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Cellulose acetate film

Because of the highly flammable nature of cellulose nitrate film, by the beginning of the 20th century manufacturers had begun to introduce cellulose acetate as a safe alternative for the plastic film base of motion picture and sheet film. Beginning with cellulose diacetate in 1909, this innovation continued with cellulose acetate propionate and cellulose acetate butyrate in the 1930s, and finally in the late 1940s with cellulose triacetate and then polyester bases. ^[1] Generally speaking, all these relatively nonflammable substitutes for nitrate are called safety film. The motion picture industry, however, continued the use of cellulose nitrate supports until the introduction of cellulose triacetate in 1948, which met the rigorous safety and performance standards set by the cinematographic industry. ^[2] However, while the poor chemical stability of the medium was unrecognized at the time, it has since become a major threat for film collections.

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1 Decay: "vinegar syndrome"
- 1.1 The progression of degradation
- 1.2 A-D testing strips
2 Preservation and storage
3 Further Reading

Decay: "vinegar syndrome"

The first instance of cellulose triacetate degradation was reported to the Eastman Kodak Company within a decade after its introduction in 1948. Stored in a hot, humid climate, this film report came from the Government of India and was followed by further reports of degradation from climates with similarly adverse conditions. These observations resulted in continuing studies in the Kodak laboratories beginning in the early 1960s. Furthermore, beginning in the 1980s, there was again a great deal of focus upon film stability following frequent reports of cellulose triacetate degradation which releases acetic acid, or vinegar. Thus, the problem became known as “vinegar syndrome.” ^[3]

The progression of degradation

In acetate film, acetyl (CH3CO) groups are attached to long molecular chains of cellulose. With exposure to moisture, heat, or acids, these acetyl groups are broken from their molecular bonds and free acetic acid is released. ^[4] While these acetic acids are released inside the plastic, it gradually diffuses to the surface, causing a characteristic vinegary smell. The decay process generally follows this pattern:

Acetic acid is released during the initial acetate base deterioration, leading to the characteristic vinegar odor. This signal marks the progression of deterioration.
Embrittlement of the plastic film base occurs in the advanced stages of deterioration, turning the formerly flexible and strong film into a weak material which can shatter with the slightest tension. These physical changes are a consequence of the fact that cellulose acetate is made up of long chains of repeating units, or polymers. When the acetic acid is released as these groups come off, the acidic environment helps to break the links between units, shortening the polymer chains and leading to brittleness.
Shrinkage is another of the physical consequences of acetate-base decomposition. With the cellulose acetate polymer chains breaking into smaller pieces, and with their side groups splitting off, the plastic film begins to shrink. In advanced stages of deterioration, shrinkage can be as much as 10%, a huge amount considering the fact that that a 1% reduction renders motion picture film unusable. Furthermore, as the acetate base continues to shrink, the gelatin emulsion of the film often does not because it is not undergoing deterioration. Thus, eventually the bonds between the emulsion and film base separate, leading to areas of buckling which are referred to by archivists as ‘channeling.’ Cut sheet films are often severely channeled in the later stages of degradation.
Another consequence of base deterioration is the appearance of crystalline deposits or liquid-filled bubbles on the emulsion. These are evidence of plasticizers, additives to the plastic base, becoming incompatible with the film base and oozing out on the surface. This discharge of plasticizers is associated with more advanced stages of degradation.
In some cases, another consequence of vinegar syndrome is the appearance of pink or blue colors in some sheet films. This is caused by dyes, called antihalation dyes, which are supposed to colorless that are incorporated into the gelatin layer. However, when acetic acid is formed during deterioration, the acidic environment of the film base cause the dyes to return to their original pink or blue color.

A-D testing strips

Developed by the Image Permanence Institute, A-D, or “acid-testing” indicator strips change color from blue through shades of green to yellow with increasing exposure to acid. According to the test User’s Guide, they were “created to aid in the preservation of collections of photographic film, including sheet and roll films, cinema film, and microfilm. They provide a nondestructive method of determining the extent of vinegar syndrome in film collections.” ^[5] These tools can be used to determine the extent of damage to a film collection and those steps which should be taken to prolong their usability.

Preservation and storage

Currently there is no practicable way of halting, much less reversing, the course of degradation. ^[6] While there has been significant research regarding a variety of factors to slow degradation such as storage in molecular sieves, temperature and moisture are the two key factors affecting the rate of deterioration. According to a table from the Image Permanence Institute, fresh acetate film stored at a temperature of 65° F and 50% relative humidity will last approximately 50 years before the onset of vinegar syndrome. However, reducing the temperature 15°, while keeping the humidity at the same level, delays the first signs by 150 years. ^[7] In general, a combination of low temperature and low relative humidity represents the optimum storage condition for cellulose acetate base films. ^[8]

During early stages of decay, the film content can be rescued by transferring it to new film stock, however once the film becomes brittle it cannot be copied in its entirety. However, because the gelatin emulsion usually stays intact during the degradation process, for sheet films it is possible to save the image by using solvents to dissolve the emulsion away from the shrunken base. Once the emulsion has been freed from the shrunken support, it can be photographed or transferred to a new support. Because of the solvents used, this is a delicate and potentially hazardous procedure and thus is quite expensive to do for a large collection. Degraded motion picture film cannot be restored in this way, but sheet films often can.^[9]

Cellulose acetate film

Additional recommended knowledge

Guide to balance cleaning: 8 simple steps

How to ensure accurate weighing results every day?

Weighing the right way

Contents

Decay: "vinegar syndrome"

The progression of degradation

A-D testing strips

Preservation and storage

Further Reading

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