Narrow pore distribution PVDF UF membrane made with safer solvent

Abstract

The present invention discloses the manufacture of a porous fluoropolymer membrane comprising a blend of N-butylpyrrolidone and a co-solvent derived from lactic acid wherein the content of N-butylpyrrolidone in the solvent system is at least 50% or more.

Description

Field of the Invention

[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; and also to a method for manufacturing a membrane and the use of the membrane for liquid filtration. Background Art

[0002] Polyvinylidene fluoride (PVDF) and polyvinylidene fluoride-co-hexafluoropropylene (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 technical application of PVDF polymers is as a raw material for producing membranes such as hollow fiber 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-treatment steps.

[0004] The applicant has found that the choice of solvent is crucial for this process and affects the properties of the resulting membrane, including but not limited to the pore size, water permeability, and mechanical strength of the membrane.

[0005] Two recent publications have described casting PVDF or PVDF-HFP resin membranes using pure NBP solvent: 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. used PVDF homopolymer (6010) to make flat sheet membranes, while Marino et al. used an HFP-VF2 copolymer. In both articles, the bubble point pore size and BPD / MPD ratio of the membranes were larger than those of the membranes described in the present invention, and the permeability was lower.

[0006] Pacheco et al. (US6126826) taught a method for preparing PVDF flat membranes with controllable pore sizes. They emphasized the advantage of controlling smaller pore sizes to improve the rejection rate during the filtration process. However, their method relied on the use of the toxic NMP solvent. There was also no clear indication in US6126826 of the ratio between the bubble point diameter and the average pore size. Their pore size values seemed to come from SEM analysis of the membrane surface. Unfortunately, SEM measurements required dry samples imaged under high vacuum, which could cause pore shrinkage and thus underestimate the true pore size in the wet state. The bubble point pressure data (using methanol wetting liquid) in US6126826 indicated that the bubble point pore size was greater than 100 nm, which was larger than the upper limit of the pore sizes obtained herein.

[0007] US 6110309 described flat membranes made using toxic solvents with relatively large pore sizes.

[0008] In the solvent field, there is an increasing demand for solvents that can replace currently used solvents (such as NMP). For polyvinylidene fluoride ("PVDF"), new solvents are needed to prepare solutions with a high PVDF content and no turbidity. The pore sizes and permeabilities of membranes prepared using new alternative solvents must be comparable to those of membranes prepared using more hazardous solvents such as NMP.

[0009] The object of the present invention is to provide a solvent system for manufacturing polyvinylidene fluoride (PVDF) membranes, which has lower toxicity than currently used solvents and can replace solvents such as N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), and N,N-dimethylformamide (DMF). The solvents currently used in polymer membrane manufacturing have hazards associated with carcinogenicity or reproductive toxicity. The European Union has recently promulgated relevant policies aimed at banning the use of toxic solvents in all industrial applications ((EC) No 1907 / 2006 and its annexes). Using safer solvents can reduce the manufacturing hazards in membrane production and help ensure the production of future polymer-based water filtration membranes.

[0010] The present invention provides a solvent system that reduces toxic solvents and does not require significant reformulation or process adjustment of existing methods. The present invention provides a safer solvent or solvent blend for casting PVDF membranes by a nonsolvent-induced phase separation process (“NIPS”). The safer solvent is N-butylpyrrolidone. The solvent blend or mixture mainly consists of N-butylpyrrolidone and uses a lactate derivative (such as ethyl lactate, methyl lactate, or N,N-dimethyl lactamide) as a minor cosolvent. These cosolvents are also less toxic and are of biological origin. It has been found that a blend of N-butylpyrrolidone and a cosolvent derived from lactic acid (with the content of N-butylpyrrolidone in the mixture being at least 50 wt% or more) can produce PVDF membranes with permeability comparable to that of membranes prepared with pure NMP solvent. Replacing NMP with NBP reduces toxic solvents in the environment. The porous PVDF filtration membranes prepared using the above solvent system have a narrow pore size distribution, with a maximum pore size (measured by a capillary flow porometer) of less than 90 nm, an average pore size ≤ 45 nm, and a water permeability ≥ 650 LMHB. The ratio of the maximum pore size to the average pore size of these membranes is less than 2.5. Compared with using pure NBP as the solvent, adding a lactate derivative can also improve the film-forming time (measured by the release time) by at least 20%. Therefore, compared with using pure NBP, this blend has previously unknown advantages. These properties may be very useful in biopharmaceutical membranes because biopharmaceutical membranes require a narrow pore distribution and high permeability to optimize the production efficiency of high-value biopharmaceuticals. Specifically, a lower ratio between the maximum pore size (commonly referred to as the bubble point pore size) and the average pore size (commonly referred to as the average pore diameter) can improve selectivity and reduce contamination by larger particles.

[0011] We found that using a blend of lactic acid solvent and NBP can shorten the film-forming time, which is another important benefit. This was observed by noting the time point when a flat membrane starts to detach from the surface of the glass plate on which it was cast after being immersed in a water bath. The pure NBP formulation takes more than 2 minutes to detach from the glass plate, while the lactic acid solvent blend continues to release within about 90 seconds after being immersed in the water bath. Without being bound by theory, we believe that the hydroxyl groups on the lactic acid solvent accelerate the phase transformation rate when in contact with water. Shortening the film-forming time is crucial for maintaining the actual production line speed during commercial production. Compared with the pure NBP formulation, the lactic acid solvent blend increases the phase transformation rate by 35%.

[0012] We further found that formulations containing both a lactic acid solvent blend and PMMA acrylic resin had a faster phase inversion rate, and the film could be separated from the glass within 1 minute. When only using NBP solvent and acrylic resin additive, the phase inversion time was longer than that maintained by the lactic acid solvent blend but faster than the NBP formulation without acrylic resin additive. Finally, we noticed that the acrylic resin blend had very high water permeability, exceeding the formulation without acrylic resin additive. Summary of the Invention

[0013] The present invention provides a coating solution comprising a novel solvent blend, which can be used to produce PVDF membranes. The main solvent in the novel solvent blend is N-butylpyrrolidone ("NBP"). The co-solvent is a lactic acid derivative, such as a lactate or a lactamide. The present invention also provides a method for preparing membranes using the coating.

[0014] Aspects of the Present Invention

[0015] Aspect 1: A coating solution for manufacturing a filtration membrane, which comprises a PVDF polymer, a water-soluble polymer and / or a hydrogel "polymer H", a solvent mixture comprising N-butylpyrrolidone and at least one water-soluble lactic acid derivative, and optionally an additive.

[0016] Aspect 2: The coating solution according to Aspect 1, wherein the solvent mixture comprises at least 50 wt% or more of N-butylpyrrolidone based on the total weight of the solvent.

[0017] Aspect 3: The coating solution according to any one or more of Aspects 1 or 2, wherein the solvent mixture comprises at least 70 wt% or more of N-butylpyrrolidone based on the total weight of the solvent.

[0018] Aspect 4: The coating solution according to any one or more of Aspects 1 to 3, wherein the lactic acid derivative comprises at least one of an ester or an amide derivative of lactic acid.

[0019] Aspect 5: The coating solution according to any one or more of Aspects 1 to 4, wherein the lactic acid derivative comprises an ester derivative of lactic acid.

[0020] Aspect 6: The coating solution according to any one or more 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.

[0021] Aspect 7: The coating solution according to any one or more 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.

[0022] Aspect 8: The coating solution according to any one or more of Aspects 1 to 7, wherein the derivative includes N,N-dimethyl lactamide, or N,N-diethyl lactamide.

[0023] Aspect 9: The coating solution according to any one or more of Aspects 1 to 8, wherein the PVDF includes a homopolymer.

[0024] Aspect 10: The coating solution according to any one or more 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.

[0025] Aspect 11: The coating solution according to any one or more of Aspects 1 to 10, wherein the PVDF is a copolymer containing HFP monomer units.

[0026] Aspect 12: The coating solution according to any one or more of Aspects 1 to 11, wherein the PVDF contains a blend of different PVDF polymers.

[0027] Aspect 13: The coating solution according to any one or more 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.

[0028] Aspect 14: The coating solution according to any one or more of Aspects 1 to 13, wherein the polymer H includes polyvinylpyrrolidone.

[0029] Aspect 15: The coating solution according to any one or more of Aspects 1 to 14, wherein the optional additive includes an acrylic resin in an amount of 1 to 20% by weight based on the total weight of the polymer P.

[0030] Aspect 16: The coating solution according to any one or more of Aspects 1 to 15, wherein the optional additive includes at least one of polymethyl methacrylate, polymethyl methacrylate copolymer, polyhydroxyacrylate, polyhydroxyalkanoate such as polylactic acid, or insoluble hydrogel polymer such as poly(2-hydroxyethyl methacrylate) or polyvinyl alcohol or combinations thereof.

[0031] Aspect 17: A method for preparing a PVDF membrane, comprising the steps of:

[0032] a. Providing the coating solution according to any one of Aspects 1 to 16;

[0033] b. Casting the coating solution into a flat sheet, hollow fiber, or tube at a temperature of 40 °C or higher, and then immersing the cast coating solution into one or more non-solvent baths to form a porous membrane;

[0034] c. Rinse the porous membrane to remove residual solvents and additives;

[0035] d. Optionally, treat the membrane with a chlorine-containing bleach to further remove additives;

[0036] e. Optionally, post-treat the membrane with a wetting agent (such as glycerol, propylene glycol or polyethylene glycol);

[0037] f. Optionally, dry the membrane.

[0038] Aspect 18: A polymer membrane prepared by the method of Aspect 17, having a maximum pore size (bubble point pore size) measured by the bubble point test of less than 90 nm; an average pore size measured by a capillary flow porosimeter of 45 nm or less, and wherein the pore size ratio between the bubble point diameter and the average pore size is less than 2.5, preferably less than 2.1, and wherein the pure water permeability measured by pressure filtration is 650 LMHB or greater.

[0039] Aspect 19: Use of the polymer membrane prepared by the method of Aspect 17 for gas or liquid filtration.

[0040] Aspect 20: Use of the polymer membrane prepared by the method of Aspect 17 for water or wastewater filtration.

[0041] Aspect 21: A method of filtering a fluid, comprising a) providing the polymer membrane of Aspect 18 and b) passing a gas or liquid through the membrane.

[0042] Aspect 22: The method according to Aspect 21, wherein the fluid is water. Description of the Drawings

[0043] Figure 1 Shows a comparison of the pore distributions of NBP solvent blend membranes with NMP.

[0044] Figure 2 Shows a comparison of the pore distributions of NBP solvent blend reinforced membranes with NMP. Detailed Description

[0045] As used herein, unless otherwise specified, percentages are by weight and the melt viscosity is measured using ASTM 3825 by a capillary rheometer at 100 seconds- 1 and 232 °C. All cited references are incorporated herein by reference. The bubble point pore size and the average pore size are measured by a capillary flow pore size analyzer using the method described in ASTM F316.

[0046] "Copolymer" is used to denote a polymer having two or more different monomer units, including terpolymers (three different comonomers) and higher-order polymers (more than three different comonomers). "PVDF" refers to polyvinylidene fluoride. "PVDF" includes homopolymers and copolymers. For example, as used herein, unless otherwise specifically stated, "PVDF" and "polyvinylidene fluoride" are used to denote both homopolymers and copolymers. "Fluoropolymer" is used to denote a polymer containing a fluorinated monomer. These polymers can be homogeneous, heterogeneous or random, and can have a gradient distribution of comonomer units.

[0047] PMMA resin is a resin containing methyl methacrylate monomer units, but may also contain other acrylate comonomers.

[0048] Hydrogel polymer refers to a three-dimensional cross-linked hydrophilic polymer that is insoluble in water. Due to the physical or chemical cross-linking of hydrophilic polymer chains, hydrogel polymers can absorb large amounts of water without dissolving.

[0049] Lactic acid derivatives refer to water-soluble co-solvents derived from lactic acid, such as lactate esters or lactamide.

[0050] A coating solution for preparing a membrane is disclosed, the coating solution comprising: polymer P, a water-soluble polymer or hydrogel "polymer H", optional other additives, and a solvent system, wherein the solvent system comprises N-butylpyrrolidone and at least one lactic acid derivative.

[0051] The coating solution for preparing the membrane comprises a polymer P selected from polyvinylidene fluoride (PVDF) homopolymers and copolymers. The term "PVDF polymer" may include a mixture of different PVDF polymers. The melt viscosity of the PVDF polymer ranges from 16 to 45 kiloPoise, preferably 25 to 42 kiloPoise, a value measured by a capillary rheometer at 230 °C and 100 s- 1 conditions.

[0052] The polymer of the present invention can be any PVDF polymer for forming a membrane by the NIPS process.

[0053] The polyvinylidene fluoride resin (PVDF) composition of the present invention comprises a homopolymer or copolymer, wherein the vinylidene fluoride units generally and preferably account for more than 70% by total weight of all monomer units in the polymer, and more preferably account for more than 85% by total weight of the units. The copolymer of vinylidene fluoride can be prepared by reacting vinylidene fluoride with one or more monomers selected from the group consisting of: vinyl fluoride, trifluoroethylene, tetrafluoroethylene; tetrafluoropropene, such as 2,3,3,3-tetrafluoropropene, E-1,3,3,3-tetrafluoropropene, Z-1,3,3,3-tetrafluoropropene, 1,1,2,3-tetrafluoropropene, 1,2,3,3-tetrafluoropropene, 1,1,3,3-tetrafluoropropene, chlorotetrafluoropropene; 3,3,3-trifluoro-1-propene, 1,2,3,3,3-pentafluoropropene, one or more partially or fully fluorinated α-olefins, such as 3,3,3,4,4-pentafluoro-1-butene, HFO-1234ze, HFO-1234yf, HFO-1233zd, hexafluoropropene, trifluoromethyl-methacrylic acid, trifluoromethyl methacrylate, partially fluorinated olefin hexafluoroisobutene, perfluorinated vinyl ethers, such as perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, perfluoropropyl vinyl ether and perfluoro-2-propoxypropyl vinyl ether, fluorinated dioxolene, such as perfluoro(1,3-dioxolene) and perfluoro(2,2-dimethyl-1,3-dioxolene), allyl, partially fluorinated allyl or fluorinated allyl monomers, such as 2-hydroxyethyl allyl ether or 3-allyloxypropanediol, ethylene, propylene. Preferred copolymers or terpolymers are formed from one or more of vinyl fluoride, chlorotrifluoroethylene, trifluoroethylene, tetrafluoroethylene (TFE) and hexafluoropropene (HFP).

[0054] The most preferred copolymer is formed from vinylidene fluoride and hexafluoropropene (HFP).

[0055] Although copolymers of all fluorine-containing monomers are preferred, non-fluorinated monomers (such as vinyl acetate, methacrylic acid and acrylic acid) can also be used to form copolymers.

[0056] Preferred copolymers include PVDF polymers comprising from about 70 wt% to about 99 wt% VDF monomer units and correspondingly from about 1 wt% to about 30 wt% HFP monomer units; more preferably VDF / HFP copolymers comprising from 1 wt% to 8 wt% HFP.

[0057] 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.

[0058] The fluoropolymer may comprise monomer units having at least one of the following functional groups: carboxylic acid, carboxylic anhydride, carboxylic ester, epoxy group (such as glycidyl group), amide, hydroxyl group, carbonyl group, mercapto group, sulfide, oxazoline, phenolic group, ester, ether, siloxane, sulfonic acid group, sulfuric acid group, phosphoric acid group or phosphonic acid group. These functional groups can be introduced by chemical reactions, which can be grafting reactions or copolymerization of fluorine-containing monomers with monomers having at least one functional group and a vinyl functional group capable of copolymerizing with the fluorine-containing monomers. Specific techniques can refer to those well-known to those skilled in the art. Examples of such monomer units with functional groups can be found in US8337725, US5415958, JP20100292594, EP247029B1, US9343744 B2, FR3079834, US20210171693, all of which patents are incorporated herein by reference.

[0059] The fluoropolymer may comprise repeating units with carboxylic acid functional groups. Monomers with carboxylic acid functional groups can be (meth)acrylic acid, such as acrylic acid, methacrylic acid, (meth)acrylic acid hydroxyethyl ester, (meth)acrylic acid hydroxypropyl ester, and (meth)acrylic acid hydroxyethyl hexyl ester. The units with carboxylic acid functional groups may also contain heteroatoms selected from oxygen, sulfur, nitrogen, and phosphorus.

[0060] Functional groups can also be introduced into the fluoropolymer by using a chain transfer agent with functional groups during the synthesis process. The chain transfer agent can be a polymer with a molar mass less than or equal to 10,000 g / mol, preferably less than or equal to 5,000 g / mol, and having the functional groups as described above. An example of such a chain transfer agent is a polymer of acrylic acid. According to a preferred embodiment, the chain transfer agent comprises a polymer of acrylic acid with a molar mass less than or equal to 10,000 g / mol, preferably less than or equal to 5,000 g / mol.

[0061] 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%, and at most 10 mol%, preferably at most 5 mol%.

[0062] Some preferred monomers with functional groups are monomers with carboxylic acid functional groups, preferably (meth)acrylic acids selected from acrylic acid, methacrylic acid, (meth)acrylic acid hydroxyethyl ester, (meth)acrylic acid hydroxypropyl ester, and (meth)acrylic acid hydroxyethyl hexyl ester.

[0063] Preferably, based on the total weight of the coating solution, the coating solution comprises 10 to 30% by weight of polymer P, more preferably 12 to 25% by weight, and most preferably 15 to 20% by weight.

[0064] Polymer H – (water-soluble polymer and / or hydrogel polymer)

[0065] A water-soluble polymer or hydrogel polymer, namely polymer H, can help regulate the viscosity of the coating solution. The main purpose of these hydrophilic polymer additives is to support the formation of pores and impart residual hydrophilicity to the film.

[0066] The water-soluble polymer can be any known water-soluble polymer. Preferred water-soluble polymers are selected from the group consisting of: polyvinylpyrrolidone (PVP); and polyalkylene oxides (commonly also referred to as polyalkylene glycols) having a molar mass of 4000 g / mol or higher. Preferred water-soluble polymers include polyvinylpyrrolidone, poly-2-ethyl-oxazoline, polyethylene glycol, poly(ethylene oxide) / poly(propylene oxide) block copolymers, and mixtures thereof. Preferred water-soluble polymers are polyethylene glycol, polyvinylpyrrolidone, and poly-2-ethyl-oxazoline. A very preferred water-soluble polymer is polyvinylpyrrolidone.

[0067] Preferred hydrogel polymers can be selected from known examples, including poly(2-hydroxyethyl methacrylate) (poly-HEMA), poly(N-isopropylacrylamide) (PNIPAM), poly(ethylene glycol methacrylate) (PEGMA), crosslinked PVP and its copolymers, and hydroxyalkyl cellulose.

[0068] When the coating solution contains 10 to 30% by weight of polymer P, the content of polymer H can be 1 to 22% by weight of the total weight of the coating solution.

[0069] In a preferred embodiment, based on the total weight of the coating solution, the coating solution contains 12 to 25% by weight of polymer P and 2 to 20% by weight of a water-soluble polymer or hydrogel polymer. In a more preferred embodiment, based on the total weight of the coating solution, the coating solution contains 18 to 20% by weight of polymer P and 8 to 16% by weight of a water-soluble polymer or hydrogel polymer.

[0070] In one embodiment, the water-soluble polymer is PVP, and the content of the PVP additive is preferably 10 - 16% based on the total weight of the coating solution.

[0071] The K value of polyvinylpyrrolidone is preferably from 10 to 120. Preferably, the K value of at least one PVP present in the composition is from 12 to 60. Polyvinylpyrrolidones with one or more different K values can be used. Polyvinylpyrrolidones can be used in combination, for example, K15 in combination with K30, or K15 in combination with K60, or K15 in combination with K90, or K30 in combination with K60. A given K value roughly corresponds to the weight-average molecular weight (using GPC / MALLS). Generally, the weight-average molecular weight of PVP with a K value of K30 is in the range of 40,000 to 80,000 g / mol, K60 generally represents a weight-average molecular weight in the range of 250,000 to 500,000 g / mol, K90 generally represents 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. The "K value" refers to the Fikentscher K value (1000k) defined by H. Fikentscher—Cellulosechemie 13, 58-64, 71-4 (1932). (US2706701) PVP polymers are commercially available, such as PVP [from BASF] or Plasdone TM , polyvinylpyrrolidone, PVP K series [all from Ashland] and are sold with reference to the K value as an indication of the molecular weight. In some embodiments, the K value is preferably 17 or less. In some embodiments, the K value is preferably 40 or less.

[0072] Additives

[0073] Optional additives may be present in the coating solution. These optional additives are well known to those skilled in the art. The total weight content of all optional additives in the coating is preferably from 0.1 to 30% by weight of the total weight of the coating, more preferably less than 15% by weight, and most preferably less than 10% by weight.

[0074] One optional additive is an acrylic resin, and its content is from 0 to 20% by weight, preferably from 1 to 20%, of the total weight of polymer P and acrylic resin in the coating solution. Another optional additive is polyethylene glycol or a polyethylene glycol copolymer with a molecular weight between 200 and 1000 Mw.

[0075] The optional acrylic resin can be one or more PMMA resins. Such PMMA resins include, but are not limited to: PMMA homopolymers, PMMA copolymer resins containing acrylate comonomers; PMMA copolymer resins containing 2-hydroxyethyl methacrylate (“HEMA”) comonomers; PMMA copolymer resins containing methoxypolyethylene glycol methacrylate “MPEGMA”; PMMA copolymer resins containing polyethylene glycol methacrylate comonomers; PMMA resins containing zwitterionic functional groups (such as sulfobetaine methacrylate); PMMA resins containing sulfonic acid groups; block copolymers composed of pure PMMA blocks and a second block containing hydrophilic comonomers (such as HEMA or MPEGMA) and hydrophobic comonomers (such as alkyl acrylates).

[0076] Adding acrylic resin to the coating can improve the permeability of the film.

[0077] Other optional additives can include (but are not limited to) inorganic salts such as lithium chloride, magnesium chloride, ferrous chloride, and aluminum chloride; quaternary ammonium salts; propylene glycol, glycerol, organic acids, molecular sieves, silica, alumina, and activated carbon. Any optional additives known to those skilled in the art can be present in the coating solution.

[0078] Solvent system

[0079] The solvent system comprises a blend of N-butylpyrrolidone and one or more co-solvents derived from lactic acid (“CSLA”), with the content of N-butylpyrrolidone in the blend being at least 50 wt% or higher, and the water solubility of all CSLA reaching or exceeding 300 g / L or being miscible with water. Preferably, the ratio of NBP:CSLA (by weight) in the mixture of N-butylpyrrolidone (NBP) and CSLA is from 50:50 to 95:5, preferably from 60:40 to 95:5, more preferably from 75:25 to 90:10.

[0080] Examples of CSLA 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.

[0081] In addition to the N-butylpyrrolidone (NBP)-lactate (CSLA) blend, the coating solution can also contain an optional co-solvent, hereinafter referred to as the optional solvent. Preferably, the optional solvent is miscible with the N-butylpyrrolidone (NBP) and CSLA blend. A small amount (less than 10 wt% of the total solvent) of the optional solvent can optionally be added together with the NBP / CSLA blend. Non-limiting examples of the optional solvent include γ-valerolactone, butyrolactone, propylene carbonate, ethyl levulinate, or other levulinate derivatives.

[0082] The total solvent in the coating solution usually accounts for 50% to 85% by weight of the total weight of the coating solution, preferably 55% to 75% by weight of the total weight of the coating solution.

[0083] Preparation of the coating solution

[0084] The method for preparing the coating solution is as follows: The polymer P and the water-soluble polymer and / or the hydrogel polymer H are added to the N-butylpyrrolidone (NBP) and CSLA blend in any order, and the polymer P and the water-soluble polymer and / or the hydrogel polymer H are dissolved according to any method known in the art. If optional additives are included, they can also be added to the solvent blend together with the polymers P and H, or can be dissolved separately and then added to the polymer dissolution mixture at any stage. The dissolution process can be assisted by raising the temperature of the coating solution and / or by mechanical operations such as stirring.

[0085] In a general method, the components are mixed with a mixer and preferably heated simultaneously to a temperature between 70 and 120 °C.

[0086] Process for preparing the membrane

[0087] In the context of the present application, a membrane should be understood as a semi-permeable structure capable of separating two fluids or separating molecular 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 geometries, such as flat sheet, spiral wound, tubular, single-hole hollow fiber, porous hollow fiber or reinforced hollow fiber.

[0088] The membrane can be produced according to the following method, which includes: providing a coating solution containing the polymer P, the water-soluble polymer or the hydrogel polymer H, any optional additives and a solvent blend; casting the membrane by extruding the coating solution; passing the extruded coating solution through a non-solvent bath / coagulant; and optionally oxidizing with a chlorine-containing bleach or other oxidant; and then washing the obtained membrane with water.

[0089] Preferably, the method for casting the membrane includes the following steps:

[0090] a. Providing a coating solution containing a PVDF resin, a water-soluble or hydrogel polymer, and an optional additive in a solvent, the solvent comprising a blend of N-methylpyrrolidone (NBP) and CSLA;

[0091] b. Degassing the coating solution of step a);

[0092] c. Extruding the coating solution,

[0093] d. Coagulating the extruded coating solution from step c) by passing it through a non-solvent bath to form a porous membrane;

[0094] e. Immersing the porous membrane in an aqueous solution;

[0095] f. Optionally, immersing the porous membrane in a sodium hypochlorite solution (0.5% - 7.5%) at a temperature of 20 - 50 °C for 4 - 24 hours, and then rinsing the porous membrane with fresh water after immersion in sodium hypochlorite;

[0096] g. Optionally, immersing in an alcohol, glycerol or ethylene glycol solution;

[0097] h. Drying the membrane.

[0098] The coating solution in step a) corresponds to the above-mentioned coating solution. The main purpose of the water-soluble polymer or hydrogel polymer is to support the formation of pores. The water-soluble polymer or hydrogel polymer can also help to regulate the viscosity of the coating solution. Although not wishing to be bound by theory, it is believed that during the coagulation step d), the water-soluble polymer will be distributed in the coagulated membrane and thus become a pore placeholder.

[0099] Degassing can be carried out at a high temperature or at room temperature, preferably between 50 and 80 °C.

[0100] In step d), the coating solution is brought into contact with a non-solvent bath (also referred to as a coagulant). In this step, the polymer P undergoes a phase inversion or coagulation to form a porous membrane structure.

[0101] In step f), the bleaching solution can be at ambient temperature or at an elevated temperature, preferably 20 °C to 50 °C.

[0102] The BPD / MPD ratio of the resulting membrane is preferably between 1.85 and 2.10. This gives the membrane selectivity, i.e., small pore size and still sufficient flux (measured by PWP).

[0103] Non-solvent

[0104] In the non-solvent / coagulant, the polymer P should have low solubility. Suitable non-solvent baths / coagulants are, for example, liquid water, water vapor, alcohols, diols, glycerol or mixtures thereof.

[0105] Suitable alcohols are, for example, monoalkanols, dialkanols or trialkanols, selected from C2 - C4 alkanols, C2 - C4 alkanediols, C3 - C4 alkanetriols, poly(ethylene oxide) with a molar mass of 100 to 1000 g / mol, as they can be used as additives in the coating solution of the present invention. A preferred non-solvent mixture is a mixture comprising liquid water and an alcohol. A preferred non-solvent is a mixture of isopropyl alcohol and water, wherein the content of isopropyl alcohol is 50 vol% to 80 vol%.

[0106] Using the coating solution and method of the present invention, flat membranes or hollow fiber membranes can be made, which can be in the form of supported membranes (cast on woven or non-woven supports) or unsupported self-standing membranes.

[0107] The membranes obtained by the process of the present 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.

[0108] The membranes described herein can be used for water and wastewater purification, biopharmaceutical processing, and food and beverage filtration.

[0109] Examples

[0110] Abbreviations and compounds used in the examples:

[0111] PWP is the pure water permeability

[0112] LMHB represents liters per square meter of membrane area per hour per bar of applied pressure (L m -2 h -1 bar -1 )

[0113] NMP represents N-methyl-2-pyrrolidone

[0114] NBP represents N-butyl-2-pyrrolidone

[0115] DML represents N,N-dimethyl lactamide

[0116] EtLac represents ethyl lactate

[0117] MeLac represents methyl lactate

[0118] General procedure

[0119] Formulations were prepared using different solvent / resin combinations, including blends with N-butyl pyrrolidone and optionally an acrylic resin additive. A control formulation (comparison) was prepared using pure NMP solvent. N-butyl pyrrolidone ( NxG) was purchased from Eastman Chemical. Dimethyl lactamide ( AMD 3L) was purchased from BASF. PVP K15 and N-methyl pyrrolidone were purchased from Thermo Scientific. 761A was purchased from Arkema. BS520 acrylic resin was provided by Trinseo.

[0120] Weigh 761A PVDF powder (16 g) was placed into an 8 oz mixing jar. 12 g of PVP K15 was added. 72 g of solvent (NBP, NBP blend, or NMP control) was added to the jar to obtain 100 g of formulation. The formulation was mixed using a overhead stirrer while heating with a heating mantle. The heating cycle was 70 °C / 30 min, 95 °C / 2 h, 105 °C / 2 h. After complete dissolution, the formulation was stored overnight in a sealed jar at 70 °C to degas.

[0121] For the formulation containing acrylic resin, BS520 was used. Preparation method: The NBP solution of BS520 (20 wt% BS520) was added to the mixture of 761A, PVP, and the remaining solvent mixture to achieve the desired formulation composition. In the formulation containing acrylic resin, the content of Kynar 761A was 15.4 wt% of the total weight of the formulation, and the content of acrylic resin was 2.1 wt% of the total weight of the formulation. The acrylic resin accounted for 12% of the total weight of PVDF and acrylic resin. To obtain 2.1% acrylic resin when using a 20 wt% NBP solution of acrylic resin, 10.5 g of the 20 wt% solution was added to the formulation.

[0122] After heating overnight, the solution was cooled to 60 °C and held for one hour, then transferred to a 6 oz polyethylene pipette bottle.

[0123] The un-reinforced film was cast on a glass plate using a 10 mil doctor blade gap on a drawdown square (8-way wet film applicator, applicator, Paul N Gardner Company). The wet film on the glass plate was immersed in a 70% isopropyl alcohol / water (v / v) bath for 1 minute, then transferred to a deionized water bath until the film peeled off. (The exposure time in 70% isopropyl alcohol was 1 minute per 10 mil coating thickness. Thicker coatings were soaked in a 70% isopropyl alcohol / 30% water (v / v) bath for a longer time. After transferring to the deionized water bath, wait until the film has a uniform haze and begins to separate from the glass before peeling. After peeling for 2 minutes, the film was transferred to a second water bath.

[0124] After collecting all the films from the formulation, they were soaked in deionized water, and the water was changed twice within two hours, then left to stand overnight in deionized water. The films were soaked in 100% isopropyl alcohol for 30 minutes and finally rinsed with deionized water.

[0125] The unsupported film was clamped in a steel frame and dried at 120 °C for 15 minutes to prevent shrinkage. The film was smooth and white flakes after drying.

[0126] Using a Hollytex nonwoven fabric sheet as the substrate, an enhanced film was similarly prepared. The wet film thickness of the enhanced film was 15 mils. The film was formed directly on the nonwoven fabric backing and not peeled off. The film was immersed in an isopropyl alcohol / water bath for 1.5 minutes. Then it was immersed in a deionized water bath for 2 minutes. Next, the film was transferred to another water bath for rinsing, with water changed twice (at 30-minute intervals), and then soaked overnight. Then it was soaked in IPA for 30 minutes, rinsed with deionized water, and finally dried. These films carry the HTX label in the solvent blend column, representing Hollytex. These enhanced films were pasted onto cardboard and dried in an oven at 60 °C for 20 minutes.

[0127] According to ASTM F316, the pore size distribution of the film was tested using a PMI automatic capillary flow porosimeter (model CFP-1500-AELHS) by capillary flow porosimetry, using Galwick wetting liquid and a 2.5 cm disc sample.

[0128] Water permeability was measured using the liquid permeation cell on the porosimeter and a 4.5 cm membrane disc sample. Three repeated permeability tests were conducted, and the results of the third cycle were used. The water permeability test was carried out in an increasing pressure range from 1.5 psi to 22 psi, with an increment of 1.5 psi, and 30 seconds were collected for each pressure step. The reported permeability was measured at 14.5 psi.

[0129] Table 1 lists the summary data for all formulations. Using a solvent blend enables a unique combination of properties. The films obtained using the solvent blend have a maximum pore size (bubble point pore size) measured by the bubble point test of less than 90 nm; an average pore size measured by the capillary flow porosimeter of 45 nm or less, a pore size ratio between the bubble point diameter and the average pore size of less than 2.5, preferably less than 2.1, and a pure water permeability measured by pressure filtration of 650 LMHB or greater.

[0130] "Release time" refers to the time required for the freshly formed film to start peeling off from the glass plate after being immersed in a pure water bath. (This is after the initial immersion in a 70% isopropyl alcohol bath.)

[0131] Table 1: Summary data for NBP mixed solvent formulation films

[0132]

[0133]

[0134] HTX = Hollytex support film.

[0135] Figure 1 and Figure 2The pore distributions of NMP control membranes and several NBP formulations were compared. The data in Table 1 compare the specifications of the bubble point diameter (BPD), mean pore diameter (MPD), BPD / MPD ratio, and water permeability. The results show that membranes with a unique combination of properties can be prepared using the novel coating solution and solvent blend, which are not possessed by the control membranes.

[0136] Figure 1 and Figure 2 The graphical data in [reference] more intuitively show the differences in pore size distribution. The control membranes have a wider pore size range, exceeding 0.1 microns and entering the microfiltration range. These larger pores will allow larger solute molecules to pass through the membrane, thus reducing the rejection range of these membranes. Figure 1 shows the pore size distribution of unsupported membranes, while Figure 2 shows an example of a supported membrane. The supported membrane was made by casting the formulation onto a Hollytex 3265 nonwoven support.

Claims

1. A coating solution for manufacturing a filtration membrane, which comprises a PVDF polymer, a water-soluble polymer and / or a hydrogel "polymer H", a solvent mixture comprising N-butylpyrrolidone and at least one water-soluble lactic acid derivative, and optionally an additive.

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

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

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

5. The coating solution according to claim 1, wherein The lactic acid derivative includes an ester derivative of lactic acid.

6. The coating solution according to 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. The coating solution according to 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 coating solution according to claim 4, wherein, The derivative includes N,N-dimethyl lactamide, or N,N-diethyl lactamide.

9. The coating solution according to any one or more of claims 1 to 8, wherein, The PVDF includes a homopolymer.

10. The coating solution according to any one or more of claims 1 to 9, wherein, The PVDF is a copolymer comprising at least one monomer unit selected from the group consisting of HFP, TFE, CTFE, VF3, VF and vinyl acetate.

11. The coating solution according to any one or more of claims 1 to 10, wherein, The PVDF is a copolymer comprising an HFP monomer unit.

12. The coating solution according to any one or more of claims 1 to 11, wherein, The PVDF comprises a blend of different PVDF polymers.

13. The coating solution according to any one or more of claims 1 to 12, wherein, The polymer H is selected from the group consisting of polyvinylpyrrolidone, poly-2-ethyl oxazoline, polyethylene glycol and combinations thereof.

14. The coating solution according to any one or more of claims 1 to 12, wherein, The polymer H includes polyvinylpyrrolidone.

15. The coating solution according to any one or more of claims 1 to 14, wherein, The optional additive comprises an acrylic resin in an amount of 1 to 20% by weight based on the total weight of the polymer P.

16. The coating solution according to any one or more of claims 1 to 14, wherein, The optional additive includes at least one of polymethyl methacrylate, polymethyl methacrylate copolymer, polyhydroxyacrylate, polyhydroxyalkanoate such as polylactic acid, or an insoluble hydrogel polymer such as polyhydroxyethyl methacrylate or polyvinyl alcohol or combinations thereof.

17. A method for preparing a PVDF membrane, which comprises the following steps: a. Providing a coating solution as described in any one or more of claims 1 to 8; b. Casting the coating solution into a flat plate, hollow fiber or tube at a temperature of 40 °C or higher, and then immersing the cast coating solution into one or more non-solvent baths to form a porous membrane; c. Rinsing the porous membrane to remove residual solvents and additives; d. Optionally, treating the membrane with a chlorine-containing bleach to further remove additives; e. Optionally, post-treating the membrane with a wetting agent such as glycerol, propylene glycol or polyethylene glycol; f. Optionally, drying the membrane.

18. A polymer membrane prepared by the method as described in claim 17, the maximum pore diameter (bubble point pore diameter) measured by the bubble point test is less than 90 nm; the average pore diameter measured by a capillary flow porosimeter is 45 nm or less, and the pore size ratio between the bubble point diameter and the average pore diameter is less than 2.5, preferably less than 2.1, and the pure water permeability measured by pressure filtration is 650 LMHB or greater.

19. Use of a polymer membrane prepared by the method as described in claim 17 or 18 for gas or liquid filtration. Use of a polymer membrane prepared by the method according to claim 17 or 18 for water or wastewater filtration.

21. A method for filtering a fluid, comprising a) providing a polymer membrane according to claim 18, and b) passing a gas or a liquid through the membrane.

22. The method according to claim 21, wherein, The fluid is water.

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