Flammability Limits of Flat Materials with Moderate Thickness in Microgravity

The flammability limits of flat materials with different moderate thickness in opposed flow were investigated to clarify the effect of sample thickness on flammability limit. Thin poly-methyl methacrylate (PMMA) sheets and polycarbonate (PC) sheets were used as samples. The thickness was varied as 0.2 mm, 0.5 mm and 1.0 mm for PMMA, and 0.1 mm, 0.2 mm, 0.5 mm and 1.0 mm for PC. The limiting oxygen concentration (LOC) of each sample was obtained by 20 s parabolic flight experiments in mild and low flow velocity condition, and by blow-off tests on ground in high flow velocity condition. The flight experiments showed that there existed significant difference in LOC according to the sample thickness especially in mild flow condition; the thicker the sample was, the larger the LOC became. We applied a hypothesis of heat penetration zone shape into the heat balance equation around the preheat zone to account the effect of sample thickness on quenching phenomena. Although the hypothesis was not confirmed by experimentally, the obtained empirical model based on the hypothesis well represented the trend of the change of LOC when the sample thickness varied not only for the PMMA and PC samples, but also for low-density polyethylene (LDPE) and high-density polyethylene (HDPE) samples with relatively thick thickness. The modified model states that increase in the sample thickness has equivalent effect to increase in the radiative heat loss and such thickness effect is amplified when the pyrolysis temperature of the material is high.