Study On Thermal Degradation,Combustion And Flame Spread Characteristics Of Aluminum Hydroxide Flame-retardant Polyester GFRP

Polyester GFRP has been widely used in high-speed train as wall panels and roof panels due to its low density,excellent thermal insulation ability,good mechanical properties,relatively low cost,and easy processing.However,the matrix of polyester GFRP—unsaturated polyester resin,has high flammability.When a fire occurs in the carriage,the heat radiation generated by the combustion of other interior materials will cause the pyrolysis of polyester GFRP,and the flame will spread rapidly to the whole carriage after the ignition of polyester GFRP.The operating environment of high-speed trains is complex,polyester GFRP may be in a high or low oxygen environment due to environmental factors such as altitude and air pressure.Therefore,an in-depth understanding of the thermal degradation,combustion and flame spread characteristics of polyester GFRP under different oxygen concentrations and external heat fluxes is of great significance for the fire risk assessment of train materials,and can provide basic data and theoretical support for fire protection design and risk assessment of high-speed trains.This paper takes the typical aluminum hydroxide flame-retardant polyester GFRP widely used in high-speed train as the research object,and studies the effects of different external environmental conditions on the thermal degradation,combustion and flame spread characteristics of aluminum hydroxide flame-retardant polyester GFRP using a combination of experimental research and theoretical analysis.The main research contents and conclusions are as follows:(1)Research on the influence of ambient atmosphere and heating rate on the thermal degradation characteristics of polyester GFRPIn order to study the thermal degradation characteristics of polyester GFRP under different ambient atmospheres and heating rates,TG-DSC and TG-FTIR-GC/MS were used to obtain the thermogravimetric properties and degradation products,and kinetic triplets were determined based on the model-free and model-fitting method.The analysis shows that thermal degradation process of polyester GFRP can be divided into two stages under nitrogen and three stages under air.The main reaction of first stage in nitrogen and air is dehydration of ATH and breakage of ester linkages,but the polystyrene chain also breaks under air,and the air causes the polyester and polystyrene to react at lower temperatures.The reaction of second stage in nitrogen and air is the scission of polyester and polystyrene chains,while the air accelerates the scission of polyester and polystyrene chains and promotes the reaction of degradation products with air to generate carbon dioxide and water.The third stage under air is the oxidation of char.The activation energies required for the first and second stage under air atmosphere are lower than those required for nitrogen.The reaction mechanism of the first stage under nitrogen is dominated by first-order reaction order model,and the second stage is dominated by the diffusion mechanism,while the reaction mechanisms of the three stages under air can all be explained by the first-order reaction order model.(2)Effects of oxygen concentration and heat flux on the combustion and smoke generation characteristics of polyester GFRPIn order to study the combustion characteristics and smoke generation characteristics of polyester GFRP in different external environments,the combustion tests of polyester GFRP under different oxygen concentrations and heat fluxes were carried out by using a fire propagation apparatus,and the smoke generation was analyzed by FTIR gas analyzer,revealing the variation law of the combustion and smoke generation characteristics of polyester GFRP under different oxygen concentrations and heat fluxes.It is found that the combustion process of polyester GFRP can be divided into five stages:the pyrolysis stage before ignition,the propagation stage after ignition,the weakened combustion stage by the hindering effect of the char layer,secondary growth stage of combustion,the edge combustion and char layer oxidation stage.At lower heat fluxes,oxygen concentration has a great influence on the ignition time and the average mass loss rate of polyester GFRP,and the influence of oxygen concentration gradually weakens with the increase of heat flux;the effect of heat flux on the ignition time and the average mass loss rate under different oxygen concentrations is similar with the above laws.Under different oxygen concentrations,there is a linear relationship between the transformation form of ignition time(1/tig)1/2,the average mass loss rate,and the average heat release rate of polyester GFRP and the heat flux.The changes of oxygen concentration and heat flux have little effect on the combustion efficiency of polyester GFRP,but have a greater effect on the fire growth rate index,fire growth index and maximum average rate of heat emission.(3)Research on the effect of oxygen concentration and heat flux on the flame spread characteristics of polyester GFRPIn order to understand the flame spread characteristics of polyester GFRP in different external environments,the vertical upward flame spread experiments of polyester GFRP under different oxygen concentrations and heat fluxes were carried out using fire propagation apparatus.The variation law of the flame spread characteristics of polyester GFRP under different oxygen concentrations and heat fluxes is revealed,and the empirical models between flame spread rate and oxygen concentration and heat flux are established by theoretical analysis.It is found that the flame spread rate increases with the increase of oxygen concentration and heat flux under most conditions,but the flame spread rates are significantly higher than the expected values at 19.00%oxygen concentration when the heat fluxes are 25 kW/m2 and 30 kW/m2 and 20.95%oxygen concentration when the heat flux is 25 kW/m2.The mechanism of this phenomenon is explained in combination with the flame spread characteristic parameter and the ignition mechanism.Except for the above three conditions under lower oxygen concentrations and lower heat fluxes,there is a power-law relationship between the flame spread rate and oxygen concentration under different heat fluxes,and the exponent of the power-law relationship is about 1.15.There is a linear relationship between the flame spread rate and external heat flux under different oxygen concentrations.The flame spread rate as a function of oxygen concentration and heat flux is(?).The external heat flux and oxygen concentration have little effect on the maximum radiation heat flux generated during the flame spread process of polyester GFRP,and the maximum radiation heat flux under different conditions is between 25 kW/m2and 30 kW/m2.But at higher heat fluxes(40 kW/m2 and 50 kW/m2)of 25.00%oxygen concentration,the maximum radiation heat flux can be increased to 35～40 kW/m2.