The effect of age

The effect of age It is difficult to find any age-related changes; some of the older films are in much better condition than some of the newer ones. In an archival situation, you do not know the previous history of the films that come into the archive. So we decided to take the same set of readings for films that had actually been produced on site at an archive and stored under archival conditions (45-50% RH and ambient temperatures). There we did see a trend: the older the material, the lower the viscosity, the higher the moisture regain, the lower the pH, and the higher the percentage insoluble polymer. So the environment does have an effect, and the age of the polymer is relevant. But, remember, archivists often don’t know where films have come from; they could have been exposed to any environment, giving a whole range of deterioration characteristics.

After looking at a series of naturally aged film stocks, we used all the same testing procedures and simulated degradation to see if we could, perhaps, predict how long a film would last under a given set of conditions. We took the same parameters and looked at what happens to film in dry conditions, in conditions of 100% RH, and in conditions of one molar acetic acid, all at a series of three different temperatures. Taking moisture regain as an example: as the temperature is increased, the onset of moisture regain is more rapid. So, increasing the temperature speeds up the rate of degradation.

That’s not new, but the effect of moisture in the environment is particularly significant, and is much more marked than the effect of temperature. At 90°C, in molar acetic acid conditions, the moisture regain C which corresponds with the loss of acetate groups and the formation of acetic acid is almost catalytic. There’s only a very, very small induction period before the acetate groups start to be lost.

What’s happening to the polymer is a complex interplay between the acetylation and the breakdown of the main chain: chain degradation is occurring and also what we call hydrolytic de-esterification C in other words, the loss of acetate groups is enhanced by the presence of moisture.