Conductive textiles

Conductive textiles K. (Kelvin) Fu, R. Padbury, O. Toprakci, M. Dirican, X. Zhang North Carolina State University, Raleigh, NC, United States

13.1

13

Introduction

Conductive textiles are textile structures that can conduct electricity. Conductive textiles can be either made using conductive fibers or by depositing conductive layers onto nonconductive textiles. Typical applications of conductive textiles include antistatic textiles, electromagnetic (EM) shielding, and e-textiles for flexible electronics. Conductive textiles can inhibit the static charge generated on fabric, to avoid uncomfortable feelings and electrical shocks. Conductive textiles can be used as shielding materials to block EM radiation that is harmful to electronics and human. With the development of flexible electronics, conductive textiles are becoming important building blocks for the design of wearable electronics for a broad range of applications. In this chapter, we discuss the fundamental principles of conductive textiles with a focus on three types of important applications including antistatic, EM shielding, and e-textiles. We will then review the recent development of advanced coating technology for conductive textiles.

13.2

Antistatic textiles

Static electricity is the buildup of electric charge on the surface of objects, which can cause many problems for textile materials and fabrics in manufacturing and handling. In dry textile process, fibers and fabrics will tend to generate electrostatic charges from friction when they are moving at high speeds on different surfaces, such as conveyer belts, transport bands, and driving cords, causing fibers and yarns to repel each other. Static electricity can also produce electrical shocks, and the “charging” can cause the ignition of flammable substances. In general, two approaches are known to prevent static electricity in textiles: one is to create a conducting surface and the other is to produce a hydrophilic surface. Therefore, it is important to design antistatic textiles to avoid the potential hazards caused by static electricity. In this section, the objective is to present the mechanism of antistatic textiles and common methods to achieve an antistatic function for durable and nondurable antistatic textiles.

Engineering of High-Performance Textiles. http://dx.doi.org/10.1016/B978-0-08-101273-4.00017-2 © 2018 Elsevier Ltd. All rights reserved.

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13.2.1 Mechanism of antistatic textiles Textile fibers including natural fibers and synthetic fibers are generally intrinsically nonconductive. Static charges are often found in synthetic fibers, especially in a dry environment. The static charges are generated by the motion of textile surfaces and during this process, negative charges are generated and accumulated on one surface and positive ones are on the other surface. The negatively charged textile material has a stronger affinity for electrons that steal from the other textile material after the two textile materials are separated. A triboelectric series can be used to predict which material will become positive or negative and how strong the effect will be among textile materials. Charging will occur if the contacting surfaces are separated. Parameters to determine charge formation include the conductivity of materials, the size topography of the textile surfaces, the separation velocity, friction, and the humidity of the environment. For textile materials, synthetic fibers have more static problems compared with natural fibers. Since most synthetic fibers, such as polyester, are hydrophobic, they have low moisture regain and have a much higher electrical resistance on the surface, which will accumulate large amount of charges on the surface and cause static charge. In comparison, natural fibers, such as cotton, have high moisture regain and exhibit higher conductivity on the surface that allows the accumulated charges to dissipate in surroundings and has no static charge concern. It is generally accepted that improving the hydrophilicity of textiles to increase the rate of static charge dissipation and applying conductive textiles to inhibit the charge generation are two effective ways to achieve antistatic textiles.

13.2.2 Antistatic textiles with hydrophilic materials Hydrophilic materials can help textiles to increase the rate of dissipation of static charges in air. Most synthetic polymer fibers are hydrophobic and they tend to generate high static charges on the surface; one simple way to reduce the static charge accumulation is to incorporate hydrophilic fibers, such as cotton and viscose, to improve the hydrophilicity of textile fabrics. The other approach is to apply hydrophilic treatment in chemical and physical ways to absorb certain amounts of water on the textile surface to obtain an antistatic effect.

13.2.3 Antistatic textiles with conductive materials Conductive textiles can achieve an antistatic function by using conductive filaments or a conductive coating to inhibit the static charge generated on fabric. Conductive filaments can be produced by adding conducting additives, such as metals and carbons, into synesthetic fibers or co-weaving metal wires with synthetic fibers to increase the conductivity of fabrics. Metals including Au, Ag, and Cu, and carbons including carbon black (CB), carbon nanotubes (CNTs), and graphenes, can be used as conducting additives for making conductive fibers or conductive coatings. Conductive coatings can be deposited onto textiles by printing, coating, and physical vapor

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deposition (PVD) methods. Details of conductive textile coatings are discussed in a later section of this chapter. Although hydrophilicity and conductivity are two strategies to achieve antistatic function, it should be noted that when we design antistatic textiles, both these two factors need to be considered to enhance maximally the antistatic effect of textiles.

13.3

EM shielding textiles

13.3.1 Mechanism of EM shielding EM radiation refers to radiations in the form of gamma rays, X-rays, infrared light, ultraviolet (UV) light, visible light, microwaves, and radio waves, in which electric and magnetic fields vary simultaneously. EM radiation travels at the speed of light in vacuum with different wavelengths or frequencies. EM radiation includes an electric and a magnetic field and they oscillate at right angles to each other. EM radiation may cause malfunctioning of electronic equipment by creating some interference. Furthermore, EM radiation with high energy, high frequency and low wavelength can also cause health issues, so EM radiation should be shielded. EM shielding is the process of restricting the diffusion of EM fields into a space. In this process, electrically and/or magnetically conductive barriers are used. Shielding is a very common method for protecting electrical equipment and human beings from radiating EM fields. This barrier can be rigid [cement for construction (Chiou et al., 1989; Xiong et al., 2011)] or flexible (i.e., textiles, gaskets). When an EM beam passes through an object, the EM beam interacts with molecules of the object and this interaction may take place as absorption, reflection, polarization, refraction, diffraction, or multiple reflections (MRs) through the object. When a barrier is exposed to an EM radiation, some of the incident energy is lost due to the shielding effect. For the EM shielding effect, absorption (A), reflection (R), and MR characteristics of the barrier are critically important. Reflection: When an electric field is applied to a sample of material, charges inside the shield tend to move. If an impedance mismatch occurs between the incident wave and the shield, the electric field cannot penetrate inside the shield and it is reflected. Reflection loss of the shield is directly proportional to the relative conductivity of the sample, and inversely proportional to the frequency and relative permeability of the sample. It would not be affected by the thickness of the shield. Multiple reflection: If the shield has a multilayer structure or a multiphase composition (i.e., composites and foams), then the incident EM wave gets reflected on the surface as well as on the inside of the shield. In practice, if shielding effectiveness due to absorption loss is higher than 10 dB, MR loss can be neglected (Roh et al., 2008). As MR loss is high for thin shields, it is of significance for measuring thin samples at low frequencies (especially in the kHz range). Absorption: If an EM wave passes through the shield, its amplitude decreases exponentially due to induced currents which later give rise to ohmic loss and material heating. Materials with electric and/or magnetic dipoles can be used as EM absorbers

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(i.e., Fe3O4). Absorption loss is independent of the type of source field and mainly depends on the physical characteristics of the shield (i.e., thickness, conductivity, permeability, etc.). For better absorption loss, a shield should possess high conductivity, high permeability, and sufficient thickness; alongside these criteria, a good shielding material should also possess low volume, low surface resistivity, high thickness, and good mechanical strength. For composite shielding materials, shielding by absorption depends on the distance between particles and/or the electrical resistivity of the composites. In contrast, shielding by reflection depends on the conductivity and thickness of the sample and the concentration of conductive filler. Mathematically, electric shielding effectiveness (S1) and magnetic shielding effectiveness (S2) can be defined as  S1 ¼ 20 log

Ein Eout



 and S2 ¼ 20 log

 Min ðdBÞ Mout

(13.1)

where “in” and “out” refer to incident and transmitted waves, respectively. E is the electrical field strength component (V m1) and M is the magnetic field strength component (H m1). As shown in Eq. (13.2), shielding effectiveness can also be expressed in terms of attenuation of power (W).  S ¼ 10 log

 Pin ðdBÞ Pout

(13.2)

Shielding effectiveness is sum of absorption loss (A), reflection loss (R) and MR losses (Geetha et al., 2009; Huang, 1995; Maity et al., 2013). This is defined in Eq. (13.3). S1 ¼ S2 ¼ S ¼ A + R + MRðdBÞ

(13.3)

Evaluation of EM shielding effectiveness is shown in Table 13.1. According to this quantitative analysis, shielding material/structure should create at least a 30 dB power loss ratio to receive an excellent-grade shielding effectiveness performance. The target value of the EM shielding effectiveness needed for commercial applications is around 20 dB.

Table 13.1

Evaluation of EM shielding effectiveness (Ozen et al.,

2013) Grade

Excellent

Very good

Good

Moderate

Fair

Percentage of EM shielding (%) Shielding effectiveness (dB)

99.9

99.9–99.0

99–90

90–80

80–70

>30

30–20

20–10

10–7

7–5

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13.3.2 Measurement of EM shielding To regulate the EM shielding effectiveness behavior of fabrics, many standards are used by device manufacturers. Among them, MIL-STD-285 (1956), MIL-G-83528 (1992), IEEE-STD 299 (97) (1997), DIN-VG-95373-15 (2016), ASTM E1851 (2015), and ASTM D 4935-99 (1999) are well known. MIL-STD-285 is the first standard related to EM shielding. It is not so accurate and repeatable due to the presence of discontinuities between the sample and the measuring enclosure, especially for smallsized samples. The frequency range is also limited (up to 400 MHz). To overcome these issues related with the standard, some modifications were made by Perumalraj et al. (2010). The IEEE-STD 299 (97) standard is suitable for large enclosures (especially, >2 m) and it has a wide range of working frequency (expandable from 9 kHz–18 GHz to 50 Hz–100 GHz). DIN-VG-95373-15 is suitable for measuring EM shielding behavior of small- to medium-sized enclosures between 30 and 200 MHz frequency ranges. ASTM D 4935-99 is a very commonly used standard for EM shielding textiles. It has a large frequency range (between 30 MHz and 1.5 GHz). There are several parameters limiting the usage of higher frequencies such as sample thickness (which should be lower than 1/100 of wavelength), requirement of device calibration, and uniform sample-antenna distance (Badic and Marinescu, 2002). Different test methods were investigated by using these standards. These are the open-field test, coaxial transmission line test, shielded box test, and shielded room tests. The open-field test method requires a large open field (30  30 m2) and there should not be any metallic or conductive object between the sample and the receiving antenna. The shielded box test is only suitable for materials in near-field conditions and its measurement range is limited (i.e., up to 500 MHz). Furthermore, inadequate contact between the sample and the shielded box may affect the repeatability and reliability of results. The shielded room test is found to improve the reliability of the shielded box test. It is possible to obtain repeatable results by using the shielded room test, especially in externally disturbing conditions. Among these test methods, the coaxial transmission line test is the most common one due to its suitability to measure small-sized, flat, and thin conductive samples in an extended range of frequency ( Jagatheesan et al., 2015).

13.3.3 Materials used in EM shielding The materials for shielding should have high electric conductivity and magnetic permeability. To this end, electrically conductive materials, conductive composite materials, ferromagnetic materials, or magnetic composite materials are used commonly in EM shielding applications. Conductive fillers such as metals, alloys, and carbonaceous materials (carbon black—CB, graphite, etc.) are common components in conductive composite materials. These materials can be used as a filler in the composite as well as shielding structure by itself (Faraday cage). The addition of conductive filler into the composite also increases its thermal conductivity. This reduces the chances of thermal and chemical degradation during processing and improves its shelf life. Different materials used in EM shielding are listed as follows:

310 l

l

l

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Ferrite (Fe3O4) and other magnetic materials (Rubacha and Zieba, 2007; Morari et al., 2011). Nonferromagnetic materials (metals, alloys, etc.). Carbonaceous materials (graphite, graphene, CNTs, carbon nanocoils (CNCs), etc.). Conductive polymers.

Ferromagnetic materials, such as Fe, Ni, Co, Fe3O4, MnFe2O4, NiFe2O4, etc., can show a spontaneous magnetization under an EM field. These materials have a high level of magnetic permeability, which helps in storing/attenuating strong magnetic fields than many other metals. Metals are the primary candidates for EM shielding due to their high electrical conductivity. They can be used as sheet, coated sheet, or wire [Cu, Ag, Au, Ni wires (Rubacha and Zieba, 2006; Sonehara et al., 2009), Ni (Sonehara et al., 2008), or Zn (Koprowska et al., 2008) coated-wires, etc.]. For high EM shielding effectiveness, the reflection mechanism is often adopted due to the presence of free electrons in metal structure. Metal coating can be applied on bulk materials, fibers, or particles, but coatings tend to suffer from their poor wear or scratch resistance (Chung, 2001). Furthermore, metals have certain shortcomings such as heavy weight, narrow breaking tolerance, oxidization in atmosphere and not being usable in corrosive media. Shyr and Shie studied the effects of conductivity, magnetic loss, and complex permittivity when using blended textiles of polyester fibers (polyethylene terephthalate, PET) with stainless steel fibers (SSF) on EM wave shielding mechanisms at EM wave frequencies ranging from 30 to 1500 MHz. Their results showed that the EM interference shielding of the SSF/PET textiles show an absorption-dominant mechanism, which is attributed to the dielectric loss and the magnetic loss at a lower frequency, and to the magnetic loss at a higher frequency, respectively (Shyr and Shie, 2012). Carbonaceous materials, such as CB, graphite, CNTs, CNCs, and graphite nanosheets, exhibit high EM shielding effectiveness due to their thermally activated carrier hopping associated with defect states. In practice, carbon-based plastic composite materials can be used as shields. Carbonaceous materials used in those composites are graphite (Morari et al., 2011), carbon nanotubes (CNT) (Park et al., 2010; Al-Saleh and Sundararaj, 2009), flexible and colloidal graphite (Chung, 2001), continuous carbon nanofiber (Luo and Chung, 1999), graphene (Wen et al., 2014), etc. Ghosh and Chakrabarti (2000) produced a CB/vulcanized rubber composite with 33 wt% CB loading. This composite showed 16 dB shielding effectiveness in the range of 8–12 GHz. Instead of a conventional polymer, the use of a conducting polymer as matrix for the composite improves shielding effectiveness by changing the percolation threshold of the composite. Agnihotri et al. showed that an addition of 0.5 wt% graphite nanoplates (GNPs) to PEDOT:PSS took the EM shielding effectiveness to  30 dB. For higher amount of GNP loadings (25 wt%), the shielding effectiveness value reached 70 dB at 0.8-mm thickness (Agnihotri et al., 2015). In another study, 1.4-mm-thick carbon fiber composites (with very high carbon content) showed higher than 70 dB shielding effectiveness at 8–12 GHz frequency range. The shielding mechanism was dominated by reflection due to the larger number of connected conduction paths in the sample (Rea et al., 2005). Luo and Chung used a carbon-matrix composite with continuous carbon fibers to improve shielding effectiveness in the frequency range from 0.3 MHz to 1.5 GHz.

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A 2.4-mm-thick composite was found to be an excellent EM shielding material with shielding effectiveness 124 dB, low surface impedance and high reflectivity (Luo and Chung, 1999). Wang et al. compared EM shielding effectiveness of printed composite films made of CB, graphite, and CNTs with different aspect ratios (300). The samples had 15 wt% of carbon fillers. CNTs with a higher aspect ratio showed better shielding effectiveness even with the small thickness of 100 μm (Wang et al., 2009). In another study, a 1-mm-thick MWCNT/PP composite film (7.5 vol% of MWCNT) showed 36.4 dB EM shielding effectiveness at 12.4 GHz. With increasing thickness and CNT content, the EM shielding effectiveness was improved. It was also observed that absorption was the primary shielding mechanism (Al-Saleh and Sundararaj, 2009). Liang et al. achieved a 21 dB shielding efficiency from 15 wt% graphene loaded-graphene/epoxy composites over a frequency range of 8.2–12.4 GHz (Liang et al., 2009). To improve dipole polarization and the hopping conductivity of composites, reduced-graphene oxide (r-GO) was also used along with Fe3O4 and polystyrene (PS). The EMI shielding effectiveness of the PS/r-GO/Fe3O4 composite was more than 30 dB in the frequency range of 9.8–12 GHz with 2.24 vol% of graphene loading (Chen et al., 2015). To observe high-temperature (200°C) performance, the r-GO/SiO2 composite was prepared with 20 wt% r-GO loading and  38 dB of EM shielding effectiveness was obtained at a 8.2–12.4 GHz frequency range (Wen et al., 2014). Intrinsically conductive polymers (ICPs): Metals are prone to corrosion, they are costly, and they possess high density. On the other hand, ICPs are macromolecules composed of conjugated double bonds. This sp2 hybridization with ionic doping makes ICPs conductive. ICPs are lightweight materials and they show resistance to corrosion. Unlike conventional polymers, ICPs are nontransparent toward EM radiations and their electrical properties can be altered. These advantages of ICPs make them attractive materials in EM shielding applications [aerospace (Naishadham, 1992), etc.]. Examples of ICPs are shown in Fig. 13.1.

Poly-p-Phenylene Vinylene

Polypyrrole H N N H

n

PEDOT:PSS o

Polythiophene *

S

Polyacetylene n

n

n

Polyaniline

N H n

Polyphenylene

PEDOT

o

o s

s o -

SO3

n *

Fig. 13.1 Common intrinsically conductive polymers.

Polyphenylene sulfide

o

S

s o n

n

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The major drawbacks of ICPs are their moderate mechanical properties and limited processability. Naishadham found that multiple layers of conductive polyacetylene (PAc) had better shielding effectiveness compared with single-layer PAc at the same thickness (Naishadham, 1992). In another study, poly(3,4-ethylenedioxythiophene) (PEDOT)/PET and polypyrrole (PPy)/PET composites were prepared by spraycoating on a PET fabric. Kim et al. observed that a PPy/PET composite with 0.3 Ω*cm volume resistivity had 35 dB EM shielding effectiveness from 50 MHz to 1.5 GHz. It was also observed that PEDOT/PET samples with 3.5 Ω*cm volume resistivity showed 14 dB shielding effectiveness at settings similar to PPy/PET samples (Kim et al., 2003). Thermal oxidation behavior of PPy can be altered by using different polymerization techniques. Kim et al. produced a dense structure of PPy coating by electrochemical polymerization. Tight packing made PPy more defensive to attack by oxygen, resulting in much better environmental stability (Kim et al., 2002; Cetiner, 2014; Avloni et al., 2007). To improve the mechanical and shielding properties of polyaniline (PANI), PANI was coated on MWCNTs and this material was proposed as a hybrid conductive filler in various thermoplastic matrices for making structurally strong microwave shielding composites (Saini et al., 2009). In addition to homopolymers of ICPs, copolymers of ICPs were also produced. For example, Saini and Choudhary synthesized a highly conductive (12.8 S cm1) copolymer of aniline:2-isopropyl aniline (CP95Ip, 95:5) with high EM shielding effectiveness values (23.2 dB) (Saini and Choudhary, 2013; Table 13.2).

13.3.4 EM shielding designs in textiles EM shielding textile materials can be found in the form of woven, knitted, or nonwoven fabric. The major components of those fabrics are fibers and yarns. To achieve an effective shielding behavior, these fibers or yarns should be electrically conductive. Conductive yarns can be made by blending conductive fibers with conventional staple fibers, twisting conductive/insulator filaments together, or conductive coating. Conductive coating by using different techniques (metal plating, etc.), compounding with conductive fillers, or just using ICPs as textile media are basic methods to create EM shielding materials. For example, conductive metallic yarn (silver, copper, etc.) can be wrapped with insulating textile materials to create hybrid yarns, which can be directly integrated into woven or knitted structures (Erdumlu and Saricam, 2015; Rau et al., 2011; Apreutesei et al., 2014).

Table 13.2

EM shielding effectiveness of different materials

Materials

Frequency

SE (dB)

Ref.

MWCNT Pani CP95Ip

15–1000 MHz 12.4–18 GHz 12–18 GHz

23 27–39 23.2

Wang et al. (2009) Saini et al. (2009) Saini and Choudhary (2013)

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The most common woven fabric designs include plain, twill, and satin patterns. Hybrid yarns or metallic wires can be integrated into these designs as warp or weft yarns. Roh et al. (2008) found that the EM shielding effectiveness of the fabric decreased with the increase in fabric openness. Das et al. analyzed the effect of material, yarn count, and number of layers of fabrics on the EM shielding effectiveness of textile fabrics. According to their results, metal sheets (copper and brass) showed much better performance than cellulose and PET sheets. With the increase in thickness, yarn diameter, weight per unit area, and the number of fabric layers, EM shielding effectiveness was also increased (Das et al., 2009). Perumalraj et al. measured the EM shield effectiveness of woven fabrics produced from copper-cotton (core-sheath) blend yarn. In their study, the effect of yarn count, weft and warp density, cover factor, weave type, copper wire diameter, and number of fabric layers on the shielding effectiveness of fabrics was analyzed in the frequency range of 20 MHz– 18 GHz. With an increase in the number of conductive fabric layers, finer yarn count, warp density, weft density, and cover factors, an increase in shielding effectiveness was observed. With an increase in copper wire diameter, a decrease in shielding effectiveness was observed (Perumalraj et al., 2009). Another shield design by using hybrid yarns is knitted fabric structure (Zhu et al., 2012; Ortlek et al., 2013). Knitted fabrics are chosen for wearable shields due to their comfort properties. Various knitted fabric designs are schematically shown in Fig. 13.2. Cheng et al. used uncommingled yarns made out of Cu wire/glass fiber/ polypropylene (PP) fibers to produce knitted fabrics for EM shielding. They measured the effect of conductive filler content and stitch density on EM shielding effectiveness in the range of 300 kHz–3 GHz. It was found that the shielding effectiveness was increased with increasing conductive filler content and stich density (Cheng et al., 2000). Perumalraj and Dasaradan produced plain, rib, and interlock fabrics made from Cu/cotton core-sheath yarns with 12.99, 13.13, and 15.22 tightness factors, respectively. They also observed that EM shielding effectiveness increased with the increase in tightness factor and stitch density of the fabric (Perumalraj and Dasaradan, 2009). Apreutesei et al. (2015) studied the knitted structures with different designs used for EM shielding. They found that half-cardigan showed the best performance in terms of EM shielding effectiveness.

(A)

(B)

(C)

(D)

Fig. 13.2 Schematic views of various knitted structures (A) plain, (B) 1/1 rib, (C) 1/2 rib, and (D) interlock (Cheng et al., 2000).

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Conductive nonwoven fabrics can be directly made from fibers by melt blowing, physical, or chemical bonding, etc. For conductive nonwoven fabric production, conductive fibers can be used directly (Ozen et al., 2013; Ozen and Sancak, 2016). Conductive coating of nonwovens is another approach to produce EM shielding nonwoven designs. Conductive coating can be done by several methods such as sputtering (Sonehara et al., 2008), plasma metallization (Koprowska et al., 2008), screen printing (Wang et al., 2009), metal coating (Ozen et al., 2016), electroless metal plating (Han et al., 2001), surface polymerization (Dhawan et al., 2002), and so on. Ozen et al. prepared needle-punched nonwoven fabrics made from PET/SS, PP/carbon, and Ag-coated PA fibers and measured the EM shielding performance in the range of 15–3000 MHz. They investigated the effect of fabric thickness, conductive fiber content, and needle-punch density on EM shielding effectiveness. In parallel with the previous research, it was observed that EM shielding effectiveness values increased with the increase in fabric thickness, conductive filler content, and needle-punch density (Ozen et al., 2013, 2016; Ozen and Sancak, 2016).

13.4

E-textiles

Electrically conductive fibers and yarns have attracted great interest because of their distinguished features including reasonable electrical conductivity, flexibility, electrostatic discharge, and EM interference protection (Tao, 2005; Liu et al., 2010). Conductive textile fibers and yarns are the primary component for wearable smart textiles introduced particularly for different applications such as sensors, EM interference shielding, electrostatic discharge, and data transfer in clothing; hence, the demand for electrically conductive fibers and yarns is evergrowing (Kim et al., 2004). The development of novel conductive fibers and yarns also becomes crucial with technological improvement in wearable electronics such as wearable displays, solar cells, actuators, data managing devices, and biomedical sensors (Coyle et al., 2007; Maccioni et al., 2006; Post et al., 2000). Application requirements play a critical role in selecting the conductivity of smart textile electronics. For some textile applications like lighting, considerable current is necessary and low ohmic (high conductivity) fibers or yarns are preferred. On the other hand, for certain sensing or heating applications lower conductivity would work better; so they require fibers or yarns exhibiting lower electrical conductivity (Cherenack and van Pieterson, 2012). Textile electronics need flexible and mechanically stable conducting materials to ensure electronic capabilities in clothing (R.F. Service, 2003; De Rossi, 2007). Table 13.3 summarizes the most common fabrication methods to produce conductive textiles at the fiber and yarn level. Metallic fibers and metallic fiber-based yarns have been used traditionally as conducting material in wearable electronics. Although metallic fibers and yarns have various advantages due to their high conductivity, availability, and relatively low production cost, some drawbacks of these materials, such as poor flexibility and bendability, heavy weight, and proneness to oxidation under

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Overview of fabrication methods to produce conductive textile fibers and yarns (Schwarz and Van Langenhove, 2013)

Table 13.3

During fabrication Posttreatments

Fiber level

Yarn level

Spinning, wire drawing, melt spinning, wet spinning Coating, ink-jet printing

Spinning, twisting Coating

ambient conditions, limit their convenience for wearable electronics (Le et al., 2013). The conventional process to fabricate metal fibers is wire drawing, which is a mechanical fabrication process. Different metal monofilaments including copper (Cu) and silver-plated copper (Cu/Ag) filaments, brass (Ms) and silver-plated brass (Ms/Ag) filaments, aluminum (Al) filaments, and copper-clad aluminum (CCA) filaments are fabricated by the Swiss company Elektrisola Feindraht AG (Escholzmatt, Switzerland) (Stoppa and Chiolerio, 2014). These metal filaments are suitable for direct use in weaving and knitting to produce electronic textiles or they can be blended with all sorts of textile fibers to fabricate electrically conductive yarns for smart-textile applications. In the last two decades, nonmetallic conductive fibers and yarns are introduced for wearable smart textile applications to ensure better textile properties such as flexibility, soft handling, drape, and washability. These nonmetallic electroconductive fibers and yarns are generally fabricated by melt spinning or wet spinning from inherently conductive polymers and their composites with conductive filler (CNTs, CB, metal powder, etc.), or by coating textile fibers or yarns with electrically conducting materials such as conductive polymers, metal powder, CB, and CNTs (Pomfret et al., 1999; Lu et al., 1996; Xue and Tao, 2005; Xue et al., 2007). Melt spinning is considered one of the most versatile methods for fabricating conductive polymeric textile fibers. To this end, high-conductivity blends of PANI, PPy with polycaprolactone, polyolefins, PS, and poly (ethyleneterephtalate) polymers are produced by dispersing conductive fillers in a thermoplastic polymer and blending them by a mechanical mixing process (Hosier et al., 2001; Ikkala et al., 1995; Yang et al., 1998). The efficiency of the melt-mixing process can be improved by varying the melting temperature, mechanical mixing time, and mixing speed. Another method for fabricating conductive fibers is wet spinning. Various wet spinning techniques have been introduced for the fabrication of conductive fibers of PANI and its derivatives. For instance, polyblend and block copolymers of PANI and poly(p-phenylene-terephthalamide) were spun from homogeneous solutions in concentrated sulfuric acid by Andreatta et al. (Andreatta et al., 1990). In another study, Mattes et al. (1997) reported PANI fibers spun from highly concentrated emeraldine base solution using the wet spinning method. On the other hand, wet spinning of inherently conductive PANI fibers in a one-step process was achieved by Pomfret et al. (2000) from the solutions of PANI protonated with

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2-acrylamido-2-methyl-1-propanesulfonic acid in dichloroacetic acid. Drawbacks for the wet-spinning production of conductive fibers are the use of large quantities of coagulation liquids and the necessity to remove solvents after spinning (Bashir et al., 2011). Although conductive polymers can be produced in the fiber and yarn form, their mechanical properties are not suitable for textile processing and applications. A more economic and efficient method of making textile fibers and yarns electrically conductive is coating conventional fibers or yarns with a layer of metals, CNTs, or conducting polymers. By using this strategy, a large variety of conductive fibers or yarns from natural and synthetic fibers with conductivity ranging from 1  106 to 1  104 S cm1 can be produced (Dubas et al., 2006; Andrews and Weisenberger, 2004; Kang et al., 2010; Yamashita et al., 2013). For instance, Guo et al. (2009) fabricated copper-plated polyester fibers successfully using the electroless deposition method (Fig. 13.3). Coating conductive polymers on the surface of natural and synthetic textile fibers or yarns is considered a versatile approach for the fabrication of conductive fibers and yarns for smart textile applications. Most studies are focused on coating traditional natural and synthetic fibers with PPy and PANI conducting polymers by using in situ solution or vapor-phase polymerization (Bhat et al., 2004; Dall’Acqua et al., 2004; Kaynak et al., 2008). Fig. 13.4 demonstrates the longitudinal SEM images of wool, cotton, and nylon 66 yarns coated with PPy using the continuous vapor polymerization method (Kaynak et al., 2008). Coating of PEDOT on the textile fibers or yarns for making them electrically conductive was also reported (Knittel and Schollmeyer, 2009). Another strategy to make the textile fibers conductive is coating their surface with CNT to utilize its high conductivity feature (Liu et al., 2008). For instance, Shim et al. (2008a) coated the surface of cotton yarns successfully with a mixture of CNTs and polyelectrolyte by using the physical padding method to introduce highly conductive yarns. On the other hand, direct sputtering or inking with metal particles or metal film could be another strategy to make the textile fibers and yarns conductive. However, this kind of coating generally results in flaking-off of the metal layer or coating from the fiber surface; so durability of metal coatings on the fiber surface remains a great challenge (Liu et al., 2010).

10 µm

500 nm

Fig. 13.3 SEM images of copper-plated polyester fibers at different magnifications (Guo et al., 2009).

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Fig. 13.4 SEM images of PPy-coated wool (A), cotton (B), and nylon 66 (C) yarns fabricated by the continuous vapor-phase method (Kaynak et al., 2008).

(A)

20 µm

(B)

30 µm

(C)

30 µm

13.5

Functional coatings

For many applications, it is the material interfaces and surfaces that provide beneficial functionality over their intrinsic bulk characteristics. Therefore, coatings provide a versatile method of modifying textiles with conductive properties (Smith, 2010). Subsequently, the textile fabric acts as a supporting structure or carrier material for the

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conductive finish (Sen, 2007). Conventional techniques such as dip coating or roll coating are typically used to apply bulk coatings in the form of a saturation or lamination that covers the entire “surface” of the textile. However, as will be presented herein, the advent of nanotechnology in textile research, the development of novel process techniques, and the advancement of inks and coating formulations affords the opportunity to apply coatings to increasingly finer structures. Therefore, the definition of a surface is highly dependent on which scale the coating can influence the underlying carrier material. The key to coating these ever finer levels of surface, such as yarns or single filaments of a bulk textile fabric, is inherent in the line-of-sight (Wei, 2009). The coating formulation is required to be able to overcome various blocking effects and penetrate the layers of complex three-dimensional (3D) substrates (Hegemann et al., 2009). Furthermore, the subsurface of the polymer that comprises the single filaments can also be infiltrated and modified with inorganic properties resulting in hybrid finishes that are neither purely organic nor inorganic ( Jena et al., 2012; Padbury and Jur, 2015). Consequently, a diverse range of unique properties and coating morphologies can be utilized to add extra functionality to textiles while maintaining their inherent properties such as flexibility, strength, and breathability. In this section, common methods of applying functional coatings to polymer substrates are introduced including examples of advanced techniques and printing methods. The field of functional coatings in polymer and textile manufacturing is extremely broad. Therefore, the objective of Section 13.5 is to provide a qualitative overview of various techniques from the perspectives of the authors. More detailed and theoretical descriptions regarding coating techniques can be found elsewhere in the literature and are referenced in Section 13.5 where applicable.

13.5.1 Percolation threshold A typical coating formulation incorporates a binder or matrix material that adheres to the fabric and provides durability with the addition of functional fillers for conductivity. Epoxies, polyacrylates, polyurethanes, and polyvinyl chlorides have been frequently used as binder materials due to their additional properties such as durability and mechanical strength (Tracton, 2005). However, the range of matrix materials is greatly expanded by the abundance of raw materials such as polymersolvent combinations and thermoplastic polymers (Smith, 2010; Sen, 2007; Tracton, 2005). Fillers of various materials such as nickel, silver, copper, and CB in the form of powders, flakes, or filaments are incorporated into the matrix with the objective of creating a conductive network (Nishikawa et al., 2010). In recent years, more novel materials have been used such as graphene and CNTs resulting in unprecedented conductivities (Bauhofer and Kovacs, 2009; Li et al., 2008; Shim et al., 2008b). Conductive networks in coatings are created by reaching a percolation threshold. Mathematically, percolation theory describes the formation of long-range connectivity in an otherwise random arrangement (Sahimi, 1994). At low concentrations, conductive fillers in a dielectric matrix are situated in random discrete locations. However, as the

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concentration of conductive filler increases, contact occurs between adjacent particles creating a conductive pathway. Therefore, the percolation threshold determines the critical concentration of filler required to maximize the conductivity of the coating as it transitions from the dielectric properties of the matrix to the conductive properties of the filler (Sahimi, 1994). Accordingly, sharp changes in conductivity are observed at concentrations close to the percolation threshold. Furthermore, percolation thresholds are also influenced by particle geometry such as rods and platelets, which lead to lower critical concentrations than spherical particles. This is due to the larger aspect ratios of rods, flakes, and platelets that span the dielectric medium enhancing contact between neighboring filler materials (Sandler et al., 2003). The development of low percolation threshold coatings is particularly important as high loadings of filler can reduce the mechanical properties of the coating and decrease the processability of the formulation (Guo et al., 2015).

13.5.2 Liquid-coating methods The most well-known method of applying functional formulations to a fabric is through the use of liquid-coating techniques. Coatings applied from this highly versatile technique can be introduced to the surface of a fabric using several methods that have subtle variations in their process procedures. In direct coating, formulations can be applied in liquid form from a chemical bath (Sen, 2007). In this process, the coating solution is introduced to the fabric by a contact roller suspended partially in the chemical bath (Sen, 2007). Meyer bar coating is an iteration of this technique and features a bar located close to the roller to remove excess solution from the fabric (Sen, 2007). Similarly, in a pad-dry-cure process, the fabric is passed through a series of rollers that guide the material directly through the chemical bath to saturate the material with the coating solution. Excess solution is removed by compressing the fabric through another set of rollers, and finally the fabric is dried or cured (Sen, 2007). The wet pickup weight and the resultant thickness and uniformity of the coating are dependent on the concentration, surface tension, and substrate characteristics, which influence the shape of the dynamic meniscus as the substrate is withdrawn from the chemical bath. Furthermore, a number of forces act on the coating solution as it is withdrawn, such as inertia and viscous drag which act to pull the solution up the substrate, and cohesive forces that push the solution outwards due to intermolecular interactions. In opposition, gravity draws the solution down while capillary action draws the solution into the fabric, depending on the contact angle of the dynamic meniscus as the fabric is withdrawn from the chemical bath (Smith, 2010). In general, higher concentrations leads to thicker coatings due to greater viscous drag and intermolecular forces, which are magnified by the withdraw speed. However, at higher velocities the coating thickness reaches a maximum due to larger shear forces and the act of gravity forcing the solution back to the chemical bath (Fang et al., 2008). While liquid-coating techniques provide a robust, standardized method of coating textile fabrics, there are a number of considerations. Firstly, liquid-based techniques often require large amounts of water or organic solvents that either needs to be

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disposed of or recovered, which increases the chemical footprint of the technique. Moreover, liquid-coating processes are energy intensive due to the several steps required to apply solutions followed by the use of large drying ovens to cure the coating or evaporate the residual solvent. However, alternative polymer chemistries that require less energy intensive curing steps or utilize fewer chemicals are available. For example, UV curable resins are widely available and UV lamps can replace energy intensive drying ovens (Sen, 2007; Tracton, 2005). Furthermore, hot-melt coatings, which are formed from a broad range of thermoplastic polymers, can be applied without the need for solvents using transfer, lamination, or hot-melt extrusion processes and cured via cooling (Sen, 2007; Tracton, 2005). Moreover, these formulations can be compounded with functional fillers to impart conductive properties.

13.5.2.1 Alternative formulations As mentioned above, the coating techniques described above are conducive to numerous conventional coating materials, such as epoxides, polyacrylates, polyurethanes, polyvinyl chlorides, and a wide variety of thermoplastic polymers. However, these techniques also lend themselves to advanced coating formulations from a wide variety of polymer-solvent combinations that address the need for better performance. The layer-by-layer (Lbl) coating technique deposits polyelectrolytes with opposite charges from solution to the surface of a material (Decher, 1997; Hammond, 2004). Specifically, polyanionic and polycationic polymers are sequentially introduced to a charged surface and fixed in place via electrostatic forces. The sequential control of polyelectrolyte layers and solution concentration affords the opportunity to apply nanoscale coatings to a broad range of substrates. The resulting coating can be used to incorporate inorganic particles (Li et al., 2010), nanoparticles (Feldheim et al., 1996), and nanofibers (Kim et al., 2011) that promote additional functionality in a textile fabric. Moreover, conventional liquid-coating techniques such as pad-dry-cure can be used to apply Lbl coatings. Rubner et al. showed how p-type-doped electrically conductive polymers and conjugated polyanions can be applied to a substrate using the Lbl technique to promote conductive properties (Cheng et al., 1994; Ferreira et al., 2016) Specifically, PPy, PANI, and poly(3-hexylthiophene) were combined resulting in conductivities of 40 S cm1 with as little as four deposited layers. In a similar fashion, polymer grafts can be anchored to the surface of a textile substrate, by chemical reaction with functional groups or methods such as atom transfer radical polymerization, and used to modify the surface functionality of a bulk fabric. Michielsen demonstrated a robust graft-site amplification method for functionalizing fiber-forming polymers such as polyamides that is compatible with conventional liquid-coating techniques (Michielsen, 1999). Using this method, fiber-forming polymers with characteristically low concentrations of intrachain functional groups can be modified with polymers that have a high concentration of functional groups, such as polyacrylic acid. Specifically, functional moieties or particles can then be glued to the graft to exploit desirable functional properties, such as electrical conductivity (Wang et al., 2014; Wu et al., 2015). Furthermore, in situ polymerization techniques have been utilized to enhance the conductive properties of textile fabrics. The work of

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Bajgar et al. (2016) and Macasaquit and Binag (2010) demonstrates the in situ polymerization of conductive polymers, such as PPy and PANI to cotton and polyester fabrics, respectively. The fabrics were coated with the monomers of the conductive polymers by immersion and subsequently dried in air. Conductivities of the order of 1–10 S cm1 were achieved using these techniques which can be improved by applying dopants such as iron (III) chloride. Conductive and semiconductor coatings can also be applied to a textile fabric using sol-gels. The sol-gel process is a wet chemical technique that forms a network gel of hydrated inorganic colloid particles via hydrolysis and polycondensation reactions (Hench and West, 1990). Initially, the solution has a low viscosity and is capable of diffusing into porous substrates such as textiles via dip coating. Over time, the solution becomes more viscous as more molecules link together via crosslinking which forms the gel component of the coating (Hench and West, 1990; Mahltig et al., 2005). The gel is allowed to solidify within the substrate and the residual liquid is removed via heat treatment resulting in a solid inorganic coating. Similarly, the hydrothermal technique can be used to process polymer fabrics through a chemical bath containing metal alkoxides or metal salts mixed with water followed by a suitable heat treatment (Cushing et al., 2004). Lee et al. (2005) utilized the hydrothermal technique to grow semiconductor zinc oxide nanowires on PET substrates combined with indium-tin-oxide (ITO) for field emission display applications.

13.5.3 Advanced coating methods Despite the convenient and wide spread use of the coating techniques described in Section 13.5.2, there are notable limitations. In the case of conventional liquid-coating techniques and materials, wet pickup and resultant coatings tend to be bulky and thick. Where minimal thickness is required, particularly at the nanoscale, it is possible to achieve this requirement by varying solution concentrations or using advanced formulations such as Lbl polyelectrolyte solutions and polymer grafts. However, these formulations require multiple time-consuming steps to introduce the coatings to a substrate surface. Furthermore, there is an increasing need for uniform coatings which are difficult to control with conventional liquid-coatings and sol-gel precursors. Therefore, the development of novel high-throughput coating techniques is of great interest to the flexible electronics and electronic textiles industries.

13.5.3.1 Physical vapor deposition PVD is a vacuum technique used to deposit a wide variety of metal, metal oxide, and metal nitride thin films onto various substrates via sputtering or evaporation (Sarakinos et al., 2010; Smentkowski, 2000). In general, collections of atoms are vaporized and deposited onto a substrate as they condense and physisorb to the substrate surface which, over time, forms a coalesced coating. During the sputtering process plasmas such as ionized argon or oxygen, are used to bombard and liberate atoms from the surface of a target which consists of the required coating material such as gold or palladium (Sarakinos et al., 2010). Subsequently, the escaped atoms are

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accelerated through the vacuum chamber toward the substrate. Similarly, the evaporation technique liberates atoms from the surface of a target; however, a subtle difference is that the target is either heated restively or with a high-energy beam, that is an electron beam, so that atom clusters have enough energy to “boil” from the surface of the target and accelerate through the vacuum chamber towards the substrate (Smentkowski, 2000). High-purity coatings can be formed from PVD techniques; however, it is a line-of-sight technique that produces nonuniform coatings on complex substrates such as textile fabrics. Evaporation and sputter techniques can also result in damage to the surface of the textile substrate through elevated temperatures or etching from contact with plasmas.

13.5.3.2 Chemical vapor deposition Chemical vapor deposition (CVD) is a thin-film deposition technique that utilizes reactions between precursors and co-reactants with a substrate surface in the vapor phase. Therefore, CVD is a vacuum technique operating below the vapor pressure of the precursors to promote a viscous gas regime that delivers the precursors to the substrate surface (Hampdensmith and Kodas, 1995; Murota et al., 2006). The reactions that take place between the precursors either directly react with the substrate or form a powder product that precipitates on the substrate surface. The former scenario results in the formation of dense inorganic films while the latter case requires a calcination step. Control of the deposition process is dependent on the process pressure, temperature, and vapor transport conditions. Typical CVD processes require high temperatures to dissociate the precursors and encourage the formation of a film. However, elevated temperatures are not favorable to a broad range of thermally sensitive thermoplastic polymers. Therefore, plasma-assisted or photoinitiated techniques can be used in place of thermal energy to promote the chemical reactions on polymeric substrates (Matthews, 2003; Cote et al., 1999). In contrast to PVD, the CVD technique has improved line-of-sight due to the mobility of vapor-phase precursors. However, it is still a challenge to achieve uniform coatings on high aspect ratio substrates or complex materials with long trenches and porous structures due to the short exposure times. Therefore, alternative gas-phase technique are available such as atomic layer deposition (ALD), that increase the precursor exposure time that promotes the formation of inorganic coatings on highly complex 3D surfaces.

13.5.3.3 Atomic layer deposition ALD is a technique used to deposit pinhole free, nanoscale coatings onto the surface of a specific substrate with atomic scale precision (Fig. 13.5). This is achieved by ALD’s unique attribute, a carefully chosen sequence of sequential self-limiting vapor-phase reactions that involve complementary precursor gases and co-reactants to promote the formation of a monolayer (Parsons et al., 2011). Multiple ALD cycles can produce thin-film coatings on a broad range of natural and synthetic polymers due to the low-temperature activation of many precursor chemistries (Puurunen, 2005). However, the coating morphology depends strongly on the polymer composition and

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Dose sequence: x(TMA/N2/H2O/N2)

N2

TMA

Direction of flow

N2 purge

H2O

Vacuum pump

Substrate

Fig. 13.5 The typical ALD process equipment consists of a hot walled vacuum reactor. Precursors are dosed into the chamber in a sequential fashion to promote self-limiting surface reactions between the precursors and the substrate.

the reaction mechanism between the polymer and the ALD precursors. In general, uniform, conformal coatings with abrupt interfaces form on polymers with a high concentration of surface functional groups such as cotton ( Jur et al., 2010). Alternatively, hybrid organic-inorganic finishes form in polymers with a lower concentration of functional groups, such as polyester. This results in subsurface diffusion followed by the chemical reaction between ALD precursors and the functional groups of the polymer (Sun et al., 2013; Padbury and Jur, 2014). The ALD technique has been used to deposit a broad range of pure metal and metal oxide conductive coatings, such as platinum, gold, tungsten, and zinc oxide, on a wide variety of polymer substrates ( Jur et al., 2011; Sweet et al., 2013; Luka et al., 2010). An area of industry that ALD has found its inadequacy is within the textile and polymer film industries. The mismatch between processing times, as textile fibers, fabrics, and films can be produced at thousands of meters per minute (mpm), creates a significant engineering challenge that historically has made it difficult to envision ALD as a practical surface modification technique for textiles. However, adaptations to the ALD process, which extend the precursor exposure conditions to exploit the infiltration of organometallic precursors into the polymer subsurface, have been developed resulting in the potential to greatly reduce the overall process time (Akyildiz et al., 2012). Roll-toroll ALD with designs based on the main characteristics of the ALD process have been developed and used commercially for niche applications (Lotus Applied Technology, 2016). Furthermore, atmospheric pressure ALD is an active and expanding area of research that enhances the viability of the ALD process (Mousa et al., 2012).

13.5.4 Printing techniques So far, the focus has been on introducing large-area coatings through conventional and advanced techniques. However, in numerous applications it is beneficial to selectively deposit conductive materials in a pattern, for example, electronic interconnects or

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antennas. The method of patterning substrates via photolithography and masks is well known in the microelectronics industry. The technique requires numerous steps from coating the substrate in a functional film and photoresist, exposing specific areas of the substrate to radiation through a patterned mask, etching exposed areas, and finally removing the resist layer leaving a patterned finish (Halbur et al., 2016). Lithography is typically used as a batch process for specialized components such as semiconductor printed circuit boards. However, continuous processes have been developed, as summarized in Table 13.4, for the high throughput manufacturing of functional printed electronic textiles or polymer films (Daniel, 2010). Typically, the printing processes can be completed with no more than two steps from applying a conductive ink followed by curing or fixing the pattern to a substrate. However, it is important to consider that preprocess photolithographic or etching techniques are required to introduce patterns to special rollers or screens, as will be discussed below. Gravure printing provides the fastest throughput through the utilization of a series of imprinted and impression rollers that transfer specialty inks to the surface of a substrate. Specific patterns are etched directly into the imprinted roller, which is typically made from copper-plated steel (Daniel, 2010). A special feature of the gravure process is that the etching process, used to produce the pattern, creates cells in the imprint roller that soak up the ink as it rotates. The depth of the cells determines the intensity of the print with deeper cells creating more intense finishes. Subsequently, the ink is transferred to the substrate through a combination of capillary forces through the substrate and pressure from the impression roller. Similarly, a further high-throughput process, flexographic printing, utilizes rollers to transfer patterns to a substrate. However, a subtle difference to gravure printing is that patterns are produced on plates made of rubber or photosensitive polymers that are temporarily attached to imprint rollers. Furthermore, the flexographic process utilizes transfer and metering rollers that pick up and store ink inside engraved cells, respectively. Accordingly, the metering roller transfers a specific quantity of ink to the imprint roller to control pattern intensity and thickness (Kipphan, 2001).

Table 13.4

Summary of printing techniques

Printing method

Viscosity (pas)

Layer thickness (μm)

Feature resolution (μm)

Throughput (m2 s21)

Gravure printing Screen printing Inkjet printing Flexographic printing

0.01–0.2

0.1–8

75

30–60

0.5–50

0.015–100

20–100

2–3

0.001–0.04 0.05–0.5

0.05–20 0.04–2.5

20–50 80

0.01–0.5 3–30

Data from Daniel, J., 2010. Printed electronics: technologies, challenges, and applications. In: International Workshop on Flexible Printed Electronics, Palo Alto Research Center, PARC, Korea, September.

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Screen printing has been enjoyed by children and adults alike to develop custom upholstery and apparel. However, this versatile printing technique provides beneficial properties to a wide range of substrates due to its compatibility with various coating viscosities resulting in different coating thicknesses and resolutions (PNEAC, 2016). The technique involves a stainless steel mesh molded over wooden or metallic screens and a patterned stencil that is produced photochemically. A wiper is moved across the mesh forcing ink through the open areas of the stencil and successively transferring the pattern to the substrate. In automatic manufacturing processes, flatbed screen prints are used in which fabric is fed on a conveyor. When the fabric is under the screen the conveyor stops and the screen automatically closes over the fabric and ink is applied through the mesh and stencil, which is transferred to the fabric (PNEAC, 2016). While flatbed screen printing possesses a low throughput, rotary screenprinting techniques have been developed which offer higher throughput. Printed patterns can be applied via inkjet printing with high resolution and varied coating thicknesses on a broad range of substrates (Wong and Alberto, 2009; Yin et al., 2010). Perhaps one of the most significant benefits of inkjet printing is that no prefabrication steps, such as the development of engraved rollers and screens, are required prior to the process. Ink drops can either be applied continuously or via drop-on-demand techniques (Yin et al., 2010). In continuous inkjet printing, a continuous stream of ink is passed through a nozzle and charged. Droplets that become charges are deflected by a suitable electric field, recovered, and recirculated. Un-charged droplets are transported to the substrate and used to form a pattern or image (Yin et al., 2010). In contrast, drop-on-demand inkjet printing takes advantage of thermal or piezoelectric responses that project droplets onto the substrate by the vaporization of the ink forming bubbles or deformation of the ink chamber, respectively (Halbur et al., 2013). In contrast with other techniques summarized in Table 13.4, inkjet printing is comparatively slow but offers the finest line widths and feature resolutions.

13.5.5 Conclusions and considerations In Section 13.5, a wide range of conventional and advanced coating techniques from liquid-coating to vapor-phase techniques have been introduced. With respect to liquid and direct coating, there are a broad range of materials available for coatings and laminations from the extensive list of thermoplastics, thermosets, and polymer-solvent combinations, many of which can be combined with conductive filler particles with the objective of creating a percolation network (Tracton, 2005). Novel formulations compatible with convention coating process techniques were also introduced providing the opportunity to incorporate conjugated conductive polymers, nanoparticles, and inorganics to the surface of a textile which further expands the realm of capability in electronic textiles. Control of process parameters such as metering, transferring, and fixing along with coating characteristics for instance, concentration and rheology, determines the thickness and uniformity of resultant coatings on a substrate. However, there are further considerations that must be taken in to account by the operator.

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Explicitly, the chemical compatibility between the coating material and the substrate determines the wettability and adhesion of the resultant coating, as introduced in Section 13.5.2. Wettability and adhesion is highly dependent on the surface tension of the two materials as well as the interfacial tension between the two components (Smith, 2010). In general, a low-surface-tension substrate can result in nonwetting behavior if the surface tension of the liquid is greater than the surface tension of the substrate. In this case, the adhesive force of the substrate is unable to overcome the cohesive forces of the coating solution. As a result, the coating formulation acts to minimize its own surface energy by beading up rather than uniformly spreading over the substrate. In the case of polymer coatings, adhesion is also impacted by the diffusion of polymer chains into the substrate. Polymer chains have greater mobility above the glass transition temperature; therefore, in the case of hot-melt coating, higher temperature can lead to stronger adhesion forces (Smith, 2010; Wei, 2009). However, diffusion and entanglement of polymer chains is also dependent on chemical compatibility between the polymers that make up the coating formulation and substrate. Low solubility parameters and repulsive forces between dissimilar polymers result in phase separation, in the same way that oil separates from water, which can result in coating delamination and reduced durability. To increase the adhesive force, high molecular weight results in greater attractive interactions between neighboring polymer chains, such as hydrogen bonds and van der Waals forces. Practically, textile fabrics are not perfectly smooth substrates; they inherently possess some surface roughness that may influence wettability, adhesion, and the uniformity of a coating. Furthermore, textile fabrics are porous which can lead to capillary action or wicking. For printing, capillary action can affect the resolution of line widths as the solution is drawn into the fabric. On the other hand, for bulk coatings, penetration into the fabric structure can enhance the mechanical properties of the substrate up to a certain limit through mechanical locking (Smith, 2010). While chemical compatibility and wettability may affect the uniformity and durability of coatings on a substrate, there are further considerations for printed patterns made from polymer-solvent formulations. Firstly, incompatibility may result in nonuniform line widths due to higher contact angles between the coating formulation and substrate surface which impacts the minimum resolution possible. Furthermore, solvents tend to evaporate quicker at the edges due to the microscopic crescent shape of the liquid-gas interface of the coating. As the solvent evaporates, liquid moves outwards to replace the liquid that has evaporated. For inks and formulations that contain a conductive particle component, coffee ring patterns can form where the flow of liquid from the canter of the drop drags particles to the outside (Yin et al., 2010; Halbur et al., 2013). The nonuniformity of the line widths and coffee ring pattern may result in lower conductivities and potential short circuits. Ultimately, the utilization of the coating and printing techniques outlined in this section is a balance between cost (i.e., capital investment and throughput), coating properties such as resolution (feature size) and thickness as well as the compatibility between substrates and coating formulations.

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Further reading Followings are list of further readings to cover products and applications of conductive textiles. Dias, T. (Ed.), 2015. Electronic Textiles, Smart Fabrics and Wearable Technology. Woodhead Publishing, Waltham, MA. Hu, L., Cui, Y., 2012. Energy and environmental nanotechnology in conductive paper and textiles. Energy Environ. Sci. 5 (4), 6423. Tao, X. (Ed.), 2015. Handbook of Smart Textiles. Springer, Singapore. Yetisen, A.K., Qu, H., Manbachi, A., Butt, H., Dokmeci, M.R., Hinestroza, J.P., Skorobogatiy, M., Khademhosseini, A., Yun, S.H., 2016. Nanotechnology in textiles. ACS Nano 10 (3), 3042–3068.