Download pdf - Smart Textile Coatings and Laminates || Conductive coatings for textiles

© 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 MΩ/�, 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

155

156 Smart textile coatings and laminates


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.

Conductive coatings for textiles 157


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



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. 10−10–105 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 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.

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



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.

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.

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.

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 Ω, and a 100 nm coating reduces the resistance to below 8 Ω on average. Gan et al. (2008) coated polyethylene terephthalate (PET) fabrics with Cu–Ni–P 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 Cu–Ni–P 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.

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 metal–polymer 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



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 beryllium–copper alloy wire (Lee et al. 2005). Although the conductive PBO yarn shows 67% the electrical conductivity of beryllium–copper, it can carry 9 A of current for over 5 minutes without failure. The polymer–metal hybrid wires would increase the life and reduce the maintenance costs of commercial and military aircraft and spacecraft. Compared with the tin-plated beryllium copper, the PBO conductor shield RG393 cables showed very similar shielding effectiveness over the fre-quency ranged from 0.1 to 10 GHz (Fig. 6.1). It is expected that the metal-lised fi bres will have many potential applications where conductive properties are required and low weight, durability and fl exibility are of critical importance.

6.3.2 Conducting polymer-coated textiles

Conducting polymers are polymeric materials that conduct electricity. They combine the electronic characteristics of metals or semiconductors with the engineering properties of polymers, hence are often referred to as ‘synthetic metals’. Electroconductive polymers have attracted a great deal of interest because of their unusual electrical properties. The sensitivity of electrical properties to external effects such as radiation, temperature and chemicals makes these polymers candidate materials for various applications includ-

60Silver coated conductive PBOCS-95 beryllium–copper alloy50

40

30

20

Shie

ldin

g e

ffectiveness (

dB

)

10

00.1–0.5 0.5–1 1–2 2–4

Frequency range (GHz)

4–8 8–10

6.1 Cable shielding effectiveness of metallised PBO and beryllium copper over the frequency range of 0.1 to 10 GHz (data source: Lee et al. 2005).



ing static electric charge dissipation (Lekpittaya et al. 2004), gas sensors (Kincal et al. 1998), biomechanical sensors (Wu et al. 2005), electrotherapy (Oh et al. 2003; Kim et al. 2004b), heating devices (Boutrois et al. 1997; Håkansson et al. 2004; Bhat et al. 2006), microwave attenuation (Kuhn et al. 1995), EMI shielding (Kaynak 1996; Kim et al. 2002; Håkansson et al. 2006, 2007), electronic textiles (Carpi and De Rossi 2005) and recovery of precious metals (Wang et al. 2007; Ding et al. 2008). Even though melt spin-ning of electroconductive fi bres is possible by blending up to 40 wt% polypyrrole with polypropylene, the conductivity of resultant fi lament is only 2.88 × 10−7 S/cm (Kim et al. 2004a), which is several orders lower than a desired conductive fi bre. Coating conducting polymers onto fi bres and fabrics results in materials which possess good mechanical and structural properties whilst retaining the electrical properties of conducting polymers. As conducting polymer coated textiles have many advantages over metal coated ones, this chapter introduces coating fi bre materials with conducting polymers, and highlights some properties of the coated textiles, as well as applications of such conductive materials.

6.4 Principles and procedures of conducting

polymer coating

As shown in Fig. 6.2, the most commonly used conducting polymers are polypyrrole (PPy), polyaniline (PANI), polythiophene and their deriva-tives. Of all known conducting polymers, PPy is the most frequently used in the commercial area due to its high electrical conductivity and prepara-tion techniques to form homo-polymers or composites that have optimal mechanical properties. It is also one of the most suitable conducting poly-mers for textile coating. Therefore, PPy is the focus of this chapter.

A thin PPy fi lm can be formed on fabrics by an electrochemical poly-merisation process, which involves the chemical oxidation of a solution containing the pyrrole monomer and a specifi c electrolyte salt. This process offers greater control of the resulting polymer thickness, morphology and degree of doping due to the in situ deposition of the polymer at the elec-trode surface. However, the PPy layer can easily be detached from the

n

Polypyrrole Polyaniline

n

N

Hn

Polythiophene

N

H

S

6.2 Structural formula of commonly used conducting polymers.



substrates, and electrochemically polymerising conducting polymers onto textiles substrates is not practical for bulk production.

It is common that insulating textile materials are coated with a conduct-ing polymer from a diluted solution or through interfacial polymerisation techniques to yield a composite material with a surface conductivity as low as a few ohms per square. Figure 6.3 shows a basic process of making con-ductive textiles through either a solution polymerisation method or a chem-ical vapour deposition method. The polymerisation of pyrrole by chemical oxidation usually is achieved in organic solvents or water. From an eco-nomic point of view, the large surface associated with fabrics is better handled through conventional dyeing and fi nishing processes. Therefore, for textile conductive coating, polymerisation in water is usually preferred, though coating in solvents may produce better products. After polymerisa-tion, the coated material should be thoroughly rinsed to remove excess deposition and oxidants from the fi bre surface. Further treatment may be conducted to improve the coating stability.

An aqueous method of conductive textiles coating can be achieved by either in situ polymerisation or two step polymerisation. When in situ polymerisation is employed, fi bre materials and all the reagents are added at the same time. Chemical polymerisation occurs in the bulk of the solu-tion, and the resulting polymers either deposit spontaneously on the surface of the immersed fi bre materials in the polymerisation solution, or precipi-tate as insoluble solids. For a better coating result, it is desirable to deposit the bulk polymerisation on fi bre surface as much as possible. This can be usually achieved by choosing an optimal reaction condition, like the reac-tant’s concentration and ratio, reaction temperature, and an appropriate treatment of the material surface to be coated (Kaynak et al. 2002; Hosseini and Pairovi 2005; Beneventi et al. 2006).

In two-step polymerisation, fi bre materials fi rst adsorb certain reagents, and then begin polymerisation with the addition of the rest of reagents. In the fi rst step, the textiles are normally subjected to heating, agitation, ultrasonic vibration or padding treatment to promote the penetration of the reagents into the textile structure. As the surface to be coated is enriched either with a monomer or an oxidising agent, a distinct advantage of this process is that the polymerisation occurs almost exclusively at the surface and possibly inside the fi bre structure. However, this process may not suit some textile materials that cannot be suffi ciently enriched with a layer of either monomer or oxidant in a separate stage preceding the

Fibre

materialsMonomer Oxidant Dopant

Conducting polymer

coated textiles+ + +

6.3 General procedure of coating a conducting polymer on textiles.



surface polymerisation. Regardless, this method is more popular than in situ polymerisation.

Coating fi bre materials with PPy has been widely reported in the litera-ture. PPy is produced by the polymerisation of the pyrrole monomer with chemical oxidants, such as FeCl3. Figure 6.4 shows schematically the process of oxidative polymerisation of pyrrole. First an electron is withdrawn from the pyrrole monomer by an oxidant, followed by the recombination of two radical pyrrole monomers to a dimer. The electron withdrawing step is repeated at the dimers with the subsequent growing of the chain to a polymer, yielding consecutively PPy as the end product of oxidative poly-merisation (Malinauskas 2001).

The electrical conductivity of conducting polymer involves the movement of charge carriers along polymer chains and hopping between polymer chains. The intra-chain charge transfer occurs when there is a rearrange-ment in the polymer chain producing the charge carriers, and an inter-chain charge transfer occurs between neighbouring polymer chains. The charge transfer allows the movement of electrons to occur over a larger distance (Bredas and Street 1985). It is generally believed that the electrical resistance of intra-chain hopping is much greater than that of inter-chain transport, as the conducting polymer fi lm/coating is an aggregation of nano-conducting polymer particles (Fig. 6.5).

When in the reduced form, dopants infl uence the conductivity by modu-lation. Doping is the creation of defects in the structure of polymer without destroying the chain. These defects in the polymer, which can be radicals, anions, cations or combinations of these, are the charge carriers. High elec-trical conductivity can be achieved by doping the conductive polymer with an appropriate dopant (Lekpittaya et al. 2004), for example, polymerisation in the presence of a sulphonate dopant yields better conductivity. Owing to the numerous types of conducting polymers and their vast molecular dif-ferences, simple modifi cations of the monomers or specifi c polymerisation conditions allow the conducting polymer coated textiles to suit different applications.

–2e–

N

N N N N

N N

+

+

N N N

–e– –H+

... ... ...

n

–2H+

6.4 Mechanism of oxidative polymerisation of pyrrole.



PPy shows good affi nity with both natural and manufactured fi bres. Textile substrates can be easily covered with a PPy layer by immersing them in a solution containing pyrrole, an oxidant and a doping agent. As polym-erisation occurs in the solution, insoluble PPy polymers are deposited onto the fi bre surface as shown in Fig. 6.6. The physical properties and mechan-ical properties of the conductive textiles may be affected by monomers, dopant anions, oxidants and synthesis parameters such as time, temperature, pH and liquor circulation in the coating vessel. During the process of the chemical oxidative polymerisation, there are usually visible changes in the colour of the polymerisation solution, and the precipitation of a dark blue or black solid polymer. Hence, the coated textile materials normally have a dark shade.

Conducting polymer coating can be conducted in the presence of excess dyes and conductive textiles with a certain colour can be produced (Kaynak

1 μm

6.5 Polypyrrole powder obtained by in situ polymer precipitation.

200 μm 1 μm

6.6 Conducting polypyrrole coated polyester fabric (left) and excessive deposits of polypyrrole on polyester fi bre during in situ polymerization (right).



et al. 2006). Certain dye molecules can be substituted as dopant anions and hence contribute to the improvement of electrical conductivity to the polymer. At the same time, dyes impart colour to the substrates, but the resultant colour will be darker than what the dye recipe intended. The coating procedure depends on the conductivity and colour of the coated fabric. It is recommended that fabric substrates be scoured before coating. Figure 6.7 shows an example of coating PPy onto scoured polyester fabrics. The advantage of adding monomer and dopant at the start of dyeing is to allow them to migrate into the fi bres, enhancing the adhesion between fi bres and PPy. The polymerisation is performed at a low temperature to minimise the formation of side products that inhibit the continuation and completion of the polymerisation reaction. This coating procedure can easily be achieved with conventional dyeing facilities. It could produce dark coloured conduc-tive fabrics with a surface resistivity of as low as hundreds ohms per square.

Chemical vapour deposition (CVD) is another method for conducting polymer coating. This method involves two steps for PPy coating to produce electroconductive composites. The fi rst step is impregnating a fabric with an oxidant and a doping agent and then drying it. The second step is apply-ing pyrrole vapour to the oxidant pre-loaded surface/substrate, yielding conducting polymer coated composite material. Kaynak et al. (2006) immersed a dyed wool fabric into a FeCl3/ethanol solution and dried at 20 °C for 10 min, followed by exposing the treated fabric to a pyrrole/nitro-gen vapour for 3 minutes. The fi nished fabric has a surface resistance of 1.2 kΩ/�.

The vapour deposition process is economical, and allows the recovery of the dopant and oxidant solutions, as well as the unreacted monomer (Dall’Acqua et al. 2006). Vapour phase coated fabrics show a highly uniform coating on the fi bre surface, as the dopant and oxidant have been applied evenly on the fi bre surface where the polymerisation occurs. This process generates nanoparticles with less aggregation, which maximises the surface area coverage and causes greater contact between polymer chains, thus resulting in a net increase in conductivity compared with aqueous coating for the same level of weight gain of conducting polymer.

Dyeing with pyrrole,

anthraquinone-2-sulphonic

acid sodium salt and dyes

(start at room temperature)

Hold at 130 °C for 30 min

Heat at

~2 °C/min

Cool at

~2 °C/min

Add FeCl3

Polymerisation at

~2 °C for 1 hour

6.7 Dyeing and coating polypyrrole on polyester fabrics.



6.5 Substrates for conductive coating

In the literature, a wide variety of materials were coated with a thin layer of conducting polymer, and pyrrole, aniline and a number of oxidants were commonly involved in the polymerisation. Among these materials, many types of fi bres have been attempted for producing conducting textiles, including polyester (Kincal et al. 1998; Kim et al. 2002; Håkansson et al. 2004; Lekpittaya et al. 2004; Lin et al. 2005; Molina et al. 2008), nylon (Oh et al. 2003; Wu et al. 2005; Ferrero et al. 2006), Lycra (Wu et al. 2005), nylon–Lycra (Håkansson et al. 2007), cotton (Lekpittaya et al. 2004; Hosseini and Pairovi 2005; Bhat et al. 2006; Varesano et al. 2009), wool (Bhadani et al. 1997; Wang et al. 2005), silk (Bhadani et al. 1997; Hosseini and Pairovi 2005; Boschi et al. 2008), cellulose derivatives (Dall’Acqua et al. 2004, 2006; Beneventi et al. 2006), polyaramide (Gasana et al. 2006) and glass (Genies et al. 1991; Kincal et al. 1998), as well as biomaterials (George et al. 2005; Ponce de Leon et al. 2008). All these materials have an affi nity for conduct-ing polymers, and a conducting polymer coating permits the production of composite textiles with improved electrical properties. The conductivity of the composite materials depends on the nature of the fi bres and the coating procedure. The fi bre selection may be determined by the intended applica-tions and desired properties of the coated materials. For example, a conducting polymer coated polyaramide woven substrate has excellent mechanical properties and thermal resistivity.

The bonding between conducting polymers and fi bres determines the tendency for the coating to delaminate. Animal fi bres may be a better choice as a substrate material for conducting polymers to attach to its surface due to the abundance of functional groups, to which the polymer molecules and dopant anion can attach themselves through ionic hydrogen and even bonds. Moreover, the scalar surface morphology of wool fi bres offers sites for adsorption of the conducting polymer. When polypyrrole was chemically polymerised onto wool fi bres and polyester monofi laments under the same coating conditions, the adhesion of the coating layer to the wool surface improved considerably; in particular, wool has better conduct-ing polymer coverage as polyester surface is smooth and has poor adhesion to the conducting polymer. Tavanai and Kaynak (2007) reported that the amount of deposited polypyrrole on the surface of PET fabrics increased as a result of the rough surface topology obtained by weight reduction through aqueous sodium hydroxide treatment. Hence fi bre surface mor-phology affects coating performance.

The process of adsorption of pyrrole at the substrate–liquid interface dictates the resulting structure of PPy. The carboxyl groups on nylon back-bones can provide a template for pyrrole monomers via hydrogen bonds, which yield a higher ordered and coherent PPy (Martin 1994). Lee and



Hong (2000) suggested that nylon polymer backbone provides a higher affi nity to pyrrole monomers, and thus was accountable for its higher conductivity and superior adhesion.

6.6 Testing of conductive coatings

6.6.1 Conductivity

It is ideal that conductive textiles have adjustable electrical properties and maintain those properties while being used. The electrical characterisation of the conductive composites is usually carried out by DC electrical con-ductivity measurements according to a standard, or a modifi ed method based on the standard. The conductivities of conducting polymer fi lms can be determined by the four-probe technique (ASTM test method D4496). The technique employs four equally spaced electrical contact points on a line with a polymer fi lm on an insulating surface. Between the outer pair of these points, a constant current is applied while the potential difference arising from this current is measured at the two inner contacts. The conduc-tivity (σ) can be calculated from the constant current (I, ampere) across the two outer points, the voltage drop (V, volt) across the two inner points, and the sample thickness (t) according to the following equation:

σπ

=ln 2

tIV

The resistance of a fi bre, yarn or narrow strip fi bre materials over a given length may be measured with the four-probe method, or using the funda-mentals of a division measurement over a known resistor and an unknown one (the sample). It can also be simply measured with a multimeter. The resistivity is normally expressed as the resistance per unit length of the test specimen, usually in Ω cm−1. Fibres and yarns are irregular materials and their dimensions vary; the coating thickness of the conducting polymer layer on the fi bre materials is not uniform and the fi bre contacts to the electrodes at different measuring points are not the same. Because of this the resistance of a fi bre or yarn is not proportional to the length of the materials, as shown in Fig. 6.8. Hence, it is often that the conductivity of these materials are characterised as resistance with an indication of distance between the electrodes for resistance measurements.

AATCC Test Method 76 is often used to test the surface resistivity of a conductive fabric. The surface resistivity is expressed as the square resist-ance, which is the resistance between two measuring points on a piece of fabric, and the unit for which is frequently expressed as Ω per square (Ω/�), to emphasise the fact that the resistance measured is independent of the distance apart of the contact points. According to the AATCC Test Method



76-1995, the surface resistivity of coated conductive fabric samples can be measured by placing two rectangular copper electrodes on the surface of the fabric specimen, then measuring the electrical resistance (R) between the electrodes, or applying a DC voltage (V, volt) between the electrodes and recording the current (I, ampere) as shown in Fig. 6.9. The surface resistivity (Rs) can be calculated according to the following formula:

RWD

RWD

VI

s Ω �( ) = ⋅ = ⋅

where W and D are the width of the conductive sample and the gap distance between the electrodes, respectively. Other methods, such as BS 6524:1984

5000

4000

3000E

lectr

ical re

sis

tance (

kΩ

)

2000

1000

00 5 10

Conducting yarn lengths (cm)

15 20

6.8 Effect of testing length on the electrical resistance of PET fi lament yarns coated with 5% of polyaniline in xylene solution. The error bars indicate standard deviation (data source: Kim et al. 2004a).

Electrode

W

D

VI

Electrode

6.9 Surface resistivity measurement.



method for determination of the surface resistivity of a textile fabric and ASTM D257-07 standard test methods for DC resistance or conductance of insulating materials, may also be used.

Gasana et al. (2006) have observed that the electrical conductivity of PPy coated polyaramide fabrics depends on current signals. The fabrics have a higher conductivity when alternating current signals were applied than the direct currents. This is because the mechanism of charge transport in PPy is electron hopping rather than electron conducting (Sato et al. 1991; Abthagir and Saraswathi 2004). The end user of conducting polymer coated fabrics should be aware that in most publications the electrical conductivity or resistivity was measured under DC power supply. The electrical proper-ties may be different for AC signal applications.

6.6.2 Coating thickness

Coating thickness and evenness affect the electrical properties of the coated material. Uneven coating will have a lowering effect on the DC conductivity while thicker polymer coating will normally lead to a higher conductivity and better electromagnetic attenuation. The coating thickness may be determined from a transmission electron microscopy (TEM) image (as shown in Fig. 6.10) of the cross-section of the coated fi bre, which was further coated with gold vapour before it was embedded in resin. Although TEM is a direct and a more precise way of measuring the coating thickness, the scope of TEM images is too small, and TEM imaging is too expensive to get a meaningful statistical result. To quickly estimate the coating

Resin

Conducting polymer

Filament

Gold

100 nm

6.10 TEM micrograph of a cross-section of polyester fi bre coated with vapour phase polymerisation.



thickness, commercial instruments for fi bre diameter measurement such as Laserscan or OFDA (optical-based fi bre diameter analyser) instrument may be used to measure and compare the mean fi bre diameter before and after coating.

Through OFDA measurements, Lin et al. (2005) revealed that the thick-ness of PPy coating on PET fabrics increases with pyrrole concentration and the PPy deposition is faster at low pyrrole concentrations than high pyrrole concentrations, as shown in Fig. 6.11. Coating thickness can also be controlled by coating time. Shorter deposition times lead to a thin inhomo-geneous coating layer and poor surface coverage. Longer deposition times will improve the surface coverage and increase the coating thickness. To further thicken the coating layer, a coated substrate may be coated repeatedly, allowing more conducting polymer to attach to the fi bre surface. However, there is a limitation to improving the conductivity of heavily coated textiles, and thick conducting polymer coating will reduce the fl exibility of the substrate.

It is worth noting that the vapour phase polymerisation shown in Fig. 6.10 took place from the exterior to the core. The diffusion and the polymerisa-tion of pyrrole into the fi bre core are slow and inhomogeneous because of the obstruction of the already present polymer. Nevertheless, the coating thickness is just an indication of polymer add-on to the fi bre. To determine the actual amount of conducting polymer attached to the fi bre/fabric, weight gain after coating may be used.

0.6

0.5

0.4

0.3

Coa

ting

thic

knes

s (μ

m)

0.2

0.0 0.5 1.0 1.5

Pyrrole concentration (mg/ml)

2.0 2.5

0.1

0.0

6.11 Effect of pyrrole concentration on the coating thickness during aqueous polypyrrole polymerisation (the molar ratio of the monomer, dopant and oxidant (FeCl3) was 1 : 0.6 : 2.5).



6.6.3 Coating fastness evaluation

It is important that conductive textiles not only have the desired electrical conductivity properties, but also acceptable quality characteristics, as well as the durability to retain acceptable conducting performance during deployment. Coatings are subjected to heat stress when they are conducting electricity, resulting in thermal expansion between the coating and sub-strate. Thus, the coating material should exhibit high ductility so that it is able to expand and contract without cracking or delaminating. When used as apparel materials, it is inevitable that the electronic textiles will be sub-jected to laundry and abrasion. The coatings may also be subjected to polymer degradation and ageing, which may result in signifi cant loss of conductivity, and reduction of the material’s effective service life. It is essen-tial to evaluate the fastness of the conductive layer as well as the effect of conducting coating on textile performance for some applications.

The coating fastness can be simply evaluated with abrasion and washing assays, and the textile industry standards or similar procedures specifi ed in the standards may be followed. Changes in conductivity, colour and surface morphology are usually tested as well. The Martindale abrasion tester has been reported for abrasion resistance measurements. By abrading a 38 mm diameter conductively coated fabric sample against an abradant fabric for a number of specifi ed cycles under a given pressure, the differences before and after abrasion can be used to quantitatively specify the abrasion fastness of the coated samples.

Washing fastness, lightfastness, colourfastness, thermal stability, etc. may also be determined using their respective textile standards, or modifi ed versions.

6.7 Properties of conducting polymer coated textiles

Conducting polymer coating not only changes the material’s conductivity but also modifi ed its physical, chemical and mechanical properties, which are important to the end products. In general, apart from imparting electri-cal conductivity, the negative impact of conducting polymer textile coating on other properties should be minimised.

6.7.1 Electrical conductivity and antistatic performance

Antistatic materials exhibit a conductivity in between not conductive and conductive, which is suffi cient to eliminate or at least to diminish static charging. Conventional textile fi bres are non-conductive materials and can easily accumulate charges, which could produce sparks to ignite fl ammable gases and vapours (Wilson 1977; Kathirgamanathan et al. 2000). Dissipating



the electrostatic charges deposited on textile products will prevent these risks. Conventional textile fabrics, such as wool, cotton and polyester, have a surface resistivity of more than 109 Ω/�, and are thus non-conductive materials. To make them antistatic materials for dissipation of static charge, they need to increase the conductivity to an adequate level.

To demonstrate antistatic performance, PPy was chemically synthesised on fabrics to produce conductive textiles. Three pairs of coated and uncoated fabrics as listed in Table 6.1 were evaluated for electrical performance. The results in Table 6.1 show that uncoated fabrics do not have a measurable conductivity (their surface resistivity exceeded 109 Ω/�), while fabrics coated with PPy can easily achieve a conductivity in the kΩ/� region, reduc-ing the static voltages by several orders, though fabric constituents (such as fi bre used, fabric thickness and weight) may affect the electroconductive effectiveness.

The surface resistivity of a PPy coated fabric can be reduced below 20 Ω/� by using optimised synthesis parameters for chemical polymerisa-tion. Therefore, coating textiles with conducting polymers is an easy approach for many antistatic applications, as conducting polymer coating can be simply carried out using most dyeing machines.

6.7.2 Frictional properties

Coating causes conducting polymer nanoparticles to cover and smooth the fi bre surface, which tends to reduce the coeffi cient of friction of the fi bres that have a rough morphology. Wang et al. (2005) examined the effects of conducting polymer coating on the fi bre frictional properties of wool and alpaca fi bres. They coated the fi bres with conducting PPy by vapour poly-merisation, and measured the degree of fi bre friction. The changes in coef-fi cient of friction of the single fi bres upon coating with the conducting polymer are presented in Fig. 6.12. It can be seen that the coeffi cient of friction is statistically signifi cant between coated and uncoated fi bres; con-

Table 6.1 Fabric information and electrical performance

Fabric sample Thickness (mm)

Weight (g/m2)

Coating Surface resistivity (kΩ/�)

Static voltage (V)

45% wool–55% polyester

0.49 170 No ∞ 10 000Yes 33.3 110

100% wool 1.05 300 No ∞ 14 000Yes 3.8 125

80% wool–20% polyester

0.45 225 No ∞ 9 000Yes 6.0 30



ducting polymer coating decreases the coeffi cient of friction. This is because the conducting polymer coating layer partially covers the scales and forms a transition hump at the scale tips, which smooths the fi bre surface.

The conducting polymer coating also reduces the directional friction effect (DFE) of wool and alpaca fi bres as shown in Fig. 6.13. This also suggests that the conducting polymer coating layer smooths the surface of animal fi bres.

6.7.3 Durability

The electrochemical stability of organic conductors is crucial especially when being used in electro-driven devices. Coating durability affects the service life of conductive textiles, as any loss of conductivity will alter the application effectiveness, in particular in situations where the electrical resistance of the conductive materials is required to be in a defi ned range. Figure 6.14 shows that the tested conductive specimens have poor abrasion

0.4

0.3

Without coating

25.8 μm alpaca

Against With

With polypyrrole coating

Sta

tic c

oeffic

ient of fr

iction

0.2

0.1

0

20.8 μm wool

Against

Scale direction

With

22.8 μm wool

Against With

0.3

0.2

0.1

Without coating

25.8 μm alpaca

Against With


Kin

etic c

oeffic

ient of fr

iction

0

20.8 μm wool

Against

Scale direction

With

22.8 μm wool

Against With

6.12 Effect of vapour coating on the frictional properties of wool and alpaca fi bres.



20

15

Without coating

25.8 μm alpaca

Static Kinetic


Directional fr

iction e

ffect (%

)

10

5

0

20.8 mm wool

Static

Type of coefficient of friction

Kinetic

22.8 μm wool

Static Kinetic

6.13 Directional friction effect of vapour coating on the wool and alpaca fi bres.

Abraded area

Unabraded area

(a) (b)

Removal of the

conductive layer

20 μm

6.14 (a) Change of fabric colour due to 2000 cycles abrasion under 9 kPa of pressure using a Martindale abrasion tester. (b) Detachment of conducting polypyrrole layer after laundering.

performance. The coated fabrics were obtained from a two step polymerisation process. After 2000 cycles of the Martindale abrasion test, the black conducting polymer coated fabric became lighter and the conductivity in the abraded area decreased by approximately 26%. Also due to abrasion, weak bond formation between conducting polymer and fi bre, and possible hygral expansion difference in moist conditions, the conducting polymer coating layer was partially removed during washing.

Huang and Liu (2006) reported a method of producing polyaniline/PET conductive composite fabrics using a multi-stage polymerisation process. They repeatedly immersed the PET fabrics in a 0.75 m solution of aniline monomer in 1m hydrochloric acid, padded and dried the fabrics. They then



initiated the polymerisation by immersion of the fabrics in a 0.17 m oxidant solution containing 1 m HCl, squeezed the fabrics with a padder and allowed the polymerisation at ambient temperature for some time. They further washed the composite fabrics with a 1 m HCl solution and then with distilled water; doped the dried fabrics with a 1 m aqueous solution of dodecyl-benzene sulphonic acid (DBSA) as secondary dopant with ethanol as a cosolvent for 20 min; followed by rinsing and drying. They claimed that such method improved the coating fastness to washing. Their results in Fig. 6.15 show that the surface resistivity of most PANI/PET composite fabrics increased 10 to several hundreds of times after repeated machine washing at 40 °C. Even for the best performed conductive fabrics they produced, the increase in resistance was at least fi ve times. This suggests that the

25 000

20 000

15 000

Cha

nge

of s

urfa

ce c

ondu

ctiv

ity (

%)

10 000

Fabric weight reduction by alkali hydrolysis (%)(a)

5 000

00

15 000

12 000

9 000

Cha

nge

of s

urfa

ce c

ondu

ctiv

ity (

%)

6 000

Amount of oxidant solution absorbed by fabric (%)(b)

3 000

0Not padding 119 100 80 60

4 9 15 19 26 30

6.15 Change of surface conductivity due to repeated machine washing at 40 °C. (a) Washfastness of conducting coating on different alkali reduction pre-treated PET fabrics. (b) Washfastness of conductive fabrics obtained by applying different oxidant solution wet pickup levels on aniline-absorbed fabrics through padding (data source: Huang and Liu 2006).



washfastness of conducting polymer coated fabrics is one of the major problems that hinder them from successful commercialisation.

For the coated conductive fabrics that are subjected to severe abrasion and washing during deployment, advanced coating methods may be used to improve the abrasion performance (Tessier et al. 2000; Garg et al. 2007). Surface treatment techniques can increase the fi bre surface energy and enhance binding interaction between the conducting polymer and the fi bre. Adequate chemical uptake and adhesion before coating can change the fi bre wettability and surface texture, and generate cross-linking radicals. The presence of free radicals induces secondary reactions, enabling the deposi-tion of solid polymeric materials with desired properties onto the substrates. Garg et al. (2007) demonstrated that atmospheric plasma glow discharge treated fabrics exhibit better hydrophilicity and increased surface energy. Plasma treatment is able to improve the ability of the substrate to bond with conducting PPy coating, resulting in better coating uniformity, abrasion resistance and conductivity.

When in use, the surface resistivity of a PPy coated fabric increases with time, as shown in Fig. 6.16. This change is almost linear for this instance. Hence, the surface resistivity of the conductive fabric may be predicted while in service, which gives a fair indication of the service life of the con-ducting polymer coated fabric-based electronic system.

It should be mentioned that the conductivity decay of the PPy coated fabrics will accelerate under elevated temperatures. Since the mechanism of conductivity degradation involves the diffusion or reaction of oxygen, an inert atmosphere will lower the rate of the conductivity loss compared with air, and the stability of the coatings can be dramatically improved by coating an encapsulating barrier, which isolates the conducting polymer from atmospheric oxygen (Kuhn et al. 1995). For example, to prevent the

25

20

15

Linear fit: R2 = 0.996

10

Change in r

esis

tance (

%)

5

00 10 20 30 40 50

Ageing time (day)

60 70 80 90 100

6.16 Change in surface resistivity of a polypyrrole coated fabric powered by 12 V DC (tested under standard atmospheric conditions).



oxidation of conductive textiles prepared with PPy coating, silicon-based coatings (polydimethylsiloxane oil) can be applied to minimise direct air contact after polymerisation (Dall’Acqua et al. 2006). The encapsulating barrier improves not only the electrical stability but also the washfastness of conductive coatings.

Conductivity decay is also due to the loss of charge carriers on the polymer backbone. Lee and Hong (2000) revealed that the exchanged counterions can stabilise the conjugated chains of PPy and local charge carriers against electrochemical oxidation. Using diluted acid can confer a coinciding structural enhancement. Such a stabilisation method may be achieved through product care, such as washing.

For high levels of fastness to washing, polymerisation may take place in the internal fi bre structure. Dall’Acqua et al. (2004) applied monomer and reagents to a cellulose substrate in an aqueous solution so that it penetrates uniformly into the core of the fi bre as it swells similarly to a non-reactive dye. Therefore, subsequent polymerisation occurred inside the fi bre, leading to improved washfastness and lightfastness (Dall’Acqua et al. 2006).

6.7.4 Tensile properties

Coating methods and substrates used affect the mechanical properties of coated fabrics. Dall’Acqua et al. (2006) evaluated pure viscose fabrics coated with PPy in vapour phase and two-step liquid phase polymerisation respectively. For vapour polymerisation, they impregnated the fabrics in aqueous solution containing both oxidant and dopant for 1 hour, dried the treated fabric for 24 hours at room temperature, followed by exposing the fabric to pyrrole vapour for 8 hours. To coat fabrics by aqueous poly-merisation, they soaked the fabrics in catalyst and dopant aqueous solution for only 30 min, then added pyrrole for polymerisation. The tensile testing results in Fig. 6.17 show that the strength and elongation at break of the fabrics dropped signifi cantly for the vapour phase coated fabrics, while the change for the liquid phase polymerisation fabrics was relative small. This is because the FeCl3 oxidises the cellulose structure more effectively in vapour phase coating than in liquid phase coating. The mechanical proper-ties of vapour phase coating samples sharply decreased by the long time treatment of oxidant and increased concentration of catalyst after drying, which damaged the fabric from the oxidation reaction.

Polymerisation mainly occurs at the outermost layer of the fi bre sub-strate. Thus, coating of conducting polymer on fi bres, fi laments, yarns and fabrics should not lead to very signifi cant changes in their tensile prop-erties if the damage of oxidant to fi bre is negligible. In most cases, coating may enhance material mechanical properties. Figure 6.18 shows an example. Conducting polymer coating on wool fi bres and yarns improved the



tenacity and breaking elongation of the fi bre materials. This is attributed to the modifi cation of the surface morphology of the wool fi bres by the depo-sition of PPy. The thin layer of conducting polymer has a reinforcing effect. The reduction in the initial modulus of the yarns upon PPy coating may be attributed to the change of frictional properties of wool fi bres due to coating (Fig. 6.12).

Evaluation of a wide range of fi bre materials suggested that the use of a textile substrate represents a convenient method for introducing mechani-cal strength, fl exibility and processability to conducting polymers for prac-tical applications. Fabrics coated with a thin layer of PPy with an optimised procedure have essentially similar mechanical properties as the textile substrate, with minor variations depending on processing conditions and fi bre types. Subjective handle assessment has indicated that the tactile prop-erties of the textiles were not signifi cantly affected by a thin coating of conductive PPy, though the coated fabrics tend to be harsher owing to the

500

400

300

Str

ength

at bre

ak (

N)

200

100

Uncoated

Weft

Warp

0

Vapour

coated

Liquid

coated

30

25

20

Elo

ngation a

t bre

ak (

%)

15

10

5

Uncoated

Weft

Warp

0

Vapour

coated

Liquid

coated

6.17 Change in mechanical properties of polypyrrole coated viscose fabrics (data source: Dall’Acqua et al. 2006).

1.6

1.4

1.2

1.0

0.8

Nor

mal

ised

tens

ile p

rope

rtie

s

0.6

0.4

0.2

Fibre

TenacityBreaking elongationInitial modulus

Yarn0.0

6.18 Tensile properties of polypyrrole coated wool fi bres and yarns (normalised by their uncoated originals).



brittleness of conducting polymers and possible bonding between fi bres. However, a thick coating of PPy caused the fabric to become stiff, resulting in a poor handle.

6.8 Applications of conductive textiles

Conducting electroactive polymers have various application fi elds such as metallisation of dielectrics, batteries, antistatic coatings, shielding of electromagnetic interferences, sensors and sensor arrays. Using textiles as the substrate materials, a thin conducting polymer coating layer is able to combine both the conductivity and the fl exibility. Since the conduct-ing polymers have a comparable degradation lifetime to textile materials, the conductive coated substrates are often employed for engineering conductive textiles. Applications of conductive substrates are varied and numerous. Fabrics coated with PPy can easily achieve high conductivity in the kΩ/� range. They reduce the static voltages by several orders. Such materials can be used for many antistatic applications. In addition to antistatic applications, they may also have other potential applications such as sensing, heating, EMI shielding, medical and sports gears, micro-wave fi lters and absorbers, etc. Some example applications are briefl y introduced here.

6.8.1 Electromagnetic interference shielding

EMI can be defi ned as the impairment of the functioning of an electronic device caused by electromagnetic radiation emanating from other nearby electronic devices. It has been found that strong electromagnetic waves hinder the normal operation of sensitive electronic devices and navigation systems. For instance, some personal electronic devices must be switched off during aeroplane taking off and landing. EMI may also cause health problems for the human body. Therefore, EMI shielding is important to protect people and electronic devices from harmful interference. In addi-tion, the refl ection of microwaves can be used to discover and locate certain objects, which has been used as a tool in military intelligence gathering. Materials absorbing microwaves can provide camoufl age effects.

In order to reduce EMI, the radiated EMI (disturbance propagated through space) needs to be fi ltered. When an incident electromagnetic wave reaches a material, it is transmitted partially through the material. The rest of the wave is attenuated by refl ection and absorption of the ma-terial. The relative shielding effi ciency of refl ection and absorption is an important determinant of its practical application. The EMI shielded by absorption rather than refl ection is more important for most applications at the present time.



Metals or metal coated surfaces possess very high EMI shielding effi -ciency based on surface refl ection, which is not ideal in some applications. On the other hand, textiles coated with electroconductive polymers are capable of not only refl ecting but also absorbing the electromagnetic waves and therefore exhibit a signifi cant advantage over the metallic shielding materials. EMI shielding by absorption can be enhanced by changing the electrical conductivities or the dielectric constants of electroconductive polymers (Kaynak 1996; Kim et al. 2002; Håkansson et al. 2006, 2007). In addition to strong absorption to the EMI signals, conducting polymer coated textiles also maintain their excellent permeability to air and mois-ture. Therefore, fl exible conductive textiles have found wide potential appli-cations in military as camoufl age material, in industry for shielding EMI and microwaves, and in daily life for blocking electromagnetic waves from electronic appliances such as mobile phones, microphones, microwave ovens and wireless communication devices (Kuhn 1997).

6.8.2 Conductive fabric circuits

Textile-based circuits offer opportunities to deploy sensors and other devices, and create large-area electrical and electronic systems. An out-standing advantage of textile circuits over traditional circuit boards is their fl exibility. They can be easily embedded in fabrics and replace conventional printed boards in electronics applications (Locher and Troster 2008). Figure 6.19 demonstrates a facile technique for patterning conducting polymer on a fabric by quenching the polymerisation reaction that occurred in the protected areas (Kaynak et al. 2006). The patterning technique can be used to fabricate patterned electronic circuits for wearable electronic textiles. The conducting pattern has a resolution of about 2 mm. This will allow a margin for chemical migrating along the pattern boundaries and reduce the

FeCl3/ethanol

treatment

Pattern

quenching

Pyrrole vapour treatment

Rinsing

Removal

Fe2+/Fe3+

FabricTreated

fabric

6.19 Flow diagram of patterning conductive textiles.



possibility of loose conductive fi bres to short the circuit. Therefore, this method best suits a circuit on a large fabric area.

Conducting polymer coating and patterning enable integration of soft circuitry into fabrics, thus eliminating the need for hardwiring and hence increasing fl exibility and wearability of the electronic textiles. As conductivity is integrated into the fi bre itself, there is no need to incorporate metallic loadings. Figure 6.20 shows an example of applying the technique for a light-emitting diode (LED) display. The LEDs will be lit up when a potential is applied through the fabric conducting areas.

6.8.3 Temperature regulation

Conducting polymer coated fabrics can be used for heat generation. Gar-ments with a heating function are desired in cold outdoor environments to keep the human body warm. The generation of heat is due to resistive heating of the conductive fabric on the passage of a current. Compared with other heating devices, the advantages of the conductive fabrics are their fl exibility, softness, temperature homogeneity, low power density on large surfaces and suppleness (Boutrois et al. 1997; Håkansson et al. 2004). They can be sewn up, cut off or pasted on substrates for a large range of applica-tions, such as aircraft de-icing, heating winter sports wear, car seats, gloves, curtains and buildings.

Increasing thermal conductivity is important for wool fabrics used in a warm environment. Wang et al. (2006) reported that by coating wool fabrics with conducting PPy improved the thermal conductivity of the PPy coated fabrics. The PPy coating parameters infl uenced the thermal conductivity of

6.20 Conducting polymer coated fabric circuitry for LED display.



the coated wool fabrics. Overall, thermal conductivity increased with the increase of electrical conductivity.

As heat can be simply generated by batteries, active cooling is more of a challenge for thermo-regulating garments than heating. Hu et al. (2005) proposed a concept of cooling with conducting polymer coated fabrics by applying the ‘Peltier effect’. It occurs whenever electrical current fl ows through two dissimilar conductors; depending on the direction of current fl ow, the junction of the two conductors will either absorb or release heat. A noticeable amount of temperature difference has been achieved using the conductive PPy coated fabric. This cooling technique is still in its infancy.

6.8.4 Substrates for metal deposition

Using the electroless metal plating method, non-conductive substrates can be deposited with metals. In the deposition process, the metal ions are reduced to metal only on specifi c areas where catalysts are present. With this method, when an aqueous [Au(III)Cl4]− gold ion solution is exposed to PPy nanoparticles or passes through PPy-coated fi bre substrates, the con-ductive PPy demonstrates an outstanding ability to reduce the gold ions electrolessly into Au(0) and deposit the Au particles on the substrates (Wang et al. 2007; Ding et al. 2008). This technique can be used for electro-less gold recovery as shown in Fig. 6.21. As nanofi bres have a large surface area and excellent permeability, PPy-coated electrospun nanofi bre mem-branes can fi lter out gold from the aqueous solution stream that continu-ously passes through the membranes. Wang et al. (2007) found that the recovery effi ciency of the PPy-coated nanofi bre textiles is affected by

10 μm

6.21 SEM image of polypyrrole coated nanofi bre membrane with gold deposits.



the membrane thickness, the permeate fl ux rate and the [Au(III)Cl4]− con-centration. A higher gold recovery yield can be achieved by using a thicker nanofi bre membrane or by reducing the solution fl ux rate.

The metallisation of textile materials also provides functionality of high electrical conductivity. Gasana et al. (2006) found that the conductivity of PPy coated polyaramide fabrics is poor under a DC current. To achieve better conduction, they further coated the PPy coated structures with a layer of metallic copper through electroless deposition. The DC electro-conductive polyaramide structures obtained after depositing copper on top of the prior deposited PPy layer showed excellent electroconductive properties, which was comparable with metallic conductivity.

6.9 Conclusions

There are many situations where conventional textile materials are required to conduct electricity. Garments with wearable electronics/devices/systems highlight the applications of such conductive textiles. Many approaches can be used to impart electroconductive properties to conventional textiles, including metallisation, conductive particles inclusion, electroless deposi-tion, electrodeposition and chemical coating. Metallised high performance hybrid polymer yarns can be stronger and lighter than metal wires, and have similar electrical performance as metal fi bres, making them an ideal replace-ment of metal wires for signal wiring and electronic textile applications.

Conducting polymer coated textiles are an economical, fl exible and light-weight alternative to traditional conductive materials. By coating textile substrates with conducting polymers, the electrical properties of conducting polymers can be combined with the strength and fl exibility of the textile substrates. The brittleness of the conducting polymer coating layer does not have a signifi cant negative impact on the fl exibility and handle of coated textile materials. PPy, PANI and polythiophene are the most commonly used polymers for conductive textile coating.

Conducting polymer coating can be easily prepared by either an aqueous method or chemical vapour phase deposition method. The simplest poly-merisation process is to mix the fi bre materials and all the reagents in one bath at the same time. While the fi bre materials are coated with the conduct-ing polymer, the in situ polymerisation process also produces precipitate of insoluble conducting polymer in the polymerisation solution. Two-step polymerisation involves allowing fi bre materials to fi rst adsorb certain rea-gents, before adding the rest of the reagents for polymerisation. This has an advantage in that the polymerisation occurs almost exclusively at the surface and possibly inside the fi bre structure. Chemical vapour phase coating is another suitable process to produce electroconductive textiles with low reagents wastes. The coating is highly uniform on the fi bre surface.



A desired coating result can usually be achieved by optimising a coating procedure.

Conducting polymers can be coated onto a wide range of fi bres, yielding new composite materials with a wide range of electrical conductivities. The conductive textiles have numerous potential applications in various fi elds such as antistatic, fabric circuits, electromagnetic interference shielding, heating and precious metal recovery, though most of the technologies are still at the developmental stage. The conductivity of coated fabrics decreases gradually when in use. In particular, the electrical conductivity drops dra-matically when the conductive coating is directly exposed to atmosphere, or is under severe service conditions such as abrasion, washing and high temperature. Though measures can be taken to slow the loss of conductiv-ity, the environmental stability and durability of electrical properties of the conducting polymer coated textiles should be considered when estimating the material service life.

The mechanical properties of conducting polymer coated textiles are affected by the oxidant treatment and polymerisation process. For fi bres that are not sensitive to oxidant, the effect of conducting polymer coating is generally small. On the other hand, if the oxidation reaction damages the fi bre structure, the mechanical properties of conducting polymer coated textiles will drop accordingly. Conducting PPy coating on animal fi bres covers the fi bre scales and smooths the fi bre surface, resulting in a reduction in the fi bre coeffi cient of friction and directional friction effect.

6.10 References

abthagir, p.s. & saraswathi, r. (2004), Charge transport and thermal properties of polyindole, polycarbazole and their derivatives, Thermochim. Acta, 424(1–2): 25–35. DOI: 10.1016/j.tca.2004.04.028.

akbarov, d., baymuratov, b., akbarov, r., westbroek, p., de clerck, k. & kiekens, p. (2005), Optimizing process parameters in polyacrylonitrile production for metal-lization with nickel, Textile Res. J., 75(3): 197–202. DOI: 10.1177/004051750507500302.

akbarov, d., baymuratov, b., westbroek, p., information, c., akbarov, r., clerck, k.d. & kiekens, p. (2006), Development of electroconductive polyacrylonitrile fi bers through chemical metallization and galvanisation, J. Appl. Electrochem., 36(4): 411–418. DOI: 10.1007/s10800-005-9085-8.

beneventi, d., alila, s., boufi , s., chaussy, d. & nortier, p. (2006), Polymerization of pyrrole on cellulose fi bres using a FeCl3 impregnation-pyrrole polymerization sequence, Cellulose, 13(6): 725–734. DOI: 10.1007/s10570-006-9077-9.

bhadani, s.n., kumari, m., gupta, s.k.s. & sahu, g.c. (1997), Preparation of conduct-ing fi bers via the electrochemical polymerization of pyrrole, J. Appl. Polym. Sci., 64(6): 1073–1077. DOI: 10.1002/(SICI)1097-4628(19970509)64:6<1073::AID-APP6>3.0.CO;2-I.

bhat, n.v., seshadri, d.t., nate, m.m. & gore, a.v. (2006), Development of conductive cotton fabrics for heating devices, J. Appl. Polym. Sci., 102(5): 4690–4695. DOI: 10.1002/app.24708.



boschi, a., arosio, c., cucchi, i., bertini, f., catellani, m. & freddi, g. (2008), Proper-ties and performance of polypyrrole (PPy)-coated silk fi bers, Fibers Polym., 9(6): 698–707. DOI: 10.1007/s12221-008-0110-5.

boutrois, j.p., jolly, r. & pétrescu, c. (1997), Process of polypyrrole deposit on textile. Product characteristics and applications, Synthetic Metals, 85(1–3): 1405–1406. DOI: 10.1016/S0379-6779(97)80294-9.

bredas, j.l. & street, g.b. (1985), Polarons, bipolarons, and solitons in conducting polymers, Accounts Chem. Res., 18(10): 309–315. DOI: 10.1021/ar00118a005.

carpi, f. & de rossi, d. (2005), Electroactive polymer-based devices for e-textiles in biomedicine, IEEE Trans. Information Technol. Biomed., 9(3): 295–318. DOI: 10.1109/TITB.2005.854514.

dall’acqua, l., tonin, c., peila, r., ferrero, f. & catellani, m. (2004), Performances and properties of intrinsic conductive cellulose-polypyrrole textiles, Synthetic Metals, 146(2): 213–221. DOI: 10.1016/j.synthmet.2004.07.005.

dall’acqua, l., tonin, c., varesano, a., canetti, m., porzio, w. & catellani, m. (2006), Vapour phase polymerisation of pyrrole on cellulose-based textile substrates, Synthetic Metals, 156(5–6): 379–386. DOI: 10.1016/j.synthmet.2005.12.021.

dhawan, a., ghosh, t.k., seyam, a.m. & muth, j.f. (2004a), Woven fabric-based electrical circuits: Part II: Yarn and fabric structures to reduce crosstalk noise in woven fabric-based circuits, Textile Res. J., 74(11): 955–960. DOI: 10.1177/004051750407401103.

dhawan, a., seyam, a.m., ghosh, t.k. & muth, j.f. (2004b), Woven fabric-based electri-cal circuits, Part I: Evaluating interconnect methods, Textile Res. J., 74(10): 913–919. DOI: 10.1177/004051750407401011.

dhawan, a., ghosh, t.k. & seyam, a. (2005), Fiber-based electrical and optical devices and systems, Textile Progress, 36(2): 1–84. DOI: 10.1533/jotp.36.2.1.64792.

dietzel, y., przyborowski, w., nocke, g., offermann, p., hollstein, f. & meinhardt, j. (2000), Investigation of PVD arc coatings on polyamide fabrics, Surface Coat-ings Technol., 135(1): 75–81. DOI: 10.1016/S0257-8972(00)00917-8.

dietzel, y., romstedt, d. & schiller, n. (2008), Metallization of textiles, Textilvered-lung, 43(7/8): 18–22.

ding, j., wang, h., lin, t. & lee, b. (2008), Electroless synthesis of nano-structured gold particles using conducting polymer nanoparticles, Synthetic Metals, 158(14): 585–589. DOI: 10.1016/j.synthmet.2008.04.019.

ferrero, f., napoli, l., tonin, c. & varesano, a. (2006), Pyrrole chemical polymeriza-tion on textiles: kinetics and operating conditions, J. Appl. Polym. Sci., 102(5): 4121–4126. DOI: 10.1002/app.24149.

gan, x., wu, y., liu, l., shen, b. & hu, w. (2008), Electroless plating of Cu-Ni-P alloy on PET fabrics and effect of plating parameters on the properties of conductive fabrics, J. Alloys Compounds, 455(1–2): 308–313. DOI: 10.1016/j.jallcom.2007.01.054.

garg, s., hurren, c. & kaynak, a. (2007), Improvement of adhesion of conductive polypyrrole coating on wool and polyester fabrics using atmospheric plasma treatment, Synthetic Metals, 157(1): 41–47. DOI: 10.1016/j.synthmet.2006.12.004.

gasana, e., westbroek, p., hakuzimana, j., de clerck, k., priniotakis, g., kiekens, p. & tseles, d. (2006), Electroconductive textile structures through electroless dep-osition of polypyrrole and copper at polyaramide surfaces, Surface and Coatings Technol., 201(6): 3547–3551. DOI: 10.1016/j.surfcoat.2006.08.128.

genies, e.m., petrescu, c. & olmedo, l. (1991), Conducting materials from poly-aniline on glass textile, Synthetic Metals, 41(1–2): 665–668. DOI: 10.1016/0379-6779(91)91153-2.



george, p.m., lyckman, a.w., lavan, d.a., hegde, a., leung, y., avasare, r., testa, c., alexander, p.m., langer, r. & sur, m. (2005), Fabrication and biocompatibility of polypyrrole implants suitable for neural prosthetics, Biomaterials, 26(17): 3511–3519. DOI: 10.1016/j.biomaterials.2004.09.037.

gregory, r.v., kimbrell, w.c. & kuhn, h.h. (1991), Electrically conductive non-metallic textile coatings, J. Industrial Textiles, 20(3): 167–175. DOI: 10.1177/152808379102000304.

håkansson, e., kaynak, a., lin, t., nahavandi, s., jones, t. & hu, e. (2004), Charac-terization of conducting polymer coated synthetic fabrics for heat generation, Synthetic Metals, 144(1): 21–28. DOI: 10.1016/j.synthmet.2004.01.003.

håkansson, e., amiet, a. & kaynak, a. (2006), Electromagnetic shielding properties of polypyrrole/polyester composites in the 1–18 GHz frequency range, Synthetic Metals, 156(14–15): 917–925. DOI: 10.1016/j.synthmet.2006.05.010.

håkansson, e., amiet, a., nahavandi, s. & kaynak, a. (2007), Electromagnetic inter-ference shielding and radiation absorption in thin polypyrrole fi lms, European Polym. J., 43(1): 205–213. DOI: 10.1016/j.eurpolymj.2006.10.001.

hearle, j.w.s. (2001), High-performance Fibres. Cambridge, Woodhead Publishing and CRC Press LLC.

holme, i., mcintyre, j.e. & shen, z.j. (1998), Electrostatic charging of textiles, Textile Progress, 28(1): 1–84.

hosseini, s.h. & pairovi, a. (2005), Preparation of conducting fi bres from cellullose and silk by polypyrrole coating, Iranian Polym. J., 14(11): 934–940.

hu, e., kaynak, a. & li, y. (2005), Development of a cooling fabric from conducting polymer coated fi bres: proof of concept, Synthetic Metals, 150(2): 139–143. DOI: 10.1016/j.synthmet.2005.01.018.

huang, h. & liu, w. (2006), Polyaniline/poly(ethylene terephthalate) conducting composite fabric with improved fastness to washing, J. Appl. Polym. Sci., 102(6): 5775–5780. DOI: 10.1002/app.23875.

jiang, s.q. & guo, r.h. (2008), Effect of polyester fabric through electroless Ni-P plating, Fibers and Polymers, 9(6): 755–760. DOI: 10.1007/s12221-008-0118-x.

jiang, s.q., newton, e., yuen, c.w.m. & kan, c.w. (2006), Chemical silver plating on cotton and polyester fabrics and its application on fabric design, Textile Res. J., 76(1): 57–65. DOI: 10.1177/0040517506053827.

jiang, s., newton, e., yuen, c.-w.m. & kan, c.-w. (2007), Application of chemical silver plating on polyester and cotton blended fabric, Textile Res. J., 77(2): 85–91. DOI: 10.1177/0040517507078739.

kathirgamanathan, p., toohey, m.j., haase, j., holdstock, p., laperre, j. & schmeer-lioe, g. (2000), Measurements of incendivity of electrostatic discharges from textiles used in personal protective clothing, J. Electrostatics, 49(1–2): 51–70. DOI: 10.1016/S0304-3886(00)00003-6.

kaynak, a. (1996), Electromagnetic shielding effectiveness of galvanostatically syn-thesized conducting polypyrrole fi lms in the 300–2000 MHz frequency range, Materials Res. Bull., 31(7): 845–860. DOI: 10.1016/0025-5408(96)00038-4.

kaynak, a., wang, l., hurren, c. & wang, x. (2002), Characterization of conductive polypyrrole coated wool yarns, Fibers Polym., 3(1): 24–30. DOI: 10.1007/BF02875365.

kaynak, a., lin, t., foitzik, r.c., wang, l. & wang, x. (2006), Fibre coating composi-tion and deposition of conductive coating on fi brous material. PCT Int. Appl. WO 2006-AU135 20060203. Australian Wool Innovation Limited and Deakin Univer-sity, Australia.



kim, m.s., kim, h.k., byun, s.w., jeong, s.h., hong, y.k., joo, j.s., song, k.t., kim, j.k., lee, c.j. & lee, j.y. (2002), PET fabric/polypyrrole composite with high electrical conductivity for EMI shielding, Synthetic Metals, 126(2–3): 233–239. DOI: 10.1016/S0379-6779(01)00562-8.

kim, b., koncar, v., devaux, e., dufour, c. & viallier, p. (2004a), Electrical and mor-phological properties of PP and PET conductive polymer fi bers, Synthetic Metals, 146(2): 167–174. DOI: 10.1016/j.synthmet.2004.06.023.

kim, s.h., oh, k.w. & bahk, j.h. (2004b), Electrochemically synthesized polypyrrole and Cu-plated nylon/spandex for electrotherapeutic pad electrode, J. Appl. Polym. Sci., 91(6): 4064–4071. DOI: 10.1002/app.13625.

kincal, d., kumar, a., child, a.d. & reynolds, j.r. (1998), Conductivity switching in polypyrrole-coated textile fabrics as gas sensors, Synthetic Metals, 92(1): 53–56. DOI: 10.1016/S0379-6779(98)80022-2.

kuhn, h.h. (1997), Adsorption at the liquid/solid interface: conductive textiles based on polypyrrole, Textile Chemist and Colorist, 29(12): 17–21.

kuhn, h.h., child, a.d. & kimbrell, w.c. (1995), Toward real applications of con-ductive polymers, Synthetic Metals, 71(1–3): 2139–2142. DOI: 10.1016/0379-6779(94)03198-F.

lee, h.s. & hong, j. (2000), Chemical synthesis and characterization of polypyrrole coated on porous membranes and its electrochemical stability, Synthetic Metals, 113(1–2): 115–119. DOI: 10.1016/S0379-6779(00)00193-4.

lee, j.-w., hart, d.l., shinn, e.t., cooley, a.j., white, e.l., macdowell, p.w., schomburg, e., slenski, g.a., watkins, a.n. & yang, r.l. (2005), Multifunctional metal/polymer hybrid fi ber for space and aerospace applications, Materials Research Society Symposium Proceedings, 851(Materials for Space Applications), Syscom Technology, Inc., Columbus, OH, USA., Materials Research Society, 177–182.

lekpittaya, p., yanumet, n., grady, b.p. & o’rear, e.a. (2004), Resistivity of conductive polymer-coated fabric, J. Appl. Polym. Sci., 92(4): 2629–2636. DOI: 10.1002/app.20270.

lin, t., wang, l., wang, x. & kaynak, a. (2005), Polymerising pyrrole on polyester textiles and controlling the conductivity through coating thickness, Thin Solid Films, 479(1–2): 77–82. DOI: 10.1016/j.tsf.2004.11.146.

locher, i. & troster, g. (2008), Enabling technologies for electrical circuits on a woven monofi lament hybrid fabric, Textile Res. J., 78(7): 583–594. DOI: 10.1177/0040517507081314.

macdiarmid, a.g. (2001), Synthetic metals: a novel role for organic polymers, Synthetic Metals, 125(1): 11–22. DOI: 10.1016/S0379-6779(01)00508-2.

malinauskas, a. (2001), Chemical deposition of conducting polymers, Polymer, 42(9): 3957–3972. DOI: 10.1016/S0032-3861(00)00800-4.

martin, c.r. (1994), Nanomaterials: a membrane-based synthetic approach, Science, 266(5193): 1961–1966. DOI: 10.1126/science.266.5193.1961.

molina, j., del río, a.i., bonastre, j. & cases, f. (2008), Chemical and electro-chemical polymerisation of pyrrole on polyester textiles in presence of phosphotungstic acid, European Polym. J., 44(7): 2087–2098. DOI: 10.1016/j.eurpolymj.2008.04.007.

oh, k.w., park, h.j. & kim, s.h. (2003), Stretchable conductive fabric for electro-therapy, J. Appl. Polym. Sci., 88(5): 1225–1229. DOI: 10.1002/app.11783.

pionteck, j. & wypych, g. (2007), Handbook of Antistatics. Toronto, Canada, ChemTec Publishing.



ponce de leon, c., campbell, s.a., smith, j.r. & walsh, f.c. (2008), Conducting polymer coatings in electrochemical technology: Part 2 – application areas, Trans. Inst. Metal Finishing, 86(1): 34–40. DOI: 10.1179/174591908X264392.

post, e.r., orth, m., russo, p.r. & gershenfeld, n. (2000), E-broidery: Design and fabrication of textile-based computing, IBM Systems J., 39(3): 840–860.

rizvi, s.a.h., crown, e.m., osei-ntiri, k., smy, p.r. & gonzalez, j.a. (1995), Electro-static characteristics of thermal-protective garments at low humidity, J. Textile Inst., 86(4): 549–558.

roh, j.-s., chi, y.-s., kang, t.j. & nam, s.-w. (2008), Electromagnetic shielding effec-tiveness of multifunctional metal composite fabrics, Textile Res. J., 78(9): 825–835. DOI: 10.1177/0040517507089748.

sato, k., yamaura, m., hagiwara, t., murata, k. & tokumoto, m. (1991), Study on the electrical conduction mechanism of polypyrrole fi lms, Synthetic Metals, 40(1): 35–48. DOI: 10.1016/0379-6779(91)91487-U.

sen, a.k. (2008), Coated Textiles: Principles and applications. Boca Raton, FL, CRC Press LLC/Taylor & Francis Group.

slenski, g.a. & walz, m.f. (2006), Novel technologies for improving wire system integrity, 9th Joint FAA/DoD/NASA Conference on Aging Aircraft, 1–19, Hyatt Regency-Atlanta, GA.

tavanai, h. & kaynak, a. (2007), Effect of weight reduction pre-treatment on the electrical and thermal properties of polypyrrole coated woven polyester fabrics, Synthetic Metals, 157(18–20): 764–769. DOI: 10.1016/j.synthmet.2007.08.008.

tessier, d., dao, l.h., zhang, z., king, m.w. & guidoin, r. (2000), Polymerization and surface analysis of electrically-conductive polypyrrole on surface-activated poly-ester fabrics for biomedical applications, J. Biomater. Sci., Polymer Edition, 11: 87–99. DOI: 10.1163/156856200743517.

varesano, a., aluigi, a., fl orio, l. & fabris, r. (2009), Multifunctional cotton fabrics, Synthetic Metals, 159(11): 1082–1089. DOI: 10.1016/j.synthmet.2009.01.036.

wang, h., ding, j., lee, b., wang, x. & lin, t. (2007), Polypyrrole-coated electrospun nanofi bre membranes for recovery of Au(III) from aqueous solution, J. Mem-brane Sci., 303(1–2): 119–125. DOI: 10.1016/j.memsci.2007.07.012.

wang, j., kaynak, a., wang, l. & liu, x. (2006), Thermal conductivity studies on wool fabrics with conductive coatings, J. Textile Inst., 97(3): 265–269. DOI: 10.1533/joti.2005.0298.

wang, l., lin, t., wang, x. & kaynak, a. (2005), Frictional and tensile properties of conducting polymer coated wool and alpaca fi bers, Fibers Polym., 6(3): 259–262. DOI: 10.1007/BF02875651.

wei, q., yu, l., ning, w. & shanhu, h. (2008), Preparation and characterization of copper nanocomposite textiles, J. Industrial Textiles, 37(3): 275–283. DOI: 10.1177/1528083707083794.

wilson, n. (1977), The risk of fi re or explosion due to static charges on textile cloth-ing, J. Electrostatics, 4(1): 67–84. DOI: 10.1016/0304-3886(77)90109-7.

winterhalter, c.a., teverovsky, j., wilson, p., slade, j., horowitz, w., tierney, e. & sharma, v. (2005), Development of electronic textiles to support networks, com-munications, and medical applications in future U.S. military protective clothing systems, IEEE Transactions on Information Technology in Biomedicine, 9(3): 402–406. DOI: 10.1109/TITB.2005.854508.

wu, j., zhou, d., too, c.o. & wallace, g.g. (2005), Conducting polymer coated Lycra, Synthetic Metals, 155(3): 698–701. DOI: 10.1016/j.synthmet.2005.08.032.