Characterization of Energy-Relevant Liquid Products from Vacuum Pyrolysis of HDPE Microplastic

In this study, the thermal degradation of high-density polyethylene (HDPE) microplastics was investigated to obtain and characterize the liquid fraction generated by vacuum pyrolysis, assessing its potential for energy valorization as an alternative fuel. The methodology included thermo-chemical characterization and mathematical modeling of HDPE based on thermogravimetric analyses conducted at heating rates of 5, 10, 15, 20, and 25 °C min–1, differential scanning calorimetry (DSC), and vacuum pyrolysis performed under the following operational conditions: 550 °C, −100 mmHg, and a residence time of 90 min. The resulting oily liquid product was characterized by gas chromatography–mass spectrometry (GC-MS) and by high-resolution mass spectrometry (HRMS) using direct infusion with electrospray ionization and atmospheric pressure chemical ionization sources. The DTG and DSC results indicated that the onset of thermal degradation of the HDPE molecular chains occurred at approximately 500 °C. The comparison between experimental and predicted data demonstrated good agreement, validating the applicability of these methods for modeling the thermal degradation kinetics. The GC-MS analysis revealed that the liquid fraction is composed mainly of hydrocarbons, particularly alkanes (saturated chains) and alkenes (unsaturated chains). Furthermore, HRMS analysis confirmed, through Van Krevelen diagrams, that the liquid product is highly heterogeneous, exhibiting a predominance of linear and saturated alkanes similar to those found in light oils. Minor contributions from Ox[H] and NxOy[H] classes were also detected, likely associated with impurities and highly condensed aromatic species formed via aromatization and polycondensation reactions during pyrolysis.