Oxidants for reducing post-process D-min increase in positive redox dye-releasing image transfer systems
This invention relates to photography, and more particularly to photographic elements and assemblages for color diffusion transfer photography employing at least one silver halide emulsion layer and a positive-working redox dye-releaser (PRDR). The photographic element contains a particular oxidant having a certain electrode potential which is located between the support and the photosensitive portion thereof. After processing, the oxidant oxidizes electron transfer agent present in the assemblage to prevent it from slowly reducing the PRDR which would otherwise cause unwanted dye release. Post-process D min  stability is thereby achieved in accordance with this invention. 
     Various formats for color, integral transfer elements are described in the prior art, such as U.S. Pat. Nos. 3,415,644; 3,415,645; 3,415,646; 3,647,437; 3,635,707; and 3,756,815, and Canadian Pat. Nos. 928,559 and 674,082. In these formats, the image-receiving layer containing the photographic image for viewing remains permanently attached and integral with the image generating and ancillary layers present in the structure when a transparent support is employed on the viewing side of the assemblage. The image is formed by dyes, produced in the image generating units, diffusing through the layers of the structure to the dye image-receiving layer. After exposure of the assemblage, an alkaline processing composition permeates the various layers to initiate development of the exposed photosensitive silver halide emulsion layers. The emulsion layers are developed in proportion to the extent of the respective exposures, and the image dyes which are formed or released in the respective image generating layers being to diffuse throughout the structure. At least a portion of the imagewise distribution of diffusible dyes diffuse to the dye image-receiving layer to form an image of the original subject. 
     Other so-called &#34;peel apart&#34; formats for color diffusion transfer assemblages are described, for example, in U.S. Pat. Nos. 2,983,606; 3,362,819 and 3,362,821. In these formats, the image-receiving element is separated from the photosensitive element after development and transfer of the dyes to the image-receiving layer. 
     In color transfer assemblages such as those described above, a &#34;shut-down&#34; mechanism is needed to stop development after a predetermined time, such as 20 to 60 seconds in some formats, or up to 3 to 10 minutes or more in other formats. Since development occurs at a high pH, it is rapidly slowed by merely lowering the pH. The use of a neutralizing layer, such as a polymeric acid, can be employed for this purpose. Such a layer will stabilize the element after silver halide development and the required diffusion of dyes has taken place. A timing layer is usually employed in conjunction with the neutralizing layer, so that the pH is not prematurely lowered, which would prematurely restrict development and dye release. The development time is thus established by the time it takes the alkaline composition to penetrate through the timing layer. As the system starts to become stabilized, alkali is depleted throughout the structure, causing silver halide development to substantially cease in response to this drop in pH. This may also cause the dye release rate to slow down. For each image generating unit, this shutoff mechanism establishes the amount of silver halide development and the related amount of dye released or formed according to the respective exposure values. 
     In color transfer assemblages employing nondiffusible PRDR&#39;s, a dye is released as an inverse function of development, i.e., dye is released by some mechanism in the non-exposed areas of the silver halide emulsion. Use of a negative-working silver halide emulsion in such a system will therefore produce a positive image in the image-receiving layer. Examples of such PRDR&#39;s are described in U.S. Pat. Nos. 4,139,379 and 4,139,389. The immobile compounds described in these patents are ballasted electron-accepting nucleophilic displacement (BEND) compounds. The BEND compound is incorporated in a photographic element is substantially incapable of releasing a diffusible dye. However, during photographic processing under alkaline conditions, the BEND compound is capable of accepting at least one electron (i.e. being reduced) from an incorporated reducing agent (IRA) and thereafter releases a diffusible dye. This occurs in the unexposed areas of the emulsion layer. In the exposed areas of the emulsion layer, however, an electron transfer agent (ETA) reduces the silver halide and becomes oxidized. The oxidized ETA is then reduced by the IRA, thus preventing the IRA from reacting with the BEND compound. The BEND compound therefore is not substantially reduced and thus no dye is released in the exposed areas. 
     After processing the photographic element described above, the electron transfer agent remains after imaging in both the exposed and nonexposed areas. A problem which occurs is that the D min  continues to increase over a period of time. This is sometimes described in the art as &#34;post-process density increase&#34;. It is believed that over a period of time, the electron transfer agent can slowly reduce the PRDR and cause this unwanted dye release. We have found a way for lessening the extent of post process density increase in PRDR systems by providing a means to inactivate the electron transfer agent after imaging. The means which we have found is to incorporate a particular oxidant in the photographic element as will be described more fully hereafter. 
     U.S. Pat. No. 4,139,379 of Chasman et al describes PRDR systems in which the present invention can be employed. In Example 2 of Chasman et al, an oxidant is employed in an interlayer in the photographic element as a scavenger. This prevents the incorporated reducing agent (or electron donor) associated with one of the emulsion layers from reacting in another emulsion layer, and thereby reduces any color contamination or &#34;cross-talk&#34; between these layers. The use of an oxidizing interlayer in this example necessitates the use of an incorporated ETA in each emulsion layer. The ETA could not be supplied from the pod since it would be oxidized by the oxidizing interlayer. The oxidant used in this invention is employed in an entirely different location in the photographic element and for an entirely different purpose. 
     In U.S. patent application Ser. No. 392,508, filed June 28, 1982 of Armour et al, now U.S. Pat. No. 4,409,315, oxidants are employed in a cover sheet in an image transfer system. It was previously thought that such oxidants should not be located in the photographic element, since they would interfere with the imaging process by oxidizing the ETA diffusing through the photosensitive layers and upset the balance of complex redox reactions taking place with the PDRD&#39;s, the IRA and the oxidized ETA. It was unexpectedly found, however, that such oxidants could be located in the photographic element without having to have an incorporated ETA in each emulsion layer, provided it is not located in the photosensitive portion thereof, without affecting image discrimination. It was found to be advantageous to have the oxidant located closer to the imaging layers and diffusing ETA for greater effectiveness. In addition, not locating the oxidant in the cover sheet also means it becomes available sooner after processing, since there are no &#34;release&#34; considerations involved with timing-layer breakdown, as there is when the oxidant is coated in the cover sheet. 
     A photographic element in accordance with the invention to be processed with an electron transfer agent comprises a support having thereon at least one photosensitive silver halide emulsion layer having associated therewith a positive-working, redox dye-releaser, and wherein the photographic element contains an oxidant which is located between the suport and the photosensitive portion of the photographic element, the oxidant having an electrode potential of from about -200 mV up to about +1200 mV versus a saturated calomel electrode at a pH of about 5 to about 6, the oxidant being capable of oxidizing the electron transfer agent after processing, and the reduced form of the oxidant being substantially incapable of reducing the positive-working redox dye-releaser. 
     A photographic assemblage in accordance with the invention to be processed with an electron transfer agent comprises: 
     (a) a photographic element as described above; 
     (b) a dye image-receiving layer; and 
     (c) a transparent cover sheet located over the layer outermost from the support. 
     The oxidants which can be employed in the invention must have an electrode potential within a defined range at a given pH, as described above. Oxidants which are too weak would be marginally or totally ineffective. Strong oxidants, even though they may reduce D min  increases, may attack the dye and cause a loss of density in D max  areas. In a preferred embodiment of the invention, the oxidants are substantially nondiffusible. 
     Any oxidant may be employed in the invention as long as it has the electrode potential as described above. In a preferred embodiment of the invention, the oxidant can be an inorganic salt, a quinone compound or an oxidizable precursor thereof capable of providing a quinone during or subsequent to processing, a peroxy acid compound or a positive halogen compound. These compounds are usually nearly colorless. They may also be blocked, if desired, to make them colorless or to prevent premature reaction. 
     Inorganic salts useful in the invention include sodium iodate, sodium periodate, sodium chlorate, sodium bromate and sodium persulfate. 
     Quinone compounds useful in the invention include 2,5- and 2,6-disubstituted, tri- and tetra-substituted hydrolytically stable quinones. Substitution is generally unrestricted and may be selected from unsubstituted or substituted alkyl or aryl, halogen, alkoxy, alkylthio and carboxyl. Such compounds include, for example, 2,5-didodecylquinone, phenyltrichloroquinone, pentadecyltrichloroquinone, 1,8-octamethylene-1,1-bis(2,4,5-tribromoquinone), tribromopentadecylquinone, 2,5-didodecyl-3-phenylsulfonylquinone, tribromooctylquinone, 2-chloro-3-pentadecylquinone, 2,5-didodecyl-3-bromoquinone, 2,5-dibromo-3,6-dioctylquinone, 2,3-dichloro-5-pentadecylquinone, 2-bromo-3,6-dioctylquinone, and N-methyl-N-octadecylcarbamoylquinone. 
     Hydroquinones are also useful in the invention provided they are oxidizable to the corresponding quinone during or subsequent to processing. Such hydroquinones at high pH can be oxidized to the corresponding quinone by a mobile, partially oxidized ETA. As the pH is lowered (by the action of the timing and neutralizing layers), the generated quinone then serves as an oxidant for the ETA. In these instances, the use of such hydroquinones behaves essentially the same as the corresponding quinones in reducing &#34;post process density increase&#34;. 
     Peroxy acid compounds useful in the invention include, for example, perbenzoic acid and m-chloroperbenzoic acid. 
     Positive halogen compounds are known in the art as compounds which are organic halogenating agents or oxidants and are described by R. Filler in Chem. Revs., 63, 22 (1963). Such compounds include, for example, N-bromosuccinimide, Chloroamine-T® (sodium p-toluenesulfonchloramide), N-chlorosuccinimide and N-bromoacetamide. 
     Especially good results are obtained in a preferred embodiment of the invention with sodium periodate, pentadecyltrichloroquinone, 2,5-didodecylquinone or 2,5-didodecylhydroquinone. The electrode potentials of these compounds versus a saturated calomel electrode at a pH of about 5 are, respectively, +1100 mV, +145 mV, +124 mV and +129 mV; and at a pH of about 6 are, respectively, +1040 mV, +85 mV, +64 mV and +64 mV. 
     The oxidants employed in the invention may be present in any concentration which is effective for the intended purpose. In general, good results have been obtained at concentrations ranging from about 0.2 to about 20 mmole per square meter of element, preferably 1 to 10 mmoles per square meter. 
     As previously described, the oxidant employed in the invention is located between the support and the photosensitive portion of the photographic element. For example, it may be located in an opaque absorbing layer or a layer adjacent thereto. 
     Any PRDR&#39;s known in the art may be employed in our invention. Such PRDR&#39;s are disclosed, for example, in U.S. Pat. Nos. 4,139,379, 4,199,354, 4,232,107, 4,242,435, 4,273,855, 3,980,479 and 4,139,389, the disclosures of which are hereby incorporated by reference. In a preferred embodiment of the invention, the PRDR is a quinone PRDR and the photographic element contains an incorporated reducing agent as described in U.S. Pat. No. 4,139,379, referred to above. In another preferred embodiment, the quinone PRDR&#39;s have the formula: ##STR1## wherein: 
     Ballast is an organic ballasting radical of such molecular size and configuration as to render the compound nondiffusible in the photographic element during development in an alkaline processing composition; 
     W represents at least the atoms necessary to complete a quinone nucleus: 
     r is a positive integer of 1 or 2; 
     R is an alkyl radical having 1 to about 40 carbon atoms or an aryl radical having 6 to about 40 carbon atoms; 
     k is a positive integer of 1 to 2 and is 2 when R is a radical of less than 8 carbon atoms; and 
     Dye is an organic dye or dye precursor moiety. 
     In a preferred embodiment of the invention, the silver halide emulsions employed are the conventional, negative-working emulsions well known to those skilled in the art. A positive image will thereby be obtained in the image-receiving layer. Use of a direct-positive emulsion will produce a negative image in the image-receiving layer. Such a negative can be used to produce positive prints if so desired. 
     The photographic element in the above-described photographic assemblage can be treated in any manner with an alkaline processing composition to effect or initiate development. One method for applying processing composition is by interjecting processing solution with communicating members similar to hypodermic syringes which are attached either to a camera or camera cartridge. The processing composition can also be applied by means of a swab or by dipping in a bath, if so desired. Another method of applying processing compositions of a film assemblage which can be used in our invention is the liquid spreading means described in U. S. Pat. No. 4,370,407 of Columbus, isued Jan. 25, 1983. 
     In another embodiment of the invention, the assemblage itself contains the alkaline processing composition and means containing same for discharge within the film unit, such as a rupturable container which is adapted to be positioned during processing of the film unit so that a compressive force applied to the container by pressure-applying members, such as would be found in a camera designed for in-camera processing, will effect a discharge of the container&#39;s contents within the film unit. 
     The dye image-receiving layer in the abovedescribed film assemblage is optionally located on a separate support adapted to be superposed on the photographic element after exposure thereof. Such image-receiving elements are generally disclosed, for example, in U.S. Pat. Nos. 3,362,819. In accordance with this embodiment of the invention, the dye image-receiving element would comprise a support having thereon, in sequence, a neutralizing layer, a timing layer and a dye image-receiving layer. When the means for discharging the processing composition is a rupturable container, it is usually positioned in relation to the photographic element and the image-receiving element so that a compressive force applied to the container by pressure-applying members, such as would be found in a typical camera used for in-camera processing, will effect a discharge of the container&#39;s contents between the image-receiving element and the outermost layer of the photographic element. After processing, the dye image-receiving element is separated from the photographic element. 
     In another embodiment, the dye image-receiving layer in the above-described film assemblage is located integral with the photographic element and is located between the support and the lowermost photosensitive silver halide emulsion layer. One useful format for integral imaging receiver photographic elements is disclosed in Belgian Pat. No. 757,960. In such an embodiment, the support for the photographic element is transparent and is coated with an image-receiving layer, a substantially opaque light-relective layer, e.g., TiO 2 , an opaque layer or layer adjacent thereto containing the oxidant described above, and then the photosensitive layer or layers described above. After exposure of the photographic element, a rupturable container containing an alkaline processing composition and an opaque process sheet are brought into superposed position. Pressure-applying members in the camera rupture the container and spread processing composition over the photographic element as the film unit is withdrawn from the camera. The processing composition develops each exposed silver halide emulsion layer, and dye images, formed as a function of development, diffuse to the image-receiving layer to provide a positive, right-reading image which is viewed through the transparent support on the opaque reflecting layer background. For other details concerning the format of this particular integral film unit, reference is made to the above-mentioned Belgian Pat. No. 757,960. 
     Another format for integral imaging receiver photographic elements in which the present invention is employed is disclosed in Canadian Pat. No. 928,559. In this embodiment, the support for the photographic element is transparent and is coated with the image-receiving layer, a substantially opaque, light-reflective layer, an opaque layer or layer adjacent thereto containing the oxidant described above, and the photosensitive layer or layers described above. A rupturable container, containing an alkaline processing composition including an ETA and an opacifier, is positioned between the top layer and a transparent cover sheet which has thereon, in sequence, a neutralizing layer and a timing layer. The film unit is placed in a camera, exposed through the transparent cover sheet and then passed through a pair of pressure-applying members in the camera as it is being removed therefrom. The pressure-applying members rupture the container and spread processing composition and opacifier over the negative portion of the film unit to render it light-insensitive. The processing composition develops each silver halide layer and dye images, formed as a result of development, diffuse to the image-receiving layer to provide a positive, right-reading image which is viewed through the transparent support on the opaque reflecting layer background. For further details concerning the format of this particular integral film unit, reference is made to the above-mentioned Canadian Pat. No. 928,559. 
     Still other useful integral formats in which this invention can be employed are described in U.S. Pat. Nos. 3,415,644; 3,415,645; 3,415,646; 3,647,437 and 3,635,707. In most of these formats, a photosensitive silver halide emulsion is coated on an opaque support and a dye image-receiving layer is located on a separate transparent support superposed over the layer outermost from the opaque support. In addition, this transparent support also contains a neutralizing layer and a timing layer underneath the dye image-receiving layer. 
     In another embodiment of the invention, the neutralizing layer and timing layer are located underneath the photosensitive layer or layers. In that embodiment, the photographic element would comprise a support having thereon, in sequence, a neutralizing layer, a timing layer and at least one photosensitive silver halide emulsion layer having associated therewith a dye image-providing material. A dye image-receiving layer would be provided on a second support with the processing composition being applied therebetween. This format could either be peel-apart or integral, as described above. 
     A process for producing a photographic transfer image in color according to the invention from an imagewise exposed phototosensitive element comprising a support having thereon at least one photosensitive silver halide emulsion layer having associated therewith a dye image-providing material as described above comprises treating the element with an alkaline processing composition in the presence of a silver halide developing agent or ETA to effect development of each of the exposed silver halide emulsion layers. An imagewise distribution of dye image-providing material is thus formed as a function of development, and at least a portion of it diffuses to a dye image-receiving layer to provide the transfer image. The electron transfer agent remaining in the photosensitive element after development is then oxidized by means of an oxidant, as described above, after processing, to prevent it from further reaction with the PRDR which would otherwise cause additional dye release over a period of time. 
     The film unit or assemblage of the present invention is used to produce positive images in single or multicolors. In a three-color system, each silver halide emulsion layer of the film assembly will have associated therewith a dye-releasing compound which releases a dye possessing a predominant spectral absorption within the region of the visible spectrum to which said silver halide emulsion is sensitive, i.e., the blue-sensitive silver halide emulsion layer will have a yellow dye-releaser associated therewith, the green-sensitive silver halide emulsion layer will have a magenta dye-releaser associated therewith and the red-sensitive silver halide emulsion layer will have a cyan dye-releaser associated therewith. The dye-releaser associated with each silver halide emulsion layer is contained either in the silver halide emulsion layer itself or in a layer contiguous to the silver halide emulsion layer, i.e., the dye-releaser can be coated in a separate layer underneath the silver halide emulsion layer with respect to the exposure direction. 
     The concentration of the dye-releasing compounds that are employed in the present invention can be varied over a wide range, depending upon the particular compound employed and the results desired. For example, a dye-releaser coated in a layer at a concentration of 0.1 to 3 g/m 2  has been found to be useful. The dye-releaser can be dispersed in a hydrophilic film-forming natural material or synthetic polymer, such as gelatin, polyvinyl alcohol, etc, which is adapted to be permeated by aqueous alkaline processing composition. 
     A variety of silver halide developing agents are useful in this invention. Specific examples of developers or electron transfer agents (ETA&#39;s) useful in this invention include hydroquinone compounds, aminophenol compounds, catechol compounds, phenylenediamine compounds, or 3-pyrazolidinone compounds. A combination of different ETA&#39;s, such as those disclosed in U.S. Pat. No. 3,039,869, can also be employed. These ETA&#39;s are employed in the liquid processing composition or contained, at least in part, in any layer or layers of the photographic element or film unit to be activated by the alkaline processing composition, such as in the silver halide emulsion layers, the dye image-providing material layers, interlayers, image-receiving layer, etc. 
     The various silver halide emulsion layers of a color film assembly employed in this invention can be disposed in the usual order, i.e., the blue-sensitive silver halide emulsion layer first with respect to the exposure side, followed by the green-sensitive and red-sensitive silver halide emulsion layers. If desired, a yellow dye layer or a yellow colloidal silver layer can be present between the blue-sensitive and green-sensitive silver halide emulsion layers for absorbing or filtering blue radiation that is transmitted through the blue-sensitive layer. If desired, the selectively sensitized silver halide emulsion layers can be disposed in a different order, e.g., the blue-sensitive layer first with respect to the exposure side, followed by the red-sensitive and green-sensitive layers. 
     The rupturable container employed in certain embodiments of this invention is disclosed in U.S. Pat. Nos. 2,543,181; 2,643,886; 2,653,732; 2,723,051; 3,056,492; 3,056,491 and 3,152,515. In general, such containers comprise a rectangular sheet of fluid- and air-impervious material folded longitudinally upon itself to form two walls which are sealed to one another along their longitudinal and end margins to form a cavity in which processing solution is contained. 
     Generally speaking, except where noted otherwise, the silver halide emulsion layers employed in the invention comprise photosensitive silver halide dispersed together with the dye releaser in gelatin or another aqueous alkaline solution-permeable polymeric binder and are about 0.6 to 7 microns in thickness; and the alkaline solution-permeable polymeric interlayers, e.g., gelatin, are about 0.2 to 5 microns in thickness. Of course, these thicknesses are approximate only and can be modified according to the product desired. The silver halide emulsions and dye releasers may also be coated in separate layers, if desired. 
     Scavengers for oxidized developing agents can be employed in various interlayers of the photographic elements of the invention. Suitable materials are disclosed on page 83 of the November 1976 edition of Research Disclosure, the disclosure of which is hereby incorporated by reference. 
     Any material is useful as the image-receiving layer in this invention, as long as the desired function of mordanting or otherwise fixing the dye images is obtained. The particular material chosen will, of course, depend upon the dye to be mordanted. Suitable materials are disclosed on pages 80 through 82 of the November, 1976 edition of Research Disclosure, the disclosure of which is hereby incorporated by reference. 
     Use of a neutralizing material in the film assemblages of this invention will usually increase the stability of the transferred image. Generally, the neutralizing material will effect a reduction in the pH of the image layer from about 13 or 14 to at least 11 and preferably 5 to 8 within a short time after treatment with alkali. Suitable materials and their functioning are disclosed on pages 22 and 23 of the July 1974 edition of Research Disclosure, and pages 35 through 37 of the July 1975 edition of Research Disclosure, the disclosures of which are hereby incorporated by reference. 
     A timing or inert spacer layer can be employed in the practice of this invention over the neutralizing layer which &#34;times&#34; or controls the pH reduction as a function of the rate at which alkali diffuses through the inert spacer layer. Examples of such timing layers and their functioning are disclosed in the Research Disclosure articles mentioned in the paragraph above concerning neutralizing layers. 
     The alkaline processing composition employed in this invention is the conventional aqueous solution of an alkaline material, e.g, alkali metal hydroxides or carbonates such as sodium hydroxide, sodium carbonate or an amine such as diethylamine, preferably possessing a pH in excess of 11, and preferably containing a developing agent as described previously. Suitable materials and addenda frequently added to such compositions are disclosed on pages 79 and 80 of the November, 1976 edition of Research Disclosure, the disclosure of which is hereby incorporated by reference. 
     The alkaline solution permeable, substantially opaque, light-reflective layer employed in certain embodiments of photographic film units used in this invention is described more fully in the November, 1976 edition of Research Disclosure, page 82, the disclosure of which is hereby incorporated by reference. 
     The supports for the photographic elements used in this invention can be any material, as long as it does not deleteriously affect the photographic properties of the film unit and is dimensionally stable. Typical flexible sheet materials are described on page 85 of the November, 1976 edition of Research Disclosure, the disclosure of which is hereby incorporated by reference. 
     While the invention has been described with reference to layers of silver halide emulsions and dye image-providing materials, dotwise coating, such as would be obtained using a gravure printing technique, could also be employed. In this technique, small dots of blue-, green- and red-sensitive emulsions have associated therewith, respectively, dots of yellow, magenta and cyan color-providing substances. After development, the transferred dyes would tend to fuse together into a continuous tone. In an alternative embodiment, the emulsions sensitive to each of three primary regions of the spectrum can be disposed as a single segmented layer, e.g., as by the use of microvessels as described in Whitmore U.S. Pat. No. 4,362,806, issued Dec. 7, 1982. 
     The silver halide emulsions useful in this invention, both negative-working and direct-positive ones, are well known to those skilled in the art and are described in Research Disclosure, Volume 176, December, 1978, Item 17643, pages 22 and 23, &#34;Emulsion preparation and types&#34;; they are usually chemically and spectrally sensitized as described on page 23, &#34;Chemical sensitization&#34;, and &#34;Spectral sensitization and desensitization&#34;, of the above article; they are optionally protected against the production of fog and stabilized against loss of sensitivity during keeping by employing the materials described on pages 24 and 25, &#34;Antifoggants and stabilizers&#34;, of the above article; they usually contain hardeners and coating aids as described on page 26, &#34;Hardeners&#34;, and pages 26 and 27, &#34;Coating aids&#34;, of the above article; they and other layers in the photographic elements used in this invention usually contain plasticizers, vehicles and filter dyes described on page 27, &#34;Plasticizers and lubricants&#34;; page 26, &#34;Vehicles and vehicle extenders&#34;; and pages 25 and 26, &#34;Absorbing and scattering materials&#34;, of the above article; they and other layers in the photographic elements used in this invention can contain addenda which are incorporated by using the procedures described on page 27, &#34;Methods of addition&#34;, of the above article; and they are usually coated and dried by using the various techniques described on pages 27 and 28, &#34;Coating and drying procedures&#34;, of the above article, the disclosures of which are hereby incorporated by reference. 
     The term &#34;nondiffusing&#34; used herein has the meaning commonly applied to the term in photography and denotes materials that for all practical purposes do not migrate or wander through organic colloid layers, such as gelatin, in the photographic elements of the invention in an alkaline medium and preferably when processed in a medium having a pH of 11 or greater. The same meaning is to be attached to the term &#34;immobile&#34;. The term &#34;diffusible&#34; as applied to the materials of this invention has the converse meaning and denotes materials having the property of diffusing effectively through the colloid layers of the photographic elements in an alkaline medium. &#34;Mobile&#34; has the same meaning as &#34;diffusible&#34;. 
     The term &#34;associated therewith&#34; as used herein is intended to mean that the materials can be in either the same or different layers, so long as the materials are accessible to one another. 
    
    
     The following examples are provided to further illustrate the invention. 
     EXAMPLE 1 
     Multicolor Photographic Test 
     A cover sheet was prepared by coating the following layers, in the order recited, on a poly(ethylene terephthalate) film support: 
     (1) an acid layer comprising poly(n-butyl acrylate-co-acrylic acid), (30:70 weight ratio equivalent to 140 meq. acid/m 2 ); and 
     (2) a timing layer comprising a 1:1 physical mixture of the following polymers coated at 4.8 g/m 2  : poly(acrylonitrile-co-vinylidene chloride-co-acrylic acid) (wt. ratio 14:79:7) and the carboxy-ester-lactone formed by cyclization of a vinyl acetate-maleic anhydride copolymer in the presence of 1-butanol to produce a partial butyl ester (ratio of acid:ester of 15:85). 
     Integral imaging-receiver (IIR) elements were prepared by coating the following layers in the order recited on a transparent poly(ethylene terephthalate) film support. Quantities are parenthetically given in grams per square meter, unless otherwise stated. 
     (1) metal containing layer of nickel sulfate .6H 2  O (0.58) and gelatin (1.1); 
     (2) image-receiving layer of poly(4-vinylpyridine) (2.2) and gelatin (2.2); 
     (3) reflecting layer of titanium dioxide (17) and gelatin (2.6); 
     (4) opaque layer of carbon black (1.9) and gelatin (1.3); 
     (5) interlayer of gelatin (0.54); 
     (6) oxidant containing layer as specified in Table 1 with gelatin (3.4) 
     (7) red-sensitive, negative-working, silver bromoiodide emulsion (1.2 silver), gelatin (1.4), cyan PRDR (0.59), incorporated reducing agent IRA (0.26), and inhibitor (0.02); 
     (8) interlayer of gelatin (2.3) and scavenger (0.43); 
     (9) green-sensitive, negative-working, silver bromoiodide emulsion (0.99 silver), gelatin (1.7), magenta PRDR (0.58), incorporated reducing agent IRA (0.28), and inhibitor (0.02); 
     (10) interlayer of gelatin (2.2) and scavenger (0.43); 
     (11) blue-sensitive, negative-working silver bromoiodide emulsion (0.99 silver), gelatin (2.2), yellow PRDR (0.55), incorporated reducing agent IRA (0.45), and inhibitor (0.01); and 
     (12) overcoat layer of gelatin (0.54). ##STR2## 
     Samples of the imaging-receiver element were exposed in a sensitometer through a graduated density test object to yield a neutral at a Status A mid-scale density of approximately 1.0. The exposed samples were then processed by rupturing a pod containing the viscous processing composition described below between the imaging-receiver element and the cover sheet described above, by using a pair of juxtaposed rollers to provide a processing gap of about 100 μm. 
     The processing composition was as follows: 
     52 g potassium hydroxide, 
     3.4 g sodium hydroxide, 
     12 g 4-hydroxymethyl-4-methyl-1-p-tolyl-3-pyrazolidinone (ETA), 
     10 g ethylenediaminetetraacetic acid, disodium salt dihydrate, 
     0.4 g lead oxide, 
     2 g sodium sulfite, 
     2 g Tamol SN® dispersant, 
     5 g potassium bromide, 
     56 g carboxymethylcellulose, 
     165 g carbon water to 1 liter. 
     Within several hours, the red, green and blue Status A density of the receiver side of the IIR&#39;s were read. After a period of 48 hours incubation at 60° C./70% RH, the densities of the same D max  and D min  areas were read again. The following results were obtained: 
     
                       TABLE 1                                                     
______________________________________                                    
          Oxidant                                                         
          Concen-                D-min                                    
Oxidant   tration         D-max/ after                                    
in        g/m.sup.2 /     D-min  Incuba-                                  
Layer 6   mmoles/m.sup.2  Fresh  tion   D-min                             
______________________________________                                    
none      --        R     1.9/0.14                                        
                                 0.76   0.62                              
(control)           G     1.9/0.12                                        
                                 0.68   0.56                              
                    B     1.7/0.14                                        
                                 0.58   0.44                              
*none     --        R       */0.16                                        
                                 0.98   0.82                              
(control)           G       */0.14                                        
                                 0.71   0.57                              
                    B       */0.14                                        
                                 0.58   0.44                              
2,5-dido- 0.54/1.2  R     1.9/0.14                                        
                                 0.46   0.32                              
decylquinone        G     1.9/0.12                                        
                                 0.39   0.27                              
                    B     1.7/0.14                                        
                                 0.37   0.23                              
2,5-dido- 1.2/2.4   R     1.9/0.15                                        
                                 0.41   0.26                              
decylquinone        G     1.9/0.13                                        
                                 0.31   0.18                              
                    B     1.7/0.15                                        
                                 0.34   0.19                              
2,5-dido- 1.6/3.6   R     1.9/0.14                                        
                                 0.29   0.15                              
decylquinone        G     1.9/0.13                                        
                                 0.24   0.11                              
                    B     1.8/0.14                                        
                                 0.29   0.15                              
2,5-dido- 2.2/4.8   R     1.8/0.14                                        
                                 0.28   0.14                              
decylquinone        G     1.9/0.12                                        
                                 0.22   0.10                              
                    B     1.7/0.14                                        
                                 0.28   0.14                              
2,5-dido- 2.2/4.8   R     1.8/0.14                                        
                                 0.29   0.15                              
decylhydro-         G     1.9/0.13                                        
                                 0.26   0.13                              
quinone             B     1.7/0.14                                        
                                 0.32   0.18                              
2,5-dido- 1.1/2.4   R     1.8/0.14                                        
                                 0.26   0.12                              
decylhydro-         G     1.9/0.12                                        
                                 0.24   0.12                              
quinone &amp;           B     1.8/0.14                                        
                                 0.30   0.16                              
2,5-dido- 1.1/2.4                                                         
decylquinone                                                              
______________________________________                                    
 *This coating also had no interlayer scavenger in layers 8 and 10. Thus, 
 the sensitometric response showed considerable color contamination.      
 
    
     The above results indicate that 2,5-didodecylquinone or its corresponding hydroquinone improve post-process D min  stability in all three imaging layers. The control without oxidant and the control without an interlayer scavenger were equally poor, thus indicating that the quinone or hydroquinone was responsible for the improvement in D min  stability. As the level of quinone oxidant was increased, greater benefit was obtained. The hydroquinone appeared only slightly less effective than the corresponding quinone. Mixtures of hydroquinone and quinone were also useful. 
     EXAMPLE 2 
     Example 1 was repeated but with the oxidants listed in Table 2. The following results were obtained. 
     
                       TABLE 2                                                     
______________________________________                                    
          Oxidant                                                         
          Concen-                D-min                                    
Oxidant   tration         D-max/ after                                    
in        g/m.sup.2 /     D-min  Incuba-                                  
Layer 6   mmoles/m.sup.2  Fresh  tion   Δ D-min                     
______________________________________                                    
none      --        R     1.9/0.14                                        
                                 0.76   0.62                              
(control)           G     1.9/0.13                                        
                                 0.68   0.56                              
                    B     1.7/0.14                                        
                                 0.58   0.44                              
Trichloro-                                                                
          0.59/1.2  R     2.1/0.15                                        
                                 0.72   0.57                              
pentadecyl-         G     2.0/0.13                                        
                                 0.64   0.51                              
hydroquinone        B     1.8/0.12                                        
                                 0.73   0.61                              
(control)                                                                 
Trichloro-                                                                
          2.1/4.8   R     2.1/0.19                                        
                                 0.72   0.53                              
pentadecyl-         G     1.9/0.12                                        
                                 0.66   0.54                              
hydroquinone        B     1.8/0.14                                        
                                 0.62   0.48                              
(control)                                                                 
Trichloro-                                                                
          2.1/4.8   R     1.8/0.15                                        
                                 0.33   0.18                              
pentadecyl-         G     1.8/0.12                                        
                                 0.28   0.16                              
quinone             B     1.7/0.15                                        
                                 0.30   0.15                              
NaIO.sub.4                                                                
          0.34/1.6  R     2.1/0.15                                        
                                 0.69   0.54                              
                    G     1.9/0.12                                        
                                 0.52   0.40                              
                    B     1.9/0.14                                        
                                 0.60   0.46                              
NaIO.sub.4                                                                
          1.0/4.8   R     2.0/0.14                                        
                                 0.23   0.09                              
                    G     1.3/0.12                                        
                                 0.18   0.06                              
                    B     1.8/0.14                                        
                                 0.26   0.12                              
NaIO.sub.3                                                                
          0.32/1.6  R     2.1/0.15                                        
                                 0.68   0.53                              
                    G     2.0/0.13                                        
                                 0.54   0.41                              
                    B     1.8/0.15                                        
                                 0.48   0.33                              
______________________________________                                    
 
    
     The above results indicate that trichloropentadecylquinone was effective in improving post-process D min  stability in all three imaging layers. The corresponding hydroquinone, however, was relatively ineffective at two levels that differed four-fold. This hydroquinone apparently had a reactivity such that it was not oxidizable under the conditions of use to the corresponding quinone. 
     While sodium periodate and sodium iodate were somewhat effective in reducing post-process D min  increase at the lower level tried, the higher level of sodium periodate provided a much greater improvement. 
     EXAMPLE 3 
     (A) A control IIR was prepared as in Example 1 except that oxidant-containing layer 6 was omitted and opaque layer 4 contained carbon (1.9 g/m 2 ) and gelatin (4 g/m 2 ). 
     (B) An IIR according to the invention was prepared similar to (A) except that layer 4 also contained 2,5-didodecylquinone at 2.2 g/m 2  (4.8 mmoles/m 2 ). 
     (C) An IIR according to the invention was prepared as in Example 1 except that oxidant-containing layer 6 was omitted, opaque layer 4 contained carbon (1.8 g/m 2 ) and gelatin (0.65 g/m 2 ), and another layer 4a was coated between layers 4 and 5 which contained 2,5-didodecylquinone (2.2 g/m 2  or 4.8 mmoles/m 2 ) and gelatin (3.4 g/m 2 ). 
     (D) An IIR according to the invention was prepared as in Example 1 except that oxidant-containing layer 6 was omitted, opaque layer 4 was split into two carbon layers, each with 0.94 g/m 2  of carbon and 0.32 g/m 2  of gelatin, and a layer of 2,5-didodecylquinone (2.2 g/m 2  or 4.8 mmoles/m 2 ) and gelatin (3.4 g/m 2 ) in-between the carbon layers. 
     Processing was as in Example 1 with the following results: 
     
                       TABLE 3                                                     
______________________________________                                    
                                 D-min                                    
                         D-max/  after                                    
        Oxidant          D-min   Incuba-                                  
IIR     Location         Fresh   tion   Δ D-min                     
______________________________________                                    
A       none       R     1.9/0.14                                         
                                 0.76   0.62                              
(control)          G     1.9/0.12                                         
                                 0.68   0.56                              
                   B     1.7/0.14                                         
                                 0.58   0.44                              
B       In Layer   R     1.7/0.14                                         
                                 0.24   0.10                              
        4          G     1.8/0.12                                         
                                 0.20   0.08                              
                   B     1.7/0.14                                         
                                 0.29   0.15                              
C       In Layer   R     1.8/0.15                                         
                                 0.26   0.11                              
        4a         G     1.9/0.13                                         
                                 0.22   0.09                              
                   B     1.8/0.15                                         
                                 0.30   0.15                              
D       In-between R     1.7/0.14                                         
                                 0.26   0.12                              
        split Layer                                                       
                   G     1.8/0.12                                         
                                 0.21   0.09                              
        4          B     1.7/0.14                                         
                                 0.30   0.16                              
______________________________________                                    
 
    
     The above results indicate that post-process D min  increase was minimized in accordance with the invention as the 2,5-didodecylquinone was positioned in other locations in the IIR, such as in the opaque carbon layer or a layer adjacent thereto. 
     The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.