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9.6Chemical Recycling of PTFE (as a Model for Other Polymers) 577 9.6Chemical Recycling of PTFE (as a Model for Other Polymers) Achim Schmidt-Rodenkirchen, Klaus Hintzer, Thorsten Gerdes 9.6.1Introduction PTFE (polytetrafluoroethylene) and other perfluorinated polymers like FEP (copoly mers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP)) or PFA (a copolymer of TFE and perfluorovinylether) are niche polymers, with a total production in 2015 of 160 kt, compared to polymers overall of 322 Mt [1]. Of the perfluorinated polymers, PTFE has the highest share, at 52% [2]. Figure 9.62About 60% (PTFE + FEP) of the total volume of produced perfluorinated polymers are suitable for chemical recycling [2] Their unique properties, including thermal and chemical stability, low dielectric constant, high dielectric strength, and low surface energy (ideal for anti-adhesive applications), turns PTFE and other perfluorinated polymers into indispensable materials [3, 4]. Due to these outstanding material properties, the market is predicted to grow by approximately 5% per year [5]. The conventional production of PTFE is a very energy-intensive procedure with many process steps, and 220 MJ is needed for 1 kg of PTFE [6]. The production of Cl2, which is needed for PTFE production among other applications, is one of the largest energy consumers worldwide [7]. Perfluorinated polymers are therefore valuable products. An overview of the energy demand for various polymers, including a ranking of PTFE, is shown in Table 9.21. 578 9Chemical Recycling Table 9.21Cumulative Energy Demand (CED) of Bulk Plastics and some Fluorinated Examples for Comparison of Energy Consumption [8] Polymer Polytetrafluoroethylene (PTFE) Polyethylene terephthalate (PET) Polyethylene (PE) Polystyrene (PS) Polyvinyl chloride (PVC) Synthetic rubber Nylon 6-6 Ethylene tetrafluoroethylene (ETFE) Cumulative energy demand (CED) Ref. MJ per kg polymer toe[a] per t polymer 220 5.2 [6] 54 1.3 [9] 65 1.5 [10] 71 1.7 [10] 53 1.3 [10] 64 1.5 [10] 163.0 3.9 [10] 150-170 3.6-4.0 [11] [a] 1 tonne of oil equivalent (toe) = 42 GJ The manufacture of TFE/HFP monomers consumes the lion's share of the energy demand in making perfluoropolymers (>99%). There have been many efforts to recover perfluorinated monomers from perfluoropolymers; all approaches were based on the finding that these polymers can be pyrolyzed at high temperatures into TFE/HFP. The use of pyrolysis to convert perfluorinated polymers back to monomers is an example of a rare phenomenon in the materials industry; only a few other polymer classes (e.g. polystyrene, polyacrylates) can be converted into monomers upon heating. Thus, perfluorinated polymers are ideal candidates for depolymerization, which provides the following benefits: High demand of resources = large savings potential Expensive material = high margin (US $20/kg) [12] Unique properties = high future potential Low volume of production = low scale for reaching industrial world scale Thermal decomposition behavior favors the formation of the monomer = high yields of monomer possible (over 90% yields of monomer/intermediate are obtainable under certain conditions) Another aspect of recycling PTFE is due to the chemical composition of the polymer. PTFE and other perfluorinated polymers consist solely of carbon and fluorine. Efficient recycling of the material can close the fluorine loop and will reduce the consumption the mineral fluorspar, which is considered as a critical raw material for the European economy [13]. All these facts make PTFE an ideal candidate for upcycling, and although these benefits are well-known, to date the 3M company is the only one to have scaled-up the process, with the opening of a production plant at Gendorf, Germany, in 2015 9.6Chemical Recycling of PTFE (as a Model for Other Polymers) 579 [14] with a capacity of a few hundred tonnes. A picture of the building, which in addition to the pyrolysis reactors also contains the mechanical treatment and cleaning, is shown in Figure 9.63. Figure 9.63 500 t/a Demonstration plant for chemical recycling of PTFE (courtesy of 3M/Dyneon) [15] 9.6.2Manufacturing Process for PTFE and Other Perfluorinated Polymers Five raw materials are needed for PTFE production, and the sequence is shown in Figure 9.64. Electrolysis of sodium chloride in water provides chlorine (Cl2) which is subsequently converted by radical substitution with methane into chloroform (CHCl3). In a parallel process, fluorspar (CaF2) is converted with highly concentrated sulfuric acid into HF and gypsum. The key starting material (CHClF2, known as R22) is then made from HF and chloroform. R22 is afterwards pyrolyzed at 900C into tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) by splitting off one HCl molecule per R22 molecule. The polymerization of the monomers takes place in an aqueous suspension or emulsion. 580 9Chemical Recycling Figure 9.64Process of conventional PTFE production with major precursors and by-products [6, 16] The consumption of the raw materials is shown in Figure 9.65. Aside from the huge amount of raw materials needed for PTFE production, some coproducts like HCl are contaminated with HF from the various process steps, which makes further application difficult. Figure 9.65Raw materials required for the R22 route [17] The energy used in producing PTFE from raw materials is shown in Figure 9.66. By far the largest amount is spent on producing TFE monomer, with less than 1% of the energy being used for the polymerization process. Consequently, if the polymer could be successfully recycled into the monomer, most of the energy can be saved. 9.6Chemical Recycling of PTFE (as a Model for Other Polymers) 581 Figure 9.66Ratio of energy demand of the major process steps on the total production process of PTFE in aqueous polymerization [6] 9.6.3Mechanism of PTFE Depolymerization The formation of the key monomer TFE is based on the recombination of two difluorocarbene (CF2) molecules. In classical TFE production, the CF2 molecule is obtained from the pyrolysis of R22. By heat treatment/pyrolysis of PTFE, the polymer is degraded back to CF2 [3]. The polymer breaks at temperatures above 500C statistically, by forming two radicals. The bond strength in the polymer chain increases from the radical along the chain (Figure 9.67). Consequently, decomposition starts at the radical end, and proceeds from there back along the chain [18]. Figure 9.67 C-C bond strength in perfluoroalkyl radicals The dependence of the rate of the depolymerization reaction on temperature and the degree of degradation is given in Figure 9.68. The influence of the temperature approximately doubles with each 10 K, which is in accordance with the Arrhenius equation (Equation 9.7). Equation 9.7 582 9Chemical Recycling The influence of substances like fillers, in this case carbon, on the degradation behavior is also interesting, and illustrates the need to perform experimental investigation to understand the decomposition of complex mixtures of PTFE, perfluorinated polymers and end-of-life (EOL) materials. Figure 9.68Thermal reaction rate vs. degree of decomposition for PTFE (--) and PTFE with 10% Carbon (--) [19] Ultimately, difluorocarbene is obtained from the depolymerization reaction. Due to the strong C-F bonding the generated molecule is stable enough to recombine into a TFE molecule before further decay. This reaction is very fast, because the activation energy Ea for the reaction of two radicals is 0 kJ/mol (Equation 9.8). Equation 9.8 Due to the reactivity of the CF2 molecule, side-reactions become relevant to achieving optimum reaction control. In Figure 9.69 the formation of homologs is shown. The side-product hexafluoropropylene (HFP) is also a valuable product, used (for example) to produce the polymer FEP, and c-C4F8 can be reconverted into TFE or HFP. Higher homologs must be avoided, along with the formation of the toxic carbonyl fluoride (fluorophosgene) by the action of oxygen. The mechanism of the formation of higher homologs, such as HFP and c-C4F8 is described in the literature [20]. 9.6Chemical Recycling of PTFE (as a Model for Other Polymers) 583 Figure 9.69Potential side-products generated during the depolymerization of PTFE via CF2 [8, 16, 20] 9.6.4Recycling PTFE and Perfluorinated Polymer Materials 9.6.4.1PTFE Production Waste The amounts of scrap, wet waste materials or off-specification materials of unfilled PTFE from manufacturers are usually in the lower percentage range. In contrast, the quantity of scrap resin from processors (generated during the finishing process) or end-use articles is usually in the range 10-30%, and in some areas it is above 50%. These high quantities are the result of very specific processing technologies for PTFE (molding, sintering, machining, and cutting). For unfilled PTFE- scrap resins, there are three established recycling paths: 1. Sintered, unfilled PTFE-scrap resin is cleaned from all contaminants and milled into certain particle size classes, which can be reused, for example, in Ram extrusion applications. This so-called "repro-PTFE" can also be mixed with virgin PTFE to a certain content; such repro-PTFE materials get specific designations and are typically used for less demanding end-use applications. 2. Clean, unfilled PTFE can be thermally degraded into low-molecular-weight PTFE, and such processes are commercially established. The thermal degradation of high-molecular-weight scrap PTFE occurs at temperatures of about 500C in ovens, kneaders, or extruders [21], and the obtained low-molecular-weight PTFE is further milled into particles a few microns in size; such materials are often called PTFE micropowders. 584 9Chemical Recycling 3. Alternatively, clean, unfilled PTFE can be degraded by high-energy radiation such as X-rays, gamma rays, or electron beams. The degradation of high- molecular-weight PTFE by electron beam irradiation is widely used commercially, and in practice continuous processes are used to improve the economics. After irradiation, the material is milled to the desired smaller particle sizes. During the irradiation, significant amounts of perfluorinated carboxylic acids are formed, which are a major concern, and as a result this recycling path may be discontinued in the future.The thermo/radiation-degraded PTFE micropowders are mostly used as additives to plastics, inks, oils, lubricants, and coatings. 9.6.4.2PTFE Composites Production Waste In contrast to clean, unfilled PTFE, for which some recycling options are well-established, and the recycling rates reach significant levels, there no large-scale recycling technologies for PTFE mixtures. This is primarily due to the presence of a large variety of different fillers (e.g., glass fibres, graphites, carbons, metals, ceramics, organic fillers, and pigments) and to the variable amounts of fillers in the PTFE compounds. Because of the large amounts of scrap streams (at least 10-30%), and considering that landfilling is not sustainable and becoming increasing heavily regulated or even banned, scrap PTFE mixtures scrap are increasingly likely to be recycled into TFE/HFP. 9.6.4.3Perfluorinated Thermoplasts Production Waste Clean, unfilled, scrap materials from manufacturing and processing of perfluorinated polymers (e.g. FEP, PFA) are generated in the low percent range. Nearly all these materials are recycled back into the different processes; the end-use properties are almost unaffected. 9.6.4.4End-of-Life (EOL) PTFE and Perfluorinated Materials In some cases, used EOL PTFE and perfluorinated thermoplasts (e.g. FEP, PFA) are recycled by special cleaning processes and end up in the repro-PTFE or PTFE micropowder markets; PFA and FEP might be reused in applications where the quality requirements are much lower. Overall, most EOL materials end up in landfills, incineration plants, or blast furnaces. PTFE is often combined with lots of fillers like carbon black, graphite, bronze, MoS2, short and long glass fibers, and colorants to achieve certain properties. Sealings contain often short glass fibers or carbon black. Typical EOL fluoropolymers are shown in Figure 9.70. 9.6Chemical Recycling of PTFE (as a Model for Other Polymers) 585 Figure 9.70Examples for recycling of end-of-life (EOL) PTFE: Pipe sealings with glass fibers (left); semi-finished product with carbon black (middle); machine-cut tubes from heat exchangers (right) Architectural materials (Figure 9.71, left) are commonly made of long fibers with a PTFE coating or laminate. Those fibers can cause problems in some process units like cyclones or reactors. Some materials contain of course unknown impurities, such as PVC in bulk material originating from the reprocessing of wires (Figure 9.71, right). In this case, the influence of chlorine on the chemical reactions and the position of elimination is yet to be investigated. Figure 9.71Architectural materials made from PTFE with long fibers (left), and polymer jacket of wires (right) 9.6.5Recycling Concepts for Perfluorinated Materials Fluoropolymers can generally be recycled according to three main categories: thermal, chemical, and mechanical. This is illustrated in Figure 9.72. The block schema for the fluorinated polymers shows the backward integration of the various recycling routes into the classic manufacturing route. The further back the integration reaches, the higher the energy required for recycling. Landfilling and thermal recycling should be avoided in future. The other recycling routes are discussed in the following sections. 586 9Chemical Recycling Figure 9.72Typical classification of polymer recycling options used for PTFE [11] 9.6.5.1Thermal Recycling With a calorific value of 5400 kJ/kg [23], PTFE and perfluorinated polymers are not suitable for generating thermal or electrical energy because the value is below the required limit of 11,000 kJ/kg for application as a surrogate fuel [23]. However, these materials can be combusted in a typical temperature range from 900C to 1000C by a commercial waste incineration plant without forming toxic organic byproducts [24]. In such an incineration process, fluoropolymers release corrosive gases (e.g. HF). For processing limited amounts of fluorinated materials in municipal waste incineration plants, the plants must be equipped with appropriate corrosion protection [5] and off-gas treatment, which is commonly done by capturing the formed HF with (for example) CaO, to generate fluorspar (CaF2) [25]. Because the calorific value of PTFE and perfluorinated polymers is below 6000 kJ/ kg, and because they are chemically inert [4, 23], not biodegradable [26] and do not emit toxic components, landfilling is allowed in accordance with local regulations. The disadvantage of this practice is the accumulation of chemical inert perfluorinated polymers in the environment [8]. 9.6.5.2Mechanical Recycling The major processing route for mechanical recycling consists of cleaning, milling, and reprocessing to repro-PTFE or even PTFE micropowder [3]. This is suitable for highly pure material without fillers. The reprocessing is done under raised pressure and temperature, commonly in RAM extrusion [23]. The physical properties can deviate significantly from virgin 9.6Chemical Recycling of PTFE (as a Model for Other Polymers) 587 PTFE, and this has to be considered for the further applications [23]. To reduce the loss of properties, repro-PTFE is often blended with virgin material [5]. Mechanical recycling of PTFE makes sense if the available input materials are sufficiently pure and do not contain fillers. However, recycling such polymers multiple times results in diminished intrinsic properties and overall lower quality [8, 22]. However, in most cases mechanical recycling does not apply to EOL materials nor to most compounds outside the processor of the semi-finished product [23]. To avoid landfilling or combustion, chemical recycling is usually the only option. 9.6.5.3Chemical Recycling of Perfluorinated Polymers As shown in Figure 9.72, chemical recycling can be integrated into a common recycling process between combustion (thermal recycling) and mechanical recycling. Chemical recycling itself embraces three subcategories. Here the following terms are used, which are general enough to cover possible sub-variants: Molecule recycling Monomer recycling Feedstock recycling By using the solubility of the polymer molecules in certain solvents, the molecules can be separated in the solvent, the impurities precipitated, and finally the molecules reused. A process for dissolving PTFE polymer composite is not yet known. In terms of fluorinated materials, the generic term "feedstock recycling" would include incineration processes for production of HF/CaF2 and CO/CO2. Because this process is very far back-integrated (i.e. recycled material streams are reintroduced into the production process far upstream) because all carbon-fluorine bonds are destroyed, it should only be considered for partially fluorinated materials when there is no other chemical recycling route available [27]. As discussed in Section 9.6.3, perfluorinated polymers can be depolymerized into the monomers TFE and HFP. This process is related to "monomer recycling". The thermal degradation of PTFE to the corresponding monomers was first described in the 1950s [28]. A continuous process for this approach was described in 1982 [29]. Further investigations were carried out [8, 16, 33] until high yields of over 90% were obtained using in a continuous fluidized-bed process. Despite the high yield of valuable products, significant amounts of HF as a by-product are produced. In addition, the depolymerization reactor and the subsequent process steps has to cope with fillers from compounds and other possible impurities. The formation and presence of HF is a unique characteristic compared to other polymers. 588 9Chemical Recycling The depolymerization process requires process units that are tolerant of formed impurities and able to stabilize the reaction mixture by carefully controlling the residence time of TFE and HFP. It is also advantageous if the crude monomer mixture can be fed into an existing TFE/HFP infrastructure, because this would reduce investment costs. Figure 9.73 gives an insight into the process design that can meet all the following process requirements: Minimum residence time of generated monomers Quick elimination of HF from the product stream Accumulation of solids in one process unit The preconditioned (milled) PTFE and EOL materials are buffered and subsequently transported into a fluidized-bed reactor (FBR) unit. In the FBR the fluoropolymer material, together with the bed material, is fluidized with superheated steam above 550C. The fluoropolymers are decomposed using flash pyrolysis, with smaller fillers (e.g. from polymer composites) being blown out of the reactor; the chemically inert bed material remains in the FBR. Downstream of the reactor, the crude gas mixture is immediately quenched by water to reduce the temperature to below 50C. The quench water is neutralized by NaOH so that HF is converted to NaF. After separating the remaining moisture from the gas flow, the raw gas can be separated into the desired components using existing distillation columns. The process steps to make the monomers consume 99.5% of the cumulative energy demand (CED) of a PTFE or FEP material [6]. Therefore, upcycling these monomers has the potential to save a lot of energy, and at the same time provide new polymers without any loss of performance. The excess water from the quenching process is fed into a wastewater treatment plant after separation of the solids. The solids were collected in one process unit and can be filtered easily from the quenching water. This quenching procedure therefore offers the following advantages: "Freezing" the chemical equilibrium of the raw gas Elimination of corrosive components from the raw gas Precipitation of solid by the droplets of the quenching nozzles Figure 9.74 gives an example of a GC analysis taken from a trial at the InVerTec miniplant running depolymerization of PTFE. The plant operates according to a patented process [30]. The chromatogram shows the valuable products TFE and HFP, along with TFE dimer, c-C4F8 (which can be used to make additional HFP). By-products in the low percentage range are C2F6, C3F6 and some C4 species. 9.6Chemical Recycling of PTFE (as a Model for Other Polymers) 589 Figure 9.73Block diagram of the recycling process via the depolymerization route Figure 9.74Example GC analysis showing the composition of the raw gas from a PTFE pyrolysis at the InVerTec miniplant [31] 590 9Chemical Recycling The control room of the miniplant in shown in Figure 9.75. For safety reasons, the miniplant itself is located in a ventilated housing in the background. Figure 9.75Control room for PTFE pyrolysis at the InVerTec miniplant, showing steam supply, control systems, and GC equipment (Courtesy of InVerTec) If materials are processed that contain fillers that are very difficult to fluidize, other reactor concepts are needed to avoid accumulation of fillers and cementing (plugging) of the fluidized bed. For that purpose, the authors [31] have developed a forced stirred moving-bed (SB) reactor, which can be installed into the depolymerization process instead of the FBR unit [32]. The continuous-bed material (which remains in the SB reactor permanently) is mixed with a stirrer at the depolymerization temperature. Fillers, which are fed into the reactor together with the polymer, are dispersed in the stirred bed, broken by the force of the moving bed, and can then be removed from the quenching system in the same way as for the FBR reactor. A picture of the laboratory reactor is shown in Figure 9.76. 9.6Chemical Recycling of PTFE (as a Model for Other Polymers) 591 Figure 9.76 The stirred-bed reactor of the InVerTec miniplant Courtesy of InVerTec) 9.6.6Environmental Aspects of the Presented Process The environmental impact of the upcycling process described above was investigated by a life-cycle assessment, compared against conventional PTFE production. The main findings are summarized in Figure 9.77. Beside the significant reduction in the CO2 footprint of the recycled material, the reduction in chlorine and hydrochloric acid is impressive. Thus, there is not only the possibility to prevent the landfilling of a chemically inert material, but also to make the polymer production much more environmentally friendly, and to close material cycles. Figure 9.77Environmental benefit from pyrolyzing perfluoropolymers back to TFE [3] 592 9Chemical Recycling With all the new recycling options, the fluoropolymer industry can establish closed loops throughout the whole value chain with close to zero emissions for perfluorinated polymers. A closed fluorine cycle (see Figure 9.78) with optimized raw material streams, sound energy balances, and low environmental burdens will be also the target for partially fluorinated materials. Sustainable Solutions for the Fluorpolymers CaF2/HF R22 TFP, HFP Products F-Polymers HTC-Process Recycling of perfluorinated materials Recycling of patially fluorinated materials HTCv2 Figure 9.78Sustainable solutions for fluoropolymers [3]. HTC-Process = PTFE depolymerization, HTCv2-Process = Conversion of partially fluorinated materials References for Section 9.6 [1] Weltweite und europische Kunststoffproduktion in den Jahren von 1950 bis 2019, Statista, https:// de.statista.com/statistik/daten/studie/167099/umfrage/weltproduktion-von-kunststoff-seit-1950/ [2] Krmer, R., Schlipf, M.: Fluorpolymere: Gesellschaftliche Megatrends geben Fluorpolymeren Aufschwung, Kunststoffe, (October 2016), pp. 110-115, https://multimedia.3m.com/mws/media/ 1430986O/article-about-fluoropolymers-gesellschaftliche-megatrends-geben-fluorpolymeren-aufschwung.pdf [3] Dams, R., Hintzer. K.: Industrial aspects of fluorinated oligomers and polymers, in Fluorinated Polymers, Volume 2: Applications, (2016), RSC Polymer Chemistry Series No. 24, The Royal Society of Chemistry [4] Carlson, P., Schmiegel, W.: Fluoropolymers, Organic, in Ullmann's Encyclopedia of Industrial Chemistry, 6th edn (2006), Bohnet, M. (Ed.), Wiley-VCH, Weinheim, Germany, pp. 10230-10269 [5] Drobny, J.G., Technology of Fluoropolymers, (2009). Taylor & Francis CRC, Press. Available online at http://books.google.de/books?id=RXexW4aEYe8C. [6] Abraham, V.: Comparative life cycle assessment and environmental impacts of PTFE production and recycling, Masters thesis, University Bayreuth, Germany 2013 [7] Schmittinger, P., et al.: Chlorine, in Ullmann's Encyclopedia of Industrial Chemistry, 6th ed., Bohnet, M. (Ed.), Wiley-VCH, Weinheim, Germany, 2006, pp. 5362-5457 [8] Jess, A., Wasserscheid, P.: Chemical Technology, Wiley-VCH, 2013 References for Section 9.6 593 [9] Shen, L., Nieuwlaar, E., Worrell, E., Patel, M.K.: Life cycle energy and GHG emissions of PET recycling: Change-oriented effects, International Journal of Life Cycle Assessment, (2011) 16, pp. 522-536 [10] Patel, M.: KEA (Cumulative Energy Demand) fr Produkte der Organischen Chemie, Working paper of project no. 104 01 123 for the German Environment Agency (Bundesumweltamt): Erarbeitung von Basisdaten zum Energieaufwand und der Umweltbelastung von energieintensiven Produkten und Dienstleistungen fr ko-Bilanzen und ko-Audits, Fraunhofer-Institut fr Systemtechnik und Innovationsforschung (FhG-ISI), Karlsruhe, Germany, 1999 [11] Weible, S., Bauer, I.: Background Report for the LCA. ETFE Construction Element. Environmental Product Declaration for ETFE construction element Project phase "LCA", 3M Dyneon, Nowofol, Vector Foiltec, 2011 [12] Ashby, M.F.: Materials Selection in Mechanical Design, Butterworth-Heinemann (Elsevier), Oxford, 2017 [13] Critical Raw Materials for the EU, Report of the ad-hoc working groups on defining critical raw materials, European Commission, 2010 [14] Hochleistungskunststoffe aus Burgkirchen, 3M Press release, (2020) https://www.3mdeutschland. de/3M/de_DE/presse-de/pressemeldungen/ganzemeldung/?storyid=f29c372a-a0ce-4ba7-96b3-75849 0c120d4 [15] SGL und Dyneon werten Fluorpolymere auf, Chemie Technik, (2017) https://www.chemietechnik. de/markt/sgl-und-dyneon-werten-fluorpolymere-auf.html [16] Aschauer, S.: Untersuchung zur Zersetzung von PTFE und PTFE-co-Polymeren in der Wirbel schicht, Diploma Thesis, University of Bayreuth, Germany, 2008 [17] 3M Dyneon Fluoropolymers: Up-cycling - Closing the Loop, 3M, 2016, https://multimedia.3m.com/ mws/media/907323O/up-cycling-fluoropolymers-brochure.pdf?fn=Up-Cycling_Brochure_EN.pdf [18] Errede, L.A.: The application of simple equations for calculating bond dissociation energies to thermal degradation of fluorocarbons, Journal of Organic Chemistry, (1962) 27, pp. 3425-3430 [19] Fock, J.: On the influence of carbon black on the thermal degradation of polytetrafluoroethylene, Journal of Polymer Science Part B: Polymer Letters, (1968) 6, pp. 127-131 [20] Puts, G., Crouse, P., Ameduri, B.: Handbook of Fluoropolymer Science and Technology, John Wiley and Sons, 2014, Chapter 5 [21] Hintzer, K., Schwertfeger, W.: Handbook of Fluoropolymer Science and Technology, (2014), John Wiley and Sons, Chapter 21 [22] Koltzenburg, S., et al.: Polymere - Synthese, Eigenschaften und Anwendungen, Springer-Verlag, Berlin, Heidelberg, 2014 [23] Recycling von Fluorpolymeren, (Technisches Merkblatt 10) (2017), pro-K Fluoropolymergroup, https://www.pro-kunststoff.de/assets/Merkbl%C3%A4tter%20und%20Co/FP%20TM-10-Recycling-von- Fluorkunststoffen.pdf [24] Henkel, R.: Gore-Tex: Keine Gefahr beim Verbrennen von PTFE, (2019), https://www.ispo.com/maerkte/ gore-tex-keine-gefahr-beim-verbrennen-von-ptfe [25] Stubenvoll, J., Bohmer, S., Szednyj, I.: Stand der Technik bei Abfallverbrennungsanlagen, Bundes ministerium fr Land- und Forstwirtschaft, Umwelt und Wasserwirtschaft, Wien, 2002 [26] Wang, L., Li, Q., Mei, T., Shi, L., Zhu, Y., Qian, Y.: A thermal reduction route to nanocrystalline transition metal carbides from waste polytetrafluoroethylene and metal oxides, Materials Chemistry and Physics, (2012) 137, pp. 1-4 [27] Grot, S., Grot, W.: Recycling of used perfluorosulfonic acid membranes, US7255798B2 [28] Madorsky, S.L., Hart, V.E., Straus, S., Sedlak, V.A.: Thermal degradation of tetrafluoroethylene and hydrofluoroethylene polymers in a vacuum, in: Journal of Research of the National Bureau of Standards, (1953) 51, pp. 327-333 594 9Chemical Recycling [29] Merkel, J.: Pyrolyse von Polytetrafluorethylen, Polyamiden und Polyethanen in der Wirbel schicht-Massen und Stoffbilanzierungen, Dissertation, Universitt Hamburg, 1982 [30] Hintzer, K., et al.: Process of making fluoroolefins by thermal decomposition of fluorinated materials (2010), 3M, WO 2010/039820 [31] Schmidt-Rodenkirchen, A., Aschauer, S., Gerdes, T., Hintzer, K., Zipplies, T., Willert-Porada, M., Rckgewinnung Fluorierter Monomere aus Reststoffen, (2010), final report 25198-31 [32] Hirschberg, M., et al.: Upcycling perfluoropolymers into fluorinated olefins (2021), 3M, WO 2021/165923 [33] BMBF-Abschlussbericht, Rckgewinnung von Calciumfluorid als Sekundrrohstoff fr Fluorpolymere, (2015), Az. 003R080D