Indian Journal of Fibre & Textile ResearchVol. 23, December 1998, pp. 223-228

Influence of additional twist on tensile behaviour offriction-spun yarns

R Chattopadhyay & A ChakrabortyDepartment of Textile Technology, Indian Institute of Technology, New Delhi 110016, India

Received 14 July 1997; accepted 9 September 1997

The influence of incorporation of additional twist on the mechanical properties of friction-spun yarnshas been studied. Both tenacity and breaking extension of the yarns improve after twisting them furtheron ring twister. The stress-strain behaviour of both OE-friction and Dref-3 yarns becomes more andmore closer to that of equivalent ring yarns as additional twist is incorporated ..

Keywords: Breaking extension, Dref-3 friction yarn, Open-end friction yarn, Twist, Yarn tenacity

1 IntroductionFriction spinning produces yarns at a much higher

speed compared to other technologies. But, the yarnsproduced have low strength'? which can be ascribedto their structures. The structure of open-end (OE)friction yarn is characterized by a low level ofmigration i.e. fibres in the yarn cross-section arepresumed to be arranged in the form of conicalannular layers'". Besides, the constituent fibres arealso highly deformed. Such a structure, therefore,becomes amenable to easy slippage during tensiledeformation, registering very little load at failure. Incase of Dref-3 yarn, the core fibres assume a straightand parallel configuration and the fibres in sheathremain wrapped in a deformed state with a varyingdegree of tightness. The projecting ends and loops ofthese fibres also make the surface more hairy. Thus,though the reinforcing effect of sheath fibres makesthe Dref-3 yam stronger than the OE-friction yarns,still it yields easily when subjected to tension andfails primarily due to slippage of fibres, ascharacterized by the rounded off or step-wisereduction in load at the failure region of the stress-strain curves (Fig. la) for cotton. Though it is lessevident for acrylic (Fig. Ib), but the SEMphotographs (Figs 2a & 2b) of the tapered failureregions of these yarns clearly indicate a slippagemode of failure. In the same photographs the ringyarns with narrow failure regions indicate actualsimultaneous rupture of fibres to be the predominantmode of failure. In the absence of self-interlockingstructure the strength of these yarns can hardly beimproved through manipulation of either machine or

process parameters as both do not change thestructure of assembled fibres significantly. Thus, ifslippage is to be prevented for bringing significantimprovement in strength, incorporation of additionaltwist on a ring-twisting machine could be a way outas it is expected to improve the structural integrity ofthe yarn. Yeung et al.6,7 have reported that the rotoryarn modified by such technique showed its tenacityand breaking extension to improve, almost to thelevel of ring yarns. Further, it also becomes tighter,cleaner and smoother due to insertion of additionaltwist. The present study was aimed at assessing theextent of improvement in tenacity expected throughincorporation of varying level of additional twist tofriction-spun yarns.

2 Materials and Methods2.1 Fibres

Acrylic fibre and two varieties of cotton fibreswere used for the present study. The characteristicsofthese fibres are given in Table 1.

2.2 Preparation of Yam SamplesThe fibres were processed on card and drawframe

to produce slivers of 3.2 ktex for cotton A and cottonBand 3.0 ktex for acrylic. The slivers from cotton Aand cotton B were converted into 59 tex and 30 texyarns respectively and both these counts were alsospun from acrylic fibres. All the yarns were producedon a three - head laboratory model Dref-3 frictionspinning machine with a pair of opening rollersrunning at a constant speed of 12000revolutions/min. The Dref-3 yarns were produced

224 INDIAN J. FIBRE TEXT. RES., DECEMBER 1998

15(0) ( b)

-- S9 l~x ~ Dr~f- 3 spun_____ S9 t~x • 0 E spun

-e- 30 t~x ) Dr~f -3 spun)(

--- 30 \~x ) OE spun /1~ / .-.... 10 . Iz / i./ .

/~ ./' ,/1>- /u /e / L,,'-'Cl i ,c:'"

, '" I>- / .., '" I

i / " I5 I \ '" I;'

I ,: i / /, I, I

I 11./,," I

/ I ""I

I il/ I

,""15 0 20 30

St fain, 0/.

Fig. I-Stress-strain curves of different friction-spun parent yarns: (a) cotton, and (b) acrylic

Fig. 2-··-$EM photographs (x 24) of the failure regions: (a) cotton yarns, and (b) acrylic yarns(Top: Dref-3 yarn; Middle: OE-friction yarn; and Bottom: Ring yarn)

with a core to sheath ratio of 60:40 and the OE-friction yams were produced on the same machinekeeping the drafting unit I inoperative. The spinningparameters of all the yams are given in Table 2.

2.3 TwistingOE-friction and Dref-3 cotton and acrylic yams

were further twisted in Z-direction on a conventionalring-twisting machine. The Z-direction was chosen

CHAKRABORTY et al.: BLEACHING OF COTION KNITIED GOODS 255

Parameter Orange HE2RStabilizer-s-A WNI NS

Red HE 8B

Table ~olour evaluation of bleached fabrics taking sodium silicate stabilized bleaching as standard

Navy Blue HRAWNI NS

DL*Da*Db*DE

1.63-1.01-2.072.82

1.23-0.45-0.454.69

1.060.840.341.38

Based on the above studies, an industrial trial wastaken on Winch with a non-silicate stabilizer (recipeVII) and results are given in Table 4. It is observedthat the yam tenacity is almost same to that obtainedin the laboratory. This is indicative of the industrialtrial being commensurate with the laboratoryexperiments. The whiteness index data also supportthe above fact.

Colour evaluation data of samples dyed afterbleaching with different peroxide stabilizers aregiven in Table 5. The sample bleached using sodiumsilicate as stabilizer was taken as the standard. Thedata show that the samples bleached using non-silicate stabilizers AWNI and NS have invariablylighter shade with all the dyes. However, the degreeof lightness varies for different dyes. The Da* andDb * values are less than 1, except in a few cases.The total colour difference DE in all these cases isgreater than 1, indicating a considerable differencein colour of these samples compared to the standard.This difference is mostly due to the depth of theshade as shown by the data. It is only in case ofOrange HE2R and Golden Yellow HER that thetonal variations also add significantly to DE value.

Table 6 shows the K/S values for the the dyedfabrics. It is seen that the K/S values for the samplesbleached in a non-silicate stabilized system arelower compared to those for the samples bleached insodium silicate stabilized system. This indicates thatthe former are lighter in shade compared to thelatter.

Table 7 shows the relative unlevelness index(RUI) values for the dyed samples. It is seen that thelevelness is excellent for the samples bleached byemploying either sodium silicate as stabilizer ornon-silicate stabilizers. Thus, it can be said that thedyeing uniformity is excellent in all the cases.

The bending length data of samples bleachedusing silicate and non-silicate stabilizers (Table 8)show that the bending length is smaller for the.

Golden Yellow HERAWNI NS AWN! NS

2.16 0.37 -0.17 2.87 3.60-0.49 -0.35 0.69 -0.89 -0.640.70 0.25 1.68 0.77 0.542.32 0.57 1.82 3.11 3.70

Table ~KlS values for dyed samples

Dye KlSvalueSodium AWNI NSsilicate

Orange HE2R 15.6210 11.3421 12.2732Red HE 8B 16.4287 14.4738 13.1246Golden Yellow HER 8.7461 8.5939 9.6350Navy Blue HR 7.7153 6.2036 5.8610

Table 7-Relative unlevelness index (RUI) for dyed samples

Dye RUI for system with stabilizerSodium AWNI NSsilicate

Orange HE2R 0.119 0.153 0.084Red HE 8B 0.101 0.220 0.181Golden Yellow HER 0.100 0.060 0.156Navy Blue HR 0.154 0.173 0.141

Table 8--Bending length of fabrics

Fabric Bending length (ern) for system withstabilizer

Silicate AWNI NS

GreyScouredWhite (bleached)Dyed

1.72.0

1.92.0

2.11.85

2.22.1

samples bleached by employing non-silicatestabilizer compared to the one bleached with silicatestabilizer. The same trend is observed in the case ofdyed fabric. Thus, it can be said that the systememploying non-silicate stabilizers give a softerhandle to the fabric, which can be assigned to theelimination of precipitate deposition as in the caseof silicates.

4 ConclusionsThe optimum peroxide concentration is 3% owf

for two-stage process' as well for single-stageprocess like scourex or modem bleaching process.A comparison of these processes shows that non-

226 INDIAN J. FIBRE TEXT. RES., DECEMBER 1998

• 12~----------------------~~ (0) ~JO'".•...... "'..... ~Z 0 ,. 0...,." st,,,8•..•

Po"",~ ~-------------:: Partn'

~ 4 L-3...1~---:-4~00:--~SO~0-~~-;tO::---:700=300

Additional Twist, tpm

.. Par",'PciAiii--------

400 500

Fig. 3-Influence of additional twist on tenacity of DE-friction yams: (a) cotton, and (b) acrylic

16• (0) (b)••- Porfnl-Z 12•..>- 591.--U 8 Porcntec:

'"I- -------------Porfnt 59If.I.

300 1.00 500 600 700 300 400 500 600 700Addlttonal Twist) tpm

Fig. 4-Influence of additional twist on tenacity of Dref-3 yams: (a) cotton, and (b) acrylic

Table 3-% Increase in tenacity of friction-spun yamscorresponding to optimum twist

Type of Type of Linear % Increase infibre friction-spun density tenacity

yam tex

Cotton A DE 59 90.0

Dref-3 59 88.2

Cotton B DE 30 66.7

Dref-3 30 138.6

Acrylic DE 30 23.4

59 21.4

Dref-3 30 40.7

59 -3.0

are helically wrapped in Z-sense, any further additionof twist in Z-direction will cause the core fibres tochange from parallel to twisted state and the sheathfibres to get wrapped further in Z-direction. Such asituation will cause the core fibres to support moreload as slippage becomes prohibitive because of thepresence of twist in it on one hand and tightlywrapped sheath fibres on the other. However, thesheath fibres themselves may contribute less towardsstrength due to overtwisting. The net effect would,

therefore, depend on the predominance of one factoron the other. The initial increase in tenacity of 30 texyarn may be ascribed to the twisted core playing amore dominant role than the obliquity effect of thesheath fibres, if at all, till the optimum twist level andvice-versa beyond that. For coarser yarns, theobliquity effect predominates right from thebeginning as the twist multiplier corresponding to theminimum added twist lies probably close to theoptimum. Besides, the structural flaws in coarseyarns are likely to' be less due to more number offibres in the yarn cross-section. Hence, the scope forimprovement through structural consolidation wouldalso be less for coarse yarns. With cotton, such flawsare likely to be more since it contains lot 'of shortfibres. Therefore, gain in strength for cotton wouldbe more.

3.3 Breaking ExtensionThe effect of additional twist on breaking

extension of OE-friction and Dref-3 cotton andacrylic yarns is shown in Figs 5 and 6. It is observedthat the breaking extension of yarns with additionaltwist is more than that of the corresponding parentyarns, except in the case of OE-friction cotton B yarn

227CHATroPADHY AY & CHAKRABORTY: FRICTION-SPUN YARNS

~24r(-a-)-----------,

'".6'" 16j)(....DO.5..ac2~en

Por..,1

~--1---o--~-=-5""tnt 30 I.x

0~3~OO~--4~0~0--~50~0--~600~I---;~0·

§9 ••

500 700

(b)

Pot",t - --- .....--~:----.nt lOin •

~I ••

300 400 600

Additional T••ist J t pm

Fig. 5-Influence of additional twist on breaking extension of OE-friction yarns: (a) cotton, and (b) acrylic

Sgt••

~••• -"fS' - --o---o..~-o...

"P"- 301••01 Par.nt! 4------------partnT--o••~en 0 L.-±__ -:+..--_±__ ~:__--:~ I300 400 500 600 700 L-::3-tOO;:----4~0~0---;5:-;!0'-;:-0-----;-6~00=--~700

Additional Twist, tpm

12~--------------------~~ (0)

.§ 8c f-~)(....

(b)

30 1••-0- - --0--~--------- --Par.nl---0-

-

Fig. 6-Influence of additional twist on breaking extension of Dref-3 yarns: (a) cotton, and (b) acrylic

(Fig. 5a). The breaking extension changes marginallyin most of the cases except cotton A OE-friction yarn(Fig. 5a) with increase in additional twist. In case ofacrylic yams (Figs 5b and 6b), the breakingextension of 59 tex yams declines when twistedbeyond 525 tpm. Incorporation of twist on ringtwister causes the fibres to follow a helical path. Asthe yam is loaded, the helices can easily get extendedand thus can contribute positively to the overallextension of the yam. With increase in twist, amarginal-rise is therefore observed. However, whentoo much twist is incorporated the freedom of fibremovement gets restricted due to higher frictionalhindrance. The fibres remain in highly strainedcondition after twisting, reducing its potential toextend further when loaded. As a result, it may evenshow a decrease in extension as observed in somecases.

3.4 Shape of Stress-Strain CurvesThe stress-strain curves of parent, twisted and

corresponding ring yams are shown in Figs 7 and8. It is observed that the stress-strain behaviour ofthe yams becomes more and more closer to that ofring yarns as additional twist is incorporated intothem. A comparison of the stress-strain curves

15r---------------.

f 10-:zu

..•...ClC.•..>- 5-

Breaking Extension) %

Fig. 7-Slress-strain curves of ring, parent and additionallytwisted friction-spun 30 lex cotton B yarns (--) ring yarn,(- - - -) Dref - 3 parent, (- - -) Dref -3 twisted, (_ ••• -) OEparent, and (_. -) OE twisted

228 INDIAN J. FffiRE TEXT. RES., DECEMBER 1998

)(••--zOJ

r>. 5-OJac•••.--

20 30

Breaking Extension, %

Fig. 8-Stress strain curves of ring, parent and additionallytwisted friction-spun30 tex acrylicyarns (--) ring yarn, (- - - -)Dref - 3 parent, (- - -) Dref -3 twisted, (- ••• -) OE parent, and(_. -) OE twisted

indicates that the Dref-3 yarns are more close to ringyarns than the OE-friction yarns.

As twist is added, the constituent fibres of theparent yarns adopt a regular helical configuration andbecome compact as well. Hence, their tensilebehaviour becomes more akin to ring yarns. In OE-friction yarns, all the fibres in the cross-section aresomewhat in twisted configuration, whereas in Dref-3 yarns, the core is in untwisted state with sheath in atwisted configuration. Besides, the fibres in OE-friction yarns are in a highly deformed state. As aresult, when the yarn is extended, OE-frictionstructure yields easily, registering less load at a givenextension level. In Dref-3 yarns, 60% of the core

fibres being straight and parallel, can support a muchhigher load at the same extension level all throughthe extension range. Hence, the stress-strain curvesof Dref-3 yarns are always much above those of theOE-friction yarns. In ring yarns, all the fibres remainin a straight and parallel configuration and thestructure is most compact. Therefore, though thefibres remain in helical paths they can support anequal or higher load at a given extension level than incase of the corresponding OE-friction and Dref-3yarns. Therefore, the stress-strain curves of ringyarns encompass both OE-friction and Dref-3 yarns.

4 Condusions4.1 Incorporation of additional twist makes the

yarns stronger. The tenacities of both cotton andacrylic OE-friction yarns respond more or lesssimilarly to the incorporation of additional twist.

4.2 In Dref-3 yarns, the response of tenacity isdifferent for fine and coarse yarns. The tenacity offine yarn first increases and then decreases whereasthat of coarse yarn decreases continuously.

4.3 The breaking extension of cotton and acrylicyarns generally increases or remains stationary,except for coarser acrylic yarns where it decreases.

4.4 As additional twist is incorporated to thefriction-spun yams, the stress-strain behaviour ofboth OE-friction and Dref-3 yarns becomes moreand more closer to that of equivalent ring yarns.

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