Reaction‐to‐fire testing of bus interior materials: Assessing burning behaviour and smoke gas toxicity

Although fire safety regulations for buses have been adapted in recent years regarding, for example, fire detection and engine fire suppression systems, the changes in regulations for bus interior materials are minimal. A comparison of fire safety regulations for interior materials in other transport sectors for trains, ships or aircraft reveals a much lower level of requirements for bus materials. Although repeated bus accidents as well as fire statistics show the danger a bus fire can pose to passengers. In particular, the combination of a fire incident and passengers with reduced mobility led to severe disasters in Germany and other European countries. To enhance the fire safety for passengers, the interior bus materials are crucial as the fire development in the bus cabin determines whether escape and rescue is possible. Against this background, bus interior materials were tested in different fire test scenarios. Measurement of a wide variety of parameters, for example, the mass loss, ignition time, smoke gas composition, heat release rate among others were carried out. Tested materials complied to the newest set of requirements. For this purpose, interior materials and their components had to be identified according to their chemical structure. Parts of the tests were funded by BASt (Federal Highway Research Institute) in the project 82.0723/2018. Experimental results show reaction‐to‐fire behaviour which lead to very limited times for escape and rescue in case of fire in a bus cabin. Based on the studies on fire behaviour and toxicity assessment, recommendations for improved fire safety regulations for interior materials could be made.


| INTRODUCTION INTO BUS FIRES
In recent years, there have been repeated bus accidents with numerous deaths and injuries. In particular, the combination of a fire incident with people with restricted mobility led to particularly severe events in Germany and other European countries. Traffic accidents or technical defects are often the cause of fires in buses. Especially, the rapid spread of fire and smoke represents a high risk potential for passengers travelling with them. 1 Often, passengers only have a few minutes to save themselves before irritating and toxic smoke products spread inside the bus. In addition, time elapses from discovering a fire until the bus is able to stop. Especially, when people are not able to save themselves quickly enough due to mobility or age restrictions, there is an imminent danger. This is demonstrated by the most serious and was initiated to develop a simplified procedure for testing the toxicity and smoke development in the event of fire of interior materials installed in buses. The scope of research included calculations for the evaluation of the toxicity of fire gases and the derivation of specifications for the formation of practicable limit values for fire smoke toxicity. The suitable measuring equipment that can be used to derive these limit values for smoke toxicity or the other physical quantities that can be used to derive these limit values has not yet been researched. A large number of scientific publications dealt with the fire safety of buses nationally and internationally in the last years. [1][2][3][4][5] Regulations for train interior materials according to European standards (EN 45545-2) are much more strict than the regulations for bus materials and offer high level fire safety for trains. 6,7 This publication is the first part of a series on research and development work on fire safety in buses. In this paper, we focus on experimental investigations in the DIN tube and on cone calorimeter tests with bus materials. The bus materials tested in this work fulfil the newest set of regulations for buses, considering the extended use of the vertical fire test for bus interior materials. Both test set-ups in the DIN tube furnace and the cone calorimeter have been modified for research purposes, for example, the initial specimen mass in the DIN tube furnace was kept constant to 1 g in all tests and an FTIR was coupled with the cone calorimeter, details are described in Section 3. This is followed by detailed considerations on assessing the burning behaviour of bus materials using the cone calorimeter, DIN tube furnace and numerical calculations on fire and smoke propagation in a bus geometry using the evaluated experimental data with commercial fluid flow solvers.  12 or AEGL values (Acute Exposure Guideline Levels) are mostly used for impact assessments. 13 With regard to the effects of exposure to pollutants, the approaches mentioned are based on four impact categories with assigned concentration ranges that can be distinguished from each other by three concentration thresholds: • first sensory effects (perceptibility), • adverse but reversible health effects (nuisance), • first irreversible or serious health effects or impairment of the possibility of escape, • life-threatening effects and fatal poisoning. In addition to that the assessment of toxicity can be achieved by the application of appropriate physiological methods or calculation models for time and dose incapacitation/other impacts: for example, fractional effective dose (FED) model [15][16][17] or N-Gas-model. 18 of the average population. The FED is determined by the following general relation: where However, it must be taken into account that a model is needed which transfers bench-scale test results to real-scale fire scenarios.
Therefore, after completion of all studies, the assignment of the measured gas concentrations from the bench-scale tests should be used to evaluate the smoke gas toxicity using (i) defined threshold values to limit toxic hazards from burning bus interior materials and (ii) application of the FED concept to assess smoke toxicity related to personal safety. Past work and similar studies on this approach can be found in literature. 22

| EXPERIMENTAL SET-UP AND INSTRUMENTATION
To proceed methodically to determine the fire smoke toxicity of materials installed in buses and coaches, appropriate fire tests had to be selected and typical materials for furnishing bus interiors were identified. As the focus was on smoke gas analysis, measurement equipment was sought that would allow coupling with an FTIR spectrometer:

| Bus interior materials
The bus seats, parts of the interior trim and the floor were identified as typical bus interior fittings (see Figure 2). These include PU, polyester (PE), acrylonitrile butadiene styrene, polyvinyl chloride, chip boards, polyamide, polyethylene, polypropylene as well as cotton, fleece, viscose, leather and synthetic leather. Various materials were made available by manufacturers for the experimental investigations: seat cushions and foams as well as samples of the seat covers and textiles from public and intercity buses.
F I G U R E 2 Bus interior of a typical German public bus in the city of Berlin.

| Experimental procedures
The cone calorimeter according to ISO 23

| Cone calorimeter with FTIR spectroscopy
The cone calorimeter allows the measurement of the HRR, mass loss, the smoke density and the smoke gas composition. Materials can be tested with different radiance levels, with or without a spark ignition.
The cone calorimeter is a standardized method that is used in the assessment of rail vehicle materials, but not for toxicity assessment and was used according ISO 5660-1. In addition to the standard procedure, an external Ansyco DX4000 FTIR spectrometer 27 was connected to the exhaust air shaft as described in ISO/TS 21397:2021.
This was used to determine the composition of the fire effluents. The F I G U R E 6 Schematic structure of the DIN tube furnace test with (1) quartz glass tube, (2) ring furnace with direction control, (3) specimen holder (glass cuvette), (4) lever for moving the (5) connection for air supply, (6) metal assembly including motor and feed speed control (7) temperature control. and the arithmetic means of the collected data were calculated. Laboratory set-up of the DIN tube furnace with a connected light extinction measurement system and a FTIR spectrometer (Ansyco DX4000) 27 at the compensation vessel is shown Figure 5. Similar studies can be found in the literature by Löhnert et al. 28 According to the German standard DIN 60695-6-1, oxygen concentrations between 5 and 21 vol% by volume and temperatures between 500 C and 600 C are characteristic of thermal oxidative decomposition. This temperature range corresponds to an radiance of less than 25 kW/m 2 and can be assigned to a self-sustaining smouldering fire. A schematic structure of the DIN tube furnace test is shown in Figure 6.
For the generation of thermal decomposition products of materials for their analytical-toxicological test, a reference body temperature must be determined in order to be able to determine the actual to the temperature on the sample using the DIN tube procedure. The The sample is removed from the quartz glass tube and the postprocessing of the furnace experiment can be carried out.

| Gas concentrations from cone calorimeter experiments
In the following, a detailed evaluation is first presented as an example using two selected samples, the PUR green foam and the textile CD. In the further explanations, only the summarised results of the investigated materials are shown in Table 1  The MARHE determined in the cone calorimeter for the foam is approximately 245 kW/m 2 . In these tests, the material showed a significantly different-lower-fire development than in the other tests. For the textiles, the MARHE was between 58 and 100 kW/m 2 . Table 1 shows the measured values for the parameters given in the Table 2 for all samples, averaged from three tests in each case.
The deviation from the mean value is also given. The foam and textiles were tested individually and in combination. The testing of the combined materials will be published in Part 2 of the publication series.
The repeatability of the tests was satisfactory, the deviations being values up to a maximum of 12%.

| Gas concentrations from DIN tube furnace experiments
In the following, concentration-time diagrams, Figures 10 and 11      The examination of the textile MB showed particularly high CO values and also the highest SO 2 concentration. It is also the only textile examined that had exceeded the limit values for formaldehyde.

| CONCLUSIONS AND RECOMMENDATIONS
The results of the bus interior tests in the cone calorimeter and the DIN tube show clearly that evaluating bus interior materials need to take three key parameters into account: HRR, smoke gas production and toxicity of the smoke gases. These three parameters allow assessment of different hazards for bus passengers: high rates of heat release lead to a rapid spread of fire within the passenger compartment, thermal tenable conditions will deteriorate more quickly as well, both limiting the time for escape. High rates of smoke production lead to reduced visibility which prevents passengers to escape. Toxicity of smoke gas components lead to reduced mobility through smoke gas inhalation which prevents escape as well. To increase safety for bus passengers in the event of fire, it is recommended to introduce limits for the rate of heat release and smoke production as well as for the toxicity of smoke gas components. One way to achieve that is to use materials which are used in trains according to EN 45545-2 as these regulations are already limit these three parameters.
Operating conditions of buses and trains are comparable, at least city buses and trams and coaches and long distance trains. However, it has to be noted that the regulations for trains take the size and conditions of train carriers into account. The volume of bus cabins tend to be smaller than train carriers.
As a conclusion an approach for buses has to consider that bus cabins are normally smaller than train compartments and would have to limit all three parameters (rate of heat release, smoke production and toxicity of smoke gas components) under flaming and smouldering conditions. These conclusions are also supported by past work in this environment. [29][30][31][32][33] This is possible either in a combination of the DIN tube with the cone calorimeter. For this case the cone calorimeter has to be used with a box according to ISO 5660-5 34 to control the oxygen supply.
For the establishment of limits of the three key parameters and as a holistic approach, bench and additional large scale experiments are required to evaluate the fire safety of interior bus materials. Bench-scale tests with seats have been already performed and are to be discussed in a following paper as well as the use of numerical calculations to assess the situation in a bus cabin by using data from small scale and bench-scale tests.

DATA AVAILABILITY STATEMENT
The data that supports the findings of this study are available on request from the corresponding author [AK]. ORCID