Low-bagging (growth) stretch denim yarn production by spinning optimization of cotton-wrapped dual-core elastane and T400 multifilament

Abstract

Cotton-wrapped elastane core yarns have been widely used in producing stretch denim fabrics due to their comfortable stretching and recovery, but they suffer from undesirable fabric growth under prolonged or repeated stress. To reduce that problem, an additional semi-elastic multifilament has been incorporated with the elastane core, called dual-core yarn. Herein, it was intended to produce well-engineered dual-core yarns possessing high elasticity with low bagging. Twenty types of cotton-wrapped elastane/T400 multifilament dual-core yarns with different combinations of elastane and T400 tension draft were produced on industrial scale in a spinning mill. Structural parameters, tensile properties and elastic recovery behavior under cyclic loading of the yarns were thoroughly studied. For an optimum combination of elastane/T400 draft, the dual-core yarn attained excellent tenacity and elongation with significantly low evenness, imperfections and hairiness values. More importantly, the results of the cyclic loading study explicitly revealed a remarkable reduction in plastic deformation and stress decay indicating low growth and high resilience of yarn after deformation. The dual-core yarn containing high strength, high elongation and low growth obtained here can have durable stretch jeans with high body movement comfort and long-lasting shape retention.

1. Introduction

The usage of core-spun yarns in producing stretch denim is currently in vogue owing to their unique qualities driven by pleasant wear comfort and form-fitting ability [1,2]. Stretch denim is usually manufactured by inserting stretch yarns in the weft direction during weaving. They are used to make clothing for formal wear, sportswear, fashion wear, casual wear and even in many types of military wear.

Concerning body movement comfort, 10–35% elasticity and high recovery power are required in denim products [3]. While in use, stretch denim fabrics prepared with single-core/elastane yarn suffer from a tendency of undesirable distortion, known as fabric growth or bagging. Bagging is a three-dimensional residual deformation occurred from the lack of dimensional stability or recovery when repeated or prolonged stress is imposed on the fabric during wear. It results in deterioration in appearance in different places of garments such as elbows, knees, pockets, hips and heels that cause dissatisfaction among wearers [4,5]. To reduce the bagging deformation of denim, a new generation of yarn, called dual-core or double-core yarn, has recently been developed [[6], [7], [8], [9]]. The dual-core yarn is comprised of three components where an elastane filament (such as Lycra®, Creora® and Inviya® I-300) and a multifilament (such as PET, PBT, PA and Lycra® T400) cores are covered with sheath staple fibers. Among various multifilament inclusion in the core, Lycra® T400 is the latest that delivers excellent performance where low-to-moderate stretch (up to 20%) is required.

Lycra® T400 fiber, an inherently elastic elastomultiester (EME) polyester fiber, is a special type of bicomponent fiber in which two different polymers are joined together within each filament. The chemical composition of commercially available T400 is 40% polyester (3 GT-type) and 60% polyester (2 GT-type), and these two different polyesters are joined together with each fiber. When exposed to heat, each component shrinks differently causing the fiber to form a permanent and regular coil which is the reason for the elastic properties of this fiber. The additional coil is developed during the dyeing and finishing process when the fiber is exposed to heat. The physical coil-like structure of T400 fiber provides low-to-moderate stretch but durable recovery [10]. T400 is mainly used in woven fabrics, usually on the weft, in a quantity higher than 20% by weight [11]. It can be used solely, twisted or core spun with other fibers like cotton to produce compound yarns [12].

The literature review provides enormous outcomes on the improvement of the properties of core-spun yarns containing single-core filament, mostly elastane. Textile researchers and producers have very recently developed dual-core yarns, and due to being a new type, there is a special focus on these yarns [4,7,8,[13], [14], [15], [16], [17], [18], [19]]. However, less significant research on optimizing the spinning parameters of dual-core yarns highlights the need for further investigations.

The work covered in this paper is the development of optimum-quality dual-core yarn comprised of a soft elastane filament surrounded by a semi-elastic multifilament core that is ultimately wrapped with cotton fibers as illustrated by the architectural view in Fig. 1. In such dual-core yarn, the soft elastane filament (Creora with stretchability 600–700%) core provides high elasticity (freedom of movement) and recovery (fit) whereas the semi-elastic multifilament (Lycra® T400 with stretchability 20–25%) reduces fabric growth (bagging) and ensures dimensional stability. Besides, elastane filament can be effectively integrated into yarn core surrounded by T400 multifilaments that reduce the possibility of off-centering of elastane.

Fig. 1.

An idealized architectural view of dual-core yarn showing an elastane filament surrounded by T400 multifilament are wrapped by sheath cotton fibers.

Without skillful engineering of dual-core yarn structure, it is not possible to achieve the desired properties of denim fabric. Much like the characteristics of elastic and semi-elastic cores, the optimization of spinning parameters has a great influence on the properties of core-spun yarns [20,21]. In this work, the impact of elastane and multifilament (T400) tension draft (hereinafter, it will be written as ‘draft’), a prime determining factor that decides the characteristics of core-spun yarns, was investigated in order to explore an optimum combination of elastane and T400 draft in terms of strength, extension and bagging deformation of dual-core yarn.

2. Materials and methods

2.1. Materials

In this work, dual-core yarns were produced on an industrial scale ring frame in a spinning mill. Cotton fibers, a blend of Indian (Shankar 6)-50% and Mali-50%, were used as sheath fibers. In spinning mills, it is a common practice to blend cotton of different origins to minimize their natural variability and to obtain a large lot size along with consistent production and quality. Elastane filament (Creora H350 originated in Vietnam) and Lycra® T400 multifilament (originated in China) were collected from Kuri & Company (Pvt.) Ltd. which is the authorized distributor of Sigma Aldrich in Bangladesh. The specifications of cotton, elastane filament and T400 multifilament are given in Table 1 and Table 2, respectively.

Table 1.

Specification of raw cotton.

Parameters	India (Shankar-6)	Mali
SCI (Spinning consistency index)	135	135
UHML (mm)	30.01	28.55
Micronaire (mtex)	161.4	167.3
Tenacity (cN/tex)	30.7	32.1
Elongation (%)	6.0	5.6
Maturity index	0.87	0.88
Uniformity index (%)	82.2	81.6
Rd (Reflectance)%	79.4	78.6
+b (Yellowness)	8.2	9.3
Short fiber index (%)	7.9	8.5
Trash count	38	36
Trash area (%)	0.56	0.3
Trash grade	4	3

Table 2.

Specification of filaments.

Filament name	Number of ends	Linear density (dtex)	Tensile Strength (cN/tex)	Elongation (%)
Elastane (Creora H350)	1a	78	7.95	570
T400 multifilament	34	83	35.4	20

^aCreora H350 is a product of five monofilaments welded together at a periodic interval to act as a single filament (shown in microscopic images in Fig. 8).

2.2. Preparation of roving from raw cotton

Cotton is a natural fiber having variability in its physical properties. Bale management is a judicious selection of cotton bales for a laydown from the population to achieve consistent performance and quality of the end product based on fiber properties [22]. Here Uster ‘Bale Manager’ software was used for 64 bales in laydown on the basis of fiber micronaire and yellowness (+b) value.

After bale management, dual-core yarns were manufactured in a cotton spinning line following the card process with the steps given in Fig. 2. The detail of the machines and technical parameters to produce roving from raw cotton are given in Table 3.

Fig. 2.

Flow chart to manufacture dual-core yarn.

Table 3.

Production details for the preparation of roving from raw cotton.

Process	Machine & Model	Delivery material	Speed
Blow Room	Trutzschler (Blendomat, 2007; SP-EM, 2007; CL-P, 2007; MX-U, 2010; CL-C3, 2009; SP-FPU, 2012)	560 ktex card mat	Chute feed to card
Carding	Trutzschler TC 06	6.5 ktex card sliver	190 m/min
Breaker Draw Frame	Trutzschler TD 09	5.7 ktex drawn sliver	650 m/min (doubling 6)
Finisher Draw Frame	Trutzschler TD 08 (with autoleveller)	5.5 ktex drawn sliver	650 m/min (doubling 6)
Simplex	Toyoda FL-100	0.98 ktex roving (TPI-1.02)	1000 rpm (Flyer rpm)

2.3. Spinning of dual-core yarn

Dual-core yarn can be produced exclusively in ring spinning system. Here, 37 tex woven grade (Cotton-system TM 5.5) dual-core-spun yarn, nowadays used industrially as weft in stretch denim fabric, samples were produced in Marzoli DTM 129, China ring spinning machine at spindle speed 12,000 rpm. Two additional feeding attachments (AMSLER, Swiss Effect Yarn System, Switzerland) are installed to feed elastane and T400 as core filament as shown in Fig. 3(i) (adopted from Ref. 17) and Fig. 3(ii). As seen in Fig. 3(i) and (ii), both elastane and T400 are delivered by a positive feed roller through which the draft of both core filaments can be controlled separately. Both core filaments are combined to the front roller nip employing a V-grooved roller where cotton fibers wrap over them. The required drafts of elastane and T400 are obtained by the speed difference between positive feed rollers and the front roller of the drafting unit.

Fig. 3.

(i). Schematic representation of the production of dual-core yarn in ring spinning frame with additional attacments for elastane and T400 multifilament feeding. (Adopted from Ref. 17)

Fig. 3(ii). (Left) Front view and (right) side view of ring spinning frame with additional feeding attachments for T400 multifilament and elastane to produce dual-core yarns.

In order to explore an optimum draft combination of elastane and T400, a good number of dual-core yarns were produced with varying filament drafts covering a wide range of T400 and elastane draft possible during spinning. For the convenience of the experiment and further data analysis, manufactured dual-core yarns were grouped into five series based on the T400 draft. Each series contains four types of yarns with four different elastane drafts as shown in Table 4. In this way, twenty types of dual-core yarn samples with five different T400 drafts (1.01, 1.07, 1.14, 1.2 and 1.3) and four different elastane drafts (3.0, 3.4, 3.8 and 4.2) were produced covering a wide range of T400 and elastane draft possible during spinning.

Table 4.

Draft combination of T400 multifilament and elastane, and consumption% of T400, elastane and cotton in dual-core yarns.

Series No.	Draft combination	T400 multifilament draft	Elastane draft	Consumption%
				T400	Elastane	Sheath cotton
Series I	Low T400 multifilament draft with variable elastane draft	1.01	3.0	22.36	7.02	70.62
			3.4	22.36	6.19	71.45
			3.8	22.36	5.55	72.09
			4.2	22.36	5.01	72.63
Series II	Medium T400 multifilament draft with variable elastane draft	1.07	3.0	20.9	7.02	72.08
			3.4	20.9	6.19	72.91
			3.8	20.9	5.55	73.55
			4.2	20.9	5.01	74.09
Series III	High T400 multifilament draft with variable elastane draft	1.14	3.0	19.63	7.02	73.35
			3.4	19.63	6.19	74.18
			3.8	19.63	5.55	74.82
			4.2	19.63	5.01	75.36
Series IV	Very high T400 multifilament draft with variable elastane draft	1.2	3.0	18.65	7.02	74.33
			3.4	18.65	6.19	75.16
			3.8	18.65	5.55	75.8
			4.2	18.65	5.01	76.34
Series V	Extremely high T400 multifilament draft with variable elastane draft	1.3	3.0	17.37	7.02	75.61
			3.4	17.37	6.19	76.44
			3.8	17.37	5.55	77.08
			4.2	17.37	5.01	77.62

In this study, dual-core yarns with elastane/T400 draft 3.8/1.2 was found to be an optimum combination in terms of strength, elongation and growth (bagging) (details are described in ‘Results & discussion’ section). Therefore, two reference single-core yarns such as (i) a single-core/elastane yarn (with elastane draft 3.8) and (ii) a single-core/T400 multifilament yarn (with T400 draft 1.2) of 37 tex were also produced to compare the data with those of dual-core yarns. In addition, 37 tex 100% cotton (non-core) commercial grade woven card yarn with TM 4.5 was also prepared as a reference sample.

Consumption% of T400 multifilament, elastane and cotton in dual-core yarn, mentioned in Table 4, were calculated by using the following formulae that are usually practiced in industries:

• (i)$\mathrm{T}400\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\%=\frac{\frac{T400denier}{T400draft}}{Resultantyarndenier}\times 100$
• (ii)$\mathrm{E}\mathrm{l}\mathrm{a}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\%=\frac{\frac{Elastanedenier}{Elastanedraft}}{Resultantyarndenier}\times 100$
• (iii)$\mathrm{o}\mathrm{t}\mathrm{t}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\%=\left[100-(\mathrm{T}400\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\%+\mathrm{E}\mathrm{l}\mathrm{a}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\%)\right]$

2.4. Measurements

Coefficient of mass variation (CVm%), imperfections and hairiness of yarns were ascertained by USTER® Tester 5 (Uster Technologies, Switzerland) in accordance with ASTM D1425 standard with the testing speed of 400 m/min. Spun yarns manufactured from staple fibers contain frequently-occurring yarn faults, referred to as imperfections. The imperfections are subdivided into three categories: thin places, thick places and neps [23]. In this study, the sensitivity thresholds of thick place (+50%), thin place (−30%), and neps (+200%) per 1000 m of yarn were considered for data analysis. Yarn hairiness (H), measured by the hairiness sensor of the evenness tester based on the optical principle, is the total length of protruding fibers (in cm) within the measurement field of 1 cm length of yarn.

Tensile properties of dual-core-spun yarns were studied by Tenso Lab-4, Mesdan, Italy operated with a constant rate of extension principle according to ASTM-2256-97 with a 50 N load cell, a gauge length of 500 mm, and a crosshead speed of 500 mm/min. All the samples were conditioned before testing according to ISO 139.

The surface morphology of the dual-core yarns with low-magnification images (shown in Fig. 5) was observed with an optical microscope Euromex BV, Model NZ 1703-M, Netherlands. High-magnification images (shown in Fig. 8) were taken through an optical microscope, Leica DM4 P, Germany.

Fig. 5.

Optical microscopic images (10 × ) of elastane/T400 draft 3.8/1.2 dual-core yarn: (a) crimp configuration of dual-core yarn without stress, (b) stressed configuration after applying load, and (c) stressed configuration after rupturing T400 and before rupturing elastane.

Fig. 8.

Optical microscopic images of (a) elastane/T400 draft 3.8/1.2 dual-core yarn (100 × ), (b) after untwisting, elastane/T400 draft 3.8/1.2 dual-core yarn (40 × ) shows perfectly-centered creora elastane in the core of T400 multifilament, (c) elastane/T400 draft 4.2/1.2 dual-core yarn (100 × ) shows off-centered elastane, and (d) elastane/T400 draft 3.8/1.3 dual-core yarn (100 × ) shows only elastane in core, T400 missing due to rupture.

Finally, the dimensional stability of dual-core yarns, the ability to retain the shape while subjected to repetitive load in use, was evaluated by imposing cyclic dynamic loading on the yarn. A number of researches on cyclic dynamic loading and extension of the elastane-contained yarn and fabric were reported in the literature [9,[24], [25], [26], [27], [28], [29], [30], [31]].

Elastic properties of dual-core yarns were evaluated by Titan 10, Universal Testing Machine, James Heal, in accordance with ISO EN 14704-3 designed principally to study the elasticity of elastic narrow fabric under cyclic loading. A 100 mm long specimen is loaded at 500 mm/min speed equivalent to a predetermined breaking load, and then the specimen is recovered to its original length at the same speed. The procedure was repeated for five consecutive cycles and at the end of the 5th cycle, the specimen was held stretched under constant load for 1 min to study the stress decay or stress relaxation with time.

For example, Fig. 4(a) exhibits a representative load-elongation curve of a dual-core yarn (elastane/T400 draft: 3.8/1.2) where its breaking load is seen to be 6.42 N. Plastic or permanent deformation of yarn is usually negligible below the yield point [32]. Hence, cyclic loading equivalent to 50% of the breaking load of yarn i.e. 3.2 N in the case of elastane/T400 draft 3.8/1.2 yarn was applied to it to produce a sufficient plastic deformation as illustrated in Fig. 4(b). Then gradual increase in extension with each loading cycle was obtained as shown in Fig. 4(c). The plastic deformation of the yarn was calculated from equation (1):

$$\mathrm{P}\mathrm{l}\mathrm{a}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{d}\mathrm{e}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}(\%)=\frac{Extension\%after5thcycle-extension\%after1stcycle}{Extension\%after1stcycle}\times 100$$ (1)

Fig. 4.

(a) Representative load-elongation curve of 37 tex dual-core yarn with elastane/T400 draft 3.8/1.2, (b) cyclic loading imposed on yarn equivalent of 50% breaking load, and (c) gradual increase of extension of yarn with successive cycles of loading.

As shown in Fig. 4(b), at the end of the 5th cycle after holding load for 1 min, the stress decay or stress relaxation, an indicator of elastic recovery, was calculated using equation (2):

$$\mathrm{S}\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{s}\mathrm{d}\mathrm{e}\mathrm{c}\mathrm{a}\mathrm{y}(\%)=\frac{F_{L\left(before\right)}-F_{L\left(after\right)}}{F_{L\left(before\right)}}\times 100$$ (2)

where F_L(before) and F_L(after) are the loads recorded before and after holding load for 1 min, respectively [9].

3. Results and discussion

3.1. Surface morphology of dual-core yarns

The surface morphology of dual-core yarns was observed by an optical microscope. Most of the dual-core yarns produced in this study showed perfect centering of core components except yarns produced with utmost elastane and T400 draft (which will be discussed in the following section). Representative microscopic images of the dual-core yarn with elastane/T400 draft 3.8/1.2 are illustrated in Fig. 5. Fig. 5(a) shows a typical crimp configuration of dual-core yarn containing elastane in the core. Fig. 5(b) is the stressed configuration of that yarn. When tensile stress was applied to that yarn, the disintegration of cotton fibers from the core part occurred followed by T400 multifilaments rupture. Rupture of T400 occurred before rupturing of elastane due to its lower extensibility than elastane (elongation at break of T400: 20% and creora elastane: 570% shown in Table 2). The image shown in Fig. 5(c) was taken after rupturing T400 and before rupturing the elastane core. The morphology of dual-core yarn viewed in the microscopic image resembles the idealized architectural view of dual-core yarn shown before in Fig. 1 where it was assumed that a perfect centering of elastane surrounded by T400 multifilament will ultimately be covered by sheath cotton fibers.

3.2. Effects of elastane/T400 draft on the tensile behavior of dual-core yarns

The tensile properties of yarn decide the performance of post-spinning operations i.e. warping, sizing and weaving as well the properties of the final product [33].

Fig. 6 shows the representative stress-strain curves of 37 tex (i) 100% cotton (non-core) yarn, (ii) single-core/elastane yarn (containing only elastane in core), (iii) single-core/T400 yarn (containing only T400 multifilaments in core) and two dual-core yarns having substantial differences in tensile properties such as (iv) low strength and low elongation (elastane/T400 draft 3.0/1.01) and (v) high strength and high elongation (elastane/T400 draft 3.8/1.2). Stress-strain curves of all 20 dual-core yarns produced with different elastane/T400 drafts are not shown in the same Figure to avoid complications in the understanding of overlapped curves.

Fig. 6.

Representative stress-strain curves of 37 tex (i) 100% cotton (non-core) yarn, (ii) single-core/elastane yarn, (iii) single-core/T400 yarn, (iv) dual-core yarn, elastane/T400 draft 3.0/1.01 and (v) dual-core yarn, elastane/T400 draft 3.8/1.2.

From the close study of respective stress-strain curves, the tenacity of 100% cotton, single-core/elastane and single-core/T400 was measured to be 23.13, 14.0 and 17.0 cN/tex, respectively. And tenacity values of all dual-core yarns (displayed in Fig. 7) are lower than that of 100% cotton yarn and nearly equal to single-core/elastane and single-core/T400 yarns covering a wide range between 10.4 and 17.4 cN/tex varied with elastane/T400 draft. Looking at Fig. 6, Fig. 7, it is obvious that the tenacity of single-core/elastane yarn, single-core/T400 yarn and dual-core yarns is much lower than that of 100% cotton yarn. Lack of elastane-cotton, T400-cotton, elastane/T400-cotton and inter-cotton cohesion due to the presence of elastane, T400 or elastane/T400 in yarns can be attributed to lower strengths of single-core and dual-core yarns than that of 100% cotton yarn.

Fig. 7.

Effect of elastane/T400 multifilament draft on the tenacity of dual-core yarns.

At the first look in Fig. 7, a gradual increase in the tenacity with the increase in combined elastane/T400 draft up to elastane draft 3.8 can be observed for all series of yarns. The increase in elastane/T400 draft means a decrease in elastane and T400, and an increase in cotton fiber percentage in yarn cross-section (the respective ratio was shown in Table 4). Higher cotton fiber percentage in yarn cross-section caused higher inter-cotton cohesion that resulted in higher tenacity values [34]. When a core-spun yarn is subjected to tensile loading, the core elastane normally gets extended, and the sheath cotton fibers wrapped around the elastane core tend to break or slip past one another before the elastane breakage. In this way, the cohesion or bonding among wrapped cotton fibers chiefly decides the overall tenacity of dual-core yarns [30,35]. In the case of dual-core yarn, both elastic core filaments get extended under loading, and T400 multifilaments rupture before rupturing elastane due to the difference in their extensibility (shown in Fig. 5).

As seen in Fig. 7, dual-core yarns produced with elastane draft 4.2 for all series showed lower strength compared to the yarns produced with elastane draft 3.8. In those cases, rupture and off-centering of elastane were observed during spinning because of the extremely high elastane draft. Produced yarns showed irregular crimp structures with huge thick and thin places. Elastane filament is structurally composed of alternatively rigid- and soft rubbery segments. The soft segments are stretched with the increase of draft during yarn preparation. When the draft was extremely high as it was 4.2 in the present case, elastane surpassed its yield stress and formed weak points in stretched soft segment parts making the elastane prone to rupture. Similarly, the utmost tension draft 1.3 imposed on T400 caused the rupture of T400 multifilament which was reflected in the lower strength of the whole series V yarns.

The reason for the variation in strength among dual-core yarns produced with the different combinations of elastane and T400 drafts was further investigated by observing the internal morphology of yarns through an optical microscope. As seen in Fig. 8(a) and (b), dual-core yarns produced with elastane/T400 draft 3.8/1.2 showed perfect centering of elastane filament covered by T400 multifilament and sheath cotton fibers. When perfect centering of elastane was ensured in dual-core yarn, both core filaments together with sheath cotton acted unitedly against the applied load during tensile testing resulting in higher tensile strength of yarns.

In the case of elastane draft 4.2, shown in Fig. 8(c), off-centering of elastane was observed for all series of yarns that occurred during spinning due to the extreme elastane draft. During tensile testing, unequal stress distribution was imposed on both core components (elastane and T400) and also lower inter-cotton cohesion originated from the separate or independent elastane and T400 core filaments resulting in lower yarn strength. Fig. 8(d) shows the elastane/T400 draft 3.8/1.3 dual-core yarn of series V where only creora elastane is visible as T400 got ruptured with excessive tension draft 1.3.

If the overall tenacity of all dual-core yarns is analyzed, elastane draft 3.8 (for all series) and T400 draft 1.2 (series IV) was found to be most favorable with regard to yarn strength. While optimizing the spinning parameters for cotton-wrapped single core/T-400 yarn in a ring frame, Akankwasa et al. achieved better yarn properties such as tenacity, evenness and hairiness at T400 draft 0.95 [36]. DuPont recommended the highest T400 draft ranging from 0.95 to 1.05 and elastane draft from 3.8 to 4.0 [37]. However, due to the difference in fineness (denier) and elasticity of the core components, the effect of core filament draft varies from material to material to yield desired yarn structure with prospective properties [37]. In the current study, elastane filament and T400 multifilament were used simultaneously as dual-core, and elastane/T400 draft 3.8/1.2 was come out to be an optimum draft combination as this yarn attained the highest tenacity 17.4 cN/tex among all dual-core yarns.

As shown by the stress-strain curves in Fig. 6, elongation at break% of 100% cotton-, single-core/elastane- and single-core/T400 yarn was measured to be 5.6%, 10.5% and 8% respectively. Elongation values of single-core/elastane and single-core/T400 yarns were distinctly higher than that of 100% cotton yarn due to the incorporation of elastic (elastane) and semi-elastic (T400) multifilament in the core yielding a proportional increase in yarn elongation (elongation of elastane: 570% and T400: 20%).

Elongation at break% of all dual-core yarns calculated from the respective stress-strain curves is shown in Fig. 9. In accordance with Fig. 6, elongation values of dual-core yarns were higher than 100% cotton yarn, and nearly equal to or quite higher than single-core/elastane and single-core/T400 yarns covering a wide range between 7.05 and 14.7% that was derived from the variations in elastane/T400 draft. For dual-core yarns, the trend in changing elongation was very similar to that of tenacity i.e. yarn elongation increased with the increase in elastane draft up to 3.8 for all series of yarns and the increase is highest with the combination of T400 draft 1.2 for series IV fibers. This is because a higher elastane draft induced a higher retraction force from its extension in the yarn which caused a higher crimp in the cotton wrapping fibers. Therefore, the elongation was higher at a higher elastane draft despite a lower elastane percentage in the yarn core [34,35,38]. Along with respective fiber extension, elongation of yarn also depends on the alignment and cohesion among fibers in yarn structure [39]. With the increase of elastane/T400 draft, cohesion among cotton fibers also increased which contributed to higher elongation as well as the tenacity of yarn.

Fig. 9.

Effect of elastane/T400 multifilament draft on the elongation at break% of dual-core yarns.

In the case of elastane draft 4.2, there was a decrease in the yarn elongation that can be ascribed to the formation of weak spots in the core elastane at the extreme elastane draft.

For series V yarns, elongation values were lower than series IV yarns. In addition, the standard deviation (SD) of elongation of all series V yarns was the highest resulting from the unstable spinning performance owing to the utmost T400 draft 1.3 as mentioned earlier.

The deviation in stretch value of the denim fabric is largely affected by the deviation of the elongation of the dual-core yarns used in it [40]. Among all, elastane/T400 draft 3.8/1.2 of series IV dual-core yarn can be considered as an optimum draft combination as it revealed the highest elongation with comparatively lower variation (SD). This yarn is expected to provide high and consistent elongation (due to low variation) in stretch denim fabric to be used in diversified applications.

3.3. Effects of elastane/T400 draft on the structural parameters of dual-core yarns

3.3.1. Coefficient of mass variation (CVm)

Unevenness of yarn is usually expressed by coefficient of mass variation (CVm%). Fig. 10 illustrates the CVm% of dual-core yarns where a range of CVm values 10.8–12.9% can be observed. CVm of 100% cotton-, single-core/elastane- and single-core/T400 yarns was measured to be 11.4, 10.6 and 11.1%, respectively (not shown in Figure). CVm of 100% cotton yarn is usually higher than core-spun yarns since cotton fiber has natural variability in its length and fineness. Conversely, elastane and T400 multifilament have a more uniform structure. The inclusion of uniform filaments in the yarn core proportionally reduces the cotton fiber% in yarn cross-section leading to lower CVm values of yarns [17].

Fig. 10.

Effect of elastane/T400 draft on the coefficient of mass variation (CVm) of dual-core yarns.

In Fig. 10, CVm of dual-core yarns, from series I to series IV, shows a decreasing trend with the increase in elastane/T400 draft and the highest reduction in CVm was found at elastane/T400 draft 3.8/1.2. An increase in elastane/T400 draft indicates a decrease of elastane/T400 and a proportional increase of cotton fiber percentage in yarn cross-section (mentioned in Table 4). The presence of higher cotton fiber in yarn cross-section with drafted elastane/T400 caused higher cohesion among cotton fibers that eventually imparted lower CVm values.

As mentioned above, frequent breakage and off-centering of elastane were observed while producing dual-core yarns with elastane draft 4.2 which resulted in higher CVm values than those of elastane draft 3.8. In addition, too high tension draft 1.3 of T400 caused the rupture of T400 multifilament and the magnitude of T400 rupture turned severe with the increase of elastane draft. Because of this reason, CVm of whole series V yarns was much higher than those of other series yarns.

Here, a draft combination of elastane/T400 3.8/1.2 for series IV yarn was obtained as most favorable in terms of CVm value of yarn.

3.3.2. Imperfections (thick, thin and neps) of dual-core yarns

Imperfections i.e. thick, thin and neps of yarns have a considerable impact on the appearance of woven or knit fabrics [41]. Thick(+50%), thin(-30%) and neps(+200%) per kilometer of dual-core yarns are shown respectively in Fig. 11, Fig. 12, Fig. 13.

Fig. 11.

Effect of elastane/T400 draft on the thick(+50%)/km of dual-core yarns.

Fig. 12.

Effect of elastane/T400 draft on the thin(-30%)/km of dual-core yarns.

Fig. 13.

Effect of elastane/T400 draft on the neps(+200%)/km of dual-core yarns.

Thick(+50%)/km of 100% cotton-, single-core/elastane- and single-core/T400 yarns was recorded as 65, 60 and 55, respectively (not shown in Figure). As seen in Fig. 11, thick places of dual-core yarns show a range of 36–78.

Thin(-30%)/km of 100% cotton-, single-core/elastane- and single-core/T400 yarns was found to be 536, 490 and 560, respectively (not shown in Figure). According to Fig. 12, dual-core yarns have a range of thin places between 441 and 760.

Neps (+200%)/km of 100% cotton-, single-core/elastane- and single-core/T400 yarns was measured to be 18, 27 and 24, respectively (not shown in Figure). As shown in Fig. 13, neps values in dual-core yarns were between 22 and 41.

Imperfections of the core-spun yarns are normally lower than that of 100% cotton yarn owing to the presence of uniform filament in the yarn core [17]. In this study, imperfections of dual-core yarns were found to be varied with varying elastane and T400 draft as found in the trend for CVm values. Thick, thin and neps values showed a significant decreasing trend with the increase of elastane/T400 draft up to elastane/T400 draft 3.8/1.2. Elastane draft 4.2 caused elastane rupture (for yarns of all series) and T400 draft 1.3 caused filament rupture (yarns of series V) during spinning. Produced yarns showed irregular crimp structure together with plenty of thick and thin places that ultimately triggered higher imperfection values in Uster Tester.

For the present study, it is summarized that by appropriate controlling of elastane/T400 draft, imperfections of dual-core yarns can be reduced remarkably. Here, the elastane/T400 draft combination 3.8/1.2 can be taken as the best combination as it resulted in the lowest thick, thin and neps values among all dual-core yarns.

3.3.3. Impact of elastane/T400 draft on the hairiness of dual-core yarns

The spun yarns, in principle, are produced by twisting the staple fibers where protruding fibers appear as fuzziness or hairiness on the yarn surface. Yarn hairiness is characterized by the amount of freely moving fiber ends protruding from the yarn surface [42].

The hairiness index of dual-core yarns is illustrated in Fig. 14 where a range of hairiness values 6.67–9.15 can be observed. The hairiness of dual-core yarn decreased with the increase of elastane/T400 draft up to 3.8/1.2 (Series I to series IV yarns). This may be interpreted with the fact that at higher elastane/T400 draft, cotton fibers got an opportunity to grip themselves more intimately which led to lower yarn hairiness. Besides, with the increase in filament draft, more retraction force was induced in the yarn structure which increases crimps in the wrapper cotton fibers resulting in the reduction in yarn hairiness. As mentioned earlier, elastane draft 4.2 (for yarns of all series) and T400 tension draft 1.3 (for yarns of series V) were so high that frequent filament rupture made the spinning process unstable. Due to too high crimps, many fibers started to protrude from the yarn body which consequently increased the yarn hairiness [34].

Fig. 14.

Effect of elastane/T400 multifilament draft on the hairiness of dual-core yarns.

Hairiness values of cotton-wrapped core-spun yarns are usually higher than the 100% cotton yarn as sheath cotton fibers cannot bind properly with the yarn body due to uncontrolled movement of sheath cotton during spinning [43]. The hairiness index of 100% cotton yarn, single-core/elastane yarn and single-core/T400 yarn was measured to be 7.48, 6.91 and 6.88, respectively (not shown in Figure). The lowest hairiness value was obtained at 6.67 for dual-core yarn with elastane/T400 draft 3.8/1.2 (Fig. 14). The outcome of the current study expressed that by manipulating elastane/T400 draft, the hairiness of dual-core yarns can be lowered than that of 100% cotton yarn as well as single-core/elastane- and single-core/T400 yarns.

It is generally known that the strength and elongation of yarn are determined by its structural parameters such as unevenness, imperfections and hairiness. In close agreement with the structural features (CVm% in Fig. 10, thick places in Fig. 11, thin places in Fig. 12, neps in Fig. 13 and hairiness in Fig. 14), the strength of dual-core yarns (Fig. 7) and more importantly elongation at break% (Fig. 9) exhibited the similar pattern. A fascinating outcome of this study is finding out an optimum combination of elastane/T400 draft 3.8/1.2 for dual-core yarns that resulted in the lowest CVm, thick, thin, neps and hairiness along with the highest strength and elongation values.

3.4. Impact of elastane/T400 draft of dual-core yarns under cyclic loading

In use, textiles are always subjected to complex, variable and intensive dynamic loads in the individual transient cycle that cause undesirable growth or bagging (especially from the motions of the knee and elbows) and loss of useful life of the garment [44]. In this context, the dynamic behavior of dual-core yarns was studied to know their growth characteristics under repetitive cyclic loading.

As explained the detailed method in the experimental section, extension% under five consecutive cycles of dynamic loading, equivalent to 50% of breaking load, of single-core/elastane yarn, single-core/T400 yarn and a representative dual-core (elastane/T400 draft 3.8/1.2) yarn are demonstrated in Fig. 15. If observed minutely, it will be visible that at the end of each loading cycle, the extension% of the yarns increased. The increase in extension is highest for the single-core/elastane yarn, lowest for the single-core/T400 yarn and in between for dual-core yarn (indicated by red up arrow symbols). Plastic deformation of yarns calculated from the extension% after the first- and 5th cycle of dynamic loading is shown in Fig. 16.

Fig. 15.

Gradual increase of extension in yarns after imposing cyclic loading equivalent to 50% breaking load.

Fig. 16.

Plastic deformation% of single-core/elastane, single-core/T400 and dual-core yarns after imposing repeated dynamic loading on yarn.

Plastic deformation refers to the change in the structure causing an undesirable permanent change in the shape of the textile product during use [25]. As seen in Fig. 16, compared with single-core/T400 yarn, the plastic deformation of single-core/elastane yarn was too high. It manifested the ability of high recovery of T400 and poor recovery of elastane in single-core yarn. In the case of single-core/elastane yarn, elastane filament in the core mostly carried the load and hence it might suffer from fatigue during repeated dynamic loading that resulted in poor recovery or resilience of the yarn. For single-core/T400 yarn, only tension draft was applied on T400 multifilament that always showed a tendency to high retraction force resulting in high recovery or low plastic deformation of yarn. Due to the presence of T400 multifilament in the core and its inherent retraction force, all dual-core yarns exhibited lower plastic deformation than single-core/elastane yarn under dynamic loading. Dual-core yarn produced with elastane/T400 draft combination 3.8/1.2 showed the lowest plastic deformation or highest resilience. As observed in Fig. 9, this yarn exhibited the highest elongation at break% too. The highest elongation originated from the high elastane draft (3.8) and high recovery derived from the high retraction force of T400 caused by its high tension draft (1.2). More clearly, T400 multifilament had low dynamic strain which contributed to the recoverability of dual-core yarns when high tension draft was applied to it.

Together with permanent deformation, the structural parameters of core-spun yarn significantly affect the stress decay or stress relaxation of yarn [24]. Stress decay is the observation of the reduction in stress while maintaining a material at a fixed deformation over a period of time and it may be used to characterize the elastic recovery of yarns [45]. While keeping the yarn in a strained condition for some finite interval of time, some amount of plastic deformation occurs that results in decaying the stress. A lower stress decay means the occurrence of lower plastic deformation of yarn which indicates the yarn is highly capable to recover to its original shape after deformation.

As displayed in Fig. 17, similar patterns like plastic deformation are observed for the stress decay of single-core/elastane yarn, single-core/T400 yarn and all dual-core yarns. Single-core/elastane yarn possesses the highest stress decay and single-core/T400 yarn possesses the lowest stress decay. For dual-core yarns, stress decay decreases with the increase of the elastane/T400 draft and the lowest stress decay is observed for elastane/T400 draft 3.8/1.2 yarn. The elastic recovery of core-spun yarn is mainly dependent on the characteristics of the elastic core component [9]. As the T400 multifilament possesses very low-stress decay or high elastic recovery, its inclusion as a core component with the elastane gives rise to the enhancement of elastic recovery of the core-spun yarns and at higher T400 draft, its retraction force becomes more effective to recover the yarn. In a similar study, cotton-wrapped core-spun yarns containing a mix of elastane and PET/PTT bi-component filament were reported to have much lower stress decay compared to the single-elastane core yarn [9].

Fig. 17.

Stress decay of single-core/elastane, single-core/T400 and dual-core yarns after imposing repeated dynamic loading on yarn.

As explained earlier, dual-core yarns produced with utmost elastane draft 4.2 (in all series) and T400 draft 1.3 (in series V) caused frequent elastane and T400 ruptures respectively during spinning. For these yarns, plastic deformation as well as stress decay was comparatively higher.

3.5. Comparative yarn properties between single-core/elastane and dual-core yarns

Finally, the properties of dual-core yarns produced in this study were compared with the same of single-core/elastane yarn that is currently used to produce stretch denim and jeans. As shown in Table 5, dual-core yarn obtained in optimum draft combination (elastane/T400 draft 3.8/1.2) showed much better results than single-core/elastane yarn in all aspects, especially higher strength and elongation along with lower plastic deformation and stress decay. Denim and jeans to be manufactured with such dual-core yarn are expected to have high strength and high elasticity with low growth or bagging deformation.

Table 5.

Comparative yarn properties between single-core/elastane and dual-core yarns.

Parameters	Single core/elastane yarn (elastane draft 3.8)	Dual-core yarn (elastane/T400 draft 3.8/1.2)
Tenacity (cN/tex)	14	17.4
Elongation at break (%)	10.5	14.7
Plastic deformation (%)	12.6	4.7
Stress decay (%)	16.2	10.8
Coefficient of mass variation, CVm (%)	10.6	10.8
Thick (+50%/km)	60	36
Thin (−30%/km)	490	441
Neps (+200%/km)	27	22
Hairiness Index	6.91	6.67

4. Conclusions

In order to explore yarn for producing stretch denim with enhanced comfort, durability and shape retention, cotton-wrapped elastane/T400 multifilament dual-core yarns were manufactured with different combinations of elastane and T400 drafts covering a wide range of T400 and elastane draft (elastane draft range: 3.0–4.2 and T400 draft range: 1.01–1.3). With the change in elastane/T400 draft, significant variations in structural parameters and properties of yarns were found. There was a draft limit up to which both tenacity and elongation of yarns increased to a remarkable extent and beyond that limit the trend turned to be reversed. The extreme elastane draft (i.e. 4.2) resulted in rupture and off-centering of elastane, and the extreme T400 draft (i.e. 1.3) caused T400 rupture during the spinning. With an optimum combination of elastane/T400 draft 3.8/1.2, the dual-core yarn exhibited highest tenacity and elongation together with lower evenness, imperfections and hairiness values. The repetitive dynamic loading study demonstrated a marked reduction in plastic deformation and stress decay indicating low growth and high recoverability after the deformation of yarn.

Compared to the single-core/elastane yarn currently being used for stretch denim, the obtained dual-core yarn in this study exhibited higher strength and elongation, lower plastic deformation and stress decay along with improved structures (CVm, imperfections and hairiness). The combination of different properties in dual-core yarn indicates its potential application to produce stretch denim garments with high body movement comfort with low fabric growth.

Author contribution statement

Ahmed Jalal Uddin: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper. Md. Abdur Rahim: Conceived and designed the experiments; Performed the experiments; Contributed reagents, materials, analysis tools or data; Wrote the paper. Sadikur Rahman: Analyzed and interpreted the data; Wrote the paper.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

Data will be made available on request.

Declaration of compeing interest

The authors declare no conflict of interest.