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Evaporative resistance evaluation

In order to evaluate the evaporative resistance, two circumstances are preferred: isothermal and nonisothermal [387]. In isothermal circumstances, the temperatures of the test plate and its ambient air are set at 35 ± 0.5°C (without fluctuating more than ±0.1 °C during testing) and the relative humidity of the ambient air is set at 40 ± 4% (without fluctuating more than ±4% during testing) by maintaining an air velocity in between 0.5 and 1 m/s (without fluctuating more than ±0.1 m/s during testing). The plate temperature is the same as the ambient air temperature, so no dry heat exchange occurs between the plate and the ambient air. In nonisothermal circumstances, various experimental parameters (the temperature of the test plate and ambient air, relative humidity of the ambient air, and the ambient air velocity) can be set the same as the experimental parameters of the thermal resistance (Rct/Rcf) evaluation (discussed in the previous section). Here, the ambient air temperature is lower than the plate’s temperature, so the dry heat loss can occur simultaneously with evaporative heat loss. In nonisothermal circumstances, the ambient air temperature, relative humidity, and velocity can be set as per researchers’ requirements. After setting up the isothermal or nonisothermal circumstances as per requirements, water is fed to the surface of the plate (to simulate the sweat- vapor on wearers’ bodies) and the guard section; then, the plate and guard section are covered with a liquid barrier (eg, untreated cellophane film, microporous poly- tetrafluoroethylene film) having a permeability index >0.7. Thereafter, a fabric or a multilayered fabric system specimen is placed on the liquid barrier-covered plate with the side normally facing the human body towards the plate; in the case of multiple layers, it is necessary to arrange the specimens on the plate as on the human body. When the fabric or fabric system specimen reaches the steady-state condition, the temperature at the plate surface (Ts) and the ambient air temperature (Ta) on the fabric surface are determined. By using the Ts and Ta, the water vapor pressure at the plate surface (Ps) and air (Pa) are recorded, respectively, by employing internationally recognized water vapor saturation tables. Then, Eqs. (5.21), (5.22) are used to evaluate the total evaporative resistance (Ret of the specimen, the liquid barrier, and the plate boundary air layer) in isothermal and nonisothermal circumstances, respectively. Here, it seems that the (Ts — Ta)- A/Rct in Eq. (5.22) can be substituted with Hc (power input) according to Eq. (5.19); thus, the simplified form of Eq. (5.22) is shown in Eq. (5.23) and can be used to evaluate the total evaporative resistance (Ret). Similar to intrinsic thermal resistance (Rcf), intrinsic evaporative resistance (Ref) of the specimen is also determined by subtracting the evaporative resistance of the liquid barrier- covered plate (Rebp) from the Ret (Eq. 5.24).

Note: The total evaporative resistance (Ret) of the specimen under nonisothermal circumstances can also be called apparent total evaporative resistance (RA). Here, the apparent term is used as a modifier to total evaporative resistance (Ret) to reflect the fact that condensation may occur within the tested fabric specimen, and the ReAt values of fabrics can only be compared to those of other fabrics measured under the same nonisothermal conditions.

where Ret=total evaporative resistance provided by the fabric specimen, liquid barrier, and air layer (kPa m2/W); A = area of the test plate (m2); Ps = the water vapor pressure at the plate surface (kPa); Pa = the water vapor pressure in the air (kPa); and HE = power input (W) to keep the plate heated at 35 ± 0.5°C when water vapor evaporates from the surface of the plate and diffuses through the test specimen into the ambient environment. The ASTM F 1868 standard is widely used to evaluate evaporative resistance because this standard can accurately simulate the metabolic heat and sweat-vapor transfer conditions present in a skin/clothing system [32]. However, this standard possesses several limitations/challenges [377,385,386]. In order to obtain consistent and accurate evaporative resistance results, it is important that the tested specimens be large enough to cover the surface of the test plate and the guard section completely to prevent any moisture transport through the edges of the specimens; the specimens must also remain flat against the plate during testing. This flat configuration will minimize the occurrence of unwanted air layers between the plate and specimens as well as within the specimens; eventually, the impact of air layers on the evaluated results of evaporative resistance can be minimized to obtain a consistent/accurate result. Some fabric specimens have a tendency to ripple, swell, or curl, or otherwise not lie flat during testing. This tendency is frequently visible in hydrophilic coating or laminated fabric specimens when they absorb water from the test plate during testing [385]. In this case, it is necessary to eliminate bubbles, wrinkles, curls, and so on, by smoothing the specimens by hand without compressing or stretching them. Thereafter, the tested specimens’ leading edges need to be carefully secured, using water vapor impermeable adhesive tapes or other devices (metal bars, magnets, etc.,) in order to consistently/ accurately evaluate evaporative resistance [386]. Additionally, water condensation may develop between the plate and the tested fabric specimen, or within the tested fabric specimen, or both; this condensation may significantly affect the evaluated results of evaporative resistance [385]. The ASTM F 1868 standard is also limited to evaluating the evaporative resistance within a range of 0.0-1 kPa m2/W [386].

Similar to the ASTM F 1868 standard, another standard, ISO 11092, is also frequently used to evaluate the evaporative resistance of any fabrics/films/battings using the following experimental parameters under steady-state condition: test plate and ambient air temperature of 35°C (isothermal circumstances), an ambient air relative humidity of 40%, and an ambient air velocity of 1 m/s (horizontal air flow with a5-10% level of turbulence) [377]. However, the unit of evaluated evaporative resistance by the ISO 11092 standard is m2 Pa/W, which is different from the unit (m2 kPa/W) mentioned in ASTM F 1868; simply multiply the ASTM based evaporative resistance by 1000 to convert it into ISO based evaporative resistance. Both standards (ASTM F 1868 and ISO 11092) only evaluate the evaporative resistance of a fabric or a fabric system that represents the resistance provided by the fabric or fabric system to the flow of sweat-vapor from wearers’ bodies to their nearby environment. However, the sweat- vapor evaporative resistance and sweat-vapor permeability of a fabric or a fabric system are inversely related [377,385]; thus, the sweat-vapor permeability can be directly evaluated to indirectly calculate sweat-vapor evaporative resistance using Eq. (5.25), where, R = sweat-vapor resistance of the fabric or fabric system (cm); Q = weight change of the tested fabric or fabric system during test period t (g); t = test period (s); A = area of the exposed test fabric or fabric system (cm2); D = diffusion coefficient of the tested fabric or fabric system (cm2/s); ДC = difference in water vapor concentration across the tested fabric or fabric system (g/cm3); and Q/At = sweat-vapor permeability that can be evaluated directly [376]. To directly evaluate the Q/At, many standard test methods (ASTM E 96, JIS L 1099, CGSB 49, and BS 7209) are available, which evaluate the water vapor permeability (WVP) or moisture vapor transmission rate (MVTR) of a fabric or a fabric system. These test standards evaluate the water flow in a unit of time through the unit area of a fabric or a fabric system under a specific condition of temperature and relative humidity [387]. In general, the WVP or MVTR can be evaluated using Eq. (5.26), where, M0 = weight of the fabric or fabric system before the test (g); M1 = weight of the fabric or fabric system after the test (g); t = time between successive weighing of the fabric or fabric system (h); and A = area of the exposed test fabric or fabric system (m2) [ 377]. In this context, a notable point is that these standards (ASTM E 96, JIS L 1099, CGSB 49, and BS 7209) did not consider that convective water vapor flows through the pores existing in the structure of a fabric or a fabric system. This convective flow can be more prominent when the pressure gradient across the fabric or fabric system is very high, or the structure of the fabric or fabric system is highly porous; this situation can affect the MVTR [392]. Keeping this point in mind, the ASTM F 2298 standard was developed in 2003; this standard covers the measurement of moisture vapor transport and gas flow properties of fabrics, membranes, or membrane laminates that are usually used in protective clothing [387]. For comparison purposes, McCullough, Kwon, and Shim [387] evaluated the WVP or MVTR using the ASTM E 96, ASTM F 2298, and JIS L 1099. They found that the evaluated WVP or MVTR from the ASTM E 96 is highly correlated (regression coefficient value 0.97) with the evaluated WVP or MVTR from the ASTM F 2298. Thus, both standards (ASTM E 96 and ASTM F 2298) can be used for the evaluation of WVP or MVTR. However, the ASTM F 2298 standard method is much faster to perform than the ASTM E 96 standard method. Furthermore, McCullough, Kwon, and Shim [387] found that the evaluated WVP or MVTR from the ASTM E 96, ASTM F 2298, or JIS L 1099 is negatively correlated with the evaluated Ret/Ref value from the ASTM F1868 (isothermal). This negative correlation is expected because the WVP or MVTR and the Ret/Ref are conceptually opposite parameters. It has also been identified that the WVP or MVTR from the JIS L 1099 is highly correlated with ASTM F 1868. Thus, both standards (JIS L 1099 and ASTM F 1868) can be a substitute for each other, However, the JIS L 1099 standard method is often preferred by fabrics/clothing manufacturers because it is quick, less cumbersome, and more cost-efficient than the ASTM F 1868. Additionally, the JIS L 1099 is also the basis for the new ISO 15496 standard, Measurement of Water Vapor Permeability of Textiles for the Purpose of Quality Control; as a consequence, the JIS L 1099 standard is more acceptable in the industry than the ASTM F 1868 standard [387].

Note: The ASTM E 96, ASTM F 2298, and JIS L 1099 standards only measure the WVP or MVTR; none of these standards evaluate the dry thermal resistance as the ASTM F 1868 standard does. Thus, any conclusion on the heat exchange between a clothed body and the ambient environment derived from the ASTM E 96, ASTM F 2298, or JIS L 1099 standards needs to be employed with caution, because both fabric attributes (Ret/Ref/WVP and Rct/thermal transmission) are needed for characterizing the heat exchange [387].

 
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