Silk
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Silk
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For other uses of this word, see Silk (disambiguation).
Silk dressesSilk is a natural protein fiber, some forms of which can be woven into textiles. The best-known type of silk is obtained from cocoons made by the larvae of the silkworm Bombyx mori reared in captivity (sericulture). The shimmering appearance for which silk is prized comes from the fibers' triangular prism-like structure which allows silk cloth to refract incoming light at different angles.
"Wild silks" or tussah silks (also spelled "tasar") are produced by caterpillars other than the mulberry silkworm (Bombyx mori). They are called "wild" as the silkworms cannot be artificially cultivated like Bombyx mori. A variety of wild silks have been known and used in China, India, and Europe from early times, although the scale of production has always been far smaller than that of cultivated silks. Aside from differences in colors and textures, they all differ in one major aspect from the domesticated varieties: the cocoons that are gathered in the wild have usually already been damaged by the emerging moth before the cocoons are gathered, and thus the single thread that makes up the cocoon has been torn into shorter lengths. Commercially reared silkworm pupae are killed before the adult moths emerge by dipping them in boiling water or piercing them with a needle, thus allowing the whole cocoon to be unraveled as one continuous thread. This allows a much stronger cloth to be woven from the silk. Wild silks also tend to be more difficult to dye than silk from the cultivated silkworm.
There is some evidence that small quantities of wild silk were already being produced in the Mediterranean area and the Middle East by the time the superior, and stronger, cultivated silk from China began to be imported (Hill 2003, Appendix C).
Silks are produced by several other insects, but only the silk of moth caterpillars has been used for textile manufacture. There has been some research into other silks, which have differences at the molecular level. Silks are mainly produced by the larvae of insects with complete metamorphosis, but also by some adult insects such as webspinners. Silk production is especially common in the Hymenoptera (bees, wasps, and ants), and is sometimes used in nest construction. Other types of arthropod produce silk, most notably various arachnids such as spiders (see spider silk).
Production methods
Sericulture: the production of cultivated silk
Silk moths lay eggs on specially prepared paper.
Eggs hatch and the caterpillars are fed fresh mulberry leaves.
After about 35 days, and 4 moltings, the silkworms are 10,000 times heavier than when hatched – they are now ready to begin spinning a cocoon.
A straw frame is placed over tray of silkworms – they begin spinning cocoons by moving their heads in a figure 8.
Liquid silk, coated in sericin, is produced in 2 of the silkworm’s glands, which is forced through spinnerets.
Sericin: water-soluble protective gum
Spinnerets: openings in silkworm’s head
As this liquid silk comes into contact with the air, it solidifies.
Within 2-3 days, the silkworm will have spun 1 mile of filament and will be completely encased in a cocoon.
After this entire process, the silkworm metamorphoses into a moth, but is usually killed by heat before it reaches the moth stage – any silkworm reaching the moth stage is used for breeding the next generation of silkworms.
Process to obtain filament silk
Cocoons that have been stifled are sorted by fiber size, fiber quality, and defects, then are brushed to find filaments.
Several filaments are gathered together and wound onto a wheel (reeling).
Each cocoon yields 1000 yards of silk filament, known as raw silk, or silk-in-the gum, fiber.
Several filaments are combined to form a yarn.
As fibers are combined and wrapped into the reel, twist can be added to hold the filaments together.
Adding twist is referred to as ‘throwing’ – resulting yarn is called thrown yarn
The type of yarn and amount of twist relate to the fabric produced.
The simplest type of thrown yarn is a ‘single’ – 8 filaments are twisted together to form a yarn
Used for filling yarns in silk fabrics, singles can have 2 or 3 twists per inch
Silk noils (silk waste): produced from the inner portions of the cocoon. It is degummed (sericin is removed) and spun like other staple fiber. Or it can also be blended with another staple fiber and is spun into yarn.
Wild silk production is not controlled. Cocoons are harvested after the moth has matured, so silk cannot be reeled – it must be spun.
Major fiber properties
Physical properties
Shape
Silk has a triangular shaped cross section whose corners are rounded.
Luster
Due to the triangular shape (allowing light to hit it at many different angles), silk is a bright fiber meaning it has a natural shine to it.
Covering power
Silk fibers have poor covering power. This is caused by their thin filament form.
Hand
When held, silk has a smooth, soft texture that, unlike many synthetic fibers, is not slippery.
Denier
4.5 g/d (dry) ; 2.8-4.0 g/d (wet)
Mechanical properties
Strength
Silk is the strongest of all the natural fibers; however it does lose up to 20% of its strength when wet.
Elongation/elasticity
Silk has moderate to poor elasticity. If elongated even a small amount the fibers will remain stretched.
Resiliency
Silk has moderate wrinkle resistance
Chemical Properties
Protein Composition
Silk is made up of GLY-SER-GLY-ALA-GLY and forms Beta pleated sheets. Interchain H-bonds are formed while side chains are above and below the plane of the H-bond network. Small residue(Gly) allows tight packing and the fibers are strong and resistant to stretching. The tension is due to covalent peptide bonds. Since the protein forms a Beta sheet, when stretched the force is applied to these strong bonds and they do not break. The 50% GLy composition means that Gly exists regularly at every other position.
Absorbency
Silk has a good moisture regain of 11%.
Electrical conductivity
Silk is a poor conductor of electricity making it comfortable to wear in cool weather. This also means however, that silk is susceptible to static cling.
] Resistance to ultraviolet light/biological organisms
Silk can become weakened if exposed to too much sunlight. Silk may also be attacked by insects, especially if left dirty.
Chemical reactivity/resistance
Silk is resistant to mineral acids. It is yellowed by perspiration and will dissolve in sulphuric acid.
Other properties
Dimensional stability
Unwashed silk chiffon may shrink up to 8% due to a relaxation of the fiber macrostructure. So silk should either be prewashed prior to garment construction, or dry cleaned. However, dry cleaning may still shrink the chiffon up to 4%. Occasionally, this shrinkage can be reversed by a gentle steaming with a press cloth. Gradual shrinkage is virtually nonexistent, as is shrinkage due to molecular-level deformation.
Uses for silk
[edit] Apparel
Silk is excellent for use in warm weather and active clothing. The silk's good absorbency makes it comfortable to wear in such conditions. Silk is also excellent in the cold because its low conductivity keeps the wearer warm.
Examples of silk clothing
Blouses
Formal Dresses
High Fashion
Negligees
Pajamas
Robes
Skirtsuits
Sundresses
Underwear
Furnishings
Silk's elegant, soft luster and beautiful drape makes it perfect for many furnishing applications.
Examples of silk furnishings
Upholstery
Wall Coverings
Window Treatments (if blended with another fiber)
Rugs
Bedding
Wall Hangings
Animal rights
As the process of harvesting the silk from the cocoon kills the larvae, silk-culture has been criticized in the early 21st century by animal rights activists on the grounds that silk production kills silkworms, and that artificial silks are available.[6] Others point out that silkworms depend upon humans for their survival, and would become extinct without humans to care for the worms and harvest the silk. [7]
Mahatma Gandhi was critical of silk production based on the Ahimsa philosophy. Ahimsa is part of the three millennial Jain philosophy of India "not to hurt any living thing," which led to development of a cotton spinning machine he distributed. Such a machine can be seen in the Gandhi Institute.
Ahimsa Silk, made from the cocoons of wild and semi-wild silk moths, is being promoted in parts of Southern India, for those who prefer not to wear silk produced which involves the death of silkworms.
Other uses
Mongols used silk as part of the under-armor garments (see Mongolian armor). Silk is so tough that it was actually used as very light armor, although its special use was to stop arrow penetration into the body. The silk would stop an arrow from penetrating far enough into the body to be lethal, and the arrow could be pulled out of the wound by tugging on the unbroken silk.[citation needed] The head of an arrow pulled out this way would not contact the body, reducing the likelihood of infection.
In addition to clothing manufacture and other handicrafts, silk is also used for items like parachutes, bicycle tires, comforter filling and artillery gunpowder bags. Early bulletproof vests were also made from silk in the era of blackpowder weapons until roughly World War I. Silk undergoes a special manufacturing process to make it suitable for use as non-absorbable surgical sutures. Chinese doctors have also used it to make prosthetic arteries. Silk cloth is also used as a material to write on.
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