Introduction

Conductive textiles have attracted great attention recently due to their potential applications in flexible and stretchable electronics, wearable devices and electronic sensors (Wang et al. 2016a; Wang et al. 2017b; Weng et al. 2016; Yildiz et al. 2016; Zeng et al. 2014). Compared to traditional conductive materials including metals and semiconductors, textiles with electrical conductivity possess many special features, such as excellent flexibility, light weight, recoverable deformation and washability (Cai et al. 2017b; Pang et al. 2016; Pu et al. 2016; Xu et al. 2016; Zhang et al. 2016; Zhong et al. 2016). Cotton fibers are widely used in textiles because of their natural softness, good comfort, heat insulation, and high hygroscopicity (Cai et al. 2017a; Li et al. 2017b; Tang et al. 2012; Zahid et al. 2017). The abundant hydroxyl groups on the surface of cotton fibers could act as active sites to combine with functional materials for preparing conductive textiles. Conductive cotton textile have promising applications in wearable displays and smart clothing. Some researchers used metal coating fiber to prepare conductive yarns (Liu et al. 2010; Yu et al. 2014a, b; Zhao et al. 2016), exhibiting good conductivity and washing performance. However, the metal layer overlay on the surface may affect the flexibility of yarns. Meanwhile, the method involving metal coating was difficult for large-scale fabrication of yarns. Many strategies have been developed to fabricate flexible electrical textiles, such as surface coating of fibers with carbon nanotubes (CNTs) and graphene nanosheets, and in situ polymerization of polypyrrole and polyaniline on fabrics (Egami et al. 2011; Gao et al. 2015, 2016; Li et al. 2015; Liang et al. 2013; Ren et al. 2017; Wei et al. 2016; Ye et al. 2016; Zhu et al. 2014).

CNTs have been used as electrically conductive materials, owing to their high surface area, low electrical resistance, low mass density and high stability. By virtue of the high power density and electrical conductivity and long life cycle (Baughman et al. 2002; Choi et al. 2016; Lee et al. 2016; Pan et al. 2010; Thostenson et al. 2001; Xu et al. 2014; Yu et al. 2014a, b), CNTs were also investigated for applications in flexible electrodes. Pure CNT yarns have been prepared and characterized (Jiang et al. 2002;Tran et al. 2009; Yu et al. 2017). Although pure CNT yarns exhibited good electrical conductivity and high strength, their resistance to bending and wear resistance were weak, which limits their applications in textile processing. Whereas, the composite yarns from the combination of CNTs and fibers show great potential in practical applications due to their notable advantages. A number of research works were focused on conductive textile fibers incorporating CNTs (Guan et al. 2016; Liu et al. 2016b; Wang et al. 2017a; Yang et al. 2014). For example, CNT/cotton yarn and fabrics were obtained by a dip-and-dry method (Li et al. 2017a; Liu et al. 2016a, 2017; Makowski et al. 2014; Thangakameshwaran and Santhoskumar 2014; Wang et al. 2016b). However, the preparation of conductive fibers by this method did not result in high durability as the CNTs reside mainly on the surface of substrate materials. The CNTs could fall off easily after the conductive fibers/fabrics were reused many times. Therefore, it is significant to develop more effective assembly method to meet the usage requirements for conductive textiles. In conventional yarn manufacturing, ring spinning is widely used to increase the cohesion force between fibers through a twisting process (Deng et al. 2011; Lima et al. 2011; Lou 2005; Soltani and Johari 2012; Xia and Xu 2013). The inter-fiber lateral force introduced during twisting can help secure surface coatings on the fibers.

Herein, the ring spinning technology was used to fabricate CNT-cotton composite yarns (CNT-CY). The CNTs were wrapped around fibers by twisting, endowing the composite yarns with electrical conductivity. The cotton roving was treated first using CNT suspension at different concentrations through the dip-and-dry process. Ring spinning was then employed to convert the CNT-cotton roving into yarns. The surface morphology, chemical structures and electrical conductivity properties of the obtained CNT-CYs were analyzed. In addition, the abrasion resistance and wash fastness (durability) were assessed.

Experimental section

Materials

Pure cotton roving was used in this study. The liner density and the twist of cotton roving were 780 tex (g/km) and 30 tpm (turns per meter), respectively. The average fineness and upper half mean length of cotton fibers were 1.65 dtex and 28 mm. High purity single-wall CNTs were provided by Nanjing XFNANO Materials Tech Co., Ltd, China.

Instruments

Scanning electron microscopy (SEM) measurements were performed with a TESCAN MIRA3 field emission SEM. A FEI Tecnai G2 F30 high-resolution transmission electron microscope (HRTEM) was used to investigate the microstructure of CNT-CY yarns. The samples were embedded in resin and left to dry. The resin blocks with samples were cut into ultra-thin sections (~ 70 nm in thickness) using a diamond knife on a Leica EM UC6 ultra-microtome machine, and then collected with cooper grids for TEM measurement. Raman analysis was performed on a Renishaw inVia Raman microscope system (Renishaw plc, Wotton-under-Edge, UK). A 50/N.A. 0.75 objective and a 785-nm near-IR diode laser excitation source (500 mW, 10%) were used in all measurements. Raman spectra were recorded using a mounted CCD camera with integration time of 10 s by single scan. The electrical resistance of different yarns samples was measured by a digital multi-meter (Keysight Truevolt 34465A). The yarn samples were fixed by two insulators and the gauge length was adjustable (as shown in Fig. S1). The resistance of different lengths yarns were measured during stretching. Yarn mechanical properties were measured using an Instron Model 5566 Materials Testing System. Tensile strength of a single CNT-CY was measured at gauge length of 250 mm with the stretching speed of 250 mm/min.

Preparation of CNT-cotton roving

The CNT-cotton roving was prepared by a dip-and-dry method, which is illustrated in Scheme 1. A stable carbon nanotube suspension was obtained through dispersing the CNTs in water followed by 30 min sonication at room temperature. The cotton roving was soaked in CNT suspension with different concentrations (0.00625, 0.0125, 0.01875, 0.025 wt%) and kept in solution for 20 min at room temperature under sonication. The CNT-cotton roving was dried in an oven at 40 °C. The cotton roving turned black from white due to adsorption of CNTs. After drying, the CNT-cotton roving was wound on a bobbin, ready for ring spinning.

Scheme 1
scheme 1

Illustration of preparation process of CNT-cotton roving

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Preparation of CNT-cotton composite yarn

The CNT-CYs were produced from the CNT-cotton roving on a ring spinning machine, which is illustrated in Fig. 1. The CNT-cotton roving was drafted to a thin strand of fibers with the desired fineness by a set of rollers rotating at different speeds, and twist was then inserted into the thin fiber strand to form a yarn. Twist insertion and yarn winding are achieved by the rapid rotation of the yarn bobbin, mounted on a spindle, which drags the newly formed yarn around the bobbin via the traveler/ring assembly. The twisting creates a lateral force that keeps the CNT coated fibers together, hence greatly enhancing the binding force between CNTs and fibers. Most CNTs were inside the yarn and CNTs could not easily come off. Meanwhile, yarns with different liner density or count were obtained by varying the speed of drafting rollers and the twist of yarn was changed by varying the speed of the spindle. Therefore, different CNT-cotton yarns were obtained by changing the spinning parameters.

Fig. 1
figure 1

Spinning process of CNT-cotton composite yarn

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Durability test to washing and abrasion resistant

In the present research, water washing durability test of the obtained CNT-CY was carried out according to AATCC Test Method 61-2006. A laundering machine (Model SW-12AII, Wenzhou Darong Textile Instrument Co., Ltd., China) was used in the washing procedure. The CNT-CY (40 cm) was washed in a rotating closed canister containing 200 mL of detergent aqueous solution (0.37 wt%) and 10 stainless steel balls. The electrical conductivity of CNT-CY was evaluated after water washing. The wear-resistance properties of yarns were measured using an FFZ622 yarn abrasion tester. The electrical resistance of yarns was measured after abrasion. The electrical conductivity properties of CNT-CYs were measured after yarn abrasion for different times.

Results and discussion

Electrical conductivity properties of CNT-cotton roving

The pristine cotton roving behaved like a nonconductor with high electrical resistance (108 Ω/cm). Coating of CNTs imparted electrical conductivity to cotton roving. Figure 2a shows the resistance of CNT-cotton roving obtained at different concentrations of CNT (from 0.00625 to 0.025 wt%). The average electrical resistance of CNT-cotton roving was 3.35 kΩ/cm after cotton roving was treated in the 0.00625 wt% CNT solution. The electrical resistance of CNT-cotton roving decreased dramatically to 0.66 kΩ/cm as the concentration of CNT solution was increased to 0.025 wt% (Fig. 2a). The number of dip-dry cycles affected significantly the electrical resistance of CNT-cotton roving. The resistance of roving decreased as increasing number of dip-dry cycles (Fig. 2b). The resistance of roving decreased to 96 Ω/cm after 4 dip-dry cycles.

Fig. 2
figure 2

Electrical resistivity of CNT-cotton roving samples corresponding to different CNT concentrations (one soaking cycle) and b different soaking cycles (0.025 wt% CNT concentration)

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Characterization of CNT-cotton composite yarn

The ring spinning technology was employed to convert CNT-cotton roving into yarns. Compared with pristine cotton yarn (Fig. 3a), CNT-CY yarn showed a similar appearance (Fig. 3c) under the same spinning parameters. CNT-CY yarns were spun continuously on the ordinary ring spinning frame. Figure 3b and d show the optical microscopic images of pristine cotton yarn and CNT-CY yarn. No obvious difference was found in morphology and structure between the two types of yarns even though CNT had been coated on cotton fibers in the CNT-CY yarn. The results indicate that the CNT coating on fibers did not hamper cotton yarn spinning.

Fig. 3
figure 3

Optical images of a pristine cotton yarn and c CNT-cotton composite yarn. Microscopic images for b pristine cotton yarn and d CNT-cotton composite yarn

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Figure 4 displays the SEM images of pristine cotton yarn and CNT-CY yarn. The pristine cotton fibers were twisted with one another in cotton yarn (Fig. 4a, b). The surface of the pristine cotton fiber was smooth without observable impurities. Similarly, a twisted structure of fibers was seen in the CNT-CY (Fig. 4c, d). In addition, some nanoscale layers were observed on the surface of the CNT-cotton fibers (Fig. 4e), which implies that CNTs have been successfully assembled on cotton fibers. The magnified SEM images (Fig. 4f) show that the continuous coating layers were formed with nanotubes on the fiber surface. Also, cross-section SEM images of CNT-CY were obtained to observe the internal structures of the treated cotton yarns (Fig. S2). The cross-section morphologies of cotton fibers remained unchanged after CNTs were coated on the cotton fibers (Fig. S2a). No crosslinking structures were founded between fibers, suggesting that the treatment with CNTs would not influence the inherent features of cotton fibers. Some CNTs were seen on the edge of fiber in yarn (Fig. S2b and c). As can be seen from the enlarged SEM images (Fig. S2d), the nanotubes were attached on the fiber edge. The SEM characterization of cross-section of CNT-CY demonstrates that some CNTs were wrapped inside yarns through the ring spinning process. Besides, EDS patterns were recorded to analyze the main elements content of the cotton yarn and CNT-CY (Fig. S3 and Table S1). The contents of C and O elements in the cotton yarn and CNT-CY were 52.06 and 47.25 wt%, respectively. Compared with pristine cotton yarns, the C element content of CNT-Cotton yarn increased to 58.70 wt%, while O element content decreased to 41.30 wt%. The visible increase of C element in CNT-Cotton yarn further proved that a large number of CNTs were adhered to the surface of cotton fibers. Fig. S4 displays the cross-section TEM images of the ultra-microtomed CNT-CY sample at different magnifications. It is seen clearly that the thin layer coated on cotton fiber surface was comprised of CNTs. The microstructure of CNTs was observed distinctly in high-resolution TEM images of the coated layer of cotton fibers (Fig. S4d).

Fig. 4
figure 4

SEM images of (a, b) pristine cotton yarns and (c–f) CNT-cotton composite yarns at different magnifications

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Furthermore, Raman scattering spectroscopy was employed to analyze the surface property of yarns. No visible scattering bands were found in the Raman spectrum of the pristine cotton yarns (Fig. S5). The spectrum of the treated cotton yarns shows three new Raman bands at 2675, 1589 and 1343 cm−1 (Fig. S5), which are attributed to G’ band, G band and D band of CNTs, respectively (Cooper et al. 2001; Dresselhaus et al. 2005; Osswald et al. 2007). The Raman data proves that the CNTs were combined effectively with cotton yarns.

The electrical conductivity properties of CNT-cotton composite yarn

The relationship of the electrical resistances of CNT-CY and CNT-cotton roving is depicted in Fig. 5a. The electrical resistance of CNT-CY yarn was proportional to that of CNT-cotton roving. Therefore, CNT-CYs with different electrical resistances can be obtained by varying the electrical resistances of roving.

Fig. 5
figure 5

a A plot of electrical resistance of CNT-cotton composite yarn as a function of electrical resistance of CNT-cotton roving. The electrical resistance of CNT-cotton composite yarn b different twist (with 50 tex of line density), c different line density (with 591 T/m of twist). d A lighted LED wired to the CNT-cotton composite yarn at 5.0 V voltage supply

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In addition to spinning, twisting increases the cohesion force between fibers, which influences the electrical conductivity of yarns. Figure 5b shows the electrical resistances of different twisted CNT-CY. The resistances of composite yarns decreased with increase in twist, due to enhanced fiber to fiber contact within the CNT-CY. Besides, the value of resistance was related linearly to measuring length of CNT-cotton yarns, indicating that the distribution of CNTs on the yarn was quite uniform. Moreover, the linear density of yarn affected the electrical resistance of yarns. The electrical resistance yarns with different liner densities were measured (Fig. 5c). Coarser yarns showed lower resistance than thinner yarns, due to higher amounts of CNTs in the coarser yarns. A light emitting diode (LED) lighted up at a voltage of 5.0 V using the CNT-CY as a connecting wire, verifying the electrical conductivity of CNT-CY (Fig. 5d).

Mechanical properties of the CNT-cotton composite yarn

Figure 6a shows the tensile behavior of the pristine cotton yarn and the CNT-CY. The cotton yarn showed a tenacity of 0.125 N/tex with a breaking elongation of 8.5%. Compared with pristine cotton yarn, the CNT-CY presented higher tenacity (0.159 N/tex), but lower breaking elongation (7.5%). The CNTs on cotton fiber surface may increase the friction between fibers and decrease the fiber slippage, hence increasing yarn tenacity and reducing yarn elongation. The CNT-CYs with good mechanical properties were used for knitting and embedding into fabrics (Fig. 6b). The enhanced mechanical property of CNT-cotton yarns facilitates its applications in wearable devices.

Fig. 6
figure 6

a Curves of tenacity versus elongation of CNT-cotton composite yarn and cotton yarn. b Conductive fabrics knitted or embedded by CNT-cotton composite yarns

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Flexible properties of CNT-cotton composite yarn

The flexibility of a yarn is important for wearable applications. The 70 tex yarn was chosen for flexibility analysis as it showed the lowest resistance. Figure 7a displays the relative resistance change (∆R/R0 or R/R0) of yarns under stretching. R0, R and ∆R present the initial resistance before stretching, the resistance after stretching, and corresponding resistance change, respectively. The electrical resistance of yarns showed a slight decrease when the CNT-CY was stretched to different levels (2, 4, 6 and 7%). The electrical resistance of CNT-CYs remained 75% of the initial electrical resistance even though the elongation increased to 7%, which indicates the composite yarns possess stable conductivity under a certain stretching condition. Furthermore, relative resistance change of the CNT-CYs was recorded for large human motions. The CNT-CYs were attached directly on a knee. When the knee performed different motions, including standing, walking and squatting, the resistance of composite yarns was collected and analyzed (Fig. 7b). The electrical resistance of yarn was almost unchanged in standing motion. The resistance change fluctuation was less than 8% under walking and squatting. The results suggest that the real human motions have a relatively small influence on the electric conductivity of CNT-cotton yarns. Additionally, it can be seen in Fig. 7c that the resistance fluctuation is less than 3.5% under bending with the increasing bending angle and knotting. Even after being bent for 100 times with a bending angle of 180°, the resistance change was less than 10%. The excellent flexible properties of CNT-CY yarns may be attributed to the combined effect of the flexibility of cotton yarns and the strong bonding force between CNTs and cotton fibers.

Fig. 7
figure 7

a Relative electrical resistance change of CNT-cotton composite yarn in the stretching and releasing. b Electrical resistance changes of CNT-cotton composite yarn in human activities. c Electrical resistance of CNT-cotton composite yarn under different deformation

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Durability of CNT-cotton composite yarn

Durability of the flexible conductive yarns is very important if they are used in functional textile clothes. The wash fastness and wear-resistance of the yarns as-prepared by our method (CNT-CY) were compared with that of yarns combined with CNTs by direct immersion (Cotton yarn-CNT). The change in electrical resistance after repeated washing of CNT-CY was small (Fig. 8a). The R/R0 of the CNT-CY was less than 1.3 after 8 washing cycles. Nevertheless, the R/R0 of Cotton yarn-CNT was around 8.3, which indicates that the CNT-CY had remarkable fastness or durability to washing. The wear-resistance of different yarns was also compared (Fig. 8b). No distinct changes for the electrical resistance of CNT-CY were observed after 20 cycles of abrasion. The R/R0 of CNT-CY was no more than 1.5 even after 100 cycles of abrasion, and around 4 after being rubbed for 400 times. While, the electrical resistance of Cotton yarn-CNT yarn changed visibly after 20 cycles of abrasion (R/R0 = 3), the R/R0 of Cotton yarn-CNT yarn reached 5 after 100 cycles of abrasion and increased to 55 after 400 cycles of abrasion. The durability of CNT-CY was much better than that of Cotton yarn-CNT yarn. The CNTs were only on the surface of the yarn in Cotton yarn-CNT yarn and then fell off easily during the abrasion process. Different from the case of Cotton yarn-CNT, the CNTs in the CNT-CY were entrapped by fibers through the twist-based spinning, which greatly improves the durability of CNT-CY.

Fig. 8
figure 8

Electrical resistance changes of CNT-CY and Cotton yarn-CNT after repeated a washing and b abrasion

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Conclusion

Conductive carbon nanotube-cotton composite yarns (CNT-CYs) have been successfully spun by a ring spinning method. The CNT-CYs exhibited good electrical conductivity, which increased with the increase in twist level and linear density of the resultant yarns. Raman spectroscopy, SEM and TEM characterizations reveal that the CNTs were assembled on the surface of cotton fibers. It was demonstrated that the CNT-CYs possessed great tensile and flexible properties. The treatment with CNTs improved the tenacity of cotton yarns. The electrical resistance of CNT-CYs changed little after repeated bending (100 times). The CNT-CYs exhibited notable durability to washing and abrasion. The electrical resistance of CNT-CYs changed only slightly under stretching and other human motions. This work provides a novel approach for manufacturing conductive, durable, and flexible yarns for potential applications in wearable electronics and smart clothing.