METHOD FOR MANUFACTURING A FIBER REINFORCEMENT COMPRISING A FLUORIDE COPOLYMER

Abstract

The invention relates to a method for manufacturing a fiber web comprising fibers of a copolymer based on the repeating motif derived from vinylidene fluoride (VDF), by electrospinning a solution of said fluorinated polymer. The invention also relates to the fiber webs obtainable by this method. Finally, the invention relates to the various applications of said fiber webs as mechanical reinforcement of polymer electrolytes in fuel cells, electrolyzers, and batteries (Figure 2).

Description

Title of the invention: METHOD FOR MANUFACTURING A FIBER REINFORCEMENT COMPRISING A FLUORIDE COPOLYMER technical field

[0001] The invention relates to a method for manufacturing a fiber web comprising fibers of a copolymer based on the repeating motif derived from vinylidene fluoride (VDF), by electrospinning a solution of said fluorinated polymer. The invention also relates to the fiber webs obtainable by this method. Finally, the invention relates to the various applications of said fiber webs intended for use in the field of new energies, in particular as mechanical reinforcement of polymer electrolytes in fuel cells, electrolyzers, and batteries. PRIOR ART

[0002] Electrospinning is an electrohydrodynamic process that allows the fabrication of polymer nanofibers from a polymer solution(s) through the application of a high electrostatic field. The submicron-diameter fibers thus obtained form a fiber web through accumulation.

[0003] Submicron diameter poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) fibers are used in various fields, such as renewable energy, due to their chemical resistance and good thermal performance. The main solvents currently used for the electrospinning of P(VDF-HFP) are N,N-dimethylacetamide (DMAC), dimethylformamide (DMF), and N-methylpyrrolidone (NMP), which are solvents harmful to health and classified as substances of very high concern (SVHC) by the European Chemicals Agency (ECHA). DMF, NMP, and DMAC are labeled "H360D - May harm the unborn child." They are therefore considered CMR solvents (Carcinogenic, Mutagenic, or Toxic to Reproduction).

[0004] P(VDF-HFP) is a semi-crystalline thermoplastic copolymer, exhibiting a complex structure depending on the conformation of the chain, thanks to five crystalline phases: a, [3, y, ô and e. The beta phase is the most polar phase and exhibits piezoelectric, pyroelectric and ferroelectric properties.

[0005] The publication by Kundu et al., “On the relevance of the polar [3-phase of poly(vinylidene fluoride) for high-performance lithium-ion battery separators”, The Journal of Physical Chemistry, 2017, vol. 47, p. 121, describes how the presence of a beta phase in the homopolymer and / or copolymer PVDF membrane is particularly interesting for applications such as battery separators, because the polarity of the high phase ratio material [3 is higher facilitating the migration of lithium ions into the separator.

[0006] The publication by Andrew et al., “Effect of electrospinning on the ferroelectric phase content of polyvinylidene difluoride fibers,” Langmuir, 2008, vol. 24, no. 3, pp. 670–672, describes the electrospinning of homopolymer PVDF solutions in DMF. By varying certain process parameters, specifically the concentration of homopolymer PVDF in the solution and the voltage applied during electrospinning, the authors obtained a web of homopolymer PVDF fibers with a crystallinity of 49% to 58%, with a beta phase content of no more than 75% in the crystalline phase. Thus, the fibers obtained have a beta phase content of 37% to 47% by weight relative to the fiber weight.

[0007] In order to improve the durability of the electrospinning technique, less toxic and bio-based solvents (in other words, alternative green solvents) constitute an appropriate strategy.

[0008] In document WO 2021 / 255389, the applicant described a process for manufacturing a non-woven fabric of homopolymer or copolymer PVDF fibers by electrospinning a solution, or a homogeneous dispersion, comprising at least 15% by weight of dimethyl sulfoxide and at least one second liquid, the second liquid having a Hansen solubility parameter of 7 MPa1 / 2 to 15 MPa1 / 2 and a boiling point strictly above 100°C. DMSO is a green solvent with a melting point of 18°C, which may limit its use in processes operating at ambient temperature, such as electrospinning, because there is a risk of the polymer solution freezing.

[0009] There is still a need to provide electrospinning processes using a liquid vehicle which is not considered, due to its constituents, to be carcinogenic, mutagenic, or toxic to reproduction.

[0010] There is also a need to provide electrospinning processes to obtain fiber webs having a higher proportion of beta phase relative to fiber weight in order to improve the ferroelectric properties of the web.

[0011] There is also a need to propose a process for obtaining a fiber veil intended for use as a mechanical reinforcement of polymer electrolyte in new energy applications (fuel cells, electrolyzers and batteries).

[0012] Another objective of the invention is, at least according to certain embodiments, to provide electrospinning processes enabling the production of fiber webs having a higher proportion of beta phase relative to the fiber weight in order to improve the ferroelectric properties of the web. Summary of the invention

[0013] The invention relates to a method for manufacturing a non-woven veil of PVDF copolymer fibers by electrospinning a solution comprising said fluorinated copolymer solubilized in a mixture of solvents, characterized in that said mixture comprises a first solvent which is gamma-valerolactone (GVL), and a second solvent which is a pure solvent or a mixture of solvents with a boiling point below 100°C.

[0014] Characteristically, the second solvent is chosen from acetone, dimethyl carbonate (DMC), methyl ethyl ketone (MEK), 2-methyltetrahydrofuran (2MTHF) or mixtures thereof.

[0015] Advantageously, none of the constituents of the solution used to solubilize the PVDF copolymer are considered to be carcinogenic, mutagenic, or toxic to reproduction (CMR).

[0016] According to one embodiment, said solution has a Brookfield viscosity at 20°C of 50 to 3000 cP, preferably of 75 to 1500 cP, and further preferably of 80 to 500 cP, as measured according to ASTM D1238-13. According to one embodiment, the PVDF copolymer represents 6% to 25% by weight, preferably 10% to 21% by weight, relative to the total weight of the solution.

[0017] The PVDF copolymer is a copolymer having a repeating motif from VDF and at least one repeating motif from a monomer other than VDF, the other monomer being chosen from the list consisting of: vinyl fluoride (VF), tetrafluoroethylene (TFE), trifluoroethylene (TrFE), a chlorofluoroethylene (CFE), a chlorodifluoroethylene, chlorotrifluoroethylene (CTFE), dichlorodifluoroethylene, a trichlorofluoroethylene, hexafluoropropylene (HFP), a trifluoropropene, a tetrafluoropropene, a chloro-trifluoropropene, hexafluoroisobutylene, perfluorobutylethylene, a pentafluoropropene, a perfluoroether, in particular a perfluoroalkylvinyl ether, ethylene, an acrylic monomer, a methacrylic monomer and mixtures thereof.

[0018] According to one embodiment, said fluorinated copolymer is the P(VDF-HFP) copolymer.

[0019] The invention further relates to a fiber veil that can be obtained by the electrospinning process, said P(VDF-HFP) veil being characterized by a crystallinity of 29 to 34% (rate determined by DSC from an enthalpy of fusion of P(VDF-HFP) of 104.5 J / g) and a percentage of beta phase in the crystalline phase between 41 and 69% (rate determined by infrared spectroscopy).

[0020] The invention also relates to the use of said PVDF copolymer fiber veil as a mechanical reinforcement of polymer electrolyte in fuel cells, electrolyzers and batteries.

[0021] The present invention overcomes the drawbacks of the prior art. The use of gamma-valerolactone for the electrospinning of PVDF copolymer is a more sustainable improvement for the manufacture of fluoropolymer fiber webs. Gamma-valerolactone is a sustainable solvent derived from biomass and is not classified as a reprotoxic CMR solvent. FIGURES

[0022] [Fig. 1] schematically represents a reactor for the preparation of an electrospinning solution, intended to be used in the process according to the invention.

[0023] [Fig. 2] shows a scanning electron microscopy (SEM) image of a web of electrospun fibers according to the invention, obtained from a mixture of gamma-valerolactone solvent and acetone (see example according to the invention). DETAILED DESCRIPTION OF THE INVENTION

[0024] In all the ranges stated in this application, terminals are included unless otherwise stated.

[0025] The fluorinated copolymer used in the process according to the invention is based on the repeating unit derived from vinylidene fluoride (VDF). The proportion of the repeating unit derived from VDF represents at least 50% molarly relative to the total number of moles of repeating units of the fluorinated polymer.

[0026] According to certain embodiments, the fluorinated copolymer comprises at least 60%, or at least 70%, or at least 75%, or at least 80%, or at least 85%, or at least 90%, or at least 95% by mole of units derived from VDF. The term "copolymer" refers to a polymer resulting from the copolymerization of at least two types of chemically different monomers, called co-monomers.

[0027] The copolymer has a repeating motif from VDF and at least one repeating motif from a monomer other than VDF, the comonomer being chosen from the list consisting of: vinyl fluoride (VF), tetrafluoroethylene (TFE), trifluoroethylene (TrFE), a chlorofluoroethylene (CFE), a chlorodifluoroethylene, chlorotrifluoroethylene (CTFE), dichlorodifluoroethylene, a trichlorofluoroethylene, hexafluoropropylene (HFP), a trifluoropropene, a tetrafluoropropene, a chloro-trifluoropropene, hexafluoroisobutylene, perfluorobutylethylene, a pentafluoropropene, a perfluoroether, in particular a perfluoroalkylvinyl ether, ethylene, an acrylic monomer, a methacrylic monomer, and mixtures thereof.

[0028] It is understood that all geometric isomers of the aforementioned fluorinated compounds are included in the above terminologies, such as: 1,1-chlorofluoroethylene (1,1-CFE), 1,2-chlorofluoroethylene (1,2-CFE), 1,2-dichloro-1,2-difluoroethylene, 1,1-dichloro-1,1-difluoroethylene and 1,1,2-trichloro-2-fluoroethylene, 3,3,3-trifluoropropene, 2-chloro-3,3,3-trifluoropropene (1233xf), 1-chloro-3,3,3-trifluoropropene (1233zd), 1,3,3,3-tetrafluoropropene, 2,3,3,3-tetrafluoropropene (or 1234yf), 3-chloro-2,3,3-trifluoropropene (or 1233yf), 3-chloro-3,3,3-trifluoropropene, 1,1,3,3,3-pentafluoropropene or the 1,2,3,3,3-pentafluoropropene.

[0029] Among the perfluoroalkyl vinyl ethers, those with the general formula Rf-O- may be mentioned. CF-CF2, Rfétant an alkyl group, preferably in Cl to C4 and for example methyl vinyl ether (MVE) or isopropyl vinyl ether (PVE).

[0030] According to some variants, said fluorinated copolymer is a mixture consisting of one or more copolymers selected from: a P(VDF-TrFE), a P(VDF-HFP), a P(VDF-TFE), a P(VDF-TrFE-CFE), a P(VDF-TrFE-CTFE), a P(VDF-TFE-CFE), and a P(VDF-TFE-CTFE).

[0031] According to one embodiment, said fluorinated polymer is the P(VDF-HFP) copolymer. The P(VDF-HFP) advantageously has a molar proportion of repeating units from the HFP of 5% to 35%. Fluorinated polymer solution

[0032] The fluorinated polymer solution comprises said polymer solubilized in a mixture of solvents, said mixture comprising a first solvent which is gamma-valerolactone (GVL), and a second solvent which is a pure solvent or a mixture of solvents with a boiling point below 100°C.

[0033] Advantageously, the second solvent is chosen from acetone, dimethyl carbonate (DMC), methyl ethyl ketone (MEK), 2-methyltetrahydrofuran (2MTHF) or mixtures thereof.

[0034] This mixture of solvents is capable of dissolving said polymer to form a true solution (i.e. single-phase or homogeneous at the molecular level); in this way it is easier to obtain a non-woven veil of PVDF copolymer fibers with a higher proportion of beta phase.

[0035] The mass content of the first solvent is between 10 and 70%, preferably between 30 and 50%, by weight of the solution. The mass content of the PVDF copolymer is between 6 and 25%, preferably between 10 and 21%, by weight of the solution.

[0036] The preparation of the electrospinning solution consists of solubilizing the PVDF copolymer in the solvent mixture.

[0037] According to one embodiment, illustrated in [Fig. 1], the first and second solvents (or solvent mixtures) are introduced into an 8L double-jacketed vertical reactor equipped with a three-bladed propeller stirrer and a condenser. The copolymer of PVDF in powder form is slowly introduced into the solvents under mechanical stirring. Dissolution is carried out hot and under stirring at a temperature between 30 and 70°C.

[0038] The hydrodynamic conditions depend on the type of agitator used. Agitators promoting axial flow are particularly preferred. The agitation time is generally from 30 minutes to 24 hours. The agitation time is preferably less than or equal to 12 hours, preferably less than or equal to 6 hours, and extremely preferably less than or equal to 4 hours.

[0039] According to one embodiment, the dissolution of the PVDF copolymer in powder form in the solvent mixture is carried out at a temperature between 30 and 70°C for a period of 1 to 5 hours.

[0040] The electrospinning solution can be electrospinned when all the particles are dissolved and the solution is clear.

[0041] According to certain embodiments, the solution, or homogeneous dispersion, has a Brookfield viscosity at a measurement temperature of 20°C to 25°C, preferably at 20°C, of ​​50 to 3000 cP, preferably 75 to 1500 cP, and even more preferably 80 to 500 cP. To measure Brookfield viscosity, a Brookfield-type viscometer such as the DV 11+ PRO can be used. Electrospinning process

[0042] The invention relates to a method for manufacturing a non-woven veil of PVDF copolymer fibers by electrospinning the solution described above, to form said veil.

[0043] The term "fiber" is meant to designate a filamentous element, which can generally be described by a diameter and a length.

[0044] The term "nonwoven fibre veil" refers to a set of fibre obtained, in particular, by assembling fibre without weaving or knitting. A complementary definition has been proposed by EDANA (European Disposal and Nonwoven Association), according to standard EN ISO 9092, as meaning made of a web of individual fibre, oriented directly or randomly, bonded by friction, cohesion or adhesion.

[0045] According to one embodiment, the electrospinning process is implemented using an electrospinning device which may be a needle, wire or cable, a cylinder, a cup or rotating disk, a cone, a helical or pyramidal system, or another system capable of producing fibers under the influence of an electric field. The installation includes a reservoir of electrospinning solution and a system for supplying the solution to an electrospinning device with a controlled flow rate. The electrospinning device is connected to a positive high-voltage generator. A metallic collecting electrode is placed facing the electrospinning device and is connected to a high-voltage negative generator or to ground. The voltage difference during electrospinning generates an electric field. Several filaments of solution are generated at the electrospinning device under the influence of the field and are guided towards the collecting electrode. The whip-like movements between the electrospinning device and the collecting electrode lead to evaporation of the solvents. The solid PVDF copolymer fibers are deposited onto a substrate placed above the collecting electrode.

[0046] Several process parameters can be adjusted to obtain a web with advantageous properties, in particular to obtain fibers of the desired size, with a substantially homogeneous appearance and minimizing the presence of polymer beads. The electrospinning parameters include, in particular:

[0047] - The voltage on the electrospinning device: between 1 kV and 80 kV;

[0048] - The voltage on the collecting electrode: between 0 kV and -40 kV;

[0049] - The distance between the electrospinning device and the collecting electrode: between 1 cm and 50 cm;

[0050] - The polymer solution flow rate: between 0.1 g / min and 20 g / min;

[0051] - The speed of the unwinding belt: between 0.01 m / min and 15 m / min;

[0052] - The relative humidity of the air in the electrospinning chamber: between 0% and 50%;

[0053] - The air temperature of the chamber: between 10 °C and 40 °C.

[0054] The fiber web passes through an oven after exiting the electrospinning chamber. The oven temperature is between 25°C and 70°C. The fiber web is then cooled to ambient temperature (25°C). This drying step allows for the evaporation of any residual solvent and may increase the crystallinity, and therefore the ferroelectric properties, of the fluorinated polymer.

[0055] The electrospun fiber veil has a crystallinity of 29 to 34% (rate determined by differential scanning calorimetry (DSC) from an enthalpy of fusion of P(VDF-HFP) of 104.5 J / g).

[0056] The percentage of beta phase in the crystalline phase is between 41 and 69% (rate determined by Fourier transform infrared spectroscopy or FTIR spectroscopy). The fibers exhibit different beta phase percentages by weight, relative to the fiber weight, the maximum of which, calculated by the product of the degree of crystallinity and the percentage of beta phase in the crystalline phase, is 21% for the fiber web made from a P(VDF-HFP) mixture in gamma valerolactone and acetone.

[0057] According to some embodiments, the beta phase content by weight relative to the fiber weight measured for PVDF copolymer fibers electrospun from gamma-valerolactone is higher than the beta phase content for PVDF copolymer fibers electrospun from other, more durable solvents and conventional solvents classified as CMR. These remarkable properties are also of interest in piezoelectric applications.

[0058] Depending on the operating conditions of the electrospinning process and particularly the distance between electrodes, the voltage and the flow rate of solution, the diameter of the electrospun PVDF copolymer fibers can be less than 200 nm.

[0059] Applications of PVDF copolymer fiber veil

[0060] According to one embodiment, the invention relates to the use of the PVDF copolymer fiber veil, described above, as mechanical reinforcement for a polymer electrolyte. The polymer electrolyte may be an ion-exchange polymer membrane in a fuel cell or electrolyzer.

[0061] According to one embodiment, the invention relates to the use of the PVDF copolymer fiber veil, described above, as a mechanical reinforcement of polymer electrolyte, said electrolyte being a polymer separator in a battery.

[0062] Depending on the electrospinning conditions chosen, PVDF copolymer fiber webs with a thickness varying between 1 and 200 pm, preferably between 2 and 100 pm, and a basis weight ranging from 0.5 g / m2 to 100 g / m2, preferably from 1 g / m2 to 50 g / m2 are produced for the field of new energies, in particular as mechanical reinforcement of polymer electrolyte in fuel cells, electrolyzers and batteries.

[0063] The veil is composed of PVDF copolymer fibers and has a weight varying according to the application. EXAMPLES

[0064] An electrospinning solution containing 14 wt% of P(VDF-HFP with 10% HFP), 43 wt% acetone, and 43 wt% gamma valerolactone was prepared in the reactor described in [Fig. 1]. The gamma valerolactone and acetone are introduced into the reactor first. The P(VDF-HFP) in powder form is slowly introduced into the solvent mixture at room temperature under mechanical stirring at 300 rpm. The thermostatically controlled bath is turned on at 50°C, and dissolution is carried out under heat and stirring for 2 to 3 hours. The electrospinning solution can be electrospinned when all the particles are dissolved and the solution is clear.

[0065] The prepared fluorinated polymer solution is electrospun with the following electrospinning parameters: - polymer solution flow rate = 3 g / min; - rotation speed of the electrospinning device = 11000 rpm; - electrode distance = 18 cm; - relative humidity of the air = 15%; - air temperature = 30°C.

[0066]

[0067]

[0068]

[0069]

[0070]

[0071] The fiber webs obtained were characterized by scanning electron microscopy as illustrated in [Fig. 2]. The average fiber diameter measured was 136 + / - 75 nm. Fiber webs were electrospun on a second electrospinning device using different solvent mixtures. The degree of crystallinity was obtained by comparing the enthalpy of fusion AHmes, measured by integrating the melting peak on a DSC thermogram, to the literature value AHiit for 100% crystalline PVDF (104.5 J / g). The following formula was used to calculate the relative crystallinity Xc: Xc = ^'100% The percentage of phase [3 in the crystalline phase of the PVDF copolymer is measured by interpretation of the spectrum obtained in Fourier transform infrared (FTIR) spectroscopy. The results obtained (degrees of crystallinity Xc, percentage of phase [3 in the crystalline phase of P(VDF-HFP) and beta phase percentage by weight in the powder and in the fiber webs) are presented in Table 1. [Tables 1] Sample Xc(%) [3-phase (%) 25:75 30 44 13 14 wt % P(VDF - HFP) / GVL + MEK 25:75 32 44 14 14 wt % P(VDF - HFP) / GVL + DMC 25:75 34 45 15 14 wt % P(VDF- HFP) / GVL + Acetone 25:75 30 68 21 The fibers exhibit different beta phase percentages by weight relative to the fiber weight, with the maximum percentage obtained being 21% for the fiber web made from a P(VDF-HFP) mixture in gamma valerolactone and acetone.

[0072]

Claims

Demands

1. A method for manufacturing a non-woven fabric of poly(vinylidene fluoride) (PVDF) copolymer fibers by electrospinning a solution comprising said PVDF copolymer solubilized in a mixture of solvents, characterized in that said mixture comprises a first solvent which is gamma-valerolactone, and a second solvent which is a pure solvent or a mixture of solvents with a boiling point below 100°C, the second solvent being selected from acetone, dimethyl carbonate, methyl ethyl ketone, 2-methyltetrahydrofuran or mixtures thereof.

2. A manufacturing process according to claim 1, wherein the mass content of the first solvent is between 10 and 70%, preferably between 30 and 50%, relative to the weight of said solution.

3. A manufacturing process according to any one of claims 1 and 2, wherein the mass content of PVDF copolymer is between 6 and 25%, preferably between 10 and 21%, relative to the weight of said solution.

4. A manufacturing process according to any one of claims 1 to 3, wherein the PVDF copolymer has a repeating motif derived from vinylidene fluoride (VDF) in a proportion of at least 50 mol% relative to the total number of moles of repeating units of the PVDF copolymer, and at least one repeating motif derived from a monomer other than VDF, the comonomer being selected from the following list: vinyl fluoride (VF), tetrafluoroethylene (TFE), trifluoroethylene (TrFE), a chlorofluoroethylene (CFE), a chlorodifluoroethylene, chlorotrifluoroethylene (CTFE), dichlorodifluoroethylene, a trichlorofluoroethylene, hexafluoropropylene (HFP), a trifluoropropene, a tetrafluoropropene, a chloro-trifluoropropene, hexafluoroisobutylene, perfluorobutylethylene, a pentafluoropropene, a perfluoroether, in particular a perfluoroalkylvinyl ether, ethylene, an acrylic monomer, a methacrylic monomer, and mixtures thereof.

5. A manufacturing process according to any one of claims 1 to 4, wherein said solution has a Brookfield viscosity at 20°C of 50 to 3000 cP, preferably of 75 to 1500 cP, and further preferably from 80 to 500 cP, as measured according to ASTM D1238-13.

6. A manufacturing process according to any one of claims 1 to 5, wherein said PVDF copolymer is the poly(vinylidene fluoride-co-hexafluoropropylene) copolymer (P(VDF-HFP)).

7. A manufacturing process according to claim 6, wherein the P(VDF-HFP) has a molar proportion in repeating pattern derived from the HFP of 5% to 35%.

8. A manufacturing process according to any one of claims 1 to 7, wherein the PVDF copolymer in powder form is dissolved in the solvent mixture at a temperature between 30 and 70°C for a period of 1 to 5 hours.

9. A manufacturing method according to any one of claims 1 to 8, implemented by means of an installation comprising: an electrospinning solution reservoir, a system for supplying the electrospinning solution to an electrospinning device connected to a positive high voltage generator, a metallic collecting electrode placed opposite the electrospinning device and connected to a negative high voltage generator or ground.

10. A process according to claim 9 wherein, at the exit of the electrospinning chamber, the PVDF copolymer fibers are subjected to a drying step in an oven at an oven temperature between 25°C and 70°C.

11. Veil to be obtained by the process according to any one of claims 1 to 10, said PVDF copolymer fiber veil being characterized by a crystallinity of 29 to 34% and by a percentage of beta phase in the crystalline phase of between 41 and 69%.

12. Use of the veil according to claim 11, having a thickness from 1 to 200 pm, preferably from 2 pm to 100 pm, and a basis weight from 0.5 g / m2 to 100 g / m2, preferably from 1 g / m2 to 50 g / m2, as mechanical reinforcement of polymer electrolyte in fuel cells, electrolyzers and batteries.

13. Use according to claim 12 as mechanical reinforcement of polymer electrolyte, said polymer electrolyte being an ion-exchange polymer membrane in a fuel cell.

14. Use according to claim 12 as mechanical reinforcement of polymer electrolyte, said polymer electrolyte being an ion-exchange polymer membrane in an electrolyzer.

15. Use according to claim 12 as a mechanical reinforcement of polymer electrolyte, said polymer electrolyte being a polymer separator in a battery.

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