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Evaluation of thermal protective performance using bench-scale tests

The thermal protective performance of a firefighter’s clothing refers to its ability to insulate the transmission of thermal energy in order to reduce the burn injuries of firefighters, and many researchers have evaluated the performance of firefighters’ clothing under various laboratory-simulated thermal exposures [22,24,25]. In order to evaluate the performance of firefighters’ clothing under various thermal exposures with varying intensities, two main types of tests are available: bench-scale tests and full-scale manikin tests [23,26,27,341]. In the bench-scale test, a specimen of fabric used to manufacture thermal protective clothing is tested under a thermal exposure of varying intensities, whereas, whole thermal protective clothing is tested in the full- scale manikin test. This full-scale manikin test is cumbersome, expensive, and difficult to carry out in the laboratory because whole pieces of thermal protective clothing are tested. Consequently, previous studies largely focused on bench-scale tests to evaluate the performance of fabrics [22,24,25]. The thermal protective performance for fabrics or their combinations can be evaluated under various thermal exposures; typically, these can be categorized as six different thermal exposures of varying intensities: radiant heat, flame, hot surfaces, molten metal substances, hot liquids, and steam [22,24,25].

Evaluation of thermal protective performance under radiant heat exposure: The thermal protective performance of fabrics under radiant heat exposure can be evaluated according to the ASTM F 1939 (or ISO 6942) test standard (Fig. 5.11). In this test standard, a fabric specimen with a dimension of 250 mm x 150 mm is vertically exposed to radiant heat generated (21 and 84 kW/m2) from a bank of quartz tubes. A TPP sensor (copper slug sensor) is placed behind the exposed fabric specimen to measure the total thermal energy passed through the fabric specimen. The accumulated energy for a certain exposure time is also calculated according to the empirical Eq. (5.13), where, tj = time value in seconds. The time value, tjntersects, represents where the measured energy (obtained from the TPP sensor) intersects with the calculated energy (obtained from the empirical Eq. 5.13). By using this time value (tintersects), the radiant heat resistance (RHR) of the fabric specimen is determined (in J/cm2) as thermal protective performance by employing Eq. (5.14).

Laboratory-simulated radiant heat exposure test described in ASTM F 1939

Fig. 5.11 Laboratory-simulated radiant heat exposure test described in ASTM F 1939.

Although the ASTM F 1939 test standard is well-established to evaluate the RHR of fabrics with continuous heating, this standard does not account for the thermal energy contained in the test specimen after radiant heat exposure has ceased. Thus, this standard does not account for thermal stored energy in fabrics. Test standard ASTM F 2702 was established to account for thermal stored energy. This test standard is similar to ASTM F 1939; however, this test standard accounts for thermal energy contained in the exposed test specimen (250 mm x 150 mm) after the standardized radiant heat exposure has ceased. Here, a radiant heat performance (RHP) value of the fabric test specimen is determined iteratively as the thermal protective performance. For this, a fabric specimen is first vertically exposed to radiant heat for a determined duration. During the exposure, the thermal energy transferred through the exposed specimen is measured by a TPP sensor (copper slug sensor). From this sensor, the total accumulated thermal energy for a particular exposure time (ti) is measured. For the same exposure time (ti), the required energy for second-degree skin burn injury is calculated by Eq. (5.15). The exposure time at which the total accumulated thermal energy meets/exceeds the required thermal energy for second-degree skin burn injury is determined. This exposure time is represented as tmax. This tmax value is further divided by 2 and is referred to as trial exposure time (ftriai) (Eq. 5.16). Subsequently, another fabric specimen is exposed to radiant heat for the ttrial time, and the energy transmitted through the specimen is measured by the sensor, with a continually measured cool phase period. From the measured total thermal energy, the ttrial value is recalculated using the same method as before. Then, the difference between the current ttrial and the previous ttrial is calculated. If the difference is 0.5 s, the current ttrial value is used to calculate the RHP according to Eq. (5.17). In both ASTM F 1939 and ASTM F 2702, an improper placement of the fabric specimen may result in an inaccurate measurement.

Furthermore, the ASTM E 1354 standard has been developed to study the fire behavior of various materials, including fabrics under radiant heat exposure with or without an external ignitor (Fig. 5.12). Here, a fabric specimen (100 mm x 100 mm) is exposed to heat flux of up to 100kW/m2. The normal specimen testing orientation is horizontal, independent of whether the end-use application involves a horizontal or a vertical orientation. However, the testing apparatus used in this standard also has the capability for vertical orientation testing in order to conduct exploratory or diagnostic study only. The heat flux is generated from a conical heater until the fabric specimen starts burning in an ambient air condition. Here, the fire behavior of the fabric specimen at a specified heat flux can be measured in terms of ignitability, heat release rates (HRR), effective heat of combustion, mass loss rates (MLR), and/or visible smoke development of fabric materials. The ignitability is determined as a measurement of time (s) from initial exposure to sustained burning (existence of flame on or over most of the specimen surface for periods of at least 4 s) of the ignited specimen; the HRR is determined by the heat evolved from the ignited specimen per unit of time; the effective heat of combustion is determined by the amount of heat evolved from per unit mass of the ignited specimen; the MLR is determined by the loss of mass of the ignited specimen per unit of time; and the visible smoke development is measured by the reduction/attenuation of transmission of a monochromatic and highly collimated light source due to smoke generated from the ignited specimen. Although this standard is widely used to measure and describe the response of fabric materials or assemblies under controlled radiant heat exposure, this standard does not incorporate by itself all parameters required for fire hazard or fire risk assessment of fabrics under actual fire conditions.

Laboratory-simulated radiant heat exposure test in ASTM E 1354

Fig. 5.12 Laboratory-simulated radiant heat exposure test in ASTM E 1354.

A great deal of research [24,29,76] used either the above-mentioned original ASTM standards with slight modifications, or the standards developed by other organizations (eg, NFPA, ISO). Sun et al. [29] and Song et al. [24] evaluated thermal protective performance according to the NFPA 1971 and modified ASTM F 1939 standards, respectively. Recently, Mandal et al. [76] evaluated performance under radiant heat exposures using the heat source described in the ASTM E 1354 standard. As a modification, Song etal. [24] and Mandal etal. [76] did not use the performance analysis as described in the original ASTM F 1939 and ASTM E 1354 standards. In these studies, the performance was evaluated in terms of time required to cause second-degree burns to the sensors (firefighters’ bodies); a fabric with a high second-degree burn time was considered a fabric with high performance. It is also notable that the above-mentioned various test standards (ASTM F1939, ASTM F 2702, ASTM E1354) evaluated the performance of fabric specimens in their relaxed state. These standards did not consider the deformation of specimens in the presence of extreme radiant heat during performance evaluation. However, this deformation may affect the transfer of thermal energy through the fabric specimen, which also affects thermal protective performance. It is necessary to design test standards which can account for the impact of a deformed fabric state on thermal protective performance.

Assessment of thermal protective performance under radiant heat exposure: Sun et al. [29] reported that the intensity of radiant heat exposure in a fire hazard can be >20 kW/m2 and that such high intensity radiant heat exposure can cause life- threatening burn injuries to firefighters. Furthermore, Perkins [93], and Song et al. [24] derived through critical analyses of the research that the protection of firefighters from low-intensity (<20 kW/m2) radiant heat exposures is equally important in a fire hazard [92]. Mandal et al. [76], Perkins [93], Sun et al. [29] and Song et al. [24] analyzed the inherently and/or chemically modified fire-retardant natural and synthetic fiber-based fabrics under laboratory-simulated radiant heat exposures to evaluate thermal protective performance; they found that fabric attributes are important to consider for providing effective protection to firefighters in radiant heat exposures. Song et al. [24] observed that layered fabrics store more thermal energy inside their structures than nonlayered fabrics. Therefore, clothing made with layered fabrics discharge more thermal energy during compression with wearers’ bodies than clothing made up of nonlayered fabrics. As this discharged energy may cause significant burns on firefighters’ bodies, the chances of burn injuries (due to stored energy) in the case of layered fabric clothing are higher than with nonlayered fabric clothing. Perkins [93], Mandal et al. [76], and Sun et al. [29] inferred that thicker or heavy-weight fabrics comprise more still or dead air inside their structures than thinner or light-weight fabrics. The authors suggest that this dead air acts as a thermal insulator and controls the thermal resistance of the fabrics. Because thicker or heavier fabrics comprise more dead air than thinner or lighter fabrics, the thicker or heavier fabrics possess higher performance and can provide a better thermal protection than thinner or lighter fabrics. Perkins [93] also found that a fabric with open constructions allows more thermal energy to pass through and lowers the performance; additionally, a fabric in touch with wearers’ bodies may also enhance the conductive transfer of thermal energy toward wearers and cause more extensive burn injuries. Kalekar and Kung [121] mentioned that opaque fabric has high transmissivity of thermal energy and causes quick burns on wearers’ bodies. Backer et al. [348] expressed concern that a fabric with high opacity can be highly ignitable under radiant heat exposure, which may aggravate a burn injury even before wearers have received sufficient thermal energy to their skin to register pain and initiate escape. Furthermore, Mandal et al. [76], and Song et al. [24] established that fabrics with low density and high thermal resistance show higher performance than fabrics with high density and low thermal resistance; this is because low-density and high thermal resistance fabrics trap more still or dead air inside their structures than high-density and low thermal resistance fabrics [29]. Sun et al. [29] also confirmed that fabric color has a negligible impact on the performance of thermal protective clothing.

Barker et al. [92,167], Song et al. [24], and several other researchers have studied the moisture effect on thermal protective performance [349]. Barker et al. [92] found that small amounts of moisture in fabric have a negative impact on the performance of thermal protective clothing under low radiant heat exposures. This reflects that the thermal conductivity of fabric increases in the presence of moisture; as a result, the performance of fabric is reduced [167]. Barker et al. [92] also stated that fabric becomes wet in the presence of more than 15% moisture, which results in an increase in the performance of the fabric. In the same context, Song et al. [24] found that wet fabric generally shows greater performance than dry or slightly damp fabric under low radiant heat exposures. They explained that the presence of high amounts of moisture inside a fabric’s structure increases the thermal resistance of the fabric, which lowers the transfer rate of radiant heat through the fabric toward the skin. Therefore, it takes longer time to generate burns on firefighters’ bodies. Lawson et al. [349] also inferred that moisture has varying levels of impact on thermal protective performance under high and low radiant heat flux. Moisture added to the test fabrics enhances performance under high radiant heat flux; however, added moisture lowers performance under low radiant heat flux. They also confirmed that the effect of moisture on performance depends upon the amount of moisture added to the test fabrics, the timing of moisture addition (before, during, or after the radiant heat exposure), and the duration of heat flux application.

Evaluation of thermal protective performance under flame exposure: In the ASTM D 4108 standard, a test device and a method were introduced to test thermal protective performance under the flame exposure (Fig. 5.13). In this test standard, a blackened TPP sensor (copper slug sensor) is placed behind a flame-exposed (84 kW/m2) fabric specimen (150 mm x 150 mm) with or without a 6.4 mm spacer. The measured thermal energy data from this sensor is used to determine the time to exceed the Stoll second-degree burn criterion. The thermal protective performance of the fabric specimen is then rated on the basis of Eq. (5.18), where, F = exposed heat flux (W/cm2); T = exposure time (s). Here, a fabric with a high performance rating is considered better than a fabric with a low performance rating. Laboratory-simulated flame exposures tests in ASTM D 4108

Fig. 5.13 Laboratory-simulated flame exposures tests in ASTM D 4108.

Although the ASTM D 4108 standard was primarily developed to compare the thermal energy insulating values of protective fabrics, this test standard confused textile scientists. In short, they misinterpreted that a measured performance rating value equaled the fabric’s ability to protect firefighters from second-degree burn injuries. However, this test standard was not designed for second-degree burn injury prediction, because it does not account for the thermal energy contained in the exposed fabric specimen after the standardized flame exposure has ceased. Due to this confusion, an ASTM F23.80 subcommittee (on flame and thermal) developed a new standard, ASTM F 2703 (Fig. 5.14). In this standard, a fabric specimen (150 mm x 150 mm) is exposed to two Meker burners and a bank of quartz tubes in order to simulate a combined flame and radiant heat exposure (84 kW/m2). Here, the unsteady state transfer of thermal energy through the test fabric specimen is measured using a TPP sensor placed behind the flame-exposed fabric specimen with or without a 6.4 mm air gap. The thermal energy contained in the test fabric specimen is assessed after the exposure has ceased. The measured thermal energy value to generate second-degree skin burn injuries is used as per the methods described in ASTM F 2702 to estimate thermal protective performance.

Laboratory-simulated flame exposures tests in ASTM F 2703

Fig. 5.14 Laboratory-simulated flame exposures tests in ASTM F 2703.

Similar to ASTM F 2703, the ASTM F 2700 standard was developed to evaluate the performance of fabrics used in thermal protective clothing. This test standard measures the unsteady state of thermal energy transfer through a fire-resistant/retardant fabric specimen (150 mm x 150 mm) with or without a 6.4 mm air gap, which is subjected to continuous exposure. As with ASTM F 2703, this test standard does not predict second-degree burn injuries. Instead, this standard calculates the heat transfer performance (HTP) value to determine thermal protective performance. The HTP value is measured according to the RHR calculation procedure discussed at the ASTM F 1939 in the previous section—“Evaluation of thermal protective performance under radiant heat exposure.” Additionally, a method described in the ASTM D 7140 standard can measure the thermal energy transfer through a fabric under an open flame exposure, within a specified period of time. Here, a fabric specimen (133 mm x 133 mm) is exposed to an open flame exposure for 60 s, and then thermal energy transferred through the specimen can be measured as thermal protective performance. This test standard is mainly used for thermal protective fabric specimens to determine their endurance by an open flame exposure. However, these protective fabrics cannot be used alone to determine their endurance. In this test standard, the fabrics are always used in conjunction with materials that demonstrate any of the following behaviors when exposed to a highly intensive open flame: breakage, charring, dripping, brittleness, ignition, melting, and shrinkage. As these behaviors are not prominent in composite fabrics (shell fabrics, thermal liners, and moisture barriers) used for thermal protective clothing, this test standard cannot be accurately used in the context of thermal protective clothing. Nevertheless, this test standard may be used to differentiate/grade/rank between various thermal protective composite fabrics based on their relative performance.

Assessment of thermal protective performance under flame exposures: Many researchers [25,75,100,101,167,200,350] have studied the thermal protective performance of inherently and/or chemically modified fire-retardant synthetic fabrics (organic and/or inorganic) in flame exposures using the previously discussed ASTM standards. Because in flame exposures mainly convection and radiant modes of heat transfer dominate, these researchers evaluated the thermal protective performance under different ratios (70:30 and/or 50:50) of convective to radiant modes. These different ratios were chosen because these closely represent the actual environment faced by firefighters. In order to simulate the 70:30 convective to radiant mode, Shalev and Barker [25,75] and Torvi and Dale [350] used the ASTM D 4108 standard test (Fig. 5.13); in the same context, Behnke [100], Shalev and Barker[25], and Lee and Barker [167] used the ASTM F 2703 standard test to simulate the 50:50 convective to radiant mode (Fig. 5.14). Shalev and Barker [25] found that the performance of thermal protective fabrics is similar under both ratios of convective to radiant modes; however, the thermal energy transfer characteristics through fabrics under the two modes are quite different. In the convective mode, thermal energy imposes on a fabric’s surface and blows through it; whereas, in the radiant mode, thermal energy directly penetrates through a fabric [36,38]. Shalev and Barker [25] corroborated that thermal energy transfer through fabric under both modes can be a complex combination of absorption, re-radiation, conduction, and perhaps forced convection. In this context, Lee and Barker [167] further found that a 100% radiant mode significantly lowers the performance of thermal protective fabrics in comparison to a combined 50:50 ratio of convective to radiant modes, with the lowest performance observed under high heat flux. This is because actual thermal energy emitted through fabric is different under both modes. It was also observed that the thermal energy transmissions through fabric in convective modes are different at high and low heat flux. This is because thermal energy more readily escapes in the air gap between a tested fabric and heat sensor at low heat flux. Additionally, the thermal energy does not directly contact the fabric at low heat flux, which results in a lower convective heat transfer coefficient through fabric. In contrast, the thermal energy directly contacts the tested fabric at high heat flux; in this situation, turbulent hot air movement on the fabric enhances the thermal energy absorptivity of the fabric. Lee and Barker [167] also concluded that the convective mode accelerates the thermal oxidation in the fabric in comparison to the radiant mode. As a result, fabric chars more in the convective mode than the radiant mode. Furthermore, Shalev and Barker [25] found that the configuration of testers in ASTM D 4108 and ASTM F 2703 standards are quite different in terms of the fabrics’ surface exposure area, angle of flame impingement, flame turbulence, and distance of the exposed fabrics from burner top; however, these different configurations have a negligible effect on the performance of thermal protective fabrics.

Barker and Lee [101], Shalev and Barker [25,75], and Mandal et al. [76] further observed that thermal protective performance in the combined flame and radiant heat exposure depends on fabric qualities [167,350]. Shalev and Barker [25] found thatfab- ric thickness has a positive association with performance under flame exposure with different ratios of convective to radiant modes. They identified the thermal resistance of thicker fabrics, which ultimately increases the performance of fabrics to be much higher than thinner fabrics’ thermal resistance [75,100,167]. However, the combined flame and radiant heat exposures store a significant amount of thermal energy inside multilayered thicker fabrics, which ultimately reduces the performances of the fabrics [351]. Furthermore, Shalev and Barker [25] and Torvi and Dale [350] stated that fabric attributes such as air permeability, density, weight, surface transfer coefficient, surface optical properties, heat capacity, conductivity, fiber to air ratio, and air void distribution mainly affect thermal energy transfer through fabrics under combined flame and radiant heat exposures, which ultimately affects the performance of thermal protective fabrics [167]. In this context, Lee and Barker [167] explained that a fabric with high density possesses higher performance than a fabric with low density, if the weights of these fabrics are similar. This is because high-density fabric indicates a high fiber-to-air ratio, thus leading to more conductive thermal energy transfer through the fabric. On the other hand, low-density fabric increases the air void distribution or air volume fraction, thus leading to less thermal energy transfer through the fabric. Recently, Mandal et al. [76] reported that a fabric with high emissivity absorbs more thermal energy than a fabric with less emissivity under flame exposures. Eventually, most of the absorbed thermal energy transmits toward wearers’ bodies and generates burns.

Lee and Barker [167] investigated the impact of moisture on the thermal protective performance of fabrics in combined flame and radiant heat exposures and found that moisture affects performance in a complex way. In low-intensity (approximately <20 kW/m2) combined flame and radiant heat exposures, the absorbed moisture inside the fabrics increases their thermal conductivity; consequently, the performance of the fabrics is reduced. On the other hand, in high intensity (approximately >20 kW/m2) flame dominant combined exposures, the convective action of the flames has an ablative effect, carrying thermal energy away from the side of the fabric exposed to the flames, which ultimately increases the performance of the fabrics.

However, the absorbed moisture inside the fabrics becomes steam that may cause scalding on firefighters’ bodies. Furthermore, the differences in thermal protective performance among bone-dry condition fabrics and standard environment (65% relative humidity) condition fabrics was 10-20% in the 50:50 combined flame and radiant heat exposures at 84 kW/m2; the moisture-related increase in performance was significantly greater in fabrics that were soaked until they contained a significant amount of water [167].

Evaluation of thermal protective performance under hot surface exposure: In the ASTM D 7024 standard, the thermal protective performance of a fabric specimen (460 mm x 205 mm) is measured in both steady state and dynamic conditions of thermal energy transfer during contact with a hot surface. The fabric specimen is sandwiched between a hot plate and two cold plates, one on either side of the hot plate. In order to measure the performance in a steady state, a constant controlled heat flux (250 W/m2) is maintained for the hot plate with a constant temperature (20°C) for cold plates. The thermal transmission coefficient or thermal resistance (R-value) is measured after a steady state is reached. The heat flux of the hot plate is varied sinusoidally with a period of 15 min to evaluate the thermal protective performance in dynamic conditions. The midpoint of the sinusoid is typically kept at 150 W/m2, and the amplitude above as well as below the midpoint is typically kept at 100 W/m2. Here, the temperature regulating factor (TRF) can be measured for thermal protective performance; the TRF is defined as the amplitude of the temperature variation of the hot plate divided by the product of the amplitude of the hot plate flux variation and the steady-state R-value. This ASTM D 7024 standard can be used to determine the overall thermal transmission coefficient due to conduction for dry specimens of textile fabrics and battings. This coefficient can be used to establish criteria for thermal parameters of textiles particularly used in the clothing industry. In Mar. 2013, this standard was temporarily withdrawn for further upgrading.

The ASTM F 1060 standard is widely used in order to evaluate the thermal protective performances of fabrics in a more realistic hot surface contact exposure (Fig. 5.15). This test standard measures the thermal insulation of a fabric specimen (100 mm x 150 mm) used in thermal protective clothing when exposed to a hot surface with a temperature up to 316°C. A TPP sensor with a weight load is placed on a fabric specimen, which in turn has been placed on a hot surface plate, to measure the thermal energy transferred through the fabric specimen during the exposure. The weight load is used to create a contact pressure up to 0.003 MPa in between the hot surface and the fabric specimen. This test standard predicts the amount of thermal energy and time required to cause second-degree burn injury. The limitation of this test standard is that it can only test the fabric specimen in a horizontal position (under a standard pressure) and does not involve any movement; hence, other test configurations (eg, testing the fabric specimen in vertical positions and/or movable conditions) need to be included in this standard [76,77,352]. The standard is also limited to short exposure because the model used to predict burn injury is limited to predictions of time-to-burn for up to 30 s. The use of this standard for longer hot surface exposures requires a different model for determining burn injury or a different basis for reporting test results. Furthermore, the thermal protection time as determined by this test standard relates to actual end-use performance only to the degree that the end- use exposure is identical to the exposure used in this test standard; that is, the hot surface test temperature must equal the actual end-use temperature and the test pressure must equal the end-use pressure [352].

Laboratory-simulated hot surface contact exposure test in ASTM F 1060

Fig. 5.15 Laboratory-simulated hot surface contact exposure test in ASTM F 1060.

Recently, a few researchers [76,77] studied fabric performance using a modified ASTM F 1060 standard. Fabric specimens were placed on a hot surface plate (400°C) under a weight load of 1 kg, and a skin simulant sensor attached to a weight load measured the thermal energy transferred through the specimens. From this measured energy, thermal protective performances provided by these fabric specimens were predicted in terms of time required to generate second-degree burns.

Assessment of thermal protective performance under hot surface exposure: According to Mandal et al. [76], fabrics become compressed in between hot surfaces and human bodies under hot surface contact exposure. Due to this compression, the gaseous air phases inside the fabrics are reduced and solid fiber phases predominate. As the thermal conductivity of a fiber is greater than air, most of the imposed thermal energy on fabrics is absorbed inside the fabrics and/or transmitted toward the human body. This transmitted thermal energy generates burns on human bodies. In this context, Mandal and Song [353] stated that the ratio of gaseous air phase to solid fiber phase inside the fabrics varies depending upon the compression characteristics of the fabrics. They observed that the ratio of gaseous air phase to solid fiber phase of compressible fabrics is lower than noncompressible fabrics. Consequently, the amount of thermal energy transfer toward human bodies is greater in compressible fabrics. As a result, thermal protective performance is reduced. Mandal et al. [76] also demonstrated that the surface frictional properties of tested fabrics have significant effect on the thermal protective performance. They confirmed that a fabric with high surface roughness can trap a good amount of dead air on the boundary layer of the fabric when coming into contact with a hot surface. This boundary air layer resists the transmission of thermal energy from the hot surface to the fabric and enhances the thermal protective performance of the fabric.

Evaluation of thermal protective performance under molten metal substances exposure: The molten metal substances contact test is conducted according to ASTM F 955 (Fig. 5.16). This test standard evaluates fabrics’ thermal resistance to molten metal substances (aluminum, brass, and iron) [354-358]. The specimen of a protective fabric (305 mm x 460 mm) is mounted over a vertically inclined (70 degree) sensor board. The sensor board (250 mm x 406 mm) is fabricated from a flame- and heat-resistant material with a thermal conductivity value of <0.15 W/m K, and this board is attached with two TPP sensors (copper slug sensors), upper and lower. Then a molten metal substance of sufficient quantity for the test is heated—the temperature of this molten substance is measured by an appropriate device, such as an optical pyrometer or any other heat-measuring device with an accuracy of at least ±14°C. Subsequently, the molten substance is poured on the fabric specimen from a pouring crucible. The pouring crucible is aligned with the specimen at a height of 305 mm, so that the majority of the molten substance stream is applied to the fabric specimen directly above the center of the upper sensor. The amount of thermal energy transmitted through the specimen, during and after molten substance exposure, is measured using the two TPP sensors. The obtained thermal energy to cause a second-degree burn injury is interpreted as the thermal protective performance under molten substances. This test standard rates fabrics which are intended for clothing protective against potential molten substance contact, for their thermal insulating properties and their reaction to the test exposure.

Laboratory-simulated molten metal substances exposure test in ASTM F 955

Fig. 5.16 Laboratory-simulated molten metal substances exposure test in ASTM F 955.

Although the ASTM F 955 standard is used to evaluate thermal protective performance under exposure to molten metal substances, this standard is very sensitive to variations in experimental variables, including the temperature of the metal, the duration of the pour, the configuration of the test stand, and the type and sensitivity of the used sensor. Thus, the results obtained from this test may not be applicable in real life exposures; however, the results can be used as a first screening to choose fabrics for thermal protective clothing. Moreover, this test is elaborate and expensive; therefore, only a few researchers have assessed protective performance under molten metal substance exposure [69,200,359-361].

Assessment of thermal protective performance under molten metal substances exposure: Benisek and Edmondson [359] evaluated the performance of fabrics used in thermal protective clothing against various molten metal substances: cast iron, three different types of steel, copper, aluminum, zinc, lead, and tin. In their study, the performance of fire-retardant/resistant wool, cotton, novoloid, aramid, glass, and asbestos fabric were evaluated. The results showed that fire-retardant wool fabric had the best performance against any molten metal substances and that fabric attributes significantly affected thermal protective performance

[360] . It has been identified that a fabric with high thermo-plasticity, high thermal conductivity, high air permeability, low softening temperature, and high flammability has low thermal protective performance; whereas, a fabric with high weight and thickness will demonstrate high thermal protective performance [361]. In this context, it is evident that a fabric possessing char formation features can provide better protection than a fabric with no char formation features. Additionally, a fabric with high smoothness cannot trap molten metal on its surface and possesses a higher thermal protective performance. As a consequence, Barker and Yener

[361] concluded that aluminized smooth fabrics, including fabrics made with fire-retardant cotton, rayon, glass, or carbon as base fibers perform particularly well in deflecting the molten metal, resulting in a higher thermal protective performance. It was also found that heavier fabrics made from inorganic materials such as ceramic and silica fibers may have equivalent performance levels to aluminized smooth fabrics [361].

Evaluation of thermal protective performance under hot liquid exposure: The performance of thermal protective fabrics under hot liquid exposure can be measured according to the ASTM F 2701 standard (Fig. 5.17). This standard is used to measure thermal energy transmission through thermal protective fabrics (woven or knit fabrics, battings, sheet structures with permeable or impermeable coating) used in firefighters’ clothing and gloves that are exposed to a hot liquid splash. In this standard, a fabric specimen (355 mm x 560 mm) is exposed to a hot liquid pour at a determined temperature, volume, pour rate, and height above the specimen. The amount of thermal energy transmitted through the fabric specimen during and after the hot liquid exposure are measured using two TPP sensors (copper slug sensors). The amount of transmitted thermal energy to cause a second-degree skin burn injury is measured as the performance of the fabric specimen. Although this is a standard method, the fabric specimen is exposed to hot liquids only at one position (45 degree); to improve results, it is suggested to conduct the test at different fabric positions (horizontal, vertical, etc.) [76,353]. This test method is also limited to the hot liquid temperatures 40°C below the flash point of the specific hot liquid used for testing. The intent of specifying the maximum temperature at 40°C below the flash point is to reduce the chances of a hot liquid fire hazard, which increases significantly at temperatures equal to or above the flash point of the liquid [362].

Laboratory-simulated hot liquid exposure test described in ASTM F 2701

Fig. 5.17 Laboratory-simulated hot liquid exposure test described in ASTM F 2701.

Recently, Mandal and Song [353], Mandal et al. [76], and Ackerman et al. [22] evaluated the thermal protective performance of inherently fire-retardant fabrics (eg, Nomex, Kevlar/PBI) under a hot liquid splash using a modified ASTM F 2701 standard (Fig. 5.18). In both studies, a fabric specimen was placed on a sensor board (aligned at a 45 degree angle) made up of nonconductive, liquid and heat-resistant material. Hot water was prepared in a circulating bath, and its temperature was maintained at 85°C using a temperature control device. The hot water was moved through a circulation system attached with a flow-control valve. This circulation process helped to warm up the water pipe to regulate water

Laboratory-simulated hot liquid exposure test in modified ASTM F 2701

Fig. 5.18 Laboratory-simulated hot liquid exposure test in modified ASTM F 2701.

temperature at 85°C. Using a tap, the hot water was passed through the water outlet. By employing a thermocouple at the front of the outlet, water temperature was constantly monitored. It was observed that the temperature at the outlet reached close to 85°C within 0.5 s immediately after opening the water tap, after which point the fabric specimen was continuously exposed to the hot water. The thermal energy transferred through the specimen was measured at direct and indirect contact points between the hot liquid and the fabric specimen using two skin simulant sensors (upper and lower sensors). This measured energy was used to calculate the time required to generate second-degree burns, which was interpreted as the thermal protective performance.

Assessment of thermal protective performance under hot liquid exposure: Ackerman et al. [22] and Mandal and Song [353] studied the performance of layered fabrics under hot liquid exposure. It was identified that fabrics’ air permeability is the most crucial aspect that affects thermal protective performance under a hot liquid splash. The researchers found that permeable fabrics have a lower performance than nonpermeable fabrics. This is because the permeable fabrics allow a rapid transfer of mass (hot liquid) through fabrics toward human bodies, which generates quick burns on the bodies. On the other hand, nonpermeable fabrics allow a negligible amount of mass transfer through fabrics toward human bodies, for a high performance of these fabrics against hot liquid [76].

It was also identified that the thermal protective performance of fabrics is dependent on the properties of hot liquids [22]. Data indicates that the thermal protective performance of fabrics is least effective when exposed to hot water among a comparison of three types of liquids: hot water, canola oil, and drilling mud. They inferred that the viscosity of water is the lowest among all these liquids; as a result, hot water easily transfers through fabrics toward firefighters’ bodies. Furthermore, the heat capacity of water is the highest among all these liquids; as a result, transferred hot water mass comprises more thermal energy than the other liquids (canola oil and drilling mud). This high thermal energy content of water generates more burns on firefighters’ bodies than the other liquids.

Evaluation of thermal protective performance under steam exposure: Although it is important to evaluate the thermal protective performance of fabrics in the presence of steam, to date no internationally recognized standard test methods exist for this evaluation purpose. Many researchers have developed their own customized instruments to evaluate the performance of fabrics in the presence of steam [22,81-83,114]. For example, researchers from the University of Alberta developed an instrument to evaluate the performance of fabrics under steam exposure (Fig. 5.19) [22]. By using this instrument, Mandal and Song [353] and Mandal et al. [76] compared the performances of inherently fire-retardant fabrics (eg, Nomex, Kevlar/PBI). In their study, they generated steam at 150°C temperature and directed this steam through a nozzle (at a pressure of 200 KPa) onto a fabric specimen placed on a specimen holder. During this exposure, the amount of thermal energy transferred through the fabric was measured by a skin simulant sensor. From this measured energy, the time required to generate the second-degree burns on firefighters’ bodies was calculated as the fabric’s thermal protective performance.

Laboratory-simulated steam exposure test using the University of Alberta steam tester

Fig. 5.19 Laboratory-simulated steam exposure test using the University of Alberta steam tester.

Assessment of thermal protective performance under steam exposure: Keiser, Becker, and Rossi [81], Keiser and Rossi [82], Keiser, Wyss, and Rossi [83], Mandal and Song [353], and Shoda, Wang, and Cheng [85] have suggested that imposed high- pressurized steam inserts into fabrics’ structure and gradually condenses. After the condensation phase, all the steam converts into hot water. This hot water generates burn injuries when it comes into contact with human bodies. These researchers identified that permeable fabrics allow more steam transfer toward human bodies than impermeable fabrics. As a result, these researchers suggested that thermal protective clothing should be impermeable in nature to effectively protect from steam exposures. Mandal and Song [353] stated that it is essential to place a moisture barrier in the fabric system in order to effectively protect from steam. This impermeable moisture barrier will immediately stop steam insertion inside the fabric system, which will ultimately decrease the chances of burns on human bodies [76].

 
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