The Scientist Looks at Cotton

James N. Grant.

Cottons grown in the different parts of the world vary in color, maturity, size, length, and strength of fibers. Soil, climate, and the species of cotton cause the variations.

Fibers may be short and coarse, as in the Asiatic varieties. They may be long and fine as in Egyptian and sea-island varieties. They may be intermediate in length and size, as in the American upland varieties, which constitute the great bulk of cotton produced in the United States.

This diversity and the demand on textile products to meet definite requirements in specific uses necessitate the classification of cotton by quality.

The conception of quality depends somewhat on individual interests. In the trade, the cotton classer judges quality by what he can see with his eyes or feel with his hands. The textile-mill superintendent thinks of quality in terms of fibers that meet requirements of his special product. The consumer judges quality by the appearance or serviceability of the product manufactured from the cotton.

In the laboratory, the textile technologist thinks of quality in terms of the chemical or physical properties of the fibers and the potentialities of those properties in putting out superior products. Because the chemical composition of most cotton fibers is nearly the same, quality is usually defined in terms of the physical properties of the fibers--length, strength, fineness, maturity, or color.

Qualities of raw cotton fibers evaluated in commercial classification are identified under such terms as staple length, character, and grade. The values assigned according to accepted scales of measurements or specified terms of description determine the relative market worth of the cotton and denote its useful attributes.

Of the many physical properties of cotton fibers, staple length is the only one assigned a concrete value in commercial classification. To the cotton classer, staple length represents the length of only a typical part of fibers he has segregated and straightened between his thumb and forefinger. From the several thousand fibers in his hands he estimates the staple length of a cotton. Uniformity in making this selection requires great skill. Evaluation is made by comparison with official standards provided for three types of cottons American upland, American Egyptian, and sea-island. Official standards of length for American upland are available in 20 intervals over the range from 3/4 to 1 1/2 inches. American Egyptian and sea-island have 4 intervals over the range from 1 1/2 to 1 3/4 inches, with provision for estimating beyond the official standards. The length evaluation is the basis for international classification of cotton for export and import. Such a selection is reasonable, because length often imparts fundamental information on other properties of the fiber, such as strength and fineness. The general tendency, for instance, is for long fibers to be very fine, with high tensile strength. The uses of fibers depend upon these basic properties.

Character of cotton fiber is based on strength, fineness, maturity, elasticity, and many other inherent physical properties, together with uniformity of fiber-length distribution. Some terms that designate character are weak, strong, soft, wasty, perished, irregular, and normal depending on the deviation from the normal cotton. Character terms are relative rather than absolute as compared to staple length, but they provide essential information where strength, dyeing qualities, or spinning properties of the fibers are the critical factors.

Evaluations of cotton fiber under grade are associated with the history of the fiber from opening of the boll until packing in the bale. Grade combines a visual classification of the color, the amount of foreign substances entangled in the fibers before and during harvest, and the evenness with which the fibers were ginned. Long exposure to intense sunlight produces changes in color and gloss. Stalks, hulls, leaves, or other foreign material entangled in the fibers may increase cleaning damage and cost before processing. High moisture content of seed cotton at the gin, as well as improperly operated ginning equipment, can result in gin damage or cause the fibers to be left in small matted tufts.

THE THREE GENERAL classifications are satisfactory in the evaluation of cotton for purchase or selection for general uses. But that type of information is of little use to breeders, who desire specific values for many physical properties of their cottons, or to manufacturers,who require correlation of physical properties of fibers with product performance. Definite values for individual fiber characteristics, such as strength, fineness, surface characteristics, maturity, and length uniformity, as well as detailed knowledge of chemical structure of the fiber, are essential to an understanding of the mechanical behavior of cotton. The characteristics determine durability, appearance, dyeing properties, and other qualities of interest to consumers.

The desire for specific values indicative of fiber-length distribution in a sample to replace dependence on judgment from visual examination has resulted in the development of several instruments for the purpose. Mechanical separation of fibers into groups of 1/8 -inch intervals is more time-consuming than optically scanning the fibers in a beam of light, but results from the mechanical method give more detailed information. Both methods are extensively used in breeding programs, in which knowledge of lengths of individual fibers and the distributions of the fibers by length is helpful in predicting the demand for a new variety. Length of individual fibers in samples of cotton range from a small part of an inch to more than the staple length. The greater part by weight is always less than the designated length. Individual fibers in varieties of American upland cottons often exceed 2 inches and in sea-island 3 1/2 inches, but the number ever attaining these unusual lengths is small. Cottons with the higher proportion of fibers of the same length group have better manufacturing characteristics the breakage due to long fibers is lessened. Experience indicates that an increase in lengths of fibers of the same weight fineness (weight of cellulose per inch of fiber), without change in strength, increases the strength of yarns by reducing the number of discontinuities between fiber ends, and that interspacing short fibers with longer ones increases yarn size without proportionally increasing its strength or durability. Extremely long fibers, however, add to processing difficulty because they have a greater tendency toward neppiness and are more difficult to separate and parallelize on textile machinery without breakage of individual fibers.

Although an increase in fiber length is often associated with an increase in yarn strength (through the fact that use is made of a higher proportion of the fiber strength corresponding to the greater length) , it is the inherent strength of the fiber that is fundamental to strength in a yarn or manufactured product.

The inherent strength of the fiber depends on the deposition of spiral layers of cellulose in the cell wall during the growth of the fiber. The angle of these spiral layers of cellulose with the fiber axis, determined from X-ray patterns of masses of fibers, is closely associated with fiber strength.

Individual fiber strengths range from 1/400 to 1/30 pound; the average is about 1/100 pound. When fibers are tested in small compressed bundles, however, their bundle tensile strengths range between 50,000 and 100,000 pounds to the square inch, the higher strength being commensurate with the tensile strength of steel. American upland varieties produce fibers intermediate in this range, with strength averaging about 78,000 pounds to the square inch.

The three common mechanical methods for determining strength are round-bundle, flat-bundle, and individual-fiber breaking load. In the round-bundle test, comparison of cottons is based on the strength per unit of the cross-sectional area of the bundle, expressed as pounds per square inch. From the flat-bundle method, the ratio of breaking load to weight of fiber tested furnishes essentially the same information as the round-bundle test but in less time. Neither of these methods gives the fundamental knowledge of elastic properties or variation between fibers obtained in the individual-fiber method.

As a complement of strength, the elastic properties of cotton fibers (determined on the individual fiber) influence their usefulness. The individual fiber elongates rapidly when load (less than 1/1000 pound) is first applied, because of its natural twists and convolutions and the kinks that result from the condition of the fiber in the boll. After the rapid initial elongation, the fiber elongates very slowly and finally breaks. At break, the length increase may be as much as 6 to 10 percent. If the load is removed before break, only a fractional part of the elongation is recovered. Strength and elongation of cotton fiber depend in general upon the variety of the plant and growth conditions. The longer fibers in a variety usually have greater strength and elongation.

In contrast to elongation and recovery, which are considered in a direction parallel to the fiber' axis, such properties as flexibility and brittleness are judged by ability of the cotton fiber to bend in directions perpendicular to the axis. Performance tests on the textile product indicate that the flexibility of cotton fiber is satisfactory and superior to that of other natural cellulosic textile fibers. However, this evaluation is a composite with other fiber properties and fabric construction.

Closely associated with other properties, but undesirable in a textile fiber, is the lack of resilience, or inability of the cotton fiber to recover rapidly from deformation, such as bending or compression.

The sizes of fibers are difficult to determine if the fiber diameter is taken as the measure, because shapes of dry fibers are highly irregular. The cross-sectional shapes assumed by fibers on exposure to the air depend on the thickness of the cellulose wall. Thick-walled fibers remain almost circular. Thin-walled fibers are approximately elliptical. Internal stresses within the walls cause the fibers to twist. The twisting produces convolutions. Such variations make fiber diameter unreliable as a measure of size.

Instead of diameter, the weight of cellulose per inch of fiber, called weight fineness, is generally accepted as a good measure of cellulose content, closely associated with size. The range in weight fineness of commercial cotton extends from 2.5 micrograms per inch in sea-island to 8.8 micrograms per inch in Asiatic varieties. American upland cottons range from 3.2 to 6.0 micrograms per inch. Special varieties, such as S X P, or United States sea-island, have weight fineness as low as 3.0 micrograms per inch, while varieties of Chinese cotton show values up to 11 micrograms per inch. A rapid approximation of weight fineness can be obtained from instruments that measure the resistance to air flow. This measure is more accurately one of fiber surface, because passage of air between fibers depends on both the fiber shape and the cross-sectional perimeter.

Textile fibers, natural or synthetic, differ in many physical and mechanical properties. Length and size of hemp, jute, and ramie fibers depend partly upon the separation of cells in the retting process, whereas the cotton fiber is a single cell. In contrast, synthetic fibers are extruded to the size and cut to the length desired by the manufacturer.