Comparison of Thermal Performance of Firefi ghter Protective Clothing at Diff erent Levels of Radiant Heat Flux Density Primerjava učinkovitosti toplotne zaščite oblek za gasilce pri različnih stopnjah sevanja toplotnega toka

The experimental work presented in this study is related to the investigation of thermal protective performance of fi refi ghter clothing, which plays a pivotal role in the fi refi ghters’ safety and performance. The fi refi ghter clothing usually consists of three layers, i.e. an outer shell, moisture barrier and thermal liner. Four samples were used for the purpose of this study. The samples were characterized on Alambeta for the evaluation of thermal resistance and thermal conductivity, respectively. Afterwards, the samples were evaluated on a thermal manikin “Maria” at room temperature to measure the insulation values. Moreover, air permeability was evaluated by using an air permeability tester. The samples were then analysed for their thermal protective behaviour in line with a lightly modifi ed ISO standard 12127, i.e. the samples were subjected toa150 °C heat plate at constant speed. In addition, transmitted heat fl ux density and percentage transmission factor of all samples were determined with the help of a radiant heat fl ux density machine at 10 kW/m2 and 20 kW/m2. It was concluded that sample 4 had higher thermal resistance and insulation values. The outer shell of sample 4 had lower air permeability values as compared to the outer shell of samples 1, 2 and 3. Similarly, the combination of the outer shell 4 and the thermal barrier 4 led to lower air permeability values as compared to the combination of the outer shell 1 and thermal barrier 1, outer shell 2 and thermal barrier 2, and outer shell 3 and thermal barrier 3. The rate of temperature rise in sample 4 occurred at a slower rate against the heated plate in comparison with samples 1, 2 and 3. Furthermore, sample 4 exhibited lower transmitted heat fl ux density and percentage transmission factor as compared to samples 1, 2 and 3.

Clothing not only serves as a barrier to the exterior atmosphere but also acts as a heat transmission channel from the human body to the surrounding atmosphere [1]. A microclimate is generated by the clothing between the human skin and air layer, which assists the thermoregulatory mechanism of the human body to maintain its temperature within a safe limit, despite the exterior environmental temperature and humidity deviating to some degree [2][3][4]. Th e exchange of heat in clothing includes conduction via the air gap and fabric layer, convection of the air gap and radiation from the fabric layer to another fabric layer [5]. In some situations, protection against fl ame and heat becomes primary precedence for a specifi c area of applications like fi refi ghting, where a shield against fl ame and thermal insulation is required [6]. Th e fi refi ghters' lives are always in continual danger when they are subjected to an escalated temperature climate, high thermal radiation, interaction with hot objects and confrontation to several types of flame, flash over being the most dangerous [7]. Th e fi refi ghter protective clothing shields the fi refi ghter from hazards like spilling of chemicals, fl ame, external radiant heat fl ux, and off ers a thermal equilibrium to their body [8]. Th e fi refi ghter protective clothing consists of three layers, i.e. an exterior shell, moisture barrier and thermal liner [8][9][10]. Th e exterior shell is made up of the substrates which do not burn or degenerate when they are confronted with the heat and fl ame. Th ese materials avert ignition when they are in contact with fl ame, and must be water repellent and permeable to water vapour. Generally, the outer shell is made up of meta-aramid (Nomex), and a combination of meta-aramid and para-aramid (Nomex III A), polybenzimidazole (PBI), Zylon. Sometimes, fl ame resilient fi nishes like Proban and Pyrovatex are employed as well. Th e moisture barrier is a microporous or hydrophilic membrane situated between the thermal liner and outer shell. Th is membrane is permeable to water vapour but impermeable to liquid water, and protects the human body from blood pathogens and chemicals in liquid form. Th is membrane is accessible in market as Gore-Tex, Proline and Cross tech, Action and Neo guard. Th e thermal liner secures the human body by delaying the external environment heat. It is made up of fl ame retardant fi bres and their blends. Th ey can be non-woven, laminated woven, quilted batting and spun laced [10][11][12]. Th e schematic diagram of a multilayer assembly is shown in Figure 1. Time is the main factor when the thermal protective performance is evaluated. An escalation in the thermal protective performance (TPP) means an increment in the duration of time for fi refi ghters to conduct their duties without enduring any severe skin burn injuries. Consequently, more time can be spent by the fi refi ghter to save lives and prevent damages instigated by fi re and heat [14][15][16].  [10,13] I -outer shell, II -moisture barrier, III -thermal liner Factors like thermal conductivity, water vapour resistance, volumetric fl ow capacity, permeability index and eff ect of air gaps can have an impact on the thermal protective performance of fi refi ghters' clothing (FFC) [17]. Th e evaluation of TPP can be performed by several tests (heat guard plate, TPP tester) [18][19][20][21][22] or the full-scale testing method (thermal manikin) [23][24]. A lot of scientifi c research in the form of numerical models and experimental studies has been conducted under various levels of radiant heat fl ux density to evaluate the thermal protective performance of FFC. Th ese studies have made use of the test methodologies like bench scale testing and full manikin test to determine the thermal protective performance of FFC under various levels of radiant heat exposure. Th e aim of this study was to investigate the thermal protective performance of diff erent FFC samples. Four diff erent sample arrangements were made. Th ese samples were tested with Alambeta, thermal manikin Maria and air permeability tester FX 3300. Th e threshold time, t (s), was measured in accordance with the ISO 12127 standard. Aft erwards, these samples were characterized with a radiant heat transmission machine (ISO 6942 method) to determine the heat transmission through a sample at 10 kW/m 2 and 20 kW/m 2 . Moreover, transmitted heat fl ux density, Q c (kW/m 2 ), percentage transmission factor, %TF(Q o ) and radiation heat transmission index (RHTI 12 and RHTI 24 ) were determined.

Materials
All fi refi ghter clothing (FFC) was provided by Vochoc Ltd (Czech Republic). Each clothing item consisted of three layers, i.e. outer layer, moisture barrier and thermal liner. Four diff erent clothing items with diff erent material combinations were used in this research. Th e material specifi cations taken from fi refi ghter clothing items (Table 1) and their arrangement in the clothing assembly are listed below ( Table 2).

Alambeta
Alambeta is a computer-controlled non-destructive device. With the help of Alambeta, the thermal properties of single layer and multilayer fabrics are determined [25][26][27]. It is non-destructive equipment which comprises of a movable hot plate attached to an ultrathin heat fl ow sensor on the top side and a lower cold plate. Th is upper heated plate falls in downward direction and makes a contact with the surface of the sample which is placed on the lower cold plate. Th e computer records the heat fl ow due to the diff erentiation in temperature between the upper heated plate and the sample on the cold plate. Th e temperature of the upper plate is held at 32 °C, where as the lower plate is kept at ambient temperature, i.e. at around 20 °C. With the help of Alambeta, characteristics like thermal conductivity, thermal diff usivity, thermal absorptivity, thermal resistance, sample thickness, and heat fl ow density and heat fl ow density ratio can be determined [28][29]. In this research, each sample was evaluated fi ve times.

Thermal manikin
A thermal manikin Maria ( Figure 2) was used to measure the thermal insulation values of fi refi ghter protective clothing samples. Th e manikin is built up of fi bre glass armed polyester shell covered with a thin nickel wire enveloped around the body to ensure the heating and temperature measurement. Th e design of shoulder, hip and knee joints was made of a circular cut to make the sitting and standing positions normal.

Figure 2: Th ermal manikin Maria with left forearm covered with sample of fi refi ghter protective clothing
During the testing, the manikin was positioned at the centre of the climatic chamber and was kept in a supporting frame, hung from the head and with the feet 0.15 m away from the fl oor. Th e manikin had 20 independent parts managed by a computer according to the association between dry heat losses and skin temperature of the human body for the conditions close to thermal comfort [29]. In our experiment, the forearm limb portion of the manikin was covered with a forearm sleeve, since the forearm limb area was much lesser as compared to the other parts of the manikin where less fabric was used.

Global method
Th e global method is a general formula for defi ning the whole body resistance. It is a conventional method which performs an overall calculation and defi nes whole body resistance. In equation1, f 1 is the relationship between the surface area of the segment I of the manikin, A i , and the total surface area of the manikin, A. To is the temperature of the operating environment in degrees centigrade (°C). T sk is the mean skin temperature in °C and . Q s,i is the sensible heat fl ux acquired by area weighing (W/m 2 ). First, the thermal insulation of a nude manikin, I a , was calculated.
Aft er subtracting I a from I T , the eff ective clothing insulation, I cle , (m 2 °C/W) was acquired.
To calculate the intrinsic thermal insulation, I cl was calculated with equation 3: where f cl is the ratio of the outer surface area of a clothed body to the surface area of a nude body.  Figure 3. Th e samples were raised to the height of 60 mm above the heated plate with the help of a dynamometer and aft erwards brought down towards the heated plate. When the distance between the heat plate and the sample was 10mm, we recorded the time and noted the temperature of the sample until there was a 10 °C rise in temperature. Aft erwards, we removed the heat source away from the sample and allowed the thermocouple and clamps to cool down for the next sample to be evaluated. Th e samples were brought towards the heated plate at the constant speed of 5mm/min [30]. Th e test procedure had to be performed on three samples to get the average value. Th e arrangement of the contact heat test is depicted in Figure 4. Th e apparatus consists of a heat plate, digital multimeter, T type thermocouple, clamps and a dynamometer: Heat plate which is VWR • ® professional hot plate developed for applications requiring exceptional accuracy, stability, and repeatability are equipped with an exclusive safety system that helps protect both the operator and sample. Digital multimeter Velleman DVM 345DI was • employed to evaluate the temperature changes in the sample. Th is device enables the user to measure AC and DC voltages, AC and DC currents, resistance, capacitance and temperature. Th e device can be interfaced with a computer and the user can also test diodes, transistors and audible continuity. T type thermocouple "UT-T" with the tempera-• ture probe test range from -40 to +260 °C with the accuracy of ±0.75% was utilized. Circular clamps were employed to hold the sample. Dynamometer was used to move the test sample • at the constant speed of 5 mm/min from fi xed distance.

Transmission of radiant heat fl ux density
Th e equipment consists of a radiation heat source, which can generate heat fl ux density of up to 80 kW/m 2 along with a calorimeter to determine the radiant heat fl ux density. Th e ISO 6942 standard was employed to measure the transportation of heat through a single layer and  [31]. Th e apparatus included a curved copper plate calorimeter placed on a non-combustible block. Th e front face of the calorimeter was layered with a thin fi lm of black paint with the absorption coeffi cient "a" greater than 0.9. Th e heating device comprised of six carbide rods, a moving frame assembly which was constantly cooled by a passage of water in cooling pipes and a removable screen. Th e fi rst step started with calibration, the position of the calorimeter was adjusted and then the calorimeter was exposed to the heating rods and the movable screen was withdrawn and returned to its original position when the temperature escalation reached 30 °C. Th e incident heat fl ux density, Q 0 , was measured. Later on, the sample was affi xed to one side of the plate of the sample holder and held in contact with the face of the calorimeter, applying the mass of 200 g. Th e movable screen was withdrawn and the starting point of the radiation head was noted. Th e movable screen was returned to its closed position aft er the temperature rise of about 30 °C. Th e time t 12 was to achieve the temperature rise of 12.0±0.1 °C and the timet 24 to achieve the temperature rise of 24±0.2 °C in the calorimeter, expressed in seconds, determined to the nearest 0.1 s. At least three samples had to be tested to get the average value [31]. Figure  5 shows the arrangement of the radiant heat testing equipment.
where M (kg) is the mass of the copper plate, C p is the specifi c heat of copper 0.385 kJ/kg°C, A(m 2 ) is the area of the copper plate, K (°C/s) is the mean rate of temperature rise in the calorimeter in the re-gion12-24 °C rise.
where Q 0 is the incident heat fl ux density (equation 7).
where R (°C/s) is the rate of the calorimeter temperature rise in the linear region and a is the absorption coeffi cient of the painted surface of calorimeter.

Evaluation of thermal properties
Th e thermal insulation of protective clothing plays a very important role in the thermal protective performance of fi refi ghter protective clothing. Th e main purpose of fi re fi ghter protective clothing is to delay the increase in temperature of the human body when they are exposed to a heat source and consequently to enhance the fi refi ghters' working time when saving lives and valuables. Th e ability of a textile substrate to conduct heat is called thermal conductivity of a textile material. A greater value of thermal conductivity indicates a greater amount of heat exchange passing through that substrate. However, the thermal conductivity of a textile substrate is determined by the physical and chemical properties of the textile substrate [32]. An increment in the relative humidity absorbed by the substrate is followed by an increase in the thermal conductivity of the textile substrate  [34]. Consequently, the more a material is hygroscopic, the better is thermal conductivity. Th ermal resistance is associated with thickness, surface weight and density. For thickness, it can be explained that at equivalent surface weights, increasing the thickness leads to an increase in the amount of air entrapped in the fabric. Th is is confi rmed by the fact that thermal resistance decreases by increasing density as higher density means less air entrapped in the textile. In consequence, a thick fabric has higher thermal resistance as compared to a light and thin textile substrate [33]. Th is is also described by the mathematic formula: R= h/λ, where R is thermal resistance, h is thickness and λ thermal conductivity. Moreover, it is infl uenced by the fabric construction parameters. Th us, a thick and heavy fabric is more insulative than a thin and light one [35]. Table 1 reveals that sample 4 had slightly greater thickness than other samples, which might be one reason for better thermal insulation and increased thermal resistance as compared to other samples. As the thickness of sample 1 was smaller than the rest of samples, sample 1 had signifi cantly lower values of thermal resistance and total thermal insulation, and clo values as compared to the rest of samples. Th is was also evident by the ANOVA test as the p-value was 7.35×10 -5 , i.e. less than 0.05, indicating a signifi cant diff erence among the samples. Furthermore, the constituent material of the substrate plays a very important role in the thermal insulation/ thermal resistance of fi refi ghter protective clothing [36]. Th e results of Alambeta in Figure 6 also support the outcomes (insulation and clo values) in Figures 7  and 8 for the thermal manikin, i.e. greater thermal insulation, which results in lower thermal conductivity and enhanced thermal resistance.

Evaluation of air permeability
As the air permeability of the moisture barrier in fi refi ghter protective clothing is zero, the evaluation of the air permeability of the outer shell and outer shell + thermal barrier was conducted. Th e air permeability of fi refi ghter protective clothing is very low, since the main task of fi refi ghter protective clothing is to protect the fi refi ghter's body from the heat in the form of radiation, convection and conduction. If the value of air permeability is very high, it decreases the thermal protective performance of fi refi ghter clothing as it allows the air to pass through the sample resulting in the temperature increase of the human body within a shorter period of time. It can be seen in Figure 9 that the outer shell of sample 4 exhibited lower air permeability values as compared to the outer shell of samples 1, 2 and 3. Figure 10 shows that sample 4 had a lower value of air permeability as compared to the rest of samples in the case of the outer shell and outer shell + thermal barrier, and this low value is supported by the high value of thermal insulation and low values of thermal conductivity as evaluated by the thermal manikin and Alambeta, respectively.

Contact heat plate test at 5mm/min exposing speed
In Table 3 and Figure 11, it can be seen that sample 4 took more time for the increment of 10 °C rise in temperature when exposed to the heat source (150 °C) at the constant speed of 5 mm/min. Furthermore, when the sample was at 10 mm distance from the heat source, the temperature at the back of sample 4 was lower as compared to other samples at the same distance. Th ere are two possible reasons for better thermal protective performance of the clothing. One is the thickness and the other is the physical and chemical properties of constituent fi bres in the fabric. In the case of sample 4, thickness was slightly higher as compared to the rest of samples; the sample had a higher percentage of meta-aramid in the outer shell, enhancing the thermal protective performance and delaying the rate of the temperature rise. Th e greater the delay in the heat transmission towards the human body, the greater is the thermal protective performance of the clothing, enabling the fi refi ghters to spend more time on duty. T c -contact temperature of hot plate T 1 -initial temperature at the back of sample when at the distance of 10 mm from hot plate T 2 -fi nal temperature at the back of a sample when there is a 10 °C rise in temperature t -threshold time for increase of 10 °C Figure 11: Contact temperature and thermal protective performance of fi refi ghter clothing at exposing speed of 5 mm/min   Figure 12 shows that in the fi rst 12 seconds, the rate of temperature rise in all samples was almost equal. However, aft erwards, the rate of temperature rise of sample 4 occurred at a much slower rate; therefore, a fl atter curve was seen. In the case of sample1, a steeper curve was observed, which indicated that the rate of temperature rise was greater as compared to the rest of samples. For samples 2 and 3, the curve pattern was very similar until the 35 th second. Aft erwards, the curve of sample 2 became slightly fl atter as compared to the curve of sample 3, indicating a slightly better thermal protective performance of sample 2 as compared to sample 3. Th e fl atter the curve, the more time was required to rise the temperature on the other side adjacent to the calorimeter, due to which the amount of heat was delayed and lower values of Q c (kW/m 2 ) and %TF(Q o ) were noted by the calorimeter. As a result, fi refi ghters are able to endure the heat for a longer period of time and perform their activities before acquiring any harmful injuries. At 20 kW/m 2 , the curve pattern of samples 4 and 1 was similar to that of the curves for 10 kW/m 2 . However, this time, the curve of sample 3 was fl atter as compared to the curve of sample 2 and both curves were overlapping each other from the time of 40-57 seconds. Aft erwards, the curve of sample 2 was slightly fl atter than the curve of sample3. It was also noticed that in Table 4, at 20 kW/m 2 , the value of Q c relatively increased for each sample as compared to the value of Q c for 10 kW/m 2 due to which steeper curves were acquired indicating the rate of temperature rise occurring at a faster rate.

Conclusion
Th e fi refi ghters's safety is infl uenced by the protective performance of fi refi ghter protective clothing. If the thermal protective behaviour of FFC can succeed in enhancing the confrontation time of fi refi ghters against radiant heat fl ux density, they will be able to save more lives and assets. Th e research showed that sample 4, which had a higher thickness value and high percentage of meta-aramid in the outer shell, displayed better thermal resistance and insulation properties as compared to the rest of samples. Th e outer shell of sample 4 depicted a lower value of air permeability and the combination of outer shell + thermal barrier of sample 4 exhibited lower thermal conductivity values with respect to other samples. Furthermore, the time of exposure to the heat plate at the constant temperature of 150 °C was longer in the case of sample 4. All these results suggest that sample 4 had slightly better thermal properties as compared to the rest of samples. A further study is required where thermal barriers would be replaced with suitable insulating materials to determine the thermal protective performance.
Additionally, the outer shell should be coated with nano-metallic particles like silver, Al 2 O 3 and TiO 2 to evaluate the thermal protective performance of FFC.