WO2007015963A1

WO2007015963A1 - Thermally developable materials with abrasion-resistant backside coatings

Info

Publication number: WO2007015963A1
Application number: PCT/US2006/028355
Authority: WO
Inventors: Thomas Jay Ludemann; Gary Elzear Labelle; Roland John Koestner; Thomas John Kub; Karissa Lynn Eckert
Original assignee: Carestream Health, Inc.
Priority date: 2005-08-01
Filing date: 2006-07-21
Publication date: 2007-02-08
Also published as: EP1910895B1; DE602006004222D1; US7153636B1; EP1910895A1; JP2009502589A

Abstract

Thermally developable materials including photothermographic and thermographic materials have an outermost backside layer that includes a combination of a polysiloxane and a smectite clay that has been modified with a quaternary ammonium compound. The resulting outermost backside layers exhibit improved abrasion resistance. The materials can also include conductive layers underneath the outermost backside layer.

Description

THERMALLY DEVELOPABLE MATERIALS WITH ABRASION-RESISTANT BACKSIDE COATINGS

FIELD OFTHE INVENTION This invention relates to thermally developable materials having outermost backside coatings with improved abrasion resistance and to methods of imaging using these materials. In particular, this invention relates thermographic and photothermographic materials.

BACKGROUND OF THE INVENTION

Silver-containing direct 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 direct thermographic imaging materials are non-photosensitive materials that are used in a recording process wherein images are generated by the direct application of thermal energy and in the absence of a processing solvent. 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 (acting as a black-and-white silver developer) for the reducible silver ions, and (c) a suitable binder. Thermographic materials are sometimes called "direct thermal" materials in the art because they are directly imaged by a source of thermal energy without any transfer of the image or image- forming materials to another element (such as in thermal dye transfer). In a typical thermographic construction, the image-forming thermographic layers comprise non-photosensitive reducible silver salts of long chain fatty acids. A preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, such as behenic acid or mixtures of acids of similar molecular weight. At elevated temperatures, the silver of the silver carboxylate is reduced by a reducing agent for silver ion (also known as a developer), whereby elemental silver is formed. Preferred reducing agents include methyl gallate, hydroquinone, substituted- hydroquinones, hindered phenols, catechols, pyrogallol, ascorbic acid, and ascorbic acid derivatives.

Some thermographic constructions are imaged by contacting them with the thermal head of a thermographic recording apparatus such as a thermal print-head of 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 Imagewise to an elevated temperature, typically in the range of from 60 to 225°C, resulting in the formation of a black-and-white image.

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

In photothermographic materials, exposure of the photosensitive silver halide to light produces small clusters containing silver atoms (Ag )_n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black- and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced.

In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a "developer," may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. At elevated temperatures, the reducible silver ions are reduced by the reducing agent. This reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the photothermographic imaging layer(s).

Differences Between Photothermography and Photography

The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.

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

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

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

Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the underlying chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials. These and other distinctions between photothermographic and photographic materials are described in Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74-75, in D. H. Klosterboer, Imaging Processes and Materials, (Neblettes' Eighth Edition), J. Sturge, V. Walworth, and A. Shepp, Eds., Van Nostrand- Reinhold, New York, 1989, Chapter 9, pp. 279-291 , in Zou et al., J. Imaging Sd. Technol 1996, 40, pp. 94-103, and in M. R. V. Sahyun, J. Imaging ScL Technol. 1998, 42, 23.

Problem to be Solved Photothermographic materials have been designed and commercialized with one or more backside (non-imaging side) layers for a number of years. Such layers are designed to have various properties including antistatic (or conductive), antihalation, improved transport, and protective properties. Efforts have been continuing to provide backside layers with improved functional properties as well as to reduce manufacturing and component costs.

For example, improved backside conductive layers are described in U.S. Patents 6,762,013 (Sakizadeh et al.), 5,368,995 (Christian et al.), and 6,689,546 (LaBeIIe et al.), EP 0 678 776 Al (Melpolder et al.), U.S. Patent Application Publications 2006/0046932 (Ludemann et al.) and 2006/0046215

(Ludemann et al.), copending and commonly assigned U.S. Serial No. 10/978,205 (filed October 29, 2004 by Ludemann, LaBeIIe, Koestner, and Chen), and U.S. Patent 7,022,467 (Ludemann et al).

Improvements in outermost backside layers have also been the focus of research in the industry, as described for example in U.S. Patent

7,018,787 (Ludemann et al.) and in copending and commonly assigned U.S. Serial No. 11/082,627 (noted above) to provide the desired transport and handling properties.

Backside coating research has also focused on improvements in resistance to abrasion and scratching since photothermographic materials are subjected to various mechanical forces during manufacture, packaging, and imaging. Yet, the resistance to abrasion may be reduced as backside coatings are changed to achieve other desired properties. For example, it is particularly noticeable that abrasion resistance deteriorates when dry coating thickness is reduced. Abrasion resistance has been addressed in the art by using crosslinked polyurethanes in the topcoat as described in U.S. Patent 5,679,505 (Tingler et al.) and formulating conductive polymers with polyurethane film- forming binders as described in U.S. Patent 6,355,406 (Majumdar et al.).

Polysiloxanes crosslinked with multi-functional isocyanates have been used to provide heat-resistant outermost backside layers in thermographic materials as described in EP 0 303 729 (Hanada et al) and Japan Kokai 07-032756 (Hanada). Copolymers of polysiloxanes with lactones have been used as backside layers of heat sensitive recording materials as described in EP 0 473 812 (Hanada et al.) While the noted innovations may provide useful improvements in certain imaging materials, there is a continuing need to improve the abrasion resistance of the outermost backside layer in thermographic and photothermographic materials even when the dry coating coverage is reduced.

SUMMARY OF THE INVENTION

To address these problems, the present invention provides a thermally developable material that comprises a support having on one side thereof, one or more thermally developable imaging layers comprising a binder and in reactive association, a non-photosensitive source of reducible silver ions, and a reducing agent composition for the non-photosensitive source reducible silver ions, and having disposed on the backside of the support, an outermost backside layer comprising one or more binder polymers, a polysiloxane, and a smectite clay that has been modified with a quaternary ammonium compound.

The present invention also provides a black-and-white photo- thermographic material that comprises a support having on one side thereof, one or more thermally developable imaging layers comprising a binder and in reactive association, a preformed photosensitive silver halide, a non-photosensitive source of reducible silver ions that comprises silver behenate, and a reducing agent composition for the non-photosensitive source reducible silver ions that comprises a hindered phenol, and having disposed on the backside of the support, a simultaneously coated outermost backside layer and a buried backside conductive layer: a) the outermost backside layer comprising a film-forming polymer, a polysiloxane, a smectite clay that has been modified with a quaternary ammonium compound, and amorphous silica particles, and b) the buried backside conductive layer being interposed between and directly adhering the support to the outermost backside layer, the buried backside conductive layer comprising a mixture of two or more polymers that include a first polymer serving to promote adhesion of the buried backside conductive layer directly to the support, and a second polymer that is different than and forms a single phase mixture with the first polymer, wherein:

1) the buried backside conductive layer has a water electrode resistivity measured at 21.1⁰C and 50% relative humidity of 1 x 10¹² ohms/sq or less, 2) the total amount of mixture of two or more polymers in the buried backside conductive layer is at least 15 weight %,

3) the buried backside conductive layer comprises a conductive non- acicular zinc antimonate,

4) the film-forming polymer of the outermost backside layer is a cellulose acetate butyrate, the second polymer of the buried backside conductive layer is a cellulose acetate butyrate, and the first polymer is a polyester, 5) the amorphous silica particles are present in an amount of from 30 to 70 mg/m² of the outermost backside layer,

6) the outermost backside layer has a dry thickness of from 1 to 3 μm,

7) the mean volume diameter of the amorphous silica particles is from 7 to 9.5 μm with a standard deviation of less than 2.2 μm,

8) the amorphous silica particles extend at least 5 μm above the surface of the dried outermost backside layer,

9) the buried backside conductive layer has a dry thickness of from 0.10 to 0.3 μm, and 10) the polysiloxane chain of the polysiloxane contains diphenyl, phenylmethyl, dimethyl, a mixture of phenylmethyl and dimethyl groups, and a mixture of (epoxycyclohexylethyl)methyl and dimethyl groups.

The present invention also provides a black-and-white photo- thermographic material that comprises a support having on one side thereof, one or more thermally developable imaging layers comprising a binder and in reactive association, a photosensitive silver halide, a non-photosensitive source of reducible silver ions, and a reducing agent composition for the non-photosensitive source reducible silver ions, and having disposed on the backside of the support, an outermost backside layer comprising a film-forming polymer and a polysiloxane. Preferably, the outermost backside layer also includes a smectite clay that has been modified with a quaternary ammonium compound and a buried backside conductive layer immediately underneath the outermost backside layer.

This invention further provides a method of forming a visible image comprising: (A) imagewise exposing a photothermographic material of this invention to electromagnetic radiation to form a latent image,

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

In alternative methods of this invention, a method of forming a visible image comprises:

(A') thermal imaging of the thermally developable material of this invention that is a thermographic material. We have found that the addition of a combination of a polysiloxane and a smectite clay that has been modified with a quaternary ammonium compound to the outermost backside layer of thermally developable materials provides improved abrasion resistance. This improvement can be achieved even when the dry coating coverage of the outermost backside layer is reduced.

DETAILED DESCRIPTION OFTHE INVENTION

The thermally developable materials described herein are both thermographic and photothermographic materials. While the following discussion will often be directed primarily to the preferred photothermographic embodiments, it would be readily understood by one skilled in the art that thermographic materials can be similarly constructed and used to provide black-and- white or color images using appropriate imaging chemistry and particularly non-photosensitive organic silver salts, reducing agents, toners, binders, and other components known to a skilled artisan. In both thermographic and photothermographic materials, the combination of a polysiloxane and a smectite clay that has been modified with a quaternary ammonium compound described herein are incorporated into the outermost backside layer on the backside of the support.

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

The thermally developable materials are particularly useful for imaging of human or animal subjects in response to visible, X-radiation, or infrared radiation for use in a medical diagnosis. Such applications include, but are not limited to, thoracic imaging, mammography, dental imaging, orthopedic imaging, general medical radiography, therapeutic radiography, veterinary radiography, and autoradiography. When used with X-radiation, the photothermo- graphic materials may be used in combination with one or more phosphor intensifying screens, with phosphors incorporated within the photothermographic emulsion, or with combinations thereof. Such materials are particularly useful for dental radiography when they are directly imaged by X-radiation. The materials are also useful for non-medical uses of X-radiation such as X-ray lithography and industrial radiography.

The photothermographic materials can be made sensitive to radiation of any suitable wavelength. Thus, in some embodiments, the materials are sensitive at ultraviolet, visible, infrared, or near infrared wavelengths, of the electromagnetic spectrum, hi preferred embodiments, the materials are sensitive to radiation at 700 nm or greater (and generally from 750 up to 950 nm). Increased sensitivity to a particular region of the spectrum is imparted through the use of various spectral sensitizing dyes. In the photothermographic materials, the components needed for imaging can be in one or more photothermographic imaging layers on one side ("frontside") of the support. The layer(s) that contain the photosensitive photocatalyst (such as a photosensitive silver halide) or non-photosensitive source of reducible silver ions, or both, are referred to herein as photothermographic emulsion layer(s). The photocatalyst and the non-photosensitive source of reducible silver ions are in catalytic proximity and preferably are in the same emulsion layer.

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

As the materials contain imaging layers on one side of the support only, various non-imaging layers may be disposed on the "backside" (non-emulsion or non-imaging side) of the support, including buried backside conductive layers, interlayers, adhesion promoting layers, and antihalation layer(s), in addition to the outermost backside layers comprising the combination of a polysiloxane and a smectite clay that has been modified with a quaternary ammonium compound described herein.

Various non-imaging layers can also be disposed on the "frontside" or imaging or emulsion side of the support, including protective firontside 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.

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

Definitions

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

"an" component refers to "at least one" of that component (for example, a smectite clay or polysiloxane described herein).

As used herein, "black-and-white" preferably refers to an image formed by silver metal. Unless otherwise indicated, when the terms "thermally developable materials," "photothermographic materials," and "thermographic materials" are used herein, the terms refer to materials of the present invention.

Heating in a substantially water-free condition as used herein, means heating at a temperature of from 50⁰C to 25O⁰C with little more than ambient water vapor present. The term "substantially water-free condition" means that the reaction system is approximately in equilibrium with water in the air and water for inducing or promoting the reaction is not particularly or positively supplied from the exterior to the material. Such a condition is described in T. H. James, The Theory of the Photographic Process, Fourth Edition, Eastman Kodak Company, Rochester, NY, 1977, p. 374.

"Photothermographic material(s)" means a dry processable integral element comprising a support and at least one photothermographic emulsion layer or a photothermographic set of emulsion layers (wherein the photosensitive silver halide and the source of reducible silver ions are in one layer and the other necessary components or additives are distributed, as desired, in the same layer or in an adjacent coated layer) that provides a black-and-white silver image. These materials also include multilayer constructions in which one or more imaging components are in different layers, but are in "reactive association." For example, one layer can include the non-photosensitive source of reducible silver ions and another layer can include the reducing composition, but the two reactive components are in reactive association with each other. By "integral", we mean that all imaging chemistry required for imaging is in the material without diffusion of imaging chemistry or reaction products (such as a dye) from or to another element (such as a received element).

"Thermographic materials" are similarly defined except that no photosensitive silver halide catalyst is purposely added or created. When used in photothermography, the term, "imagewise exposing" or "imagewise exposure" means that the material is imaged using any exposure means that provides a latent image using electromagnetic radiation. This includes, for example, by analog exposure where an image is formed by projection onto the photosensitive material as well as by digital exposure where the image is formed one pixel at a time such as by modulation of scanning laser radiation.

When used in thermography, the term, "imagewise exposing" or "imagewise exposure" means that the material is imaged using any means that provides an image using heat. This includes, for example, by analog exposure where an image is formed by differential contact heating through a mask using a thermal blanket or infrared heat source, as well as by digital exposure where the image is formed one pixel at a time such as by modulation of thermal print-heads or by thermal heating using scanning laser radiation.

"Catalytic proximity" or "reactive association" means that the reactive components are in the same layer or in adjacent layers so that they readily come into contact with each other during imaging and thermal development. "Emulsion layer," "imaging layer," "thermographic emulsion layer," or "photothermographic emulsion layer" means a layer of a thermographic or photothermographic material that contains the photosensitive silver halide (when used) and/or non-photosensitive source of reducible silver ions, or a reducing composition. Such layers can also contain additional components or desirable additives. These layers are on what is referred to as the "frontside" of the support.

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

"Simultaneous coating" or "wet-on-wet" coating means that when multiple layers are coated, subsequent layers are coated onto the initially coated layer before the initially coated layer is dry. Simultaneous coating can be used to apply layers on the frontside, backside, or both sides of the support.

"Transparent" means capable of transmitting visible light or imaging radiation without appreciable scattering or absorption. The phrases "silver salt" and "organic silver salt" refer to an organic molecule having a bond to 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.

The term "buried layer" means that there is at least one other layer disposed over the layer (such as a "buried" backside conductive layer).

The terms "coating weight," "coat weight," and "coverage" are synonymous, and are usually expressed in weight per unit area such as g/m².

As is well understood in this art, for the chemical compounds herein described, substitution is not only tolerated, but is often advisable and various substituents are anticipated on the compounds used in the present invention unless otherwise stated. Thus, when a compound is referred to as "having the structure" of a given formula, any substitution that does not alter the bond structure of the formula or the shown atoms within that structure is included within the formula, unless such substitution is specifically excluded by language. As a means of simplifying the discussion and recitation of certain substituent groups, the term "group" refers to chemical species that may be substituted as well as those that are not so substituted. Thus, the term "alkyl group" is intended to include not only pure hydrocarbon alkyl chains, such as methyl, ethyl, π-propyl, t-butyl, cyclohexyl, iso-octyl, and octadecyl, but also alkyl chains bearing substituents known in the art, such as hydroxyl, alkoxy, phenyl, halogen atoms (F, Cl, Br, and I), cyano, nitro, amino, and carboxy. For example, alkyl group includes ether and thioether groups (for example CH₃-CH₂-CH₂-O-CH₂- and CH₃-CH₂-CH₂-S-CH₂-), haloalkyl, nitroalkyl, alkylcarboxy, carboxyalkyl, carboxamido, hydroxyalkyl, sulfoalkyl, and other groups readily apparent to one skilled in the art. Substituents that adversely react with other active ingredients, such as very strongly electrophilic or oxidizing substituents, would, of course, be excluded by the skilled artisan as not being inert or harmless.

Research Disclosure (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 Photocatalyst

As noted above, photothermographic materials include one or more photocatalysts in the photothermographic emulsion layer(s). Useful photo- catalysts are typically photosensitive silver halides such as silver bromide, silver iodide, silver chloride, silver bromoiodide, silver chlorobromoiodide, silver chlorobromide, and others readily apparent to one skilled in the art. Mixtures of silver halides can also be used in any suitable proportion. Silver bromide and silver iodide are preferred. More preferred is silver bromoiodide in which any suitable amount of iodide is present up to almost 100 mol % iodide. Even more preferably, the silver bromoiodide comprises at least 70 mole % (preferably at least 85 mole % and most preferably at least 90 mole %) bromide (based on total silver halide). The remainder of the halide is either iodide or chloride and iodide. Preferably, the additional halide is iodide. Silver bromide and silver bromoiodide are most preferred, with the latter silver halide generally having up to 10 mol % silver iodide.

In some embodiments of aqueous-based photothermographic materials, higher amounts of iodide may be present in homogeneous photo- sensitive silver halide grains, and particularly from 20 mol % up to the saturation limit of iodide as described, for example, U.S. Patent Application Publication 2004/0053173 (Maskasky et al.).

The silver halide grains may have any crystalline habit or morphology including, but not limited to, cubic, octahedral, tetrahedral, orthorhombic, rhombic, dodecahedral, other polyhedral, tabular, laminar, twinned, or platelet morphologies and may have epitaxial growth of crystals thereon. If desired, a mixture of grains with different morphologies can be employed. Silver halide grains having cubic and tabular morphology (or both) are preferred.

The silver halide grains may have a uniform ratio of halide throughout. They may also have a graded halide content, with a continuously varying ratio of, for example, silver bromide and silver iodide or they may be of the core-shell type, having a discrete core of one or more silver halides, and a discrete shell of one or more different silver halides. Core-shell silver halide grains useful in photothermographic materials and methods of preparing these materials are described in U.S. Patent 5,382,504 (Shor et al.). Iridium 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). hi some instances, it may be helpful to prepare the photosensitive silver halide grains in the presence of a hydroxytetrazaindene (such as 4-hydroxy- 6-methyl- 1,3,3 a,7-tetrazaindene) or an N-heterocyclic compound comprising at least one mercapto group (such as l-phenyl-5-mercaptotetrazole) 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 halides be preformed and prepared by an ex-situ process. With this technique, one has the possibility of more precisely controlling the grain size, grain size distribution, dopant levels, and composition of the silver halide, so that one can impart more specific properties to both the silver halide grains and the resulting photothermographic material.

In some constructions, it is preferable to form the non-photo- sensitive source of reducible silver ions in the presence of ex-situ-pvepared silver halide. In this process, the source of reducible silver ions, such as a long chain fatty acid silver carboxylate (commonly referred to as a silver "soap" or homogenate), is formed in the presence of the preformed silver halide grains. Co-precipitation of the source of reducible silver ions in the presence of silver halide provides a more intimate mixture of the two materials [see U.S. Patent

3,839,049 (Simons)] to provide a material often referred to as a "preformed soap."

In some constructions, it is preferred that preformed silver halide grains be added to and "physically mixed" with the non-photosensitive source of reducible silver ions. Preformed silver halide emulsions can be prepared by aqueous or organic processes and can be unwashed or washed to remove soluble salts. Soluble salts can be removed by any desired procedure for example as described in U.S. Patents 2,618,556 (Hewitson et al.), 2,614,928 (Yutzy et al), 2,565,418 (Yackel), 3,241,969 (Hart et al.), and 2,489,341 (Waller et al.). It is also effective to use an in-situ process in which a halide- or a halogen-containing compound is added to an organic silver salt to partially convert the silver of the organic silver salt to silver halide. Inorganic halides (such as zinc bromide, zinc iodide, calcium bromide, lithium bromide, lithium iodide, or mixtures thereof) or an organic halogen-containing compound (such as N-bromo- succinimide or pyridinium hydrobromide perbromide) can be used. The details of such in-situ generation of silver halide are well known and described in U.S. Patent 3,457,075 (Morgan et al.).

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

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

The silver halide grains used in the imaging formulations can vary in average diameter of up to several micrometers (μm) depending on the desired use. Preferred silver halide grains for use in preformed emulsions containing silver carboxylates are cubic grains having an average particle size of from 0.01 to 1.5 μm, more preferred are those having an average particle size of from 0.03 to 1.0 μm, and most preferred are those having an average particle size of from 0.03 to 0.3 μm. Preferred silver halide grains for high-speed photothermographic use are tabular grains having an average thickness of at least 0.02 μm and up to and including 0.10 μm, an equivalent circular diameter of at least 0.5 μm and up to and including 8 μm and an aspect ratio of at least 5:1. More preferred are those having an average thickness of at least 0.03 μm and up to and including 0.08 μm, an equivalent circular diameter of at least 0.75 μm and up to and including 6 μm and an aspect ratio of at least 10:1.

The average size of the photosensitive silver halide grains is expressed by the average diameter if the grains are spherical, and by the average of the diameters of equivalent circles for the projected images if the grains are cubic or in other non-spherical shapes. Representative grain sizing methods are described in Particle Size Analysis, ASTM Symposium on Light Microscopy, R. P. Loveland, 1955, pp. 94-122, and in C. E. K. Mees and T. H. James, The Theoiγ of the Photographic Process, Third Edition, Macmillan, New York, 1966, Chapter 2. Particle size measurements may be expressed in terms of the projected areas of grains or approximations of their diameters. These will provide reasonably accurate results if the grains of interest are substantially uniform in shape.

The one or more light-sensitive silver halides are preferably present in an amount of from 0.005 to 0.5 mole, more preferably from 0.01 to 0.25 mole, and most preferably from 0.03 to 0.15 mole, per mole of non-photosensitive source of reducible silver ions. Chemical Sensitization

The photosensitive silver halides can be chemically sensitized using any useful compound that contains sulfur, tellurium, or selenium, or may comprise a compound containing gold, platinum, palladium, ruthenium, rhodium, indium, 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, 1977, Chapter 5, pp. 149-169. Suitable conventional chemical sensitization procedures are also described in U.S. Patent 1,623,499 (Sheppard et al.), U.S. Patents 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.), and 5,759,761 (Lushington et al.), and EP 0 915 371Al (Lok et al). Mercaptotetrazoles and tetraazindenes as described in U.S. Patent

5,691,127 (Daubendiek et al.), cited herein, can also be used as suitable addenda for tabular silver halide grains.

Certain substituted and unsubstituted thiourea compounds can be used as chemical sensitizers including those described in U.S. Patent 6,368,779 (Lynch et al.).

Still other additional chemical sensitizers include certain tellurium- containing compounds that are described in U.S. Patent 6,699,647 (Lynch et al.), and certain selenium-containing compounds that are described in U.S. Patent 6,620,577 (Lynch et al.). Combinations of gold(III)-containing compounds and either sulfur-, tellurium-, or selenium-containing compounds are also useful as chemical sensitizers as 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 sulfur-containing compounds that can be used in this fashion include sulfur-containing spectral sensitizing dyes. Other useful sulfur-containing chemical sensitizing compounds that can be decomposed in an oxidized environment are the diphenylphosphine sulfide compounds described in copending and commonly assigned U.S. Patent Application Publication 2005/0123870 (Simpson et al).

The chemical sensitizers can be present in conventional amounts that generally depend upon the average size of the silver halide grains. Generally, the total amount is at least 10^"10 mole per mole of total silver, and preferably from 10^" to 10^"" mole per mole of total silver for silver halide grains having an average size of from 0.01 to 2 μm.

Spectral Sensitization

The photosensitive silver halides may be spectrally sensitized with one or more spectral sensitizing dyes that are known to enhance silver halide sensitivity to ultraviolet, visible, and/or infrared radiation (that is, sensitivity within the range of from 300 to 1400 nm). Non-limiting examples of spectral sensitizing dyes that can be employed include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, holopolar cyanine dyes, hemicyanine dyes, styryl dyes, and hemioxanol dyes. They may be added at any stage in chemical finishing of the photothermo graphic emulsion, but are generally added after chemical sensitization is achieved. Suitable spectral 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.), 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.), can be used in the practice of the invention. Useful spectral sensitizing dyes are also described in Research Disclosure, December 1989, item 308119, Section IV mdResearch Disclosure, 1994, item 36544, section V.

Teachings relating to specific combinations of spectral sensitizing dyes also include 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, 10^"7 to 10^"2 mole per mole of silver halide.

Non-Photosensitive Source of Reducible Silver Ions

The non-photosensitive source of reducible silver ions in the thermally developable materials is a silver-organic compound that contains reducible silver(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 50°C or higher in the presence of an exposed photocatalyst (such as silver halide, when used in a photothermographic material) and a reducing agent composition.

The primary organic silver salt is often a silver salt of an aliphatic carboxylic acid (described below). Mixtures of silver salts of aliphatic carboxylic acids are particularly useful where the mixture includes at least silver behenate.

Useful silver carboxylates include silver salts of long-chain aliphatic carboxylic acids. The aliphatic carboxylic acids generally have aliphatic chains that contain 10 to 30, and preferably 15 to 28, carbon atoms. Examples of such preferred silver salts include silver behenate, silver arachidate, silver stearate, silver oleate, silver laurate, silver caprate, silver myristate, silver palmitate, silver maleate, silver fumarate, silver tartarate, silver furoate, silver linoleate, silver butyrate, silver camphorate, and mixtures thereof. Most preferably, at least silver behenate is used alone or in mixtures with other silver carboxylates.

Silver salts other than the silver carboxylates described above can be used also. Such silver salts include silver salts of aliphatic carboxylic acids containing a thioether group as described in U.S. 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 141Al (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.), and silver salts of heterocyclic compounds containing mercapto or thione groups and derivatives as described in U.S. Patents 4,123,274 (Knight et al.) and 3,785,830 (Sullivan et al.).

It is also convenient to use silver half soaps such as an equimolar blend of silver carboxylate and carboxylic acid that analyzes for 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.

Sources of non-photosensitive reducible silver ions can also be core-shell silver salts as described in U.S. Patent 6,355,408 (Whitcomb et al.) that is cited herein, wherein a core has one or more silver salts and a shell has one or more different silver salts, as long as one of the silver salts is a silver carboxylate. Other useful sources of non-photosensitive reducible silver ions are the silver dimer compounds that comprise two different silver salts as described in U.S. 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.).

Organic silver salts that are particularly useful in organic solvent- based thermographic and photothermographic materials include silver carboxylates (both aliphatic and aromatic carboxylates), silver benzotriazolates, silver sulfonates, silver sulfosuccinates, and silver acetylides. Silver salts of long- chain aliphatic carboxylic acids containing 15 to 28 carbon atoms and silver salts of benzotriazole are particularly preferred. Organic silver salts that are particularly useful in aqueous based thermographic and photothermographic materials include silver salts of compounds containing an imino group. Preferred examples of these compounds include, but are not limited to, silver salts of benzotriazole and substituted derivatives thereof (for example, silver methylbenzotriazole and silver 5-chloro- benzotriazole), silver salts of 1,2,4-triazoles or 1-H-tetrazoles such as phenyl- mercaptotetrazole as described in U.S. Patent 4,220,709 (deMauriac), and silver salts of imidazoles and imidazole derivatives as described in U.S. Patent 4,260,677 (Winslow et al.). Particularly useful silver salts of this type are the silver salts of benzotriazole and substituted derivatives thereof. A silver salt of a benzotriazole is particularly preferred in aqueous-based thermographic and photothermographic formulations.

Useful nitrogen-containing organic silver salts and methods of preparing them are described in copending and commonly assigned U.S. Patent 6,977,139 (Zou et al.). 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. Such co-precipitated salts are described in copending and commonly assigned U.S. Patent 7,008,748 (Hasberg et al). The one or more non-photosensitive sources of reducible silver ions are preferably present in an amount of from 5% to 70%, and more preferably from 10% to 50%, based on the total dry weight of the emulsion layers. Alternatively stated, the amount of the sources of reducible silver ions is generally from 0.001 to 0.2 mol/m² of the dry photothermographic material (preferably from 0.01 to 0.05 mol/m²).

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

Reducing Agents

The reducing agent (or reducing agent composition comprising two or more components) for the source of reducible silver ions can be any material (preferably an organic material) that can reduce silver(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 reducing agents are preferred. An "ascorbic acid" reducing agent (also referred to as a developer or developing agent) means ascorbic acid, complexes, and derivatives thereof. An "ascorbic acid" reducing agent means ascorbic acid, complexes, and derivatives thereof. Ascorbic acid reducing agents are described in a considerable number of publications including U.S. 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-rhamnoascorbic 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-thϊol type ascorbic acid, as described in EP 0 573 700Al (Lingier et al.), EP 0 585 792Al (Passarella et al.), EP 0 588 408Al (Hieronymus et al.), U.S. Patents 2,688,549 (James et al.), 5,089,819 (Knapp), 5,278,035 (Knapp), 5,376,510 (Parker et al.), 5,384,232 (Bishop 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. Additionally useful are the ascorbic acid reducing agents described in U.S. Patent Application Publication 2005/0164136 (Ramsden et al.). Also useful are the solid particle dispersions of certain ascorbic acid esters that are prepared in the presence of a particle growth modifier that are described in U.S. Patent Application Publication 2006/0051714 (Brick et al.). When a silver carboxylate silver source is used in a photothermo- graphic material, one or more hindered phenol reducing agents are preferred. In some instances, the reducing agent composition comprises two or more components such as a hindered phenol developer and a co-developer that can be chosen from the various classes of co-developers and reducing agents described below. Ternary developer mixtures involving the further addition of contrast enhancing agents are also useful. Such contrast enhancing agents can be chosen from the various classes of reducing agents described below.

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

One type of hindered phenol includes hindered phenols and hindered naphthols. This type of hindered phenol includes, for example, 2,6-di-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-bemzylphenol 2-benzyl-4-methyl- 6-t-butylphenol, and 2,4-dimethyl-6-(l '-methylcyclohexyl)phenol. 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(hydroxyphenyl)- methanes such as, bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane (CAO-5), 1,1 '-bis(2-hydroxy-3,5-dimethylρhenyl)-3,5,5-trimethylhexane (NONOX^® or PERMANAX^® WSO), and l,l'-bis(2-hydroxy-3,5-dimethylphenyl)isobutane (LOWINOX^® 22IB46). Such hindered phenols can be obtained from a number of commercial sources or prepared using known starting materials and conditions. One process for making hindered phenols [such as 2,2'-(2-methylpropylidene)- bis(4,6-dimethyl)phenol] at higher yields includes the reaction of an appropriate dimethylphenol (such as 2,4-dimethylphenol) with an aldehyde (such as wo-butyraldehyde) at relatively low temperature (for example from 20 to 5O⁰C and certainly at less than 90⁰C) in a highly acidic reaction medium (for example, containing sulfuric acid).

Mixtures of hindered phenol reducing agents can be used if desired, such as the mixture of a hindered phenol and a hindered bis-phenol described in U.S. Patent 6,413,712 (Yoshioka et al.).

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 al.). 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. Patent 6,100,022 (Inoue et al.). Yet another class of co-developers includes substituted acrylonitrile compounds such as the compounds identified as HET-01 and HET-02 in U.S. Patent 5,635,339 (Murray) and CN-Ol through CN-13 in U.S. Patent 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, hydroxylamines, alkanolamines and ammonium phthalamate compounds as described in U.S. Patent 5,545,505 (Simpson), hydroxamic acid compounds as described for example, in U.S. Patent 5,545,507 (Simpson et al.), N-acylhydrazine compounds as described in U.S. Patent 5,558,983 (Simpson et al.), and hydrogen atom donor compounds as described in U.S. Patent 5,637,449 (Harring et al.). When used with a silver carboxylate silver source in a thermographic material, preferred reducing agents are aromatic di- and tri-hydroxy compounds having at least two hydroxy groups in ortho- or pαrα-relationship on the same aromatic nucleus. Examples are hydroquinone and substituted hydroquinones, catechols, pyrogallol, gallic acid and gallic acid esters (for example, methyl gallate, ethyl gallate, propyl gallate), and tannic acid.

Particularly preferred are catechol-type reducing agents having no more than two hydroxy groups in an ørt/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, and phenyl-

(3,4-dihydroxyphenyl)ketone. 3,4-Dihydroxybenzonitrile is also useful. 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.

Other Addenda

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

Suitable stabilizers that can be used alone or in combination include thiazolium salts as described in U.S. Patents 2,131,038 (Brooker) 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), the urazoles described in U.S. Patent 3,287,135 (Anderson), sulfocatechols as described in U.S. Patent 3,235,652 (Kennard), the oximes described in GB 623,448 (Carrol et al.), polyvalent metal salts as described in U.S. Patent 2,839,405 (Jones), thiuronium salts as described in U.S. Patent 3,220,839 (Herz), palladium, platinum, and gold salts as described in U.S. Patents 2,566,263 (Trirelli) and 2,597,915 (Damshroder), compounds having -SO₂CBr₃ groups as described in U.S. Patents 5,369,000 (Sakizadeh et al.), 5,464,747 (Sakizadeh et al.) 5,594,143 (Kirk et al.), 5,374,514 (Kirk et al.), and 5,460,938 (Kirk et al.). Stabilizer precursor compounds capable of releasing stabilizers upon application of heat during imaging can also be used, as described in U.S. Patents 5,158,866 (Simpson et al.), 5,175,081 (Krepski et al.), 5,298,390 (Sakizadeh et al.), and 5,300,420 (Kenney et al.).

In addition, certain substituted-sulfonyl derivatives of benzo- triazoles may be used as stabilizing compounds as described in U.S. Patent 6,171,767 (Kong et al).

"Toners" or derivatives thereof that improve the image are desirable components of the thermographic materials. These compounds, when added to the imaging layer, shift the color of the image from yellowish-orange to brown-black or blue-black. Generally, one or more toners described herein are present in an amount of from 0.01% to 10% (more preferably from 0.1% to 10%), based on the total dry weight of the layer in which the toner is included. Toners may be incorporated in the thermographic layer or in an adjacent non-imaging layer. Compounds useful as toners are described in U.S. Patents

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

Additional useful toners are substituted and unsubstituted mercaptotriazoles as described in U.S. Patents 3,832,186 (Masuda et al.), 6,165,704 (Miyake et al.), 5,149,620 (Simpson et al.), 6,713,240 (Lynch et al.), and 6,841,343 (Lynch et al.).

Phthalazine and phthalazine derivatives [such as those described in U.S. Patent 6,146,822 (Asanuma et al.)], phthalazinone, and phthalazinone derivatives are particularly useful toners. A combination of one or more hydroxyphthalic acids and one or more phthalazinone compounds can be included in the thermographic materials. Hydroxyphthalic acid compounds have a single hydroxy substituent that is in the meta position to at least one of the carboxy groups. Preferably, these compounds have a hydroxy group in the 4-position and carboxy groups in the 1- and 2-positions. The hydroxyphthalic acids can be further substituted in other positions of the benzene ring as long as the substituents do not adversely affect their intended effects in the thermographic material. Mixtures of hydroxyphthalic acids can be used if desired.

Useful phthalazinone compounds are those having sufficient solubility to completely dissolve in the formulation from which they are coated. Preferred phthalazinone compounds include 6,7-dimethoxy-l-(2H)-phthalazinone, 4-(4-pentylphenyl)- 1 -(2H)-phthalazinone, and 4-(4-cyclohexylphenyl)- l-(2H)-phthalazinone. Mixtures of such phthalazinone compounds can be used if desired. This combination facilitates obtaining a stable bluish-black image after processing. In preferred embodiments, the molar ratio of hydroxyphthalic acid to phthalazinone is sufficient to provide an a* value more negative than -2 (preferably more negative than -2.5) at an optical density of 1.2 as defined by the CIELAB Color System when the material has been imaged using a thermal print- head from 300 to 400°C for less than 50 milliseconds (50 msec) and often less than 20 msec. In preferred embodiments, the molar ratio of phthalazinone is to hydroxyphthalic acid 1 : 1 to 3 : 1. More preferably the ratio is from 2: 1 to 3 : 1. In addition, the imaged material provides an image with an a* value more negative than -1 at an optical density of 1.2 as defined by the CIELAB Color System when the above imaged material is then stored at 7O⁰C and 30% RH for 3 hours. Thermal solvents (or melt formers) can also be used, including combinations of such compounds (for example, a combination of succinimide and dimethylurea). Known thermal solvents are disclosed in U.S. Patents 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). The thermographic materials can also include one or more image stabilizing compounds that are usually incorporated in a "backside" layer. Such compounds can include phthalazinone and its derivatives, pyridazine and its derivatives, benzoxazine and benzoxazine derivatives, benzothiazine-dione and its derivatives, and quinazoline-dione and its derivatives, particularly as described in U.S. Patent 6,599,685 (Kong). Other useful backside image stabilizers include anthracene compounds, coumarin compounds, benzophenone compounds, benzotriazole compounds, naphthalic acid imide compounds, pyrazoline compounds, or compounds described in U.S. Patent 6,465,162 (Kong et al.) and GB 1,565,043 (Fuji Photo). The direct thermographic materials may also include one or more additional polycarboxylic acids (other than the hydroxyphthalaic acids noted above) and/or anhydrides thereof that are in thermal working relationship with the sources of reducible silver ions in the one or more thermographic layers. Such polycarboxylic acids can be substituted or unsubstituted aliphatic (such as glutaric acid and adipic acid) or aromatic compounds and can be present in an amount of at least 5 mol % ratio to silver. They can be used in anhydride or partially esterifled form as long as two free carboxylic acids remain in the molecule. Useful polycarboxylic acids are described for example in U.S. Patent 6,096,486 (noted above). The photothermographic materials can also include one or more image stabilizing compounds that are usually incorporated in a "backside" layer. Such compounds can include phthalazinone and its derivatives, pyridazine and its derivatives, benzoxazine and benzoxazine derivatives, benzothiazine dione and its derivatives, and quinazoline dione and its derivatives, particularly as described in U.S. Patent 6,599,685 (Kong). Other useful backside image stabilizers include anthracene compounds, coumarin compounds, benzophenone compounds, benzotriazole compounds, naphthalic acid imide compounds, pyrazoline compounds, or compounds described in U.S. Patent 6,465,162 (Kong et al), and GB 1,565,043 (Fuji Photo).

Phosphors are materials that emit infrared, visible, or ultraviolet radiation upon excitation and can be incorporated into the photothermographic materials. Particularly useful phosphors are sensitive to X-radiation and emit radiation primarily in the ultraviolet, near-ultraviolet, or visible regions of the spectrum (that is, from 100 to 700 nm). An intrinsic phosphor is a material that is naturally (that is, intrinsically) phosphorescent. An "activated" phosphor is one composed of a basic material that may or may not be an intrinsic phosphor, to which one or more dopant(s) has been intentionally added. These dopants or activators "activate" the phosphor and cause it to emit ultraviolet or visible radiation. Multiple dopants may be used and thus the phosphor would include both "activators" and "co-activators."

Any conventional or useful phosphor can be used, singly or in mixtures. For example, useful phosphors are described in numerous references relating to fluorescent intensifying screens as well as U.S. Patents 6,440,649 (Simpson et al.) and 6,573,033 (Simpson et al.) that are directed to photothermographic materials. Some particularly useful phosphors are primarily "activated" phosphors known as phosphate phosphors and borate phosphors. Examples of these phosphors are rare earth phosphates, yttrium phosphates, strontium phosphates, or strontium fluoroborates (including cerium activated rare earth or yttrium phosphates, or europium activated strontium fluoroborates) as described in U.S. Patent Application Publication 2005/0233269 (Simpson et al.).

The one or more phosphors can be present in the photothermo- graphic materials in an amount of at least 0.1 mole per mole, and preferably from 0.5 to 20 mole, per mole of total silver in the photothermographic material. As noted above, generally, the amount of total silver is at least 0.002 mol/m². While the phosphors can be incorporated into any imaging layer on one or both sides of the support, it is preferred that they be in the same layer(s) as the photosensitive silver halide(s) on one or both sides of the support.

Binders

The photosensitive silver halide (if present), the non-photosensitive source of reducible silver ions, the reducing agent composition, and any other imaging layer additives are generally combined with one or more binders that are generally hydrophobic or hydrophilic in nature. Thus, either aqueous or organic solvent-based formulations can be used to prepare the thermally developable materials of this invention. Mixtures of either or both types of binders can also be used. It is preferred that the binder be selected from predominantly hydrophobic polymeric materials (at least 50 dry weight % of total binders).

Examples of typical hydrophobic binders include polyvinyl acetals, polyvinyl chloride, polyvinyl acetate, cellulose acetate, cellulose acetate butyrate, polyolefins, polyesters, polystyrenes, polyacrylonitrile, polycarbonates, methacrylate copolymers, maleic anhydride ester copolymers, butadiene-styrene copolymers, and other materials readily apparent to one skilled in the art. Copolymers (including terpolymers) are also included in the definition of polymers. The polyvinyl acetals (such as polyvinyl butyral, polyvinyl acetal, and polyvinyl formal) and vinyl copolymers (such as polyvinyl acetate and polyvinyl chloride) are particularly preferred. Particularly suitable hydrophobic binders are polyvinyl butyral resins that are available under the names MOWITAL^® (Kuraray America, New York, NY), S-LEC^® (Sekisui Chemical Company, Troy, MI), BUTVAR^® (Solutia, Inc., St. Louis, MO) and PIOLOFORM^® (Wacker Chemical Company, Adrian, MI).

Hydrophilic binders or water-dispersible polymeric latex polymers can also be present in the formulations. Examples of useful hydrophilic binders include, but are not limited to, proteins and protein derivatives, gelatin and gelatin-like derivatives (hardened or unhardened), cellulosic materials such as hydroxymethyl cellulose and cellulosic esters, acrylamide/methacrylamide polymers, acrylic/methacrylic polymers polyvinyl pyrrolidones, polyvinyl alcohols, poly(vinyl lactams), polymers of sulfoalkyl acrylate or methacrylates, hydrolyzed polyvinyl acetates, polyacrylamides, polysaccharides and other synthetic or naturally occurring vehicles commonly known for use in aqueous- based photographic emulsions (see for example, Research Disclosure, item 38957, noted above). Cationic starches can also be used as a peptizer for tabular silver halide grains as described in U.S. Patents 5,620,840 (Maskasky) and 5,667,955 (Maskasky).

Hardeners for various binders may be present if desired. Useful hardeners are well known and include diisocyanate compounds as described in EP 0 600 586 Bl (Philip, Jr. et al.), vinyl sulfone compounds as described in U.S. Patent 6,143,487 (Philip, Jr. et al.) and EP 0 640 589 Al (Gathmann et al.), aldehydes and various other hardeners as described in U.S. Patent 6,190,822 (Dickerson et al.). The hydrophilic binders used in the photo thermographic materials are generally partially or fully hardened using any conventional hardener. Useful hardeners are well known and are described, for example, in T. H. James, The Theory of the Photographic Process, Fourth Edition, Eastman Kodak Company, Rochester, NY, 1977, Chapter 2, pp. 77-8.

Where the proportions and activities of the thermally developable materials require a particular developing time and temperature, the binder(s) should be able to withstand those conditions. When a hydrophobic binder is used, it is preferred that the binder (or mixture thereof) does not decompose or lose its structural integrity at 120°C for 60 seconds. When a hydrophilic binder is used, it is preferred that the binder does not decompose or lose its structural integrity at 150°C for 60 seconds. It is more preferred that the binder not decompose or lose its structural integrity at 177°C for 60 seconds.

The polymer binder(s) is used in an amount sufficient to carry the components dispersed therein. Preferably, a binder is used at a level of from 10% to 90% by weight (more preferably at a level of from 20% to 70% by weight) based on the total dry weight of the layer. It is particularly useful that the thermally developable materials include at least 50 weight % hydrophobic binders in both imaging and non-imaging layers on both sides of the support (and particularly the imaging side of the support). Support Materials

The thermally developable materials comprise a polymeric support that is preferably a flexible, transparent film that has any desired thickness and is composed of one or more polymeric materials. They are required to exhibit dimensional stability during thermal development and to have suitable adhesive properties with overlying layers. Useful polymeric materials for making such supports include polyesters [such as poly(ethylene terephthalate) and poly(ethylene naphthalate)], cellulose acetate and other cellulose esters, polyvinyl acetal, polyolefins, polycarbonates, and polystyrenes. Preferred supports are composed of polymers having good heat stability, such as polyesters and polycarbonates. Support materials may also be treated or annealed to reduce shrinkage and promote dimensional stability.

It is also useful to use 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.). Another support comprises dichroic mirror layers as described in U.S. Patent 5,795,708 (Boutet). Opaque supports can also be used, such as dyed polymeric films and resin-coated papers that are stable to high temperatures. Support materials can contain various colorants, pigments, antihalation or acutance dyes if desired. For example, the support can include one or more dyes that provide a blue color in the resulting imaged film. Support materials may be treated using conventional procedures (such as corona discharge) to improve adhesion of overlying layers, or subbing or other adhesion- promoting layers can be used.

Thermographic and Photothermographic Formulations and Constructions

An organic solvent-based coating formulation for the thermographic and photothermographic emulsion layer(s) can be prepared by mixing the various components with one or more binders in a suitable organic solvent system that usually includes one or more solvents such as toluene, 2-butanone (methyl ethyl ketone), acetone, or tetrahydrofuran, or mixtures thereof. Methyl ethyl ketone is a preferred coating solvent.

Alternatively, the desired imaging components can be formulated with a hydrophilic binder (such as gelatin, a gelatin-derivative, or a latex) in water or water-organic solvent mixtures to provide aqueous-based coating formulations.

Thermally developable materials can contain plasticizers and lubricants such as poly(alcohols) and diols as described in U.S. 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 and 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.), cited herein, describes various means of modifying photo thermographic materials to reduce what is known as the "woodgrain" effect, or uneven optical density.

Layers to promote adhesion of one layer to another are also known, as described in U.S. 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 in U.S. Patent 5,928,857 (Geisler et al.).

Layers to reduce emissions from the material may also be present, including the polymeric barrier layers described in U.S. 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 6,746,831 (Hunt).

Mottle and other surface anomalies can be reduced 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, for example in U.S. Patent 5,621,983 (Ludemann et al.). The thermally developable materials can also include one or more antistatic or conductive layers on the frontside of the support. Such layers may contain metal oxides as described below, or other conventional antistatic agents known in the art for this purpose such as soluble salts (for example, chlorides or nitrates), evaporated metal layers, or ionic or conductive polymers such as polythiophenes and those described in U.S. Patents 2,861,056 (Minsk) and 3,206,312 (Sterman et al.), or insoluble inorganic salts such as those described in U.S. Patent 3,428,451 (Trevoy), electroconductive underlayers such as those described in U.S. Patent 5,310,640 (Markin et al.), electronically-conductive metal antimonate particles such as those described below and in U.S. Patent 5,368,995 (Christian et al.), electrically-conductive metal-containing particles dispersed in a polymeric binder such as those described in U.S. 5,547,821 (Melpolder et al.), and fluorochemicals that are described in numerous publications.

The photothermographic and thermographic materials may also usefully include a magnetic recording material as described in Research Disclosure, Item 34390, November 1992, or a transparent magnetic recording layer such as a layer containing magnetic particles on the underside of a transparent support as described in U.S. Patent 4,302,523 (Audran et al.).

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

Additionally, one or more acutance dyes may be incorporated into one or more frontside imaging layers.

Dyes useful as antihalation and acutance dyes include squaraine dyes as 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 as described in EP 0 342 810Al (Leichter), and cyanine dyes as described in U.S. Patent 6,689,547 (Hunt et al.).

It is also useful to employ compositions including acutance or antihalation dyes that will decolorize or bleach with heat during processing, as described in U.S. Patents 5,135,842 (Kitchin et al.), 5,266,452 (Kitchin et al.), 5,314,795 (Helland et al.), 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 antihalation compositions can include an infrared radiation absorbing compound such as an oxonol dye or various other compounds used in combination with a hexaarylbiimidazole (also known as a "HABI"), or mixtures thereof. HABI compounds are described in U.S. 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 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). In some embodiments, the thermally developable materials include a surface protective layer over one or more imaging layers.

The thermally developable formulations can be coated by various coating procedures including wire wound rod coating, dip coating, air knife coating, curtain coating, slide coating, slot-die 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). A typical coating gap for the emulsion layer can be from 10 to 750 μm, and the layer can be dried in forced air at a temperature of from 2O⁰C to 100⁰C. 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 or more, as measured by an X-rite Model 361/V Densitometer equipped with 301 Visual Optics, available from X-rite Corporation, (Granville, MI). Subsequently to or simultaneously with application of the emulsion formulation to the support, a protective overcoat formulation can be applied over the emulsion formulation.

Preferably, two or more layer formulations are applied simultaneously to a support using slide coating, the first layer being coated on top of the undercoat layer while the undercoat layer is still wet. The first and second fluids used to coat these layers can be the same or different solvents. Simultaneous coating can be used to apply layers on the frontside, backside, or both sides of the support. In other embodiments, a "carrier" layer formulation comprising a single-phase mixture of 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 emulsion layer formulation and any additional frontside overcoat formulations.

Backside Compositions and Layers

In one embodiment, the thermally developable material comprises a support having on one side thereof, one or more thermally developable imaging layers comprising a binder and in reactive association, a non-photosensitive source of reducible silver ions, and a reducing agent composition for the non-photosensitive source reducible silver ions, and having disposed on the backside of the support, an outermost backside layer comprising one or more binder polymers, a polysiloxane, and a smectite clay that has been modified with a quaternary ammonium compound.

It is also preferred that the outermost backside layer be immediately adjacent a buried backside conductive layer (defined below). That is, there are no layers disposed between them.

In another embodiment the thermally developable material is a photothermographic material and comprises a support having photothermographic imaging chemistry on one side thereof, and having disposed on the backside of the support an outermost backside layer comprising one or more binder polymers and a polysiloxane. Preferably, the outermost backside layer also includes a smectite clay that has been modified with a quaternary ammonium compound and a buried backside conductive layer immediately adjacent the buried backside conductive layer. Polysiloxanes, also referred to as silicone fluids are liquid compounds known for their ability to provide coatings with lubricating, mar-resistant, and antiblocking properties. Polysiloxanes may also provide such coatings with reduced coefficient of friction. Their properties are believed to be a function of low intermolecular forces between the silicone polymer chains. Polysiloxanes have the general Structure (I) below.

The simplest polysiloxane compounds are polydimethylsiloxanes where all R groups are methyl. Polysiloxanes can be modified with various substituents as the terminal R groups (that is, R₁, R₂, R₃, R₆, R7, and R₈), with various substituents as R groups attached to silicon within the polysiloxane chain (that is R₄, and R₅), or with various substituents in both portions. In addition, different siloxane groups, each with different substituents can be incorporated within the polysiloxane chain. Some common representative terminal substituent groups include methyl, hydrogen, hydroxy, hydroxyalkyl (such as vinyl, hydroxymethyl), alkylamino (such as aminopropyl), and alkylcarboxy groups (such as carboxypropyl). Preferred terminal groups include methyl and alkylamino groups. Common substituent groups within the polysiloxane chain include alkylamino, phenyl, benzyl, perfluoroalkyl (such as trifluoropropyl), and alkyl groups (such as methyl, ethyl, butyl, hexyl, octyl, and octadecyl groups). The alkyl groups may be further substituted, such as with cycloalkyl and epoxycycloalkyl groups. Preferred substituent groups within the polysiloxane chain include diphenyl, phenylmethyl, dimethyl, a mixture of phenylmethyl and dimethyl groups, or a mixture of (epoxycyclohexylethyl)methyl and dimethyl groups. It is preferred that the polysiloxanes be soluble in organic solvents and particularly those solvents used in coating the outermost backside layer. A preferred solvent is MEK (methyl ethyl ketone) and mixtures of MEK with other organic solvents such as methanol. It is further preferred that the polysiloxanes have a molecular weight greater than 25,000 and preferably from 25,000 to 35,000.

The polysiloxanes are present in the outermost backside layer in an amount of at least 0.5 % and up to 8%, and preferably from 0.5 to 6 %, and more preferably from 1.5 to 6% based on total layer dry weight. The polysiloxanes can be obtained from a number of commercial sources such as those listed below in the section entitled Materials and Methods for the Experiments and Examples.

Preferably, the outermost backside layer also contains a smectite clay that has been modified with a quaternary ammonium compound (also known as an ammonium salt). Smectite clays are a family of clays that swell or exfoliate when immersed in water or certain other polar organic liquids. The useful smectite clay can be naturally-occurring or synthetically-prepared. Useful naturally-occurring smectite clays include bentonite and hectorite. Useful synthetically-prepared smectite-type clays include montmorillonite, beidellite, hectorite-saponite, and stevensite. Typically, synthetically-prepared clays are smaller in lateral dimension and therefore possess a smaller aspect ratio. Synthetically-prepared clays are purer and are of narrower size distribution, compared to naturally-occurring clays and may not require any further purification or separation before modification. Mixtures of two or more of such clays (both naturally-occurring and synthetically-prepared) can be used in this invention.

A preferred smectite clay is montmorillonite, an aluminum magnesium silicate clay having the approximate formula

R⁺o_.33(Al,Mg)₂Si₄θ_!o(OH)₂-n(H₂θ), where R⁺ includes one or more of the cations Na⁺, K⁺, Mg²⁺, NH₄ ⁺, and Ca²⁺ and possibly others. (See Merck Index, Ninth Ed., Merck & Co., Rahway, NJ.). It can be modified by replacing the R⁺ groups.

The smectite clays useful in this invention have been modified by exfoliation and exchange of a portion of the R⁺ groups with various quaternary ammonium compounds. This allows the clays to be more easily dispersed in organic solvents. In general, a dispersion of the clay and an ammonium compound are heated for a suitable period time to exchange cations such as Na⁺, K⁺, Mg²⁺, NH₄ ⁺, and Ca²⁺ with those of the quaternary ammonium compound. Such clays are commercially available from Southern Clay Products under the trade-names CLOISITE^® and CLAYTONE^®. In many CLOISITE^® clays the ammonium compounds are hydrogenated tallow amines. The modified smectite clay is generally present in the outermost backside layer in an amount of from 0.5 to 5 weight %, and preferably from 1 to 3 weight %, based on the total dry binder weight. Further details of the use of such modified clays are provided in U. S . Patent 7,018,787 (noted above).

In preferred embodiments, the thermally developable materials also have at least one buried conductive layer on the backside (non-imaging side) of the polymeric support along with one or more additional backside overcoat layers the outermost of which comprises the polysiloxane and the smectite clay that has been modified with a quaternary ammonium compound described above.

The buried backside conductive layer can include one or more conductive components, such as 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), conductive polymers such as polyanilines, polyindoles, and polythiophenes as described in U.S. Patent 5,747,412 (Leenders et al.), electroconductive underlayers as described in U.S. Patent 5,310,640 (Markin et al.), fluorochemicals such as Fluorad^® FC-135 (3M Corporation), ZONYL^® FSN (E. I. DuPont de Nemours & Co.), as well 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.), 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.), U.S. Patent Application Publications 2006/0046214 (Ludemann et al.) and 2006/0046215 (Ludemann et al.), copending and commonly assigned U.S. Serial No. 10/978,205 (filed October 29, 2004 by Ludemann, LaBeIIe, Koestner, and Chen), and U.S. Patent 7,022,467 (Ludemann et al). Combinations of these materials can also be used.

Preferably, the buried conductive layer on the backside (non-imaging side) of the support includes a conductive material such as nanoparticles or clusters of metal oxides such as TiO₂, SnO₂, Al₂O₃, ZrO₂, In₂O₃, ZnO, TiB₂, ZrB₂, NbB₂, TaB₂, CrB₂, MoB, WB, LaB₆, ZrN, TiN, TiC, WC, HfC, HfN, ZrC, acicular and non-acicular zinc antimonate (ZnSb₂O₆), indium-doped tin oxide, antimony-doped tin oxide, aluminum-doped zinc oxide, tungsten trioxide, vanadium pentoxide, molybdenum trioxide, and niobium-doped titanium oxide. Conductive metal oxide nanoparticles are preferred, non-acicular metal antimonates are more preferred. In one construction, where the support is a polyester resin, the buried backside conductive layer formulation comprises a single-phase mixture of two or more polymers and a suitable conductive metal oxide such as non-acicular metal antimonate particles. A preferred binder is a single phase mixture of a polyester resin as a first polymer and a polyvinyl acetal such as polyvinyl butyral or a cellulose ester such as cellulose acetate butyrate as a second polymer. A preferred polyvinyl butyral is a polyvinyl butyral having a molecular weight of at least 8,000 and less than 30,000. A preferred cellulose ester is cellulose acetate butyrate (CAB) having a molecular weight of 70,000.

The weight ratio of "first" polymer to "second" polymer in the buried backside conductive layer is generally from 10:90 to 60:40, and preferably from 20:80 to 50:50. A most preferred polymer combination is of polyester and cellulose acetate butyrate having a weight ratio of 40:60.

The buried backside conductive layer can also include still other polymers that are not defined herein as first or second polymers. These additional polymers can be either hydrophobic polymers or organic-soluble hydrophilic polymers. Some hydrophilic polymers that may be present include, but are not limited to, proteins or polypeptides such as gelatin and gelatin derivatives, polysaccharides, gum arabic, dextrans, polyacrylamides (including polymethacrylamides), polyvinyl pyrrolidones and others that would be readily apparent to one skilled in the art.

The metal oxide particles (such as the preferred non-acicular metal antimonate nanoparticles) generally comprise from more than 40 and up to 85% (preferably from 55 to 75%) by weight of the dry buried backside conductive layer. Thus, the weight % of the polymer mixture in the dry buried backside conductive layer is from 15 to 60 weight % and preferably from 25 to 45 weight %). Another way of defining the amount of metal oxide particles is that they are generally present in the buried backside conductive layer in an amount of from 0.05 to 1 g/m (preferably from 0.10 to 0.3 g/m ) of the dry layer coverage. Mixtures of different types of metal oxide particles can be used if desired. The optimum ratio of total binder to conductive particles can vary depending upon the specific conductive metal oxide particles and binders used, the conductive particle size, the coverage of conductive particles, the dry thickness of the conductive layer, and the dry thickness of adjacent layers. One skilled in the art would be able to determine the optimum parameters to achieve the desired conductivity and adhesion to adjacent layers and/or support.

The metal oxide particles (such as the preferred non-acicular metal antimonate nanoparticles) are generally present in an amount sufficient to provide a backside water electrode resistivity (WER) of 1 x 10¹² ohms/sq or less and preferably 1 x 10¹¹ ohms/sq or less at 7O⁰F (21.1°C) and 50% relative humidity.

Optional components of the buried backside conductive layer include materials that may improve coatability or adhesion, crosslinking agents (such as diisocyanates), surfactants and shelf-aging promoters.

Various methods describing the preparation of buried backside conductive layers are described in U.S. Patent 6,689,546 (LaBeIIe et al.) in U.S. Patent Application Publications 2006/0046932 (Ludemann et al.) and 2006/0046215 (Ludemann et al.), and in copending and commonly assigned U.S. Serial Number 10/978,205 (all noted above).

The buried backside conductive layers containing metal oxides are generally coated out of one or more miscible organic solvents including, but not limited to, methyl ethyl ketone (2-butanone, MEK), acetone, toluene, tetrahydrofuran, ethyl acetate, or any mixture of any two or more of these solvents. These hydrophobic organic solvents may contain a small amount (less that 20%, and preferably less than 10%) of a hydrophilic organic solvent such as methanol or ethanol.

The buried backside conductive layers and at the least one outermost backside layer can be simultaneously (wet-on-wet) coated using various coating procedures such as wire wound rod coating, dip coating, air knife coating, curtain coating, slide coating, or slot-die coating, extrusion coating. These procedures are the same as those described above for the thermographic and phototheraiographic imaging layers.

The buried backside conductive layer may be relatively thin. For example, it can have a dry thickness of from 0.05 to 1.1 μm (preferably from 0.10 to 0.3 μm, and most preferably of from 0.2 to 0.3 μm). The thin "buried" backside conductive layers are useful as "carrier" layers. The term "carrier layer" is often used when multiple layers are coated using slide coating and the buried backside conductive layer is a thin layer adjacent to the support.

In one preferred embodiment, the buried backside conductive layer is a carrier layer containing non-acicular metal antimonate particles and is directly disposed on the support without the use of primer or subbing layers, or other adhesion-promoting means such as support surface treatments. Thus, the support can be used in an "untreated" and "uncoated" form when a buried backside conductive carrier layer is used. The carrier layer formulation is applied simultaneously with application of these other backside layer formulations and is thereby located underneath these other backside layers.

In another preferred embodiment the outermost backside layer also comprises amorphous silica particles. It is more preferred that such amorphous silica particles have a narrow particle size distribution. The use of amorphous silica particles having a narrow particle size distribution in the outermost backside layer provides thermally developable imageable materials with lower haze while maintaining the surface roughness necessary to reduce blocking and provide efficient feeding into image processors. Further details describing the use of amorphous silica particles in outermost backside layers of thermally developable materials not containing polysiloxanes is described in copending and commonly assigned U.S. Serial Number 11/082,627 (filed March 1, 2005 by Philip, Ludemann, Koestner, and LaBeIIe). The roughness obtained from the amorphous silica particles is dependent on the diameter of the particles, the loading, and the thickness of the binder layer to which they are added. We believe this a function of the amount of the amorphous silica particle that is above the surface of the outermost backside layer. For example, for an outermost backside layer dry thickness of from 2 to 2.5 μm, preferred particles are those with a mean volume diameter from 7 to 9.5 μm with a standard deviation of less than 2.2 μm. The amorphous silica particles can be employed at a coating weight of 20 to 100 mg/m² and preferably from 30 to 70 mg/m². It is preferred that the mean volume diameter of the amorphous silica be at least 3 μm greater than the dry thickness of the outermost backside layer. It is more preferred that the amorphous silica particles extend at least 5 μm above the surface of the dried outermost backside layer. It is also preferred that the standard deviation divided by the mean particle size is less than 0.28 μm. The outermost backside layer generally has a dry thickness of from 1 to 3μm, preferably from 2 to 2.5 μm. Stated another way, the outermost backside layer generally has a dry coating weight of from 0.5 to 3 g/m² and preferably from 0.5 to 2.5 g/m².

Silica particle size can be measured by several methods. One popular method is by light scattering. A variety of companies manufacture instruments capable of measuring particle size. Among them are Microtrac, Inc. (Montgomeryville, MD) and Horiba Jobin Yvon Inc, Edison, NJ.

Methods that are typically used to conduct separation of particles in size ranges down to the micrometer size (that is, to classify particles) include, but are not limited to, settling chambers utilizing various impingers, centrifugal, separators, cyclone separators, and impingement separators. A particularly preferred method of separating particles by size is centrifugal air classification. This technique is described for example at <http://www.ccetechnologies.com> and <http://www.malvern.co.uk/ProcessEng/processes/classification/overview.htm>.

The relationship of the buried backside conductive layer(s), and the support or layer or layers immediately adjacent is important because the types of polymers and binders in these layers are designed to provide excellent adhesion to one another as well as acceptably dispersing the conductive metal oxide, the polysiloxane and smectite clay that has been modified with a quaternary ammonium compound, and/or other layer components are readily coated simultaneously, and affect the ability of the backside layers to reduce blocking. The layer directly disposed over the buried backside conductive layers is known herein as a "backside overcoat" layer, and can be known as a "backside protective" layer. Preferably it is the "outermost backside" layer and contains both the polysiloxane, and the smectite clay that has been modified with a quaternary ammonium compound described above. This backside overcoat layer comprises a film-forming polymer. The buried backside conductive layer immediately underneath comprises the conductive metal oxide (such as non-acicular metal antimonate particles) in a mixture of two or more polymers that includes a "first" polymer serving to promote adhesion of the buried backside conductive layer directly to the polymeric support, and a "second" polymer that is different than and forms a single-phase mixture with the first polymer and that promotes adhesion to the backside overcoat layer. For example, when the support is a polyester film, then a preferred mixture of polymers in the conductive layer is a single-phase mixture of a polyester resin and a polyvinyl acetal such as a polyvinyl butyral or cellulose ester such as cellulose acetate butyrate. It is preferred that the film-forming polymer of the backside overcoat layer be of the same class or at least be compatible with the second polymer of the buried backside conductive layer. Preferred film-forming polymers of the backside overcoat layer are polyvinyl butyral or cellulose acetate butyrate. In another embodiment, the buried backside conductive layer is disposed between the backside overcoat layer and an "undercoat" layer directly adhering to the support. In this embodiment, the backside overcoat layer is again directly above the buried backside conductive layer and is again known herein as a backside overcoat layer, an "interlayer," or a "protective" layer. This backside overcoat layer comprises a film-forming polymer. It can be the outermost backside layer or have further layer(s) disposed thereon. Preferably it is the outermost layer and contains the polysiloxane and the smectite clay that has been modified with a quaternary ammonium compound described herein. The conductive layer immediately beneath the backside overcoat layer comprises a conductive metal oxide (such as non-acicular metal antimonate particles) dispersed in polymer that serves to promote adhesion of the buried backside conductive layer to the backside overcoat layer as well as to the undercoat layer immediately beneath it. This undercoat layer is directly adhered to the polymeric support. The second, adhesion promoting layer directly adhered to the support comprises a mixture of two or more polymers. The first polymer serves to promote adhesion of the undercoat layer directly to the polymeric support. The second polymer serves to promote adhesion of the undercoat layer to the buried backside conductive layer.

It is preferred that the film-forming polymer of the backside overcoat layer be of the same class or at least be compatible with the second polymer of the buried backside conductive layer. It is also preferred that the polymer of the buried backside conductive layer be of the same class or compatible with the second polymer of the undercoat layer.

It is preferred that the undercoat, adhesion-promoting, layer use a single phase mixture of a polyester resin as a "first" polymer and a polyvinyl acetal such as polyvinyl butyral or a cellulose ester such as cellulose acetate butyrate as a "second" polymer." A preferred binder for the backside overcoat layer is cellulose acetate butyrate.

In yet another embodiment, the buried backside conductive layer is disposed between a backside overcoat layer and an undercoat layer directly adhering to the support. In this embodiment, the backside overcoat layer is again directly above the buried backside conductive layer and is again known herein as a backside overcoat layer, an "interlayer" or a "protective layer." This backside overcoat layer comprises a film-forming polymer. It can be the outermost backside layer or have further layer(s) disposed thereon. Preferably it is the outermost layer and contains the polysiloxane and the smectite clay that has been modified with a quaternary ammonium compound. The buried backside conductive layer immediately beneath the backside overcoat layer comprises a conductive metal oxide (such as non-acicular metal antimonate particles) in a mixture of two or more polymers, a "first" polymer that serves to promote adhesion of the buried backside conductive layer to the undercoat layer, and a "second" polymer that serves to promote adhesion of the buried backside conductive layer to the backside overcoat layer. It is preferred that the film-forming polymer of the backside overcoat layer, and the "second" polymer of the buried backside conductive layer are either compatible or are the same or different polyvinyl acetal resins. A preferred film-forming polymer of the backside overcoat layer is cellulose acetate butyrate. It is also preferred that the polymer of the second, adhesion promoting, layer and the first polymer of the buried backside conductive layer are the same or different polyester resins.

Representative "first" polymers can be chosen from one or more of the following classes: polyvinyl acetals (such as polyvinyl butyral, polyvinyl acetal, and polyvinyl formal), cellulosic ester polymers (such as cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose acetate propionate, hydroxy- methyl cellulose, cellulose nitrate, and cellulose acetate butyrate), polyesters, polycarbonates, epoxies, rosin polymers, polyketone resin, vinyl polymers (such as polyvinyl chloride, polyvinyl acetate, polystyrene, polyacrylonitrile, and butadiene-styrene copolymers), acrylate and methacrylate polymers, and maleic anhydride ester copolymers. The polyvinyl acetals, polyesters, cellulosic ester polymers, and vinyl polymers such as polyvinyl acetate and polyvinyl chloride are particularly preferred, and the polyvinyl acetals, polyesters, and cellulosic ester polymers are more preferred. Polyester resins are most preferred. Thus, the adhesion-promoting polymers are generally hydrophobic in nature. Representative "second" polymers include polyvinyl acetals, cellulosic polymers, vinyl polymers (as defined above for the "first" polymer), acrylate and methacrylate polymers, and maleic anhydride-ester copolymers. The most preferred "second" polymers are polyvinyl acetals and cellulosic ester polymers (such as cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose acetate propionate, hydroxymethyl cellulose, cellulose nitrate, and cellulose acetate butyrate). Cellulose acetate butyrate and polyvinyl butyral are particularly preferred second polymers. Of course, mixtures of these second polymers can be used in the buried backside conductive layer. These second polymers are also soluble or dispersible in the organic solvents described above.

It is preferred that the "first" and "second" polymers are compatible with each other or are of the same polymer class. One skilled in the art would readily understand from the teaching herein which polymers are

"compatible with" or "of the same class" as those film-forming polymers. For example, it is most preferred to use a single phase mixture of a polyester resin as a "first" polymer and a polyvinyl acetal such as polyvinyl butyral or a cellulose ester such as cellulose acetate butyrate as a "second" polymer." Many of the film- forming polymers useful in the backside overcoat layer are described in other places herein (for example, binders used in imaging layers and or other conventional backside layers).

The backside overcoat layer may also include other addenda commonly added to such formulations including, but not limited to: shelf life extenders, antihalation dyes, colorants to control tint and tone, magnetic recording materials to record data, UV absorbing materials to improve light-box stability, and coating aids such as surfactants to achieve high quality coatings, all in conventional amounts.

In one preferred embodiment, when used in a photothermographic material, the backside overcoat layer includes an antihalation composition, such as those antihalation compositions described above.

Imaging/Development

The thermally developable materials can be imaged in any suitable manner consistent with the type of material, using any suitable imaging source (typically some type of radiation or electronic signal for photothermographic materials and a source of thermal energy for thermographic materials). In some embodiments, the materials are sensitive to radiation in the range of from at least 100 nm to 1400 nm, and normally from 300 nm to 850 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). If necessary, sensitivity to a particular wavelength can be achieved by using appropriate spectral sensitizing dyes.

Imaging can be carried out 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 phosphor emitted radiation (particularly X-ray induced phosphor emitted radiation), 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 such as described in Research Disclosure, item 38957 (noted above). 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.). The photothermographic materials also can be indirectly imaged using an X-radiation imaging source and one or more prompt-emitting or storage X-radiation 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 nm), 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 photothermographic material at a suitably elevated temperature, for example, at from 5O⁰C to 250°C (preferably from 80°C to 200°C and more preferably from 100°C to 200°C) for a sufficient period of time, generally from 1 to 120 seconds. Heating can be accomplished using any suitable heating means. A preferred heat development procedure for photo thermographic materials includes heating at from 130°C to 165⁰C for from 3 to 25 seconds. Thermal development is carried out with a phototherrnographic material in a substantially water-free environment and without application of any solvent to the material.

When imaging direct thermographic materials, the image may be "written" simultaneously with development at a suitable temperature using a thermal stylus, a thermal print-head or a laser, or by heating while in contact with a heat-absorbing material. The thermographic materials may include a dye (such as an IR-absorbing dye) to facilitate direct development by exposure to laser radiation. Thermal development of either thermographic or photothermo- graphic materials is carried out with the material being in a substantially water- free environment and without application of any solvent to the material.

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

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

(C) positioning the imaged, heat-developed photothermographic or thermographic material between a source of imaging radiation and an imageable material that is sensitive to the imaging radiation, and (D) thereafter exposing the imageable material to the imaging radiation through the visible image in the exposed and heat-developed photothermographic material to provide an image in the imageable material.

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

Materials and Methods for the Experiments and Examples:

All materials used in the following examples are readily available from standard commercial sources, such as Aldrich Chemical Co. (Milwaukee Wisconsin) unless otherwise specified. All percentages are by weight unless otherwise indicated. The following additional methods and materials were used. Many of the chemical components used herein are provided as a solution. The term "active ingredient" means the amount or the percentage of the desired chemical component contained in a sample. All amounts listed herein are the amount of active ingredient added unless otherwise specified.

CAB 381-20 is a cellulose acetate butyrate resin available from Eastman Chemical Co. (Kingsport, TN). It has a butyryl content of 37%, an acetyl content of 13.5%, a hydroxyl content of 1.8%, and a polystyrene equivalent number average molecular weight determined by gel permeation chromatography of 70,000. CLAYTONE^® APA is a montmorillonite that has been modified by reaction with an organic quaternary ammonium salt. It is available from Southern Clay Products (Gonzales, TX). (http://www.claytone.net.).

CLOISITE^® 15A and CLOISITE^® 2OA are natural montmorillonites that have been modified by reaction with a dimethyl, dihydrogenated-tallow, quaternary ammonium chloride. Both are available from Southern Clay Products (Gonzales, TX). (http://www.nanoclay.com).

DMS-MFPS is dimethylsiloxane-co-methyl(3,3,3-trifluroropropyl) siloxane is available from Aldrich Chemical Company. Dow DC 200 is a polydimethyl siloxane available from Dow

Corning (Midland, MI). It can be obtained with various viscosities that reflect the molecular weight, (http://www.dowcorning.com/)

Dow DC510-500cst is a polyphenylmethylsiloxane.

Dow DC710-500cst is a polyphenylmethylsiloxane. CELNAX^® CX-Z641M is an organosol dispersion containing 60% of non-acicular zinc antimonate nanoparticles in methanol. It was obtained from Nissan Chemical America Corporation (Houston, TX).

Gelest DMS-A31 is an aminopropyl-terminated polydimethyl- siloxane available from Gelest Inc. (Morrisville, PA) (http://www.gelest.com). Gelest DMS-B25 is a carboxypropyl-terminated polydimethyl- siloxane.

Gelest DMS-S31 is a silanol-terminated polydimethylsiloxane.

Gelest DMS-V3 l is a vinyl-terminated polydimethylsiloxane.

Gelest ECMS-227, ECMS-327, and ECMS-924 are all (epoxycyclohexylethyl)methylsiloxane dimethysiloxane copolymers.

Gelest PDV-0525 is a vinyl-terminated diphenylsiloxane-dimethyl- siloxane copolymer.

Gelest PMM- 1043 is a phenylmethylsiloxane-dimethylsiloxane copolymer. PMHS is polymethylhexyl siloxane and is available from Scientific

Polymer Products (Ontario, NY). PMODS is polymethyloctaldecyl siloxane and is available from Aldrich Chemical Company.

PMOS is polymethyloctyl siloxane and is available from Scientific Polymer Products (Ontario, NY).

PS-563 is a carboxypropyl terminated polydimethylsiloxane and is available from United Chemical Technologies (Bristol, PA).

Mayer Bars are ¹A inch diameter Type 303 Stainless Steel coating rods and are available from R.D. Specialties, Inc. (Webster, NY).

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

SYLOID^® 74X6000 is a synthetic non-spherical amorphous silica that is available from Grace-Davison (Columbia, MD).

VITEL^® PE-2700B LMW is a polyester resin available from Bostik, Inc. (Middleton, MA).

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

Abrasion Resistance Test:

The abrasion resistance of a coating is an indication of the integrity of the coated layer. The test is both subjective and comparative. The rounded tip of a tongue depressor is held at a 45° angle and pushed along the top of the coating. The abrasion resistance of the surface is rated according to the amount of marring and coating removed. Each sample is tested 6 times using the same tongue depressor. The tongue depressor is wiped after each test, and the results recorded as the average of the 6 values. Anew tongue depressor is used for each sample. Abrasion resistance is rated on a scale of 0 to 5 with 0 being worst. A rating of 2 or greater is preferred. A rating of 3 or greater is more preferred.

Level 0 - 100% of the coating removed. Level 1 - greater that 75% of the coating removed.

Level 2 - 50 to 74% of the coating removed.

Level 3 - 25 to 50% of the coating removed.

Level 3.5 - 10 to 20% of the coating removed.

Level 4 - Surface is marred, 1 to 5% of the coating removed. Level 4.5 - Surface is slightly marred.

Level 5 - Surface is not marred.

Coefficient of Friction Test

Dry View film is placed within cartridges holding up to 125 sheets. The friction between the frontside (imaging side) and backside of the film is crucial to obtaining good stack quality of the film.

The coefficient of friction (COF) was measured between the frontside of one piece of photothermographic film and the backside of a second piece of photothermographic film using a SP-102B-3M90 Extended Capability Slip/Peel Tester (available from IMASS, Inc., Accord, MA). The test was carried out according to ASTM method D 1894 "Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting."

In this test, a first piece of photothermographic film was attached to a stainless steel platen. The backside of the film was in contact with the platen and the frontside was exposed. A second piece of photothermographic film measuring 2.5 x 6 in (6.4 cm x 15.2 cm) was attached to a moveable sled so that the backside of the second piece of film was in contact with the frontside of the first piece of film. A 398 g weight was added to the top of the sled. The sled was pulled at 90 in/min (229 cm/min) so that the backside of the second piece of film slid against the frontside of the first piece of film. The coefficient of friction was determined by the following equation.

COF = F / N = attractive force/normal force = meter reading/sled weight Comparative Example 1

Buried Backside Conductive Layer Formulation:

A buried backside conductive layer formulation containing zinc antimonate clusters was prepared as described in copending and commonly assigned U.S. Serial No. 10/978,205 (noted above) and also described below.

A dispersion was prepared by adding 16.88 parts of MEK to 7.92 parts of CELNAX^® CX-Z641M (containing 60% non-acicular zinc antimonate solids in methanol - 4.75 parts net). The addition took place over 15 minutes. Strong stirring was maintained for 15 minutes. A polymer solution was prepared by dissolving 0.35 parts of

VITEL^® PE-2700B LMW and 1.40 parts of CAB 381-20 in 41.25 parts of MEK.

The polymer solution was added to the CELNAX^® dispersion over 15 minutes with strong mixing. An additional 32.21 parts of MEK was then added over 5 minutes. Mixing was continued for 10 minutes. The final formulation had a viscosity of 5 cP (centipoise) and a specific gravity of 0.84.

Outermost Backside Layer Formulation:

An outermost backside layer formulation was prepared by mixing the following materials:

MEK 52.37 parts MeOH 34.92

CAB 381-20 12.17 parts

SYLOID^® 74X6000 0.38 parts

Antihalation Dye BC- 1 0.16 parts

The buried backside conductive layer formulation and outermost backside layer formulation were simultaneously coated onto one side of a 7 mil (178 μm) blue tinted poly(ethylene terephthalate) support using a precision automated multilayer slide coater equipped with an in-line dryer. The backside coatings were dried at approximately 93⁰C for 3 minutes. The coating weight of the backside conductive layer was 0.02 g/ft² (0.21 g/m²). Three thicknesses of the outermost backside layer were coated. These samples had no thermally developable coatings on the frontside. To determine the abrasion resistance of the backside overcoat after imaging, the samples were subjected to processing conditions by passing them through a heated rotating drum thermal processor at 122⁰C for 15 seconds.

The results are shown below in TABLE I. All samples had 'poor abrasion resistance. The abrasion resistance decreased as the thickness decreased. Samples tended to have better abrasion resistance after processing.

TABLE I -Abrasion Resistance Properties of Outermost Backside Layer

Sample Dry Thickness Dry Coating Weight Abrasion Resistance Abrasion Resistance Coefficient of

(μm) (g/m²) Before Processing After Processing Friction

C-M 4.8 4.0 2 3.5 0.93

C-l-2 3.6 3.0 1.5 2.5 0.92

C-l-3 2.4 2.0 1 1 0.88

OO

Inventive Example 1

A buried backside conductive layer formulation was prepared as described in Comparative Example 1.

Outermost Backside Laver Formulation:

MEK 52.37 parts

MeOH 34.92

CAB 381-20 12.17 parts

SYLOID^® 74X6000 0.38 parts

Antihalation Dye BC-I 0.16 parts

CLOISITE^® 15A 0.24 parts

Polysiloxane See Table II 10% solids in MEK In this Example, CLOISITE^® 15 A was present at 2% by weight to that of CAB 381-20. Three polysiloxanes were used. The buried backside conductive layer and outermost backside layers were simultaneously coated onto the backside of a photothermographic material using the techniques described above in Comparative Example 1. The dry thickness of the buried backside conductive layer was also the same as in Comparative Example 1 (0.21 g/m²). The dry thickness of the outermost backside layer was at 2.4 μm (2 g/m²). The abrasion resistance of each sample was determined as described above. The results, shown below in Table II, demonstrate the significant improvement in abrasion resistance when polysiloxane and clay are incorporated in the outermost backside layer. The data of Sample C- 1-3 of

Comparative Example 1 is repeated for comparison. The results further show that a reduction in coefficient of friction is not necessary to improve abrasion resistance. TABLE II -Abrasion Resistance Properties of Outermost Backside Layer

Sample Polysiloxane Polysiloxane as a Abrasion Abrasion Coefficient of % of CAB Resistance Resistance After Friction 381-20 Before Processing Processing

C- 1 -3 -Comparative None None 1 1 0.88

1-1 -Inventive Gelest DMS-A31 1.2 4.0 4.5 0.87

1-2-Inventive Gelest DMS-A31 1.4 4.0 4.5 0.91

1-3 -Inventive Gelest DMS-A31 1.6 4.5 4.5 0.91

1-4-Inventive Dow DC 200-350cst 1.0 1.5 2.0 0.84

1-5-Inventive Dow DC 200-350cst 1.6 4.0 4.0 0.89

I 1-6-Inventive Dow DC 200-350cst 3.0 4.5 4.5 0.72

Os O 1-7-Inventive Dow DC 200-1000cst 1.0 3.0 3.0 0.88

1-8-Inventive Dow DC 200-1 OOOcst 1.6 3.0 4.0 0.88

1-9-Inventive Dow DC 200-1000cst 3.0 3.8 4.5 0.70

Inventive Example 2:

This example shows the effect of adding CLOISITE® 15Ato the outermost backside layer of photothermographic materials containing either Dow DC 510-500cst or Gelest DMS-A31. The same buried conductive backside formulation as in Inventive Example 2 was used, but the level of CLOISITE^® 15 A was varied.

New lots of raw materials were used. The results, shown below in TABLE III demonstrate that the abrasion resistance can be significantly improved with addition of CLOISITE^® 15 A. This experiment gave lower coefficient of friction then the previous experiment due to normal variations from coating runs and changes in raw materials. Addition of clay did lower the coefficient of friction. There was no significant correlation between coefficient of friction and abrasion resistance improvement when comparing all three experiments.

TABLE III -Abrasion Resistance Properties of Outermost Backside Layer

Sample Polysiloxane Polysiloxane as a Cloisite 15 A as a Abrasion Resistance Coefficient of

% of CAB 381-20 % of CAB 381-20 Before Processing Friction

2-1 -Inventive GelestDMS-A31 1.5 None 3.0 0.71 2-2-Inventive GelestDMS-A31 1.5 2.0 4.5 0.62 2-3 -Inventive DowDC510-500cst 6.0 None 3.5 0.60 2-4-Inventive DowDC510-500cst 6.0 1.0 4.0 0.55 2-5-Inventive DowDC510-500cst 6.0 2.0 4.5 0.57 2-6-Inventive DowDC510-500cst 6.0 3.0 4.5 0.56

Os 2-7-Inventive DowDC510-500cst 6.0 None 3.0 0.63 2-8-Inventive DowDC510-500cst 6.0 2.0 4.5 0.54 2-9-Inventive DowDC510-500cst 7.0 None 3.0 0.65 2-10-Inventive DowDC510-500cst 8.0 None 4.0 0.53

Inventive Example 3:

A buried backside conductive layer formulation, prepared as described in Inventive Example 1, was coated onto a 7 mil (178 μm) blue tinted polyethylene support using a #5 Mayer Rod and dried at for 3 minutes at 93⁰C. The dry coating weight of the buried backside conductive layer was 0.21 g/m².

Outermost backside layer formulations were prepared using the same formulation as in Inventive Example 1. Seven polysiloxanes were evaluated, each at two concentrations. The concentration of CLOISITE^® 15 A was kept constant at 2% by weight to that of CAB 381-20.A11 samples contained 7 weight % solids and were coated above the backside conductive layer using a laboratory scale automated knife coater equipped with an in-line dryer. Samples were dried for 3 minutes at 93⁰C. The dry coating weight of the outermost backside layer was 2.0 g/m².

Samples were tested for abrasion resistance as described above. The results, shown below in TABLE IV, demonstrate that abrasion resistance can be significantly improved with the addition of both a clay and a polysiloxane.

TABLE IV- Abrasion Resistance Properties of Outermost Backside Layer

Sample Polysiloxane Polysiloxane as a Cloisite^® 15A as a Abrasion Resistance % of CAB 381-20 % of CAB 381-20 before Processing

3-1 -Comparative none N/A 2.0 1.3

3-2-Inventive Dow DC510-500cst 1.5 2.0 3.7

3 -3 -Inventive Dow DC510-500cst 6.0 2.0 4.3

3-4-Inventive Gelest DMS-A31 1.5 2.0 4.5

3-5-Inventive Gelest DMS-A31 6.0 2.0 4.5

3-6-Inventive Gelest DMS-B25 1.5 2.0 3.3

S 3-7-Inventive Gelest DMS-B25 6.0 2.0 4.5

3-8-Inventive Gelest DMS-S31 1.5 2.0 3.3

3-9-Inventive Gelest DMS-S31 6.0 2.0 4

3-10-Inventive Gelest DMS-V31 1.5 2.0 3.5

3-11 -Inventive Gelest DMS-V31 6.0 2.0 3.7

Sample Polysiloxane Polysiloxane as a Cloisite 15A as a Abrasion Resistance

% of CAB 381-20 % of CAB 381-20 before Processing

3-12-Inventive Gelest PDV-0525 1.5 2.0 4

3-13-Inventive Gelest PDV-0525 6.0 2.0 4.5

3-14-Inventive Gelest PMM-1043 1.5 2.0 3.7

3-15-Inventive Gelest PMM-1043 6.0 2.0 4

ON

Inventive Example 4

A buried backside conductive layer formulation, prepared as described in Inventive Example 1, was coated onto a 7 mil (178 μm) blue tinted polyethylene support using a #5 Mayer Rod and dried at for 3 minutes at 93⁰C. The dry coating weight of the buried backside conductive layer was 0.21 g/m². Outermost backside layer formulations were prepared using the same formulation as in Inventive Example 1. Seven polysiloxanes either having different substituents within the siloxane chain or having different terminal groups were evaluated, each at two concentrations. The concentration of CLOISITE^® 15 A was kept constant at 2 weight % that of CAB 381-20. All samples were coated above the backside conductive layer using a laboratory scale automated knife coater equipped with an in-line dryer. Samples were dried for 3 minutes at 93°C. The dry coating weight of the outermost backside layer was 2.0 g/m².

The results, shown below in TABLE V, demonstrate that abrasion resistance can be significantly improved with the addition of both a clay and a polysiloxane. Polymethyloctyldecyl siloxane (PMODS) is insoluble in MEK and did not improve abrasion resistance. Polymethyloctylsiloxane (PMOS) and polymethylhexyl siloxanes (PMOHS) are partially soluble in MEK and although the resultant coatings had a patterning on the surface of the film they impart improved abrasion resistance to the film when present at either 1.5 or 6 weight % to that of CAB-381-20.

TABLE V -Abrasion Resistance Properties of Outermost Backside Layer

Sample Polysiloxane Polysiloxane as a Cloisite 15A as a Abrasion % of CAB % of C AB Resistance before

Processing

4-1 -Comparative none N/A none 0.0

4-2-Inventive Dow DC510-500cst 1.5 2.0 3.2 4-3-Inventive Dow DC510-500cst 6.0 2.0 4.5

4-4-Inventive Gelest DMS-A31 1.5 2.0 4.5 4-5-Inventive Gelest DMS-A31 6.0 2.0 4.5

4-6-Inventive PMODS 1.5 2.0 0.0

4-7-Inventive PMOS 1.5 2.0 4.0

4-8-Inventive PMOS 6.0 2.0 4.5

4-9-Inventive PMHS 1.5 2.0 4.0

4-10-Inventive PMHS 6.0 2.0 4.5

Sample Polysiloxane Polysiloxane as a Cloisite^® 15A as a Abrasion % of CAB % of CAB Resistance before Processing

4-11 -Inventive DMS-MFPS 1.5 2.0 2.8

4-12-Inventive DMS-MFPS 6.0 2.0 4.0

4- 13 -Inventive PS-563 1.5 2.0 3.5

4-14-Inventive PS-563 6.0 2.0 3.7

4-15-Inventive DMS-MFPS 1.5 2.0 3.3

OO

4-16-Inventive DMS-MFPS 6.0 2.0 4.5

4-17-Inventive Gelest ECMS-227 1.5 2.0 3.5

4-18-Inventive Gelest ECMS-227 6.0 2.0 4.0

4-19-Inventive Gelest ECMS-327 1.5 2.0 3.9

4-20-Inventive Gelest ECMS-327 6.0 2.0 4.0

4-21 -Inventive Gelest ECMS-924 1.5 2.0 3.5

4-22-Inventive Gelest ECMS-924 6.0 2.0 4.0

Samples 4-11 and 4-15 are replicates. Samples 4-12 and 4-16 are replicates.

Example 5:

This example shows that abrasion resistance can be significantly improved with the combination of a polysiloxane and a smectite clay.

A buried backside conductive layer formulation was prepared as described in Comparative Example 1. An outermost backside layer formulation was prepared as described in Inventive Example 1 but the concentration of CLOISITE^® 15 A was varied. The formulations were simultaneously coated using a precision automated multilayer slide coater equipped with an in-line dryer. Samples were dried for 3.5 minutes at 93⁰C. The dry coating weight of the buried backside conductive layer was 0.2 g/m². The dry coating weight of the outermost backside layer was 2.0 g/m².

The results, shown below in TABLE VI, demonstrate that addition of polysiloxanes having a wide variety of functional groups significantly improves abrasion resistance of outermost backside coatings. The combination of a polysiloxane and a smectite clay modified with a quaternary ammonium compound provides additional improvement

TABLE VI

Sample Polysiloxane Ratio of Polysiloxane Ratio of Cloisite Abrasion Resistance to CAB 15Ato CAB before Processing

5-1 -Inventive Dow DC510-500cst 6.0% 2.0% 4.2

5-2-Inventive Gelest DMS-S31 6.0% None 3.0

5-3 -Inventive Gelest DMS-S31 6.0% 2.0% 3.5

5-4-Inventive Gelest DMS-V31 6.0% None 3.5

5-5-Inventive Gelest DMS-V31 6.0% 2.0% 4.3

5-6-Inventive Dow DC710-500cst 6.0% 2.0% 2.3

5-7-Inventive Dow DC710-500cst 8.0% 2.0% 4

Example 6:

An experiment was run to determine the level of clay necessary to provide improved abrasion resistance as well as to determine if various other smectite clays modified with a quaternary ammonium compound provide improved abrasion resistance.

A buried backside conductive layer formulation was prepared as described in Comparative Example 1. An outermost backside layer formulation was prepared as described in Inventive Example 1 but the concentration of CLOISITE^® 15A was replaced with other clays. The formulations were simultaneously coated using a precision automated multilayer slide coater equipped with an in-line dryer. Samples were dried for 3.5 minutes at 93⁰C. The coating weight of the outermost backside layer was 2.0 g/m². All outermost backside layer formulation contained Dow DC510-500cst polysiloxane at 6 weight % to that of CAB 381-20. The concentration of clay was varied as shown in TABLE VII.

The results, shown below in TABLE VI, demonstrate that a ratio of clay to binder resin of between 1 and 2% is necessary to provide improved abrasion resistance.

TABLE VII

Sample Clay Used Ratio of Abrasion Resistance Clay to CAB Before Processing

6- 1 -Inventive Cloisite^® 2OA 1% 1.5 6-2-Inventive Cloisite^® 2OA 2% 3.85 6-3 -Inventive Claytone^® APA 2% 3.5

Inventive Example 7:

This example shows the effect of varying the level of polysiloxane on sheet-to-sheet surface scratching. Such scratching is caused by one sheet of coated film rubbing against an adjacent sheet in the course of feeding and transport through an automated imaging device. Excessive scratches that are visible in an image are objectionable.

A buried backside conductive layer formulation was prepared as described in Comparative Example 1. An outermost backside layer formulation was prepared as described in Inventive Example 1 but the concentration of polysiloxane DC510-500cst was varied. The formulations were simultaneously coated using a precision automated multilayer slide coater equipped with an in-line dryer. Samples were dried for 3.5 minutes at 93°C. The dry coating weight of the buried backside conductive layer was 0.2 g/m . The dry coating weight of the outermost backside layer was 2.0 g/m". Four hundred sheets of each coating were fed, imaged, and developed in a commercially available Eastman Kodak Model 8610 medical imaging printer. All sheets were printed to a uniform an optical density of 1.80. Sixty sheets were randomly selected and the number of scratches in the region of each sheet impacted by the feed mechanism was determined using a Vidar Diagnostic Pro Plus medical image scanner from Vidar Systems Corporation (Herndon, VA). The scanner counts the number of scratched pixels in the 1.80 optical density image.

The results, shown below in TABLE VIII, demonstrate that higher levels of a polysiloxane, such as DC510-500cst, can cause an increase in the propensity for film scratching. An amount of polysiloxane DC510-500cst 6% by weight or less to that of CAB 381-20 and preferably about 3.0% by weight to that of CAB381-20 provides a coating with an acceptable number of scratches while maintaining the improved abrasion resistance described in Inventive Example 4. TABLE VIII - Scratch and Abrasion Resistance Properties of Outermost Backside Layer

Sample Polysiloxane Ratio of Ratio of Imager Scratch Abrasion Resistance

Polysiloxane to Cloisite^® 15A Pixels Before Processing

CAB to CAB

7-1 -Inventive Dow DC510-500cst 3.0% 2.0% 306 4.5

7-2-Inventive Dow DC510-500cst 6.0% 2.%0 1228 3.5

OJ

Claims

CLAIMS:

1. A thermally developable material that comprises a support having on one side thereof, one or more thermally developable imaging layers comprising a binder and in reactive association, a non-photosensitive source of reducible silver ions, and a reducing agent composition for said non-photosensitive source reducible silver ions, and having disposed on the backside of said support, an outermost backside layer comprising one or more binder polymers, a polysiloxane, and a smectite clay that has been modified with a quaternary ammonium compound, and a buried backside conductive layer underneath said outermost backside layer.

2. The material of claim 1 said outermost backside layer further comprising amorphous silica particles.

3. The material of claim 1 wherein the standard deviation divided by the mean volume diameter of said amorphous silica particles is less than 0.28.

4. The thermally developable material of claim 2 wherein the mean volume diameter of said amorphous silica particles is at least 3 μm greater than the dry thickness of said outermost backside layer.

5. The thermally developable material of claim 2 wherein said outermost backside layer has a dry thickness of from 2 to 2.5 μm and the mean volume diameter of said amorphous silica particles is from 7 to 9.5 μm with a standard deviation of less than 2.2 μm.

6. The thermally developable material of claim 2 wherein said silica particles are present in said outermost backside layer in an amount of from 20 to lOO mg/m².

7. The material of claim 1 wherein said outermost backside layer comprises a polyvinyl acetal or cellulosic polymer as a binder.

8. The material of claim 1 wherein said outermost backside layer has a dry thickness of from 1 to 3 μm.

9. The material of claim 1 said buried backside conductive layer being interposed between and directly adhering said support to said outermost backside layer, and having a water electrode resistivity measured at 21.1°C and 50% relative humidity of 1 x 10¹² ohms/sq or less.

10. The material of claim 9 wherein said buried backside conductive layer comprises a metal oxide that is present in said buried backside conductive layer in an amount of from 0.05 to 1 g/m² distributed within one or more binder polymers that are present in an amount of from 15 to 60 weight %.

11. The material of claim 9 wherein said buried backside conductive layer has a dry thickness of from 0.05 to 1.1 μm.

12. The material of claim 9 wherein said buried backside conductive layer and said outermost backside layer have been formulated in organic solvents and have been simultaneously coated.

13. The material of claim 9 wherein said outermost backside layer comprises a cellulosic ester polymer and said buried backside conductive layer comprises a single phase mixture of a polyester and a polyvinyl acetal or a cellulose ester.

14. The material of claim 1 wherein said polysiloxane is present in said outermost layer an amount of 0.5 to 6 weight %.

15. The material of claim 1 wherein said polysiloxane has the following Structure (I):

wherein one or more of the terminal groups of said polysiloxane is methyl or alkylamino.

16. The material of claim 1 wherein the siloxane chain of said polysiloxane contains diphenyl, phenylmethyl, dimethyl, a mixture of phenyl- methyl and dimethyl groups, or a mixture of (epoxycyclohexylethyl)methyl and dimethyl groups.

17. The material of claim 1 wherein said non-photosensitive source of reducible silver ions is a silver salt of an aliphatic carboxylic acid or a mixture of silver salts of aliphatic carboxylic acids, at least one of which is silver behenate.

18. The material of claim 1 that is a non-photosensitive thermographic material.

19. The material of claim 1 that is a photothermographic material further comprising a photosensitive silver halide.

20. A black-and-white photothermographic material that comprises a support having on one side thereof, one or more thermally developable imaging layers comprising a binder and in reactive association, a preformed photosensitive silver halide, a non-photosensitive source of reducible silver ions that comprises silver behenate, and a reducing agent composition for said non-photosensitive source reducible silver ions that comprises a hindered phenol, and having disposed on the backside of said support, a simultaneously coated outermost backside layer and a buried backside conductive layer: a) said outermost backside layer comprising a film-forming polymer, a polysiloxane, a smectite clay that has been modified with a quaternary ammonium compound, and amorphous silica particles, and b) said buried backside conductive layer being interposed between and directly adhering said support to said outermost backside layer, said buried backside conductive layer comprising a mixture of two or more polymers that include a first polymer serving to promote adhesion of said buried backside conductive layer directly to said support, and a second polymer that is different than and forms a single phase mixture with said first polymer, wherein:

1) said buried backside conductive layer has a water electrode resistivity measured at 21.1⁰C and 50% relative humidity of 1 x 10¹² ohms/sq or less,

2) the total amount of mixture of two or more polymers in said buried backside conductive layer is at least 15 weight %,

3) said buried backside conductive layer comprises a conductive non- acicular zinc antimonate,

4) said film-forming polymer of said outermost backside layer is a cellulose acetate butyrate, said second polymer of said buried backside conductive layer is a cellulose acetate butyrate, and said first polymer is a polyester,

5) said amorphous silica particles are present in an amount of from 30 to 70 mg/m² of said outermost backside layer,

6) said outermost backside layer has a dry thickness of from 1 to 3 μm,

7) the mean volume diameter of said amorphous silica particles is from 7 to 9.5 μm with a standard deviation of less than 2.2 μm, 8) said amorphous silica particles extend at least 5 μm above the surface of said dried outermost backside layer, 9) said buried backside conductive layer has a dry thickness of from 0.10 to 0.3 μm, and

10) said polysiloxane chain of said polysiloxane contains diphenyl, phenylmethyl, dimethyl, a mixture of phenylmethyl and dimethyl groups, or a mixture of (epoxycyclohexylethyl)methyl and dimethyl groups.

21. A method of forming a visible image comprising: (A) imagewise exposing the material of claim 1 that is a photothermo- graphic material to electromagnetic radiation to form a latent image, (B) simultaneously or sequentially, heating said exposed photothermo- graphic material to develop said latent image into a visible image.

22 A black-and-white photothermographic material that comprises a support having on one side thereof, one or more thermally developable imaging layers comprising a binder and in reactive association, a photosensitive silver halide, a non-photosensitive source of reducible silver ions, and a reducing agent composition for said non-photosensitive source reducible silver ions, and having disposed on the backside of said support, an outermost backside layer comprising a film-forming polymer, a polysiloxane, and a smectite clay that has been modified with a quaternary ammonium compound, and a buried backside conductive layer underneath said outermost backside layer.

23. The black-and-white photothermographic material of claim

22 said buried backside conductive layer being interposed between and directly adhering said support to said outermost backside layer, and having a water electrode resistivity measured at 21.1 °C and 50% relative humidity of 1 x

10 ohms/sq or less.

24. The material of claim 23 wherein said buried backside conductive layer and said outermost backside layer have been formulated in organic solvents and have been simultaneously coated.

25. The material of 22 wherein said polysiloxane is present in an amount of from 1.5 to 6% based on the total dry weight of said film-forming polymer.

26. The material of claim 7 wherein said polysiloxane is present in an amount of from 1.5 to 6% based on the total dry weight of said polyvinyl acetal or cellulosic polymer binder.

27. The material of claim 1 wherein said polysiloxane is present in an amount of from 1.5 to 6% based on the total dry weight of said binder polymers.