Indian Journal of Textile Research

Vol. 11, December 1986, pp.208-214

Radial Packing Density of Rotor- and Ring-spun Yarns

S M ISHTIAQUE

Department of Textile Technology, Indian Institute of Technology, New Delhi 110016, India

Received 7 April 1986; accepted 28 April 1986

The radial packing density of rotor-spun yarn was measured on the basis of the theory reported earlier by the author [J ndian

J Text Res, 11 (1986) 26]. The effect of yarn twist and count on the fibre packing density of rotor-spun yarn in comparison with

tbat of ring-spun yarn was studied. The packing density of both the yarns over the yarn cross-section was not uniform and

increased with yarn twist. In rotor yarn it was higher near the yarn axis and less towards the outer surface of yarn in comparison

with that of ring-spun yarn. The packing density of rotor-spun yarn was less than that of ring-spun yarn, which is one of the

causes of the lesser strength of rotor-spun yarn. For the same material and twist coefficient, the packing density of coarse yarnwas less than that of finer yarn.

Keywords: Radial packing density, Ring yarn, Rotor yarn, Yarn twist

I Introduction

Irregularity of structure in a yarn can to a greatextent influence the properties and thus the quality ofthe yarn. Although the longitudinal irregularity ismore 'important in this respect, irregularity in thedistribution of fibre in yarn cross-section can alsosignificantly affect yarn properties.

Although several authors have suggested that thepacking density of fibres in yarn is likely to be non­uniform, none have measured the extent of non­unifo~mity in rotor-spun yarn and compared it withthat of ring-spun yarn. Morton 1 indicated that, forviscose rayon and acetate staple-fibre yarns of viscosecounts and twist, the density of fibre packing over theyarn cross-section was not uniform. Riding andTreloar2 and RidingJ, using Morton's techniques toinvestigate the structure of continuous filament yarns,showed that packing density was not uniform acrossthe section but decreased from a point placed two­thirds of the yarn radius from the axis of yarn. Ridingand Treloar2, however, proposed that this decrease inpacking density at yarn surface for continuous filamentyarns arose primarily from the non-circularity of yarnperiphery rather than true reduction in density. Someinformation about packing of fibres in worsted yarnwas also given by Hickie and Chaikin4•

This paper describes the measurement of fibrepacking density of yarn on the basis of the theory dealtwith'elsewhere5• This paper also discusses the effect ofyarn twist and count on fibre packing density of rotor­spun yarn in comparison with that of ring-spun yarn.

2 Materials and Methods2.1 Materials

Details offibres used and the processing parametersare given in Table I. The second draw frame passage

208

sliver of 3.2 ktex from polyester and viscose fibres was

used. Yarns were spun on BD 200 and Autocoro unitsat a rotor speed of 60,000 rpm and an opening rollerspeed of 6,000 rpm. Seven rotor and five ring-spun

yarns were produced from the same material underidentical conditions for a comparative study.

2.2 Sample Preparation

The cross-section of yarn was studied microscopi­

cally, the technique of soft cross-section being used. Acoating of polyvinyl acetate was applied to the yarn toprevent any deformation of the fibres while cutting theyarn cross-section on a microtome. After drying, thesample was embedded into a mixture of molten wax andparaffin by pouring the mixture into a receptacle in themiddle of which the yarn had been positioned. Thethickness of the cross-section was kept at 25 J.lm, which

was found by experience to give a satisfactory viewduring observation under a microscope. Xylene wasused as the immersion liquid, as it has good opticalproperties and also dissolves the remaining wax in the

sample. Some cross-sections of yarns are shown in Fig.I. For each yarn, 60 cross-sections were cut forobservation.

2.3 Digitization of Coordinates of Fibres inYarn Cross-section

Normally, a transparent Perspex template, on whichare engraved concentric circles, is used for measuringthe area of fibres in a yarn cross-section. The templateis centred visually over the photograph of the yarncross-section and fixed in position. The fibre area ineach successive zDne is determined by means of aplanimeter. The method depends absolutely on visualcentring of the template over the photographs. Toavoid this problem a new device was used to calculatethe area of fibres in different zones.

"' I

ISHTIAQUE:RADIAL PACKING DENSITY OF ROTOR- AND RING- SPUN YARNS

Table I-Table I-Characteristics of Rotor-spun Yarn

Yarn2

34567

Fibre parameter

ViscoseDiolenTesilTesilDiolenViscoseViscose

Length (I), mm

40323838323434

Fineness (1), tex

0.160.170.170.170.170.1450.145

Specific density (p), kg m -31500136013601360136015001500

Fibre diam (d), Jim

1.712.612.612.612.611.111.1

Yarn parameler Fineness (1), tex

19.220.121.520.535.534.637.7

Twist (Z), m-1

7848127471150784561407

Twist coefticient (a), m -I ktexll356.260.057.786.284.759.645.7

Analysis Width of class interval (h), mm

0.0080.0080.0080.0080.0100.0100.010

Theoretical area of fibres in yarn cross-section (5,), mm1

0.012800.014780.015780.015090.026130.023070.02508

Experimental area of fibres in yarn cross- section (5.), mm1

0.010780.012240.012980.013140.023040.019880.02184

Radius of yarn (r), mm

0.0910.0950.1000.09550.13250.12600.1370

Packing density of yarn, II

0.4140.4310.4130.4580.4170.3980.370

20

5 =2TCh2 I (i-~)flie i= 1

Then the experimental area of fibres in yarn cross­

section (5e) is:

The ratio of the area of fibres in class-interval to the

total area of class-interval expresses the packingdensity of that class-interval. It is therefore possible todetermine the packing density of fibres in all class­

intervals and the required picture of radial packingdensity of individual cross-sections of yarn can beobtained hs shown in Fig. 2.

where h is the width of class-interval; and Pi' thepacking density in i-th class-interval.

2.4 Radial Packing Density of Yarn

If the total area of fibres in yarn cross-section is

divided into different class-intervals we get the area offibres in individual class-intervals as described earlier5.

The area of i-th class interval can be expressed as:

To rtxord the coordinate of fibres in the cross­

section, a commercially used device Digimet,developed in SVUT, Czechoslovakia, was used. Thedevice consists of a digital frame, central unit and atape-punching device. With this, it is possible to recordthe coordinates of fibres directly from the projectedcross-sections of yarn from the screeen of a digitalframe and transfer the message to the central unit,which indicates the X and Ycoordinates on tape. Thusobtained centre coordinates of fibres of any particularcross-section are used for measuring the centre of massof yarn cross-section and finally the area of fibres indifferent zones.

The radial packing density of yarn was determined 3 Results and Discussionon the basis of the theory described by the present The radial packing of yarn demonstrates theauthoru. All calculations were performed on a Hewlett - mechanics of yarn building. Figs 3 and 4 show thatPackard 9810 A table calculator with recorder. This curves of radial packing density of all measured yarns

program is capable of giving the behaviour of radial have good resemblance. The fibre packing density ispacking density of individual yarn cross-sections. The not uniform across the yarn cross-section, nor it isfinal results were obtained as tables and graphs. maximum at yarn core. This confirms the law of non­

linear compressibility offibres. For all measured yarnsthe maximum packing density occurs at a pointbetween 1/4th and 1/3rd of yarn radius from the yarnaxis. The results indicate that yarns of different countsand twist range are slightly hollow at their centre. Thisphenomenon was also observed by Hickie and Chaikinin the case of wool worsted yarns and by Neckar7 in thecase of ring-spun yarn.

In ring-spinning, fibres are transferretl from a thinribbon to a roughly circular shape and usually fibresare gripped at the nip of the front roller. The outermostfibres are strained in tension, and unless there is

excessive yarn tension, the core is subject to

2TC r,h

and the area occupied by fibres in this class-intervalcan be expressed as:

2TCh2 (i - 1) {Ii

209

INDIAN J. TEXT. RES., VOL. 11, DECEMBER 1986

Fig. l-Cross-sections of rotor polyester yarns ( x 256) [ta) tex. 21.46; tWI't. 747 m - I

and (b) tex. 35.5: twist, 7~4 m - I]

compression. In one case, the stress is relieved byshortening the path length of fibre in yarn and in theother case by lengthening it. The net result is that fibresdepart from a purely helical conformation to give aninterlocking structure. The insertion of twist will causethese fibres to be longitudinally strained, so that therewill be competition among fibres to reach the positionof minimum strain at the centre or axis of yarn. The

210

extent of this movement will necessarily be limited by

the number or physical states of fibres competing ..Thus, because of steric hindrance it is highly likely thatthe yarn would have a slightly hollow centre which givesless packing density near yarn axis as shown in Fig. 4.

Of all the rotor yarn properties studied, twist has avery significant effect, attributed mainly to thestructure of the yarn. A comparison of the structure of

ISHTIAQUE:RADIAL PACKING DENSITY OF ROTOR- AND RING- SPUN YARNS

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211

INDIAN J. TEXT. RES., VOL. 11, DECEMBER 1986

1·0

Rotor-spun yarn"Yarn NO.1

0·8

0·6

0·4

0·2

o0·04 ().oe

o NO.2

A No.3

o NO.4

.No.5

ANo.6

• NO.7

Ring-spun yarn+ Yarn NO.1

0·12 0·16

0·8

r,mmFig. 3-Radial packing density of rotor-spun yarns

0·16

--..-:~

0·112Q.oe

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Fig. 4~ Radial packing dcm,ilY 01 nng-,pun yarn,

Q.04o

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0·6.o--~_~ + Yarn NO.1

I~

X No.2o NO.3t:. NO.4\ \\

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ISHTIAQUE:RADIAL PACKING DENSITY OF ROTOR- AND RING- SPUN YARNS

Table 2-Characteristics of Ring-spun Yarn

Yarn No.2

345

Yarn parameter Fineness (1), tex19.619.619.820.235.1

Twist (Z), m-1

6697947301080736

Twist coefficient (a), m-J ktexl{J48.657.853.580.279.0

AnalysisWidth of class interval (h), mm

0.0080.0080.0080.0080.010

Theoretical area of cross-section (51)' mml

0.013070.014410.014560.014850.02581

Experimental area of fibres in yarn cross-section (5), mml

0.012550.013790.013820.014450.02529

Radius of yarn (r), mm

0.0870.0900.0910.0880.128

Packing density of yarn, Jl

0.5270.5420.5310.5940.491

Yarn materials (designated as 1-5) and their fibre properties are same as in Table I

ring-spun yarn wjth rotor yarn shows that the fibreforming the yarn core has less twist in ring-spun yarnthan surface fibres and the yarns are pressed tightlytogether by wrapping of outer fibres. But rotor yarnspossess a single helical structure, and the fibres aredistributed from the inner core to the outer core

without fibre migration occurring; the fibres aretwisted more uniformly as a result of which the innercore receives more compression. The inner zone issubjected to a variety of fibre tensions in spinning andthe migrated structure will be somewhat similar to thatof ring-spun yarn. It can be shown from the curve ofradial packing density that the inner zone has lesspacking density. The intermediate zone may extend tothe solid boundary of the yarn, and in this zone, thepacking density first increases and later linearlydecreases with radius. Also, because of fibre slip, thecore will be twisted differently from the intermediatezone. The outer zone will contain hairs and wrapperfibres which have been picked up on the collectingsurface and in this region, packing density is less incomparison with that of ring-spun yarn.

It was reported that the rotor twist is always greatelthan that calculated from rotor speed and withdrawal

speed of yarn. Sonill observed that machine twist staysonly on yarn surface and that towards yarn axis itincreases; at Ij3rd of yarn radius the twist is 30-40 %more than machine twist. Owing to a higher twist inthis zone, packing density is also higher (Fig. 3). Thevariation of twist in yarn body can also be explained by

the variation of packing density in yarn cross-section.The effect of twist on yarn packing density (Tables I

and 2) shows that as yarn twist increases, the packingdensity of yarn also increases. The increase in packingdensity of yarn with twist can be explained as due to:(i) fibre inclination on yarn axis; and lii) effect oftransverse pressure with twist.

The first case can be observed only because of a

higher inclination of fibres on yarn axis. It is wel1known that inclination of fibres increases with twist.

Owing to greater inclination, the total area of fibres inyarn cross-section wil1 be more. Hence the greaterpacking density of yarn.

The second case can be explained on the basis offibre compactness owing to increase of twist. Sotransverse forces will increase towards the inner layerwhich gives a more compact structure. Owing toincrease of twist, the diameter of yarn wil1 alsodecrease. So both the factors reinforce each other to

increase the packing density of yarn.Figs 5-9 show the characteristic difference in the

behaviour of radial packing density of ring- and rotor­spun yarns. The packing density of ring-spun yarn ismore towards yarn surface and less towards yarn axisin comparison with that of rotor yarn. This againconfirms that the rotor yarn possesses more twist in theintermediate zone and, as a result, the packing densityis more in this zone; yarn surface has less twist and amore open structure owing to less transverse force onsurface and gives less packing density in this zone incomparison with that of ring-spun yarn.

,·0

0·8

0-'2

Fig. 5-Characteristic difference in behaviour of radial packingdensity of yarn No. I

213

INDIAN J. TEXT. RES., VOL. 11, DECEMBER 1986

"I

I

0'20

o Ring-spun YC1rn

Q Rotor- spun yarn

r,mm

10-05

0.__

0,'

0·6

0·2

0-16,

r,mm

100'

0.8[ lI

"l~ o Ring-spun yarn

I

C Rotor-spun yarn

I

0·'pi

0·2

0"

0"

r,mm

Fig. 9-Characteristic difference in behaviour of radial packing

density of YlJrn No.5

Rotor-spun yarns 4 and 5' of polyester fibres andyarns I and 6 of viscose fibres were spun withapproximately the same twist coefficient. As shown in

Table I, yarn 5 has less packing density than yarn 4 andyarn 6 has less packing density than yarn I. The samepattern is also found in ring-spun yarns 4 and 5 (Table2). Hence it can be concluded that, for the same twist

coefficient and material, coarse yarn has less packingdensity than finer yarn. This can be explained on thebasis of relation between packing density of yarn andits radius.4 Conclusions

4.1 The packing density of rotor-spun yarn is lessthan that of ring-spun yarn, which is one of the causesof lesser strength of rotor-spun yarn.

4.2 The packing density of rotor-spun yarn is morenear the yarn axis and less towards the outer surface of

yarn in comparison with that of ring-spun yarn.4.3 Yarns of different counts and twists have a

slightly less packing density near the yarn axis.4.4 The maximum packing density occurs at a point

approximately one-quarter to one-third of yarn radiusfrom yarn axis.

4.5 The packing density over the yarn cross-section isnot uniform.

4.6 The packing density of yarn increases with yarntwist.

4.7 For the same material and twist coefficient,

coarse yarn has less packing density than finer yarn.References

I Morton W E, Text Res J, 26 (1965) 325.

2 Riding G and Treloar L R G, Text Res J, 20 (1959) 366.3 Riding G. J Text I nst. 50 (1959) 425.

4 Hickie T S and Chaikin M, J Text Inst, 65 (1974) 433.

5 Ishtiaque S M, Indian J Text Res, II (1986) 26.6 Ishtiaque S M, Ph.D. thesis, University of Mechanical and Textile

Engineering. Liberec, Czechoslovakia, 1983.

7 NeckatB, Texi ResJ. 46 (1976) 545.

8 Soni M K, Infernal sfructure of yarn, Ph.D. thesis, University of

Mechanical and Textile Engineering, Liberec, Czechoslo­vakia, i979.

0'110·11

o R~yam

D Rotor-1oplI'I var"

',"""0004

o

._ 0-4I'l.

o Ring-.pun yarn

D Rotor-spun yarn

0.•

,ir08

0·2

Fig. 8-Characteristic difference in behaviour of radial packingdensity of yarn No.4

As given in Tables I and 2, the packing density ofrotor-spun yarn is less than that of ring-spun yarn in allthe observed cases. The reason is the more openstructure, which does not give an interlocked structureas found in the case of ring-spun yarn because ofgreater migration. So the lesser packing density ofrotor-spun yarn may be one of the causes of lesserstrength of such yarn in comparison with that of ring­spun yarn.

f'i

Fig. 7-Characteristic difference in behaviour of radial packingdensity of yarn No.3

Fig. 6-Characteristic difference in behaviour of radial packing

density of yarn No.2

214