Nitrocellulose

Nitrocellulose (Cellulose nitrate, guncotton) is a highly flammable compound formed by nitrating cellulose (e.g. through exposure to nitric acid or another less powerful nitrating agent). It was used until World War II as a smokeless propellant, replacing gunpowder. Other uses include plastics, films and membranes for Western blot.

When dissolved in ether or other organic solvents, the solution is called collodion, which has been used as a wound dressing and carrier of topical medications since the U.S. Civil War.

Collodion was also used as the carrier for silver salts in some very early photographic emulsions, particularly spread in thin layers on glass plates. To this day it is used in Compound W Wart Remover as a carrier of salicylic acid, the active ingredient.

Table of contents

1 Guncotton 2 Nitrate Film 3 Other Uses

Guncotton

It was first observed by Pelouze in 1838 and more properly developed and also named by Christian Friedrich Schönbein in Basle around 1846. His preparation method was the first to be widely imitated - one part of fine cottonwool to be immersed in fifteen parts of an equal mix of sulphuric and nitric acids. After two minutes the cotton was removed and placed in cold water and washed to set the esterification level and remove all acid residue. It was then slowly dried at a temperature of less than 100 °C.

The process uses the nitric acid (2HNO₃) to convert the cellulose (C₆H₁₀O₅) into cellulose nitrate (C₆H₈(NO₂)₂O₅) and water. The sulphuric acid is present to prevent the water produced in the reaction from diluting the concentrated nitric acid.

The power of guncotton meant that it was adopted for blasting and as a projectile force, it has around six times the gas generation of an equal volume of gunpowder and produces less smoke and less heating. However the sensitivity of the material during production led to the British, Prussians and French discontinuing manufacture within a year. Further research indicated that the key was the very careful preparation of the cotton, unless it was very well cleaned and dried it was liable to spontaneously explode.

The British also developed a much lengthier manufacturing process, with the washing and drying times each extended to 48 hours and repeated eight times over. The acid mixture was also changed to two parts sulfuric acid to one part nitric acid.

Nitrate Film

Nitrocellulose itself was used as the first flexible film base beginning with Eastman Kodak products in August, 1889. It was used through 1933 for X-ray films, where the hazard of its flammability was most acute, and continued to be used for motion picture films thorugh 1951. It was replaced by safety film, with an acetate base. The use of nitrocellulose flim for motion pictures led to the widespread requirement for fireproof projection rooms with wall coverings made of asbestos. A cinema fire caused by ignition of nitrocellulose film stock was a central plot element in the Italian film Cinema Paradiso. Nitrocellulose film base manufactured by Kodak can be identified by the presence of the word "Nitrate" in (usually) red letters between perforations; acetate film manufactured during the era when nitrate films were in use was marked "safety" or "safety film" between perforations in (usually) yellow letters.

Color negative film was never manufactured with a nitrate base, nor was 8mm or 16mm motion picture film.

Other Uses

Depending on the manufacturing process, the nitrocellulose is esterified to a varying degree. Table tennis balls and some photographic films, for example, have a fairly low esterification level and burn rather slowly with some charred residue. The first cine films were made using nitrocellulose; decades later, it was discovered that nitrocellulose gradually decomposed, releasing nitric acid, which further catalysed the decomposition. Salvageing old films which were undergoing this accelerating self-catalysed disintegration became a major problem for film archivists. Cool temperatures can delay these reactions indefinitely.