Smart Textile Coatings and Laminates || Conductive coatings for textiles

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  • Woodhead Publishing Limited, 2010

    6Conductive coatings for textiles

    L. WA NG, RMIT University, Australia and X. WA NG and T. LI N, Deakin University, Australia

    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.

    Key words: conductive coating, electronic textiles, conducting polymers, polypyrrole, surface resistivity.

    6.1 Introduction

    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.

    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 >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).

    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

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    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.

    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.

    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.

    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.

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    6.2 Methods for imparting conductivity on textiles

    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.

    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.

    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

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    surface properties; thus coating conductive materials on the fi bre surface is most desirable.

    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.

    6.3 Main types of conductive coating

    6.3.1 Metal coating

    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 on