Effective depolymerization of polyethylene plastic wastes under hydrothermal and solvothermal liquefaction conditions

Depolymerization of polyethylene (PE) is one of the most challenging tasks in the chemical upcycling of PE-based plastic wastes because the disassociation of the stable carbon–carbon bonds in PE is only possible at a very high reaction temperature. The thermal liquefaction of PE cable plastic waste in a stainless-steel batch reactor was thoroughly evaluated in this study. The effect of different liquefaction methods (hydrothermal liquefaction (HTL), ionic liquids catalyzed HTL, and solvothermal liquefaction (STL)) on the yields of product fractions (oil products, solid residue, and gas) and the properties of the oil products were examined. At 350 °C and 90 min reaction duration, the conversion (%) of 75.43%, the oil yield of 39.33%, the energy recovery rate of 39.7%, the higher heating values (HHV) of 43.83 MJ/kg for the oil samples, and the lower boiling range molecular distribution were obtained by the solvothermal liquefaction method with acetone as a solvent. The HHV of the oil samples obtained in the STL method (43.28–43.83 MJ/kg) is comparable to that of gasoline (HHV − 43.4 MJ/kg). The contribution of the solvent to the depolymerization reaction was mainly the dissolution and dispersion of feedstock by solvation, therefore reducing thermal cracking temperature through enhanced mass and thermal energy transfer. In thermal liquefaction, solvent and feedstock had a low level of solvolysis reactions, so the depolymerization reaction mainly follows thermal cracking. The main reaction path is the random scission of PE molecules during heat treatment, with a low level of polymerization, cyclization, and radical recombination reactions, which occurred through the free radical mechanisms. This work has demonstrated the feasibility of a very promising technique for effective chemical upcycling of polyethylene-based plastics.