The responsible disposal of end-of-life vehicles (ELVs) is a significant environmental challenge. A substantial portion of ELV waste consists of plastics, which often end up in landfills. The increasing use of plastics in vehicles, driven by fuel efficiency goals, exacerbates this issue. Traditional recycling methods for ELV waste plastics are economically unviable due to the high cost of separating different plastic types. This study addresses this problem by exploring the upcycling of mixed ELV waste plastics into flash graphene (FG), a valuable material with numerous applications. The use of flash Joule heating (FJH) offers a solvent-free, water-free method with the potential for significant environmental benefits compared to existing graphene production methods. The overarching goal is to demonstrate the feasibility of transforming a significant waste stream into a high-value product, contributing to a more circular economy and reducing environmental impact. The specific objectives include demonstrating the FJH upcycling of ELV-WP into FG, enhancing vehicle polyurethane foam (PUF) composites with the produced FG, performing a prospective life cycle assessment (LCA) comparing FJH to other graphene production methods, and showing the continuous upcycling of FG-enhanced composites.

Existing literature highlights the challenges in managing ELV waste plastics. Traditional recycling struggles with the heterogeneous nature of ELV plastics and the economic burden of separating different polymer types. While some progress has been made with pyrolysis methods, these often require complex catalysts and inert atmospheres and are not effective with mixed plastic streams. Academic interest often focuses on polypropylene due to its prevalence in automotive applications. Current strategies often lack economic incentives, with the low cost of virgin polymers hindering widespread adoption of recycling initiatives. Governmental regulations, while aiming to improve ELV material recovery, have often fallen short of their goals. Meanwhile, the automotive industry is exploring 'green' polymers and sustainable reinforcements, but these do not address the massive existing volume of ELV waste plastic. Recent research has demonstrated the potential of FJH to convert mixed plastic waste into FG, which is attractive due to its high value and versatility. This prior work lays the foundation for the current study, expanding on this promising upcycling approach to ELV-WP.

The study employed a custom-designed dual-capability flash Joule heating (FJH) station capable of both low-current (LC) and high-current (HC) discharges. ELV waste plastic (ELV-WP), obtained from Ford F-150 trucks, was ground to 1 mm particle size and mixed with 5 wt% metallurgical coke to enhance conductivity. The LC-FJH process (1–25 A, 10–16 s) carbonized the plastic mixture, while the HC-FJH process (∼200 A, <1 s) converted the carbonized material into FG. The process was characterized by Raman spectroscopy, powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and Brunauer-Emmett-Teller (BET) gas adsorption to assess the quality, purity, and properties of the ELV-WP-FG. The dispersibility of ELV-WP-FG in aqueous solution was compared to that of commercially available graphene. For composite fabrication, ELV-WP-FG was incorporated into polyurethane foam (PUF) at different weight percentages (0.01%, 0.025%, 0.05%, and 0.1%). The mechanical properties (Young's modulus, compressive strength, tensile strength, tear resistance) and acoustic properties (noise absorption) of the resulting ELV-WP-FG-PUF composites were evaluated. Differential scanning calorimetry (DSC) and cross-sectional scanning electron microscopy (SEM) were used to examine the interaction between the FG and the PUF matrix. The continuous upcycling capability was demonstrated by re-flashing the ELV-WP-FG-PUF composite to recover FG. Finally, a prospective life cycle assessment (LCA) compared the environmental impact of FJH with traditional graphene synthesis methods (ultrasonication and chemical exfoliation), considering cumulative energy demand (CED), global warming potential (GWP), and cumulative water use (CWU).

The FJH process successfully converted ELV-WP into high-quality turbostratic FG, with yields ranging from 19% to 24%. Raman spectroscopy confirmed the high quality of the FG, exhibiting intense and narrow 2D peaks and a high 2D/G ratio, indicative of minimal defects and optical decoupling between layers. XRD and XPS analysis confirmed the successful conversion and removal of impurities. ELV-WP-FG demonstrated superior dispersibility compared to commercial graphene. Incorporation of ELV-WP-FG into PUF composites resulted in significant improvements in mechanical properties, with increases in Young's modulus (up to 34%) and compressive force deflection at 50% strain (up to 19%). The acoustic absorption of the composites was also enhanced, particularly at low frequencies (up to 30% increase at 200 Hz). DSC analysis revealed increases in the glass transition temperature (Tg) of the PUF composites, indicating strong interactions between the FG and the polymer chains. SEM imaging showed the dispersion of FG within the PUF matrix, although some aggregation was observed at higher FG loadings. The continuous upcycling of the FG-PUF composites back into high-quality FG was successfully demonstrated. The prospective LCA indicated that the FJH method significantly reduced CED, GWP, and CWU compared to traditional graphene synthesis methods. For example, compared to ultrasonication, FJH resulted in an 88% reduction in CED, an 85% reduction in GWP, and a 93% reduction in CWU. The largest environmental impact of FJH was associated with electricity consumption, suggesting the potential for further improvement by using renewable energy sources.

The findings of this study demonstrate the successful upcycling of a substantial waste stream (ELV-WP) into a high-value material (FG) using a sustainable and efficient process (FJH). The enhanced mechanical and acoustic properties of the FG-PUF composites highlight the practical value of this approach for automotive applications. The significant reduction in environmental impact compared to traditional graphene synthesis methods, as shown by the LCA, strongly supports the sustainability advantages of the proposed method. The ability to continuously recycle the FG-PUF composite back into FG further contributes to the circularity of the process. The results address the growing need for sustainable waste management strategies in the automotive industry and the broader context of circular economy principles. The improved dispersibility of the FG compared to commercial samples is noteworthy, suggesting advantages in composite processing. However, there is room for optimization, specifically addressing the observed aggregation at higher FG loadings to further enhance composite performance. The study provides a compelling example of how advanced materials synthesis can contribute to waste reduction and resource recovery.

This research successfully demonstrates the upcycling of end-of-life vehicle waste plastics into high-quality flash graphene using flash Joule heating. The resulting graphene enhanced the properties of polyurethane foam composites, and the entire process is shown to be continuously upcyclable. A life cycle assessment confirms significant environmental advantages over traditional methods. Future work should focus on optimizing the process for even higher yields and exploring other potential applications of the produced graphene.

The LCA conducted in this study is prospective and based on modeled data. While it provides a valuable comparison of different synthesis methods, the actual environmental impact of the FJH method may vary depending on the specific implementation and energy sources used. Additionally, the study focused on a specific type of ELV waste plastic. Further research is needed to determine the applicability of the method to a wider range of plastic compositions and waste streams. The aggregation observed at higher graphene loadings in the PUF composites suggests the need for further optimization of the dispersion and incorporation methods.