Colour Films and Paper Developers
The most important colour developers, derivatives of which are used in both colour negative film and colour paper, are p-phenylenediamines.
Eastman Kodak Co. introduced both the film and paper developer formulations, which are now the industry-standards. They are known as the C41 process for colour negative film, and the RA-4 for colour paper.
The dyes needed for colour film and colour paper are different because the application is different. In colour film the processed negatives are often stored in paper or cardboard wallets, where glue from the wallets can react with the dyes. Humidity during storage may also be an issue. They are also designed for use with transmitted light.
While there are chemicals that could be potential developers, p-phe-nylenediamines have been proven to be the most useful for producing dyes. These developer molecules must donate an electron to the silver ion during the process of becoming oxidised. An alkyl group ortho to the coupling amino group has been found to facilitate this electron donation, thereby increasing the developer activity. Most of these p-phenyl-enediamine compounds cause dermatitis, the sensitivity to which increases with prolonged exposure. These materials are therefore treated with caution, particularly during the manufacturing process.
The synthesis of many colour p-phenylenediamine developers is described by Bent et al.2 in their 1951 paper. Of the 54 developers or so that are described two of them are used in products. They are known as CD4, used as the industry-standard for processing colour negative film, and CD3, used as the industry-standard for processing colour paper. CD4 was first described a patent from a German company now known as Agfa-Gavaert.3'4 The synthesis of CD4, as described by Bent et al., is given in Figure 5.
The colour paper developer, CD3, is also used in motion picture film and was patented first by Weissberger.5 Bent et al. report that the synthesis of this compound is through the nitroso intermediate outlined in Figure 5. In his patent, Weissberger et al. claimed that substitution into the aryl ring afforded dyes (on oxidation and subsequent reaction with couplers) whose hue could be changed. They commented:5
... these compounds may be substituted in the aromatic ring with other groups including ... they have a tendency to alter the colour of the final dye image and the colour may be controlled in this way ...
The resultant dye hue, or colour, is but one of a number of tests that are listed in the paper published by Bent et al. They carried out tests on each of the 54 or so p-phenylenediamine developers that they report, namely
• half wave potential
• development rates
in aniline at 130-135 deg C
Zn dust
C2H^ /CH2CH2OH
c HCl sodium nitrite
C2H5 CH2CH2OH
CH3 NO .HCl
Figure 5 The synthesis of CD4
coupling efficiencies biological assays.
The half wave potential provides a measure of the free energy change of the oxidation potential of the developer. A higher free energy for a given compound is indicative of the tendency of the developer to release an electron, which is a mandatory process for a developer. Using the Lewis-Randall convention,6 a good developer will exhibit a more positive half wave potential compared with a poor developer. Figure 6 compares CD3 and CD4 with the N,N-diethyl parent developer.
These results suggest that there is no difference between the three compounds in terms of their ability to release electrons.
Development rates for the three compounds were measured in coatings as the time taken to attain an optical density of 0.2 above fog (the density produced from an unexposed emulsion), using a light source with an exposure of logE 1.75. The measurements (as 1/time, units of min-1) for the three developers listed in the order shown in Figure 6 were 0.80, 0.38 and 0.67, respectively, suggesting that the N,N-diethyl developer is more efficient in terms of development rates.
Coupling efficiencies were measured using a standard coupler, in this case 2-cyanoacetyl-coumarone. The technique used was to evaluate the eluted dye after chromatographic separation of the mixture produced upon reaction between the coupler and oxidised colour developer. All three of the developers listed in Figure 6 showed comparable coupling efficiencies.
The above evidence would suggest that all three developers could be used for any application, provided that the dye hue and dye stability were acceptable for the given application. The determining factor was the biological assay tests. The basic test was to evaluate the developers for skin sensitisation in guinea pigs. CD3 and CD4 proved to be less dermatitic than did the parent compound listed in Figure 6. This test is of paramount importance because the developer solutions are handled in areas where air extraction may not be possible, and potentially in large vats for the larger photofinishing companies.
Earlier on in this chapter the development rate of CD3 was reported as being considerably lower than either CD4 or the parent N,N0-diethyl
Halfwavepotential[2]
C2H5 /C2H5
C2H5 /C2H5
C2H5 /CH2CH2NHSO2CH3 N
-190
-190
C2H5 CH2CH2OH
Figure 6 A comparison of colour developers p-phenylenediamine. The developer solution made using CD3 for colour paper applications has been enhanced by the addition of benzyl alcohol - see Chapter 5 for further details. Indeed developers are used in conjunction with other chemicals to form a developer solution, which is only one step in the development of a colour negative film or paper. Figure 7 outlines the use of the various solutions in some of the commercial processing kits that are available.
Figure 8 shows the effect on the film components when the film is processed through the various solutions. The chemical composition of the various solutions is covered in Chapter 5.
In broad outline, a latent image is formed on exposure to light. During colour development the silver halide crystals that have been exposed to light are converted to silver, at the same time that the image dye is formed from the oxidised colour developer and the relevant coupler. The bleach step converts the silver back to silver ions, and the fix step removes the silver ions. Unreacted coupler molecules that remain in the film are harmless as they are not coloured.
Figure 8 is not intended to be to scale, nor does the diagram contain all of the layers that would be found in any colour product, for example
Colour Negative |
Colour Prints from |
Colour |
Black-and- |
Black-and- |
Film |
Colour Negatives |
Reversal/ Chrome Film |
White Negatives |
White Prints |
process C-41 |
process RA-4 |
process E-6 |
— |
— |
developer |
developer |
first developer |
developer |
developer |
bleach |
bleach-fix |
wash |
stop |
stop |
wash |
stabiliser or wash |
reversal bath |
fix |
fix |
fixer |
dry |
colour developer |
rinse |
wash |
wash |
pre-bleach |
washing aid |
fibre: | |
stabiliser or final |
wash |
developer | ||
rinse |
bleach |
drying aid |
stop | |
dry |
fixer wash final rinse wash dry |
fix rinse washing aid wash |
Figure 7 Processing steps for several product families
Figure 7 Processing steps for several product families
"Q
"af~ a ca o Ad o ao a o a ^ û_o o a oa o oa a
A Latent image
A Latent image
Bleach Fix yellow dye cloud © magenta dye cloud © cyan dye
Figure 8 Chemical changes during the development steps the yellow filter layer has been omitted. Chapters 9-12 cover film and paper design in much more detail.
The chemical formulations of some of the commercially available processing solutions are covered in Chapter 5.
References
1. L.F.A. Mason, Photographic Processing Chemistry, Focal Press Ltd., London, 1975, ISBN 0-240-50824-6.
2. R.L. Bent, J.C. Dessloch, F.C. Duennebier, D.W. Fassett, D.B. Glass, T.H. James, D.B. Julian, W.R. Ruby, J.M. Snell, J.H. Sterner, J.R. Thirtle, P.W. Vittum and A. Weissberger, J. Amer. Chem. Soc., 1951, 73, 3100.
3. US 2,108,243 Agfa-Gavaert, B. Wendt.
5. US 2,193,015, Eastman Kodak Co., A. Weissberger.
6. G.N. Lewis and M. Randall, Thermodynamics, McGraw Hill Inc., New York, 1923.
CHAPTER 5
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