CHAPTER XX
ARTIFICIAL SILK—ITS PRODUCTION AND USE IN THE MANTLE INDUSTRY
The history of the artificial silk industry, since its foundation about the year 1890, illustrates curiously the rapidity with which isolated facts, of apparently merely academic interest, are seized upon and adapted to the needs of modern civilisation. It is during this period, especially, that the bonds between science and industry, in a dozen different directions, have been drawn so close that to-day it is in many cases impossible to differentiate the two. The pure science of to-day is the technology of to-morrow—and not always even of to-morrow, but of to-day. But we have moved even beyond this; the industrial needs of the day are creating and extending our science at a rate which shows how relatively poor a stimulus has been the mere desire for knowledge. Such has been the history of the artificial silk industry. No sooner had Chardonnet shown that the preparation of a new fabric was not only possible but profitable, than a thousand aspects of the problem were taken up. Patents were taken out on all sides—the majority, as usual, valueless, one or two of great importance. Companies were formed, factories built, machines invented; numberless applications were proposed, mostly again worthless, whilst patient research and innumerable experiments have carried one or two suggestions to a successful place in practice. Among these has been the adaptation of artificial silk to the manufacture of mantles, which will be outlined in the present chapter. Before taking up this question, however, a short account of the manufacture of the fabric itself must be given.
Chardonnet Process.
—In the Chardonnet process, an account of which was published about 1890, continuous fibres are obtained by forcing through tiny jets a viscous solution of collodion, or nitrocellulose, as it has been misnamed, in a mixture of ethyl alcohol and ether. In the original form of the process, the solution was forced into water, which, by removing the alcohol and ether, caused an instantaneous coagulation of the surface, so that a filament was obtained which could be wound directly on to a spool. More generally, however, the jets deliver the solution into a chamber through which warm air is passed; this is equally effective in removing the solvents and causing surface coagulation, and the filaments are woven directly into threads of ten to forty strands, according to the purpose for which the fabric is required, fifteen to twenty being used for silk from which mantles are to be made. On account of its inflammability, the thread is denitrated by means of a solution of ammonium sulphide.
The raw material for the process is cellulose, usually in the form of cotton. Treatment of this with a suitable mixture of concentrated sulphuric and nitric acids replaces some of the hydroxyl groups by the ‘nitrate radicle,’ NO₃, a mixture of various nitrates of cellulose being formed, in which the so-called tetra-, penta-, and hexa-nitrates predominate.[525] The product, cellulose nitrate or collodion, very closely resembles the original cellulose in appearance and structure. It is washed thoroughly to free it from traces of acid—which render it liable to explode spontaneously—and after drying, dissolved in the minimum quantity of the mixed solvents;[526] the solution is filtered from insoluble impurities through wads of cotton, pressures of thirty to sixty atmospheres being required. This filtration purifies and thoroughly mixes the solution, so that perfect uniformity is obtained in the product. The glass jets through which the solution is now forced, under a pressure of forty to fifty atmospheres, have a diameter of 0·08 mm., but the threads obtained contract on the removal of the solvents, so that fibres of 0·01-0·02 mm. are formed.
[525] The cellulose esters are usually named as if they were derived from a compound C₁₂H₂₀O₁₀, the formula for cellulose being (C₆H₁₀O₅)n. Thus the formation of the ‘hexa-nitrate’ would be represented—
C₁₂H₂₀O₁₀ + 6HNO₃ = C₁₂H₁₄O₄(NO₃)₆ + 6H₂O.
[526] In the Lehner process, in which collodion is also used, larger quantities of solvent are employed, so that much more dilute solutions are obtained; these require low pressures to form the thread, which is then hardened chemically.
Chardonnet probably began his work about 1885. It is interesting to observe that an Englishman, Swan, had proposed in 1883 to use a solution of collodion in acetic acid, fabrics prepared by his process being shown at the London Exhibition of 1884.[527]
[527] Vide Böhm, Zeitsch. angew. Chem. 1912, 25, 657. There is no account of this process in the English patent literature.