The effect of an enclosed environment

So far we had looked at all the constituents and at the fact that the film is in a can; but what about the fact that, because the film is in a can, it is in a smaller environment than if it was just on a shelf? We ran infra-red spectra, which give a trace corresponding to particular groups in the molecule.

We looked at some cinematograph films, and the first thing we noticed was that the moisture content was changing significantly: it was taking up moisture. We already knew this, but the loss of acetate groups did not appear to be particularly great. Not many acetate groups are actually being lost in a film ageing from new to loss of image.

We were lucky enough to get hold of a triacetate sculpture that had stood in an open environment for 20 or 30 years. It was exhibiting the formation of acetic acid - the vinegar syndrome - but, in this case, the degradation patterns were quite different. It didn’t appear that the sculpture was taking up moisture but that, in the place of the acetate groups on the polymer backbone, we were actually substituting hydroxyl groups in some way, and there was a massive loss of acetic acid. So it seems that in an open environment we are promoting loss of acid, and in a closed environment we are setting up some kind of equilibrium that prevents further loss of acid.

We decided to check that by artificial ageing techniques and found that, in an open environment, the onset of moisture regain (which corresponds to loss of acetate groups) is much greater. The onset is much more rapid than in an enclosed environment, where the loss of acetate groups will be suppressed. Unfortunately, however, we seem to have this Catch 22 situation whereby, although we’re subduing loss of acetate groups and de-esterification and forming less acetic acid than in an open environment, the polymer chain is breaking up instead. In an open environment we see very little decrease in viscosity or loss of tensile properties compared to an enclosed environment where it appears that, because we’re actually containing the acid impurities within the film can, they can serve to break down the main polymer chain which loses its viscosity and will also lose its tensile properties.