Flame Resistance Performance of Silicone Pad for Application in Railway Industry

Abstract

This study investigates the applicability of eco-friendly silicone materials with improved flame retardancy as interior materials for Korean urban railway vehicles, focusing on developing nonslip pads for seats made of non-combustible materials. Fire safety standards vary worldwide, necessitating country-specific testing and analysis. For application to the interior of railway vehicles in Korea, technical standards for the flame-retardant performance of railway vehicles were evaluated, and nonslip pads for seats were tested by comparing two types of flame-retardant silicone. In addition to fire property testing on a specimen basis, experimental verification was performed on a full chair assembly including silicone pads. Passenger comfort testing through pressure measurements was also conducted alongside fire safety performance testing The actual fire test showed that the maximum average heat release rate value was about 20% lower than the standard’s upper limit. Using flame-retardant silicone pads enhances fire safety and passenger comfort, satisfactorily meeting the required performance standards for Korean railway vehicles.

1. Introduction

Each country implements different safety standards for key elements during fire incidents in railway vehicles, including materials used inside and outside the vehicle. Major internal and external elements include passenger seats, interior panels, flooring, insulation, connecting passageways, wiring, and the vehicle’s exterior. Organic foam is essential for the comfort of passengers sitting for long periods. However, its flammability can produce large amounts of smoke and toxic gases during a fire, complicating evacuation and potentially causing passengers to suffocate. For example, subway fires in Baku and Daegu resulted in 289 and 189 fatalities, respectively [1,2]. Consequently, the field of fire safety extensively researches the characteristics of railway fire incidents worldwide, including heat release rates, toxicity indexes, and smoke density.

For fire safety performance analysis, conducting full-scale fire tests on actual railway vehicles is the most definitive method, but it is costly. Therefore, research on fire safety evaluation has been conducted based on each country’s railway vehicle safety standards, using full-scale tests, scaled model experiments, or numerical simulation methods. Lee et al. conducted a full-scale fire test on an intercity train in Korea, analyzing fire safety performance based on heat release rates, temperature measurements, and photos of window damage [3]. Kim et al. assessed the fire safety of a tilting train body made of composite materials developed in Korea through large-scale model experiments [4]. Ingason used a 1/10 scale train for fire experiments, calculating the heat release rate by considering combustion characteristics based on the number of windows, fuel weight, and lining material type [5]. Chen et al. investigated the influence of geometric structure, combustion performance, fire load, and ventilation conditions on the heat release rate of passenger and dining compartments in Chinese high-speed trains [6]. Capote et al. used a cone calorimeter (bench-scale test) and FDS to study fire occurrence patterns in passenger train compartments, performing experiments on heat release rates [7]. Hu et al. conducted numerical simulations of fires in railway compartments [8]. White et al. aimed to understand the growth and spread of fire inside trains in Australia through large-scale experiments, measuring and analyzing temperature, ventilation flow, heat flux, and smoke density [9]. Luo et al. compared FDS simulation results and experimental data on the internal structure of railway vehicles according to the U.S. fire safety standard NFPA 286 [10], concluding that removing doors reflects non-conservative conditions and thus is not recommended [11]. Full-scale fire tests and numerical simulations are also widely used for analyzing tunnel fire incidents. Zhou et al. analyzed the heat release rate during a fire in a newly constructed high-speed train tunnel in China [12]. Rie and Ryu used the Fire Dynamics Simulator (FDS) to evaluate fire safety for the GTX subway line in Korea, analyzing heat release rates, CO concentrations, and visibility during a fire to improve evacuation conditions through an optimized smoke extraction system, and proposed strategies for various evacuation scenarios [13]. Zisis et al. employed the Large Eddy Simulation (LES) model to simulate airflow and the evacuation process in a suburban train and a 1.5 km long rectangular tunnel after a 20 MW diesel fire, exploring factors that significantly affect evacuation safety [14].

Railway vehicle fire regulations in various countries commonly require fire safety performance for materials used in components susceptible to fires. Therefore, country-specific standards necessitate specimen-level material testing and fire safety analysis for railway vehicles and parts. This is crucial because significant differences exist in the flammability test requirements for transit seating across different countries and transportation sectors [15,16]. For example, recent railway projects in Europe, the Middle East, and Southeast Asia specify EN 45545-1 [17] in their Requests for Proposals (RFPs) regarding fire safety for railway vehicles. EN 45545 is a fire protection standard developed for implementation across Europe since 2016, integrating existing fire safety regulations from various European countries. EN 45545-2 [18] details 68 types of tests based on installation location and product type, outlining specific fire test requirements. For products outside these categories, the standard requires a separate evaluation process to determine the testing methods [19,20,21]. In Korea, following the 2003 Daegu subway arson incident, which resulted in the highest casualties (192 deaths and 148 injuries) since 1966, fire safety requirements for railway vehicle technology standards were strengthened. The body and interior equipment of trains are primarily made from non-combustible materials, but interior panels, seats, connecting curtains, flooring, insulation, and wiring must meet indoor equipment fire safety standards. The testing procedures and evaluation methods are well defined. However, when the standard was initially established in 2014, only the eight major components deemed critical for fire safety were required to undergo fire testing. This poses challenges in applying fire safety evaluations to new products, such as nonslip pads for train seats [22].

As the types and test methods of parts specified in railway vehicle fire safety requirements differ from country to country, extensive research has been conducted on specimen fire property testing and fire safety evaluation of materials used for various parts. For electrical wires, Rao et al. summarized methods for assessing the fire safety of wiring and other materials used in railway vehicles, testing heat release rate, smoke density, and toxicity index using cone calorimeters according to international standards, and analyzed the results [23]. They classified DW, seating materials, FRP, silicone rubber, and curtain fabrics according to their maximum average heat release rate (MARHE) values. Bi et al. conducted combustion characteristic tests according to the international standard ISO 9705-1 [24] on key components of high-speed train carriages to provide guidance on the fire protection design of high-speed trains. They found that the fire resistance performance of window glass has a significant impact on train fire safety [25]. In addition, studies in the United States and Europe have evaluated the fire safety of floor assemblies in railway vehicles, providing guidelines for design modifications and optimal fire safety design [26,27]. Specifically, for interior and exterior materials, the heat release rate, smoke generation rate, and fire performance have been measured and analyzed using a cone calorimeter, considering the materials and finishes used for various types of passenger trains [28,29].

Fire safety analysis and evaluation are also essential for applying materials that enhance the flame retardancy of existing materials in railway vehicles. Skripinets et al. developed an epoxyurethane mastic with flame-retardant and vibration damping properties for use on internal metal surfaces of railway vehicles. They analyzed the damping performance of the mastic by examining its flame-retardant properties, viscoelasticity, and dynamic mechanical properties [30]. Zhu et al. conducted experiments to improve the fire performance of sandwich composites using inflatable mats and evaluated their applicability in the railway industry by applying EN 45545. The experiments assessed heat release rate, smoke density, toxicity, and mechanical performance after fire, demonstrating excellent flame-retardant performance. This suggests applicability in the railway industry and shows potential for improving the thermal and mechanical performance of sandwich composites [31]. Such research is ongoing not only in the railway field but also in other transportation sectors, such as buses [32,33].

With the significant use of foam in railway vehicle seats for passenger comfort, it is necessary to use foam materials with higher flame retardancy. Currently, urethane foam treated with expensive flame retardants is mainly used, but the application of more efficient flame-retardant materials should be considered to improve cabin safety. Silicone is an eco-friendly material with advantages such as a wide temperature range, resistance to weather conditions, electrical insulation, and non-toxicity. These advantages are especially noticeable in extreme environments, such as high and low temperatures, where they resist heat and cold even at temperatures as high as 200 °C and as low as −90 °C while maintaining elasticity [34]. This provides a unique competitive advantage over other materials [35,36,37]. Additionally, research is being actively conducted to enhance flame retardancy by adding various fillers to silicone-based polymers or using silicone as a coating material [38,39,40,41]. Silicone foam is used in all nuclear power plants in Korea except Kori Nuclear Power Plant Unit 1. The flame retardancy of silicone materials is particularly superior to that of other materials, enhancing their value as flame retardants in the construction field. Recently, the importance of flame-retardant silicone has been further emphasized, focusing on controlling the spread of flames and toxic gases during fires in high-rise buildings [42].

In this study, we explored the applicability of silicone materials with enhanced flame retardancy as interior materials for Korean urban railway vehicles, aiming to develop nonslip pads to prevent slipping on seats made of non-combustible materials. For application to Korean trains, we analyzed flame-retardant technical standards for railway vehicles and tested nonslip pads made of flame-retardant silicone under development to ensure that they met the required performance standards. It was shown that fire safety and passenger comfort can be improved with the use of nonslip pads made from flame-retardant silicone for railway vehicle chairs. This was developed through mechanical performance analysis, testing not only flame-retardant properties but also the comfort of the seating surface in the railway field.

2. Materials and Methods

2.1. Design and Fabrication of Railway Cabin Seat Assemblages with Silicone Pads

In this study, tests were conducted on two types of flame-retardant silicone samples (A and B), as summarized in

Table 1. Components of each sample.

	Common Silicone	Flame-Retardant Silicone A	Flame-Retardant Silicone B
Components	- High-tear silicone rubber - Pigment - Hardener	- Silicone rubber - Inorganic filler, etc. - Flame-retardant (metal complex) - Pigment - Hardener	- High-tear silicone rubber - Inorganic filler, etc. - Flame-retardant (metal complex) - Pigment - Hardener

. The proportions of additives that improved the flame-retardant properties, apart from silicone, which was the main component, were adjusted. The specimens were manufactured by HRS Co., Ltd., and tested according to railway vehicle technical standards to examine their applicability in cabin seats for urban railway vehicles in Korea.

To study the suitability of the nonslip silicone pads made from the manufactured specimens for cabin seats, cabin seat assemblages were designed, and molds were fabricated for the tests. Figure 1a shows the four silicone pad shapes that were designed considering the contact area between the seat surface and the human body, whereas Figure 1b shows the molds that were prepared for use in cabin seat assemblages. A protrusion in the form of an undercut was considered to facilitate the attachment of the seating surface of a seat made of PC to a railway carriage. Figure 1c shows the surface shape of the manufactured test seat.

2.2. Flame Retardancy Test

In Korea, the MARHE, smoke density, and toxicity index are among the standards for materials used in the interiors of railway vehicles, and these standards must be met by silicone materials. The MARHE test method follows ISO 5660-1 (Reaction to fire test—heat release, smoke production, and mass loss rate—Part 1: heat release rate (cone calorimeter method) [43] and smoke production rate (dynamic measurement)). Figure 2a shows the cone calorimeter test equipment. The MARHE calculation was based on the formula defined in Equation (1), whereby the data were collected in 2 s intervals. MARHE was considered the maximum value of the average rate of heat emission (ARHE) calculated within 20 min. The radiant heat conditions of the added cone heater were set to (25 or 50) kW/m² depending on the type of interior material.

$$ARHE\left(t_n\right)={\displaystyle \frac{{\displaystyle \sum _{i=2}^n\left(t_i-t_{i-1}\right)\times \frac{q_i+q_{i-1}}{2}}}{t_i-t_1}}$$

(1)

The smoke density (D_s) was measured according to ASTM E 662 (Standard Test Method for Specific Optical Density of Smoke Generated by Solid Materials) [44] using the equipment shown in Figure 2b. D_s was measured as the change in light transmittance of the smoke as a function of the amount of smoke generated during the combustion of a sample. The smoke density is calculated as follows:

$$D_s=G\left[{\log }_{10}\left({\displaystyle \frac{100}{T}}\right)+F\right]$$

(2)

where $G=V/AL$; $V$, $A$, $L$, and $T$ are the volume of the chamber, exposed area of the sample, length of the light path through smoke, and percentage of light transmittance measured by the light-sensing device, respectively, and $F$ is the filter condition constant. The toxicity index is quantified in accordance with BS 6853 Annex B.2 (Code of practice for fire precautions in the design and construction of passenger carrying trains) using the equipment shown in Figure 2c, and the thermal radiation intensity applied to the test body should be maintained at 25 kW/m². The toxicity index refers to the comparative concentrations of the main gases (carbon monoxide, nitrogen compounds, etc.) generated during the combustion of a sample to the reference values.

2.3. Human Pressure Distribution Test

Riding comfort is an important factor in designing train car seats. A human pressure distribution test was conducted to explore the effect of the shape of the silicone pad attached to the train chair on ride comfort. The experimental equipment (X3 Pro, XSENSOR Technology Co., Calgary, Canada) was a pressure distribution test system developed by the XSENSOR Company, as shown in Figure 3a. It is used to measure and image the pressure on the human body owing to the influence of the environment in various fields [45,46,47]. For the experiment, as shown in Figure 3b, the height of the chair prototype (Figure 1c) manufactured for each shape was set by installing the legs as in an actual train. After installing the pressure-sensing pad on the seat surface, the person selected as a passenger sat down, and the pressure value measured on the PC connected to the pad was saved as an image. The experiment selected three passengers representing 5%, 50%, and 95% of the standard Korean weight range.

2.4. Flame Retardancy Test of Cabin Seat Assemblage

In addition to the sample unit test, additional fire tests were required when silicone materials were applied in railroad car seats, as shown in Figure 2d. The test method followed EN 45545-2 (Railway applications—fire protection on railway vehicles Part 2: requirements for fire behavior of materials and components), Annex B (fire testing method for seating), and EN 45545-2, which included the details of the square burner, ignition angle, and installation location of the seat, as well as the preparation of the test specimen, conditions for the end of the test, and reporting of the test results.

3. Results and Discussion

3.1. Thermal Resistance Analysis of Silicone Pads

First, the MARHE was obtained from the ARHE calculated using the heat release rate (HRR) according to ISO 5660-1 [43] of the fabricated samples with thicknesses of 1 and 2 mm. If it is thinner than 1 mm, there is a risk of damage from vandalism by passengers; if it is thicker than 2 mm, fastening it to the chair becomes difficult. Therefore, two thicknesses were considered. At 25 kW/m², which is the radiant heat condition for evaluating interior materials such as seat covers and pads, non-flame-retardant silicone was ignited, and the calorific value was measured; however, the flame-retardant silicone did not ignite. In the case of the non-flame-retardant specimens, the 1 mm thick specimen showed that the MARHE value increased by approximately 20% compared with that for the 2 mm thick specimen (

Table 2. Comparison of MARHE at incident heat flux of (25 and 50) kW/m².

kW/m2	Common Silicone: 1 mm	Common Silicone: 2 mm	Flame-Retardant Sheet A: 1 mm	Flame-Retardant Sheet A: 2 mm	Flame-Retardant Sheet B: 1 mm	Flame-Retardant Sheet B: 2 mm
25	28.1		32.9		Non-ignited
50	58.9	62.6	47.4	51.0	40.2	44.5

). Figure 4a confirms that the time required to reach the maximum heat release rate was delayed by approximately 50 s. In addition, the ARHE value was larger for the 2 mm thick specimen (Figure 4b), which seemed to be due to the increase in the volume of the combustible base material. In addition, both the non-flame-retardant and flame-retardant silicone materials were ignited at 50 kW/m², which is the radiant heat condition applied to seat bodies and other commonly used interior materials. Figure 4b shows the HRR and AR-HE results; the calorific values are summarized in Table 2. At both 25 and 50 kW/m², MARHE degradation due to flame retardation was observed.

Figure 5 shows the ignition characteristics of the 1 mm thick specimens. Compared with the non-flame-retardant specimens, the calorific values of the flame-retardant A and B specimens were significantly reduced, and the combustion process almost ended ap-proximately 200 s after ignition for both flame-retardant specimens, whereas the combustion process of the non-flame-retardant specimens continued until approximately 500 s. The MARHE value of the non-flame-retardant specimen was 58.9 kW/m², and the MAR-HE values of the flame-retardant A and B specimens were 47.4 and 40.2 kW/m², respectively, indicating a significant reduction in the calorific value. Figure 6 shows the ignition characteristics of the 2 mm thick specimens. Similar to the 1 mm thick specimens, the calorific values of the flame-retardant A and B specimens were lower than those of the non-flame-retardant specimens, but the effect was lower than for the 1 mm thick specimens. This was also attributed to the increase in the amount of base material that could be combusted. The MARHE value for the non-flame-retardant, flame-retardant A, and flame-retardant B material was 62.6, 51.0, and 44.5 kW/m², respectively. According to the above results, regardless of the thickness of the specimen, the composition of the flame-retardant B specimen showed a superior flame-retardant effect compared with that of the flame-retardant A specimen. It was also demonstrated that the thicker the silicone layer was, the higher the combustion rate and calorific value. Considering the results comprehensively, sample B showed a lower maximum heat release rate than sample A for both 1 and 2 mm thicknesses; therefore, it was regarded as having low exothermic characteristics.

3.2. Smoke Density and Toxicity Analyses of Silicone Pads

For the smoke density test, the applicable technical standard for railway vehicles provides smoke density values for 1.5, 4, and 10 min after the beginning of the measurement. The flame retardancy of the interior materials was determined based on the measurement times. As shown in the table below, smoke density was regulated for seats and flooring materials for up to 4 min, and for interior boards for up to 10 min. In this case, the combustion of flammable materials during a fire is affected by radiant heat from the surrounding flames and by direct heating from the flames. Therefore, the test conditions of both the non-flaming mode (sample combustion condition owing to radiant heat) and the flaming mode (sample combustion owing to radiant heat and flame) were considered.

In contrast to the results showing a high MARHE with increasing thickness, the experimental results confirmed that as the thickness increased, the smoke density decreased under both non-flaming and flaming conditions. The samples were considered to have a sufficient heat resistance because they showed no ignition results under the 25 kW/m² condition, which was the same as the smoke density radiant heat condition. Considering that the smoke density measurement was performed under 25 kW/m² radiant heat conditions, as shown in Figure 2b, the greater the thickness, the higher the thermal resistance of the sample, leading to a higher burnup of the sample, is considered to be lowered. From

Table 3. Comparison of smoke density of various silicone specimens.

Material	Thickness	Testing Mode	Average Smoke Density
			1.5 min	4 min	10 min	Maximum
Flame-retardant sheet A	1 mm	Non-flaming mode	27.1	161	226	230
		Flaming mode	28.7	101	119	120
	2 mm	Non-flaming mode	6.3	162	280	284
		Flaming mode	16.3	73.7	106	108
Flame-retardant sheet B	1 mm	Non-flaming mode	34.8	234	339	341
		Flaming mode	36.9	138	143	147
	2 mm	Non-flaming mode	4.7	99	231	265
		Flaming mode	10.8	76.6	123	124

, it can be concluded that the observed smoke density in the non-flaming mode was higher than that in the flaming mode, which is attributed to intense incomplete combustion, which results in the formation of smoke particles.

Considering the Korean technical standards for railway vehicles, it is estimated that both samples A and B with 2 mm thickness can be safely used as interior materials (seats, flooring, etc.); however, the smoke density up to 10 min after smoke generation exceeds 200. Consequently, the specimens were found to be unsuitable for application in interior panels because they adhered to only part of the requirements. Sample B showed a lower maximum heat release rate than sample A for each mixing ratio considered; however, the results of the different ratios did not differ significantly in terms of smoke density.

To confirm the degree of toxicity of the combustion gas of the silicone material according to railway vehicle technical standards, a toxicity index test was performed on samples A and B. After measuring the concentrations of the eight gaseous components (CO₂, CO, NO_x, SO₂, HCl, HF, HBr, and HCN) generated by combustion under the conditions specified in the test method (BS 6853 Annex B.2), the ratio and sum of the reference values (i.e., immediately dangerous to life or health (IDLH) for an exposure time of 30 min) defined in

Table 4. Reference values of BS 6853.

Gas	Reference Values (mg/g or g/m2)
Carbon dioxide (CO2)	14,000
Carbon monoxide (CO)	280
Hydrogen fluoride (HF)	4.9
Hydrogen chloride (HCl)	15
Hydrogen bromide (HBr)	20
Hydrogen cyanide (HCN)	11
Nitrogen dioxide (NOx)	7.6
Sulfur dioxide (SO2)	53

in BS 6853 were calculated as follows:

$$R={\displaystyle \sum }_x{r}_x$$

(3)

where $r_x=c_x/f_x$, $c_x$, $f_x$, and $r_x$are the measured values of the x component, the reference value, and the relative toxicity value of each component, respectively.

As a result, CO₂, CO, and NO_x were detected, but their toxicity index (R) values were lower than 0.2, as summarized in

Table 5. Toxicity index results of various specimens.

	Sheet A: 1 mm	Sheet A: 2 mm	Sheet B: 1 mm	Sheet B: 2 mm
R Value	0.149	0.147	0.147	0.162

, indicating that the measured values met the criteria given in

Table 6. Toxicity index (R value) criteria for interior materials for railway application (KRTS-VE-Part 51-2016(R1)).

Fire Safety Requirements	Pass Criteria
	Hazard Class 1	Hazard Class 2	Hazard Class 3	Hazard Class 4
Interior plate	≤3.6	≤2.7	≤1.6	≤1.6
Seat cover	≤3.2	≤2.7	≤2.3	≤2.0
Seat cushion	≤3.6	≤3.6	≤3.2	≤3.2
Seat body	≤3.2	≤2.7	≤2.3	≤2.0
Floor	≤5.0	≤4.0	≤3.0	≤3.0

. In contrast to the test method of ASTM E 662 [44], the test method of BS 6853 Annex B.2 involves the direct combustion of the sample; it proceeds through the application of 25 kW/m² of radiant heat, similar to ISO 5660-1 [43]. After measuring the calorific values of samples A and B, it was concluded that the lack of ignition resulted in a low smoke generation and low smoke toxicity.

3.3. Shape Effect of Silicone Pad on Seat Comfort

The resultant data shown in Figure 7 were obtained from the body pressure distribution test according to the passenger weight and shape of the silicone pad. When silicone pads were applied, the pressure distribution was lower for all body weights. The lighter the weight, the greater was the effect of applying the silicone pad, whereas the heavier the weight, the lower was the decrease in the average pressure and peak pressure, which is due to the relatively low stiffness of the silicon material compared with the PC material. For the lightest 5% percentile (Figure 7a), there was no significant difference in the shape of the body pressure distribution. However, when the silicone pad was applied, the peak pressure decreased significantly, and the average and peak pressures of design #2 improved the most. For the weight percentiles 50 and 95% (Figure 7b,c), the average and peak pressures decreased slightly, and there was no difference based on the shape of the silicone pad. From the body pressure distribution shape of design #3, the pressure gradient along the interface between the chair and pad appears to be very large, which is known to be a factor leading to a low riding comfort [48]. Because designs #1 and #2 show a lower level of pressure distribution for weights percentiles 5% and 50% and a relatively high uniformity in the pressure distribution for the weight percentile 95%, they can be considered suitable for the shape of the silicone pad.

3.4. Thermal Resistance Analysis on Cabin Seat Assemblage

After evaluating the flame-retardant performance of the sample unit, a fire test was conducted by applying silicone pads to the seat assembly to estimate the fire safety of an actual assembly. The test was conducted using a room-corner tester according to ISO 9705-1 (Reaction to fire tests: room-corner test for wall and ceiling lining products—Part 1: Test method for a small room configuration), as shown in Figure 2d [24]. The evaluation was conducted according to the procedure in EN 45545-2 Annex B. Figure 8 shows actual experimental photographs obtained in accordance with the combustion test procedure. The sample was combusted in compliance with the following rules. A burner with a power of approximately 7 kW according to the standard was installed and maintained at a distance of 10 mm from the surface of the seat and maintained for 180 s. Subsequently, the amount of heat emitted during the experiment (20 min) was measured. The assemblage sample was prepared using a flame-retardant PC seat, onto which two types of silicone pads with different shapes were applied.

In both silicone pads (types A and B), a separate ignition phenomenon did not occur after burning with the burner flame for 180 s, as shown in Figure 9. A residual flame of PC plastic, not silicone, was observed, and the penetration phenomenon caused by the melting of the seat surface was also noted. As summarized in

Table 7. Comparison of MARHE and HRR depending on the shape of silicone pads.

	MARHE (kW)	Max HRR (kW)	Reference
Design 1	4.42	10.89	MARHE ≤ 20 kW (KRTS Hazard class 4)
Design 2	3.49	8.94

, the measured MARHE values for types A and B were 4.42 and 3.49 kW, respectively, both of which are less than 20 kW, which is the risk level prescribed by the railway vehicle technical standard. The key factor was the high flame retardancy of the silicone pad; however, its shape did not play a significant role. Although type B met the safety standards, when exposed to a high-intensity fire, a penetration phenomenon was observed owing to the melting of the seat plastic (PC) exposed between the silicone pads. The application of silicone pads to seats can enhance functionality (nonslip, seating comfort, etc.) and also contribute to the improvement of safety in the passenger compartments of railway vehicles.

3.5. Fire Safety Assessment of Silicone Pad for Railway Vehicle

To use flame-retardant silicone as a seat interior material for Korean railway vehicles, measurements of MARHE, smoke density, and toxicity index for each test piece are required, in addition to fire tests for seat assemblies. However, the fire safety evaluation targets stipulated by Korean railway vehicle technical standard [49] (KRTS-VE-Part51-2017(R1)) include covers, cushions, bodies, and assemblies, excluding nonslip silicone pads. As shown in

Table 8. Fire safety assessment requirements for Korean railway vehicles (KRTS-VE-Part 51-2017(R1)).

Requirements		Test Standards	Test Items	Pass Criteria for Each Hazard Class
				[1]	[2]	[3]	[4]
Seats	Cover	ISO 5660-1 [43]	MARHE (kW/m2, @ 25 kW/m2)	≤75	≤50	≤50	≤50
		ASTM E 662 [44]	Ds (1.5 min)	≤150	≤125	≤100	≤100
		ASTM E 662 [44]	Ds (4.0 min)	≤300	≤250	≤200	≤200
		BS 6853 Annex B.2 [50]	Toxic index (R)	≤3.2	≤2.7	≤2.3	≤2.0
	Cushion	ISO 5660-1 [43]	MARHE (kW/m2, @ 25 kW/m2)	≤75	≤50	≤50	≤50
		ASTM E 662 [44]	Ds (1.5 min)	≤175	≤175	≤125	≤100
		ASTM E 662 [44]	Ds (4.0 min)	≤300	≤300	≤200	≤175
		BS 6853 Annex B.2 [50]	Toxic index (R)	≤3.6	≤3.6	≤3.2	≤3.2
	Body	ISO 5660-1 [43]	MARHE (kW/m2, @ 50 kW/m2)	≤90	≤90	≤90	≤60
		ASTM E 662 [44]	Ds (1.5 min)	-	-	-	≤100
		ASTM E 662 [44]	Ds (4.0 min)	≤300	≤300	≤200	≤200
		BS 6853 Annex B.2 [50]	Toxic index (R)	≤3.2	≤2.7	≤2.3	≤2.0
	Assembly	EN 45545-2 Annex B [18]	MARHE (kW)	≤75	≤55	≤45	≤20

, requirements vary for each part; applying the standard for covers, cushions, or bodies to nonslip silicone pads allows for more relaxed standards for each item. Therefore, this study selected and applied the harshest conditions for each test item, finding that all were satisfactory, as they did not exceed standard values. Therefore, the proposed seat is applicable in passenger compartments of urban railway vehicles.

Both flame-retardant silicone specimens A and B, with a thickness of 2 mm, did not ignite at 25 kW/m² but ignited at 50 kW/m², which is the more severe radiant heat condition applied to seat bodies and other interior components. The calorific value was measured, and the obtained value of 44.5–51.0 kW/m² satisfied the fire requirements (≤ 60 kW/m²) of the seat body. Smoke density (DS (4 min), ≤175) and toxicity index (≤2.0) were 73.7–162 and 0.147–0.162, respectively, indicating that all the criteria were met. Moreover, the value of MARHE in the seat assemblage test was 3.49–4.42 kW, which is well below the standard value of 20 kW required by the hazard class.

4. Conclusions

In this study, we developed a nonslip pad for railway seats using eco-friendly silicone material with improved flame retardancy. Fire safety standards vary worldwide, requiring country-specific testing and analysis. For application to the interiors of railway vehicles in Korea, technical standards for flame-retardant performance were evaluated, and nonslip pads for seats were tested by comparing two types of flame-retardant silicone. Most of Korea’s railway vehicle technical standards are similar to Europe’s EN 45545. However, after the Daegu subway fire accident, stricter conditions are required for some performance aspects, and the standard is not applied to parts other than those specified. Due to areas of ambiguity, a detailed analysis was performed.

After confirming that the material satisfies fire safety requirements through specimen-based fire property testing, an actual chair assembly with silicone pads of four shapes was manufactured. The optimal shape was selected through passenger comfort experiments, and a full-scale fire test was conducted. By using flame-retardant silicone pads developed through analysis of fire safety and mechanical performance, fire safety and passenger comfort are improved, meeting the fire safety performance standards required for Korean railway vehicles.