Cotton

Harvesting And Storage

Introduction Ginning Spinning Quality Of Cotton

Ginning

• Cotton picked from the plant is in the form of seed cotton (kapas), and before the fibers can be used for any purpose, they have to be separated from the seed, the process of separation is called ‘Ginning’. It is the first mechanical process to which cotton is subjected. As the fibers are fairly firmly attached to the surface of the seed, it requires certain amount of force to detach them. Unless this force is tampered in some way, the fibers are likely to be torn irregularly and their texture impaired. Thus ginning is not only an essential process in the utilisation of cotton, but also an important one, as any damage to the fibers at this stage cannot be rectified later.

Poor ginning may be due to some of the following causes

• Excessive dampness or dryness of cotton during ginning.
• Inefficient ginning machinery
• Improper machine setting and speeds, and
• Inadequate cleaning of seed cotton before ginning
• The gin-damaged lint usually consists of entangled and matted fibres, cut fibres, excessive foreign matter and motes, helps hull pieces, etc. Such cotton not only produces excessive waste, but also presents considerable difficulties during processing, increases labour costs produces weaker and less uniform yarns, and detracts from the appearance of the finished product.
• For quick and accurate estimation of ginning percentage, CIRCOT has designed a portable type ginning machine for use by ginners, breeders, seed industry, farmers and traders. Two versions of the design are viz., the foot operated kisan gin and CIRCOT Laghu Otain Yantra (CLOY gin).

Kisan gin

• This is a pedal operated machine in which a pair of counter rotating rollers is employed to pinch and pull out fibres and seeds.
• The machine consists of 2 cylindrical rollers, the lint differ, the lint slide and the drive system. The capacity of the machine is about 1 kg of kapas per hour.

CIRCOT Lalghu otai Yantra (CLOY gin)

• This power operated machine works on the principle of Macorthy gin. A chrome leather roller, fixed knife, moving knife and seed grid are the main components that accomplish separation of fibres from the seed. The machine is driven by a single phase 1 hp motor. The capacity of the machine is 8 kg kapas/hr.

Influence of humidity in ginning

• In view of the large variations in the atmospheric humidity prevailing in various ginning factories the study conducted by CIRCOT revealed that the tests at relative humidities ranging from 30 per cent to 80 per cent however, showed no significant difference in the ginning percentage.

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Spinning

• For the comparison and estimation of spinning performance of different cottons, it is necessary that a standard procedure is adopted as the basis for evaluation for this purpose the most widely used quality characteristic is the lea strength.
• The product of the count of the yarn and its lea strength is known as count strength product (CSP) and is normally used to indicate the spinning value of cotton.

The methods used for the estimation of spinning value includes.

• Judge the performance by the lea length or CSP, all cottons being spun into one common count with the same twist multiplier. This method is adopted in Egypt. The samples of different cottons are spun to 603 count under identical conditions with a twist multiplier of 3.6 and then ranked according to CSP.
• Spin all cottons into known counts and then assess the spinning performance by a comparison of the lea strength at different counts, followed in USA.
• Spin the cotton into two or three counts and express the spinning value in terms of the highest standard count (MSC) having a standard strength, this being arrived at from the lea strength or CSP values of the counts actually spun.

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Quality Of Cotton

• Fibre quality is mostly governed by that of the yarn from which it is woven, and since the quality of the yarn in turn depends upon the properties of the fibers from which it is spun. The quality of raw cotton which is generally judged from the physical characteristics of the fibre is an important factor.
• Cotton possesses many qualities to make it a good textile fibre. Its dominant position as a textile raw material is primarily due to its versatility for a wide range of end-uses. The quality of cotton is mainly assessed by staple length.

Staple length

• Average length of individual cotton fibre. Longer the staple - better the quantity.
• Short staple : 19.5mm and below
• Medium staple : 20.0mm - 21.5mm
• Superior staple : 22.0mm - 24.0mm
• Long staple : 24.5mm - 26.5mm
• Superior long staple : 27.0mm - 29.5mm
• Extra superior long staple : 30.0mm and above

Ginning Percentage

• Out turn of lint to seed cotton expressed as percentage by weight normally varies between 24 - 38%.
• GP %= Weight of lint/Weight of seed cotton x 100

Spinning Quality

• Depends on staple length, fineness and strength of fibre Expressed in counts. Count is the no. of hanks (one hank = 840yds) found in one pound of yarn.

Fineness

• Related to staple length
• Expression of the weight per unit length of fibre
• Influenced by soil climate and management practices.

Colour

• Varies from reddish tint to bright shining white Brighter the colour - better the quality

Touch

• Coarseness or finess of lint to touch

Cleanliness

• Lint should be free from impurities.

Neppiness

• Defect of yarn due to tiny knots
• No uniformity in the thread
• Yarn is weak

Fibre maturity

• A sample of lint corrected at maturity is of 3 types:
• Mature (ripe)
• Half mature (Half ripe)
• Un mature (Un ripe)
• Ripe fibres have thickened walls and good convulsion (twist)
• Un ripe fibres have thin walls lakh of twist and weak, with a tendency to break up during manufacture.

Strength of fibre

• Estimated by means of a fibre-testing machine by clasping the ends of a single fibre between the jaws of machine and applying the strain gradually. The breaking strength of fibre depends upon its area of cross-section, test length, type of testing instrument used, the rate of loading etc., It also depends upon the relative humidity of the atmosphere.
• The tensile strength of fibre varies for 50,000 to 1,25,000 pounds per square inch. Fine cottons tend to have greater tensile strength than the short and coarse cottons.

Convolutions (Twists)

• The uniform distribution of the convolutions helps to give better inter- fibre grippage. Convolutions confer the following additional advantages
• Make the fibre equally flexible in all directions.
• Prevent close packing of fibres in yarn and hence give better cover in cloth.
• As the frictional contact of adjacent fibres is reduced, it lessens the risk of electrification, if any.
• The number of convolutions depends on the ratio of cell-wall thickness to ribbon width.
• The number of convolutions per inch varies from about 150 for Indian cottons to about 300 for Sea Island cottons.

Surface Friction

• The spinning quality of a textile fibre depends not only on its staple length and fineness, but also in its ability to offer sufficient frictional resistance.
• The effectiveness of friction depends upon the nature of the fibre surface and normal pressure between fibres due to twist.

Hygroscopicity

• Cotton absorbs moisture from the surrounding atmosphere depending on its temperature and relative humidity.
• Moisture had a marked effect on the tensile strength, elasticity and other properties of the fibre.

Rigidity

• In processing cotton, the fibres have to be twisted to make a yarn. The more rigid a fibre is, the greater is the force required to twist it and vice versa. Modules of rigidity is defined as the ratio of the tangential force per unit area to the angle of twist produced. It depends upon the shape of cross section and the wall thickness of the fibre.
• Temperature and relative humidity have a great influence on fibre rigidity. At room temperature, the rigidity of cotton fibre is six times that in an atmosphere saturated with moisture.

Elasticity

• Changes in length and volume as well as shears or twists produced by applied stresses are all included in the elastic properties.
• Cotton fibres are fairly elastic, though they exhibit both the primary creep and the secondary creep to some degree.

Plasticity

• Cotton is relatively non- plastic. Finishing process like shrinking depend on the increase in the plasticity of cotton fibres as they swell in water at elevated temperatures.

Cotton quality requirements

Count-wise Pattern of Yarn Production

• The main trends observed from data given.
• Yarn production in coarse counts (1s to 10s) has come down during recent years.
• The production in fine and superfine counts (41s and above) has increased during the years.
• The production in the 31s' - 40s' count range has also increased during recent years.
• The bulk of yarn production of about 71% comprises of counts in the range of 11s to 40s.
• The above trends indicate that while attention has to be given for producing cottons suitable for finer counts, greater efforts have to be made to produce cottons suitable for the count ranges of 11s to 40s.

Blending with Man-Made Fibres

• Although the preference for cotton in apparel fabrics is increasing as against fabrics made from purely synthetic fibres, the use of blends of cotton and synthetic fibres is expected to continue for various reasons. The proportion of blended yarns has increased to 13% in recent years.
• Prior to 1975, imported cottons from Egypt, Sudan, etc., were used for blending with polyester. Research work in CIRCOT showed that superior quality cottons developed in our country like MCU 5, Sujatha, Suvin, Hybrid 4, Varalaxmi, DCH 32, etc. are quite useful for blending. Our cottons, however, need improvement in respect of fibre maturity and trash content. Cottons used for blending should have good fibre strength and extensibility.

Deficiencies in fibre quality

• There are a few important deficiencies that add to lowering the quality of our cottons. We have to pay special attention to eliminate/minimise them so that the general quality of our cotton improves.

Variability in Fibre Quality

• A frequent complaint both from Indian and foreign users of our cottons is about the variability in quality observed even in the same lot of cotton. Leaving aside the factors such as admixture of seeds in cultivation and mixing up of varieties at marketing centers or at ginning factories, there are other genuine reasons like suitability of land/soil for the variety, level of crop management and attention paid to crops by different farmers, etc., which contribute to variability in quality of cottons.
• Some zoning system along with supply of good quality seeds and other inputs may have to be considered at least for superior quality varieties and hybrids like MCU 5, Hybrid 4, Hybrid 6, DCH 32 etc,

Fibre Strength for OE Spinning (open end)

• As it has been stated earlier that fibre strength plays a very important role in deciding the yarn quality in OE spinning system. Since our spinning industry is being modernised more rapidly at present by installing OE systems, we have to keep in mind the special requirement of fibre strength and orient our research programmes accordingly.

Fibre Immaturity

• Many of the long and extra long staple cottons, particularly interspecific hybrids, contain a high proportion of immature fibres.
• This mars the yarn quality, appearance and also leads to problems in dyeing. It is therefore desirable that cottons have 75% - 80% mature fibres. Ideally, we should aim at 80% mature fibres in our selection programmes.

Stickiness in cotton

• The presence of 'honey dew' in cotton (secretion by sucking pests like aphid, whitefly, etc.,) results in stickiness on account of which severe problems are faced in ginning and subsequent spinning as the fibres stick to the rollers in these machines.
• Further, the honeydew sometimes leads to black coloured mould formation resulting in loss of quality.

Motes

• Motes or undeveloped seeds with immature fibres constitute another source adding to lowering of yarn quality. The problem is encountered more in hybrid cottons. Although some of them are removed during processing, quite a few escape and enter the yarn and give rise to objectionable faults.
• Such faults in yarn have to be removed first by a separate process before producing quality fabrics on modern, high-speed knitting or weaving machines, which add to the cost of production. Therefore, this problem has to be kept in mind while selecting parents for evolving hybrid cottons.

Trash, Other Contaminants and Seed Coat Bits

• The average trash content in Indian cottons is about 6% - 7%, which is very high as compared to that in cottons from other countries in the world. It varies from as low as 2% in suvin and MCU 5 to as high as 18% in Wagad, V 797 etc., from saurashtra (India). Besides trash from plant origin, several extraneous materials contaminate our cottons.
• In our country cotton is harvested by hand picking and as such there is no justification absolutely for high trash content- care is necessary for general cleanliness at all levels starting from picking, storage, transport to market, storage at marketing centre till ginning.

Range of Variability of some Economic Characters of 4 different cultivated species of cotton

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