Cotton

Submitted by-Lili

M.Sc (C.T) 3rd sem.

cottonTextile classification-As it is obtained from a plant it is classified as a

natural, cellulose, seed, mono-cellular, staple fiber.

The macro structure of cotton-Under a microscope, a cotton fiber appears as a

very fine, regular fiber.. It ranges in length ½” to 2 ¾ “, depending upon

the quality of the fiber. It is the finest in common use, ranges 11µm to

22µm .

Fiber morphology

The fiber length to breadth ratio of cotton ranges from about 6000:1 for the longest and best types, to about 350:1 for the shortest coarsest cotton type. the greater the ratio, the more readily can the cotton fibers be spun into yarn.

The color of cotton fibers depends on its type, environment, soil and climatic conditions under which it is grown.

The microscopic appearance of cotton-

Under the microscope, the cotton fiber looks like a twisted ribbon or a collapsed and twisted tube. These twists or convolutions identify the cotton fiber under the microscope.

The seed end of the fiber is quite irregular, having been torn, during ginning, from the epidermis or skin of the cotton seed.

The main part of the fiber, about 3/4th to 5/6th of its length, is regular, with a thick fiber wall, a

canal along the center of the fiber called the lumen, and about 16 convolutions/cm.

fiber tip is less than 1/4th of the fiber length. At this end, the fiber tapers to a cylindrical, pointed tip,

and has no convolutions.

The convolutions are formed after the cotton boll bursts open, the limp, sap-filled cotton seed hairs begin to dry out, their cell walls collapse inward, decreasing the size of the lumen. When the cotton seed hairs cease shrinking, twisting and collapsing inward, they become the valuable, convoluted cotton fiber

The micro-structure of cotton fiber-

Its cross-section is oval, compared with the normal hexagonal plant cell. However, like all plant cells, cotton has a distinct cuticle, well developed primary and secondary walls, and a lumen.

The cuticle is the’ very-outside’ or ‘skin’ of the cotton fiber. It is composed of a waxy layer (pectin) only a few molecules thick. The waxy nature of the cuticle enables it to adhere tenaciously to the primary wall of the fiber.

The primary cell wall, which is immediately underneath the cuticle, is about 200nm thick. It is composed of very fine threads of cellulose, called fibrils.

These fibrils are about 20nm thick, but length is not known. The fibrils spiral at about 70° to the fiber axis. This spiraling imparts strength to the primary cell wall and hence, to the

Secondary cell wall, which forms the bulk of the fiber. Concentric layers of spiraling, cellulosic fibrils, not unlike the growth rings of trees, make up the secondary wall.

The hollow canal, running the length of the fiber, Is called the lumen.

The lumen was once the central vacuole of the growing cotton fiber. It was full of cell sap, which was composed of a dilute, aqueous solution of proteins, sugars, minerals and cell-waste products.

When the sap evaporated, its constituents remained behind to contribute to the color of the cotton fiber

This caused the fiber to collapse inward resulting in the characteristics kidney-shaped cross-section of the cotton fiber.

The polymer system-

The cotton polymer-The cotton polymer is a linear, cellulose polymer. The

repeating unit in the cotton polymer is cellobiose which consists of two glucose units.

The cotton polymer consists of about 5000 cellubiose units, i.e. degree of polymerization.

The most important chemical groupings on the cotton polymer are hydroxyl groups(OH) and methylol groups(CH2OH).

The polymer system of cotton-

Cotton is a crystalline fiber. Its polymer system is about 65-70% crystalline and, correspondingly, about 35-30% amorphous.

cotton polymers are, in the main, well oriented and probably no further apart then 0.5nm , in the crystalline regions. This is maximum distance across which hydrogen bonds can form between polymers.

Hydrogen bonds are the most dominant and most important forces of attraction present in the polymer system of cotton. For this reason, Van der Waal’s forces which are also present have little relevance.

Physical properties-

TenacityThe strength of cotton fibers is attributed to the good alignment of its long polymers.

And its polymer system is about 70% crystalline, the countless, regular, hydrogen bond formations between adjacent polymers, and the spiraling fibrils in the primary and secondary cell wall.

It is one of the few fibers which gains strength when wet.

this occurs because of a temporary improvement in polymer alignment in the amorphous regions of the polymer system, which leads toapproximate 5% increase in fiber tenacity.

Elastic –plastic nature-

The cotton fiber is relatively inelastic because of its crystalline polymer system, and for this reason cotton textiles wrinkle and crease readily.

Bending and crushing of cotton textile materials places considerable strain on fibers polymer system.

These become weak points in the polymer system, and hence weak areas in the cotton fiber structure.

Causes cotton textile materials to crease and wrinkle readily.

Hygroscopic properties

The cotton fiber is very absorbent,

Owing to the countless polar –OH groups in its polymers, these attract water molecules, which are also polar.

Aqueous swelling of the cotton fiber is due to a separation or forcing apart of polymers by the water molecules in the amorphous regions only.

The hygroscopic nature ordinary prevents cotton textile materials from developing static electricity.

Thermal properties-

Cotton fibers have the ability to conduct heat energy, minimizing any destructive heat accumulation. Thus they can withstand hot ironing temperatures.

Excessive application of heat energy causes the cotton fiber to scorch, char and burn. This is an indication that cotton is not thermoplastic.

Chemical properties-

Effect of acids

Cotton fibers are weakened and destroyed by acids. Acidic conditions hydrolyze the cotton polymer at the glycoside

oxygen atom, which links the two glucose units to form the cellobiose unit.

Mineral or inorganic acids, being stronger that organic acids, will hydrolyze the cotton polymer more rapidly.

Effect of alkali-

Cotton fibers are resistant to alkalis and are relatively unaffected by normal laundering. The resistance is attributed to the lack of attraction between the cotton polymers and alkalis.

Mercerization without tension, or slack mercerization, causes the cotton fibers to swell; that is, increase in thickness, and contract in length.

The swelling is thought to be due to alkali molecules, or radicals, entering the amorphous regions of the fibers polymer system

Mercerization under tension, which can only be carried out on cotton yarn or fabric, little swelling or fiber contraction occurs. The fiber emerges with increased tenacity and with a distinct, though subdued luster.

Mercerizing also causes the fiber surface to become smooth and more regular, enabling it to reflect incident light more evenly.

Effect of bleaches-

The most common bleaches used on cotton textile materials are sodium hypochlorite and sodium per borate.

Sodium hypochlorite bleaches cotton textile materials at prevailing room temperature. However, bleaching with sodium per borate is more effective when the laundry solution exceeds 50°C in temperature.

These two bleaches are examples of oxidizing bleaches, which is mostly used for cotton textile materials. Oxidizing bleaches are so called because they liberate oxygen which does the actual bleaching

Manufacturing of cotton Introduction- cotton is grown in different parts of

world, where the climate is mild. The most favorable conditions for cotton belt are North America, Egypt. The largest cotton growing areas are India, China, United States, Egypt etc.

Manufacture of cotton-

1. Growing and harvesting- cotton grows on bushes 3-4 feet height, the blossoms or flowers appears, falls off and the ball begins its growth, inside the ball are the seeds from which the cotton fibers grow.

• This is also called seed hair. When ball is ripe, it splits open and the fluffy white cotton stands out from the ball.

The cotton is picked up with the help of machine or by hand. Cotton gets discolored and dirty if it is kept on plant for longer duration.

2.Ginning- in this process seeds are removed and fibers are pressed into bales. This process is carried out by a machine. Seeds are used for the production of oil, soaps and cosmetics. The fiber at this stage is called LINT.

Ginning3. Bailing- cotton is compressed or pressed into bales. These

are wrapped with jute cocking and cotton is then supplied to mills in bales.

4.Opening and picking- opening refers to opening of the bale and loosening and

fluffing of the fiber which have been tightly packed into bales.

Layers of fibers from several bales are fed into an opener. The fibers emerge from the opener like a fluffy mass, i.e. 10-15 times as bulky as compared to the bale.

The picking machine continues the loosening and cleaning of the fibers and a thick white sheet is formed which is called LAP.

5.Carding-

In this process the lap passes between two cylinders covered with clothing, which is a heavy fabric with many specialty bent wires. Individual fibers are straightened and made somewhat parallel.

In this machine cotton is thoroughly cleaned off all embedded dirt and foreign matter. There is no sorting of long and short fibers. The fibers emerges from the carding rolls as a thin sheet

6.Combing- it removes the short fibers and keeps rest of the

fibers nearly parallel. As much as ¼ of the fiber may be combed out as waste which are used for other purposes.

The fibers that remain here are long staple, good quality fibers. These are used for manufacturing cotton. Combed fibers are fine, uniform, strong and smooth.

7.Slivering- The thin sheet of fibers laid parallel to each

other after combing. These are given a slight pull and then twisted into one inch thick, long sliver. The slivers are collected in a dram

8.Drawing-The slivers are fed into the drawing rolls at the same time.

This is a continuation of the blending which started in the opening process.

The drawing frame consists of four sets of cells, each of which travels at a faster speed then the previous set. The difference in the speed causes elongation of the sliver and reduction in the diameter.

9. Roving- this is similar to drawing but the diameter of the sliver reduces further and slight twist is given. Then it is wound on the bobbin.

10.Spinning & weaving- in factories cotton is spun into yarns on large spinning machines. Afterwards it is fed to the spools which are sent for weaving to the weaving mills.

11.Dyeing & finishing- the woven fabric is dyed with different colors and then finishing is given to the fabric t improve its appearance and to remove certain drawbacks or limitations.

The bobbins are place in the spinning frame, where these are passed through several sets of rollers running at successively more speed and finally drawn out to yarn of the desired size.

Thank you