NITROCELLULOSE

also called Cellulose Nitrate, a mixture of nitric esters of cellulose, and a highly flammable compound that is the main ingredient of modern gunpowder. Nitrocellulose is a fluffy white substance that retains some of the fibrous structure of untreated cellulose. It is not stable to heat, and even carefully prepared samples will ignite on brief heating to more than about 150 C (300 F). When nitrocellulose decomposes, it forms products that catalyze further decomposition; this reaction, if not stopped in time, results in an explosion. In 1838 T.-J. Pelouze discovered that cotton could be made explosive by dipping it in concentrated nitric acid. C.F. Schnbein in 1845 improved the manufacturing method by dipping cotton in a mixture of nitric and sulfuric acids and then washing the substance to remove the excess acid. The form of nitrocellulose known as guncotton came into use as an ingredient of gunpowder in the 1860s. The early history of its use was punctuated by many disastrous explosions, however, and in 1868 Sir Frederick Augustus Abel showed that the methods then prevalent for washing nitrocellulose after nitration did not remove enough acid. Paul Vieille added special stabilizers to nitrocellulose to neutralize the catalytically active decomposition products; the first stable and reliable propellant, smokeless powder, resulted from his work and became the main form of gunpowder. The manufacturing process uses either cotton linters or wood pulp as the source of cellulose. These materials are treated in a mixture of nitric and sulfuric acid at a ratio of one part cellulose to 30 parts acid. Most of the spent acid is removed by centrifuging the moist nitrocellulose, which is washed in a large quantity of water and is then subjected to boiling in acidified water to eliminate the remaining unstable by-products of nitration. The material is then pulped to disintegrate the cellulose fibres, and a subsequent washing removes the last traces of acid. Wood pulp is the preferred material for nitrocellulose used as a gun propellant. If cotton is treated until almost all of the cellulose molecules' hydroxyl groups have been esterified but the molecules' structure has not yet been broken down, the result is guncotton. This form of nitrocellulose has more than 13 percent nitrogen and is soluble in acetone but not in ether and alcohol. It is used for propellants, either alone or combined with nitroglycerin. Less completely nitrated celluloses are called collodion cotton or pyroxylin and are inferior to guncotton in explosive properties. Collodion with a nitrogen content of not more than 12 percent is used chiefly for lacquers and celluloid plastics. Materials with a nitrogen content of about 11.5 percent were once used as artificial silk but have been replaced by other materials such as viscose rayon. This same material was used for manufacturing photographic film until safety film made of cellulose acetate plastics became popular.