Enhanced Performance and Structural Dynamics of CNT/PU Wrapped Yarns

Abstract

Carbon nanotube (CNT) yarn-based wearable technologies have garnered interest, due to their excellent electrical conductivity, high aspect ratio and excellent flexibility. However, their low abrasion resistance and restricted elongation are always problems that significantly limit their industrial applicability. This paper describes a wrapping technique by winding the CNT yarn onto PU filaments, adjusting the yarn’s winding turns at 400, 1600, and 12,000 t/m per minute. By using a twist rate of 900 t/m and winding turns at 12,000 t/m, a conductive core–shell structure with the CNT yarn as skin yarn and the PU yarn as core yarn was achieved. The CNT/PU yarn exhibited consistent elctrical resistance over a strain range of 30%. Furthermore, with a low trigger voltage of 3.5 V, the surface temperature of the wrapped yarn could reach 84.8°C, showing stable electrothermal performance and a slight temperature fluctuation in the stretching or bending states. The CNT/PU yarns have a wide range of applications in the field of human motion monitoring and electrical heating.

Introduction

Smart textiles, due to the convergence of contemporary technology and the textile sector, have garnered significant attention. Different from conventional textiles, these innovative fabrics integrate sophisticated electrical components and smart technologies.^1–3 Notably, in addition to their inherent comfort and convenience^4–8 the smart textiles equipped with strain-sensing capabilities exhibit considerable potential across diverse domains, including medicine, fashion, sports, and beyond, carrying substantial social and economic implications. Presently, fundamental materials such as carbon nanotubes (CNTs), graphene, carbon black (CB), conductive polymers, metallic nanoparticles, and nanowires are important in the development of flexible strain-sensing textiles. ⁹ Because of their restricted stretchability and sensing range, conventional strain sensors, mainly semiconductors and metals, are limited in stretchability and sensing range and do not meet the requirements of wearable strain sensors. So researchers have directed their efforts toward the development of wearable, flexible strain sensors characterized by elastic and pliable properties. ¹⁰

Carbon nanotube fibers, composed of carbon nanotubes, ¹¹ hold immense application potential in smart textiles due to their high aspect ratio, low density, exceptional flexibility, mechanical prowess, electrical conductivity, and chemical stability.^12–14 In spite of their high strength, carbon nanotube fibers typically require surface coating or entanglement composites with other fibers to enhance their mechanical and weavable properties due to their small diameter, weak abrasion resistance, limited fiber strength, and weak weavable qualities.^15–19 Lin et al. ²⁰ achieved success in crafting spandex-based strain sensors utilizing a composite of conductive materials, carbon black and functionalized carbon nanotubes. Adjusting the conductive network structure and interfacial interactions achieved substantial alterations in the tunneling distance and the number of conductive pathways. The sensor had an impressive strain sensing range of up to 200%, with adjustable strain coefficients ranging from 5 to 1,40,238. Meanwhile, Wang et al. ²¹ successfully embedded highly conductive single-walled carbon nanotubes into an elastic cotton/polyurethane core yarn by a self-designed coating method in order to continue to improve the strain sensing performance. Due to the wrapping structure of the cotton/spandex yarn and the reinforcing effect of the single-walled carbon nanotubes, the composite yarn was able to withstand up to 300% strain, and could be cycled nearly 300,000 times at 40% strain without any significant breakage. Gao et al. ²² achieved the preparation of a highly elastic conductive fiber, based on carbon nanotubes and polyurethane with a skin–core structure, showcasing a sensing range exceeding 350% strain and a coefficient of 166.7 at 350% strain, facilitating the detection of subtle strains.

Within the realm of carbon nanotube-wrapped yarns, the winding twist of the outer yarn to the core yarn significantly influences the yarn’s appearance and properties, but that has received insufficient attention in existing research. As shown in Figure 1, this study uses spandex yarns as core yarns and CNT as skin yarns in the creation of entangled yarns. It validates the advantages of the entangled yarns by comparative analysis with unentangled CNT yarns. Furthermore, we meticulously investigate the impact of “winding twist” and “yarn twist” on the mechanical and electrical properties of wrapped yarns. We selected the wrapped yarns with better overall performance and further investigated to see if they could stably maintain their electrothermal properties under external forces to expand their potential in electrothermal applications.

Figure 1.

Schematic illustration of the manufacture procedure of CNT/PU yarn.

Experiment

Materials and Equipment

The materials used in this study include copper wire with a purity exceeding 99.9% (Biling), PU yarn with a diameter of 140D (Zhuji City Shenfeng Chemical Fiber Factory), CNT yarn sourced from Suzhou Jiedi Nano Technology Co. Ltd, conductive silver paint from Elcometer, and UHU transparent strong adhesive obtained from Pfizer (Guangzhou) Stationery Co. These selections represent a comprehensive range of components employed in the experimental procedures.

Preparation Method

Creation of Carbon Nanotube Strands with Varying Degrees of Twist

Carbon nanotube yarns with different twists were prepared by a homemade yarn twisting device in the laboratory. One end of the carbon nanotube yarn was fixed to the motor chuck, and the other end was pressed with weights and connected to an external power supply with a voltage of 1.5 V and a current of 0.75 A. The motors provided different twists at different speeds, and carbon nanotube yarns with twists of 300, 600, 900, and 1200 t/m were prepared respectively. Since the fineness of the carbon nanotube yarns was too small, the direct use of the e-textile strength machine for the hold-up test might lead to slippage, so it was necessary to use a sample card for sample preparation and then conduct subsequent tests.

The following are the steps in the sample creation process: cut a 60 × 20 mm sample card with a 10 × 10 mm square hole in it. The yarn is placed on the middle axis of the sample card in order to keep it straight. Attach the yarn to the copper wire by dripping silver glue. It takes the conductive silver paint at least 4 h to set. To make the connection between the yarn and the sample card stronger, apply another layer of universal glue once the silver glue has dried. Add spacers on the square holes every 5 mm then fold the ends of the sample card in half. Before testing, allow the universal adhesive to fully set for over 12 h at room temperature. Since the conductivity of the copper wire and silver glue is far higher than that of the carbon nanotube yarn, the effect of their resistance can be ignored.

Creation of CNT/PU-Wrapped Yarns with Varied Wrapping Twists

CNT/PU-wrapped yarns were fabricated using a tunable entangled yarn preparation device, using seven PU yarns with various twisting rates (300 t/m). The device facilitated entanglement degrees of 400, 1600, and 12,000 t/m, resulting in the formation of entangled CNT/PU yarns. This method significantly improved breaking strength and elongation.

Skin yarns were CNT yarns (twist rate: 1200 t/m), while wrapping yarns were twisted PU yarns. The procedure involved cutting a suitable length of PU yarn, securing both ends to a motor chuck, and using an external power source (1.5 V, 0.75 A) to control motor speed (0.343 rpm). The precise methodology ensured the controlled preparation of CNT/PU-wrapped yarns with distinct wrapping twists.

Production of Wrapped Yarns Using CNT Yarns with Varied Twist Rates

On the basis of the previous experiment, we investigated the critical twist factor, namely the state in which the skin and core yarn spirals are in contact with each other without squeezing or separating. Next, we further investigated the entangled yarns prepared from CNT yarns with different twists under the condition of the critical twist factor. Specifically, we used 300, 600, 900, and 1200 t/m CNT yarns to prepare entangled yarns with an entanglement twist of 12,000 t/m.

Testing and Characterization

Yarn Diameter Calculation

We employed a high-definition CCD measuring microscope (GP-300C) from Kunshan High Quality Precision Instrument Co., Ltd to measure the width and diameter of the CNT yarn and to observe its surface morphology. We also used an OD micrometer (0–25 mm) from Shanghai Hengqiang Co., Ltd to measure the thickness of the CNT.

Mechanical Performance Testing

The mechanical characteristics of CNT yarns were tested using an electronic fabric strength machine (XS (08F2) series; Shanghai Xusai Instrument Co., Ltd.). The machine’s settings were determined by the accompanying computer software. To ensure test accuracy and repeatability, the sample card was cut along the central dotted line before the test. The parameters for CNT yarn were configured with a 10 mm spacing and a stretching speed of 1 mm/min, while those for wrapped yarn were set at a 10 mm spacing and a stretching speed of 20 mm/min.

Electrical Performance Testing

We used a digital multimeter, the KETSIGHT Model 34461A (Agilent Technologies, Inc.), to measure the electrical characteristics of CNT yarns. The probes of the multimeter were attached to each of the samples’ two copper wires and we noted the resistance of the CNT yarns as it changed during stretching.

Thermal and Electrical Performance Evaluation

Both ends of the sample were clamped on a motor chuck and heat was generated in the sample by setting a fixed voltage using two wires with cleats, one end connected to the copper wire of the sample and the other end connected to the positive and negative terminals of an external power supply. The temperature was also monitored in real time with the help of an infrared camera model fotric225 and AnalysIR software. The test procedure consisted of waiting for 10 s to obtain a data plot, then starting the external power supply, energizing it for 3 min and finally switching it off to allow the sample to cool.

The analysis was divided into three parts: an electrothermal test at constant voltages (1, 1.5, 2, 2.5, 3, and 3.5 V) that recorded temperature changes after the samples were energized; an electrothermal test at fixed voltages (3 V) that involved stretching the wrapped yarn at 10%, 20%, and 30% and recording temperature changes along with a comparison graph; and an electrothermal test at fixed voltages (3 V) that involved bending the wrapped yarn at angles (45°, 90°, and 180°) that recorded temperature changes and documented the average resistance after stabilization under different bending states, as shown on a graph.

Results and Discussion

CNT Yarns with Different Twist Levels

Surface Morphology

Use measurement microscopy to track changes in the surface morphology of CNT yarns during the twisting process. Figure 2(a–e) and (g) demonstrates the uneven, flat black films that the untwisted CNT yarns present. As the twist increased, the diameter of the CNT yarn steadily dropped. This was explained by the fact that the twist increased the holding force between the fibers and lowered the inter-fiber gap. Simultaneously, the yarn’s surface morphology transformed from a flat ribbon to a cylindrical shape, eventually becoming homogenized. The surface of the CNT yarn seemed smooth and somewhat metallic when seen under a microscope.

Figure 2.

Macro-morphology diagrams of CNT yarns with different twist levels: (a) 0 t/m, (b) 300 t/m, (c) 600 t/m, (d) 900 t/m, (e) 1200 t/m. (f) Stress–strain diagrams, and (g) conductivity/mean diameter diagrams.

Mechanical Properties

Figure 2(f) shows that the stress and strain of the yarn increase significantly with an increase in twist. At a twist rate of 1200 t/m, the stress energy of CNT yarn reaches 485.2 MPa, which is a 198% increase compared with that of untwisted CNT yarn, while the strain energy under this condition reaches 17.67%, which is a 392% increase. The reason is that twisted CNT yarn is more compact, which reduces fracture non-simultaneity and improves the tensile strength of yarn.

Electrical Properties

From Figure 2(g), it can be seen that the electrical conductivity of CNT yarn increases gradually with the increase in twist. When the twisting degree is 1200 t/m, the conductivity is 1617.01, which is 161.27% higher than that of the untwisted CNT yarn. This is because after twisting, the carbon nanotube fibers inside the yarn are in closer contact with each other, and more conductive pathways are formed, resulting in an increase in conductivity. In summary, 1200 t/m is selected as the twist rate for the next wrapped CNT yarn.

CNT/PU-Wrapped Yarns with Different Wrapping Twists

Surface Morphology

From Figure 3(a–c), it can be clearly seen that the skin yarns are uniformly wrapped around the core yarns. The wrapping angles are 4°, 61°, and 84° when the wrapping twists are 400, 1600, and 12,000 t/m, respectively. By increasing the wrapping twist, the wrapping angle becomes larger, the length of CNT yarns wrapped around the core yarn gradually increases, and its wrapping structure gradually becomes a spring-helical structure. In order to further analyze the surface morphology of the entangled yarns, they were stretched at 100%, 200%, and 400%, respectively. As shown in Figure 3(a1–c3), the pitch of the leather yarn increases with the increase of the tensile strain, and the neighboring spirals are separated from each other.

Figure 3.

Surface morphology of CNT/PU-wrapped yarns: (a) 400 t/m, (b) 1600 t/m, and (c) 12,000 t/m. Plots of different tensile states: (a1–c1) 100% tensile, (a2–c2) 200% tensile, and (a3–c3) 400% tensile. (d) Stress–strain diagram of CNT/PU-wrapped yarn, (e) resistance change rate of CNT yarn, and (f) resistance change rate of CNT/PU-wrapped yarn.

Mechanical Properties

The mechanical properties of CNT/PU entangled yarns and seven pure PU yarns were tested at 400, 1660, and 12,000 t/m. As shown in Figure 3(d), the breaking strength of the CNT/PU wrapped yarns was significantly higher than that of the pure PU yarns, and the breaking strength of the wrapped yarns gradually decreased with the increase in the wrapping twist. This is because in the stretching process, only the core yarn spandex provides the strength at first. However, with the gradual increase in stretch, the skin yarn will also be stretched, and then the skin yarn and the core yarn will provide strength together. In the case of the 12,000 t/m wrapped yarn, it is the core yarn that breaks first, whereas for the 400 t/m wrapped yarn and the 1600 t/m wrapped yarn, the skin CNT yarn breaks at 25.01% and 359.43% of strain, respectively, and subsequently, with the increase in strain, the strength is provided only by the spandex core yarn. This is due to the fact that the length of the CNT yarns that are wrapped around the spandex core yarns increases with increasing wrapping twist. Consequently, skin yarns in wrapped yarns with smaller wrapping twists are the first to straighten out and interact with the core yarns, resulting in a greater degree of strength. Additionally, skin yarns break under the same strain conditions, resulting in a greater degree of strength.

Electrical Properties

The twist of CNT yarn with the biggest stress–strain after twisting is 1200 t/m, as shown in Figure 3(e). This is because, despite the CNT yarn being twisted repeatedly between the fibers and the internal structure of the yarn becoming compact, the twist angle and the direction of the carbon nanotube bundles are different from the axial direction of the yarn due to the large twist rate of the twisted CNT yarn at 1200 t/m, making a small resistance change under this twist rate. This increases the contact between the carbon nanotube bundles in the yarn and forms more conductive pathways. However, at 1200 t/m, the axial direction of the yarn and the twistback angle and direction of the carbon nanotube bundles alter, causing a small change in the resistance of the CNT yarn. It is in a specific strain range, but compared to other groups, it has a higher strain and a larger resistance change rate. The resistance change rate is the smallest because the internal fibers will be twisted and embraced by the twisting at 600 t/m, slightly improving both strength and electrical conductivity. However, because of the small twist, the yarn will remain partially flat, partially cylindrical, and non-uniform. Due to this, a slight strain can easily become loose and result in a diminished conductive network as well as a considerable change in resistance within a small strain when pulled by an external force.

Due to the excellent mechanical strength of CNTs, wrapping them around spandex yarns can significantly increase the overall yarn strength to resist external tensile forces. The electrical properties of CNT/PU-wrapped yarns with different wrapping angles were observed during stretching. As shown in Figure 3(e), the resistance change rate of the 12,000 t/m wrapped yarn drops to a lesser extent after 30% strain, and its sensing coefficient is much smaller than that of the other two groups of wrapped yarns. This suggests that, in line with the design expectation of tensile wires, the spring-helix wrapped construction may achieve tensile performance and has a small resistance change rate under large strain conditions. As a result, we decided to wind at a twist rate of 12,000 revolutions per minute.

Based on CNT/PU-Wrapped Yarns with Different Twist Levels

Surface Morphology

Figure 4(a–d) shows the surface morphology of CNT/PU-wrapped yarns at different twist levels under the Critical twist factor. Due to the holding force between the CNT bundles, the fineness of the CNT yarn decreases with the increase in twist, and yarn uniformity is improved. As the fineness of the skin yarn decreases and the wrapping twist increases, the length of the wrapped skin yarn increases on the same length of core yarn, and the width of each helix gradually becomes uniform. In terms of critical entanglement angles, 300, 600, 900, and 1200 t/m correspond to 7000, 8000, 10,000, and 12,000 t/m, respectively.

Figure 4.

Surface morphology plots of CNT/PU-wrapped yarns with different twist levels: (a) 300 t/m, (b) 600 t/m, (c) 900 t/m, (d) 1200 t/m. (e) Stress–strain plots, (f) conductivity/resistance plots, and (g) resistance variability plots at 30% strain.

Mechanical Properties

As Figure 4(e) illustrates, testing was done at the crucial entanglement twist level on the mechanical characteristics of CNT/PU entangled yarns with varying twist levels. The findings indicate that the wrapped yarn’s stress increases abruptly at 1200 t/m but steadily reduces with increasing twist. The outcomes are displayed in Figure 4(e). This is because the skin yarn becomes more in touch with the core yarn and becomes stiffer as a result of the loss in fineness, which causes both strain and stress to increase. CNT yarns are better suited with a twist of 900 t/m due to the high strain demand.

Electrical Properties

Based on the previous experiments, the most suitable twist rate of CNT yarn at the same wrapping angle was further investigated. As shown in Figure 4(f–g), the twist of 900 t/m has the maximum electrical conductivity. The resistance change rate of the wrapped yarn with this twist tends to be close to zero in the 30% strain range, indicating that the resistance change rate remains stable in this tensile strain range, and it has good electrical and thermal properties as expected.

Electrical and Thermal Properties

The ideal parameters are finally chosen for the ensuing experimental investigation of electrical and thermal properties in order to further explore whether the electrical and thermal properties of this parameter are stable, based on the force electric characteristics mentioned above. Excellent electrical and thermal capabilities are possessed by CNT yarn. When a certain voltage is passed, the electrons inside the yarn will move along the direction of the external electric field, and the electrons will be converted into thermal energy through impact, friction, etc., and the law satisfies Joule’s law with the following formula:

$$Q=\frac{U^2}{R}t=I^2\mathrm{Rt}$$

Where Q is the heat generation (J), U is the voltage (V) passed through the external power supply, R is the real-time resistance of the yarn (Ω), t is the energization time (s), and I is the current (A) passed through.

Effect of Different Voltages on the Performance of CNT/PU-Wrapped Yarns with Different Twist Levels

Figure 5(a) demonstrates the constant voltage electrothermal performance of 900 t/m CNT/PU entangled yarn at the critical twist factor. As monitored by the infrared camera, it can be seen that the overall surface of the outer yarn is reddening after energization. The temperature of the entangled yarn increased rapidly and reached a steady state after 15 s of energization, with a maximum temperature of 84.8°C. After 3 min of energization, the temperature returned to room temperature within 10–15 s when the power was turned off. This is due to the fast thermal response rate of the carbon nanotube bundles, which have a very large length-to-diameter ratio and high heat transfer performance in the radial direction. Therefore, once the power is disconnected, the heat generated by the material can be dissipated in a very short time. In the figure, with the increase of the set constant voltage, the surface temperature of the wrapped yarn starts to increase in a gradient, and when the constant voltage is 4 V, the temperature on the surface of the wrapped yarn is more than 110°C, which will lead to the fusion phenomenon of the spandex core yarns, destroy the wrapped structure and lead to the degradation of performance. Therefore, in order to keep the winding structure normal, the surface temperature of the winding yarn after energization should not exceed 100°C.

Figure 5.

Electro-thermal diagrams of CNT/PU-wrapped yarns: (a) constant voltage electro-thermal properties, (b) electro-thermal properties of different stretch states, (c) different bending angle electric heat map: 0°, 45°, 90°, 180°, and (d) temperature/time diagrams of different bending angles.

Effect of Different Stretching States on the Properties of CNT/PU-Wrapped Yarns with Different Twist Levels

Figure 5(b) shows the real-time temperature of the entangled yarns under different stretching states at 3 V. With the increase in stretch, the temperature of the surface of the wrapped yarn has a slight decrease. This is because as the tensile strain increases, the pitch of the yarn increases, the neighboring helixes are separated from each other, the formed conductive pathway is destroyed, and the resistance of the yarn increases. When the voltage and energizing time are certain, the resistance becomes larger, which will reduce the heat generation. The highest temperatures in different stretched states were 84.3°C, 82.7°C, 82.1°C, and 81.4°C, respectively. When compared with the unstretched state, the temperature of the stretched yarn decreased by only 3.44%, indicating that the wrapped yarn retains a stable electrical and thermal performance at 30% strain, which is typical of stretched wires.

Effect of Different Bending Degrees on the Properties of CNT/PU-Wrapped Yarns with Different Twist Levels

Figure 5(c–d) demonstrates the real-time temperature of entangled yarns at 3 V at various bending angles. As the bending angle increases, the surface temperature of the entangled yarn increases, owing to the fact that as the degree of bending increases, the yarn tare gradually forms a top-thinned and bottom-dense structure. Consequently, neighboring spirals are able to contact each other more closely at the lower end of the bending portion, resulting in a more effective conductive network and reduces yarn resistance. When the voltage and energizing time are constant, heat generation will increase as the resistance decreases. The maximum steady state temperatures at different bending angles were 85.7°C, 86.9°C, 87.5°C, and 90.6°C, and the temperature only increased by 5.72% when bending to 180°. This indicates that the electrical and thermal properties of CNT/PU-wrapped yarns are stable at a certain bending angle.

Conclusion

In this study, we prepared CNT/PU entangled yarns with different entanglement parameters, and then selected the yarns with the best mechanical and electrical properties for the electro-thermal performance test to ensure their stability under a variety of conditions. The study concludes: the mechanical and electrical properties of CNT yarns were significantly improved by the twisting method, especially at 1200 t/m. The strain stress of CNT yarns was significantly increased, the breaking strength improved by 198%, and the strain increased by 392%. An entanglement structure was introduced, and CNT/PU entangled yarns were prepared, among which the spandex entangled yarns entangled with 900 t/m CNT yarns showed the best performance. The elongation at break reached 1156.45%, and the strength increased by 160.05% at the critical winding twist. According to the design specification, the 900 t/m CNT/PU wrapped yarn’s electrical characteristics demonstrated a stable resistance shift at the critical wrapping twist. According to the electrical and thermal properties of the 900 t/m CNT/PU entangled yarn, its surface temperature is controlled by the external voltage, but it should be kept within 100°C to prevent the structure from becoming damaged. It is notable that the temperature change of the yarn is relatively small under different stretching and bending conditions, demonstrating excellent thermal and electrical properties. Overall, the CNT/PU entangled yarns exhibit excellent electrothermal properties under different strains and external deformation states, showing a wide range of applications.