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Monday, February 12, 2018

Cotton Fiber: The King of Natural Fiber

Cotton fiber and its structure:
Cotton fiber is one of the most versatile fibers, derived from the seed hair of a bushy annual plant. Cotton provides about 50% of the world’s textile fiber. The fibers are the outer covering of the seeds, which themselves yield valuable oil, used for the production of cooking oil or margarine or as a constituent of animal feeds.

Cotton was used extensively in antiquity, with evidence of cotton utilization within India and China stretching back millennia. Cotton truly became westernized with the onset of the power loom and the explosion and dominance of cotton fabrics led to the fiber being known as king cotton. This domination faded with the emergence of synthetic materials, but cotton continues to hold more than 50% share for apparel and textile goods. For many apparel products, cotton is an indicator of quality and 100% cotton shirts carry a significant premium more than comparable synthetics and blended materials.

Cotton fibers are single-cell filaments that are harvested from plants belonging to the genus Gossypium. The cotton fibers are formed around a seed boll that has reached maturity to aid distribution and to protect the delicate seed. An individual cotton fiber appears as a contorted tube with a kidney bean shape with convolutions along the length. Typical mature cotton has a hollow-ribbon cross section. This unique twisted shape creates a unique handle and appearance, allowing it to bend with freedom due to this shape structure. The high stiffness of cotton fiber means it can be readily processed in to high count yarns using the ring spinning process. Cotton can vary in length, diameter, and maturity and these factors will determine the quality of a cotton and which process and yarn type is most likely.

The cotton fiber is almost exclusively constructed from cellulose, which comprises around 90% of the total mass of the fiber. The noncellulosic material includes proteins, inorganics, pectins, and waxes. Cellulose appears in all plants to various extents, but it is the structure and organization of these cellulose chains that give cotton excellent mechanical properties and chemical resilience.

The cotton bolls are harvested and processed to remove trash and plant matter followed by a series of homogenizing steps prior to spinning. The cotton processing chain as shown in Figure-1, is extensive, comprising multiple steps all designed to improve the uniformity and quality of a cotton staple.

Processing chain for cotton
Figure-1: Processing chain for cotton
The quality of a cotton fiber is intrinsically linked to the fiber price. The factors affecting quality are length, fineness, color, strength, and uniformity, which were traditionally benchmarked against reference samples by human appraisers to determine the premium or discount a particular grade would command. However, the use of objective appraisal techniques is becoming the norm and most national trading bodies have developed a set of objective standards for the aforementioned properties along with trash and nep (tangled fiber balls) content. The development of the high-volume instrument (HVI) in the 1980s created an objective means of trading cotton, improving quality in the mill, and feeding valuable information back to cotton growers, leading to an overall increase in the average length (and quality) of Australian upland cotton since the introduction of the HV.

For the designer, there are additional aspects to consider in regards to processing. Chemical modification of cotton has been well established and can create different design features. Mercerization is a process of converting cellulose I to cellulose II via the treatment with sodium hydroxide. This changes the appearance of the cotton fibers, the subsequent yarn, and the final fabric. Cotton also has a fine sheen of surface wax (0.4%–1.2%), which acts as a natural lubricant to limit the level of fiber breakage during the intensive opening, gilling, and carding phases. This wax is a fatty hydrophobic compound that also renders the cotton fiber impermeable to water and imparts a greasy handle on the finished fabric.

For a crisper handle and improved dyeing, the wax must be removed in a scouring process. This will often be followed by a bleaching and mercerization step to improve the whiteness and luster of the fiber. These steps are typically done after fabric formation. Along with dyeing, it is these finishing processes that are water and energy intensive, generating waste products that must be handled accordingly.

Production of cotton fiber:
The traditional varieties of cottons cultivated in this country (Indian subcontinent) in the past belonged to the species of Gossypium arboreum and Gossypium herbaceum, called generally as desi cottons. The varieties belonging to these species have inherently low yield and are shorter and coarser than varieties belonging to Gossypium hirsutum or American Upland types. Cottons belonging to Gossypium barabadense have extremely long and fine fibres and are grown mostly in Egypt and Sudan. In 1947–1948, the proportion of Indian cottons belonging to G. hirsutum was hardly 3% of the overall production, whereas today the cultivation of Hirsutum cottons has risen to above 50% of the overall area. The area under Barabadense, negligible even in 1970, has also started rising steadily. Apart from these, one more reason for the improvement in our cotton situation has been the introduction of a number of hybrid varieties of cotton which are known for their phenomenal increased yields compared to those of the older varieties.

The cotton plant grows in moderately hot climate and is widely distributed over the world. There are many varieties which produce fi bres of different physical properties. The best qualities reaching the world market for superfi ne counts come from Egypt, however, the quantity grown are much less. A slightly shorter length quality is grown in India, China and Pakistan. It is of course possible to adapt the best qualities to various parts of the world so that cotton can be grown practically anywhere, provided the climate is suitable for the purpose.

The biggest cotton producers as on date are India, China, USA, Pakistan and Brazil. Table-1 gives the top cotton producing countries for the year 2016–2017. In 2016–2017, India has emerged as the largest producer of cotton.

top cotton producing countries
Table-1: Top cotton producing countries
The qualities vary mainly in colour and staple length. American cotton is generally white as compared to Egyptian cotton which is light cream. Indian cotton is of a light grey colour. India is the only country growing all lengths of cotton starting from the lowest to the highest, but the majority of them have staple length usually not exceeding 25.4 mm, whereas Egyptian and American cottons grow up to 63.5 mm long. The lustre of the fibres also varies, Egyptian cotton being the most highly lustrous. It has also been found possible to grow coloured cotton (green, grey and brown) and such varieties are utilized in the USSR. It may be interesting to note that 90 mm length cotton fi bre variety was obtained in Brazil by crossbreeding Sesido cotton with Peru-pima cotton (Textile Research Journal, 1980, 50 (12), 752).

The cotton plant belongs to the malaceae or mallow family. It is a perennial plant, with a taproot, cultivated since very ancient times (in the Nile valley by the Egyptians and in Central America by the Aztecs), for its textile fibres that come from the long bristles that encircle the seeds. It is perennial, that is, it produces seeds year after year. But after a time the value of cotton deteriorates both in quality and quantity. The best results are obtained by cultivating it as an annual crop. This has also the advantage of enabling the cotton grower to improve his stock by the introduction of new strains.

Planting begins at various times in different regions, depending on the climate. In India it is from April to August. The cotton seed is planted 1–10 cm deep in rows 1 m apart of the prepared land, depending on the moisture and the type of soil. If the soil is sufficiently warm and moist, young plants push up through the surface of the soil in 8–12 days. If it is too dry then germination may not take place and if it is wet and cold, then germination is slow and uncertain. The leaves and stems of the young plants are very tender, of fleshy colour and are smooth and oily. The leaves then become dark olive green and the plants grow rapidly. Within 40 days plant begins to form flower stalks. Flowering takes another 30 days. The opened flower is yellowish white on the first day when pollination occurs; it turns to pink on the next day when fertilization takes place and the petals of the flowers fall on the third day. The immature seeds thus formed, grow rapidly and the large cotton boll matures in 40–50 days.

Cotton fibres grow inside the closed pod. Pod contains cotton seeds, which are wrapped up in young, actively growing hairs. It is interesting to note that each cotton seed may produce as many as 20,000 fibres on its surface, and a single boll may contain 150,000 fibres or more. The fibres grow from the surface of the seeds from a single cell, which lengthens to a thin walled tubular structure of maximum length. The fibre wall is then thickened by daily growth of rings of cellulose. Harvest occurs 6–7 months after seeding and the fibres are separated from the seeds (shelling). The fibres are assembled into balls for treatment in the textile industry. When the seeds are nearly ripe, the pod bursts open, and the cotton hairs project, forming a white fluffy mass which are usually accommodated in four sections in the pod. This is termed as boll. The fibres now complete their ripening, after which their cell contents gradually dry up, leaving the commercial cotton fibre. The drying of the cell contents under the influence of the sun is a very important factor in producing one of the chief properties of cotton, namely; its characteristic natural twist called convolution. The number of convolutions varies according to the quality of cotton, e.g. Sea Island 300, Egyptian-230, American-190, Indian-I50 per 25 mm. Cotton fibres are picked from the boll either by hand picking method or by machine picking method.

After the cotton has been gathered, the cotton fibres are separated from the seeds by a process known as ginning, in which it is passed between rotating knife roller or saw toothed discs, whereby cotton fibres are pulled off the seed. The seeds are then collected to be utilized for the manufacture of cotton seed oil, and the residue used as fodder. The fibres collected after the ginning process will have their upper free end tapering to a point, while basal end is open when it has been cut off from the seed. Fragments of the seed coat, leaves or stem, and accidental dirt may also be present in the ginned cotton.

The ginned cotton is graded, depending on the quality of the fibre, before pressing them into bales. The fibres which are packed into bales are then dispatched to the spinning mills (see Fig-2).

Flow chart showing cotton fibre production
Figure-2: Flow chart showing cotton fibre production
Types of Cotton according to production:
There are four types of cotton. They are
  1. Bt Cotton
  2. Organic cotton
  3. Colored cotton
  4. GM (Generally Modified) Cotton,
Bt-Cotton:
Bt cotton is a genetically modified crop. Just that. Follow the explanation to understand the need of such a crop and how it is exactly done. Bt stands for Bacillus thuringiensis - a soil bacterium which contains a toxic gene called Bt gene.

Bt-Cotton pests attack these crops causing extensive damage. Pest attack accounts for the major cause of reduced production. There are about 162 species of insects, which are known to devour cotton at various stages of growth, of which 15 are considered to be key pests. Among the insects, cotton bollworms are the most serious pests in India. Over the past 40 years, many pests have developed resistance to pesticides.

The only successful approach to engineering crops for insect tolerance has been the addition of Bt-Cotton toxin, a family of toxins originally derived from soil bacteria. Thus genetically engineered form of natural cotton is produced by inserting a synthetic version of a gene from the naturally occurring soil bacterium Bacillus thuringiensis, into cotton. The primary purpose is to induce the plant to produce its own BT toxin to destroy the bollworm, a major cotton pest. The gene causes the production of BT toxin in all parts of the cotton plant throughout its entire lifespan. When the bollworm ingests any part of the plant, the BT cotton toxin pierces its small intestine and kills the insect. The BT toxin contained by the BT crops is no different from other chemical pesticides, but causes much less damage to the environment. These toxins are effective against a variety of economically important crop pest but pose no hazards to non-target organisms like mammals and fish. Three BT crops are now commercially available: corn, cotton and potato. The main selling points of Bt-Cotton are the reductions in pesticides to be sprayed on a crop and the ecological benefits which stem from that.

Organic cotton:
Research work at the Central Institute for Cotton Research was translated into an organic cotton production technology, focusing on ‘on-farm’ recycled soil amendments and biorational pest management. Organic cotton is also known as green cotton, environmentally friendly cotton or biological cotton.

As per the National Organic Standards Board, USA, Organic agriculture is defined as an ecological production management system that promotes and enhances biodiversity, biological cycles and soil biological activity. It is based on minimal use of ‘off-farm’ inputs and no management practices that restore maintain and enhance ecological harmony.

Organic cotton is grown and processed without toxic chemicals like pesticides, fungicides, plant growth regulators and synthetic fertilizers that can be absorbed easily when in contact with the user’s skin. Pesticides, fertilizers and chemicals used to grow and process conventional cotton fabrics may go directly to the user’s blood stream, which consequently affects the body organs and tissues. Table-2 gives the advantages of growing cotton organically.

Advantages of cultivating cotton organically
Table-2: Advantages of cultivating cotton organically
Organic production systems replenish and maintain soil fertility, reduce the use of toxic and persistent pesticides and fertilizers and build biologically diverse agriculture. To increase organic matter in the farm crop rotation/ crops used for mulching, green manuring, manure/compost should be used in balanced proportion. Intercropping is used where secondary crops (often sunflowers or millet) are grown between and around small plots of cotton. To control weeds, use of green manuring, crop mulching and hand weeding is recommended.

To control pest and diseases use of resistant varieties, follow timely sowing, intercultural operations, cultivation of intercrop and trap crops, use of pheromone traps, use of parasites and predators, use of bio-control that is Bacillus thuringiensis, use of herbal oils and recommended pesticides (pyrethrum, retanon) is done. There are rules for organic cotton processing and preparing fabrics.

Agencies giving certification should ensure that in the land which is to be used for organic cotton farming, from last three years there is cultivation without pesticides and chemical products or artificial fertilizers. If there is used of chemicals and pesticides at the moment than one has to wait for a period of three years before the chemical residues on the farm disappears. Organic cotton is not a variety; it is the more of certification of the non-toxicity in soil of the agricultural land and the produce from it.

The ingredients for success need to be applied together are:
  1. Suitable measures to improve and maintain soil fertility;
  2. Establishment of crop rotation and crop diversity, fostering natural balance;
  3. Selection of varieties suitable to the conditions (soil, availability of irrigation, market requirements);
  4. Appropriate types and amounts of manures at the right time;
  5. Timely crop management such as intercultural operations, weeding and irrigation;
  6. Careful monitoring of the crop and sufficient protection against pests according to the concept of economic threshold level;
  7. Timely and proper picking of the cotton;
  8. Sufficient documentation for inspection and certification;
  9. Capacity building and experimenting for continuous improvement.
Coloured cotton:
There is worldwide concern about the growing health hazard associated with the use of textiles dyed with the azo-group and other synthetic dyes. A new trend has emerged in the cotton market scenario as the natural coloured cotton has caught the fancy of crop scientists. Natural colour cotton is a naturally pigmented fibre that grows in shades of green, brown and beige.

The cotton plants are also hardy and naturally resistant to pests, eliminating the need for environmentally damaging pesticides and insecticides. The whole process from planting to growing to spinning to readymade garments uses no dyes, toxic or harmful materials. It is 100% pure and natural. It is the most environmentally green product possible and creates a beautiful, breathable, pure fabric with a soft, delicate feel. The natural colour is due to the plant’s inherent (non-modified) genetic properties. University of California and Bureau Veritas testing have shown that it compares favourably to white cotton fabrics in a study evaluating abrasion, resistance, dimensional stability and pilling resistance.

Colour grown cotton naturally grows in shades of creams, greens and browns so no further dye is required. The renewed interest in research on colour cotton is an off-shoot of the growing consumer driven movement as it is an eco-friendly alternative to dye-based textiles. Earlier the interest was slim in this due to low economic spin-off and inferior quality of cotton, but researchers now have come up with commercial varieties by incorporating desirable strains through breeding techniques.

The natural coloured cotton is much more favourable and desirable as compared to the chemically dyed cotton as in the later case colour fades when treated with detergents. Recently an indigenous cotton plant “Gossypium arboreum” is identifi ed that possessed medium stable length and good strength, which earlier was the major drawback of coloured cotton. Various shades of brown and green have also been identifi ed in “Gossypium hirsutum” and “Gossypium barbadense” variety of cotton.

Since naturally coloured cotton does not have pesticides, chemicals, viz. bleaches, dyes, fewer allergies and respiratory problems are found. Naturally coloured cotton feels softer to the skin and pleasant smell. Naturally coloured cotton is still relatively rare commodity because it requires specialized harvest techniques and facilities, making it more expensive to harvest than normal white cotton.

Yields of naturally coloured cotton are typically lower and the fibre is shorter and weaker but has softer feel than the more commonly available ‘white’ cotton.

References:

  1. Textile and Clothing Design Technology by Tom Cassidy, Parikshit Goswami
  2. Introduction to Textile Fibres by H. V. Sreenivasa Murthy
  3. https://en.wikipedia.org/wiki/Cotton
  4. https://www.statista.com
  5. https://www.fibl.org/
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