Silk, the most elegan
t fiver, was
discovered in China by Empress Si Ling Chi when she was sitting beneath a
mulberry tree in the palace garden enjoying a cup of tea. Suddenly, a cocoon,
which had been attached to the tree, fell into the Empress’ tea cup. Attempting
to remove it, she was fascinated to discover a very fine thread started to
unravel.
The filaments of silk that form the
cocoon are triangular in shape and are composed of two fibers of fibroin and a
supporting matrix of sericin. Sericin makes up 20% to 30% of the filaments and
acts as a support to glue the cocoon together and also as a lubricant during
spinning.
Fibroin has an average diameter of about
10 to 18 microns and is a protein that abounds in non-polar amino acids such as
glycine and alanine. Whereas sericin is a protein that is water-soluble and
composed of amino acids such as serine and aspartic acid. The sericin that envelopes
the filaments is highly tenacious and requires high temperature alkaline
processing to remove it completely in order to obtain raw silk. The tenacity
and gum-like quality of sericin itself makes if a good candidate for biomedical
joining and sealing applications.
Physical properties
Silk fibers have a triangular cross
section with rounded corners. This allows light to hit at many different angles,
so silk is a bright fiber and has a natural shine. It has a smooth, soft
texture that is not slippery, unlike many synthetic fibers. Its denier is 4.5
g/d when dry and 2.8 – 4.0 g/d when wet. Silk is one of the strongest natural
fibers but loses up to 20% of its strength when wet. It has a good moisture
regain of 11%. Its elasticity is moderate to poor. if elongated even a small
amount it remains stretched. It can be weakened if exposed to too much
sunlight. It may also be attacked by insects, especially if left dirty.
Silk is a poor conductor of electricity
and thus susceptible to static cling.
Unwashed silk chiffon may shrink up to 8%
due to a relaxation of the fiber macrostructure. So silk shoulder either be
pre-washed prior to garment construction, or dry cleaner. Dry cleaning may
still shrink the chiffon up to 4%. Occasionally, this shrinkage can be reversed
by a gentle steaming with a press cloth. There is almost no gradual shrinkage
or shrinkage due to molecular-lever deformation.
Silk is sensitive to higher temperatures.
It should be, therefore, subjected to ironing at moderate temperature. Silk is
sensitive to light than any other natural fibers. Yellowing of the fiber is
generally accompanied by photo-degradation that is mainly occurred due to
action of UV radiation of light.
Chemical properties
Action of Acids: Hot concentrated acids
readily decomposes silk. The degree of hydrolysis is much greater with acid
than with alkali and is pH dependent. Weak (5% solution) and cold hydrofluoric
acid does not have any harmful action on silk. Dilute organic acids show little
effect on silk at room temperature.
Action of Alkali: Dilute alkali
temperature does not cause any considerable chemical and structural damage of
silk, but the luster of the fiver may be decreased depending on the exposure
time. Caustic soda, when it is hot and strong, dissolves the silk fiber.
Action of oxidizing and reducing agent:
Silk fibroin is not severely affected by hydrogen peroxide solution. The action
of chorine solution on the silk fibroin is more harmful than does the solution
of hypochlorite. The reducing agents that are commonly found in use in textile
processing such as hyposulphite, sulfurous acids and their salts do not
exercise any destructive action on the silk fiber.
Uses of Silk
Silk’s good absorbency makes it
comfortable to wear in warm weather and while active. Its low conductivity
keeps warm air close to the skin during cold weather. It is often used for
clothing such as shirts, blouses, formal dresses, high fashion clothes,
negligees, pajamas, robes, skirt suits, sun dresses and underwear.
Silk’s elegant, soft luster and beautiful
drape makes it perfect for many furnishing applications. It is used for
upholstery, wall coverings, window treatments (if blended with another fiber),
rugs, bedding and wall handing. Silk is also used for parachutes, bicycle
tires, comforter filling and artillery gunpowder bags.
Early bulletproof vests were made from
silk in the era of black powder weapons until roughly world war I. A special
manufacturing process makes it suitable as non-absorbable surgical sutures. Chins
doctors have used it to make prosthetic arteries. Silk cloth is also used as a
material to write on.
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