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. 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.