silk fiber

Silk
It is also called Queen of fibers.
It is natural protein fiber of animal origin. Instead of being
grown in the form of hair, it is produced by insects in the
form of continuous fine strand of fibers called as filament,
to build cocoons. There are about 300~400 varieties of
moth but only a few are profitable to collect.

Silk
Silk producing Countries:
Most historians agree that silk and sericulture (the
production of silkworms) had their origin in china
nearly 2500 BC.
Now there are over 40 countries on the world map
producing silk. China is the largest producer and
supplier of silk followed by India and then Brazil.
* Production of silkworm is called as Sericulture.

Silk
Types of silk ( w. r. t) silkworms:
The silk is obtained from the cocoons of certain
caterpillars (larvae), they build cocoons to protect
themselves.
Most common types are :
a. Cultivated silk spun by Bombyx mori silkworm.
b. Wild silk spun by certain wild species of silk worm.

Silk
Life cycle in sericulture
Hatching of egg:
The first stage in sericulture process is the production
of silk worm eggs in a controlled environment.
Female moth deposits about 400 eggs at a time.
Hatching of eggs is normally done by spreading eggs
over shallow trays in the

Silk
Eggs are kept in hatching shed, which is closely
regulated at 80 ⁰F for about 8 to 10 days.
Finally tiny eggs are hatched into Larvae
(caterpillars). At this point, the Larva is about
quarter of an inch.

Silk
Feeding period:
Once hatched, the
larvae are placed in
huge trays and fed
huge amount of
chopped mulberry
tree leaves.

Silk
During this stage silkworm do nothing but eat.
After about 30 to 35 days since it was hatched, it stops
eating and changes color to greenish white and filled
with liquid silk.
It is about 10,000 times heavier than when it hatched.
The silkworm is now ready to spin a silk cocoon.

Silk
Spinning the
cocoon:
The silk worm now
starts spinning of
cocoon and liquid
silk secretions
extrude from two
glands.

This viscous, clear liquid is forced through the
openings, on the mouthpart of larva, harden on
exposure to air and form twin filaments.
A second pair of glands nearby secretes a gummy
binding fluid called sericin which binds two
filaments (fibrions) together steadily over the
next 3 to 4 days, the silkworm rotate its body
into a figure- 8 movement over some 300,000
times constructing a cocoon and producing
about a mile of silk filament.

Silk
The caterpillar then changes from larva to pupa and in this
condition it sleeps for about 18 to 20 days. Then if left
undisturbed it transforms into moth.
If pupa is not killed it will transform into moth and becomes
fully awake and strives to emerge from cocoons . It will push
itself against the wall of cocoon by breaking some of the
obstructing fibers.
The continuous silk filament is thus broken up into
thousands of short pieces.
Within the few days of making its cocoons, the silk
producers kill the pupa in the cocoon.

Silk
The usual method of killing are;
1. Immersing the cocoons in the steam for few minutes.
2. By baking cocoons in hot oven.
3. By placing the cocoons in the boiling water.
4. By freezing the cocoons to death.

Processes:
 Reeling
 Throwing
 Degumming

Silk
Reeling of silk:
The process of unwinding the filament from the
cocoon is called reeling.
This process is carried out in a building called filature
or reeling plant.
The cocoons are soaked in boiling water to soften the
sericin gum that is holding the filaments in place.

Silk
The cocoons are taken out and loose fluffy silk on
outside is cleaned off which is then used in production
of spun silk yarn.
The cocoons are again placed in hot water basin and
labor use a brush or broom to push cocoons up and
down in the water until some loose end of filament
becomes attached to broom .
The filament is drawn out gently; the cocoons tumbles
around in water and gradually unreel itself.
* A single filament of silk is very fine (12~30μm)

Silk
Throwing:
It is a process in
which two or more
multifilament strands
are combined and
twisted together to
form heavier threads.

Silk
Degumming:
The process of removing the sericin from raw silk or
thrown silk is called Degumming.
The natural sericin gum is normally left on silk during
reeling, throwing and weaving. It acts as a size which
protects the filaments and fibers from mechanical
injury.
The gum is removed from finished yarn by boiling
with soap and water.

Cross section of raw silk is roughly elliptical. Figure shows that triangular
twin fibroin filaments, covered by sericin, face each other. The beauty,
softness and luster of silk are due to the triangular cross section of the silk
filament. After degumming process, twin fibroin filaments separates into
individual filaments giving finer and more lustrous fiber. It lacks longitudinal
features along the longitudinal-section

Silk
Silk fabrics woven with sericin have a characteristic
stiffness of handle and dull in appearance.
After degumming silk acquire beautiful luster.
One third of weight of fabric is lost when sericin gum
is removed in this way.
Raw silk with gum still on filament is called hard silk.

Silk
Spun silk:
Short length of silk fibers obtained from the various
silk waste are not used in reeled silk and in making
thrown silk yarns but these are spun into yarns just like
cotton, linen or wool yarns.
Spun silk yarn are strong but they are less lustrous
than reeled silk yarn and not as strong and elastic as
reeled silk filament yarn.

Sources of staple silk :
Pierced cocoons
Floss
Double cocoons

Silk
Silk worm diseases:
The silk worm is subject to number of severe diseases
the most common of which is “Pebrine”
Black spots appear on the body of silk worm.
The infected silkworm kill off by the disease before
they can spin

Cross Sectional view
(Triangular)
Longitudinal view

Properties of silk fiber:
Tensile strength:
Silk is strong fibre. Its strength is due to linear
structure and very crystalline polymer.
Usually 30.9 – 44.1 cN/tex
* Which one is more stronger Silk or Steel???
Elongation:
Silk filament has elongation at break 20-25 %.
Specific gravity:
Its specific gravity is 1.25

Elastic properties:
Silk is considered to be more plastic than elastic.
Because it’s a crystalline polymer it does not allow
polymer movement which could only occur in
amorphous region.
Its handle is regarded as soft because of the
smooth, even, and regular surface of silk filament.
Effect of moisture:
It absorbs moisture. Its moisture regain is 11%.
Degummed silk will swell as it takes up moisture.

Effect of heat:
Silk will withstand higher temperature of 140 degree
centigrade and decompose at 165 degree centigrade.
Effect of age:
Silk may suffer a gradual loss of strength if stored not
carefully.
Effect of sunlight:
Silk starts decomposition by exposure to strong
radiation of sunlight.
Effect of acids:
Dilute acids should be used.

Effect of alkalis:
Light alkali like ammonia cause little damage.
Effect of insects:
It can be readily attacked by
clothes moth or
carpet beetles.
Effect of microorganism:
Silk is resistant to mildew and bacteria
unless it is left for some period of time
under extreme conditions.

ADVANTAGES OF SILK FABRIC:
 Luxurious hand (the feel of a fiber, yarn, or fabric to
the wearer)
 Excellent drape (a fabric’s ability to fold while worn)
 Wonderful luster (reflection of light on fabric)
 Hydrophilic
 Stain resistant
 Strong but lightweight

DISADVANTAGES OF SILK FABRIC:
 Fair abrasion and resiliency
 Turns yellow if bleached
 Poor resistance to exposed sunlight
 Expensive
 Degrades over time with exposure to oxygen, making it
difficult to preserve

Applications of silk fiber:
Dietary
Pharmaceutical
Biomedical
Cosmetic
Automobile
House Building
Anti-bacterial
* Consult the article given for detailed study of silk fiber applications (will be included in exam)

silk fiber

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silk fiber