Aqueous dispersion of polymer containing vinylidene fluoride and trifluoroethylene

Abstract

The present invention relates to an aqueous dispersion of a fluoropolymer comprising repeating units derived from vinylidene fluoride (VDF) in an amount of 60 mol % to 82 mol % and repeating units derived from trifluoroethylene (TrFE) in an amount of 18 mol % to 40 mol %, based on the total number of moles of repeating units, wherein the fluoropolymer has a thermodynamically ordered structure improved to a ferroelectric phase.

Description

[Technical Field]

[0001] This application claims priority to European Patent Application No. 20170585.2, filed April 21, 2020, the entire contents of which are incorporated herein by reference for all purposes.

[0002] The present invention relates to novel copolymers of vinylidene fluoride and trifluoroethylene having improved electrical properties, a process for their preparation in aqueous media in the absence of fluorinated surfactants, and their use as piezoelectric, ferroelectric, dielectric, or pyroelectric materials in electrical / electronic devices. [Background technology]

[0003] Copolymers of vinylidene fluoride and trifluoroethylene have been utilized and developed for use in electrical / electronic devices (e.g., transducers, sensors, actuators, ferroelectric memories, capacitors) due to their ferroelectric, piezoelectric, pyroelectric and dielectric behavior / properties, with the piezoelectric behavior being particularly well-known.

[0004] As is well known, the term piezoelectric refers to the ability of a material to exchange electrical for mechanical energy, and vice versa. Electromechanical response is believed to be essentially related to dimensional changes during deformation or pressure oscillations. The piezoelectric effect is reversible in that materials that exhibit the direct piezoelectric effect (generation of electricity when stress is applied) also exhibit the inverse piezoelectric effect (generation of stress and / or strain when an electric field is applied).

[0005] Ferroelectricity is the property of a material whereby it exhibits spontaneous electric polarization, the direction of which can be switched between equivalent states by the application of an external electric field.

[0006] Pyroelectricity is the ability of certain materials to generate an electric potential when heated or cooled. Indeed, as a result of this change in temperature, positive and negative charges migrate to opposite ends (i.e., the material becomes polarized), and therefore an electric potential is established.

[0007] The piezoelectric, pyroelectric, and ferroelectric properties in copolymers of vinylidene fluoride and trifluoroethylene are generally understood to be related to a particular crystalline habit, the so-called ferroelectric or beta phase, in which the hydrogen and fluorine atoms are arranged to give the largest dipole moment per unit cell.

[0008] Said VDF-TrFE copolymers are known in the art and are described inter alia in U.S. Pat. No. 4,778,867 (PRIES SEYMOUR (US)) 18 / 10 / 1988, U.S. Pat. No. 4,708,989 (THOMSON CSF (FR)) 24 / 11 / 1987, U.S. Pat. No. 4,784,915 (KUREHA CHEMICAL IND CO LTD (JP)) 15 / 11 / 1988, U.S. Pat. No. 4,173,033 (DAIKIN IND LTD (JP)) 30 / 10 / 1979.

[0009] Generally speaking, the techniques for producing these VDF-TrFE copolymers may be based on suspension polymerization, i.e., using an organic initiator in an aqueous phase under temperature and pressure conditions such that VDF exists in the supercritical phase, to produce a coarse-particle slurry that precipitates from the aqueous polymerization medium immediately after production. Nevertheless, suspension polymerization methods require high pressures, and there are safety concerns associated with operating TrFE under such harsh conditions, where TrFE may suffer from explosive behavior, making handling quite troublesome on an industrial level. Since TrFE is considered to have deflagration / explosion behavior similar to tetrafluoroethylene (TFE), the opportunity to limit the polymerization pressure represents a significant advantage in safety management.

[0010] Therefore, techniques based on aqueous emulsion polymerization have been investigated because they allow for production under milder conditions, and furthermore, they provide stable dispersions of VDF-TrFE polymer particles at high throughputs with limited trifluoroethylene (TrFE) partial and total pressures, and pose few environmental concerns.

[0011] Furthermore, access to aqueous dispersions of latex or more generally VDF-TrFE polymers makes it possible to open up processing / modification opportunities for coating / casting techniques based on solvent-free methods, which are attracting more and more attention in the field.

[0012] However, VDF-TrFE copolymers obtained from latexes produced by prior art aqueous emulsion polymerization processes require the use of fluorinated surfactants, which are undesirable components from an environmental point of view.

[0013] Another problem with VDF-TrFE copolymers obtained from latexes produced by prior art aqueous emulsion polymerization processes is that they generally have reduced piezoelectric, pyroelectric and ferroelectric performance compared to, for example, suspension polymerized VDF-TrFE copolymers.

[0014] Here, optimization of the piezoelectric, pyroelectric, or ferroelectric effect requires maximizing the thermodynamic order of the ferroelectric phase to achieve improved piezoelectric, pyroelectric, or ferroelectric performance with more structured crystalline domains, which contradicts the results obtained by emulsion polymerization.

[0015] WO 2018 / 065306 to Solvay Specialty proposes a solution to this second problem, which describes an emulsion polymerization method for producing latexes of copolymers of VDF and TrFE, and optionally other comonomers, in which the polymer has a thermodynamically ordered structure in the ferroelectric phase, resulting in: (i) Parameter Xc (%) defined as follows:

number

[0016] The Xc parameter mentioned above relates to the thermodynamic order of the ferroelectric phase, and the polymers obtained in this way have a particularly high thermodynamic order of the ferroelectric phase, which correlates with improved electrical properties. However, the method described in WO 2018 / 065306 still requires the use of a fluorinated surfactant according to formula (I): [ka] wherein X1, X2, and X3, which are equal to or different from each other, are independently selected from H, F, and C groups optionally containing one or more catenary or non-catenary oxygen atoms. 1~6 (per)fluoroalkyl groups; L represents a bond or a divalent group; R F is a divalent fluorinated C 1~3 is a bridging group; and Y is an anionic functional group.

[0017] Therefore, there remains a need in the art for aqueous dispersions of copolymers comprising VDF and TrFE repeat units that can be produced in aqueous media free of fluorinated surfactants under conditions as smooth as prior art emulsion polymerizations, and which polymers have high thermodynamic order in the ferroelectric phase, resulting in electrical properties that match or are improved over prior art polymers.

[0018] The invention described herein now provides materials and methods that meet these needs.

[0019] BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the values ​​(%) of the parameter Xc for samples according to the invention and comparative samples as a function of the percentage proportion of VDF repeat units in the polymer. Summary of the Invention

[0020] The present invention relates to an aqueous dispersion comprising particles of a polymer (polymer F), wherein the polymer F is - repeating units derived from vinylidene fluoride (VDF) in an amount of 60 mol% to 82 mol% based on the total number of moles of repeating units; - repeating units derived from trifluoroethylene (TrFE) in an amount of 18 mol % to 40 mol % based on the total number of moles of repeating units; - optional repeat units derived from at least one additional monomer different from VDF and TrFE; Including, - the particles have a number average particle size comprised between 200 and 5000 nm, measured using laser scattering according to ISO 13321; The polymer (F) has a thermodynamically ordered structure in a ferroelectric phase, so that (i) Parameter Xc (%) defined as follows:

number

[0021] This inequality reflects a higher value of the parameter Xc for a given polymer composition relative to prior art dispersions, including those described in the referenced WO 2018 / 065306 cited in the "Background" section above. As discussed above with respect to the cited prior art documents, this higher value of Xc corresponds to a higher thermodynamic order in the ferroelectric phase, which translates into improved electrical properties as shown in the Experimental Section.

[0022] The present invention further relates to a method for producing an aqueous dispersion of polymer particles having the above-mentioned characteristics, which comprises polymerizing, in an aqueous reaction medium, 60 mol % to 82 mol % of vinylidene fluoride (VDF), based on the total number of moles of monomers, 18 mol % to 40 mol % of trifluoroethylene (TrFE), based on the total number of moles of monomers, and optionally at least one additional monomer different from VDF and TrFE, in the presence of a radical initiator selected from the group consisting of persulfates, the polymerization is carried out at a total pressure comprised between 25 and 35 bar and at a temperature comprised between 75°C and 95°C, - the resulting dispersion has a solids content of 1 to 30% by weight, the aqueous reaction medium is free of fluorinated surfactants, preferably free of added surfactants; Regarding the method.

[0023] The applicant has surprisingly found that the method detailed above allows the production of aqueous dispersions of VDF-TrFE polymers under smooth conditions and with high throughput, especially at relatively low TrFE partial and total pressures, which polymers have improved structure / structural order of the ferroelectric phase as measured using the Xc(%) parameter defined above, and consequently improved piezoelectric, pyroelectric and ferroelectric performance, as well as improved electrical properties, compared to prior art VDF-TrFE polymers. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a plot of the parameter Xc (%), hereafter also called the crystallinity order parameter, as a function of the VDF content in mol %. Data points marked with π correspond to inventive examples 1-3. Data points marked with λ correspond to comparative examples CA1-CA3. Data points marked with X correspond to comparative examples CB1-CB3. The dotted line corresponds to Xc%=-1.89(100-CM)+99, and the dashed line corresponds to Xc%=-1.89(100-CM)+100. DETAILED DESCRIPTION OF THE INVENTION

[0025] As mentioned above, the present invention relates to an aqueous dispersion containing particles of a fluorinated polymer (F). The polymer (F) of the present invention contains 60 to 82 mol %, preferably 65 to 80 mol %, more preferably 68 to 80 mol % of repeating units derived from VDF and 18 to 40 mol %, preferably 20 to 35 mol %, more preferably 20 to 32 mol % of repeating units derived from TrFE. The percentages are based on the total number of repeating units.

[0026] The polymer (F) of the present invention may further comprise repeating units derived from one or more fluoromonomers other than VDF and TrFE, such as, inter alia, hexafluoropropylene, tetrafluoroethylene, chlorotrifluoroethylene, etc., or repeating units derived from one or more non-fluorinated monomers, such as, inter alia, acrylic or methacrylic monomers, more particularly, repeating units derived from at least one hydrophilic (meth)acrylic monomer (MA) of the following formula: [ka] (In the formula, R1, R2, and R3 are equal to or different from each other and independently represent a hydrogen atom or a C1-C3 hydrocarbon group; R OH is a hydroxyl group or a C1-C5 hydrocarbon moiety containing at least one hydroxyl group).

[0027] Non-limiting examples of hydrophilic (meth)acrylic monomers (MA) are, inter alia, acrylic acid, methacrylic acid, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate; hydroxyethylhexyl (meth)acrylate.

[0028] The monomer (MA) is more preferably - Formula: [ka] 2-hydroxypropyl acrylate (HPA) of any of the following formulas: [ka] - acrylic acid (AA) of the formula: [ka] - A mixture of these is selected from among:

[0029] If an additional comonomer different from VDF or TrFE is present, the polymer (F) of the invention advantageously comprises: - repeating units derived from vinylidene fluoride (VDF) in an amount of 60 mol% to 82 mol% based on the total number of moles of repeating units; - repeating units derived from trifluoroethylene (TrFE) in an amount of 18 mol % to 40 mol % based on the total number of moles of repeating units, and - repeat units derived from at least one additional monomer different from VDF and TrFE in an amount of 0.1 to 5 mol % Includes.

[0030] Nevertheless, polymers consisting essentially of repeat units derived from VDF and TrFE are generally preferred.

[0031] The melt flow index (MFI) of the polymer (F) will be selected by the skilled artisan in relation to the processing technique chosen to obtain the final part (for example a film or sheet).

[0032] Nevertheless, it is generally understood that polymer (F) will advantageously have an MFI measured according to ASTM D1238 (230°C / 5Kg) of between 0.5 and 500 g / 10 min, preferably between 0.5 and 200 g / 10 min, more preferably between 0.5 and 10 g / 10 min.

[0033] The aqueous dispersions of the present invention preferably do not contain fluorinated surfactants, which are undesirable from an environmental standpoint, and more preferably do not contain any added surfactants.

[0034] As stated above, the aqueous dispersion of the present invention comprises particles of polymer (F) having a parameter Xc (%), i.e., a crystallinity order parameter, such that the aforementioned inequality is satisfied.

[0035] From a technological point of view, this crystallinity order parameter is a measure of the structural and conformational order of the ferroelectric phase; the higher the Xc parameter, the more structured and more ordered the ferroelectric crystalline phase.

[0036] As a result, polymers (F) from the aqueous dispersions of the present invention have improved ferroelectric performance resulting from the ferroelectric crystalline phase, with higher structural order resulting in better ferroelectric performance over polymers from latexes that can be produced using prior art techniques.

[0037] The method for preparing the aqueous dispersion detailed above comprises polymerizing 60 mol % to 82 mol % of vinylidene fluoride (VDF), based on the total moles of monomers, 18 mol % to 40 mol % of trifluoroethylene (TrFE), based on the total moles of monomers, and an appropriate amount of at least one optional additional monomer different from VDF or TrFE, in an aqueous reaction medium to obtain the above-mentioned polymer F. The important parameters to be controlled for the reaction to proceed properly in the absence of a fluorinated surfactant, preferably in the absence of other added surfactants, are the selection of initiator, temperature, total pressure, and total polymer concentration.

[0038] Indeed, to obtain polymer F with a sufficiently high thermodynamic order of the ferroelectric phase, it is essential to carry out the reaction at a total pressure comprised between 25 and 35 bar, preferably between 27 and 32 bar, and at a temperature comprised between 75 and 95° C., preferably between 82 and 90° C., in the presence of a persulfate radical initiator, preferably chosen from sodium, potassium and ammonium persulfate. The ratio between the amount of aqueous reaction medium and the amount of monomers must also be such that the resulting dispersion has a solids content of between 1 and 30% by weight, preferably between 10 and 30%, more preferably between 15 and 30%.

[0039] The radical initiator is preferably included in the aqueous reaction medium at a concentration ranging from 0.001 to 20 weight percent of the reaction medium.

[0040] The polymerization can be carried out in the presence of a chain transfer agent. The chain transfer agent can be selected from those known in the polymerization of fluorinated monomers, such as ketones, esters, ethers, or aliphatic alcohols having 3 to 10 carbon atoms, such as acetone, ethyl acetate, diethyl ether, methyl tert-butyl ether, isopropyl alcohol, etc.; chloro(fluoro)carbons having 1 to 6 carbon atoms, optionally containing hydrogen, such as chloroform or trichlorofluoromethane; and bis(alkyl)carbonates (wherein alkyl has 1 to 5 carbon atoms), such as bis(ethyl)carbonate or bis(isobutyl)carbonate. The chain transfer agent can be fed to the polymerization medium at the start, continuously, or in discrete amounts (stepwise) during the polymerization, although continuous or stepwise feeding is preferred.

[0041] The described method results in the formation of an aqueous dispersion of polymer (F) as described. The number-average particle size of the particles of polymer F, as measured by laser scattering according to ISO 13321, typically ranges from 200 to 5,000 nm, preferably from 300 to 1,000 nm. Dispersions prepared by the method of the present invention typically have a number-average particle size slightly larger than prior art latexes, which require the use of fluorinated surfactants. Such dispersions are typically stable and can be redispersed by stirring, as known to those skilled in the art, although in some cases the dispersed particles may tend to settle. After being formed in the aqueous dispersion of the present invention, polymer F can be separated from the aqueous medium by known separation and / or coagulation techniques applied to dispersions and latexes, such as freezing and thawing the dispersion (which results in coagulation of the dispersed particles), followed by mechanical separation of the polymer, washing with deionized water, and finally drying the polymer. Alternative methods, such as filtration and centrifugation, are known to those skilled in the art.

[0042] The present invention also relates to the use of the above-mentioned polymer (F) as a ferroelectric, piezoelectric, dielectric or pyroelectric material in electrical / electronic devices.

[0043] Non-limiting examples of said devices are transducers, sensors, actuators, ferroelectric memories, capacitors, among others.

[0044] The polymer (F) is generally included in the device in the form of a substantially two-dimensional part (for example a film or sheet).

[0045] The film or sheet can be manufactured by standard techniques such as extrusion, injection molding, compression molding and solvent casting.

[0046] The two-dimensional article may be further subjected to post-processing treatments, such as, for example, annealing, stretching, biorientation, etc., in order to enhance the ferroelectric, piezoelectric, dielectric, or pyroelectric behavior, among others.

[0047] The two-dimensional article can be subjected to a high poling electric field, obtained by poling cycles, to adjust the polarization, remnant polarization and maximum displacement current measured in the coercive field, in real time by a high voltage and data acquisition computer controlled system, among other things. Embodiments of this process are described in ISNER-BROM, P., et al., “Intrinsic piezoelectric characterization of PVDF copolymers: determination of elastic constants,” Ferroelectrics, 1995, vol. 171, pp. 271-279; BAUER, F., et al., “Very high pressure behavior of precisely-poled PVDF,” Ferroelectrics, 1995, vol. 171, pp. 95-102; and U.S. Pat. No. 4,611,260 (DEUTSCH FRANZ FORSCH INST(FR)) 9 / 09 / 1986 and U.S. Pat. No. 4,684,337 (DEUTSCH FRANZ FORSCH INST(FR)) 4 / 08 / 1987, the disclosures of which are incorporated herein by reference.

[0048] If the disclosure of any patent, patent application, or publication incorporated herein by reference contradicts the statement of this application to the extent that it may render a term unclear, the statement shall control.

[0049] The present invention will be described in more detail hereinafter with reference to the following examples, the purpose of which is illustrative only and is not intended to limit the scope of the invention.

[0050] Polymerization Examples According to the Invention Example 1: 582 / 39 Copolymer VDF: 70 mol% - TrFE: 30 mol% A vertical autoclave made of AISI 316 steel, equipped with baffles and a stirrer operating at 150 rpm, was charged with 56 L of demineralized water. The temperature was then brought to 85 °C, and upon reaching this temperature, 160 mL of pure ethyl acetate and VDF were introduced in such an amount that the VDF partial pressure reached 6.9 bar (absolute). A gaseous mixture of VDF-TrFE in a nominal molar ratio of 70 / 30 was then added via a compressor until a total pressure of 30 bar was reached. The composition of the gaseous mixture present at the autoclave head was analyzed by GC. At the start of the polymerization, the gas phase was found to consist of 75.7 mol% VDF and 24.3 mol% TrFE. Next, 660 mL of a solution of sodium persulfate (NaPS) in demineralized water at a concentration of 14.4 wt% was added.

[0051] The polymerization pressure was maintained constant by feeding the VDF-TrFE mixture. When 20,000 g of the mixture had been fed, the feeding mixture was discontinued, the agitation was reduced to 50 rpm, the reactor was cooled to room temperature, and the aqueous dispersion was removed. The aqueous dispersion was then frozen for 48 hours and then thawed to extract the polymer. After thawing, the coagulated polymer was washed with demineralized water and dried at 80°C for 48 hours.

[0052] Example 2: 582 / 34 Copolymer VDF: 75 mol% - TrFE: 25 mol% 14.2 L of demineralized water was introduced into a horizontal AISI 316 steel autoclave equipped with a stirrer operating at 90 rpm. The temperature was then brought to 70 °C. Upon reaching this temperature, 500 ml of a 6.6 wt. % aqueous ethyl acetate solution (chain transfer agent) and VDF were introduced in such an amount that the VDF partial pressure reached 7.35 bar (absolute). A gaseous mixture of VDF-TrFE in a nominal molar ratio of 75 / 25 was then added via a compressor until a total pressure of 30 bar was reached. The composition of the gaseous mixture present at the autoclave head was analyzed by GC. At the start of polymerization, the gas phase was found to consist of 81.6 mol.% VDF and 18.4 mol.% TrFE. 240 ml of a 7.2 wt.% solution of sodium persulfate (NaPS) in demineralized water was then introduced.

[0053] The polymerization pressure was maintained constant by feeding the VDF-TrFE mixture. When 3900 g of the mixture had been fed, the feed mixture was discontinued, the agitation was reduced to 15 rpm, the reactor was cooled to room temperature, and the aqueous dispersion was removed. The aqueous dispersion was then frozen for 48 hours and then thawed to extract the polymer. After thawing, the coagulated polymer was washed with demineralized water and dried at 80°C for 48 hours.

[0054] Example 3: 582 / 77 Copolymer VDF: 80 mol% - TrFE: 20 mol% A vertical autoclave made of AISI 316 steel, equipped with baffles and a stirrer operating at 500 rpm, was charged with 3.4 L of demineralized water. The temperature was then brought to 80 °C, and upon reaching this temperature, 200 mL of a 6.6 wt. % aqueous ethyl acetate solution and VDF were introduced in such an amount that the VDF partial pressure reached 7.75 bar (absolute). A gaseous mixture of VDF-TrFE in a nominal molar ratio of 80 / 20 was then added via a compressor until a total pressure of 30 bar was reached. The composition of the gaseous mixture present at the autoclave head was analyzed by GC. At the start of polymerization, the gas phase was found to consist of 86.1 mol.% VDF and 13.9 mol.% TrFE. Next, 200 mL of a 7.2 wt.% solution of sodium persulfate (NaPS) in demineralized water was added.

[0055] The polymerization pressure was maintained constant by feeding the VDF-TrFE mixture. When 1000 g of the mixture had been fed, the feeding mixture was discontinued, the agitation was reduced to 50 rpm, the reactor was cooled to room temperature, and the aqueous dispersion was removed. The aqueous dispersion was then frozen for 48 hours and then thawed to extract the polymer. After thawing, the coagulated polymer was washed with demineralized water and dried at 80°C for 48 hours.

[0056] X a Example of emulsion polymerization in the presence of a cyclic surfactant of formula (V) where =NH4 and a radical initiator (comparative example) [ka] Example CA1 - Copolymer VDF-TrFE 70 / 30 (molar ratio) 3.5 L of demineralized water was introduced into a vertical autoclave made of AISI 316 steel, equipped with baffles and a stirrer operating at 570 rpm. The temperature was then brought to the reaction temperature of 85°C, and once this temperature was reached, the X detailed above was added. a50 g of a 34% wt / wt solution of a cyclic surfactant of formula (V) = NH4 dissolved in distilled water and VDF in an amount sufficient to achieve a VDF partial pressure of 6.9 bar (absolute) were introduced. Next, a gaseous mixture of VDF-TrFE in a nominal molar ratio of 70 / 30 was added via a compressor until a total pressure of 30 bar was reached. The composition of the gaseous mixture present at the autoclave head was analyzed by GC. At the start of the polymerization, the gas phase was found to consist of: 75.9 mol% VDF, 24.1 mol% TrFE. Next, 50 ml of a solution of sodium persulfate (NaPS) in demineralized water at a concentration of 3% by volume was introduced. The polymerization pressure was maintained constant by feeding the above VDF-TrFE mixture. After 500 g of the mixture had been introduced, the mixture was discontinued and the pressure was allowed to drop to 15 bar (absolute) while maintaining the reaction temperature constant. The reactor was then cooled to room temperature, and the latex was removed. The polymer was then extracted by freezing the latex for 48 hours and then thawing it. After thawing, the coagulated polymer was washed with demineralized water and dried at 80°C for 48 hours.

[0057] Comparative Example CA2 - Copolymer VDF-TrFE 75 / 25 (molar ratio) The same procedure as in Ex. CA1 was followed, but initially VDF at 7.35 bar (absolute pressure) was introduced and supplemented with a gaseous mixture of VDF-TrFE in a nominal molar ratio of 75 / 25 until a set total pressure of 30 bar was reached, and the feeding of said mixture was continued to maintain the set pressure value.

[0058] Comparative Example CA3 - Copolymer VDF-TrFE 80 / 20 (molar ratio) The same procedure as in Ex. CA1 was followed, but initially 7.8 bar (absolute) of VDF was introduced and topped up with a gaseous mixture of VDF-TrFE in a nominal molar ratio of 80 / 20 until a set total pressure of 30 bar (absolute) was reached, giving an initial composition throughout: 81.9 mol % VDF, 18.1 mol % TrFE, and the feeding of said mixture was continued to maintain the set pressure value.

[0059] Example of emulsion polymerization in the presence of PFPE surfactant and inorganic initiator (comparison) Example CB1 - Copolymer VDF-TrFE 70 / 30 (molar ratio) 3.5 L of demineralized water were introduced into a horizontal autoclave made of AISI 316 steel, equipped with baffles and a stirrer operating at 570 rpm. The temperature was increased to a reaction temperature of 85° C. Once this temperature was reached, 32.5 g of a microemulsion prepared according to Example 1 of US Pat. No. 7,122,608 (SOLVAY SOLEXIS SPA) were introduced.

[0060] Microemulsion A was prepared as follows: 4.83 g of NaOH was dissolved in 32.83 g of deionized water in a glass reactor equipped with a stirrer under gentle stirring. 52.35g of CF3O(CF2-CF(CF3)O) m’ (CF2O) n’ -CF2COOH (wherein m' / n'=20, the number average molecular weight is 434, there is no fraction with a molecular weight greater than 700, and there is 9% by weight of a fraction with a molecular weight comprised between 600 and 700), 10 g of the formula CF3O(CF2-CF(CF3)O) m’ (CF2O) n’ -Galden from CF3 (R) where m' / n'=20 and has a number average molecular weight of 760; was added.

[0061] Then, 6.95 bar (absolute pressure) of vinylidene fluoride was introduced. A gaseous mixture of VDF-TrFE in a nominal molar ratio of 70 / 30 was added via a compressor until a set pressure of 30 bar (absolute pressure) was reached. The gas phase was found to consist of 76.3 mol% VDF and 23.7 mol% TrFE by GC. Next, 60 mL of aqueous ammonium persulfate solution (1 wt%) was introduced via a metering system. The polymerization pressure was maintained constant by feeding the above-mentioned monomer mixture; when 2% of the mixture (target set at 288 g) had been fed, the temperature was reduced to 105 °C. At this point, the feed was interrupted, and the pressure was allowed to drop to 15 bar (absolute pressure) while maintaining the temperature constant. The reactor was cooled to room temperature, and the latex was removed. The polymer was then extracted by freezing the latex for 48 hours and then thawing it. After thawing, the coagulated polymer was washed with demineralized water and dried at 80°C for 48 hours.

[0062] Comparative Example CB2 - Copolymer VDF-TrFE 75 / 25 (molar ratio) The same procedure as in Ex. CB1 was followed, but initially 7.35 bar (absolute) of VDF was introduced and topped up with a gaseous mixture of VDF-TrFE in a nominal molar ratio of 75 / 25 until a set pressure value of 30 bar (absolute) was reached, giving an initial composition throughout: 82.2 mol % VDF, 17.8 mol % TrFE, and the feeding of said mixture was continued in order to maintain the set pressure value.

[0063] Comparative Example CB3 - Copolymer VDF-TrFE-80 / 20 (molar ratio) The same procedure as in Ex. CB1 was followed, but initially 7.8 bar (absolute) of VDF was introduced and topped up with a gaseous mixture of VDF-TrFE in a nominal molar ratio of 80 / 20 until a set pressure value of 30 bar (absolute) was reached, giving an initial composition throughout: 81.7 mol % VDF, 18.3 mol % TrFE, and the feeding of said mixture was continued in order to maintain the set pressure value.

[0064] Table 1 reported below summarizes the main physical and thermodynamic properties of the dispersions and polymers obtained from the examples.

[0065] In Table 1, T c , ΔH c , T m , and ΔH m are the Curie transition temperature, the enthalpy associated with the Curie transition, the melting temperature, and the enthalpy of fusion of the polymer, respectively, determined by differential scanning calorimetry according to ASTM D3418 and ASTM D3418. X c The parameters are:

number

[0066] Particle size is the number average particle size (nm) of the polymer particles in the aqueous dispersion removed from the reactor at the end of the polymerization process, measured by laser scattering according to ISO 13321. Dry content is the weight percent of polymer contained in the aqueous dispersion removed from the reactor at the end of the polymerization process.

[0067] [Table 1]

[0068] Referring now to the graph in Figure 1, which reports the plot of the parameter Xc (%) as a function of the VDF content (mol %) of polymer (F) using the data in Table 1, it can be seen that polymer F produced according to the invention all have higher Xc values ​​than polymers with the same amount of VDF produced by prior art methods.

[0069] This higher level of Xc reflects improved electrical properties, as can be seen from the data reported in Table 2. The data reported in the table were measured on films made from the polymers obtained in the examples (see below in the Test Methods section). Comparing polymers with the same content of VDF in mole % (direct comparison of Ex.1 according to the invention with comparative examples Ex CA1 and Ex CB1, direct comparison of Ex.2 according to the invention with comparative examples Ex CA2 and Ex CB2, and similarly for Example 3), it can be seen that the polymers according to the invention have improved properties for all reported electrical properties.

[0070] [Table 2]

[0071] Test Method Thermal property measurements (j) Curie transition temperature (T C ) and the enthalpy associated with the Curie transition (ΔH c ) measurement. T c The Curie transition temperature (ΔH) is the temperature at which a transition between the ferroelectric and paraelectric phases occurs in a ferroelectric material. The Curie transition temperature is determined as the first endothermic peak appearing in a DSC thermogram during the second heating cycle, or as understood in accordance with the ASTM D 3418 standard. c is the enthalpy associated with this first-order transition (Curie transition), determined in the second heating cycle of the DSC thermogram as detailed above, and applies mutatis mutandis to the first endothermic peak appearing in said second heating cycle, following the instructions contained in the ASTM D 3418 standard, with modifications as necessary. DSC analyses were performed using a Perkin Elmer Diamond DSC instrument, employing a heating rate of 10°C / min in the second heating cycle as specified in ASTM D 1238.

[0072] (jj) Enthalpy of fusion (ΔH m ) and melting temperature (T m ) measurement The enthalpy of fusion (ΔH) was determined by differential scanning calorimetry (DSC) according to ASTM D 3418 using a Perkin Elmer Diamond DSC instrument. m ) and melting temperature (T m ) is found.

[0073] Piezoelectricity Measurement (i) Film preparation A 20% w / w polymer solution was prepared in methyl ethyl ketone and films were cast onto glass substrates by doctor blade technique using an Elcometer Automatic Film Applicator, Model 4380.

[0074] The polymer layer thus cast was dried under vacuum at 100°C for 2 hours. The thus obtained and dried film was printed with 12 1 cm x 1 cm patterns as electrodes on both sides of the polymer film by inkjet printing technology using poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) purchased from Agfa-Gevaert under the trade name ORGACON® as the conductive material. The thickness of the sample was measured using a Mitutoyo micrometer.

[0075] (ii) Annealing of the film The film obtained as detailed above was placed in a vented oven set at an internal temperature of 135° C. After 1 hour, the oven was switched off and allowed to cool to room temperature for 5 hours.

[0076] (iii) Film poling The films were poled using an LC Precision poling device combined with a RADIANT High Volate Interface, which generates electric fields up to 10 KV. The annealed films were placed in a polarization cell where an electric field of 150 V / micron or 200 V / micron was applied across the annealed specimen.

[0077] (iv) Measurement of the piezoelectric coefficient (d33) The piezoelectric coefficient (d33) values ​​were measured using a PIEZOMETER PM300 instrument by placing the poled samples obtained above in the strain gouges of the instrument, which induced the film under vibration at 110 Hz at room temperature. d33 is reported as μC / N.

[0078] (v) Leakage current measurement at 100V / μm Leakage current refers to the gradual loss of energy from a charged capacitor. It is measured using a polarization cell that applies 100V / micron to the electrodes of the capacitor and measures the energy loss after 5 seconds.

[0079] (vi) Measurement of the dielectric constant of the film The values ​​of the dielectric constant [k] were derived from direct measurements of the dielectric capacitance with a piezometer system provided by Piezotest. All capacitance values ​​were measured at 110 Hz.

number

[0080] (vi) Ferroelectric hysteresis measurement (P r , P max ) Hysteresis measurements were performed by poling the annealed films in an electric field ranging from 80 V / micron to 250 V / micron, obtaining a hysteresis curve, and determining the maximum and remnant polarization from the curve. max is the maximum polarization that can be achieved with the maximum applied electric field, and P r is the residual polarization (also called remnant polarization) in the sample after the applied electric field is removed.

[0081] (viii) Coercive force field (Ec). The coercive field is the minimum voltage required to initiate the orientation of dipoles in the polymer film, extrapolated from the hysteresis loop as Ec, where the polarization is zero.

[0082] (ix) Breakdown voltage and maximum poling field (BKD and MPV) The breakdown voltage (BKD) is the minimum voltage at which a portion of an insulator becomes conductive and breaks down when subjected to poling. The maximum poling voltage (MPV) is the maximum voltage applied to a polymer specimen, typically kept 10-15% below the BKD for copolymers and 20-25% below the BKD for terpolymers as a safety margin for testing, and is the voltage at which the polarization value is not further improved by the electric field.

Claims

1. An aqueous dispersion comprising particles of a fluoropolymer (polymer F), said polymer F comprising: repeat units derived from vinylidene fluoride (VDF) in an amount of 75 mol % to 82 mol % relative to the total number of moles of repeat units, repeat units derived from trifluoroethylene (TrFE) in an amount of 18 mol % to 25 mol % relative to the total number of moles of repeat units; - optional repeat units derived from at least one additional monomer different from VDF and TrFE; Including, - said particles have a number average particle size comprised between 200 and 5000 nm, measured using laser scattering according to ISO 13321; The polymer F has a thermodynamically ordered structure in the ferroelectric phase, so that (i) The parameter Xc (%), which is defined as follows: [Equation 1] [In the formula, ΔH ｃ is the enthalpy associated with the Curie transition between the ferroelectric and paraelectric phases, determined in J / g in the second heating scan by DSC at a heating rate of 10°C, and ΔH ｍ is the enthalpy of fusion determined in J / g according to ASTM D3418]; (ii) the content of VDF repeat units relative to the total number of moles of repeat units (expressed in mole %), expressed as CM; The relationship between is the following inequality: Xc (%)>a*(100-CM)+b where a=-1.89 and b=99, the aqueous dispersion is free of fluorinated surfactants; Aqueous dispersion.

2. (i) the relationship between said parameter Xc (%) and (ii) the content of VDF repeat units (expressed in mole %) relative to the total number of moles of repeat units, expressed as CM, satisfies the following inequality: Xc (%)>a*(100-CM)+b (wherein a=-1.89 and b=100) The aqueous dispersion of claim 1.

3. 3. The aqueous dispersion according to claim 1 or 2, wherein the polymer (F) comprises repeating units derived from one or more fluoromonomers other than VDF and TrFE, or repeating units derived from one or more non-fluorinated monomers.

4. An aqueous dispersion described in claim 1 or 2, wherein the polymer (F) contains repeating units derived from hexafluoropropylene, tetrafluoroethylene, or chlorotrifluoroethylene, or repeating units derived from a (meth)acrylic monomer.

5. The polymer (F) has the following formula: 【Chemical 1】 wherein R1, R2, and R3 are equal to or different from one another and independently represent a hydrogen atom or a C 1 -C 3 hydrocarbon group; and R OH is a hydroxyl group or a C 1 -C 5 hydrocarbon moiety containing at least one hydroxyl group.

3. The aqueous dispersion according to claim 1, comprising a repeating unit derived from at least one hydrophilic (meth)acrylic monomer (MA) of the formula:

6. The polymer (F) is repeating units derived from VDF in an amount of 75 mol% to 80 mol% and repeating units derived from TrFE in an amount of 20 mol% to 25 mol% based on the total number of moles of repeating units; - optional repeat units derived from at least one additional monomer different from VDF and TrFE in an amount of 0 to 5 mol %; 3. The aqueous dispersion of claim 1 or 2, comprising:

7. 7. The aqueous dispersion according to claim 1, wherein the melt flow index (MFI) of the polymer (F), measured according to ASTM D 1238 (230°C / 5 kg), is from 0.5 to 500 g / 10 min.

8. The aqueous dispersion of any one of claims 1 to 7, wherein the aqueous dispersion does not contain added surfactants.

9. 8. A method for producing an aqueous dispersion of polymer particles according to any one of claims 1 to 7, comprising polymerizing, in an aqueous reaction medium, 75 to 82 mole % of vinylidene fluoride (VDF), based on the total number of moles of monomers, 18 to 25 mole % of trifluoroethylene (TrFE), based on the total number of moles of monomers, and optionally at least one additional monomer different from VDF or TrFE, in the presence of a radical initiator selected from the group consisting of persulfates, the polymerization is carried out at a total pressure comprised between 25 and 35 bar and at a temperature comprised between 75 and 95°C, the resulting dispersion has a solids content of 1 to 30% by weight, - the aqueous reaction medium is free of fluorinated surfactants, method.

10. 10. The method of claim 9, wherein the aqueous reaction medium does not contain added surfactants.

11. 11. The method according to claim 9 or 10, wherein the number average particle size of the polymer particles, measured by laser scattering according to ISO 13321, is comprised between 200 and 5000 nm.

12. A process according to any one of claims 9 to 11, wherein the polymerization is carried out at a total pressure comprised between 27 and 32 bar and at a temperature comprised between 82°C and 90°C.

13. 13. The method of any one of claims 9 to 12, wherein the aqueous dispersion has a solids content of from 10% to 20% by weight.

14. A fluoropolymer obtained by separating the aqueous medium from the aqueous dispersion according to any one of claims 1 to 7.

15. A film or sheet comprising the fluoropolymer of claim 14.

16. 15. A method of manufacturing an electrical / electronic device, said method comprising using the fluoropolymer of claim 14 as a ferroelectric, piezoelectric, dielectric or pyroelectric material in said electrical / electronic device.

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