Positive-acting photothermographic materials

Abstract: Positive-acting photothermographic elements suitable in graphic arts and medical imaging comprising a photosensitive medium comprising a reducible silver source, a photo-acid generator, a binder and a reducing system for silver ion comprising a reducing agent for silver ion in which exposure of the element to actinic radiation causes the production of acidic species in the exposed regions which inhibit reduction of the silver source by the reducing system.

Description: This invention relates to imaging materials and in particular to black & white, positive-acting photothermographic materials. There is a growing need for a range of positive-acting imaging materials able to complement negative-acting Dry Silver materials, particularly for application in graphic arts and medical imaging. For such uses, prospective imaging materials should desirably exhibit the following characteristics: (i) contact (10 5 to 10 3 erg/cm 2 ) or higher speeds, (ii) dry processing, preferably using standard Dry Silver processing conditions, and (iii) thermally stable for archival storage purposes. The prior art has long sought to provide suitable positive-acting photothermographic materials, examples of which include those materials disclosed by British Patent Nos. 1156933, 1172425, 1507829, 2022277 and 2195463; European Patent Nos. 223587, 301539, 320020 and 362827; U.S. Patent Specification Nos. 3589901, 4075017, 4124387, 4587198, 4753862, 4761360, 4772541, 4800149, 4814252 and 4865942, and Japanese Patent Nos. 53-120520, 57-089750, 57-101832, 58-040543, 58-040544, 60-030931, 61-107243, 61-183460, 61-188535, 61-022841, 62-187837, 62-178742, 63-034536 and 63-330064. Research Disclosure No. 125, September 1974, pages 40-41, Item 12535, Direct-positive photothermographic system, discloses a process of developing a direct-positive image comprising imagewise exposing to radiant energy a photothermographic element comprising a support having thereon a layer comprising (1) a radiation-sensitive composition comprising a free-radical source and a nucleating agent in association with (2) an oxidation-reduction image-forming combination comprising a silver salt oxidizing agent and an organic reducing agent with (3) an activator-toning agent and (4) a polymeric binder and heating to a temperature within the range of about 80°C to about 250°C. During imagewise exposure, preferably to actinic-radiation such as UV radiation, the nucleating agents are bleached by the free-radical source to form compounds which are inactive as reducing agents for silver ions. Upon heating at from about 80°C to about 250°C for about 0.5 to about 30 seconds, the nucleating agents in the unexposed areas react with the oxidizing agent to form physical development centres which are acted upon by the reducing agent to form a positive image. The nucleating agents are preferably bis-heterocyclic compounds which irreversibly bleach in the presence of the free radical source. No protection is sought for positive-acting photothermographic elements in which the photosensitive medium contains a nucleating agent as described above. EP-A-0537912 discloses positive-acting photothermographic elements having a photosensitive medium comprising a dispersion of a reducible silver source and a reducing system for silver ion and a photocurable composition. The photocurable composition comprises a free radical curable resin and a photoinitiator having an absorbance to radiation in the wavelength range of 340 to 440nm. In those areas of the element exposed to radiation, the photoinitiator promotes curing of the free radical curable resin, thereby increasing the glass-transition temperature of the resin in those regions, and effectively immobilising or otherwise preventing the reducing system from reacting with the reducible silver source during subsequent thermal processing. Examples of suitable photoinitiators include onium salts, such as iodonium and sulphonium salts, which may be used alone or in combination with a sensitiser, e.g., oxonol dyes, 1,4-dihydropyridines and triarylpyrazolines. The present invention seeks to provide alternative positive-acting photothermographic materials. According to the present invention there is provided a positive-acting photothermographic element having a photosensitive medium comprising a reducible silver source, less than 0.75% by weight of silver halide based on the weight of the reducible silver source, a photo-acid generator, a binder and a reducing system for silver ion comprising a reducing agent for silver ion in which exposure of the element to actinic radiation causes the production of acidic species in the exposed regions which inhibit reduction of the silver source by the reducing system, with the proviso the photosensitive medium is free of nucleating agents. The photothermographic element of the invention comprises a dispersion of a reducible silver source, a reducing system for silver ion and a photo-acid generator, as well as optional additives, such as coating aids and other adjuvants, in one or more binder layers. In those areas of the element exposed to radiation, the acidic species produced by the photo-acid generator inhibit subsequent thermal reduction of the silver source to give well-defined positive images. The reducing system comprises at least a compound capable of reducing silver ion to silver metal. In its simplest form it consists of a reducing agent for silver ion (i.e. a developer), but preferably it also comprises one or more compounds, known in the photothermographic art as toners, which enhance the effect of the developer. It should be noted that the photothermographic elements of the invention do not require the presence of light-sensitive silver halides; indeed, the absence of such materials is preferred from the point of view of pre- and post-imaging stability. The elements contain less than 0.75% by weight of the silver source is in the form of silver halides, more preferably less than 0.5% is in the form of silver halides. Most preferably, the elements are essentially free of silver halides. The photothermographic elements of the invention are suitable for use in a wide range of imaging fields including contact printing, although they find particular utility in the production of daylight-handleable, dry-processed duplicating films, and are found to have good pre- and post-exposure stability. The invention also relates to a method of generating a positive image comprising (a) imagewise exposing a photothermographic element of the invention to radiation to produce a latent image and (b) heating the exposed element to develop the latent image. The photothermographic elements of the invention usually comprise at least two binder layers within which are dispersed the relevant photothermographic chemistry. Normally, the reducible silver source is contained in one layer and the reducing agent for silver ion in a separate layer. When a toner is also present, at least one and more preferably both of the reducing agent and toner are contained in a layer separate from that containing the reducible silver source. In one embodiment, the photothermographic element comprises a support having on one surface thereof, a first binder layer comprising the reducible silver source and a second binder layer comprising the reducing agent for silver ion and the photo-acid generator. Alternatively, the photo-acid generator may be included in the first binder layer together with the reducible silver source, or in both the first and second binder layers. The latter is preferred, as the presence of the photo-acid generator in the layer containing the reducible silver source is found to enhance the sensitivity of the element. The photothermographic element may optionally be provided with an inert barrier layer overlying the other layers. Suitable barrier materials include coatings of water-soluble polymers, such as gelatin, poly(vinyl alcohol), poly(vinyl pyrollidone) etc., or organo-soluble polymers, such as cellulose esters, optionally containing one or more surfactants, and laminated sheets of a transparent material, e.g., polyester, optionally treated with a surfactant or other coating aids. Generally, each layer of the photothermographic element is coated at a wet thickness of from 25 to 250 µm, with a typical value of about 150 µm, and comprises from 1 to 90%, preferably 5 to 50% by weight of solids in the layer. The reducible silver source may comprise any material which contains a reducible source of silver ions. Silver salts of organic and hetero-organic acids, particularly long chain fatty carboxylic acids (comprising from 10 to 30, preferably 15 to 25 carbon atoms) are preferred. Complexes of organic or inorganic silver salts in which the ligand has a gross stability constant for silver ion of between 4.0 and 10.0 are also useful. Examples of suitable silver salts are disclosed in Research Disclosure Nos. 17029 and 29963 and include: salts of organic acids, e.g., gallic acid, oxalic acid, behenic acid, stearic acid, palmitic acid, lauric acid and the like; silver carboxyalkylthiourea salts, e.g., 1-(3-carboxypropyl)thiourea, 1-(3-carboxypropyl)-3,3-dimethylthiourea and the like; complexes of silver with the polymeric reaction product of an aldehyde with a hydroxy-substituted aromatic carboxylic acid, e.g., aldehydes, such as formaldehyde, acetaldehyde and butyraldehyde, and hydroxy-substituted acids, such as salicylic acid, benzilic acid, 3,5-dihydroxybenzilic acid and 5,5-thiodisalicylic acid; silver salts or complexes of thiones, e.g., 3-(2-carboxyethyl)-4-hydroxymethyl-4-thiazoline-2-thione and 3-carboxymethyl-4-methyl-4-thiazoline-2-thione; complexes or salts of silver with nitrogen acids selected from imidazole, pyrazole, urazole, 1,2,4-triazole and 1H-tetrazole, 3-amino-5-benzylthio-1,2,4-triazole and benzotriazole; silver salts of saccharin, 5-chlorosalicylaldoxime and the like; and silver salts of mercaptides. The preferred silver source is silver behenate. The reducible silver source generally comprises from 5 to 70, preferably from 7 to 45% by weight of its binder layer. The reducing agent for the silver source may comprise any of the conventional photographic developers known in the art, such as phenidone, hydroquinones and catechol, although hindered phenols are preferred. Examples of suitable reducing agents are disclosed in U.S. Patent Nos. 3770448, 3773512 and 3893863 and Research Disclosure Nos. 17029 and 29963 and include: aminohydroxycycloalkenone compounds; esters of amino reductones as developing agent precursors; N-hydroxyurea derivatives; hydrazones of aldehydes and ketones; phosphoramidophenols; phosphoramidoanilines; polyhydroxybenzenes, e.g., hydroquinone, t-butylhydroquinone, isopropylhydroquinone, and (2,5-dihydroxyphenyl)methylsulfone; sulfhydroxamic acids; sulfonamidoanilines; 2-tetrazolylthiohydroquinones, e.g., 2-methyl-5-(1-phenyl-5-tetrazolylthio)hydroquinone; tetrahydroquinoxalines, e.g., 1,2,3,4-tetrahydroquinoxaline; amidoximes; azines; a combination of an aliphatic carboxylic acid aryl hydrazide and ascorbic acid; a combination of a polyhydroxybenzene and a hydroxylamine, a reductone and/or a hydrazine, hydroxamic acids; a combination of an azine and a sulfonamidophenol; an α-cyanophenylacetic acid derivative; a combination of a bis-β-naphthol and a 1,3-dihydroxybenzene derivative; 5-pyrazolones; sulfonamidophenol reducing agents; 2-phenylindane-1,3-dione and the like; 1,4-dihydropyridines, such as 2,6-dimethoxy-3,5-dicarbethoxy-1,4-dihydropyridine; bisphenols, e.g., bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane, bis(6-hydroxy-m-toly)mesitol, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 4, 4-ethylidenebis(2-t-butyl-6-methylphenol) and 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane, UV sensitive ascorbic acid derivatives and 3-pyrazolidones. The preferred developers are hindered phenols of general formula (I): in which; R represents hydrogen or an alkyl group, generally comprising up to 5 carbon atoms, e.g., -C 4 H 9 . The reducing agent is generally present in an amount from 2 to 15% by weight of its binder layer. The presence of a toner, sometimes referred to as a tone modifier, is not essential to the construction, but is highly preferred. The precise mode of action of toners is not well understood, but it is believed that they catalyse the reaction between the developer and silver ions. They also influence the physical form of the silver metal produced in the reaction and hence the appearance ("tone") of the developed image. Examples of suitable toners are disclosed in Research Disclosure No. 17029 and include: imides, e.g., phthalimide; cyclic imides, pyrazolin-5-ones and a quinazolinone, such as succinimide, 3-phenyl-2-pyrazolin-5-one, 1-phenylurazole, quinazoline and 2,4-thiazolidinedione; naphthalimides, e.g., N-hydroxy-1,8-naphthalimide; cobalt complexes, e.g., cobaltic hexammine trifluoroacetate; mercaptans, e.g., 3-mercapto-1,2,4-triazole; N-(aminomethy)aryl dicarboximides, e.g., N-(dimethylaminomethyl)phthalimide; a combination of blocked pyrazoles, isothiuronium derivatives and certain photobleach agents, e.g., a combination of N,N'-hexamethylene bis(1-carbamoyl-3,5-dimethylpyrazole), 1,8-(3,6-dioxaoctane)bis(isothiuronium trifluoroacetate) and 2-(tribromomethylsulfonyl) benzothiazole); merocyanine dyes, such as 3-ethyl-5-[(3-ethyl-2-benzothiazolinylidene)-1-methylethyl-idene]-2-thio-2,4-oxazolidinedione; phthalazinone, phthalazinone derivatives or metal salts of these derivatives, such as 4-(1-naphthyl)phthalazinone, 6-chlorophthalazinone, 5,7-dimethoxyphthalazinone and 2,3-dihydro-1,4-phthalazinedione; a combination of phthalazinone and a sulfinic acid derivative, e.g., 6-chlorophthalazinone plus sodium benzene sulfinate or 8-methylphthalazinone plus sodium p-tolylsulfinate; a combination of phthalazinone plus phthalic acid; a combination of phthalic acid and imidazole or benzimidazole; a combination of phthalazine (including an adduct of phthalazine and maleic anhydride) and at least one compound consisting of a phthalic acid, a 2,3-napthalene dicarboxylic acid or an o -phenylene acid derivative and anhydrides thereof, e.g., phthalic acid, 4-methylphthalic acid, 4-nitrophthalic acid and tetrachlorophthalic anhydride; quinazolinediones, benzoxazine or naphthoxazine derivatives, rhodium complexes, e.g., ammonium hexachlororhodate (III), inorganic peroxides and persulfates, e.g., ammonium peroxydisulfate; benzoxazine-2 ,4-diones, e.g., 1,3-benzoxazine-2,4-dione; pyrimidines and asym-triazines, e.g., 2,4-dihydroxypyrimidine and tetrazapentalene derivatives, e.g., 3,6-dimercapto-1,4-diphenyl-1H,4H-2,3a,5,6a-tetrazapentalene. The preferred toner is phthalazinone: optionally in combination with tetrachlorophthalic acid or its anhydride. The toner, when present, is generally included in amounts of from 0.2 to 12% by weight of its binder layer. A vast number of known compounds and mixtures, such as diazonium salts, phosphonium salts, sulphonium salts, e.g., triphenyl sulphonium salts and iodonium salts or halogen compounds, quinone diazide sulfochlorides, trichloromethylpyrones, bis(trichloromethyl)-s-triazines, e.g., 2-(p-methoxystyryl -4,6-bis(trichloromethyl)-s-triazine, and organometal/organohalogen combinations, may be used as radiation-sensitive constituents forming or splitting-off acids during irradiation of the element. Compounds containing no basic substituents are preferred. Suitable diazonium salts are compounds having an absorption range between 300 and 500 nm, which are known to be suitable for diazotype purposes. As a rule, the above mentioned diazonium, phosphonium, sulfonium and iodonium compounds are used in the form of salts soluble in organic solvents, normally as the separation products of complex acids, such as tetrafluoroboric acid, hexafluorophosphoric acid hexafluoroantimonic acid and hexafluoroarsenic acid. Alternatively derivatives of positive working quinone diazides may be used. From this group of compounds, naphthoquinone-1,2-diazide-4-sulfochloride is preferred, because, during its exposure, three acid functions are formed which give rise to a relatively high degree of intensification. In principle, all organic halogen compounds known as free radical forming photoinitiators, for example, those containing more than one halogen atom attached to a carbon atom or an aromatic ring, may be used as halogen containing radiation-sensitive compounds capable of forming a hydrohalic acid. Examples of such compounds are disclosed in U.S. Patent Nos. 3515552, 3536489 and 3779778 and in German Offenlegungsschrift No. 2243621. Furthermore, certain substituted trichloromethypyrones, such as those described in German Offenlegungsschrift No. 2610842 may also be used, as well as 2-aryl-4,6-bis-trichloromethyl-s-triazines. Examples of suitable photo-acid generators include: 4-(di-n-propyl-amino)-benzene-diazoniumtetrafluoroborate, 4-p-tolylmercapto-2,5-diethoxybenzene-diazonium hexafluorophosphate 4-p-tolymercapto-2,5-diethoxybenzene-diazonium tetrafluoroborate; diphenylamine-4-diazonium sulfate, 4-methyl-6-trichloromethyl-2-pyrone, 4-(3,4,5-trimethoxystyryl)-6-trichloromethyl-2-pyrone, 4-(4-methoxystyryl)-6-(3,3,3-trichloro-propenyl)-2-pyrone, 2-trichloromethylbenzimidazole, 2-tribromomethylquinoline, 2,4-dimethyl-tribromoacetylbenzene, 3-nitro-1-tribromoacetylbenzene, 4-dibromoacetylbenzoic acid, 1,4-bis-dibromomethylbenzene, tris-dibromomethyl-s-triazine, 2-(6-methoxy-naphth-2-yl)-4,6-bistrichloromethyl-s-triazine, 2-(naphth-1-yl)-4,6-bis-trichloromethyl-s-triazine, 2-(4-ethoxyethyl-naphth-1-yl)-4,6-bis-trichloromethyl-s-triazine, 2-(benzopyran-3-yl)-4,6-bis-trichloromethyl-s-triazine, 2-(4-methoxy-anthracene-1-yl) -4,6-bis-trichloromethyl-s-triazine, and 2-(phenanthr-9-yl)-4,6-bis-trichloromethyl-s-triazine. The quantity of the photo-acid generator may vary widely depending on its chemical nature and the exact composition of the photosensitive medium. The photo-acid generator must however, be present in an effective amount to inhibit thermal development in the exposed region(s) of the element. Generally, the photo-acid generator is present in an amount of from 0.5 to 30%, preferably 1 to 20%, more preferably 2 to 15% by weight, based on the total weight of the solids. The preferred photo-acid generators, for use in the present invention are iodonium and sulphonium salts. The iodonium salt may comprise any of the photosensitive iodonium salts known to the art, for example, those iodonium salts disclosed in U.S. Patent Specification Nos. 3729313, 3741769, 3808006, 4026705, 4228232, 4250053, 4701402, and 4769459. Either a single iodonium salt or a combination of two or more iodonium salts may be used. Iodonium salts are compounds having a positively charged iodine atom bearing two covalently bonded carbon atoms and any anion. Aliphatic iodonium salts are not normally thermally stable at temperatures above 0°C. However, stabilised alkyl phenyl iodonium salts, such as those disclosed in Chemical Letters (1982) pp. 65-6, are stable at ambient temperatures and may be used in the invention. The preferred compound are diaryl, aryl-heteroaryl and diheteroaryl iodonium salts in which the carbon-to-iodine bonds are formed by the aryl or heteroaryl groups. Sulphonium salts are compounds having a positively charged sulphur atom bearing three (or more) covalently bonded carbon atoms and an anion. The preferred sulphonium salts are aryl, aryl-heteroaryl and heteroryl sulphonium salts in which the carbon-to-sulphur bonds are formed by the aryl or heteroaryl groups. Suitable photosensitive iodonium and sulphonium salts are represented by general formulae (II) and (III) below: (II) I + -(A) 2 X - (III) S + -(A) 3 X - in which; each A independently represents an aromatic or heteroaromatic group which may be linked together to include the iodine or sulphur atom within a ring structure, and X - is an anion such that HX is an acid of p Ka not greater than 3. The aromatic group(s) represented by A, generally comprises from 4 to 20, preferably 4 to 14 and more preferably 4 to 10 constituent atoms, and may be selected from aromatic carbocyclic rings, e.g., phenyl or naphthyl, and aromatic heterocyclic rings including thienyl, furanyl and pyrazolyl, and may optionally possess one or more substituents selected from alkyl groups comprising up to 5 carbon atoms, e.g., methyl, alkoxy groups comprising up to 5 carbon atoms, e.g., methoxy, halogen atoms, such as chlorine, bromine, iodine and fluorine, carboxy groups comprising up to 5 carbon atoms, cyano and nitro groups, or any combination thereof. Condensed aromatic-heteroaromatic groups, e.g., 3-indolinyl, may also be present. Examples of suitable iodonium salts include: Most of the iodonium salts are known, they may be readily prepared and some are commercially available. The synthesis of suitable iodonium salts is disclosed in F.M. Beringer et al ., Journal of the American Chemical Society, 80 , 4279(1958). The preferred photo-acid generator is a sulphonium salt having a nucleus of the formula: in which; X - is as defined previously. Many anions are useful as the counter-ion in the onium salt, provided the acid from which the anion is derived has a pKa of less than 3, preferably less than 1. Suitable inorganic anions include halide anions, HSO 4 - , and halogen-containing complex anions, e.g., tetrafluoroborate, hexafluorophosphate, hexafluoroarsenate and hexafluoroantimonate. Suitable organic anions include those of the formulae R 1 CO 2 - and R 1 SO 3 - where R 1 represents an alkyl or aryl group, either of which may be substituted, for example: CF 3 CO 2 - , CC1 3 CO 2 - , CH 3 SO 3 - , CF 3 SO 3 - , CH 3 C 6 H 4 SO 3 - The photothermographic chemistry of the element is typically applied to the support in a binder. A wide range of binders may be employed in the various layers of the photothermographic element. Suitable binders are transparent or translucent, are generally colourless and include natural polymers, synthetic resins, polymers and copolymers and other film forming media such as: gelatin, gum arabic, poly(vinyl alcohol), hydroxyethyl cellulose, cellulose acetate, cellulose acetate butyrate, poly(vinyl pyrrolidone), casein, starch, poly(vinyl chloride), copoly(styrene-maleic anhydride), copoly(styrene acrylonitrile), copoly(styrene-butadiene), polyvinyl acetals, e.g., poly(vinyl formal) and poly(vinyl butyral), polyesters, polyurethanes, phenoxy resins, poly(vinylidene chloride), polyepoxides, polycarbonates, poly(vinyl acetate), cellulose esters and polyamides. The binders may be coated from aqueous or organic solvents or an emulsion. The binders may optionally comprise cross-linking components, as described in our copending British Patent Application No. 9121789.3 of even date corresponding to EP-A-0537912. Suitable cross-linkers include polyfunctional acrylates, such as hydantoin hexaacrylate, trimethylolpropane trimethacrylate and the like. In situations where photolysis of the photo-acid generator also produces free radicals, photocuring may occur in the light-struck areas, which in turn hinders the diffusion of the developer or toner towards the silver salt, and may provide an improved image differential. However, this is not an essential feature of the invention. Examples of acid generators which also produce radicals on photolysis include iodonium salts, sulphonium salts and trichloromethyltriazines. The photothermographic elements of the invention are prepared by simply coating a suitable support or substrate with the one or more binder layers containing the photothermographic chemistry and, optionally, the barrier layer. Each layer is generally coated from a suitable solvent using techniques known in the art. In one highly preferred embodiment, the photothermographic element comprises a support having coated on one surface thereof a first binder layer comprising the silver salt of an organic or a hetero-organic acid, e.g., silver behenate, and the photo-acid generator, preferably a sulphonium salt, and a second binder layer comprising a developer, preferably a hindered phenol. A toner is preferably included in the second binder layer. In alternative embodiments, the order of coating the first and second binder layers may be reversed and the photo-acid generator may be incorporated into the second binder layer, either instead of or in addition to its inclusion in the first. Exemplary support include materials such as paper, polyethylene-coated paper, polypropylene-coated paper, parchment, cloth and the like; sheets and foils of such metals as aluminium, copper, magnesium and zinc; glass and glass coated with such metals as chromium, chromium alloys, steel, silver, gold and platinum; synthetic polymeric materials such as poly(alkyl methacrylates), e.g., poly(methyl methacrylate), polyesters, e.g., poly(ethylene terephthalate), poly(vinylacetals), polyamides, e.g., nylon, cellulose esters, e.g., cellulose nitrate, cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate, and the like. A variety of conventional additives such as surfactants, anti-oxidants, stabilisers, plasticisers, ultraviolet absorbers, coating aids etc., may be used in the preparation of the photographic elements of the invention. It is not essential for the photothermographic elements of the invention to comprise a separate support since each binder layer, together with the photothermographic chemistry may be cast to form a self-supporting film. The supports can be sub-coated with known subbing materials such as copolymers and terpolymers of vinylidene chloride with acrylic monomers (such as acrylonitrile and methyl acrylate) and unsaturated dicarboxylic acids (such as itaconic acid or acrylic acid); carboxymethyl cellulose, polyacrylamide; and similar polymeric materials. The support can also carry a filter or antihalation layer, such as one comprising a polymer layer with a dye therein which absorbs the exposing radiation after it passes through the radiation-sensitive layer. This eliminates unwanted reflection from the support. The photosensitive media may also contain a sensitiser in order to increase the sensitivity of the system towards radiation of a wavelength longer than that of the intrinsic U.V./blue sensitivity. By suitable selection of the sensitiser a photothermographic element may be prepared which is sensitive to radiation of a selected wavelength band within the general range 300 to 1000nm, the particular wavelength and the width of the band depending upon the absorption characteristics of the sensitiser. A wide variety of sensitisers are known in the art, but best results are obtained when the sensitiser is free from strongly basic groups that may scavenge the photogenerated acid. Depending on the intended use, it may also be desirable for the sensitiser and/or photo-acid generator to bleach during the exposure/development cycle. For example, if the final image is to be used as a contacting mask in a subsequent imaging process (e.g., of a printing plate), it is vital that the non-image areas are transparent to near-UV and visible light, e.g., in the range 350 to 450 nm, and especially at wavelengths >380nm. Examples of bleachable sensitisers include: xanthene dye complexes, as disclosed, e.g., in U.S. Patent Specification No. 4924009, in which a xanthene dye, such as rose bengal, eosin, erythrosin, a fluorescein dye or an ester thereof, is complexed with the photo-acid generator; bleachable 3-substituted coumarin compounds, e.g., as disclosed in U.S. Patent Specification No. 4147552, such as 7-diethylamino-5',7'-dimethoxy-3,3'-carbonylbiscoumarin, 3,3'-carbonylbis-5,7-dimethoxycoumarin, 7-diethylamino-3,3'-carbonylbiscoumarin and 7-diethylamino-7'-methoxy-3,3'-carbonylbiscoumarin; dialkoxyanthracenes, e.g., 9,10-diethoxyanthracene; 1,4-dihydropyridine sensitisers such as those disclosed at the XIIIth Symposium on Photochemistry, e.g., 3,5-bis(methoxycarbonyl)-2,6-dimethyl-4-phenyl-1,4-dihydropyridine; and 1,3,5-triarylpyrazolines of the formula: in which; Ar represents an aryl group. In use, the photothermographic elements of the invention are imagewise exposed to radiation of an appropriate wavelength to photolyse the photo-acid generator, thereby creating a latent image. Following exposure, the element is dry processed by heating at temperatures of from 90 to 150°C for 5 to 60 seconds, preferably 125 to 130°C for 5 to 15 seconds, using techniques known in the art for the processing of negative-acting Dry Silver materials, to develop a positive image. After exposure and heating to achieve the black/white image a further exposure allows for the image to be fixed for duplication purposes. The invention will now be described with reference to the following Examples. A glossary of common abbreviations etc., is present below for ease of comprehension. Cellulose Acetate Solution = a solution of cellulose acetate (8.3 wt.%) in acetone 29.2 wt.%) and methyl ethyl ketone (62.5 wt.%) CAB 381-20 = Cellulose acetate butyrate, commercially available from Kodak Ltd. Sensitiser TPP = 1,3,5-triphenylpyrazoline DH = 3,5-bis(methoxycarbonyl)-2,6-dimethyl-4-phenyl-1,4-dihydropyridine DEA = 9,10-diethoxyanthracene Photo-acid generators Reducible silver Source Silver Behenate Full soap : 12.3% by weight silver behenate; 65.3% by weight methyl ethyl ketone; 21.8% by weight toluene and 0.5% by weight BUTVAR B-76 (poly(vinyl butyral)). Silver Soap Stock Solution: Silver Behenate Full Soap (40g); CAB-381 (4g); TMPTA (1g) and methyl ethyl ketone (17g). Reducing Agent Toner TCPA = tetrachlorophthallic anhydride "FX512" (3M Co), "CAB 381-20" (Kodak Ltd), "ATM 11" (Ancomer Chemicals Ltd.) "NU-ARC" and "BUTVAR B-76" are all trade names/designations. Example 1 The following photothermographic elements were prepared in accordance with the invention. Each element generally comprises a bottomcoat containing the reducible silver source (silver behenate) and a topcoat containing the developer. Photothermographic Element 1 Bottomcoat Silver Behenate (Full Soap) = 20 g HHA = 2g CAB 381-20 = 2g Iodonium Salt A = 0.16 g TPP = 0.007g Methyl ethyl ketone (MEK) = 8g Topcoat Cellulose Acetate solution = 15g Developer A = 0.4g Toner A = 0.04g Sulphonium Salt A = 0.43g Photothermographic Element 2 Bottomcoat Silver Behenate (Full Soap) = 30g CAB 381-20 = 1.5g TMPTA = 4.5 g TPP = 0.005g Oxonol Dye A = 0.0045g MEK = 3g Iodonium Salt A = 0.2g Topcoat Cellulose Acetate solution = 10g Developer A = 0.3 g Toner A = 0.03g Sulphonium Salt A = 0.40 g Photothermographic Element 3 Bottomcoat Silver Behenate (Full Soap) = 20g TMPTA = 3g CAB 381-20 = 1g Iodonium Salt A = 0.15 g TPP = 0.01g MEK = 4g Topcoat Cellulose Acetate solution = 15g Developer = 0.45g Toner = 0.045 g Sulphonium Salt A = 0.53g Elements 4 to 6: prepared as per Photothermographic Elements 1 to 3 but omitting the photo-acid generator (Sulphonium Salt A) from the topcoat. Photothermographic Element 7 Bottomcoat Silver Behenate (Full Soap) = 10g BUTVAR B-76 = 1g Topcoat Cellulose Acetate solution = 10g Developer A = 0.3g Toner A = 0.03g Sulphonium Salt A = 0.40g Photothermographic Element 8 Bottomcoat Silver Behenate (Full Soap) = 10g CAB 381-20 = 0.5g TMPTA = 1.5g Topcoat Cellulose Acetate solution = 10g Developer A = 0.3g Toner A = 0.03g Sulphonium Salt A = 0.40g Photothermographic Element 9 Bottomcoat Silver Soap Stock Solution = 8g Sulphonium Salt A = 0.16g Topcoat Cellulose Acetate solution = 10g Developer A = 0.15g Toner A = 0.03g sulphonium Salt A = 0.2g Photothermographic Element 10 Bottomcoat Silver Soap Stock Solution = 20g Sulphonium Salt A = 0.4g DH = 0.016g Topcoat Cellulose Acetate solution = 10g Developer A = 0.15g Toner A = 0.03g sulphonium Salt A = 0.2g Photothermographic Element 11 Bottomcoat Silver Soap Stock Solution = 20g Sulphonium Salt A = 0.42g TPP = 0.0047g Topcoat Cellulose Acetate solution = 10g Developer A = 0.15g Toner A = 0.03g sulphonium Salt A = 0.2g Photothermographic Element 12 Bottomcoat Silver Behenate (Full Soap) = 20 g CAB 381-20 = 2g TMPTA = 1g Sulphonium Salt A = 0.5g TPP = 0.006g MEK = 8g Topcoat Cellulose Acetate solution = 10g Developer A = 0.15g Toner A = 0.02g Photothermographic Element 13 Bottomcoat Silver Behenate (Full Soap) = 20g CAB 381-20 = 2g TMPTA = 1g Sulphonium Salt A = 0.5g TPP = 0.006g MEK = 8g Topcoat Cellulose Acetate solution = 10g Developer A = 0.15g Toner A = 0.04g Photothermographic Element 14 Bottomcoat Silver Soap Stock Solution = 20g Sulphonium Salt A = 0.42g TPP = 0.0047g Topcoat Cellulose Acetate solution = 15g Developer A = 0.45g Toner A = 0.045g sulphonium Salt A = 0.45g Photothermographic Element 15 Bottomcoat Silver Soap Stock Solution = 20g Sulphonium Salt A = 0.42g TPP = 0.0047g Topcoat Cellulose Acetate solution = 15g Developer A = 0.225g Toner A = 0.045g sulphonium Salt A = 0.45g All coatings were applied using a knife edge coater to an unsubbed polyester support (100 µm) at 150 µm wet thickness unless otherwise stated. All UV exposures were performed using a 6kW NU-ARC UV lamp and the optical density measurements using a DT 1405 transmission densitometer. D LogE curves were plotted using the results from step-wedge exposures. Example 2 Individual samples of Photothermographic Elements 1 to 6 were each exposed to 50 units of UV radiation and thermally processed for 10 seconds at various temperatures. The optical densities of the exposed (Dmin) and unexposed (Dmax) areas are tabulated in Tables 1 to 3 below. Thermal Processing Temperature (°C) Element 1 Element 4 Dmin Dmax Dmin Dmax 110 0.04 0.04 0.04 0.04 115 0.04 0.05 0.04 0.05 120 0.04 0.11 0.05 0.10 125 0.04 0.66 0.12 0.74 130 0.09 1.95 0.45 0.84 135 0.39 3.64 1.52 3.22 Thermal Processing Temperature (°C) Element 2 Element 5 Dmin Dmax Dmin Dmax 105 0.04 0.06 0.04 0.08 110 0.04 0.09 0.05 0.12 115 0.04 0.45 0.08 0.43 120 0.04 0.91 0.23 1.21 125 0.10 1.94 0.87 2.12 130 0.37 2.76 1.71 2.84 Thermal Processing Temperature (°C) Element 3 Element 6 Dmin Dmax Dmin Dmax 105 0.03 0.06 0.03 0.06 110 0.04 0.08 0.04 0.10 115 0.04 0.21 0.04 0.28 120 0.04 0.99 0.06 0.88 125 0.06 1.82 0.26 2.25 130 0.12 2.68 0.97 2.98 In Elements 4 to 6, silver behenate was present in a photocurable bottom layer in which the photoinitiator (an iodonium salt) was also an acid generator, and developer was present in the top layer. Elements 1 to 3 were identical except for additional acid generator (sulphonium salt) in the top layer. As can be seen from Tables 1 to 3, both sets of coatings gave useful images but those containing the extra photo-acid generator (Elements 1 to 3) showed lower Dmin and higher Dmax. All the images produced were positive and neutral in colour. The UV/vis spectra of samples of Elements 3 and 6 processed at 120 and 125°C respectively were measured (see Figures 1a and 1b) and clearly show a loss of absorption across the region 360 to 700 nm. Example 3 Individual samples of Photothermographic Elements 7 and 8 in which the only photo-active component, was the photo-acid generator, i.e., no photocurable components, were each exposed to 50 units of UV radiation and thermally processed. The optical densities of the exposed (Dmin) and unexposed (Dmax) areas are tabulated in Table 4 below. Thermal Processing Temperature (°C) Element 7 Element 8 Dmin Dmax Dmin Dmax 105 0.04 0.04 0.04 0.05 110 0.05 0.06 0.04 0.07 115 0.05 0.09 0.04 0.20 120 0.05 0.21 0.04 1.04 125 0.05 0.70 0.05 2.35 127 0.06 2.57 - - 130 0.08 3.59 0.06 3.48 135 - - 0.12 3.74 All the images produced were positive and neutral in colour. The D LogE curves for both elements are similar (see Table 5 below) and show that both constructions have a high gamma with little optical changes after step level 6. The D LogE curve for Element 7 is shown in Figure 2. Step Level D LogE Results (Optical Density) Element 7 Element 8 Dmin (0) 0.05 0.05 1 0.14 0.15 2 0.44 0.61 3 1.14 1.60 4 1.71 2.17 5 2.06 2.22 6 2.04 2.34 7 1.99 2.34 8 2.39 2.42 9 2.33 2.42 10 2.15 2.54 11 2.12 2.53 12 2.10 2.58 13 - 2.57 Example 4 This Example demonstrates the thermal ageing properties of Element 7. Individual strips were imaged using a continuous-tone wedge, thermally processed and then stored at 50°C. The change in density of each step level was then measured. The results are tabulated in Table 6 below. Element 7 Step Level 0 6 days Dmin (0) 0.06 0.07 1 0.08 0.10 2 0.13 0.15 3 0.31 0.36 4 0.75 0.82 5 0.94 1.14 6 1.11 1.40 7 1.27 1.59 8 1.61 2.07 9 1.58 1.86 10 1.63 2.11 11 1.53 1.84 These results show that Element 7 is almost unchanged after thermal ageing for 6 days at 50°C and indicates the possibility that images could be prepared which are suitable for long-term archival storage. Example 5 This Example demonstrates the effect of using a sensitiser in conjunction with the photo-acid generator to increase the sensitivity of the system. The sensitisers tested were a 1,4-dihydropyridine derivative (DH) and a 1,3,5-triarylpyrazoline (TPP). Individual samples of Photothermographic Elements 9 to 11 were each exposed to 50 units of UV radiation and thermally processed. The D LogE results are described in Table 7 and Figure 3. Step Level Element 9 (No sensitiser) Element 10 (DH) Element 11 (TTP) Dmim 0 0.06 0.04 0.06 1 0.11 0.05 0.05 2 0.16 0.06 0.05 3 0.27 0.08 0.05 4 0.50 0.12 0.06 5 0.71 0.23 0.07 6 1.03 0.51 0.11 7 1.00 0.68 0.19 8 1.04 0.88 0.32 9 1.06 1.04 0.61 10 1.09 1.18 0.99 11 1.09 1.41 1.15 12 1.25 1.32 1.25 13 1.23 1.34 1.25 The above results show that adding a sensitiser to the photosensitive medium increases the sensitivity of the Element. This was particularly true of Element 11 containing the triarylpyrazoline sensitiser, since this composition was able to give good positive images after an exposure of only 7.5 units of UV radiation (see Table 8 for results. Thermal Processing Temperature (°C) Element 11 Dmin Dmax 115 0.04 0.06 120 0.04 0.27 125 0.05 0.99 130 0.10 2.16 135 0.14 2.61 Example 6 As the photolysis of a sulphonium salt generates both radicals and acid which can oxidise the developer or inhibit the thermographic reaction, two experiments were performed to investigate the effect of changing the toner or developer content on the photosensitive media. In the first experiment, the toner content was doubled and the effect on the sensitivities of the construction measured. The results obtained are tabulated in Table 9 below. Step Level D LogE Results (Optical Density) Element 12 Element 13 Dmin (0) 0.08 0.05 1 0.08 0.08 2 0.07 0.12 3 0.09 0.21 4 0.12 0.36 5 0.16 0.74 6 0.29 0.93 7 0.50 1.13 8 0.72 1.43 9 0.93 1.74 10 1.07 1.80 11 1.18 1.89 12 1.19 1.89 13 1.17 1.96 These results show that increasing the toner content decreases the sensitivity of the construction (see Figure 4). This would suggest that the imagewise production of acid is inhibiting the thermographic development of the silver behenate. In the second experiment, the developer content was doubled. As the radicals which can oxidize the developer have a short lifetime (i.e., they can be quenched by oxygen), the photo-acid generator was incorporated into both the top and bottomcoats. The results obtained are tabulated in Table 10 below. Step Level D LogE Results (Optical Density) Element 14 Element 15 Dmin (0) 0.06 0.07 1 0.05 0.07 2 0.05 0.07 3 0.05 0.07 4 0.06 0.07 5 0.07 0.08 6 0.11 0.09 7 0.19 0.12 8 0.32 0.29 9 0.61 0.55 10 0.99 0.89 11 1.25 1.19 12 1.15 1.35 13 1.26 1.49 The results show that Elements 14 and 15 (see Figure 5) have similar sensitivities to Element 12 & 13 and confirm that inhibition of the thermographic development of the silver behenate is caused by the imagewise production of acid. EXAMPLE 7 This example shows how the sensitivity of the photothermographic elements of the invention may be improved by the provision of a topcoat (or barrier layer) of cellulose acetate. Element 16 was prepared from the following formulations:- Bottomcoat Silver Behenate (Full Soap) = 20g BUTVAR B-76 = 2g tetrachlorophtalic anhydride = 0.004g Developer A = 0.3g Middlecoat BUTVAR B-76 = 1.5g Methyl ethyl ketone = 5g toluene = 5g Sulphonium Salt A = 0.6g DEA = 0.008g Toner A = 0.015g Topcoat Cellulose Acetate Solution These formulations were coated at 100, 150 and 100 µm wet thickness respectively, each coating being dried for 3 minutes at 80°C before adding the next. An analogous 2-layer element (Element 17) was prepared as follows:- Bottomcoat Silver Behenate (Full Soap) = 30g BUTVAR B-76 = 3g tetrachlorophthalic anhydride = 0.006g Developer A = 0.45 g Topcoat Cellulose Acetate Solution = 30g Toner A = 0.045g DEA = 0.024g Sulphonium Salt A = 1.5g Each formulation was coated at a wet thickness of 150 µm and dried for 3 minutes at 80°C. Samples of each element were UV-exposed to 5 units (Element 16) and 10 units (Element 17) and thermally processed at different temperatures. The optical densities of the exposed (Dmin) and unexposed (Dmax) areas are tabulated in Table 11 below. Thermal Processing Temperature (°C) Element 16 Element 17 Dmin Dmax Dmin Dmax 115 0 0 0 0.01 120 0 0 0 0.10 125 0 0.07 0 0.63 130 0 0.39 0.01 0.98 135 0 0.80 0.05 2.59 140 0.01 1.56 0.11 2.72 145 0.01 1.67 - - Element 16 incorporating the optional barrier layer showed very low Dmin (and acceptable Dmax) with only 5 units exposure. Samples of both elements were given 10 units exposure through a step wedge and processed at 130°C (Element 17) and 144° (Element 16) for 10 seconds. The DlogE curves were plotted, and are shown in Figure 6. The greater speed and contrast of Element 16 is readily apparent. Example 8 This Example demonstrates the use of an iodonium salt as a photo-acid generator in combination with 9, 10-diethoxyanthracene (DEA) as a sensitiser. Element 18 was prepared from the following formulations:- Bottomcoat Silver Behenate (Full Soap) =20g BUTVAR B-76 = 2g Developer A = 0.15g TCPA = 0.004g Middlecoat BUTVAR B-76 = 1.5g methanol = 5g Toner A = 0.015g methyl ethyl ketone = 5g Iodonium Salt B = 0.15g DEA = 0.008g Topcoat Cellulose Acetate Solution These formulations were coated at 150 µm wet thickness on a vesiculated polyester support. Samples of Element 18 were UV-exposed to 25 units and then thermally processed at different temperatures to give the results shown in Table 12 below. Thermal Processing Temperature (°C) Element 18 Dmin Dmax 125 0.02 0.03 130 0.04 0.08 135 0.04 0.11 140 0.03 0.24 145 0.05 0.31 The results show that an image, albeit rather faint, is produced using the combination of iodonium and 9,10-diethoxyanthracene. Processing at higher temperatures may provide clearer images. Example 9 This Example demonstrates the use of Triazine A as a photo-acid generator. Element 19 was prepared from the following formulations:- Bottomcoat Silver Behenate (Full Soap) = 20g BUTVAR B-76 = 2g Developer A = 0.15g TCPA = 0.004g Middlecoat BUTVAR B-76 = 1.5g toluene = 5g Toner A = 0.015g methyl ethyl ketone = 5g Triazine A = 0.2g Topcoat Cellulose Acetate Solution These formulations were coated at 150 µm wet thickness on a vesicular polyester support. Individual samples of Element 18 were UV-exposed to 25 units and then thermally processed at different temperatures to give the results shown in Table 13 below. Thermal Processing Temperature (°C) Element 19 Dmin Dmax 120 0.00 0.13 125 0.03 0.58 130 1.30 1.30 The results show that, by processing the exposed samples at lower temperatures, it is possible to produce a (faint) positive image. However when processing at higher temperatures, the whole element darkens. The images produced by this particular triazine compound are not stable as they will bleach on exposure to UV light owing to the production of chlorine radicals.

Claims: A positive-acting photothermographic element having a photosensitive medium comprising a reducible silver source, less than 0.75% by weight of silver halide based on the weight of the reducible silver source, a photo-acid generator, a binder and a reducing system for silver ion comprising a reducing agent for silver ion in which exposure of the element to actinic radiation causes the production of acidic species in the exposed regions which inhibit reduction of the silver source by the reducing system with the proviso the photosensitive medium is free of nucleating agents. A positive-acting photothermographic element as claimed in Claim 1 comprising a support having coated thereon a first layer comprising the reducible silver source and binder and a second layer comprising the reducing agent for silver ion, the photo-acid generator and binder. A positive-acting photothermographic element as claimed in Claim 1 comprising a support having coated thereon a first layer comprising the reducible silver source, the photo-acid generator and binder and a second layer comprising the reducing agent for silver ion and binder. A positive-acting photothermographic element as claimed in any one of Claims 1 to 3 in which the reducible silver source comprises a silver salt or complex of an organic or a hetero-organic acid. A positive-acting photothermographic element as claimed in Claim 4 in which the reducible silver source is silver behenate. A positive-acting photothermographic element as claimed in any preceding Claim in which the reducing agent for silver ion is phenidone, hydroquinone, catechol or a hindered phenol. A positive-acting photothermographic element as claimed in Claim 6 in which the reducing agent is a hindered phenol having a nucleus of general formula (I): in which; R represents hydrogen or an alkyl group generally comprising up to 5 carbon atoms. A positive-acting photothermographic element as claimed in any preceding Claim in which the reducing system is in a layer separate from the reducible silver source and comprises a toner selected from phthalazinone, phthalazine or phthalic acid, optionally in combination with tetrachlorophthalic acid or its anhydride. A positive-acting photothermographic element as claimed in any preceding Claim in which the photo-acid generator is selected from sulphonium salts, iodonium salts, phosphonium salts, diazonium salts, quinone diazide sulfochlorides, bis(trichloromethyl)-s-triazines, trichloromethylpyrones and mixtures of organohalogen and organometal compounds. A positive-acting photothermographic element as claimed in Claim 9 in which the photo-acid generator is an iodonium salt of general formula (II) or a sulphonium salt of general formula (III): (II) X - I + -(A) 2 (III) X - S + -(A) 3 in which; each A independently represents an aromatic group which may be linked together to include the iodine or sulphur atom within a ring structure, and X - is an anion such that HX is an acid of p K a not greater than 3 on photolysis. A positive-acting photothermographic element as claimed in Claim 9 or Claim 10 in which the photo-acid generator comprises a cation selected from: and an anion selected from C1 - , Br - , SbF 6 - , PF 6 - , BF 4 - , AsF 6 - , HSO 4 - , CF 3 SO 3 - , CF 3 CO 2 - , CC1 3 CO 2 - , CH 3 SO 3 - and CH 3 C 6 H 4 SO 3 - . A positive-acting photothermographic element as claimed in any preceding Claim in which the medium further comprises a sensitiser selected from xanthene dyes, 3-substituted coumarins, dialkoxyanthracenes, 1,4-dihydropyridines, 1,3,5-triarylpyrazolines and oxonol dyes. A method of generating a positive image comprising (a) imagewise exposing a photothermographic element as claimed in any one of Claims 1 to 12 to radiation to produce a latent image and (b) heating the exposed element to develop the latent image.

Inventors: ELLIS RICHARD J C O MINNESOTA, ELLIS, RICHARD J., Ellis, Richard J., c/o Minnesota 3M Research Ltd.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1992
Main classification: G03C1
IPC codes: G03C1, G03C8, G03C5