An experimental study of improving fabric appearance of denim by using low torque singles ring spun yarns

Abstract

This paper presents an experimental study on the improvement of denim fabric appearance by reducing the residual torque of weft yarns. In this paper, 100% cotton low torque yarns of linear density 84 tex and 58 tex with different twist levels were produced by a new method of producing low torque yarns on a modified ring spinning machine. The results from the experiments show that the low torque yarns possess lower yarn snarling turns. Based on the mechanism analysis of ‘small snake’ pattern, the low torque ring yarns were applied to the denim fabric to improve fabric smoothness appearance. The experimental study demonstrates that the ‘small snake’ pattern of the denim fabrics has been greatly decreased by using the low residual torque ring yarns as weft yarns. Objective measurements of the low stress mechanical and surface properties of denim fabrics using low torque and conventional yarns were conducted. The results show that the denim fabric using the low torque ring yarns as weft yarns presents a smoother and more even surface. Moreover, a multiple comparisons LSD test of fabric properties indicates that there is no significant difference on tensile strength and tearing strength between fabric samples using the low torque ring yarns and conventional ring yarns with normal twist level as weft yarns, respectively. In addition, the experimental results demonstrate that the ‘small snake’ pattern of the fabric decreased with the reduction of weft yarn snarling.

Keywords

denim fabric ‘small snake’ pattern fabric appearance low torque yarn yarn torsional buckling

Denim is perhaps the most used item of clothing and is the only fabric which has been produced in such large quantities. Conventionally, denim is a heavy 3/1 twill fabric made from 100% cotton with indigo-dyed warp yarn and undyed weft yarn. Through modern technology, denims have been improved in quality and provide a satisfactory level of comfort and durability to consumers. However, the surface of denim fabric has the possibility of becoming uneven after garment washing. A group of white and light color warp streaks forms what is usually called a ‘small snake’ pattern on the fabric surface. It affects the fabric’s appearance, smoothness, and soft handle and thus the aesthetics and comfort of apparel made from it. The main factor contributing to the formation of a ‘small snake’ pattern is the yarn residual torque, particularly on weft yarns. After yarn dyeing and sizing, the warp yarn residual torque is greatly reduced while the weft yarn still possesses higher yarn residual torque. Moreover, when fabrics are subject to water, the local yarn buckling is probably activated which distorts the fabric and results in the formation of ‘small snake’ pattern on the denim surface. As we know, when a cotton yarn is dyed with indigo, a ring dyeing effect is created. The outer layer of the warp yarn is coated with indigo and the core of the yarn remains undyed, which gives the indigo denim the unique characteristic of washdown effect. During washing, the raised parts (warp yarn) of fabric surface receive a greater washing effect than the other parts of the fabric. Some portions of surface fibers and dyes of the raised parts of fabric surface are rubbed off by abradant; thus the ‘fade look’ of denim becomes uneven. Therefore, the reduction of weft yarn residual torque probably reduces the yarn torsional buckling and brings benefits to the internal stress balance in the denim fabric, thus improving the surface smoothness appearance of denim. Although research work has been conducted on the effects of yarn twist or yarn torque on woven fabric properties and appearance as well as the color property of cellulase-treated cotton denim and the effect of washing technology on the surface appearance of denim fabric,^1–10 studies of the effect of yarn residual torque on the ‘small snake’ pattern of denim have not been reported. In this study, we tried to improve denim fabric appearance by reducing the residual torque of weft yarns.

Recently, a spinning technology to produce low torque ring spun yarns has been developed, which is a modified ring spinning technology with a modification system installed on the conventional ring spinning machine.^11–17 Through processing, the yarn structure can be modified and the yarn residual torque decreased significantly. In this study, 100% cotton low torque yarns of linear density 84 tex and 58 tex yarns with different twist levels were produced using the modified ring spinning technology, and properties of low torque yarns were tested and compared to properties of conventional ring spun yarns particularly on yarn residual torque. Using the low torque yarns and conventional yarns as weft yarns, 3/1 twill denim fabrics were designed and woven by a rapier loom. After enzyme pumice washing, the fabric appearance was evaluated and the fabric physical properties, such as tensile strength and tear strength, as well as the low stress mechanical and surface properties of the fabric were measured. The relationship between the weft yarn snarling and the fabric ‘small snake’ pattern was also investigated in this study.

Mechanism analysis of ‘small snake’ pattern

Yarn torsional buckling

Torsional buckling probably takes place when the yarn has high residual torque under low tension which leads yarn into a helical configuration. If the yarn is treated as a solid cylindrical elastic rod, the maximum torque [Mm] that can exist in an elastic rod of length l without producing torsional buckling is given by¹⁸

M_{m}^{2} = \frac{4 (EI)^{2} π^{2}}{l^{2}} + 4 (EI) P

(1)

where EI is the bending rigidity of the yarn and P is the applied tensile force. The buckle initiating torque is dependent on the bending rigidity of the yarn, the length of the specimen, as well as the magnitude of applied tension.

For denim fabric made from 100% cotton with indigo-dyed warp yarn and undyed weft yarn, the warp yarn has lower yarn residual torque due to yarn dyeing and sizing while the weft yarn still possesses higher yarn residual torque. Moreover, there exists interaction of warp yarn torque and weft yarn torque in woven fabric.⁵ When the denim fabric undergoes finishing washing, warp and weft yarns forming the denim fabric are immersed in water, which will greatly increase their torsional energy. With such wetting, if the yarn residual torque exceeds the buckle initiating torque, the yarn torsional buckling which had been subdued before will take over, distorting the yarn structure, and thus distorting the fabric structure into a ‘small snake’ pattern effect on the fabric surface. Figure 1 shows denim fabric surface in the close up view. From Figure 1, the gray fabric does not display obvious yarn buckling on the fabric surface, and after the fabric was immersed in water and underwent enzyme pumice washing, the torsional energy of yarns was greatly released and the local yarn buckling was activated, which distorted the fabric and resulted in the formation of ‘small snake’ pattern on the denim surface. Therefore, the reduction of weft yarn residual torque does reduce the yarn torsional buckling and thus improves the surface smoothness appearance of denim.

Figure 1.

Denim fabric surface in a close up view: (a) gray fabric and (b) enzyme pumice washed fabric.

Effect of the weft yarn torque on the warp yarn torque

For denim fabric, the weft yarn torque probably has an effect on the warp yarn residual torque due to the higher weft yarn torque compared with that of warp yarn as well as the interaction of yarn residual torque between warp and weft yarns. In order to investigate this effect by which weft yarn residual torque affects ‘small snake’ pattern on the fabric surface, assumptions must be made. If it is assumed that the warp yarn and weft yarn at the crossover points closely contact each other, the adapted model for the warp yarn float can be a fixed-ends rod with the weft yarn contacting at the point B, as shown in Figure 2. In addition, the yarn is assumed to behave as a cylindrical elastic rod and the force acting at the contacting region between the warp yarn and weft yarn is assumed to be a point force only.

Figure 2.

Schematic diagram of a fixed-ends rod for warp yarn with weft yarn contacting at point B: (a) original state and (b) state after weft yarn rotating.

The torsional deformation of a circular rod under a torque is well treated in the classical literature.¹⁹ For woven fabric, the twisted warp yarn and weft yarn possess residual torques. Thus the torsional deformations of the warp yarn and weft yarn shown in Figure 2(a) can be given by

φ_{1} = \frac{M_{1} L_{1}}{G_{1} I_{1}}

(2)

φ_{2} = \frac{M_{2} L_{2}}{G_{2} I_{2}}

(3)

where

φ_{1}

and

φ_{2}

are the angles of twist of the warp and weft yarns, respectively,

M_{1}

and

M_{2}

are the torques of the warp and weft yarns, respectively,

L_{1}

and

L_{2}

are the lengths of the warp and weft yarns, respectively,

I_{1}

and

G_{1}

are the warp yarn polar moment of inertia and shear modulus of elasticity respectively, and

I_{2}

and

G_{2}

are the weft yarn polar moment of inertia and shear modulus of elasticity respectively. In Figure 2(a),

γ_{1}

and

γ_{2}

are the helix angles of the deformed surface of the warp and weft yarns, respectively.

Compared with warp yarns, weft yarns possess higher yarn residual torques in denim fabric due to warp yarn sizing and dyeing. When the highly unstable weft yarns from the torsional point of view are woven into a fabric which is then immersed in water and if there is no enough restraining matrix present, the torsional energy of weft yarns will be released. As a result, the weft yarn probably rotates around its yarn axis to some extent and then reaches a new equilibrium condition of torque with its restraining matrix. If it is assumed that the original weft yarn torque is $M_{2}$ and the corresponding angle of twist of the weft yarn is $φ_{2}$ , and after the weft rotation the yarn torque decreases to $M_{2}'$ and the angle of twist reduces to $φ_{2}'$ , the difference of the $φ_{2}$ and $φ_{2}'$ can be given by

Δ φ_{2} = φ_{2} - φ_{2}' = \frac{(M_{2} - M_{2}') L_{2}}{G_{2} I_{2}}

(4)

where

Δ φ_{2}

is the additional angle of twist of the weft yarn due to weft yarn rotating.

It is further assumed that the pressure at the contact point of warp and weft yarns (point B in Figure 2) is high and no relative rotating slippage occurs. Hence, the warp yarn probably rotates around its axis due to the reactive torque from the weft yarn, as shown in Figure 2(b). The additional angle of twist of the warp yarn due to weft yarn rotating mainly depends on the additional angle of twist of the weft yarn and the inclination angle of warp yarn to the perpendicular line of weft yarn.²⁰ The additional angle of twist of the warp yarn can be expressed as

Δ φ_{1} = C \times Δ φ_{2}

(5)

where

Δ φ_{1}

is the additional angle of twist of the warp yarn,

Δ φ_{2}

is the additional angle of twist of the weft yarn, and C is the transmission coefficient between the rotation of weft yarn and warp yarn. In Figure 2(b),

γ_{AB}'

γ_{BC}'

, and

γ_{2}'

are the helix angles of the deformed surface of the warp yarn sections AB and BC and weft yarn in the state after weft yarn rotating, respectively.

In the present model shown in Figure 2(a), the relative angle of twist of the warp yarn between A and B, and B and C can be obtained as follows:

φ_{AB} = φ_{BC} = \frac{M_{1} b_{1}}{G_{1} I_{1}}

(6)

After the warp yarn rotates around its axis due to the reactive torque from the weft yarn, the angle of twist of the warp yarn between A and B, and B and C are given by

φ_{AB}' = φ_{AB} + Δ φ_{1}

(7)

φ_{BC}' = φ_{BC} - Δ φ_{1}

(8)

where

φ_{AB}'

and

φ_{BC}'

are the relative angles of twist between A and B, and B and C respectively. Substituting for

Δ φ_{1}

from equation (5),

φ_{AB}

and

φ_{BC}

from equation (6), and from equation (4) gives

φ_{AB}' = \frac{M_{1} b_{1}}{G_{1} I_{1}} + C \times \frac{(M_{2} - M_{2}') L_{2}}{G_{2} I_{2}}

(9)

φ_{BC}' = \frac{M_{1} b_{1}}{G_{1} I_{1}} - C \times \frac{(M_{2} - M_{2}') L_{2}}{G_{2} I_{2}}

(10)

Hence, the yarn residual torque of warp yarn sections AB and BC are expressed as

M_{AB}' = M_{1} + \frac{G_{1} I_{1}}{b_{1}} \times C \times \frac{(M_{2} - M_{2}') L_{2}}{G_{2} I_{2}}

(11)

M_{BC}' = M_{1} - \frac{G_{1} I_{1}}{b_{1}} \times C \times \frac{(M_{2} - M_{2}') L_{2}}{G_{2} I_{2}}

(12)

From equations (11) and (12), it can be seen that $M_{AB}'$ and $M_{BC}'$ are dependent on the warp yarn residual torque $M_{1}$ as well as the magnitude of torque release in weft yarn $(M_{2} - M_{2}')$ , the transmission coefficient between the rotation of weft yarn and warp yarn C, the length of weft yarns L₂ and the length of warp yarn sections AB and BC b₁, and the warp yarn torsional rigidity $G_{1} I_{1}$ and the weft yarn torsional rigidity $G_{2} I_{2}$ .

Let Δ M_{1} = \frac{G_{1} I_{1}}{b_{1}} \times C \times \frac{(M_{2} - M_{2}') L_{2}}{G_{2} I_{2}}

Then equations (11) and (12) can be written:

M_{AB}' = M_{1} + Δ M_{1}

(13)

M_{BC}' = M_{1} - Δ M_{1}

(14)

After receiving the reactive torque from weft yarn, the residual torque of warp yarn section (AB)

M_{AB}'

is equal to the sum of the original warp yarn residual torque

M_{1}

and the additional warp yarn torque

Δ M_{1}

. Because the warp yarn torque is greatly reduced after yarn dyeing and sizing, it is reasonable to assume that the original warp yarn residual torque

M_{1}

may be equal or less than the buckle initiating torque M_m. However, the yarn residual torque of warp yarn section (AB) probably increases to

M_{AB}'

due to the effect of weft yarn torque on warp yarn torque which probably exceeds the buckle initiating torque. Therefore, reducing weft yarn torque M₂ will decrease the additional warp yarn torque

Δ M_{1}

and thus the yarn residual torque of warp yarn section (AB)

M_{AB}'

. As a result, the possibility as well as the magnitude of the warp yarn buckling may be decreased, which endows denim a more smoothness surface and thus decreases the effect of ‘small snake’ pattern on denim fabric.

Experimental details

Yarn and fabric sample preparation

With twist levels of 438, 396, 365, 333, and 313 (tpm) and twist levels 523, 473, 448, 411, and 386 (tpm), 84 tex and 58 tex conventional yarns and low torque yarns were produced on the modified Zinser 319 Ring Spinning Machine, respectively. Table 1 shows the details of the yarns. A carded cotton roving of 952 tex was used in this study. The cotton fibers had a macronaire value of 4.0, a 2.5% span length of 28.7 mm, a bundle strength of 22 cN/tex, and a breaking extension of 5.3%. All parameters were measured by Spinlab 900.

Table 1.

Specifications of yarn samples

Yarn type	Linear density (tex)	Twist (tpm)
Low torque ring spun yarn	84	313
		333
		365
		396
		438
Conventional ring spun yarn	84	313
		333
		365
		396
		438
Low torque ring spun yarn	58	386
		411
		448
		473
		523
Conventional ring spun yarn	58	386
		411
		448
		473
		523

Using the low torque yarns and conventional yarns as weft yarns, 12 denim fabrics were produced by a Picanol rapier loom. The fabric specifications are listed in Table 2. From fabric Sample 1 to Sample 6, 84 tex conventional ring slub yarns were used as warp yarns while 84 tex low torque yarns and conventional ring yarns were used as weft yarns in Samples 1–3 and 4–6, respectively. For Samples 7–12, 84 tex conventional ring yarns were used as warp yarns while 58 tex low torque yarns and conventional ring yarns were used as weft yarns in Samples 7–9 and 10–12, respectively. After weaving, the fabric samples underwent enzyme pumice washing.

Table 2.

Fabric specifications

Fabric sample no.	Fabric content		Yarn linear density (tex)		Yarn twist (tpm)		Fabric density (threads/cm)		Weave
Fabric sample no.	Warp	Weft	Warp	Weft	Warp	Weft	Warp	Weft	Weave
1	C	L	84	84	438	313	24	19	3/1 twill
2	C	L	84	84	438	333	24	19	3/1 twill
3	C	L	84	84	438	396	24	19	3/1 twill
4	C	C	84	84	438	333	24	19	3/1 twill
5	C	C	84	84	438	365	24	19	3/1 twill
6	C	C	84	84	438	438	24	19	3/1 twill
7	C	L	84	58	523	411	27	17	3/1 twill
8	C	L	84	58	523	448	27	17	3/1 twill
9	C	L	84	58	523	473	27	17	3/1 twill
10	C	C	84	58	523	411	27	17	3/1 twill
11	C	C	84	58	523	448	27	17	3/1 twill
12	C	C	84	58	523	523	27	17	3/1 twill

Note: L and C represent the low torque ring spun yarn and the conventional ring spun yarn respectively.

Test methods

All yarn and fabric samples were conditioned for at least 24 hours under standard conditions (20 ± 2℃ and 65 ± 2% RH) and then tested for yarn and fabric properties.

A yarn snarling test was built to measure yarn twist liveliness or residual torque by using a principle similar to ISO Standard 3343-1984. In this test, the yarn is drawn from a yarn package holder through a tension device to ensure consistent tension (0.06 cN/tex). Then the yarn is secured by the two screws and a weight of 0.003 cN/tex is added to the bottom. After being prepared, the yarn sample is put into a water bath for yarn relaxation until it reaches equilibrium. The numbers of snarling turns per 25 cm length of yarn were measured to indicate the yarn residual torque. Thirty readings are taken on each sample.

Yarn tensile properties were tested on an Uster Tensorapid instrument at a speed of 5000 mm/min and a specimen length of 500 mm. Fifty readings for each sample were recorded. The hairiness of yarns at different length groups was measured using a Zweigle G566 hairiness tester. Each yarn bobbin measures three different parts which are small bobbin, middle bobbin and full bobbin, and the average value for each bobbin was calculated. The test speed was 50 m/min and the pre-tension was 4.9 cN.

Fabric smoothness appearance in terms of ‘small snake’ pattern was evaluated subjectively with reference to AATCC 124. The evaluation of fabric smoothness appearance is based on following five grade scales: 5 – no ‘small snake’; 4 – slight ‘small snake’; 3 – moderate ‘small snake’; 2 – severe ‘small snake’; 1 – very severe ‘small snake’. A grade midway between those whole-number standard is also assigned to represent the fabric appearance if the appearance of the test specimen warrants it. Three specimens were taken for each fabric sample. Three observers rated each test specimen independently.

After enzyme pumice washing, the fabric tensile strength and tearing strength were measured following ASTM D5034 and ASTM D1424, respectively. For each fabric sample, the specimens were taken both in the warp and weft directions. Five specimens, in either fabric direction, were tested for tensile and tearing strengths. Data were analyzed by one-way analyses of variance to compare the sample means. The significance level was set at 0.05 level and a multiple comparisons LSD test was performed using the SPSS statistical software.

Fabric low-stress mechanical and surface property measurements were conducted on KES-FB Tester. The tensile behavior and shear behavior of two fabric samples were studied on KES-FB1, while the bending behavior of fabrics was investigated on a KES-FB2, the compression properties were measured with KES-FB3 and surface roughness and friction were identified by KES-FB4. All of the 17 parameters describing fabric mechanical properties were measured following the test procedure. Five specimens were tested both in warp and weft directions and the mean values were determined.

Results and discussion

Yarn properties and comparison

Comparison of yarn snarling between low torque ring yarn and conventional ring yarn

As mentioned above, the weft yarn residual torque is believed to be one of the prominent factors contributing to the ‘small snake’ pattern of denim fabric. Here, the low torque ring yarns were efficiently spun on the modified ring spinning system. The comparisons of snarling between low torque and conventional yarns are presented in Figures 3 and 4. It is obvious that the snarling of low torque yarns is much lower than that of conventional yarns with normal twist level (438 tpm for 84 tex and 523 tpm for 58 tex), even compared with yarns of the same twist level. In the case of low torque yarns (84 tex) with twist level 333 tpm and (58 tex) with twist level 411 tpm, when compared to conventional yarns with normal twist level, the low torque yarns have snarling reductions of around 40% from 39 to 24 turns/25 cm for 84 tex and around 32% from 47 to 32 turns/25 cm for 58 tex, respectively. Conventional yarns can also achieve yarn snarling reduction to some extent by using lower twist level, but the level of snarling reduction is less than that of low torque yarn. As shown in Figure 3, the conventional yarn (84 tex) with twist level 333 tpm only has snarling reduction of about 15% compared to conventional yarns with normal twist level. The figures also show that the snarling of low torque yarns decreases with reducing yarn twist. It is known that in staple yarns, the twist is essential to hold the fibers together and to impart some degree of cohesiveness to the structure. As the twist is inserted, the fibers are bound a helical path along the yarn, and fiber internal stress and strain are created in the yarn. Since the yarn twist is one of the most important factors governing the magnitude of yarn torque and the tendency of yarn to snarl, decreasing yarn twist tends to decrease the fiber internal stress and strain built in the yarn, thus reduce the yarn torque. Moreover, the structural features of low torque yarn such as deformed non-concentric helix, local reversion of helix direction for some fiber segments and distribution of most fibers close to yarn center may result in the further reduction of the yarn torque in low torque yarn.¹⁷

Figure 3.

Comparison of yarn snarling between low torque and conventional yarns (84 tex).

Figure 4.

Comparison of yarn snarling between low torque and conventional yarns (58 tex).

Comparison of yarn tenacity and elongation between low torque ring yarn and conventional ring yarn

Yarn strength is one of the major yarn properties in evaluating yarn performance for modification and further textile processing. The comparisons of tenacity between low torque and conventional yarns are shown in Figures 5 and 6. From the comparison, it is easy to see that the tenacity of low torque yarns is only slightly lower than that of conventional yarn with normal twist level (438 tpm for 84 tex and 523 tpm for 58 tex). The lowest value of yarn tenacity of low torque yarn is around 16.5 cN/tex among the six 84 tex yarn samples and the yarn tenacity is around 15.4 cN/tex for 58 tex low torque yarn with twist level 411 tpm, which is acceptable for industrial application. As we know, the snarling decrease of conventional ring yarn can also be achieved through reducing yarn twist level. However, the yarn tenacity will decrease concomitantly. Figures 5 and 6 show that the yarn tenacity decreases as yarn twist level decreases and that the level of tenacity reduction occurs at a faster rate for the conventional yarns than for low torque yarns, especially in lower yarn twist level. In the 84 tex conventional yarn with twist level 313 tpm, the value of yarn tenacity is only 14.91 cN/tex, which may result in the increase of yarn end break in yarn spinning and bring problems in the following yarn processing, weaving and fabric properties. Evidences obtained from yarn structure studies on low torque yarns indicated that the majority of fibers in the low torque yarns follow a configuration of deformed non-concentric conical helixes with the axis of helix varying significantly in space and most fibers tend to be distributed close to the yarn center, and alter their radial positions frequently with relatively greater migration amplitude, which offer an explanation for the good yarn strength of low torque yarn when low twist level was adopted.^12,13,17 As for the yarn elongation, the low torque yarn has a slightly lower elongation than that of the conventional ring yarn, as shown in Figures 7 and 8.

Figure 5.

Comparison of yarn tenacity between low torque and conventional yarns (84 tex).

Figure 6.

Comparison of yarn tenacity between low torque and conventional yarns (58 tex).

Figure 7.

Comparison of yarn elongation between low torque and conventional yarns (84 tex).

Figure 8.

Comparison of yarn elongation between low torque and conventional yarns (58 tex).

Figure 9.

Comparison of yarn hairiness between low torque and conventional yarns (84 tex).

Comparison of yarn hairiness between low torque ring yarn and conventional ring yarn

Yarn hairiness was also evaluated for low torque and conventional yarns. As shown in Figures 9 and 10, the hairiness of low torque yarns is much lower than that of conventional yarns. In the case of low torque yarns (84 tex) with twist level 333 tpm and (58 tex) with twist level 411 tpm, the hairiness of 84 tex low torque ring yarn is 61% less than that of the conventional ring yarn while the hairiness of 58 tex low torque ring yarn is significantly reduced by 29%. It is most likely that the reduction of hairiness in the low torque yarns was achieved by improving the fiber distribution in the spinning triangle, the false twisting operation, the improved fiber migration and the compact yarn structure. The reduced hairiness of the low torque ring yarns could be beneficial for the quality of yarns and the resultant products while the higher yarn hairiness of conventional yarns in lower twist levels probably deteriorates the fabric appearance.

Figure 10.

Comparison of yarn hairiness between low torque and conventional yarns (58 tex).

Fabric properties and appearance

Comparison of ‘small snake’ pattern of denim fabrics using low torque and conventional ring yarns as weft yarns

In order to improve fabric appearance and evaluate the effect of weft yarn torque on the ‘small snake’ pattern of the denim fabric, six fabric samples were produced by using the low torque yarns as weft yarns, and other six fabrics with the conventional ring yarns in the same or similar twist levels as wefts were also woven for the comparison of fabric properties and appearance. Photographs of denim fabric appearance using low torque yarns and conventional yarns as wefts are shown in Figures 11–14. Figure 15 shows the grade of ‘small snake’ of twelve samples.

Figure 11.

Photographs of denim fabric appearance (84 tex low torque yarn as weft).

Figure 12.

Photographs of denim fabric appearance (84 tex conventional yarn as weft).

Figure 13.

Photographs of denim fabric appearance (58 tex low torque yarn as weft).

Figure 14.

Photographs of denim fabric appearance (58 tex conventional yarn as weft).

Figure 15.

Grade of ‘small snake’ pattern of the denim fabrics.

As shown in Figure 15, the appearance of denim fabrics improved significantly when using the low torque yarns as weft yarns due to the lower yarn residual torque. With normal twist level (438 tpm for 84 tex and 523 tpm for 58 tex) yarns as weft yarns, conventional fabrics have severe ‘small snake’ pattern, while fabrics using the low torque yarns as weft yarns only have moderate, or even slight ‘small snake’ pattern on the fabric surface. Moreover, using slub yarns as warp yarns in fabric Samples 1–6, the fabrics made by low torque yarns as wefts show a more prominent slub effect in the warp direction of denim due to less ‘small snake’ pattern and better smoothness of fabric. When using lower twist level (333 tpm for 84 tex and 411 tpm for 58 tex), that is, lower yarn residual torque, the conventional fabrics also achieve better fabric appearance with slight ‘small snake’ pattern on the denim surface such as Sample 4.

In addition, the experimental results demonstrate that the ‘small snake’ pattern of the fabric decreases with the reduction of weft yarn snarling. Figures 16 and 17 show the relationship between weft yarn snarling and fabric ‘small snake’ pattern of the twelve fabric samples. Among fabric Samples 1–6, with the lowest weft yarn snarling, fabric Sample 1 has the best fabric smoothness appearance, while fabric Sample 6 has the severest ‘small snake’ pattern because of the highest weft yarn snarling among the six weft yarn samples (84 tex) in this study. Similarly, fabric Sample 7 presents the slight ‘small snake’ pattern and Sample 12 shows server ‘small snake’ pattern on denim surface among six fabrics using the low torque yarns (58 tex) and conventional yarns as weft yarns. The possible reason is that reducing yarn residual torque, particularly weft yarn residual torque may result in the reduction of the magnitude of yarn buckling and its unevenness, which endows denim a more smooth surface and thus decreases the problem of ‘small snake’ pattern on denim fabric. Following the same trend, Figures 16 and 17 also show the difference of effect of yarn snarling on the fabric ‘small snake’ between fabrics using low torque yarns and conventional yarns as weft yarns. For example, with higher yarn snarling, the Sample 4 achieves higher ‘small snake’ grade than that of Sample 3. Based on the experiment results and analysis above, it can be confirmed that the weft yarn residual torque is the main factor contributing to the ‘small snake’ pattern of denim fabric, but other yarn properties also make contributions to the fabric appearance to some extent. After the modification of the yarn structure, the low torque yarn not only differs from conventional ring yarn in terms of yarn residual torque but also other mechanical properties and surface properties.¹²

Figure 16.

The relationship between weft yarn snarling and fabric ‘small snake’ pattern (84 tex weft yarn).

Figure 17.

The relationship between weft yarn snarling and fabric ‘small snake’ pattern (58 tex weft yarn).

Comparison of tensile strength and tearing strength of denim fabrics using low torque and conventional ring yarns as weft yarns

Fabric tensile strength and tearing strength are important for the durability of garment. Particularly, denim will undergo hard garment washing such as stone washing or enzyme and stone washing. During this process, denim fabric should have enough strength to endure the process. More importantly, after garment washing, denim fabric should still have better strength to meet the needs of the end use. Figures 18–21 present the results of tensile strength and tearing strength of denim fabrics using the low torque yarns and conventional yarns as weft yarns. In this study the appearance of denim fabric is improved through using low torque yarns as weft yarns, hence the analysis of fabric will be placed on the fabric properties in the weft direction. One-way analysis of variance was performed on the data in order to test and assess if there was a significant difference between the means of samples with different types of weft yarns at 0.05 level. Compared to normal denim fabric produced in the factory such as Sample 6 and Sample 12, the fabric appearance is improved at equal or more than one grade in the Samples 1, 2, 4, 7, 8, and 10. Therefore, the means of those samples were compared and the results of multiply comparison LSD test are given in Table 3. It is obvious that there is no significant difference on tensile strength and tearing strength between fabric samples using the low torque yarns as weft yarns and conventional fabrics with normal twist level (438 tpm for 84 tex and 523 tpm for 58 tex) yarns as wefts. However, the tensile strength and tearing strength of fabrics using conventional lower twist yarns as weft yarns shows a significantly decrease compared to the normal conventional yarn fabric and the majority of fabric samples using low torque weft yarns. The possible reason is that the structure of lower twist conventional yarn is easy to become loose during winding and weaving processes, which results in a further decrease of yarn tensile stress, while the low torque yarn can still maintain its twist structure at lower twist due to its better fiber migration and cohesion in the yarn. Therefore, even though the fabric appearance can be improved greatly using lower twist conventional yarns as weft yarns, the weft tearing strength and tensile strength loss of fabric are serious.

Figure 18.

Fabric tensile strength in warp direction.

Figure 19.

Fabric tensile strength in weft direction.

Figure 20.

Fabric tearing strength in warp direction.

Figure 21.

Fabric tearing strength in weft direction.

Table 3.

Results of multiple comparisons LSD test

Fabric sample no.		Tearing strength in weft direction		Tensile strength in weft direction
Fabric sample no.		Mean difference (I-J)	Sig.	Mean difference (I-J)	Sig.
6	1	−0.820	0.733	19.900	0.364
	2	−0.080	0.973	24.280	0.272
	4	8.860*	0.002	48.840*	0.036
4	1	−9.680*	0.001	−28.940	0.193
	2	−8.940*	0.002	−24.560	0.266
	6	−8.860*	0.002	−48.840*	0.036
12	7	0.940	0.573	13.320	0.414
	8	−0.960	0.565	−0.060	0.997
	10	7.860*	0.000	88.960*	0.000
10	7	−6.920*	0.001	−75.640*	0.000
	8	−8.820*	0.000	−89.020*	0.000
	12	−7.860*	0.000	−88.960*	0.000

The mean difference is significant at the 0.05 level.

Comparison of the low stress mechanical and surface properties of denim fabrics using low torque and conventional ring yarns as weft yarns

The comparison of the low stress mechanical and surface properties of denim fabrics was conducted between two fabric samples, Sample 2 using the low torque yarns as weft yarns and Sample 6 using conventional yarns as weft yarns. The results for low stress mechanical and surface properties of the fabrics measured by KES-FB Tester are listed in Table 4. From Table 4, it can be seen that the denim fabric using low torque ring yarns as wefts gives lower values in the mean deviation of friction (MMD) as well as geometrical roughness (SMD) compared with the fabric sample with conventional ring yarns as wefts, which indicates that the smoothness and the evenness of the low torque yarn fabric surface is better than that of the conventional yarn fabric probably due to the decrease of ‘small snake’ pattern on denim fabric surface when the low torque yarns were adopted. The test results also show that the denim fabric using low torque ring yarns has higher linearity of compression (LC) as well as slightly lower bending rigidity (B) and shear stiffness (G) than that of the conventional yarn denim fabric.

Table 4.

Fabric mechanical characteristics measured by KESF

Fabric sample no.	Tensile				Bending		Shear			Surface			Compression			Thickness	Weight
Fabric sample no.	EMT (%)	LT	WT (gf.cm /cm²)	RT (%)	B gf.cm²/cm)	2HB (gf.cm /cm)	G(gf/cm. deg)	2HG (gf/cm)	2HG5 (gf/cm)	MIU	MMD	SMD (µm)	LC	WC (gf.cm/cm²)	RC(%)	T(mm)	W(mg/cm²)
2	7.83	0.798	15.50	35.85	0.498	0.695	4.18	10.74	15.70	0.258	0.024	3.95	0.393	0.340	33.77	1.513	44.750
6	7.78	0.822	15.80	36.94	0.529	0.697	4.59	11.58	14.72	0.258	0.045	4.25	0.325	0.342	35.04	1.603	45.750

Conclusion

In this study, a mechanism analysis of ‘small snake’ pattern formation and an experimental investigation on the improvement of denim fabric appearance by using low torque singles ring spun yarn were conducted. The analysis indicates that reducing weft yarn torque probably decreases the possibility or the magnitude of the warp yarn buckling, which endows denim a more smoothness surface and thus decreases the effect of ‘small snake’ pattern on denim fabric.

Based on the fact that the main factor contributing to the formation of ‘small snake’ pattern on the denim fabric surface is the yarn residual torque particularly on weft yarns, the low torque yarns produced by a newly developed spinning technology were applied on the denim fabric to improve fabric smoothness appearance. The results of the investigation show that the low torque ring spun yarn possesses lower yarn snarling and less yarn hairiness as well as normal yarn tenacity compared to that of the conventional ring spun yarn. The experimental study also demonstrates that the appearance of the denim fabrics is greatly improved by using the low torque yarns as weft yarns as a result of the lower yarn residual torque. Thus the fabric shows less ‘small snake’ pattern on the denim fabric surface. The test results of the low stress mechanical and surface properties of fabric also show that the denim fabric using the low residual torque ring yarns as weft yarns presents smoother and more even surface. Moreover, a multiple comparisons LSD test of fabric properties manifests that there is no significant difference on tensile strength and tearing strength between fabric samples using the low torque yarns as weft yarns and conventional fabrics with normal twist level yarns as wefts, while the weft tearing strength and tensile strength loss of fabric are serious using low twist conventional yarns as weft yarns. In addition, a result of investigating the relationship between weft yarn snarling and fabric ‘small snake’ pattern indicates that the ‘small snake’ pattern of the fabric decreases with the reduction of weft yarn snarling.

Footnotes

Funding

This work was supported by a grant from the Innovation and Technology Commission of Hong Kong SAR Government and Central Textiles (H.K.) Limited, Chip Tak Weaving Factory Limited, Fountain Set (Holdings) Limited and Perfecta Dyeing Printing and Weaving Works Limited (Grant No. ITS/055/02).

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