Abstract
This study presents a fabrication method for functional commingled yarns and prepared conductive knitted fabrics for shielding electromagnetic waves and electrostatic discharge. Stainless steel filament was used as core yarn, polyethylene terephthalate filament or silver yarn was used as wrapped yarn producing polyethylene terephthalate/polyethylene terephthalate/stainless steel filament or silver/silver/stainless steel filament commingled yarns via filaments hollow spindle spinning system and then knitting into silver/stainless steel composite fabric. The effects of cycle number and metal content on air permeability, surface resistance, and electromagnetic shielding properties of resultant knitted fabrics were discussed. Besides, influences of number of layers and lamination angle on electromagnetic shielding were also investigated intensively. The result shows that, conductive composite fabrics made by silver/silver/stainless steel filament commingled yarns and 450D polyethylene terephthalate plied filaments had higher surface resistance of 3.4 Log(Ω/sq) and 5.6 Log(Ω/sq), respectively, in coursewise and walewise directions. Electromagnetic shielding varied with number of layer, lamination angle, cycle number, and metal content. When six layers of conductive knitted fabrics were laminated with 45°, electromagnetic shielding reached 15 dB at 1–3 GHz frequency. The highest air permeability, 317.6 cm3/cm2/s, occurred at single-layer conductive composite fabric.
Introduction
As the usage of electrical and electronic devices has grown rapidly, many derived problems such as electrostatic and electromagnetic wave have emerged recently. A large amount of electrostatic and electromagnetic accumulations cause machinery operation problems and human health risks [1]. Inappropriate systemic design for electromagnetic shielding (EMSE) possibly generates electromagnetic interference (EMI). Besides, ionizing radiation has higher energy, able to break covalent bonds forming ions which results in biosystem damage. Non-ionizing radiation has the ability to heat up tissue temperature [2,3]. During the Second World War, many reports on cancer and sterility originated from electromagnetic radiation had been chronicled for radar workers and military personnel. Much early research about illness in power station operators had been covered in Eastern European countries. In recent years, several international research groups had extensively examined thousands of new scientific studies and together described the rising pathology of electrical diseases [4].
Generally, DNA damage easily brings about cancer. However, unlike ionizing radiation, high-frequency energy cannot cause DNA damage among cells immediately. Causes of cancer or enhancing cancer-causing effects of known chemical carcinogens have not been found in animals [5–7]. Current proofs only proved that wireless notebook computer radiation generated DNA fission. Most studies showed that mobile phone usage was accompanied with malignant brain tumor or affected male fertility [8].
Among studies on EMI shielding property, metal fiber, carbon material, and conductive polymer have become the common conductive materials. Woven fabric, knitted fabric, and nonwoven textiles are usually used as EMSE materials. In 2011, Grabowska et al. [9] fabricated hybrid yarn with damping braided structure, and found that hybrid yarn composed of series-connected bunch of yarns and conductive solenoid coils prevented from intensive electromagnetic field. Bhat et al. [10] coated PANi on the surface of cotton fabric to improve shielding capacity, and indicated that EMSE was influenced by uniformity of PANi; therefore, the insulation region leaked electromagnetic waves, weakening electromagnetic shield. Roh et al. [11] studied EM shielding effectiveness (SE) of metal composite fabrics by changing size and geometry of metal grid size, and pointed out that overall EMSE increased with metal content and shielding frequency depended on aspect ratio of metal grid. Conductive yarns such as stainless steel and copper wires have high stiffness, which make them difficult to weave. Therefore, Cheng et al. [12] compounded stainless steel and copper wires with glass yarn and polypropylene textured yarn, fabricating into uncommingled yarn using a hollow spindle spinning system. In this study, we fabricate commingled yarns using filaments hollow spindle spinning system and then knitted them into composite fabrics. The stainless steel filaments (SFs) and silver yarns were used as wrapped yarn to form more flexible fabric after knitting compared to other woven fabric structures, in order to improve flexibility and electrical properties. Effects of cycle number and metal content on surface resistance and air permeability, as well as effects of number of layers and lamination angle on EMI SE were explored for the resulting conductive composite fabric.
Experimental
Yarn preparation
This study used SF (50 µm diameter, provided by Yuen Neng Co., Ltd., Taiwan) as core material, and used polyethylene terephthalate (PET) filament (75D, from Yi Jinn, Taiwan) as well as silver yarn (150D) as the wrap material to fabricate SF/PET/PET or SF/Silver/Silver commingled yarn using filaments hollow spindle spinning system. The characteristics of used materials are listed in Table 1. During the process, the winding speed was 8000 rpm; the warp number was, respectively, 9 turns/cm whose codes were, respectively, named as D9. Therein, D shows the double-wrapped structure of yarn, and the number indicates the warp number. More details about the fabrics are given in Table 2. Figure 1 shows the picture of the SS yarn, constituting silver yarn as wrapped yarn and stain steel filaments as core yarn.
Image observations of the SS commingled yarn. Characteristics of the materials. PET: polyester filament yarns; SY: silver yarns; SF: stainless steel filaments. Specification and characteristic of conductive commingled yarns. PET: polyester filament yarns; SY: silver yarns; SF: stainless steel filaments. Characteristics of the knitted fabrics.
Knitted fabric preparation
The knitted fabrics used in this study were successfully produced on a flat knitting machine (14G, KH-626TS, Kauo-Heng, Taiwan) using the commingled yarns and the PET. In order to increase lubrication during fabrication process, both commingled yarns and PET filament (75D) were fed into weft bobbin at the same time. In this research, 450D PET plied filament was used to make the commingled yarns having certain intervals, which resulted in different metal densities. Hence, six types of complex knitted fabrics SP-D9, SPP1-D9, SPP2-D9, SPP3-D9, SPP4-D9, and SP-SS were fabricated in this study, where SP means the comingled yarns and PET filament (75D), P means stands for the inserted 450D PET plied filament, the number represents the cycle number (ratio of alternative number for SP and P yarns, respectively), D9 and SS mean the type of commingled yarns. Figure 2(a) shows the picture of the SP-SS knitted in which the wrapping number of the commingled yarns is 9 turns/cm. During processing, two groups of yarn feeder were used during processing. The first feeder was incorporated into SP, while the second was fed into P yarns containing 450D PET plied filament as shown in Figure 2(b). In addition, SP yarn fed from first feeder and P yarn from second feeder were considered as the control group, thus, respectively, producing SP-SS knitted fabric. During the knitting process, one cycle for knitted fabrics is defined as yarn feeder moving back and forth for one time, meaning that two courses of loops were produced after one-cycling knitting as shown in Figure 2(b). As the cycles of the second feeder increase, the amount of metal content decreases accordingly.
(a) SP-SS knitted fabric image and (b) structural diagram of conductive knitted fabrics.
Test
When EMI shielding fabrics were made into clothing, air permeability is an important evaluation index in terms of comfort properties. Air permeability was performed by TEXTEST FX3300 tester according to ASTM D0730. The testing sample was 25 × 25 cm2. Ten samples were tested to determine the average of air permeability.
Surface resistance was measured by RT-1000 tester (CHIEN HSING Co., Ltd., Taiwan) on the basis of JIS L1094. The sample was placed on the insulating plate, and then a tester was pressured on the sample with a 2.27 kg load on electrodes to make sure if the probes had a good contact with the surface of the specimen.
EMSE test used EM-2107A clamper and scanned in the frequency range from 300 k to 3 GHz. The electromagnetic wave was far-field plane. EMSE of reference sample was firstly measured, named as SERef, SE of knitted fabric was SELoad. Figure 3 shows the size of reference specimen and load specimen. The actual SE of resultant knitted fabric is defined as SELoad subtracting SERef. SE is defined as the ratio of electromagnetic field strength measured without ( Reference specimen and load specimen.
Results and discussion
Effects of cycle number and metal content on surface resistance
Figure 4 shows the effect of cycle number on the surface resistance of composite fabric. PET insulation content at testing area changes with cycle number, influencing on electrical resistance. SP-D9 fabric had the lowest surface resistance. Knitted fabric was formed by yarn loop interlacing, and coursewise loops were composed of successional conductive commingled yarn. Therefore, electrical current transferred quickly and surface resistance would be decreased. With the increase of PET filament content, walewise loops produced larger insulating interstices and the conducting path became discontinuous; hence, surface resistance was increased. Ultimately, the surface resistance difference between SP-D9 and SPP4-D9 fabric was 4.58 Log(Ω/sq) and 4.42 Log(Ω/sq), respectively, in the coursewise and walewise.
Surface resistance of SP-D9 knitted fabric with D9 yarn when changing the cycle number of SP commingled yarns and P plied yarns.
Figure 5 shows the comparative surface resistance of SP-SS and SP-D9 knitted fabrics. Insertion of PET plied filaments decreased metal content among knitted fabrics. Comparatively, SP-SS composite fabric possessed the lowest surface resistance. This is because at the same testing area, conductivity promoted and transferred well with increase in metal content, thus resulting in decreased surface resistance. A part of yarns in walewise loops was disconnective and discontinuous, but yarns in coursewise loop showed a continuous route. As a result, composite fabric had lower coursewise surface resistance than the walewise.
Surface resistance of SP-SS and SP-D9 knitted fabrics.
Effects of cycle number, metal content, layer number, and lamination angle on EMSE
Figure 6 shows the effect of cycle number on EMSE of five-layer knitted fabric laminating at 0°. Metal content decreased at unit area varying from SP to SPP4, and meanwhile the electrical conductivity lowered resulting in reduced EMSE. EMSE at low frequency reached above 10 dB, and the highest value was achieved above 20 dB. In addition, a large coverage of metal enhanced the EMI shielding effect. This is because smaller interstices of metal filaments increased the possibility of reflection and absorption to electromagnetic wave. However, EMSE at high frequency was below 10 dB, and reached 10 dB at only few certain frequency range. This is due to the fact that although metal filament attenuated electromagnetic wave because of its magnetic and conductive effects, the high-frequency waves had still been leaked out displaying limited damping capacity because of interspace among knitted fabrics.
Effect of cycle number on EMSE of five-layer knitted fabric (SP–SPP4-D9) laminating with 0° at frequency of 300 k–3 GHz.
Figures 7 and 8 show EMSE of multiple layers of SP-D9 knitted fabrics laminating at 0° and 45°. Figure 9 shows the schematic of fabrics with different lamination angles. For single-layer fabric, EMSE achieved only about 10 dB in low-frequency range. This is because knitted fabric had low loop density and more interspaces, which was incapable of intercepting more electromagnetic waves. As the layer number increased, EMSE improved evidently. In addition, 45° lamination had higher EMSE than 0° lamination, which is because 45°-laminated fabrics had larger metallic coverage, higher loop density, and smaller interspaces between fabrics. EMSE of knitted fabric was 7 dB–10 dB at 1 GHz incident frequency, reaching moderate shielding level which satisfies application in the general life. When comprehensively analysed, it was noted that EMSE reached above moderate shielding level when knitted fabrics were surpassing three layers. By changes of lamination angles, the highest EMSE of SP-D9 knitted fabric achieved 18–20 dB at an incident frequency of 1.2 GHz and 2 GHz, shielding about 90% electromagnetic wave. This SE is 30% higher than that of double-layer plain knitted fabric at 0.86–0.96 GHz and 1.75–1.85 GHz frequency as indicated by Palamutcu et al. [13], and can be applied on EMSE protective garments.
Effect of layer number on EMSE of SP-D9 knitted fabric laminating with 0° at 300 k–3 GHz frequency. Effect of layer number on EMSE of SP-D9 knitted fabric with different layers (1, 3, and 5 layers) and 45° lamination angle at 300 k–3 GHz frequency. Schematic of three-layer knitted fabrics laminating with (a) 0° and (b) 45°.
Figures 10 and 11 show EMSE of different layers of SP-SS and SP-D9 knitted fabrics with 0° and 45° lamination. Comparatively, SP-SS knitted fabric had higher EMSE than SP-D9 fabric. This is because the silver content at unit area increased and then improved the electromagnetic damping. Ultimately, five-layer SP-SS knitted fabric had −12.02 dB, −12.65 dB, −7.39 dB EMSE, which are better than the five-layer SP-D9 knitted fabric, −14.65 dB, −11.49 dB, −9.24 dB, respectively, when the incident frequencies were 900, 1800, and 2450 MHz and the lamination was 45°. Five-layer composite fabrics satisfy the EMI shielding requirements in people's livelihood.
Effect of metal content among commingled yarn on EMSE with different layers (1, 3, and 5 layers) and 0° lamination angle at 300 k–3 GHz frequency (wrap yarn: PET filament and silver yarn). Effect of metal content among commingled yarn on EMSE of SP knitted fabric with different layers (1, 3, and 5 layers) and 45° lamination angle at 300 k–3 GHz frequency (wrap yarn: PET filament and silver yarn).
Effects of cycle number, metal content, and layer number on air permeability
Figure 12 shows the effect of layer number (one, three and five layers) on air permeability of composite fabric. With an increase of layer number, EMSE increased but air permeability decreased, indicating discomfort to human body. Evidently, single-layer knitted fabrics had the highest air permeability, reaching above 200 cm3/cm2/s. Air permeability decreased with layer number due to smaller interstices between layers which makes air transmitting through fabrics difficultly.
The effect of layer number on the air permeability of SP-D9 knitted fabric.
Figure 13 shows the effect of cycle number on air permeability of composite fabric. With an increase of cycle number, air permeability decreased and PET filaments content increased at unit area. Therefore, air permeability presented the lowest value at the maximum content of PET filaments. This is because PET filament was coarser than commingled yarn, and knitted fabric became fluffier at higher content of PET filaments, resulting in more difficult air transmission. Finally, single-layer SP knitted fabric had the highest air permeability of 317.6 cm3/cm2/s.
Effect of cycle number on air permeability of single-layer knitted fabric.
Conclusion
This study used SF as core yarn, PET filament and silver yarn as wrapped yarn fabricating stainless steel/PET/PET and stainless steel/silver/silver commingled yarns using a hollow spindle spinning machine, and then knitting them into conductive composite fabric by an autonomic plate knitting machine. Experimental result shows that when wrap yarn was transformed into silver yarn, surface resistance of stainless steel/silver/PET knitted fabric had the lower surface resistance as 3.4 Log(Ω/sq) in walewise direction. The air permeability of single-layer knitted fabric reached above 180 cm3/cm2/s. With an increase of cycle number of PET plied yarns, air permeability of conductive composite fabric decreased slightly. When wrap yarn was changed into silver yarn, metal content of composite fabrics increased and then improved the EMI shielding property. Ultimately, five-layer SP-SS knitted fabric had −12.02 dB, −12.65 dB, −7.39 dB EMSE, which are better than the five-layer SP-D9 knitted fabric, −14.65 dB, −11.49 dB, −9.24 dB, when the incident frequencies were 900, 1800, and 2450 MHz and the lamination was 45°. The resulting laminated conductive fabrics effectively act as shielding cloth to protect humans from electromagnetic wave radiation.
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) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was financially supported by National Science Council of the Taiwan under Contract NSC-102-2622-E-468-001-CC3.