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.

fibre fineness

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

yarn testing procedure in spinning

YARN TESTING

INTRODUCTION:
Yarn occupies the intermediate position in the manufacture of fabric from raw material. Yarn results are
therefore essential, both for estimating the quality of rawmaterial and for controlling the quality of
fabric produced. The important characteristics of yarn being tested are,

1. yarn twist
2. linear density
3. yarn strength
4. yarn elongation
5. yarn evenness
6. yarn hairiness etc.

SAMPLING:
In order that the results obtained are reproducible and give reliable information about the material,
the sampling must be true and representative of the bulk lot. The sampling procedure should be designed
to take account of and to minimise the known sources of variability such as the variation between
spindles, the variation along the length of the bobbin, etc. The procedure for sampling and the number
of test carried out are given under each characteristic.

AMBIENT CONDITIONS FOR YARN TESTING:
Some textile fibres are highly hygroscopic and their properties change notably as a function of the moisture
content. Moisture content is particularly critical in the case of properties, i.e yarn tenacity,
elongation, yarn evenness, imperfections, count etc. Therefore conditioning and testing must be carried out
under constant standard atmospheric conditions. The standard atmosphere for textile testing involves a
temperature of 20+-2 degree C, and 65+-2% Rh. In tropical regions, maintaining a temperature of 27+-2 degree C,
65+-2%RH is legitimate. Prior to testing, the samples must be conditioned under constant standard
atmospheric to attain the moisture equillibrium. To achieve this it requires at least 24 hours.

TWIST:

· "Twist is defined asthe spiral disposition of the components of yarn, which is generally expressed
as the number of turns per unit length of yarn, e.g turns per inch, turns per meter, etc.

· Twist is essential to keep the component fibres together in a yarn.

· The strength, dyeing, finishing properties, the feel of the finished product etc. are all dependent
on the twist in the yarn.

· With increase in twist, the yarn strength increases first , reaches a maximum and then decreases.

· Depending on the end use, two or more single yarns are twisted together to form "plied yarns" or
"folded yarns" and a number of plied yarns twisted together to form "cabled yarn".

· Among the plied yarns, the most commonly used are the doubled yarns, wherein two single yarns of
identical twist are twisted together in a direction opposite to that of the single yarns.

· Thus for cabled and plied yarns, the direction of twist and the number of turns per unit length of
the resultant yarn as well as of each component have to be determined for a detailed analysis.

· Direction of twist is expressed as "S"-Twist or "Z"-Twist. Direction depends upon the direction of rotation
of the twisting element.

· Twist take up is defined as, "The decrease in length of yarn on twisting, expressed as a percentage
of the length of yarn before twisting.

LINEAR DENSITY OR COUNT OF YARN:

· The fineness of the yarn is usually expressed in terms of its linear density or count.

· There are a number of systems and units for expressing yarn fineness. But they are classified as follows

DIRECT SYSTEM:

1. English count(Ne)
2. Metric count(Nm)
3. French count(Nf)

INDIRECT SYSTEM:

1. Tex
2. Denier

1. Ne : No of 840 yards yarn weighing in One pound
2. Nm : No of one kilometer yarn weighing in One Kilogram
3. Nf : No of one kilometer yarn weighing in 0.5 kilogram
4. Tex : Weight in grams of 1000 meter(1 kilometer) yarn
5. Denier: Weight in grams of 9000 meter(9 kilometer) yarn

· For the determination of the count of yarn, it is necessary to determine the weight of a known length
of the yarn. For taking out known lengths of yarns, a wrap-reel is used. The length of yarn reeled off depends upon the count system used.

· Another factor which determines the length of yarn taken for testing is the type of balance used.
Some balances like quadrant balance, Beesley's blanace have been specially designed to indicate the yarn
count directly from tests on specified short lengths of yarn and are very useful for determining the
counts of yarn removed from the fabrics. The minimum accuracy of balance required is 0.001mg

· One of the most important requirements for a spinner is to maintain the average count and count variation
within control. The term count variation is generally used to express variation in the weight of a lea
and this is expressed as C.V.%. This is affected by the number of samples and the length being considered
for count checking. While assessing count variation, it is very important to test adequate number of leas.
After reeling the appropriate length of yarn, the yarn is conditioned in the standard atmosphere
for testing before it's weight is determined.

· The minimum number of sample required per count is 20 and per machine is 2.

YARN STRENGTH AND ELONGATION:

· Breaking strength, elongation, elastic modulus, resistance abrasion etc are some important factors which
will represent the performance of the yarn during actual use or further processing. Strength testing
is broadly classified into two methods

1. single end strength testing
2. skein strength or Lea strength

Tensile strength of single strands of yarn:

· During routine testing, both the breaking load and extension of yarn at break are usually recorded for
assessing the yarn quality. Most of the instruments record the load-elongation diagram also.

· Various parameters such as initial elastic modulus, the yield point, the tenacity or elongation at any stress
or strain, breaking load, breaking extension etc can be obtained from the load-extension diagram.

· Two types of strengths can be determined for a yarn

1. Tensile strength -load is applied gradually
2. Ballistic strength - applying load under rapid impact conditions

· Tensile strength tests are the most common tests and these are carried out using either a single strand
or a skein containing a definite number of strands as the test specimen.

· An important factor which affects the test results is the length of the specimen actually used for
carrying out the test. The strength of a test specimen is limited by that of the weakest link in it.If
the test specimen is longer, it is likely to contain more weak spots, than a shorter test specimen. Hence
the test results will be different for different test lengths due to the weak spots.

· The amount of moisture in the yarn also influences the test results. Cotton yarn when fully wet show
higher strength than when dry, while opposite is the case with viscose rayon yarns. Hence, to eliminate the
effect of variation due to moisture content of the yarn, all yarn strengrth tests are carried out,
after conditioning in a room where the standard atmospheric condition is maintained.

· The rate of loading as determined by the "time-to-break", which is the time interval between the
commencement of the application of the load and the rupture of the yarn, is an important factor , which
determines the strength value recorded by using any instrument. The same specimen will show a lower
strength when the time-to-break is high, or higher when the time-to-break is low.

· The instruments used for determining the tensile strengh are classified into three groups, based
on the principle of working.

1. CRT - Constant rate of traverse
2. CRE - Constant rate of extension
3. CRL - Constant rate of loading

· In the instruments of CRE type, the application of load is made in such a way that the rate of elongation
of the specimen is kep constant. In the instruments of the CRL type,the application of load is made
in such a way that the rate of loading is constant througout the duration of the test. This type of
instruments are usually preferred for accurate scientific work. In the CRE and CRL types of instruments,
it is easy to adjust the "time-to-break" while this adjustment is not easy in the CRT types of instruments.

· The uster Tensorapid applies the CRE principle of tensile testing. Constant Rate of Extension describes
the simple fact that the moving clamp is displaced at a constant velocity. As a result, the specimen between
the staionary and the moving clamp is extended by a constant distance per unit of time and the force
required to do so is measured. Apart fron single values, this instrument also calculates mean value
coefficient of variation and the 95% confidence range of maximum force, tenacity,elongation and work done

· The total coefficient of variation describes the overall variability of a tested lot, i.e the within-sample
variation plus the between-sample variation. If 20 individual single-end tensile test are performed
on each of ten bobbins or packages in a sample lot, the total coefficient of variation is calculated
from the pooled data of the total number of tests that were carried out.

· In tensorapid, the breaking tenacity is calculated from the peak force which occurs anywhere
between the beginning of the test and the final rupture of the specimen. The peak force or maximum force is
not identical with the force measured at the very moment of rupture. The breaking elongation is calculated
from the clamp displacement at the point of peak force. The elongation at peak force is no identical with the elongation at the very moment of rupture(elongation at rupture).

· The work to break is defined as the area below the stress/strain curve drawn to the point of
peak force and the corresponding elongation at peak force. The work at the point of peak force
is not identical with the work at the very moment of rupture.

· To compare tensorapid test results with other results,

1. a measurement must be performed according the CRE princple
2. testing speed must be exactly 5 m/min
3. the gauge length or the length of the specimen should be 500 mm
4. the pretension should be 0.5 cN/tex

· There are two fundamental criteria which affect the compatibility between different measurements
of tensile yarn properties.

1. testing conditions, i.e the testing principle(CRE,CRL), testing speed, gauge length, and pre-tensioning.
2. the second criteria,which also affects the magnitude of the differences, relates to the specific
   stress/strain characteristic of the yarn itself, which is determined by the fibrous materials, the
   blend ratio, and the yarn construction.

Skein strength or Lea strength:

The skein breaking strength was the most widely used measure of yarn quality in the cotton textile industry.
The measurement of yarn quality by this method has certain drawbacks. Firstly, in most of the subsequent
processing, such as winding, warping or weaving, yarn is used as single strand and not in the form of
a skein except occasionally when sizing ,bleaching, mercerising or dyheing treatments are carrried out
on hanks. Secondly, in the method used for testing skein strength, the rupture of a single strand at a weak
place affects the result for the whole skein. Further, this method of test does not give an indication
of the extensibility and elastic properties of a yarn, the characters which play and important role
during the weaving operations. However, since a large size sample is used in a skein test as against
that in a single strand test, the sampling error is less. The skein used for strength test can be used
for determination of the linar density of the yarn as well.

· In addition to the factors influencing the yarn strength, the size of the skein(lea) will affect to a
large extent the strength recorded. The usual practice is to use a lea(120 yards) of yarn prepared by
winding 80 turns on a wrap-reel having a perimeter of 1.5 yards(54 inches), so that during a test, there
are 160 strands of 27 in.(") length. There are different systems in use. But the actual breaking strength
recorded on the machine would depend on the type of skein used as both the number of strands and
test length may differ. The instruments most commonly used for this test is CRT type, where the
bottom hook moves at 12 inches per min.

· After findingout skein strength, broken skeins are also weighed to determine the linear density.
The most common skein used is the lea and the results of lea strength tests are expressed as C.S.P.,
which is the product of the linear density(count)of the yarn in the English system (Ne) and the lea breking
strength expressed in lbs. In view of the fact that C.S.P. is much less dependent on yarn count
than on strength, especially when count diffferences are small, C.S.P. is the mostg widely used
measure of yarn qauality.