Thermal degradation of virgin and waste plastics: Estimation of kinetic and thermodynamic parameters using model-free iso-conversional methods

Kinetic triplets—apparent activation energy (E_a), pre-exponential factor (A), and the reaction model—were estimated for the thermal degradation of three primary virgin and waste plastics, as well as mixed plastics waste. Model-free iso-conversional FWO, KAS, Starink, Kissinger and Vyzovkin and Friedman methods were employed for the kinetic analysis. The apparent activation energy was determined by the integral methods as 206.5-209.1 kJ mol⁻¹ and 195.6-198.8 kJ mol⁻¹ for the virgin and waste high-density polyethylene (HDPE), 211.6-214.1 kJ mol⁻¹ and 183.1-186.6 kJ mol⁻¹ for the virgin and waste polypropylene (PP), 144.0-163.1 kJ mol⁻¹ and 159.7-167.1 kJ mol⁻¹ for the virgin and waste polystyrene (PS), and 173.6-178.9 kJ mol⁻¹ for the mixed plastics waste respectively. E_a was found to follow the trend HDPE > PP > PS with higher values for HDPE and PP virgin samples than that for their waste and marginally smaller for the virgin PS than its waste. Degradation of all plastic samples followed Avrami-Erofeev equation with n varying between 1.0 and 1.8. The effect of conversion on E_a suggested the degradation of both virgin and waste HDPE to consist of multiple parallel reactions while that of others to be more complex. FTIR analysis of the evolved gases was used to explain the possible reaction mechanism. A small difference between the enthalpy change and the apparent activation energy (6-7 kJ mol⁻¹) for all plastic samples indicated favourable pyrolysis reactions. The estimated kinetic parameters and thermodynamic properties showed the stability of different plastics as HDPE > PP > PS.