Document zQB6rn3B0mXe3gQM17aJyBenn

Fluorinated PA for Fluoropolymer Production Fluoropolymers are specialty performance materials that have a unique combination of performance properties. Fluoropolymers play a critical role in achieving important societal goals such as clean energy, electrification of vehicles and advancements in electronics making the continued use of fluoropolymers vital in numerous segments of the global economy. Fluoropolymers have, furthermore, been demonstrated to be safe for their intended uses and meet the criteria to be considered Polymers of Low Concern (PLC).1 Fluoropolymer synthesis consist of a multistep process, typically radical emulsion polymerization, that must be optimized, including choosing the correct polymerization aids (PA) to achieve the necessary quality of fluoropolymers, including fluoroelastomers, for high performance applications. The PA can affect may attributes of the polymerization and resulting produced fluoropolymer including unwanted chain termination leading to formation of unintentional fluorinated byproducts, molecular weight variations, changes in monomer incorporation and other attributes critical to quality. Table 1 list several key properties that need to be considered and critical to implementation of fluoropolymers emerging applications. Radical polymerization of fluoropolymers and fluororadical interactions have been well-documented highlighting the criticality of these intermediates when using perfluorinated monomers.2,3,4,5 For this reason, it is critical and necessary that the PA is stable under the production conditions used to produce much of the worlds many fluoropolymers, including fluoroelastomers, currently on the market. Table 1. Example of Critical Go/No-Go Polymer Property Criteria for Candidate development. PTFE Fine Powder Molecular Weight (proxy Standard Specific Gravity, SSG Melting Temperature Color Extrusion Pressure Melt Creep Viscosity Comonomer Content PTFE Dispersion Melting Temperature Foaming Skinning Particle Size Critical Cracking Thickness Internal Use Only It has been reported that the introduction of molecules or unintentional impurities containing hydrogen, chlorine, or bromine atoms will lead to chain transfer and/or chain termination (inhibition) during their use with fluororadicals.5 These heightened interactions are a result of the electron-withdrawing effects of fluorine, producing highly electrophilic carbon radicals.5 In fact, most organic, non-fluorinated, molecules even a low concentrations manifests as inhibition or chain transfer producing low molecular weight molecules or hydrocarbon grafted fluorinated residuals. Several fundamental studies have looked at the introduction of hydrogen containing molecules to fluorinated (macro)radicals systems.3,4,6 For example, DuPont/Chemours has published extensively on trying to map the fundamental activity of these fluororadical interactions.7,8 In addition, rate constants for hydrogen atom abstraction by model perfluoroalkyl radicals have been studied and compared to analogous non-fluorinated alkyl radicals by the Dolbier group.4 These same principles and modeled reactions would apply if nonfluorinated surfactants, hydrogen atom donors, are used as PA. Typically, non-fluorinated PAs would be expected to have numerous C-H bonds and as mentioned are prone to abstraction by highly reactive fluororadicals of the growing fluoropolymer chain.3 In one case, the hydrogen abstraction can occur between the PA and fluororadical yielding hydrogen-capped species and grafted fluorinated PA adducts. Hydrocarbon radicals are well-known to react with fluoromonomers.4,5,9 This polar low molecular weight adducts would be classified as PFAS and are the unique consequence of utilizing hydrogen donors like non-fluorinated PAs. The inhibition or termination caused by these side reactions has negative affects to polymer properties but also to overall polymer yield witnessed by reaction rate decreases. The use of stable, perfluorinated polymerization aids like HFPO-DA, owning to the stability of the C-F bond, inhibits attack by the growing free radicals, For PTFE polymerizations limits undesired side reactions like chain transfer-to-surfactant that occur when using a hydrocarbon or partially fluorinated processing aid. This deleterious side reaction leads to extractable residues and decreases polymer MW.10,11 Chain transfer or chain termination can also lead to slow reaction rates, comonomer incorporation issues and overall lack of control. Modifications to help overcome these deficiencies can produce higher oligomer of PFAS compound such as H-Capped Carboxylic Acids, Diacids, and H-Capped Sulfonic Acids (when certain sulfur species are present). Figure 1 (residual summary) and 2 (total ion chromatograph, TIC) illustrate the residual profiles for the baseline sample (HFPO-DA) and hydrocarbon surfactant. The data illustrated for sodium lauryl sulfate (SLS) in Figure 1 and 2 offers an example of the residuals and profiles observed when hydrocarbon polymerization aids are used in place of fully fluorinated aids. As observed in Figure 2, the increased number of peaks observed in the hydrocarbon TIC is attributed to several classes of PFAS compounds mentioned above. These profiles have been observed in a wide variety of NFS tested by Chemours. Internal Use Only Residue (ppb on polymer basis) H-(CF2)x-SO3H HOOC-(CF2)x-COOH H-(CF2)xCOOH PPA Related Fluorinated Figure 1. Residual Summary Hydrocarbon PA (SLS) Raw Dispersion 71,680 N/A N/A 1,884,000 HFPO-DA Raw Dispersion <LOQ 489 511 N/A Figure 2. Total Ion Chromatograph Many PTFE applications rely precise control of specific properties that are extensively on polymer fibrillation that is made possible with an ultra-high polymer molecular weight (Mn ~10^7).2 Figure 3 illustrates the molecular changes with the usage of a hydrocarbon PA as measured by the polymers melting temperature (DSC) with the higher peak melting temperature correlating to higher molecular weight and lower chain transfer. Fibrillation and mechanical strength enabled by higher molecular weights is needed to generate high flexural strength, tensile strength, impact strength and dielectric breakdown strength needed in critical applications. In addition, PTFE materials are also used in extreme environments, for example at temperatures above 300 degrees C. The chain transfer-to-surfactant side reaction can also lead to an incorporation of hydrocarbon into the polymer backbone that, when exposed to elevated temperatures, can degrade into carbonaceous color formers and defect points. PTFE materials made using HFPO-DA are not plagued by this decrease in performance. The use of non- Internal Use Only fluorinated surfactants containing abstractable hydrogen atoms leads to increased chain transfer and limits the critical properties mentioned above. Figure 3. Melting Temperature by DSC. For example, the control of the polymerization is critical to meet the requirements of low dielectric constant, low tangent loss, thermal and chemical stabilities in 5G type (semicon, high frequency data transmission technologies) applications. Fluoropolymers are second to none among all the polymeric materials and most fluoropolymers used in these applications today, made with HFPO-DA, need to be further treated with fluorinating agent to remove chain ends and low-level residuals. The conversion to C-F structures is required to meet the end-use applications commercially practiced today.12 Any presence of the undesired structures, especially intentionally added like non-fluorinated PAs, would deteriorate the properties. The use of NFS certainly introduces more than enough undesired chemical structures, including but not limited to the structures mentioned above to compromise the properties, especially the low tangent loss properties. The deteriorated properties cannot be reversed without more significant fluorination effort or at all. FEP has been tested with multiple partial fluorinated and non-fluorinated surfactants. The presence of anything beyond a very limited number of highly protected C-H bonds has consistently resulted in unacceptable levels of chain transfer that have negatively impacted polymerization rate, molecular weight, comonomer incorporation, color formation when melt processing, formation of residuals, and even the ability to initiate the reaction if present at the beginning of the reaction. Figure 4 illustrates the impact of hydrocarbon PA on FEP molecular weight and MIT flex life which are key properties that need to be met for successful application development. Based on the information gathered by Chemours, chain transfer due to hydrocarbon addition retards HFP/Vinyl Ether incorporation coupled with the tendency towards low Mw, cause by CT, Internal Use Only resulting in lower than required flex-life for critical applications. These changes brought by a NFS result in poor physical properties of the resulting product and impracticality of implementing in existing production facilities Figure 4. Impact of hydrocarbon on critical parameters for fluoropolymers. The impact of a hydrocarbon surfactant on PFA polymerization is like that for FEP, it results in increased and unacceptable levels of chain transfer. The increased chain transfer has significant negative implications on reaction rate, molecular weight, comonomer incorporation, color formation during melt processing, formation of residuals, and can even prevent a reaction from being initiated or stop it from proceeding depending on concentration. For many PFA applications, stress crack resistance is a critical property. The afore mentioned increase in chain transfer due to a hydrocarbon surfactant reduces stress crack resistance, as measured by MIT flex life. This reduction in stress crack resistance makes it inadequate for use in applications such as handling aggressive chemicals used in semiconductor processing fabs. PFA is currently widely Internal Use Only specified as the only material that can meet the needs in many applications in these fabs due to its high purity, chemical resistance, and thermal stability. A significant reduction in stress crack resistance would leave the semiconductor industry with no known material to meet the requirements in the most demanding applications. As described, the PA can affect may attributes of the polymerization and resulting produced fluoropolymer including unwanted chain termination leading to formation of unintentional fluorinated byproducts, molecular weight variations, changes in monomer incorporation and other attributes critical to quality. Therefore, a robust process for hydrocarbon selection must be implemented and include all aspects of the process from initiation to residual formation to final properties of the resulting materials. It has been illustrated, that addition of hydrocarbon containing molecules can alter the properties and it is critical and necessary that the PA is stable under the production conditions used to produce much of the worlds many fluoropolymers, including fluoroelastomers, currently on the market. References: 1) Henry, B. J.; Carlin, J. P.; Hammerschmidt, J. A.; Buck, R. C.; Buxton, L. W.; Fiedler, H.; Seed, J.; Hernandez, O. A critical review of the application of polymer of low concern and regulatory criteria to fluoropolymers. Integrated Environmental Assessment and Management 2018, 14 (3), 316-334. DOI: 10.1002/ieam.4035. 2) Ebnesajjad, S. Fluoroplastics Volume 2: Melt Processible Fluoropolymers: Chapter 8 Polymerization and Finishing Melt-Processible Fluoropolymers. ; Elsevier, 2016. 3) US Patent #6642186-B2. 2003. 4) Zhang, L.; Cradlebaugh, J.; Litwinienko, G.; Smart, B. E.; Ingold, K. U.; Dolbier, J. W. R. Absolute rate constants for some hydrogen atom abstraction reactions by a primary fluoroalkyl radical in waterElectronic supplementary information (ESI) available: Tables of kinetic data and plots of kinetic data. See http://www.rsc.org/suppdata/ob/b3/b313757k. Organic & Biomolecular Chemistry 2004, 2 (5). DOI: 10.1039/b313757k. 5) Puts, G. J.; Crouse, P.; Ameduri, B. M. Polytetrafluoroethylene: Synthesis and Characterization of the Original Extreme Polymer. Chemical Reviews 2019, 119 (3), 17631805. DOI: 10.1021/acs.chemrev.8b00458. 6) Brinker, K. C.; Bro, M. I. (assigned to E.I. du Pont deNemours and Company) Process for polymerizing perfluorinatedmonomers. GB805115, 1958. Internal Use Only 7) Feiring, A. E.; Krespan, C. G.; Resnick, P. R.; Smart, B. E.;Treat, T. A.; Wheland, R. C. (assigned to E.I. du Pont de Nemoursand Company) Hydrofluorocarbon solvents forfluoromonomerpolymerization. US5182342, 1993. 8) Carlson, D. P. (assigned to E.I. du Pont de Nemours andCompany) Tough stable tetrafluoroethylene-fluoroalkyl perfluorovin-yl ether copolymers. US3642742, 1972. 9) Puts, G.; Venner, V.; Amduri, B.; Crouse, P. Conventional and RAFT Copolymerization of Tetrafluoroethylene with Isobutyl Vinyl Ether. Macromolecules 2018, 51 (17), 6724-6739. DOI: 10.1021/acs.macromol.8b01286. 10) Benning, A. F. (assigned to E.I. du Pont de Nemours) Fluorinated aliphatic phosphates as emulsifying agents for aqueous polymerizations. US2559749, 1951. 11) Punderson, J. O. (assigned to E.I. du Pont de Nemours and Company) Polymerization Process. US3391099, 1968. 12) Patent Appilication #WO2023063339A1. Internal Use Only