Patent Publication Number: US-5837439-A

Title: Siloxane nonagglomerating antifoamants

Description:
FIELD OF THE INVENTION 
     This invention relates to the manufacture of silver halide and other components of photographic imaging systems. Specifically it relates to materials that can be used to inhibit the formation or stability of foam, and more specifically it pertains to antifoamants that are compatible with oxidized gelatin. 
     BACKGROUND OF THE INVENTION 
     During manufacture of components of silver halide-based photographic products, vigorous stirring of gelatin-containing solutions is often required. During precipitation of silver halide emulsions, for example, intense agitation of the aqueous gelatin reaction medium must be maintained in order to rapidly disperse incoming AgNO 3  and halide reactant solutions. One frequently encountered consequence of such mixing is air entrainment and generation of foam, which can lead to severe inhomogenieties, handling difficulties, and ultimately to defects in resulting photographic products. For these reasons, reduction or elimination of foam in various manufacturing steps is often accomplished by addition of antifoam materials that either prevent its formation or cause it to dissipate. 
     Ideal antifoamants would be effective over the temperature range of use, be nonhazardous, and cause no other adverse effects. In particular, they should not degrade the quality of the component in which they are used, nor of any of the other components with which they are combined in the ultimate manufacture of desired photographic products. Antifoamants which have been disclosed (Research Disclosure #36929 January, 1995) include the following compounds which are of low water solubility: 1) block oligomeric glycol compounds of polyethylene oxide (A) and polypropylene oxide (B) wherein the amount of B is much greater than that of A, 2) di- and mono-alkyl or alkenyl esters of polyethylene glycol (of relatively low molecular weight), 3) mixtures of paraffinic hydrocarbon oils and polyethylene glycol esters, with or without hydrophobic-treated silica particles and/or insoluble bis-alkyl amide compounds, 4) poly(dimethylsiloxane) fluid emulsified in water, and 5) poly(alkylene oxide)-modified poly(dimethylsiloxane) fluids. 
     Maskasky, U.S. Pat. No. 4,713,320 discloses that use of low methionine containing gelatin having less than 30 micromoles of methonine per mole of gelatin in precipitation of AgBr or Ag(Br,I) tabular photographic emulsions results in thinner grains and a lower rod population compared to emulsions made under similar conditions but with nonoxidized gelatin. However, we have found that some antifoamants can interact with oxidized gelatin to produce agglomerated silver halide grains, which renders the resulting photographic emulsions completely unsuitable for product use. 
     In copending and coassigned application U.S. Ser. No. 08/594,611-Daubendiek et al filed Feb. 2, 1996, there are disclosed several antifoamants suitable for use with low methionine gelatin. 
     PROBLEM TO BE SOLVED BY THE INVENTION 
     There is a need for improved methods of forming silver halide grains with reduced agglomeration of the grains during formation. There is a particular need for a method of forming such grains in low methionine containing gelatin used in forming thin grains. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to overcome disadvantages of prior solutions for forming of silver halide emulsions and methods of forming silver halide emulsions. 
     Another object of the invention is to obtain thin tabular silver bromide or silver bromoiodide grains. 
     A further object is to provide improved antifoamant control during silver halide grain formation. 
     Another further object is to provide for formation of silver halide grains with low agglomeration during formation. 
     These and other objects of the invention are generally accomplished by a solution for forming silver halide emulsions comprising water, a gelatin containing less than 30 micromoles of methionine per gram of gelatin, and at least one antifoamant of Formula I 
     
         (H.sub.3 C).sub.3 Si--(OSi(CH.sub.3).sub.2).sub.m --OSi(CH.sub.3).sub.3(I) 
    
     wherein m is an average number of dimethyl siloxane units. 
     In another embodiment there is provided a method of forming silver halide grains comprising providing a solution for forming silver halide emulsions comprising water, a gelatin containing less than 30 micromoles of methionine per gram of gelatin, and at least one antifoamant of Formula I 
     
         (H.sub.3 C).sub.3 Si--(OSi(CH.sub.3).sub.2).sub.m --OSi(CH.sub.3).sub.3(I) 
    
     wherein m is an average number of dimethyl siloxane units. 
     In a further embodiment of the invention there is provided an emulsion comprising silver halide grains, water, and at least one antifoamant of Formula I 
     
         (H.sub.3 C).sub.3 Si--(OSi(CH.sub.3).sub.2).sub.m --OSi(CH.sub.3).sub.3(I) 
    
     wherein m is an average number of dimethyl siloxane units. 
     ADVANTAGEOUS EFFECT OF THE INVENTION 
     The invention provides a solution or reaction medium that allows formation of improved silver halide emulsions with low silver halide grain agglomeration. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention has numerous advantages over the prior processes. The antifoamants of the invention provide for formation of thin tabular silver bromide and silver bromoiodide grains without substantial agglomeration. Further, the antifoamants of the invention allow improved formation of such thin grains with a high recovery rate at lower cost. The antifoamants allow a more robust process that does not require as precise control as when other antifoamants are utilized. 
     We have found, surprisingly, that certain antifoamants with the particular chemical structures of Formula I avoid this agglomeration and are, therefore, preferred. 
     We have found that compounds of Formula I are effective antifoamants for use in precipitating photographic emulsions in aqueous gelatin containing low levels of methionine (e.g., oxidized gelatin). These compounds provide antifoam action while also avoiding agglomeration of the AgX particles. In Formula I the value for the average number of dimethyl siloxane units, m, is between 50 and 1000. A preferred value of m is between 50 and 300 for good antifoam performance with least use of the antifoamant. 
     The antifoamants of the invention find a preferred use in the reaction media for formation of silver halide grains to be used in photography. With formation of silver halide grains the process begins with nucleation of exceedingly fine grains which are then grown to a larger, useful size. The grains need to be maintained separate during growth in order for the process to result in separate crystalline grains. Grains that have agglomerated are not effective for photographic use. Further they may lead to clogging of equipment, as well as undesirable photographic properties. The problem of agglomeration has been found to be particularly of concern with the use of low methionine gelatin, a common form of which is oxidized gelatin. Further it has been found to be a particular problem when the grains contain iodide, when they are formed under conditions of high suspension density and/or high ionic strength, and when their grain volumes are large (&gt;ca. 0.005 μm 3 ). We have found, surprisingly, that the invention antifoamants avoid agglomeration during silver halide grain formation and, therefore, are very much preferred for formation of silver halide grains. Thin tabular grains, typically founded in oxidized gelatin, find particular advantage in the use of the antifoamant solutions of the invention. By thin, it is meant that grains have a thickness of less than 0.07 micrometers and a diameter of greater than 0.7 micrometers. The thickness of the thin grains is greater than 0.01 micrometers. These emulsions generally contain silver bromide with a small amount of silver iodide up to about 15 percent. Such grains are preferred for use in the formation of color films of high speed and high quality. 
     The grains formed by the method of the invention and in the solution of the invention may be further treated by the known finishing methods to chemically sensitize and spectrally sensitize the grains. The antifoamant compound containing solutions of the invention do not interfere with the later finishing of the grains formed therein. Further, any of the various antifoggants, dopants, sensitizers, and supersensitizers are compatible with the antifoamant compounds in the invention. Other variations in the formation of silver halide emulsions, such as set forth in Sections I, II, III, and IV of Research Disclosure #36544 of September 1994 also may be utilized with the antifoamant compound containing solutions and the method of the invention. 
     The invention is illustrated with the following examples. 
     Emulsion A 
     Illustration of the Problem to be Solved 
     Ag(Br,I) Ultrathin Tabular Emulsion. Banded I structure with the first 75% being 3% I and the last 25% being 12% I. 
     A vessel equipped with a stirrer was charged with 4.8 L of water containing 3.00 g of lime processed bone gelatin, 3.29 g NaEr, sufficient sulfuric acid to adjust pH to 5.4 at 39° C., and 0.47 cc of Antifoamant 1. Nucleation was accomplished by a balanced simultaneous 4 sec addition of 2.5M AgNO 3  and halide (98.5% NaBr, 1.5% KI) in sufficient quantity to form 0.01068 moles of Ag(Br,I). Following nucleation, the reactor gelatin was quickly oxidized by addition of a 50 cc water solution that was ca. 0.05% NaOCl and the temperature was raised to 54° C. in 9 min. After a hold time of 9 min at 54° C., 100 g of oxidized, lime-processed, bone gelatin, dissolved in 2.76 L of water containing 1.29 cc of Antifoamant 1, was added. Twenty-four minutes after onset of nucleation, the pBr was decreased to 1.69 by addition of NaBr over a period of one minute. Twenty-five minutes after onset of nucleation, growth was begun during which 2.8M AgNO 3 , 2.8M NaBr, and a 0.088M suspension of AgI were added in proportions to maintain an iodide level of 3 mole-% in the growing crystals. During growth of this AgBr.97I.03 phase (a total of 7.00 moles) flow of reactants was first held constant for 80 min, at the end of which time pBr was decreased to 1.39. Next flow of reactants was accelerated 14× over the next 70 min. In the final growth segment, flow of AgNO 3 , NaBr, and AgI were decreased to 0.29× that at the end of the previous segment, and then accelerated 1.60×. A more concentrated AgI suspension (0.390M) was used in growth of this last 2.33 moles, and proportions of AgNO3 and AgI were regulated so that this last 2.33 moles was 12 mole-% I. After the final growth segment was completed, the emulsion was cooled to 40° C., and it was coagulation washed, then pH and pBr were adjusted to storage values of 6 and 3.1, respectively. 
     The resulting emulsion was examined with an optical microscopy at 150× magnification, and found to contain large agglomerates, which resulted from large numbers of tabular grains clumping together. In some cases it appeared that grains which were thus clumped, eventually grew as one large irregularly shaped particle. Such large agglomerates are highly undesirable in photographic emulsions, and this is the problem which this invention solves. 
     Antifoamant 1 (A-1)=Dioleate ester of polyethylene glycol, 9-10 EO, where 9-10 EO refers to a mean number of 9-10 ethylene oxide groups in the polyethylene glycol, and oleate is from oleic acid. 
     Emulsions B and C 
     Demonstration of the cause of agglomeration. 
     Emulsion B 
     Precipitation with nonoxidized lime processed bone gelatin 
     This emulsion was precipitated exactly as in Emulsion A, except the oxidization step after nucleation was omitted, and nonoxidized, rather than oxidized, lime-processed bone gelatin was used throughout. 
     Examination of the resulting emulsion by optical microscopy at 150× magnification revealed no agglomerates. A sample of the emulsion was also examined by scanning electron micrography (SEM), and mean grain area was determined using a Summagraphics SummaSketch Plus sizing tablet that was interfaced to an IBM personal computer. The mean diameter of the tabular grain population was 1.61 μm (COV=42), and more than 98% of the projected area was provided by tabular crystals. The emulsion did contain a considerable number of small non tabular grains so that only 60 number-% were tabular. When the total population was sized, the mean diameter was 1.06 μm and the COV was 81. Grain thickness was determined using a dye adsorption technique: The level of 1,1&#39;-diethyl-2,2&#39;-cyanine dye required for saturation coverage was determined, and the equation for surface area was solved assuming the solution extinction coefficient of this dye to be 77,300 L/mole cm and its site area per molecule to be 0.566 nm 2 . This approach gave a value of 0.113 μm. Thus the mean aspect ratio of the tabular population was ca. 14. 
     Emulsion C 
     Precipitation with oxidized, lime-processed bone gelatin but without antifoamant. 
     This emulsion was precipitated exactly as in Emulsion A, except that no antifoamant was used. 
     Examination of the resulting emulsion by optical microscopy at 150× magnification revealed no agglomerates. A sample of the emulsion was also examined by scanning electron micrography (SEM) and mean grain area was determined using a Summagraphics SummaSketch Plus sizing tablet that was interfaced to an IBM personal computer. The mean diameter of the tabular grain population was 1.90 μm (COV=51), and more than 99.9% of the projected area was provided by tabular crystals. The emulsion contained very few non tabular grains so that by number the tabular grains accounted for 95% of the total population. When the total population was sized, the mean diameter was 1.83 μm and the COV was 55. Grain thickness was determined using a dye adsorption technique: The level of 1,1&#39;-diethyl-2,2&#39;-cyanine dye required for saturation coverage was determined, and the equation for surface area was solved assuming the solution extinction coefficient of this dye to be 77,300 L/mole cm and its site area per molecule to be 0.566 nm 2 . This approach gave a value of 0.061 μm. The mean aspect ratio of the tabular population was thus ca. 31. 
     
                       TABLE 1                                                     
______________________________________                                    
Effect of A-1 and Gelatin Type on Agglomeration and Aspect Ratio          
                          Anti-          Aspect                           
Example Emulsion Gelatin  foamant                                         
                                Agglomerates                              
                                         Ratio                            
______________________________________                                    
1 (Com- A        Oxidized A-1   yes      --                               
parison)                                                                  
2 (Com- B        Non      A-1   no       14                               
parison)         oxidized                                                 
3 (Com- C        Oxidized none  no       31                               
parision)                                                                 
______________________________________                                    
 
    
     Summary of the Problem and its Cause 
     Precipitation of Ag(Br,I) emulsions in the presence of oxidized gelatin the antifoamant whose structure is the dioleate ester of polyethylene oxide (9-10 units) result in an unacceptable agglomeration of the resulting grains, as shown in Example 1. This agglomeration is caused by interaction of the cited antifoamant and oxidized gelatin, as shown in Examples 2 and 3. Neither of the approaches used in Examples 2 or 3 is desirable: Nonoxidized gelatin causes a highly undesirable thickness increase; in fact, the emulsion prepared with nonoxidized gelatin is almost 2× as thick as that obtained when the precipitation is formulated with oxidized gelatin, and this effect is evident in the aspect ratios tabulated in Table 1. Precipitation without antifoamant is feasible at laboratory scale, but only under severely limited stirring conditions in larger reactors used in manufacturing. In fact, as a practical matter, commercial emulsions to be prepared in large reactors with oxidized gelatin require a compatible antifoamant. 
     Antifoamant Comparisons. 
     Comparisons were made between antifoamants A-1 and A-5 using unwashed portions of Emulsion C, which was prepared with oxidized gelatin in absence of added antifoamant. At the end of the making procedure, samples of this antifoamant-free liquid emulsion were placed in separate vessels. The first compatibility test (ST-1) involved simple addition of individual antifoamants to the respective vessels (concentration matching that of A-1 in Emulsion A and B at the end of precipitation) while stirring the contents at 40° C. After stirring ca. 20 min, a portion of each antifoamant-treated sample was examined with a 250× optical microscope. If no agglomeration was observed, the mixture was carried forward to the next test (ST-2), which involved increased ionic strength. This was done by addition of sufficient NaNO 3  to increase the concentration to 2× that which prevailed at the end of precipitation of Emulsion C, due to the presence of the NaNO 3  by-product of the silver halide formation reaction. After stirring the resulting emulsion with antifoamant and additional NaNO 3 , it was again examined at 250× magnification. If no agglomeration was observed at this point, the mixture was carried forward for an additional test (ST-3). In this case additional Ag(Br,I) was precipitated by balanced AgNO 3  and halide salt addition at a molar flow rate to precipitated Ag(Br,I) ratio that matched that at the end of the initial precipitation. After forming an amount of new Ag(Br,I) equal to 1/3 that placed in the vessel at the start of the antifoamant test, the emulsion was examined again at 250× magnification. If no agglomeration was observed at this point, a complete 12.67 mole precipitation was carried out with the candidate antifoamant, using a procedure like that for the original emulsion, but with the cited increase in batch size. In the case of antifoamant A-5a, the comparison was on the basis of antifoamant behavior in the 12.67 mole precipitation. Results are given below in Table 2. 
     
                       TABLE 2                                                     
______________________________________                                    
               Agglomeration Detected                                     
                                      12.67 mole                          
Antifoamant                                                               
        Bxamples     ST-1   ST-2 ST-3 Precipitation.                      
______________________________________                                    
A-1     4 (comparison)                                                    
                     yes    --   --   --                                  
A-5     5, 6, 7, 8 (invention)                                            
                     no     no   no   no                                  
 A-5a   9 (invention)                                                     
                     --     --   --   no                                  
A-6     100 (comparison)                                                  
                     yes    --   --   --                                  
______________________________________                                    
 A-5 = Emulsified poly(dimethylsiloxane) in watersoluble resin (Dow Cornin
 X23396SF) having a number average molecular weight of 6,400 by a size    
 exclusion chromatography method.                                         
 A5a = Poly(dimethylsiloxane) (Dow Corning 200 Fluid, 1000CS Viscosity    
 Grade, added as 100% liquid of a number average molecular weight of 18,00
 by a size exclusion chromatography method.                               
 A6 =  Poly(ethylene oxidepropylene oxide)butoxy!-modified                
 polydimethylsiloxane (Silwet L720, manufactured by OSi Specialties) with 
 molecular weight of 12,000 according to the product bulletin SC906B      
 &#34;silicone surfactant&#34;.                                                   
 
    
     As is evident from data in Table 2 poly(dimethylsiloxane) antifoamants provide agglomerate-free emulsions when used in precipitations carried out in oxidized gelatin, whereas neither the dioleate ester of polyethylene glycol (9-10 EO) nor  poly(ethylene oxide-propylene oxide)butoxy!-modified poly(dimethylsiloxane) did. These results are not obvious on the basis of prior art. First of all, there is no indication in the prior art of the existence of the problem of antifoamant-induced agglomeration of emulsion grains in oxidized gelatin. Beyond that there is no suggestion that an poly(dimethylsiloxane) antifoamant would obviate this difficulty. Moreover,  poly(ethylene oxide-propylene oxide)butoxy!-modified poly(dimethylsiloxane) compound that is related to invention antifoamants leads to the undesired agglomeration. 
     In summary, invention antifoamants with these preferred characteristics are unexpectedly found to be suitable for use in precipitating silver halide photographic emulsions in the presence of oxidized gelatin: 
     Chemical structure: 
     Poly(dimethylsiloxane) compounds of the following formula, with m preferably 50-300: 
     
         (H.sub.3 C).sub.3 Si--(OSi(CH.sub.3).sub.2 -).sub.m --OSi(CH.sub.3).sub.3. 
    
     In summary, antifoamants with these characteristics are unexpectedly found to be suitable for use in precipitating silver halide photographic emulsions in the presence of oxidized gelatin: 
     The useful concentration range for the invention antifoamants is 0.02 to 10 grams per liter, with the preferred range being 0.1 to 5 grams per liter for effective control of agglomeration, as at this range it is most effective as an antifoamant and does not cause agglomeration. 
     The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.