Submitted by Al …
In part one, we discussed the dangers of nitrate film; namely high flammability and spontaneous combustion. As a result of numerous fires caused by these unstable nitrate films, safety film was invented. Instead of using the highly flammable plasticized nitrocellulose, or celluloid, film base was made from cellulose acetate starting in the 1930s for X-ray stock and cellulose triacetate from 1948 for motion picture film. Acetate film manufactured during the era when nitrate films were still in use was marked 'Safety' or 'Safety Film' along one edge in dark letters.
At first, cellulose acetate film was considered the answer to the film industry’s problems. About 10 years after the invention of cellulose triacetate in 1948, it was discovered that this type of film was chemically unstable and subject to a different type of deterioration. This material releases acetic acid, the key ingredient in vinegar and responsible for its acidic smell. The problem became known as vinegar syndrome.
The decay process 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.
The plastic film base becomes brittle. This occurs in the advanced stages of deterioration, weakening the film and causing it to shatter with the slightest tension.
Shrinkage also occurs during this process. In advanced stages of deterioration, shrinkage can be as much as 10%. There have been some reports of film 35mm wide shrinking to almost 17mm.
As the acetate base shrinks, the gelatin emulsion of the film does not shrink, because it is not undergoing deterioration. The emulsion and film base separate, causing buckling, referred to by archivists as channeling. Sheet films are often severely channeled in the later stages of degradation.
Crystalline deposits or liquid-filled bubbles appear 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 a sign of advanced degradation.
In some cases, pink or blue colors appear in some sheet films. This is caused by anti-halation dyes, which are normally colorless and incorporated into the gelatin layer. When acetic acid is formed during deterioration, the acidic environment causes the dyes to return to their original pink or blue color.
Microfilm made of acetate plastic prior to 1980 is also subject to vinegar syndrome. All master microfilm created today uses a polyester plastic backing. Under proper storage conditions, today's film has a 500 year Life Expectancy (LE). The older acetate films have no more than a 100 year LE. Since some of this film is now over 60 years old, we are beginning to see more and more vinegar syndrome degradation.
If you're unsure of your films composition, try to tear the leader strip widthwise. If you can, it's acetate; if you can't, it is polyester. Acetate films may show little or no evidence of deterioration, but they're still living on borrowed time. Even if you decide to leave things as they are, you'll need to monitor your film. An easy way to do this is to place acid-detection test strips next to the film and check them periodically. Changes in the strips' color indicate an increase in acetic acid. ALL acetate film will eventually succumb to vinegar syndrome.
Believe it or not, vinegar syndrome is contagious. This is why you need to keep deteriorating films away from uninfected films. You can sometimes slow degradation by placing desiccants such as silica gel or activated charcoal (also known as "molecular sieves") in the film container to absorb moisture and acid. However, this isn't a set-it-and-forget-it strategy; the sieves must be checked regularly and replaced as needed. Consider this a stopgap measure you can use while looking for a longer-term solution.
As with nitrate films, the vinegar syndrome degradation problem exists with safety film made for the amateur home market as well as the commercial market. Amateur photographers and collectors alike would be well advised to check on their collections sooner rather than later.
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