The most important plant products after foodstuffs and timber are fiber crops. They exert economic, social, and political influence both at the local level and on an international scale. Throughout the world millions of people are employed in growing them, and major industries are based on processing the fibers into yarns and textile fabrics.
Synthetic fibers have made significant inroads into the textile market in recent years but—particularly since the increase in the price of petroleum, the base material for many synthetic fibers—the natural products remain competitive. Genetic modifications to the fiber plants have improved yields. And compared with their chief “natural” competitors—fibers made from wood pulp derived from timber-most vegetable fiber crops mature in months rather than the decades required by trees.
There are three main types of vegetable fibers, classified as soft, hard, or short. The soft (or bast) fibers are extracted from the stems of plants and include jute, flax (linen), hemp, ramie, and kenaf. Hard fibers, such as manila hemp, sisal, henequen, and New Zealand flax, are gathered from the leaves for providing hard cordage or brush and have suffered more from the impact of synthetic fibers (which can be made stronger). Cotton, the world’s most important vegetable fiber, belongs to the third group, the short fibers, produced from the “hair” on the seeds; they also include kapok and coir (from coconut).
Fiber structure and characteristics
The bast and hard fiber strands are bundles of numerous overlapping, parallel-fiber cells. The cell walls consist of minute microfibrils (the smallest units visible under a microscope), which are composed of cellulose chains linked by hydrogen bonds. The orientation of the microfibrils determines each fiber’s elasticity.
Cotton is unique among the natural fibers. Each fiber is a single tubelike cell made up of 20-30 layers of cellulose. When the cotton boll (fruit capsule) opens, the fibers dry into twisted ribbons with spiraling microfibrils.
A fiber must be at least 100 times longer than it is wide to be suitable for spinning into thread for weaving. Although its length measures up to 3,000 times greater than its width, cotton is relatively short—only one-third of the length of flax—and can pose problems for the spinner and weaver. Fibers of the finest Sea Island cotton (Cossypium barbadense) are up to 1.5 inches (38 millimeters) longer than those of coarse Indian blanket cotton (G. herbaceum). Natural twist in the fibers also contributes to quality. Flax (Linum usitatissimum) has nodes that help the fibers lock together during spinning, but cotton has up to 2,000 natural twists (in alternate directions) to every inch.
Tensile strength, flexibility, and elasticity are important if a fiber is to withstand processing stress. Good colorfastness and resistance to decay are needed if it is to be marketable. The newly harvested fiber also has to be quick to clean if it is to be economically produced. Ramie (Boehmeria nivea), for example, has never competed with cotton or linen because it is difficult to clean. The selling price, however, may be largely determined by the amount of moisture the final fabric will absorb. Cotton’s tubular structure allows it to take up and release water rapidly, which makes it useful for toweling, comfortable to wear, and receptive to dyes and bleaches.
Cotton—from fiber to fabric
When a cotton boll bursts, four or five tufts of fiber emerge, each containing several seeds; it is then harvested. The fiber, or lint, is sent to a cotton gin to be separated from the seeds and is packed into 480-pound (218-kilogram) bales. When it is ready for processing, the bale is opened, beaten to loosen the fibers, then formed into a sheet, known as a lap, on a scutching machine. Different qualities of cotton are often blended at this stage to ensure a uniform yarn. The roll of fiber is straightened on a high-speed carding machine (a revolving spiked cylinder), which disentangles and cleans the fibers until they form a thin web.
The length of time the cotton remains in the carding machine determines the cleanness and quality of the fiber.
Only the longer fibers produce good-quality cotton, so the short fibers (known as noils) are extracted by combing and spun into cheaper yarns. The long fibers emerge as a sliver of strong, smooth material. This sliver passes through a series of rollers, which draw it out into a finer strand and twist it into roving.
Spinning involves elongating and twisting the cotton to the required thickness and length, then transferring it onto bobbins for weaving. The ring frame, invented by John Thorp in the United States in 1828, drafts and twists simultaneously and is used for large quantities. Spinning speeds have recently been significantly increased by the openended technique, often referred to as O-E spinning, which uses high-speed rotor disks for twisting the yarn. More delicate qualities are manufactured on a spinning mule, which drafts, spins, and twists in three separate operations. Doubled yarns, for thread and hosiery, are produced by twisting several yarns together. Moistened roving produces a cleaner, more compact thread, but the correct moisture level is crucial.
Three types of yarn are produced: warp, or longitudinal threads; weft, or filling yarn for crosswise lacing, with less twist; and knitting yarns. The warp and weft are woven together on high-speed looms in modern factories. The Sulzer loom allows a wider fabric to be produced and at higher speeds, and it has introduced a more efficient method of weft insertion.
After weaving, further processing is sometimes needed. To increase the luster, strength, and absorbency, some fabric is mercerized.
This technique, devised in 1850 by John Mercer, a self-taught chemist, involves dipping the tensioned cloth into a cold concentrate of caustic soda or liquid ammonia for two to three minutes, then washing it off. Finally, the cloth may be scoured to remove dirt or oil, bleached with sodium hypochlorite or hydrogen peroxide, and colored with synthetic dyes.
The immediate future promises interesting new developments, such as the shuttleless loom, cotton grafted with polymers to produce a cloth with new properties, and greater automation in the finishing stages.
Soft or bast fibers
Bast fibers come from the soft stem or phloem of dicotyledonous plants. They are made up of cell bundles cemented together by nonfibrous gummy substances composed of lignin, pectin, and cellulose. Before the fibers can be extracted from the stem, the gums must be softened, dissolved, then washed away. This process, known as retting, involves immersing the plant stalks in water or exposing them to dew. Bacteria ferments the woody tissues, and enzymes dissolve the binding pectic substances until the fibers can easily be separated.
Known also as linen after its plant of origin, Linum usitatissimum, flax is the strongest of the vegetable fibers. Linen garments estimated to be at least 3,500 years old have been recovered from Egyptian tombs. Such resistance to decay and strength made flax the main source of cloth until the arrival of cotton in about 1800. Compared with cotton, however, flax’s lack of elasticity caused its popularity to suffer, particularly because it cannot withstand the stresses of high-speed power looms; today it is reserved for high quality, durable, but expensive, household cloth.
The Soviet Union produces more than half of the world’s total supply of fiber flax. But the best qualities of linen come from Ireland and Belgium, where the river water in which the flax is retted is thought to impart a particularly desirable, rich, creamy color and texture to the linen.
Extracting the fiber
Flax stems are pulled, rather than cut, from the soil by machines that bundle the stems for retting. The better grades of Belgian flax may be retted twice—first for two and one-half days then, after rinsing and drying, for a further day at 90° F. (32° C).
After retting, the stems are split by mechanical breaking rollers and then passed to a turbine scutcher. There the stems are beaten by blades to separate the fibers and crush the pith. The long fibers are combed and all the short fibers removed for coarse-fiber production. Following this so-called hackling process, the long fibers are drawn out through rollers to form a sliver, and then wound on to bobbins for spinning.
Wet spinning is employed for fine linen and warp yarns. The roving is passed through hot water to soften the fibers and to allow them to slip easily over each other. New spinning methods have been devised for flax, including a twistless process that uses the plant’s natural pectins to bind the fibers. However, ring spinning on a flyer machine remains the most economical method.
The flax is then bleached and woven into a range of fabrics, from heavy canvas to fine linen for handkerchiefs. Flax also lends itself to blends with other fibers, such as cotton, Tery-Iene (Dacron), wool, and acrylics, so that the durability of linen can be combined with the lower cost and greater elasticity of other materials.
Flemp is extracted from the stem of the well-known drug-producing plant, Cannabis sativa. Coarser than flax and very resistant to rotting, it is manufactured into marine cordage, heavy-duty tarpaulins, and, until recently, sailcloth. The higher-quality, water-retted fiber, which turns a creamy white, can be used for finer textiles (one-third of the population of South Korea wears hemp clothing). Unlike flax, hemp is cut in the field after the flowering season. Summer hemp contains better fiber than winter hemp and is normally water-retted to form finer cloth. Flemp is processed in the same way rather than being machine hackled.
Also known as China grass and rhea, ramie is the longest, broadest, and one of the strongest textile fibers, but it is very costly to process. Unlike flax, hemp, and jute, it is not retted and the fiber is taken from the stalk by hand. The bark is scraped off in ribbons, which are cleaned in a strong soda solution. They are left to ferment in hydrochloric acid, then returned to the soda to soften and remove the gum. Once the fibers have been extracted, combed, and sorted, ramie is processed in the same way as flax.
Most jute is white and comes from the Indian herbaceous annual Corchorus capsuiaris. (Cor-chorus olitorius yields the rarer russet-colored tossa jute.) Although jute is the cheapest natural textile fiber and is easy to dye and spin, it is weak, deteriorates quickly as a result of its low cellulose content, and does not bleach well.
The finer qualities are spun into yarn and processed like flax, but 75 per cent of jute is made into burlap, sacking, twine, and carpet backings. As a packaging material, it has difficulty competing with kenaf, roselle, and synthetics such as polypropylene, largely because there has been little mechanical progress in India, the center of the jute industry.
Kenaf, which derives from the Asian plant Hibiscus cannabis, is now used increasingly as a substitute for wood pulp. It is stronger, lighter, more water-resistant, and—because the stems are less woody—is better suited to mechanized fiber separation than is jute.