Pyrolysis is recognized as a promising technology for waste plastics management. Although there have been many studies on pyrolysis of waste plastics, there is still a lack of in-depth research on the mechanism of synergistic effect between mixed plastics and the mechanism of product formation. In this paper, based on the pyrolysis characteristics of Polystyrene, Polyethylene, and mixed plastics (Polystyrene/Polyethylene), it is demonstrated that a synergistic effect exists in the co-pyrolysis of Polystyrene/Polyethylene and affects the pyrolysis behavior and pyrolysis products. It was found that polystyrene chain segments containing C=C double bonds, generated from the pyrolysis of polystyrene, initiated the pyrolysis of polyethylene during the polystyrene/polyethylene co-pyrolysis, resulting in the termination pyrolysis temperature of the co-pyrolysis being advanced by 19.8 K. Due to the reduction in the termination pyrolysis temperature by 19.8 K, the average activation energy of the co-pyrolysis was reduced by about 14%. Compared with the weighted values of single-component plastics (Polystyrene and Polyethylene), the actual oil production of co-pyrolysis increased by 9.7% to 89.80%. At the same time, the content of low molecular weight Styrene and Toluene in pyrolysis oil increased by 12.3% and 1.65%, respectively. This study provides a useful and comprehensive reference for realizing the closed cycle of “from plastics to plastics”.

Pyrolysis technology, as a method for recycling waste polystyrenes (WPs), is widely regarded as an effective means to achieve the high value reutilization of WPs due to its environmental friendliness and the renewability of the resources used. However, in the conventional pyrolysis process for WPs, relatively high temperatures are often required to induce pyrolysis. This process not only consumes a significant amount of energy but also leads to complex and variable product compositions due to the high pyrolysis temperatures. Therefore, there is an urgent need to develop a high-value-added pyrolysis process that can lower the pyrolysis temperature of WPs and regulate its products, achieving the efficient conversion of WPs. This paper proposes a high-value “threshold temperature pyrolysis process” based on the relationships between pyrolysis temperature, threshold activation energy, and the conversion rate of WPs. The study found that under a heating rate of 10 K/min, when the conversion rate of WPs reaches 0.3, the maximum activation energy required for the entire pyrolysis process is approximately 223 kJ/mol, corresponding to a pyrolysis temperature of 673.15 K. Therefore, conducting isothermal pyrolysis at this temperature is expected to achieve the efficient conversion of WPs. The experimental results show that, compared to the conventional pyrolysis of WPs, the threshold temperature of the pyrolysis process not only lowers the pyrolysis temperature by 40 K but also regulates the distribution of pyrolysis products and the composition of pyrolysis oil, leading to a 7%wt increase in the yield of the pyrolysis oil, reaching 89.3%wt. Meanwhile, the relative content of low-molecular-weight aromatic hydrocarbons (Toluene, Styrene, and α-Methylstyrene) in the pyrolysis oil increases by 7.4%wt, which also suggests that the threshold temperature of the pyrolysis process promotes the shift in pyrolysis oil towards lighter fractions. These findings provide a solution for energy saving, emissions reductions, and the efficient conversion of WPs.