Flexible membranes

Abstract

The present invention pertains to membranes made of compositions comprising vinylidene fluoride polymers and certain acrylic polymers having improved flexibility, to a process for their manufacture, and to their use in applications where outstanding flexibility is required.

Description

Flexible membranesCross reference to previous applications

[0001] This application claims priority filed on 2024-07-26 in Europe with Nr 24315359.0, the whole content of this application being incorporated herein by reference for all purposes.Technical Field

[0002] The present invention pertains to membranes made of compositions comprising vinylidene fluoride polymers and certain acrylic polymers having improved flexibility, to a process fortheir manufacture, and to their use in applications where outstanding flexibility is required.Background Art

[0003] Fluoropolymers are known in the art to be excellent materials used in a variety of applications and environments due to their intrinsic chemical resistance and good mechanical properties.

[0004] Polyvinylidene fluoride (PVDF) is a polymer which is known, in particular, for its high chemical stability, which is associated with an excellent mechanical strength.

[0005] Nevertheless, PVDF presents the disadvantage of lacking high flexibility, and this limits its use in applications where this property is required, such as, for example, in membranes.

[0006] It is known to improve the flexibility of PVDF by incorporation of monomer units derived from halogenated comonomers such as, for example, chlorotrifluoroethylene.

[0007] Japanese patent JP60023688 (Kureha) discloses the manufacture of homogeneous thermoplastic copolymers of vinylidene fluoride (VDF) and of chlorotrifluoroethylene (CTFE), containing an amount ranging from 2 to 15% by weight of chlorotrifluoroethylene, by copolymerisation with delayed and modulated injection of CTFE into all of the VDF, so as to take into account the relative reactivity of VDF and of CTFE. The resulting homogeneous copolymers exhibit a flexibility which is increased when compared with PVDF, but their melting temperature is markedly lower than that of PVDF, and this constitutes a serious disadvantage.

[0008] The addition of certain methacrylate polymers to PVDF membranes has been described in US 4618533, with the aim of improving certain properties of the membranes, in particular related to the use in water filtration.

[0009] US 2018 / 355206 describes a binder having utility in a lithium-ion battery, the binder comprising at least one polyvinylidene fluoride and at least one acrylic copolymerincluding monomers bearing functional groups which have an affinity for metals or are capable of fixing to the metals.

[0010] In some cases a very flexible polymers of VDF would be required, especially flexible at low temperatures where cracks could appear due to its high crystallinity.

[0011] The Applicant has developed a PVDF-based film that exhibits good flexibility, has a high strength and can be used in certain applications.Summary of invention

[0012] It is thus an object of the invention a fluoropolymer film that comprises, preferably consists of: a) at least one fluorinated copolymer [polymer (F)] that comprises:(i) recurring units derived from vinylidene fluoride (VDF);(ii) recurring units derived from at least one vinyl monomer (MA) of formula (I):wherein each of Ri, R2and R3, equal to or different from each other, are independently selected from a hydrogen atom and a C1-C3 hydrocarbon group, and Rx is a C1-C20 hydrocarbon moiety comprising at least one functional group selected from the group consisting of hydroxyl, carboxyl, epoxy, ether (-O-), ketone (-C=O-), epoxy, per-carbonate (-O-CO-O-), ester (-OCO-), ester phosphate, ether group, nitrile and anhydride; wherein the vinyl monomer (MA) is present in an amount of from 0.05 to 2 % by moles of with respect to the total moles of recurring units of polymer (F); and b) at least one a (meth)acrylate polymer [polymer (A)] that comprises:(I) recurring units derived from methyl methacrylate (MMA);(II) recurring units derived from at least one C4-Ci8(meth)acrylate monomer.

[0013] In a second instance, the present invention pertains to a process of preparing the fluoropolymer film as above defined, said process comprising the following steps:(A) providing at least one solution of polymer (F) in a solvent (S);(B) providing at least one solution of polymer (A) in a solvent (S);(C) mixing the solutions obtained in step (A) and in step (B);(D) processing the mixture in the form of a film; and(E) drying the film provided in step (C).Detailed description

[0014] In the context of the present invention, the use of parenthesesbefore and after symbols or numbers identifying formulae or parts of formulae has the mere purpose of better distinguishing that symbol or number with respect to the rest of the text; thus, said parentheses could also be omitted.

[0015] By the term “recurring unit derived from vinylidene fluoride” (also generally indicated as vinylidene difluoride 1 ,1 -difluoroethylene, VDF), it is intended to denote a recurring unit of formula -CF2-CH2-.

[0016] The polymer (F) comprises recurring units derived from vinylidene fluoride (VDF) and recurring units derived from at least one vinyl monomer (MA) of formula (I).

[0017] The term "vinyl monomer" as employed herein may comprise recurring units derived from one or more than one vinyl monomer (MA) as above described. In the rest of the text, the expressions " vinyl monomer (MA)" is to be intended, both in the plural and the singular, that is to say that they denote both one or more than one vinyl monomer (MA).

[0018] In one embodiment of the invention, the monomer (MA) is a compound of formula (H):wherein each of R1 and R2 have the meanings as above defined, R3 is hydrogen, and ROH is a hydrogen or a C1-C18 hydrocarbon moiety comprising at least one functional group selected from the group consisting of hydroxyl, carboxyl, epoxy, ether (-O-), ketone (-C=O-), epoxy, per-carbonate (-O-CO-O-), ester (-OCO-), ester phosphate, ether group, nitrile and anhydride; more preferably, each of R1 , R2, R3 are hydrogen, while ROH has the same meaning as above detailed.

[0019] The monomer (MA) of formula (II) is more preferably selected among:- hydroxyethylacrylate (HEA) of formula:2-hydroxypropyl acrylate (HPA) of either of formulae:- acryloyloxyethyl succinate (AES) of formula- (meth) acryloyloxypropyl succinic acid (APS) and mixtures thereof.

[0020] Most preferably, the monomer (MA) of formula (II) is AA, HEA, 2-carboxyethyl (meth) acrylate and / or (meth) acryloyloxyethyl succinate.

[0021] In another embodiment of the invention, the monomer (MA) is a compound of formula (III):R1R2C=CR3-O-C(O)-R4(III) wherein R1, R2and R3, equal to or different from each other, are independently selected from a hydrogen atom, a halogen atom, and a C1-C5 hydrocarbon group, and wherein R4is a C3-C20 linear or branched hydrocarbon chain moiety, optionally comprising at least one functional group selected from a hydroxyl, a carboxyl, an epoxide, an ester phosphate and an ether group.

[0022] Non-limitative examples of monomers (MA) of formula (III) include, notably: - vinyl laurate (VL) of formula- succinic acid vinyl ester of formula

[0023] Preferred vinyl monomers (MA) are notably:- acrylic acid (AA),- (meth)acrylic acid,- 2-carboxyethyl (meth) acrylate,- 3-butenoic acid,- (meth) acryloyloxyethyl succinate,- (meth) acryloyloxypropyl succinate,- 3-(allyloxy)propanoic acid,- hydroxyethyl (meth)acrylate,- hydroxypropyl(meth)acrylate,- hydroxyethylhexyl(meth)acrylate,- (meth)acrylonitrile,- succinic anhydride,- vinyl laurate (VL),- vinyl neodecanoate (VnD),- succinic acid vinyl ester, and mixtures thereof.

[0024] Polymer (F) may still comprise other moieties such as defects, end-groups and the like, which do not affect nor impair its physico-chemical properties.

[0025] Polymer (F) is semi-crystalline. The term semi-crystalline is intended to denote a polymer (F) which possesses a detectable melting point. It is generally understood that a semi-crystalline polymer (F) possesses a heat of fusion determined according to ASTM D 3418 of advantageously at least 0.4 J / g, preferably of at least 0.5 J / g, more preferably of at least 1 J / g.

[0026] Polymer (F) is preferably a linear copolymer, that is to say, it is composed of macromolecules made of substantially linear sequences of recurring units from VDF monomer and (MA) monomer; polymer (F) is thus distinguishable from grafted and / or comb-like polymers.

[0027] Polymer (F) comprises at least 0.05 % by moles, more preferably at least 0.1 % by moles, even more preferably at least 0.2 % by moles of recurring units derived from said vinyl monomer (MA).

[0028] Polymer (F) comprises preferably at most 2 % by moles, more preferably at most 1.8 % by moles, even more preferably at most 1.5% by moles of recurring units derived from said vinyl monomer (MA).

[0029] In a preferred embodiment of the invention, in polymer (F) the recurring units derived from vinylmonomer (MA) of formula (I) are comprised in an amount of from 0.1 to 1 % by moles with respect to the total moles of recurring units of polymer (F).

[0030] The polymer (F) has advantageously an intrinsic viscosity, measured in dimethylformamide at 25 °C, of above 0.15 l / g and at most 0.60 l / g, preferably in the range of 0.20 - 0.50 l / g, more preferably comprised in the range of 0.25 - 0.50 l / g.

[0031] The polymer (F) may further comprise recurring units derived from one or more fluorinated comonomers (CF) different from VDF.

[0032] By the term “fluorinated comonomer (CF)”, it is hereby intended to denote an ethylenically unsaturated comonomer comprising at least one fluorine atoms.

[0033] Non-limitative examples of suitable fluorinated comonomers (CF) include, notably, the followings:(a) C2-C8fluoro- and / or perfluoroolefins such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP), pentafluoropropylene and hexafluoroisobutylene;(b) C2-C8 hydrogenated monofluoroolefins, such as vinyl fluoride; 1 ,2- difluoroethylene and trifluoroethylene;(c) perfluoroalkylethylenes of formula CH2=CH-Rra, wherein Rrois a Ci-C6perfluoroalkyl group;(d) chloro- and / or bromo- and / or iodo-C2-C6fluoroolefins such as chlorotrifluoroethylene (CTFE).

[0034] In one embodiment of the invention, polymer (F) comprises from 0.1 to 10.0% by moles, preferably from 0.3 to 5.0% by moles, more preferably from 0.5 to 3.0% by moles of recurring units derived from said fluorinated comonomer (CF).

[0035] In one preferred embodiment of the invention, the polymer (F) comprises recurring units derived from:- at least 70% by moles, preferably at least 75% by moles, more preferably at least 85% by moles of vinylidene fluoride (VDF),- from 0.1 % to 1% by moles, of a vinyl monomer (MA) of formula (I);- optionally from 0.1 to 10.0% by moles, preferably from 0.3 to 5.0% by moles, more preferably from 0.5 to 3.0% by moles of recurring units derived from at least one fluorinated comonomer (CF).

[0036] The polymer (F) may be obtained by polymerization of a VDF monomer, at least one monomer (MA) and optionally at least one comonomer (CF) either in suspension in organic medium, according to the procedures described, for example, in WO 2008 / 129041 , or in aqueous emulsion, typically carried out as described in the art (see e g. US 4,016,345, US 4,725,644 and US 6,479,591).

[0037] The procedure for preparing the polymer (F) in suspension comprises polymerizing in an aqueous medium in the presence of a radical initiator the vinylidene fluoride (VDF) monomer, monomer (MA) and optionally comonomer (CF), in a reaction vessel, said process comprising- continuously feeding an aqueous solution comprising monomer (MA); and- maintaining the pressure in said reactor vessel exceeding the critical pressure of the vinylidene fluoride.

[0038] During the whole suspension polymerization run, pressure is maintained above critical pressure of vinylidene fluoride. Generally, the pressure is maintained at a value of more than 50 bars, preferably of more than 75 bars, even more preferably of more than 100 bars.

[0039] The expressions "continuous feeding", “adding continuously” or "continuously feeding" means that slow, small, incremental additions the aqueous solution of vinyl monomer (MA) take place until polymerization has concluded.

[0040] The polymer (F) thus obtained has a high uniformity of monomer (MA) distribution in the polymer backbone, which advantageously maximizes the effects of the modifying monomer (MA) on both adhesiveness and / or behaviour of the resulting copolymer.

[0041] When the recurring units derived from monomer (MA) in polymer (F) comprise functional groups that can be hydrolysed, polymer (F) can be further subjected to a step of hydrolysis. In particular, a polymer (F) comprising recurring units (MA) of formula (I) wherein Rx is a C1-C20 hydrocarbon moiety comprising at least one functional group selected from nitrile and anhydride can be subjected to hydrolysis to provide a polymer (F’) wherein the nitrile or anhydride groups are at least partially hydrolyzed to the corresponding carboxylic acid groups.

[0042] The at least one (meth)acrylate polymer (A), different from polymer (F), is a polymer comprising recurring units derived from methyl methacrylate (MMA) and recurring units derived from at least one C4-C18 (meth)acrylate monomer.

[0043] The C4-Ci8(meth)acrylate monomer may optionally include one or more functional group(s) selected from the group consisting of double bonds, hydroxyl, carboxyl, epoxy, ether (-O-), ketone (-C=O-), epoxy, per-carbonate (-O-CO-O-) and ester (-OCO-); or a five- to six-membered heterocycle.

[0044] Non-limited examples of C4-Ci8(meth)acrylate monomers are n-butyl (meth)acrylate, 2-ethoxy ethyl (meth)acrylate, tert-butyl (meth)acrylate, pentyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate), lauryl (meth)acrylate, cycloalkyl (meth)acrylate, like cyclohexyl(meth)acrylate, phenyl (meth)acrylate, polyalkylene glycol acrylate.

[0045] In a preferred embodiment of the present invention, polymer (A) is a polymer comprising recurring units derived from methyl methacrylate and recurring units derived from n-butyl acrylate (BA).

[0046] In another preferred embodiment of the present invention, polymer (A) is a polymer comprising recurring units derived from methyl methacrylate, recurring units derived from n-butyl acrylate (BA) and recurring units derived from octyl acrylate (OA).

[0047] In still another preferred embodiment of the present invention, polymer (A) is a polymer comprising recurring units derived from methyl methacrylate, recurring units derived from n-butyl acrylate (BA) and recurring units derived from lauryl acrylate.

[0048] The recurring units derived from methyl methacrylate (MMA) and the recurring units derived from at least one C4-C8(meth)acrylate monomer are present in polymer (A) in an amount ratio between 50:50 to 70:40, preferably in a 60:40 ratio.

[0049] Polymer (A) may further include up to 15 % by moles of additional recurring units derived from at least one vinyl monomer (MA) of formula (II) as above defined.

[0050] In one embodiment of the present invention, polymer (A) is a polymer comprising recurring units derived from methyl methacrylate, recurring units derived from n- butyl acrylate (BA) and recurring units derived from (meth)acrylic acid.

[0051] The (meth)acrylic polymer (A) is prepared by polymerizing a mixture of methyl methacrylate (MMA), at least one C4-Ci8(meth)acrylate monomer, and optionally at least one vilyl monomer (MA) of formula (II).

[0052] The polymer (A) for use in the composition (C) of the present invention preferably has a number average molecular weight (Mn) of at least 1 kDa, for example between 1 and 200 kDa. More preferably, the polymer (A) has a number average molecular weight (Mn) between 15 and 150 kDa.

[0053] The polymer (A) for use in the composition (C) of the present invention preferably has a ratio between weight average molecular weight (Mw) and number average molecular weight (Mn) of about 1 to 4.

[0054] Polymer (A) may suitably contain from 1 to 50, preferably 3 to 40, and more preferably 10 to 30 % by weight of at least one C4-Ci8(meth)acrylate monomer.

[0055] The preferred fluoropolymer film of the present invention comprises the polymer(A) in an amount comprised between 5 and 50 % by weight, preferably between 10 and 40% by weight, more preferably between 15 and 30% by weight.

[0056] The Applicant has demonstrated that the fluoropolymer films of the present invention show improved flexibility in comparison with films including VDF polymers only, and are flexible enough to be used in products that undergo dynamic flexing at low temperatures, such as flexible secondary batteries.

[0057] The fluoropolymer film of the invention may be prepared by casting or by compression moulding.

[0058] The fluoropolymer film of the invention may be prepared by casting by a process comprising the following steps:(A) providing at least one solution of polymer (F) as defined above in a solvent (S);(B) providing at least one solution of polymer (A) as defined above in a solvent (S);(C) mixing the solutions obtained in step (A) and in step (B) to provide a composition (C);(D) processing the composition (C) in the form of a film; and(E) drying the film provided in step (D).

[0059] The process for preparing the fluoropolymer film may further include a step of submitting the dried film obtained in step (E) to a compression step to obtain a film of the suitable thickness.

[0060] The solvent (S) may preferably be an organic polar one, examples of which may include: N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, hexamethylphosphamide, dioxane, tetrahydrofuran, tetramethylurea, triethyl phosphate, and trimethyl phosphate. These solvents may be used singly or in mixture of two or more species.

[0061] Under step (E) of the process of the invention, drying may be performed either under atmospheric pressure or under vacuum. Alternatively, drying may be performed under modified atmosphere, e.g. under an inert gas, typically exempt notably from moisture (water vapour content of less than 0.001% v / v).

[0062] The drying temperature will be selected so as to effect removal by evaporation of one or more solvents (S) from the film of the invention.

[0063] The fluoropolymer film of the invention may alternatively be prepared by compression moulding the mixture of polymer (F) and polymer (A) a temperature of at least 200°C between two foils of an inert support.

[0064] Preferably, the inert support is a metal support.

[0065] In a preferred embodiment, the fluoropolymer film of the invention may be prepared by compression moulding between two foils of aluminium at a temperature of about 240°C.

[0066] Suitably, the fluoropolymer film of the invention has a thickness comprised between 10 and 200 micrometers, preferably between 20 and 50 micrometers, more preferably between 30 and 40 micrometers.

[0067] The fluoropolymer film of the invention is particularly suitable for use in electrochemical devices, in particular in secondary batteries.

[0068] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

[0069] The invention will be now described with reference to the following examples, whose purpose is merely illustrative and not intended to limit the scope of the invention.

[0070] EXPERIMENTAL PART

[0071] Raw materials

[0072] Polymer (A-1): Plastistrength® L1000, copolymer of MMA / BA commercially available from Arkema.

[0073] Polymer (F-1): VDF-AA (0.6% by moles) polymer having an intrinsic viscosity of 0.40 l / g in DMF at 25°C.

[0074] Determination of intrinsic viscosity of polymer (F)

[0075] Intrinsic viscosity (q) [dl / g] was measured using the following equation on the basis of dropping time, at 25°C, of a solution obtained by dissolving the polymer (F) in N,N-dimethylformamide at a concentration of about 0.2 g / dl using a Ubbelhode viscosimeter:where c is polymer concentration [g / dl], qris the relative viscosity, i.e. the ratio between the dropping time of sample solution and the dropping time of solvent, qspis the specific viscosity, i.e. qr-1 , and F is an experimental factor, which for polymer (F) corresponds to 3.

[0076] DSC analysis

[0077] DSC analyses were carried out according to ASTM D4591 standard; the melting point relating to the second heating run (Tf2) was determined at a heating rate of 10°C / min.

[0078] General Preparation of films

[0079] Polymers films were prepared in dry-room, at -10°C as dew point, from polymer solution in 1-Methyl-2-pyrrolidinone at concentration of 6% w / w; casted onto glass support and dried in dynamic vacuum oven at 90°C to obtain dried films between 25-45 pm of thickness.

[0080] Mechanical property measurements

[0081] Mechanical properties of polymer cast films were measured on ASTM D638

[0082] Type specimen, using a UTM (universal testing machine) with a load cell of 0.0 KN, at room temperature.

[0083] Strain at break and Young Modulus of the films of polymers were calculated according to ASTM D638.

[0084] Results come from the average of 5 specimens.

[0085] The results are shown in Table 1 .Table 1

Claims

ClaimsClaim 1. A fluoropolymer film that comprises, preferably consists of: a) at least one fluorinated copolymer [polymer (F)] that comprises:(i) recurring units derived from vinylidene fluoride (VDF);(ii) recurring units derived from at least one vinyl monomer (MA) of formula (I):wherein each of Ri, R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom and a C1-C3 hydrocarbon group, and Rx is a C1-C20 hydrocarbon moiety comprising at least one functional group selected from the group consisting of hydroxyl, carboxyl, epoxy, ether (-O-), ketone (-C=O ), epoxy, per carbonate (-O-CO-O-), ester (-OCO-), ester phosphate, ether group, nitrile and anhydride; wherein the vinyl monomer (MA) is present in an amount of from 0.05 to 2 % by moles of with respect to the total moles of recurring units of polymer (F); and b) at least one a (meth)acrylate polymer [polymer (A)] that comprises:(I) recurring units derived from methyl methacrylate (MMA);(II) recurring units derived from at least one C4-Ci8(meth)acrylate monomer.Claim 2. The fluoropolymer film according to claim 1 , wherein the vinyl monomer (MA) is a compound of formula (II):wherein each of R1 and R2 have the meanings as above defined, R3 is hydrogen, and ROH is a hydrogen or a C1-C18 hydrocarbon moiety comprising at least one functional group selected from the group consisting of hydroxyl, carboxyl, epoxy, ether (-O-), ketone (-C=O-), epoxy, per-carbonate (-O-CO-O-), ester (-OCO-), ester phosphate, ether group, nitrile and anhydride.Claim 3. The fluoropolymer film according to claim 1 , wherein the vinyl monomer (MA) is a compound of formula (III):R1R2C=CR3-O-C(O)-R4(III) wherein R1, R2and R3, equal to or different from each other, are independently selected from a hydrogen atom, a halogen atom, and a C1-C5 hydrocarbon group, and wherein R4is a C3-C20 linear or branched hydrocarbon chain moiety, optionally comprising at least one functional group selected from a hydroxyl, a carboxyl, an epoxide, an ester phosphate and an ether group.Claim 4. The fluoropolymer film according to anyone of claims 1 to 3, wherein the vinyl monomer (MA) is selected from the group consisting of:- acrylic acid (AA),- (meth)acrylic acid,- 2-carboxyethyl (meth) acrylate,- 3-butenoic acid,- (meth) acryloyloxyethyl succinate,- (meth) acryloyloxypropyl succinate,- 3-(allyloxy)propanoic acid,- hydroxyethyl (meth)acrylate,- hydroxypropyl(meth)acrylate,- hydroxyethylhexyl(meth)acrylate,- (meth)acrylonitrile,- succinic anhydride,- vinyl laurate (VL),- vinyl neodecanoate (VnD),- succinic acid vinyl ester, and mixtures thereof.Claim 5. The fluoropolymer film according to claim 1 wherein polymer (F) further comprises recurring units derived from one or more fluorinated comonomers (CF) different from VDF, wherein said comonomer (CF) is selected from the group consisting of:(a) C2-C8fluoro- and / or perfluoroolefins such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP), pentafluoropropylene and hexafluoroisobutylene;(b) C2-C8 hydrogenated monofluoroolefins, such as vinyl fluoride; 1 ,2- difluoroethylene and trifluoroethylene;(c) perfluoroalkylethylenes of formula CH2=CH-Rf0, wherein Rrois a Ci-C6perfluoroalkyl group;(d) chloro- and / or bromo- and / or iodo-C2-C6fluoroolefins such as chlorotrifluoroethylene (CTFE).Claim 6. The fluoropolymer film according to any one of the preceding claims, wherein polymer (F) comprises recurring units derived from:- at least 70% by moles, preferably at least 75% by moles, more preferably at least 85% by moles of vinylidene fluoride (VDF),- from 0.1% to 1% by moles, of a vinyl monomer (MA) of formula (I);- optionally from 0.1 to 10.0% by moles, preferably from 0.3 to 5.0% by moles, more preferably from 0.5 to 3.0% by moles of recurring units derived from at least one fluorinated comonomer (CF).Claim 7. The fluoropolymer film according to any one of the preceding claims, wherein the C4-C18 (meth)acrylate monomer in polymer (A) is selected from the group consisting of n-butyl (meth)acrylate, 2-ethoxy ethyl (meth)acrylate, tert-butyl (meth)acrylate, pentyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate), lauryl (meth)acrylate, cycloalkyl (meth)acrylate, like cyclohexyl(meth)acrylate, phenyl (meth)acrylate, polyalkylene glycol acrylate.Claim 8. The fluoropolymer film according to any one of the preceding claims, wherein polymer (A) is a polymer comprising recurring units derived from methyl methacrylate and recurring units derived from n-butyl acrylate (BA).Claim 9. The fluoropolymer film according to any one of the preceding claims, wherein the recurring units derived from methyl methacrylate (MMA) and the recurring units derived from at least one C4-Ci8(meth)acrylate monomer are present in polymer (A) in an amount ratio between 50:50 to 70:40, preferably in a 60:40 ratio.Claim 10. The fluoropolymer film according to any one of the preceding claims, wherein polymer (A) further includes up to 15 % by moles of additional recurring units derived from at least one vinyl monomer (MA) of formula (II)wherein each of R1 and R2 have the meanings as above defined, R3 is hydrogen, and ROH is a hydrogen or a C1-C18 hydrocarbon moiety comprising at least one functional group selected from the group consisting of hydroxyl, carboxyl, epoxy, ether (-O-), ketone (-C=O-), epoxy, per-carbonate (-O-CO-O-), ester (-OCO-), ester phosphate, ether group, nitrile and anhydride.Claim 11 . The fluoropolymer film according to any one of the preceding claims, wherein the polymer (A) is comprised in the polymer film in an amount between 5 and 50 % by weight, preferably between 10 and 40% by weight, more preferably between 15 and 30% by weight.Claim 12. The fluoropolymer film according to any one of the preceding claims which has a thickness comprised between 10 and 200 micrometers, preferably between 20 and 50 micrometers, more preferably between 30 and 40 micrometers.Claim 13. A process of preparing the fluoropolymer film according to any one of claims 1 to 12, said process comprising the following steps:(A) providing at least one solution of polymer (F) in a solvent (S);(B) providing at least one solution of polymer (A) in a solvent (S);(C) mixing the solutions obtained in step (A) and in step (B);(D) processing the mixture in the form of a film; and(E) drying the film provided in step (C).Claim 14. The process according to claim 13, which further include a step of submitting the dried film obtained in step (E) to a compression step to obtain a film of the suitable thickness.Claim 15. The process according to anyone of claims 13 or 14, wherein the solvent (S) may preferably be an organic polar one, examples of which may include: N-methyl-2- pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, hexamethylphosphamide, dioxane, tetrahydrofuran, tetramethylurea, triethyl phosphate, and trimethyl phosphate, or mixtures thereof.Claim 16. Use of the fluoropolymer film according to any one of claims 1 to 12 in electrochemical devices, in particular in secondary batteries.

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