Cellulose

Main component of the acetate and nitrate bases used in film manufacture.

Cotton wool is pure cellulose.

The introduction of flexible photographic films in the last decade of the 19th century was made possible by advances in the technology of cellulosic plastics, in which the natural polymer cellulose (see diagram) was chemically modified. Plastics with various properties could be obtained by modifying cellulose in different ways – for example, attaching the nitro (ONO ₂ ) group to the cellulose molecule yielded cellulose nitrate, the first plastic to be used for flexible film support.

Polymeric substances like cellulose are composed of long chains of repeating individual units. In the case of cellulose, chains of 500 to 2000 units are typical, and each individual unit has 3 possible sites (the OH groups evident the diagram) where it can be chemically modified.

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The so-called ‘sidegroups’ attached to cellulose are very important in determining the properties of the resulting plastic. The well known explosive flammability of cellulose nitrate film is a consequence of the fact that oxygen from the nitro side group (ONO ₂ ) is readily available to sustain combustion. Other attributes such as- physical properties, solubility in various solvents, and water absorbency are also dependent on the nature of the side groups.