US3874946A

US3874946A - Photothermographic element, composition and process

Info

Publication number: US3874946A
Application number: US443892A
Authority: US
Inventors: Lorenzo F Costa; Allan James A Van; Frank Grum
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1974-02-19
Filing date: 1974-02-19
Publication date: 1975-04-01
Anticipated expiration: 1992-04-01
Also published as: GB1495335A; DE2506815A1; FR2261555B1; JPS54165B2; DE2506815B2; CA1045876A; JPS50120328A; CH609160A5; DE2506815C3; FR2261555A1; BE825716A

Abstract

A stabilizer precursor comprising a compound which is a photolytically active polybrominated organic compound comprising (a) a 2,4-bis(tribromomethyl)-s-triazine, (b) a polybromoalkylsulfonyl compound having an aromatic chromophore group, or (c) a combination of the described stabilizer precursors, in a photothermographic element or composition comprising photographic silver halide provides improved postprocessing stability of the element or composition. The described stabilizer precursors are useful in photothermographic materials, for example, comprising photographic silver halide and an oxidation-reduction image-forming combination comprising a heavy metal salt oxidizing agent with a reducing agent. A developed visible image is provided after imagewise exposure by overall heating the photothermographic material to moderately elevated temperatures.

Description

United States Patent 91 [111 3,874,946 Costa et al. Apr. 1, 1975 1 PHOTOTHERMOGRAPHIC ELEMENT, Primary E.\'aminerNorman G. Torchin COM POSITION AND PROCESS Inventors: Lorenzo F. Costa; James A. Van

Allan; Frank Grum, all of Rochester, NY.

Eastman Kodak Company, Rochester, NY.

Filed: Feb. 19, 1974 Appl. No.: 443,892

Assignee:

References Cited UNITED STATES PATENTS 6/1972 de Mauriac 96/1 14.1 X

Assistant Examiner-Alfonso T. Suro Pico Attorney, Agent, or Firm-R. E. Knapp [57] ABSTRACT A stabilizer precursor comprising a compound which is a photolytically active polybrominated organic compound comprising (a) a 2,4-bis(tribromomethyl)-striazine, (b) a polybromoalkylsulfonyl compound having an aromatic chromophore group, or (c) a combination of the described stabilizer precursors, in a photothermographic element or compostion comprising photographic silver halide provides improved postprocessing stability of the element or composition. The described stabilizer precursors are useful in photothermographic materials, for example, comprising photographic silver halide and an oxidationreduction image-forming combination comprising a heavy metal salt oxidizing agent with a reducing agent. A developed visible image is provided after imagewise exposure by overall heating the photothermographic material to moderately elevated temperatures.

19 Claims, No Drawings PHOTOTHERMOGRAPHIC ELEMENT, COMPOSITION AND PROCESS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to certain photolytically active polybrominated stabilizer precursors and photothermographic elements, compositions and processes using said precursors to provide improved stable images. In one of its aspects it relates to a photothermographic element comprising (a) photographic silver halide in association with (b) an oxidation-reduction imageforming combination comprising (i) a heavy metal salt oxidizing agent with (ii) an organic reducing agent, (c) a polymeric binder for the layer and (d) a photolytically active polybrominated organic compound stabilizer precursor as described. In another of its aspects it relates to a photothermographic composition comprising photographic silver halide, a polymeric binder and a photolytically active polybrominated organic compound stabilizer precursor as described. A further aspect relates to such photographic compositions which are photothermographic materials. A further aspect relates to a method of developing a stable image in a photothermographic element comprising the described photographic silver halide in association with the oxidation-reduction image-forming combination and the described stabilizer precursors.

2. Description of the State of the Art It is known to obtain an image in a photosensitive element by so-called dry processing with heat. The photothermographic element employed for providing such an image can contain a reducing agent, a light-sensitive silver salt of an organic acid, such as silver behenate, as an oxidizing agent and a low concentration of photographic silver halide. Such photothermographic materials are described, for example, in U.S. Pat. No. 3,457,075 of Morgan, et al., issued July 22, 1969; U.S. Pat. No. 3,152,904 of Sorensen, et al., issued Oct. 13, 1964; British Specification No. 1,161,777 published Aug. 20, 1969; U.S. Pat. No. 3,392,020 of Yutzy, et al., issued July 9, 1968; U.S. Pat. No. 3,707,377 of Tiers, et al., issued Dec. 26, 1972 and in Research Disclosure, January, 1973, pages 16-21.

In a photothermographic material one of the main difficulties involves post-processing stability. Because heat developable photographic elements are suitable for so-called dry processing with heat and are designed to eliminate a fixing step which normally would remove undeveloped silver, it is necessary that a means be provided for post-processing stabilization to enable roomlight handling.

Several means have been proposed to answer the need for post-processing stability of photothermographic elements. These include, for example, (1) washing the element with water to remove undeveloped silver salts, (2) heating the element to release Bronstead or Lewis acid such as HCl, borontrifluoride or HF from compounds such as mnitrobenzenesulfonyl chloride, para-toluenesulfonic acid urea addition complex or pacetamidobenzenediazonium fluoroborate and (3) chelation of the oxidizing agent with, for example, salicylaldoxime or benzotriazole as described in U.S. Pat. No. 3,152,904 of Sorensen, et al., issued Oct. 13, 1964. Another method proposed for solving the problem of post-processing instability is to provide a photothermographic combination in which the oxidationreduction image-forming combination is on one sheet of material and the latent image-forming photographic silver halide on a separate sheet. The sheets are separated after imagewise exposure such as described in U.S. Pat. No. 3,152,904. A further means proposed for stabilization involves swabbing a 1% solution of phenylmercaptotetrazole onto the surface of the overall heated photothermographic material or rubbing benzotriazole into the surface of the photothermographic element. These proposed means for stabilization are not useful for large volume handling of heat developable photographic materials.

It has also been proposed to provide stabilized images in heat developable, photographic materials by treating the developed image with a solution containing certain thiol or thione compound stabilizers. This is described, for example, in U.S. Pat. No. 3,617,289 of Ohkubo, et al., issued Nov. 2, 1971. One of the compounds proposed for this solution stabilization is l-phenyl-S- mercaptotetrazole. Unfortunately, the addition of this compound to unexposed photographic silver halide compositions provides undesired desensitization at concentrations which produce stabilization and toning of a developed image.

Another means of stabilization of an image in a photothermographic material is described in copending U.S. application Ser. No. 249,260 of Hiller, filed May 1, 1972 which is a continuation-in-part application of U.S. Ser. No. 43,171 of Hiller, filed June 3, 1970, now abandoned and corresponding to Belgian Pat. No. 768,071 published July 30, 1971. Certain stabilizer precursors which are azole thioethers or blocked azoline thiones are employed in photothermographic ma terials according to the description in the applications. An example of a stabilizer precursor proposed in photothermographic materials is 5- methoxycarbonylthio- 1 -phenyltetrazole. U nfortunately, this compound does not always provide the desired increased stability in the developed image.

Another means for post-processing stabilization of certain photothermographic materials is described in U.S. Pat. No. 3,707,377 of Tiers, et al., issued Dec. 26, 1972. Certain polyhalogenated organic oxidizing agents such as tetrabromobutane and tribromoquinalidine are described in certain heat developable photographic materials to provide post-processing stabilization. Unfortunately, tetrabromobutane and tribromoquinaldine do not provide the desired stabilization in certain photothermographic materials. For instance, photothermographic elements containing tribromoquinaldine cannot be expected to be processable above about 50C. without undesired decomposition of the tribromoquinaldine. As illustrated in the following comparative examples, the photolytically active polybrominated organic compound stabilizer precursors according to the invention provide surprisingly improved stabilization without significantly adversely affecting maximum density and without undesired decomposition.

l-lalogenated organic compounds have been employed in photographic materials for various purposes, such as antifoggants. l-lalogenated organic antifoggants for photographic emulsions are described, for example, in U.S. Pat. No. 3,128,187 of Segal, et al., issued Apr. 7, 1964; U.S. Pat. No. 3,232,762 of Ford, et al., issued Feb. 1, 1966; U.S. Pat. No. 2,732,303 ofMorgan, et al.,

issued Jan. 24, 1956; US Pat. No. 3,835,581 of Tinker, et 211., issued May 20, 1958 and US. Pat. No. 3,271,154 of Dersch, issued Sept. 6, 1966. None of these patents, however, indicate that the photolytically active polybrominated organic compound stabilizer precursors according to the invention can be employed in photothermographic materials to provide improved post-processing stability.

Photographic materials which do not contain photographic silver halide and comprising tribromomethylsulfonyl derivatives of certain heterocyclic compounds such as benzothiazole compounds and benzimidazole compounds are described in Canadian Pat. No. 831,465 issued Jan. 6, 1970. This patent, however, does not described photothermographic materials containing photographic silver halide.

There has been a continuing need for photothermographic materials comprising an image-forming combination containing photographic silver halide which have improvedpost-processing image stability, that is reduced background density and no significant decrease in desired maximum density when employing a halogen-containing stabilizer precursor.

SUMMARY OF THE INVENTION It has been found according to the invention that the described improvements are provided in a photothermographic element or composition as described by employing as a bromine containing organic stabilizer precursor a polybromo compound represented by the formula:

wherein R is alkyl containing 1 to 6 carbon atoms,

R is hydrogen or a bromine atom,

R is a bromine atom or a carbamoyl group, and

R is an aromatic chromophore group which has the property of imparting to said bromine compound stabilizer precursor the ability to absorb electromagnetic radiation having a wavelength between about 250 and 385 nm; and at least one of R R and R is or contains a bromine atom.

The described improvements are provided, for example, in a photothermographic element comprising a support having thereon a layer comprising (a) photographic silver halide in association with (b) an oxidation-reduction image-forming combination comprising (i) a heavy metal salt oxidizing agent, preferably a silver salt oxidizing agent, with (ii) an organic reducing agent, (c) a polymeric binder for the layer and (d) a bromine compound stabilizer precursor as described.

A latent image in the described photothermographic element or composition can be developed by overall heating the element or composition to moderately elevated temperatures.

DETAILED DESCRIPTION OF THE INVENTION A variety of bromine compound stabilizer precursors, as described within the above formulas, can be employed according to the invention to provide improved post-processing stability without significantly adversely affecting desired properties of the photothermographic material. 1 Different tests can be employed to determin whether compounds or materials are useful as stabilizer precursors as described depending upon the particular photothermographic element, composition, desired im- 7 age, processing conditions and the like. One test which can be employed for certain photothermographic elements is as follows: I

A silver behenate-behenic acid dispersion (A) is prepared by ball-milling the following components for '72? hours:

silver behenate 168.0 g behenic acid 64.0 g poly(viny1 butyral) 120.0 g lithium stearate 16.4 g acetone-toluene (1:1 parts by volume) 2.0 i

A photothermographic element is prepared by combining 141 m1 of the described dispersion (A) with the following addenda, mixing thoroughly and coating at 0.63 g of composition per square decimeter on a suitable paper support:

acetone solution containing 0.01% 18.0 ml by weight 3-ethyl-5[(3-ethyI-2- benzothiazolinylidene 1 -methylethylidene]-2-thio-2,4-oxazolidinedione acetone solution containing 10% by 24.0 ml weight 2,6-dichloro-4-benzenesulfonamidophenol poly(vinyl butyral)-silver 22.0 ml bromoiodide emulsion (6 l./mole Ag, 6 mole 7: l, 100 g. polymer/mole Ag) acetone-toluene (1:1 by volume) 44.0 ml

organic solvent solution, typically 1 1.0 ml

acetone, containing polybromo stabilizer precursor according to the invention (typically organic solvent solution containing about 3% by weight of stabilizer precursor according to the invention) to'the moiety which upon combination with silver ions or atoms prevents instability due to roomlight expo-. sure. The exact mechanism of stabilization is not fully understood. It is believed, however, that upon imagewise exposure of a photothermographic material containing photographic silver halide to actinic radiation, latent image specks of metallic silver are formed in the photographic silver halide remaining in the background areas of the photothermographic element to produce unwanted background printup, especially after subsequent overall heating. It is believed that the bromine from the described stabilizer precursors of the invention is at least in part released photolytically and attacks and destroys the latent image metallic silver sites before they produce printup, that is before background fog is produced, without attacking the developed silver image to any significant degree. It is believed that the photolytically released bromine comprises free radicals which reoxidize the latent image silver atoms in the photographic silver halide to silver ions.

An advantage of the bromine containing stabilizer precursors according to the invention is that they have sufficient thermal stability for use in the described photothermographic materials. This is illustrated in the following comparative examples.

One useful class of stabilizer precursors according to the invention within the described formulas comprises a compound wherein described R and R are each bromine atoms and R is phenyl, naphthyl or benzothiazole.

The term aromatic chromophore group as employed herein is intended to mean a group which imparts to the bromine compound stabilizer precursor containing the group the ability to release at least one bromine atom when exposed to electromagnetic radiation having a wavelength between about 250 and 385 nm. A variety of aromatic chromophore groups can provide this desired property. Especially useful aromatic chromophore groups include phenyl, naphthyl or benzothiazole. These groups can contain substituent groups which do not adversely affect the stabilizing action of the stabilizer precursors according to the invention, such as methyl, ethyl and the like.

The term polybromo or polybrominated compound as employed herein is intended to mean a compound as described containing two or more bromine atoms.

Useful stabilizer precursors as described include, for example, the following compounds:

2-tribromomethylsulfonylbenzothiazole 2,4-bis(tribromomethyl)-6-methyltriazine 2,2-dibromo-2-phenylsulfonylacetamide The described polybrominated organic compounds can be prepared by methods known in the art. For example, the parent organic compound can be brominated employing conventional bromination processes. An example preparation is the preparation of 2- tribromomethylsulfonylbenzothiazole which comprises adding dropwise at 5 to 10C. 150 milliliters of bromine to a stirred solution of 240 grams of sodium hydroxide in 2.5 liters of water. Upon completion of the addition, a solution of l 10 grams of 2- carboxymethylmercaptobenzothiazole and 50 grams of sodium bicarbonate in 1.25 liters of water is added dropwise. Stirring is continued for 3 hours and the resulting white mixture is allowed to stand overnight at room temperature. The resulting white solid can be collected by filtration and recrystallized from acetic acid to provide the desired 2-tribromomethylsulfonylbenzotriazole having a melting point of 168170C.

Another example ofa useful preparation is the preparation of 2,4bis(tribromomethyl)-6-methyltriazine in which a mixture of 140 grams of 2,4,6trimethyltriazine and 520 grams of sodium acetate dispersed in 2,300 milliliters of acetic acid is heated to 70C. while adding 520 milliliters of bromine slowly with stirring over a 2- hour period. The temperature is raised to 90C. for one hour and the mixture is allowed to stand about 16 hours. After standing, 2 liters of water is added and the solid mixture resulting is collected by filtration and recrystallized from acetonitrile. The desired 2,4-

6 bis(tribromomethyl)-6-methyltriazine has a melting point of l53l55C.

Some halogenated organic compound impurities can be present in the resulting compositions. These impurities can be separated, if desired, before mixing the desired brominated compound stabilizer precursor with the described photothermographic materials.

The described polybrominated stabilizer precursors according to the invention are useful in a variety of silver halide photothermorgaphic materials.

The useful concentration of stabilizer precursor according to the invention will depend upon different factors such as the particular photothermographic ele- .ment, particular components of the photothermographic material, desired image, desired image stability and the like. A useful concentration of stabilizer precursor according to the invention is about 0.002 mole to about 0.75 mole of the described stabilizer precursor per mole of total silver in the photothermographic material. An especially useful concentration is about 0.01 mole to about 0.5 mole of the described stabilizer precursor per mole of total silver in the photothermographic material. When combinations of stabilizer precursors are employed according to the invention, the total concentration of stabilizer precursors is within the described concentration range. The optimum concentration useful can be determined based on the described factors.

Typical photothermographic materials in which the stabilizer precursors of the invention are useful are described, for example, in U.S. Pat. No. 3,457,075 of Morgan, et al., issued July 22, 1969; U.S. Pat. No. 3,152,904 of Sorensen, et al., issued Oct. 13, 1964; U.S. Pat. No. 3,429,706 of Shepard, et al., issued Feb. 25, 1969; U.S. Pat. No. 3,672,904 of deMauriac, issued June 27, 1972 and Research Disclosure, January, 1973, pages 16-21. The stabilizer precursors of the invention are especially useful in photothermographic materials comprising (a) photographic silver halide in association with (b) an oxidation-reduction image-forming combination comprising (i) a silver salt of a long-chain fatty acid, such as silver behenate, with (ii) a phenolic reducing agent, (6) a poly(vinyl butyral) binder for the layer, (d) a toner, also known as an activator-toning agent or known as a toner-accelerator, comprising 1- (2H)-phthalazinone, succinimide or N-hydroxy-1,8- naphthalimide, and (e) a spectral sensitizing dye.

An especially useful embodiment of the invention is a photothermographic element comprising a support having thereon a layer comprising (a) photographic silver halide in association with (b) an oxidationreduction image-forming combination comprising (i) silver behenate, with (ii) a sulfonamidophenol reducing agent, (c) a poly(vinyl butyral) binder for the layer, (d) a toner comprising l-(2H)-phthalazinone, succinimide or N-hydroxy-l,8-naphthalimide, and (e) a stabilizing concentration of 2-tribromomethylsulfonylbenzothiazole, 2,4-bis(tribromomethyl)-6-methyltriazine or 2,2-dibromo-2 -phenylsulfonylacetamide.

The described photothermographic materials according to the invention comprise a photosensitive component which is photographic silver halide. In the photothermographic materials it is believed that the latent image silver resulting from the photosensitive salt acts as a catalyst for the described oxidationreduction image-forming combination. A typical concentration range of photographic silver halide is from about 0.01

mole to about 20.0 moles of photographic silver halide per mole of silver salt oxidizing agent, for example, per mole or silver behenate. Other photosensitive silver salts can be used in combination with the photographic silver halide if desired. Useful photosensitive silver salts include silver-dye complexes. Preferred photographic silver halides are silver chloride, silver bromide, silver bromoiodide, silver chlorobromoiodide or mixtures thereof. Very fine-grain photographic silver halide is especially useful although coarse or fine-grain photosensitive silver halide can be employed if desired. The photographic silver halide can be prepared by any of the procedures known in the photographic art. Such procedures and forms of photosensitive silver halide are described, for example, in the Product Licensing Index, Volume 92, December, 1971, publication 9,232 on page 107, paragraph I. The photographic silver halide according to the invention can be unwashed or washed, can be chemically sensitized, can be protected against the production of fog and stabilized against loss of sensitivity during keeping, as described in the above Product Licensing Index publication.

The photothermographic elements and compositions according to the invention comprise an oxidationreduction image-forming combination which contains a heavy metal salt oxidizing agent, preferably a silver salt oxidizing agent, especially a silver salt of a longchain fatty acid. Such silver salt oxidizing agents are resistant to darkening upon illumination. Typically useful silver salts of long-chain fatty acids are those containing about 17 to 30 carbon atoms. Compounds which are useful silver salt oxidizing agents include: silver behenate, silver stearate, silver oleate, silver laurate, silver hydroxystearate, silver caprate, silver myristate and silver palmitate. Silver salts which are not silver salts of long-chain fatty acids can be employed in combination with the silver salts of long-chain fatty acids. Such silver salt oxidizing agents which are useful include, for example, silver benzoate, silver benzotriazole, silver terephthalate, silver phthalate and the like. Examples of other heavy metal salt oxidizing agents are gold stearate, mercury behenate and gold behenate. Combinations of oxidizing agents are also useful.

The described photothermographic elements and compositions can comprise various organic reducing agents. Useful reducing agents which can be employed with the described stabilizer precursors include substituted phenols and naphthols, for example, bis-beta naphthols. Suitable bis-beta-naphthols include, for example, 2,2'-dihydroxy-l,l -binaphthyl, 6,6'-dibromo- 2,2'-dihydroxy-l,l -binaphthyl or 6,6-dinitro-2,2 dihydroxy-l ,l '-binaphthyl. Other reducing agents which can be employed in photothermographic elements, according to the invention, include polyhydroxybenzenes such as hydroquinone, alkyl-substituted hydroquinones such as tertiary butylhydroquinone, methylhydroquinone, 2,5-dimethylhydroquinone and 2,6-dimethylhydroquinone; catechols and pyrogallols, chloro-substituted hydroquinones such as chlorohydroquinone or dichlorohydroquinone; alkoxy-substituted hydroquinone such as methoxyhydroquinone or ethoxyhydroquinone; aminophenol reducing agents, such as 2,4-diaminophenols and methylaminophenols; ascorbic acid reducing agents such as ascorbic acid, ascorbic acid ketals and ascorbic acid derivatives; hydroxylamine reducing agents; 3-pyrazolidone reducing agents such as l-phenyl-3-pyrazolidone and 4-methyl-4- hydroxymethyl-l-phenyl-3-pyrazolidone and the like. Combinations of reducing agents can be employed if desired.

Especially useful reducing agents which can be employed in the photothermographic materials according to the invention are sulfonamidophenol reducing agents as described in Research Disclosure, January,

1973, pages 16-21. One especially useful class of sulfonamidophenol reducing agents is represented by the formula: 1

wherein R is phenyl, naphthyl, methylphenyl, thi-. enyl, quinolinyl, thiazyl, or alkyl containing 1 to 4 car- I bon atoms, such as methyl, ethyl, propyl or butyl', R is hydrogen, R SO Nl-l, alkoxy containing 1 to 4 carbon atoms, such as methoxy, ethoxy, propoxy and butoxy, bromine or chlorine; R is hydrogen, bromine, chlorine, alkyl containing 1 to 4 carbon atoms, as described, or alkoxy containing 1 to 4 carbon atoms, such as methoxy, ethoxy and propoxy. R, R and/or R can contain substituent groups which do not adversely affect the reducing properties of the described sulfonamidophenol reducing agents or the desired sensitometric properties of the photothermographic elements and materials of the invention. Examples of substituent groups which can be present are alkyl containing 1 to scribed, for example, in Belgian Pat. No. 766,590 is- 1 sued June 15, 1971 and in Research Disclosure, January, 1973, pages 16-21. Combinations of toning agents can be employed in the photothermographic materials according to the invention if desired. Typical toning agents include, for example, phthalimide, N- hydroxyphthalimide, N-potassium phthalimide, succinimide, N-hydroxy-l ,S-naphthalimide and N- hydroxysuccinimide. In some cases other toningagents. i

can be employed such as l-(2H)-phthalazinone, ,2- acetylphthalazinone and the like.

It is desirable, in some cases, to employ a combination of stabilizer precursors in the photothermographic materials of the invention.

In addition to combinations of stabilizers within the 1 described formula according to the invention, other image stabilizers or stabilizer precursors can in some cases be employed with the stabilizer precursors of the invention. Typical stabilizer precursors which can be employed in combination with the stabilizer precursors of the invention include, for example, azole thioethers and blocked azoline thione stabilizer precursors as described in Belgian Pat. No. 768,071 issued July 30,

1971, and in copending US. application Ser. No. 435,806 of Burness, et al., filed concurrently herewith.

A photothermographic element or composition as described according to the invention can contain various colloids and polymers alone or in combination as vehicles, binding agents and in various layers. Suitable materials can be hydrophobic or-hydrophilic. They are transparent or translucent and include both naturallyoccurring substances such as proteins, for example, gelatin, gelatin derivatives, cellulose derivatives, polysaccharides such as dextran, gumarabic and the like; and synthetic polymeric substances such as water soluble polyvinyl compounds like poly(vinyl pyrrolidone), acrylamide polymers and the like. Other synthetic polymeric compounds which can be employed include dispersed vinyl compounds such as in latex form and particularly those which increase dimensional stability of photographic materials. Effective polymers include waterinsoluble polymers of alkyl, acrylates and methacrylates, acrylic acid, sulfoalkyl acrylates,methacrylates and those which have crosslinlcing sites which facilitate hardening or curing as well as those having recurring sulfobetaine units as described in Canadian Pat. No. 774,054. Especially useful high molecular weight materials and resins include poly(vinyl butyral), cellulose acetate butyrate, poly(methyl methacrylate), poly(vinyl pyrrolidone), ethylcellulose, polystyrene, poly(vinyl chloride), chlorinated rubber, polyisobutylene, butadiene-styrene copolymers, vinyl chloride-vinyl acetate copolymers, copolymers of vinyl acetate, vinyl chloride andmaleic acid and poly(vinyl alcohol). The useful concentration of reducing agent accord ing to the invention will vary depending upon the particular photothermographic element, desired image, processing conditions, particular stabilizer precursor employed and the like. A useful concentration of reducing agent is typically from about 0.2 mole to about 2.0 moles of reducing agent per mole of photographic silver halide. A useful concentration of reducing agent in relationship to oxidizing agent, such as silver behenate or silver stearate, is typically from about 0.01 mole to about moles of reducing agent per mole of silver salt of a long-chain fatty acid, such as per mole of silver behenate, in the photothermographic material. Reducing agents can be employed in combination. When combinations are employed, the total concentration of reducing agent is typically within the described concentration range.

Photothermographic materials according to the invention can contain development modifiers that function as speed-increasing compounds, hardeners, antistatic layers, plasticizers and lubricants, coating aids, brighteners, spectral sensitizing dyes, absorbing and fil ter dyes, also as described in the Product Licensing Index, Volume 92, December, 1971, publication 9232, pages 1071 10.

The photothermographic elements according to the invention can comprise a wide variety of supports. Typical supports include cellulose nitrate film, cellulose ester film, poly(vinyl acetal) film, polystyrene film, poly(ethylene terephthalate) film, polycarbonate film, film supports as described in U.S. Pat. No. 3,634,089 of Hamb, issued Jan. 11, 1972 and U.S. Pat. No. 3,725,070 of Hamb, eta1., issued Apr. 3, 1973 and related films or resinous materials, as well as glass, paper, metal and the like supports which can withstand the processing temperatures employed according to the invention. Typically a flexible support is employed.

The photothermographic compositions and other compositions according to the invention can be coated on a suitable support by various coating procedures including dip coating, air knife coating, curtain coating or. extrusion coating using hoppers such as described in U.S. Pat. No. 3,681,294 of Beguin issued June 15, 1954. If desired, two or more layers can be coated simultaneously such as described in U.S. Pat. No. 2,761,791 of Russell, issued Sept. 4, 1956 and British Pat. No. 837,095. r

Spectral sensitizing dyes can be used in the described photothermographic elements and compositions of the invention to confer additional sensitivity to the ele- -ments and compositions of the invention. Useful sensitizing dyes are described, for example, in the Product Licensing Index, Volume 92, December, 1971, publication 9232, pages 107-110, paragraph XV.

After imagewise exposure of a photothermographic element according to the invention, typically a visible light, the resulting latent image can be developed merely by overall heating the element to moderately elevated temperatures. This overall heating merely involves heating the described element overall to a temperature within the range of from about C. to about 250C. such as for about 0.5 to about 60 seconds. By increasing or decreasing the length of time of heating, a higher or lower temperature within the described range can be employed depending upon the desired image, particular reducing agent, and the like. Typically, a lower processing temperature is desired. A preferred processing temperature range is from about C. to about C. A developed and stabilized image is typically produced within several seconds, such as from about 0.5 seconds to about 60 seconds.

Any suitable means can be used for providing the desired processing temperature range. The heating means can be a simple hot plate, iron, roller or the like.

Processing is usually carried out under ambient conditions of pressure and humidity. Conditions outside normal atmospheric pressure and humidity can be employed if desired.

The described stabilizer precursor according to the invention can be in any suitable location in the photothermographic material according to the invention which provides the desired stabilized image. If desired, one or more of the components of the photothermographic element according to the invention can be in one or more layers of the element. For example, in some cases, it can be desirable to include certain percentages of the reducing agent, toner, image stabilizer precursorand/or other addenda in a protective layer over the photothermographic element. This in some cases can reduce migration of certain addenda in the layers of the photothermographic element.

The described stabilizer precursors according to the invention can be used in, for example, photothermographic materials comprising photographic silver halide as described in U.S. Pat. No. 3,679,426 of Youngquist, issued July 25, 1972; U.S. Pat. No. 3,667,958 of Evans, issued June 6, 1972; U.S. Pat. No. 3,667,959 of Bojara and deMauriac, issued June 6, 1972; Belgian Pat. No. 766,590 issued June 15, 1971; U.S. Pat. No. 3,672,904 of deMauriac, issued June 27, 1972; Belgian Pat. No. 772,371 issued Oct. 15, 1971 and U.S. application Ser. No. 272,832 of Evans and McLaen, filed July 18, 1972.

EXAMPLE 1 This is a comparative example.

light for 72 hours. The sensitometric data resulting from this stability test are also given in following Table I.

EXAMPLE 2 The procedure described in Example 1 is repeated with the exception that the described composition contained ll milliliters of an acetone solution containing 3% by weight of 2-tribromomethy1sulfonylbenzo-- thiazole; and, the acetone-toluene solvent mixture was reduced to 33 milliliters.

A silver behenate-behenic acid dispersion A is pre- 15 pared by ball-milling the following components for 72 hours:

The resulting photothermographic element was im.-. agewise exposed and then overall heated as described in Example 1. It was then exposed to 1,500 ft. candlessilver behenate 168.0 g behinrc afijdt fig-g g of light for 72 hours also as descr1bed in Example 1. p0 y vmy u yra g lithium Stem: 164 g The resultlng sensitometric data are given in following acetone-toluene (1:1 parts by volume) 2.0 I Table I.

' TABLE 1 Fresh 72 Hour Print-Up Relative* Re1ative* Example Stabilizer Speed Contrast Speed Contrast Dmax min max I min no 100 1.87 0.01 1.24 78 1.62 0.22 1.24 2 yes 52 1.49 0.01 1.22 52 1.29 0.01 1.13

measured at 0.3 above Dmin The above results demonstrate that compounds such as 2-tribromomethylsulfonyl-benzothiazole are extremely efficient image stabilizers for photothermographic materials. Note that print-out density due to'roomhght handlmg is prevented over a 72 hour period.

A photothermographic element is prepared by combining 141 ml. of the above-described dispersion (A) with the following addenda, mixing thoroughly and coating at 0.63 g of composition/dm on a suitable paper support:

Acetone solution containing 0.01% 18.0 ml

by weight 3-ethyl-5-{(3-ethyl-2- benzothiazoliny1idene)-1-methy1- ethylidene]-2-thio-2,4-oxazo1idinedione Acetone solution containing 10% by 24.0 ml

weight 2.6-dich1oro-4-benzenesu1fonamidophenol Po1y(vinyl butyral)-silver bromoiodide 22.0 ml

emulsion. (6 l./mole Ag, 6 mole 7r 1.

100 g polymer/mole Ag) Acetone-toluene (1:1 by volume) 44.0 ml

The following composition is coated over the element at a coverage of 0.43 g/dm' thereby providing a protective layer containing approximately 10.76 mg cellulose acetate/dm of support.

cellulose acetate 5.0 g acetone-dichloromethane 1:1 parts by volume) 200.0 ml

The resulting photothermographic element is imagewise exposed for 1 second to tungsten light and then overall heated by contacting the photothermographic element for 2 seconds on a metal block heated to 150C. The sensitometric results for the resulting developed image are given in following Table I. The processed element is then subjected to 1,500 ft. candles of J1 EXAMPLE 3 A silver behenate dispersion is prepared by ballmilling the following components for hours:

silver behenate 50.0 g poly(vinyl butyral) 15.0 g methyl isobutyl ketone 500 ml poly( vinyl butyral)-silver bromide emulsion (6 liters of emulsion per mole of silver, 100 g. of polymer per mole of silver) acetone solution containing 0.66 60.0 ml milligrams of 2-(5,5-dicyano- 4-phenyl-2,4-pentadienylidene)- 1-ethy1naphtho[ 1,2-d1-thiazoline (hold one hour at room temperature) silver behenate dispersion as 500.0 ml described above acetone solution containing 50.0 ml 10% by weight succinimide (hold one hour at room temperature) acetone solution containing 180.0 ml 1 14% by weight poly(vinyl butyral) acetone solution containing 6.3% by weight 4-benzenesulfon-. amidophenol acetone solution containing 5% by weight 2,4-dihydroxybenzophenone -Continued tended to mean a compound according to the invention I j I 300 I which increases stability in the photothermographic z gtgg m material prior to imagewise exposure. 2% by volume silicone i surfactant (Silicone AFJO The data in Table ll indicates that the T80 treated which isatrade name of coatings do not provide useful images with the deii s i Electric scribed element, processing time and temperature listed. It is believed that the undesired results with TBQ are acetone Solution Containing ml due at least in part to undesired decomposition of TBQ 2.28% by weight anhydrous o T d Erbium iodide (hold one hour at temperatures above 50 C. h s is demonstrate n at room temperature) the following examples. Photolytic efficiency of stabi- I lizer precursors according to the invention can also be The fesultmg photothefmogfaphlc element tested employing the procedure in the following examovercoated with the following composition at 0.25 mili liliters per square decimeter:

EXAMPLE 4 POM4'4, (hexahydmmmetham 50 g The thermal stability of proposed stabilizer precurindamhlidcne)diphenylgne carbonate] sors can be determined by dissolving about 5 millih] h 95 O 1 grams of the proposed stabilizer precursor and about 5 m milligrams of cadmium iodide in 5 milliliters of 2- acetone .0 m butanone and immersing the sample in a thermostati- Smmne Surfactant (L522 Sold 05 ml cally controlled water bath while raising the temperaby the Union Carbide Co., USA.) ture of the water bath slowly from room temperature to 75C. over a period of 6 hours of continuous obser- After drying, the photothermographic element is sepvation. The temperature at which each sample proarated into several equal parts. One part is maintained duces color is recorded. Those proposed stabilizer preas a control and the remaining parts are dipped quickly cursors which provide a color below about 60C. are into a solution of grams per liter of tribromoquinalconsidered insufficiently stable for purposes of the indine, referred to herein as TBQ, in 2-butanone and air vention. The results are recorded in the following Table dried. Control and comparative examples of the de- 30 Ill. scribed photothermographic element are also prepared The proposed stabilizer precursors can also be qualiin the same manner by dipping film strips into the soltively evaluated for relative photolytic efficiency by vents alone or into a solution of 2-tribromomethylsulpreparing solutions in the manner described for the fonylbenzothiazole, also referred to herein as TSB, rethermal stability test in Example 4. The solution is exspectively. Each film sample is exposed imagewise for posed to actinic light from an unfiltered xenon arc lamp 5 minutes to red light from the zero order light of a califor 5 seconds at a distance of 10 centimeters. The denbrated monochromator when protected with a filter sity of the color produced in the solution is observed combination that permits only radiation of wavelengths and an arbitrary reading of the density qualitatively is longer than 590 nanometers to pass. The respective given to each solution of excellent, good, fair or poor. samples are then overall heated as indicated in the fol- The relative photolytic efficiency ratings are given in lowing Table II employing the listed time and temperah f n i T bl [IL ture conditions. In Table II the term con is used to The following compounds are tested: identify untreated film samples used as controls and the term S-C is used to identify solvent treated control Compound A 2-tribromomethylsulfonylbenzothiazole samples. (T53) The described polylerominated organic compounds Compound B m. according to the invention are useful in the phototherchloropro n uinolinium pmographic element according to Example 3 as an incumluenesulfmme bation antifoggant. An incubation antifoggant is in- Compound C 2,4-bis(tribromomethyl)-6-methyl- TABLE II Compound Development Development Descriptor Time (Min) Temperature (C) Observations TBQ 5 53 A weak image is formed TBQ 5 No image TBQ 5 No image TBQ 5 No image TBQ 2 No image S-C 2 90 A poor negative image is formed Con 2 90 A negative image is formed TBQ 2 No image TSB 2 100 A good negative image is formed S-C 2 100 A good negative image is formed Con 2 100 A good negative image is formed TBQ 2 H0 No image TSB 2 1 10 Good negative image (better than control) S-C 2 Good negative image Con 2 l 10 Good negative image -Continued s-triazine (referred to herein as BMT) Compound D bis(tribromomethyl)sulfoxide Compound E bis(tribromomethyl)sulfone Compound F alpha,alpha,alphatribromoquinaldine (TBQ) TABLE III Decomposition Photolytic Compound Temperature (C) Efficiency A Above 75 Good B 50 Poor C Above 75 Excellent D 73 Excellent E 60 Excellent F 50 Good The tests reported in the above table indicate that compounds A and C are especially useful because their decomposition temperature is above 75C.

EXAMPLE 5 The procedure described in Example 2 is repeated with the exception that the compound 2,4- bis(tribromomethyl)-6-methyl-s-triazine (BMT) is employed in place of 2-tribromomethylsulfonylbenzothiazole (TSB). Results similar to those in Example 2 are obtained.

EXAMPLE 6 Fivestrips of a photothermographic paper, i.e., 3M Company Dry Silver Paper Type 7743, are treated as follows:

Sample No.

l Untreated Original (Control I) 2 Solvent treated (Control 2) 3 TBQ treated 4 BMT treated 5 TSB treated The solvent treated sample is prepared by stroking the front surface of the 3M Company Dry Silver Paper Type 7743 once with a camels hair brush which has been dipped into 2-butanone.

The TBQ and BMT treated samples are prepared in the same mannere as the solvent treated sample except, in place of the solvent, a solution is used comprising grams per liter of the stabilizer precursor in the solvent.

The TSB treated sample is prepared as described above except that the concentration of the solution was only 9 grams of TSB per liter.

The samples are dried overnight in the dark and treated in a fadeometer the next day. The fadeometer is a standard testing apparatus which provides exposure to intense light. The total reflected densities were measured from each sample at 550 nanometers after 4 hours and after 28 hours fading in the high intensity xenon arc fadeometer. The measured densities are recorded in the following Table IV.

TABLE iV Reflection density after Sample No. Fading Time (Hours) Compound Control Solvent TBQ BMT TSB"? *It is believed that these densities would be lower if the treating solution were as concentrated as the treating solutions of the other samples.

3M Company Dry Silver Paper Type 7743 comprises 7 a paper support having thereon a photothermographic layer comprising a toner which is 1-(2H)- phthalazinone with 2,2'-methylenebis(4-methyl-6- tertiarybutylphenol), silver behenate, silver bromide, aluminum oxide, and tetrabromobutane. The photothermographic layer is overcoated with a vinylacetate and vinylacetate-vinyl chloride copolymer layer. Tetrabromobutane is employed as a stabilizer.

The invention has been described in detail with particular reference to preferred embodiments thereof,

but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

What is claimed is:

1. In a photothermographic element comprising a prising (i) a heavy metal salt oxidizing agent with (ii) an organic reducing agent, (c) a polymeric binder for the layer and (d) a bromine compound stabilizer precursor, the improvement wherein said bromine compound stabilizer precursor comprises a compound rep resented by the structure:

wherein R is alkyl containing 1 to 6 carbon atoms, R is hydrogen or a bromine atom, R is a bromine atom or a carbamoyl group, R is an aromatic chromophore group which has the property of imparting to said bromine compound wherein said bromine compound stabilizer presursor is 2,4-bis(tribromomethyl)-6-methyltriazine.

5. A photothermographic element as in claim 1 wherein said bromine compound stabilizer precursor is 2,Z-dibromo-2-phenylsulfonylacetamide.

support having thereon a layer comprising (a) photographic silver halide in association with, (b) an oxidation-reduction image-forming combination com 6. A photothermographic element as in claim 1 comprising about 0.002 mole to about 0.75 mole of said bromine compound stabilizer precursor per" mole of total silver ,said ,photothermographic element.

7. A photothermographic element comprising a support havingthereon a layer comprising a. photographic silver halide in association with b. an oxidation-reduction image-forming combination comprising i. silver behenate, with ii. a phenolic reducing agent,

c. a poly(vinyl butyral) binder for the layer,

d. a toner comprising phthalazinone, succinimide or N-hydroxy-l ,8-naphthalimide,

e. a spectral sensitizing dye, and

f. about 0.015 to about 0.50 mole of 2- tribromomethylsulfonylbenzothiazole per mole of total silver in said element.

8. A photothermographic composition comprising photographic silver halide, a polymeric binder and a bromine compound represented by the formula:

wherein Rlis alkyl containing 1 to 6 carbon atoms, R s hydrogen or a bromine atom, R is a bromine atom or a carbamoyl group,

R is an aromatic chromophore group which has the property of imparting to said bromine compound stabilizer precursor the ability to absorb electromagnetic radiation having a wavelength between about 250 and 385 nm; and at least one of R, R and R is or contains a bromine atom.

9. photothermographic composition as in claim 8 wherein said bromine compound is 2- trlbromomethylsulfonylbenzothiazole.

l0. photothermographic composition as in claim 8 comprising about 0.002 mole to about 0.75 mole of said bromine compound per mole of total silver in said composition.

1.1. In a photothermographic composition comprising (a) photographic silver halide in association with (b) an ox dation-reduction image-forming combination comprising (i) a heavy metal salt oxidizing agent with (u) a reduclng agent, (c) a polymeric binder and (d) a bromine compound stabilizer precursor, the improvement wherein said bromine compound stabilizer prefursor comprises a compound represented by the strucure:

wherein R is alkyl containing 1 to 6 carbon atoms, R s hydrogen or a bromine atom, R [S a bromine atom or a carbamoyl group,

R is an aromatic chromophore group, which has the property of imparting to said bromine compound stabilizer precursor the ability to absorb electromagnetic radiation having a wavelength between about 250 and 385 nm; and at least one or R, R

. and R is or contains a bromine atom.

12. A photothermographic composition as in claim 11 wherein R is phenyl, naphthyl or benzothiazolyl.

13. A photothermographic composition as in claim 11 wherein said bromine compound stabilizer precursor is 2-tribromomethylsulfonylbenzothiazole.

14. A photothermographic composition as in claim 11 wherein said bromine compound stabilizer is 2,4-

, bis(tribromomethyl)-6-methyltriazine.

15. A photothermographic composition as in claim 11 wherein said bromine compound stabilizer precursor is 2,2-dibromo-2-phenylsulfonylacetamide.

16. A phototheimographic composition as in claim 11 comprising about 0.002 mole to about 0.75 mole of said bromine compound stabilizer precursor per mole of total silver in said composition.

17. A photothermographic composition comprising a. photographic silver halide in association with b. an oxidation-reduction image-forming combination comprising i. silver behenate, with ii. a phenolic reducing agent, 0. a poly(vinyl butyral) binder for the composition, (1. a toner comprising phthalazinone, succinimide or N-hydroxy-1,8-naphthalimide, e. a spectral sensitizing dye, and f. about 0.015 to about 0.50 mole of 2- tribromomethylsulfonylbenzothiazole per mole of total silver in said composition.

18. A method of developing an image in a photothermographic element comprising a support having thereon a layer comprising (a) photographic silver halide in association with (b) an oxidation-reduction image-forming combination comprising (i) a silver salt oxidizing agent with (ii) a reducing agent, (c) a polymeric binder and (d) a bromine compound stabilizer precursor comprising a compound represented by the structure:

wherein R is alkyl containing 1 to 6 carbon atoms,

R is hydrogen or a bromine atom,

R is a bromine atom or a carbamoyl group,

R is an aromatic chromophore group which has the property of imparting to said bromine compound stabilizer precursor the ability to absorb electromagnetic radiation having a wavelength between about 250 and 385 nm; and at least one of R, R and R is or contains a bromine atom,

comprising overall heating said element to about C. to about C.

19. A method as in claim 18 of developing an image in a photothermographic element comprising a support having thereon a layer comprising a. photographic silver halide in association with 19 20 b. an oxidation-reduction image-forming combinae. a spectral sensitizing dye, and I tion comprising f. about 0.015 to about 0.50 mole of 2- i. silver behenate, with' tribromomethylsulfonylbenzothiazole per mole of ii. a phenolic reducing agent, total silver in said photothermographic element, c. a poly(vinyl butyral) binder, 5 comprising overall heating said element to about 80C. I

d. a toner comprising phthalazinone, succinimide, or to about 170C. for about 0.5 to about 30 seconds.

N-hydroxy-l,8-naphthalimide,

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3 87Lt,9h6

DATED April 1, 1975 INVENTOR(S) Lorenzo F. Costa, James A. VanAllan and Frank Grum It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Abstract, line 7, "compostion" should read --composition--.

Column 1, line 33, "light-sensitive should readlight-insensitive-.

Column 2, lines LB-A6, "tribromoquinalidine" should read tribromoquinaldine Column 6, line 10, photothermorgaphic" should read --photothermographic-- Line 3M, delete the after "Shepard" Line 65, oxidationreduct-ion" should read oxidation-reduction Column 7, line 16, "9,232" should read 9232 Column 10, line 6, "3,681,294" should read --2,68l,29Lt---.

Column 15, line 5%, "manners" should read -manner- Column 16, line 63, "presursor" should read --precursor Column 18, line 18, "phototheimographic" should read --photothermographic-- Signed and Scaled this twenty-third D ay Of September I 9 75 RUTH C. MASON C MARSHALL DANN .Ilrvslmg ()j'j'iu'r (mnmisztr'mu'r uj'lau'nls and Trademarks

Claims

1. In a photothermographic element comprising a support having thereon a layer comprising (a) photographic silver halide in association with, (b) an oxidation-reduction image-forming combination comprising (i) a heavy metal salt oxidizing agent with (ii) an organic reducing agent, (c) a polymeric binder for the layer and (d) a bromine compound stabilizer precursor, the improvement wherein said bromine compound stabilizer precursor comprises a compound represented by the structure:

2. A photothermographic element as in claim 1 wherein R3 is phenyl, naphthyl or benzothiazolyl.

3. A photothermographic element as in claim 1 wherein said bromine compound stabilizer precursor is 2-tribromomethylsulfonylbenzothiazole.

4. A photothermographic element as in claim 1 wherein said bromine compound stabilizer presursor is 2,4-bis(tribromomethyl)-6-methyltriazine.

5. A photothermographic element as in claim 1 wherein said bromine compound stabilizer precursor is 2,2-dibromo-2-phenylsulfonylacetamide.

6. A photothermographic element as in claim 1 comprising about 0.002 mole to about 0.75 mole of said bromine compound stabilizer precursor per mole of total silver in said photothermographic Element.

7. A photothermographic element comprising a support having thereon a layer comprising a. photographic silver halide in association with b. an oxidation-reduction image-forming combination comprising i. silver behenate, with ii. a phenolic reducing agent, c. a poly(vinyl butyral) binder for the layer, d. a toner comprising phthalazinone, succinimide or N-hydroxy-1, 8-naphthalimide, e. a spectral sensitizing dye, and f. about 0.015 to about 0.50 mole of 2-tribromomethylsulfonylbenzothiazole per mole of total silver in said element.

9. A photothermographic composition as in claim 8 wherein said bromine compound is 2-tribromomethylsulfonylbenzothiazole.

10. A photothermographic composition as in claim 8 comprising about 0.002 mole to about 0.75 mole of said bromine compound per mole of total silver in said composition.

11. In a photothermographic composition comprising (a) photographic silver halide in association with (b) an oxidation-reduction image-forming combination comprising (i) a heavy metal salt oxidizing agent with (ii) a reducing agent, (c) a polymeric binder and (d) a bromine compound stabilizer precursor, the improvement wherein said bromine compound stabilizer precursor comprises a compound represented by the structure:

12. A photothermographic composition as in claim 11 wherein R3 is phenyl, naphthyl or benzothiazolyl.

14. A photothermographic composition as in claim 11 wherein said bromine compound stabilizer is 2,4-bis(tribromomethyl)-6-methyltriazine.

17. A photothermographic composition comprising a. photographic silver halide in association with b. an oxidation-reduction image-forming combination comprising i. silver behenate, with ii. a phenolic reducing agent, c. a poly(vinyl butyral) binder for the composition, d. a toner comprising phthalazinone, succinimide or N-hydroxy-1, 8-naphthalimide, e. a spectral sensitizing dye, and f. about 0.015 to about 0.50 mole of 2-tribromomethylsulfonylbenzothiazole per mole of total silver in said composition.

18. A METHOD OF DEVELOPING AN IMAGE IN A PHOTOHERMOGRAPHIC ELEMENT COMPRISING A SUPPORT HAVING THEREON A LAYER COMPRISING (A) PHOTOGRAPHIC SILVER HALIDE IN ASSOCIATION WITH (B) AN OXIDATION-REDUCTION IMAGE-FORMING COMBINATION COMPRISING (I) A SILVER SALT OXIDIZING AGENT WITH (II) A REDUCING AGENT, (C) A POLYMERIC BINDER AND (D) A BROMINE COMPOUND STABILIZER PRECURSOR COMPRISING A COMPOUND REPRESENTED BY THE STRUCTURE:

19. A method as in claim 18 of developing an image in a photothermographic element comprising a support having thereon a layer comprising a. photographic silver halide in association with b. an oxidation-reduction image-forming combination comprising i. silver behenate, with ii. a phenolic reducing agent, c. a poly(vinyl butyral) binder, d. a toner comprising phthalazinone, succinimide, or N-hydroxy-1,8-naphthalimide, e. a spectral sensitizing dye, and f. about 0.015 to about 0.50 mole of 2-tribromomethylsulfonylbenzothiazole per mole of total silver in said photothermographic element, comprising overall heating said element to about 80*C. to about 170*C. for about 0.5 to about 30 seconds.