Introduction
Cotton is a soft fiber that grows around
the seeds of the cotton plant. The fibre is most often spun into thread and
used to make a soft, breathable textile. Cotton is a valuable crop because only
about 10% of the raw weight is lost in processing. Once traces of wax, protein,
etc. are removed, the remainder is a natural polymer of pure cellulose. This
cellulose is arranged in a way that gives cotton unique properties of strength,
durability, and absorbency. Each fiber is made up of twenty to thirty layers of
cellulose coiled in a neat series of natural springs. When the cotton boll
(seed case) is opened the fibers dry into flat, twisted, ribbon-like shapes and
become kinked together and interlocked. This interlocked form is ideal for spinning
into a fine yarn. Cotton is composed of pure cellulose, a naturally occurring
polymer. Cellulose is a carbohydrate, and the molecule is a long chain of
glucose (sugar) molecules.
3-D model of cellulose
History
Cotton has been used to make very fine
lightweight cloth in areas with tropical climates for millennia. Some
authorities claim that it was likely that the Egyptians had cotton as early as
12,000 BC, and they have found evidence of cotton in Mexican caves (cotton
cloth and fragments of fibre interwoven with feathers and fur) which dated back
to approximately 7,000 years ago. There is archaeological evidence that people
in South America and India domesticated independently different species of the
cotton plant thousands of years ago. The earliest written reference is to
Indian cotton. Cotton has been grown in India for more than three thousand
years, and it is referred to in the Rig-Veda, written in 1500 BC. A thousand
years later the great Greek historian Herodotus wrote about Indian cotton:
“There are trees which grow wild there, the fruit of which is wool exceeding in
beauty and goodness that of sheep. The Indians make their clothes of this tree
wool.” By the end of the 16th century BC, cotton had spread to warmer regions
in Americas, Africa and Eurasia.
Cotton Cultivation:
Cultivation of cotton requires a long
free period, plenty of sunshine and a moderate rainfall usually
24-48".Heavy rain destroys the crop of cotton.All these conditions and
optimal period of usually six months, enable the plant to be mature.These
conditions are usually found in between north and south areas.
However cotton is cultivated in stages given below...
Selection of seed; Selection of seed is very important as it controls the following
factors
- Quality of Cotton
- Length of fibers
Resistance to various
cotton diseasesTherefore A proper seed of required properties is selected.
Preparation of land; It is very important for cotton
cultivation to plough the land.This plougning is done three to four times &
then water is supplied to the land that improves the deficiencies of many
organic compounds in the land.
After that Cotton seed is usually sown in the spring. Blooms appear on the
plant from 80 to 110 days after planting. The blooms are creamy white or yellow
when they first appear. From 12 hours to 3 days after the blooms have appeared
and they have changed their color as pink, lavender or red. After this blooms
have fallen off the plant leaving the developing boll on the stem. After 50 to
80 days pod mature and bursts open so the cotton is ready to be picked.
Now lets find out how this happens in steps..
Formation of the fiber:
The cotton fibers form long hairs
attached to the seeds inside the boll. As the plant grows growth of fiber also
take place and therefore the length of the fiber also increases. The individual
cotton fiber is a seed-hair consisting of a single cell. It grows from the
outer skin of the cotton seed.
Each cotton seed may produce as 20,000 fibers on its surface. A single boll
will contain 150,000 fibers or more.
In some varieties of cotton, tiny fibers can be detected on the embryo seed one
or two days before flowering. Other verities begin their fiber production a day
or two after flowering.
Fiber Growth:
In early six days, the growth of the fiber is
comparatively slow. Then for the next fifteen days it is much more rapid. The
fiber may reach a length equal to 2,000 times its diameter during this three
week growing period. Then for next three days it grows more slowly again
until the lengthwise growth sudden stop. During its period of rapid elongation,
the cotton fiber is in the form of a thin walled tube of cellulose with one end
closed and other attached to the seed. It is filled with protoplasm and liquid
nutrients which have been drawn from the main supply vessel of the plant.
When it stops lengthwise growth, the cotton fiber begins to strength its
internal structure. Layers of cellulose are added one after another to the thin
cellulose membrane from inside the cell. Each day sees a new layer deposited,
creating a structure similar in cross section to the growth ring in a tree. The
cotton fiber, however adds its layer by depositing cellulose from the liquid
inside the fiber. The innermost layers are the youngest ones, whereas the
outermost layers are the youngest in a tree.
Each growth ring in the cotton fiber corresponding to a day of growth and
cellulose deposition.
Cotton grown in artificial light switched off and on develops ring like the
natural fiber. The cellulose is laid down in the form of spiral fibrils or tiny
threads, some 1,000 or more to each ring .The deposition of cellulose continues
for about 24 days, so that mature cotton constructed from thousands of fibrils
of cellulose arranged in spiral form. During its period of growth, cotton is
compressed tightly into the limited place available inside the boll. As the
cell wall thick, the fibers are fixed in their distorted portion.
Effect of Growth
Conditions:
Cotton yield is affected by the conditions
under which the plant has grown before it flowered. The quality of the fiber is
affected by the conditions it experiences during the fiber are developing
inside the boll.Shorts fibers are due to poor growth in the initial phase of
development. Thin-walled fibers are due to slowed down development of the wall
in the second phase. The deposition of cellulose also effect.
Neps:
The immature, thin walled fiber can cause trouble in several ways. They do not
dye as dark as the thick walled fibers. They break easily causing greater
losses of waste fiber during processing. They are more flexible than the
thick-walled fiber, so that they bend and tangle more easily forming Neps.
Harvesting and picking:
After the full Growth of fiber on seed,
picking is done by either mechanically or by hand.In hand picking cotton bolls
are picked up by hand practice and put into the bags over shoulders of the
pickers.These bags are made up of poly ester fibers therefore these small poly
ester fibers may mix in cotton fibers.But in mechanical piking
this is done by mechine so this is time saving as compered to first one.But
there is a drawback of this technique that mechine can break down the fibers
and also while piking some part of plant or boll remains with the fibers.Usually
hand picking is referred in the areas where labor is easily available,
while in progressed countries mechanical picking is done.
The Ginning Process:
Ginning is the process in which fibers are
separated from cotton seeds.This is done by different gin mechines.The Process
involves following steps.
Stage1 Seed Cotton
Conditioning and Cleaning:
Seed cotton - seeds with fiber
still attached-usually arrives at the gin in large trailers or modules used for
hauling it from the field and for storing it until ready for ginning. From the
storage area, conveyor pipes transfer it to the various stages of the ginning
process using large volumes of air to make the flow of the cotton easier and
faster.
Typically, seed
cotton is first dried in large driers using heated air to reduce its moisture
content. A cylinder cleaner then removes the leaves and other small trash from
the seed cotton by shaking it with spiked cylinders, while conveying it across
a screen with small openings that sift the trash released from the seed cotton
by the impact action of the cylinders.
Next, a stick machine removes any large sticks or hulls (the dried bolls that
form a shell around cotton as it grows) with revolving channel saws. These saws
grab the seed cotton and whip it over metal bars to sling off its trash. If the
seed cotton requires additional drying and cleaning, gins will often run it
through another drier and another cylinder cleaner and stick machine.
Stage2 Ginning the Seed
Cotton:
The seed cotton is now
ready for ginning.Cotton is conveyed to the roller gin, while upland cottonsare
conveyed to the saw gin for separation of seed and fiber. After being ginned,
the cotton fiber is often referred to as lint.
Lints: are the long cotton fibers that are
recovered from seed during ginning.
Linters: Cotton linters are very short fibers that
remains with seeds after ginning, that are recovered from seeds by special
delinting techniques.
Stage3 Lint
Cleaning:
Lint cleaners remove the small
trash from the ginned lint left behind by the cylinder cleaner and stick
machines. Saw-lint cleaners grab the lint with a cylinder saw and whip it over
metal bars to dislodge its trash. Lint cleaning of roller ginned cotton usually
involves a combination of three machines: a cylinder cleaner, an impact cleaner
which uses cylinders to agitate and release the trash from the lint, and an
air-jet cleaner which removes the trash from the lint using high velocity air.
Stage4 Packaging the
Lint:
In the final
stage, a bale press compresses the ginned lint into bales that weigh between
450 and 500pounds. The bales are then wrapped with a protective covering, ready
for delivery to the warehouse where they are sold to the various textile mills.
Types of ginning:
There are two types of
ginning.
- Roller ginning
- Saw ginning
Roller
ginning:
The saw gin did not replace the roller gin entirely.
Although much slower, the roller gin is gentler to the cotton than the saw gin,
whose serrated saws tend to break more of its fibers. Roller gins, therefore,
continue to be used for ginning Pima cotton to protect its extra-long staple or
length, a desirable quality that increases the value of the cotton fiber.
Even though the roller gin continued to be used for ginning extra-long-staple
cottons, its low ginning rate made it too expensive to maintain and operate. In
an effort to increase its ginning capacity, several gins were developed that
improved the roller ginning rate, though not significantly. It wasn't until the
1960's, when the rotary-knife gin was developed at the Southwestern Cotton
Ginning Research Laboratory in cooperation with gin manufacturers and private
ginneries, that roller gins became more efficient.
The rotary-knife gin improved on the McCarthy gin invented by Fones McCarthy in
1840. It retained the McCarthy Gin's ginning roller and stationary knife, but
replaced its reciprocating knife with a rotary knife that greatly increased the
roller ginning rate. Compared to the McCarthy Gin which produces only 1/4 bale
of lint an hour per stand, the rotary-knife gin can produce up to 1 1/2 bales
of lint an hour per stand.
Cross section of the rotary-knife roller gin. Seed cotton (A) slides down the
feeder apron (B) and enters the gin between the ginning roller (C) and the
rotary knife (D). The ginning roller, which rotates constantly and is held
tightly against the stationary knife (E) pulls the lint (F) under the
stationary knife. The seeds (G), too big to pass under the stationary knife,
are swept away by the rotary knife which spins in the opposite direction of the
ginning roller, and fall onto a seed belt that delivers them to a collection
bin.
Saw Ginning:
Whitney's saw gin,
which removed the seeds from the cotton fibers with a spiked cylinder that
pulled the lint through wooden slots too narrow for the seeds to pass through,
was 50-100 times faster than hand ginning. It was especially effective in
separating the hard-to remove seeds in Upland cottons, making these
short-staple cottons more economical to produce than before. With Whitney's saw
gin, farmers were finally able to mass produce Upland cottons to satisfy
consumer demand, and the South had a new export commodity that contributed to
the economic development of this nation.
Modern research has improved on an old concept, and saw ginning remains the
most efficient way to gin cotton. The spikes in Whitney's saw gin have been
replaced with circular serrated saws that have made it even more effective for
separating seed and fiber. Today's saw gin can produce up to 15 bales of lint
an hour per stand.
Structure:
Cotton fiber consists
of following parts..
Primary Wall:
Primary wall is a tough, protective layer
that formed the shell of the fiber during its early days of growth inside the
boll. Seen under the microscope, the surface of the cotton fiber is wrinkled
like a prune. These surface wrinkles are caused by shrinkage as the fiber has
dried.
Cotton is beaten in water under the special conditions and short fragments of
the primary wall slip of like sleeves. Chemical analysis of primary wall shown
that it contains wax, protein, pectinaceous substances and cellulose. When
these non cellulosic materials are removed chemically the cellulose fibrils can
be seen with the help of electron microscope as a felt like mat of tiny
threads. These cellulose fibrils are between 1/40th and 1/100th of a micron in
diameter, and consist of many long cellulose molecules held tightly alongside
one another by natural forces of attraction that are exerted between close
packed molecules.
These tiny fibrils of cellulose are, like the fiber itself, strongest in a
longitudinal direction. The crisscross network of fibrils forming the primary
wall of the cotton fiber is an arrangement that confers great strength. The
outer wall of the fiber is able to resist force from any direction, although it
does not have the immerse longitudinal strength that would be available if all
the cellulose fibrils were arranged side by side along the longitudinal axis of
the fiber.
Tough skin effect of the primary wall is noticeable when cotton is
immersed in solution. In cuprammonium hydroxide, the cotton fiber swells up
into a distended tube which is tied at intervals like a string of sausages.
This is probably due to the resistance of the primary wall to the effects of
the solvent. As the cuprammonium hydroxide penetrate and swells the secondary
cellulose inside the fiber, the primary wall ruptures ( break or burst
suddenly) and is rolled back into tight ligatures that resist the swelling
effects of the solvent. This resistance on the part of the primary wall is
largely due to the matted arrangement of the cellulose fibrils forming the
network of the wall. Immature fiber which have little secondary cellulose, are
swelled only tightly by solvent.
Secondary Wall:
The inner or
secondary layer of cellulose forms the bulk of the cotton fiber. This is the
cellulose that is laid down during the second stage of fiber growth.
The growth ring arrangement of
the cellulose in the secondary wall can be seen when swollen fiber are examined
microscopically. With the help of the electron microscope, the organization of
the fibrils can be followed. The fibrils of the secondary wall are packed
together in a near parallel arrangement. The layers of fibrils lie in spiral
formation along the fiber, the direction of the spiral often reversing in the
same layer.
The secondary wall is almost pure cellulose and represents about 90% of
the total fiber weight. The bulk of the cellulose in the cotton fibers
therefore arranged in the form of fibrils that are packed alongside each other.
They are aligned as spirals running lengthwise through the fiber. The fibrils
are therefore able to exert high strength in a longitudinal direction in the
fiber bestowing immense longitudinal strength on the fiber itself.
Lumen:
When the cotton
fiber is alive and growing, it is distended by pressure of the liquid nutrients
and protoplasm inside it. As the fiber dies and collapses, the liquid
disappears leaving an almost empty channel running lengthwise through the
center of the fiber. This center canal is the lumen.
In a mature cotton fiber, the
deposition of cellulose in the secondary wall may have been so heavy as to
leave very little lumen at all. The dry fiber looks like a solid rod rather
than a tube. On the other hand an immature fiber may have so little secondary
cellulose that the lumen is wide and distinct.
When the cellulose of the primary and
secondary walls are dissolved by powerful solvents, a thin membrane of
protoplasm is left behind. This is the dried up residue of the material that
were dissolved in the watery liquid inside the living fiber. It contains a
colored substance, the end chrome, which gives the cotton its natural color.
Cotton has been grown experimentally in which the lumen contains brown or green
pigments which act as a natural dyes. In a natural cotton fiber, the lumen
almost completely collapsed represents a considerable volume of unoccupied
space. It enables the cotton fiber to absorb water by capillary attraction and
so has an important influence on the properties of cotton as a textile.
The lumen, however, forms only a part of
the unoccupied space in a cotton fiber. The cellulose walls are porous and can
absorb water.
The fibrils of cellulose forming the fiber walls are compact and relatively not
allowing water to penetrate, the sub-microscopic spaces between them form
capillaries that make the cellulose network porous. Large surfaces are exposed
by the fibrils network, and water is able to penetrate by capillary attraction.
It has been estimated that as much as 20-41%of the volume
of a cotton fiber consists of unoccupied space. One third of this is accounted
for the lumen and the rest is provided by the spaces between the fibrils in the
fiber walls. In general, the coarser varieties of cotton are more porous than
the compact, finer cotton.
Immature Fiber:
Immature cotton fiber can be recognized by the thinner
cell walls. The dry, immature fiber does not show the oval or kidney shape
cross section; instead, the fiber tube collapses into thin ribbon that curls
into a variety of distorted shapes. Often the immature fiber is u-shaped in
cross-section. These thin-walled, immature fibers may not twist as they
collapse on drying. They seldom have the pronounced convolutions that are so
typical a feature of the mature fiber.
Properties:
Mechanical properties
Elongation:
Cotton has elongation at break of 5-10%.
Elasticity:
Cotton is a relatively inelastic rigid fiber. At 2% it has elastic recovery of
74%, at 5% extension, he elastic recovery is 45%.
Tenacity:
Tenacity is 3.0-5.0 g/den and tensile strength is 2800-8400kg/cm2.
Effect of Moisture:
Cotton fiber absorbs water and its strength increase. These fiber become 20%
more strong than dry fiber. At 100% humidity (humidity means amount of moisture
in air) cotton has an absorbency of 25-27%. Standard moisture in cotton is
8.5%. Standard absorbency of cotton is 65Rh+2 at 20c+2.
Action of Water:
The dry cotton fiber constructed from its fibrils of cellulose. Cellulose is
tightly held together by bounds.
Water, however, penetrate into the cellulose network
of cotton fiber. It makes it way into the capillaries, spaces between the
fibrils and chemical links in the cellulose molecules. In this way water
molecules tend to force the molecules of cellulose apart Lessing the force that
held the cellulose molecules together and destroying some of the rigidity of
the entire cellulose structure.
Water acts, in this way, as a plasticizer for
cotton. The cellulose molecules in wet cotton are so well lubricated by the
water molecules that the fibers become quite plastic and easily deformed. The
effect of pressure upon the cotton is identical with its effect upon any other
plastic—it changes it shape.
Cotton fibers water and they swell. The swelling of cotton
yarns and fabrics in water is accompanied by some shrinkage.
Mercerization:
A most valuable application of this swelling of cotton is in the process
known as mercerization. By treating cotton with caustic soda the fibers could
be made to swell. This caused an overall shrinkage in the fabric as the strain
between the threads release themselves. At the same time, fabric became much
strong and more easily dyed.
H.A Lowe found that by holding the
cotton so as to prevent its shrinking during mercerization it developed the
beautiful luster that we nom associated with the process.
During mercerization under these
conditions the cotton fibers regain their original circular cross-section due
to the swelling of the cellulose, and they tend to lose their
convolutions.
As they are held fast the whole time, they can not take up the strains by
shrinking; the plastic cellulose deforms and the cotton develops a smoother
surface than it had before .After washing and dying, the mercerized cotton
fiber retains his smooth cylindrical form.
Now-a-days, cotton yarn is mercerized to
improve its lustrous appearance and dyeing qualities. When the cotton contains
a high proportion of thin-walled immature fibers, mercerization will swell
these fibers and make them dye more like mature fiber.
Mercerized cotton is chemically little different
from ordinary cotton, It remains almost pure cellulose, but in a different
physical form. It is more reactive, and takes 12% of water from the air,
compared with its usual 6-8%.
H2SO4 and other acids are being used to swell
cotton and yield effects that resemble those of mercerization. Cotton is passed
rapidly through strong H2SO4, so that the material is in contact with the acid
for only a few seconds. Acid finishing of this sort is adapted to provide a
number of different effects.
Cotton fabric is made more transparent
or more wool like in this way. It is given an organdie finish which is
permanent and the fabric regains its characteristic stiffness after washing and
ironing.
Many other acids and salts are used for
treating cotton, and the details of such processes are often undisclosed. The
effect produce depend on the conditions used and on the nature of the fabric
itself. In most cases, however, it is the swelling of the cotton fiber that is
the most important factor.
During mercerization, the cellulose of the cotton fiber is swollen rapidly, and
the fiber becomes almost a solid cylinder of cellulose. The effect remains even
arter the cotton has been washed and neutralized. The drawing shows cross section
of cotton yarns.
Effect of Heat:
Cotton has an excellent resistance to degradation (Degradation means peocess of
being made worse or broken down) by heat. It brgin to turn yellow after several
hours at 120c, and decompoces markedly at 150c, as a result of oxidation.
Cotton is severely damage after a few minutes at 240c
Cotton burns readily in air.
Effect of Age:
Cotton shows only a small loss of strength when stored carefully. After fifty
years of storage, cotton may differ only slightly from fiber a year or two
old.
Effect of Sunlight:
There is gradual loss of strength when cotton is exposed to sunlight, and the
fibers turn yellow. Much damage caused by the ultra –violet light and by the
shorter waves of visible light.
Physical Property
Color:
The color of cotton is normally creamy white, is affected greatly by the
conditions under which it is produced. If the fiber is left too long in the
boll before being picked, it may turn grey or bluish white.
Length:
The length of cotton fiber is 1000-3000 times its diameter.
Shape:
The shape of cotton fiber is U or kidney type in cross section with lumen in
centre.
Rd Yellow:
Rd means Reflected Deviation. We check how much light is reflected from cotton
surface. It should be high than yarn will give better appearance.
Yellowness/+b:
When yellowness of the cotton is more, the grade becomes lower and which
produces weaker and inferior yarn. So, it also affects the yarn quality.
Luster:
Cotton fibers have a natural luster (means soft grow or shine) which is due to
natural polish on the surface of cotton. Surface smoothers and shape of the
fiber both control the luster of fiber. High luster is provided by the fiber of
nearly circular cross section.
Micro-structure
The wall of the fiber varies in thickness. It consists of two main section, the
primary wall or cuticle forming the outer layer, and the secondary wall forming
the inner layer.
1 micron= 1/1,000mm= 1/25,000 in. approx
Chemical Properties
Chemical composition of Cotton fiber:-
Cellulose 94% Range 88%-96%
Protein 1.3% Range 1.1%-1.9%
Pectin substance 1.2% Range 0.7%-1.6%
Waxes 0.6% Range 0.4%-1%
Trash content 1.2% Range 0.7%-1.6%
Sugar content 0.3% Range 0.1%-1%
Organic acid 0.8% Range 0.5%-1%
Pigment traces
Others 1.4%
Effect of Acid:
Cotton is attacked by the hot dilute acids or cold concentrated acids.It is not
affected by cold weak acids.
Effect of Alkalis:
Cotton swells in caustic alkali but is not damage. It can be washed repeatedly
in the soap solution without taking harm.
Effect of Organic Solvent:
Cotton has a high resistance to normal solvents but is dispersed by the copper
complexes cuprammonium hydroxide and cupriethylene diamine, and also
concentrated ( 70% ) H2SO4.
Insects:
Cotton is not attacked by moth grubs or beetles.
Cotton in Use
1-Cotton fabric has a pleasant feel. They are cool in hot weather. Cotton is
stronger in wet. This property is due to cotton stability in water and alkaline
solution.
2-The resistance of cotton to washing and wear is matched by the permanence of
many cotton dyes. Cotton can be dyed easily and cotton color will remain same
after repeated washing.
3-Cotton cellulose is not affected by moderate heat, so that cotton fabrics can
be ironed with a hot iron without damage. Cotton fabrics come up crisp and
fresh on ironing.
4- Cotton fiber is fairly rigid and stiff so cotton yarns and fabrics are not
flexible. They are, however, more flexible than linen (Flax).
5-To achieve flexibility, cotton can be spun into fine yarn and made into a
tight woven fabric. Poplins and voles are made in this way.
6-Cotton fibers are smoother, stiffer and straighter than wool and they do not
make up so readily into air- entrapping fabrics. But special cellulose waves
can be used to create the air cells that provide warmth and the surface of
cotton fabrics can be raised to form an air-filled pile. Molletons and
flannelettes are cotton fabric of this sort.
7-The versatility of cotton has made it into the most widely used of all
textile fibers. Cotton is made into every type of garment and house hold
fabric. Heavy cotton yarn and material are used for tyre cords and industrial
fabric of all description.
8-Cotton is widely used in making rainwear fabrics. It can be woven tightly to
keep out the driving wind and rain, yet the fabric will allow perspiration to
escape. Special rainwear materials are oven in such a way that water swells the
cotton fibers and close up the interstices in the cloth.
9-Much of the comfort of a textile material depends upon its ability to absorb
and desorbs moisture. A garment that does not absorb any moisture at all will
tend to feel clammy as perspiration condenses on it from the skin. Cotton
fibers, however, are able to absorb appreciable amount of moisture, and having
done so they will get rid of it just as readily to the air. Cotton garments are
therefore comfortable and cool, passing on perspiration from the body into the
surrounding air. No matter how tightly woven a cotton fabric may be, it will
permit the body to breath in this way.
10-This absorbency of cotton makes it an excellent material for house-hold
fabrics such as sheets and towels too.
11-The cotton fiber itself is dimensionally stable. A made-up cotton garment
may shrink to some extent due to the tension introduced by spinning and
weaving, but the fiber itself does not contribute significantly to any
shrinkage.
This so called relaxation shrinkage caused by the easing of strains set up
during spinning and weaving, can be overcome by a treatment called Rigmel and
Sanforized shrunk cotton fabrics are compression-shrunk in this way; they are
dimensionally stable and will neither stretch nor shrink more than 1% in either
direction.
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