CA2170586A1 - Deoxyribonucleic acid (dna) as an antifoggant and print stabilizer for photothermographic elements - Google Patents

Abstract

Deoxyribonucleic acids (DNA) compounds have been found to function as antifoggants and post-processing print stabilizers in photothermographic elements.

Description

Patent DocketNo. 51626CAN8A
DEOXYRIBONUCLEIC ACID (DNA) AS AN ANTIFOGGANT
AND PRINT STABI~,~7,~,R FOR PHOTO'l'~KMOGRAPHIC
ELEMENTS

BACKGROUND OF THE INVENTION
Field of Invention:
This hlve~lion relates to novel, heat-developable photothermographic ele~ s and in particular, it relates to deoxyribonucleic acids (DNA) compounds as antifoggants and post-proces~ing print stabilizers for photothermographic elements.
Background of the Art:
Silver halide-co.~ g photothermographic im~gin~ materials (i.e., heat-developable photographic elementc) processed with heat, and without liquid development, have been known in the art for many years. These elements are also known as "dry silver" compositions or emulsions and generally co",p,;se a support having coated thereon: (a) a photosensi~i~/e compound that generates silver atoms when irrat~i~te-l; (b) a non-photosensitive, reducible silver source; (c) a redllcing agent (i.e., a developer) for silver ion, for cAa-,.ple the silver ion in the non-photo-sensitive, reducible silver source; and (d) a binder.
The photosensitive compound is generally photographic silver halide which must be in catalytic pro~i" ily to the non-photosensitive, reducible silver source.
Catalytic pl~i""ly requires an intim~te physical association ofthese two materials so that when silver atoms (also known as silver specks, clusters, or nuclei) aregenerated by irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that silver atoms (Ag) are a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proxill~ily with the non-photosensitive, reducible silver source in a number of di~erel-l fashions. The silver halide may be made "in si~u, " for example by adding a halogen-30 co~ g source to the reducible silver source to achieve partial met~theci~ (see, .
for e,~",Fle, U.S. Patent No. 3,457,075); or by coprecipitation of silver halide and the reducible silver source (see, for example, U.S. Patent No. 3,839,049). The silver halide may also be made "ex silu" and added to the organic silver salt. The addition of silver halide grains to photothermographic materials is described in ResearchDisclosure, June 1978, Item No. 17029. It is also reported in the art that when silver halide is made ex situ, one has the possibility of controlling the composition and size of the grains much more precisely, so that one can impart more specific~)rope,lies to the photothermographic cle-..P,-~I and can do so much more cons:~t~ ly than with the in situ technique.
The non-photosencitive, reducible silver source is a compound that contains silver ions. Typically, the ple~"ed non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecularweight are generally used. Salts of other organic acids or other organic compounds, such as silver im;d~7O1ates, have been proposed. U.S. Patent No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photo-sensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag).
The imagewise distribution of these clusters is known in the art as a latent image.
This latent image is generally not visible by ordinary means. Thus, the photo-sensitive emulsion must be further processed to produce a visible image. This isaccon.pli~l-ed by the reduction of silver ions which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, (i.e., the latent image). This produces a black and white image. In photographic elements, the silver halide isreduced to form the black-and-white image. In photothermographic elements, the light-i~-ce-.~;l;./e silver source is reduced to form the visible black-and-white image while much ofthe silver halide lelllainS as silver halide and is not reduced.
In photothermographic elements the reduçin~ agent for the organic silver salt, often re~"ed to as a "developer," may be any compound, preferably any organic compound, that can reduce silver ion to met~llic silver. At elevated -3- 21 705~6 tctnpet~tLIres, in the presence of the latent image, the non-photosensitive reducible silver source (e.g., silver behenate) is reduced by the reducing agent for silver ion.
This produces a negative black-and-white image of elçmçnt~l silver.
While conventional photographic developers such as methyl gal!ate, hydro-S quinone, substitllted-hydroquinones, catechol, pyrogallol, ascorbic acid, and ascorbic acid derivatives are useful, they tend to result in very reactive photo-thermographic formulations and fog during preparation and coating of photo-therrnographic elç~ As a result, hindered phenol developers (i.e., red~lcine agents) have traditionally been prefe.,ed.
As the visible image in black-and-white photothermographic elements is usually produced entirely by el~ silver (Ag), one cannot readily decrease the amount of silver in the emulsion without reducine the maximum image density.
However, reduction of the amount of silver is often desirable to reduce the cost of raw materials used in the emulsion and/or to enhance performance. For example, toning agents may be incorporated to improve the color of the silver image of the photothermographic elements as desc-il,ed in U.S. Patent Nos. 3,846,136;
3,994,732; and 4,021,249.
Another method of incl eas;l-g the maximum image density in photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming or dye-releasing compounds in theemulsion. Upon im~ein~ the dye-forming or dye-releasing compound is oxidized, and a dye and a reduced silver image are simultaneously formed in the exposed region. In this way, a dye-enhanced black-and-white silver image can be produced.
Dye enhanced black-and-white silver image forming elçm~ntc and processes are described in U.S. Patent No. 5,185,231.
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic elements differcignifiç~ntly from conventional silver halide photographic elements which require wet-procçccing In photothermographic imaging elem~nt~, a visible image is created by heat as a result of the reaction of a developer incorporated within the element. Heat is esse~ for development and tell,pel~l~res of over 100C are routinely required. In contrast, conventional wet-processed photographic imaging elementc require processing in aqueous procesQ;ng baths to provide a visible image (e.g., developing and fixing baths) and development is usually performed at a more moderate S tc~llpe~alure (e.g., 30-50C).
In photoll-el mographic elementc only a small amount of si!ver halide is used to capture light and a dill~renl form of silver (e.g., silver behenate) is used to generate the image with heat. Thus, the silver halide serves as a catalyst for the development of the non-photosçncitive, reducible silver source. In contrast, 10 conventional wet-processed black-and-white photographic elements use only oneform of silver (e.g., silver halide) which, upon development, is itself converted to the silver image. Additionally, photothermographic elementc require an amount ofsilver halide per unit area that is as little as one-hundredth of that used in conventional wet-processed silver halide.
Photothermographic systems employ a light-insçnQitive silver salt, such as silver behen~te~ which participates with the developer in developing the latent image. In contrast, photographic systems do not employ a light-incen.citive silver salt directly in the image-forming process. As a result, the image in photothermo-graphic elem~ents is produced p-i"~a~ily by reduction ofthe light-insensitive silver 20 source (silver bellel-~te) while the image in photographic black-and-white elements is produced primarily by the silver halide.
In photothermographic elements, all of the "chemistry" of the system is inco. ~,oraled within the element itself. For example, photothermographic elements incorporate a developer (i.e., a reduçin~ agent for the non-photosensitive reducible 25 source of silver) within the elem~nt while conventional photographic elements do not. The incorporation of the developer into photothermographic elements can lead to increased formation of "fog" upon coating of photothermographic emulsions.
Even in so-called instant photography, the developer chemistry is physically separated from the photosensitive silver halide until development is desired. Much 30 effort has gone into the plepa-~tion and m~nuf~ctllre of photothermographic elemçnt~ to minimi7e formation of fog upon coating, storage, and post-processingaging.
Similarly, in photothermographic elenlente the unexposed silver halide inherently re,..ains after development and the ~lement must be stabilized against further development. In contrast, the silver halide is removed from photographicelements after development to prevent further im~p,ing (i.e., the fixing step).
In photothermographic elempnts the binder is capable of wide variation and a number of binders are useful in preparii~g these ele-m~nts. In contrast, photographic ~ e ~s are limited almost exclusively to hydrophilic colloidal binders such as gelatin.
Because photothermographic elen-ent.~ require thermal processir p they pose dirrer~.lt considerations and present di.~tinctly difrere--l problems in m~n~lf~ctl.re and use. In addition, the effects of additives (e.g., stabilizers,&n~i~oggallls, speed enh~ncers, s~n.~iti7ers, supe-~e~ , etc.) which are intended to have a direct effect upon the im~ging process can vary depending upon whetherthey have been incorporated in a photothermographic element or incorporated in aphotographic clcll~enl.
Because of these and other differences, additives which have one effect in conventional silver halide photography may behave quite dilI~renlly in photo-thermographic elements where the underlying chen istry is so much more complex.
For example, it is not uncommon for an antifoggant for a silver halide systems to produce various types of fog when incorporated into photothermographic elements.Distinctions between photothermographic and photographic elements are desc, il,ed in Imaging Processes and Materials (Neble~e's Eighth Edition); J.
Sturge et al. Ed; Van Nostrand Reinhold New York, 1989; Chapter 9 and in Unconven~ional Imaging Proce.s;se.s; E. Brinckman et al, Ed; The Focal Press:
London and New York: 1978; pp. 74-75.
Various techniques are typically employed to try and gain higher sensitivity in a photothermographic material. In efforts to make more sensitive photothermo-graphic ~le~ one of the most difficult parameters to ~ inl~ at a very low level is the various types of fog or Dmin. Fog is spurious image density which ._ appears in non-imaged areas of the element after development and is often reported in sensitometric results as Dmin. Phototherrnographic emulsions, in a manner similar to photographic emulsions and other light-sensitive systems, tend to suffer fromfog.
Tr~ditiQnslly, photothermographic elern~nts have suffered from fog upon coating. The fog level of freshly prepared photothermographic elen~nts will be rere, led to herein as initial fog or initial Dmin.
In addition, the fog level of photothermographic elements often rises as the material is stored, or "ages." This type of fog will be refe, l ed to herein as shelf-aging fog. Adding to the difficulty offog control on shelf-aging is the fact that the developer is incollJor~ted in the photothermographic elçrnent This is not the case in most silver halide photographic systems. A great amount of work has been done toimprove the shelf-life characteristics of photothermographic elements.
A third type of fog in photothermographic systems results from the instability of the image and/or background after processing. The photoactive silver halide still present in the developed image may continue to catalyze formation of met~llic silver during room light handling or post-processing exposure such as in graphic arts contact frames. This is known as "post-processing fog" or"silver print-out." Without having acceptable resistAnce to fog, a commercially useful material is difficult to prepare. Thus, there exists a need for "print stabilizers" to stabilize the unreacted silver halide. Various techniques have been employed to improve sensitivity and ..-~ in resistance to fog.
In color photothermographic elements, often unreacted dye forming or dye rele~cing compounds may slowly oxidize and form areas of color in the unexposed areas. In these ele .~ s stabilizers are often added to reduce "leuco dye backgrounding."
The addition of separate post-processing print stabilizers or stabilizer precursors provides the desired post-proceqsing stability. Most often these are sulfur-con~Ail-ing compounds such as mercaptans, thiones, and thioethers as desc,ibed inResearchDisclosure, June 1978, item 17029. U.S. PatentNo.
4,245,033 describes sulfur compounds of the mercapto-type that are development 7 2i 70586 I ~ ;.lrai~le.~ of a photothermographic system. See also U.S. Patent Nos. 4,837,141 and 4,451,561. Mesoionic 1,2,4-triazolium-3-thiolates as fixing agents and silver halide 7~ li7ers are described in U.S. Patent No. 4,378,424. Substituted 5-"lc,capto-1,2,4-triazoles, such as 3-amino-5-benzothio-1,2,4-triazole, used as5 post-processing stabilizers are desc, ibed in U. S. Patent Nos. 4,128,557; 4,137,079;
4,138,265; and Research Disclosure 16977 and 16979.
Some of the problems with these stabilizers include thermal fogging during procçssin8 or losses in photographic sensitivity, maximum density, or contrast at effective stabilizer concentrations.
St~bili7er precursors have blocking or modifying groups that are cleaved during processing with heat, light, and/or alkali. This provides an active stabilizer that can cGr,lbh-e with the photoactive silver halide in the unexposed areas of the photographic material to form a light- and heat-stable complex. For example, in the pl esence of a: ~b.li~e- precursor in which the blocking group on a sulfur atom is 15 cleaved upon processing, the resulting silver mercaptide will be more stable than the silver halide to light, and heat.
Blocking groups that are thermally-sensitive have also been used. These blocking groups are removed by heating the imaging material during processinE
For example, U.S. Patent No. 5,158,866 describes the use of omega-substituted 20 2-propionamidoacet~l or 3-propionamidopropionyl stabilizer precursors as post-processing stabilizers in photothermographic elements. U.S. Patent No. 5,175,081desclibes the use of certain azlactones as stabilizers. U.S. Patent No. 5,298,390 desvlil,es the use of certain alkyl sulfones as blocked compounds capable of rele~in~ stabilizers with heat. U.S. Patent No. 5,300,420 describes the use of 25 certain nitriles as blocked compounds capable of releasing stabilizers with heat.
Various disadvantages attend these dillerenl blocking techniques. Highly basic solutions that are necess~.y to cause unblocking of the alkali-sensitive blocked derivatives are corrosive and i"iLating to the skin. With photographic stabilizers that are blocked with a heat-removable group, it is often found that the liberated 30 reagent or by-product can react with other components of the photothermographic c4..e~.1 and cause adverse effects. Also, premature release of the stabilizing moiety within the desired time during proce~ing may occur, res~ltine in fogging ofthe emulsion or loss of sensitivity.
DNA and its decomposition products are known photographic restrainers which differ from one another in efficiency. As described by Steignl~nn (Sci. Indust.
5 Photogr. 1964, 35, 145) and ~mm~nn-Brass (J. Photogr. Sci. 1972, 20, 37), the physical and chemical ripening of silver halide is more strongly inhibited by partially deco...?osedDNA than by the undestroyed double-strand DNA or the mono-nucleotides and mononucleosides. Also the products of full decomposition of the DNA have a slow lt~l~ainil-g action smaller than that of the intact DNA. Likewise, 10 the oligonucleotides have little or even no effect on ripening. It is also known (G.B.Tagliafico, J. Photogr. Sci. 1968, 17, 17) that when DNAis completely deco-l.pGsed into oligonucleotides, has no re~ltaining effect on the chemical lipening of an ammoniacal silver bromide emulsion either. These experiments indic~te that evidently the optimum rei~llaining of DNA can be obtained with its15 partially decomposed molecules. A recent study (H.Hermel and A. Huttner J. ImagSci. Tec. 1992, 36 f3~, 287) shows that partially decomposed DNA acts as antifoggant and sel,s;lizer in the chemical ripening process. Similar results have been observed when partially decomposed DNA prepared by ultrasonic treatment was added to an X-ray emulsion, a negative emulsion, a direct positive emulsion, and a 20 reprolith emulsion during the chemical ripening process.
There is a continued need for improved stabilizer compounds that inhibit all types of fog and do not have any detrimental effects on the photothermographic element.

SUMMARYOFTHEINVENTION
The present invention provides heat-developable, photothermographic clelllenls which are capable of providing high photospeed; stable, high density images of high resolution and good sl-al~.ness, and good shelf stability.
The present invention provides photothermographic elements coated on a 30 support wherein the photothermographic element comprises:
(a) a photosensitive silver halide;

, .
(b) a non-photosensitive, reducible source of silver;
(c) a reducing agent for the non-photosensitive, reducible source of silver;
(d) a binder; and 5 (e) DNA.
When the photothermographic element used in this invention is heat developed, preferably at a temperature offrom about 80C to about 250C (176F
to 482F) for a duration of from about 1 second to about 2 minutç~ in a subsl~i~tially water-free condition after, or sim~llt~neously with, imagewise exposure, a black-and-white silver image is obtained.
The reduçing agent for the non-photosensitive silver source may be any conventional photographic developer such as methyl gallate, hydroquinone, substituted-hydroquinones, catechol, pyrogallol, ascorbic acid, and ascorbic acid derivatives. However, it is prefe"ed that the reduc.ing agent be a hindered phenol developer. Further, the reducing agent may optionally comprise a compound capable of being oxidized to form or release a dye. Preferably the dye-forming material is a leuco dye According to the present invention, one or more DNA compounds is added either to the emulsion layer(s) or to a layer or layers ~djacç~t to the emulsionlayer(s). Layers that are adjacçnt to the emulsion layer(s) may be, for example,protective topcoat layers, primer layers, interlayers, opacifying layers, ~ntih~l~tion layers, barrier layers, auxiliary layers, etc. It is prefel l ed that the DNA compound be present in the photothermographic emulsion layer or topcoat layer The present invention also provides a process for the formation of a visible image by first exposing to ele~;l, or"agnetic radiation and thereafter heating the inventive photothermographic element described earlier herein.
The present invention also provides a process comprising the steps of:
(a) exposing the inventive phototherrnographic element described earlier herein to electromagnetic radiation, to which the silver halide grains ofthe Ple nel-t are sensitive, to generate a latent image;

-10- 2l7o5~6 (b) heating the exposed elernent to develop the latent image into a visible image;
(c) positioning the Plement with a visible image thereon between a source of ultraviolet or short wavelength visible radiatio~l energy and an ultraviolet or short wavelength radiation photosen.~itive imageable merli~lm; and (d) lLeleaner exposing the image~ble medium to ultraviolet or short wavelength visible radiation through the visible image on the ele ~nl thereby absoll ing ultraviolet or short wavelength visible radiation in the areas ofthe element where there is a visible image and lr~nc-~ g ultraviolet or short wavelength visible radiation through areas of the element where there is no visible image.
The photothermographic element may be exposed in step (a) with visible, h.r ~red, or laser radiation.
The photothermographic elements of this invention may be used to prepare black-and-white, monochrome, or full color images. The photothermographic element of this invention can be used, for example, in conventional black-and-white or color photothermography, in electronically generated black-and-white or colorhardcopy recording, in the graphic arts area (e.g., phototypesetting), in digital proofing, and in digital radiographic imaging The element of this invention provides high photospeeds, provides strongly absorbing black-and-white or color images, and provides a dry and rapid process.
He~ting in a subsl~n~ ly water-free condition as used herein, means heating at a tenlpel ~L~Ire of 80 to 2S0C. The term "subst~nti~lly water-free condition"
means that the reaction system is approxi.l.alely in equilibrium with water in the air, and water for ind~lcing or promoting the reaction is not particularly or positively supplied from the exterior to the element Such a condition is described in T. H. James, The Theory of ~he Photographic Process, Fourth Edition, ~cmill~n 1977, page 374.
As used herein:

"photothermographic element" means a construction comprising at least one photothermographic emulsion layer and any supports, topcoat layers, image-receiving layers, blocking layers, ~ntihql~tion layers, subbing or p~h~ling layers, etc;
"emulsion layer" means a layer of a photothermographic element that co.ltah~s the non-photosensitive silver source and the photosensitive silver halide;
"ultraviolet region of the spectrum" means that region of the spectrum less than or equal to about 400 nm, prel~.~bly from about 100 nm to about 400 nm.
More preferably, the ultraviolet region ofthe spectrum is the region between about 190 nm and about 400 nm;
"short wavelength visible region ofthe spectrum" means that region ofthe spectrum from about 400 nm to about 450 nm;
il~laied region ofthe spectrum" means from about 750 nm to about 1400 nm; "visible region ofthe spectrum" means from about 400 nm to about 750 nm; and "red region of the spectrum" means from about 640 nm to about 750 nm. Preferably the red region ofthe spectrum is from about 6S0 nm to about 700 nm.
Other ~Cpect~ advantages, and benefits of the present invention are apparenl from the detailed des~;liplion, e~a..,pl~s, and claims.

DETAILED DESCRIPTION OF THE INVENTION
IncG.~,o,ation of DNA into a photothermographic element appears novel.
We have found that addition of DNA, preferably Calf Thumus DNA, Herring DNA
and Salmon DNA to the silver emulsion layer or topcoat layer of a photothermographic element reduces shelf-age fog and improves post-processing stability of photothermographic elements, preferably black-and-white photothermographic elements.
In contrast, when DNA was added during the preparation of the homogenate (in a manner analogus to that used in wet silver halide), a completely fogged photothermographic element was obtained on coating.
The DNA compounds of the present invention typically comprise from about 0.1 wt% to 50 wt% of the dried layer of the photothermographic element in -- which they are placed. They may be incGl~olaled directly into the silver-cont~ining layer, into an adjacent layer, or an image-receiving layer. The DNA compounds ofthe present invention are especially useful in photothermographic elements and constructions for prepalalion of black-and-white, monochrome, and full color 5 images.
The amount~ of the above-desc- il,cd DNA stabilizer compounds that are added to the photothermographic element of the present invention may be varied depending upon the particular compound used, upon the type of emulsion layer (e.g., black-and-white vs. color), and whether the stabilizer is located in the 10 emulsion layer, topcoat layer, or image-receiving layer.
Photothermographic elements of the invention may contain other stabilizers or stabilizer precursors in combination with the DNA stabilizer compounds of theinvention, as well as other additives in combination with the compounds of the invention such as shelf-life stabilizers, toners, development accelerators, and other 15 image-modifying agents.
The Photosensitive Silver Halide As noted above, the present invention includes a photosen~itive silver halide.
The photosensitive silver halide can be any photosensitive silver halide, such as silver bromide, silver iodide, silver chloride, silver bromoiodide, silver chlorobromo-20 iodide, silver chlorobromide, etc. The photosensitive silver halide can be added tothe emulsion layer in any fashion so long as it is placed in catalytic proxi"~ily to the organic silver compound which serves as a source of reducible silver.
The silver halide may be in any form which is photosensitive incl~lding~ but not limited to cubic, octahedral, rhombic dodec~edral, onhorhombic, tetrahedral,25 other polyhedral habits, etc., and may have epitaxial growth of crystals thereon.
The silver halide grains may have a uniform ratio of halide throughout; they may have a graded halide content, with a continuously varying ratio of, for example, silver bro". ~e and silver iodide; or they may be of the core-shell-type, having a discrete core of one halide ratio, and a discrete shell of another halide ratio. Core-30 shell silver halide grains useful in photothermographic element~ and methods ofpr~pa~ing these materials are desc,il.ed in U.S. Patent No. 5,382,50~. A core-shel silver halide grain having an iridium doped core is particularly pl~lled. Iridium doped core-shell grains ofthis type are described in U.S. Patent Application Serial number 08/239,984 (filed May 9, 1994).
The silver halide may be p~ e~)a~ ed ex situ, (i.e., be pre-formed) and mixed 5 with the organic silver salt in a binder prior to use to prepare a coating solution.
The silver halide may be pre-formed by any means, e.g., in accordance with U.S.
Patent No. 3,839,049. For example, it is effective to blend the silver halide and organic silver salt using a homogenizer for a long period of time. Materials of this type are often r~fe"ed to as "pre-formed emulsions." Methods of preparing these 10 silver halide and organic silver salts and manners of blending them are described in ResearchDisclosure, June 1978, item 17029; U.S. Patent Nos. 3,700,458 and 4,076,539; and Jap~nese Patent Application Nos. 13224/74, 42529/76, and 17216/75.
It is desirable in the practice of this invention to use pre-formed silver halide 15 grains of less than 0.10 ~lm in an infrared sen~;l;7e~i, photothermographic element. It is also prere,led to use iridium doped silver halide grains and iridium doped core-shell silver halide grains as disclosed in U.S. Patent Application Serial Nos.
08/072,153, and 08/239,984 described above.
Pre-formed silver halide emulsions when used in the element of this 20 invention can be unwashed or washed to remove soluble salts. In the latter case, the soluble salts can be removed by chill-setting and le~c.hing or the emulsion can be coa~lPtion washed, e.g., by the procedures described in U.S. Patent Nos.
2,618,556; 2,614,928; 2,565,418; 3,241,969; and 2,489,341.
It is also effective to use an in si~u process, i.e., a process in which a 25 halogen-co"~ g compound is added to an organic silver salt to partially convert the silver of the organic silver salt to silver halide.
The light sensitive silver halide used in the present invention can be employed in a range of about 0.005 mole to about 0.5 mole; preferably, from about 0.01 mole to about 0.15 mole per mole; and more preferably, from 0.03 mole to 30 0.12 mole per mole of non-photosensitive reducible silver salt.

.. ~ .
Sen~it~e- ~
The silver halide used in the present invention may be chemically and spectrally se~ ed in a manner similar to that used to sç~ e conventional wet-processed silver halide photographic materials, or state-of-the-art heat-developable 5 photothermographic rle-..e~
For example, it may be çhemic.~lly ser~s;~;ied with a chemical seneiti~ing agent, such as a compound cont~inine sulfur, selenium, tellurium, etc., or a compound conlA ~ g gold, pl~tin-~m pall~ m mthçni-lm, rhodium, iridium, etc., a re~uci~ agent such as a tin halide, etc., or a colllbination thereof. The details of 10 these procedures are described in T.H. James, The Theory of the Pho~ographic Process, Fourth Edition, Chapter 5, pp. 149 to 169. Suitable chemical sensiti7~tion procedures are also desclosed in Shepard, U.S. Patent No. 1,623,499; Waller, U.S.
Patent No. 2,399,083; McVeigh, U.S. Patent No. 3,297,447; and Dunn, U.S. Patent No. 3,297,446.
Addition of sensili~ g dyes to the photosensitive silver halides serves to provide them with high sensitivity to visible and hlrl ared light by spectral se~ ation. Thus, the photosensitive silver halides may be spectrally sensitized with various known dyes that spectrally sel~c;l;~ç silver halide. Non-limiting eAa-llples of sent;~ g dyes that can be employed include cyanine dyes, 20 merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, holopolar cyanine dyes, hemicyanine dyes, styryl dyes, and hemioxanol dyes. Of these dyes,cyanine dyes, merocyanine dyes, and complex merocyanine dyes are particularly useful. Suitable sens;l;~.;n~ dyes are described, for example in U.S. Patent Nos, 3,719,495 and 5,393,654.
An appropliate amount of sellc;l;~ dye added is generally about I o-l to 10 l mole; and p.eferdbly, about io 3 to 10'3 moles per mole of silver halide.
Supersens;li~e. ~
To get the speed of the photothermographic çlemçnts up to maximum levels and further ellh~l~ce sensitivity, it is often desirable to use supel~en~ e,~. Any supe~se~s~ er can be used which increases the sensitivity. For example, prefel,ed infia~d sllp~.~er~;l;ie~ are described in U.S. Patent Application Serial No.

_15_ 21 70586 08/091,000 (filed July l3, 1993) and include heteroaromatic l"e~caplo compounds or heteroaromatic disulfide compounds of the formula:
Ar-S-M
Ar-S-S-Ar 5 wherein: M replese.-ts a hydrogen atom or an alkali metal atom.
In the above noted sUpersçneiti7ers~ Ar repl esents a heteroaromatic ring or fused heteroarol"alic ring co~-lAi~ one or more of nitrogen, sulfur, oxygen, selen: ~m or tellurium atoms. P-eferably, the heteroaromatic ring is benzimidazole, naplltl~nlidazole, benzoll-iazole, naphthothiazole, benzoxazole, naphthoxazole, lO bel~oselen~.ole, benzotellurazole, im;d~7.ole, oxazole, pyrazole, triazole, th;~li~7s~1e, tetrazole, triazine, pyrim:1ine, pyridazine, pyrazine, pyridine, purine, quinoline or q inq7.olinone However, other heteroaromatic rings are envisioned under the breadth of this invention.
The heteroaromatic ring may also carry substituçnts with examples of 15 pler~"ed substit~lents being selected from the group consisting of halogen (e.g., Br and Cl), hydroxy, amino, carboxy, alkyl (e.g., of 1 or more carbon atoms, preferably 1 to 4 carbon atoms) and alkoxy (e.g., of I or more carbon atoms, preferably of 1 to 4 carbon atoms.
Prerelled supe,~en~ are 2-",ercaptobell7.;... d~7ole, 2-mercapto-20 5-methylben7.imidazole, 2-mercaptobenzothiazole, and 2-mercaptobenzoxazole.
The supersensitizers are used in general amount of at least 0.001 moles of sen~ Ç~ per mole of silver in the emulsion layer. Usually the range is between 0.001 and 1.0 moles of the compound per mole of silver and prere,ably between O.Ol and 0.3 moles of compound per mole of silver.
25 The Non-Photosensitive Reducible Silver Source The present invention incl~ldçs a non-photosensitive reducible silver source.
The non-photosensitive reducible silver source that can be used in the present invention can be any compound that contains a source of reducible silver ions.
Preferably, it is a silver salt which is comparatively stable to light and forms a silver 30 image when heated to 80C or higher in the presence of an exposed photocatalyst (such as silver halide) and a reduçing agent.

Silver salts of organic acids, particularly silver salts of long chain fatty carboxylic acids, are prefelled. The chains typically contain 10 to 30, preferably 15 to 28, carbon atoms. Suitable organic silver salts include silver salts of organic compounds having a carboxyl group. Examples thereof include a silver salt of an 5 a!;ph?tic carboxylic acid and a silver salt of an aromatic carboxylic acid. Preferred e~a",ples ofthe silver salts of aliphatic carboxylic acids include silyer behen~te, silver stearate, silver oleate, silver laureate, silver caprate, silver myristate, silver palmit~tç, silver maleate, silver fumarate, silver tartarate, silver furoate, silver linoleate, silver butyrate, silver camphorate, and mixtures thereof, etc. Silver salts 10 that can be substituted with a halogen atom or a hydroxyl group also can be effectively used. ~l ef~l l ed c ,.~ ples of the silver salts of aromatic carboxylic acid and other carboxyl group-cont~inin~ compounds include: silver benzoate, a silver-substituted ben7.03~ç, such as silver 3,5-dihydroxybenzoate, silver o-methyl-ben7.o~tç, silver m-methylbenzoate, silverp-methylbenzoate, silver 2,4-dichloro-15 benzoate, silver acet~midobenzoate, silverp-phenylbenzoate, etc.; silver gallate;
silver t~nn~te; silver phthalate; silver terephth~l~te; silver salicylate; silver phenylacet~te; silver pyromellilate; a silver salt of 3-carboxymethyl-4-methyl-4-thiazoline-2-thione or the like as described in U.S. Patent No. 3,785,830; and a silver salt of an aliphatic carboxylic acid cont~inin~ a thioether group as described in 20 U.S. Patent No. 3,330,663.
Silver salts of compounds co~ nin~ mercapto or thione groups and derivatives thereof can also be used. Prel~"ed examples of these compounds include: a silver salt of 3-mercapto-4-phenyl-1,2,4-triazole; a silver salt of 2-"lcrcaplobel~i",idazole; a silver salt of 2-mercapto-5-aminothia~i~7.ole; a silver 25 salt of 2-(2-ethylglycolamido)benzothiazole; a silver salt of thioglycolic acid, such as a silver salt of a S-alkylthioglycolic acid (wherein the alkyl group has from 12 to 22 carbon atoms); a silver salt of a dithiocarboxylic acid such as a silver salt of dithio^cetic acid; a silver salt of thioamide; a silver salt of 5-carboxylic-1-methyl-2-phenyl-4-thiopyridine; a silver salt of mercaptotriazine; a silver salt of 30 2-me,~idplobçn70xazole; a silver salt as described in U.S. Patent No. 4,123,274, for example, a silver salt of a 1,2,4-l"elcdptothiazole derivative, such as a silver salt of 3-amino-5-benzylthio-1,2,4-thiazole; and a silver salt of a thione compound, such as a silver salt of 3-(2-carboxyethyl)-4-methyl-4-thiazoline-2-thione as disclosed in U.S. Patent No. 3,201,678.
Furthermore, a silver salt of a compound co.~ ing an imino group can be 5 used. ~ere.,~,d examples ofthese compounds include: silver salts of benzotriazole and substitllted derivatives thereof, for exa.,.yle~ silver methylbenzotriazole and silver 5-chlorob&.~olliazole, etc.; silver salts of 1,2,4-triazoles or 1-H-tetrazoles as des~,il,ed in U.S. Patent No. 4,220,709; and silver salts of imidazoles and imidazole derivatives.
Silver salts of acetylenes can also be used. Silver acetylides are described in U.S. PatentNos. 4,761,361 and 4,775,613.
It is also found convenient to use silver half soaps. A prere.led example of a silver half soap is an equimolar blend of silver behenate and behenic acid, which analyzes for about 14.5 % silver and which is prepared by precipilalion from an aqueous solution of the sodium salt of commercial behenic acid.
Transparent sheet elements made on transparent film backing require a trans~,arel-t coating. For this purpose a silver behenate full soap, containing not more than about 15 % of free behenic acid and analyzing about 22 % silver, can be used.
The method used for making silver soap emulsions is well known in the art and is dicrlQsed in Research Disclosure, April 1983, item 22812, ~esearch Disclosure, October 1983, item 23419, and U.S. Patent No. 3,985,565.
The silver halide and the non-photosensitive reducible silver source that form a starting point of development should be in catalytic proximity, i.e., reactive accori~tion. "Catalytic proximity" or "reactive association" means that they should be in the same layer, in adjacent layers, or in layers separated from each other by an inte....e~ e layer having a thickness of less than I micrometer (1 llm). It is prere. led that the silver halide and the non-photosensitive reducible silver source be present in the same layer.

-18- 2i 70586 Photothermographic emulsions con~ g pre-formed silver halide in accordance with this invention can be sen.~iti7.ed with chemical sensitizers, or with spectral sf n~ ;7e~ ~ as described above.
The source of reducible silver generally constitutes about 5 to about 70 %
5 by weight ofthe emulsion layer. It is preferably present at a level of about 10 to about 50 % by weight of the emulsion layer.
The Reducing Agent for the Non-Photosensitive Reducible Silver Source When used in black-and-white photothermographic eleln~nt.~ the reducing agent for the organic silver salt may be any compound, preferably organic 10 compound, that can reduce silver ion to metallic silver. Conventional photographic developers such as phenidone, hydroquinones, and catechol are useful, but hindered bisphenol reducin~ agents are plere-,ed.
A wide range of reduçing agents has been disclosed in dry silver systems includ~ amidoximes, such as phenylamidoxime, 2-thienylamidoxime and 15 p-phenoxy-phenylamidoxime; azines, such as 4-hydroxy-3,5-dimethoxybenz-aldehydea7.ine; a con-binalion of aliphatic carboxylic acid aryl hydrazides and ascorbic acid, such as 2,2'-bis(hydroxymethyl)propionyl-~-phenylhydrazide in co,..bh~alion with ascorbic acid; a combination of polyhydroxybenzene and hydroxylamine; a reductone and/or a hydl~n~e, such as a co..,bination of 20 hydroquinone and bis(ethoxyethyl)hydroxylamine, piperidinohexose reductone, or formyl-4-methylphenylhydrazine; hydloxamic acids, such as phenylhydlo~amic acid,p-hydroxyphenylhydroxamic acid, and o ~laninellydroxamic acid; a combination of azines and sulfonamidophenols, such as phenothiazine withp-ben~el1esulfonamido-phenol or 2,6-dichloro-4-benzel)esLIlfonamidophenol; a-cyanophenylacetic acid 25 derivatives, such as ethyl a-cyano-2-methylphenylacet~te, ethyl a-cyano-phenyl-~cet~te; a co-llbination of bis-o-naphthol and a 1,3-dihydroxybenzene derivative, such as 2,4-dihydroxybenzophenone or 2,4-dihydroxyacetophenone; 5-pyrazolones such as 3-methyl-1-phenyl-5-pyrazolone; reductones, such as dimethylaminohexose reductone, anhydrodihydroa---il-ohexose reductone, and anhydrodihydro-30 piperidone-hexose reductone; sulfonamidophemol reducin~ agents, such as 2,6-dichloro-4-bel-7~ esulfonamidophenolandp-benzenesulfonamidophenol;

indane-1,3-diones, such as 2-phenylindane-1,3-dione; chromans, such as 2,2-dimethyl-7-~-butyl-6-hydroxychrol-lan; 1,4-dihydropyridines, such as 2,6-dimeth~xy-3,5-dicarbethoxy-1,4-dihydropyridine; ascorbic acid derivatives, such as 1-ascorbylp~lmit~te, ascorbylstearate; unsaturated aldehydes and ketones;
5 certain 1,3-ind~-e~iones, and 3-pyrazolidones (phenidones).
IIindered bisphel-ol developers are compounds that contain only one hydroxy group on a given phenyl ring and have at least one additional substituent located ortho to the hydroxy group. They differ from traditional photographic developers which contain two hydroxy groups on the same phenyl ring (such as is 10 found in hydroquinones). Hindered phenol developers may contain more than onehydroxy group as long as they are located on diaère~l phenyl rings. Hindered phenol developers in~ de, for example, binaphthols (i.e., dihydroxybinaphthyls), biphenols (i.e., dihydroxybiphenyls), bis(hydroxynaphthyl)methanes, bis(hydroxy-phenyl)...~ nes, hindered phenols, and naphthols.
Non-limiting represe-.lati~e bis-o-naphthols, such as by 2,2'-dihydroxyl-I-b;~-~rhll.yl, 6,6'-dibromo-2,2'-dihydroxy-1,1'-binaphthyl, and bis(2-hydroxy-l-naphthyl)methane. For additional compounds see U.S. Patent No. 5,262,295 at column 6, lines 12- 13, incorporated herein by I ere~ ence.
Non-limiting leprese.,lative biphenols include 2,2'-dihydroxy-3,3'-di-t-butyl-5,5-dimethylb;l)he,-yl; 2,2'-dihydroxy-3,3',5,5'-tetra-t-butylbiphenyl;
2,2'-dihydroxy-3,3'-di-~-butyl-5,5'-dichlorobiphenyl; 2-(2-hydroxy-3-~-butyl-5-methylphenyl)-4-methyl-6-n-hexylphenol; 4,4'-dihydroxy-3,3',5,5'-tetra-~-butyl-biphenyl; and 4,4'-dihydroxy-3,3',5,5'-tetramethylbiphenyl. For additional compounds see U.S. Patent No. 5,262,295 at column 4, lines 17-47, incorporated herein by rerelel-ce.
Non-limiting representative bis(hydroxynaphthyl)methanes include 2,2'-methylene-bis(2-methyl-1-naphthol)lllelhane. For additional compounds see U.S. Patent No. 5,262,295 at column 6, lines 14-16, incorporated herein by reference.
Non-limiting replesenlalive bis(hydroxyphenyl)meth~nes include bis(2-hydroxy-3-t-butyl-5-methylphenyl)meth~ne (CAO-5); 1,1 -bis(2-hydroxy-._ .
3,5-di~ Lylphenyl)-3,5,5-ll illlelllylhexane (Pel lllanaXTM or NonoxT~);
1,1 '-bis(3,5-tetra-~-butyl-4-hydroxy)rnethqne; 2,2-bis(4-hydroxy-3 -methylphenyl)-plopane, 4,4-ethylidene-bis(2-t-butyl-6-methylphenol); and 2,2-bis(3,5-dimethyl-4-hydroA~ henyl)propane. For additional compounds see U.S. Patent N.o.
5 5,262,295 at column 5 line 63 to column 6, line 8 incorporated herein by reference.
Non-limiting represelllative hindered phenols include 2,6-di-~-butylphenol;
2,6-di-~-butyl-4-methylphenol; 2,4-di-t-butylphenol; 2,6-dichlorophenol;
2,6-dimethylphenol; and 2-t-butyl-6-methylphenol.
Non-limiting leprese.llali~/e hindered naphthols include 1-naphthol;
10 4-methyl-1-naphthol; 4-methoxy-1-naphthol; 4-chloro-1-naphthol; and 2-methyl-l-n~phthol For additional compounds see U.S. Patent No. 5,262,295 at column 6, lines 17-20, incor~,olaled herein by reference.
The reducin~ agent should be present as 1 to 10% by weight ofthe imaging layer. In multilayer elements, if the reduçing agent is added to a layer other than an 15 emulsion layer, slightly higher proportions, of from about 2 to 15%, tend to be more desirable.
The Optional Dy~Forming or Dye-Releasing Compound As noted above, the reducing agent for the reducible source of silver may be a compound that can be oxidized directly or indirectly to form or release a dye.Leuco dyes are one class of dye-forming compound that form a dye upon oxidation. Any leuco dye capable of being oxidized by silver ion to form a visible image can be used in the present invention. Leuco dyes that are both pH sensitive and oxidizable can also be used, but are not preferred. Leuco dyes that are sensitive only to changes in pH are not included within scope of dyes useful in this invention because they are not oxidizable tD a colored form.
As used herein, a "leuco dye" or "blocked leuco dye" is the reduced form of a dye that is generally colorless or very lightly colored and is capable of forming a colored image upon oxidation of the leuco or blocked leuco dye to the dye form.
Thus, the blocked leuco dyes (i.e., blocked dye-rele~ing compounds), absorb lessstrongly in the visible region of the electromagnetic spectrum than do the dyes. The reslllt~nt dye produces an image either directly on the sheet on which the dye is formed or, when used with a dye- or image-receiving layer, on the image-receiving layer upon diffusion through emulsion layers and interlayers.
Repr~senlali~e classes of leuco dyes that can be used in the photothermo-graphic elemçnt~ of the present invention includç, but are not limited to: chromo-S genic leuco dyes, such as inrlo~niline, indophenol, or azomethine leuco dyes;imid~7.ole leuco dyes, such as 2-(3,5-di-~-butyl-4-hydroxyphenyl)-4,5-diphenyl-imid~7.ole, as described in U.S. Patent No. 3,985,565; dyes having an azine, diazine, oxazine, or tlli~7ine nucleus such as those described in U.S. Patent Nos. 4,563,415;
4,622,395; 4,710,570; and 4,782,010; and benzylidene leuco compounds as desclil,cd in U.S. Patent No. 4,923,792.
Another pre~" ed class of leuco dyes useful in this invention are those derived from azo..,etl~ e leuco dyes or indoaniline leuco dyes. These are often refe. led to herein as "chromogenic leuco dyes" because many of these dyes are useful in conventional, wet-processed photography. Chromogenic dyes are preparedby oxidative coupling of ap-phenylenedi~mine compound or ap-aminophenol compound with a photographic-type coupler. Reduction of the corresponding dye as des~,,ibed, for e,.a".ple, in U.S. Patent No. 4,374,921 forms the chromogenicleuco dye. Leuco chromogenic dyes are also described in U.S. Patent No.
4,594,307. Cyan leuco chromogenic dyes having short chain carbamoyl protecting groups are des~,ibed in European Laid Open Patent Application No. 533,008. For areview of chromogenic leuco dyes, see K. Venkataraman, The Chemis~ry of Synthetic Dyes, ~c~denlic Press: New York, 1952; Vol. 4, Chapter VI.
Another class of leuco dyes useful in this invention are "~ld~7ine" and "ket~7.ine" leuco dyes. Dyes ofthis type are described in U.S. Patent Nos.
4,587,211 and 4,795,697. Ben_ylidene leuco dyes are also useful in this invention.
Dyes ofthis type are described in U.S. Patent No. 4,923,792.
Yet another class of dye-releasing compounds that form a diffusible dye upon oxidation are known as pre-formed-dye-release (PDR) or redox-dye-release (RDR) compounds. In these compounds, the redllçing agent for the non-photo-sensitive, reducible source of silver releases a mobile pre-formed dye upon oxi~tion. Examples of these compounds are disclosed in Swain, U.S. Patent No.
4,981,775.
Further, other image-fo",l,ng compounds where the mobility of the compound having a dye part cl-anges as a result of an oxidation-reduction reaction 5 with silver halide, or an organic silver salt at high te"")e~lure can be used, as described in J~pq~-ese Patent Application No. 165,054/84.
Still further the red~lcin~ agent may be a compound that releases a conventionql photographic dye coupler or developer on oxidation as is known in the art.
The dyes formed or released in the various color-forming layers should, of course, be Ji~renl. A difference of at least 60 nm in reflective maximum absorbance is prere, red. More preferably, the abso,l,ance maximum of dyes formed or released will differ by at least 80-100 nm. When three dyes are to be formed, two should preferably differ by at least these minimum~, and the third should preferably differ from at least one of the other dyes by at least 150 nm, and more preferably, by at least 200 nm. Any red-lcing agent capable of being oxidized by silver ion to form or release a visible dye is useful in the present invention as previously noted.The total amount of optional leuco dye used as a reducin~ agent used in the present invention should prefe,~bly be in the range of 0.5-25 wt%, and more prere,~bly, in the range of 1-10 wt%, based upon the total weight of each individual layer in which the reduGing agent is employed.
The Binder The photosen~itive silver halide, the non-photosensitive reducible source of silver, the red~lcin~ agent, and any other ~ddend~ used in the present invention are generally added to at least one binder. The binder(s) that can be used in the present invention can be employed individually or in combination with one another. It isp~e~"ed that the binder be selected from polymeric materials, such as, for example, natural and synthetic resins that are sufficiently polar to hold the other ingredients in sollltion or suspension.
A typical hydrophilic binder is a llansl)arenl or tr~n~iUcent hydrophilic colloid. F.y~mples of hydrophilic binders include: a natural substance, for example, a protein such as gelatin, a gelatin derivative, a cellulose derivative, etc.; a poly-saccharide such as starch, gum arabic, pullulan, dextrin, etc.; and a synthetic polymer, for ~,~a~ ule, a water-soluble polyvinyl compound such as polyvinyl alcohol, polyvinyl pyrrolidone, acrylamide polymer, etc. Another example of a hydrophilic binder is a dispersed vinyl compound in latex form which is used for the purpose of increasing dimensional stability of a photographic element.
Examples of typical hydrophobic binders are polyvinyl acetals, polyvinyl chloride, polyvinyl aC~te, cellulose acetate, polyolefins, polyesters, polystyrene, polyacrylonitrile, polycarbonates, meth~crylate copolymers, maleic anhydride ester copolymers, butadiene-styrene copolymers, and the like. Copolymers, e.g., terpolymers, are also in~luded in the definition of polymers. The polyvinyl acetals, such as polyvinyl butyral and polyvinyl formal, and vinyl copolymers such as polyvinyl acetate and polyvinyl chloride are particularly prefe"ed.
Although the binder can be hydrophilic or hydrophobic, preferably it is hydrophobic in the silver cont~ining layer(s). Optionally, these polymers may beused in cGmbinalion of two or more thereof.
The binders are preferably used at a level of about 30-90 % by weight of the emulsion layer, and more prere, ~bly at a level of about 45-85 % by weight. Where the proi)o, lions and activities of the reducing agent for the non-photosensitive reducible source of silver require a particular developing time and te",pe,~ture, the binder should be able to withstand those conditions. Generally, it is prefe-~ed that the binder not decompose or lose its structural integrity at 250F
(121C) for 60 seconds, and more plere"ed that it not decompose or lose its structural integrity at 350F (177C) for 60 seconds.
The polymer binder is used in an amount sufficient to carry the components dispersed therein, that is, within the effective range of the action as the binder. The effective range can be appropriately deterrnined by one skilled in the art.
~hotothermographic Formulations The forrnulation for the photothermographic emulsion layer can be prepared by dissolving and dis~e, ~ing the binder, the photosensitive silver halide, the non-photosensitive reducible source of silver, the red~lçin~ agent for the non-photo-sensitive reducible silver source, and optional additives, in an inert organic solvent, such as, for eAan,ple, toluene, 2-butanone, or tetrahydrofuran.
The use of "toners" or derivatives thereof which improve the image, is highly desirable, but is not eSsenti~l to the element. Toners can be present in an S amount of about 0.01-10 % by weight of the emulsion layer, preferably about 0.1-10 % by weight. Toners are well known compounds in the photothermographic art, as shown inU.S. PatentNos. 3,080,254; 3,847,612; and 4,123,282.
EA~IIIPIeS of toners include: phth~limide and N-hydroxyphth~limide; cyclic imides, such as succinim; ~e, pyræoline-5-ones, quinazolinone, 1-phenyluræole, 10 3-phenyl-2-pyrazoline-S-one, and 2,4-thiazolidinedione; naphth~limides, such as N-hydroxy-1,8-nqrhth~lirnide; cobalt complexes, such as cobaltic hex~mine trifluor~acet~te; ~--elcaptans such as 3-n.ercal)to-1,2,4-triazole, 2,4-dimercapto-pyrimidine, 3-mercapto-4,5-diphenyl-1,2,4-triazole and 2,5-dimercapto-1,3,4-thia-diæole; N-(aminomethyl)aryldicarboximides, such as (N,N-dimethylaminomethyl)-15 phthalimide, and N-(dimethylaminomethyl)naphthalene-2,3-dicarboximide; a combination of blocked pyrazoles, isothiuronium derivatives, and certain photo-bleach agents, such as a combination of N,N'-heAa~"elhylene-bis(l-carbamoyl-3,5-dimethylpyræole), 1,8-(3,6-diazaoctane)bis(isothiuronium)trifluoroacetate, and 2-(tribrolno...ell.ylsulfonyl benzothiazole); merocyanine dyes such as 3-ethyl-20 5-1(3-ethyl-2-benzothiæolinylidene)-1-methyl-ethylidene]-2-thio-2,4-o-æolidine-lione; phth~l~7inone, phth~l~7inone derivatives, or metal salts or these derivatives, such as 4-(1-naphthyl)phthalazinone, 6-chlorophth~1~7inone, 5,7-dimethoxy~ h~l~7inone, and 2,3-dihydro-1,4-phthalazinedione; a combination of phthqls7ine plus one or more phthalic acid derivatives, such as phthalic acid, 25 4-methylphth~lic acid, 4-nitrophthalic acid, and tetrachlorophthalic anhydride, quinæolinediones, benzoxæine or naphthoxæine derivatives; rhodium complexes functioning not only as tone modifiers but also as sources of halide ion for silver halide formation in silu, such as ammonium hexachlororhodate (III), rhodium bromide, rhodium nitrate, and potassium hexachlororhodate (III); inorganic 30 peroxides and persl~lf~tes such as ammonium peroxydislllfate and hydrogen peroxide; b~-n70~r~7ine-2,4-diones, such as 1,3-benzoxæine-2,4-dione, 8-methyl--25- 21 70~6 1,3-ben~oA~ e-2,4-dione, and 6-nitro-1,3-benzoxazine-2,4-dione; pyrimidines and asym-triazines, such as 2,4-dihydroxypyrimidine, 2-hydroxy-4-aminopyrimidine, and azauracil; and tetraazapentalene derivatives, such as 3,6-dimercapto-1,4-diphenyl-1~,4H-2,3a,5,6a-tetraazapentalene and 1,4-di-(o-chlorophenyl)-3,6-dimercapto-S lH,4H-2,3a,5,6a-telr~azapent~lene.
The photothermographic elements used in this invention can be further protected against the additional production of fog and can be further stabilizedagainst loss of sensitivity during storage. While not necess~ry for the practice of the invention, it may be advantageous to add mercury (II) salts to the emulsion layer(s) 10 as an antifoggant. Plefelled mercury (II) salts for this purpose are mercuric acetate and mercuric bromide.
Other suitable antifoggants and stabilizers, which can be used alone or in co-,.bil~alion with the DNA stabilizers of this invention include the thiazolium salts described in U.S. Patent Nos. 2,131,038 and U.S. Patent No. 2,694,716; the azaindenes described in U.S. Patent Nos. 2,886,437; the triazaindolizines described in U.S. Patent No. 2,444,605; the mercury salts described in U.S. Patent No.
2,728,663; the urazoles described in U.S. Patent No. 3,287,135; the sulfocatechols desc-ibed in U.S. Patent No. 3,235,652; the oximes described in British Patent No.
623,448; the polyvalent metal salts described in U.S. Patent No. 2,839,405; the thiuronium salts described in U.S. Patent No. 3,220,839; and p~ dillm, platinum and gold salts desc,ibed in U.S. Patent Nos. 2,566,263 and 2,S97,915. Stabilizerprecursor compounds capable of releasing stabilizers upon application of heat during development can also be use in combination with the stabilizers of this invention. Such precursor compounds are described in, for example, U.S. Patent Nos. S,158,866, 5,175,081, 5,298,390, and 5,300,420.
Photothermographic elements of the invention can contain plasticizers and lubricants such as polyalcohols and diols ofthe type described in U.S. Patent No.
2,960,404; fatty acids or esters, such as those described in U.S. Patent Nos.
2,588,765 and 3,121,060; and silicone resins, such as those described in BritishPatentNo. 95S,061.

Photothermographic elements cont~ining emulsion layers described herein may contain matting agents such as starch, titanillm dioxide, zinc oxide, silica, and polymeric beads inclllcling beads of the type described in U.S. Patent Nos.
2,992,101 and 2,701,245.
Emulsions in accordance with this invention may be used in photothermo-graphic cle~ s which contain anti~t~tic or conductinp layers, such as layers that comprise soluble salts, e.g., chlorides, nitrates, etc., evaporated metal layers, ionic polymers such as those described in U.S. Patent Nos. 2,861,056, and 3,206,312 orinsoluble inorganic salts such as those described in U.S. Patent No. 3,428,451.
The photothermographic elements of this invention may also contain electrocollductive under-layers to reduce static electricity effects and improvell~,ls~ o,l throuch processinp equipment. Such layers are described in U.S. Patent No. 5,310,640.
Photothermographic Constructions The photothermographic elements of this invention may be constructed of one or more layers on a support. Single layer elements should contain the silverhalide, the non-photosensitive, reducible silver source, the reducing agent for the non-photosensitive reducible silver source, the binder as well as optional materials such as toners, ac~lt~nce dyes, coating aids, and other adjuvants.
Two-layer constructions should contain silver halide and non-photo-sensitive, reducible silver source in one emulsion layer (usually the layer adjacent to the support) and some of the other ingredients in the second layer or both layers.
Two layer constructions comprising a single emulsion layer coating cont~ining all the h~gred;enls and a protective topcoat are also envisioned.
~Ulticolor photothermographic dry silver elements can contain sets of these bilayers for each color or they can contain all ingredients within a single layer, as described in U.S. Patent No. 4,708,928.
Barrier layers, preferably comprising a polymeric material, can also be present in the photothermographic element of the present invention. Polymers forthe barrier layer can be selected from natural and synthetic polymers such as gelatin, _ polyvinyl alcohols, polyacrylic acids, sulfonated polystyrene, and the like. Thepolymers can optionally be blended with barrier aids such as silica.
Photothermographic emulsions used in this invention can be coated by various coating procedures inc~ lin~ wire wound rod coating dip coatin~ air knife 5 coatin~ curtain co~ting or extrusion coating using hoppers of the type described in U.S. Patent No. 2,681,294. If desired, two or more layers can be çoated simultaneQusly by the procedures described in U.S. Patent Nos. 2,761,791;
5,340,613; and British Patent No. 837,095. Typical wet thickness of the emulsionlayer can be about 10-150 micrometers (~m), and the layer can be dried in forcedair at a tempe.~t-lre of about 20-100C. It is plererled that the thickness of the layer be selected to provide maximum image densities greater than 0.2, and, more prerer~bly, in the range 0.5 to 4.5, as measured by a MacBeth Color DensitometerModel TD 504 using the color filter co,."~lern~nt~ry to the dye color.
Photothermographic elements according to the present invention can contain l S acutance dyes and antihalation dyes. The dyes may be incorporated into the photo-thermographic emulsion layer as acutance dyes according to known techniques. Thedyes may also be incorporated into antihalation layers according to known techniques as an antihalation backing layer, an antihalation underlayer or as anovercoat. It is prefel, ed that the photothermographic elements of this invention 20 contain an ~ntihql~tion coating on the support opposite to the side on which the emulsion and topcoat layers are coated. ~ntih~l~tion and acut~nce dyes useful in the present invention are described in U.S. Patent Nos. 5,135,842; 5,226,452;
5,314,795.
Development conditions will vary, depending on the construction used, but 25 will typically involve heating the photothermographic element in a substantially water-free condition after, or simultaneously with, imagewise exposure at a suitably elevated te.npe,dl~lre. Thus, the latent image obtained after exposure can be developed by heating the element at a moderately elevated temperature of, from about 80C to about 250C (176F to 482F), preferably from about 100C to 30 about 200C, (212F to 392F), for a sufficient period of time, generally about 1 second to about 2 minutes When used in a black-and-white elenlent, a black-and-white silver image. When used in a monochrome or full-color element, a dye imageis obtained sirnult~rleously with the formation of a black-and-white silver image is obtained. Heating may be carried out by the typical heating means such as an oven, a hot plate, an iron, a hot roller, a heat generator using carbon or titanium white, or 5 the like.
If desired, the imaged l~le...cnt may be subjected to a first heating step at a temperature and for a time sufficient to intensify and improve the stability of the latent image but insufficient to produce a visible image and later subjected to a second heating step at a temperature and for a time sufficient to produce the visible 10 image. Such a method and its advantages are described in U.S. Patent No.
5,279,928.
The Support Photothermographic emulsions used in the invention can be coated on a wide variety of supports. The support, or substrate, can be selected from a wide15 range of materials depending on the imaging requirement. Supports may be llar~sparenl or at least tr~nsl~lce~lt. Typical supports include polyester film, subbed polyester film (e.g.,polyethylene terephth~l~te or polyethylene naphth~l~te), cellulose acetate film, cellulose ester film, polyvinyl acetal film, polyolefinic film (e.g., polethylene or polypropylene or blends thereof), polycarbonate film and 20 related or resinous materials, as well as glass, paper, and the like. Typically, a flexible support is employed, especially a polymeric film support, which can be partially acetylated or coated, particularly with a polymeric subbing or primingagent. Plere-led polymeric materials for the support include polymers having good heat stability, such as polyesters. Particularly prefel ed polyesters are polyethylene 25 terephth~l~te and polyethylene naphth~l~te.
A support with a backside resistive heating layer can also be used photo-thermographic imap;ing systems such as shown in U.S. Patent No. 4,374,921.
The Image-Receiving Layer When the reactants and reaction products of photothermographic systems 30 that contain compounds capable of being oxidized to form or release a dye remain in contact after im~in~, several problems can result. For example, thermal development often forms turbid and hazy color images because of dye contamin~tion by the reduced met~llic silver image on the exposed area of the emulsiorl In addition, the resutting prints tend to develop color in unimaged background areas. This is often referred to as "leuco dye backgroundin~g." This S "background stain" is caused by slow post-processing reaction between the dye-for""ng or dye-rele~cing compound and reduçing agent. It is therefore desirable to , ar.srer the dye formed upon imaging to a receptor, or image-receiving layer.
Thus, the photothermographic elc..~nl may further comprise an image-receiving layer. Images derived from the photothermographic elements employing compounds capable of being oxidized to form or release a dye, such as, as for ~x- n~le, leuco dyes, are typically transferred to an image-receiving layer.
If used, dyes generated during thermal development of light-exposed regions of the emulsion layers migrate under development conditions into the an image-receiving or dye-receiving layer wherein they are retained. The dye-receiving layer may be composed of a polymeric material having affinity for the dyes employed.
Necessd, ily, it will vary depending on the ionic or neutral characteristics of the dyes.
The image-receiving layer can be any flexible or rigid, transparent layer made of thermoplastic polymer. The image-receiving layer preferably has a thicl~ness of at least 0.1 mm more pre~,~bly from about 1-10 mm, and a glass transition tel"pe, alure (T8) of from about 20C to about 200C. In the present invention, any thermoplastic polymer or cor"bination of polymers can be used, provided the polymer is capable of absorbing and fixing the dye. Because the polymer acts as a dye mordant~ no additional fixing agents are re~uired.
Thermoplastic polymers that can be used to prepare the image-receiving layer include polyesters, such as polyethylene terephth~l~tes; polyolefins, such as poly-ethylene; cellulosics, such as cellulose acetate, cellulose butyrate, cellulose propionate; polystyrene; polyvinyl chloride; polyvinylidine chloride; polyvinyl ~e~te; copolymer of vinyl chloride-vinyl acetate; copolymer of vinylidene chloride-acrylonitrile; copolymer of styrene-acrylonitrile; and the like.
The optical density of the dye image and even the actual color of the dye image in the image-receiving layer is very much dependent on the characteristics of the polymer of the image-receiving layer, which acts as a dye mordant, and, as such, is capable of absorbing and fixing the dyes. A dye image having a reflection optical density in the range of from 0.3 to 3 .S (preferably, from 1.5 to 3 .5) or a tr~n~mic~ion optical density in the range of from 0.2 to 2.5 (preferably, from 1.0 to 5 2.5) is des;rab'e The image-receiving layer can be forrned by dissolving at least one thermo-plastic polymer in an organic solvent (e.g., 2-butanone, acetone, tetrahydrofuran) and applying the resvltin~ solution to a support base or substrate by various coating methods known in the art, such as curtain coatinp~ extrusion co~tin~, dip coating, 10 air-knife coating~ hopper co~tin~ and any other coating method used for coating solutions. After the solution is coated, the image-receiving layer is dried (e.g., in an oven) to drive off the solvent. The image-receiving layer may be strippably adhered to the photothermographic element. Strippable image-receiving layers are described in U.S. Patent No. 4,594,307.
Selection of the binder and solvent to be used in preparing the emulsion layer si~nific-~ntly affects the sl, ;ppability of the image-receiving layer from the photosen~itive çleme-lt Preferably, the binder for the image-receiving layer is impermeable to the solvent used for coating the emulsion layer and is incompatible with the binder used for the emulsion layer. The selection of the preferred binders 20 and solvents results in weak adhesion between the emulsion layer and the image-receiving layer and promotes good sl~ ippability of the emulsion layer.
The photothermographic element can also include coating additives to improve the strippability of the emulsion layer. For example, fluoroaliphatic poly-esters dissolved in ethyl acetate can be added in an amount of from about 25 0.02-0.5 wt% of the emulsion layer, preferably from about 0.1-0.3 wt%. A
ret)rese"tative example of such a fluoroaliphatic polyester is "Fluorad FC 431", (a fluorinated surfactant available from 3M Company, St. Paul, MN). Alternatively, a coating additive can be added to the image-receiving layer in the same weight range to enhance strippability. No solvents need to be used in the stripping process. The 30 sLIippable layer preferably has a del~nnin~ting re.si~t~nce of 1 to 50 g!cm and a -31- 21 70~86 tensile ~l,en~ at break greater than, prefetably at least two times greater than, its de~. n~ tin~ resict~nce.
Plerelably, the image-receiving layer is adj~c-ent to the emulsion layer in order to f~cilit~te ll ~n~rel of the dye that forms after the imagewise exposed S emulsion layer is subjected to thermal development, for example, in a heated shoe-and-roller-type heat processor.
Photothermographic multi-layer constructions con~ ing blue-sensitive emulsions co.~ ing a yellow dye-ro""-h~g or dye-rele~ing compound can be ovel-coaled with green-sensitive emulsions cor~l~ining a magçnt~ dye-forming or 10 dye-re~ e compound~ These layers can in turn be overcoated with a red-sensitive emulsion layer cont~ining a cyan dye-forming or dye-rele~ing compound.~m~ing and heating to form or release the yellow, magenta, and cyan dyes in an imagewise fashion. The dyes so formed or released may migrate to an image-receiving layer. The image-receiving layer can be a pel ".ane..l part of the construction or it can be removable, "i.e., strippably adhered," and subsequently peeled from the construction. Color-forming layers can be m~int~ined distinct from each other by the use of functional or non-functional barrier layers between thevarious photosen~itive layers as described in U.S. Patent No. 4,460,681. False color address, such as that shown in U.S. Patent No. 4,619,892, can also be used rather than blue-yellow, green-magenta, or red-cyan relationships between sensitivity and dye formation or release. False color address is particularly useful when im~ging is performed using longer wavelength light sources, especially red or near infraredlight sources, to enable digital address by lasers and laser diodes.
If desired, the dyes formed or released in the emulsion layer can be tranSrel l ~ d onto a separately coated image-receiving sheet by placing the exposed ermJl~;on layer in intimate face-to-face contact with the image-receiving sheet and heating the resulting composite construction. Good results can be achieved in this second embodiment when the layers are in uniform contact for a period of time ofabout 0.5-300 seconds at a temperature of about 80-220C.
In another embodiment, a multi-colored image can be plepared by super-imposing in register a single image-receiving sheet successively with two or more -32- 21 7~586 imagewise c.~l,oscd photothermographic e!en~entc each of which forms or releasesa dye of a dilrerel-t color, and heating to transfer the thus formed or released dyes as desc,il,ed above. This method is particularly suitable for the production of color proofs especially when the dyes formed or released have hues that match the S internationally agreed standards for color reproduction (Standard Web Offset Printing colors or SWOP colors). Dyes with this property are disc!osed in U. S.
Patent No. 5,023,229. In this embodiment, the photothermographic elements are preferably all senciti7ed to the same wavelength range regardless of the color of the dye formed or rele~ed. For example, the ele-.lf.~ls can be sensitized to ultraviolet 10 radiation with a view toward contact exposure on conventional printing frames, or they can be sF~ ed to longer wavelengtllc, especially red or near infra-red, to enable digital address by lasers and laser diodes. As noted above, false color address is again particularly useful when im~ping is performed using longer wavelength light sources, especi~lly red or near infrared light sources, to enable digital address by 15 lasers and laser diodes.
Use as a Photomask As noted above, the possibility of low absorbance of the photothermo-graphic el~-..Pnl in the range of 350-450 nm in non-imaged areas f~cilit~tes the use of the photothermographic elements of the present invention in a process where 20 there is a subsequent exposure of an ultraviolet or short wavelength visible radiation sensitive imageable medium. For example, jmqgin~ the photothermographic element with coherent radiation and subsequent development affords a visible image. The developed photothermographic element absorbs ultraviolet or short wavelength visible radiation in the areas where there is a visible image and transmits ultraviolet 25 or short wa~relcny,lll visible radiation where there is no visible image. The developed elem~nt may then be used as a mask and placed between an ultraviolet or short wavelength visible radiation energy source and an ultraviolet or short wavelength visible radiation photosensitive imageable medium such as, for example, a photopolymer, diazo compound, or photoresist. This process is particularly usefi~l 30 where the im~ ble medi~lm con-plises a printing plate and the photothermo-graphic ele-..c-~ serves as an imagese~ g film.

Objects and advantages of this invention will now be illustrated by the following ex~mples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention.
s EXAMPLES
All materials used in the following ~,.a",ples are readily available from standard co,--m~. ,;al sources, such as Aldrich Chemical Co. (Milwaukee, WI). All pereen1~s are by weight unless otherwise indic~ted~ The following additional 10 terms and materials were used.
AcryloidTM A-21 is a poly(methyl meth~crylate) polymer available from Rohm and Haas, Philadelphia, PA.
ButvarTM B-76 is a poly(vinyl butyral) resins available from Monsanto Company, St. Louis, MO.
CA 398-6 is a cellulose acetate polymer available from F~stn~ Chemical Co., Kingsport, TN.
CAB 171-lSS cellulose acetate butyrate polymer available from Eastman Chemical Co., Kingsport, TN.
CAOô 5 is bis(2-hydroxy-3-~-butyl-5-methylphenyl)methane, an antioxidant available from Rohm and Haas, Philadelphia, PA. It is a hindered phenol red~lçing agent (i.e., a developer) for the non-photos~n~itive reducible source of silver. CBBA is 2-(4-chlorobenzoyl)l,en~oic acid.
MEK is methyl ethyl ketone (2-butanone).
MMBI is 5-methyl-2-mercapto~en,;~ d~7Qle.
4-MPA is 4-methylphthalic acid.
NonoxTM is 1,1-bis (2-hydroxy-3, 5-dimethylphenyl)-3, 5, 5-trimethylhexane [CAS RN=7292-14-0] and is available from St. Jean PhotoChemicals, Inc., Quebec.
It is a hindered phenol red~lc.in~ agent (i.e., a developer) for the non-photosçn.~itive reducible source of silver. It is also known as PermanaxTM WSO.
PHZ is phth~l~7ine PHP is pyridinium hydrobromide perbromide.

~34~ ~1 70586 TCPAN is tetrachlorophthalic anhydride.
THDI is DesmodurTM N-100, a biuretized hexamethylenediisocyanate available from Mobay Chemical Corporation.
Se~ e Dye-l is described in U.S. Patent No. 3,719,495 and has the 5 structure shown below.

CH3cH2O~

~N ~3 Se~ g Dye-2 is described in U.S. Patent No. 5,393,654 and has the structure shown below.

(CH2)sCO2 (CH2)sCO2H
Calf Thymus DNA is Type I: sodium salt "highly polymerized" from calf thymus and is a white fibrous preparation containing less than 3% protein. It was obtained from Sigma Chemical Company (St. Louis, MO).
Herring DNA is the sodium salt degraded free acid from herring sperm. It was also oblailled Sigma Chemical Company, St. Louis, MO.
Salmon DNA is a crude nucleoprotamine from salmon testes. It as also oblained Sigma Chemical Company, St. Louis, MO.
All DNA samples were kept refrigerated until used.
Example 1 A 13.6 wt% emulsion of silver behenate/behenic acid half soap was made in acetone by homogenization. To 201.5 g ofthis emulsion was added 1.12 g of ButvarTM B-76 polyvinyl butyral and the mixture was stirred 30 minutes. Three 1.00 mL aliquots of a solution of 10.0 g zinc bromide in 100.0 mL methanol were added sequentially with stirring for 10 mimltes after each addition. Toluene (66.66 g) was added and the mixture was stirred for an additional 15 minutes A
solution (2.40 mL) conl~il.in~ 4.00 g of pyridine in 100 mL of 2-butanone was added with continued stirring for 15 minutes The mixture was allowed to stand for 4 hours at room telllpe.al~lre.
To this emulsion was added 31.75 g of ButvarTM B-76 and the emulsion stirred for 30 rninutes This was followed by the addition of 2.73 mL of a solution of 1.33 g N-bromosuccinimide in 100 mL metl~anol. CAOTM 5 (4.20 g) was added with stirring for 5 minutes AcryloidTM A-21 (27.22 g) was added with stirring for 5 minutes.
The following steps were carried under green safe lights. A 6.00 mL aliquot of a solution of 0.03 g of Sens;l;~;ng Dye-1 dissolved in 25.00 mL of methanol and 75 mL of toluene was added to the above emulsion, and the mixture was stirred for 5 minutes. At this stage, the viscosity of the resultant emulsion should be between 180 and 220 centipoise. If the viscosity is greater than 220 centipoise, acetoneshould be added to bring the viscosity into the appropriate range. The photothermo-graphic emulsion was coated at 4.4 mil (112 ~lm) wet thic~ness onto paper and dried at 180F (82.2C) for one minuteto give a dry coating weight of 1.25 gm/ft2.
A master batch oftopcoat solution was p,epared by mixing: 164.728 g of acetone, 82.350 g of 2-butanone, 33.300 g of methanol, 13.500 g of CA 398-6 cellulose a~et~t~, 1.542 g of phthalazine, 1.068 g of 4-methylphthalic acid, 0.636 g oftetrachlorophthalic acid, and 0.800 g oftetrachlorophthalic anhydride.
Three levels of each DNA compound was evaluated by addition to a 7.00 g aliquot of the master batch of topcoat solution before coating. The samples werethen placed in an ultrasonic bath for various times. All solutions were decænted to remove any compounds that were not soluble.
The topcoat formulation was coated at 2.8 mil (71 ~lm), wet thickness, on top of the photothermographic emulsion layer and dried for 3 minutes at 70C to provide a photothermographic elernent having a dry coating weight of 0.24 gm/ft2.
The photothermographic elements were stored in the dark at room t~.,-p~rhl-lre for I day and for 7 days before im~ging These s~n-ples are referred to herein as 1 day and 7 day naturally aged sa"~ples The coated paper was imaged by exposing a sample with a photometric sensitometer equipped with an F?~tm~n Kodak #101 tungsten light source. A~er exposure, the strips (I inch x 7 inches; 2.5 cm x 17.8 cm) were processed at 250F
(121C) by heating for 6 seconds in a hot rolt processor.
Sensitometry measure",e"ls were made on a custom-built computer-sc~nned densitometer and are believed to be co"~pa,able to measurements obtainable from co"~n-erc;ally available densitometers.
Sensitometric results for the I day naturally aged samples, shown below, demonstrate that addition of DNA compounds provides improved stabilization against shelf-aging fog.
1 Day Shelf-Aging Stability Results ComPound/Amount Time (min) Dmin Dmax SPeed Calf DNA
0.0000 g 0 minlltes 0.54 1.81 0.75 0.0042 g 0.28 1.79 0.72 0.0168 g 0.29 1.78 0.71 0.0840 g 0.31 1.79 0.62 0.0000 g 15 min~ltes 0.54 1.81 0.75 0.0042 g 0.34 1.80 0.70 0.0168 g 0.29 1.78 0.71 0.0840 g 0.23 1.82 0.63 0.0000 g 60 minutes 0.54 1.81 0.75 0.0042 g 0.37 1.80 0.70 0.0168 g 0.29 1.70 0.71 0.0840 g 0.27 1.80 0.63 0.0000 g 120 minutes 0.54 1.81 0.75 0.0042 g 0.38 1.77 0.75 0.0168 g 0.32 1.78 0.68 0.0840 g 0.30 1.80 0.71 Salmon DNA
O.OOOOg 0 minlltes 0.49 1.82 0.63 0.0042 g 0.37 1.81 0.78 0.0168 g 0.39 1.82 0.75 0.0840 g 0.43 1.82 0.66 0.0000 g 15 minutes 0.54 1.82 0.76 0.0042 g 0.34 1.82 0.67 0.0168 g 0.30 1.83 0.67 0.0840 g 0.29 1.82 0.67 21 705~6 Herring DNA
0.0000 g 0 minutes 0.49 1.82 0.63 0.0042 g 0.37 1.82 0.65 0.0168 g 0.39 1.78 0.69 0.0840 g 0.42 1.79 0.71 0.0000 g 15 minutes 0.54 1.81 0.75 0.0042 g 0.34 1.85 0.67 0.0168 g 0.32 1.80 0.68 0.0840 g 0.31 1.80 0.66 10 Speed is log E (E in ergs/cm2) co~re;"~onding to a density of 0.6 above Dmin. In these s~ F'es, the lower the speed number, the "faster" the photospeed ofthe paper.
S~npl~s were allowed to naturally age for 7 days in the dark at room te"")e, alure before im~ging They were then exposed and processed. Sensitometric15 results, shown below, demonstrate that addition of DNA compounds provide improved stabilization against shelf-aging fog. Samples without DNA (0.0000 g) gave the highest values of Dmin. As the time of sonication increased, increased amounts of added DNA gave lower values for Dmin.
7 Day Shelf-Aging Stability Results ComPound/Amount Time (min) Dmin Dma~ SPeed^
Calf DNA
0.0000 g 0 min~ltes 0.62 1.70 0.79 0.0042 g 0.21 1.70 0.73 0.0168 g 0.23 1.76 0.74 0.0840 g 0.33 1.79 0.69 0.0000 g 15 minutes 0.62 1.70 0.79 0.0042 g 0.28 1.81 0.75 0.0168 g 0.26 1.81 0.75 0.0840 g 0.21 1.82 0.75 0.0000g 60 minutes 0.62 1.70 0.79 0.0042 g 0.28 1.80 0.70 0.0168 g 0.26 1.70 0.71 0.0840 g 0.21 1.80 0.63 0.0000 g 120 minutes 0.62 1.70 0.79 0.0042 g 0.27 1.80 0.78 0.0168 g 0.22 1.80 0.79 0.0840 g 0.22 1.70 0.73 Salmon DNA
O.ooO0 g 0 minlltec 0.65 1.80 0.76 0.0042 g 0.25 1.82 0.67 0.0168 g 0.30 1.80 0.80 0.0840 g 0.33 1.84 0.83 0 0000 g 15 min~ltes 0.65 1.80 0.76 0.0042 g 0.37 1.80 0.64 0.0168 g 0.30 1.79 0.79 0.0840 g 0.25 1.80 0.80 Herring DNA
0.0000 g 0 minutes 0.65 1.80 0.76 0.0042 g 0.25 1.82 0.76 0.0168 g 0.37 1.79 0.81 0.0840 g 0.40 1.80 0.97 0.0000 g 15 minutes 0.65 1.80 0.78 0.0042 g 0.30 1.86 0.75 0.0168 g 0.27 1.84 0.79 0.0840 g 0.25 1.86 0.88 20 Speed is log E (E in ergs/cm2) co,.esl)onding to a density of 0.6 above Dmin. In these samples, the lower the speed number, the "faster" the photospeed of the paper.
The sa.,.ples that had not been sonicated and had been naturally aged for 1 day were used to test post-processing print stability. The optical density of the 25 samples was measured on a Macbeth TR 924 Densitometer using the visible filter.
The sa",pl~s were then placed in a light chamber at room te,nperalure for 72 hours at 100 foot-candles ill.lmin~tion After 72 hours the samples were removed and their optical density reme~cured on the Macbeth TR 924 Densitometer.
The sensitometric results, shown below, demonstrate that the DNA
30 compounds of this invention provide improved post-processing print stability. In all caes, the post-procescing print stability improved as the amount of DNA compoundwas incteased. For cAalnplc, the addition of calf DNA resulted in post-processing print stability improvements of from 25% (upon addition of 0.0042 g) to 62%
(upon addition of 0.0840 g). ~Dmin is Dmin (Final) - Dmin (Initial).

Post-lrr.~ces~ing Print Stability Com~ound/Amount ~ Dmin Calf DNA
0.16 0.0042 g 0.12 0.0168 g 0.10 0.0840 g 0.07 Salmon DNA
O.OOOOg 0.15 0.0042 g 0.13 0.0168g 0.11 0.0840 g 0.10 Herring DNA
O.0000g 0.15 0.0042 g 0.15 0.0168 g 0.13 0.0840 g 0.12 Example 2 A silver halide-silver behenate dry soap was prepared by the procedures described in U.S. Patent No. 3,839,049. The silver halide totaled 9% of the total silver while silver behenate comprised 91% of the total silver. The silver halide was a 0.055 ~m silver bromoiodide emulsion with 2% iodide.
The following steps were carried under green safe lights: A photothermo-graphic emulsion was prepared by homoge~ g 300 g of the silver halide-silver behen~te dry soap described above with 525 g toluene, 1675 g of 2-butanone, and 50 g of ButvarTM B-76. The homogenized photothermographic emulsion (510 g) was cooled to 55F (12.8C) with stirring. A solution of 0.63 g of pyridinium hydrobromide pe,l,ro",ide (PHP) in 3.16 g of meth~nol was added and stirring h;ned for 2 hours. The addition of 3.25 mL of a calcium bromide solution (prep~,ed by dissolving 1.00 g CaBr2 in 10 mL of methanol) was followed by 30 mimltes of stirring. ButvarTM B-76 polyvinyl butyral (108.5 g) was added and theemulsion stirred for 20 minutes. The mixture was allowed to stand for 16 hours at 55F (12.8C).
The following steps were carried under infrared safe lights: To the stirred emulsion was added a solution of of 0.37 g of 5-methyl-2-mercaptoben7imi-1~7.ole (I~I), 4.19 g of 2-(4-chlorobenzoyl)benzoic acid (CBBA), and a solution of 0.07 g of Se~.s;~ g Dye 2 in 25.10 g of methanol. The emulsion was allowed to stir for 20 minutes at which time 15.9 g of NonoxTM was added followed by 1.00 gof THDI in 7.0 g 2-butanone.
A master batch oftopcoat solution was prepared by mixing 510 g of 2-butanone, 60.00 g of meth~nol, 48.00 g of CAB 171-15S cellulose acetate butyrate, 1.08 g oftetrachlolopl-lhalic acid, 1.62 g of 4-methylphthalic acid, 3.30 g of phthsl~7ine, and 1.92 g of AcryloidT~ A-21. Three levels of each DNA compoundwas evaluated by addition to a 14.00 g aliquot of the master batch of topcoat solution before coating The samples were then placed in an ultrasonic bath for various times. All solutions were decqnted to remove any compounds that were notsoluble.
The photothermographic emulsion and and topcoat formulations were coated onto a 7 mil (176 ~lm) polyethylene terephth~l~te support using a dual-knife coater. This apparatus consists of two knife coating blades in series. The support was cut to a length suitable to the volume of solution used, and after raising the hinged knives, placed in position on the coater bed. The knives were then lowered and locked into place. The height of the knives was adjusted with wedges controlled by screw knobs and measured with electronic gauges. Knife # 1 was raised to a clea. ~nce corresponding to the thickness of the support plus the desired wet thic~ne~s ofthe emulsion layer (layer #1). Knife #2 was raised to a height equal to the desired thic~ness of the support plus the desired wet thickness the emulsionlayer (layer #1) plus the desired wet thi~nç~s of the topcoat layer (layer #2).
The photothermographic emulsion layer was coated at a wet thicl~ness of 4.3 mil (109 ~m) above the support to give a dry coating weight of 1.93 gm/ft2. The topcoat was coated over the photothermographic emulsion layer at a wet thicknessof 5.1 mil (130 )lm) above the support to give a dry coating weight of 0.24 gm/ft2.
The photothermographic element was dried for four minutes at 175F (79.4C).
The photothermographic element was imaged by exposing with a laser sensitometer with an infrared light source (813 nm). After exposure, the strips (1 inch x 7 inches; 2.5 cm x 17.8 cm) were processed at 250F (121C) by heatingfor 15 seconds on a hot roll processor.
Sensitometric measurements were made on a custom-built computer-scanned densitometer and are believed to be cor -parable to measurernenl~
obt~ ble from co.. crcially available densitometers.
Sc.-~;lo~ ,l-ic results for 1 day naturally aged s~..ples, shown below, demonstrate that incorporation of DNA compounds into photothermographic elements provides improved Dmin stability toward shelf-aging fog.
1 Day Shelf-Aging Stability Results ComPound/Amount Time (min) Dmin Dmax Speed^
Calf DNA
0.0000 g 0 minutes 0.52 4.20 1.74 0.0042 g 0.16 4.18 1.65 0.0168 g 0.20 4.18 1.71 0.0840g 0.29 4.18 1.73 0.0000 g 30 n inutes 0.52 4.30 1.76 0.0042 g 0.28 4.16 1.66 0.0168 g 0.17 4.10 1.74 0.0840 g 0.16 4.18 1.76 Speed is log 1/E (E in ergs/cm2)+4 corresponding to a density of 1.00 above Dmin. In these samples, the higher the speed number, the "faster" the film.
Sensitometric results for the 7 day naturally aged samples, shown below, again de~on~l~ate addition of DNA to the photothermographic element provides improved stabilization against shelf-aging fog. Samples without DNA (0.0000 g) gave high levels of fog (Dmin > I .0) and did not give an image. As the time of sonication inc~ased, increased amounts of added DNA gave lower values for Dmin.

.
7 Day Shelf-Aging Stability Results ComPound/Amount Time (min)Dmin Dmax SPeed^
Calf DNA
0.0000 g 0 minutesCompletely Fogged -----0.0042 g 0.28 4.08 1.60 0.0168 g 0.29 4.05 1.65 0.0840 g 0.31 4.00 1.70 0.0000 g 30 mi~ utesCompletely Fogged -----0.0042 g 0.30 4.10 1.62 0.0168 g 0.25 4.04 1.70 0.0840 g 0.24 4.16 1.74 Speed is log 1/E (in ergs/cm2)+4 corl~")onding to a density of 1.00 above Dmin.
In these samples, the higher the speed number, the "faster" the film.
The samples that had been naturally aged for 1 day were used to test post-processinp print stability. The optical density of the samples were measured on a Macbeth TR 924 Densitometer using the additive blue filter. Samples were then placed in a heat and light ch&",ber controlled to 45C and 20% RH for 10 hours at 1200 foot-candles illumin~tion. A~er 72 hours the samples were removed and theiroptical density remeasured on the Macbeth TR 924 Densitometer.
Sensitometric results, shown below, demonstrate that the DNA compounds of this invention provide improved post-processing print stability. In all cases, the post-proces~ g print stability improved as the amount of DNA compound was increased. The observed improvements by incorporating DNA were from 13%
(upon adddition of 0.0042 g) to 22% (upon addition of 0.0840 g). ~Dmin is Dmin (~inal) - Dmin (Initial).
Post-P~r~sing Print Stability ComPound/Amount Initial Dmin ~ Dmin Calf DNA
0.0000 g 0.52 0.22 300.0042 g 0.16 0.19 0.0168 g 0.20 0.17 0.0840 g 0.29 0.15 Reasonable modifications and variations are possible from the foregoing di~clos ~re without depa, ling from either the spirit or scope of the present invention 35 as defined by the claims.

Claims (17)

1. A photothermographic element comprising a support bearing at least one photosensitive image-forming photothermographic emulsion layer comprising;
(a) a photosensitive silver halide;
(b) a non-photosensitive, reducible source of silver;
(c) a reducing agent for the non-photosensitive, reducible source of silver;
(d) a binder; and (e) DNA.

2. The photothermographic element according to Claim 1 wherein the silver halide is silver bromide, silver chloride, silver iodide, silver chlorobromide, silver bromoiodide, silver chlorobromoiodide, or mixtures thereof.

3. The photothermographic element according to Claim 1 wherein said non-photosensitive, reducible source of silver is a silver salt of a C1 to C30 carboxylic acid.

4. The photothermographic element according to Claim 3 wherein said non-photosensitive silver source is silver behenate.

5. The photothermographic element according to Claim 1 wherein said reducing agent is a compound capable of being oxidized to form or release a dye.

6. The photothermographic element according to Claim 5 wherein said compound capable of being oxidized to form or release a dye is a leuco dye.

7. The photothermographic element according to Claim 1 wherein said binder is hydrophilic.

8. The photothermographic element according to Claim 1 wherein said binder is hydrophobic.

9. The photothermographic element according to Claim 1 wherein said reducing agent is a hindered phemol.

10. The photothermographic element of Claim 9 wherein said hindered phenol is selected from the group consisting of binaphthols, biphenols, bis(hydroxy-naphthyl)methanes, bis(hydroxyphenyl)methanes, hindered phenols, and naphthols.

11. The photothermographic element of Claim 10 wherein said hindered phenol is a bis(hydroxyphenyl)methane.

12. The photothermographic element of Claim 1 wherein said DNA is selected from the group consisting of Calf DNA, Salmon DNA, and Herring DNA.

13. A process for the formation of a visible image comprising exposing the photothermographic element of Claim 1 to light to form a latent image and subsequently heating said exposed element.

14. A process comprising the steps of:
(a) exposing the photothermographic element of Claim 1 to electro-magnetic radiation, to which the silver halide grains of the element are sensitive, to generate a latent image;
(b) heating the exposed element to develop the latent image into a visible image;
(c) positioning said element with a visible image thereon between a source of ultraviolet or short wavelength visible radiation and an ultraviolet or short wavelength visible radiation photosensitive imageable medium; and (d) then exposing said ultraviolet or short wavelength visible radiation sensitive imageable medium to ultraviolet or short wavelength visible radiation through said visible image on said element, thereby absorbing ultraviolet or short wavelength visible radiation in the areas of said element where there is a visible image and transmitting ultraviolet or short wavelength visible radiation where there is no visible image on said element.

15. The process of claim 14 wherein said imageable medium is a resist developable, ultraviolet or short wavelength visible radiation sensitive imageable medium.

16. The process of claim 14 wherein said exposing of said element in step (a) is done with a red or infrared emitting laser or red or infrared emitting laser diode.

17. The process of claim 14 wherein said ultraviolet or short wavelength visible radiation sensitive imageable medium is a printing plate, a contact proof, or a duplicating film.