Abstract
This paper presents a study of the electromagnetic shielding characterization of woven fabrics, produced with two different types of conductive yarns, namely silver-containing (Ag/PA/Co) core yarns and silver-containing (Ag/PA-Co) blended yarns. The effect of various yarn and fabric properties, such as yarn count, core filament count, blend ratio, weft density, electrical resistivity, yarn type and wave frequency, on the electromagnetic shielding effectiveness was investigated. The results have shown that the shielding effectiveness can be tailored by changing the yarn and fabric parameters and also there are significant differences between the electromagnetic shielding characteristics and performances of the fabrics produced with different yarn groups. Such fabrics are future promising for both daily and professional uses, since they combine high shielding effectiveness performance and the comfort properties of conventional fabrics.
Keywords
With the rapid growth of electrical and electronic devices and accessories, which emit electromagnetic (EM) energy in different frequency bands, exposure to EM radiation has increased. 1
The use of electronic equipment causes EM radiation, which can be harmful for the performance of electronic and electrical equipment and also for human health. Therefore, it has become essential to shield against all sources of EM energy.1,2
When an EM wave enters an organism, it causes heat to come out by vibrating the molecules. 3 The heat developed on the tissues is mainly a result of the Foucault currents flowing in the liquids of the cells. Foucault currents, like all electric currents, generate heat as well as EM forces. Foucault currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field, due to relative motion of the field source and conductor or due to variations of the field with time. This can cause a circulating flow of electrons, or current, within the body of the conductor. These circulating eddies of current have inductance and thus induce magnetic fields. These fields can cause repulsive, attractive, propulsion and drag effects. The stronger the applied magnetic field, or the greater the electrical conductivity of the conductor, or the faster the field changes, then the greater the currents that are developed and the greater the fields produced. 4
To prevent the negative effects of EM waves, shielding is necessary. EM shielding can be described as prevention of EM radiation transmission by a material. 5 It can also be described as the restriction of the effect of EM fields between two areas. 6
Regardless of its type and structure, a shielding material is characterized by its shielding effectiveness. 7 The decibel (dB) is mostly used as the unit of EM shielding. A high shielding effectiveness value relates to a good protection level against EM waves. 8
Textile materials can be used as EM shields, when conductive or magnetic properties are added to the structure. 7 It is well known that the textile materials are in general extremely good electrical isolators. The majority of the fibers are made from natural or synthetic polymers with very high electrical resistivity. However, the electric conductivity is a desired property of the textiles for certain applications, such as electrical heating, protection from EM radiation, transfer of signals, etc. Therefore, many attempts have been made in order to produce electrically conductive fibers. 9
Electrically conductive textiles are preferred for shielding applications most times due to their good shielding properties and numerous advantages compared to metal foils or metallic grids, such as low weight, elasticity, porosity, air permeability, corrosion resistance, low cost and comfort to the wearer. 7
Textiles with EM shielding function have a broad range of application areas. Particularly in military and civil applications, the importance of protection against the EM radiation is very high. An electrically conductive fabric placed over various objects can hide them from the radar detection activity. Also, a costume made of electrically conductive fabrics can protect the human body in the case of its exposition in strong EM fields. 10
The objective of this research is to investigate the shielding behavior of woven fabrics produced with two different groups of conductive yarns, namely silver-coated polyamide filament containing core spun yarns and silver-coated polyamide fiber blended ring spun yarns. The resultant yarns will be named as Ag/PA/Co core yarns and Ag/PA-Co blended yarns, respectively, in the rest of the paper for ease. The effects of various yarn and fabric parameters, such as yarn count, core filament count, blend ratio, weft density, electrical resistivity, yarn type and wave frequency, on EM shielding properties are discussed.
Experimental details
Yarn and fabric production
Production of conductive yarns was carried out on a ring spinning frame. As the first stage of this study, Ag/PA/Co core yarns were produced by using silver-coated polyamide filaments as core material and cotton as sheath material. Production of core yarns was performed by the help of a core yarn apparatus. The apparatus was placed on the drafting system of the ring spinning frame, in order to integrate the conductive Ag/PA filament in the cotton sheath. More precisely, the core yarn apparatus was placed on the delivery rollers, and thus the conductive filament was fed to the core into the spinning triangle, before twisting. In the structure of conductive Ag/PA/Co core yarns, silver-coated polyamide filaments of 22, 44 and 77 dtex were used as core materials. Conductive core yarns were produced in three different counts: 49 tex, 26 tex and 17 tex.
In the second stage, Ag/PA/Co blended conductive yarns with the same yarn count as the core yarns were produced by combining silver-coated polyamide fibers with non-conductive cotton fibers. For the production of conductive blended yarns, slivers of silver-coated polyamide staple fibers were blended with cotton slivers on a draw frame in four different blend ratios, namely 9/91, 15/85, 30/70 and 45/55%. The mentioned blend ratios were chosen in order to compare the shielding effectiveness performances of the blended yarns with the core yarns having the same conductive material ratio and yarn count, as can be seen in Table 1. The results of this comparison are shown in Figure 1(a) and (b). Then, the blends were fed to the ring spinning frame after passing through the roving frame.
Shielded rooms with (a) transmitter and (b) receiver antennas.
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Types and properties of the weft yarns
After the yarn production, Ag/PA/Co core yarns and Ag/PA-Co blended yarns were integrated to the woven fabric structure in the weft, with three different weft densities, namely 8, 16 and 24 picks/cm, for the evaluation of their electromagnetic shielding effectiveness (EMSE) properties. Ne 20/2 100% cotton ring spun yarns were used as warp yarns, with 12 ends/cm. For the fabric structure, 3/1 twill was chosen because of better grouping of yarns. Types and properties of the weft yarns investigated are shown in Table 1.
Electromagnetic shielding measurements
The shielding properties of the fabrics produced were tested in the frequency range of 200 MHz to 5.8 GHz, which covers emission frequencies of the electrical house devices, mobile phones and some of the medical equipment and more. Tests were performed by using an anechoic chamber test system according to the EN50147-1 standard.
During the measurements, the sample was placed between a signal generator and a receiver. The signal from the generator was first amplified and then sent to the antenna. The EM radiation from the antenna was directed to the sample. A part of the radiation was reflected and the remaining was transmitted through the sample to the receiving antenna. Schematic drawing of the test system is shown in Figure 2.
Schematic drawing of the test system.
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According to the basic principle of the EMSE test, the number of transmitted signals were measured and the EM field blocked by the sample was calculated.
Usually, the attenuation provided by a material is measured by taking into account the sent and the received powers. Ten times the logarithm of the ratio of the two amounts gives the attenuation in dB. However, the experimental setup allows only a part of the EM radiation to pass through the testing aperture to the receiving antenna. Therefore, instead of attenuation the shielding effectiveness was used. The shielding effectiveness involves the power received by the antenna without the sample and with sample. The measurement value of the EM wave was expressed in dBmV, and then the reduction of the EM wave was obtained from the difference of the two measurements, in dB.
The results were evaluated statistically by using the SPSS software program. Analysis of variance (ANOVA) tests and SNK (Student–Neuman–Kleus) tests for subgroup analyses were performed at a 95% confidence interval. For the evaluation of metallic core ratio and the resistance of the core filaments on the shielding effectiveness, the Pearson correlation test was performed at a 99% confidence interval.
Electrical resistivity measurements
Electrical resistivities of the fabrics produced with Ag/PA-Co blended yarns were measured according to the ASTMD 257 standard by using an electrical resistivity test fixture. Surface resistivity (ρ) was described as the resistance to leakage current along the surface of an insulating material and it was reported in ohms per square centimeter (ohm/cm2). The electrical resistivity was measured between two parallel electrodes in contact with the specimen surface and separated by a distance equal to the contact length of the electrodes.
Regarding the fabrics produced with Ag/PA/Co core yarns, not very stable results were obtained from the surface resistivity measurements, since the conductive filament in the core was covered with non-conductive cotton fibers. Therefore, instead of surface resistivity, the resistivity of the core filament was considered for this group of fabrics. Resistivity values were received from the producing company as 4700, 1500 and 500 ohm/km for 22 dtex, 44 dtex and 77 dtex Ag/PA filaments respectively. 11
Results and discussion
The shielding behavior of the fabrics investigated in this study was evaluated separately for 200–950 MHz and 1.2–5.8 GHz frequency ranges, due to different shielding characteristics in higher and lower frequencies.
Ag/PA/Co core yarns
Electromagnetic shielding effectiveness (EMSE) of the fabrics produced with Ag/PA/Co core yarns, in the 200–950 MHz frequency range
Electromagnetic shielding effectiveness (EMSE) of the fabrics produced with Ag/PA/Co core yarns, in the 1.2–5.8 GHz frequency range
Figure 3(a) and (b) show the variation of the average shielding effectiveness of the woven fabrics produced with Ag/PA/Co core yarns in 200–950 MHz and 1.2–5.8 GHz frequency ranges, respectively.
Variation of electromagnetic shielding effectiveness (EMSE) with frequency and weft density for the fabrics produced with Ag/PA/Co core yarns: (a) 200–950 MHz; (b) 1.2–5.8 GHz frequency ranges.
The effect of frequency on the shielding effectiveness was found to be statistically significant for both of the frequency ranges. It can be seen in Figure 4(a) that, in the 200–950 MHz frequency range, the shielding effectiveness decreased with increasing frequency. The highest results were obtained at 200 MHz, as 40 dB in this frequency range. The same trend was observed also in the 1.2–5.8 GHz frequency range. At 1.2 GHz frequency, the highest results were obtained at 35 dB. In general the measurements performed in the 200–950 MHz frequency range showed higher results compared to the 1.2–5.8 GHz measurements. This is caused by smaller wavelength in higher frequencies, according to the formula f = c/λ, where f is the frequency, c is the speed of light and λ is the wavelength of the EM wave.
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Change of electromagnetic shielding effectiveness (EMSE) with metallic core fineness and core resistance for the fabrics produced with Ag/PA/Co core yarns: (a) 200–950 MHz; (b) 1.2–5.8 GHz frequency ranges.
The results of statistical analyses have shown that weft density is a significant factor effecting the shielding effectiveness, and also that the shielding effectiveness increased with increasing weft density for both frequency ranges. While the weft density increases, the conductive fiber amount in the unit area of the fabric and weft cover factor increase, thus conductive yarns become closer to each other, resulting in a tighter conductive net. This result is supported by previous studies in the literature.1,12 An increase in conductivity of the fabric increases the EMSE. Although this trend was not observed in 400 and 600 MHz frequencies, where lower results were obtained for fabrics with 24 picks/cm, it was observed clearly in higher frequency ranges, especially for 2.2, 3, 4 and 5 GHz frequencies. The shielding values were closer to each other in 1.2, 2.2, 5 and 5.8 GHz frequencies.
Another important yarn parameter determining the conductive material inclusion in the core yarns is the core fineness. Results of statistical analyses have shown that the fineness of the core filament has a statistically significant effect on shielding effectiveness for both frequency ranges. In the content of this research, the effect of the resistance of the core filament on the shielding effectiveness was also investigated for the fabrics produced with core yarns. Figures 5(a) and 4(b) show the effects of core fineness and core resistance of Ag/PA filaments on the shielding effectiveness values of woven fabrics, for 200–950 MHz and 1.2–5.8 GHz frequency ranges, respectively. The average shielding effectiveness for each core fineness was calculated by taking the average of the EMSE measurement results of the fabrics produced with all the three yarn counts and three weft densities produced with the same core. It can be seen in the figures that, when the filament becomes coarser the resistance decreased and the shielding effectiveness increased due to improved conductivity. This result explains the increase in the shielding effectiveness with increasing core filament count. The same trend was found to be valid for both frequency ranges. As can be seen in Figure 5(a), the shielding effectiveness increased with increasing thickness of the core filament in the 200–950 MHz frequency range. The highest results were obtained with 77 dtex core count, as 30 dB on average. This can be explained by there being a higher amount of conductive fibers in the yarn. Statistical analyses of the subgroups have shown that in the 1.2–5.8 GHz frequency range the difference between 44 and 77 dtex filaments was not statistically significant, whereas the difference between 22 dtex and these two was statistically significant, and the values obtained with 22 dtex were lower than the others. The highest results were obtained with 44 dtex core fineness as 25 dB on average in this frequency range. It can be concluded that thicker filaments in the core result in better shielding effectiveness due to the higher amount of conductive material per unit area of fabrics woven with such yarns and thus increased conductivity of the fabrics.
Change of electromagnetic shielding effectiveness (EMSE) with metallic core ratio for the fabrics produced with Ag/PA/Co core yarns: (a) 200–950 MHz; (b) 1.2–5.8 GHz frequency ranges.
Results of the statistical analyses have shown that yarn count has a significant effect on shielding effectiveness of woven fabrics produced with Ag/PA/Co core yarns for both of the frequency ranges. In the 200–950 MHz frequency range, statistical analyses of the subgroups have shown that there was not a significant difference between the results of fabrics produced with 49 and 26 tex yarns counts, whereas the results obtained with 17 tex yarns were significantly higher than the other groups. Highest results were obtained with 17 tex yarns as 29.5 dB. For the 1.2–5.8 GHz frequency range, results of the subgroup analyses have shown that the fabrics produced with 26 and 17 tex did not have a statistically significant difference, whereas the results obtained from fabrics produced with 49 tex yarns were significantly lower. The highest results were obtained with 17 tex yarns as 25 dB. As a general trend it was observed in both frequency ranges that finer yarn counts resulted in higher shielding effectiveness results for the same weft density and core fineness. This result might have been caused by two things. The first might be that a finer yarn count with the same core fineness results in a higher ratio of conductive material in the yarn. Secondly, a finer yarn count with the same filament has a thinner cotton sheath layer on top of the conductive filament, which increases the possibility of contacts between the conductive materials, especially in higher weft densities.
Figure 6(a) and (b) show the relationship between the core ratio (%) by weight in the yarn produced with 22, 44 and 77 dtex Ag/PA filaments in 49, 26 and 17 tex yarn counts and shielding effectiveness performances of the fabrics woven from these yarns for 200–950 MHz and 1.2–5.8 GHz frequency ranges, respectively. It can be seen clearly in the figures that the shielding effectiveness increased with increasing percentage core ratio in the yarn for both low and high frequency ranges. Even though in the low frequency range there is a drop in shielding effectiveness with the 77 dtex core and 17 tex yarn count and in the high frequency range slightly lower results were obtained with the 77 dtex core compared to the 44 dtex core, these drops, which might be due to the selectivity of the EM wave, do not change the general trend, since they are less than 1.5 dB. As previously mentioned, EMSE increases as the yarn becomes finer and the core filament get thicker.
Variation of electromagnetic shielding effectiveness (EMSE) with frequency and weft density for the fabrics produced with Ag/PA-Co blended yarns: (a) 200–950 MHz; (b) 1.2–5.8 GHz frequency ranges.
Ag/PA-Co blended yarns
Electromagnetic shielding effectiveness (EMSE) of the fabrics produced with Ag/PA/Co blended yarns, in the 200–950 MHz frequency range
Electromagnetic shielding effectiveness (EMSE) of the fabrics produced with Ag/PA/Co core yarns, in the 1.2–5.8 GHz frequency range
The effects of weft density and frequency on the shielding effectiveness of woven fabrics having Ag/PA-Co blended yarns as weft yarns are shown in Figure 6(a) and (b) for 200–950 MHz and 1.2–5.8 GHz frequency ranges, respectively. As can be seen in Figure 7(a), in the lower frequency range the shielding effectiveness was inclined to increase up to 400 MHz and then to decrease with increasing frequency. In the higher frequency range, it was observed that the shielding effectiveness decreased with increasing frequency, even though there was a peak at 4 GHz, as can be seen in Figure 7(b). Both figures clearly show that the shielding effectiveness improved with increasing weft density for the whole frequency range, as a result of increased weft cover factor and higher conductive material in the unit area of the fabric amount and thus increased conductivity in the fabrics. The highest results were obtained with 24 picks/cm weft density at the 400 MHz frequency as 26 dB for the lower frequency range and again with 24 picks/cm weft density at the 4 GHz frequency as 25 dB for the higher frequency range. It was found that the weft density was a significant factor for core yarns, as can be seen in Figure 3(a) and (b), but the effect of this factor seems to be more clear and significant in the blended yarns, as can be seen in Figure 7(a) and (b). This fact can be explained by the differences between the structures of core and blended yarns; more precisely, how the conductive material is placed in the yarn structure. In the blended yarns the conductive fibers are spread homogenously in the whole yarn, whereas in the core yarns the conductive filament, covered with non-conductive cotton fibers, lies in the inner region of the yarn. Therefore, especially in higher weft densities, there are more possibilities for interconnections of the conductive materials in the blended yarns and the increase in the weft density increases these interconnections and enhances the shielding effectiveness.
Change of electromagnetic shielding effectiveness (EMSE) with electrical resistivity (ρ) and conductive fiber ratio for the fabrics produced with 26 tex Ag/PA-Co blended yarns: (a) 200–950 MHz; (b) 1.2–5.8 GHz frequency ranges.
The blend ratio also has a significant effect on the shielding effectiveness of fabrics produced with Ag/PA-Co blended yarns as weft yarns. In both frequency ranges the shielding effectiveness improved with increasing ratio of Ag/PA fibers in the yarn, due to increasing amount of conductive material per unit area of fabric and thus improved electrical conductivity. With the yarns made out of 100% Ag/PA fibers, 26 dB shielding effectiveness was achieved for the lower frequency range and 24 dB for the higher frequency range. Following 100%, the highest results were obtained with 30% Ag/PA fiber ratio as 18.6 dB in the lower frequency range and 22.6 dB in the higher frequency range, which is comparable to the result obtained with 100% ratio. This means that in the higher frequency range a considerable saving in conductive fiber amount can be achieved.
Electrical resistivity results have shown that the electrical resistivity of the fabrics decreases with increasing ratio of conductive fibers in the blend, resulting in higher conductivity and better shielding effectiveness, both in lower and higher frequency ranges. Figure 8(a) and (b) show the effects of the metallic fiber ratio and electrical resistivity of the yarns on EMSE of the fabrics produced with 26 tex Ag/PA-Co blended yarns in 200–950 MHz and 1.2–5.8 GHz frequency ranges, respectively. As can be seen in the figures, electrical resistivity decreased with increasing ratio of Ag/PA fibers in the blend, due to increasing conductivity, and this resulted in a considerable improvement in the shielding effectiveness of the fabrics. This trend was found to be valid for both of the frequency ranges. The results of statistical analyses have also supported that the electrical resistivity of the fabrics had a significant effect on the shielding effectiveness.
Change of electromagnetic shielding effectiveness (EMSE) with yarn count and electrical resistivity (ρ) for the fabrics produced with Ag/PA-Co 15% blended yarns: (a) 200–950 MHz; (b) 1.2–5.8 GHz frequency ranges.
The results of this research have shown that the blended yarns exhibited totally different shielding behaviors in means of yarn count. The shielding effectiveness decreased with increasing yarn count, which means that higher results were obtained with coarser blended yarns, whereas the opposite trend was observed in Ag/PA/Co core yarns of the same count. This result of the blended yarns can be explained by the increasing cover factor and the decreasing distance between the conductive yarns, when the yarn becomes coarser in the same weft density. Resistivity measurements also supported this result. As can be seen in Figure 9(a) and (b), with the same Ag/PA-Co fiber ratio, surface resistivity (ρ) increased and thus the conductivity and shielding effectiveness decreased as the yarn became finer. The highest results were obtained with 49 tex yarns as 20 dB in the lower frequency range and 21 dB in the higher frequency range.
Comparison of electromagnetic shielding effectiveness (EMSE) for Ag/PA/Co and Ag/PA-Co yarns with the same amount of conductive material: (a) 200–950 MHz; (b) 1.2–5.8 GHz frequency ranges.
In order to investigate the effect of yarn type, Ag/PA/Co core and Ag/PA-Co blended yarns having the same conductive material ratio in the yarn structure were compared. Results are given in Figure 1(a) and (b) for 200–950 MHz and 1.2–5.8 GHz frequency ranges, respectively. As can be seen in the figures, with the same yarn count and conductive material amount, Ag/PA/Co core yarns have shown higher results compared to Ag/PA-Co blended yarns for both frequency ranges. The reason for this result is that in the core yarn structure the conductive line continued in the filament form in the core, whereas in blended yarns the conductive line was interrupted by non-conductive cotton fibers. This result proves that the yarn structure has a significant effect on the shielding effectiveness.
For the fabrics produced with core yarns, in general higher results were obtained in lower frequency ranges, compared to higher frequency ranges. This was explained by a smaller wavelength in the higher frequency ranges. This tendency was found to be opposite in the fabrics produced with blended yarns, where higher results were obtained in higher frequencies, due to the difference in the yarn structure and thus the shielding mechanism.
Conclusion
In this paper, the possibilities for the use of textile materials for the purpose of EM shielding in the 200 MHz–5.8 GHz frequency range were investigated. Two different types of conductive yarns, namely silver-containing core (Ag/PA/Co) yarns and silver-containing blended (Ag/PA-Co) yarns, were studied. The effects of various yarn and fabric properties, such as yarn count, core filament count, blend ratio, weft density, electrical resistivity, yarn type and wave frequency, on the EMSE were investigated. The results have shown that there are significant differences between the EM shielding characteristics and performances of the fabrics produced with different yarn groups. It was also observed that it is possible to control the shielding effectiveness by tailoring the yarn and fabric parameters. In general, shielding effectiveness increased with increasing weft density for all the fabric types in both frequency ranges. The results of the fabrics produced with core yarns have shown that shielding effectiveness improved with coarser cores and finer yarn counts. The shielding effectiveness of the fabrics produced with blended yarns improved with increasing ratio of the conductive fibers in the blend and also with coarser yarn counts. Shielding effectiveness decreased with increasing frequency in both of the ranges, due to a smaller wavelength in higher frequencies, as was explained in the Results and discussion section.
As can be seen in Table 2, in the lower frequency range, the highest results were obtained from the fabric that was produced with Ag/PA/Co core yarns, with 24 picks/cm weft density, 26 tex yarn count and 77 dtex core fineness as 48 dB at 200 MHz. In the higher frequency range the highest results were obtained with 16 and 24 picks/cm weft density, 17 tex yarn count and 77 dtex core fineness as 37 dB at 1.2 GHz, as it can be seen in Table 3.
In this study, up to 48 dB shielding effectiveness was obtained. These types of fabrics show promise for future use in daily and professional applications, since they offer high shielding effectiveness performance in a broad frequency range without lacking wear comfort, which most of the fabrics produced with metallic yarns do. Application areas could include children’s garments, garments for the flight crew in airplanes, gowns for medical staff who are exposed to radiation and more. Further investigations are needed to explore these fields.
Footnotes
Funding
This was supported by Ege University (Project number 08 -Müh-039), as a scientific project. The anechoic chamber test system and the electrical resistivity test fixture were bought with the financial support of the State Planning Organization of Turkey.