-..-.-----.----.--- -- •.- ---~--

Indian Journal of Textile ResearchVo1.9, March 1984, pp.13-18

Dimensional Stability of Plain Weft Knitted Fabrics

I C SHARMA, R NAGPAL & S SOOD

The Technological Institute of Textiles, Bhiwani 125022

Received 27 May 1983; accepted 16 December 1983

The dimensional stability of plain weft knitted fabrics made from 2/30' and 2/40' grasi crimp and acrylic yarns has beeninvestigated under three different conditions of relaxation, viz. dry relaxation, wet relaxation and tumble drying. It is foundthat in dry- and wet-relaxed states, the structure depends on the stitch length and yarn diameter. The plain weft knittedstructure has been found to be a rationally determinate structure in the fully relaxed state, as in any other state th~ nature of theknitted loop is dependent on the physical properties of yarn, mechanical processing and knitting variables. The values of Kc,

Kw, K, and KI (constants of proportionality for courses/in, wales/in, stitch density and stitch length respectively) have beenfound to be constant, predictable and independent of yarn or machine variables in fully relaxed state. In any other fabric state,these values may vary considerably and have little commercial value. Fabric thickness has been found to be independent ofloop length, but dependent on yarn diameter in fully relaxed state.

Munden 1 and Doyle2 established that the length of theyam in the knitted loop plays a major role indetermining the dimensions of a knitted fabric.Although the knitting variables, such as yarn tensionand fabric take-down tension, may appear to besignificant at the stage at which the fabric is removedfrom the machine, a subsequent relaxation process canlead to a state in which the dimensions are completelyindependent of these variables. But we find that whenthe fabric is removed from the machine, there are noforces acting on it any longer, as the loops attempt tocounteract the stresses imposed during knitting.However, since each loop interacts with the adjoiningloops, complete recovery is not physically possible.With the increasing degree;:of relaxation, the fabric willtend towards the minimum energy state. For thisreason, four different relaxation techniques werechosen in the present study to seeas to which techniqueis suitable for reaching a stable configuration.

From a theoretical study, Munden and Postle3concluded that for dry-relaxed fabrics, the values ofKw, Kc and K. (constants of proportionality forwales/in, courses/in and stitch density respectively)depend upon the yarn diameter. Shanahan and Postle4predicted that for full relaxed fabrics, both Kc and Kw

are functions of I/d, where d is the yarn diameter.Hepworth and LeafS have recently proposed a

theoretical model for full relaxed fabrics in jammedcondition. This model takes account of geometricaljamming between adjacent loops and predicts thatdimensions vary with d/I. In addition to thesetheoretical analyses, some interesting experimentalstudies on the subject have also been reported.

Wolfaardt and Knapton6 showed that the value of

K. is dependent on the yarn linear density for woollenfabrics, although no similar conclusions were drawnfor Kc and Kw. Knapton et al.7 showed that after tenlaundering cycles, the value of Kc for cotton fabricsshows some dependence on fabric tightness (measured

by JT I, where T is the linear density in tex, and I, thestitch length in inches, although KI was relativelyconstant.

It is observed that apparently conflictingconclusions can be drawn from the reports publishedso far. So, due to insufficient experimental evidence,two different counts, viz. 2/30' and 2/40" were chosenin the present study to see the variation in thedimensions of plain fabrics in various relaxed stateswith change in stitch length and yarn diameter. Thus,the effect of yarn diameter on relaxed dimensions isconsidered.

Materials and MethodsFabrics were knitted from acrylic fibres of 2 denier

and 51 mm staple length and grasi crimp fibres of 1.75denier and 51 mm staple length. Yarns of low twistliveliness (twist multiplier, 2.5) were selected tominimize the spirality effect, which is a problem,particularly with plain fabrics, and no single yarnswere used for this reason. In addition, two counts, viz.2/30' and 2/40', were taken for both acrylic and grasicrimp fibres.

Fabrics were knitted on a circular knitting machineof 15gauge and 7.5 in diameter. 336 needles of needlenumber 44 were used on the knitting machine. Fourdifferent stitch lengths were chosen to produce a widerange of fabric cover factors from a given yarn.

13

INDIAN J. TEXT. RES., VOL. 9, MARCH 1984

Fig. 1- Relationship between courses/in and reciprocal of stitchlength for 2/30' acrylic fabrics

Results and Discussion

The dimensional stability of the weft knitted plainfabrics has been investigated under three differentconditions of relaxation, viz. dry relaxation, wetrelaxation and tumble drying. The process ofrelaxation for a knitted fabric involves change in theinternal force situation for the structure so as to bringabout an equilibrium state of minimum internalenergy. Consequently, redistribution of yarn withinthe loop takes place.

Effect of courses/in-The values of courses/in areplotted against the reciprocal of stitch length for 2/30sacrylic fabrics (Fig. 1) obtained by measuringcourses/in after every relaxation treatment. It is

52 3

I/STITCH LENGTH, in

o

ex:

~ 15<fluJ<fl

ex:~o 10u

J: 20uz

30r-F.R.-FULLY RELAXED

W.R.(b)-WET RELAXED IN BOILING WATERWR(a)-WET RELAXED IN WATER AT

ROOM TEMP25I-D.R._DRY RELAXED

Tumble drying-The samples were dipped in waterfor 24 ~r. After soaking, they were hydro extracted andthen tumble dried in a tumble drier for 60 min. At thisstage, the dimensions of the samples were measured.

Stitch lengths were measured on a Shirley crimptester with an end load of 109. Fabric thickness wasmeasured on Essdiel thickness gauge tester with a totalload of 20 g f/cm 2, which is equivalent to a pressure of1.96 x 10-3 N/mm2. Different relaxation techniqueswere used to bring the fabric to minimal energy level.

Dry relaxation-After being knitted, the tubularfabrics were laid flat for several days to facilitaterecovery from the stresses imposed during knitting.Prior to testing, all the samples were conditioned instandard atmosphere (RH, 65% and temperature, 27±2°C). Sample lengths of approx. 15in were cut and asquare of lOx lOin was marked on each sample with afine waterproof ink. The samples were then laid flat for48 hr, aWterwhich they were in dry-relaxed state. Thedry-relaxed dimensions were measured at this stage.

Wet relaxation (a)-The samples were placed intrays containing water and 0.5% Lissapol at roomtemperature. Only minimal agitation of samples tookplace. They were allowed to soak for 24 hr and thendried under standard conditions. At this stage, thedimensions of the samples were measured.

Wet relaxation (b)-The samples were wetted with1% Lissapol in boiling water for 10 min by placing thefabric on a perforated sheet to prevent distortion. Theywere then dried in a circulating air oven at 115°Cfor 30min. At this stage, the dimensions of the fabrics weremeasured.

Table I-A verage Val~es of Kc, Kw and K, for Fabrics Knitted from Acrylic and Grasi Crimp Yarns

Yarn

CountDry relaxed Wet relaxed AWet relaxed BTumble dried

KcAcrylic

2/30'4.56 (-4.5)* 4.77(-2.75)5.56 (-1.6)6.07 (-0.7)

Acrylic

2/40'4.18 (-5.0) 4.47 (-4.5)5.46 ( - 3.8)6.16(-1.0)Grasi crimp

2/30'4.0 (- 5.5) 4.54 (-4.5)5.43 (- 3.0)6.14 (-0.6)

Grasi crimp

2/40'3.71 (-6.0) 4.75 (-4.8)5.41 (-2.75)6.15 (-1.2)

KwAcrylic

2/30'4.23 (- 3.7) 4.36 (-2.2)4.65 ( - 1.00)4.83 ( - 0.85)Acrylic

2/40'3.83 (-4.5) 4.07 ( - 3.75)4.59 ( - 1.75)4.82 (-0.00)Grasi crimp

2/30'3.79 (-4.6) 4.15 (- 3.6)4.61 (-2.1)4.81 (-0.75)Grasi crimp

2/40'3.55 (-6.5) 4.33 (-4.6)4.59 ( - 3.25)4.83 (-1.2)

K,

Acrylic

2/30'19.32 (-7.0) 20.83 ( - 2.5)25.86 ( - 2.4)29.36 (-18)

Acrylic

2/40'16.1 (-125) 18.24 (-120)25.14 (-43)29.75 (-25)Grasi crimp

2/30'15.22 (-119) 18.8 (-140)25.12 (-81)29.56 (-43)Grasi crimp

2/40'1~.27 (-150) 20.73 ( - 110)24.85 (- 80)29.72 (-18)

*Values in parentheses show the magnitude of intercept.

14

:3 6 9 12-- 15 18 21 24

, I (STITCH LENGTH)2,in

F.R.-FULLY RELAXED

W.R.(bl-WET RELAXED I N BOILING

700 WATER W.R.(bl

W.Rlal-WET RELAXED I N WATER n'b )V.Rlal600 AT ROOM TEMP. <:S"! ~.,'b #D.R.

D.R.-DRY RELAXED ./ ,;:s,500 ?>'b , •• ,'"

~ (;jOj 'b"'J-;:~.in 400 ,'IY~ ~., ~ ~0300 [0": '<::iOj~::I: OJ"'3 '(-:.

~'200 .•••'1; 1<:>"

~ 0,1.1"

100 ..••~,Oj'

Fig. 3- Relationship between stitch density and reciprocal ofsquare of stitch length for 2/30' acrylic fabrics

reliable method for obtaining a fabric in undistortedstate and brings a stable configuration.

Effect of stitch density(S)- The method involvingmeasurement of stitch density is more accurate thanthe other methods for studying fabric variables. Theuse of stitch density or number ofloops per unit area offabric is preferred to linear measurements, since it isless affected by distortion. This is because increase inlength, produced by longitudinal stresses, iscompensated to a certain extent by decrease in width.The values of stitch density for 2/305 acrylic fabric areplotted against the reciprocal of square of stitch lengthin Fig.3. A similar trend has been observed for 2/405acrylic and 2/305 and 2/405 grasi crimp fabrics, except aslight variation in the magnitude of intercepts. It isobserved that for the different ranges of construction,the stitch density varies linearly with the reciprocal ofsquare of stitch length and a constant K5 is obtained.The average values of K5 are given in Table 1.

The results obtained are in agreement with thosebased on Munden's relation, according to which for aknitted fabric, the stitch density or the number ofloopsper unit area is inversely proportional to the square ofstitch length. It is also observed from these results thatthe values of K5 are approximately the same for fabricsknitted from both acrylic and grasi crimp yarns. Thevalues of K5 show more spread in dry relaxed state thanin tumble dried state, confirming that tumble drying ismore reliable for obtaining a fabric in undistorted stateand brings in a stable configuration.

Effect of yarn diameter-According to Doyle 2 , thevalues of courses/in, wales/in and stitch density areindependent of the yarn material, structure and systemof knitting, but dependent on loop length. Therelationships are: courses/in = Ke/l , wales/in = Kw/l,and stitch density = K5/P, where I is the stitch lengthand Ke, Kw and Ks are constants. These relationshipsare indeed linear, but the lines plotted do not passthrough the origin, as predicted. Instead of this, theymake an intercept. In fully relaxed state (tumble

FOR WET REL AXA T ION ( b )Y =4·65X-l·0 ,'= 0'995

FOR WET RELAXATION (a )Y=4·36X -2'5,' =0· 959

2 3 4

l/STITCH LENGTH,in

25'FR._FULLY RELAXED

WR.(bl -WET RELAXED INBOILING WATER

20~W.R'(al-WET RELAXED IN WATERAT ROOM TEMP.

D.R.-DRY RELAXED

15

ru~10a:wa.

~ 5

~

Fig. 2-Relationship between wales/in and reciprocal of stitchlength for 2/30' acrylic fabrics

SHARMA et al.:eMENSIONAL STABILITY OF PLAIN WEFT KNITTED FABRICS

observed that for the different ranges of construction,courses/in and the reciprocal of stitch length arelinearly related and a constant value of Ke is obtained.A similar trend has been observed for 2/305 and 2/405

grasi crimp fabrics, except a slight difference in themagnitude of intercepts. The results obtained are inagreement with those obtained on the basis ofMunden's relation, according to which courses/in isinversely proportional to the stitch length.

It is also observed that the values of Ke areapproximately the same for fabrics knitted fromacrylic and grasi crimp yarns. The average values of Ke

and the intercepts obtained from the experimentalresults are given in Table 1. The values of Ke showmore spread in dry-relaxed state as compared to that intumble dried state, confirming that tumbl~ drying is amore reliable method for obtaining a fabric inundistorted state and brings a stable cortfiguration.

Effect of wales/in-The values of wales/in areplotted against the reciprocal of stitch length foracrylic fabrics (Fig.2) determined by measuringwales/in after every relaxation treatment. It isobservedthat for the different ranges of construction,/wales/inand the reciprocal of stitch length are linearly relatedand a constant value of Kw is obtained. A similar trendhas been observed for grasi crimp fabrics, except aslight variation in the magnitude of intercepts. Theaverage values of Kw and the intercepts obtained fromthe experimental results are given in Table 1. Theresults obtained are again in agreement with thoseobtained on the basis of Munden's relation, accordingto which wales/in is inversely proportional to the stitchlength.

It is also observed that the values of Kw areapproximately the same for fabrics knitted from bothacrylic and grasi crimp yarns. Also, the values of Kw

show more spread in dry-relaxed state than in tumbledried state, confirming that tumble drying is a more

15

INDIAN 1. TEXT. RES., VOL. 9, MARCH 1984

.-2/305ACRYLICA-2/405ACRYLICo-2!305GRA5ICRIM P+-2/405 GRASIC RI MP

Fig. 4-Relationship between loop shape factor and reciprocal ofstitch length for acrylic and grasi crimp fabrics: (A) Dry-relaxed state

. and (B) Fully relaxed state

T.D ..!

T.D.-TUMBLE DRIEDD.R.-DRY RELAXED

T.D.-TUMBLE DRIED

D.R- DRY RELAXED

000 o.. ..

~D.R.o

• -2/305 T.D.-TUMBLE DRIED

0-2/40s D.R DRY RELAXED

. 0 •

o,~D.R. 0

15

20

.- 2/305

35, 0-2/40530

.- 2/305o ~2/405I

6,5

3-5

3·0

5,0

In" 25

4,5~" 4·0

5'0

::;, ,o.R.~0'5 0'6 0-7 0"8 O·g ',0 ',1 '·2 ',3

COVER FACTOR

Fig. 6- Relationship between Kc and cover factor for acrylic yarns

2-51 I I I I I0·5 O£ 07 0,8 0·9 ',0 1-1 1·2 "3

COVE R FACTOR

&01- T.D. ,,0.°. 0. •u 5·5"

Fig. 5- Relationship between Ks and cover factor for acrylic yarns

101 I t I I I I I05 0'6 0'7 0,8 0,9 ',0 ,., 1'2 1'3

COVER FACTOR

The values of Ks are plotted against cover factorvalues for dry relaxed and fully relaxed acrylic yarns(2/30s and 2/40") in Fig.5. It is observed that thedifference in the values of Ks for the two counts isnoticeable but small. However, in dry relaxed state,this difference is not apparent and there is a definitedependence of K. on cover factor; the value of Ks

increases proportionately with increase in the value ofcover factor. In tumble dried state, this difference isnegligibly small, giving a horizontal line parallel to theaxis, i.e. the relationship is constant.

Effect of relaxation treatments on fabric length andwidth dimensions-The length and width dimensionsof the plain knitted fabrics are dependent on the coverfactor as seen in Figs 6-8, where the values of Kc, Kw

and Kl are plotted against cover factor for acrylicyarns. It is observed that Kw is proportional to - p

(cover factor) and Kc is proportional to +q (coverfactor), where p and q are constants that acquiredifferent values for the different fabric relaxation

Fig. 7-Relationship between Kw and cover factor for acrylic yarns

(8)

ci.Ua:.- ',3o>-

'<J. '2LL

g: "'1 )<l: (A

3i ',0

g, (}9o...J

0'82 1 l. ~ I'{STITCH LENGTH. in

drying), powever, the value of this intercept isminimu~ and is considered negligible. Nutting andLeafSassumed that a generalized fabric geometry mustinclude an additional term, proportional to yarndiameter, to account for this observed disparity. Theexperimental results shown in Figs.I-3 confirm thisand show that the intercepts are dependent on yarndiameter. :The magnitude of the intercept is more for2/40s yarn as compared to that for 2/30s yarn.However, in the fully relaxed state (tumble drying),these intercepts almost disappear and the effect ofdiameter is nullified.

It may be concluded that (i) Doyle's original theoryis true only when the fabrics are in their fully relaxedstate, and (ii)interpolated intercepts found in any otherrelaxed state a,re due not to diameter factor but to ahigh unrelaxed state of fabric internal energy.

Effect of loop shape factor (courses per in/wales perin)- Loop shape factor is a measure of the ratio of theloop width to the loop length. This ratio, by geometryof loop, should be constant for fabrics in thecompletely stable configurati9n. The ratio is, however,critically affected by any fabric distortion, since suchdistortion causes increase in one parameter withdecrease i~ the other.

The ratio of courses/in to wales/in is plotted againstthe reciprocal of stitch length for dry relaxed andtumble dried acrylic and grasi crimp fabrics in Fig.4. Inthe case of dry relaxation, individual points show agreater spllead from the average value. However, thespread is minimized in the case of tumble drying andshows a constant relationship, indicating that the fullrelaxation treatment (tumble drying) is more reliablefor obtaining a fabric in undistorted state and leads toa stable configuration.

Effect of cover factor-Cover factor (ratio of theprojected area of the threads to the total area) is given

by lr::- (where I is the stitch length; and c, the count).Iyc

16

SHARMA et of.: DIMENSIONAL STABILITY OF PLAIN WEFT KNITTED FABRICS

1·2

'" l-l5~ °

1·01 . & I I I I -. I0'5 ()06 0,7 <HI 0-9 1'0 1-1 1'2 1'3

COY ER FACTOR

D.R.....L­0'300'24 0,26 0'28

STITCH LENGTH, in0'22

FR.-FULLY RELAXEDW.R.(b)-WET RELAXED'· I N BOlLI NG WATE RW.R.(a)-WET RELAXED IN WATER AT ROOM TEMP.

.': ~D'R'- DRY RELAXED

• 0'038~ 0 v 0 0 F.R.

~ 0.036 ~W.R.(b):;: 0'034 "u ~ W.R(a)...~ 0-032a:1Il

if: 0'030

0·0280·20

Fig. 9- Relationship between fabric thickness and stitch length for2/36' grasi crimp fabrics

e

° ° ~1·05

e-2/305 T.D.-TUMBLE DRIED

to-2/405 D.R-DRY RELAXED

1'3T.D.-.l"\ n ° °

1·25 e e - ~ • _

Fig. 8- Relationship between KI and cover factor for acrylic yarns

Fig. 10-Relationship between fabric thickness and stitch lengthfor 2/30' acrylic fabrics

Fig. 11- Relationship between fabric weight and reciprocal ofstitch length for dry relaxed acrylic fabrics

for the dry relaxed and fully relaxed acrylic fabrics inFigs 11and 12respectively. It is evident that weight perunit area is linearly related to the reciprocal of stitchlength, as predicted by Doyle and Munden. Hence, forgross weight differences, such as between summer and

5

I I0·24 0-26 0·28 0,30

STITCH LENGTH. in

1

l{STITCH LENGTH. in

~W.R~b)

~.(O, D.R.

-<>-----<:>------O- __ --()-_F.R.

0·0280'20 0·22

o

...:r<!)

UJ

~7Sua:1Il<{u. 50

.':vi 0,038l/l~ 0·036>t:u:;: 0,034...

~ 0·0321Il

if: 0-030

125

'"E

-0;100

150

states compared. These values decrease progressivelyas the fully relaxed state is approached, but even in thisstate, values of p and q are finite, though small.

This relationship is more obviously pronounced inFig.8, where the values of loop shape factor Kl areplotted against those of cover factor for 2/30' and 2/40'acrylic yarns. For dry relaxed fabrics, the value of Klvaries from 1.02 to 1.15 for 25.4% change in stitchlength, whereas in the fully relaxed state, thesedifferences are considerably reduced. The value of Klvaries linearly with cover factor in the dry relaxed state(although the spread is higher) with a markeddependence on yarn count, i.e. K, is not a constant, assuggested by Munden, but is directly proportional tothe cover factor. However, in the fully relaxed state, Klis independent of yarn count at any value of coverfactor. Again, K, is not a constant, though the changein its value is very small compared with that in the dryrelaxed state.

Effect of loop length on fabric thickness(t)- Thefabric thickness data are plotted against loop lengthsfor the dry, wet and fully relaxed grasi crimp andacrylic fabrics in Figs 9. and 10 respectively. It isobserved that there is a definite decrease in fabric

thickness with increase in loop length in both dry andwet relaxed conditions. The measured values ofthickness in these states for any loop length areconsiderably different, indicating dependence of fabricthickness on the relaxed condition of the fabric. Thevalue of the slope of the curve for the wet relaxed fabricis lower than that for the dry relaxed fabric. In the fullyrelaxed state, however, the thickness is constant at t

=0.0374 in for 2/30' grasi crimp fabrics and at t

= 0.0386 in for 2/30' acrylic fabrics and is independentof the loop length. This is in agreement with themathematical model of Peirce9. It may be concludedthat in fully relaxed state, thickness is constant and isdependent on yarn diameter.

Effect of fabric weight per unit area- Fabric weight(g/m2) is plotted against the reciprocal of stitch length

17

INDIAN J. TEXT. RES., VOL. 9, MARCH 1984

3 The ratio of courses/in to wales/in (loop shapefactor) shows a greater spread from the average valuein the case of dry relaxation. The ratio, however,stabilizes with full relaxation, indicating that the fullrelaxation treatment brings about a stable con­figuration in the fabric.

4 In dry relaxed state, the values of Ke,Kw,Ks and Ktshow dependence on cover factor; the values increaseproportionally with increase in the value of coverfactor. However, in fully relaxed state, they show verylittle dependence on cover factor.

5 In dry and wet relaxed states, there is a definitedecrease in fabric thickness with increase in looplength. In fully relaxed state, thickness is, however,constant and shows dependence on yarn diameter.

6 Weight per unit area of fabric varies inversely withthe length of yarn knitted into the stitch.2 3 4

I/STITCH LENGTH, in

100

o

200

225

125

NE

--...'",..:- 175J:':?w~~ 150a:lD<lu.

Fig. 12-Relationship between fabric weight and reciprocal ofstitch length for fully relaxed acrylic fabrics

winter weight fabrics, a change in yarn count isnormally specified. For smaller weight adjustments,attempts can be made to lighten the fabric by knittingmore slafkly, since for a slacker fabric length isincreased, more easily by the take-down tension.However, a greater shrinkage in fabric length willoccur as the fabric is wetted out.

Conclusions

I For fabrics made from different yarns and counts,the courses/in and wales/in vary inversely with thelength of yarn knitted into the stitch. Stitch density ornumber of loops per unit area of fabric is inverselyproportional to the square of stitch length.

2 The magnitude of intercept increases withdecrease In yarn diameter. However, in fully relaxedstate, the~e intercepts almost disappear and the effectof diameter is then nullified.

AcknowledgementThe authors are thankful to Prof. R.C.D. Kaushik,

Director, TIT, Bhiwani, for permission to publish thispaper.

References

I Munden D L, J Text /nst, 50 (1959) T 448.2 Doyle P J, J Text /nst, 44 (1953) 561.3 Munden D L & Postle R, J Text /nst, 58 (1967) 532,4 Shanahan W J & Postle R, Text Res J, 40 (1970)'656.5 Hepworth B & Leaf GAY, J Text Inst, 67 (1976) 241.6 Wolfaardt C & Knapton J J F, SA WTR/ Technical Report

No.121,1969.7 Knapton JJ F, Truter E Y & Aziz A K M A, J Text /nst,66(1975)

413.

8 Nutting T S & Leaf GAY, J Text /nst, 55 (1964) T 45.9 Peirce F T, Text Res J, 17 (1947) 123.

10 Gowers eN & Hurt F N, J Text /nst, 4 (1978) 108.II Knapton J J F, Ahrens F J, Ingenthron W W & Fong W, Text

Res J, 38 (1968) 999.12 Hurley R B, Text Res J, 36 (1966) 980.