I ndian Journal of Fibre & Textile Research Vol. 29, December 2004, pp. 4 1 2-41 8

Influence of process variables o n characteristics o f modal siro-spun yarns using Box -Behnken response surface design

R V M Gowda', M Sivakumar & M S Senthil Kannan

Department of Texti le Technology, Bannari Amman Institute of Technology, Sathyamangalam 638 40 1 , I ndia

Received 7 May 2003; revised received and accepted 5 January 2004

The influence of process variables on characteristics of modal siro-spun yarns has been studied using Box-Behnken response surface design. The characteristics of these yarns have been evaluated and compared with those of equivalent double-rove spun yarns. I t i s observed that the increase in strand spacing from 6 mm to 10 mm increases the unevenness and decreases the hairiness, strength and extension of modal s iro-spun yarns. The traveller mass and spindle speed show varying effects on characteristics of siro-spun yarns. Taking the effects of all the three experimental factors into account, it has been found that the yarn unevenness is minimum and elongation is maximum at lower levels of factors while the hairiness i s minimum a t their higher levels. The yarns exhibit maximum tenacity and fewer imperfections a t middle levels o f the factors. Hence, the strand spacing of 8 mm, traveller mass of 1 /0 and spindle speed of 17000 rpm are found to produce siro yarns of nominal count (30 tex) with optimum quality. Further, the siro-spun yarns are found to be relatively inferior in respect of unevenness and superior as regards the hairiness when compared with the equivalent double-rove spun yarn.

Keywords: Box-Behnken design, Modal fibre, Siro-spun yarn, Yarn quality IPC Code: Int. Cl.7 D02G 3/00, GO I N 33/36

1 Introduction The passion for new textile structures in terms of

comfort, appearance and fashion has been continuously leading to the development of numerous types of fibres. Modal is a new cellulose-based fibre manufactured by Lenzing AG, Austria. As compared to cotton, modal has higher breaking strength while it is wet ' . It exhibits the comfort of cotton and the brilliance of silk and, therefore, is becoming more popular now-a-days in knitwear, innerwear, T-shirts, terry towel, etc2•

The modal fibres presently are being spun into medium and fine counts on ringframe. Being a new fibre, its performance on other spinning systems also needs to be studied to exploit its potential for diversified end uses. The siro spinning developed by CSIRO combines spinning and doubling operations in a single stage. It has been very well established and found to be economical in the production of two-strand (two-fold) worsted yarns, which have been used successfully in place of conventionally produced two-fold yams in weaving sector3 . Therefore, the scope of this spinning

"To whom all the correspondence should be addressed. Phone: 221 289; Fax: +9 1 -4295-223775; E-mai l : rvm�owda@rediffmail.com

system should be exploited for the production of knitting and hosiery yams also.

The important process parameters that i nfluence siro-spun yarn characteristics are strand spacing, spindle speed and traveller mass. There has been a great deal of study on the effect of these parameters on characteristics of yams spun from cotton and other fibres4-9 However, most of these studies were carried out at lower spindle speeds and focused on investigating the effects of individual factors without considering the interaction amongst them. Owing to the recent increase in the rate of production on ring frame, the results observed on lower spindle speeds may not hold good with those expected at higher speeds. Hence, an investigation on the effect of these parameters, especially a higher spindle speed and its interaction with strand spacing and traveller mass, on siro-spun yam properties is essential . Further, there is hardly any mention i n l iterature on siro spinning of modal and the resultant yarn characteristics. In view of these factors, the present work was aimed at studying the performance of modal fibre in siro spinning to produce a yarn suitable for knitting, and to investigate the effect of individual process variable and their interaction effects on siro-spun yarn characteristics using Box-Behnken response surface design. The characteristics of these yams have also

GOWDA et al.: CHARACTERISTICS OF MODAL SIRO-SPUN YARNS 4 1 3

been evaluated and compared with those of equivalent double-rove spun yams.

2 Materials and Methods

2.1 Materials

Modal fibres of 38 mm and 1 .5 denier, procured from Mis Lakshmi Mills Ltd, Coimbatore, in the form of rovings, each of 1 .45s Ne (393 .67 tex), were used .

2.2 Methods

2.2.1 Production of Yarn Samples

The roving separators (guides) of 6, 8 and 1 0 mm were prepared from a thin sheet of iron. The drafting system on a G 511 ringframe was modified to fix the roving separators, one near the back rollers' nip and the other near the middle rollers' nip in the back zone of the drafting system. The roving traverse mechanism was stopped and locked in a central position and the two rovings were fed through the separators to the drafting system. The Box-Behnken designlO has been used to plan the experiments for producing siro yarns of nominal count (30 tex) using the key process variables, viz. strand spacing, traveller mass and spindle speed. The Box-Behnken design was chosen to reduce the number of experiments. The various factors and their coded levels are given in Table 1 . The experimental runs based on the factor-level combination are shown in Table 2. Besides siro yarns, an equivalent double-rove spun yarn (spun with 0 mm spacing between the rovings; 1 .2 break draft; 1 7000 rpm spindle speed; EM 1 10 traveller mass; 3 mm spacer; and 3 . 1 twist multiplier) was also produced using the same roving.

2.2.2 Measurement of Yarn Characteristics

All the yarn samples were evaluated for important characteristics to investigate the influence of process variables on yarn quality. The yams were tested for strength and elongation on Uster Tensorapid 3 at a gauge length of 500 mm and an extension rate of 5000 mrnlmin with 30 tests per sample. The unevenness, thin places (-30%, -40%, -50%), thick places (+35%, +50%, + 1 00%), neps (+200%) and hairiness of yarns were measured on Uster Tester 3 at a test speed of 400 rnlmin. Two tests were conducted per sample. The results were entered into the SYST AT package and analyzed. Regression equations and contours were generated to observe the linear, quadratic and interactive effects of process variables on yarn characteristics.

Table I - Factors and levels

Factor Level

- 1 0 + 1

Strand spacing (X,), mrn 6 8 10 Traveller mass (X2) 2/0 I/o

Spindle speed (X.!), rpm 1 6000 17000 18000

Table 2-Factor-level combinations for siro yarn spinning

Expt Coded level Actual value No. Xl Xl X3 Xl X2 X3

1 - I - 1 0 6 2/0 17000 2 -I + 1 0 6 1 1 7000 3 + 1 -1 0 10 2/0 1 7000 4 + 1 + 1 0 1 0 I 17000 5 - 1 0 - I 6 I/o 16000 6 -1 0 + 1 6 I /o 1 8000 7 + 1 0 - 1 1 0 I/o 16000 8 + 1 0 + 1 10 1/0 18000 9 0 - I - I 8 2/0 16000 1 0 0 - I + 1 8 2/0 1 8000 1 1 0 + 1 -I 8 I 16000 1 2 0 + 1 + 1 8 1 8000 1 3 0 0 0 8 I/o 17000 1 4 0 0 0 8 I /o 1 7000 1 5 0 0 0 8 I/o 1 7000

3 Results and Discussion The results for various characteristics of all the

yarn samples are given in Table 3 and the regression equations and regression coefficients are given in Table 4. The results and their interpretation discussed here are valid only within the experimental range of process variables used in the present work.

3.1 Yarn Unevenness

It can be observed from the regression equation given in Table 4 that the yam unevenness increases with the increase in strand spacing and traveller mass (Fig. O. This is true for all the three levels of spindle speed. It can be observed from Fig. 2 that the increase in strand spacing increases the strand length and stra.nd angle that result in loss of few fibres (sucked into pneumafi l tube) randomly - a phenomenon observed clearly on ringframe - which leads to increase in yarn unevenness.

Keeping the strand spacing constant, the increase in traveller mass and spindle speed is found to increase the yarn U% (Fig. 3) . The increase in spindle speed and traveller mass may increase the strand length due to greater downward pull (increased yam tension and tension in the strands) which can be understood from Fig. 2. The increased strand length results in loss of

4 1 4 INDIAN J . FIBRE TEXT. RES., DECEMBER 2004

Table 3 - Characteristics of siro-spun yarns

Expt U% Thin �Iaces Thick �Iaces Neps Tenacity Extension-No. -30% -40% -50% +35% +50% + 100% +200% cN/tex at-break

%

I 7.00 65 0 0 39 I I I 26 23.72 9.7 1 2 7.34 75 23 37 30 1 5 1 0 32 24.86 9. 18 3 7 .40 102 10 0 4 1 1 5 5 2 1 24.90 9.20 4 7.29 92 28 19 26 10 5 25 23.92 8.90 5 7.06 52 0 0 2 1 5 2 1 25.77 10.28 6 7 .34 88 0 0 4 1 I I 4 24 24.80 9.60 7 7.22 65 0 0 1 5 4 0 1 5 24. 1 5 9.35 8 7.04 70 0 0 2 1 2 0 14 25.36 9.53 9 7.20 59 0 0 42 1 8 4 28 23.54 9.58 10 7. 1 6 48 0 0 35 1 0 0 24 25.69 9.50 I I 7.30 68 0 0 38 1 5 2 28 25 . 1 2 9.59 1 2 7.23 88 5 1 3 1 1 9 8 26 24.26 9.59 1 3 7.05 50 I 0 24 9 2 1 9 24.67 9.75 14 7.28 6 1 0 0 34 8 I 14 25.25 9.87 1 5 7.29 62 0 0 57 1 2 4 29 24.92 9.59 16a 6.89 42 0 0 29 1 0 24 25.55 9.93

"Double-rove spun yarn

Yarn characteristic

U% Thin places

(-30%) Thin places

(-40%) Thin places

(-50%) Thick places

(+35%) Thick places

(+50%) Thick places

(+ 100 %) Neps

(+200%) Hairiness

Tenacity cN/tex

Extension-at­break, %

Table 4-Regression equations and regression coefficients for yarn characteristics

Regression equation

Z = 7.205+0.026*XI+0.050*X2+O.054*X2*X2 - 0.038*X,*X, -0. I I 5*X,*XI - 0. 1 1 3*XI *X2 Z = 56.538+4.625* XI+7 .625* X2+6.250* X3+ 1 O·058* X2* X2 -7.750* XJ* XI -2.000* XI *

X2+ 1 3 .058* XI* XI+7.750* X2* X) Z = 0.333+0.625* XI+7.000* X2+O.625* X3+6.8333* X2* X2+

1 .250* XI* X2+5.583* XI* XI+1 .250* X2* X3 - 5.9 17* X3* X) Z = - 2.250* XI+7. 125* X2+7. 1 25* X2* X2 - 4.500* XI* X2+

6.875* XI * XI+0.250* X2* X3 - 6.875* X3* X3 Z = 38.333 - 3 .500* XI - 4.000* X2+3.833* X2* X2 - 0. 1 67* XI* XI -5.667* X3* Xr3.500* XI * X)

Z = 9.667 - 1 .375* XI+0.625* X2+6.542* X2* X2 -3.458* XI* XI - 0.708* X3* X3 - 2.000* XI* X,+3.000* X2* Xr2.250* XI* X2

Z = 2.333 - 0.750* XI+ 1 .875* X2+O.625* X3+2.583* X2* Xz+ 0.333* XI* XI - 1 .4 1 7* X,* X3 - 0.750* X I* X3+2.500* X2* X3 -2.250* X I* X2

Z = 20. 1 54 - 3.5* XI+ 1 .5 * X2 - 1 .269* XI* X2+6.73 1 * X2* X2 -I * XI* X)

Z = 3 .2 1 9 - 0.09 1 * X2 + 0.064* X1 - 0.045* XI - 0. 1 0 1 * X2* X2 -0.054* XI* XI + 0.068* X2* X3 + 0. 1 05* X I* X2

Z = 24.947 - 0. 100* XI+0.034* Xz+0. 194* X3 - 0. 1 07* XI* XI -0.480* X2* Xz+0. 1 85* X3* X)+0.540* X3* XI - 0.540* X!* .\"2 -0.753* X2* X3

Z = 9.7 10 - 0.224* XI - 0.270* X2 - 0.25 1 * X) - 0.482* X2* X2+O.2 1 5 * XI* X3 - 0.337* X2* X,+0.058* XI* X2

Hairiness (H)

3.28 2.98 2.94 3.06 3. 1 6 3.22 3 . 1 2 3 . 1 6 3.22 3 .29 2 .8 1 3. 1 5 3.20 3 .2 1 3.25 3.90

Regression coefficient

0.8 1 7 0.883

0.87 1

0.85 \

0.661

0.95 1

0.968

0.824

0.932

0.959

0.967

some good fibres that vary during the course of yarn formation and this causes higher yarn U%.

Further, for a given level of traveller mass, the increase in strand spacing and spindle speed is found to increase the yarn unevenness (Fig. 4). This can be explained on similar basis as given above. Finally, it is evident from Table 5 that the yarn unevenness is luw at lower levels of al l the three experimental factors.

samples of equivalent siro-spun yams. This clearly shows that the yam becomes irregular as the spacing between the two strands increases.

For the double-rove spun yarn, the unevenness is found to be lower as compared to that of all the

3.2 Imperfections

3.2. 1 Thill Places

It is clear from Fig. 5 that the thin places (-50%) first decrease and then increase as the strand spacing increases from 6 mm to 1 0 mm. The thin places are fewer at a strand spacing of 8 mm. This can be attributed to the fact that the initial increase in strand

GOWDA et at.: CHARACTERISTICS OF MODAL SIRO-SPUN YARNS 415

1.0 .---_=----.--......... --.....

0.5

-1.0 �������L_�U - 1.0 �.S 0 O.S

Strand Spacing

Fig. 1 - U% at spindle speed of 16000 rpm

Fp

1.0

Fig. 2 - A schematic representation of spinning geometry in siro spinning [A - convergence point, s - strand spacing (nun), I -strand length (nun), 8 - strand angle, F. - strand tension, Fp - yam tension, strand spacing (s) = 21 sin(8/2), strand length (I) = sqrt.{ (sl2)2 + h2 } , yam tension (Fp) = 2Fscos(8/2), and if Fp=l, then Fs =1/{2 cos (8/2) }]

1.0 ,....--T"'T--��.,....T"""'t-"T""'t

o

o 0.5 1.0 Traveller Mass

Fig. 3 - U% at strand spacing of 6 nun

� .�

0.5

� 0

] rn -0.5

- 0.5 o 0.5 Spindle Speed

Fig. 4 - U% at traveller mass of 2/0

1.0

spacing results in better ' trapping-in of surface fibres from both the strands and therefore the fibre loss becomes minimum, which, in tum, decreases the number of thin places. A further increase in strand spacing to 1 0 mm leads to increased strand length and strand angle that lead to a random loss of fibres, resulting in more thin places. The thin places are also found to increase with the increase in traveller mass and spindle speed (Fig. 6). The heavier traveller mass and higher spindle speed generate greater yarn tension and strand tension, which along with variation if any, may lead to stretching of strands and hence more thin places.

.

The effect of process variables on thin places at -30% ,\and -40% sensitivity levels is more or less similar to that observed for thin places at -50%.

3.2.2 Thick Places

The thick places at all three sensitivity levels show a mixed trend in respect of the effect of process variables. However, the thick places at +50% and + 1 00% follow the trend which is more or less similar to that for thin places.

3.2.3 Neps

For a given level of traveller mass, the neps (+200%) show a downward trend with the increase in strand spacing (Fig. 7). The increased strand spacing results in better trapping-in of fibres or even smaller neps in the yarn body due to the interlocking between

-the two strands and hence shows fewer neps in the yarn.

The effect of spindle speed and traveller mass on neps is not definite and depends on the interactive effect between the process variables, which can be

4 1 6 INDIAN J . FIBRE TEXT. RES., DECEMBER 2004

Table 5-Values of yarn characteristics deduced from regression equations

Factors U% Thin places Thick places

Xl - 1 0

+1

CIl CIl '" ::E .... .!l Q) >-",. �

X2 X3 (-50%)

- \ -1 6.92 2 0 0 7.2 1 0

+1 +1 7 . 10 8

0

-1.0 �--��--�--��--� -1.0 - 0.5 0 O.S

Strand Spacing 1.0

(+50%)

1 2 10 10

Fig. 5 - Thin places (-50%) at spindle speed of 17000 rpm

0.5 "0 11) 11) 0.. tn 11) 0 ::a .::

.s. tn

-0.5

-].0 �----��--�--����� - 1.0 - 0.5 o O.S 1.0

Traveller Mass

Fig. 6 - Thin places (-50%) at strand spacing of 6 nun

understood from the regression equation for neps (Table 4).

Finally, i t is clear from the values given in Table 5 that the yarn samples produced at middle levels (8 mm, 1/0, 1 7000 rpm) of the three process variables have fewer imperfections.

3.3 Hairiness

For a given spindle speed, the increase in strand spacing and traveller mass decreases the yarn hairiness (Fig. 8) . The increased strand spacing results

Neps Tenacity Extension-at- Hairiness (+200%) cN/tex break, % (H)

27 20 23

CIl CIl '" ::E .... .!l Q) >-'" �

CIl � ::E ..... .!l Q) >-'" �

23.7 9.9 3.31 25.0 9.7 3.22 23.0 8.4 3 . 16

1.0

26 0.5

0

-0.5

-1.0 � ____ ....... __ .;;;:a"",---..;;_�_�...., -1.0 -0.5 0 0.5

Strand Spacing

Fig. 7 - Neps at spindle speed of 1 7000 rpm

0

�.5

1.0

-1.0 ....... ---........ --.L.......IL.....I.--I--'.&...L-L.-.J -1.0 -0.5 0 0.5 1.0

Strand Spacing

Fig. 8 - Hairiness index at spindle speed of 1 6000 rpm

in greater strand length and strand angle that lead to better trapping-in of the surface fibres from both the strands at the convergence point and hence shows lower hairiness of the yarn. This effect was also observed by Cheng et al. 7 for cotton yarn. This also coincides with the observation that the hairiness for double-rove spun yarn is higher than that for the equivalent siro-spun yarns (Table 3) . Also, the increased strand spacing causes loss in some short fibres (which generally form hairs), resulting in lower

GOWDA et al.: CHARACTERISTICS OF MODAL SIRO-SPUN YARNS 4 17

hairiness. The increase in traveller mass increases the strand tension that leads to increased strand length. The increased strand length perhaps causes better trapping-in of fibres at the convergence point and therefore reduced hairiness of yarn. The spindle speed seems to have no significant effect on the yarn hairiness (Fig. 9). Finally, i t can be concluded that the higher levels of experimental factors ( 1 0 mm, 1 , 1 8000 rpm) produce yarns with lower hairiness.

3.4 Tenacity and Extension-at-break

For a given spindle speed, the yarn tenacity and extension-at-break decrease with the increase in strand spacing (Figs 10 and 1 1 ) . This can be attributed to the observed fact that the i ncreased strand spacing causes loss in fibres due to the suction . of few fibres by pneumafil , which obviously results in lower strength and elongation of yarn.

1.0

bl) 0.5 r:: '0 '" P-m 0 ." r:: '" b m

�.5

-1.0 -1.0

l • ,

. • -0.5 0 0.5

Spindle Speed

3.1

3.2

3.2

3.3

3.3

1.0

Fig. 9 - Hairiness index at traveller mass of 2/0

1.0 .----r---r----r----,

24.6 __ --------1 0.5

-1.0 ':-..L--l."--.1-.....L..L-..L._II...-.......I.--I -0.5 0 0.5 1.0

Strand Spacing

Fig. 1 0 - Tenacity at spindle speed of 1 8000 rpm

With the increase in traveller mass, the yarn tenacity and extension-at-break first increase and then decrease. The maximum yarn tenacity is obtained by using a traveller mass of 1 10. The i nitial increase in traveller mass from 2/0 to 1 10 increases the yarn tension and strand tension that cause the stretching of fibres, leading to better fibre orientation and therefore higher strength of yarn. A further increase in traveller mass i ncreases the thin places, as discussed earlier, and hence lowers the strength and extension of yarn.

The increase in spindle speed is found to decrease the yarn tenacity and extension-at-break (Figs 1 2 and 1 3) . Probably, the generation of relatively more thin places at higher spindle speeds is responsible for the reduced yarn tenacity and extension.

Finally, it can be inferred from the data in Table 5 that the yarn strength reaches maximum at the middle levels (8 mm, 1 10, 1 7000 rpm) of the experimental factors, while the yarn extension-at break

0.5

-0.5

-1.0 �_..L..L-..::...._.t::;.. __ .....Ior:�--I -1.0 -0.5 0 1.0

Strand Spacing

Fig. 1 1 -Extension at spindle speed of 1 6000 rpm

1.0 .-----r---T"""---y-"T"""--.

-1.0

Fig. 1 2 - Tenacity at traveller mass of I

4 1 8 INDIAN J . FIBRE TEXT. RES., DECEMBER 2004

1.0 ,.....--r---r---T"'""----,

-1.0 '--_.-...'--:a.....�.a....I_ ........... _L--I -1.0 -OOS 0 OoS

Spindle Speed

Fig. 1 3 -Extension at traveller mass of 1/0

1.0

cqntinuously decreases as the process parameters shift towards the higher levels.

3.5 Characteristics of Double-rove Spun Yarn

Table 3 reveals that the double-rove spun yarn produced by no spacing between the two rovings exhibits relatively lower unevenness and higher hairiness as compared to the equivalent siro-spun · yarns. In respect of tensile characteristics, no appreciable differences are observed between the two types of yarns.

4 Conclusions 4.1 An increase in strand spacing from 6 mm to

1 0 mm is found to increase the unevenness and decrease the hairiness, strength and extension of modal siro-spun yarns.

4.2 The traveller mass and spindle speed have varying effects on characteristics of siro-spun yarns.

4.3 Considering the combined effect of all the three process variables, the strand spacing of 8 mm,/ the traveler mass of 110 and the spindle speed of 1 7000 rpm are found to produce siro yarns of optimum quality.

4.4 The siro-spun yarns are relatively inferior in respect of unevenness and superior as regards the hairiness when compared to the equivalent double­rove spun yarn.

Acknowledgement The authors are thankful to Mis Lakshmi Mills Ltd,

Coimbatore, for providing Modal roving samples and to Mis ShivaTex Yarn, Dindigul, for offering testing services.

References Nevell T P, in Cellulosic Dyeing, edited by John Shore (Society of Dyers and Colorists), 1995.

2 Lenzing modal fibre for all end uses, Tecoya Trend ( Tecoya Trend Publications Pvt. Ltd, Mumbai, India), 2000.

3 www.siro-spun.com. 4 Leary R H, Text Asia, April ( 1 997) 72. 5 Bhatnagar S, Man-made Text India, August ( 1989) 296. 6 Ishtiaque S M, Sharma I C & Sharma S, Indian J Fibre Text

Res, 1 8 ( 1 993) 1 16. 7 Cheng K P S & Sun M N, Text Res J, July ( 1 998) 520. 8 Cheng K P S, How Y L & Sun M N, Text Asia, December

( 1992) 70. 9 Bhatnagar S & Sharma S K, Man-made Text India, March

( 1 989) 95. 10 Box G E P & Behnken D W, Technometrics, 2 (4) ( 1 960)

455.