Fluoropolymer fibre

Abstract

The present invention pertains to a process for manufacturing one or more fluoropolymer fibers, said process comprising the following steps: (i) providing a liquid composition [composition (C1)] comprising: at least one fluoropolymer comprising at least one hydroxyl end group [polymer (F.sub.OH)L and a liquid medium comprising at least one organic solvent [solvent (S)]; (ii) contacting the composition (C1) provided in step (i) with at least one metal compound [compound (M)] of formula (I) here below: X.sub.4mAY.sub.m (I) wherein X is a hydrocarbon group, optionally comprising one or more functional groups, m is an integer from 1 to 4, A is a metal selected from the group consisting of Si, Ti and Zr, and Y is a hydrolysable group selected from the group consisting of an alkoxy group, an acyloxy group and a hydroxyl group, thereby providing a liquid composition [composition (C2)]; (iii) submitting to at least partial hydrolysis and/or polycondensation the composition (C2) provided in step (ii) thereby providing a liquid composition [composition (C3)] comprising at least one fluoropolymer hybrid organic/inorganic composite; (iv) processing the composition (C3) provided in step (iii) by electrospinning thereby providing one or more fluoropolymer fibers; (v) drying the fluoropolymer fiber(s) provided in step (iv); and (vi) optionally, submitting to compression the fluoropolymer fiber(s) provided in step (v) at a temperature comprised between 50 C. and 300 C. The invention also pertains to a process for the manufacture of said fluoropolymer fiber(s) and to uses of said fluoropolymer fiber(s) in various applications.

Claims

1. A process for manufacturing one or more fluoropolymer fibres, said process comprising: contacting a liquid composition (C1) comprising: at least one fluoropolymer comprising at least one hydroxyl end group [polymer (F.sub.OH)], and a liquid medium comprising at least one organic solvent (S); with at least one metal compound (M) of formula (I):
X.sub.4mAY.sub.m(I) wherein X is a hydrocarbon group, optionally comprising one or more functional groups, m is an integer from 1 to 4, A is a metal selected from the group consisting of Si, Ti and Zr, and Y is a hydrolysable group selected from the group consisting of an alkoxy group, an acyloxy group and a hydroxyl group, thereby providing a liquid composition (C2); submitting composition (C2) to at least partial hydrolysis and/or polycondensation, thereby providing a liquid composition (C3) comprising at least one fluoropolymer hybrid organic/inorganic composite; processing composition (C3) by electrospinning, thereby providing one or more fluoropolymer fibres; drying the fluoropolymer fibre(s), thereby providing one or more dried fluoropolymer fibres; and optionally, submitting to compression the dried fluoropolymer fibre(s) at a temperature comprised between 50 C. and 300 C., thereby providing compressed fluoropolymer fibres.

2. The process according to claim 1, wherein polymer (F.sub.OH) comprises recurring units derived from at least one fluorinated monomer and at least one comonomer comprising at least one hydroxyl end group [comonomer (MA)].

3. The process according to claim 2, wherein comonomer (MA) is at least one comonomer of formula (II-A): ##STR00013## wherein R.sub.1, R.sub.2 and R.sub.3 are hydrogen atoms and R.sub.OH is a C.sub.1-C.sub.5 hydrocarbon moiety comprising at least one hydroxyl group.

4. The process according to claim 1, wherein polymer (F.sub.OH) is selected from the group consisting of: polymers (F.sub.OH-1) comprising recurring units derived from at least one comonomer (MA) as defined above, from at least one per(halo)fluoromonomer selected from tetrafluoroethylene (TFE) and chlorotrifluoroethylene (CTFE), and from at least one hydrogenated monomer selected from ethylene, propylene and isobutylene, optionally containing one or more additional comonomers; and polymers (F.sub.OH-2) comprising recurring units derived from at least one comonomer (MA) as defined above, from vinylidene fluoride (VDF), and, optionally, from one or more fluorinated monomers different from VDF.

5. The process according to claim 1, wherein composition (C2) is obtainable by reacting at least a fraction of the hydroxyl group(s) of the polymer(s) (F.sub.OH) with at least a fraction of the hydrolysable group(s) Y of the compound(s) (M), said composition (C2) comprising: at least one grafted fluoropolymer [polymer (Fg)] comprising pendant groups of formula -AY.sub.m1X.sub.4m, wherein X is a hydrocarbon group, optionally comprising one or more functional groups, m is an integer from 1 to 4, A is a metal selected from the group consisting of Si, Ti and Zr, and Y is a hydrolysable group selected from the group consisting of an alkoxy group, an acyloxy group and a hydroxyl group, a liquid medium comprising at least one organic solvent (S), and optionally, residual amounts of at least one compound (M) of formula (I):
X.sub.4mAY.sub.m(I) wherein X is a hydrocarbon group, optionally comprising one or more functional groups, m is an integer from 1 to 4, A is a metal selected from the group consisting of Si, Ti and Zr, and Y is a hydrolysable group selected from the group consisting of an alkoxy group, an acyloxy group and a hydroxyl group.

6. The process according to claim 1, wherein the one or more fluoropolymer fibres are assembled, thereby providing either a bundle of fluoropolymer fibres or a fluoropolymer mat.

7. A fluoropolymer fibre obtainable by the process according to claim 1.

8. A fluoropolymer fibre comprising: at least one fluoropolymer hybrid organic-inorganic composite comprising fluoropolymer domains consisting of chains obtainable by the polymer (Fg) and inorganic domains consisting of residues obtainable by the compound (M), optionally, at least one compound (M) of formula (I):
X.sub.4mAY.sub.m(I) wherein X is a hydrocarbon group, optionally comprising one or more functional groups, m is an integer from 1 to 4, A is a metal selected from the group consisting of Si, Ti and Zr, and Y is a hydrolysable group selected from the group consisting of an alkoxy group, an acyloxy group and a hydroxyl group, and optionally, at least one derivative obtainable by at least partial hydrolysis and/or polycondensation of at least one compound (M) of formula (I).

9. A fluoropolymer mat obtainable by the process according to claim 1.

10. The fluoropolymer mat according to claim 9, comprising fluoropolymer fibres and/or bundles of fluoropolymer fibres, dried fluoropolymers fibres and/or bundles of dried fluoropolymers fibres or compressed fluoropolymers fibres and/or bundles of compressed fluoropolymers fibres as provided in claim 1.

11. The fluoropolymer mat according to claim 10, wherein said fluoropolymer mat is a non-woven fabric.

12. The fluoropolymer mat according to claim 9, wherein said fluoropolymer mat has a porosity ranging from 10% to 90%, based on the total volume of the fluoropolymer mat.

13. A multilayer assembly comprising: at least one fluoropolymer mat according to claim 9, and at least one substrate layer, wherein at least one surface of said fluoropolymer mat is adhered to at least one surface of said substrate layer.

14. An electrochemical device comprising the fluoropolymer mat according to claim 9.

15. A filtration membrane comprising the fluoropolymer mat according to claim 9.

16. An electrochemical device comprising the multilayer assembly according to claim 13.

17. A filtration membrane comprising the multilayer assembly according to claim 13.

18. The fluoropolymer mat according to claim 12, wherein said fluoropolymer mat has a porosity ranging from 50% to 70%, based on the total volume of the fluoropolymer mat.

19. The fluoropolymer fibre of claim 8, wherein the fluoropolymers fibre consists of: at least one fluoropolymer hybrid organic-inorganic composite consisting of fluoropolymer domains consisting of chains obtainable by the polymer (Fg) and inorganic domains consisting of residues obtainable by the compound (M), optionally, at least one compound (M) of formula (I) here below:
X.sub.4mAY.sub.m(I) wherein X is a hydrocarbon group, optionally comprising one or more functional groups, m is an integer from 1 to 4, A is a metal selected from the group consisting of Si, Ti and Zr, and Y is a hydrolysable group selected from the group consisting of an alkoxy group, an acyloxy group and a hydroxyl group, and optionally, at least one derivative obtainable by at least partial hydrolysis and/or polycondensation of at least one compound (M) of formula (I).

Description

DETAILED DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic representation of the reaction of at least a fraction of the hydroxyl group(s) of a polymer with at least a fraction of the hydrolysable group(s) Y of the compound(s) (M).

(2) FIG. 2 is a schematic representation of a fluoropolymer hybrid organic/inorganic composite which includes fluoropolymer domains (2) consisting of chains obtainable by polymer (Fg) and inorganic domains (1) consisting of residues obtainable by compound (M).

(3) The invention will be now described in more detail with reference to the following examples whose purpose is merely illustrative and not limitative of the scope of the invention.

(4) Raw Materials

(5) Polymer (F.sub.OH-A): VDF-HEA copolymer (HEA: 0.8% by moles) having a melt flow index of 9.3 g/10 min (ASTM D1238, 5 Kg, 190 C.) and a melting point of 166 C.

(6) Polymer (F.sub.OHB): VDF-HFP-HEA terpolymer (HFP: 2.3% by moles; HEA: 1% by moles) having a melt flow index of 15 g/10 min (ASTM D1238, 2.16 Kg, 190 C.) and a melting point of 154 C.

(7) SOLEF 6008 PVDF homopolymer having a melt flow index of 6.0 g/10 min (ASTM D1238, 2.16 Kg, 230 C.) and a melting point of 172 C.

(8) Electrospinning Apparatus and General Procedure

(9) Electrospinning was carried out by using an in-house apparatus composed of a high voltage power supply (Spellman, SL 50 P 10/CE/230), a syringe pump (KD Scientific 200 series), a glass syringe, a stainless-steel blunt-ended needle (inner diameter=0.84 mm) connected with the power supply electrode, and a static grounded plate aluminium collector (1010 cm.sup.2). The polymer solution was dispensed, through a PTFE tube, to the needle vertically placed on the collecting plate. Electrospinning was performed at relative humidity in the range 25-30% and a temperature around 20 C. When not specified, electrospinning experiments were always performed immediately after solution preparation (with solution kept at 20 C.).

(10) Determination of SiO.sub.2 Content

(11) The amount of SiO.sub.2 in the fluoropolymer hybrid organic/inorganic composite was measured by Energy Dispersive Spectroscopy (EDS) analysis of Silicon (Si) and Fluorine (F) elements on micrographs obtained from Scanning Electron Microscopy (SEM).

(12) The SiO.sub.2 content was determined by using the following equation (1):
SiO.sub.2[%]=([SiO.sub.2]/([SiO.sub.2]+[F]))100(1)
wherein [SiO.sub.2] and [F] from equation (1) are calculated using the following equations (2) and (3), respectively:
[SiO.sub.2]=((Si.sub.EDS60)/28)(2)
[F]=((F.sub.EDS64)/38)(3)
wherein: Si.sub.EDS and F.sub.EDS are the weight % of Si and F obtained by EDS, 60 is the molecular weight of SiO.sub.2, 28 is the atomic weight of Si, 64 is the molecular weight of CH.sub.2CF.sub.2, and 38 is the atomic weight of two F elements.

(13) Determination of Ionic Conductivity

(14) The membrane was put between two stainless steel electrodes and sealed in a container.

(15) The resistance of the membrane was measured and the ionic conductivity () was calculated using the following equation:
Ionic conductivity()=d/(R.sub.bS)
wherein d is the thickness [cm] of the film, R.sub.b is the bulk resistance [cm] and S is the area [cm.sup.2] of the stainless steel electrode.

(16) Measurement of the Average Diameter of the Fibre

(17) Fibre morphology was observed with a Philips 515 scanning electron microscope (SEM) at an accelerating voltage of 15 kV. Prior to SEM analysis, samples were sputter-coated with gold. The diameter of the fiber was evaluated by averaging the data obtained by the measurement on about 100 fibres in the SEM image at 10000 magnification.

(18) Measurement of the Thickness of the Mat

(19) The thickness of the mat obtained by electrospinning was determined with a digital micrometer from Mahr equipped with a probe head 908H with a diameter of 12 mm. A force of 0.1 N was applied to the probe head, and the thickness was obtained by averaging the measurements on five locations on the mat. The procedure was repeated by applying to the probe head a force of 0.3 N and of 0.8 N. The thickness of the mat has then been calculated as the intercept at a force of 0 of the linear regression of the values obtained for each force applied.

(20) Measurement of the Compressibility of the Mat

(21) The compressibility of the mat was determined by the slope of the linear regression obtained from the thickness versus force values. Such defined compressibility, in %/N, expresses the percent reduction of the thickness of the mat obtained by applying 1 N to the probe head.

(22) The higher the compressibility value, the softer and thus more difficult to handle the mat thereby provided.

(23) Measurement of the Porosity of the Mat

(24) The porosity of the mat measured by weighting a square specimen of the membrane by using the following equation:
Porosity [%]=100[1w/(abt)]
wherein w is the weight [g], a and b [cm] are the sides of the specimen, t is thickness of the mat [cm] and [g/ml] is the density of composite. In the Examples here below, was 1.78 g/ml.

(25) Measurement of the Dimensional Stability of the Mat

(26) Two 11 cm square specimens were cut from the mat, immersed for 1 minute in N,N-dimethylformamide (DMF) and then dried for 24 hours in an oven. The shrinkage of the mat was measured as the average variation of the dimensions of these two square specimens before and after treatment in DMF.

Example 1Manufacture of a Fluoropolymer Mat

(27) In a glass vial containing a magnetic PTFE stir bar, a fluoropolymer composition was provided, said composition comprising: 10% by weight, based on the total volume of the composition, of the polymer (F.sub.OH-A), and a 70:30 by volume mixture of acetone and dimethyl sulfoxide. The composition thereby provided was stirred at 300 rpm for 40 minutes at room temperature. Then, tetraethoxysilane (TEOS) was added drop-wise to the stirred solution. The polymer (F.sub.OH-A)/TEOS ratio in the composition was kept at 0.38 by weight.

(28) The content of OSiO inorganic domains, calculated assuming complete TEOS hydrolysis/polycondensation, was 43% by weight referred to the total solid content of the mixture. The stirring was kept for other 10 minutes at room temperature.

(29) To promote the hydrolysis/polycondensation of the TEOS, 3 mg of a 37% w/v solution of HCl were added to the vial. The solution was stirred overnight at room temperature at 300 rpm and, just before electrospinning, at 300 rpm at 40 C. for 10 minutes.

(30) A mat was then manufactured by processing by electrospinning within 24 hours, according to the procedure detailed hereinabove, the fluoropolymer composition thereby provided at an applied voltage of 19 kV, a flow rate of 0.01 ml/min and a distance between the syringe tip and the collector of 15 CM.

(31) The mat thereby provided was then dried at 150 C. for 3 hours in an oven.

(32) The average diameter of the fibre was 21040 nm.

(33) The content of OSiO inorganic domains was 28% by weight, as measured by SEM/EDS analysis.

(34) The thickness of the mat was 535 m.

(35) The compressibility of the mat was 41%/N.

(36) The porosity of the mat was 85%.

(37) The ionic conductivity of the mat was 1.910.sup.3 S/cm.

(38) The mechanical properties of the mat at 23 C. are set forth in Table 1 here below:

(39) TABLE-US-00001 TABLE 1 Elastic Yield Yield Breaking Breaking Modulus Stress Strain Stress Strain [MPa] [MPa] [%] [MPa] [%] 365 10 4.4 14.4 28

(40) The mechanical properties were measured at 23 C. according to ASTM D638 standard procedure with a specimen type V at a grip distance of 25.4 mm, a L0 of 21.5 mm and a speed of testing of 1-50 mm/min. The test was started at a speed of 1 mm/min to determine the elastic modulus and then the speed was moved to 50 mm/min to determine the other properties listed in Table 1.

Example 2Manufacture of a Fluoropolymer Mat

(41) The fluoropolymer mat obtained in Example 1 was placed in a press at 200 C. for 30 minutes under a pressure of 1.5 ton.

(42) After thermal treatment, the thickness of the mat was 222 m. The compressibility of the mat was 17%/N.

(43) The porosity of the mat was reduced to 64% but the membrane advantageously kept its porous structure.

(44) The ionic conductivity of the mat was 1.0210.sup.4 S/cm.

Example 3Manufacture of a Fluoropolymer Mat

(45) In a glass vial containing a magnetic PTFE stir bar, a fluoropolymer composition was provided, said composition comprising: 13% by weight, based on the total volume of the composition, of the polymer (F.sub.OHB), and a 80:20 by volume mixture of acetone and dimethyl sulfoxide.

(46) The composition thereby provided was stirred at 300 rpm for 40 minutes at room temperature. Then, tetraethoxysilane (TEOS) was added drop-wise to the stirred solution. The polymer (F.sub.OH-B)/TEOS ratio in the composition was kept at 0.67 by weight.

(47) The content of OSiO inorganic domains, calculated assuming complete TEOS hydrolysis/polycondensation, was 30% by weight referred to the total solid content of the mixture. The stirring was kept for other 10 minutes at room temperature.

(48) To promote the hydrolysis/polycondensation of the TEOS, a 0.1 M solution of HCl was added to the vial in a HCl:TEOS molar ratio of 2:1. The solution was stirred at room temperature for 10 minutes.

(49) A mat was then manufactured by processing by electrospinning within 48 hours, according to the procedure detailed hereinabove, the fluoropolymer composition thereby provided at an applied voltage of 20 kV, a flow rate of 0.01 ml/min and a distance between the syringe tip and the collector of 20 cm.

(50) The mat thereby provided was then dried at 150 C. for 3 hours in an oven.

(51) The average diameter of the fibre was 28080 nm.

(52) The content of OSiO inorganic domains was 30% by weight, as measured by SEM/EDS analysis.

(53) The thickness of the mat was about 20 m.

(54) The shrinkage of the mat was less than 5%.

Example 4Manufacture of a Fluoropolymer Mat

(55) A mat was manufactured by processing by electrospinning, according to the procedure detailed hereinabove, the fluoropolymer composition provided in Example 3 onto a porous polyethylene film having a porosity of 40% and a thickness of about 25 m adhered to the static grounded plate aluminium collector of the electrospinning apparatus.

(56) The plate was moved during the electrospinning processing through a boustrophedon trajectory.

(57) The mat so obtained was dried at 80 C. for 3 hours in an oven and then placed in a press at 80 C. for 2 minutes under a pressure of 1 ton. The peeling strength of the fluoropolymer mat to the porous polyethylene film was higher than the mechanical strength of the mat.

(58) The ionic conductivity of the mat was 1.810.sup.4 S/cm.

(59) After two hours, the shrinkage of the mat was less than 5% as compared with the dimensions of the uncoated porous polyethylene film.

(60) The wettability of the mat in an electrolyte solution was 4.2 times higher than the wettability of the uncoated porous polyethylene film. The wettability was measured by using the Wicking test: a 505 mm mat specimen was placed vertically in contact with an electrolyte solution and, after 40 minutes, the wetting level of the mat by the electrolyte solution was recorded. The higher the wetting level of the mat after 40 minutes versus a reference, the higher the wettability of the mat by the electrolyte solution.

COMPARATIVE EXAMPLE 1

Manufacture of a Fluoropolymer Mat

(61) The same procedure as detailed under Example 1 was followed but using Polymer (F.sub.OH-A) without adding TEOS.

(62) The average diameter of the fibre was 22040 nm.

(63) The thickness of the mat was 968 m.

(64) The mat thereby provided was then tested as in Example 2. A continuous film with no porous structure was obtained.

Comparative Example 2Manufacture of a Fluoropolymer Mat

(65) The same procedure as detailed under Example 1 was followed but using SOLEF 6008 PVDF homopolymer.

(66) The average diameter of the fibre was 20050 nm.

(67) The thickness of the mat was 354 m.

(68) The compressibility of the mat was 59%/N.

(69) The content of OSiO inorganic domains was 10% by weight, as measured by SEM/EDS analysis.

(70) The ionic conductivity of the mat was 2.510.sup.3 S/cm.

Comparative Example 3Manufacture of a Fluoropolymer Mat

(71) The same procedure as detailed under Example 1 was followed but using SOLEF 6008 PVDF homopolymer without adding TEOS.

(72) The average diameter of the fibre was 21040 nm.

(73) The mat thereby provided was then tested as in Example 2. A continuous film with no porous structure was obtained.

Example 5Use of the Fluoropolymer Mat as Separator in a Lithium-Ion Battery

(74) A coin cell was prepared by placing the electrospun membrane as prepared according to Example 1 between a Lithium metal negative electrode and a positive electrode containing LiFePO.sub.4 as active material, SOLEF 5130 PVDF as binder and Super P Li conductive carbon black. The coin cell was filled with 200 l of Selectilyte LP30 electrolyte consisting of a 1 M solution of LiPF.sub.6 in ethylene carbonate/dimethyl carbonate (1:1 weight ratio).

(75) The discharge capacity values of the coin cell so obtained at different discharge rates are set forth in Table 2 here below:

(76) TABLE-US-00002 TABLE 2 Average Discharge Rate [mAh/g] [%] 5 Discharge 5D 58.2 37.1 2 Discharge 2D 116.5 74.2 1 Discharge D 133.6 85.0 0.33 Discharge D/3 149.0 94.9 0.2 Discharge D/5 151.3 96.4 0.1 Discharge D/10 155.0 98.7 0.05 Discharge D/20 154.8 98.6

(77) It has been found that a self standing fluoropolymer mat is advantageously provided by the process according to the invention, wherein at least a fraction of the hydroxyl groups of the polymer (F.sub.OH) is reacted with at least a fraction of the hydrolysable groups Y of the compound (M), said mat advantageously having a high content of inorganic domains while exhibiting, after drying and, optionally, calendering steps, outstanding thermo-mechanical resistance properties up to temperatures of about 300 C.

(78) It has been also found that a self standing fluoropolymer mat is advantageously provided by the process according to the invention, wherein at least a fraction of the hydroxyl groups of the polymer (F.sub.OH) is reacted with at least a fraction of the hydrolysable groups Y of the compound (M), said mat advantageously exhibiting lower compressibility values and successfully maintaining, after drying and, optionally, calendering steps, its fibrous structure and thus its inherent porosity.