|The History of Cotton|
Cotton has a long history: it has been cultivated and used to make fabrics for at least 7,000 years. It may have existed in Egypt as early as 12,000 B.C. Fragments of cotton fabrics found by archeologists in Mexico (Tehuacan Caves) are estimated to be 5,000-7,000 years old. In the Mohenjo Daro civilization of the Indus River Valley (what is now Pakistan), cotton was being grown, spun and woven into cloth 5,000 years ago. At about the same time, natives of Egypt’s Nile valley were making and wearing cotton clothing. Arab merchants brought cotton cloth to Europe about 800 A.D. When Columbus arrived on the American continent in 1492, he found cotton growing in the Bahama Islands. While the history of Egyptian cotton may be the oldest, a genuine search for the history of cotton farming and production would help us understand that various peoples of the world had independently started using cotton thousands of years ago
Not Just Fabric; It’s Food and Feed
Botanically SpeakingSeparating Fiber from the Cotton Boll
Making the Yarn
Making Fabric: Weaving and Knitting
Dyeing, Printing, and Finishing
The Fabric of Our Lives®*
Even after 7,000 years, cotton remains the most marvelous fiber in human life. It is noted for its versatility, appearance, performance, and comfort. From all types of apparel, to space suits, to sheets and towels, and tarpaulins and tents, cotton is still nature’s wonder fiber that provides thousands of useful products. No other fiber even comes close. Consequently, cotton is the single best selling fiber in the world, outselling all man-made fibers combined.
And one shouldn’t underestimate the productive capacity of the fiber! A bale of cotton (a little less than 500 pounds in weight) is sufficient to make 700 luxury bath towels or 300,000 $100 bills (yes, cotton makes up 75% of a dollar bill).
In terms of production, China, USA, India, Pakistan, and Brazil are the world leaders. In terms of mill use, the rankings go to China, India, Pakistan, USA, and Turkey. As for raw cotton exports, at 14 million bales a year (approx. 7 billion pounds), the United States exceeds the rest of the world combined. Annual business revenue stimulated by cotton in the U.S. economy exceeds $120 billion, making cotton the number one value-added crop in the country.
[* Cotton Incorporated, a U.S. based corporation, has proprietary rights over this slogan.]
Not Just Fabric; It’s Food and Feed
Cotton actually is two crops: fiber and seed. An often-overlooked component of the crop, about two thirds of the harvested crop is composed of the seed, part of which is fed whole to livestock. The remaining seed is crushed to separate its three products: oil, meal, and hulls. Cottonseed oil is a common component of many food items, used primarily as a cooking oil, shortening, and salad dressing. The oil is used extensively in the preparation of such snack foods as crackers and cookies. Limited quantities also go into soaps, pharmaceuticals, cosmetics, textile finishes and other products. The meal and hulls are used as livestock, poultry and fish feed, and as fertilizer.
Cultivated cotton is a perennial shrub, genus Gossypium (Malvaceae or Mallow family). There are some forty recognized species, though all of the economically-relevant cotton production revolves around four species.
G. arboreum and G. herbaceum are referred to as Old World Cotton, whereas G. barbadense and G. hirsutum are called New World Cotton.
Separating Fiber from the Cotton Boll
The cotton gin is where cotton fiber is separated from the cotton seed. [When Eli Whitney patented his first “gin” towards the end of the eighteenth century, he was referring to the mechanical capacity of the “engine.”] The first step in the ginning process is when the cotton is dried to reduce moisture and improve the fiber quality. Then it runs through cleaning equipment to remove leaf trash, sticks and other foreign matter.
Saw gins may be used for cotton varieties with shorter staple or fiber length. This process involves the use of circular saws that grip the fibers and pull them through narrow slots. The seeds are too large to pass through these openings, resulting in the fibers being pulled away from the seed. Long fiber cottons are ginned using a roller gin because saw gins can damage their delicate fibers. The roller gin was invented in India centuries ago and this concept is still used in modern gins. Long staple cottons, like Pima, separate from the seed more easily than the short-stapled Upland varieties. A roller gin uses a rough roller to grab the fiber and pull it under a rotating bar with gaps too small for the seed to pass.
The raw fiber, now called lint, goes through a press where it is compressed into bales. A bale weighs slightly less than 500 pounds of fiber. Samples then may be taken from each bale to classify (grade) the lot according to fiber strength, fineness, length, uniformity, color, and trash content using human expertise as well as equipment such as high volume instrumentation (HVI).
Making the Yarn
Lint from several bales is mixed and blended together with the help of computers for production according to fiber properties. This ensures that the new high-speed automated feeding equipment performs at peak efficiency and that fiber properties are consistent. The blended lint is blown by air from the feeder through chutes to cleaning and carding machines that separate and align the fibers into a thin web. The web of fibers at the front of the card is then drawn through a funnel-shaped device called a trumpet, providing a rope-like strand called a sliver.
Roving frames draw or draft the slivers out even more thinly and add a gentle twist as the first step in ring spinning of yarn. Ring spinning machines further draw the roving and add twist making it tighter and thinner until it reaches the yarn thickness or “count” needed for weaving or knitting fabric. The yarns can be twisted many times per inch.
Open-end spinning, with rotors that can spin five to six times as fast as a ring spinning machine, are becoming more widespread. In open-end spinning, yarn is produced directly from sliver, thus eliminating the roving process. Other spinning systems, such as air jet and Vortex, have also eliminated the need for roving, as well as addressing the key limitation of both ring and open-end spinning, which is mechanical twisting. These systems use compressed air currents to stabilize the yarn. By removing the mechanical twisting methods, air jet and Vortex methods are more productive than their predecessors.
After spinning, the yarns are tightly wound around bobbins or tubes and are ready for fabric forming. Ply yarns are two or more single yarns twisted together. Cord is plied yarn twisted together.
Making Fabric: Weaving and Knitting
Weaving is the oldest method of making yarn into fabric. While modern methods are more complex and much faster, the basic principle of interlacing yarns remains unchanged. On the loom, lengthwise yarns called the warp form the skeleton of the fabric. They usually require a higher degree of twist than the filling yarns that are interlaced widthwise.
Traditionally, cloth was woven by a wooden shuttle that moved horizontally back and forth across the loom, interlacing the filling yarn with the horizontally, lengthwise warp yarn. Modern mills use high-speed shuttle less weaving machines. Some carry the filling yarns across the loom at rates in excess of 2,000 meters per minute.
The rapier-type weaving machines have metal arms or rapiers that pick up the filling thread and carry it halfway across the loom where another rapier picks it up and pulls it the rest of the way. Other types employ small projectiles that pick up the filling thread and carry it all the way across the loom. Still other types employ compressed air to insert the filling yarn across the warp. Besides speed, the modern machine is also more versatile and quieter than its older counterpart.
There are three basic weaves with numerous variations, and cotton can be used in all of them: (1) the plain weave, in which the filling is alternately passed over one warp yarn and under the next, is used for gingham, percales, chambray, batistes and many other fabrics; (2) the twill weave, in which the yarns are interlaced to form diagonal ridges across the fabric, is used for sturdy fabrics like denim, gabardine, herringbone and ticking; (3) the satin weave, the least common of the three, produces a smooth fabric with high sheen. Used for cotton sateen, it is produced with fewer yarn interlacings and with either the warp or filling yarns dominating the face of the cloth.
Knitting, on the other hand, is a method of constructing fabric by using a series of needles to interlock loops of yarn. Lengthwise rows of these loops, comparable to the warp yarn in woven goods, are called wales. Crosswise rows, comparable to filling yarns, are known as courses. There are many similarities in knitting done by hand and machine, but there are also some significant differences.
A hand knitter uses two needles forming one stitch at a time. Depending on the width of fabric desired, a modern knitting machine might use over 2,500 needles. Most cotton is knit on circular machines which have needles fixed to the rim of a rotating cylinder. As the cylinder turns, the needles work their way from stitch to stitch producing a tubular fabric. Its width is regulated by the size of the cylinder, which usually ranges from 9 to 60 inches in diameter.
Instead of a single cone of yarn, a knitting machine may have up to four cones per inch of fabric width. For example, a machine with a 32-inch cylinder can have over 2,700 needles and 128 cones of yarn feeding simultaneously.
The flat knitting machine is another basic type. Designed with a flat bed, it has dozens of needles arranged in a straight line and produces a knit fabric that is flat, similar to woven fabric. A flat knitting machine makes over one million stitches a minute, and can be set to drop or add stitches automatically in order to narrow or widen the fabric at certain points to conform to specific shapes. Knitting machines can be programmed to produce a wide variety of fabrics and shapes.
Dyeing, Printing and Finishing
Cotton fabrics, as they come from the loom in their rough, unfinished stages, are known as greige goods or grey cloth. Most undergo various finishing processes to meet specific usage requirements. Besides spinning and weaving, some mills also dye or print their fabrics and finish them. Others sell greige goods to converters who have the cloth finished in independent plants. Computers are being used increasingly in the dyeing, printing, and finishing processes to formulate and match colors with greater speed and accuracy.
In its simplest form, finishing includes cleaning and preparing the cloth, dyeing or printing it and then treating it to enhance performance characteristics. To produce a smooth surface in preparation for dyeing and finishing, the greige goods are passed rapidly over gas-fired jets or heated copper plates to singe off lint and loose threads.
Moving at speeds that can be greater than 200 yards a minute, the material is scoured and bleached in a continuous process that involves the use of hydrogen peroxide. The time for the chemicals to do the preparation reactions occurs from piling the fabric on conveyor belts that pass through steaming chambers, or stacking in large steam-heated, J-shaped boxes before the goods are withdrawn from the bottom.
If a more lustrous cloth is desired, the goods are immersed under tension in a caustic soda solution and then later neutralized. The process, called mercerizing, causes the fiber to swell permanently. This gives the fabric a silken sheen, improves its strength and increases its affinity for dye. Mercerizing also can be done at the yarn stage.
The most commonly used processes for imparting color to cotton are piece dyeing and yarn dyeing.
In piece dyeing, which is used primarily for fabrics that are to be a solid color, a continuous length of dry cloth is passed full-width through a trough of hot dye solution. The cloth then goes between padded rollers that squeeze in the color evenly and removes the excess liquid. In one variation of this basic method, the fabric, in a rope-like coil, is processed on a reel that passes in and out of a dye beck or vat.
Yarn dyeing, which occurs before the cloth is woven or knitted, is used to produce checks, plaids, woven stripes and other special effects. Blue dyed warp yarns, for example, are combined with white filling yarns in denim construction.
One of the most commonly used yarn dyeing methods is package dyeing. In this system, yarn is wound on perforated cylinders or packages and placed on vertical spindles in a round dyeing machine. Dye solution is forced alternately from the outside of the packages inward and from the inside out under pressure.
Printing colored designs on cotton cloth is similar to printing on paper. Prevalent methods include “wet” printing such as flat-bed screen, rotary screen, and engraved roller (80% of present-day printing is flat/rotary screen; rotary alone accounts for over 60% of the market practice). “Dry” printing, such as transfer printing, involves transfer calendars and paper. Digital printing is the newest technique using CAD systems and equipment such as wax jet / inkjet printing. This method is still in its infancy and as yet has less than 1% of the market.
In engraved-roller method, long runs of the same fabric design are produced on a machine operating at speeds between 50 to 100 yards a minute. As many as of 10 different colors can be printed in one continuous operation. A typical printing machine has a large padded drum or cylinder, which is surrounded by a series of copper rollers, each with its own dye trough and doctor blade that scrapes away excess dye. The number of rollers varies according to the fabric design, since each color in the design is etched on a separate roller. As the cloth moves between the rotating drum and rollers under great pressure, it picks up color from the engraved area of each roller in sequence. The printed cloth is dried immediately and conveyed to an oven that sets the dye.
The automatic screen-printing method, though slower than roller printing, has the advantage of producing much larger and more intricate designs, elaborate shadings and various handcrafted effects. In flat bed screen-printing, the fabric design is reproduced on fine mesh screens, one for each color. On each screen, the areas in the design that are not to be penetrated by the dye are covered with lacquer or some other dye-resistant coating. The screens are coated with dye on the back and mounted in the proper sequence above a flat bed. As a belt carries the fabric along from screen to screen, a squeegee or roller presses the dye through the open area of the screen onto the fabric. The new flat bed machines can have speeds of up to 1,200 yards per hour for a fabric with a 36-inch design repeat.
The recently developed rotary screen printing machines have production speeds of up to 3,500 yards an hour. The system combines roller and screen printing, utilizing perforated cylinders instead of flat screens. The color paste is fed inside the cylinders and a small metal roller forces the color through the pores of the cylinder onto the fabric which is moving continuously under the cylinders. As many as 16 colors can be printed on one fabric using this method. Use of this technique is increasing since the screens or cylinders can be produced less expensively than the engraved copper rollers used in roller printing.
Finishing, as the term implies, is the final step in fabric production. Hundreds of finishes can be applied to textiles, and the methods of application are as varied as the finishes. Different types of finishing processes include, but are not limited to: washing, drying, shrink control, needle-punching, napping, shearing, backcoating, and stain repellent finishes such as Scotchguard™ and Teflon.™ A finish often contributes to a fabric's "feel" or "hand." It may also contribute such characteristics as bulk or loft, and resistance to abrasion or stains. For example, washing a fabric adds softness and loft, whereas backcoating a fabric adds durability.
Commercial complexities aside, cotton has indeed been the fiber of our lives for thousands of years.