CA1193128A - Heat developable imaging materials and process - Google Patents

Abstract

HEAT DEVELOPABLE IMAGING MATERIALS AND PROCESS
Abstract of the Disclosure A heat developable photographic materiel comprising in reactive association (a) photosensitive silver halide, (b) an image-forming combination comprising (i) a complex of silver with a nitrogen acid selected from the group consisting of imidazole, pyrazole, urazole, 1,2,4-triazole and 1H-tetrazole nitrogen acids or combinations thereof, with (ii) an organic reducing agent, and (c) a polymeric binder, provide developed images without the need for processing solutions or baths. An image can be developed in this heat developable material by merely heating the material to moderately elevated temperatures. Other addenda employed in heat developable materials can be employed with the heat developable photographic materials described.

Description

Bac~g olmd of -the Invention Field of the Invention This invention relates to heat developable imaging materials and processes for developing an image ernploying in reactlve association (a) photosensitlve silver halideg (b) an image forming cornbination comprising (i) cer-tain complexe~
of silver with certain nitrogen acids with (ii) an organic reducing agent, and (c) a polymeric binder. In one of its aspects it relates to a heat developable photographic element comprising a support having thereon the described combination of materialsO In ano-ther aspect i-t relates to a heat developable photographic composition comprising the described imaging combination. A further aspect of the invention relates to a process of developing an image in a heat developable photographic element containing the described imaging combination.

escription of the State of the Art It is known to obtain an image in an imaging material, especially a pho-tographic imaging material, by so-called dry processing with heat~ These materials are sometimes described as heat developable photographic materials or pho-tothermographic materials. Such heat developable photographic materials after imagewise exposure are heated -to provide a developed image in the absence of separate processing solutions or baths. Typical heat developable imaging materials or photothermographic materials are described, for example~ in U.SO Paten-t 3gl52~904 of Sorensen et al, issued October 13, 1964; u.s. Patent 3,457,o75 of Morgan et alg issued July 22, 1969, u.s. Patent 3,152,903 of Shepard et al, issued October 13g 1964; U S. Patent 3~392~020 of Yutzy et al, issued July 9, 1968, and British Specification lg 161,777 published August 20~ 1969.

The most commonly employed silver salts in such heat developable photographic materials are silver salts of long-chain fatty acids, such as silver behenate. It has been desirable to replace such silver salts of long-chain fatty acids to enable use o~ aqueous or hydrophilic compos~tions which further ena~le the use Or conventional silver halide technology in heat developable photographic materials. Use of silver behenate as a so-called source of silver in such heat developable materials is not particularly compatible with 10 aqueous formulations of' silver halide materials. Other silver salts or comp]exes have been proposed for such heat developable photographic materials. These include 3 for example, silver salts of benzotriazole, silver salts of saccharin and related silver salts or complexes.
These are described, for example, in heat developable photographic rnaterials and U.S. Patent 3,617,289 of Ohkubo et al, issued October 2, 1971; u. s . Patent 3,666,477 of Goffe, issued May 30, 1972, u.s. Patent 3,672,904 of deMauriac, issued June 27, 1972; U.S. Patent 3,832,185 of Masuda et al, 20 issued August 27, 1974; British Specification 1,205,500 published September 16, 1970; U.S. Patent 3,689,270 of Anderson et al, issued September 5, 1972; and German Off'enlegungsschrif't 2,326,865 published December 69 1973.
While many of these silver complexes or salts can provide an image in such heat developable photographic materials, they often provide undesirable processing temperature latitude~
undesired photographic speed, undesired image tone, or a combination of` one or more of these problems.
It is desirable in many cases to spectrally sensitize 30 heat developable photographic materials to enable exposure to exposure means containing other than the blue region of the visible spectrum. Difficulty is often encountered in spectrally ' ~7 3~2~

sensiti~ing photosensitive silver halide to be used in heat developable materials. While in many cases some degree of spectral sensi-tization can be obtained~ it is often insufficient for most photographic purposes. It has been desirable to provide a heat developable photographic material based on aqueous photographic silver halide technology which permits use of a broad range of spectral sensitizing dyes.
It has been desirable also to provide heat developable photographic materials which enable use of commonly employed silver halide developing agents in the photographic material. Heat developable photographic materials often have used unconventional reducing agents which require costly processes of preparation. It has been desirable to eliminate this problem with a heat developable photographic material which better enables use of conventional developing agents, such as hydroquinone developing agents and ascorbic acid developing aeents. Typical reducing agents which have been used in heat developable photographic materials are described~
for example, in U.S Patents 3,672,904 of deMauriac, issued June 27, 1972. ~ommercially available photothermographic materials have used such reducing agents as 2g2'-methylenebis (4-methyl-6-tertiary butyl phenol) which is not a conventional silver halide developing agent.
Photographic materials which are not heat developable photographic materials are known containing nitrogen acids. Nitrogen acids, for example, are described in photogxaphic materials in Belgian Pa-tent 7gO,955 issued May 3, 1973 and U.S. Patent 33933,507 of von Konig et al, issued January 20, 1976. Neither of these references~ however, suggest answers to the described problems in heat developable photographic materials.

3~2~

Heavy metal salts o~ azoles are also known in thermographic materials, that is materials in which the visible image is formed by imagewise heatingg not by a photographic process. Such heavy metal salts of azoles are described, for example, in thermographic materials in U S Patent 3,767,414 of Huffman et al, issued Oc-tober 23, 1973. The described thermographic materials, however, do not enable the advantages encountered with photosensitive materials~
Silver salts of a range of organic compounds are known for various purposes in photographic materials.
Such uses of silver salts are describedg for example, in U.S. Pa-tent 2,3539754 of Peterson, issued July 18, 1944; and U.S. Patent 3,794g496 of Manhart, issued February 26, 1974.
There has been a continuing need for heat developable photographic materials which have the degree of photosensitivity provided by a photographic silver halide material and which enable the use of conventional gelatino silver halide emulsion technology.
This need has been especially true ~or heat developable photographic materiaIs which enable a processing temperature below about 170C~

Summary o~ the Invention It has been found according to the invention that the described advantages can be provided in a heat developable photographic material comprising in reactive association (a) photosensitive silver halide, (b) an image-forming combination comprising ~i) a complex of silver with a nitrogen acid selected from the group consisting of imidazole, pyrazole, urazole 3 1,2,4-triazole and lH-tetrazole nitrogen acids and combinations thereof, wi-th (ii) an organic reducing agent and (c) a polymer:Lc binder. Especially useful 3~

complexes of silver with a nitrogen acid as described are those complexes with 1,2,L~-triazole and lH-tetrazole nitrogen acids because these enable wider proces6ing temperature lat~tude and provide higher speed photographic materials.
An image in the described heat developable materials can be provided by merely heating -the exposed material for several seconds at moderately elevated temperatures such as a temperature within the range of about 120C to about 180C. No processing solutions or baths are required to provide a developed i~lage.

Detailed Description of the Invention An important embodiment of the invention is a heat developable photographic element comprising a support having thereon in reactive association components (a~, (b) and (c) as descrlbed. A range o~ complexes of silver with the described nitrogen acids are useful ln the heat developable photographic materials. A nitrogen acid as described herein is intended to include those compounds which have the moiety -NH- in the heterocyclic nucleus. Especially useful complexes of silver with nitrogen acids are silver complexes of lH-te-trazole nitrogen acids represented by the formula:

HN/N~N
_ I Ag l N
R

wherein Rl is hydrogen, al~yl containing l to 20 carbon atoms~
typically l to 5 carbon atoms~ such as methyl~ ethyl, propyl, blltyl and pentyl, or NH2. Examples of especially useful lH-tetrazole ni.trogen acids are:
lH-tetrazole, dodecyltetrazole and 5~n-b-utyl-lH-tetrazole.

~ 6 3~

Other especially useful silver complexes of nitrogen acids are silver complexes of 1~2,4-triazole nitrogen acids represen-ted by the formula:

H
N ~ N
_ ~ N _ Ag wherein R is hydrogen or NH2. This includes silver complexes of 1,2,4-triazole and amino-1,2,4-triazole.
Other classes of useful complexes of silver with nitrogen acids include silver complexes of urazoles, pyrazoles and imidazoles. These classes, in some cases, are not as effective as the preferred classes of complexes of silver salts with nitrogen acids. A usefu] complex is a silver complex of a urazole nitrogen acid represented by the formula:

H
~ Ag H - NH

Another class of complex of silver with a nitrogen acid is a silver complex of pyrazole represented by the formula:

~ ~ ~ Ag A further complex of silver with a nitrogen acid is a silver complex of an imidazole represented by 3~

the formula:

Ag and ~ g wherein Rl is H, CH3 or N02 and R2 is H.

Combinations of the descri-bed complexes of silver with nitrogen acids are also usef`ul. The optimum combination can be de-termined based on such factors as the desired image, other components in the heat developable photographic materialg processing conditions, and the like.
Preparation of the described ~itrogen acids can be carried out with procedures known in the art. Alsog preparation of complexes of silver with the described nitrogen acids can . .
be carried out using procedures known in the art. The complexes of silver with nitrogen acids can be prepared either in-situ, that is in combination with other componen-ts of the heat developable photographic materialsSor ex-situ, that ls separate from other components of the heat developable photo-graphic materials. In most instances, the preparation will be separate from the other components of the photographic materials based on ease of control o~ preparation and storage capability~ A complex of silver with lg2,~-triazole and its preparation is describedg for example3 in Chemical Abstracts~ Volume 217 page 3051~ and Cheml~al Abstracts, _ Volume 52S page l999. Preparation of the silver salt or complex of imidazole is described, for example, in _emical ~bstrac~sg Volume 52, page l999 also. The preparation of a complex of silver with a nitrogen acid in many ca.ses will merely involve mixing a source of silver ior~ such as silver nitra-te~with the desired nitrogen acid in a suitable solvent, such as an aqueous solvent. Purification of the resulting product can be carried out using methods known in the art. Another example of a preparation of a comp]ex of silver with a nitrogen acid is the preparation of complexes of silver with lH--tetrazo]es~
such as 5-methyl, 5-ethyl and 5-isopropyl derivatives of lH-tetrazole, in the Tour ~ anic Chemis~, Volurne 15, beginning at page 1082 and the Journal o-f Organic Chemis-try3 Volume 18,beginning at page 1003.
The term "complex" as used herein is intended to include any type of bonding or complexing mechanism which enables the resulting material to provide imaging properties in the described image-forming combination~ In some instances the exact bonding of the described complex of silver with a nitrogen acid is not fully understood. Accordingly, the -term "complex"
is intended to include salts and other forms of bonding which enable the desired image-forming combination to provide the desired image in the described heat developable photographic material. The term "complex" is intended to include wha-t may have been known in the past as salts. The term "complex" is intended to include neutral complexes or non-neutral complexes.
An advantage of the described heat developable materlal and process of the invention is that aqueous forrnulations, such as aqueous gelatino photographic emulsions 3 are especially useful. The heat developable photographic materials according to the invention comprise a photosensitive component _ g _ which is photosensitive sîlver halide. The photosensitive silver halide is especially useful due to its increased photosensitivity compared to other photographic components.
A typical concentration of pho-tosensitive silver halide in a heat developable photographic element according to the inven-tion is wlthin the range of about 0.05 to about 3 moles of photosensitive sil~er halide per mole of silver complex in the heat developable photographic material. For example, a typically preferred concentration range of pho-tographic sil~er halide is about 0.1 to about 0.5 moles of photo-graphic silver halide per mole of silver complex ~n the described heat developable p'notographic material. Other photographic materials can be used in combination with the described photosensiti~e silver halide if desired. ~or exampleg usef'ul photosensitive silver salts can include silver dye complexes such as those descr-Lbed in U.S. Patent 3,647,439 of Bass9 issued March 7, 1972. Preferred pho-tographic silver halides are silver chloride, silver bromide, silver bromoiodide, silver chlorobromoiodide or mixtures thereof`. For purposes of the invention, silver iodide is also considered to be a useful photographic silver halide. Very fine-grain pho-tographic silver halide is useful although coarse or fine-grain pho tO-graphic sllver halide can be employed if desired. The photo-graphic sil~er halide can be prepared by any of the procedures known in the photographic art especially -those procedures which involve the preparation of photographic silver halide gelatino emulsions. Useful procedures and forms of photographic silver halide for purposes of the invention are described9 for example9 in the Product Licensing Index, ~lolume 92, December9 1971, pub]lcation 9232 on page 10'7~ paragraph I. The photographic silver halide as described can be lmwashed or washed~ can be chemically ~ ~3~

sensitized using chemical sensitization procedures known in the art, can be protected against production of fog and stabilized against loss of sensitivity during keeping as described in the above Product Licensing Index publlcation.

._ The described heat deve10pable photographic materials can comprise a variety of reducing agents, especially organic reducing agents which are typical photograp'hic silver halide developing agents. These reducing agents are organic reducing agents. Combinations of organic reducing agents, typically silver halide developing agents, can be useful~
Reducing agents which are especially useful are silver halide developing agents including polyhydroxy benzenes, such as hydroquinone~ alkyl-substituted hydroq-uinones, including tertiary-butyl hydroquinone, methylhydroquinone, 2,5-dimethylhydroquinone, and 2,6-dimethylhydroquinone; catechol and pyrrogallol developing agents, chloro-substituted hydroquinones such as chlorohydroquinone or dichlorohydroquinon.e; alkoxy-substituted hydroquinones such as methoxyhydroquinone or ethoxyhydro~uinone; aminophenol reducing agents such as 2,4-diaminophenols and methylaminophenols, ascorblc acld developingagents such as ascorbic acid, ascorbic acid ketals and ascorbic acid derivatives; hydroxylamine reducing agents; 3-pyrazolidinone reducing agents such as l~phenyl-3-pyrazolidinone and 4-methyl-~-hydro~ymethyl-l-phenyl-3-pyrazolid-lnone; reductone reducing agents, such as 2-hydroxy-5-methyl-3-plper-ldino~

2-cyclopentanone; gallic acid ester reducing agen~ts such as methylgallate; sulfonamido phenol. reducin~ a.gent~ such as the sulfonamidophenol reducing agents described in Research Disclosure~ January 19'73, pages 16-21; phenylenediamlne silver

3 halide de-veloping agents such as paraphenylenediamine and the like. Especially useful heat developable photographic materials include those whlch contain an organic reducing agent selected f'rom the group consisting of ~3~

hydroquinone, ascorbic acid~ pyrrogallolg gallic acid ester, and phenylenediamine silver halide developing aKents and combinations thereo~O
A use~ul concentration of organic reducing agent in a heat developable photographic ma-terial as described is typically within -the range of about 0.1 moles to ~bout 3 moles of organic reducin~ a~ent per mole o~ sil~er complex.
An especially useful concentratlon of organic reducing agent is within the range of about 0.5 -to about 1.5 moles of reducing agent per mole of sllver complex. The optimum concentration of reducing agent can be determined based upon such factors as the desired image, other components in the heat developable photographic material, processing conditions and the like.
The heat developable photographic materials according to the invention can contain various colloids and polymers alone or in combination as vehicles5 binding agents, and in various layers. Sui-table materials as described are preferably hydrophilic materials although some hydrophobic materia]s can be useful. The colloids and polymers are transparent or translucent and include both naturally occurring substances such as proteins, for exampleg gelatin, gelatin derivatives, cellulose derivatives, polysaccharides, such as dextran, and the like; and synthetic polymeric substances such as water soluble polyvinyl compounds like poly(vinyl pyrrolidone)g acrylamide polymers and the like. Other synthetic polymeric compounds which can be employed include dispersed ~inyl compounds such as in latex form and particularly those which increase dimensional stability of pho-tographic materials. Effective polymers include high molecular weight materials, polymers and reslns which are compatible with the described complex of silver with a nitrogen acid and other componen-ts of the hea-t developable ~3~

photographic material. Especially useful material~ include gelatin, poly(vinyl pyrrolidone)~ and poly(vinyl alcohol).
Other useful polymeric materials as binders include:
(a) Copolvvmer of acrylamide and l~vinylimidazole, 90 10 weight ratio.
(b) Copolymer of acrylamide and 2-acetoacetoxyethyl methacrylate~ 9802 weight ratio~
Combinations of the described colloids and polymers can also be used.
The heat developable photographic materials according to the invention can contain an image toner in order to provide a more neutral or black tone image upon processing. The optimum toning agent will depend upon such -factors as the particular heat developable photographic material~ the desired image, particular processing conditions and the like. In some cases certain lmage toning agents provide much be-tter results wi-th certain complexes of silver with nitrogen acids than with other silver salts. A simple screening test can be used to select a useful image toning agent. One such test compr-Lses that described in following Example 13. The silver complex of the nitrogen acid in that example can be replaced with other complexes of silver salts with nitrogen acids to select the optimum image toning agent for the particular complex of silver with the nitrogen acid.
In this test the most useful toning agent is -typically that toning agent which provides a ratio of (a) visible maximum densi-ty to (b) blue light maximum density exceeding a value of 0.~70 A ~ariety of toning agents are also folmd to act as development accelerators producing up to a 1.0 log E speed increase in the photographic material. Especially useful toning agents are selected from the group consis-ting of 1,2,4-triazole, lHr-te-trazole, thiouracil and 1,3g4~thiad-lazole ~3~

toning agents. Examples of preferred toning agenks are 5-amino-1~3,4-thiaaiazole-2--thiolj 3-mercapto-1,2,L~--triazole and bls(dimethylcarbamyl) disulfide~ Especially useful toning agents are -those which provide a black tone image.
A range of concentration of toning agen-t is useful in a heat developable photographic material according to the inventlon. ~ typically useful concentratlon o~ toning agent is within the range of about 0.01 to about 0.1 moles of -toning a~ent per mole of silver complex in the heat developable photographic ma-terial. The optimum concentration of toning agent ~ill depend upon such factors as the particular heat developable photographic material, processing conditionsg desired image and the like.
It is often useful to include a me]t forming compound in the heat developable photographic materials according to the invention in order to provide an improved developed image.
The term "melt-forming compound" as employed herein is intended to mean a cornpound which upon heating to the described processing temperature provides an improved reaction medium3 typically a molten medium, wi-thin which the described image-forming combination can provide better image development. The exact nature of the reaction medium at processing tempera-tures described is not fully understood, however, it is believed tha-t at the reaction temperatures a melt occurs which permits the reaction components to better interact. Useful melt-forming compounds are typically separate components from the image~forming combina-tion, although the image-forming combination can enter into the melt formation. T~pically useful rnel-t forming compounds are amides, imldes~ cyclic ureas and -triazoles which are compatible with the other components of the heat developable photographic materials. Useful melt-forming compounds are described~ for example, in U.S Patent 3,438,776 of Yudelson issued April 15, ]969.

_ lL~ _ ~3~

A variety of melt-forming compounds are useful including, for exampleg formamide~ acetamide, propionamide~ 2-pyrrolidione3 and 1,3-dimethylurea. The 1,3-dimethylurea is especially useful with the described preferred complexes of silver with nitrogen acids.
A range of concentration of melt-forming compound is useful in the heat developable photographic materials described. A typically useful concentration range of rnelt-forming compound is about 0 5 to about 2 parts by weight of melt-forming compound per gram of the silver complex. The optimum concentration of melt-forming compound will depend upon the particular heat developable photographic material 3 desired image~ processin~ conditions and the like.
Spectral sensltizing dyes can be useful in the described elements and compositions of the invention to confer additional sensi-tivity to the elements and composi-tions.
Useful sensitizing dyes are described, for example, in the Product Licensing Index, Volume 92, December 1971~ publication 9232, pages 107-110, paragraph XV. An advantage of the ~0 heat developable photographic materials according to the invention is that a wider range of spectral sensitizing dyes is useful than with many other heat developable photographic materials. This in part is due to the use of conventional silver halide emulsions in the heat developable materials according to the inventionO
A preferred heat developable photographic material according -to the invention contains a sPectral sensitizi~ dye selected from the group consisting o~ anionic cyanine dyes~
anionic merocyanine dyes and combinations of these dyes.

Examples of especially useful cyanine dyes and merocyanine dyes include:

~ 9~7~

(a) S ~ ~-C2~5 N ~ , ~0 ~ S

(b) S CH3 ~

(CH2)2 (CH2)2 +
CH - S03 Na~ CH - S03 Na While the optimum concentration of the components of a heat developable photographic material according to the in~ention will depend upon a variety of factors as described, an especially useful heat developable photographic element according to the invention comprises, for each mole of photographic silver halide, 5 to 20 moles of the described complex and 3 to 15 moles of the described organic reducing agent.
The complex of silver with the described nitrogen acid can contain a range of molar ra-ti.o of the nitrogen acid to silver ion in the complex. The optimum ratio of the nitrogen acid to silver as silver ion will depend upon the particular nitrogen acid, the particular heat developable photographic material, processing conditions and the like. However~ the molar ratio of the nitrogen acid to silver as silver ion in the complex is usefully within the range of 1:1 to 3:1. This molar ratio of nitrogen acid to silver as silver ion can be determined using methods known in the photographic art. An especiall~
useful molar ratio of the described nitrogen acid to silver as silver ion is 1~5 1.o 2i~

The heat developable photographic materials also can contain a range o~ pAg. The pAg can be measured using conventional calomel and Ag-Ag~l electrodes, connected to an Orion digital pH-meter. The typical pAg in a heat developable photographlc material according to the invention is within -the range of about 2 to about 6, with a pre~erred range of pAg being 3.5 to 5 5~ The optimum pAg will depend upon the described factors, such as the particular heat developable photographic materialg desired image, processing condi-tions and the like.

The heat developable photographic materials according to the invention typically have a pH range ~hich is on the acid side of neutralg that is a pH of less than 7. A typically useful pH for a heat developable photographic material according to the invention is within the ran~e of about 2 to about 6, with the preferred range being about 3.5 to 5Ø
It is desirable to employ a stabilizer or a stabilizer precursor in the described heat developable materials according to the invention to improve post-processing image stabilityO
In some cases the complexes, as describedgare stable after processing. However, in the case of photographic silver halideg it can be desirable to stabilize the silver halide a~ter processing in order to avoid post processing print-out. A
variety of stabilizer or stabillzer precursors are use~ul in a heat developable photographic material as described.
The stabilizers or stabilizer precursors can be used alone or in combinationO Typical useful stabilizers or stabilizer precursors are azole thioethers and blocked azoline thione stabilizer precursors such as described in Belgian Patent 7~8,071,issued July 309 197~ and 4-aryl-1-carbamyl-2-tetrazoline-5-thione 3o stabilizer precursorsgsuch as described in U.S. Patent 3,893~859 o~ Burness et alg issued July 8, 1975. Other useful stabilizers or stabilizer precursors include those described in U.S. Patent 3,839~041 of Hiller lssued ~ctober 1, 1974;
U.S. Patent 3~844,788 of Burness et al issued October 29, 1974 and U.S. Patent 33877,940 of Ericson issued April 15~ 1975.
Photolytically activated polyhalogenated organic compounds can be usedg but these compounds have been found to be less desirable than other stabilizers or stabilizer precursors. Such photo-lytically activated polyhalogenated organic compounds are described, or example, in U.S. Patent 3~874,91~6 of ~osta et al, issued April 1, 1975 and U.S. Patent 3,707,377 of I'iers et al7 issued December 28, 1972.
A range of concentration of stabillzer or stabilizer precursor can be useful in the described heat developable materials. An optimum concentration of stabilizer or stabilizer precurso-r will depend upon such factors as the particular heat developable material, processing conditions, desired stability of image and the like. A typically useful concentration range of stabilizer or stabilizer precursor is about 0.1 to about 20 moles of stabillzer or stabilizer precursor per mole of photographic silver halide in the heat developable material. A preferred concentration of stabilizer or stabilizer precursor is within the range of about 4--to about 8 moles o~ stabilizer or stabilizer precursor per mole of photographic silver halide in the heat developable material as described It is in some cases useful to have a so-called overcoat layer on the heat developable photographic element according to the invention to reduce fingerprinting and abrasion marks.
The overcoat layer can be one or more of the describecl polymers which are also useful as binders. Howe-ver, other polymeric materials which are compatible with the heat developable layer and can tolerate the processlng temperat-ures employed - 18 ~

~q~

according to the invention can be use~ul. Such other binders or polymeric materials include, ~or instance, cellulose acetate.
Combinations o~ polymeric materials can be useful ~or overcoat purposes if desired~
The heat developable materials according to the invention can contain other addenda such as development modi~iers that function as speed-increasing compounds, hardeners, plasticizers and lubricants, coating aids, brighteners, absorbing and filter dyes, antistatic materials or layers, and the like. These are described, for exampleg in the Product Licer~sing Indexg Volume 92~ December 1971~ publication 9232~ pages 107 llOo The heat developable elements according to the invention can comprise a variety o~ supports which can tolerate the processing temperatures employed according to the invention.
Typical suppor-ts include cellulose ester film, poly(vinyl acetal) ~ilm, pol~(ethylene terephthalate) film~ polycarbonate f`ilm and polyester film supports as described in U.S. Patent 3~634~ 389 of Hamb issued July 11~ 1972 and U.S. Patent 3~725~070 of Hamb et al issued ~pril 3~ 1973~ Related film and resinous support materials, as well as glass, paper, metal and the llke supports which can ~lithstand the processing temperatures described are also use~ul. Typically, a flexible support is most useful.
The compositions according to the invention can be coated on a suitable support by various coating procedures known in the photographic art including dip coating, airkni~e coating, curtain coating or extrusion coating using hoppers such as described in U.SO Patent 3~681~294 of Beguin issued June 15~ 1954~ I~ desired, two or more layers can be coated simul-taneously such as described in U.S. Patent 2~761~791 O~
Russell issued Septernber 4~ 1956 and British Patent 837~ 095 - 19 ~

published Jlme 9~ 1960.
The described components of the hea-t developable materials according to the invention can be in any suitable location in the heat developable element which provides the desired image. For example~ if desired one or more components of the heat developable element according to the invention can be in one or more layers of -the element. ~[n some cases, it can be desirable to include certain percentages of the described reducing agents, image stabilizer or s-tabili~er precursors and/or other addenda in a protective layer over the heat developable element. In some cases this can reduce migration of certain addenda between layers of the described element.

It is necessary that the photosensitive silver halideg as described, and other components in the image-forming com-bination according to the invention be in reactive association with each other in order to provide the desired image. The term "in reactive association" as employed herein is intended -to mean that the photographi~ silver halide and the image-forming combination are in a loca-tion wi-th respec-t to each other which enables the desired processing and provides a more useful developed image. It is believed that the latent image formed from imagewise exposure of the photosensiti~e silver halide acts as a catalyst for -the image-forming combina-tion containin~ the complex of silver with the described ni-trogen acid.

It is believed that this enables a lower processing -tempera-ture for image formation -than other~ise would be possible. While the exact nature of the reaction mechanisms and image formation in the developable material according to the invention is not fully understood, it is believed that the reaction is an ~3~

amplification reaction enabled by tne catalytic effect of thelatent image silver. Accordingly, the term "in reactive association" is intended to mean that the components are in a location with re~spect to each other which enable this desired lower processing temperature.
If desired, other heat developable photographic materials can be used in combination wi-th the heat developable photographic materials according to -the invention.

The other heat developable photographic materials must be compatible with and not adversely affect the image formation in a heat developable material according to the inventionO
For example, a heat developable photographic element can comprise respectively a support having thereon a heat developable photo~
graphic layer comprising a complex of s~lver with a nitrogen acid according to the invention and a separate layer containing a dif~erent photothermographic material containing pho-~graphic silver halide as a component with other necessary imaging materials.
An example of such a heat developable ma-terial is one containing a layer contiguous to the layer containing the complex of silver with a nitrogen acid and which separate layer contains photo-graphic silver halide in reactive association with a silver salt o~ certain heterocyclic thione compounds and an organic reducing agent. A useful material containing such a silver salt of certain heterocyclic thione compounds is described in U.S. Patent 3,893,860 of Sutton et al issued July 8, 1975.
In some cases the silver salt of the heterocyclic thione compound as described in U.S Patent 3,893,860~ can be used in the same layer as the described complex of silver with a nitrogen acid. The optimum concentrations and ratios of components in such a layer will depend upon the desired image~

~ 3~
'~ ~ .......

particular components o~ the heat ~evelopable material~
processing conditions and the likeO
An especially useful embodiment of the invention is a heat developable photographic element comprising a support having thereon in reactive association (a) a gelatino photo-sensi-tive silver halide emulsiong (b) an image-forming combination comprising (i) a complex of silver with dodecyl tetrazole, and (ii) an ascorbic acid developing agen~ such as ascorbic acid, (c) a gelatino binder3 (d) a toning agent consisting essentially of 5-amino 1,3,4-thiadiazole-2-thiol or 3-mercapto-1,2,4-triazole, and (e) a melt-forming compound consisting essentially of l,3-dimethylurea.
Another especially useful embodiment of the invent~on is a heat developable photographic element comprising a support having thereon in reactive association (a) a gelatino photosensitive silver halide emulsion~ (b) an image-forming combination comprising (i) a complex of silver with 3~amino-1~2,4-kriazole and (ii) a hydro~uinone developing agent, (c) a gelatino binder, (d) a toning agent consisting essentially of 5-amino-1,3,4 thiadiazole-2-thiol or 3-mercapto-1,2,4-triazole~
and (e) a me~t-forming compound consisting essentially of 1~3-dimethylurea.
Various imagewise exposure means are useful with the heat developable materials according to the invention. The materials according to the invention are typically sensitive to the ultraviolet and blue regions of lthe spectrum and exposure means which provide this radiation are preferredO Typically, however, if a spectral sensitizing dye is employed in the heat developable materials~ exposure means using other ranges of the ~ 22 '~f.~

spectrum are useful. Typlcally, a photosensitive el~ment according to the invention is exposed imagewise with a visi'ble light source such as a tungs-ten lamp3 although other ~ources of radiation are useful such as lasers~ electron beams and the like.
A visible image can be developed in a heat developable materia~ as described, within a shor-t time after imagewise exposure3 merely by unif'ormly heating the heat developable material to moderately elevated temperatures ~or example3 the heat developable element can be 'neated after imagewise exposure to a temperature within the range o~ about 120C to about 180C. Heating is carried out until a desired image is developed~ typically within about 1 to about 90 seconds, such as within about 1 to about 30 seconds. The heat developable material according to the invention is preferably heated to a -temperature within the range of about 140C to about 165C until a desired image is obtained, typically for about 1 to about 30 seconds.
Another embodiment of the invention is a process of developing an lmage in a heat developable photographic element compr~sing a support having thereon in reactive association (a) photosensitive silver halide~ (b) an image-forming combina-tion comprising (i) a complex o~ silver with a nitrogen acid, as descrlbed5 with (ii) an organic reducing agent~ also as described, and (c) a polymeric binder, comprising heating the described element tc a temperature wi-thin the range of about 120C to about 180C until a desired image is obtained, such as for about 1 to about 30 secondsO
A preferred process is a process of developing an image in a heat developable photographic element comprising a support having thereun in reactive association (a) a gelatlno photosensitive silver halide emulsion, (b) an image-f'orming ~ 23 -~3~

combination comprlsing (i~ a complex of silver with dodecyl te-tra-zole~ and (ii) an ascorbic acid developing agent3 (c) a gelatino binder~ (d) a toning agent consisting essentiall~ of 5-amino-1,3,4-thiadiazole-2-thiol or 3 mercapto-1,2~4-triazole, and (e) a melt-forming compound consis-ting essen-tially of 1,3-dimethylurea, comprising heating the described element -to a temperature within t'ne range of about 140C to about 155C
until a desired image is developed, such as f'or a'bou-t 1 to abou-t 30 seconds.
Another especially useful process is a process of developing an image in a heat developable photographic element comprising a support having thereon in reactive association (a) a gelatino photosensitive silver halide emulsion, (b) an image-forming combination comprising (î) a complex of silver with 3-amino-1~2,L~-triazole, and (ii) a hydroquinone developing agent, (c) a gelatino binder$ (d) a toning agent consis-ting essentially of` 5-amino-1,3~-thiadiazole-2--thiol or 3-mercapto-1,2,4-triazole, and (e) a melt-~orming cornpound consisting essentially of 1~3-dimethylurea, comprising heating the described element to a temperature within the range of about 140C to about 165C until a desired image is developed, such as for about 1 to abou-t 30 seconds~
The heat developable materials according to the invention are useful for forming a negative or positive image.
The ~ormation of a negative or positive image will depend primarily upon the selection of the par-ticular photosensi-tive silver halide. One class of use~u'l photosensitive silver halide materials is the class of direct-positive photographic sllver halide materia]s designed to produce pos:L-tive images.

l2~

In-ternal image silver halide emulsions can be used for this purpose, such as those described in U.S. Patent 2,592,250 of Davey et al issued April ~, 1952; U.S. Patent 3,206,313 of Porter et al issued September lL~, 1965, u.s. Patent 3,367,778 of Berriman et al issued February 6, -L968; and U.S. Patent 3,L~47,927 of Bacon et al issued June 3, 1969.
If desired mixtures of surface and internal image silver halide emulsions can be used as described in U.S.
Patent 2,9969382 of Luckey et al issued April lL~3 1961.
~lthough it is often undesirable,due to the lack of control in preparation, the described photographic silver halide can be prepared in situ in the described material according to the invention. Such a method of preparation of photographic silver halide _ situ in a photothermographic material is described, for example, in U.S. Patent 3,457,075 of Morgan et al issued July 22, 1969.
Processing according to the invention is usually carried out under ambient conditions of pressure and humidity.
Pressures and humidity outside normal a-tmospheric conditions 20 can be employed if desired; however, normal atmospheric condi-tions are preferred.
~ variety of means can be ernployed to provide the necessary heating of the described hea-t developable materials to provide a developed imageO The heating means can be a simple ho-t plate, iron, roller or the like.
It is desirable in some ins-tances to include a development accelerator in a heat developable pho-tographic materlal as described. Development accelerator as used herein is intended to include those compounds which increase the rate of development to provide a developed image or lead ~3~

-to increased irnage density in the reslllting image. Compounds whlch provide these desired properties o:~ten are compounds which act as humectants. An example of a heat developable photographic elernent containing such a developmen-t accelerator is: a photographic eLement containing in reactive association (a) a photographic gelatino silver bromoiodide emulsion~
(b) an image-forming combination comprising (i) a complex of si]ver with 3-amino~1,2,4-triazole (containing a ratio of nitrogen acid to silver as silver ion in the complex of 1:1), with (ii) 2,5-dihydroxyphenylacetic acid as a developing agent, and (c) gelatin as a polymeric binder. In addition~
a development accelerator is included in the material at a concentration ~hich provides about 50 milligrams of developmen-t accelerator per square foo-t of support of the photographic elementO Compounds which provide a degree of development acceleration in this material are ethylene glycol, triethylene glycolg 1~2-propanediol, dipropylene glycol, 2~2'~oxydiethylenol, tetraethylene glycol9 -tetrahydrothiophene-l,l-dioxide and acetamide. Of these compounds tetrahydrothiophene l,l-dioxide is especially useful because it provides minimal loss in incubation stability and tends to provlde a reduction in the increase of print-out density upon processing.
The following examples are included for a further unders-tanding of the in~ention~

Exam~le 1 - Heat developable material containing complex of 'rT~er with ~ ria701e. ~ ~~
A solution of 5.5 grams of 1~2~4-triazole and 8.o grams of deionized photographic gelatin in 20 milliliters of a mixture of 90~ by volume ethanol and 10~ by volume methanol and 80 milliliters of distilled water was prepared A solu-tion ~3~

of 13.6 grams o~ silver nitrate in lO0 ~illlliters of distilled water was added to the resulting solution using a blending procedure. The blending procedure consisted of rapidl~
adding the si.lver nitrate solution to the lg2~L~-triazole solution in a jacketed mechanical blender at ].lO-120~ (corresponding to 43 to 49~C). The mixing speed of the mechanical blender was then slowly increased to provide a rapid mixing action and the mixture was blended at that mixi.ng speed f`or 45 minutes.
The final weight of the composition was 2.5 kilograms per 0 sil~er mole, the pH was 1.4 and the pAg was 3.1.
The resulting photographic composition was coated on a resin coated paper support using a kn-ife-blade coating procedure to provide a wet coating thickness of 4 mils.
The coating preparation for this purpose contained the following composition:

dispersion containing complex of silver 2.0 ml with 1,2,4-triazole (prepared as described above) composition consisting of 1 part by 1.2 ml volume of silver bromide gelatino e~ulsion to 9 parts by volume distilled water lO~ by weight deionized gelatin 1.2 ml hydroquinone (5~ by weight in water) 1~7 ml saponin surfactank O.Z ml distilled water 1.7 ml The photographic element was imagewise exposed for 20 seconds on a commercial sensitometer to provide a developable latent image. The resulting image was developed by contac-ting the exposed element on a heated metal block at ]50C for lO secondsO
A developed image was produced having a green-black color on a light gray background.
The procedure was repeated wherein a porkion of -the described dispersion containing the complex of` silver with 1~2g4-triazole was adJusted to a pH of 6.o with ammonium hydroxlde to provide a pAg of 7.1. A coating formulation ldentical _ ~7 ~3~8 to that described above was prepared and coated on the described paper support. m e resulting photographic element was then imagewise exposed f'or 20 seconds as described to provide a developable latent image. The resultlng image was developed by contacting the exposed element with a heated metal block at 1)~0C f'or 5 seconds. A developed image resulted having a maximum density of o.87 and a minimal density of 0 27.
e image tone of the developed image was brown-black and exhibited 7 visible developed steps on a conventional exposure scale. The coating unif'ormity and image density was improved as a result of' the dispersion pH being adjusted to 6.o.

Example 2 - Fleat developable photo~ra~hic material containing a~complex of silver with 3-amino-1~2,~-triazole.
The procedure described in Example 1 was repeated with the exception that 5.0 grams of 3-amino-1,2,~-triazole was used in place of 1,2,l~-triazole. The f'inal pH of the photographic cornposition was 3.5 and the pAg was 5.0 prior to imagewise exposure.
The composition used for coating contained the following components:

dispersion containing the complex of' 2.0 ml silver with 3-amino-1,2,~-triazole silver bromide emulsion (as described 1.2 ml in Example 1) 5~ by weight hydroquinone in water 1.7 ml 0.5~ by weight Surfactant lOG in water 0.2 ml (Surfactant lOG is a reaction product of an alkyl phenol with glycidol; and, is a trade name of' and commercially available f'rom the Olin Corporation, UoS~A~ ) distilled water 2.9 ml The composition was coated on the describe~ paper support at a wet coating thickness of ~ mils.

- 2~ -The resulting pho-tographic elemen-t was imagewise exposed for 20 seconds on a commercially available sensitometer to provide a developable latent imageO The resulting image was developed by contac-ting the photographic element with a metal block at 140~ for 5 seconds. A developed image was produced having a ma~imum density of 0.75 and a minimum density of 0.12. The developed image had a brown-black tone with 7 visible density steps.

Example 3_- Illustration of processing lati-tudeO
A heat developable photographic element was prepared as described in Examp]e 2. The resulting photographic element was divided into 6 parts each of which was imagewise exposed for 20 seconds on a commerc~ally available sensitometer to provide a developable latent image. Each el~ment was then heated by contacting the element at the temperature designated in following Table I for 5 seconds. The image developed in each case is described in following Table I.

~ ~D L-- L-- ~ L-- L--m m m m m m H

a~
~1 C~J ~ ~) c~ ~ C~l ~ ~ ~( ~ ~ C~l O O O O O ~
~D L~ LQ LQ LQ LQ
~D L-- L-- L-- L-- L--O O O O O O

o o o o o o O L~ O LQ O
Q L~ ~ ~D
~1 3o -3~28 This illustrates that over a 20C tempera-ture range, that is between 145C and 165C, neither the maximum density nor the photographic speed significantly changed while only a 0.09 increase in minimum density w~s produced. It also illustrates no observable change in image -tone within this processlng tempera-ture range, Example 4 Addition of toning agent.
~ dispersion was prepared containing a complex of silver with 3-amino-1,2,L~-triazole as described in Example 2.
A coa-ting formulation was prepared containing the following components:

dispersion containing complex of 2.6 ml silver with 3~amino-1~2,4-triazole silver bromide gelatino emulsion 0.32 ml (as described in Example 1) 5% by weight hydroquinone in water 1.7 ml 0.5~ by weight Surfactant lOG in 0.2 ml water distilled water 2.9 ml The described formulation was coated on a paper support as described in Example 1 at a wet coating thickness of L~ mils.
The resulting photographic element was imagewise exposed on a commercially available sensitometer ~or 20 seconds to provide a developable latent image. The resulting image was developed by contacting the photographic element with a heated metal block at 160C for 5 seconds. This produced a developed image having a maximum density of o.L~8 and a minimum density of 0.10, The developed image had a brown -tone. This was used as a control to compare with photographic elements containing a toning agent, - 31 -~3~

A coatin~ identical to the above was prepared with the exception that 0.51 milliliters of 3-mercapto 1,2~LL~triazole (0.25~ by weight in rnethanol) was added to the coatin~
formulation prior to coating on the paper support ~nd 2.4 milliliters of disti'lled water was used~ The resulting photo-graphic element was imagewise exposed as described above and processed using the same tempera-ture and time also as described.
The resulting developed image had a maximum density of 0.78 and a minimum density of 0.10. The image tone of the developed image was black on a white background.
This illustrates that maximum density and image tone were improved by adding the toning agent, 3-mercapto-132~4-triazole~to the described photographic element.

Example 5 - Spectral sensitizer.
A dispersion containing a complex of silver with a nltrogen acid was prepared as described in Example 2.
A photographic element was prepared as described in Example 2. The photographic element was imagew~se exposed for 20 seconds in a cornrnercially available sensitometer as descrlbed in Example 2 to provide a developable latent image. The element was then heated by contacting it with a metal block at a temperature of lL~0C' ~or 5 seconds. A developed image was produced having a maximum density to blue light of o.98 and a minimum den~ity to blue light of 0.10 with 6 visible steps.
The procedure was repeated with t'he exception -that o.L~6 milliliters of a 0.01~ by weight a~ueous solution of the spectral sensitizing dye represented by -the formula:

-CH - C ~ 0 ~ S

C2H~

~3~Z~

was added to -the silver ha]ide emulsion prior to the addition of the remaining described addenda of' the photographic composition. The composition was coated on a paper support as described in Example 2. The resulting photographic element was imagewise exposed for 5 seconds in a commercial sensitometer to provide a developable latent image as described above.
The image was developed by con-tacting -the element with a metal block at 14~C for 2 seconds~ The resulting developed image had a maximum density to blue light of 1.2 and a minimum densi-ty to blue light of 0O3 with 10 visible steps.
This illustrates that the photographic speed or response to white light exposure was doubled by the addition of the described spectral sensitizing dye.

6 ~ developable photographic film.
___ ~ _ The dispersion and coating composition as described in Example 2 were coated at a wet thickness of 7 mils on a gel-subbed poly(ethylene terephthalate) film support. The resulting developable photographic film was imagewise exposed in a commercial sensitome-ter for 10 3 seconds to provide a developable latent image. The resulting exposed element was heated for 2 seconds by contacting it with a metal block a-t a temperature of 160C. The resulting developed image had a brown-black color and a maximum density of 1~8 with a minimum density of 0.2.
A heat developable photographic f'ilm was prepared as described above wi-th the except-lon that the photographic layer prior to imagewise exposure was overcoated with the following composition:

10% by weight deionized photographic 1.7 ml 3 gelatin in water 5~ b~ weight hydroquinone in water 1.7 ml 0.5~ by weight in water of Surfactant 0.2 ml lOG
dis-tilled water L~.L~ rnl 3~

The coating composition was applied a-t a we-t coating thickness of 4 mils. Tne overcoat was dr:Led at L~9c (120F). The heat developable photographic element was imagewise exposed and the resulting image developed as described above.
The resulting developed image had a maximum transmission density o~ 1.6~ and a minimum transmission density of 0.16.
The resulting image had improved clarity over the unovercoa-ted material.

Example 7 ~ Heat developable_material containi,~
]0 a m ~
A complex of silve~ was prepared by dissolving 3.36 grams of 5-n-propyl-lH-tetrazole (0.03 moles) in 100 milliliters of distilled water. A solution of 5.10 grams (0.03 moles) of silver nitrate was prepared by dissol~ing the silver nitrate in 100 millili-ters of distilled water. The silver nitrate solution was added, with stirring and under safelight conditions,to the described tetrazole solution~ A whi-te precipitate resulted and was filtered with suction and washed well with distilled water followed by washing with ace-tone.
The resulting product was air dried to provide 5.96 grams of a whi-te chunky solid. This solid was analyzed and found it contained L~9.1~ silver, 25 9~ nitrogen, 2.9~ hydrogen and 21.o~ carbon. The calculated values for ~gCL~H7N4 were 49 LI~ silver, 25.6~ nitrogen, 3~19~ hydrogen and 21.9~ carbon.
A dispersion was prepared by combining 4.4 grams of the described white solid with 4.0 grams of deioni~ed photo-graphic gelatin in 20 milliliters of a solvent consisting of 90~ by volume ethanol and 10~ by volume methanol. The resulting composition was brought to a total weight of 100 grams (5 kilograms _ 31~ _ per mole o~ silver) wi-th distilled waterO The composition was then dispersed by sonic vibration means for 5 minutes in a water jacketed container. The thoroughly mixed material had a final pH of 5.9 and a pAg of 5~9.
A coating composition was prepared by mixing the followir.g components:

dispersion containing a complex of 2.0 ml silver with 5-n-propyl-lH tetra%ole as described above silver bromide er.~ulsion (l to lO parts 102 ml by ~olume water) 5~ by weight hydro~uinone in water 1.7 ml 0.5~ by weigh-t Surfactant lOG in 0.2 ml water distilled water 2.9 ml The resulting composition was coated at a wet coating thickness of 4 mils on a paper support. The resulting photo~
graphic element was imagewise exposed for 20 seconds with a commercially available sensitometer to provide a developable latent image. The resulting image was developed by con-tacting the e]ement with a metal block at a temperature of 160~C for 5 seconds. This provided a developed brown image on an off-white background with 7 visible density steps.

~xample 8 - Heat developable photographic materia] containing -a-complex o~ s~Tver w~tn ~-n-~utyl~ etrazole.
A complex of silver with 5-n-butyl-lH-te-trazole was prepared employing the same procedure for preparation of a complex of silver as described in Example 7 with the exception that 2.52 grams of 5-n-butyl-lH-tetrazole and 3. L~o grams of silver nitrate were used in place of the described tetrazole and silver nitrate concentration of Example 7, The yield of the resulting product was LL 043 grams, The product was analyzed and found to contain 46.1~ silver, 24.2~ nitrogen, 4.1~ hydrogen and 25.7~ carbon. This corresponded to a calculated amount for AgC5HgN4 of 46.4~ silver3 24.0~ nitrogen~ 3.8~ hydrogen and 25.8~ carbonO
The resulting product was used for preparation of a dispersion as described in Example 7 using 2~33 grams of the described complex of silver with 5-n-butyl-lH-tetrazole~
The final pH of the coating composition was 6.o and the pAg was 5.8.
A heat developable photographlc element was prepared by mixing a coating formulation the same as the composition described in Example 7 with -the exception tha-t the dispersion contained the complex of silver with 5-n butyl-lH-tetrazole rather than 5-n-propyl-lH-tetrazole. The resulting heat developable photographic element was imagewise exposed as described in Example 7 and the resulting latent image was developed at the temperature also described ln Example 7 The resulting developed image had a red-brown color on a cream colored background with 7 visible density steps~

Exam~le 9 - Ascorbic acid as a developing agent.
. . . ~
A dispersion identical to tha-t described in Example 8 was prepared containing the complex of silver with 5-n-butyl-lH-tetrazole.
A coa-ting composition was prepared by mixing the following componen-ts:

dispersion con-taining complex silver 2.0 ml with ~-n-bu-tyl-lH-tetrazole 3 silver bromide gelatino emulsion 1.2 ml (diluted 1:10 parts by weigh-t with water) 5~ by weigh-t ascorbic acid in water 1.7 ml ~ 36 -0.5~, Surfactant lOG ln water 0.2 ml distilled water 2.9 ml The resulting composition was coated on a paper support at a wet coating thickness of 4 mils. The heat developable photographic element was then exposed imagewise for 20 seconds on a commercial sensitome-ter to provide a developable latent image. The resulting latent image was developed by contacting the photographic element with a metal block at a temperature of 160C for 5 seconds. A developed image resulted having a red-brown color on a light yellow background with 8 visible density steps.

Example lO - Use of melt-forming compound.
A heat developable photographic element (control) was prepared identical to that described in Example 9. The heat developable photographic element was imagewise exposed and the resulting latent image developed as described in Example 9. The resulting developed image had a ma~imum density to blue light of 0.40 and a minimum density -to blue light of o.o6. Processing to provide this image was by con-tacting the element on a heated metal block at 140C for 5 seconds.
~ composition was prepared containing a melt-forming compound which was 1~3-dimethylurea~ The coating composition contained the following components:

dispersion containing complex of silver 2.0 ml with 5-n-butyl-lH-tetrazole (prepared as described in Example 8) silver bromide gelatino emulsion 1.2 ml (diluted 1:10 parts by weight with water) 5~ by weight 1,3-dimethylurea in 1.7 ml distilled water 0.5~ by weight Surfactant lOG in 0.2 ml water distilled -water 1.2 ml 3.~2~

The resulting photographic element was imagewise exposed for 20 seconds on a commercial sensitometer to provlde a ~evelopable laten-t image. Upon processing, as described3 a developed image was produced having a maximum density to blue light of 1.02 and a minimum denslty to blue light of 0.18. The image was brown in tone with 8 visible density steps.
It was observed that image densi-ty was increased when the meit-~orming compound 1~3-dimethylurea was added to the heat developable materialO

Example 11 - Reversal photographic silver halide emulsion in ~ tne neat ~eve~Lop~Te materlaT.
A dispersion containing a complex of silver with 5-n-butyl-lH-tetrazole was prepared identical to that described in Example 8. A photographic composition was prepared by mixing the following components:

dispersion containing complex of silver 2.0 ml with 5-n-butyl-lH-tetrazole photographic silver bromoiodide emulsion 1.8 ml designed for providlng reversal images (silver bromoiodide gelatino emulsion which was surface chemically sensitized with reduction and gold chemical sensi-tization and contained iridium internal latent image sites) (diluted 1:5 parts by weight with water) 5~0 by weight ascorbic acid in water 1.7 ml 5~ by weight 1,3-dimethylurea in water 1.7 ml 0.5~o by weight surfactantloG in water 0.2 ml distilled water o.6 ml The resulting composition was coated on a paper suppor-t at a wet coating thickness of 4 mils~ The resulting heat developable photographic elemen-t was imagewise exposed for 10 seconds on a commercially available sensitome-ter -to provide a ~ 38 -hea-t developable latent image. The resulting latent image was developed by contacting the photographic element with a metal block at a temperature of 150~ for 5 seconds. A
distinct reversal image was developed. The image was white on a brown-blac~ background. I'he developed image had 7 visible density steps in the image area.

Exam~le 12 - Stabilizer precursor in a heat de~elopable ~~ ~~ materia-l ~ ~
o A dispersion was prepared containing a complex of sil~er with 3-amino-lg2,4-triazole. The ratio of silver as silver ion to the 1,2,4-triazole T~as 1:1.
A photographic composition was prepared by mixing the following components:

dispersion containing the complex of silver with 3-amino-lg2,~-triazole (the dispersion contained 1.0 kilograms of deionized photographic gelatin per mole of silver) 2.0 ml silver bromoiodide gelatino emulsion 0.7 ml (containing 1.79 kilograms of emulsion per mole of silver:diluted 1:5 parts by weight with water) 5~ by weight hydroquinone in water 1.7 ml o.5~0 Surfactant lOG in water 0~2 ml distilled water 3 ~L ml The photographic composition was coated at a wet thickness of 4 mils on a paper support. This provided a heat developable photographlc element. This was used as a controlO
A heat developable photographlc material was prepared in the same manner as described with -the exception that the ~hoto~raphic composition prior to coating on t~Je paper l~ppor1 contained 3.1 ml of an aqueous solution containing 5y~ by weigh-t of the s-tabilizer precursor, ~ hydroxyethyl isothiouronium tri-chloroacetate and contained 0.28 milliliters of dlstilled wa-ter~

3 ' ~

The resulting photographic composition containing the trichloroaceta-te compound was also coated at a wet thickness of 4 mils on a paper support~
Both the control photographic element and the photographic element containing the trichloroaceta-te compound were imagewise exposed in a commercially available sensitometer for 20 seconds to provide a developable latent image. Both photographic materials were then developed by contacting the element with a heated metal block at 140-165C for 30 seconds.
The densities of the resultlng developed images were analyzed and the resulting photographic materials were then placed in a container for 24 hours and exposed to 130 foot-candles of illumination during this period. After this time, the densities of the image were again read with a densitometer.
r~le results obtained are given in the following Table II.

~3~

~ ¦ co (~) L~'\ r~ t~ O
. ~1~ ~D LS~ LL~ ~ ~
h ~ O O O O O O

cd t~ L~ ~ r--I L~ C() ~ r; r--I r~ r~ r; O

~ L~l C~ O~ ~ O N
~ C~ r--~ O
~ 1~ ' 5~
~3 t~ L~ 0 O L--~ ~O ~
~ r~ r~ r~ r--l r~ O

r~ ) L'~) ~ r--I r~
. ~3 ~O ~O ~0 ~O ~O LS~
~ 1~1 r~ r~ r--I r--I r~l C~ ~C
~i3 L~ O C~l ~ ~Y) C\l . ~ ~ ~ L5~ LL~ LS~ L5 r~ ~I r--I r--I r--I r--I

O r--I r--I CU (!~ O
t~) L'~
~1 O O O O O O
a $~
X
~3 C~ C~ LL~ ~ O LL~
LS~
r~ rl r--l r--I r--I O

C~
O L~ O L~ O LS~
a) ~ ~ LL~ L5~ ~0 r--I r I r~ r~ r-l r~

~3~

The above table indicates that the photographic material containing the isothiuronium salt pro~ides diminished print-out denslties without a severe loss in maximum density of the deve]oped image.
Isothiuronium stabilizer precursors which are useful for the heat developable photog~aphic materials as descrlbed are illustrated by those compounds descrihed in U.S.
Patent 3,301~678 of Humphlett et al issued January 31a 1967.

Example 13 - Toning agents in heat developable materials containing compLexes -o~-siLver with dodecy~L tetrazole.
A heat developable photographic element was prepared similar to that described in the prior Example 8 relating to the complex of silver with dodecyl tetrazole. The heat developable photographic element contained 5.5 milligrams per square decimeter of silver from the complex of silver with dodecyl tetrazole; 1.1 milligrams per square decimeter of silver as silver bromoiodide (silver bromoiodide gelatino emulsion containing 3 mole ~ iodide); 10.1 milligrams per square decimeter of ascorbic acid3 10.1 milligrams per square decimeter of 1~3-dime-thylurea; 0.1 milligrams per s~uare decime-ter of the -toning agent listed in the following Table III; and 10.1 milligrams per square decimeter of diionized photographic gela-tin with a minor concentra-tion of Surfactant lOG. The resulting heat developable photographic elemen-t was imagewise exposed to llght in a commercial sens.L-tometer to pro~ide a de-velopab:Le latent image. The resulting image was developed by contacting the photographic element with a metal block at a temperature of 155C for 3 seconds. The resulting image had a m~ximum and minimum density value as listed in following Table III.

The difference between the maximum densi-ty to visible light and -the maximum density to blue light is listed ln Column B of Table -[II. The most useful toning agents were considered to be - ~2 -3~

those which provlded a ratio of maximum density to visiblelight compared to maximum density to blue light exceeding a value of o.87. Compounds which did not have this value are considered as comparative examples.

- 1~3 ~

2 ;a .~w~

tlD ~D
~D bD O o o o t~
bDC~ I ~D I
o , o , ~ o ~, ~ ~ C~ ~ C~ C~
P s , a) o ~ ~ ~1 a) ~ s ~3 V ~ ~ D ~ Q bD bD O
Q I ~ I r O .~
~) j j c) 5 5 ~, s 5~ ~ o ~ o ~ o a) o a) ~ o ~ s~ o o ~ s~ c ~ c~ c ~
~ ~ ~ Q ,0 !~ S~ P P ~1 0 bD ,1~ bD

,~ ~ U~ ~ ~ U~ ~ ~ ~ U~ CU U~ C~ U~ O
L~ CO CO CO U~ C~CO ~ 1~ CO CO CO
m \
, ~ o o o o o o o o o o o o o o ~ ~ ~ ~ ~ ~ ~O CO o C~ ~ ~ o CO
~ O O ~CO ~ C~l ~O ~ 1~ ~ O co C~) ~ o ~ o o ~(Y) o o o o o ~ o o u~ l \ \ \ \ \ \ ~ \ \ \ \ \ \
cq a~ C0C~lU~ N ~J H O rJI~ H CJ`, O ~0 C~l O.~ ~ O ~ ~ C~C~l O
~ ~H
h rP r~l H~ O H H O H H H H H H H
r~
~ ~\
H O c~ ~C
H CQ ~ L~CO ~ ~ U~ CO1--~o CO CU CO ~0 ~1 0~0 ~UU~ cr~ r-J Lr~O O C~ LO H
(Y) . . o ~ O
r~f V r~\ \ \ \ \ , \ \ \ \ \ \ O O
E~ o r~ .C0 C~ O~O~ ~ ~ C0 C0 H ~ O O O O H O r--Ir--I O O r~ O O

r~ 'CS
r~
~CS O
r~ r~
~ ~S
~ ,~C,)rCS I O
rcS ~ ~S~Sr I ~I P
s:~ ~ p c) a~
~ -1~ S-lC) S r~ o ~tS
O ~ .r-l r-l ~ C) Q ~ , c) rcs r I
O O rcs ,~ r~r-l O ~ O
V O rcS N ~>~ ,~,' ~ N r ir--l N
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~U CU ~ ~ ~O(r) L~ CU r~

_ L~5 _ Example 14 Comparative exam~le wi~h the silver sal-t ~of~silver benzotriazole.
The procedure described ln Example 1 was repeated with the exception tha-t 4.8 g of benzotriazole and 800 g deionized photographic gelatin in 50 ml of a mixture of 90~
by volume ethanol and 10~ by volume methanol and 75 ml dis-tilled water was prepared. To this was added using a blend:lng procedure 6.8 g of AgN03 in 50 ml distilled water, The final pH of' the dispersion was 1.2 and the pAg 2.6.
The coating composition contained the following components:

dispersion containing the complex of 2.0 ml silver with benzotriazole silver bromide emulsion (described 1.2 ml in Example 1) 5~0 by weight hydroquinone in water 1.7 ml 0.5~ b~ weight Surfactant lOG in water 0.2 ml distilled water 2.9 ml The above composition was coated on the described paper support at a wet coating thickness of 4 mils.
The photographic composition was imagewise exposed for 20 seconds on a commercially available sensitometer to provide a developable latent image.
A dispersion of 3-amino-1~2~4--triazole was prepared as in Example 2 with the exception that 3.4 g of 3-amino-1,2g4-triazole and 6.8 g AgN03 was used. The final pH was 1.8 and the pAg 1.9. The weight of dispersion was 5.0 ~g/Ag mole.
The coating composition was identical to that prepared in Example 2. The imagewise exposure was the same as given above for the photographic composition containing the silver complex of benzotriazole.

L~6 3~

The resulting images were developed by contacting the exposed element with heated metal blocks set at the temperatures given in Table IV for a period of 5 or 2 seconds, The resulting blue-light densi-ties are listed in Table IV.

~ ~3~ ~
~L , ~g ~ CO O C~l . .~ o o C) .
a) l o o o o o o X F~
`P. ~
i' O ~ri ~I V E~
c~ q) ~D
~r~ 0-~
N U~
I ~1 a ~C LO ~0 0~ ~ L~ L~
~) O ~3 ~ O 0 ~1 0 0 C~ ~
~ l ~1 ~1 ~ r-l ~I r-l F~

L~ ~s~ L~
. ,~
C) ~ O
~a) l ooooo ~n F~
~1 OL~
V
a) rl E~
o N ~D
~n U~
O
N C) X O ~) ~I C~
5~ o ~ ~I C\~ ~r) c O
l O O O O O
F~

o O Ll~ O LO O
LS~ L~
~1 r~he above table demonstrates the grea-ter densities obtainable from the composition containin~ the silver complex of 3-amino-1,2~4-triazole in less than half the processing time as compa,red to the silver complex of benzotriazole.
The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood tha-t variations and modi,ications can be effected within the spiri~ and scope of the invention.

Claims (58)

What is claimed is:

1. In a heat-developable, photographic element comprising a support having thereon in a polymeric binder and in reactive association (a) photosensitive silver halide, and (b) an image-forming combination comprising (i) a complex of silver with a nitrogen acid, with (ii) an organic reducing agent for said complex of silver, the improvement wherein said nitrogen acid of said complex is a nitrogen acid selected from the group consisting of imidazole, pyrazole, urazole, 1,2,4-triazole and 1H-tetrazole nitrogen acids and combinations thereof.

2. In a heat-developable, photographic element comprising a support having thereon in a polymeric binder and in reactive association (a) photosensitive silver halide, and (b) an image-forming combination comprising (i) a complex of silver with a nitrogen acid, with (ii) an organic reducing agent for said complex of silver, the improvement wherein said complex of silver with a nitrogen acid is represented by the formula:

wherein R is hydrogen or NH2; and wherein the molar ratio of said nitrogen acid to silver as silver ion in said complex is within the range of 1:1 to 3:1.

3. In a heat-developable, photographic element comprising a support having thereon in a polymeric binder and in reactive association (a) photosensitive silver halide, and (b) an image forming combination comprising (1) a complex of silver with a nitrogen acid, with (ii) an organic reducing agent for said complex of silver, the improvement wherein said complex of silver with a nitrogen acid is represented by the formula:

wherein R1 is hydrogen, alkyl containing 1 to 20 carbon atoms, or NH2; and wherein the molar ratio of said nitrogen acid to silver as silver ion in said complex is within the range of 1:1 to 3:1.

4. A heat-developable, photographic element as in claim 1 wherein said photosensitive silver halide is a gelatino photographic silver halide emulsion.

5. A heat-developable, photographic element as in claim 1 wherein said organic reducing agent is selected from the group consisting of hydroquinone, ascorbic acid, pyrogallol, gallic acid ester, and phenylenediamine silver halide developing agents and combinations thereof.

6. A heat developable, photographic element as in claim 1 wherein said polymeric binder is a gelatino binder.

7. A heat-developable, photographic element as in claim 1 also comprising a toning agent.

8. A heat-developable, photographic element as in claim 1 also comprising a toning agent selected from the group consisting of 1,2,4-triazole, 1H-tetrazole, thiouracil and 1,3,4-thiadiazole toning agents.

9. A heat-developable, photographic element as in claim 1 also comprising a 5-amino-1,3,4-thiadiazole-2-thiol toning agent.

10. A heat-developable, photographic element as in claim 1 also comprising a 3-mercapto-1,2,4-triazole toning agent.

11. A heat-developable, photographic element as in claim 1 also comprising a bis(dimethylthiocarbamyl) disulfide toning agent.

12. A heat-developable, photographic element as in claim 1 also comprising a separate melt-forming compound.

13. A heat-developable, photographic element as in claim 1 also comprising a separate melt-forming compound consisting essentially of l,3-dimethylurea.

14. A heat-developable, photographic element as in claim 1 also comprising a spectral sensitizing dye selected from the group consisting of anionic cyanine dyes, anionic merocyanine dyes and combinations of said dyes.

15. A heat-developable, photographic element as in claim 1 comprising for each mole of said photographic silver halide, 5 to 20 moles of said complex and 3 to 15 moles of said organic reducing agent.

16. A heat-developable, photographic element comprising a support having thereon in a gelatino binder, in reactive association, (a) a gelatino photosensitive silver halide emulsion, (b) an image-forming combination comprising (i) a complex of silver with dodecyltetrazole and (ii) an ascorbic acid silver halide developing agent, (c) a toning agent consisting essentially of 5-amino-1,3,4-thiadlazole-2-thiol or 3-mercapto-1,2,4-triazole, and (d) a melt-forming compound consisting essentially of 1,3-dimethylurea.

17. A heat-developable, photographic element comprising a support having thereon in a gelatino binder, in reactive association, (a) a gelatino photosensitive silver halide emulsion, (b) an image-forming combination comprising (i) a complex of silver with 3-amino-1,2,4 triazole, and (ii) a hydroquinone silver halide developing agent, (c) a toning agent consisting essentially of 5-amino-1,3,4-thiadiazole-2-thiol or 3-mercapto-1,2,4-trizole, and (d) a melt-forming compound consisting essentially of 1,3-dimethylurea.

18. A heat-developable, photographic element as in claim 1 wherein the molar ratio of said nitrogen acid to silver as silver ion in said complex is within the range of 1:1 to 3:1 parts acid pAg in said element is within the range of 2 to 6.

19. In a heat-developable, photographic composition comprising in a polymeric binder (a) photosensitive silver halide, and (b) an image-forming combination comprising (i) a complex of silver with a nitrogen acid, with (ii) an organic reducing agent for said complex of silver, the improvement wherein said nitrogen acid of said complex is a nitrogen acid selected from the group consisting of imadazole, pyrazole, urazole, 1,2,4-triazole and 1H-tetrazole nitrogen acids and combinations thereof.

20. In a heat developable, photographic composition comprising in a polymeric binder and in reactive association (a) photosensitive silver halide, and (b) an image-forming combination comprising complex of silver with a nitrogen acid, with (ii) an organic reducing agent for said complex of silver, the improvement wherein said complex of silver with a nitrogen acid is represented by the formula:

wherein R is hydrogen or NH2; and wherein the molar ratio of said nitrogen acid to silver is silver ion in said complex is within the range of 1:1 to 3:1.

21. In a heat-developable; photographic composition comprising in a polymeric binder and in reactive association (a) photosensitive silver halide, and (b) an image-forming combination comprising (i) a complex of silver with a nitrogen acid, with (ii) an organic reducing agent for said complex of silver, the improvement wherein said complex of silver with a nitrogen acid is represented by the formula:

wherein R1 is hydrogen, alkyl containing 1 to 20 carbon atoms, or NH2; and wherein the molar ratio of said nitrogen acid to silver as silver ion in said complex is within the range of 1:1 to 3:1.

22. A heat-developable, photographic composition as in claim 19 wherein said photosensitive silver halide is a gelatino photographic silver halide emulsion.

23. A heat-developable, photographic composition as in claim 19 wherein said organic reducing agent is selected from the group consisting of hydroquinone, ascorblc acid, pyrogallol, gallic acid esters, and phenylenediamine silver halide developing agents, and combinations thereof.

24. A heat-developable, photographic composition is in claim 19 wherein said polymeric binder is a gelatino binder.

25. A heat-developable, photographic composition as in claim 19 also comprising a toning agent.

26. A heat-developable, photographic composition as in claim 19 also comprising a toning agent selected from the group consisting of 1,2,4-triazole, 1H-tetrazole, thiouracil and 1,3,4-thiadiazole toning agents.

27. A heat-developable, photographic composition as in claim 19 also comprising a 5-amino-1,3,4-thiadiazole-2-thiol toning agent.

28. A heat-developable, photographic composition as in claim 19 also comprising a 3-mercapto-1,2,4-triazole toning agent.

29. A heat developable, photographic composition as in claim 19 also comprising a bis(dimethylthiocarbamyl) disulfide toning agent.

30. A heat-developable photographic composition as in claim 19 also comprising a separate melt-forming compound.

31. A heat-developable, photographic composition as in claim 19 also comprising 1,3-dimethylurea at a separate melt-forming compound.

32. A heat-developable, photographic composition as in claim 19 also comprising a spectral sensitizing dye selected from the group consisting of anionic cyanine dyes, anionic merocyanine dyes, and combinations of said dyes.

33. A heat-developable, photographic composition as in claim 19 comprising for each mole of said photographic silver halide, 5 to 20 moles of said complex and 3 to 15 moles of said organic reducing agent.

34. A heat-developable, photographic composition comprising in a gelatino binder, (a) a gelatino photosensitive silver halide emulsion, (b) an image-forming combination comprising (i) a complex of silver with dodecyltetrazole, and (ii) an ascorbic acid silver halide developing agent, (c) a toning agent consisting essentially of 5-amino-1,3,4-thiadiazole-2-thiol or 3-mercapto-1,2,4-triazole, and (d) a melt-forming compound consisting essen-tially of 1,3-dimethylurea.

35. A heat-developable, photographic composition comprising in a gelatino binder, (a) a gelatino photosensitive silver halide emulsion, (b) an image forming combination comprising (i) a complex of silver with 3-amino-1,2,4-triazole, and (ii) a hydroquinone silver halide developing agent, (c) a toning agent consisting essentially of 5-amino-1,3,4-thiadiazole-2-thiol or 3-mercapto-1,2,4-triazole, and (d) a melt-forming compound consisting essen-tially of 1,3-dimethylurea.

36. A heat-developable, photographic composition comprising in a polymeric binder, (a) photosensitive silver halide, and (b) an image-forming combination comprising (i) a complex of silver with a nitrogen acid selected from the group consisting of imidazole, pyrazole, urazole, 1,2,4-triazole, and 1H-tetrazole nitrogen acids and combinations thereof with (ii) an organic reducing agent for said com-plex of silver;
wherein the molar ratio of said nitrogen acid to silver as silver ion in said complex is within the range of 1:1 to 3:1 and pAg in said composition is within the range of 2 to 6.

37. A process of developing an image in an exposed, heat-developable, photographic element comprising a support having thereon in a polymeric binder, in reactive association, (a) photosensitive silver halide, and (b) an image-forming combination comprising (i) a complex of silver with a nitrogen acid selected from the group consisting of imidazole, pyrazole, urazole, 1,2,4-triazole and 1H-tetrazole nitrogen acid and combinations thereof with (ii) an organic reducing agent for said complex of silver, comprising heating said element to a temperature within the range of about 120°C to about 180°C.

38. A process as in claim 37 comprising heating said element to a temperature within the range of about 120°C to about 180°C for about 1 to about 30 seconds.

39. A process as in claim 37 wherein said complex of silver with a nitrogen acid is represented by the formula wherein R is hydrogen or NH2; and wherein the molar ratio of said nitrogen acid to silver as silver ion in said complex is within the range of 1:1 to 3:1.

40. A process as in claim 37 wherein said complex of silver with a nitrogen acid is represented by the formula wherein R1 is hydrogen, alkyl containing 1 to 20 carbon atoms, or NH2; and wherein the molar ratio of said nitrogen acid to silver as silver ion in said complex is within the range of 1:1 to 3:1.

41. A process of developing an image in an exposed, heat-developable, photographic element comprising a support having thereon in a gelatino binder, in reactive association, (a) a gelatino photosensitive silver halide emulsion, (b) an image-forming combination comprising (i) a silver salt of dodecyltekrazole, and (ii) an ascorbic acid silver halide developing acid, (c) a toning agent consisting essentially of 5-amino-1,3,4-thiadiazole-2-diol or 3-mercapto-1,2,4-triazole, and (d) a melt-forming compound consisting essentially of 1,3-dimethylurea, comprising heating said element to a temperature within the range of about 140°C to about 165°C.

42. A process of developing an image in an exposed, heat-developable, photographic element comprising a support having thereon in a gelatino binder, in reactive association, (a) a gelatino photosensitive silver halide emulsion, (b) an image-forming combination comprising (i) a complex of silver with 3-amino-1,2,4-triazole, and (ii) a hydroquinone silver halide developing agent, (c) a toning agent consisting essentially of 5-amino-1,3,4-thiadiazole-2-thiol or 3-mercapto-1,2,4-triazole, and (d) a melt-forming compound consisting essen-tially of 1,3-dimethylurea, comprising heating said element to a temperature within the range of about 140°C to about 165°C.

43. A heat-developable, photographic element comprising a support having thereon in a polymeric binder and in reactive association (a) photosensitive silver halide, (b) an image-forming combination comprising (i) a complex of silver with an imidazole nitrogen acid, with (ii) an organic reducing agent for said com-plex of silver.

44. A heat-developable, photographic element as in Claim 43 wherein said photosensitive silver halide is a gelatino photographic silver halide emulsion.

45. A heat-developable, photographic element as in Claim 43 wherein said polymeric binder is a gelatino binder.

46. A heat-developable, photographic element as in Claim 43 also comprising a toning agent.

47. A heat developable, photographic element as in Claim 43 also comprising a spectral sensitizing dye selected from the group comprising essentially of anionic cyanine dyes, anionic merocyanine dyes and combinations of said dyes.

48. A heat-developable, photographic element as in Claim 43 comprising for each mole of said photographic silver halide, 5 to 20 moles of said complex and 3 to 15 moles of said organic reducing agent.

49. A heat-developable, photographic element as in Claim 43 wherein the molar ratio of said imidazole nitrogen acid to silver as silver ion in said complex is within the range of 1:1 to 3:1 and pAg in said element is within the range of 2 to 6.

50. A heat-developable, photographic composition comprising in a polymeric binder (a) photosensitive silver halide, and (b) an image-forming combination comprising (i) a complex of silver with an imidazole nitrogen acid, with (ii) an organic reducing agent for said com-plex of silver.

51. A heat-developable, photographic composition as in Claim 50 wherein said photosensitive silver halide is a gelatino photographic silver halide emulsion.

52. A heat-developable, photographic composition as in Claim 50 wherein said binder is a gelatino binder.

53. A heat-developable, photographic composition as in Claim 50 also comprising a toning agent.

54. A heat-developable, photographic composition as in Claim 50 also comprising a spectral sensitizing dye selected from the group consisting of anionic cyanine dyes, anionic merocyanine dyes, and combinations of said dyes.

55. A heat-developable, photographic composition as in Claim 50 comprising for each mole of said photo-graphic silver halide, 5 to 20 moles of said complex and 3 to 15 moles of said organic reducing agent.

56. a heat-developable, photographic composition comprising in a polymer binder, (a) photosensitive silver halide, and (b) an image-forming combination comprising (i) a complex of silver with an imidazole nitrogen acid, with (ii) an organic reducing agent for said com-plex of silver, wherein the molar ratio of said imidazole nitrogen acid to silver as silver ion in said complex is within the range of 1:1 to 3:1 and pAg in said composition is within the range of 2 to 6.

57. A process of developing an image in an exposed, heat-developable, photographic element comprising a support having thereon in a polymeric binder, in reac-tive association, (a) photosensitive silver halide, and (b) an image-forming combination comprising (i) a complex of silver with an imidazole nitrogen acid, and (ii) an organic reducing agent for said com-plex of silver, comprising heating said element to a temperature and for a time sufficient to effect imagewise deposition of silver from said complex.

58. A process as in Claim 57 comprising heating said element to a temperature within the range of about 120°C to about 180°C for about 1 to about 30 seconds.