Indian Journal of Fibre & Textile Research Vol. 25, December 2000, pp. 277-283

Retained strength of air-spliced yarn -Rupture process and effect of test length

S Sengupta'

National Institute of Research on Jute and Allied Fibre Technology, 1 2 Regent Park, Calcutta 700 040, India

Received 9 August 1999; revised received and accepted 19 April 2000

Air-spliced yams have been prepared from carded cotton, combed cotton, polyester-cotton, cotton-acrylic and acrylic hosiery yams by repeated cutting and splicing of yams and an attempt has been made to explain the tensile failure process of these spliced yams. For cotton, acrylic and polyester-cotton blended yams, the stick-slip rupture is generally observed in spliced zone, whereas it is catastrophic in normal and spliced yams which break in normal zone. The splicing zone of cot­ton-acrylic blended yam, however, does not exhibit pronounced stick-slip rupture, instead a long creep or plastically de­formable region is observed when the spliced retained strength (SRS) is higher. The study has been extended to understand the effect of yam test length on spliced retained strength and it is found that with higher test length(50 cm) the SRS is higher than that with lower test length( l6 cm) when the splicing position is kept in the middle of the test length during tensile test­ing.

Keywords: Acrylic yam, Air-jet splicing, Cotton yam, Cotton-acrylic yam, Polyester-cotton yam, Spliced retained strength, Spliced rupture position

1 Introduction

Now-a-days, knot free yarn is a prerequISIte for weaving or knitting high quality fabrics in high speed modern machines1 ,2. Hence, during winding, the defects of the yarns are not only detected by the pre­set yarn clearer but also removed by cutting. After cutting or bobbin changing, the yarn-ends, instead of being knotted, are spliced together by a suitable splicer on the winding machine itself. Of the different types of splicer, the pneumatic ones are the most popular3.4. In the present study, an attempt has been made to explain the tensile failure process of various spliced yarns produced from carded cotton, combed cotton, polyester-cotton, cotton-acryl ic and acrylic hosiery yarns. The effect of yarn test length on splice retention strength of different yarns is also studied.

2 Materials and Methods

2.1 Materials

Three types of cotton fibres, namely F-4 14, S-6, J-34, and two types of synthetic fibres, namely polyester and acrylic, were used to prepare carded cotton, combed cotton, polyester-cotton (52:48), cotton-acrylic (60:40) and acryl ic hosiery yarns which were further used to prepare the spliced yarns.

• Phone : 42 1 2 1 15 ; Fax : 002 1 -033-47 1 2583; E-mail : nirjaft@wb.nic.in

2.2 Methods

2.2.1 Measurement of Fibre Properties

All the cotton fibre properties, except maturity coefficient, were determined on Spinlab 900 HVI fibre tester. Maturity coefficient was measured by sodium hydroxide swelling methods. For synthetic fibres, the details . of cut length and denier were supplied by the manufacturers and the tensile properties were measured by Instron tensile tester using 2 cm test length and 1 cm/min crosshead speed. Average of ten tests was taken. The physical properties of various cotton and synthetic fibres are given in Tables 1 and 2 respectively. 2.2.2 Measurement of Yarn Properties

The yarns of different counts were prepared in ring spinning system using Laxmi-Rieter spinning line and s liver blending system. Count CV% was measured

Table I-Physical properties of cotton fibres

Property F-4 I 4 S-6 J-34

2.5% Span length,mrn 26.80 29.50 26.70

Uniformity, % 49.40 50.00 48.50

Bundle strength, cN/tex 26.26 26.66 26.(:n

Elongation, % 5.70 5 .80 5.40

Fineness, denier 1 .60 1 .27 1 .42

Maturity coefficient 0.82 0.8 1 0.80

278 INDIAN J. FIBRE TEXT. RES., DECEMBER 2000

according to IS : 1 3 15 . Uster imperfections and Classimat faults were tested as described in their respective manuals6•7 . The tensile properties were mearsured in Uster tensorapid tester using 50 cm test length and 10 cm/min crosshead speed. The physical properties of yarns are shown in Tables 3 and 4.

2.2.3 Preparation of Spliced Yarns

Spliced yarns were prepared by repeated pneumatic splicing of parent hosiery yarns in a particular head of Schlafhorst 138 GKG model autoconer using DZ1 prism. The overlapping was approximately 16 mm which was fixed for DZ1 prism. For all the yarns, the opening and splicing air pressure was 5 bar. In all the cases, the opening and splicing blasts were 380 ms and 1 62 ms respectively.

Table 2-Physical properties of polyester and acrylic fibres

Property Polyester

Cut length, mm 38.00

Linear density, denier 1 .50

Strength,cN/tex 35.28

Elongation, % 25.00

Property

Acrylic

38.00

1 .50

33.9 1

29.30

2.2.4 Determination of Spliced Retained Strength (SRS)

The spliced retained strength (SRS) of the yarn was determined using the fol lowing relationship:

Table 3--Physical properties of 20 tex yams Yam

Cotton Carded Combed

Polyester-cotton Cotton-acrylic Acrylic

Mixing 1 00% F-4 1 4 65% F-4 1 4 52% Polyester 60% J-34 1 00%

Twist multiplier" Count CY, % U % Thin places/km (-50 %) Thick places/km (+50%) Neps/km (200%) Classimat faults/ 1 00 km

. AI B I C I DI A3 B3 C2 D2

" tpcm = Twist multiplier / tex· l12

Property

Mixing

Linear density, tex Twist multipl ier" Count CY, % U % Thin places/km (-50%) Thick places/km (+50%) Neps/km (200%) Classimat faults / 1 00 km

A I B I CI D I A3 B3 C2 D2

a tpcm = Twist multiplier / tex'v,

37.97 1 .78

1 2 .90 23

273 269

2 1 4 I I

35% S-6

34.6 1 1 .82

1 0.03 I

5 1 44

1 08 6

36% Combed F-4 14 40% Acrylic 1 2% Combed S-6

3 1 .75 3 1 .84 1 .56 1 .80 9.72 9 .80 2 7

20 1 4 49 1 3

1 23 253 9 1 8

Table 4--Physical properties of yams Yam

Cotton Polyester-cotton Carded Combed

1 00% J-34 65% F-4 1 4 52% Polyester 35% S-6 36% Combed F-4 1 4

1 2% Combed S-6 1 7 1 5 1 5 38.09 34.43 32.88

2.04 1 .67 1 .79 1 4.30 1 \ . l 8 1 0.80 57 5 5

472 63 50 572 87 72

744 1 44 1 85 22 5 9

Acrylic

25. 1 6 1 .82

12 . 1 7 40 38 34

325 29

Cotton-acrylic

60% J·34 40% Acrylic

1 7 3 1 .83

1 .83 1 0.02 9

20 27

29 1 20

SENGUPTA : RETAINED STRENGTH OF AIR-SPLICED YARN 279

SRS %= ___ S�pl_ic_e_d--=y_a_rn_s_tr_en--=g::....t_h __ X 100 Normal (non - spliced) yarn strength

Spliced retained strengths of all the yarns were measured on Uster tensorapid tester (Tables 5 and 6). Two test lengths 16 cm and 50 cm were chosen. The crosshead speed was 1 0 cm/min. Fifty tests of each of the yarn samples for each test length were carried out and their mean breaking loads calculated (Table 6).

During testing, the spliced zone of the yarn was kept at the middle of the test length. After rupture, the broken ends of the yarns were examined to determine whether the yarn breakage had occurred at the spliced zone or not. The percentage of yarn breakage, occuring at the spl iced zone, is expressed as: Spliced rupture position (SRP), %

Number of yam samples broken at spliced zone Total number of yarn samples tested

Thus, if all the yam ruptures occur at the spliced zone, the value of SRP is 100%.

Increase in spliced retained strength with the increase in test length from 1 6 cm to 50 cm is computed and expressed as:

Increase in SRS, % = SRS50 - SRSl6 x lOO

SRSl6 The values of SRP and the increase in SRS of four

different yarns are given in Table 6.

2.2.5 Tensile Rupture Process of Normal and Spliced Yarns

The rupture process of all the 20 tex yarns was determined by tracing complete load-elongation curve of each test sample of each yarn category. For each yarn category, thirty-five normal (non-spl iced) and one hundred spliced yarns were tested. The test length and crosshead speed were 50 cm and 10 cm/min respectively. The yarns were tested on Uster tensorapid tester, keeping the spliced zone at the middle of the test length. The tensile value and breakage zone of each test specimen of spliced yarn were noted. The spliced yarns which broke at normal zone were, at first, segregated and then the breaking strength values of the spliced yarn specimens which broke at the spliced zone were noted in decending order. Half of the population size of this spliced yam strength series, having breaking strength values higher than that of the mid value, was grouped together while the other half of the series, having lower breaking strength values, was separately grouped.

Table 5--Spliced retained strength of 20 tex yarns Type of yarn Mean breaking Spliced retained Number of

load, N strength, % breaks, % Non-spliced 2.69

Carded cotton Spliced, Gr I 2 .28 84.76 30 Spliced, Gr II 2.08 77.52 35 Spliced, Gr III \ .78 66. 1 7 35

Non-spliced 2 .77 Combed cotton Spliced, Gr I 2.47 89. 1 7 2 8

Spliced, Gr I I 2.28 82.3 1 36 Spliced, Gr III \ .88 62.87 36

Non-spliced 3.75 Polyester-cotton Spliced, Gr I 3 .38 90. 1 3 24

Spliced, Gr I I 3 .2 1 85.60 38 Spliced, Gr I I I 2.48 66. 1 3 3 8

Non-spliced 2.27 Cotton-acrylic Spl iced, Gr I \ .82 80. 1 7 20

Spliced, Gr I I 1 .66 73 .20 40 Spliced, Gr I I I \ ,44 63.28 40

Non-spliced 3 .08 Acrylic Spliced, Gr I 2.57 83.44 1 2

Spliced, Gr II 2 ,48 80.57 44 Spliced, Gr I I I 2. \ 0 68.29 44

280 INDIAN 1. FIBRE TEXT. RES., DECEMBER 2000

Table 6--Effect of test length on tensile behaviour of spliced yam Yam type Test length Mean breaking SRS Increase in Mean SRP

cm load, N Carded cotton Non-spliced 1 6 2.40

Spliced 1 6 1 .58 Non-spliced 50 2.32 Spliced 50 1 .73

Combed cotton Non-spliced 1 6 2. 1 9

Spliced 1 6 1 .58 Non-spliced 50 2. 1 7 Spliced 50 1 .7 1

Polyester-cotton Non-spliced 1 6 2.97

Spliced 1 6 2.39 Non-spliced 50 2.92 Spliced 50 2.53

Cotton-acrylic Non-spliced 1 6 2.27

Spliced 1 6 1 .66 Non-spliced 50 2. 1 1 Spliced 50 1 .68

SRS--Spliced retained strength; and SRP-Spliced rupture position

Later, the master load-elongation curves of normal yarns (N), spliced yarns which broke at normal zone (Or. I), spliced yarns which broke at spliced zone with comparatively higher breaking load (Or. II), and spliced yarns which broke at spliced zone with comparatively lower breaking load (GrIll) were drawn (Fig. 1 ) . The load-elongation curves of each of the non-spliced and spliced yarns were traced till the yarn got completely ruptured. The values of mean breaking load, SRS and number of breaks for each group of the spliced yarn are given in Table 5.

3 Results and Discussion

The synthetic fibres are longer, stronger and more extensible as compared to cotton fibres (Tables 1 and 2). Moreover, the acrylic fibre is weaker but more extensible than the polyester. It is quite expected that all these variations in the fibre properties in addition to the yarn characteristics and properties (Tables 3 and 4) have a great influence on strength realization of spliced zone of the yarns. In view of this, an attempt has been made to study the rupture process of spliced yarns made out of various fibres and their blends.

The pneumatic splicing of yarn joins the two broken ends of the yarns by twisting, wrapping and intermingling (tucking) of the fibres of which the yarn is made, as explained by Kaushik et al.8-10 • They suggested that the spliced protion is composed of two

% SRS, %

65.83

74.56 1 3 .26

72. 1 4

78.80 9.23

80.47

86.64 7.67

73. 1 3

79.62 8.87

3 ".---...,.,..---.... <aJ N

4 6 8 1 0 Extension, %

elongation, % % 5.40 4.86 1 00 5 .84 4.92 66.5 4.63

4.49 97 4.80 4.55 7 1 8.93

8 .46 98 9.33 8.7 1 76.5 5 .84

5 .7 1 95.5 6.34 4.42 75

Extension. %

Fig. 1- Load-elongation curves of non-spliced and spliced yams. (a) combed cotton, (b) carded cotton, (c) polyester-cotton (52:48), (d) acrylic, and (e) cotton-acrylic (60:40) [N- normal or non­spliced yams, 1- spliced yarns which break at normal zone, 11-spliced yarns which break at spliced zone with comparatively higher breaking load, and I I I- spliced yarns which break at spliced zone with lower breaking load]

SENGUPTA ; RETAINED STRENGTH OF AIR-SPLICED YARN 28 1

zones, namely spliced zone and transition zone, and that the structure of the yarn in the spliced zone is quite different from that of the normal yarn. Therefore, a suitable test length is to be standardized to study the tensile properties of the spliced zone of the yarn.

The master load-elongation curves of 20 tex spliced and normal yarns are shown in Fig. I . Moreover, all the tests of a particular spliced yarn are grouped as : spliced yarns which break at normal zone (Gr. I), spliced yarns which break at spliced zone with comparatively higher breaking load (Gr. II) and spliced yarns which break at spliced zone with comparatively lower breaking load (GrIll). The SRS and number of breaks of all the above groups are shown in Table 5 . During the analysis of rupture behaviour of spliced yarns, it was observed that maj ority of the tests show the break at spliced zone. The remaining breaks occur in the non-spliced zone inspite of the presence of spliced zone at the middle of the test length. This is apparently due to the wide difference in the structure of spliced zone and normal zone. The spliced zone, which creates the highly entangled zone without much thinning, does not allow much slippage between the fibres but facilitates redistribution of fibres to make the portion strong enough to sustain the load and, at the same time, redistributes and pr'j'agates the developed stress to other weakest zone of the yarn, resulting in stress concentration and consequently breakage in this zone.

Fig. l shows that the rupture process of spliced yarn varies considerably depending on the properties of constituent fibres and the breaking zone i .e. whether the yarn breaks at non-spliced portion or spliced portion. The non-spliced yarn (N) and Gr I of all the yarns studied show the elastically deformable region in the initial stages due to the resistance to slippage and then break catastrophically. But the load of Gr I yarns is always lower compared to that of the non-spliced yarns at almost all extentions, with the ultimate rupture at an early extention. This implies a slippage and rearrangement between the spliced fibres of Gr I yarns in the early stage of extention, which leads to breakage of the weakest zone of non-spliced portion. The fibre slippage in spliced zone at the initial part of extention increases successively from Gr I to Gr II and Gr III yarns as is evident from the lower slope of the curves (Fig. I ) . However, the rupture process also differs in Gr II and Gr III yarns, i.e. within the yarns where the

breaks occur at spliced zone. In all the yarns studied, except the cotton-acrylic blended yarns, the ruptures show stick-slip effect. Due to criss-cross arrangement of constituent fibres in spliced zone, some fibres, which have already reached their breaking extention, rupture at the rupture point and instantaneously the load value falls as the fibres start slipping. During this, the increase in lateral pressure due to the structural consolidation and the entangled unbroken fibres arrest further slippage. The load value thus increases again. This occurs several times and gives step-wise breaks which continue to much higher extention. In the rupture of non-spliced zone, due to different arrangement of fibres in normal yarn and higher structural consolidation during rupture, there is no slippage after the breakage of some fibres as other fibres bear the load successfully. But as soon as the majority fibres break, the remaining fibres cannot sustain the load. Hence, the catastrophic break occurs. Polyester-cotton blended yarn, quite expectedly, shows higher stick-slip effect than cotton yarns due to the presence of high strength and high extensible polyester fibre (Tables 1 and 2) which controls the slow breaking and slipping of cotton fibre reflected in a high frequency in step breaks. Moreover, the rupture of Gr III polyester­cotton yarns show a considerable increase in load after the initial rupture. The initial rupture, in this case, is basically the rupture of majority of cotton fibres and after that the polyester fibres bear the load, resulting in a considerable increase in load value. It is evident from the curves that the frequency of steps in stick-slip effect reduces much in Gr III yarn as compared to that in Gr II yarn due to the lack of sufficient entanglement during splicing, resulting in the slippage of fibres. However, acrylic-cotton spliced yarn shows a different nature of rupture process. It shows a large plastically deformable region or creep, following the end point and then an abrupt catastrophic break. As cotton fibre has lower extensibility and higher modulus compared to acrylic fibre (Tables 1 and 2), some cotton fibres, which have already been strained to the breaking point during tensile deformation, rupture but the rupture process propagates up to much higher extention due to slippage and continuous sustained breakage of cotton fibre. Gr III yarn shows lower plastically deformable region or creep than Gr II yarn, but after rupture the load reduces slowly, which is an evidence of the slippage of fibres. Moreover, some breaks in Gr III yarn

282 INDIAN 1 . FIBRE TEXT. RES., DECEMBER 2000

indicate a mere slippage between the fibres of two joining ends, resulting in very low SRS.

Table 5 shows that the SRS of Gr I yarn is about 90% for combed cotton and polyester-cotton blended yarns, whereas it is about 82% for carded cotton, acrylic and acrylic blended yarns. This reduction in SRS is apparently because of the high yarn faults (Table 3). Moreover, in the all-acrylic spliced yarn, only 1 2% breaks are at non-spliced zone, whereas carded cotton spliced yarn shows the maximum non­spliced zone break (30%). The length distribution and properties of fibres (Tables 1 and 2) as well as yarn properties (Table 3) may be the reason for this difference in occurrence of non-spliced breaks of spliced yarn . The ranges of SRS for Gr II and III yarns are 73 - 85% and 62 - 68% respectively considering all types of yarns with 35 - 45% breaks. The lowering of SRS in Gr III yarn compared to that in Gr II yarns is basically due to the insufficient entanglement of fibres, which leads to marked slippage reflected in the rupture process.

Table 6 shows the effect of test length on tensile behaviour of four different spliced yarns. In all the yarns studied, SRS at 50 cm test length is always higher than that at 1 6 cm test length. This observation, quite unexpectedly, does not follow the 'weak link effect ' of Peirce l l . Incidentally, the same phenomenon, i .e . higher tensile load in higher test length, was also observed by Bhattacharya et al. 1 2 in the case of DREF-III spun polyester-acryl ic (70:30 and 80:20) blended yarns at 10 cm and 50 cm test lengths. This is apparently due to the difference in the structures of spliced zone and normal zone. The stress development, redistribution, propagation and decay behaviour during tensile deformation of spliced yarn are expected to differ from normal zone due to the above-mentioned structural difference. It is also observed that the increase in SRS at 50 cm test length over that at 1 6 cm test length is the highest ( 1 3 .26%) in carded cotton yarn. This increase in SRS reduces in both combed cotton and polyester-cotton blended yarns successively . This may be the effect of constituent fibre variation (Tables 1 and 2) and yarn count variation. It is evident from the results of SRP that at 1 6 cm test length, almost all the breaks occur at the spliced zone. On the other hand, at 50 cm test length, a significant number of non-spliced breaks are observed in spite of the presence of spliced zone in the middle of the test length. The incidence of the

non-spliced break in spliced yarn is maximum in carded cotton yarn. These SRP values show that in the long test length, the stress development and its redistribution at the spliced zone lead to the propagation of stresses from spliced zone to the weak zone of the normal yarn, resulting in the stress concentration and consequently rupture at that zone instead of spliced zone. Higher U%, imperfections, faults and strength CV% (Table 3) are responsible for lower SRS and SRP of carded cotton yarn. The SRS of combed cotton yarn is higher than that of carded cotton yarn due to the presence of longer and finer cotton (S-6) fibres (Table 1) . For the same reason, polyester-cotton yarn shows higher SRS .

4 Conclusions

The analysis of the rupture process of the spliced yarn reveals that in about 70-80% of the total tests, the breaks are observed at spliced zone while in the rest of the tests, the breaks are observed at non­spliced zone of the spliced yarn. These non-spliced zone breaks in the spliced yarn improve the retained strength in comparison with 100% spliced zone breaks. For cotton, acrylic and polyester-cotton blended yarns, the stick-slip rupture is generally observed in spliced zone, whereas it is catastrophic in normal and spliced yarns which break in normal zone. Splicing zone of c�tton-acrylic blended yarn, however, does not exhibit pronounced stick-slip rupture, instead a long creep or plastically deformable region is observed when spliced retained strength is higher. Though the breaking load and extension of spliced yarns are lower than those of the non-spliced yarns, the stick-slip rupture or creep in the spliced zone at a high extension is useful for processabi lity in the subsequent stages.

Some breaks having a very low load value, included in the Gr III category, show a mere slippage between fibres of two joinning ends. Such type of breaks may be reduced by optimisation of splicing process.

Fibre characteristics and yarn properties have a influence in acertaining the strength retention after splicing, e.g. the higher SRS of combed cotton and polyester-cotton blended spliced yarns. Spliced retained strength at 50 cm test length is higher than that at 1 6 cm test length. This is an useful finding in view of the processabi lity of spliced yarn in downstream processes.

SENGUPTA : RETAINED STRENGTH OF AIR-SPLICED YARN 283

Acknowledgement

The author is grateful to Dr B P Sarkar, Head, MPD and to (Late) A Majumder, Principal Scientist of NIRJAFf, for their keen interest, help and suggestions

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