|Impact of Environmental Pollution on the Preservation of Archives and Records: A RAMP Study (UNESCO, 1988)|
|5.0 The nature and degradation from pollution of materials in archives|
As cellulose is not conveniently soluble in common liquids and has therefore been converted to become so, by exchanging some of the side hydroxyl (OH) groups with nitrate or acetate groups, the substitution need not: be complete, the extent of it affects the properties of these modified natural polymers. The conversions also cause some shortening of the long cellulose chains. The products are however usefully soluble in common organic solvents. Hence it becomes possible to manipulate these materials as plastics, and by casting or extrusion to form films and fibres of great importance.
The degradation of cellulose nitrate, particularly in the form of a film base, is well known, thought it is also to be expected in collodion photography and in the decay of pyroxylin (cellulose nitrate) impregnated book cloths, which have been widely used in bookbinding. The immediate concern is to recognise that as it decays through hydrolysis, nitric acid is produced in situ. This causes the acidity to increase and the process accelerates. More nitric acid is made and diffuses into the environment. Its powerful oxidising properties made it even more undesirable. Negatives of this material filed in an archive will then be a source of internal pollution. There are serious fire risks if such films are kept, the acids may also corrode metal cans. Extreme care and vigilance is necessary in identifying and separating these highly flammable items. The fire hazard makes it desirable to find other cellulose esters which are much less flammable, that is 'Safety film' .
Cellulose acetates were developed and have found similar uses as a film base, plastic mouldings materials and textile fibres. The ester groups show essentially the same hydrolysis reaction as the cellulose nitrate, but the acetic acid produced is neither a strong acid nor an oxidising agent. It is possible that the developing acidity assists in the breakdown of the cellulose chain, and its archival quality is being questioned. With a moderate degree of conversion the cellulose all-acetate is known to break down in cans of film, giving a strong smell of vinegar when the can is opened. The acetic acid is volatile.
Exposure to sulphur dioxide or oxides of nitrogen will cause degradation as they do with cellulose. Fundamentally there is little reason to think that the fully esterified cellulose tri-acetate would rot hydrolyse. As such cine, microfilm, microfiche and magnetic tapes have been made with cellulose acetate bases there is an obvious concern for these formats. However there are other causes of embrittlement as well as the breakdown of the polymer structure.
None of the cellulose derivatives are particularly flexible, nor are their long molecules easily folded and twisted, for they are built of large rings, each bearing bulky side groups which obstruct the molecular movements. Such materials need to be softened by plasticisers, usually required in considerable quantities, perhaps up to 30%. These substances may diffuse out of the plastic film and migrate into adjacent material or evaporate. They may cause no obvious harm, but as they were chosen to have some solvency and a low volatility they may cause images made with dyes to become a little blurred, eg on colour film. Most of the plasticisers are esters, generally of weak organic acids, however some used in film bases are phosphates. Should these hydrolyse in the course of time then phosphoric acid will be produced. It is not very volatile and as it is a stronger acid than the organics it would be expected to contribute to the further degradation of the polymers. Papers laminated with cellulose acetate may therefore be further degraded by the contact. Hot lamination would be expected to be even more active in promoting degradation. If the papers had been treated with alkaline buffer before lamination that might be enough to deal with the acids generated.
As all these cellulose esters are based on cellulose they will be expected to behave in a similar manner when exposed to oxidative pollution.
It is also probable that these materials might carry residual acids (e.g. sulphuric) used in the original production which would shorten their life and make them more active as internal sources of degradation.