Monofilament-reinforced hollow fiber membrane with scalloped lumen

Abstract

A hollow fiber membrane is formed by embedding a braid having a spiral open weave of monofilaments only, to avoid a whiskering problem. The open weave is characterized by contiguous, circumferential, rhomboid-shaped areas of polymer film separated by monofilament. When the braid is supported on a plasticized PVA cable having a scalloped periphery, the braid can be infiltrated with membrane polymer which, when coagulated, embeds the braid positioning it around the lumen. The embedded spiral weave, free of any circumferentially constricting monofilament, allows the membrane to be biaxially distensible. 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. An outside-in hollow fiber symmetric membrane comprising, tubular flexible braid of monofilament embedded as a monolayer in a tubular film of membrane polymer so as to form and reinforce a scalloped lumen of e asymmetric membrane having a periphery comprising plural outwardly convex circular segments or lobes interconnected by valleys in a cross-sectional plane perpendicular to a longitudinal axis of the asymmetric membrane, each having a nadir, the tubular flexible braid, free of multifilament yarn, comprising, from 6 to 24 separate monofilaments in the range from 25-250 denier (g/9000 meters), woven in a spiral open weave having an air permeability greater than 10 cc/sec/cm.sup.2 at 1.378, to provide contiguous rhomboids areas of the membrane polymer bounded by monofilaments, the spiral open weave being woven with the separate monofilaments at axially oppositely directed spiral angles in the range from 20?-60? from the longitudinal axis, so as to be free of a restricting circumferential filament such that the asymmetric membrane is biaxially distensible under pressure from within, prior to being damaged; the tubular film of membrane polymer, including the tubular flexible braid embedded therein, having a wall thickness in the range from 0.2 mm-0.6 mm, such that the asymmetric membrane is adapted for use as a microfiltration or ultrafiltration liquid separation membrane, wherein the lumen has scalloped cross-section of plural interconnected arcuate segments including adjacent, outwardly convex arcs, or segments of contiguous circles, interconnected, one to another at their circumferences, wherein the symmetric membrane has a wall thickness measured between the outer surface of the asymmetric membrane and the nadir of a valley, hv, greater than the wall thickness measured between the outer surface of the asymmetric membrane and the upper surface of a lobe's crest, hc, that is, hv/hc>1, and a shortest distance between the nadir of the valley and a nearest one of the monofilaments to the nadir of the valley is greater than a distance between the lobe's crest and a nearest one of the monofilaments to the lobe's crest.

2. The outside-in hollow fiber asymmetric membrane of claim 1 wherein the scalloped lumen has a pitted polymer surface, and the monofilament is selected from the group consisting of poly(vinylidene fluoride, polycarbonate, polystyrene, polyester, polyolefin, polyamide, poly(methylmethacrylate), poly(vinyl chloride) and glass fiber.

3. The outside-in hollow fiber asymmetric membrane of claim 1 wherein monofilaments overlying one another at intersections are spaced apart relative to each other and the membrane polymer is formed therebetween.

4. The outside-in hollow fiber asymmetric membrane of claim 1 wherein intersections of the monofilaments, wherein one overlies another, are the only locations where the monolayer has two superimposed monofilaments.

5. The outside-in hollow fiber asymmetric membrane of claim 1 wherein the volume of the monolayer of monofilament braid occupies a minor portion of the annular volume of the asymmetric membrane.

6. The outside-in hollow fiber asymmetric membrane of claim 2 wherein the scalloped lumen is defined by from 3-6 interconnected lobes and has a hydraulic diameter in the range from 0.5 mm-1.5 mm.

7. The outside-in hollow fiber asymmetric membrane of claim 5 having an adhesive strength greater than 15 Kgf/cm.sup.2, a bubble point greater than 4 bar and a greater than 95% rejection of 0.03 ?m polystyrene beads ire an aqueous dispersion.

8. The outside-in hollow fiber asymmetric membrane of claim 1 wherein the air permeability of the spiral open weave is greater than 100 cc/sec/cm.sup.2 at 1.378 kPa.

9. An outside-in hollow fiber asymmetric membrane comprising, a tubular flexible braid of monofilament embedded as a monolayer in a tubular film of membrane polymer so as to form and reinforce a scalloped lumen of the asymmetric membrane having a periphery defined by two diametrically opposed outwardly convex lobes in a cross-sectional plane perpendicular to a longitudinal axis of the asymmetric membrane, the tubular flexible braid, free of multifilament yarn, comprising, from 6 to 24 separate monofilament in the range from 25-250 denier (g/9000 meters), woven in a spiral open weave having an air permeability greater than 10 cc/sec/cm.sup.2 at 1.378 kPa, to provide contiguous rhomboid areas of the membrane polymer bounded by monofilaments, the spiral open weave being woven with the separate monofilaments at axially oppositely directed spiral angles in the range from 20?-60? from the longitudinal axis, so as to be free of a restricting circumferential filament such that the asymmetric membrane is biaxially distensible under pressure from within, prior to being damaged; the tubular film of membrane polymer, including the tubular flexible braid embedded therein, having a wall thickness in the range from 0.2 mm-0.6 mm, such that the asymmetric membrane is adapted for use as a microfiltration or ultrafiltration liquid separation membrane, wherein the lumen has a scalloped cross-section of plural interconnected arcuate segments including adjacent, outwardly convex arcs, or segments of contiguous circle interconnected, one to another at their circumferences, wherein the asymmetric membrane has a wall thickness measured between the outer surface of the asymmetric membrane and a nadir of a valley, hv, greater than the wall thickness measured between the outer surface of the asymmetric membrane and the upper surface of a lobe's crest, hc, that is, hv/hc>1, and a shortest distance between the nadir of the valley and a nearest one of the monofilaments to the nadir of the valley is greater than a distance between the lobes crest and a he rest one of the monofilaments to the lobe's crest.

10. The outside-in hollow fiber asymmetric membrane of claim 1 wherein intersections of the monofilaments, wherein one overlies another, are at locations corresponding to the upper surfaces of the crests of the lobes in the cross-sectional plane perpendicular to the longitudinal axis.

11. The outside-in hollow fiber asymmetric membrane of claim 1 wherein intersections of the monofilaments, wherein one overlies another, are at locations corresponding to the upper surfaces of the crests of the lobes and the nadirs of the valleys in the cross-sectional plane perpendicular to the longitudinal axis.

12. The outside-in hollow fiber asymmetric membrane of claim 9 wherein intersections of the monofilaments, wherein one overlies another, are at locations corresponding to the upper surfaces of the crests of the lobes in the cross-sectional plane perpendicular to the longitudinal axis.

13. The out hollow fiber asymmetric membrane of claim 9 wherein terminal portions of adjacent arcs of the two diametrically opposed lobes of the membrane are separated by a distance which is less than 25% of the diameter of the arcs defining the lobes, and has a hydraulic diameter in the range from 0.75 mm-1.5 mm.

Description

BRIEF DESCRIPTION OF THE DRAWING

(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 tri-lobe PVA 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 some of which contact the lobes of the tri-lobe cable.

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

(6) FIG. 5 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 some of which form a boundary of a double-barreled lumen formed by the opposed lobes of a twin-lobed cable.

(7) FIG. 6 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 which together form a boundary of a four-lobed, or quadric-lobed, lumen formed by a four-lobed, or quadric-lobed, cable.

(8) FIG. 7 illustratively duplicates a photomicrograph showing macrovoids around the periphery of the lumen where trapped air was released, and cross-sections of the filaments which were displaced within the polymer film when the membrane was forcibly sectioned.

(9) FIG. 8 illustrates the process steps for forming the membrane starting with a flexible, plasticized scalloped PVA cable sheathed in an open weave braid, to produce a membrane which is wound on a spool in a winder bath.

(10) FIG. 9 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 OF PREFERRED EMBODIMENTS

(11) Supporting the braid as it is woven on the cable:

(12) Referring to FIG. 1, there is shown a sheathed cable SC comprising a scalloped 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 difluoride) (PVDF), polycarbonate, polystyrene, polyester, polyolefin, polyamide, poly(methyl methacrylate), 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. The tubular shape of the braid is provided by contact of filaments 12 at points which may create small clearances or spaces 14 on the surfaces of the lobes after the braid is woven.

(13) FIG. 2 is a photomicrograph of a portion of the sheathed cable showing how twelve (12) filaments 12 snugly embrace the cable 30, often with the clearances 14 less than 50 ?m, between the underside of the filaments and curved surfaces near the contact points on the cable; and showing much larger spaces 14 between the underside of the filaments and other curved surfaces away from and on either side of the contact points. The small clearances, if any, result from relaxation of the woven braid after it is removed, and prior to coating it with polymer 11. The polymer 11 infiltrates all such spaces 14 and 14.

(14) 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.

(15) FIG. 3 illustrates the positioning of the filaments in the membrane covering the cable viewed in the vertical x-y plane. Some of the twelve filaments contact, or nearly contact the crests of the lobes 21 of the lumen 16; other filaments are in contact with each other at points 15 where they overlie one and another at an intersection; still other filaments are close to their intersection points above the valleys 19 between contiguous lobes. The coagulated polymer film 11 provides a relatively deep tubular layer essentially free of macrovoids. A photomicrograph of a sliced cross-section of the tri-lobe (or three-lobed) membrane showing details of its physical structure is provided in FIG. 7. The integrity of the tubular layer overlying and embedding the filaments, is the key to the excellent performance characteristics of the tri-lobe membrane, as evidenced by the data presented in Tables 1 & 2 below.

(16) FIG. 4 illustrates the membrane 20 as it was formed on the tri-lobe cable 30 which was subsequently dissolved. Some of the contact points of the filaments and the upper surfaces of the lobes of the cable, show that some filaments get slightly pulled away from the surfaces, leaving the small clearances 14 due to relaxation of the filaments after the braid is woven, and due to forces encountered in the environment of the coating nozzle. Other filaments leave spaces 14 because the filaments are above the valleys 19. All these spaces get filled with polymer.

(17) FIG. 5 illustrates a double-barreled membrane 42, in which the polymer is coagulated to form the tubular film 11 with a lumen 43 having twin diametrically opposed lobes 44 in the x-y plane, on a double-barreled cable which has been dissolved, in a manner analogous to the membrane formed as described for FIG. 3. When such a lumen of maximum cross-sectional area is desired, irrespective of the number of lobes, but particularly with a maximized twin-lobe lumen, with lobes positioned side-by-side and diametrically opposed, the integrally extruded double-barreled cable used to produce the lumen, is necessarily connected by a small thickness of PVA to maintain the integrity and strength of the cable when it is extruded; and, the lobes are not seen to be tangential relative to each other. The lobes are spaced apart by the thickness s, which results in the terminal ends of the adjacent arcs also being similarly spaced apart. The distance s is preferably less than 25% of the diameter of the circles (and arcs) defining the lobes.

(18) FIG. 6 illustrates the membrane 45, in which the polymer is coagulated to form the tubular film 11 with a lumen 46, having four radially equispaced lobes 47 in the x-y plane, on a four-barreled cable which has been dissolved, in a manner analogous to the membrane formed as described for FIG. 3.

(19) FIG. 7 is a photomicrograph of a sliced cross-section of a membrane having a tri-lobe lobe lumen formed by coagulated film 11, showing macrovoids or pits 17 where small air bubbles, in the range from 1-100 ?m, more typically from 5-50 ?m, were trapped near the boundary of the lumen, mainly in the valleys between the lumen's interconnected lobes, when the wall of the membrane was formed. The trapped air was then released, when the cable supporting the reinforcing braid was dissolved, leaving the pits as the air bubbles' fingerprints. The generally smaller microvoids 18, typically in the range from 5-20 ?m, around the filaments, resulted when the filaments were moved in the matrix polymer by impact of the sectioning blade. A sub-circumferential layer of peripheral microvoids 22, typical for a membrane formed with PVDF/NMP, is formed when the liquid polymer coagulates. This peripheral layer is shown as dots, directly below the circumference, in FIG. 7.

(20) It is seen that the filaments 12 overlap at intersections 15 of the embedded filaments 12 of the braid over which the membrane 20 is formed on the cable 30. When the braid of filaments 12 is woven over the cable 30, there are clearances 14 and 14, which range in diameter from 5 ?m-0.1 mm or more, depending upon how closely the surfaces of the filaments, or whether the filaments overlie a valley 19 between contiguous lobes. The amount of such clearance depends upon the number of lobes of the cable, its diameter and other factors. Polymer infiltrates the clearances and when coagulated forms the film 11.

(21) 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.

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

(23) Referring to FIG. 8, 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 23 over guide rolls 24 and 25 to a coating nozzle 50.

(24) The dope is prepared by mixing from 10?30% by weight of the PVDF with from 70?90% 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.

(25) The covered cable SC is fed from unwinder 23 and over guide rolls 24 and 25 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.

(26) The dope is metered through coating nozzle 50 to produce a 400 ?m thick film with the braid embedded in the bottom of the film. The dope is then coagulated at a temperature of 30?50? C. in an aqueous coagulation bath 60 and fed over guide rolls 61 and 62 to a cleaning bath 70. The wash water in a first cleaning bath 70 is maintained at a temperature of 40?80? C. for from 0.5?1.5 min to dissolve and wash out the residual NMP from the membrane.

(27) 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 60?80? 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.

(28) Accordingly, as illustrated in FIG. 9, a bundle 26 of about 2500 cut sections of SC each about 2.5 m long, is hung vertically in a cable-dissolving tank 27 into the top of which is introduced 60?80? 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.

(29) When the concentration of PVA in the wash water leaving tank 27 is <0.5% the bundle 26 is removed from the tank 27. Because too many pores of the membranes are still clogged, the bundle 26 is mounted horizontally in a second pore-cleansing tank 28 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 29 through piping 31 overnight, then drained through drain piping 33. 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.

(30) 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 and contact 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.

(31) The diameter of plasticized PVA cable with a scalloped periphery 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 0.1?1.8 mm, preferably 0.5?1.5 mm, to provide a braid reinforced membrane having an average wall thickness in the range from 0.2-0.5 mm. The resulting lumen is non-circular and the wall thickness of the membrane formed is non-uniform.

(32) 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.

(33) 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.

(34) 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.

(35) 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).

(36) 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

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

(38) 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

(39) 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 scalloped through-apertures (holes) and the average diameter of each aperture 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.

(40) 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, degrades P-17 and P-05)

Example 3

(41) 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 finishes its coagulation. The cross-section of the braid is schematically illustrated in FIG. 3.

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

Example 4

(43) 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.

Example 5

(44) 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 19 g/min a braid is woven at the same spiral angle, over a tri-lobe cable having a average diameter of 0.85 mm using 12 nylon monofilaments, each of 100 denier (0.1 mm) to yield a membrane with the average diameter of the lumen 0.93 mm, and a nominal outer diameter of 1.85 mm.

Comparative Example

(45) 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).

(46) Evaluation of Physical Properties:

(47) 1. Water Permeability

(48) (i) A membrane having a length of 200 mm is folded in half and inserted in an acrylic tube having a diameter of 10 mm and a length of 100 mm. At one end of the tube, the membrane, near both of its ends is sealed with epoxy (or urethane) leaving the lumen in each end open. The other end of the tube is left open and mounted in a water permeability testing apparatus.

(49) (ii) Distilled water held under pressure, is discharged from a pressurized vessel into the tube at a pressure of 1 bar (14.7 psig or 100 kPa) so as to force water through the wall of the membrane and discharge the permeate into a collection beaker. The water permeability is obtained by weighing of the permeate collected over a specified period of time.

(50) 2. Adhesion Strength:

(51) (i) a fiber 50 mm long is inserted for 10 mm of its length near one end, into a 10 mm inside diameter polyurethane tube 50 mm long.

(52) (ii) the 10 mm of fiber in the polyurethane tube is potted using epoxy (or urethane)

(53) (iii) 10 mm of the other end of the fiber is wrapped with paper so as not to damage it and the wrapped end is inserted into one of the jaws of an Instron (UTM) tensilometer, the gage length of which was set at 70 mm. Any material providing suitable gripping without damaging the membrane may be substituted for the paper. When the tube is secured in the other jaw of the machine, the fiber is to be taut so as not to be suddenly elongated when the Instron is in operation.

(54) (iv) 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.

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

(56) 3. Bubble Point

(57) (i) Use the same membrane sample prepared for the water permeability test, mount the tube in the water permeability testing apparatus, then immerse the tube including the membrane in a water bath.

(58) (ii) The testing apparatus is connected to a source of nitrogen under pressure and, with the membrane immersed, the tube is pressurized with nitrogen.

(59) (iii) Adjust the pressure to the tube in stages with a pressure regulator, through a range from 0 bar (atmospheric) to 8 bar, holding the pressure in increments of 0.5 bar for 60 seconds.

(60) (iv) Record the pressure when a nitrogen bubble forms on the surface of the membrane, or alternatively, the pressure at which the membrane ruptures.

(61) (v) The recorded pressure is the bubble point.

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

(63) UV [using a Perkin Elmer Lambda 25 UV/vis spectrometer]

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

(65) (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.

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

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

(68) A styrene bead dispersion was prepared by mixing polystyrene beads having a size of 0.03 ?m in thrice distilled (or reverse osmosis) water to provide a dispersion having a concentration of 0.05% polystyrene beads, with enough surfactant to prevent styrene beads from sticking together, and agitating the mixture for 1 hr.

(69) (v) pouring the dispersion into a pressure vessel and under a pressure of 0.5 bar, forcing the polystyrene beads through the membrane and

(70) (vi) collecting the permeate over a period of 1 minute.

(71) (vii) setting a reference line for the thrice distilled water using a UV-Visible spectrometer and measuring the absorbance of the feed solution, then measuring the absorbance of the permeate collected.

(72) (viii) The % rejection can be obtained by using a UV-Visible spectrometer and be calculated by the following formula:
Rejection (%)=(1?C.sub.permeate/C.sub.feed)*100

(73) C.sub.feed: absorbance of the feed solution:

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

(75) A rejection ratio 90% or more is deemed useful and pore size of the membrane can be estimated indirectly by using dispersions of beads having various diameters ranging from 20 nm-100 nm.

(76) 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 6 0.03 98 Example 4 2.05 1.25 800 18 52 5.5 0.03 97 Example 5 1.8 0.93 750 15 50 7 0.03 97 Comparative 2.1 0.85 600 12 31 1.5 0.04 95 Example

(77) 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.

(78) Further, the data for the membrane with the tri-lobe lumen, Example 5, show a bubble point about 20% higher than those obtained with membranes having circular lumens, Examples 3 and 4.

(79) Weight Advantage of Monofilament Braid Membrane

(80) Equal lengths (1 m) of a membrane made as described in Example 3, 4 and 5, 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.

(81) TABLE-US-00003 TABLE 2 Membrane weight after 30% Membrane weight Membrane weight Glycerine after water intake after drying (g/m.sup.2) treatment (g/m.sup.2) (g/m.sup.2) Example 3 108 181 360 Example 4 115 186 356 Example 5 119 193 371 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 43% 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.

(82) 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.

(83) Having thus described the monofilament membrane having a scalloped lumen, and the process for making the membrane, 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.