Kinetic and thermodynamic study of micron waste polypropylene thermal degradation

How to properly dispose of waste polymers is a recognized challenge all over the world. Thermal degradation is currently recognized as a promising method for recycling polymer waste into fuels or products with high energy density without polluting the environment. In the present study, the thermal degradation characteristics, kinetics, thermodynamic parameters, and volatiles of a representative and extremely widely-used polymer (micron waste polypropylene [PP]) pyrolysis in nitrogen were investigated. The results indicate that the thermal degradation of micron waste polypropylene can be considered as a one-step reaction with merely one distinct peak on the reaction rate curves. The peak and average reaction rates decrease with the heating rate. The most appropriate reaction model to characterize the thermal degradation is g(α) = 1−(1−α)^1/4. The average values of activation energy and pre-exponential factor are 128.76 kJ/mol and 6.79 × 10⁹ min⁻¹, respectively. The kinetic parameters obtained in this study are all larger than those of PP with the particle size of millimeters or larger. The predicted thermogravimetric curves of thermal degradation are in good agreement with the experimental results. The changes of enthalpy, Gibbs free energy, and entropy show that the thermal degradation of micron waste polypropylene is a non-spontaneous and endothermic reaction. In addition, the concentrations of all volatiles in descending order are: H₂O > Esters (COO)  > CO₂ > Alkanes (CH₃) > R₂CCH₂ > Olefins (CC) > Alcohols (ROH) > Methylene group > CO.