Abstract:
A method is disclosed for washing silver halide photographic emulsions, including desalting and/or concentrating, based on depletion phase separation mechanism, wherein phase separation is effected by the addition of water-soluble non-interactive and non-adsorbing carboxyl polymers. The process involves the separation of the supernatant fluid, containing the undesirable water soluble salts and the added phase separating agents, from the washed and condensed silver halide phase for desalting and redispersion of the latter.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the method of preparing silver halide photographic emulsions utilizing water-soluble non-interactive carboxyl polymers and non-adsorbing polymers as desalting agents for the removal of the undesired dissolved salts and/or further concentration of the emulsions at the ambient pH of the prepared emulsions. 
     BACKGROUND OF THE INVENTION 
     Silver halide photographic emulsions are usually prepared by reacting an aqueous solution of halide salt with silver salt in the presence of a protective colloid, e.g. gelatin, to produce silver halide nuclei. After physical ripening to the desired grain size and size distribution, the emulsions are subjected to chemical and spectral sensitization. Generally, in the process of manufacturing a photographic silver halide emulsion, the silver halide emulsion is usually subjected to desalting to remove water-soluble salts such as excessive silver halides, alkali nitrate and ammonium salts after completion of physical ripening. Prior to or during the chemical and spectral sensitization, the resulting water-soluble salts, e.g. sodium nitrate and excess halide during the preparation of silver halide emulsion, should be removed to prevent deleterious effects on final coating applications. It is also desirable to concentrate the washed emulsions for subsequent addition of other photographically active components. 
     The desalting methods include a noodle method, a dialysis method, and a flocculation precipitation method. Of these methods, the flocculation precipitation method is extensively put into practical use. 
     The earliest method of removing the extraneous salts is by noodle washing (U.S. Pat. Nos. 2,527,260 and 3,396,027), wherein the prepared emulsion is chilled set and broken into small fragments and subjected to a continuous water flow to remove the salt by osmosis. This technique requires a large volume of water and is very time consuming, resulting in extensive swelling of the gelatin and dilution of the remelted emulsion. 
     Another washing method employs the precipitation of silver halide particles by the addition of large amounts of inorganic salts, e.g. sodium or magnesium sulfates, etc. (U.S. Pat. No. 2,618,556). The interface separating the supernatant fluid and the sediment silver halide particle in such case is not well-defined, resulting in difficulty for the removal of the supernatant fluid and the loss of silver halide grains. Small molecule organic salts, e.g. sulfonated benzene, naphthalene, or their condensates with formalin, or alkyl sulfates (U.S. Pat. No. 2,527,260; GB Patent Nos. 967,624; 945,334; 1,053,670), were also employed as coagulating agents. The formation of insoluble complex between the negative charge of the coagulant and the positive amino groups of gelatin at a pH lower than the isoelectric point of gelatin, results in phase separation and coagulation of the solid silver halide particles. 
     Anionic polymers were also used as coagulants to generate phase separation similar to those described above by small molecule coagulants. These polymers include sulfated poly(vinyl alcohol) (U.S. Pat. No. 3,867,154); poly(vinyl sulfonate) (GB Patent No. 967,624); poly(styrene sulfonate) or its copolymers (U.S. Pat. No. 3,168,403); other sulfonated polymers (U.S. Pat. Nos. 3,241,969; 3,137,576); the copolymers of carboxylate-containing monomers, such as acrylate, methacrylates, and maleic acids (U.S. Pat. Nos. 2,565,418; 4,087,282; 4,990,439; 5,411,849; 5,486,451; Japanese 62/32445; European Patent No. 88120367.3; GB Patent No. 1,121,188). By lowering the pH of the emulsions below the isoelectric point of gelatin, complexes between the polymers and gelatin, as well as the gelatin-coated silver halide particles, are formed and separated from the clear supernatant which contains most of the soluble salts. Similar to the above anionic polymers are the modified gelatin derivatives, e.g. the covalent reaction products of gelatin with carboxylic or sulfonic acid chlorides, carboxylic anhydrides, etc. (U.S. Pat. Nos. 2,614,928; 2,614,929; 2,614,931; 3,359,110; 3,867,154; 5,411,849). The insolubility of these modified gelatin coagulants at a pH below the isoelectric point of gelatin causes precipitation of silver halide particles, and hence the soluble salt in the supernatant can be removed by decanting or centrifugation. In all the aforementioned precipitation methods, pH lowering is necessary to bring about flocculation. The extraneous ionic coagulants remain in the silver halide bottom phase, resulting in difficulty in redispersing and increase in viscosity of the subsequently redispersed emulsion and also imparting adverse effects on the photographic performance of the silver halide emulsions such as fogging. 
     Two other physical separation methods for the removal of soluble salts are based on membrane techniques, e.g. ultrafiltration and electrodialysis (U.S. Pat. No. 5,523,201) by use of semipermeable membranes and ion exchange membranes, respectively. Membrane fouling and the lengthy time required for desalting and difficulty in further concentration of the emulsion are possible drawbacks of these processes. 
     Depletion phase separation in polymer lattices containing non-adsorbing polymers have been studied extensively. Several theories have been proposed in recent years (For general references, see &#34;Polymers at Interfaces&#34; by G. J. Fleer, M. A. Cohen Stuart, J. M. H. M. Scheutjens, T. Cosgrove, and B. Vincent, Chapman &amp; Hall, 1993; &#34;Polymeric Stabilization of Colloidal Dispersion&#34; by D. H. Napper, Academic Press, 1983) to explain such phenomena. Similar behavior is also observed with non-ionic surfactant micelles (e.g. see Progr. Colloid Polym Sci., 100, 201 (1996); Colloids and Surfaces, vol. 28, 1(1987)). The depletion phase separation is known in synthetic lattices to cause particle instability. 
     No working process has been described that will allow complete washing of emulsion without the need for a pH adjustment which adds to the complexity of the process and results in fogging. 
     SUMMARY OF THE INVENTION 
     The object of this invention is to provide a method of preparing light-sensitive silver halide emulsions, including all grain sizes and morphologies, by using as desalting agents, i.e., non-interactive carboxyl polymers, to remove the excess salts and water-soluble by-products without any pH adjustment. The phase separation is operated by a depletion phase separation mechanism, wherein most of the desalting agents added are excluded from the bottom silver halide phase and remain in the clear supernatant liquids containing the extraneous unwanted salts for subsequent removal. Another objective of this invention is to provide a method of concentrating the washed and redispersed emulsions for subsequent chemical and spectral sensitization. The redispersed emulsions thus obtained are devoid of the excess salts and the phase separating agents used. 
     In the present silver halide emulsions, the added carboxyl polymers are soluble in aqueous salt solution containing gelatin and should not form complexes with gelatin, nor interact with the surface-coated gelatin to bring about &#34;bridging&#34; and flocculation of the particles. The added polymers are excluded from the sedimented silver halide phase and remain in the salt-containing supernatant liquids for subsequent removal. More importantly, the depletion phase separation is effected at the ambient pH of the prepared emulsion without any pH adjustment. In particular, the separated silver halide phase forms a gel-like network structure even at 40° C. This gel-like bottom phase is easy to be separated from the supernatant liquids and can be subjected to further washing with water without the loss of silver halide grains. In all cases, the volume of the bottom silver halide phase is much smaller than that of the supernatant liquid so that the concentration of the washed emulsion can be adjusted with further addition of aqueous gelatin solution. The redispersed emulsion is devoid of the phase separation agents used so that any possible deleterious effects on the photosensitive silver halide emulsions can be minimized. 
     DETAILED DESCRIPTION OF THE INVENTION 
     With the commonly used ionic coagulating agents, e.g. the sulfated, sulfonated, or carboxylated small molecules or polymers, or the modified gelatin, the coagulants added remain with the silver halide particles in the precipitated phase. Furthermore, pH lowering below the isoelectric point of gelatin (i.e., pH&lt;5) is generally required to bring about coagulation. The coagulated phase is usually difficult to handle because of the higher viscosity of the precipitated phase caused by complex formation between the anionic sites of the coagulants and the positive amino groups on gelatin. The most severe problem is the loss of speed (photoactivity) frequently associated with the presence of ionic polymer when its amount exceeds 10 g of ionic moiety/mole of silver. 
     In the present invention, non-interactive and non-adsorbing, carboxyl polymers are used as the flocculating agents to cause depletion phase separation. The polymers have minimum or no interaction with gelatin or gelatin-coated silver halide grains and are excluded from the particle phase once a certain critical concentration of the added flocculant is reached. This critical concentration for phase separation may be related to the molecular weights or coil dimensions of the polymers. Because phase separation is a result of osmotic pressure imposed by the dissolved polymer upon the particles causing the latter to aggregate, separation can be conducted at the same pH of the prepared emulsion; therefore no pH adjustment is necessary for such separation. Since the polymers do not adsorb onto the surface of the particles, a minimum amount of the extraneous phase separation agent is retained in the sedimented silver halide particle phase, and thus any adverse effects on the photographic performance of the light-sensitive silver halide grains can be reduced. 
     The sedimented silver halide particle phase has a gel-like network structure even at 40° C., hence the loss of silver can be minimized during separation of the supernatant liquid from the silver halide phase by decanting or by low speed centrifugation. The integral gel-like characteristics of the silver halide phase also render further washing with water for the removal of any physically entrapped polymer or residual salts relatively easy. The volume of the sedimented silver halide phase is generally about 20 times less than that of the supernatant liquid so that the concentration of the final redispersed emulsion can be achieved to any desired level. 
     Any silver halide emulsion with a range of grain size from 0.1 micron to several microns may be subjected to the present washing procedure. The concentration of the silver halide particle in the initially prepared emulsion suitable for the present washing procedure may range from 0.5% to 20%, preferably from 5% to 10%. Further washing, if desired, may be conducted with de-ionized water. In addition, the washing procedure using the present non-interactive polymers may be applied to all types and grain morphologies of silver halide emulsions, including iodide, chloride, bromide, bromoiodide, chloro-bromide, etc. 
     There are many carboxyl polymers which can be chosen as the phase separation agents in the present invention, as long as they are non-interactive in the presence of gelatin or gelatin-coated silver halide particles. Since the agents added are mostly excluded from the silver halide phase, the adverse effects on the photographic performance of the final washed emulsions frequently encountered by the use of conventional ionic coagulants can be greatly reduced. On the contrary, the residual amount of the non-interactive polymers which are physically entrapped in the washed emulsion may impart advantageous features to the final coated film, such as stabilization ability, plasticization, and enhanced physical resistance to abrasion. 
     The non-interactive and non-adsorbing polymers used in the present invention may include any commercially available weakly ionized carboxylated polymers as long as they do not react with gelatin or gelatin-coated silver halide particles in the normal pH range for emulsion preparation (pH=5.3-5.6). They may include all polymers which can be synthesized by any prior art in polymer synthesis, such as free radical or ionic polymerization or polycondensation, or step-growth polymerization. 
     In a preferred embodiment, the physicochemical nature of the weakly ionized carboxylate containing polymers, suitable as the phase separation agent in the present invention, can be characterized by the following measurable parameters. 
     1) The stock solutions contain from 5%-40% (w/w) in concentration of the carboxyl polymers at pH equal or greater than 5.6, 
     2) The concentration of the polymer stock solution in a silver halide emulsion may range from 5% to 40% (w/w), preferably from 5% to 30% (w/w), depending on the molecular weight and polymer coil dimensions. The viscosity of the stock solution may range from 10 cp to 10,000 cp, preferably from 100 cp to 1,500 cp; 
     3) The molecular weight of the water-soluble polymer preferably ranges from 1×10 3  to 1×10 7 , more preferably from 1×10 4  to 5×10 5 . The radius of gyration of the polymer may range from 1.5 nm to 200 nm, preferably from 3 nm to 100 nm; 
     4) The critical concentration of polymer required for phase separation to occur in a silver halide emulsion may preferably range from 0.1% to 20%, preferably 0.5 to 15%, depending on the molecular weight and radius of gyration of the polymer, more preferably from 1% to 4%, i.e. 0.5 to 2.0 times the concentration of gelatin in the pre-washed emulsions. 
     5) The polymer is non-interactive in the presence of free gelatin in aqueous salt solution, such that the viscosity of the mixed solution containing the polymer and gelatin is not higher than the weight average of the viscosities of the polymer and gelatin (if it interacts with gelatin, it forms an insoluble complex and brings down silver halide with it); 
     6) The polymer is non-interactive in the presence of free gelatin in aqueous salt solution, such that the specific optical activity of the gelatin solution is unaltered by the addition of the polymer; 
     7) The polymer is non-interactive in the presence of free gelatin in aqueous salt solution, such that the light scattering intensity of the mixture is not greater than the weight average of the scattering intensities of the individual components; and 
     8) The polymer is preferably non-adsorbing and repulsive to the surface of the gelatin-coated silver halide particle surface, such that the adsorption of the polymer on the particle cannot be detected by conventional analytical techniques, e.g., by aqueous size exclusion chromatography for measuring the adsorbed amount by UV or RI detectors, or by photon correlation spectroscopy (i.e., dynamic light scattering or quasi-elastic light scattering) for measuring the particle size increase upon addition of polymers. 
     Some examples of weakly ionized carboxyl-containing polymers of the present invention, the stock solution of which are at pH of 5 to 6 are: 
     
         ______________________________________partially hydrolyzed poly(acrylamide), such as the Cyanamer-                        (1)  P21 supplied by Cytee-Industries Inc.  sodium carboxymethyl cellulose(CMC-Na, Aldrich) with     (2)  various degrees of substitution (e.g. DS = 0.7)  copolymers of maleic acid with vinyl methyl ether (PMVE/MA)   (3)                         or ethylene (EMA, Zeeland Chemicals Inc.)                         polyacrylic acid (PAA)                                   (4)  Sodium poly(acrylate)______________________________________ 
    
     Furthermore, water-soluble copolymers consisting of any combination of the monomers mentioned in the above homopolymers or with other vinyl comonomer containing heterocyclics, such as N-vinyl oxazolidone and N-vinyl lactams are also included for this application. 
    
    
     EXAMPLES 
     Examples of the present invention are described in detail below. This invention is not limited to the specific types, sizes, and shapes of the silver halide grains. Three types of silver halide emulsions were prepared as described in the following examples and used to demonstrate the application of the present invention employing various phase separating agents. 
     Example 1 (Cubic Silver Chloro-bromide Emulsion) 
     Emulsion EM01--A silver halide cubic emulsion, containing 70 mol % chloride and 30 mol % bromide ions was prepared by the conventional double-jet precipitation procedures (see &#34;Typical and preferred color paper, color negative, and color reversal photographic elements and processing&#34;, Research Disclosure, Item 37038, February 1995, disclosed anonymously). The emulsion grains were found to be monodisperse with an average size of 0.15 μm. At the end of the double-jet precipitation, the emulsion was deionized and concentrated by the standard ultrafiltration procedure. The emulsion was subsequently treated with chemical and spectral sensitization by standard procedures commonly used in the industry. This emulsion is referred to as EM01. 
     Emulsion EM02--Another emulsion was precipitated by the same method used for EM01. The pre-washed emulsion at the end of the double-jet precipitation (10 moles of silver halide) was referred to as EM02. This emulsion was divided into several portions (575 g each containing 0.345 moles of silver halide) in stainless steel beakers each containing a magnetic stirring device and thermostated in a 40° C. water bath. Each portion of the emulsion was subjected to the washing process using various polymers as listed in Table 1a. The emulsion EM02 was subsequently treated with chemical and spectral sensitization by the same procedures used for EM01. 
     Summarized in Table 1a are the formulations for phase separation using various polymers as the desalting agents for the small cubical EM02 emulsion, each formulation requires 575 g of the pre-washed emulsion at the ambient pH of 5.6. The molecular weights tested for each polymer are listed under column 2. The weights of each polymer with specific stock solution concentration are shown under column 3. The last column lists the critical polymer concentration required for phase separation (Cps) of an emulsion sample (575 g). Contrary to the present results an interactive polymer such as sodium poly(styrene sulfonate) under similar conditions cannot bring about acceptable phase separation. 
     Table 1b shows the results of various photographic performance for the EM02 emulsion treated with some of the polymers as the desalting and concentrating agents. Included for comparison are the results for the ultra-filtration check EM01 emulsion. As evident from these data, the polymer-washed EM02 emulsions yielded excellent photographic performance in comparison with the check EM01 sample. 
     
                       TABLE 1a______________________________________Formulations for Phase Separation with 575 g of EM02 Emulsion (40°.)                           Polymer Concentration    g of Polymer (%) for Phase Separation  Polymer MW Stock (%) (Cps)______________________________________Cyanamer-P21     2 × 10.sup.5             100(10%)    1.67  CMC-Na     9 × 10.sup.4          100(10%)                1.67                          EMA       1 × 10.sup.5                         100(10%)                1.67  PMVE/MA     4 × 10.sup.4          100(10%)                1.67                          PAA       3 × 10.sup.5                         87.5(10%)               1.50______________________________________ 
    
     
                       TABLE 1b______________________________________Photographic Results for the Polymer-Washed EM02 Emulsions               Silver                Grain Size  Polymer          Fog       Density    Speed         Contrast                                     (μm)______________________________________CMC-Na   0.05   3.08      124   2.1     0.15  EMA       0.05      3.16      125    2.0      0.15  PMVE/MA   0.07      3.3        128    1.93     0.15  UF(EM01)  0.05      3.35      125     1.84     0.15______________________________________ 
    
     Example 2 (Cubic Silver Chloride Emulsion) 
     Emulsion EM03--A silver chloride cubic emulsion was prepared by the conventional double-jet precipitation procedures similar to that of emulsion EM02. The emulsion grains were found to be monodisperse with an average size of 0.75 μm. This pre-washed emulsion is refereed to as EM03. The formulations for the desalting process using various polymers are listed in Table 2. 
     
                       TABLE 2______________________________________Formulations for Phase Separation with 575 g of EM03 Emulsion (40°.)                 g of Polymer                          Polymer Concentration (%)  Polymer             MW              Stock (%)    for Phase Separation                          (Cps)______________________________________Cyanamer P-21     2 × 10.sup.5             75(10%)    1.30  CMC-Na         9 × 10.sup.5  75(10%)               1.30  EMA            1 × 10.sup.5  75(10%)               1.30______________________________________ 
    
     Example 3 (Tabular Silver Bromo-iodide Emulsion) 
     Emulsion EM04--A silver bromo-iodide emulsion of tabular morphology was prepared by the conventional double-jet precipitation (see U.S. Pat. No. 5,476,760). The dimensions of the emulsions grain are 2.3 μm×0.12 μm. The formulations for the desalting process using various polymers are listed in Table 3. 
     
                       TABLE 3______________________________________Formulations for Phase Separation with 575 g of EM04 Emulsion (40°.)                 g of Polymer                          Polymer Concentration (%)  Polymer            MW              Stock (%)                for Phase                          Separation (Cps)______________________________________Cyanamer P-21     2 × 10.sup.5             75(10%)    1.30  CMC-Na          9 × 10.sup.4     75(10%)              1.30                         EMA             l × l0.sup.5                        65(10%)              1.15______________________________________ 
    
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.