Preservation & Technical Research
The Effect of Decomposing Cellulose Triacetate Motion Picture Film Base on the Gelatin Emulsion and Silver Image
It has been observered that in extreme cases of cellulose triacetate (CTA) film base decomposition, known as "Vinegar Syndrome", the image structure may deteriorate to form a "noisy" image.
 Original image |
 Representation of deterioration |
However, it is possible that these effects may start to occur at earlier stages of decomposition. While this may not be readily visible when examining a film over a bench, even a minor change in the images edge structure can have a significant effect on the reproduction of sound.
Anecdotal evidence gathered during the "Operation Newsreel" examination of Australian newsreels indicated that sound negatives were more likely to decompose than other film components.
In conjunction with the State Library of Queensland we are examining the conditions, due to base decomposition, that lead to changes in the structure of the silver image within the emulsion. In particular this project is looking for the physical condition of the emulsion at the point where changes in the silver image affect the modulation transfer function of the film.
Gelatin
The stable structure of gelatin, as it is found in a photographic emulsion under "normal" conditions, is that of long tightly coiled chains. In this state the gelatin is at it's iso electric point (IEP) and has a pH of approximately 4.8. At the IEP the coils have an equal balance of amino groups and carboxylic groups. This provides a net neutral charge along the coils.
As the pH moves away from the IEP towards a more acidic condition, such as is found in decomposing CTA, the amino groups become predominant to give a net positive charge (+), if the move is towards a more basic or alkali condition then the carboxylic groups become the predominant species and a net negative charge (-) results (1, 2).
from S.E. Sheppard, R.C. Houck & C. Dittmar,
J. Phys. Chem., 46 158 (1942)
As the charge on the coils changes to either a net positive or net negative condition, the coils will repel each other and uncoil slightly. This causes the gelatin to increase in solubility and significantly swell.
Hypothesis
We are examining the hypothesis that the change in image structure is due to reticulation of the gelatin caused by unrestrained swelling of the emulsion. This swelling is brought about by a lowering of the pH due to decomposition acids.
Reticulation of Gelatin Silver Halide Emulsion (3).
Local reticulation of the emulsion surrounding a plasticiser blister
Reticulation is attributed to excessive swelling of the gelatin emulsion (1, 3). As the emulsion swells the stresses built up are relieved by a "folding" of the gelatin. This has been shown to cause a change in the silver image as the silver moves towards areas of lower tension within the gelatin (3).
As part of the hypothesis it is felt that the most likely acid causing this unrestrained swelling is diphenyl phosphate. A strong acid produced by the hydrolysis of triphenyl phosphate, the plasticiser used in CTA film (4).
In the example shown on the left, a localised change in the image structure, consistent with reticulation, surrounds a ruptured blister of plasticiser. Since there is no indication of reticulation in other parts of the image it seems most likely that it is the hydrolysed plasticiser that is causing the effect.
Further Work
100' samples of CTA films will have 8-12KHz test tone sound tracks printed and the test tones recorded from each sample. These recordings will be kept as the reference standard. Polyester based control samples will also be produced.
The samples will be aged under accelerated conditions to the onset of "Vinegar Syndrome". At this point some test samples will be further aged under drier conditions while the rest will continue aging under the higher relative humidities. At regular intervals the samples will be examined for signs of image decay by:
- replaying the track and noting any changes in the signal
- microdensitometry.
To determine whether it is the acetic acid or diphenyl phosphate that is causing the swelling, resolution test frames will have the emulsion removed and then the emulsion will be readhered onto a glass support. Acetic acid or diphenyl phosphate solutions will be applied to each frame. The degree of swelling and any changes in the image structure will be observed.
References
- The Theory of the Photographic Process 4th Ed. - T.H. James; McMillan 1977.
- Modern Photographic Processing - G. Haist; Wiley-Interscience 1979.
- Transactions of the Society of Motion Picture Engineers Vol 11 No:32 - Behaviour of Gelatin in the Processing of Motion Picture Film.
- Investigation of the Archival Stability of Cellulose Triacetate Film: The Effect of Additives to CTA Support - Y. Shinagawa, M. Murayama & Y.Sakaino; Polymers in Conservation.
