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Harnessing High-Density-Polyethylene-Derived Liquid as a Model Solvent for the Co-Liquefaction of Low-Rank Coals: Toward Sustainable Mesophase Pitch for Making High-Quality Carbon Fibers from Waste Plastics
Summary
Despite its title referencing polyethylene recycling and mesophase pitch production, this paper studies an industrial chemistry process for converting waste plastic into carbon fiber precursors via coal liquefaction — not microplastic pollution or environmental health. It examines how hydrogen-rich liquid derived from HDPE plastic can improve coal-to-carbon conversion, and is not relevant to microplastics or human exposure.
The accumulation of polyolefin waste, particularly high-density polyethylene (HDPE), presents a growing environmental challenge due to limited recycling options and poor end-of-life recovery. This study explores a strategy to convert HDPE into mesophase pitch (MP), a valuable carbon precursor, by integrating polyolefin recycling with the mild solvolysis liquefaction (MSL) of low-rank coals. HDPE was first hydrogenolyzed into a hydrogen-rich aromatic liquid (HDPE-liquid), which was then used as the liquefaction solvent. Under identical conditions (400 °C, 60 min), Utah Sufco coal co-liquefied with HDPE-liquid produced tar that formed mesophase pitch with a higher mesophase content (84.5% vs. 78.6%) and a lower softening point (~302 °C vs. >350 °C) compared to pitch from conventional tetralin (THN). The approach was extended to Illinois #6 and Powder River Basin coals, increasing the mesophase content from 12.4% to 32.6% and 17.8% to 62.1%, respectively. These improvements are attributed to differences in tar composition: HDPE-derived tars had lower terminal methyl (Hγ) contents, reducing cross-linking during thermal upgrading. This work demonstrates that HDPE-derived liquids can act as functional solvents for coal liquefaction, enabling an effective route to recycle polyolefin waste into durable carbon products, while also reducing reliance on fossil-based solvents for mesophase pitch production.
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