Silk fibers
Silk is a protein fiber made from silkworms and is the only natural
fiber that is a filament fiber. Silk fibers spun by several species of
arthropods have existed naturally for hundreds of millions of years.
Silk fibers are produced from various types of ectodermal glands in the
mites, spiders, and several groups of insects. Commercial silk is
obtained from the cocoons spun by certain caterpillars (larvae of moths
and butterflies) before pupation. Until the discovery of nylon and other
synthetic fiber polymers, the silk of domestic silkworm, Bombyx mori,
was economically and, at the time of war, also strategically important
commodity.
Silk is an animal fiber, produced by caterpillars belonging to the genus
Bombyx. A single silk filament is the product of a series of stages
derived from the cultivation of mulberry trees for feed to the
propagation of the domesticated silkworm, Bombyx mori. During the
caterpillar phase, the worm wraps itself in a liquid protein secreted by
two large glands in its head. This secreted protein hardens upon
exposure to the air.
The resulting filament is bonded by second secretion, sericin, which
forms a solid sheath or cocoon. Under natural conditions, a moth
eventually breaks through the cocoon. In sericulture, the larva is
killed in the cocoon by steam or hot air in the chrysalis stage before
its metamorphosis. Sustained heat processing softens the hardened
sericin so that the filament can be unwound.
The silk filament is a continuous thread of great strength measuring
from 500-1500 meters in length. Single filaments are too thin for
utilization. For production purposes, several filaments are combined
with a slight twist into one strand. This process is known as “silk
reeling or filature”. Silk is a premium priced agricultural commodity,
although its sheer volume is less than one percent of the market for
natural textile fibers. The international demand for high-quality silk
has multiplied. Appropriate cocoon-drying techniques and reeling
operations are vital to supplying good quality silk.
Physical Characteristics of Cocoon
The silk glands of the Bombyx mori are structured like tubes consisting
of a Posterior, Middle and Anterior section. The Posterior is long and
thin. The Middle is short with a diameter measuring 3-4 mm. The Anterior
is extremely thin, leading to the spinneret in the head of the larvae
from which the silk is excreted.
Color :
Color is a characteristic particular to the species. It is the presence
of pigments in the sericin layers, which cause the color. This color is
not permanent and washes away with the sericin during the degumming
process. There are diverse hues of color including but limited to
white, yellow, yellowish green and golden yellow.
Shape :
Cocoon shape, as color, is peculiar to the given species. Generally,
the Japanese species is peanut-shaped, the Chinese elliptical, European a
longer elliptical and the polyvoltine species spindle-like in appearance. Hybrid cocoons assume a shape midway between the parents.
Wrinkle : The deflossed
cocoon has many wrinkles on its surface. Wrinkles are coarser on the
outer layer than within the interior layer. It is recognized that
coarse wrinkled cocoons reel poorly.
Cocoon Weight :
The most significant commercial feature of cocoons is weight. Cocoons
are sold in the marketplace based on weight as this index signals the
approximate quantity of raw silk that can be reeled. Pure breeds range
from 2.2 to 1.5 g, while hybrid breeds weight from 1.8 to 2.5 g.
Thickness/ Weight of Cocoon Shell :
The thickness of the cocoon shell is not constant and changes according
to its three sections. The central constricted part of the cocoon is
the thickest segment, while the dimensions of the expanded portions of
the head are 80 to 90 percent of the central constricted. The weight of
the silk shell is the most consequential factor as this measure
forecasts raw silk yield.
Hardness or Compactness :
Cocoon hardness correlates to shell texture and is affected by cocoon
spinning conditions. The degree of hardness also influences air and
water permeability of cocoons during boiling. A hard shell typically
reduces reliability (during the cocoon reeling process), while a
soft-shell may multiply raw silk defects. In short, moderate humidity
is preferred for good quality cocoons.
Shell Percentage :
It is essential to quantify the ratio of the weight of the silk shell
versus the weight of the cocoon. This value gives a satisfactory
indication of the amount of raw silk that can be
reeled from a given quantity of
fresh cocoons under the transaction. In newly evolved hybrids,
recorded percentages are 19 to 25 percent, where male cocoons are higher
than female cocoons.
Raw Silk Percentage :
The normal range is 65 to 84 percent for the weight of the cocoon shell
and 12 to 20 percent for the weight of the whole fresh cocoon.
Filament Length :
Filament Length determines the workload, rate of production, evenness
of the silk thread and the dynamometric properties of the output. The
range of total length is from 600 to 1 500 m of which 80 percent is
reliable while the remainder is removed as waste.
Reliability :
Reliability is defined as the fitness of cocoons for economically
feasible reeling. Reliability is greatly affected by careful action
during cocoon spinning, drying, storage, pre-processing, reeling machine
efficiency and operator skill. The measured range is from 40 to 80
percent with serious deviations depending on the type of cocoon.
Size of Cocoon Filament : The measure denier expresses the
size of silk thread. A denier is the weight of 450 m length of silk
thread divided into 0.05 g units. At the coarsest section of cocoon
filament from 200 to 300 meters, the denier increases. Once more these
dimensions become finer and finer as the process approaches the inside
layer. The average diameter of cocoon filament is 15 to 20 microns for
the univoltine and bivoltine species.
·
Defects : A series of minor defects may be found in cocoon
filaments such as loops, split-ends, fuzziness, nibs, and hairiness.
While these defects are observed among silkworm varieties, mounting
conditions seem to contribute to their incidence. These filament defects
directly affect raw silk quality.
Lousiness :
Hair-like projections in the silk fiber are called Lousiness. Another
factor promoting lousiness is mounting of over-mature larvae. When
fabrics woven with these defects are dyed, it looks as if the fabric is
covered with dust or is a paler shade than the rest. In fact, the
protruding fibril is more transparent and has a lesser capacity to
absorb dyes.
Silk Reeling
Silk Reeling is the process by which a number of cocoon baves are reeled
together to produce a single thread. This is achieved by unwinding
filaments collectively form a group of cooked cocoons at one end in a
warm water bath and winding the resultant thread onto a fast-moving
reel. Raw silk reeling may be classified by direct reeling method on a
standardly sized reel,
indirect method of reeling on small reels, and the transfer of reeled silk from small reels onto standardly sized reels on a re-reeling machine. The last technique is primarily applied in modern silk reeling processes.
indirect method of reeling on small reels, and the transfer of reeled silk from small reels onto standardly sized reels on a re-reeling machine. The last technique is primarily applied in modern silk reeling processes.
Hand Spinning Wheel
This primitive spinning apparatus is operated by two hands – one to
drive the wheel and the other to feed in cocoons. One end of the reeling
thread is wound onto each wheel, while cocoons are boiled in a separate
pot.
Automatic Reeling Machine
In raw silk production, the continuing increase in labor costs has
mandated automation. Around 1950, the Automatic reeling machine, which
controls the number of reeling cocoons per thread, was invented. Shortly
thereafter, it was replaced by a second Automatic reeling machine,
which could automatically control the size of the reeling thread.
The Automatic reeling machine mechanizes the processes of groping ends,
picking ends; cocoon feeding to reeling thread and separation of dropped
end cocoons during the reeling process. The efficiency of the Automatic
reeling machine compares favorably with the manual Mult-ends reeling
machine.
The Automatic reeling machine though built to replace manual reeling
still requires manpower for problems with the reeling thread, which must
be corrected by hand. A moderate amount of cooked cocoons are carried
to the newly cooked cocoon feeder and then removed into the groping end
part.
The end groped cocoons go to the picking end part and the correctly
picked end cocoons are dispensed to the cocoon supplying basket which
continuously rotates around the reeling basin on an endless chain belt.
Usually, the reeling method is classified into the fixed cocoon
feeding system and moving cocoon-feeding system.
Silk Production
Cocoon Composition
The composition of the whole cocoon is defined as the cocoon shell, pupa
and cast off skin. The pupa makes up the largest portion of its weight.
Note that much of the cocoon content is water. Therefore it is
necessary to remove the water to improve the cocoon filament for reeling
and to better preserve the cocoon over a long period.
Composition
|
Description
|
Composition of Cocoon Shell
|
The silk filament forming the cocoon shell is composed of two brins (proteins) named fibroin and
covered by silk gum or sericin. The amount of sericin ranges from 19 to 28 percent according to the type of cocoon.
|
Structural Features of Silk
|
|
Physical and Chemical Properties
|
|
Hygroscopic Nature
|
11 percent is the accepted moisture regain coefficient
for silk; the mercantile weight of silk is derived based on this factor. |
Effect of Light
|
Continuous exposure to light weakens silk faster than cotton or wool. Raw silk is more resistant
to light than degummed silk. |
Electrical Properties
|
Silk is a poor conductor of electricity and accumulates a static charge
from friction. This trait can render it difficult to handle in the
manufacturing process. This static charge can be dissipated by high
humidity or by maintaining an R.H. of 65 percent at 25ºC.
|
The action of Water
|
Silk is a highly absorbent fiber, which readily becomes impregnated with
water. Water, however, does not permanently affect silk fiber. Silk
strength decreases about 20 percent when wet and regains its original
strength after drying. The fiber expands but does not dissolve when
steeped in warm water. Note that the fiber will also absorb dissolved
substances present in water.
|
Effect of Heat
|
If white silk is heated in an oven at 110ºC for 15 minutes, it begins to
turn yellow. At 170ºC, silk disintegrates and at its burning points
releases an empyreumatic odor.
|
Degradation by Acids, Alkalis
|
Treatment of silk fibers with acid or alkaline substances causes hydrolysis of the peptide linkages.
The degree of hydrolysis is based on the pH factor, which is at minimum between 4 and 8.
Degradation of the fiber is exhibited by the loss of tensile strength or change in the viscosity of the
solution. |
Proteolytic Enzymes
|
Proteolytic enzymes do not readily attack fibroin in a fibrous form
apparently because the protein chains in silk are densely packed without
bulky side chains. Serious degradation may be caused by water or steam
at 100ºC.
|
Oxidation
|
Oxidizing agents may attack proteins in three possible points. Hydrogen
peroxide is absorbed by silk and is thought to form complexes with amino
acid groups
and peptide bonds.
|
Other Agents
|
Chlorine attacks fibroin more vigorously than does sodium hypochlorite. The oxidation is mainly at the tyrosine residues.
|
Cocoon Quality
|
A Series of natural circumstances will produce variations in cocoon quality. Some of
the most noteworthy include:
|
No comments:
Post a Comment