Emulsion preparation using simultaneous chemical and spectral sensitization in the presence of a salt

Abstract: A method is disclosed for the preparation of a photographic silver halide emulsion comprising the steps of precipitation, chemical ripening and spectral sensitization, characterized in that said chemical ripening step and said spectral sensitization step are performed simultaneously and in the presence of an water-soluble metal salt the anion of which can form with a silver cation a salt with a solubility product higher than 10 -7.5 . Improved sensitometric characteristics and aptitude for rapid processing are obtained.

Description: 1. Field of the invention. The present invention relates to a method for the preparation of a photographic silver halide emulsion with improved characteristics. 2. Background of the invention. One of the most evident and permanent goals of photographic silver halide emulsion technology consists in ever further improving sensitivity without rising fog and without being obliged to increase the average emulsion grain size which would result in increased granularity. This longterm goal is known by photographic scientist as optimization of the grain-fog-sensitivity relationship. The term sensitivity can apply to the intrinsic blue or near-ultraviolet sensitivity or to the sensitivity in a defined spectral region. Another more recent goal of emulsion technology is providing emulsions which are suitable for so-called rapid processing. This means emulsions which can be processed in a relative short time without disadventageous side-effects like unsufficiently developed exposed silver halide, unfixed rest silver halide or residual dye stain. Still another goal of emulsion technology consists in performing the consecutive steps of emulsion preparation starting from the precipitation stage up to the final coating in a rapid, efficient and cost-effective way, in the ideal case even in a continuous on-line way. EP-A-291 339 claims the addition of a spectral sensitising dye during the desalting step after silver halide precipitation. US-A-5 141 846 discloses the advantageous effect achieved by the addition of a spectral sensitising dye and a silver halide complexing agent during the chemical ripening of silver halide (111) grains. One of the numerous research directions tried in the past in order to achieve one or more of these goals implied the addition of a water-soluble salt in some particular stage of the emulsion preparation. For instance, US 2,839,405 discloses the use of several metal salts for improvement of fog and stability of bromoiodide emulsions. The salt is added after chemical and spectral sensitization. Jones US 2,843,490 describes the addition of a divalent metal salt during spectral sensitization by a meso-substituted carbocyanine dye ; sensitivity and stability are claimed to be improved. US 3,804,634 discloses the combination of a merocyanine dye and a divalent cadmium salt. US 3,837,862 claims a combination of a cyanine and a merocyanine dye ; some particular inorganic salts can be present. US 3,865,598 deals with the addition of bromides or iodides to an emulsion containing a J-band forming green sensitizer. The salt is added after the chemical ripening and before the spectral sensitization. It is believed that a halide conversion takes place, whereafter the sensitizer adsorbs better at the newly formed conversion zones. Unexamined Japanese patent publication (Kokai) 57-104926 claims increased sensitivity and lower fog by the combined presence of some specified salts and some particular thioethers. US 5,130,212 discloses a process combining the presence of a water-soluble calcium salt with a cyanine and a merocyanine dye. Some scientific studies deal with the mechanism of the effect of salt in emulsion preparation. So Z. Jin-Wei et al, Photog. Sci. Photochem (1). 37, (1989) and T. Matsubara et al, J. Imag. Sci., 35(4), 274, (1991) established that the addition of inorganic salts to spectral sensitizers in solution can lead to the formation of aggregates. Further it is known from photographic literature that the addition of inorganic salt or the change in polarity of the medium can influence the strenght of adsorption (e.g. H.J. Price, J. Imag. Sci., 31(3), 130 (1987) and Y. Suda et al, J. Soc. Photogr. Sci. Techn. Jpn, 50(4), 281 (1987)). The addition of salts can also improve the storage properties and the sensitivity of the prepared material. Price also established that for some cyanine dyes it is necessary to have a certain amount of calcium ions present in order to obtain the right degree of aggregation. He showed that a little amount of calcium added to deionized gelatin results in a much better adsorption. The present invention constitutes a further extension and improvement on the teachings of the use of water-soluble salts during emulsion preparation. It is an object of the present invention to provide a method for the preparation of silver halide emulsions with improved sensitometric characteristics. It is another object of the present invention to provide a method for the preparation of silver halide emulsions which are suitable for rapid processing. It is still a further object of the present invention to provide a method by which the total emulsion preparation sequence starting from the precipitation up to the final coating can be performed in a rapid and cost-effective way. 3. Summary of the invention. The objects of the present invention are realized by providing a method for the preparation of a photographic silver halide emulsion comprising the steps of precipitation, chemical ripening and spectral sensitization, characterized in that said chemical ripening step and said spectral sensitization step are performed simultaneously and in the presence of a water-soluble metal salt the anion of which can form with a silver cation a salt with a solubility product higher than 10 -7.5 . In a preferred embodiment the salts resulting from the precipitatation, e.g. potassium nitrate, are removed and then during the simultaneous chemical and spectral sensitization step a water-soluble metal salt is added in order to establish a desired optimal concentration. In a most preferred embodiment the cation of the metal salt is divalent or polyvalent. The advantages of the present invention are most perspicuous when cyanine dyes are used as spectral sensitizers. 4. Detailed description of the invention. In principle the soluble salts produced during emulsion precipitation, usually potassium nitrate, can be used as the water-soluble metal salt of the present invention and stay present while performing the simultaneous chemical ripening and spectral sensitization steps. However in a more preferred embodiment the precipition salts are removed by a conventional washing procedure, e.g. a procedure involving flocculation, several washings and redispersion, or an ultrafiltration procedure. Thereafter the concentration of the water-soluble metal salt present during the simultaneous chemical and spectral sensitization step can he freely chosen in order to establish a desired optimal ionic strenght of the reaction mixture. It is essential for the present invention, that, contrary to prior art, the chemical ripening and the spectral sensitization are really performed simultaneously, and that the water-soluble metal salt is also present from the start of that simultaneous process. This means in practice that the spectral sensitizing dye(s), the chemical ripening agent(s) and the water-soluble salt(s) are added together to the emulsion or, to work more practically, with short intervals after each other. In principle any addition order can be followed. However preferred sequences are (a) first adding sensitizing dye(s), then salt(s) and finally ripening agent(s), or (b) first salt(s), then dye(s) and finally ripening agent(s). The light-sensitive silver halide emulsions can be spectrally sensitized with methine dyes such as those described by F.M. Hamer in "The Cyanine Dyes and Related Compounds", 1964, John Wiley & Sons. Dyes that can be used for the purpose of spectral sensitization include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, hemicyanine dyes, styryl dyes and hemioxonol dyes. However the preferred class of sensitizing dyes for use in accordance with the present invention is the class of cyanine dyes, and most preferably polar cyanine dyes poorly adsorbing on a silver halide grain surface. Together with the sensitizing dye a so-called supersensitizer can be added. Suitable supersensitizers are i.a. heterocyclic mercapto compounds containing at least one electronegative substituent as described e.g. in US 3,457,078, nitrogen-containing heterocyclic ring-substituted aminostilbene compounds as described e.g. in US 2,933,390 and US 3,635,721, aromatic organic acid/formaldehyde condensation products as described e.g. in US 3,743,510, cadmium salts, and azaindene compounds. In principle a mixture of more than one sensitizing dye can be used. Some especially useful cyanine dyes are represented below : The fact that the advantages of the present invention are most perspicuous when employing polar poorly adsorbing cyanine dyes can readily be explained as follows. Apolar dyes due to their nature tend to be rapidly adsorbed at the emulsion grains and the use of high ionic strenght coming from an extra salt will be of little or no value in further promoting this adsorption. However due to the fast adsorption this type of dyes will be predominantly adsorbed at the grains in the proximity of the region of addition ; this can result in a heterogeneous spectral sensitization giving rise to a non-ideal photographic response. On the other side polar dyes adsorb slowly and they will be homogeneously mixed before substantial adsorption occurs. However using them causes a slow-down in the over-all emulsion production process due to their slow adsorption kinetics. The practice of the present invention can solve this paradox. Slowly adsorbing dyes can be added and after a short period to allow homogeneous mixing the ionic strenght can be increased by the addition of the water-soluble metal salt forcing in this way the dye to adsorb on the grain. If these conditions are maintained up to the final coating the dye will experience difficulty in desorbing from the grain and the desired photographic effect will be reached. In this way an emulsion production process can be realized which is rapid, efficient and cost-effective, one of the objects of the present invention. The water-soluble salt in connection with the present invention can be all kinds of salt the anion of which can form with a silver cation a salt with a solubility product higher than 10 -7.5 , which is considerably higher than the solubility products of the silver halides. By doing so no anion conversion can occur at the emulsion grain surface. Examples of useful salts include magnesium sulphate, calcium nitrate, lithium sulphate, sodium sulphate, potassium sulphate, sodium nitrate and potassium nitrate, in decreasing order of preference. Salts with an divalent or polyvalent cation are preferred, because salts of this type need to be added in a lower molar concentration in order to reach a desired level of ionic strenght or conductivity than it is the case with monovalent cations. In this way the finally coated emulsion layer in not overloaded with salt. The most preferred salts are magnesium sulphate and calcium nitrate. In principle, mixtures of two or more salts can be used. Preferably the salts are added as an aqueous solution, e.g. a 0.5 M to 1.3 M solution. The optimal concentration of sensitizing dye and water-soluble salt shell be established by experiment for each particular combination of dye and salt under consideration. Usualy a dye concentration is chosen as to obtain a theoretical grain coverage between 10 and 200 percent. Preferably the amount added of the water-soluble salt is established as to obtain a concentration between 0.001 and 1.5 M, and most preferably between 0.01 M and 1.0 M. The formation of the silver halide grains itself occurs according to well-known conventional techniques. They can be prepared by mixing the halide and silver solutions in partially or fully controlled conditions of temperature, concentrations, sequence of addition, and rates of addition. The silver halide can be precipitated according to the single-jet method, the double-jet method, or the conversion method. The resulting silver halide particles may have a regular crystalline form such as a cubic or octahedral form or they may have a transition form. They may also have an irregular crystalline form such as a spherical form or a tabular form, or may otherwise have a composite crystal form comprising a mixture of said regular and irregular crystalline forms. The silver halide grains may have a multilayered grain structure. According to a simple embodiment the grains may comprise a core and a shell, which may have different halide compositions and/or may have undergone different modifications such as the addition of dopes. Besides having a differently composed core and shell the silver halide grains may also comprise different phases inbetween. The average size of the silver halide grains may range from 0.05 to 1.0 micron, preferably from 0.2 to 0.5 micron. The size distribution of the silver halide particles of the photographic emulsion can be homodisperse or heterodisperse. A homodisperse size distribution is obtained when 95% of the grains have a size that does not deviate more than 30% from the average grain size. The silver halide emulsion can be chemically ripened as described i.a. in "Chimie et Physique Photographique" by P. Glafkides, in "Photographic Emulsion Chemistry" by G.F. Duffin, in "Making and Coating Photographic Emulsion" by V.L. Zelikman et al, and in "Die Grundlagen der Photographischen Prozesse mit Silberhalogeniden" edited by H. Frieser and published by Akademische Verlagsgesellschaft (1968). As described in said literature chemical sensitization can be carried out by effecting the ripening in the presence of small amounts of compounds containing sulphur e.g. thiosulphate, thiocyanate, thioureas, sulphites, mercapto compounds, and rhodamines. The emulsions can be sensitized also by means of gold-sulphur ripeners or by means of reductors e.g. tin compounds as described in GB 789,823, amines, hydrazine derivatives, formamidine-sulphinic acids, and silane compounds. In a preferred embodiment conventional gold-sulphur ripening agents are used. The halide composition of the silver halide emulsions prepared according to the method of this invention is not specifically limited and may be any composition selected from silver chloride, silver bromide, silver iodide, silver chlorobromide, silver bromoiodide, and silver chlorobromoiodide. However the method of the present invention is particularly suited for the preparation of emulsions containing substantially no iodide, and most particularly for rather sensitive silver bromide emulsions containing substantially no iodide. As it is known to those skilled in the art the presence of small mole percents of iodide (1 % to 5 %) in the grain and especially at the grain surface promotes the adsorption of sensitizing dyes. On the other side the presence of iodide slows down the chemical sensitization and the development and fixation speed. This could render the coated emulsion unsuitable for rapid processing, resulting in insufficient development, unfixed rest silver halide and residual dye stain. The practice of the present invention offers the possibility of preparing emulsions substantially free of iodide, suited for rapid processing, and yet obtaining a spectral sensitivity equal to a comparable emulsion containing iodide, as will proved by the examples furtheron. The silver halide emulsion for use in accordance with the present invention may comprise compounds preventing the formation of fog or stabilizing the photographic characteristics during the production or storage of photographic elements or during the photographic treatment thereof. Many known compounds can be added as fog-inhibiting agent or stabilizer to the silver halide emulsion. Suitable examples are i.a. the heterocyclic nitrogen-containing compounds such as benzothiazolium salts, nitroimidazoles, nitrobenzimidazoles, chlorobenzimidazoles, bromobenzimidazoles, mercaptothiazoles, mercaptobenzothiazoles, mercaptobenzimidazoles, mercaptothiadiazoles, aminotriazoles, benzotriazoles (preferably 5-methyl-benzotriazole), nitrobenzotriazoles, mercaptotetrazoles, in particular 1-phenyl-5-mercapto-tetrazole, mercaptopyrimidines, mercaptotriazines, benzothiazoline-2-thione, oxazoline-thione, triazaindenes, tetrazaindenes and pentazaindenes, especially those described by Birr in Z. Wiss. Phot. 47 (1952), pages 2-58, triazolopyrimidines such as those described in GB 1,203,757, GB 1,209,146, JP-A 75-39537, and GB 1,500,278, and 7-hydroxy-s-triazolo-[1,5-a]-pyrimidines as described in US 4,727,017, and other compounds such as benzenethiosulphonic acid, benzenethiosulphinic acid and benzenethiosulphonic acid amide. Other compounds that can be used as fog-inhibiting compounds are metal salts such as e.g. mercury or cadmium salts and the compounds described in Research Disclosure N° 17643 (1978), Chapter VI. The fog-inhibiting agents or stabilizers can be added to the silver halide emulsion prior to, during, or after the ripening thereof and mixtures of two or more of these compounds can be used. Besides the silver halide another essential component of a light-sensitive emulsion layer is the binder. The binder is a hydrophilic colloid, preferably gelatin. Gelatin can, however, be replaced in part or integrallly by synthetic, semi-synthetic, or natural polymers. Synthetic substitutes for gelatin are e.g. polyvinyl alcohol, poly-N-vinyl pyrrolidone, polyvinyl imidazole, polyvinyl pyrazole, polyacrylamide, polyacrylic acid, and derivatives thereof, in particular copolymers thereof. Natural substitutes for gelatin are e.g. other proteins such as zein, albumin and casein, cellulose, saccharides, starch, and alginates. In general, the semi-synthetic substitutes for gelatin are modified natural products e.g. gelatin derivatives obtained by conversion of gelatin with alkylating or acylating agents or by grafting of polymerizable monomers on gelatin, and cellulose derivatives such as hydroxyalkyl cellulose, carboxymethyl cellulose, phtaloyl cellulose, and cellulose sulphates. The gelatin can be lime-treated or acid-treated gelatin. The preparation of such gelatin types has been described in e.g. "The Science and Technology of Gelatin", edited by A.G. Ward and A. Courts, Academic Press 1977, page 295 and next pages. The gelatin can also be an enzyme-treated gelatin as described in Bull. Soc. Sci. Phot. Japan, N° 16, page 30 (1966). The gelatin can contain calcium or can be substantially free of calcium. The binders of the photographic element, especially when the binder used is gelatin, can be hardened with appropriate hardening agents such as those of the epoxide type, those of the ethylenimine type, those of the vinylsulfone type e.g. 1,3-vinylsulphonyl-2-propanol, chromium salts e.g. chromium acetate and chromium alum, aldehydes e.g. formaldehyde, glyoxal, and glutaraldehyde, N-methylol compounds e.g. dimethylolurea and methyloldimethylhydantoin, dioxan derivatives e.g. 2,3-dihydroxy-dioxan, active vinyl compounds e.g. 1,3,5-triacryloyl-hexahydro-s-triazine, active halogen compounds e.g. 2,4-dichloro-6-hydroxy-s-triazine, and mucohalogenic acids e.g. mucochloric acid and mucophenoxychloric acid. These hardeners can be used alone or in combination. The binders can also be hardened with fast-reacting hardeners such as carbamoylpyridinium salts as disclosed in US 4,063,952. When the silver halide emulsions are used in colour photographic material the typical ingredients for this type of material are present, e.g. colour forming components, masking compounds, development inhibitor releasing (DIR) compounds, etc. These ingredients are usually added in the form of a dispersion, if needed containing so-called oilformers. The photographic emulsion can be coated on a support by means of any of the conventional coating techniques, e.g. dip coating, air knife coating, extrusion coating, slide hopper coating and curtain coating. Usualy before coating the composition containing the emulsion has to be diluted. In the practice of this invention the coating composition is preferably diluted, not with mere water, but with a salt solution e.g. the same salt as used during the simultaneous chemical and spectral sensitization. In this way the ionic strenght can be maintained at a constant level so that no desorption of the dye from the grain will occur before coating. Beside the light sensitive emulsion layer(s) the photographic material can contain several non light sensitive layers, e.g. an anti-stress top layer, one or more backing layers, and one or more intermediate layers eventually containing filter dyes or antihalation dyes that absorb scattering light and thus promote the image sharpness. Suitable light-absorbing dyes are described in i.a. US 4,092,168, US 4,311,787, DE 2,453,217, and GB 7,907,440. One or more backing layers can be provided at the non-light sensitive side of the support. This layers which can serve as anti-curl layer can contain i.a. matting agents e.g. silica particles, lubricants, anti static agents, light absorbing dyes, opacifying agents, e.g. titanium oxide and the usual ingredients like hardeners and wetting agents. The photographic material may further comprise various kinds of surface-active agents in the photographic emulsion layer or in at least one other hydrophilic colloid layer. Suitable surface-active agents include non-ionic agents such as saponins, alkylene oxides e.g. polyethylene glycol, polyethylene glycol/polypropylene glycol condensation products, polyethylene glycol alkyl ethers or polyethylene glycol alkylaryl ethers, polyethylene glycol esters, polyethylene glycol sorbitan esters, polyalkylene glycol alkylamines or alkylamides, silicone-polyethylene oxide adducts, glycidol derivatives, fatty acid esters of polyhydric alcohols and alkyl esters of saccharides; anionic agents comprising an acid group such as a carboxy, sulpho, phospho, sulphuric or phosphoric ester group; ampholytic agents such as aminoacids, aminoalkyl sulphonic acids, aminoalkyl sulphates or phosphates, alkyl betaines, and amine-N-oxides; and cationic agents such as alkylamine salts, aliphatic, aromatic, or heterocyclic quaternary ammonium salts, aliphatic or heterocyclic ring-containing phosphonium or sulphonium salts. Such surface-active agents can be used for various purposes e.g. as coating aids, as compounds preventing electric charges, as compounds improving slidability, as compounds facilitating dispersive emulsification, as compounds preventing or reducing adhesion, and as compounds improving the photographic characteristics e.g higher contrast, sensitization, and development acceleration. Preferred surface-active coating agents are compounds containing perfluorinated alkyl groups. The photographic element of the present invention may further comprise various other additives such as e.g. compounds improving the dimensional stability of the photographic element, UV-absorbers, spacing agents and plasticizers. Suitable additives for improving the dimensional stability of the photographic element are i.a. dispersions of a water-soluble or hardly soluble synthetic polymer e.g. polymers of alkyl(meth)acrylates, alkoxy(meth)acrylates, glycidyl (meth)acrylates, (meth)acrylamides, vinyl esters, acrylonitriles, olefins, and styrenes, or copolymers of the above with acrylic acids, methacrylic acids, Alpha-Beta-unsaturated dicarboxylic acids, hydroxyalkyl (meth)acrylates, sulphoalkyl (meth)acrylates, and styrene sulphonic acids. The support of the photographic material may be opaque or transparent, e.g. a paper support or resin support. When a paper support is used preference is given to one coated at one or both sides with an Alpha-olefin polymer, e.g. a polyethylene layer which optionally contains an anti-halation dye or pigment. It is also possible to use an organic resin support e.g. cellulose nitrate film, cellulose acetate film, polyvinylacetal film, polystyrene film, polyethylene terephthalate film, polycarbonate film, polyvinylchloride film or poly-Alpha-olefin films such as polyethylene or polypropylene film. The thickness of such organic resin film is preferably comprised between 0.07 and 0.35 mm. These organic resin supports are preferably coated with a subbing layer which can contain water insoluble particles such as silica or titanium dioxide. The photographic silver halide emulsions can be used in various types of black-and-white or colour photographic elements such as in photographic elements for graphic arts, e.g. camera materials, scan materials, image-setting materials and contact materials, and for so-called general amateur and professional photography, diffusion transfer reversal photographic elements, cinamatographic recording and duplicating films, and radiographic recording and duplicating films. In a preferred embodiment the emulsions prepared according to the present invention are sensitive tabular grain emulsions containing no iodide which are used in an X-ray recording film suited for rapid processing. The photographic materials are to be exposed according to their particular composition and application. Possible exposure sources include tungsten light, xenon light, halogen bulbs, metal-halogen lamps, CRT devices, X-ray sources and laser exposure. The processing cycle of the expose photographic materials conventionally includes a development step, a fixing step, a washing step and a drying step. In case of colour materials a bleaching or bleach-fixing step is required. In case of black-and-white development the developing solution, or the material itself in case of so-called "activation development", contains a developing agent, e.g. such well-known substances like 1,4-dihydroxy benzene compounds such as hydroquinone, chlorohydroquinone, bromohydroquinone, toluhydroquinone, morpholinemethyl hydroquinone and sulfohydroquinone. Apart from the main developing agent an auxiliary developing agent can be used. Typical auxiliary developing agents include 3-pyrazolidone developing agents, e.g. 1-phenyl-3-pyrazolidone, 1-phenyl-4,4-dimethyl-3-pyrazolidone, 1-phenyl-4-methyl-4'-hydroxymethyl-3-pyrazolidone, and N-methyl-p-aminophenol sulphate. In case of colour development p-phenylenediamine derivatives are employed as developing agents. In the fixing or bleach-fixing solution usually ammonium or sodium thiosulphte is employed as fixing agent. Useful bleaching agents for bleach or bleach-fixing baths are ferricyanide ions, persulphate ions and complexes of iron(III) and polyaminocarboxylic acids, e.g. iron(III)-ethylenediaminetetraacetic acid mono sodium salt. The following examples illustrate the present invention without however being limited thereto. EXAMPLES EXAMPLE 1 A tabular silver bromoiodide emulsion, containing 1 mole % of AgI and 99 mole % of AgBr, was precipitated using the double jet technique. The thus obtained tabular grain emulsion, containing 75 grams of gelatin pro mole of AgNO 3 , had an average sphere-equivalent diameter of 0.63 micron. A second emulsion, a pure AgBr tabular grain, was manufactured in a similar way by leaving out the iodide during the precipitation process. The average sphere-equivalent diameter of this emulsion was 0.63 micron too. The emulsions were chemically and spectrally sensitised in the presence of sensitizing dye anhydro-5,5'-dichloro-3,3'-bis(n.sulfobutyl)-9-ethyloxacarbocyanine hydroxide (dye 1), chloroauric acid, sodium thiosulphate, toluenethiosulfonic acid, carboxy-Phenidone and potassium thiocyanate and in the presence or absence of a salt (see table 1). The order of addition was first salt, then dye and finally ripening agents. To prepare a coating solution the following ingredients were added per mole of silver halide : 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene (0.29 grams), sorbitol (9.10 grams), polyethylacrylate (MW = 1000000 ; 14.50 grams), 1,3-dihydroxybenzene (3.05 grams), dextrane (MW = 10000 ; 31.00 grams). The emulsion was coated on one side of a polyethylene support at a silver coverage of 3.5 g/m 2 , expressed as AgNO 3 . A protective layer was applied containing gelatin at a coverage of 1.1 g/m 2 , polymethylmethacrylate, formaldehyde hardener and conventional wetting agents. Samples of the photographic materials were exposed by green light of 540 nm during 0.02 seconds using a continuous wedge, and were then processed in a rapid processing cycle composed of following steps : loading (3.4 sec.), developing : 23.4 sec./ 35°C., cross-over : 3.8 sec., fixing : 15.7 sec./ 35°C., cross-over : 3.8 sec., rinsing : 15.7 sec./ 20°C., cross-over + drying : 32.2 sec. : total time : 98.0 sec. Composition of developer : concentrated part A : potassium hydroxide 55.9 g potassium sulphite (65 % solution) 255 g ethylenediaminetetraacetic acid-tetrasodium-trihydrate 8.4 g diethyleneglycol 111.6 g potassium carbonate 128.5 g hydroquinone 120 g 5-methylbenzotriazole 0.36 g water to make 1 l pH adjusted to 11.8 concentrated part B : acetic acid 39.5 g 1-phenyl-3-pyrazolidone 6.0 g 5-nitro-indazole 1.0 g diethyleneglycol to make 100 ml concentrated part C : glutardialdehyde (50 % solution) 21.5 g potassium metabisulphite 36.0 g water to make 100 ml starter : water 15.0 ml acetic acid (96 % solution) 4.0 ml potassium bromide 4.0 g Before the start of the processing the three parts and the starter were mixed in the following ratio : 250 ml of part A, 700 ml of water, 25 ml of part B, 25 ml of part C and 25 ml of starter. The pH of this mixture was then 10.1 at 25 °C. Composition of the fixer : concentrated part A : ammoniumthiosulphate (78 % solution) 661 g sodium sulphite 54 g boric acid 25 g sodium acetate trihydrate 70 g acetic acid 40 g water to make 1 l pH adjusted to 5.3 by acetic acid concentrated part B : acetic acid 10 g sulphuric acid 13 g aluminium sulphate (34 % solution) 27 g water to make 250 ml The ready-to-use fixer was made by mixing 200 ml of concentrated part A, 750 ml of water and 50 ml of concentrated part B. The pH of this mixture was 4.3 at 25 °C. After processing the density as a function of the light dose was measured and therefrom were determined : fog level (with an accuracy of 0.01 density point), the relative speed (sens) at a density of 1 above fog (reference was set to a speed of 100). n° % I % cov. mole salt/l sens fog 1 0 200 0.063 161 0.01 2 0 100 0.063 142 0.07 3 0 200 0 137 0.06 4 0 100 0 100 (ref) 0.10 5 1 200 0 165 0.03 6 1 100 0 142 0.09 From table 1 it is clear that, due to the addition of salt (magnesiumsulfate) during the simultaneous ripening and spectral sensitization period, the fog decreases and the sensitivity increases. In manufacturing the emulsion this way, it is possible to eliminate the iodide and still get a material with a comparable sensitivity as a material containing an emulsion with iodide but without addition of a water-soluble salt. EXAMPLE 2 In these experiments the emulsions 1, 3 and 5 of example 1 were coated on both sides of the support and subjected to a heat treatment (3 days in 57 °C / 34 RH) for simulation of ageing. Exposure and processing was identicxal to example 1. The sensitometic results of fresh samples (1) were compared to those of the treated samples (2) in table 2 : n o % I salt (mole/l) sens 1 . fog 1 sens 2 fog 2 1 0 0.063 112 0.043 113 0.11 3 0 - 100 (ref) 0.086 98 0.20 5 1 - 110 0.051 109 0.11 The relative fog increase is about the same for all three samples. However the absolute fog value and the sensitivity of invention emulsion 1 remain after treatment as good as for the control emulsion 5 with 1 % of iodide. EXAMPLE 3 This example illustrates the importance of the simultaneous presence of ripening agents, sensitizing dye and water-soluble salt, as it is the case in the present invention. A cubic AgBrI-grain was ripened under different conditions with thiosulfate and gold, after treatment with toluene thiosulfonic acid, in the presence of sensitizing dye 1 and in the presence or absence of calcium nitrate at a concentration of 0.083 mole/l. The emulsions were exposed, processed and evaluated as in the previous examples. In table 3 the sensitivities were compared for the different possible cases : salt during ripening sensi during ripening salt before coating sensi before coating Sens. a no no no yes 106 b no yes no no 100 (ref) c no no yes yes 109 d yes no no yes 96 e no yes yes no 120 f yes yes no no 130 It is clear from these examples that the highest sensitivity is obtained in case (f), where the ripening is carried out in the presence of salt and sensitizing dye. EXAMPLE 4 Two tabular AgBrI emulsions prepared in a similar way as in the previous examples with repectively 0,1 % or 0,6 % iodide were ripened in the presence of a spectral sensitiser dye 1 or dye 2 at a grain coverage of 75 % and in the presence or absence of a salt, being magnesium sulphate or calcium nitrate (see table 4). In the presence of salt an ionic strenght of 0.25 was established. In the chemical sensitisation process, a p.carboxyphenylmercaptotetrazole derivative was also present and the Phenidone derivative was left out. The material was exposed by a halogen bulb (0.06 s) directly after coating (without drying) and with the use of a V535 filter and a suitable density filter (density = 2). THe processing and evaluation was identical to the previous examples. n o of dye % iodide sens. salt fog 1 0.1 100 (ref) - 0.01 2 0.1 96 - 0.01 1 0.6 130 - 0.01 2 0.6 109 - 0.01 1 0.1 238 Mg 0.03 2 0.1 238 Ca 0.01 1 0.6 241 Ca 0.02 2 0.6 243 Mg 0.04 The table illustrates the enormous increase in green sensitivity due to the addition of a salt. The iodide content and the nature of dye and salt cause little additional difference. EXAMPLE 5 In this experiment a tabular AgBr emulsion was ripened in a way identical to example 4 in the presence of a sensitising dye (dye 1) and of a colourless non-sensitizing dye (A) with following formula : In some cases (see table 5) a salt (magnesium sulfate) was added. Exposure, processing and evaluation was the same as in previous examples with the exception that the sensitivity measured was 0.2 above fog. no % dye 1 % dye A salt (mole/l) sens 1 75 25 0 100 (ref) 2 150 50 0 120 3 75 25 0.063 112 4 150 50 0.063 127 5 50 50 0 108 6 100 100 0 58 7 50 50 0.063 119 8 100 100 0.063 71 From these data it is obvious that there is always a clear increase in sensitivity by adding the salt. In choosing the right combination of colourless dye, sensitising dye and salt, it is also possible to bring the amount of sensitizing dye to one third and still obtain a good sensitivity (compare cases 2 and 7). This is very advantageous in trying to solve the problem of residual dye stain occuring after rapid processing. In a fast test (5 seconds fixation in a sodium thiosulphate fixer, 5 seconds washing) sample 2 showed a residual dye stain about twice as dense as sample 7.

Claims: Method for the preparation of a photographic silver halide emulsion comprising the steps of precipitation, chemical ripening and spectral sensitization, wherein said chemical ripening step and said spectral sensitization step are performed simultaneously, characterised in that said simultaneous chemical and spectral sensitization step is performed in the presence of at least one water-soluble metal salt the anion of which can form with a silver cation a salt with a solubility product higher than 10 -7.5 . Method according to claim 1 wherein the soluble salts produced during precipitation are removed by a washing step before performing said simultaneous chemical and spectral sensitization in the presence of said at least one water-soluble metal salt the anion of which can form with a silver cation a salt with a solubility product higher than 10 -7.5 . Method according to claim 1 or 2 wherein said spectral sensitization is performed by using a cyanine spectral sensitizing dye. Method according to claim 3 wherein said cyanine spectral sensitizing dye is a cyanine dye poorly adsorbing to the emulsion grain. Method according to any of claims 1 to 4 wherein the cation of said water-soluble metal salt the anion of which can form with a silver cation a salt with a solubility product higher than 10 -7.5 has a valence of at least two. Method according to any of claims 1 to 4 wherein said water-soluble salt is chosen from the list of magnesium sulphate, calcium nitrate, lithium sulphate, sodium sulphate, potassium sulphate, sodium nitrate and potassium nitrate. Method according to any of claims 1 to 6 wherein the spectral sensitizing dye(s) used for performing said spectral sensitization, the ripening agent(s) used for performing said chemical ripening, and said at least one water-soluble metal salt are added with short intervals after each other in the order : first dye(s), then salt(s) and finally ripening agent(s). Method according to any of claims 1 to 6 wherein the spectral sensitizing dye(s) used for performing said spectral sensitization, the ripening agent(s) used for performing said chemical ripening, and said at least one water-soluble metal salt are added with short intervals after each other in the order : first salt(s), then dye(s) and finally ripening agent(s). Method for making a photographic material comprising the steps of - preparing a silver halide emulsion according to the method of any of claims 1 to 8; and - coating said silver halide emulsion on a support. Method according to claim 9 wherein said emulsion contains no substantial amount of iodide. Method according to claim 10 wherein said emulsion containing no substantial amount of iodide is a silver bromide emulsion.

Inventors: DE KEYZER RENE, DEPREZ LODE, DE KEYZER, RENE, DEPREZ, LODE, De Keyzer, René, c/o Agfa-Gevaert N.V., Deprez, Lode, c/o Agfa-Gevaert N.V.
Applicants: AGFA GEVAERT NV, AGFA-GEVAERT N.V.
Application year: 1993
Main classification: G03C1
IPC codes: G03C1