Transcript Silk fibre - TexTile Come

SILK
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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.
Wild silk
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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.
Macro structure
Silk is a natural protein filament. It’s density is 1.34 g/cm cube
which makes it a medium weight fibre.
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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.
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Shiny silk filaments are revealed when sericin is removed in
mild alkaline solution.
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Silk is a fine, regular, translucent filament. It may be 600m
long but average about 300 m in length.
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Its cross-section may vary from 12µm to 30µm. this gives fibre
length to breath ration in excess of 2000:1.
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The beauty and softness of silk is due to the triangular crosssection 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.
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Micro-structure
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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.
Silk polymer
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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%.
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.
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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 it’s 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 it’s 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 100°C.
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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:
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Chemical properties
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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.