Nitrocellulose Process.—The development of rayon as a textile really began with the work of Count Hilaire de Chardonnet, who is frequently called the "Father of the Rayon Industry." Char donnet began experiments on the making of an artificial textile in 1878, and in 1884 produced his first fibre from a nitrocellulose solution of pulp derived from mulberry leaves, coagulating the filaments in heated air. At the Paris Exposition in 1889 he ex hibited articles made from these artificial fibres and secured the financial backing to build a factory at Besancon, France, where the first commercial production of rayon began in 1891. Chardonnet's process was modified somewhat by Lehner, who used a solution of alcohol instead of warm air to coagulate the filaments.
The Chardonnet process for making rayon is used to a very small extent today. Cellulose in the form of loose purified cot ton linters is treated in a pot or tank with a mixture of concen trated nitric and sulphuric acids to form nitrocellulose of ap proximately 11.8% nitrogen content. The contents of the nitrating pot are discharged into a centrifuge and wrung for several minutes to remove the bulk of the adhering nitrating acids. The nitro cellulose is next subjected to a series of washing treatments and is then boiled in a dilute acid solution to destroy the unstable sulphuric acid esters. Finally it is centrifuged to a moisture content of approximately 25% and in this condition is dissolved In a mixture composed of 4o parts alcohol and 6o parts ether. The solution, known by the classic name collodion, is then deaer ated and filtered before being spun.
There are two methods of spinning nitrocellulose yarn, wet spinning and dry spinning, but the wet spinning system has never achieved great commercial importance. In the dry spinning process, the collodion is forced through individual glass capillary tubes and the filaments are drawn up and collected on a spool. During the passage of the filaments from the capillary tubes to the spool, all of the ether and a large part of the alcohol are evapor ated, the vapours being drawn away by vacuum to a recovery system.
In the form in which it is thus spun, the yarn is still chemically nitrocellulose and must be denitrated in order to regenerate the cellulose. This is accomplished by treating the yarn, under care fully controlled conditions, in a solution of sodium hydrosulphide or ammonium sulphide. The yarn is then washed, bleached, and dried.
The nitrocellulose process has the advantage of being compara tively simple, with a stable spinning solution and without involv ing complicated timing of chemical reactions. The yarn also is
comparatively strong when spun, which reduces waste from injury during handling (especially important in the production of fine deniers). But the cost of the denitration operation and the com paratively slow spinning speed (much smaller output per machine than in other processes) have caused the use of this process to decrease in recent years, until in 1939 less than 1% of the world's rayon output was produced by the nitrocellulose process.
Cuprammonium Process.—A second process for the production of artificial fibres from cellulose, the cuprammonium process, was patented in 1890 by a French chemist, Louis Henri Despeissis. The discovery that cellulose could be dissolved in a cuprammonium solution and then precipitated with sulphuric acid is usually credited to Schweitzer (1857), although there is some evidence that John Mercer had already performed this operation. And in 1882 Weston had patented a process for making filaments from a cuprammonium solution of cellulose, but, like Swan's discoveries in the nitrocellulose field, these filaments were developed in connection with the manufacture of electric light bulbs and were not exploited for textiles. Despeissis himself did not put his patent into commercial production. Several German chemists (H. Pauly, M. Fremery, E. Bronnert, and Urban) developed modifica tions and improvements on this process and put it into commercial production about 1898. This process of rayon production was com paratively expensive and was falling into disuse when Elsaesser further developed the method of stretch spinning (which had been patented by Edmund Thiele) and put the new process into produc tion in 1919. Stretch spinning permitted the manufacture of very fine filaments and revived the use of the cuprammonium process.
Purified cotton linters are commonly used as the cellulose base for the manufacture of rayon by the cuprammonium process. These linters are mixed with copper hydroxide or carbonate in a shredding machine and the salts washed out of the mixture in a hydraulic press. The dissolving of the cellulose mixture in an aqueous solution of ammonia, containing some sodium hydroxide and usually some glucose or lactic acid, takes place at a constant temperature. This solution must be prepared under exact chemical control to give a spinning solution of the proper viscosity and cellulose content. The solution is then carefully filtered and stored in tanks to remove all air bubbles (air bubbles in a spin ning solution would cause a break in the filament being spun).