Triethyl phosphate / N-methylpyrrolidone mixed solvent for PVDF membrane production

Abstract

The present invention discloses a method for producing a PVDF membrane, comprising a dope solution for producing the membrane comprising a blend of TEP and NMP as a solvent system, wherein the PVDF resin comprises a homopolymer resin or a copolymer of PVDF and at least one of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene, and tetrafluoropropene.

Description

[Technical Field]

[0001] The present invention relates to a dope solution comprising at least one polymer P, at least one water-soluble or hydrogel polymer, and a triethyl phosphate / N-methylpyrrolidone blend, as well as a method for producing a membrane and the use of this membrane for water treatment. [Background technology]

[0002] Polyvinylidene fluoride (PVDF) and polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) copolymers are high-performance polymers used in a variety of technical applications due to their mechanical properties and chemical and thermal stability. Polyvinylidene fluoride (PVDF) has limited solubility in many common solvents.

[0003] One of the main technical applications of PVDF polymer is as a raw material for the production of membranes, such as hollow fiber membranes. The process for producing PVDF membranes involves dissolving the PVDF polymer in a solvent, coagulating the PVDF polymer from the solvent, and further post-processing steps. The choice of solvent is critical to the process and affects the properties of the resulting membrane, including, but not limited to, its mechanical strength, elasticity, water permeability, and pore size.

[0004] There are many publications that describe the use of pure triethyl phosphate as a solvent for casting PVDF membranes by the NIPS phase inversion process. For example, Poly. Jnl., 42, (2006), 158; Liu et al., J. Mem. Sci., 375, (2011), 1; Chang et al., J. Mem. Sci., 539, (2017), 295. In the patent literature, Chidlow (U.S. Patent No. 5,565,153) describes the use of TEP as a solvent for casting flat-sheet PVDF membranes without the use of cosolvents.

[0005] Herczeg (U.S. Patent Application Publication No. 2004 / 050791) teaches the use of NMP to produce PVDF membranes in the presence of "non-solvent" additives, with TEP listed as one of the "non-solvents." U.S. Patent Application Publication No. 2004 / 050791 does not define "non-solvent," nor does it provide examples of PVDF formulations and NMP-TEP blends.

[0006] There is literature on DMAC and TEP for producing PVDF hollow fiber membranes (Garcia-Fernandez et al., J. Mem. Sci. 468 (2014) 324), but no testing has been done on NMP.

[0007] In the field of solvents, there is a continuing demand for alternatives to solvents currently used in certain applications (e.g., NMP). In the case of polyvinylidene fluoride (PVDF), new solvents must be able to prepare solutions that allow high PVDF content without turbidity. For membranes made from new solvents, achieving membrane quality is critical. In particular, the water permeability of such membranes must be as high as possible while maintaining strength and elasticity, as measured by elongation at break.

[0008] The objective of this invention is to provide a solvent system that is less toxic than currently used solvents in the manufacturing process of polyvinylidene fluoride (PVDF) membranes. The objective of this invention is to reduce the use of hazardous solvents, such as N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), and N,N-dimethylformamide (DMF), in membrane production. Solvents currently used in polymer membrane production pose risks related to carcinogenicity and reproductive toxicity. Recently, the European Union (EU) has implemented a policy to phase out the use of toxic solvents for all industrial applications (EC No. 1907 / 2006 and subsequent annexes). The use of safer solvents will help ensure the production of water filtration membranes in the future. The use of safer solvents can reduce manufacturing hazards in membrane production.

[0009] The present invention provides a solvent system that reduces toxic solvents and does not require significant formulation changes or process adjustments from current methods. Specifically, the present invention focuses on membrane casting via the non-solvent-induced phase separation (NIPS) method using PVDF resin, NMP solvent, and blended additives such as polyvinylpyrrolidone and polyethylene glycol.

[0010] They found that blending NMP with triethyl phosphate (TEP), a less toxic solvent than NMP, can produce PVDF membranes with properties comparable to those made with pure NMP solvent. This blend reduces the use of hazardous solvents and represents a significant cost savings for manufacturers, as TEP is approximately half the price of NMP.

[0011] Thus, a dope solution for producing membranes with reduced toxic solvents and a method for producing membranes have been discovered. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] U.S. Patent No. 5,565,153 [Patent Document 2] US Patent Application Publication No. 2004 / 050791 [Patent Document 3] US Patent Application Publication No. 2004 / 050791 [Non-patent literature]

[0013] [Non-Patent Document 1] Poly.Jnl.,42,(2006),158 [Non-patent document 2] Liu et al., J.Mem.Sci.,375,(2011),1 [Non-patent document 3] Chang et al., J.Mem.Sci.,539,(2017),295 [Non-patent document 4] Garcia-Fernandez et al., J.Mem.Sci.468(2014)324 Summary of the Invention [Problem to be solved by the invention]

[0014] The inventors have developed a blend of TEP and NMP to replace pure NMP as a solvent for producing PVDF hollow membranes with similar properties. This solvent blend reduces the amount of hazardous solvent (NMP) used in the manufacturing process and reduces costs due to the lower price of TEP compared to NMP. [Means for solving the problem]

[0015] Aspects of the invention Aspect 1: A dope solution comprising a PVDF resin, a water-soluble polymer or a hydrogel polymer, and optionally an additive in a solvent, wherein the solvent comprises a blend of triethyl phosphate (TEP) and N-methylpyrrolidone (NMP).

[0016] Aspect 2: The dope solution of Aspect 1, wherein the weight ratio of TEP to NMP is 75:25 to 1:99.

[0017] Aspect 3: The dope solution according to aspect 1, wherein the weight ratio of TEP to NMP is 75:25 to 30:70.

[0018] Aspect 4: The dope solution according to Aspect 1, wherein the weight ratio of TEP to NMP is 70:30 to 50:50, preferably 65:35 to 50:50.

[0019] Aspect 5: The dope solution according to any combination of Aspects 1 to 4, wherein the amount of PVDF is 12% by weight to 25% by weight, preferably 18% by weight to 20% by weight, based on the total weight of the dope solution.

[0020] Aspect 6: The dope solution according to any combination of Aspects 1 to 5, wherein the PVDF resin has a melt viscosity of 18 to 45 kpoise, preferably 25 to 42 kpoise.

[0021] Aspect 7: The dope solution according to any combination of Aspects 1 to 6, wherein the PVDF resin comprises a homopolymer resin.

[0022] Aspect 8: A dope solution according to any combination of Aspects 1 to 6, wherein the PVDF resin comprises a copolymer of VDF and at least one of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene, or tetrafluoropropene.

[0023] Embodiment 9: The dope solution according to any combination of embodiments 1 to 8, wherein the PVDF polymer comprises a mixture of PVDF polymers having different viscosities.

[0024] Aspect 10: A dope solution according to any combination of Aspects 1 to 9, wherein the optional additive comprises an acrylic resin in an amount of 0 to 20% by weight, preferably 1 to 20% by weight, based on the total weight of the polymer P and the acrylic resin in the dope solution.

[0025] Aspect 11: A dope solution described in any combination of Aspects 1 to 10, wherein the optional additive comprises an acrylic resin selected from the group consisting of PMMA resin; PMMA copolymer resin containing an acrylic acid ester comonomer; PMMA copolymer resin containing a hydroxyethyl methacrylate comonomer; PMMA copolymer resin containing a methoxy-polyethylene glycol methacrylate; PMMA resin containing zwitterionic functional groups; PMMA resin containing sulfonic acid groups; and block copolymers comprising a pure PMMA block and a second block containing both a hydrophilic comonomer such as HEMA or PEGMA and a hydrophobic comonomer such as an alkyl acrylate.

[0026] Aspect 12: The dope solution according to any combination of Aspects 1 to 11, wherein the dope solution has a viscosity of 50,000 to 250,000 cps, preferably 80,000 to 180,000 cps, when measured with a Brookfield viscometer using a #7 spindle at 70° C. and 50 RPM.

[0027] Aspect 13: A dope solution according to any combination of Aspects 1 to 12, wherein the water-soluble hydrogel polymer comprises at least one of polyvinylpyrrolidone, polyethylene oxide, polyethylene oxide / polypropylene oxide block copolymer, or a mixture thereof.

[0028] Aspect 14: A dope solution described in any combination of Aspects 1 to 12, wherein the hydrogel polymer comprises at least one of poly(hydroxyethyl methacrylate) (polyHEMA), poly-N-isopropylacrylamide (PNIPAM), polyethylene glycol methacrylate (PEGMA), cross-linked PVP, or copolymers thereof.

[0029] Embodiment 15: A method for casting a membrane, comprising the steps of: (a) preparing a dope solution (PS) containing a PVDF resin, a water-soluble or hydrogel polymer, and optionally an additive in a solvent containing a blend of triethyl phosphate and N-methylpyrrolidone, with a weight ratio of TEP to NMP of 75:25 to 1:99; (b) degassing the dope solution of step (a); (c) pressing the dope solution; (d) passing the extruded dope solution through a non-solvent bath to form a porous membrane; (e) immersing the porous membrane in water; (f) optionally, immersing the porous membrane in an aqueous solution of sodium hypochlorite (0.5% by weight to 7.5% by weight); (g) optionally rinsing the porous membrane with fresh water after soaking in sodium hypochlorite; (h) conditioning the porous membrane with a wetting agent.

[0030] Aspect 16: The method of aspect 15, wherein the weight ratio of TEP to NMP is from 75:25 to 1:99.

[0031] Aspect 17: The method of aspect 15, wherein the weight ratio of TEP to NMP is 75:25 to 30:70.

[0032] Aspect 18: The method of aspect 15, wherein the weight ratio of TEP to NMP is from 70:30 to 50:50, preferably from 65:35 to 50:50.

[0033] Aspect 19: The method according to any combination of Aspects 15 to 18, wherein the amount of PVDF is 12% by weight to 25% by weight, preferably 18% by weight to 20% by weight, based on the total weight of the dope solution.

[0034] Aspect 20: The method according to any combination of Aspects 15 to 19, wherein the PVDF resin has a melt viscosity of 18 to 45 kpoise, preferably 25 to 42 kpoise.

[0035] Embodiment 21: The method of any combination of embodiments 15-20, wherein the PVDF resin comprises a homopolymer resin.

[0036] Aspect 22: The method described in any combination of Aspects 15 to 20, wherein the PVDF resin comprises a copolymer of VDF and at least one of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene, or tetrafluoropropene.

[0037] Embodiment 23: The method of any combination of embodiments 15 to 22, wherein the PVDF polymer comprises a mixture of PVDF polymers having different viscosities.

[0038] Aspect 24: The method of any combination of Aspects 15 to 23, wherein the optional additive comprises an acrylic resin additive in an amount of 0 to 20% by weight, preferably 1 to 20% by weight, based on the total weight of polymer P and acrylic additive in the dope solution.

[0039] Aspect 25: The method of any combination of aspects 15 to 24, wherein the optional additive comprises an acrylic resin, the acrylic resin being selected from the group consisting of polymethyl methacrylate (PMMA) resin; PMMA copolymer resin containing an acrylic ester comonomer; PMMA copolymer resin containing a hydroxyethyl methacrylate comonomer; PMMA copolymer resin containing methoxy-polyethylene glycol methacrylate; PMMA copolymer resin including a polyethylene glycol methacrylate comonomer; PMMA resin including zwitterionic functional groups; PMMA resin including sulfonic acid groups; and block copolymers including a pure PMMA block and a second block including both a hydrophilic comonomer such as HEMA or PEGMA and a hydrophobic comonomer such as an alkyl acrylate.

[0040] Aspect 26: The method of any combination of aspects 15 to 25, wherein the non-solvent bath described in step (d) comprises at least one of water; a mixture of water and a solvent such as NMP or TEP; a mixture of water and an alcohol; a mixture of water and glycerol; or a mixture of water and propylene glycol.

[0041] Aspect 27: The method described in any combination of Aspects 15 to 26, wherein the humectant is selected from the group consisting of glycerol, propylene glycol, butylene glycol, hexylene glycol, and mixtures or aqueous solutions thereof.

[0042] Aspect 28: A method according to any combination of Aspects 15 to 27, wherein in step (c), the dope solution is extruded through a hollow fiber die (a tube with an orifice design) while injecting bore fluid into the bore of the porous membrane.

[0043] Aspect 29: The method of aspect 28, wherein the bore fluid comprises a mixture of water, TEP, and NMP.

[0044] Aspect 30: The method of Aspect 29, wherein the water, TEP, and NMP are in a weight ratio of 50 to 80% water, 20 to 40% TEP, and 0 to 25% NMP, based on the total weight of the water, TEP, and NMP; preferably, the weight ratio is 50 to 70% water, 15 to 25% TEP, and 5 to 25% NMP.

[0045] Embodiment 31: The method of any combination of embodiments 15-30, further comprising (d2) passing the extruded dope solution through a second non-solvent bath after the first non-solvent bath.

[0046] Aspect 32: The method of aspect 31, wherein the second non-solvent bath is selected from the group consisting of water, a mixture of water and a surfactant, a mixture of water and glycerol, a mixture of water and propylene glycol, and a mixture of water and polyethylene glycol.

[0047] Aspect 33: A method according to any combination of Aspects 15 to 32, wherein the dope solution is extruded onto a hollow braided fiber.

[0048] Aspect 34: The method of any combination of Aspects 15 to 33, wherein the dope solution is extruded onto a porous or non-porous support.

[0049] Aspect 35: A method described in any combination of Aspects 15 to 34, wherein the water-soluble polymer comprises at least one of polyvinylpyrrolidone, polyethylene oxide, polyethylene oxide / polypropylene oxide block copolymer, or a mixture thereof.

[0050] Aspect 36: The method of any combination of Aspects 15-34, wherein the hydrogel polymer comprises at least one of polyhydroxyethyl methacrylate (polyHEMA), poly-N-isopropylacrylamide (PNIPAM), polyethylene glycol methacrylate (PEGMA), cross-linked PVP, or copolymers thereof.

[0051] Aspect 37: A membrane produced by the method of any combination of Aspects 15 to 36, wherein the membrane has a pure water permeability of 100 to 1000 LMHB, a mechanical strength in the range of 4.0 to 6.0 MPa, and an elongation at break in the range of 90 to 200%, preferably 100 to 200%.

[0052] Embodiment 38: Use of a membrane produced by the method of any combination of embodiments 13 to 36 for the filtration of water, including drinking water, wastewater, industrial process water, and membrane bioreactors.

[0053] Embodiment 39: Use of the dope solution of any combination of embodiments 1-14 to produce a membrane.

[0054] Embodiment 40: The use of embodiment 39, wherein the membrane is used for filtration of water, including drinking water, wastewater, industrial process water, and membrane bioreactors. [Brief explanation of the drawings]

[0055] [Figure 1] FIG. 1 is a graph of elongation versus % NMP in the dope solution shown in Table 2. [Figure 2] FIG. 2 is a graph of mechanical strength versus % NMP in the dope solution shown in Table 2. DETAILED DESCRIPTION OF THE INVENTION

[0056] Detailed Description of the Invention In this specification, unless otherwise specified, % is % by weight and melt viscosity is measured by capillary rheometry at 100 sec and 232°C using ASTM 3825. All cited documents are incorporated herein by reference.

[0057] "Copolymer" means a polymer having two or more different monomer units, including terpolymers (three different comonomers) and higher order polymers (three or more different comonomers). "PVDF" means polyvinylidene fluoride. "Polymer" is used to refer to both homopolymers and copolymers. For example, as used herein, "PVDF" and "polyvinylidene fluoride" are used to refer to both homopolymers and copolymers unless otherwise specified. "Fluoropolymer" means a polymer containing a fluorinated monomer. Polymers may be homogeneous, heterogeneous, or random, and may have a gradient in the distribution of comonomer units.

[0058] Hydrogel polymers are three-dimensionally cross-linked hydrophilic polymers that are insoluble in water and can absorb large amounts of water without dissolving due to physical or chemical cross-linking of the hydrophilic polymer chains.

[0059] The dope solution for producing the membrane comprises a polymer P, a water-soluble polymer or a hydrogel polymer, optionally other additives, and a solvent system.

[0060] The dope solution for producing the membrane comprises a polymer P selected from the group consisting of polyvinylidene fluoride (PVDF) homopolymers and copolymers. The term "PVDF polymer" is intended to include mixtures of different PVDF polymers. The PVDF polymer has a melt viscosity in the range of 18 to 45 kpoise, preferably 25 to 42 kpoise.

[0061] The polymer of the present invention can be any PVDF fluoropolymer used to form membranes by the NIPS process. Particularly useful fluoropolymers include, but are not limited to, PVD homopolymers and copolymers in which the majority of the monomer units are a combination of vinylidene fluoride with comonomers such as hexafluoropropylene, chlorotrifluoroethylene, or vinyl fluoride.

[0062] The polyvinylidene fluoride (PVDF) composition of the present invention comprises any of a homopolymer, copolymer, terpolymer, or higher polymer of vinylidene fluoride formed by polymerizing vinylidene fluoride (VDF), and the vinylidene fluoride units typically account for preferably 70% or more, more preferably 75% or more, of the total weight of all monomer units in the polymer. Copolymers, terpolymers and higher polymers of vinylidene fluoride include copolymers, terpolymers and higher polymers of vinylidene fluoride with vinyl fluoride, trifluoroethene, tetrafluoroethene; tetrafluoropropenes such as 2,3,3,3-tetrafluoropropene, E-1,3,3,3-tetrafluoropropene, Z-1,3,3,3-tetrafluoropropene, 1,1,2,3-tetrafluoropropene, 1,2,3,3-tetrafluoropropene, 1,1,3,3-tetrafluoropropene, chlorotetrafluoropropene, 3,3,3-trifluoro-1-propene, 1,2,3,3,3-pentafluoropropene, one or more partially or fully fluorinated α-olefins such as 3,3,3,4,4,4-pentafluoro-1-butene, hexafluoropropene, trifluoromethacrylate, tetrafluoropropenes such as tetrafluoropropenes ..., tetrafluoropropenes, tetrafluoropropenes, tetrafluoropropenes, tetrafluoropropene The copolymers can be prepared by reacting vinyl fluoride, trifluoromethyl methacrylate, partially fluorinated olefin hexafluoroisobutylene, perfluorinated vinyl ethers such as perfluoromethyl vinyl ether, perfluorovinyl ethers such as perfluoroethyl vinyl ether, perfluoroethyl vinyl ether, perfluoro-n-propyl vinyl ether, perfluoro-2-propoxypropyl vinyl ether, fluorinated dioxoles such as perfluoro(1,3-dioxole), perfluoro(2,2-dimethyl-1,3-dioxole), allyl monomers, partially fluorinated allyl monomers, or fluorinated allyl monomers such as 2-hydroxyethyl allyl ether or 3-allyloxypropanediol, ethene, and propene with one or more monomers selected from the group consisting of vinyl fluoride, trifluoroethene, tetrafluoroethene (TFE), and hexafluoropropene (HFP). The most preferred copolymers are formed with hexafluoropropene (HFP).One preferred PVDF is Kynar® PVDF from Arkema.

[0063] Although all copolymers containing fluorinated monomers are preferred, non-fluorinated monomers such as vinyl acetate, methacrylic acid and acrylic acid can also be used to form copolymers at levels up to 5% by weight based on polymer solids.

[0064] Preferred copolymers are VDF containing about 70 to about 99% by weight of VDF monomer units and, correspondingly, about 1 to about 30% by weight of HFP monomer units; and VDF containing about 70 to about 99% by weight of VDF and, correspondingly, about 1 to about 30% by weight of trifluoroethylene monomer units.

[0065] Blends of polyvinylidene fluoride polymers are also contemplated as part of this invention, including functionalized and non-functionalized fluoropolymers, PVDF homopolymers and PVDF-HFP copolymers, and PVDF polymers having different melt viscosities.

[0066] Preferably, the dope solution has a polymer P content of 10 to 30 wt %, more preferably 12 to 25 wt %, most preferably 18 to 22 wt %, based on the total weight of the dope solution.

[0067] Water-soluble / hydrogel polymers The soluble or hydrogel polymers may be useful for adjusting the viscosity of the dope solution. The main purpose of these hydrophilic additives is to support pore formation and impart residual hydrophilicity to the membrane.

[0068] The water-soluble polymer may be any known water-soluble polymer. Preferred water-soluble polymers are selected from the group consisting of polyvinylpyrrolidone (PVP); polyethylene glycols having a molecular weight of 200 to 1000 Mw; and polyalkylene oxides having a molar mass of 4000 g / mol or more. Preferred water-soluble polymers include polyvinylpyrrolidone, polyethylene oxide, polyethylene oxide / polypropylene oxide block copolymers, and mixtures thereof. Preferred water-soluble polymers are polyethylene glycol and polyvinylpyrrolidone. A highly preferred water-soluble polymer is polyvinylpyrrolidone.

[0069] Preferred hydrogel polymers can be selected from known examples including poly(hydroxyethyl methacrylate) (polyHEMA), poly-N-isopropylacrylamide (PNIPAM), polyethylene glycol methacrylate (PEGMA), high K PVP, cross-linked PVP, and copolymers thereof.

[0070] When the dope solution contains 10-30 wt % of polymer P, the amount of water-soluble or hydrogel polymer can be 1-22 wt %, based on the total weight of the dope solution.

[0071] In a preferred embodiment, the dope solution contains 12 to 25 wt % of polymer P and 2 to 20 wt % of the water-soluble polymer or hydrogel polymer, based on the total weight of the dope solution. In a more preferred embodiment, the dope solution contains 18 to 20 wt % of polymer P and 8 to 16 wt % of the water-soluble polymer or hydrogel polymer, based on the total weight of the dope solution.

[0072] In one embodiment, the amount of PVP additive is preferably 10-16%, more preferably 14-15%, based on the total weight of the dope solution.

[0073] The polyvinylpyrrolidone preferably has a K value of 10 to 120. Preferably, at least one PVP present in the composition has a K value of 12 to 60. One or more polyvinylpyrrolidones with different K values can be used. Polyvinylpyrrolidones can be used in combination, such as K15 and K30, K15 and K60, K15 and K90, or K30 and K60. A given K value roughly corresponds to the weight-average molecular weight (using GPC / MALLS). Typically, a PVP with a K value of K30 has a weight-average molecular weight in the range of 40,000 to 80,000 g / mol, K60 generally has a weight-average molecular weight in the range of 250,000 to 500,000 g / mol, and K90 generally has a weight-average molecular weight in the range of 1,000,000 to 1,400,000 g / mol. K15 generally has a K value in the range of 7,000 to 20,000 g / mol. The term "K value" refers to the Fikentscher K value (1000 k) defined by H. Fikentscher in Cellulosechemie 13, 58-64, 71-4 (1932) (U.S. Pat. No. 2,706,701). PVP polymers are commercially available as products such as Luvitec (registered trademark) PVP (manufactured by BASF) or Plasdone (trademark), Povidone, and PVP K series (all manufactured by Ashland), and are sold with reference to their K value as an index of molecular weight.

[0074] In some embodiments, the amount of polyethylene glycol (PEG) is 0-10% by weight based on the total weight of the dope solution.

[0075] In some embodiments, PEG is used, preferably having a molecular weight of 200-1000 Mw, although in some embodiments the molecular weight can be higher.

[0076] additives In one embodiment, the optional additive is an acrylic resin in an amount of 0 to 20% by weight, preferably 1 to 20% by weight, based on the total weight of polymer P and acrylic resin in the dope solution.

[0077] The optional acrylic resin can be one or more of PMMA resin; PMMA copolymer resin containing an acrylate comonomer; PMMA copolymer resin containing a hydroxyethyl methacrylate comonomer; PMMA copolymer resin containing methoxy-polyethylene glycol methacrylate; PMMA copolymer resin containing a polyethylene glycol methacrylate comonomer; PMMA resin containing zwitterionic functional groups such as sulfobetaine methacrylate; PMMA resin containing sulfonic acid groups; or a block copolymer composed of a pure PMMA block and a second block containing both a hydrophilic comonomer such as HEMA or PEGMA and a hydrophobic comonomer such as an alkyl acrylate.

[0078] Other additives include, but are not limited to, inorganic salts such as lithium chloride, magnesium chloride, ferrous chloride, and aluminum chloride; quaternary ammonium salts; propylene glycol, glycerol, organic acids, molecular sieves, silica, aluminum oxide, activated carbon, and the like.

[0079] Solvent System The solvent system comprises a blend of triethyl phosphate (TEP) and N-methylpyrrolidone (NMP). Preferably, the blend of triethyl phosphate (TEP) and N-methylpyrrolidone (NMP) has a weight ratio of TEP:NMP of 75:25 to 1:99, preferably 75:25 to 30:70, and more preferably 70:30 to 50:50. In some embodiments, the weight ratio is 65:35 to 50:50.

[0080] A preferred embodiment is a blend range of TEP:NMP of 75:25 or less. At ratios higher than 75:25 (e.g., 90 TEP:10 NMP), mechanical strength and elongation decrease.

[0081] The dope solution may contain an optional co-solvent (hereinafter referred to as co-solvent) in addition to the triethyl phosphate / N-methylpyrrolidone blend.

[0082] Preferred are cosolvents that are miscible with the triethyl phosphate / N-methylpyrrolidone blend. Suitable cosolvents are, for example, selected from dimethylformamide, dimethyl sulfoxide, sulfolane, N-ethyl-2-pyrrolidone, Nn-butyl-2-pyrrolidone, N,N-dimethyl-2-hydroxypropanoic acid amide, N,N-diethyl-2-hydroxypropanoic acid amide, ethyl lactate, methyl lactate, Cyrene™ (dehydroglucosenone), ethyl levulinate, and 2-pyrrolidone.

[0083] The total amount of solvents (TEP, NMP and co-solvents) in the dope solution is generally 50 to 85 wt %, preferably 55 to 75 wt %, of the total weight of the dope solution.

[0084] In a preferred embodiment, no more than 15% by weight of the dope solution is co-solvent, based on the weight of the total amount of all solvents in the dope solution.

[0085] In the most preferred embodiment, no co-solvents are used in the dope solution, and the triethyl phosphate / N-methylpyrrolidone blend is used solely as the solvent system.

[0086] Preparation of dope solution The dope solution can be prepared by adding polymer P and the water-soluble polymer and / or hydrogel polymer in any order to a triethyl phosphate (TEP) / N-methylpyrrolidone (NMP) blend and dissolving polymer P and the water-soluble polymer and / or hydrogel polymer according to any process known in the art. The dissolution process can be assisted by increasing the temperature of the dope solution and / or by mechanical manipulation such as stirring.

[0087] In one common method, the ingredients are blended in an overhead mixer, preferably at 30-300 rpm, for 4-12 hours while heating, preferably to a temperature of 70-120°C.

[0088] Preferably, the final dope solution has a final viscosity of 50,000 to 250,000 centipoise, preferably 80,000 to 180,000 cps, as measured on a Brookfield viscometer using a #7 spindle at 70° C. and 50 RPM.

[0089] Membrane manufacturing method In the context of this application, a membrane is understood to be a semipermeable membrane structure capable of separating two fluids or separating molecules and / or ionic components or particles from a liquid. Membranes act as selective barriers, allowing some particles, substances, or chemicals to pass through while retaining others. Membranes can have a variety of shapes, including flat sheets, spiral wound, tubular, single-hole hollow fibers, perforated hollow fibers, and reinforced hollow fibers.

[0090] The membranes can be made according to a method that includes providing a dope solution containing polymer P, a water-soluble polymer or hydrogel polymer, and a solvent blend, casting a membrane by extruding the dope solution, passing the extruded dope solution through a non-solvent bath / coagulant, and optionally oxidizing and washing the resulting membrane.

[0091] Preferably, the method for casting a membrane comprises the steps of: (a) preparing a dope solution (DS) comprising a PVDF resin, a water-soluble or hydrogel polymer, and optionally additives in a solvent comprising a blend of triethyl phosphate and N-methylpyrrolidone; (b) degassing the dope solution of step (a); (c) Press the dope solution; (d) solidifying the extruded dope solution of step c by passing it through a non-solvent bath to form a porous membrane; (e) Immersing the porous membrane in an aqueous solution; (f) optionally, immersing the porous membrane in an aqueous sodium hypochlorite solution (0.5% to 7.5%) at a temperature of 20 to 50°C for 4 to 24 hours, and then rinsing the porous membrane with fresh water after the sodium hypochlorite immersion; (g) optionally immersed in an alcohol solution; (h) Conditioning the fibers with a wetting agent.

[0092] The dope solution in step (a) corresponds to the dope solution described above. The main purpose of the water-soluble polymer is to support the formation of pores. The water-soluble polymer or hydrogel polymer also helps to adjust the viscosity of the dope solution. In the solidification step (d), the water-soluble polymer disperses in the solidified film and maintains the position of the pores.

[0093] Degassing can be carried out at elevated temperature or room temperature, preferably at 50 to 80°C.

[0094] In step (d), the dope solution is contacted with a non-solvent bath, also called a coagulant, which causes the polymer P to coagulate and form a membrane structure.

[0095] In step (f), the solution can be at room temperature or at an elevated temperature, preferably 20°C to 50°C.

[0096] The hollow fiber membranes of the present invention can be assembled into hollow fiber membrane modules by adapting any of the assembly techniques or methods known in the art. Procedures for making such modules using hollow member fibers are well known and are described, for example, in the following publications, each of which is incorporated by reference in its entirety for all purposes: U.S. Pat. Nos. 8,728,316; 8,679,337; 8,636,904; 8,307,991; 8,225,941; 8,042,695; 7,749,381; 7,704,393; 7,316,754; 7,160,455; 6,682,652; and 6,331,248; U.S. Patent Application Publication No. 2003 / 0038075; and Mat et al., Current Opinion in Chemical Engineering, Vol. 4, May 2014, pp. 18-24.

[0097] The process of casting hollow fiber membranes from a dope solution involves forcing the dope solution through a hollow fiber die (a tube with an orifice design) while injecting a solvent mixture into the membrane bore, and then drawing the fiber through a non-solvent bath. Typically, the membrane is collected on a collection reel.

[0098] In the process of the present invention, the bore fluid is preferably a mixture of water, TEP and NMP, preferably in the ratios (by weight) of 50-80% water, 20-40% TEP and 0-25% NMP, more preferably 50-70% water, 15-25% TEP and 5-25% NMP.

[0099] In some embodiments, the dope solution is heated to 40-80°C.

[0100] In some embodiments, the bore fluid is heated to 40-60°C.

[0101] In some embodiments, the non-solvent bath is pure water.

[0102] In some embodiments, the water non-solvent bath is heated to 40-60°C.

[0103] In some embodiments, the hollow fiber die is heated to 40-90°C, preferably 50-70°C.

[0104] In some embodiments, the fiber is immersed in the non-solvent bath for 2 to 20 meters.

[0105] The process of the present invention can optionally have a second non-solvent bath following the first non-solvent bath.

[0106] In some embodiments, the second non-solvent bath is at room temperature.

[0107] The second non-solvent bath (or the single non-solvent bath, if used) can be selected from the group consisting of pure water, a mixture of water and a surfactant, a mixture of water and glycerol, a mixture of water and propylene glycol, or a mixture of water and polyethylene glycol.

[0108] Non-solvent The polymer P must have low solubility in the non-solvent / coagulant. Suitable non-solvent baths / coagulants are, for example, liquid water, water vapor, alcohols, glycols, glycerol, or mixtures thereof.

[0109] Suitable alcohols include, for example, mono-, di-, or trialkanols selected from the group consisting of C2-C4 alkanols, C2-C4 alkanediols, C3-C4 alkanetriols, and polyethylene oxides having a molar mass of 100 to 1000 g / mol, which can be used as additives in the dope solution of the present invention. A preferred mixture of non-solvents is a mixture containing liquid water and an alcohol, more preferably a mixture containing liquid water and an alcohol optionally used as an additive in the dope solution of the present invention. A preferred non-solvent is liquid water.

[0110] The dope solution and method of the present invention can be used to produce flat sheet membranes. The dope solution is cast onto a support, which is then contacted with a bath.

[0111] In a preferred embodiment, steps (e) to (h) are carried out. In addition to oxidation, washing is carried out to remove the water-soluble polymer and form pores. The washing step is carried out after the oxidation.

[0112] Any oxidizing agent can be used for the oxidation, with water-soluble oxidizing agents such as sodium hypochlorite and hydrogen peroxide being preferred.

[0113] The resulting membrane has a high tensile strength, preferably 3.5 MPa or more, preferably 4 MPa or more, a high elongation of 95% or more, preferably 100% or more, and a PWP (pure water permeability) of 190 LMHB (liters per hour-meter squared-bar) or more.

[0114] The membranes obtained by the process of the present invention can be used for any separation purpose, such as water treatment applications, treatment of industrial and municipal wastewater, desalination of seawater and brackish water, dialysis, plasma decomposition, food processing, etc. [Example]

[0115] Abbreviations and compounds used in the examples: PWP pure water penetration NMP N-methyl-2-pyrrolidone TEP Triethyl Phosphate

[0116] Preparation of dope solution for film formation Example 1:50:50TEP-NMP Kynar® MG15 (PVDF homopolymer resin, 35-39 kpoise, 57 g) (available from Arkema) and polyvinylpyrrolidone (K30 grade, 45 g) were weighed into a 16 oz. mixing jar. Triethyl phosphate (99 g) and N-methylpyrrolidone (99 g) were added to the powders. A mixer blade was inserted into the jar and secured in place with the cover. The mixture was heated to 95°C with a heating mantle while stirring with an overhead stirrer at 75 rpm for 6 hours. A clear, yellow, viscous solution resulted. The jar was sealed with a cap and placed in a 70°C oven overnight. The viscosity of this solution was measured to be 126,000 cps.

[0117] [Table 1]

[0118] Example 2: 70:30TEP-NMP. The same as Example 1, except that the amounts of triethyl phosphate and N-methylpyrrolidone were changed to those shown in Table 1.

[0119] Example 3: 80:20TEP-NMP (comparison). The same as Example 1, except that the amounts of triethyl phosphate and N-methylpyrrolidone were changed to those shown in Table 1.

[0120] Example 4:60:40TEP-NMP. The same as Example 1, except that the amounts of triethyl phosphate and N-methylpyrrolidone were changed to those shown in Table 1.

[0121] Example 5:67:33TEP-NMP. The same as Example 1, except that the amounts of triethyl phosphate and N-methylpyrrolidone were changed to those shown in Table 1.

[0122] Example 6: 90:10TEP-NMP (comparison). The same as Example 1, except that the amounts of triethyl phosphate and N-methylpyrrolidone were changed to those shown in Table 1.

[0123] Example 7: 100% NMP (comparison). Same as Example 1, except that the solvent was 100% N-methylpyrrolidone, and the amounts were changed to those in Table 1.

[0124] Example 8: 100%TEP (comparison). Same as Example 1 except 100% TEP solvent was used.

[0125] Film casting and post-processing Hollow fiber casting The following procedure was applied to cast all of the above formulations: The casting line consisted of temperature-controlled tanks to hold the membrane formulation (dope) and bore fluid. The dope and bore fluid were pumped using a high-precision delivery gear pump. The membrane formulation was charged to a temperature-controlled dope tank (approximately 70°C) and allowed to stand for 15 minutes before casting. The dope transfer line to the spinneret was maintained at the same temperature as the dope tank. The spinneret was independently heated to approximately the same temperature. The bore fluid was charged to a temperature-controlled bore fluid tank at approximately 60°C. The non-solvent water bath was heated to approximately 60°C. The take-up speed of the collection reel was approximately 12 m / min. The take-up reel was immersed in a room temperature water bath to wash the fiber.

[0126] After casting, the membranes were soaked in water overnight, then treated with aqueous sodium hypochlorite (1.5%) at 45°C for 6 hours, and then soaked at room temperature overnight. The membranes were then rinsed twice with fresh water and tested for permeability.

[0127] Membrane test results Mechanical testing was performed on an Instron 4201 universal testing frame (per ASTM D638) equipped with a fiber holder designed to wrap the fiber around a spool, preventing damage to the delicate hollow fiber caused by standard tensile bar grips. The gap spacing was 100 mm and the strain rate was 50 mm / min. -1 The fibers were tested in a water-wet state.

[0128] To test pure water permeability, five membrane loops approximately 35 cm long were potted with epoxy resin in a test module. The water flow was measured at 0.5 bar, with a 15-minute purge before measurement, followed by a 1 m -2 h -1 bar -1 , normalized to LMHB.

[0129] Table 2 shows the permeability and mechanical properties of the casting formulations.

[0130] [Table 2]

[0131] Table Data When the properties of membranes produced using solvent blends were tested, it was surprising to find that membranes with high mechanical strength and high elongation could be obtained at levels above 50% TEP. When TEP was used as the sole solvent (100% TEP), elongation and mechanical strength were poor. When up to 70% of the NMP was replaced with TEP, the resulting membranes were found to have elongations of at least 95%, preferably 100%, mechanical strengths of at least 3.5, preferably at least 4 MPa, and LMHBs of 190 or greater, which were equal to or better than those of 100% NMP membranes.

Claims

1. A doped solution comprising a PVDF resin, a water-soluble polymer or a hydrogel polymer, and optionally an additive in a solvent, wherein the solvent comprises a blend of triethyl phosphate (TEP) and N-methylpyrrolidone (NMP).

2. The doped solution according to claim 1, wherein the weight ratio of TEP to NMP is 75:25 to 30:

70.

3. The doped solution according to claim 1, wherein the amount of PVDF is 12% to 25% by weight, preferably 18% to 20% by weight, based on the total weight of the doped solution.

4. The doped solution according to claim 1, wherein the melt viscosity of the PVDF resin is 18 to 45 kilopoise, preferably 25 to 42 kilopoise.

5. The doping solution according to claim 1, wherein the PVDF resin comprises a homopolymer resin.

6. The doped solution according to claim 1, wherein the PVDF resin comprises a copolymer of VDF and at least one of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene, or tetrafluoropropene.

7. The doped solution according to claim 1, wherein the PVDF polymer comprises a mixture of PVDF polymers having different viscosities.

8. The doped solution according to claim 1, wherein the optional additive contains 0 to 20% by weight, preferably 1 to 20% by weight, of acrylic resin based on the total weight of polymer P and acrylic resin in the doped solution.

9. The doped solution according to claim 1, wherein the water-soluble polymer comprises at least one of polyvinylpyrrolidone, polyethylene oxide, polyethylene oxide / polypropylene oxide block copolymer, or a mixture thereof.

10. A method for casting a film, including the following steps: (a) Prepare a doped solution (PS) containing PVDF resin, a water-soluble polymer or hydrogel polymer, and optionally an additive in a solvent containing a blend of triethyl phosphate and N-methylpyrrolidone, with a TEP to NMP weight ratio of 75:25 to 1:99; (b) Degas the doping solution from step (a); (c) Press the dope solution; (d) Pass the extruded doped solution through a non-solvent bath to form a porous membrane; (e) Immerse the porous membrane in water; (f) Optionally, immerse the porous membrane in an aqueous sodium hypochlorite solution (0.5% to 7.5% by weight); (g) Optionally, after immersing in sodium hypochlorite, rinse the porous membrane with fresh water; (h) Conditioning the porous membrane with a wetting agent.

11. The method according to claim 10, wherein in step (c), the doping solution is extruded through a hollow fiber die (tube in the orifice design) while injecting bore liquid into the bore of the membrane.

12. The method according to claim 11, wherein the bore fluid comprises a mixture of water, TEP, and NMP.

13. The method according to claim 12, wherein the water, TEP, and NMP are in a weight ratio of 50-80% water, 20-40% TEP, and 0-25% NMP based on the total weight of the water, TEP, and NMP; preferably, the weight ratio is 50-70% water, 15-25% TEP, and 5-25% NMP.

14. The method according to claim 10, wherein the doping solution is extruded onto a hollow braided fiber.

15. The method according to claim 10, wherein the doped solution is extruded onto a porous support or a non-porous support.

16. The film is manufactured by any one of claims 10 to 15, and the film has a pure water permeability of 100 to 1000 LMHB, a mechanical strength of 4.0 to 6.0 MPa, and an elongation at break of 90 to 200%, preferably 100 to 200%.

17. Use of a membrane manufactured by any one of claims 10 to 15 for filtering drinking water, wastewater, industrial process water, and water including membrane bioreactors.

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