Triethyl phosphate / n-methylpyrrolidone solvent blends for preparing PVDF membranes

Abstract

The invention discloses a membrane casting solution for membrane manufacturing, which comprises a blend of TEP and NMP as a solvent system in the process of preparing a PVDF membrane, and PVDF resin comprises a homopolymer or a copolymer of VDF and at least one of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene or 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 blend of triethyl phosphate / N-methylpyrrolidone, and to a method for preparing a membrane and the use of the membrane for water treatment. Background Art

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

[0003] One major technical application is the use of PVDF polymers as raw materials for the production of membranes (e.g., hollow fiber membranes). The process of producing PVDF membranes includes dissolving the PVDF polymer in a solvent, condensing 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 the mechanical strength, elasticity, water permeability, and pore size of the membrane.

[0004] There are many publications describing the use of pure triethyl phosphate as a solvent for casting PVDF membranes via the NIPS phase inversion process, such as Lin et al., Euro. 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 (US5565153) describes the use of TEP as a solvent for casting flat sheet PVDF membranes, but without any co-solvent.

[0005] Herczeg (US2004050791) teaches the use of NMP in the presence of a "non-solvent" additive to prepare PVDF membranes, in which TEP is listed as one of the "non-solvents." US2004050791 does not provide a definition of "non-solvent" nor examples of PVDF formulations and NMP-TEP blends.

[0006] There is a reference to the preparation of PVDF hollow fiber membranes with DMAC and TEP (García-Fernández et al., J. Mem. Sci. 468 (2014) 324), but NMP was not tested.

[0007] In the field of solvents, there is a constant need for solvents that can replace currently used solvents, such as NMP, in specific applications. For polyvinylidene fluoride (PVDF), new solvents should allow the preparation of solutions with a high PVDF content and without turbidity. For membranes produced from new solvents, it is important to achieve the quality of the membranes; in particular, the water permeability of such membranes should be as high as possible, while maintaining strength and elasticity, measured by elongation at break.

[0008] The object of the present invention is to provide a solvent system for polyvinylidene fluoride (PVDF) for use in processes for preparing membranes which is less toxic than the solvents currently used. The object of the present 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 manufacturing. Solvents currently used in polymer membrane manufacturing have hazards associated with carcinogenicity or reproductive toxicity. The European Union has recently enacted policies to ban the use of toxic solvents in all industrial applications. [(EC) No 1907 / 2006 and subsequent annexes] The use of safer solvents helps to ensure the future production of water filtration membranes. The use of safer solvents reduces manufacturing hazards in membrane production.

[0009] The present invention provides a solvent system that reduces toxic solvents and does not require extensive reformulation or process adjustments to existing methods. Specifically, we focus on the non-solvent induced phase separation (NIPS) process for casting films using PVDF resin, NMP solvent, and formulation additives such as polyvinyl pyrrolidone and polyethylene glycol.

[0010] Research has found that a blend of triethyl phosphate (TEP), a less toxic solvent than NMP, and NMP can produce PVDF membranes with comparable performance to membranes made with pure NMP solvent. The blend reduces the use of harmful solvents and can save manufacturers significant costs because TEP is about half the price of NMP.

[0011] Thus, a casting dope for preparing a membrane having reduced toxic solvents and a method of preparing the membrane have been discovered. Summary of the invention

[0012] We have developed a blend of TEP and NMP that can replace pure NMP as a solvent for preparing PVDF hollow membranes with similar properties. This solvent blend reduces the amount of harmful solvent (NMP) used in the manufacturing process and reduces costs due to the lower price of TEP relative to NMP.

[0013] Aspects of the Invention

[0014] Aspect 1: A casting solution comprising a PVDF resin, a water-soluble or hydrogel polymer, and optional additives in a solvent, wherein the solvent comprises a blend of triethyl phosphate (TEP) and N-methylpyrrolidone (NMP).

[0015] Aspect 2: The casting solution as described in Aspect 1, wherein the weight ratio of TEP to NMP is 75:25 to 1:99.

[0016] Aspect 3: The casting solution as described in Aspect 1, wherein the weight ratio of TEP to NMP is 75:25 to 30:70.

[0017] Aspect 4: The casting solution as described in Aspect 1, wherein the ratio of TEP to NMP is 70:30 to 50:50, preferably 65:35 to 50:50.

[0018] Aspect 5: The casting solution according to any combination of aspects 1 to 4, wherein the amount of PVDF is 12 wt% to 25 wt%, preferably 18 wt% to 20%, based on the weight of the total casting solution.

[0019] Aspect 6: The casting solution according to any combination of Aspects 1 to 5, wherein the melt viscosity of the PVDF resin is 18 to 45 kpoise, preferably 25 to 42 kpoise.

[0020] Aspect 7: The casting solution according to any combination of aspects 1 to 6, wherein the PVDF resin comprises a homopolymer resin.

[0021] Aspect 8: The casting 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 tetrafluoropropylene.

[0022] Aspect 9: The casting solution according to any combination of aspects 1 to 8, wherein the PVDF polymer comprises a mixture of PVDF polymers of different viscosities.

[0023] Aspect 10: The casting 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% based on the total weight of polymer P and acrylic resin in the casting solution.

[0024] Aspect 11: A casting solution of any composition of Aspects 1 to 10, wherein the optional additive comprises an acrylic resin, the acrylic resin being selected from the group consisting of: PMMA resin; PMMA copolymer resin containing an acrylate comonomer; PMMA copolymer resin containing a hydroxyethyl methacrylate comonomer; PMMA copolymer resin containing methoxypolyethylene glycol methacrylate; PMMA copolymer resin containing a polyethylene glycol methacrylate comonomer; PMMA resin containing zwitterionic functional groups; PMMA resin containing sulfonic acid groups; and block copolymers consisting of a pure PMMA block and a second block containing a hydrophilic comonomer such as HEMA or PEGMA and a hydrophobic comonomer such as an alkyl acrylate.

[0025] Aspect 12: The casting solution according to any combination of aspects 1 to 11, wherein the viscosity of the casting solution is 50,000 to 250,000 centipoise, preferably 80,000 to 180,000 cps, as measured by a Brookfield viscometer at 70°C, 50 RPM, and #7 rotor.

[0026] Aspect 13: The casting solution according to any combination of aspects 1 to 12, wherein the water-soluble hydrogel polymer comprises at least one of polyvinyl pyrrolidone, polyethylene oxide, polyethylene oxide / polypropylene oxide block copolymer or a mixture thereof.

[0027] Aspect 14: A casting solution as any combination of Aspects 1 to 12, wherein the hydrogel polymer comprises at least one of polyhydroxyethyl methacrylate (polyHEMA), poly-N-isopropylacrylamide (PNIPAM), polyethylene glycol methacrylate (PEGMA), cross-linked PVP or a copolymer thereof.

[0028] Aspect 15: A method for casting a film, comprising the following steps:

[0029] a. providing a casting solution (PS) comprising a PVDF resin, a water-soluble or hydrogel polymer and optional additives in a solvent, wherein the solvent comprises a blend of triethyl phosphate and N-methylpyrrolidone, wherein the weight ratio of TEP to NMP is 75:25 to 1:99;

[0030] b. degassing the casting solution of step a);

[0031] c. Extruding the casting liquid;

[0032] d. passing the extruded casting solution through a non-solvent bath to form a porous membrane;

[0033] e. soaking the porous membrane in water;

[0034] f. Optionally, the porous membrane is immersed in a sodium hypochlorite solution (0.5 wt % -7.5 wt %);

[0035] g. optionally rinsing the membrane with fresh water after soaking in sodium hypochlorite;

[0036] h. Condition the film with a wetting agent.

[0037] Aspect 16: The method of aspect 15, wherein the weight ratio of TEP to NMP is 75:25 to 1:99.

[0038] Aspect 17: The method according to aspect 15, wherein the weight ratio of TEP to NMP is 75:25 to 30:70.

[0039] Aspect 18: The method according to aspect 15, wherein the ratio of TEP to NMP is 70:30 to 50:50, preferably 65:35 to 50:50.

[0040] Aspect 19: The method according to any combination of aspects 15 to 18, wherein the amount of PVDF is 12 wt% to 25 wt%, preferably 18% to 20%, based on the weight of the total casting solution.

[0041] Aspect 20: The method of any combination of aspects 15 to 19, wherein the melt viscosity of the PVDF resin is 18 to 45 kpoise, preferably 25 to 42 kpoise.

[0042] Aspect 21: The method of any combination of aspects 15 to 20, wherein the PVDF resin comprises a homopolymer resin.

[0043] Aspect 22: The method of 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 tetrafluoropropylene.

[0044] Aspect 23: The method of any combination of aspects 15 to 22, wherein the PVDF polymer comprises a mixture of PVDF polymers of different viscosities.

[0045] Aspect 24: The method of any combination of aspects 15 to 23, wherein the optional additive comprises an acrylic resin in an amount of 0 to 20% by weight, preferably 1 to 20% based on the total weight of polymer P and acrylic resin in the casting solution.

[0046] Aspect 25: A method of any composition of Aspects 15 to 24, wherein the optional additive comprises an acrylic resin selected from the group consisting of polymethyl methacrylate (PMMA) resins; PMMA copolymer resins containing acrylate comonomers; PMMA copolymer resins containing hydroxyethyl methacrylate comonomers; PMMA copolymer resins containing methoxypolyethylene glycol methacrylate comonomers; PMMA copolymer resins containing polyethylene glycol methacrylate comonomers; PMMA resins containing zwitterionic functional groups; PMMA resins containing sulfonic acid groups; and block copolymers comprising a pure PMMA block and a second block containing a hydrophilic comonomer (e.g., HEMA or PEGMA) and a hydrophobic comonomer (e.g., alkyl acrylate).

[0047] Aspect 26: A method as 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 (e.g., NMP or TEP); a mixture of water and an alcohol; a mixture of water and glycerol; or a mixture of water and propylene glycol.

[0048] Aspect 27: The method according to 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.

[0049] Aspect 28: The method according to any combination of aspects 15 to 27, wherein in step c), the casting solution is extruded through a hollow fiber die (hole-in-tube design) while a bore liquid is injected into the pores of the membrane.

[0050] Aspect 29: The method of aspect 28, wherein the bore fluid comprises a mixture of water, TEP, and NMP.

[0051] Aspect 30: The method of aspect 29, wherein the ratio of water, TEP and NMP is 50 to 80% water, 20-40% TEP and 0-25% NMP based on the total weight of water, TEP and NMP, preferably the ratio is 50-70% water, 15-25% TEP and 5-25% NMP.

[0052] Aspect 31: The method according to any combination of aspects 15 to 30, further comprising the step d2) passing the extruded casting solution through a second non-solvent bath after the first non-solvent bath.

[0053] 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 glycerin, a mixture of water and propylene glycol, and a mixture of water and polyethylene glycol.

[0054] Aspect 33: The method of any combination of aspects 15 to 32, wherein the casting solution is extruded onto the hollow braided fibers.

[0055] Aspect 34: The method of any combination of aspects 15 to 33, wherein the casting solution is extruded onto a porous or non-porous support.

[0056] Aspect 35: The method of any combination of aspects 15 to 34, wherein the water-soluble polymer comprises at least one of polyvinyl pyrrolidone, polyethylene oxide, polyethylene oxide / polypropylene oxide block copolymer, or a mixture thereof.

[0057] Aspect 36: A method as in any combination of aspects 15 to 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 a copolymer thereof.

[0058] Aspect 37: A membrane produced by a method according to any combination of Aspects 15 to 36, wherein the pure water permeability of the membrane is between 100 and 1000LMHB, the mechanical strength is in the range of 4.0 to 6.0 MPa, and the elongation at break is in the range of 90 to 200%, preferably in the range of 100 to 200%.

[0059] Aspect 38: Use of the membrane produced by the method of any combination of Aspects 13 to 36 for water filtration including drinking water, wastewater, industrial process water, and for membrane bioreactors.

[0060] Aspect 39: Use of the casting solution according to any combination of aspects 1 to 14 for preparing a membrane.

[0061] Aspect 40: The use of Aspect 39, wherein the membrane is used for water filtration including drinking water, wastewater, industrial process water, and for membrane bioreactors. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 is a graph of the elongation of the casting solutions cited in Table 2 as a function of % NMP.

[0063] Figure 2 is a graph of the mechanical strength of the casting solutions cited in Table 2 as a function of % NMP. DETAILED DESCRIPTION

[0064] As used herein, unless otherwise indicated, percentages are by weight and melt viscosity is measured by capillary rheometer at 100 seconds using ASTM 3825. -1 and 232° C. All cited references are incorporated herein by reference.

[0065] "Copolymer" refers to a polymer having two or more different monomer units, including terpolymers (three different comonomers) and polymers (more than three different comonomers). "PVDF" refers to polyvinylidene fluoride. "Polymer" may refer to a homopolymer or a copolymer. For example, as used herein, unless otherwise specifically stated, "PVDF" and "polyvinylidene fluoride" refer to both homopolymers and copolymers. "Fluoropolymer" refers to a polymer comprising a fluorinated monomer. The polymer may be homogeneous, heterogeneous or random, and may have a gradient distribution of comonomer units.

[0066] Hydrogel polymers refer to three-dimensional cross-linked hydrophilic polymers that are insoluble in water. Due to the physical or chemical cross-linking of the hydrophilic polymer chains, hydrogel polymers can absorb large amounts of water without dissolving.

[0067] The casting solution for preparing the membrane comprises polymer P, a water-soluble or hydrogel polymer, optionally other additives and a solvent system.

[0068] The casting solution for preparing the membrane comprises a polymer P selected from polyvinylidene fluoride (PVDF) homopolymers and copolymers. The term "PVDF polymer" shall include mixtures of different PVDF polymers. The melt viscosity of the PVDF polymer ranges from 18 to 45 kpoise, preferably from 25 to 42 kpoise.

[0069] The polymer of the present invention can be any PVDF fluoropolymer used to form a membrane by the NIPS process. Particularly useful fluoropolymers include, but are not limited to, PVDF homopolymers and copolymers, the majority of the monomer units of the copolymer being vinylidene fluoride combined with a comonomer such as hexafluoropropylene, chlorotrifluoroethylene, or vinyl chloride.

[0070] The polyvinylidene fluoride resin (PVDF) composition of the present invention comprises homopolymers, copolymers, terpolymers, and high polymers of vinylidene fluoride prepared by polymerizing vinylidene fluoride (VDF), wherein the vinylidene fluoride unit usually and preferably accounts for more than 70% of the total weight of all monomer units in the polymer, and more preferably accounts for more than 75% of the total weight of the units. Copolymers, terpolymers, and higher polymers of vinylidene fluoride can be prepared by reacting vinylidene fluoride with one or more monomers, the one or more monomers being selected from vinyl fluoride, trifluoroethylene, tetrafluoroethylene; tetrafluoropropylene, such as 2,3,3,3-tetrafluoropropylene, E-1,3,3,3-tetrafluoropropylene, Z-1,3,3,3-tetrafluoropropylene, 1,1,2,3-tetrafluoropropylene, 1,2,3,3-tetrafluoropropylene, 1,1,3,3-tetrafluoropropylene, chlorotetrafluoropropylene; 3,3,3-trifluoro-1-propylene, 1,2,3,3,3 pentafluoropropylene, one or more partially or fully fluorinated α-olefins For example, 3,3,3,4,4-pentafluoro-1-butene, hexafluoropropylene, trifluoromethylacrylic acid, trifluoromethyl methacrylate, partially fluorinated olefin hexafluoroisobutylene, perfluorinated vinyl ethers, such as perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, perfluoro-n-propyl vinyl ether and perfluoro-2-propoxypropyl vinyl ether, fluorinated dioxoles, such as perfluoro(1,3-dioxole) and perfluoro(2,2-dimethyl-1,3-dioxole), allyl, partially fluorinated allyl or fluorinated allyl monomers, such as 2-hydroxyethyl allyl ether or 3-allyloxypropylene glycol, ethylene, propylene. Preferred copolymers or terpolymers are formed from one or more of vinyl fluoride, trifluoroethylene, tetrafluoroethylene (TFE) and hexafluoropropylene (HFP). The most preferred copolymer is formed from hexafluoropropylene (HFP). A preferred PVDF is Arkema's PVDF.

[0071] Although copolymers containing perfluorinated monomers are preferred, non-fluorinated monomers such as vinyl acetate, methacrylic acid, and acrylic acid may also be used to form the copolymers in amounts up to 5 weight percent based on polymer solids.

[0072] Preferred copolymers are VDF containing from about 70 to about 99 weight percent VDF monomer units and correspondingly from about 1 to about 30 weight percent HFP monomer units, and from about 70 to about 99 weight percent VDF and correspondingly from about 1 to about 30 weight percent trifluoroethylene monomer units.

[0073] Mixtures of polyvinylidene fluoride polymers are also contemplated as part of the present invention, including functionalized fluoropolymers and non-functionalized polymers, PVDF homopolymers and PVDF-HFP copolymers, and PVDF polymers having different melt viscosities.

[0074] Preferably, the casting solution comprises 10 to 30 wt. % of polymer P, based on the total weight of the casting solution, more preferably 12 to 25 wt. %, most preferably 18-22 wt. %.

[0075] Water-soluble / hydrogel polymers

[0076] Water-soluble or hydrogel polymers can help adjust the viscosity of the casting solution. The main purpose of these hydrophilic additives is to support pore formation and impart residual hydrophilicity to the membrane.

[0077] The water-soluble polymer can be any known water-soluble polymer. Preferred water-soluble polymers are selected from the group consisting of polyvinyl pyrrolidone (PVP), polyethylene glycol with a molecular weight of 200 to 1000 Mw, and polyalkylene oxides with a molar mass of 4000 g / mol or more. Preferred water-soluble polymers include polyvinyl pyrrolidone, polyethylene oxide, polyethylene oxide / polypropylene oxide block copolymers and mixtures thereof. Preferred water-soluble polymers are polyethylene glycol and polyvinyl pyrrolidone. Very preferred water-soluble polymers are polyvinyl pyrrolidone.

[0078] Preferred hydrogel polymers may be selected from known examples including polyhydroxyethyl methacrylate (polyHEMA), poly-N-isopropylacrylamide (PNIPAM), polyethylene glycol methacrylate (PEGMA), high K value PVP, cross-linked PVP and copolymers thereof.

[0079] When the casting solution contains 10 to 30 wt% of polymer P, the amount of water-soluble or hydrogel polymer may be 1 to 22 wt% based on the total weight of the casting solution.

[0080] In a preferred embodiment, the casting solution contains 12 to 25 wt % of polymer P and 2 to 20 wt % of water-soluble or hydrogel polymer based on the total weight of the casting solution. In a more preferred embodiment, the casting solution contains 18 to 20 wt % of polymer P and 8 to 16 wt % of water-soluble or hydrogel polymer based on the total weight of the casting solution.

[0081] In one embodiment, the amount of the PVP additive is preferably 10-16%, preferably 14-15%, based on the total weight of the casting solution.

[0082] The K value of polyvinyl pyrrolidone is preferably 10 to 120. Preferably, the K value of at least one PVP present in the composition is 12 to 60. One or more polyvinyl pyrrolidones with different K values ​​can be used. Polyvinyl pyrrolidones can be used in combination, for example, K15 is combined with K30, or K15 is combined with K60, or K15 is combined with K90, or K30 is combined with K60. A given K value roughly corresponds to the weight average molecular weight (using GPC / MALLS). The weight average molecular weight of PVP with a general K value of K30 is in the range of 40,000 to 80,000 g / mol, K60 generally indicates a weight average molecular weight in the range of 250,000 to 500,000 g / mol, and K90 generally indicates a weight average molecular weight in the range of 1 million to 1.4 million g / mol. K15 is generally in the range of 7,000 to 20,000 g / mol. "K value" refers to the Fikentscher K value (1000k) defined by H. Fikentscher—Cellulosechemie 13, 58-64, 71-4 (1932). (US2706701) PVP polymers are available as commercial products, for example PVP [from BASF] or Plasdone TM , povidone, PVP K series [all from Ashland] and are sold with reference to the K value as an indication of molecular weight.

[0083] In some embodiments, the amount of polyethylene glycol PEG is 0-10 wt % based on the total weight of the casting solution.

[0084] In some embodiments, PEG is used and its molecular weight is preferably between 200 and 1000 Mw. In some embodiments, the molecular weight can be higher.

[0085] additive

[0086] In one embodiment, the optional additive is an acrylic resin, and its amount is 0-20% by weight, preferably 1-20% based on the total weight of the polymer P and the acrylic resin in the casting solution.

[0087] The optional acrylic resin can be any one or more of the following resins: PMMA resin; PMMA copolymer resin containing acrylate comonomer; PMMA copolymer resin containing hydroxyethyl methacrylate comonomer; PMMA copolymer resin containing methoxy-polyethylene glycol methacrylate; PMMA copolymer resin containing polyethylene glycol methacrylate comonomer; PMMA resin containing zwitterionic functional groups, such as sulfobetaine methacrylate; PMMA resin containing sulfonic acid groups; block copolymers consisting of pure PMMA blocks and a second block, wherein the second block contains a hydrophilic comonomer (such as HEMA or PEGMA) and a hydrophobic comonomer (such as alkyl acrylate).

[0088] Other additives may include, but are not limited to, inorganic salts such as lithium chloride, magnesium chloride, ferrous chloride, aluminum chloride, etc.; quaternary ammonium salts; propylene glycol, glycerol, organic acids, molecular sieves, silica, alumina, and activated carbon.

[0089] Solvent system

[0090] The solvent system comprises a blend of triethyl phosphate (TEP) / N-methylpyrrolidone (NMP). Preferably, the triethyl phosphate (TEP) / N-methylpyrrolidone blend comprises a weight ratio of TEP:NMP of 75:25 to 1:99, preferably 75:25 to 30:70, more preferably 70:30 to 50:50. In some embodiments, the weight ratio is 65:35 to 50:50.

[0091] The preferred embodiment is that the blending range of TEP:NMP is 75:25 and below. When the ratio is higher than 75:25 (for example, 90TEP:10NMP), the mechanical strength and elongation will decrease.

[0092] In addition to the triethyl phosphate / N-methylpyrrolidone blend, the casting solution may contain an optional auxiliary solvent, hereinafter referred to as a co-solvent.

[0093] Preferred are co-solvents that are miscible with the triethyl phosphate / N-methylpyrrolidone blend. Suitable co-solvents are selected, for example, from dimethylformamide, dimethyl sulfoxide, sulfolane, N-ethyl-2-pyrrolidone, N-n-butyl-2-pyrrolidone, N,N-dimethyl-2-hydroxypropionamide, N,N-diethyl-2-hydroxypropionamide, ethyl lactate, methyl lactate, Cyrene TM (dehydroglucose ketone), ethyl levulinate and 2-pyrrolidone.

[0094] The total solvent [TEP, NMP and co-solvent(s)] in the casting solution usually accounts for 50 to 85 wt % of the total casting solution weight, preferably 55 to 75 wt % of the total casting solution weight.

[0095] In a preferred embodiment, no more than 15 wt % of the casting solution is a co-solvent, based on the total weight of all solvents in the casting solution.

[0096] In a most preferred embodiment, no co-solvents are used in the casting solution and the triethyl phosphate / N-methylpyrrolidone blend is the only solvent system used.

[0097] Preparation of casting solution

[0098] The polymer P and the water-soluble polymer and / or the hydrogel polymer can be dissolved by adding them to the triethyl phosphate (TEP) / N-methylpyrrolidone (NMP) blend in any order and according to any method known in the art. The dissolution process can be supported by increasing the temperature of the casting solution and / or by mechanical manipulation such as stirring.

[0099] In a general method, the components are mixed with an overhead mixer, preferably at 30-300 rpm, and preferably while heated to a temperature between 70 and 120°C, for 4 to 12 hours.

[0100] Preferably, the final viscosity of the final casting solution is 50,000 to 250,000 centipoise, preferably 80,000 to 180,000 cps as measured by Brookfield viscometer at 70°C, 50 RPM, #7 spindle.

[0101] Membrane preparation process

[0102] In the context of the present application, membrane is understood to be a semipermeable structure capable of separating two fluids or separating molecules and / or ionic components or particles from a liquid. The membrane acts as a selective barrier, allowing some particles, substances or chemicals to pass through while retaining others. The membrane can have various geometric shapes, such as flat plates, spirally wound, tubular, single-hole hollow fibers, porous hollow fibers or enhanced hollow fibers.

[0103] The membrane can be produced according to the following method, which includes: providing a casting solution comprising polymer P, a water-soluble polymer or a hydrogel polymer and a solvent blend, casting a membrane by extruding the casting solution, passing the extruded casting solution through a non-solvent bath / coagulant, and optionally oxidizing and washing the obtained membrane.

[0104] Preferably, the film casting method comprises the following steps:

[0105] a. providing a casting solution (DS) comprising a PVDF resin, a water-soluble or hydrogel polymer, and optional additives in a solvent, wherein the solvent comprises a blend of triethyl phosphate and N-methylpyrrolidone;

[0106] b. degassing the casting solution of step a);

[0107] c. Extruding the casting liquid;

[0108] d. by passing the casting solution extruded in step c through a non-solvent bath to condense the casting solution to form a porous membrane;

[0109] e. immersing the porous membrane in an aqueous solution;

[0110] f. Optionally, the porous membrane is immersed in a sodium hypochlorite solution (0.5% -7.5%) at a temperature of 20-50°C for 4-24 hours, and then the porous membrane is rinsed with fresh water after the sodium hypochlorite soaking;

[0111] g. optionally soaked in an alcohol solution;

[0112] h. Condition the fibers with a wetting agent.

[0113] The casting solution in step a) corresponds to the casting solution described above. The main purpose of the aqueous solution polymer is to support the formation of pores. Water-soluble or hydrogel polymers can also help adjust the viscosity of the casting solution. In the coagulation step d), the water-soluble polymer is distributed in the coagulated film and thus becomes a placeholder for the pores.

[0114] Degassing can be carried out at elevated temperature or at room temperature, preferably between 50 and 80°C.

[0115] In step d), the casting solution is contacted with a non-solvent bath (also called a coagulant). In this step, the polymer P coagulates and forms a membrane structure.

[0116] In step f), the solution may be at ambient temperature or at elevated temperature, preferably from 20°C to 50°C.

[0117] The hollow fiber membrane according to the present invention can be assembled in a hollow fiber membrane module by adopting any such assembly technique or method known in the art. The use of hollow component fibers to make such modules is well known and is described, for example, in the following publications, each of which is incorporated herein by reference in its entirety for all purposes. U.S. Patent 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.

[0118] The process of casting hollow fiber membranes from a casting solution involves extruding the casting solution through a hollow fiber die (tube-in-hole design) while simultaneously injecting a mixed solvent into the pores of the membrane and stretching the fiber through a non-solvent bath. Generally, the membrane is then collected onto a collection reel.

[0119] For the method of the present invention, the bore fluid is preferably a mixture of water, TEP and NMP, preferably in a ratio (by weight) of 50-80% water, 20-40% TEP, 0-25% NMP, more preferably 50-70% water, 15-25% TEP, 5-25% NMP.

[0120] In some embodiments, the casting solution is heated to between 40 and 80°C.

[0121] In some embodiments, the well fluid is heated to between 40 and 60°C.

[0122] In some embodiments, the non-solvent bath is pure water.

[0123] In some embodiments, the water non-solvent bath is heated to between 40 and 60 °C.

[0124] In some embodiments, the hollow fiber mold is heated to between 40 and 90°C, preferably between 50 and 70°C.

[0125] In some embodiments, the fiber is immersed in the non-solvent bath for 2-20 meters.

[0126] The process of the present invention may optionally have a second non-solvent bath following the first non-solvent bath.

[0127] In some embodiments, the second non-solvent bath is at room temperature.

[0128] The second non-solvent bath (or the single non-solvent bath, if used) may 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.

[0129] Non-solvent

[0130] In the nonsolvent / coagulant, the polymer P should have a low solubility. Suitable nonsolvent baths / coagulants are, for example, liquid water, water vapor, alcohols, glycols, glycerol or mixtures thereof.

[0131] Suitable alcohols are, for example, monoalkanols, dialkanols or trialkanols selected from C2-C4 alkanols, C2-C4 alkanediols, C3-C4 alkanetriols, polyethylene oxides, and have a molar mass of 100 to 1000 g / mol, as they can be used as additives in the casting solution of the present invention. Preferred non-solvent mixtures are mixtures comprising liquid water and alcohols, more preferably mixtures comprising liquid water and alcohols optionally used as additives in the casting solution of the present invention. Preferred non-solvents are liquid water.

[0132] Flat sheet membranes can be prepared using the casting solution and method of the present invention.The casting solution is cast onto a support which is then contacted with a bath.

[0133] In a preferred embodiment, process steps e) to h) are performed, oxidation and washing are performed in order to remove water-soluble polymers and form pores. Oxidation is followed by a washing step.

[0134] For oxidation, any oxidizing agent may be used. Preferred are water-soluble oxidizing agents such as sodium hypochlorite or hydrogen peroxide.

[0135] The resulting membrane preferably has a high tensile strength - greater than 3.5 MPa, preferably equal to or greater than 4 MPa and a high elongation - equal to or greater than 95%, preferably equal to or greater than 100% elongation and a PWP (pure water permeability) greater than 190 LMHB (liters per hour-square meters-bar).

[0136] The membranes obtained by the process of the invention can be used for any separation purpose, such as water treatment applications, industrial or municipal wastewater treatment, seawater or brackish water desalination, dialysis, plasmolysis, food processing.

[0137] Example

[0138] Abbreviations and compounds used in the examples:

[0139] PWP pure water permeability

[0140] NMP N-Methyl 2-pyrrolidone

[0141] TEP Triethyl Phosphate

[0142] Preparation of film casting solution

[0143] Example 1: 50:50 TEP-NMP

[0144] Will MG15 (PVDF homopolymer resin, 35-39 kpoise, 57 g) [available from Arkema, Inc.] and polyvinyl pyrrolidone (K30 grade, 45 g) were weighed into a 16 oz mixing jar. Triethyl phosphate (99 g) and N-methyl pyrrolidone (99 g) were added to the powder. The agitator blade was inserted into the jar and secured in place with a lid. The mixture was stirred at 75 rpm with an overhead stirrer while heating to 95°C with a heating mantle for six hours. A clear yellow viscous solution was obtained. The jar was sealed with a lid and placed in an oven at 70°C overnight. The viscosity of the solution was measured to be 126,000 cps.

[0145] Table 1

[0146]

[0147]

[0148] Example 2: 70:30 TEP-NMP Except that the amounts of triethyl phosphate and N-methylpyrrolidone were changed to those in Table 1, the rest were the same as Example 1.

[0149] Example 3: 80:20 TEP-NMP - Comparative Example Except that the amounts of triethyl phosphate and N-methylpyrrolidone were changed to those in Table 1, the rest was the same as Example 1.

[0150] Example 4: 60:40 TEP-NMP Except that the amounts of triethyl phosphate and N-methylpyrrolidone were changed to those in Table 1, the rest were the same as Example 1.

[0151] Example 5: 67:33 TEP-NMP Except that the amounts of triethyl phosphate and N-methylpyrrolidone were changed to those in Table 1, the rest were the same as Example 1.

[0152] Example 6: 90:10 TEP-NMP (Comparative Example) Except that the amounts of triethyl phosphate and N-methylpyrrolidone were changed to those in Table 1, the rest were the same as Example 1.

[0153] Example 7: 100% NMP (Comparative Example) Except that the solvent is 100% N-methylpyrrolidone and the amount is changed to that in Table 1, the rest is the same as Example 1.

[0154] Example 8: 100% TEP (Comparative Example) Except for using 100% TEP solvent, everything else is the same as Example 1.

[0155] Film casting and post-processing

[0156] Hollow fiber casting

[0157] The following process is applicable to casting all the above formulations. The casting line consists of a temperature-controlled tank for holding the membrane preparation (casting solution) and the bore solution. The casting solution and the bore solution are pumped using a gear pump to achieve high-precision dispensing. The membrane preparation is loaded into the temperature-controlled casting solution tank (approximately 70°C) and allowed to stand for 15 minutes before casting. The casting solution transfer line to the spinneret is maintained at the same temperature as the casting solution tank. The spinneret is heated independently to approximately the same temperature. The bore solution is loaded into the bore solution tank and the temperature is controlled at approximately 60°C. The water non-solvent bath is heated to approximately 60°C. The take-up speed on the collection reel is approximately 12 meters / minute. The take-up reel is immersed in a room temperature water bath to help clean the fiber.

[0158] After casting, the membranes were soaked in water overnight, then treated with sodium hypochlorite solution (1.5%) at 45°C for 6 hours, and then soaked overnight at room temperature. The membranes were then rinsed twice with fresh water before testing the permeability.

[0159] Membrane test results

[0160] Mechanical testing was performed on an Instron 4201 Universal Test Frame (following ASTM D638) equipped with a fiber holder designed to hold the fiber wound on a bobbin. This prevents standard tensile bar fixtures from damaging the delicate hollow fibers. The gap spacing was 100 mm and the strain rate was 50 mm min -1 The fibers were tested while wetted with water.

[0161] To test pure water permeability, 5 membrane loops of approximately 35 cm in length were encapsulated in a test module with epoxy resin. The water flow was measured at 0.5 bar, with a 15-minute purge before reading and then normalized to 1 m -2 ·h -1 bar -1 ,LMHB.

[0162] Table 2 shows the permeability and mechanical properties of the casting formulations.

[0163]

[0164]

[0165] Tabular Data

[0166] When testing the properties of films made using solvent mixtures, it was surprisingly found that films with high mechanical strength and high elongation could be obtained at levels above 50% TEP. Using TEP as the sole solvent (100% TEP) resulted in poor elongation and mechanical strength. We found that replacing up to 70% of NMP with TEP resulted in films with elongation of at least 95%, preferably 100%, mechanical strength of at least 3.5, preferably at least 4 MPa, and LMHB greater than 190, which is comparable to or better than films containing 100% NMP.

Claims

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

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

99.

3. The casting solution as claimed in claim 1, wherein the weight ratio of TEP to NMP is 75:25 to 30:

70.

4. The casting solution as claimed in claim 1, wherein the ratio of TEP to NMP is 70:30 to 50:50, preferably 65:35 to 50:

50.

5. The casting solution as claimed in claim 1, wherein the amount of PVDF is 12 wt% to 25 wt%, preferably 18 wt% to 20 wt%, based on the weight of the total casting solution.

6. The casting solution as claimed in claim 1, wherein the melt viscosity of the PVDF resin is 18 to 45 kpoise, preferably 25 to 42 kpoise.

7. The casting solution of any one or more of claims 1 to 6, wherein the PVDF resin comprises a homopolymer resin.

8. The casting solution according to any one or more of claims 1 to 6, wherein the PVDF resin comprises a copolymer of VDF and at least one of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene or tetrafluoropropylene.

9. The casting solution of any one or more of claims 1 to 6, wherein the PVDF polymer comprises a mixture of PVDF polymers of different viscosities.

10. The casting solution as claimed in any one or more of claims 1 to 6, 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 polymer P and acrylic resin in the casting solution.

11. The casting solution of any one or more of claims 1 to 6, wherein the optional additive comprises an acrylic resin selected from the group consisting of a PMMA resin; a PMMA copolymer resin containing an acrylate comonomer; a PMMA copolymer resin containing a hydroxyethyl methacrylate comonomer; a PMMA copolymer resin containing a methoxy-polyethylene glycol methacrylate; a PMMA copolymer resin containing a polyethylene glycol methacrylate comonomer; a PMMA resin containing zwitterionic functional groups; a PMMA resin containing sulfonic acid groups; and a block copolymer consisting of a pure PMMA block and a second block containing a hydrophilic comonomer such as HEMA or PEGMA and a hydrophobic comonomer such as an alkyl acrylate.

12. The casting solution of any one or more of claims 1 to 6, wherein the viscosity of the casting solution is 50,000 to 250,000 centipoise, preferably 80,000 to 180,000 cps as measured by Brookfield viscometer at 70°C, 50 RPM, #7 rotor.

13. The casting solution of any one or more of claims 1 to 6, wherein the water-soluble hydrogel polymer comprises at least one of polyvinyl pyrrolidone, polyethylene oxide, polyethylene oxide / polypropylene oxide block copolymers, or mixtures thereof.

14. The casting solution of any one or more of claims 1 to 6, 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.

15. A method for casting a film, comprising the following steps: a) providing a casting solution (PS) comprising a PVDF resin, a water-soluble or hydrogel polymer and optional additives in a solvent, wherein the solvent comprises a blend of triethyl phosphate and N-methylpyrrolidone, wherein the weight ratio of TEP to NMP is 75:25 to 1:99; b) degassing the casting solution of step a); c) extruding the film casting solution; d) passing the extruded casting solution through a non-solvent bath to form a porous membrane; e) soaking the porous membrane in water; f) optionally soaking the porous membrane in a sodium hypochlorite solution (0.5 wt % - 7.5 wt %); g) optionally rinsing the membrane with fresh water after soaking in sodium hypochlorite; h) Conditioning the fibers with a wetting agent.

16. The method of claim 15, wherein the weight ratio of TEP to NMP is from 75:25 to 1:

99.

17. The method of claim 15, wherein the weight ratio of TEP to NMP is from 75:25 to 30:

70.

18. The method of claim 15, wherein the ratio of TEP to NMP is from 70:30 to 50:50, preferably from 65:35 to 50:

50.

19. The method of claim 15, wherein the amount of PVDF is 12 to 25 wt%, preferably 18 to 20 wt%, based on the weight of the total casting solution.

20. The method of claim 15, wherein the melt viscosity of the PVDF resin is 18 to 45 kpoise, preferably 25 to 42 kpoise.

21. The method of any one or more of claims 15 to 20, wherein the PVDF resin comprises a homopolymer resin.

22. The method of any one or more of claims 15 to 20, wherein the PVDF resin comprises a copolymer of VDF and at least one of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene, or tetrafluoropropylene.

23. The method of any one or more of claims 15 to 20, wherein the PVDF polymer comprises a mixture of PVDF polymers of different viscosities.

24. The method according to any one or more of claims 15 to 20, wherein the optional additive comprises an acrylic resin in an amount of 0 to 20% by weight, preferably 1 to 20% based on the total weight of polymer P and acrylic resin in the casting solution.

25. The method of any one or more of claims 15 to 20, wherein the optional additive comprises an acrylic resin selected from the group consisting of polymethyl methacrylate (PMMA) resins; PMMA copolymer resins containing acrylate comonomers; PMMA copolymer resins containing hydroxyethyl methacrylate comonomers; PMMA copolymer resins containing methoxy-polyethylene glycol methacrylate comonomers; PMMA copolymer resins containing polyethylene glycol methacrylate comonomers; PMMA resins containing zwitterionic functional groups; PMMA resins containing sulfonic acid groups; and block copolymers comprising a pure PMMA block and a second block containing a hydrophilic comonomer such as HEMA or PEGMA and a hydrophobic comonomer such as an alkyl acrylate.

26. The method of any one or more of claims 15 to 20, 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.

27. The method according to any one or more of claims 15 to 20, wherein the humectant is selected from the group consisting of glycerol, propylene glycol, butylene glycol, hexylene glycol and mixtures thereof or aqueous solutions.

28. The method according to any one or more of claims 15 to 20, wherein in step c) the casting solution is extruded through a hollow fiber die (hole-in-tube design) while the bore solution is injected into the pores of the membrane.

29. The method of claim 28, wherein the bore fluid comprises a mixture of water, TEP and NMP.

30. The process of claim 29, wherein the ratio of water, TEP and NMP is 50 to 80% water, 20-40% TEP and 0-25% NMP, preferably 50-70% water, 15-25% TEP and 5-25% NMP, based on the total weight of water, TEP and NMP.

31. The method of any one or more of claims 15 to 20, further comprising the further step d2) passing the extruded casting solution through a second non-solvent bath after the first non-solvent bath.

32. The method of claim 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 glycerin, a mixture of water and propylene glycol, and a mixture of water and polyethylene glycol.

33. The method of any one or more of claims 15 to 20, wherein the casting solution is extruded onto hollow braided fibers.

34. The method of any one or more of claims 15 to 20, wherein the casting solution is extruded onto a porous or non-porous support.

35. The method of any one or more of claims 15 to 20, wherein the water soluble polymer comprises at least one of polyvinyl pyrrolidone, polyethylene oxide, polyethylene oxide / polypropylene oxide block copolymers, or mixtures thereof.

36. The method of any one or more of claims 15 to 20, 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.

37. A membrane produced by the method of any one or more of claims 15 to 20, wherein the pure water permeability of the membrane is between 100 and 1000LMHB, the mechanical strength is in the range of 4.0 to 6.0MPa, and the elongation at break is in the range of 90 to 200%, preferably in the range of 100 to 200%.

38. Use of a membrane produced by the method of any one or more of claims 15 to 20 for filtration of water including drinking water, waste water, industrial process water and in membrane bioreactors.

39. Use of a casting solution as claimed in any one or more of claims 1 to 14 for producing a membrane.

40. The use according to claim 39, wherein the membrane is used for water filtration including drinking water, waste water, industrial process water and for membrane bioreactors.

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