Understanding the role of plastic additives in the pyrolysis of plastic wastes: Impact of additives on product selectivity, kinetics, and mechanistic pathways

Pyrolysis offers a promising solution to converting plastic waste into high-value products. However, the impact and mechanism of plastic additives in regulating the pyrolysis of different plastic types remain unclear. To address this gap, this study first investigates the differences in structure, thermal stability, and decomposition kinetics of five common chlorine-free plastics and their corresponding virgin polymers. Subsequently, microwave catalytic pyrolysis is employed as the core methodology to analyze the distribution and composition of pyrolysis products under a fixed condition (microwave power of 800 W and a plastic-to-HZSM-5 catalyst ratio of 8:3). The results indicate that Na₂SO₄ and CaCO₃ are the main additives used in HDPE and PP plastics. Na₂SO₄, with a higher thermal conductivity than HDPE, enhances heat transfer, resulting in a faster pyrolysis rate for shopping bags (HDPE plastic). CaCO₃ adsorbs intermediates formed during the pyrolysis of water pipes (HDPE plastic), altering the reaction pathway and promoting chain scission. Consequently, monocyclic aromatic hydrocarbons (MAHs) increase in shopping bags and water pipes, with relative contents of 81.51 % and 79.96 % in liquid oil, respectively, compared to 67.99 % in virgin HDPE. Conversely, the highly branched structure of PP leads CaCO₃ to promote cross-linking reactions during the pyrolysis of woven bags (PP plastic), resulting in macromolecular formation. This increases activation energy and decreases the MAH relative content in the pyrolysis products. The variations in pyrolysis characteristics between LDPE, PS, and PET plastics and their polymers are primarily attributed to differences in morphology, impacting the pyrolysis rate and degree of molecular chain breakage.