Home Engineering Thermal Protective Clothing for Firefighters
Structural properties (weave/knit design and porosity)
Fabrics can be classified into three categories: woven, knitted, and nonwoven (Fig. 7.10). Among all these fabrics of the same weight, nonwoven fabrics hold more dead air in their structure. Therefore, nonwoven fabrics are largely used to produce thermal insulators, which are subsequently used in thermal protective clothing . Cost- effective nonwoven fabrics act as an excellent thermal insulator by lowering conductive and convective thermal energy transfer through fabric. However, these nonwoven fabrics are not a good thermal insulator due to their regular fiber surface area, especially where radiative thermal energy transfer predominates . Thus, many researchers have suggested some approaches to reduce the radiative thermal energy transfer through nonwoven fabrics [394,534,535]. Qashou, Tafreshi, and Pourdeyhimi  studied the radiative thermal energy transfer in nonwoven fabrics. Their study found that increasing the fiber volume in these fabrics can reduce the transient rate of radiative thermal energy transfer for a fixed fiber diameter. However, fiber diameter has negligible influence on the unsteady transfer of radiative thermal energy through these fabrics for a fixed fiber volume. Recently, Barker and Heniford  reported thatnon- woven fabrics constructed of high surface area based finer and nonround fibers show high thermal insulation by reducing the radiative heat transfer through the fabric. They also stated that nonwoven fabrics comprising a tortuous fiber path show high thermal insulation. Here, the tortuous path created by the fibers reduces the radiative heat transfer by increasing the radiative absorption and scattering by the component fibers within the nonwoven fabric. Gibson et al.  also investigated the impact of fiber size on the thermal insulation of nonwoven fabric. They found that lowering the fiber size helps to enhance the thermal insulation of the fabric by reducing the radiative heat transfer through the fabric. However, < 1 qm fiber diameter is inefficient because it is too small to interact with thermal radiation. They also found that nanofiber nonwoven fabric alone cannot be a good thermal insulator, but nanofibers can be used with large diameter fibers to enhance the thermal insulation of a nonwoven fabric. Barker and Heniford  also found that oxidized poly-acrylonitrile nonwoven fabric has more thermal insulation than aramid nonwoven fabric, even though both fabrics have the same weight. This is because the thickness of poly-acrylonitrile nonwoven fabric is higher than aramid nonwoven fabric at the same weight. A notable precaution is that a nonwo- ven fabric holds lower amounts of dead air during compression, which may lower the thermal insulation of the fabric. Additionally, nonwoven fabrics have very poor compression recovery, which may permanently lower the thermal insulation of the fabrics after long compression . Furthermore, aknitted fabric traps more dead air inside its structure than a woven fabric because a knitted fabric has loops in its structure. In comparison, a woven fabric structure is mainly developed through interlacement of warp and weft so no loops are present in its structure, and it traps a lesser amount of dead air than knitted fabric. Thus, the thermal insulation characteristic of knitted fabric is usually much higher than woven fabric [407,537,538]. However, by varying the number of ends/wales per inch or pick/course per inch, the thermal insulation characteristics of a fabric can also be varied .
The thermal insulation characteristics of a fabric also differ depending upon the design of woven and knitted fabrics [2,395,514]. For instance, frequent interlacing between warp and weft occurs in a plain weave fabric in comparison to a 5-harness satin weave fabric (Fig. 7.11). Due to greater interlacing, the plain weave fabric traps more dead air than the satin weave fabric. This causes greater insulation in the plain weave fabric than the satin weave fabric. In this context, Frydrych, Dziworska, and Bilska  studied the thermal insulation characteristics of plain and twill weave fabrics made of 100% cotton and 100% Tencel yarn. They found that the plain weave fabrics made of cotton and Tencel yarns have the lowest thermal insulation. It was also evident that types of finish on fabric do affect the thermal insulation of plain and twill fabrics. For example, a twill weave fabric with a desizing finish shows the lowest thermal insulation. However, this thermal insulation characteristic can be enhanced by resin finishing. Cotton fabric with a starch finish holds higher thermal insulation than cotton fabric with an elastomeric finish. Similarly, Matusiak and Sikorski  found that the thermal insulation of plain weave fabric is lower than twill and hopsack weave fabrics when all fabrics have the same density and are made of yarn with the same linear densities. Furthermore, Mao and Russel  mentioned that the thermal insulation of knitted fabric can be modified by varying its design. They stated that three-dimensional knitted spacer fabric can trap more dead air than ordinary knitted fabric, which results in higher thermal insulation. This thermal insulation can be enhanced by attaching films or preformed fabrics on the surface of a spacer fabric. Mao and Russel  also found that a mechanically integrated wool fiber surface on spacer knitted fabrics can drastically enhance the thermal insulation of a spacer fabric. This enhancement of thermal insulation is attributed to the reduction of convection through the fabric’s cross-section and an increase in reflection of radiation from the fabric surface.
Fig. 7.10 Woven, knitted, and nonwoven fabric structures.
Fig. 7.11 Plain and satin weave fabric design.
Fabric porosity also affects thermal insulation characteristics [394,405,539]. Depending upon the weave/knit design, the porosity of a fabric varies. For example, the porosity of knitted fabrics is higher than woven fabrics and the porosity of plain weave fabrics is much higher than twill weave fabrics . Generally, a fabric with a moderately porous structure (Fig. 7.12) can trap more dead air than a lesser or nonporous structure; this greater amount of trapped air causes thermal insulation. However, a highly porous fabric structure may allow thermal energy to penetrate under radiant heat and/or flame exposures, which can lower the thermal insulation characteristics of the fabric [101,538].
Fig. 7.12 Porous fabric structure.
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