GB2138960A - Photographic diffusion transfer processes - Google Patents

Abstract

A method of forming a photographic diffusion transfer image comprising exposing a photosensitive silver halide transfer, processing it so that an image diffuses to an adjacent image-receiving element and separating the image-receiving element from the photosensitive layer characterised in that the silver halide emulsion layer is coated on one side of a paper support that is coated on the same side with an underlayer containing light-absorbing pigment so that its reflection density is greater than 1.0 and which layer is chemically isolated from the silver halide layer. this invention can provide lithographic printing plates with improved dot quality and exposure latitude.

Description

SPECIFICATION Photographic diffusion transfer processes This invention relates to photographic diffusion transfer processes.

Photographic light-sensitive materials ofen contain light-absorbing materials coated as an antihalation layer. In the case of a two-sheet diffusion transfer process, carbon black is often contained in a layer coated immediately below the emulsion. This reduces light scatter and prevents undesirable reflections occurring between the base and coated layer interfaces, which cause additional diffuse exposure in the silver halide emulsion. Improvements in image quality, notably first generation halftone dot quality and exposure latitude, are observed as the level of antihalation protection is increased up to the maximum reflection density obtainable on the particular support used. These improvements, however, are gained with a subsequent reduction in overall photographic speed.

In commercially available products using white pigmented polyethylene coated paper base the problem of dot quality has been partially solved by introducing a small amount of carbon black into the white pigmented polyethylene or completely solved by coating a gelatinous layer containing a larger amount of carbon black between the support and the silver halide emulsion layer.

It is known, however, that the presence of carbon in a silver halide material has an adverse effect on its photographic performance. Impurities in the carbon black can act as fog centres and, in a diffusion transfer system, act as nucleating centres for solution physical development, which lowers the efficiency of silver transfer to the receiver sheet. This reduces the maximum optical transmission density obtained if the receiver consists of a proofing material containing suitable silver-reducing nuclei coated on a paper support, and therefore reduces sensitometric contrast. So although carbon black acts to reduce light scatter, its fogging properties result in poorer halftone dot quality than that achieved using other non-fogging antihalation agents.It is possible to use other pigments or dyes to act as antihalation agents, but because of the very high covering power afforded by carbon black, it is desirable to find ways of incorporating a fairly large amount of carbon blackwithout adversely affecting photographic performance.

U.S. Patent 3,900,323 proposes the addition of water-soluble salts of heavy metals such as cadmium for prevention of fog due to the use of carbon black. U.K. Patent 2,076,170 proposes the use of force-oxidised carbon black and a subsequent washing technique to remove the sulphur impurities. Both these patents refer to applications that include light-absorbing layers containing carbon black, notably wash-off materials and integral dye-diffusion transfer.

It can be seen therefore that inventive activity has been concentrated on making the carbon black contained in gelatinous layers less harmful.

The present invention enables the use of optimum quantities of carbon black to be used without encountering the disadvantages associated with the use of carbon black in gelatin layers referred to above.

According to the present invention there is provided a method of forming a photographic diffusion transfer image comprising exposing a photosensitive silver halide layer, processing it so that an image diffuses to an adjacent image-receiving element and separating the image-receiving element from the photosensitive layer characterised in that the silver halide emulsion layer is coated on one side of a paper support that is coated on the same side with an underlayer containing light-absorbing pigment such that its reflection density is greater than 1.0 and which layer is chemically isolated from the silver halide layer.

Preferably the reflection density of the pigment layer is from 1.5 to 2.2, preferably 1.9 to 2.1. The pigment layer may be chemically isolated by coating in or beneath a hydrophobic polymer layer. In a preferred embodiment it comprises a polyolefin, preferably polethylene, layer having carbon black dispersed therein.

In such a layer essentially each particle of carbon black is surrounded by polyethylene thus chemically isolating it from the silver halide layer.

The required reflection density will normally be achieved by employing from 0.2 to 0.6 g/nj2 of carbon black of the lamp black type.

The present method is particularly suited to graphic arts materials, especially to the preparation of lithographic printing plates.

The nature of the silver halide emulsion, the image-receiving layer, the supports, processing solutions to be used together with any addenda to any of the above are known in the art and have been described, for example, in Research Disclosure Item 17643, December 1978 published by Industrial Oportunities, Havant, Hampshire.

The following Examples are included for a better understanding of the invention.

Example I - Three-layer donor coatings Coatings were made of an ortho-sensitised 99.6:0.4 chloroiodide emulsion with 0.3 ,ummean edge length cubic grains on a resin-coated paper support containing carbon black in the polyethylene layer providing a reflection density of 2.05. An underlayer containing developing agents and gelatin was coated between the base and emulsion (Coating 1). As a control, a sample of the same emulsion was coated onto white (i.e.

non-carbon) resin-coated (RC) paper support (as described n 8ritish Specification 1,111,684) in the presence of an underlayer additionally containing a level (0.5 g carbon black per m2) to give an equal reflectance to the first coating (Coating 2). A thin gelatin-only supercoat was applied to both coatings to reduce abrasion sensitivity. An unexposed sample of each coating was processed in a monobath for 6 seconds and subsequently laminated to a receiver sheet containing a mixture of nuclei (to catalyse physical development) and toning agents dispersed in gelatin, coated on RC paper support, for 60 seconds at 2300. The receiver was then stripped apart from the donor, rinsed and dried.Measurements of both the amount of silver transferred and also that remaining on the donor were made by X-ray fluorescence and the results are summarised in Table 1. Optical transmission density measurements of the silver transferred to the receiver, made on a Macbeth TR 524 densitometer, are also included for completeness.

TABLE 1 SILVER (mg m-2 COATING DENSITY ON DONOR RECEIVER RECEIVER 1 300 690 3.20 2 (control) 659 458 1.97 The results show the improvements to be gained in silver salt transfer to the receiver (hence higher density) when the carbon black is chemically isolated from the silver halide layer.

Example 2 Coating 1 of Example 1 was compared to a similar commercially available material coated on a paper support itself having a grey-pigmented polyethylene coating to a reflection density of 0.64 (Coating 3). The commercial material was processed using its own processing liquid and receiver.

Each coating was exposed by contact to a test object consisting of bar charts of various spatial frequencies. The exposure latitude represents the variation in exposure which still provides resolution at the spatial frequency specified. The results are tabulated below.

TABLE 2 Exposure Reflection Spatial Frequency Latitude COATING Density (line pairs/mm) (log rel E) 1 2.05 6 2.25 11 0.70 3 0.64 6 0.4 11 0.22 It can be seen that increasing the reflection density of the polyethylene layer from 0.64 to 2.05 gives a large increase to exposure latitude.

Example 3 The same photographic silver halide emulsion used in Example 1 was coated at the same coating weight on three paper supports. Coating 4 was on paper coated with polyethylene containing white pigment, Coating 5 was on the same support coated with a gelatinous layer containing carbon black and Coating 6 was on paper coated with polyethylene containing carbon black.

The coatings were tested as described in Example 2 and the results are tabulated below.

TABLE 3 Exposure latitude (log rel E) Level of antihalation at a spatial agent : In terms of frequency of 6 COATING Reflection Density 1/mm 4 0.17 0.40 5 1.36 0.60 6 2.05 1.00 The results clearly show the advantages to be gained by increasing the density of the support.

Claims (9)

1. A method of forming a photographic diffusion transfer image comprising exposing a photosensitive silver halide layer, processing it so that an image diffuses to an adjacent image-receiving element and separating the image-receiving element from the photosensitive layer characterised in that the silver halide emulsion layer is coated on one side of a paper support that is coated on the same side with an underlayer containing light-absorbing pigment such that its reflection density is greater than 1.0 and which layer is chemically isolated from the silver halide layer.

2. A method as claimed in claim 1 in which the reflection density of the pigment layer is from 1.5 to 2.2, preferably 1.9 to 2.1.

3. A method as claim 1 or 2 in which the pigment layer is isolated by coating in or beneath a hydrophobic polymer layer.

4. A method as claimed in claim 3 in which the hydrophobic polymer comprises a polyolefin.

5. A method as claimed in claim 4 in which the hydrophobic polymer layer comprises polyethylene having carbon black dispersed therein.

6. A method as claimed in claim 5 in which the polyethylene layer contains 0.2 to 0.6 g/m2 carbon black of the lamp black type.

7. A method as claimed in any of claims 1-6 in which the image-bearing receiving element is suitable for use as a lithographic printing plate.

8. A method according to claim 1 substantially as described herein and with reference to the Examples.

9. An image-bearing receiving element produced by a method according to any of claims 1-8.