Abstract
A core-spun yarn containing an ultrafine copper wire for wearable electronics-oriented applications has been manufactured using a modified vortex spinning system for the first time. The copper wire is fed into the spinning nozzle through a groove on the surface of the top front roller and an orifice through the fiber guiding member in sequence. Scanning electron micrographs confirm that the copper wire locates in the core region and is tightly wrapped by the helical staple fibers of the outer layer in the core-spun yarn, owing to the special yarn formation mechanism of the vortex spinning system. The vortex core-spun yarn containing a copper wire has a strength higher by 86.6% and a breaking extension lower by 70.2% compared to the copper wire, while its strain sensitivity in the workable strain range is not affected by either the yarn manufacturing process or the existence of staple fibers. The vortex core-spun yarn containing a metal wire could be a promising candidate for the conductive tracks of wearable electronics due to its improved structure, durability, and comfort.
Conductive tracks play an important role in connecting discrete electronic components integrated in electronic textiles. They can be fabricated by printing conductive polymers or nanosized metallic particles onto the surface of fabrics. Printed conductive tracks suffer from electrical failures caused by microcracks resulting from cyclic loadings and delamination due to shear stresses or mismatched physical properties. 1 Due to their high conductivity and good mechanical properties, metal wires can work as conductive tracks by embedding or integrating them into fabrics using well-established textile technologies, such as weaving and knitting. Li and Tao 2 reported a fabric circuit board (FCB) fabricated by integrating a polyurethane film-coated ultrafine copper wire with a core diameter of 50 µm into a knitted elastic fabric. Since the copper wire is in a three-dimensional looped structure, the variation of its configuration can adapt to the large stretching deformation of the FCB so that its strain is always kept at a very small value. As a result, the electrical resistance of the FCB remains stable and its fatigue life can reach more than 1 million loading cycles at 20% maximum strain. This technique is promising not only for the excellent and reliable electro-mechanical properties of the fabricated FCB, but also the potential for its large-scale mass production. Nevertheless, the as-produced FCB still has the following problems: (a) as worn on the human body, there is a great possibility of short circuit resulting from the contact between adjacent metal wire loops in the deformed FCB, since the thin polyurethane coating on the wire is prone to be peeled off due to frictions; (b) the metal wire is liable to be damaged during its interaction with the mechanical components in the manufacturing process of the FCB or under impact loading from hard objects; (c) the interaction between the human body and the FCB is less friendly, since the metal wire may cause discomfort to human skin. Although composite yarns in which a metal wire is wound around a spun yarn have already been reported for smart-textiles applications, 3 the metal wire is still exposed outside in this type of conductive yarn and the conductive fabrics produced with them still potentially suffer from the aforementioned problems.
Core-spun yarn has a two-component structure with a continuous filament as the core that is wrapped by an outer layer of short-staple fibers that serves as the sheath. A core-spun yarn with a metal wire as the core covered by short-staple fibers not only possesses the electrical property of the metal wire but also the good wear comfort, strength, surface appearance, and, most importantly, the protection for the metal wire provided by the staple fibers in the outer layer. As a result, it has become an excellent candidate for the conductive tracks of electronic textiles. When producing a core-spun yarn on a ring spinning machine, 4 it is difficult to control the location of the core filament and keep it in the yarn center due to the intense fiber migration, resulting in wrapping defects. 5 The ring spinning method is also disadvantageous due to its low production rate and long process. A number of studies on producing core-spun yarn with a metal wire core using friction spinning have also been reported.6–9 However, friction spinning is currently only suitable for producing coarser yarn count, which restricts the application of the resultant core-spun yarn in wearable electronics. Vortex spinning is a newly developed technology that inserts twist into the fiber strand by means of a high-speed swirling airflow in a nozzle 10 at a maximum production rate of 500 m/min. Due to the special yarn formation mechanism of vortex spinning, the resultant yarn has a layered structure consisting of parallel core fibers wrapped by an outer layer of helical fibers. 11 This makes the vortex spinning technology extremely suitable for producing core-spun yarns in which the core filament can be evenly wrapped by the staple fibers in the outer layer and is not prone to deviate from the yarn center provided that the filament is accurately located in the center of the yarn during the spinning process. Unfortunately, few studies have paid attention to producing core-spun yarns using the vortex spinning technology,12,13 partly due to the difficulty in accurately controlling the position of the core filament in the yarn. This short communication reports for the first time a core-spun yarn with a metal wire as the core manufactured by a modified vortex spinning system that can accurately and effectively control the position of the core filament during the manufacturing process. The as-manufactured vortex core-spun yarn containing a metal wire is aimed at replacing the bare metal wires to function as conductive tracks of the FCB for electronic textile-oriented applications.
Manufacture of the core-spun yarn on the modified vortex spinning system
Viscose rayon fibers (Shandong Shenghe Textile Co. Ltd, China) with a mean length of 38 mm and a fineness of 1.5 dtex are used as the staple fibers for the core-spun yarn and are converted into roving with a linear density of 0.68 g/m. An ultrafine enameled copper wire (Jiangshan Hengchang Electric Wire Co. Ltd, China) with a core diameter of 50 µm is adopted as the core filament for the core-spun yarn. The copper wire is coated with a thin polyurethane film with a thickness of about 5 µm. The resistance per unit length of the copper wire is 8.8 Ω/m. An experimental vortex spinning machine is used for producing the core-spun yarn containing the copper wire. As shown in Figure 1(a), the single strand of roving is fed from the back of the machine into the drafting device consisting of three pairs of rollers. The roving comes out of the front rollers after being attenuated by the drafting device, which has a total draft ratio of 53. At the same time, the copper wire is fed to the drafting device from behind the front rollers with a specifically designed core filament feeding device. The feeding device mainly consists of a pair of discs for controlling the tension of the copper wire, a sensor (TSP-100, Hans Schmidt & Co. GmbH, Germany) for measuring the tension, a pulley and two rods with a hole for guiding the copper wire. In order to prevent the deformation, damage, or even fracture of the fine copper wire due to the high gripping pressure at the front roller nip, a groove of a rectangular cross-section is cut on the surface of the top front roller along the feeding path of the copper wire. The copper wire is then delivered through the front roller nip in this groove. The roving is delivered in the ungrooved area at a lateral distance (several millimeters) away from the groove in order to prevent the mingling of the roving with the copper wire.
Schematic diagrams of (a) the spinning process and (b) the nozzle structure of the modified vortex spinning system.
As the copper wire and the drafted strand of staple fibers come out of the front rollers, they are fed into an air-jet nozzle. As shown in Figure 1(b), the design of the nozzle employed in this research has been modified compared to that adopted in the commercialized vortex spinning system. The major modification is that an orifice for feeding the core filament is provided through the fiber guiding member in the nozzle entrance. The orifice has a diameter of 250 µm and is coaxial with the nozzle axis. The drafted staple fibers enter the nozzle along the spiral surface on the fiber guiding member. The spirally delivered staple fibers then converge in the nozzle chamber with the copper wire coming out of the orifice and the leading portions of the staple fibers are pulled into the passage of the spindle by the copper wire. Meanwhile, high-speed air currents are ejected into the nozzle chamber through four injectors that are located around the chamber and are tangential to the nozzle wall, thus creating a swirling airflow inside the nozzle chamber. The trailing portions of the staple fibers are then separated and expanded over the tip of the spindle from the force of the swirling airflow. They then whirl and wrap around the copper wire, forming a core-spun yarn. The resultant core-spun yarn is delivered out of the nozzle from the passage of the spindle and is wound onto a yarn package. Since the copper wire has the additional functions of guiding the staple fibers into the spindle and preventing the upward propagation of the twist, the guiding needle that is employed in the commercial vortex spinning system can be eliminated. The air pressure is set to be 6 × 105 Pa. The yarn delivery speed of vortex spinning can be practically as high as 500 m/min. However, due to the limited performance of the experimental machine, a delivery speed of 220 m/min has been adopted in the experiment. The produced yarn has a linear density of 32 tex.
Structure of the vortex core-spun yarn containing a copper wire
Scanning electron micrographs of the vortex core-spun yarn containing the copper wire were obtained using a scanning electronic microscope (S-4800, Hitachi, Japan) to study the longitudinal structure of the yarn, as shown in Figure 2. It can be clearly observed from Figure 2(a) that the yarn consists of two layers of staple fibers. The staple fibers are parallel to the yarn axis in the core region and are regularly wrapped by an outer layer of helical ones. The copper wire is completely embedded in the staple fibers in the core region so that it is not observed in the micrograph. The copper wire is covered by these core staple fibers and they are both tightly bound by the wrapper fibers. Therefore, the copper wire can be well protected by the staple fibers, thus creating a firm yarn structure. The wrapper fibers exhibit almost equal wrapping angles and there are few wild fibers.
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The appearance and structure of the yarn specimen shown in Figure 2(b) are different from those of the yarn specimen shown in Figure 2(a). Few core staple fibers have been observed in the core region of the yarn. Most of the staple fibers exhibit helical configurations and directly wrap around the copper wire that is located in the yarn core. Therefore, the structure of the yarn resembles that of a ring yarn. There are also some wrapper-wild fibers that have wrapping angles different from the main wrapping angle of the yarn, either tightly or loosely wrapping around the yarn. The presence of the wrapper-wild fibers leads to an irregularly wrapped structure of the yarn. Some wild fibers protruding from the main body of the yarn can also be observed. These wild fibers are either short or form a loop, which are typical characteristics of vortex spun yarn. Figure 3 shows a cross-sectional view of the yarn under an optical microscope (VHX-100 E, Keyence, Japan). When preparing the yarn cross-section microtomy, the yarn specimen was first embedded in the epoxy resin. After the resin was solidified, the resin with the yarn was cut into slices with a thickness of 500 µm, which were then polished for the microscopic observation. It can be observed from Figure 3 that the copper wire locates at the core of the composite yarn with its eccentricity less than 1/3. The wire is completely covered by the outer layer of staple fibers.
Scanning electron micrographs of the vortex core-spun yarn containing an ultrafine copper wire. Microscopic image of the cross-section of the vortex core-spun yarn containing a copper wire.
Electro-mechanical properties of the vortex core-spun yarn containing a copper wire
Tensile tests of both the copper wire and the vortex core-spun yarn were conducted on a mechanical testing system (Model YG 061, Laizhou Electronic Instrument Co. Ltd, China). A sample length of 500 mm and a test speed of 5000 mm/min were adopted in the test according to the ASTM D2256 standard. When performing the tensile tests, the electrical resistance of both the copper wire and the core-spun yarn was measured by the four-probe method using a Keythley 2000 multimeter. Fifteen specimens were tested for each sample to obtain the average value. All the tests were carried out at the standard laboratory condition of 20 ± 2℃ and 65 ± 2% relative humidity. The measured electro-mechanical properties of the copper wire and the vortex core-spun yarn are shown in Figure 4. A typical load–extension curve of the vortex core-spun yarn is shown in Figure 4(a) and is compared with that of a copper wire. The load–extension curves of all the copper wire specimens exhibit negligible variations except for the difference in their breaking extensions. The ultrafine copper wire has an average breaking load of 6.57 cN. Due to its good ductility, the copper wire exhibits an average breaking extension of 17.1%. The load increases rapidly and linearly to about 4 cN when it is stretched below a small extension of about 0.8%. With further stretching, the plastic deformation of the copper wire takes place. After an extension of about 5% is reached, the load increases marginally with the continuous increase of the length of the wire until the breaking point is reached.
Electro-mechanical properties of the copper wire and the vortex core-spun yarn: (a) typical load–extension curve and resistance variation of a vortex core-spun yarn in comparison with a copper wire (line: load–extension curve; dot: resistance variation; ΔR: resistance change; R0: initial resistance); (b) comparison of breaking load, breaking extension, and strain sensitivity of the copper wire and the vortex core-spun yarn.
Comparatively, the vortex core-spun yarn has a strength higher by 86.6% than the copper wire, since the load is shared by both the copper wire in the yarn core and the staple fibers in the outer layer. However, the breaking extension of the core-spun yarn is greatly reduced by 70.2% compared to the copper wire. When the core-spun yarn is extended by a load, the outer layer of staple fibers is strained to the breaking point first, since it is less extensible than the copper wire in the core. After the layer of staple fibers breaks, the copper wire supports the total load, which is higher than the load that it can bear. This results in the rupture of the copper wire and a much lower breaking extension of the core-spun yarn.
Figure 4 also illustrates the measured results of the relative change in resistance of both the copper wire and the vortex core-spun yarn when they are strained. The measured results are linearly fitted to obtain the strain sensitivity, that is, the relative change in resistance per unit strain. The vortex core-spun yarn shows an average strain sensitivity of 2.17, which is the same as that of the copper wire. This confirms that the electrical property of the copper wire remains intact within the range of its breaking extension in the vortex core-spun yarn after processing.
Conclusions
A core-spun yarn containing an ultrafine copper wire for wearable electronics-oriented applications is successfully manufactured using a modified vortex spinning system. The copper wire locates in the core region and is tightly wrapped by the helical short-staple fibers of the outer layer in the vortex core-spun yarn. The staple fibers are expected to provide an effective protection to the copper wire core and to prevent the potential occurrence of short circuits resulting from the contact between the adjacent conductive tracks. The friendliness of the interaction between the human and the conductive track is also expected to be improved due to the improved surface structure, feel, and appearance provided by the short-staple fibers. The vortex core-spun yarn containing a copper wire has a strength higher by 86.6% and a breaking extension lower by 70.2% compared to the copper wire. The strain sensitivity of the vortex core-spun yarn in the workable strain range is not affected by either the yarn production process or the existence of the staple fibers.
Footnotes
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (Grant number 11302048), the Shanghai Pujiang Program (Grant number 16PJ1400400) and the Fundamental Research Funds for the Central Universities (Grant number EG2017016) and DHU Distinguished Young Professor Program.