US 2968558 A
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United States Patent THERMO-REVERSIBLE GELS AND PHOTO- GRAPHIC EMULSIONS PREPARED THERE- WITH Alain Charles Andr Clavier and Jacques Pouradier,
Vincennes, France, assignors to Eastman Kodak Contpany, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Oct. 19, 1955, Ser. No. 541,547
Claims priority, application France Dec. 8, 1954 3 Claims. (Cl. 96- 114) This invention relates to new products useful in the photographic art and a method of preparing them.
Gelatin has ordinarily been employed as the protective colloid for the light-sensitive silver halides in photographic emulsions or as components of other photographic layers (filter, antihalation, overcoating etc., layers) due to its ability to hold these salts in suspension, its good water permeability, its insolubility in photographic developing solutions at ordinary temperatures and its ability to form a thin coating of the light-sensitive silver halide emulsion or other component upon a film support. Other protective colloids have been suggested for use instead of gelatin, but in most cases, these products are not so satis factory as gelatin as far as one or several of the abovementioned properties are concerned.
The most common method of preparing silver-halide dispersions is by reacting a water-soluble silver salt, such as silver nitrate, and a water-soluble halide, such as potassium bromide, in an aqueous solution of a peptizing agent, usually gelatin. There results from this operation a dispersion of the silver halide in an aqueous solution of the gelatin or other peptizing agent, which solution also contains salts resulting as a by-product of the reaction. In processes of making silver-halide dispersions and emulsions using the natural product, gelatin, the emulsion maker has in the past been limited in the scope of his operations by the physical properties of that material. For instance, concentrations of silver halide and of gelatin have had to be used which are not necessarily most desirable in photographic products. Also, in reducing the proportion of soluble salts in the emulsion to a minimum many difficulties have been met with. It has been the practice previously, where gelatin has been used as the dispersing material, to remove the soluble materials therefrom by setting the gelatin dispersion, noodling and then Washing the noodles by osmosis in cold water having a hardening agent therein. Under such conditions, a minimum gelatin concentration of approximately 4% is necessary, and this limitation has hampered the operations of the emulsion maker in his preparation of emulsions of this type.
Besides, the use of gelatin in silver halide photographic emulsions and other photographic materials is not satisfactory from other points of view. Thus, gelatin can be attacked by animal and plant organisms. It may be attacked by insects, especially in tropical climates or by molds or bacteria, especially under very humid conditions and at temperatures conducive to their growth. Moreover, because of its natural source, gelatin varies in its properties from one batch to another. Further, the preparation of gelatin sufficiently pure for photographic use demands much care.
One object of our invention is to provide new products useful in the photographic art, containing a thermoreversible gel which may be a total or partial substitute for gelatin but which is without the above-mentioned undesirable properties of gelatin. Another object of our invention is to provide a method of preparing these new products. A further object of our invention is to provide silver halide photographic emulsions in which the silver halide is suspended in the thermo-reversible gel referred to. A still further object of our invention is to provide a procedure for preparing useful thermal reversible gel compositions. Other objects of the invention will appear herein.
The new products relating to our invention comprise a thermoreversible gel of a high polymer having a major proportion of acid and nitrile groups.
In one embodiment of the invention, the high polymer present in the product in the form of a thermoreversible gel contains recurring units of the following general formula:
wherein R is a hydrogen or halogen atom or an alkyl su'bstituent group, and recurring units of the following general formula:
(II) G 112- C R- wherein R is a hydrogen or halogen atom or an alkyl substi-tuent group, the ratio of recurring units of Formula II to recurring units of Formula I being 1:1 to 6:1 and, preferably, between 1.5:1 and 3:1.
More particularly, the high polymer may be, for instance, a copolymer of acrylic acid with acrylonitrile containing about one acrylic acid unit for one to two acrylonitrile units.
It has been found that the above-defined high polymers form thermoreversible gels which have the property of precipitating in a reversible manner for pH. values under 5 and to melt at temperatures which can be altered as desired within a broad range by merely controlling the conditions of preparation within the limits specified hereinafter. Besides, the properties of the gels are similar to those of gelatin, as far as rigidity, transparency, swelling of the dried gel in cold water and hardening properties are concerned.
The above properties, and other properties as indicated 7 hereinafter, make these gels particularly apt to be substituted for gelatin, either totally or partly, especially for photographic applications. The copolyrners prepared from the same monomers under conventional conditions,
will generally not produce gels, and similar products in p the form of a solution or in unmixed form can be used only when mixed with gelatin. It should be noted, however, that the new gels of our invention, are compatible with gelatin in any proportions. A method of preparing useful thermoreversible gels is described herein.
The layer containing the macropolymers in the form of a gel can be any of the layers which constitute a photographic product such as filter, anti-halation, overcoating, intermediate etc., layers. Such layers are transparent and they can be hardened or made insoluble by means of various reagents, such as salts of divalent metals, e.g. calcium.
The layer can also be a light sensitive silver halide emulsion. In such a layer, the gel of macropolymer is used, for instance, as the binder or carrier for the silver halides. Such a gel is particularly advantageous since the layer of which it constitutes the binder is permeable to the photographic processing baths generally used for processing gelatino-silver halide emulsions, and still re mains strong enough to resist the mechanical action of these baths.
The gel of macropolymer can be present in the emulsion layer, either as a peptizing agent for the silver halides, or as a binder for the silver halides. It can be present both as a peptizing agent for silver halides and as a binder, or only as a peptizing agent for the silver halides, the binder being hydrophilic colloid, such as gelatin, or as a binder for silver halides peptized by conventional dispersing agents, such as gelatin, various gelatin derivatives such as those mentioned in the US. Patent 2,525,753 of August 13, 1947, hydrolyzed starch acetate, ethanolamine cellulose acetate, copolymers of maleic anhydride and vinyl acetate, alkyl celluloses, and the like.
The support of the product of our invention may be of any usual base such as paper or conventional photographic film bases made from a cellulose ester or a synthetic resin. On the nature of the support depends whethor an underlayer is necessary or useless between the support and the layer. For instance, an emulsion layer can be adjacent to the support provided the latter has no unfavorable eifect on the' emulsion. The emulsions may contain chemical and/ or optical sensitizers as usual emulsions do.
Our invention is not limited to the above-mentioned products but is generally applicable to all photographic products usually comprising a layer to which it was necessary, in the prior art, to incorporate for some purpose gelatin or a substitute therefor. In products prepared in accordance with our invention gelatin is partly or totally replaced by a gel of a macropolymer of the type specified above. Advantageously, this gel is substituted for all or the major part (at least 50%) of the gelatin, thanks to the thermoreversible properties of such a gel. In some instances, it may be advantageous to add to the gel a small quantity of gelatin as an auxiliary agent in view, for instance, of the action of the gelatin on the photographic properties proper of the product in the case of a light sensitive layer.
Among other examples illustrative of the new products prepared in accordance with our invention, the bichromated light-sensitive layers, receiving layers for dye imbibition printing, receiving layers used in reversal-transfer processes, and the like may be mentioned.
The preparation of new products in accordance with our invention is illustrated in Examples III to VII which should not be considered as exhaustive of our invention. The gels used in these examples are described in Exampies I and II.
EXAMPLE I Two liters of water, 96 cc. of concentrated acrylic acid, 174 cc. of acrylonitrile and, as the catalyst, a solution of 2 g. of potassium persulfate and 2 g. of sodium metabisulfite in 100 cc. of water were successively put into a vessel equipped with a mechanical stirrer and maintained at 60 C. Stirring was continued at the same temperature for 4 hours, and a few drops of hydrochloric acid were then added to enable the macropolymer to settle to the bottom of the vessel, in the form of a compact white paste. The macropolyrner was separated from the liquid and thoroughly washed with water.
The resulting copolymer was dissolved in water, and a solution of caustic soda was added as dissolution was proceeding in such a way that the pH of the solution does not rise above and is comprised between 6 and 10. The dissolution is faster at about 60 C. The proportion of water added was about ten times that of the macropolymer. Upon cooling, there was obtained a transparent gel having a melting point of about 60 C.
EXAMPLE II A three-liter two-necked flask was placed on a water bath at 65 C. One neck Was fitted with a dropping funnel. A thermometer was introduced through the other neck. In-the center was placed a stirrer surrounded by a condenser. Two liters of water were introduced in the flask, and when the temperature reached 65 C., 80 cc. of acrylic acid nitrile, 54 cc. of acrylic acid and'a solution of 1.6 g. of potassium persulfate and 1.6 g. of sodium metabisulfite in 100 cc. of water, were successively introduced into the flask. Polymerization was virtually completed within one hour.
The polymer suspension was set by adding a few drops of hydrochloric acid. The liquid was removed and the polymer was dissolved in an aqueous ammonia-solution, or in water containing a normal solution of caustic soda so that the pH of the solution should not exceed 10 or 11. The pure polymer, having no free radicals and photographically inert, was obtained upon a second acid precipitation. This polymer contains 365/1000 (by weight) of acid groups and 635/1000 of nitrile groups. It is soluble at pH values above 4.5 and insoluble at lower values of pH. The solution is extremely clear. The temperature of gelification is 27 C. for a concentration of 6% and increases by 6 C. increments when the concentration increases by increments. The gel is markedly more resilient, but less rigid than gelatin. In the liquid form its viscosity is slightly greater than that of gelatin sol.
EXAMPLE III On the back side of a film having an emulsion layer on the other side, there was coated a sol of a polymer prepared under the conditions of Example I and corresponding to a gel whose melting point is about 50 C. for an 8% concentration. Owing to the high melting temperature, the layer sets very rapidly which enabled coating to be carried out at a faster speed than the usual for coating of a gelatin backing layer.
EXAMPLE IV To a melted gel prepared as described in Example I, there were simultaneously added with rapid stirring in darkness, an aqueous solution of silver nitrate and an aqueous solution of one or more alkaline halides (bromide, chloride and/or iodide); a few drops of an acid (sulfuric acid for instance) were added in order to reduce the pH to 24 whereupon the emulsion precipitated. After washing the pH was adjusted to about 67 in order to obtain an emulsion ready for coating onto a photographic base.
EXAMPLE V A polymer was prepared under the conditions of Example II. 2 g. of this polymer were dissolved in 300 cc. of water adjusted to a pH of 5.5 and the mixture was heated at 40 C. 20 g. of potassium bromide dissolved in 60 cc. of water were added, and the temperature was maintained at 40 C. A solution of 19.5 g. of silver nitrate dissolved in 100 cc. of water was prepared in a separate vessel and maintained at 40 C. In darkness the solution of silver nitrate was added to the solution of potassium bromide and the whole mixture was maintained at 60 C. for five minutes; then, 30 g. of dry polymer were added with stirring and the dissolution was rapidly completed. Stirring was continued for a quarter of an hour at 60 C. After this time the mixture was cooled and set. The resulting emulsion was coated on a paper support. Photographic tests showed that the emulsion presented no fog effect and its speed was equivalent to that of a similar emulsion prepared with an inert gelatin.
EXAMPLE VI EXAMPLE VII The following composition was coated on a grained zinc plate Copolymer g 7 Ammonium bichromate g 3.5 Water; cc 100 The layer was exposed for three minutes through a negative illuminated by a mercury vapor lamp (Mazda MA 500). A relief image was obtained by development with hot water. The copolymer employed in this example was obtained by polymerizing at 65 C. for two hours a mixture of 160 cc. of acrylic acid nitrile and 106 cc. of glacial acrylic acid in 4000 cc. of water, in the presence of a mixture of 3.2 g. of potassium persulfate and 3.2 g. of sodium metabisulfite.
Generally the products of our invention can be prepared by any known process useful in preparing corresponding products containing gelatin except that the above copolymers are substituted for gelatin. The new products of our invention as shown in the preceding examples do not cause any fog to appear. They have no unfavorable action on the speed and contrast of the photographic emulsions. They are useful not only for emulsion layers but also for the other photographic layers such as backing layers as described in Example III, overlayers, intermediate layers, anti-halation layers, etc.
The macropolymer useful in the form of a gel in the products of our invention can be prepared by various means as indicated in the following examples and the various properties of thermoreversible gels can be adjusted to obtain the most advantageous properties of the product desired by varying within the limits indicated the nature of the initial monomers, the relative proportions of the monomers, the type of the polymerization, the polymerization medium, the dilution of the monomers, the nature and proportion of the catalyst, the polymerization temperature and the like.
As regards the nature of the initial monomers acrylic acid and acrylonitrile are preferably used in view of their availability but other monomer mixtures can be used such as methacrylic acid and acrylonitrile or methacrylonitrile, acrylic acid and methacrylonitrile, etc. A small proportion (such as up to of a third copolymerizable ethylenic monomer can be added to the mixture of the basic monomers in order to alter the properties of the gel. For instance, the addition of a small proportion of acrylamide to the initial monomers improves the adherence to glass of the thermoreversible gels of our invention which may be similar to that of gelatin. Likewise, the addition of a small proportion of vinyl acetate will result in gels having an increased rigidity while the addition of a small proportion of styrene will modify the solubility properties of the gels. Also, it may be desired to have recurring units reacting under the conditions of use of the new products of our invention introduced in the molecule of the macropolymer, for instance, recurring units apt to couple with the oxidation product of a primary aromatic amine to form a dye.
The proportions of the two initial monomers are such that the copolymer formed will substantially comprise one acid group for l to 6 nitrile groups and preferably for 1.5 to 3 nitrile groups. Of course, the proportions of the monomers mixed for the polymerization vary according to the conditions under which the polymerization is carried out as illustrated in the following examples.
The macropolymers can be obtained by copolymerizations of various conventional types or by conversion of the functional groups of a homopolymer. Where copolymerizations were used the macropolymers are prepared in a homogeneous phase in a medium wherein the monomers are soluble and the polymers insoluble. An acrylonitrile-acrylic acid copolymer can be obtained by partial hydrolysis of an acrylonitrile polymer.
As regards the polymerization medium, an aqueous solution having a pH of about 3 is preferred. Preferably, the pH is adjusted to that value by mere addition of acrylic acid. The dilution varies from 5 to parts of water by Weight for one part of the monomer (by weight). However, other polymerization media can also be used such as ethyl alcohol, for instance. The catalyst is preferably a mixture of equal proportions of potassium persulfate and sodium metabisulfite but other usual catalysts can also be used such as benzoyl peroxide or a mixture of hydrogen peroxide and ferrous chloride. The quantity of the catalyst used is generally comprised between and 7 of the weight of the monomers.
The polymerization is generally carried out at a temperature comprised between 20 and C.
The copolymers are generally obtained in an aqueous acid medium. All of them are insoluble in such a medium and at the end of the polymerization a white compact paste is obtained. In the case of alkaline hydrolysis of polyacrylonitrile, however, the polymer is in dissolved form. In each instance the polymer is precipitated by adjusting the pH to a value which is at most equal to S, for instance, by the addition of sulfuric acid or hydrochloric acid. The polymer can then be washed with an excess of water whereby the traces of present catalyst, the non-reacted monomers and the low-molecular portions are removed. After the polymer has been washed, it is dissolved in water to which a base has been added. When a strong base is used such as caustic soda or potash the lye is added dropwise during the neutralization of the acid groups of the polymer. If a mild base is used such as ammonia, an excess of the base can be added at once. The polymer is then stirred. For concentrations above it is necessary to heat the polymer during dissolution; this does not have a detrimental effect owing to the good heat stability of the polymers.
Gelification is obtained by cooling the solution obtained as above. The temperature of gelification depends on the composition of the copolymer, as well as on the method of preparing the same and its concentration.
The following examples which should not be considered as limitative of the scope of our invention illustrate methods of preparing gels useful in the products of our invention.
EXAMPLE VIII One liter of water, cc. of glacial acrylic acid, 200 cc. of acrylonitrile and a catalyst solution of 1 g. of potassium persulfate and 1 g. of sodium metabisulfite in 100 cc. of water were successively introduced in a vessel equipped with a stirrer and maintained at a temperature of 50 C. The temperature was maintained and stirring was continued for four hours; then, a few drops of hydrochloric acid were added. The polymer, which settled at the bottom of the container in the form of a white compact paste, was separated and carefully washed with water.
A quantity of water forty times as great as the quantity of the copolymer by weight and 50 cc. of 10 N ammonia were added to the copolymer thus obtained. The mixture was heated to boiling temperature to ensure rapid dissolution and this temperature was maintained until the ammonia smell was no longer noticeable. Upon cooling there was obtained a gel melting at about 50 C. The polymer was dried in the form of the ammonia salt.
EXAMPLE IX Thermoreversible gels were obtained from the copolymers listed in Table I by acidifying these copolymers and washing and dissolving them in an alkaline solution as described in Examples I and VIII.
All the copolymers of the present example are acrylic acid-acrylonitrile copolymers prepared by using a mixture of equal proportions of potassium persulfate and sodiummetabisulfite as the catalyst. Column I indicates the ratio of the weight of pure glacial acrylic acid to the weight of acrylonitrile, column II the Weight of the catalyst, in hundredths of the total Weight of the monomers, column III the amount of water added to the monomers in terms of cubic centimeters by gram of monomer, column IV the polymerizing temperature, and column V the polymerizing time in hours, that is to say the time during which the polymerizing conditions indicated were maintained.
Table I I II III IV V C. 0. as 4.3 as as 3 0. 91 3.3 s. 1 60 3 0. 74 3.3 8.1 3 0. 59 3.3 a. 1 e0 3 0. 45 as 8.1 60 3 0. 74 0.8 8.1 24 s 0.74 1.6 8.1 40 4 0. 74 0.7 8.1 70 3 0. 67 3.3 8.1 60 3 0. 66 1.6 3.1 50 2 EXAMPLE X By proceeding as in Example II, but using a mixture of 0.4 g. of potassium persulfate and 0.4 g. of sodium metabisulfite as the catalyst, all other conditions being similar, the temperature of gelification of the resulting product was 32 C. for a concentration of 3.5% and increased by C. for every increase of the concentration by EXAMPLE XI A macropolymer similar to that described in Example II was obtained by proceeding as in Example I! except that acrylic nitrile was not added until after polymerizing acrylic acid for one hour. Simultaneously with acrylic acid nitrile 0.5 g. of potassium persulfate and 0.5 g. of sodium metabisulfate were added. Sixty cc. of acrylic acid nitrile were suflicient to produce a copolymer giving a gel whose properties were similar to those of the gel described in Example II.
EXAMPLE XII This example illustrates the partial hydrolysis of a polymer of acrylic acid nitrile. To one part of waterinsolublc polymerized acrylic acid nitrile in the form of a fine powder were added one part of caustic soda and 100 parts of Water. The mixture: was refluxed for one hour. The acrylic acid nitrile dissolved. The mixture in the flask was adjusted to a. pH of about 9 and filtered. The polymer was obtained in the liquid part by the addition of hydrochloric acid to a pI-l 2.5. Depending on the greater or lesser degree of the hydrolysis the polymer gives gels whose melting temperatures are more or less high and which are all the more rigid as the initial acrylonitrile polymer had a greater molecular weight.
EXAMPLE XIII Thermoreversible gels were obtained from polymers prepared as described in Example II except that the catalyst indicated in Example II was replaced by a solution of l g. of benzoyl peroxide in 50 cc. of ethanol and/or 'water was replaced by an equal quantity of ethanol.
EXAMPLE XIV A thermoreversible gel was obtained from a polymer prepared as described in Example II except previous neutralizing of the acrylic acid, adjusting of the initial pH of water to 10 and the use of 1 cc. of hydrogen peroxide and 2.5 g. of ferrous chloride as the catalyst.
EXAMPLE XV Following the operating method described in Example II polymerization Was carried out at 30 C. in 1000 cc. of Water by successively adding 87 cc. of acrylic acid nitrile and 48 cc. of acrylic acid, then 0.4 g. of potassium persulfate and 0.4 g. of sodium metabisulfite in 50 cc. of water. The resulting polymer was similar to the polymer of Example II and gave a gel of lower rigidity wlnch melts at 45 C. for a concentration of /100; the melting temperature increases by 6 for every increase of the concentration by 4 a The viscosity of the sol is smaller than that of the sol obtained from the polymer prepared in Example H.
8 EXAMPLE XVI By proceeding as in Example II except that 5 g. of acrylamide dissolved in 20 cc. of water were added to the monomers, a gel was obtained having similar properties as that of Example II and having also glass adherence properties similar to those of gelatin.
EXAMPLE XVII By proceeding as in Example II a copolymer was prepared from 90 cc. of acrylic acid, 90 cc. of acrylic acid nitrile and 15 cc. of vinyl acetate. When polymerization was completed and the pH adjusted to 10, the polymer Was hydrolyzed by heating to boiling temperature for one hour and subsequently treated as described in the above examples to obtain a gel. The resulting gel has a greater rigidity then gelatin and melts at 36 C. for a concentration.
EXAMPLE XVIII By proceeding as described in Example II a copolymer was prepared from cc. of acrylic acid, cc. of acrylic acid nitrile and 10 cc. of styrene. The resulting polymer swells and is soluble in benzene and water-acetone mixture.
Depending on the conditions of the preparation thermoreversible gels can be obtained having gelification temperatures which range between 5 C. and 80 C. for concentrations comprised between 0.5% and 20%. Compared with photographic gelatin having a gelification temperature between 20 C. and 35 C. for concentrations between 2% and 8% certain gels useful for carrying out the invention have substantially the same gelificati-o temperature as gelatin for the same concentrations while other gels which may be used in accordance with our invention have much higher or lower gelification tem peratures for the same concentrations or are more concentrated or diluted for the same gelification temperature. Example XIX illustrates this possibility of obtaining gels having different melting points in the same concentration.
EXAMPLE XIX By proceeding as indicated in Example II gels were prepared from acrylic acid nitrile and acrylic acid in proportions indicated in Table II with the indicated proportions of catalyst (mixture of equal proportions of potassium persulfate and sodium metabisulfite). The polymerization was carried out in two liters of water at the temperatures indicated.
Table II Acrylic Acrylic Polymeriza- Relerencc No. of the gels nitrile, Acid, Satalyst, g. tion Temcc. cc. perature,
93 41 l 6 65 80 5t 1 6 65 80 54 0 1 65 186 82 3 2 50 93 41 l 6 50 80 54 0 1 50 For a concentration of 7% and pH 6, gel No. 1 melts at 24 C.; No. 2 at 35 C. and No. 3 at 39 C.
For a concentration of 2% and pH 6, gel No. 4 melts at 20 C., No. 5 at 44 C. and No. 6 at 67 C.
Likewise, gels can be prepared which will melt at a predetermined temperature in different concentrations.
Depending on the conditions of the preparation, it is possible to obtain gels having different rigidities and more particularly it is possible to obtain gels having the same gelification temperature but different rigidities from macropolymers of different compositions. The rigidity of the gels varies depending in particular upon the temperature at which it is measured, the composition, and the method of preparation of the macropolymer. For
instance, if the rigidity is measured at 15 C. below the melting point, it increases from 1 to 20 when to 10% (by total weight of the monomer) of subsequently hydrolyzed vinyl acetate is introduced into the polymer at the beginning of the polymerization. When the proportion of vinyl acetate added reaches 10% of the total weight of the monomers, the rigidity of the corresponding gel is twice that of gelatin measured under the same conditions.
Similarly the viscosity of the sols corresponding to the gels which may be used in accordance with the invention depends upon the conditions of the preparation. Viscosities are measured at various concentrations, at a temperature 10 C. above the melting point of the gel, and it appears that these viscosities are slightly greater than that of gelatin measured under similar conditions:
If the viscosity of gelatin is referred to as having the relative value of 1, the viscosities for different polymers and different concentrations vary between 1.5 and 6 in relative value.
It is known that at the isoelectric point of gelatin the physical and mechanical properties of the latter such as gelification, rigidity, viscosity, turbidity and the like are at a maximum or a minimum. On the other hand, the gels used in accordance with the invention show in their unhardened form the valuable characteristic to have substantially constant properties between pH and pH 9. Under pH 5 they undergo reversible precipitation, that is to say, they can recover their original state if the pH is readjusted to its previous value, an advantageous property for certain photographic applications.
Among the other advantages of the gels used in accordance with the invention mention must be made of the fact that the corresponding sols produce practically no foam while gelatin solutions readily produce foam and air bubbles. Moreover, while an aqueous gelatin solution is very susceptible to bacterial action, the products of the invention either in the form of gels or sols under the same conditions are not attacked. Some of these products can be preserved for several months at room temperature in the form of gels or sols without undergoing hydrolysis nor any apparent deterioration. Adding the products of the invention to a gelatin sol or gel will delay the bacterial attack on gelatin and under usual conditions substantially eliminates it when the weight of the product of the invention added to gelatin is equal to the weight of gelatin. Similarly, the sols of the invention can be maintained for several hours at boiling temperature without showing considerable deterioration.
All these properties made the gels of the invention suitable for use in all the photographic fields of use of gelatin. Moreover, While most of these gels are compatible in all proportions with gelatin, they can be used in various mixtures with the latter especially for masking or delaying the undesirable characteristics of gelatin such as susceptibility to bacterial attack as indicated above.
Also the gels described are useful in applications of gelatin other than in the photographic field. The invention is of course not limited to the embodiments described which are only illustrative.
1. A photographic material comprising a dispersion of silver halide in a thermoreversible water solution of a polymer essentially consisting of the following recurring units:
CH2C R- 000M and --CH2CR- M being a substituent selected from the group consisting of alkali metal and ammonium, the ratio of the second unit to the first unit being in the range of 1:1 to 6:1 said polymer being essentially the sole gel-forming ingredient and binder for the silver halide.
3. A method of preparing a silver halide dispersion which comprises reacting silver nitrate with an alkali metal halide in a thermoreversible water solution of a polymer which essentially consists of the following recurring units:
R and R being selected from the group consisting of hydrogen, halogen and lower alkyl and M being selected from the group consisting of alkali metal and ammonium, the ratio of the recurring units of formula II to those of formula I being within the range of 1:11 to 6:1 said polymer being essentially the sole gel-forming ingredient and binder for the silver halide.
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