PVDF UF membranes with narrow pore distributions made with safer solvents

Abstract

The present invention discloses a porous fluoropolymer membrane product comprising a blend of N-butylpyrrolidone and a lactic acid-derived co-solvent, with at least 50% N-butylpyrrolidone in the solvent system.

Description

[Technical Field]

[0001] The present invention relates to a membrane dope solution comprising at least one polymer P, at least one water-soluble or hydrogel polymer H, and a solvent blend of N-butylpyrrolidone ("NBP") and a lactic acid derivative, as well as a method for making the membrane and its use in liquid filtration. [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 technical uses of PVDF polymer is as a raw material for the production of membranes such as hollow fiber membranes and flat sheet 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.

[0004] Applicants have found that the choice of solvent is critical to the process and affects the properties of the resulting membrane, including, but not limited to, the membrane's pore size, water permeability, and mechanical strength.

[0005] Two recent publications describe the use of pure NBP solvent to cast PVDF or PVDF-HFP resin membranes: Marino et al., Journal of Membrane Science 542 (2017) 418-429 (https: / / doi.org / 10.1016 / j.memsci.2017.08.038) and Russo et al., Polymers 2021, 13, 2579 (https: / / doi.org / 10.3390 / polym13152579). Russo et al. fabricated flat-sheet membranes using PVDF homopolymer (6010), whereas Marino et al. used HFP-VF2 copolymer. In both papers, the membranes had larger bubble point pore sizes and BPD / MPD ratios than those described in this invention, and both had lower permeability.

[0006] Pacheco et al. (U.S. Pat. No. 6,126,826) teach a method for producing controlled-pore PVDF flat-sheet membranes. They emphasize the benefit of controlling pore size to a small size to improve filtration rejection. However, this method relied on the use of a toxic NMP solvent. It is also unclear in U.S. Pat. No. 6,126,826 what the ratio of bubble-point pore size to average pore size was. The pore size values ​​appear to have been obtained from SEM analysis of the membrane surface. Unfortunately, SEM measurements require dry samples imaged under high vacuum, which can cause pore shrinkage and therefore may underestimate the true pore size in the wet state. The bubble-point pressure data in U.S. Pat. No. 6,126,826 (performed with a methanol wetting solution) indicate bubble-point pore sizes of 100 nm or greater, which is larger than the upper pore size limit obtained here.

[0007] US Pat. No. 6,110,309 describes sheet membranes made using toxic solvents and having large pore sizes.

[0008] In the solvent field, there is a constant demand for alternatives to currently used solvents such as NMP. For polyvinylidene fluoride ("PVDF"), new solvents are needed to prepare clear solutions with high PVDF content. Membranes formed using new alternative solvents should have comparable properties in terms of pore size and permeability compared to those formed using more hazardous solvents such as NMP.

[0009] The present invention aims to provide a solvent system for producing polyvinylidene fluoride (PVDF) membranes that is less toxic than currently used solvents, such as N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), and N,N-dimethylformamide (DMF). Currently, solvents used in polymer membrane production pose risks associated with carcinogenicity and reproductive toxicity. Recently, the European Union (EU) enacted a policy to phase out the use of toxic solvents in all industrial applications (EC No. 1907 / 2006 and subsequent annexes). The use of safer solvents reduces manufacturing hazards in membrane production and helps ensure the future production of polymer-based water filtration membranes.

[0010] The present invention provides a solvent system that eliminates toxic solvents and does not require substantial formulation or process adjustments from current methods. The present invention provides a safer solvent or solvent blend for casting PVDF membranes via the non-solvent-induced phase separation process ("NIPS"). The safer solvent is N-butyrylpyrrolidone. The solvent blend or mixture contains a majority of N-butylpyrrolidone with a minor cosolvent, such as ethyl lactate, methyl lactate, or N,N-dimethyl lactamide. These cosolvents are also less toxic and bio-based. It has been found that blends of N-butylpyrrolidone and lactic acid-derived cosolvents containing at least 50% by weight or more of N-butylpyrrolidone can produce PVDF membranes with permeability comparable to membranes produced with pure NMP solvent. Replacing NMP with NBP reduces the amount of hazardous solvents in the environment. Porous PVDF filtration membranes fabricated using the above solvent system exhibit a narrow pore size distribution, with a maximum pore size of less than 90 nm (measured by capillary flow porometry), an average pore size of 45 nm, and a hydraulic permeability of greater than 650 LMHB. The ratio of maximum pore size to average pore size in these membranes is less than 2.5. Furthermore, the addition of the lactic acid derivative improves membrane formation time (measured by stripping time) by at least 20% compared to pure NBP as the solvent. Thus, this blend offers previously unknown advantages over the use of pure NBP. These properties are highly useful for biopharmaceutical membranes, where narrow pore distributions and high permeabilities are desired to optimize the manufacturing productivity of expensive biologics. Specifically, a low ratio of maximum pore size (commonly referred to as bubble point pore size) to average pore size (commonly referred to as mean pore diameter) improves selectivity and reduces contamination by large particles.

[0011] An additional important advantage was found in the use of lactate solvent and NBP blends: shortening the film formation time. This was observed by recording the time at which the flat-sheet film began to peel from the surface of the glass plate on which it was cast after immersion in a water bath. While the pure NBP formulation took more than two minutes to peel from the glass plate, the lactate solvent blend consistently peeled off within approximately 90 seconds of immersion in the water bath. Without being limited by theory, it is believed that the hydroxyl groups in the lactate solvent increased the phase transition rate upon contact with water. Reducing film formation time is important for maintaining practical production line speeds in commercial manufacturing processes. The lactate solvent blend increased the phase transition rate by 35% over the pure NBP formulation.

[0012] Furthermore, the formulation using both the lactate solvent blend and PMMA acrylic resin showed an even faster phase transition, with the film separating from the glass in 1 minute. With NBP solvent and acrylic resin additive alone, the phase transition time remained longer than with the lactate solvent blend, but was faster than with the NBP formulation without the acrylic resin additive. Finally, the acrylic resin blend showed significantly higher water permeability, exceeding that of the formulation without the acrylic resin. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] U.S. Patent No. 6,126,826 [Patent Document 2] U.S. Patent No. 6,110,309 [Non-patent literature]

[0014] [Non-Patent Document 1] Marino et al., Journal of Membrane Science 542(2017)418-429(https: / / doi.org / 10.1016 / j.memsci.2017.08.038) [Non-patent document 2] Russo et al., Polymers 2021,13,2579;https: / / doi.org / 10.3390 / polym13152579 Summary of the Invention [Problem to be solved by the invention]

[0015] The present invention provides a dope solution containing a novel solvent blend that can be used to produce PVDF membranes. The primary solvent in the novel blend is N-butylpyrrolidone (NBP). The co-solvent is a lactic acid derivative, such as a lactate ester or lactate amide. The present invention also provides a method for using the dope to produce membranes. [Means for solving the problem]

[0016] Aspects of the present invention

[0017] Aspect 1: A dope solution for producing a filtration membrane, comprising a PVDF polymer, a water-soluble polymer and / or hydrogel "Polymer H", a solvent mixture comprising N-butylpyrrolidone and at least one water-soluble lactic acid derivative, and optional additives.

[0018] Aspect 2: 2. The dope solution of embodiment 1, wherein the solvent mixture comprises at least 50 wt % or more of N-butylpyrrolidone, based on the total weight of the solvents.

[0019] Aspect 3: 3. The dope solution of claim 1 or 2, wherein the solvent mixture comprises at least 70 wt % or more of N-butylpyrrolidone, based on the total weight of the solvents.

[0020] Aspect 4: Aspect 4. The dope solution according to any one of Aspects 1 to 3, wherein the lactic acid derivative includes at least one of an ester derivative or an amide derivative of lactic acid.

[0021] Aspect 5: 5. The dope solution according to any one of aspects 1 to 4, wherein the lactic acid derivative includes an ester derivative of lactic acid.

[0022] Aspect 6: Aspect 6. The dope solution according to any one of Aspects 1 to 5, wherein the lactic acid derivative is selected from the group consisting of methyl lactate, ethyl lactate, propyl lactate, N,N-dimethyl lactamide, and N,N-diethyl lactamide.

[0023] Aspect 7: 7. The dope solution according to any one of aspects 1 to 6, wherein the lactic acid derivative is selected from the group consisting of methyl lactate, ethyl lactate, propyl lactate, and combinations thereof.

[0024] Aspect 8: Aspect 8. The dope solution according to any one of aspects 1 to 7, wherein the lactic acid derivative comprises N,N-dimethyl lactamide or N,N-diethyl lactamide.

[0025] Aspect 9: Aspect 9. The dope solution according to any one of aspects 1 to 8, wherein the PVDF comprises a homopolymer.

[0026] Aspect 10: The dope solution according to any one of Aspects 1 to 3, wherein the PVDF is a copolymer containing at least one monomer unit selected from the group consisting of HFP, TFE, CTFE, VF3, VF, and vinyl acetate.

[0027] Aspect 11: 11. The dope solution according to any one of aspects 1 to 10, wherein the PVDF is a copolymer containing HFP monomer units.

[0028] Aspect 12: 12. The dope solution of any one of embodiments 1-11, wherein the PVDF comprises a blend of different PVDF polymers.

[0029] Aspect 13: 13. The dope solution according to any one of aspects 1 to 12, wherein the polymer H is selected from the group consisting of polyvinylpyrrolidone, poly-2-ethyloxazoline, polyethylene glycol, and combinations thereof.

[0030] Aspect 14: 14. The dope solution according to any one of embodiments 1 to 13, wherein the polymer H comprises polyvinylpyrrolidone.

[0031] Aspect 15: Aspect 15. The dope solution of any one of aspects 1 to 14, wherein the optional additive comprises an acrylic resin in an amount of 1 to 20 wt %, based on the total weight of polymer P.

[0032] Aspect 16: 16. The dope solution of any one of the preceding aspects, wherein the optional additive comprises at least one of polymethyl methacrylate, polymethyl methacrylate copolymer, polyhydroxyl acrylate, polyhydroxyalkanoate such as polylactic acid, or an insoluble hydrogel polymer such as polyhydroxyethyl methacrylate or polyvinyl alcohol, or a combination thereof.

[0033] Aspect 17: A method for producing a PVDF membrane, comprising the steps of: (a) preparing a dope solution according to any one of aspects 1 to 16; (b) casting the dope solution into a flat sheet, hollow fiber, or tubular form at a temperature of 40°C or higher, and then immersing the cast dope solution in one or more non-solvent baths to form a porous membrane; (c) rinsing the porous membrane to remove residual solvent and additives; (d) optionally treating the membrane with a chlorine bleach to further remove the additive; (e) optionally post-treating the membrane with a wetting agent such as glycerol, propylene glycol, or polyethylene glycol; (f) optionally drying the membrane; A method comprising:

[0034] Aspect 18: 18. A polymeric membrane produced by the method of embodiment 17, wherein the maximum pore size (bubble point pore diameter) is less than 90 nm as measured by a bubble point test, the average pore size is 45 nm or less as measured by capillary flow porometry, the pore size ratio of the bubble point pore diameter to the average pore size is less than 2.5, preferably less than 2.1, and the pure water permeability is 650 LMHB or more as measured by pressure filtration.

[0035] Aspect 19: 18. Use of a polymeric membrane produced by the method according to embodiment 17 for filtration of gases or liquids.

[0036] Aspect 20: 18. Use of a polymeric membrane produced by the method according to embodiment 17 for filtering water or wastewater.

[0037] Aspect 21: A method for filtering a fluid, comprising: (a) providing the polymeric membrane of embodiment 18; and (b) passing a gas or liquid through the polymeric membrane.

[0038] Aspect 22: 22. The method of embodiment 21, wherein the fluid is water. [Brief explanation of the drawings]

[0039] [Figure 1] Figure 1 shows the pore size distribution of NBP versus NMP solvent blend membranes. [Figure 2] Figure 2 shows the pore size distribution of NBP versus NMP solvent blend reinforced membranes. DETAILED DESCRIPTION OF THE INVENTION

[0040] Unless otherwise specified, all percentages in this specification are by weight, and melt viscosities are measured by capillary rheometry using ASTM 3825 at 100 sec and 232°C. All cited documents are incorporated herein by reference. Bubble point pore size and average pore size were measured by capillary flow porometry using the method described in ASTM F316.

[0041] "Copolymer" is used to mean a polymer having two or more different monomer units, including terpolymers (three different comonomers) and higher order polymers (more than three different comonomers). "PVDF" means polyvinylidene fluoride. "PVDF" includes both homopolymers and copolymers. For example, as used herein, "PVDF" and "polyvinylidene fluoride" are used to mean both homopolymers and copolymers unless otherwise specified. "Fluoropolymer" is used to mean a polymer containing a fluorinated monomer. The polymer may be homogeneous, heterogeneous, or random, and may have a gradient in the distribution of comonomer units.

[0042] PMMA resin is a resin that contains methyl methacrylate monomer units, but may also contain other acrylate comonomers.

[0043] Hydrogel polymers refer to 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.

[0044] Lactic acid derivatives refer to water-soluble cosolvents derived from lactic acid, such as lactate esters and lactate amides.

[0045] A dope solution for use in the production of membranes is disclosed, which comprises a polymer P, a water-soluble polymer or hydrogel "polymer H", optionally other additives, and a solvent system, which comprises N-butylpyrrolidone and at least one lactic acid derivative.

[0046] 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" may include a mixture of different PVDF polymers. The PVDF polymer has a melt viscosity in the range of 16 to 45 kpoise, preferably 25 to 42 kpoise, as measured by a capillary rheometer at 230°C and 100 sec-1.

[0047] The polymer of the present invention can be any PVDF polymer used to form membranes by the NIPS process.

[0048] The polyvinylidene fluoride (PVDF) composition of the present invention includes either a homopolymer or a copolymer, and the vinylidene fluoride units typically account for preferably 70% or more, more preferably 85% or more, of the total weight of all monomer units in the polymer. The copolymer of vinylidene fluoride is a copolymer of vinylidene fluoride and one or more monomers selected from the group consisting of vinyl fluoride, trifluoroethene, and 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, and chlorotetrafluoropropene; partially or fully fluorinated α-olefins such as 3,3,3-trifluoro-1-propene, 1,2,3,3,3-pentafluoropropene, and 3,3,3,4,4,4-pentafluoro-1-butene; HFO-1234ze, HFO-1234yf, and HFO The copolymers can be prepared by reacting ethylenediamine-1233zd, hexafluoropropene, trifluoromethyl methacrylate, trifluoromethyl methacrylate, the 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 monomers, partially fluorinated allyl monomers, or fluorinated allyl monomers such as 2-hydroxyethyl allyl ether or 3-allyloxypropanediol, ethene, or propene. Preferred copolymers or terpolymers are formed with one or more of vinyl fluoride, chlorotrifluoroethylene, trifluoroethene, tetrafluoroethene (TFE), and hexafluoropropene (HFP).

[0049] The most preferred copolymer is formed from vinylidene fluoride and hexafluoropropene (HFP).

[0050] Copolymers containing all fluorinated monomers are preferred, although non-fluorinated monomers such as vinyl acetate, methacrylic acid, acrylic acid, etc. may also be used to form the copolymers.

[0051] A preferred copolymer is a PVDF polymer 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 more preferably a VDF / HFP copolymer containing 1 to 8% by weight of HFP.

[0052] Blends of polyvinylidene fluoride polymers are also contemplated as part of this invention, including blends of functionalized fluoropolymers with non-functionalized polymers, blends of PVDF homopolymers with PVDF-HFP copolymers, and PVDF polymers having different melt viscosities.

[0053] The fluoropolymer may comprise monomeric units bearing at least one of the following functional groups: carboxylic acid, carboxylic anhydride, carboxylic ester, epoxy group (such as glycidyl), amide, hydroxyl, carbonyl, mercapto, sulfide, oxazoline, phenol, ester, ether, siloxane, sulfonic acid, sulfate, phosphoric acid or phosphonic acid group. This functional group can be introduced, according to techniques well known to those skilled in the art, by chemical reaction which can be grafting or copolymerization of a fluoromonomer with a monomer bearing at least one of the functional groups and a vinyl function copolymerizable with the fluoromonomer. Examples of such monomer units having functional groups are described in U.S. Pat. No. 8,337,725, U.S. Pat. No. 5,415,958, JP 2010-0292594 A, EP 247,029, U.S. Pat. No. 9,343,744, FR 3,079,834, and U.S. Patent Application Publication No. 20210171693, all of which are incorporated herein by reference.

[0054] The fluoropolymer may contain a repeating unit having a carboxylic acid functional group. Examples of the monomer having a carboxylic acid functional group include acrylic acid, methacrylic acid, and (meth)acrylic acids such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and hydroxyethylhexyl (meth)acrylate. The unit having a carboxylic acid functional group may further contain a heteroatom selected from oxygen, sulfur, nitrogen, and phosphorus.

[0055] Functional groups can also be introduced into the fluoropolymer by a chain transfer agent having functional groups when used during the synthesis process. The chain transfer agent can be a polymer having a molar mass of 10,000 g / mol or less, preferably 5,000 g / mol or less, and having functional groups as described above. An example of this type of chain transfer agent is a polymer of acrylic acid. According to a preferred embodiment, the chain transfer agent comprises a polymer of acrylic acid having a molar mass of 10,000 g / mol or less, preferably 5,000 g / mol or less.

[0056] When the fluoropolymer contains functional groups, the content of functional monomer units in the fluoropolymer is at least 0.01 mol %, preferably at least 0.1 mol %, at most 10 mol %, preferably at most 5 mol %.

[0057] Preferred monomers having a functional group are those having a carboxylic acid function, preferably of the (meth)acrylic acid type, selected from acrylic acid, methacrylic acid, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate and hydroxyethylhexyl (meth)acrylate.

[0058] Preferably, the dope solution contains 10 to 30% by weight of polymer P, more preferably 12 to 25% by weight, most preferably 15 to 20% by weight, based on the total weight of the dope solution.

[0059] Polymer H: (Water-soluble polymer and / or hydrogel polymer)

[0060] Polymer H, a water-soluble or hydrogel polymer, may help adjust the viscosity of the dope solution. The main purpose of these hydrophilic polymer additives is to support pore formation and impart residual hydrophilicity to the membrane.

[0061] The water-soluble polymer may be any known water-soluble polymer. Preferred water-soluble polymers are selected from the group consisting of polyvinylpyrrolidone (PVP); and polyalkylene oxides (commonly referred to as polyalkylene glycols) having a molar mass of 4000 g / mol or more. Preferred water-soluble polymers include polyvinylpyrrolidone, poly-2-ethyloxazoline, polyethylene glycol, polyethylene oxide / polypropylene oxide block copolymers, and mixtures thereof. Preferred water-soluble polymers are polyethylene glycol, polyvinylpyrrolidone, and poly-2-ethyloxazoline. A highly preferred water-soluble polymer is polyvinylpyrrolidone.

[0062] Preferred hydrogel polymers can be selected from known examples including polyhydroxyethyl methacrylate (polyHEMA), poly-N-isopropylacrylamide (PNIPAM), polyethylene glycol methacrylate (PEGMA), cross-linked PVP and copolymers thereof, and hydroxyalkyl cellulose.

[0063] When the dope solution contains 10 to 30% by weight of polymer P, the amount of polymer H can be 1 to 22% by weight based on the total weight of the dope solution.

[0064] 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.

[0065] In one embodiment, the water-soluble polymer is PVP, and the amount of PVP additive is preferably 10-16% based on the total weight of the dope solution.

[0066] 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 10,000 to 1.4 million g / mol. K15 generally ranges from 7,000 to 20,000 g / mol. "K value" refers to the Fikentscher K value (1000k) as defined by H. Fikentscher, Cellulosechemie 13, 58-64, 71-4 (1932) (U.S. Pat. No. 2,706,701). PVP polymers are commercially available, such as Luvitec® PVP (manufactured by BASF) or Plasdone® Povidone and PVP K series (both manufactured by Ashland), and are sold with reference to their K value as a measure of molecular weight. In some embodiments, a K value of 17 or less is preferred. In some embodiments, a K value of 40 or less is preferred.

[0067] additives

[0068] The dope solution may contain optional additives known to those skilled in the art. The total weight content of all optional additives in the dope is preferably 0.1 to 30 wt %, more preferably less than 15 wt %, and most preferably less than 10 wt %, based on the total weight of the dope.

[0069] 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 the polymer P and the acrylic resin in the dope solution. Another optional additive is polyethylene glycol or a polyethylene glycol copolymer having a molecular weight of 200 to 1000 Mw.

[0070] The optional acrylic resin can be one or more PMMA resins, including, but not limited to, PMMA homopolymer, PMMA copolymer resins containing an acrylic ester comonomer, PMMA copolymer resins containing a hydroxyethyl methacrylate ("HEMA") comonomer, PMMA copolymer resins containing methoxypolyethylene glycol methacrylate ("MPEGMA"), PMMA copolymer resins containing a polyethylene glycol methacrylate comonomer, PMMA resins containing zwitterionic functional groups such as sulfobetaine methacrylate, PMMA resins containing sulfonic acid groups, and block copolymers composed of a pure PMMA block and a second block containing both a hydrophilic comonomer such as HEMA or MPEGMA and a hydrophobic comonomer such as an alkyl acrylate.

[0071] Adding acrylic resin to the dope increases the permeability of the membrane.

[0072] Other optional additives 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, aluminum oxide, activated carbon, etc. Any additive known to those skilled in the art may be included in the dope solution.

[0073] Solvent System

[0074] The solvent system comprises a blend of N-butylpyrrolidone and one or more co-solvents derived from lactic acid ("CSLA"), wherein the N-butylpyrrolidone in the blend is at least 50% by weight, and all CSLA has a water solubility of 300 g / L or greater or is miscible with water. Preferably, the blend of N-butylpyrrolidone (NBP) and CSLA comprises an NBP:CSLA ratio (by weight) of 50:50 to 95:5, preferably 60:40 to 95:5, and more preferably 75:25 to 90:10.

[0075] Examples of CSLAs derived from lactic acid include, but are not limited to, methyl lactate, ethyl lactate, propyl lactate, N,N-dimethyl lactamide, or N,N-diethyl lactamide.

[0076] The dope solution may contain an optional co-solvent (hereinafter referred to as optional solvent) in addition to the N-butylpyrrolidone (NBP)-lactate (CSLA) blend. Preferred is an optional solvent that is miscible with N-butylpyrrolidone (NBP) and the CSLA blend. A small amount of optional solvent, less than 10% by weight of the total solvent, can be optionally added along with the NBP / CSLA blend. Examples of optional solvents include, but are not limited to, gamma-valerolactone, butyrolactone, propylene carbonate, ethyl levulinate, or other levulinic acid derivatives.

[0077] The total amount of solvent in the dope solution is generally 50 to 85% by weight, preferably 55 to 75% by weight, of the total weight of the dope solution.

[0078] Preparation of dope solution

[0079] The dope solution can be prepared by adding polymer P and water-soluble polymer and / or hydrogel polymer H to N-butylpyrrolidone (NBP) and a CSLA blend in any order and dissolving polymer P and water-soluble polymer and / or hydrogel polymer H according to any process known in the art. If any additives are included, they can be added to the solvent blend of polymers P and H, or they can be dissolved separately and added to the polymer dissolution mix at any stage. The dissolution process can be assisted by increasing the temperature of the dope solution and / or by mechanical manipulation such as stirring.

[0080] In one common method, these ingredients are blended in a mixer while heating, preferably to a temperature of 70-120°C.

[0081] Membrane manufacturing process

[0082] 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 while retaining others. Membranes can have various shapes, such as flat sheets, spiral wound, tubular, single-hole hollow fibers, perforated hollow fibers or reinforced hollow fibers.

[0083] The membranes can be made according to a process that includes providing a dope solution containing polymer P, water-soluble or hydrogel polymer H, optional additives, and a solvent blend, extruding the dope solution, passing the extruded dope solution through a non-solvent bath / coagulant, optionally oxidizing with chlorine bleach or another oxidizing agent, and then casting the resulting membrane with water.

[0084] Preferably, the process for casting a membrane comprises the steps of: (a) preparing a dope solution containing PVDF resin, a water-soluble or hydrogel polymer, and optionally additives in a solvent containing N-butylpyrrolidone (NBP) and a CSLA blend; (b) degassing the dope solution of step (a); (c) extrusion of 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 a sodium hypochlorite solution (0.5% to 7.5%) at a temperature of 20 to 50°C for 4 to 24 hours, and rinsing the porous membrane with fresh water after the sodium hypochlorite immersion; (g) optionally immersing in an alcohol, glycerol or glycol solution; (h) Drying the membrane.

[0085] The dope solution of step (a) corresponds to the dope solution described above. The main purpose of the water-soluble or hydrogel polymer is to support the formation of pores. The water-soluble or hydrogel polymer may also help to adjust the viscosity of the dope solution. Without wishing to be limited by theory, it is believed that during the coagulation step (d), the water-soluble polymer becomes dispersed in the coagulated film and thus acts as a ground holder for the pores.

[0086] Degassing can be carried out at elevated temperature or room temperature, preferably at 50 to 80°C.

[0087] In step (d), the dope solution is contacted with a non-solvent bath, also called a coagulant, which causes a phase inversion or coagulation of the polymer P, forming a porous membrane structure.

[0088] In step (f), the bleaching solution can be at room temperature or at an elevated temperature, preferably between 20°C and 50°C.

[0089] The resulting membrane preferably has a BPD / MPD ratio between 1.85 and 2.10, which provides a membrane that is selective, has a small pore size, and has sufficient flux as measured by PWP.

[0090] Non-solvent

[0091] It is desirable that the polymer P has 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.

[0092] Suitable alcohols include mono-, di-, or trialkanols selected from the group consisting of C2-C4 alkanols, C2-C4 alkanediols, C3-C4 alkanetriols, and polyethylene oxides with molar masses 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. A preferred non-solvent is a mixture of isopropanol and water, with an isopropanol content of 50 to 80% by volume.

[0093] Flat sheet or hollow fiber membranes can be produced using the dope solutions and methods of the present invention in either supported (cast onto a woven or nonwoven substrate) or unsupported freestanding membrane formats.

[0094] 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.

[0095] The membranes described herein can be used in water and wastewater purification, biopharmaceutical processing, and food and beverage filtration. [Example]

[0096] Abbreviations and compounds used in the examples: PWP is pure water permeable. LMHB is liters per square meter of membrane area per hour per bar of applied pressure (L m -2 h -1 bar -1 ) means NMP means N-methyl-2-pyrrolidone. NBP means N-butyl-2-pyrrolidone. DML means N,N-dimethyllactamide. EtLac means ethyl lactate. MeLac means methyl lactate.

[0097] General Procedure

[0098] Formulations were made with various solvent and resin combinations, including blends with N-butylpyrrolidone and the optional use of acrylic resin additives. A control formulation (Comparative Formulation) was made using pure NMP solvent. N-butylpyrrolidone (Tamisolv® NxG) available from Eastman Chemical Co. Dimethyl lactamide (Agnique® AMD 3L) available from BASF. PVP K15 and N-methylpyrrolidone available from ThermoScientific. Kynar® 761A available from Arkema. BS520 acrylic resin was supplied by Trinseo.

[0099] Kynar® 761A PVDF powder (16 g) was weighed into an 8 oz mixing jar. 12 g of PVP K15 was added. 72 g of solvent (either NBP, NBP blend, or NMP control) was added to the jar to yield 100 g of formulation. The formulation was mixed in an overhead mixer while being heated with a heating mantle. The heating cycle was 70°C / 30 min, 95°C / 2 hr, 105°C / 2 hr. After complete dissolution, the formulation was stored in a sealed jar at 70°C overnight to degas.

[0100] For formulations containing acrylic resin, ALTUGLAS® BS520 was used. These were prepared by adding a BS520 NBP solution (20% by weight BS520) to a mixture of Kynar® 761A, PVP, and the remainder of the mixed solvent to achieve the desired formulation composition. For formulations using acrylic resin, the amount of Kynar 761A was 15.4% by weight, and the acrylic resin was 2.1% by weight of the total formulation. The acrylic resin was 12% of the combined weight of PVDF and acrylic resin. When using a 20% by weight acrylic resin NBP solution, 10.5 g of this 20% by weight solution was added to the formulation to obtain 2.1% acrylic resin.

[0101] After heating overnight, the solution was cooled to 60° C. over 1 hour and transferred to a 6 oz polyethylene dropper bottle.

[0102] Unreinforced films were cast onto glass plates using a 10-mil blade gap on a drawdown square (8-pass wet film applicator, Gardco® applicator, Paul N Gardner Company). The wet film on the glass plate was immersed in a 70% isopropanol / water (v / v) bath for 1 minute, then transferred to a deionized water bath and immersed until the film peeled off. The exposure time to 70% isopropanol was 1 minute per 10 mils of coating thickness. Thicker films were immersed longer in the 70% isopropanol / 30% water (v / v) bath. The film was transferred to a deionized water bath and waited until it had a uniform haze and began to peel off from the glass. The film was then transferred to a second water bath 2 minutes after peeling.

[0103] After all membranes were removed from the formulation, they were immersed in two changes of deionized water over two hours, then left overnight in deionized water. The membranes were then immersed in 100% isopropanol for 30 minutes and finally rinsed with deionized water.

[0104] To prevent shrinkage, the unsupported membrane was clamped to a steel frame and dried at 120° C. for 15 minutes. The membrane was dried to a smooth, white sheet.

[0105] Reinforced membranes were also fabricated using Hollytex nonwoven sheets as a backing. A 15 mil wet film thickness was used for the reinforced membranes. The membranes were cast directly onto the nonwoven backing sheet and not peeled off. They were placed in an isopropanol / water bath for 1.5 minutes. They were then immersed in a deionized water bath for 2 minutes. The membranes were then transferred to a water bath for rinsing, with two changes of water (30 minutes apart), and then soaked overnight. They were then immersed in IPA for 30 minutes, rinsed with deionized water, and then dried. These membranes are labeled HTX, meaning Hollytex, in the solvent mixture column. The reinforced membranes were attached to cardboard and dried in a 60°C oven for 20 minutes.

[0106] The pore size distribution of the membranes was tested by capillary flow porometry using Galwick wetting fluid on 2.5 cm disk samples using a PMI automated capillary flow porometer, model CFP-1500-AELHS, in accordance with ASTM F316.

[0107] Water permeability was measured using a porometer liquid permeability cell for 4.5 cm membrane disks. Three replicate permeability tests were performed, with the results of the third cycle being used. Water permeability tests were performed over an increasing pressure range of 1.5 psi to 22 psi, with 1.5 psi increments and a 30-second recovery at each pressure step. The reported permeability was obtained at 14.5 psi.

[0108] Summary data for all formulations tested is provided in Table 1. The use of solvent blends provides a unique combination of features. Membranes obtained using solvent blends have a maximum pore size (bubble point pore diameter) of 90 nm or less as measured by bubble point testing, an average pore size of 45 nm or less as measured by capillary flow porometry, a pore size ratio of bubble point pore diameter to average pore diameter of 2.5 or less, preferably 2.1 or less, and a pure water permeability of 650 LMHB or more as measured by pressure filtration.

[0109] The "peel time" is given as the time it takes for the nascent film to begin to lift off the glass plate after immersion in a pure water bath (this is after the first immersion in a 70% isopropanol bath).

[0110] [Table 1]

[0111] HTX=Hollytex support membrane

[0112] Figures 1 and 2 compare the pore distributions for the NMP control membrane and several NBP formulations. The data in Table 1 compares specifications for bubble point pore size (BPD), mean pore size (MPD), BPD / MPD ratio, and water permeability. These results demonstrate that the use of novel dope and solvent blends results in membranes with a unique combination of characteristics not met by the comparative membranes.

[0113] The graphical data in Figures 1 and 2 further illustrate the differences in pore size distribution. The pore sizes of the comparative membranes are greater than 0.1 um, extending into the microfiltration range. These larger pores allow larger solute molecules to pass through, narrowing the membrane's rejection range. Figure 1 shows the pore size distribution of an unsupported membrane, while Figure 2 shows an example of a supported membrane. Supported membranes were prepared by casting the formulation onto a Hollytex 3265 nonwoven support.

Claims

1. A dope solution for producing a filtration membrane, comprising a PVDF polymer, a water-soluble polymer and / or hydrogel "Polymer H", a solvent mixture comprising N-butylpyrrolidone and at least one water-soluble lactic acid derivative, and optional additives.

2. 2. The dope solution of claim 1, wherein the solvent mixture comprises at least 50% by weight or more of N-butylpyrrolidone based on the total weight of the solvents.

3. 2. The dope solution of claim 1, wherein the solvent mixture comprises at least 70% by weight or more of N-butylpyrrolidone based on the total weight of the solvents.

4. The dope solution according to claim 1 , wherein the lactic acid derivative comprises at least one of an ester derivative or an amide derivative of lactic acid.

5. The dope solution of claim 1 , wherein the lactic acid derivative comprises an ester derivative of lactic acid.

6. 5. The dope solution of claim 4, wherein the lactic acid derivative is selected from the group consisting of methyl lactate, ethyl lactate, propyl lactate, N,N-dimethyl lactamide, and N,N-diethyl lactamide.

7. 6. The dope solution of claim 5, wherein the lactic acid derivative is selected from the group consisting of methyl lactate, ethyl lactate, propyl lactate, and combinations thereof.

8. The dope solution of claim 4 , wherein the lactic acid derivative comprises N,N-dimethyl lactamide or N,N-diethyl lactamide.

9. The dope solution according to any one of claims 1 to 8, wherein the PVDF contains a homopolymer.

10. 10. The dope solution according to claim 1, wherein the PVDF is a copolymer containing at least one monomer unit selected from the group consisting of HFP, TFE, CTFE, VF3, VF, and vinyl acetate.

11. 11. The dope solution according to claim 1, wherein the PVDF is a copolymer containing an HFP monomer unit.

12. The dope solution of any one of claims 1 to 11, wherein the PVDF comprises a blend of different PVDF polymers.

13. 13. The dope solution according to claim 1, wherein the polymer H is selected from the group consisting of polyvinylpyrrolidone, poly-2-ethyloxazoline, polyethylene glycol, and combinations thereof.

14. The dope solution according to any one of claims 1 to 12, wherein the polymer H comprises polyvinylpyrrolidone.

15. The dope solution according to any one of claims 1 to 14, wherein the optional additives include an acrylic resin in an amount of 1 to 20% by weight based on the total weight of the polymer P.

16. 15. The dope solution of claim 1, wherein the optional additive comprises at least one of polymethyl methacrylate, polymethyl methacrylate copolymer, polyhydroxyl acrylate, polyhydroxyalkanoate such as polylactic acid, or an insoluble hydrogel polymer such as polyhydroxyethyl methacrylate or polyvinyl alcohol, or a combination thereof.

17. 1. A method for producing a PVDF membrane, comprising the steps of: (a) preparing a dope solution according to any one of claims 1 to 8; (b) casting the dope solution into a flat sheet, hollow fiber, or tubular shape at a temperature of 40° C. or higher, and then immersing the cast dope solution in one or more non-solvent baths to form a porous membrane; (c) rinsing the porous membrane to remove residual solvent and additives; (d) optionally treating the membrane with a chlorine bleach to further remove the additive; (e) optionally post-treating the membrane with a wetting agent such as glycerol, propylene glycol, or polyethylene glycol; (f) optionally drying the membrane.

18. (g) A polymer membrane produced by the method of claim 17, having a maximum pore size (bubble point pore diameter) of less than 90 nm as measured by a bubble point test, an average pore size of 45 nm or less as measured by capillary flow porometry, a pore size ratio of the bubble point pore diameter to the average pore diameter of less than 2.5, preferably less than 2.1, and a pure water permeability of 650 LMHB or more as measured by pressure filtration.

19. 19. Use of a polymer membrane produced by the method of claim 17 or 18 for filtration of gases or liquids.

20. 19. Use of a polymer membrane produced by the method according to claim 17 or 18 for the filtration of water or wastewater.

21. 20. A method for filtering a fluid, comprising: (a) providing a polymeric membrane according to claim 18; and (b) passing a gas or liquid through said polymeric membrane.

22. 22. The method of claim 21, wherein the fluid is water.

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