Large particle size fluoropolymer latex preparation

Abstract

Disclosed is a shear stable latex comprising a fluoropolymer, a particle size modifier, and a surfactant; the latex has a solids content of at least 15% by weight. Wherein the ratio of the particle size modifier to the surfactant is equal to or greater than 2, based on molar ratios.

Description

Technical Field

[0001] The invention describes the preparation of large particle size fluoropolymer latexes. Large particle size fluoropolymer latexes can be prepared by emulsion polymerization using one or more fluoromonomers, surfactants, particle size modifiers, and initiators. These large particle size fluoropolymer latexes can be used in battery separator coatings. Background Art

[0002] The primary particle size of commercial grade PVDF made by emulsion polymerization is 100 to 400 nm. There is no PVDF shear-stable latex on the market that exceeds 400 nm. There is a need in the industry to increase the adhesion of battery separators to electrodes without reducing the porosity of the battery separator, which will reduce the movement of lithium ions and limit the usefulness of the battery. Increasing the particle size of PVDF binders can provide a solution to this problem.

[0003] As is well known in the art, at higher polymer concentrations, latex is more difficult to stabilize. In addition, it is known that various salts make latex unstable. It is also well known that particle size is larger tends to make latex unstable and limits higher polymer concentrations. It is necessary to provide a shear-stable latex with a larger particle size fluoropolymer. It is unexpected that the present invention provides good latex stability. In fact, the specific utility of the present invention is highlighted by the following: in the presence of a particle size modifier containing a functionalized fluoropolymer with a high solid content (based on the total reaction mixture, more than 15 weight %, preferably more than 20 weight %) in the latex, a shear-stable latex with a large particle size can be prepared.

[0004] The inventors have found that by using specific particle size modifiers when preparing fluoropolymers, the primary particle size of the fluoropolymers can be increased to greater than 400 nm, compared to the same polymerization process without the particle size modifier. The use of these particle size modifiers makes it possible to prepare fluoropolymer shear-stable latexes containing high levels of dispersed polyvinylidene fluoride and having a primary particle size of 400 nm or greater. This result is unexpected because fluoropolymers prepared by emulsion polymerization typically result in particles less than 400 nm. Summary of the invention

[0005] The present invention provides a shear-stable latex comprising a fluoropolymer, a particle size modifier, and a surfactant; the latex has a solid content of at least 15% by weight, wherein the ratio of the particle size modifier to the surfactant is equal to or greater than 2, based on a molar ratio.

[0006] In the latex, the volume average particle size of the fluorine-containing polymer is greater than 400 nm and less than 3000 nm, preferably greater than 450 nm and less than 2000 nm, and the volume average particle size is measured by light scattering.

[0007] In the case of a multimodal particle size latex, at least 20%, more preferably at least 30%, most preferably at least 35% of the total number of fluoropolymer particles in the latex have a volume average primary particle size greater than 475 nm and less than 2000 nm, more preferably greater than 500 and less than 2000 nm.

[0008] The present invention provides a method for preparing a fluorine-containing polymer having a large particle size. The method comprises:

[0009] (a) contacting an aqueous mixture comprising a particle size modifier, a surfactant, and a free radical initiator with a monomer feed comprising one or more fluorine-containing monomers;

[0010] (b) initiating polymerization of the one or more fluorinated monomers to form a shear-stable fluorinated polymer latex.

[0011] wherein the surfactant comprises a non-fluorinated surfactant; and wherein the fluoropolymer is thermoplastic and comprises at least 71 weight percent vinylidene fluoride.

[0012] Embodiments of the present invention include: Embodiment 1 is an aqueous latex, and the aqueous latex comprises:

[0013] A surfactant, a particle size modifier, and a fluoropolymer; wherein the surfactant comprises at least one of the following alkyl sulfonates: C7-C20 1-alkyl sulfonates, C7-C20 2-alkyl sulfonates, C7-C20 1,2-alkyl disulfonates, and mixtures thereof; wherein the particle size modifier comprises MX, wherein M is an alkali metal, or NH 4 , preferably an alkali metal, and X is a halogen; wherein the ratio of particle size modifier to surfactant is equal to or greater than 2, based on a molar ratio, wherein the fluoropolymer concentration is at least 15 weight percent, preferably at least 20 weight percent, based on the total weight of the aqueous latex, wherein the volume average particle size of the fluoropolymer in the latex is greater than 400 nm and less than 3000 nm, preferably greater than 450 nm and less than 2000 nm, the volume average particle size being measured by light scattering, wherein the latex is shear stable, as measured by the Latex Shear Stability Test, wherein after 30 minutes at 2500 rpm and 25°C, the viscosity is less than 100 cps.

[0014] Embodiment 2 is the aqueous latex as described in embodiment 1, wherein the volume average particle size of the fluoropolymer in the latex is greater than 500 nm and less than 1500 nm, as measured by light scattering.

[0015] Embodiment 3 is the aqueous latex of any one or more of the preceding embodiments, wherein the fluoropolymer comprises at least 50 wt % vinylidene fluoride.

[0016] Embodiment 4 is the aqueous latex of any one or more of the preceding embodiments, wherein the fluorinated monomer comprises hexafluoropropylene.

[0017] Embodiment 5 is the aqueous latex as described in any one or more of the preceding embodiments, wherein M is an alkali metal.

[0018] Embodiment 6 is the aqueous latex as described in any one or more of the preceding embodiments, wherein M is selected from the group consisting of Na, Cs, and Li.

[0019] Embodiment 7 is an aqueous latex as described in any one or more of the preceding embodiments, wherein X is Cl or Br, preferably Cl.

[0020] Embodiment 8 is an aqueous latex as described in any one or more of embodiments 1 to 4, wherein the particle size modifier comprises NaCl, CsCl, LiCl, or NH 4 At least one of Cl.

[0021] Embodiment 9 is an aqueous latex as described in any one or more of embodiments 1 to 4, wherein M is lithium, sodium, cesium, or NH 4 , and X is Cl.

[0022] Embodiment 10 is the aqueous latex as described in any one or more of the preceding embodiments, wherein the molar ratio of particle size modifier to surfactant is at least 3.

[0023] Embodiment 11 is the aqueous latex as described in any one or more of the preceding embodiments, wherein the molar ratio of particle size modifier to surfactant is at least 2, at most 7.8, preferably at most 7.0.

[0024] Embodiment 12 is the aqueous latex of any one or more of embodiments 1-4, 7, 10, or 11, wherein when M is cesium, the fluoropolymer exhibits a unimodal particle size distribution.

[0025] Embodiment 13 is the aqueous latex of any one or more of embodiments 1-4, 7, 10, or 11, wherein when M is sodium or lithium, the fluoropolymer exhibits a multimodal particle size distribution.

[0026] Embodiment 14 - A method of increasing the volume average particle size of a fluoropolymer, the method comprising:

[0027] (a) contacting an aqueous mixture comprising a surfactant, a particle size modifier, a monomer feed comprising one or more fluorinated monomers, and a free radical initiator feed; and

[0028] (b) initiating polymerization of the one or more fluorinated monomers to form a shear-stable fluorinated polymer latex;

[0029] wherein the surfactant comprises an alkyl sulfonate selected from the group consisting of C7-C20 1-alkyl sulfonates, C7-C20 2-alkyl sulfonates, C7-C20 1,2-alkyl disulfonates, and mixtures thereof;

[0030] The particle size modifier comprises MX, wherein M is an alkali metal, or NH 4 , and X is a halogen, and

[0031] wherein the ratio of particle size modifier to surfactant is 2 or greater on a molar ratio basis.

[0032] Embodiment 15 is the method of embodiment 14, wherein the fluorine-containing monomer comprises vinylidene fluoride.

[0033] Embodiment 16 is the method of any one or more of embodiments 14 to 15, wherein the fluorine-containing monomer comprises hexafluoropropylene.

[0034] Embodiment 17 is a method as described in any one or more of embodiments 14 to 16, wherein the alkyl sulfonate is selected from: C8-C12 1-alkyl sulfonate, C8-C12 2-alkyl sulfonate, C8-C12 1,2-alkyl disulfonate, and mixtures thereof.

[0035] Embodiment 18 is a method as described in any one or more of embodiments 14 to 16, wherein the surfactant comprises an alkyl sulfonate selected from the group consisting of 1-octane sulfonate, 2-octane sulfonate, 1,2-octane disulfonate, 1-decane sulfonate, 2-decane sulfonate, 1,2-decane disulfonate, 1-dodecane sulfonate, 2-dodecane sulfonate, 1,2-dodecane disulfonate, and combinations thereof.

[0036] Embodiment 19 is the method of any one or more of embodiments 14 to 16, wherein the alkyl sulfonate comprises 1-octane sulfonate.

[0037] Embodiment 20 is the method of any one or more of embodiments 14 to 16, wherein the alkyl sulfonate is sodium alkyl sulfonate, potassium alkyl sulfonate, or ammonium alkyl sulfonate, or a mixture thereof.

[0038] Embodiment 21 is a method as described in any one or more of embodiments 14 to 20, wherein the particle size modifier is MX, wherein M is a metal, or NH 4 , and X is a halogen.

[0039] Embodiment 22 is the method of any one or more of embodiments 14 to 20, wherein M is an alkali metal, or NH 4 .

[0040] Embodiment 23 is the method of any one or more of embodiments 14 to 20, wherein M is selected from Na, Cs, and Li.

[0041] Embodiment 24 is the method of any one or more of embodiments 14 to 23, wherein X is Cl or bromide, preferably Cl.

[0042] Embodiment 25 is a method as described in any one or more of embodiments 14 to 20, wherein the particle size modifier comprises NaCl, CsCl, LiCl, or NH 4 At least one of Cl.

[0043] Embodiment 26 is the method of any one or more of embodiments 14 to 25, wherein the fluoropolymer comprises a copolymer comprising vinylidene fluoride and hexafluoropropylene monomer units.

[0044] Embodiment 27 is the method of any one or more of embodiments 14 to 26, wherein the fluoropolymer comprises at least 75 wt% vinylidene fluoride units.

[0045] Embodiment 28 is the method of any one or more of embodiments 14 to 27, wherein the free radical initiator comprises a persulfate.

[0046] Embodiment 29 is the method of any one or more of embodiments 14 to 28, wherein after step (b), the weight percent of the functionalized fluoropolymer in the latex is at least 15 weight percent, preferably at least 20 weight percent of the latex.

[0047] Embodiment 30 is the method of any one or more of embodiments 14 to 21, 24, 26-29, wherein when M is cesium, the fluoropolymer exhibits a unimodal particle size distribution.

[0048] Embodiment 31 is the method of any one or more of embodiments 14 to 21, 24, 26-29, wherein when M is sodium or lithium, the fluoropolymer exhibits a multimodal particle size distribution.

[0049] Embodiment 32 is a method of making a multimodal fluoropolymer dispersion, the method comprising:

[0050] (a) contacting an aqueous mixture comprising a surfactant, a particle size modifier, with a monomer feed comprising one or more fluorinated monomers and a free radical initiator; and

[0051] (b) providing sufficient heat and pressure to effect polymerization of the one or more fluorinated monomers to form a fluorinated polymer dispersion;

[0052] wherein the surfactant comprises at least one alkyl sulfonate selected from the group consisting of C7-C20 linear 1-alkyl sulfonate, C7-C20 linear 2-alkyl sulfonate, C7-C20 linear 1,2-alkyl disulfonate, and mixtures thereof; and comprises at least 50% by weight of vinylidene fluoride,

[0053] The particle size modifier comprises MX, wherein M is lithium, sodium, or NH 4 , and X is Cl, wherein the ratio of the particle size modifier to the surfactant is greater than 2 on a molar basis.

[0054] Embodiment 33 is the use of the aqueous latex as described in any one or more of embodiments 1 to 13 in lithium ion battery applications, preferably as a separator coating or electrode binder. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 is a graph showing the effect of particle size modifier / surfactant molar ratio on latex particle size. DETAILED DESCRIPTION

[0056] The present invention provides a shear stable fluoropolymer latex having a solids content of at least 20% and a volume average primary particle size of 400 nm or more, preferably 450 nm or more, most preferably greater than 500 nm. The present invention also provides a method for preparing a shear stable fluoropolymer latex having a volume average primary particle size of 400 nm or more, preferably 450 nm or more, most preferably greater than 500 nm, wherein the fluoropolymer of the present invention is a thermoplastic. Shear stability is determined using the Latex Shear Stability Test Method described herein.

[0057] The fluoropolymer is prepared as an aqueous dispersed polymerization reaction mixture (typically referred to as an emulsion or latex) that includes one or more surfactants and uses one or more free radical initiators.

[0058] Optionally, the polymerization for preparing the fluoropolymer is carried out in the presence of a chain transfer agent to adjust the molecular weight, in the presence of a buffer to maintain the desired pH range during the polymerization, and in the presence of an antifouling agent to reduce or eliminate adhesion of the polymer to the inner surface of the polymerization vessel.

[0059] The term "fluoropolymer" as used for the purposes of the present invention means a polymeric material comprising at least 71% by weight of fluorinated monomer units. Suitable fluorinated monomers are described below. The remaining units may be one or more of the following: fluorinated monomers, ethylene, propylene, (meth)acrylates, (meth)acrylic acid, or other monomers known to copolymerize with fluorinated monomers.

[0060] In a preferred embodiment, the fluorine-containing polymer of the present invention comprises vinylidene fluoride and may be a homopolymer or copolymer of vinylidene fluoride. Preferably, at least 71% by weight of the fluorinated monomer units are vinylidene fluoride.

[0061] The fluoropolymer may be a homopolymer, a copolymer, a terpolymer, or a polymer derived from more than three monomers. The term copolymer as used herein means a polymer comprising two or more different monomer units. They are generally thermoplastic, where "thermoplastic" means being able to be formed into a shape by applying heat and (usually) pressure (such as in molding and extrusion processes). Exemplary polymers prepared by the method of the present invention include: polyvinylidene fluoride homopolymer; copolymers, terpolymers, and higher polymers having a vinylidene fluoride content of at least 71% by weight and typically at least 75% by weight. In some exemplary embodiments of the present invention, there may be a level of up to about 99% by weight of VDF monomer units. According to the present invention, specific preferred fluoropolymers include, for example: copolymers of vinylidene fluoride with hexafluoropropylene, or tetrafluoroethylene, or trifluoroethylene, and terpolymers of vinylidene fluoride with tetrafluoroethylene and hexafluoropropylene, or with tetrafluoroethylene and trifluoroethylene. Other copolymers and terpolymers may contain fluorine-containing monomers different from the monomers listed above, combined with vinylidene fluoride. Suitable examples of the other fluorine-containing monomers used according to the present invention will be further described below.

[0062] Surfactants

[0063] The surfactant used in the polymerization comprises at least one alkyl sulfonate. As used herein, the term "alkyl sulfonate" and terms ending with the term "sulfonate" refer to alkali metal salts, ammonium salts, or monoalkyl, dialkyl, trialkyl, or tetraalkyl substituted ammonium salts of alkyl sulfonic acid or alkyl disulfonic acid. Sodium alkyl sulfonate, potassium alkyl sulfonate, and ammonium alkyl sulfonate, or mixtures of any of them are generally used.

[0064] Preferably, the surfactant used in the polymerization comprises at least one alkyl sulfonate selected from the following: C7-C20 1-alkyl sulfonate, C7-C20 2-alkyl sulfonate, C7-C20 1,2-alkyl disulfonate, and mixtures thereof; more preferably, the alkyl sulfonate is selected from: C8-C12 1-alkyl sulfonate, C8-C12 2-alkyl sulfonate, C8-C12 1,2-alkyl disulfonate, and mixtures thereof. Preferably, the alkyl sulfonate is sodium alkyl sulfonate, potassium alkyl sulfonate, or ammonium alkyl sulfonate, or mixtures thereof. Preferably, the alkyl sulfonate is linear. One or more alkyl sulfonates can be used in the present invention.

[0065] Exemplary alkyl sulfonates include, but are not limited to, 1-octane sulfonate, 2-octane sulfonate, 1,2-octane disulfonate, 1-decane sulfonate, 2-decane sulfonate, 1,2-decane disulfonate, 1-dodecane sulfonate, 2-dodecane sulfonate, 1,2-dodecane disulfonate.

[0066] 1-Octane sulfonate is a preferred surfactant.

[0067] Fluoromonomer

[0068] The term "fluorinated monomer" as used in accordance with the present invention means a fluorinated and ethylenically unsaturated monomer capable of participating in a free radical polymerization reaction. The fluorinated monomer used in accordance with the present invention may consist solely of vinylidene fluoride, or it may contain any of the various fluorinated monomers known in the art. According to the present invention, suitable fluorinated monomers include at least one fluorine atom, and may, for example, incorporate fluorinated alkyl, fluoroalkoxy, or vinyl type fluorine atoms.

[0069] Suitable exemplary fluorinated monomers for use in accordance with the present invention include vinylidene fluoride (VDF), tetrafluoroethylene (TFE), trifluoroethylene (TrFE), chlorotrifluoroethylene (CTFE), 1,2-difluoroethylene, perfluorobutylethylene (PFBE), hexafluoropropylene (HFP), vinyl fluoride (VF), pentafluoropropylene, 2,3,3,3-tetrafluoropropylene, trifluoropropylene, fluoro(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), longer chain perfluoro(alkyl) vinyl ethers such as fluoro(ethyl) ...(methyl) vinyl ether (PMVE), perfluoropropyl vinyl ether (PPVE), perfluorobutyl vinyl ether (PBVE), longer chain perfluoro(alkyl) vinyl ethers such as fluoro(ethyl) vinyl ether (PEVE), and perfluoro-2-propoxypropyl vinyl ether, Vinyl ethers, one or more of partially or fully fluorinated alpha 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 dioxoles, such as perfluoro(1,3-dioxole) and perfluoro(2,2-dimethyl-1,3-dioxole) (PDD), partially or fully fluorinated C4 and higher alpha olefins, partially or fully fluorinated C3 and higher cycloolefins, partially fluorinated allylic monomers, or fluorinated allylic monomers, and combinations thereof.

[0070] In a preferred embodiment, VDF is used in combination with at least one fluorine-containing monomer selected from the group consisting of tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), and hexafluoropropylene (HFP).

[0071] Particle size modifier

[0072] The particle size modifier comprises MX, wherein M is an alkali metal or NH 4 , preferably an alkali metal, and X is a halogen. Preferably M is an alkali metal. Preferably X is Cl or Br, preferably Cl.

[0073] Exemplary M include Na, Cs, and Li.

[0074] Exemplary particle size modifiers include NaCl, CsCl, LiCl, or NH 4 Cl.

[0075] Preferably, M is lithium, sodium, cesium, or NH 4 , and X is Cl.

[0076] The molar ratio of particle size modifier to surfactant is equal to or greater than 2, preferably 3 or greater, on a molar ratio basis.

[0077] In some embodiments, the molar ratio of particle size modifier to surfactant is at least 2 and at most 7.8, preferably between 2 and 7.

[0078] In one embodiment, when M is cesium, the fluoropolymer exhibits a unimodal particle size distribution.

[0079] In other embodiments, when M is sodium or lithium, the fluoropolymer exhibits a multimodal particle size distribution.

[0080] Free Radical Initiators

[0081] According to the present invention, the free radical initiator suitable is a compound or a combination of compounds that can provide a free radical source spontaneously or by exposure to heat or light. The free radical initiator is added to the reaction mixture in an amount sufficient to initiate the polymerization reaction and maintain the polymerization reaction at a desired reaction rate. Suitable non-limiting types of initiators include: persulfates, peroxides, peroxy bicarbonates / esters, azo compounds, and redox systems, which are all well known in the art. As used herein, the term "ionic initiator" means a free radical initiator including at least one salt containing a metal cation and / or ammonium or substituted ammonium cation. The term "radical" and the expression "radical" refer to a chemical substance containing at least one unpaired electron.

[0082] Preferred free radical initiators include persulfates, such as sodium persulfate, potassium persulfate, or ammonium persulfate. The amount of persulfate added to the reaction mixture (based on the total weight of monomers added to the reaction mixture) is typically from about 0.005 to about 1.0 weight percent, based on the total weight of monomers used in the reaction.

[0083] The free radical initiator may comprise an organic peroxide, such as an alkyl, dialkyl, or diacyl peroxide, a peroxybicarbonate, and a peroxyester, or a mixture thereof. A preferred dialkyl peroxide is di-tert-butyl peroxide (DTBP), which may be added to the reaction mixture in an amount of about 0.01 to about 5 wt % based on the total monomers, and preferably added in an amount of about 0.05 to about 2.5 wt %, based on the total weight of the monomers used in the reaction. A preferred peroxybicarbonate initiator is di-n-propyl peroxybicarbonate, and diisopropyl peroxybicarbonate, which may be added to the reaction mixture in an amount of about 0.5 to about 2.5 wt % based on the total monomers. Peroxyester initiators include tert-amyl peroxypivalate, tert-butyl peroxypivalate, and succinic acid peroxide.

[0084] The free radical initiator may comprise an azo initiator, such as 2,2'-azobis(2-methylpropionamidine) dichloride.

[0085] The free radical initiator may comprise a redox system. "Redox system" means a system comprising an oxidant, a reductant, and optionally a promoter as an electron transfer mediator. Oxidants include, for example, persulfates; peroxides such as hydrogen peroxide; hydroperoxides such as tert-butyl hydroperoxide and cumene hydroperoxide; and oxidizing metal salts such as ferric sulfate. Reductants include, for example, sodium formaldehyde sulfoxylate, sodium and potassium sulfites, ascorbic acid, bisulfites, metabisulfites, and reduced metal salts. The promoter is a component in a redox system that is capable of reacting with both the oxidant and the reductant in different oxidation states, thereby accelerating the overall reaction. Promoters include, for example, transition metal salts such as ferrous sulfate. In a redox system, the oxidant and the reductant may be used in an amount of about 0.01 to about 0.5% by weight, based on the total weight of the monomers used in the reaction. The optional promoter may be used in an amount of about 0.005 to about 0.025% by weight, based on the total weight of the monomers used in the reaction. Redox systems are described in GSMisra and UDN Bajpai, Prog. Polym. Sci., 1982, 8(1-2), pp. 61-131.

[0086] Chain transfer agent

[0087] Chain transfer agents may be added to the polymerization mixture to adjust the molecular weight of the product. They may be added in one portion at the beginning of the reaction, or gradually or continuously during the entire reaction. If present, the amount and mode of addition of the chain transfer agent depends on the activity of the specific reagent used, as well as on the desired molecular weight of the polymer product. The chain transfer agent added to the polymerization reaction is typically about 0.05 to about 5 weight percent, more typically about 0.1 to about 2 weight percent, based on the total weight of the monomers used in the reaction.

[0088] Oxygen-containing compounds such as alcohols, carbonates, ketones, esters and ethers can act as chain transfer agents. Examples of oxygen-containing compounds that can be used as chain transfer agents include: isopropanol, ethyl acetate, methyl acetate, diethyl carbonate, acetone, ethanol, n-propanol, acetaldehyde, propionaldehyde, ethyl propionate. Other classes of compounds that can act as chain transfer agents in the polymerization of halogen-containing monomers include, for example, halogenated hydrocarbons and hydrogen-containing halogenated hydrocarbons, and chlorinated hydrocarbons such as carbon tetrachloride. Simple alkanes or branched alkanes such as ethane, propane or 2-ethylhexane can also be used as chain transfer agents.

[0089] Buffer

[0090] The polymerization reaction mixture may optionally contain a buffer to maintain a controlled pH throughout the polymerization reaction. The pH is typically controlled in the range of about 3 to about 8 to minimize undesirable color development in the product.

[0091] The buffer may comprise an organic or inorganic acid or an alkali metal salt thereof, or a base or salt thereof having at least one pK in the range of about 4 to about 10, typically in the range of about 4.5 to about 9.5. a Value and / or pK b Exemplary buffers suitable for use according to the present invention include phosphate buffers and acetate buffers, which are well known in the art.

[0092] When using persulfate (e.g., potassium persulfate) as the free radical initiator, a buffer is particularly useful. In this case, a preferred buffer is sodium acetate. The preferred amount of sodium acetate buffer is from about 50% by weight to about 150% by weight, based on the weight of the initiator added to the reaction. In a typical exemplary embodiment, the initiator feed comprises approximately equal weights of potassium persulfate and sodium acetate in an aqueous solution.

[0093] Antifouling agent

[0094] Antifoulants such as paraffin waxes or hydrocarbon oils are typically optionally added to the reaction mixture to minimize or prevent adhesion of the fluoropolymer to reactor components. Any long chain saturated hydrocarbon wax or oil can perform this function. Prior to forming the fluoropolymer, the oil or wax is added to the reactor in an amount sufficient to minimize the formation of polymer adhesion to reactor components.

[0095] Polymerization process

[0096] The overall process may be as follows: initially add deionized water, surfactant, and particle size modifier to the reactor, followed by deoxygenation (removal of oxygen). The reactor may be a pressurized polymerization reactor equipped with an agitator and heat control means. The stirring may be constant, or the stirring may be performed so as to optimize the process conditions during the process of preparing the stabilizer. After the reactor reaches the desired temperature, a certain amount of fluorinated monomer and optional comonomer is added to the reactor. The ratio of the monomer and comonomer may be constant throughout the polymerization, or may vary during the polymerization process. The initiator solution is fed to the reactor at a suitable flow rate to maintain the desired reaction rate. After the desired amount of monomer is reached, the monomer feed may be stopped. Unreacted monomer may be discharged, and the obtained latex may be collected through a discharge port or by other collection means. The latex may be kept in an aqueous medium for subsequent application or use.

[0097] The preparation of the fluoropolymer according to the present invention is generally carried out in a pressurized reactor equipped with an efficient stirring system using equipment known in the art. The pressure for polymerization can be selected from a wide range of pressures, from about 280 to about 20,000 kPa, depending on the capacity of the reaction equipment, the selected initiator system, and the monomer composition used. The polymerization pressure is generally about 2,000 to about 11,000 kPa, and most typically about 2,750 to about 6,900 kPa. The polymerization temperature can vary from about 20°C to about 160°C, depending on the selected initiator system, and is generally about 35°C to about 130°C, and most typically about 65°C to about 95°C.

[0098] Characterization of latex

[0099] The present invention provides a latex comprising a fluoropolymer that is shear stable as measured by the Latex Shear Stability Test Method, wherein the latex is agitated at 2500 rpm for 30 min as described in the Test Methods section (below). If under these test conditions, the latex maintains a viscosity of 100 cps or less, the latex is shear stable.

[0100] The volume average particle size of the latex of the present invention is greater than 400 nm, preferably greater than 450 nm, and most preferably greater than 500 nm. The volume average particle size of the latex of the present invention is less than 3 microns.

[0101] In the case of a latex having a multimodal particle size distribution, at least 20% or more, preferably at least 30%, most preferably at least 35%, of the total number of fluoropolymer particles in the latex have a volume average primary particle size greater than 475 nm and less than 2000 nm, more preferably greater than 500 and less than 2000 nm, or greater than 525 nm and less than 2000 nm.

[0102] The solid content of the shear stable latex may be greater than 20 wt%, preferably greater than 22 wt%.

[0103] In the case of using the method of the present invention, the melt viscosity of the fluoropolymer may be 50 kpoise or greater.

[0104] The following examples are provided to illustrate the practice of the invention and should not be construed as limiting the scope of the claims.In the examples, deionized water and ACS reagent grade ingredients were used unless otherwise stated.

[0105] Example

[0106] Test Method

[0107] Light scattering test method for latex particle size: The particle size of latex particles was measured using a Nicomp CW380 particle size analyzer (light scattering). The volume average particle size was used.

[0108] Latex shear stability test method

[0109] (a) Filter 450.0 g of latex sample through a 125 μm pore size screen. Add 0.5 g of defoamer ( Foamex 840 from Evonik).

[0110] (b) Pour the sample into a 500 mL container. Agitate the sample at room temperature using a Caframo Universal overhead stirrer (model BDC3030), monitoring the latex while agitating to note any changes in consistency. Run the agitator for 30 minutes, or until the latex stops moving or flocculates.

[0111] (c) After stirring for 30 minutes, the latex was filtered through a 125 micron pore size screen. The floccules collected on the screen (if present) were weighed. The Brookfield viscosity of the filtered latex was measured at 25° C. using a Brookfield viscometer (Model DV-II+Pro, spindle #34, 35 rpm).

[0112] (d) The latex sample is considered shear stable if the collected flocculate (wet flocculate) is less than 1.0 wt% (4.5 g) of the total latex and the Brookfield viscosity after 30 minutes of agitation is less than 100 cps.

[0113] Melt viscosity (MV): ASTM method D3835-16 (capillary rheology). At 232°C, 100s -1 The measurement results are reported below. Values ​​are reported in kiloPoise (kP).

[0114] Solids - Weigh the latex sample. Dry the sample at 100°C for 24 hours and weigh the dried sample. % Solids = Dry weight / Total weight.

[0115] Four sets of examples were prepared. The synthesis parameters and latex characterization are summarized in the following table.

[0116] General procedure for preparing large particle size shear-stable fluoropolymer latex

[0117] 4500g of deionized water, surfactant, and particle size modifier are added to 2 gallons of autoclaves. The autoclave is stirred at 72rpm, heated to 83°C, and pressurized to 650psi (4481KPa), using HFP and vinylidene fluoride. The feeding of 2.0 wt % KPS aqueous solution is started with 180.0mL / h. Once pressure drop (which indicates that polymerization has been initiated) is started, the KPS feed rate is reduced to 25.0mL / h, and the pressure is maintained by additional VDF and HFP feeds. Feeding is continued in this way until the amount of desired VDF and HFP is reached. The reaction temperature is maintained at 83°C for another 30 minutes. Subsequently, the pressure is spontaneously reduced for 10 minutes, at which point the reactor ventilation reaches atmospheric pressure and is cooled to room temperature. The product is discharged from the reactor.

[0118] All examples were completed following the above-mentioned procedures. The table lists the monomers, surfactants, and particle size modifiers used in each example.

[0119] Effect of Particle Size Modifier Level

[0120] The effect of particle size modifier level in preparing large particle size "LPS" shear stable latex was studied for two particle size modifiers (NaCl and CsCl). As shown in Tables 1A and 1B, when the modifier / surfactant ratio was 5.3, LPS shear stable latex (PS>400nm) was obtained. When the modifier / surfactant ratio reached 7.9, the reaction mixture flocculated. As the modifier / surfactant ratio increased, the particle size increased linearly, as shown in Tables 1A and 1B. Figure 1 as shown in .

[0121] This trend was similar for samples using CsCl and SOS. When the CsCl / SOS ratio was 4.9, LPS shear-stable latex (>400 nm) was obtained. When the CsCl / SOS ratio reached 7.2, the reactants flocculated.

[0122] Table 1A: Summary of large particle size fluoropolymer latexes prepared using different particle size modifier levels

[0123]

[0124]

[0125] Table 1B: Summary of large particle size fluoropolymer latexes prepared using different particle size modifier contents

[0126]

[0127] Figure 1 Particle size is shown as a function of modifier / surfactant ratio. As this ratio increases, so does the volume average particle size.

[0128] Effect of Different Particle Size Modifiers: Tables 2A and 2B show that the particle size of fluoropolymer latex can be increased using specific halide salts. At this ratio, CaCl 2 The corresponding batch of latex flocculates, and FeSO 4 The particle size of the present invention cannot be achieved.

[0129] Table 2A: Summary of large particle size fluoropolymer latexes prepared using various particle size modifiers

[0130]

[0131] Table 2B: Summary of large particle size fluoropolymer latexes prepared using various particle size modifiers

[0132]

[0133] Effects of different surfactants

[0134] Tables 3A and 3B show that surfactants other than SOS, such as triblock polyethylene glycol from BASF, cannot be used to prepare LPS shear-stable latexes.

[0135] Table 3A: Summary of Large Particle Size Fluoropolymer Latexes Prepared Using Alternative Surfactants

[0136]

[0137] Table 3B: Summary of Large Particle Size Fluoropolymer Latexes Prepared Using Alternative Surfactants

[0138]

[0139] The LPS shear-stable latex prepared using different particle size modifiers can have different particle size distributions. 4 The LPS shear-stable latex prepared by Cl can have a multimodal distribution. The LPS shear-stable latex prepared by CsCl has a unimodal distribution.

Claims

1. Water-based latex, including: surfactants, particle size modifiers, and fluoropolymers; wherein the surfactant comprises at least one of the following alkyl sulfonates: C7-C20 1-alkyl sulfonates, C7-C20 2-alkyl sulfonates, C7-C20 1,2-alkyl disulfonates, and mixtures thereof, The particle size modifier comprises MX, wherein M is an alkali metal or NH 4 , preferably an alkali metal, and X is a halogen, Wherein the ratio of particle size modifier to surfactant is equal to or greater than 2, based on a molar ratio, wherein the fluoropolymer concentration is at least 15 wt %, preferably at least 20 wt %, based on the total weight of the aqueous latex, wherein the volume average particle size of the fluorine-containing polymer in the latex is greater than 400 nm and less than 3000 nm, preferably greater than 450 nm and less than 2000 nm, and the volume average particle size is measured by light scattering, wherein the latex is shear stable as measured by the Latex Shear Stability Test wherein the viscosity is less than 100 cps after 30 minutes at 2500 rpm and 25°C.

2. The aqueous latex of claim 1, wherein in the latex, the fluorine-containing polymer has a volume average particle size of greater than 500 nm and less than 1500 nm, the volume average particle size being measured by light scattering.

3. The aqueous latex of claim 1, wherein the fluoropolymer comprises at least 50 weight percent vinylidene fluoride.

4. The aqueous latex according to claim 1, wherein the fluorine-containing monomer comprises hexafluoropropylene.

5. The aqueous latex according to claim 1, wherein M is an alkali metal.

6. The aqueous latex of claim 1, wherein M is selected from the group consisting of Na, Cs, and Li.

7. The aqueous latex according to claim 1, wherein X is Cl or Br, preferably Cl.

8. The aqueous latex of claim 1, wherein the particle size modifier comprises NaCl, CsCl, LiCl, or NH 4 At least one of Cl.

9. The aqueous latex of claim 1, wherein M is lithium, sodium, cesium, or NH 4 , and X is Cl.

10. The aqueous latex of claim 1, wherein the molar ratio of particle size modifier to surfactant is at least 3.

11. The aqueous latex of claim 1, wherein the molar ratio of particle size modifier to surfactant is at least 2, at most 7.8, preferably at most 7.

0.

12. The aqueous latex of claim 1, wherein when M is cesium, the fluoropolymer exhibits a unimodal particle size distribution.

13. The aqueous latex of claim 1, wherein when M is sodium or lithium, the fluoropolymer exhibits a multimodal particle size distribution.

14. A method for increasing the volume average particle size of a fluoropolymer, the method comprising: a) contacting an aqueous mixture comprising a surfactant, a particle size modifier, with a monomer feed comprising one or more fluorinated monomers and a free radical initiator feed; and b) initiating polymerization of the one or more fluorinated monomers to form a fluorinated polymer shear-stable latex; wherein the surfactant comprises an alkyl sulfonate selected from the group consisting of C7-C20 1-alkyl sulfonates, C7-C20 2-alkyl sulfonates, C7-C20 1,2-alkyl disulfonates, and mixtures thereof; The particle size modifier comprises MX, wherein M is an alkali metal, or NH 4 , and X is a halogen, and wherein the ratio of particle size modifier to surfactant is 2 or greater on a molar ratio basis.

15. The method of claim 14, wherein the fluorine-containing monomer comprises vinylidene fluoride.

16. The method of claim 14, wherein the fluorine-containing monomer comprises hexafluoropropylene.

17. The method of claim 14, wherein the alkyl sulfonate is selected from the group consisting of C8-C12 1-alkyl sulfonate, C8-C12 2-alkyl sulfonate, C8-C12 1,2-alkyl disulfonate, and mixtures thereof.

18. The method of claim 14, wherein the surfactant comprises an alkyl sulfonate selected from the group consisting of 1-octane sulfonate, 2-octane sulfonate, 1,2-octane disulfonate, 1-decane sulfonate, 2-decane sulfonate, 1,2-decane disulfonate, 1-dodecane sulfonate, 2-dodecane sulfonate, 1,2-dodecane disulfonate, and combinations thereof.

19. The method of claim 14, wherein the alkyl sulfonate comprises 1-octane sulfonate.

20. The method of claim 14, wherein the alkyl sulfonate is sodium alkyl sulfonate, potassium alkyl sulfonate, or ammonium alkyl sulfonate, or a mixture thereof.

21. The method of claim 14, wherein the particle size modifier is MX, wherein M is a metal, or NH 4 , and X is a halogen.

22. The method of claim 14, wherein M is an alkali metal, or NH 4 .

23. The method of claim 14, wherein M is selected from the group consisting of Na, Cs, and Li.

24. The method of claim 14, wherein X is Cl or bromide, preferably Cl.

25. The method of claim 14, wherein the particle size modifier comprises NaCl, CsCl, LiCl, or NH 4 At least one of Cl.

26. The method of claim 14, wherein the fluoropolymer comprises a copolymer including vinylidene fluoride and hexafluoropropylene monomer units.

27. The method of claim 14, wherein the fluoropolymer comprises at least 75 weight percent vinylidene fluoride units.

28. The method of claim 14, wherein the free radical initiator comprises a persulfate.

29. The method of claim 14, wherein after step (b), the weight percent of functionalized fluoropolymer in the latex is at least 15 weight percent, preferably at least 20 weight percent of the latex.

30. The method of claim 14, wherein when M is cesium, the fluoropolymer exhibits a unimodal particle size distribution.

31. The method of claim 14, wherein when M is sodium or lithium, the fluoropolymer exhibits a multimodal particle size distribution.

32. A method of making a multimodal fluoropolymer dispersion, the method comprising: a) contacting an aqueous mixture comprising a surfactant, a particle size modifier, with a monomer feed comprising one or more fluorinated monomers and a free radical initiator; and b) providing sufficient heat and pressure to effect polymerization of the one or more fluoromonomers to form a fluoropolymer dispersion; wherein the surfactant comprises at least one alkyl sulfonate selected from the group consisting of C7-C20 linear 1-alkyl sulfonate, C7-C20 linear 2-alkyl sulfonate, C7-C20 linear 1,2-alkyl disulfonate, and mixtures thereof; and comprises at least 50% by weight of vinylidene fluoride, The particle size modifier comprises MX, wherein M is lithium, sodium, or NH 4 , and X is Cl, wherein the ratio of the particle size modifier to the surfactant is greater than 2 on a molar basis.

33. Use of the aqueous latex of claim 1 in lithium ion battery applications, preferably as a separator coating or electrode binder.

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