Monofilament-reinforced hollow fiber membrane with scalloped lumen
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.
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 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 2 , a bubble point greater than 4 bar and a greater than 95% rejection of 0.03 μm 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 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 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.