Synthetic Fibres

Rayon staple fibre is manufactured in the

same way as rayon filament yarn, differing only in the spinning and processing tech niques. The greatest volume of staple fibre is made by the viscose process, but it is also manufactured to an increasing extent by the acetate and cuprammonium processes. The spinning of staple fibre differs from the spinning of continuous filament yarn in that many more filaments are spun from one spinneret, which often contains thousands of minute orifices. As the filaments emerge from the spinning bath (or from the spinning chamber in the acetate process), they are collected in the form of tow, or a rope of filaments. In the acetate process, where no further treat ment of the filaments is necessary, the tow is cut into short fibres of the desired length and baled for shipment. In the viscose and cuprammonium processes, where the filaments require purification after spinning, the tow may be cut immediately as it comes from the spinning machine and all further processing done in bulk form. An alternative method is to process the filaments in the form of tow, passing them through the various baths and over rollers. The tow is cut at the last stage, while still wet, and the fibres opened up and dried. Still another method, which may be used by all processes, is to transform the tow directly into roving, break ing the filaments by a tearing or draft action, and keeping the staple filaments parallel.

Staple fibre has gained wide use through its adaptability and the variety of characteristics which may be imparted to it. The staple may be of any uniform size and length desired, so as to adapt it to the machines on which it is to be spun (the cotton, woollen, worsted, or schappe spinning system). The crimp of the fibre can be varied by differences in processing, by the use of resin finishes, or by mechanical means. The lustre may be bright, semi-dull, or dull, or a mixture of these lustres. Special strong staple is made, similar to the strong filament yarns for tire cords, while hollow-filament staple is supplied for other purposes. The variety of textures and dyeing effects which may be obtained by mixing rayon staple with other fibres, as well as the many successful fabrics of rayon staple alone, account for the tapid adoption of this new textile. It has been particularly valuable to countries which seek textile self-sufficiency (see following section Growth of Rayon Production).

Other Rayon Products.—Other rayon products, in addition to filament yarn and staple fibre, are rayon monofil (artificial horsehair), visca, and rayon waste. Rayon monofil is, as the name implies, a single round filament of comparatively large cross section, resembling horsehair. Visca is a similar single' fila ment, but flat rather than round, being spun through a rectangu lar instead of a round orifice. Both these products are used

chiefly in the millinery industry. Rayon waste includes spinning waste from the yarn producers, thread waste and fabric ends from rayon weavers and knitters, clips from garment cutters, and rags. Rayon waste is used chiefly for admixture with wool in the woollen and worsted industries.

Raw Materials.—The principal raw material consumed in the manufacture of rayon is cellulose, which is converted into a solution and spun into filaments. Cellulose is very plentiful, occurring in the cell structure of trees and plants. But only cotton linters and wood pulp made from spruce or western hemlock have been widely used for the production of rayon. The advantages of these sources are their abundance and consequent cheapness and the uniform quality of the cellulose obtainable from them. Pulp for rayon production has also been made from southern slash pine in the United States, beechwood in Germany, and reeds in Italy and Japan.

The wood pulp used in the United States is a special chemical grade of bleached sulphite pulp. Rayon-grade pulp contains about 90 to 95% alpha cellulose, with very rigid specifications as to the amounts of other cellulose and impurities which may be tol erated. The wood pulp is prepared with great care from selected materials and its quality is controlled by frequent chemical and physical tests. Wood pulp is used primarily in the viscose process of rayon manufacture. It is estimated that the world consump tion of wood pulp by the rayon industry in 1939 amounted to approximately i,ioo,000 short tons.

Cotton linters are the small fibres which cling to the cotton seed after the long staple, or lint cotton, has been removed. These short fibres are separated from the seed by a process simi lar to ginning. Those used for the rayon industry are selected with great care, after physical and chemical examination of their quality and suitability for the process in which they are to be used. These linters are purified and bleached under controlled conditions which guard against deterioration or contamination of the cellulose, giving a final product which is about 98% pure alpha cellulose. Cotton linters are the principal source of cellulose for the cuprammonium, nitrocellulose, and acetate processes, and for certain special grades of viscose rayon.

The rayon industry also consumes great quantities of chemicals (see the discussion of the individual processes of manufacture for the chemicals consumed in them). These chemicals also must pass rigid tests for uniformity and freedom from impurities which might affect the quality of the manufactured yarn.