US3372031A - Direct positive silver halide element with contiguous colloid layer containing developing agent - Google Patents

Description

United States Patent 3,372,031 DIRECT POSITIVE SILVER HALIDE ELEMENT WITH CON TIGUUUS COLLOID LAYER CON- TAINING DEVELOPING AGENT John Harold Baylis and Edward Cyril Dodd, Ilford, Essex, England, assignors to iiiord Limited, Essex, England, a British company No Drawing. Filed July 30, 1964, Ser. No. 386,391 Claims priority, application Great Britain, Aug. 28, 1963, 34,090/63 6 Claims. (1. 96-76) ABSTRACT OF TEE DISCLOSURE There is provided a process for the production of a photographic material. On a support, such as paper or the the like, is deposited a layer of a direct positive emulsion which is a fogged gelatino silver halide emulsion containing at least 95 mole percent of silver chloride and a desensitising agent effective to sensitise the Herschel effect to produce direct positive characteristics. When the emulsion is fully dried, a layer of a water permeable colloid and a developing agent for the emulsion is deposited and dried in less than 50 seconds. Alternately a thin waterpermeable layer may be placed between the emulsion and developer layers.

This invention relates to photographic materials and processes and more particularly to photographic materials which are adapted to be processed to yield positive images directly.

It is known that photographic emulsions which, on exposure and development, yield a positive image directly may be made by including in a silver chloride emulsion (which may contain a small proportion of silver bromide or silver iodide, e.g. up to mol percent) a small proportion of a desensitising compound and fogging the emulsion by light or by chemical means.

The foregoing method of producing a direct positive photographic material has received wide recognition and is extensively practised on a commercial scale. In fact, however, the actual production of such materials in a quality suitable for commercial use is by no means simple. It has been found in particular that not all desensitising compounds are effective and that the selection of a desensitising compound which will yield products of really good quality is one of considerable difficulty. However, materials of very good quality have been produced by the processes defined in British patent specifications Nos. 871,- 938, 873,937, 875,887, 905,237 and 907,367.

A known process of producing photographic prints is to employ as the print material a material which carries a layer of photographic silver halide emulsion which contains a photographic developing agent, imagewise expose the material, treat it with an alkali bath in order to activate the development (a so-called activator .bath) thereby producing a silver image in the material, and then treat the material with a bath (a so-called stabiliser bath) containing a substance which, by complexing with the residual silver halide in the material inhibits its lightsensitivity, thus stabilising the photograhic print thus obtained.

It is possible to carry out a process of the foregoing type by feeding the exposed print material through a system of rollers which apply to it, successively, the activator and stabiliser solutions; thus rendering possible the production of prints in a rapid and reliable manner. In this form the process is of value in document reproduction.

However, the print obtained is necessarily a negative record of the original document and if a positive record is required it is necessary to make a print from an intermediate negative record first obtained.

It would obviously be desirable to combine with a print process as just described the use of a direct positive photographic material since this would enable stable positive prints to be obtained in a single operation. However, the method would then require the inclusion in the photographic emulsion of both the desensitiser necessary to achieve the direct positive result and also the developer necessary to the success of the print process referred to. This, unfortunately, is impracticable because the desensitisers used are essentially oxidising compounds and are incompatible with the developers which are reducing compounds.

It has now been discovered, however, that very satisfactory direct positive materials which may be employed in the print process referred to can be made by a modification of the structure and composition of the photographic material.

According to the present invention there is provided a phtographic material which comprises, on -a support, a layer of a direct positive photographic emulsion which is a fogged gelatino silver halide emulsion of which at least mol percent of the silver halide is silver chloride, and which contains a desensitising agent effective to sensitise the Herschel effect to produce direct positive characteristics, and a separate layer of a water-permeable colloid containing a developing agent for the photographic emulsion.

The layers may be located in either order on the support and may be contiguous or separated by a thin waterpermeable colloid layer. The support may be paper or film as conventionally used for photographic materials.

In the production of the photographic material it is essential to avoid migration of developer into the photographic emulsion layer and to this end it is preferable that the first layer applied to the support should be substantially dried before the second layer is applied. The provision of a simple thin layer of water-permeable colloid between the emulsion layer and the developer-containing layer assists in avoiding risk of migration of developer and the technique of drying down the layers as rapidly as possible is also advantageous in this respect.

The photographic emulsion layer may be formulated in any manner known per se for direct positive emulsions relying on the Herschel effect and is preferably an emulsion prepared as described in any of British patent specifications Nos. 871,938, 873,937, 875,887, 905,237 and 907,367. It will for preference usually be a fine-grained gelatino silver chloride emulsion fogged by treatment with formaldehyde and containing the desensitiser in a proportion of about 1 to 10 grams per gram mol of silver present. It is found generally preferable to apply the emulsion to film or paper base at a silver coating weight of 0.8 to 2.0 grams silver per square metre and a gelatin coating weight of 1 to 10 grams per square metre.

The developer-containing layer is preferably a gelatin layer and the developer used is preferably hydroquinone. It is preferably laid down, optionally after the application of a thin gelatin layer to the emulsion surface, at a gelatin coating weight of 1.0 to 1.5 grams per square metre and a hydroquinone coating weight of 0.3 to 0.8 gram per square metre.

The following example will serve to illustrate the invention:

Example 4.4 litres of a washed photographic emulsion containing 188 grams of silver chloride and adjusted to pH 10 was treated with 12 ml. of a 4% by Weight aqueous solution of formaldehyde. The emulsion was heated at 52 C. for 35 minutes and was then neutralised by the addition of citric acid and cooled. There was then added the compound l(2-pyridyl)2(4-pyridyl)ethylene bisbenzyl bromide at the rate of 3.3 grams per gram mol of silver present. The pH of the emulsion was adjusted to 6.0 and the emulsion was coated on paper base and dried in 70 to 100 seconds. The silver coating weight was 1.3 to 1.5 grams per square metre.

There was then applied to the dried emulsion layer a gelatin solution containing hydroquinone, of pH 5.2, at a coating weight of 1.2 grams per square metre of gelatin and 0.5 gram per square metre of hydroquinone. The coated layer was dried in 30 to 50 seconds.

The material thus prepared was then imagewise exposed with light not containing ultra-violet or blue Wavelengths. It was then processed by activation and stabilisation by passing it through a roller system applying to it, in succession, thin layers of an aqueous activator solution containing 60 grams per litre of sodium hydroxide and 50 grams per litre of sodium sulphite (cryst.), and an aqueous stabiliser solution containing, per litre, 280 grams of ammonium thiocyanate, 5 grams of sodium sulphite (anhydrous), 32 grams of sodium metabisulphite, 5 grams of 1- phenyl-S-mercaptotetrazole and 200 mol of industrial ethyl alcohol.

There was thus obtained a direct positive print of the original subject of high contrast and excellent quality.

It is to be noted that the material also provides excellent prints by conventional processing using a fiXing bath.

We claim as our invention:

1. A process for the production of a photographic material which comprises applying to a support a first layer of a direct positive photographic emulsion which is a fogged gelatino silver halide emulsion of which at least 95 mol percent of the silver halide is silver chloride and which contains a desensitizing agent efiective to sensitise the Herschel efiect to produce direct positive characteristics,

drying the first layer, applying to the dried first layer a second layer of an aqueous solution of water-permeable colloid and a developing agent for the photographic emulsion and drying the said second layer in less than 50 seconds.

2. The process of claim 1 wherein a thin water-permeable colloid layer is applied to the emulsion layer and dried prior to applying the colloid and developing agent layer.

3. The process of claim 1 wherein the developing agent of the second layer is hydroquinone, and the colloid is gelatin.

4. The process of claim 3 wherein the gelatin and hydroquinone are applied at coating weights of 1.0 to 1.5 g. and 0.3 to 0.8 g. respectively, each per square metre of support area.

5. The process of claim 1 wherein the first layer is a fine-grained gelatino silver chloride emulsion fogged by treatment with formaldehyde and containing a sensitiser for the Herschel elfect in a proportion of 1 to 10 g. per gram mol of silver halide present.

6. The process of claim 1 wherein the first emulsion layer is applied at a coating weight corresponding to 0.8 to 2.0 g. of silver and 1 to 10 g. of gelatin, each per square metre of support area.

References Cited UNITED STATES PATENTS 2,541,472 2/1951 Kendall et a1. 96101 2,614,927 10/1952 Broughton et al. 9'695 2,834,676 5/1958 Stanley et al 96-29 NORMAN G. TORCHIN, Primary Examiner.

J. TRAVIS BROWN, Examiner.