Shear stable aqueous fluoropolymer dispersions

Abstract

The present invention relates to shear stable aqueous fluoropolymer suspensions, preferably polyvinylidene fluoride (PVDF), having a particle size of less than 120 nm, a solids content of greater than 25% by weight, a melt viscosity of greater than 20 kP, a shear stability at 1500 rpm of greater than 20 minutes, and to a process for preparing said fluoropolymer suspensions.

Description

[0001] Statement regarding federally funded research or development

[0002] This application was completed with government support under DE-EE0009106 granted by the Department of Energy. The government owns certain rights to this invention. Invention Field

[0003] This invention relates to shear-stable aqueous fluoropolymer dispersions and methods for preparing such dispersions.

[0004] background:

[0005] Fluorocarbon polymers have been widely used in recent years. For example, polyvinylidene fluoride (PVDF), as a homopolymer, copolymer, or alloy, is a melt-processable resin that is shaped into polymer structures through various processes, such as extrusion, injection molding, fiber spinning, extrusion blow molding, and blown film forming. Fluorocarbon polymers are also used as polymer processing aids due to their low surface energy and phase behavior. Most fluorocarbon polymers are prepared via emulsion polymerization followed by energy-intensive separation steps; therefore, they have a large carbon footprint.

[0006] Fluorocarbon polymers, especially PVDF, are increasingly important as binders in lithium-ion battery electrodes in the renewable energy sector due to their excellent electrochemical resistance, superior adhesion, and flexibility. PVDF has been found to be a useful binder for forming electrodes used in non-aqueous electrolysis devices. U.S. Patent Nos. 5,776,637, 6,200,703, and 9,434,797 (all incorporated herein by reference) describe PVDF binder solutions in organic solvents and powdered electrode materials for forming electrodes used in non-aqueous batteries. Conventional solvent casting processes use large amounts of N-methyl-2-pyrrolidone (NMP) solvent as a dispersion medium. Driven by environmental and safety concerns, there is a need to produce high-quality electrodes without using large amounts of organic solvents.

[0007] Meanwhile, fluorocarbon polymers have become increasingly important as external coatings due to their excellent resistance to UV-A and UV-B radiation and many corrosive chemicals. Applying PVDF-based coatings, which typically involve high levels of organic solvents, requires these solvents to dissolve acrylics and disperse the PVDF. These organic solvents often pose safety, health, and environmental hazards and are therefore frequently regulated. Organic solvents are generally toxic and flammable, necessitating special production controls to mitigate risks and reduce environmental pollution from them. Furthermore, a significant carbon footprint is associated with the use of solvent-based coatings, which is environmentally unfriendly.

[0008] The polymerization kinetics of fluorinated monomers differ significantly from those of non-fluorinated monomers. Therefore, fluorinated emulsifiers have traditionally been used in the processing of fluorinated emulsions. Fluorinated emulsifiers (often referred to as "permanent chemicals") are environmentally persistent and are prohibited in many jurisdictions. Non-fluorinated emulsifiers are known for use in the polymerization of fluoropolymers. However, they do not provide dispersions with the shear stability offered by this invention.

[0009] Aqueous dispersions of fluoropolymers are highly unstable, tending to solidify under shear and undergo hard settling during storage. The main difference between fluoropolymers and acrylic dispersions stems from the high specific gravity of fluoropolymers, which varies with their crystallinity; for example, PVDF has a specific gravity ranging from 1.74 to 2.00 g / cm³. 3 The specific gravity of PTFE varies between 2.00 and 2.35 g / cm³. 3 The temperature varies between these two values. Therefore, due to their specific gravity, fluoropolymer dispersions tend to settle rapidly. Furthermore, the amorphous phases of fluoropolymers have low glass transition temperatures (Tg); for example, PVDF and PTFE have Tgs of -40°C and -73°C, respectively. Consequently, when fluoropolymer particles in an aqueous dispersion collide with each other due to Brownian motion or shear mixing, these particles tend to agglomerate, eventually leading to dispersion coagulation and rendering it unusable.

[0010] There is a need for more shear-stable aqueous dispersions of fluoropolymers, which can be used to formulate or prepare coatings and membranes from water-based formulations that are more environmentally friendly and sustainable while maintaining the excellent performance of solution-cast coatings and membranes. Therefore, there is a need to improve the shelf-stability and shear stability of aqueous fluoropolymer dispersions so that they can be successfully used in water-based applications. Summary of the Invention

[0011] This invention relates to aqueous fluoropolymer dispersions, comprising:

[0012] a. Fluoropolymers with an average particle size of less than 120 nm,

[0013] b. Solid content greater than 25% by weight

[0014] c. Melt viscosity greater than 20 kPa, and

[0015] d. Shear stability at 1500 rpm is greater than 20 minutes.

[0016] This invention also relates to an emulsion method for preparing the fluoropolymer dispersion. The amounts of emulsifier, stabilizer, and initiator used during polymerization are all important for obtaining a stable fluoropolymer dispersion. This invention provides a stable fluoropolymer dispersion, and also provides a method for synthesizing the stable fluoropolymer dispersion in an aqueous reaction medium, comprising: a) forming an aqueous emulsion comprising at least two nonionic emulsifiers and at least one fluorinated monomer; b) initiating polymerization of the fluorinated monomer using a desired initiator; and c) continuously feeding at least one stabilizer after polymerization has begun, during the reaction, or after polymerization has ended. This method uses at least two emulsifiers, based on ethylene glycol esters of acrylic acid and nonionic block copolymers containing at least two blocks selected from polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. A stabilizer is also used to produce the aqueous fluoropolymer dispersion of this invention.

[0017] The aqueous fluoropolymer dispersion can be used directly in sustainable processes without separation. Products made with aqueous fluoropolymers are more environmentally friendly and sustainable than similar organic solvent-based processes; that is, to manufacture electrodes for non-aqueous electrochemical devices, such as batteries and double-layer capacitors, or coatings on different substrates.

[0018] The fluoropolymer dispersions of the present invention do not contain fluorinated surfactants, which are commonly referred to as "permanent chemicals". Detailed Implementation

[0019] Aspect 1 of the present invention provides an aqueous fluoropolymer dispersion comprising a fluoropolymer.

[0020] The fluoropolymer described herein has an average particle size of less than 120 nm, and is subjected to a temperature of 230°C and a time of 100 seconds. -1 The melt viscosity is at least 20 kPa or higher.

[0021] The dispersion has a solids content of at least 25% by weight and a Brookfield viscosity of less than 1000 cP after shearing at 1500 rpm for at least 20 minutes.

[0022] Embodiment 2 is an aqueous fluoropolymer dispersion as described in Embodiment 1, wherein the fluoropolymer contains at least 60% by weight of VDF monomer units, preferably at least 70% by weight of VDF monomer units.

[0023] Embodiment 3 is an aqueous fluoropolymer dispersion as described in Embodiment 1 or 2, wherein the fluoropolymer contains at least a comonomer selected from the following: HFP, CTFE, PMVE, PPVE, TFE, 2,3,3,3-tetrafluoropropylene, monomer units, preferably HFP.

[0024] Embodiment 4 is an aqueous fluoropolymer dispersion as described in any one of Embodiments 1 to 3, wherein the average particle size is less than 100 nm.

[0025] Embodiment 5 is an aqueous fluoropolymer dispersion as described in any one of Embodiments 1 to 4, comprising (1) an acrylic-based ethylene glycol ester, preferably, the acrylic-based ethylene glycol ester comprising at least one of polyethylene glycol acrylate (PEGA), polyethylene glycol methacrylate (PEGMA), polypropylene glycol acrylate (PPGA), or polypropylene glycol methacrylate (PPGMA); (2) a nonionic block copolymer emulsifier comprising at least two blocks of polyethylene glycol, polypropylene glycol, and / or polytetramethylene glycol; and (3) a stabilizer.

[0026] Embodiment 6 is an aqueous fluoropolymer dispersion as described in Embodiment 5, wherein the fluoropolymer comprises polyvinylidene fluoride having at least 60% by weight of VDF monomer units, and the acrylic-based ethylene glycol ester is selected from: polyethylene glycol acrylate (PEGA), polyethylene glycol methacrylate (PEGMA), polypropylene glycol acrylate (PPGA), and polypropylene glycol methacrylate (PPGMA); and the nonionic block copolymer emulsifier comprises at least one polyethylene glycol block and at least one polypropylene glycol block.

[0027] Embodiment 7 is an aqueous fluoropolymer dispersion as described in any one of Embodiments 1 to 6, wherein the fluoropolymer comprises at least one non-fluorinated monomer having at least one reactive double bond and an ionic moiety.

[0028] Embodiment 8 is an aqueous fluoropolymer dispersion as described in any one of Embodiments 5 to 7, further comprising a stabilizer, wherein the stabilizer comprises a non-fluorinated oligomer, the non-fluorinated oligomer comprising at least one ionic portion, preferably an acidic portion.

[0029] Embodiment 9 is an aqueous fluoropolymer dispersion as described in any one of Embodiments 5 to 8, further comprising a stabilizer, wherein the stabilizer comprises at least one of an alkyl sulfate or an alkyl sulfonate, and wherein the alkyl group is a C6 to C18 alkyl group.

[0030] Embodiment 10 provides a method for preparing a shear-stable aqueous fluoropolymer dispersion with a particle size of less than 120 nm, comprising:

[0031] (a) An aqueous reaction medium, at least one fluorinated monomer, at least one acrylic-based ethylene glycol ester, and at least one nonionic emulsifier are provided in a reaction vessel, said nonionic emulsifier having at least two blocks selected from polyethylene glycol, polypropylene glycol, and / or polytetramethylene glycol, each block having 2 to 200 repeating units.

[0032] (b) The fluorinated monomer in (a) is polymerized in the presence of at least one initiator, wherein the amount of initiator added to the polymerization is at least 1500 ppm, based on the weight of the fluorinated monomer added to the polymerization.

[0033] (c) Add at least one stabilizer to the fluoropolymer dispersion during or after polymerization.

[0034] Embodiment 11 is the method as described in Embodiment 10, wherein the fluorinated monomer comprises vinylidene fluoride monomer.

[0035] Embodiment 12 is the method as described in Embodiment 10 or 11, wherein the diol-based acrylic emulsifier comprises at least one of polyethylene glycol acrylate (PEGA), polyethylene glycol methacrylate (PEGMA), polypropylene glycol acrylate (PPGA), and polypropylene glycol methacrylate (PPGMA).

[0036] Embodiment 13 is the method as described in any one of Embodiments 10 to 12, wherein the amount of the diol-based acrylic emulsifier added to the polymerization is 0.05 to 5% by weight, preferably 0.1 to 2% by weight, based on the total fluorinated monomers added to the polymerization.

[0037] Embodiment 14 is a method as described in any one of Embodiments 10 to 13, wherein the stabilizer comprises at least one of alkyl sulfate or alkyl sulfonate.

[0038] Embodiment 15 is a method as described in any one of Embodiments 10 to 13, wherein the stabilizer is selected from: C6-C18 alkyl sulfonates, C6-C18 alkyl sulfates, C6-C18 alkyl disulfonates, C6-C18 alkyl pyrosulfonates, and mixtures thereof.

[0039] Embodiment 16 is the method as described in Embodiment 14, wherein a stabilizer comprising at least one of alkyl sulfates or alkyl sulfonates is added after polymerization is completed or after at least 50% by weight of the fluorinated monomer has been fed into the reactor, preferably after at least 75% by weight of the fluorinated monomer has been fed into the reactor.

[0040] Embodiment 17 is the method as described in Embodiment 14, wherein at least one alkyl sulfate or alkyl sulfonate stabilizer is added in an amount of 0.1 to about 5% by weight, based on the total weight of the fluorinated monomers added to the polymerization.

[0041] Embodiment 18 is a method as described in any one of Embodiments 10 to 13, wherein the stabilizer comprises an oligomer containing an ionic monomer unit having an acid group, preferably a carboxylic acid group.

[0042] Embodiment 19 is the method as described in any one of Embodiments 10 to 13, wherein the stabilizer is selected from: oligomers of polyacrylic acid, oligomers of poly(meth)acrylic acid, oligomers containing maleic acid monomer units, and co-oligomers of any one or combinations thereof.

[0043] Embodiment 20 is the method as described in Embodiment 18, wherein the stabilizer is added after at least 30% by weight of the fluorinated monomer has been fed into the reactor, preferably after at least 50% by weight of the fluorinated monomer has been fed into the reactor.

[0044] Embodiment 21 is the method as described in Embodiment 18, wherein a stabilizer for the oligomer is added in an amount of 0.1 to about 2.5% by weight, and the oligomer contains ionic monomer units having acid groups based on the total fluorinated monomers added to the polymerization.

[0045] Embodiment 22 is a method as described in any one of Embodiments 10 to 13, wherein the stabilizer comprises a nonfluorinated monomer having at least one reactive carbon-carbon double bond and an ionic moiety, preferably the ionic moiety being an acid moiety or a salt thereof.

[0046] Embodiment 23 is the method as described in any one of Embodiments 10 to 13, wherein the stabilizer is selected from: acrylic acid, methacrylic acid, styrene sulfonate, alkyl methacrylate phosphate, 2-acrylamide-2-methylpropanesulfonic acid, β-carboxyethyl acrylate, and combinations thereof.

[0047] Embodiment 24 is the method as described in Embodiment 22, wherein during polymerization, preferably after at least 15% by weight of the fluorinated monomer has been fed into the reactor, preferably after at least 30% by weight of the fluorinated monomer has been fed into the reactor, a stabilizer comprising a non-fluorinated monomer having at least one reactive carbon-carbon double bond and an ionic moiety is added.

[0048] Embodiment 25 is the method as described in Embodiment 22, wherein a stabilizer of a nonfluorinated monomer having at least one reactive carbon-carbon double bond and an ionic moiety is added in an amount of 0.1 to about 1.5% by weight, based on the total monomers added to the polymerization.

[0049] In embodiments of the method, the stabilizer may comprise one or more of the following: an alkyl sulfate or alkyl sulfonate, an oligomer comprising an ionic monomer having an acid group, a nonfluorinated monomer having at least one reactive carbon-carbon double bond and an ionic moiety, or a combination thereof.

[0050] Another embodiment of the present invention is an aqueous fluoropolymer dispersion comprising (1) a vinylidene fluoride polymer having at least 60% by weight of vinylidene fluoride monomer units; (2) an acrylate-based ethylene glycol ester, preferably comprising at least one of polyethylene glycol acrylate (PEGA), polyethylene glycol methacrylate (PEGMA), polypropylene glycol acrylate (PPGA), or polypropylene glycol methacrylate (PPGMA); (3) a nonionic block copolymer emulsifier comprising at least two blocks of polyethylene glycol, polypropylene glycol, and / or polytetramethylene glycol; and (4) a stabilizer.

[0051] The fluoropolymer described herein has an average particle size of less than 120 nm, and is subjected to a temperature of 230°C and a time of 100 seconds. -1 The melt viscosity below is at least 20 kPa or higher, and

[0052] The dispersion has a solids content of at least 25% by weight and a Brookfield viscosity of less than 1000 cP after shearing at 1500 rpm for at least 20 minutes. The stabilizer may comprise a non-fluorinated oligomer containing at least one ionic moiety, preferably an acid moiety, or the stabilizer may comprise at least one of an alkyl sulfate or an alkyl sulfonate, wherein the alkyl group is C6-C18. The fluoropolymer may comprise at least one fluorinated monomer having at least one reactive double bond and an ionic moiety. Detailed Implementation

[0053] All references cited in this application are included in this paper by way of citation.

[0054] As used herein, percentages are weight percentages (wt%) unless otherwise stated, and molecular weights are weight-average molecular weights (Mw) unless otherwise stated. Amounts expressed in "ppm" are based on weight. Molecular weights were determined by gel permeation chromatography (GPC) using PMMA (polymethyl methacrylate) standards.

[0055] The melt viscosity was determined according to ASTM D3835 using a capillary rheometer at 230°C for 100 seconds. -1 and 4 seconds -1 The measurements were taken at the shear rate.

[0056] The term "fluoropolymer" refers to polymers and copolymers containing at least 50 mol% fluorinated monomer units (including polymers having two or more different monomers, including, for example, terpolymers). These copolymers may be homogeneous, heterogeneous, or random, and may have a gradient distribution of comonomer units.

[0057] "Copolymer" is used to refer to polymers having two or more different monomer units, including terpolymers and more complex polymers. "Polymer" can refer to both homopolymers and copolymers.

[0058] “PVDF” means polyvinylidene fluoride, which includes both homopolymers and copolymers unless otherwise stated. “VDF” means vinylidene fluoride.

[0059] A dispersion is a polymer particle dispersion or suspension in water. It does not contain an oil phase or other organic phase.

[0060] Solids content refers to the amount of substance retained after drying of the dispersion. The solids content of aqueous dispersions was measured by gravimetric method using an HG63 moisture analyzer from Mettler Toledo.

[0061] This invention provides a shear-stable fluoropolymer dispersion with a particle size less than 120 nm, a solids content greater than 25%, and a melt viscosity greater than 20 kPa. The aqueous fluoropolymer dispersion of this invention is suitable for direct use in applications such as coatings on various substrates or casting aqueous electrodes for lithium-ion batteries, without the use of any fluorinated surfactants. Therefore, the fluoropolymer dispersion of this invention is prepared using a combination of particle size, suitable emulsifiers, and stabilizers (described in detail below).

[0062] The present invention also provides a method for preparing the fluoropolymer dispersion of the present invention. A general method for synthesizing the stable fluoropolymer of the present invention includes: a) forming an aqueous emulsion comprising at least one nonionic diol-based acrylic emulsifier, at least one nonionic emulsifier, and at least one fluorinated monomer; b) initiating polymerization of the fluorinated monomer using a persulfate initiator; and c) feeding at least one stabilizer simultaneously during the reaction after polymerization has begun, or adding a stabilizer after polymerization has ended.

[0063] Aggregation process

[0064] The polymerization reaction according to the invention can be carried out by loading a reactor with water (preferably deionized water), at least one nonionic glycol-based acrylic emulsifier, at least one nonionic emulsifier (different from the glycol-based acrylic emulsifier), at least one fluorinated monomer, and optionally a chain transfer agent and / or an antifouling agent. Air can be purged from the reactor before introducing the fluorinated monomer. Water is typically added to the reactor before it reaches the desired initial temperature, but other materials may be added before or after the reactor reaches the temperature. At least one persulfate radical initiator is added to initiate and maintain the polymerization. Additional monomers may be optionally added to replenish consumed monomers, and other materials may be optionally added during the polymerization process to maintain the reaction and control the properties of the final product. Stabilizers are typically added during the polymerization process or after polymerization is complete.

[0065] Fluorine monomers

[0066] Fluorinated monomers are used to obtain the fluorinated polymers of the present invention. As used in this invention, "fluorinated monomer" refers to a fluorinated monomer having an unsaturated carbon-carbon double bond capable of undergoing free radical polymerization.

[0067] Preferably, the fluoropolymer of the present invention comprises a vinylidene fluoride polymer having at least 60% VDF units, preferably 70% VDF units.

[0068] Vinylidene fluoride copolymers comprise copolymers containing at least 60% by weight vinylidene fluoride copolymerized with at least one comonomer. Fluorinated comonomers may be selected from: tetrafluoroethylene (TFE), trifluoroethylene (TrFE), chlorotrifluoroethylene (CTFE), perfluorobutylethylene (PFBE), hexafluoropropylene (HFP), vinyl fluoride (VF), pentafluoropropylene, 2,3,3,3-tetrafluoropropylene, trifluoropropylene, fluorinated (alkyl) vinyl ethers, such as perfluoroethyl vinyl ether (PEVE) and perfluoro-2-propoxypropyl vinyl ether, perfluoromethyl vinyl ether (PMVE), perfluoropropyl vinyl ether (PPVE), perfluorobutyl vinyl ether (PBVE), long-chain perfluorinated vinyl ethers, one or more partially fluorinated or Fully fluorinated α-olefins, such as 3,3,3-trifluoro-1-propene, 2-trifluoromethyl-3,3,3-trifluoropropene, 1,2,3,3,3-pentafluoropropene, 3,3,3,4,4-pentafluoro-1-butene, hexafluoroisobutylene (HFIB), fluorinated dioxolane, such as perfluoro(1,3-dioxolane) and perfluoro(2,2-dimethyl-1,3-dioxolane) (PDD), partially fluorinated or perfluorinated C4 and above α-olefins, partially fluorinated or perfluorinated C3 and above cyclic olefins, partially fluorinated or perfluorinated allyl monomers or fluorinated allyl monomers, and combinations thereof.

[0069] Other monomer units in these polymers may include any monomer containing polymerizable C=C double bonds that can copolymerize with VDF monomers. Additional monomers may be 2-hydroxyethyl allyl ether, 3-allyloxypropanediol, allyl monomers, ethane, propylene, acrylic acid, or methacrylic acid.

[0070] Preferred fluorinated comonomers are tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, 2,3,3,3-tetrafluoropropylene, fluoroethylene, pentafluoropropylene, perfluoromethyl vinyl ether, and perfluoropropyl vinyl ether. Hexafluoropropylene is the most preferred.

[0071] emulsifier

[0072] Emulsifiers are used in this invention.

[0073] The emulsifiers used in this invention include, but are not limited to, nonionic block copolymers comprising at least two blocks selected from polyethylene glycol, polypropylene glycol, and polytetramethylene glycol, wherein each block has 2 to 200 repeating units, preferably 5 to 100. These emulsifiers do not have any carbon-carbon double bonds capable of undergoing free radical polymerization. These emulsifiers do not have any acrylate or methacrylate groups. The end groups in these block copolymers are preferably selected from hydrogen, hydroxyl, carboxyl, ester, ether, and / or hydrocarbon groups. Examples of block copolymers are polyethylene oxide (PEO) and polypropylene oxide (PPO), arranged in an AB (diblock) or ABA triblock structure, with each block having 2 to 200 repeating units, preferably 5 to 100. Particularly preferred emulsifiers in this group contain blocks of PEG and PPG, such as poly(propylene glycol)-block-poly(ethylene glycol)PPO-PEO, poly(propylene glycol)-block-poly(ethylene glycol)-block-poly(propylene glycol) "PPO-PEO-PPO", or block-poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) "PEO-PPO-PEO". These nonionic block emulsifiers do not contain any reactive double bonds; therefore, they cannot undergo free radical polymerization.

[0074] The second group of non-fluorinated, nonionic emulsifiers that can be used in this invention includes, but is not limited to, acrylate-based ethylene glycol esters containing unsaturated carbon-carbon double bonds capable of free radical polymerization, preferably acrylate or methacrylate groups, and segments of polyethylene glycol (PEG), polypropylene glycol (PPG), polytetramethylene glycol (PTMG), or combinations thereof, with repeating units preferably between 3 and 100, more preferably between 3 and 50. Examples of acrylate-based ethylene glycol esters used in this invention include, but are not limited to, polyethylene glycol acrylate (PEGA), polyethylene glycol methacrylate (PEG-MA), polypropylene glycol acrylate (PPGA), polypropylene glycol acrylate (PPGA), and polypropylene glycol methacrylate (PPGMA).

[0075] The nonionic acrylic-based glycol ester emulsifier of the present invention should have a chemical structure that has properties such as water solubility and produces a fluoropolymer dispersion with a particle size of less than 120 nm.

[0076] The total amount of emulsifier used is 100 ppm to 5% by weight, preferably 100 ppm to 3% by weight, more preferably 1000 ppm to 1% by weight, based on the total fluorinated monomers added to the polymerization.

[0077] During the polymerization process, the nonionic emulsifier of the present invention can be added entirely before polymerization, continuously fed during polymerization, or partially fed before polymerization and then fed during polymerization.

[0078] Initiator

[0079] The terms "initiator" and "radical initiator" and "free radical initiator" refer to chemicals capable of providing a source of free radicals, which can be spontaneously induced or induced by exposure to heat or light. The amount of initiator added to the reaction mixture (in ppm, based on the total weight of the fluorinated monomers added to the polymerization) can be from 1500 ppm to 1.0 wt%, preferably from 2000 ppm to 1.0 wt%. The free radical initiator may contain persulfates, such as sodium persulfate, potassium persulfate, lithium persulfate, or ammonium persulfate. The amount of persulfate added to the reaction mixture (based on the total weight of the monomers added to the reaction mixture) can, for example, be from about 1500 to about 1.0 wt%, preferably from 2000 ppm to 1.0 wt%. The terms "radical" and "free radical" refer to chemical substances containing at least one unpaired electron. The free radical initiator is added to the reaction mixture in an amount sufficient to initiate and maintain the polymerization rate. The order of addition can vary depending on the desired process and the characteristics of the dispersion emulsion. In some embodiments of the invention, the initiating system does not contain a reducing agent, and most preferably, no reducing agent is used.

[0080] The free radical initiator may comprise a redox system. Those skilled in the art will understand that a "redox system" refers to a system comprising an oxidant, a reductant, and optionally a promoter as an electron transport medium. Oxidants include, for example, persulfates, peroxides, and oxidizing metal salts (e.g., ferric sulfate). Reductants include, for example, sodium formaldehyde sulfoxylate, sodium sulfite, potassium sulfite, ascorbic acid, bisulfite, metabisulfite, and reducing metal salts. The promoter is a component of the redox system and is capable of reacting with both the oxidant and reductant under different oxidation states, thereby accelerating the overall reaction. Promoters include, for example, transition metal salts, such as ferrous sulfate. In the redox system, the oxidant and reductant may be used in amounts from about 0.01 to about 0.5% by weight, based on the total fluorinated monomers added to the polymerization. Optional promoters may be used in amounts from about 0.005 to about 0.025% by weight, based on the total fluorinated monomers. The redox system is described, for example, in GS Misra and UDN Bajpai, Prag. Polym. Sci., 1982, 8(1-2), pp. 61-131.

[0081] stabilizer

[0082] The term "stabilizer" refers to a class of molecules or oligomers (low molecular weight water-soluble polymers with a molecular weight less than 10,000 g / mol) that provide stability to particles of fluoropolymers (e.g., PVDF) in aqueous media and prevent aggregation or flocculation of fluoropolymer particles during shearing or vigorous mixing. Unexpectedly, we have found that the shear stability of the fluoropolymer dispersion is significantly improved when the stabilizer of the present invention is introduced into the dispersion. The stabilizer of the present invention is described below.

[0083] For ease of handling, stabilizers can be used in solutions such as aqueous solutions.

[0084] The first type of stabilizer is a molecule possessing both hydrophobic and hydrophilic portions, which enables it to stabilize and disperse hydrophobic molecules and aggregates of hydrophobic molecules in aqueous media. Examples of stabilizers include alkyl or aryl groups linked to sulfonic acid, sulfate, phosphonic acid, phosphate, or carboxylic acid groups. The acid groups are usually in salt form.

[0085] The preferred group of first-class stabilizers for the fluoropolymer dispersions of the present invention includes alkyl sulfate and alkyl sulfonate stabilizers. The terms "alkyl sulfate stabilizer" or "alkyl sulfonate stabilizer" refer to stabilizers having an alkane group as its hydrophobic portion, preferably containing an alkane group of C6 to C18 and an alkyl sulfate or alkyl sulfonate group as its hydrophilic portion. This alkane group is free of any fluorine. Preferred first-class stabilizers are in salt form, preferably having an alkali metal (i.e., lithium, sodium, or potassium), an ammonium ion, or an alkyl-substituted ammonium ion as a counter ion.

[0086] Examples of alkyl sulfate stabilizers used in this invention include, but are not limited to, ammonium, lithium, sodium, or potassium salts of alkyl sulfates. Examples of alkyl sulfate stabilizers include, but are not limited to, C6 to C18 alkyl sulfates, such as dodecyl sulfate and octyl sulfate. Examples include, but are not limited to, sodium dodecyl sulfate (SLS), potassium dodecyl sulfate (KLS), ammonium dodecyl sulfate, lithium dodecyl sulfate (ALS), sodium laureth sulfate, sodium octyl sulfate, potassium octyl sulfate, ammonium octyl sulfate, lithium octyl sulfate, and mixtures thereof.

[0087] Examples of alkyl sulfonate surfactants include, but are not limited to, C6-C18 alkyl sulfonates and C6-C18 alkyl disulfonates, and mixtures thereof. Typical counterions of alkyl sulfonate surfactants include, but are not limited to, sodium, potassium, lithium, ammonium, or alkyl-substituted ammonium. For example, C8-C12 alkyl sulfonates can be used, such as octyl sulfonates, octyl disulfonates, decyl sulfonates, decyl disulfonates, dodecyl sulfonates, dodecyl disulfonates, and combinations thereof. For example, octyl sulfonates can be sodium octyl sulfonate, potassium octyl sulfonate, ammonium octyl sulfonate, alkyl-substituted ammonium octyl sulfonate, and lithium octyl sulfonate. For example, sodium octyl sulfonate (SOS) is a stabilizer used in this invention.

[0088] Alkyl sulfate and alkyl sulfonate stabilizers are used in amounts from about 0.01 to about 10% by weight, based on the total fluorinated monomers added to the polymerization. Preferably, they are used in amounts from about 0.1 to about 5% by weight, more preferably from about 0.1 to 2.5% by weight, based on the total fluorinated monomers. These stabilizers are added after polymerization is complete or after at least 50% by weight of the fluorinated monomers have been fed into the reactor, preferably after at least 75% by weight of the fluorinated monomers have been fed into the reactor.

[0089] The second type of stabilizer is an oligomer comprising at least one repeating monomer unit having at least one ionic moiety, preferably an acid moiety. The oligomer is prepared in the same manner as the polymer using the monomer unit, but the oligomer chain length is significantly shorter (less than 200). Therefore, it exhibits higher water solubility than similar polymers and readily adsorbs onto the surface of fluoropolymer particles. The oligomers of the present invention have a chain length of less than 200 repeating monomer units. Examples of this group of stabilizers include, but are not limited to, oligomers of polyacrylic acid, oligomers of polymethacrylic acid, oligomers of maleic acid, and copolymers thereof. The second type of stabilizer is used in an amount of about 0.1 to about 5% by weight, based on the total monomers. Preferably, it is used in an amount of about 0.2 to about 2% by weight, based on the weight of the total fluorinated monomers added to the polymerization. Preferably, the second type of stabilizer is added after at least 30% by weight of the fluorinated monomers have been fed into the reactor, more preferably after at least 50% by weight of the fluorinated monomers have been fed into the reactor.

[0090] The third type of stabilizer is a non-fluorinated ionic comonomer, which can be optionally co-fed with VDF during polymerization after the start of polymerization. These stabilizers have at least one unsaturated carbon-carbon double bond capable of undergoing free radical polymerization and contain an ionic moiety. The ionic moiety on the comonomer can be a carboxyl group, phosphate group, phosphonate group, sulfonate group, sulfonic group, or -OH group.

[0091] Examples of this group of stabilizers include, but are not limited to: sodium salts of acrylic acid, methacrylic acid, styrene sulfonic acid, sodium 1-allyloxy-2-hydroxypropane sulfonate, alkyl methacrylate phosphate, alkyl acrylate phosphate, allyl ether ammonium phosphate, 2-acrylamido-2-methylpropanesulfonic acid, β-carboxyethyl acrylate, ethyl methacrylate phosphate, etc. The third type of stabilizer is used in an amount of about 0.05 to about 5% by weight, based on the total fluorinated monomers added to the polymerization. Preferably, it is used in an amount of about 0.1 to about 2% by weight, based on the weight of the total fluorinated monomers added to the polymerization. The third type of stabilizer is added during polymerization, preferably after at least 15% by weight of the fluorinated monomers have been fed into the reactor, and more preferably after at least 30% by weight of the fluorinated monomers have been fed into the reactor. The third type of stabilizer can be added continuously or intermittently when VDF is polymerized in the presence of an initiator.

[0092] Adding the stabilizer of this invention during polymerization can slow down the polymerization rate, depending on the rate and type of addition to the polymerization medium. This can be compensated for by adding additional initiators to the polymerization to maintain the reaction rate.

[0093] The resulting polymer dispersion

[0094] According to ASTM D3835, at 230°C and reported for 100 seconds... -1 At the shear rate, the melt viscosity of the fluoropolymer obtained by the disclosed method is greater than 20 kpoise, and the solid content of the dispersion is greater than 25% by weight.

[0095] The solid content of the dispersion of the present invention is 25 to 60% by weight.

[0096] The particle size of the fluoropolymer particles in the dispersion is in the range of 20 to 120 nm, preferably 50 to 120 nm, and more preferably 50 to 100 nm.

[0097] Example

[0098] melt viscosity

[0099] Melt viscosity measurements were performed on the fluoropolymer of the present invention. Measurements were taken at 230°C according to ASTM D3835, and the viscosity was measured over 4 seconds. -1 And 100 seconds -1 The report was given at the specified shear rate.

[0100] Shear stability

[0101] Shear stability of aqueous fluoropolymer dispersions was measured at room temperature using a 48mm A164 dispersion blade (manufactured by CaframoLab Solution). 750 mL of dispersion was placed in a 1-liter wide-mouth flask, the dispersion blade was lowered to approximately one-third of the dispersion height from the bottom of the flask, and the rpm setting was 1500 rpm. The stabilization time was recorded when the dispersion's fluidity decreased and it became gel-like [Bruch's viscosity exceeded 1000 cP at room temperature (22°C) and with an LV-4 rotor]. When the Brinell viscosity was 1000 cP or higher, the dispersion ceased to be a fluid and behaved more like a gel. To pass the shear stability test, the sample had to withstand 20 minutes of shear. If the Brinell viscosity was less than 1000 cP after 20 minutes at 1500 rpm, the sample was considered shear-stable. Brinell viscosity was measured at room temperature (22°C) using an LV-4 rotor.

[0102] Particle size

[0103] SEM images were acquired using a Hitachi SU 8010 SEM, and particle size was directly measured from the SEM images by averaging over 50 particles.

[0104] The PPGMA used in the examples is polypropylene glycol methacrylate with a number average molecular weight (Mn) of 300 to 500 (Sartomer® SR604 from Arkema).

[0105] Particle size, melt viscosity, solids content, and shear stability were measured for the examples and comparative examples and are reported in Table 1.

[0106] Comparative Example 1: Homopolymer

[0107] Add 4000 g of water and a predetermined amount of nonionic emulsifier Pluronic® 31Rl (from BASF) to a 7.5 L stainless steel reactor. Purge the mixture with nitrogen and stir for 0.5 h. Raise the reactor temperature to 105°C to steam-off. Seal the reactor while continuing stirring and set the reactor temperature to the desired reaction temperature. Load the reactor with vinylidene fluoride to a pressure of 650 psig; add an aqueous initiator solution containing 1 wt% potassium persulfate and 1 wt% sodium acetate at a rate of 360 g / h to initiate the reaction. The feed rate of this initiator solution is set to approximately 20 g / h to maintain a good reaction rate for the remainder of the reaction. Maintain the reaction pressure at 650 psig by adding vinylidene fluoride (as needed). After adding a total of 2000 g of VDF to the reactor, stop the monomer feed. Continue stirring for 10 minutes and maintain the temperature. Stop stirring and heating. After cooling to room temperature, purge the remaining gas and empty the dispersion from the reactor through a stainless steel mesh.

[0108] It does not contain acrylic-based glycol ester emulsifiers. This comparative example shows large particle size and low stability.

[0109] Comparative Example 2: Homopolymer

[0110] The following comparative examples are based on the teachings of U.S. Patent Nos. 8,338,518 and 8,765,890.

[0111] Add 4200 g of water and a predetermined amount of nonionic emulsifier Pluronic® 31Rl (from BASF) and PPGMA to a 7.5 L stainless steel reactor. Purge the mixture with nitrogen and stir for 0.5 h. Raise the reactor temperature to 105°C to purge steam. Seal the reactor and continue stirring, setting the temperature to the desired reaction temperature. Load the reactor with vinylidene fluoride to a pressure of 650 psig; add an aqueous initiator solution containing 2 wt% potassium persulfate and 2 wt% sodium acetate at a rate of 500 g / h to initiate the reaction. The feed rate of this initiator solution is set to approximately 60 g / h to maintain a good reaction rate for the remainder of the reaction. Maintain the reaction pressure at 650 psig by adding vinylidene fluoride (as needed). After introducing a total of 1700 g of VDF into the reactor, stop the monomer feed. Continue stirring for 10 minutes and maintain the temperature. Stop stirring and heating. After cooling to room temperature, purge the remaining gas and drain the dispersion from the reactor through a stainless steel mesh.

[0112] This comparative example shows the particle size, but the lack of stabilizer resulted in poor stability.

[0113] Comparative Example 3: Copolymer (Pilot Plant)

[0114] 400 lbs of deionized water, 270 g of PLURONIC 31R1 (a non-fluorinated nonionic emulsifier from BASF), and 0.42 lbs of ethyl acetate were loaded into an 80-gallon stainless steel reactor. After evacuation, stirring was initiated at 23 rpm, and the reactor was heated. Once the reactor temperature reached the desired setpoint of 100°C, VDF and HFP monomers were introduced into the reactor, with HFP comprising 13.8% by weight of the total monomers. Subsequently, the reactor pressure was increased to 650 psi by adding approximately 30 lbs of total monomers. The polymerization reaction was initiated by adding 4.0 lbs of an initiator solution consisting of 1.0 wt% potassium persulfate and 1.0 wt% sodium acetate. During initiation, the HFP to VDF ratio was adjusted to achieve an HFP content of 4.3% of the total monomers in the feed. The rate of further initiator solution addition was also adjusted to achieve a polymerization rate of approximately 70 lbs / h. VDF and HPF continued copolymerization until approximately 150 lbs of monomers were introduced into the reactants. HFP feed was stopped, but VDF feed continued until approximately 172 lbs of total monomer were fed into the reactor. VDF feed was then stopped, allowing the batch to react at the reaction temperature and consume the remaining monomer under reduced pressure. After 40 minutes, initiator feed and stirring were stopped, the reactor was cooled, vented, and the dispersion was recovered. The initiator comprised approximately 555 ppm of total monomer. Subsequently, 10 wt% of an aqueous SLS solution (stabilizer) was added to the resulting dispersion to achieve 2500 ppm of SLS, based on the total weight of the fluorinated monomers added for polymerization. SLS = Sodium dodecyl sulfate.

[0115] Comparative Example 3 had large particle sizes and used a nonionic emulsifier (PLURONIC) with at least two blocks, but did not use an acrylic-based glycol ester. This resulted in poor stability even with the use of 2500 ppm of stabilizer.

[0116] Comparative Example 4

[0117] Add 3000 g of water and a predetermined amount of PPGMA to a 7.5 L stainless steel reactor. Purge the mixture with nitrogen and stir for 0.5 h. Raise the reactor temperature to 105°C to purge steam. Seal the reactor, continue stirring, and set the reactor temperature to the desired reaction temperature. Load the reactor with vinylidene fluoride to a pressure of 650 psig; add an aqueous initiator solution containing 2 wt% potassium persulfate and 2 wt% sodium acetate at a rate of 500 g / h to initiate the reaction. The feed rate of this initiator solution is set to approximately 60 g / h to maintain a good reaction rate during the remainder of the reaction. Maintain the reaction pressure at 650 psig by adding vinylidene fluoride (as needed). After adding a total of 1700 g of VDF to the reactor, stop the monomer feed. Continue stirring for 20 minutes and maintain the temperature. Stop stirring and heating. After cooling to room temperature, purge the remaining gas and drain the dispersion from the reactor through a stainless steel mesh.

[0118] Examples 1-4

[0119] Add 4200 g of water and a predetermined amount of nonionic emulsifier Pluronic® 31Rl (from BASF) and PPGMA to a 7.5 L stainless steel reactor. Purge the mixture with nitrogen and stir for 0.5 h. Raise the reactor temperature to 105°C to purge steam. Seal the reactor while continuing stirring, and set the temperature to the desired reaction temperature. Load the reactor with vinylidene fluoride to a pressure of 650 psig; add an aqueous initiator solution containing 2 wt% potassium persulfate and 2 wt% sodium acetate at a rate of 500 g / h to initiate the reaction. The feed rate of this initiator solution is set to approximately 60 g / h to maintain a good reaction rate during the remainder of the reaction. Maintain the reaction pressure at 650 psig by adding vinylidene fluoride (as needed). After adding a total of 1500 g of VDF to the reactor, stop the monomer feed. Continue stirring for 10 minutes and maintain the temperature. Stop stirring and heating. After cooling to room temperature, purge the remaining gas and drain the dispersion from the reactor through a stainless steel mesh. Subsequently, 10% by weight of an aqueous SLS solution was added to the resulting dispersion to achieve an SLS content of 2000 ppm, based on the weight of the fluorinated monomers added for polymerization.

[0120] Example 5

[0121] Add 4200 g of water and a predetermined amount of nonionic emulsifier Pluronic® 31Rl (from BASF) and PPGMA to a 7.5 L stainless steel reactor. Purge the mixture with nitrogen and stir for 0.5 h. Raise the reactor temperature to 105°C to purge steam. Seal the reactor and continue stirring, setting the temperature to the desired reaction temperature. Load the reactor with vinylidene fluoride to a pressure of 650 psig; add an aqueous initiator solution containing 2 wt% potassium persulfate and 2 wt% sodium acetate at a rate of 500 g / h to initiate the reaction. The feed rate of this initiator solution is set to approximately 60 g / h to maintain a good reaction rate for the remainder of the reaction. Maintain the reaction pressure at 650 psig by adding vinylidene fluoride (as needed). After adding a total of 1500 g of VDF to the reactor, stop the VDF feed and add 273 g of HFP monomer to the reactor, followed by adding VDF until the total VDF reaches 1500 g. Continue stirring for 10 minutes and maintain the temperature. Stop stirring and heating. After cooling to room temperature, the remaining gas was vented, and the dispersion in the reactor was discharged through a stainless steel mesh. Subsequently, 10% by weight of an aqueous SLS solution was added to the dispersion to bring the SLS content to 2000 ppm, based on the amount of fluorinated monomers added for polymerization.

[0122] Example 6

[0123] Add 3000 g of water and a predetermined amount of nonionic emulsifier Pluronic® 31Rl (from BASF) and PPGMA to a 7.5 L stainless steel reactor. Purge the mixture with nitrogen and stir for 0.5 h. Raise the reactor temperature to 105°C to purge steam. Seal the reactor while continuing stirring, and set the temperature to the desired reaction temperature. Load the reactor with vinylidene fluoride to a pressure of 650 psig; add an aqueous initiator solution containing 2 wt% potassium persulfate and 2 wt% sodium acetate at a rate of 500 g / h to initiate the reaction. The feed rate of this initiator solution is set to approximately 60 g / h to maintain a good reaction rate for the remainder of the reaction. Maintain the reaction pressure at 650 psig by adding vinylidene fluoride (as needed). After adding a total of 800 g of VDF to the reactor, feed the reactor with a 10% aqueous solution of a (Type II stabilizer) (e.g., polyacrylic acid with a weight average molecular weight of ~4000) at a rate of 200 mL / h while continuing to feed VDF. When the total VDF feed reaches 1700g, stop adding more VDF and polyacrylic acid solution. Continue stirring for 10 minutes while maintaining the temperature. Stop stirring and heating. After cooling to room temperature, vent the remaining gas and discharge the dispersion from the reactor through a stainless steel mesh.

[0124] Mv refers to melt viscosity.

[0125] Example 7

[0126] Add 3000 g of water and a predetermined amount of nonionic emulsifier Pluronic® 31Rl (from BASF) and PPGMA to a 7.5 L stainless steel reactor. Purge the mixture with nitrogen and stir for 0.5 h. Raise the reactor temperature to 105°C to purge steam. Seal the reactor while continuing stirring, and set the temperature to the desired reaction temperature. Load the reactor with vinylidene fluoride to a pressure of 650 psig; add an aqueous initiator solution containing 2 wt% potassium persulfate and 2 wt% sodium acetate at a rate of 500 g / h to initiate the reaction. The feed rate of this initiator solution is set to approximately 60 g / h to maintain a good reaction rate for the remainder of the reaction. Maintain the reaction pressure at 650 psig by adding vinylidene fluoride (as needed). After adding a total of 800 g of VDF to the reactor, feed a 1% (w / w) aqueous solution of a Class III stabilizer [e.g., alkyl methacrylate phosphate (Sipomer® PAM 4000 from Solvay)] at a rate of 100 mL / h while continuing to feed VDF. When the total VDF feed reaches 1700 g, stop adding additional VDF and PAM 4000 solution. Continue stirring for 10 minutes and maintain the temperature. Stop stirring and heating. After cooling to room temperature, vent the remaining gas and drain the dispersion from the reactor through a stainless steel mesh.

[0127] Table 1

[0128]

[0129] *Based on total fluorinated monomers (VDF+HFP).

Claims

1. An aqueous fluoropolymer dispersion comprising a fluoropolymer, The fluoropolymer described herein has an average particle size of less than 120 nm, and is subjected to a temperature of 230°C and a time of 100 seconds. -1 The melt viscosity is at least 20 kP or higher, wherein the solid content of the dispersion is at least 25% by weight, and the Brinell viscosity of the dispersion is less than 1000 cP after shearing at 1500 rpm for at least 20 minutes.

2. The aqueous fluoropolymer dispersion of claim 1, wherein the fluoropolymer comprises at least 60% by weight of VDF monomer units, preferably at least 70% by weight of VDF monomer units.

3. The aqueous fluoropolymer dispersion of claim 1, wherein the fluoropolymer comprises at least a comonomer selected from the group consisting of HFP, CTFE, PMVE, PPVE, TFE, 2,3,3,3-tetrafluoropropylene, preferably HFP.

4. The aqueous fluoropolymer dispersion of claim 1, wherein the average particle size is less than 100 nm.

5. The aqueous fluoropolymer dispersion according to any one or more of claims 1 to 4, comprising (1) an acrylic-based ethylene glycol ester, preferably, the acrylic-based ethylene glycol ester comprising at least one of polyethylene glycol acrylate (PEGA), polyethylene glycol methacrylate (PEGMA), polypropylene glycol acrylate (PPGA) or polypropylene glycol methacrylate (PPGMA); (2) a nonionic block copolymer emulsifier comprising at least two blocks selected from polyethylene glycol, polypropylene glycol and polytetramethylene glycol; and (3) a stabilizer.

6. The aqueous fluoropolymer dispersion of claim 5, wherein the fluoropolymer comprises polyvinylidene fluoride having at least 60% by weight VDF monomer units, the acrylic-based ethylene glycol ester is selected from: polyethylene glycol acrylate (PEGA), polyethylene glycol methacrylate (PEGMA), polypropylene glycol acrylate (PPGA), and polypropylene glycol methacrylate (PPGMA); and the nonionic block copolymer emulsifier comprises at least one polyethylene glycol block and at least one polypropylene glycol block.

7. The aqueous fluoropolymer dispersion of claim 5 or 6, wherein the fluoropolymer comprises at least one non-fluorinated monomer having at least one reactive double bond and an ionic moiety.

8. The aqueous fluoropolymer dispersion of claim 5 or 6, wherein the stabilizer comprises a non-fluorinated oligomer, the non-fluorinated oligomer comprising at least one ionic portion, preferably an acidic portion.

9. The aqueous fluoropolymer dispersion of claim 5 or 6, wherein the stabilizer comprises at least one of an alkyl sulfate or an alkyl sulfonate, wherein the alkyl group is a C6 to C18 alkyl group.

10. A method for preparing a shear-stable aqueous fluoropolymer dispersion with a particle size less than 120 nm, comprising: a) Providing an aqueous reaction medium, at least one fluorinated monomer, at least one acrylic-based ethylene glycol ester, and at least one nonionic emulsifier in a reaction vessel, said nonionic emulsifier having at least two blocks selected from polyethylene glycol, polypropylene glycol, and / or polytetramethylene glycol, each block having 2 to 200 repeating units. b) Polymerizing the fluorinated monomer in (a) in the presence of at least one initiator, wherein the amount of initiator added to the polymerization is at least 1500 ppm, based on the weight of the fluorinated monomer added to the polymerization; and c) Add at least one stabilizer to the fluoropolymer dispersion during or after polymerization.

11. The method of claim 10, wherein the fluorinated monomer comprises vinylidene fluoride monomer.

12. The method of claim 10, wherein the glycol-based acrylic emulsifier comprises at least one of polyethylene glycol acrylate (PEGA), polyethylene glycol methacrylate (PEGMA), polypropylene glycol acrylate (PPGA), or polypropylene glycol methacrylate (PPGMA).

13. The method of claim 10, wherein the amount of the diol-based acrylic emulsifier added to the polymerization is 0.05 to 5% by weight, preferably 0.1 to 2% by weight, based on the total fluorinated monomers added to the polymerization.

14. The method of any one or more of claims 10 to 13, wherein the stabilizer comprises at least one alkyl sulfate or alkyl sulfonate.

15. The method of any one or more of claims 10 to 13, wherein the stabilizer is selected from: C6-C18 alkyl sulfonates, C6-C18 alkyl sulfates, C6-C18 alkyl disulfonates, C6-C18 alkyl pyrosulfates, and mixtures thereof.

16. The method according to any one or more of claims 10 to 13, wherein at least one alkyl sulfate or alkyl sulfonate stabilizer is added after polymerization or after at least 50% by weight of the fluorinated monomer has been fed into the reactor, preferably after at least 75% by weight of the fluorinated monomer has been fed into the reactor.

17. The method of any one or more of claims 10 to 13, wherein at least one alkyl sulfate or alkyl sulfonate stabilizer is added in an amount of 0.1 to about 5% by weight, based on the total weight of the fluorinated monomers added to the polymerization.

18. The method of any one or more of claims 10 to 13, wherein the stabilizer comprises an oligomer containing an ionic monomer unit having an acid group, preferably a carboxylic acid group.

19. The method of any one or more of claims 10 to 13, wherein the stabilizer is selected from: oligomers of polyacrylic acid, oligomers of poly(meth)acrylic acid, oligomers containing maleic acid monomer units, and co-oligomers of any one or combinations thereof.

20. The method according to any one or more of claims 10 to 19, wherein the stabilizer is added after at least 30% by weight of the fluorinated monomer has been fed into the reactor, preferably after at least 50% by weight of the fluorinated monomer has been fed into the reactor.

21. The method of any one or more of claims 10 to 13, wherein a stabilizer for the oligomer is added in an amount of 0.1 to about 2.5% by weight, and the oligomer contains ionic monomer units having acid groups based on the total fluorinated monomers added to the polymerization.

22. The method of any one or more of claims 10 to 13, wherein the stabilizer is added during polymerization and comprises a nonfluorinated monomer having at least one reactive carbon-carbon double bond and an ionic moiety, preferably the ionic moiety being an acidic moiety or a salt thereof.

23. The method of any one or more of claims 10 to 13, wherein the stabilizer is added during polymerization and is selected from: acrylic acid, methacrylic acid, styrene sulfonate, alkyl methacrylate phosphate, 2-acrylamide-2-methylpropanesulfonic acid, β-carboxyethyl acrylate, and combinations thereof.

24. The method of claim 22, wherein during polymerization, preferably after at least 15% by weight of the fluorinated monomer has been fed into the reactor, and more preferably after at least 30% by weight of the fluorinated monomer has been fed into the reactor, a stabilizer comprising a nonfluorinated monomer having at least one reactive carbon-carbon double bond and an ionic moiety is added.

25. The method of claim 22, wherein a stabilizer comprising a nonfluorinated monomer having at least one reactive carbon-carbon double bond and an ionic moiety is added in an amount of 0.1 to about 1.5% by weight, based on the total monomers added to the polymerization.

26. An aqueous fluoropolymer dispersion comprising (1) a vinylidene fluoride polymer having at least 60% by weight of vinylidene fluoride monomer units; (2) an acrylate-based ethylene glycol ester, preferably the acrylate-based ethylene glycol ester comprising at least one of polyethylene glycol acrylate (PEGA), polyethylene glycol methacrylate (PEGMA), polypropylene glycol acrylate (PPGA), or polypropylene glycol methacrylate (PPGMA); (3) a nonionic block copolymer emulsifier comprising at least two blocks of polyethylene glycol, polypropylene glycol, and / or polytetramethylene glycol; and (4) a stabilizer, wherein the fluoropolymer has an average particle size of less than 120 nm and is stable at 230°C for 100 seconds. -1 The melt viscosity is at least 20 kP or higher, and the solid content of the dispersion is at least 25% by weight, and the Brinell viscosity of the dispersion is less than 1000 cP after shearing at 1500 rpm for at least 20 minutes.

27. The aqueous fluoropolymer dispersion of claim 26, wherein the stabilizer comprises a non-fluorinated oligomer, the non-fluorinated oligomer comprising at least one ionic portion, preferably an acidic portion.

28. The aqueous fluoropolymer dispersion of claim 26, wherein the stabilizer comprises at least one of an alkyl sulfate or an alkyl sulfonate, wherein the alkyl group is a C6 to C18 alkyl group.

29. The aqueous fluoropolymer dispersion of claim 26, wherein the fluoropolymer comprises at least one nonfluorinated monomer having at least one reactive double bond and an ionic moiety.

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