Smart Textile Coatings and Laminates || Conductive coatings for textiles

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<ul><li><p> Woodhead Publishing Limited, 2010</p><p>6Conductive coatings for textiles</p><p>L. WA NG, RMIT University, Australia and X. WA NG and T. LI N, Deakin University, Australia</p><p>Abstract: Having electrical conductivity is often a prerequisite for many smart and intelligent textiles. Conventional textiles are non-conductive materials. One of the approaches to make a textile item electrically conductive is conductive coating. This chapter introduces some practical methods to impart conductivity to textiles, including metal coating and conducting polymer coating. It highlights the principles, coating methods, performance and applications of textile materials coated with conducting polymers such as polypyrrole.</p><p>Key words: conductive coating, electronic textiles, conducting polymers, polypyrrole, surface resistivity.</p><p>6.1 Introduction</p><p>Human intelligence requires effi cient transfer of electrical signals amongst a complex network of nerve cells within our brain: having electrical conductivity is often a prerequisite for many smart and intelligent textiles also. However, conventional textiles are non-conductive materials. One way to make a textile item electrically conductive is through the use of a conductive coating.</p><p>For conventional non-conductive textiles, build-up of static charge is very undesirable. Static electricity is generated when two unlike materials make contact and are then separated from each other, for instance by friction or rubbing of garments. If textiles are insulators (surface resistivity &gt;1000 M/, megaohm per square), very high static charges may build up, meaning that the materials could pick up excessive dust, possibly ignite a fl ammable or explosive atmosphere, damage electronic equipment, or induce a charge on a nearby non-earthed object. Electrostatic discharge from textiles has been a concern in a wide range of situations (Rizvi et al. 1995). Textiles with a certain degree of electrical conductivity are thus desired for many antistatic applications (Holme et al. 1998; Pionteck and Wypych 2007).</p><p>The demand for electroconductive fi bres and textiles is growing rapidly not only in antistatic applications but also in other areas such as sensing, data transfer, monitoring, corrosion protection and electromagnetic </p><p>155</p></li><li><p>156 Smart textile coatings and laminates</p><p> Woodhead Publishing Limited, 2010</p><p>interference (EMI) shielding. Depending on applications, fabrics or yarns may be required to have different levels of electrical conductivity. For example, highly conductive fl exible yarns may be required to fabricate textile-based electronic circuitry and user interface elements, which offer an alternative to traditional printed circuit boards. Relatively low conduc-tivity textiles may be used for static dissipation. Textiles of high electrical conductivity are of great interest in innovative product development.</p><p>Embedding electronic circuits and wearable devices into garments, or smart electronic textiles, is a growing trend for improving the quality of life (Carpi and De Rossi 2005; Dhawan et al. 2005; Locher and Troster 2008). Such garments integrate embedded electronics and sensing devices for a range of functions including physiological monitoring, communications, harvesting of energy, passive/active thermal management, casualty care, protection and entertainment (Carpi and De Rossi 2005; Dhawan et al. 2005; Winterhalter et al. 2005; Locher and Troster 2008). Dhawan et al. (2005) reviewed fi bre and textile-based electrical and optical devices and systems including some commercial products and product concepts of electronic textiles, which demonstrated that research in this area is paving the way for the development and commercialisation of fully integrated electronic textiles with intelligent built-in features.</p><p>Improving the quality and effi ciency of healthcare, both at home and in hospitals, is becoming more and more important for patients and society at large. Many smart sensors and fi rst aid and communication devices can be integrated into clothing for monitoring, reporting and enhancing the comfort and security of patients. Apart from smart textiles in healthcare, sportswear and functional fabric-based products that incorporate integrated electron-ics have also been developed and commercialised. Such products are capable of interfacing with or accommodating entertainment and commu-nications devices including iPods, mobile phones, laptop computers, and interactive jackets, bags, business suits and fabric keyboards.</p><p>The aesthetic quality is an important factor when integrating smart ma-terials and devices into conventional textiles. The electronic textiles should be fl exible, lightweight, comfortable, economical and durable, and more importantly they require electroconductive materials, such as conductive yarns and fabrics. To make fl exible substrates conduct electricity, many methods have been developed (Gregory et al. 1991; Dietzel et al. 2000, 2008; Akbarov et al. 2005, 2006; Carpi and De Rossi 2005; Wang et al. 2005; Kaynak et al. 2006; Gan et al. 2008; Roh et al. 2008; Sen 2008; Wei et al. 2008). This chapter briefl y introduces some practical methods to impart conductivity to textiles, including metal coating and conducting polymer coating. It highlights the principles, coating methods, performance and ap-plications of textile materials coated with conducting polymers such as polypyrrole.</p></li><li><p> Conductive coatings for textiles 157</p><p> Woodhead Publishing Limited, 2010</p><p>6.2 Methods for imparting conductivity on textiles</p><p>Metal fi laments and yarns made from brass, stainless steel, aluminium, copper or nickel are commonly used in electronic fabrics, in particular as part of embedded electronic circuitry (Post et al. 2000; Dhawan et al. 2004a,b; Winterhalter et al. 2005; Locher and Troster 2008; Roh et al. 2008) because they are low cost, widely available and have a good strength, biological inertness and reliable electroconductive properties. However, metal fi bres are heavier than polymer-based textile fi bres, and fabrics containing metal fi bres are not as fl exible as fabrics made from conventional fi bres. Once they are deformed, they are unlikely to recover, like most conventional fabrics do, to their initial shape without applying a force. Metal fi bres add bulk and weight to a garment, which makes the garment uncomfortable to wear.</p><p>Carbon fi laments have reasonable electrical conductivity. They can be incorporated into fabrics mainly for antistatic purposes. Conductive yarns can also be made by wrapping a metallic ribbon or foil around a high strength fi bre in a helix shape. The metallic ribbons, such as copper, silver or gold foil, impart high conductivity to fi bre-based materials, while the helix arrangement provides a certain degree of fl exibility and extensibility for the ribbon, and the core fi lament provides the strength while in service. Fabrics made from such yarns have better fl exibility than metal wires. However, the fragility of metal foil may lead to breaking of the electrical continuity.</p><p>Adding conductive particles, such as silver particles or carbon nanotubes, during fi lament spinning can modify the fi lament electrical property. The conductivity of the modifi ed fi lament depends on the weight ratio of the conductive particles. When the adjacent conductive particles are far apart, the fi lament cannot effectively conduct electricity. Although composite fi bres containing carbon nanotubes can be spun by dispersing and properly orienting them in polymers, producing highly conductive fi bres would require boosting the nanotube percentage to as much as 20% due to structure homogeneity problems and the aggregation of conductive materi-als. Similarly, fi bres containing conductive carbon/graphite can be spun, but the carbon content in the fi bre needs to exceed 30% for good conductivity. The inclusion of conductive particles in the fi bre spinning process is not an ideal approach. The conductive materials can be more effectively utilised if they are confi ned to the fi bre surface. Instead of using a metal fi bre or a fi lament, coating fi bres with high percentage carbon and/or metallic com-pound inclusion is also cost effective because the conductive substance needed for coating is small in quantity, especially when expensive metals such as gold, platinum or silver are used. In addition, the multifunctional properties needed for intelligent textiles are mainly determined by the </p></li><li><p>158 Smart textile coatings and laminates</p><p> Woodhead Publishing Limited, 2010</p><p>surface properties; thus coating conductive materials on the fi bre surface is most desirable.</p><p>Another practical technique to make electroconductive fi bre materials is to coat conducting polymers onto fi bre or fabric surface. Conducting poly-mers can be produced to have a wide ranging modulation of their electrical conductivity, from insulating to highly conductive, i.e. 1010105 S/cm (Mac-Diarmid 2001). Their conductivity value can be adjusted by incorporating specifi c concentrations of dopant during polymerisation. Conducting polymers are brittle and insoluble, and cannot be readily melted for fi bre extrusion. Hence, it is not practical to spin conductive fi bres and textiles from these conducting polymers. Instead, coating is more commonly used to impart conductivity to non-conductive fi bre materials, including both natural fi bres and synthetic materials. Conducting polymers can combine the electronic characteristics of metals with the engineering properties of polymers.</p><p>6.3 Main types of conductive coating</p><p>6.3.1 Metal coating</p><p>Most methods for imparting conductivity to fl exible substrates rely on coating a conductive layer of metals, conducting polymers or other conduc-tive materials such as conductive paints and lacquers onto their surface. Coating precious metals on textile surfaces have had a long history for aesthetic or protective purposes. The metal layer on fi bres can also fi nd new applications in electrical conductivity, in addition to protective and decora-tive effects. Textile metallisation technologies, which include vacuum depo-sition, ion plating, electroplating and electroless plating, deposit metallic particles on the textile surface and create metallic-coated textiles (Dietzel et al. 2000, 2008; Akbarov et al. 2005, 2006; Lee et al. 2005; Gasana et al. 2006; Jiang et al. 2006, 2007; Jiang and Guo 2008; Wei et al. 2008). Vacuum metallisation and ion plating are processes that deposit evaporated metal particles onto the substrate surface. The former method deposits metal vapour on substrates in vacuum, and the materials to be vaporised can be solids in any form and purity. The ion plating evaporates the molecules of metal materials and accelerates the ionised metal particles onto the sub-strate surface in an inert gas. The ion plating technology has higher density and stronger adhesion when compared with vacuum deposition.</p><p>An electroplating process, also referred to as electrodeposition, coats textile materials with a layer of metal particles such as copper, silver, gold, nickel, cadmium, chromium, zinc and alloy by means of an electrical current. It is performed in an electrolytic cell which contains an electrolyte and two electrodes. The anode (positive electrolyte) is the metal source, while the </p></li><li><p> Conductive coatings for textiles 159</p><p> Woodhead Publishing Limited, 2010</p><p>cathode (negative electrolyte) is the material to be coated. Under a low-voltage current, ions in the electrolyte move towards the cathode, and they are converted into metal form electrochemically and deposited onto the textile surface.</p><p>Electroless plating, also known as chemical or auto-catalytic plating, is a plating method that does not involve the use of external electric energy. It involves several simultaneous chemical reactions in an aqueous solution to reduce metallic ions to metal on non-conductive textile materials. Electroless plating can be performed by autocatalytic plating or ion exchange plating depending on the metal to be coated.</p><p>Metal-coated textile materials are breathable and lightweight compared with metal fi bre fabrics. Their electrical conductivity can be tailored by controlling the coating thickness and choosing the right metals. They provide functions such as antistatic properties, shielding against EMI and radiofrequency interference, conducting electricity and presenting a bril-liant decorative effect. The conductive fabrics can be used by the military for camoufl age applications. As some metals resist microbial attacks, metal coated textiles are good materials for antimicrobial applications, such as wall coverings and barrier curtains. Non-conductive yarns and fabrics chem-ically coated with gold or silver nanoparticles also offer very high levels of electrical conductivity and silver metallised fabrics are ideal for making fabric circuitry. Metallised fabrics can also be used in protective clothing for protection against intense radiant heat, as textile surface coated with aluminium provides a high degree of heat refl ection.</p><p>Wei et al. (2008) reported that copper sputter coating on the surfaces of polypropylene (PP) nonwovens reduced the surface electrical resistance signifi cantly, and the resistance decreased as the coating thickness increased. Tested at 1 cm longitudinal intervals, a 20 nm coating can give a surface resistance of about 157 , and a 100 nm coating reduces the resistance to below 8 on average. Gan et al. (2008) coated polyethylene terephthalate (PET) fabrics with CuNiP alloy using the electroless plating method, and demonstrated that the plating parameters affect the properties of alloy-coated fabrics. Conductive fabrics with high EMI shielding effectiveness could be prepared at an optimum condition. A CuNiP alloy deposit weight of 40 g/m2 produced a shielding effectiveness of more than 85 dB over the 100 MHz to 20 GHz frequency range.</p><p>For electronic equipment and wiring in aeronautics and space applica-tions, low weight, improved fl exibility and high strength are paramount. With electronics confi ned to small spaces and subject to severe stress, copper wire is prone to break (Slenski and Walz 2006). A hybrid metalpolymer fi lament can replace copper wire for signal wiring. The polymer wire is manufactured by covering a high performance polymer fi lament such as PBO (poly-phenylene benzobisoxazole), which has a tensile strength </p></li><li><p>160 Smart textile coatings and laminates</p><p> Woodhead Publishing Limited, 2010</p><p>higher than 7 GPa and decomposition temperature in excess of 600 C (Hearle 2001), with a metallised coating (nickel, copper, silver or gold), resulting in an electro-conductive and yet fl exible polymer/metal hybrid yarn. The polymer wire is more than two times stronger and seven times more resistant to dynamic cut-through at high temperature (200 C), and approximately 27% lighter than the current comparable size of state-of-the-art high strength CS-95 berylliumcopper alloy wire (Lee et al. 2005). Al...</p></li></ul>

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