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Cotton is a major crop in the United States with a retail value of cotton fiber and seed products exceeding $55 billion in 1995. Cotton is still the most important textile fiber in the world despite market inroads made by synthetic fibers such as polyester. Research at the Southern Regional Research Center has made major contributions to cotton's retention of its market share. Fabrics from cotton are comfortable to wear and can be dyed a wide range of attractive colors.
Cotton plants are perennials grown as annuals in the southern half of the United States. Each cotton fiber is an elongated cell, or seed hair, that grows from the seed in a closed seedpod called a boll. When the boll opens, these tubular fibers are exposed to air, lose moisture, and collapse to a flattened, twisted structure. The mature cotton fiber is actually a dead, hollow cell wall composed almost entirely of cellulose. The lengths of single cotton fibers vary, but are generally about one inch.
It is important to understand the relationship between the structure of this unique natural fiber and its properties. Many of its features are too small to be seen without microscopes. Electron microscopes, which use electrons rather than visible light to produce magnified images, are especially useful in studying cotton structure.
This scanning electron micrograph of a coiled fiber shows the extreme difference between its length and width dimensions and the flattened, twisted areas formed when the fiber dried.
At higher magnification the fiber, the twists and wrinkles can be seen even better.
A bundle of cut fibers shows variations in the structures of fiber cross sections. Since cotton fibers are natural products, they are not all the same. As they grow, their hollow tubes fill with cellulose. When cotton is harvested, some fibers contain more cellulose than others. The fibers with nearly full tubes have somewhat bean-shaped cross sections. Fibers with tubes that are not filled with cellulose are flatter.
Cotton fibers are twisted into yarns and woven into fabrics. This is an illustration of a typical plain-weave fabric.
At a higher magnification note how fibers lie parallel to form yarns.
Cotton fibers are mostly cellulose, a polymer of many individual glucose molecules. Although molecules are very small their shapes can be studied with different methods such as x-ray diffraction and computerized modeling. These pictures are of computer models (made with the CHEM-X software). They show three views of space filling models of a fragment of cellulose with eight glucose units (real cellulose may contain 20,000 or more glucose residues). Also shown is a "ball and stick" model. These cellooctaose molecules are slightly over 4.0 nanometers (nm) long, 0.45 nm thick and 0.8 nm wide.
Cellulose Segment
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