In this work, attention is focussed on the mechanisms of dye degradation in the dark. This is the most probable degradation mechanism because the motion picture films stored in the archive are seldom exposed to light. Information for this review was gathered from Tuite (1979), and the review articles of Bergthaller (2002c) and Theys and Sosnovsky (1997).
The 1979 Tuite review article is, incredibly, the last published review article fully devoted to photographic dye degradation.
There are a number of reactions occurring in the dark that lead to dye fading. These involve either the dyes themselves or the residual couplers. Indeed, in the chromogenic Investigations into the Degradation of Photographic Materials 181
Fig. 12. Yellow couplers. a-Pivaloyacetanilide type (top) and benzoylacetanilide type (bottom). R and R1are various organic moieties, and ballast can be various long-chain aliphatic groups (from Theys and Sosnoysky, 1997). Reprinted with permission from:
Theys, R.D., Sosnovsky, G., 1997. Chemistry and color photography. Chem. Rev.97, 83–132. © 1997 American Chemical Society.
Fig. 13. Two couplers of the pyrazolinone class: 3-arylamino substituted (left) and 3-acylamino substituted (right). R, R1, and R2are various organic moieties (from Theys and Sosnoysky, 1997). Reprinted with permission from: Theys, R.D., Sosnovsky, G., 1997.
Chemistry and color photography. Chem. Rev.97, 83–132. © 1997 American Chemical Society.
Fig. 14. Magenta-forming pyrazolotriazole couplers (from Theys and Sosnoysky, 1997).
Reprinted with permission from: Theys, R.D., Sosnovsky, G., 1997. Chemistry and color photography. Chem. Rev.97, 83–132. © 1997 American Chemical Society.
photographic processes, the colour couplers are ballasted in the emulsion and are not washed away after processing the film. This means that the unused couplers will stay in the emulsion and can undergo reactions leading to colour products. This is the reason why chromogenic processes (processes in which the colour couplers are not incorporated into the emulsion but are precipitated in the three layers during the processing steps, for example, Kodakchrome) have traditionally a high dark stability.
An important reaction related to magenta dye instability is the reaction involving the residual magenta coupler shown in Fig. 16 and resulting in the yellowing of the non- image areas. This reaction leads to a yellow-coloured compound, either in the dark or in the presence of light. The product is methynylbis coupler in the dark and the azo-dye in the light. This problem was solved with the substitution of two of the ortho positions of the 1-phenyl ring (Tuite, 1979).
Another important reaction occurs between the residual magenta coupler (3-acylamino pyrazolone couplers) and the magenta dye to form a variety of colourless products. Using aldehydes in the final processing bath solved this problem. Indeed, the cross-linked couplers do not react with the magenta dye. Later, magenta couplers (anilino pyrazolone) were devised, which showed much lower tendency to react with their magenta dyes and were used in paper prints (Tuite, 1979). Nevertheless, the 3-acylamino pyrazolone couplers were the most common couplers used in camera-use films such as colour negatives, colour reversal films, and colour films for motion pictures at least up to 1988 (Sakanoue and Furutachi, 1988) because of their image quality. Sakanoue and Furutachi (1988) found that the degree of dye decomposition relates to the pKa value of the coupler and that the use of low-pKa couplers could eliminate the necessity of formaldehyde from the processing solu- tion. They proposed a fading mechanism, described in Fig. 17.
A third important class of reactions involving magenta dyes is the reaction with resid- ual thiosulphate (from the fixing bath) (Miyagawa and Shirai, 1985; Kurosaki et al., 1988). In these articles, the authors report the reductive fading pathway, the structures of the dye, and the colourless adduct.
Investigations into the Degradation of Photographic Materials 183
Fig. 15. Cyan-forming phenol (right) and naphthol (left)-type couplers (from Theys and Sosnoysky, 1997). Reprinted with permission from: Theys, R.D., Sosnovsky, G., 1997.
Chemistry and color photography. Chem. Rev.97, 83–132. © 1997 American Chemical Society.
The result of the innovations on magenta couplers is that none of the modern (post- 1979) Kodak colour photographic materials that use traditional processing are magenta dye limited for dark stability. They can be limiting for light stability. Adding an UV absorbing layer in the prints in front of the magenta layer circumvents this problem.
The yellow dyeis the least stable dye in the dark because of hydrolytic reactions occur- ring both in acidic and alkaline environments. The acidic hydrolytic attack occurs at the azomethine linkage, while alkaline hydrolysis occurs at the keto linkage (Tuite, 1979). The final processing pH of a film is chosen to minimise these two reactions. The reaction shown in Fig. 18 is considered the most important dark fading reaction occurring when a film suffers from vinegar syndrome.
Indeed, yellow fading was monitored for years at Kodak as an indication of the onset of the degradation of the cellulose triacetate base (vinegar syndrome). Tuite (1979) reports that the dark fading characteristic of the yellow dye for 10% dye loss ranges from 3 years for Kodak Ektachrome 40 Movie Film (type A, EM-25 process) to 32 years for Kodak Fig. 16. Reactions involving residual magenta coupler (from Tuite, 1979). Reprinted with permission of IS & T: The Society for Imaging Science and Technology sole © owners of the Journal of Applied Photographic Engineering.
Ektachrome 50 Professional Film 6118 (Daylight, E-8 process). As far as light stability is concerned, the largest single improvement came from the simple change from a benzoyl group to a pivalyl group on the yellow coupler (Tuite, 1979).
The cyan dyeis the dye most stable to light. The principal undesired dark reaction is the reduction of the residual cyan coupler to the colourless leuco form of the dye (Fig. 19). Other reducing agents that have not been removed completely can also cause this or similar reac- tions: e.g. retained thiosulphate from the fix bath, ferrous ion from a poorly regenerated bleach fix bath, and ballasted hydroquinones that are used as incorporated interlayer scavengers.
This problem was solved by a change in the colour-developing agent from CD-2 to CD-3 and by a change in the dye structure, namely the use of phenol-type couplers with amide group in the 2.5 position of the ring (Tuite, 1979; Theys and Sosnovsky, 1997).
The masking dyes that gives the orange colour to negatives is very stable in the dark (Bergthaller, 2002c).
Photochemical degradation involves absorption of light by a dye to generate both singlet- and triplet-excited states. Aerial oxidation of the excited molecules forms undesir- able by-products. Stabilisation of the dye involves intercepting both the actinic radiation with a UV absorber and one or more of the reactive intermediate with a quencher such as nickel dibutyldithiocarbamate. Bergthaller (2002c) proposes a degradation trail common for both dark fading and photo fading. He assumes that under appropriate conditions the detachment of the developer fragment from a dye cloud could be started more readily from a sequence of single electron transfer and proton transfer to the azomethine bond. This idea is based on the fact that, in many cases, increased resistance of dyes to dark stability has resulted from efforts directed towards improved light stability.
Investigations into the Degradation of Photographic Materials 185
Fig. 17. Proposed mechanism for the reaction between magenta 3-acylamino pyrazolone coupler and magenta (from Sakanoue and Furutachi, 1988)
The atmospheric constituents, which most often cause colorants to fade, are oxides of sulphur and nitrogen, and ozone. Atmospheric oxygen can also be a signifi- cant contributor to the fading of certain types of yellow and magenta dyes. In contrast, it was found that oxygen could inhibit the fading of cyan dyes (Theys and Sosnovsky, 1997).
Fig. 18. Yellow dye hydrolysis under acid (top) and alkaline (bottom) conditions (from Tuite, 1979).