Key issues related to thermal protective clothing
Some of the key issues related to performance evaluation and designing of thermal protective fabric/clothing are discussed in this section. These key issues need to be thoroughly researched and resolved to develop cost-efficient and high-performance thermal protective clothing that can provide better protection and comfort for firefighters.
Thermal protective performance evaluation
Firstly, although previous researchers extensively evaluated the thermal protective performance of fabrics under various flame and radiant heat exposures, their studies are limited with respect to the intensity of the exposures [24,25,29,75,76]. Most of these studies evaluated performance at an intensity of 84 kW/m2 [25,75,76]. In fact, presently existing international test standards to evaluate the performance of fabrics/ clothing are all set at an intensity of 84 kW/m2. At this point, it can be concluded that previous researchers’ studies do not properly simulate the intensity of exposures to evaluate thermal performance, because the minimum intensity of an actual standard fire hazard (structural fire or wildland fire) is very high (~130 kW/m2) [27]. Furthermore, Barker [11] suggested that firefighters are frequently exposed to low-intensity exposures (<20 kW/m2) for long durations. Although these low-intensity long- duration exposures are insufficient for the damage of thermal protective clothing, these exposures can cause significant burns on firefighters. However, no standard is presently available to evaluate the thermal protective performance under low- intensity flame and radiant heat exposures. As a result, the recommendation from previous researchers on the performance of thermal protective clothing may not be accurately applicable for protection from actual fire hazard scenarios. It is recommended to conduct consistent research on the development of new thermal protective performance test standards to fulfill requirements for protection from actual fire hazard scenarios.
Secondly, presently available test standards mainly used copper calorimeters/sen- sors to evaluate thermal protective performances. However, a copper sensor causes uncertain heat loss or underestimates thermal protective performance under prolonged exposures; hence, this sensor is not accurate in all conditions [27]. It is necessary to select a sensor carefully to accurately evaluate thermal protective performance. Contextually, it is necessary to remember that most of the available sensors in the market (TPP sensor, embedded sensor, skin simulant sensor, PyroCal sensor, and
Thermal Protective Clothing for Firefighters. http://dx.doi.org/10.1016/B978-0-08-101285-7.00008-3
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water-cooled Prototype sensor) to evaluate thermal protective performance are unsuitable for long duration high/low intensive fire exposures. In this regard, the water-cooled Prototype sensor is found more suitable than the other sensors. However, given the nature of this sensor, the application of the water-cooled Prototype sensor is cumbersome and can produce errors in terms of the actual heat flux evaluation for skin burn injury prediction. Additionally, the physical structure (eg, weight, thickness) and design (eg, coolant systems, sealant applications) of the existing Prototype sensor needs improvement for an accurate performance evaluation in long-duration intensive exposures [331,332]. It reflects that the development of a sensor that can measure the heat flux under long duration is necessary for protective clothing study. The advancement of technology (eg, optical high power laser, fiber optic, nanotube, radiometry, black body) can provide the opportunity to develop a powerful sensor; this can be helpful for the effective evaluation of thermal protective performance of firefighters’ clothing [578-582]. Moreover, thermal protective performance evaluation standards mainly predicted the time required to generate second-degree burns on firefighters’ bodies; these skin burn prediction results are interpreted as thermal protective performance. In this skin burn prediction process, the Stoll Curve is typically used [129,131,364]. Although the Stoll Curve is simple to implement [364], it is limited to a certain range of exposure time and heat flux [129]. Eventually, the Stoll Curve needs to extrapolate when the heat flux and exposure time falls beyond this range. Thus, the skin burn prediction results obtained from the Stoll Curve are critical in terms of accuracy [134]. Furthermore, the Stoll Curve was developed by assuming rectangular heat flux exposures [364]. However, the heat flux profiles behind thermally exposed fabrics or clothing may not always be rectangular. Any variation from rectangular heat flux invalidates the use of the Stoll Curve to predict second-degree skin burns [134].
Thirdly, all the presently available standardized thermal protective performance tests are based on a transmitted energy approach. Here, the amount of thermal energy transferred through fabric is measured by a sensor placed behind a thermally exposed fabric/clothing specimen, and this measured thermal energy is used to calculate thermal protective performance of the fabric/clothing [24]. These tests did not consider the thermal energy stored inside the tested fabric in a thermal exposure to evaluate thermal protective performance [11]. However, previous researchers confirmed that multilayered thick fabric can store large amounts of thermal energy in a thermal exposure, and the discharge of this stored energy (naturally or forcefully if the fabric is suddenly compressed) can lower the thermal protective performance of firefighters’ clothing [24,100,351]. Hence, it is necessary to evaluate the thermal protective performance of fabrics/clothing by considering stored energy as well. Keeping this in mind, a thermal protective performance test apparatus was developed to study the energy stored in thermal protective clothing during radiant heat exposure and the contribution of stored energy to firefighter burn injuries [92,583]. However, this test apparatus can only simulate a situation in which the hot thermal protective clothing suddenly compresses against firefighters’ bodies (such compression can be common due to repetitive flexing of firefighters’ limbs or due to compression of the firefighters’ clothing against fixed surfaces while on duty). Recently, Song et al. [24] evaluated thermal protective performance under radiant heat exposure considering the energy stored in a fabric. They used the instrument of ASTM F 2731 to evaluate the thermal protective performance for different single-layered, double-layered, and triple-layered dry and moistened thermal protective fabrics. By using this instrument, they exposed a selected fabric specimen under radiant heat exposures, after which the exposed specimen was kept for cooling for a fixed period of time. In this fixed cooling period, the specimen released the energy it stored during the exposures. Here, the release of thermal energy was either natural or carried out with compression. In Song’s study, a second-degree burn time was predicted at the end of the cooling period by iteratively changing the exposure time. Here, a new terminology—“minimum exposure time” (MET) to cause a second-degree burn—was coined. They identified that stored thermal energy contributes a large part of the total energy required to cause a second-degree burn injury, and the stored energy effect is more prominent in triple-layered fabrics. They also concluded that moisture accumulated in fabrics, compression, and air gaps (between tested fabrics and burn prediction sensors) have a significant effect on MET [584]. Although Song’s study was comprehensively performed to evaluate and identify factors affecting thermal protective performance by considering the energy stored in fabrics, they tested fabrics only under radiant heat exposures. However, firefighters are not only exposed to radiant heat; they can also be exposed to flame, hot surfaces, molten metal substances, hot liquids, and steam. Thus, it is essential to develop the tester to evaluate thermal protective performance using a stored energy approach under other exposures as well. Furthermore, it is essential to standardize thermal protective performance test methods using a stored energy approach under all types of thermal exposures.
Fourthly, the standardized bench-scale and full-scale tests were developed to evaluate the thermal protective performance of dry conditioned fabric; however, these tests do not focus on moistened fabric. In this regard, it is evident that firefighters often work in water-spray environments and/or kneel and crawl through puddles; this situation saturates the outer surface of the firefighters’ clothing with water. Additionally, on-duty firefighters perspire heavily, which causes internal moisture load inside the firefighters’ clothing. Here, the accumulated water or moisture inside the thermal protective fabric/clothing plays a significant role on thermal protective performance by changing the thermal conductivity and heat capacity of the fabric under different types of thermal exposures and intensities. It is also evident that the accumulated moisture may convert into steam depending upon the intensity or types of thermal exposures, which may result in steam burns on firefighters’ bodies [24,92,349]. Thus, it is essential to evaluate the thermal protective performance of moistened fabric/clothing. Although some researchers have attempted to evaluate the thermal protective performance of moistened fabric [24,92], there is a need to develop standard methods to evaluate the thermal protective performance of moistened fabrics/clothing under different types of thermal exposures. A major obstacle in the development of such testing methodologies is the lack of basic understanding of how moisture is absorbed in clothing when exposed, either to perspiration from a sweating firefighter, or to water from a fire ground source and how absorbed moisture affects thermal protection [11,27].
Finally, bench-scale and full-scale thermal protective performance tests are destructive in nature. This means fabric/clothing specimens used in these tests get destroyed every time they are tested in thermal exposures. As thermal protective fabric/ clothing is expensive, this emphasizes the need to develop nondestructive tests that can evaluate thermal protective performance without destroying the specimens [502]. In this regard, NFPA developed a nondestructive test method (NFPA 1851) for the selection, care, and maintenance of thermal protective clothing. In this test method, the shell fabric, thermal liner, and moisture barrier are exposed to normal light, chemicals (water-alcohol), and water pressure (1 psi), respectively. Here, the behaviors of the tested fabrics are visually observed to determine their performance for selection and maintenance. It can be inferred that this test method is subjective; hence, there is a need to objectively evaluate the thermal protective performance of thermal protective clothing using a nondestructive test.
