Conductive textiles 13webhost.bridgew.edu/ebrush/CHEM 489 PDF/Journal C… · · 2018-01-2413.3.2...

13Conductive textiles

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

Conductive textiles are textile structures that can conduct electricity. Conductive tex-

tiles can be either made using conductive fibers or by depositing conductive layers

onto nonconductive textiles. Typical applications of conductive textiles include anti-

static textiles, electromagnetic (EM) shielding, and e-textiles for flexible electronics.

Conductive textiles can inhibit the static charge generated on fabric, to avoid uncom-

fortable 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 technol-

ogy 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 con-

veyer 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 anti-

static textiles.

Engineering of High-Performance Textiles. http://dx.doi.org/10.1016/B978-0-08-101273-4.00017-2

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306 Engineering of High-Performance Textiles

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. Param-

eters 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 apply-

ing 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 char-

ges 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 accu-

mulation is to incorporate hydrophilic fibers, such as cotton and viscose, to improve

the hydrophilicity of textile fabrics. The other approach is to apply hydrophilic treat-

ment 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 car-

bons, 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. Conduc-

tive coatings can be deposited onto textiles by printing, coating, and physical vapor

Conductive textiles 307

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 fac-

tors 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 elec-

tric 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, elec-

trically 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 mul-

tiple reflections (MRs) through the object. When a barrier is exposed to an EM radi-

ation, 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 compo-

sition (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 EMwave passes through the shield, its amplitude decreases expo-

nentially 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


(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, per-

meability, 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 com-

posites. 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

Tab

201

Gra

Perc

shie

Shie

(dB

S1 ¼ 20 logEin

Eout

� �and S2 ¼ 20 log

Min

Mout

� �dBð Þ (13.1)

re “in” and “out” refer to incident and transmitted waves, respectively. E is the
whe
electrical field strength component (V m�1) andM is the magnetic field strength com-

ponent (H m�1). As shown in Eq. (13.2), shielding effectiveness can also be expressed

in terms of attenuation of power (W).

S¼ 10 logPin

Pout

� �dBð Þ (13.2)

lding effectiveness is sum of absorption loss (A), reflection loss (R) andMR losses
Shie
(Geetha et al., 2009; Huang, 1995; Maity et al., 2013). This is defined in Eq. (13.3).

S1 ¼ S2 ¼ S¼A +R+MR dBð Þ (13.3)

uation of EM shielding effectiveness is shown in Table 13.1. According to this
Eval
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.

le 13.1 Evaluation of EM shielding effectiveness (Ozen et al.,3)

de Excellent

Very

good Good Moderate Fair

entage of EM

lding (%)

99.9 99.9–99.0 99–90 90–80 80–70

lding effectiveness

)

>30 30–20 20–10 10–7 7–5


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 stan-

dard 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 small-

sized 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 enclo-

sures (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 measur-

ing 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 reli-

ability 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 per-

meability. To this end, electrically conductive materials, conductive composite mate-

rials, ferromagnetic materials, or magnetic composite materials are used commonly in

EM shielding applications. Conductive fillers such as metals, alloys, and carbona-

ceous materials (carbon black—CB, graphite, etc.) are common components in con-

ductive 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. Dif-

ferent materials used in EM shielding are listed as follows:


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

ductivity. 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 reflectionmechanism is often adopted due to the presence of free electrons inmetal

structure. Metal coating can be applied on bulk materials, fibers, or particles, but coat-

ings tend to suffer from their poor wear or scratch resistance (Chung, 2001). Further-

more, 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 per-

mittivity when using blended textiles of polyester fibers (polyethylene terephthalate,

PET) with stainless steel fibers (SSF) on EMwave shielding mechanisms at EMwave

frequencies ranging from 30 to 1500 MHz. Their results showed that the EM interfer-

ence 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 com-

posite materials can be used as shields. Carbonaceous materials used in those compos-

ites are graphite (Morari et al., 2011), carbon nanotubes (CNT) (Park et al., 2010;

Al-Saleh and Sundararaj, 2009), flexible and colloidal graphite (Chung, 2001), con-

tinuous 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 effec-

tiveness 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 con-

nected 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.


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 com-

posite 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 compos-

ites, 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 radia-

tions 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.

Polypyrrole

Polyaniline

Polythiophene

S

S

SO3

NH

NH

NH

n

n

n n

n

n

n

n

ss

s

-

o

oo

ooo

**

Poly-p-PhenyleneVinylene

Polyphenylene

Polyphenylenesulfide

Polyacetylene

PEDOTPEDOT:PSS

Fig. 13.1 Common intrinsically conductive polymers.


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 spray-

coating 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 proper-

ties 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 cm�1) 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 nonwo-

ven 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. Con-

ductive 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 15–1000 MHz 23 Wang et al. (2009)

Pani 12.4–18 GHz �27–39 Saini et al. (2009)

CP95Ip 12–18 GHz �23.2 Saini and Choudhary (2013)


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 mate-

rial, 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 thick-

ness, 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. mea-

sured 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 den-

sity, 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, respec-

tively. 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).


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, con-

ductive fiber content, and needle-punch density on EM shielding effectiveness. In par-

allel 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, flexibil-

ity, 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 ever-

growing (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 elec-

tronics. For some textile applications like lighting, considerable current is neces-

sary 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, avail-

ability, and relatively low production cost, some drawbacks of these materials, such as

poor flexibility and bendability, heavy weight, and proneness to oxidation under

Table 13.3 Overview of fabrication methods to produce conductivetextile fibers and yarns (Schwarz and Van Langenhove, 2013)

Fiber level Yarn level

During

fabrication

Spinning, wire drawing, melt spinning, wet

spinning

Spinning,

twisting

Posttreatments Coating, ink-jet printing 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 mechan-

ical 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, Switzer-

land) (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 flexibil-

ity, 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 mate-

rials 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 con-

centrated 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


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�10�6 to

1�104 S cm�1 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) fab-

ricated copper-plated polyester fibers successfully using the electroless deposition

method (Fig. 13.3). Coating conductive polymers on the surface of natural and syn-

thetic 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 poly-

mers 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 intro-

duce 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).

20 µm

30 µm

(B)

(A)

(C)

30 µm

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).


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). Sub-

sequently, the textile fabric acts as a supporting structure or carrier material for the


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 lam-

ination 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 def-

inition 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 fila-

ments 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). Fur-

thermore, 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). Con-

sequently, a diverse range of unique properties and coating morphologies can be uti-

lized 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 sub-

strates 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 conductiv-

ity. Epoxies, polyacrylates, polyurethanes, and polyvinyl chlorides have been fre-

quently 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 polymer-

solvent 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 anotherwise randomarrangement (Sahimi, 1994).At lowconcentrations, conductive

fillers in a dielectric matrix are situated in random discrete locations. However, as the


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 ver-

satile 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 chem-

ical 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 sub-

strate, and cohesive forces that push the solution outwards due to intermolecular inter-

actions. 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 con-

centrations 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


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 coat-

ing 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 with-

out 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 numer-

ous 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 tex-

tile 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 cm�1 with as little as four deposited layers.

In a similar fashion, polymer grafts can be anchored to the surface of a textile sub-

strate, 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 poly-

mers 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


Bajgar et al. (2016) and Macasaquit and Binag (2010) demonstrates the in situ poly-

merization 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 cm�1 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 solu-

tion 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 hydro-

thermal 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 formu-

lations such as Lbl polyelectrolyte solutions and polymer grafts. However, these for-

mulations 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 sub-

strate surface which, over time, forms a coalesced coating. During the sputtering pro-

cess 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


accelerated through the vacuum chamber toward the substrate. Similarly, the evapo-

ration technique liberates atoms from the surface of a target; however, a subtle differ-

ence 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 reac-

tions 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 cal-

cination 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. How-

ever, 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 com-

plex 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). How-

ever, the coating morphology depends strongly on the polymer composition and

TMA

H2O

N2

Direction of flow

Substrate

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

N2 purgeVacuumpump

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, uni-

form, conformal coatings with abrupt interfaces form on polymers with a high con-

centration of surface functional groups such as cotton ( Jur et al., 2010).

Alternatively, hybrid organic-inorganic finishes form in polymers with a lower con-

centration 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 poly-

mer 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 signif-

icant 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 thepotential togreatly reduce theoverall process time (Akyildiz et al., 2012).Roll-to-

roll ALDwith 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


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 sum-

marized 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 con-

sider 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 sub-

strate. 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 sub-

strate 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 uti-

lizes 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

0.01–0.2 0.1–8 75 30–60

Screen

printing

0.5–50 0.015–100 20–100 2–3

Inkjet printing 0.001–0.04 0.05–20 20–50 0.01–0.5Flexographic

printing

0.05–0.5 0.04–2.5 80 3–30

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


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 screen-

printing 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 con-

tinuous 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, respec-

tively (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 lam-

inations 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 provid-

ing 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.


Explicitly, the chemical compatibility between the coating material and the

substrate determines the wettability and adhesion of the resultant coating, as intro-

duced in Section 13.5.2. Wettability and adhesion is highly dependent on the sur-

face 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 sur-

face 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 poly-

mers that make up the coating formulation and substrate. Low solubility param-

eters and repulsive forces between dissimilar polymers result in phase separation,

in the same way that oil separates from water, which can result in coating delam-

ination 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 fab-

rics are not perfectly smooth substrates; they inherently possess some surface

roughness that may influence wettability, adhesion, and the uniformity of a coat-

ing. 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, pen-

etration into the fabric structure can enhance the mechanical properties of the sub-

strate up to a certain limit through mechanical locking (Smith, 2010).

While chemical compatibility and wettability may affect the uniformity and dura-

bility of coatings on a substrate, there are further considerations for printed patterns

made from polymer-solvent formulations. Firstly, incompatibility may result in non-

uniform line widths due to higher contact angles between the coating formulation and

substrate surface which impacts the minimum resolution possible. Furthermore, sol-

vents 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 out-

wards 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 liq-

uid 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 tex-

tiles. 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.




















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