HOLLOW FIBER MEMBRANE INCORPORATING A SURFACTANT AND PROCESS FOR PREPARING SAME

Abstract: Improved asymmetrical microporous hollow fibers incorporating a low molecular weight surfactant and the process for the production of the hollow fibers are disclosed. The hollow fibers have significantly improved flux and rewetting characteristics. The process involves passing a polymeric solution through an outer annulus of die to create an annular stream and a precipitating fluid through the inner orifice of the die creating a stream within the annular stream resulting in hollow fiber formation. The process further involves incorporating a low molecular weight surfactant into the hollow fibers at any one of several stages during the process.

Description: HOLLOW FIBER MEMBRANE INCORPORATING A SURFACTANT AND PROCESS FOR PREPARING SAME SPECIFICATION TO ALL WHOM IT MAY CONCERN: Be it known that we, Louis C. Cosentino, a resident of Plymouth, Hennepin county, Minnesota, Robert T. Hall II, a resident of Welch, Dakota county, Minnesota, Robert G. Andrus, a resident of New Hope, Hennepin county, Minnesota, Paul D. Brinda a resident of Robbinsdale, Hennepin county, Minnesota and Randal M. Wenthold, a resident of Belle Plain, LeSuer county, Minnesota, all citizens of the United States have invented new and useful improvements in a HOLLOW FIBER MEMBRANE INCORPORATING A SURFACTANT AND PROCESS FOR PREPARING SAME of which the following is a specification. Background of the Invention 1. Field of the invention, This invention relates generally to improved asymmetrical microporous hollow fibers incorporating a low molecular weight surfactant and to the process for the production of the hollow fibers. In particular, the invention relates to asymmetrical, microporous hollow fibers having improved flux and rewetting characteristics. The process involves passing a polymeric solution through an outer annulus of die to create an annular stream and passing a precipitating fluid through the inner orifice of the die creating a stream within the annular stream resulting in hollow fiber formation. The process further involves incorporating a low molecular weight surfactant on or into the hollow fibers at any one of several steps during the fiber manufacturing process. 2. Description of the Related Art. Microporous, hollow fibers are polymeric capillary tubes having an outside diameters equal to about 1 mm or less and whose walls function as semipermeable membranes. The fibers are useful in separation processes involving transport mainly through sorption and diffusion. Such processes include dialysis, including hemodialysis, ultrafiltration, hemofiltration, blood separation, drug release in artificial organs and water filtration. These applications have various requirements including pore size, strength, biocompatibility, cost and speed of production and reproducibility. Conventional art hollow fibers for these uses have typically included regenerated cellulose materials and modified polyacrylonitrile material. However, it is difficult to control the porosity and pore size of these fibers, and for some applications, composite membranes consisting of an ultra-thin layer contiguous with a more porous substrate are needed to provide the necessary strength. Conventional art hollow fiber membranes have also been prepared from hydrophobic polymers such as polysulfones and polyaromatic polyamide polymers. The hydrophobic nature of the polymer presents difficulties in using these membranes in aqueous systems, and therefore, hydrophilic polymers have been incorporated directly into the fibers. For example, Klein, et al., U.S. Patent No. 4,051,300, discloses a process for the preparation of hollow microporous fibers capable of withstanding from 600 psi to 2000 psi applied pressure without collapse. However, the process disclosed by Klein is slow and time consuming. Further, fibers so prepared are designed only to be used as the support structure of a final composite membrane. The actual selective membrane is applied as an ultrathin coating to this support structure in an additional step or steps. Heilmann, U.S. Patent No. 4,906,375, discloses a process comprising wet spinning a polymer solution made up of a solvent, about 12 to 20 wt.% of a first, hydrophobic polymer and 2-10 wt.% of a hydrophilic polyvinylpyrrolidone polymer and simultaneously passing through a hollow internal core a precipitant solution comprising an aprotic solvent in conjunction with at least 25 wt.% non-solvent. However, the hollow fiber membranes so produced have limitations in hydrophilicity, water flux, etc. and by their very nature are limited in use to dialysis applications. In a separate line of development of fiber membranes, surfactants have been incorporated into membrane manufacturing processes. For example, Wrasidlo et al., U.S. Patent No. 4,432,875, discloses film or fiber membranes comprising a hydrophobic polymer and, baked onto the membrane a polymeric, high molecular weight surfactant. The polymeric *•*. surfactant apparently takes the place of the hydrophilic polymer in the Klein and Heilmann references. The fiber produced using the Wrasidlo process, however, is limited to sheet membranes and is not able to be adapted to the manufacture of hollow fiber membranes. Further, the "baking on" of the surfactant in Wrasidlo results in a fiber that is costly to manufacture, thus making the fiber's use economically impractical for smaller companies. Moreover, most conventional art fibers utilize glycerol to impart the rewetting and flux characteristics of the fiber. However, the addition of glycerol to the fiber makes the fiber costly to manufacture. Further, the glycerol must be thoroughly rinsed prior to use or it will contaminate the piping system. This makes glycerol-coated fibers inefficient, costly and time consuming for the end user. In addition, if the glycerol were not used in the fiber, the fiber would have a much lower flux rate. While the conventional art fibers discussed above are useful in many applications, there is and always has been a trade-off among properties including tensile strength, elasticity, porosity, flux, and sieving characteristics including molecular size cutoff, solute clearance, etc. Thus, new membranes are constantly needed which can offer advantages in particular applications with given property requirements. The fiber membranes discussed above each have their own particular advantages and disadvantages, e.g., Klein teaches a fiber which can withstand high pressures present in reverse osmosis systems, Heilmann discloses a fiber which is tailored to dialysis systems, having lower flux and more stringent sieving properties, and Wrasidlo discloses membranes for reverse osmosis and filtration processes. However, not one of these references teach a hollow, microporous membrane which is equally suitable in processes such as hemo filtration, hemc-dialysis and water purification. A hollow fiber membrane that could be applied across a large range of applications would provide a decided advantage over conventional art fiber membranes. Additionally, a new and useful process is needed that will ensure that a low cost hollow fiber is available to both large and small companies. Specifically, a new and useful process is needed that allows the incorporation of low molecular weight surfactants into hollow fiber membranes that does not require the use of high temperatures to ensure the incorporation of the surfactant into and/or onto the fibers. Further, a new and useful hollow fiber is needed that can be autoclaved repeatedly without the loss of the rewetting characteristic and one which does not rely on glycerol for rewettability. Summary of the Invent on It is an object of the hollow fiber membrane incorporating a surfactant and the process for preparing the same provided in accordance with the present invention to solve the problems outlined above that have heretofore inhibited the successful production of a cost-efficient fiber having a broad range of applications. The microporous hollow fiber membrane in accordance with the present invention enables the use of a unique hollow fiber that, as will be shown, has greatly improved flux and rewetting characteristics than conventional art fibers. "Flux," as used herein, is a measure of the volume of water passed by the membrane under pressure for a given time and area. "Rewetting" and similar words such as rewettable, etc., as used herein, is a description of the ability of a membrane to maintain a significant level of flux after cycles of wetting and drying the membrane. The hollow fiber includes about 75 to 99 dry wt.% of a hydrophobic polysulfone polymer, about 0.1 to 20 dry wt.% of a hydrophilic polyvinylpyrrolidone polymer and about 0.001 to 10 dry wt.% of a low molecular weight surfactant. The hollow fiber membrane has a flux of at least about 5 x lO- 5 mL/min/cm 2 /mmHg and maintained significant flux characteristics for up to at least five use and drying cycles. In addition, as will be shown, the fiber has superior rewetting characteristics, "rewetting" being defined as the ability of the fiber's flux to continuously return after at least five drying and wetting cycles. In addition, the invention includes a method of manufacturing the fibers. This process includes the steps of (a) forming an annular liquid by passing a polymeric solution comprising about 5 to 25 wt.% of a hydrophobic polysulfone polymer and about 1 to 25 wt.% of a hydrophilic polyvinyl¬ pyrrolidone polymer dissolved in an aprotic solvent and having a viscosity of about 100 to 10,000 cps through an outer annular orifice of a tube-in-orifice spinneret, (b) passing a precipitating solution comprising about 0.1 to 100 wt.% of an organic solvent and about 0.1 to 100 wt.% of water into the center of the annular liquid through the inner tube of the spinneret, (c) passing the polymer precipitate through the atmosphere or an augmented atmosphere, (d) quenching the polymer precipitate in a bath to form a hollow fiber, (e) contacting the polymer precipitate with a solution comprising about 0.01 to 10 wt.% of a low molecular weight surfactant, and (f) taking up the fiber at a rate of about 125 - 250 ft/min. A second embodiment incorporates the low molecular weight surfactant solution into the polymeric solution prior to precipitation (step (a) above) while a third embodiment contacts the surfactant solution with cut and formed bundles of hollow fibers. One of the advantages of the present invention is that hollow fibers treated with surfactant retain, as will be shown, their "rewetting" character after repeated washing and autoclaving without the use of glycerol. Another advantage of the present invention is that the surfactant may be incorporated on and/or into the hollow fiber without the need to covalently bond the fiber as in heat bonded surfactant to fiber. More significantly, the present invention provides a ready-to-use, rewettable hollow fiber membrane without the use of glycerol. These and other objects and advantages of the present invention will become apparent during the course of the following detailed description and appended claims. The invention may best be understood with reference to the accompanying drawings, disclosure and examples wherein an illustrative embodiment is shown. Brief Description of the Drawings Figure 1 is a side elevational diagram with parts cut away depicting the process of the present invention; Figure 2 is a side elevational detail view of the dry- jet wet spinning spinneret used in the process of the present invention; Figure 3 is a fragmentary sectional detail view of the orifices of the spinneret. Detailed Description of the Preferred Embodiment The invention is directed to a microporous, hollow fiber that includes a hydrophobic polymer, a hydrophilic polymer and a low molecular weight surfactant. The hydrophobic polymer is preferably a polysulfone polymer, polyethersulfone, poly(arylsulfone) , poly(aryl ether sulfone) or a poly(phenylsulfone) . The polysulfone polymer is preferably a poly(arylsulfone) . More preferably, the polysulfone polymer is a poly(oxy-1,4-phenylene sulfonyl-1,4-phenyleneoxy-l, 4- phenyleneisopropylidene-1,4phenylene) polymer having the formula (-OC 6 H 4 C(CH 3 ) C 6 H 4 S0C 6 H -) n with the accompanying structure: available from Amoco Chemicals Corp. (Atlanta, Georgia) ) under the UDEL mark; or a polyether sulfone having the formula (-0-C 6 H 4 S0C 6 H 4 -) with the accompanying structure: (ID available from ICI Americas, Inc. (Wilmington, Delaware) under the VICTREX mark; or or a poly(arylsulfone) available from Amoco Chemicals Corp. (Atlanta, Georgia) under the RADEL mark, or a mixture thereof. Most preferably, the polysulfone polymer is a polysulfone of formula (I) . The polysulfone polymers preferably have a molecular weight of about 20,000 to 100,000. More preferably, the molecular weight is about 55,000 to 65,000 and most preferably, the molecular weight is about 60,000 to 65,000. If the molecular weight of the polymer is greater than about 100,000, the viscosity of the polymeric solution may become too great for processing. On the other hand, if the molecular weight of the polysulfone polymer is less than about 20,000, the viscosity of the polymeric solution may become too low to produce a fiber and any fiber formed may be too weak for processing. The hydrophilic polymer not only supplies hydrophilicity to the hollow fiber membrane but also markedly improves its porosity as well. The hydrophilic polymer may be water soluble cellulose, starch derivatives, polyvinylpyrrolidone, polyethylene glycols. Preferably, the hydrophilic polymer is polyvinylpyrrolidone ("PVP"). The PVP generally consists of recurring units of the formula (-C (C 4 H 6 NO)HCH 2 -) n with the accompanying structure: The PVP is useful to increase the solution viscosity of a polymeric spinning solution or dope. Further, this polymer is water soluble and the majority of the polymer may be dissolved from the formed fiber to increase its porosity. As some of the PVP may remain in the fiber, the fiber's wettability by an aqueous media is increased. However, to further increase the wettability of the fiber by an aqueous media, the fiber incorporates a low molecular weight surfactant. This surfactant may be amphoteric, zwitterionic, nonionic, anionic, cationic, or the surfactant may include a mixture of surfactant types. A representative, non- limiting list of useful amphoteric surfactants includes lauroamphocarboxyglycinate, e.g., MIRANOL 2MHT MOD available from Miranol, Inc. (Dayton, New Jersey) or synergistic constituents thereof. A representative, non-limiting list of useful zwitterionic surfactants includes B-N-alkylaminopropionic acids, N-alkyl-B-iminodipropionic acids, fatty acid imidazoline carboxylates, N-alkyl betaines, sulfobetaines, sultaines, and amino acids (e.g., asparagine, L-glutamine, etc.). A representative, nonlimiting list of useful nonionic surfactants include alkoxylated alkylamines, ethanol, isopropanol, methanol glycerine, alkylpyrrolidones, linear alcohol alkoxylates, difunctional block copolymer surfactants with terminal secondary hydroxyl groups, difunctional block copolymers with terminating primary hydroxyl groups, fluorinated alkyl esters, N- alkylpyrrolidones, alkoxylated amines, and poly(methylvinylether/maleic anhydride) derivatives. Other suitable surfactants would include oligomeric or non-monomeric species containing a C12-18 aliphatic and/or aromatic hydrophobic moiety and a hydrophilic functionality within the same molecule. A representative, non-limiting list of anionic surfactants include aromatic hydrophobic based acid esters and anionic flourochemical surfactants. A representative, non- limiting list of cationic surfactants includes methylbis- hydrogenated tallow amido-ethyl, 2-hydroxy-ethyl ammonium methyl sulfate, water soluble quaternized condensate polymers, and cocoalkyl bis (2-hydroxyethyl) methyl, ethoxylated chlorides. Preferably, the surfactant is an aromatic hydrophobic based acid ester, an alkoyxlated fatty amine, an alkoxylated alkylamine or a lauroampho-diacetate/sodium trideceth sulfate. More preferably, the surfactant is an alkoxylated cocoamine. Most preferably, the surfactant is an ethoxylated (2-15 EO) cocoamine. The preferred embodiment utilizes a low molecular weight surfactant. Thus, useful surfactants are generally non- polymeric and/or oligomeric surfactants having molecular weights of less than about 2,000. Preferably, the surfactant has a molecular weight of about 300 to 1,500, more preferably, about 700 to 1,200, and most preferably about 800 to 1,000. If the molecular weight of the water soluble surfactant is too high, longer soaking and rinsing times would be required in most cases, interfering with the efficiency of the process. On the other hand, if the molecular weight of the water soluble surfactant is too low the surfactant may wash off too quickly resulting in an unwettable fiber. Naturally however, this is contingent upon the particular surfactant's critical micelle concentration (CMC) which allows one to draw comparisons between theoretical monolayer coverage (i.e surfactant to surface area) and performance and solvating characteristics such as dependence on pH, dissolved solids which affect the efficiency of a given surfactant toward fiber coverage. For reasons of cost and effectiveness, it is preferred that the final fiber prior to potting contains from substantially about 0.001% to 10.0% by weight of the surfactant (1.0 X 10-Sg to O.lg of surfactant/lg of fiber), more preferably from substantially about 0.1% to 2.0% by weight of the surfactant (O.OOlg to 0.02g of surfactant/lg of fiber) and most preferably from substantially about 0.1% to 0.5% by weight of the surfactant (O.OOlg to 0.05g of surfactant/lg of fiber) when the surfactant is incorporated into the polymeric dope solution in accordance with the present invention. When the surfactant is contacted with the formed fiber in the quenching bath series or when bundles of fibers are soaked in the surfactant solution, it is preferred that the final fiber prior to potting contains from substantially about 0.001% to 10.0% by weight of the surfactant (1.0 X 10-5g to O.lg of surfactant/lg of fiber) , more preferably from substantially about 0.1% to 2.0% by weight of the surfactant (O.OOlg to 0.02g of surfactant/lg of fiber) and most preferably from substantially about 0.1% to 0.5% by weight of the surfactant (O.OOlg to 0.05g of surfactant/lg of fiber). It is also preferred that the final fiber after potting contains amounts of surfactant substantially equal to those designated above. It should be noted that the actual concentration of surfactant in the soaking solution is dictated by processing restraints. At higher concentrations, flush time needed to remove excess surfactant from the fiber is increased. At lower concentrations, longer soaking times are required to obtain effective membranes. The surfactant interacts with the hollow fiber to become associated with the fiber probably through an absorption and/or adsorption phenomenon, i.e., the surfactant is co- miscible with and/or absorbed on the fiber surface. The inventors hypothesize that this is most likely accomplished by hydrogen bonding, dipole-dipole attractions and Van der Walls forces. There is no evidence to suggest that covalent bonding is involved. In addition, the inventors hypothesize that the surfactant utilized in accordance with the present invention may act to change the conformational nature of the polymer thus imparting the superior characteristics. The hollow fiber membranes in accordance with the present invention have improved flux characteristics as discussed previously. The fiber surface may well be modified with surfactant so as to reduce inter and/or intra molecular surface tension and/or water wettability. This may enable the opening of previously closed pore structure helping to account for increased water flux across the membrane. When the treated fibers were examined under high magnification scanning electron microscopy (SEM) , no apparent change in fiber structure was noted. Further, it also appears that the effective molecular size cutoff is numerically increased using the membranes of the present invention. For example, using bovine serum albumin (BSA) as a molecular marker (0.5 g/L), a BSA (in reverse osmosis water) rejection test showed a significant increase in the effective pore size for the surfactant treated fiber. A non- treated fiber, on the other hand, showed approximately 99% BSA rejection as opposed to treated fibers which showed a 70% rejection of BSA. The surfactant does not wash out of the fiber completely, even with repeated use and drying cycles. The hollow fiber membranes may be formed using tube-in- orifice spinning procedures as disclosed in the copending, commonly assigned application Serial No. 07/684,585, filed April 1, 1991 entitled "Improved Fiber Spinning Process for the Preparation of Asymmetrical Microporous Hollow Fibers" the disclosure of which is hereby incorporated by reference. In particular, the hydrophobic polymer and hydrophilic polymer are formed into a polymeric solution comprising an aprotic solvent. An aprotic solvent is a solvent which is not proton-releasing under processing conditions, i.e., having non-acidic or non- ionizable hydrogen atoms. Preferably, this solvent is also soluble in water. A representative, non-limiting list of aprotic solvents useful in the invention includes dimethylformamide (DMF) , dimethylsulfoxide (DMSO) , dimethylacetamide (DMA) , n-methylpyrrolidone and mixtures thereof. Preferably, the solvent is DMA. Depending on the desired property of the hollow fiber, a small amount of another solvent may be added instead of using a pure aprotic solvent. Preferably the additional solvent is a lower alcohol. This may enhance the precipitation of the polymer in the fiber formation. Preferably, about 11-25 wt.%, more preferably, about 14-16 wt.%, and most preferably, about 15 wt.% of the fiber forming hydrophobic polymer are dissolved in the aprotic dimethylacetamide solvent. When less than about 11 wt.% of the hydrophobic polymer is used, the fibers formed are not strong enough to withstand the stresses involved in the high speed process of the present invention. On the other hand, when the level of hydrophobic polymer exceeds about 25 w.%, a fiber having inferior hydraulic properties is produced. The hydrophilic polymer is dissolved in the solvent at a concentration of about 0.1-5 wt.%, more preferably, about 2-4 wt.%, and most preferably, about 3 wt.%. When the hydrophilic polymer is included in the dope solution above about 5 wt.%, the resulting fibers are stiff and difficult to manufacture. A similar result is seen when the amount of hydrophilic polymer is less than about 0.1 wt.%. The polymeric solution has a viscosity of about 700- 2300 cps, preferably about 1400-1700 cps, and most preferably, about 1500 cps at 25°C, as measured on a Brookfield viscometer. The solution is preferably filtered to remove any entrained particles (contaminants or undissolved components) to prevent apparatus blockage. The polymeric solution is spun from the outer, annular orifice of a tube-in-orifice spinneret. A precipitating solution is delivered to the tube of the spinneret. The precipitating solution preferably includes a protic solvent, an aprotic solvent and water and combinations thereof. To some extent, the composition of the precipitating solution affects the porosity, clearance, tensile strength, wall thickness, inner and outer diameters and flux properties of the fiber. The practitioner of ordinary skill in the art will recognize that the precipitating solution compositions outlined in the following tables are helpful to direct the practitioner in selecting a useful formulation for a desired fiber end use. The selection of particular components and proportions is obviously up to the practitioner. The composition of the precipitating solution effective to produce a hollow fiber membrane for use in hemodialysis is illustrated below in Table I. TABLE I More Most Preferred Preferred Preferred Lower alcohol 30-90wt.% 65-90wt.% 75-85wt.% Water 10-35wt.% 10-35wt.% 10-35wt.% Aprotic Solvent Precipitating solutions effective to produce a hollow fiber membrane for use in a hemofilter operation may comprise the components in proportions as illustrated in Table II. TABLE II More Most Preferred Preferred Preferred Lower alcohol 30-90wt.% 50-85wt.% 80-85wt.% Water 10-35wt.% 10-30wt.% 15.25wt.% Aprotic solvent 0-50wt.% 5-35wt.% 10-20wt.% Precipitating solutions effective to produce a hollow fiber membrane for use in a blood filter to separate red blood cells from higher molecular weight materials may comprise the components in proportions as illustrated in Table III. TABLE III More Most Preferred Preferred Preferred Lower alcohol 30-90wt.% 30-60wt.% 35-45wt.% Water 10-35wt.% 10-30wt.% 15-25wt.% Aprotic solvent 0-50wt.% 20-50wt.% 35-45wt.% Precipitating solutions effective to produce a hollow fiber membrane for use in water filtration may comprise the components in proportions as illustrated in Table IV. TABLE IV More Most Preferred Preferred Preferred Lower alcohol 30-98wt.% 30-60wt.% 35-45wt.% Water 2-35wt.% 2-30wt.% 2-25wt.% Aprotic Solvent 0-90wt.% 20-50wt.% 35-45wt.% The above tables are merely offered to guide the practitioner in formulating fiber precipitation solutions. Indeed, the practitioner may decide that it is advantageous to operate in a "Preferred" range for one component while operating in a "Most Preferred" range for another. Representative, non-limited examples of lower alcohols include methanol, ethanol, n-propanol, is-propanol, n-butanol, t- butyl alcohol, iso-propanol, n-butanol, t-butyl alcohol, isobutyl alcohol or a mixture thereof. Preferably, the alcohol comprises methanol, ethanol, n-propanol, isopropanol, n-butanol or a mixture thereof. Various polyols, lower alcohols, glycerine etc. and/or aqueous solutions of inorganic salts may also be used. More preferably, the alcohol comprises isopropanol. The water which may be used in the precipitating liquid may be tap water, deionized water or water which is a product of reverse osmosis. Preferably the water is deionized water which has first been treated by reverse osmosis. The aprotic solvent used in the precipitating solution may again be dimethylformamide (DMF) , dimethylsulfoxide (DMSO) , dimethylacetamide (DMA) , n-methylpyrrolidone and mixtures thereof. Preferably, the aprotic solvent is the same as that used in the polymeric fiber forming solution. Most preferably, the aprotic solvent is DMA. The proportions of the alcohol, water and aprotic solvent which make up the precipitating solution influence the morphology, clearance, permeability, selectivity, etc. of the hollow fiber membrane. It is generally preferred that the proportion of water in the precipitating solution remain relatively low, about 2 to 35 wt.%, to ensure that a fiber having desirable characteristics is produced. If the precipitating liquid contains less than about 2 wt.% water, the resultant precipitation of the polymers may be too slow to form a fiber. On the other hand, a precipitating liquid that has a concentration of water greater than about 35 wt.% may result in a fiber having decrease flux with a small pore size. As indicated above, the polymeric dope is pumped, filtered and directed to the outer, ring orifice of a tube-in- orifice spinneret. At the same time, the precipitating liquid is pumped to the inner coaxial tube of the spinneret. These two solutions are then delivered from the spinneret in a manner such that the polymer dope forms an annular sheath surround a flow of precipitating liquid within the annulus. Preferably, the spinneret head is maintained at a temperature of about 5-85°C, more preferably, about 15-25°C, and most preferably, about 18°C. The polymeric dope is subjected to a pressure of about 0- 1400 kPa, more preferably, about 140-1000 kPa, and most preferably, about 350-850 kPa. In a preferred embodiment, the polymer dope is spun through a ring orifice having an outside diameter of about 0.018 to 0.040 inches (about 460 to 1.016 microns) and an inside diameter of about 0.008 to 0.010 inches (about 200 to 254 microns) . At the same time, precipitating liquid is pumped through the tube of the spinnerette at a pressure of about 0- 1000 kPa, preferably about 0-100 kPa, and most preferably, about 1-20 kPa. In a preferred embodiment, the precipitating liquid or diluent solution is delivered through a tube having an outside diameter of substantially about 0.010 inches (about 254 microns) and an inside diameter of substantially about .005 inches (about 127 microns) . In a preferred embodiment, in order to produce a hollow fiber having an approximately 380 micron outside diameter and an approximately 280 micron inside diameter, the polymer dope is delivered to the spinnerette at a rate of substantially about 1.0-10 mL/min, more preferably, about 2-5 mL/min, most preferably, about 3 mL/min, and the precipitating liquid is delivered at a rate of at least about 1.0-10 mL/min, more preferably, about 2-5 mL/min, and most preferably, about 2-3 mL/min. The spinnerette is oriented in a manner such that fiber production is driven by fluid flow and by removal from the spinnerette by gravity effects. Preferably, the fiber emerges from the spinnerette and is pulled by gravity and the take-up speed in a nearly vertical direction downwards. In order to provide satisfactory fibers in the practice of the invention, laminar fluid flow should be maintained both within the spinneret head and the spun fluids which interact to precipitate the fiber. If turbulent flow is present in the spinneret head, especially within the channels which convey the polymeric dope, gas pockets may develop and ultimately form large voids in the spun fiber. Turbulent flow within the spun fluids may also result in voids within the fiber. It is helpful to visualize the spinnerette dimensions by resort to ratios of the annular orifice for passage of the polymeric dope and the coaxial tubular orifice for passage of the diluent or precipitating solution. One helpful ratio is the ratio of the cross-sectional area of the annular orifice to tubular orifice. Preferably, the ratio is greater than about 1:1, more preferably, the ratio is about 2:1 to 5:1, and most preferably, the ratio of the annular orifice to tubular orifice cross-sectional area is about 3:1 to 4:1. Another helpful dimensional ratio is the annular ring thickness to tube inside diameter. Preferably, the ratio is greater than about 1:1, more preferably, the ratio is about 2:1 to 7:1, and most preferably, the ratio of the annular ring thickness to tube inside diameter is about 3:1 to 6:1. A third helpful dimensional ratio is the outside diameter of the annular orifice to tube inside diameter. Preferably, this ratio is greater than about 2:1, more preferably, the ratio is about 3:1 to 10:1, and most preferably, the ratio of the annular outside diameter to tube inside diameter is about 5:1 to 8:1. As the fiber emerges from the spinneret, it drops in a substantially downward vertical direction over a distance of about 0.1-lOm, more preferably, about .5 to 2.0 m, and most preferably, about 1.0 to 1.5m. This allows the precipitating liquid to substantially precipitate the polymer in the annular dope solution forming the solid fiber capillary before it is immersed in a quenching solution. Between the spinneret and the quenching bath, the fiber drops through the atmosphere, air, air with a particular relative humidity, an augmented atmosphere, e.g., a mixture of air or air with a particular relative humidity and a gas, an inert gas, or a mixture thereof. Preferably, for ease in processing and to produce a high quality fiber, the fiber drops through air maintained at a temperature of 0°C to 100°C, more preferably, the air is maintained at a temperature of 5°C to 50°C and most preferably at 15°C to 25°C. Preferably the air is also maintained at a relative humidity of substantially about 10% to 90%, more preferably from substantially about 20% to 80% and most preferably from substantially about 40% to 65%. This gaseous atmosphere may be relatively stagnant, or there can be fluid flow. Preferably, the flow rate is sufficient to allow complete air change over in the spinning environment once every 30 minutes. In one preferred embodiment, the gas flow is about 10 L/min. Next, the fiber is submerged in a tank comprising water and 0-10 wt.% other materials. Again, the water may be tap, deionized water, or the product of a reverse osmosis process. The temperature of the quenching bath is preferably between about 0° to 100°C, more preferably, about 15°C to 45°C, and most preferably, about 35°C. The water temperature directly affects the performance of the fiber. Lower temperatures can reduce the flux of the resulting fiber. Increasing the quenching bath temperature can increase the flux of the fiber. The fiber is preferably immersed in the quenching bath for a period of about 0.1 to 10 seconds, preferably about 0.1 to 5 seconds, and most preferably, about 1 second. This residence time permits the full precipitation of the hydrophobic polymer to form the microporous hollow fiber. The quenching bath also helps to remove the excess, unprecipitated polymers as well as some of the hydrophilic polymer, the water soluble solvent and precipitating liquid. After the quenching bath, the fiber may be further rinsed to remove additional unprecipitated polymers and solvents. This rinsing may be accomplished in a water bath arrangement. Preferably, the additional rinse is achieved in a water bath having a water temperature of about 0°C-100°C, more preferably, about 15°C-45°C, and most preferably, about 35°C. The fiber is then wound on a take-up reel. The take-up reel is preferably rotating at a speed such that the fiber is being wound at about 90-150% of the rate at which it is being formed at the spinneret. More preferably, the fiber is being wound at a rate substantially equal to that at which it is being produced. In other words, the fiber is taken up with enough speed (i) to create a fiber of the desired size and (ii) to apply sufficient tension to the fiber such that it will remain taut in the take-up guide unaffected by ambient air currents, i.e. there is no "draft." The surfactant may be incorporated into or onto the hollow fiber membrane through a number of mechanisms. The polymeric spinning solution itself may comprise about 0.01 to 10 wt.% of surfactant. In other useful embodiments, about 0.01 to 10 wt.% of a surfactant may be incorporated into the quenching bath, rinse bath, take-up reel bath, a surfactant bath or any other process step wherein the gelled tube or precipitated hollow fiber is contacted with an aqueous or organic solution, or both. In another embodiment, the fiber is cut and formed into bundles that are then soaked in a surfactant solution. Preferably, the hollow fiber membrane or gelled polymeric solution has a contact time with a surfactant solution of less than about 10 seconds. If the surfactant is incorporated into the quenching bath, rinse bath or take-up reel bath, the fiber's residence time in the solution is about 4 to 48 hours, in another embodiment, the fibers are cut and bundled prior to soaking in the surfactant solution for less than 72 hours, more preferably for less than 30 hours and most preferably for less than 24 hours. The surfactant solution may be contacted with the gelled polymer or polymeric precipitate at a temperature of about 0°C to 100°C. More preferably, the hollow fiber or polymeric precipitate is contacted with a surfactant solution at a temperature of about 20°C to 50°C, and most preferably at about 40°C to 50°C. The hollow fibers may then be dried by any method appropriate to general manufacturing procedures including but not limited to air, heat, vacuum, or any combination thereof. The hollow fibers may be further processed to form useful articles including hemodialyzer cartridges, hemofilters, blood filters, water filters, etc., having improved performance levels. Detailed Description of the Drawings The process of the present invention may be generally described by referring to the drawings. A polymeric dope solution 12 including a polysulfone polymer and polyvinylpyrrolidone polymer dissolved in an aprotic solvent is prepared in a mixing vessel 14. The solution is then filtered in a filter press 16 and delivered by means of a pump 18 to a dry-jet wet spinning spinneret apparatus 20. This apparatus is discussed in further detail below. Simultaneously, a diluent or precipitating solution 22 is prepared in a second mixing vessel 24 from water and a lower alcohol. The diluent solution 22 is also delivered to the spinneret apparatus 20 by means of pump 26. The dope solution 23 and diluent solution 22 are spun from the spinneret apparatus 20 to form a hollow fiber 28. The hollow fiber 28 drops through a volume of gaseous fluid 30 which is enclosed within a pipe 32 until the fiber reaches the surface of a quenching bath 34. Water is circulated through the quenching bath 34 in an overflow manner, i.e., a continuous flow of water 36 is supplied to the quenching bath 34, and the excess fluid overflows and is removed, e.g., at 38. The fiber 28 is then directed out of the quenching bath 34 and is wound on a take-up reel 40 which is immersed in a second, rinsing bath 42, and the excess fluid overflows the bath and is removed, e.g., at 46. The hollow fiber 28 thus produced may then be removed from the take-up reel 40 and further processed. An example of further processing includes cutting fibers 28 to a uniform length, bundling them and drying them in any conventional manner. Referring to Figures 2 and 3, the details of a spinneret head 102 which is part of the dry-yet wet spinning spinneret apparatus 20 is illustrated. The dope solution 12 enters through a dope port 104, is directed to an annular channel 106, and flows out of an annular orifice 18 in a generally downward direction. The diluent solution 22 enters the spinneret head 102 through a diluent port 110, is directed through an inner channel 112 and flows out through a tubular orifice 114 which is in a generally concentric orientation with respect to the annular orifice 108. Again, in a preferred embodiment of the present invention, the hollow fibers 28 formed may be cut into bundles (not shown) of a constant length and soaked in an aqueous surfactant solution (not shown) as discussed above. Water flux is determined by a test developed in-house. Specifically, the water flux is measured on test mat size (0.02 to 0.08m 2 ) bundles which are potted in a polycarbonate cylindrical case. A transmembrane pressure of 5 psi is maintained across the unit as reverse osmosis water is pumped through one of two side ports (one side port clamped off) , exiting out one of two end ports (one end port clamped off) . The water is collected via graduated cylinder on a timed basis to determine flux. Drying of the membrane may be accomplished by circulating dry air through and around the hollow fiber membranes. The flux of a fiber which has been cycled in this manner can be compared to its original values to determine the membrane's rewettability. The process is repeated in duplicate to insure reproducibility. Examples The following specific examples which contain the best mode, can be used to further illustrate the invention. These examples are merely illustrative of the invention and do not limit its scope. Exa l 1 A polymer solution was prepared by dissolving 15.1% by weight of a polysulfone polymer having a molecular weight of about 60,000 to 65,000 and 2.8% by weight of PVP having a K- value of about 80 to 87 in 81.6% dimethylacetomide with 0.5% by weight of an exthoxylated (15 EO) cocoamine surfactant. The material was filtered and then pumped into a tube-in orifice spinnerette at a rate of 3.5-3.7 ml/min at a temperature of about 65-72°F. A diluent solution containing 40% by weight isopropanol, 40% by weight DMAC and 20% by weight deionized, reverse osmosis water was delivered to the spinnerette at a temperature of 65-72°F and at a rate of 2.5-2.6 ml/min. The polymeric dope solution was delivered through the outer, annular orifice of the spinnerette having an outside dimension of about .037 inches and an inside dimension of about .010 inches. The diluent was delivered through a tube orifice within the annular orifice having an inside diameter of about .005 inches. The spinnerette head was maintained at about 70°F by means of a water bath or run without a water bath and maintained at room temperatures 68-74°F. The spinnerette discharged the column of dope solution downward through air at a temperature of 68-80°F and relative humidities of 20-60%. The fiber dropped through this controlled environment 1.1 meters into a reverse osmosis quenching water bath which was maintained constant at 90-100°F. Reverse osmosis water was pumped into the quenching bath resulting in overflow. The fiber was pulled at approximately 10 RPM into a second bath containing reverse osmosis water maintained at a temperature of 90-100°F. The fiber was removed from the take-up wheel, cut and formed into bundles containing approximately 2,100 fibers of about 30.5 cm. The fiber bundles were soaked in reverse osmosis water with overflow maintained at 46-58°C. The bundles were centrifuged and dried at 38-50°C in a convection oven. Example 2 A polymer solution was prepared by dissolving 16.2% by weight of a polysulfone polymer having a molecular weight of about 60,000 to 65,000 and 2.8% by weight of PVP having a K- value of about 80 to 87 in 81.6% dimethylacetomide with 0.5% by weight of an ethoxylated (15 EO) cocoamine surfactant. The material was filtered and then pumped into a tube-in orifice spinnerette as in Example 1. A diluent solution containing 98% by weight isopropanol, 0% by weight DMAC and 2% by weight deionized, reverse osmosis water was delivered to the spinnerette at a temperature of 65-72°F and at a rate of 2.8-3.0 ml/min. The polymeric dope solution was delivered through the outer, annular orifice of the spinnerette having an outside dimension of about .037 inches and an inside dimension of about .010 inches. The diluent was delivered through a tube orifice within the annular orifice having an inside diameter of about .005 inches. The spinnerette head was maintained at about 70°F by means of a water bath or run without a water bath and maintained at room temperatures 68-74°F. The spinnerette discharged the column of dope solution and diluent downward through air at a temperature of 68-80°F and relative humidities of 20-60%. The fiber dropped through this controlled environment 1.1 meters into a reverse osmosis quenching water bath which was maintained constant at 90- 100°F. Reverse osmosis water was pumped into the quenching bath resulting in overflow. The fiber was pulled at approximately 10 RPM into a second bath containing reverse osmosis water maintained at a temperature of 90-100°F. The fiber was removed from the take-up wheel, cut and formed into bundles containing approximately 2,100 fibers of about 30.5 cm. The fiber bundles were soaked in reverse osmosis water with overflow maintained at 46-58°C. The bundles were centrifuged and dried at 38-50°C in a convection oven. Example 3 A polymer solution was prepared by dissolving 15.1% by weight of a polysulfone polymer having a molecular weight of about 60,000 to 65,000 and 2.8% by weight of PVP having a K- value of about 80 to 87 in 82.1% dimethylacetomide. The material is filtered and then pumped into a tube-in orifice spinnerette at a rate of 3.5-3.7 ml/min at a temperature of about 65-72°F. A diluent solution containing 80% by weight isopropanol, 0% by weight DMAC and 20% by weight deionized, reverse osmosis water was delivered to the spinnerette at a temperature of 65-72°F and at a rate of 2.5-2.6 ml/min. The polymeric dope solution was delivered through the outer, annular orifice of the spinnerette having an outside dimension of about .020 inches and an inside dimension of about .010 inches. The diluent was delivered through a tube orifice within the annular orifice having an inside diameter of about .005 inches. The spinnerette head was maintained at about 70°F by means of a water bath or run without a water bath and maintained at room temperatures 68-74°F. The spinnerette discharged the column of dope solution and diluent downward through air at a temperature of 68-80°F and relative humidities of 20-60%. The fiber dropped through this controlled environment 1.5 meters into a reverse osmosis quenching water bath which was maintained constant at 90- 100°F. Reverse osmosis water was pumped into the quenching bath resulting in overflow. The fiber was pulled at approximately 20 RPM into a second bath of reverse osmosis water maintained at a temperature of 90-100°F. The fiber was removed from the take-up wheel, cut and formed into bundles containing approximately 6,000 fibers of about 30.5 cm. The fibers were then placed for 24 hours in a static soak tank containing 1% by weight of an ethoxylated (15 EO) cocoamine surfactant and water maintained at 68°F to 100°F. The bundles were centrifuged and dried at 38-50°C in a convection oven. Example 4 A polymer solution was prepared by dissolving 15.1% by weight of a polysulfone polymer having a molecular weight of about 60,000 to 65,000 and 2.8% by weight of PVP having a K- value of about 80 to 87 in 82.1% dimethylacetomide. The material is filtered and then pumped into a tube-in orifice spinnerette at a rate of 3.5-3.7 ml/min at a temperature of about 65-72°F. A diluent solution containing 90% by weight isopropanol, 0% by weight DMAC and 10% by weight deionized, reverse osmosis water was delivered to the spinnerette at a temperature of 65-72°F and at a rate of 2.5-2.6 ml/min. The polymeric dope solution was delivered through the outer, annular orifice of the spinnerette having an outside dimension of about .020 inches and an inside dimension of about .010 inches. The diluent was delivered through a tube orifice within the annular orifice having an inside diameter of about .005 inches. The spinnerette head was maintained at about 70°F by means of a water bath or run without a water bath and maintained at room temperatures 68-74°F. The spinnerette discharged the column of dope solution and diluent downward through air at a temperature of 68-80°F and relative humidities of 20-60%. The fiber dropped through this controlled environment 1.5 meters into a reverse osmosis quenching water bath which was maintained constant at 90- 100°F. Reverse osmosis water was pumped into the quenching bath resulting in overflow. The fiber was pulled at approximately 110% of the rate at which it is being formed into a second bath containing 1% by weight of an ethoxylated (15 EO) cocoamine surfactant and reverse osmosis water maintained at a temperature of 90-100°F. The fiber was removed from the take-up wheel, cut and formed into bundles containing approximately 6,000 fibers of about 30.5 cm. The bundles were centrifuged and dried at 38- 50°C in a convection oven. Example 5 A polymer solution was prepared by dissolving 15.1% by weight of a polysulfone polymer having a molecular weight of about 60,000 to 65,000 and 2.8% by weight of PVP having a K- value of about 80 to 87 in 82.1% dimethylacetomide. The material is filtered and then pumped into a tube-in orifice spinnerette at a rate of 3.5-3.7 ml/min at a temperature of about 65-72°F. A diluent solution containing 90% by weight isopropanol, 0% by weight DMAC and 10% by weight deionized, reverse osmosis water was delivered to the spinnerette at a temperature of 65-72°F and at a rate of 2.5-2.6 ml/min. The polymeric dope solution was delivered through the outer, annular orifice of the spinnerette having an outside dimension of about .020 inches and an inside dimension of about .010 inches. The diluent was delivered through a tube orifice within the annular orifice having an inside diameter of about .005 inches. The spinnerette head was maintained at about 70°F by means of a water bath or run without a water bath and maintained at room temperatures 68-74°F. The spinnerette discharged the column of dope solution and diluent downward through air at a temperature of 68-80°F and relative humidities of 20-60%. The fiber dropped through this controlled environment 1.5 meters into a reverse osmosis quenching water bath which was maintained constant at 90- 100°F. Reverse osmosis water was pumped into the quenching bath resulting in overflow. The fiber was pulled at approximately 20 RPM into a second bath containing 1% by weight of an ethoxylated (2 EO) cocoamine surfactant and reverse osmosis water maintained at a temperature of 90-100°F. The fiber was removed from the take-up wheel, cut and formed into bundles containing approximately 6,000 fibers of about 30.5 cm. The bundles were centrifuged and dried at 38- 50°C in a convection oven. Test Data The membranes made from the examples above were measured for flux, rewettability and diffusional flow rates. The water flux was measured on test mat size (0.02 to 0.08m 2 ) bundles which were potted in a polycarbonate cylindrical case. A transmembrane pressure of 5 psi was maintained across the unit as reverse osmosis water was pumped through one ""of two side ports (one side port clamped off), exiting out one of two end ports (one end port clamped off) . The water was collected via graduated cylinder on a timed basis to determine flux. Drying of the membrane was accomplished by circulating dry air through and around the hollow fiber membranes. The flux of fibers cycled in this manner were compared to their original values to determine the membrane's flux and rewettability characteristics. The hollow fiber membranes were also tested for diffusional air flow, a method of determining the integrity of a membrane. When dry, air flow easily through the pores in the membrane; when wet, air does not flow through an intact membrane. Membranes were wet with reverse osmosis water to fill the pores. A transmembrane pressure equal to 30 psi was applied to the upstream side of the membrane. Air which diffuses through is measured to determine the integrity of the membrane. Test 1 A filter module comprising approximately 1.4 m 2 of membrane prepared in accordance with Example 1 was tested in accordance with the previously disclosed flux test. The module produced a water flux of .0026 mL/min/mmHg/cm 2 * The module also produced a diffusional air flow of 20 mL/min at 30 psi inlet air pressure. Test 2 A filter module comprising approximately 1.4 m 2 of membrane fabricated in accordance with Example 2 was tested for its ability to re-wet upon successive dryings. Each wet dry cycle trial consisted of the following steps: 1) flux test 2) diffusional flow test 3) air dried by blowing 20°C air through the lumen for 24 hours A filter module comprising approximately 1.4 m 2 of membrane fabricated in accordance with Example 3 was tested for its rewetting characteristics and its ability to remove bovine serum albumin from blood. Each rewetting test trial consisted of the following steps: 1) flux test 2) air dried by flowing 20°C air through the lumen for 24 hours 3) rewet The results are tabulated below: Flux Trial 1 .000428 Trial 2 .000446 Trial 3 .000457 Trial 4 .000454 Trial 5 .000476 The bovine serum albumin rejection rate was 80%. Test A filter module comprising approximately 3.0 m 2 of membrane fabricated in accordance with Example 4 was tested for its initial flux rate and diffusional flow characteristics. The module produced a water flux of .00146 and a diffusional air flow of 53 mL/min. Test 5 A filter module comprising approximately 3.0m 2 of membrane fabricated in accordance with Example 5 was tested for its initial flux rate and diffusional flow characteristics. The module produced a water flux of .00092 and a diffusional air flow of 51 mL/min. Although the description of the preferred embodiment has been presented, it is contemplated that various changes, including those mentioned above, could be made without deviating from the spirit of the present invention. It is therefore desired that the present embodiment be considered in all respects as illustrative, not restrictive, and that reference be made to the appended claims rather than to the foregoing description to indicate the scope of the invention.

Claims: We claim: 1. A microporous hollow fiber membrane having improved flux and rewetting characteristics, the fiber comprising: (a) about 75 to 99 wt.% of a hydrophobic polysulfone polymer; (b) about 0.1 to 20 wt.% of a hydrophilic polyvinylpyrrolidone polymer; (c) about 0.001 to 10 wt.% of a low molecular weight surfactant; wherein said fiber has a flux of at least about 5 x lO- 5 mL/min/cm2/mmHg and wherein said fiber rewets by maintaining said flux for at least five use and drying cycles. 2. The membrane of claim 1 wherein the polysulfone polymer comprises a polymer of the formula: 3. The membrane of claim 1 wherein the polysulfone polymer comprises a polyethersulfone polymer. 4. The membrane of claim 1 wherein the polysulfone polymer comprises a polyarylsulfone polymer. 5. The membrane of claim 1 wherein the surfactant is selected from the group consisting of a nonionic, anionic, or amphoteric surfactant or a mixture thereof. 6. The membrane of claim 5 wherein the surfactant is selected from the group consisting of an aromatic hydrophobic based acid ester, an alkoxylated alkylamine, a lauroampho- diacetate/sodium trideceth sulfate, or a mixture thereof. 7. The membrane of claim 6 wherein the surfactant comprises an alkoxylated fatty amine. 8. The membrane of claim 7 wherein the alkoxylated fatty amine surfactant is selected from the group consisting of an alkoxylated cocoamine or an ethoxylated (2-15 EO) cocoamine. 9. The membrane of claim 1 wherein the surfactant is selected from the group consisting of an aromatic hydrophobic based acid ester, an alkoxylated alkylamine, a lauroampho- diacetate/sodium trideceth sulfate, or a mixture thereof. 10. The membrane of claim 1 wherein the surfactant comprises an alkoxylated fatty amine. 11. The membrane of claim 1 wherein the alkoxylated fatty amine surfactant is selected from the group consisting of an alkoxylated cocoamine or an ethoxylated (2-15 EO) cocoamine. 12. A process for the manufacture of an improved microporous hollow fiber membrane having improved flux and rewetting characteristics, the process comprising the steps of: (a) passing, through an outer annular orifice of a tube-in-orifice spinneret, a polymeric solution comprising about 5 to 25 wt.% of a hydrophobic polysulfone polymer and about 1 to 25 wt.% of a hydrophilic polyvinylpyrrolidone polymer dissolved in an aprotic solvent and having a viscosity of about 100 to 10, 000 cP to form an annular liquid; (b) passing, through the inner tube of the spinneret, into the center of the annular liquid a precipitating solution comprising: (i) about 0.1 to 100 wt.% of an organic solvent; and (ii) about 0.1 to 100 wt.% of water as a nonsolvent, wherein the precipitating solution interacts with the polymeric solution to form an annular polymer precipitate; (c) dropping the polymer precipitate through the atmosphere or an augmented atmosphere; (d) quenching the polymer precipitate in a bath to form a hollow fiber; (e) contacting the polymer precipitate with a solution comprising about 0.001 to 10 wt.% of a low molecular weight surfactant; and (f) taking up the fiber at a rate of about 90 to 150% of the rate at which it is formed; wherein the fiber has a flux of at least 5 x 10-5 mL/min/cm2/mmHg and rewets by maintaining said flux for at least five use and drying cycles. 13. The process of claim 12 wherein the polysulfone polymer comprises a polymer of the formula: 14. The process of claim 12 wherein the polysulfone polymer comprises a polyethersulfone polymer. 15. The process of claim 12 wherein the polysulfone polymer comprises a polyarylsulfone polymer. 16. The process of claim 12 wherein the aprotic solvent is selected from the group consisting of dimethylformamide (DMF) , dimethylsulfoxide (DMSO) , dimethylacetamide (DMA) , N- methylpyrrolidone and mixtures thereof. 17. The process of claim 12 wherein the organic solvent is selected from the group consisting of dimethylformamide (DMF) , dimethylacetamide (DMA), isopropanol and mixtures thereof. 18. The process of claim 12 wherein the precipitating solution comprises 40 wt.% dimethylacetamide, 40 wt.% isopropanol, and 20 wt.% water. 19. The process of claim 12 wherein the precipitating solution comprises 40 wt.% dimethylformamide, 40 wt.% isopropanol, and 20 wt.% water. 20. The process of claim 12 wherein the precipitating solution comprises from about 90-98 wt.% isopropanol and 10-2 wt. % water. 21. The process of claim 12 wherein the surfactant is selected from the group consisting of a nonionic, anionic, amphoteric surfactant or mixtures thereof. 22. The process of claim 21 wherein the surfactant is selected from the group consisting of an aromatic hydrophobic based acid ester, an alkoxylated alkylamine, a lauroampho- diacetate/sodium trideceth sulfate, or a mixture thereof. 23. The process of claim 22 wherein the surfactant comprises an an alkoxylated fatty amine. 24. The process of claim 23 wherein the alkoxylated fatty amine surfactant is selected from the group consisting of an alkoxylated cocoamine or an ethoxylated (2-15 EO) cocoamine. 25. The process of claim 12 wherein the surfactant is selected from the group consisting of an aromatic hydrophobic based acid ester, an alkoxylated alkylamine, a lauroampho- diacetate/sodium trideceth sulfate, or a mixture thereof. 26. The process of claim 12 wherein the surfactant comprises an an alkoxylated fatty amine. 27. The process of claim 12 wherein the alkoxylated fatty amine surfactant is selected from the group consisting of an alkoxylated cocoamine or an ethoxylated (2-15 EO) cocoamine. 28. The process of claim 12 further comprising cutting and forming the fibers into bundles. 29. The product made in accordance with the process of Claim 12. 30. A process for the manufacture of an improved microporous hollow fiber membrane having improved flux and rewetting characteristics, the process comprising the steps of: (a) passing, through an outer annular orifice of a tube-in-orifice spinneret, a polymeric solution comprising about 5 to 25 wt.% of a hydrophobic polysulfone polymer and about 1 to 25 wt.% of a hydrophilic polyvinylpyrrolidone polymer dissolved in an aprotic solvent and having a viscosity of about 100 to 10,000 cP to form an annular liquid; (b) passing, through the inner tube of the spinneret, into the center of the annular liquid a precipitating solution comprising: (i) about 0.1 to 100 wt.% of an organic solvent; and (ii) about 0.1 to 100 wt.% of water as a nonsolvent, wherein the precipitating solution interacts with the polymeric solution to form an annular polymer precipitate; (c) dropping the polymer precipitate through the atmosphere or an augmented atmosphere; (d) quenching the polymer precipitate in a bath to form a hollow fiber; (e) taking up the fiber at a rate of about 90 to 150% of the rate at which it is formed; (f) cutting and forming the fibers into bundles; and (g) contacting the bundles with a solution comprising about 0.001 to 10 wt.% of a low molecular weight surfactant; wherein the resultant fiber has a flux of at least 5 x 10-5 . mL/min/cm2/mmHg and rewets by maintaining said flux for at least five use and drying cycles. 31. The process of claim 30 wherein the bundles are soaked in the surfactant solution for at least about 10 seconds. 32. The process of claim 30 wherein the bundles are soaked in the surfactant solution for about 18 to 24 hours. 33. The process of claim 30 wherein the surfactant solution is maintained at about 0°C to 50°C. 34. The process of claim 33 wherein the bundles are soaked in the surfactant solution for at least about 10 seconds. 35. The process of claim 33 wherein the bundles are soaked in the surfactant solution for about 18 to 24 hours. 36. The process of claim 30 wherein the organic solvent is selected from the group consisting of dimethylformamide (DMF) , dimethylacetamide (DMA), isopropanol and mixtures thereof. 37. The process of claim 30 wherein the precipitating solution comprises 40 wt.% dimethylacetamide, 40 wt.% isopropanol, and 20 wt.% water. 38. The process of claim 30 wherein the precipitating solution comprises 40 wt.% dimethylformamide, 40 wt.% isopropanol, and 20 wt.% water. 39. The process of claim 30 wherein the precipitating solution comprises from about 90-98 wt.% isopropanol and 10-2 wt. % water. 40. The process of claim 33 wherein the organic solvent is selected from the group consisting of dimethylformamide (DMF) , dimethylacetamide (DMA), isopropanol and mixtures thereof. 41. The process of claim 33 wherein the precipitating solution comprises 40 wt.% dimethylacetamide, 40 wt.% isopropanol, and 20 wt.% water. 42. The process of claim 33 wherein the precipitating solution comprises 40 wt.% dimethylformamide, 40 wt.% isopropanol, and 20 wt.% water. 43. The process of claim 33 wherein the precipitating solution comprises from about 90-98 wt.% isopropanol and 10-2 wt. % water. 44. The product made in accordance with the process of Claim 30. 45. A process for the manufacture of an improved microporous hollow fiber membrane having improved flux and rewetting characteristics, the process comprising the steps of: (a) passing, through an outer annular orifice of a tube-in-orifice spinneret, a polymeric solution comprising about 5 to 25 wt.% of a polysulfone polymer, about 0.001 to 10 wt.% of a low molecular weight surfactant and about 1 to 25 wt.% of a polyvinylpyrrolidone polymer dissolved in an aprotic solvent, and having a viscosity of about 100 to 10,000 cP to form an annular liquid; (b) passing, through the inner tube of the spinneret, into the center of the annular liquid a precipitating solution comprising: (i) about 0.1 to 100 wt.% of an organic solvent; (ii) about 0.1 to 100 wt.% of water as a nonsolvent, wherein said precipitating solution interacts with the polymeric solution to form an annular polymer precipitate; (c) dropping the polymer precipitate through the atmosphere or an augmented atmosphere. (d) quenching the polymer precipitate in a bath to form a hollow fiber; and (e) taking up the fiber at a rate of about 90 to 150% of the rate at which it is formed; wherein the fiber has a flux of at least about 5 x 10-5 mL/min/cm2/mmHg and rewets by maintaining said flux for at least five use and drying cycles. 46. The process of claim 45 wherein the polysulfone 47. T e process o c a m 45 w ere n t e po ysu one polymer comprises a polyethersulfone polymer. 48. The process of claim 45 wherein the polysulfone polymer comprises a polyarylsulfone polymer. 49. The process of claim 45 wherein the aprotic solvent is selected from the group consisting of DMF, DMA, DMSO, N- me-thylpyrrolidone and mixtures thereof. 50. The process of claim 45 wherein the organic solvent is selected from the group consisting of dimethylformamide (DMF) , dimethylacetamide (DMA), isopropanol and mixtures thereof. 51. The process of claim 45 wherein the precipitating solution comprises 40 wt.% dimethylacetamide, 40 wt.% isopropanol, and 20 wt.% water. 52. The process of claim 45 wherein the precipitating solution comprises 40 wt.% dimethylformamide, 40 wt.% isopropanol, and 20 wt.% water. 53. The process of claim 45 wherein the precipitating solution comprises from about 90-98 wt.% isopropanol and 10-2 wt. % water. 54. The process of claim 45 wherein the surfactant is selected from the group consisting of a nonionic, anionic, or amphoteric surfactant or a mixture thereof. 55. The process of claim 54 wherein the surfactant is selected from the group consisting of an aromatic hydrophobic based acid ester, an alkoxylated alkylamine, a lauroampho- diacetate/sodium trideceth sulfate, or a mixture thereof. 56. The process of claim 55 wherein the surfactant comprises an an alkoxylated fatty amine. 57. The process of claim 56 wherein the alkoxylated fatty amine surfactant is selected from the group consisting of an alkoxylated cocoamine or an ethoxylated (2-15- EO) cocoamine. 58. The process of claim 45 wherein the surfactant is selected from the group consisting of an aromatic hydrophobic based acid ester, an alkoxylated alkylamine, a lauroampho- diacetate/sodium trideceth sulfate, or a mixture thereof. 59. The process of claim 45 wherein the surfactant comprises an an alkoxylated fatty amine. 60. The process of claim 45 wherein the alkoxylated fatty amine surfactant is selected from the group consisting of an alkoxylated cocoamine or an ethoxylated (2-15 EO) cocoamine. 61. The product made in accordance with the process of Claim 45.

Inventors: ANDRUS ROBERT G, BRINDA PAUL D, COSENTINO LOUIS C, HALL ROBERT T II, WENTHOLD RANDAL M, ANDRUS, ROBERT, G., BRINDA, PAUL, D., COSENTINO, LOUIS, C., HALL, ROBERT, T., II, WENTHOLD, RANDAL, M.
Applicants: MINNTECH CORP, MINNTECH CORPORATION
Application year: 1993
Main classification: B01D71
IPC codes: B01D67, B01D69, B01D71, D01D5, D06M13, D06M101