Fabric assistance gives the percentage excess clothstrength over the strength of a band of thread&equal innumber to the number in the cloth strip, i.e. it is thedifference between the strength of a band of threadswhen tested without transverse threads and thestrength of the same number of threads while in afabric.

In the present investigation, the effect of differentweft twist multipliers on the structural and mechanicalproperties of plain-woven finished fabrics has beenstudied.

Materials and Methods

Materials-Polyester and viscose were mixed in theratio 65:35. Warp and weft of2/34s count were used inthe preparation of the fabric samples. Five samples ofsingle weft with twist multipliers of3.1, 3.3, 3.5,4.0 and4.5 were prepared by using twist wheels of 44,41,39,34and 30 teeth on a Tex-Maco ring frame. The plied weftyarn already running was at the single's twistmultiplier of 3.1. For successful preparation of fabricsamples, it was decided to continue running of thedoubled yarn since the constant doubling twistimparted would have nearly the same effect on all theweft yarn samples prepared. Five samples of fabric,with the same warp but weft with varied twistmultipliers, were prepared on a Cimmco automaticloom, with a reed space of 60 in., and fabric sett of 60ends per inch and 52 picks per inch.

Test methods-Before testing, all the fabric sampleswere conditioned in standard atmosphere (RH, 65%;temp., 27 ± 2°C) for 48 hr. Standard test methods3, aslaid down by the Indian Standards Institution , wereused to determine yarn and fabric characteristics. Allthe yarn properties were measured for the foldedyarns.

Indian Journal of Textile Research

Vol. 10, March 1985, Pp. 22-25

Effect of Different Weft Twist Multipliers on the Structuraland Mechanical Properties of Plain-Woven Finished Fabrics

I C SHARMA, NARENDER GUPTA, SANJIV MEHAN, AKHIL KUMAR & NAND KUMAR GAUR

The Technological Institute of Textiles, Bhiwani 125022

Received II June 1984; accepted 24 September 1984trrhe effect of different weft twist multipliers on the structural and mechanical properties of plain-woven finished fabrics has

been investigated. Fabrics with 5 different twist multipliers in the single weft (3.1,3.3,3.5,4.0 and 4.5) were woven using 2/34spolyester-viscose (65: 35) warp and weft, keeping the other constructional parameters identical. The fabric assistance, fabricstrip strength, yarn and fabric tensile strength, abrasion resistance, stiffness, thickness, air-permeability, crease recovery andyarn crimp of the fabrics were determined. With increase in twist level, the yarn packing coefficient and inter-fibre frictionincreased initially. Beyond the optimum twist level, the fibre rupture reduced the strength and abrasion resistance because ofan insignificant contribution of such fibres towards these properties. Similar trends were observed for fabric stiffness, creaserecovery, air-permeability, and fabric assistance . ./ -,..,/

Fabric tensile properties depend, to a great extent, onthe twist multiplier of the yarn used, because twist iscorrelated, both positively and negatively, with theyarn strength. Tensile strength, one of the mostimportan,t characteristics of a woven textile, is ameasure bf the coherence of the fabric, and without auseful degree of coherence, other properties are of littlevalue. A broad correlation exists between the strengthof a cloth and its end use, and the tensile strength andabrasion resistance of a cloth may be used to predictserviceability. Every use to which a fabric may be putdemands a minimum tensile strength, which can bedetermined experimentally. The main justification isthat it provides a comprehensive check on most of thefeatures Of cloth construdion and finish. While theeffect of some of these features is important andobvious, the effect of others is negligible.

The mdst important factors are the total quantity ofmaterial in the direction under test and its quality. Theintegral strength of a fibre in a cross-section of a cloth

gives an upper limit to the strength that could possiblybe expected. Gregoryl noted that the realized strengthof a fabric is only about 50% of this strength. Anotherapproach can be made through yarn strength on theassumption that the cloth strength might beapproximately equal to the integral strength of allyarns in the direction tested, i.e. warp-way or weft­way. The yarn strength is likely to be a more usefulguide than the integral fibre strength. The latter doesnot represent the actual yarn strength as 60% of fibrescontribute their fibre strength at rupture to the yarn atthe optimum level of twist. Regarding the relationshipbetween the cloth strength and integral yarn strength,Taylor2 observed that the strength of a yarn increaseswhen it is put in a fabric because of the assistance fromadjacent yarns at cross-over points.

22

,'-

,..,

SHARMA et at.: EFFECT OF WEFT TM ON PROPERTIES OF PLAIN-WOVEN FINISHED FABRICS

The instruments used for carrying out the tests togive quantitative prediction of fabric properties andthe standardized sample sizes used are given in Table 1.

The strips were drawn from a place at least I in.away from the selvedge. They were cut out of thespecimen, 0.5 in. wider and 5 in. longer than the sizespecified (7 x 2 in.), and the side threads were ravelledout to yield a strip of correct width.

The weft flexural rigidity was calculated by using thefollowing relationship.

G = 3.39 Wl x L3 mg-cm.

where Fa is the fabric assistance; Fs> the mean fabricstrip strength; and Ms> the mean fabric strip bandstrength.

The total fabric assistance was measured by taking

the geometric mean of warp and weft fabricassistances:

~Total assistance = (warp assistance x weft assistance)2'

The total crimp was calculated as:

Total crimp (%)=(warp crimp%)t-; (.weft crimp%)t

g=(732 G/gT) kg/cm2

Table I-Properties Tested along with Instruments Usedand Sample Sizes

o F,-MsFa, %- X 100

Ms

where G is the flexural rigidi ty; W l, the cloth weight inoz/sq.yd; and L, the bending length in cm.

The relationship used for the calculation of bendingmodulus (g) is

Results and Discussion

Data on tensile strength, abrasion resistance, air­permeability, crease recovery and stiffness are given inTable 2.

Data on fabric assistance, mean strip strength, mean

strip band strength, and crimp percentage are given inTable 3.

Tensile strength-Table 2 shows that as the twistmultiplier in weft increases, the tensile strength of thefabric in the weft direction increases almost linearly till4.0 twist multiplier. With a further increase in wefttwist multiplier, the tensile strength falls. The resultsare supported by similar trends observed in the case ofyarn tensile strength for the varying weft twistmultipliers. This behaviour may be explained asfollows:

With increase in twist multiplier, the yarn packingcoefficient is increased owing to increase in inter-fibrefriction. But, beyond an optimum twist level,apparently 4.0 in this case, the fibre rupture reduces thetensile strength owing to an insignificant contributionof these fibres. This result is in agreement with that ofSreenivasan and Shankaranarayana4, who found thatcount strength product first increases and then falls toa minimum diameter. The diameter of yarnprogressively decreases with increase in twistmultiplier.

Crease recovery-With an increase in twist

multiplier the caricature of the yarn undergoes achange, as the fibres are more firmly wrapped aroundthe core of the yarn. The resulting yarns tend tobehave more stiffly. As a result, fabrics made of suchyarns used in weft show an increased crease resistance.Therefore, recovery of crease results in a compara­tively less time. The observed increase in the creaserecovery angle may also be explained likewise. Beyond4.0 twist multiplier, the crease recovery angledecreases.

Abrasion resistance-Abrasion resistance has been

described by TaitS as the most important single factorin wear. The fabric resistance to abrasion, as may be

Sample size

12 ~ 1 in.

7 x 2 in.

3 x I in.

10 threads

of 10 in.9 x I in.

Gauge, 24in.

7 x 2 in

Shirley stiffnesstester

Good Brands tensile

strength tester

Instrument

Fabric thickness

tester

Air permeability

tester Type

ATL-2/FF­12/METEFEM,

BudapestShirley crease

recovery tester

BFT abrasion testingmachine

-do-

Good Brands tensile

strength testerGerman strength

tester FMGW-500

Fabric

Fabric air

permeability

Fabric assistance

Crease recovery

angleStiffness

Bending length

where g 1 is the cloth thickness in thousands of an inch

and G, the flexural rigidity in mg-cm.Fabric assistance, both warp-way and weft-way,

was calculated by using the following relationship:

Fabric

Air permeabilityFabric thickness

Abrasion resistanceYarn

Property

Tensile strengthYarn

23

seen from Table 2, increases with increase in weft twistmultiplier, falling rapidly for 4.5 twist multiplier, even

below thr number of cycles to abrasion obtained for3.5 twist multiplier. Similar results were obtained foryarn abrasion resistance. This may be explained asfollows:

With increase in the fibre interaction, the initial

binding effect due to twist on the yarn increases, andfor twist multipliers beyond 4.0, decreases because ofdisintegration due to fibre rupture.

Fabricstiffness-Table 2 shows that the bendinglength increases with increase in twist multiplier in theweft. Consequently, the flexural rigidity and bendingmodulus (calculated by using the fabric samplethickness) also increase. This is because the increased

stiffness of yarn impedes the bending of the fabricunder its own weight in the fabric. This result is in closeagreement with the results of Cobper6 and Cusick 7.

According to Cooper, the stiffness of yarn increaseswith increase in twist, while the latter shows that

increased bending length results in higher fabricstiffn~ss.

Air petmeability- Table 2 shows that the air­permeabmty increases with increase in the weft twistmultiplier' initially, and falls rapidly beyond theapparent optimum twist level. The increase is becauseof increas~ in the packing coefficient of the yarn. Thefall in the curve beyond 4.0 twist multiplier is becauseof the fibre rupture of the yarn. This result is in close

24

agreement with that of Sreenivasan andShankaranarayana4, who found that yarn diameterdecreases with incrl:ase in twist and CV% of diameter

falls up to minimum diameter. These workersexplained that spiral diameter is lesser than the yarndiameter and the ratio tends to approach unity as twistincreases, up to the optimum level of twist.

Fabric assistance- Table 3 shows that the crossingthreads provide positive assistance to the strip strengthin all the samples, because in all cases, the strip strengthper thread is greater than the band strength per thread.It is also seen that the fabric assistance in both warpand weft directions increases with increase in twist

multiplier in the weft. This can be explained as follows.In the case of an unwoven band of threads, the ruptureof the individual threads takes place according to theirextensibilities. The weaker yarns extend more andcause an uneven extension in the band when the load is

applied. According to Morton 8, this would cause asmall relative movement between adjacent threads.When crossing thrl;'<ldsare present, this phenomenon isreduced and occurs at short intervals, so that when the

load is applied on a fabric sample, each weak spotdemands assistance from a number of threads on either

side. Fabric assistance also increases in both warp andweft directions as the warp density increases for theabove stated reasons.

The total fabric assistance also shows the same

pattern as is shown by the warp and weft assistance for

SHARMA et al.: EFFECT OF WEFT TM ON PROPERTIES OF PLAIN-WOVEN FINISHED FABRICS

initial levels of twist but falls for twist multiplier

beyond 4.0 because of the fibre rupture in the weftyarns and consequential drop in the strength.

Conclusions

(1) With increase in twist multiplier of weft, thefabric tensile strength increases almost linearly at first.Beyond the optimum twist multiplier of 4.0, thestrength reduces. Since not much strength is requiredin the weft, yarn with 4.0 twist multiplier may be usedas warp in the fabric.

(2) With an increase in twist multiplier in weft, thefabric crease recovery increases initially and then falls.Crease recovery in suitings in the warp direction is ofutmost importance and is dependent on weft used. Itmay therefore, be concluded that since it is desired tomaintain the length-wise crease in the suiting fabrics,the twist multiplier in the weft should be low.

(3) The resistance to abrasion of the fabric increases

witn mcrease in welt twist multiplier upto 4.0 and thenfalls rapidly for higher twist multipliers.

(4) The bending length, flexural rigidity andbeHd~ng modulus increase with initial increase in thetwist and then fall for twist multipliers beyond theoptimum twist level. The fabric drape is characterizedby this.

(5) The increase in twist multiplier from 3.1 to 4.0

results in an increase in the fabric permeability to airand with the further increase in the twist multiplier the

air-permeability falls rapidly.(6) The contribution of weft to the warp in the fabric

increases with increase in twist multiplier but, owing to

reduced yarn strength, beyond the optimum twistlevel, shows a decrease.

AcknowledgementThe authors wish to express their thanks to Prof. R

C D Kaushik, Director, TIT, Bhiwani, for permission

to publish this paper. They are also greatly obliged toBhiwani Textile Mills, Bhiwani, for assistance

extended in the preparation of the fabric samples.

References

I Gregory J, J Text Inst, 44 (1953) L-515.2 Taylor H M, J Text Inst, 50 (1959) T-161.3 Indian standard specifications, IS:6359 (Indian Standards

Institution, New Delhi) 1971.4 Sreenivasan K & Shankaranarayana K S, Text Res J, 31 (1961)

746.

5 Tait J H, cited in Textile fibres, yarns and fabrics by Ernest RKaswell.

6 Cooper D N E, Text Res J, 30 (1960) 150-151.7 Cusick C E, J Text Inst, 56 (1965) T-596.8 Morton W E, J Text IlIst, 25 (1934) P-262.

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