Long Way Education

By Eman Abdallah Kamel

Cotton is so important. In our daily lives, cotton plays a significant role; here’s how it’s grown, spun, woven, printed, and finished, as well as its uses.

Cotton

Cotton is an important component of many countries’ industrial and agricultural sectors. Cotton is used for a wide range of products, including coats, jackets, trousers, shirts, underwear, furnishings, bedspreads, window shades, towels, sheets, pillowcases, and linens. Cotton is also used in the manufacture of medical supplies and industrial threads.

The major cotton-producing countries are China, India, the United States, Brazil, Pakistan, Uzbekistan, Turkey, Australia, Mexico, and Argentina. The top exporters of cotton are India, the United States, China, Brazil, and Pakistan.

Let’s learn more about this amazing fiber.

Cotton Species

There are four cultivated species of cotton: Gossypium arboreum, G. herbaceum, G. hirsutum, and G. barbadense.

• The species Gossypium arboreum and G. herbaceum are known as Asiatic cotton because they are grown in Asia.
• G. hirsutum is known as American cotton or upland cotton. These species account for about 90% of global production.
• G. barbadense refers to Egyptian cotton, Sea Island cotton, Peruvian cotton, or high-quality cotton.

Manufacturing Stages

The manufacturing stages of cotton include,

• Cultivation and Development,
• Ginning Process,
• Spinning,
• Weaving,
• Dying,
• Printing,
• Finishing.

1. Cultivation and Development

Cotton seeds contain all of the organs necessary for producing a small seedling. The seed is rounded at one end, the chalaza, and pointed at the other end, the micropyle. The tip of the primary root, the radical, faces the fine stratum. The precursors of the stem and cotyledons are visible inside the seed. The chalaza is the main site of water and oxygen uptake during germination. The primary root head is the first part of the plant that emerges through the microparticle.

The cotyledons that will feed the new seedling are folded inside the seed, with the seedling underneath ready to elongate and push the seedling through the soil. Gossypol glands throughout the inner part of the seed also appear in the tissues of the developing plant.

Germination begins when the seed absorbs water and oxygen through the chalaza after planting. Inactive tissues swell, and cells begin to divide and grow. Through the micropyle, the radicle emerges, turns downward, and extends deeper into the soil, providing a taproot that provides water and nutrients throughout the plant’s life. The hypocotyl extends from the radicle and forms an arch that begins to push through the soil. Seedling emergence normally occurs between 4 and 14 days after planting. The hypocotyl straightens out as it emerges from the surface, a process known as epigeal germination. After the cotyledons are pulled through the soil surface, they unfold and reveal the epicotyl and the apical meristem that will be the source of the next growth.

Did You Know?

The cotyledons play two roles during germination. Before they unfold, they provide stored food for the germinating seedlings. When the cotyledons unfold, they produce chlorophyll, turn green, and produce energy through photosynthesis.

Warm, moist soil produces cotton the fastest. Low temperatures (below 60 degrees F) or low soil moisture can slow germination. Physical impedance, such as crusting, does not inhibit germination, but it may prevent the hypocotyl from erupting.

By the time the cotyledons emerge, the taproot may be as deep as 10 inches. Many factors impede root growth and development, such as cold soil, low soil pH, seedling disease, water stress, and herbicide infestation. During the early stages of cotton growth, any hindrance to root development can result in a weak crop.

During the growth process, the rootstock of the cotton plant becomes the main root from which the lateral roots begin to grow. Lateral roots and terminal roots together form the basal root system. Then, other higher-order roots develop from this primary root system. These highly arranged roots have a functional life of about 3 weeks. The cotton plant has growth points, both at the top of the main stem and on fruiting branches. These growth points allow the plant to grow upward and outward simultaneously. Cotton can grow very tall under unrestrained conditions. A growth regulator, such as mepiquat chloride (C₇H₁₆ClN), is generally applied to cotton to reduce internode elongation, especially in well-fertilized irrigated cotton.

The first plant structures to appear on the main stem are the leaves of the main stem. The main stem, branches, and leaves are formed at the attachment points of the main stem, called nodes. A new node of the apical meristem is produced at a rate of every 3 days.

The main stem leaves provide carbohydrates for fruit development. The fruit produced near the main stem will receive more carbohydrates from the leaves of the main stem than the fruit produced at distant locations.

The fruit bud begins to form at the beginning of the fruit branch. The development of fruits occurs with the growth of steadily aging leaves. As the leaves grow and mature, photosynthesis occurs at a variable rate. Cotton leaves reach their maximum photosynthetic capacity at about 20 days of age, after which they decline.

As the reproductive growth of the cotton plant increases, this is done with the support of the aging foliage canopy. Early aging of the cotton leaf canopy due to reduced fertility, water stress, and other stresses reduces the photosynthetic ability of the crop. When pollen reaches the stigma, it germinates into the pollen tube. The pollen tube grows through the style, the micropyle, and into the chamber of the ovum, where fertilization occurs.

Bolls begin to develop after pollination. It requires nearly 50 days to “open” after pollination. There are three stages in the development of a boll: enlargement, filling, and maturation.

1. Enlargement: The enlargement stage of boll development lasts nearly 3 weeks. During this time, the fiber is a thin-walled tubular structure. On the seed coat, a single epidermal cell develops into a fiber. Lack of water, extreme temperatures, and nutrient deficiencies, especially potassium, can all reduce fiber length.
2. Filling: The boll-filling stage begins during the fourth week after flowering. During this time, the elongation of the fibers stops, and the formation of the secondary wall of the fibers begins. Cellulose is deposited within the stretched fibers every 24 hours to fill the space of
   the stretched fibers. The deposition of cellulose in the fiber cell is also sensitive to environmental conditions. The boll-filling stage continues into the sixth week after pollination.
3. Maturation: This stage begins when the boll reaches its full size and maximum weight. During this stage, the maturation of fibers and seeds occurs, as does boll dehiscence. The capsule walls of the boll dry out, causing the close cells of the dorsal suture to contract unevenly.
   This contraction causes the suture between the walls of the carpel to split, and the boll opens.

2. Ginning

The cottonseed is moved to the gins to separate the lint from the seed. Cotton first passes through dryers to reduce moisture content, then through cleaning equipment to remove foreign substances. These processes facilitate processing and improve fiber quality. Cotton is then air-conveyed to gin stands, where circular saws are used to pull lint through closely spaced ribs that prevent the seed from passing through. By air blasts or rotating brushes, the lint is removed from the saw teeth and then compressed into bales weighing nearly 500 pounds. Afterward, the cotton is moved to a warehouse for storage until it is shipped to textile factories.

After lint is baled, samples from each bale are classified according to fiber strength, length, uniformity of length, color, non-fibrous content, and fineness. Scientific quality control checks are performed periodically to ensure that the accuracy of the classifier is maintained.

3. Spinning

In a textile mill, machines open bales. The lint is then mixed and cleaned by blowing and beating. The short lint is separated and sold for use in other industries. The best fibers are usually about 1 inch to 1 ¾ inches long. The mixed cotton goes into a carding machine to further clean the fibers and make them lay side by side. The carding machine makes the fibers into a smooth, untwisted rope called a sliver. Eight strands of silver are blended in the drawing process. The speed of drawing has dramatically increased over the past few years. These days, it can reach 1,500 feet per minute. Roaming frames draw the slivers out thinner and add a nice touch as the first step in the ring-spinning of yarn.

Ring spinning machines draw the roving and add the twist, making it tighter and thinner until it reaches the thickness of the thread needed for the fabric. The threads can be twisted multiple times per inch. Ring-spinning frames continue to play a role in many countries, but open spinning, with vanes that can spin five to six times the speed of a ring-spinning machine, is becoming more prevalent.

In open spinning, the yarns are produced directly from the sliver. Spinning devices take the fibers from the sliver and rotate them up to 2,500 revolutions per second.

In air-jet and vortex systems, compressed air is used to stabilize the yarn. Compared to other short-staple spinning methods, these methods are faster and more productive. After spinning, the yarns are wrapped tightly around bobbins or tubes and are ready to weave.

Did You Know?

Ply yarns are single or more strands that are twisted together. The cord is wrapped in yarn twisted together.

4. Weaving

Weaving is the oldest method of turning threads into fabric. Cotton yarns are woven into fabrics by loom machines. Modern looms run at great speeds, interlacing the length-wise yarns (warp) and crosswise yarns (filling). On the loom, lengthwise yarns (warp) form the skeleton of the fabric. They usually require a higher degree of twist than the crosswise (filling) yarns that are interlaced widthwise.

Modern mills use high-speed weaving machines. Some looms can run filling threads at speeds exceeding 2,000 meters per minute.

Weaver-type rapier machines use metal arms to pick up the filling thread halfway across the loom, and another rapier picks it up and pulls it the rest of the way. In other types, the filling thread is carried across the loom by small projectiles. There are three basic weaves: plain, twill, and satin.

1. Plain weaving: In this process, the filling is alternately passed over one warp thread and under the next warp thread.
2. The twill weave: The yarns are interlaced to form diagonal ridges across the fabric. This method is used for solid fabrics such as denim and gabardine.
3. Satin weave: Produces a smooth, high-gloss fabric. It is produced using fewer interlocking threads, and the warp threads dominate the “face” of the fabric.

5. Dyeing

Cotton is commonly dyed through two methods: piece dyeing and yarn dyeing.

1. Piece dyeing: This process is mainly used for solid-colored fabrics, in which a continuous length of dry fabric is passed its full width through a basin of hot dye solution.

Next, the fabric is rolled between lined rollers, which evenly apply pressure to the color and remove excess liquid. In this method, the fabric is processed in a rope-like coil on a bobbin that passes in and out of the dye vat or peck.

2. Yarn dyeing: A thread dyeing technique is used to create cotton plaids, checks, woven lines, and other special effects before the fabric is woven. For example, blue-dyed warp threads are combined with white-filling threads to make denim.

6. Printing

There are three methods of cotton printing: roller printing, automatic flat-screen printing, and rotary screen printing.

1. Roller print machine: Drum printing machines can produce long runs of fabric at a speed of 50 to 100 yards per minute. It is possible to print up to ten different colors simultaneously with a drum printing machine.

In a typical printing machine, the cylinder is surrounded by a series of copper rollers, each with its own dye trough and blade to remove excess dye.

Since each color in the design is engraved on its bobbin, the number of bobbins varies depending on the design. As the fabric moves between the rotating drum and bobbins under great pressure, it picks up color from the dug area of each bobbin sequentially. The printed fabric is dried immediately and transferred to a dye-fixing oven.

2. Automatic flat-screen printing: This method is slower than cylinder printing, but it produces much larger and more complex designs.

In this process, each color of a fabric design is reproduced on a fine mesh screen. Dye-resistant paint or varnish is applied to areas of the screen where the dye should not penetrate.

Screens are painted with dye on the back and installed in the correct order over the flatbed. As the fabric is moved from one screen to another by the belt, a squeegee or roller presses dye through the open areas of the screen onto the fabric.

3. Rotary screen printing: Currently, rotary screen printing is being used to combine the above two methods. It combines cylinder and screen printing, using perforated cylinders instead of flat screens. It is possible to print 16 colors on a single canvas using this method. In this process, a small metal cylinder pushes the color paste through the drum’s holes onto the fabric after the color paste is fed into the rollers. A benefit of this method is that screens or cylinders are produced at a lower cost than copper-embossed copper spools used for cylinder printing.

7. Finishing

There are several ways to finish textiles. Cotton fabrics are likely to be completed in different ways than other fabrics.

Cotton fabrics can have different finishes that affect how they look and feel. Finishes also add other properties, such as durability, water resistance, flame resistance, shrinkage control, etc.

Sources

• fao.org
• pubchem.ncbi.nlm.nih.gov/compound/Mepiquat-chloride

©Eman Abdallah Kamel, 2022

About the author: Eman is a writer and textile engineer. She obtained her bachelor’s degree in textile science from the Faculty of Applied Arts.

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