(188e) Characterization and Modeling of the Thermal Degradation of Fluoropolymers into the Gaseous Evolution of per- and Poly-Fluoroalkyl Substances (PFAS)

Fluoropolymers have the potential to thermally break down into long chain per- and polyfluoroalkyl substances (PFAS) and/or polychlorofluoro- substances to the environment. [1] Recently, the U.S. Environmental Protection Agency (EPA) released a technical update on the use of incineration to manage PFAS waste streams by incineration at high temperatures, upwards of 1000 to 1400 °C. [2] The EPA’s Contaminant Candidate List now includes several PFAS subtypes with thousands of other distinct chemicals for identifying additional PFAS that could potentially require future regulation under the Safe Drinking Water Act. [3] In order to understand the environmental impact, it is important to characterize and study the gas phase thermal breakdown fragmentation and desorption products that evolve during the combustion of fluoropolymers. Research on the efficacy of incineration is limited, but recent models predict that the combustion of C₁ and C₂ PFAS results in the formation of the products of incomplete combustion (PICs) such as CF₂ and CF₃ radicals [4]. In our research study, we analyzed the thermal breakdown and degradation products of polytetrafluorethylene (PTFE), polyvinylidene fluoride (PVDF), and polyvinylidene hexafluoropropylene (PVDF-HFP) using Thermogravimetric Analysis from 20 to 1000 °C, coupled to Mass Spectrometry (TGA-MS). The TGA-MS data provides a signature for possible gaseous degradation products that evolved at specific temperatures. The separation of individual byproducts and isolation of gaseous byproducts remains a challenge experimentally, however, atomistic modeling can lead to predictive chemical reaction pathways and thus provide insight into the initial fragmentation of the fluoropolymers during thermal breakdown.

Building on the gas phase characterization from TGA-MS results, process modeling with ChemCad and Aspen Plus was utilized to study the mass and energy balance for the combustion of the fluoropolymers of interest (such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) with hexafluoropropylene (HFP)). The process is modeled in a Gibbs Reactor in atmospheric combustion conditions with varying oxygen (air) flow rates and temperatures, ranging in temperatures from 250 to 1000 °C. For example, the process feed stream was specified to study the breakdown of PVDF with its constituent monomers which include HFP to form perfluorobutyl ethylene (PFBE), chlorotrifluoroethylene (CTFE), and perfluoropropyl vinyl ethers (PPVEs) reacting with varying air flow rates of stoichiometric oxygen (­O₂) and nitrogen (N₂). In addition, Density Functional Theory (DFT) calculations were also used to gain additional mechanistic understandings on the thermal degradation of relevant fluoropolymer structures. Overall, this study is focused on the instantaneous characterization of the gaseous reaction products from the combustion of fluoropolymers using mass spectrometry. The mechanistic modeling provides context to the kinetic and energetic pathways for the potential thermal breakdown occurring in the TGA-MS studies. Experimental and modeling results together inform the process design modeling for large scale energy and mass balance calculations.

KEYWORDS: Per- and polyfluoroalkyl substances (PFAS), Gas Phase Characterization, Fluoropolymers, Thermal Gravimetric Analysis Coupled Mass Spectrometry, Thermal Degradation Analysis, Combustion, Process Design, Aspen Plus, CHEMCAD.

CONTACT: Dr. Enoch A. Nagelli, Dept of Chemistry and Life Science, United States Military Academy, West Point, New York 10996. Email: enoch.nagelli@westpoint.edu TEL: 845-938-3904.

REFERENCES

1. David A. Ellis, Scott A. Mabury, Jonathan W. Martin and Derek C. G. Muir “Thermolysis of fluoropolymers as a potential source of halogenated organic acids in the environment” Nature 412, 321–324 (2001). https://doi.org/10.1038/35085548.

2. U.S. Environmental Protection Agency. (2020, February). Per- and Polyfluoroalkyl Substances (PFAS): Incineration to Manage PFAS Waste Streams. https://www.epa.gov/chemical-research/technical-brief-and-polyfluoroalky... and https://www.epa.gov/chemical-research/status-epa-research-and-developmen....

3. U.S. Environmental Protection Agency. (2022, October). EPA Issues Final List of Contaminants for Potential Regulatory Consideration in Drinking Water, Significantly Increases PFAS Chemicals for Review https://www.epa.gov/ccl/contaminant-candidate-list-5-ccl-5

4. Jonathan D. Krug, Paul M. Lemieux, Chun-Wai Lee, Jeffrey V. Ryan, Peter H. Kariher, Erin P. Shields, Lindsay C. Wickersham, Martin K. Denison, Kevin A. Davis, David A. Swensen, R. Preston Burnette, Jost O.L. Wendt, and William P. Linak (2022) “Combustion of C1 and C2 PFAS: Kinetic modeling and experiments” Journal of the Air & Waste Management Association, 72:3, 256-270, DOI: 10.1080/10962247.2021.2021317