Cuprammonium yarn today is usually spun by the stretch spinning method. The spinning solution is pumped through the spinneret, or nozzle, into a funnel filled with water which removes most of the ammonia and part of the copper, slowly coagulating the yarn. While in this plastic state, the filaments are stretched to the desired fineness. Emerging from the funnel of clear water, the filaments are completely solidified by a bath of mild sulphuric acid, which also removes most of the copper. The remaining acid and copper sulphate are removed by sprinkling with water. The yarn is washed in a soap bath, dried, and then treated with a second soap bath to soften the yarn, dried again, and packaged.
Rayon yarn made by this process can be spun with extremely fine filaments, finer than silk, and has great uniformity and flexibil ity. The yarn may be used twisted or untwisted, and fabrics made of it have the ability to withstand the same heat in ironing as comparable cotton fabrics. The quantity of yarn produced by the cuprammonium process was 4% of the total world production in 1939.
Viscose Process.—A third process for the production of rayon grew out of the researches on cellulose derivatives by C. F. Cross and E. J. Bevan. These men discovered and patented in 1892 a viscous solution resulting from the dissolution of cellulose xan thate in dilute caustic soda. The manufacture of a textile fibre from this solution was undertaken by C. H. Stearn and C. Topham while they were engaged in the spinning of lamp filaments of this material, but the technical problems of finding a proper solution and a satisfactory method for spinning the yarn required years of research. In 1898 Topham discovered the ageing process in pre paring the spinning solution, and in 1902 he invented his spinning box, a revolving cylinder in which the very tender filaments were coiled by centrifugal force as they were drawn from the spinning bath. In the same year (1902) Max Mueller discovered that it was possible to convert the cellulose xanthate into regenerated cellulose at the same time that the spinning solution was being coagulated into filaments, instead of by a separate operation. This discovery, permitting the spinning of a yarn strong enough to be wound directly on a spool, greatly stimulated the development of the viscose process. The commercial production of rayon yarn by the viscose process was started in 1905, the use of this process rapidly increasing and surpassing the other methods of making rayon.
In the manufacture of rayon by the viscose process, a special chemical grade of bleached sulphite wood pulp is usually used, although purified cotton linters or a mixture of linters and wood pulp may be used. Sheets of this purified pulp are steeped for about two hours in 18% caustic soda solution. Hydraulic rams then remove the excess caustic until the weight of the wet cellu lose, known as alkali-cellulose, is three times its original dry weight, thus insuring a set proportion of caustic to cellulose.
The alkali-cellulose is shredded to a fine "crumb" and stored in steel cans to age, after which it is treated with carbon bisulphide. The resulting product, cellulose xanthate, is dissolved in a solu tion approximately 4% caustic soda to form the viscose solution.
The preparation of this solution takes place under very exact time and temperature control. Many batches of this solution are blended in large tanks to insure uniformity. If a yarn of sub dued lustre is desired, small quantities of inert foreign mate rials, such as titanium dioxide or mineral oil, are incorporated in the viscose solution. The solution is then forced through a series of pressure filters to remove all dirt and other undesirable particles and is stored in tanks to deaerate and ripen for several days at a controlled temperature, until it is of the proper vis cosity for spinning.
The spinning solution, which contains approximately 7.25% cellulose and 6.5% sodium hydroxide, is extruded by pressure through the minute holes of nozzles, known as spinnerets, into a coagulating bath of warm water containing approximately o% sulphuric acid, 20% sodium sulphate, o.8% zinc sulphate, and 4% glucose. This bath must be kept circulating in large volume to keep its chemical content from being appreciably changed by the reaction between the acid and the spinning solution. The flow of the spinning solution through each spinneret into the spinning bath is regulated by a small pump, thus controlling the uniformity of the yarn. The yarn passes through the spinning bath at the rate of 7o to roo metres a minute. The spinnerets are made of a platinum-gold alloy, with the individual holes in a typical spin neret being about o•rmm. in diameter.