Monofilament-reinforced hollow fiber membrane

Abstract

A hollow fiber membrane is formed by embedding a braid having a spiral open weave of monofilaments only, to avoid a whiskering problem common in prior art multifilament braid-supported tubular membranes. The open weave is characterized by contiguous, circumferential, rhomboid-shaped areas of polymer film separated by monofilaments. When the braid is supported on a plasticized PVA cable it can be infiltrated with membrane polymer which, when coagulated embeds the braid positioning it around the lumen. The spiral weave, free of any circumferentially constricting monofilament, when embedded in film, allows the membrane to be biaxially distensible. In other words, the membrane has give not only in the axial or longitudinal direction but also in the radial direction. Give in the radial direction permits soiled membranes to be backwashed under higher pressure than in a comparable braid which is not radially distensible.

Claims

1. A process for embedding a monofilament, tubular open-weave braid of monofilaments in a membrane polymer, comprising, weaving an open tubular braid having a recurring rhomboid pattern of synthetic resinous monofilaments in the denier range from 25-250, directly over the surface of and snugly contacting a core cable of poly vinyl alcohol plasticized with from 5-20 weight percent of plasticizer, the cable having a diameter chosen to provide a lumen of desired diameter in the range from 0.5-2.0 mm; coating the tubular braid with a membrane-forming dope in a coating nozzle until the dope infiltrates into an area below the surface of filaments overlying the cable to form the lumen; pulling the cable and braid together through the coating nozzle; coagulating the dope to form a semipermeable membrane embedding the braid as a monolayer which, together with polymer surrounding it, defines the lumen of the membrane; washing in hot water until at least 99% of the plasticized PVA is removed, and, further washing with an aqueous oxidizing agent chosen from sodium hypochlorite (NaOCl), hydrogen peroxide and potassium hypochlorite (KOCl) to make an asymmetric membrane which tests for eluted water having a total organic carbon (TOC) of <0.5 ppm without damaging the lumen of the membrane formed.

2. The process of claim 1 wherein the semipermeable membrane has a wall thickness in the range from 0.2-0.6 mm thick.

3. The process of claim 1 wherein the concentration of NaOCl in the aqueous oxidizing agent is in the range from 0.1-0.5% NaOCl and its temperature is in the range from 20 C.-80 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing and additional objects and advantages of the invention will best be understood by reference to the following detailed description, accompanied with schematic illustrations of preferred embodiments of the invention, in which illustrations like reference numerals refer to like elements, and in which:

(2) FIG. 1 is a front vertical perspective view, of a braid supported on a cylindrical support referred to as a cable, prior to being infiltrated with a dope.

(3) FIG. 2 is a photomicrograph of a sheathed cable at a 50 magnification showing the cable 30 over which twelve monofilaments 12, each 100 denier, are spirally woven at an angle of about 35 to the longitudinal z-axis of the cable.

(4) FIG. 3 is a schematic cross-sectional view of the membrane in the x-y plane at right angle to the axial z-axis showing filaments of a monolayer of braid woven with 12 filaments overlying one another.

(5) FIG. 4 is a perspective isometric staggered cross-sectional view schematically illustrating the filaments snugly covering the cable which is to be dissolved, leaving a small clearance between the cable and filaments filled with coagulated polymer, so as to form the lumen of the membrane after coagulation.

(6) FIG. 5 illustrates the process steps for forming the membrane starting with a flexible, plasticized PVA cable sheathed in an open weave braid to produce a membrane which is wound on a spool in a winder bath.

(7) FIG. 6 illustrates the transfer of a bundle of fibers cut from the spool taken from the winder bath, which bundle is given a finishing treatment with dilute aqueous sodium hypochlorite (NaOCl).

DETAILED DESCRIPTION

(8) Supporting the Braid as it is Woven on the Cable:

(9) Referring to FIG. 1, there is shown a sheathed cable SC comprising a cable 30 over which is spirally woven a braid 10 comprising monofilaments (or filaments) 12. The filaments 12 are made from a synthetic resinous material (filament polymer) which is insoluble in permeate to be filtered through the membrane to be formed. The filament polymer is preferably selected from the group consisting of poly vinylidene fluoride (PVDF), polycarbonate, polystyrene, polyester, polyolefin, polyamide, poly meta acrylate, poly vinyl chloride and glass fiber. Filaments 12, typically all of the same denier, are wound at the same spiral winding angle greater than 20, preferably in the range from 20-60 relative to the longitudinal axis of the mandrel by a custom-built braiding machine using twelve (12) cones modified to hold and discharge a filament less than 250 m in diameter; some filaments, typically alternate filaments, are wound at an axially, oppositely directed angle from each other to provide interlaced filaments in what is commonly referred to as a diamond weave. A large winding angle indicates the filament is wound closer to the x-y plane (a transverse orientation); a small winding angle indicates the filament is more axially aligned as it is wound.

(10) As stated above, FIG. 2 is a photomicrograph of a portion of the sheathed cable showing how twelve (12) filaments 12 snugly embrace the cable 30 leaving small clearances 13 between the underside of the filaments and the cable, into which spaces polymer 20 infiltrates. The location of the braid relative to the cable in the polymer defining the lumen formed when the cable is dissolved, is thus fixed. Its location relative to the wall thickness of the membrane, for a stated diameter of the lumen, can only be manipulated by increasing or decreasing the nominal outer diameter of the membrane.

(11) FIG. 3 illustrates a cross-section of the coagulated film 11 showing the overlap at intersections of the embedded filaments 12 of the braid forming the membrane 20 on the cable 30. When the braid of filaments 12 is woven over the cable 30, there is a small clearance 14 in the range from 0.05-0.2 mm between the surfaces of the overlying filaments 12 into which clearance coagulated film 11 infiltrates. Though an open-weave filament is expected to be weaker than a prior art weave of multifilament, the resulting open-weave membrane retains its tubular configuration without collapsing under suction pressure exerted during filtration, and has excellent peel strength of at least 15 kgf/cm.sup.2. The braid, with the cable removed, has an air permeability >(greater than) 100 cc/sec/cm.sup.2 at a P of 1.378 kPa. The monofilament construction of the braid ensures stability and a minimal moisture regain, much less than that of a comparable multifilament braid; and the unique open-weave of the braid 10 makes it unnecessary to preshrink it to ensure its stability.

(12) FIG. 4 illustrates the coagulated membrane 20 on the cable 30 after the film 11 infiltrates onto the surface of cable 30, filling the spaces (clearance shown on an exaggerated scale) 14 between the inner surfaces of the filaments 12 and the cable 30.

(13) Hollow Fiber Membrane and Process for Making It:

(14) Referring to FIG. 5, there is schematically shown a flowsheet for the process of making a braided PVDF membrane wherein a sheathed cable SC (tubular open-weave braid 10 of twelve filaments 12, each 100 denier, covering cable 30) is fed from braid un-winder 21 over guide rolls 22 and 23 to a coating nozzle 50.

(15) The dope is prepared by mixing from 1030% by weight of the PVDF with from 7090% by weight of N-methylpyrrolidone (NMP) in a dope tank 40 in which the dope is blanketed with an inert gas, preferably nitrogen, from cylinder 41. The dope may be prepared by any conventional method and may include suitable additives, if needed. The dope is prepared by mixing 20 wt % of PVDF (product name: Solef 1015) and 80% of N-methylpyrrolidone (NMP) at 60 C. at a temperature in the range from 30 C.100 C., preferably 40 C.70 C. so that the viscosity of the dope is in the range from 30,000 cps 60,000 cps at 30 C.

(16) The covered cable SC is fed from unwinder 21 and over guide rolls 22 and 23 to a coating nozzle 50. Cable 30 is an extrudate of PVA plasticized with 10% glycerine. The cable has a diameter of 0.75 mm; the filaments are woven at a weave angle of 35 to sheathe the cable 30 with the open-weave tubular braid.

(17) The dope is metered through coating nozzle 50 to produce a 400 in thick film with the braid embedded in the bottom of the film. The dope is then coagulated at a temperature of 3050 C. in an aqueous coagulation bath 60 and fed over guide rolls 61 and 62 to a cleaning bath 70. The wash water in cleaning bath 70 is maintained at a temperature of 4080 C. for from 0.51.5 min to dissolve and wash out the residual NMP from the membrane.

(18) The membrane 20 on cable 30 leaves over guide roll 73 and is cleaned in a second cleaning bath 76 maintained at a temperature of 6080 C. after which the cleaned membrane leaves under guide roll 74, and is captured on a winder 80 in a winder bath 81 of diluted 50% aqueous glycerine. The diluted glycerine prevents an upper layer of wrapped membrane from sticking to a contiguous lower layer. The purpose of the winder is to store the washed membrane and the cable still supporting it, until the membrane can be cut into short sections, approximately the length desired for building a desired module, and freed from the plasticized PVA cable.

(19) Accordingly, as illustrated in FIG. 6, a bundle 25 of about 2500 cut sections each about 2.5 m long, is hung vertically in a cable-dissolving tank 26 into the top of which is introduced 6080 C. hot water until the bundle is submerged. As the plasticized PVA dissolves, it flows downwards through the lumens of the membranes because the density of a saturated solution of PVA is about 1.33. The water contaminated with PVA collects in the bottom of the tank as dissolved PVA and is removed.

(20) When the concentration of PVA in the wash water leaving tank 26 is <0.5% the bundle 25 is removed from the tank 26. Because too many pores of the membranes are still clogged, the bundle 25 is mounted horizontally in tank 27 into which an aqueous solution of from 0.1-0.5% NaOCl solution at from 20 C.-80 C., preferably from 40 C.-60 C., is introduced to remove the remaining PVA and other contaminants which would restrict flow of permeate into the lumen. The solution is continuously recycled by pump 28 through piping 29 overnight, then drained through drain pipe 31. The bundle of membranes, each with a lumen having a diameter of 0.8 mm, now free of PVA and other contaminants which clogged the pores of the membrane, is transferred to a module-building facility.

(21) The monolayer of braid 10 is thus embedded in coagulated PVDF film 11 which has excellent permeability and is essentially insoluble in water. The polymer acquires a pattern of rhomboid areas 13 (see FIG. 1) generated by the embedded braid, each area bounded by monofilaments 12. These areas 13 provide direct unobstructed radial passage of permeate into the lumen 16. Radial passage refers to the path of permeation from the surface of the membrane 20 to the lumen 16. To control the pattern, and also to provide greater strength, the filaments 12 may be sonically or thermally welded at intersections 15. The intersections 15 are the only locations where the filaments overlap each other. The area of each of the zones depends upon the openness of the weave, the diameter of the filament used and the spiral angle of the weave.

(22) The diameter of plasticized PVA cable used is chosen according to the desired diameter of the lumen (inner diameter of the membrane). Typically, the cable, whether one or more, has an average diameter in the range from 0.11.8 mm, preferably 0.51.5 mm, to provide a braid reinforced membrane having an average wall thickness in the range from 0.2-0.5 mm. If more than one cable is used to minimize the amount of plasticized PVA used and to accelerate dissolution of the core, as may be done to make a membrane with a relatively large diameter, >2.2 mm, and a lumen having a relatively large diameter, >1.2 mm, the cables are tightly bundles so as to be in fluid-tight contact with one and another. The resulting lumen is non-circular and the wall thickness of the membrane formed is non-uniform.

(23) Preferred braided membranes have a bubble point of at least 2 bar. The novel membrane has an adhesion strength of more than 15 kgf/cm.sup.2 typically from 12 to 20 kgf/cm.sup.2.

(24) The open-weave monolayer braid 10, embedded in the polymer film 20, unexpectedly provides excellent permeability and resistance to damage. The embedded monolayer eliminates a whiskering problem common to braids woven with one or more multifilament yarns.

(25) The recurring open areas in the open-weave braid provide circumferential, interconnected rhomboid or diamond-shaped loops 13 lying in the vertical (as shown in FIG. 1) axial (z-axis) direction, and, no filament is deployed circumferentially in a generally planar circle in the x-y plane. Since there is no constriction in the radial direction, not only the braid but also the membrane may be biaxially distended under sufficient internal fluid pressure prior to being damaged. By biaxially distended is meant that the open-weave braid allows not only substantial longitudinal extension of the membrane, such as might occur during backwashing, but also allows substantial radial distension of the membrane which typically does occur during backwashing. As would be expected, the longitudinal expansion of the embedded braid is much less than that of the braid itself, but much greater than that of a prior art multifilament braid coated with the same polymer.

(26) The rhomboid pattern 13 is retained when the intersections of filaments 12 are welded. The pattern may be more close-knit (not shown) so that it provides a membrane with smaller radially open passages to the lumen, each of smaller area relative to an area defined by the relatively loosely knit braid (shown).

(27) Prior art braids, woven with multifilament yarn, have at least some of the yarn forming an essentially continuous circle in the x-y plane, thus resulting in constricting any radial distension of the braid. Prior art tubular multifilament braids are therefore inelastic in the radial direction. This constricting property is retained even when the braid is embedded in an elastic polymer film. Pressure exerted from within a prior art multifilament tubular braided membrane, cannot and does not increase the diameter of the braid, thus making it susceptible to damage. In contrast, in addition to the longitudinal extension one would expect of the elastic properties of a polymer-embedded, open-weave monofilament reinforced tubular membrane, such properties allow the membrane to expand or distend radially, when sufficient pressure provides a radial distending force. Consequently, a relatively higher pressure than normally used, sufficient to distend the membrane but insufficient to damage it, may be used to backwash the membrane.

Example 1

(28) Duplication of feeding a monofilament braid as described in the Yoon et al publication No. US 2009/0206026:

(29) A monofilament braid of 100 denier (0.1 mm) nylon monofilament was spirally woven on a 1.0 mm cable of metal wire, alternate filaments being woven at opposed spiral angles of 30. The sheathed cable was placed on a 2.54 cm diameter rubber roller rotating at 30 rpm. The braid was crumpled on the cable and could not be advanced. The speed of the roller was reduced to 15 and then 5 rpm. In no case was the braid advanced without damage. The roller speed was then increased to 40 rpm. The braid was crumpled.

Example 2

(30) The following three grades of PVA available from Kuraray were each melt extruded in a single screw Hankook Model M-65 extruder fitted with a 65 mm diameter screw having a length/diameter ratio of 22. The barrel temperature is 195 C. and the die temperature 160 C. The die is provided with 18 through-apertures (holes) and its diameter is 1.6 mm. The air quenching length for the PVA cable is 2 m in 25 C. air for 2 seconds. The drawing ratio is 1.5:1.

(31) TABLE-US-00001 Extrusion Temp 195 C. Fully hydrolyzed (F-05 and F-17) degrades Intermediate hydrolyzed (M-17) degrades Partially hydrolyzed (P-24, P-20, P-17 and P-05) degrades

Example 3

(32) A braid is formed by weaving 12 filaments, each of 100 denier nylon, at a spiral angle of 35 over a plasticized PVA cable having a diameter of 0.75 mm using a custom-built braiding machine. The sheathed cable is pulled through a coating nozzle into which a dope, prepared as described above to have a viscosity of 43,000 cps at 30 C., is flowed at an output rate of 11 g/min. The dope infiltrates the braid, coats the cable and embeds the braid. The membrane is coagulated in a water bath at 45 C. and washed as shown in FIG. 5. The wall thickness of the membrane is 400 m the braided monofilaments forming a monolayer around the lumen which has essentially a little larger diameter than that of the dissolved cable, namely 0.8 mm, because PVA cable is swollen in the coagulation bath and cleaning bath before the membrane finish its coagulation. The cross-section of the braid is schematically illustrated in FIG. 3.

(33) The physical properties of the membrane made in Example 3 above, are given in Table 1 below.

Example 4

(34) In a manner analogous to that described in Example 3 above, a dope, prepared as described above to have a viscosity of 43,000 cps at 30 C., is flowed at an output rate of 16 g/min a braid is woven at the same spiral angle, over a cable having a diameter of 1.1 mm using 12 nylon monofilaments, each of 100 denier (0.1 mm) to yield a membrane with a 1.25 mm lumen, and a nominal outer diameter of 2.05 mm.

Comparative Example

(35) In a manner analogous to that described in Example 3 above, a braid is woven at the same spiral angle, without using a cable, with twenty four PET multifilament yarns each 300 denier/96 filament (a single filament is superfine, about 3 denier) and having an inner diameter of 0.85 mm; the braid was embedded in the same polymer solution to provide a wall thickness of 650 m (0.65 mm, but the membrane film thickness is about 100 m).

(36) Evaluation of Physical Properties:

(37) 1. Water Permeability

(38) (i) A membrane having a length of 200 mm is folded once to insert in an acrylic tube having a diameter of 10 mm and a length of 100 mm. One end of the membrane is sealed with epoxy (or urethane). The other end of membrane is open.

(39) (ii) The open end as described is mounted in a water permeability testing apparatus.

(40) (iii) A liquid in a pressure vessel is discharged when a pressure is applied to the liquid, and the discharged liquid flows into the tube. A membrane is hung at the end of the tube. The water permeability is obtained by measuring the amount of permeated liquid from the membrane sample.

(41) (iv) filling the tube with water and hanging the sealed part of the membrane on the beaker to collect the permeate.

(42) (v) applying a pressure of 1 bar to the vessel containing water therein and measuring the amount of water discharged from the acryl tube.

(43) (vi) measuring the weight of permeate in the beaker and calculating water permeability by measuring the amount of discharged water.

(44) 2. Adhesion strength:

(45) (i) preparing a hollow fiber membrane having a length of 50 mm

(46) (ii) preparing a urethane tube having a length of 50 mm and a diameter of 10 mm

(47) (iii) put 10 mm of the membrane in the urethane tube and potting with epoxy (or urethane)

(48) (iv) A gage length for Instron(UTM) was 70 mm. 10 mm of the end of the membrane was wrapped with a paper so as not to break. Any material providing suitable gripping without deteriorating the membrane can be used instead of the paper. When the membrane is secured by Instron, the membrane should be straight from the upper part to the bottom part. Further, upper/bottom grip should not be swayed during the operation of Instron.

(49) (v) The crosshead speed was 50 mm/min, The maximum tensile stress is divided by its unit area, so the maximum tensile stress is registered as the adhesion strength.

(50) The average elongation at break is registered as the elongation.

(51) 3. Bubble Point

(52) (i) Use the same sample prepared for the water permeability test, except that the tube including a membrane is dipped in the water bath.

(53) (ii) When the membrane is wetted, the pressure vessel was charged with nitrogen instead of the water.

(54) (iii) adjusting a pressure regulator of nitrogen from 0 bar to 8 bar at an interval of 0.5 bar with holding 60 seconds to hold its pressure.

(55) (iv) measuring the pressure when the air bubble is formed on the surface of the membrane or burst at once.

(56) (v) The pressure at which the air bubble or burst of the membrane appears is registered as the bubble point.

(57) 4. Percent (%) Rejection of Particles

(58) UV [Using a Perkin Elmer Lambda 25 UV/Vis Spectrometer]

(59) (i) preparing two strands of the hollow fiber membrane having a length of 100 mm

(60) (ii) inserting the membrane in an acrylic tube having a internal diameter 10 mm and a length of 100 mm; sealing one end of the membrane with paraffin(or urethane). The other end of the membrane is potted in the acrylic tube to prepare a sample.

(61) (iii) mounting the sample in a water permeability testing apparatus

(62) (iv) preparing a solution for measuring rejection ratio, as follows:

(63) A styrene bead dispersion was prepared by mixing thrice distilled water, styrene beads having a size of 0.03 m and surfactant to prevent styrene beads from sticking together and agitating the mixture for 1 hr.

(64) (v) pouring the styrene bead dispersion in a pressure vessel and passing the styrene bead solution through the membrane under a pressure of 0.5 bar and collecting the solution passed through the membrane after 1 minute.

(65) (vi) sampling a water base (thrice distilled water or RO water) and a feed dispersion (the styrene bead dispersion).

(66) (vii) setting a base line of the base solution (thrice distilled water or RO water) using a UV-Visible spectrometer and measuring the absorbance of the feeding solution, then measuring the absorbance of a sample passed through the membrane.

(67) (viii) The % rejection can be obtained by using a UV-Visible spectrometer and be calculated by the following formula:
Rejection (%)=(1Cf/Cp)*100

(68) C.sub.feed : absorbance of the feeding solution:

(69) C.sub.permeate: absorbance of a sample passed through the membrane

(70) According to the formula, 90% or more of the rejection ratio is useful and pore size of the membrane can be estimated indirectly.

(71) TABLE-US-00002 TABLE 1 Water Adhesion Elongation Bubble Outer Diam. Inner Diam. Permeability Strength at break Point Pore Size Rejection (mm) (mm) (LMH/Bar) (Kgf/cm.sup.2) (%) (bar) (m, SEM) Ratio (%) Example 3 1.6 0.8 800 17 51 >5 0.03 98 Example 4 2.05 1.25 800 18 52 >5 0.03 97 Comparative 2.1 0.85 600 12 31 1.5 0.04 95 Example

(72) It is evident from the Table 1 above, that the pore sizes for each of the membranes are essentially the same, as one would expect. However, the water permeability of the membrane with multifilament yarn in the braid, is only 75% of the membrane with the monofilament braid, its bubble point is lower than 33%, and its elongation at break is 66% lower than that of the membrane with the monofilament monolayer braid.

(73) Weight Advantage of Monofilament Braid Membrane

(74) Equal lengths (1 m) of a membrane made as described in Example 3 and 4, and a membrane made with a multifilament braid described in the Comparative Example above are dried so as to contain less than 1% by weight of water. Each membrane was then soaked in 30% glycerine solution for 24 hours and dried at 30 C. convection oven for 4 hours and weighed. The membranes were thereafter soaked in water for 24 hours, then weighed. The results are given in Table 2 below.

(75) TABLE-US-00003 TABLE 2 Membrane weight Membrane weight Membrane weight after 30% after water after drying Glycerine treatment intake (g/m.sup.2) (g/m.sup.2) (g/m.sup.2) Example 3 108 181 360 Example 4 115 186 356 Comparative 255 385 516 Example
It is evident from the results above, that the multifilament braid retains more than double the amount of glycerine, and about 68% more water than the monofilament membrane. Such increased weight is magnified when several thousand membranes are assembled in a module, making it more difficult to insert and remove modules in a purification system.

(76) Particularly with respect to the efficacy of removal of the plasticized PVA in the membranes before they are assembled into modules and placed in service, all three membranes test routinely for <0.5 ppm TOC being typically <0.3 ppm TOC using the prescribed KWWA (Korea Water and Wastewater Works Association) F 106 test. This confirms that essentially all the plasticized PVA has been removed.

(77) Having thus described the monofilament membrane, and the process for making it, in detail, and illustrated both with specific examples of the best mode of each, it will be evident that we have provided an effective solution to an unrecognized problem. It is therefore to be understood that no undue restrictions are to be imposed, and our invention not restricted to a slavish adherence to the details set forth herein.