What Makes Cotton Good?

THE FARMER who has an indifferent local market will say that quality in cotton means a high yield and ease in picking. To the cotton broker, quality may mean above-average classer’s grade and staple length. The technologist may define it as a strong, reasonably long, and fine cotton. The spinner’s definition may include a smooth running, uniform cotton that produces a nep-free, strong yarn. The finisher looks for something that will bleach and dye uniformly. The consumer wants a product that will survive repeated washing and ironing.

All these are abstractions that are difficult to measure, but even so the cotton breeder has been able to perfect new, productive varieties that embody these diverse and poorly defined qualities.

The starting point is the development of rapid and reliable instruments and techniques to measure the properties of fiber and relate them to use value. Studies of the past decade have centered around cooperative regional plantings that would give us material for investigating the way in which fibers are formed, the period of fiber elongation, and how the cell wall develops. The investigations have aided greatly in interpreting influences of heredity and environment on the properties of fiber and how they affect the quality of the finished product.

At least four characteristics have a major bearing on quality of cotton: Length, strength, fineness, and structure of the fiber wall. The cotton breeder and the industry can now measure these factors quickly.  Laboratory technicians can compile accurate data that will show the relation of specific qualities to the strength and appearance of yarn, and other manufacturing needs; evaluate the influence of heredity and place or condition of growth on the development of these properties; help the cotton breeder or the cotton grower make improvements in these qualities, and guide the manufacturers in choosing and using such properties to make goods of desired quality. The cotton breeder must study thousands of selections to obtain the best combinations of agronomic and fiber quality factors. He can now get accurate information on the fiber properties of these selections; in the past he had to depend on “feel,” judgment, and guess.

Fiber length is quickly measured by a photoelectric instrument known as the Fibrograph. Fiber strength is measured by the Pressley strength tester. Fiber fineness up to now has been measured tediously by weighing fibers of known lengths. A new instrument, the Arealometer, provides a rapid way to measure fineness, expressed as specific surface or cm²/mgr.  The cell-wall structure is measured by X-ray technique and expressed as X-ray angles: The greater the angle between the fibrils or cellulose strands and the long axis of the fiber, the lower the fiber strength, and vice versa.

Cotton fiber is made up of a multitude of fine, threadlike strands of crystalline cellulose glued together by other cellulose molecules that are not a part of the crystals, that is, amorphous cellulose. These threads are placed in the cell walls in a spiral like the strands of a rope. The cellulose strands reverse their spirals at intervals along the fiber, and the pitch at which they are deposited in the fiber can be determined by the X-ray.  The angle between the long axis of the strands and that of the fiber is indicated by the size of the arc that is measured as the X-ray angle.

There are other properties, largely subdivisions of the four major ones listed. They cannot yet be measured so quickly, or they lack definite end points or precision of determination. Hence their exact importance in fiber quality is not so clearly established. For example, cell-wall thickness, one of the attributes of fineness, is hard to measure, and is reported unsatisfactorily as “percent of fiber with a given lumen-wall ratio.”  To some extent, the extremely thin-walled fibers and perhaps the oversized thick-walled fibers influence the processing, finishing, and wearability or utilization of the product. New and improved methods of measuring fiber wall thickness and fiber perimeter are needed before their effects can be accurately evaluated.

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.

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.

H. D. Barker, a pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering, has devoted most of his life to cotton research in the Department. He has spent about 12 years in Haiti making scientific studies of cotton and other plants of that country.

FOR FURTHER READING
Anderson, Donald B., and Kerr, Thomas: Growth and Structure of Cotton Fiber, Industrial and Engineering Chemistry, volume 30, pages 48-54, 1938.
Hertel, K. L.: Cotton Fiber-Length Determination Using the Fibrograph, American Society for Testing Materials Bulletin, pages 25-27, August 1942.
Pearson, Norma L.: Neps in Cotton Yarns as Related to Variety, Location, and Season of Growth, U. S. D. A. Technical Bulletin 878, 1944.
Pressley, E. H.: A Cotton Fiber Strength Tester, American Society for Testing Materials Bulletin, pages 13—17, October 1942.
Sisson, W. A.: X-ray Analysis of Textile Fibers: Part V, Relation of Orientation to Tensile Strength of Raw Cotton, Textile Research, volume 7, pages 425-431, 1937.
Sullivan, R. R., and Hertel, K. L.: Surface per Gram of Cotton Fibres as a Measure of Fibre Fineness, Textile Research, volume 11, pages 30-38, 1940.

RESEARCH ON COTTON has proved that different varieties of it behave alike in some ways, but there are differences in structure, strength, fineness, and uniformity of the fiber of different varieties and species. Soil moisture, temperature, fertility, and other factors also have a bearing, but variety is by far the most important factor in cotton. In some cases, the strength of 22s yarn from different selections of about the same length and staple differ as much as 40 percent.

Manufacturers are now buying cotton on the basis of variety and they select varieties that meet their specific needs. To help cotton breeders select and increase seed for a desired purpose, the Department makes available to them a fiber and spinning-test service. The breeding work and studies of inheritance by the Department in cooperation with State experiment stations have produced strains that surpass many now being grown. In the above picture, a plant breeder selects individual plants from the breeding plot for yield, quality, and other superior characteristics.

A new strain developed at the Shafter, Calif., Experiment Station yields fiber 50 percent stronger than the variety now widely grown in that area.  Another, developed at State College, N. Mex., is 35 percent stronger than other local strains. Other good, strong strains are being perfected elsewhere and will go into production when they have been proved well adapted and better than common varieties in yield, resistance to diseases, insects, and so on.  In the photo at left, J. B. Dick of the Stoneville, Miss., station, studies the characteristics of new varieties of cotton.

The place where cotton is grown also affects its quality. Tests have demonstrated that the variety of cotton that produced the strongest yarn when grown at Florence, S. C., also produced the strongest yarn when grown at Stoneville, Miss., and College Station, Tex. Likewise, the kinds that produce weak yarn at one location will produce weak yarn when grown elsewhere.

In order to get mass production of the better varieties of cotton, a one-variety community plan was initiated by the Department some years ago under which large supplies of seed are made available to all growers in a community or area. All growers in a neighborhood agree to plant the same kind of cotton so that the community gin (below) can limit its operations to the adopted variety. Thus, there is no mixing of seed or lint at the gin; it is easy to maintain pure seed stocks and at the same time supply a large volume of uniform cotton. In 1945, over 5½ million acres of the more than 7 million acres in one-variety production were limited to only four types.

Breeders of cottonseed are also cooperating in the one-variety program. They are reducing the number placed on the market and sometimes keep the original variety names even when the kind is improved.

Another step toward reducing the number of varieties is the breeding of new high-yielding strains of medium-length staple, good spinning quality, and equal adaptation to wilt or non-wilt soils. Thus, the same variety can be used for standardized production in much larger areas than heretofore in the eastern area, regardless of the presence of wilt; higher yields and better cotton are produced.

Almost 40 percent of our acreage and more than 45 percent of the production in 1945 were in one-variety communities. As communities are combined into larger areas growing a single kind of cotton, the cotton trade and manufacturers can obtain increasingly large pure lots of cotton of the same variety. In Georgia and Alabama, where careful estimates have been made, the extra income to growers above what they make under usual methods averages up to $11 an acre.

The cotton breeding work of the Department is under the direction of H. W. Barre, head of the Division of Cotton Crops and Diseases, Bureau of Plant Industry, Soils, and Agricultural Engineering. Other phases of this important American crop are being studied in the Department and elsewhere—marketing of cotton, for example, the pests that attack it, new uses, fertilization, and economics of cotton production.

COTTON-TESTING EQUIPMENT

Aids to the cotton breeder and grader are instruments that quickly and accurately measure cotton fiber for length, strength, and structure.  Above, left, Chester Chew measures fiber length with the fibrograph. At right Marion Simpson runs a test on the Pressley fiber-strength machine, while (lower, left) Martha Chamblin measures fiber structure with the X-ray diffraction unit. Earl E. Berkley (lower, right) tests resiliency of cotton to determine its ability to resist injury from packing or crushing. An article by Dr. Berkley appears on page 369 [above -ASC].