Indian Journal of Textile ResearchVol. 14,June 1989, Pp. 71-76

Influence of twist and tightness factor on physical characteristics of plainknitted fabrics made from acrylic-viscose rotor-spun yarns

Reo Kaushik, K R Salhotra" & G K TyagiThe Technological Institute of Textiles, Bhiwani 125021, India

Received 14 July 1988; accepted 3 October 1988

The physical characteristics of plain knits made from acrylic-viscose rotor-spun yams of varying twistand tightness factors have been investigated. Courses/em, wales/em and stitch density do not change withfibre composition or yam twist but increase appreciably with the increasing tightness factor irrespectiveof the relaxation treatment. Dry-relaxed knits from acrylic-majority yam are more porne to pillingbut theincidence of pilling reduces considerably with increasing twist. Fabrics knitted with low-tightness factorexhibit lower area shrinkage.

..•...,.... ..

Keywords: Acrylic-viscose yarn, Knitted fabric, Rotor-spun yarn, Tightness factor, Yarn twist

1 IntroductionIn spite of the lower strength and more twist liveli-

ness of rotor-spun yarns, many knitters prefer themto ring-spun yarns because of the better cover due totheir response to steaming and shrinkage. In fact, thelower strength of rotor yarn is more than compen-sated by its less lint-shedding, fewer knots, fewerweak places and slubs, and less clogging of knittingneedles as shown by their superior knitting perform-ance':", Twist liveliness, which leads to fabric spiral-ity, can be reduced by decreasing the yarn twist or byefficient setting of yarn" - 5.lt appears from the litera-ture that rotor spinning offers no guarantee for ac-ceptability of knitting yarn. But, if good manufactur-ing and quality control methods are used, yarns su-perior than those obtained by the conventionalspinning methods can be produced. Some research-ers have reported that the rotor yarns present inter-esting possibilities in terms of knitting performanceand fabric quality. However, not much has been re-ported for knits of acrylic and its blend, especiallywith viscose rayon. This paper discusses the use ofacrylic-viscose rotor-spun yarns for weft knitted fa-brics.

2 Materials and Methods2.1 Preparation of Yarn and Fabric Samples

All the yarns of 29.5 tex, with S twist, were madefrom different blends of acrylic and viscose rayon fi-bres at different twist factors on an Ingolstadt RotorSpinner RU 11/RU 80 (model 4602). The rotor and

'Indian Institute of Technology, New Delhi 110016. India.

opening roller speeds were kept at 40,000 and 6000rpm respectively. The rotor diameter was 56 mm.The specifications of fibres used are given in Table I.

The Krenzler TK-l model sinker top knitting ma-chine was used to make plain knitted fabrics of vary-ing tightness with the following specifications: diam.,3.5 in; feeder one; machine rpm, 350; gauge, 18; andneedles, 198. To bring the fabrics to minimum ener-gy level, all knits were subjected to dry and full-re-laxation treatments. For studying the physical char-acteristics of knits from yarns of varying composi-tions and twists, the fibre proportions,. twists andtightness factors were decided in accordance withthe Box and Hunter's factorial design".

2.2 Dry-Relaxation TreatmentAfter knitting, the fabrics were laid flat for fifteen

days to facilitate recovery from stresses imposedduring knitting. All the knits were tested for dry-re-laxed dimensions after conditioning the fahrics in astandard atmosphere of 65% RH and 2rc.2.3 Full-Relaxation Treatment

The fabric samples, wetted in water contauung0.2% Lissapo1 NI for 45 min at 50°C, were washed

Table I-Specifications of acrylic and viscose rayon fibresFibre Fibre Fibre Bundle Breaking Fibretype length denier strength extension stiffness

mm g/dcn 'y., g/den

3R 2.0 1.67 3<).53 7.n3R 2.0 I.) 7 13.27 11.2

AcrylicViscose

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INDIAN 1. TEXT. RES .. VOL. 14.1UNE 1989

in a side padd\c machine (Bibbys of Halifax, Eng-land). The paddle machine, agitating at 25 rpm, wasrun for I h. The fabric samples were then removed,hydroextractcd, and tumble-dried in a Kamsin dryerat oooe for I h. The samples were then cooled bypassing cold air for 5 min and then laid on the floorfor 48 h under standard atmospheric conditions.

2.4 Factorial DesignIn factorial design approach" (wherein many fac-

tors arc varied simultaneously to find out the indi-vidual as well as interaction effect) the variables areselected at five levels, viz. - 1.082, - I, 0, + 1 .and+ 1.682. The response Yis given by a second orderpolynominal, i.e.

I. I.

Y= bo + I b. X; + II<

I o,jXi Xj

;= Ii= I i= I

where Xi is the ith variable; k, the number of var-iables; and b.; bi and bij are the regression coeffi-cients associated with the variables.

To find out the regression coefficients, the re-sponse Yhas to be found out using different experi-mental combinations of variables under investiga-tion. For the case of three variables, the experimentalplan is given in Table 2 and the actual levels of thethree variables, viz. blend proportion, twist multipli-er and tightness factor, are given in Table 3.

2.5 Tests

Courses/em. wales/em and stitch density of dry-and fully-relaxed fabrics were measured by a count-ing glass. Fabric thickness was recorded by R&Bcloth thickness tester. Pilling propensity was mea-sured on a fabric pilling tester (Foricorn, Bombay)and the shrinkage of fully-relaxed knits was estimat-ed by the BSI method".

2.6 Experimental Analysis

The results of all the responses for the various ex-perimental combinations were fed to an IBM com-puter. Suitable computer programmes were deve-loped for calculating the regression coefficients. On-ly significant terms were considered fOI further anal-ysis of results after testing the regression coefficientsfor significance at <)5°/., confidence level. The re-sponse surface equations for different fabric charac-teristics along with the correlation coefficients be-tween the experimental values and the calculatedones (obtained from the response surface equations)are given in Tab\c 4. Maps between a response anddifferent variables were drawn manually using 125combinations obtained hy the computer.

72

3 Results and Discussion3.1 Courses and Wales per Centimetre

The effects of fibre composition, yarn twist andtightness factor on cour.ses/cm and wales/em areshown in Figs 1 and 2. It is seen that an increase ineither acrylic content or yam twist does not affectthe courses or wales per centimetre. However, bothincrease gradually with the increasing tightness fac-tor, the increase being of lower magnitude in wales/cm. Since length-shrinkage always occurs on dry-re-laxation for all kinds of knits, width-shrinkage is oc-casional and occurs only for tight structures. Afterfull-relaxation treatment, as the fabric approachesthe strain-free state, there is further increase incourses and wales per centimetre irrespective of thefibre proportion, yam twist and fabric tightness.

Table 2-Experimental plana for three variablesSI. No. Levels of variables

XI x~ xJ

-\ -\ -12 I -I -1J -I I -14 I I -\

5 -I -I I6 I -I 17 -I \ 1R I 1 \

9 - 1.682 o 010 + \.682 0 0II 0 -1.682 012 () + 1.682 013 0 () - 1.68214 0 0 + 1.68215 0 0 016 () 0 017 0 0 018 0 0 019 0 0 020 0 0 0

"Box and Hunter's design for three variables.

Table 3- Actual values corresponding to coded levelsLevels XI

(Fibrecornposition)"

(Twistmultiplier)

2.653.003.504.004.35

XJ

(Tightnessfactor)11.1312.5014.5016.5017.86

- \.682

- 1.0000.000

+ 1.000+ 1.682

o205080

100

"Acrylic 0;',.

KAUSHIK et at: lHAKACI t.KIS IICS Ut- PLAIN KNITlt:.1J t-AHKICS

ResponseTable 4-Response surface equations for various fabric characteristics

Response surface equation

Dry-relaxed state Fully-relaxed state Dry-relaxed Fully-relaxedstate state

Courses/em 11.6943 + 1.3537 x, + 0.5381 xj 11.9306 + 1.335xJ + 0.5528xi 0.8020 0.8170Wales/em 11.0679 + 0.227 3x, + 0.26 72x~ 11.1921 + 0.5339xJ +0.1726xi 0.8929 0.9231

- O.1505x1 x, +0.1307~ +0.1866x~Stitch density 130.3919 + 20.1916x, + 7.960 I~ 136.1162 + 21.6522xJ + 8.55~ 0.8437 0.8372Fabric thickness, mm 0.8218 +0.0493x1 +0.0097x, + 0.7989 + 0.0704x1 + 0.0154xz + 0.9353 0.0645

0.0 113x~ 0.0 10xi + 0.0097 X} - 0.0137 XzX,Weight, g/m ' 148.5495 + 11.2721 x, + 4.4062x~ + 153.7179 + 6.0074 tz + 11.0358x) 0.8010 0.8630

7.2706x~ + 5.0628~ + 8.3873.x3Pilling propensity 13.1289 + 2.3728xl - O.7089x, 15.0272+ 1.664xl - 0.6856xz 0·8999 0.9192(No. of piles) - 0.628x~ - 0.75x1x, - 1.0x,x, - 1.6873x, - 1.2369xi - 0.7066x~

-1.5x1x,Shrinkage, 'Yo 4.3454 + 5.8685x1 + 1.166Xz 0.9581

- 2.7879x; + 2.1862~

17

Dry - relaxed0 A,-X,

16 6. 8, -X2D (t -X3

15 Fully- relaxed

• A2-XlE • 82 -Xzu

14 • (z -X3-.~Vl C2 CIwVl 13IXOJa'-' Az. II

12

A., &.

llr--~ •........-

10L-~ ~ L-~~--~- 1. 662 - 1 0 + 1 + 1.662

VARIABLE X

Fig.. 1- Effect of fibre composition. yarn twist and tightnessfactor Oil courses/em

However, the change is less in knits made from high-twist yarns than in knits made from low-twist yarns.

3.2 Stitch DensityFig. J shows the effect of fibre composition, yarn

twist and tightness factor on stitch density. It is ob-served that change in either fibre composition oryarn twist causes no change in stitch density, the lat-ter, however, increases with an increase in fabrictightness in a quadratic manner owing to an increasein no. of loops/em". On full-relaxation, at all levels offibre composition and yarn twists, stitch density in-creases for tighter structures and decreases for slackstructures, which may be accounted for by the width

16Ory- relaxedD Al -- Xl

15 6. Bl -X2D (1 -- X3

Fully - relax ed14 • A2-Xl

E • 62 - X2u • (2 - X3

<;

VIw-'-c 12~

10L-__L-__~ ~ __-L ~-1.682 -1 o

VARIABLE x

Fig. 2 - Effect of fibre composition, yarn twist and tightnessfactor on wales/ern

expansion occurring at lower levels of tightness fac-tor. For fully-relaxed knits, stitch density is alwayshigher than that for dry-relaxed knits because of theshrinkage in area of the knits over its dry-relaxedstate.

3.3 Fabric ShrinkageFig. 4 shows that fabric shrinkage (%) of fully-re-

laxed knits decreases with decreasing tightness fac-tor because at a lower tightness factor, the knits arcslack and may be able to recover much more afterthe removal from the machine. The slacker knits,therefore, exhibit lower shrinkage after full-relaxa-tion treatment. Besides tightness factor, yam twist

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INDIAN J. TEXT. RES., VOL. 14,JUNE 19H9

also seems to make a significant contribution to fa-bric shrinkage. Fig. 4 shows a slight decrease in arealshrinkage followed by a significant increase with theincreasing twist: The increase in fabric shrinkagemay be attributed to the decreased resistance tobuckling owing to an increase in incidence of wrap-per fibres which increases with the increasing twist.

• 230" Ory- relaud-•• o Al-Xl~0 6 Bl-XZ~ 210 D (, -)(3••E fu lIy- relaxedu • A2-X,- 190•• • B2-X2:I • (2 -X3~~.9

170>-~VI 150z•••CI

A'l.8z~u 130 Al·81~•...VI

110- t , 682 -I 0 ., .1.6&2

VARIABLE X

Fig. 3-Effect of fibre composition, yarn twist and tightnessfactor on stitch density

1~.----------------------.

9

Fully-relaxed

• A2-X,12 • B2-X2

• (2-X3

.u~<::.:: 6za:J:VI 3<~ A2a:<

w

a: XCD< -3<L.

-6

-9~------------------------~Fig. 4-Effect of fibre composition. yarn twist and tightness

factor on fabric shrinkage

74

Hari et al" have also reported an increase in yarn re-sidual shrinkage with. increase in twist.

3.4 Fabric ThicknessFig. 5 shows that the fabric thickness in dry-re-

laxed state increases with increase in either acryliccontent or yarn twist, probably due to the lower spe-cific gravity of acrylic fibre and twist contraction,which, in turn, cause an increase in yarn bulk and fa-bric thickness .

Apart from fibre composition and yarn twist,tightness factor also seems to affect the fabric thick-ness, as is evident from the' response surface equa-tion in Table 4. Fabric thickness increases with an in-crease in tightness factor except in loose knits. Thismay be explained by the fact that as the tightness fac-tor increases, the no. of loops/em- increases, which,in turn, causes an increase in stitch density and thick-ness. Further, it is not possible to comment on the in-itial shape of the curve between fabric thicknessandtightness factor due to the interaction between dif-ferent variables and the limited nature of this study.

3.5 Fabric WeightYarn twist and tightness factor significantly affect

the weight per unit area of acrylic-viscose weft-knit-ted fabrics (Fig. 6). Except loose knits, all fabrics ex-hibit an increase in weight per unit area with an in-crease in tightness factor. This may be attributed tothe fact that as the tightness factor increases, the no.of loops/em? increases, leading to an increase institch density and fabric weight per unit area. An in-crease in yarn twist also results in an increase in fa-bric weight, probably due to yarn contraction. Fur-ther, fabrics knitted from either high-twisted yarns.or with more tightness factor possess more weight

1. 00~-_r!I~~~

o A,-X,A2

0.95 '" B '-X20 ( ,-X3

E Fully- r e 10 x~ AlEo • A2 - ·X,O.}O

A B2 -x 2vo • (2-X 3vou.oz 0·85'"'-'-:I:>-

'-'0·80

-a;(IJ« 0.75u-

0.70':- __ '-- __ -'- .1....---' ,- , ·682 - , 0

VARIABLE

• , .1·68 Z

X

Fig. 5-Effect of fibre composition. yarn twist and tightnessfactor on fabric thickncsx

KAUSHIK et af.: CHARACTERISTICS OF PLAIN KNITTED FABRICS

per unit area after full-relaxation treatment owing tothe relaxation shrinkage.

3.6 Fabric Pilling Propensity

Fig. 7 shows t,hat the dry-relaxed knits made fromyarns with higher proportions of acrylic fibre exhibitmore pilling tendency. The number of piles in-creases because the fibres in the yarns come out in-stead of breaking owing to the higher strength, ex-tensibility and low stiffness of acrylic fibre, resultingin more pile formation.

•...i3 170

•..• 160

'"ex>..:.•.

140- 1. 6!-;;8::;-2----:_1'--....JO~--.IL--.~1.-6 8-2---.J

VARIABLE X

Fig. 6-Effect of fibre composition. yarn twist and tightnessfactor on fuhric weight

22Dry- relaxed

0 A,-Xl6 B,-XZ

ZO a (I-X3Fully -relaxed

• Az-Xl18 • BZ-XZ Al• (Z-X3VI...,....•

16c,

.•.0

'"....CD:l:~:z

8_1.6~8~Z-_I~--~O~--.~,-.71.~6~87Z---J

VARIABLE X

Fig . .7-ElTel·t of fibre composition. yarn twist and tightnessfactor on pillin!! propcnsiry

On the other hand, the number of piles decreasesconsistently with increase in yarn twist. The dec-rease may be partly due to an increase in inter-fibrecohesion and partly due to an increase in incidenceof wrapper fibres, which have freedom to move onthe core of the yarn instead of coming out".

Earlier studies 10 have indicated that the pillingtendency reduces with the increase in tightness fac-tor. In the present investigation, within the experi-mental range (11.13-17.86) of tightness factor. an in-crease in the number of piles followed by a decreasewith the increasing tightness factor has been ob-served. This increase may be attributed to the morefree movement of yarns owing to the more unstabil-ity of the loosely knitted fabrics, enabling the yarnsto secure more loosely in the body of the fabric.However, a further increase in tightness factor nolonger allows further yarn movement owing to an in-crease in fabric cover, causing a reduction in pile for-mation.

Further, the transient response of fabrics with full-relaxation treatment in comparison with dry-relaxedstate shows a decrease in pile formation irrespectiveof fibre proportion and twist. This is attributed to thecombined effect of increased fabric stability and fi-bre buckling which occurs because each componentis exposed to this treatment.

4 Conclusions".1 Courses/em, wales/em and stitch density do

not change with fibre composition or yarn twist hutincrease appreciably with the increasing tightnessfactor irrespective of the relaxation treatment.

4.2 In fully-relaxed state. slack knits exhibit lowerarea shrinkage. However. an increase in yarn twistresults in a decrease in shrinkage followed hy an in-crease with further increase in twist.

".J Fabric thickness increases with the increasingacrylic content or yarn twist and is generally higherfor fully-relaxed knits. It reflects the same trend asfabric weight with tightness factor irrespective of therelaxation treatment.

••.4 Fibre composition has no effect on fabricweight in dry-relaxed slate. The fabric weight dec-reases initially and then increases with an increaseeither in yarn twist or tightness factor for dry- andfully-relaxed knits.

4.5 Dry-relaxed knits made from yarns with high-er proportions of acrylic fibre exhibit more pillingtendency. The incidence Ofpilling reduces consider-ably with an increase in twist and tightness factor af-ter the full-relaxation treatment.

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INDIAN J. TEXT. RES., VOL. 14,JUNE 1989

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76

7 Methods of test for textiles, British Standard Institute, Lon-don (1974) 4/140.

8 Hari P K, Balasubramanian p. Sengupta A K & Chavan R B,Text ReSJ, 55(1985) 122.

9 Hari P K, Proceeding, seminar on rotor spinning (Indian In-stitute of Technology, New Delhi) 1981,142.

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