After spinning, the yarn is washed free of the adhering spin ning bath solution, desulphurized by dissolving out the precipi tated sulphur, bleached (optional), washed, treated with oil or soap to soften it, and dried.
Three systems of spinning viscose rayon are in use—the centri fugal, bobbin, and continuous processes. In the centrifugal pro cess, which is more familiarly known as pot spinning, the yarn passes through the spinning bath, up over a rotating glass drawing wheel called a "godet," then downwards into a cylinder revolving on its vertical axis at high speed. This cylinder is called the pot (Topham spinning box) and the entering yarn is coiled by centrifugal force on the wall of the pot. The revolution of the pot, in addition, imparts a twist to the filaments and throws off through perforations in the wall of the pot much of the spinning bath solution which saturates the yarn. The size of the filaments is controlled by the relation between the rate at which the spin ning solution is pumped through the spinneret and the speed of the drawing wheel over which the yarn passes, the filaments being drawn to the desired fineness as they coagulate. The cake of yarn from the pot, which represents about one pound of finished yarn, is then wound into skeins, in which form it passes through the various purification treatments mentioned above. In recent times, developments have been made whereby some, or all, of these treatments are given the yarn in the cake form.
In bobbin spinning, the yarn is wound on a perforated bobbin as it comes from the bath and remains on the bobbin through the subsequent liquid purification treatments.
The most recent commercial development is the "continuous" process, whereby the yarn passes from the spinning bath, which is located at the top of the machine, down over a series of rotating spools arranged in a vertical bank. Each spool represents a stage in the processing treatment of the yarn, the thread running many times around each spool in a spiral fashion. Near the bottom of the machine the yarn is dried around a heated spool and wound upon a twisting spindle.
The approximate poundage consumption of raw materials per pound of viscose-process rayon yarn is as follows: wood pulp and cotton linters 1.15 to 1-25, caustic soda 1.4, carbon bisulphide 0.35,
sulphuric acid 1.7 to 1.9, glucose 0.5, and all other chemicals 0.4. In addition, from ioo to 2oogal. of pure soft water are re quired in the production of a pound of viscose rayon yarn. These figures represent gross consumption and do not take into account the recovery of chemicals used in the process of manufacture.
Rayon manufactured by this process has an affinity for the same dyes as cotton. Fabrics made of this yarn will withstand about the same amount of heat from a hot iron as will cotton fabrics of corresponding yarn size. Variations in the process of making yarn by the viscose process have been developed to give yarns with special qualities. An unusually strong yarn, made especially for the manufacture of tire cord fabric but also finding other uses, is now manufactured by subjecting the filaments to extensive stretching as they are spun. Slub yarns for the weaving of shantung-type fabrics are produced by fluctuating the rate at which the viscose solution is pumped through the spinneret. Con tinuous filament yarns with a fuzzy or roughened texture have also been made by cutting or abrading some of the outer filaments of the yarn.
The viscose process is the most widely used of all the methods of making rayon, accounting for 83% of the total world production of rayon in 1939. The greatest advantage of this process is the abundance and relative cheapness of the raw materials required.
Acetate Process.—The fourth method for making rayon, the cellulose acetate process, was very much later than the others in entering the field of commercial production. Cellulose acetate was produced by Naudin and Schutzenberger in 1865. Cross and Bevan experimented with it and patented a process for its produc tion in 1894. Bronnert, in Germany, experimentally made a textile thread from it in 1899. In the United States, A. D. Little, W. H. Walker, and H. S. Mork were granted a patent for producing textile fibres of cellulose acetate (this patent was eventually put into limited commercial production by the Lustron Company about 1919). The important discovery that cellulose acetate modified by partial saponification (treatment with caustic) was soluble in acetone, instead of the chloroform necessary to dis solve ordinary cellulose acetate, was made by G. W. Miles in 1903.