Thermal Protective Performance of Turnout Gear at High Flux Environment

*Rumeel Ahmad Bhutta and Sengkwan Choi*

## **Abstract**

Thermal protective performance (TPP) tests are conducted at a heat flux of 84 kW*=*m<sup>2</sup> to evaluate a firefighter's turnout gear performance. The test criterion used is based on a study conducted by Behnke in 1984. However, an average heat flux of 200 kW*=*m<sup>2</sup> has been documented in the literature over the past decade. Henceforth, experiments are conducted on the turnout gear at a higher heat flux level of 126 kW/ m2 , which represents a typical heat flux during the initial phase of a fully developed fire. The analytical analysis provides insights into air gaps, the effect of smoke layers on radiative attenuation and the thermal decomposition of the fabric layers. Numerical techniques were employed to introduce a survival curve to effectively gauge the degree of spatial burn at variable levels of heat fluxes, moisture effects and superficial burns. The sustainability of high resistance fabrics in terms of toxicity of flame retardants and recycling of these textiles have been discussed. Bio-composites as flame retardants are being introduced to replace traditional flame-retardant chemicals.

**Keywords:** firefighter, high heat flux, survival curve, thermal protective clothing, turnout gear

#### **1. Introduction**

Turnout gear is worn to safeguard individuals employed in hazardous fields like firefighting, high-speed sports, the oil and gas industry and the military. The protective gear can be either thermal protective clothing for firefighting, chemical resistant clothing for the oil and gas industry or puncture- and cut-resilient clothing for the military [1]. Thermal protective clothing is crucial for firefighters because fire is a common occurrence and poses a significant threat to people's safety. Firefighter protective clothing can be divided into two types: station wear, which is a single-layer garment worn while at the fire station, and turnout gear, which is a multiple-layer garment worn while responding to a fire incident [2]. The turnout gear is a combination of three fabric layers: an outer shell (OS), a moisture barrier (MB) and a thermal liner (TL).

The outer shell is the primary layer in a multi-layered clothing system and is part of a family of fabrics that are naturally resistant to fire [3, 4]. It has a highly organized structure of aromatic groups attached to a backbone chain, which prevents it from

breaking down into smaller molecules and producing less smoke when exposed to fire [5, 6]. When this aramid fabric group is combined with other fire-resistant materials using lamination, coating or bonding, it creates a moisture barrier. This porosity of the membrane is a determining factor in moisture control, that is, it is permeable to sweat vapor and acts as a barrier for water droplets. To make a thermal liner, a batting fabric made from fire-resistant fibers is sewn or quilted onto a face fabric, which is also made using fire-resistant fibers. Its main function is to reduce heat dissipation, provide comfort and release metabolic heat away from the wearer's body.

The maximum evaluation criterion for thermal protective clothing was proposed by Behnke [7]. He exposed swatches of the protective fabric made of Nomex®IIIA to several incident fluxes. The amount of incident flux that replicated the appearance from damaged clothing was estimated to be 84 kW*=*m<sup>2</sup> <sup>≈</sup>2 cal*=*cm<sup>2</sup> ð Þ<sup>s</sup> for 8 seconds of exposure. This estimated value was proposed to be a representation of flashover.

The conditions of a fire, whether it's inside a building or in a forest, can vary from one incident to another due to the unpredictable nature of the materials that are burning [8]. That is why firefighters' protective clothing is designed to be effective under a range of heat levels that are representative of several types of fires, including flashovers, backdrafts, fully developed fires, and forest fires. These garments are classified into routine, hazardous and emergency categories [9, 10]. Recent experiments [11–19] reported an average of 200 kW*=*m2 heat flux under extreme conditions in buildings and other enclosed spaces. These are higher levels of fire severity than previously measured. Henceforth, a new upper limit of 126 kW*=*m<sup>2</sup> is recommended, and the performance of turnout gear is tested at this level.

It corelates with the building data collected by the end of the past decade which emphasize modern construction trends in the housing sector [20, 21]. According to this data, the home area has increased by approximately 232*:*3 m2 in 2008, as compared to 144 m<sup>2</sup> in 1973. The 26% of homes built in 2008 were bigger than 278*:*7 m2; additionally, a growth of 56% in the construction of 2-storey homes (1-level above ground floor, US standard) was reported. Also, the current housing incorporates taller ceilings of 4.3–6.1 m, than the traditional 2.4 m. All these factors contribute to increased area and volume. As a result, the smoke spread during a fire incident remains above the ongoing fire; hence, more oxygen is available, causing the fire to grow rapidly and violently. This fire spread is also linked to the decorations and contents in the accommodation that have transitioned from natural to synthetic materials, such as cotton being replaced by polyurethane foam [22].

The effectiveness of a multi-layered protective garment can be evaluated using either a full-scale manikin test or a laboratory bench-scale test using the TPP rating system [23–25]. A full-scale manikin test assesses the performance of the entire garment against a convective/radiative thermal load, while a bench-scale test is performed on small patches of the garment. To capture thermal load on the skin, fullscale manikin testing facilities have been developed using a network of over 100 sensors distributed across the manikin's surface [26–30]. However, due to the high cost of full-scale tests, bench-scale tests are preferred as they are more affordable and can still replicate real-world room fires. Standards for bench-scale tests have been developed for diverse types of thermal loads, such as convective, radiant or a mix of both. The assessment criteria such as ISO 9151 [31], ISO 6942 [32], ASTM F2702 [33], ISO 17492 [24], ASTM F2703 [34] and NFPA 1971 [35] vary depending on the heat source and are representative of several types of fire incidents. The data from these experiments are then input into analytical models to predict skin burns.

#### **1.1 Skin burns**

The skin layers of interest in burn degree evaluation are the epidermis, dermis and subcutaneous. The second layer, dermis, is of the most significance as its temperature profile corelates to second-degree burns. The human body maintains a skin temperature of 37°C during daily activities. Pain is induced when the body temperature increases by 8°C, which occurs at a temperature of 44°C. A first-degree (superficial) burn occurs at 48°C, and a second-degree burn occurs with continued exposure to heat at 55°C. At 72°C, the skin tissues cannot sustain thermal equilibrium and are destroyed. Two widely accepted criteria are applied to assess this damage, Stoll's and Henrique's.

Stoll's criterion is applied to evaluate protection against second-degree burns [36]. It involves recording temperature data from a benchtop test, and the intersection of the experimental curve with Stoll's documented experimental curve predicts tolerance time. This criterion assumes a rectangular heat pulse, and its validity depends on this assumption.

In contrast, Henrique's is based on a chemical rate process and uses temperature and time to determine skin damage [37]. Skin-simulant sensors are used in modern thermal manikin experiments to predict third-degree burns using Henrique's burn integral, Ω, as shown in Eq. 1. The values of physical constant have been presented in **Table 1**. While Stoll's criterion is easy to use, Henrique integral has shown more accurate predictions for incident heat flux range not covered experimentally by Stoll. Henceforth, it is a preferred choice when assessing superficial burns at heat fluxes above 41 kW*=*m2.

$$\Omega = \int\_0^t \text{Pexp}\left(-\frac{\Delta \mathcal{E}}{\text{RT}}\right) \,\text{d}t \tag{1}$$

Ω, Henrique integral, second-degree burn occurs when *Ω* is unity

ΔE, the activation energy (J/mol) P, a pre-exponential factor

T, time-dependent absolute temperature of the basal layer.


#### **Table 1.**

*Henrique burn model physical constant values [8, 38].*

## **2. Experimental methodology**

To conduct novel tests on the thermal protective garment subjected to a fire load of 126 kW*=*m2, a purpose-built experimental apparatus was designed by cooperation between the Korean Conformity Laboratories (KCL) and Ulster University (UU), as shown in **Figure 1**. It comprises a radiant panel, a specimen assembly and a trolley. Two-layered halogen quartz tubes can output a consistent radiant flux of 126 kW*=*m<sup>2</sup> for more than 60 seconds. More details on the development, configuration and limitations of the test apparatus can be found in the preliminary study of KCL [39].

The experimental setup consists of an outer shell (OS), a moisture barrier (MB), a thermal liner (TL) and a substrate. The addition of a substrate represents closed-back configuration with variations in the air gap between the skin and the innermost layer [40, 41]. The temperature measurement at the back surface of the thermal liner is vital. Therefore, four type-K thermocouples were prepared to record the thermal liner back surface temperature. Constant contact is needed between the thermocouples and thermal liner back surface, for which sewing of the thermocouple was first implemented, as suggested by the National Institute of Standard and Technology (NIST) [42]. However, it was difficult to keep the bead in contact with fabric; plus, sewing made it demanding to retain the stripped wire away from the fabric surface. As four thermocouples are required to be placed in proper contact with the back surface, sewing proved to be impractical. Additionally, the application of heat-resistant tape is not recommended [42]. To overcome this, a new mechanism was developed based on pressure contact.

Thermocouple beads were soldered to a copper sheet of circular cut with a thickness of 0.35 mm. Thermocouple wires ran across the slits on a rod made of high thermal-resistant plastic. It was then put through circular holes in the substrate. The distance between the copper sheet and the circular rod was kept to greater than 6 mm to ensure proper air circulation. In this manner, four probes were prepared and attached at four distinct points of the fabric back surface. This method of pressure contact can record the spatial variations of the temperature, and it has been validated [43].

**Figure 1.** *Bench-scale apparatus.*

*Thermal Protective Performance of Turnout Gear at High Flux Environment DOI: http://dx.doi.org/10.5772/intechopen.114293*

**Figure 2.** *Turnout gear layup for experimental studies.*

Three types of fabric configurations, Type A, B and C as described in **Figure 2**, were examined at two different exposure levels of 84 and 126 kW*=*m2, representative of flashover and post-flashover. Specimens were exposed to the incident radiant flux for three different exposure times, of 10, 20 and 25 seconds. Type-A configuration is of a conventional layup. It is commercially available in the market and, therefore, is treated as a benchmark to access the performance of Type B and C. A 2 mm (2A) air spacing is ensured between the fabric layers and 6.5 mm between TL and substrate as per research standards [29, 41, 42, 44].
