| Metro trains, due to their speediness, convenience, comfort, environment protection and energy conservation, play an increasingly important role to efficiently solve the transportation problems in congested cities. However, metro trains are generally operating underground or inside the tunnels. The escape and rescue of numerous passengers aboard are considerably difficult in case of fire, leading to terrible casualties, property loss and environmental pollution. Thus, increasing attention has been devoted to the fire safety of metro trains in recent years. Understanding the decomposition and combustion characteristics of metro train interior materials under different external conditions is a significant aspect of fire safety of metro trains, and can provide essential data and a theoretical basis for fire safety design, fire fighting and rescue.The fixed flammable materials in the carriage interior of metro trains mainly comprise the floor coverings and seats. Typical metro train interior materials namely commercial flame-retardant ethylene-propylene-diene monomer rubber (flame-retardant EPDM rubber) generally used as floor coverings and fibre-reinforced phenolic composite (FRP composite) generally employed as seats were selected for the present study. The effects of external conditions on the thermal decomposition characteristics, reaction mechanisms and volatile products of thermal decomposition process and combustion characteristics of the above two typical metro train interior materials were investigated. The impacts of the specimen thickness on the combustion characteristics of FRP composite were established. The kinetic parameters and flammability properties are obtained. The major work and conclusions of this thesis are summarized as follows:Effects of heating rate, temperature and environmental atmosphere on decomposition characteristics of flame-retardant EPDM rubber and FRP composite were revealed:(1) Both of the (thermogravimetric) TG and (differential thermogravimetric) DTG curves would move to the higher temperature region with an increase of heating rate. The total mass loss varied little with heating rate. In the case of flame-retardant EPDM rubber, the maximum DTG declined with heating rate in the case of N2 atmosphere, while there were no definite variation trend between the maximum DTG and heating rate in the case of air atmosphere. In the case of FRP composite, the maximum DTG remained also constant with heating rate in N2 atmosphere, while the maximum DTG declined with heating rate in the case of air atmosphere. The decomposition process of flame-retardant EPDM rubber may be mainly divided into three stages. The decomposition process of FRP composite may be mainly divided into two stages. (2) In the case of isothermal conditions, the total mass loss of flame-retardant EPDM rubber increased with the applied temperature. The total mass loss of FRP composite in N2 atmosphere generally increased with the applied temperature. However, the total mass loss of FRP composite in air atmosphere decreased with the applied temperature under the isothermal conditions of 403-433 K and maintained almost constant under the isothermal conditions of 713-863 K. (3) The decomposition would be accelerated in air atmosphere in comparison with that in N2 atmosphere. In the case of flame-retardant EPDM rubber, another peak DTG occurred in the case of air atmosphere behind the last peak DTG in the case of N2 atmosphere. In addition, the maximum and average DTGs as well as the total mass loss in air atmosphere were all less than those in N2 atmosphere. Both of the average activation energy obtained from non-isothermal tests and overall activation energy acquired from isothermal tests in N2 atmosphere were larger than those in air atmosphere. In the case of FRP composite, the mass would increase with temperature in the temperature range of approximately 450-600 K in air atmosphere in non-isothermal experiments. Moreover, increase of mass with time was also demonstrated under the isothermal conditions of temperature range of 403-433 K in air atmosphere, while it did not occur to FRP composite in N2 atmosphere. The maximum and average DTGs as well as the total mass loss in N2 atmosphere were all less than those in air atmosphere. The average activation energy obtained from non-isothermal tests in N2 atmosphere was larger than that in air atmosphere. However, the overall activation energy acquired from isothermal tests was less than that in air atmosphere.Reaction mechanism and volatile products of thermal decomposition process of flame-retardant EPDM rubber and FRP composite under different environmental atmosphere were presented:(1) In the case of flame-retardant EPDM rubber, The decomposition of aluminum hydroxide, magnesium hydroxide and EPDM rubber mainly contributed to the occurrence of the three stages, as noted above. The decomposition of aluminum hydroxide occurred from about 500 to 673 K and 500 to 653 K under inert and oxygenous atmosphere respectively. Magnesium hydroxide was decomposed in the temperature range of 500-753 K and 500-703 K under inert and oxygenous atmosphere respectively. The decomposition of aluminum hydroxide and magnesium hydroxide generated large amount of water. A small quantity of small aliphatic hydrocarbon molecules and alkylbenzenes was released in the temperature range of about 500-700 K and 500-680 K under inert and oxygenous atmosphere respectively, which may be due to the scission of the branched chain from the chain backbone of EPDM rubber and the decomposition of other organic materials in the specimen. Large amount of hydrocarbon molecules was released with the breakage of the chemical bond in the chain backbone of EPDM rubber and further decomposition of other organic materials in the specimen under temperatures above 700 K and 680 K under inert and oxygenous atmosphere respectively. Meanwhile, large amount of carbon dioxide was released under temperatures above 730 K and 600 K under inert and oxygenous atmosphere respectively. Large quantity of carbon monoxide was indicated under temperatures above 730 K in the case of oxygenous atmosphere, while no carbon monoxide was present in the case of inert atmosphere. Toxic gases such as hydrogen fluoride, formaldehyde, hydrogen chloride, formic acid, sulfur dioxide, carbon disulfide, hydrogen bromide, aniline, etc. were released from about 700 and 650 K under inert and oxygenous atmosphere respectively. In addition, great importance should be attached to the appearance of carbon monoxide and hydrogen cyanide in the case of oxygenous atmosphere owing to their high toxicity. (2) In the case of FRP composite, additional crosslinking between phenol and its derivatives and breakage of the crosslinks mainly resulted in the occurrence of the two stages, as noted above. The first stage corresponded to the temperature range of about 400-638 K and 400-600 K under inert and oxygenous atmosphere respectively. In the first stage, large amount of bisphenol and its derivatives was released. Micromolecules such as hydrogen, methane, water, acetylene, ethylene, formaldehyde, methyl alcohol, carbon dioxide, carbon monoxide, formic acid, butane, methylphenol, etc. were also generated. The second stage corresponded to the temperature range of about 638-1100 K and 600-900 K under inert atmosphere and oxygenous atmosphere respectively. In the second stage, a large number of benzene, phenol and their derivatives such as methylbenzene, methylphenol, xylenol were indicated. A small quantity of water was also generated with the stripping off of hydroxymethyl and hydroxyl from chain backbone. In addition, formic acid and aniline were also released. More carbon dioxide and carbon monoxide were released in the second stage than those in the first stage. It should be noted that sulfur dioxide appeared in the case of oxygenous atmosphere, while no sulfur dioxide was present under inert atmosphere.Effects of external heat flux on combustion characteristics of flame-retardant EPDM rubber and FRP composite were demonstrated. The impacts of specimen thickness on combustion characteristics of FRP composite were established:(1) In the case of flame-retardant EPDM rubber, three different decomposition regions may be identified according to the applied external heat flux: (1) region 1 (external heat flux≤35 kW/m2) with almost no crack of the generated char layer; (2) region 2 (35 kW/m2< external heat flux≤45 kW/m2) with crack of part of the formed char layer; (3) region 3 (external heat flux> 45 kW/m2) with complete crack of the formed char layer. Six thermal decomposition stages were noted in both of region 2 and 3:initial decomposition before ignition (Stage Ⅰ), accelerated decomposition after ignition (Stage Ⅱ), subdued decomposition with massive char formation (Stage Ⅲ), further decomposition with cracking of the char layer (Stage Ⅳ), the second subdued decomposition with a small quantity of char production (Stage Ⅴ) and the non-flaming oxidation (Stage Ⅵ). However, only four thermal decomposition stages (Stage Ⅳ and Ⅴ disappeared) constituted the thermal decomposition process in region 1. In the cases of external heat fluxes larger than 35 kW/m2, two peak heat release rate (HRR), a quasi-steady stage between the first and second peak HRR and two peak effective heat of combustion (EHC) were noted. In addition, the second peak EHC was much larger than the first peak EHC. The transformed ignition time, peak and average mass loss rate (MLR), peak and average HRR, the HRR in the quasi-steady stage and fire growth index (FGI) all increased linearly with external heat flux. The total heat release (THR) increased linearly with the external heat flux from 25 to 50 kW/m2 and maintained almost constant in the cases of 50,55,60 and 65 kW/m2. The thermal thickness was demonstrated to be proportional to the ratio of the density of the specimen and the applied external heat flux. (2) In the case of FRP composite, six thermal decomposition stages were also noted:initial decomposition before ignition (Stage Ⅰ), accelerated decomposition after ignition (Stage Ⅱ), subdued decomposition with massive char formation (Stage Ⅲ), further decomposition with cracking of char layer (Stage Ⅳ), the second subdued decomposition with a small quantity of char production (Stage Ⅴ) and the non-flaming oxidation (Stage Ⅵ). The ignition time increased with thickness. The difference of the ignition time between the cases of FRP composite with different thickness was shortened with increase of external heat flux. The mass loss factor decreased with thickness, while the average MLR increased with thickness. The average HRR of FRP composite in the case of 3 mm was larger than that of 5 mm, and the average HRR of FRP composite with thickness of 8 mm was the highest. The THR increased with thickness. The transformed ignition time, peak and average MLR, maximum and average HRR and FGI all increased linearly with external heat flux. The thermal thickness increased linearly with the ratio of the density of the specimen and the applied external heat flux.The flammability properties of flame-retardant EPDM rubber and FRP composite were deduced and validated:Based upon the correlations between the characteristic cone calorimeter data and external heat flux along with theoretical analyses. The flammability properties including the critical heat flux (CHF), the minimum heat flux, the ignition temperature, the heat of gasification and the heat of combustion were deduced and validated. |
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