IMAGEABLE ARTICLES HAVING DYE SELECTIVE INTERLAYERS

Abstract: Copolymers of acrylonitrile and vinylidene chloride are used as selective diffusion interlayers in dye-diffusive photothermographic imaging constructions. The interlayers have sufficient permeability to be useful for constructions in which cationic dyes must be diffused from an emulsion layer into an image-receiving layers, but have insufficient permeability to the unoxidized leuco dye precursor. Use of these interlayers in multicolor dye-diffusive imaging constructions increases image stability and color separation in the final image.

Description: IMAGΈABLE ARTICLES HAVING DYE SELECTIVE INTERLAYERS FIELD OF THE INVENTION This invention relates to photothermographic materials and in particular, it relates to dry silver systems capable of producing colored images. BACKGROUND OF THE ART Imaging systems capable of producing multiple colors often rely upon barrier interlayers to separate adjacent imaging chemistries. In some cases, such as those in which a dye image is formed and diffused to a receiving layer, the interlayer must not only separate imaging chemistries, but be permeable to dyes as well. Dry silver based imageable articles are one type of imaging system that employ interlayers in this manner. Dry silver compositions or emulsions are photothermographic compositions and contain a Iight-insensitive, reducible silver source; a light-sensitive silver source; and a reducing agent for the light-insensitive, reducible silver source. The light-sensitive material is generally photographic silver halide which must be in catalytic proximity to the light-insensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the Iight-insensitive, reducible silver source by the reducing agent. It has been long understood that silver halide is a catalyst for the reduction of silver ions and that the silver-generating, light-sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as by partial metathesis of the light-insensitive, reducible silver source with a halogen-containing source; coprecipitation of silver halide and light-insensitive, reducible silver source material; and other methods that intimately associate the silver halide and the silver source. In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms. The image-wise distribution of these clusters of silver atoms is known in the art as a latent image as this latent image generally is not visible by ordinary means and the light-sensitive article must be further processed in order to produce a visual image. The visual image is produced by the catalytic reduction of silver ions which are in catalytic proximity to the silver halide grains bearing the latent image. In color dry silver imaging systems, a leuco dye or other color forming system is typically incorporated as a reducing agent for the light-insensitive, reducible silver source, generally in combination with a spectral sensitizcr for the silver halide. The leuco dye is oxidized upon development, thereby giving a colored image. In full color constructions, spectrally sensitized emulsion layers are typically coated onto a substrate and separated by one or more barrier interlayers. Residual silver stain is a major problem with certain dry silver color constructions known in the art. This has been overcome by causing the developed dye image to diffuse from the dry silver layer to an image-receiving layer that is then stripped from the emulsion layer (s). In this case, a barrier interlayer must serve the dual roles of separating the chemistries of neighboring emulsion layers and allowing diffusion of the dye image under thermal processing conditions. In addition to the dye image, other components of the emulsion layers also diffuse to the image-receiving layer under thermal processing conditions. In this regard, unoxidized leuco dyes from the emulsion layers tend to diffuse to the image-receiving layer where they oxidize over time, leading to low print stability and poor color separation. Therefore, there is a need for interlayers which allow dye images, but not neutral leuco dyes, to diffuse to the image-receiving layer. Depending on the particular ingredients of a given dry silver layer, the development may be best carried out, for example, under acidic or basic conditions. When multiple dry silver layers with incompatible developing chemistries are employed, it is very difficult to keep development conditions within one dry silver layer from affecting the development of nearby or adjacent dry silver layers. As a result, it is often advantageous to coat dry silver layers with different developing conditions on opposite sides of a transparent substrate. U.S. Pat. No. 4,594,307 discloses a heat developable photographic material that produces a pure and stable dye image by the oxidation-reduction reaction between a reducible, Iight-insensitive organic silver salt and a leuco dye reducing agent wherein the dye formed is transferred to an image-receiving layer by continuing the heating for development in order to separate the dye formed from the silver images and other residual chemicals. The generation of color dry silver images has been accomplished using microencapsulated constructions and tri-pack (yellow/magenta/cyan) multilayer constructions, such as those disclosed in U.S. Pat. Nos. 4,883,747 and 4,923,792. The cited patents above employed Gantrez S-97 polystyrene, Vinol 523 partially hydrolyzed polyvinyl alcohol, and Butvar B-76 polyvinyl butyral as barrier interlayers. These constructions generally have substantial silver and sensitizer stain present that affects the image color separation. The stain problem can be overcome by causing the developed dye image to diffuse from the image-forming layers into a receptor layer that is subsequently stripped from the rest of the construction. The success of this type of approach depends in large part on the ability of barrier interlayers positioned between the image-forming layers of the tri-pack construction to selectively permit migration of the image forming dyes while controlling the migration of other image-forming layer components, particularly unoxidized neutral leuco dyes. U.S. Pat. Nos. 4,021,240; 4,460,681; and 5,077,188 disclose the use of multiple image-forming layers separated by barrier interlayers. SUMMARY OF THE INVENTION By the present invention, it has been discovered that copolymers of acrylonitrile and vinylidene chloride are useful as interlayers in dye-diffusive imageable articles employing cationic and neutral dyes and that these copolymers have the ability to selectively allow cationic dye diffusion while hindering neutral leuco dye diffusion. In one embodiment, the present invention provides imageable articles comprising: (a) an image-forming layer comprising a leuco dye which forms a cationic dye upon oxidation; (b) an image-receiving layer; and (c) a polymeric interlayer interposed between the image-forming and image-receiving layers, the polymeric interlayer comprising an acrylonitrile-vinylidene chloride copolymer. In another embodiment, the present invention provides imageable articles comprising a substrate coated on one side thereof with an image-receiving layer, the image-receiving layer having coated thereon at least one image-forming layer separated from the image receiving layer by a polymeric interlayer, the polymeric interlayer comprising a copolymer of acrylonitrile and vinylidene chloride and the image-forming layer comprising a neutrally charged leuco dye which forms a cationic dye upon oxidation. In still another embodiment, the present invention provides a method of purifying a mixture comprising a neutral leuco dye and a cationic dye, the method comprising heating the mixture comprising neutral leuco dye and cationic dye while in intimate contact with a layer comprising a copolymer of acrylonitrile and vinylidene chloride, thereby causing the cationic dye to diffuse through the layer away from the neutral leuco dye. By the phase "intimate contact", it is meant that the mixture containing the neutral leuco and cationic dyes must be in direct contact with the layer, typically an interlayer, comprising the acrylonitrile-vinylidene chloride copolymer. For example, the dye-containing mixture may be overcoated with the polymeric layer. In another possible arrangement, the dye-containing mixture is applied to one substrate and the polymeric layer is applied to another substrate and then the two structures are laminated together such that the dye-containing mixture and the polymeric layer are in direct contact with one another. DETAILED DESCRIPTION OF THE INVENTION The present invention provides imageable articles having improved image stability which comprise: (a) an image-forming layer comprising a neutral leuco dye which generates a cationic dye upon oxidation; (b) an image-receiving layer; and (c) a polymeric interlayer interposed between the image-forming and image- receiving layers, the polymeric interlayer comprising a copolymer of acrylonitrile and vinylidene chloride. While single color applications are envisioned, the greatest benefit of the present invention may be obtained in multicolor or full color applications. These latter applications typically comprise a substrate coated with a dye receiving layer, the dye receiving layer having coated thereon a plurality of imaging layers separated by polymeric interlayers. At least one of said interlayers comprises a copolymer of acrylonitrile and vinylidene chloride. Alternatively, the image-receiving layer may be supplied as an external component carried on a second substrate that is brought into contact ( i.e., laminated) with a first substrate bearing an image-forming layer during processing such that the dye image is transferred from the first substrate to the image-receiving layer. In that case, the laminated construction constitutes an imaged construction according to the present invention. Image-Forming Layer The image forming layer may be of any type known in the imaging art in which a colored dye image is formed by oxidation of a neutral leuco dye to form a cationic dye. In a preferred embodiment, the image-forming layer(s) comprised) a dry silver composition which comprises an intimate mixture of a light-sensitive silver halide; a light-insensitive, reducible silver source such as a silver salt of an organic acid (e.g., silver behenate, silver benzimidazolate, or silver saccharine); and an auxiliary reducing agent. As used herein the phrase "auxiliary reducing agent" refers to an additional reducing agent (e.g., phenol, hindered phenol, methyl gallate, catechol, pyrogallol, hydroquinone, etc.) for the light-insensitive, reducible source of silver in addition to the leuco dye which also functions as a reducing agent for silver ion. Normally, dry silver compositions further comprise a spectral sensitizer. Such a mixture is usually prepared in a solvent as a dispersion that is spread as a layer on a suitable substrate. When dry, the layer is exposed to a light image and thereafter, a reproduction of the image is developed by heating the coated substrate. Imaging layer(s) of the present invention may comprise a single coated layer or a plurality of sequentially coated sublayers in which the various components are dispersed. In cases where the imaging layers comprise a plurality of sublayers, the sublayer containing the silver halide is referred to as an emulsion layer. Silver Halide Silver halides known in the art for use in photothermography are useful in the present invention and include, but are not limited to, silver chloride, silver chlorobromide, silver chloroiodide, silver bromide, silver iodobromide, silver chloroiodobromide, and silver iodide. The silver halide used in the present invention may be used as is. However, it may be chemically sensitized with a chemical sensitizing agent such as compounds of sulfur, selenium, or tellurium, etc.; compounds of gold, platinum, palladium, rhodium or iridium, etc.; a reducing agent such as tin halide, etc.; or a combination of the foregoing. Details thereof are described in James, T.H. The Theory of the Photographic Process, Fourth Ed.; MacMillan: New York, 1977; pp. 149-169. The light sensitive silver halide used in the present invention is preferably employed in the range of about .01-15 weight percent, and more preferably in the range of about 0.1 to 10 weight percent, based upon the total weight of each imaging layer in which the silver halide is present. Sensitizer The sensitizer employed in the dry silver composition may be any dye known in the photographic art to spectrally sensitize silver halide. Non-limiting examples of sensitizing dyes that can be employed include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, holopolar cyanine dyes, hemicyanine dyes, styryl dyes, and hemioxonol dyes. Of these dyes, cyanine dyes, merocyanine dyes, and complex merocyanine dyes are preferred. An appropriate amount of sensitizing dye added is generally in the range of from about 10 "10 to 10 "1 mole, and preferably from 10 "8 to 10 "3 mole per mole of silver halide. Light-Insensitive, Reducible Organic Silver Salt ' 5 The light-insensitive, reducible organic silver salt that can be used in the present invention is a silver salt that is comparatively stable to light and which forms a silver image by reacting with the above disclosed leuco compound or an auxiliary reducing agent that is coexisting with the leuco compound, if desired, when it is heated to a temperature of above about 80° C, and preferably, above 10 about 100° C in the presence of exposed silver halide. Suitable organic silver salts include silver salts of organic compounds having a carboxyl group. Preferred examples thereof include silver salts of aliphatic and aromatic carboxylic acids. Preferred examples of silver salts of aliphatic carboxylic acids include silver behenate, silver stearate, silver oleate, silver laurate, silver 15 caproate, silver myristate, silver palmitatc, silver alcale, silver fumarate, silver tartarate, silver Iinoleate, silver butyrate, silver camphorate, and mixtures thereof, etc. Silver salts that are substituted with a halogen atom or a hydroxyl group can also be effectively used. Preferred examples of silver salts of aromatic carboxylic acids and other carboxyl group-containing compounds include silver 20 benzoate, a silver-substituted benzoate such as silver 3,5-dihydroxybenzoate, silver ø-methylbenzoate, silver m-methylbenzoate, silver -methylbenzoate, silver 2,4-dichlorobenzoate, silver acetamidobenzoate, silver /7-phenyl benzoate, etc., silver gallate, silver tannate, silver phthalate, silver tcrephthalate, silver salicylate, silver phenylacetate, silver pyromellitate, silver salts of 25 3-carboxymethyI-4-methyl-4-thiazoline-2-thiones or the like as disclosed in U.S. Pat. No. 3,785,830 and silver salts of aliphatic carboxylic acids containing a thioether group as described in U.S. Pat. No. 3,330,663. Silver salts of compounds containing mercapto or thione groups and derivatives thereof can also ' be used. Preferred examples of these compounds include silver 3-mercapto-4- 30 phenyl-l,2,4-triazolate, silver 2-mercaptobenzimidazoIate, silver 2-mercapto-5- aminothiadiazolate, silver 2-(S-ethyIglycolamido)benzothiazolate; silver salts of thioglycolic acids such as silver salts of S-alkyl thioglycolic acids (wherein the alkyl group has from 12 to 22 carbon atoms); silver salts of dithiocarboxylic acids such as silver dithioacetate, silver thioamidoate, silver l-methyl-2-phenyl- 4-thiopyridine-5-carboxylate, silver triazinethiolate, silver 2-sulfidobenzoxazole; and silver salts as disclosed in U.S. Pat. No. 4,123,274. Furthermore, silver salts of a compound containing an amino group can be used. Preferred examples of these compounds include silver salts of benzotriazoles, such as silver benzotriazolate; silver salts of alkyl-substituted benzotriazoles such as silver methylbenzotriazolate, etc.; silver salts of halogen-substituted benzotriazoles such as silver 5-chlorobenzotriazolate, etc.; silver salts of carboimidobenzotriazoles, etc.; silver salts of 1 ,2,4-triazoles and 1-H-tctrazoles as described in U.S. Pat. No. 4,220,709; silver salts of imidazoles; and the like. The silver halide and the organic silver salt that form a starting point of development should be in reactive association (i.e., in the same layer, in adjacent layers, or in layers separated from each other by an intermediate layer having a thickness of less than 1 micron). It is preferred that the silver halide and the organic silver salt are present in the same layer. The silver halide and the organic silver salt that are separately formed in a binder can be mixed before use to prepare a coating solution, but it is also effective to blend both of them in a ball mill for a long time. Further, it is effective to use a process which comprises adding a halogen-containing compound in the organic silver salt prepared to partially convert the silver of the organic silver salt to silver halide. Methods of preparing these silver halide and organic silver salts and manners of blending them are described in Research Disclosure No. 17029 and U.S. Pat. No. 3,700,458. The Iight-insensitive, reducible source of silver is preferably present in an amount of from 0.1 to 50 weight percent, and more preferably from about 1-5 weight percent, based upon the total weight of each imaging layer(s) in which the silver source is present. A suitable coating amount of the light-sensitive silver halide and the organic silver salt employed in the present invention is in a total from 50 mg to 10 g/m calculated as an amount of silver, as disclosed, for example, in U.S. Pat. No. 4,478,927. Leuco Dye Suitable leuco dyes for use in the present invention are compounds that oxidize to form a dye image. In practice of the present invention, at least one imaging layer must comprise a leuco form of a cationic dye and at least one other imaging layer must comprise a leuco form of a neutral dye. Preferred neutral leuco dyes are phenolic leuco dyes such as 2-(3,5-di-/-butyl-4-hydroxyphenyI)-4,5-3-diphenylimidazole or bis(3,5-di-/ -butyl-4-hydroxyphenyI)phenylmethane. Such phenolic leuco dyes useful in practice of the present invention are disclosed in U.S. Pat. Nos. 4,374,921; 4,460,681; 4,594,307; and 4,780,010. The leuco dyes used in the present invention may be any colorless or lightly colored compound that forms a visible dye upon oxidation. The compound must be oxidizable to a colored state. Compounds that are both pH sensitive and oxidizable to a colored state arc useful, but not preferred, while compounds only sensitive to changes in pll are not included within the term "leuco dyes" since they are not oxidizable to a colored form. The dyes formed from the leuco dye in the various color-forming layers should, of course, be different. A difference of at least 60 nm in reflective maximum absorbance is preferred. More preferably, the absorbance maximum of dyes formed will differ by at least 80-100 nm. When three dyes are to be formed, two should preferably differ by at least these minimums and the third should preferably differ from at least one of the other dyes by at least 125 and more preferably, by at least 150 nm. Any leuco dye capable of being oxidized by silver ion to form a visible dye is useful in the present invention as previously noted. Leuco dyes such as those disclosed in U.S. Pat. Nos. 3,442,224; 4,021,250; 4,022,617 and 4,368,247 are also useful in the present invention. Other leuco dyes may be used in imaging layers as well, for example, benzylidene leuco compounds disclosed in U.S. Pat÷ No. 4,923,792. The reduced form of the dyes must absorb less strongly in the visible region of the electromagnetic spectrum and be oxidized by silver ions back to the original colored form of the dye. Benzylidene dyes have extremely sharp spectral char¬ acteristics giving high color purity of low gray level. The dyes have large extinction coefficients, typically on the order of 10 to 10 5 and possess good compatibility and heat stability. The dyes are readily synthesized and the reduced leuco forms of the compounds are very stable. The dyes generated by the leuco compounds employed in the photothermographic elements of the present invention are known and are disclosed, for example, in The Colour Index; The Society of Dyes and Colourists: Yorkshire, England, 1971; Vol. 4, p. 4437; and Venkataraman, K. Tlie Chemistry of Synthetic Dyes; Academic Press: New York, 1952; Vol. 2, p. 1206; U.S. Pat. No. 4,478,927, and Hamer, F.M. Tiie Cyanine Dyes and Related Compounds; Interscience Publishers: New York, 1964; p. 492. The leuco compounds may readily be synthesized by techniques known in the art. There arc many known methods of synthesis from precursors since the reaction is a simple two-step hydrogen reduction. Suitable methods are disclosed, for example, in: F.X. Smith et al. Tetrahedron Lett. 1983, 24(45), 4951-4954; X. Huang. L. Xe, Synth. Commun. 1986, 1<J(13) 1701-1707; H. Zimmer et al. J. Org. Chem. 1960, 25, 1234-5; M. Sekiya et al. Chem. Pharm. Bull. 1972, 20(2), 343; Ibid 1974, 22(2), 448; and T. Sohda et al. Chem. Pharm. Bull. 1983, 31(2) 560-5. Further, as other image forming materials, materials where the mobility of the compound having a dye part changes as a result of an oxidation-reduction reaction with silver halide, or an organic silver salt at high temperature, can be used, as disclosed in Japanese Patent Application No. 165054 (1984). Many of the above-disclosed materials are materials wherein an image-wise distribution of mobile dyes corresponding to exposure is formed in the light-sensitive material by heat development. Processes of obtaining visible images by transferring the dyes of the image to a dye fixing material (diffusion transfer) have been described in the above-described cited patents and Japanese Patent Application Nos. 168,439 (1984) and 182,447 (1984). The total amount of leuco dye utilized in the present invention should preferably be in the range of 1-50 weight percent, and more preferably in the range of 5-20 weight percent, based upon the total weight of each individual layer in which the leuco dye is(are) employed. When the heat developable light-sensitive material used in this invention is heat developed in a substantially water-free condition after, or simultaneously with, image-wise exposure, a mobile dye image is obtained simultaneously with the formation of a silver image either in exposed areas or in unexposed areas with exposed light-sensitive silver halide. The light-sensitive silver halide and the organic silver salt oxidizing agent used in the present invention are generally added to at least one binder as disclosed herein below. Further, the dye releasing redox compound is dispersed in the binder. The binder(s) that can be used in the present invention can be employed individually or in combination with one another. The binder may be hydrophilic or hydrophobic. A typical hydrophilic binder is a transparent or translucent hydrophilic colloid, examples of which include a natural substance, for example, a protein such as gelatin, a gelatin derivative, a cellulose derivative, etc., a polysaccharide such as starch, gum arabic, pullulan, dextrin, etc., and a synthetic polymer, for example, 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 material. Preferably, the polymeric binder is present in an amount in the range from 1-99 weight percent, and more preferably, from 20-80 weight percent in each layer in which the polymeric binder is employed. The coating amount of the binder used in the present invention is 20 g or less per m 1 ; preferably, 10 g or less per and more preferably, 7 g or less per m 2 . The preferred photothermographic silver containing polymer is polyvinyl butyral, but ethyl cellulose, methacrylate copolymers, maleic anhydride ester copolymers, polystyrene, and butadiene-styrene copolymers can be used (where applicable) according to the particular solvents used. In the photographic light-sensitive material and the dye fixing material utilized in the present invention, the photographic emulsion layer and other binder layers may contain inorganic or organic hardeners. It is possible to use chromium salts such as chromium alum, chromium acetate, etc.; aldehydes such as formaldehyde, glyoxal, glutaraldehyde, etc.; N-methylol compounds such as dimethylolurea, methylol dimethylhydantoin, etc.; dioxane derivatives such as 2,3-dihydroxydioxane, etc.; active vinyl compounds such as 1,3,5- triacryloylhexahydro-s-triazine, l,3-vinyIsulfonyI-2-propanol,etc; active halogen compounds such as 2,4-dichloro-6-hydroxy--?-triazine, etc.; and mucohalogenic acids such as mucochloric acid, mucophenoxychloric acid, etc.. Image-Receiving Layer Dyes generated during thermal development of light-exposed regions of the emulsion layers migrate under development conditions into a dye receiving layer wherein they are retained. The dye receiving layer may be composed of a polymeric material having affinity for the dyes employed. Necessarily, it will vary depending on the ionic or neutral characteristics of the dyes. Examples of organic polymeric materials used in the dye receiving material of this invention include polystyrene having a molecular weight of 2,000 to 85,000, polystyrene derivatives having substituents with not more than 4 carbon atoms, polyvinyl cyclohexene, polydivinylbenzene, polyvinylpyrrolidine, polyvinylcarbazole, polyallylbenzene, polyvinyl alcohol, polyacetals such as polyvinyl formal and polyvinyl butyral, polyvinyl chloride, chlorinated polyethylene, polytrifluoroethylene, polyacrylonitrile, poly(N,N- dimethylallyiamide), polyacrylates having a //-cyanophenyl group, a pentachlorophenyl group or a 2,4-dichIorophenyI group, poly(acryl chloroacrylate), poly (methyl methacrylate), poly (ethyl methacrylate), poly(propyl methacrylate), poly(isopropyl methacrylate), poly(isobutyl methacrylate), poly(/erf-butyl methacrylate), poly(cyclohexyl methacrylate), polyethylene glycol dimethacrylate, poly(cyanoethyl methacrylate), polyesters such as polyethylene terephthalate, polysulfone Bisphenol A polycarbonate, polycarbonates, polyanhydrides, polyamides, and cellulose acetate. The synthetic polymers described in "Polymer Handbook", 2nd edition (edited by J. Brandrup and E. H. Immergut, published by John Wiley and Sons, Inc.) are also useful. These polymeric substances may be used singly or a plurality of them may be used in the form of a copolymer. Interlayers Interlayers employed in the present invention are selected from polymeric materials that are selectively permeable to the dyes used to form the developed image. They are preferably coated from solvents in which the previously coated emulsion layer is not soluble. At least one of the interlayers employed in the present invention must be a copolymer of acrylonitrile and vinylidene chloride. The copolymer may be either a block or random copolymer. The mole ratio of acrylonitrile to vinylidene chloride should be from about 0.2 to 0.95:1; preferably, from about 0.4 to 0.9:1; and most preferably, from about 0.5 to 0.8:1. Interlayers of the present invention provide a means for improving image stability and optionally color separation of cationic dyes. This is accomplished by interposing interlayers comprising copolymers of acrylonitrile and vinylidene chloride between image-forming layer(s) containing leuco dyes (that oxidize to form cationic dyes) and the image-receiving layer. In this arrangement, the cationic dyes preferentially diffuse to the image-receiving layer relative to the neutral, unoxidized leuco dyes. The lessened amount of neutral leuco dye in the image-receiving layer improves long term print stability. In a preferred embodiment of the present invention, the first coated interlayer comprises a copolymer of acrylonitrile and vinylidene chloride copolymer and the first coated imaging layer contains a leuco dye that forms a cationic dye upon oxidation. These polymers can be used as interlayers in a construction of an at least two color, and preferably of an at least three color photothermographic color recording system. This type of construction with the proper solvent selection is conducive to the use of simultaneous multiple coating techniques with good color separation. It also enables the simultaneous thermal development of at least two or more individual color-forming photothermographic systems having different chemistries, but similar thermal properties. Preferably, the interlayers employed in the imageable articles of the present invention should be impermeable to the solvent employed in any layers subsequently coated onto it. The test for determining if an interlayer polymer is impermeable to the solvent of the next layer can be simply performed. First, a layer containing a sensitized, halidized silver salt of a fatty carboxylic (for example 10-32 carbon atoms, preferably 12-28 carbon atoms) acid and poly(vinyI butyral) polymer is coated onto a suitable substrate. A second coating comprising a candidate interlayer polymer is applied after the first coating has dried. The last layer contains the appropriate solvent, a color forming developer, and toning agent reactant. The dried coatings are given an excessive light exposure and then heated for 60 seconds at 120°-130°C. The test is positive if no color or image is formed. The imageable elements of the present invention optionally may be overcoated with a protective coating. Suitable materials for the protective coating include, but are not limited to, polymers that are insoluble in aqueous systems, soluble in some organic solvents, and impervious to certain other organic solvents. The "barrier" polymer, which is the fourth layer and preferably contains the color reactants, is normally a methyl methacrylate polymer (preferably a hard polymer or copolymer with a Tukon hardness of 20 or more), copolymer, or blend with other polymers or copolymers (for example, copolymers with /i-butyl acrylate, butyl methacrylate, and other acrylates such as acrylic acid, methacrylic acid, acrylic anhydride, and the like), polystyrene, or a combination of a polyvinyl chloride terpolymer with a butadiene-styrene copolymer. The barrier layer may be crosslinked also. This would be preferably done by the inclusion of a latent or activatable crosslinking agent. Crosslinking could then be effected after coating. The theory of this process is essentially the same for a light-sensitive material comprising a negative emulsion and a light-sensitive material comprising a direct positive emulsion and only differs in that the portion to be developed is an exposed area in one and an unexposed area in the other. Accordingly, even when a direct positive emulsion is used, a dye image providing good color reproducibility is obtained in the same way as in the case of a negative emulsion. Heating in a substantially water-free condition, as used herein, means heating at a temperature of 80° to 250° C. The term "substantially water-free condition" means that the reaction system is in equilibrium with water in the air, and that water for inducing or promoting the reaction is not particularly or positively supplied from exterior to the element. Such a condition is described at page 374 of "The Theory of the Photographic Process", 4th Edition (T. H. James, published by Macmillan Co.). The coating solution used in the present invention may be prepared by separately forming a silver halide and an organic silver salt oxidizing agent and mixing them before use. It is also effective to mix the two in a ball mill for a long period of time. Another effective method comprises adding a halogen- containing compound to the prepared organic silver salt oxidizing agent and forming silver halide by the reaction of the halogen-containing compound with silver in the organic silver salt oxidizing agent. The various layers comprising the imageable articles of the present invention may contain surface active agents for various purposes; for example, as coating aids or for prevention of electrical charging, improvement of lubricating properties, emulsification, prevention of adhesion, improvement of photographic properties (for example, acceleration of development providing hard tones or sensitization), etc. For example, it is possible to use nonionic surface active agents such as saponin (steroid), alkylene oxide derivatives (for example, polyethylene glycol/polypropylene glycol condensates, polyethylene glycol alkyl ethers or polyethylene glycol alkylaryl ethers, polyethylene glycol esters, polyethylene glycol sorbitan esters, polyalkylene glycol alkyl amines or amides, polyethylene oxide adducts of silicone, etc.), glycidol derivatives (for example, alkenylsuccinic acid polyglycerides, alkylphenol polyglycerides, etc.), polyhydric alcohol aliphatic acid esters or saccharide alkyl esters, etc.; anionic surface active agents containing acid groups such as a carboxyl group, a sulfo group, a phospho group, a sulfate group, a phosphate group, etc., such as alkylcarboxylic acid salts, alkylsulfonic acid salts, alkylbenzenesulfonic acid salts, alkylnaphthalenesulfonic acid salts, alkyl sulfuric acid esters, alkylphosphoric acid esters, N-acyl-N-alkyltaurines, sulfosuccinic acid esters, sulfoalkyl polyoxyethylene alkyl phenyl ethers, polyoxyethylene alkylphosphoric acid esters, etc.; ampholytic surface active agents such as amino acids, aminoalkylsulfonic acids, aminoalkylsulfuric acid esters or phosphoric acid esters, alkyl betaines, amine oxides, etc.; and cationic surface active agents such as alkylamine salts, aliphatic or aromatic quaternary ammonium salts, heterocyclic quaternary ammonium salts such as pyridinium salts, imidazolium salts, etc., aliphatic or heterocyclic phosphonium salts, aliphatic or heterocyclic sulfonium salts, etc. Of the above-disclosed surface active agents, polyethylene glycol type nonionic surface active agents having a repeating unit of ethylene oxide in their molecules are often preferably incorporated into the light-sensitive material. It is particularly preferred that the molecule contains 5 or more of the recurring units of ethylene oxide. The light-sensitive material used in the present invention may contain, if desired or necessary, various additives known for heat developable light-sensitive materials and may have a layer or layers other than the light-sensitive layer such as, for example, an antistatic layer, an electrically conductive layer, a protective layer, an intermediate layer, an antihalation layer, a strippable layer, etc. The imageable articles of the present invention are coated on a substrate. Suitable substrates include rigid and flexible substrates; metals (for example, steel and aluminum plates, sheets, and foils); films or plates composed of various film-forming synthetic or high polymers including addition polymers (for example, polyvinylidene chloride, polyvinyl chloride, polyvinyl acetate, polystyrene, polyisobutylene polymers, and copolymers thereof)* and linear condensation polymers (for example, polyethylene terephthalate, polyhexamethylene adipate, and polyhexamethylene adipamide/adipate); nonwoven wood byproduct based substrates such as paper and cardboard; and glass. Substrates may be transparent or opaque. Especially useful substrates are films of cellulose acetate films such as cellulose triacetate or diacetate, films of polyamides derived from a combination of heptamethylenediamine and terephthalic acid, a combination of fluorenedipropylamine and adipic acid, a combination of hexamethylenediamine and diphenic acid, and a combination of hexamethylenediamine and isophthalic acid, films of polyesters derived from a combination of diethylene glycol and diphenylcarboxylic acid and a combination of bis- -carboxyphenoxybutane and ethylene glycol, a polyethylene terephthalate film, and a polycarbonate film. The films may be modified; for example, polyethylene terephthalate films modified by such modifiers as cyclohexane dimethanol, isophthalic acid, methoxypolyethylene glycol, or 1,2-dicarbomethoxybenzenesuIfonic acid are effective. The substrate used for the light-sensitive material in the present invention is one that has good dimensional stability at the processing temperature. The polyesters described in U.S. Pat. No. 3,634,089 are preferably used. More preferably, a polyethylene terephthalate film is used. If necessary, two or more layers may be applied at the same time as disclosed in U.S. Pat. No. 2,761,791 and British Pat. No. 837,095. In the present invention, the latent image obtained after exposure of the heat-sensitive material can be developed by heating the material at a moderately elevated temperature of, for example, about 80° to about 250°C, for about 0.5 second to about 300 seconds. By increasing or decreasing the heating time, the temperature may be higher or lower within the above range. Temperatures in the range of about 110° to about 160°C are especially useful. Heating may be carried out by typical heating means such as a hot plate, an iron, a hot roller, a heat generator using carbon or titanium white, or the like. The imageable articles of the present invention are prepared by coating methods generally known in the art and disclosed in U.S. Pat. Nos. 4,452,883 and 2,761,791 and British Patent No. 837,095. Heating for transfer of the dyes can be effected by using the same heating means as exemplified for the heat development. To increase the quality of the dye image transferred to the dye receiving layer, it is preferred to prevent an increase in fogging by the occurrence of unnecessary development during dye transfer. For this purpose, it is especially effective to include a compound that reacts with the silver halide and/or can have the silver halide adsorbed thereon as a development stopping agent and/or an antifoggant in any one of the layers constituting the dye receiving material. Such a compound is preferably included in the dye receiving layer or a layer provided above the dye receiving layer, such as a protective layer, because it rapidly inhibits excessive development of the light-sensitive layer during transfer of the dye by heating and a sharp and clear dye image can be obtained. Such compounds include, for example, a nitrogen- containing heterocyclic compound, preferably a 5- or 6-membered heterocyclic compound containing a nitrogen atom. The following non-limiting examples further illustrate the present invention. EXAMPLES The materials used in the following examples were obtained from standard commercial vendors, such as Aldrich Chemical Co. (Milwaukee, WI), unless otherwise noted. The dyes referred to in the following examples have the following structural formulae: Red Sensitizing Dye A (prepared according to U.S. Pat. No. 3,719,495) Oil Blue A Basic Blue 3 Solvent Yellow 56 Example 1 This example describes preparation of acrylonitrile-vinylidene chloride copolymers utilized in the present invention. The monomers were distilled from calcium hydride under nitrogen prior to polymerization. A solution of 60 ml vinylidene chloride, 33 ml acrylonitrile, and 160 mg azobis(isobutyronitrile) was heated to reflux for two days. The resultant solution was precipitated by dropwise addition to methanol. The precipitated polymer was filtered, redissolved in tetrahydrofuran, and reprecipitated by addition to methanol. Following filtration, the purified copolymer was dried in vacuo. The resultant polymer had the following composition: 50 moI% styrene and 50 moI% vinylidene chloride. The conversion of monomers was about 4%. Example 2 Diffusion of dyes through copolymer films of the invention were conducted as follows: The copolymers were coated from a 10 wt% solution in tetrahydrofuran as a 1 mil thick layer on a transparent substrate (type 0R477400 primed polyester, 3M Company, St. Paul, MN) and oven dried at 80°C. A second layer consisting of 10 wt% poly(vinyI butyral) and 1 wt % dye in methanol was coated onto the first layer. The coatings were air dried. The coatings were then heated to 140° C for varying times. Absorbance measurements were made and the top layer was removed by repeated application and removal of Scotch brand Magic transparent tape (3M Company, St. Paul, MN), and the absorbance of the coated substrate was measured. The percent dye diffusion was calculated as: % Dye Transfer = A^^ , ayer) / A^^ , ayers) x 100% Dyes used for evaluation of polymer diffusivity were Oil Blue A (DuPont Company, Wilmington, DE), Basic Blue 3 (a cationic oxazine dye), and Solvent Yellow 56 (an azo dye). The results for several copolymers are presented in Table 1. Table 1 Examples 3 and 4 A 15% solution of a copolymer of vinyl chloride and vinyl acetate (9:1) in methyl ethyl ketone was coated at a wet thickness of 0.08 mm onto an opaque polyester film substrate and dried in an oven at a temperature of 80° C for five minutes to form an image-receiving layer. A dispersion of silver behenate half soap (1 mole silver behenate to 1 mole behenic acid, 10% solids) in toluene was made by a homogenization process. A 110 g portion of the 10% half soap dispersion was diluted with 380 g ethanol. Then 0.4 g poly (vinyl butyral) was added to the dilute dispersion and dissolved. Mercuric bromide (10 ml of a solution containing 1.8 g mercuric bromide in 100 ml methanol) was added to the dispersion with stirring. Additional poly(vinyI butyral) (26 g) having a poly(vinyI alcohol) content in the range of 9-13% was added to the dispersion, hereinafter referred to as Dispersion A. TO 3M Fluorad FC-431 surfactant (3 drops, a fiuorochemical coating additive employed as a stripping agent) was added to a 25g aliquot of Dispersion A and mixed. The resultant dispersion was coated onto the image-receiving layer at a wet thickness of 0.08 mm and dried in an oven at a temperature of 80° C for 5 min to form a strippable blank emulsion layer. The following polymer solutions were coated over the blank emulsion layer at a wet thickness of 0.08 mm and dried in an air oven at a temperature of 80° C for 5 min to form an interlayer: Comparative Example A: An interlayer coating solution was prepared by dissolving 3.5% of a copolymer of vinylidene chloride and vinyl chloride (1:20, obtained from Scientific Polymer Products, Inc.) in tetrahydrofuran. Example 3: An interlayer coating solution was prepared by dissolving 3.5% of a copolymer of polyacrylonitrile and vinylidene chloride (1:4) in tetrahydrofuran. Example 4: An interlayer coating solution was prepared by dissolving 3.5% of a copolymer of polyacrylonitrile and vinylidene chloride (1:1) in tetrahydrofuran. Cya n leuco dye (0.3g , 3 , 6-b is ( d ieth ylam in o) -9- (4- methyIbenzoyl)phenoxazine obtained from Hodogaya Chemical as L-704) was dissolved in 3 ml toluene. The solution was mixed with 1 ml of a solution containing 0.005 g of a red sensitizing Dye A in 150 ml toluene and 50 ml methanol, 0.1 g 4-methylphthaIic acid, and a 25 g aliquot of Dispersion A. The resulting mixed dispersion was coated onto the interlayer at a wet thickness of 0.13 mm and dried in an oven at a temperature of 80° C for 5 min to form a cyan emulsion layer. Strips were then cut from the resultant coated sheets and exposed to light in an EG&G sensitometer through a Wratten 25 red filter for 10 "3 sec to produce heat-developable latent images in the emulsion layer. The exposed sheets were heat-developed at a temperature of 138 C on a heat blanket for 30 sec. The portion of the element containing the photothermographic emulsion layers and the interlayer was then stripped away from the image-receiving layer. Cyan dye images corresponding to the red light exposed area of the sheets were observed to have been transferred to the image-receiving layer. Additionally, N-bromosuccinimide solution (0.08 g in 50 ml acetone and 50 ml toluene) was dropped (approximately 0.015 ml) on the non-exposed area of the image receiving layer. The leuco dye which had been transferred to the image receiving layer through the emulsion layers and the interlayer was oxidized by the N-bromosuccinimide solution and formed cyan dye. The treated area was dried at room temperature for 30 min. The optical density of the oxidized dye by the oxidizing agent was measured by the densitometer using a red filter to determine the diffusion of the leuco dye. The results are given in Table 2. Table 2 Sample Optical Density of Exposed Optical Density of Non- Area (corresponds to exposed area (corresponds diffusion of oxidized dye) to diffusion of leuco dye) 1.52 0.44 0.91 The results in the preceding table show that the copolymers of Examples 3 and 4 decrease the diffusion of neutral leuco dye while permitting the diffusion of the oxidized (cationic) form of the dye relative to Comparative Example A. Reasonable modifications and variations are possible from the foregoing disclosure without departing from either the spirit or scope of the present invention as defined by the claims.

Claims: WHAT IS CLAIMED IS: 1. An imageable article comprising: (a) an image-forming layer comprising a neutral leuco dye which generates a cationic dye upon oxidation; (b) an image-receiving layer, and (c) a polymeric interlayer interposed between said image-forming and image-receiving layers, said polymeric interlayer comprising an acrylonitrile-vinylidene chloride copolymer. 2. The imageable article according to Claim 1 wherein said imaging layer further comprises a light-insensitive, reducible silver source; a light- sensitive silver halide; a polymeric binder; and a sensitizer. 3. The imaging article according to Claim 2 wherein said light- insensitive, reducible silver source comprises a silver salt of an aliphatic carboxylic acid. 4. The imaging article according to Claim 3 wherein said light- insensitive, reducible silver source comprises silver behenate. 5. The imaging article according to Claim 2 wherein said light- sensitive silver halide comprises silver bromide. 6. The imaging article according to Claim 2 wherein said imaging layer further comprises toning agent. 7. The imaging article according to Claim 1 wherein the mole ratio of acrylonitrile to vinylidene chloride in said interlayer is from about 0.2 to 0.95:1. 8. The imaging article according to Claim 7 wherein the mole ratio of acrylonitrile to vinylidene chloride in said interlayer is from 0.4 to 0.9:1. 9. A dry silver photothermographic element comprising a substrate coated on one side thereof with an image-receiving layer, said image-receiving layer having coated thereon at least one image-forming layer separated from said image receiving layer by a polymeric interlayer, said polymeric interlayer comprising an acrylonitrile-vinylidene chloride copolymer and said image-forming layer comprising a neutral leuco dye that forms a cationic dye upon oxidation. 10. The dry silver photothermographic element according to Claim 9 wherein said image-forming layer further comprises a light-insensitive, reducible silver source; a light-sensitive silver halide; a polymeric binder; and a sensitizer. 11. The dry silver photothermographic element according to Claim 10 wherein said light-insensitive, reducible silver source comprises a silver salt of an aliphatic carboxylic acid. 12. The dry silver photothermographic element according to Claim 11 wherein said light-insensitive, reducible silver source comprises silver behenate. 13. The dry silver photothermographic element according to Claim 10 wherein said light-sensitive silver halide comprises silver bromide. 14. The dry silver photothermographic element according to claim 10 wherein said imaging layer further comprises toning agent. 15. The dry silver photothermographic element according to Claim 9 wherein the mole ratio of acrylonitrile to vinylidene chloride in said interlayer is from about 0.2 to 0.95:1. 16. The dry silver photothermographic element according to Claim 15 wherein the mole ratio of acrylonitrile to vinylidene chloride in said interlayer is from about 0.4 to 0.9:1. 17. A method of purifying a mixture comprising a neutral leuco dye and a cationic dye, said method comprising heating said mixture which is in intimate contact with a layer comprising an acrylonitrile-vinylidene chloride copolymer, thereby causing said cationic dye to diffuse through said layer away from said neutral leuco dye. 18. The method of Claim 17 wherein the mole ratio of acrylonitrile to vinylidene chloride in said interlayer is from 0.2 to 0.95:1. 19. The method of Claim 18 wherein the mole ratio of acrylonitrile to vinyl chloride in said interlayer is from about 0.4 to 0.9:1. 20. The method of Claim 17 wherein said heating is conducted at a temperature in the range of 80°-250°C.

Inventors: ISHIDA TAKUZO, PREMENTINE GLENN S, ISHIDA, TAKUZO, PREMENTINE, GLENN, S.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1993
Main classification: G03C8
IPC codes: G03C1, G03C8