cotton cheracteristic and Ph effect of cotton

COTTON

Cotton has been cultivated for over 5000 years in tropical and sub-tropical countries and the usage of its conversation into yarn and fabric dates back to times immemorial. The highly refined and artistic of dyeing and printing of cotton fabrics was developed in the Indian subcontinent and China to high level of sophistication much before the industrial revolution started in the west, Even now when the man-made fibres have made great advancement in production and quality, “the King cotton” is continuing to maintain its acceptance and popularity. This is due to cotton’s highly versatile and desirable properties of pleasant handle, wear comfort strength, easy maintenance and above all modest price. Cotton is grown in the entire continent but major cotton producing countries, in order of quantity is

USA

China

India

Pakistan

Uzbekistan

Turkmenistan

Turkey

Australia

Egypt

Argentina

Russia

Egyptian and upto certain extent American cotton is of finer and longer fibres whereas Indian, Pakistani, Russian and American cotton is slightly inferior type.

GROWTH

The cotton plant belongs to botanical genus Gossypium and the fibre grows as a single cell from the cuter layer of the seed. After blossoming of the flower, a green pod form the increases to the size of a walnut and is commonly termed as boll. After 30 to 50 days of flowering, the boll bursts open and displays a white fluffy mass of fibres. After the sap in the sap in the fibres is dried, the seed fibres are picked up manually or mechanically.

COTTON PLANTATION

Cotton is generally grown in moderately hot climate cotton plant is produces seeds tear of cotton deteriorates both in quality and quantity so new qualities is done from April to August depending upon climate. The seeds are planted 1 to 10cm deep in rows and about one meter apart moisture and type of soil.

Young plants come up through surface of soil in about 8-12 days. Plants now grow and within 40 days they begin to form flower stalk. Flowering takes another 30 days. The cotton fibres grows inside closed “pod” which id grown in cotton flower. The pods cotani cotton seeds, wrapped up in young activity growing hairs.

There are about 20,000 fibres around one seed and about 15,000 or more fibres in a pod. When seeds are nearly riped the pod bursts open and cotton hairs project.

These cotton fibres along with their seeds picked up from boll. Either by hand pickig or by machine picking method.                                                     

GINNING

The process used to separate fibres from seed of cotton is called ginning. Some of the non-fibrous impurities are also removed during ginning. The fibres are about one third of weight of the seed cotton and are separated from the seed by a process of ginning. In this operation, the seed cotton is conveyed by grooved leather rollers against two knives that cut the fibres off the seed leaving a fuzzy layer of very short fibres. In the saw gin process, seeds are pulled away from the seed by rows of revolving saw-toothed bands. For transporting cotton to the spinning mills the fibres are pressed into compact rectangular packages called bales. The bale weight is from 170 to 220 kg each. The short fibres left on the seeds after ginning are known as linters. The linters are no use for spinning into yarns but after shaving off by the disinterring machines these can be converted into regenerated cellulosic rayons or nitrocellulose for use as explosively.

MORPHOLOGHY

On drying of cotton boll, the fibre shrinks, its cross-section changes from circular to bean shape it develops irregular convolution lengthwise.

Fibre dimensions vary considerably according to their varieties and may differ in length from 6cm in fineness from 10 to 20 micro meter longer and finer cotton with light color are values much higher than the coarser qualities. A fully developed and mature cotton fibre has four characteristic regions they are discussed below:

(a)    CUTICLE: This consists of a thin outer layer composed of waxes and pectic materials but its true nature is not known. Main function of this layer is to protect the fibre from atmospheric.

(b)   PRIMARY WALL: This layer is formed during initial growth of fibre and encloses nucleus and protoplasm that are essential for every living cell. It consists of cellulose and pectin in the form of fibrils and is laid longitudinally along the fibre axis. Its thickness is only about 1/100 the of the overall width of fibre.

(c)    SECONDARY WALL: This part of the fibre is formed in the second phase of growth and is nearly 90% of the weight. On viewing the fibre under a microscope after swelling in the cuprammonium solution, successive layers of cellulose is visible that are termed as daily growth rings. These layers are fibril in structure and are deposited in a spiral form but the spiral changes direction frequently. This reversal is responsible for development of convolutions during drying of the fibres. The convolutions range between 8 to12 per millimeter and provide point of adhesion between fibres that contribute to strength of the yarn spun from these.

(d)   LUMEN: This canal runs through the entire fibre and carries minerals essential for its growth. It keeps on shrinking with building up of the secondary wall and at the time of drying, it is only 1/20^th of the fibre. The lumen contains proteins, minerals ad color pigments.

(e)    CONVOLUTION: Convolution is the natural twist and is one of the chief properties of cotton. Convolutions are ribbon like twists which are produced during drying of the cotton cell content under the influence of sun. the number of convolutions in a cotton fibre varies according to its quality. Such as Egyptian cotton has about 230, American has 190 and Indian/ Pakistan cotton has about 125 convolution per inch. Convolution of cotton fibre gives soft feeling to the body, while using the cotton fabric.

THE POLYMER SYSTEM OF COTTON

Cotton polymer has very well oriented molecules. It has dominant OH hydroxyl groups. Cotton fibre is a crystalline fibre having 65-70% crystalline regions I its polymer.

CONSTITUENTS OF RAW COTTON:

Raw cotton contains the usual constituents of a living vegetable cell but their quantities vary according to the variety of cotton. The approximate composition of raw cotton is as under:

Cellulose Oil and wax Pectin’s Proteins Mineral matter (Ash) Other organic matter

88.0-96.0% 0.4-1.0% 0.7-1.2% 1.1-1.9% 0.7-1.6% 0.5-1.0%

It will be obvious that on scouring and bleaching the cotton materials, about 6-7% weight consisting of waxes pectin, protein etc. will be lost due to removal of their non cellulosic components.

COTTON QUALITY

Quality of cotton fibre is mainly concerned with its staple length, fineness and colour.

(a)STAPLE LENGHTH: Staple length of cotton grown countries is below;

COUNTRY	STAPLE LENGTH
Indian	Short in length about 25mm or less
Pakistani	Short in length about 25mm or less
Egyptian	Can grow upto 65mm in length
Brazilian	Can be upto 90mm by cross breeding

(B) FINENESS: Egyptian cotton is finest than the other cotton grown countries

(C) COLOUR: Colour of cotton by country wise is give below;

American	Normally white
Egyptian	Light cream colour and has high luster
Pakistani and Indian	Light grey colour

PHYSICAL PROPERTIES

a)      TENACITY: Cotton has a tenacity of 3-5 gpd (grams/denier). It is a strong fire due to good alignment of its long polymer. Cotton gains strength when wet. It is about 25% stronger in wet condition. This is due to reason that the polymers  improved giving strength to the cotton fiber.

b)      ELASTIC PLASTIC NATURE: Cotton is in-elastic due its crystalline polymer ststem and thet is the reason that fabrics crease and wrinkle easily

c)      HYGROSCOPIC NATURE: Cotton is very obsorbent due to OH group in its polymer which attracts wter molecules being polar. Cotton has a moisture regain of 8.5%. Because of its ghroscopic nature cotoon does ot develop static charges. Since the cotton is very absorbent. Its fabrics feels crispy due to the reason that the cotton fibres rapicly absorb the moisture from finger.

d)      THERMAL PROPERTIES: cotton conducts heat energy from body that’s why cotton fabric have a cool feeling. And that’s why excessive heat burns it.

·        The action of pH and chemical agents on cellulose

a)      EFFECT OF ACIDS: Cotton is not attacked by or effected by cold and weak acids. However if it is treated by hot dilute or cold concentrated acids it disintegrates.

EFFECT OF ALKALIES: Cotton has excellent resistance to alkalies.it swells in caustic soda but is not damaged. Cotton is mercerized. Mercerization is a process which is fabric is carried out by treating the cotton fabrics with caustic soda, which gives luster to fabric, it must be noted thet mercerization is done only to the cotton fabrics. Since the cotton is resistant to alkalies. It can be washed repeatedly in soap solution without any damage. From the chemical standpoint, the essence of this is process is in the absorption of alkali with the formation of alkali cellulose. Caustic soda combines with cellulose forming a molecular compound according to following equation:

C6H7o2(OH)3 + NaOH                     C6H7O2(OH)3NaOH

b)       

c)EFFECT OF HEAT: cotton has an excellent resistance to degradation by heat. It begins to turn yellow after several hours at 120c (It is called the scorching). Cellulose withstands short-term heating at a temperature of 180-200°C. A temperature above 275°c, intensive decomposition of cellulose lakes place with the formation of liquid and gaseous products of different composition. At 400 – 450 degree centigrade all gaseous decomposition products disappear and a hard residue (carbon) remains. It burns readily in air giving a smell like paper burning.

d)   EFFECT OF INSECTS: Cotton is not attacked by moths or beatles.

e)Action of reducing and oxidizing agents

Reducing agents have no effect on cellulose, while oxidizing agents readily convert it to oxycellulose. For chemical treatment of fibrous materials, large use is made of various oxidizing agents: sodium hypochlorite, hydrogen peroxide, sodium chlorite etc, and such acids that are ca of oxidizing, as for instance, nitric acid.

            Effect of light and atmospheric conditions

Under the action of light, cellulose is oxidized by atmospheric oxygen and due to photo oxidation, oxycellulose is formed as a result of which the strength of the cellulose is considerable reduced, the copper and iodine numbers are correspondingly increased, and the viscosity of cuprammonium solution is reduced.

           Effect of microorganism on cellulose

If the moisture content in fibres is over 9% and the relative humidity is over 75-85%. Some bacteria and mildew fungi may cause cellulose decay.

SPINNIG.

1. Ring spinning
2. Open-end spinning
3. Compact spinning

Mechanism of weaving video

denim manufacturing process video

dyeing process for denim "ROPE DYEING AND SLASHER DYEING" PROCESS

 Denim Fabric
Denim Fabrics woven of 100% cotton would be very strong and durable.
Traditionally Blue Denim is warp faced cotton fabric with 3 x 1 twill construction with warp being dyed in a solid colour and weft left un-dyed. The look and quality of the Denim Fabric shall improve after dyeing, the process of which differs from plant to plant. Normally the process of dyeing dictates the technology of Denim manufacturing.
The dyeing for Denim Fabric happens at the sizing stage. Generally there are two most popular methods of dyeing Denim Fabric. They are:
o Rope Dyeing
o Sheet Dyeing
 

This process eliminates a few intermediate processes of the rope dyeing. The yarn sheet is washed with chemicals such as caustic and washing soda and after squeezing the excess water; the yarn sheet is allowed to pass through Dyeing Troughs one time for oxidation and development of dye on yarn. After dyeing, the dyed yarn is washed again with fresh water for two-three times and finalIy squeezed before allowing it to pass through 36 drying cylinders.
Even today Denim Fabric without Indigo Dyeing is not called authentic Denim. Initially when Denim Fabric entered the fashion market, Denim manufacturers were using Natural Indigo Dye, which was costly and giving a natural finish. Though Synthetic Indigo Dye has gradually replaced Natural Indigo Dye, some unorganised manufacturers still prefer the latter and attract premium after branding them “Natural Dye Used”.

Dyeing: The unique feature in the manufacturing of denim fabric is the dyeing of the warp yarn through a long chain Indigo Dye Range. The logs of yarns from the warping process called "ball warps" are loaded at the entry end of the range and are processed through a series of boxes which contain dye to build the shade and through boxes that rinse the yarn of excess dye. Between boxes, the ropes are exposed to air in a process known as skying, where oxidation or fixing of the dye takes place.
At the end of the range, the yarn is passed over steam heated dry cans for drying. Process controls are located throughout critical components of the Indigo Range and processing is monitored and controlled. When the yarn leaves the dry cans, an in-line color monitoring system measures its shade and provides immediate feedback.

MORRISON ROPE DYEING MACHINE.

        

        

Features.
Total length of yarn sheet in machine = 700 meter
Speed of machine = 60 meter per min (average used 25-40 meter per min)
Scour box = for pre treatment and can be used for sulphur dying
Skying rollers = 35-40 rollers
Skying time after every dye bath = 1-2 minutes or depend on the speed of the machine
Total number of dye bath = 8 baths
Steamer = for the fixation of sulpher dyes
Total number of washing baths = 3 washing bath (2 for cold and 1 for hot wash)
Total number dryers = 36 (4 TEFLON coated and 32 are stain less steel)

Usually in US Denim mills 12 – 24 ropes are simultaneously process on the rope dyeing machine. Prior to dyeing, the ropes are boiled out and treated with caustic-soda and wetting agent to remove from the cotton oil, impurities which could influence the fastness for the dye.

To dye with indigo, the ropes are immersed into the dye-bath. To dye in rope 30 – 60 seconds immersion (20 meters yarn) and 60 - 180 seconds are required for the oxidation of the Indigo dyestuff to ensure that also ends in the centre of the rope are equally dyed. Please note that squeezing pressure is important 5 tons as fastness of colour and shade depends on even squeezing pressure. The comparatively long immersion and oxidation time requires a comparatively expensive equipment of machinery.

In order to obtain the required deep shade of blue colour the ropes are 5 – 6 times immersed in a sequence of dye boxes with an oxidation range then so called skying after each dye box. (Indigo belongs to the group of the vat dyes which is water-soluble in reduced solution and becomes an insoluble pigment when oxidized.

Having passed the dyeing and oxidation rage the ropes are guided through 2 or 3 washing boxes to wash off excessive loss pigments in the last box softener are added to ease the opening of the ropes. They are dried in series of cans. The dried ropes which contain 380 – 420 ends are then deposited into large coilers Rebeaming with 300 – 380 ends per rope is easer. These coilers are placed behind the long chain beamer where the Rebeaming and opening of the ropes takes place. In order to guarantee even yarn tension through Rebeaming on to a back beam ready for sizing the ropes are guided over a tension device which is placed approx. in 10 -11 meters distance from the long chain beamer. Broken ends which very really happen process of the rope are repaired at this process stage. Initially these machines were supplied without yarn stop motion but are available now a days on special request. This is of major importance as lost ends, fluff, 3 – tail ends and yarn remnants can cause inferior performance in weaving.

The so prepared beck beams are now sized in a sizing machine preferably with 2 size boxes. The size pick up varies between 8 – 10%. In Europe mainly modified starches with binders are used, whilst in USA certain low % of PVA is applied sin combination with starches by some companies. Depending on the final finishing process (washed denim) with no filler also CMC gives excellent performance in weaving. Special size mixes for soft denim will be discussed separately. We recommend however not to use PVA for sizing of denim as a surface of denim may show a leather skinned appearance.

Process Control of Rope Dyeing for denim.

1. Concentration of Hydrosulphite
It is measured by “tytano meter”. It should be from 1.5 gpl to 2.5gpl , or by redox potential of dye bath which should be from -730 mV to -860 mV.

2. Caustic Soda or pH value

Should be from 11.5-12.5

3. Dye concentration in Dye bath

it is measured by spectrophotometer. It should be in g/l


Guidelines

High Indigo Concentration --> Shade is greener and lighter
Low Indigo Concentration --> Shade is dull and Red.

High pH or Caustic Concentration --> Redder and lighter
Low pH or caustic concentration --> greener and darker

Dipping Time

Longer the dipping time, better will be the penetration and lesser will be the ring dyeing effect. It varies from 15-22 seconds.

Squeeze Pressure
High pressure will lead to lower wet pick up and result in lesser color and better penetration. At rope dyeing, squeeze pressure is 5-10 tonnes, ie. wet pick up is as low as 60%. Hardness of squeeze roller is about 70-75 deg. shores. It sqeeze rolls are too hard then there are chances of slippage and uneven yarn tension.. If squeeze rollers are too soft then shading will occur. Surface of the squeeze rolls should be ground twice a year.

Airing Time

It should be 60-75 seconds. Longer airing time results in high tension on the yarn and subsequent processes will become difficult.

Drying

Insufficient or unevenly dried yarns will result in poor rebeaming

Calculation of Replenishing Dye feed/min

Conc. of stock vat is g/l= 90
range speed in yards/min=25
count = 7s
totoal ends = 4100

Wt of yarn dyed /min= (4100*25*1000)/(7*840*202)= 7924 gms
shade desired = 2%
Amount of dye to be replenished/min= 158.5 gms

Effect of pH

At pH of 10.5 to 11.5, there will be formation of more monophenolate ions, which lead to higher color yield, as strike rate of the dye to the yarn bundle is very high, and wash down activities will be very good.

At pH higher than this, dye penetration will be less and wash down characteristics are also poor.

Testing

1. Alkalanity in Dye Bath Liquor

Pipet 10.0 ml of vat liquor into 100ml of distilled water in a 150 ml beaker. place under continuous agitation and insert the electrodes of a pH meter caliberated at pH 7.0 with standard buffer solution.

Titrate with tenth normal HCl ( 0.1 HCl) to pH 7.0 (ml = A)

calculate
g/l of NaOH = A *0.40

2. Hydro in Dye bath Liquor

Add 2 ml of 37% HCHO to 150 ml beaker. Add 2 ml of dye range liquor . Add 6 ml of 25% glacial acetic acid solution prepared by diluting 1 part acid with 3 parts water. Add 2 ml of starch/KI indicator. Add ml of water. Titrate with 0.046 N ( prepared by diluting 460 ml of 0.1 N Iodine to one liter ) solution until the color changes from emarald green to bluish purple.

G/l of hydro= mo fo 0.046N of Iodine

Importance of High Concentration of Free Hydrosulphite

The clearest shades with minimum reddish streaks are observed at by relatively high conc. of hydrosulphite. On the other side, with lack of hydrosulphite, the leuco indigo is less dissolved and thereby adheres to a greater extent to the fibres. With lack of hydrosulphite furthermore, the amount of unreduced dyestuff by oxidation at the upper level of the liquor and through activiation of unfixed dyestuff, gets separated from the fibrous material would constantly rise as the reducing agent for creating leucoform would be missing. Under these circumstances a reddish bronze like shade results due to dispersion of not reduced dyestuff in the yarn. The min. proportion of hydrosulphite should be around 1.3 to 1.5 gpl in case of rope dyeing and 3-4 gpl in case of sheet dyeing. Also to avoid the lack of hydrosulphite or Indigo at certain places in the immersion, vat, the whole quantity of the liquor should be circulated 2-3 times every hour.

Reaction Time

At very short reaction time, an adequate liquor exchange ( i.e. the amount of chemicals consumed and replaced by fresh addition of reduced indigo) is not assured. This has a negative influence on dyeing and depth of dye penetration. In addition to this the time available for diffusion of dyestuff until oxidation commences is too short. To ensure an even and good depth of dye penetration by dyeing in several passages, the reaction time should be 20-30 sec. for each vat (eg. at a speed of 20m/min for a reaciton time of 10 seconds, the immersion path should be maximum 3.3 meters).

A reaction time exceeding 60 seconds should be avoided as the amount of dyestuff again get reduced and released may again supersede that of additionally take up dye stuff, resulting in higher shades.

Softening Agent: 8 g/lit

Drying: Rest humidity should be 30% and then sized.

Indigo Dye Range                                                                             

Sizing of yarn in Set/ Beam to Beam Position.
The object of Sizing is to improve the strength of yarn by chemically binding the fibres with each other and also improve upon its friction resistance capacity by chemically coating the surface of yarn/fibres. Further, number of threads in warpers beam sheet is very less against number of threads required in whole width of fabric. Hence multiplication of sheets by drawing yarns together from many warp beams and again making one sheet is also performed on sizing machine. On sizing, normally, 8-12 % size material on warp thread is applied. This improvement in strength and frictional resistance characteristic of warp yarn is essential because during weaving, yarn has to undergo severe strain & stress as well as frictional operations.

Rope dayeing process for denim

Process Control of Rope Dyeing for denim

1. Concentration of Hydrosulphite

It is measured by vatometer. It should be from 1.5 gpl to 2.5gpl , or by redox potential of dye bath which should be from -730 mV to -860 mV.

2. Caustic Soda or pH value

Should be from 11.5-12.5

3. Dye concentration in Dye bath

it is measured by spectrophotometer. It should be in g/l

Guidelines

High Indigo Concentration --> Shade is greener and lighter

Low Indigo Concentration --> Shade is dull and Red.

High pH or Caustic Concentration --> Redder and lighter

Low pH or caustic concentration --> greener and darker

Dipping Time

Longer the dipping time, better will be the penetration and lesser will be the ring dyeing effect. It varies from 15-22 seconds.

Squeeze Pressure

High pressure will lead to lower wet pick up and result in lesser color and better penetration. At rope dyeing, squeeze pressure is 5-10 tonnes, ie. wet pick up is as low as 60%. Hardness of squeeze roller is about 70-75 deg. shores. It sqeeze rolls are too hard then there are chances of slippage and uneven yarn tension.. If squeeze rollers are too soft then shading will occur. Surface of the squeeze rolls should be ground twice a year.

Airing Time

It should be 60-75 seconds. Longer airing time results in high tension on the yarn and subsequent processes will become difficult.

Drying

Insufficient or unevenly dried yarns will result in poor rebeaming

Calculation of Replenishing Dye feed/min

Conc. of stock vat is g/l= 90

range speed in yards/min=25

count = 7s

totoal ends = 4100

Wt of yarn dyed /min= (4100*25*1000)/(7*840*202)= 7924 gms

shade desired = 2%

Amount of dye to be replenished/min= 158.5 gms

Effect of pH

At pH of 10.5 to 11.5, there will be formation of more monophenolate ions, which lead to higher color yield, as strike rate of the dye to the yarn bundle is very high, and wash down activities will be very good.

At pH higher than this, dye penetration will be less and wash down characteristics are also poor.

Testing

1. Alkalanity in Dye Bath Liquor

Pipet 10.0 ml of vat liquor into 100ml of distilled water in a 150 ml beaker. place under continuous agitation and insert the electrodes of a pH meter caliberated at pH 7.0 with standard buffer solution.

Titrate with tenth normal HCl ( 0.1 HCl) to pH 7.0 (ml = A)

calculate

g/l of NaOH = A *0.40

2. Hydro in Dye bath Liquor

Add 2 ml of 37% HCHO to 150 ml beaker. Add 2 ml of dye range liquor . Add 6 ml of 25% glacial acetic acid solution prepared by diluting 1 part acid with 3 parts water. Add 2 ml of starch/KI indicator. Add ml of water. Titrate with 0.046 N ( prepared by diluting 460 ml of 0.1 N Iodine to one liter ) solution until the color changes from emarald green to bluish purple.

G/l of hydro= mo fo 0.046N of Iodine

Importance of High Concentration of Free Hydrosulphite

The clearest shades with minimum reddish streaks are observed at by relatively high conc. of hydrosulphite. On the other side, with lack of hydrosulphite, the leuco indigo is less dissolved and thereby adheres to a greater extent to the fibres. With lack of hydrosulphite furthermore, the amount of unreduced dyestuff by oxidation at the upper level of the liquor and through activiation of unfixed dyestuff, gets separated from the fibrous material would constantly rise as the reducing agent for creating leucoform would be missing. Under these circumstances a reddish bronze like shade results due to dispersion of not reduced dyestuff in the yarn. The min. proportion of hydrosulphite should be around 1.3 to 1.5 gpl in case of rope dyeing and 3-4 gpl in case of sheet dyeing. Also to avoid the lack of hydrosulphite or Indigo at certain places in the immersion, vat, the whole quantity of the liquor should be circulated 2-3 times every hour.

Reaction Time

At very short reaction time, an adequate liquor exchange ( i.e. the amount of chemicals consumed and replaced by fresh addition of reduced indigo) is not assured. This has a negative influence on dyeing and depth of dye penetration. In addition to this the time available for diffusion of dyestuff until oxidation commences is too short. To ensure an even and good depth of dye penetration by dyeing in several passages, the reaction time should be 20-30 sec. for each vat (eg. at a speed of 20m/min for a reaciton time of 10 seconds, the immersion path should be maximum 3.3 meters).

A reaction time exceeding 60 seconds should be avoided as the amount of dyestuff again get reduced and released may again supersede that of additionally take up dye stuff, resulting in higher shades.

Softening Agent: 8 g/lit

Drying: Rest humidity should be 30% and then sized.

Addition of chemicals

1. Red Tinge: reduce addition of NaOH, increase slightly Na2S2O3

2. Darkish Red: increase Hydro

3. Light Greenish: decrease Hydro

4. Dark Green: Increase Caustic

FIBRE TESTING PROCEDURE

FIBRE TESTING

IMPORTANCE OF RAWMATERIAL IN YARN 

MANUFACTURING

Raw material represents about 50 to 70% of the production cost of a short-staple yarn. This fact is sufficient to indicate the significance of the rawmaterial for the yarn producer. It is not possible to use a problem-free raw material always , because cotton is a natural fibre and there are many properties which will affect the performance. If all the properties have to be good for the cotton, the rawmaterial would be too expensive. To produce a good yarn with this difficulties, an intimate knowledge of the raw material and its behaviour in processing is a must.

Fibre characteristics must be classified according to a certain sequence of importance with respect to the end product and the spinning process. Moreover, such quantified characteristics must also be assessed with reference to the following

• what is the ideal value?
• what amount of variation is acceptable in the bale material?
• what amount of variation is acceptable in the final blend

Such valuable experience, which allows one to determine the most suitable use for the raw material, can only be obtained by means of a long, intensified and direct association with the raw material, the spinning process and the end product.

Low cost yarn manufacture, fulfilling of all quality requirements and a controlled fibre feed with known fibre properties are necessary in order to compete on the world's textile markets. Yarn prodcution begins with the rawmaterial in bales, whereby success or failure is determined by the fibre quality, its price and availability. Successful yarn producers optimise profits by a process oriented selection and mixing of the rawmaterial, followed by optimisation of the machine settings, production rates, operating elements, etc. Simultaneously, quality is ensured
by means of a closed loop control system, which requires the application of supervisory system at spinning and spinning preparation, as well as a means of selecting the most sutable bale mix.

BASIC FIBRE CHARACTERISTICS:
A textile fibre is a peculiar object. It has not truly fixed length, width, thickness, shape and cross-section. Growth of natural fibres or prodction factors of manmade fibres are responsible for this situation. An individual fibre, if examined carefully, will be seen to vary in cross-sectional area along it length. This may be the result of variations in growth rate, caused by dietary, metabolic, nutrient-supply, seasonal, weather, or other factors influencing the rate of cell development in natural fibres. Surface characteristics also play some part in increasing the variablity of fibre shape. The scales of wool, the twisted arrangement of cotton, the nodes appearing at intervals along the cellulosic natural fibres etc.

Following are the basic chareteristics of cotton fibre

• fibre length
• fineness
• strength
• maturity
• Rigidity
• fibre friction
• structural features

STANDARD ATMOSPHERE FOR TESTING:
The atmosphere in which physical tests on textile materials are performed. It has a relative humidity of 65 + 2 per cent and a temperature of 20 + 2° C. In tropical and sub-tropical countries, an alternative standard atmosphere for testing with a relative humidity of 65 + 2 per cent and a temperature of 27 + 2° C,
may be used.

FIBRE LENGTH:
The "length" of cotton fibres is a property of commercial value as the price is generally based on this character. To some extent it is true, as other factors being equal, longer cottons give better spinning performance than shorter ones. But the length of a cotton is an indefinite quantity, as the fibres, even in a small random bunch of a cotton, vary enormously in length. Following are the various measures of length in use in different countries

• mean length
• upper quartile
• effective length
• Modal length
• 2.5% span length
• 50% span length

Mean length:
It is the estimated quantity which theoretically signifies the arithmetic mean of the length of all the fibres present in a small but representative sample of the cotton. This quantity can be an average according to either number or weight.

Upper quartile length:
It is that value of length for which 75% of all the observed values are lower, and 25% higher.

Effective length:
It is difficult to give a clear scientific definition. It may be defined as the upper quartile of a
numerical length distribution
eliminated by an arbitrary construction. The fibres eliminated are shorter than half the effective length.

Modal length:
It is the most frequently occurring length of the fibres in the sample and it is related to mean and median for skew distributions, as exhibited by fibre length, in the follwing way.

(Mode-Mean) = 3(Median-Mean)

where,
Median is the particular value of length above and below which exactly 50% of the fibres lie.

2.5% Span length:
It is defined as the distance spanned by 2.5% of fibres in the specimen being tested when the fibres are parallelized and randomly distributed and where the initial starting point of the scanning in the test is considered 100%. This length is measured using "DIGITAL FIBROGRAPH".

50% Span length:
It is defined as the distance spanned by 50% of fibres in the specimen being tested when the fibres are parallelized and randomly distributed and where the initial starting point of the scanning in the test is considered 100%. This length is measured using "DIGITAL FIBROGRAPH".

The South India Textile Research Association (SITRA) gives the following empirical relationships to estimate the Effective Length and Mean Length from the Span Lengths.

Effective length = 1.013 x 2.5% Span length + 4.39
Mean length = 1.242 x 50% Span length + 9.78

FIBRE LENGTH VARIATION:
Eventhough, the long and short fibres both contribute towards the length irregularity of cotton, the short fibres are particularly responsible for increasing the waste losses, and cause unevenness and reduction in strength in the yarn spun. The relative proportions of short fibres are usually different in cottons having different mean lengths; they may even differ in two cottons having nearly the same mean fibre length, rendering one cotton more irregular than the other.It is therefore important that in addition to the fibre length of a cotton, the degree of irregularity of its length should also be known. Variability is denoted by any one of the following attributes

1. Co-efficient of variation of length (by weight or number)
2. irregularity percentage
3. Dispersion percentage and percentage of short fibres
4. Uniformity ratio

Uniformity ratio is defined as the ratio of 50% span length to 2.5% span length expressed as a percentage. Several instruments and methods are available for determination of length. Following are some

• shirley comb sorter
• Baer sorter
• A.N. Stapling apparatus
• Fibrograph

uniformity ration = (50% span length / 2.5% span length) x 100
uniformity index = (mean length / upper half mean length) x 100

SHORT FIBRES:
The negative effects of the presence of a high proportion of short fibres is well known. A high percentage of short fibres is usually associated with,
- Increased yarn irregularity and ends dddown which reduce quality and increase processing costs
- Increased number of neps and slubs whiiich is detrimental to the yarn appearance
- Higher fly liberation and machine contttamination in spinning, weaving and knitting operations.
- Higher wastage in combing and other oppperations.
While the detrimental effects of short fibres have been well established, there is still considerable debate on what constitutes a 'short fibre'. In the simplest way, short fibres are defined as those fibres which are less than 12 mm long. Initially, an estimate of the short fibres was made from the staple diagram obtained in the Baer Sorter method

Short fibre content = (UB/OB) x 100

While such a simple definition of short fibres is perhaps adequate for characterising raw cotton samples, it is too simple a definition to use with regard to the spinning process. The setting of all spinning machines is based on either the staple length of fibres or its equivalent which does not take into account the effect of short fibres. In this regard, the concept of 'Floating Fibre Index' defined by Hertel (1962) can be considered to be a better parameter to consider the effect of short fibres on spinning performance. Floating fibres are defined as those fibres which are not clamped by either pair of rollers in a drafting zone.

Floating Fibre Index (FFI) was defined as

FFI = ((2.5% span length/mean length)-1)x(100)

The proportion of short fibres has an extremely great impact on yarn quality and production. The proportion of short fibres has increased substantially in recent years due to mechanical picking and hard ginning. In most of the cases the absolute short fibre proportion is specified today as the percentage of fibres shorter than 12mm. Fibrograph is the most widely used instrument in the textile industry , some information regarding fibrograph is given below.

FIBROGRAPH:
Fibrograph measurements provide a relatively fast method for determining the length uniformity of the fibres in a sample of cotton in a reproducible manner.

Results of fibrograph length test do not necessarily agree with those obtained by other methods for measuring lengths of cotton fibres because of the effect of fibre crimp and other factors.

Fibrograph tests are more objective than commercial staple length classifications and also provide additional information on fibre length uniformity of cotoon fibres. The cotton quality information provided by these results is used in research studies and quality surveys, in checking commercial staple length classifications, in assembling bales of cotton into uniform lots, and for other purposes.

Fibrograph measurements are based on the assumptions that a fibre is caught on the comb in proportion to its length as compared to toal length of all fibres in the sample and that the point of catch for a fibre is at random along its length.

FIBRE FINENESS:
Fibre fineness is another important quality characteristic which plays a prominent part in determining the spinning value of cottons. If the same count of yarn is spun from two varieties of cotton, the yarn spun from the variety having finer fibres will have a larger number of fibres in its cross-section and hence it will be more even and strong than that spun from the sample with coarser fibres.

Fineness denotes the size of the cross-section dimensions of the fibre. AS the cross-sectional features of cotton fibres are irregular, direct determination of the area of croo-section is difficult and laborious. The Index of fineness which is more commonly used is the linear density or weight per unit length of the fibre. The unit in which this quantity is expressed varies in different parts of the world. The common unit used by many countries for cotton is microgrammes per inch and the various air-flow instruments developed for measuring fibre fineness are calibrated in this unit.

Following are some methods of determining fibre fineness.

• gravimetric or dimensional measurements
• air-flow method
• vibrating string method

Some of the above methods are applicable to single fibres while the majority of them deal with a mass of fibres. As there is considerable variation in the linear density from fibre to fibre, even amongst fibres of the same seed, single fibre methods are time-consuming and laborious as a large number of fibres have to be tested to get a fairly reliable average value.

It should be pointed out here that most of the fineness determinations are likely to be affected by fibre maturity, which is an another important characteristic of cotton fibres.

AIR-FLOW METHOD(MICRONAIRE INSTRUMENT):
The resistance offered to the flow of air through a plug of fibres is dpendent upon the specific surface area of the fibres. Fineness tester have been evolved on this principle for determininG fineness of cotton. The specific surface area which determines the flow of air through a cotton plug, is dependent not only upon the linear density of the fibres in the sample but also upon their maturity. Hence the micronaire readings have to be treated with caution particularly when testing samples varying widely in maturity.

In the micronaire instrument, a weighed quantity of 3.24 gms of well opened cotton sample is compressed into a cylindrical container of fixed dimensions. Compressed air is forced through the sample, at a definite pressure and the volume-rate of flow of air is measured by a rotometer type flowmeter. The sample for Micronaire test should be well opened cleaned and thoroughly mixed( by hand fluffing and opening method). Out of the various air-flow instruments, the Micronaire is robust in construction, easy to operate and presents little difficulty as regards its maintenance.

FIBRE MATURITY:

Fibre maturity is another important characteristic of cotton and is an index of the extent of
development of the fibres. As is the case with other fibre properties, the maturity of cotton fibres varies not only between fibres of different samples but also between fibres of the same seed. The causes for the differences observed in maturity, is due to variations in the degree of the secondary thickening or deposition of cellulose in a fibre.

A cotton fibre consists of a cuticle, a primary layer and secondary layers of cellulose surrounding the lumen or central canal. In the case of mature fibres, the secondary thickening is very high, and in some cases, the lumen is not visible. In the case of immature fibres, due to some physiological causes, the secondary deposition of cellulose has not taken sufficiently and in extreme cases the secondary thickening is practically absent, leaving a wide lumen throughout the fibre. Hence to a cotton breeder, the presence of excessive immature
fibres in a sample would indicate some defect in the plant growth. To a technologist, the presence of excessive percentage of immature fibres in a sample is undesirable as this causes excessive waste losses in processing lowering of the yarn appearance grade due to formation of neps, uneven dyeing, etc.

An immature fibre will show a lower weight per unit length than a mature fibre of the same cotton, as the former will have less deposition of cellulose inside the fibre. This analogy can be extended in some cases to fibres belonging to different samples of cotton also. Hence it is essential to measure the maturity of a cotton sample in addition to determining its fineness, to check whether the observed fineness is an inherent characteristic or is a result of the maturity.

DIFFERENT METHODS OF TESTING MATURITY:
MATURITY RATIO:
The fibres after being swollen with 18% caustic soda are examined under the microscope with suitable magnification. The fibres are classified into different maturity groups depending upon the relative dimensions of wall-thickness and lumen. However the procedures followed in different countries for sampling and classification differ in certain respects. The swollen fibres are classed into three groups as follows

1. Normal : rod like fibres with no convolution and no continuous lumen are classed as "normal"
2. Dead : convoluted fibres with wall thickness one-fifth or less of the maximum ribbon width are classed as "Dead"
3. Thin-walled: The intermediate ones are classed as "thin-walled"

A combined index known as maturity ratio is used to express the results.

Maturity ratio = ((Normal - Dead)/200) + 0.70
where,
N - %ge of Normal fibres
D - %ge of Dead fibres

MATURITY CO-EFFICIENT:
Around 100 fibres from Baer sorter combs are spread across the glass slide(maturity slide) and the overlapping fibres are again separated with the help of a teasing needle. The free ends of the fibres are then held in the clamp on the second strip of the maturity slide which is adjustable to keep the fibres stretched to the desired extent. The fibres are then irrigated with 18% caustic soda solution and covered with a suitable slip. The slide is then placed on the microscope and examined. Fibres are classed into the following three categories

1. Mature : (Lumen width "L")/(wall thickness"W") is less than 1
2. Half mature : (Lumen width "L")/(wall thickness "W") is less than 2 and more than 1
3. Immature : (Lumen width "L")/(wall thickness "W") is more than 2

About four to eight slides are prepared from each sample and examined. The results are presented as percentage of mature, half-mature and immature fibres in a sample. The results are also expressed in terms of "Maturity Coefficient"

Maturity Coefficient = (M + 0.6H + 0.4 I)/100 Where,

M is percentage of Mature fibres
H is percentage of Half mature fibres
I is percentage of Immature fibres

If maturity coefficient is

• less than 0.7, it is called as immature cotton
• between 0.7 to 0.9, it is called as medium mature cotton
• above 0.9, it is called as mature cotton

AIR FLOW METHOD FOR MEASURING MATURITY:

There are other techniques for measuring maturity using Micronaire instrument. As the fineness value determined by the Micronaire is dependent both on the intrinsic fineness(perimeter of the fibre) and the maturity, it may be assumed that if the intrinsic fineness is constant then the Micronaire value is a measure of the maturity

DYEING METHODS:
Mature and immature fibers differ in their behaviour towards various dyes. Certain dyes are preferentially taken up by the mature fibres while some dyes are preferentially absorbed by the immature fibres. Based on this observation, a differential dyeing technique was developed in the United States of America for estimating the maturity of cotton. In this technique, the sample is dyed in a bath containing a mixture of two dyes, namely Diphenyl Fast Red 5 BL and Chlorantine Fast Green BLL. The mature fibres take up the red dye preferentially, while the thin walled immature fibres take up the green dye. An estimate of the average of the sample can be visually assessed by the amount of red and green fibres.

FIBRE STRENGTH:
The different measures available for reporting fibre strength are

1. breaking strength
2. tensile strength and
3. tenacity or intrinsic strength

Coarse cottons generally give higher values for fibre strength than finer ones. In order, to compare strength of two cottons differing in fineness, it is necessary to eliminate the effect of the difference in cross-sectional area by dividing the observed fibre strength by the fibre weight per unit length. The value so obtained is known as "INTRINSIC STRENGTH or TENACITY". Tenacity is found to be better related to spinning than the breaking strength.

The strength characteristics can be determined either on individual fibres or on bundle of fibres.

SINGLE FIBRE STRENGTH:
The tenacity of fibre is dependent upon the following factors

chain length of molecules in the fibre orientation of molecules size of the crystallites distribution of the crystallites gauge length used the rate of loading type of instrument used and atmospheric conditions

The mean single fibre strength determined is expressed in units of "grams/tex". As it is seen the the unit for tenacity has the dimension of length only, and hence this property is also expressed as the "BREAKING LENGTH", which can be considered as the length of the specimen equivalent in weight to the breaking load. Since tex is the mass in grams of one kilometer of the specimen, the tenacity values expressed in grams/tex will correspond to the breaking length in kilometers.

BUNDLE FIBRE STRENGTH:
In practice, fibres are not used individually but in groups, such as in yarns or fabrics. Thus, bundles or groups of fibres come into play during the tensile break of yarns or fabrics. Further,the correlation between spinning performance and bundle strength is atleast as high as that between spinning performance and intrinsic strength determined by testing individual fibres. The testing of bundles of fibres takes less time and involves less strain than testing individual fibres. In view of these considerations, determination of breaking strength of fibre bundles has assumed greater importance than single fibre strength tests.