35 CHAPTER 3 MATERIALS AND METHODS 3.1 INTRODUCTION Electrically conducting and/or ferromagnetic materials in combination with fibres and textiles are proven to be effective in shielding against electromagnetic radiation. Fine wires of copper, steel or silver blended with textile fibres are the widely studied materials for electromagnetic shielding. Composite yarns containing metallic wires and textile fibres produced by friction spinning, core spinning or twisting are converted into fabrics for EMI shielding. Blending textile fibres with metallic wires improves the textile characteristics and process performance. One find it hard to weave a metallic wire in normal weaving machine rather a composite yarn, similarly for knitting also. However, metal coated synthetic fibre yarns are commercially available with good electrical conductivity and textile characteristics, and one example is silver coated yarn. Electroless process is a method used to deposit metal layers on fabric surface which gives good surface conductivity. Metal powder coating on plastics and textiles is also a technique practised for making packaging with EMI shielding. But the surface resistivity is high for these materials as non-conducting binders are used besides smaller particle size of metal powders. This research work deals with making EMI shield fabrics from metallic wires, composite yarns and metal coated yarns and to study the effect of process variables on EMI shielding. In addition, an attempt was also
36 made to develop embroidered fabrics using metal coated yarn and screen printing of metal powders like ferrous and aluminium on fabrics for EMI shielding application. 3.2 MATERIALS Metals with good electrical conductivity are good materials for EMI shielding. In this research work, metal based fibres, metal composite yarns and metallic fillers were used to develop woven, knitted, embroidered or printed fabrics for shielding against electromagnetic radiation. 3.2.1 Materials Chosen for Developing EMI Shield Fabrics Steel multi-filament yarn was chosen because the ferromagnetic metals/alloys with large relative permeability r ) offer higher attenuation (Perambur S. Neelakanta, 1995) especially at low frequencies. Commercially available steel yarn and also highly conducting silver coated nylon were procured to develop woven fabric samples in different weaves and pick densities. Besides highly conducting metallic wires of copper and silver coated copper were utilized to produce composite yarn by spinning or twisting to develop fabric samples by weaving or knitting. Commercial grade silver coated yarns with different conductivity levels were employed to develop grid patterns by embroidery technique on woven knitted and mesh type textile substrates to characterize EMI shielding. Mesh type nylon and fabrics are ideal for shielding curtains and screens which are optically transparent to certain extent. In yet another approach, fine metallic powders of ferrous and aluminum were applied to fabric surface by screen printing using a binder.
37 Knitted cotton fabrics, PET woven fabric and PE/PU nonwoven fabrics were used as substrates. 3.3 METHODS 3.3.1 Developing Metal Composite Yarn Samples Short staple ring spinning machine was employed to spin core spun yarn from silver coated copper metallic wire and cotton roving. Optimum process conditions for draft, twist and spindle speed were chosen. A ring twister was used to produce composite yarn comprising copper metallic wire and polyester yarn. 3.3.2 Developing Woven Fabrics for EMI Shielding A lab model computerized rigid rapier loom was employed to weave fabrics from metallic wire or metal composite yarns. Single end warping machines was employed to prepare weaver s beam containing 3 metre length of warp. Different woven fabrics were produced from commercial grade stainless steel filament, silver coated nylon and from the composite yarns developed by core spinning and ring twisting. The effect of pick spacing and weave of fabrics on EMI shielding was studied. 3.3.3 Developing Knitted Fabrics for EMI Shielding Knitted fabrics were developed from silver coated copper wires by a process known as plating along with PET yarn. Lab model knitting machine was employed for the purpose. Knitted fabrics were also produced from core spun yarn in different loop lengths and the effect of various constructional parameters on EMI shielding was investigated.
38 3.3.4 Fabrics Embroidered with Conducting Threads for EMI Shielding Silver coated nylon yarns with different levels of electrical conductivity were used for developing embroidered patterns in stripe, square and check designs on woven, knitted and mesh fabrics from computerised embroidery machine. Effect of electrical resistance and pattern on EMI shielding of embroidered fabrics was analysed. 3.3.5 Screen Printing of Fabrics with Metallic Powders for EMI Shielding Metallic fillers such as fibres and particles, graphite, carbon black and Carbon Nanotubes (CNT) are used in polymer composites for EMI shielding. In this study, ferrous and aluminium powders were used in the printing paste containing acrylic binder for screen printing on fabrics. Ferrous filler has been chosen for its ferromagnetic properties and aluminium for its good electrical conductivity. Effect of various concentration levels of metal powder on EMI shielding was interpreted. 3.3.6 Testing for EMI Shielding Effectiveness Different test methods are followed for evaluating the SE of EMI shields. IEEE Std-299, MIL-STD 285 and modified, ASTM E1851, ASTM D4935 are some of the standards which provide measurement test procedures and methods as well as performance levels and maximum allowable limits as pointed out by Salvatore et al (2008). Ketalin et al (2011) developed a device to measure the electro smog of different fabrics and studied the reproducibility of results. For evaluating SE of thin materials like fabrics, the most preferred ASTM D4935 standard was followed for most of the studies. The correlation between the modified MIL-STD 285 and ASTM D4935 was
39 analysed in the case of steel yarn fabrics. The frequency selected for modified MIL-STD 285 and ASTM D4935 is in the range of 30 1000 MHz and 30 1500 MHz respectively. 3.3.7 Testing Physical and Mechanical Properties of Fabrics Some of the structural, physical and mechanical properties of fabrics like thickness, air permeability, bursting strength related to the knitted and printed fabrics were evaluated as per standard test methods. 3.3.8 Testing for Antibactertial Activity Antibacterial activity of knitted fabrics consisting silver coated copper wire was evaluated as per AATCC 100 and AATCC 147 method since EMI fabrics used in hospital environment may cause cross infection. 3.3.9 Scanning Electron Microscopy Surface morphology of knitted and printed fabrics was examined by scanning electron microscopy at different magnification levels.