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Specific Components of Silver Dye-Bleach Materials - Dyes, Other Basic Components, Development, Bleaching of Dyes and Excessive Silver, Components of the Bleach Bath

layer agent yellow azo

Dyes

Image dyes are very specific components of silver dye-bleach materials. In early experiments, several classes of dyes were investigated but in practice only azo dyes have been used.


Image dyes have to fulfill a number of fundamental requirements to work:

Optimal spectral absorption in the coated layer and the final image.

Irreversible bleaching to colorless reaction products on processing.

Good water solubility of the dye for coating and of the bleach products, which have to be washed out at the end of processing for a stable final picture.

No diffusion into neighboring layers, neither during coating nor processing.

No influence on the viscosity of coating solutions.

No influence on the sensitization of the silver halide grains.

Reproducible association of the dye molecules in the gelatin matrix.

The important requirement of water solubility and diffusion fastness becomes possible mainly by controlled association of the azo molecules, but a delicate problem exists. The association of dye molecules is often coupled with a spectral shift and change in light stability. Every change in the manufacturing procedure or a batch change of coating components can influence the spectral behavior of the dyes.

Non-conjugated bis-azo components are used in Ilfo-chrome as yellow and magenta dyes; conjugated compounds as cyan dyes.

Other Basic Components

Basic components such as gelatin, wetting agents, and silver halide emulsions, which are sensitized to the appropriate spectral range, are comparable to chromogenic color materials.

Development

Normal black and white developers of the super additive type on the base of hydroquinone-phenidone are used for silver dye-bleach materials. The goal is a low contrast silver image, since the transformation into the dye image in general leads to a contrast increase. Special additives to silver dye-bleach developers are mentioned in the section on Additives.

Bleaching of Dyes and Excessive Silver

The most important step in silver dye-bleach processing is the transformation of the initial silver image into a dye image. The relatively noble silver metal has to act as a reducing agent. The bleach bath contains iodide to lower the redox potential of silver and a strong acid. The overall equation for the bleach reaction is

R-N = N-R’ + 4Ag + 4H + 4 I - ? R-NH 2 + H 2 N-R’ + 4AgI

R = substituted aromatic rest of the dye molecule

Whereby the reaction takes place in two steps, leading primarily to a hydrazo compound followed by the cleavage to the amines as a final product (see below). The bleach bath has to be strongly acidic, as the reaction consumes protons and the bleach products are aromatic amines that have to be protonated for dissolution. The pH of practical bleach solutions is 0-1. It is established by sulfuric acid.

Silver and dyes are fixed in the gelatin layer; a direct exchange of electrons for the bleach reaction is excluded. Therefore, a redox catalyst is needed for the transport of electrons from silver to the azo compound. 1,4-diazines, mostly quinoxalines, are used for this purpose. In acid solution they are reduced reversibly to radical cations and 1,4-dihydroqui-noxalines in fast reactions.

The 1,4-dihydroquinoxaline reacts with an azo group in a first step to the corresponding hydrazo compound and in a second step under formation of two amines. On the other side, the 1,4-dihydroquinoxaline is unstable and rearranges in a reversible (relatively slow) reaction (half-life 1 to several seconds) to the more stable tautomeric 1,2-dihydroquinoxa-line, which does not react directly with azo compounds.

The quinoxaline is reduced in the bleach bath at the silver surface and diffuses a radical or dihydroquinoxaline to the azo dye, which in turn is reduced to amines under regeneration of the quinoxaline. The relative redox potentials of the bleach components are represented in Figure 42.

The standard potential of silver is reduced from 800 mV/NHE to -150mV/NHE by addition of 0.1M KI. Silver metal is now a strong reducing agent able to reduce the bleach catalyst. Azo compounds can be reduced to the corresponding amines in irreversible reactions at around 400mV/NHE. During dye bleaching a spherical hole without dye is formed around every silver grain.

Simultaneously silver is transformed into silver iodide. The growth of the bleach holes becomes slower with increasing bleach time, because more reducing agent is needed with increasing radius of the hole.

The range of the active reducing agent 1,4-dihydroquinoxa-line is limited by the tautomerization reaction to the inactive 1,2-dihydroquinoxaline. An aromatic nitro-compound is added to the bleach bath as a mild oxidizing agent. This agent also limits the reach of the dihydroquinoxaline and is used to oxidize any excess of silver. The electron transfer from silver to the nitro compound is also catalyzed by quinoxaline. The addition of an oxidizing agent to the bleach bath makes a separate silver bleach bath superfluous (see below).

During dye bleaching all silver is transformed into very insoluble silver iodide.

Components of the Bleach Bath

A strong acid, normally sulfuric acid 0.1 to 0.2 M with a pH of 0 to 1.

Potassium iodide as a strong silver ligand.

An aromatic nitro compound as a mild oxidizing agent to bleach any excess of silver metal, normally m-nitrobenzene-sulfonic acid is used.

A quinoxaline as bleach catalyst.

A bleach accelerator.

A reducing agent to prevent the oxidation of iodide to iodine. Ascorbic acid has been used for this purpose. The normally used bleach accelerator (see below) has reducing properties and makes the addition of another reducing agent unnecessary.

Fixing

The last step of silver dye-bleach processing is the fixing or dissolution of all silver compounds. This processing step is slower and less efficient than fixing in conventional color photographic systems. The reason is the low solubility of silver iodide and the continuously increasing concentration of iodide during the utilization of the fixing bath. High concentrations of ammonium thiosulfate are used and certain organic solvents help increase the fixing speed.

Figure 45 shows a microscopic cross-section through a silver dye-bleach with copy material developed for graphic applications and fast processing. Gelatin interlayers separate the dye layers containing the appropriately sensitized silver halide crystals. These empty layers are needed to suppress unwanted long distance bleaching in neighboring dye layers (bleach coupling). The interlayer between yellow and magenta contains a yellow filter to eliminate the blue light that could expose the silver halide in the magenta and cyan layer (exposure coupling). The yellow filter layer contains some yellow dye and a small amount of colloidal silver; just enough to bleach the yellow dye completely in the bleach bath. A transparent gelatin layer is coated on top to physically protect the photographic layers. Figure 45 shows the material in a wet and therefore swollen state after development and before bleaching.

High quality silver dye-bleach print materials used for slide copies are coated with supplemental emulsion layers on top of every dye layer to increase the practical speed and to reduce the contrast. Distant bleaching down to the lower neighboring dye layer is desirable. Distant bleaching to the upper dye layer over the empty interlayer (so-called bleach coupling) is impossible. This kind of silver dye-bleach material contains higher concentrations of dyes and therefore has no need for a supplemental yellow filter.

Figure 46 shows the situation after a white exposure and development. From the top one can see an empty gelatin protection layer, a blue-sensitive emulsion layer, a yellow layer containing blue-sensitive silver halide emulsion, an intermediate masking layer containing some colloidal silver (not visible), a green-sensitive emulsion layer, a magenta layer containing green-sensitive silver halide emulsion, an empty intermediate layer, a red-sensitive emulsion layer, and a cyan layer containing red-sensitive silver halide emulsion below a gelatin undercoat and the polyester base.

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