Base polymer decomposition
General Cellulose Ester Decomposition
Cellulose ester polymers all suffer from decomposition reactions that will ultimately cause the long molecular chains that form the film base to break apart. The decomposition reactions are similar for both cellulose nitrate and cellulose acetate polymers although the actual by-products and the way these react with the rest of the film may be different. The mechanism of this decomposition is believed to proceed along the following path (cellulose triacetate shown):
By-products from these reactions are acidic and the acids formed then catalyse, or accelerate, the reaction further. Once initiated this reaction is irreversible.
During decomposition of the film base the nitro groups detach themselves from the cellulose base. These groups, as nitrogen oxides, may initially form nitrous acid and then nitric acid by combining with water in the base or emulsion. Nitric acid is a strong acid and consequently the pH can be very low.
The nitric acid produced acts upon the metallic silver that forms the image and eventually converts it into a colourless silver salt, most likely silver nitrate. The acid formed also accelerates the rate of decomposition of the film base. It has been suggested that the acid initiates a digestion reaction of the cellulose in the film base which breaks down the cellulose into sugar-like compounds. This is known colloquially as 'nitrate honey’. The film blocks into a solid mass and finally the base completely disintegrates and turns into a brown powder. Table 3.1 and Fig 3.4 summarise the stages of nitrate decomposition.
|D1||Bleaching of the silver image|
|D2||The film becomes sticky|
|D3||Bubbles of nitrate 'honey’ appear on the surface of the reel|
|D4||The film sticks into a solid mass|
|D5||The film falls into a brown powder|
|Table 3.1||Figure 3.4|
The bleaching of the image can occur while the film base is still apparently in quite good condition.
Cellulose Acetate (di & tri)
Because the majority of the holdings of most archives are on cellulose acetate based film the major concern of film archives is the decomposition of cellulose acetate known as 'Vinegar Syndrome’. The acid is released within the base but slowly diffuses to the surface causing the noticeable 'vinegar’ odour. Another symptom often noticed in the later stages of 'Vinegar Syndrome’ is the appearance of crystal growths on the surface of the film (Fig 3.5).
These crystals are the plasticisers forced out of the base by a change in the acid content of the base. Analysis has shown that these crystals are mainly triphenyl phosphate (TPP), the most commonly used plasticiser for cellulose triacetate used in film.
In migrating to the surface of the film the plasticisers will form very small holes in the surface of the emulsion.
Occasionally the plasticisers may form small blisters between the emulsion and the base. In extreme cases these will cause the gelatin to rupture and leave a significant hole in the image.
Brittleness and shrinkage are two of the main features of the effect of 'Vinegar Syndrome’. As the level of free acid in the film’s base increases the linkages between the cellulose units can break causing a shortening of the cellulose polymer chains. This lowers the tensile strength of the base. Shrinkage also occurs as acid migrates to the surface. The base can shrink to a far greater degree than the gelatin emulsion. When this occurs the emulsion layer will shear away from the base and either buckle on the surface or break away from the film in flakes. Early diacetate based films first indicate decomposition by a smell of naphthalene (similar to mothballs). This is the plasticiser, monochloro-naphthalene, being forced from the base by increasing acidity. All other aspects of decomposition of diacetate bases are identical to triacetate although ultimately there are fewer acetyl groups overall to form acetic acid.
Acetic acid is a relatively weak acid and will not cause bleaching of the silver image. However acetic acid is linked to accelerated dye fading in colour films. The shift in pH brought about by decomposition will affect the gelatin. Some degradation of the image structure may occur as the film deteriorates due to softening of the emulsion.
Polyester film can decompose with a mechanism very similar to cellulose esters. However this reaction under room temperatures is so slow that it can be considered non-existant. To accelerate this reaction heat or ultra violet light is required.
However there are two potential problems that can occur with polyester base film. Firstly 'core set’ or memory. If film is kept coiled for extended periods of time the base polymer cold flows to reduce the tension and will acquire a high degree of curl. This effect may be minimised by reversing the films wind.
Secondly if the film becomes damp and the emulsion blocks together then the strength of the bond between emulsion layers can be sufficient to cause the polyester to delaminate when it is unwound. Polyester is very strong longitudinally but quite weakly bound as layers (Fig 3.6).
With sufficient force it is possible to cause the film to split and separate. Even if the film does not come apart completely the area that has delaminated will appear as a serious blemish on the image (Fig 3.7).