MEMBRANES MADE USING FINE POWDERS

Abstract

This invention allows for the production of high strength and high permeability TIPS membranes using extractable fillers with fine powder PVDF grades.

Claims

1. A composition for TIPS membranes comprising 30 to 50% PVDF, 15 to 25% fine powder extractable filler having an average particle size of between 1 to 250 nm, 35 to 55% organic latent solvent, and 0 to 10% additives, wherein the PVDF has a heat of melting Delta H of from 45 to 55 l/gm on the second heat D3418 (DSC), and the percent of reverse units by NMR is from 4.6 to 5.8.

2. The composition of claim 1 wherein the PVDF powder has a melting point of between 160 to 170 C on second heat.

3. The composition of claim 1 wherein the PVDF powder comprises from 30 to 45 wt percent of the composition and the organic latent solvent comprises from 35 to 42 wt percent of the composition.

4. The composition of claim 1 wherein the PVDF is a homopolymer or copolymer comprising at least 95 weight % vinylidene fluoride.

5. The composition of claim 1 wherein the fine powder extractable filler is selected from the group consisting of including fumed silica, zinc oxide, aluminum oxide, zirconium oxide, iron oxide, calcium carbonate, and combination thereof.

6. The composition of claim 1 wherein the latent solvent is selected from the group consisting of Diethyl phthalate, dibutylphthalate, dibutylsebacate, acetyl-tributylcitrate, tributylcitrate, acetyl-triethylcitrate and combinations thereof.

7. A method for producing a porous membrane comprising the steps of (i) feeding the composition of claim 1, to an extruder, (ii) extruding the melted product to form a structure, (iii) Extracting the solvent from the structure with an organic solvent preferably alcohol, (iv) Extracting the filler with acid or base, and (v) washing the structure with pure water to produce a porous membrane.

8. A method for producing a porous membrane comprising the steps of (a) pre-blending fine powder PVDF having a D50 of 3 to 15 micron, fine powder extractable filler having average particle sizes of between 1 to 250 nm and latent solvent to produce a free flowing powder blend, wherein the free flowing powder blend contains 15-30% by weight of a latent solvent, (b) feeding the free flowing powder blend to an extruder or kneader wherein the free flowing powder blend is melted to produce a melted blend, (c) feeding an additional aliquot of latent solvent down-stream in the extruder or kneader into the melted blend to produce a melted product, (d) extruding the melted product to form a structure, (e) extracting the solvent from the structure with organic solvent, preferably alcohol, and (f) extracting the filler with acid or base, to produce a porous membrane.

9. The method of claim 8, wherein the PVDF has a heat of melting Delta H of from 45 to J/gm on the second heat ASTM D3418 (DSC), and the percent of reverse units by NMR is from 4.6 to 5.8.

10. The method of claim 8, further comprising step (g) of washing the structure with water after step (f).

11. The method of claim 8, wherein the amount of the additional aliquot of latent solvent is from 5 to 50 wt %, based on the total weight of the material prepared in (a).

12. The method of claim 8, wherein in step (a) the fine powder PVDF and the fine powder extractable filler are first blended together followed by the addition of the latent solvent.

13. The method of claim 8, further comprising the steps of (c2) extrude out solid pellets from step (c), and (c3) Feeding the pellets to a second extruder.

14. The method of claim 8, wherein the structure exiting step (d) extrudes into a water bath.

15. The method of claim 8, wherein the PVDF is a homopolymer or copolymer comprising at least 95 weight % vinylidene fluoride.

16. The method of claim 8, wherein the fine powder extractable filler is selected from the group consisting of including fumed silica, zinc oxide, aluminum oxide, zirconium oxide, iron oxide, and calcium carbonate and combination thereof.

17. The method of claim 7 or 8, wherein the fine powder extractable filler comprises fumed silica.

18. The method of claim 8, wherein the fine powder extractable filler comprises zinc oxide.

19. The method of claim 8, wherein the latent solvent is selected from the group consisting of dimethyl phthalate, diethylphthalate, dibutylphthalate, dioctylphthalate, diethylhexylphthalate, dibutylsebacate, triethylcitrate, acetyl-triethylcitrate, tributylcitrate, acetyl-tributylcitrate, glycerol triacetate (Triacetin), glycerol tributyrate (Tributyrin), propylene carbonate, diphenylearbonate, butyllevulinate, n-octylpyrrolidone, benzoic acid esters such as methyl benzoate and ethyl benzoate, phosphoric acid esters such as triphenyl phosphate, tributyl-phosphate, and tricresyl phosphate dimethyl succinate, diethyl succinate, gamma valerolactone, and mixtures thereof.

20. The method of claim 8, wherein the latent solvent is selected from the group consisting of Diethyl phthalate, dibutylphthalate, dibutylsebacate, acetyl-tributylcitrate, tributylcitrate, acetyl-triethylcitrate, triethylcitrate, and combinations thereof.

21. (canceled)

22. A membrane made by the method of claim 8, wherein the membrane has a flowrate of at least 800 lmhb and a tensile strength of at least 8 MPa.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0063] FIG. 1 Schematic of on embodiment of the inventive process.

[0064] FIG. 2 Graph showing the relationship of solids content on water permeability and on mechanical strength.

DETAILED DESCRIPTION OF THE INVENTION

[0065] PVDF Particle size is measured by Microtrac Laser Particle size analyzer (using Laser diffraction).

[0066] Free flowing means the ability to be fed uniformly (at consistent kg/hour rate) through a powder feeder (such as K-Tron K-CL-FSF KT20 or KT35 on D5 platform gravimetric feeders) without bridging.

[0067] As used herein, unless otherwise indicated, percentages are weight percent, melt viscosity is measured using ASTM 3825 at 100 sec-1 and 232C and all references cited are incorporated herein by reference.

[0068] PVDF means a Polyvinylidene fluoride polymer (homopolymer or copolymer). Fine Powder PVDF means a Polyvinylidene fluoride polymer produced by emulsion polymerization and having a D50 powder size of 3-15 micron.

[0069] Fine powder extractable filler means a filler with an average particle size of 1 to 250 nm that is capable of being removed from the formed article (such as a membrane) generally using a solvent, heat, or chemical degradation using acid or base. Preferably the filler has a surface area of greater than 20 m2/g. It is understood that the particle size of the fine powder extractable filler refers to the size of the primary particle.

[0070] This invention relates to a composition having a fine particle size range of PVDF resin, a method of making the composition, and a method of making a membrane from the composition via a TIPS process.

[0071] This invention allows to the production of uniform porosity membranes using extractable fillers and fine particle PVDF. This invention solves the problem by pre-blending fine powders PVDF, fine powder extractable filler, and latent solvent while maintaining a free flowing powder blend where the pre-blend comprises from 15-30% by weight of the latent solvent. Never has this solution been identified prior to our invention. In addition adding a significant portion of solvent to the powder—maintains a reasonable viscosity for compounding without excessive shear heating.

[0072] We have discovered a unique process to produce a compound to allow the production of uniform pore size distribution TIPS membrane while overcoming the handling issues noted in previous patents such as an intractable paste formation. This invention utilizes a pre-blend of emulsion grade PVDF powder with extractable filler, and a portion of the latent solvent.

[0073] The compositions covered by this invention comprises: [0074] a) Fine Powder PVDF (3-15 micron D50 by laser diffraction): 30% to 50% by weight [0075] b) Fine powder extractable filler with an average particle size of 1 to 250 nm: 15-25% by weight [0076] c) Organic Latent Solvent or Solvent blend—35 -55% by weight [0077] d) optionally additives—0 to 10%

[0078] Preferably, the PVDF/filler powder blend is agitated to produce a uniform blend, and then the latent solvent fluid is gradually added while blending to disperse it effectively to achieve a pre-blend containing 15 to 30 wt percent latent solvent. Alternatively, any order of addition is acceptable as long as the composition is maintained as a free flowing powder. Depending on the type and size of the extractable filler, the fluid addition level (wt %) is controlled to keep good powder flow without caking in a typical Loss-in-weight (LIW) powder feeder system. This flowable pre-blend is fed into an extruder such as a twin-screw or co-kneader extruder to produce a melt. The remainder of the latent solvent (additional aliquot needed for the final formulation) is added down-stream via a loss-in-weight (LIW) liquid feeder (such as K-Tron K-ML-D5-P gravimetric liquid feeder) after the pre-blend is in a melted state. In this way, a uniformly dispersed filler in a melted product can be achieved without excessive heat generation (due to the presence of the latent solvent in the feed powder pre-blend-(initial addition), and the remainder of the latent solvent (additional aliquot) added down-stream in the extruder/kneader. Without the addition of the latent solvent to the powder pre blend excessive shear heating can take place. With this invention due to the powder pre blend step—a uniform distribution of the filler is achieved. The downstream addition of the additional aliquot of latent solvent should occur at a temperature that allows easy incorporation of that latent solvent (preferably between 130 C-230 C) with a LIW feeder system to closely control the final formulation of the compound to be extruded subsequently into a hollow fiber or flat sheet membrane. FIG. 1 shows a schematic embodiment of the inventive process.

[0079] The resultant formulation can be pelletized by strand or underwater cutting and can then in a second step be extruded using a single screw extruder, equipped with a gear pump and capillary die into a hollow fiber membrane, In this case the extruder should be run at proper temperature profile and processing conditions to prevent premature phase separation and leach out of the solvent.

[0080] Alternatively, the twin-screw could be equipped with a gear pump and a membrane die to produce the membrane directly.

Polymer

[0081] The polymer of the invention can be any fluoropolymers polymer used for forming membranes by the TIPS process. Especially useful fluoropolymers include, but are not limited to the homo—and copolymers having a majority of monomer units being either vinylidene fluoride or vinyl fluoride, ethylene tetrafluoroethylene (ETFE), and ethylenechloro trifluoroethylene (ECTFE). Polyvinylidene fluoride containing copolymers are the most preferred. The invention will use polyvinylidene fluoride as an exemplary fluoropolymer, but one of skilled in the art can easily envision using polyvinyl fluoride, ETFE, ECTFE and other similar polymers with the same parameters described.

[0082] The polyvinylidene fluoride resin (PVDF) composition of the invention is preferably a homopolymer made by polymerizing vinylidene fluoride (VDF), copolymers, terpolymers and higher polymers of vinylidene fluoride wherein the vinylidene fluoride units comprise is typically greater than percent of the total weight of all the monomer units in the polymer, and more preferably, comprise greater than 75 percent of the total weight of the units. It is possible however especially when copolymers are made with tetrafluoroethylene (TFE) that the VDF could be as low as 25 weight percent of the total monomers. Copolymers, terpolymers and higher polymers of vinylidene fluoride may be made by reacting vinylidene fluoride with one or more monomers from the group consisting of vinyl fluoride , trifluoroethene, tetrafluoroethene, one or more of partly or fully fluorinated alpha-olefins such as 3,3,3-trifluoro-1-propene, 1,2,3,3,3 pentafluoropropene, 3,3,3,4,4-pentafluoro-1-butene, hexafluoropropene, trifluoromethyl-methacrylic acid, trifluoromethyl methacrylate, the partly 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), allylic, partly fluorinated allylic, or fluorinated allylic monomers, such as 2-hydroxyethyl allyl ether or 3-allyloxypropanediol, ethene, propene. Preferred copolymers or terpolymers are formed with vinyl fluoride, trifluoroethene, tetrafluoroethene (TFE), and hexafluoropropene (HFP). Most preferred copolymers are formed with hexafluoropropene (HFP). One preferred PVDF is Kynar™PVDF by Arkema.

[0083] While an all fluoromonomer containing copolymer is preferred, non-fluorinated monomers such as vinyl acetate, methacrylic acid, and acrylic acid, may also be used to form copolymers, at levels of up to 5 weight percent based on the polymer solids.

[0084] Preferred copolymers are of VDF comprising from about 71 to about 99 weight percent VDF, and correspondingly from about 1 to about 29 percent TFE; from about 71 to 99 weight percent VDF, and correspondingly from about 1 to 29 percent HFP (such as disclosed in U.S. Pat. No. 3,178,399); and from about 71 to 99 weight percent VDF, and correspondingly from about 1 to 29 vsieight percent trifluoroethylene.

[0085] Another preferred embodiment provides for copolymers of VDF and TFE are envisioned where the TFE is in the range of 25 to 75 weight percent with the remainder being VDF.

[0086] Another Preferred embodiment provides for terpolyrners such the terpolymer of VDF, HFP and TFE, and the terpolyrner of VDF, trifluoroethene and TFE. The contemplated terpolyrners could have from 24 to 75 weight percent VDF, the HFP or trifluoroethene content could range from 1-40 weight percent and the TFE content could range from 24 to 75 weight percent. A preferred terpolymer is 45-55% IFE, 25-35% VDF and 10-20% HFP.

[0087] Mixtures of polyvirtyliderie fluoride polymer is also envisioned as part of the invention, including functionalized polymers with non-functionalized polymers, and polymers having different melt viscosities.

[0088] The fine powder PVDF preferably has a D50 particles size range of 3 to 15 micron as measure by Microtrac laser diffraction.

[0089] The PVDF has a heat of melting of Delta H of 45 to 55 J/gm on the second heat according to ASTM D3418 (DSC), and the percent of reverse units by NMR is between 4.6 to 5.8.

[0090] Preferably, the PVDF powder has a melting point of between 160 to 170 C on second heat (ASTM D3418).

Latent Solvent

[0091] The polymer particles are blended with latent solvents, to form the free-flowing, powders. Latent solvents are organic liquids which do not dissolve (less than 5% by weight soluble) or substantially swell the fluoropolymer resin at room temperature, but will dissolve the fluoropolymer resin at elevated temperatures.

[0092] Solvents that will dissolve the polymer are not preferred, and this will lead to a viscosity increase. Useful latent solvents for the invention include, but are not limited to, dimethyl phthalate, diethylphthalate, dibutylphthalate, dioctylphthalate, diethylhexylphthalate, dibutylsebacate, triethylcitrate, acetyl-triethylcitrate, tributylcitrate, acetyl-tributylcitrate, glycerol triacetate (Triacetin), glycerol tributyrate (Tributyrin), propylene carbonate, diphenylcarbonate, butyllevulinate, n-octylpyrrolidone, benzoic acid esters such as methyl benzoate and ethyl benzoate, phosphoric acid esters such as triphenyl phosphate, tributyl-phosphate, and tricresyl phosphate; dimethyl succinate, diethyl succinate, gamma valerolactone, and mixtures thereof.

[0093] Preferred latent solvents are Diethylphthalate, dibutylphthalate, dibutylsebacate, acetyl-tributylcitrate, tributylcitrate, acetyl-triethylcitrate, triethylcitrate, and mixtures thereof.

Extractable Fillers

[0094] Extractable fillers are used in the invention. Preferably, the fillers have average particle sizes in the range of 1 to 250 nanometers. Preferably, the fillers have average particle sizes in the range of 1 to 100 nanometers, and more preferably from 1 nm-50 nanometers. Particle size can be seen by scanning electron microscope. Examples of extractable filler include add or base extractable pore-formers which are typically hydrophobic such as silica, aluminum oxide, zirconium oxide, zinc oxide, iron oxide and calcium carbonate. Water extractable fillers are also possible by using such salts as water extractable compounds such as metallic salts (lithium, calcium and zinc salts). Preferred fillers include fine inorganic oxide powders (with average particle size 1-100 nm) including fumed silica, zinc oxide, aluminum oxide, zirconium oxide, iron oxide, and calcium carbonate.

Other Additives

[0095] Before extraction of the solvent and fillers in the TIPS process, the composition may comprise additional additives. In addition to the fluoropolymer and solvent and extractable filler, one or more other additives may be added to the membrane composition, typically at from 0 to 10 weight percent, preferably at 1 to 10 weight percent and more preferably from 5 to 10 weight percent, based on the based on the weight percent of fluoropolymer. Typical additives include, but are not limited to, acrylic resin polymers, polymethylmethacrylate (PMMA), PMMA copolymers, poly-2-ethyioxazoiine, polyvinylacetate, polyethylene glycol, poiyyinyl alcohol, polyvinylpyrrolidone, poly-2-ethyloxazoline, polymethylvinylketone, polymethylmethacrylate-co-ethylacrylate, polymethylmethacrylate-co-butylacrylate, polymethymethacrylate-co-butylacrylate-co-hydroxyethylmethacrylate, polymethylmethacrylate-co-butylacrylate-co-methoxypolyethyeleneglycol-methacrylate, polymethylmethacrylate-co-methacrylic acid, polymethylmethacrylate-co-butylacrylate-co-methacrylic acid, polymethylmethacrylate-co-aminopropane sulfonic acid, polymethylmethacrylate-co-aminopropanesulfonic acid sodium salt, PMMA-zwitterion copolymers such as poly-methylmethacrylate-co-sulfobetaine methacrylate, polymethylmethacylate-co-phosphorylcholinemethacrylate, polymethymethacrylate-co-carboxybetaine methacrylate, polymethylmethacrylate-graft-vinylpyridine-sulfobetaine; and combinations thereof.

[0096] The composition formed by this inventive process can be formed into porous membranes by extrusion followed by thermally induced phase separation (TIPS).

[0097] Thermally Induced Phase Separation (TIPS) is one of two primary methods to make porous phase inversion membranes. TIPS is a form of polymer melt processing, in that a polymer material is melted with a diluent plasticizer or latent solvent to form a homogeneous melt. In the melt, the polymer and diluent are fully miscible. Upon cooling, the solubility of the polymer drops and it phase separates into a solid phase. When cast into an appropriate form factor (e.g. sheet, film, tube, hollow fiber) the thermal phase separation produces a porous structure.

[0098] Unlike the non-solvent induced phase separation process (NIPS) the latent solvents used in TIPS will not dissolve the polymer at room temperature. Heating close to the polymer melting point is needed to make a homogeneous solution. In the TIPS process, crystallization of the polymer as it cools is the driving process for phase separation, unlike non-solvent exchange in the NIPS process. Due to the crystallization process in TIPS, TIPS membranes have higher crystallinity than NIPS membranes and therefore higher strength.

[0099] TIPS membrane (thermal controlled- being significantly faster than NIPS process) have a uniform pore size throughout the structure whereas NIPS membrane being diffusion control produces a gradient in pore size through the membrane (asymmetric pore size distribution).

[0100] Furthermore, the use of a high temperature extrusion process allows much higher (as much as 2× or more) polymer solids in compared to a NIPS process. The higher polymer solids content also helps increase the mechanical strength of the TIPS membranes compared to NIPS membranes.

[0101] While high mechanical strength is an inherent property of TIPS membranes, they can suffer lower permeability compared to NIPS membranes. To improve permeability of TIPS membranes, extractable inorganic fillers are often used as part of the formulation and then extracted after the membranes are cast. By use of the inorganic fillers, TIPS membranes can achieve both higher strength and higher water permeability than typical NIPS membranes.

[0102] Therefore, desirable processes for making TIPS membranes should be able to utilize both higher polymer solids and extractable inorganic fillers to achieve the desired properties of high mechanical strength and high permeability.

[0103] The TIPS process is described above, and is the preferred process for forming a membrane using the powder blend of the invention. The powder pre-blend of the invention is free flowing at room temperature, allowing for the transfer of the powder pre-blend in the into the extruder and ultimately forming a membrane using a TIPS process.

[0104] The porous membranes can be in the form of flat sheets, supported sheets, tubes, or hollow fibers or supported hollow fibers.

[0105] The final dry thickness of the membranes of this invention are generally between 50 to 500 microns, and preferably from 100 to 300 microns. This can be measured using a cryofractured membrane in a scanning electron microscope, or an optical microscope using a calibrated eyepiece or sizing software.

EXAMPLES

Tensile Testing Method

[0106] Mechanical testing was done on an Instron 4201 universal test frame equipped with a fiber holder designed to wrap fibers around a spool. This prevented damage to the delicate hollow fibers by standard tensile bar grips. The gap spacing was 100 mm with a strain rate of 50 mm min.sup.−1. Fibers were tested in the wet state. Replicates of five fibers were run and averaged.

Water Permeability

[0107] To test pure water permeability, 5 loops of membrane approximately 30-40 cm long each were potted at one end into a ¾″ OD clear PVC tube of 50 cm length. The tube was filled with deionized water through the open end and then connected to a water permeation test manifold. Water permeability was measured in dead-end flow, outside-in through the fibers, at 0.5 bar pressure. The surface area of the membranes was determined by measuring fiber OD with an optical microscope, calculating the circumference, and the multiplying by the exposed length of fibers in the test module.

Example 1

[0108] The final formulation to be produced in weight percent ingredients: 38% Kynar 761 (Melt viscosity of between 26 to 29 KPoise at 100 sec-1, at 230 C) fine powder/emulsion grade PVDF, 20% ZnO (Azo 66) and 42% dibutylsebacate (DBS).

[0109] The Kynar 761 and ZnO were added to a high speed Henschel mixer. These were preblended at 500 rpm for 2 minutes. Then the DBS (20% by weight of the combined dry powder) was gradually added through the port with the mixer running at 500 rpm over 5 minutes. The resultant powder was free flowing.

[0110] A 30 mm ZSK twin-screw extruder with a 36:1 L/D barrel was set up with a specific screw design to allow for the additional liquid DBS that was needed to be injected down stream with a loss-in-weight positive displacement liquid injection pump. The temperatures for the extruder were set to 190 C, the screw rpm at 200 rpm. The free-flowing powder was fed to the rear of the twin-screw at 10 lbs/hr while the DBS was fed down-stream at a rate of 3.8 lbs/hr to achieve the final formulation for this compound as noted above. The extrudate was strand pelletized using a cold water bath. The torque during compounding was measured at 22% and the melt temperature was 190 C, showing that there was no significant shear heating—even with a high viscosity PVDF such as Kynar 761 with a melt viscosity at 100 sec-1 and 235 C of 27 kpoise using ASTM D3835. The resultant pellets were analyzed for dispersion quality by extracting the DBS with Alcohol, then analysing by SEM for pore size and dispersion quality of the ZnO. In addition the ZnO was extracted with 2 molar sulfuric acid for 4 hrs.

[0111] The results show that fine powder produced using emulsion process can be uniformly fed to the extruder with addition of latent solvent added down stream to make the final formulation and can be pellitized.

Example 2

[0112] Pellets prepared in example 1 were fed into a 1 inch single screw extruder with barrier screws and 24 to 1 L/D and 3 to 1 compression ratio. The extruder was run at 25 rpm with the tempreture profile shown in the following table.

TABLE-US-00001 Conditions Barrel 1 ° F. 300 Barrel 2 ° F. 350 Barrel 3 ° F. 370 Clamp 370 Adapter 360 Die 1 ° F. {Top} 360 Die 2 ° F. {Bottom} 360 RPM 25 Roll Speed 10 ft/min Roll Temp ° F. 60

[0113] Using the above equipment we produced 6″ wide film with thicknesses from 0.004 to 0.008 inch thick using an 8″ wide vertically mounted coat hanger die with a die gap of 0.01 inch. The films were cooled in a three roll stack with chrome polished rolls set at 60 F.

[0114] In the next step films will be soaked in alcohol and washed and then soaked in acid and washed to form a membrane.

[0115] A uniform pore structure is observed by SEM and or capillary flow porometry.

[0116] A uniform distribution of Zinc oxide is observed by SEM.

[0117] FIG. 1 shows the schematic procedure for preparation of the films for production of the membranes as described in examples 1 and 2.

Example 3: (Comparative)

[0118] A 500 g slurry of PVDF resin in diethylphthalate was prepared with a fine powder PVDF with a series of resins with a D50 particle size as measured by Microtrac of 10 micron. The mixture contained 45% PVDF resin and 55% diethylphthalate. Diethylphthalate was first weighed out into a mixing jar, followed by addition of PVDF resin. The mixture was stirred up for 1 minute using a hand wisk to disperse the solids. The mixtures were allowed to sit for 2 hours to fully wet with solvent. The mixtures were then mixed again for 1 minute using a hand held electric powered wisk mixer. This more completely blended the resins into the solvent. The result was an intractable paste that could not be fed to a twin-screw extruder.

Example 4: (Comparative)

Casting of TIPS Membranes with Inorganic Filler Additive

[0119] A series of experiments were run using solvent and fine powder PVDF using the equipment described below.

[0120] The following example describes a batch process for casting TIPS membranes using a formulation made of fine powder PVDF resin, latent solvent, but without any inorganic filler additive.

[0121] In a 300 ml jacketed stainless steel mix vessel is blended PVDF fine powder resin and diethyly phthalate. The amounts used are listed in the table below. The mixture is heated to 200 C with stirring at 60-65 rpm by an internal overhead stirrer while under nitrogen. Dissolution of the resin is confirmed by withdrawing samples of the melt after 1.5 hrs, revealing a clear solution. Once dissolution is complete, the tank temperature is lowered to 170-180 C to be closer to the crystallization temperature. A tube in orifice spinneret is used to cast a hollow fiber from the melt. The molten solution of PVDF resin is pumped by a gear pump into the heated die (170-180 C) while ambient temperature diethylpthalate is pumped through the lumen of the fiber. The fiber is cast into an ambient temperature water bath through an air gap of 7 mm and collected on a reel with low tension. After collecting, the fiber is soaked in alcohol to remove the solvent and then water to remove the ethanol. No further post treatment was performed. Fibers were tested for water permeability by gluing into small plastic tubes and measuring water flux at bar with outside-in water flow. Mechanical testing was performed on an Instron test unit. Data for water permeability and mechanical strength are listed in the table.

[0122] These results show an inverse relationship between solids content and water permeability, while showing an increase in mechanical strength with increasing solids content. Thus, this method has limitations in the flux vs strength properties of these fibers.

TABLE-US-00002 Concentration of PVDF Water permeability Tensile resin (wt %) Imbh Strength MPa 25 1200 2.27 30 625 2.78 32.5 385 4.32 35 184 5.47

[0123] These experiments show the normal “trade-off” where without filler—if you get high strength—you get low permeability. See also FIG. 2.

Example 5: (Comparative)

[0124] Blends were prepared by mixing powders and solvent blends in a blender. Different combinations of mixing sequences were tried. The goal was to make a free flowing powder. Visual inspection was done to see if powder was free flowing

[0125] Powder blend could be classified into three types: [0126] Free flowing powder that easily poured out of blender [0127] Chunky powder that could be poured out but in larger crumbly chunks, similar to cottage cheese

[0128] Paste that could not be poured out and had to be scooped out.

[0129] Formulation:

TABLE-US-00003 Kynar 761 powder 40 g Dioctylphthalate/dibutylphthalate 35 g (90% DOP, 10% DBP) Zano20 (zinc oxide nano-powder) 25 g

[0130] This composition was used for all of these tests.

[0131] Test 1: Mix all together in blender

[0132] Mixed for 15 seconds twice. Blender strained by the end of the mix due to thickness of the mixture. Result was a paste that did not pour out of blender. It had to be scooped out.

[0133] Test 2: Premix Kynar powder and solvent, then add Zano 20

[0134] Mixed 40 g of Kynar and 15 g of solvent blend. Blended 2×15 seconds. Free flowing damp powder formed.

[0135] Added all 25 g of Zano to the 761/solvent blend and mixed 2×15 seconds. A free flowing damp powder resulted.

[0136] Add 10 g more of the solvent mix and blended 2×15 seconds. The powder clumped up and congealed into a paste. It could not be poured out of the blender. It had to be scooped out.

[0137] Test 3: Premix Zano 20 and solvent

[0138] Mixed all Zano with all of the solvent in the blender and mixed 2×15 seconds. A fluid slurry formed. Added Kynar 761 powder and blended again, 2×15 seconds. A thick paste formed that could not be poured out of the mill.

[0139] These results (Test 1—test 3) show that it is not possible to prepare a premix with fine powder PVDF such as Kynar 761

Example 6: (Comparative)

[0140] Mixed Solef 6010 powder resin and Zano 20

[0141] Duplicating test 2 of Comparative example 5 with PVDF made by suspension with larger Particle size distribution (D50 of 115 micron), Solef 6010 powder (40 g) was blended with Zano (25 g) in blender, 2×15 second mix cycles. A free flowing powder mix resulted. Added solvent to this powder blend and then mixed again, 2×15 second cycles. The final blend could easily be poured out of the blender.

[0142] The results also show that with suspension grade larger particle size PVDF, the mixture is flowable. This shows that suspension grade PVDF does not behave in the same manner as emulsion grade PVDF.