Post-activation type dry image forming material

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This invention relates to a dry image forming material. More particularly, the present invention relates to a post-activation type dry image forming material which is nonphotosensitive under normal lighting conditions but is rendered so photosensitive by preliminary heating (heat activation) as to be capable of recording thereon a latent image and therefore, after activation, can form thereon a visible image by only a dry process comprising the step of image-wise exposure to light and the step of heat development, and which is improved in spectral sensitivity without being spoiled in such properties characteristic of imaging materials of post-activation type as capability of being stored even in a light room before image formation. If a raw image forming material which is not yet subjected to image formation can be stored even in a light room like a raw dry image forming material according to the present invention, it is possible that, after information is recorded in the form of an image on a desired area of the image forming material, new information can be recorded in the form of an additional image on another desired area of the image forming material at some convenient time. As desired, a copy of the material having the first information can be obtained by means of a copying machine such as a duplicator or a printer. It is needless to say that updated information can be recorded in the form of a further image on still another desired area of the image forming material as far as the image forming material has blank areas in which it is not yet subjected to image formation. Since the dry image forming material of the present invention is increased in spectral sensitivity, it can form thereon an image by photographing even a manuscript having a colored background or a colored image. Despite the increased sensitivity, the image forming material of the present invention is so stable as to enable additional information to be recorded thereon in a practical sense.

As silver salt photographic materials on which an image can be formed by a dry process only, there have heretofore been proposed dry image forming materials comprising an organic silver salt oxidizing agent, a reducing agent for silver ions and a catalytic amount of a silver halide. Examples of such dry image forming materials include post-activation type dry image forming materials disclosed in U.S. Pat. Nos. 3,802,888 and 3,764,329, and Japanese Patent Publication Nos. 41967/1978 and 5685/1979, wherein the incorporation of a spectral sensitizing dye into a dry image forming material is mentioned. These proposed post-activation type dry image forming materials, however, are defective either in that they are so poor in light or storage stability of the raw image forming materials as to undergo marked deterioration in their photographic or sensitometric characteristics when stored under normal lighting conditions or when exposed to high temperatures even if stored in the dark, or in that they are too insufficient in sensitivity to be used in photographing. As the storage stability of a raw image forming material is increased, the sensitivity of the material is decreased, whereas the storage stability of a raw image forming material is decreased as the sensitivity of the material is increased. Therefore, none of the conventional post-activation type dry image forming materials are so good in both stability and sensitivity as to meet such a demand that the materials record thereon necessary information and even additional information in the form of visible images at desired places and times by a time-saving dry process only.

It is, therefore, an object of the present invention to provide a practically usable post-activation type dry image forming material excellent enough in both stability and sensitivity to meet the above-mentioned demand and capable of recording thereon additional information.

It is well known to those skilled in the art that it is very difficult to apply the knowledge and technique acquired and the various additives used in the field of wet process silver halide photographic materials to the field of dry process photographic or image-forming materials comprising as the essential components an organic silver salt oxidizing agent, a reducing agent for silver ions and a silver halide catalyst because both types of the materials are quite different in components and mechanism of image formation. It is particularly noted that post-activation type dry image forming materials, which are required to be capable of being exposed to or stored under normal lighting conditions substantially without undergoing deterioration in their photographic or sensitometric characteristics, are quite different in conditions of storage and image formation from the wet process photographic materials and even common heat-developable dry image forming materials of the already photosensitive type which are never exposed to light prior to use in image formation. Therefore, it is quite impossible to anticipate whether or not an additive suitable for the wet process photographic materials or the common dry image forming material of the already photosensitive type can be used successfully in post-activation type dry image forming materials. In fact, it is rather natural that such an additive be unable to be used in a post-activation type dry image forming material since incorporation of the additive into the post-activation type dry image forming material quite often results in low light or storage stability of the raw dry image forming material. For example, as is disclosed in Japanese Pat. Publication No. 25498/1974 and U.S. Pat. No. 3,933,507, incorporation of a spectral sensitizing dye into a system comprising an organic silver salt oxidizing agent and a reducing agent for silver ions but no photosensitive silver halide results in a dry image forming material of the already photosensitive type. As will be easily understood from this instance, a spectral sensitizing dye incorporated into an image forming system has a tendency to impart photosensitivity to the system. Accordingly, in the case of a post-activation type dry image forming material which is required to be capable of being exposed to light prior to use and of being preliminarily heated prior to image-wise exposure to light substantially without deterioration of its photographic or sensitometric characteristics and not to undergo substantial fogging, a spectral sensitizing dye, if incorporated into the material, generally downgrades drastically the light or storage stability of the raw image forming material and renders the material more subject to fogging.

On the other hand, in the case of wet process silver halide photographic materials and common heat-developable dry image forming materials of the already photosensitive type, spectral sensitizing dyes as commonly used in these materials are not necessarily required to be stable to light irradiation and heating since the materials are never exposed to light and heat prior to use for image formation. In fact, unstable spectral sensitizing dyes are widely used in the above-mentioned materials.

Almost all of the attempts to use in post-activation type dry image forming materials spectral sensitizing dyes as commonly used in the wet process silver halide photographic materials and the common heat-developable dry image forming materials of the already photosensitive type have failed either because the spectral sensitizing dyes adversely affects the stability of the resulting raw post-activation type dry image forming materials so that they cause the resulting dry image forming materials to undergo fogging at the step of preliminary heating prior to light exposure and the escalation of the fogging at the step of heat development, or because the spectral sensitizing dyes themselves are so decomposed or bleached in the post-activation type dry image forming materials as not to exhibit any spectrally sensitizing capability from the outset, or as to lose their spectrally sensitizing capability too rapidly to be practically useful.

It is, therefore, another object of the present invention to provide a post-activation type dry image forming material of the character as described before, which is spectrally sensitized, but neither substantially undergoes such fogging caused by light irradiation or heating prior to use for image formation as will be attributable to the inclusion of a spectral sensitizing dye, nor involves any substantial loss of the spectrally sensitizing capability of a spectral sensitizing dye owing to the decomposition or bleaching of the spectral sensitizing dye.

The foregoing and other objects, features and advantages of the present invention will be apparent to those skilled in the art from the following detailed description and appended claims taken in connection with the accompanying drawing in which:

FIG. 1 is a spectral sensitivity curve of a post-activation type dry image forming material according to the present invention prepared in Example 31; and

FIG. 2 is a spectral sensitivity curve of a comparative post-activation type dry image forming material prepared in Comparative Example 31.

We have made extensive and intensive investigations with a view to developing a post-activation type dry image forming material spectrally sensitized in a practical sense and having high sensitivity as well as excellent light or storage stability of raw material to find that such a post-activation type dry image forming material can be obtained when it includes peculiar oxidizing compounds [components (d) and (e) mentioned below] capable of providing a strongly oxidizing atmosphere unlike those in the case of wet process silver halide photographic materials and common heat-developable dry image forming materials of the already photosensitive type, and a specific spectral sensitizing dye having a unique chemical structure. We have completed the invention based on such a finding.

More specifically, in accordance with the present invention, there is provided a post-activation type dry image forming material comprising:

(a) a silver salt of long chain fatty acid having 16 or more carbon atoms;

(b) a reducing agent for silver ions;

(c) a silver halide component including silver iodide or a silver halide-forming component capable of forming a silver halide component including silver iodide by the reaction thereof with said silver salt of long chain fatty acid (a);

(d) an oxidizing agent for free silver;

(e) a photoreactive halogeno oxidizing agent;

(f) a binder; and

(g) a spectral sensitizing dye consisting of at least one member selected from the group consisting of those compounds represented by the following formulae: ##STR1## wherein each Z independently is sulfur or selenium, Y is hydrogen or methyl, each R independently is hydrogen or chlorine, and M is hydrogen, HN(C.sub.2 H.sub.5).sub.3 or sodium.

The post-activation type dry image forming material of the present invention is improved in stability of the material prior to heat activation, i.e., stability of the raw image forming material, as well as sensitivity. The reason for this is believed to be as follows. In the post-activation type dry image forming material, the silver halide component present therein as the component (c) prior to heat activation of the material is photochemically reduced by the action of light during the storage of the raw image forming material in a light room to form free silver in part in the silver halide crystals. The free silver thus formed, if left as it is, would exert a catalytic activity for promoting the reduction reaction of the silver salt of long chain fatty acid (a) capable of making a visual change. However, the free silver is oxidized by the action of the free silver-oxidizing agent (d) to reconvert the free silver into the original silver halide, leading to extinguishment of the above-mentioned catalytic activity to be exerted by the free silver. The free silver-oxidizing agent (d) having served to oxidize the free silver to the silver halide, itself, is in a reduced state. The reduced free silver-oxidizing agent (d), in turn, is oxidized by the photochemical action of the photoreactive halogeno oxidizing agent (e) so that it is returned to the original state to regenerate the capacity of oxidizing free silver. As is clearly understood from the foregoing explanation, a main reason for the excellent stability of the raw image forming material (material prior to heat activation) is believed to be that the component (d) reduced by having served to reconvert the light-produced free silver into the silver halide component is regenerated (oxidized) by the action of the component (e) during the course of storage.

Silver iodide is superior in thermal stability to silver bromide and silver chloride. Further, the free silver formed in silver iodide crystals has a low redox potential and therefore is easily oxidized as compared with those produced in silver bromide crystals and silver chloride crystals. Accordingly, the inclusion of silver iodide in the silver halide component (c) enables the free silver photochemically produced in part in silver halide crystals to be easily oxidation-reconverted into the original silver halide by the action of the free silver-oxidizing agent (d). Both of the above-mentioned low redox potential of the free silver formed in silver iodide crystals and the thermal stability of silver iodide contribute to excellent stability of the raw material of the present image forming material containing silver iodide.

Under a strongly oxidizing atmosphere provided in the post-activation type dry image forming material of the present invention, those compounds of the formula (I), (II), (III) or (IV) usable as the spectral sensitizing dye (g) do not act in favor of imparting a photosensitivity to the material, which also contributes, together with the high stability of the compounds themselves, to excellent stability of the raw material.

It is needless to say that an light- or heat-unstable silver source for silver image formation is unsuitable for use in the post-activation type dry image forming material of the present invention. For instance, when a silver complex of imidazole, pyrazole, urazol, 1,2,4-triazole or 1H-tetrazole, or a silver salt of lower fatty acid is used in place of the silver salt of long chain fatty acid (a), the resulting dry image forming material is unsatisfactory in stability of the raw material.

As the silver salt of long chain fatty acid (a) having 16 or more carbon atoms to be used in the post-activation type dry image forming material of the present invention, there can be mentioned, for example, silver palmitate, silver margarate, silver stearate, silver arachidate, silver behenate, silver cerotate and silver melissinate. Silver behenate is most useful. They may be used either alone or in combination. The silver salt of long chain fatty acid may be used in an amount of about 0.1 to about 50 g/m.sup.2, preferably 1 to 10 g/m.sup.2 of the support area of the present image forming material.

As the reducing agent for silver ions (b) to be used in the post-activation type dry image forming material of the present invention, there is used an organic reducing agent which has a suitable reducing ability that, when heated, it reduces the non-photosensitive long chain fatty acid silver salt (a) with the aid of catalysis of the free silver produced in the exposed portions of the activated dry image forming material to form a visible silver image. Examples of the silver ion-reducing agents include monohydroxybenzenes such as p-phenylphenol, p-methoxyphenol, 2,6-di-tert-butyl-4-methylphenol and 2,5-di-tert-4-methoxyphenol; polyhydroxybenzenes such as hydroquinone, tert-butylhydroquinone, 2,6-dimethylhydroquinone, chlorohydroquinone and catechol; naphthols such as .alpha.-naphthol, .beta.-naphthol, 4-aminonaphthol and 4-methoxynaphthol; hydroxybinaphthyls such as 1,1'-dihydroxy-2,2'-binaphthyl and 4,4'-dimethoxy-1,1'-dihydroxy-2,2'-binaphthyl; phenylenediamines such as p-phenylenediamine and N,N'-dimethyl-p-phenylenediamine; aminophenols such as N-methyl-p-aminophenol and 2,4-diaminophenol; sulfonamidophenols such as p-(toluenesulfonamido)phenol and 2,6-dibromo-4-(p-toluenesulfonamido)phenol; methylenebisphenols such as 2,2'-methylenebis(4-methyl-6-tert-butylphenol), 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), 2,2'-methylenebis[4-methyl-6-(1-methylcyclohexyl)phenol], 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane and 2,6-bis(2'-hydroxy-3'-tert-butyl-5'-methylbenzyl)-4-methylphenol; 3-pyrazolidones such as 1-phenyl-3-pyrazolidone and 4-methyl-4-hydroxymethyl-1-phenyl-3-pyrazolidone; and ascorbic acids. A suitable reducing agent may be chosen depending on the kind of silver salt of long chain fatty acid (a) employed in combination therewith. Preferred are phenols. More preferred are hindered phenols in which one or two sterically bulky groups are bonded to the carbon atom or carbon atoms contiquous to the hydroxyl group-bonded carbon atom to sterically hinder the hydroxyl group. Such hindered phenols have a high stability to light and, hence, the use thereof is effective for assuring a high storage stability of the raw dry image forming material of the post-activation type. As examples of such hindered phenols, there can be mentioned 2,6-di-tert-butyl-4-methylphenol, 2,2'-methylenebis(4-methyl-6-tert-butylphenol), 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), 2,4,4-trimethylpentylbis(2-hydroxy-3,5-dimethylphenyl)methane (i.e., 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane), 2,6-methylenebis-(2-hydroxy-3-tert-butyl-5-methylphenyl)-4-methylphenol, 2,2'-methylenebis[4-methyl-6-(1-methylcyclohexyl)phenol] and 2,5-di-tert-butyl-4-methoxyphenol. These reducing agents may be used either alone or in combination. The suitable amount of the reducing agent is usually in the range of from 0.1 to 3 moles per mol of the silver salt of long chain fatty acid (a).

It is requisite for the purpose of the present invention that the silver halide component (c) or the silver halide component formed from the silver halide-forming component (c) include therein silver iodide. In order for the silver iodide to exert a sufficient effect for the purpose, it is preferable that silver iodide be included in an amount of at least 30 mole % based on the silver halide component. The more preferable amount of silver iodide is at least 50 mole % based on the silver halide component. From the viewpoint of sensitivity of the image forming material, the silver halide component is desired to contain, besides silver iodide, at least 2 mole %, based on the silver halide component, of silver bromide and/or silver chloride, although the silver halide component may include only silver iodide, i.e. 100 mole % of silver iodide. Furthermore, from the viewpoint of stability of the raw image forming material, it is desirable that the silver halide component contain, besides silver iodide, silver bromide rather than silver chloride. Therefore, the most preferred silver halide component consists of silver iodide and silver bromide. In this case, silver iodide and silver bromide may be provided in the form of either a mixture thereof or mixed crystals thereof. The molar ratio of silver iodide to silver bromide may be preferably 30/70 to 98/2, more preferably 50/50 to 95/5. The preferred amount of the silver halide component to be used is 0.1 to 20 mole %, based on the amount of the silver salt of long chain fatty acid (a).

Silver iodide and any other silver halide(s) as prepared by the known method commonly employed in the field of photograhic film manufacture may be formulated as the silver halide component (c), together with other components such as the silver salt of long chain fatty acid, into a composition for providing the image forming coating or layer of the dry image forming material, as is disclosed in U.S. Pat. No. 3,152,904. Alternatively, silver iodide and any other silver halide(s) can be prepared in situ either in a composition for providing the image forming coating of the dry image forming material or in the coated image forming layer of the dry image forming material, by the reaction of a silver halide-forming component (c) with part of the long chain fatty acid silver salt (a), as is disclosed in U.S. Pat. No. 3,457,075. The latter mode is preferred in which silver iodide and any other silver halide(s) are formed by the reaction between the long chain fatty acid silver salt (a) and the silver halide-forming component (c), which is a kind of halogenating agent.

As the suitable halogenating agent to be used as the silver halide-forming component (c) in the latter mode mentioned above, there can be mentioned organic halides of elements belonging to Group IV, V or VI of the periodic table and having an atomic number of 14 or more, the preferred elements being Ge, Sn, P, Bi, Te and Se, (see U.S. Pat. No. 4,113,496); halogen molecular species and complexes thereof (see U.S. Pat. No. 4,173,482); organic N-haloamides containing a unit of the formula -CONX- wherein X is bromine or iodine (see U.S. Pat. No. 3,764,329); aryl-halomethanes (see U.S. Pat. No. 4,188,266); and metal halides. They may be used either alone or in combination. Preferred are halogen molecular species and complexes thereof, and organic N-haloamides, from the viewpoint of stability of the raw image forming material. Specific examples of the halogenating agent include compounds respectively represented by the formulae: ##STR2## In the above formulae, X is bromine or iodine. Further specific examples of the halogenating agent include iodine, bromine, iodine bromide, a complex of triphenyl phosphite and iodine, a complex of p-dioxane and iodine, a complex of p-dioxane and bromine, N-bromo(or -iodo)phthalimide, N-bromo(or -iodo)succinimide, N-bromo(or -iodo)phthalazinone, N-bromo(or -iodo)acetamide, N-bromo(or -iodo)acetanilide and .alpha.-bromo(or -iodo)diphenylmethane. Still further specific examples of the halogenating agent include CoX.sub.2, NiX.sub.2, MgX.sub.2, BaX.sub.2, RbX, CsX, TeX.sub.2, TeX.sub.4 and AsX.sub.3. In these formulae, X is bromine or iodine. From the viewpoint of stability of the raw image forming material as well as sensitivity, the preferred halogenating agents for forming silver iodide are iodine and N-iodosuccinimide. Complexes of iodine such as a complex of triphenyl phosphite and iodine and a complex of p-dioxane and iodine are also preferred. In the case of N-iodosuccinimide, it is preferred that a solution of N-iodosuccinimide in an alcohol such as methanol or ethanol, which has been previously prepared, be incorporated into an emulsion for the desired dry image forming material. From the viewpoint of sensitivity as well as stability of the raw image forming material, the preferred halogenating agents for forming silver bromide are N-bromosuccinimide, cobalt dibromide, nickel dibromide and .alpha.-bromodiphenylmethane. The amount, in equivalents, of the halogenating agent to be used may be equal to or more than the desired amount, in equivalents, of the silver halide component.

The oxidizing agent for free silver (d) to be used in the post-activation type dry image forming material of the present invention has a capacity of oxidizing free silver produced during the storage of the material, thereby contributing to improving the stability of the raw material. As examples of the free silver-oxidizing agent (d), there can be mentioned compounds of divalent mercury (Hg.sup.++), compounds of trivalent iron (Fe.sup.+++), compounds of trivalent cobalt (Co.sup.+++), compounds of divalent palladium (Pd.sup.++) and sulfinic acid compounds. As examples of the compounds of divalent mercury, there can be mentioned mercuric salts of aliphatic carboxylic acids such as mercuric acetate and mercuric behenate; mercuric salts of aromatic carboxylic acids such as mercuric benzoate, mercuric m-methylbenzoate and mercuric acetamidobenzoate; mercuric halides such as mercuric bromide and mercuric iodide; mercuri-benzotriazole; and mercuri-phthalazinone. Preferred are mercuric acetate, mercuric bromide and mercuric iodide. As examples of the compounds of trivalent iron, there can be mentioned a complex of trivalent iron and acetylacetone and a complex of trivalent iron and bipyridyl. As examples of the compounds of trivalent cobalt, there can be mentioned a complex of trivalent cobalt and acetylacetone and a complex of trivalent cobalt and o-phenanthroline, and cobaltic halides such as cobaltic iodide and cobaltic bromide. Examples of the compounds of divalent palladium include a complex of divalent palladium and acetylacetone, and palladium (II) halides such as palladium (II) iodide and palladium (II) bromide. As examples of the sulfinic acid compound, there can be mentioned n-octylsulfinic acid and p-toluenesulfinic acid. As the component (d), the compounds of divalent mercury are most preferred. The preferred amount of the component (d) is in the range of from 0.01 to 10 mole % based on the silver salt of long chain fatty acid.

As described before, the oxidizing agent for free silver [component (d)] is reduced by serving to oxidize the free silver produced during the storage of the raw image forming material into the original silver halide. The thus reduced oxidizing agent for free silver, in turn, is oxidized, under lighting conditions, by the action of the photoreactive halogeno oxidizing agent (e), whereby it is effectively returned to the original state in which the component (d) has a capacity of oxidizing free silver. The photoreactive halogeno oxidizing agent is such a halogeno compounds as can generate free radicals of halogen upon light exposure. Preferred examples of such a halogeno compound are halogenated organic compounds having bromine- and/or iodine-carbon linkages.

Whether or not a given halogeno compound is suitable for use as the component (e) in the present invention can be determined, for example, by the following photoreaction test.

1 Mole of silver behenate [suitable as the silver behenate is one which has been synthesized in a mixed solvent (1:5-5:1 by volume) of water and at least one water-soluble or partially water-soluble alcohol having 3 to 8 carbon atoms], 450 g of polyvinyl butyral and 0.25 mole of said given halogeno compound for use as the "photoreactive halogeno oxidizing agent" are dissolved into a mixed solvent (2:1 by weight) of methyl ethyl ketone and toluene, and then formed into a film according to an ordinary casting method.

The film thus formed is tested with respect to the following two requirements. When the film satisfies both of the requirements, the halogeno compound (for use as the photoreactive oxidizing agent) employed is suitable for the purpose of the present invention.

Requirement 1: when the film is examined by X-ray diffractometry, the peak due to silver bromide (2.theta.=31.0.degree.) or the peak due to silver iodide (2.theta.=23.7.degree.) should not substantially be observed [the relative intensity of said peak is less than about 10 when the relative intensity of the peak due to silver behenate (2.theta.=12.1.degree.) is defined as 100].

Requirement 2: subsequently, the film is irradiated with a light (0.5 mW/cm.sup.2) emitted from a black lamp in an atmosphere having a temperature of 50.degree. C. and a relative humidity of 80% for 2 hours, and then examined by X-ray diffractometry again. The peak due to silver bromide (2.theta.=31.0.degree.) or the peak due to silver iodide (2.theta.=23.7.degree.) should be substantially observed [the relative intensity of said peak is about 10 or more when the relative intensity of the peak due to silver behenate (2.theta.=12.1.degree.) is defined as 100].

In the above test, the values of 2.theta. are those of diffraction peaks obtained by using CuK.sub..alpha. line. In the present invention, as the apparatus for X-ray diffractometry is used an apparatus of Rotor Unit type (RU-200 PL type) manufactured and sold by Rigaku Denki Kabushiki Kaisha, Japan.

Specific examples of halogeno compounds capable of being used as the photoreactive halogeno oxidizing agent (e) include .alpha.,.alpha.,.alpha.',.alpha.'-tetrabromo-o-xylene, .alpha.,.alpha.,.alpha.',.alpha.'-tetrabromo-m-xylene, ethyl .alpha.,.alpha.,.alpha.-tribromoacetate, .alpha.,.alpha.,.alpha.-tribromoacetophenone, .alpha.,.alpha.,.alpha.-tribromo-p-bromotoluene, 1,1,1-tribromo-2,2-diphenylethane, tetrabromomethane, meso-1,2,3,4-tetrabromobutane, 2,2,2-tribromoethanol, 2,2,2-tribromoethylcyclohexyl carbamate, 2,2,2-tribromoethylphenyl carbamate, 2,2,2-tribromoethyl benzoate, 2,2,2-tribromoethyl ethylcarbamate, 2-methyl-1,1,1-tribromo-2-propanol, bis(2,2,2-tribromoethoxy) diphenylmethane, 2,2,2-tribromoethyl stearate, 2,2,2-tribromoethyl-2-furoate, bis(2,2,2-tribromoethyl) succinate, 2,2,2-tribromoethyl phenylsulfonate, 2,2,2-tribromoethoxytrimethyl-silane, 2,2,2-tribromo-1-phenylethanol, 2,2,2-tribromoethyldiphenyl phosphate, 1,2-diiodoethane and iodoform. They may be employed either alone or in combination. Of them, the bromo compounds are preferred since they give little coloring and improved stability to the resulting raw dry image forming material. Especially preferred are .alpha.,.alpha.,.alpha.',.alpha.'-tetrabromo-o-xylene, .alpha.,.alpha.,.alpha.',.alpha.'-tetrabromo-m-xylene, ethyl .alpha.,.alpha.,.alpha.-tribromoacetate, .alpha.,.alpha.,.alpha.-tribromo-p-bromotoluene, .alpha.,.alpha.,.alpha.-tribromoacetophenone, 1,1,1-tribromo-2,2-diphenylethane, 2,2,2-tribromoethanol and meso-1,2,3,4-tetrabromobutane. Most preferred are .alpha.,.alpha.,.alpha.',.alpha.'-tetrabromo-o-xylene and .alpha.,.alpha.,.alpha.',.alpha.'-tetrabromo-m-xylene. The amount of the photoreactive halogeno oxidizing agent (e) is preferably 2.5 to 40 mole %, based on the silver salt of long chain fatty acid (a).

As the binder (f) to be used in the dry image forming material of the present invention, there can be mentioned, for example, film-forming polymer materials such as, polyvinyl butyral, polymethyl methacrylate, cellulose acetate, polyvinyl acetate, cellulose acetate propionate, cellulose acetate butyrate, vinyl chloride-vinyl acetate copolymers, polyvinyl alcohol, polystyrene, polyvinyl formal and gelatin. Polyvinyl butyral is the most preferred binder. They may be used either alone or in combination. It is preferred that the binder be used in such an amount that the weight ratio of the binder to the long chain fatty acid silver salt (a) is in the range of from about 0.1 to about 10.

The spectral sensitizing dye (g) to be used in the dry image forming material of the present invention is selected from those compounds each represented by one of the general formulae (I), (II), (III) and (IV) mentioned hereinbefore. The amount of the component (g) is preferably in the range of from 0.001 to 1 mole %, based on the component (a).

Specific examples of the component (g) include the following compounds: ##STR3##

According to need, the post-activation type dry image forming material of the present invention may comprise in addition to the foregoing essential components a variety of additives such as a chemical sensitizer, a toner for a silver image and/or a development promotor.

Among chemical sensitizers which improve the sensitivity of the dry image forming material of the present invention, those liable to spoil greatly the storage stability of the dry image forming material prior to the use thereof are not preferred. As chemical sensitizers which substantially improve the sensitivity of but not substantially spoil the storage stability of the dry image forming material of the present invention, there can be mentioned, for example, amide compounds as disclosed in Japanese Patent Application Laid-Open Specification No. 7914/1976, e.g. 1-methyl-2-pyrrolidone; quinoline compounds as disclosed in Deutsche Offenlegungsschrift No. 2,845,187 and represented by the following general formula: ##STR4## wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4 R.sub.5 and R.sub.6, each independently, is a hydrogen atom, an aryl group selected from phenyl and naphthyl groups unsubstituted or substituted with methyl, methoxy or halogen, a C.sub.1 -C.sub.10 straight or branched chain alkyl group, a C.sub.1 -C.sub.4 alkoxyl group, an aralkyl group selected from benzyl and phenethyl groups unsubstituted or substituted with methyl, methoxy or halogen, a hydroxyl group, a cyano group, a carboxyl group, a C.sub.2 -C.sub.5 alkoxycarbonyl group, a nitro group, an amino group or a carbamoyl group, and D is a hydrogen atom, a hydroxyl group or an amino group; and 3-pyrazolin-5-one compounds as disclosed in Deutshe Offenlegungsschrift No. 2,934,751 and represented by the following formula: ##STR5## wherein R.sub.7 is a hydrogen atom, a C.sub.1 -C.sub.5 straight or branched chain alkyl group, an unsubstituted or substituted phenyl group or an unsubstituted or substituted C.sub.3 -C.sub.8 cycloalkyl group, R.sub.8 is a C.sub.1 -C.sub.5 straight or branched chain alkyl group, an unsubstituted or substituted phenyl group or an unsubstituted or substituted C.sub.3 -C.sub.8 cycloalkyl group, and R.sub.9 and R.sub.10 are the same or different and each represents a hydrogen atom, a C.sub.1 -C.sub.5 straight or branched chain alkyl group, an unsubstituted or substituted phenyl group or an unsubstituted or substituted phenylalkyl group having a C.sub.1 -C.sub.5 straight or branched chain alkyl moiety. They may be used either alone or in combination preferably in an amount of 5 to 50 mole %, based on the silver salt of long chain fatty acid (a). Specific examples of 3-pyrazolin-5-one compounds include 2-phenyl-3-pyrazolin-5-one, 1-(p-iodophenyl)-2,3-dimethyl-3-pyrazolin-5-one, 2,3,4-triphenyl- 3-pyrazolin-5-one, 1-phenyl-2,3-dimethyl-3-pyrazolin-5-one, 1,3-diethyl-2-phenyl-3-pyrazolin-5-one, 2,3-dimethyl-1-ethyl-4-isopropyl-3-pyrazolin-5-one, 2-o-tolyl-3-methyl-4-ethyl-3-pyrazolin-5-one, 2-cyclohexyl-3-pyrazolin-5-one, 2-methyl-1,3-diphenyl-3-pyrazolin-5-one and 1-cyclohexyl-2,3-dimethyl-3-pyrazolin-5-one. The post-activation type dry image forming material of the present invention, even when containing a chemical sensitizer as mentioned above for improving the sensitivity, exerts the effects as intended in the present invention.

As the toner for a silver image, there can be mentioned, for example, phthalazinone, zinc acetate, cadmium acetate, phthalimide and succinimide. They may be used either alone or in combination. The amount of the toner for a silver image is preferably in the range of from 1 to 100 mole %, based on the component (a).

The preferred method of preparing the dry image forming material of this invention is described by way of example as follows. An silver salt of long chain fatty acid is dispersed in a binder-forming polymer solution by means of a ball mill, a homogenizer, a mixer, a sand mill or the like. To the resulting dispersion are added the other essential components and optionally various additives. The composition thus obtained is applied