Mechanism of accelerated concurrent flame spread over glass fiber reinforced unsaturated polyester resin composites with ATH/MH retardants under external radiation

Abstract

This paper reports an experimental investigation of concurrent flame spread over composites, for the first time, with combined flame retardants and external radiation factors. Glass fiber reinforced unsaturated polyester resins without flame retardants (GFUPR) as well as those containing aluminum hydroxide (ATH) and magnesium hydroxide (MH), which were utilized widely in modern high-speed train, were systematically explored. Infrared measurement was employed to track the displacement of pyrolysis front. Because of the dilution of H2O generated during the decomposition of ATH and MH, the flame temperature (Tf) of GFUPR-ATH/MH is thought to be lower than that of GFUPR. Nonetheless, the most noteworthy finding is that the flame spread rate, flame preheat length and flame height for GFUPR-ATH/MH could be larger than that of GFUPR if concurrent airflow velocity exceeds 0.5 m/s under high external radiation. To interpret this unexpected while practically important phenomena, theoretical analysis based on the heat/mass transfer theory is established to explore the general mechanism. It is concluded that in the transition from natural-convection to forced-convection, the different in the flame convection in burning zone between the sample with high Tf (i.e. GFUPR) and the sample with low Tf (i.e. GFUPR-ATH/MH), will decrease rapidly. It is primarily because the sample with low Tf is earlier to enter in the region where forced-convection is controlled as the airflow velocity increases. The critical parameter (Fr2Re)-1 = 1 could be used to characterize this transition. As a result, in forced-convection region, the role of thermal conductivity of the fuel-gas mixture (kmix) may emerge, that the sample with high kmix but low Tf (i.e. GFUPR-ATH/MH) may impart more heat to burning zone than the sample with low kmix but high Tf (i.e. GFUPR), generating more combustible gases in the burning zone hence larger flame heat feedback to preheat zone. Moreover, this effect will be reinforced by the larger flame preheat length of GFUPR-ATH/MH, due to the temperature dependency of molecular diffusivity. This mechanism analysis is well corroborated by the experimental results. This new finding may facilitate understanding of the effect of flame retardants on the heat/mass transfer mechanism in flame spread under concurrent airflow and external radiation.