The degradation and stabilisation of cellulose triacetate cinematograph film

Edge, Michele (1990) The degradation and stabilisation of cellulose triacetate cinematograph film. Doctoral thesis (PhD), Manchester Polytechnic.

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Abstract

Cellulose triacetate support cinematograph films exhibit degradation when stored under archival (dark storage) conditions. Degradation is manifested by the evolution of acetic acid and white crystalline surface deposits with accompanying image erosion. The purpose of this study has been to identify and prevent any degradation mechanisms which may operate in such circumstances. With respect to this under both natural (archival) and accelerated ageing conditions it has been shown that there is a direct correlation between moisture regain, acidity and decrease in viscosity of the support material. Two degradation processes have been identified utilising these techniques along with FTIR and FTNMR. The first is an acid catalyzed hydrolytic de-acetylation of ester functionality and the second is an acid catalyzed oxidative scission of the cellulose backbone. With respect to both mechanistic routes, the emulsion layer, which is acutely sensitive to surrounding moisture from the atmosphere, serves as a source of protoic catalyst (H30+). Here redistribution of moisture in the emulsion occurs to adjacent cellulose triacetate support. As a consequence a moisture gradient exists throughout the length and width of wound cinematograph film. Degradation apparently grows into the reel from the outer circumference and sprocket areas. The presence of hydroperoxides, which have been identified within the film materials, supports the existence of an oxidative degradation pathway. It has also been shown that iron is absorbed, from metal storage cans, into the support via facile uptake as the material becomes progressively more hygroscopic. Subsequently, iron then acts as a powerful catalyst in accelerating the degradation, probably by catalyzing the decomposition of active peroxides. This is confirmed by doping films with iron. The emulsion layer also serves to impart some stability to the support material by scavenging any acid degradation products and in so doing its gelatin constituent undergoing hydrolysis. It also likely serves as a diffusion tarrier to oxygen attack. The support plasticiser, triphenyl phosphate, also enhances film stability. Other than improving resistance to moisture uptake it possibly complexes with any metal ions taking part in the degradation and behaves to some extent as an antioxidant. As the cellulose triacetate support degrades the plasticiser is rendered incompatible with it and is pushed to the surface where it crystallises out as a white deposit. The stabilisation of cellulose triacetate support cinematograph film has also been examined using a number of potential stabiliser systems. These include acid scavengers, primary hindered phenolic antioxidants, secondary hindered piperidine stabilisers, metal dialkyldithiocarbamates, metal deactivators, secondary thioesters and phosphites. Stabilisation efficiency was assessed by measuring percentage retention in viscosity of the polymer support. None of the stabiliser systems was found to be effective when used alone, indicating that more than one type of mechanism is operative in degradation. Effective stabilisation is achieved through a synergistic tris system of stabilisers namely, Tinuvin 770, Irganox MD1024 and Sodium Phenyl Phosphinate. This combination has the ability to scavenge acetic acid and free radicals, and deactivate any peroxides and transition metal ions.