Formal kinetics of polystyrene pyrolysis in non-oxidizing atmosphere

► Global reaction of polystyrene thermal degradation in nitrogen is the single-step one of autocatalytic type. ► Activation energy of 168kJ/mol is fairly constant in a wide range of conversions. ► Nth order reaction model is unable to predict reaction rates for different heating rates. ► Pyrolysis combustion flow calorimetry is a viable tool for thermal analysis of flammable polymers. Previously reported kinetic data on polystyrene thermal degradation are inconsistent, and this may be a potential source of error in modeling the ignition and burning of the polymer composite materials containing polystyrene. To derive formal kinetic model of polystyrene thermal degradation, pyrolysis combustion flow calorimetry (PCFC) has been applied in this work. The heat release rate-temperature dependencies were measured at four heating rates of 0.25, 0.5, 1.0 and 2.0°C/s under nitrogen flow, and the kinetic parameters were derived by means of the model-free isoconversional method, the peak value method, the method of Kissinger and the model fitting non-linear optimization method. The single-step global reaction model has been demonstrated to have a constant activation energy of 168kJ/mol in a wide range of conversions. The autocatalytic reaction type has been established by evaluating the dependence of the kinetic function on the conversion derived from the measurement data. Thereby developed kinetic model has been validated against a variety of data sets including PCFC measurements made in this work, published TGA measurements, and isothermal experimental data. The model reproduced the experimental data to a reasonable accuracy for different temperature programs. The nth order reaction model was demonstrated to be unable to predict reaction rates for a range of different heating rates although it could be optimized for a single temperature program. Use of the nth order reaction has been shown to be a reason of obtaining unrealistically high apparent activation energies, reported for polystyrene degradation in the literature. The importance of processing multiple heating rate data to avoid misleading results is highlighted.