In the silver salt diffusion transfer process an imagewise exposed photosensitive silver halide material is placed in contact with a receiving layer in the presence of a developing agent and an alkaline processing composition containing a silver halide solvent. Development occurs imagewise while the silver halide solvent brings silver ions into solution in the undeveloped areas. The silver ions migrate to the receiving layer and these in the presence of development nuclei, undergo a process of physical development thus producing a silver image in the receiving layer complementary to. that formed in the silver halide layer. In one form of the process, the exposed silver halide material is placed in face-to-face contact with a separate receiving sheet in the presence of a viscous processing solution. After processing the "sandwich'' is peeled apart leaving the receiving sheet bearing the desired image. The silver precipitation nuclei are commonly formed from colloidal suspensions of certain metals or metal sulphides. The present invention is primarily concerned with nickel sulphide nuclei.

Receiver sheets are currently prepared by a process in which an aqueous composition containing nickel sulphide silver precipitation nuclei is coated onto a support and dried. It has been observed that the longer this composition is held prior to coating, the more the properties of the finished receiving layer deteriorate, e.g. the maximum image density obtainable becomes less.

It has now been found that suspensions of nickel sulphide nuclei can be stabilised by the addition of a reducing agent. Reducing agents are, of course, widely used in photography. Examples of such uses are as silver halide developing agents, as developer solution preservatives preventing premature oxidation by atmospheric oxgen, as electron transfer agents and interlayer scavengers. For example, US Patents 3,765,889, 4,514,488 and 4,585,725 all describe processing solutions for the silver salt diffusion process which may contain hydroxylamine compounds. It is to be noted that such compounds only come into contact with the development nuclei during processing and after imagewise exposure of the silver halide layer and certainly not during the coating of the layer containing the nuclei as do the reducing agents employed in the present invention.

According to the present invention there is provided an aqueous composition comprising a colloidal suspension of nickel sulphide particles and a stability-improving amount of a reducing agent.

In one embodiment the reducing agent is a sulphite, e.g. sodium or potassium sulphite whereas in another embodiment it is a hydroxylamine compound of the formula:

R ^X \

wherein R1 and R2 are each independently hydrogen or an alkyl, alkoxyalkyl or alkoxyalkoxy group, or an acid salt thereof. Alternatively, combinations of such reducing age may be used. The groups R 1 and R2 when not hydrogen preferably contain 1—4 carbon atoms. Examples of groups which may be represented are methyl, ethyl,

propyl, methoxyethyl, ethoxyethyl, methoxyethoxy and ethoxyethoxy. The acid salts may be sulphates, chlorides, thiocyanates or toluene sulphates.

The reducing agents may be water-soluble or insoluble and coated with the nuclei in solution or dispension respectively. It will be understood that reducing agents which react with water, e.g. borohydrides will not be suitable for the present invention.

Examples of particularly preferred hydroxylamines of formula (I) include hydroxylamine sulphate and diethyl-hydroxylamine.

The amount of reducing agent to be used may be optimised by experiment (as is shown in the

Examples below) but, as a guide, a coverage in the

2 range 0.5 to 1000 mg/m may be used. Hydroxylamine sulphate is preferably used at a coverage of from 20

2 to 350 mg/m , while sulphite ions are preferably

2 used at 1 to 7 mg/m and diethylhydroxylamine at 30

2 to 650 mg/m . The nuclei themselves will usually be

2 coated, at a coverage in the range 1.5 to 3 mg/m

(as NiS).

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

EXAMPLE 1 Nickel sulphide nuclei are formed by precipitation in a 12% (w/w) solution of gelatin from sodium sulphide (75g/l) and nickel nitrate (35g/l).

The dispersion is then stabilised by the addition of silver iodide and other solutions made to provide a coating composition having the formula:

Gelatin 40 gram

Nickel Sulphide 0.063

Silver iodide 0.06

5-phenyl-2-mercaptoxadiazole 0.17 5-methylbenztriazole 0.044

Water to 1 litre

This composition was divided into four 250ml portions and placed in polystyrene containers. Hydroxylamine sulphate (HAS) was added to the melt in accordance with Table 1 below. The suspensions were stirred magnetically for 5 minutes to fully dissolve and mix the HAS. The containers were sealed and stored at 4°C.

The properties of the nickel sulphide colloidal suspensions were measured soon after making and then at varying time intervals over a period of approximately 3 weeks as described below.

The following solutions were used: Solution A — nickel sulphide colloidal suspension as described above at 40°C.

Solution B - 1:1 mixture of Kodak PMT II activator and 0.01M Silver nitrate at 20°C. All readings were carried out at wavelength of 660nm. The rate of development of the nuclei was measured by injecting equal volumes of solutions A and B into the cell of a spectrophotometer, which was at a temperature of 40°C. The rate of change of absorbance with respect to time was recorded and from this the rate and D ax were calculated.

Rate was taken as the gradient of the steepest part of the curve. Dmax was the distance measured (cm) between maximum and minimum absorbance. For relative optical density (O.D.) measurements the spectrophotometer was set to zero with a 1cm cell containing distilled water in the reference beam. A 1cm cell containing solution A was then placed in the sample beam and the absorbance

reading taken. The results are shown in Table 2.

TABLE 2

In order to assess the stability of the solutions, a decomposition value is calculated. initial rate (Ri)

^Decomp ' ⁼ final rate (Rf)

The calculated rates are shown in Table 3

The higher the decomp. value the lower the stability. Thus, it can be seen that addition of HAS considerably increases the stability of the solutions. An improvement in the initial and final Dmax was also noted with solutions containing HAS.

EXAMPLE 2

Two layer coatings in which the nuclei dispersion contained three different reducing agents at varying concentrations on both transparent and opaque supports were made using coating solutions as follows:

Nuclei : 3.2 ml/ t ² nickel sulphide colloidal suspension + 3.6 ml/L 20% formaldehyde

Supercoat : 88%% ggeellaattiinn aatt 11..0088 mmll//fftt ²² ++ 22%% TRITON X 100 (a non-ionic surfactant)

Test images were formed in the fresh and incubated receiving sheets (incubated at 4°C for 30 days). Measurements were made of resolution (width of resolvable lines in μm) and density. The results are shown in Table 4 in which R—Res and T—Res mean reflection and transmission resolution and R-Den and T—Den similarly refer to density _. .

It is believed that R-Den can be taken as a measure of the speed of silver transfer while T—Den is taken as a measure of quantity of silver transferred. These factors must be considered in conjunctin with measurements of resolution which are indicative of image quality.

TABLE 4

REDUCING AGENT CONC.mg/m' AGE R-Res T-Res R-Den T-Den

Control None

HAS 69.0

Sulphite 95.6

DHA* 276.0

*DHA is Diethylhydroxylamine

The results show that improvements (compared to the control) are seen in R-Den or T-Den or both without unduly affecting image quality as indicated by resolution.

EXAMPLE 3

The procedure of Example 2 was repeated but with a 180 day incubation period. The results are shown in Table 5.

TABLE 5

REDUCING CONC. AGE R-RES T-RES R-DEN T-DEN AGENT mg/m

Control None

HAS 34.5

HAS 69.0

As in Example 2, improvements are seen in R—Den and/or T—Den without unduly affecting image quality.