Theoretical analysis of the yarn fracture mechanism of self-twist jet vortex spinning

Abstract

According to the yarn mechanism of self-twist jet vortex spinning, this article analyzes the structure and the fracture mechanism of self-twist jet vortex spinning yarn. Combined with experiments, this article established that the fiber in self-twist jet vortex spun yarn has self-twist, which increases the mutual contact area and the cohesion between the fibers in the yarn. This is helpful to improve the evenness and tensile properties of jet vortex spun yarn. The self-twist jet vortex spinning can keep the high spinning speed of the jet vortex spinning at the same time. The research on self-twist jet vortex spinning lays the foundation for the research and the development of jet vortex spinning.

Keywords

processing fabrication production quality spinning winding/unwinding testing

In 1995, the Murata Company in Japan first proposed the jet vortex spinning technology, which set MVS (Murata Vortex Spinning) as the representative. It has a high spinning speed, while the breaking strength of the yarn is low. Since then, how to improve the breaking strength of jet vortex spun yarn has become the object of many research scholars. Rozelle¹ described the jet vortex spinning process and the yarn mechanism. His research was mainly focused on the process parameters, yarn structure and airflow in the flow field. Pei et al.² studied the effect of the nozzle hole pressure and the distance from the guide pin to the hollow spindle top on polyester yarn strength by experiments. Yu and Yu³ studied the effect of the hollow spindle cone angle on the yarn performance. Zou et al.^4,5 studied the effect of nozzle structural parameters and the velocity of the airflow at the nozzle hole exit on the flow field characteristics by numerical calculation, and obtained the relationship between the rotational airflow and twisting degree. Kyaw et al.⁶ determined the proportion of core fiber, wrapped fiber and floating fiber in jet vortex spun yarn, by comparing the structure of ring yarn, rotor spun yarn and MVS yarn. Oxenham⁷ used untwisting to verify the structure of jet vortex spun yarn. His studies have shown that MVS yarn has the appearance of ring-spun yarn, and has more wrapped fibers than jet spinning yarn. Xue et al.⁸ proposed a patent of self-twist hollow spindle. They grooved on the inner surface of the hollow spindle inlet by laser technology to achieve fiber self-twist, which could achieve the goals of reducing noil and improving yarn strength. However, the lack of systematic analysis and reasonable approach makes the effect not obvious. This article researched the self-twist jet vortex spun yarn structure and the fracture mechanism based on the yarn formation mechanism and analyzed the influence of different hollow spindle surface friction coefficients on the yarn breaking strength by experiments. During the spinning process, the self-twist of fiber will be wound into the yarn body. This research is significantly helpful to improve the jet vortex spun yarn breaking strength, and provide more comprehensive support for the design of the key components of the self-twist jet vortex spinning system.

Yarn formation mechanism

During the jet vortex spinning process, the compressed air through the nozzle holes goes tangentially into the twisting chamber of the main nozzle, along the inner wall of the nozzle, and flows along the chamber formed by the outer wall of the hollow spindle and the inner wall of nozzle chamber. The vortex formed flows downwards spirally. As Figure 1⁹ shows, the compressed air generates negative pressure at the entrance of the hollow spindle. The fiber bundle is sucked into the hollow spindle along the spiral curved surface and the guide pin after drafting. The front part of the fiber is formed of the yarn core. The fiber end becomes the free fiber end after going into the hollow spindle. Under the high-speed rotating airflow, the free fiber ends lay flat on the wall of the still hollow spindle, presenting an umbrella shape. The free fiber ends slide on the wall of the still hollow spindle and wrap the core fiber. Finally, the yarn is formed.
Figure 1.
The jet vortex spinning twisting process.

Different from jet vortex spinning, the hollow spindle of self-twist jet vortex spinning has been grooved by laser on the surface, as shown in Figure 2. The grooves can increase the surface friction coefficient of the hollow spindle. When the free fiber end rotates on the hollow spindle surface, the friction force becomes larger.
Figure 2.
The self-twist jet vortex spinning twisting process.

The friction torque applied on the fiber during its rotation around the hollow spindle can be obtained as follows
$T_{f} = f \cdot r_{f}$
(1)
where T_f (N·mm) the friction torque, f (N) is the frictional force and r_f (mm) is the fiber radius.

When the friction torque is large enough to overcome the torsional stiffness of fiber R_t, the free fiber end incurs a rolling moment on the surface of the hollow spindle. The fiber obtains torsional deformation. The critical condition of self-torsional deformation is
$T_{f} \geq R_{t}$
(2)

The torsional deformation will become larger and larger when the free fiber end rolls on the surface of the hollow spindle. The torsional deformation is called self-twist in this paper. The self-twist will be translated in to the yarn finally. In the yarn, the self-twist is elastic deformation and it will reach elastic recovery to a certain degree. The elastic recovery will make the yarn structure more compact and the cohesion between the fibers larger. The self-twist of the fibers will make the mutual contact area and the friction between the fibers larger.

Yarn structure

Yarn structure is an external manifestation of yarn mechanism and it can link up the process parameters and yarn performance. Basal and Oxenham¹⁰ observed the jet vortex spun yarn structure by scanning electron microscopy. They also observed and classified the fibers, which had different configurations, by charge-couple device (CCD) camera and tracer fiber technology. They established that the jet vortex spun yarn was constituted of core fibers in the center and wrapped fibers on the periphery.

The structure of self-twist jet vortex spun yarn is analyzed by tracer fiber technology. The self-twist jet vortex spinning hollow spindle is applied on the MVS861 jet vortex spinning machine. Dark red polyester fibers are added into the sliver as tracer fibers during the twisting process. The dark red polyester fiber is 38 mm × 1.56 dtex, provided by the Wujiang Jingyi Group. The conformations of the same core fiber in different locations are recorded continuously using a Hi-Scope three-dimensional video microscope at 160 times magnification. Then, the core fiber conformations in different locations are connected sequentially by the image mosaic technology, as shown in Figure 3. The results show that the structure of the self-twist jet vortex spun yarn is similar to that of the jet vortex spun yarn. The core fibers are parallel and straight. The wrapped fibers distribute out of the core fibers. There is a very clear boundary between the two kinds of fibers.
Figure 3.
Spatial conformation of self-twist jet vortex spun yarn.

This article compared the structures of jet vortex spun yarn and self-twist jet vortex spun yarn using a digital microscope. The yarn sample of jet vortex spinning and self-twist jet vortex spinning under the same process parameters are supported by the Wujiang Jingyi Group. Before testing, the specimens should be put into a 125–135℃ saturated steam for 20–30 min, for the heat treatment. The heat treatment can help the yarn keep the original shape, without being changed by external forces. The tested specimens are six bobbins from different spindles, which have 10 replications per bobbin when tested. The magnification of the yarn is 250 times. The main morphological characteristics of the yarns observed are shown in Figure 4. Untwisting the yarn, the outer fibers helically wrap the inner parallel fibers, and the spiral state is spaced apart. A single viscose fiber is removed carefully. The magnification of the fiber is 750 times. The main morphological characteristics of the fibers observed are shown in Figure 5. The specimen was tested using digital microscope KH-7700, according to GB/T5324-2009.¹¹
Figure 4.
Details of the yarn structure: (a) yarn structure of the jet vortex spun yarn; (b) yarn structure of the self-twist jet vortex spun yarn.

Figure 5.
Details of fiber morphology: (a) fiber morphology of the jet vortex spun yarn; (b) fiber morphology of the self-twist jet vortex spun yarn.

From Figure 4, the outer layers of the jet vortex spun yarn and the self-twist jet vortex spun yarn are both similar to that of the ring spinning yarn. For the jet vortex spun yarn in Figure 4(a), the twist angle of the outer layer fibers is 120°, measured by experiment. For the self-twist jet vortex spun yarn in Figure 4(b), the twist angle of the outer layer fibers is 150°, measured by experiment; it is slightly larger than that of jet vortex spun yarn. The evenness of the self-twist jet vortex spun yarn is much better than that of jet vortex spun yarn; the wrapped fibers are distributed unevenly in Figure 4(a). According to the larger friction coefficient, during the self-twist jet vortex spinning process, the tangential velocity near the hollow spindle surface decreases, so that the rotational angular velocity of the free end is reduced. Thereby, it increases the twist angle of the outer layer fibers.

Figure 5 shows the single viscose fiber from the self-twist jet vortex spun yarn. Figure 5(b) shows apparent twist. When the increasing hollow spindle surface friction force is enough to overcome the torsional rigidity of the fiber, the fiber end will roll on the surface of the hollow spindle under the high-speed rotating airflow, which results in self-twist.

Fracture mechanism

When the fiber is stretched to be broken, the stress analysis is as shown in Figure 6. The axial tensile distribution of the fiber is shown in Figure 7. The normal compressive stress q acting on the core fiber mainly comes from the wrapped fibers and is assumed to be uniform. When the jet vortex spun yarn is stretched by the applied force,¹² the core fibers will slip. In addition, there will be a friction force between the core fibers. The friction force obeys the Amontons Grillaume low.¹³ The tension at each head end of the fiber is zero. Further away from the head end, due to the accumulation of friction force, the tension becomes higher. When the friction force accumulates to reach the breaking strength of the fiber, the tension will not increase any more. According to the definition, the slipping length of the two head ends l_c can be calculated as follows
$2 π r_{f} \cdot l_{c} \cdot q \cdot μ + aA = π r_{f}^{2} \cdot σ_{b}$
(3)
where l_c (mm) is the slipping length, q (N) is the normal compressive stress acting on the fiber surface, μ is the surface friction coefficient of the fiber, A is the friction mutual contact area, α (N/mm²) is the constant associated with the roughness and the hardness of the material, A (mm²) is the friction mutual contact area and $σ_{b}$ (N/mm²) is the fracture stress of the fiber.
Figure 6.
Stress analysis when the fiber in the jet vortex spun yarn is stretched.

Figure 7.
Axial tension distribution of the fiber.¹⁴

The length of the fibers in jet vortex spun yarn is assumed to be the same as L (mm). The total number of the fibers is N; the proportions of the slipping fibers and the breaking fibers in the yarn are related to the slipping length l_c (mm). The number of slipping fibers N_S is
$N_{S} = N \frac{2 l_{c}}{L}$
(4)
when the yarn breaks, there is a potential fracture surface BB’, and BC = B’C’, as shown in Figure 8. The breaking strength P of the jet vortex spun yarn is constituted of two parts: one is the friction force of the slipping fibers; the other is the breaking strength of the breaking fibers
$P = N_{S} (2 π r_{f} \cdot l_{c} \cdot q \cdot μ + aA) + (N - N_{S}) π r_{f}^{2} \cdot σ_{b}$
(5)

Figure 8.
Slipping fibers and breaking fibers in the jet vortex spun yarn.

In the self-twist jet vortex spun yarn, the self-twist will reach elastic recovery, which will make the normal compressive stress q increase. The self-twist of fibers will make the friction mutual contact area A increase. According to equation (3), when the normal compressive stress q and the friction mutual contact area A increase at the same time, the slipping length lc will be reduced. According to the formula (4), when the slipping length l_c decreases and the number of the slipping fibers N_S will be reduced. According to equation (5), when the number of slipping fibers N_S decreases, the number of breaking fibers will increase. The breaking strength of self-twist jet spinning vortex yarn will increase when the number of breaking fibers increases. Therefore, the self-twist jet vortex spinning can retain a high spinning speed of the jet vortex spinning, and improve the breaking strength of yarn effectively.

Experimental details

Combined with experiments, this article analyzes comparatively the influence of different hollow spindle groove micro-feature structures on the yarn performance in order to test and verify that the self-twist jet vortex spinning can improve the yarn properties effectively. The spinning machinery and raw materials used in the experiment of this article are provided by the Wujiang Jingyi Group.

Basic experimental parameters

According to the existing analysis and exploration results,¹⁵ a more representative treatment scheme of the hollow spindle grooves is chosen, based on the MVS No.861 original hollow spindle. Because the material of the hollow spindle is ceramic, we choose a laser line to groove the hollow spindle. The laser, whose width is 0.03 mm, is chosen after comparing the effect of lasers of different types. The region of the grooves ranges from 1 to 5.25 mm away from the hollow spindle top plane. Every groove is processed 20 times by the laser. The width of the groove is 0.1 mm, and the depth of the groove is 0.1 mm. The structure parameters of the hollow spindle groove are shown in Table 1. The hollow spindle groove structures are shown in Figure 9.
Table 1.
Structure parameters of the hollow spindle groove

Hollow spindle types Angle between the groove and the hollow spindle top plane Groove number

A — —

B 90° 20

C 90° 40

Figure 9.
Hollow spindle groove structures: (a) A; (b) B; (c) C.

In the experiment, the specimen of viscose yarn is as shown in Table 2, and the spinning process parameters are shown in Table 3. The raw material is viscose fiber, 38 mm × 1.33 dtex. The quantity of the sliver is 21 g/5 m; the unevenness of the sliver is ≤2%; the moisture regain of the sliver is 11.0%. The environment of the workshop is under standard atmosphere. The relative humidity is 65 ± 3%, and the temperature is 20 ± 2℃.
Table 2.
Specimens of viscose yarn in the experiment

Hollow spindle types 19.7 tex 14.7 tex

A A-1 A-2

B B-1 B-2

C C-1 C-2

Table 3.
Spinning process parameters

Parameters 19.7 tex 14.7 tex

Pressure of nozzle (MP) 0.55 0.55

Total draft 207 223

Main draft 33 35

Feeding ratio 0.96 0.96

Winding ratio 1.02 1.02

Distance from front roller to hollow spindle (mm) 20 20

Roller center distance (mm × mm) 43 × 45 43 × 45

Diameter of hollow spindle channel (mm) 1.1 1.1

Spinning speed (m/min) 360 320

Comparison of yarn evenness

The type of yarn evenness test instrument is a YG139D,¹⁶ according to standard FZ/T1086-2000.¹⁷ The yarn pre-tension of different counts is adjusted by (0.5 ± 0.1) cN/tex. During the yarn evenness test, the test speed is 400 m/min. The sampling time is 3 min. The tested specimens are six bobbins from different spindles that have 10 replications per bobbin when tested.

As it can be seen in Table 4, the thick place, thin place and nep are quantities within 1 km, the sensitive gears of which are +50%, −50% and +200%, respectively. The CVm value of B-1 is 4.2% smaller than that of A-1. The CVm value of C-1 is 1.5% larger than that of A-1. The CVm value of B-2 is 3.7% smaller than that of A-2. The CVm value of C-2 is 4.8% larger than that of A-2. The U value of B-1 is 4.3% smaller than that of A-1. The U value of C-1 is 1.8% larger than that of A-1. The U value of B-2 is 2.3% smaller than that of A-2. The U value of C-2 is 8.1% larger than that of A-2. The slub, detail and neps of B are all significantly fewer than those of A; the slub, detail and neps of C are all significantly greater than those of A. The self-twist jet vortex spinning can improve the evenness of the jet vortex spun yarn effectively. Within a certain range, the surface friction coefficient of the hollow spindle is larger, and then the torsion deformation of the fiber is larger. The friction and the cohesion between the fibers in the yarn are larger, and the yarn structure is more compact; the yarn evenness is more uniform. When the groove number is increased (from B to C), the surface friction coefficient of the hollow spindle is too large for C. During the free fiber end rolling on the grooves, the victory of the free fiber end will be too slow. The number of free fiber end rotations is fewer in the same time period. The twist factor of the spun yarn is lower. The wrapped effort of the fibers on the yarn is too small to catch the fibers. During the spinning process, the rate of the dropping fiber under the high-speed rotating airflow rises. So, yarn evenness is significantly deteriorated.
Table 4.
Yarn evenness of MVS861 and MVS870

Yarn types CVm/% U/% Slub/ ind·km⁻¹ Detail/ ind·km⁻¹ Neps/ ind·km⁻¹

19.7 tex

A-1 13.65 10.83 4 12 10

B-1 13.07 10.36 3 4 2

C-1 13.85 11.02 6 13 12

14.7 tex

A-2 15.57 12.21 20 57 33

B-2 15.00 11.93 12 22 8

C-2 16.31 13.20 22 59 38

Yarn tensile properties

The specimen is tested using a YG068M automatic yarn tensor, according to standard GB/T398-2008.¹⁸ Before testing, the specimen should be humidified for 48 hours in a constant temperature and humidity laboratory. The yarn pre-tension of different counts is adjusted by (0.5 ± 0.1) cN/tex. The clamp distance is 0.5 m. The tested speed is 500 mm/min. The tested specimens are six bobbins from different spindles that have 10 replications per bobbin when tested. The mean ± SD (standard deviation) is mainly decided by the test equipment and environment.¹⁹

As Table 5 shows, the breaking strength of B-1 is about 6.9% more than that of A-1; the breaking strength of C-1 is about 30.1% less than that of A-1; the breaking strength of B-2 is about 2.9% more than that of A-2; the breaking strength of C-2 is about 4.0% more than that of A-2; the breaking elongation of B-1 is about 6.5% less than that of A-1; the breaking elongation of C-1 is about 3.1% more than that of A-1; the breaking elongation of B-2 is about 7.3% less than that of A-2; the breaking elongation of C-2 i about 3.1% more than that of A-2; the self-twist jet vortex spun yarn 40^S B-2 increases by about 6.5%. The breaking work and breaking tenacity have a similar tendency to the breaking strength and the breaking elongation. During the spinning process, the self-twist effect of the fiber free end is wound into the yarn. The self-twist of fibers will make the normal compressive stress q and the friction mutual contact area A increase, which will reduce the slipping length l_c of the yarn. The breaking strength of self-twist jet spinning vortex yarn will increase as the number of slipping fibers N_S decreases. When the surface friction coefficient of the hollow spindle is too large, as in C, the wrapped effort of the fibers will be too small. The yarn tensile properties will be significantly deteriorated. Within a certain range, self-twist jet vortex spinning can improve the tensile properties of jet vortex spun yarn, which can expand the application areas of the yarn.
Table 5.
Results of yarn tensile properties

Yarn types Breaking strength/cN Breaking elongation/% Breaking work /cN·cm Breaking tenacity/ cN·tex⁻¹

19.7 tex

A-1 238.6 (11.5) 12.3 (5.8) 837.2 12.1

B-1 255.2 (11.2) 11.5 (5.6) 933.8 12.9

C-1 226.3 (12.1) 12.7 (6.1) 803.1 11.7

14.7 tex

A-2 166.7 (10.3) 12.3 (5.9) 559.3 11.2

B-2 175 (10.2) 11.4 (5.4) 577.6 11.8

C-2 160.1 (10.9) 13.1 (6.3) 548.2 10.6

Data in parentheses are coefficients of variation (CV%).

T-test

To comment on the degree of difference between the average values of breaking strength of A-1 and the average values of breaking strength of B-1, the statistic t is calculated as follows
$t = \frac{{X_{1}}^{-} - {X_{2}}^{-}}{\sqrt{\frac{\sum x_{1}^{2} + \sum x_{2}^{2}}{n_{1} + n_{2} - 2} \times \frac{n_{1} + n_{2}}{n_{1} \times n_{2}}}}$
(6)
where ${\bar{X}}_{1}$ is the average value of A-1, ${\bar{X}}_{2}$ is the average value of B-1, n₁ is the number of A-1, n₂ is the number of B-1 and n₁ = n₂ = 60.

The degree of freedom is df = n – 1 = 59, and t(59)_0.05 = 2.001 from the t critical values. It can be obtained by calculating that t = 2.365. So we can see that t ≥ t(59)_0.05. According to this, grooving on the hollow spindle has a statistically significant effect on the yarn property.

Conclusion

This paper analyzed the structure and the fracture mechanism of self-twist jet vortex spun yarn based on its yarn mechanism. Compared with jet vortex spinning, the hollow spindle of self-twist jet vortex spinning has been grooved by laser on the surface, which can increase the surface friction coefficient. The free fiber end incurs a rolling moment on the surface of the hollow spindle, resulting in the self-twist of the fiber. The self-twist in the yarn will reach elastic recovery, which will make the normal compressive stress q and the friction mutual contact area A increase. According to equations (3)–(5), the number of slipping fibers N_S will be reduced when the slipping length l_c decreases. The breaking strength of self-twist jet spinning vortex yarn will increase with the number of slipping fibers decreasing.

Hollow spindle types	Angle between the groove and the hollow spindle top plane	Groove number
A	—	—
B	90°	20
C	90°	40

Hollow spindle types	19.7 tex	14.7 tex
A	A-1	A-2
B	B-1	B-2
C	C-1	C-2

Parameters	19.7 tex	14.7 tex
Pressure of nozzle (MP)	0.55	0.55
Total draft	207	223
Main draft	33	35
Feeding ratio	0.96	0.96
Winding ratio	1.02	1.02
Distance from front roller to hollow spindle (mm)	20	20
Roller center distance (mm × mm)	43 × 45	43 × 45
Diameter of hollow spindle channel (mm)	1.1	1.1
Spinning speed (m/min)	360	320

Yarn types	CVm/%	U/%	Slub/ ind·km⁻¹	Detail/ ind·km⁻¹	Neps/ ind·km⁻¹
19.7 tex
A-1	13.65	10.83	4	12	10
B-1	13.07	10.36	3	4	2
C-1	13.85	11.02	6	13	12
14.7 tex
A-2	15.57	12.21	20	57	33
B-2	15.00	11.93	12	22	8
C-2	16.31	13.20	22	59	38

Yarn types	Breaking strength/cN	Breaking elongation/%	Breaking work /cN·cm	Breaking tenacity/ cN·tex⁻¹
19.7 tex
A-1	238.6 (11.5)	12.3 (5.8)	837.2	12.1
B-1	255.2 (11.2)	11.5 (5.6)	933.8	12.9
C-1	226.3 (12.1)	12.7 (6.1)	803.1	11.7
14.7 tex
A-2	166.7 (10.3)	12.3 (5.9)	559.3	11.2
B-2	175 (10.2)	11.4 (5.4)	577.6	11.8
C-2	160.1 (10.9)	13.1 (6.3)	548.2	10.6

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Fundamental Research Funds for the Central Universities (grant number CUSF-DH-D-2015002), the Fundamental Research Funds for the Central Universities (grant number 2232014D3-01), the Shanghai Natural Science Foundation (grant number 13ZR1400900), the Keygrant Project of the Chinese Ministry of Education (grant number 113027A) and the Natural Science Foundation of China (grant number 51573095).

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