1.2 Electromagnetic properties of textile materials

Traditional textile materials are mostly dielectric materials and are important electrical insulation materials. The electromagnetic properties of textile materials include electrical conductivity, dielectric properties, electrostatic and magnetic properties.

Figure 4.

The morphology of one kind of the yarn. (a) The appearance of the yarn and (b) The distribution of fibers in the yarn.

Figure 5. The type of the fabric. (a) Woven fabric; (b) Knitted fabric and (c) Nonwoven fabric.

slender bodies with the continuous homogeneous internal structure. But actually, they have a wide variety of cross-sectional shapes, and section shape changing along the length, heterogeneous internal structure, with the porosity form. According to the source of the fibers, they can be divided into natural fibers and chemical fibers. Fibers such as cotton, hemp, silk, and wool are the natural fibers with the longest

The cross-sectional morphology of natural fibers. (a) Wool fiber; (b) Cotton fiber; (c) Silk fiber; (d) Hemp

The longitudinal morphology of natural fibers. (a) Wool fiber; (b) Cotton fiber; (c) Silk fiber; (d) Hemp fiber.

Traditional engineering materials Transitional material Textile materials

Homogeneous state Solid state Discontinuous state

Smooth surface Soft, fillable loose structure Surface texture No buckling state Flexion or non-buckling Multiple buckling state

Rigidity Flexible Flexible

Dense, non-permeable Dense or porous Porous

The difference between textile materials and traditional engineering materials.

history.

180

Figure 3.

fiber.

Figure 1.

Table 1.

Figure 2.

Basic process of textile processing.

Electromagnetic Materials and Devices

#### 1.2.1 Conductive properties

The electrical conductivity of textile materials is expressed as specific resistance. There are usually three representations: volume specific resistance, mass specific resistance and surface specific resistance (Table 2).

According to the law of resistance, the resistance R of the conductor is proportional to the length L of the conductor, inversely proportional to the cross-sectional area S, and related to material properties. That is,

$$R = \rho\_V \cdot \frac{L}{S} \tag{1}$$

1.2.2 Dielectric properties

Electromagnetic Function Textiles

DOI: http://dx.doi.org/10.5772/intechopen.85586

1.2.2.1 Dielectric constant

constant of the materials.

1.2.2.2 Dielectric loss

larger than tan δ.

Acrylic staple fiber (deoiled)

Table 3.

183

1.2.3 Electrostatic performance

Fiber Dielectric

The dielectric constant of common textile fibers.

The dielectric constant of dried fiber is 2–5 at the frequent of 50 or 60 Hz. The

As the relative dielectric constant of water is several tens of times larger than that of the dry textile material, the dielectric constant of the fiber is different when the moisture regain or moisture content of the textile material is different. The presence of frequency, temperature, and impurities also changes the dielectric

A physical process in which a dielectric converts a portion of electrical energy into thermal energy under the action of an electric field is known as dielectric loss. The magnitude of the dielectric loss is related to the applied electric field frequency, electric field strength, fiber constant, and dielectric loss angle. In unit time, the heat

of the applied electric field (Hz); E is the external electric field strength (V/cm);

The dielectric constant of dry textile material generally is 2–5, for which tan δ is equal to 0.02–0.05. The dielectric constant of water is 20–80, for which tan δ is 0.15–1.2. Therefore, the higher the moisture content of the textile material, the

The specific resistance of textile materials with dielectric properties is generally high, especially for synthetic fibers with low hygroscopicity, such as polyester and acrylic fibers. Under normal atmospheric conditions, the mass specific resistance is

<sup>P</sup> <sup>¼</sup> <sup>0</sup>:556<sup>f</sup> � <sup>E</sup><sup>2</sup> � <sup>ε</sup><sup>r</sup> � tan <sup>δ</sup> � <sup>10</sup>�<sup>12</sup> (4)

(deoiled)

2.8 — —

); f is the frequency

Fiber Dielectric

constant (ε)

2.3

dielectric constant of the liquid water is 20 and the adsorbed water is 80. The dielectric constants of common textile fibers measured at the frequency of 1 kHz

and the relative humidity of 65% are shown in Table 3.

energy P produced per unit volume of fiber is

and tan δ is the tangent of the dielectric loss angle δ.

where P is the power consumed by the electric field (W/cm<sup>3</sup>

constant (ε)

Viscose wire 15 Polyester staple fiber

The high moisture regain of cotton and viscose leads to its high dielectric constant.

Cotton 18 Acetate 4.0 Wool 5.5 Nylon staple fiber 3.7 Viscose fiber 8.4 Nylon yarn 4.0

Acetate staple fiber 3.5 Polyester staple fiber 4.2

where ρ<sup>V</sup> is the resistivity or volume specific resistance and its unit is Ω cm, and it is the physical expression that indicates the electrical conductivity of a material.

For textile materials, the cross-sectional area or volume is not easy to measure; so, we usually use the mass specific resistance ρ<sup>m</sup> rather than the volume specific resistance ρ<sup>V</sup> to indicate the conductivity of textile materials, especially for the fibers and yarns.

$$
\rho\_m = d \cdot \rho\_V \tag{2}
$$

where ρ<sup>m</sup> represents the mass specific resistance, and the unit is Ω cm/cm<sup>2</sup> . d is the density of the material in 1 g/cm<sup>3</sup> . In actual measurement, the moisture content of the fiber or the relative humidity of the air has a great influence on its electrical resistance. The dried textile fibers have extremely poor electrical conductivity, and their mass specific resistance is generally bigger than 10<sup>12</sup> Ω cm/cm<sup>2</sup> . For most textile materials, there is an approximate relationship between the moisture content M and the mass specific resistance ρ<sup>m</sup> of the textile materials in the range of 30–90% relative humidity:

$$\lg \rho\_m = -n \lg M + \lg K \tag{3}$$

where n and K are experimental constants.


#### Table 2.

Mass specific resistance of textile materials.
