The degradation of cellulose acetate base motion picture film.
The natural degradation of processed black and white cellulose acetate based 35mm
motion picture film material has been investigated using 1H and 13C NMR
spectroscopy. According to current standards only this material is considered to be an
archival recording medium. It has been recognised that understanding the relationship
between structure and physical properties would provide knowledge to help inhibit
motion picture film degradation. A correlation between observed changes in molecular
structure and physical property changes namely, % moisture regain, acidity and
viscosity has been obtained.
Results of these studies indicate that the primary mode of degradation is hydrolysis of
ester groups leading to deacetylation of the C6 acetyl group in the polymer chain. On
further deactylation, the C3 acetyl group is lost in preference to the C2 acetyl group.
Therefore, it can be concluded that a decrease in viscosity retention and an increase in
acidity concurs with deacetylation of the polymer chain. On this basis mechanisms of
deacetylation would predominate since oxidative chain-scission would take place
through the C2 acetyl group which is less accessible.
NMR spectroscopy also reveals that the plasticisers, namely triphenyl phosphate and
diethyl phthalate incorporated into cellulose acetate base, influence its stability. Under
archival conditions the triphenyl phosphate plasticiser appears not to undergo
degradation but becomes increasingly incompatible with the cellulose acetate base.
Diethyl phthalate however was seen to degrade as the cellulose acetate base itself
A more detailed understanding of the macromolecular structure of cellulose acetate
based motion picture film has been obtained with the use of Molecular Modelling and
Graphics studies using QUANTA software. This supports the NMR study indicating
that the C6 and C3 acetyl groups are clearly more exposed in the polymer matrix and
therefore, are more likely to be lost as hydrolysis of the polymer chain occurs.
Modelling studies also supported reduced stability of the polymer in the presence of
plasticisers. When plasticisers were docked within the polymer matrix distortions of the
polymers chains were observed as chains were pushed apart and dihedral angles
The role of the emulsion in film degradation was also examined by electrophoresis.
This separation of proteins has shown that under archival conditions, naturally aged
gelatin emulsion layers show a decrease in molecular weight. This has been attributed
to the acid catalysed deacetylation of the cellulose acetate base lowering the pH of the
film and inducing hydrolysis of gelatin. Only when considerable amounts of acid have
been evolved are changes to the emulsion observed.
Finally, the effects of storage containers were characterised using viscosity and acidity
measurements. Accelerated ageing studies at 50%RH 70°C to emulate archival storage
conditions revealed that in the early stages of degradation polyethylene and
polypropylene cans were more detrimental to film stability than the more
traditional metal can. The plastic cans were shown to promote oxidative degradation of
the film and polypropylene was shown to lead to higher levels of peroxide in films than