The flexibility and toughness of the fiber are undoubtedly among the more important factors in the processing, finishing, and use of many garments and fabrics, but they are difficult to measure and are usually measured directly only where their effects are pronounced, such as, for example, in tire cords. Fortunately, however, cell-wall structure as interpreted from the X-ray angle seems related to flexibility and toughness.

We have found that a significant correlation exists between X-ray angles and the percentage of increase in skein strength of 36s, two-ply yarn, over twice the skein strength of 36s singles yarn. This, together with other preliminary data on flexibility, fatigue resistance, and the response of certain varieties in tire cord, indicates that a fiber that has good tensile strength and a large X-ray angle would be preferred to one with a higher tensile strength but a small X-ray angle.

Cell-wall structure as measured by the X-ray angle is closely associated with fiber strength as measured by the Pressley index. In fact, X-ray measurements can be substituted for directly obtained fiber-strength measurements in predicting spinning performance except where weather damage occurs to the fibers. Peculiarly enough, deteriorated fibers that have little strength can still be used for reliable X-ray measurement for cell-wall structure. Advantage is taken of this fact to evaluate the potential fiber strength of the cotton breeder's selections when bad picking weather has made direct fiber-strength measurements valueless to the breeder in informing him whether or not he has succeeded in obtaining desired fiber-strength combinations. Primarily, however, X-ray measurements are of greatest value when used together with fiber-strength measurements, not in place of them.

Norma L. Pearson, of the Department, has shown that varieties that characteristically produce long fiber tend to produce fine fiber—that is, the greater the inherited fiber length, the smaller the fiber. If, however, the length of a given variety is increased by the growing conditions such as unusually rich, moist soil, the relationship is reversed—the longer the fiber, the coarser it becomes. - Similarly, above-average inherited fineness and length tend to give neppy yarns of poor appearance, whereas environmentally induced fineness, usually associated with reduced fiber length, results in smooth yarn of good appearance. There are exceptional conditions where environmentally induced fineness has nothing to do with reduced length and good-yarn appearance. If the fibers are thin-walled, because of frost or other damage to the plant or boll after the fibers have fully elongated, but before the fiber wall is mature, such fineness may or may not be associated with shorter fiber and good-yarn appearance.

It is evident that the use value—that is, the quality of the textile fabricated from cotton—depends on the interaction of fiber properties. The size of the yarn is determined mostly by the length and size or fineness of the fiber, while the use value is in turn influenced by the fiber strength and structure. Strong fabrics must be made of reasonably strong fiber, whatever its length, but fabrics, like tire cord, that are flexed a great deal, may require fibers that can stand bending. The strength of coarse to medium-sized singles yarns can be largely accounted for by two properties, fiber length and strength. In the finer yarns and those where appearance of the yarn is important, fiber fineness is of increasing importance. In plied yarns requiring strength, toughness, and flexibility, added significance is attached to cell-wall structure as measured by the X-ray.

Within a variety, stronger yarn may be expected from a shorter fiber if the shorter length is induced by stress during growth. When comparing one variety with another under varying conditions of growth, the skein strength is usually greater as the length increases, although unusual fiber strength may overcome length differences, reversing this relationship. For certain varieties, Pima and S X P cottons, for example, there exists a negative relationship between fiber length and skein strength because the shorter S X P cotton is stronger; therefore, it is not surprising that the shorter but stronger fibers produced by stress during growth give a stronger yarn than the longer but weaker fibers from the same varieties. Thus, while fiber quality research has demonstrated that fiber properties are modified by growth conditions and other environmental conditions, varieties are characterized by distinctive combinations of fiber properties that should be recognized in marketing and manufacturing operations.

Recent research has by no means unlocked all of the secrets of cotton quality, but much has been done to help further improve the quality of the world's dominant fiber, to utilize more effectively varying fiber properties, and to improve the competitive position of American cotton.

THE AUTHORS

E. E. Berkley is a West Virginian who, for the past 10 years, has done research for the Department on the structure and strength of plant fibers. At present he is a fiber technologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering. Dr. Berkley is the author or co-author of numerous articles on the structure of cotton fiber and related subjects. In 1945 he was appointed associate editor of the Textile Research Journal, and in 1946 he was elected to the Executive Committee of the Division of Cellulose Chemistry of the American Chemical Society.