Indi an Journal of Fibre & Textile Research Vol. 28, March 2003, pp. 23-28

Direct determination of yarn snarliness

A Primentas"

Schoo l of Textile Industr ies, The University of Leeds, Leeds LS2 9JT, UK

Received 5 October 2001; revised received and accepted J J March 2002

The formation of snarls that takes pl ace in the yarns in post-spinning processes, such as winding, warping, weavi ng and knitting, has been studied. This de fect is due to the excessive ya rn twist liveli ness (tors ional buckling) and causes great trouble to thc manufac turers of yarns and fabric s. Biased test ing techniques are used for the measurement of this defect. A snarling testing apparatus PRIANIC and an unbiased testing technique have been developed and are reported. The invest igati on of factors such as yarn package form, yarn twist factor, time elapsed between production and processing stages of yarns and yarn condi ti oning show the significant role they play in the reduction of yarn snarliness.

Keywords: Knitting, Snarliness, Twist li ve liness, Warping, Weaving, Winding, Yarn package, Yarn twist factor

1 Introduction The twisted shape of the fibres forming a yarn

blings to the fore two of their main physico-mechanical properties: torsional rigidity and torsional buckling. The higher the torsional rigidity, the stronger is the tor­sional buckling effect, also known as yarn twist liveli­ness. It is the tendency of a newly spun yarn to untwist spontaneousli , a detrimental yarn characteristic hav­ing two representatives - the snarliness that occurs in the yarns and the spirality that appears in knitted fab­rics. On the contrary, the crepe phenomenon, appears in special woven fabrics, is considered as a beneficial result of the yarn twist liveliness.

Yarn twist liveliness is affected by the twist factor, yarn fineness2 and retractive forces , which, in turn, are determined by the torsional and bending stresses in the fibres3

, and the torque generated during yam twisting. The latter is constituted by three components, viz. tor­sion, bending and tension of the fibres4

, whereas its level depends on the degree of fibre migration that takes places. The twist liveliness of fal se-twisted and set yarns depends on their structure and, in particular, on the properties of the helical parts of their filamentss.

1.1 Snarl

The snarl is due to yarn torque and takes place dur­ing the torsional buckling effect, where the yarn retires into itself and simultaneously is twisted in the opposite

a Present address: Department of Textile Engineering, Faculty of Applied Technologies, TEl of Piraeus, 250 Thivon & P. Ralli, Athens 12244, Greece Phone: + 3-2 10-4515137; Fax: + 3-2 10-4122977 ; E-mail: aprim@ teipir.gr

twist direction6. Newly produced yams exhibit a great

tendency to untwist when freed from restraint. When such a yarn is prevented from untwisting, by being held at both ends approaching each other, it forms an arc. The nearer to U-shape this arc gradually comes, the easier is snarl formation (Fig. 1), whereas in highly twisted yarns, snarls are formed very fast before an arc appearance (Fig. 2). Recent research7 has shown that the torque required for a yarn to snarl is independent to the yarn twi st level. Snarls appear in thinner yarn parts where twist accumulation occurs and/or in a likel y part of the yam when the numerical difference (yarn weight minus untwisted stress) has a negative value.

Many problems originate from the yarn snarliness and appear in post-spinning yarn processes. In yarn winding, a snarly yarn might be accumulated behind the finger guide (knife) of an electronic slub catcher, resulting in yarn breakage and worsening its quality.

Fig . I- Snarl formation of a normally twi sted ya rn

24 INDIAN 1. FIBRE TEXT. RES. , MARCH 2003

! j

! ~- ----------

I

----~~- ........... - -.---........ ~. -- -.- -- - .

Fig.2-Snarl fo rmatio n of a hi ghly twisted ya rn

Due to lack of tensions, the snarl s are fo rmed in single yarns as they are withdrawn over the top of bobbins during the fo lding-twisting processes and especially in two-for-one twisting where thi s problem becomes more serious. In weaving, snarls in weft yarns appear when the shuttle returns to the shed from the box. Some of these snarls are entrapped into the cloth and do not open out even when the weft is subseq uentl y tensioned as picking proceeds, making necessary the cloth mending for their removal. As the hanks made from highl y twisted yarns are removed from the reel swift , they shrin k and fo rm numerous snarl s, causing great trouble during their positioning on the rods (buttons) of the hank-dyeing machi ne. Moreover, during the winding of these dyed yarn hanks to cones, the formed snarls can cause snatching, tension peaks and yarn breakage .

It is a virtue for a yarn, with its optimum twist factor, to have the minimum snarliness level that ca n be achieved by several ways such as storing9 at adequately high temperature and relative humidity (70-75 %) or by steaming lO that resu lts in a fast yarn relaxation.

1.2 Existing Methods for Testing Yarn Snarli ness Several workers3.11. 14 , in an endeavour to measure

yarn snarliness, have described various methods that are more or less based on the same principle. In these methods, a light mass object is suspended fro m the

midd le of a known length of yarn. As soon as the two yarn ends are brought together, a loop (snarl ) is formed as rotated unti l reaching the immobility state. The number of turns of the loop and/or the measurement of the di stance between the two yarn ends,s

.' 6 give the yarn snarliness level. The drawback of these methods is the constraint of the snarl formation in a specific yarn pl ace, a fact that does not permit the drawing of true conclusions about the objecti ve snarling tendency of the yarns. Furthermore, there is confusion about the mass of the hanged object and the length of the yarn samples. Also , there is no mention about yarn pretension.

Al l the above high lighted the necess ity of using a simple new method and an apparatus for the measurement of snarliness, overcoming most of the sources of errors and shortcomings of the earl ier methods. Therefore, in the present work , a snarl ing testing apparatus PRIANIC and an unbi ased test ing technique have been developed and are reported.

2 Materials and Methods 2.1 Snarling Testing Apparatus PRIANIC

The apparatus shown in Figs 3 and 4 cons ists of a base where two stands are firmly fixed. A th readed rod with two ball bearings is placed on the top of these stands. A motor on the base drives the threaded rod where beneath the latter there is a scale with li near metre (l00 cm). On the right side of the apparatus , a movable part ( J) with clamp jaw and a pointer is in contact with the threaded rod , whereas a stationary jaw (2) with a special yarn pretension system ex ists on the left side. The pretension system cons ists of an adjustable co unterbalance and a graduated rod (0-200 g) that is based on the TEX system, with tension uni t of tex/2 ± 10% g.

2.1.1 Principle

The operation principle of the dev ice is simple. A yarn length of one metre is clamped on the two jaws of the dev ice. The movab le part approaches the stati onary one as it is driven by the rotat ing threaded rod. The yarn figures a U-shape loop and according to its twi st li ve liness, a snarl is fo rmed.

2.1.2 Testing Procedure The device is set to its ini tial cond it ions. The

movable part (1) is returned to the ri ght side of the device, where the pointer indicates the value 100 on the scale. As the linear density of the yarn is known, the appropriate adjustmenr is made on the pretension system. After discarding a substanti al quantity of yarn

PRIMENTAS: DIRECT DETERMINATION OF YARN SNARLINESS 25

Fig.3-Snarling testing apparatus "PRIANIC"

Guide Movable Jaw (1) Pulley·

\ / Threaded Rod

Yarn Specimen

Scale

Pointer

Fig.4-Schematic presentation of the snarlin g testing apparatus "PR1ANIC"

from a bobbin, one metre yarn specimen is fastened in the grip of the stationary j aw (2) wh ile holding the other end of the yarn to prevent the loss of twist. The yarn is stretched until no contact of the graduated rod pointer of the pretension system can be detected and fastened in the grip of the movable part (1) . By switching on the motor, the threaded rod rotates , forcing the movable part to move towards the stationary jaw. Thus, the yarn becomes loose and figures a U-shape loop. By the time the first snarl is formed, the operation of the device is suspended . The pointer of the stopped movable part indicates a value (e.g . 73.6 cm) that represents the snarliness value of the examined yarn specimen. The higher the value, the more snarly is the yarn.

Ten yarn specimens from each sample are considered as the most appropriate for testing. The atmospheric testing conditions must be 65 ± 2 % RH at 20 ± 2 DC under which the yarn samples must be kept for conditioning for 24 h.

2.1.3 Advantages of the Apparatus and the Method

The PRIANIC apparatus has the ad vantage of the

Fig.5-Effect of yarn unwinding method on yarn twi st (a-over­end, b-sideways, and c-under-end)

standard operation speed. The movable jaw moves towards the stationary jaw at a constant speed of 0.5 cmls, giving the total maximum time of 200 s for testing a dead yarn. In the existing various apparatus and methods, there is no reference about the speed of the movable jaw. Moreover, the testing time using the PRIANIC apparatus has been reduced remarkably in rel ation to the time of the other methods that ranged approximately between 10 min and 15 min . Furthermore, the apparatus is supplied with a proper

yarn pretension system (tex/2 ±1O% g) that is strongly recommended for testing yarn characteristics under dynamic conditions. The most important feature of the PRIANIC device and the accompanied method is that the yarn forms freely a snarl in the most likely part of it, confirming its unbiased testing principle. The snarliness value given by the measurement of the distance between the two yarn ends at the starting point of the snarl formation makes this method unrivalled.

2.2 Yarn Unwinding

The simple experiment shown in Fig. 5 was carried out to investigate the effect of the yarn unwinding manner on yarn twist and twist liveliness. Two parallel roving stripes were manually wound on a cop. The over-end unwi nding added some twist in thi s assembly. The added number of turns per unit length of the unwound assembly with Z direction was equal to the figured spira ls of that length on the cop (Fig. Sa). On the contrary, the under-end unwinding (Fig. 5c) showed exactly the same behaviour apart from that the twist direction was opposite, i.e . S direction. When the assembly was unwound sideways, no

26 IND IAN J. FIBRE TEXT. RES ., MARCH 2003

a lterat ion in its structure was observed (Fig. 5b). The over-end unwinding of the yarns from their

cop package adds most of the times some twist and probably increases slightly the twist li ve liness of the yarns. It was shown that this twi st increase was negligible when referred to yarn from cone packages and therefore did not affect the twist li ve liness7

. Thus, it was decided fo r the various experiments to adopt the convenient, generally accepted and widely used over-end yarn unwinding.

2.3 Preparation of Yarn Samples and Testing of Snarliness 2.3.1 Long-Staple Yarns

Yarn samples of 32 tex were produced from 100% acry lic long-staple fibres with four different twist fac tors. Measurements of the yarn snarliness of these samp les were made on the PRIANIC appara tus, with set pretension of 15 g, every two days for a period of 20 days . The obtained results are shown in Fig. 6, which clearly shows the signi ficant role of the storage time on the re laxation of the twi st liveliness .

2.3.2 Short-Staple Yarns Experiments were carried out to examine any

alterations of the snarliness level of short-stap le yarns due to the poss ible effect of the following four factors:

(i) the shape of the yarn package (cop, cone); (i i) the twist factor of the yarn samp les; (iii ) the time elapsed from the yarn production up

to the snarliness testing; and (iv) the atmospheric conditions in wh ich the yarn samples were stored. Cotton sing les ring-spun Z-twisted yarns of

nominal linear densities 29 tex and 39 tex were produced and wound o nto cops and cones. For both these yarn linear densities, three nominal twist factors (32, 34 and 39 turns.cm,l.tex V

,) were chosen. Quantities of these six yarn samples were made up

into two lots. One of these lots was kept in a standard testing laboratory environment of 20±2 °C and 65 % RH and was referred to as conditioned lot. The other lot was kept in environment of non-standard conditions and was referred to as unconditioned.

After the yarn samples production, their linear densities and twist levels were tested and the means are presented in Table 1. The effect of the yarn package form of the unconditi oned lot, the first day after the yarn production, on the yarn snarliness is presented in Fig. 7. Table 2 shows the readings of yarn snarli ness, taken from all the yarn samples of both lots after one and one hundred days elapsed from the yarn production day.

3 Results and Discussion From Fig. 6 it is obvious that the increased twi st

level of the long-staple yarn samples of both linear densities resulted in an increase of their snarliness . On the other hand, a ll the four yarn samples showed the dramatic decrease of the snarliness tendency on the very first two days after the ir production. Thi s

98

96

E u,94 (/) (/) Q) c 'E 92 co c

(f)

~ 90 >-

88

· 1 112 Twist factor, turns.cm .tex

/-+- 24.0 --- 27.5 . 1 -.- 29.3 --+- 30.8

86+--------,--------,--------,--------,

o 5 10 Time , days

15

Fig.6- Effect of yarn storage on snarl iness value

20

Table 1- Effect of yarn package on short-staple yarn tw ist

Yarn Linear density Twist, turns.n'-' Actual twist factor

sample tex A B C turns.cm·'.tex in

S, 29.3 603.9 592.7 598.3 32.4

S2 29.3 656.5 63 1.2 643.9 34.9

S3 29.4 7 11.2 7 11.5 711.3 38.6

S4 39.4 522.3 505 .5 5 13.9 32.3

S5 38.9 545.6 526.8 536.2 33.4

S6 39.5 632.0 625 .6 628.8 39.5

A-Yarn on a cop package; B-Yarn on a cone package; and C-Average value of cop and cone yarn twi st read ings

PR IMENTAS: DIRECT DETERMINATION OF YARN SNARLINESS 27

Table 2- Effect or yarn package, condi tioning and conditioning time on short-staple yarn snarlincss

Yarn Linear density Twist factor Yarn Yarn snarliness ,cm sample tex · 1 Yl turns. cm .tex package C D

I" 100" I " 100 "

A 78.2 72.3 67 .0 52.6 S, 29.3 32.4

43.8 41.9 43 .7 4 1.6 B

A 74.0 70.7 78.3 73.0 S2 29.3 34.9

55.7 49.3 59.9 52. 1 B

A 88. 1 86.8 89.9 80.4 S) 29.4 38.6

67.0 64.4 61.9 59.5 B

A 76.1 75.0 78.6 69. 1 S4 39.4 32.3

39.4 37 .7 56.4 37.3 B

A 79.9 74.9 73 .5 67 .6 Ss 38.9 33.4

58.8 49.3 45.1 4 1.2 B

A 89.6 88. 1 90.5 82.4 S6 39.5 39.5

77.2 76. 1 65.1 52.5 B

A-Y3rn on a cop package; 13-Yarn on a cone package; C-Unconditioncd yarn; and D--Conditioncd yarn. a Number of days intcrvcned between yarn production and testing.

Yam 29 In Yam 39 lex 100

~ 90 oCone

80

5 70

60 "' "' " 50 c

~ 40 c III 30 E .. 20 >-

10

32.4 34.0 38.6 32.3 33.4 39.5

Twist Factor . tums.cm·' .texY.

Fig.7- Erfect of the yarn packagc form on yarn snarli ness

reduction in snarliness became smoother as the time from the production day was steadily elapsed. It could be also stated that yarn snarliness followed an almost linear trend.

The data in Tables 1 and 2 aBd Fig. 7 show that al­though the differences in twist amount were very small between cops and cones of the same yarn sam­ples, the yarns from the cones in all the cases exhib­ited a relatively high reduction (20-40%) in snarli­ness. This was probably due to the tension applied to the yarn resulting from the winding speed and ten­sioning devices on the winding machine. This tension seemed to extend slightly the yarn, resulting in a rear­rangement of the structure of the yarn cross-section. Moreover, the same tension may be responsible for the significant reduction in torque produced by the twist insertion during the spinning process.

With an intervened time of 100 days from the production day , the yarns from conditioned cones showed, in most of the cases, smaller tendency to form snarls. Th is may be due to the combination of various factors such as the more efficient conditioning of the yarn during the wi nding stage where the most important factor was likely to be the wind , and the application of longitudinal tensile stress on the yarn. Furthermore, a cross-wou nd cone had a much more open structure. The better conditioning of the yarn in the cone form might also be due to lower tension exist ing in the yarn for ming coils around the yarn package, resulting in a softer package when compared with the cop form of the yarn package. Furthermore, on comparing the readings obtained by measuring the yarn snarl iness after a period of 100 days with those obtained one day after the yarn production, it can be seen that the time factor contributed slightl y to the snarli ness reduction of unconditioned yarns. On the other hand, following the appropriate stat ist ical analysis that was based on the fac torial des ign analysi s of variance of these data, it could be stated that, in some cases, apart the great significance of the factor yarn package form, the fac tors conditioning and time were of noticeable importance for yarn relaxation with a significance level above 95 %.

4 Conclusions Twist li veliness remall1s a great problem,

manifesting itself as snarls in the yarns and spirality in

28 INDIAN J. FIBRE TEXT. RES ., MARCH 2003

knitted fabrics. The relative effectiveness of the various apparatus used for determining yarn snarliness and their disadvantages, concerning the adoption of the biased testing, led to the development of the snarling testing apparatus PRIANIC in an attempt to overcome most of the limitations of the earlier instruments. The validity of this instrument as well the adopted simple testing method was shown after testing a numerous series of yarns at different twist factors. Because this apparatus is still in the embryonic stage of development, many improvements need to be made. These could be: (i) an electronically adjusted pretension system that will be helpful mainly during the yarn stretching test. This test is considered as a sequential part of the snarl test. Except the presence of the advanced pretension system, the reverse movement of the movable bracket at various speeds is considered as essential; and (ii) the use of a counter with a rotating grip fixed on the movable bracket offers the opportunity for yarn twist testing. The twist alteration of the examined yarn will give the capability for examining the character of the new yarn status during the snarling test.

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