Cellulose triacetate

During the 1930s, cellulose triacetate was mixed with diacetate to make
film base by Eastman Kodak and some of these films are difficult to
determine. In 1951, all nitrate base film was replaced by pure cellulose
triacetate film base. This is the safety film that is most used today,
sometimes just called cellulose acetate or just acetate.
It can still be burnt, but requires a higher temperature. It is thought
that some cellulose nitrate fires have been made more serious by the
presence of cellulose acetate film that burns less easily but for
longer.
It is subject to shrinking, although to a lesser extent and rate than
nitrate, and to serious chemical decay in time, known as "vinegar
syndrome". This decay is a hydrolysis and acetic acid is one of the
products. As the acetic acid increases and the pH of the film is lowered,
the hydrolysis rate increases. Diacetate is more easily hydrolysed than
triacetate but it seems that once the process begins the progress is
autocatalytic.
Vinegar syndrome decay has been attributed to improper storage
conditions of high temperature and humidity resulting in the hydrolysis
of plasticisers to phosphoric acid creating the initial acid condition,
but other factors may be at work.
Vinegar syndrome has been shown to be influenced by the presence of iron
in cans, as rust particles and as iron oxide magnetic sound tracks on
the film. Films with sound tracks such as sprocketed magnetic tape and
COMMAG 16mm colour news film are more prone to vinegar syndrome decay
than conventional film.
A number of storage techniques have been devised to remove the acetic
acid from the film can atmosphere as it is produced. These usually
consist of particular can, special bags or small chemical containers
placed in the can with the film, for example the FICA system and the
Kodak Molecular Sieve. Until the efficiency of these procedures is well
established, most archives try to use as low a temperature as practical.
However, it seems reasonable to assume that as low a temperature as can
be maintained is the best storage condition.