Process for manufacturing fluoropolymer membranes

Abstract

The present invention pertains to a process for manufacturing a fluoropolymer membrane, said process comprising the following steps: (i) providing a composition [composition (C)] comprising, preferably consisting of: at least one fluoropolymer [polymer (F)], a water-soluble liquid medium [medium (M.sub.ws)] comprising, preferably consisting of at least one solvent selected from the group consisting of diesters of formula (I-.sub.de), esteramides of formula (I-.sub.ea) and diamides of formula (I-.sub.da); R.sup.1 (O)CO-A.sub.de-OC(O)R.sup.2 (I-.sub.de) R.sup.1O(O)C-A.sub.ea-C(O)NR.sup.3R.sup.4 (I-.sub.ea) R.sup.5R.sup.6N(O)C-A.sub.da-C(O)NR.sup.5R.sup.6 (I-.sub.da) wherein: R.sup.1 and R.sup.2, equal to or different from each other, are independently selected from the group consisting of C.sub.1-C.sub.20 hydrocarbon groups; R.sup.3, R.sup.4, R.sup.5 and R.sup.6, equal to or different from each other, are independently selected from the group consisting of hydrogen, C.sub.1-C.sub.36 hydrocarbon groups, possibly substituted, being understood that R.sup.3, R.sup.4, R.sup.5 and R.sup.6 might be part of a cyclic moiety including the nitrogen atom to which they are bound, said cyclic moiety being possibly substituted and/or possibly comprising one or more than one additional heteroatoms, A.sub.de is a C.sub.3-C.sub.10 divalent alkylene group comprising one or more ether oxygen atoms, A.sub.ea and A.sub.da, equal to or different from each other, are independently C.sub.3-C.sub.10 divalent alkylene groups, optionally comprising one or more ether oxygen atoms and/or one or more functional side groups; (ii) processing the composition (C) at a temperature of at least 100 C. thereby providing a film; (iii) cooling the film provided in step (ii) to a temperature below 50 C.; (iv) contacting the film provided in step (iii) with a non-solvent medium [medium (M NS)] thereby providing a fluoropolymer membrane; and (v) optionally, drying the fluoropolymer membrane provided in step (iv).

Claims

1. A process for manufacturing a fluoropolymer membrane, said process comprising: processing a composition (C) at a temperature of at least 100 C. thereby providing a film, wherein composition (C) comprises: at least one fluoropolymer [polymer (F)], a water-soluble liquid medium (M.sub.WS) that is free from dimethyl sulphoxide (DMSO) and comprises at least one solvent selected from the group consisting of diesters of formula (I-.sub.de), esteramides of formula (I-.sub.ea) and diamides of formula (I-.sub.da):
R.sup.1(O)CO-A.sub.de-OC(O)R.sup.2(I-.sub.de)
R.sup.1O(O)C-A.sub.ea-C(O)NR.sup.3R.sup.4(I-.sub.ea)
R.sup.5R.sup.6N(O)C-A.sub.da-C(O)NR.sup.5R.sup.6(I-.sub.da) wherein: R.sup.1 and R.sup.2, equal to or different from each other, are independently selected from the group consisting of C.sub.1-C.sub.20 hydrocarbon groups; R.sup.3, R.sup.4, R.sup.5 and R.sup.6, equal to or different from each other, are independently selected from the group consisting of hydrogen, optionally substituted C.sub.1-C.sub.36 hydrocarbon groups, wherein R.sup.3 and R.sup.4 optionally form a cyclic moiety including the nitrogen atom to which they are bound, and wherein R.sup.5 and R.sup.6 optionally form a cyclic moiety including the nitrogen atom to which they are bound, wherein each cyclic moiety is optionally substituted and/or optionally comprises one or more than one additional heteroatoms, A.sub.de is a C.sub.3-C.sub.10 divalent alkylene group comprising one or more ether oxygen atoms, A.sub.ea and A.sub.da, equal to or different from each other, are independently C.sub.3-C.sub.10 divalent alkylene groups, optionally comprising one or more ether oxygen atoms and/or one or more functional side groups; cooling the film to a temperature below 50 C. to form a cooled film; contacting the cooled film with a non-solvent medium (M.sub.NS) thereby providing a fluoropolymer membrane; and optionally, drying the fluoropolymer membrane.

2. The process according to claim 1, wherein polymer (F) comprises recurring units derived from at least one fluorinated monomer (F).

3. The process according to claim 1, wherein polymer (F) is selected from the group consisting of: polymers (F-1) comprising recurring units derived from vinylidene fluoride (VDF) and, optionally, from at least one fluorinated monomer different from VDF; and polymers (F-2) comprising recurring units derived from at least one fluorinated monomer 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, in amounts of from 0.01% to 30% by moles, based on the total amount of TFE and/or CTFE and said hydrogenated monomer(s).

4. The process according to claim 1, wherein the medium (M.sub.WS) comprises at least one solvent selected from the group consisting of esteramides of formula (I-.sub.ea) and diamides of formula (I-.sub.da), wherein A.sub.ea in formula (I-.sub.ea) and A.sub.da in formula (I-.sub.da), equal to or different from each other, are branched C.sub.3-C.sub.10 divalent alkylene groups.

5. The process according to claim 1, wherein the medium (M.sub.WS) comprises at least one solvent selected from the group consisting of diesters of formula (I-.sub.de), wherein A.sub.de in formula (I-.sub.de) is a C.sub.3-C.sub.10 divalent alkylene group comprising one or more ether oxygen atoms.

6. The process according to claim 5, wherein the medium (M.sub.WS) comprises at least one solvent selected from the group consisting of diesters of formula (II-de):
R.sup.1(O)CO(CH.sub.2CH.sub.2O).sub.nCH.sub.2CH.sub.2OC(O)R.sup.2(II-.sub.de) wherein n is an integer comprised between 1 and 2 and R.sup.1 and R.sup.2, equal to or different from each other, are C.sub.1-C.sub.20 alkyl groups.

7. The process according to claim 1, wherein composition (C) is processed by using casting techniques.

8. The process according to claim 7, wherein composition (C) is processed by casting onto a flat supporting substrate thereby providing a flat film.

9. The process according to claim 7, wherein composition (C) is processed by casting onto a tubular supporting substrate thereby providing a tubular film.

10. The process according to claim 1, wherein cooling is performed upon exposure of the film to air having a relative humidity higher than 10% at a temperature below 50 C.

11. The process according to claim 1, wherein cooling is performed by contacting the film with a liquid medium at a temperature below 50 C.

12. The process according to claim 1, wherein medium (M.sub.NS) comprises water and/or at least one solvent selected from the group consisting of diesters of formula (I-.sub.de), esteramides of formula (I-.sub.ea) and diamides of formula (I-.sub.da).

13. The process according to claim 1, wherein the fluoropolymer membrane is dried at a temperature of at least 30 C.

14. A fluoropolymer membrane obtainable by the process according to claim 1.

15. The fluoropolymer membrane according to claim 14, said membrane being a symmetric membrane.

16. A filtration membrane comprising the fluoropolymer membrane according to claim 14.

17. A separator membrane for electrochemical devices comprising the fluoropolymer membrane according to claim 14.

18. The fluoropolymer membrane according to claim 14, said membrane being an asymmetric membrane.

19. The process according to claim 1, wherein composition (C) consists of at least one polymer (F), and the water-soluble liquid medium (M.sub.WS), wherein medium (M.sub.WS) consists of at least one solvent selected from the group consisting of diesters of formula (I-.sub.de), esteramides of formula (I-.sub.ea) and diamides of formula (I-.sub.da).

20. The process according to claim 19, wherein the medium (M.sub.WS) consists of at least one solvent selected from the group consisting of diesters of formula (II-de):
R.sup.1(O)CO(CH.sub.2CH.sub.2O).sub.nCH.sub.2CH.sub.2OC(O)R.sup.2(II-.sub.de) wherein n is an integer comprised between 1 and 2 and R.sup.1 and R.sup.2, equal to or different from each other, are C.sub.1-C.sub.20 alkyl groups.

Description

EXAMPLE 1

(1) A 15% by weight solution was prepared by dissolving 15 g of SOLEF 1015 PVDF in 85 g of TEGDA at 160 C. for 2 hours under mechanical stirring. Then, the solution was cast over a glass slide kept at 30 C. to form a film having a thickness of about 200 m. The film was then allowed to solidify in air for 120 min at 30 C.; after this solidification time, the film and the glass slide were immersed for one day in a deionized water bath to extract the TEGDA. Then, the film was dried for two days in air at 50 C.

(2) The membrane so obtained had a porosity of 75%, an average pore diameter of 0.11 m, a water permeability of 2342 L/hm.sup.2, a stress at break of 2.1 MPa and an elongation at break of 471%.

EXAMPLE 2

(3) A porous membrane was prepared according to Example 1 by dissolving 20 g of SOLEF 1015 PVDF in 80 g of TEGDA.

(4) The membrane so obtained had a porosity of 72%, an average pore diameter of 0.08 m, a water permeability of 665 L/hm.sup.2, a stress at break of 0.9 MPa and an elongation at break of 63%.

EXAMPLE 3

(5) A 30% by weight solution was prepared by dissolving 300 g of SOLEF 1015 PVDF and 50 g of PVP in 850 g of RHODIASOLV POLARCLEAN solvent at 160 C. for 2 hours under mechanical stirring. After keeping the homogeneous solution so obtained at 160 C. for 3 hours without mixing for degassing, the solution was extruded through a spinneret maintained at 160 C., said spinneret comprising an outer tube and an inner tube having a diameter of 1.53 mm and 0.6 mm, respectively, by a gear pump under a nitrogen pressure of 0.3 MPa to a coagulation bath maintained at 15 C. A diluent was introduced into the inner orifice of the spinneret at 160 C. to act as lumen. The bore flow and the dope flow rate were 12 mL/min and 10 mL/min, respectively.

(6) The hollow fibers thereby provided were then immersed in water at 25 C. in order to ensure complete removal of the lumen and then washed with a 4000 ppm solution of sodium hypochlorite buffered to pH 7 in order to remove PVP.

(7) The hollow fiber so obtained had a porosity of 81%, a water permeability of 750 L/hm.sup.2, a stress at break of 1.0 MPa and an elongation at break of 72%.

COMPARATIVE EXAMPLE 1

(8) A 15% by weight solution was prepared by dissolving 15 g of SOLEF 1015 PVDF in 85 g of ATBC at 180 C. for 2 hours under mechanical stirring. Then, the solution was cast over a glass slide kept at 30 C. to form a film having a thickness of about 200 m. The film was then allowed to solidify in air for 30 min at 50 C.; after this solidification time, the film and the glass slide were immersed overnight in 1 It. of ethanol to extract the diluent. Then, the film was dried for two days in air at 40 C. The membrane so obtained had a porosity of 76%, an average pore diameter of 0.56 m, a water permeability of 3200 L/hm.sup.2, a stress at break of 1.0 MPa and an elongation at break of 9%.

COMPARATIVE EXAMPLE 2

(9) A 20% by weight solution was prepared by dissolving 20 g of SOLEF 1015 PVDF in 80 g of ATBC at 180 C. for 2 hours under mechanical stirring. Then, the solution was cast using the small casting machine over a glass slide kept at 30 C. to form a film having a thickness of about 200 m. The film was then allowed to solidify in air for 30 min at 50 C.; after this solidification time, the film and the glass slide were immersed overnight in 1 It. of ethanol to extract the diluent. Then, the film was dried for two days in air at 40 C.

(10) The membrane so obtained had a porosity of 76%, an average pore diameter of 0.35 m, a water permeability of 2550 L/hm.sup.2, a stress at break of 0.9 MPa and an elongation at break of 14%.

(11) As shown in Table 1 here below, the membranes obtained according to the process of the invention as notably exemplified by Examples 1, 2 and 3 advantageously have outstanding water permeability properties combined with outstanding mechanical properties, in particular enhanced elongation at break values, while having a relatively low average pore diameter.

(12) TABLE-US-00001 TABLE 1 Average pore Water Stress Elongation Porosity diameter permeability at break at break Run [%] [m] [L/h m2] [MPa] [%] Ex. 1 75 0.11 2342 2.1 471 Ex. 2 72 0.08 665 0.9 63 Ex. 3 81 750 1.0 72 C. Ex. 1 76 0.56 3200 1.0 9 C. Ex. 2 76 0.35 2550 0.9 14