Cotton Fibre Quality Issues-In Brief

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Cotton Fibre Quality Issues in Brief

Cotton & Yarn Quality Co-Relation:

Instead of buying any cotton available at lowest price, spinning it to produce yarn of highest count possible & selling yarn at

any market in random, it is advisable to locate a good market where Yarn can be sold at highest price & select a Cotton which

has characteristics to spin yarn of desired specifications for that market.

Essential Characteristics of cotton quality & characteristics of yarn quality are given from detailed experimental investigations.

Some of important conclusions which help to find co-relation between Yarn quality & Cotton quality are given below:

ð Staple Length : Finer yarn count as well as stronger yarn is spun from longer fibre which assures higher prices.

ð Strength: : Stronger fibres give stronger yarn. Further, processing speeds can be higher so that higher productivity is achieved with less end-breakage.

ð Fibre Fineness : Finer Fibers produce finer count of Yarn & it also helps to produce stronger Yarns.

ð Fibre Maturity : Mature fibres give better yarn evenness. There are fewer end-breakages & better dye absorbency is additional benefit.

ð Uniformity Ratio : If ratio is higher. Yam is more even & there are reduced end-breakages.

ð Elongation : A better elongation value helps to reduce end-breakages in spinning & hence higher productivity with low raw material waste.

ð Non-Lint Content : Low Trash% reduces process waste in Blow Room & cards. There is less chances of yarn defects.

ð Sugar Content : Higher Sugar Content creates fibre stickiness & create processing problem of licking in machines.

ð Moisture Content : If Moisture Content is more than standard (8.5%), invisible loss is higher. But if it is less, then there gets a tendency for fibre brittleness resulting in frequent yarn breakages.

ð Feel : Smoother feel Cotton produces smoother yarn which has potential for weaving better fabric.

ð Class : Cotton having better grade in classing produces less process waste & yarn gets better appearance.

ð Grey Value : Rd. of calorimeter is higher it means it reflects light better & Yarn give better appearance.

ð Yellowness : Higher yellowness value deteriorates fibre grades & they produce weaker & inferior yarn.

ð Neppiness : Neppiness creates due to entanglement of fibres in ginning process of immature fibres which are sorted out by careful processing But, Neps due to immature fibre presence in end products cause higher yarn defects

level.

An analysis can be made of Yarn properties which are directly attributed to cotton quality.

ð Yarn Count : Higher Count of Yarn .is produced by longer, finer & stronger fibres.

ð C.V. of Count : Higher Fibre Uniformity & lower short fibre% is beneficial to keep C.V (Co-efficient at lowest.

ð Tensile Strength : This is directly related to fibre strength. Longer Length of fibre will also help to produce stronger yarns.

ð C.V. of Strength : Is directly related CV of fibre strength.

ð Elongation : Is beneficial for weaving efficiently. Fibre with better elongation has positive impacts.

ð C.V. of Elongation: C.V. of Yarn Elongation can be low when C.V. of fibre elongation is also low.

ð Mars Variation : This property directly related to fibre maturity and fibre uniformity.

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ð Hairiness : Is due to faster processing speeds and high level of very short fibers,

ð Dyeing Quality : Will defend on Evenness of Yarn and marketing of cotton fibres.

ð Brightness : Yarn gives brighter appearance if cotton grade is higher.

Quality Standard for Ring frame Cop-Combed Cotton

Sl.No Yarn characteristic Reqrd value for 30s Other counts 01 Average count 30 ( 29.6 - 30.4) Nominal count plus or minus 1.3% 02 Count C.V% less than 1.5 Less than 1.5% 03 Twist Multiplier 3.5 - 3.6 3.5 - 3.6 04 TPI C.V% less than 2.5 less than 2.5% 05 U% 9.2 - 9.8 5 - 10 % Uster Stat. value 06 -50% thin place / 1000m less than 4 5 - 10 % Uster Stat. value 07 -30% thin place / 1000m less than 650 5 - 10 % Uster Stat. value 08 +50% thick place / 1000m less than 30 5 - 10 % Uster Stat. value 09 +200 Neps / 1000m less than 50 5 - 10 % Uster Stat. value 10 Total Imperfection / 1000 m less than 85 5 - 10 % Uster Stat. value 11 RKM ( tenacity) gm/tex more than 16.5 more than 16.5 12 RKM C.V% less than 7.5 % 5 - 10 % Uster Stat. value 13 Elongation % more than 5.5 more than 5.5 14 Hairiness H 4.0 - 4.5 < 50% value of Uster Statistics 15 Hairiness Standard Deviation less than 1.5 25% Uster stat value 16 Objectionable classimat faults(short & long) less than 1 per 100 km less than 1/100km 17 Total classimat faults less than 150 5 - 10 % Uster Stat. value 18 H1- thin faults less than 5 per 100 km 5 - 10 % Uster Stat. value 19 shade variation on cones in UV lamp No Shade Variation No Shade Variation

Cotton Quality Specifications:

Length- The most important fibre quality is Fibre Length

Staple classification Length mm Length inches Yarn Fineness Yarn Count Short Less than 24 15/16 -1 Coarse Below 20 Medium 24- 28 1.1/132-1.3/32 Medium Count 20s-34s Long 28 -34 1.3/32 -1.3/8 Fine Count 34s - 60s Extra Long 34- 40 1.3/8 -1.9/16 Superfine Count 80s - 140s

ð Spinning Count depends not only on staple length but also on fineness & processing machinery.

ð Length is measured by hand stapling or Fibro graph for 2.5% Span Length

ð 2.5%SL (Spun Length) means at least 2.5% of total fibres have length exceeding this value.

ð 50% SL means at least 50% of total fibres have length exceeding this value.




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Length Uniformity

Length Uniformity is calculated by 50SL x 100 / 2.5 SL. Significance of UR (Uniformity Radio) is given below:

ð UR% Classification : 50-55

ð Very Good : 45-50

ð Good : 40-45

ð Satisfactory : 35-40

ð Poor Below : 30 Unusable M= 50% SL

UHM SL - Average value of length of Longest of 50% of Fibers

UI Uniformity Index

UI M/UHM

Interpretation of Uniformity Index- Now Uniformity is measured by HVI

U.Index Classification UHM Classification Below 77 Very low Below 0.99 Short 77-99 Low 0.99-1.10 Medium 80-82 Average 1.11-1.26 Long 83-85 High Above 1.26 Extra Long Above 85 Very High

Fibre Strength

Fibre Strength, next important quality is tested using Pressley instrument & value is given in ‘000 lbs/inch2 (1000 psi). For

better accuracy, Stelometer is used & results are given in gm/tex. Lately, strength is measured in HVI & result is given in terms

of gm/tex.

Interpretation of Strength value is given below:

Classification gm/tex Weak Below 23 Medium 24-25 Average 26-28 Strong 29-30 Very Strong Above 31

Strength is essential for stronger yarns and higher processing speeds.

ð Fibre Fineness & maturity are tested in a conjunction using Micronaire Instrument.

ð Finer Fibers give stronger yarns but amenable for more neppiness of Yarn due to lower maturity.




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ð Micronaire values vary from 2.6 to 7.5 in various varieties.

Fineness & Maturity

Micronaire value is referred to evaluate fibre fineness & its suitability for spinning particular yarn count. As it is a combined

result of fibre fineness & maturity, it cannot be interpreted, property for ascertaining its spinning value which is taken in

conjunction with standard value of Calibrated Cotton value. Table below explain that micronaire value goes up along with

maturity but declines with fibre thickness. An Egyptian cotton variety-3 sample of High, Low & Medium maturity were taken

& tested which are given below:

Maturity Micronaire Perimeter Maturity Maturity Ratio High 4.3 52.9 85.1 1.02 Medium 4.0 54.4 80.1 0.96 Low 3.9 54.7 79.3 0.95

Here, Micronaire Value of 4.3 is higher than 3.9 of lower maturity. Greek Cotton was tested & results are as below:

High 3.8 57.0 75.1 0.88 Medium 3.5 54.9 70.7 0.84 Low 3.2 55.2 65.8 0.80

Micronaire Value of 3.8 is higher than 3.2 of low maturity cotton. US Cotton was tested & results are as below:

High 4.1 64.4 75.9 0.87

Medium 3.4 62.1 68.0 0.80 Low 2.7 59.8 56.1 0.67

Hence, it is essential to know what Micronaire value is good for each variety of Cotton.

Maturity Ratio Classification 1.00 and above Very Mature 0.95 - 1.0 Above Average 0.85 - 0.95 Mature 0.80 - 0.85 Below Average Less than 0.80 immature

Cotton Grade

Cotton grade is determined by evaluating colour, leaf & ginning process. Higher grade ensures better yarn appearance &

reduced process waste. Colour is determined by using Nickerson-Hunter Calorimeter that gives values Rd (Light or Dark) &

+b(Yellowness).




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White Colour- American Upland Cottons are classified According to Grades as Given Below

S.No Grade Symbol Code 1 Good Middling GM 11 2 Strict Middling SM 21 3 Middling M 31 4 Strict Low Middling SLM 41 5 Low Middling LM 51 6 Strict Good Ordinary SGO 61 7 Good Ordinary GO 71 8 Below Grade

Similar grading is done for Light Spotted, Spotted, Tinged and Yellow Stained Cottons. PIMA cottons are graded I to 9

Yarn Testing

Yarn occupies mid-position in manufacture of fabric from raw material. So, yarn test resuls are essential, both for estimating

raw material quality & for controlling fabric produced. 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 results obtained are reproducible & give reliable information about material, sampling must be true &

representative of bulk lot. Sampling procedure is designed to take account of & to minimise known sources of variability such

as variation between spindles, variation along bobbin length, etc. Sampling procedure & number of tests carried out are given

under each characteristic:

Suitable Conditions for Yarn Testing

Some textile fibres are highly hygroscopic & their properties change notably as a function of moisture content which is

particularly critical in case of yarn tenacity, elongation, yarn evenness, imperfections, count etc. So, conditioning & testing is

carried out under constant standard atmospheric conditions. Standard atmosphere for textile testing involves a temperature of

20+-2ºC, & 65+-2% RH%. In tropical regions, maintaining a temperature of 27+-2ºC, 65+-2%RH is legitimate. Prior to

testing, samples must be conditioned under constant standard atmospheric to attain moisture equilibrium & that requires at

least 24 hours.




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Twist

"Twist is defined as spiral disposition of components of yarn, which is usually expressed as number of turns/unit length of

yarn, e.g turns/inch, turns/meter, etc. Twist is essential to keep component fibres together in a yarn. Strength, dyeing,

finishing properties, feel of finished product etc. are all dependent on twist in yarn. With increase in twist, yarn strength

increases first, reaches a maximum & then decreases. Depending on end use, 2 or more single yarns are twisted together to

form "plied yarns" or “folded yarns" & a number of plied yarns twisted together to form "cabled yarn". Among plied yarns, the

most commonly used are doubled yarns, where in 2 single yarns of identical twist are twisted together in a direction opposite

to that of single yarn. Thus for cabled & plied yarn, twist direction & number of turns/unit length of resultant yarn as well as

of each component have to be determined for a detailed analysis. Twist direction is expressed as "S" Twist or "Z" Twist

depending on direction of rotation of twisting element. Twist take up is defined as, "The decrease in yarn length on twisting,

expressed as a percentage of yarn length before twisting:

Linear Density or Yarn Count

The fineness of yarn is usually expressed in terms of its linear density or count. There are a number of systems & units for

expressing yarn fineness. But they are classified as follows:

a) Direct System:

1. English count(Ne) : No of 840 yards yarn weighing in 1 lbs 2. Metric count(Nm) : No of 1 km yarn weighing in 1 km 3. French count(Nf) : No of 1 km yarn weighing in 0.5 km

b) Indirect System:

1. Tex : Weight in gm of 1000m yarn 2. Denier : Weight in gm of 9000m yarn

Determination of Yarn Count

For determination of count of yarn, it is necessary to determine weight of a known length of yarn. For taking out known

lengths of yarns, a wrap-reel is used. The length of yarn reeled off depends on count system used. Another factor which

determines length of yarn taken for testing is type of balance used. Some balances like quadrant balance, Beesley's balance is

specially designed to indicate yarn count directly from tests on specified short lengths of yarn & is very useful for determining

yarn counts removed from fabrics where minimum balance accuracy required is 0.001mg. One of the most important

requirements for a spinner is to maintain average count & count variation within control. Count variation is usually used to

express variation in weight of a lea & this is expressed as C.V. % which is affected by number of samples & length being

considered for count checking. While assessing count variation, it is very important to test adequate number of leas. After

reeling appropriate length, yarn is conditioned in standard atmosphere for testing before its weight is determined. Minimum

number of sample required per count is 20 & per machine are 2.




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Yarn Strength & Elongation:

Breaking strength, elongation, elastic modulus, resistance abrasion etc are some important factors which represent

performance of yarn during actual use or further processing. Strength testing is broadly classified into 2 methods:

1. Single end strength testing 2. Skein strength or lea strength

Tensile Strength of Single Yarn During routine testing, both breaking load & extension of yarn at break are usually recorded for assessing yarn quality. Most

of instruments record load-elongation diagram also. Various parameters such as initial elastic modulus, yield point, tenacity or

elongation at any stress or strain, breaking load, breaking extension etc is obtained from load-extension diagram. 2 types of

strengths are 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 & these are carried out using either a single strand or a skein containing a

definite number of strands as test specimen. An important factor which affects test results is length of specimen actually used

for carrying out test. Strength of a test specimen is limited by that of weakest link in it. If test specimen is longer; it is likely to

contain more weak spots, than a shorter specimen. Hence test results are different for different test lengths due to weak spots.

Yarn moisture content also influences test results. Cotton yarn when fully wet, show higher strength than when dry, while

opposite is case with viscose rayon yarns. Hence, to eliminate effect of variation due to moisture content of yarn, all yarn

strength tests are carried out after conditioning in a standard atmospheric condition is maintained. The rate of loading as

determined by "time-to-break", which is time interval between commencement of application of load & yarn rupture, is an

important factor, which determines strength value recorded by using an instrument. Same specimen shows a lower strength

when time-to-break is high or higher when time-to-break is low. Instruments used for determining tensile strength are

classified into 3 groups, based on working principle:

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

In instruments of CRE type, application of load is made in such a way that elongation rate of specimen is kept constant. These

types instruments load application is made in such a way that rate of loading are constant throughout test duration. These

types of instruments are usually preferred for accurate scientific work. In CRE & CRL types of instruments, it is easy to adjust

"time-to-break" while this adjustment is not easy in CRT types of instruments. Uster Tensorapid applies CRE principle of tensile

testing. Constant Rate of Extension describes simple fact that moving clamp is displaced at a constant velocity. As a result,

specimen between stationery & moving clamp is extended by a constant distance/unit time & force required to do so is

measured. Apart from single values, this instrument also calculates mean value coefficient of variation & 95% confidence

range of maximum force, tenacity, elongation & work done. Total coefficient of variation describes overall variability of a




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tested lot, i.e within-sample variation plus between-sample variation. If 20 individual single-end tensile tests are performed on

each of 10 packages in a sample lot, total coefficient of variation is calculated from pooled data of total number of tests that

were carried out. In tensorapid, breaking tenacity is calculated from peak force which occurs anywhere between beginning of

test & final rupture of specimen. Peak force or maximum force is not identical with force measured at very moment of

rupture. Breaking elongation is calculated from clamp displacement at point of peak force. Elongation at peak force is no

identical with elongation at very moment of rupture (elongation at rupture). Work to break is defined as area below

stress/strain curve drawn to point of peak force & corresponding elongation at peak force. Work at point of peak force is not

identical with work at very moment of rupture. To compare tensorapid test results with other results:

ð A measurement must be performed according CRE principle ð Testing speed must be exactly 5m/min ð Gauge length or length of specimen should be 500mm ð Pre-tension should be 0.5cn/tex

2 fundamental criteria affect compatibility between different measurements of yarn tensile properties.

1. Testing conditions, i.e testing principle (CRE, CRL), testing speed, gauge length, & pre-tensioning.

2. The 2nd criteria, which also affects magnitude of differences, relates to specific stress/strain characteristic of yarn

itself, which is determined by fibrous materials, blend ratio, & yarn construction.

Skein Strength or Lea Strength:

Skein breaking strength was the most widely used measure of yarn quality in cotton textile industry. Yarn quality

measurement by this method has certain drawbacks. Firstly, in most of subsequent processing, such as winding, warping or

weaving, yarn is used as single strand & not in form of a skein except occasionally when sizing, bleaching, mercerising or

dyeing treatments are carried out on hanks. Secondly, in method used for testing skein strength, rupture of a single strand at a

weak place affects result for whole skein. Further, this method of test does not give an indication of extensibility & elastic

properties of a yarn, characters which play & important role during weaving operations. However, since a large size sample is

used in a skein test as against that in a single strand test, sampling error is less. The skein used for strength test can be used for

determination of linear density of yarn as well. In addition to factors influencing yarn strength, size of skein (lea) affects to a

large extent strength recorded. Usual practice is to use a lea (120yds) of yarn prepared by winding 80 turns on a wrap-reel

having a perimeter of 54”, so that during a test, there are 160 strands of 27" length. There are different systems in use. But

actual breaking strength recorded on machine depending on type of skein used as both number of strands & test length may

differ. The most commonly used instruments for this test is CRT type, where bottom hook moves at 12”/min. After finding out

skein strength, broken skeins are also weighed to determine linear density. The most common skein used is lea & results of lea

strength tests are expressed as C.S.P., which is product of linear density (count) of yarn in English system (Ne) & lea breaking

strength expressed in lbs. In view of fact that C.S.P. is much less dependent on yarn count than on strength, especially when

count differences are small, C.S.P. is the most widely used measure of yarn quality.




Cotton Fibre Quality Issues-In Brief

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