Water Vapor Transmission and Electromagnetic Shielding Characteristics of Stainless Steel/ Viscose Blended Yarn Woven Fabrics

Abstract
In this study, stainless steel/viscose blended yarn was prepared and different structured woven Fabrics were prepared for studying the moisture transmission and electromagnetic shielding behaviour. By doubling viscose spun yarn with SS filament yarn, the SS/viscose blended yarn was prepared. The woven fabrics were made in a sample loom using viscose yarn and SS/viscose blended yarn. By changing the metal content, thread density and conductive fibre proportions at different levels, the developed fabrics were analyzed for maximum shielding effectiveness in the frequency of 300 kHz to  1.5 GHz. The fabric having conductive threads in warp and weft directions showed larger shielding effec- tiveness (SE) compared to fabric having conductive threads in one direction. The increase in weft density, proportions of conductive threads (in weft direction) and metal content increases the shielding level of fabric. The highest SE of 56 dB was observed for plain woven fabric compared to 3/1 twill, 2/2 twill and 2/2 basket fabrics in the frequency of 700 MHz. The influence of environmental factors such as relative humidity and pH on shielding behaviour of fabrics were also studied. As the relative humidity was increased, the SE was also increased. The fabric treated with acidic (or) basic condition exhibited better SE than the fabric in neutral condition. Similarly, air permeability and water vapour transmission characteristics of the developed conductive fabrics were also analyzed. The air permeability of the fabric was higher when the metal content in the fabric was low. The fabric having more floats showed higher air permeability compared to fabrics with less floats. Similarly, the water vapour transmission rate was also high for long float fabrics. The developed conductive fabrics could be used as wall covering and personal protective clothing in defense industry.

Introduction
In recent times, electrical and electronic equipments operate at wide frequency range of electromagnetic waves due to the developments in 21st century which is IT century and information revolution period. Hence, exposing of the electronic devices to unwanted electromagnetic radiation has increased. However, exposure for long period is hazardous that creates many problems in humans like headaches, brain tumors, fatigue, alzheimer’s disease, dizziness, miscarriage, allergies, leukemia, cancer and sleep problems.1 When the electric and electronic components are exposed to such radiation for longer duration, it may damage the exposed device or lead to improper functioning of the device. Hence, electromagnetic interferences generally create problems in the electronic devices as well as affect the biological life of a person by creating harmful effect on people, animal and environment. For example, the radiations from microwave ovens, Wi-fi and X-rays have disturbing effect on functioning of living organisms. For example, radiofrequency (RF) electromagnetic waves may interact with biological tissue through a number of mech- anisms such as excitation of molecular vibrations by RF fields, changes in protein conformation and changes in binding ligands such as ca2+ to cell receptor proteins.2 When an electromagnetic (EM) wave enters into an organism, it creates vibration between molecules and releases the heat. As a result, obstruction to the generation of DNA and RNA cells occurs which damages the cell growth and causes harmful effects.3,4 Usually, the RF field interact with cells by means of thermal or non-thermal mechanisms. In thermal mechanism, the change in temperature is observed in the biological tissue by exposing to RF fields. This is due to the changes caused in biochemical reactions as a result, energy transfer in tissue which ultimately increases its temperature. For example, the specific absorption rate recorded for the tissue density of 1 × 103 kg/m3 and resistivity 1 Ωmis 10 W/kg.2

Generally, the electromagnetic radiation is classified in two ways (i.e), natural and manmade. Natural source of electromagnetic radiation is the sun. The man-made sources which emits  the electromagnetic waves are electronic typewriters, electric motors,

television, cathode ray tube (CRT), electronic calculators, digital printers, radars, personal computers, internet modems, magnetic resonance imaging, FM/AM radio, base stations, mobile phones (900 to 1800 MHz),Wireless LAN (1–2 GHz), satellite commutations (4– 8 GHz),radar communication (1 to 10,000 MHz),microwave oven (2450 MHz) and other electric and electronic items.5–7 Out of these ranges, uses of mobile phone is very wide.8 Similarly, other commercial bands such as 900 MHz GSM,  1800 MHz GSM and 2400 MHz Wi-Fi in which most of the electronic products operate.9 Hence, shielding/ arresting the EM radiation which disturbs the functioning of nearby system (or) living being is essential. As the EM radiation has both electric and magnetic field,it is necessary to attenuate both the field.10  In order to arrest the electromagnetic radiation, different materials have been adopted as shielding materials such as metal foams, foils, ferrite materials, carbon nanotubes (CNT), graphene, carbon, etc.11  The demands for light weight and flexible shielding materials make the conductive fabrics as most prominent electromagnetic barrier materials.12 The shielding behaviour of conductive woven fabrics depends on various fabric related parameters such as number of layers, metal content, number of apertures, aperture size and its distribution, fabric thickness, yarn count and thread density.3

In a study, Perumalrajetal.13 prepared copper based conductive core-spun yarn with cotton as sheath material. They observed that shielding efficiency depends on the diameter of the copper wire. As wire diameter increases, the shielding effectiveness (SE) decreases due to high bending rigidity of yarn which results in more open spaces inside the fabric. They also reported that number of fabric layers, thread density and cover factor of the fabric have positive impact on the shielding level of the fabric sample. As the pick density increases, the SE is also increased due to increase in copper content per square meter of fabric. Similarly, large cover factor leads to more number of threads covered per unit area in the sample and hence higher SE is observed. In case of weave is considered, twill woven shows higher SE than plain woven fabric due to more float length. As a result, it leads to grouping of yarn and hence lower porosity and SE are observed in the frequency range of 600 – 18000 MHz. However, this is not true for low frequency range i.e. 20– 200 MHz where plain woven fabric shows better SEthantwill woven fabric. On the other hand, Ozdemir and ozkur14 developed the core-sheath yarns having stainless steel in core and cotton fibre as sheath material. The various grid structures have been formed by introducing conductive yarns in certain intervals. It is observed that cellular woven fabric shows higher SE than 3/1 twill woven fabrics when the weft yarns are horizontal to the antenna polarization.

Similar to core-sheath conductive yarns, the shielding behaviour of conductive fibre blended yarns have been investigated for shielding effectiveness. Liu et al. 6 prepared stainless steel and cotton blended yarn (15%/85%) and made the plain, twill and sateen woven fabrics. The developed fabric samples were tested for SEin the frequency range of 1300 MHz to 3500 MHz using waveguide testing method. It is observed that the cover factor of the fabric increases, the SE is also increased regardless of yarn linear density. This is due to presence of more contact points by yarn hairiness. When the cover factor reaches the stable value, no further increase in SE is observed. In case of weave structure is considered, the SE of plain weave is better than that of twill and satin weaves. As the float makes the loose and large interstices in the fabric, the EM wave leak is observed.15 In another study, Das et al.16 prepared SS/PET hybrid yarn (80:20) using stainless steel and polyester fibers and developed the woven fabric for shielding the frequency range of 300 kHz to 1500 MHz. This study revealed that shielding level could be improved by increasing the content of conductive yarn, pick density and number of fabric layers. When the fabric density increases, the SE is also increased. In addition, more float length of yarn helps in grouping of threads which reduces the porosity and increases the shielding level.

Similarly, the conductive clothing produced from textured steel yarns were studied by Ozdemir et al.10 The developed crepe woven fabrics having conductive yarn floats in the diagonal manner shows good SE of 25 dB around 1800 MHz. Similarly, the 3/1 twill fabric shows higher SE of 15 dB than 2/2 twill woven fabrics in the frequency of 2400 MHz despite the average float length is same. As the conductive yarn floats are seen in diagonal or symmetrical manner, which increases the interactions among the con- ductive yarns and hence higher SE is observed. When the density of conductive weft yarn is decreased, no significant change in SE is observed for most of the weaves. The de- veloped fabric can be used for attenuating the EM signal from 1800 GSM, Wi-Fi, ra- diophone, and baby monitors. In order to compare the shielding behaviour of different types of yarn structures, Su and Chern et al.17 produced different types of conductive yarns such as cover yarn, core yarn and plied yarn from SS fiber and PET filament. The different structured woven fabrics made from these yarns were studied for the shielding behaviour in the frequency range of 9 kHz to 3000 MHz. It is observed that the increase in thread density increases the amount of SS content in the fabric which enhances the shielding level. It is also observed that when the conductive yarn exists in one direction of the fabric shows lesser SE compared to the fabric having conductive yarns in both the directions of the fabric. The 1/1 plain and 2/2 twill woven fabrics have smaller square mesh than 3/1 twill fabric which is narrow and longer. Hence, it exhibits better electrical conducting net and good shielding level. Compared to all three types of yarns, the core- yarn has SS filaments in the inner region of yarn which is straight and their distance is shortest. Hence, the core-yarn fabric shows highest SE than other two yarns.

Most of the developed woven shielding fabrics can be used as protective clothing to safeguard the humans from EM radiation in hospitals, industrial workplaces, etc. Hence, the comfort, flexibility and other properties of the fabrics are also important. Several research works have been carried out on comfort, mechanical and shielding behaviour of metal fibre blended conductive fabrics. As proposed by Palanisamy et al.,18 the comfort properties of stainless steel blended yarn shielding fabric is essential for certain applications. The bending moment of the fabric is decreased for the increase in metal fibre content. Likewise, the air permeability and SE of the fabric are also larger for the increase in metal content. Since, the metal fibres are finer compared to PP fibres, the diameter of the yarn having more metal content is less and hence higher air permeability is observed compared to the yarn of lower metal content. So, presence of larger pores in the fabric imparts higher air and water vapour permeability. In another study, Huang et al.19 prepared an electromagnetic wave shield from core-spun yarns having stainless steel (SS) filaments wrapped Ge fibers for functional garments and bedding. Using ring frame, different twist levels of core-spun yarns such as 8 to 12 TPI were prepared. It is observed that 12TPI yarn fabric shows the highest tensile strength of 5.0 N. Similarly, the air permeability of the fabric is also increased when the TPI of the yarn is increased. This is due to presence of lower diameter yarns by high twist levels which increases the air permeability. However, the highest SE of -41.3 dB is observed for the fabric having 9 TPI yarns.

On the other hand, Jaiveer et al.20  prepared the silver/golden coated zari wrapped polyester multifilament yarns for developing the conductive shielding fabrics. The de- veloped fabrics were tested for air permeability and shielding behaviour. The fabrics having three different pick densities and with various fabric structures such as plain, 2/1 twill, 2/2 twill, 5 end satin and honey comb were developed. It is observed that as the float length increases, the electrical resistivity of sample is decreased due to presence of less crimp. Similarly, an increase in amount of conductive yarn in the fabric leads to higher shielding level. In case of silver and gold coated yarn fabric, honeycomb structure shows highest SE than other weave structures due to presence of more floats. As a result, grouping of yarns occur, that leads to reduced porosity and larger thickness of fabric. Similarly, the effect of relative humidity (RH) on SE of fabrics were also studied. The fabric saturated with 100% RH shows high SE due to increased electrical conductivity. Despite, the effect of thickness on air permeability of the samples has not been studied. In another study, Bedeloglu21 prepared the acrylic and SS based hybrid yarn fabrics for shielding the EM radiation and also tested the air permeability, pilling resistance, thermal resistance and the flexural rigidity of developed hybrid fabrics. As reported in the study, the plain woven fabrics exhibits larger SE of 20 dB especially at high frequencies and higher thermal absorptivity compared to twill woven fabrics. However, the air permeability of twill hybrid yarn fabrics is higher than plain fabrics. The presence of wire in the hybrid yarn makes it tighter and hence more air permeability is observed. In case of abrasion resistance, the SS wire-based hybrid yarn fabric shows low fiber loss (%) and high abrasion resistance (%). Similarly, the use of SS wire-based yarn shows high pilling and thermal resistance resistances compared to 100% acrylic yarnfabrics. In case of shielding behaviour, the plain woven fabric exhibits the SE of 20 dB which is higher than that of twill woven fabrics at high frequencies. Since the developed fabric provides adequate flexibility and comfort characteristics, it could be used for protection, safety and military applications.

In another study, the influence of moisture content of cotton and polyester knitted fabrics on shielding efficiency has been reported.22 It is seen that drying time and type of liquid media have significant effect on SE. It is observed that cotton treated with acidic sweat shows SE of 0.9 dB at 1.5 GHz with a moisture content of 206%. The short falls in the current research is that the effect of metal content and weave structure on air per- meability and water vapour transmission characteristics of viscose/SS blended yarn fabrics have not been investigated. In addition, the effect of pH and moisture content on shielding behaviour of viscose/SS yarn fabrics are also not studied in detail. In the current research, shielding level of stainless steel filament and viscose spun yarn combined hybrid yarn is not investigated. Some of the research works23,24 have reported the effect of fabric parameters such as grid openness, weave structure and fabric pore size on shielding effectiveness. The finding reported by them in few cases are controversial, for example, the effect of weave structure on shielding effectiveness. Some research studies concluded that shielding behaviour is directly proportional to float of the fabric whereas few research works reports the opposite way.13,16,19 Hence, it is essential to investigate the effect of weave structure on electromagnetic shielding behaviour of fabrics. Hence, in this study, effect of conductive yarn content, fabric parameters (weave type, thread density) and environmental conditions (moisture content and pH) on SE have been investigated. The air permeability and water vapour transmission characteristics of the fabrics have also been tested for analyzing the comfort of the fabrics.

Materials and methods
Specification of the yarns
The SS multifilament yarn (14/1×90/200Z/316L) of 108 tex purchased from China 3LTEX was used to produce the conductive fabric. The purchased filaments have inherent oxidation resistance, heat and corrosion resistance, good washability and flexibility which leads to development of comfortable textiles. The 100 % viscose spun yarn of 32 tex produced in LR/6 ring spinning machine was also used. Since, the viscose possesses good moisture absorption properties, it could help in analyzing the moisture transmission char- acteristics of fabrics as well as effect of relative humidity on  shielding behaviour. For producingtheviscose/stainless steelblended yarn, theviscose spun yarn was doubledwith SS filament. The properties of SS multifilament yarn and viscose spun yarn are listed in Table 1.
The microscopic images of yarns captured with the help of Nikon microscope are shown in Figure 1 at the magnification level of 4.5x.
The viscose/SS filament doubled yarn was used as a conductive yarn for preparing the woven fabric samples. The viscose spun yarn was used as a nonconductive thread in the fabric. In order to achieve the required shielding level, the minimum amount of con- ductive thread can be combined with viscose yarn in different proportions in the warp and weft directions of the fabric. The CCI manufactured sample loom was used to prepare 4 different woven fabric samples such as 1/1plain, 2/2 twill, 3/1 twill and basket weave fabric samples. The thread density in warp and weft directions were maintained constantly for all the fabric samples. However, the proportion of conductive yarns were varied. Table 2 shows the conductive fabrics prepared in this study and the proportions of conductive yarns presence in the fabrics.

Table  1.  Specifications of SS multifilament yarn and viscose spun yarn.

Figure 1.  Microscopic views of (a) multifilament stainless steel yarn (b) ring-spun viscose yarn (c) viscose and stainless steel doubled yarn (d) SS/viscose blended fabric. Preparation of woven fabric
samples.

Measurement of air permeability and water vapour transmission rate of fabric samples
The TEXTEST FX3300 air permeability tester was used to measure the air per- meability of developed fabric samples according to ASTM D 737-96 standard.25 The air permeability of fabric was calculated by measuring the rate of airflow passing through a known surface area of 5.93 inch2  under a predetermined air pressure difference of two surfaces. The value of air permeability of fabric is denoted as cm3/ cm2/s.
The water vapour transmission rate of fabric samples was assessed according to ASTM E96by26 using W3/060 water vapour transmission tester. In this tester, with a known surface area of 33 cm2, the vapour transmission rate is measured. Before testing of fabric samples, conditioning of samples were carried out at standard at- mospheric conditions having 65% relative humidity and 27°C temperature for 24 h. The results of air permeability and water vapour transmission rate of fabric samples are shown in Table 3. Totally, 5 number of samples were taken for analysis for each testing. The fabrics developed in this study could be used as shielding cloth to at- tenuate the EM radiation as well as antistatic cloth for various applications. Hence, the air permeability and water vapour transmission characteristics of fabrics have to be investigated.

Table 2.  Specifications of developed fabric samples.

Where, P11- 1/1Plain weave; T3/1- 3/1 twill weave; T2/2- 2/2 twill weave; B22- 2/2 Basket weave.

Measurement of electromagnetic shielding effectiveness
The developed samples were tested for electromagnetic shielding effectiveness according to ASTM D4935 in the vector network analyzer using coaxial transmission line method. This method is very effective in measuring the shielding behaviour of planar material at plane and near field electromagnetic radiation. Figure 2 shows the image of vector network analyzer used in this study. The shielding testis performed in the frequency range of 300 kHz to 1.5 GHz as most of the devices operate in this frequency ranges. In this method, two flanges were used to fix the material in the coaxial transmission line and shielding behaviour was measured using E5061 A vector network analyzer (Agilent technology). Two types of specimens were fabricated i.e, reference and load samples as per the ASTM standard and their SE values were measured in decibel (dB). The following equation is used to measure the total loss (SE) incurred by the fabric samples.
SE(dB) = 10log(P1 /P2 ) = -20log(S21 )
Where, P1 is power transmitted in dB; P2 is the power incident in dB. S21 is the scattering parameter associated with port 1 and port 2.

Table 3.  The levels of conductive yarn content, float, thickness, air permeability and water vapour transmission rate (WVTR) of developed fabric samples.

Figure 2.  Vector network analyzer with coaxial transmission line holder (a & b) used for SE measurement.

Influence of fabric conditioning and pH on fabric shielding behaviour
In order to assess the effect of fabric conditioning on shielding behaviour, three different relative humidity were used. The fabric was conditioned in environment chamber at 0% relative humidity for 60 minutes. Similarly, other fabrics were conditioned at different atmosphere for achieving the 65% and 100% relative humidity. Then, the fabric samples were wrapped with plastic film in the chamber and then placed in the vector network analyzer for assessing the SE. For studying the effect of pH condition on shielding behaviour of fabric samples, they were treated with 3 different chemical conditions such as acid, alkali and neutral conditions. For example, to achieve the acidic condition (pH-3), the fabric was dipped in 100% glacial acetic acid for 30 minutes and then it was taken out and kept in oven at 95°C for 60 min. For achieving the alkali condition (pH-11), fabric was dipped in sodium hydroxide flakes for 30 min and then it was taken out and kept in oven at 95°C for 60 min for drying.

Statistical analysis
The statistical analysis was carried out using one-way Analysis of Variance (ANOVA) to study the effect of fabric parameters and treatments on shielding effectiveness of fabrics. The significance level or p value taken for the analysis was 0.05. If the value of p is equal to or less than 0.05, then the process is considered to be significant which confirms its effect on shielding behaviour of fabrics.

Results and discussion
Effect of stainless steel content and fabric structure on air permeability
As the air permeability of fabric is one of the important factor in deciding the fabric comfort, the effect of fabric thickness on air permeability was analyzed. As the thickness of plain woven fabric is increased from 0.541 mm to 0.569 mm, the air permeability of the fabric is decreased from 70.3 to 61.3 cm3/cm2/sec (Table 3). This is not true for all the fabrics because the fabric structure and conductive yarn content also play significant role in deciding the air permeability of fabrics. Hence, the effects of stainless steel content and structure of fabric on air permeability were investigated. Figure 3 shows the influence of weave structure and metal content on air permeability of fabrics.
As observed in Figure 3, as the metal content increases in the fabric, the air per- meability is decreased. This is due to presence of multifilament stainless steel yarn, which shows decreased intrafibre pore resulting in lower air permeability. The findings of this study is contrast to the results reported by Palanisamy et al.18They reported that small metal fibre diameter makes the fabric more porous which results in higher air perme- ability. Similarly, the influence of fabric weave structure on air permeability was analyzed. As observed in Figure 3, fabric weave has influence on air permeability nature of the fabrics. The plain woven structure shows lowest air permeability compared to twill and basket woven structures due to presence of smaller aperture size by the interlacement of yarns in the fabric. Hence, plain woven fabric shows less thermal comfort compared to twill and basket structures. The highest air permeability of 125 cm3/cm2/s is observed for 3/1 twill fabric structure. This is due to openness of the fabric caused by more floats. In order to confirm the effect of weave type on air permeability of fabric, statistical analysis

Figure 3.  Influence of fabric structure and metal content on air permeability.

Table 4.  ANOVA test results on effect of weave structures on air permeability of fabric.

Where, SS - Sum of squares, df – Degrees of freedom, MS – Mean square, F – f value, P – p value was carried out. Table 4 shows the ANOVA test values obtained to study the effect of weave type on air permeability of fabric. As observed in Table 4, the calculated Fcalc value 54.4 which is greater than Fcric value of 4.06 at 5% significance level with the degrees of freedom of 3. In addition, the p-value is less than 0.05. Hence, fabric structure has significant effect on air permeability of the fabric.
The same observation was reported by Bedeloglu.21 He observed that air permeability of twill structured acrylic and SS based hybrid yarn was higher than plain fabrics. This is due to presence of SS wire in the hybrid yarn which makes the yarn to be tighter that leads to higher air permeability.

Effect of metal content and fabric structure on water vapour transmission rate
The water vapour permeability of fabrics having varying metal contents and fabric structures were analyzed. The effect of metal content on water vapour transfer rate of fabric is shown in Figure 4. As observed in Figure 4, the structure of the fabric influences the water vapour transmission behaviour.

Figure 4.  Effect of metal content on water vapour transfer rate of fabric.

Table 5.  ANOVA test for effect of metal content on water vapour permeability.

Similar to air permeability, the water vapour transmission rate is higher for long float fabrics. The twill 3/1 fabric exhibits the water vapour transmission rate of 6856 g/m2/day which is highest than that of matt and plain fabrics. Hence, twill fabric could provide more thermal comfort than other fabric structures. This is similar to the findings reported by Palanisamy et al.,18 where larger pores presence in the fabric provide higher water vapour permeability. Similarly, the effect of metal content on water vapour transmission rate of fabric samples were also investigated. As the metal content is increased from 10 wt% to 22.5 wt%, the water vapour permeability of fabric sample is decreased. In order to confirm the effect of metal content on water vapour transfer, one-way ANOVA was carried out. Table 5 shows the ANOVA test results obtained for studying the effect of metal content on water vapour permeability.
As observed in Table 5, the calculated Fcalc value 6.50 which is greater than Fcric value of 4.25 at 5% significance level with the degrees of freedom of 2. In addition, the p-value is less than 0.05. Hence, metal content has significant effect on water vapour permeability of the fabrics. Similarly, the effect of areal density on air permeability and water vapour transmission characteristics of developed fabrics was analyzed. From Table 3, it is

Figure 5.  Influence of conductive yarn proportions on shielding effectiveness of different woven structures.

observed that as the areal density of 2/2 twill woven fabric increases, the air permeability and water vapour transmission rate of fabrics are decreased. The twill fabric with areal density of 117 g/m2  shows the air permeability of 100.7 cm3/cm2/s and water vapour transfer rate of 6241 g/m2/day. However, this not applicable for plain woven and basket woven fabrics due to change in yarn floats and proportion of SS fibre content in the fabric.

Effect of proportion of conductive yarns in weft way on shielding behaviour
Figure 5 shows the conductive yarn proportions and their influence on SE of different structuredfabrics. The fabrichaving0%metal content(NC fabric)is transparentto EM waves and did not stop the EM radiation. But, when the conductive thread is introduced in warp way inthe ratioof1:4(SS:Viscose)inplain woven fabric,theobserved SE is11.2dB at1200MHz (Figure5(a)). Inorder to enhance the shieldinglevel,the conductive SS threadis introducedin warp and weft way of the woven fabrics. When the SS filament is introduced in weft way of plain woven fabric in the ratio of 1:3 (SS:Viscose), the SE is improved to 14.2 dB (Figure 5(a)). Similar observations were made for 2/2 matt, 2/2 twill and 3/1 twill fabrics.
The proportions of conductive SS filaments in weft directions of the fabric were varied such as 0%, 10%, 16.25% and 22.5% by placing the conductive yarn in the ratios of0/0, 4/0, 4/3 and 4/7 respectively. As seen in Figure 5(a), plain woven fabric with increasing conductive yarn proportions from 1:7 to 1:3 in the weft direction shows higher SE. This is due to formation of Journal of Industrial Textiles

smaller conductive grid in the fabric which enhances the attenuation of EM signals.27 As the amountofSS yarn isincreasedfrom0wt%to22.5wt%, the SE isalsoincreasedfrom49.61dB to 56.52 dB in the frequency of 700 MHz. Similarly, by increasing the conductive yarn proportionsfrom1:7to1:3for2/2basket weave, theSE is increasedfrom57.38dBto61.88dB (Figure5(b))duetosmallergrid size. TheSE of2/2twill weave is also increasedfrom 48.61dB to 56.61 dB for the increasing proportions of metal contents. The same trend is observed for 3/1 twill weave fabrics as seen in Figure 5(d). Compared to all woven structures, the largest increasing trend is observed for 3/1 twill woven fabrics due to float effect.
Jaiveer et al.20 and Das et al.16 also reported the effect of metal content on SE and obtained the same trend fortwill and basket woven fabrics. In addition, as the incident
frequency increases, the SE of the fabric is also increased up to 700 MHz, and then it is

Table 6.  ANOVA- Single factor analysis on the effect of conductive yarn proportions on SE.

Where, SS - Sum of squares, df – Degrees of freedom, MS – Mean square, F – f value, P – p value

Figure 6.  Effect of picks per inch on shielding effectiveness of plain woven fabric. Effect of weave type on shielding behaviour.

decreased due to skin effect.28 In order to confirm the effect of conductive yarn proportion on SE, One-way ANOVA was carried out. The ANOVA test results are shown in Table 6.     FromtheANOVAresults(Table6),itisobservedthatthepvalueislesserthan0.05. Hence, it is confirmedthat conductiveyarn proportionhas significant effect on shielding levelofthefabric.

Effect of pick density on electromagnetic shielding effectiveness of fabrics
For studying the effect of pick density on electromagnetic shielding effectiveness, the plain woven fabrics with three different pick densities such as 20, 30 and 40 picks per inch (PPI) were prepared. The amount of SS content presence in the warp way conductive yarn is 10wt% for all the fabrics. The shielding levels of fabric samples having the pick densities of20, 30 and 40 PPI are shown Figure 6. As observed in Figure 6, an increase in pick density increases the shielding effectiveness of the fabric. In fact, a small increase in pick density exhibits slight improvement in shielding effectiveness. However, at higher pick density, larger SE is observed due to presence of more SS content in the fabric. The obtained results are similar to copper based conductive core-sheath yarn fabrics reported by Perumalrajetal.13 They observed that pick density is directly proportional to shielding effectiveness. As the metal content increases for a square meter of fabric, the SE is also enhanced.
Jaiveer et al.20 reported that honeycomb structure shows highest SE followed by 5-end satin, 2/2 twill, 2/1 twill and plain woven fabrics. This is not true for all blended yarn fabrics. In order to study the effect of different weave structure on SE, four different fabrics such as plain, 3/1 twill, 2/2 twill and 2/2 basket were analyzed. Based on the yarn floating pattern, the aperture of the fabric gets changed. Figure 7 shows the effect of weave type on shielding behaviour of fabrics in the frequency of 700 MHz.
As observed in Figure 7, the trend of shielding behaviour is same for 3/1 twill, 2/2 twill and basket woven fabrics. However, the shielding behaviour of plain woven fabric is different from other fabric structures. At 700 MHz frequency range, the plain woven fabric shows the highest SE of 56 dB. For 3/1 twill fabric, the maximum SE of 53 dB is observed in the frequency of

Figure 7.  Effect of weave type on shielding level in the frequency of 700 MHz.

Figure 8.  Effect of relative humidity on shielding effectiveness of fabric.

700MHz. For Basket weave, theobserved SE(45dB)is least comparedto other woven fabrics. This is due to presence of larger aperture size which leads to penetration of more EM waves. Despite the cover factor is same as observed in Table 3, highest shielding effect is observed for plain woven fabric compared to other fabrics. This may be due to change in aperture size and shapes because of yarn interlacement patterns.29,30 The obtained results are contradictory to the findings reported by Jaiveer et al.20 However, the similar findings were reported by Liu and Wang.15 They reported that plain woven fabric exhibits highest SEthan twill and satin woven fabrics. This is due to presence of more interlace points so that the fabric gets closed structure and exhibits high shielding effect. In case of satin woven fabric, least number of interlace points are observed and hence fabric becomes more opened which exhibits poor shielding effect. The same observations are confirmed in this study. Hence, it can be observed that the fabric constituting with more interlace points develop the small apertures and fabric becomes closer. As a result, a higher shielding effect is observed compared to the fabric with less floats. The same trend is observed for all the fabrics irrespective of the metal content.

Effect of moisture on shielding behavior of fabrics
As viscose spun yarn has good moisture content, the effect of moisture on shielding behaviour of fabric has been investigated in this study. In order to analyze the effect of moisture content or relative humidity on shielding effectiveness, samples conditioned with three different relative humidity levels such as 0% RH (fabric kept in oven at 100°C for 65min.), 65% RH (fabric at normal testing condition) and 100% (fabric in saturated condition) were prepared. The shielding levels of prepared fabric samples are shown in Figure 8. As observed in Figure 8, an increase in relative humidity from 0% to 100%, increases the shielding effectiveness of fabrics. The sample conditioned with 100% relative humidity (B22-WT -40-4/7) exhibits better shielding value (43.6 dB) than that of samples condi- tioned with 65% RH (40.5 dB) and 0% RH (38.5 dB). This is due to presence of more moisture in the fabric which increases the electrical conductivity subsequently the shielding level of the fabric. The similar observations were reported by Palanisamy et al.22
They observed that the SE of cotton fabric increases from 0.1 dB to 0.7 dB as the moisture content is increased from 25% to 205%. The same trend is found in this study.

Effect of pH on shielding behavior of fabrics
For studying the effects of acidic and basic conditions of fabric on SE, fabric samples with three different conditions such as pH of3 (fabric dip in 100% glacial acetic acid for 30 min and kept oven at 95°C for 60 min),pH of 7 (fabric test at normal condition) and pH of 11 (fabric dip in sodium hydroxide flakes for 30 min and kept oven at 95°C for 60 min) were prepared. As AISI 316L standard stainless steel is used in this study, it has very good resistance to acids and alkalis.31  The shielding behaviour of acidic and basic conditions of fabrics are analyzed and the results are shown in Figure 9. As seen in Figure 9, the B22-WT-40–4/7 fabric sample with basic condition exhibits almost same shielding effectiveness compared to the sample of acidic condition. However, the shielding effectiveness of untreated fabric is less than the treated fabric sample.22 This

Figure 9.  Effect of pH conditions on shielding effectiveness of fabrics.

confirms that when the pH of treated fabric is changed, the shielding effectiveness of fabric is also changed. This is due to presence of charges due to acid and base conditions which improves the conductivity of fabric samples. In a study by Palanisamy et al.,22 acidic and alkaline treated samples shows similar shielding behaviour at 1.5 GHz which is proved in this study as well.

Conclusion
The electromagnetic shielding characteristics and water vapour transmission behaviour of stainless steel/viscose blended yarn woven fabrics were investigated in this study. The air permeability of the fabric decreases with the increase in SS content of the fabric. The plain woven fabric shows lowest air permeability compared to twill and basket woven fabrics. The fabrics with more floats such as twill and satin woven fabrics exhibit higher air permeability. The water vapour transmission characteristics of developed shielding fabrics were also studied. The twill 3/1 fabric exhibits the water vapour transmissionrate of6856 g/ m2/day which is highest than that of matt and plain fabrics. This is not true when the metal content of fabric is increased. Likewise, the shielding behaviour of all the developed fabrics were analyzed using vector network analyzer inthe frequency range of300 kHz to 1.5 GHz. The peak shielding value is obtained in the mid frequency of 700 MHz which is due to the resonance effect. The fabric having conductive yarns in both warp and weft directions shows higher SE than the fabric having conductive yarns only in warp direction. When the proportions of conductive yarns are changed in weft direction, the SE of the fabric is also changed. Similarly, the effect ofpick density on SE offabric isalso investigated. As the pick density of plain woven fabric is increased from 20 PPI to 40 PPI, the SE of fabric is also increased from 40 to 44 dB. With respect to fabric structures, the plain woven fabric shows highest SE of 56 dB than 3/1 twill (53 dB) and other fabrics at 700 MHz. Hence, it can be observed that SS and viscose blended fabric having more interlace points is suitable for shielding the EM radiation. The effect of relative humidity on shielding behaviour of fabric was also studied. The fabric sample conditioned with 100% relative humidity shows increased SE compared to the fabrics conditioned with 65% and 0% RH. In case of pH is considered, the acidic and alkaline treated fabrics show better SE compared to untreated fabric sample. The developed fabrics in this study could be used as radiation protection clothing and shielded wall covering for high security rooms.