CHAPTER 7 DEVELOPMENT OF METALLIC WIRE/CORE SPUN YARN BASED KNITTED FABRICS FOR EMI SHIELDING

Transcription

1 86 CHAPTER 7 DEVELOPMENT OF METALLIC WIRE/CORE SPUN YARN BASED KNITTED FABRICS FOR EMI SHIELDING 7.1 INTRODUCTION Knitted structures are progressively built-up from row after row of intermeshed loops. The newly-fed yarn is converted into a new loop in each needle hook. The needle then draws the new loop head first through the old (fabric) loop, which it has retained from the previous knitting cycle. The needles, at the same time, release, (cast-off or knock-over) the old loops so that they hang suspended by their heads from the feet of the new loops whose heads are still held in the hooks of the needles. A cohesive knitted loop structure is thus produced by a combination of the intermeshed needle loops and yarn that passes from needle loop to needle loop. This investigation focuses on the effect of metallic wire based knitted fabric construction on electromagnetic shielding behaviour. The study involves production of metal wire based core spun yarn by ring spinning and production of knitted fabrics by plating of metallic wire and also using core spun yarn. Fabrics were produced in different geometry (loop length, thickness and weight/unit are) to study their effect on EMI shielding. Plated knit structure is produced by introducing bare metallic wire along with other yarn in the feeder. A plated structure contains loops composed of two (or more) yarns, usually with differing physical properties. Each has been separately supplied through its own guide or guide hole to the needle hook, in

2 87 order to influence its respective position relative to the surface (technical face and technical back of the fabric). Plating (as an all-over effect or on selected stitches) may be used to produce surface interest, coloured patterns, open-work lace or to modify the wearing properties of the structure (David Spencer 2001). Knitting of core spun yarn is easier showing an improved knitting performance with reduced friction at various guiding elements when compared to knitting a bare metallic wire. EMI shielding is very critical in the hospital environment. EMI may interfere with other life saving electronic devices like EEG, MRI, physiotherapy equipments etc, causing malfunction or failure which might be fatal. Electromagnetic shielding fabrics, especially when used in hospital environment to protect sensitive medical devices and the personnel are a source of microbial cross infection. It is well documented that silver ions have the ability to kill many pathogenic microorganisms. Since knitted fabrics developed for this study comprise silver coated copper wire, an assessment of antibacterial activity as per AATCC 147 and AATCC 100 test method was carried out on fabrics. 7.2 CORE SPINNING OF METAL COMPOSITE YARN Core spinning is a process by which fibers are twisted around an existing yarn either filament or staple-spun yarn, to produce a sheath-core structure in which the already formed yarn is the core. Core yarns are usually two-component structures, one forming the yarn core and the other the covering. Generally, a continuous filament yarn is used for the core and staple fibers as the sheath covering. Core yarns are usually used to enhance functional properties of fabrics, such as strength, durability, and, in the case of an elasticated core, stretch-comfort (Carl Lawrence 2003). For this research

3 88 work, core spun yarn was produced in ring spinning machine using a silver coated copper wire and cotton roving. Ring spun conductive core yarn was produced in a computerized miniature short staple ring spinning machine (Trytex) from commercially available silver coated copper wire and cotton roving. Schematic core spinning is illustrated in Figure 7.1. Pre tension Conductive core yarn Cotton roving Front roller Core spun yarn Figure 7.1 Schematic Core Spinning Process Cotton roving with linear density of 226 tex was fed from the back roller of the drafting zone and the metallic wire under a pretension 0.5 cn/tex using a disc type tensioner was fed to the front roller. Drafted fibres of cotton roving and metallic wire merges at the front roller nip and delivered as core spun yarn. A twist multiplier (English count) of 3.1 was maintained with a spindle speed of 8000 RPM. Physical characteristics of metal wire and ring spun core yarn and its components are given in Table 7.1.

4 89 Table 7.1 Physical Characteristics of Metallic Wire and Ring Spun Core Yarn Description Metallic wire (core) Cotton roving (sheath) Ring spun core yarn Material Silver coated copper wire 100% cotton 30% metal wire and 70% cotton fibres by weight Count 120 dtex 227 tex 38 tex Diameter of yarn mm - - Conductivity (S.m/mm 2 )/linear resistance (Ohm/m) 58.5/ DEVELOPMENT OF KNITTED FABRIC SAMPLES Plated structure knitted fabric samples were produced by feeding the metallic wire and cotton yarn simultaneously to the needles. Another set of single jersey fabric samples with differing loop length were produced from core spun yarn Knitting by Plating of Metallic Wire Metallic wire was incorporated in to the knitted fabric by a process known as Plating. Metallic wire linear density of 120 dtex along with cotton yarn of 16.4 tex were fed to the knitting machine through two separate yarn guides positioned in such a way to feed to the same needle hook for producing single jersey plated fabric. A lab model tubular knitting machine with cylinder diameter of 3.75 inch having 294 needles was employed to knit samples.

5 Knitting of Metal Composite Core Spun Yarn Single jersey knitted fabrics in two different loop lengths from core spun conductive yarn were produced. Loop length of fabric is altered by adjusting the index dial of the knitting machine. The same lab knitting machine was used to knit core yarn. 7.4 TESTING OF PHYSICAL AND MECHANICAL PROPERTIES OF KNITTED FABRICS Various physical and mechanical properties of knitted fabrics like loop length, weight and thickness, metal wire content by weight, air permeability and bursting strength were evaluated by standard test methods and procedures. Air permeability of the fabrics was tested as per ASTM D test method. The bursting strength of fabric was evaluated as per ASTM D3786 test method using hydraulic diaphragm tester. The specifications and properties of knitted fabrics are furnished in Table 7.2. surface morphology of the plated and core spun yarn knitted fabrics were studied by scanning electron microscopy (SEM) in different levels of magnification. 7.5 MEASURING ELECTROMAGNETIC SHIELDING EFFECTIVENESS OF KNITTED FABRICS There are several systems for measuring the plane-wave shielding effectiveness of materials, such as a shield room, a coaxial transmission line holder and time domain signals (Cheng 2000). In this study, EMSE testing of fabrics due to plane wave (far field condition) was carried out as per ASTM D4935 standard using a network analyzer (Agilent 5061A ENA Series RF Network Analyzer) supported by VBA (Visual Basic for Application) macro

6 91 program connected to a coaxial transmission set up to hold specimen as shown in Figure 7.2. Figure 7.2 EMSE Test Setup as per ASTM D4935 Standard The range for frequency sweep was from 30 MHz to 1500 MHz. The purpose of shielding effectiveness (SE) test is to determine the transmission due to introducing a material between the source and signal analyzer. SE is determined by measuring the electric field strength levels with both load (E L ) and reference (E R ) specimens; this is with and without shielding material, respectively as in Equation (7.1): SE = 20 log 10 ( E L /E R ) = (db) L (db) R (7.1) Scattering (S) parameters of the two port measuring system, S 11 (Reflection) and S 21 (Transmission) were recorded in graph form to analyse the shielding behavior of fabrics. S-parameters are used to evaluate how signals are reflected by and transferred through the specimen. Two types of loss are encountered by an electromagnetic wave striking a metallic surface. The wave is partially reflected from the surface, and the transmitted (non-reflected) portion of the wave is attenuated as it passes through the

7 92 shield. This latter effect, called absorption or penetration loss, is the same in either the near or the far field and for electric or magnetic fields. Reflection loss, however, is dependent on the type of field, and the wave impedance. Reflectance (S 11 ) parameter otherwise known as reflection (return) loss is the ratio of reflected to incident signal and transmission (S 21 ) parameter is the ratio of transmitted to incident signal. Both parameters are expressed in log magnitude giving negative values. With S 21 in db, its negative is Insertion Loss or Shielding Effectiveness, and represents the loss suffered in the transmission. The total shielding effectiveness of a solid material with no apertures is equal to the sum of the absorption loss (A) plus the reflection loss (R) plus a correction factor (B) to account for multiple reflections in thin shields. Total shielding effectiveness therefore can be written as, S = A + R + B db (7.2) All the terms in Equation (7.2) must be expressed in decibels. The multiple reflection factor B can be neglected if the absorption loss A is greater than 9 db. From a practical point of view, B can also be neglected for electric fields and plane waves (Henry W. Ott. 2009). In this study, the plots for S 11 and S 21 parameters were obtained for the frequency range of 30 MHz to 1500 MHz. 7.6 ASSESSMENT OF ANTIBACTERIAL ACTIVITY Electromagnetic shielding fabrics, especially when used in hospital environment to protect sensitive medical devices and the personnel are a source of cross infection. It is well documented that silver ions have the ability to kill many pathogenic microorganisms. Since knitted fabrics developed for this study comprise silver coated copper wire, an assessment of antibacterial activity as per AATCC 147 and AATCC 100 test method was

8 93 carried out on fabrics. Fabric knitted by plating of metallic wire (SJP) and core spun yarn knitted fabrics (SJC) were evaluated for antibacterial activity AATCC 147 Test Method The Parallel Streak Method has filled a need for a relatively quick and easily executed qualitative method to determine antibacterial activity of diffusable antimicrobial agents on treated textile materials. As per the standard test procedure, two test strains namely, E.coli: 2.7 X 10 9 cfu/ml and S. aureus: 2.4 X 10 9 cfu/ml were used for the study. All the plates containing the control and specimen samples with the test strains were observed for zone of bacteriostasis after 24 hours of incubation. Measuring the zone of inhibition is a qualitative method of evaluating the antimicrobial efficacy AATCC 100 Test Method This test method provides a quantitative procedure for the evaluation of the degree of antibacterial activity. Quantitative evaluation also provides a clearer picture for possible uses of such textile materials. Figure 7.3 Broths Used for AATCC 100 Test Method

9 94 With the same inoculum (E. coli: 2.7 X 10 9 cfu/ml and S. aureus: 2.4 X 10 9 cfu/ml), three different culture flasks namely un-inoculated, inoculated with and without fabric for each bacteria was prepared as in Figure 7.3 and incubated for 18 hours. From the bacterial count assessed using the template, the reduction percentage was calculated taking control (without fabric) into consideration. 7.7 RESULTS AND DISCUSSION Physical and Mechanical Properties of Knitted Fabrics Some of the physical and mechanical properties of metallic wire plated and core spun yarn knitted fabric samples are given in Table 7.2. In spite of the smaller loop length, plated fabric has lower values for thickness and weight/unit area because of the finer cotton yarn used for plating. The smaller loop length of the plated fabric (SJP) resulted in higher metal wire content in the fabric. Similarly, between the core spun yarn fabrics, a smaller loop length of closely knit fabric (SJC) has higher metal wire content than open knit fabric (SJO). The advantage of using conductive core spun yarn is its better knitting performance besides protecting the core component. Air permeability of SJP fabric is approximately three fold when compared with core spun yarn knitted fabrics. Improved cover caused by core spun yarn structure resulted in lower air permeability of core spun yarn knitted fabrics. Bursting strength of plated fabric is lower by approximately 20% than core spun yarn knitted fabrics. This could be due to the lower strength of the individual yarn in the plated structure when compared to the core spun yarn strength. Fabric weight (grams/ m2) in the case of plated knitting is lower due to finer yarn counts in spite of the smaller loop length.

10 Table 7.2 Physical and Mechanical Properties of Knitted Fabrics Description Single jersey plated fabric (SJP) Single jersey close knitted (SJC) Single jersey open knitted (SJO) Yarn Type and Linear Density Metallic wire 120dtex, Cotton yarn 16.4 tex Core spun yarn 37.9 tex Core spun yarn 37.9 tex Wales per inch Courses per inch Loop Length (cm) Fabric Weight (gram/m 2 ) Metal Wire Content of Fabric (gram/m 2 ) Air Permeability cm 3 / sec/cm 2 Bursting Strength (Kg/ cm 2 )

11 96 The surface morphology of plated fabric was studied by SEM (Figure 7.4). The metallic wire mono filament (indicated by an arrow) is fully exposed on the surface of the fabric. The voids formed between the loops as seen from Figure 7.4 resulted in higher porosity of plated fabric. Metal Wire Figure 7.4 SEM Image of Fabric Knitted by Plating of Metallic Wire The SEM image of core spun yarn fabric knitted with a larger loop length is shown in Figure 7.5. Figure 7.5 SEM Image of Core Spun Yarn Knitted Fabric

12 97 It is evident from Figure 7.4 that porous structure of plated jersey fabric resulted in significantly higher air permeability (Table 7.2) when compared with core spun yarn knitted fabric with improved cover (Figure 7.5). The core spun yarn fabrics showed comparatively higher bursting strength values than plated fabric which could be attributed to the improved cover and the use of core spun yarn Shielding Effectiveness of Knitted Fabrics Shielding behaviour of Plated knit fabrics The S 11 and S 21 parameters of fabrics knitted by plating technique using silver coated copper wire along with cotton yarn are depicted in Figures 7.6 and 7.7. Figure 7.6 S 11 (Reflection) Plot of SJP Fabric From Figure 7.6, the values of reflection loss (RL) is close to 0 db from 30 to 180 MHz and again from 240 to 912 MHz meaning reflection loss is close to 100% where the maximum reflection occurs. In between there is

13 98 a peak of -15dB at 210 MHz which corresponds to reflected signal amplitude of approximately 18% of incident signal indicating the lowest reflection. The cause for this peak is attributed to the resonance due to fabric geometry. Then RL decreases from -3 db to -8 db in the frequency range of 912 to 1060 MHz which means reflected signal amplitude is reduced from 70% to 40% of incident signal. RL increases to -5 in the region of 1130 to 1350 MHz and finally touches -3 db at 1500 MHz. The reflection loss at the interface between two media is related to the difference in characteristic impedances between the media. S 11 plot (Figure 7.6) is in agreement with reflection loss for plane waves are greater at low frequencies and for high conductivity material. Impedance mismatch between free space (air in the case of coaxial transmission) and shield decreases as the frequency increases resulting in lower reflection loss. S 21 parameter of SJP fabric is shown in Figure 7.7. At lowest frequency of 30 MHz, transmission is -100 db meaning shielding effectiveness (SE) or attenuation is very close to 100% or %. A steep increase in transmission is noticed upto 210MHz, however safer 99% shielding is obtained in the frequency range MHz. The -40 db SE mark is the preferred limit for solving many of EMI problems associated with radiated emission. S 21 values between -40 and -20 db is maintained in the frequency range of MHz indicating shielding in the range of 99 to 90%. From 870 MHz frequency, transmission increases slowly from -20 db to reach highest value of -9 db at 1065 MHz. This suggests that the maximum electromagnetic wave transmission through fabric is 35% of incident signal resulting in the lowest SE obtained at 1065 MHz. From this point onwards, transmission is almost flat with very low rate of decrease and finally touches - 13 db at 1500 MHz indicating transmitted signal amplitude of 22% of the incident signal. In total, maximum amplitude of transmitted wave through SJP fabric is 35% of incident signal at 1065 MHz.

14 99 Figure 7.7 S 21 (Transmission) Plot of SJP Fabric The drop in shielding at higher frequencies is mainly due to lower reflection loss as seen from Figure 7.6. Shielding is absorbent dominant at higher frequencies with low reflection component. When an electromagnetic wave passes through a medium, its amplitude decreases exponentially and decay occurs because currents induced in the shield produce ohmic losses and heating of the material. General expression for absorption A loss is given by the Equation (7.3). A 3.34t f r r db (7.3) where t is the shield thickness in inches, f is the frequency in Hertz and r and r are relative permeability and conductivity of the shield material respectively. It is understood from the Equation (7.3) that absorption loss is proportional to thickness and frequency.

15 Shielding behaviour of close knit metal wire based core spun yarn fabrics Figure 7.8 represents the reflectance behaviour of SJC fabric. Reflection loss is maximum and close to zero upto 250 MHz resulting in more than 99% of incident signal being reflected and a peak of -8 db at 300 MHz indicates approximately 40% reflection of incident signal. Compared to SJP fabric, there is a shift in the peak towards higher frequency which may be attributed to the presence of dielectric (cotton) material in core spun metallic wire. From 330 MHz to 765 MHz, the reflection loss lies between 0 and 1 db. RL starts declining at 912GHz and attains a minimum value of -19 db at 1080 MHz resulting in approximately 89% of signal entering the fabric. From then onwards, the reflection loss increases steadily and touches -3 db at 1500 MHz which corresponds to reflected signal amplitude of 70% of the incident. Compared to SJP fabric, SJC fabric showed lower reflection loss at high frequencies. Figure 7.8 S 11 Plot of SJC Fabric

16 101 Figure 7.9 shows the shielding characteristics of SJC fabric. Transmission is -80 db at 30 MHz and reaches the lowest value of -90 db at 80 MHz resulting in maximum shielding of nearly 99.99%. From 80 MHz, transmission increases steeply to a high value of -7 db at 290 MHz. Minimum electromagnetic shielding of 99% (-40 db mark) is maintained in the region of MHz. The trend reverses showing a decline in transmission from 330 MHz and touches -35 db recording a second maximum shielding of approximately 98% at 545 MHz and then slowly increases to the highest transmission of -3 db offering the lowest SE at 1060 MHz which corresponds to a shielding of 30% of incident signal. In the frequency region of MHz and below -20 db line, minimum of 90% shielding is maintained. From 1050 MHz, transmission slowly descends to -10 db at 1500 MHz resulting in transmitted signal amplitude of 30% of incident signal. In the case of SJC fabric, maximum amplitude recorded for transmission is 70% of incident signal at 1060 MHz. Figure 7.9 S 21 Plot of SJC Fabric

17 Shielding behaviour of open knit metal wire based core spun yarn fabrics Figure 7.10 is the plot for S 11 parameters of SJO fabric. RL of SJO fabric is close to zero mark almost similar to SJC fabric upto 250 MHz. The RL peak of -22 db at 300 MHz a lower reflection compared to SJP and SJC fabric resulting in only 8% of incident signal amplitude got reflected. Higher reflection loss is justified by the lower metal wire content of SJO fabric as seen from Table 7.2. Figure S 11 Plot of SJO Fabric The frequency at which the resonance peak occurs is identical for both the core spun yarn fabrics. RL is almost flat and between 0 to -2dB in the frequency range of MHz as like other fabrics. Reflected wave amplitude is 78% of the incident at -2 db. RL declines from 765 MHz and attains the lowest value of -23 db at 1065 MHz where reflected wave amplitude is only 7% of incident wave. From then onwards, there is a steady increase of RL to around -4 db at 1500 MHz. Both core spun yarn knitted

18 103 fabrics showed lower RL spread over a wider frequency range in the high frequency region compared to SJP fabric. Figure 7.11 demonstrates the transmission characteristics of SJO fabric having largest loop length and lowest metal wire content. Lowest transmission is -83 db at 30 MHz which is the highest attenuation obtained for the fabric. Out of three fabrics studied, the maximum attenuation is lower for SJO fabric which is caused by low metal wire content as seen from Table 7.2. Transmission moves up steeply and attains a value of -8dB which is identical to SJC fabric. Maximum -40dB mark is maintained in the frequency region of MHz which is similar to SJC fabric. Transmission decreases from 330 MHz and reaches -34dB at 545MHz and then slowly increases to the highest value of -5dB at 1060MHz. A transmission of -20dB and below is maintained in the frequency region of MHz. From 1060MHz, it decreases gradually to nearly -10dB at 1500MHz resulting in transmitted wave amplitude of 30% of incident signal which is similar to SJC fabric. Figure 7.11 S 21 Plot of SJO Fabric

19 104 There is no marked difference observed in the shielding behaviour between SJC and SJO fabrics and only a marginal increase in maximum shielding and a little wider electromagnetic spectrum giving TL of -20 db or low is observed in the case of SJC fabric which could be attributed to its higher metal wire content by weight (Table 7.2). SJP fabric exhibited better shielding because of the electrical inter-connectivity of loops formed by bare metallic wire used for plating. While comparing the plots of S 11 and S 21 it is observed that SE values are lower wherever the resonance in reflection (inverted peak) or lower RL occurs. Unlike the SJP fabric, SJC and SJO fabrics show resonance peaks in wider band width for reflection (Figures 7.8 and 7.10) in the higher frequency region which could be caused by the presence of core spun yarn. It was established (Ben Munk 2000) that the size, shape and thickness of conducting element and the dielectric thickness are responsible for resonance in a particular frequency. In this study, loop size, fabric thickness and the presence of dielectric (cotton) are responsible for resonance. It was reported by Jung Sim Roh et al (2008) that capacitive coupling at higher frequencies creates a conductive mesh network. This is in good agreement with the comparable shielding effectiveness of core spun yarn fabrics and plated fabrics at high frequencies. 7.8 ANTIBACTERIAL ACTIVITY OF KNITTED FABRICS Zone of bacteriostasis as per AATCC 147 was not noticed on plates after 24 hour of incubation against the two test strains, Escherichia coli (ATCC 11229) and Staphylococcus aureus (ATCC 6538). However, as per AATCC 100 test method, the bacterial reduction was 60% and 63% for plated and core spun yarn knitted fabrics respectively against E. coli but the reduction was zero against S. aureus. It is inferred that silver coat on copper wire is not leaching out on plates in AATCC 147 test method resulting in no bacteriostasis but the silver is able to form enough ionic concentration in

20 105 broth of AATCC 100 test method resulting in 60 63% reduction against E. coli which is Gram negative bacteria However the concentration of silver ions is not enough to act against S. aureus which is a Gram positive bacteria. 7.9 CONCLUSION Single jersey knitted fabrics in different configuration were produced from plating technique of metallic wire as well as knitting of metallic wire based core spun yarn produced for the purpose. Air permeability of plated knit fabric was significantly higher by 200% than core spun yarn due to the porous structure as studied by SEM. Core spun yarn knitted fabrics demonstrated 20% increase in bursting strength than plated knit fabric because of the improved cover resulting from core spun yarn. Smaller loop length of fabrics resulted in higher metal content by weight and hence higher electromagnetic shielding. Plated knit fabric exhibited better electromagnetic shielding than core spun yarn fabrics due to its higher metal wire content and improved electrical connectivity of loops formed by bare metallic wire. All knitted fabrics exhibited transmission of -40 db or lower (Equivalent SE of 40 db or above) in the low frequency range of 30 MHz to 250 MHz which solves many of the practical EMI related problems. Core spun yarn knitted fabrics showed lower reflection loss in wider bandwidth at high frequencies. Fabric with larger loop size (SJO) demonstrated the lowest reflection loss because of lower metal wire content. Metal wire based fabrics showed a moderate antibacterial activity of 60-63% bacterial reduction against E-coli as per AATCC 100 test method due to the presence of silver coating. Knitted fabrics comprising silver coated copper metallic wire and textile fibre can be tailored to make curtains, window screens, antielectrosmog apparel, packagings and enclosures for electric/electronic devices to shield against radio frequency interferences. With antimicrobial property these fabrics are ideal for shielding medical devices.