Lustre.—The lustre of rayon yarn may be made bright, semi-dull, or dull, as desired. The bright lustre is "natural" to rayon yarn, the smooth Surface of the yarn reflecting the light. The incorporation of inert foreign materials, such as titanium di oxide or mineral oil, in artificial filaments reduces the lustre by virtue of the fact that the body of the filament becomes a heterogeneous, rather than a homogeneous, system. In bright yarn, light rays are affected by only one refractive influence, while in a delustred filament the light rays are affected by two or more refractive influences. In selecting the material to be used for delustring purposes, the chemist always seeks an inert, white, or colourless compound whose refractive index is as far removed as possible from that of regenerated cellulose (1.53) or cellulose acetate (1•47). The substance must also be so finely divided that it will not weaken the filament or clog the minute holes of the spinneret.
The development of these delustring methods, during 1926 and 1927, was the work of Glover and Heaven in England and Sing master in the United States. The discovery of the delustring process gave a great impetus to the growth of the rayon industry, tremendously increasing the variety of fabrics which could be ob tained by the use of rayon yarn, either by itself or in combination with other fibres.
Dyeing.—The dyeing of rayon yarn, at first beset with diffi culties, has been improved by the development of new techniques and new dyes, until rayon fabrics may now be. dyed with as fast and as even colours as any other fibre. Acetate rayon, being a chemical compound of cellulose and not affected by the usual textile dyes, required special research. But with dyes which have been created specifically for acetate rayon, this fibre no longer offers difficulties, while its resistance to ordinary cotton type dyestuffs makes it especially useful in creating cross-dye effects with other fibres.
Viscose, nitrocellulose, and cuprammonium rayon take the same types of dyes as cotton, though they differ from cotton and from each other in their relative affinity for specific dyes. This variation, while it may be overcome by dyeing techniques, is used frequently to obtain cross-dye effects with cotton, with other types of rayon, or with silk or wool, permitting a wide range of combinations of two or more fibres. Many dyes show a greater fastness on rayon than on cotton.
The dyeing qualities of the rayon yarns themselves have been greatly improved as the result of research. Unevenness in dyeing
was found to result from straining or stretching the yarn during spinning or processing, thus affecting the surface structure and consequently its affinity for dyes. Very careful control of the tension under which rayon is handled, especially when wet, has removed this difficulty. The great precautions which the manu facturers of rayon now take to insure absolute uniformity of raw materials, manufacturing procedure, and final processing produce a yarn with excellent dyeing qualities.
Rayon may also be spun-dyed, i.e., dyes may be incorporated in the spinning solution to produce a yarn of the desired colour. The variations in the style demand for colours, however, make this process somewhat impractical for commercial use except in the case of black yarn.
Staple Fibre.—In addition to continuous filament yarn, rayon is also produced in the form of short lengths of staple fibre, similar to cotton or wool. The wide-spread use of rayon in this form is a comparatively recent development, although it has been known for many years. From the beginning of rayon production, the waste from the spinning and processing of rayon filament yarn has been mixed with other fibres and spun, usually by the woollen or worsted systems of spinning. This waste was merely a by product and its volume was small.
The first manufacture of short rayon filaments as a primary product was in Germany during the World War. Rayon filaments were cut to suitable short lengths for spinning on cotton and wool machinery, to supplement the supply of the natural fibres. After the War, while rayon filament yarn was being rapidly improved and its markets expanding, the production of rayon staple fibre was neglected, and it was not until the late 1920S that interest in this other rayon product was revived. Since that time the growth of the rayon staple fibre industry has been phenomenal, the pro duction of staple in 1939 being nearly equal to the production of filament yarn. Italy, Germany, and Japan have been leaders in the increased production of rayon staple fibre, substituting it for cotton to increase their self-sufficiency and decrease the necessity for using their foreign exchange for the purchase of cotton. But the growth of staple fibre in the United States and Great Britain has also been rapid. In these countries, the new fabrics and style effects made possible by rayon staple have been very popular and the use of rayon staple fibre has given cotton mills an opportunity to enter new fabric fields.