A new look into the pyrolysis of polystyrene

Pyrolysis of polystyrene has been investigated in the temperature range 400-650 °K and up to 40% conversion. In this study, we used polystyrene particles (around 140 μm) suspended in an electric field and heated by radiation (very fast heating rates). The main advantage of this method is that kinetic interpretation is straightforward and unique because chemical reactions are not coupled with heat and mass transfer processes. Due to the fast heating, the initial structure of the polymer is not modified and therefore the results relate to the initial chemical structure and intrinsic rate. The following properties were determined in real time: mass, diameter, density, optical properties, and temperature. From mass loss rate versus inverse temperature, and activation energy of 103.1 kJ/mol was determined. This value is in the low spectrum of activation energies measured by other methods. From the behavior of the squared diameter with time, it was found that pyrolysis obeyed a d² law. Density was found to remain constant with extent of reaction, indicative of a non-charring process. The novelty of this study is the ability to determine the absorption coefficient of the particle (from the photophoretic force measured in this study). This enabled a look into the condensed phase during reaction, in real time. The absorption coefficient did not change during the first 8% of conversion, then it increased gradually with conversion. Since the absorption coefficient is directly related to the chemical structure and extent of cross-linking, constancy indicates that pyrolysis occurred without changes in the chemical structure. The gradual increase of the absorption coefficient (above 8% conversion) indicates that significant changes occurred during pyrolysis, mostly by increasing the molecular weight or cross-linking of the polystyrene chains.