|Cotton fibers in the scanning electron microscope|
vegetable natural fiber
Cotton ( gossypium )
|Fiber length||15-56 mm|
|Fiber diameter||12-35 µm|
|density||1.51 g / cm³|
|tensile strenght||287-800 MPa|
|Specific tensile strength||15–55 cN / tex (dry)|
|modulus of elasticity||4.5–11 GPa (dry)|
|Elongation at break||6–10% (dry)|
|Specific electrical resistance||10 7 Ωcm|
|Thermal conductivity||0.54 W / (m K)|
|Water absorption||8th %|
|Chemical resistance||alkali but not acid resistant|
The cotton fiber is a natural fiber obtained from the seed hairs of the plants of the genus cotton ( Gossypium ) is obtained. In the system of natural fibers, cotton is therefore one of the seed fibers. As an extension of its epidermis , the seed of the cotton forms longer hairs called lint and three to five days after flowering very short hair called linters. Only the long fibers , mostly spun into thin threads, are used for textiles, while the linters are only suitable for cellulose products .
Development and construction
The cotton fiber consists of a single cell , the primary wall of which grows out of the seed of the cotton until the fiber finally expands. This is then filled by a secondary wall. This is followed by a tertiary wall and finally a cavity, the lumen . In the cell plasma , cellulose is synthesized in an enzymatic complex through the chaining of glucose . About 40–100 cellulose molecules combine to form elementary fibrils. In these, the cellulose is in highly ordered crystal lattices. Several elementary fibrils, also called micelles, combine to form microfibrils , which in turn form macrofibrils.
The structure of the primary, secondary and tertiary walls is very different. The cellulose content is highest in the secondary wall, while the primary wall, which is a few tenths of a micrometer thick, only contains about 5% of the total cellulose content of the fiber. Besides cellulose, it mainly consists of pectins and waxes . There is little cellulose and a lot of impurities in the tertiary wall. It practically fulfills the function of a “filter” for the fiber.
The table shows the chemical composition of the primary wall, the entire fiber and any impurities such as accompanying vegetal substances, especially seed coats.
|component||Cotton fiber (%)||Primary wall (%)||Vegetable accompanying substances (%)|
|Pectins / pectinates||-||12||-|
The arrangement of the fibrils in the three walls is also very different. While the fibril strands are very irregularly arranged in the primary wall, in the secondary wall they are crossed in the manner of a helix structure and in the tertiary wall they are strictly parallel to the fiber axis.
The figure opposite shows a schematic cross section through a cotton fiber. The surface structure of the cotton fibers is flat, twisted and loop-like. The color of the fibers varies from creamy white to dirty gray, depending on the manufacturing or processing process. In contrast to many other natural fibers, cotton has only extremely low levels of lignin or pectin and only a very small amount of hemicellulose of around 5.7%. Thus the cotton fiber consists almost exclusively of highly crystalline cellulose , in addition to the wax layer of the cuticle .
When processing the cotton, only around 10% of the raw weight is lost. When the wax, protein and other plant residues are removed, a natural polymer made of cellulose remains . The special arrangement of the cellulose gives the cotton a high tear resistance. Each fiber consists of 20-30 layers of cellulose in a twisted structure. When the cotton ball - the fruit cluster of the cotton plant - is opened, the fibers dry and interlock. This shape is used for spinning into a very fine yarn.
Cotton is very absorbent and can absorb up to 32% of its weight in moisture without dripping. However, once cotton fabrics get wet, they dry slowly. In addition, cotton also has a high dirt and oil absorption capacity, but is also able to release them again. Cotton fabrics are very skin-friendly (they don't “scratch”) and have an extremely low allergy potential . These properties make them interesting for the textile industry.
Cotton is insoluble in water and more tear-resistant when damp or wet than when dry. The strengths and stiffnesses of the cotton fiber are lower than those of the bast fiber , the elasticity being significantly higher. The fibers are alkali but not acid resistant. Cotton is susceptible to attack by microorganisms , but it is quite resistant to moths and other insects. Cotton is easily flammable but can be boiled and sterilized.
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