Studies on elastane-cotton core-spun stretch yarns and fabrics: Part I Yarn characteristics

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1 Indian Journal of Fibre & Textile Research Vol. 38, September 2013, pp Studies on elastane-cotton core-spun stretch yarns and fabrics: Part I Yarn characteristics A Das a & R Chakraborty Department of Textile Technology, Indian Institute of Technology, New Delhi , India Received 24 July 2012; revised received and accepted 22 September 2012 The present paper reports the interaction effect of elastane stretch, proportion of elastane core and twist multiplier on physical and mechanical properties of stretchable elastane-cotton core-spun yarns. The core component is 78 dtex elastane filament and the sheath consists of cotton. The core-spun yarns are produced in a modified ring frame. A three-variable factorial design technique, proposed by Box & Behnken has been used to study the combined interaction effect of the above variables on various yarn characteristics, like tenacity, breaking elongation, yarn-to-metal friction, elastic recovery and hairiness. It is observed that the above parameters have significant impact on all the yarn characteristics except breaking elongation. Keywords: Core-spun yarn, Elastane filament, Elastic recovery, Hairiness, Yarn-to-metal coefficient of friction 1 Introduction Stretchable fabrics are now-a-days used widely in garment industries and the demand for these products is increasing day by day due to their wear comfort characteristics 1. The stretchable yarns are produced using core spinning technique on a modified ring frame, siro spinning, air entangling, hollow spindle spinning, rotor spinning and friction spinning 2-7. Each system has its own features. The conventional ring spinning is simple and economical. Core-spun yarn is produced at ring frame by combining a continuous strand through the delivery rollers and the staple fibres through normal drafting arrangement. The component fed through the delivery rollers is usually known as the "core", and the other component is known as the "sheath" that forms the outer cover. The core may be of continuous filament yarn or of spun yarn. The modified ring spinning frame included a positive feed roller as elastane delivery unit and a V-groove guide to feed the elastane filament to front roller. The elastane filament is stretched between the positive feed roller and front roller, which will provide elasticity in core-spun yarn. The basic requirement to produce an elastic core-spun yarn is to stretch a spandex thread before it enters the spinning unit 8. This action provides elasticity in the final yarn a Corresponding author. apurba65@gmail.com by retraction of the elastane core when stress is removed, thus compacting and bulking the spun yarn cover. The core-spun yarn can be extended to the point where the non-elastic sheath portion of the yarn is stretched to its limit, thus resisting further extension of the core-spun yarn 9. Su et al. 10 studied the structural and performance characteristics of elastic core-spun yarns. They have produced 19.7 tex elastic core-spun yarn from 44.4 dtex/4f spandex filament as the core and cotton fibers as the sheath. In order to improve yarn performance, they examined the yams cross-sectional structure and investigated the effect of draw ratio and feed-in angle of the spandex on the structure and performance of yarn. Su et al. 11 used spandexes of three degrees of fineness as core material with cotton fibers as the sheath to spin fine elastomeric yarns. They have reported that the production of a finer elastomeric yarn with a uniform structure is feasible. Adeli et al. 12 carried out structural evaluations of elastic core-spun yarns and fabrics under tensile fatigue loading. They have measured the tensile properties of fatigued yarns by a tensile tester 3 and the work carries detailed study on mechanical characteristics of high elastic core-spun yarn made in rotor spinning with spandex as core. Miao et al. 13 stated that at a low pre-tension level the core filament could migrate from the centre to the outer layers and back to the centre repeatedly during core yarn

2 238 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 2013 spinning, which has been described as Barber Pole Effect. Sawhney et al. 14 and Miao et al. 13 have also reported that to enhance sheath slipping resistance the filament pre-twist should be opposite to the sheath twist direction in ring spinning. The effects of various parameters on bulk characteristics and opening performance of fibres are also reported 15-17, which directly or indirectly influences characteristics of yarns and fabrics. Recently various elastane filaments have been developed to enhance soft stretch with renewed comfort by reducing pressure on the body. When the focus is on freedom, for active sportswear and certain other types of intimate apparel, these yarns accommodate extreme movement without restraint as they maintain garment s original fit and appearance. Many of the fabrics use 3-30% elastane content, depending on the type of the garment. Blending cotton and spandex is one of the most popular combinations in use, and is especially predominant in this segment of the fashion industry. The various characteristics of elastane core-spun yarns have been a subject of investigation for the last few decades. Knowledge about the characteristics of yarn is essential for predicting the characteristics of fabric and the behaviour of its end products. The high rate of growth in production of core-spun yarns has led to a substantial increase in research aimed at establishing links between yarn characteristics and production parameters. Most of the fabrics made from these yarns are mainly used in swimsuits/bathing suits, net bodysuits, lingerie, ski pants, slacks, leggings, socks, compression garments such as cycling shorts, surgical hose, wrestling singlet, etc. The properties of elastane core-spun yarns can be varied mainly by varying three independent variables, such as elastane stretch, twist multiplier and proportion of elastane component in core. The primary objective of this study is to predict the characteristics of the elastane core-spun yarns and fabrics. The present study deals with the characteristics of yarn, like tensile characteristics, elastic recovery, hairiness and yarn-to-metal coefficient of friction. A three factorial Box & Behnken 18 technique has been used to produce yarn samples, and the combined influence of elastane stretch, proportion of elastane core and twist multiplier on yarn properties and plain woven fabric properties is investigated in detail. The present paper is confined to the characterization of yarn properties based on the selected factorial design. 2 Materials and Methods 2.1 Materials Cotton fibre with mean fineness of 1.4 dtex, 2.5% span length of 30.8 mm and tenacity of 29.6 cn/tex were used to produce roving of 650 tex. The core component was 78 dtex elastane filament. 2.2 Methods Design of Experiment and Yarn Preparation The core-spun yarns were produced in a ring spinning frame [LMW5/1 model] with a special attachment for making the core-spun yarn. A threevariable factorial design, proposed by Box & Behnken 18 (Table 1) was used to investigate the combined interaction of process parameters (i.e. proportion of elastane core, elastane stretch and twist multiplier) on various characteristics of yarn. The proportion of core and sheath and the twist multiplier were adjusted by changing various twist and draft change pinions. The elastane stretch was changed by setting the draft controller knob of the PINTER elastane core attachment which was specially attached to the ring spinning machine for making the core-spun yarns. The sliver from the finisher draw frame was converted to roving and then the roving and elastane filament were fed to ring spinning machine to produce the core-spun yarns with predetermined elastane stretch, proportion of elastane core and twist multiplier. The yarn sample numbers in this study are referred by using their standard order in the design. The linear Table 1 Box and Behnken design for three variables Run No. Elastane stretch Yarn TM Proportion of elastane core, % (A) (B) (C)

3 DAS & CHAKRABORTY: ELASTANE-COTTON CORE-SPUN YARNS & FABRICS: PART-I 239 density of elastic core-spun yarn is measured using following equation: T csy = T sh + T s /S d... (1) where T csy is the linear density of elastic core-spun yarn (tex); T sh, the linear density of the covered cotton (tex); T s, the linear density of the spandex (tex); and S d, the draw ratio (or stretch) of the spandex during spinning. Table 2 shows the actual values of the three variables corresponding to coded levels Evaluation of Physical Properties of Yarn The testing of tensile properties of the yarns was carried out using Instron tensile tester (Model 4301). The specimen length under test was taken as 100mm and the testing speed was maintained at 300 mm/min. Lawson-Hepmhill friction tester was used to measure yarn-to-metal frictional coefficient. The elastic recovery of yarn was measured by hanging a standard mass with the yarn sample of a particular length for certain time. After that the mass was released and the length Table 2 Actual values of variables corresponding to coded levels Variables Coded levels Elastane stretch, (A) Yarn TM, (B) Proportion of elastane core, % (C) was measured again. Then elastic recovery of yarn samples was measured by following equation: Elastic recovery (%) = [Recovered extension/imposed extension] (2) The hairiness of yarn was measured by Zwiegle hairiness tester with yarn withdrawal speed of 50m/min and pretension of 3cN/tex for all the yarn samples. The hairiness was tested over a length of 100m. The S 3 value, obtained in this instrument, is mainly the number of hairs equal to or more than 3mm present in 100 m length of yarn. The properties of yarns and the response surface equations are given in Tables 3 and 4 respectively.100% cotton reference yarn was produced with equivalent linear density of 34.5 tex and twist multiplier of Results and Discussion 3.1 Tenacity and Breaking Elongation of Yarn The contour plots (Fig. 1) and the response surface equation (Table 4) show the interactive relations between process variables and yarn tenacity. Table 4 also shows that the process variables are highly correlated with the yarn tenacity. No specific trend was observed in breaking elongation for any of the process variables. It is evident from Figs 1(a) and 1(b) that with the increase in elastane stretch the yarn Table 3 Physical properties of elastane core-spun yarns Sl No. Experimental variables Yarn characteristics Elastane stretch (A) Yarn TM (B) Proportion of elastane core, % (C) Linear density tex Tenacity cn/tex Breaking elongation % Yarn-to-metal coefficient of friction Elastic recovery % Hairiness (S 3 )

4 240 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 2013 Table 4 Response surface equations of yarn properties Yarn characteristics Response surface equation R 2 Tenacity A B C A B A C B C A B C Yarn-to-metal coefficient of friction E-003 A B+3.750E-003 C-2.500E-003 A B-5.000E A C E-003 B C E-003 A E-003 B E-003 C 2 Elastic recovery A B C A B B C A C Yarn hairiness A B C A B A C B C A B C 2 Fig. 1 Relationship between process parameters and yarn tenacity tenacity initially increases and then drops. The same trend is observed for all the levels of twist multiplier [Fig. 1(a)] and elastane core content [Fig. 1(b)]. Babaarslan 19 reported that in elastane core-spun yarn, most of the loading stress is mainly taken up by relatively lesser extensible sheath component, in the present case cotton sheath fibres. With the initial increase in elastane stretch ratio, the proportion of wrapped cotton fibre in the yarn cross-section increases. So, there is more cotton sheath fibre taking up the load in core-spun yarn when elastane stretch increases initially. When the elastane stretch ratio is increased further, the stretchable range of elastane component decreases and the breakage of core-spun yarn takes place when the sheath cotton fibres are stretched straight up to their breaking point during extension. Similar trend was also observed by Dang et al. 20 in wool/spandex core-spun yarn. It can be observed from Fig. 1(a) that the increase in twist also results in initial increase and subsequent drop in tenacity of elastane core-spun yarn. This is mainly due to the fact that at lower twist level the cotton sheath fibres are wrapped loosely and cohesion between the elastane core and sheath fibres are lower 20. With the increase in twist level the cohesion between core elastane filament and sheath cotton fibres increases, this results in increase in yarn tenacity. As the twist increases further, the cohesion between elastane filament and sheath cotton fibres becomes too much. The high twist results in higher angle of alignment of sheath cotton fibres which results in lower load sharing of fibres due to obliquity effect. Also the contribution of core elastane filaments towards yarn strength reduces due to insertion of higher twist in the filament. It can be observed from Fig. 1(b) that with the decrease in proportion of elastane core content, the yarn tenacity in general increases. The same trend is valid for all the levels of elastane stretch and twist multiplier. This may be due to the fact that less core content means higher amount of load bearing cotton sheath fibres in the yarn cross section. So, more contribution of strength is imparted by the sheath fibres. The contribution of core to the strength of the yarn is very less. Mostly during tensile testing, failure of the structure occurs due to failure of the sheath fibres. Moreover, higher core % results in slip between core and sheath, resulting in lower tenacity of elastane core-spun yarn. 3.2 Yarn-to-Metal Friction The interactive relations between process variables and yarn-to-metal friction are shown in contour plots

5 DAS & CHAKRABORTY: ELASTANE-COTTON CORE-SPUN YARNS & FABRICS: PART-I 241 (Fig. 2) and the response surface equation in Table 4. It is evident from Table 4 that the process variables are highly correlated with the yarn-to-metal friction. Figure 2(a) shows the effects of elastane stretch and twist multiplier on yarn-to-metal coefficient of friction of elastane core-spun yarns. It is evident that at constant level of elastane stretch, with the increase in the amount of twist the yarn-to-metal coefficient of friction decreases. This may be due to the fact that by increasing the level of twist in the core-spun yarn the yarn becomes compact and the surface becomes harder. This results in lesser deformation in yarn surface during contact with other surface. So, lesser contact area between yarn and metal surface results in lower coefficient of friction. For fibrous materials, the coefficient of friction is dependent on the area of contact 21. It is evident from Figs 2(a) and 2(b) that the increase in elastane stretch results in increase in yarn-to-metal coefficient of friction. This may be due to the fact that the higher elastane stretch results in generation of greater bulkiness due to loop formation by the sheath cotton fibres after relaxation. The bulky yarn results in greater contact area with other surface, which, in turn, shows in higher yarn-to-metal coefficient of friction. It can be observed from Fig. 2(b) that the increase in proportion of elastane core results in increase in yarn-to-metal coefficient of friction. The increase in proportion of elastane core from 10% to 20% is achieved by reducing the number of cotton sheath fibres proportionally, because the same elastane core filament is used. Therefore, increase in elastane content means the decrease in number of covering sheath fibres. So, with the increase in proportion of elastane core, i.e. decrease in number of sheath fibres in the yarn cross-section, there is more chance of sheath fibres to get buckled. This is due to the fact that the average buckling force per sheath cotton fibre increases with the increase in elastane content. Hence, the buckled sheath fibres result in a softer cover on the yarn surface. The softer yarn surface causes higher yarn-to-metal coefficient of friction. The reason for this has already been discussed. 3.3 Elastic Recovery The contour plots in Fig. 3 and the response surface equation in Table 4 show the interactive relations between process variables and elastic recovery of yarn. It has been observed that the elastic recovery of Fig. 2 Relationship between process parameters and yarn-tometal friction Fig. 3 Relationship between process parameters and yarn elastic recovery

6 242 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 2013 core-spun yarn is much higher than the normal cotton yarn as elastane is used as filament in core. It is clear from Fig. 3(a) that the increase in yarn twist reduces in elastic recovery. This may be due to the fact that as the yarn twist increases, the sheath cotton fibres are wrapped with the elastane core firmly, resulting in reduced stretchability of the core-spun yarn. Due to this compact yarn structure at higher twist, the mobility of the elastane core also reduces. The trend is just opposite to that reported by Dang et al. 20 in case of wool/spandex core-spun yarn. It can be observed from Figs 3(a) and (b) that the increase in elastane stretch increases the elastic recovery significantly. This is mainly due to the fact that higher elastane stretch during yarn production process results in more ability of the yarn for retraction. The same trend is observed for all the level of yarn twist and elastane core content. Figure 3(b) shows that the proportion of elastane core, within the present experimental range, has no significant impact on elastic recovery. 3.4 Hairiness of Yarn It has been observed that the hairiness (S 3 ) value for core-spun yarn is much higher than normal cotton yarn (for normal cotton yarn the S 3 value is 1030). This is due to the fact that during manufacturing of elastane core-spun yarn there is a possibility of uncontrolled movement of staple sheath. So, the sheath fibres are not properly bound into the yarn structure. The interactive relations between process variables and yarn hairiness are shown in Fig. 4 and the response surface equation in Table 4. It can be observed from Fig. 4(a) that the increase in the level of twist reduces the hairiness of elastane core spun yarn. This is mainly due to the fact that at lower level of twist the sheath fibres are not wrapped properly with the elastane core and the chances of fibres coming out of the yarn body increase after relaxation. On the other hand, with the increase in level of yarn twist the sheath fibres are bound properly with the core and chances of these sheath fibres coming out of the yarn body are lesser. It is evident from Figs 4(a) and (b) that the increase in elastane stretch increases the hairiness of yarn, which is mainly due to the fact that the increase in elastane stretch causes more distortion in arrangements of sheath fibres. Due to this the sheath fibres tend to come out of the body of the yarn and thus the hairiness increases. It can be observed from Fig. 4(b) that the increase in the proportion of elastane core increases the hairiness. Fig. 4 Relationship between process parameters and yarn hairiness The reason for this trend is that with the increase in the proportion of elastane core component the buckling tendency of sheath fibres increases. The reason for this has already been discussed. 4 Conclusion The physical characteristics of elastane core-spun yarns are significantly influenced by the process parameters like, elastane stretch, proportion of elastane core and twist multiplier. With the increase in elastane stretch the yarn tenacity initially increases and then drops. The same trend is observed for all the levels of twist multiplier and elastane core content. The similar trend, i.e. initial increase and subsequent drop in tenacity of elastane core-spun yarn, has been observed when the level of yarn twist increases. With the decrease in proportion of elastane core content the yarn tenacity, in general, increases and the same trend is valid for all the levels of elastane stretch and twist multiplier. The yarn-to-metal coefficient of friction decreases with the increase in amount of twist and with the decrease in elastane stretch. The increase in proportion of elastane core results in increase in yarnto-metal coefficient of friction. The elastic recovery

7 DAS & CHAKRABORTY: ELASTANE-COTTON CORE-SPUN YARNS & FABRICS: PART-I 243 of elastane core-spun yarn is much higher than the normal cotton yarn. The increase in yarn twist decreases the elastic recovery. The elastic recovery increases significantly with the increase in elastane stretch. The hairiness (S 3 ) value for elastane core-spun yarn is much higher than normal cotton yarn. The increase in the level of twist reduces the hairiness of elastane core-spun yarn, but the increase in elastane stretch and proportion of elastane core component increases the hairiness of yarn. References 1 Matsuo T, Res J Text Apparel, 8(1)(2004) Bhortakke M K, Nishimura T & Matsuo, Text Res J, 69(2)(1999) Lin J H, Chang C W, Lou C W & Hsing W H, Text Res J, 74(2004) Merati A A, Konda F, Okamura M & Marui E, Text Res J, 68(4)(1998) Ruppenicker G F, Harper R J, Sawhney A P & Robert K Q, Text Res J, 59(1)(1989) Sawhney A P S, Robert K Q & Ruppenicker G F, Text Res J, 59(9)(1989) Sawhney A P S, Ruppenicker G F & Robert K Q, Text Res J, 59(4)(1989) Zhang H, Xue Y & Wang S, Res J Text Apparel, 9(3)(2005) Gazi Ortlek H & Ulku S, Text Res J, 77(2007) Su C I, Maa M C & Yang H Y, Text Res J, 74(2004a) Su C I, Yang & Ying H, Text Res J, 74(12)(2004b) Adeli B, Ghareaghaji A A & Shanbeh M, Text Res J, 81 (2)(2011) Miao M, How Y L & Ho S Y, Text Res J, 66(1996) Sawhney A P S, Ruppenicker G F, Kimmel L B & Robert K Q, Text Res J, 62(1992) Das A, Kothari V K & Balaji M, J Text Inst, 2007, 98(3)(2007) Ishtiaque S M, Das A & Chaudhari S, Indian J Fibre Text Res, 28 (2003) Ishtiaque S M, Das A & Chaudhari S, Indian J Fibre Text Res, 28 (2003) Box G E P & Behnken D W, Technometric, 2(4)(1960) Babaarshlan O, Text Res J, 71(4)(2001) Dang M, Zhang Z & Wang S, Fibres Polym, 7(4)(2006) Gupta B S & Mogahzy Y E E, Text Res J, 61(9)(1991) 547.