The Effect of Single and Dual-Core Yarns Produced with Different Core Materials on the Elasticity and Recovery Properties of Denim Fabrics

Elastic structures are preferred for improving the elasticity and recovery of the fabrics. Bagging, which is a three-dimensional fabric deformation, is an undesirable appearance of fabrics. The lack of dimensional stability or recovery after pressure on the fabrics causes bagging deformation. In recent years, denim manufacturers prefer double-core yarns to cope with the problem of bagging. In this study, various types of weft yarns were produced by using different core materials in different combinations. Double-core and single-core ring-spun yarns (Ne 18, ae 4,4) were used as weft yarns in weaving. Yarn and fabric samples were conditioned under standard atmospheric conditions and measured according to the related test standards. Yarns were tested and evaluated for important physical and mechanical properties such as evenness, imperfections, tenacity, breaking elongation, hairiness and yarn liveness. For denim fabric samples, all tests were performed after 3 consecutive domestic washing processes. Test results showed that there are significant differences between fabric properties depending on the weft yarn characteristics. By using double-core weft yarns in denim fabric production, consumers can have stretch jeans for improving body movement comfort while exhibiting low growth and shrinkage values.


INTRODUCTION
Consumers want to look good and feel comfortable in their clothes. The minimum elasticity requirements of the fabrics can be determined with the values of stretch that are encountered during sitting, bending or flexing of knees, hips and elbows. While body is standing at rest, the stretching value is zero and during different actions, back flex is 13-16%, elbow flex (lengthwise) is 35-40%, seat flex is 25-30% and knee flex (lengthwise) is about 35-45%. So, for improving body movement comfort in denim jeans, at least 10-35% elasticity is needed. The amount of extension depends on the end use of the fabric. The percentage of the stretch for loose fitting (comfort stretch) is 30% in course way -15% in wale way while it is 60% in course way -35% in wale way for form fitting (semi-support). This is generally accomplished by incorporating elastic fibres into the yarn structure [1]. Depending on body movements, the three-dimensional deformation occurring in certain parts (knee, elbow etc.) of the garments, known as bagging, is an undesirable appearance [2]. The lack of dimensional instability or recovery after prolonged or repeated pressure on the fabrics causes bagging deformation [3]. In recent years, double-core yarns have begun to be produced, especially for denim jeans, to overcome the mentioned problem. Core-spun yarns consist of two different components; a staple sheath and a filament core. If the core filament is an elastic polyurethane filament, it is known as "soft-core yarn" whereas if it is a rigid filament, yarns are known as "hard-core (rigid) yarn" on the market. In recent years, alternatively, multicomponent core-spun yarns, called double-core yarns, are developed in order to cope with the recovery problems of elastane. Double-core (dual-core) yarns are composed of three components; an elastic polyurethane filament (such as Lycra®, Creora® or Inviya® I-300) and a multifilament (such as Lycra® T400®) that are used in the core, which is then covered by a staple sheath. Double-core yarns are used for high quality denim fabrics [4]. Typically, as fabric stretch increases, the tendency for fabric growth or shrinkage increases as well. However, with double-core yarns, by combining elastane and multifilament, higher elasticity and recovery, lower growth and shrinkage values can be achieved [5]. Although it has high production costs compared to soft-core yarns, double-core spun yarns are preferred because of the high value-added denim jeans. Many researchers have been focused on core-spun yarn properties [6][7][8][9][10]. Sarıoğlu and Babaarslan investigated the effects of different filament fineness and yarn count on fatigue behavior of rigid core-spun yarns containing PET textured filament yarn [11]. Telli et al., focused on the usage of core and double-core yarns containing tungsten for electromagnetic shielding [12]. Akankwasa et al. focused on the mechanical properties of cotton/T400® knitted fabrics and reported that fabrics knitted from cotton/T400® yarns are advantageous in breaking elongation, bursting strength and air permeability compared to 100% cotton yarns [13]. Cataloglu stated that, alternative to elastane, using bi-component polyester fibres and poly butylene terephthalate (PBT) has inspired interest with regarding to higher strength due to better chemical resistance, better recovery, dimensional stability and elasticity properties [14]. Sarıoğlu et al. analysed the effect of the false-twist textured polyester-filament yarns with different filament fineness, used as a core component of composite yarn on residual torque [15]. Denim fabrics were produced by using multifilament polyester yarns with different filament numbers and it was reported that yarns that have higher filament number might be problematic concerning elasticity, growth and elastic recovery [16]. Ertaş et al. analysed the effect of the density changes in the use of the double-core threads used in denim fabrics [17]. Bedez Ute analysed the mechanical and dimensional properties of the denim fabrics produced with double-core and core-spun weft yarns with different weft densities [18]. In another study, they investigated the effects of elastane linear density on the mechanical properties of double-core and single-core yarns, spun with different core materials [19]. Many researchers have examined the bagging properties of the fabrics. Ozdil evaluated stretch and bagging properties of denim fabrics containing different rates of elastane [20]. El-Ghezal et al. focused on the effect of elastane ratio and the finishing process on the elasticity of denim made of weft yarns that were cotton covered core-spun yarns containing elastane, having the same twist factor and various elastane ratios [21]. The performance of stretch denim fabrics under the effect of repeated home laundering practices was studied by Kan et al. [22]. Doustar et al. investigated the effect of weave design and fabric weft density on the bagging tendency of woven fabrics using FAST system [23]. In this study, the effects that weft yarn type (double-core, soft-core and hard-core) and core material have on the elasticity and recovery properties of the denim fabrics were investigated.

Materials and Methods
With the aim of determining the eff ect of core material and core yarn producti on methods on the elasti city and recovery properti es of denim fabrics, diff erent types of weft yarns were produced. Double-core and core-spun yarns were used as weft yarns in weaving. Core-spun yarns were produced with two components; a staple sheath (cott on) and a fi lament core (EME, PES or elastan). Double-core (dual-core) yarns were produced from three components; elastane fi lament and a multi fi lament (EME or PES) used in the core, covered by a staple sheath (cott on) ( Figure 1). The fi neness of the fi rst core (elastane) and the second core (PES or EME) are 44 dtex and 55 dtex, respecti vely. Their draft rati os are 3,4 and 1,06, respecti vely. 100% cott on Ne 10/1 Ring slub warp yarns were processed in a conventi onal denim process; which includes rope dyeing, rope opening, sizing etc.   The weaving process was performed with fi ve diff erent weft yarns (Table 1) and the other producti on parameters were kept constant. Twill 3/1 woven fabrics were treated according to standard denim fi nishing procedures. Denim fabric producti on process parameters are given in Table 2.   [24]. Fabric test methods are given in Table 3.  Table 4, figures a and b. The apparatus used is a stretch testing instrument, which consists of a part with a fixed clamp at the top and a separate clamp to attach the weight at the bottom of the test sample. Two lines with the distance of 25 cm are marked on the sample (O1) and then stretched under tension for a certain time (Table 4, figure c). A mass of 3 lbs (1.35 kg) was used for applying tension to the denim fabric specimen so as to determine the fabric stretch and fabric growth after applying specific tension. Initially, the specimen is cycled three times to the fixed load with each cycle taking 5 s and then the load is hanged for 30 minutes (Table 4, figure d). After applying tension for 30 min (Table 4, figure e), the resulting distance between the lines is measured, which is A (mm). After the tension is removed and allowing 60 minutes for relaxation, fabric growth is found by measuring the relaxed distance between the lines: distance B (mm). Calculations are explained with an example in Table 4.

RESULTS AND DISCUSSION
The effects of the core material and weft yarn characteristics on the yarn evenness, yarn hairiness, breaking strength and elongation values, as well as the fabric test results, are given in Table 5 and Table 6. Weft and warp shrinkage values were measured after 3 home launderings.   Weft and warp densiti es of the fabrics and fabric weight increased aft er washing. Denim fabrics' weight values, woven with diff erent weft yarns, were close to each other before washing, but aft er washing, the fabrics containing EME had the lowest and the EME + elastane based double-core weft yarns had the highest values. As expected, due to higher weft shrinkage values, fabrics produced with weft yarns containing elastane have higher fabric weight and weft & warp densiti es than hard-core weft yarns. It is remarkable that in general more weft breaks were observed when hard-core weft yarns were used in the producti on of denim fabric. Weft and warp densities of the fabrics and fabric weight increased after washing. Denim fabrics' weight values, woven with different weft yarns, were close to each other before washing, but after washing, the fabrics containing EME had the lowest and the EME + elastane based double-core weft yarns had the highest values. As expected, due to higher weft shrinkage values, fabrics produced with weft yarns containing elastane have higher fabric weight and weft & warp densities than hard-core weft yarns. It is remarkable that in general more weft breaks were observed when hard-core weft yarns were used in the production of denim fabric.  Before washing, denim fabrics woven with weft yarns containing EME had the lowest fabric elasti city and soft -core weft yarns containing elastane had the highest fabric elasti city ( Figure 2). Aft er washing, elasti city values of all the samples decreased and again denim fabrics produced with soft -core yarns showed the highest elasti city values, whereas fabrics having core-spun weft yarns containing PES fi lament in the core showed the lowest values, as expected. The elasti city values of the denim fabrics woven with double-core yarns are found lower than the elasti city values of the fabrics woven with soft -core yarns, although higher than the elasti city values of the fabrics woven with hard-core weft yarns.
However, the high elasti city of the fabrics is not enough for customer sati sfacti on alone, growth values should also be at the desired level. The growth values of the denim fabrics woven with double-core yarns are found lower than the growth values of the fabrics woven with soft -core yarns. On the other hand, denim fabrics produced with hard-core weft yarns have lower growth values than the fabrics produced with doublecore and soft -core weft yarns (Figure 3). Fabrics containing soft -core weft yarns showed the highest growth values whereas EME based core-spun weft yarns showed the lowest values.

CONCLUSION
Modern consumers want to look good and feel comfortable in their clothes. For this reason, they expect them, especially denim jeans, to provide body movement comfort, allow freedom of movement, reduce burden and body shaping. The opposite is actually contrary to the spirit of denim jeans. In recent years, denim manufacturers prefer double-core yarns because they provide elasti city with the reduced bagging problem. In this study, the eff ects of weft yarn type (double-core, soft -core and hard-core) and core material on the elasti city and recovery properti es of denim fabrics were investi gated. It can be concluded that, by using double-core weft yarns in denim fabric producti on, consumers can have stretch jeans with improved body movement comfort (more than 30% stretch) that exhibit low growth and shrinkage values at the same ti me. Although double-core yarns have higher producti on costs compared to soft -core yarns, they are preferred in high quality denim jeans since they give sati sfactory results in terms of both appearance and comfort.
[ woven with double-core yarns are found lower than the elasticity values of the fabrics woven with softcore yarns, although higher than the elasticity values of the fabrics woven with hard-core weft yarns. showed the highest growth values whereas EME based core-spun weft yarns showed the lowest values.

CONCLUSION
Modern consumers want to look good and feel comfortable in their clothes. For this reason, they expect them, especially denim jeans, to provide body movement comfort, allow freedom of movement, reduce burden and body shaping. The opposite is actually contrary to the spirit of denim jeans. In recent years, denim manufacturers prefer double-core yarns because they provide elasticity with the reduced bagging problem. In this study, the effects of weft yarn type (double-core, soft-core and hard-core) and core material on the elasticity and recovery properties of denim fabrics were investigated. It can be concluded that, by using double-core weft yarns in denim fabric production, consumers can have stretch jeans with improved body movement comfort (more than 30% stretch) that exhibit low growth and shrinkage values at the same time. Although double-core yarns have higher 6