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Review article special feature: New technology and materials From the comprehensive introduction lecture meetings of Kanto and Kansai branches Characteristics, Recent Development Trends, and Applications of Fluoroelastomers Satoko YASUDA*, Fritz SIMEON Demystifying Properties of Fluoroelastomers and Their Current and Potential Future Applications Satoko YASUDA* and Fritz SIMEON (AGC Inc. AGC Chemicals, Polymer Development Office, 10 Goikaigan, Ichiha- ra-shi, Chiba 290-8566 JAPAN) mailto:-@agc.com Fluoropolymers are unique specialty polymers that have superior characteristics such as water & oil repellency and superior resistance to various chemicals and weather, making them both reliable and highly suitable for many crit- ical applications. Among other elastomeric materials, fluoroelastomers have excellent properties. The fluorinated elas- tomers have an outstanding chemical resistance and heat resistance allowing to be used continuously at extreme tern- perature conditions for many industrial applications. In this article, the unique chemical and functional structures of fluoroelastomers are demystified. In addition, the current applications and potential prospects of fluoroelastomers are discussed. (Received on February, 29, 2020) Key Words:Fluoroelastomer, Fluororubber, Heat Resistance, Chemical Resistance, FKM, FEPM, FFKM 1. Introduction Fluoropolymers and fluoroelastomers are known for their high performance and high cost. However, However, by understanding their functions correctly and making appropriate choices, they can be materials that can be used stably for a long time This paper describes the mechanism of fluoropolymer functionality, and introduces the types, chemical characteristics, application examples, and future prospects of fluororubber. introduce the types, chemical characteristics, application examples, and future prospects of fluororubber. 2. What is a fluoropolymer? The fluoropolymer is a polymer characterized by excellent heat resistance, chemical resistance, flame retardancy, water and oil repellency, electrical insulating property, weatfher resistance, and the like. Among these characteristics, heat resistance, flame resistance, and chemical resistance are due to the strength of the C-F bond. The electronegativity of fluorine is the highest among all atoms, and its atomic radius is second only to hydrogen. Therefore, the C-F bond distance is short, the bond energy is large, and it is the most difficult bond to break (Figure 1). As a result, the stereostructure of the Tomoko Yasuda works in the Polymer Development Division, Basic Technology Development Department, Chemicals Company at AGC Co., Ltd. (10 Goi Kaigan, Ichihara City, Chiba Prefecture, 290-8566). She completed a major in Chemistry and Life Chemistry in the Graduate School of Advanced Science and Engineering at Waseda University in 2012. That same year, she joined Asahi Glass Co., Ltd. (now AGC Co., Ltd.) and has been with the company ever since. Her specialties are fluoroelastomer and fluororesin. Simeon Flitz; works in Polymer Development Division, Basic Technology Development Department, Chemicals Company at AGC Co., Ltd. (10 Goi Coast, Ichihara City, Chiba Prefecture, 290-8566). He completed a Ph.D. in Chemical and Pharmaceutical Engineering from the Singapore-MIT Alliance Program at the National University of Singapore in 2008. He joined Asahi Glass (now AGC Co., Ltd.) in 2018 and has been with the company ever since. His specialties are polymers, separation, electrochemistry, environment and energy, carbon dioxide storage, and biomaterials. 202 ( 24) Demystifying Properties of Fluoroelastomers and Their Current and Potential Future Magazines of the Japanese Rubber Association polymers with high chemical resistance even with low fluorine content, and allows control over their physical and chemical properties. Figure 1 Structure of fluorine polymer polymer, with its C-H or C-C bonds, is concealed by the presence of fluorine atoms, leading to the manifestation of excellent heat resistance and chemical stability.1) Fluoropolymers exhibit excellent water and oil repellency as well as insulating properties due to their low polarizability. Despite their high electronegativity and large polarity, the short bond distance results in low polarizability. This weakens intermolecular interactions and reduces the intermolecular cohesive energy, leading to the water and oil repellency characteristics. Moreover, the extremely low polarizability results in a low dielectric constant and high insulation properties. Table 1 presents the van der Waals radii, bond lengths with carbon, binding energies, and polarizabilities with carbon of each atom.1, 2) It is generally said that a high fluorine content in a polymer improves these properties, but it is also known that specific properties may manifest differently depending on the polymer's structure. For example, PTFE and ETFE resins have a completely symmetrical molecular structure where C-F polarizations cancel each other out, resulting in a zero dipole moment. As a result, they exhibit low dielectric constant and good electrical insulation properties. On the other hand, PVDF resin has an asymmetric structure that generates intramolecular dipoles and makes it a polar polymer. As a result, it dissolves and swells in polar organic solvents (Figure 2). On the other hand, PVDF resin, with its similar structure, possesses an asymmetric arrangement, leading to the generation of intramolecular dipoles and resulting in its polarity as a high-polarity polymer. In fluoropolymers, the properties are not determined solely by the fluorine content, but also significantly influenced by the structure. Thus, optimizing the type of monomer used and the polymer structure enables the creation of 3. Types, Physical Properties, and Applications of Fluoroelastomers Fluoroelastomers are a general term for rubbers that contain fluorine atoms in their molecules, and are the most heat-resistant and oil-resistant synthetic rubbers. There are three main types of commercially available fluoroelastomers: Fluoroelastomer based on vinylidene fluoride-hexafluoropropylene copolymer (FKM), Fluoroelastomer based on tetrafluoroethylene-propylene copolymer (FEPM), and Fluoroelastomer based on tetrafluoroethylene-perfluorovinyl ether copolymer (FFKM). These are classified based on the monomers used to form their structures. Fluoroelastomers are categorized as non-diene rubbers that do not contain double bonds, and possess superior heat resistance above 200C and excellent oil resistance, positioning them as high-performance materials (Figure 3).3) Due to their exceptional resistance to heat, oil, and chemicals, fluoroelastomers find extensive applications in various industries, including the chemical industry, machinery, resource excavation, semiconductor manufacturing equipment, automobiles, railways, and aviation. They are commonly utilized as materials for seals, hoses, wire coatings, and others. These exceptional properties enable their effective use in demanding operating conditions where other rubbers are not suitable. 3. 1 FKM FKM is the most representative type of fluoroelastomer and is a rubber that uses VdF (vinylidene fluoride) and HFP (hexafluoropropylene) as monomers. FKM is primarily classified into five categories. It is classified based on the constituent monomers and is divided into Type 1 to Type 5. The classification of FKM and its main product names are shown in Table 2. FKM was first commercialized as "Viton" by DuPont in the 1950s. The most commonly used Type 1 binary FKM is a copolymer of VdF (vinylidene fluoride) and HFP (hexafluoropropylene). The second most commonly used Type 2 ternary FKM is further copolymerized with TFE (tetrafluoroethylene). FKM possesses characteristics of fluoroelastomers such as heat, oil, and chemical resistance. It also has properties such as good processability and excellent compression permanent deformation. Table 1 Comparison of Fluorine and Other Atoms H F Cl Van der Waals radius () 1.20 1.35 1.80 C-X bond length () 1.09 1.32 1.77 Binding energy (kJ/mol) of C-X. Polarizability (10-24cm3) of C-X 416 487 323 0.66 0.68 2.58 ETFE PVDF Fig. 2 ETFE has a nonpolar structure, and PVDF has a polar structure Fig. 3 Rubber Positioning ( 203 ) 203 Volume 93 Number 6 (2020) Tomoko Yasuda and Simeon Fritz In particular, the Type 1 binary FKM has many grades of polyol cross-linking systems, which provide excellent processability. After Type 3, the materials are specialized for improving physical properties and use peroxide cross-linking as the main cross-linking method. Type 3 is copolymerized with PMVE (perfluoromethyl vinyl ether) to improve lowtemperature flexibility. Type 4 contains propylene, improving alkali and amine resistance. Type 5 also has high alkali and amine resistance, and was improved using a different approach than Type 4. These materials are selected based on the intended application. 3. 2 FEPM Tetrafluoroethylene-propylene (TFE-P) copolymer fluorinated elastomers belong to the category of FEPM. Due to differences in chemical structure, FEPM exhibits significantly different properties from the general-purpose fluoroelastomer FKM mentioned earlier, particularly in terms of base resistance, steam resistance, and insulation properties. Despite having a much lower fluorine content of around 56% compared to FKM, FEPM exhibits excellent properties due to its structure, in which the hydrocarbon monomer propylene is protected by adjacent TFE. Binary FEPM is composed of TFE and propylene, while ternary FEPM also contains VdF copolymerized with them (Type 4 of the aforementioned FKM). 1) History of developing FEPM Around 1970, when DuPont first released FKM and Daikin Industries, Ltd. began producing and selling FKM in Japan, Asahi Glass (now AGC) focused on developing fluorinated elastomers using TFE as a raw material. By copolymerizing propylene with TFE, the resulting polymer structure became non-polar and had unique characteristics that differed from FKM. This TFE-P copolymer fluorinated elastomer was commercialized as "AFLAS," and due to its unique properties, such as excellent chemical resistance and insulation properties that are 100 times better than FKM, it has been adopted in heat-resistant insulated wire components for transportation, industrial seal materials, and oil drilling-related components, among others. In this way, FEPM was differentiated from FKM based on differences in properties, and the two are still separated by their respective applications. 2) Mechanism of base resistance of FEPM FEPM is a fluoroelastomer with excellent base resistance. Figures 4 and 5 show the molecular structures of FEPM and FKM, respectively. FKM has VdF, and the -CF3 group derived from the neighboring HFP is a strong electron-withdrawing group, so that the H of VdF has a low electron density and a positive charge, and the de-HF reaction easily occurs in the presence of bases (especially amines). As a result, double bonds are formed in the polymer, and the main chain is attacked by chemicals and disintegrates, causing the so-called rubber curing and degradation phenomenon. In contrast, FEPM has an electron-donating -CH3 group in propylene, resulting in high electron density around the surrounding H atoms and a structure that does not undergo dehydrofluorination. The hydrogen atoms in propylene are protected by the fluorine atoms of TFE, making them resistant to attack by basic chemicals. Therefore, the presence or absence of the VdF structure is one of the indicators to determine the base resistance of fluoroelastomers. Among sealing materials, FEPM is commonly used in alkaline and amine environments because the structure containing VdF is not suitable for use in high alkaline and amine environments. Classification FKM Type 1 FKM Type 2 FKM Type 3 FKM Type 4 FKM Type 5 FEPM FFKM Table 2 Classification and composition of fluoroelastomer Constituent Monomers VdF/HFP VdF/HFP/TFE VdF/TFE/PMVE VdF/TFE/Propylene VdF/HFP/TFE/ PMVE/Ethylene TFE/Propylene TFE/PMVE Main Product Name Viton,Tecnoflon,DAI-EL Viton,Tecnoflon,DAI-EL Viton,Tecnoflon,DAI-EL AFLAS Tecnoflon AFLAS Kalrez, AFLAS FFKM 3.3 FFKM FFKM is a fully fluorinated elastomer made from copolymerization of tetrafluoroethylene and perfluoroalkyl vinyl ether (TFEPA-VE), with PMVE commonly used as the monomer within the PAVE class of monomers. Fluorine (F) in TEF protects the bond. CH3 F Does not undergo defluorination with amines.. Electron-donor property H F F F H F F C C C C C C Undergoes defluorination with amines. C Electron-donor property H F F CF3 H F F Bond is weak Fig.4 The mechanism by which FEPM does not decay even in a basic (amine) environment Fig.5 Structure and fluorine content of fluoroelastomer 204 ( 26) Demystifying Properties of Fluoroelastomers and Their Current and Potential Future Magazines of the Japanese Rubber Association 1) Development History of FFKM is being pursued to enhance low-temperature flexibility.5) It is expected to broaden The development of FFKM began with DuPont's introduction of Kalrez in the 1960s as a high-performance rubber that could withstand harsh environments where FKM could not. However, FFKM itself is highly the temperature range of its applications. Further development of materials that achieve all low-temperature resistance, heat resistance, and chemical resistance is anticipated in the future. expensive due to its complete fluorination, and processing it requires advanced technical expertise. As a result, DuPont marketed Kalrez in the form of molded products. Later, other companies developed FFKMs with improved processability, and sales of the polymer form began, resulting in increased processing flexibility. Asahi Glass introduced FFKM to the market in the 2000s, which is distinguished by its superior mechanical properties compared to other FFKMs achieved through increasing its molecular weight while maintaining processability. 2) Features and applications of FFKM 4. 3 Improvement in processability of FEPM (AGC's development trend) In 2015, In 2015, AFLAS 400E and AFLAS 600S, incorporating new cross-linking sites, were developed as new FEPMs (Table 3). These grades significantly improved the cross-linking speed, which had been a challenge in previous FEPMs, and also improved compressive permanent deformation and colorability (Figure 7). In AFLAS 600S, mold release properties were also significantly improved. This new product contributes to shortening the process time and improving yield in the processing stage, and has started to expand into new applications that were previously not applicable. In The absence of C-H bonds in the polymer results in excellent chemical resistance, particular, the improvement in cross-linking speed and the introduction of making it the rubber with the highest level of heat and chemical resistance. Since the new cross-linking sites allow for composition with other materials that were heat resistance of the polymer body is also high, it is possible to impart heat resistance to withstand an environment of 300C or more by selecting an previously difficult, specifically adhesion and lamination, and it is expected to expand the range of applications6). appropriate vulcanization system. FFKM is used in various applications that require high functionality, such as the chemical industry, resource drilling, and aerospace. Additionally, FFKM is widely used as packing material in semiconductor manufacturing equipment due to its high plasma resistance, and it is expected to continue expanding in the rapidly growing semiconductor marketplace. 4.4 FFKM with Improved Physical Properties (AGC's Development Trend) Development of FFKM is also progressing, and AGC is developing polymers and compounds by combining its proprietary small molecule synthesis and polymerization technologies. Recently, AGC has commercialized the PM-1200 grade of AFLAS FFKM, which has improved low-temperature flexibility and suppressed 4. Recent developments in fluoroelastomer hardness change at low temperatures while maintaining heat resistance (Table 4). 4.1 Improvement of amine resistance in FKM In around 2013, a new structure of fluoroelastomer, VdF[-HFO-1234yf] (2,3,3,3tetrafluoropropene), which improved amine resistance, was developed (Figure 6) 4). This is an example of utilizing HFO-1234yf monomer, which has never been used in commercially available fluoropolymers. The improvement of amine resistance, which is a weak point of FKM, is a merit as it is less likely to degrade in low concentration of amine. This mechanism is due to the fact that the electron density of the H in VdF is not as low as in general FKM, and it has a structure that makes it less likely to undergo dehydrofluorination. 4.2 Low-temperature Flexibility Improvement of FKM Fluoroelastomer is limited in its use in low-temperature environments due to its higher glass transition temperature (Tg) compared to other rubbers. Therefore, the development of low-temperature improved FKM, which incorporates MOVE (a perfluorovinyl ether monomer with a -O-CF2-O- structure), Table 3. Comparison between the conventional product AFLAS and the new products AFLAS 400E and 600S Grade 100S/H 150E 200P 150C 400E 600S Crosslinking system Peroxide Peroxide Peroxide Electron beam Electron beam, Peroxide Peroxide Applications/Features Seal for high- temperature and heavy-duty application Wire coating, tubing Low-temperature improvement (3-way FEPM:FKM Type 4) Wire coating High-speed crosslinking, extrusion-grade, colorable High-speed cross-linking, improved compression set and mold releasability, colorable Fig.6 Composition of amine-resistant rubber ( 205 ) Fig. 7 Comparison of cross-linking speeds between conventional AFLAS and new product 400E Table4 Conventional AFLAS FFKM and new products Grade PM-1100 PM-1200 PM-3000 Crosslinking system Peroxide Peroxide Peroxide Applications/Features Standard grade Reduced hardness changes in the low temperature range High heat resistance grade Therefore, the development of low-temperature improved FKM, which incorporates MOVE (a perfluorovinyl ether monomer with a -O-CF2-O- structure), is being pursued to enhance low-temperature flexibility. It is expected to broaden the temperature range of its applications. 205 Volume 93 Number 6 (2020) Tomoko Yasuda and Simeon Fritz It is expected that material development that leverages the strengths of each company will continue in the future. 6. Summary Fig. 8 Comparing FFKM Temperature-Based Hardness Changes The AFLAS FFKM PM-1200 grade is now being used in temperatures below room temperature, where it was difficult to use AFLAS FFKM PM-1100 (Figure 8). 5. Future Prospects of fluoroelastomer The applications of fluoroelastomer have become increasingly diverse, and there is a growing demand for various functions. As a result, development is expected to continue to meet these demands. For example, it is anticipated that development will focus on high-value-added materials with improved heat resistance and mechanical properties, as well as highly functional non-perfluorinated rubber materials that fall between FFKM and FKM or FEPM. Specialized materials aimed at specific functions are also expected to be developed. * * This paper has presented an overview of commercially available fluoroelastomer and recent development trends. As discussed, there are various types of fluoroelastomer within this category, and their rubber properties differ significantly d epending on their structure. Therefore, selecting the appropriate fluoroelastomer for a specific application is crucial. References 1) Yamabe, M. (ed.) , "Tokoton Yasashii Fusso no Hon [The Easy-toUnderstand Book on Fluorine]", Nikkan Kogyo Shimbunsha: Tokyo, 2012; pp. 12-13. 2) Satokawa, T. (ed.), "Fusso Jyushi Handobukku [Fluorine Resin Handbook]", Nikkan Kogyo Shimbunsha: Tokyo, 1990; pp. 625-626. 3) Drobny, J. G., "Fluoroelastomers Handbook, 2nd ed."; Elsevier, 2016; pp. 3-4. 4) Morikawa, T.; Washino, K; Morita, S.; Fukuoka, S.; Doi, M.; Yokotani, S.; Furuya, T.; Terada, J. "Kokai Tokkyo Koho [Japanese unexamined patent application publication, 2013-216915, 2013. 5) SOLVAY Homepage. https://www.solvay.jp/ja/binaries/TecnoflonFKM-Peroxide-Curable_JA-234347.pdf (accessed Feb 29, 2020). 6) AGC Homepage. https://www.agc.com/innovation/library/detailhtml/1198159_4283.html (accessed Feb 29, 2020). * * * 206 ( 26)