Home Engineering Thermal Protective Clothing for Firefighters
Yarn properties (type, twist, and configuration) have an impact on the thermal insulation characteristics of fabrics [409,521-523].
There are different types of yarn available for manufacturing fabrics. These are, namely, texturized yarn, spun yarn, rotor yarn, and filament yarn. It has been found that texturized, spun, and rotor yarn have protruding fibers on their surface (Fig. 7.6). These protruded fibers trap dead air and increase the thermal insulation of the yarn. The surface of filament yarn is smooth; consequently, filament yarn cannot trap much dead air on its surface. Thus, the thermal insulation characteristics of filament yarn-based fabric are lower than the texturized/spun/rotor yarn-based fabrics [521,523,524].
Fig. 7.6 Surfaces of texturized and filament yarns.
In the case of all types of yarn except filament yarn (spun yarn, rotor yarn, and texturized yarn), a bunch of fibers is twisted along its axis to produce the yarn. The structure of a yarn is highly dependent on the amount of twists (turns per inch) applied to it. It has been observed that the structure of a highly twisted yarn is more compact than a less twisted yarn (Fig. 7.7). Due to this compact structure, ahighly twisted yarn traps less dead air on its surface than a loosely twisted yarn. This situation results in greater thermal insulation for a loosely twisted yarn-based fabric than a highly twisted yarn-based fabric. However, the movement of trapped air may occur freely (natural convection) in a very loose yarn structure, which may lower the thermal insulation characteristics of the fabric .
Fig. 7.7 Structures of loose-, medium-, and high-twist yarn.
As a bunch of fibers are twisted to produce a yarn, the arrangement of fibers in the twisted yarn affects the thermal insulation [409,522]. If the fibers lie parallel to each other and are then twisted to produce a yarn, this configuration of yarn shows higher thermal insulation than a twisted yarn produced from a bunch of nonparallel fibers (Fig. 7.8). This is because a yarn with a parallel fiber arrangement can trap more dead air than a yarn with a nonparallel fiber arrangement. This trapped dead air enhances the thermal insulation characteristics of fabric made from a yarn with a parallel fiber arrangement. Also, a yarn with a nonparallel fiber arrangement cannot trap dead air properly; in turn, the thermal insulation characteristics of fabric made from a yarn with nonparallel fiber arrangement are reduced .
Fig. 7.8 Configuration of yarn with parallel and nonparallel fibers arrangement.
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