The products formed by the action of nitric acid are usually considered to be nitrocelluloses. It would appear that they are more correctly described as cellulose-nitrates, for analysis indicates the presence of the NO3 group, which is characteristic of nitrates, and not of the NO2 group, which is the feature of nitro bodies in general. Further, nitro compounds, when subject to the action of reducing agents, are converted into amido compounds, as is the case, for instance, with nitro-benzene, C6H5NO2, into aniline, C6H5NH2, or with nitro-naphthalene, C10H7NO2, which changes into naphthylamine,
But the nitric acid derivatives of cellulose are not capable of conversion by reducing agents into similar amido compounds. They have the following properties, which accord more closely with nitrates than with nitric bodies: alkalies remove the nitric acid; cold sulphuric acid expels the nitric acid, cellulose sulphates being formed; boiling with ferrous sulphate and hydrochloric acid causes the elimination of the nitric acid as nitric oxide (on which reaction a method for determining the degree of nitration of gun cotton is based). It is best therefore to consider them as cellulose nitrates. Several well-characterised cellulose nitrates have been prepared, but is an exceedingly difficult matter to obtain any one in a state of purity, the commercial articles being always mixtures of two or three. Those that are best known and of the most importance are the following:—
Cellulose Hexa-nitrate, C6H4O5(NO3)6. This forms the principal portion of the commercial explosive gun cotton, and is made when a mixture of strong nitric acid and strong sulphuric acid is allowed to act on cotton at from 50 to 55° F. for twenty-four hours. The longer the action is prolonged, the more completely is the cotton converted into the nitrate, with a short duration the finished product contains lower nitrates. This hexa-nitrate is insoluble in ether, alcohol, or in a mixture of those solvents, likewise in glacial acetic acid or in methyl alcohol.
Cellulose Penta-nitrate, C6H5O5(NO3)5, is found in explosive gun cotton to a small extent. When gun cotton is dissolved in nitric acid and sulphuric acid is added, the penta-nitrate is thrown down as a precipitate. It is not soluble in alcohol, but is so in a mixture of ether and alcohol, it is also slightly soluble in acetic acid. Solutions of caustic potash convert it into the di-nitrate.
Cellulose Tetra-nitrate, C6H6O5(NO3)4, and Cellulose Tri-nitrate, C6H7O5,(NO3)3, form the basis of the pyroxyline or solu
ble gun cotton of commerce. It has not been found possible to separate them owing to their behaviour to solvents being very similar. These nitrates are obtained by treating cotton with nitric acid for twenty or thirty minutes. They are characterised by being more soluble than the higher nitrates and less inflammable. They are freely soluble in a mixture of ether and alcohol, from which solutions they are precipitated in a gelatinous form on adding chloroform. Acetic ether, methyl alcohol, acetone and glacial acetic acid, will also dissolve these nitrates.
Cellulose Di-nitrate, C6H8O5(NO3)2, is obtained when cellulose is treated with hot dilute nitric acid, or when the high nitrates are boiled with solutions of caustic soda or caustic potash. Like the last-mentioned nitrates it is soluble in a mixture of alcohol and ether, in acetic ether, and in absolute alcohol. The solution of the pyroxyline nitrates in ether and alcohol is known as collodion, and is used in photography and in medical and surgical work.
One of the most interesting applications of the cellulose nitrates is in the production of artificial silk. Several processes, the differences between which are partly chemical and partly mechanical, have been patented for the production of artificial silk, those of Lehner and of Chardonnet being of most importance. They all depend upon the fact that when a solution of cellulose nitrate is forced through a fine aperture or tube, the solvent evaporates almost immediately, leaving a gelatinous thread of the cellulose nitrate which is very tough and elastic, and possesses a brilliant lustre. Chardonnet dissolves the cellulose nitrate in a mixture of alcohol and ether, and the solution is forced through fine capillary tubes into hot water, when the solvents immediately evaporate, leaving the cellulose nitrate in the form of very fine fibre, which by suitable machinery is drawn away as fast as it is formed. Lehner's process
is very similar to that of Chardonnet. Lehner uses a solution of cellulose nitrate in ether and alcohol, and adds a small quantity of sulphuric acid; by the adoption of the latter ingredient he is able to use a stronger solution of cellulose nitrate, 10 to 15 per cent., than would otherwise be possible, and thereby obtains a stronger thread which resists the process of drawing much better than is the case when only a weak solution in alcohol and ether is employed. By subsequent treatment the fibre can be denitrated and so rendered less inflammable.