US 3591379 A
Description (OCR text may contain errors)
United States Patent U.S. Cl. 96-50 13 Claims ABSTRACT OF THE DISCLOSURE Photographic elements such as, for example, X-ray film, comprising a support, one or more photographic silver halide layers, and one or more protective overcoat layers and a composition of matter which has utility in such overcoat layers are disclosed. The photographic elements are characterized by having a good combination of photographic properties such as, for example, sensitivity and contrast, resistance to abrasion and low haze; and are more particularly characterized in the use of at least one overcoat layer having the disclosed composition comprising gelatin, and at least one other hydrophilic colloid and finely divided colloidal silica or silica gel having an average particle size below 50 millimicrons.
BACKGROUND OF THE INVENTION Field of the invention This invention relates to photographic materials and their preparation. In another aspect, it relates to a composition of matter which has been found to be particularly desirable as a protective overcoat layer for silver halide emulsion layers in photographic elements. This invention also relates to photographic elements which have one or more of such overcoat layers. In another embodiment, the invention relates to an improved X-ray film having overcoat layers comprising the composition.
The prior art The use of gelatin, a hydrophilic colloid, as the binding agent or vehicle in photographic silver halide emulsions and elements is well known. Attempts have been made to substitute various synthetic and natural materials for all or a part of the gelatin to improve certain photographic and physical properties. However, improvement in one property is generally achieved at the expense of another and often more important property. For example, the use of gelatin and at least one other hydrophilic colloid as the vehicle or binding agent in a silver halide emulsion layer increases the covering power, thus yielding a higher density upon development for the same amount of silver per unit area. Such layers do not have good abrasion resistance and must therefore usually be coated with an overcoat layer. It has also been found that overcoat layers, containing gelatin and mixtures of gelatin and other hydrophilic colloids are suitable for this purpose. I have disclosed the concept of adding hydrophilic colloids to the gelatin vehicle or binding agent in either or both the silver halide emulsion layer or the overcoat layer and a specific application of this concept is shown in my US. application Ser. No. 506,831 filed Nov. 18, 1965. Elements of the type disclosed in the aforementioned application have improved resistance to abrasion particularly when processed in automatic processing equipment, for example, of the type described in Russell and Kunz U.S. Pats. 3,147,090 issued Sept. 1, 1964, and 3,025,779 issued Mar. 20, 1962. The need for increased resistance, such as is afforded by the addition of at least two other colloids besides gelatin to the overcoat layer, is particularly acute where the element is to be processed by such machines because of the 3,591,379 Patented July 6, 1971 great amount of roller contact in such machines and also because the higher temperatures under which such machines operate causes a softening of the gelatin layers. However, the addition of hydrophilic colloids to the gelatin vehicle, whether in the emulsion layer or overcoat layer, produces an undesirable increase in haze in that layer which renders the processed element or film more difficult to read. This haze can be ascribed to a light scattering within that layer and/or at the interfaces of that layer.
The prior art has attempted to solve this problem by the use of heavy gelatin overcoatsz such as taught by US. Pat. 3,05 8,826 of Meerkamper, Bank and Umberger, issued Oct. 16, 1962. The overcoat solution of the prior art does reduce the amount of haze but unfortunately creates a new problem since the use of such coats requires a substantial increase in the amount of time required to dry the processed unit. This is, of course, particularly objectionable in photographic elements which, because of the particular photographic materials used, are designed for and/ or require rapid processing. This heavy overcoat is particularly objectionable in X-ray film designed for machine processing since one of the primary functions of such machines is to decrease processing time.
It would, therefore, be highly desirable to provide the art with photographic elements having a layer or layers containing gelatin and at least one, and for best results at least two, other hydrophilic colloidsand thus possessing good photographic properties, abrasion resistance, and also low haze-which do not require an increase in processing time.
In an aspect of the invention of this application, such elements are obtained by the use of an overcoat layer comprising gelatin, at least one other hydrophilic colloid, and colloidal silica having an average particle size below 50 millimicrons. The addition of coarse particulate materials including silica to gelatin vehicles as inert matting agents is known to the art. However, the silica used for this purpose has a particle size on the order of 500 millimicrons and increases rather than decreases haze.
It is also known to the art to add small particle size silica to gelatin emulsions and overcoat layers-but not to overcoat layers comprising gelatin and at least one other hydrophilic colloid-as antistatic agents; note, for example, US. Pat. 3,053,662 of Mackey and Rice, issued Sept. 11, 1962.
OBJECTS AND SUMMARY It is, therefore, an object of the invention to provide photographic elements having good photographic properties, resistance to abrasion, and low haze, which can be rapidly processed and also to provide means for obtaining such elements. It is a further object to provide X-ray film having such photographic and physical properties which can be rapidly processed.
It is a still further object of the invention to provide a composition of matter comprising gelatin and at least one other hydrophilic colloid and finely divided colloidal silica or silica gel which can be used 'as an overcoat layer in such elements or X-ray film to achieve low haze without requiring an increase in processing time.
It is another object of the invention to provide photographic elements having at least one layer comprising gelatin and a hydrophilic colloid which is 'a carboxymethylated protein and having at least one overcoat layer containing gelatin and at least one other hydrophilic colloid and finely divided colloidal silica or silica gel. It is still a further object to provide such photographic elements in which said layer comprising carboxymethylated pnotein comprises another hydrophilic colloid which is a water-soluble vinyl polymer.
Additional objects will be disclosed and will become apparent, and in part obvious, from the following description and the appended claims.
SUMMARY In accordance with my invention, I have found that the above and other objectives can be attained by the use of an overcoat layer comprising gelatin and at least one other hydrophilic colloid and finely divided silica gel or colloidal silica having an average particle size below 50 millimicrons.
In summary, the overcoat composition of my invention comprises gelatin, a second hydnophilic colloid, and colloidal silica or silica gel in a weight ratio of 0.05 to 0.7 gram of the secondhydrophilic colloid per gram of gelatin and 0.05 to 2 grams of colloidal silica per gram of gelatin. For best results, the quantity of colloidal silica should be in the range of from 0.2 to 0.5 gram of colloidal silica per gram of gelatin. It should, of course, be appreciated that the overcoat composition is applied to the silver halide or other layer, dispersed within a volatile solvent or carrier, usually water, and that such dispersions are encompassed by the invention.
In summary, the photographic element of the invention comprises a support, one or more silver halide layers covering either side or both sides of the support, and one or more protective overcoat layers of the invention covering at least one of said silver halide layers and usually at least all of the silver halide layers which would otherwise form the exterior surface of the photographic element. In a further embodiment of the invention, at least one of the layers comprises as a second hydrophilic colloid in addition to gelatin a carboxymethylated protein and in a still further embodiment of the invention at least one of the layers comprises in addition to gelatin both a carboxymethylated protein and a water-soluble vinyl polymer.
DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS In practicing the instant invention, finely divided silica gel or colloidal silica is added to an overcoat composition comprising gelatin and at least one other hydrophilic colloid. I have found that such an addition surprisingly results in the production of a low haze photographic element having a good combination of photographic properties and abrasion resistance and which does not require an increase in processing time.
The overcoat composition of the invention can be conveniently prepared by admixing colloidal silica or silica gel to a solvent dispersion, usually an aqueous dispersion, of gelatin and at least one other hydrophilic colloid in a sufficient quantity, based on the quantity of dry gelatin, to provide from 0.05 to 2 grams and for best results from 0.20 to 0.5 gram of silica gel or colloidal silica per gram of gelatin (dry basis). It must be stressed that in order for the silica gel or colloidal silica to be effective in reducing haze it must have an average particle size below 50 millimicrons. This is necessary because coarse particlesized silica will not reduce haze and will in many cases actually increase the amount of light scattering and thus resultin an increase in haze rather than a decrease. In practicing the invention, any of the silica gels or colloidal silicas can be used so long as it possesses the required particle size. For purposes of simplicity, the haze reducing agent will hereafter and in the claims be referred to as colloidal silicathis should be taken as referring to any of the silica family having the required particle size regardless of the degree of hydration. One form of colloidal silica which is particularly suitable in the practice of the invention is the modified (by the chemical bonding through oxygen atoms of an amphoteric metal, such as aluminum, to silicon atoms in the surface of the silica particle) form described in US. Pat. 2,892,797 of Alexander and Iler issued June 30, 1959, and sold under the 4 trademark Ludox AM by E. I. du Bont de Nemours and Company.
Considering the photographic element of the invention in greater detail, such elements comprise (1) a support, (2) one or more photographic (silver halide) layers, and (3) one or more protectve overcoat layers. The photographic element can also contain other auxiliary layers such as are generally found in photographic elements, for example, subbing layers, antihalation layers, etc.
The particular support material used in the invention is not critical though desirably the support material should be firm, stable, inert, transparent, and preferably flexible. The following materials are suitable support materials: cellulose nitrate film, cellulose ester film, polyvinyl acetal film, polystyrene film, polyethylene terephthalate film and other polyester film as well as glass, paper, metal, wood and the like. Supports such as paper which are coated with a-olefin polymers, particularly polymers of a-olefins containing two 101 more carbon atoms, as exemplified by polyethylene, polypropylene, ethylenebutene copolymers, and the like, give good results. The above materials are, of course, only explanatory and not limiting, as other materials having the desired properties can be used. Further, it is contemplated that additional suitable support materials will be developed in the future. As before mentioned, the support can be coated on either or both sides.
Any of the gelatin photographic silver halide emulsions can be employed in practicing this invention, including, for example, photographic silver halide emulsions used in X-ray and camera films and the like. Suitable photographic emulsions contain silver halides such as silver chloride, silver bromide, silver bromoiodide, silver chloroiodide, silver chlorobromide and the like. Particularly good results are obtained with silver bromoiodide emulsions in which the average grain size of the silver bromoiodide crystals is in the range of about 0.5 to about 5 microns.
The photographic emulsions used in the elements described herein can be chemically sensitized with compounds of the sulfur group, noble metal salts such as gold salts, reduction sensitized with reducing agents, and combinations of these.
The photographic silver halide emulsions can also contain additional additives, particularly those known to be beneficial in photographic emulsions. For example, the emulsion can contain spee increasing compounds, for ex ample, onium salts, such as quaternary ammonium or ternary sulfonium salts, polyalkylene glycols, thioethers, and the like. The photographic silver halide emulsions can be stabilized with mercury compounds, azeindenes, quaternary benzothiazolium compounds, hydroxy substituted aromatic compounds, and the like.
The photographic silver halide elements described herein can contain absorbing dyes in the emulsion layer or in an auxiliary layer such as a layer coated between the support and the emulsion layer or the absorbing dyes can be included in both the emulsion and the auxiliary layer. These elements can also contain coarse inert particles such as those often employed as matting agents in photographic elements. Suitable materials of his type include, for example, particles of silicon dioxide, glass, stare, polymethyl methacrylate, and the like. Such inert particles are often included in an emulsion layer and/or auxiliary layers coated over and/or under an emulsion propionates of the type described in U.S. Pat. 3,133,816 of Ben-Ezra issued May 19, 1964. The emulsions and photographic elements described herein can contain incorporated developing agents such as polyhydroxy benzenes, aminophenols and 1,3-pyrazolidones. The photographic emulsions can also contain spectral sensitizers such as cyanines, merocyanines, complex (trinuclear) cyanines and complex (trinuclear) merocyanines, styryls and hemicyanines. These emulsions can be blue sensitive emulsions or they can be orthochromatic, panchromatic or X-ray emulsions.
Considering now the emulsion layer and protective overcoat layer together for purposes of describing the binding agent or vehicle which is used in each and which is basically the same with the exception of the particular additives used in each.
The binding agent or vehicle employed in the overcoat layers of this invention and also desirably in the silver halide emulsion comprises gelatin which is a hydrophilic colloid, and at least one hydrophilic colloid other than gelatin in the protective overcoat. The second hydrophilic colloid usually is present at a concentration of about to about 70% by weight based on the gelatin (dry basis) used in the binding agent. One of the hydrophilic colloids which can be conveniently employed for this purpose is a carboxymethylated protein. carboxymethylated proteins which can be employed in the practice of this invention include those described in U.S. Pat. 3,011,890 of Gates et al. issued Dec. 5, 1961. Some of the naturally occurring amino containing vegetable and animal proteins which can be carboxymethylated and used in practicing this invention include, for example, soybean protein, casein, globulin, and the like. The carboxymethylation of such proteins can be conveniently accomplished by reaction upon the protein with an alpha halo fatty acid such as bromoacetic acid at a pH of 9-12, such as is obtained by the presence of an alkaline buifer material in the reaction mass. The degree of carboxymethylation is subject to variation but will usually be in the range of about 5 to about 25% by weight. Many proteins such as gelatin can ordinarily be carboxymethylated without any prior treatment and used in practicing this invention. However, some proteins contain naturally occurring sulfur compounds which are desirably destroyed or removed by oxidation prior to carboxymethylation. A suitable method which can be for this purpose is treatment with hydrogen peroxide as described in U.S. Pat. 2,691,582 of Lowe and Gates issued Oct. 12, 1954. As already indicated, the carboxymethylated protein can be used in combination with a diiferent hydrophilic colloid other than gelatin. In the preferred case, the combination of hydrophilic colloids other than gelatin is a mixture of a carboxymethylated protein, e.g., carboxymethylated casein, with a water-soluble vinyl polymer, e.g., polyacrylamide.
As noted above, in practicing this invention, preferably a third ingredient of the binding agent is a hydrophilic colloid which is different from both the gelatin and the carboxymethylated protein. This hydrophilic colloid should not contain substantially any free amino or carboxy groups and be compatible With gelatin. The preferred hydrophilic colloid which is difierent from both the gelatin and the carboxymethylated protein is preferabl a water-soluble vinyl polymer such as a water soluble polyacrylamide of the type described in U.S. Pat. 3,271,158 of Allentofl and Minsk issued Sept. 6, 1966. While particularly efiicacious results are obtained with water-soluble polyacrylamide, other hydrophilic colloids can be used in the binding agent with carboxymethylated protein and gelatin. Such other hydrophilic colloids which can be employed with gelatin and the carboxymethylated protein and with or in place of the preferred polyacrylamide, include any of the hydrophilic water-permeable colloids, particularly the materials generally employed in the preparation of photographic silver halide emulsions as binding materials or vehicles, as exemplified by colloidal albumin, cellulose derivatives, synthetic resins, particularly water-soluble vinyl polymers and the like. Specific examples include water-soluble polymers such as polysaccharides, e.g., dextran, as disclosed in U.S. Pat. 3,063,838 of Jennings issued Nov. 13, 1962; vinyl polymers, e.g., poly-N-vinyl pyrrolidones, as disclosed in U.S. Pat. 3,043,- 697 of Forsgard issued July 10, 1962; polyvinyl alcohol derivatives, e.g., acid derivatives such as succinoylated polyvinyl alcohol, as described in U.S. Pat. 3,165,412 of Minsk and Abel issued Jan. 12, 1965; proteins, protein derivatives, as described in U.S. Pat. 2,852,382 of Illingsworth, Dann and Gates issued Sept. 16, 1958; cellulose derivatives, e.g., hydroxyethyl cellulose, as disclosed in U.S. Pat. 3,003,878 of Illingsworth and Minsk issued Oct. 10, 1961; and like compounds.
Also the emulsion and/or overcoat layer can contain hydrophilic colloids which are acrylic interpolymers, i.e., those interpolymers prepared from polymerizable acrylic monomers containing the characteristic acrylic group Such polymers are conveniently prepared by the interpolymerization of an acrylic monomer with at least one dissimilar monomer which can be another acrylic monomer or some other dilferent polymerizable ethylenically unsaturated monomer. Preferably these acrylic interpolyrners are compatible with gelatin and have a T g of less than 20 C. (Tg can be calculated by differential thermal analysis as disclosed in Techniques and Methods of Polymer Evaluation, Volume 1, Marcel Dekker Inc., New York, 1966.) A similar increase in physical and photographic properties is not achieved when polymers having a Tg of above about 20 C. are employed in the emulsions. Interpolymers which can be used comprise units of an alkyl acrylate such as, for example, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylates (e.g., n-butyl or t-butyl acrylates), amyl acrylates, hexyl acrylates and the like. Acrylic interpolymers containing units of acrylic acid or a sulfoester acrylate are especially useful. Typical polymers of this type are copoly(butyl acrylate-acrylic acid), copoly(methyl acrylate-acrylic acid), copoly(ethyl acrylate-acrylic acid), copoly(butyl acrylate-sulfopropyl acrylate) and the like. High ratios of solubilizing groups, such as the acrylic acid groups or the sulfoester groups produce a more soluble solution type polymer with respect to water carrier solvents. Mixtures of the more soluble solution type polymers and the latex polymers can be used, if desired, to achieve the desired emulsion characteristics. The acrylic interpolymer generally comprises at least 25% to about 75% by weight of the binder vehicle for the emulsion and from about 75 to about 25 by weight gelatin of the total binder.
The silver halide layers, gelatin overcoat or other layers in the photographic elements of this invention can also contain plasticizers or lubricating materials such as long chain fatty acids, silicone resins, N-alkyl B amino propionates, palmityl palmitate, and the like. Further, the emulsion layers and other layers present in photographic elements made according to this invention can be hardened with any suitable hardener such as aldehyde hardeners, aziridine hardeners, hardeners which are derivatives of dioxane, oxypolysaccharides such as oxystarch, oxy plant gums, inorganic hardeners such as. chromium salts, hardeners having reactive vinyl groups such as the vinyl sulfonyl ethers and the like.
The photographic element is prepared in the conventional manner by coating the support with the emulsion and overcoat layers in the sequence required to give the desired photographic element layer combination. It should be noted that the particular sequence is not critical to the invention and can be varied in any manner to give the particular combination dictated by the use to which the element is to be put.
The gelatin overcoats of this invention are particularly efiective when used with medical X-ray films having emulsion layers containing about 300-600 milligrams of silver per square foot and about 100-600 milligrams of hydrophilic colloid per square foot. The characteristics of processed materials of this type are improved to a considerable degree when such materials containing the overcoats of this invention are processed through roller transport processors, particularly when hardening developers are used as described in Barnes and Rees US. application Ser. No. 561,265, filed June 28, 1966.
Further, although, as is evident from the above, the silver halide layer will be usually applied as what is referred to by the art as an emulsion, it can also be applied by other means, such as, for example, by vapor deposition.
A further understanding of the invention can be had from the following examples.
EXAMPLE 1 This example illustrates the reduction in the haze which can be obtained in a photographic element comprising a support, silver halide emulsion layer, and an overcoat layer comprising gelatin and at least one other hydrophilic colloid, by the addition of finely divided colloidal silica to said overcoat layer. In this example, a coarse grain silver bromo-iodide used in medical X-ray film is coated on a polyethylene terephthalate film support at a coverage of 470 milligrams per square foot. Based on the weight of silver, the emulsion contains 100 grams gelatin, 11 grams carboxymethyl casein (a hydrophilic colloid), and 22 grams of polyacrylamide per gram mole of silver. For purposes of comparison, samples are then taken of the silver emulsion coated film and each sample is then respectively coated with an overcoat layer, at the same coverage, having the composition set forth below in Table I. The samples are then exposed on an intensity sensitometer and then processed in an automatic processing machine of the type described in US. Pat. 3,147,090 of Russell and Kunz issued Sept. 1, 1964, using a hardening developer of the type described in US. Pat. 3,232,761 of Allen and Burness issued Feb. 1, 196 6, with a total processing time of 7 minutes. The process samples are then tested for haze .(percent of light scatter). The results of these tests are summarized in the following Table 1.
1B of Example 1, but using a coarse silica, having an average particle size of 500 millimicrons, in place of the colloidal silica. The sample is then'exposed, processed and tested for haze in the same manner as in Example 1. This sample is observed to possess not only more haze than sample 113 but also more haze than sample 1A (i.e'., the sample not containing silica).
In addition to producing processed coatings having low haze, the colloidal silica containing overcoat layers of the invention also have good coatability and produce photographic materials having increased speed, improved abrasion resistance, improved resistance to water spotting during processing, reduced tackiness, improved hardness, increased electrical conductivity and reduced electrical charging when in contact with X-ray intensifying screens.
Although the invention has been described in consider} able detail with reference to certain preferred embodiments thereof, it will be understood that variations and modifications can be effected without departing from the spirit and scope of the invention as described hereinbefore and as set forth in the appended claims.
1. In a photographic element comprising a support, at least one silver halide layer, and at least one overcoat layer protecting the exterior surface of at least one silver halide layer which is furthermost from said support, the improvement wherein at least one overcoat layer comprises gelatin and at least one other hydrophilic colloid and colloidal silica having an average particle size below millimicrons, said colloidal silica being present in a weight ratio range with respect to gelatin of from about 0.05 to about 2 grams of colloidal silica per gram of gelatin (dry basis).
2. The element of claim 1 wherein at least one of said other hydrophilic colloids is a carboxymethylated protein.
3. The element of claim 2 where the carboxymethylated protein is present in a concentration of from 0.05 to 0.7 gram per gram of gelatin.
4. The element of claim 3 wherein the carboxymethylated protein is carboxymethyl casein.
5. The element of claim 2 wherein said other hydrophilic colloid also comprises a water-soluble vinyl polymer.
6. The element of claim 2 wherein one of said other TABLE I Percent Overcoat composition (dry coverage, mg./lt. haze decrease Carboxy- Haze from ad Colloidal l methyl Polyaeryl- (percent dition of silica, Gelatin, casein, amide, light colloidal Sample mgJft. nag/ft. mgJit. mg./1t. scatter) silica 1A 91 13 13 24. 6 36 91 13 l3 l4. 8 40 2A 91 38 31. 3 36 91 38 27. 9 11 A 91 38 20. 3 36 91 38 14. 4 29 and Iler, issued June 30, 1959,
From the above table it is readily apparent that the addition of colloidal silica to the overcoat layer produces an advantageous reduction in haze, especially so where the overcoat layer in addition to gelatin contains two other hydrophilic colloids (e.g., carboxymethyl casein and polyacrylamide). Further, although the silver halide emulsion layer used for the above samples also contains, in addition to gelatin, carboxymethyl casein and polyacrylamide, similar results are obtained where the silver halide layer contains only a single hydrophilic colloid (e.g., gelatin).
Similar reductions in haze are obtained with gelatinsilica overcoat layers containing additional hydrophilic colloids other than carboxymethyl casein and/or polyacrylamide.
EXAMPLE 2 An additional sample is prepared following the same having an average particle size less than 50 millimlcrons and sold under the trademark Ludox AM by E. I. du Pont de N emours dz O0.
hydrophilic colloids also comprises a water-soluble acrylic interpolymer and wherein said acrylic interpolymer is present in a concentration of from about 25 percent to about percent based on gelatin.
7. The element of claim 1 wherein said weight ratio range of colloidal silica is from about 0.2 to 0.5 gram of colloidal silica per gram of gelatin.
8. The photographic element of claim 1 wherein the support is coated on both sides with at least one silver halide layer.
9. The phototgraphic element of claim 1 wherein the support material is an inert, firm, flexible, stable, transparent material selected from the group consisting of cellulose acetates and polyethylene terephthalates.
10. The photographic element of claim 1 wherein the silver halide layer comprises gelatin, carboxymethylated procedure and with the same composition as with sample 75 casein and polyaerylamide.
11. In an X-ray element comprising a support, at least one silver halide layer on each side of said support, and wherein at least one overcoat layer protects the exterior surface of the silver halide layer on each side of said support which layer is :Eurthermost from said support; the improvement wherein at least one overcoat layer comprises gelatin and at least one other hydrophilic colloid and colloidal silica having an average particle size below 50 millimicrons, said colloidal silica being present in a weight ratio range with respect to gelatin of from about 0.05 to about 2 grams of colloidal silica per gram of gelatin (dry basis), and wherein at least one silver halide layer on each side of said support comprises gelatin, carboxymethylated casein and polyacrylamide.
12. The X-ray element of claim 11 wherein at least one auxiliary layer is interposed between at least one of said silver halide layers furthermost from said support and an overcoat layer protecting that silver halide layer.
13. The X-ray element of claim 11 wherein said silver 10 halide layers comprise silver bromoiodide and wherein each side of said element has a silver coverage of about from 300 to 600 milligrams per square foot.
References Cited UNITED STATES PATENTS NORMAN G. TORCHIN, Primary Examiner M. F. KELLEY, Assistant Examiner US Cl. X.R. 9667, 87