Cotton

24 06 2010

King Cotton.  The cotton textile industry has perhaps been studied as much as any industry in history, and the fiber itself is so important that it’s traded as a commodity.  “In high cotton” means to be wealthy, somebody can be out of his “cotton picking mind”, and  “to cotton” has even become a verb!  Today the range of uses for cotton has expanded so much (only 35% of the global harvest ends up in textiles) and there are so many issues surrounding cotton – from government subsidies, to GMO cotton to the intense chemical cultivation needed to produce conventional cotton – that I want to say at the outset that I just want to discuss  cotton without putting a value judgment on the plant, which after all is a pretty incredible natural gift to us!

Cotton is the world’s most popular natural fiber.    The fruit of the plant, better known as the cotton boll,  provides the fiber – the fiber of a thousand faces and almost as many uses, the fibers which the ancients called “white gold” because it was so valuable. 

Successful cultivation of cotton requires a long frost-free period, plenty of sunshine, and moderate rainfall, usually from 600 to 1200 mm (24 to 48 inches).  In general, these conditions are met within the seasonally dry tropics and subtropics in the Northern and Southern hemispheres, but  lots of the cotton grown today is cultivated in areas with less rainfall than cotton needs, so 70% of cotton crops are irrigated.

Today, cotton is cultivated in around 130 countries – but only six countries (China, Brazil, India, Pakistan, the USA and Uzbekistan) account for more than 80% of total production.  It is one of the world’s most widely produced crops and uses about 2.5% of the world’s arable land area.  Cotton cultivation is fundamental to the economies of many developing countries; according the International Cotton Advisory Committee (ICAC), around 20 million farms depend on cotton.

Cotton fibre grows on the seed of a variety of plants of the genus Gossypium, a member of the Hibiscus family. Of the four cotton species cultivated for fibre, the most important are :

  • G. hirsutum (also known as Upland cotton or Mexican cotton), which originated in Mexico and produces 90% of the world’s cotton.  Upland cotton is white, 2.1 – 3.2 cm long.
  • G. barbadense (also known as Pima cotton), of Peruvian origin, which accounts for 5% of the world’s cotton.  Pima cotton is longer than Upland, 3.5 – 4.1 cm, and more costly.  Special features of Pima cotton are its luster and extreme softness. Types of Pima cotton:
    • Egyptian cotton is 3.8 to 4.4 cm long, yellow brown in color, and grown only in Egypt.
    • Sea Island, longest of all the cotton fibers (3.5 to 6.4 cm) and the most expensive.  Yellow in color.  Grown in SC and GA coast.

All parts of the cotton plant are useful.  When seed cotton is ginned, more seed than fiber is produced.  For every kilogram of fibre produced, each cotton plant  produces 1.65 kg of seed.  The cottonseed is crushed in order to separate it into its three products – oil, meal and hulls. Cottonseed oil (about 20% of the harvested plant) is used primarily for shortening, cooking oil and salad dressing and in lots of snack foods. The meal and hulls that remain are very high quality proteins, and  are used either separately or in combination as livestock, poultry and fish feed and as fertilizer.  We do not think of cotton as a potential source of food, and for good reason. The seeds of the cotton plant are rife with a potent poison called gossypol that attacks both the heart and liver. Only the multi-chambered stomachs of cattle and other hooved animals can cope with this poison, relegating cottonseed to a role as animal feed.

The fiber, or lint, which is used in making cotton cloth is almost pure cellulose. Linters – the short fuzz on the seed – provide cellulose for making plastics, explosives and other products. Linters also are incorporated into high quality paper products and processed into batting for padding mattresses, furniture and automobile cushions.  As a refined product, cotton linters have medical, cosmetic and other uses.

Cotton planting, harvesting and spinning can be done in a highly mechanized way – or it can be done entirely by hand.  Only about 30% of the world’s cotton production is harvested by machines.  Conventional cotton stripping machines use rollers equipped with alternating bats and brushes to knock the open bolls from the plants into a conveyor.

A second kind of stripper harvester uses a broadcast attachment that looks similar to a grain header on a combine. All harvesting systems use air to convey and elevate the seed cotton into a storage bin referred to as a basket. Once the basket is full, the stored seed cotton is dumped into a boll buggy, trailer or module builder.

Today, nearly all cotton is stored in “modules”, which look like giant loaves of bread. Modules allow the cotton to be stored without loosing yield or quality prior to ginning. Specially designed trucks pick up modules of seed cotton from the field and move them to the gin.

Modern gins place modules in front of machines called module feeders. Some module feeders have stationary heads, in which case, giant conveyors move the modules into the module feeder.  The module feeders literally break the modules apart and “feed” the seed cotton into the gin. Once in the cotton gin, the seed cotton moves through dryers and through cleaning machines that remove the gin waste such as burs, dirt, stems and leaf material from the cotton. Then it goes to the gin stand where circular saws with small, sharp teeth pluck the fiber from the seed.

From the gin, fiber and seed go different ways. The ginned fiber, now called lint, is pressed together and made into dense bales weighting about 500 pounds.  Producers usually sell their cotton to a local buyer or merchant who, in turn, sells it to a textile mill either in the United States or a foreign country.

The seed usually is sold by the producer to the gin. The ginner either sells the seed for feed or to an oil mill where the linters (a byproduct of the oil mill – don’t confuse this with the ginned fiber, called lint) are removed in an operation very much like ginning. Linters are baled and sold to the paper, batting and plastics industries, while the seed is processed into cottonseed oil, meal and hulls.

At the textile mill, the bales are opened by machines, and the lint is mixed and cleaned further by blowing and beating. The short lint that comes out usually is separated and sold for use in other industries. The best part of the lint consists of fibers about 1 inch to 1 ¾ inches long.

The mixed and fluffed-up cotton goes into a carding machine which cleans the fibers some more and makes them lie side by side. The combing action of the carding machine finishes the job of cleaning and straightening the fibers, and makes them into a soft, untwisted rope called a sliver (pronounced sly-ver).

On modern spinning frames, yarn is made directly from the sliver. The spinning devices take fibers from the sliver and rotate it up to 2,500 revolutions in a second twist that makes fibers into a yarn for weaving or knitting into fabrics.

After all of that,  the yarn is ready to be woven into fabric.

Cotton, as an intensely studied commodity, has a variety of grades, usually dependent on the length of the cotton staple fibers.  Cotton quality is judged based on the grade, color, length of the fibers and the character:

  • Grade: determined by the major or minor brightness of the fibers, by the more or less white color and the presence of particles of the leaf or other extraneous substances.
  • Color: color can differ greatly, from white to grey, but also reddish, tawny, chamois colored varieties.
  • Length: the most important attribute, and this category is divided into two parts:
    • Long fiber (long staple) measures more than 28 mm
    • Short fiber (short staple) does not reach the length of 18 mm; an intermediate category of 18 – 28 mm (such as the US Uplands cotton) constitutes 60% or more of world production
  • Character or micronaire: partly connected with origin, variety and maturity but a cotton of good character is that whose fibers are the most strong and robust (so as to resist traction and breakage); homogenous and uniform (to produce few losses in working) and have a complete physical-chemical constitution (so as to give the cotton mass notable solidity and compactness, smoothness and silkiness).  Cotton fiber fineness is defined as mass per unit of length,  the term millitex (for milligram per kilometer) is used; and upland cottons have millitex values between 150 and 200.  Ideal maturity ratios are around 0.8

The traditional method of establishing cotton quality is by visual hand classing. Professional classers hand class bale by bale and visually define color, grade, leaf content, preparation, maturity, and incidence of defects. In other words, the overall visual characteristics of the cotton is sampled. In addition, the classers randomly pull the staple to evaluate the length of the fibers.

A certain degree of subjectivity is involved in this method of classifying cotton. Samples are judged against grade boxes that are produced to establish standards. The Universal Standards, established each year by the USDA, define standards: Good Middling, Strict Middling, Middling, Strict Low Middling, Low Middling, Strict Good Ordinary, Good Ordinary. Other terms that are used to describe variations in color include: Light Spot, Spotted, Tinged, Grey, Dull, etc. Inferior cotton is denominated as Below Grade. Specific machines are then used randomly, as a complement, to measure micronaire and strength (Pressley). Cotton negotiated under this methodology is referred to as ‘sold on description’.

The more modern method of ascertaining quality is with a High Velocity Instrument (HVI). With these machines more than 1000 samples can be tested per day. These machines are generally not as accurate as hand classing with respect to color, grade and leaf, however, the results are more objective and very effective for measuring, staple, strength, uniformity, short fiber content and elongation which have gained much more importance. These machines do not have the capability of measuring the overall preparation of the cotton. Nonetheless, the trend is for textile mills to require HVI results when purchasing cotton.

CHARACTERISTICS:

Cotton fabrics are very comfortable to wear due to their soft hand, lovely drape and other characteristics.  They are easy to handle and sew.

Cotton has excellent absorbing capabilities.  “Absorbent” cotton will retain 24-27 times its own weight in water and is stronger  wet than dry. This fiber absorbs and releases perspiration quickly, thus allowing the fabric to “breathe”.

Cotton can stand high temperatures and takes dyes and printing inks easily.

Cotton is washable but does shrink if it has not been treated with a shrink resistant finish.  Boiling and sterilizing temperatures can also be used on cotton without disintegration.  Colored cotton garments retain their color longer if they are washed in warm or cool water. Cotton fabrics can be bleached but too much bleaching could weaken the fibers.  Sunlight  harms cotton by causing it to oxidize and turn yellow.

Cotton wrinkles very easily, but can be ironed at relatively high temperatures. However, there are many cotton garments on the market that have been treated with wrinkle resistant finishes or blended with polyester to give it wash and wear properties..

Mercerized cotton is treated to permanently straighten the cotton fibers which then becomes a smooth, rod-like fiber that is uniform in appearance with a high luster.


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One response

17 07 2012
Jass smith

I must say that overall I am really impressed with this information about cotton.It is easy to see that you are impassioned about your writing. I think Egyptian Cotton is best cotton in the world.

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