Nitrocellulose (also known as cellulose nitrate, flash paper, flash cotton, guncotton, pyroxylin and flash string, depending on form) is a highly flammable compound formed by nitrating cellulose through exposure to a mixture of nitric acid and sulfuric acid. One of its first major uses was as guncotton, a replacement for gunpowder as propellant in firearms. It was also used to replace gunpowder as a low-order explosive in mining and other applications. In the form of collodion it was also a critical component in an early photographic emulsion, the use of which revolutionized photography in the 1860s.
Production
The process uses a mixture of nitric acid and sulfuric acid to convert cellulose into nitrocellulose. The quality of the cellulose is important. Hemicellulose, lignin, pentosans, and mineral salts give inferior nitrocelluloses. In precise chemical terms, nitrocellulose is not a nitro compound, but a nitrate ester. The glucose repeat unit (anhydroglucose) within the cellulose chain has three OH groups, each of which can form a nitrate ester. Thus, nitrocellulose can denote mononitrocellulose, dinitrocellulose, and trinitrocellulose, or a mixture thereof. With fewer OH groups than the parent cellulose, nitrocelluloses do not aggregate by hydrogen bonding. The overarching consequence is that the nitrocellulose is soluble in organic solvents such as acetone and esters; e.g., ethyl acetate, methyl acetate, ethyl carbonate. Most lacquers are prepared from the dinitrate, whereas explosives are mainly the trinitrate.
The chemical equation for the formation of the trinitrate is
3 HNO3 + C6H7(OH)3O2 C6H7(ONO2)3O2 + 3 H2O
The yields are about 85%, with losses attributed to complete oxidation of the cellulose to oxalic acid.
Use
The principal uses of cellulose nitrate is for the production of lacquers and coatings, explosives, and celluloid.
In terms of lacquers and coatings, nitrocellulose dissolves readily in organic solvents, which upon evaporation leave a colorless, transparent, flexible film. Nitrocellulose lacquers have been used as a finish on furniture and musical instruments.
Guncotton, dissolved at about 25% in acetone, forms a lacquer used in preliminary stages of wood finishing to develop a hard finish with a deep lustre. It is normally the first coat applied, sanded and followed by other coatings that bond to it.
The explosive applications are diverse. Relative to coatings applications, the nitrate content is higher for propellant applications typically. For space flight, nitrocellulose was used by Copenhagen Suborbitals on several missions as a means of jettisoning components of the rocket/space capsule and deploying recovery systems. However, after several missions and flights, it proved not to have the desired explosive properties in a near vacuum environment. In 2014, the Philae comet lander failed to deploy its harpoons because its 0.3 grams of nitrocellulose propulsion charges failed to fire during the landing.
Nail polish is made from nitrocellulose lacquer, as it is inexpensive, dries quickly, and is not damaging to skin.
Other uses
Collodion, a solution of nitrocellulose, is used today in topical skin applications, such as liquid skin and in the application of salicylic acid, the active ingredient in Compound W wart remover.
Laboratory uses
Membrane filters made of a mesh of nitrocellulose threads with various porosities are used in laboratory procedures for particle retention and cell capture in liquid or gaseous solutions and, reversely, obtaining particle-free filtrates. A nitrocellulose slide, nitrocellulose membrane, or nitrocellulose paper is a sticky membrane used for immobilizing nucleic acids in southern blots and northern blots. It is also used for immobilization of proteins in western blots and atomic force microscopy for its nonspecific affinity for amino acids. Nitrocellulose is widely used as support in diagnostic tests where antigen-antibody binding occurs; e.g., pregnancy tests, U-albumin tests, and CRP tests. Glycine and chloride ions make protein transfer more efficient. Radon tests for alpha track etches use nitrocellulose. Adolph Noé developed a method of peeling coal balls using nitrocellulose. It is used to coat playing cards and to bind staples together in office staplers.