Classifications

• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03C—PHOTOSENSITIVE MATERIALS FOR PHOTOGRAPHIC PURPOSES; PHOTOGRAPHIC PROCESSES, e.g. CINE, X-RAY, COLOUR, STEREO-PHOTOGRAPHIC PROCESSES; AUXILIARY PROCESSES IN PHOTOGRAPHY
  • G03C1/00—Photosensitive materials
  • G03C1/005—Silver halide emulsions; Preparation thereof; Physical treatment thereof; Incorporation of additives therein
  • G03C1/015—Apparatus or processes for the preparation of emulsions
• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03C—PHOTOSENSITIVE MATERIALS FOR PHOTOGRAPHIC PURPOSES; PHOTOGRAPHIC PROCESSES, e.g. CINE, X-RAY, COLOUR, STEREO-PHOTOGRAPHIC PROCESSES; AUXILIARY PROCESSES IN PHOTOGRAPHY
  • G03C1/00—Photosensitive materials
  • G03C1/005—Silver halide emulsions; Preparation thereof; Physical treatment thereof; Incorporation of additives therein
  • G03C1/04—Silver halide emulsions; Preparation thereof; Physical treatment thereof; Incorporation of additives therein with macromolecular additives; with layer-forming substances
  • G03C1/047—Proteins, e.g. gelatine derivatives; Hydrolysis or extraction products of proteins

Definitions

• ABSTRACT A method is disclosed of preparing washed chemically sensitized silver halide emulsions according to which washing occurs by coagulation based on the use of acid-coagulable gelatin derivatives, wherein between coagulation washing and chemical sensitization the pH of the emulsion is raised to a value above 8. By this pH raise it is possible to increase the speed, gradation and maximum density.
• the present invention relates to the preparation of photographic gelatino silver halide emulsions.
• the preparation of a silver halide emulsion basically consists of a number of stages which are as follows:
• the photographic emulsion comprises by-products of the grain formation and growth stage, excess halide, ammonia or acid, etc. which should be removed by washing so that at the chemical ripening or digestion stage a practically neutral (pH from about 6 to about 7) silver halide emulsion is obtained which is substantially free from undesirable water-soluble compounds.
• the most conventional method of washing consists in cooling the emulsion so that it sets to a fairly soft jelly, comminuting it into small fragments, usually by shredding to produce noodles, and washing the fragments by suspending them in water.
• the water has to be changed frequently or continuously until the emulsion fragments are sufficiently free from the undesirable products.
• This washing method is very time consuming and allows no opportunity for concentrating the emulsion as the emulsion fragments swell during washing and the viscosity drops.
• Modern methods of emulsion washing are based on the principle of causing the gelatin to coagulate or flocculate so that it settles together with the silver halide and thus separates from the aqueous solution comprising the soluble by-products.
• the supernatant solution is decanted or removed by some other method and the settled gelatin-silver halide mixture is redispersed, after washing of the precipitate, in water or aqueous gelatin.
• the flocculation method offers a number of advantages; in particular, a very high percentage of the unwanted salts may be removed in one stage and concentrated silver halide emulsions can be prepared.
• the gelatin-silver halide mixture can be coagulated by addition of salts e.g. sodium and ammonium sulphate.
• salts e.g. sodium and ammonium sulphate.
• a large amount of the salt is required to obtain complete flocculation and this salt should be removed by repeated washing.
• an additional amount of salt is added in order to harden the emulsion which may make it difficult to redisperse the coagulum.
• Water-miscible organic solvents into which the gelatin is insoluble can also be used to bring about flocculation, but this method is expensive and the precipitate contains salts that are insoluble in the organic solvent used.
• organic polymeric or non-polymeric sulphonic acids or sulphuric acids for example naphthalene disulphonic acids, polystyrene sulphonic acid and derivatives, long-chain alkyl sulphonic acids and sulphuric acids e.g. the well-known anionic wetting agents, etc.
• the silver halide may be precipitated by reaction of a watersoluble silver salt solution and a water-soluble halide solution in an aqueous solution of the acid coagulable gelatin derivative or in an aqueous solution of gelatin to which an acid coagulable gelatin derivative is added in sufficient proportion to impart acid-coagulable properties to the entire mass.
• the silver halide may be precipitated in an aqueous solution of gelatin whereupon the dispersion is treated optionally after physical ripening with an organic reagent e.g.
• the dispersion comprising silver halide and gelatin derivative is acidified to a pH of approximately 4.5 or less, e.g. between 3 and 4.5.
• the coagulum formed may be separated from the liquid by several techniques e.g. by decanting the supernatant liquid followed by washing of the coagulum. The washed coagulum is then redispersed to form photographic emulsions for the subsequent finishing and coating operations.
• the present invention therefore, provides a method of preparing photographic silver halide emulsions in which an emulsion incorporating an acid-coagulable gelatin derivative is prepared and wherein the undesirable byproducts of silver halide grain formation and physical ripening are removed by lowering the pH (e.g. to a value in the range from about 3 to about 4.5) to bring about coagulation and by washing the coagulum and the coagulum is redispersed and the silver halide emulsion formed is subsequently chemically sensitized, the method being characterized in that some time between the conclusion of the washing operation and the chemical sensitization the pH of the silver halide emulsion is raised to a value above 8.
• the pH e.g. to a value in the range from about 3 to about 4.5
• coagulation washing of silver halide emulsions based on the use of acidcoagulable gelatin derivatives is well known in the art of silver halide emulsion preparation.
• the silver halide is preferably precipitated in an aqueous solution of the gelatin derivative itself or in an aqueous solution of both normal gelatin and a gelatin derivative the said derivative being present in a proportion sufficient to impart acid-coagulable properties to the entire mass.
• the gelatin derivative may be added after the stage of emulsification in normal gelatin, and even after the physical ripening stage, provided it is added in an amount sufficient to render the whole coagulable under acid conditions.
• the precipitation may occur in an aqueous solution of gelatin and an acid coagulable gelatin derivative may be formed in situ by adding an organic reagent as hereinbefore referred to before or after physical ripening.
• an organic reagent as hereinbefore referred to before or after physical ripening.
• acid-coagulable gelatin derivatives suitable for use in accordance with the present invention can be found e.g. in the United States Patent Specifications referred to above. Particularly suitable are phthaloyl gelatin and N-phenylcarbamoyl gelatin.
• the silver halide emulsion comprising acid-coagulable gelatin derivative is acidified e.g. by means of dilute sulphuric acid, citric acid, acetic acid, etc. (e.g. to a pH within the range of about 3 to about 4.5) so as to effect coagulation.
• the coagulum formed may be removed from the liquid by any suitable means, for example the supernatant liquid is decanted or removed by means of a siphon, whereupon the coagulum is washed out once or several times.
• a temperature is used generally comprised between about 35C and about 70C, dependent on the desired grain size and thus emulsion speed to be obtained, coagulation may occur at a temperature comprised between about 10C and about 50C. It is preferred to lower the temperature to about lO30C before coagulation and thus before adjusting the pH to the coagulation point.
• Washing of the coagulum may occur by rinsing with mere cold water.
• the first wash water is preferably acidified to lower the pH of the water to the pH of the coagulation point.
• washing may be effected by redispersing the coagulum in water at elevated temperature using a small amount of alkali e.g. sodium or ammonium hydroxide, recoagulating by addition of an acid to reduce the pH to the coagulation point and subsequently removing the supernatant liquid. This redispersion and recoagulation operation may be repeated as many times as is necessary.
• alkali e.g. sodium or ammonium hydroxide
• the coagulum is redispersed to form a photographic emulsion suitable for the subsequent finishing and coating operations by treating, preferably at a temperature within the range of about 35 to about 60C, with the required quantity of water, normal gelatin and, if necessary, alkali for a time sufficient to effect a complete redispersal of the coagulum.
• normal gelatin which is preferably used
• other known photographic hydrophilic colloids can also be used for redispersion e.g. a gelatin derivative as referred to above, albumin, agar-agar, sodium alginate, hydrolysed cellulose esters, polyvinyl alcohol, hydrophilic polyvinyl copolymers, etc.
• the coagulation washed silver halide emulsion is alkalized to a pH value above 8, in the stage between the last washing of the coagulated silver halide emulsion and the chemical sensitization i.e. before, during or after redispersion of the coagulated silver halide emulsion.
• redispersion may be effected at a pH-value above 8, or, preferably, after redispersion of the silver halide emulsion, the pH is adjusted to a value above 8.
• the silver halide emulsion after having raised the pH above 8, may be neutralized (pH from about 6 to about 7) immediately, the emulsion is generally kept at the alkaline pH for some minutes e.g. from 5 to 30 minutes before neutralisation whereupon it is digested to the optimum relation between fog and sensitivity in a known manner at a temperature generally comprised between about 35C and about 60C.
• the method for preparing photographic silver halide emulsions comprises the following steps:
• Digestion or chemical sensitization of the silver halide emulsion may occur according to any of the accepted procedures e.g. as described on page 107 of the December 1971 issue of Product Licensing lndex published by Industrial Opportunities Limited, Havant, England and in the patent literature referred to therein. It may be digested in the presence of small amounts of sulphur-containing compounds such as allyl thiocyanate, allyl thiourea, sodium thiosulphate, etc.
• the emulsion may also be sensitized by means of reductors for instance tin compounds as described in United Kingdom Pat. Specification No. 789,823 filed Apr. 29, 1955 by Gevaert Photo-Producten N.V., polyamines e.g.
• emulsions may also be chemically sensitized by the combined use of these chemical sensitizers.
• Photographic silver halide emulsions of all kinds may be prepared by the process of the invention, which include negative as well as direct-positive silver halide emulsions, coarse-grain as well as fine-grain silver halide emulsions.
• the silver halides may be silver bromide, silver chloride or silver chlorobromide and may comprise a small amount up to of silver iodide.
• Raising the pH to a value above 8 at a stage of emulsion preparation intermediate the conclusion of coagulation washing and the chemical ripening results in an increase of the speed and gamma as well as maximum density, which makes the method of the invention of particular importance in the preparation of highly sensitive ammonia emulsions e.g. silver bromoiodide ammonia emulsion as used in the preparation of X-ray films.
• the method of the present invention permits to markedly reduce noble metal sensitization. Before coating on a support, any one or more of the common so-called coating finals may be added to the photographic silver halide emulsions prepared in accordance with the present invention.
• coating finals include spectral sensitizers, colour couplers, antifoggants and emulsion stabilizers, coating aids, plasticizers, light-absorbing dyes, hardeners, development modifiers, etc. a survey of which can be found on pages 107-109 of the December 1971 issue of Product Licensing lndex, published by Industrial Opportunities Limited, Havant, England.
• the emulsions may comprise lactams e.g. caprolactam in order to reduce possible pressure desensitization.
• the silver halide emulsions prepared in accordance with the present invention may be coated on the wide variety of supports known for use in photographic silver halide elements which include cellulose nitrate film, cellulose acetate film, poly(vinyl acetal) film, polystyrene film, po
• Paper supports may be used which are partially acylated or coated with baryta and/or an a-olefin polymer, particularly a polymer of an a-olefin containing from 2 to 10 C-atoms such as polyethylene, polypropylene, ethylenebutylene copolymers and the like.
• Emulsion A a The silver halide was precipitated in aqueous phthaloyl gelatin (amino groups of gelatin acylated for about 90 to I00 percent) by addition of an aqueous solution of halide and an ammoniacal aqueous solution of silver nitrate at a temperature of 44C.
• the silver halide grains were physically ripened at 44C in a conventional way until the desired grain-size, grain size distribution and grain sensitivity was reached.
• the emulsion was coagulated by adjusting the pH of the emulsion to 3.6 by means of sulphuric acid.
• the silver halide emulsion settled rapidly and the supernatant liquid was decanted.
• the coagulated silver halide emulsion was redispersed in water "and sufficient inert gelatin was added to obtain a ratio of gelatin to silver halide (expressed as silver nitrate) of 0.4.
• the pl-l-value was adjusted to 6.5 and the pAg was adjusted to a value corresponding to an E.M.F. of+ mV (Ag/saturated calomel electrode).
• the emulsion obtained was chemically sensitized by addition of a sulphur and gold sensitizer and digesting at 47C until the optimum sensitivity-fog relation was reached.
• Emulsion B was prepared in the same way as emulsion A with the difference however that:
• the pH-value of the redispersed emulsion was adjusted to 9.2 by means of sodium hydroxide and kept at this value for 10 minutes whereupon the pH-value was adjusted to 6.5 by means of citric acid and the pAg was adjusted to a value corresponding with an E.M.F. of 70 mV (Ag/saturated calomel electrode),
• the emulsion obtained was chemically sensitized by addition of a sulphur and gold sensitizer and digestion at 47C until the optimum sensitivity-fog relation was reached.
• the amount of noble metal sensitizer (potassium aurithiocyanate) had to be reduced markedly i.e. from 6.7 mg to 2.4 mg for an amount of silver halide corresponding to 500 g of silver nitrate.
• the emulsions A and B were coated under identical circumstances on a polyethylene terephthalate support and overcoated with a gelatin antistress layer comprising formaldehyde as hardening agent.
• the results attained are listed in the following table.
• the speed values were measured at density 0.1 above fog (S,) and 1 above fog (S they are relative values with respect to the speed of emulsion A for which a value 100 has been given.
• redispersion of the coagulated emulsion and the chemical ripening results in an increase of the gradation, speed and maximum density (compare results of B with those of D). They further show that the increase in gradation, speed and maximum density is higher when reducing the temperature before coagulation (compare results of B with those of C).
• This emulsion was prepared in exactly the same way as emulsion B of example 1.
• Emulsion C This emulsion was prepared in exactly the same way as emulsion B of example 1 with the only difference.
• Emulsion B was chemically sensitized as described for emulsion B of example 1 (2.4 mg of potassium aurithiocyanate per amount of silver halide corresponding to 500 g of silver nitrate) whereas emulsion B was chemically sensitized as described for emulsion A of example 1 (6.7 mg of potassium aurithiocyanate per amount of silver halide corresponding to 500 g of silver nitrate).
• Emulsions B and B were further treated as described in Example 1.
• Method according to claim 1 comprising the steps of:

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Citations (3)

• 1972
  • 1972-05-12 GB GB2243372A patent/GB1414782A/en not_active Expired
• 1973
  • 1973-05-05 CH CH632473A patent/CH568580A5/xx not_active IP Right Cessation
  • 1973-05-07 FR FR7316413A patent/FR2184649B1/fr not_active Expired
  • 1973-05-08 CA CA170,662A patent/CA1000102A/en not_active Expired
  • 1973-05-08 DE DE2323151A patent/DE2323151A1/de active Pending
  • 1973-05-11 US US359512A patent/US3873322A/en not_active Expired - Lifetime
  • 1973-05-11 JP JP48052458A patent/JPS4950919A/ja active Pending
  • 1973-05-11 BE BE1005034A patent/BE799395A/xx unknown

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