COTTON
Cotton has been cultivated for over 5000 years in tropical and sub-tropical countries and the usage of its conversation into yarn and fabric dates back to times immemorial. The highly refined and artistic of dyeing and printing of cotton fabrics was developed in the Indian subcontinent and China to high level of sophistication much before the industrial revolution started in the west, Even now when the man-made fibres have made great advancement in production and quality, “the King cotton” is continuing to maintain its acceptance and popularity. This is due to cotton’s highly versatile and desirable properties of pleasant handle, wear comfort strength, easy maintenance and above all modest price. Cotton is grown in the entire continent but major cotton producing countries, in order of quantity is
Egyptian and upto certain extent American cotton is of finer and longer fibres whereas Indian, Pakistani, Russian and American cotton is slightly inferior type.
GROWTH
The cotton plant belongs to botanical genus Gossypium and the fibre grows as a single cell from the cuter layer of the seed. After blossoming of the flower, a green pod form the increases to the size of a walnut and is commonly termed as boll. After 30 to 50 days of flowering, the boll bursts open and displays a white fluffy mass of fibres. After the sap in the sap in the fibres is dried, the seed fibres are picked up manually or mechanically.
COTTON PLANTATION
Cotton is generally grown in moderately hot climate cotton plant is produces seeds tear of cotton deteriorates both in quality and quantity so new qualities is done from April to August depending upon climate. The seeds are planted 1 to 10cm deep in rows and about one meter apart moisture and type of soil.
Young plants come up through surface of soil in about 8-12 days. Plants now grow and within 40 days they begin to form flower stalk. Flowering takes another 30 days. The cotton fibres grows inside closed “pod” which id grown in cotton flower. The pods cotani cotton seeds, wrapped up in young activity growing hairs.
There are about 20,000 fibres around one seed and about 15,000 or more fibres in a pod. When seeds are nearly riped the pod bursts open and cotton hairs project.
These cotton fibres along with their seeds picked up from boll. Either by hand pickig or by machine picking method.
GINNING
The process used to separate fibres from seed of cotton is called ginning. Some of the non-fibrous impurities are also removed during ginning. The fibres are about one third of weight of the seed cotton and are separated from the seed by a process of ginning. In this operation, the seed cotton is conveyed by grooved leather rollers against two knives that cut the fibres off the seed leaving a fuzzy layer of very short fibres. In the saw gin process, seeds are pulled away from the seed by rows of revolving saw-toothed bands. For transporting cotton to the spinning mills the fibres are pressed into compact rectangular packages called bales. The bale weight is from 170 to 220 kg each. The short fibres left on the seeds after ginning are known as linters. The linters are no use for spinning into yarns but after shaving off by the disinterring machines these can be converted into regenerated cellulosic rayons or nitrocellulose for use as explosively.
MORPHOLOGHY
On drying of cotton boll, the fibre shrinks, its cross-section changes from circular to bean shape it develops irregular convolution lengthwise.
Fibre dimensions vary considerably according to their varieties and may differ in length from 6cm in fineness from 10 to 20 micro meter longer and finer cotton with light color are values much higher than the coarser qualities. A fully developed and mature cotton fibre has four characteristic regions they are discussed below:
(a) CUTICLE: This consists of a thin outer layer composed of waxes and pectic materials but its true nature is not known. Main function of this layer is to protect the fibre from atmospheric.
(b) PRIMARY WALL: This layer is formed during initial growth of fibre and encloses nucleus and protoplasm that are essential for every living cell. It consists of cellulose and pectin in the form of fibrils and is laid longitudinally along the fibre axis. Its thickness is only about 1/100 the of the overall width of fibre.
(c) SECONDARY WALL: This part of the fibre is formed in the second phase of growth and is nearly 90% of the weight. On viewing the fibre under a microscope after swelling in the cuprammonium solution, successive layers of cellulose is visible that are termed as daily growth rings. These layers are fibril in structure and are deposited in a spiral form but the spiral changes direction frequently. This reversal is responsible for development of convolutions during drying of the fibres. The convolutions range between 8 to12 per millimeter and provide point of adhesion between fibres that contribute to strength of the yarn spun from these.
(d) LUMEN: This canal runs through the entire fibre and carries minerals essential for its growth. It keeps on shrinking with building up of the secondary wall and at the time of drying, it is only 1/20th of the fibre. The lumen contains proteins, minerals ad color pigments.
(e) CONVOLUTION: Convolution is the natural twist and is one of the chief properties of cotton. Convolutions are ribbon like twists which are produced during drying of the cotton cell content under the influence of sun. the number of convolutions in a cotton fibre varies according to its quality. Such as Egyptian cotton has about 230, American has 190 and Indian/ Pakistan
THE POLYMER SYSTEM OF COTTON
Cotton polymer has very well oriented molecules. It has dominant OH hydroxyl groups. Cotton fibre is a crystalline fibre having 65-70% crystalline regions I its polymer.
CONSTITUENTS OF RAW COTTON:
Raw cotton contains the usual constituents of a living vegetable cell but their quantities vary according to the variety of cotton. The approximate composition of raw cotton is as under:
Cellulose Oil and wax Pectin’s Proteins Mineral matter (Ash) Other organic matter | 88.0-96.0% 0.4-1.0% 0.7-1.2% 1.1-1.9% 0.7-1.6% 0.5-1.0% |
It will be obvious that on scouring and bleaching the cotton materials, about 6-7% weight consisting of waxes pectin, protein etc. will be lost due to removal of their non cellulosic components.
COTTON QUALITY
Quality of cotton fibre is mainly concerned with its staple length, fineness and colour.
(a)STAPLE LENGHTH: Staple length of cotton grown countries is below;
COUNTRY | STAPLE LENGTH |
Indian | Short in length about 25mm or less |
Pakistani | Short in length about 25mm or less |
Egyptian | Can grow upto 65mm in length |
Brazilian | Can be upto 90mm by cross breeding |
(B) FINENESS: Egyptian cotton is finest than the other cotton grown countries
(C) COLOUR: Colour of cotton by country wise is give below;
American | Normally white |
Egyptian | Light cream colour and has high luster |
Pakistani and Indian | Light grey colour |
PHYSICAL PROPERTIES
a) TENACITY: Cotton has a tenacity of 3-5 gpd (grams/denier). It is a strong fire due to good alignment of its long polymer. Cotton gains strength when wet. It is about 25% stronger in wet condition. This is due to reason that the polymers improved giving strength to the cotton fiber.
b) ELASTIC PLASTIC NATURE: Cotton is in-elastic due its crystalline polymer ststem and thet is the reason that fabrics crease and wrinkle easily
c) HYGROSCOPIC NATURE: Cotton is very obsorbent due to OH group in its polymer which attracts wter molecules being polar. Cotton has a moisture regain of 8.5%. Because of its ghroscopic nature cotoon does ot develop static charges. Since the cotton is very absorbent. Its fabrics feels crispy due to the reason that the cotton fibres rapicly absorb the moisture from finger.
d) THERMAL PROPERTIES: cotton conducts heat energy from body that’s why cotton fabric have a cool feeling. And that’s why excessive heat burns it.
· The action of pH and chemical agents on cellulose
a) EFFECT OF ACIDS: Cotton is not attacked by or effected by cold and weak acids. However if it is treated by hot dilute or cold concentrated acids it disintegrates.
EFFECT OF ALKALIES: Cotton has excellent resistance to alkalies.it swells in caustic soda but is not damaged. Cotton is mercerized. Mercerization is a process which is fabric is carried out by treating the cotton fabrics with caustic soda, which gives luster to fabric, it must be noted thet mercerization is done only to the cotton fabrics. Since the cotton is resistant to alkalies. It can be washed repeatedly in soap solution without any damage. From the chemical standpoint, the essence of this is process is in the absorption of alkali with the formation of alkali cellulose. Caustic soda combines with cellulose forming a molecular compound according to following equation:
C6H7o2(OH)3 + NaOH C6H7O2(OH)3NaOHb)
c)EFFECT OF HEAT: cotton has an excellent resistance to degradation by heat. It begins to turn yellow after several hours at 120c (It is called the scorching). Cellulose withstands short-term heating at a temperature of 180-200°C. A temperature above 275°c, intensive decomposition of cellulose lakes place with the formation of liquid and gaseous products of different composition. At 400 – 450 degree centigrade all gaseous decomposition products disappear and a hard residue (carbon) remains. It burns readily in air giving a smell like paper burning.
d) EFFECT OF INSECTS: Cotton is not attacked by moths or beatles.
e)Action of reducing and oxidizing agents
Reducing agents have no effect on cellulose, while oxidizing agents readily convert it to oxycellulose. For chemical treatment of fibrous materials, large use is made of various oxidizing agents: sodium hypochlorite, hydrogen peroxide, sodium chlorite etc, and such acids that are ca of oxidizing, as for instance, nitric acid.
Effect of light and atmospheric conditions
Under the action of light, cellulose is oxidized by atmospheric oxygen and due to photo oxidation, oxycellulose is formed as a result of which the strength of the cellulose is considerable reduced, the copper and iodine numbers are correspondingly increased, and the viscosity of cuprammonium solution is reduced.
Effect of microorganism on cellulose
If the moisture content in fibres is over 9% and the relative humidity is over 75-85%. Some bacteria and mildew fungi may cause cellulose decay.
SPINNIG.
- Ring spinning
- Open-end spinning
- Compact spinning
