Modeling and Validating Analytic Relations for Electromagnetic Shielding Effectiveness of Fabrics with Conductive Yarns

*Ion Razvan Radulescu, Lilioara Surdu, Emilia Visileanu, Cristian Morari and Marian Costea*

## **Abstract**

Electromagnetic (EM) radiation may be harmful for human's health and for functioning of electronic equipment. The field of Electromagnetic Compatibility approaches various solutions to tackle this problem, while shielding of the radiation is one of the main solutions. Since the development of spinning technology for producing conductive yarns for fabrics, textile electromagnetic shields have become a valuable alternative to metallic shields. Their main advantages are given by the flexibility, the low weight and the good mechanical resistance, as well as by the possibility to precisely design the shield. The scientific literature includes several analytic relations for estimating the electromagnetic shielding effectiveness (EMSE), in case of woven fabrics with conductive yarns, which may be modeled as a grid of electric conductors. This book chapter tackles three different analytic models for estimating EMSE, which are useful to predict this functionality in the design phase of fabrics. The analytic relations are subsequently comparatively validated by EMSE measurements via TEM cell equipment of two woven fabrics with conductive yarns out of stainless steel and silver with a grid of 4 mm. Results of validated analytic relations are used for the approach of designing textile shields with regard to final application requirements.

**Keywords:** fabrics, yarns, stainless steel, silver, shielding effectiveness, modeling, validating

## **1. Introduction**

Textile materials have reached in the last two decades a lot of additional functionalities in connection to new application fields [1]. Keywords such as technical textiles or smart textiles are increasingly gaining popularity among end users since various products are already available on the market. One important domain of technical textiles are fabrics destined for the construction sector – BUILDTECH, having electromagnetic (EM) shielding properties [2].

The research field of EM shielding fabrics combines at least two major disciplines, namely textile science and electromagnetic compatibility. As such, this chapter tackles interdisciplinary research with a lot of potential applications. In order to understand why EM shielding functionalities are relevant for textile materials, one has to consider the huge amount of EM radiation of our environment these days, caused by telecommunication or other electric energy sources [3].

Several studies prove that EM radiation is harmful for human beings [4]. Nonionizing radiation produce a heating of the cellular tissues of the human body with negative impact on the health and may produce cancer [5]. Although the scientific studies have not done a strict correlation between radiation and deterioration of human's health, it is evident that a causality does exist [6].

Another problem are undesired radiation sources which cause interference with electronic devices. EM Interferences (EMI) are a deep topic of electromagnetic compatibility (EMC) science and one of the most applied solutions is shielding [7]. By shielding of interferences between electronic devices a proper functioning is ensured [8].

Shielding of EM radiation has been done traditionally by use of metallic plates. Due to their outstanding electric conductivity and the formation of Eddy currents, which produce an opposite EM field, the incident EM field is attenuated. The Shielding Effectiveness (EMSE) may be considered as basic quantity defining this attenuation. One of the most applied relation for measurement of EMSE is (1) [9]:

#### *EMSE* ¼ **10 log <sup>10</sup>** *Power of the incident field Power of the transmitted field* ¼ ¼ *Reflection* þ *Absorption* þ *Multiple Reflections* (1)

However, although metallic plates have most effective EMSE properties due to their excellent electric conductivity properties, advanced technical textile materials are also used for such applications. The evolution of spinning technologies [10, 11] and the possibility to produce electric conductive fibers and yarns to be inserted into the fabrics structure, have made possible the manufacturing of textile electromagnetic shields. Either being inserted as yarns within the woven or knitted fabrics or as fibers within nonwoven fabrics, these novel textile materials render electric conductivity properties. The fabric structure with inserted conductive yarns may be designed according to weaving/knitting principles of textile science, having various yarn counts, fabric densities, weaves or float repeats [12].

When compared to metallic plates, electrical conductive textiles are flexible, lightweight and have a good mechanical resistance, and have as well the possibility to precisely design the shield for a certain application. Fabrics are currently preferred in many shielding applications over metallic plates, for the lower costs and the adaption in shape and size to the protected area. The insertion of conductive yarns into the fabric structure may yield various geometric patterns. One of the most common pattern is the grid pattern, which is obtained by inserting conductive yarns in warp and weft system of a woven fabric. Although such conductive woven fabrics do not reach the EMSE of metallic plates, they are used due to their special, mentioned properties.

Modeling of EMSE for electrical conductive materials was a task given by the necessity to estimate the material's properties in the design phase. Manufacturing EM shields without a prior estimation of their EMSE properties requires subsequent physical determinations and an ongoing loop of manufacturing and measurement. This undesired loop may be overcome by modeling of the desired property, in order to be able to estimate the property's values without manufacturing in view of the end application requirements. As such, important resources of time, materials, energy and men power are saved.

*Modeling and Validating Analytic Relations for Electromagnetic Shielding Effectiveness… DOI: http://dx.doi.org/10.5772/intechopen.95524*

Modeling may be done by two main principles [13]:


A simple mechanistic method to model the shielding effectiveness achieved through conductive plane shields was provided by Schelkunoff, and was named impedance method [7]. This method performs an analogy of the discontinuity of shield impedance with a lossy section within long bifilar transmission lines. The impedance method is considered to estimate sufficiently precise shielding effectiveness, by using following geometric and electric parameters: thickness of the shield, electric conductivity and magnetic permeability of the shield, over a specified frequency domain. Shielding is considered to occur due to three mechanisms: reflection loss of the wave due to mismatch of impedance between air and shield (electric conductivity and magnetic permeability are parameters with high sensitivity), absorption loss due to heat loss of the EM wave within the shield (thickness of shield is parameter with high sensitivity) and multiple reflection correction term due to re-reflection of the EM wave within the shield (1). In case of shields with grid structure such as the woven fabrics with conductive yarns, adaptations of the impedance method were accomplished, by introducing additional parameters of the fabric structure, such as the distance between conductive yarns or the conductive yarn's diameter. For modeling of EMSE by woven fabrics is a growing field of research, up-to-date contributions may be found in [14, 15]. A model based on the circuit method for near field electromagnetic waves was described in [16]. The circuit method was developed by Kaden and is considered next to the impedance method, main mechanistic model to estimate EMSE for electromagnetic shields, based on geometric and electric parameters [7].

This book chapter tackles modeling of EMSE for grid structures of woven fabrics with inserted conductive yarns, comparatively by mechanistic models of different analytic relations [17–20], with the purpose of being able to estimate EMSE in the design phase of the textile shielding products.

#### **2. Materials and methods**

This subchapter describes the analytic relations for estimation of the EMSE and the fabric samples with inserted conductive yarns and their properties, destined for validation.

#### **2.1 Electric and geometric parameters for fabrics**

**Figure 1** presents the main geometric parameters of woven textile structures: *h* - fabric thickness [m].

*a* - distance between conductive yarns (ratio between conductive and nonconductive yarns-float repeat) [m].

*d* – optical diameter of the conductive yarn [m].

The following electric parameters apply for the textile materials:


**Figure 1.**

*(a) and (b) – Fabric structures and geometric parameters.*

**ε** ¼ **ε0εr**, where:

ε<sup>0</sup> – Electric permittivity of vacuum ε<sup>0</sup> = 1/(36π\*10<sup>9</sup> ) F/m.

ε<sup>r</sup> – Relative electric permittivity [1].

μ<sup>y</sup> – Magnetic permeability of the metallic yarns [H/m].


μ<sup>0</sup> – Magnetic permeability of vacuum μ<sup>0</sup> = 4π\*10�<sup>7</sup> H/m.

μ<sup>r</sup> – Relative magnetic permeability [1].


$$\mathfrak{G}\_{\mathfrak{Y}} = \frac{1}{\sqrt{\pi \mathfrak{f} \mathfrak{u}\_{\mathfrak{Y}} \mathfrak{o}\_{\mathfrak{Y}}}} \tag{2}$$

δ<sup>f</sup> – Skin depth of the fabric [m], expressed as:

$$\mathfrak{G}\_f = \frac{1}{\sqrt{\pi f \mathfrak{u}\_f \mathfrak{o}\_f}} \tag{3}$$

#### **2.2 Materials: the fabrics and their properties**

Two types of fabrics with inserted conductive yarns made of stainless steel (F1) and silver (F2) were manufactured for validating the analytic relations of EMSE. The fabrics have similar structures, plain weave and a distance between conductive yarns of 5 mm, while F1 is based on stainless steel Bekinox BK50/2 yarns and F2 is based on silver Statex yarns. Cotton yarns of Nm50/2 were set as basic yarns. Both fabrics have been manufactured on rapier weaving looms, by inserting the conductive yarns in warp and weft system, with float repeat 6:2. **Table 1** presents their designed structure and their physical-mechanical and electric properties.

**Figures 2** and **3** present pictures of the weaving loom and the warp beam at the company SC Majutex SRL (www.majutex.ro), used for manufacturing the textile woven fabrics F1 and F2.

\* The electric conductivity of the yarns was computed considering the linear electric resistance of the yarns and the cross-section of the yarns, based on the measured optical diameter – relation (4).


*Modeling and Validating Analytic Relations for Electromagnetic Shielding Effectiveness… DOI: http://dx.doi.org/10.5772/intechopen.95524*

#### **Table 1.**

*Textile structures and physical-mechanical & electric properties.*

**Figure 2.** *Weaving loom.*

$$
\sigma\_{\mathcal{Y}} = \frac{l}{\mathcal{R}\_l \cdot \mathcal{A}} \tag{4}
$$

Where:

σ<sup>y</sup> – electric conductivity of the yarns [S/m].


\*\* The relative magnetic permeability of the composite yarn respectively fabric was computed considering the relation presented in [21].

$$
\mu\_R = \mathbf{M}\_{de} \mathbf{M} \mathbf{M}\_{de} \mu\_{\text{MM}} \tag{5}
$$

Where:

*Mde* - the equivalent percentage of bulk material from total volume; *MMde* � the equivalent percentage of the magnetic material from bulk material; μ*MM* – relative magnetic permeability of the magnetic material;

A relative magnetic permeability of stainless steel was set to μ*MM* = 40, according to [22], while *Mde* and *MMde* were computed according to the textile structure of yarn and fabric.

Ferromagnetic properties were computed for fabric F1 with inserted Bekinox stainless steel yarns, since fabric F2 with inserted Statex silver yarns has no ferromagnetic properties (**μ<sup>r</sup>** ffi **1**).

The Bekinox BK50/2 stainless steel yarn is a spun yarn with 80% content of cotton fibers and 20% content of Bekinox VS stainless steel fibers, thus *MMde* ¼ **0***:***20**.

For fabric F1, the cover factor of the woven fabric was considered for computing the parameter *Mde* ¼ **0***:***96** (in one of our previous research studies [23]) and the ratio between conductive and basic yarns for computing the parameter *MMde* ¼ **0***:***33** (**Table 2**).

\*\*\*The electric conductivity of fabrics was computed based on the measurement of the electric resistance of a piece of fabric (**Figure 4**) and relation (6).

The electric resistance *R* was measured via digital Ohmmeter and thickness *h* was measured via digital caliper for (6).

$$
\sigma = \frac{1}{R} \cdot \frac{L}{l \cdot h} \tag{6}
$$

**Figure 3.** *Warp beam.*


**Table 2.**

*Computation of relative magnetic permeability.*

*Modeling and Validating Analytic Relations for Electromagnetic Shielding Effectiveness… DOI: http://dx.doi.org/10.5772/intechopen.95524*

**Figure 4.** *Experimental setup for measurement of fabric's electric conductivity*

## **2.3 Measured EMSE of the woven fabric samples**

EMSE may be measured according to two main principles with following standardized methods:


EMSE of both fabrics F1 and F2 was measured via a TEM cell measurement system, including signal generator, amplifier and oscilloscope, according to standard ASTM ES-07. **Figure 5a**) presents the scheme of a TEM cell and the washer-shaped textile sample, while **Figure 5b**) presents a picture of the TEM cell.

**Figure 6** presents the diagram of EMSE to frequency on logarithmic scale with measured values for F1 and F2 in the frequency domain 0.1–1000 MHz.

The samples were coated on the edge with silver paint, in order to ensure a good electric conductivity at connection points to the TEM cell (**Figure 7**).

These experimental measurements of the fabrics with inserted conductive yarns of stainless steel (F1) and silver (F2), were done in order to validate two analytic relations for estimation of EMSE.

**Figure 5.** *(a) and (b) TEM cell for EMSE measurement according to standard ASTM ES-07.*

**Figure 6.** *EMSE measured for F1 and F2.*

**Figure 7.** *(a) Sample F1 for TEM cell. (b) Sample F2 for TEM cell.*

## **2.4 Methods: the analytic relations**

The analytic relations of modeling EMSE are valid under certain physical premises:


transmission lines, represents the basic source of all adapted relations for EMSE modeling;

• The validation of EMSE analytic relations may be done by measurements of EMSE via Transverse Electromagnetic (TEM) cell system.

## *2.4.1 Impedance method with correction factors for meshed materials*

For flexible, meshed materials like woven fabrics with inserted conductive yarns, the shielding effectiveness in given by Eq. (7) [17]:

$$\text{EMSE} = \mathbf{A}\_{a} + \mathbf{R}\_{a} + \mathbf{B}\_{a} + \mathbf{K}\_{1} + \mathbf{K}\_{2} + \mathbf{K}\_{3} \tag{7}$$

Where:

Aa = attenuation introduced by a particular discontinuity, dB.

Ra = aperture single reflection loss, dB.

Ba = multiple reflection correction term, dB.

K1 = correction term to account for the number of like discontinuities, dB.

K2 = low-frequency correction term to account for skin depth, dB.

K3 = correction term to account for coupling between adjacent holes, dB.

**Term Aa**. A premise for this relation is that the frequency of the incident wave is below the cut-off frequency, given by: *f <sup>c</sup>* ¼ *c=***λ***c*, with the cut-off wavelength 2.0 times of the maximum rectangular opening. With *a* ¼ **5 mm** and **λ***<sup>c</sup>* ¼ **2** *a* ¼ **10 mm**, it results a cut-off frequency of *f <sup>c</sup>* ¼ **30 GHz**. Our frequency domain limit being 1 GHz, this premise is fulfilled.

$$\mathbf{A}\_{\mathfrak{a}} = \mathbf{27.3} \left( \frac{\hbar}{a} \right) [\mathbf{dB}] \tag{8}$$

Where:

*h* = depth of opening (fabric thickness) [cm].

*a* = width of the rectangular opening perpendicular to E-field (distance between conductive yarns) [cm].

**Term Ra.** The aperture single reflection loss term depends upon both the impedance of the incident wave and the shape of the aperture.

$$R\_d = 20\log\_{10}\left(\frac{1+4K^2}{4K}\right)[\text{dB}]\tag{9}$$

Where

$$K = \text{j6.}\,\text{69} \cdot \text{10}^{-5} \text{f} \cdot \text{a} \tag{10}$$

for rectangular apertures and plane waves,

*f* = frequency in MHz and.

*a*, the same significance as above and also expressed in [cm].

**Term Ba**. The multiple reflection term is given by relation (11):

$$\mathbf{B}\_{\rm{d}} = \mathbf{20} \log\_{10} \left( \mathbf{1} - \frac{\left( \mathbf{K} - \mathbf{1} \right)^{2}}{\left( \mathbf{K} + \mathbf{1} \right)^{2}} \mathbf{10}^{-\frac{A\_{\rm{d}}}{10}} \right) \mathbf{[dB]} \tag{11}$$

$$\text{valid for}: \mathbf{A}\_d < \mathbf{15} \,\text{dB} \tag{12}$$

For sample F1 and F2 *a* ¼ **5 mm** and with *h***<sup>1</sup>** ¼ **0***:***55 mm**, respectively *h***<sup>2</sup>** ¼ **0***:***49 mm**, it result *Aa* ≈**3***:***003 dB** and the premise (12) is fulfilled.

**Term K1**. For a source distance from the shield that is large compared with the aperture spacing, the correction term for the number of discontinuities is given by relation (13):

$$K\_1 = -\mathbf{10}\log\_{10}(\mathbf{S}\cdot\mathbf{n})\,\mathrm{[dB]}\tag{13}$$

Where:

*S* = area of each hole (sq cm).

*n* = number of holes/sq. cm.

For fabric samples F1 and F2: *<sup>S</sup>* <sup>¼</sup> **<sup>0</sup>***:***25 cm2** and *<sup>n</sup>* <sup>¼</sup> **<sup>4</sup>***=***sq cm**, thus *<sup>K</sup>***<sup>1</sup>** <sup>¼</sup> **<sup>0</sup>** may be neglected. Moreover the term K1 can be ignored for sources close to the shield, which is the case of the TEM cell.

**Term K2.** The skin depth correction term is introduced for the reduction of EMSE when the skin depth becomes comparable to the screening wire diameter or the dimension between apertures. An empirical relation was developed for the skin depth correction term (14, 15):

$$K\_2 = -20\log\_{10}(1 + 35p^{-2.3})\tag{14}$$

Where:

$$p = d/\mathfrak{G}\_{\mathfrak{Y}} \tag{15}$$

As a computation example, the skin depth of silver yarns of fabric sample F2 – with a higher electric conductivity of the yarns than F1, is given by relation (16):

$$\delta\_{\mathfrak{Y}} = \frac{\mathbf{63}}{\sqrt{f}} \,\mathrm{[mm]} \tag{16}$$

For frequency *<sup>f</sup>* <sup>¼</sup> **1 MHz** we obtain *<sup>δ</sup><sup>y</sup>* <sup>¼</sup> **<sup>63</sup>** � **<sup>10</sup>**�**<sup>3</sup> mm***:*

The term K2 is the single correction factor of the analytic relation sum which encounters the electric parameter of the yarns (electric conductivity and magnetic permeability), within the relation of skin depth. It is thus a factor with high sensitivity on the overall EMSE relation. The electric parameters were considered for the conductive yarn (not for the fabric), since the ratio *p* ¼ *d=***δ***<sup>y</sup>* is a property of the yarn.

**Term K3**. Attenuation is relatively high when apertures are tight and the depth of the openings is small compared to the aperture width. This is the result of coupling between adjacent holes and especially important for small openings. The relation is given by (18):

$$K\_3 = 20\log\_{10}(\text{coth}A\_a/8.686) \text{[dB]} \tag{17}$$

#### *2.4.2 Impedance method with correction between foil and grid*

The paper [18] and the handbook [19] propose an analytic relation for EMSE, based on impedance method, with correction between the fabric as a foil and the fabric as a grid. Since at low frequencies, the wavelength λ is much larger than the distance of the aperture opening *a,* the incident radiation sees the fabric as a foil (thin electric materials in relation to the skin depth). At higher frequencies, when

*Modeling and Validating Analytic Relations for Electromagnetic Shielding Effectiveness… DOI: http://dx.doi.org/10.5772/intechopen.95524*

the wavelength is comparable with the aperture opening, the grid structure of the fabric becomes relevant (thick electric materials). As such, an exponential function, depending on the frequency *f*, the dimension of the aperture *a* and the constant *C*, was proposed to balance the two relations of EMSE for foil and grid structure - Eq. (18).

$$\text{EXSE} = \exp\left(-\mathbf{C}a\sqrt{f}\right)(\text{EXSE})\_{\text{foil}} + \left[\mathbf{1} - \exp\left(-\mathbf{C}a\sqrt{f}\right)\right](\text{EXSE})\_{\text{grid}} \tag{18}$$

Where:

*a* = Distance between conductive yarns [mm].

*f* = frequency of EM field [MHz].

*C* = constant.

Analytic relation (18) was firstly introduced for metalized textiles by [18]. The two analytic relations for (EMSE)foil – the shielding effectiveness of a metallic foil of the same thickness (*h*) as the fabric and (EMSE)grid – the shielding effectiveness of an aperture (of size *L*� *D*), subjected to a plane wave radiation are given by Eqs. (19, 20) and are valid for metallized textiles:

$$(\text{EMSE})\_{full} = 20 \log\_{10} \left\{ \left[ \exp \left( \frac{h}{\delta} \right) \right] \left[ \frac{\left( \mathbf{1} + k \right)^2}{4k} \right] \left[ \mathbf{1} - \frac{\left( k - \mathbf{1} \right)^2}{\left( k + \mathbf{1} \right)^2} \exp \left( -\frac{2h}{\delta} \right) \right] \right\} \tag{19}$$

$$\left(\left(\text{EMSE}\right)\_{grid} = \mathbf{100} - \mathbf{20}\,\log\_{10}(a\cdot f) + \mathbf{20}\log\_{10}[\mathbf{1} + \ln\left(a/s\right)] + \mathbf{30}\mathbf{D}/a \quad \text{(20)}\right)$$

where:

*k* ¼ *Z***0***=Zm* (ratio of wave impedance to shield impedance).

*h* – thickness of the fabric [m].

δ – skin depth of the material [m].

*a* – maximum distance between conductive yarns [mm].

*s* – minimum distance between conductive yarns [mm].

*D* – depth of the aperture [mm].


The constant *C* is derived by equaling the two relations (EMSE)foil = (EMSE)grid and considering the frequency of three skin depths 3δ, as the point below current is negligible in a foil, (95% of the current flows within 3δ) [18].

The Eqs. (18–20) were derived by contribution of [20] in order to be applied for fabrics with inserted conductive yarns. Following conditions apply for the proposed Eqs. (19–20) to be valid for fabrics with inserted conductive yarns:

a*:* σ ≫ ωε

This condition applies to conductive materials for which ωε is negligible as compared to σ (in metals the condition is practically always fulfilled). This leads to a simplification of *EMSE* relations through the simplification of the shield material impedance definition:

$$Z\_m = \sqrt{\frac{\mathbf{j}\bullet\boldsymbol{\mu}}{\sigma + \mathbf{j}\bullet\boldsymbol{\sigma}}} \approx \sqrt{\frac{\mathbf{j}\bullet\boldsymbol{\mu}}{\sigma}}\tag{21}$$

Sample F1 has an electrical conductivity σf1 = 41.5 S/m and the electrical conductivity of sample F2 is σf2 = 589 S/m. Since it is considered that the fabrics have the electric permittivity of vacuum (ε<sup>0</sup> = 1/36π\*10<sup>9</sup> F/m), the term ωε has a value of 0.055 at a frequency of 1 GHz. Therefore, this condition is fulfilled for both samples in the analyzed frequency range (100 kHz–1 GHz), since the difference between the two terms is of three orders of magnitude for sample F1 (41.5 > > 0.055) and four orders of magnitude for sample F2 (589 > > 0.055).

## *b: Zm* ≪ *Z***<sup>0</sup>**

This condition corresponds to a situation when there is a substantial mismatch between the wave impedance and shield impedance. When this condition is fulfilled, *EMSE* relations greatly simplify as the ratio between the wave impedance and the shield impedance becomes much greater than 1 (*k* > > 1). Thus, Eq. (19) becomes:

$$(\text{EMSE})\_{full} = 20 \log\_{10} \left\{ \left[ \exp \left( \frac{h}{\delta} \right) \right] \left[ \frac{k}{4} \right] \left[ 1 - \exp \left( -\frac{2h}{\delta} \right) \right] \right\} \tag{22}$$

By using the simplified version of *Zm*, the following values are obtained for the impedance of the two textile samples: 0.21 Ω for F1 and 0.037 Ω for F2 at a frequency of 100 kHz, and 21 Ω for F1 and 3.66 Ω for F2 at 1 GHz. The lowest impedance ratio is *k* ≈ 18 for sample F1 at 1 GHz which is still much greater than 1. *k* = 103 for sample F2 at 1 GHz. Therefore, we can say that this condition is fulfilled for both samples in the analyzed frequency range (100 kHz–1 GHz). Note that sample F1, which has lower electric conductivity, fulfills this condition by two degrees of order only for the frequency domain 100 kHz-74.5 MHz. For higher frequencies, the EMSE relation is still valid for F1 but with a greater error, according to [20].

#### *c. h* < 3δ (Thin materials)

It refers to the fact that the thinner the material the higher the reflections from the second interface of the material and thus the re-reflection term in *EMSE* relations is more significant. The amplitude of the incident wave will decrease with about 95% at a distance of three skin depths (3δ) from the first interface inside the material. From the skin depth definition, **<sup>δ</sup>** <sup>¼</sup> **<sup>1</sup>***<sup>=</sup>* ffiffiffiffiffiffiffiffiffiffiffi **π***f***μσ** p , one can see that it is lower for materials with higher conductivity and at higher frequencies. For sample F2 (which has a higher conductivity than F1), δ ≈ 0.66 mm at 1 GHz and 3δ ≈ 1.98 mm, which is lower than the fabric thickness (*h* = 0.55 mm) showing that the condition is fulfilled.

These conditions are needed to express relations (19-20) in case of fabrics with inserted conductive yarns according to [20] and are valid both for F1 and F2.

$$\begin{aligned} (\text{EMSE})\_{foil} &= \mathbf{168.14} + \mathbf{20} \log\_{10} \left( \sqrt{\frac{\sigma\_r}{f \mu\_r}} \right) + \mathbf{8.6859} \frac{h}{\delta} \\ &+ \mathbf{20} \log\_{10} \left| \mathbf{1} - e^{-2h/\delta} e^{-j2\mu t} \left( \frac{\mathbf{Z}\_0 - \mathbf{Z}\_m}{\mathbf{Z}\_0 + \mathbf{Z}\_m} \right)^2 \right| \end{aligned} \tag{23}$$

where β is the phase constant.

$$(\text{EMSE})\_{grid} = \mathbf{158.55} - \mathbf{20} \log\_{10}(a \cdot f) - \mathbf{20} \log\_{10} \sqrt{n} \tag{24}$$

*Modeling and Validating Analytic Relations for Electromagnetic Shielding Effectiveness… DOI: http://dx.doi.org/10.5772/intechopen.95524*

**σ***<sup>r</sup>* was considered in Eq. (23) the relative electric conductivity of the fabric in relation to the electric conductivity of copper **<sup>σ</sup>***Cu* <sup>¼</sup> **<sup>5</sup>***:***<sup>8</sup> <sup>10</sup><sup>7</sup> S***=***m**. The Eq. (24) is valid for electrically thin materials and square apertures.

*C* is a constant for woven fabrics with conductive yarns computed in [20], in relation to the electric conductivity of the fabric and the number of apertures of the maximum length of the washer-shaped sample for the TEM cell **Table 3**:

$$\mathbf{C} = \mathbf{1}.\mathbf{972} \cdot \mathbf{10^{-5}} \sqrt{\sigma\_f \cdot \mathbf{n}} \tag{25}$$

And

$$m = \frac{l\_c}{a} \tag{26}$$

With:

*n* – number of apertures.

*l*<sup>c</sup> – maximum length of fabric for washer-sized sample within TEM cell.

*a* – distance between conductive yarns.


**Table 3.**

*Number of apertures and electric conductivity of fabric to compute constant* C *according to [20].*

The parameter *lc* is needed according to [9] in order to compute the number of apertures of the maximum linear distance. The number of apertures on maximum linear distance are only relevant for EMSE of multiple apertures [9]. *C* is a factor with high sensitivity.

However, relation (24) is valid under certain conditions [20]:


**Figure 8.**

*The maximal distance of the sample for TEM cell meant to compute the number of apertures.*

**Figure 8** presents the scheme of a washer-shaped sample of TEM cell, with the longest distance, for counting the maximal number of apertures.

The measurements show the values: *lc* = 75 mm and *n* = 18 for F1 and F2. These values were used to compute constant *C* of Eq. (25), in order to be used in general equation of EMSE (18), with Eq. (23) for (EMSE)foil and Eq. (24) for (EMSE)grid.

#### **3. Validation of the two analytic relations by the two samples**

Both the analytic relation of impedance method with correction factors (described at 2.4.1), and the analytic relation with correction between foil and grid (described at 2.4.2), were applied for sample F1 and sample F2, by introducing the electric and geometric parameters of the samples and generating the related EMSE diagrams.

**Figures 9** and **10** present the modeled and the measured values in case of the relation of impedance method with correction factors for F1 and F2.

**Figures 9** and **10** show a good modeling of the impedance method with correction factors, both for the fabric with stainless steel yarns and the fabric with silver yarns. The analytic relation (7) shows for the fabric with stainless steel yarns a maximal difference to the measured values of about 10 dB at the frequency of 10 MHz and has for the frequency domain of 0.1–1 MHz and 100–1000 MHz, close values with maximal differences of 2–5 dB. Same analytic relation shows for the fabric with silver yarns a maximal difference to the measured values of 5 dB, especially in the end points of the frequency domain, namely 0.1 MHz and 1 GHz. **Figures 11** and **12** present the modeled and the measured values in case of the relation with correction between foil and grid for F1 and F2.

**Figures 11** and **12** show a good estimation of EMSE by relation with correction factor between foil and grid, both for the fabric with stainless steel yarns and the fabric with silver yarns. The analytic relation (18) shows a mean difference to the

**Figure 9.** *F1 (stainless steel) – Modeled (7) and measured.*

*Modeling and Validating Analytic Relations for Electromagnetic Shielding Effectiveness… DOI: http://dx.doi.org/10.5772/intechopen.95524*

**Figure 10.** *F2 (silver) – Modeled (7) and measured.*

**Figure 11.** *F1 (stainless steel yarns) – modelled (18) and measured.*

measured values of 5–10 dB on the frequency domain for F1 and a mean difference to the measured value of 9–10 dB on the frequency domain for F2.

The two methods for estimating EMSE in case of fabrics with inserted conductive yarns, relay on different principles of analytic relations. Both methods use as input parameters the electric and geometric parameters of the fabric. These electric

**Figure 12.** *F2 (silver) – modelled (18) and measured.*

and geometric parameters were presented with same notations for the two methods, since the scientific literature tackles the methods individually and presents the parameters with various notations [17–20].

*Two types of fabrics with inserted conductive yarns, having similar structure, but different raw materials for the conductive yarns, namely stainless steel and silver, were used to validate EMSE by the two methods, based on experimental data.*

Some of the parameters of the two methods have greater significance in overall estimation of EMSE, some have less significance. This aspect is tackled by the sensitivity analysis.

As such, for the first method, all factors of relation (7) do not depend on frequency, neither on the electric parameters of the fabrics, except of the factor *K2*, which is expressed as ratio between yarn diameter *d* and yarn skin depth **δ***<sup>y</sup>* . *K2* is the single factor depicting the electric parameters, since the relation of skin depth for the conductive yarn of the fabric includes frequency, magnetic permeability and electric conductivity. By analyzing influence of each factor for general outcome of EMSE, it results that factor *K2* has the most significant influence and this is due its characterization of the electric parameters of the fabrics.

The second method aims to balance the relations of EMSE for the foil and for the grid, depending on frequency and the related thin or thick electric material. Main parameter with significant influence in this regard is the constant *C*. This constant depicts the balance between the EMSE of the foil and EMSE of the grid. Constant *C* was computed according to [20] depending on electric conductivity of the fabric and number of apertures on the longest distance of the washer-shaped fabric sample for the TEM cell (according to standard ASTM ES-07).

The geometric parameters of fabrics have for both methods an important significance too, however with less sensitivity than the electric parameters.

Another aspect to be tackled is the tolerance of the EMSE measurement via TEM cell system according to standard ASTM ES-07. The contour of the washer-shaped

*Modeling and Validating Analytic Relations for Electromagnetic Shielding Effectiveness… DOI: http://dx.doi.org/10.5772/intechopen.95524*

samples was coated with silver paste in order to ensure a good electric connection to the metallic parts of the TEM cell. Since EMSE was measured as ratio between the electric power values without and with material present and all measurement devices have they own tolerance (network analyzer, amplifier, TEM cell connection), the overall measurement error could be estimated at 3–4 dB.

As such, EMSE values were compared between models of estimation based on electric a geometric parameters of the fabric and experimental measurements. It can be stated that the validation approach has an indicative character. By comparing validation results of the two analytic relations (7) and (18) we may conclude that analytic relation (7) has closer values to experimental data. However, both relations have close values to the experimental values of EMSE and may be used to predict this special functionality of fabrics. Design of fabrics for electromagnetic shielding applications may be supported by both methods.

#### **4. Specified requirements of electromagnetic shielding textiles**

The designer of fabrics may relate on the document [FTTS-FA-003 v2], in order to evaluate what the needed requirements are for fabrics destined for EM shielding. This document states the limits of EMSE for fabrics of professional and general use (**Tables 4** and **5**).

According to the specification, F2 (silver yarns) has for professional use a good EMSE in the frequency domain of 0.1–100 MHz and a moderate EMSE in the frequency domain of 100–1000 MHz. Moreover, the same fabric sample F2 has for general use an excellent EMSE in the frequency domain of 0.1–100 MHz and a very good EMSE in the frequency domain of 100–1000 MHz.

The sample F1 (stainless steel yarns) has professional use a fair EMSE in the frequency domain of 100–1000 MHz, while for general use the same sample has a very good EMSE. For general use in the frequency domain of 0.1–100 MHz, sample F1 has good EMSE.

These considerations prove that the achieved fabric sample may be successfully used as electromagnetic shields for various applications, with different grades of EMSE on the frequency domain.


#### **Table 4.**

*Class I – Textiles for professional use.*


#### **Table 5.**

*Class II – Textiles for general use.*

### **5. Discussion on structure–property relationship of EM shielding fabrics**

The two proposed models have high relevance in product development for EM shielding applications. By having set the target EMSE value and the frequency of the application (**Tables 4** and **5**), one of the first choices is selection of the raw material for the conductive yarn (stainless steel, silver, copper etc.). The quantity (mass) of conductive yarn leads to a cost-effectiveness calculation for the shielding fabric. A second fabric parameter to be designed is the distance between conductive yarns *a*. A tighter distance between conductive yarns *a*, means a higher EMSE, according to both methods (7) and (18). On the other hand, too tight conductive yarns are critical, due to technological limitations on non-conventional weaving machines destined for insertion of conductive yarns with metallic content.

Other fabric parameters, such as weave, density and yarn count are third choice to design EM shields. Although, none of the presented models depicts these parameters, some other important properties of fabrics, such as mechanical resistance, flexibility and drape highly depend on these parameters.

Basically, the proposed models offer a quick overview of the functionality of shielding, especially useful when starting designing a fabric.

#### **6. Conclusion**

This book chapter tackles electromagnetic shielding as an important functionality to be achieved by textile materials. In order to be estimate this functionality, quantified by the electromagnetic shielding effectiveness (EMSE), the book chapter proposes two analytic models from the scientific literature [7, 20]. The approach of modeling EMSE offers to the textile designer relevant insight on the relation between fabric properties and shielding functionality. Most relevant electric and geometric parameters of woven fabric shields were considered within the analytic models, such as: fabric thickness, conductive yarn's diameter, distance between conductive yarns, electric conductivity and magnetic permeability of yarns and fabrics.

The two analytic relations were meant to be validated by experimentally measured woven fabrics with inserted conductive yarns out of stainless steel and silver. EMSE was measured for both these fabrics via TEM cell system, according to the standard ASTM ES-07. The experimental results show that both analytic relations have a good estimation of EMSE with a maximum difference on the frequency domain 0.1–1000 MHz of 5–8 dB. Moreover, the achieved fabrics show effective EMSE values according to the requirements specifications of textile shields for general and professional use.

In conclusion, the proposed analytic models do support the textile engineer in designing woven fabrics, to achieve desired shielding functionality. Some technological aspects regarding textile processing should be also considered, like limitations of inserting the metallic yarns on non-conventional weaving machines, achieving desired mechanical resistance properties by modifying yarns density of fabrics and overall cost-effectiveness of using expensive yarns with metallic content. This contribution was possible by interdisciplinary knowledge of textile field and electromagnetic compatibility field.

*Modeling and Validating Analytic Relations for Electromagnetic Shielding Effectiveness… DOI: http://dx.doi.org/10.5772/intechopen.95524*

## **Author details**

Ion Razvan Radulescu<sup>1</sup> \*, Lilioara Surdu<sup>1</sup> , Emilia Visileanu<sup>1</sup> , Cristian Morari<sup>2</sup> and Marian Costea<sup>3</sup>


\*Address all correspondence to: razvan.radulescu@incdtp.ro

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

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[2] Harlin A, Ferenets M. Introduction to conductive materials. In: Mattila H, editor. Intelligent textiles and clothing. Woodhead Publishing Limited in association with The Textile Institute, 2006. p.217–238. ISBN-10: 1–84569–162-8.

[3] Costea M. Methods and instruments to ensure electromagnetic immunity. AGIR Publishing House Bucharest, 2006, ISBN 973–720–041-1

[4] Syaza S.K.F., et al, Non-ionizing radiation as threat in daily life. Journal of Fundamental and Applied Sciences, 2017, 9(2S), p. 308–316, http://dx.doi. org/10.4314/jfas.v9i2s.21

[5] The Interphone study [Internet]. 2011. Available from: https://interphone .iarc.fr/ [Accessed: 2020-09-23]

[6] WHO - Radiation: Electromagnetic fields [Internet]. 2016. Available from: https://www.who.int/ news-room/q-adetail/radiation-electromagnetic-fields [Accessed: 2020-09-23]

[7] Schwab A., Kuerner W. Electromagnetic compatibility. AGIR Publishing House Bucharest, 2013, ISBN 978–973–720-359-5

[8] Paul C. R., Chapter 1: Introduction to Electromagnetic compatibility. In: Introduction to Electromagnetic compatibility, Wiley Inter-science, 2006. ISBN-10: 0–471–75500-1

[9] Ott H. W. Chapter 6 Shielding. In: Electromagnetic compatibility engineering. Wiley Inter-science, 2009. p. 238. ISBN 978–0–470-18930-6

[10] BekinoxBK yarns [Internet], 2020, Available from: https://www.bekaert. com/-/media/Files/Download-Files/ Basic-Materials/Textile/Bekaert-Bekinox-BK-yarns.pdf [Accessed: 2020-10-23]

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[16] Rădulescu, I. R., Surdu, L., Visileanu, E., Costea, M., PĂTRU, I., & Voicu, V. (2018). Modelling and testing the electromagnetic near field shielding effectiveness achieved by woven fabrics with conductive yarns. Industria Textila, 69(3), 169–176.

[17] Keiser B. Chapter 6: Shielding. In: Principles of Electromagnetic Compatibility, ARTECH House, 1985, Standard Book Number: 0–89006–065-7

[18] Henn, A.R.; Crib, R.M. Modeling the shielding effectiveness of metallized fabrics. In Proceedings of the International Symposium on

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Electromagnetic Compatibility, Anaheim, CA, USA, August 1992.

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[20] Neruda M., Vojtech L., Electromagnetic Shielding Effectiveness of Woven Fabrics with High Electrical Conductivity: Complete Derivation and Verification of Analytical Model, In: Materials, 2018, 11, 1657, https://doi. org/10.3390/ma11091657

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[23] Surdu, L., Visileanu, E., Rădulescu, I.R., Sandulache, I., Mitran, C., Mitu, B., Stancu, C., Ardeleanu, A., Research regarding the cover factor of magnetron sputtering plasma coated fabrics, In: Industria Textila, 2019, 70, 2, p. 154– 159, http://doi.org/10.35530/ IT.070.02.1618

Section 4 Application Areas

## **Chapter 9**

## Face Mask: A Novel Material for Protection against Bacteria/Virus

*Thilagavathi Govindharajan and Viju Subramoniapllai*

## **Abstract**

Facemask is defined as a loose-fitting device which creates a physical barrier between the mouth and nose of the individual wearing mask and likely pollutants in the immediate environment. Evolution of severe viral respiratory infectious agents such as pandemic COVID-19, severe acute respiratory syndrome, pandemic influenza and avian influenza has driven the use of protective face masks by public and health workers. In this chapter, characteristics features and uses of different types of masks are discussed. Characteristics of various nonwoven technologies for manufacturing face masks are also discussed. Test methods and recent developments in face masks are briefly covered.

**Keywords:** Face Mask, Bacteria, Virus, Filtration

## **1. Introduction**

Facemasks are in general used for reducing breathing exposure to airborne particles such as virus and bacteria that may be connected with a wide range of health effects [1]. Facemasks are considered to impede or reduce the spread of airborne particles that causes annoying health issues. Facemask is defined as a loose-fitting device which creates a physical barrier between the mouth and nose of the individual wearing mask and likely pollutants in the immediate environment [1]. Based on the use, face mask is commonly characterized as medical, isolation, dental and surgical masks. Usually a face mask is made-up of flat or pleated fabric with one to three layers, which in turn secured to the head with ear loops [1, 2]. An evolution of severe viral respiratory infectious agents such as pandemic COVID-19, severe acute respiratory syndrome, pandemic influenza and avian influenza has driven the use of protective face masks among public and health workers [3].

The face masks worn must prevent the infectious microbes such as viruses and bacteria from penetrating through the fabric structure. Hence it is recommended to produce fabric structures with pore sizes lesser than the microbe size. If the size of the microbes is known, then the fabrics can be manufactured according to the requirements. **Tables 1** and **2** shows the specific viruses and bacteria and associated diseases. From **Tables 1** and **2**, fabric producers can get an idea regarding the pore size required in face masks for filtering viruses and bacteria respectively [4]. This chapter discusses the different types of face masks, filtration mechanisms, manufacture and characteristic features of various face masks. Test methods and recent developments in face masks are also covered.


#### **Table 1.**

*Size of some highly infectious disease viruses [4].*


#### **Table 2.**

*Size of some disease-causing bacteria [4].*

## **2. Different types of masks**

Masks are fabricated based on the size of pores and particles desired to be filtered out, which is mainly determined by the health and medical professionals. Different types of masks as per the characteristics features and specific uses are discussed below.

#### **2.1 Dust mask**

Dust masks are in general flexible and these masks are developed to provide protect against dust, molds, pollens and other irritants. They normally not provide protection against any pathogens and therefore they should not be used for viral protection [3].

#### **2.2 Single-use face mask**

Single-use face masks are disposable and commonly used for single application. They are usually made from wood pulp tissue paper or single layer nonwoven fabric and are very thin. They are normally used for providing protection against larger dust particles, at construction sites and in other similar industries. It is not recommended to use such type of face masks for protection against covid-19 [3].

## **2.3 Surgical mask**

Surgical mask is defined as a loose-fitting and disposable device that creates a physical barrier between mouth and nose of the wearer and the probable pollutants in the immediate environment [3]. Surgical mask is normally composed of 3 layers. The innermost layer contains an absorbent material which absorbs moisture while the wearer is breathing. The middle layer is made of melt blown nonwoven which work as a filter, and the outer layer repels liquids [5]. The surgical mask prevents the splashes, larger-particle droplets or sprays with a diameter above 100 mm. It also controls the spread of respiratory secretion and a person's saliva to others. The Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2 virus) is spherical, though slightly pleomorphic, has a diameter of 60–140 nm. These masks will not be able to prevent inhalation of very small particles existing in the air and therefore it does not offer full protection against pathogens [3].

## **2.4 N95 respirator**

*N95 respirator*s are normally non-oil resistant and also termed as electrets filter. The word N95 denotes that these types of face masks can filter at least 95% of aerosols at particle size 0.3 μm [6, 7]. It is reported N95 respirators sometimes do not offer adequate protection against the aerosol particles which are lesser than 300 nm. Hence protection given by certain N95 respirator masks can also drop below 95%, during high inhalation flow rates [6]. The N95 respirators of different industries has varied performance mainly based on the penetrating particle's size. The N95 respirator is made up of four main layers namely inner layer, support layer, filter layer, and layer mask filter layer from inside to outside of it. In addition, a ventilator fan is mounted on the outer layer of N95 to enhance the breathing performance. N95, N99 and N100 masks filter the corona virus effectively [6]. International regulation suggests that the surgical masks and N95 respirator are not advisable to be worn continuously for more than 4 hours and 8 hours respectively, if not it becomes soiled, damaged or contaminated.

## **2.5 P100 respirator**

This is a sort of filtering face piece respirator. It is highly oil-proof and the filtration efficiency of the aerosol particles is 99.97%. From the comparisons made by the researchers regarding the filtration efficiency of N95 and P100 respirators, it was found that there was no considerable gap present in the permeability values before use. Whereas results after post exercise were more expedient for the P100 respirator, on the other hand, N95 failed the post-exercise criterion [6]. Moreover, owing to the likely effects on the breathing resistance, face seal, and moisture retention during the use and hard work, there is the threat of reshaping the face mask. P100 face masks could able to maintain their shape in humid and high temperature compared to the N95 [8].

## **2.6 Full face respirator**

Full-face respirators are constructed using rigid plastic material which consists of both apparent part and central port part [9]. Such masks are used for the management of breathing problems and sleep troubles by providing respiration to patients [6]. Face covering part is made from flexible elastomeric material that fits well to cover the anatomy of face. Due to the elastic nature, straps aids in generating adequate force which gives good adherence of mask on to the wearer's face.

However, this arrangement not works well if the person rolls during sleep. In such situations, masks are removed from the wearer, which disconnects the sealing between mask and face of the wearer and thus leads to poor protection [10].

#### **3. Mechanisms of filtration**

The filtration mechanism of filter fabrics is mainly based on physical filtration mechanism namely interception, inertial impact, diffusion, gravitation, and electrostatic attraction [4]. During a particle filtration process, an interception takes place as the particle radius is equal to or greater than the fiber-particle distance (within 0.1 to 1 μm particle size) [4]. The particles are instantly arrested outside the face masks as soon as it comes in contact with and attaches to the fiber following the air streamline around the fiber. Inertia impact occurs when a particle size is bigger than 1 μm with a larger mass, which are not capable of going after the arc pathway of the air streamline crashes into the fiber. For the particles in high velocity, it becomes harder to penetrate through the pores of the mask sieve and to arrive at wearer. The diffusion mechanism [2, 3] makes the particles to diverge from the actual flow lines arbitrarily when they are close to fibers, mainly for particles sizes which are lesser than 0.1 μm. In the filter fabrics the electrostatic attraction occurs through the electrostatically charged mats [3]. By means of electrostatic driven adsorption the oppositely charged particles are attracted. The electrostatic attraction is efficient for capturing sub-micron particles with no increase in pressure drop [3]. On the other hand, the filtration state will alter when the fibers of the filter fabrics are within nanoscale. By means of charging techniques namely corona charging, tribocharging, and electrospinning the aerodynamic behavior of airflow around the filter can be made [4]. Nevertheless, the charges will decompose steadily during use or long-term storage.

#### **4. Polymeric materials used in face masks**

Fibers are the tiniest component of a textile structure and their characteristic features are mainly dependent on their physical and chemical properties. Fibers with irregular surfaces/irregular cross-sections have the capability to trap particles efficiently than the fibers with smooth surface/regular cross-sections. Cotton is characterized by convolutions, ideal for capturing small particles. Viscose rayon fibers have striations, which offer certain irregularity to the fibers surface. However, these striations are not as effective in inhibiting particle movements as in cotton fibers. Synthetic fibers such as polypropylene, polyester and nylon intrinsically possess smooth surface structure and round cross-sections that permit particles to certainly slide by the fiber. The smooth and round cross-section also creates capillary forces between fibers. Synthetic fibers with different and irregular cross-sections can be manufactured by using suitable spinnerets during fiber manufacture process and irregularity in the fiber structure can be made by means of texturisation process. Polyester fibers with longitudinal channels possess good wicking property. Fiber length is another factor influencing the fabric barrier properties. Longer fibers are not much effective in capturing small particles than shorter fibers [10].

Commonly used materials for face masks applications includes polypropylene, polyesters, polyamides polyphenylene oxide and polycarbonates. Trifluorochloroethylene is a kind of fluorinated polymers is also used for face mask applications. In general, hydrophobic and thermoplastic polymers are preferred. One of the main advantage of using polypropylene fiber in face mask is its *Face Mask: A Novel Material for Protection against Bacteria/Virus DOI: http://dx.doi.org/10.5772/intechopen.98604*

nonabsorbent properties. Antibacterial face mask has been prepared from polypropylene by treating it with dimethyl-dioctadecyl-ammonium bromide. The face mask showed good resistance against bacteria [6]. Madsen et al. [11] analyzed the filtration efficacy of face masks made from polypropylene, polyester, glass fiber and cellulose material. The polypropylene showed highest filtration efficiency followed by polyester, glass and cellulose material. The filtration efficiencies of several commonly available fabrics from silk, cotton, flannel, chiffon, and synthetic fibers has been evaluated by Konda et al. [12]. It is observed that for particle sizes of <300 nm, the filtration efficiencies ranged from 5–80% whereas, for particle sizes >300 nm, filtration efficiencies ranged from 5–95%. It is also noted that the face mask produced from cotton-flannel, cotton-chiffon and cotton-silk showed more than 80% filtration efficiency for particle size less than 300 nm and also showed 90% filtration efficiency for particle size greater than 300 nm. Recently, micro fibers and nanofibers are gaining greater attention due to its unique features.

## **5. Technology of Manufacturing Face mask**

The filtration efficiency of a face mask depends on several factors including fiber type, fiber's cross-sectional shape, technology of manufacture and web's structure. There are several methods to develop fibrous matt that can be used as filters in masks. Face masks are generally prepared from nonwoven fabrics as nonwoven fabrics offers superior filtration efficiencies than woven or knitted fabrics owing to their randomness and 3D structure which permits higher thickness which increases the particle travel distance. Layered nonwoven structures can also be used for the development of face masks. The key nonwoven technology for manufacturing face masks includes melt blown, spun bonding and electrospinning [3]. **Table 3** shows the characteristics of various nonwoven technologies for manufacturing face masks [3].


#### **Table 3.**

*Characteristics of various nonwoven technologies for manufacturing face masks [3].*

#### **5.1 Melt blown technology**

In this technique, a high-velocity air blows a molten thermoplastic polymer from an extruder die with hundreds of tiny nozzles onto a conveyor or take-up screen to structure a fine fibrous and self-bonded web. The fibers produced via such method have relatively small diameters (1–5 mm) and thus, the pore size is much smaller, this makes them superior in filtration performance [13]. Hence, this technology is mostly used for the development of nonwoven fabrics for various filtration applications such as respirators, surgical face masks, liquid filters, cartridge filters and clean room filters. By introducing electrostatic charges and superabsorbent polymer in melt blown polymer fibers it is possible to improve the particle capturing efficiency, air moisture absorption capability and hygienic comfort [3].

Electrostatic-assisted melt blown fabric has been developed by applying electrostatic field directly to the meltblown spinning head [14]. The mean pore diameter of polypropylene microfibers formed using this technique reduced from 1.69 to 0.96 mm, this also resulted in a narrow fiber size distribution. The mean average pore size of conventional melt blown and electrostatic-assisted melt blown fabric were 33.415 mm and 29.285 mm, respectively [14]. The application of electrostatic force resulted in decreased pore size and mean fiber diameter for electrostaticassisted melt blown fabric. It is also observed that electrostatic-assisted melt blown fabric had better filtration efficiency than conventional meltblown fabric [14]. For 0.3 mm particle size, the filtration efficiencies of conventional melt blown fabric and electrostatic-assisted melt blown fabric were 40.65% and 50.82%, respectively. Also, for 1 mm particle size, the filtration efficiencies were 73.98% and 86.44% and for 2.5 mm particle sizes, the filtration efficiencies were 95.35% and 98.96%, respectively [3, 14]. With reduced fiber diameter, the pore size also reduces and the even distribution of fibers per unit area increases this gives the particles additional chance to adhere to the nonwoven fabric [3]. Furthermore, electrostatic-assisted melt blown fabric shows good adsorption ability than conventional fabrics. Melt blown technology has also been used to develop N95 respirator mask filters and surgical mask filters. The melt blown fabrics are the most important filter of face masks that arrests the bacteria as well as microbes from entering or exiting the mask. Polypropylene is the most widely used material to produce surgical face mask. The porosity ranges from 75 to 95% and the basis weight of polypropylene web is around 5–1000 g/m2 . Polycarbonate, polystyrene, polyethylene, or polyester compositions are also used to prepare face masks [3].

#### **5.2 Spun bonding technology**

In this process, molten polymer is extruded onto a conveyor belt via spinneret. Extrusion is a process where the polymeric chips are generally fed into an extruder containing a rotating screw in a heated barrel in which polymer chips are mixed, then melted and finally pumped through a die, that gives the fibers a uniform thin profile and can also endure higher temperatures. Prepared fibers are then quenched by a cool air, and are collected on a conveyor belt and then are additionally bonded by means of thermal, mechanical or chemical means to form the spun bonded nonwoven fabric. The developed spun bonded fabric have random fibrous structure with a weight ranging from 5 to 800 g/m2, the thickness of the web range from 0.1 to 4.0 mm, and the diameter of the fiber vary from 1 to 50 mm [15]. When the fiber diameter decreases, the pore size of the nonwoven fabric decreases; resulting in more even allotment of fibers which increases the particle capturing efficacy. Polymers such as polyester, polypropylene, polyethylene, nylon, polyurethane, etc. are appropriate polymers for the spun bonding process [16]. Among different

polymers, isotactic polypropylene is one of the most extensively used polymer for spunbonded nonwoven fabric manufacture as polypropylene is relatively economical and offers the highest yield (fiber per kilogram). The polypropylene based nonwoven products offers lowest density [3].

### **5.3 Electro spinning**

Electro spinning is a method wherein a polymer based solution is discharged with the help of electric field. The polymeric fluid becomes finer as it passes through the electromagnetic field and deposits on a plate ensuing the production of nonwoven nano-fibrous web. The negative charge is supplied at the collector and positive charge is supplied at the nozzle making the fluid jet to acquire charge at the nozzle tip for the creation of Taylor cone. Charged polymeric fluid is extruded through the nozzle and are drawn by the electrostatic force [17]. As the jet speed up and thins in the electrical field, radial charge repulsion results in splitting the primary jet into numerous filaments, known as "splaying', thereby manufacturing polymeric nanofibers which are electrically charged which are collected at the collector. By means of the subsidiary jets formed, the mean fiber diameter is determined which ranges from 100 nm to 500 nm and the structure and properties of nanofibers is mainly affected by electrostatic forces and viscoelastic behavior of the polymer [18].

In order to examine the relationship between the fiber diameter and volume charge density, a model for the fluid jet inside the electric field has been developed by Fridrikh et al. [19]. The model predicts a terminal jet diameter, which is a result of balance between normal stresses owing to surface charge repulsion and surface tension. It can be found from electrical current, flow rate and the surface tension of the fluid. In another study polyacrylonitrile polymer was used to produce patterned nanofibrous membranes through electro spinning technique [20]. The bulged bubble template was used as collector for manufacturing the patterned membrane. They noted that though the pressure drop reduced in the range of 151.7 to 24.7 mm H2O, the filtration efficiency reduced only in the range 99.94% to 96.33% when compared to nanofibrous filter [20].

In air filtration applications, reducing the pressure drop and maintaining filtration efficiency play an important role in patterned nanofibers manufacture. The results suggest that electrospinning could be a possible option to produce nanofibrous membranes which can be used in filters to manage hazardous ultrafine particles such as viruses. Electro spun nanofibrous nonwoven mats can be effectively used for the filtration of submicron and nanoparticles for enhanced health protection from different contaminants (e.g. coronavirus). Lackowski et al. [21] reported that these nonwoven mats encompass higher filtration efficiency for nano and submicron particles, which are superior than high efficiency particulate air filters having a filtration efficiency of 99.97% for the particle of size 0.3 micron. The filter created by an electrospinning process is generally charged and due to the presence of electrostatic force it has very high filtration efficiency. The commonly used polymeric materials includes polyvinyl chloride (PVC) and poly vinylidene fluoride (PVDF) [3].

## **6. Recent developments in face masks**

#### **6.1 3D-printed masks**

Polypropylene due to its unique features such as easy processability, printability, recyclability, mechanical integrity and low cost, it is normally used for various

technical and industrial applications [4, 6]. Alternatively, styrene-(ethylenebutylene)-styrene is a polymeric elastomer which has low processing temperature and low distortion during extrusion. Thus, the combination of polypropylene and styrene-(ethylene-butylene)-styrene can be used for the processability of 3D printed N95 face masks. Furthermore, it could manage the thermoplastic elastomeric ratio tailoring the elasticity and flexibility of the 3D model material to have a fitted face masks. Accordingly, 3D printing procedure is suitable to produce stable and biocompatible N95 masks that are similar to industrial brands [22]. Swennen et al. [23] fabricated a re-usable face mask by employing 3D printing procedure based on the materials and methods (3D imaging and 3D printing). In a 3D protective face mask there are two 3D-printed reusable polyamide composite components (a filter membrane support and a face mask) and two disposable components (filter membrane and head fixation band). The 3D modeling of the masks can be made quickly using computer-aided design. Cai et al. [24] developed a new technology for enhancing the wearing comfort and fit by using a three-dimensional laser scanning method. Acrylonitrile butadiene styrene plastic using the 3D printing method is employed to make the face seal prototypes.

## **6.2 The nostril filters**

The use of nostril filters is an added innovative approach to protect individuals from airborne allergenic particles. The nasal filters located inside the nasal passages are thought to prevent airborne particles coming to the respiratory system. The conventional nasal filters are usually from non-woven web, woven nontoxic mesh, or porous filters. It reduces every day runny nose and sneezing by an average 12% and 45%, respectively [4]. Electrospinning technology enabled nasal filters are more effective in catching nanoparticles before entering into the host and also provides features such as flexibility and minimum pressure drop [25]. Nanofiber nasal filter (NNF) has been prepared by overlaying a carbon filter substrate onto electrospun nylon nanofibers [26]. The filtering efficiency of the filter is more than 90% for particles greater than 1 μm and 50% efficiency for particles less than 0.5 μm. These filters have immense potential in personal protective equipment against exposure to ultrafine particles [4].

## **6.3 Transparent mask**

There is a communication difficulty between the deaf-mute patient and the doctor while wearing a mask. The filter fabric with high optical lucidity can be worn for personal protective equipment while making lip-reading available. On the other hand, there are some challenges in creating a transparent mask and maintaining the filtration efficiency [4]. Electrospinning technology was employed to develop patterned nanofiber air filters with high optical transparency and effective particulate matter 2.5 capture capability [27]. Developed patterned nanofibers showed high particulate matter 2.5 filtration efficiency of 99.99% and high porosity (*>*80%) with 69% transmittance. Liu et al. developed bilayer electrospun nanofibrous mat from poly (methylmethacrylate) and polydimethylsiloxane. They nonwoven mat showed particulate matter filtration efficiency of over 96% with high optical transmittance 86% [28]. The transparent nanofiber filter is reusable with the ability to retain high particulate matter removal efficiency even after five washing cycles. In a recently study, biodegradable face mask with a hierarchical structure and transparent look has been prepared by printing polylactic acid polymer struts on a polylactic acid nanofiber web by means of electrospinning and 3D printing technology [4, 28]. Hello Mask has been developed by researchers via electrospinning technology [29].

The transparent face mask contains very fine membranes with a pore size of around 100 nanometers, this offers efficient protection against pathogens.

## **7. Common tests used for respirators and surgical masks**

The ASTM standards given by FDA, as the certified standard in the US. Standards ASTM F2100–11 (2011), indicates the performance necessities for the respirators and the face masks. The face masks are classified depending on the performance according to various testing namely breathability, bacterial filtration efficiency, flammability, fluid resistance etc. **Table 4** shows the ASTM F2100–11 levels of protection for face mask selection.

## **7.1 Bacterial filtration efficiency**

Bacterial filtration efficiency (BFE) can be performed as per American society of testing and materials (ASTM) F2101 protocol. This test procedure determines the ability of the face masks to prevent the penetration of microorganisms generated through various activities namely sneezing, coughing and speech. As per this this test standard, the fabric face mask is placed between an aerosol chamber and a six-stage cascade impactor. The aerosol from *Staphylococcus aureus* is fetched into the chamber and using vacuum it is allowed to pass through the mask material. The air flow rate during testing is maintained at 28 L/min. For a minute duration,


**Table 4.**

*ASTM F2100–11 levels of protection for face mask selection.*

*Staphylococcus aureus* aerosol is passed to the nebulizer. Subsequently, cascade impactor and the air pressure is allowed to pass through the sample for two minutes. The concentration of the *Staphylococcus aureus* aerosol plays important role, hence it requires to be monitored and may be kept at 2200 ± 500 CFU per test. The mean value of diameter of the bacteria aerosol and the geometric standard deviation must be in the range of 3.0 ± 0.3 μm and 1.5 respectively.

$$BFE = 100 \,\,\frac{(\text{C} - \text{F})}{\text{C}} \,\tag{1}$$

where C and F indicates the amount of bacteria colonies present in the control and in the presence of the filter, respectively.

#### **7.2 Particulate filtration efficiency**

Particulate filtration efficiency method can be determined as per American society of testing and materials (ASTM) F2299 protocol, and denotes the quality of the surgical masks. The test methods measure the quality of the face masks for filtering the particles with various sizes. According to the FDA regulation certificate, the Particulate filtration efficiency test can be performed using the 0.1 μm polystyrene latex particles. The utilization of latex spheres provides an accurate test for determining a submicron efficiency performance. The polystyrene latex particles have been suspended in water, and the aerosols were produced using the particle generator, which is generally adaptable and can offer the favorable particles concentration. Particle counter downstream can be used to count the particles. The concentration of the aerosol can be attuned from 10,000 to 15,000 particles by feeding through the drying chamber by means of HEPA filtered air. As per the FDA protocols, the used particles are not charge neutralized.

#### **7.3 Viral filtration efficiency**

The viral filtration efficiency is normally measured for some face mask such as N95 and N99. In fact this test procedure is similar to ASTM F2101 followed for determining bacterial filtration efficiency. In this method, the bacteriophage ΦX174, contaminates the bacteria namely *Escherichia coli* is utilized as the experiment virus which is aerosolized to produce virus holding water droplets with approximate size of 3:0 ± 0:3 μm. In this method, the agar plates are first inoculated with *Escherichia coli. As a result*, the bacterial cells are lysed to produce plaques and hence parts in contact with the viral droplets become clear. The viral filtration efficiency can be determined by calculating the amount of bacteria colonies present in the control and in the presence of the filter as explained in bacterial filtration efficiency method.

#### **7.4 Fluid resistance**

This test method predicts the capability of the masks and respirators to lower the squirted synthetic blood or sprayed fluid which can go through the outer layer of the mask and transmit through the inner part by altering the pressure. As per ASTM F 1862, the penetration resistance capability of the medical face mask is calculated using the high-velocity synthetic blood, which is in contact with the surface layer of the face mask (in a particular time between 0 s and 2.5 s). Some factors such as

viscosity polarity, the structure, surface tension, and the relative hydrophobicity or hydrophilicity of the face mask material have shown considerable effect on the penetration and the wetting of the body fluids. By regulating the surface tension of synthetic blood, the wetting properties of blood can be simulated which must be lower than the surface tension range for body fluids, blood excluding saliva approximately in the range of 0.042 Nm\_ 1 to 0.060 Nm\_ 1.

## **7.5 Flame resistance**

The hospitals contain various sources of the oxygen, heat, and fuel, the ASTM F2100–11 standards needs an assessment concerning the flame resistance for face masks. The used material for face masks should not contain any hazards for the consumer, and their flammability characteristics should not be high. The sample mask used for the flammability test should not be flamed or remain in flame after five seconds from burning. The flame spread test determines the time required for the flame to reach the sample in 5 inch distance. "Class 1" corresponds to group of the materials, which have standard flame resistance, and they are appropriate for the use in face masks and respirators.

## **7.6 Differential pressure (Delta-P)**

The differential pressure test provides information regarding airflow resistance of the face masks and their breathability characteristics. While performing the test, air passes through the face mask in a controlled way, and different pressures are determined for the inner and outer layers of the face mask. The ratio of differential value to the surface area (cm<sup>2</sup> ) of the face mask is used to determine the breathability, where high Delta P values denotes a harder breath for the consumers. ΔP can be measured through following relationship.

ΔP = *PM/*4*.*9, where PM denotes the mean value of the differential pressure of the face mask, in Pa. As per ASTM F2100–11, the minimum value for Delta P must be lower than 5.0 mm H2O/cm2 (or not more than 49 Pa). The Delta P values which are lesser than 0.2 or greater than 0.5 are not considered as standard values for the common surgical use.

## **8. Conclusions**

Wearing a mask is a main factor to retard the spread of the virus. A summary of different types of face masks is discussed in relation to the structure and performance in filtering out the bacteria and viruses. Comparison of properties of nonwoven technology such as melt blown, spun bonding and electro spinning for manufacturing face masks has been discussed. Several researchers are currently making large efforts to innovatively develop 3D-printed respirators, transparent and nostril filters, responding to COVID-19. The progression of advanced masks and respirators will play a critical role in providing protection against COVID-19.

*Textiles for Functional Applications*

## **Author details**

Thilagavathi Govindharajan\* and Viju Subramoniapllai Department of Textile Technology, PSG College of Technology, Coimbatore, Tamilnadu, India

\*Address all correspondence to: thilagapsg@gmail.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Face Mask: A Novel Material for Protection against Bacteria/Virus DOI: http://dx.doi.org/10.5772/intechopen.98604*

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## **Chapter 10**
