An instrument for characterization of the thermal and optical properties of charring polymeric materials

Abstract

A test instrument was developed and used to characterize the changes in the thermal and optical properties of charring polymeric materials when exposed to radiative heat fluxes. The system is based on a bench-top emissometer apparatus, which was developed originally to simultaneously measure the surface temperature and spectral properties of materials at elevated temperatures and was modified in this work to study charring polymers. An advantage of using a modified emissometer is that the front surface temperature is measured optically instead of with a thermocouple. Time-resolved measurements of the front surface temperature, along with thermocouple measurements of the interior and back surface temperatures can provide information on the changes in thermal conductivity and thermal diffusivity with the extent of charring, assuming that the heat of gasification is known. In addition, this apparatus measures the changes in the material's spectral emissivity and functional group composition as it chars. Data are presented for experiments on two different samples of 3.2-mm-thick polyurethane, heated using radiant fluxes from 22 to 32 kW/m 2 . Analysis of the surface temperature and emissivity data indicates that the samples undergo a transition from a volume to a surface absorber during initial irradiation. Quantification of this behavior will be a critical element in predicting the flammability limits of these materials.