SILK

1
SILK

• Silk is a natural protein fibre, some forms of
  which can be woven into textiles. The best-known
  type of silk is obtained from the cocoons of
  the larvae of the mulberry silkworm Bombyx
  mori reared in captivity (sericulture). The
  shimmering appearance of silk is due to the
  triangular prism-like structure of the silk
  fibre, which allows silk cloth to refract
  incoming light at different angles, thus
  producing different colors.

2
Wild silk

• A variety of wild silks, produced
  by caterpillars other than the mulberry silkworm
  have been known and used in China, South Asia,
  and Europe since ancient times. However, the
  scale of production was always far smaller than
  that of cultivated silks. They differ from the
  domesticated varieties in color and texture, and
  cocoons gathered in the wild usually have been
  damaged by the emerging moth before the cocoons
  are gathered, so the silk thread that makes up
  the cocoon has been torn into shorter lengths.
  Commercially reared silkworm pupae are killed by
  dipping them in boiling water before the adult
  moths emerge, or by piercing them with a needle,
  allowing the whole cocoon to be unraveled as one
  continuous thread. This permits 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.

3
Macro structure

• Silk is a natural protein filament. Its density
  is 1.34 g/cm cube which makes it a medium weight
  fibre.
• raw silk strand consists of two silk filaments
  encased by a protein called sericin. This sericin
  gives raw silk a coarser handle. Sericin is also
  very weather resistant since it can withstand
  prolonged weather exposure.
• Shiny silk filaments are revealed when sericin
  is removed in mild alkaline solution.
• Silk is a fine, regular, translucent filament.
  It may be 600m long but average about 300 m in
  length.
• Its cross-section may vary from 12µm to 30µm.
  this gives fibre length to breath ration in
  excess of 20001.
• The beauty and softness of silk is due to the
  triangular cross-section of silk filament. As
  silk filament is tightly twisted and angle of
  reflection changes continuously. As a result, the
  intensity of reflected light is broken resulting
  in a soft subdued luster.

4
Micro-structure

• The irregular, random, longitudinal view of silk
  filaments are not sufficiently distinctive to
  identify silk under the microscope. The
  triangular cross-section can be used to identify
  silk. This appearance is due to the slit-like
  opening of the silk secreting glands, one being
  located on either side within the mouth of the
  silk moth larvae.
• Silk is a coagulating stream of fibroin solution,
  and has no identifiable micro-structure. In this
  regard it resembles the man-made fibres.

5
Silk polymer

• Silk polymer is linear, fibroin polymer. fibroin
  is the name of protein which constitutes silk.
• Silk polymer is composed of 16 different amino
  acids. Three amino acids called Alanine, glycine
  and serine make up about fourth-fifth of silk
  polymer.
• Silk polymer is not composed of any group
  containing sulphur. Hence it does not contain any
  disulphide bonds.
• The silk polymer occur only in beta
  configuration. Silk polymer is about 140 nm long
  and 0.9 nm thick.
• Silk is considered as being composed of layers of
  folded, linear polymers. That explained the high
  crystallinity i.e. its crystalline regions are
  65-70 and amorphous region are 30-35.

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Silk fibre

• Silk polymer system contains no disulphide
  bonds.
• Like wool the repeating unit of silk is amino
  acid.
• Important chemical grouping is of silk polymer is
  peptide groups.
• The major forces of attraction are thought to be
  hydrogen bonds. These are only effective across a
  distance less than 0.5 nm.

7
Properties

• Tenacity
• Silk is strong fibre. Its strength is due to its
  beta-configuration, linear structure and very
  crystalline polymer. these two factors permit
  many more hydrogen bonds to be formed in a much
  more regular manner. Usually 30.9 44.1 cN/tex.
  Wet strength is 75 to 85 prcnt of dry strength.
• When wet silk loses strength. This is due to
  water molecules hydrolyzing a significant number
  of hydrogen bonds.
• Elastic nature
• Silk is considered to be more plastic than
  elastic. Because its a crystalline polymer it
  does not allow polymer movement which could occur
  in amorphous region.
• Its handle is regarded as soft because of the
  smooth, even, and regular surface of silk
  filament.
• Hygroscopic nature
• Because its a crystalline polymer, its less
  absorbent than wool.
• Thermal properties
• Silk is more sensitive to heat than wool. This
  due to the lack of covalent linkages in the
  polymer system. The salt linkages, hydrogen bonds
  tend to break when temperature reaches 100C.

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• Moisture regain is 11.
• Specific gravity is 1.25.
• Effect of sunlight
• Sunlight tends to encourage the decomposition of
  silk by atmospheric oxygen.
• Electric properties

9
Chemical properties

• Acid causes immediate breakdown of silk polymer
  because there are no covalent linkages between
  silk polymers.
• Alkaline solution cause silk filament to swell.
  Initially alkali will separate the silk polymers
  from each other. Prolonged exposure will
  hydrolyze peptide bond, resulting in polymer
  degradation and complete destruction of silk
  polymer. So yellowing of white or dulling of
  colored textile occurs during laundering. It is
  due to rearrangement of polymers.
• Effect of bleach is same as wool.
• The resistance to environment is not as good as
  that of wool. This low resistance is due to lack
  of covalent cross linkages.