Cotton Fiber

Cotton fiber is a cellulosic fiber. It has worldwide popularity for its variety of use. Cotton fiber is the most used fibers for producing various types of fabric through all over the world. Cotton Fabrics are comfortable to wear than the other fibers product. It is comfort to wear in all the season.

Like all the textile fibers, cotton has its own physical and chemical properties which are required to know for better processing in spinning, weaving, knitting, dyeing, printing as well as finishing. Here, I have written about physical and chemical properties of cotton fiber.

CHARATERISTICS OF COTTON

Cotton, as a natural cellulosic fiber, has a lot of characteristics, such as;

• Comfortable Soft hand
• Good absorbency
• Color retention
• Prints well
• Machine-washable
• Dry-cleanable
• Good strength
• Drapes well
• Easy to handle and sew

4. End Uses of Cotton:

• Apparel - Wide range of wearing apparel: blouses, shirts, dresses, children’s wear, active wear, separates, swimwear, suits, jackets, skirts, pants, sweaters, hosiery, neckwear.

• Home Fashion - curtains, draperies, bedspreads, comforters, throws, sheets, towels, table cloths, table mats, napkins

The Production Process

1.     In spring, the acreage is cleared for planting. Mechanical cultivators rip out weeds and grass that may compete with the cotton for soil nutrients, sunlight, and water, and may attract pests that harm cotton. The land is plowed under and soil is broken up and formed into rows.

2.     Cottonseed is mechanically planted by machines that plant up to 12 rows at a time. The planter opens a small furrow in each row, drops in seed, covers them, and then packs more dirt on top. Seed may be deposited in either small clumps (referred to as hill-dropped) or singularly (called drilled). The seed is placed 0.75 to 1.25 in (1.9 to 3.2 cm) deep, depending on the climate. The seed must be placed more shallowly in dusty, cool areas of the Cotton Belt, and more deeply in warmer areas.

3.     With good soil moisture and warm temperature at planting, seedlings usually emerge five to seven days after planting, with a full stand of cotton appearing after about 11 days. Occasionally disease sets in, delaying the seedlings' appearance. Also, a soil crust may prevent seedlings from surfacing. Thus, the crust must be carefully broken by machines or irrigation to permit the plants to emerge.

4.     Approximately six weeks after seedlings appear, "squares," or flower buds, begin to form. The buds mature for three weeks and then blossom into creamy yellow flowers, which turn pink, then red, and then fall off just three days after blossoming. After the flower falls away, a tiny ovary is left on the cotton plant. This ovary ripens and enlarges into a green pod called a cotton boll.

5.     The boll matures in a period that ranges from 55 to 80 days. During this time, the football-shaped boll grows and moist fibers push the newly formed seeds outward. As the boll ripens, it remains green. Fibers continue to expand under the warm sun, with each fiber growing to its full length—about 2.5 in (6.4 cm)—during three weeks. For nearly six weeks, the fibers get thicker and layers of cellulose build up the cell walls. Ten weeks after flowers first appeared, fibers split the boll apart, and cream-colored cotton pushes forth. The moist fibers dry in the sun and the fibers collapse and twist together, looking like ribbon. Each boll contains three to five "cells," each having about seven seeds embedded in the fiber.

Most steps involved in the production of cotton have been mechanized, including seeding, picking, ginning, and baling. Samples are taken from the bales to determine the quality of the cotton.

6.     At this point the cotton plant is defoliated if it is to be machine harvested. Defoliation (removing the leaves) is often accomplished by spraying the plant with a chemical. It is important that leaves not be harvested with the fiber because they are considered "trash" and must be removed at some point. In addition, removing the leaves minimizes staining the fiber and eliminates a source of excess moisture. Some American crops are naturally defoliated by frost, but at least half of the crops must be defoliated with chemicals. Without defoliation, the cotton must be picked by hand, with laborers clearing out the leaves as they work.

7.     Harvesting is done by machine in the United States, with a single machine replacing 50 hand-pickers. Two mechanical systems are used to harvest cotton. The picker system uses wind and guides to pull the cotton from the plant, often leaving behind the leaves and rest of the plant. The stripper system chops the plant and uses air to separate the trash from the cotton. Most American cotton is harvested using pickers. Pickers must be used after the dew dries in the morning and must conclude when dew begins to form again at the end of the day. Moisture detectors are used to ensure that the moisture content is no higher than 12%, or the cotton may not be harvested and stored successfully. Not all cotton reaches maturity at the same time, and harvesting may occur in waves, with a second and third picking.

8.     Next, most American cotton is stored in "modules," which hold 13-15 bales in water-resistant containers in the fields until they are ready to be ginned.

9.     The cotton module is cleaned, compressed, tagged, and stored at the gin. The cotton is cleaned to separate dirt, seeds, and short lint from the cotton. At the gin, the cotton enters module feeders that fluff up the cotton before cleaning. Some gins use vacuum pipes to send fibers to cleaning equipment where trash is removed. After cleaning, cotton is sent to gin stands where revolving circular saws pull the fiber through wire ribs, thus separating seeds from the fiber. High-capacity gins can process 60, 500-lb (227-kg) bales of cotton per hour.

10.                        Cleaned and de-seeded cotton is then I 0 compressed into bales, which permits economical storage and transportation of cotton. The compressed bales are banded and wrapped. The wrapping may be either cotton or polypropylene, which maintains the proper moisture content of the cotton and keeps bales clean during storage and transportation.

11.                        Every bale of cotton produced in the United States must be given a gin ticket and a warehouse ticket. The gin ticket identifies the bale until it is woven. The ticket is a bar-coded tag that is torn off during inspection. A sample of each bale is sent to the United States Department of Agriculture (USDA) for evaluation, where it is assessed for color, leaf content, strength, fineness, reflectance, fiber length, and trash content. The results of the evaluation determine the bale's value. Inspection results are available to potential buyers.

12.          After inspection, bales are stored in a carefully controlled warehouse. The bales remain there until they are sold to a mill for further processing.

Quality Control

Cotton growing is a long, involved process and growers must understand the requirements of the plant and keep vigilant lookout for potential problems. Pests must be managed in order to yield high-quality crops; however, growers must use chemicals very carefully in order to prevent damage to the environment. Defoliants are often used to maximize yield and control fiber color. Farmers must carefully monitor moisture levels at harvesting so bales will not be ruined by excess water during storage. Soil tests are imperative, since too much nitrogen in the soil may attract certain pests to the cotton.

Expensive equipment such as cotton planters and harvesters must be carefully maintained. Mechanical planters must be set carefully to deposit seed at the right depth, and gauge wheels and shoes must be corrected to plant rows at the requisite spot. Similarly, improperly adjusted machinery spindles on harvesting machines will leave cotton on the spindle, lowering quality of the cotton and harvesting efficiency. A well-adjusted picker minimizes the amount of trash taken up, rendering cleaner cotton.

Byproducts/Waste

There is much discussion regarding the amount of chemicals used in cotton cultivation. Currently, it is estimated that growers use, on average, 5.3 oz (151 g) of chemicals to produce one pound of processed cotton. Cotton cultivation is responsible for 25% of all chemical pesticides used on American crops. Unfortunately, cotton attracts many pests (most notably the boll weevil) and is prone to a number of rots and spotting, and chemicals are used to keep these under control. There are concerns about wildlife poisoning and poisons that remain in the soil long after cotton is no longer grown (although no heavy metals are used in the chemicals). As a result, some farmers have turned to organic cotton growing. Organic farming utilizes biological control to rid cotton of pests and alters planting patterns in specific ways to reduce fungicide use. While this method of cultivation is possible, an organically grown crop generally yields less usable cotton. This means an organic farmer must purchase, plant, and harvest more acreage to yield enough processed cotton to make the crop lucrative, or reduce costs in other ways to turn a profit. Increasingly, state university extension services are working with cotton farmers to reduce chemical use by employing certain aspects of biological control in order to reduce toxins that remain in the land and flow into water systems.

Repeat unit of cellulose

The current consensus regarding cellulose crystallinity (X-ray diffraction) is that fibers are essentially 100% crystalline and that very small crystalline units imperfectly packed together cause the observed disorder.

The density method used to determine cellulose crystallinity is based on the density gradient column, where two solvents of different densities are partially mixed. Degree of Crystallinity is, then, determined from the density of the sample, while densities of crystalline and amorphous cellulose forms are known (1.505 and 1.556 respectively). Orientation of untreated cotton fiber is poor because the crystallites are contained in the micro fibrils of the secondary wall, oriented in the steep spiral (25-30^o) to the fiber axis.

 Cotton length of different countries:

01. China cotton:                Length: 1" to 1.25"

02. Uzbekistan cotton:        Length: 1" to 1.25"

03. Europe cotton:              Length: 1" to 1.25"

04. USA cotton:                  Length: 1.25" to 1.75"

05. Egypt cotton:                Length: 2" to 2.5" (Best Quality Cotton)

06. India cotton:                Length: 1.00"

 07. Pakistan cotton:          Length: 1.00"

 

Physical Properties of Cotton:

Physical properties of cotton fibers are given below:

1.     Color: The color of cotton fiber could be white, creamy white, bluish white, yellowish white or grey.

2.     Tensile Strength: Cotton is moderately strong fiber. It has a tenacity of 3-5 gm/den. The strength is greatly affected by moisture; the wet strength of cotton is 20%, which is higher than dry strength.

3.     Elongation at break: Cotton does not stress easily. It has an elongation at break of 5-10%.

4.     Elastic Recovery: Cotton is inelastic and rigid fiber. At 2% extension it has an ER of 74% and at 5% extension it has an ER of 45%.

5.     Specific Gravity: Specific gravity is 1.54.

6.     Moisture Regain (MR %): Standard moisture regain is 8.5.

7.     Effect of Heat: Cotton has an excellent resistance to degradation by heat. It begins to turn yellow after several hours at 120⁰C and decomposes marked by at 150⁰C. As a result of oxidation, cotton is severally damaged after few minutes at 240⁰C. Cotton burns in air.

8.     Effect of Sun Light: There is a gradual loss of strength when cotton is exposed to sun light and the fiber turn yellow. The degradation of cotton by oxidation is done when heat is promoted and encouraged. By sun light much of the damage is caused by UV-light and by the shorten weaves of visible light.

9.     Effect of Age: Cotton shows a small loss of strength when stored carefully. After 50 years of storage cotton may differ only slightly from the new fibers.

Chemical Properties of Cotton:

Cotton is a natural cellulosic fiber and it has some chemical properties. Chemical properties of the cotton fiber are given below:

1.     Effect of Acids: Cotton is attacked by hot dilute acids or cold concentrated acids which it disintegrates. It is not affected by acids.

2.     Effects of Alkalis: Cotton has an excellent resistance to alkalis. It swells in caustic alkalis ( NaOH) but it does not damaged. It can be washed repeatedly in soap solution without any problem.

3.     Effect of Organic Solvent: Cotton has high resistance to normal cleaning solvents. Cotton is dissolved by the copper complexes, such as cuprammonium hydroxide, cupriethylene diamine and concentrated 70% H₂SO₄.

4.     Effects of Insects: Cotton is not attacked by moth-grubs or beetles.

5.     Effect of Micro Organism: Cotton is attacked by fungi and bacteria. Mildews will feed on cotton fabric, rotting and weakling the materials. Mildews and bacteria will flourish on cotton under hot and humid condition. They can be protected by impregnation with certain types of chemicals. Copper Nepthenate is one of the chemical.

I think above physical and chemical properties will help us to be careful about the processing of cotton in the different stages of processing from spinning to garments making.

 Harvesting

Cotton can either be picked by hand or by machines. Manual picking is slow but better preserves fiber characteristics of cotton. Boll opening is the first action on the fiber which pushes fibers from the place where they were embedded for weeks

before being exposed to the external conditions. The boll opening action is gentle and thus has no effect on the fiber quality. However, a longer stay of the open bolls in the field may change the color and also make the fibers shrink, thus affecting

the three most important fiber characters, i.e. length, strength and micronaire. One character may be affected more than the other if there is frequent dew. Such an effect cannot be eliminated as all bolls do not open at the same time and some open

bolls have to stay in the field for days and sometime even weeks. In the case of  and picking, it is possible to pick open bolls at frequent intervals, and weather effects on the fiber, after bolls have opened, can be minimized. In China  Mainland) this effect is minimized as land holdings are so small that the majority of the growers who have planted cotton on about 1/10^th of a hectare, can go many times to the field to pick few open bolls. In slightly bigger plots, fiber quality is preserved through a number of pickings during the season. 3-4 pickings/season are very common in many countries where cotton is hand picked. Hirsutum cottons can hold locks for a longer period of time without letting the seedcotton fall on the ground. But, if cotton is not picked for weeks and months, it slowly gets loose and ultimately falls on the ground. In G. arboreum varieties, burs do not possess  nough holding force to keep seedcotton sticking in the burs. It is easy to pick such a cotton but requires more frequent pickings. In China (Mainland), India, Myanmar and Pakistan, where such cottons are grown on significant area, as many as 8-10 picks are very common. Thus arboreum varieties are not suitable for machine picking as locks fall to the ground quickly. Upland cotton locks which fall to the ground are usually loose, but arboreum locks remain more or less intact. G. barbadense types are almost like hirsutum cottons. Most varieties belonging to G. herbaceum are very difficult to pick because of the position of burs after opening. Bolls are smaller and locks after opening are positioned such that each lock has to be picked separately. Varietal differences do exist within all species. About 30% of world production is machine picked. Australia, Israel and the USA are the only countries in the world where all cotton is picked by machines. In terms of percentage, a substantial amount of cotton is picked by machines in Bulgaria,

Greece and Spain. The percentage of machine picking and hand picking in the 10 largest cotton producing countries of the world is given in table 1. A high percentage of cotton in all republics of the former Soviet Union used to be machine picked before the republics became independent countries in 1991. Due to the lack of maintenance facilities for pickers, machine picking has been declining in almost all republics. Kazakhstan and Kirghizstan have improved the maintenance acilities, and it is assumed that now about 70-80% of total production is machine picked in these two countries. Since 1991, hand picking has been increasing in Azerbaijan, Tajikistan, Turkmenistan and Uzbekistan, and it is estimated that now 60-70% of the total area is hand picked. Among the major cotton producing countries of the world, Argentina and Brazil are the only countries where machine

picking is expected to increase in the next few years. A prototype two row stripper machine was designed in Argentina which is commercially produced now. The model Sapucay 492 is claimed to be less expensive, and the operational cost is also

lower compared with other machines on the market. Distance between rows can be adjusted between 0.85-1.0 meter and picking efficiency ranges from 85-90%. Last year a number of these machines were imported into Brazil. Turkey may also

introduce machine picking in the next few years because of a labor problem which is becoming acute. In Pakistan, by end of 1980’s, shortage of labor had become a problem because of doubling production in seven years. Machine picking would

probably have been introduced by now if production had not suffered due to leaf curl virus disease. Partly, the labor problem was tackled through various social incentives to the picking labor. There are many issues to be resolved before machine picking is adopted in any country. However, some experimentation work has already been performed on machine picking in Pakistan. Turkey is comparatively more prepared to adopt machine picking. In addition to some major cotton producing countries mentioned in table 1, all cotton is picked manually in Cameroon, Chad, Côte d’Ivoire, Iran, Madagascar, Mali, Myanmar, Philippines, Senegal, Sudan, Syria, Thailand, Togo, Uganda and Vietnam. However, at least some cotton is picked by machines in the following countries (table 2).

Cost of Harvesting

The ICAC also undertook another study in 1995 on the cost of harvesting cotton in various countries. The data are available for 57 sets of cotton production conditions in 31 countries. The harvesting cost which included stick cutting and slashing

was calculated in various countries in various ways. It is calculated by machine hours, weight of seed cotton picked, mandays and cost of picking a unit area. While machine hours applied to mechanical picking only, weight of seedcotton and mandays are most commonly used for calculating the cost of picking cotton.

 Defoliation

For machine picking it is necessary that there are no green leaves on the plant. Under natural conditions, leaves are shed with age but formation of the abscission layer between the main stem/branch and leaf petiole is stimulated by low night temperatures. If the carbohydrate accumulation is slow and leaves are not shed naturally, application of defoliants becomes necessary. Some varieties are more susceptible to low temperatures and have a greater tendency to shed leaves compared with others. If the leaves are not shed and a time has come to pick cotton, it becomes necessary to shed leaves artificially through application of defoliants. Defoliants also need to be applied if there are green bolls along with open leaves.

Ginning

The ICAC undertook a survey of the cotton production practices which was published in October 1996. The database has information on 35 countries which represent about 90% of world production during 1996/97. Information on ginning was also available for other countries. On the basis of 37 countries, only about 15% of world cotton production is ginned on roller gins. Almost all cotton is saw ginned in most countries with the exception of the following countries, table 5.

There are some roller gins in Argentina, Australia, Brazil, China (Mainland), Pakistan and Syria and about 1-2% of total production is ginned on roller gins. In Australia, 3 gins are mixed roller/saw combinations and they are all in New South

Wales. About 2/3 of total production in Sudan is of medium staple cotton but it is ginned on roller gins. Ginning efficiency of the machines in Sudan is also dependent on the type of cotton ginned. It takes longer to gin the same quantity of Acala types compared with Barakat varieties. The first saw gin in Sudan may start working in May 1997. In Pakistan, about 150,000 tons of G. arboreum seedcotton is never accounted for in the gins and is usually ginned at small home roller gins

for domestic use. In India and Iran, where G. arboreum and G. herbaceum are grown on significant area, a large quantity of short staple cottons is ginned at home for domestic use. The saw gin stands could be small and large thus affecting the machine efficiency. Similarly, the roller gins could be of various output. The ICAC collected information on the number of roller and saw gins installed in each country (table 6). However, the information was not available from all countries.

 Cost of Ginning

In many countries custom ginning is not available. The growers sell their seedcotton to a middle man who buys cotton either for himself or on commission basis for the ginner. Once cotton is delivered to the gin it is the property of the ginner and he is responsible for the profit and loss on the sale of lint and seed. Under such circumstances it is difficult to estimate the cost of ginning and the data on ginning cost is not available from all countries. Anyhow, the ginning cost from countries where custom ginning is done is the actual cost to the grower. While some countries provided detailed item wise cost of ginning, a number of others provided an estimated cost of ginning all the seedcotton from a unit area. Cost of ginning is given in table 7. Cost of ginning given here also includes transportation cost to the ginning factory, fee for classing and grading of cotton, if any, and cost of other expenses related to ginning. Value of seed has not been deducted from the ginning cost. Ginning is most expensive in Spain, probably due to the subsidy to be paid to the growers. Compared with the total cost of producing cotton, ginning is an expensive operation in Argentina, Bolivia, Pakistan, Paraguay, Philippines and Zimbabwe. In Israel, ginning of hirsutum costs US$158/ton of lint. Ginning of Pima cotton is expensive and costs US$240/ton of lint. Ginning is least expensive in China (Mainland) because of ginning in the public sector under the control of Bureau of Cotton and Jute of the All China Federation of Supply and Marketing Cooperatives.

Percentage of Hand and Machine Picking in 10 Major Countries

Country Hand Picking Machine Picking

Argentina 25 75

Australia 100

Brazil 90-95 5-10

China (Mainland)100

Greece 8 92

India 100

Pakistan 100

Turkey 100

USA 100

Uzbekistan 60-70 30-40