Cellulose Nitrates

nitrate esters of cellulose with the general formula [C₆H₇O₂(OH)_3—x(ONO₂)_x]_n where x = 1–3; they form a white, fibrous, loose mass very similar to cellulose in appearance. The most important characteristics of cellulose nitrates are the degree of substitution or the nitrogen content, which to a considerable extent determine the physicomechanical, chemical, and technological properties of the polymer. The major types of cellulose nitrates that have practical uses (the nitrogen content is given in parentheses) are colloxylin (10.7–12.2 percent), pyroxylin no. 2 (12.2–12.5 percent), and pyroxylin no. 1 (13.0–13.5 percent). A special type of cellulose nitrate was discovered by D. I. Mendeleev and called collodion (12.4 percent).

The density of cellulose nitrates is 1.58–1.65 g/cm³. The average degree of polymerization of colloxylin is 150–600 (molecular weight, 37,500–150,000), and of pyroxylins, 1,000–2,000 (molecular weight, 250,000–500,000). The solubility of cellulose nitrates depends on the nitrogen content; acetone is a universal solvent for the industrial brands. Cellulose nitrates are insoluble in water and such nonpolar solvents as benzene, carbon tetrachloride, and gasoline. They are not resistant to the action of acids and bases. Dilute mineral acids cause slow hydrolysis; bases saponify them and break them down.

Cellulose nitrates have low resistance to the atmosphere and to heat. Upon heating, they begin to decompose even at 40°–60°C, and the rate of decomposition increases rapidly with increasing temperature. Upon rapid heating, decomposition may result in a flash and explosion. The addition of stabilizers (diphenylamine and urea derivatives) to cellulose nitrates provides improved stability.

Cellulose nitrates are produced by nitration of pulverized dried cellulose by a nitrating mixture. The product is repeatedly washed with water and sodium carbonate solution and again with water and, if necessary, is dehydrated (for example, with ethyl alcohol). The commercial product is stored with a water or ethyl alcohol content of 20–40 percent.

The uses of cellulose nitrates are determined by the nitrogen content. Colloxylin is used in the production of celluloid, plastics (etrols), cellulose nitrate lacquers, glues, and enamels. It is used in small quantities as a base for motion-picture and still film. Pyroxylins are used in the production of smokeless powder, dynamites, and other explosives. The main drawback of cellulose nitrates is their flammability. They are being replaced by cellulose acetates and synthetic polymers.

Cellulose nitrates were one of the first polymeric materials; they were first obtained by the French chemist H. Braconnot in 1832.