Indian Journal of Fibre & Texti le Research Vol. 3 1 , June 2006, pp. 286-292

Influence of sheath structure on twist and diameter of dref-III polyester-wool blended friction-spun yam

S K Sinha" Department of Textile Technology, National Institute of Technology, lalandhar 144 0 I I , India

and R Chattopadhyay

Department of Textile Technology, Indian Institute of Technology, New Delhi 1 1 0 0 1 6, India

Received 31 January 2005; accepted 31 March 2005

Dref-III polyester-wool blended friction yams having different sheath composi tions and structures were spun successful ly on a friction spinning machine keepi ng polyester in core. The sheath composition and structure were changed by vary i ng core content and selectively placing polyester and wool i n different layers of sheath. The location of the constituent fibres i n sheath was found to affect both twist and yarn d iameter. Al l wool sheath resulted in a bulky yarn with low twist.

Keywords: Core-sheath ratio, Dref-Ill yam, Friction spinning, Polyester, Wool

IPC Code: Int. CI .8 D02G3/00

1 Introduction Twist is a mean to hold the fibre assembly together

in spun yarns. There has been a great deal of research on twisting process and its influence on yarn structure and properties in friction spinning. 14 The mechanism of twist insertion l ies in the creation of torque field between friction drum and fibres. In dref-III spinning system, core-sheath type yarns are produced using two independent streams of fibres. The core fibres are supplied to the spinning zone through a conventional apron drafting system, whereas the sheath fibres are carried by a pair of opening rollers and through a fibre transport channel onto the friction drums. The two drums rotate in the same direction so that m the nip the drum surfaces move in opposite directions creating a torque field on the fibre assembly lying between them.5-8 It causes the core to be false twisted and the fibres delivered by the opening rollers to be wrapped or twisted around the core.9

There are many factors that affect yarn twist. The "twist insertion has teen reported to be influenced by the factors, such as humidity, fibre type, fibre fineness, yarn diameter, friction ratio (ratio between drum speed and yarn delivery rate), fibre sleeve diameter, fibre depositing angle on the drum surface,

"To whom al l the correspondence should be addressed. E-mail : sinhask@ni.l:i.ac. in

etc.6-7• 9· 1 1 Merati and Okamural2 observed that a wide transport channel outlet gives higher twist efficiency, while a narrow channel outlet fai ls to provide enough torque to twist the yarn effectively. The twist and its distribution in friction-spun yarn have been found to be dependent on the process parameters, like core­sheath ratio and the compatibility of the core and sheath components. I . 6-7. 1 0· 1 1 . 13 Sengupta et at.3 studied the effect of core-sheath ratio and the amount of spin finish on dref-III yarn properties. They reported that the twist decreases with the increase i n sheath percentage, irrespective of amount of spin finish applied. The properties of yarn were also stated to be dependent on the frictional properties of core and sheath components. Lord and Ruse4 studied the twist distribution in dref-I1I yarn and reported an intermittent leakage of twist from the yarn tai l . They observed a sort of sl ip/stick phenomenon of the yarn tail , which is believed to cause variation in twist especially in the sheath.

The rotational speed of the sheath fibres in the yarn formation zone essentially depends upon the rotational speed of the drums and its transmission to the sheath fibre assembly. As a consequence the twist in the yarn is determined by the ratio of the friction drum speed and delivery rate. A certain amount of slippage is unavoidable between the rotating sleeve

SINHA & C HATTOPADHYAY: DREF-III POLYESTER-WOOL B LENDED FRICTION-SPUN YARN 287

forming sheath and the friction drums at the point of twist insertion since the fibres are not mechanically gripped firmly between the drums but rather pressed against the drums pneumatically . This slippage would depend upon the coefficient of friction between the sleeve and the spinning drums, and the transverse or normal force with which the sleeve is pressed against the drums. This normal force will be obviously influenced by the magnitude of suction pressure acting through drums.7, '2, ' 5 I t has been shown that the magnitude of suction air pressure affects the twist efficiency and structure of the friction-spun yarn.7. 1 2

In dref-I I I yarn, the sheath is laid in the form of a series of concentric layers one after another around the core. It usually comprises fibres coming from five independent s l ivers, forming five layers. There exists a unique possibility to selectively place the fibres originating from slivers at different locations within the sheath by simply changing the relative position of slivers at the feed point in drafting unit-I I . The fibre constituents of the individual slivers could also be changed to get some novelty effects. Twist is the most dominant yarn structural parameter that affects yarn properties such as temi le, bending, torsional, bulk, etc. The full potential of twisting in friction spinning is also not realized due to possible s l ippage of the fibre assembly on the friction drum as stated earlier. The aim of the present investigation is to study primarily the effect of sheath structure in terms of location of its components and their proportion on twist and diameter of polyester-wool blended yarn.

2 Materials and Methods Polyester fibre of staple length 48 mm and fineness

1 .2 denier ( I I micron) was processed on Lakshmi Rieter C 1 /3 card and RSB 85 1 draw frame to produce a sliver of 3 .45 g/m.

Two passages of draw frame were given to the sliver. Wool sliver having 54 mm mean length and 25 micron fineness was procured from industry. The wool sliver was drawn once on a draw frame to bring its linear density to 2 . 1 8 g/m. Appropriate number of slivers was creeled on the machine. The yarn core was made of polyester fibres only by feeding the polyester s li'/er through the drafting unit-I. Wool and polyester s livers were selectively positioned in the drafting unit-II to create three different sheath compositions. Figure 1 is a schematic representation of the yarn formation zone where arrival position of sheath fibres from five different s livers on it has been shown; P and W indicate polyester and wool fibres

p Delivery

---... End

Structure A Core

Sliver feed from drafting unit - II

P W Delivery

---•• End

Drum nip Core

Structure B

Sliver feed from drafting unit - II

Drum nip Core

Structure C

___ ... Delivery

End

Fig. I-Schematic representation of yarn tail and direction of sheath fi bre approach

respectively. Based on the composition of sheath and its relative position, the yarn structure was classified into following three categories:

( i) Sheath containing polyester and wool i n the i nner and outer layer respectively is designated as structure A .

( i i ) Sheath containing polyester, wool and polyester in alternate layers is designated as structure B .

( i i i ) All wool sheath i s designated as structure C. Eighty-one yarn samples of 33 tex were spun by

varying delivery rate, friction drum speed and core ­sheath ratios. The delivery speeds used were 1 20, 1 50, and 1 80 rnImin; friction drum speeds were 3500, 4000 and 4500 rpm; and core - sheath ratios were 50: 50, 60: 40 and 70: 30. The details of yarn count, core -sheath ratio, over al l blend ratios and percentage of fibres in different sheath layers are stated in Table 1 .

288 INDIAN J. FIBRE TEXT. RES., JUNE 2006

The change in core sheath - ratio resulted in nine different yarn structures on the basis of location of constituent fibres in the sheath and their weight proportions. A schematic representation of the yarn cross-sections is shown in Fig. 2.

. 2.1 Measurement of Yarn Twist

The twist measurement was carried out following the twist-to-break principle. A yarn of 254 mm length

was gripped between the jaws of the twist tester and twisted in the direction of original twist of the yarn. Twisting was continued till the yarn broke and the number of turns required to break the yarn (N I ) was noted. The test was repeated with twisting in the opposite direction to that of the original twist and again the number of turns required to break the yarn (N2) was noted. The yarn twist was then calculated by using the following relationship '6:

Table I-Structure detai Is of yarns

Structure Yarn Core count Layer I

tex (Innermost)

33 P,o P IO 33 P,o PIO 33 P,o WIO

33 P60 Pg 33 P60 Pg 33 P60 Wg

33 P70 P6 33 P70 P6 33 P70 W6

Sheath Structure Core sheath Layer 2 Layer 3 Layer 4 Layer 5 ratio

PIO P IO W IO W IO 50:50 WIO WIO WIO P IO WIO WIO WIO W IO

Pg Pg Wg Wg 60:40 Wg Wg Wg Pg Wg Wg Wg Wg

P6 P6 W6 W6 70:30 W6 W6 W6 P6 W6 W6 W6 W6

Overall polyester/ wool ratio

80:20 70:30 50:50

84: 1 6 76:24 60:40

88: 1 2 82: 1 8 70:30

P - Polyester fibre and W - Wool fibre. Subscript numbers i ndicate weight % of corresponding fihres in the yarn at their designated locations.

Structure A Structure B Structure C Core sheath ratio

50: 50

60: 40

70: 30

Fig. 2-Pictorial representation of the cross-section of yarn structures [Dark and l ight bands represent wool and polyester fibres respectively. Figures within the band indicate over al l polyester-wool ratio in the yarn cross section]

SINHA & CHATTOPADHYAY: DREF-III POLYESTER-WOOL B LENDED FRICTION-SPUN YARN 289

Twist (turns / m) = (N2 - N J ) x 39.37.

The number of observations per sample was between 35 and 45 so as to have 5% error of estimation. The average twist was then calculated.

2.2 Measurement of Yarn Diameters

The yarn was carefully removed from the package and wound on a board under constant low tension. The wound board was brought under a microscope and the diameter readings were taken. At least 1 00 readings were taken per sample at different places on the yarn.

3 Results and Discussion 3.1 Twist 3. 1 .1 Illfluellce of Process Parameters

Table 2 shows the variation in twist with the change in different parameters. It is observed that the twist in the three yarn structures A, B, and C are quite different from each other irrespective of drum speed, delivery speed and core sheath ratio . The twist in structure A is always maximum, whereas in structure C it is minimum. Twist in structure B shows intermediate values. The range of twist for all the drum and delivery speed combinations chosen was 589-826 turns/m for structure A, 368-507 turns/m for structure B and 1 8 1 to 339 turns/m for structure C.

For any particular structure, with the i ncrease in core-sheath ratio from 50 : 50 to 60 : 40 the twist does not decrease much. But a further increase in core­sheath ratio to 70 : 30 ratio causes a significant

reduction in twist. With the increase in drum speed the twist usually increases. However, this rising tendency for structure C is less pronounced. Twist also decreases with the increase in delivery speed for all the three structures.

In dref-3 yarn, the core fibres fed from drafting unit- l are false twisted by the two drums and the fibres fed by the opening rollers wrap around this false twisted core, forming the sheath. Theoretically, the core should show no twist but part of the core twist remains trapped 9 since during untwisting action of the core the wrapped fibres do not allow the core to rotate freely and loose twist completely. The measured twist is, therefore, an average of total twist present in the yarn cross-section.

The wrapping action of sheath fibres is the result of a torque field created on them by the two drums. The sheath fibres as a result rotate around the core and get wrapped. The sheath fibres originate from five different slivers fed through drafting unit-2. Since a significant lateral movement of fibres within the transport channel is not expected, one can expect a one to one correspondence between the position of the sliver in the feed and appearance of its constituent fibres around the yarn core. The fibres from the extreme left feed sliver (positioned towards the entry end of the drum) will form the first sheath layer adjacent to the core and as soon as this segment moves forward a second layer consisting of fibres from the next sliver is deposited on the top of it. The yarn segment keeps on receiving fibres in the form of layer after layer as it moves towards the exit end.

Table 2-Yarn twist

Structure Yarn twist. turns/m 3500a 4000a 4500a

1 20b 1 50b 1 80b 1 20b 1 50b 1 80b 1 20b 1 50b 1 80b

For 50:50 core sheath ratio AI 759 707 667 80 1 74 1 673 826 759 701 B I 470 432 395 49 1 44 1 409 507 463 430 C I 3 1 5 290 27 1 328 301 279 339 3 1 1 289

For 60:40 core sheath ratio A2 745 690 653 797 730 669 8 1 1 74 1 690 B2 46 1 428 389 488 437 404 50 1 460 425 C2 27 1 243 222 289 255 23 1 30 1 268 243

For 70:30 core sheath ratio A3 70 1 645 589 73 1 653 60 1 738 697 64 1 B3 4 1 5 39 1 368 459 4 1 3 379 469 438 400 C3 23 1 2 1 1 1 8 1 24 1 222 1 98 249 229 209

a Friction drum speed in rpm. b Del ivery speed in mlmin.

290 INDIAN J. FIBRE TEXT. RES., JUNE 2006

Fibres from extreme right hand side feed s liver form the last sheath layer. The profile of the yarn segment in the yarn formation zone (hereafter called yarn tail) is, therefore, tapered in nature.

The outer surface of the yarn tail consists of either polyester or wool fibre except for structure C, where it is only wool . In structure type A, the first three layers adjacent to the core consist of polyester fibres and subsequent two outside layers are made of wool. In structure type B, the first layer adjacent to the core and the outermost layer are made of polyester and the intermediate wool layers are sandwiched in between them. In structure type C, the entire sheath is made of wool fibres only.

The actual twist to be received by the yarn would depend upon the average diameter of the yarn segment between the drums i .e. yarn tai l and the friction between the outer surface of yarn and friction drum. This friction, in turn, depends upon (i) the coefficient of friction between the fibres in the outer layer of the yarn tail & the friction drum and (i i) the normal force acting on it as a result of pressure difference due to suction acting through the perforations of the friction drums. The relationship of pressure difference (flp) and airflow (F) across a fibre mass packed into a consiant volume chamber as reported 1 7 earlier is given below:

flp k1Js2m2 L2y = ---' __ --co.... F (yAL - m)

where k is the factor of form; 11, the viscosity of air; s, the mean specific surface of fibres; m, the total mass of fibres; y, the fibre density ; L, the length of the section of the chamber in which fibres are packed; and A, the area of the section.

When all other parameters are constant, the pressure difference (flp) would be proportional to the total specific surface area of fibres. The specific surface area, in turn, is inversely related to fibre diameter. Finer the fibre, the lesser is the diameter and more would be the specific surface area. In the present wool-polyester blended yarn, the polyester i s finer than wool. Hence, more is the proportion of polyester, more will be the specific surface area of the fibre agglomerates on the friction drum and hence more would be the pressure difference.

For structure A, the proportion of polyester is above 80% and hence the total specific surface area of the fibres in the yarn tail wil l be much h igher i n comparison to other two structures. As a result, flp

(the normal force on the yarn tail against the friction drum) would be more. Besides, 3/5th of the yarn tail length is surrounded by polyester while i t i s undergoing twisting between the drums. As a result, the tail would be efficiently twisted by the friction drum and results in a higher value of twist. In the case of structure C, the overall proportion of wool in the yarn is highest (30, 40 and 50%). Wool is crimpy and resi lient fibre and its average fineness (25 micron) i s higher than polyester ( 1 1 micron). Hence, the pressure difference (flp), across the yarn tail during spinning, would be much less and so would be the normal force. The entire yarn tai l is surrounded by wool only as all the feed slivers from drafting unit -2 are wool only . The absolute bending rigidity for wool i s greater than that for polyester as it i s coarser. Hence, the wrapping of wool around the yarn core would be more difficult. Therefore, the twisting will be most inefficient, resulting in minimum value of twist. For structure B, the overall polyester proportion is in between 70% and 82% and hence twist is higher than that expected in structure C.

As the core content increases, the sheath fibres are made to wrap over a larger core diameter. As a result, number of wraps or twist around the core decreases. When delivery speed is raised, keeping drum speed constant, twist declines as for every single rotation of the drum, i .e. the twisting element, more yarn is withdrawn. Similarly, when drum speed is increased, keeping delivery constant, the twist rises as more rotation of the drum takes place for same yarn delivery.

One interesting fact to observe is the twist i n structures B I and C3 having an overall polyester -wool ratio 70:30 and A I and B3 having overall polyester- wool ratio around 80:20 in the yarn cross­section. Hence, the twist difference cannot be entirely ascribed to the difference in proportion of polyester and wool in the yarn. Structure C3 shows much less twist than structure B I . The range of twist for B I and C3 structures is 395-507 turns/m and 1 8 1 -249 turns/m respectively. S ince i n C3 structure the entire sheath consists of wool fibre only, wrapping by successive wool layers will be difficult and twist wil l be low. Simi larly for structures A I and B3• the twist lies in the range of 826-667 turns/m and 469-368 turns/m respectively. In A I structure, the last two layers are made of wool and the three layers adjacent to the core (equivalent to 30% in mass) are made of polyester whereas in structure B3. three wool layers (equivalent to 1 8% in mass) are sandwiched between two

SINHA & CHA lTOPADHY A Y: DREF-III POLYESTER-WOOL BLENDED FRICTION-SPUN YARN 29 1

polyester fibre layers (equivalent to 6% each i n mass). The wrapping action by 30% polyester fibres being very intimate in A I structure, more twist remains trapped in the yarn. For B3 structure, the polyester proportion in sheath being only 1 2% and that too being distributed in equal proportion (6% each) in the first and last l ayers, the trapping of core fibre twist is not significant. The false twisted core looses twist as i t emerges from the twisting zone. Hence, the overall twist in the yarn becomes less.

3. 1.2 Illfluellce of Frictioll Ratio

It will be interesting to know how the friction ratios affect twist. Friction ratio is defined as the ratio of surface speed of friction drum and delivery speed. In a way i t represents machine twist, as surface speed of the drum is responsible in rotating the yarn end. The change in delivery speed and drum speed resul ts in different ratios ranging between 2 .7 and 5 .3 . It i s observed from Fig. 3 that with the increase in friction ratio the twist also increases in all the structures and A type structure shows highest twist followed by structure B and structure C.

A higher friction ratio indicates a higher drum speed relative to delivery and hence a higher twist i s expected. The response of different structures to twisting action has already been explai ned.

3.2 Yarn Diameter

It is observed from the Table 3 that the yarn diameter changes with the change in sheath composition and structure. The diameter is minimum for structure A and maximum for structure C for all core-sheath ratios, irrespective of drum and delivery

speeds. Yam diameter of structure B takes an intermediate value. The diameter increases marginally with the increase in delivery speed for all the core sheath ratios. For a given sheath structure i t remains more or less same, irrespective of drum speed.

Wool fibres being stiffer (due to being coarser) more crimpy and resilient are difficult to wrap around the core tightly. As a result, they exert lesser transverse pressure on core fibres. Hence, whenever they are placed at the sheath (as in structure C) the overall consolidation of the yarn is less and hence the diameter becomes maximum. Structure A has minimum proportion of wool that l ies in the outermost layer of the sheath (20, 1 6 and 1 2%). However, three l ayers of sheath adjacent to the core consist of polyester fibres and their total amount is 30%, 24% and 1 8% for three different structures Aj , A2 and A3. S ince polyester gets easily intimately wrapped around core, the sufficient consolidation of

2.7 3.14 3.3 3.5 3.8 4.12 4.24 4.71 5.3 Friction ratio

Fig. }-Effect of friction ratio on yarn twist

__ A1 __ B1 __ C1 . ___ A2 -.rB2 __ C2 -+- A3 - B3 - C3

Table 3 -Diameter of yarn

Structure Yarn diameter , mm 3500" 4000" 4500"

1 206 1 506 1 80" 1206 1 506 1 806 1 206 1 50" 1 806

For 50:50 core sheath ratio A) 0.3 1 0.32 0.34 0.3 1 0.32 0.33 0.3 1 0.32 0.33 B ) 0.36 0.37 0.37 0.36 0.36 0.37 0.35 0.36 0.37 C ) 0.4 1 0.42 0.44 0.4 1 0.42 0.43 0.40 0.42 0.42

For 60:40 core sheath ratio A2 0.32 0.33 0.35 0.32 0.33 0.34 0.3 1 0.32 0.33 B2 0.36 0.37 0.38 0.36 0.37 0.38 0.36 0.37 0.37 C2 0.38 0.43 0.44 O.4 l 0.42 0.44 0.4 1 0.42 0.43

For 70:30 core sheath ratio A3 0.33 0.34 0.35 0.33 0.34 0.35 0.33 0.34 0.34 B3 0.38 0.38 0.39 0.37 0.38 0.39 0.37 0.38 0.38 C3 0.44 0.44 0.48 0.43 0.42 0.47 0.43 0.44 0.46

" Friction drum speed in rpm. b Del ivery speed in m/min.

292 I NDIAN 1. FIBRE TEXT. RES., lUNE 2006

the structure takes place before wool fibre wraps them. As a result, the yarn diameters are min imum for A type structures. Structure B has two thin layers of polyester in sheath positioned at the outermost and innermost sheath layers. The structural consolidation is therefore not as good as in structure A. Hence, the d iameters of these structures take intermediate values.

The mass proportion of wool i s same ( i .e . 30%) in structure C3 having 70:30 core-sheath ratio and in structure B 1 having 50:50 core-sheath ratio. However, the yarn d iameters averaged over friction drum and delivery speeds are 0.44mm and 0.36mm respectively. Therefore, the diameter i s not entirely dependent on the proportion of wool. Location of the constituents within the cross-section plays a significant role. In case of structure C3. 70% polyester in core does not get effectively wrapped by 30% wool in sheath, whereas the core consisting of 50% polyester in the case of structure B I gets effectively wrapped first by 1 0% polyester and then by 30% wool and at last by 1 0% polyester again. The effective wrapping consolidates the core, thereby showing lesser yarn diameter.

4 Conclusions 4.1 The twist in the dref-III yarn reduces with the

i ncrease in del ivery speed and reduction i n drum speed for all the three structures. The diameter increases with the increase in delivery speed and reduces with the increase in drum speed.

4.2 Sheath contain ing 1 00% wool reduces the level of twist retained by the yarn, which might negatively

affect i ts mechanical properties though the yarn looks more bulky.

4.3 The twist and diameter differences in three different structures cannot be entirely ascribed to the difference in proportion of polyester and wool i n the yarn . Location of the constituents within the cross­section and thei r mass proportion plays a signi ficant role.

References 1 Salhotra K R, Chattopadhyay R, Dhamija S & Kaushik R C

D, Indian J Fibre Text Res, 27 (2002) 1 22. 2 Salhotra K R, Chattopadhyay R, Kaushik R C D, & Dhamija

S, J Text Inst, 90( I ) ( 1 999) 637. 3 Sengupta A K, Chattopadhyay R, Venkatachelapathi G S &

Padmanabhan A R, Melliand Textilber, 73( 1 992) 224. 4 Thierron W, Technical Report No. 578 ( South Africa Wool

and Textile Research I nstitute), April 1 986. 5 Alagha M 1, Oxen ham W & Iype C, Text Res J, 64(4) ( 1 994)

1 85 . 6 Balasubramanian N, Indian J Fibre Text Res, 1 7( 1 992) 246. 7 Konda F, Okamura M & Merati A A, Text Res J, 66(7)

( 1 996) 446. 8 Lord P R, 100 C W & Ashizaki T, J Text Illst, 78(4) ( 1 987)

234. 9 Lawrence C A, Fllndall/ellfal.\' of SpUII Yam Technology

(CRC Press) , 2003. 1 0 Ishtiaque S M & Swaroopa T K , Asian Text J , 1 0 ( 1998) 343. I I Ishtiaque S M & Aggarwal D, J Text Inst, 9 1 (2000) 546. 1 2 Merati A A & Okamura M , Text Res J, 70( I I ) (2000) 974. 1 3 Rust 1 P & Lord P R , Text Res J , 6 1 ( 1 1 ) ( 1 99 1 ) 645. 1 4 Lord P R & Rust 1 P , J Text Inst, 82(2) ( 1 990) 2 1 1 . 1 5 Merati A A , Konda F , Okamura M & Marui E , Text Res J,

67(9) ( 1997) 643. 1 6 Booth 1 E , Textile Testing, 3'd edn (Butturworths, London),

1 974. 1 7 Bona M , Textile Quality -Physical Methods of Product alld

Process COlltrol, 2nd edn (Texi l ia, Istituto er la, Italy), 1 995.