Kinetic pyrolysis of poly(methyl methacrylate) and its effects on the ignition delay time.

摘要

An experimental technique is developed in order to capture a phenomenon related to micro-gravity combustion, the piloted ignition mixing time, in normal gravity conditions. The technique is based on investigating the degradation of a large particle size material, 0.5mm diameter PMMA by Thermogravimetry (TG)/Mass Spectrometry (MS)/Differential Thermal Analysis (DTA). The experimental results were incorporated in a numerical model previously developed for determining the piloted ignition delay time of a solid combustible impulsively exposed to an incident heat flux and prescribed forced flow. The results indicated, that due to mass transport effects and a rate limiting process controlled by diffusion, the theoretical surface temperature over-predicted the experimental surface temperature, beginning at 285°C, thus, this temperature was deemed the pyrolysis temperature. The overshoot time period commencing from the pyrolysis temperature to the ignition is deemed the mixing time. The over predicted surface temperature is attributed to mass transfer reduction of the volatile fuel emanating from the surface detected by the TG/MS. The over predicted numerical surface temperature is attributed to applying the forced convection heat transfer coefficient, which corresponds to negligible mixing time.; TG/MS/DTA measurements indicate that due to the large particle size, mass transport is impacting the degradation in both inert and oxidative environments. The diffusion process has inhibiting effects on the weight loss rate and the rate of production of gases, these inhibiting effects are enhanced with the increase of oxygen content. A net reduction of 40% in the weight loss rate was experienced by enhancing the oxygen content from 5% to 21% O ₂ in N₂. Furthermore, in an oxidative environment compared to that of inert environment, diffusion is initiated at lower temperatures, and was shifted to higher temperatures with lowering the heating rates. The MS data showed abrupt changes in the rate of production of gases that corroborated the hypothesis that degradation is impacted by mass transport effects. DTA profiles indicated that in an inert environment the reaction is endothermic with value of 2.8 kJ/g, while in oxidative environment the reaction is exothermic.