Indian Journal of Fibre & Textile Research Vol. 28, March 2003 , pp. 60-64

Spirality of weft-knitted fabrics: Part II-Methods for the reduction of the effect

A Prime ntas"

Schoo l of Textile Industri es, T he Un iversity of Leeds, Leeds LS2 9JT, UK

Received 5 October 2001; revised received and accepted II Ma rch 2002

The effec t of yarn steam setting and fabric washing on spi rali ty has been studied and a brie f description of a ya rn treatment, based on the fa lse twisti ng process, worked out towards the solut ion of th is problem is given. The results indi cate that a possible method for the reduction o f the spi rality effect should inc lude a real reduct ion in the twi st livel iness or torque existing in the yarns.

Keywords: Knitted fab ri c, Spirality, Steam setting

1 Introduction Several techniques dealing with yarn and fabric

processing have been adopted to overcome spirality. The most suitable method for producing spirality-free single jersey fabrics is by knitting two-fo lded yarns l

.S

,

where the opposing . torsional fo rces in the sing les yarns and the resulted fo lded yarn are counterbalanced. Although the use of folded yarns, instead of sing les yarns, improves the fabric characteri sti cs2

.6

.7

, due to the problems regarding the appropri ate relation between their twist levels, fabrics may exhi bit spirality that would be in the direction of the res idual twist8. Garments such as T-shirts, knitted from fo lded yarns, become heavier than those produced by using sing les yarns. The two-folded yarns require the production of finer or lower linear density singles yarns to produce lighter fabrics, resulting 111 a dramati c increase in their production cost.

When two singles yarns of equal twist level, equi valent twist li veliness and opposite twist directions are fed to a feeder of a kn itting machine, their tendency to di stort the knitted loops towards the one or the other direction (S or Z ) is neutrali zed and a straight fabric appears I.J,9. 10 (Fig. I a). The production

of yarns of similar twist liveliness is rather di ffi cult and any exhibited spirality of the resultant fabric

, Present address: Department of Textile Enginee ring, Faculty o f Applied Technologies, T El of Piraeus, 250 Th ivon & P. Ralli , Athens 122 44, Greece. Phone : + 3·210-45 15 137; Fax: + 3-2 10-4 122977; E-mail : aprim @teipir.gr

assumes the twi st directi on of the yarn with the highest twist liveliness. Thi s method, similar to pl ating2

, is an effecti ve technique for keeping the spirality to a minimum level. Furthermore, knitting alternate ends of S- and Z- twisted yarns with equi valent twist li ve liness will produce an overall

. I ' f f b . II 12 . h . I d spira Ity- ree a n c ' Wit an Jrregu ar an uneven texture, presenting a cockling or a herring-bone9

effec t on the fabric surface (Fig. lb). Although this method is not suitable fo r the producti on of cotton or wool pl ain weft knitted fabricsJ

, it is commo nl y used in the manufacturing of stockings from fine nylon yarns lJ

, where the hen'ing-bone effect g ives the fabric a greater potential for length-wise stre tch. The last two afore-mentioned techniques are labour intensive since it is necessary to mark clearly the yarn twist direction on every yarn package I and to inspect them in order to avo id the mixing of the yarn types during the fabri c production.

Probably the first attempt for chemical correction o f spirality was made on fabrics produced from crossbred worsted yarns 14 . Thjs detrimental effect disappeared as such cloths were cold crabbing treated using cold sodium sulphide solution, which cau~ed partial decomposition of the wool fi bres, resulting in the weakening of the fa bric.

Research involving merceri zation treatment of cotton yarns showed 45% reduction in yarn twist live liness and 10- 13% reduction in the spi rality of dry-relaxed fabrics IS. On the other hand , a tremendous reduction of 20-30% in spirality was observed when merceri zati on took pl ace in the fabric state, where a

PRIMENTAS: SPIRALITY OF WEFT-KNITTED FABRICS: PART II 61

(a) (b)

Fig. I- Single jersey weft knitted fabrics made of S- and Z- twisted yarns feeding: (a) same feeder, (b) alternate feeders (NOIe the "herring-bone" effect!)

larger reduction in loop asymmetry occurred 16. Although mercerization is an efficient wet-relaxation process, giving the best results compared to

h 171 8 . . I I' f h ot ers ' , It IS not a comp ete so utlon or t e spirality of single knitted fabrics.

The blending of a small percentage of low-melt polyester fibres with cotton4 and the heat treatment of the produced yam resulted in a reduced spiral distorted fabric with an unpleasant texture since the yarn and fabric became rather stiff. Although a resin treatmentS of knitted fabrics reduced their spirality and improved their dimensional stabi lity, appearance and handle, it caused the weakening of cotton fabrics.

Twist setting or relaxation improves the mechanical stability of yarns l.4.13,19.20 as it relieves the

stresses set up in textile fibres by twisting and ensures the standstill of the twist liveliness of even highly twisted yarns while retaining their twi st level. The twist setting methods involve storage of yarn packages at adequately high temperature and relative humidity for a proper period of time2

1.22 or placement

of the yarn packages in a perfectly enclosed chamber where the regulated humid air (heated air and moisture) is forced into the chamber for a given

period of timen . Steaming at 100 °C and normal atmospheric pressure is practically equivalent to boiling in water without the disadvantages of the latter (e.g. the motion of boiling water gives a felting effect in the case of wool yarns). The proper and even steam penetration to all parts of a yarn package is the main advantage of the modern sophisticated vacuum autoclave steamers. Although the steaming process seems to be simple, great care must be exercised to avoid possible damage such as yellowing of the white mixture yarns and colour bleeding and staining in dyed yarns22

.24

. The yarn steam setting process does

not ent ire ly e liminate the un twisting torque of the set yarns and its effectiveness lasts up to the time that the yarns and the knitted fabrics are processed with a wet treatmene,7.IO. 13.2S. Especially for wool yarnsl7, the

yarn water setting, which involves the placing of the yarn packages in a vesse l with boiling water and plunging the packages into cold water and drying, is more effective as it causes less discoloration of the dyed yarnsl.l x. This method is not su itable for the treatment of cotton yarns26.

The spirality of single-jersey knitted fabrics may be temporarily corrected by low levels of fabric strain , achieved by using a former during steam process ing

. 137 18 F d f b . h ' h or presslllg···. or worste a rlCS, Ig temperature steaming of the fabric reduces spirality often after scouring but not after dyeing27. Boarding or calendering of the fabric under ordinary finishin g conditions are only transient methods because as soon as the fabric is wet, during subsequent washing or scouring, it regains its sp iral ity , resulting in a consequent loss in appf'arance and discomfort in wear. In many cases, the fabric returns to the distorted shape, it might have had, if it was produced from the same but unset yarns 13 .

2 Materials and Methods Experiments were carried out to investigate the

effect of the yarn steaming process on yarn properties and to confirm the effect of the steaming process on the temporary reduction of the spirality angle of fabr ics produced from steamed yarns.

Z-twisted yarn samples of various twist factors, produced for a series of experiments28

, were sp li t up into two groups. The yarn quantities of the first group were wound on perforated cones providi ng a means for easier penetration of the steam I n the yarn

62 INDI AN J. FI BR E T EXT. RES. , MARCH 2003

package. These bobbins were pl aced in a Sanderson steam vacuum autoslave for the setting process . The

yarns were s teamed at 105 °C under a pressure of approximate ly 69 kN' .m·2 (kPa) fo r 60 min. After thi s, the set ya rn samples of the f irs t group and the unset yarns of the second group were left to re lax in room condition s for 48 h. Fi na lly, a ll the yarn sample ' were

stored in standard atmospheri c condit ions (20±2 °C

and 65±2 % RH ) fo r four days. The yarn properties such as linear dens ity, tenacity,

snarliness, hairin es~,_ ~venness and fri ctio n of both the set and un set samples were tested on relati ve tes ting dev ices. T he means of the read ings are g iven in Tabl e 1.

A WILDT one- feeder weft knitting machine with a gauge E I4 was used fo r the prod ucti on of the knitted fab ric samples . T he ti ghtness factors were K 1 = 1.11 5

v, · 1 d K I 293 v, -I' tex .mm an 2 =. tex .mm lo r the ya rns w ith linear densiti es of 29 tex and 39 tex respec ti ve ly. T he fabric samples we re washed on a do mesti c washing mac hi ne ru nning o nce a full washing cycle. A transparent sheet arid a protracto r were used for the d irect measurement o f the spirality ang le of re laxed fabri cs before and after the washing treatment. The results obta ined are presented in Fig. 2.

3 Results and Discussion 3.1 Effect of Yarn Steiiming on Yarn Properties and Jtabric

Spirality As it can be seen from T able 1, it is c lear that the

steaming process did not affect the line3r dens ity o f both 29 tex and 39 tex yarn samples. TJ-iE linear density of 29 tex set yarn samples with low twi st

factors (32.4 and 34.9) was marg ina lly higher than that of the unset yarns . Their differences were not

significant. Co nsidering the diffe rences shown by set and unset

yarns 'in terms of tenacity , it could be said that the steaming process weakened the yarn and that the red uctio n in strength was probably due to the water abso rpti on that occurred during steaming. When the fibres absorb water they change in dimensions, swelling transversely and ax ia lly, and become more ro unded and the fibre slippage is more li kely to occur, resulting in quicker ya rn breakage. A nother poss ible reason fo r thi s fa ll in tenacity is consistent with the idea caused by the breakage of the strong hydrogen bonds fo rmed between chain molecules in the non­crystalline regio ns as we ll the weak hydrogen bonds formed between the fib res26

. Moreover, as Bchr27 has stated , the setting o r tex til e fibre structures is unders tood to be the loosening o f c ross and auxili ary va lency bobds between fibre mo lecules and the

30 OUnse\ BW . SeIBW 29 lex o UnselAW

25 ea Set AW

39 tex

~ -0 20

'0 en ~ 15

.q ~ 10

." c.. til

324 349 386 32.3 33.4 395

Twist fac~or. turns.cm"' .tcx '"

Fig.2-Effect of yarn steaming and fabri c washing on spirality angle CBW- Before washing and A W- Arte r wash ing)

Table I-Effec l of steam setting on ya rn properties

AClUall inear Twisl fac tor Steam Tenacity Snarliness Hai riness Regularity Coe ffi cie nt of density, tex turns.cm-l.tex y, selli ng cN.lex -1 cm hai rs.m·l CV O/O fri ction Crt)

29.5 32.4 N 12.55 43 .5 61.80 14.22 0.24 1

30.1 32.4 Y 12.40 7.0 63.0 1 12.40 0.252

29.2 34.9 N 15. 11 55.2 57.00 13.63 0.239

30.2 34.9 Y 12.66 9.3 52.42 13.33 0.289

29 .8 38.6 N 16.43 61.7 49.29 14.3 1 0.242

30.0 38.6 Y 14.13 12.2 47.62 13.69 0.260

40.3 32.3 N 13.56 39.4 90.62 13.15 0.249

39.7 32.3 Y L 1.99 5.6 75.56 13.3 1 0.268

40.3 33 .4 N 14.55 44.1 72.59 12.09 0. 246

39.8 33 .4 Y 13. 15 7.8 60.54 12.79 0.264

39.9 39.5 N 16. !5 56.9 63 .9 1 12.32 0.250

40.6 39.5 Y 13.82 I 1.5 49.97 12.52 0.~2

N-Unset yarn ; and Y - Sel yarn.

PRIMENTAS:"SPIRALlTY OF W EFT- KNITTED FABRICS: PART II 63

reorientation of these chain molecules followed by a new, tensionless alTangement of same at a higher degree of orientation . Despite 16% reduction in tenacity exhibited by one of the yarn samples (TF 34.9, 29 tex) , in practical terms it was considered as not of great importance.

In the case of snarliness, the theory claiming the very significant effect of the steaming on yarn twi st liveliness seems to be confirmed. The set yarns showed a substantial reduction In their twist liveliness.

Comparing the hairiness of the set and unset yarns, it could be stated that the steaming process resulted in a less hairy yarn appearance. This was probably due to hydrogen bond formation, bringing the protruding fibres (hairs) closer to the main body of the yarn . Furthermore, for both the yarn linear densities , as the twist factor increased, the hairiness decreased, a trend that was shown by both the set and unset yarns.

As the yarn evenness is concerned, the exhibited di ffere nces shown between set and unset yarns seemed to be irrelevant to the effect of steaming.

The set yarns exhibited higher coefficient of friction. Thi s may be due to the removal of wax, applied in the yarn during winding, by the steaming process. Another possible explanat ion could be that the steaming process made the yarns stiffer. Thus, as the yarns were passing around the metal surface of the yarn abras ion-friction testing device, from the

Amonton's law Tz = ~ xel' tJ , the tension T2 increased

due to the extra tension that might app l ied to the yarn to overcome its bending stiffness. The above reasoning comes in sharp contrast to the statement of Araujo and Smith4 who claimed that steam set yarns exhibi t an improved knittab ili ty as the instabili ty created by yarn bend ing during knitting is reduced.

From Fig. 2, it is clear that the yarn steaming in a ll yarn cases resulted in less spi ral fabrics . The spirality reduction was very significant for the fabrics produced from the set yarns prior to washing and less significant after washing. It must be pointed out that a great reduction can only be achi eved by the optimum yarn steaming conditions.

3.2 Application of False Twisting Process on Short-staple Yarns

The results of tbe previous experiment28 confirmed that a less twist-lively or lower torque yarn produces less distorted knitted fabric as the spirality is concerned. Therefore, it was decided to find another way to reduce or disturb the torque of short-stapl e

yarns to determint whether this could affect the spirality . This torque di sturbance could be achieved by applying the principle of false twisting process . In this process, although there is no twist alteration since the inserted twist in the upstream part of the yarn is removed as thi s yarn part passes through the false twi sting device, a change in the yarn torque may occur. A specially designed hollow spindle with two apertures (Fig. 3) was used for the purposes of this experiment29

. The spindle was rotating by means of a . motor at a constant speed of 2000 revol uti ons per minute. The whole assembly was mounted on the WILDT circul ar knitting machine that was used for the previous experimental work. After a series of tests, the yarn passage through the spindle's apertures in the manner shown in Fig. 4 was applied.

From the principle of the false twisting process it is known that as a yarn passes through the rotating dev ice, the inserted twist in the upstream yarn part is cancelled by the twist in the downstream yarn part due to their opposite directi ons. To entrap the very fast changes of the yarn to rque due to this twi sting­detwisting process, the feeding end of the experimental spind le was located very close to the knitting zone. Many possible combinatio l~ s of the speed ratio spindle/knitting machine and the spindle position in regards to the knitting zone were tried. [n some cases, the yarn breaks occurred as the yarn was almost fully detwi sted in its downstream part ,

·'-u---~m=~~O;--TH}-·(H:H:H I (iUH.k

Fi g. 3- Arrangement of the experimental spindle used for the app lication of the fal se twi sting process on a knitting mach ine

I I

I\QJL I

Fig.4-Yarn passage through the experimental false twisting spi ndle

64 INDIAN J. FIBR E TEXT. RES., MARCH 2003

whereas in some other cases the formation of numerous snarls in the upstream yarn part took place and an irregular loop distortion in the fabric appeared. In the rest of the trial s, no particular effect on the spirality of the fab rics was detected .

The fal se twi sting method for torque di sturbance or alteration of short-staple yarns proved insufficient in spite o f the fact that thi s process, used and reported by Ngor30

, had an effect on cotton/polyester (67/33) blend where there was a balance of torque in the yarn and the fabri c showed zero spira lity. A poss ible explanation to thi s result is that the heat of the heater on a FfT machine could affect the polyes te r, a thermoplastic material.

4 Conclusions Many methods have been developed to overcome

the spira lity of weft knitted fabr ics . The most common ly applied, but ex pens ive one, method is the use of two-folded ya rns. The widcly used ya rn steam­set process merely red uces the spira lity rathe r than preventing its appea rance. Mo reover, the washing of the fabrics produced from steam-set yarns increases their small spira lity angle appeared before washing. Attempts to adopt the false twi sting process fo r torque disturbance or twist alteration of short-staple yarns proved unsuccess ful. A poss ibl e method for the reduction of the spirality effect should include a real reduction of the twi st li ve liness or torque ex ist ing In the short-stap le ring-spun ya rns.

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