EP0565152A1

EP0565152A1 - A silver salt diffusion transfer process

Info

Publication number: EP0565152A1
Application number: EP93200656A
Authority: EP
Inventors: Jules c/o Agfa-Gevaert N.V. Berendsen
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 1992-04-10
Filing date: 1993-03-08
Publication date: 1993-10-13
Anticipated expiration: 2013-03-08
Also published as: EP0565152B1; DE69321507D1; JP3136224B2; JPH0627621A

Abstract

The invention provides a silver complex diffusion transfer reversal process in which an information-wise exposed photographic silver halide emulsion layer material is transformed by a silver solvent into soluble silver complex compounds which are allowed to diffuse into an image receiving element and are reduced therein with a developing agent in the presence of an alkaline solution to form a silver image. The invention is characterised in that the silver halide emulsion layer includes a polyoxyethylene thioether and the alkaline solution contains a tertiary alkanolamine and is substantially free of primary and secondary alkanolamines, and preferably contains a tertiary alkanolamine. The process achieves improved long-term ambient storage and/or usage of the alkaline solution without loss of overall process quality.

Description

1. Field of the invention.

The present invention relates to a silver complex diffusion transfer reversal (DTR) process.

2. Background of the invention

The principles of the DTR process are known and have been described, for example, in United States Patent US 2352014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde - The Focal Press - London and New York (1972). In the DTR-process, non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called "silver solvent" into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent generally in the presence of physical development nuclei. to form a silver image having reversed image density values with respect to the silver image obtained in the exposed photographic material.
The developing agent or a mixture of developing agents can be present in an alkaline processing solution and or in the photographic silver halide emulsion layer material. In case the developing agent or a mixture of developing agents is contained in the photographic silver halide emulsion material, the processing solution can be merely an aqueous alkaline solution that initiates and activates the development.
The silver solvent, mostly sodium thiosulphate, may be supplied from the non-light-sensitive image-receiving element as mentioned above, but it is normally at least partly already present in the alkaline processing solution.
The alkaline processing solution usually contains sufficient alkaline substances to bring the pH above 10. e.g. sodium hydroxide, sodium carbonate and a great many other compounds which can raise the pH, for example borax, tertiary sodium phosphate, lithium hydroxide and amines, particularly alkanolamines.
The use of primary and secondary alkanolamines in the alkaline solution is sufficient to generate a pH of above 10 as required, but these materials suffer from the disadvantage that on storing and/or using the solutions in open ambient conditions, a reaction with the carbon dioxide of the air may occur leading to a consumption of the alkanolamines and the failure of the solutions to maintain the required pH.
It has been proposed in European Patent EP-A-398435 to replace the primary and secondary alkanolamines with tertiary alkanolamines. While the pH can be maintained by the use of this proposal, it has been found that the developing speed is reduced, thereby resulting in a loss of quality of the overall process as perceived by the user.
The DTR-process initially intended for office copying purposes has found now wide application in the graphic art field, more particularly in the production of screened prints from continuous tone originals. For the latter purpose it is particularly important that the processing characteristics remain steady for a large set of prints and that the gradation, optical density (transmission density in the case of film material and reflection density in the case of opaque material e.g. paper material) and the neutrality of the colour tone (black) of the screen dots in the screened prints satisfy graphic art standards which are particularly high compared with normal copying.

3. Summary of the invention.

It is an object of the present invention to provide a silver complex diffusion transfer reversal process in which the long term ambient storage and/or usage of the alkaline solution is achievable without loss of overall process quality. The man skilled in the art would have directed his attention to the composition of the alkaline solution in order to achieve this objective. Surprisingly however, we have found that this objective can be achieved by a modification of the composition of the silver halide emulsion.
Other objects and advantages of the invention will become apparent from the description that follows.
According to the invention, there is provided a silver complex diffusion transfer reversal process in which an information-wise exposed photographic silver halide emulsion layer material is transformed by a silver solvent into soluble silver complex compounds which are allowed to diffuse into an image receiving element and are reduced therein with a developing agent in the presence of an alkaline solution to form a silver image, characterised in that the silver halide emulsion layer includes a polyoxyethylene thioether and the alkaline solution contains a tertiary alkanolamine and is substantially free of primary and secondary alkanolamines.

4. Detailed description of the invention.

Thus, in general terms, the invention lies in the combination of two features, namely an alkaline solution containing a tertiary alkanolamine and which is substantially free of primary and secondary alkanolamines and a silver halide emulsion which comprises a polyoxyethylene thioether compound.
We are aware of British Patent GB 1455413 which discloses the preparation and use of polyoxyethylene thioethers as silver halide emulsion activators in various photographic processes. It has not previously been proposed however that such activators can be used in silver complex diffusion transfer processes wherein the alkaline processing solution contains a tertiary alkanolamine and that is free of primary and secondary alkanolamines. Furthermore it has been found that other known activators for photographic processes such as e.g. onium type activators are not effective for improving speed and image quality of a DTR-image obtained with an alkaline processing liquid containing a tertiary alkanolamine and no primary or secundary alkanolamine.

The image receiving layer

The DTR-image can be formed in the image-receiving layer of a sheet or web material being a separate element with respect to the photographic silver halide emulsion material or in a so-called single-support-element, also called mono-sheet element or unitary DTR-material, which contains at least one photographic silver halide emulsion layer and the image-receiving layer in waterpermeable relationship therewith, e.g. on top of each other or separated by a thin waterpermeable stripping layer or alkali-degradable interlayer as described e.g. in US-P 3,684,508 or wherein the photographic silver halide emulsion layer is optically masked from the image-receiving layer, e.g. with a white waterpermeable pigment layer as described e.g. in US-P 3,607,270 and 3,740,220.
The image receiving layer comprises for best imaging results physical development nuclei normally in the presence of a protective hydrophilic colloid, e.g. gelatin and or colloidal silica.
Preferred development nuclei are sulphides of heavy metals e.g. sulphides of antimony, bismuth cadmium, cobalt, lead, nickel, palladium, platinum, silver and zinc. Especially suitable development nuclei are NiS.Ag₂S nuclei as described in US-A 4,563,410. Other suitable development nuclei are salts such as e.g. selenides, polyselenides, polysulphides, mercaptans and tin (ll) halides. Heavy metals or salts thereof and fogged silver halide are suitable as well. The complex salts of lead and zinc sulphides are active both alone and when mixed with thioacetamide, dithiobiuret and dithiooxamide. Heavy metals preferably silver, gold, platinum, palladium and mercury can be used in colloidal form.
The transfer behaviour of the complexed silver largely depends on the thickness of the image-receiving layer and the kind of binding agent or the mixture of binding agents used in the nuclei containing layer. In order to obtain a sharp image with high spectral density the reduction of the silver salts diffusing into the image receiving layer must take place rapidly before lateral diffusion becomes substantial.
An image-receiving material satisfying said purpose is described in published European Patent Specification EP 306561 and is particularly suitable for being processed with an aqueous alkaline processing liquid according to the present invention.
The image-receiving layer and/or other hydrophilic colloid layer of an image-receiving layer and/or other hydrophillic colloid layer of an image-receiving material used in a DTR-process according to the present invention may have been hardened to some extent to achieve enhanced mechanical strength. Appropriate hardening agents for hardening the natural and/or synthetic hydrophilic colloid binding agents in the image-receiving layer include e.g. formaldehyde, glyoxal , mucochloric acid, and chrome alum. Other suitable hardening agents for hardening the hydrophillic colloid binding agents in the image-receiving layer are vinylsulphonyl hardeners, e.g. as described in Research Disclosure 22 507 of Jan. 1983.
In the process of the present invention the image-receiving material can be used in the form of roll film or sheet film or in the form of a filmpack e.g., for in-camera-processing.

The silver halide emulsion material

The image-receiving material can be used in conjunction with any type of photographic silver halide emulsion material suited for use in diffusion transfer reversal processing. The silver halide emulsion material may contain one or more hydrophilic colloid - silver halide emulsion layers.
In the photographic material to be processed after exposure with a processing solution according to the present invention, whether or not in combination with a DTR-image-receiving material, the hydrophilic colloid silver halide emulsion layer can be coated from any photosensitive silver halide emulsion comprising a hydrophilic colloid binder, which usually is gelatin. The weight ratio of hydrophilic colloid binder to silver halide expressed as equivalent amount of silver nitrate to binder is e.g. in the range of 1:1 to 10:1.
The photosensitive silver halide used in the present invention may comprise silver chloride, silver bromide, silver bromoiodide, silver chlorobromoiodide and the like, or mixtures thereof. To obtain a sufficiently high rate of solution of the silver halide and a satisfactory gradation necessary for graphic purposes a silver halide emulsion mainly comprising silver chloride is used preferably. Thus, usually the silver halide which is used is silver chloride, but a proportion of silver bromide may also be present. We have found that the present invention is especially beneficial when the silver halide present in the silver halide emulsion comprises silver chloride together with at least 5 mole% silver bromide.
The silver halide emulsions may be coarse or fine grain and can be prepared by any of the well known procedures e.g. single jet emulsions such as Lippmann emulsions, ammoniacal emulsions, thiocyanate - or thioether-ripened emulsions such as those described in US A 2 222 264, 3 320 069 and 3 271 157. Surface image emulsions may be used or internal image emulsions may be used such as those described in US A 2 592 250, 3 206 313 and 3 447 927. If desired, mixtures of surface and internal image emulsions may be used as described in US A 2 996 382.
The silver halide particles of the photographic emulsions may have a regular crystalline form such as cubic or octahedral form or they may have a transition form. Regular-grain emulsions are described e.g. in J. Photogr. Sci ., Vol. 12, No. 5, Sept./Oct. 1964 pp. 242-251. The silver halide grains may also have an almost spherical form or they may have a tabular form (so-called T-grains), or may have composite crystal forms comprising a mixture of regular and irregular crystalline forms. The silver halide grains may have a multilayered structure having a core and a shell the silver halide grains may comprise also different halide compositions and metal dopants in between.
The average size of the silver halide grains may be of at least 0.1 µm, preferably within the range of 0.2 to 1.2 µm, and the size distribution can be homodisperse or heterodisperse. A homodisperse size distribution is obtained when 95% of the grains have a size that does not deviate more than 30% from the average grain size. We have found however that the present invention is especially beneficial when the average (by weight) silver halide grain size is relatively large, such as more than 0.3 microns, preferably from 0.5 to 2.0 microns.
Apart from negative-working silver halide emulsions that are preferred for their high light-sensitivity, use can be made also of direct-positive silver halide emulsions that produce a positive silver image.
Either in combination with one or more developing agents, or not, the silver halide emulsions may contain pH controlling ingredients, and other ingredients such as stabilizers, antifogging agents, development accelerators, wetting agents, and hardening agents for gelatin.
The silver halide emulsion coated side of the photographic material can be provided with a top layer that contains hydrophilic colloids that form a waterpermeable layer. Such top layer is usually free of gelatin. Its nature is such that it does not inhibit or restrain the diffusion transfer of the complexed silver but acts e.g. as an anti-stress layer. Appropriate hydrophilic binding agents for such top layer are e.g. methyl cellulose, the sodium salt of carboxymethyl cellulose, hydroxyethyl cellulose, hydroxyethyl starch, hydroxypropyl starch, sodium alginate, gum tragacanth, starch, polyvinyl alcohol, polyacrylic acid, polyacrylamide, poly-N-vinyl pyrrolidone, polyoxyethylene, and copoly(methylvinylether/maleic acid). The thickness of this layer depends on the nature of the colloid used and the required mechanical strength. Such layer if present may be transferred at least partially to the image-receiving layer without deleterious action on the image formation.
The development and diffusion transfer can be initiated in different ways e.g. by rubbing with a roller that has been wetted with the alkaline processing liquid e.g. acts as meniscus coater, by wiping with an absorbent means e.g. with a plug of cotton or sponge, or by dipping the material to be treated in the liquid composition. Preferably, they proceed in an automatically operated apparatus such as the COPYPROOF (registered trade name of AGFA-GEVAERT NV. Belgium) type CP 38, CP 380, CP 42 or CP 530 processors. The DTR-process is normally carried out at a temperature in the range of 10°C to 35°C.

The polyoxyethylene thioether compound

The silver halide emulsion layer includes a polyoxyethylene thioether. In preferred embodiments of the invention, the polyoxyethylene thioether comprises recurring units of the following formula:

wherein R represents a substituted or unsubstituted aliphatic, aromatic or heterocyclic group. By way of example, R in the foregoing formula (a) may be an aliphatic, carbocyclic aromatic or heterocyclic group carrying one or more of the following substituents: alkyl , aralkyl, hydroxyalkyl, alkoxyalkyl, alkylthioalkyl, acyloxyalkyl (the alkyl groups of such substituents preferably carrying at most 5 C-atoms), cycloalkyl e.g. cyclohexyl, aryl, e.g. phenyl and phenyl substituted with alkyl, alkoxy and alkyl-thio.
We especially prefer a compound of the formula:

where n is from 2 to 50.
Preferred polyoxyethylene thioethers have a molecular weight of between 300 and 7500, most preferably between 500 and 5000.
Such polyoxyethylene compounds can be derived from polyepihalo-hydrins e.g. polyepichlorohydrin and polyepibromohydrin including ethers and esters thereof by reaction with aliphatic, carbocyclic aromatic, or heterocyclic mercaptides. When using a preparation method of that type, polyoxyalkylene compounds can be prepared which in addition to the structural units of the above formula (a) comprise recurring units of the following formula (c):

wherein X is halogen due to incomplete conversion of the polyepihalohydrin. It has been found that it is advantageous to carry out development in the presence of polyoxyethylene compounds comprising units of formulae (a) and (c) but at least 20 mole % of randomly distributed recurring units of formula (a) should be present.
In the preparation of the polyoxyethylene compounds, the molecular weight of the polyepihalohydrins, ethers or esters thereof may vary within wide limits. Dimeric compounds may be used as well as commercially available polyepihalohydrins which possess generally a molecular weight of at most 3000. However, polyepihalohydrins of higher molecular weight, as described in Jl. Polymer Sci. 40 (1959) p. 571, may also be employed.
The concentration of the polyoxyethylene thioether in the silver halide emulsion layer is from 0.1 to 50g, preferably from 1 to 10g, polyoxyethylene thioether per mole of silver halide present in the emulsion layer.

The alkaline processing solution

The alkaline solution includes one or more tertiary alkanolamines, optionally together with further alkalis. Especially suitable tertiary alkanolamines are e.g. (I) N-methyl-diethanolamine, (II) N,N-dimethylethanolamine or (III) 3-(N,N-diethylamino)propane-1,2-diol. The total concentration of components (I), (II) and/or (III) in the alkaline solution is ideally from 0.3 mol/l to 1.5 mol/l.
Said tertiary alkanolamine(s) may be used in combination with a minor amount i.e. not more than 0.2 mol. preferably less than 0.05 mol of an inorganic base per litre, e.g. not more than 2g/l of sodium hydroxide to bring the pH of the processing liquid in the range of 10.5 to 13 without a substantial increase in CO₂-absorption.
For ecological reasons and to avoid a decrease in swelling of the hydrophilic colloid binder of the materials to be processed the present processing liquid is preferably completely free from phosphate ions.
The optimum pH of the processing liquid according to the present invention depends on the type of silver halide emulsion material to be developed, intended development time and processing temperature.
For the DTR-process a silver solvent is indispensable. It may be supplied from the non-light-sensitive image-receiving element, but it is normally at least partly present already in the alkaline processing solution.
The silver solvent, which acts as a complexing agent for silver halide, preferably is a water-soluble thiosulphate or thiocyanate, e.g. sodium, potassium or ammonium thiosulphate or thiocyanate or mixtures thereof. When present in the alkaline processing solution, the molar amount of thiosulphate compound is preferably in the range of 0.03 to 0.13 mol/1.
The developing agent is preferably present in the alkaline solution, but may alternatively or additionally be present in the silver halide emulsion.
The silver halide developing agent used in the process and processing liquid according to the present invention is preferably a p-dihydroybenzene compound, e.g. hydroquinone, methyl hydroquinone or chlorohydroquinone, preferably in combination with an auxiliary developing agent being a 1-phenyl-3-pyrazolidinone-type developing agent and/or p-monomethylaminophenol. When fairly low gradation images for continuous tone reproduction have to be produced preference is given to developing agent combinations as described in US-P 3,985,561 and 4,242,436.
Preferably hydroquinone-type developing agents are present in the processing liquid according to the present invention in an amount of 0.05 to 0.25 mol per litre. 1- Phenyl-3-pyrazolidinone type developing agents may be present in an amount of 1.8 x 10⁻³ to 2.0 x 10⁻² mol per litre. Particularly useful 1-phenyl-3-pyrazolidinone developing agents are 1-phenyl-4-4-dimethyl-3-pyrazolidinone, and 1-phenyl-4-monomethyl-3-pyrazolidinone. The latter type of developing agents is advantageously present in the image receiving element.
When incorporated in the photographic material, the developing agent(s) can be present in the silver halide emulsion layer or are preferably present in a hydrophilic colloid layer in water-permeable relationship therewith, e.g. in the antihalation layer adjacent to the silver halide emulsion layer of the photosensitive element.
The alkaline processing solution preferably also contains (a) silver image toning agent(s) providing a neutral (black) image tone to the DTR-produced silver image in the image-receiving material.
For DTR-processing the aqueous alkaline processing solution according to the present invention may comprise toning agent(s) in a concentration in a range e.g. from 30 mg to 200 mg per litre. Other additives are thickening agents, e.g. hydroxyethylcellulose and carboxmethylcellulose, fog inhibiting agents, e.g. potassium bromide, potassium iodide and a benzotriazole, calcium-sequestering compounds, wetting agents, e.g. block copolymers of ethyleneoxide and propylene oxide, anti-sludge agents, and hardeners including latent hardeners.

EXAMPLES

The invention will now be further illustrated in the following non-limiting examples. In these examples the following polyoxyethylene thioethers were used.

Compound 1:

Molecular weight is approximately 2832 g/mol. "n" is approximately 21.

Compound 2:

Molecular weight is approximately 2411 g/mol. "n" is approximately 21. These compounds were prepared following the methods described in British Patent GB 1455 413

Example 1

Preparation of negative working silver halide emulsion photosensitive materials

A paper support having a weight of 110 g/m² being coated at both sides with a polyethylene layer was coated at one side with an orthochromatically sensitized negative working silver halide emulsion layer containing an amount of silver chlorobromide (7.5 mol % bromide, 1.2 mol % iodide) equivalent to 1.4 g/m² of silver nitrate. The average grain size of the emulsion was 0.15µm with a variance of 0.46. The emulsion layer further contained hydroquinone, 1-phenyl -4-methyl -pyrazolidin-3-on and pyrocathecol as developing agents. Three additional photosensitive materials were prepared as described above with the difference that Compound 1 was added in different amounts as listed in Table 1. Table 1

Material g of compound 1 per 100g of AgNO₃

A 0

B 0.150

C 0.300

D 0.450

Preparation of image-receiving material (A1).

One side of a paper support having a weight of 110 g/m² and being coated at both sides with a polyethylene layer was coated at a dry coverage of 2 g/m² with an image-receiving layer containing silver-nickel sulphide nuclei and gelatin. This layer was applied by slide hopper coating so that the nuclei were in an undermost coating of 1.3 g gelatin per m² and a top layer was provided of 0.7 g of gelatin per m².

Exposure procedure

The photographic materials were exposed through a sensitometric wedge in a contact exposure apparatus operating with a light source having a colour temperature of 3200^oK.

DTR-transfer procedure

The exposed photographic materials were pre-moistened with the hereinafter defined processing liquid 1, the contact time with said liquid being 6 seconds before being pressed together with an image-receiving material as defined above. The transfer processor employed was a COPYPROOF (Registered Trade Name of Agfa-Gevaert N.V.) type CP 380. The transfer contact times were 30, 45 seconds at 16^o and 24^o C processing temperature.

Evaluation

All wedge prints were measured on a densitometer MACBETH (Registered Trade Name) type 1R 924 behind visual filter, having following wavelength (nm)/optical density (D) characteristics : 700nm/D = 0; 600nm/D = 0.2; 500nm/D = 1.25; 420nm/D = 3.0.
For the DTR-prints obtained on paper base image-receiving materials maximum reflection density was measured (D_R), and the gamma value (maximum gradient of the straight line portion of the sensitometric curve). The reflection density measurement proceeded according to American National Standard for Photography (Sensitometry) ANSI P112.17 1985.

Composition of processing solution

INGREDIENT
Hydroxyethylcellulose (g)	1
EDTA (g)	2
Na₂SO₃ (anhydrous) (g)	45
Na₂S₂O₃ (anhydrous) (g)	14
KBr (g)	0.5
1-Phenyl-5-mercapto-tetrazole (g)	0.08
1-(3,4 dichlorophenyl)-1-H-tetrazole-5-thiol (g)	0.04
DMEA (ml)	30
MDEA (ml)	35
Water up to	1 litre
EDTA = ethylenediaminetetraacetic acid tetrasodium salt DMEA = dimethylethanolamine MDEA = methyldiethanolamine

Table II

Processing of photosensitive materials A, B, C and D with image receiving material A₁ at 16^o C in processing solution 1.
Material	D_R			Gradation
	30''	15''	60''	30''	45''	60''
A	0.94	1.18	1.52	0.35	0.67	1.08
B	1.11	1.74	1.60	0.87	1.28	1.49
C	1.03	1.52	1.69	0.84	1.36	1.62
D	0.92	1.59	1.72	0.97	1.43	1.67

Table III

Processing of photosensitive materials A, B, C and D with image receiving material A₁ at 24^o C in processing solution 1.
Material	D_R			Gradation
	30''	15''	60''	30''	45''	60''
A	1.15	1.60	1.85	0.61	0.91	1.15
B	1.49	1.72	1.93	1.26	1.40	1.85
C	1.37	1.75	1.96	1.15	1.57	2.14
D	1.51	1.81	1.97	1.31	1.75	2.67

From Tables I and II can be seen that the coatings B to D according to the invention show faster transfer characteristics than the reference coating A.

Example 2

Preparation of positive working silver halide emulsion photosensitive material

A film support having a thickness of 100µm was coated at one side with an antihalation layer containing carbon black and titanium dioxide dispersed in gelatin. On said antihalation layer an orthochromatically sensitized positive working emulsion layer containing an amount of silverchlorobromide (9.3 mol % bromide) equivalent to 2.65 g/m² of silver nitrate was coated. The average grain size of the emulsion was 0.38 µm with a variance of 0.20. The emulsion was fogged by ripening with a gold compound and a reductor. The obtained photosensitive material was called E. A photosensitive material (F) similar to E but for the addition of 1g/m² of Compound 2 was made.

Preparation of image-receiving material (A₂)

A subbed polyethylene terephthalate film support was coated at both sides at a dry coverage of 1.8 g/m² with an image-receiving layer containing silver-nickel sulphide nuclei dispersed in gelatin. This layer was applied by slide hopper coating so that the nuclei were in an undermost coating of 1.4 g gelatin per sq.m. A top layer was provided of 0.4 g gelatin per m².

Exposure procedure.

DTR-transfer procedure and Evaluation were identical to Example I but for the following differences:

1) The processing liquid I was replaced by processing liquid II having a composition as shown below
2) The transfer contact time were 45, 60, 75 and 90 seconds at 24^oC processing temperature.

Composition of processing liquid II

Ingredient
hydroxyethylcellulose (g)	1.5
EDTA (g)	2.0
Na₂SO₃ (anhydrous) (g)	49
Na₂S₂O₃ (anhydrous) (g)	13
KBr (g)	0.5
hydroquinone (g)	13
1 phenyl-4-methyl-pyrazolidin-3-on (g)	4.7
1-phenyl-5-mercapto-tetrazole (g)	0.08
1-(3,4-dichlorophenyl)-1-H-tetrazole-5-thiol (g)	0.035
DEDHPA (ml )	44.1
NaOH (50%) (ml)	7.47
Water up to	1 litre
DEDHPA = 3-(N,N-diethylamino)propane-1,2-diol

Table IV

Processing of photosensitive materials E and F with image-receiving material A₂ at 24^o C using processing liquid II.
Material	D_TR
	45''	60''	75''	90''
E	2.0	2.7	3.4	3.9
F	2.6	3.4	3.8	4.4

It is clear that the photosensitive material F according to the invention shows better transfer characteristics than the comparative photosensitive material E.

Claims

A silver complex diffusion transfer reversal process in which an information-wise exposed photographic silver halide emulsion layer material is transformed by a silver solvent into soluble silver complex compounds which are allowed to diffuse into an image receiving element and are reduced therein with a developing agent in the presence of an alkaline solution to form a silver image, characterised in that the silver halide emulsion layer includes a polyoxyethylene thioether and the alkaline solution contains a tertiary alkanolamine and is substantially free of primary and secondary alkanolamines.
A process according to claim 1, wherein the silver halide emulsion layer contains silver halide having an average grain size of more than 0.3 microns, preferably from 0.5 to 2.0 microns.
A process according to claim 1, wherein the silver halide present in the silver halide emulsion comprises silver chloride together with at least 5 mole % silver bromide.
A process according to claim 1, wherein the concentration of the polyoxyethylene thioether in the silver halide emulsion layer is from 0.1 to 50g, preferably from 1 to 10g, polyoxyethylene thioether per mole of silver halide present in the emulsion layer.
A process according to Claim 1, wherein the polyoxyethylene thioether has a molecular weight of less than 7500 preferably between 500 and 5000.
A process according to claim 1, wherein the polyoxyethylene thioether comprises recurring units of the following formula:
wherein R represents a substituted or unsubstituted aliphatic, aromatic or heterocyclic group.
A process according to claim 6, wherein the polyoxyethylene thioether comprises a compound of the formula:
where n is from 2 to 50.
A process according to claim 1, wherein said tertiary alkanolamine is (I) N-methyl-diethanolamine, (II) N,N-dimethyl-ethanolamine or (III) 3-(N,N-diethylamino)propane-1,2-diol.