EP0733940A1

EP0733940A1 - Reduction of spot image defects caused by metal impurities in photographic film

Info

Publication number: EP0733940A1
Application number: EP96102725A
Authority: EP
Inventors: James A. Jr. c/o SM Company Gavney; John L. c/o 3M Company Hardiman
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: Eastman Kodak Co
Priority date: 1995-03-22
Filing date: 1996-02-23
Publication date: 1996-09-25
Also published as: JPH08262614A

Abstract

A photographic element comprising a silver halide emulsion in a hydrophilic colloidal binder coated on at least one side of a support layer, said emulsion having both a phosphate and a 5-sulfosalicylic acid therein, displays reduced spotting from spurious metal particle contamination.

Description

1. Field of the Invention

The present invention relates to photographic elements, and in particular to photographic elements which would contain spots due to the presence of metallic impurities, particularly as metal particles, in the photographic emulsion.

2. Background of the Art

The surfaces of silver halide crystals used in forming a photographic image are susceptible to modification by very low concentrations of impurities such as polyvalent metal ions. Such species may reduce a small part of the silver ions at the crystal surface to metallic silver. These silver particles can influence the image formation upon development by causing the development of fog, which is developed silver where no light struck the silver halide.
A known method of controlling or eliminating such defects is the addition of chelating or sequestering agents. These are chemicals which bond to metal ions so strongly that they may remove them from reaction with the photographic element. However, they may hold the metal in a particular valence state which then causes enhancement of the spot defect or its reversal from one form to its reverse. Minute metallic particles can perturb the image formation process in another way by modifying the development process itself if the minute particles have oxidation or reduction potentials which can increase or decrease the development of silver halide in their immediate proximity.
U.S. Patent 2,239,284 discloses the use of phosphates in developing solutions or in photographic gelatin emulsions to improve images.
U.S. Patent No. 3,258,338 discloses phosphoric compounds, including phosphates as gelatin softening compounds in silver halide emulsion layers.
U.S. Patent No. 3,300,312 discloses the use of sulfosalicylic acid in photographic elements to reduce spotting from metallic particles.
U.S. Patent No. 3,332,777 discloses the use of phosphates to flocculate silver halide gelatin emulsion layers.
U.S. Patent No. 3,443,951 discloses the use of phosphoric acid esters in photographic elements to prevent spot formation caused by metal particles.
U.S. Patent 4,340,665 discloses the use of phosphate and amine complexing agents in photographic elements to reduce spot formation caused by iron contamination.
We have found a mixture of such chelating agents which act together to eliminate or reduce the number or intensity of such defects, reducing defects from both light and dark spots.

Summary of the Invention

We have found that by combining two particular types of reagents, both white and black spots can be eliminated or greatly reduced from photographic elements without significantly influencing the sensitometry of the film in any other way. Each reagent by itself can eliminate one form of defect but also undesirably influence the sensitometry. However, the combination of phosphates and sulfosalicylic acid (or salts thereof) can be shown to eliminate or reduce the number or intensity of both forms of defect without any change in sensitometry.

Detailed Description of the Invention

There are several stages in the manufacture of photographic film when contamination by very small metallic particles can occur and often the exact source of the contamination cannot be traced. The particles may be introduced onto the polyester base during its manufacture or transportation or the contamination may happen during the coating of the silver halide emulsion layer onto the polyester. In normal manufacturing processes, all possible precautions are taken to prevent or minimize such contamination, but the amount of contaminant needed to produce a visible effect is so small that there can still be visible problems even with precautions being taken.
The surfaces of silver halide crystals used in forming a photographic image are susceptible to modification by very low concentrations of impurities such as polyvalent metal ions. Such species may reduce a small part of the silver ions at the crystal surface to metallic silver. These silver particles can influence the image formation upon development by causing the development of fog; that is developed silver where no light struck the silver halide.
Minute metallic particles can perturb the image formation process in another way by modifying the development process itself. A particle in a high oxidation state can react with and reduce the hydroquinone of the developer thus reducing its activity in that location leading to a lower developed density than expected. Such an effect can be seen in the image as a white spot. A particle of metal in a lower oxidation state can act by itself as a developer for silver halide in an additive way with the hydroquinone and give higher density than in the surrounding area. This will give a black spot in the image. The terms 'white' or 'black' spot are relative terms merely meaning that the spot appears whiter or blacker than the surrounding non-contaminated area of the image.
Contamination by a particle of a polyvalent metallic species can give both a white and a black spot in the same place when the defect may be seen as a central black spot surrounded by a more diffuse white spot. We believe the formation of such spots is caused when oxygen and water react with a metallic iron particle to make iron II hydroxide. This is a reducing agent which enhances development in its immediate surroundings by reducing silver halide to silver. This reaction also produces iron III hydroxide which diffuses laterally and there acts as an oxidising agent to reduce the development activity of the hydroquinone developer, thus producing a diffuse area of lower developed density. The combination of black center and white surround has been given the name "donut defect".
Spot defects in developed image can be unacceptable in many uses of photographic films, but can be especially critical when the film is being used as graphics arts and medical X-ray films. In the former field of technology, the film is used to generate a half tone image composed of small dots, and extraneous dots can lower the quality of the film for the subsequent printing process. Medical image films are continuous tone, but the presence of spot defects is not acceptable for at least two reasons. White spots or regions of slightly lower density than their surroundings can be indications of serious medical problems and so erroneous generation of white spots could lead to faulty diagnoses. The examination where the presence of white spots is most critical and therefore most unacceptable is that of mammography where such small areas of lower density can be taken as evidence of microcalcifications. Small black spots are less critical in the diagnosis process but are still objectional since they distract the radiologist from his view of the pathological image.
We have found that silver halide emulsions, and especially silver bromide emulsions sensitized to respond to infra-red light and intended for exposure to a solid state infra-red laser are especially sensitive to the presence of microscopic particles of metallic contaminants. Depending on the exact nature of the particle and probably its valency the observed defect can be either a white or black spot or a donut. None of these defects are acceptable to the radiologists who use the film.
Microscopic examination of the areas where spot defects can be seen have shown that there is frequently a small particle of metal, often iron, between the polyester base and the silver halide layer.
After consulting the extensive literature on corrosion of metals, especially iron, we added small amounts of phosphate ions to the silver halide layer to "passivate" the surface of any iron particles present as contaminants. We have found that passivation is a better solution than chelation or sequestration as taught in the prior art because while the phosphate passivates the iron to prevent it from forming donuts it does not form an iron complex whose redox potential enhances the formation of one form of defect over the other.
One disadvantage to the use of phosphates in infra red sensitized films is the change in sensitometry, especially speed increase, caused by their addition in amounts sufficient to prevent donut defects even when the change in pH they cause is neutralized.
Another disadvantage is that phosphates do not prevent the formation of white spots if there is present a contaminant which can form such low density areas.
Study of a large range of different metal chelating agents showed that one such, 5-sulfosalicylic acid, was able to prevent the formation of white spots on the film.
A combination of phosphate and 5-sulfosalicylic acid (or its salt thereof as with ammonium, alkali metal cations, e.g., sodium, potassium, etc.) was found to reduce the number or intensity of black spots, white spots and donuts.
In addition the mixture of the two did not cause a change in sensitometry.
Specifically we have found that a mixture of 0.12 to 0.70 grams of phosphate mixed with 0.28 to 2.54 grams of 5-sulfosalicylic acid per mole of silver halide can remove almost all spot defects from an infra-red sensitive film used for medical diagnosis. Even more preferable is a mixture of to 0.12 grams of phosphate with 0.56 grams of 5-sulfosalicylic acid per mole of silver bromide.
Phosphates suitable for use in the practice of this invention are the phosphate salts of ammonium or alkali metals:
M3PO4, where M is the ammonium or metal ion.
Any of the various types of photographic silver halide emulsions may be used in the practice of the present invention. Silver chloride, silver bromide, silver bromoiodide, silver chlorobromide, silver bromochloride, silver bromochloroiodide,silver chlorobromoiodide and mixtures thereof may be used for example.
Any configuration of grains, cubic, orthorhombic, hexagonal, tabular, epitaxial or mixtures thereof may be used. These emulsions are prepared by any of the well known procedures, e.g., single or double jet emulsions as described by Wietz et al., U.S. Patent 2,222,264, Illingsworth, U.S. Patent 3,320,069, McBride, U.S. Patent 3,271,157 and U.S. Patents 4,425,425 and 4,425,426.
The silver halide emulsions of this invention may be unwashed or washed to remove soluble salts by products. In the latter case the soluble salts can be removed by chill setting and leaching or the emulsion can be coagulation washed e.g., by the procedure described by Hewitson et al., U.S. Patent 2,618,556; Yutzy et al., U.S. Patent 2,614,928; Yackel, U.S. Patent 2,565,418; Hart et al., U.S. Patent 3,241,969; ad Waller et al., U.S. Patent 2,489,341.
Silver halide emulsions of this invention can be protected against the production of fog ad stabilized against changes in sensitivity during keeping by the addition of antifoggants and stabilizers alone or in combination, these can include the thiazolium salts described in Staud, U.S. Patent 2,131,038 and Allen U.S. Patent 2,694,716; the azaindines described in Piper, U.S. Patent 2,886,437 and Heimbach U.S. Patent 2,444,605; the mercury salts described in Allen, U.S. Patent 2,728,663; the urazoles described in Anderson, U.S. Patent 3,287,135; the sulphocatechols described in Kennard,U.S. Patent 3,235,652; the oximes described in Carrol et al., British Patent 623,448; nitron; nitroindazoles; the polyvalent metal salts described in Jones, U.S. Patent 2,839,405; the thiuronium salts described in Herz, U.S. Patent 3,220,839; and the palladium, platinum and gold salts described in Trivelli, U.S. Patent 2.566,263 and Damschroeder, U.S. Patent 2,597,915.
Silver halide emulsions in accordance with this invention can be dispersed in various hydrophilic colloids alone or in combination as vehicles or binding agents. Suitable hydrophilic materials include both naturally occurring substances such as proteins, for example gelatins derived animal bones and hides by the acid or liming process and chemically modified gelatins e.g. (phthalated, succinylated etc.) cellulose derivatives, polysaccharides, such as dextran, gum arabic and the like; and synthetic substances such as water soluble polyvinyl compounds, e.g. poly(vinyl pyrrolidone), acrylamide polymers or other synthetic polymeric compounds such as dispersed vinyl compound in latex form, and particularly those that increase the dimensional stability of photographic materials. Suitable synthetic polymers include those described, for example, in U.S. Patents 3,142,568 of Nottorf; 3,193,386 of White; 3,062,674 of Houck, Smith and Yudelson; 3,220,844 of Houck, Smith and Yudelson; Ream and Fowler, 3,287,289; and Dykstra, U.S. Patent 3,411,911; particularly effective are those water insoluble polymers of alkyl acrylates and methacrylates, acrylic acid, sulfoalkyl acrylates or methacrylates, those which have cross linking site which facilitate hardening or curing and those having recurring sulfobetaine units as described in Canadian Patent 774,054.
Photographic silver halide emulsions of this invention can be dispersed in colloids that may be hardened by various organic and inorganic hardeners, alone or in combination, such as the aldehydes, ketones, carboxylic and carbonic acid derivatives, sulfonate esters, sulfonyl halides, and vinyl sulfones, active halogen compounds, epoxy compounds, aziridines, active olefins, isocyanates, carbodiimides, mixed function hardeners such as oxidized polysaccharides, e.g., dialdehyde starch, oxyguargum, etc.
Emulsions in accordance with this invention can be used in photographic elements which contain antistatic or conducting layers, such as layers that comprise soluble salts, e.g. chlorides, nitrates etc., evaporated metal layers, ionic polymers such as those described in Minsk, U.S. Patents 2,861,056 and 3,206,312 or insoluble inorganic salts such as those described in Trevoy, U.S. Patent 3,428,451.
The photographic emulsions of this invention can be coated on a wide variety of supports. Typical supports include polyester film, subbed polyester film, poly(ethylene terephthalate) film, cellulose ester film, poly(vinyl acetal) film, poly carbonate film, and related resinous materials, as well as glass, metal paper and the like. Typically, a flexible support is employed, especially a paper support, which can be partially acetylated or coated with baryta and /or an alpha-olifin polymer, particularly a polymer of an alpha-olefin containing 2 to 10 carbon atoms such as polyethylene, polypropylene, ethylenebutene co-polymers and the like.
Emulsions of this invention can contain plastisizers and lubricants such as polyalcohols, e.g., glycerin and diols of the type described in Milton, U.S. Patent 2,960,404; fatty acids or esters such as those described in Robins, U.S.Patent 2,588,765 and Duane, U.S. Patent 3,121,060; and silicone resins such as those described in DuPont British Patent 955,061.
The photographic emulsions as described herein can contain surfactants such as saponin, anionic compounds such as the alkylarylsulfonates described in Baldsiefen, U.S. Patent 2,600,831 fluorinated surfactants, and amphoteric compounds such as those described in Ben-Ezra, U.S. Patent 3,133816.
Photographic elements containing emulsion layers as described herein can contain matting agents such as starch, titanium dioxide, silica, zinc oxide, polymeric beads including beads of the type described in Jelley et al., U.S. Patent 2,992,101 and Lynn, U.S. Patent 2,701,245.
Emulsions of the invention can be utilized in photographic elements which contain brightening agents including stilbene, triazine, oxazole and coumarin brightening agents. Water soluble brightening agents can be used such as those described in Albers et al., German Patent 927,067 and McFall et al., U.S. Patent 2,933,390 or dispersions of brighteners can be used such as those described in Jansen, German Patent 1,150,274 and Oetiker et al., U.S. Patent 3,406,070.
Photographic elements containing emulsion layers according to the present invention can be used in photographic elements which contain light absorbing materials and filter dyes such as those described in Sawdey, U.S. Patent 3,253,921; Gaspar, U.S. Patent 2,274,782; Carrol et al., U.S Patent 2,257,583 and Van Campen U.S. Patent 2,956,879. If desired the dyes may be mordanted, for example as described in Milton and Jones, U.S. Patent 3,282,699.
Contrast enhancing additives such as hydrazines, rhodium, iridium, and combinations thereof are also useful.
Photographic emulsions of this invention can be coated by various coating procedures including dip coating, air knife coating curtain coating or extrusion coating using hoppers of the type described in Beguin, U.S. Patent 2,681,294. If desired two or more layers may be coated simultaneously by the procedures described in Russell, U.S. Patent 2,761,791 and Wynn British Patent 837,095.
The silver halide photographic elements can be used to form dye images therein through the selective formation of dyes. The photographic elements described above for forming silver images can be used to form dye images by employing developers containing dye image formers, such as color couplers, as illustrated by U.K. Patent 478,984; Yager et al., U.S. Patent 3,113,864; Vittum et al., U.S. Patents 3,002,836, 2,271,238 and 2,362,598. Schwan et al., U.S. Patent 2,950,970; Carroll et al., U.S. Patent 2,592,243; Porter et al., U.S. Patents 2,343,703, 2,376,380 and 2,369,489; Spath U.K. Patent 886,723 and U.S. Patent 2,899,306; Tuite U.S. Patent 3,152,896 and Mannes et al., U.S. Patents 2,115,394, 2,252,718 and 2,108,602, and Pilato U.S. Patent 3,547,650. In this form the developer contains a color developing agent, e.g., a primary aromatic amine which in its oxidized form is capable of reacting with the coupler (coupling) to form the image dye. Also, instant self developing diffusion transfer film can be used.
The dye forming couplers can be incorporated in the photographic elements, as illustrated by Schneider et al., Die Chemie, Vol. 57, 1944 p. 113, Mannes et al., U.S. Patent 2,304,940, Martinez U.S. Patent 2,269,158, Jelley et al., U.S. Patent 2,376,697, Fierke et al., U.S. Patent 2,801,171, Smith U.S. Patent 3,748,141, Tong U.S. Patent 2,772,163, Thirtle et al., U.S. 2,835,579, Sawdey et al., U.S Patent 2,533,514, Peterson U.S. Patent 2,353,745, Seidel U.S. Patent 3,409,435, and Chen Research Disclosure, Vol. 159, July 1977, Item 15930. The dye forming couplers can be incorporated in different amounts to achieve differing photographic effects. For example, U.K. Patent 923,045 and Kumai et al., U.S Patent 3,843,369 teach limiting the concentration of coupler in relation to the silver coverage to less than normally employed amounts in faster and intermediate speed emulsion layers.
The dye forming couplers are commonly chosen to form subtractive primary (i.e., yellow, magenta and cyan) image dyes and are non-diffusible, colorless couplers, such as two and four equivalent couplers of the open chain ketomethylene, pyrazolone, pyrazolotriazole, pyrazolobenzimidazole, phenol and naphthol type hydrophobically ballasted for incorporation in high boiling organic (coupler) solvents.
The couplers may be present either directly bound by a hydrophilic colloid or carried in a high boiling organic solvent which is then dispersed within a hydrophilic colloid. The colloid may be partially hardened or fully hardened by any of the variously known photographic hardeners. Such hardeners are free aldehydes, U.S. Patent 3,232,764, aldehyde releasing compounds, U.S. Patents 2,870,013 and 3,819,608, s-triazines and diazines U.S. Patents 3,325,287 and 3,992,366, aziridines, U.S. Patent 3,217,175, vinylsulfones, U.S. Patent 3,490,911, carboimides and the like may be used.
Other conventional photographic addenda such as coating aids, spectral sensitizers, antistatic agents, accutance dyes, antihalation dyes, antifoggants, stabilizers, latent image stabilizers, antikinking agents, lubricating agents, matting agents ad the like may also be present.

Examples

It is possible to simulate the formations of spot defects of the type observed in the manufacture of photographic film by mixing into the emulsion immediately before coating a very fine dispersion of iron particles that are on the order of 15 to 18µm. In these examples, this method was used to ensure that a high level of iron would be present and make discernible the compounds that were capable of removing the spot defects caused by their presence.
The infrared sensitizing dye; dye A; used in the examples was: 3-ethyl-5,6dimethyl-2-(2-(3-(2-(3-ethyl-5,6-dimethyl-2-benzoxazole)-ethylene)-2-chlorocyclopent-1-enyl)-ethylene)benzoxazolium iodide.

Example 1

A silver bromide emulsion in gelatin was prepared using crystals about 0.24µm diameter. The emulsion was chemically sensitized by N-methyl thiosuccinamate and sodium tetra chloro aurate. After chemical sensitization the stabilizer tetra azaindene was added and the emulsion kept in cold storage until needed for coating.
Early screening tests for an adequate anti-spot agent involved potassium phosphate, sodium meta-phosphate, DTHC, EDTA, oxalate, and SSCA. These potential sequestering agents were studied in a range of 0.0033 to 0.0167 mole/mole of Ag.
The emulsion was prepared for coating by adding the styryl triazine supersensitizer "Leucophor BCF" from the Sandoz company, the infra red sensitizing dye, dye A, and surfactants to give good coating quality. Also added were poly ethyl acrylate latex as gelatin extender and formaldehyde as gelatin hardener. The emulsion was coated onto 7 mil polyester at a coating weight of about 2.0 grams of silver per square meter.
After coating and drying the film was tested for its sensitometry by exposure to a variable amount of light from a laser diode operating at 820nm shining through a 0-3 neutral density wedge. The film was processed for 90 seconds total time in a 3M processor type 515 using X-ray developer and fixer from the Eastman Kodak Company marketed under the name RP X-Omat chemistry. Speed expressed as relative log exposure at density of 1.0 above D_min.

The detection of spot defects was by exposing and developing a sheet of film to a density of about 1.0 and visually observing and counting spots when the film was placed on a light box such as is used in radiology. It was discovered that the potassium phosphate removed the black spots and Sulfosalicylic Acid removed the white spots (Table 2). The rest of the compounds tested either had no effect on the spots or in some cases intensified the contrast and number of the black or white spots.

TABLE 1

Moles Potassium Phosphate Mole Ag	Moles Sulfosalicylic Acid Mole Ag	Speed (@ 1 Month Aging)
0	0	3.007
0.00055	0	3.009
0.00110	0	3.043
0.00220	0	3.029
0.00330	0	3.042
0	0	2.993
0	0.00220	2.938
0	0.00330	2.938
0	0.01000	2.937
0	0	2.997
0.0056	0.00110	2.989
0.00055	0.00138	2.994
0.00055	0.00165	3.017
0.00055	0.00193	3.005
0.00055	0.00220	3.005
0	0	3.004

Based on results, potassium phosphate and Sulfosalicylic Acid were examined in combination in an effort to obtain an anti-spot agent that could effectively remove black and white spots without effecting the sensitometry.

Example 2

A silver bromide emulsion in gelatin was prepared using crystals about 0.24µm diameter. The emulsion was chemically sensitized by N-methyl thiosuccinate and sodium tetrachloroaurate. After chemical sensitization the stabilizer tetra azaindene was added and the emulsion kept in cold storage until needed for coating.

The phosphate and sulfosalicylic acid were added either separately or in combination before spectral sensitization. It can be seen from the sensitometric data presented in Table 1 that the phosphate would increase the speed with increased amounts, but that the sulfosalicylic acid would slightly reduce the speed. The combination of 0.00055 mole phosphate and a range of sulfosalicyic acid around 0.002 mole/mole of Ag did not effect the speed of the resultant film.

TABLE 2

AGENT	APPEARANCE EFFECTS
Potassium Phosphate	Made black spots much lighter, did not affect white spots.
Sodium Meta-Phosphate	Made black spots some lighter, but not as much as Potassium Phosphate, did not affect white spots.
DTPA	Changed black spots to white spots. Failed to affect white spots.
DTHC	Failed to affect spots.
EDTA	Changed black spots to white spots. Changed white halo around black spots to black.
Oxalate	Changed black spots to white spots. Changed white halo around black spots to black.
Sulfosalicylic Acid	Very effective at reducing white spots, changed intensity of black spots to gray.
Potassium Phosphate/SCA	Most effective for both black and white spots. Eliminated most white spots. Reduced intensity of black spots to a very faint gray.

The emulsion was prepared for coating by adding the styryl triazine supersensitizer "Leucophor BCF" from the Sandoz company, the infra red sensitizing dye, dye A, and surfactants to give good coating quality. Also added were poly(ethylacrylate) latex as gelatin extender and formaldehyde as gelatin hardener. The emulsion was coated onto 7 mil polyester at a coating weight of about 2.0 grams of silver per square meter.
Prior to coating approximately 0.05 g of the iron particles were added to the emulsion. After coating and converting, individual sheets were inspected and the levels of spots counted. It was discovered that the combination of phosphate and sulfosalicylic acid greatly reduced both the white spots and the black spots.
After coating and drying the film was tested for its sensitometry by exposure to a variable amount of light from a laser diode operating at 820nm shining through a 0-3 neutral density wedge. The film was processed for 90 seconds total time in a 3M processor type 515 using X-ray developer and fixer from the Eastman Kodak Company marketed under the name RP X-Omat chemistry.
The detection of spot defects was by exposing and developing a sheet of film to a density of about 1.0 and visually observing and counting spots when the film was placed on a light box such as is used in radiology.

Claims

A photographic element comprising a silver halide emulsion in a hydrophilic colloidal binder coated on at least one side of a support layer, said emulsion having both a phosphate salt of an alkali metal or ammonium and a 5-sulfosalicylic acid or salt thereof therein.
The element of claim 1 wherein said phosphate is present in said emulsion layer in an amount of 0.12 to 0.70 grams of phosphate per gram mole of silver halide.
The element of claims 1 or 2 wherein said 5-sulfosalicylic acid is present in said emulsion in an amount of at least 0.28 to 2.54 grams per gram mole of silver halide.
The element of claims 1 or 2 wherein said emulsion is spectrally sensitized to the infrared region of the electromagnetic spectrum between 720 and 1000 nm.
The element of claim 1 wherein said phosphate is selected from the group consisting of an alkali metal phosphate.
The element of claim 1 wherein said phosphate has the formula M₃PO₄, wherein M is ammonium or a monovalent metal ion.
The element of claims 1 or 2 wherein said silver halide emulsion is a silver bromide emulsion.