EP0517989A1 - Chemical substance useful as accelerator in a silver dye bleaching process - Google Patents

Abstract

The chemical substance is intended to be added to baths used in the processes for discolouring diazoic elements in the presence of silver during the processing of photosensitive products. It enables these processes to be speeded up. It is derived from pi -deficient heterocyclic compounds having a nucleophilic or hypernucleophilic character.

Description

The main drawbacks of the azomethine dyes constituting the usual color photographic images are their lack of light stability and their undesirable chromatic absorption which alter the colorimetric rendering of the final proofs. Diazo dyes, on the other hand, do not have these defects at such Their light stability is considerable and their immense variety makes it possible to select primary dyes giving rise to a very high chromatic rendering.

The idea of photographic systems based on such compounds dates back to the last century, but it was not until much later that sensitive surfaces involving the use of diazo dyes were put on the market. (1,2,3 ).

• 1) P.Glafkidès.Chimie et Physique Photographiques.P.Montel Ed.
• 2) T.H.James.The Theory of the Photographic Process.Macmillan Publishing Co.Inc. New York.Collier Macmillan Publishers London.
• 3) Photographic Processing Chemistry L.F.A Mason. Focal Press.London.

Their only singularity of treatment is based on the fact that it is possible to destroy a diazo dye proportional to the amount of metallic silver that is oxidized in its immediate vicinity. (1,2 and 3 give many references to this respect.)

• 1) P.Glafkidès.Chemie et Physique Photographiques.P.Montel Ed.
• 2) THJames.The Theory of the Photographic Process.Macmillan Publishing Co. Inc. New York. Macmillan Publishers London Necklace.
• 3) Photographic Processing Chemistry LFA Mason. Focal Press.London.

A photosensitive product based on the use of these diazo dyes consists of the superposition of three sensitive layers on the three primaries, each containing a diazo dye, uniformly distributed, and of color complementary to the chromatic sensitivity of the layer considered.

• 1) Développement par un révélateur classique d'une image argentique,donc négative, dans chacune des couches sensibles après exposition à la lumière.
• 2) Blanchiment de l'argent métallique ainsi créé en même temps que l'on détruit une quantité proportionnelle de colorant se trouvant au voisinage du métal.
• 3) L'image colorée obtenue lors de la phase précédente est donc positive et il ne reste plus qu'à éliminer par fixage les sels d'argent résiduels,layer et sécher pour obtenir une épreuve photographique finale.De ces trois étapes de traitement il est évident que celle du blanchiment,sur laquelle repose en fait tout le procédé, est la plus délicate.

The principle of treatment of such a photographic product is as follows:

• 1) Development by a conventional developer of a silver image, therefore negative, in each of the sensitive layers after exposure to light.
• 2) Whitening of the metallic silver thus created at the same time as a proportional quantity of dye is found in the vicinity of the metal.
• 3) The colored image obtained during the previous phase is therefore positive and all that remains is to remove by fixing the residual silver salts, layer and dry to obtain a final photographic print. It is obvious that that of bleaching, on which the whole process is based, is the most delicate.

Chemistry of bleaching silver and dyes:

The overall (well known) reaction of this system is:

The concentration of H⁺ ions is therefore an essential element of this reaction.

The conversion silver / silver ion being reversible it is necessary to complex the silver ions by a powerful ligand like the iodide ion. However the overall reaction is slow and it is possible to considerably increase its kinetics by the addition of a catalyst.

The best catalysts are heterocycles belonging to pyrazine derivatives of quinoxalines or phenazines.

As it is easy to develop a photosensitive silver surface and also to fix it at high speed, it is clear that the limiting phase of the process is the acceleration of the destruction of the dyes because the rapid whitening of the silver is very easy. The present invention therefore relates to this last point as will be explained below.

Test Protocol:

• A) Light exposure: It was carried out using a flash battery identical to that found on automatic shooting booths, while rigorously retaining the same energy conditions as the flashes of lightning during the tests.
• B) Processing: This, like the exposure, was carried out using a rotary machine or using the latest equipment consisting of a roller machine capable of performing processing at very high speed.

Reagents used:

C) Developer (ready-to-use solution): Distilled water 700 cc Anhydrous sodium sulfite 60g Borax decahydrate 20g Potassium bromide 1.8g Ascorbic acid 10 g Phenidon Z 3 g Hydroquinone 15g Benzotriazole 0.8 g Photo quality optical brightener 3 g Distilled water qs 1litre pH 9.80 to 0.05 near at 20 ° C. B) Silver and dye bleaching bath: Distilled water 800 cc Pure sulfuric acid 40 cc Sodium salt of metanitrobenzenesulfonic acid 8 g Potassium iodide 10 g Thiol (Mercaptosuccinic acid) 3 g Catalyst (2,3,6 trimethylquinoxaline) 2 g Distilled water qs 1 liter pH below 0.30 C) Fixing bath: Ammonium thiosulfate 125g Potassium metabisulfite 15g Potash detergent to adjust the pH Distilled water qs 1 liter pH at 20 ° C 6.90 to 0.10 near.

For each photographic test, the exposure conditions were kept constant with regard to the power of the flashes of flash and the treatment solutions were changed as well as the rinsing waters.

In the rotary machine used to conduct the tests, the usual sequence of treatment solutions is two tanks of developer, three tanks of bleaching, one tank of rinsing water, three tanks of fixative then tanks of water to the output from the paper. The processing temperature is 35 ° C and the total machine cycle time is approximately four and a half minutes.

• a) un blanc,la porte de la cabine de prise de vue étant ouverte durant toute la la durée du traitement.
• b) un fond de cabine,cette dernière étant vide,porte fermée.
• c) prise de vue d'un modéle,toujours la même personne.
• d) photographie d'une gamme chromatique.

The following tests were carried out for each test:

• a) a blank, the door of the shooting booth being open throughout the duration of the treatment.
• b) a cabin bottom, the latter being empty, door closed.
• c) shooting of a model, always the same person.
• d) photograph of a chromatic scale.

The following test conditions were chosen:
temperature 33 ° C.
sequence: two tanks of developer, two tanks of bleaching, one tank of rinsing water, three tanks of fixer then water until the exit of the strip. Same duration of the cycle of the machine. In the case of tests at high speed (two minutes and forty five seconds) the temperature was raised to 35 ° C, all other things being equal. Under these conditions, using the usual treatment solutions, the white is gray due to the lack of whitening of the dyes, the cabin background has an appearance. gray with a strong dominant cyan, the character is dark on a cyan background as well as the chromatic range which presents a deep imbalance.

The goal to be achieved is therefore in these conditions to find a substance, which, added to the bleaching bath, will allow the normal appearance of the tests to be restored.

Many authors have studied the probable mechanism of the action of the catalyst with regard to the bleaching of silver and that of the dyes simultaneously.

• A) Le catalyseur en solution très acide a un atome d'azote quaternisé qui à ce titre sert d'intermédiaire d'oxydo-réduction en transportant un proton sur un atome qui passe ainsi à l'état d'ion argent.Cette réaction est réversible d'où la nécessité d'avoir un complexant puissant de l'ion argent en milieu très acide.
• B) A la suite de cette première réaction le catalyseur se trouve sous forme d'un radical chargé positivement qui va se dismuter pour donner un agent réducteur actif et le catalyseur de départ.

It seems very likely that we have the following series of reactions:

• A) The catalyst in very acid solution has a quaternized nitrogen atom which as such acts as an oxidation-reduction intermediary by transporting a proton on an atom which thus passes to the state of silver ion. reversible hence the need for a powerful complexing agent of the silver ion in a very acid medium.
• B) Following this first reaction, the catalyst is in the form of a positively charged radical which will disproportionate to give an active reducing agent and the starting catalyst.

seconde étape

All of these reactions are shown below:
first stage

second step

Considering that this is the real mechanism of money laundering and dyes it seemed to us that there was no possibility of acting on the first reaction because in the presence of a powerful ligand of the silver ion like the iodine ion this can only be done in one direction and relatively quickly.

One can thus act, for a given catalyst, only on the second reaction either by increasing the quantity of the catalyst, which does not give good results besides, or by introducing another body likely to shift the balance of the second reaction from left to right.

A first idea would therefore be to seek to eliminate the H⁺ ions produced by the second reaction using a strong organic base such as for example:
1,5 Diazabicyclo [4,3,0] non-5- ene (DBU) or even 1,8 Diazabicyclo [5,4,0] undec-7-ene. The tests carried out with these products as elsewhere with other strong organic bases have been found to be completely negative.

It is therefore clear that the only remaining possibility is to introduce a Lewis base, a nucleophilic or if possible hypernucleophilic molecule which makes it possible to "transport" the proton of the second reaction from the left to the right so as to transform the derivative charged with the catalyst. active dihydrogen derivative and starting catalyst thus ensuring a rapid return of the bleaching system to the initial state.

• 4) F.A.Cotton.Chemical Applications of Group Theory;john Willey & Sons,Inc,New York
• 5) D.S.Schonland.Molecular Symmetry.Ed française Gauthier-Villars Paris.
• 6) Jerry March Advanced Organic Chemistry.John Wiley & Sons New York.
• 7) Newkome & Paudler Contemporary Heterocyclic Chemistry.John Wiley & Sons New York.

We can also think that such a body must belong to a point group of relatively high symmetry to have a favorable distribution of its molecular orbitals. (4,5,6 and 7)

• 4) FACotton.Chemical Applications of Group Theory; john Willey & Sons, Inc, New York
• 5) DSSchonland.Molecular Symmetry. French Ed Gauthier-Villars Paris.
• 6) Jerry March Advanced Organic Chemistry. John Wiley & Sons New York.
• 7) Newkome & Paudler Contemporary Heterocyclic Chemistry. John Wiley & Sons New York.

Under the given conditions, i.e. in a very acid medium, the electron donor groups can only be alkyl groups acting on a starting structure already favorable and particularly stable in such a medium. It therefore seemed to us that only the π-deficient heterocycles constituted from this point of view the best starting structures. We will call in what follows "accelerators" the bodies which in the presence of the catalyst give a very notable acceleration of the kinetics of bleaching of the dyes and bring back the bands above mentioned in their normal appearance.

The first compound of interest is therefore pyridine, the action of which has already been mentioned in numerous publications but the accelerating action of which is very insufficient. We must therefore seek an improvement of this potential accelerator by introducing electron donor groups which do not may be that alkylated substituents, such as the methyl radical, ortho or para-toluidine do not give good results (meta-toluidine is not suitable for reasons of molecular orbitals and this a priori).

The introduction of two methyl radicals in ortho of pyridine as in 2-6 Lutidine then gives a sensitive accelerator but not yet perfect; the other lutidines, whose point group of molecular symmetry is therefore lower, do not lead, as expected, no interesting body.

After pyridine, we can also look for other derivatives such as meta or para-aminopyridine which have no donor groups and whose action is zero, but alkylated derivatives such as ortho or especially paradimethylaminopyridine, hypernucleophilic body. conversely provides an already extremely powerful accelerator to the dose of 5.10⁻³ mole per liter while orthodimethylaminopyridine undoubtedly has an accelerating effect but less marked than in the case of the para derivative, probably because of its lower degree of symmetry. We can also look for homologs as in the quinoline or acridine group, however, without appreciable gain.

The second series of compounds to be studied are derived from pyridazine, pyrimidine and pyrazine and related; two pyridazine substances can be immediately excluded for which it is known that the bond between the two nitrogen atoms is mainly simple (8) and pyrimidine for which, from the start, the electronic distribution seems unfavorable (8). We know that in pyrazine the interactions 1-2 n⁺σ, n⁻σ * are strong and lead to a significant separation of the levels n⁺ and n⁻ (9).

• 8)A.R.Katritzky & C.W.Rees Comprehensive Heterocyclic Chemistry Vol.3 Part.2B.
• 9) T.A.Albright,J.K.Burdett,M.H.Whangbo Orbital Interactions in Chemistry .John Wiley

We are therefore led to study the pyrazine derivatives likely to be powerful accelerators. Methyl derivatives of low symmetry do not give very active molecules, again because of their lack of symmetry, against tetramethylpyrazine is also a powerful accelerator at low doses of the order of 5.10⁻³ mole per liter. The whites obtained are very pure and the chromatic balance completely normal in the tests as described above. This corresponds to the fact that the molecule has a high symmetry (point molecular group D2h).

• 8) ARKatritzky & CWRees Comprehensive Heterocyclic Chemistry Vol.3 Part.2B.
• 9) TAAlbright, JKBurdett, MHWhangbo Orbital Interactions in Chemistry. John Wiley

A simple preparation of tetramethylpyrazine from diacetyl was developed on this occasion.

Preparation of tetamethylpyrazine:

Wait one hour, then pour solution A into solution B and bring the whole, with stirring, to reflux for three hours. After cooling, bring to ten liters with distilled water and correct the pH if necessary. sulfuric acid diluted or with a decimolar solution of sodium hydroxide so as to obtain a value of approximately 5. We filter, wash, air dry. The body obtained is dihydrotetramethylpyrazine which oxidizes in air to give the sought product, tetramethylpyrazine with a yield greater than 75% of theory compared to the starting diacetyl.

The simultaneous use of two accelerators gives good results but does not however bring any particular advantage in other words there is no synergistic effect.

The other nucleophilic compounds that we have tried have not given interesting results essentially because of their instability in very acidic aqueous medium while others have only a theoretical interest because they are too expensive or too difficult to manufacture.

It is good to note the important role that symmetry plays in the molecules we mentioned. Example C2v structure of paradimethylaminopyridine which is an accelerator of choice while the counterpart in ortho, less symmetrical is also much less active.

Claims (4)

Chemical substance intended to be added to the baths used in the discoloration processes of the diazo elements in the presence of silver during the treatment of photosensitive products, characterized in that it is derived from π-deficient heterocycles of nucleophilic or hypernucleophilic nature and allows an acceleration of these processes. Substance according to claim 1, characterized in that it has a high degree of symmetry. Substance according to claim 1, characterized in that it has a high stability in aqueous and acid medium. Substance according to claim 2, characterized in that its symmetry is of the D2h or C2v type.