But the full development of this process on a commercial scale IA as the work of Henri and Camille Dreyfus, who made use of the acetone-soluble compound discovered by Miles. During the World War, the Dreyfus brothers• built a large factory at Spondon, Eng land, for the production of cellulose acetate to be used as non inflammable "dope" for covering aeroplane wings. A similar plant was started by them in the United States. When the end of the war destroyed the demand for the cellulose acetate "dope," Drey fus turned to the production of rayon as the most promising use for the large plant he had built, and after several years of research a commercially successful spinning process was evolved.
In making rayon by the acetate process, purified cotton linters which have been soaked in acetic acid are treated under carefully regulated conditions with a mixture of acetic anhydride and glacial acetic acid, together with a small quantity of concentrated sulphuric acid as a catalyst. A brine jacket around the reaction vessel keeps the temperature of the reaction under control. The complete conversion of the cellulose into a transparent solution takes from 5 to 8 hours. This "primary" or tri-acetate cellulose is diluted with water and acetic acid, and allowed to stand for Dr) to 20 hours at an increased temperature, during which tine the "primary" acetate is partially saponified to the "secondary" or acetone-soluble state. When this reaction has reached the desired stage, the secondary acetate is precipitated by pouring the solu tion into a large tank of cold water. The cellulose acetate flakes are washed in warm water several times, until free of acid, and dried.
The spinning solution is prepared by dissolving the dried cellu lose acetate flakes in acetone in a closed mixer until the solu tion contains approximately 25% cellulose acetate. From 4 to 12 hours, depending upon the type of mixing device, are required to dissolve the cellulose acetate completely. For dull yarns, a small amount of titanium dioxide is introduced into the machine during the dissolution period. The spinning solution is blended with other batches in a large storage tank, forced through a series of pressure filters, and stored to remove all air bubbles.
In the spinning operation, the acetone solution of cellulose ace tate is forced through the many fine holes of the spinneret down ward into an enclosed spinning chamber which is known as a cell. A rising current of warm air virtually completely evaporates the acetone by the time the filaments are drawn out at the bottom of the cell, and the yarn is ready for twisting and packaging for sale.
In producing a pound of acetate rayon, the pounds of raw materials required are approximately as follows : bleached cotton linters 0.70, acetic anhydride 2.40, glacial acetic acid 6.00, sulphuric acid 0.07, acetone 4.00, and titanium dioxide 0.02, as well as about i,000gal. of water. Taking into consideration the recovery of chemicals used in the manufacturing process, the net poundage consumption per pound of yarn is about as follows: bleached cotton linters 0.70, acetyl groups which combine with the linters plus 0% loss during recovery (expressed as acetic acid ) 0.54, sulphuric acid 0.07, and acetone lost during processing and recovery 0-40. The recovery of the acetone and the dilute acetic acid is vital to successful commercial production of rayon by this process.
Acetate yarn differs from the other rayon yarns in that it is a cellulose compound instead of pure regenerated cellulose. For this reason it has certain characteristics peculiar to itself. The suppleness and resilience of this yarn contribute good draping qualities and feel or "hand" to fabrics. It is more sensitive to heat and cannot stand too hot an iron ; this same quality makes it possible to give a fabric made of acetate rayon a permanent moire effect. Acetate rayon absorbs less moisture than the other types of rayon and, therefore, when wet, swells less and retains more of its original strength. The dyeing of acetate rayon is accomplished with special dyes which have been developed for it, while the fact that acetate rayon is not affected by ordinary dyes makes it very popular for cross-dyeing.
The production of rayon by the acetate process, though late in entering the field, has increased rapidly until in 1939 it accounted for 13% of the world production and 3o% of the United States production. Although the raw materials used in this process have a high initial cost and the necessary recovery processes are ex pensive, there are many advantages in the simple spinning opera tion. The yarn needs no washing or other finishing operations, which not only eliminates the expense of such processing, but in creases the proportion of first-grade yarn, inasmuch as the yarn is not exposed to injury by handling during finishing processes. Such spinning waste as does occur may be dissolved in acetone and mixed with the spinning solution to be re-spun.