This invention relates to the protection of photographic silver images against the action of peroxides, It is known that processed silver halide emulsion photographic materials and especially micro¬ films are susceptible to the formation of microscopic spot blemishes when stored under adverse conditions. The defect is described in an article entitled 'Microscopic Spots in Processed Microfilm Their Nature and Prevention' published in Photographic Science and Engineering, Vol. 7, No. 5. p. 253 , 19 3. Oxidation- reduction reactions between silver and peroxides are believed to be the cause of the defects.

Various methods have been described for the prevention or reduction of microscopic ageing defects. Conditions for the processing and storage of microfilm are given in the British Standards Institution Recommendations BS1153-. February 1975 'The Processing and Storage of silver-gelatin type Microfilm' and in the American National Standard ANSI PHI. 3-1976

•Practice for the Storage of Processed Safety Photo _¬ graphic Film' .

The present invention provides a solution to the problem of peroxide microspots which is simple, of

low cost and permanent.

According to the present invention there is provided a method of protecting silver images formed in a photographic material against attack by peroxides comprising contacting the material with a catalyst or the decomposition of hydrogen peroxide or a precursor of such a catalyst.

In one embodiment of the present invention finely divided, preferably colloidal, particles of the catalyst are incorporated into the photographic material during manufacture in a layer on the same side of the support as a photosensitive silver halide layer. The catalyst may be incorporated in any layer in an amount which does not interfere with the sensitometric properties of the material.

Preferably the catalyst is incorporated in a layer or layers adjacent the silver halide emulsion layer For example, the catalyst may be incorporated in a sub _¬ layer between the emulsion layer and the support and/or in an overlayer on the other side of the emulsion layer.

The catalyst may comprise gold, palladium or platinum but is preferably manganese dioxide. Colloidal manganese dioxide may be incorporated into a photographic silver halide material in amounts of from 50 to 500

2 2 -mg/m , preferably from 100 to 300 mg/m . As indicated above it may be located in a sub-layer and/or overlay and the amounts used are preferably from 50 to 250 mg/m in the sublayer and from 5 to 50 mg/m in the overlayer.

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As mentioned above the preferred catalyst particles are colloidal. The size of colloidal particles may vary widely, e.g. 10 ^" ' - 10~^ cm, as is known but o o sizes up to 500 A or even 1000 A are possible with the o preferred particles being no larger than 150 A.

The thickness of such layers may vary widely but thicknesses in the range 0.5 to 3.0 μm for the overlayer and 0.5 to 5.0 μm for the sub-layer are preferred. In an alternative embodiment, the catalyst can be coated in a hydrophilic binder over a layer already containing a silver image.

In the above embodiment the catalyst is present in a layer containing hydrophilic binder which is prefer- ably gelatin, an acylated gelatin, polyvinyl alcohol or mixtures thereof.

Photographic materials containing manganese dioxide catalyst should not be subjected to prolonged contact with acid fixer solutions containing sulphite ions as this can dissolve the catalyst and remove it from the material. The use of an alkaline fixer, e.g. an alkaline thiosulphate solution is therefore preferred.

In another embodiment of the present invention the silver image is treated with an aqueous solution of a catalyst precursor. The precursor is then converted to the catalyst either by a comparatively slow oxidative process involving atmospheric oxygen and/or peroxides or

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by a further processing step. The treatment with the catalyst precursor solution may, for example, take place as a final processing step or at any time after processing has otherwise been completed. Since the catalyst precursor is water-soluble any processing after its application to the photographic material will normally be avoided as this would tend to remove it.

The preferred catalyst precursor is a water- soluble manganese (II) salt, e.g. manganous sulphate, nitrate or chloride. The solution preferably contains from 0.02 to 0.25, more preferably 0.025 to 0.1, moles manganese (II) salt per litre and optionally contains non-phosphate buffers to maintain the solution between pH 6.5 and 7.5. The time of treatment may vary widely between, for example, 1 secon up to several minutes preferably from 5 to 30 seconds. The temperature of the treatment may also vary widely and, apart from its usual relation¬ ship with the time of treatment, is not considered critic The catalyst precursor solution preferably contains a wetting agent to provide an even distribution of catalyst precursor in the photographic material and to avoid the formation of salt spots on drying. Any wetting agent may be used, e.g. an anionic, cationic or non-ionic wetting agent.

Water-soluble compounds known to absorb strongly and stabilize filamentary silver images may also be included in the solution, for example nitroge^cα^tainiji

heterocyclic compounds and thiols, e.g. imidazole and l-phenyl-5-mercaptotetrazole.

The present invention further provides a photosensitive photographic material comprising a support bearing at least one silver halide emulsion layer which has incorporated in a layer adjacent thereto finely divided, preferably, colloidal, particles of a catalyst for the decomposition of hydrogen peroxide. The photographic materials protected from peroxide attack by the present invention are preferably microfilm materials but may be any photographic material, for example as described in Research Disclosure, December

1971, Item 9232 published by Industrial Opportunities

Ltd., Havant, Hampshire, U.K. The following Examples are included for a better understanding of the invention.

Example 1

To a stirred solution of 1 gm potassium permanganate in 90 mis of demineralised water at pH 7.5 was rapidly added a solution of 15 mis of 20 volume (6%) hydrogen peroxide in 90 mis of demineralised water adjusted to pH 7.5. The reaction was stirred for 30 minutes and then 20 mis of a 1096 gelatin solution added.

After dialysis for 3 days in semi-permeable membrane tubing the colloidal manganese dioxide content of the solution was found to be 0.15 .

50 mis of the above colloidal manganese dioxide solution were added to 450 mis of a 1096 gelatin solution

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adjusted to pH 6.5. The gelatin solution was then coated over a silver-containing gelatin layer on a poly¬ ethylene terephthalate film at the rate of 1.3 mis/sq.ft p to give a MnO _p laydown of 2.10 _. mg/m . Acceleration tests using hydrogen peroxide as described in Henn and Wiest - Photographic Science and Engineering, Vol. 7 , Number 5, September - October 1963 , page 253j failed to induce spotwise attack on such coati whereas similar control coating without manganese dioxid showed the expected spotwise attack. Example 2

20 is of 0.15% colloidal manganese dioxide • solution were added to 80 mis of a 6% polyvinyl alcohol solution and this solution was coated at a rate of 2.0 mls/sq.ft. over an existing layer of silver- containing polyvinyl alcohol coated on polyethylene terephthalate film base to give a final laydown of 6.46 mg MnOp per sq. metre.

Such layers showed increased resistance to oxidation and no tendency to undergo spotwise attack when compared with coatings notcontaining MnOp in the accelerated tests described in Example 1. Example 5

A photographic microfilm material was prepared by coating on a polyethylene terephthalate film base, a dye-containing anti-halation underlayer containing p colloidal manganese dioxide coated at 151 mg/m and gela

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p at 1.90 g/m , a fine grain negative emulsion layer and a gelatin supercoat containing colloidal manganese dioxide coated at 9 mg/m 2 and gelatin at 0.89 g/m2.

Another photographic microfilm material was prepared by coating on a polyethylene terephthalate film base, a dye-containing anti-halaticn underlayer containing p colloidal manganese dioxide coated at 176 mg/m and p gelatin at 1.90 g/m , a fine grain negative emulsion p layer coated at 1.77 g Ag/ and a gelatin supercoat at 0.89 g/m .

Samples of the experimental films were exposed either to a uniform neutral density of about 2.0 or a standard test image. These were then processed in a RECORDAK "PR0STAR" processor, model HDVR, under the following conditions;

(i) 75 s development at 30°C in a developer of the formula:

^• p_-methylaminophenol sulphate 2 g sodium sulphite (anhydrous) 90 g hydro uinone 8 g sodium carbonate 45 g potassium bromide 5 g water to 1 litre

(ii) 37 s wash at 28°C. (iϋ) 75 s fix at 28°C in a solution containing: Sodium thiosulphate 200 g/l

Sodium sulphite 20 g/l

(iv) 37 s wash at 28°C. (v) Dry at 50°C.

The processed films were incubated at 49°C, 8496 RH in an atmosphere containing 150 ppm hydrogen peroxide for 64 hours.

The control experimental film showed severe microspot attack whilst all samples of the experimental films containing colloidal manganese dioxide in the underlayer and supercoat were unaffected. At higher peroxide concentrations (600 ppm) none of the samples showed microspot attack but the control film showed a significant loss of density after 3 days. The film, containing manganese dioxide was unaffected. Example 4

The following treatment baths were prepared: Formulation 1

Manganous sulphate (4H O) 11.5 g Water to 1 litre Formulation 2

Manganous nitrate (4Hp0) 12.55 g Water to 1 litre Formulation 5 Imidazole 3.41 g

Hydrochloric acid (0.2M solution) 121.5 ml

Manganous chloride (4H 0) 9.9 g

Water to 1 litre

Accelerated tests using acid peroxide as described in Example 1 on processed photographic films treated with the stabilising solutions described above, showed a marked resistance to microspot formation.

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