Nitrocellulose is a historic film base, out of production for more than half a century now. Produced of cellulose nitrate, despite its obvious advantages (mechanical durability and flexibility), it is highly flammable. The film base made on the basis of nitrocellulose is subject to irreversible and spontaneous degradation. During decomposition, when air humidity is low and the temperature rises above 45°C, film can undergo spontaneous combustion. Nitrocellulose fire is very dangerous since it does not need oxygen for combustion. Due to the risk of fire, screening of NITRATE films is not common practice.

FILM BASE SELECTION

Nitrocellulose film base, popularly known as celluloid, was made of cellulose. Easily inflammable foil was produced through pouring a thin layer of solution, e.g. on a smooth metal surface. The main raw material had to be very clean. Cellulose was most often used, acquired from short, unspinnable fibres that grow on cotton seeds – linters. In stone centrifuges pure cellulose was acted against with a mix of nitric acid and sulphuric acid in precisely indicated conditions of concentration and temperature. Thus, the process of esterification occurred, consisting mainly in the replacement of hydroxide groups of cellulose with acid residues of nitric acid.

The addition of concentrated sulphuric acid was mixed with water to support the process of the creation of ester. Esterification could take a less or more complete course, resulting in a mix of cellulose nitrate with varied content of acid residues (NO3). The described reaction led to the creation of both highly esterified cellulose used to produce smokeless powder, and a more weakly esterified cellulose used to produce film. The degree of esterification depended on the strength of used acids and their period of activity. Washed mix of cellulose nitrates was subjected to the effect of alcohol and ether, then camphor, plasticisers, stabilisers and highly caustic solvents were added. The result was transparent untarnishable foil of very high flexibility and durability. Dense honey-like solution was poured onto smooth metal drums or onto polished and chrome-plated metal tapes. The following step consisted in blowing and heating the solution layer in order to make the solvents evaporate quickly. The thickness of dry celluloid varied from 0.12 mm to 0.14 mm. After separating from the drum or metal tape, foil was dried again and rolled into rolls, 110 cm wide and 300 m long. In this in this simple way that nitrocellulose film base came into being.

References:

M. Iliński, Możliwość wytwarzania taśmy filmowej w Polsce, Kinotechnik, Issue 2, Warsaw.