Spectroscopic determination of the optical constants and radiative properties of black PMMA for pyrolysis modeling

In-depth radiation absorption significantly affects the ignition time and burning rate of flammable translucent materials. To improve the accuracy of flammability predictions by pyrolysis models, the spectral dependency of radiation sources and the depth of the computational cell in the sample should be considered. This paper provides the absorption spectra of black Poly(methyl methacrylate) (PMMA) in a wide spectral range of 0.25 to 25 μm by measuring the high-resolution spectral transmissivity of thin samples. Compared to clear PMMA, black PMMA has a higher spectral absorption coefficient in the UV–Vis–NIR region (<2.3μm). We extracted the complex index of refraction of black PMMA from the spectral absorption coefficient by applying the Kramers–Kronig transform (KK-transform). Furthermore, to investigate the effect of PMMA temperature on its absorption, the absorbance of the sample was measured with FTIR-ATR for the temperature up to the polymer's melting temperature. The results showed that the radiation absorption of PMMA may increase for higher material temperature. Implementing the measured optical constants, simulated temperature profiles in a layer of PMMA subjected to the heat flux of 15 and 28 kW/m² are obtained, showing good agreement with the experimental data. Finally, a set of data for the effective absorption coefficient as a function of source temperature and depth is proposed for the black PMMA to improve the gray modeling of radiation for pyrolysis simulations.