Thermal Decomposition of Polyethylene for Carbon Nanomaterials Synthesis via Atmospheric-Pressure Microwave Plasmas

Polyethylene (PE) constitutes a significant portion of plastic waste, representing approximately 30% of nondegradable plastics. Efficiently utilizing waste PE is essential to address plastic pollution. This study proposed the thermal decomposition of powdered PE using atmospheric microwave plasmas, with processing time under 1 s. Gas temperature was measured in the plasma using optical emission spectroscopy (OES) method as 4780 K. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to examine the surface morphology of the products, which indicated that the average size of the nanoparticles produced was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$29~\pm ~0.58$</tex-math> </inline-formula> nm, with the majority of particles having a diameter below 50 nm. The X-ray diffraction (XRD) and Raman spectroscopy confirmed the formation of few-layer graphene within the reaction products (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\leq 5$</tex-math> </inline-formula> layers). Based on the characterization results, the reaction mechanism was subsequently analyzed. The experimental results indicate that microwave plasma thermal decomposition can convert PE solid material into carbon nanomaterials under atmospheric pressure in an ultrafast, one-step process, offering new possibilities for large-scale industrial preparation of graphene materials and PE waste treatment.