Abstract
To overcome the poor wear resistance and weaving difficulties of carbon nanotube (CNT) yarn, a new type of functional yarn was prepared by a wrapping-melting method. In this method, CNT yarn was wrapped by polypropylene (PP) yarn first, and then a 15 V voltage was applied on both ends of the yarn. The electrically heated CNT yarn melts the wrapped PP yarn and forms a uniformly compacted PP wrapped CNT (PWC) yarn. The study showed that the PWC-15V yarn can retain the good electrical properties of CNT yarn with an electrical conductivity of 4643.7 S/m. Besides, it can work as a multifunctional yarn with strain sensing and electrical heating. As a strain sensor, it has a highest gauge factor of 1.06 and good stability with 100 cyclic loading. As an electrical heater, the PWC yarn can be heated to about 50°C with over 9 V applied voltage. Moreover, the washable PWC yarn also demonstrates good flexibility and improved wear resistance, which indicates that it has great potential for wearable electronics.
Introduction
As a multifunctional fiber material with high strength, high toughness and high electrical conductivity, carbon nanotube (CNT) yarn is an ideal candidate as an embedded fiber for intelligent textiles.1–3 CNT yarn is a macroscopic aggregate of CNT. 4 The tensile strength of the untwisted yarn is up to 300 MPa, and the tensile strength of the yarn increases obviously after twisting or wrapping. 5 However, the diameter of the common CNT yarn made from dry spinning is only about 10 μm. Such a tiny diameter causes the breaking force of the CNT yarn to be less than 2 N. 6 Meanwhile, it has poor wear resistance, which makes the subsequent weaving process difficult to carry out for utilization in smart textiles. 7 Therefore, it is necessary to improve the wear resistance of CNT yarn for the weaving process.
In these decades, the strategy of combining CNT yarn with other commercial fibers has been adopted, which has been demonstrated as an effective way to enhance the comprehensive properties. For example, Chen et al. 8 wrapped two layers of the CNT yarn tightly on the surface of the elastic yarn in the opposite wrapping direction with angles of 80° and 100°. The wrapped CNT yarn showed only 1.65% resistance change under a tensile strain of 45%. Foroughi et al. 9 combined spandex with multiple CNT yarn using a parallel twisting process, further feeding them onto a circular knitting machine. The knitted fabric has high elasticity with a breaking strain of up to 900% and a tensile strength close to 86 MPa. In addition, some scholars coated various polymers such as poly vinyl alcohol 2 and polydimethylsiloxane10,11 on the surface of CNT yarn. The coating layer acts like sizing, significantly increase the mechanically properties of the CNT yarn. However, this structure also has a negative influence on the softness of the yarn.
This study proposes and fabricates a novel polypropylene-wrapped CNT (PWC) yarn with a partial coating. First, PP filaments are helically wrapped around a CNT core yarn. The wrapped yarn is then electrified to heat the PP filaments via the electrothermal heating effect of the CNT core. The PWC yarn is synthesized under optimized voltage conditions. At elevated voltages, the PP partially melts on the CNT yarn surface, forming a dense textured structure. The morphological structure, mechanical properties, and abrasion resistance of the PWC yarn are systematically characterized and analyzed. Furthermore, the yarn is integrated into textiles to monitor its sensing performance. Compared to pristine CNT yarns, the PWC yarn exhibits superior abrasion resistance and flexibility, demonstrating significant potential for applications in wearable electronic textiles.
This study proposed and fabricated a novel polypropylene-wrapped CNT (PWC) yarn with a partial coating. First, PP filaments were helically wrapped around a CNT core yarn. The wrapped yarn was then electrified to heat the PP filaments via the electrothermal heating effect of the CNT core. The PWC yarn was synthesized under optimized voltage conditions. At elevated voltages, the PP partially melted on the CNT yarn surface, forming a dense textured structure. The morphological structure, mechanical properties, and abrasion resistance of the PWC yarn were systematically characterized and analyzed. Furthermore, the yarn was integrated into textiles to monitor its sensing performance. Compared to pristine CNT yarns, the PWC yarn exhibited superior abrasion resistance and flexibility, demonstrating significant potential for applications in wearable electronic textiles.
Materials and Methods
The CNT yarn was provided by Suzhou Nanotechnology Research Institute, the model is SCNCF. PP filament with a finesse of 700 D was bought from Yancheng Huan-Yu engineering Fiber Co., Ltd.
Preparation of Polypropylene Wrapped CNT (PWC) Yarn
The process for producing the yarn is shown in Figure 1. At first, PP filament was wrapped on CNT yarn using a home-made wrapping twisting device. Then the wrapped yarn was subjected to voltages at the two ends of the core CNT yarn. The electric-heated CNT yarn partially melt the wrapped PP filament by controling the applied voltage. The preparation process is divided into two steps. The first step involves preparing a wrapping structure consisting of a core of CNT with a covering of PP fibers. The second step involves connecting the two ends of the CNT to the power supply using copper wire. According to the principle of electrothermal heating, the CNT becomes the heat source when the voltage is applied. The PP fibers near the heat source begin to melt when they are exposed to the temperature higher than its melting point. Due to heat dissipation, the temperature decreases gradually outward. Because the temperature does not exceed the melting point, the outermost layer of PP fibers retains its fiber state. The PWC yarn is completed when the melted PP becames a coating after the power is cut off.
A schematic representation of the preparation of the CNT wrapped yarn.
Characterization and Measurement
The surface morphology and cross-section structure of PWC yarn were observed using an optical microscope (model GP640S from Kunshan Gaopin Precision Instrument Co., Ltd). The breaking tension of the PWC yarn was tested using a single fiber tensile tester (Xusai Instrument Co., Ltd, model: XS (08) XG) equipped with a strain gauge, at a rate of 1 mm/min. Electromechanical properties of the PWC yarn were obtained by in situ testing of electrical resistance using a device from Agilent Technology Co., Ltd (model: 34450A) while subjecting the yarn to strain. An infrared camera (Thermal Imaging Technology Co., Ltd, model: S/N225) was used to observe the temperature changes of the PWC yarn under different conditions, such as knotting and bending, when energized. The wear resistance of the yarn was evaluated by rubbing it against a rotating grinding wheel. By controlling the size of the voltage, the rotation speed of the grinding wheel is stabilized at 80 r/min. The PWC yarn was attached at one end and across the grinding wheel and a 200 g weight was hung to keep the yarn straight at all times. The time of grinding wheel rotations was counted until the yarn broke. To assess washability, the yarn was washed with a soap concentration of 4 g/L at 40°C for five cycles. After each washing cycle, the resistance of the yarn was tested.
Results and Discussion
Analysis of Surface Morphology
Figure 2 shows the surface morphology of CNT yarn, PP filaments and PWC yarn with different applied voltage. Both CNT yarn (Figure 2(a)) and PP filaments (Figure 2(b)) show uniform structure with diameter of around 130 µm (Figure 2(d)) and 280 µm (Figure 2(e)) separately. By wrapping PP filaments around CNT yarn, PWC yarn has an increased diameter of 338 µm (Figure 2(c) and (f)). The surface of CNT yarn was completely covered by uniform wrapped PP filaments. After applying different voltages on the inner CNT yarn, the PWC yarn was heated by Joule heating, which makes the PP filaments melt to a certain degree. For PWC-10V yarn, the PP filaments that contact with CNT yarn experienced slight shrinkage, which broke the stable and uniform structure of the PWC yarn and resulted in a hairy surface morphology. With increasing the voltage to 15 V, the PP filaments that contact with CNT yarn started to melt, which gave the PWC-15V yarn a composed structure with melted PP connecting to CNT yarn and fibrous PP wrapping on the surface. When 20 V voltage was applied on PWC yarn, all of the PP filaments melted and this led to a rough and hard coating surface. Since PWC-15V has a compact structure and soft handle as well, it is a perfect candidate for wearable electronics.
Microscopic image of CNT yarn, PP filament, and PWC yarn with different applied voltages. Microscopic image of: (a) CNT yarn, (b) PP filament, (c) PWC yarn in lower magnification, (d) CNT yarn, (e) PP filament, (f) PWC yarn, (g) PWC-10V, (h) PWC-15V, and (i) PWC-20V in higher magnification.
Electrical Conductivity and Strain Sensitivity
The current–voltage curves for CNT yarn and PWC-15V yarn are shown in Figure 3(a). The resistance of both CNT yarn and the PWC-15V yarn is increased linearly. The resistance of PWC-15V yarn is greater than that of the CNT yarn. This is attributed to the presence of the non-conductive PP filament, which raises the overall resistance of the PWC-15V yarn. Samples ranging from 1 cm to 5 cm were processed, and the resistance value was averaged for each sample. The relationship between the resistance and length of CNT yarn and PWC-15V yarn is depicted in Figure 3(b). The resistance of CNT yarn shows a slight increase after wrapping, indicating that the wrapping process has little effect on the resistance of the CNT yarn. The relationship between resistance change rate and strain for CNT yarn and PWC-15V yarn is illustrated in Figure 3(c). It shows that the PWC yarn can retain the good electrical properties of CNT yarn with an electrical conductivity of 4643.7 S/m. The resistance of CNT yarn increases linearly, yielding a gage factor (GF) of 1.05. In the case of PWC-15V yarn, the resistance change is smaller when the strain is less than 4%. Beyond 4% strain, relative sliding between CNT bundles occurs, leading to weakening of the force between CNT. The resistance change pattern resembles that of CNT yarn. The GF of PWC-15V is calculated to be 1.06 within a strain range of 4–18%, which is comparable to that of CNT yarn, indicating that the wrapping process does not significantly affect the sensing performance of the yarn. Figure 3(d) shows the rate of resistance change of PWC-15V yarn drawn during 5%, 10%, and 15% strain cycles. The speed of the strain sensor was controlled at 100 mm/min. The corresponding ΔR/R values were 2.1%, 7.6%, and14.2%, respectively, as the strain increased, which shows an approximately proportional relationship with the strain, also ensuring the recognition of the PWC-15V yarn sensor for different strains.
The electrical performance of CNT and PWC-15V functional yarn. (a) The current–voltage curves for CNT yarn and PWC-15V yarn.(b) Resistance of the dyed CNT yarn and PWC-15V yarn. (c) Electrical resistance of the CNT yarn and PWC-15V yarn with different strains.(d) The resistance variation rate of PWC-15V yarn is stretched under 5%, 10%, and 15% strain.
Electrothermal Performance
Due to the exceptional electrical conductivity of carbon nanotubes, researchers have successfully developed highly conductive yarns using various methods. The conductivity of PWC-15V functional yarn is derived from CNT yarn.12–15 Figure 4(a) illustrates the voltage–temperature relationship between PWC-15V functional yarn and CNT yarn. As the voltage increases, the temperature of both PWC-15V functional yarn and CNT yarn also increases, indicating that both yarns possess certain electrothermal properties. The temperature rise curve of PWC-15V functional yarn is linear and stable with increasing voltage. At the same voltage, the CNT yarn exhibits a higher temperature due to the presence of a PP filament outside the PWC-15V yarn, which has poor thermal conductivity, resulting in a lower detected temperature than that of the CNT yarn. Infrared tests of PWC-15V functional yarn at different voltages demonstrate that the PP-wrapped PWC-15V yarn does not affect its own electrical conductivity, and the temperature of the PWC-15V yarn remains uniform. Figure 4(b) depicts the temperature response of CNT yarn and PWC-15V yarn. Initially, when not heated within 0–10 s, the temperature remains at approximately room temperature (about 25°C). Upon turning the power on, the yarn heats up rapidly, with CNT yarn reaching approximately 38°C in about 30 s, and PWC-15V yarn reaching approximately 30°C. The heating rate of the PWC-15V functional yarn is nearly identical to that of the CNT yarn. As power continues, the temperature gradually approaches equilibrium, fluctuating around the highest value. Upon voltage release, the yarn quickly returns to room temperature within a few seconds. These results indicate that wrapping does not affect the thermal response of PWC-15V yarn, which exhibits excellent thermal conductivity, enabling rapid heating and cooling. The equilibrium surface temperature of the PWC-15V functional yarn is lower than that of the carbon nanotube yarn due to the inherent good thermal conductivity of the PP, which dissipates the temperature of the carbon nanotube yarn’s core layer to the external environment.
Analysis of yarn thermal performance test results. (a) Electrothermal temperature as a function of applied voltage. (b) Electrothermal temperature curves of PWC-15V yarn as a function of time. (c) Electrothermal temperature curves of PWC-15V yarn with the switched voltage from 1 to 9 V.
To assess the electrothermal cycling stability of PWC-15V yarn, the temperature variation of PWC-15V functional yarn was investigated at voltages ranging from 1 to 9 V. Figure 4(c) presents the temperature variation of the PWC-15V yarn during cyclic changes in voltage. After 100 cycles, the maximum temperature stabilizes at approximately 49.5 ± 0.5°C, and the minimum temperature stabilizes at approximately 26 ± 0.5°C. This cyclic voltage–temperature relationship indicates that PWC-15V functional yarn exhibits good electrical and thermal stability.
Mechanical Properties and Washability
Figure 5(a) shows the stress–strain curves of the three yarn. When the CNT yarn was wrapped by the PP yarn as the core yarn, the strength increased two-fold and the flexibility of the yarn was not affected, and it could conduct electricity normally in the bent and knotted state (Figure 5(b)). The carbon nanotube yarn, due to its nanoscale morphology structure, shows a transient fracture during friction, and the structural design of PWC-15V functional yarn effectively protects the carbon nanotube yarn.
Analysis of the results of yarn mechanical and washing performance. (a) Stress–strain curves of CNT and PWC-15V yarn. (b) Diagram of PWC-15V yarn tied and bent. (c) The relationship between the wear resistance time of PWC yarn and the preparation voltage. (d) The relationship between current and voltage after washing.
To evaluate the wear resistance of the yarn, PWC yarns prepared with different voltage were put on a rotating grinding wheel. Figure 5(c) shows the wear resistance time when the yarn broke. For PWC yarn prepared without applied voltage, the yarn broke after rotating the wheel for 190 s. With increased applying voltage on PWC yarn, the PWC-5V and PWC-10V and PWC-15V yarn showed increased wear resistance. Among them, the abrasion time for PWC-15V yarn is 314 s, which improved by 60.51% compared with the abrasion time of PWC yarn. During yarn abrasion, the fracture of yarn is caused by the friction force between the yarn and the grinding wheel. For PWC yarn, the outer layer of PP filament was worn off first, followed by the inner layer of PP filament wearing off. However, for PWC-5V yarn and PWC-10V yarn, the inner layer of PP filament has partially melted, which enhanced the wear resistance of the yarn. For PWC-15V, more of the PP filament melted with increased inner temperature, caused by increased applying voltage. This makes a much more compact and structure between PP filament and CNT yarn and results in an optimum abrasion time.
For functional yarn, poor washability is a persistent issue in the manufacturing process. After experimental simulation of the washing process, the currents at different voltages were measured. As shown in Figure 5(d), the slope of the curve from the first to the fifth wash did not change and almost exactly overlapped. After washing PWC-15V functional yarn 30 times, the resistance showed only a slight increase from 120.72 to 129.04 Ω. The results showed that the increase of washing times slightly increased the resistance of PWC-15V functional yarn, but it had almost no effect on the function of PWC-15V yarn.
Applications of PWC Yarn in Textiles
To explore the strain sensing property of the functional yarn, the PWC-15V yarn was embedded in a knitted fabric. The relationship between strain and resistance change rate of the fabric is demonstrated in Figure 6(a). The resistance change rate shows no significant change when the strain is less than 7%. When the yarn is embedded in the knitted structure, it is in a bent state. When the strain is less than 7%, the yarn is stretched from bending to straight, resulting in a small amount of strain. As the strain exceeds 7%, the PWC-15V functional yarn is fully stretched, and the resistance changes with the stretching of the CNT yarn, confirming the sensing properties of the PWC-15V fabric.
Sensing performance test results of PWC-15V functional yarn. (a) Relationship between resistance change rate and strain of the sensing fabric and (b) relationship curve between resistance change rate and time for a human joint, demonstrating the sensing properties of the fabric.
To evaluate the sensing resolution and dynamic response, the textile sensor was integrated into the metacarpophalangeal joint for real-time monitoring of finger flexion (Figure 6(b)). Quantitative analysis of the normalized resistance change (ΔR/R) revealed a strong positive correlation between sensor response and joint flexion angle, achieving a maximum ΔR/R value of 2.5 at 90° flexion. Notably, the sensor exhibited rapid signal recovery upon returning to the neutral position, demonstrating minimal hysteresis and excellent repeatability. This angular resolution capability, combined with low signal drift, confirms the system’s precision in detecting complex biomechanical motions. This indicates that the sensor can accurately detect finger bending with very low hysteresis. The functional yarn can be used to monitor of human behavior.
Conclusion
In this study, PWC yarn was synthesized by encapsulating a carbon nanotube (CNT) core yarn with a polypropylene (PP) sheath through an electrothermal coating process. By applying controlled electrical heating, the PP sheath achieves uniform adhesion to the CNT core, effectively mitigating interfacial slippage between the two components. This approach results in a densely structured PWC yarn with enhanced dimensional homogeneity and surface integrity, which collectively improve its abrasion resistance and textile processability.
The experimental results confirm that the PWC functional yarn exhibits robust strain-sensing capabilities. With an electrical conductivity of 4643.7 S/m, the yarn achieves a temperature of approximately 50°C under a 9 V applied voltage through Joule heating. Furthermore, it demonstrates a high gauge factor and exceptional cyclic stability over 100 loading–unloading cycles. Remarkably, the yarn maintains consistent conductivity even when bent or knotted, and its resistance remains stable after repeated washing—a critical feature for wearable applications. The synergistic integration of polypropylene, which contributes mechanical durability, and carbon nanotubes, which enable superior electrical performance, positions this composite yarn as a promising candidate for advanced applications in smart textile sensing and next-generation wearable electronics.
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
