Upcycling end-of-life vehicle waste plastic into flash graphene

The study addresses the challenge of managing end-of-life vehicles (ELVs), particularly the large fraction of plastics that are not recovered and typically landfilled after depollution, dismantling, and shredding. With the rise in plastic use in vehicles for weight reduction, mixed and contaminated ELV waste plastics (ELV-WP) are difficult and uneconomical to recycle using conventional approaches that rely on segregation and clean streams. The research objective is to demonstrate a scalable, solvent- and water-free upcycling route that converts mixed ELV-WP directly into high-value turbostratic flash graphene (FG) via flash Joule heating (FJH), evaluate its performance as a reinforcement in automotive polyurethane foam (PUF), assess the potential for circularity by re-flashing FG-containing composites back into graphene, and compare the environmental burdens of this approach to established graphene production methods via a prospective cradle-to-gate life cycle assessment (LCA).

Global ELV management practices vary, but a significant portion of vehicle mass beyond metals is landfilled, with ELV plastics being the largest non-recycled fraction. Previous recycling efforts focus on single polymers such as polypropylene (e.g., bumpers) and often rely on pyrolysis with complex catalysts, inert atmospheres, and relatively clean streams—approaches that struggle with mixed and contaminated ELV plastics. Economic incentives for ELV plastic recovery remain weak due to low virgin polymer prices. The automotive sector explores sustainable materials (e.g., cellulose fibers, waste textiles) for new vehicles, yet legacy vehicles contain massive quantities of ELV-WP that remain problematic. Recent advances showed that mixed waste plastics can be converted to graphene via flash Joule heating, yielding high-quality turbostratic graphene efficiently and at low projected energy cost. Graphene’s mechanical, electrical, thermal, and chemical properties have driven its use as a composite additive across applications, potentially reducing host material usage and environmental footprints. This background motivates an upcycling approach that bypasses sorting, tolerates mixed plastic streams, and generates high-value products to improve recycling economics.

Feedstock and preparation: Mixed ELV waste plastics (ELV-WP) from Ford F-150 trucks (bumpers, gaskets, carpets, mats, seating, door casings) were received post-depollution and dismantling, then shredded. Large plastic pieces were ground to ~1 mm particle size using a hammer mill without washing, sorting, or separation. The ground ELV-WP comprised a mixture including PVC, ABS, PP, PC, PA, POM, PU, and PE. For improved conductivity during FJH, 5 wt% finely ground metallurgical coke (metcoke) was added to the ELV-WP (95 wt%). Flash Joule Heating (FJH) process: A custom dual-mode FJH system performed sequential low-current (LC) and high-current (HC) discharges within a quartz tube reactor equipped with graphite electrodes and vents for outgassing. The ELV-WP/metcoke mixture initially had ~500 Ω resistance. LC-FJH: constant 208 V, current increasing from ~1–25 A over 10–16 s as the polymer carbonized and resistance dropped to ~10 Ω; temperature ~2300 K (IR measured). About 30% of the original plastic mass remained as highly carbonized solid; volatile products (e.g., H2/C1–3/C4–6 gases and condensable waxes/oils) evolved and could be collected. HC-FJH: ~200 A at an initial ~150 V for <1 s, rapidly heating to ~3000 K and converting the carbonized material to turbostratic graphene. HC step yielded ~85% mass recovery from the carbonized intermediate for an overall theoretical yield of ~25% of the raw ELV-WP. Across 20 batches, 19–24% yields were realized, producing 11 g of ELV-WP-derived FG (ELV-WP-FG). Characterization of FG: Raman spectroscopy (532 nm) confirmed turbostratic graphene via an intense, narrow 2D peak (~2690 cm−1), lower D peak (~1350 cm−1), high 2D/G ratio, single-Lorentzian 2D fit, and presence of TS1/TS2 bands (1875/2025 cm−1) with absence of the M band. From 225 spectra, average 2D/G was 0.81, D/G 0.58, and 2D FWHM 54 cm−1. Powder XRD showed broadened (002) and (100) peaks at 26.1° and 43.3° shifted from graphite (26.6°), indicating turbostratic stacking. XPS survey indicated ~98% C with minor O; high-resolution C1s showed sp2/sp3 bonding and π–π* transition at ~291 eV. TEM revealed sheet sizes averaging 13.8 ± 7.1 nm with increased interlayer spacing (0.358 nm vs 0.334 nm for graphite) and rotational disorder (SAED). TGA under air showed a single mass loss at 500–650 °C. BET surface area was 60 m2 g−1 with pore volume 0.23 cm3 g−1 and mesoporosity. Dispersibility testing via UV–Vis in 1 wt% Pluronic-F127 aqueous medium after sonication/centrifugation showed ELV-WP-FG reaching 0.35 mg mL−1 at an initial loading of 3 mg mL−1, about twice that of a commercial physically exfoliated graphene benchmark. PUF composite fabrication and testing: ELV-WP-FG was dispersed into the polyol component (with water, surfactant, catalysts, crosslinker) by shear mixing (3 min at 1500 rpm), followed by addition of diisocyanate and brief mixing (12 s). The mixture was molded (30.5 × 30.5 × 5.1 cm3), heated at 65 °C for 7 min, post-cured at 65 °C for 30 min, and rested 24 h. Loadings were 0.01, 0.025, 0.05, and 0.1 wt%. Mechanical and thermal properties were measured per ASTM standards (D3574 Tests A, L, C, E; D624 Die C). DSC (−90 to 100 °C, N2) assessed transitions; SEM imaged foam cross-sections; contact angles assessed hydrophobicity; acoustic absorption was measured using a two-microphone impedance tube (ISO 10534-2). Prospective LCA: A cradle-to-gate LCA (functional unit: 1 kg graphene powder) compared FJH from ELV-WP to two graphite-based routes: (i) ultrasonication-based physical exfoliation (sonication, centrifugation, rinse/dry) and (ii) chemical exfoliation via modified Hummers oxidation (KMnO4/H2SO4/H3PO4), followed by hydrazine reduction and drying. Impact categories were cumulative energy demand (CED), 100-year global warming potential (GWP), and cumulative water use (CWU). Background data were sourced from Argonne’s GREET (GREET.Net and spreadsheets). The cut-off approach assigned no upstream burdens to ELV-WP (post-shredder residual) since depollution/dismantling/shredding are attributed to metal recovery; transportation, plant overheads (HVAC, lighting), equipment manufacture, and waste treatment/disposal were excluded. Assumptions and inventories are detailed in Supplementary materials.

- FJH upcycling performance: Mixed, unsorted ELV-WP was converted directly to turbostratic flash graphene. Typical batch outcomes: LC step carbonized ~30% mass; HC step converted carbonized material with ~85% recovery for overall 19–24% yield (~25% theoretical) of graphene from raw ELV-WP. Temperatures reached ~2300 K (LC) and ~3000 K (HC) within seconds. - Graphene quality: Raman showed strong 2D peak, low D, and turbostratic signatures (TS1/TS2, no M). From 225 Raman spectra: average 2D/G = 0.81; D/G = 0.58; 2D FWHM = 54 cm−1. XRD displayed broadened (002) at 26.1° and (100) at 43.3°, shifted from graphite. XPS indicated ~98% carbon with minimal oxygen; C1s sp2/sp3 and π–π* features present. TEM indicated sheet sizes of 13.8 ± 7.1 nm and interlayer spacing of 0.358 nm. TGA showed oxidation onset/degradation at 500–650 °C in air. BET surface area was 60 m2 g−1 with 0.23 cm3 g−1 pore volume. - Dispersibility: ELV-WP-FG dispersed to 0.35 mg mL−1 in a surfactant-assisted aqueous system at 3 mg mL−1 initial loading, about 2× higher than a commercial physically exfoliated graphene comparator. - PUF composite enhancements: At 0.1 wt% FG, Young’s modulus increased by up to 34% relative to control. Compressive force deflection at 50% strain increased by ~19% even at 0.01 wt% loading. Acoustic absorption increased at low frequencies (50–300 Hz), with up to ~30% higher absorption at 200 Hz, and a sharp increase from 300–3000 Hz. Foam density changed little across loadings. Contact angle rose from 88.2° (control) to 101.6° at 0.1% FG, indicating increased hydrophobicity. DSC showed Tg increase from ~65 °C (control) to ~72 °C with FG; beta transition shifted from −63 to −60 °C. SEM suggested some FG aggregation at higher loading (0.1%). - LCA outcomes (per 1 kg graphene powder): Versus ultrasonication exfoliation, FJH reduced CED by ~88%, GWP by ~85%, and CWU by ~93%. Versus chemical exfoliation/reduction, FJH reduced CED by ~80%, GWP by ~80%, and CWU by ~97%. In FJH, >96% of burdens were from electricity use during flashing, implying further reductions via renewable energy. Major contributors for comparators were ethanol and aqueous acids/oxidants/rinsing steps. - Circularity: FG-containing PUF composites could be re-flashed back into high-quality FG of comparable characteristics. XPS and N1s features indicated nitrogen incorporation from PUF, suggesting N-doped FG upon re-flashing.

The work demonstrates a solvent- and water-free, rapid, and scalable route to upcycle heterogeneous ELV waste plastics directly into high-value turbostratic graphene without sorting. The produced FG exhibits properties (turbostratic stacking, good dispersibility) that enable effective incorporation at very low loadings into automotive PUF, improving mechanical stiffness, compressive performance, hydrophobicity, and low-frequency sound absorption. The ability to re-flash FG-containing composites back to FG indicates a closed-loop pathway that preserves material utility and potentially reduces demand for virgin fillers. The prospective cradle-to-gate LCA shows substantially lower energy use, climate impacts, and water use for FJH compared with common physical and chemical exfoliation routes, largely because FJH avoids solvents, lengthy thermal treatments, and aqueous workups. These results support the central hypothesis that FJH can both valorize mixed ELV plastics and deliver environmental benefits over incumbent graphene manufacturing pathways, positioning FJH as a promising technology for circular materials management in the automotive sector and beyond.

This study establishes that mixed, unsorted ELV waste plastics can be rapidly upcycled to high-quality turbostratic flash graphene via a dual-stage flash Joule heating process. The resulting graphene enhances polyurethane foam performance at low loadings and can itself be recovered by re-flashing FG-containing composites, enabling continuous upcycling. A prospective cradle-to-gate LCA indicates large reductions in cumulative energy demand, greenhouse gas emissions, and water use relative to ultrasonication and chemical exfoliation routes. Future work should pursue process scale-up and yield optimization, renewable electricity sourcing to further lower impacts, recovery and valorization of volatile coproducts from the LC step, deeper evaluation of composite performance across broader formulations and loadings, and expanded LCAs that include transport, plant burdens, and end-of-life management for comprehensive sustainability assessments.

- The LCA employed a cut-off approach attributing no upstream burdens to ELV plastics (post-shredder residual) and excluded transportation, plant overheads, capital equipment, and waste treatment/disposal; results are therefore prospective and may shift with broader system boundaries. - Lack of standardization in graphene quality definitions and production practices complicates direct comparisons across studies and suppliers. - Sonication- and chemical-based comparator routes vary widely in solvents, surfactants, processing times, and assumed dispersion concentrations; selected literature values may not represent all implementations. - Composite results showed potential nanoparticle aggregation at higher FG loadings, which may limit property gains without further dispersion optimization. - Some mechanical properties showed limited or variable changes across loadings; optimization of formulation and processing could be required for consistent improvements. - Batch-scale FJH yields (19–24%) leave room for improvement; scalability, process control, and continuous operation strategies were not fully evaluated here.