Converting plastic waste pyrolysis ash into flash graphene
Graphical abstract
Introduction
Pyrolysis is a form of chemical recycling that leads to homolysis of plastic waste (PW) polymers in order to extract energy-rich gases, fuel oils, waxes, naphtha, and virgin monomers [[1], [2], [3]]. A byproduct of this PW beneficiation process (“beneficiation” is a treatment that increases the value of a feedstock by removing a low value waste stream) that can reach up to 20 wt% is carbon-rich pyrolysis ash (PA) that currently has low or negative value [4]. Chemical recycling is a promising method toward the formation of a circular plastic economy, but it may not be economically viable without political or commercial incentives and it struggles to compete with less expensive virgin oil and commodity monomers [5,6]. Through the upcycling of negative value byproducts to high value products, a circular plastics economy and decreased use of virgin chemical feedstocks might be achieved.
Graphene is a 2D material that has interesting mechanical, chemical, thermal, and electrical properties [[7], [8], [9], [10], [11]]. The use of graphene in composite materials shows particular promise since graphene-containing nanocomposites generally show significantly enhanced mechanical, electrical, and thermal properties [12,13]. Over the past decade, bulk graphene synthesis has involved a top-down method of converting graphite to graphene through physical or chemical exfoliation [14], or reduction of graphene oxide, which itself comes from graphite [15]. These synthetic methods are expensive, resource intensive, difficult to scale and yield varying qualities of graphene or graphene nanoplatelets [16]. Recently, we reported a scalable, bottom-up, low-cost synthesis of flash graphene (FG) [17]. The technique uses flash Joule heating (FJH) to heat carbonaceous materials to temperatures over 3000 K in ∼100 ms, producing >90% yields of high quality turbostratic FG (tFG). The high temperatures of FJH result in high purity tFG, since much of the non-carbon atoms are removed through sublimation. Here, we use PA to produce tFG, then demonstrate the use of PA derived tFG for enhancement of poly(vinyl alcohol) (PVA) composites as well as Portland cement composites.
Section snippets
Chemicals and materials
PA was graciously provided by Shangqiu Zhongming Eco-Friendly Equipment Co., Ltd in Shangqiu City, Henan, China. PA was ground using a mortar and pestle before use. PVA (80% hydrolyzed, 9000–10,000 Mw) was obtained from Sigma-Aldrich and used without further purification or characterization. Pluronic-F127, a non-ionic surfactant, was obtained from Millipore-Sigma. Commercial graphene samples used for comparison were obtained from Tianyuan Empire Materials & Technologies, Shatin, Hong Kong and
Synthesis and characterization of FG
tFG was synthesized from PA that resulted from the industrial pyrolysis of polypropylene PW at 450 °C. As part of the FJH process, the pulse voltage, duration, and initial sample resistance all impact the internal temperature reached and therefore control the sheet size, thickness, elemental purity, and defect concentration of the tFG. Hence, optimization of these parameters is essential to produce high-quality tFG.
Low-quality graphene typically exhibits a large D peak at 1350 cm−1, signifying
Characterization of tFG nanocomposite
PVA was chosen as the example composite system since it is a biodegradable, non-toxic, and biocompatible polymer [26] that is being extensively researched for medical applications, fuel cell polymer electrolyte membranes, and environmentally friendly packaging material [27]. However, PVA has not seen widespread use for consumer applications because of its poor mechanical properties, water solubility, and hydrophilicity.
Addition of graphene to synthesize polymer nanocomposites is often
Conclusion
A method to convert PW PA into tFG is reported. The tFG was used to enhance the mechanical and physical properties of PVA, needing no further functionalization or purification. A 50% increase in failure strain, as well as a 500% decrease in water uptake was observed, compared to neat PVA. The results demonstrate an environmentally friendly path for the reduction of PW by economically incentivized chemical recycling through the upcycling of PA to high value tFG, as well as the enhancement of
CRediT authorship contribution statement
Kevin M. Wyss: Conceptualization, Investigation, Data curation, Writing - original draft, Writing - review & editing. Jacob L. Beckham: Investigation, Writing - original draft. Weiyin Chen: Investigation. Duy Xuan Luong: Investigation. Prabhas Hundi: Investigation. Shivaranjan Raghuraman: Investigation. Rouzbeh Shahsavari: Writing - original draft, Supervision, Writing - review & editing. James M. Tour: Conceptualization, Writing - original draft, Supervision, Writing - review & editing,
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Universal Matter Inc. has licensed from Rice University the FJH approach to graphene. J.M.T. is a stockholder in Universal Matter, but not an employee, officer or director. Potential conflicts of interest are mitigated through regular disclosures to and compliance with Rice University’s Office of Sponsored Programs and Research Compliance. C-Crete Technologies
Acknowledgement
K.M.W. and J.L.B. acknowledge the National Science Foundation Graduate Research Fellowship Program for graciously providing financial support. The Air Force Office of Scientific Research (FA9550-19-1-0296) and the DOE-NETL (DE-FE0031794) funded this work. Dr Bo Chen assisted with XPS analysis.
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