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For any technical consultancy in Textiles and Nonwovens Email N.Balasubramanian or phone 9869716298




Articles by Dr. N.Balasubramanian


ba1ja2@yahoo.co.uk


SPINNING
NONWOVENS
Count variation in yarnYarn IrregularityYarn HairinessIntimacy of Mixing and Blend VariationDevelopments in DraftingTips to get full benefits from Modernisation
Rotor Spinning - Influence of opening roller and transport tube parameters, fibre integration and wrapper fibres Influence of rotor, navel and winding parameters on yarn quality and performance in rotor spinningModern developments in rotor spinning to improve economics, productivity, elecrtical energy saving over ring spinningUpgradation and Diversification of Rotor SpinningMerits and Limitations of cotton fibre length measuring instruments>Polyester vsPolypropyleneDifferential Hooking By FibrographEffect of Feeding ConditiQns at the Ringframe Upon Yarn Quality and Spinning PerformanceTranslation of strengthStrength and elongation of polyester cotton and polyester cotton blends at different stages of processingFriction Spinning a critical ReviewMaturity by Micronaire
Drawing Sliver irregularityB L curveMedium and Long term variations of Rotor yarnsInfluence of rotor speed and diameter on properties of Rotor yarnsTop roller Weighting apron spacing and top roller settingUpgrading by Apron drafting
Core spinning AbstractEnergy SavingCharacteristics of Imperfections
Technical consultancy in Spinning NonwovensEnd breakage rateTesting of man made fibres and filamentsNonwoven moulded carpetsArticleYarn diameter specific volume and packing densityTesting of cotton for fineness and maturityarticles on Nonwovens
Importance of count variability in yarn hardly needs any emphasis. Higher count variability invariably leads to higher strength variability. The weak patches in the yarn lead to frequent end break in further processing, which often reaches annoying levels leading to rejection of bobbins and cones. In latest autoconers, which have settings for rejecting bobbins with count of yarn exceeding beyond certain limits of the nominal, processing of yarns with even slightly high count variations becomes extremely difficult. Winding efficiency reaches unacceptably low levels with such yarns. Higher count variability especially of medium to long length range results in moire like appearance in fabric and increases warp way streaks and weft bars. Ring cuts and soiled ring packages is another problem with higher count CV. To overcome this, wider clearance is kept between ring diameter and full package leading to lower doff weights. With higher count variability, percentage of bobbins exceeding tolerance limits of nominal count increases, leading to sales rejections and market complaints. In shuttleless looms, problem of weft tear is encountered during weaving when count of weft changes abruptly beyond certain limits at the time of pirn change. High count variations in weft are also a cause of warp way fabric creases in processed fabrics like dyed poplins1. Dependence on Length Variability of count depends upon the length of the yarn used for estimating count. Though 120 yd. or lea is normally used for estimating yarn count, sometimes half leas are used especially in coarse polyester blend counts to keep strength measurement within the capacity of strength tester. In very fine counts like 120s, two leas are weighed together to estimate count to achieve better accuracy in weighment. It is well known that CV of count decreases with increase in length but the rate of reduction decreases with increase in length. CV of half lea will be1.414 (i.e.; �� 2) of full lea, if there is no serial correlation between adjoining leas. But there is usually a positive serial correlation because of long term variations. So CV of half lea will be 1.2 to 1.3 times the CV of full lea. The same rule holds good in the case of slivers and rovings. CV of 5yd wrapping of roving is much lower than 1.732 (i.e.; ��3) times CV of ��15 yd. wrapping in inter. The former is usually around 1.4 to 1.5 times that of latter. Tracing the Source of Count Variation Location of source of count variability will be greatly facilitated if wrappings and estimate of CV from the same are based on corresponding wrapping lengths of material at different stages. Thus wrappings and CV of wrappings may be based on 5yd instead of the traditional 15 yd length at Inter. 5 yd length at Inter after drafting will be closer to 120 yd length in yarn than 15 yd length and CV estimates based on the former will be more helpful to show if ringframe is contributing to additional variation. At drawframes, CV of wrappings based on 0.5 yd length will be more useful from the same consideration. Estimation of CV of such lengths can be obtained from modern Evenness testers like Uster tester 3. Norms for CV It is beneficial to set norms for CV not only at Ringframes but also at different stages of processing. Two sets of Norms, one with and another without autoleveller at Drawframe are given in Table 1
Table 1: Norms for CV at different stages
Material Wrapping Length Yd Without Autoleveller With Autoleveller
Yarn 120 2.5 - 3 1.5 - 2
Roving 15 1.5 - 1.8 0.9 - 1.2
Roving 5 2.2 - 2.7 1.3 - 1.8
Drawframe 5 0.7 - 0.8 0.4 - 0.5
Drawframe 1.0 - 1.2 0.5 - 0.7
Drawframe 1 1.5 - 1.6 0.9 - 1.1
Drawframe 0.5 1.9 - 2.1 1.3 - 1.5
Contribution from various processes Ring frame Contribution to count variation from ringframe comes from Variation in mechanical draft between frames Slippage of top roller Stretch of material in creel Variation in Mechanical Draft Variations in mechanical draft come from the use of different change pinions on frames of the same make and drafting system. Some common causes for this defective practice are Lack of sufficient stock of change pinions Using change pinions differing by one tooth on the two sides of a frame, ostensibly to achieve an average count close to nominal. This should be discouraged as it increases variability in count between frame sides. Frames of different makes and drafting systems are used for spinning the same count but the same mechanical draft is not kept on them, Slippage of strand under top rollers arises because of inadequate weighting or improper grip. Count variation is therefore reduced upon conversion of older version of top arms to later versions with higher pressures2, 3. Higher frequency for cot buffing, higher starting diameter for cot up to 30 cm are therefore helpful to reduce count variation4, 5. Disturbances to weighting also comes from worn out springs, leakage of air and improper seating of plunger on rib in pneumatic drafting, leading to count variation. One of the reasons for stretch of strand in the creel is low roving twist. The level of creel breaks can assess this. Misalignment of creel roving bobbin in relation to the creel roving guide is another contributory factor to stretch. Improper location of creel guide rod in relation to the bobbin can also cause stretch. If located too high or too low, stretch takes place when roving unwinds from top most or bottom most portion of roving bobbin. Group Control of Count Group control of count should invariably be preferred to individual frame control as a basis for changing the change pinion except under special circumstances. For this purpose, average count from wrappings taken from all ringframes working on a count should be determined and if the count exceeds the preset limits even after a repeat wrapping, pinion change should be done on all frames. The only exception for this is when frames or drafting systems differ widely in age. Under such conditions, slippage of strand occurs under drafting rollers with older frames and drafting systems leading to a lower actual draft at the same mechanical draft5. In one study2 the mechanical draft had to be reduced from 36.5 to 32.6 to get the same yarn count with the same back material when drafting system was changed from SKF PK211E to SKF PK225. Speedframe Most of the problems enumerated above like variations in mechanical draft between frames and slippage of strand under top roller are sources of count variation at speedframe. In addition, the variation in stretch from start to full position is an added source of variability. Stretch variation during build can be estimated accurately, only if a correct method of estimating wrapping is followed. Traditionally roving bobbin is placed on the top of self-weighted top roller driven by wrapping block, with a spindle inside while preparing wrapping. This method has the drawback of slippage of roving between the self- weighted top roller and wrapping drum. The amount of slippage increases from full bobbin to empty bobbin, often to the extent of 5 - 6 %, leading to errors in estimation of wrapping variation from start to full. A better and more accurate method, free from such error, is to suspend the bobbin from a bobbin holder located at the back of wrap block. The roving drawn from the bobbin is passed through the nip of self-weighted top roller and wrapping drum. Actual studies in the mill showed that wrapping estimated by this method is 5 - 10 % coarser than the one obtained by the traditional method. Draw Frame Major contribution to yarn count variation comes from drawframe. Primarily there are two sources of variability 1. Medium term variations in the sliver and 2. Long term variations in the average hank between frames and between shifts. Medium Term Variations Variations in the length region between 0.2 to 0.7 m, depending on the yarn count and hank organisation influence directly count CV as it corresponds to length of lea divided by draft between drawframe and ring frame. Contribution to this variability comes from irregularities in breaker sliver, which do not get reduced by doubling in finisher. Long Term Variations Long term variability in the hank of drawing sliver arises from variation in average weight of lap weight in lap feed systems and weight of feed sheet to card in chute feed systems, that occur from shift to shift. The piano feed regulating motion in scutcher and pressure switch control in chute feed to card operate on the basis of volumetric control and the degree of openness of the material therefore affects the extent of control. In piano feed belt shift would occur when the sheet becomes bulkier (because of better openness) even though weight per unit length remains the same. Likewise, the pressure developed in the chute would be higher for the same weight with well-opened material. Thus it is necessary to standardise the evenness roller to inclined lattice setting in scutcher and speed of beaters and pressure switch setting in the chute, based on factors like type of cotton, whether the feed is directly from bales or from a sandwich mixing from pre-opened material. This will minimise variations caused by openness of material. The variation in average hank of card sliver from shift to shift have less chance of getting reduced from doubling at draw frame, as slivers made in one shift do not get doubled with those in the next shift. But the variability due to this can be kept down by increasing storage capacity of laps and card sliver cans so that material made at longer intervals of time get doubled. But most of the mills do not have such storage space. Moreover material stored for long lengths run the risk of contamination with dust and fly. Autoleveller Modern draw frames are equipped with autoleveller and sliver monitor to reduce both type of variability mentioned above. The autoleveller brings down the variability in sliver lengths beyond 0.3 - 0.5 m. Further, the rate of reduction in CV of wrapping with increase in wrapping length is steeper with autolevelled product. This will be clear from Fig.1, where the variance length curves of Finisher drawing slivers made on the same drawframe with and without autoleveller are compared. The amount reduction in CV with autoleveller increases as wrapping length increases from 0.5 to 3 m. Variance length curve of Breaker drawing sliver (without autoleveller) fed to this draw frame is also shown plotted in the same Fig. The figure shows that doubling in the draw frame brings down the CV with length 0.5 m and about and the order of reduction increases with length. So the variance length curves become steeper from Breaker to Finisher. The short term irregularity in sliver is however not reduced by doubling or autolevelling. Norms for CV of wrapping of drawing sliver at various lengths with and without Autoleveller are given in Table 1. Functioning of autoleveller gets affected by zero setting, obstruction to free movement of scanning roller, improper functioning of servo motors, drives, solenoids and related parts. Leveling action of autoleveller has therefore to be checked at regular intervals. A method usually followed in the mills is to feed 9 ends and 7 ends of sliver and check the wrapping of outgoing sliver. The extent of deviation of hank from nominal under such circumstances is taken as leveling sensitivity of autoleveller. In normal practice, however, such wide variations in input hank seldom occur and the effectiveness of autoleveller in leveling lower order of variations would be of greater interest. The best method of judging this is by comparing CV of 0.5, 1 and 3 m lengths of material with and without autoleveller. Sliver Monitor Sliver monitor consists of a sensor fitted to the calendar rollers of draw frame to check the hank of the outgoing sliver If the actual hank deviates from the nominal by more than the preset limits the drawframe will be stopped. The limits of deviations from nominal hank at which drawframe will be stopped can be varied. It is normally advisable to set it at +/- 2 %. In some makes, sliver monitor can also be set to stop drawframe if CV of wrapping goes beyond certain limits. If the drawframe stoppage due count exceeding preset limits is high then the hank of breaker sliver should be checked to find if it is deviates considerably from normal. If this is not the case, then the functioning of autoleveller has to be checked. Thus a step by step approach should be followed. Mechanical Condition of Draw Frame Poor mechanical condition of draw frame, coupled with inadequate weighting for top rollers have sometimes been traced as a source of count variation. When older versions of Whitin drawframes were replaced by later versions of Laxmi Rieter drawframes, among other noticeable benefits found by the mills, was a significant reduction in count CV of yarn6 as shown in Table 2.
Table 2: Effect of mechanical condition of Drawframe on Count and Strength CV of yarn (30s)
Drawframe Type Age Mechanical Condition CV of
Count%
CV of
Strength%
Mixing1 Mixing2 MIxing1 Mixing2
Whitin J5 Old Unsatisfactory 4.9 3.8 11.4 8.2
Laxmi Rieter DO2S New Good 4.4 3.0 8.1 4.7
Card Sliver Though card sliver is characterised by high variability of wrapping, its contribution to yarn count CV is not that significant because doublings in drawframe reduce the variations substantially. Autolevelling at card is therefore not found very beneficial in bringing down count CV of yarn. This will be clear from Table 3 where card slivers made with and without autoleveller were processed up to ringframe and variability at different stages assessed.
Table 3: Effect of Autolevelling at Card on Yarn Count CV % (20s)
U % of Card sliver CV % of 1 yd of Card sliver CV % of 1yd of Finisher Drawing sliver Count CV % of Yarn
With Autolevelling 3.8 1.2 1.3 2.6
Without Autolevelling 4.1 7.0 1.6 3.2
Though autolevelling at card brings down CV of 1 yd wrapping of card sliver from 7 to 1.2, CV of 1 yd of finisher drawing sliver comes down only marginally from 1.6 to 1.3 and count CV of yarn drops marginally from 3.2 to 2.6. U % of card sliver also does not have much effect on count CV of yarn. This will be clear from a study where cards giving low U % and wrapping CV % and those giving high U % and wrapping CV % were separately channelised up to yarn. The results (Table 4) show that even with large differences in U % and wrapping CV % of card sliver, there is no significant difference in yarn count CV %.
Table 4: Effect of card sliver irregularity on Yarn count CV % (20s)
Property Cards with low sliver variability Cards with high sliver variability
Card sliver U % 4.3 7.2
CV of 6 yd wrapping of card sliver 6.1 7.3
CV of Yarn count % 2.8 2.9
Comber Sliver High comber sliver U % arising from piecing wave can affect yarn count CV, particularly if single post comber drawframe passage is used. This will be clear from the following study where combers giving low and high U % were channelised up to ring frame separately and checked for yarn count CV. The results given in Table 5 show that yarn count and strength variations are higher with yarns made from combers with higher variability.
Table 5: Effect of comber sliver U % on variability of count and strength of Yarn (40s)
U % of comber sliver CV of yarn count % CV of yarn lea strength %
3.7 1.75 4.12
6.9 2.78 7.96
Medium Term Variations in Yarn Count variation is traditionally used to indicate variability in 120 yd lengths. Variability in shorter lengths are also equally important though they do not form part of quality monitoring activity in most of the mills. Variability of weights in lengths in the region of 0.5 to 10 m affect the appearance of fabric, lead to moire like defects and contribute to weft bars and warp way streaks. U % of drawing sliver has a direct influence on the variations in these lengths. This will be clear from the results of a study in a mill given in Table 6.
Table 6. Contribution of drawing sliver U % on medium term variations in yarn (20s)
Draw Frame
settings,
mm Fr/Back
U % of Drawing sliver CSP U % CV of 1 m % CV of 3 m % CV of 10 m % CV of 120 yd %
49 - 56 6 2044 15.6 8.0 6.5 4.6 2.5
44 - 53 4.6 2116 13.8 6.2 5.2 4.0 2.6
Drawing sliver U % was brought down substantially in the mills by optimising roller settings. When this material was channelised up to ring frame, medium term variations in the yarn as indicated by CV of 1 to 10 m lengths came down markedly though CV of 120 yd length showed little improvement. The fabric appearance also improved markedly. This shows the importance of monitoring CV of yarn count in length regions 1 to 10 m. This is conveniently obtained in later model Uster Evenness testers like UT3. References: 1. Warpway Creases in fabrics N.Balasubramanian and C.Chatterjee, Indian Textile J., 1996 Oct, p66 2. Benefits from modernisation of ring frames from second generation Top arms M.Balakrishnan and N.Balasubramanian, BTRA Scan 1980 Sept., XI, p4 3 Yarn quality improvements from second generation Top arms at Ring Frame G.Janakiraman and N.Balasubramanian, BTRA Scan, 1987 Dec, XVIII, p 9 4.Cot buffing at Speed Frame - A useful measure to reduce count variation S.K.Sett, G.V.Aras and N.Balasubramanian, BTRA Scan, 1984 March, XV, p 4 5.Contribution of Ring Frame drafting condition to yarn count variability in fine counts N.Balasubramanian and G.Janakiraman, Indian J. of Fibre & Textile Research, 1990, 15, p 198 6. Influence of mixing characteristics and Draw Frame condition on yarn quality G.V. Aras and N.Balasubramanian, BTRA Scan, 1982 June, XIII, p 10
  • Yarn Irregularity - Concept and measurement
    N.Balasubramanian
    Retired Joint Director & Consultant,
    Importance of Irregularity
    Irregularity is the most important quality characteristic of yarn, its importance arising from the following factors ● Irregularity has a profound influence on appearance of yarn and fabric. More regular the yarn, better will be the appearance and aesthetic value of the product. As a result, better sale value can be achieved. With advent of Uster Evenness tester and Uster standards, importance of irregularity has become even higher. Buyers insist on yarns meeting certain Uster standards (such as 25% or 5%). In export market, especially in developed countries, Yarns have to meet Uster 5 or 10% standards. Mills who produce consistently more regular yarn get a premium in the selling price
    ● Regularity contributes to a smoother feel. In apparel and most of other textiles, smoothness is most desired characteristic. Sale value of fabric is dependent, among other things, on smoothness.
    ● Regular yarns will have fewer weak places and, as yarn breaks at weakest place, will have a better strength. Better strength realisation from fibre can be achieved if regularity of yarn is improved. It is for this reason good mills, which produce consistently more regular yarn, are able to produce a yarn of much higher strength from the same cotton
    ● Because of the lower incidence of weak places, fewer end breaks are encountered with regular yarns in weaving preparatory, weaving and knitting. Efficiency in these processes is improved leading to higher productivity. Importance of regularity will be appreciated from the better performance achieved by rotor spun yarns in weaving processes compared to ring spun yarns. Rotor yarns have a lower strength than ring yarns but have a better regularity (short, medium and long term). Though mean strength is lower, strength of weak place is higher in rotor yarn than ring yarns and so it performs better.
    ● Fabric defects and rejections are critically influenced by irregularity of yarns. Periodic and quasi periodic irregularities in yarn result in warp way streaks and weft bars in woven fabrics leading to fabric rejections1,2. Yarn defects like slubs, crackers, long thick places and long thin places downgrade the fabric and cause considerable value loss. Mills which produce more regular yarns therefore get better realisation and contribution and as a result higher profitability
    Types of Irregularity
    ○ Weight per unit length
    Variation in weight per unit length is the basic irregularity in yarn. All other irregularities are dependent on it. This is because weight per unit length is proportional to fibre number i.e; number of fibres crossing a section of yarn. Variations in fibre number is the factor influenced by drafting. So any improvement in drafting or spinning will first reflect in improvement in variability of weight per unit length. Count variation in yarn has been found to have profound e ffects on weaving and finishing. High count variation has been a cause of weft way tear in Sulzer looms. Weft count variation has been found to cause warp way creases in finished fabric3.
    ○ Diameter
    Variability in diameter is important because of its profound influence on appearance of yarn. Variations in diameter are more easily perceived by eye. Latest models of evenness testers have therefore a module for determining diameter variability. Diameter variability is however caused by weight variability. As twist has tendency to run into thin place, variability in weight gets exaggerated in diameter variability.
    ○ Twist
    Twist variation is important because of its influence on performance of yarn and fabric dyeability and defects. Soft ends are a major cause of breaks in weaving preparatory and loom shed, arising from twist variations. Soft twisted yarns take more dye and so uneven dyeing is caused by high twist variation. Weft bars and bands are also caused by low twisted yarns. Twist variations come from slack spindle tapes, jammed spindles. A certain amount of twist variation is also present along the chase of cop.
    ○ Strength
    Importance of strength variation is easy to appreciate. Yarn breaks at the weakest element and so yarns with high strength variability will result in high breakages in further processes. Strength variability is partly dependent upon count variability and partly upon spinning conditions and mechanical defects.
    ○ Hairiness
    High variation in hairiness leads to streaky warp way appearance and weft bars in fabric. More light will be reflected from portions of weft where hairiness is more and this leads weft bands. High hairiness disturb warp shed movement in weaving and result in breaks, stitches and floats. Among other factors, worn out rings and travellers, vibrating spindles, excessive ballooning and variation in humidity in spinning room cause variations in hairiness from bobbin to bobbin
    ○ Colour
    Variations in colour of yarn cause batch to batch variation in fabric colour, which leads to rejects. This is particularly critical in cloth marketed to garment units. Variations in colour of yarn and fabric are caused by variations in colour of cottons used in mixing. Larger lot sizes made from a large number of bales help to mitigate this problem. Checking of cotton and mixing for colour will also minimise large variations in colour. HVI testing has therefore a module for checking colour in cotton and Uster�s latest models have a module to detect colour variations in yarn.
    ○ Fibre ends per unit length
    Irregularity in thickness or weight/unit length arises primarily from short term variations in fibre end density in the yarn or sliver. We will first consider the case where all fibres are of the same length, which is the case with man made fibres and fibres are straight and parallel to the axis of sliver/yarn. Consider two sections of sliver A and B separated by a distance l.
    Fig 1:Arrangement of fibres in a yarn

    All fibres that have their left hand ends in AB will cross B and no other fibres will cross B. Number of fibres crossing section B (which is equal to thickness or weight/unit length) is given by
    n l
    Where n = number of fibre ends per unit length or fibre end density and l is fibre length.
    .Variability in thickness of yarn is therefore dependent of short term variations in fibre end density. This formula is a particular case of more complicated relation connecting fibre end density and thickness.
    Variable fibre length
    Number of fibres of length l crossing section B, N , is the number of fibres whose left hand ends lie in AB. This is given by
    N = n l f(l)dl
    where
    n = fibre end density and f(l) = frequency of fibres of length l.
    The total number of fibres is given by
    =∫0lmaxn f(l) dl
    where
    Irregularities in fibre end density with wavelength less than fibre length get masked by the fibre length and do not show up or show with reduced amplitude in thickness irregularity4,5.. Further, with variable fibre length, short length periodicity in left hand ends gets considerably reduced in amplitude with right hand ends. Let wavelength of fibre end density in left hands be = λ Mean fibre length = L
    Amplitude of periodicity in left hand fibre ends = r1
    Amplitude of periodicity in thickness = R
    Amplitude of periodicity in right hand ends =r2
    Fig shows how the amplitude of periodicity in fibre end density in left hand ends gets reduced in the amplitude of thickness and of fibre right hand ends,when wavelength is lower than fibre length.
    amplitude reduction Fig 2. Reduction in amplitude of periodicity of left hand ends, in thickness and rigth hand ends

    When λ is less than fibre length L, amplitude of periodicity in thickness reduces to 0.15 and that of right ends to .01 of the amplitude of periodicity in left hand ends. This shows the advantage of variable fibre length in curbing short term periodicities as there is a reversal in direction of drafting from one process to next and fibre front ends gets fed as rear ends. This advantage is not there with man made fibres with constant staple length. And it is for this reason we encounter weft bars in man made fibre material due to short term periodicities in fibre ends introduced at speed frame2.
    Range
    Measurements of weights or diameter are classified in number of rows, each column containing about 4, 5, 6,or 7 items as shown below
    x11x12x13x14
    x21x22 ......
    ............
    ............
    xn1 ......xn4
    .
    Range, which is the difference between maximum and minimum value is determined in each row. Mean range,rm for all the rows is then calculated. Range% = (rm/xm)100. Higher the range%, higher the variability. If the irregularity is random, there is a relationship between range and standard deviation, σ. Let mean range be rm. Then σ = rm/a. The value of a is dependent upon up on the number of units in a row and its values are shown in Table 1 below.
    TABLE 1
    Number in a row235 61012
    A1.128112.0592.3262.5323.0783.25
    To check the accuracy of this method, S.D was calculated from range for the 3 sets of date given in the paper by Burkhalter7. The data was rearranged in 7 rows with 6 weights in each row. The estimated and actual S.D. (σ) are given in Table 2 below
    Table 2 Comparison of SD estimated from Range and actual
    Set NoRangeestimated from rangeActual
    10.240.0950.1
    262.372.87
    30.4560.1800.165
    The agreement is reasonably good for mill work.
    � Mean Deviation%
    Let the individual weights of specimen be as given in Table 3.
    Table 3
    Weight of specimen DeviationSquare of Deviation
    x1x1 -xm(x1 -xm)2
    x2x2 -xm (x2 -xm)2
    .....
    .....
    .....
    .....
    xnxn - xm(xn -xm)2
    . Deviation of each specimen weight from mean is determined, ignoring the sign. Let mean be xm
    Mean Deviation %,
    MD% =1xm×(∑n1|x - xm|/n) ×100 Mean deviation % is measured by linear integrator of Uster venness tester. If a recorder chart of weight variations is available, Mean deviation is given by the area enclosed by the chart/ average value. The area can be determined by planimeter6. ● Standard Deviation Deviation is squared as shown in 3rd column of table and the average of square of deviation is determined. Its square root is determined as shown below.
    σ = (1/(n-1)×√∑n1(xi - xm)2
    Burkhalter7 has suggested a quick method for estimating standard deviation. Since 67% of data normally fall within � σ , he suggests that standard deviation can be estimated with good approximation by determining half the difference between values within which 67% of data lie. However this approximation will hold good only for normal or near normal distribution.
    ● Coefficient of Variation, CV% Coefficient of Variation, CV% = . CV is a better measure of irregularity than MD as it gives greater weightage to portions moved far away from mean. However MD is much easier to calculate and there is a fairly good correlation between CV and MD. Later models of Evenness testers are equipped with linear and quadratic integrator and give both U% and CV%.
    In Evenness testers like, Uster, continuous measurement of weight/unit length/mass is carried out where
    x = instantaneous value of mass
    = Mean value
    In such a case
    MD% = 1xmT0T|xi - xm|
    CV% = 1x(√∫0T(xi - xm)2 dt ×100
    U% or CV% measured by Uster Evenness tester is a weighted running average of variability of specimen, the weighting being more for lengths of specimen which have just passed through the tester( i.e; weighting is inversely proportional to the time that has elapsed after the passage of material through the measuring head) ● If irregularities are random and follow normal distribution as shown in Fig 3 below) then CV = 1.25 MD.
    normal
    Fig 3 : Normal distribution

    � If frequency distribution is triangular as shown in Fig 4 below, CV/MD is lower than 1.25
    Triangular
    Fig 4: Triangular distribution

    ● If frequency distribution is skewed as shown in Fig 5 , then CV/MD is greater than 1.25
    skewed Fig 5: Skewed distribution
    ● If irregularity distribution is wholly periodic(Fig 6 ) then CV/MD = 1.117
    periodic
    Fig 6 : Periodic distribution

    ● As normal yarns contain extra irregularities due to drafting waves and mechanical faults, CV = 1.27 to 1.4 times of MD. Since the type of frequency distribution does not vary much from yarn to yarn, it is safe to estimate CV by multiplying MD by 1.3 Typical frequency distribution of a normal yarn in relation to ideal yarn with random distribution is shown in Fig 7.
    Idealand normal Fig 7 Comparison of normal yarn with ideal yarn

    ● Original models of Uster integrators were determining MD (U%) and CV with a certain amount of approximation. Later with improvements in electronics, later models were designed to determine CV precisely. But U% is still estimated with a certain amount of approximation. So the relationship between CV and U depends upon the model of Uster Evenness tester and type of irregularity in yarn as shown in Table 3 below.
    Table 4 Type of irregularity Model of Evenness tester CV/U Symmetric with a single peak GGP (very old model), Uster1, 2 and 3 ≈1.25 Symmetric with 2 or more peaks GGP ≈1.25 Uster 1, 2, 3 and 4 > 1.25 Asymmetric GGp, Uster 1, 2, 3, and 4 >1.25 with a higher conversion factor for Uster 1, 2, 3 and 4
    ● Exceeding Frequency
    This is defined as the frequency f with which a particular linier density is exceeded in a unit length of yarn and is given by
    f = A/L
    where A= number of times a specified value of CV% is exceeded in a given length L of yarn. Number of end breaks in subsequent processes is related to exceeding frequency.
    ● Fraction Defective/Deviation rate
    This estimates the percentage of material that lies beyond certain limits away from the mean on either side. Weights removed far away from the mean are mainly responsible for end breaks and rejections. Deviation Rate % = Where li is the length of ith piece which falls beyond prescribed limits and L is total length of yarn under study
    One of the conditions stipulated in purchase of yarn is that not more than 5% of bobbins will have a count beyond mean � specified percentage. Supplies that do not meet this specification are liable to rejection. The following examples show that yarns with a lower variability will be able to meet more stringent requirements.
    ● Example
    Count = 30s. The supplier�s requirement is 30s � 1.5. ● Mill A - CV of count = 2.5%
    Standard Deviation, σ = 0.75
    % bobbins falling outside 30 � 1.5 = 5 (From Table 5 below) Mill B - CV of Count = 5%
    SD σ = 1.5
    % of bobbins with count outside 30 � 1.5 = 31.7.(Table 5) Mill A which has a lower CV% also has fewer % bobbins falling beyond � 1.5 of mean. This shows the utility of checking the % of bobbins falling outside certain limits on either side of mean.
    5:Proportion of values beyond a limit with Normal distribution
    Number of SD on each side of mean Proportion of values lying outside limits%
    0100
    0.524460
    131.7
    1.64510
    1.965
    24.56
    32.7
    ● Irregularity Testers
    ○ Visual Assessment
    An estimate of yarn irregularity is obtained by winding the yarn on a black board and comparing it with ASTM standards. A good correlation is found between yarn appearance grade and irregularity as measured by Uster Evenness tester8 ○ Cut and weigh Method
    Yarn is wound on a drum and pieces of known length are cut with the help of a template. The cut pieces are weighed on a sensitive balance and CV% is estimated. This is a time consuming method and is adopted only in special research program. Moreover accuracy has to be maintained in cutting and weighing. Only advantage is it is a direct method and does not involve any assumptions.
    ○ Mechanical Type tester
    Tongue and Groove tester
    This is used mainly for slivers and rovings. This consists of two rollers one with a tongue and another with a groove, in between which the sliver is passed. Tongue roller moves up and down depending upon the weight of sliver. The roller movement is amplified recorded by pen recorder and integrated to find the CV%. Groove width ranges from � in for coarse, � in for medium and 1/16 in for fine slivers. This is also time consuming. Further errors are introduced because of variation in compactness of sliver. A similar type of instrument was converted to a microprocessor based one by De9. The displacement of the lever carrying the tongue roller is picked up by transducer to convert displacement into DC voltage. This is converted to a 8 bit digital word by a converter and digitally processed to estimate CV. The equipment is designed to determine variance length curve of a sliver. Good agreement with Uster Evenness tester is reported.
    � Shoe type tester
    This is mainly for yarns. Yarn is passed between a hard steel shoe and a a half round ball fitted on a light lever suitably fulcrumed in the middle. The diameter variations in yarn cause the lever to move up and down which is amplified by means of a light falling on a mirror fitted on the lever. This is amplified and CV% is estimated by an integrator
    � Photo Electric tester
    A light source is made to fall on yarn, running in front of the slit. The image of yarn is made to fall on a photo cell. The amount of light falling on photo cell varies depending upon diameter of yarn. The current generated by photocell is amplified and integrated.
    � Capacitance type tester
    The material is passed between two plates of a condenser at a known speed. Capacity of condenser varies depending upon weight variations in the material. The voltage generated is amplified and a continuous record of variations is recorded on a recorder. An integrator is used to determine MD%(U%) or CV%. Uster and Fielden walker testers are the most commonly used equipments based on this principle. Merits of this system are that it attempts to measure a quantity proportional to weight per unit length of material. Condenser slots of different sizes are used for slivers, rovings and yarns.
    The results of yarn irregularity measurement are affected by ● Fibre inclination � Fibre inclination in yarn may vary from place to place. This will affect the cross-sectional area for the same number of fibres in cross-section
    ● Fringe Effect - The fields of force between the plates are curved at the edges and as a result the material is detected before it enters the space between the condenser plates. This affects linearity of mass voltage relationship leading to errors in measurement. This can be overcome by incorporating a guard plate prior to normal condenser.
    ● Filling proportion (λ) - This is the ratio between material and air space and is given by λ = (d/D)100. This should be kept much lower than 1. Otherwise relation between change in capacitance and mass of material becomes non linear. Locher10 showed that the change in capacity of a condenser as a result of material of dielectric constant ε is given by
    1/C =1/C1 + 1/C2
    where C1 = C0 ε D/(D-d), C2 = C0 ε D/d. Since ε0 = 1 and δ C = C - C0 δ C/C0 = (ε - 1)/(( ε ((1/λ) - 1) + 1). When ε is much lower than 1, δ C/C0 is close to λ .Relation between change in Capacitance and dielectric constant is shown in Fig 8 below
    Fig 8 : Relation between change in capacitance and dielectric constant

    At relatively high dielectric constants (above10) and low filling proportion, capacitance of condenser (and instrument reading) is primarily affected by weight per unit length variation of material and is not much affected by variations in dielectric constant, caused by variations in blend proportion and moisture content. Textile materials usually have a dielectric constant above 12 and therefore variations in dielectric constant have only a small effect on measured value. However Hearle and Walker11 showed that only cotton and viscose have dielectric constants near 10 and wool has a value between 6-7 and synthetics less than 5. Therefore for these materials variations in RH and blend proportion are likely to affect instrument results. ● Inadequate conditioning and variations in relative humidity during testing.
    Foster12 showed that the error due to uneven moisture content can be kept down if following precautions are taken
    1. If testing room humidity is below 67%, its variations should not exceed 16% during conditioning and if it is above 67%, the variation should be below 8%.
    2. Yarn and roving bobbins should be conditioned in testing room with a free circulation of air from all sides of each bobbin. Conditioning for yarn may be about an hour, for roving about 2 - 3 hours. Slivers should be tested immediately without conditioning, after discarding a few layers on the top. Relative humidity and temperature vary between production area and testing room. Therefore irregularity increases with conditioning in test room, because of uneven penetration of moisture into inner sliver layers.
    A typical study13 showed sliver irregularity increases with conditioning time(TABLE 6) . Conditioning is therefore not recommended while testing slivers.
    Table 6: Increase in U% of finisher drawing sliver with conditioning time
    . Conditioning timeU% CV(1m)%
    Without conditioning2.51.31
    1hr2.991.42
    4hr3.271.51
    8hr3.851.49
    24hr3.881.39
    ● Shape factor of material - Fibre assembly should be as close to cylindrical as possible and should not be asymmetric. A slight twist is given to multi filament yarns prior to testing for this purpose. ● Material should be kept away from both plates or kept in touch with one of the plates.
    Principle of operation of Uster Integrator
    Grosberg and Palmer14 showed that in simple terms, the operation of Uster integrator is based on the use of resistance - Capacitance circuits. The integrator consists of two such circuits as shown in Fig 9 below
    Fig 9 : Integrator circuit of Uster Evenness tester
    A voltage ft is developed upon insertion of material in between measuring plates of the condenser in the main instrument. This is impressed upon R1C1 circuit. R1 and C1 are very large and as a result, condenser C1 charges slowly to the mean value of fluctuating voltage ft. The voltage across R1 at any time is equal to the deviation of ft from the mean value. The voltages across R1 are impressed across R21 without taking sign into account in the case of linear integrator. In the case of Quadratic integrator, the deviations are squared and impressed across R2'. The voltage across C2 charges slowly to the mean value of deviation(in linear integrator) and mean value of square of deviation (in case of Quadratic integrator) and is displayed as irregularity value. Thus the equipment measures CV(8mm, kv/α1) where v is material speed α1 is time constant of R1C1 circuit and k is a constant.
    Improvements in Uster Evenness Tester
    Over the years Uster has brought out several improved models like Uster 3 and Uster 4 and lately Uster 5. The important additional features in these are
    1. Maximum testing speed has been increased up to 800m/min
    2. Variance length curve is automatically determined and the resolution is increased. Variance length from cut lengths of 2 cm to 600 m can be measured in latest models
    3. Spectrogram range is vastly increased (from 1/5 of measured length to a maximum of 2200 m) and spectrograms of different bobbins can be placed side by side for comparison in a 3D plot
    4. Frequency distribution
    5. Module for hairiness measurement is incorporated. A homogenous ray of parallel light falls on the yarn and rays of scattered light from protruding hairs reach the detector and provide a measure of hairiness. Yarn hairiness is expressed in terms of a single value
    6. Module for determination of diameter variation by use of an opto electronic sensor is incorporated. A parallel infra red light beam falls on the yarn and the image is evaluated by line scan camera and corresponding algorithms. The measuring sensor determines absolute mean diameter (over a measuring field of .3mm) and diameter variations CV, variance length curve and spectrogram.. The roundness of the yarn and density of yarn are also measured
    7. Facility to measure various characteristics of slub yarns made on a fancy slub yarn ring frame. Slub frequency, spectrograms of base and slub yarn and simulation of yarn in woven and knitted fabric and on yarn board are useful features
    8. Presence of dust, trash and foreign fibres is estimated
    9. Manual involvement in testing substantially reduced. Automatic transfer of yarn takes place from the package changer and insertion of yarn into the desired slot is also done automatically.
    Prototypes
    QQM3 is a mobile Evenness measuring equipment15 which can measure yarn evenness at the production stage. It has 2 optical sensors of 2 mm width and uses infra red light for projection of yarn on the sensor. and measuring speed is limited to 300m/min.Comparison of evenness by optical and capacitance testers has been done by Sparavigna et al;16 A good agreement is found between measurements of diameter by Uster 4 and QQM and image analyser. Determination of diameter from Uster 4 data shows that it follows bimodal distribution as against unimodal obtained by the instrument. Further CV estimated from extracted data from Uster4 is higher than that given by the instrument.Carvalho17 et al; employ a mass parametirzation system using1 mm capacitance sensors. New parameters like integral deviation ratio and signal processing techniques were used to characterize irregularity apart from U%, CV%, spectrogram and faults. Ewald and Landstreet18 use beta radiation to measure uniformity of slivers, rovings and yarns. Thalium204 is used as Beta ray source, ionized airfilled chamber is used as detector and an electronic integrator is used to determine CV.
    � Pneumatic Method
    Yarn is passed through a narrow tube into which a stream of air is forced. Air flow rate is then measured. Air flow rate is affected by the mass of yarn in the tube. Variation in air flow rate is therefore a measure of irregularity. � Acoustic Method
    Yarn is moved through a sound field between a sound generator and a pick up device. Time taken for sound waves to move across the gap is measured electronically. Transit time of sound is dependent upon the weight of yarn in the gap.
    � Conductometric method
    Yarn is passed through a bath of electrically conducting fluid. When a DC voltage is applied against a section of yarn, current is produced proportional to the conductivity and therefore to weight per unit length of the section. The variability in current is therefore used as a measure of irregularity.
    While CV and MD are the common measures of irregularity, a complete picture of irregularity is obtained by 1 Coorelogram 2 Varaince Length Curve and 3 Spectrogram
    ● Correlogram
    A number of papers have been published on using correlogram for characterizing irregularity19,20,21.
    If r(x) is the Autocorrelation coefficient of thickness( or weight per unit length)of yarn at points 'x' apart, then the plot of r(x) against x is Correlogram.
    For determining auto correlation coefficient, thickness or weight/unit length is measured at specified intervals, say �s,� along the length of yarn. The results are rearranged as shown below
    Table 9
    ● t1●t1+s
    ● t2●t2+s
    ● t3●t3+s
    ● ..●..
    ● ..●..
    ● ..●..
    ● ..●..
    ● ..●..
    ● ti●ti+s
    ● ..●..
    ● ..●..
    ● ..●..
    ● ..●..
    ● ..●..
    ● ..●..
    ● ..●..
    ● tn●tn+s
    Auto correlation coefficient (rs) of thickness between points separated by distance s is given by rs =
    n(Σti×ti+s - Σt i×Σti+s)
    divided by
    [(n×Σti2 - (Σti)2)×(n×Σti+s2 - (Σti+s)2)]0.5
    A plot of rs against s for various values of s gives the correlogram.
    For lengths below fibre length, correlogram is determined by the length distribution of fibre. Since part of the fibres are common to both crossections, there will be a positive auto correlation coefficient as indicated by principle of common elements.
    Fig 10 : Arrangement of fibres in yarn
    Consider two sections of sliver, A and B, along the length of sliver distance x apart. Let AC= l. All fibres of length l that have their right hand ends lying in BC will cross both sections A and B. Their number is = (l - x ) n f(l)dl where
    n = fibre end density
    f(l) dl = proportion of fibres of length l
    Total number of fibres is given by
    Total number of fibres crossing A is given by
    xlm(l - x)n f(l) dl Correlation coefficient between points x apart (rx), is given by
    l max is maximum fibre length and f(l) is frequency of length l.
    The shape of correlogram in the region below fibre length is shown in Fig 11
    Fig 11 : Correlogram in the region below fibre length
    However, normal yarns contain drafting waves and other irregularities. Drafting wave is a damped wave with successive amplitudes diminishing. The correlogram of a normal yarn is more likely to be as shown in Fig 12
    Fig 12 : Correlogram of normal yarn
    Correlogram can be determined by measuring thickness or weight per unit length of yarn at successive intervals of preset distance. Recorder chart can be used for this purpose. The date is rearranged to determine serial correlation. A computer program can be used to determine the correlogram from the data. Uster Evenness tester can also used for determining correlogram22. The yarn after passing through the slot is taken round a pulley placed by the side and passed through the slot again. By altering the distance of pulley from the slot the distance between two successive points can be varied.
    ● Variance Length curve
    Variance length curve is a graph relating variance of weight of length of yarn and the length. This is a very useful tool in characterising the various types of irregularities in yarn and will assist in locating processes which require improvement. A number of papers have appeared on Variance length curve23,24,25,26,27,28 Irregularity in a yarn is given by Variance(A L). Here L denotes the total length of yarn taken for study and �A� denotes the length of pieces cut from the yarn and weighed. IfA is varied keeping L constant, variance length curve obtained is known as �Between length� curve or B-L curve. On the other hand, if A is kept constant and L is varied, the curve obtained is known as �Within length� curve or V-L curve. Total variance of yarn, B(0) or V(∞) is given by
    B(0) = V(∞) = B(L) + V(L).
    With increase in length, B(L) reduces initially slowly, then rapidly and afterwards slowly to reach a value asymptotically. V(L is a mirror image of B(L). In Uster tester, within length is given by kv/α1. Therefore U% increases with speed as shown in Fig 13
    Fig 13 : Effect of material speed on U%
    At lengths shorter than longest fibre length, B-L curve is influenced by fibre length distribution of fibre, as some fibres will be common to both sections. Variance length curve is related to correlogram of yarn as indicated below B(a) = B(0)×(2/l2)×∫(a - x) rxdx
    where denotes auto correlation coefficient of thickness between points x distance apart, B(a) = Variance of weight of lengths �a� long. When �a� is than lt, where lt is half the length biassed mean fibre length of fibre,lc = length biassed mean length, 2lt and If there are no other irregularities( no extra auto correlation coefficients) . B(a) = B(0)
    where is mean fibre length. For lengths longer than longest fibre length,
    B(a) = B(0 � ])
    Where Vt = square of coefficient of variation of fibre length in tuft (Fibrograph) diagram This enables one to find out how much the actual variance length curve deviates from the ideal.
    Fig 14 shows the variance length curve for random yarn. determined from the above equations
    Fig 14 : Variance length curve of random yarn
    Variance length curve shows the amount of short, medium and long term variations present in a yarn. While short term variations come from ring frame, medium term variations arise from roving frame and long term variations from drawframe. Grosberg29 showed that B-L curve of worsted yarn can be predicted from the total variance of slivers at different stages of processing. Malatinszky and Grosberg30 found good agreement between B-L curve predicted by the above method and actual for worsted yarns. Balasubramanian31 applied this method to cotton yarns and found it to give too large a value compared to actual value. In a later article an explanation32 is given why this method is not applicable to cotton yarns . The assumption that fibre ends are arranged at random at very short lengths (of the order of L/D) does not hold good for cotton slivers and hence the prediction of B-L curve of yarn from variance of intermediate products is not possible.
    Measurement
    Cut and Weigh method
    Cut and weigh method is the classical method for determining variance length curve. The material is wound under constant tension on a drum of known circumference and templates of different sizes 1.e; .01, .02, .05, 0.1 0.2 m are used to cut the yarn into required lengths and the pieces are weighed on a sensitive balance. To minimize errors due to any periodicity coinciding with diameter of drum, pieces were collected from random rows and weighed. For longer lengths, the material is wound on a drum of 1 m circumference and single cut is made across the width to get pieces of 1 m length and cuts were made after 5 or 10 wraps to get pieces of 5 and 10 m lengths. A special arithmetic procedure was suggested by Grosberg and Palmer26 for getting an unbiased estimate of variance. Bandyopadyay33 suggested a further improvement taking into account variation of local means. As per this method B(x,y) = - W) Van Zwet28 has proposed a quicker method of estimation of B-L curve from the the points read from recorder chart. Nienhuis suggested that CB(G) curve enables easier comparisons between material from different stages of processing than CB(L) curve, where G is the weight of yarn of length L. O�coneel34 et al modified the Pasfic Evenness tester to record the measured signals in a magnetic tape and feeding the tape to a digital computer to determine weight per unit length and its variance. The method was used to determine variance length curve of yarn. Moderate agreement is found between the measurements of the instrument and cut and weigh method for medium length range
    Cut and weigh method is highly time consuming and so evenness testers were equipped with facility to determine variance length. The old Uster Evenness testers were fitted with �inert� setting which is used to determine variance length curve. Grosberg and Palmer14 examined the integrator circuit and showed that an additional resistance R0 and condenser C0 are introduced in the circuit ( as shown by dotted lines in Fig ) when service selector is changed to �inert� test. The voltage across C0 is average voltage over a time of kv/α0. The average of this is the voltage developed across C1, and the deviations from average are the voltage across R1. The mean value of the deviations (or square of deviations) are estimated by the voltage across C2. By varying the material speed from 1 to 100 m per min, �Between lengths� can be varied and variance length curve estimated. Grignet and Monfort further improved upon this method27. A fairly close agreement is found between B-L curves estimated by Uster tester and cut and weigh methods.
    ● Spectrogram
    Spectrogram is a fourier analysis of variations present in the material. The amplitude of the variations are sorted as per their wavelength and plotted as a amplitude vs wavelength curve in one diagram.
    Constant Staple length
    With random fibre arrangement the amplitude vs wavelength is given by equation below
    Where
    ( λ) = amplitude
    l = fibre length
    λ = Wavelength
    The spectrogram of yarn of constant staple length shows two peaks, one at 2.7 l and another smaller one at l which is a lower harmonic.
    Spectrogram of staple fibre Yarn Fig 15 : Spectrogram of staple fibre yarn (with random fibre arrangement)
    The spectrogram of a yarn with variable fibre length due to random fibre arrangement has a �hump� whose maximum wavelength lies in the region of 2.5 to 3 times fibre length.
    Spectrogram Fig 16 : Spectrogram of cotton and woolen yarn
    On the top of it, waves introduced by drafting waves is superimposed. Wavelength of drafting wave is also 2.5 to 3 times fibre length and so in normal yarn the "hump" is more pronounced depending upon the amplitude of drafting wave. Thus spectrogram of cotton yarns has a hump at 6 to 9cm, of woolen yarn at about 20 -25 cm. OE rotor yarns have a peak at a slightly lower length than ring yarns because fibres are curled with hooks leading to a lower projected length on yarn axis. Spectrogram has a hump at 3.5 times the fibre length with cotton roving and 4 times the fibre length with cotton drawing sliver. When periodic variation is present, spectrogram will show a sharp peak at the point corresponding to wavelength of periodicity. The defect should occur at least 25 times during material passae to be shown as a peak. Serious types of periodic faults are distinguished by the amount by which the peak exceeds the base spectrogram. Anything in excess of 50% is likely to show as fabric defect. Intensity of the defect, correspondence between its wavelength and fabric width determine the occurrence of weft bars. Old Uster tester GGP has 35 filters with each filter encompassing a certain wavelength. Spectrogram is therefore a stepped curve with 35 steps Later models have a higher number of filters, 54-55 in Uster 1 and 2 Latest model Uster testers have increased filters up to 80 and the wavelength determination up to 2560 m. Further, 3 dimensional plots of spectrograms from 10 bobbins can be displayed side by side which helps in identifying the defective bobbins. The amplitude in the spectrogram chart is a function of the variance calculated from the amplitude of the wave. Diana Germanova- Krasteva35 have developed a method for calculating the amplitude and wavelength from the CV% values and spectrogram chart of Uster 3 tester. So spectrogram is a useful tool for detecting the periodicities in the material. It also gives their wavelength which can be used to trace the cause of periodicity and rectify it.
    Fast fourier transforms36 are used to determine spectrograms to detect the presence of sinisoidal waves caused by eccentricity in drafting roller. Fast Walsh Hadamond transform is used for determining rectangular patterns such as flattened top roller. Fast impulse frequency transform is used to detect impulse type of defects such as those caused by presence of trash in rotor groove or broken draft gear tooth. Sinusoidal irregularity produces only a single peak. Rectangular, triangular and impulse type of irregularity produce not only fundamental but also harmonics. Jun and Korobov37 suggested suggested a new method for determining spectrogram based on multiresolution analyser that has greater precision and higher speed than Fourier transform. Matsumoto38 et al; examined irregularity of sliver by estimating bispectra. The bispectrum of ideal sliver and two pseudo ideal slivers made of fibre clusters were calculated using computer simulation program. Tallant and Patureau39 suggested an improved method of calibrating Uster spectrograph by using a tape with square wave instead of sinusoidal wave provided by manufacturer. The advantages of this method are that it can evaluate the performance of more channels and can detect harmonics from fundamental even when wavelength of fundamental is beyond the range of spectrograph. Sharieff40 et. Al; used spectral analysis to separate yarn irregularity into periodic, semi periodic and random components. Periodic component increases non linearly with the extent of eccentricity in roller and draft and quasi periods also an show a increase .
    Imperfections
    These represent outlayers of variations in yarn which have a profound influence on appearance of yarn and fabric and performance of yarn41. Uster imperfection tester measures three types of imperfections viz; Thin places, Thick places, Neps. Nep is differentiated from a thick place by the length of the defect. The instrument evaluates nep as a thick place whose length is shorter than 4mm and longer thick places are evaluated as thick. Even so some imperfections are found to get counted as both thick place and neps. Gupta42 concluded that the instrument overestimates imperfections as a result of double counting() However Gupte43 showed after careful examination of imperfections that thick places and neps occur together in many instances. This is because the presence of foreign matter and fibre cluster (that get counted as nep) causes disturbance to smooth fibre movement in drafting and results in thick places and therefore there is no overestimation.. 4 classes of each of these imperfections are measured as indicated in Table below.
    Table 10
    Thin(%)Thick (%)Nep (%
    -30+35+140
    -40+50+200
    -50+70+280
    -60+100+400
    However -50% setting for thin places, +50% setting for thick places and +200% for neps are customarily used settings for ring yarns. For Open end yarns +280% setting for neps is recommended. Thin places are influenced by fibre properties, combing and drafting conditions. Thick places are influenced by fibre properties, blow room, carding and combing and drafting conditions. Neps are influenced by fibre properties and blow room, carding and combing conditions. Gupte and Balasubramanian44 found that CV% by quadratic integrator is better correlated with thin and thick places than U% by linear integrator. Further CV% has a better correlation with thin places than thick places. Gupte and Balasubramanian45 found that while actual diameter of thin place agrees with calculated, that of thick place is much higher than calculated. Twist at thin place is 150% and that at thick place is 70% of normal place. Tenacity of thin place is lower while that of thick place is same as normal portion
    Faults
    Table11 below shows the difference between imperfections and faults.
    Table 11
    ImperfectionsFaults
    imperfections are frequent occurring defects ( 100 � 5000 per 1000 metres)Faults are seldom occurring defects (200 -5000 per lakh metre)
    Imperfections affect the appearance of yarn and fabric
    Faults affect appearance of and sale value of fabric and lead to end breaks in winding and weaving and knitting
    Imperfections are too small to necessitate clearing in winding.Clearing of objectionable faults is desirable in winding
    Yarn fault classification system like Uster Classimat has facility to detect faults as per their size and length. ● Short Length faults
    16 categories of short length faults are measured by Classimat.
    Table12
    Short Length Faults/TD> /TD> /TD> /TD>
    Size of Fault/TD>A - 0.1 to 1cm/TD>B - 1 to 2cm/TD>C � 2 to 3cm/TD>D - above 4cm/TD>
    +100 to +150%/TD>A1/TD>B1/TD>C1/TD>D1/TD>
    +150 to +250%/TD>A2/TD>B2/TD>C2/TD>D2/TD>
    +250 to +400%/TD>A3/TD>B3/TD>C3/TD>D3/TD>
    above +400%/TD>A4/TD>B4/TD>C4/TD>D4
    ● Very short thick places are caused by the presence of seed coats, broken seeds, trash in the case of cotton and cut fibres in synthetics. Medium size short thick places are caused by embedded fluff, spun-in loose lint among others. Faults of 4cm and above are caused by slubs, undrafted ends, bad piecings etc.
    ● Long Thick Faults
    5 catergories of long thick faults are measured by classimat.
    E - Above 8cm length and +100% and above crosssection size.
    Table 13
    Size of fault8 - 32cmabove 32cm
    +45 - +100%FG
    Long thick places of E category are known as 'Spinner's double' and are caused by lashing of an end with adjoining end at roving or ring frame and over end piecing at ring frame. F and G faults are due to drafting defects at earlier stages, sliver splitting in creel at roving and long overlap of sliver at the time of sliver break at draw frame.
    ● Long Thin Faults
    4 types of thin places are measured
    Table 14
    Size of Fault8 - 32cmabove 32cm
    -30 to -45%H1I1
    -45 to -75%H2I2
    Long thin faults are due to raw material defects, drafting faults in roving and drawing, split sliver in creel of roving, mal functioning of stop motion at drawframe. Longer and thicker size faults are easily perceivable in fabric. So A4, B4, C3, C4, D3, D4 are termed as objectionable faults. Quality conscious buyers insist that these faults are below 1 for one lac metre. German and Japanese buyers consider even C2 and D2faults as objectionable. E, H2, I1, I2 faults are also objectionable as they are easily perceivable in woven and knitted fabrics, lead to fabric rejections and high end breaks during subsequent processing. Clearer settings in winding should be set to remove these faults. Keisokki classifault trichord flex has an extended range of fault measuring capability.
    Neps and slubs - 25 classes
    Thick places � 15 classes
    Thin places � 15 classes
    Foreign fibre faults � 25 classes
    Class limits for fault limits can be freely chosen by user. Histogram analysis of faults is an added feature that helps the mills to locate the problem areas. Both capacitance and optical sensors are used.
    Analysis of characteristics of faults will give useful clues as to their source of origin. This is facilitated by the provision to set the classimat to cut the yarn at the occurrence of preset fault. Mukhopadhyay and Balasubramanian46 found count variation to be higher in the fault portion. Twist level is higher in thin faults like H1, H2, I1, I2 and higher at thick faults like E and F. This is because of tendency of twist to flow to thin place. Tenacity is lower at E fault. Though thin faults are weaker, their tenacity is comparable to normal portion. Agarwal47 et al found that C3, C4 and D faults have a lower strength even after sizing and are therefore a major source of warp breaks in weaving.. Mukhopadhyay48 et. Al; found that while the diameter of thin faults lies close to that prescribed in Classimat, thick fault diameter exceeds that prescribed limits.
    Online measurement
    Online measuring equipments are increasingly used by quality conscious and export oriented mills. Sensors fitted on winding units measure faults, imperfections and evenness of yarns. Continuous variation charts of yarn mass/diameter are also displayed. Such systems have the merit that the entire production is monitored and enable rapid corrective action. Loepfe offers yarnmaster, based on optical measurement, which detects off standards with respect to CV%, imperfections, count and hairiness and stops winding machine. It has facilities for detection and removal of long thick and thin faults, neps, coloured fibres, transparent filaments and online simulation of fault appearance in fabric. Uster Quantum Classifault can operate on production winding machine as well as laboratory winding machine. Apart from the 23 thick faults and 4 thin faults by capacitance principle, it has an optical sensor for measuring 27 classes of foreign material faults caused by contamination. Further yarn count is monitored and if count beyond certain pre-set limits, the bobbin is rejected thereby ensuring no rejections due to count beyond limits. Optimum clearing curve is provided based on scatter plot of faults. Faults remaining in the package after clearing is also estimated.
    A comparison of evenness and imperfections by Uster tester and Corolab fitted on rotor machine showed good correspondence between the two measurements in the case of CV%49. The former is based on capacitance while the latter is based on optical measurement. But imperfections measured by corolab was found to be higher than that by Uster. Yarn type, weaving or knitting influences the relationship between thick places measured on the two equipments.
    Random Fibre Arrangement
    No discussion on concept of irregularity will be complete without a reference to irregularity due to random fibre arrangement. In order to produce a yarn without any irregularity, there should be a mechanism to put a new fibre as soon as one fibre ends. But there is no such mechanism in currently available spinning systems. The best that can be done is to arrange fibres randomly. In Blow room and Carding, fibres are randomly placed and so the best possible arrangement in card sliver will be one with random arrangement. Even if it were possible to produce a card sliver with fibres arranged one behind another, this arrangement will be destroyed by doubling in drawframes, as slivers are fed side by side without regard to position of fibre ends. In other words, doubling at drawframe randomises fibre arrangement further. A certain amount of minimum irregularity is likely to be present due to random fibre arrangement as shown by Martindale50. Since fibre length is very small compared to yarn length, the probability that a particular fibre is present in a certain cross-section is very small and the probability of its non occurrence in the cross-section is very large. Under such conditions, the number of fibres in cross-section will vary as per Poisson distribution. For a Poisson distribution, standard deviation, σ is equal to square root of Mean.
    If n = mean number of fibres in a cross-section
    σ = √n
    CV%= (σ/n)×100
    =100/√n
    A certain amount of irregularity is also caused by variations in weight per unit length along the length of a fibre and between fibres. If CVf is the irregularity due fibre weight variations then
    CV %= √((100/√n)2+ (CVf/√n)2).
    A good deal of variation exists between material to material in variability of fibre weight per unit length but roughly CVf may be taken as 30% for cotton, 50% for wool and 10% for man made fibres. Upon putting these values in the above equation
    CV% =106/√n for cotton
    112/√n for wool and
    102/√n for man made fibres. This means that that irregularity due to random fibre arrangement will increase as n decreases. If C= Yarn Count(Ne) and M= Micronaire value of fibre, D= Denier of fibre
    n=15000/(C×M )
    =5314.5/(C×D )
    CV due to random arrangement will therefore be very low with Drawing sliver and increases slightly at roving and becomes noticeable in yarn. In fine counts CV% due to random fibre arrangement will be a substantial part of the yarn irregularity. This also means that CV% of yarn can be brought down by using finer fibres in place of coarser fibres. Van den Abelle51 pointed out that in actual processing arrangement of fibres in a yarn is not exactly random because of certain limitations for formation of extreme thick places. An alternative model where fibres are arranged in m lanes is prposed so that yarn thickness no where exceeds m fibres. Variance for such a yarn is given by V2 =1002/n(1 - (n/m) ) A similar reasoning is also given by Picard52
    The assumption is more of academic in nature and will not affect significantly the value for random irregularity.
    Index of Irregularity
    Index of irregularity53(I) is the measure, proposed by Huberty, to find out the extent to which actual irregularity deviates from that due to random. Index of Irregularity I =CVa/CVr where CVa is actual measured irregularity and CVf is random irregularity. A higher value means that there is more scope for improving the processes.
    Linhart54 has proposed an alternative index to characterize irregularity of yarn. Bias and standard error of Huberty�s coefficient and new index have been calculated.
    References
    1 , Weft bars and Weft way defects, N.Balasubramanian and S.Sekar Indian Textile J, 1982 Dec,p101,
    2. Weft bars in man- made fibre blends due to torsional vibration of back rollers at speed frame ,S.K.Sett and N.Balasubramanian, J. Textile Association,1985,Nov, p 183
    3. Influence of weft count variation on warp way creases in fabrics, N.Balasubramanian and C.Chatterjee, I. Textile Journal, 1996 Oct p 66
    4.Relaation between fibre end density and thickness, N.Balasubramanian, J. Textile Institute, 1965, 56, T342,
    5. Some calculations relating to arrangement of fibres in slivers and rovings, G.A.R. Foster, J.Gregory and J.R., Womersley, J. Textile Institute, 1945, 36, T311
    6. Planimeter provides easy way for evaluation of evenness tests, Textile Industr, 1949, 113, Jan, p 92
    7. A rapid approximation to Stanadard deviation and Coefficient of variation, J.H. Burkhalter Textile Research J., 1958, 28, p 91
    8. A comparative study of visual assessment of yarn irregularity with Uster Evenness results, N.Balasubramanian, Indian Cotton Growing Review, 1964, March
    ,9. Microprocessor based sliver unevenness analyser, S.K. De, Textile Research J, 1981, 51, p426
    10. H.Locher, J. Textile Institute, 1953, 44, p648
    11.Testing of the irregularity of blended yarns using apparatus of dielectric-capacitance type, J.W.S. Hearle and P.H. Walker, Testing of irregularity of blended yarns using apparatus of dielectric- capacity type, J. Textile Institute, 1953, p811
    12. G.A.R.Foster, J. Textile Institute, 1957, 48, T109
    13.Effect of conditioning on U% of sliver, N. Balasubramanian and G. Janakiraman, I, Textile Journal, 1994, June, p 67
    14. The use of the Zellweger irregularity tester for finding the variance length curve of worsted yarns, P.Grosberg and R.C. Palmer, J. Textile Institute, 1954, 45, T275
    15. Characterisation of yarn irregularity and variation in yarn diameter, J. Militky, S. Ibrahim, D. Kremenakova and R. Mishra, 2010 Beltwide Cotton conferences, New Orliance, 4-10 Jan 2010
    16. Beyond capacitative systems with optical measurements for yarn evenness evaluation, A. Sparavigna, E. Broglia and S.Lugli, Mechatronics, 14, 10, 2004, p1163
    17. Yarn Evenness parameters- A new approach, V. Carvalho, J.L. Monteiro, F.O. Soares and R.M. Vasconcelos, Textile Research J, 78, p119
    18. Uniformity Analysis of Yarns, Rovings, and Slivers Using Beta Radiation P.R. Ewald and C.B. Landstreet, Textile Research J, 1957, 27, p49
    19. Use of correlograms for measuring yarn irregularity, D.R. Cox and M.W.Townend, J. Textile Institute, 1951, 41, p145,
    20. J.L.Spencersmith and H.A.C. Todd, Suppl. J. royal Statist. Soc, 1941, 7, p131, 21. G.A.R. Foster, Suppl, J. Roy, Stat., Soc., 1946, 8, p42
    22. A simple method of plotting correlograms of worsted yarns, W.C.Onions and Selwood, , J. Textile Institute, 1956, 46, T, 127 and T 603
    23. M.W. Townsend, J. Textile Institute, 1949,40, p566 24. Calculation of variance length curve for ideal sliver, H. Olerup, J. Textile Inst., 1952, 43, p290,
    25. Calculation of variance length curve from length distribution of fibres Part I & II H.Breny, J. Textile Inst., 1953, 44, p1 and p10,
    26. On the determination of the B(L) curve by cutting and weighing, P. Grosberg and R.C. Palmer, J.Textile Inst., 1954, 45, T291,
    27. Modification to the calculation of B(L) curvd by the inert test method. J.Grignet and F. Monfort, J. Text., Inst., 1958, 49, T706,
    28. C.J. Vanzwet, A method for calculation of CB(L) curve, J. Textile., Inst, 1955, 46, p794
    29. Medium and long term variations of a yarn I, P.Grosberg, J. of Textile Institute, 1955, 46, T301
    30. Medium and long term variation of yarn II, J. Textile Institute, P.D.Malatinszky and P.Grosberg 1955, 46, T310
    31. Contribution to the study of B-L curve of cotton yarns, N.Balasubramanian, Textile Research J, 1963, 33, p697
    32. B-L curve of Cotton yarns, N.Balasubramanian, Textile Research J, 1965, 35
    33.S.B. Bandyopadhyay, J. Textile Institute, 1955, 46, T63
    34 Unevenness (Length Variance) of a Worsted Yarn by Cutting-and-Weighing and the Use of the Pacific Tester, R. A. O�conell, F. J. AhRrens, and R. J. Martsch, Textiles Research J, 1975, 45, p 596
    35. Spectral Analysis of the Mass Irregularity of Slivers using Uster Tester, Diana Germanova-Krasteva, Fibres and Textiles Eastern Europe, July/Sept. 2003, 41,p42
    36. Yarn irregularity parametrisation using optical sensors . V.Carvalho, P. Cardoso, M.Belsley and R.M.Vasconsilos and F.O.Soares, FIBRES & TEXTILES in Eastern Europe,
    January/March 2009, Vol. 17, No. 1 (72) pp. 26-32 37. Calculational method for spectrogram and variance length curve of yarn based on multiresolution analysis, M.Jun and N.A.Korobov, 6th international conf on parallel and distributed technology computing applications and technologies (PDCATOS), 2005, p993
    38. Bispectra of Sliver Irregularities, Y. Matsumoto, K. Toriumi, I. Tsuchiya and K. Harakawa, Textile Research Journal,,1991,61, p334
    39. Harmonic Response of the Uster Spectrograph, J.D. Tallant and M.A. Patureau, Textile Research J, 1968, 38, p 208
    40. Spectral Analysis of Yarn irregularity and Its Relationship to Other Yarn Characteristics, I.Sharieff , S. G. Vinzanekar and T. Narasimhan, Textile Research Journal, 1983, 53, p 606
    41. Improving the yarn appearance and imperfections in the mills, N.Balasubramanian and K.Viswanathan, J. Textile Association, 1977, June, p67
    42.An interesting observation oncounting of neps and thick places by Uster imperfection indicator, A.K.Gupta, J.T.I, 1986, 77, p354
    43. A.A.Gupte, Ph.D. Thesis, Bombay University, 1993
    44. Influence of imperfections on U% and CV% of yarn, A.A.Gupte and N.Balasubramanian, BTRA Scan, 1991, Septr., 22, p4
    45. Characteristics of yarn imperfections in cotton and blended yarns, A.A. Gupte and N.Balasubramanian, Indian J. Fibre and Textile Research1988, 13, p192
    46. Optical Analysis of Classimat Faults, D.Mukhopadhyay and N.Balasubramanian, BTRA Scan, 1993, 24, No 2, p1
    47. Evaluation of Classimat Faults for Their Performance in weaving S. K. Aggarwal, P. K. Hari and T.A. Subramanian, Textile Research J, 1987, 57, p735
    48. A.Mukhopadhyay, I. C. Sharma, and K. Dasgupta, Textile Research J, 2002, 72, p 178
    49. A Comparison of Yarn Evenness and Imperfection Data, J. B. Price, T. A. Clamari and W.R. Meredith, Textile Research J, 2002, 72, p810
    50. Theory of random fibre arrangements in worsted yarns, J.G. Martindale, J. Textile Inst., 1945, 36, T33
    51. A.M Van den Abellee., J. Textile Inst, 1951, 42, P162
    52. M.C. Picard, 1951, 42, T503
    53. A. Huberty, Proc. I. W.T.O. Techn. Comm. 1, 55, 194,
    54. On Huberty�s coefficient of irregularity of yarns and its estimators, H.Linhart, Textile Research J, 1965, 35, p1055
  • Yarn Hairiness

    Hairiness of Yarns

    N.Balasubramanian
    Retd. Joint Director(BTRA) and Consultant, Tel No 91 22 25280767, Mobile 9869716298
    Protruding fibres, loops from the surface of yarn and loosely wrapped wild fibres constitute hairiness. Hairiness is a unique feature of staple fibre yarns that distinguishes it from filament yarns. Hairiness is generally regarded as undesirable because of the following factors
    1. 1. It adversely affects the appearance of yarns and fabrics. Hairiness is one of the factors that go to determine the appearance grade of the yarn. Higher hairiness downgrades the appearance. Hairiness in yarns leads to fuzzy and hazy. According to Uster 15% of fabric defects and quality problems stem from hairiness. appearance of fabric. Warp way streaks and weft bars are caused by high hairiness and variation in hairiness. Periodic variation in hairiness has been traced to be a cause for alternate thin and thick bands in fabrics
    2. It affects performance of yarn in subsequent stages. Adjoining warp threads cling together in the loom shed because of long hairs in yarn, thereby resisting separation of sheet during shedding. This leads to more warp breaks and fabric defects like stitches and floats.
    3. Excessive lint droppings in sizing, loom shed and during knitting are encountered with hairy yarns because of shedding of hairs and broken hairs.
    4. In printed goods, prints will be hazy and lack sharpness if yarn is hairy.
    5. In sewing breakages will be high with hairy yarns and removal of hairiness by singeing is invariably practised.
    6. Pilling tendency will be more with higher hairiness. Is hairiness undesirable
    In spite of these drawbacks, hairiness has some beneficial effects. It adds to the textile character of the fabric and contributes to comfort , liveliness, skin friendliness and warmth. This will be apparent from a comparison of fabrics made from filament yarn and staple fibre yarn of the same type of fibre and count.Fabric made from filament yarn will have �plastic� feel. Warmth found in woolen and flannel fabrics is to some extent due to hairiness. Thus a certain amount of hairiness is desirable but beyond that it causes problems enumerated above. Measurement of Hairiness Hairiness consists of protruding fibres, looped fibres and loosely wrapped wild fibres. Subjective Methods Yarns can be graded for hairiness by comparison of appearance. Relative levels of two yarns can be easily done by comparison of full bobbins. Grading for hairiness can also be done by wrapping the yarn on a black board and comparing them. Uster has developed yarn hairiness grade standard boards. This will assist in grading of yarns. Paired comparisons by a number of unbiased observers can determine statistically significant differences in hairiness by estimating coefficient of consistency. Microscopic Methods Before instruments were developed, hairiness was measured by viewing the yarn under a microscope. Barela1 was one of the earliest to use this method. Image of yarn is projected on a screen and number of protruding hairs and loops in a known length are counted. Length of protruding hairs is also measured with the help of micrometer eye piece scale. From this total length of hairs per unit length is determined. Onions and Yates2 used photographic standards to grade the hairiness of projected yarn image. Pillay3 projected the yarn on Projectina microscope and counted the number of protruding ends and loops in 10 mm length. Length of protruding fibres is measured by a curvimeter on a tracing of yarn image. Jedryka4 took photographs of yarn image under microscope with a magnification of 50x. The boundary levels of yarn were marked. Four zones parallel to the boundary line were drawn on either side. The lines were equidistant with space between adjoining lines being kept as half the diameter of yarn. Hairiness is determined by the number of intersections of fibre with the lines marking the zone on either side. This method gives the hairiness as per the length of the hairs. About 50 to 100 yarn samples were examined from which average hairiness is determined. Major difficulty in microscopic methods is in indetifying the boundary of yarn. Looped fibres, wild fibres, low twisted portions, variation in yrn diameter and cross-section smudge the boundary. High variation in hairiness is found both within and between bobbins and as a result large number of yarn specimens have to be examined to get a fairly reliable estimate. This makes the method laborious and time consuming.
    Photelectric method
    Several instruments are available for measurement of hairiness based on photoelectric method.
    Shirley- Atlas Hairiness Tester
    A measuring head consisting of a photocell placed close to the yarn counts the number of interruptions made by the protruding hairs to an LCD beam. The measuring head is infinitely adjustable from 0 to 10mm from the surface of yarn. This enables measurement of hairiness as per the length of hairs. Yarn is driven by nip rollers at 50 to 300 m/min by an electronic variable speed drive. Latest version is operated by a pc. Continuous chart of hairiness can also be obtained through a recorder or printer.
    Zweigle Hairiness Tester
    This also uses a measuring head with a photocell and a laser light source. The instrument measures hairiness of 9 length zones from 1 to 15mm in a single run of the yarn. The equipment is controlled by a pc which carries out statistical analysis of the results. An automatic bobbin changer up to 24 bobbins is available which makes the instrument fully automatic.
    Meiners Dell Hairiness Tester
    The instrument measures simultaneously hairiness of hair lengths from 1 to 10 mm in steps of 1mm. A single run of the yarn gives hairiness for all lengths at a selected speed. A portable model is also available which enables on line measurement of hairiness. This equipment will be useful in spotting out spindles higher hairiness thereby enabling prompt corrective action.
    Changling Hairiness Tester
    This uses a laser light source and a sensitive integrated photocell for measuring number of projecting hairs. Measurements of hair number for lengths from 1 to 9mm is possible.
    Uster tester
    Uster Evenness tester has a hairiness attachment The measuring field consists of homogenous rays of parallel light from a infra red light source. Yarn is placed in this field.. Scattered light from the protruding hairs of yarn reach an optical sensor which converts it into an electronic signal. The body of yarn itself is dark as it is not transparent. Hairiness thus measured is an estimate of total length of protruding fibres in a cm length and is termed as Hairiness index. Hairiness index of 4 means that the total protruding length of hairs in 1cm length is 4cm. While this method has the merit that it gives a single index to characterize the hairiness, it has the drawback that it does not provide information on long length and short length hairs separately. Thus two yarns may have the same hairiness index but one may have more long hairs and fewer short hairs than other. Since long hairs are more objectionable than short hairs, information on the level of hairs as per their length will be more useful. Even so there is a good correlation is observed between hairiness by Shirley hairiness tester and Uster tester5. Uster tester further gives coefficient of hairiness over measured lengths 1cm(normal) 10, 100, 300, 10000, 5000 cm or in other words variance length curve of hairiness. Presence of periodicity in hairiness can also be determined by spectrogram of hairiness.
    Premier Electronic tester
    Premier qulaicenter which is similar to Uster tester has an attachment to measure hairiness by hair count as well as Hairiness index method. ASTM standard D5647-01(1995) gives a standard method for measuring hairiness with photoelectric instruments. This will be helpful to minimize variations from laboratory to laboratory.
    Weighing Technique
    Difference in the mass of yarn before and after singeing6 is used as a measure of hairiness. Flaw in this method is that a large amount of yarn has to be singed to get an accurate estimate. Moreover singeing does not fully remove fully projecting hairs particularly the shorter length hairs.
    Factors influencing Hairiness
    Raw Material
    Fibre length, short fibre content, fineness and rigidity are the most important properties of fibre that influence hairiness7. Number of fibre ends per unit length increases as fibre length reduces and as each fibre end is a potential source of hairiness, yarns from shorter fibres are more hairy.. As a result any process from picking to ginning to opening of cotton that results in fibre breakages will increase hairiness in yarns. This is supported by the study of Mclister and Rogers8 who found yarns spun from spindle harvested cotton had less hairiness than that from stripper harvested cotton. Spindle harvesting, being gentler than stripper harvesting, results in fewer fibre breakages thereby leading to less hairy yarn. Hairiness increases with coarseness of fibre because of higher resistance to twisting . For the same reason yarns from fibres with higher flexural and torsional rigidity have higher hairiness. With wool fibres with higher curvature result in less hairines. Xungai Wang, Lungi Cheng and Bruce McGregor9 found hairiness increases with increase in cashmere in wool/cashmere blends because of this reason. Jackowski10 found blended yarns have higher hairiness than yarns made from component fibres. On the contrary Barella and Vigo11 found that hairiness was more in polyester than cotton yarn and 50/50 blend of these fibres has an intermediate value. The discrepancy between these findings may be because of difference in fibre fineness and length of the blended fibres. Fibres prone to static generation generally result in more hairy yarns because of repulsion of fibres. This is the reason why yarns from polyester and other synthetic fibres have higher hairiness than those from natural fibres.
    Yarn Parameters
    Count and twist have considerable influence on hairiness. Coarser yarns have more hairiness than finer yarns because of higher number of fibres in cross-section in the former. Yarn hairiness chart has therefore close correspondence with irregularity chart with coarser regions having more hairiness than finer portions. Hairiness reduces with increase in twist12 because shorter spinning triangle and more effective twisting in of surface fibres into yarn. As a result hairiness is more in hosiery yarns.
    Process Parameters
    Preparatory
    Pillay3 found fibre parallelisation has significant influence on hairiness. Hairiness reduces with increase in number of drawframe passages. With more draw frame passages, fibre orientation is increased and fibre hooks are reduced. As a result fibre extent along the length of strand is increased which is the reason for reduction in hairiness. For the same reason combed yarns have less hairiness than carded yarns. Further with combing short fibre content is reduced which is another reason why hairiness is reduced. A compact roving by use of front zone condenser will bring down hairiness as this will strand width at ring frame
    Ring Frame
    Strand width at the front roller nip has the maximum influence on hairiness. This is because twist will closer to front roller nip and spinning triangle will be smaller and fibres in selvedge will be integrated better into yarn. As will be discussed later compact spinning was developed based on recognition of this. A coarser roving hank and higher ring frame draft will therefore increase hairiness. This is confirmed by studies by Pillay3.
    Spindle speed
    Higher spindle speed is generally found to increase hairiness13,14. This is bcause of the larger balloon at higher speed. With a larger balloon, traveler tilt will be more and this will reduce the space available for traveler movement. Moreover yarn will dash against separator with bigger balloon.
    Doff position
    Doff position and chase position have a significant influence on hairiness. Krishnaswamy, Paradkar and Balasubramanian15 compared the hairiness of yarns at different doff positions in mills and found hairiness was higher at cop bottom position as shown in Fig1. This is because of larger balloon found at cop bottom which increases traveler tilt and causes dashing of yarn against separator. In some cases an increase in hairiness is found towards the end of doff.
    Spindle speed
    Higher spindle speed is generally found to increase hairiness[33,34]. This is because of the larger balloon at higher speed. With a larger balloon, traveller tilt will be more and this will reduce the space available for yarn passage and there will be chafing and abrasion of yarn. Twist flow at lappet will also be reduced. Moreover yarn will dash against separator with bigger balloon thereby generating hairiness. Chaudhuri[35] however found that hairiness is not affected by spindle speed in acrylic yarns.
    Doff position
    Doff position and chase positions have a significant influence on hairiness. Krishnaswamy, Paradkar and Balasubramanian[36,37] compared the hairiness of yarns at different doff positions in mills and found hairiness to be higher at cop bottom position as shown in Fig1. This is because of larger balloon found at cop bottom, which increases traveller tilt and causes dashing of yarn against separator. In some cases, an increase in hairiness is found towards the end of doff.
    Hairiness at different doff positions
    Fig 1: Effect of doff position on hairiness

    Chase position
    Hairiness is more at the shoulder and reduces progressively towards the nose of the chase as shown in Fig2. The balloon is bigger at shoulder and traveller tilt is more. Yarn also dashes against separator. Both these factors increase hairiness. The periodic variation in hairiness in the chase, thus caued, is sometimes a source of hairiness[38,39]
    Hairiness at different chase psitions
    Fig 2 : Effect of chase positon on hairiness

    A mill was experiencing high fabric rejections due to weft bar. The weft bar consisted of alternate thick and thin bands with varying amplitude and periodicity. Thick and thin band portions did not show any difference in pick spacing, count, twist or diameter of yarn. But the hairiness of yarn was found to be markedly higher in thick band portion compared to thin band portion as shown in Table I
    Table 1 Hairiness in thick and thin bands
    Number of fibres beyondThick BandThin band
    4d36.926.8
    8d13.28.5

    The yarn from thick band is found to be more hairy. As a result, hairs in this portion cover interspaces between yarns and more light gets reflected leading to a denser appearance. Length of yarn in one period of thick and thin band was found to be close the length of yarn in one chase of ring bobbin. The cloth was woven on Sulzer loom where 2 splits were made side by side. Cloth width in each split was close to one half of the length of yarn wound during chase movement of ring rail. As a result, yarn from shoulder regions goes to one split and that from nose goes to the next split. But as the yarn made during one half of chase movement is slightly longer than cloth width in one split, a gradual shift in poisoning of yarns from shoulder to the nose region takes place. Yarns from shoulder and nose regions group together alternately in the fabric leading to formation of thick and thin bands.
    Traveller
    Weight, profile and type of cross-section of traveller have critical influence on hairiness.
    Weight
    Heavier traveller up to a limit reduces hairiness[40] because of improved flow of twist to front roller nip. As a result pilling of knitted material reduces. Higher tension associated with heavier traveller will also help to firmly twist the surface fibres into yarn.
    Profile
    Elliptical traveller has a low bow size and as a result limited space is available for passage of yarn. Chafing of yarn will therefore be more resulting in increased hairiness. C shape traveller has a high bow size, which provides ample space for passage of yarn. Hairiness will be least with this traveller. But as centre of gravity is higher with C traveller it results in unstable flight and traveller fly especially at high speeds. Further traveller profile does not match with profile of anti wedge rings, which leads to unsteady traveller flight and rapid wear. As a compromise, Clip and EM1 and EM2 travellers were developed. While having an elliptical profile these travellers have a higher bow size than elliptical. Hairiness will therefore be lower with these travellers compared to elliptical without compromising on speed. Bow size becomes more critical when rings are worn out.
    In one mill hairiness was high on 44s warp yarn, due to worn out condition of rings. The rings were No1 flange antiwedge and traveller used was HRW clip. As change of ring will take time, and since spindle speeds were not high, C type traveller was used in place of clip. A marked reduction of hairiness was found[41] with �C� as will be seen from Fig 3. As C type traveller wears out fast, traveller replacement cycle has to be accelerated.

    C traveller

    Fig 3 : Comparison of hairiness with clip and 'C' type travellers, 1 - 'Hairy' bobbin with clip traveller 2 - Good bobbin with 'C' traveller

    Cross-section
    Round wire or half round wire cross-section will give less hairiness than flat wire. This is because of reduced frictional resistance to yarn movement by the former.
    Applicant of lubricant to traveller
    Application of specially developed lubricant to the traveler[36,37,42] has been found helpful in reducing hairiness by 20 � 30%. The reduction is more prominent immediately after application of lubricant and gradually reduces with passage of time. Application of lubricant once in 6-9 days is therefore necessary to good full benefits. It is important to ensure while choosing the lubricant that it does not stain the yarn. BTRA has developed lubricants meeting this requirement.
    Coated Travellers Travellers with coatings, such as silver and ceramic coating and chromium plating, are available for reducing traveller wear and for extending traveller-changing frequency. Because of their smooth finish, friction between yarn and traveller is reduced, which brings down hairiness. Usta and Canoglu40 found that with heavier travellers, silver coating brings down hairiness.

    Traveller Changing Frequency
    Hairiness is found to increase over the traveller replacement cycle because of traveller wear. Rate of increase is initially slow but after a point of time becomes rapid as shown in Fig For hosiery, sewing threads and p/c blends, where low hairiness is desired, traveler replacement frequency has to be kept low.Hairiness increases with traveler wear because of unstable traveler flight and flutter which accompanies it.
    Traveller changing cycle
    Fig 4 : Variation of hairiness over traveller changing cycle

    New Traveller Design
    Newer Ring and traveler designs like Spicon of BRT and Orbit by Rieter have been developed to reduce traveler wear.With such designs the traveler in its running position lies in a plane close to resultant of all forces acting on it and as a result traveler tilt is minimum. As a result hairiness is lower with such ring/traveler combinations. Moreover round wire cross-section could be used without compromise on speeds.

    Ring
    Flange No, type and wear influence hairiness considerably
    Flange No
    Higher flange number gives more space for passage of yarn and will reduce hairiness. But taveller wear will be more and higher speeds cannot be achieved in finer counts. Normally No 2 flange should be used up to 20s count and No1 flange should be used for counts 30s and above. For bringing down hairiness No 2 flange may be used in counts of border range.

    Wear and tear
    Worn out ring is a major cause of hairiness and variation in hairiness in mills[43]. When wear is pronounced, the bobbins are highly hairy and exhibit whisker like defects. Rings where not changed for 9 years in a mill and were extremely worn out. Upon replacing the rings a substantial reduction of hairiness is seen[43]
    When rings are more than 3 years old, hairiness starts increasing. Replacement of rings will bring significant reduction in hairiness. Some typical results are given in Table2 to show the effect of ring life on hairiness.

    Table3:Effect of ring life on hairiness
    MillCountAge of ring yearsHairiness Hairs/m
    A31s P/C426.4
    New21.7
    40s P/V47.7
    New4.6
    B60s HT T/C511.0
    12.9
    Yarns spun on pilot plant ring frame give lower hairiness[36] and higher strength[44] than those on mill�s ring frame though same roving bobbins were used as feed material. This is because rings in pilot plant ring frame are worn out to a lesser extent than mills ring frame because of less running.
    Lappet
    Abrasion against lappet is a source of hairiness. This gets aggravated when lappet is grooved or is worn out. Some manufacturers have come out with glass finish lappet, which minimizes friction and thereby reduces hairiness. Height of lappet above the ring bobbin has to be optimised to reduce not only end breaks but also hairiness. If lappet to bobbin tip distance is high, balloon will be longer. This will reduce twist flow and also increase area of contact between yarn and lappet. As a result hairiness will be higher. Controlled studies[36] have shown a lower hairiness with reduction in lappet height. Care should however be taken to ensure that yarn does not touch bobbin tip while lowering lappet height.
    Disturbed Spindle Centering
    Disturbed spindle centering is one of the major causes for the spindle-to-spindle variation in hairiness. On spindles where cantering is disturbed, hairiness is found to be higher and upon accurate centering hairiness comes down significantly[37]. When spindle is not centered traveller movement is not smooth because of peak tensions in yarn. Traveller tilts and flutter also increases leading to higher hairiness.
    Separator
    Plastic separator will increase hairiness because of static generation. Disturbed, slanting and bent separators generate hairiness because of excessive dashing of balloon on separator.
    Spindle and bobbin vibrations
    Vibration of spindle arises because of worn out spindle tip and bearing. Bobbin vibrations arise not only from spindle vibration but also from eccentricity in bobbin and improper fit. When bobbin vibrates hairiness increases because of uneven traveller flight.
    Plastic Bobbin
    Plastic bobbins generally give more hairiness than wooden bobbins especially with polyester blend yarns. This is because of static generation.
    Relative Humidity
    Recommended humidity in ring frame department is 55 � 60%. At higher humidity levels, fibres tend to stick to drafting rollers resulting in protruding hairs and loops. At low humidity levels static generation causes repulsion of fibres, particularly with p/v and p/c blends, leading to more hairiness.
    Winding
    Hairiness increases in winding[45, 46,47,48,49]. This is because of abrasion of yarn against tension disc, guide eyes, balloon breakers and winding drum. Extent of increase varies from 50 to 150%. Extent of increase in hairiness increases with winding speed46. Lang et al[48, 49] showed, through a theoretical analysis, that hairiness increase takes place mainly at tension discs because of frictional resistance offered by disc surface to projecting hairs. As the yarn moves forward, these fibres get pulled out of yarn. Loosely bound surface fibres may also become projecting hairs because of rubbing action. The authors used a parameter K and a critical length to estimate the effect of winding on hairiness increase. Friction coefficient between yarn and friction disc has the maximum influence on K. Increase in initial tension of yarn will reduce generation of hairiness.
    A very interesting finding of practical significance is that initial level of hairiness in ring yarn has considerable influence on the extent of increase in hairiness in winding[50]. Ring bobbins judged to be more hairy and less hairy were selected from a ring frame in a mill spinning 60s. The yarns were separately wound on Autoconer. Hairiness of ring yarns and wound yarns are given in Table3.

    Table3 : Increase in hairiness upon winding with yarns of different levels of hairiness
    Type of YarnLess hairy yarnMore hairy Yarn
    3mm5mm6mm7mm3mm5mm6mm7mm
    Ring Yarn3.0.18.06.04201.7.33.10
    Wound Yarn13.6.9.22.1218.91.71.48.19

    Short length hairs increase by 4-4.5 times with winding with �less hairy� yarns. But with �more hairy� yarns, short length hairs do not increase with winding. Long length hairs however show an increase with winding with both �less hairy� and �more hairy� bobbins.
    Longer hairs being on the surface of yarn are more likely to come in contact with tension disc and so get pulled out because of frictional resistance. This is the reason why they increase with both type of yarns. With � more hairy yarns� the surface of yarn body and short length hairs are well buried under long length hairs and therefore do not come into contact with the tension disc. There is therefore no generation of short length hairs at tension disc and short length hairs therefore do not show an increase with winding with such yarns. Moreover, with �hairy� yarns, most of the fibres whose ends are loosely anchored on the surface have already developed into hairs in ring frame itself because of abrasion at traveller/ring junction. Therefore there are fewer such fibres that can develop into in to hairs during winding. This is not the case with �less hairy� yarns as there are many loosely anchored fibres in the yarns. The work of Dash et al[51] supports this winding. The authors found that hairiness increase in winding is more in compact yarns (which have low hairiness) than normal yarns.
    Modifications to reduce hairiness
    Compact Spinning

    As pointed out earlier, major cause for hairiness is the spinning triangle at front roller delivery point, which restricts flow of twist up to nip. In compact spinning, a condensing zone is introduced after normal drafting zone, as shown in Fig 5. As a result, the strand width becomes closer to yarn diameter and the size of spinning triangle is considerably reduced. Selvedge fibres get fully integrated into yarn and projecting fibres are markedly reduced.
    Fig 5 : Comparison of Compact and Normal spinning

    Several manufacturers have developed compact spinning based on different versions of condensing system. Zinser Aircom Tex Condensing zone has a pair of rollers in front of regular drafting. The top roller drives an apron, which has a set of perforations in the middle. The drafted strand is guided underneath the apron. The perforations consist of elliptical and circular pores. The apron runs over a profile tube, which has a slot in the region S (Fig 6) through which suction is applied. The strand follows the perforation track of the apron thereby getting condensed.
    Zinzer Air com
    Fig 6 :Zinser Air Com Tex Compact spinning

    Suessen EliTe
    The condensing zone consists of a lattice apron, located at the bottom and driven by the delivery top roller as shown in Fig 7. The apron runs over a profile tube, which has a slot S, at the middle. Suction is applied through the slot. Front top roller of drafting system drives the delivery top roller through a gear. The diameter of delivery roller is slightly higher than front roller of drafting system, due to which the fibres in the strand are delivered in a straightened condition. Air drawn through inclined slot causes rotation of fibres around their axis, which contributes to better integration of short fibres into strand.
    Suessen Compact
    Fig 7 : Seussenelite Compact Spinning

    Rieter comforspin system
    A perforated drum replaces the front bottom roller of drafting (fig 8). A stationery insert, I, with a specially designed suction slot, in the middle over a length S, is located inside the drum. Apart from the normal top roller a second nip roller, with weighting, is also placed on the drum. Condensation of strand takes place in the zone between the top roller and nip roller As a result of suction inside the drum, the fibres follow the suction slot and get condensed. An air guide element ensures that suction operates in the slot area. The system is suitable only cottons beyond 1.07 inch length and is therefore applicable for finer counts.
    Reiter comforspin
    Fig 8 : Rieter ComforSpin Compact Spinning

    Toyota RX240NewEST
    Condensing unit is similar to Suessen and consists of a pair of delivery rollers and a perforated apron running over a suction tube with a suction slot. Condensing takes place as the perforations span a narrow width. Delivery rollers, driven by gear, drive the apron. Each condensing unit covers 4 spindles and can be conveniently dismantled. Retrofitting to Toyota normal ring frameRX240New is possible.
    Rotorcraft
    Instead of suction, Rotorcraft and Lakshmi machine works make use of a magnetic compacting system to condense the strand. Front bottom roller supports two top rollers in between which a magnetic compactor is placed. The compactor is pressed against the bottom roller by permanent magnets. The shape of the compactor enables condensation of strand. The main merit of this system is that it can be retrofitted into an existing ring frame, which should bring down the cost. Further power required to produce air suction and costs associated with it are reduced.
    Cognotex
    Compact spinning machine is similar to Rieter Comforspin and is designed for long staple fibres like wool. Angled balloon rollers are used as front rollers in the compacting zone to accommodate longer fibres.
    Officine Gaudino
    This is also for long fibres. Instead of suction, mechanical compacting is done. A smooth bottom front roller and angled top roller are located in front of drafting zone. The axle of top roller is in a slanting position in relation to axle of bottom roller. These rollers run at a slightly slower speed than the front roller of drafting. The negative draft, thus created, together with offset top roller create a false twisting action, which condenses the strand. The system can be retrofitted to an existing ring frame. A noticeable feature of the system is the much lower cost (only 20% higher than normal ring frame) compared to other compacting systems (where costs are 200-250%higher than normal).
    Considerable reduction in hairiness by compact spinning has been reported by many authors[52,53,54,55,56,57,58]. The extent of reduction varies with the type of raw material, current levels of hairiness, type of compact spinning, count and twist factor. On an average reduction in hairiness is about 10 � 30 % in Uster Hairiness index and about 50 � 80 % in S3 values. Reduction in hairiness is more with short staple cottons and those with high short fibre content than with long staple cottons and those with low short fibre content[59,60]. As mentioned earlier there is a significant negative correlation between hairiness and fibre length and fibre length uniformity in normal yarns. But the correlation reduces with compact spinning because of higher order of reduction in hairiness with short staple and non - uniform cottons[60]. With long staple cottons with high uniformity ratio, there is not much reduction in hairiness with compact spinning.
    Fibre length distribution of projecting hairs follows an exponential distribution[61,62,24]. While Barella[61,62] found that data falls in 2 or 3 different segments, Wang et al[24] found that in the case of worsted yarns, the data fits in one single curve. Hairiness is plotted against hair length based on the data of Celik et al[56] and Nikolic et al[54] in Figs 9 and 10.
    Hairiness length variation
    Fig 9 Variation of hairiness with hair length in woolen yarn
    Hairiness length variation cotton
    Fig 10 Variation of hairiness with hair length in cotton yarn
    Hairiness reduction with compact spinning becomes more marked above 2 mm length. Slope of hairiness versus hair length on semi logarithmic scale is not linear but appears to be curvilinear especially in the case of cotton yarns. It could be considered as made up of several linear segments with varying slopes as proposed by Barella and Manich[62]. In the case of woolen yarns, the departure from linearity is only slight.
    Extent of reduction in hairiness obtained with compact spinning with different hair length is given in Fig 11. Hairiness reduction with compact
    Fig 11 : Reduction of hairiness with compact spinning for various hair lengths
    Reduction in hairiness increases with hair length in all the cases and the increase is more prominent beyond 2mm length. This means that long length hairs are more effectively reduced in compact spinning. Long length hairs are generally more objectionable from the point of view of appearance and breakages and performance in subsequent stages. Compact spinning is therefore preferable with high quality apparel material and modern weaving units with shuttleless looms.
    Strength of yarn improves by 5 to15% and elongation by 5-8% in compact spinning. Strength improvement is more prominent at low twists and in 100% cotton yarns[63,64] In normal yarns projecting fibres do not fully contribute to yarn strength. When these fibres are fully integrated into yarn as in compact spinning their contribution to yarn strength and elongation improves. This is the reason for the increase in strength and elongation of yarn in compact spinning. Strength improvement is also more with short staple cottons and those with high short fibre content because of higher order of reduction in hairiness. Basal and Oxenham[63] examined the structure of compact and normal yarn using a tracer fibre technique and image analysis application version 3.0.Compact yarns have a high rate and amplitude of migration, which is likely to improve inter fibre friction. This may be another reason for the higher strength of these yarns. Yarns from compact spinning, with a lower twist factor, have a lower hairiness and comparable strength with normal yarn with normal twist factor[56,64]. This means that higher productivity can be achieved in compact spinning which should enable rapid payback of investment. Carded compact yarns have a lower hairiness and broadly comparable strength with combed normal yarn[64]. An interesting feature is that carded compact yarns have a higher strength than combed normal yarns with cottons having low short fibre content. But with cottons having high short fibre content, combed normal yarn has higher strength than carded compact yarns. Overall, there appears to be scope for reducing comber noil with compact spinning. This provides another avenue for getting rapid return on investment. However, it must be noted that normal combed yarns have a lower Uster U%, imperfections and better appearance than carded compact yarns This is because improvements from combing arise not only from removal of short fibres but also from removal of neps and parellelisation. Except in some isolated cases, there is no reduction in Uster evenness and imperfections in compact spinning. This is because drafting conditions determine irregularity and condensing the strand has little effect on it.
    Comparison of Compact Spinning systems
    Of the three systems, condensing zone is used after normal drafting zone in Zinser and Suessen. In Rieter, condensing is done on the front bottom roller of drafting system itself. A bigger diameter front bottom roller (drum) is used for this purpose and this increases the setting length in front zone. This restricts the use of this system for long staple cottons. Between Zinser and Suessen, condensation of strand takes place right up to delivery point in the Suessen system. In zinser, strand traverses a small distance after release from condensing zone before it reaches delivery nip. This can result in partial loss of condensation. But the suction slot is much wider and bigger in Zinser than in Suessen and Rieter systems. This improves the condensation and minimizes choke up of perforations. Further, suction level is also more in Zinser. Hairiness reduction is found to be better with Suessen system compared to Zinser in the work of Nikolic et al[53] (shown in Fig 10). This could be because condensation takes place right up to delivery nip in the Suessen system. However, contrary results were found by Goktepe et al64 who have compared the three compacting systems*. They found the system with apron at the top (Zinser), had the lowest hairiness value. Compact spinning system with apron at the bottom (Suessen) has the highest hairiness value and the system where front bottom roller is replaced by drum (Rieter) has an intermediate hairiness value in coarse to medium counts (21s and 31s). This may be because of bigger suction area and suction pressure in Zinser system. High variations in hairiness and higher irregularity were also observed in the system with apron at the bottom due to choke up of fibre and dust under perforations. Rieter system however gives the lowest hairiness value in medium to fine yarn (41s) Apart from hairiness reduction and strength improvement, compact spinning offers some other benefits which may help to pay back the higher cost of investment.
    Compact spinning has however the following limitations
    Solo Spinning
    Woolmark, CSIRO and WRONZ have developed Solospun system for reducing hairiness in wool and wool blend yarns. This is a clip on attachment to front top roller of drafting, which holds solospun rollers, shown in Fig 12. Solospun roller is placed just below the front top roller and rests on bottom roller and loaded by means of a spring. The surface of solospun roller is made up of 4 segments separated by land, which is flush with the roller surface. There are a series of slots that are offset in each segment. Twist flow to front roller is intermittently blocked by this arrangement and as a result drafted strand is split in to a number of sub strands. Twist passing through the solospun roller twists the sub strands. Upon emerging from solospun roller, the sub strands are twisted into yarn by final twist. As a result the protruding fibres are entrapped into the yarn and final yarn has fewer protruding fibres. Twisting principle of solospun yarn has been studied by Cheng et al[65]. Solospun worsted yarns have significantly lower number of S3 hairs and hairiness index[66]. Hair length distribution of solospun yarn follows exponential distribution.
    Solo spinning
    Fig 12: Solospun spinning system

    Wrapped yarn
    Filament wrapping is another means to reduce hairiness by binding the protruding fibres on the yarn body. The filament is fed, through a guide fitted above front top roller, to the strand emerging from front roller. Twist introduced during spinning will wrap the filament round the yarn body. Stop motion has to be fitted to each spindle to stop the production in the event of filament/roving break or exhaust.
    False twisting in winding
    Though compact spinning reduces hairiness, the benefits obtained are partially lost in winding as hairiness increases in winding as shown in Fig 13. The extent of increase is also more with compact yarns.
    False twist in wining
    Fig 13: Increase in hairiness with winding

    By passing the yarn through an air jet nozzle unit or any other false twisting arrangement prior to winding, hairiness can be reduced and several manufacturers have come out with such systems. Fig 14 shows an outline of an air jet nozzle unit in winding.

    Fig 14: Air jet nozzle in winding

    The swirling air in the air jet unit wraps the protruding fibres around the yarn body. The merits of such attachments compared to compact spinning are
    Muratec has developed an air jet device known as Perla A, which is fitted on their Process coner 21C. Savio has developed a �hairless device� consisting of air jet nozzle. The geometry and shape of the air jet has to be designed carefully to achieve entangling and wrapping effect of protruding fibres. Muratec has also developed another unit Perla D where instead of air nozzle, a disc driven by servomotor, is used to give false twist to yarn. Winding speed up to 1800 and 1200 m/m can be achieved with Perla A and Perla D respectively. The benefits obtained in compact spinning can be maintained by using such a false twisting unit in winding, as shown in Fig 13. Hairiness increases in normal winding but with an air jet suction unit hairiness in wound yarn is maintained close to that in ring bobbin. Several authors have reported significant reduction in hairiness by air jet nozzle in winding[67,68,69]. Air pressure and type and size of air nozzle have influence over the extent of reduction in hairiness. Higher air pressure and lower orifice angle give greater reduction in hairiness[68,69,70]. Strength is unaffected and a marginal deterioration in evenness of yarn is found with air jet unit. Computer fluidic dynamics was used to simulated airflow pattern inside the jets[71]. Air velocity in the core of the jet leads to wrapping of protruding fibres over the body of yarn. Two-phase model used to simulate wrapping of protruding hairs on yarn was attempted by Zeng et al[70]. This showed that yarn hairiness reduction occurs because of wrapping of hairs on yarn. Optimum nozzle pressure of 1.5 - 2.5 bar and a jet orifice angle of 40 � 500 were found by this model. Too high a nozzle pressure and very low nozzle angle increase air recirculation zone and are therefore not advisable. It is not clear if the wrapped hairs will stay wrapped or will get released upon abrasion at next stage.
    Air jet unit at Ring Frame
    Studies have also been made to incorporate an air jet unit between lappet and front roller of ring frame[72,73,74,75,76]. Yarn from front roller nip passes through the nozzle and compressed air is forced through an orifice into the nozzle. The air vortex produced by the air current winds the protruding fibres on the yarn body due to false twisting. After the yarn emerges from the air jet unit, real twist binds the wrapped fibres firmly into yarn. Starter yarn is fed to the lower end of the air jet unit and airflow sucks the yarn through it to facilitate piecing. Air pressure, orifice size and angle have considerable influence on hairiness reduction[70]. Higher air pressure and smaller orifice size reduce hairiness to a greater extent but such nozzles are not preferable from working point of view. Too high nozzle pressure and low orifice angle increase air recirculation zone and so affect wrapping action. An air pressure of o.5kgf/cm2 is adequate to reduce hairiness[75]. Nozzle distance of 10cm from front roller nip and nozzle angle of 450 gives best results in terms of yarn quality[75,76]. Nozzle which produces air current in the same direction as that of yarn twist gives greater reduction in hairiness than that where air vortex is in opposite direction to yarn twist[73]. On the other hand, airflow movement against the direction of yarn movement gives more reduction in hairiness[75]. Yarn evenness deteriorates significantly with the use of air jet in ring spinning. Fabrics made from yarns with air jet attachment as weft show a lower pilling tendency, which arises from lower hairiness. Use of air jet in ring spinning is still at experimental stage and most of the studies reported are on laboratory model spin tester. The effect of air jet unit on end breakage rates and piecing time, have to be still studied. No commercial versions of ring frame with air jet unit have so far been developed.
    Relative merits of Compact Spinning and Air Jet in Winding
    It would useful to consider the relative merits of compact spinning and Air jet unit in winding. Compact spinning involves much higher investment and retrofitting is also not possible in most of the systems. It should therefore be considered in products where hairiness plays a critical role like hosiery, high value apparels, sewing threads.
  • At the same time, compact spinning has the merit that it improves strength and elongation of yarn apart from hairiness. Twist in the yarn can be reduced enabling higher ring frame productivity. Higher spindle speed is also possible, as end breakages will be reduced.
  • Power cost will be higher with compact spinning
  • The benefits from compact spinning in terms of reduced hairiness is partially lost in winding as hairiness increases in winding
  • Air jet unit in winding involves lower cost, as retrofitting of the unit to an existing winding machine is possible.
  • Strength and elongation do not improve with air jet unit. Slight deterioration in evenness and imperfections is also seen with some yarns.
  • Thus to get full benefits from compact spinning, air jet unit should be incorporated in winding Hairiness in different Spinning systems
    Hairiness is higher in ring spinning than rotor, air jet and air vortex spinning systems Reasons for high hairiness in ring spinning have been discussed earlier.
    Rotor Spinning
    Fibres are well controlled in rotor and get attached to sweeping tail of yarn. Number of protruding ends is therefore lower but protruding loops are higher[77]. Fibres are more densely packed near the centre and mean fibre position is closer to the center[78] in rotor yarn. Wrapper fibres bind any fibre protruding outside. As a result, hairiness is lower in rotor yarn than ring yarn[19]. Rotor yarns have about 20% lower hairiness than ring yarns[79]. Higher twist and higher rotor speed reduce hairiness[15]. Plain naval with smooth surface reduces hairiness. Spiral shaped ceramic naval reduces hairiness[80]. Naval with groove which is used for introducing false twist, increases hairiness because of rough surface.
    Air Jet Spinning
    In Murata air vortex yarns, core fibres have no twist and are wrapped by surface fibres. More than half of yarn cross-section is made of wrapper fibres. Uniformity of wrapping of wrapper fibres and higher number of wrapper fibres results in low hairiness in air vortex yarns. Soe et al81 found lowest hairiness, for hair length 3mm and above, with air vortex yarns followed by rotor yarns and ring yarns. As a result weight loss in enzymatic treatment is highest in ring spun yarns followed by open end and air jet yarns[82]. Higher delivery speed in air vortex spinning[83,84] increases hairiness. Higher nozzle pressure reduces hairiness[83,84] but adversely affects evenness of yarn. Higher nozzle angle and smaller spindle diameter reduce hairiness in air vortex spinning[83].
    Air jet spinning is the first version of jet spinning and air vortex is the second improved version by Murata. In air jet spinning, edge fibres on the selvedge of drafted strand form wrapper fibres. In air vortex spinning, fibres in the selvedge region of emerging strand (which is about half of the total number of fibres) are thoroughly separated leading to higher number of wrapper fibres. Hairiness is therefore lower in air vortex spinning compared to air jet spinning[85]. While hairiness increases with cotton content in air jet, no such trend is found in air vortex spinning. So air vortex is more suitable for cotton rich blends.
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    67.M.Miao and X.Wang, Reducing yarn hairiness with an air-jet arrangement in winding, Textile Research J., 1997, 67, p481
    68.K.P.Chellamani, D.Chattopadhyay and K.Kumaraswamy, Yarn quality improvement with an air jet arrangement in cone winding, Indian J., of Fibre and Textile Research, 2000, 25, p289
    69.S.S.Salem and M.Azam, Impact of air jet nozzle pressures and winding speed at autocone on imperfections and hairiness of 20s cotton yarn, Pakistan Textile J., 2004, March
    70.Y.Zeng and C.W.Yu, Numerical and experimental study on reducing yarn hairiness with jet ring and jet wind, Textile Research J., 2004,74, March, p222
    71.R.S.Rengaswamy, V.R.Kothari, A.Patnaik, A.Ghosh and H.Punekar, Reducing yarn hairiness in winding by means of jet: optimization of jet parameters, yarn linear density and winding speed, Autex Textile J., 2005, 5, Septr., p128
    72.K.P.S.Cheng and C.H.L.Li, Jet Ring spinning and its influence on yarn hairiness, Textile Research J., 2002, 72, p1079
    73.K.Ramachandran and B.S.Dasaradan, Design and fabrication of air jet nozzles for air vortex ring spinning system to reduce hairiness of yarn, IS(I) JournalTX, 2003, 84, Aug, p6
    74. X.Wang, M.Miao and Y.L.How, Studies in jet ring spinning Part I Reducing hairiness with jet ring, Textile Research J., 1997, 67, p253
    75.R.S.Rengaswamy, V.K.Kothari, A.Patnaik, and H.Punekar, Airflow simulation in nozzle for hairiness reduction in ring spun yarns Part I Influence of airflow direction, nozzle distance and air pressure, J., of Textile Institute, 2006, 97, Issue 1, p89
    76.A.Patnaik, R.S.Rengaswamy, V.K.Kothari, and H.Punekar, Airflow simulation in nozzle for hairiness reduction in ring spun yarns Part II Influence of nozzle parameters, J., of Textile Institute, 2006, 97, Issue 1, p97
    77.J.Lunnenschloss and E.Hummel, Textil Praxis, 1968, 23, p591
    78.Y.Huh, Y.R.Kim and W.Oxenham, Analysing structural and physical properties of ring, rotor and friction spun yarns, Textile Research J., 2002, 72, Feb, p56
    79. G.M.E.Kog and T.Ogulata, Textil Teknik, 1999 Septr., p88
    80.Anon, Fibres and Textiles in Eastern Europe, 2006, July/Septr., 14, p357
    81.A.K.Soe, M.Takahose, M.Nakajima and T.Matsuo, Structure and property of MVS yarns in comparison to ring and rotor yarns Textile Research J., 2004, 74, p819
    82.A.Khoddomi, M.Siavashi, S.A.H.Ravandi and M.Morshed, Enzymatic hydrolysis of cotton fabrics with weft yarns produced by different spinning systems, Iranian Polymer J., 2002, 11, No2, p99
    83.G.Basal and W.Oxenham, Effect of some process parameters on the structure and properties of vortex spun yarns, Textile Research J., 2006, 76, p492
    84.H.G.Ortlec and S.Ulku, Effect of some variables on properties of 100%cotton vortex spun yarns, Textile Research J., 2005, 75, p458
    85.G.Basal and W.Oxenham, Vortex spun yarn vs air-jet spun yarns, Autex Research J., 2003, 3 No3

    Batt manufacture
    The polymer chips laid in hopper are melted inside an extruder. The molten material is filtered and is pumped by a spinning pump through a spinneret to form a bunch of filaments. As the fabric is wide two or three spinnerets are laid side by side to increase the number of filaments. The emerging filaments are quenched by a stream of cold air (Fig 22) and are attenuated mechanically or pneumatically to orient the molecules so as o increase the strength. There are two methods of batt formation
    1.Partial orientation
    2.Full orientation
    The strength improvement by partial orientation is adequate for many products like coverstock for diapers and hygiene materials. With partial orientation higher production rates can be achieved. Partial orientation is achieved pneumatically by an air generation unit. However for products like geotextiles, carpet backing, roofings and industrial products, full orientation is achieved by drawing the filaments over heated godets, with draw ratio of 1 : 3 or 4 followed by pneumatic acceleration. The filaments are then passed through a pneumatic air gun where high velocity air is forced through a constricted area of low pressure. This helps to achieve high uniformity and cover. Electrostatic charge is applied to keep the filaments separate. Batt formation takes place by random and uniform deposition of filaments on a moving perforated conveyor. The batt is then taken forward for bonding. Lay down with preferential direction in cross or machine direction is also possible. Suction is provided under the conveyor to improve randomisation. Fig 22 outlines batt formation in spunbonding.
             Spunbonding
          Fig :22 Spunbonding
    Recofil system developed by Reifenhauser is the most widely used system of spunbonding. Latest Reicofil model 4 can produce light weight material for hygienic products at speeds of 800 meters/min. Energy consumption of 1 � 1.2 kwh/kg for melting and conveying the raw materials. Other systems are �Docan system� and �Lutrafil sysem�. Reiter spunbond system claims a production capacity of 5000 tons/year/beam and energy consumption less than 1kwh/kg Hills and Fre specialise in spunbonding equipment from bi and multi-component material and claim speeds over 5000 metres/min, In the latest developments, special emphasis is laid on energy conservation and reuse of waste made in production.Ratio between he components can be varied from 50 : 50 to 85 : 15. Spunbonding from bi component and multi-component fibres provides products with unique properties. The different components are melted in separate extruders and passed through separate spin assembly and afterwards combined to pass through special spinneret orifices to form bi or multi-component filaments Special spinneret orifices are available to form core/sheath or side by side and segmented pie profile bi-component structures.
    The relative merits of spun bonding and staple fibre bonding are given in Table 5 below.
    Table 5 : Relative merits of filament bonded and staple fibre bonded Nonwovens
    Spun BondingStaple fibre bonding
    Higher strengthLower Strength
    Lower ElongationHigher Elongation
    Higher Uniformity in thickness and gsmLower uniformity in thickness and gsm
    Lacking in textile character and feelHas good textile character and feel
    Higher tear strengthLower tear strength
    Products are normally of low to medium gsm (20 � 250)Wide range of products from low to medium to high gsm are available (20 � 1500)
    Less flexibility in regard to raw material. Generally line is suitable for either polypropylene or polyester or Nylon or bicomponent fibre. Some latest models however claim flexibility in regard to fibre All types of raw material can be processed in the same lineAll types of fibres, manmade and natural can be made in the same line
    High Plant capacity in terms of productionLow to medium capacity in terms of production
    High capital investmentLow to Medium capital investment
    Major Applications
    1.Coverstock for diapers and hygiene products
    2.Surgical materials
    3.Carpet backing
    4.Bedding and furniture
    5.Geotextiles
    6.Roof Materials and other construction material
    7.Filters
    8.Industrial products
    Meltblown Nonwoven
    Melblown nonwoven is batt formed of ultra fine filaments deposited on a screen. Polymer is extruded through a die consisting of several hundred holes.(Fig 23) Streams of hot air at temp of 220 to 3400C, passing from an outer channels of the die, rapidly attenuate the extruded polymer at 0.5 o 0.7 times the speed of sound, to form extremely fine micro filaments. The filaments are quenched by cool air flowing from the contours of the die and blown on to collector screen thus forming a self bonded nonwoven batt. The fibres are bonded together because of interlacing action of deposition and thermal bonding by the hot air. The meltblown material has low strength and is often used in combination with other nonwoven material. Because of the ultra fine size of the filaments and increase in specific surface area, meltblown material has a high filtration efficiency for gas and liquids and particles.
    Barrier material is formed by combining meltblown material with spunbonded. Absorbents are made by combining cellulose or wood pulp with meltblown material Meltblown material sandiwiched between two layers of spunbonded materials, known as sms nonwovens are widely used as hospital gowns, hygiene, and filtration products. SMMS are also sometimes made.
    Meltblown
            Fig 23 Meltblown Process
    Process Checkpoints
    1. Viscosity of polymer chips should be checked in viscosity meter.
    2.The spinnerets and other parts of the spinning system must be thoroughly cleaned of polymer build-up at periodic intervals and at the time pack change. When polymer build up contaminates the parts, production has to be brought down to maintain satisfactory processing. The quality of product also gets adversely affected by presence of defects like stripes, bars and doglegs. Traditional cleaning consists of 1 burning the hardened polymer by placing the spinneret in a furnace 2. Cleaning in an ultrasonic bath of caustic soda and 3. spray bath cleaning. This method is time consuming and there is risk of spinneret damage. The vacuum pyrolysis process of cleaning is much more rapid and ensures defect free thorough cleaning
    3. Spinnerets should be examined under magnification for wear and tear. Preferably, an automated spinneret examination system should be used to inspect the spinneret at the time of purchase and during use to ensure that the design specifications are maintained as this speeds up the process. If the holes of the spinneret show signs of wear they should be refurbished.
    References
    1. Nonwovens � Technoogy of manufacture and properties
    A.K. Rakshit, A.N. Desai and N. Balasubramanian, Proceedings of Symposium on Nonwovens, BTRA, 1987, Feb
    2. A comparison of woven and nonwoven geotextiles,
    J. Perfetti, Meliand Textilber (Eng. Edn.), 1985, March, p207
    3.Woven and Nonwoven medical/surgical materials
    D.V.Parikh, T.A.Calamari, A.P.S.Sawhney, N.D.Sachinwala, W.R.Goynes, J.M.Hemstreet, and T. Van Hoven, International Nonwovens J, 1999 Spring
    4. Development of a geocomposite canal liner � BTRA�S experience
    A.N.Desai and N.Balasubramanian, J. Textile Association, 1991 Jan, p187
    5. http://www.inda.org/wet-laid.html
    6. Relative merits of polypropylene and polyester
    N.Balasubramanian, Indian Textile J, 2004, 32, p32
    7. Influence of processing conditions on functional properties of high loft structures
    A.N.Desai and N.Balasubramanian, Indian J of Fibre and Textile Research, 1990, 15, p169
    8. Opening size and water permeability of nonwoven geotextile
    A.K.Rakshit and N.Balasubramanian, Indian Textile J., 1991 June, p 26
    9. Indian Textile J, 2009, July, p 71
    10. http://www.texdata.com/content/0082e.pdf
  • Nonwovens- Bonding by Needle punching
    2. Nonwovens –Bonding by Needle-punching
    N.Balasubramanian
    Retired Joint Director, BTRA & Consultant, 022 25280767
    Relative merits of Nonwovens over wovens and various methods of batt formation have been discussed in an earlier article1. The batt has to be bonded to impart strength and dimensional stability. The following are the various methods of bonding widely prevalent in the industry.
    •Needle Punching
    • Hydroentanglement
    • Thermal Bonding
    • Chemical Bonding
    • Stitch Bonding
    • Ultrasonic Bonding
    In addition spun bonding and meltblown nonwovens are nonwovens made directly from filament extrusion. In this article, we will discuss various factors that affect quality and performance of needle-punched fabrics and various developments in the needles and the machine.
    Needle Punching
    Needle punching is by far the most versatile and commonly used method of bonding accounting for 20 -25 % of the nonwovens. Further, needle punched fabric forms the base in making many composites. Needle punching is carried out passing a number of needles with barbs, mounted in a board, through the batt at a high reciprocating speed.. The machine consists essentially of a needle board, stripper plate and stitching plate, Needles are arranged in a number of rows (up to 23-26) in a needle board with about 1500 to 5000 needles per 1 m working width. Board size ranges from 200 to 320m working width. Stripper and stitching plates are perforated so that needles pass through them during up and down movement of each stroke. The stroke varies from 30 to 60 mm. The needles are usually triangular in cross-section with barbs at the three edges. As the needle penetrates through the batt, the barbs carry fibres with them thereby causing mechanical entanglement of fibres as shown in Fig 1 below.
    Fig 1 This gives strength and dimensional stability to the batt. Upon needle punching, the batt becomes thinner, stiffer and stronger. The extent of bonding depends upon punch density, depth of needle penetration, needle type, shape and size, barb shape, size and angle and fibre characteristics. Depth of penetration of needle is the extent to which the needle has penetrated through the stitching plate as indicated in Fig 2. With increase in depth of penetration, more barbs go through the batt/felt and more fibres are transported in vertical direction.
    Fig 2 : Depth of Penetration
    Stitch density or punch density denotes the number of stitches or punches per square cm made by needles on the felt . Punch density is dependent upon the number of needles per board, number of boards, stroke frequency and delivery rate. Stroke frequency is indicated by the number of up and down strokes made by the needle board per min. This is usually 1000 in old machines and has gone up progressively upto 3000 in latest models.
    Net Advance
    Advance made by felt per stroke, mm = Needle punching machines also differ in regard to the direction of punching. In down stroke machines punching takes place from top to down while in up stroke machine punching takes place from bottom to top as shown in Fig 3
    Fig 3 : Up and Down Punching
    In addition, number of boards in a machine varies from 1 to 4. Number of needles per 1 metre width and production rate increase with number of boards. Latest model high production machines have 4 boards with up to a maximum of 20000 needles/m width. Width of needle punching machine ranges from 2 to 10 m.
    Process Factors
    The important process factors that affect quality of needle punched nonwovens are
    1. Type of batt preparation
    2. Punch density
    3. Depth of penetration
    4. Needle characteristics
    5. Feeding System
    6. Batt quality
    7. Fibre characteristics
    Batt Preparation
    The major methods of batt prepation and factors affecting batt quality have been discussed in an earlier article1.
    Punch density
    Punch density is determined by the number needles per metre width, production rate and stroke frequency.
    Punch Density = = Thus punch density can be increased by 1. increasing the needles per board 2. increasing stroke frequency at a given delivery rate and 3. reducing delivery rate at a given stroke frequency. Number of needles per metre width is determined by the make of the machine. Production rate can be increased by increasing net advance and or punching rate. There has been a substantial increase in needle density over the years contributing to marked increase in production rate. Number of needle boards per machine also varies with the model. With increase in number of boards, there will be more needles per metre width and consequently higher punch densities at a given production rate or higher production rate at a given punch density can be obtained.
    Needle punching is usually done in 2 stages viz; pre-needling and finish needling. This gives the best results in terms of uniformity as punching has been done in a gradual manner. However, later models have dispensed with pre needling machine. Gradual reduction of thickness of batt, optimum control by means of adjustable gap between conveyors and close feeding of material to needle are required to minimize uneven draft and irregularity in product. Comprssive batt feeder by Dilo and RDF/FFS and DFS of Oerlikon Neomag, and SFD and DCIN batt feeder of Asselin, are equipped with such features.
    Several authors have studied the effect of process parameters in needle punching on thickness, strength, elongation and other physical properties of Nonwovens,2,3,4,5,6,7,8,9. Apart from straightforward methods of varying process parameters, central composite rotatable experiment design and Taguchi methods have also been employed to study the effect of needling conditions on fabric properties9,12, Taguchi method with grey relational analysis is useful in getting multiple property optimization. Artificial neural network modeling has also been used to predict the effect of web gsm, punch density and depth of penetration on properties of nonwoven15 and the prediction shows good agreement with actual results.
    Fibre Properties
    For successful needle punching fibre length has to be beyond 40 mm to assist in entanglement. Strength and dimensional stability of nonwoven improves as length increases up to 80cm. Beyond that, strength improvements are marginal because of fibre breakages. With increase in fibre length, fibre entanglement during needle punching improves resulting in less slippage and higher strength (Fig 4). With reduction in fibre denier (tex), web becomes more compact and strength improves because of better entanglement due to increased specific surface area4,11,13. However, improvement is marginal beyond a point because very fine fibres are more susceptible to breakages. Further opening size, which is a critical property in geotextiles reduces11 with fineness because of compactness (Fig 5). So for filter and geotextile applications fibre denier has to be properly chosen to meet pore size requirements. Studies on mixture of fine and coarse fibre showed that compressibility, recovery and energy loss% increases with increase in fine fibre and later decreases16. Though not as important as in woven, fibre strength has also a significant influence on increasing strength of nonwovens. Contribution of fibre strength to nonwoven is, however, very low ranging between 5 – 15 %. With increase in ratio of kinetic to static fibre friction, nonwoven exhibits stick-slip tendency during load application. With increase in fibre friction, strength improves because of reduced fibre slippages and greater fibre carrying capacity of needles. Sometimes solutions are sprayed on the fibres to improve fibre friction With higher fibre crimp, consolidation improves resulting in better dimensional stability and strength of needle punched nonwovens. Rayon fibres consolidate more easily than courtelle or wool4. Fibre cross-section shape influences compression and recovery17. Initial thickness and thickness loss are highest with trilobal followed by round and hollow cross-section with polyester nonwovens. Hollow cross-section showed least % compression and minimum compression resilience. Nylon fibre has circular cross section while nomax has cocoon shape cross section. Nylon has therefore a greater packing factor when repeatedly pressed by hot press18 . Breaking strength and modulus increase with packing factor with nomax but decrease with nylon because of heat deterioration of latter upon subjecting it to repeated hot press action. Other properties like bending rigidity and antistatic properties have also some influence on properties of needle punched material. Jute has some inherent advantages as raw material because of its strength, rugged appearance, biodegradability and lower cost. However jute content should be kept within 30% in Jute/polypropylene blend, otherwise wear life and appearance of carpet will suffer19. By sandwiching jute nonwoven in between two layers of polypropylene nonwoven and needle punching, the loss in abrasion resistance and wear life can be minimized. This is because jute is hidden inside and does not come in contact with abrading material. In plain layered carpets made out of jute and polypropylene, needle punching in finish needling should be done from polypropylene side so that top surface is free from jute fibres.
    Resin bonded jute or waste cotton ‘Namda’ is commonly used as an underpad to needle-punched carpet in automobiles as floor covering to reduce vibration and noise. Needle- punched floor covering from kenaf, jute and cotton blended with polypropylene and polyester with an underpad of polyurethane foam or soft cotton nonwoven reduces noise significantly in automobiles20 Kevlar, carbon, steel, teflon and glass fibres can also be needle punched to produce composites used in bullet proof fabrics, insulation,filtration, reinforcement and fire resistant products.
    Fig 4 : Effect of denier on tensile strength
    Fig 5 : Effect of fibre denier on Opening size
    Thickness and gsm With increase in punch density, weight per unit area(gsm) and thickness of fabric show a progressive reduction2,3 and dimensional stability of fabric improves5. The extent of reduction is initially more marked and progressively reduces afterwards. In needle punching, barbs in the needle push the fibres from horizontal to vertical direction as shown in Fig 1 . The fibre displacement leads to drafting of the felt. As a result gsm and thickness of felt decreases. The effect of these factors increases with punch density. Beyond a certain level of punch density, the fabric resists further compression and so rate of thickness and gsm reduction becomes less. The thickness of felt is lower with finer fibre than coarser fibre because of better cosolidation1 but the effect is more pronounced at lower punch density. Density of the fibre also influences thickness. With lower density, thickness will be higher because of increased fibre diameter. Thickness will also be higher with crimped fibres and hollow fibres. Effect of punch density on weight/unit area and thickness are shown in Figs 6 and76
    . Fig 6 Effect of punch density on weight g/cm2
    Fig 7: Effect of punch density on thickness
    As strength of fabric is affected by variations in gsm, breaking tenacity is usually estimated. This is given by
    Tenacity
    Tenacity of the fabric is critically affected by punch density. In cross direction, tenacity initially increases with punch density, reaches a peak and thereafter falls2,21.. In machine direction tenacity increases continuously with punch density but the rate of increase is initially is steep and reduces progressively afterwards. The general nature of tenacity vs punch density relationship is shown in Fig 8
    Fig 8: Effect of punch density on breaking tenacity
    Tensile strength increases with punch density because of greater amount of interlocking of fibres. But the nature of the curve is slightly different between cross direction(CD) and machine(MD) direction. This is because in card – crosslapper preparation, fibres are preferentially oriented in cross direction. As a result CD strength is higher than MD at all punch densities. Higher interlocking of fibres, at higher punch density, helps to reduce fibre slippage and increase the contribution of fibre strength to the nonwoven strength. But beyond a point, increase of punch density does not increase interlocking much and at the same fibres get broken or damaged by the repeated passage of barbs5. This is the reason why there is an optimum punch density at which maximum strength is found in cross direction. But in machine direction, such an optimum is not found and strength continues to increase with punch density though the rate of increase diminishes. This is because majority of fibres slip rather than break during breakage of nonwoven in machine direction because of low orientation of fibres. With increase in punch density, extent of slippage gets reduced. Further CD/MD tenacity ratio decreases with punch density as shown in Fig 9
    Fig 9 : Effect of punch density on CD/MD ratio
    During needle punching drafting of batt takes place leading to orientation of fibres from cross to machine direction. With increase in punch density, draft increases and the reorientation of fibres increases. As a result CD/MD ratio reduces as shown in Fig 9
    Elongation
    Effect of punch density on elongation is shown in Fig 10 .Elongation is higher in machine direction than cross direction because fibre slippage during break is more in the former. Elongation reduces with punch density but the effect is more pronounced at low punch densities. With increase in punch density fibre entanglements become stronger, slippage of fibres is reduced and as a result elongation comes down. Beyond a certain level of punch density reduction of fibre slippage is reduced and so elongation is not much affected.
    Fig 10 : Effect of punch density on elongation
    Tear Strength
    Effect of punch density on tear strength follows the same pattern as tenacity2.
    Depth of penetration
    With increase in depth of penetration, tenacity increases initially but afterwards levels off2,3(Fig 11). With increase in depth of penetration more fibres pass through the batt and to a greater depth. As a result interlocking of fibres is increased, leading to higher strength. But beyond a point, increase of depth of penetration does not improve interlocking much and moreover causes damage to fibres as more barbs pass through the batt. As a result strength improvement levls off beyond a certain level of depth of penetration. Strength is higher in cross direction than machine direction at all depths of penetration because of preferential orientation of fibres in cross direction.
    Fig 11 : Effect of depth of penetration on tenacity
    Pore Size
    Pore size or opening size is an important parameter in nonwoven geotextiles. This determines separation function of the material of different size of gravels and its ability to to retain soil particles without clogging. Dry sieving using well graded spherical glass particles is one of the methods used for determining pore size.Percentage of particle retained after sieving for a known time is determined for different particle sizes. A plot of particle retained % against particle size is drawn from which the opening size at which 90% of particle is retained 090 is determined. The effect of punch density on poresize is shown in Fig 12 for fibres of different denier11.
    Fig 12 : Effect of punch density on opening size
    With increase in punch density, the fabric becomes more compact and as a result opening size reduces. The rate of reduction reduces with punch density. The fabric becomes more compact with finer fibre resulting in lower opening size.
    Compression and recovery
    With increase in punch density and depth of penetration, compressibility initially decreases sharply and afterwards reaches asymptotically a value. Recovery follows a reverse trend, increasing with punch density2. This is because the material becomes more compact and stiff with increase in punch density and depth of penetration. However in jute, compressional behavior decreases initially and later increases9. Recovery decreases with increase in punch density and depth of penetration9 because of greater compactness . Needle punched fabrics from staple fibre have a higher compressional energy loss than spun bonded22. This is because of the bulkiness of former. Needle punched nonwovens are more compressible with higher fatigue compression recovery than thermal bonded, wet laid and woven because of 3 dimensional fibre arrangement and higher bulkiness23. With hollow polyester fibres, cross laid batt has higher compressibility and lower recovery compared to parallel laid24. This means fibre orientation in the batt has influence on compressional behavior.
    Sound transmission loss decreases initially and later increases10 with punch density with jute.
    Air and water permeability
    Air and water permeability are broadly unaffected by punch density2, 11. Though pore size is reduced, fabric becomes more compact one compensating for other. Water flow rate through nonwovens could be predicted by finite element analysis25 with good agreement with actual measurements.
    Stiffness and Abrasion resistance
    With increase and punch density and depth of penetration fabric becomes thinner and more compact and as a result more stiff14. However at higher levels of punch density and depth of penetration stiffness reduces because of fibre breakages. Abrasion resistance improves with punch density and depth of penetration because of compactness and improved fibre integration into the product. Sandwiching hollow polyester between two layers of normal polyester improves abrasion resistance without increasing stiffness15.
    Bursting strength and puncture resistance
    Bursting strength and cone puncture resistance decrease with punch density2 because of reduction in thickness. Liquid absorption rate of cotton needle punched nonwoven increases with punch density and net advance26. Water absorbency and thickness of jute/polypropylene needle punched nonwovens increase with jute content27
    Thermal Conductivity
    Thermal conductivity increases with reduction in punch density as material is less compact and more air is entrapped28. On compressing the material by putting a weight over it, thermal conductivity is reduced. Thermal insulation is highest with 9 barbed needle and higher ceramic content in a multilayered needle punched nonwoven made up of ceramic and glass fibres29. On the other hand higher glass content increases radiative transmission of heat30. Radiation thermal conductivity increases with reduction in area density, gsm and thickness. Increase in pore size increases radiation thermal conductivity31
    Needle Design
    Needle design and shape have considerable influence on the property of nonwoven. Needle consists of 3 parts as shown in Fig 13. 1.Crank 2. Crank shaft and 3 Working blade. In double reduced needle, there is an intermediate blade between crank shaft and working blade (Fig:13). While the cross section of crank and crank shaft are circular, cross section of blade is triangular. The working blade has barbs on the the three apexes and the barbs are staggered. Essentially the barbs transport the fibres through the batt and carry out interlocking.
    The chief properties of the needle that influence nonwoven quality are
    • Gauge
    • Type of barb
    • Barb spacing
    • Barb angle
    • Kick up
    • Length of working blade
    • Cross section of working blade
    Fig 13 : Needle elements
    Gauge
    Gauge of the needle has to be chosen based on the denier(fineness)_ of the fibre. Finer barb has to be used with finer denier. Broad guidelines regard to the gauge of the barb to be used for different fineness is given in Table 1 below.
    Table 1 Gauge of needle in relation to denier of fibre Gauge Diameter,mm Above 30d 18 1.2 19 1.1 20 .95 21 .90 30d 23 0.85 25 0.80 28 0.75 18 – 20 d 30 0.70 32 o.65 12 – 18 d 34 0.60 3 – 12 d 36 0.55 38 0.50 1.5 D 40 0.45 42 0.40 Type of Barb
    Common type of barbs are conventional and diepressed types. Barbs are cut inside the blade with a chisel in conventional barb. As a result, the barb has a pronounced kick up and sharp edges. Fibres are therefore raked during needle punching causing disturbance and damage to fibres and sometimes fibre breakages. Further, the kick up wears away during use resulting in rapid change in needling efficiency and product quality.However, the products made from such needles are more bulky and lofty as a result of the kick up.Diepressed needle was developed to overcome these drawbacks (Fig 14). The barbs in the die pressed needle are formed on a die and so have rounded and smooth edges. Further a controlled amount of kick up can be had with these needles. As a result fibre breakages and damage are considerable reduced with die pressed needle. The products made are also more compact and have lower thickness. The strength is more and products are more smooth with these needles. Now a days, die pressed needles are invariably preferred for all products except for waddings and high lofts where conventional barb is used.
    Fig 14 : Conventional and diepressed needles
    Barb Spacing
    Needles of 4 barb spcings are commonly available, as discussed below and illustrated in Fig 15: Depth of penetration with different barb spacing is given in Table 2
    Regular Barb
    This is the most common type of needle and is available from 13 to 46 gauge. Spacing between barbs is highest, 6.3 mm. Uniform packing distribution of fibres and smooth surface are achieved from top to bottom with this needles because of large spacing between barbs. RB is invariably preferred in pre needling and for high lofts and waddings as it results in higher thickness
    Medium Barb (MB)
    Medium barb improves needling efficiency and results in more compact products and has a barb spacing of 4.8mm. This needle creates more distinct fibre tufts in the felt and as a reult results in slightly inferior surface appearance. This needle is therefore preferred in waste fibre felts used as underlays in carpets and shoddy. If regular barb is used for such material, the fibres will be pushed out of the felt and rapid fibre accumulations will be found on stitiching plate. MB allows more fibres to be carried per stroke with less penetration depth as a result of the reduced spacing between barbs. At times MB is also used in synthetic leather or shoe felts in intermediate needling.
    Close Barb
    This needle is invariably preferred in finish needling. Spacing between barbs is 3.3mm Its main merit is that it allows all the barbs to pass through the felt with a relatively low penetration depth.The resuction in depth prevents the first few barbs from pushing fibres too far below the stitching plate and thus cause a fuzzy surface. When fibres are pushed out of the main body, they will not be contributing to the strength. Further, draft during needle punching will be lower with closed barb. So close barb needle results in a more compact and stronger nonwoven than regular barb. Needle breakages is also lower with close barb as needles travel less distance. However, close barb should not be used in preneedling as it creates distinct fibre bundlesat the bottom and variations in density from top to bottom.
    High density Barb
    These needles have an extremely close barb spacing (1.3 mm). This combined with the short distance between first barb and needle tip enables all barbs to enter the felt with only 8.3mm penetration depth. This needle is preferred in finish needling with thin products. HDB barbs with extremely low kick up or no kpick up are commonly used to get very smooth surface combined with high punch density. These needles are preferred for manufacture of synthetic leather, automotive package tray, spun bonds, automotive trunk liner, shoe and hat felts. Because of of low depth of penetration, needle deflection and breakages are lower.
    Fig 15 : Needles of different barb spacing
    Table 2 Depth of penetration with different barb spacing Type of Needle Penetration depth
    RB 23.3 mm MB 19.1 mm CB 14.8 mm HDB 8.3
    Barb Angle
    Fig 16 below shows the barb angle and kick up in a needle
    Fig 16 : Barb Angle
    Barbs with a lower barb angle facilitate needle penetration as fibres slip out of the barb and as a reult loftier and thicker products are formed. However, strength of the product is lower because of fibre slippage,
    Kickup
    Higher kickup rakes up the fibres without contributing to interlocking.Fibre damages are therefore more and further fabric surface is rough and strength is lower.However, thickness is more and hence higher kickup is preferred in high lofts and waddings.
    Working blade length
    Lower blade length reduces needle deflection and breakages and result in higher strength.Shorter blade lengths are therefore preferred in finish needling.
    Working blade length varies from 11 to 30mm length. Longer working blade lengths are preferred for thicker products but they are more prone to deflection and breakage
    Needle Crosssection
    Needle crosssection is usually triangular. However, newer designs have been developed for certain applications like geotextiles for getting higher strength.
    1. Pinch blade
    Pinch blade has 2 barbed edges with a diamond shaped crosssection (Fig 17).Needle apexes have an angle of 30 0 as against 600 in normal needle.Consequent to this narrow radius, fibres tend to wrap round the barb face firmly with reduced fibre slippage. Fibre carrying capacity of barb is increased and strength is therefore hogher. Moreover, each barb carries separate fibre fringes and the two barbs work independently. It is futher possible to orient the needle in such a way to reduce damage in machine direction or cross direction scrim cloth threads.Further damage to both warp and weft threads can be minimised by orienting the barb at 45 0. Pinch blade is eminently suitable for geotextiles and filters Pinch blades are preferred in geotextiles and filter fabrics. Pinch blade is however more aggressive than triangular and is therefore not preferred for smooth fabrics
    2. Star Blade
    Star blade has either 3 or 4 barbs.(Fig 17) The 3 edged star blade by Gros-Beckart has a slightly concave working area and a more acute edge angle which results in better grip of fibres and intensive bonding. The cross sectional area is lower by 8% compared to conventional needle. The blade is available primarily in 32 to 38 gauge. The additional barb in the 4 edged star blade in Gros-Beckart’s 4star and Foster, increases the fibre carrying capacity and interlocking by about 14% The angle between the outer edges of barb is 600 which increases carrying capacity of barb. Tensile strength and abrasion resistance of fabric is improved by 20 to 30% as a result with this barb. In addition, productivity is increased by 20%.Star blade is preferred with geottextiles particularly from spunbonds and automotive interiors and heavy duty materials.
    Fig 17 : Blade cross-sections of pinch and star blades
    Number of barbs
    Normal needle has 9 bars 3 on each apex. However, to have a smother fabric, needles with 3 or 6 barbs are preferred with each apex having 2 or 1 barb instead of 3. For extremely smooth fabrics 2 barb needle is recommended.
    Blade Style
    Flexing strength is an important characteristic that determines the needle's ability to withstand flexing forces.Flexing loads are high with waste and regenerated fibres and cotton and with rigid materials like aramid and at high punch densities. With normal needles of cylindrical crossection, needle breakages will be high and quality of product will be inferior under such conditions. Needle with conical shape has been developed to increase flexing strength. With conical shape, needle with a lower crosssection first enters the batt when resistance is low and the larger diameter enters afterwards when the resistance is higher Fig 18). So bending and breakage of needle is reduced. Gros-Beckert has developed an improved version of conical needles with a lower diameter, termed as Gebecon. It is claimed to have even higher flexing strength and breakage resistance. The working part of the needle is thinner, more flexible and has extended wear life
    Fig 18 : Needle cross-sections
    Quadro needle
    Quadro needle by Singer has a working blade in rhombic form. (Fig 19 ). The barbs are in two opposite edges that are most distant from one another. The barb arrangement with a coarse shank results in more intensive transportation of fibres with a smooth surface. It is possible to increase strength in machine or cross direction depending upon the orientation of fibres in the batt.Damage to scrim threads is also less.Quadro needles are preferred in geotextiles, filters, blankets and needle felts with support material.
    Fig 19 : Quadro Needle
    Needles for fine and micro fine fibres
    Gebecon needles by Grosbeckert have been developed for fine and micro fine fibres
    Orientation of Barbs
    By changing the orientation of barb in relation to orientation of fibres in felt needling efficiency, strength and surface appearance of product can be altered. We will first consider the case with a needle with one barb. This is often used to minimize damage to scrim cloth.
    Fig 20 : Arrangement of Barbs in relation to material Flow.
    The arrangement of barbs given in Fig 20 B is desirable when a smooth surface free from holes is desired. The fibres slip out of the barbs and so interlocking will be poor. But holes will not be prominent due to insertion of needle. This arrangement is preferred for syntheic leather, fibre fills, spunbonds and carpet backing. The arrangement in Fig 20 A will be preferred when higher strength is required. The fibres move into the barb and the carrying capacity of barb is increased with this arrangement.
    We will now consider the more common triangular needle which has barbs on all the three edges. Fig 21 shows two types of arrangement of needles in relation fibre flow.
    Fig 21 : Orientation of needles in relation to fibre flow With arrangement in Fig 21 B two barbs are favourably disposed to the fibre flow and will have high fibre carrying capacity and so this arrangement is normally preferred. But if needle marks are to be minimized arrangement in Fig 21 A may be used at the cost of slight reduction in strength.
    Scrim Cloth
    Scrim cloth is sandwiched between 2 layers of nonwoven and needle punched in making certain products like filter fabrics and blankets. Use of scrim improves dimensional stability, strength and life. Cotton, polyester, blends and HDPE woven tapes are used as scrim The cloth should be of sufficiently open construction, otherwise, threads will be pushed out of the fabric in the form loops and show as defects. Stringent specifications are prescribed in export for nonwoven filters. Calendering and heat setting of needle punched material have been found helpful in meeting export specifications with polyester filters32. Orientation of barb is even more important in products involving a scrim cloth. During needling such products, the needle barb cause damage to the warp and weft threads and lower the strength. Further, the barbs push the threads out of the felt and cause rolled up nep like defects. The most efficient way to minimize damage to threads is to have needle with barb only on one edge and orient it so that the barb is in most unfavourable position as shown in Fig 22A . Damage to scrim is minimum with such an arrangement but interlocking will be poor as there is only one barb. For improving interlocking without damage to scrim, barbs on 2 edges is used (Fig 22 B)
    Fig 22 : Orientation of barbs in triangular needle to avoid damage to scrim
    Alternately to increase interlocking arrangement, pinch needle with 2 barbs ( Fig 23) is used. The barbed edges are perpendicular to fibre flow and parallel to warp threads. Damage to warp threads are minimized while fibre carrying capacity of barbs are increased. By turning the barbs by 900 damage to weft threads can be minimized. The interlocking will be lower as the barbs are less favourably placed to incoming fibre. Pinch blade is more suitable for avoiding damage to scrim cloth as it has only 2 barb edges and at the same time a higher fibre carrying capacity. By arranging the barbs parallel to warp threads as shown in Fig 23 A, damage to warp threads is minimized and by arranging the barbs parallel to weft threads as shown in Fig 23 B, damage to weft threads is minimized. By keeping the barbs at 450 to warp and weft, damage to both is minimized.
    Fig 23 Orientation of barbs in pinch needle to avoid damage to scrim
    Teardrop shaped needle with a single barb is offered by Grosbeckert to minimize damage to scrim cloth (Fig 24).
    Fig 24: Teardrop Needle
    Needle Point
    Different type of needle points available are given in Fig 25 . Needle point is never totally sharp and there is slight rounding of the edges. Extent of rounding or its radius is based on the product. Nonwovens made from monofilaments get damages with needle with a sharp point. Rounded ballpoints are used with such products. Rounded points and polished points are preferred with papermakers felts, filters and blankets. Chisel point needle has been developed by Foster for needle punching into a support base/backing consisting of foam. Needle point has a chisel shape instead of conical. Buckling up foam is avoided by this needle.
    Fig 25 : Needle point
    Fibre damage
    Some amount of fibre damage and breakages takes place during needle punching. Damage can be minimized by using diepressed needles and needles with low kick up, fewer number of barbs and regular barb (RB). Fibre breakages and loss in strength with wool fibres increase with needle punch density, depth of penetration and kick up of the barb33
    Needle punching of cotton
    Needle punching of cotton is difficult because of lower fibre length and higher frictional and penetration resistance. Special needles have been developed by various manufacturers for needle punching of cotton (Fig 27). Fig 26 : Needle for cotton Essentially 3 changes have made in needle design 1. Working blade needle length is reduced from 30 mm to 22 mm 2. Shape of the needle is conical as against cylindrical 3. Needle point length is increased from 3 to 5 mm and further the point is polished Long point length and conical shape help in penetration while shorter working blade length contributes to stability and minimizes needle damage Results of typical study comparing the results of cotton products made by using conventional and cotton needle are shown in Table 3 below Table 3 : Improvements in Strength by using needle for cotton Type of needle Gsm Tenacity gms /tex Elongation % CD/MD MD CD MD CD Cotton 114 0.6 0.67 98 69 1.1 Conventional 100 0.36 0.36 96 75 1.0 Tenacity of product is improved substantially by using cotton needles in place of conventional Surface Coating Chromium and nickel plated needles have been developed for improving wear life particularly with hard fibres like aramid, glass, steel etc. Polished working blade is also available to facilitate easy penetration. Special surface treatment with titanium nitride and metallurgical treatment have been given in Gebedur needles by Gros-Beckert to improve wear life. Needle punching Force Needles experience maximum resistance and high punching force as the needles enter the batt34. Once the needles have entered the batt, needle punching force drops as needles penetrate further, because of passages created upon penetration. Punching force increases with area density, gsm, fibre length, finness and crimp34,35,36. Higher specific area with finer fibres increases resistance to penetration and punching force. Dynamic punching forces experienced by individual rows of needles showed that penetration force increases with needle location in the board33. Penetration force increases from feed side to delivery side up to a maximum and then decreases. Location of peak penetration force moves from feed to delivery side as punch density increases. While fabric breaking energy increases with needle energy, tearing strength reduces. Dynamic needle punching force in random velour machine (discussed later) is influenced by loom vibration, inertia of needle, rigidity of brush conveyor and the resistance offered by batt38. Transport of fibres during needle punching Fibre transport through layers during needle punching was examined by introducing coloured fibres and counting them39. Still and cine photos indicated that fibre extension takes place during reorientation caused by needle punching40. Fibre slippage then takes place leading to fibre transport. Fibre in the top layer of batt are preferentially picked by barb than those in inner layers41. Fibre transport increases with punch density and depth of penetration. Large barbs and cylindrical needles promote fibre transport.
    Needle punching of Woven fabrics
    Needle punching of woven fabrics is also sometimes carried out for 1. Roughening the surface to facilitate coating 2. To create special appearance and fuller texture. In such cases, round point needles are preferred to minimize damages
    Needle Replacement
    After a period of running, barbs wear out and a few needles break and the quality will deteriorate as a result. If needle replacement is done every 30 million cycles, a marked reduction in quality will be seen as shown by solid line in Fig 27 up to the point of next needle replacement, followed by a marked improvement in quality. To minimize this, rotation of needles is done by some nonwoven units. Phased replaced of needles in a needle board is a better method to achieve uniform quality over a long period as shown by dotted line in Fig 27.
    Fig 27 : Uniformity of quality with phased replacement of needle Section wise replacement of needles as shown in Fig 28 will maintain more uniform and stable quality. The needle board is divided widthwise into 3 sections identified as A, B, C. After 10 million strokes, all needles in section A (which has completed 30million cycles) are replaced. At the same time, broken needles in section B and C are replaced by good needles removed from A. After another 10 million strokes, all needles in section B are replaced and as before broken needles in section A and C are replaced by good needles removed from section B. The process is repeated every 10 million cycles. By following this practice, more consistent and uniform quality as shown by dotted line in Fig 27 is obtained.
    . Fig 28 : Phased replacement of Needle Modern Developments
    1. Elliptical movement of needle board In conventional needling machine, the material stops at the time of needle penetration. As soon as the needles leave the material, it is accelerated from zero to throughput speed. Uneven drafting and shrinkage are caused by this. To overcome this drawback elliptical movement of needle is employed in Hyperpunch needle machine. by Dilo and Muliti motion drive by Oerlikon neomag. Elliptical movement consists of a synchronised vertical and horizontal movement of needle beam. The horizontal movement reduces the speed difference between needles and material. To enable needle beam to follow the movement of material, stripper and stitching blades are slotted. To reduce speed difference between needle beam and material, horizontal stroke has to be adjusted as per the throughput speed. Such adjustment facility is available in Dilo loom HV. Elliptical movement reduces draft in MD and shrinkage in cross direction and improves regularity of product in pre-needling. In finish needling, it enables higher throughput speeds up to 150m/min at 3000 strokes/min with less needle breakage and improved fibre entanglement and higher strength. These looms are ideally suited for needling spunbonds and papermaker felts at high speeds.
    2 Hyperlace punching
    Hyperlace machine, developed as an alternative to spunlace technology, consists of several cyclo punching machines where a transalatory and circular moving path is provided for needle. As a result high density needling is achieved with small dimensional changes. 4 cyclo punching needle looms in tandem have about 20000 needles per meter width. Each needle has just one barb with .02 mm depth. Single fibre is transported by needle during each stroke thereby increasing the entanglement. Fine fibres of 1.5 to 3 denier can be processed. Production rate up to 2000 strokes per min and 100 metres per min delivery rate are claimed on light weight fabrics of 25 to 90 gsm.
    2. Hi Needle punching
    H1 needle punching technology by Fehrer claims superior nonwovens by oblique needle penetration. This is achieved through an asymmetrically curved needle zone with straight needle passage. Better fibre orientation, randomization and entanglement are achieved because of longer needle path in this loom. Surface and mechanical properties of 3 polyester/ cotton blends made on this loom have been reported42
    Needle board
    Needle removal and replacement have been automated by most of the manufacturers. Fraunhifer and Oerlikon Neomag have developed a software to simulate needle punch pattern which helps to optimize position of needles in needle board. Uniformity and freedom from streakiness are claimed with this. Needle positioning can be made as per customer requirements. Singer has a semi automated version. Pre-insertion and pre-positioning of needles is done by hand but pressing to ultimate position is done by machine. Removal of needle is done by pushing and extracting. Problems in Needle punching Some common problems faced in Needle punching and the solutions are given in Table 4 below Table 4 : Problems and Remedies in Needle punching Problems Solutions Accumulation of fibres, neps, clusters and plastic beads in Needles and Needle board 1. Increase the frequency of needle board cleaning 2. Use longer needles 3. Stripper plate should be set as close as possible to material while allowing free passage. Stripper plate should be inclined slightly forward to allow for a larger gap on the inlet side than outlet side 4. Check if scrim cloth is pushed through nonwoven in which case needles with lower number of barbs and with proper orientation should be used Horizontal lines across the fabric It occurs at certain net advances. Slowly vary the stroke frequency while keeping delivery speed constant till the horizontal lines disappear. Longitudinal lines along the fabric These are caused by broken or damaged needles. Rusted needles Special carbon steel needles and coated needles are less prone to corrosion. Spraying of needle boards with silicone based lubricants like WD40 will minimize corrosion Ensure compressed air used for cleaning is dry. Structure Needle Punching Structure needle punching was developed for making carpets with designs, fuller in appearance and thereby add value to the product. Common designs are ribs, velours, geometrical patterns and worm screw chenille. Designed carpets are preferred in wall coverings and floor carpets used in auditoriums, museums, theatres and other public places and in high priced automobiles. Nonwoven batt first pre needled and finish needled with a low punch density, is used as feed material for structuring. Fork needles are used to form a pile. The prongs at the tip of the needle push the loops from the base of the carpet into the openings between lamellae of stitching support. The stitching support consists of a grid of lamellae (Fig 29).The loops formed in a number of parallel row and are transported between stripper plate and lamellae as shown in Fig 30 Fig 29 Lamelle grid in Structure needling. Fig 30 : Loop formation in Structure needling Needle design of fork needle is given in Fig 31. As in the case of normal needle , Fork needles are available as 1 Single reduced 2. Double reduced and 3 Cylindrical or Conical shape( Fig 31). Dimensions of the needle of different gauges are given in Table 5 below. Table 5 : Dimensions of different parts of needle for different gauges Gauge Cranked Shaft (mm) Intermediate Blade(mm) Working Blade(mm 15 1.83 16 1.62 1.50 17 1.35 1.35 18 1.20 19 1.10 20 0.95 22 0.90 25 0.85 30 0..75 36 0.55 Fig 31 Design of Fork Needle Important parameters of fork needle which determine appearance of fabric, are fork depth and width shown in Fig 32. Fork width normally varies from 20 to 40 mm and depth from 20 to 50 mm. Fork depth should be selected based on the thickness of the carpet and depth of pile required. Fork width is determined based on width of pile required. Fig 32 : Parameters of Fork Needle Rib or velour By proper selection of needles it is possible to get rib or velour effect on the same machine. If the needle board has longitudinal and transverse grooves rib or cord effect can be obtained by the same needle by different positioning in the needle board as shown in Fig 33. Fig 33 : Different positioning of needles to get rib and velour effects Multiple geometrical designs can also be produced by inserting the needles in the board as per design and with computer program control of stripper and lamella plates. Structured carpets have a low strength and so have to be coated at the back with a binder. Machines to produce light weight carpets with random velours have also been developed like dilour of Dilo and Superlooper of Oerlikon Neomag and SDV 2A 50S of Assalin. These are preferred as floor carpets by some automobile units as they give an appearance similar to wovens. These looms use crown needles. These needles have 3 or 4 barbs at the same level (Fig 34). The needles are arranged in the board in a random pattern to produce velours with non linear appearance. Velours are formed by the penetration of crown needles into the batt transported by a brush conveyer( Fig 35). Penetration depth is high and as a result densely piled uniform velours are formed on the batt as it leaves the conveyor. Low area density carpets up to 100 gsm can be produced out of fine fibres Stroke frequency ranges from 1200 to 1600/min. Fine fork needles can also be used. Gros-Bekert has developed fine fork needles to cater production of structured nonwovens from ultra fine fibres in the range 1.1 to 3.3 dtex for producing random velour products. Fig 34 : Crown Needle Fig 35 : Random Velour Process References 1. N. Balasubramanian, Nonwovens - Battformation, Indian Textile J., 2009 Dec, p33 2.A.K. Rakshit, A.N.Desai and N.Balasubramanian, Engineering needle-punched nonwovens to achiceve 3.J.Lunenschloss and W.Albrocht Nonwoven Bonded Fabrics, Elis Harwood, London, 1985 4 J.W.S. Hearle, and M.A. Sultan, A study of needled fabrics Part III The influence of fibre type and dimensions, J. Textile Institute, 1968, 59, p137 5. J.W.S. Hearle, M.A. Sultan and T.N. Choudhary, A study of needled fabrics Part II Effects of needling process, J. Textile Institute, 1968, 59, p 103 6. G.J.I. Igwe and P.A. Smith, Meliand Textilberishte(Engl. Edn), 1985, 66, p626 7. G.J.I. Igwe and P.A. Smith, Meliand Textilberishte(Engl. Edn)1986, 67, p 624 8. F. Scardino, J. Ind Fabrics, 1986, 4, p26 9. S. Sengupta, P.Roy and P.K. Mujumdar, Effect of punch density and depth of penetration gsm on compressional behavior of jute needlepunched nonwoven using central composite rotatable experimental design, Indian J Fibre and Textile Research, 2008, _33, p411 10. S.Sengupta, Indian J Fibre and Textile Research, Modelling on sound transmission loss of jute needle punched nonwoven fabrics using cetral composite rotatable experimental design, 2010, 35, p293 11. N.Balasubramanian, A.K.Rakshit and V.K.Patil, Opening size and water permeability of Nonwoven geotextiles, Indian Textile J, 1991 June, p26 12. C.J.Kuo,T.Su, and C.Tsai, Optimisation of needlepunching process for nonwoven fabrics with multiple quality characteristics by grey based taguchi method, Fibres and Polymers, 2007, 8, No6, p654 13. A.Watanabe, M.Miwa, T.Yokoi and A. Nakayama, Fatigue behavior of nonwoven under hot press conditions- Part IV Effect of fibre fineness on mechanical properties, Textile Research J, 1998, 68, p77 14. V.K.Midha, Study of stiffness and abrasion resistance of needle-punched nonwovens, J Textle Institute, 2011, 102, p106 15. A.Rawal, A. Majumdar, S. Anand and T. Shah, Predicting properties of needle-punched nonwovens using artificial neural network, J. Applied Polymer science, 2009, June, 112, p3572 16. V.K. Kothari and A.Das, Compressional behavior of layered needle-punched nonwoven geotextile, Geotextiles and Geomembranes, 1993, 12, p179 17. S.Debnath and M.Madhusudanam, Compression properties of polyester needle-punched fabric, J Engineering fibres and fabrics, 2009, 4, issue 4, p14 18. A.Watanabe, M.Miwa, A. Takena and T.Yokoi Fatigue behavior of nonwoven under hot press conditions Part II geometric structure of fibre cross-section, Textile Research J, 1995, 65, p247 19.A.N. Desai and N.Balasubramanian, Process optimization in the development of structure needle-punched jute-based nonwoven carpets, Indian J Fibre and Textile Research, 1995, 20, p181 20. .D.V.Parikh, Y.Chen and L.Sun, Reducing automotive interior noise with natural nonwoven floor covering systems, Textile Research J, 2006, 76, p813 21. A.Watanabe, M.Miwa, T.Yokoi and A. Nakayama, Fatigue behavior of nonwoven under hot press conditions Part V Effect of punch density on mechanical properties, Textile Research J, 1998, 68, p171 22. V.K. Kothari and A. Das Compressional behavior of nonwoven geotextiles, Geotextiles and Geomembranes, 1992, 11, p235 23 A.Watanabe, M.Miwa, A. Takena and T.Yokoi Fatigue behavior of nonwoven under hot press conditions, PartIII- Effect of fabric structure on compression behavior, Textile Research J, 1996, 66, p669 24. V.K. Medha, R. Alagiriswamy and V.K. Kothari, Indian J Fibre and Textile Research, 2004, 29, p391 25..A.Patnaik and R.D. Anandjwala, Modelling water permeability in needle-punched nonwovens using finite element analysis, South African conference on computationaland applied mechanicsSacomos, Capetown, 2008, March, p26 26. P.Kiekens and M.Zamfir, Nonwovens from cotton fibres for absorbant products obtained by needle-punching process, Autex Research J, 2002, 2, No 4 27. S.Debnath and M. Madhusoothanan, Water absorbancy rate of jute polypropylene blended needle-punched fabrics, J of Industrial textiles, 2010, 39, p213 28. S.M. Hosseini Varkiyani, H. Rahimzadeh, H. Bafekrpoor and A.A.A. Jeddvi, Influence of punch density and fibre blends on thermal conductivity of nonwoven, The Open Textile J, 2001, 4, p1 29. M. Mohammadi, P. Banks-Lee andP. Ghadini, Determining effective thermal conductivity of multilayered nonwoven fabrics, Textile Research J, 2003, 73, p802 30. M. Mohammadi, P. Banks-Lee andP. Ghadini, Determining radioactive heat transfer through heterogenous multilayered nonwoven materials, Textile Research J, 2003, 73,p 896 31. R.Vallabh, P.Banks-Lee and M.Mohammadi, Determination of radiation thermal conductivity, J. Eng Fibres and Fabrics, 2008, 3, p46 32. N.Balasubramanian, A.K.Rakshit and V.K. Patil, Some critical manufacturing parameters affecting properties of nonwoven dust filters, Indian J Fibre and Textile Research, 1993, 18, p8) 33. M.Miao, M.E.Classey and M.Rastogi, An experimental study of needled nonwoven process-Part III Fibre damage due to needling Textile Research J, 2004, 74, p485 34. J.W.S. Hearle, M.A.I. Sultan, J Textile Institute, A study of needled fabric –Part VI The measurement of punching force during needling, 1968, 59, T237 35. J. Lunenschloss, Meliand Textil International, 53, p144 36. C. Goswamy, T. Beck and F.L. Scaridino, Influence of fibre geometry on the punching-force characteristics of webs during needle felting, Textile Research J, 1972, 42, p605 37 A.M. Seyam, Application of on-line monitoring dynamic forces experienced by needles during formation of needled fabric, Int. Nonwoven J, 1999, 8, p 55 38. H.Mashroteh and M.Zarrebini, Analysis of punching force during random-velour needling, Textile Research J, 2011, , 81, p471 39. J.W.S.Hearle and T.N. Choudhari, J Textile Institute, 1969, 60, T478 40. J.W.S. Hearle, A.T. Purdy and A.T. Jones, A Study of needle actionduring needle-punching, J Textile Institute, 1973, 64, T617 41. M.Miao, An Experimental Study of the Needled Nonwoven Process Part II: Fiber Transport by Barbed Needles, Textile Reseearch J, 2004, 74, p394 42. Surface and Mechanical Property Measurements of H Technology Needle-Punched Nonwovens, C. Roedel and S. S. Ramkumar,,Textile Research J, 2003,73, p381
    3.Nonwoven – Bonding by Thermalbonding
    N.Balasubramanian*
    Retd. Jt. Director, BTRA and Consultant
    Thermal bonding represents one of the important methods of making nonwovens which possesses many advantages. 1. It is an environmentally clean technology as no chemicals are used, 2. Has high production rates and requires low space. 3. No water pollution as in chemical bonding 4. The products made from cellulosic are free from allergy to skin. In products close to skin like diapers, sanitary napkins and medical applications it is preferred. 5. The products have a soft feel and are absorbent and permeable. 6. There is no weight loss after washing as in chemical bonding 7. Insulation products and composites can be made 8. It is ideal for making waddings and paddings used in winter clothing However, this technology has not much headway in India. The reasons for this are there are no indigenous machine manufactures. Further, low melt fibres and powders have to be imported. In this paper, different methods of manufacturing various types of thermal bonded nonwovens are described. Influences of fibre properties of base and low melt fibre and process parameters on the properties of nonwovens are covered based on the research and development work carried out. Different types of bicomponent dibres and their merits over moncomponent low melt fibres are covered. I There are two major types of thermal bonding 1. Hot calender bonding 2. Through hot air bonding. The former makes stiffer, thinner and stronger products and the latter loftier thicker and low strength products. For both types of products, batt preparation is the same. A small percentage of low melt binder fibre or powder is added to the regular fibre at mixing stage and the material is passed through opening and carding and crosslapping or airlaying techniques to form a batt. Belt calendar bonding makes products with characteristic between the two. Calender Bonding The batt is passed through the nip of a pair of calendar rollers (Fig 1) with pressure applied on top roller. One or both calender rollers are heated to a temperature above the melting point of binder fibre. The low melt fibre softens or melts to form bonds between the fibres. Heating of calender rollers is usually by oil. Oil, heated in a separate unit by gas burner or electric heater, is force circulated through the calender rollers by a centrifugal pump. Temperature regulators are provided in heating unit to ensure uniform temperature. Afterwards, the bonded material is passed through cooling rollers, known as quenching, to form a cohesive bonded fabric. If the material is wound without cooling, there will be unrelieved stresses caused by tension. This may lead to shrinkage during subsequent use. Both calendar rollers may be plain or one of them may be engraved. The product obtained with engraved roller will be softer as bonding takes place only at the engraved points but it will have lower strength. One or both rollers may be Teflon quoted to minimize sticking of material to hot roller. Instead of direct passage through nip, S passage around the bottom roller to the nip is sometimes done (Fig 1) for increasing heating time especially for heavy material. Calender roller diameter ranges from 40 cm to 70 cm, length from 1.5 to 5m and pressure from 100 to500 KN. Normal pressure range is 30 to 150 kp/cm Material speed depends upon the type of fibre and gsm and usually ranges from 25 to 100 m/min . Fig 1 : Calender Bonding To improve the bonding, some manufactures offer 3 calender rollers with double nip bonding (Fig 2 ). After passing through the nip of top and middle roller, the batt passes around the middle roller and emerges from the nip of middle bottom roller after a second bonding action. Top and middle rollers are usually heated while bottom roller is not heated. This system is used for heavier products like geotextiles and for giving special effects. In 3 calender bonding engraved rollers with different engravings can used in the top and bottom roller to product with enhanced designs. Fig 2 : 3 Roller calendar Bonding Type of Loading Since pressure is applied at the sides of calender rollers, there will be a deflection of the roller leading to more pressure at the sides than at the centre. To obtain uniform pressure across the width, either of the following options are used 1.Top and bottom rollers at kept at an angle Adjustable calendar pressure can be obtained by X crossing of calendar rollers to get uniform pressure across the width. CX calenders are used to get varying amounts of offsetting as per the pressure. The offsetting is adjusted electrically by an encoder. 2. Top roller is made slightly convex. As pressure is applied at the edges, the roller straightens out giving uniform grip over the width. But this method has the drawback that the extent of concavity cannot be varied, which is required with varying pressure. 3. Thermo hydrein calender by Ramisch Kleinwefers. Deflection of calendar rollers is compensated by the application of thin roller shelf with internal pressure in the vicinity of the nip of calendar rollers1. The internal pressure is generated by a system of double piston bearing elements and oil from a hydraulic unit. The cylinder pistons produce the required force. With constant internal pressure, nip pressure is likely to be constant. 4 ‘S’ roll by Andritz Kusters In the ‘S’ roll, annular gap between rotating steel tube and fixed axle is separated by seals into two semi circular chambers. Hydraulic pressure by oil is applied in the chamber facing the nip in such a way that the pressure is in linear ratio to forces in cylinder. As a result deflection is automatically compensated and uniform and infinitely variable force is maintained across the full width. In the latest model direct drive is provided to this roll. 4. Texcol Hycon by Andritz Kusters The deflection controlled calendar rollers have a flexiroll sleeve made of polyamide. The sleeve is supported by hydrostatic pistons. An oil film transfers infinitely adjustable forces to the sleeve to obtain uniform pressure across the width. In addition, pressing width can be easily adjusted by means of supporting elements. Preheating Preheating of web helps to achieve better bonding of inner layers. It also helps to keep calendar roller temperature lower thereby minimizing sticking tendency. Preheating is usually done by infra red heaters. Area Bonded Nonwovens Bonding with low melt fibre Area bonded nonwovens refer to those made with both calendar rollers smooth A small proportion of low melt binder fibre is mixed with normal fibre and passed though batt formation machinery in the usual manner. The batt is then bonded by passing through calender rollers. The effect of process parameters on such nonwovens are discussed below. Calender Roller Pressure Rakshit Patil and Balasubramanian2 carried out detailed investigations on effect of process parameters in thermal bonded viscose nonwovens made by mixing a small proportion of low melt fibre. With increase in calender roller pressure, thickness reduces initially at a faster rate, later slowly (Fig:3 ). Strength also increases initially up to critical pressure, beyond which improvements are marginal (Fig 4 ). Initial strength improvement is because of melting of binder fibres and bonding of the material. Beyond a critical pressure, fibres get flattened and damaged and strength improvements by bonding are offset by damage to fibres, Elongation reduces with pressure because of reduced fibre slippage due to bonding (Fig 5 ) Strength is higher and elongation lower in CD than MD with card – crosslapping because of preferential orientation fibres in cross direction. Bursting strength increases initially but later levels out because of stiffness by bonding. Fig 3 : Effect of Calender pressure on thickness Fig 4 : Effect of calender pressure on strength Fig 5 : Effect of calendar pressure on elongation Binder fibre concentration With increase in binder fibre %, thickness reduces, strength improves and elongation reduces steeply (Figs 6, 7 and 8). Extent of bonding improves with increase in binder fibre % which explains the above results. But as binder fibre tenacity is much lower than normal fibre, increase of binder fibre % beyond a limit does not improve strength. Fig 6 : Effect of binder fibre % on thickness Fig 7 : Effect of binder fibre % on strength Fig 8 : Effect of binder fibre % on elongation Temperature With increase in temperature of calender, strength improves initially, afterwards levels off and drops beyond a certain value. Initial increase is because of better bonding. Increase of temperature beyond the melting point of binder fibre does not improve the bonding. On the contrary, it damages the fibre leading to reduction in strength. Material Speed With increase in material speed strength decreases as the time of contact at the nip reduces with lower bonding action. Base Fibre properties Winchester and Whitewell3 found that crimp, linear density and staple length are the important properties of viscose fibre affecting rupture, elasticity and handle of nonwoven. Finer fibre denier increases strength and reduces air permeability. Staple length increases wet strength, tear strength and abrasion resistance. Crimp increases elongation and reduces air permeability. Binder fibre types, % of binder fibre and bonding conditions have significant influence on the properties of nonwoven. Polyester binder fibre results in loftier and less stiff nonwoven than vinyon binder fibre. The findings of this work have however the limitation that thermal bonding has been done by hot plate pressing under static conditions and not by calender bonding Type of binder fibre Binder fibre with higher birefringence and higher crystallinity and molecular orientation forms weak and brittle bonds and result in lower strength4. This is because of insufficient polymer flow and fibrillation. On the other hand binder fibre with lower birefringence and less developed morphology results in better bonding. Further high production rates can be obtained with such fibres because of shorter time taken by fibre to soften and melt. Though these studies were made with point bonded nonwovens made from polypropylene, they should also hold good with area bonded nonwovens made by adding binder fibre to normal fibre. Quench Rate Higher quench rate increases the strength and elongation of the material. This is because of recrystallisation and lower crystal size. However, at high quench rates stress concentration takes place leading to brittleness and reduced strength. Single nip and double nip Though double nip point (Fig 2) should increase strength because double boding action, this is not always realized. As the material is in contact with hot middle roller for 1800 rotation, the binding points get stretched, especially with stiffer fibres like cotton and broken leading to lower strength particularly with thinner material. Double nip bonding is generally preferred with heavier fabrics. Low melt monocomponent fibres Low melt fibres are based on polyolefin (polyethylene and Polypropylene), polyamides or polyester. Polyvinyl chloride , vinyl acetate copolymers and cellulose acetate are also sometimes used as binder fibre. Modifications with commonomers and additives help to lower the melting temperature of normal synthetic fibres for development of low melt fibres.. The fibres are copolymers with less developed morphology and lower crystallinity. As a result, the fibres have low strength. A good low melt fibre should have lower melt viscosity and good affinity/ adhesive property to base fibres. Further, it should solidify quickly upon emerging from heat treatment. The melting point ranges from 1100 to 1500C. Grilene KE 150 and 170 copolyester (Foss), vinyl chloride vinyl acetate copolymer (Wacker MP), Exelto polyolefin fibre (Exelto), low melt polyamide (Premiere), FSI 101 polyethylene, FSI 0200 polyvinyl alcohol, 0502 ethylene vinylacetate, 0800 polyester fibre (Fibre Science.inc), tergal 190 PETP (Tergal) are some common low melt fibres. Biodegradable binder fibres have the merit that they permit recycling of nonwovens. Some important biodegradable fibres are cellulose acetate from celanese, polyactic acid (from corn) by Dow –Cargill, Bio PET by Dupont, PTAT copolyester by Eastman Chemicals, Bicomponent fibres Bicomponent fibres produce more uniform bonding and improved strength. Bicomponent fibres consist of two polymers, one of normal type and another low melt type made from the same spinneret with both components in the same filament. Different types of bicomponent fibres are shown in Fig 9. Skin core, also known as sheath/core, types are shown at the left and right side of figure while side by side types are shown at the centre. In the skin core type, core is a normal and skin is made of low melt component. The two can be placed concentric as in left hand side or eccentric as in right hand side. In the side by side type, shown in centre, one half is normal and other half is low melt type. When the batt is heated to the melting point of low melt component, bonding takes place because of melting of this component. The main advantage of bicomponent fibre is uniform bonding throughout the material as the low melt component is part of the fibre. As a result, low variability in thickness, strength and elongation is obtained. Another advantage is a wider range of bonding temperature. Since the core is a normal material, strength of bonded material is higher. Further the product is softer compared to that made from mono component. Fig 9 : Common types of bicomponent fibres .With concentric skin core type, better strength is obtained while with eccentric skin core type higher bulkiness is obtained. Skin core type is also sometimes used for improving water absorbency, dyeability and soil resistance, these properties being provided by skin component. Side by side bicompomponent is used for making bulkier products because of the difference in shrinkage of the components. Trilobal type skin core type of bicomponent fibres are also available (Fig 10) Fig 10 : Bicomponent fibre with trilobal crossection Some of the common types of skin core fibres are 1 1100 amorphous coPET/PET 2.1800 amorphous coPET/PET 3. 1500 melt crystalline coPET/PET 4. PA/PET 5. PE/PP or PET 6. HDPE/PET 6. Easter coPolyester/PP 7. PETP/copolyester Islands in Sea bicomponent A number of pieces of one polymer (island) are placed in a sea of another polymer (Fig 11) . The island polymer has a higher strength and lower elongation than sea polymer. The sea has a lower melting point to ensure bonding without affecting the island. Polypropylene, nylon and polyester polymer usually form the island. Sea matrix are made of polyvinyl alcohol or water soluble polystyrene fibre. Fig 11 : Islands in sea bicomponent fibre Island/sea bicomponent fibre made from 75/25 N6 and PE resulted in stronger bonds than moncomponent N6 upon thermal bonding of spunbonds5. A strong interface occurs between sea and island polymers, the weaker PE holds the structure together and helps to transfer the stress to stronger islands. Segmented pie structure This is shown in Fig 12. There are 16 segments where alternate wedges are made of nylon and polyester. The fibres are carded and with the help of high pressure jet of air or water passed through the web fibres are split. The resulting entanglement results in a strong fabric. The reason for using polyester and nylon in the wedges is that they have a poor adhesion to each other and so under the action of jet of water, splitting easily occurs. Pie wedge fibres are used to make micro fibres. The main application for this is in synthetic suedes and leather and in technical wipes. Fig 12 : Hollow pie wedge fibre Thermal Bonded Cotton nonwovens Thermal bonding has several merits for application to cotton for making nonwovens. Because of its lower length, cotton is difficult to needle punch. Chemical bonding causes allergy and is not suitable for close to skin and medical products. Thermal bonding is therefore widely used for making cotton nonwovens. The factors affecting thermal bonding of cellulosic fibres has been reviewed earlier by Desai and Balasubramanian1. Area Bonded Since low melt fibres are imported and not available easily, studies were made to make cotton thermalbonded nonwovens by using locally available polypropylene (PP) fibre as a binder fibre6. Melting point of PP fibre is 1600 to 1700 and calendar rollers are heated to this point. As in the case normal low melt binder fibre, thickness reduces with increase in PP. Absorbency reduces with increase in PP content (Fig 13) but the rate of reduction is more with light weight nonwovens.. Absorbency is higher with heavier products except at high PP levels. Strike through time, which measures the time taken by distilled water to pass through nonwoven, increases with increase in PP (Fig 14). Strike through time is also more with heavier weight nonwovens Rewet property, which determines the ability of nonwoven to keep the skin of wearer dry, increases with PP with low weight and reduces with PP with heavier weight nonwoven (Fig 15). Fig 13 Effect of binder fibre%(PP) on absorbency Fig 14 : Effect of binder fibre% on strike through time, sec Fig 15 : Effect of Binder fibre % on rewet % Point bonded cotton nonwovens Point bonded nonwovens refer to those made with engraved rollers. They have much lower strength than area bonded as only part of the area is bonded but are softer. Use of ordinary cellulose acetate(OCA), plasticized cellulose acetate (PCA), easter/PP bicomponent and PE/PET bicomponent as binders in thermal spot bonding of cotton has been investigated byH.Rong7,8, High strength absorbent cotton thermal spot bonded can be produced using 50% Easter/PP bicomponent fibre as binder. Easter/PP bicomponent fibre enables bonding at lower calender temperature and gives higher strength than Easter unicomponent Shape of the bond point becomes smoother and well defined as bonding temperature increases. Cellulose acetate as binder fibre has the merit of biodegradability as against synthetic binder fibre. However cellulose acetate has high melting temperature and this poses difficulty when used as binder. This can be overcome by pretreating it with water. This lowers the softening temperature of cellulose acetate and improves significantly strength of cotton/cellulose acetate thermal bonded nonwovens9.Melting point of cellulose acetate can also brought down by treatment with solvent vapours 10,11. Higher strength and bonding at lower temperature can be obtained as a result. Cotton bonded with cellulose acetate has good biodegradability and could be easily degraded by microbial attack. Investigations are reported on thermal bonded nonwovens for short-wear-cycle applications. Thermal bonded composites made with cotton on one side or both sides with 41 – 75% content have excellent wicking rates, wetting, absorbency and retention properties with handle similar to hydroentangled or knitted products12. Thorough mixing of binder fibre with cotton is required to get optimum tensile properties13. Nonwoven mouldable automobile carpets can also be made by blending cotton, kenaf and flax with biodegradable binder polymers PTAT and PTAT/PP, Biopet and PE/PET, PLA and PCA bicomponent fibre. Such products have the advantage of being environmentally biodegradable14 and trimmings made during manufacture can be reused. Statistical prediction equation has been developed for determining optimum process conditions to achieve peak loads of cotton thermal bonded nonwovens15. Point Bonded Nonwovens from synthetics One of the calendar rollers is engraved to make point bonded nonwovens for making smoother products. Both calendar rollers can also be engraved to produce novonette pattern in the product. A helical pattern of lands and grooves on both rollers results in a diamond point. Point bonded nonwovens can be made from polypropylene fibres without addition of low melt fibre. Fibre Orientation Fibre orientation in the input material, known as orientation distribution function (ODF), has marked influence on tensile properties of such material. Random orientation of fibres in the input material results in maximum strength because of improved bonding arising from higher number of cross over points16. With preferentially oriented fibres, number of bonds will be less and strength will be lower. ODF also determines the nature of break of nonwoven. Fabrics tear across the preferred fibre direction when load is applied in machine or cross direction. ODF determines the structural changes and deformation that take place during loading of nonwoven. Bonding temperature has influence only on the point of failure16,17. ODF and anisotrophy of bond pattern determine the load elongation behavior of nonwovens. Orthotropic theory has been used for predicting the effect of mechanical conditions in nonwovens with preferred orientation distribution. Theory shows good agreement with experimental results18. Bond point Geometry Bond point geometry, area of the tip, concentration and % bond point area are some of the critical factors that influence properties of point bonded nonwovens. Generally bond point area should be 10 – 25 % of total area and bond point concentration should be 100 to 500 per square inch. Fig 16 : Bond point parameters Important bond point parameters are the length and width of the bond point tip, and side angle (Fig 16). Very low bond point concentration results in low strength and poor life of product. Smith19 et al; found that concentration beyond a point will result in stiff products. Lower bond point concentration increases elongation and decreases flexural rigidity of polyethylene point bonded nonwovens. Increase in angle of bond point increases tenacity. Measured bond area is found to be higher than engraved roll pattern possibly because of transmission of temperature at the periphery. Computer based study however shows that with increase in % bond area, tensile strength, energy to break and breaking elongation increase except with weak bond points20 Numerical method, based on finite element analysis, for prediction of nonwoven tensile behavior from bond point design and process parameters has been attempted by Mueller and Kochmann21. Rapid modeling of different point geometries and layouts is possible using this method and this will help manufacturers in developing design. Process Parameters Warner22 showed that unlike in area bonded, calender roller pressure does not have much effect on strength with point bonded material. Pressure however aids in compacting the web and facilitates plastic flow at melting temperature and reduces thickness. Melting temperature of polypropylene is increased by 150 with application of pressure due to clapeyron effect. Heat of deformation under pressure also increases the temperature substantially. Melting temperature at the centre of web is more important than that at the nip of calender rollers in order to get a good bond. However, heat transfer from calender nip to the centre of fabric is insufficient to increase temperature to melting point as conduction heating plays only a limited role in transferring heat from nip of rollers to the centre of fabric. Some diffusion also occurs but its contribution is much lower than flow. Because of these reasons, pressure does not have much effect on strength. The main function of pressure is to facilitate flow of polymer which mechanically locks the fibres upon solidification. Bond area, Bond size and bond temperature have got maximum effect on tensile properties23,24,, . Tensile modulus and shear increases with temperature and bond area23 . Fig. 17 shows the effect of temperature on strength of point bonded nonwovens Fig 17 : Effect of temperature on strength of point bonded nonwovens Shrinkage of fabric also increases with temperature25. Polypropylene melts over a range of temperatures and not at one specific value and this range decreases with increase in process speed. If bonding is done at low temperature, fibres can be teased out of the bonded portion. Breakage of nonwoven then occurs due to inter-fibrillar slippage. At high temperature beyond melting point, fibre damage occurs and the bonds are also brittle and weak at the periphery. With such material failure occurs at the periphery of bond spot by breakage of fibres. Mechanism of Breakage At high temperatures, a steep morphological gradient is found at the boundary of bonding area. Rapid loss in molecular orientation occurs from bridging fibres to bond edge, as a result of which most breakages occur in this region during breakage of nonwoven26. Fibres close to the bond periphery have a lower strength and elongation than in rest of unbonded area. Unequal strain distribution is therefore found in spot bonded nonwovens and as result only 10 – 15 % of fibre strength is realized. However, with polyethylene point bonded nonwovens fail due to disintegration of bond area rather than fibre breaking at perimeter19.Studies with polarised laser microspectroscopy showed that birefringence decreases to half its original value at the bonding point27,28 while it is unaltered in the non-bonded portion. Higher bonding temperature and longer bonding time result in larger differences in morphology at bonding region. While density at bond point is lower than that at bridge fibres, in polyester, opposite trend is found in polypropylene. Image acquisition equipment have been used by some workers to examine structural changes of thermal point bonded nonwovens during deformation29.This enables determination of orientation distribution, bond spot strain, unit bond repeat pattern strain as a function of macroscopic deformation.. Mode of application of deformation influences type of failure of material. Structural changes and microscopic deformation of bond spot are determined by initial orientation distribution function of the material17. Temperature of bonding has little influence on this. To get a clue to the low fibre strength utilization in thermal bonding, thermal bonding of a pair of fibres was carried out by passing them through a hot calendar roller30. This showed fibre strength reduces significantly upon bonding. The reduction is primarily because of heating and not because of pressure. This study confirmed that stronger bonds are formed with lower birefringence fibres even at lower temperatures. Morphology of Fibre Morphology of fibre has a marked influence on properties of thermal bonded material. Fibres with higher crystallinity and higher molecular orientation form weak and brittle bonds. This is the consequence of poor polymer flow compounded by fibrillation of fibres2,31 .Fibres with lower crystallinity and lower molecular orientation form stronger bonds and stronger fabrics. This is reinforced by a study where polypropylene fibres with varying molecular orientation were made on a pilot extrusion plant and made into point bonded nonwovens by Andrreassen et al;32Wei et al;25. Tensile strength of fabrics made from such fibres is found to be primarily influenced by molecular weight distribution and molecular orientation as determined by the conditions under which fibre is made primarily. Tensile strength of nonwoven increases with decreasing draw ratio and increase in extrusion temperature, though these conditions result in lower fibre strength. Fibres with low orientation or higher elongation also result in higher strength and flexural rigidity of nonwoven because of better load sharing33, . While increase of strength is desirable it should not lead to higher flexural rigidity as it makes the product stiff and board like. Changes in thickness due to bonding with poorly oriented fibre are lower25. T196 polypropylene fibre which has a sheath core structure with a lower refractive index at sheath, lower modulus and higher elongation than normal T101 performs better and result in higher strength in nonwoven. This is because T196 fibre produces stronger bonds at lower temperature, where strength degradation is minimum34. Computer simulation Computer model and simulation techniques have been used to predict stress strain relationship of point bonded polyester nonwovens. Bond layout, fabric density, orientation and curl of fibre and bond tensile properties were fed to the computer and a program used to determine stress strain curve during deformation35.Computer simulation program restricted to two dimensional nonwovens has been developed by Britton36 et al;. Mechanical properties of fibres laid in a web and held together by some means are first programmed. The changes in the fabric system, as it undergoes distortion, are computed. In another study an image simulation system is used to obtain an image of nonwoven using web density, fibre properties, unit cell size and bond properties and the effect of structural and process variables on the mechanical properties of nonwoven was examined37. Demirci38 et al; have developed a simulation technique to visualise a nonwoven composed of nonuniformly oriented fibres and bond points made from deformable fibres. Dynamic response of such a simulated material to process parameters bond points is determined using finite element software. Such studies have the merit that optimum process parameters can be obtained without extensive experimental investigations. Testing Conditions Effect of specimen length and width on strength of nonwovens in conventional tensile test and cyclic loading have been investigated by Hou et al;39. Longer specimen length generally result in higher modulus but sample width has insignificant effect. Cyclic loading reveals that the material is initially elastic but later on shows plastic behahiour. Applications Light to medium weight products in the range 30 – 100 gsm are made from calendar thermal bonded nonwovens. Main applications are 1. Interlining 2. Coverstock for diapers 3. Filter material 4. Sanitary napkins 5. Wipes 6. Tea bags 7. Trims for automobile 8. Geotextiles 9. Surgical gowns and caps 10. Face masks 11. Automobile headliner 12. Wipes 13. Shoe composites 14. Computer disc Belt Calendering In belt calendaring nonwoven batt is passed between a heated roller/drum and a heat resistant silicone coated blanket. Pressure is applied at the exit end by the guide roller on the hot roller. Nonwoven is in contact with the hot roller for longer time of 1- 10 sec and pressure is much lower, up to 9 N (9kp/cm), compared to hot calender bonding. The products are thicker, flexible, permeable and weaker than calender bonded material. By varying the speed, temperature and pressure special effects can be given. Patterned bonding can be obtained by using a patterned blanket. Heated drum from 0.5 to 2 m, width up to 6 m and speeds up to 100 m/min are available. Machines with 2 heated drums in tandem are available where fabric is bonded on one side in first drum and on other side in next. Embossed calender Bonding The web is passed though a pair of calendar rollers, one of which is male patterned metal roller and other matched female patterned felt roller. Many geometrical shapes including designs are used in embossed roller (Fig 18). Fig 18 : Embossed Calender Bonding Embossed calendar bonding is used for making baby wipes, coverstock, automobile products and special fabrics and clothings Through Hot Air Bonding Through hot air bonding is used to produce loftier products used as waddings in quilts, blankets and winter clothing and insulation material. The batt made out of normal and a small % of low melt fibre is passed over a slotted traversing lattice and is taken through a hot air chamber. The batt can also be guided between two belts through the chamber. Distance between the belts is adjustable as per the product specifications. Monocomponent low melt fibre or bicomponent fibres can also be used as binder fibre. Hot air at melting temperature of low melt fibre is drawn though the batt from the bottom as well as top as shown in Fig 19 through suction by a fan. Binder fibre melts and forms droplets on normal fibre and bonding takes place. Hot air chamber is in number of sections based on the capacity. Positive pressure from one side and negative pressure from opposite side is provided in each section to ensure even passage of hot air through the material. The direction of air supply is adjustable separately in each section. Air supply to the chamber is both from left and right to ensure uniform temperature. The same air after passage though batt is reheated and used for conserving energy. Temperature measuring units are placed in each chamber to facilitate monitoring and control. Temperature across the width is maintained with an accuracy of ± 1.5 % . Direct heating systems by gas or indirect heating by steam or thermal oil is used for heating air. The material then passes through a cooling chamber and a bulky product emerges. When products of lower width than the belt are made, systems are provided for lateral closing of air to minimize loss of air. Products made by this method are lofty, open, soft absorbent and porous. Width range is from 2 to 7m and material speed is up to 100 m/min. Fig 19: Through Hot air Bonding Fleissner uses a perforated drum, over which batt is taken round. Hot air is sucked from the end of drum through a radial fan through the perforations and passed through the batt to form a bonded material (Fig 20). Low melt fibre melts flows to the point of contact between fibres and forms the bond. After bonding the material becomes loftier and bulky. The material is soft, absorbent, resilient and has insulation value. Flow of air can be adjusted as per requirement. Working width up to 7m and fabric thickness up to 50 mm and speeds up to 1000 m/min are claimed. Fig 20 : Through Hot air bonding over perforated Drum Products made by through hot air bonding can be given cold calendering to make more compact products. Such products are softer, flexible and more extensible than hot calendered nonwovens. Struto Technology Struto technology consists of vertical lapper to make batt from card web. This is then taken through a hot air chamber for bonding. Vertical arrangement of fibres is achieved by this technology which improves loftiness. Thickness up to 40mm and production rates up to 100 m/min are possible.Jirsak40 et al ; found that compression properties of high lofts made from batts prepared from vertical lapper were significantly better than those made from conventional crosslapper. Further hollow polyester fibres bonded with newly developed binder fibres from Teijin using Struto technology have compression property close to polyurethane foam. Air laying Air laying method of batt preparation has also a 3 dimensional random arrangement of fibres. With through hot air bonding, these products will be loftier than card crosslapping batts. Fibre properties and process parameters Type of fibre, % of low melt fibre, temperature of hot air and duration of exposure are some of the important parameters that should be optimized to get most desirable high- loft. Desai and Balasubramanian41 investigated the influence of fibre crimp and type and process parameters on properties of high loft nonwovens from polyester fibres. Hollow and high crimp fibres result in thicker and bulkier product but loose their bulkiness most rapidly with cyclic loading compared to normal fibres. Hollow fibre high lofts are more compressible than normal fibre material and at the same time have higher insulation value. Increase of low melt fibre content and duration of bonding reduces specific compressibility and resilience (Fig 21 and 22). Thickness of the high loft is an important parameter that determines thermal resistance. Thermal insulation improves with thickness. The specific stress of such material increases with low melt fibre content, temperature and duration of exposure. The products are found to have good uniformity. If needle punching is done prior to through hot air bonding, a higher strength is obtained but the product becomes thinner and stiffer. Fig 21 Resilence vs low melt fibre% Fig 22 : Specific Compressibility vs low melt fibre % Hong42 et al found that PP through air bonded material has lower friction coefficient and withstands abrasion compared to polypropylene thermal bonded, tencel spunlace and cotton spunlace and therefore can be used as coverstock material for diapers. Its quick drying characteristic is another advantage. However consumers prefer cellulosic spunlace compared to surfactant treated polypropylene. Studies have been made to make nonwovens from kenaf by through hot air bonding43.Through hot air bonded kenaf has a soft feel and is lofty. If needle punching is done prior to hot air bonding, the product becomes stiffer. A statistical approach was employed to determine optimum bonding process parameters for getting minimum pore size of through hot air bonded nonwoven by Wang and Gong44. Lin45 et al; showed that bulk density of nonwoven has more influence on thermal conductivity and limiting oxygen index than thermal consolidation and processing methods. Nonwovens with good thermal insulation and low flammability can be made from FR polyester hollow fibre and low melt polyester fibre by thermal pressing of batt made out of them. Modelling thru air bonding process showed that the time needed to heat and melt the fibre decreases with increase in porosity of product and velocity of hot air46. Fibre orientation factor has maximum influence on bond formation. Bonding temperature and fibre diameter have a small influence on time required for bonding. Computational fluid dynamics modeling has been employed to examine the effect of fibre properties and process parameters on properties of bonded product47. Fibre orientation in the input material has a marked influence on the anisotropy of strength with through hot air bonded material48. These products have a good stability and elasticity. A model was developed to estimate initial tensile response of such products based on orientation averaging and Poisson’s ratio. Applications Medium weight products in the range 70 to 200 gsm are made by through hot air bonding. Common applications are 1. Wadding or padding used in winter clothing 2. Quilting 3. Filters 4. Seat cushions 5. Sound proofing material 6. Composites Thermal Bonding using powder Thermal bonding is also carried by using a low melt powder in place of fibre. Powders from polyethylene, low density polyamide, vinyl acetate/chloride copolymers are commonly used . The powder is added in the mixing stage in a controlled manner. It is also sprayed on the web or nonwoven fabric for making powder coated and mouldable products like automobile carpets (Fig 23). In airlaying fibre abd powder are dropped on the laying lattice.After application of powder the material passes through an infra red heating chamber and passes through a pair of calender rollers to form laminated products which can then be moulded to shape. Powder bonding can be made on one side (Fig 23) or both sides (Fig 24) Powder bonding has a wider range of temperature for bonding than low melt fibre. Fig 23 : Powder spray bonding Fig 24 : Double sided spray bonding An LDPE film or thermoplastic fabric can also be introduced over the material before calendaring. Powder bonder is suitable in open structures or mouldable materials and composites. The powder should withstand washing and chemical treatments and should not emit any fumes or odour during use. Applications 1. Interlings for collar and outer wear 2. Shoe inlays 3. Automobile carpets 4. Insulation material 5. Building material 6. Waddings and quilts Radiation Bonding Infra red lamps are used to provide the radiant energy to bond the Nonwovens with binder. While binder is melted there is no harmful effect on base fibre. Bonding takes place when material comes out of radiation chamber. This method has the advantage of low energy and capital costs. References 1. A.N. Desai and N.Balasubramanian, Critical factors affecting properties of thermal-bonded Nonwovenswith special reference to cellulosic fibres, Indian J Fibre and Textile Research, 1994, 19, p209 2. A.K. Rakshit, V.K.Patil and N.Balasubramanian, Studies on thermally bonded viscose Nonwovens, Resume of papers at 32nd Joint Technological conference, SITRA, 1991, June, p 215 3. S.C. Winchester and J.C. Whitewell, Studies on nonwovens Part I – A multivariable approach, Textile Research J, 1970, 40, p 458 4. S.Chand, G.S.Bhatt, J.E. Spruiell and S. Malkan, Role of fibre morphology in thermal bonding, International Nonwoven Journal, 2002 Fall, 11, p12 5. N. Fedorova, S. Verenich, and , B.Pourdeyhimi, . ‘Strength optimization of thermally bonded Nonwovens, J of Engineered fibres and Fabrics, 2007, 2, issue 13. 6, A.N.Desai, K.R.Bhatnagar and N.Balasubramanian, Influence of cotton content and area density on the properties of cotton thermal-bonded nonwovens, Indian J of fibre and Textile Research, 1994, 14, p 251 7. H. Rong, Structure and properties of cotton based biodegradable/compostable nonwovens, Ph.D. Thesis, University of Tennessee, Knoxville, 2004, May. 8. H.Rong and G.S.Bhatt, Preparation and properties of cotton-easter nonwovens, International Nonwovens J, 20003 Summer, 12, p53 9. X. Gao, R.E. 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