Definitions

nitrocellulose

nitrocellulose

[nahy-truh-sel-yuh-lohs]
nitrocellulose, nitric acid ester of cellulose (a glucose polymer). It is usually formed by the action of a mixture of nitric and sulfuric acids on purified cotton or wood pulp. The extent of nitration and degradation (breaking down) of the cellulose is carefully controlled in order to obtain the desired product. When cotton is treated so that nearly all of the hydroxyl groups of the cellulose molecule are esterified, but with little or no degradation of the molecular structure, the nitrocellulose formed is called guncotton. Guncotton resembles cotton in its appearance. Extremely flammable, it explodes when detonated and is used in the manufacture of explosives. Guncotton is insoluble in such common solvents as water, chloroform, ether, and ethanol. If the nitration is not carried to completion (the point at which about two thirds of the hydroxyl groups are esterified), the soluble cellulose nitrate pyroxylin is formed.

Nitrocellulose (also: cellulose nitrate, flash paper) is a highly flammable compound formed by nitrating cellulose through exposure to nitric acid or another powerful nitrating agent. When used as a propellant or low-order explosive, it is also known as guncotton.

Guncotton and gunpowder

Henri Braconnot discovered in 1832 that nitric acid, when combined with starch or wood fibers, would produce a lightweight combustible explosive material which he named xyloïdine. A few years later in 1838 another French chemist Théophile-Jules Pelouze (teacher of Ascanio Sobrero and Alfred Nobel) treated paper and cardboard in the same way. He obtained a similar material he called nitramidine. Both of these substances were highly unstable, and were not practical explosives.

However, Christian Friedrich Schönbein, a German-Swiss chemist, discovered a more practical solution around 1846. As he was working in the kitchen of his home in Basle, he spilled a bottle of concentrated nitric acid on the kitchen table. He reached for the nearest cloth, a cotton apron, and wiped it up. He hung the apron on the stove door to dry, and as soon as it was dry there was a flash as the apron exploded. His preparation method was the first to be widely imitated — one part of fine cotton wool to be immersed in fifteen parts of an equal blend of sulfuric and nitric acids. After two minutes the cotton was removed and washed in cold water to set the esterification level and remove all acid residue. It was then slowly dried at a temperature below 100°C. Schönbein collaborated with the Frankfurt professor Rudolf Böttger, who had discovered the process independently in the same year. By a strange coincidence there was even a third chemist, the Braunschweig professor F. J. Otto, who had also produced guncotton in 1846 and was the first to publish the process, much to the disappointment of Schönbein and Böttger. (Itzehoer Wochenblatt, 29 October 1846, columns 1626 f.)

The process uses the nitric acid to convert the cellulose into cellulose nitrite and water:

3HNO3+ C6H10O5 → C6H7(NO2)3O5 + 3H2O
The sulfuric acid is present to protonate the nitric acid to form the nitronium ion.

The power of guncotton made it suitable for blasting. As a projectile driver, it has around six times the gas generation of an equal volume of black powder and produces less smoke and less heating. However the sensitivity of the material during production led the British, Prussians and French to discontinue manufacture within a year.

Jules Verne viewed the development of guncotton with optimism. He referred to the substance several times in his novels. His adventurers carried firearms employing this substance. Most notably, in his From the Earth to the Moon, guncotton was used to launch a projectile into space.

Further research indicated that the key was the very careful preparation of the cotton: unless it was very well cleaned and dried, it was likely to explode spontaneously. The British, led by Frederick Augustus Abel, also developed a much lengthier manufacturing process at the Waltham Abbey Royal Gunpowder Mills, patented in 1865, 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.

Guncotton remained useful only for limited applications. For firearms, a more stable and slower burning mixture would be needed. Guncotton-like preparations were eventually prepared for this role, known at the time as smokeless powder.

Guncotton, dissolved at approximately 25% in acetone, forms a lacquer used in preliminary stages of wood finishing to develop a hard finish with a deep luster. It is normally the first coat applied, sanded, and followed by other coatings that bond to it.

Production

Nitrocellulose is made either using concentrated sulfuric/nitric acid or sulfuric acid/potassium nitrate. Cotton is generally used as the cellulose. The cellulose is added to the acid mix to nitrate. After the cellulose has finished nitrating it is washed and dried. Nitrocellulose is stored wet so it can't be accidentally lit or explode.

Nitrate film

Nitrocellulose was used as the first flexible film base, beginning with Eastman Kodak products in August, 1889. Camphor is used as plasticizer for nitrocellulose film. It was used until 1933 for X-ray films (where its flammability hazard was most acute) and for motion picture film until 1951. It was replaced by safety film with an acetate base.

The use of nitrocellulose film for motion pictures led to a widespread requirement for fireproof projection rooms with wall coverings made of asbestos. Famously, the US Navy shot a training film for projectionists which included footage of a controlled ignition of a reel of nitrate film which continued to burn even when fully submerged in water. Due to public safety precautions, the London Underground forbade transport of nitrate films on its system until well past the introduction of safety film. A cinema fire caused by ignition of nitrocellulose film stock (foreshadowed by an earlier small fire) was a central plot element in the Italian film Cinema Paradiso. Today nitrate film projection is usually highly regulated and requires extensive precautionary measures including extra projectionist health and safety training. Additionally, projectors certified to run nitrate films have many containment strategies in effect. Among them, this includes the chambering of both the feed and takeup reels in thick metal covers with small slits to allow the film to run through. Furthermore, the projector is modified to accommodate several fire extinguishers with nozzles all aimed directly at the film gate; the extinguishers automatically trigger if a piece of flammable fabric placed near the gate starts to burn. While this triggering would likely damage or destroy a significant portion of the projection components, it would prevent a devastating fire which almost certainly would cause far greater damage. In addition, projection rooms may be required to have automatically operating metal covers for the projection windows, preventing the spreading of a fire to the auditorium.

It was discovered decades later that nitrocellulose gradually decomposes, releasing nitric acid which further catalyzes the decomposition (eventually into a still-flammable powder or goo). Low temperatures can delay these reactions indefinitely. It is estimated that the great majority of films produced during the early twentieth century were lost forever either through this accelerating, self-catalyzed disintegration or studio warehouse fires. Salvaging old films is a major problem for film archivists (see film preservation).

Nitrocellulose film base manufactured by Kodak can be identified by the presence of the word Nitrate in dark letters between the perforations. Acetate film manufactured during the era when nitrate films were still in use was marked Safety or Safety Film between the perforations dark letters. Letters in white or light colors are print-through from the negative.

Color negative film was never manufactured with a nitrate base (Technicolor used monochrome negatives), nor were 8 mm or 16 mm motion picture film stocks.

Uses

  • 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, due to its non-specific affinity for amino acids. Nitrocellulose is widely used as support in diagnostic tests where antigen-antibody binding occur, e.g. pregnancy tests, U-Albumin tests and CRP.
  • 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. To this day it is used in Compound W Wart Remover as a carrier of salicylic acid, the active ingredient.
  • Collodion was also used as the carrier for silver salts in some very early photographic emulsions, particularly spread in thin layers on glass plates.
  • Magician's flash paper, sheets of paper or cloth made from nitrocellulose, which burn almost instantly, with a bright flash, and leave no ash.
  • Radon tests for alpha track etches
  • Nitrocellulose lacquer was used as a finish on guitars for most of the 20th century and is still used on some current applications. Manufactured by (among others) Dupont, the paint was also used on automobiles sharing the same color codes as many guitars including Fender and Gibson brands.
  • Nitrocellulose lacquer is also used as an aircraft dope, painted onto fabric-covered aircraft to tauten and provide protection to the material.
  • As a transportation medium for one-time pads, thus making the disposal of the pad complete, secure, and efficient.

Other uses

Depending on the manufacturing process, nitrocellulose is esterified to varying degrees. Table tennis balls, guitar picks and some photographic films have a fairly low esterification level and burn comparatively slowly with some charred residue. See celluloid.

References

See also

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