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The Influence of Thread Twist on Alterations in Fibers’ Mechanical Properties Abstract

In order to design high-quality threads, it is necessary to know the properties of threads and fibers, as well as the loadings and deformations which may occur during the sewing process. Thread properties depend on the mechanical properties of the fiber and the constructional parameters of the thread and its surface treatment, which directly influence sewing performance. The mechanical properties of a thread primarily depend on the fiber mechanical properties and the amount of twist. Knowledge of the thread dynamic loadings during the sewing process, depending on the number of turns and the lubrication method, is important for planning the required processing properties of the thread. This paper presents research into the influence of thread twist and the lubrication method on the mechanical properties and dynamic load of PES core-spun thread and its fibers. Research into the mechanical properties of the different twisted and surface treated threads, and separated fibers was carried out for this purpose. The influence of thread dynamic load during a sewing process was also researched regarding any alterations in the mechanical properties of the threads and separated fibers. Analyses of the results show that the amount of twist depends on the mechanical properties of the thread and its constituent fibers, whilst the method of surface treatment is based on the specific mechanical properties of the thread. A dynamic load causes greater or smaller thread deformations, which is reflected in changes in the thread and fiber mechanical properties. The occurred changes depend on dynamic load, amount of twist, and the lubrication method, which is confirmed with statistical analysis of the measured results.

Key words sewing thread, fiber, amount of twist, surface treatment, dynamic load, mechanical properties

Textile Research Journal Vol 76(2): 134–144 DOI: 10.1177/0040517506057424

Andreja Rudolf1 and Jelka Geršak Textile Department, Faculty of Mechanical Engineering University of Maribor, SI-2000 Maribor, Slovenia

1 Corresponding author: University of Maribor, Faculty of Mechanical Engineering, Textile Department, Smetanova ulica 17, SI-2000 Maribor, Slovenia. Tel: +386-2-220-7964; fax: +386-2220-7990; e-mail: [email protected]

www.trj.sagepub.com © 2006 SAGE Publications

The Influence of Thread Twist on Alterations in Fibers’ Mechanical Properties A. Rudolf, et al. When twisting, the fibers parallel to the yarn axis incline over a defined angle and turn around the yarn axis. This happens at the effective tensile force because of the torsion moment the fibers’ rotation around the yarn axis and the compression forces on the fibers towards the yarn core. Fibers become closer, adhesion between the fibers in the yarn increases and yarn fineness decreases [1, 2]. The stated loadings of the thread cause loadings of the fibers, which increases with the amount of twist. A core-spun thread is made of a multifilament core, which is covered with spun fibers that were twisted around the core. A filament core gives core-spun thread its necessary strength, while the fibers from the coat assure appropriate thread softness and resistance to abrasion, and protects the filaments against dynamic and thermal loadings during the sewing process. When planning such highquality thread it is necessary to define the mechanical behavior of the core-spun thread and determine the optimal degree of twist amount, which will assure suitable mechanical and processing properties for the thread. Numerous authors have studied the influence of twist on the mechanical properties of fiber, yarn and thread. They have been researching mechanical properties such as: initial elasticity modulus, breaking tenacity and breaking extension besides the relationship between tension–extension using the tensile test for spun and filament yarns and the threads, respectively, as well as the relationship between fiber and yarn strength [3–8]. Surface treatment of the thread is very important for stitch formation without defects, since incorrect and irregular treatment can cause additional thread loadings during the sewing process. Therefore, the thread lubricating agent must first of all fulfill two conditions; it must enable the uniform and smallest possible friction; as well as protecting the thread against sewing needle heat. The thread must be treated with an appropriate lubricating agent, which will act as a lubricating component, enable easy and uniform running of the thread under the dynamic conditions of the sewing process when sliding over the guiding elements and working mechanisms [9–11].

Thread Dynamic Load The dynamic loadings of thread during a sewing process have a negative influence on its processing properties. In the stitch formation process, especially the needle thread is exposed to tensional, frictional and bending loadings [11– 13]. Such severe conditions, in combination with the heat generated in the sewing needle, can reduce the initial strength of a thread. This is the one of the causes of thread breakages and damage [13]. The thread must, therefore, have such properties that it can overcome dynamic and thermal loadings, which originate due to the cyclic loadings

135

during high-speed sewing. They must however, not cause greater changes in thread strength. Analyses of the stitch formation process and the forces acting shows that the thread is exposed to numerous loadings on its way from the package to the formed stitch. This is reflected in the form of strength alteration. Studies on the thread strength reduction of the sewing threads have been an important aspect of assessing their performance during high-speed sewing. The effects of certain thread properties, machine parameters, and number of fabric plies have been investigated by many authors [14– 17]. Crow and Chamberlain [14] were two of the earliest researchers to investigate thread-strength reduction. More recently, Webster and Laing [32] investigated the effect of repeated extension and recovery on the physical properties of lockstitch seams. Studies of dynamic tension in sewing threads have been carried out by numerous authors [12, 15–24]. They found out that alteration in thread strength is closely connected to thread passages over the guiding elements of the sewing machine and, because of this, friction and bending originates between the needle thread and touching areas, as well as friction with the bobbin thread [12, 20, 21]. Furthermore, it is well known that the highest thread tension force occurs at the moment of stitch tightening [15, 20, 21]. Sundaresan et al. [22–24] discovered that the range of thread strength alteration is a function of acting dynamic loadings on the thread and its ability to offer resistance against the applied load. Any alteration in the thread’s breaking tenacity can be assigned to alterations in the fiber mechanical properties in regard to dynamic loading and/or in regard to alterations in thread structure. The majority of these loadings are cyclic by nature, and therefore, cause thread and fiber fatigue. On the basis of the above descriptions, the final value of any breaking tenacity and extension cannot be one of the main characteristics for projecting thread properties. The thread, however, must have appropriate visco-elastic properties, which are important for its behavior at the lower loadings, so that they can transmit dynamic conditions when sewing. It is, therefore, important for the sewing process that the threads’ dynamic tension, which is defined as a quotient between tensile force and thread linear density, does not exceed the value of the tension at the yield point. Tensile force which, when acting on the thread in the stitch formation process, only causes elastic thread deformation. Provided this dynamic thread tension exceeds the tension at the yield point, it then comes to the first plastic thread deformations, which are reflected through changes in the threads’ visco-elastic properties [12]. Therefore, the tension at the yield point must be as high as possible, so that thread can overcome dynamic loadings during the sewing process.

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Table 1 Properties of the applied single yarn, threads and separated fibers. Thread type

Y T1A T2A T3A T1G T2G T3G

Linear density Tt (tex)

Number of turns Tm (t/m)

Nominal

Actual

Nominal

Actual

Friction coefficient µ

13.5 13.5 × 2 13.5 × 2 13.5 × 2 13.5 × 2 13.5 × 2 13.5 × 2

13.37 30.43 30.96 31.52 28.75 28.97 29.51

– 800 1036 1200 800 1036 1200

939.21 779.68 1124.40 1334.72 752.96 1098.96 1378.64

0.20 0.22 0.21 0.20 0.22 0.23 0.22

Methodology An investigation was carried out to determine any influence of the number of turns and the lubrication method on the mechanical properties of the thread and its fibers together with the influence of the dynamic loadings on alterations in the mechanical properties of the thread and fibers. Core-spun thread was used for this research, produced from 100% polyester, with nominal linear density 13.5 tex × 3 and final twist in Z-direction with different numbers of turns, namely t1 = 800 t/m–1, t2 = 1036 t/m–1 and t3 = 1200 t/m–1. Twisted and dyed threads were then surfacetreated using the ‘apparatus lubrication’ procedure (specimens with types T1A, T2A, T3A), and the ‘galette lubrication’ procedure (specimens with types T1G, T2G, T3G). The basic properties of the prepared threads, and their fibers, are listed in Table 1. Research was carried out on a fabric produced from 100% wool in twill weave with a surface mass of 196 g/m2, warp density 26 ends/cm and weft density 23 picks/cm. The sample of length 80 cm was joined from two layers and sewn using seam type 1.01.01 [25], stitch type 301 [26], stitch length 2.5 cm, Singer sewing needle with metric number Nm 80 at stitching speed 4000 rpm, with a previously appointed thread pretension, which assured a balanced seam. Measurements of the mechanical properties were carried out on analyzed threads and their fibers before and after the sewing process. The fibers were separated from the filament core of the thread. The needle thread was carefully taken out of the seam after sewing and individual fibers were separated from the thread core for measuring any alterations in the mechanical properties. On the basis of the measured results and using a special computer program, the average curve tension-extension and the viscoelastic parameters of the thread were calculated [27]. The measurements of the fibers’ mechanical properties were carried out on a Vibrodyn 400 electronic feeding dynamometer, connected with the Vibroskop 400 linear density measuring device. Using a suitable program, the dyna-

Breaking tenacity σb (cN/tex)

Breaking extension εb (%)

Thread

Fiber

Thread

Fiber

42.09 39.10 37.85 37.74 43.24 42.88 40.03

63.2 64.8 64.0 62.0 64.0 63.6 61.8

13.29 18.42 19.01 19.98 18.85 19.18 20.23

19.7 20.6 21.5 22.3 20.4 20.6 21.9

mometer records linear density, breaking force and breaking elongation, and automatically calculates the breaking tenacity, breaking extension and elasticity modulus E0 at 1% extension. Static pretension was measured with the Coats Tensile Meter instrument. Stitching speed was controlled by a tachometer. Measurements of the thread tensile force were carried out on a Brother DB2 B737-913 sewing machine, using a specially developed device for measuring thread tensile force. The measurement results are given as average values of the 50 characteristic tensile force peaks at the moment of stitch tightening. Thread dynamic tension was calculated for comparison with tension at the yield point. Statistical analysis was carried out, based on F and t test [28]. Using these tests we investigated whether the mean values of the breaking tenacity and breaking extension were statistically significant with respect to the number of turns, the lubrication method and the dynamic load at 95% level of confidence. Picture analysis of the threads and fibers before and after the sewing process was carried out using a Ziess Axiotech 25 HD microscope. Analysis was based on determining the nature of damage seen on the tested threads and fibers. Micrographs were recorded at different magnifications. In this way it was possible to define the place along the thread, and the fiber damage regarding the interlace of the threads in the seam. All measurements were carried out under the standard testing conditions according to standard ISO 139 [29]. Tensile loading of the threads was measured according to standard ISO 2062 [30] and tensile loading of the fibers according to standard ISO 5079 [31].

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Table 2 Measurement results of the mechanical properties of the yarn, surface treated threads and separated fibers. Analyzed properties of the threads and fibers σb Breaking tenacity σb (cN/tex)

Breaking extension εb (%)

Single yarn T1A Y

Thread type T2A

T3A

T1G

Separated fibers from yarn and threads T2G

T3G

Y

T1A

T2A

T3A

T1G

T2G

T3G

42.09 39.10 37.85 37.74 43.24 42.88 40.03

63.2 64.8 64.0 62.0 64.0 63.6 61.8

s

3.66

2.95

2.18

2.17

2.97

2.59

2.79

4.3

2.4

2.0

2.3

3.1

2.7

2.3

CV

8.71

7.56

5.76

5.74

6.87

20.84 13.83

6.8

3.8

3.2

3.7

4.9

4.2

3.7

εb

13.29 18.42 19.01 19.98 18.85 19.18 20.23

15.4 20.6 21.5 22.3 20.4 20.6 21.9

s

0.63

CV

0.40

0.46

0.40

0.39

0.54

0.44

1.8

1.6

1.5

1.6

1.8

1.4

1.7

4.73

2.17

2.44

2.02

2.05

1.25

2.18

11.5

7.7

7.0

7.1

8.8

6.9

7.5

Tension in yield point σy (cN/tex) 6.62

5.87

5.58

5.24

6.13

5.97

5.62

–

–

–

–

–

–

–

Extension in yield point εy (%)

1.27

2.14

2.43

2.37

2.11

2.29

2.39

–

–

–

–

–

–

–

Elasticity modulus E0 (cN/tex)*

5.35

2.89

2.43

2.29

3.04

2.72

2.41

457

122

84

76

130

88

74

Extension ε0 (%)

0.54

0.95

1.16

1.06

0.94

1.04

1.06

–

–

–

–

–

–

–

Modulus E1 (cN/tex)

1.41

0.73

0.75

0.68

0.80

0.78

0.75

–

–

–

–

–

–

–

Extension ε1 (%)

2.99

4.67

5.21

5.26

4.64

5.00

5.33

–

–

–

–

–

–

–

Modulus E2 (cN/tex)

4.65

3.66

3.46

3.25

3.89

3.85

3.39

–

–

–

–

–

–

–

Extension ε2 (%)

10.43 15.52 16.06 17.09 15.35 16.41 17.10

–

–

–

–

–

–

–

* Elasticity modulus E0 of the fibers was measured at 1% elongation.

Results and Discussion Influence of the Number of Turns and the Lubrication Method Measurement results for the mechanical properties of the analyzed yarn and threads, as well as the mechanical properties of the separated fibers from yarn and threads show the important influence of the number of turns on the mechanical properties of the threads and fibers (Table 2). This also confirms the statistical analysis of the measured results (Table 3). On the basis of the measured results it was discovered that the values of the breaking tenacity σb, modulus E0, E1, E2 and the tension at the yield point σy decreased with an increasing number of turns, whereas the breaking extension increased with the number of turns (Figure 1). Values for the breaking tenacity of the thread changed from 37.74 to 39.10 cN/tex for the apparatuslubricated thread and from 40.03 to 43.24 cN/tex for galette-lubricated thread. In addition, this thread with the highest nominal number of turns 1200 t/m was perceived to have the highest fall in breaking tenacity. A similar trend in breaking tenacity decrease with increasing numbers of turns was also indicated for separated fibers, as shown in Figure 2(a). An increase in breaking extension with the number of turns was also shown for the threads and fibers,

as shown in Figures 1(b) and 2(b). Furthermore, the elasticity modulus E0 showed a drastic fall with an increasing number of turns for the threads and their fibers, as shown in Figures 1(c) and 2(c). The values of the modulus E1 and E2 indicate that the continuing slopes of the tension-extension curve fell with an increasing number of turns (Table 2). The tension value at the yield point is highest for apparatus and galette-lubricated threads, at the nominal number of turns 800 t/m (Figure 1(d)). Comparison of the measured results for the mechanical properties of the yarn, threads and separated fibers shows that the individual fiber has the higher values for all mechanical properties, as was expected. The most interesting cognizance is that the mechanical behavior of the analyzed core-spun yarn and threads is similar to the behavior of the separated filament fibers. This means that any decrease in breaking tenacity, elasticity modulus, and tension at the yield point comes with an increasing number of turns because of the fibers’ loadings. According to the amount of twist the orginated shear and compressive forces probably caused critical thread loading, and weakened fibers in their initial elastic regions. This is reflected firstly in the intensive reduction of the elasticity modulus E0. On the basis of this analysis it can be seen that the nominal number of turns 800 t/m gives the apparatus, as well as the galette-lubricated thread, their highest values for

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Table 3 Statistical analyses of the significant differences of the mean values of the breaking tenacity and breaking extension regarding the number of turns and the lubrication method. Test magnitude

Breaking tenacity

Influence of the number of turns

Breaking extension

Breaking tenacity Influence of the lubrication method Breaking extension

Thread type

Fcalc.

T1A – T2A

1.838

T1A – T3A

1.860

T2A – T3A

1.012

Ftab.

1.88

H(0): σ1 = σ 2

tcalc.

confirmed

2.411

confirmed

2.627

confirmed

0.251

confirmed

0.648

confirmed

5.568

ttab.

H(0), α = 0.05 rejected

1.98

rejected not rejected

T1G –T2G

1.316

T1G – T3G

1.127

T2G – T3G

1.168

confirmed

5.289

rejected

T1A –T2A

1.340

confirmed

6.824

rejected

T1A –T3A

1.020

confirmed

19.415

T2A – T3A

1.313

confirmed

11.149

rejected

T1G –T2G

1.935

unconfirmed

3.426

rejected

T1G – T3G

1.305

confirmed

17.067

1.88

1.88

1.88

not rejected 1.98

1.98

1.98

rejected

rejected

rejected

T2G – T3G

1.483

confirmed

10.764

rejected

T1A – T1G

1.011

confirmed

7.082

rejected

T2A – T2G

1.412

confirmed

10.529

T3A – T3G

1.669

confirmed

4.584

T1A – T1G

1.074

T2A – T2G

1.345

T3A – T3G

1.192

1.88

1.88

confirmed

1.514

confirmed

1.637

confirmed

2.967

1.98

rejected rejected not rejected

1.98

not rejected rejected

Note: If null hypothesis H(0): µ1 = µ2 is rejected, the differences between mean values are significant.

breaking tenacity, elasticity modulus and, especially, tension at the yield point, which is decisive for its good resistance against loadings in the sewing process. Furthermore, it can be seen that the lubrication method has an influence on the mechanical properties of the threads (Figure 1), while essential changes in the mechanical properties of the thread filament core-separated fibers were not registered (Figure 2). Statistical analysis shows the significant influence of the lubrication method first of all on the thread breaking tenacity (Table 3). Research into the influence of the lubrication method on the analyzed thread mechanical properties indicates that apparatuslubricated threads show lower values. The lower values for the apparatus-lubricated thread mechanical properties are probably a reflection of the lubricant penetration between the fibers from the coat and, thus, a greater amount of imbibed lubricant. This is reflected in the higher values for thread linear density, which moved from 30.43 to 31.52 tex, in comparison with the galette-lubricated thread, which registered lower values of linear density, namely from 28.75 to 29.51 tex (Table 1). On the basis of this, it can be considered that apparatus-lubricated threads produce lower values for actual linear density. Penetration of the lubricating agent between the fibers in the coated area, at

the same time, reduces friction between the fibers, which is reflected in the lower values for modulus E0, E1, E2 and the tension at the yield point σy of the apparatus-lubricated threads compared to the corresponding values for the galette-lubricated threads. This also show the values of breaking forces for differently lubricated threads, as can be seen in Table 1.

Alterations in Mechanical Properties after the Sewing Process Table 4 give the results for the static pretension, tensile force and dynamic tension of the analyzed threads at a stitching speed 4000 rpm. The values for dynamic tension, which is a quotient between tensile force and thread linear density, show that, for different twisted threads, the dynamic tension falls with an increasing number of turns. This is probably a reflection of the greater smoothness of more twisted thread. Alternatively, the dynamic tension results of the analyzed threads show some influence from the lubrication method. Comparisons between the apparatus-lubricated and galette-lubricated threads show lower values of dynamic tension for apparatus-lubricated threads. The only exception is the apparatus-lubricated thread with

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Figure 2 Influence of the number of turns and the lubrication method on the mechanical properties of the fibers: (a) breaking tenacity; (b) breaking extension; (c) elasticity modulus E0.

Figure 1 Influence of the number of turns and the lubrication method on the mechanical properties of the thread: (a) breaking tenacity; (b) breaking extension; (c) elasticity modulus e0; (d) tension at the yield point.

type T1A, which also has the highest friction coefficient value, static pretension and tensile force. Measurement results for the mechanical properties of the analyzed threads and separated fibers after the sewing process are presented in Table 5. These measurement results show the influence of dynamic loadings, amount of twist, and the lubrication method on the mechanical properties of the threads and fibers. Analysis of the measured results for the thread mechanical properties shows that loading in the sewing process

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Table 4 Results of the static pretension, tensile force and dynamic tension of the analyzed threads at stitching speed 4000 rpm. Thread type

Tensile force Ft

Static pretension Fst (N)

Average value Ft (N)

Standard deviation s (N)

Variation coeff. CV (%)

Dynamic tension σD (cN/tex)

1.6 1.4 1.4 1.4 1.9 1.6

1.24 1.08 1.07 1.13 1.13 1.10

0.06 0.08 0.08 0.09 0.08 0.09

4.88 7.63 7.66 8.34 7.19 8.56

4.08 3.48 3.40 3.93 3.90 3.74

T1AS T2AS T3AS T1GS T2GS T3GS

Table 5 Measurement results of the mechanical properties analyzed threads after the sewing process at stitching speed 4000 rpm. Thread type Analyzed properties of the threads and fibers

Threads

Separated fibers from threads

T1AS

T2AS

T3AS

T1GS

T2GS

T3GS

T1AS

T2AS

T3AS

T1GS

T2GS

T3GS

σb Breaking tenacity σb (cN/tex) s CV

37.41

35.40

35.55

39.61

37.99

37.28

64.5

63.8

62.8

64.2

58.1

59.7

2.66

2.46

2.26

2.42

2.81

2.53

2.2

2.08

2.3

3.4

8.2

3.1

7.12

6.94

6.37

6.10

7.39

6.78

3.3

3.3

3.7

5.3

14.1

5.3

εb Breaking extens sion εb (%) CV

16.73

16.60

18.02

16.81

16.77

18.76

18.1

18.7

19.4

16.8

17.2

18.5

0.47

0.58

0.50

0.64

0.67

0.51

1.6

1.5

1.5

1.6

2.8

1.6

2.83

3.48

2.76

3.82

4.02

2.71

8.8

8.0

7.5

9.7

16.3

8.6

Tension in yield point σy (cN/tex) 5.52

5.38

5.04

5.87

5.97

5.61

–

–

–

–

–

–

Extension in yield point εy (%)

2.56

2.78

2.80

2.70

2.92

3.09

–

–

–

–

–

–

Elasticity modulus E0 (cN/tex) *

2.23

2.04

1.84

2.25

2.14

1.88

307

136

113

302

141

95

Extension ε0 (%)

1.32

1.54

1.47

1.40

1.64

1.61

–

–

–

–

–

–

Modulus E1 (cN/tex)

1.19

1.26

1.08

1.27

1.39

1.07

–

–

–

–

–

–

Extension ε1 (%)

4.72

4.80

5.15

4.84

4.88

5.54

–

–

–

–

–

–

Modulus E2 (cNtex)

3.65

3.40

3.15

3.79

3.55

3.34

–

–

–

–

–

–

Extension ε2 (%)

13.84

13.54

14.94

13.78

13.46

15.59

–

–

–

–

–

–

* Elasticity modulus E0 of the fibers was measured at 1% elongation.

causes the decrease of the breaking tenacity values, breaking extension, tension at the yield point and, especially, the elasticity modulus, E0 (Tables 2 and 5). It also shows the greatest alteration after the sewing process for all analyzed threads (Table 6). The influence of the thread’s dynamic loadings on alterations in its mechanical properties also confirms the statistical analysis of the measured results (Table 7). A similar influence of dynamic load in the sewing process also shows alterations in the mechanical properties for separated fibers (Table 6). A decrease in breaking extension values and an increase in the elasticity modulus was perceived for the fibers of the analyzed threads, but no great changes resulted in breaking tenacity.

Investigations into alterations in the thread mechanical properties after the sewing process show that they depend on the amount of twist. The apparatus-lubricated thread with type T1AS and nominal number of turns 800 t/m, still has, after the sewing process, the highest breaking tenacity value, tension at the yield point and modulus E0 and E2. A similar dependence is also shown for the galette-lubricated thread with type T1GS, which only has a little lower tension value at the yield point. The influence of the amount of twist on the behavior of the thread in the sewing process is statistically significant for almost all samples (Table 7). Analysis of the measured results for the mechanical properties of the fibers separated from the threads after

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Table 6 Alteration in the mechanical properties of analyzed threads and separated fibers after loading in the sewing process at a stitching speed of 4000 rpm.

Thread type

Breaking tenacity alteration ∆σb (%)

T1AS T2AS T3AS T1GS T2GS T3GS

Breaking extension alteration ∆ε (%)

Elasticity modulus alteration ∆E0 (%)

Tension at yield point alteration ∆σy (%)

Thread

Fiber

Thread

Fiber

Thread

Fiber

Thread

4.31 6.46 5.79 8.41 11.42 6.86

0.46 0.31 –1.29 –0.31 8.65 2.40

9.21 12.70 9.81 10.83 12.57 7.28

12.14 13.02 13.00 17.65 16.50 15.53

22.75 15.90 19.78 25.77 21.48 22.14

–151.64 –61.90 –48.68 132.31 –60.23 –28.38

6.03 3.51 3.82 4.11 0.10 0.18

Note: Minus sign signifies increase of particular mechanical property.

Table 7 Statistical analyses of the significant differences of the mean values regarding the dynamic load, number of turns and the lubrication method after the sewing process. Test magnitude

Thread type

Fcalc.

Ftab.

H0: σ1 = σ 2

tcalc.

Breaking tenacity

T1A – T1AS T2A – T2AS T3A – T3AS T1G – T1GS T2G – T2GS T3G – T3GS

1.230 1.176 1.093 1.512 1.175 1.226

1.88 1.88 1.88 1.88 1.88 1.88

confirm confirmed confirmed confirmed confirmed confirmed

1.858 5.267 4.930 6.714 9.027 5.152

Breaking extension

T1A – T1AS T2A – T2AS T3A – T3AS T1G – T1GS T2G – T2GS T3G – T3GS

1.398 1.553 1.519 2.766 1.575 1.322

1.88 1.88 1.88 1.88 1.88 1.88

confirmed confirmed confirmed unconfirmed confirmed confirmed

19.186 17.273 21.408 19.255 19.504 15.344

T1AS – T2AS T1AS – T3AS T2AS – T3AS

1.18 1.38 1.18

1.88

confirmed confirmed confirmed

3.93 3.77 0.32

1.98

rejected rejected not rejected

T1GS – T2GS T1GS – T3GS T2GS – T3GS

0.74 0.91 1.23

1.88

confirmed confirmed confirmed

3.08 4.71 1.31

1.98

rejected rejected not rejected

T1AS – T2AS T1AS – T3AS T2AS – T3AS

0.67 0.90 1.34

1.88

confirmed confirmed confirmed

1.21 13.22 13.19

1.98

not rejected rejected rejected

T1GS – T2GS T1GS – T3GS T2GS – T3GS

0.91 1.60 1.77

1.88

confirmed confirmed confirmed

0.36 16.78 16.54

1.98

not rejected rejected rejected

Braking tenacity

T1AS – T1GS T2AS – T2GS T3AS – T3GS

1.22 0.76 0.80

1.88

confirmed confirmed confirmed

4.32 4.87 3.61

1.98

rejected rejected rejected

Breaking extension

T1AS – T1GS T2AS – T2GS T3AS – T3GS

0.54 0.73 0.96

1.88

confirmed confirmed confirmed

0.79 1.33 7.39

1.98

not rejected not rejected rejected

Influence of the dynamic load

Breaking tenacity Influence of the number of turns Breaking extension

Influence of the lubrication method

Note: If null hypothesis H(0): µ1 = µ2 is rejected, the differences between mean values are significant.

ttab.

1.98

1.98

H(0), α = 0.05 not rejected rejected rejected rejected rejected rejected rejected rejected rejected rejected rejected rejected

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the sewing process show the influence of twist amount on alterations in mechanical properties, because the separated fibers from threads with types T1AS and T1GS, have the highest values for breaking tenacity and elasticity modulus E0 (Table 5). This means that the stated threads, in spite of having the highest dynamic tension during the sewing process, still kept the highest values for mechanical properties after loadings, which is essential for any further applied thread properties in the formed stitch. Research to determine the influence of amount of twist showed that a greater number of turns offer greater smoothness to the thread and, thus, lower tensile force values when sewing. The relation between the lower number of turns and the higher breaking tenacity, yield point tension, elasticity modulus, and lower breaking extension, shows that those threads with lower number of turns assure better production properties. Analysis of the results for tension at yield point σy and the thread dynamic tension σD at the moment of stitch tightening (Tables 4 and 5) shows that dynamic tension does not exceed tension value at the yield point. This is also reflected in a small change in tension at the yield point for all analyzed threads after sewing (Table 6). The fact is that the thread dynamic tension during the sewing process exceeds the value of elasticity modulus E0, which is probably the reason for alterations in the mechanical properties after the sewing process. Furthermore, analysis of the measured results shows the important influence of the lubrication method on the mechanical properties of the thread and fibers, after the sewing process. A greater alteration in breaking tenacity is caused for galette-lubricated threads (Table 6), which is also confirmed by statistical analysis (Table 7). Separated fibers from galette-lubricated threads also show greater breaking tenacity alteration, namely to 8.65%, whereas the apparatus-lubricated threads only reach a breaking tenacity alteration value of 1.29%. The alteration in breaking extension for the separated fibers, which moves from a value of 12.14 to 13.02% for apparatus-lubricated threads and for galette-lubricated threads from 15.53 to 17.65%, also shows the influence of the lubrication method. Alteration in elasticity modulus E0 moves from a value of 15.90 to

22.75% for apparatus-lubricated threads, whereas it is greater for galette-lubricated threads, namely from 21.14 to 25.77%. Fibers undergo greater alteration in elasticity modulus E0. On this basis, it can be considered that galette-lubricated threads are less resistant to dynamic loadings than apparatus-lubricated threads.

Picture Analysis The results of the picture analyses of the threads, before and after dynamic loading in the sewing process, show that the amount of twist influences the fineness and smoothness. In Figure 3 the thread damage, caused by tensional and frictional forces, is visible during the sewing process. Damage was indicated especially at the needle and the bobbin thread interlace points, which is reflected in the forms of thread turn motions and the loosening structure of the thread, as well as in the fibers projecting out of the thread and their entanglements. Furthermore, on the basis of picture analysis results for separated fibers, it can be seen, that damage has already been done during their production and processing into yarn and thread, respectively. Different structural fiber damage is indicated, namely local softening, mechanical damage and fiber folding. On this basis, the fiber structural damage analyses after the sewing process also indicate, besides the mentioned damage, axial splitting as in Figure 4(a), surface fiber peeling (Figure 4(b)) and transverse and longitudinal cracks (Figure 4(c)). Damages, which were seen especially at the interlace points of the needle and the bobbin thread can be attributed to the tensional, frictional and bending loadings of the thread during the sewing process. It is well known that axial splitting is the continuation of transversal crack caused by tensional fatigue because of additional shear tension at the top of the crack. It is almost always parallel with the fiber axis for polyester fiber. Transversal and longitudinal cracks can be attributed to strong thread bending in the area of the sewing needle eye and the interlacing points of the needle and bobbin thread, whereas the fiber surface peeling is a reflection of frictional thread overloading, on account that this leads to rubbing of the fibers forming the thread.

Figure 3 Appearance of the analyzed threads after sewing at 50 × magnification: (a) thread with type T2GS; (b) thread with type T1AS; (c) thread with type T3GS.

The Influence of Thread Twist on Alterations in Fibers’ Mechanical Properties A. Rudolf, et al.

143

Figure 4 Appearance of the analyzed fibers separated from threads after sewing at 500 × magnification: (a) thread with type T1GS; (b) thread with type T2GS; (c) thread with type T2GS.

Conclusions Research into the mechanical properties of twisted corespun threads has shown that technologically conditional forces during thread twisting also influence the thread properties. The influence of twist shows that the increase in the number of turns, results in a decrease in breaking tenacity values, elasticity modulus and tension at the yield point, and an increase in breaking extension. The described trend also follows for the separated fibers. It is established that the researched core-spun thread corresponds to the lowest nominal number of turns 800 t/m, because it is assigned to the highest breaking tenacity, elasticity modulus and especially tension at the yield point, which is fundamental for processing thread properties during the sewing process. Furthermore, this analysis of the results shows that the lubrication method imparts specific mechanical properties to the thread. It showed that the lubrication method has an influence on the fibers from the sheath of the core-spun thread, because there are no essential changes in the mechanical properties of the fibers from the core. The differences in values for the mechanical properties of the differently lubricated threads can be attributed to the amount of imbibed lubricating agent between the fibers from the thread sheath, which is also reflected by the change in linear density and friction between the fibers. The higher linear density of the apparatus-lubricated threads in contrast to the galette-lubricated threads shows that apparatuslubricated threads, because of their greater amount of imbibed lubricating agent, produce lower values for actual linear density. This is also confirmed by the breaking force values of the differently lubricated threads. Research into the sewing performance of the analyzed threads, was shown by dynamic tension evaluation and alteration in their mechanical properties after the sewing process, that the resulting alterations in the mechanical properties are a consequence of thread loadings and its fibers during the sewing process. On the basis of picture analysis it is clear that fiber damage, which originates during the sewing process as a consequence of tensional, frictional and bending deformation of the fibers in the thread, shows a reduction of the breaking tenacity, break-

ing extension, initial elasticity modulus and tension at the yield point. A comparison between the analyzed threads shows that the threads with types T1AS and T1GS, in spite of the highest dynamic tension during the sewing process, still have the highest breaking tenacity values and elasticity modulus E0. The same holds for the separated fibers from the threads. On the basis of this it can be expected that threads, which offer the highest, and for the sewing process, the needed resistance in its initial area, will also offer good resistance against further loadings. Research on the influence of lubrication methods on thread behavior during the sewing process has shown that smaller alterations in mechanical properties were achieved for apparatus-lubricated threads, because of better resistance to dynamic loading.

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