Study of the decomposition kinetics, pyrolysis products, and even reaction mechanisms plays an important role for the development of polymer recycling. In the present research, the kinetics of virgin and waste polypropylene (PP) and lowdensity polyethylene (LDPE) were studied by a modified Coats−Redfern method. Afterward, thermal cracking of them in a semibatch reactor under atmospheric pressure in nitrogen has been investigated. Both virgin and waste plastics are decomposed at 420−460 °C, and the products have been characterized using GC, FT-IR, 1 H NMR, and GC-MS. The reaction path and the degradation mechanism for the thermal cracking of polymer in this study were also discussed. The lower activation energy of waste PP and LDPE indicates that waste plastics degrade at lower initial temperature and may favor mild conditions. Due to the short residence time, the higher gaseous and liquid yields were obtained for virgin PP and LDPE. A large amount of residues for waste polymer indicates that it is a favorable way to degrade waste plastics in a semibatch reactor without further separation. Chain scission reactions are the predominant degradation mechanism in this thermal cracking process. The significant content of unsaturated hydrocarbons in PP thermal cracking products shows the intramolecular hydrogen transfer and β-scission reactions are predominant. In the case of LDPE, intermolecular hydrogen transfer and β-disproportionate reactions also occur. For thermal cracking polymer in a semibatch reactor under atmospheric pressure, the high yields of gasoline (C 6 −C 12 ) and diesel (C 13 −C 22 ) fraction in liquid products confirm that it is a desirable way to realize waste plastics recovery.

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Cocracking behaviors of polyethylene/polypropylene and polyethylene/hydrocarbon mixtures have been investigated using a closed batch reactor/tube reactor, followed by Thermogravimetric Analysis (TGA) at certain heating rate from ambient temperature to 873 K. In part I, the cocracking of HDPE, LDPE, and PP was divided into two stages for intensive study: polyolefin cracking at closed batch/tube reactor was performed using nonisothermal/isothermal method, and further, the thermogravimetric study of polyolefin and the cracked products from polyolefin cracking was realized using nonisothermal method. Presence of synergistic effect is observed for cocracking of polyolefin mixtures, and such synergistic effect not only exists in mild cracking using closed batch reactor/tube reactor but also exists in the thermal degradation process of thermogravimetric analysis. The kinetic studies were performed according to Coats and Redfern method for first-order reaction. It is found that the pyrolysis process can be described by one first-order reaction and PE/PP mixture can decompose at a lower temperature than PE, where the activation energy of PE/PP can significantly decreased compared with that of HDPE. The results also indicate that interactions between HDPE and PP are mainly related to experimental conditions: temperature, residence time, and the degree of mixing for PE/PP. Thus, closed batch reactor with agitation equipment is useful for thermal cracking of polyolefin to produce more light oils.