Monofilament-reinforced hollow fiber membrane
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.
1. 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 infiltrating the braid so as to form and reinforce the membrane's lumen, the 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 kPa, to provide contiguous rhomboid areas of the membrane polymer bounded by monofilaments, the rhomboid areas having a pair of opposed acute included angles, and the spiral weave being woven with filaments 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 allowing the braid and membrane to be biaxially distensible under pressure from within, prior to being damaged;
such that the asymmetric membrane is adapted for use as a microfiltration or ultrafiltration liquid separation membrane.
2. The asymmetric membrane of claim 1 wherein monofilament is selected from the group consisting of polyvinylidene fluoride, polycarbonate, polystyrene, polyester, polyolefin, polyamide, poly meta acrylate, poly vinyl chloride and glass fiber.
3. The asymmetric membrane of claim 1 wherein monofilaments overlying one another at intersections are movable relative to each other before the film is formed.
4. The 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 asymmetric membrane of claim 1 wherein the volume of the monofilament in the monolayer of monofilament braid occupies a minor portion of the annular volume of the membrane.
6. The 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 in an aqueous dispersion.