Cotton Fibre

Cotton is a natural fibre, which grows from the surface of seeds in the pods, or bolls, of a bushy mallow plant. It is composed basically of a substance called cellulose. 

Fig. Cotton Fibre

In this article, we will discuss the following points about cotton fibre;

• Chemical Structure of Cotton
• History of Cotton
• Kinds and types of cotton
• Cultivating and harvesting of cotton
• Cotton Ginning
• Byproducts of cotton
• Cotton Fiber Morphology
• Classing or grading of cotton
• Physical and chemical properties of cotton fibre

Chemical structure of Cotton:

• Cotton fibre consists of Cellulose. The repeating unit of cellulose is;

Chemical Structure of Cotton Fiber

History of Cotton:

Cultivation of cotton has been done since about 5000 years ago. According to archaeologists, cotton has been cultivated and is being used for textile purposes in the Indus Valley from about 2100 B.C., in Mexico by 3500 B.C. and in the USA by 500 B.C. The cotton fibre was introduced in the Mediterranean countries by Arab merchants and Crusaders. After the Industrial Revolution and the invention of textile machinery, the production and manufacturing of cotton products started in England in 1730. Now cotton is the most widely used fiber due to its versatility, durability and comfortability.

Kinds and Types of Cotton:

The characteristics of cotton fibre grown in different regions of the world also differ from each other. This Variation in cotton fibers is usually caused by the different growth conditions of different areas such as soil, water and fertilizers etc. Therefore the different kinds of cotton are recognized by the name of their geographical area where it is produced. The characteristics of different kinds of cotton are given as following;

1. Sea Island Cotton:

This type of cotton is grown on the island of the coast of Carolina and along the Coast of Southern states. Its fibre is 40mm – 50mm long. This kind of cotton fibre is highly valued due to its good length, fineness and uniformity.

2. Egyptian Cotton:

Egyptian cotton is grown along the delta of the River Nile and its length range is approximately (32-38mm). Its fibre quality is also very good in respect of length and uniformity. It is mostly used in the manufacturing of knitted undergarments, towels and fine mercerized fabrics.

3. Upland Cotton:

It is also called American Upland Cotton because it is grown and cultivated in the United States of America.  The upland cotton is fairly white and strong and its length range is from (22mm to 32 mm).

4. American Pima Cotton:

American Pima cotton is mainly grown in the Valley of Texas, Mexico, Arizona and California.  Its fibre is strong, fine, lustrous and creamy white in colour. Its staple length range is from (35mm-38mm). American Pima cotton fibre is used in high-quality fabrics where silky smoothness and lustrous appearance are required.

5. Peruvian Cotton:

Peruvian cotton is found in the West Indies and central South America with a length range of approximately (30mm-33mm). Its fibre is moderately smooth and mostly used to produce blend yarns with wool fibre.

6. Asiatic Cotton:

Asiatic cotton is usually grown in Asian cities like India, China, Pakistan, Bangladesh etc. Asiatic cotton has a shorter fibre length approximately 1 inch (25mm) and is usually coarser than other types of cotton fibres.

Cotton Cultivation:

Cotton is usually grown in warmer climates, due to which its cultivation is being carried out in the southern part of Pakistan, India, the United States, China etc. Cotton requires almost two hundred days of continuous warm weather with adequate moisture and sunlight. In the United States, usually in March or April, carefully selected cotton seeds are planted in rows, whereas in Pakistan it is cultivated during monsoon days of July and August.

Harvesting of Cotton:

When the secondary cell wall of cotton bolls is fully developed after cultivation then the fully ripe cotton bolls are picked. There are two methods used for the picking of cotton bolls;

1. Hand Picking
2. Mechanical Picking

1. Hand-picking:

• Cotton picking by hand is still practical in nearly all countries.
• The common practice in hand picking is to pick the seed cotton out of the boll and put it into a long sack, which hangs over the picker's shoulders and is dragged along the ground as he/she moves forward.
• Each picker keeps his seed cotton separate and is paid according to the weight of cotton which was picked by him and then sacks of cotton are dropped down into a truck which takes it to a ginning factory.
• An experienced person can pick 300 lbs of seed cotton per day under normal conditions. About 1500 lbs of seed cotton are required to give a bale of 500 lbs of lint cotton.

2. Mechanical Picking:

• Both the cost and shortage of hand-picking labour here stimulated the invention of many devices and machines for the picking of seed cotton.
• Many different principles have been tried in developing a machine to pick the lint out of the boll as is done by hand picker.
• The machine that has had the most attention in recent years is the “spindle type machine”.
• These are tractor-type motor vehicles designed to pick the cotton from either one or two rows at the same time.
• As the machine moves down the rows special shoes (plant compressors) guide the cotton plants into the machine. There are two groups of spindles which revolve in a horizontal plane.
• The spindles are mounted on endless chains which move backwards at the same rate of speed at which the picker machine moves forward.
• In this way, the spindle remains in a stationary position with respect to the plant for some time. When the cotton is withdrawn from cotton plants by spindles then cotton is removed from the spindles by a roller called the doffer. Then it is carried towards the large collecting bag or basket called a hopper.
• If the cotton is too dry then it will stick and wound around spindles. This difficulty is overcome by a spray which wet the spindles as they move forward into the picking zone.
• This type of mechanical picker can pick 1000-1500 lbs of seed cotton per hour.

Disadvantages of Mechanical Picking:

1. A mechanical picker cannot differentiate between clean and trashy cotton or between seed cotton and leaf or pieces of bolls. Thus mechanical picking collects the seed cotton along with an excess amount of trash. Therefore a lower grade cotton is obtained by mechanical picking as compared to hand picking.
2. Another difficulty is that all bolls in the field are not open at the same time and when mechanical picking is done, the immature bolls are also injured by the rotating spindles.
3. Leaves and other vegetable parts of the plant may crushed during the process of mechanical picking and can stain the cotton fibers.

Ginning and Baling:

After picking the ginning of cotton fibre is carried out, in which the cotton seeds are separated from the cotton fibres. Then the ginned cotton fibres are well packed in polypropylene bags in the form of rectangular bales and tied with iron bands. These cotton bales weigh about 500 pounds (225kg) each.

1. Seed cotton enters the ginning machine.
2. Roll throws seed cotton against bars.
3. Saw teeth take cotton up and against bars.
4. Bars near saws on both sides let lint pass through but hold back seed.
5. The seed falls into a conveyor.
6. The lint on the saw tooth is struck by a blast of air.
7. Blown into lint cotton conveyor pipe.

Byproducts of Cotton:

During the cleaning process of raw cotton at the cotton mill, valuable byproducts are also obtained that amount approximately to one-sixth of the entire income derived from the cotton plant. The main byproducts are;

1. Cotton Linters
2. Hulls
3. Inner seeds

1. Cotton Linters:

Linters are the short, fuzzy hair-like fibres that remain on the seeds after they have been separated from the fibre in the cotton ginning process. They are used in the manufacture of rayon and acetates, plastics, shatterproof glass, photographic film and fast-drying lacquers and for other purposes.

2. Hulls:

Apart from the cotton linters, the hulls are also obtained as a byproduct which is the outer part of the cotton seeds. The hulls are rich in nitrogen, an important plant food, and are used as fertilizer in the manufacture of paper, plastics and cattle feed and as a base for explosives.

3. Inner seeds:

The cottonseed oil is also obtained from the inner seed of the hull, which can be used in the manufacturing of cooking oils and soap. The residue of the inner seed can also be used as cattle feed.

Cotton Fiber Morphology:

It consist in basically three parts;

1. The Microscopic Appearance of Cotton
2. The Micro-structure of Cotton
3. Polymer System of Cotton Fibre

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 convolution identify the cotton fiber under microscope.
• Fiber has two ends, seed ends attached to the seed and the second end that is free. After removal of seed, the seed end have been torn during ginning from the epidermis or the skin of the cotton seed.
• There is a canal along the centre of the fiber called lumen and filled with liquid nutrients, water, protein and protoplasm before bursting of cotton boll. When the cotton boll bursts and opens, the liquid inside the lumen dries and the cell walls collapsed inward, decreasing the size of the lumen.
• Due to twisting and collapsing it surface becomes convoluted having 50-60 convolutions per centimetre.
• The convolutions give cotton fiber an uneven surface, which increase inter-fiber friction and enables fine cotton yarns of adequate strength to be spun.
• The appearance of the cotton fiber cross-section under microscope is referred to a kidney shaped. This shape occurs from the inward collapse of the cotton fiber when it dries out.
• The countless air space, which exists because of the convolutions and kidney, shaped cross-section of the cotton fibre increase the moisture absorbency of cotton textile materials thus making them more comfortable to wear.

The Micro-structure of Cotton Fibre:

The cotton fibre has four distinct parts;

1. Cuticle
2. Primary Cell Wall
3. Secondary Cell Wall
4. Lumen

1. Cuticle:

The Cuticle is the skin/upper surface of the cotton fibre. It consists of wax. The waxy nature of the cuticle enables it to adhere tenaciously to the primary cell wall of the fibre. The inert nature of this cotton wax protects the rest of fibre against chemical and other degrading agents.

2. Primary Cell wall:

The Primary Cell Wall is immediately under the cuticle. This layer is composed of very fine threads of cellulose called fibrils. There are about 1000 or more fibrils on each ring. These fibrils are arranged in the form of spirals. The deposition of cellulose continues about for twenty-four days, so that each mature cotton fibre can be regarded as being constructed from thousands of fibrils of cellulose arranged in the from of spirals. The spiral angle of fibril on primary wall is at 70° to the fibre axis.

3. Secondary Cell Wall:

Beneath the primary cell wall lies the Secondary Cell Wall, which forms the bulk of the fibre. Concentric layers of spiralling, cellulosic fibril, make up the secondary cell wall. The spiral angle of fibril on secondary cell wall is at 25° to 45°. Much of the strength and stability of cotton fibre, and hence of yarns and fabrics may be attributed to these spiralling fibrils.

4. Lumen:

The hollow canal running the length of the fibre is called the lumen. Its walls are innermost, concentric layers of spirals of the secondary cell wall. The lumen is filled with liquid nutrients having aqueous solutions of proteins, sugars, minerals and cell waste products. As the liquid is evaporated the pressure inside the fibre became less than the atmospheric pressure on the outside. This caused the fibre to collapse inward resulting in characteristic kidney-shaped cross-section of the cotton fibre.