Particularly useful phosphors are those that emit radiation

having a wavelength of from 300 to 450 nm and preferably radiation having a wavelength of from 360 to 420 nm.

Preferred phosphors useful in the phosphor intensifying screens include one or more alkaline earth fluorohalide phosphors and especially the rare earth activated (doped) alkaline earth fluorohalide phosphors. Particularly useful phosphor intensifying screens include a europium-doped barium fluorobromide (BaFBr ₂ :Eu) phosphor. Other useful phosphors are described in U.S. Patent 6,682,868 (Dickerson et al.) and references cited therein.

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 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. All percentages are by weight unless otherwise indicated. The following additional materials were prepared and used.

BYK-022 is a defoamer and is available from Byk-Chemie Corp. (Wallingford, CT).

BZT is benzotriazole. AgBZT is silver benzotriazole. NaBZT is the sodium salt of benzotriazole.

CELVOL ^® V203S is a polyvinyl alcohol and is available from Celanese Corp. (Dallas, TX).

SPP 3000 is a partially hydrolyzed (88%), low molecular weight (3000 Mw) poly( vinyl alcohol) available from Scientific Polymer Products. (Ontario, NY).

TRITON ^® X-114 is a nonionic surfactant and is available from Dow Chemical Coip. (Midland MI).

TRITON ^® X-200 is an anionic surfactant that is available from Dow Chemical Corp. (Midland MI).

ZONYL ^® FSN is a nonionic fluorosurfactant that is available from E. I. DuPont de Nemours & Co. (Wilmington, DE). It is a fluorinated poly- ethyleneoxide alcohol.

ZONYL ^® FS 300 is a nonionic fluorosurfactant that is available from E. I. DuPont de Nemours & Co. (Wilmington, DE).

Compounds A-I and A-2 are described in U.S. Patent 6,605,418 (noted above) and are believed to have the following structures:

(A-I)

(A-2).

Bisvinyl sulfonyl methane (VS-I) is l,l '(niethylenebis(sulfonyl))- bis-ethene and is described in EP 0 640 589 Al (Gathmann et al.). It is believed to have the following structure:

^^ SO ₂ - CH ₂ - SO ₂ .^>

(VS-I).

Blue spectral sensitizing dye SSD-I is believed to have the following structure:

(SSD-I).

Chemical sensitizer Compound SS-Ia is described in U.S. Patent 6,296,998 (Eikenberry et al.) and is believed to have the structure following structure:

(SS-Ia).

Compound T-I is 2,4-dihydro-4-(phenylmethyl)-3H-l ₅ 2 _! 4-triazole- 3-thione. It is believed to have the structure shown below. It may also exist, as the thione tautomer. The silver salt of this compound is referred to as AgT-I. The sodium salt of this compound is referred to as NaT-I .

(T-I)

GoId(III) chemical sensitizer Compound GS-I is believed to have the following structure.

(GS-I)

Developer D-I is L-ascorbic acid 6-0-palmitate and is available from Aceto Corp. (Lake Success, NY). It is believed to have the structure shown below

(D-I)

Synthesis of Boron Compounds:

Trialkyl borates can be prepared by the reaction of boric acid with an alcohol, under conditions that promote the removal of the formed water. (See for example, H. Steinberg and D. L. Hunter Industrial and Engineering Chemistry, 1957, 49, 174-181.) The reaction of an alcohol with a boronic acid gives the corresponding ester under analogous conditions. Reaction of a diol with a boronic acid gives products such as B-3, B-4, B-5, and B-7.

Preparation of Compounds B-8 and B-9:

To a solution of 0.2 g of potassium hydroxide in 100 ml of ethanol under a nitrogen atmosphere, was added 22.75 g of paraformaldehyde and the mixture stirred for 30 minutes until all the materials dissolved. Cooling to — 8°C, was followed by dropwise addition of 79.64 g of diethanolamine in 50 ml of ethanol over 25 minutes. A solution of 83.43 g of resorcinol in 110 ml of ethanol was then added over 50 minutes, followed by addition of 10 ml of ethanol and 79.64 g of trimethyl borate. After warming to room temperature for 2.5 hours, the mixture was slowly heated to reflux for 2 hours. Upon cooling and standing overnight at room temperature, the crude product was removed by filtration and washed with isopropanol. Multiple fractional recrystallizations from isopropanol/water gave two main products, 32.90 g of Compound B-8, mp > 285°C, and 5.66 g of Compound B-9, mp 268-275°C. NMR and MS data were in accord with the assigned structures. Preparation of Compound B-IO:

To a mixture of 10.43 g of disopropanolamine, 9.57 g of 2,4-dimethylphenol, and 100 ml of ethanol was added 9.53 g of 37% formalin solution and 4.47 g of potassium hydroxide. The mixture was then heated at reflux under nitrogen for 43 hours. Hydrochloric acid (3%) was added to achieve a pH 1 , the mixture washed 3 times with ethyl acetate, and the acid layer neutralized to pH 7.5 with sodium hydroxide. The solution was extracted 3 times into ethyl acetate, dried over sodium sulfate, filtered and concentrated in vacuo to give 6.23 g of a yellow oil. This material was dissolved in 200 ml of toluene and 1.33 g of boric acid was added. The mixture was heated to reflux under nitrogen under a Dean-Stark trap for 9.5 hours, the solution was decanted from a gummy residue, concentrated in vacuo, and recrystallized from isopropanol/heptane to give 2.92 g of Compound B-10, mp 153 - 159°C. NMR and MS data were in accord with the assigned structure.

Preparation of Compound B-Il :

A mixture of 16.2 g of 2,4-dimethylphenol, 100 ml of ethanol, 13.92 g of diβthanolamine and 10.7 g of 37 % formalin solution was heated at reflux for 20 hours. Paraformaldehyde (1.98 g) was then added. Heating for an additional 3 hours was followed by addition of 7 ml of diethanolamine. Heating was continued for 2 hours. Upon cooling, the mixture was concentrated in vacuo, and 12.7 g of boric acid and 300 ml of toluene added. The mixture was heated at reflux under a Dean-Stark trap for 6 hours. Upon cooling the filtrate was separated from the solids and retained. The solids were boiled with 150 ml of toluene, filtered while hot, and the filtrate cooled and the crude product collected by filtration. The retained filtrate was concentrated in vacuo to give additional crude product. The combined products were recrystallized from toluene to give Compound B- 11 , mp 181-184°C. NMR and MS data were in accord with the assigned structure. Preparation of Compounds B-13 and B-15:

A mixture of 40 ml of ethanol, 8.49 g of paraformaldehyde, and 68 mg of potassium hydroxide was stirred at room temperature until dissolved. The solution was cooled in ice water, and 29.75 g of diethanolamine in 20 ml of ethanol were added dropwise over 20 min. A solution of 24.84 g of 4-hexyl- resorcinol in 35 ml of ethanol was then added dropwise over 30 min. Stirring for an additional 30 minutes was followed by addition of 31.2 g of trimethyl borate over 5 min. After warming to room temperature for 1 hour, and heating at reflux for 3.5 hours, the mixture was cooled in ice for 2.5 hours. Filtration afforded 3.66 g of Compound B- 15 as a white powder, mp 262 -263 ⁰ C (dec). NMR and MS data were in accord with the assigned structure.

Trimethyl borate (4 ml) was added to the filtrate and the reaction mixture was heated at reflux for 4 hours. Upon cooling, and concentration by solvent removal in vacuo, 200 ml of ethyl acetate was added. The mixture was heated, cooled, and filtered to give crude Compound B-13. Recrystallization from

ethyl acetate/methanol gave 15.29 g of Compound B-13, mp 199 - 202°C. NMR and MS data were in accord with the assigned structure.

Preparation of compound B- 14:

Amixture of 13.83 g of 2,4-di-fcrt-amylphenol, 38 ml of ethanol, 3.31 g of potassium hydroxide, 6.20 g of diethanolamine, and 1.80 g of paraformaldehyde were heated at reflux under nitrogen for 1 hour. Ethanol (100 ml) was added and the mixture heated at reflux for an additional 2.5 hours. Upon cooling, the solvent was removed in vacuo, 130 ml of IN hydrochloric acid were added, the mixture extracted 3 times with ethyl acetate, dried over sodium sulfate, filtered, and concentrated in vacuo to give 21.0 g of material. Addition of 500 ml of toluene and 3.67 g of boric acid was followed by heating at reflux under a Dean-Stark trap under nitrogen for 2.5 hours. The reaction mixture was filtered hot. Compound B- 14 crystallized upon cooling, mp 211-214 ⁰ C. NMR and MS data were in accord with the assigned structure. Preparation of Compound B- 16:

Amixture of 30 ml of 37 % formalin and 50 ml of methanol was cooled to 4°C, 42.0 g of diethanolamine in 50 ml of methanol was added, followed by 22.0 g of hydroquinone. After 22 hours at room temperature, 41.6 g of trimethyl borate was added and the mixture heated at reflux for 4.5 hours. The solid was filtered off and recrystallized twice from water to give 10.35 g of Compound B-16, mp > 275°C. NMR and MS data were in accord with the assigned structure.

Preparation of Compound B-18:

Amixture of 10.52 g of 2,4-dihydroxybenzophenone, 110 ml of ethanol, 11.36 g of diethanolamine, and 3.24 g of 37 % formalin were heated at reflux for 4.5 hours. Addition of 11.2 g of trimethyl borate was followed by stirring at room temperature for 18 hours, and then by heating at reflux for 5 hours. After concentration by solvent removal in vacuo, the residue was crystallized from isopropanol. Two recrystallizations from ethanol gave 4.22 g of Compound B-18. NMR and MS data were in accord with the assigned structure.

Example 1: Preparation of Aqueous-Based Phototfaermographic Material:

An aqueous-based photothermographic material of this invention was prepared in the following manner.

Preparation of Developer D-I Dispersion: A solid particle dispersion of Developer D-I was prepared by combining 20 weight % L-ascorbic acid 6-O-palmitate, 2 weight% CELVOL™ V203S polyvinyl alcohol), 0.6 weight % TRITON ^® X-114 surfactant, 0.02 weight % of BYK-022, and 77.38 weight % of high purity water. The mixture was milled with 0.7 mm zirconium silicate ceramic beads until the dispersion achieved a median particle size of approximately 0.45 μm (micrometers) as measured by light scattering. This required 7 hours. Examination of the final dispersion by transmitted light microscopy at IOOOX magnification showed well-dispersed particles, all below 1 μm.

Preparation of Compound VS- 1 Dispersion: A solid particle dispersion of Compound VS-I was prepared by combining 20 weight % VS-1, 2.4 weight % SPP 3000 polyvinyl alcohol) and 77.6 weight % of deionized water. The mixture was milled with 0.7 mm zirconium silicate ceramic beads for 90 minutes at 2000 rpm using a micro media mill as described in U.S. Patent 5,593,097 (Corbin). Preparation of Water Insoluble Boron Compound Dispersion:

A solid particle dispersion of Compound B-6 was prepared by combining 10 weight % Compound B-6, 0.68 weight % TRITON ^® X-200 surfactant and 89.32 weight % deionized water. The mixture was milled with 0.7 mm zirconium silicate ceramic beads for 90 minutes at 2000 rpm using a micro media mill as described in U.S. Patent 5,593,097 (noted above).

This procedure was also used to mill other water-insoluble boron compounds.

Preparation of Silver Benzotriazole (AgBZT) Dispersion:

Aqueous gelatin dispersions of silver benzotriazole (AgBZT) emulsion was prepared as follows:

A stirred reaction vessel was charged with 900 g of lime-processed gelatin and 6000 g of deionized water. Solution A containing 216 g/kg of benzotriazole, 710 g/kg of deionized water, and 74 g/kg of sodium hydroxide was prepared. The mixture in the reaction vessel was adjusted to a pH of 9.0 with 2.5N sodium hydroxide solution. The pAg of the reaction vessel was adjusted by addition 0.7Og of Solution A. The temperature of the reaction vessel was maintained at approximately 50°C.

Solution B containing 362 g/kg of silver nitrate and 638 g/kg of deionized water was also prepared. Solutions A and B were then added to the reaction vessel by controlled double-jet addition at a constant silver nitrate flow rate of 25 ml/min for 20 minutes while maintaining constant pAg and pH in the reaction vessel. The flow rate of Solution B was then accelerated over 41 minutes to 40 ml/min, while maintaining constant pAg and pH throughout the process. The flow rate of Solution B was further accelerated over 30.5 minutes to 80 ml/min during the last stage of the process while maintaining constant pAg and pH in the reaction vessel.

The resulting silver benzotriazole dispersions were washed by conventional ultrafiltration process as described in Research Disclosure, Vol. 131, March 1975, Item 13122. The pH of the dispersions was adjusted to 6.0 using 2.0N sulfuric acid. Upon cooling, the dispersions solidified.

Preparation of Ultra-Thin Tabular Grain Silver Halide Emulsion:

A reaction vessel equipped with a stirrer was charged with 6 liters of water containing 2.1 g of deionized oxidized-methionine lime-processed bone gelatin, 3.49 g of sodium bromide, and an antifoamant (at pH = 5.8). The solution was held at 39°C for 5 minutes. Simultaneous additions were then made of 50.6 ml of 0.3 molar silver nitrate and 33.2 ml of 0.448 molar sodium bromide over 1 minute. Following nucleation, 3.0 ml of a 0.1 M solution of sulfuric acid was added. After 1 minute 15.62 g sodium chloride plus 375 mg of sodium thiocyanate were added and the temperature was increased to 54°C over 9 minutes. After a 5-minute hold, 79.6 g of deionized oxidized-methionine lime-processed bone gelatin in 1.52 liters of water containing additional

antifoamant at 54 ⁰ C were then added to the reactor. The reactor temperature was held for 7 minutes (pH = 5.6).

During the next 36.8 minutes, the first growth stage took place (at 54°C), in three segments, wherein solutions of 0.3 molar AgNO ₃ , 0.448 molar sodium bromide, and a 0.16 molar suspension of silver iodide (Lippmann) were added to maintain a nominal uniform iodide level of 3.2 mole %. The flow rates during this growth stage were increased from 9 to 42 ml/min (silver nitrate) and from 0.73 to 3.3 ml/min (silver iodide). The flow rates of the sodium bromide were allowed to fluctuate as needed to affect a monotonic pBr shift of 2.45 to 2.12 over the first 12 minutes, of 2.12 to 1.90 over the second 12 minutes, and of 1.90 to 1.67 over the last 12.8 minutes. This was followed by a 1.5 minute hold.

During the next 59 minutes the second growth stage took place (at 54°C) during which solutions of 2.8 molar silver nitrate, and 3.0 molar sodium bromide, and a 0.16 molar suspension of silver iodide (Lippmann) were added to maintain a nominal iodide level of 3.2 mole %. The flow rates during this segment were increased from 10 to 39.6 ml/min (silver nitrate) and from 5.3 to 22.6 ml/min (silver iodide). The flow rates of the sodium bromide were allowed to fluctuate as needed to affect a monotonic pBr shift of 1.67 to 1.50. This was followed by a 1.5 minute hold. During the next 34.95 minutes, the third growth stage took place during which solutions of 2.8 molar silver nitrate, 3.0 molar sodium bromide, and a 0.16 molar suspension of silver iodide (Lippmann) were added to maintain a nominal iodide level of 3.2 mole %. The flow rates during this segment were 39.6 ml/min (silver nitrate) and 22.6 ml/min (silver iodide). The temperature was linearly decreased to 35°C during this segment. At the 23 ^rd minute of this segment a 50 ml aqueous solution containing 0.85 mg of an indium dopant (K ₂ [Ir(5-Br-thiazole)Cl ₅ ]) was added. The flow rate of the sodium bromide was allowed to fluctuate to maintain a constant pBr of 1.50.

Atotal of 8.5 moles of silver iodobromide (3.2% bulk iodide) were formed. The resulting emulsion was washed using ultrafiltration. Deionized lime-

processed bone gelatin (326.9 g) was added along with a biocide and pH and pBr were adjusted to 6 and 2.5 respectively.

The resulting emulsion was examined by Scanning Electron Microscopy. Tabular grains accounted for greater than 99% of the total projected area. The mean ECD of the grains was 2.522 μm. The mean tabular thickness was 0.049 μm.

This emulsion was spectrally sensitized with 3.31 mmol of blue spectral sensitizing dye SSD-I per mole of silver halide. This dye quantity was split 80%/20% with the majority being added before chemical sensitization and the remainder afterwards. Chemical sensitization was carried out using

0.0085 mmol of sulfur chemical sensitizer (Compound SS-Ia) and 0.00079 mmol per mole of silver halide of gold(III) chemical sensitizer (Compound GS-I) at 6O ⁰ C for 6.3 minutes.

Preparation of Photothermographic Materials: Component A: Silver benzotriazole (AgBZT) and gelatin (35% gelatin/65% water) were placed in a beaker and heated to 50°C for 15 minutes to melt the material. A 5% aqueous solution of 3-methylbenzothiazolium iodide was added. Mixing for 15 minutes was followed by cooling to 40°C. The sodium salt of benzotriazole was added and the mixture was stirred for 15 minutes. Compound NaT-I was then added. Mixing for 15 minutes was followed by addition of 2.5 N sulfuric acid to adjust the pH to 5.5. ZONYL ^® FSN surfactant and either Compound A-I or Compound A-2 were then added in the amounts shown in TABLE I.

Component B: A portion of the tabular-grain silver halide emulsion prepared above was placed in a beaker and melted at 4O ⁰ C.

Component C: Succinimide, dimethyl urea, pentaerythritol, and Compounds A-2 and VS-I were added as dry materials to water and heated to 50 ⁰ C. Water soluble boron compounds were dissolved at this stage in the warm aqueous solution. The dispersed developer D-I was added to the above solution.

Component D: A dispersion of water insoluble boron compounds was prepared as described above and added at this point.

Coating of Photothermo graphic Materials:

Components A, B, C, and D were mixed immediately before coating to form a photothermographic emulsion formulation. Each formulation was coated as a single layer on a 7 mil (178 μm) transparent, blue-tinted poly(ethylene terephthalate) film support using a conventional automated knife coating machine. The coating gap was adjusted to achieve the dry coating weights shown below in TABLE I. Samples were dried at 120°F (48.9 ⁰ C) for 8 minutes. Comparative Sample 1-1 -C contained no boron compound. It served as a control. Inventive Samples 1-2-1 and 1-3-1 had compound B-6 or compound B-4 added respectively, as dispersions. The amount of active boron compounds B-6 and B-4 added is shown below in TABLE II.

TABLE I - Photothermographic Emulsion Dry Composition

Compomen t Material Dry Coating

Weight [g/m ² ]

A Silver (from AgBZT) 1.56

A Lime processed gelatin 1.58

A 3-Methylbenzothiazolium Iodide 0.078

A Sodium benzotriazole (NaBZT) 0.092

A Compound NaT-I 0.087

A ZONYL ^® FSN surfactant 0.046

B Silver (from AgBrI emulsion) 0.24

C Succinimide 0.13

C 1 ,3-Dimethylurea 0.45

C Pentaerythritol 0.58

C Phthalazine Compound A-2 0.071

C Compound VS-I 0.065

C Dispersed Developer D-I 4.38

D Dispersed Boron Compound See TABLE II

Evaluation of Photothermographic Materials:

The resulting photothermographic films were imagewise exposed for 10 ^"2 seconds using an EG&G flash sensitometer equipped with a P- 16 filter and a 0.7 neutral density filter. Following exposure, the films were developed by heating on a flat-bed processor for 18 seconds at 150°C to generate continuous tone wedges.

Densitometry measurements were made on a custom built computer-scanned densitometer and meeting ISO Standards 5-2 and 5-3 and are believed to be comparable to measurements from commercially available densitometers. Densities of the wedges were then measured with a computer

densitometer 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). Dmin is the density of the non-exposed areas after development and it is the average of the eight lowest density values. These samples provided initial Dmin, Dmax, and "Relative

Speed-2." Relative Speed-2 was determined at a density value of 1.00 above Dmin. Speed values were normalized. Sample 1-1 -C, which contained no additive, was assigned a relative speed value of 100.

Natural Age Keeping: Non-imaged samples were stored in a black polyethylene bag for

2 months at ambient room temperature and relative humidity to determine their Natural Age Keeping properties. The samples were then imaged and compared with the freshly imaged samples.

TABLE II

Sample Invention (I) or Boron Amownt of

Comparison (C) Compound Boron Compoωiαd

1-1-C C None None

1-2-1 I B-6 0.22

1-3-1 I B-4 0.29 The results, shown below in TABLE III, demonstrate that after

2 months of storage, photothermographic materials incorporating dispersed boron compounds B-6 and B-4 exhibited improved Natural Age Keeping. This is shown by their reduced increase in Dmin (ΔDmin) when compared with a control sample containing no boron compound. Incorporation of boron compounds appears to have little if any effect on Dmax.

TABLE HI

Sample Relative Initial NAK Initial NAK Speed-2 Dmin Dmax

2 Month 2 Month 2 Month 1 I Month Dmin ΔDmin Dmax ΔDmax

1-1-C 100 0.344 0.367 +0.023 3.25 2.70 -0.55 1-2-1 83 0.339 0.328 -0.011 3.10 2.60 -0.50 1-3-1 130 0.352 0.361 +0.009 3.35 2.81 -0.54

Example 2: Evaluation of Additional Water Insoluble Boron Compounds:

Preparation of Photothermographic Materials:

Components A, B and D: Components A ₃ B and D were prepared by the procedure in Example 1.

Components C: Component C was prepared as described in Example 1 except for the substitution of Phthalazine Compound A-I for A-2.

Coating and Evaluation of Photothermographic Materials:

The components were formulated, coated, and dried as described in Example 1. The coating gap was adjusted to achieve the dry coating weights shown below in TABLE IV. Comparative Sample 2-1 -C contained no boron compound. It served as a control. Inventive Samples 2-2-1 to 2-7-1 had a dispersed boron compound at the dry weight shown below in TABLE V.

TABLE rV — Photothermographic Emulsion Dry Composition

Component Material Dry Coating

Weight [g/m ² ]

A Silver (from AgBZT) 1.43

A Lime processed gelatin 1.45

A 3-Methylbenzothiazolium Iodide 0.072

A Sodium benzotriazole (NaBZT) 0.085

A Compound NaT-I 0.080

A ZONYL ^® FSN surfactant 0.042

B Silver (from AgBrI emulsion) 0.22

C Succinimide 0.12

C 1,3-Dimethylurea 0.42

C Pentaerythritol 0.54

C Phthalazine Compound A-I 0.059

C Compound VS-I 0.059

C Dispersed Developer D-I 4.01

D Dispersed Boron Compound See TABLE V

TABLE V

Sample Invention (I) or Borom Amount of Comparison (C) Compound Boron Compound [g/m ² ]

2-1 -C C None None

2-2-1 I B-9 0.26

2-3-1 I B-2 0.39

2-4-1 I B-5 0.24

2-5-1 I B-3 0.22

2-6-1 I B-28 0.29

2-7-1 I B-27 0.25

The coated materials were imaged, developed, and evaluated as described in Example 1. The results, shown below in TABLE VI, demonstrate that after 2 months of storage, photothermographic materials incorporating boron compounds exhibited improved Natural Age Keeping. This is shown by their reduced increase Dmin (ΔDmin) when compared with a control sample containing no boron compound. Incorporation of boron compounds appears to have little if any effect on Dmax.

TABLE VI

Sample Relative Initial NAK Initial NAK Speed-2 Dmin Dmax

2 Month 2 Month 2 Month 2 Month Dmin ΔDmiu Dmax ΔDmax

2-1 -C 100 0.292 0.321 +0.029 2.68 2.40 -0.28

2-2-1 107 0.289 0.301 +0.012 2.84 2.37 -0.47

ON 2-3-1 105 0.306 0.301 -0.005 2.67 2.25 -0.32

2-4-1 110 0.298 0.310 +0.012 2.65 2.35 -0.30

2-5-1 110 0.298 0.311 +0.013 2.67 2.35 -0.32

2-6-1 110 0.295 0.314 +0.019 2.71 2.42 -0.29

2-7-1 95 0.291 0.307 +0.016 2.55 2.30 -0.25

Example 3: Evaluation of Water Soluble Boron Compounds:

Preparation of Photo thermographic Materials:

Components A and B: Components A and B were prepared by the procedures in Example 1.

Component C: Component C was prepared as described in Example 1 except for the substitution of Phthalazine Compound A-I for A-2.

Coating and Evaluation of Photothermo graphic Materials:

Components A, B, and C were formulated and coated as described in Example 1. The coating gap was adjusted to achieve the dry coating weights shown below in TABLE VII. Comparative Sample 3-1 -C contained no boron compound. It served as a control. Inventive Samples 3-2-1 to 3-5-1 contained a water soluble boron compound or a combination of two water soluble boron compounds at the dry weights shown below in TABLE VIII.

TABLE VII - Photothermographic Emulsion Dry Composition

Component Materials Dry Coating

Weight [g/m ² ]

A Silver (from AgBZT) 1.56

A Lime processed gelatin 0.99

A 3-Methylbenzothiazolium Iodide 0.078

A Sodium benzotriazole (NaBZT) 0.092

A Compound NaT-I 0.087

A ZONYL ^® FSN surfactant 0.046

B Silver (from AgBrI emulsion) 0.24

C Succinimide 0.13

C 1,3-Dimethylurea 0.16

C Pentaerythritol 0.58

C Phthalazine Compound A-I 0.065

C Compound VS-I 0.065

C Water Soluble Boron Compound See TABLE VIII

C Dispersed Developer D-I 4.38

TABLE VIII

Sample Invention (I) or Boron Amount of Comparison (C) Compound Boron Compound [g/m ² ]

3-1-C C None None

3-2-1 ϊ B-I 0.016

3-3-1 I B-I 0.033

3-4-1 I B-l + B-17 0.033 + 0.21

3-5-1 ϊ B-17 + B-8 0.21+ 0.19

The coated materials were imaged, developed, and evaluated using the same procedures detailed in Example 1. The results, shown below in TABLE IX, demonstrate that after 2 months of storage, photothermographic materials incorporating boron compounds exhibited improved Natural Age Keeping. This is shown by their reduced increase in Dmin (ΔDmin) when compared with a control sample containing no boron compound. Some lowering of Dmax was found.

TABLE IX

Sample Relative Initial NAK Initial NAK Speed-2 Dmin Dmax

1 Month 1 Month 1 Month 1 Month Dmin ΔDmin Dmax ΔDmax

3-1-C 100 0.301 0.327 +0.026 2.88 2.65 -0.23

3-2-1 87 0.288 0.304 +0.016 2.87 2.50 -0.37

^ 3-3-1 87 0.294 0.297 +0.003 2.83 2.43 -0.40

O

3-4-1 123 0.293 0.289 -0.004 3.01 2.29 -0.72

3-5-1 100 0.298 0.315 +0.017 2.86 2.51 -0.35

A further advantage of the boron compounds of the present invention is an improvement in post-processing stability as measured by a Desktop

Print Stability Test and a Dark Stability Test. 70/80 Desktop Print Stability Test: Imaged and developed samples of each film were illuminated with 100 foot-candles (1076 lux) at 70 ⁰ F (2Ll ⁰ C) and 80% RH (relative humidity) for 24 hours. The Drain of each sample was measured both before and after illumination using an X-Rite ^® Model 301 densitometer (X-Rite Inc., Grandville, MI) equipped with a visible filter having a transmittance peak at 530 am. The change in visual density in the non-imaged areas, reported as ΔDmin, was calculated by subtracting the initial Dmin of the sample from the Dmin of the sample after being subjected to light, heat, and humidity.

Dark Stability Test: Developed samples were also evaluated in a post-processing Dark Stability Test. An X-Rite point densitometer was used to read the initial Dmin. The samples were then exposed to 1076 lux of light in a controlled environment at 7O ⁰ F and 50% RH (relative humidity) for 2 hours. The test samples were then packaged in an aluminum envelope and heat-sealed. The envelope was placed in an oven at 120 ⁰ F (48.8 ⁰ C) and 50% RH oven for 48 hours. The change in Dmin (ΔDmin) was calculated by subtracting the Dmin of the initial sample from the Dmin of sample after testing.

The results, shown below in TABLE X ₅ demonstrate that photo- thermographic materials incorporating boron compounds exhibited both improved Desktop Print Stability and Dark Stability. This is shown by their reduced increase Dmin (ΔDmin) when compared with a control sample containing no boron compound.

TABLEX

Sample Boron Amount of 70/80 Desktop Dark Stability Compound Boron Compound Print Stability [ΔBminf

[g/m ² ] [ΔDmin]

3-1-C None None 0.35 0.11

3-2-1 B-I 0.016 0.35 0.09

3-3-1 B-I 0.033 0.22 0.10

3-4-1 B-l+B-17 0.033+0.21 0.13 0.08

I 3-5-1 B-17 + B-8 0.21+0.19 0.19 0.10 to

Example 4: Evaluation of Boron Compounds within a Gelatin Overcoat:

Preparation of Co-Precipitated AaBZT/AgT-1 :

A co-precipitated AgBZT/AgT-1 emulsion was prepared as described in copending and commonly assigned U.S. Serial Number 10/935,384 (noted above).

A stirred reaction vessel was charged with 900 g of lime-processed gelatin, and 6000 g of deionized water. The mixture in the reaction vessel was adjusted to a pH of 8.9 with 2.5N sodium hydroxide solution, and 0.8 g of Solution A (prepared below) was added to adjust the solution vAg to 8OmV. The temperature of the reaction vessel was maintained at approximately 5O ⁰ C.

Solution A was prepared containing 216 g/kg of benzotriazole, 710 g/kg of deionized water, and 74 g/kg of sodium hydroxide.

Solution B was prepared containing 362 g/kg of silver nitrate and 638 g/kg of deionized water. Solution C was prepared containing 336 g/kg of T-1 , 70 g/kg of sodium hydroxide and 594 g/kg of deionized water.

Solutions A and B were then added to the reaction vessel by conventional controlled double-jet addition. Solution B was continuously added at the flow rates and for the times given below, while maintaining constant vAg and pH in the reaction vessel. After consumption of 97.4% total silver nitrate solution (Solution B), Solution A was replaced with Solution C and the precipitation was continued. Solution B and Solution C were added to the reaction vessel also by conventional controlled double-jet addition, while maintaining constant vAg and pH in the reaction vessel. The AgBZT/AgT-1 co-precipitated emulsion was washed by conventional ultrafiltration process as described in Research Disclosure, Vol. 131, March 1975, item 13122. The pH of AgBZT/AgT-1 emulsions was adjusted to 6.0 using 2.0N sulfuric acid. Upon cooling the emulsion solidified and was stored.

Time Solution B Flow Rate

[minj [mi/min]

Flow Rate 1 20 25

Flow Rate 2 41 25-40

Flow Rate 3 30 40-80

Preparation of Photothermo graphic Materials:

Component A: A portion of the AgBZT/ AgT-I mixed crystal emulsion prepared above and hydrated gelatin (35% gelatin/65% water) were placed in a beaker and heated to 50°C for 15 minutes to melt the material. A 5% aqueous solution of 3-methylbenzothiazolium iodide was added. Mixing for 15 minutes was followed by cooling to 40°C. The sodium salt of benzotriazole was added and the mixture was stirred for 15 minutes. Mixing for 15 minutes was followed by addition of 2.5N sulfuric acid to adjust the pH to 5.0. ZONYL ^® FS 300 surfactant and compound A-2 were then added.

Component B and C: Components B and C were prepared as described in Example 1 except that Phthalazine Compound A-2 was not added to component C.

Overcoat Formulation:

A stock aqueous gelatin overcoat formulation was prepared.

Gelatin and water were first soaked at room temperature and then melted at 40°C.

ZONYL ^® FS 300 surfactant was added. 1,3-Dimethylurea was added to the solution and dissolved with stirring at 40°C. The bulk overcoat solution was divided into several portions.

Water soluble boron compounds for Examples 4-2-1 to 4-6-1 were added and dissolved at 40°C with stirring. Dispersed boron compound, B-23, was added to the overcoat solution for Example 4-7-1. The dispersion of Compound VS-I was added just prior to coating.

Coating and Evaluation of Photothermo graphic Materials:

Components A, B, and C were formulated, coated, and dried as described in Example 1. The photothermographic emulsion and each overcoat formulation were simultaneously dual knife coated onto a 7-mil (178 μm) transparent, blue-tinted poly(ethylβne terephthalate) film support using a conventional automated knife coating machine. The formulations and coating gaps for each layer were adjusted to achieve the dry coating weights for the photothermographic and overcoat layers shown below in TABLE XI. Comparative Sample 4-1 -C contained no boron compound. It served as a control, Inventive Samples 4-2-1 to 4-6-1 contained water soluble boron compounds dissolved in the overcoat formulation and coated at the dry coating weight shown below in TABLE XII. Sample 4-7-1 contained water dispersed boron compound B-23.

TABLE XI - Photothermographic Emulsion Dry Composition

Component Materia! Dry Cøatmg

Weight [g/m ² ]

Photothermographic Formulation

A Silver (from AgBZT/AgT-1) 1.53

A Lime processed gelatin 1.28

A 3-Methylbenzothiazolium Iodide 0.076

A Sodium benzotriazole (NaBZT) 0.090

A ZONYL ^® FS 300 surfactant 0.022

A Phthalazine Compound A-2 0.078

B Silver (from AgBrI emulsion) 0.27

C Succinimide 0.16

C 1,3-Dimethylurea 0.35

C Pentaerythritol 0.50

C Dispersed Developer D-I 4.38

Overcoat Formulation

E Lime processed, deionized gelatin 1.61

E VS-I Dispersion 0.093

E 1,3-Dimethylurea 0.26

E ZONYL ^® FS 300 surfactant 0.038

E Water soluble Boron Compounds See TABLE XII

E Dispersed Boron Compound (B-23) See TABLE XII

TABLE XII

Sample Invention (I) or Boron Amount of Comparison (C) Compound Boron Compound

[g/m ² ]

4-1 -C C None None

4-2-1 I B-I 0.033

4-3-1 I B-I 0.068

4-4-1 I B-8 0.099

4-5-1 I B-8 0.20

4-6-1 I B-29 0.067

4-7-1 ϊ B-23 0.094

The coated materials were imaged, developed, and evaluated as described in Example 1. The results, shown below in TABLE XIII, demonstrate that after 2 months of storage, photothermographic materials incorporating boron compounds exhibited improved Natural Age Keeping. This is shown by their reduced increase in Dmin (ΔDmin) when compared with a control sample containing no boron compound. Some samples also showed a small improvement in (ΔDmax).

TABLE Xiπ

Sample Relative Initial NAK Initial NAK Speed-2 Dmin Dinax

1 Month I Month 1 Month 1 Month Dmin ΔDmin Dmax ΔDmax

4-1-C 100 0.317 0.486 +0.169 3.33 2.87 -0.46

4-2-1 95 0.317 0.377 +0.060 3.36 3.14 -0.22

4-3-1 81 0.303 0.314 +0.011 3.12 2.72 -0.40

4-4-1 89 0.313 0.351 +0.038 3.52 3.14 -0.38

4-5-1 89 0.312 0.346 +0.033 3.47 3.10 -0.37

4-6-1 72 0.315 0.354 +0.039 3.14 2.77 -0.37

4-7-1 73 0.307 0.379 +0.072 3.17 2.87 -0.30

Example 5: Use of Boron Compounds in the Overcoat Layer:

The following example demonstrates the use of boron compounds in the overcoat layer to improve Desktop Print Stability.

Aqueous-based photothermographic materials of this invention were prepared in the following manner.

Preparation of Photothermographic Materials:

Component A: Silver benzotriazole (AgBZT, described in Example 1) and gelatin (35% gelatin/65% water) were placed in a stainless steel can and heated to 50°C for 15 minutes to melt the material. A 5% aqueous solution of 3-methylbenzothiazolium iodide was added with stirring and held for 10 minutes. The sodium salt of benzotriazole was added and the mixture was stirred for 5 minutes. Compound NaT-I was added and the mixture was stirred for 5 minutes. The mixture was then cooled to 40°C and stirred for 30 minutes. The pH was adjusted to 5.6 with 2.5 N sulfuric acid. ZONYL ^® FSN surfactant was then added.

Component B: A portion of the tabular-grain silver halide emulsion was placed in a stainless steel can and melted at 4O ⁰ C.

Component C: Component C was prepared by first adding the dry materials to water and heating to 40°C until dissolved. Components A, B, and C were mixed immediately before coating to form a photothermographic material. The photothermographic material was coated as a single layer on a 7 mil (178 μm) transparent, blue-tinted poly(ethylene terephthalate) film support using a knife coater to form an imaging layer having the dry composition shown below in TABLE XIV. Samples were dried for 8 minutes at 110°F (43.3°C). After drying the coating was cut into 12.7 cm x 45.8 cm sheets.

TABLE XIV - Phototliermographic Composition

Component Material Dry Coating

Weight [g/m ² ]

A Silver (from AgBZT) 1.63

A Lime processed gelatin 1.89

A 3-Methylbenzothiazolium iodide 0.076

A Sodium Benzotriazole (NaBZT) 0.090

A NaT-I 0.047

A ZONYL ^® FSN surfactant 0.051

B Silver (from AgBrI emulsion) 0.37

C Succinimide 0.12

C 1,3-Dimethylurea 0.15

C Compound A-I 0.059

C Compound VS-I 0.088

C L-ascorbic acid 1.72

The resulting photothermographic material sheets were imagewise exposed for 10 ^"3 seconds using an EG&G flash sensitometer equipped with a Wratten 47 filter and a 1.7 neutral density filter. Following exposure, the sheets were thermally developed using a heated rotating drum for 18 seconds at 150 ⁰ C.

Preparation of Gelatin Overcoats

A stock aqueous gelatin overcoat formulation was prepared. Gelatin and water were first soaked at room temperature and then melted at 40°C. ZONYL ^® FSN surfactant was added with stirring.

The bulk overcoat solution was divided into 5O g portions. Water soluble boron compounds were added and dissolved at 40°C with stirring to prepare Samples 5-2-1 and 5-3-1. A sample containing no boron compound was also prepared and served as a comparison. Sheets of exposed and thermally developed photothermographic imaging material were placed on a stainless steel vacuum plate and heated to

40°C. Gelatin overcoat solutions prepared above were coated over the photo- thermographic emulsion layer using a knife coater. Coating and drying were carried out in an exhaust hood with the doors nearly closed so that the increased air flow accelerated drying. Samples were allowed to dry at. room temperature and uncontrolled humidity for 3 minutes. The composition of the overcoat layer is shown below in TABLE XV.

TABLE XV - Gelatin Overcoat Compositions

Component Material Coatisag Weight

[g/m ² l

D Lime processed gelatin 1.08

D ZONYL ^® FSN surfactant 0.061

D Boric acid (Compound B-I) See TABLE XVI

D Sodium tetraborate decahydrate See TABLE XVI

Evaluation of Desktop Print Stability Imaged and developed samples of each film with applied overcoat were illuminated with 460 lux of fluorescent light at 7O ⁰ F (21 ⁰ C) and 50% RH for 24 hours and the change in density in the Dmin areas (ΔDrnin) was measured. This is a 70/50 Desktop Print Stability Test. The Dmin of each sample was measured both before and after illumination using an X-Rite ^® Model 310 densitometer (X-Ri te Inc., Grandville, MI) equipped with a visible filter having a transmittance peak at 530 nm. The change in visual density in the non-imaged areas, reported as ΔDrnin, was calculated by subtracting the initial Dmin of the sample from the Dmin of the sample after being subjected to light, heat, and humidity.

The results, shown below in TABLE XVI demonstrate that photothermographic materials incorporating boron compounds exhibited improved Desktop Print Stability. The boron compounds, applied in this way, did not alter the processed image already on the sheet.

TABLE XVI

Sample Invention (I) or Overcoat Coating Weight Δ©min Comparison (C) Additive lg/m ² ]

5-1-C Comparison None — +0.21

5-2-1 Invention Boric acid 0.30 +0.02

5-3-1 Invention Sodium 0.37 +0.06 tetraborate decahydrate