FINENESS and Maturity are Important Elements in Cotton-Fiber Quality

Strength of cotton fiber is an important factor in the strength of yarns and fabrics, although in the past its importance may have been over-emphasized. It is generally less recognized that fineness and maturity of fiber are also important elements which materially influence the strength and other properties of the manufactured products.

Fineness refers to the width or the cross-sectional size of the fiber.  This differs greatly among fibers of American upland cotton (fig. 30).  Methods of measuring fiber fineness generally involve determination of either the so-called "diameter" (in the case of cotton fibers the "ribbon width"), or of the weight per unit of fiber length (approximately proportional to the average area of cross section of the fiber wall). In general, the latter determination is the more advantageous as the resulting measure is more nearly comparable with that for yarn fineness.

Maturity, on the other hand, refers to the fiber-wall thickness, or, more accurately, to the ratio of actual wall thickness to the maximum wall thickness that is possible if the cotton fiber were permitted to reach its maximum growth. Figure 31 shows American upland fibers of varying thickness of walls. It will be evident that due to different degrees of fineness, actual wall thickness may vary for the same degree of maturity.

Fineness has long been recognized as an important element of quality in wool, silk, and more recently, rayon. It has not received the same recognition in the case of cotton. Possibly the close relationship between fiber fineness and staple length in cotton made the distinctive effects of these two properties less noticeable, since the longer staples generally meant finer fibers. The distinction was demonstrated by studies in which long-staple sea-island cotton, which possesses the greatest degree of fineness of any cotton, was cut into shorter lengths to simulate 15/16- and 1-inch cottons of natural growth which are normally less fine. The 22s yarn spun from the 1-inch staple cut from this sea-island cotton showed an average skein strength of 146 pounds, a figure 51 percent higher than the average of a large number of American upland cottons naturally of this staple length and 27 percent higher than the strongest yarn ever manufactured from this staple length group in the spinning laboratory of the Bureau of Agricultural Economics.

FIGURE 30.—Variations in fineness of fibers from American upland cotton. a, very fine mature fiber; b, fine mature fiber; c, medium mature fiber; d, coarse mature fiber. X 475

FIGURE 31.—Variations in maturity of fibers from American upland cotton. a, very immature or thin-walled fiber; b, immature or thin-walled fiber; c, mature or normally developed fiber; d, abnormally matured or over-thickened fiber. X 475

The relationship of fiber fineness to length, however, holds only in a general way. Fineness of fiber has been found to vary materially from fiber to fiber of the same length, and from length to length of the same sample; it varies also with variety, soil, and growth conditions of the plant.

Why Fineness is Important

Fineness is important (1), because it determines the pliability of the fiber; that is, its ease of bending. Anyone who has examined yarns or fabrics made of fibers such as sisal, hemp, jute, and horsehair recognizes their stiffness and coarseness and their general lack of adaptability and usefulness for certain purposes. For example, cloth made of such coarse fibers is not very suitable for clothing; it is heavy, harsh, and irritating to the skin. With increasing coarseness of fibers, rigidity and stiffness increase much more rapidly than does the size of the fiber. For example, for a given shape of fiber if the size is doubled, the rigidity and stiffness is approximately quadrupled. If the size is tripled, the rigidity is increased nine times. Thus fineness, as measured by the weight per unit of fiber length, has a magnified influence on the flexibility of the fibers and presumably also on the softness and flexibility of yarns and fabrics made from them.

Fineness of the fiber is important (2), because it determines the average number of fibers in yarn of given count and in turn the yarn strength. This is because the count or size of a yarn is based on the weight per unit of length and a definite length always contains a definite weight of fibers. Therefore, the finer the fibers, the greater the average number in sections of the yarn. The average number of fibers per section of yarn seems to influence yarn strength in three ways: (1) Through their greater flexibility, the finer fibers, when twisted, have greater binding power and the frictional potentialities can be used to greater degree. (2) A given number of fine fibers will make a finer yarn than the same number of coarse fibers. In the illustration above cited of the short-staple cottons made artificially from sea island, the 15/16-inch cut fiber could be spun easily into 60s yarn of very good strength, an achievement not previously duplicated, so far as is known, with cottons of this natural staple length. Frequently cottons of 1⅛ or even 3/16 inches in staple are spun into 60s yarn only with difficulty. The success of the results with the sea island was undoubtedly associated with the larger average number of fibers in the section of yarn than would have been present in the usual cotton of 15/16-inch staple length.  (3) The surface substance of the fibers seems to be stronger than the interior substance, due to a "skin effect", and consequently the finer fibers, having proportionately more sur- face, should contribute greater strength to the yarn.

   Fineness of cotton fibers is dependent on two major factors.  One of these is the natural or inherited tendency of the fibers.  Just as some breeds of horses are naturally larger than other breeds, so some species and varieties of cotton have naturally larger, coarser fibers than other species and varieties. For example, sea-island cotton belonging to a different species than the usual American upland varieties has naturally a finer fiber. It is entirely probable that the natural fineness of cotton fibers may be materially altered by breeding.

Growth Factors in Fineness

   The second factor that determines the fineness of cotton fibers is that of growth. Al factors such as soil, moisture, plant food, climate, and the like, which affect plant growth neay be expected to influence also the thickness of the fiber wall. This is the effect of maturity on fineness. During its first 25 to 30 days of growth a cotton fiber elongates rapidly but its walls remain very thin, The type of growth then changes and during the next 25 to 30 days the length changes but little, but the walls thicken by increase of their secondary deposit. If this second period of growth is arrested, or if the climatic conditions restrict it, the fiber will not produce as thick a wall as it otherwise would have done. If only a small amount of secondary deposit is laid down, the wall will be thin and the fiber relatively immature and fine. However, if conditions of growth are favorable, deposition of cell-wall substance will continue and the wall will become thicker and the fiber relatively more mature and coarser. Relatively fewer of the well-developed mature fibers will be required in the cross section of a yarn of given size, than of the lesser developed, immature fibers.

Although cotton fibers from varieties that normally produce medium or coarse fibers may be fine as a result of immaturity alone, this type of fineness is not necessarily advantageous from the standpoint of ease of spinning and quality of yarn. Too great fineness from this cause may introduce distinct difficulties into the spinning processes, and contribute to nep formation and to unsatisfactory dyeing properties of yarn and fabric. Thus, while a given degree of fineness corresponds always to the same average number of fibers in a yarn of given size, there is a qualitative difference in fineness that depends upon the thickness of the fiber walls. Because of the flattened form of its cross section, an immature fiber should be, theoretically, much less rigid or stiff than a mature fiber of the same wall cross section. Perhaps this explains the seemingly greater tendency for thin-walled cotton fibers to form neps as compared with thick-walled fibers.

From the theoretical standpoint and assuming identical composition, it might be assumed that a yarn made from immature fibers should possess the same strength as one made from mature fibers, fineness and other factors being the same. Or, if the greater flexibility of the thin-walled fibers is advantageous, the yarn made from immature fibers might be even the stronger. Limited observations indicate that this relationship is by no means simple and that considerable work will have to be done before the relationship of fiber maturity to yarn strength can be determined.

ROBERT W. WEBB and CARL M. CONRAD, Bureau of Agricultural Economics.