Indian Journal of .Fibre & Textile Research Vol. 36, March 2011, pp. 35-41

Electroless nickel plated composite textile materials for electromagnet compatibility

R Perumalraj a Bannari Amman Institute of Technology, Sathyamangalam 638 401, India and B S Dasaradan Department of Textile Technology, P.S.G. College of Technology, Peelamedu, Coimbatore 641 004, India

Received 16 April 2010; revised received and accepted 8 June 2010

A study on electromagnetic shielding effectiveness of electroless nickel plated copper core with sheath composite fabrics through Taguchi design and ANOVA has been reported. The electromagnetic shielding effectiveness of these conductive composite fabrics has been measured in the frequency range 200 - 1000 MHz using network analyzer equipment. It is observed that with an increase in palladium chloride and nickel (II) sulphate concentrations, the shielding effectiveness increases. The ANOVA results show that the time and temperature are negligible factors as compared to other factors.

Keywords: Composite fabric, Core yarn, Copper, Dref II Spinning, Electromagnetic wave, Nickel plating, Polyester, Shielding

1 Introduction most polymer composites used for household Electromagnetic shielding provides protection by electrical and electronic devices are electrically reducing signals to levels at which they no longer insulating and transparent to electromagnetic radiation affect equipment or can no longer be received 1. This and electrostatic discharge (ESD). The potential is achieved by reflecting and absorbing the radiation. health hazards (e.g. cancer) associated with exposure Polymeric composites are extensively used as passive to electromagnetic fields 12-17 are also the matter of and active elements in some electrical circuit concerned. To shield and limit against EMI and ESD, components in different technological applications 2, 3 conductive polymer composites started replacing due to their light weight, easy process ability, coated materials for various shielding applications in flexibility, corrosion resistance and cost-effectiveness. the electrical and electronic industries, especially for But elastomers and plastics contain very low electronic household materials. This trend has been concentrations of free charge carriers. They are driven mainly because of the better characteristics of electrically non-conductive and transparent to these polymers in terms of ESD, shielding from EMI, electromagnetic radiation. To provide conductivity thermal expansion, density, and chemical (corrosion and shielding from Electromagnetic Interference and oxidation resistance) properties 18-22 . However, (EMI), incorporation of fillers of high intrinsic conductive polymers have rigid characteristics owing conductivity such as particulate carbon blacks, carbon to their chemical conformation of benzene rings, and graphite fibres, or metal powder to the polymer making their formation difficult. A few studies have matrix is required 4,5 . The amount of electrically reported conductive knitted fabrics reinforced conductive filler required to impart high electrical composites as electromagnetic shielding effectiveness conductivity to an insulating polymer can be (EMSE) and ESD materials. Previously, Cheng et al . 23-26 dramatically decreased by the selective localization of fabricated some reinforced the filler in one phase, or best at the interface of a polypropylene composites for use as EMSE and ESD continuous two-phase polymer blend 6-11 . Unfortunately, materials, and indicated that the knitted fabric reinforced polymer composites are suitable for —————— aTo whom all the correspondence should be addressed. making complex shaped components and application E-mail: [email protected] in electromagnetic shielding 36 INDIAN J. FIBRE TEXT. RES., MARCH 2011

The coating of fabrics is a textile finishing process parameters to improve the EMSE of copper core yarn that adds to the value and improves functions 27 . With with polyester sheath composite fabric through the advent of synthetic fibres, there are now less Taguchi design and ANOVA. expensive ways to make metallic fabrics in the twenty-first century. Metallized fabric, in particular, 2 Materials and Methods has an identity and a quality of brightness that can The fabrics from complex copper core spun create beautifully reflected and lustrous images. The (179 tex) with polyester sheath (copper wire diameter, reflective surfaces can offer a unique appearance as a 0.12mm and core: sheath ratio of copper and result of this technical process and metalized fabrics polyester, 23 : 77) were produced on Dref-II friction are very popular in both the contemporary consumer spinning machines. These copper core / polyester market and in technical applications. Metalized fabric sheath yarns were converted into plain woven fabric has been produced by a laminating process between and the fabric was then treated with electroless nickel metal powder or foil and fabric with binder 28 . plating as per Taguchi design to improve the EMSE Vacuum deposition and sputtering coating processes of composite fabric. In the EN plating process, the have been applied to polyester fabric with aluminium, driving force for the reduction of nickel metal ions titanium and stainless steel 29 and a silver-plated and their deposition is supplied by a chemical product was usually produced by electroplating on the reducing agent in solution. This driving potential is surface of conductive materials 30,31 . essentially constant at all points of the surface of the Electroless plating is a chemical reduction process component, provided the agitation is sufficient to which depends upon the catalytic reduction of a ensure a uniform concentration of metal ions and metallic ion in an aqueous solution containing a reducing agents. The EN plating process was reducing agent, and the subsequent deposition of the basically divided into five main stages, namely pre- metal without the use of electrical energy. The metals cleaning, sensitization, activation, electroless nickel capable of being deposited by electroless plating deposition and post-treatment. In the electroless include nickel, cobalt, copper, gold, palladium nickel plating process, unless otherwise stated, all and silver. chemicals used are of A R grade. Electroless nickel (EN) is an alloy of 88-99% 2.1 Pre-treatment nickel and the balance with phosphorous, boron, or a To facilitate the plating of metal particles on the few other possible elements. EN coatings, therefore, fabric surface, pre-treatment was carried out. The can be tailored to meet the specific requirements of an processes of pre-treatment consist of pre-cleaning, application with the proper selection of the nickel's sensitization and activation. alloying element(s) and their respective percentages in the plated layer. All varieties of EN, including Pre-cleaning composite EN coatings, can be applied to numerous Pre-cleaning is done for operating the electroless substrates including metals, alloys, and nonconductors nickel solution so that required properties are obtained with outstanding uniformity of coating thickness to in the fabric. Additives and/or contaminants can have complex geometries. a profound effect on performance, as will operate Diligent control of the solution's stabilizer content, conditions. In this process, all fabric samples were pH, temperature, tank maintenance, loading, and pre-cleaned in a 2% non-ionic detergent at pH 7 and freedom from contamination are essential for its temperature 40°C for 20 min. Deionised water was reliable operation. EN solutions are highly surface then used to rinse the pre-cleaned samples. Triton X- area dependent. Surface areas are introduced to the 100 as a non-ionic detergent was used. solution by the tank itself, in-tank equipment, immersed substrates, and contaminants. Continuous Sensitisation filtration, often sub-micron, of the solution at a rate of Sensitisation is a process of catalyzing the non- atleast ten turnovers per hour is always recommended conductive substrate. This enables the fabric surface to avoid particle contamination which could lead to to act as a catalyst for the deposition of nickel. The solution decomposition or imperfections in the plated auto-catalysation is obtained through this process. In layer. In this study, attempts have been made to the case of sensitizing fabric surfaces, the cleaned optimize the electroless nickel plated process fabric samples were subjected to the surface sensitizer PERUMALRAJ & DASARADAN: ELECTROLESS NICKEL PLATED COMPOSITE TEXTILE MATERIALS 37

(mixture of 5 g/L stannous chloride and 5M/L hydrochloric acid) with slow agitation for 10 min at 25°C and 1 pH. In sensitization, non-conductive substrate absorbs the stannous Sn 2+ ions from the stannous solution.

Activation Activation is carried out for activating the fabric surface for the nickel deposition. The objective of this process is same as that of sensitization. In this process, a activator solution is used. The sensitized fabrics were rinsed with deionised water subsequently and then immersed in the activator solution [palladium (II) chloride, hydrochloric acid (conc. 37%) and boric acid] at pH 2 and 25°C for 5 min in order to achieve Fig. 1—Palladium cluster enveloped by stannous chloride layers surface activation. The activated fabrics were rinsed solutions results in electroless nickel plating as with deionised water afterwards. demonstrated. After this, nickel particles are adhered In activation, nucleation effect is performed on the on the substrate surface. sensitized substrate in the acid palladium chloride solution (i.e. nucleate solution), where the sensitized 2.3 Post-treatment 2+ substrate carries stannous (Sn ) ions and reacts with Post-treatment is done to achieve the required 2+ palladium (Pd ) ions. The divalent tin on substrate properties on the fabric surface after deposition. In surface is eliminated due to the fact that the sensitized this process, the cleaned nickel-plated fabrics were tin on substrate surface reacts with the palladium salt cured by steaming machine directly at 150°C for 1 in the activation solution. Figure 1 clearly illustrates min. All electroless nickel-plated fabric samples were that the stannous chloride layers envelope the conditioned under standard atmospheric pressure at palladium metal clusters, which is lifted up on 65% ± 2% relative humidity and 21°C ±1° C for at substrate surface. least 24 h before further evaluation.

2.2 Deposition 2.4 Experimental Design Deposition involves reducing the nickel salt into Taguchi method 32,33 was applied to identify the nickel, i.e. the metal ions are reduced as metals by the optimum level of electroless nickel process reducing agent. In case of electroplating, electric parameters for electromagnetic shielding current is used for the reduction process. In this effectiveness performance characteristic individually. deposition, sodium hypophosphite monohydrate is Taguchi method replicates each experiment with the used as reducing agent. During the deposition stage, aid of an outer array that deliberately includes the the nickel plating solution was employed in the sources of variation that a product would come across electroless nickel plating tank, which was composed while in service. Such a design is called a minimum of nickel (II) sulphate hydrate, tri-sodium salt sensitivity design or a robust design. The robust dehydrate, ammonium chloride, a few drops of design method is called Taguchi method. sodium hydroxide (conc. 10%) and sodium To achieve the optimum design factor setting, hypophosphite monohydrate. The activated fabrics Taguchi advocated a combination of two stage were immersed in the nickel plating solution at pH 9 process. First step is related to the selection of with various temperature and time, right after the robustness seeking factors and the second step is metallising reaction of electroless nickel plating. related to the selection of adjustment factors to In deposition, nickel sulphate (metal salt), sodium achieve the desired target performance. Various hypophosphite (reducing agent) and stabilizer in stages in the experimental design have been dealt by strong alkaline comprise a complex nickel plating many researchers in the past. In other experimental bath. The autocatalytic reaction between the nickel designs, uncontrollable factors are kept under ions and the reducing agent in the nickel plating observation during experimentation, whereas Taguchi 38 INDIAN J. FIBRE TEXT. RES., MARCH 2011

methods include those factors in the experimentation experiments were conducted according to the trial to make the design a robust one in the form of signal- conditions specified in L 9 OA. A total of 36 to-noise ratios (S/N). In robust design, sensitivity to experiments (three repetitions at each trial condition) noise is minimized by seeking combinations of design was conducted. parameter settings. The most appropriate S/N ratio Using Taguchi analysis and analysis of variance can be selected depending upon the properties of (ANOVA), the optimal performance parameters were interest (Table 1) for both scaling factors and determined and the optimal values were predicted. adjusting factors. The average values of performance characteristics at The experiments using the controlled and each level and against each parameter were calculated uncontrolled variables at different levels (Table 2) were and are given in Table 3. carried out randomly to avoid the systematic errors. The selection of appropriate orthogonal array (OA) is 2.5 Electromagnetic Shielding Effectiveness Test a critical step in Taguchi’s experimental design. The The EMSE of the conductive fabric was calculated OA selected should satisfy the following criterion: by following ASTM D4935 – 99 test methods. The basic characteristic of the conductive fabric is its Degrees of freedom (DOF) of OA > Total DOF required attenuation property. Attenuation of the Therefore, L 9 Taguchi orthogonal array and 3 electromagnetic energy is a result of the reflection, levels were selected to assign various columns. The absorption and multi-reflection losses caused by a Table 1 — Signal-to-noise (S/N) ratio and its significance 32, 33 specific material inserted between the source and the Case S/N ratio receptor of the radiated electromagnetic energy. 2 2 Attenuation caused by a material is characterized, Target is the best S/N ( θ) = 10 log 10 ( τ / s ) 2 depending on the measuring method used, by the two Small-the-better S/N ( θ) = -10 log 10 (yi / n) 2 Larger-the-better S/N ( θ) = 10 log 10 [(1/yi ) / n] quantities, namely shielding effectiveness (SE) and Binary scale (GO/NO-GO) S/N ( θ) = 10 log 10 (p/1-p) insertion loss (A). p= proportion of good products

Table 2—Controlled and uncontrolled variables 2.5.1 Shielding Effectiveness Shielding effectiveness (SE) is defined as the ratio Controlled variable Uncontrolled Factor Notation variable of electromagnetic field strength ( E0) measured with and without the tested material ( E ) when it separates Level Level Level Noise Noise 1 34 1 2 3 level 1 level 2 the field source and the receptor, as shown below :

Palladium chloride A 17.5 29.2 40.9 - - SE = E0 / E1 grains/gal Nickel (II) sulphate In decibels, B 642.6 759.4 876.2 - - grains/gal Time, min C 5 10 20 - - SE dB = 20log E0 / E1

0 Temperature, C D 30 40 50 - - This depends on the distance between the source N1 N2 Frequency, MHz E - - - (High) (Low) and the receptor of electromagnetic energy. In the far N3 N4 field zone, it characterizes the attenuation of the Aerial density F - - - (High) (Low) electromagnetic wave. The measurement carried out Table 3 — Experimental test setup for optimization of electromagnetic shielding effectiveness Expt. No. Palladium chloride Nickel (II) sulphate Time Temperature N1 N2 N1 N2 S/N grains/gal grains/gal min 0C N3 N3 N4 N4 ratio 1 17.5 642.6 5 30 28 25 26 27 28.44 2 17.5 759.4 10 40 30 26 27 28 28.83 3 17.5 876.2 20 50 34 30 31 32 30.01 4 29.2 642.6 10 50 36 33 34 35 30.74 5 29.2 759.4 20 30 37 34 35 36 30.99 6 29.2 876.2 5 40 41 38 39 40 31.92 7 40.9 642.6 20 40 45 42 43 44 32.76 8 40.9 759.4 5 50 42 39 40 41 32.14 9 40.9 876.2 10 30 44 40 41 43 32.45 PERUMALRAJ & DASARADAN: ELECTROLESS NICKEL PLATED COMPOSITE TEXTILE MATERIALS 39

in the near field zone characterizes the attenuation effectiveness for the electric or magnetic field component only.

2.5.2 Insertion Loss Insertion loss (A) is a measure of the losses (or attenuation) in a transmitted signal caused by the tested material being inserted into the measuring channel. The insertion loss can be measured using the following relationship 34 :

A = U0 / U1

In decibels, Fig 2—Effects of control factors AdB = 20 log ( U0 / U1) carried by palladium chloride in combination with where U0 is the channel output voltage without the HCl and boric acid. Hence, the main control factors tested material; and U1, the same voltage with the involved in this process are palladium chloride, nickel tested material. sulphate, time and temperature.

A spectrum analyzer with coaxial transmission 3.1 Effects of Palladium Chloride Concentration on EMSE equipment was used to measure the EMSE of The palladium has a dominant influence on EMSE the copper core conductive fabric at frequency ranges of the electroless nickel plated copper core yarn 20-200MHz, 200 MHz - 1GHz and 1-18GHz. composite fabrics. Figure 2 shows the effects of

2.6 Test Procedure palladium chloride on copper core/ polyester sheath The tests were carried out in an anechoic chamber. yarn composite fabric. With an increase in Anechoic chamber is a room in which no acoustical concentration of palladium chloride, a general reflections or echoes exist. The floor, walls and ceilings increase in EMSE is observed due to nickel chemical of these rooms are lined with a metallic substance to deposition. Electroless nickel plating is the process of prevent the passage of electromagnetic waves. Radio depositing metallic nickel by controlled chemical frequency signal was generated by a signal generator reactions catalyzed by the metallic nickel being and it was transmitted through an antenna outside the deposited. It produces a film of uniform thickness chamber. Signal from the signal generator was with very good conductivity. The amount of nickel measured by spectrum analyzer with antenna inside the deposition improves the absorption and reflection of chamber. The first measurement (calibration) was electromagnetic wave. The absorption loss (dB) is carried out without the test fabric. The tests in the proportional to the square root of the product of the different frequency ranges were conducted. permeability and the conductivity of the electroless nickel plated copper core yarn composite fabric. If a 3 Results and Discussion magnetic material is used in place of a good Higher shielding effectiveness value indicates the conductor, there is an increase in the permeability and better electromagnetic shielding effectiveness from a decrease in the conductivity. It increases the nickel plated copper core fabric and hence larger - the absorption loss, since the permeability increases more - better S/N is employed to optimize the process than the conductivity decrease for the reflections loss. parameter of electroless nickel plating. Table 3 shows The total loss through a nickel plated copper core/ the orthogonal array of the experimental setup polyester sheath yarn composite fabric is the sum of followed by shielding effectiveness and the relevant loss due to absorption and that due to reflections by S/N ratio. Figure 2 shows the effects of electroless the composite fabric. The ANOVA carried out for nickel plating process parameter on shielding EMSE of fabric shows the dominating effects on effectiveness values in terms of S/N ratio. The palladium chloride concentration (Table 4). efficiency of electromagnetic shielding depends on the amount of nickel deposited over the fabric surface. 3.2 Effect of Nickel (II) Sulphate on EMSE This amount of deposited nickel depends an surface The nickel (II) sulphate concentration has a activation. The activation of the fabric surface is significant influence on EMSE of the electroless 40 INDIAN J. FIBRE TEXT. RES., MARCH 2011

Table 4 — ANOVA factors effects and F value of response variable for electromagnetic shielding Control factor Degree of F before Empty or pooled F after Nature Optimum % Factor freedom pooling F=<1.5 pooling level effects Palladium chloride, grains/gal 2 44 No - Dominant 0.7 88 Nickel (II) sulphate, grains/gal 2 3 No 35 Significant 15 7 Time, min 2 1 Pooled 3 Neutral/negligible - 3 Temperature, 0 C 2 1 Pooled - Neutral/negligible - 2 nickel plated copper core / polyester sheath yarn selected in the study and various levels of the design composite fabrics. Figure 2 shows the effect of nickel parameters and their interactions. Confirmation test (II) sulphate concentration on EMSE of composite carried out with optimum parameters for the response fabric with incident frequency from 200MHz to variable shows the closer results to that of original 1000MHz. The ANOVA carried out for EMSE of results and does not show any significant difference at fabric shows the significant effect of nickel (II) 95% confidence levels. The values obtained in sulphate concentration. Metal deposition reduces the confirmation tests along with the original values for nickel salt into nickel. The nickel particles are adhered response variables, considered in the study, are given on the substrate surface mainly by autocatalytic below: reducing agent in the nickel plating solutions. The Original result : 45 absorption and reflection of electromagnetic waves are Confirmation result : 44 improved by nickel particles. Significance at 95% confidential

3.3 Effect of Time on EMSE level (Yes/No) : No The ANOVA carried out for EMSE of fabric shows Scaling factors : A1 and B3 the neutral / negligible effect of time. Figure 2 shows Adjustment factors : C3 that the increase in the shielding effectiveness of the fabric with the increase in time is due to metal 4 Conclusion adhesive performance on fabric surface but it slightly The electroless nickel plated copper core/ polyester drops at 10 min and hence the processing time factor sheath yarn composite fabrics provide an attenuation is neutral/ negligible as compared to other factors. of 25-45 dB at the frequency range 200 - 1000 MHz. Hence, these fabrics can be used to shield the house- 3.4 Effects of Temperature on EMSE hold electrical and electronic applications, such as The ANOVA carried out for EMSE of fabric shows FM/AM radio, wireless phone, cellular phone and the neutral / negligible effect on temperature. It is computer that operate up to 1000 MHz frequency observed from Fig. 2 that the optimum performance 0 ranges. It is observed that of EMSE occurs at 40 C due to the fact that the (i) with an increase in the palladium chloride and optimized metal deposition occurs at this temperature nickel (II) sulphate, an increase in nickel deposition and the nickel particles move to distribute uniformally and electromagnetic shielding has been observed in on the composite fabrics. 200 - 1000 MHz frequency range. ANOVA carried out for the signal-to-noise ratio of (ii) the ANOVA carried out for EMSE of various combinations of the design and noise factors electroless nickel plated copper core / polyester sheath shows the predominant controlling nature of yarn composite fabric shows the neutral / negligible palladium chloride, nickel (II) sulphate, time and effects on time and temperature factors. temperature (Table 4). ANOVA results also show the dominating effects of palladium chloride as compared References to nickel (II) sulphate, time and temperature. Effect 1 Hombach V, Meier K, Burkhardt M, Kuhn E & Kuster N, of individual variables in terms of their average IEEE Trans, MicrowaveTheory Technology, 44 (1996) 1865. 2 Perumalraj R, Dasaradan B S, Anbarasu R, Arokiaraj P & Leo S/N ratio along with the % factors effects and F Harish S Electromagnetic shielding effectiveness of copper values (pooled and unpooled) further confirm the core-woven fabrics, J Tex Inst , 100 (6) (2009) 512-524. dominant nature. 3 Masi J V, Dixon D S & Avoux M, Proceeding, IEEE Confirmation tests in the Taguchi method International Symposium on EMC (IEEE, Atlanta), 1987, 183. 4 Perumalraj R & Dasaradan B S, Electromagnetic shielding supplements assures the validity of the results effectiveness of copper core yarn knitted fabrics, Indian J obtained in the experimental designs, orthogonal array Fibre Text Res , 34 (2) (2009) 149-154. PERUMALRAJ & DASARADAN: ELECTROLESS NICKEL PLATED COMPOSITE TEXTILE MATERIALS 41

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