Fabric properties

There are various fabric properties which affect the thermal insulation characteristics of fabric. These properties can be categorized as surface, structural (weave/knit design, porosity), physical (weight, thickness, and density), thermal (thermal conductivity or diffusivity, heat capacity), and moisture accumulation [24,25,29,93,101,350].

Surface properties

Generally, a fabric absorbs more thermal energy if the wavelength of a thermal spectrum aligns in the infrared or ultraviolet region [526]. More specifically, it has been found that all polymeric fabric absorbs more thermal energy at wavelengths >3 pm. On the other hand, the absorbency rate is lower at wavelengths < 3 pm. A dark color fabric absorbs more thermal energy in comparison to a light color fabric if the wavelength of the thermal spectrum aligns in the visible range. Regardless of the color, other surface properties affect the absorbency of thermal energy; hence, they affect the thermal insulation characteristics of the fabric [527]. When thermal energy is imposed on multiphase (solid and gaseous) fabrics, the incident thermal energy may partially be reflected, and/or absorbed, and/or transmitted depending on the fabric system’s emissivity; the emissivity (e) of a fabric strongly depends on the nature of its surface optical properties [25,35]. The surface optical properties are influenced by the method of fabrication, finishing, temperature/thermal cycling, and chemical reactions during thermal exposures. A fabric with maximum emissivity (e ~ 1) reflects less thermal energy than a fabric with minimum emissivity (e ~ 0); therefore, thermal energy imposed on fabrics with high emissivity is mostly absorbed inside the fabric and/or transmitted to the skin [25,76,528].

Based on the previous discussion, it is necessary to know the reflectance, absorbance, and transmittance of a fabric along with the wavelength of the thermal spectrum in order to thoroughly understand the thermal insulation characteristics of the fabric

[529] . Many researchers measured the reflectance, absorbance, and transmittance of fabric by their own developed methods [36,348,530]. Backer et al. [348] and Ross

[530] evaluated the reflectance, absorbance, and transmittance of Nomex and PBI fabrics using the Beckman DK-2A reflectometer with a Xenon source. Torvi [36] used another method to evaluate the transmittance of Nomex-IIIA and Kevlar/PBI fabrics. In this method, Torvi exposed fabric specimens to a high heat flux source for 10 min and evaluated the changes in transmissivity of the specimens just before and after the exposure. He concluded that no significant changes occurred in transmissivity of the fabric before and after the exposures. He also suggested that the transmittance of a protective fabric is associated with the extinction coefficient that can be calculated using Eq. (7.1), where, у = extinction coefficient (m^-1), т = transmissivity, and L_fab = fabric thickness (m). Morse et al. [531] also evaluated the reflectance, absorbance, and transmittance of Nomex and PBI fabrics using a Gier Dunkle integrating sphere reflectometer with a Beckman DK-2A spectrophotometer between the wavelengths of 0.5-2.5 pm (Table 7.1). This method enabled the measurement of thermal energy transmissions

Table 7.1 Surface optical properties of Nomex and PBI

in all directions and yielded integrated values for individual wavelengths. These authors also used a high and low reflectance backing with the fabric and a reflectometer to determine the directional reflectance and transmittance. It was found that thermal energy transmission is very high beyond the wavelength of 2.5 ^m. It is also established that the aluminization of the fabric surface is one of the common methods used to change the reflectance, absorbance, and transmittance of a fabric [532]. This aluminization can increase the reflectance of a protective fabric up to 90% under a purely radiant heat exposure; hence, this fabric can provide an effective protection for wearers in such an exposure. However, the aluminization process cannot provide an effective protection in flame exposure because a greater transfer of conductive thermal energy and/or ignition of the aluminum laminate may occur in this exposure. Additionally, an aluminized fabric is stiff, costly, sensitive to soiling/soot, and most importantly inefficient to transmit wearers’ sweat-vapor toward their ambient environment. Thus, an aluminized fabric may cause a greater heat stress and discomfort to wearers [25,75].

Furthermore, surface roughness is another important property to consider for effective thermal insulation. This can be evaluated by the Kawabata Evaluation System surface tester. If the surface roughness of a fabric is high, this type of fabric holds more dead air on its surface than a fabric with low surface roughness. This can be reasoned according to the boundary air layer theory, which states that a solid surface traps air when moving air comes in contact with a solid surface (fiber/yarn). If the surface roughness is high, it will trap more dead air; in turn, it will provide a greater insulation under all types of thermal exposures [403,404]. Another notable point is that a fabric with a rough surface causes greater contact resistance, especially when it comes in contact with a hot surface (Fig. 7.9). Due to higher contact resistance, the transfer of conductive thermal energy becomes lower. This situation ultimately increases the thermal insulation characteristics of the fabric [353]. Fabric surface roughness also plays an important role in exposure to molten metal splash. It has been found that a fabric with a rough surface may trap molten metal on thermal protective clothing [69,200]. This situation ultimately enhances the conductive thermal energy transfer

Fig. 7.9 Effect of fabric surface roughness on insulation under hot surface exposures.

from thermal protective clothing to wearers’ bodies. Therefore, a fabric with a smooth surface is recommended for effective protection from molten metal splash.