Evaluation of the Thermophysical Properties of Poly(MethylMethacrylate): A Reference Material for the Development of a flammability Test for Micro-Gravity Environments

Abstract

A study has been conducted using PMMA (Poly(methyl methacrylate)) as a reference material in the development process of the Forced Flow and flame Spread Test (FIST). This test attempts to establish different criteria for material flammability for micro-gravity environments. The FIST consists of two tests, ignition and flame spread tests, that provide a series of material “fire” properties that jointly provide important information on the flammability of a material. This work deals with the former. PMMA was chosen as a reference material mainly because of its well characterized properties. Evaluation of the ignition delay time as a function of a suddenly imposed external heat flux can be described by a known relationship with the minimum surface temperature at which piloted ignition can occur. The ignition temperature can be obtained from the experimental determination of a critical heat flux for ignition and the total convective heat transfer coefficient. And by assuming the absorptivity to be approximately unity, a material constant can be found, and is often referred to as the thermal inertia. The ignition temperature is addressed by splitting the ignition process into the time required to initiate thermal decomposition of the material and the ignition delay time. The present work provides an independent evaluation of the evolution of the thermal properties of PMMA, as a function of temperature. The thermophysical properties were determined by using the time to ignition and time to pyrolysis approached as obtained from the FIST. Discrepancies between these two approaches were resolved by defining a mixing time and a minimum average fuel concentration for ignition. Independent evaluation of the density, thermal conductivity and specific heat serve to correlate the property values of obtained from the FIST.