Cellulose nitrate is a synthetic polymer derived from cellulose, the main component of plant cell walls. It holds a significant place in industrial history as one of the very first synthetic polymers developed, often considered the precursor to the modern plastics industry. Known commercially as nitrocellulose, collodion, and celluloid, its unique properties allowed it to transition from explosive use to widespread consumer and industrial applications.
Chemical Composition and Manufacturing
Cellulose nitrate is produced through a chemical modification process called nitration. Purified cellulose fibers, such as cotton linters or wood pulp, are treated with a powerful mixture of concentrated nitric acid and sulfuric acid. The sulfuric acid absorbs the water produced during the reaction, preventing the dilution of the nitric acid.
This process replaces the hydroxyl (-OH) groups on the cellulose molecule with nitrate (-NO2) groups, chemically transforming the structure into a cellulose nitrate ester. The resulting material’s properties, including its stability and flammability, are determined by the degree of nitration, which is measured by its nitrogen content. A lower nitrogen content, typically between 10.5% and 12.6%, yields a product suitable for lacquers and plastics, while a highly nitrated form above 12.6% is used for propellants and explosives.
Key Historical Applications
One of the most significant early applications was the creation of Celluloid, a thermoplastic material made by combining cellulose nitrate with camphor as a plasticizer. Celluloid was the first plastic that could be molded and shaped using heat and pressure, leading to its use in countless consumer goods. It was commonly used to imitate expensive natural materials like ivory and tortoiseshell for items such as billiard balls, combs, collars, and jewelry.
The material also revolutionized the fields of photography and cinema, where it was adopted as the base for early motion picture film and X-ray film, often referred to as “nitrate film.” Its transparency, strength, and flexibility made it ideal for the new medium of moving images, and it remained the industry standard until the 1950s. Before its use in film, a highly nitrated form was dissolved to create collodion, a viscous liquid used initially as a liquid bandage and later for wet-plate photography.
The most energetic application of the compound was in the military as “guncotton,” a highly nitrated form first patented in 1846. Guncotton was a replacement for black gunpowder, and its development allowed for the creation of smokeless powder, a more stable and efficient propellant for firearms and artillery.
Inherent Instability and Degradation
The primary hazard of cellulose nitrate stems from its inherent chemical instability and its nature as an oxidizing agent. The nitrate groups within the polymer structure contain oxygen, meaning the material does not require atmospheric oxygen to sustain combustion. This property makes cellulose nitrate fires extremely difficult to extinguish once they have started.
Over time, cellulose nitrate objects, particularly nitrate film, undergo a self-catalyzed breakdown known as decomposition. This degradation process is accelerated by exposure to high temperatures, ultraviolet light, and high humidity. As the material breaks down, it releases nitrogen oxide gases, which react with moisture in the air to form acidic compounds like nitric acid. The presence of these acidic gases further promotes the decomposition of the remaining material in a vicious cycle.
In advanced stages of deterioration, the material can spontaneously ignite, even without an external heat source, due to the accumulation of heat from the exothermic degradation reaction. The gases released from burning cellulose nitrate, including nitrogen dioxide, are also highly toxic and can be fatal if inhaled.
Modern Status and Niche Uses
Due to its significant instability and flammability, cellulose nitrate has been largely replaced in most historical applications by safer alternatives. For instance, the film industry transitioned to less hazardous materials like cellulose acetate and polyester bases for motion picture film.
Despite its replacement in consumer goods, cellulose nitrate still maintains specific niche uses today under controlled conditions. It is utilized as a propellant in military and rocketry applications, where its energetic properties are purposefully harnessed. In the civilian sector, its excellent film-forming ability makes it a component in certain lacquers and varnishes, particularly for wood finishes and automotive paints. Furthermore, porous cellulose nitrate membranes are widely used in laboratory settings for filtration and as a substrate for immobilizing biological materials in diagnostic tests.