HOLLOW FIBER MEMBRANE INCORPORATING A POLYIMIDE AND PROCESS FOR PREPARING SAME

Abstract: The invention relates to asymmetric hollow fiber separative membranes based on polyimide polymer. The membranes are dry-wet spun into hollow fiber from a 15-25 % solution of a highly polar polyimide polymer using a precipitating solution in the lumen.

Description: HOLLOW FIBER MEMBRANE INCORPORATING A POLYIMIDE AND PROCESS FOR PREPARING SAME Backaround of the Invention 1. Field of the Invention. This invention relates generally to improved asymmetrical, microporous, hollow fibers incorporating a polyimide and to the process of hollow fiber production. In particular, the invention relates to asymmetrical, microporous, hollow fiber membranes with low water or blood leachable impurities. 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. 2. Descrintion 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 where ultra-pure water is needed such as in the electronic and pharmaceutical industries. Each of these applications has various requirements including pore size, strength, biocompatibility, cost and speed of production and reproducibility. Given the varying uses to which this fiber may be applied, it is highly desirable that the hollow fiber membrane have as little leachable impurities as possible in water, blood, from 0% to saturated solutions of NacL in water, and other similar type of aqueous solutions. It is also typically desired that the membranes be easily or immediately wettable by water, blood and other types of aqueous solutions without the need for costly polymer additives, post fiber-formation treatments with wetting agents or both. Related art hollow fibers for the aforementioned 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 ultrathin layer contiguous with a more porous substrate are needed to provide the necessary mechanical strength. Related art hollow fiber membranes have also been prepared from hydrophobic polymers such as polysulfones, aromatic polyamides and polyimides, and polyamide-imides. However, the hydrophobic nature of these polymer presents difficulties with wetting these membranes when used in aqueous systems. Therefore, hydrophilic polymers such as polyvinyl alcohol, polyvinyl acetate co-polymers, polyvinylpyrrolidone and polyvinylpyrrolidine have typically been incorporated directly into the fibers to achieve a hydrophilic fiber that wets easily. Alternatively, polyethylene glycol, glycerol and/or a variety of surfactants have been incorporated directly into the fibers or used post-fiber formation to achieve wettability. For example, U.S. Patent No. 4,906,375 to Heilman, 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 to achieve wettability. The two polymer solutions are simultaneously passing through a hollow internal core with 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 have limited useful applications. U.S. Patent No. 4,432,875 to Wrasidlo et al., discloses reverse osmosis fiber membranes made from specific polyimide structures. Baked onto the membrane is a polymeric, high molecular weight surfactant. The polymeric surfactant apparently takes the place of the hydrophilic polymer Heilmann reference and is used to increase the wettability of the resultant fiber membrane. The fiber produced using the Wrasidlo process, however, is limited to sheet membranes that have a porosity significantly different than microporous 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. U.S. Pat. No. 3,719,640 to Le et al., discloses linear polymers of polyamide-imides having a specific formulation containing a quaternizable nitrogen atom. When nitrogen is quaternized, the polymer becomes hygroscopic and may be used as separatory membranes in such processes as desalination. U.S. Pat. No. 4,900,449 to Kraus et al., discloses the use of polyimide polymers for pleated flat sheet type membranes. The membranes and process described are limited in use to flat sheet membranes for water filtration applications. Such membranes have less than one-half the surface area available for filtration as the filter membranes of the present invention. While the related 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. None of the aforementioned references teach an asymmetric, microporous, hollow fiber membrane which is equally suitable in processes such as hemofiltration, plasma fitration, hemodialysis and water purification and which does not use some type of polymer "additive" to render the resultant membrane hydrophilic. A hollow fiber membrane that could be applied across a large range of applications would provide a decided advantage over related art fiber membranes. Additionally, a new and useful process is needed that will ensure that a uniformly porous, cost-performance effective hollow fiber is available to large and small companies alike. Specifically, a new and useful membrane and process is needed that is chemically inert to blood and water solutions, or both, within the normal blood pH range of 7.35-7.45 and also be rewettable after repeated sterilizations. It is also desirable that leachable additives such as surfactants and/or hydrophilic polymers are completely absent from the resultant fiber because residual toxic substances are a major concern when hollow fiber membranes are used in medical applications or applications involving the semiconductor industry. Such a hollow fiber membrane would provide a significant advantage over related art membranes. Summary of the Invention It is an object of the hollow fiber membrane incorporating a polyimide 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, immediately wettable fiber without the use of PVP or surfactants, which maintains its rewettability after numerous steam and/or chemical sterilizations and which has a broad range of applications. The process of manufacturing a microporous hollow fiber membrane in accordance with the present invention enables the use of a unique hollow fiber membrane that, as will be shown, is chemically inert to aqueous solutions and/or blood, is rewettable after repeated steam and/or chemical sterilizations of at least 6-7 times, has superior clearance, sieving, and water permeability characteristics and is usable over a wide range of applications. The hollow fiber membrane of the present invention includes about 15-25 wt.% of a fiber forming polymer selected from the group of polyimides and is characterized by the absence of polymer additives which increase wetability, wherein the hollow fiber membrane can be made so that is has a pore size range such that it rejects 100% of molecules (sieving coefficient of 0.0) having a molecular weight greater than about 65,000 daltons and rejects 0.0% of molecules (sieving coefficient of 1.0) having a molecular weight of about 6,000 daltons and less, and rejects from about 35% to 0.0% of molecules (sieving coefficient of 0.65 to 1.0) having a molecular weight of 17,000 daltons; and wherein at a blood flow rate of 300 mL/min and 1.35m2 of active surface area, the fiber has clearance rates of 225-270 for urea, 200-250 for creatinine, 170-225 for phosphate, and 125-150 for Vitamin B12 and wherein the fiber has high sieving coefficients of 0.0 for albumin, 0.65-1.0 for myoglobin, and 1.0 for inulin. In addition, the invention includes a method of manufacturing the fibers. This process includes the steps of (a) dissolving the undegraded polyimide in the appropriate solvent system (b) forming an annular liquid by passing the polymeric solution comprising about 15-25 wt.% of a highly polar polyimide dissolved in an organic solvent and having a viscosity of about 1500-5000 cps through an outer annular orifice of a tube-in-orifice spinneret, (c) passing a precipitating solution comprising about 65-99 wt.% of an organic solvent and about 35-1 wt.% of water into the center of the annular liquid through the inner tube of the spinneret, (d) passing the polymer precipitate through the atmosphere or an augmented atmosphere, (e) quenching the polymer precipitate in a bath to form a hollow fiber; and (f) taking up the fiber at a rate of about 40-70 m/min. The most significant advantage of the present invention is that the hollow fiber membranes so formed immediately wet with aqueous solutions without the use of PVP, glycerine, or other additives. This results in an economical fiber with a homogeneous sponge structure that requires no further mechanical, chemical or other treatment to establish aqueous solution wettability. 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 Drawinas 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 dryjet 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; Figure 4A is an enlarged, microscopic, cross-sectional view of the hollow fiber membrane in accordance with the present invention illustrating the "homogeneous sponge-like" structure; Figure 4B is a greatly enlarged view thereof taken from the area enclosed by box 4B in Figure 4A; Figure 5 is an enlarged detailed view of the hollow fiber membrane in accordance with the present invention illustrating the homogenous sponge-like structure taken at a 450 angle of cross-section; ; Figure 6A is an enlarged, microscopic cross-sectional view of prior art hollow fiber membranes illustrating "voids"; Figure 6B is a greatly enlarged detail view thereof taken from the area enclosed by box 6B in Figure 6A; Figure 7 is an enlarged, microscopic cross-sectional view of hollow fiber membranes with voids. Detailed Descrlption of the Drawinas The process of the invention may be generally determined in view of Figures 1-3. A polymeric dope solution 12 comprising a polysulfone polymer and a polyvinyl pyrrolidone 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. This diluent solution is also delivered to the spinneret apparatus 20 by means of pump 26. The dope solution 12 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 wheel 40 which is immersed in a second, rinsing bath 42. Again, a continuous flow of water 44 is supplied to the 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 wheel 40 and further processed. An example of further processing includes cutting the fibers 28 to a uniform length, bundling them and drying them in any conventional manner. A detail of a spinneret head 102 which is a part of the dry-jet wet spinning spinneret apparatus 20 is illustrated in Figures 2 and 3. The dope solution 12 enters through a dope port 104, is directed to an annular channel 106, and flows out of an annular orifice 108 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. Detailed Description of the Preferred Embodiment The invention is directed to an asymmetrical microporous, hollow fiber membrane that includes a polyimide polymer that is highly polar. The pore size of the membrane and the molecular weight cutoff will vary depending on the application, i.e. water filtration, ultrafiltration, hemofiltration, plasma filtration (plasmapheresis), etc. However, we define microporous to mean membranes having a pore size ranging from about 0.001 m to 0.5 m and more preferably from about .005m to about 0.2 m. We also define "flux" or water permeability" to mean a measure of the volume of water passed by the hollow fiber membrane under pressure for a given time and area. "Rewetting" and similar words such as rewettable, rewettability, etc., as used herein, is a description of the ability of a membrane to maintain a particular level of flux or water permeability after either cycles of wetting and drying the membrane or after steam or chemical sterilization. "Asymmetric" means that the pore size of the fiber varies from smaller to larger from the inner barrier layer to the outer sponge-like layer, respectively. "Uniformly porous" and "sponge-like" means that the porosity of the hollow fiber membrane is homogeneous throughout. In addition, "solvents with respect to the polymer" are typically aprotic solvents while "non-solvents with respect to the polymer" are typically protic solvents. "Anti-solvent" is a nonsolvent with respect to the polymer and is used herein when referring to additional nonsolvents that are added to the polymeric solution. "Nonsolvents," on the other hand, are also nonsolvents with respect to the polymer, but is used herein when referring to nonsolvents added to the precipitating solution. The membrane is particularly well suited for medical applications where the membrane will come into contact with blood because it is biocompatible, does not activate complement, and has the remarkable ability to exhibit high sieving coefficients for middle molecules such as B2 microglobulins and myoglobins. When used as a membrane for dialysis applications, the membrane has a pore size ranging from substantially about 0.001 m to substantially about 0.01m with the average pore size being from substantially about 0.003m to substantially about 0.005 m. Also surprisingly, the membrane is equally suited in all filtration applications for its unique ability to completely remove existing endotoxin from the solution being filtered. When this unique membrane is used as a water filter, the pore size preferably ranges from about 0.005m to about 0.5 m with an average pore size of from about 0.05m to about 0.1 m. When used as a plasma filtration membrane the maximum pore size ranges from substantially about 0.1 m to substantially about 0.2 m The highly polar polymer in accordance with the present invention is preferably an aromatic polyimide that when precipitated as a membrane is immediately wettable without the use of polymer additives or surfactants. The preferred polyimide in accordance with the present invention is disclosed in U.S. Pat. No. 3,708,458 to Alberino which is hereby incorporated by reference. The polyimide is prepared from benzophenone-3,3',4,4' - tetracarboxylic acid dianhydride and a mixture of 4,4'-methylenebis(phenyl isocyanate) and toluene diisocyanate (2,4- or 2,6-isomer) of mixtures thereof. The polyimide includes the recurring group: EMI11.1 wherein 10% to 90% of the R groups are EMI12.1 and the remaining R groups include either EMI12.2 The aromatic iso- and diisocyanates may be substituted by their amine analogs. The CAS Registry No. of the preferred polyimide is 58698-66-1. The polyimide is available from Lenzing Corp. (Austria) under the P-84 and/or HP P-84 (high purity) marks. In an alternative embodiment, a polymer based on the phenyl-indane diamine; 5(6)-amino-1-(4'-aminophenyl)-1,3trimethylindane with a CAS Registry No. of 62929-02-6 may be used. The alternative embodiment polymer is available from Ciba-Geigy Corporation (Hawthorne, N.Y.) under the "Matrimid 5218" mark. The structure of the polymer repeating unit is believed to consist of: EMI13.1 The alternative preferred embodiment may be prepared by the methods disclosed in U.S. Pat. No. 3,856,752. The polyimide polymers useful in accordance with the present invention preferably have a molecular weight of about 30,000 to 125,000 daltons. More preferably, the molecular weight is about 35,000 to 115,000 daltons and most preferably, the molecular weight is about 40,000 to 105,000 daltons. As stated previously, no additional additives, such as polyvinylpyrrolidone, polyethylene glycol, glycerine, cellulose or starch derivatives or amphoteric, zwitterionic, nonionic, anionic, or cationic surfactants, are needed to produce a hollow fiber membrane that wets immediately upon contact with blood, water and other aqueous solutions and maintains the rewettability for at least 6-7 sterilizations by steam or chemicals. Because no additional polymers are needed to make the resultant fiber wettable, the choice of solvents for use as the precipitating solution is critical in influencing the hydrophilicity, structure and porosity of the fiber. In addition, the elimination of additives in the polymeric dope solution decreases and virtually eliminates all but trace amounts of solids and/or oxidizable material that is leachable from the resultant fiber. Further, the structural integrity of the resultant hollow fiber membrane is more stable after the removal of the solvent and/or antisolvents and nonsolvents. Initially, the polyimide polymer is dissolved in a solvent. Preferably, this solvent is also miscible with water. A representative, non-limiting list of solvents useful in the invention includes dimethylformamide (DMF), dimethylsulfoxide (DMSO), dimethylacetamide (DMA), n-methylpyrrolidone, and mixtures thereof. Preferably, the solvent is DMF, an aprotic solvent. Depending on the desired properties of the hollow fiber, a small amount of an antisolvent may be added in small quantities to the primary solvent that is used. The addition of an antisolvent in the polymer forming solution will enhance the desired precipitate characteristics of the polymer during fiber formation. For example, adding acetic acid in the amount of 4-7 wt.% ensures that the fiber has a uniform sponge-like structure, free of voids, large vacuous spaces extending from the inner membrane wall to the outer membrane wall that can permit the passage of large molecular weight molecules if the void pierces the inner and/or outer membrane wall. Alternatively, additional amounts of solids may be added to the polymer solution up to 25.0 wt.% to solve this problem. The homogeneous, sponge-like structure may also be achieved in accordance with the process and formulations described herein. Figure 4 depicts a cross section of a hollow fiber membrane in accordance with the present invention magnified 130x taken on a Hitachi S-800 scanning electron microscope. Figure 4B which is a 10X magnification (1300X) of the area enclosed by box 4B in Figure 4A and illustrates the "uniform sponge-like structure" 200 of hollow fiber membranes in accordance with the present invention. Figure 5 is a 10,000X view taken at a 450 angle of cross-section of hollow fibers in accordance with the present invention showing the outer membrane wall 210 and the sponge-like inner composition 215. "Voids" 220, which characterize many hollow fiber membranes, may be seen by referring to Figures 6A (130X) and 6B (1300x). The absence of voids in the formed hollow fiber membrane results in a mechanically stronger fiber with enhanced diffusion rates. Preferably, about 15-25 wt-%, more preferably, about 16-20 wt-%, and most preferably, about 17-19 wt-% of the fiber forming polyimide polymer is dissolved in the dimethylformamide solvent. When less than 15 wt-% of the polyimide polymer is used, the fibers formed may not be strong enough to withstand the stresses involved in the high speed process in accordance with the method of the present invention. In addition, test data regarding sieving and clearance characteristics are not reproducible because the fibers lack the desirable uniform sponge-like structure. Further, the fibers lack integrity due to the weakness from the voids in the fiber walls. Higher polyimide solids may be employed in organic solvent systems if spinneret housings, feed lines, and polymer solution tanks are heated. Upon heating, the viscosity of the polymer solution is lowered, allowing otherwise unusable polymer solution formulations to be spun. Depending upon the composition of the precipitating solution the skilled practitioner chooses, heating and/or cooling the system may influence the morphology and performance characteristics of the resultant fiber membrane. The polymeric solution has a viscosity of about 15005000 cps, preferably about 2000-4000 cps, and most preferably about 3500-3800 cps at 250C, as measured on a Brookfield (LV) 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 includes a solvent with respect to the polymer and a non-solvent with respect to the polymer or a variety of non-solvents. The composition of the precipitating solution is critical because it affects the porosity, degree of uniform sponge-like structure, clearance, tensile strength, wall thickness, inner and outer diameters and flux properties of the fiber. For example, as the weight percent of the solvent with respect to the polymer increases, fiber formation is impaired and is characterized by a "glassy" weaker structure and it becomes increasingly difficult to "pull" the fiber. Conversely, as the weight percent of the solvent with respect to the polymer decreases and the weight percent of water and/or other non solvents with respect to the polymer increases, voids are seen in the fiber structure which may allow high molecular weight molecules to pass through the fiber if they pierce the outer membrane wall. This may best be seen in Figure 7 which illustrates a fiber cross-section magnified 130X with voids 221 that resulted from using a precipitating solution with an increased weight percent of non-solvent with respect to the polymer. In addition, as the weight percent of water and/or other non-solvents with respect to the polymer increases, a low pore density on the outer fiber wall and a tighter closed inner wall with a low flux is seen. It will therefore be appreciated by those skilled in the art that the selection of the composition of the precipitating solution is crucial. The composition of the precipitating solution effective to produce a hollow fiber membrane for use in hemodialysis, as well as, water filters, autologous blood filters, and plasma filters is illustrated below in Table I. Table I More Most Preferred Preferred Preferr ed Solvent with respect to polymer 50-99 wt.% 60-95 wt.% 75-90 wt. Water 35-1 wt.% 30-5 wt.% 20-10 wt. Add' 1 Non-Solvents with respect to polymer 15-0 wt.% 10-0 wt.% 5-0 wt. The table above is merely offered to guide the practitioner in formulating precipitating solution 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. In addition, depending on which formulation of precipitating solution the practitioner selects, he or she may also vary the percent solids in the polymer solution to obtain a fiber of the desired characteristics. The water which may be used in the precipitating solution may be tap water, deionized water or water which is a product of reverse osmosis. Preferably the water has first been treated by reverse osmosis. As stated previously, the solvent (with respect to the polymer) used in the precipitating solution is dimethylformamide (DMF), dimethylsulfoxide (DMSO), dimethylacetamide (DMA), nmethylpyrrolidone and mixtures thereof. Preferably, the solvent is the same as that used in the polymeric fiber forming solution. More preferably, the solvent is DMA or DMF. Most preferably, the solvent is DMF. Additional combinations of solvents and non-solvents, which may or may not contain salts, may be used so long as they are miscible with dimethylformamide, dimethylsulfoxide, dimethylacetamide, n-methylpyrrolidone and mixtures thereof. A representative, non-limiting list of non-solvents (with respect to the polymer) that may be used in the precipitating solution are acetic acid, isopropanol, water, glycerol, acetic anhydride, and ethanol. The proportions of the water, and other non-solvents (e.g. alcohol) which may make up the precipitating solution influence the morphology, clearance, permeability, and selectivity characteristics of the hollow fiber membrane. In particular, the total absence of a solvent with respect to the polymer in the precipitating solution may result in a small number of pores in the fiber wall as well as lower flux. Further, water is clearly an important ingredient in the precipitating solution used in this membrane formation process. Because the addition of water affects the performance characteristics of the resultant fiber membrane it is generally preferred that the proportion of water in the precipitating solution be about 1-35 wt.%, to ensure proper fiber performance characteristics. Less than about 10 wt.% of water may result in the polymeric solution precipitating too slowly forming a fiber with increased pore size. This is desirable to form a fiber for use in water filters but would not, for example, form a fiber suitable for use as a dialyzer fiber. Conversely, a concentration of water greater than about 35 wt.% results in a fiber with lower pore density on the outside and a tighter closed inner wall with a general decrease in flux. However, when the proportion of water falls within 1-35 wt.%, we see enhanced uniformity in the desirable sponge-like structure and the hollow fiber membrane is characterized by the complete absence of voids. This uniformity results in more overall uniform flux with respect to all types of filters and tighter controls with respect to molecular cutoffs in dialyzer applications. Initially, the highly polar polymer is diluted in DMF. Depending on the desired properties and characteristics of the hollow fiber, a small amount of a non-solvent (with respect to the polymer) (also called anti-solvents) other than water may be added instead of using pure DMF solvent. This may enhance the precipitation of the polymer in the fiber formation. For example, the addition of 4-7 wt.% glacial acetic acid to the polymer/DMF solution enhances the uniform sponge-like structure of the resultant fiber and the fiber is further characterized by the complete absence of voids. The polymeric dope solution is pumped, filtered and directed to the outer, ring orifice of a tube-in-orifice spinneret. At the same time, the precipitating solution 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 solution within the annulus. Preferably, the spinneret head is maintained at a temperature of about 5-850C, more preferably, about 15-250C, and most preferably, about 220C. The polymeric dope is subjected to a pressure of about 01400 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 280 microns). At the same time, precipitating solution is pumped through the tube of the spinneret 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 solution 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 .004 to .005 inches (about 100 to 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 spinneret at a rate of substantially about 1.0-10 mL/min, more preferably, about 2-5 mL/min, most preferably, about 3-4.5 mL/min, and the precipitating solution 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 spinneret is oriented in a manner such that fiber production is driven by fluid flow and by removal from the spinneret by gravity effects. Preferably, the fiber emerges from the spinneret 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 for the polymeric solution and the precipitating solution which interact to precipitate the formed 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 spinneret 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 3:1 to 25:1, and most preferably, the ratio of the annular orifice to tubular orifice cross-sectional area is about 4:1 to 15:1. 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 1.5:1 to 7:1, and most preferably, the ratio of the annular ring thickness to tube inside diameter is about 2:1 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 4:1 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 to 10m, more preferably, about 0.5 to 2.0 m, and most preferably, about 0.5 to 1.5m. This allows the precipitating solution 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 1000C, more preferably, the air is maintained at a temperature of 50C to 500C and most preferably at 150C to 250C. Preferably the air is also maintained at a relative humidity of substantially about 10% to 99%, 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. In an alternative embodiment, the fiber may be dropped directly into the quenching bath. 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 polyimide polymer to form the microporous hollow fiber. After the quenching bath, the fiber may be further rinsed to remove any remaining 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 or, in other words, at approximately about 150-230 ft/min (about 45-70 m/min) 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 takeup guide unaffected by ambient air currents, i.e. there is no "draft." 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. For example, at a 300 mL/min flow rate, a clearance rate of at least about 225 mL/min is possible for urea; at least about 200 mL/min for creatinine; and at least about 125 mL/min for Vitamin B12. The flux rate possible with the fibers of the present invention is preferably greater than 500 mL/hr/mmHg/mê, more preferably is between 500-1000 mL/hr/mmHg/mê, and most preferably is greater than 1000 mL/hr/mmHg/m2. The sieving coefficient for BSA is preferably less than about 0.01, and most preferably is about 0.0. Sieving coefficients for myoglobulin were between about 0.65 and 1.0. Typical clearance rate data for fibers formed in accordance with the present invention are as follows: Flow Rate Urea Creatinine Phosphate B-12 Cyto C 200 mL/m 175-200 165-200 155-195 110-130 125-185 300 mL/m 225-290 200-270 170-250 125-150 140-265 400 mL/m 250-320 215-305 195-280 125-160 150-255 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. Example 1 A polymeric dope solution was formed by dissolving 17.5 wt.% of P-84 in dimethylformamide. The material was filtered and then pumped to a tube-in-orifice- spinneret at a rate of 4.50 mL/min and at a temperature of 240C. Simultaneously, a precipitating solution consisting of 80 wt.% dimethylformamide and 20 wt.% reverse osmosis deionized water was mixed, filtered and delivered to the spinneret at a temperature of 24 C and a rate of 2.75 mL/min. The polymeric dope solution was delivered through the outer, annular orifice of the spinneret, which orifice had an outside dimension of about 0.022 to 0.025 inches (about 560 m) and an inside dimension of about 0.010 inches (about 254 m). The precipitating solution was delivered through a tube orifice within the annular orifice, which tube orifice had an inside diameter of about 0.005 inches (about 127 m). The spinneret head was maintained at 240C. The spinneret discharged the polymeric solution and precipitating solution downward into ambient atmosphere for a distance of about 1.5 meters into a quenching bath maintained at 320C. Formed fiber material was wound on a take-up reel at a rate of 70 m/min. The fiber was then removed from the take-up wheel, cut, bundled, soaked in a water bath at 320C for 10 hours, dried and tested. Test Data &num;1 Fiber membranes prepared by the method recited in Example 1 had sieving coefficients of 0.0 for albumin, .82 for myoglobin and 1.0 for inulin. These fibers had the surprising advantage of having high sieving coefficients for middle molecules (molecular weights of from about 5,000 daltons to 25,000 daltons) such as B2 microglobulins and myoglobins. Flow Rate Urea Creatinine Phosphate B-12 Cyto C 200 mL/m 179.4 164.9 156.5 125.1 129.9 300 mL/m 225.0 198.5 182.6 140.2 143.0 400 mL/m 244.8 212.5 208.7 149.3 146.8 Example 22 The method for preparing fiber as in Example 1 was repeated using a precipitating solution of 81 wt.% DMF and 19 wt.% deionized water. Test Data &num;2 Resultant fiber membranes had sieving coefficients of 0.0 for albumin, 0.79 for myoglobin, and 1.0 for inulin. Flow Rate Urea Creatinine Phosphate B-12 Cyto C 200 mL/m 188.1 178.3 166.7 119.8 156.9 300 mL/m 249.6 223.4 212.5 136.6 178.7 400 mL/m 281.5 246.7 233.5 139.6 184.0 Example 3 The method employed in Example 1 was repeated using 17.0 wt.% of the P-84 polyimide polymer and 83 wt.% DMF. The precipitating solution comprised 81 wt.% DMF and 19.0 wt.% deionized water. Sieving coefficients were similar to the Test Data obtained for Examples 1 and 2 above for albumin and inulin with a sieving coefficient of 0.77 for myoglobulin. Test Data &num;3 Blood F1. Urea Creatinine Phosphate B12 Cytochrome C 200 mL/m 190.7 178.4 166.7 124.8 162.9 300 mL/m 255.2 232.45 228.0 141.5 185.7 400 mL/m 287.3 256.9 240.0 145.3 188.8 Example 4 Fibers for use in a water filter were manufactured in the following manner. A polymeric dope solution was formed by dissolving 19.0 wt.% of Matrimid 5218 in 81.0 wt.% DMF. The material was filtered and then pumped to a tube-in-orifice spinneret at a rate of 2.9 mL/min at a temperature of 23 C. Simultaneously, a precipitating solution consisting of 85.5 wt.% DMF and 14.5 wt.% water was mixed, filtered and delivered to the spinneret at a temperature of 230C and a rate of 3.0 mL/min. The polymeric dope solution was delivered through the outer, annular orifice of the spinneret having an outside diameter of 940 m and an inside diameter of 254 m. The precipitating solution was delivered through a tube orifice within the annular orifice having an inside diameter of about 127 m. The spinneret head was maintained at about 230C. The spinneret discharged the column of polymeric solution and precipitating solution downward for a distance of about 0.81 m into a quenching water bath maintained at a temperature of 350C. The fiber was wound on a take-up reel at a rate of about 45m/min. Cut bundles were soaked in a 46 C water bath for 16 hours. Fiber bundles were dried and tested. Based on a 0.05 m2 test mat, at 5 psi, water permeability was calculated to be 500 mL/hr/mmHg/mê. Example 5 Fibers for use in a plasma filter were manufactured in the following manner. The method for preparing fiber as in Example 25 was repeated using a polymeric dope solution consisting of 16.75% P-84 polymer and 83.25 wt.% DMF. The precipitating solution included 85.5 wt.% DMF and 14.5 wt.% deionized water. Fibers had a sieving coefficient of 0.65 using a 0.1% solution of fluorescein isothiocyanate dextran (Sigma), a molecular weight marker of approximately 500,000 Daltons. Water permeability was in excess of 900 mL/hr/mmHg/m2. Example 6 Fibers for use in a water filter were manufactured in the following manner. A polymeric dope solution was formed by dissolving 16.75 wt.% P-84 polymer in 83.25 wt.% DMF. The material was filtered and then pumped to a tube-in -orifice spinneret at a rate of 4.5 mL/min at a temperature of 230C. Simultaneously, a precipitating solution consisting of 85.5 wt.% DMF and 14.5 wt.% water was mixed, filtered and delivered to the spinneret at a temperature of 230C and a rate of 3.0 mL/min. Fibers were further processed in accordance with the method of Example 4. A water filter (1.5 m2 of fiber) containing the fibers manufactured using the above formulation was tested for water permeability. At 8.6 psi, filters had a water permeability of 1020 mL/hr/mmHg/mê. At 10.0 psi, filters had a water permeability of 1320 mL/hr/mmHg/m2. Example 7 Fibers for use in water filters were prepared in the following manner. A polymeric dope solution was formed by dissolving 15.2 wt.% P-84 polyimide polymer in 79.80 wt.% DMF and 5.0 wt.% glacial acetic acid. The material was filtered and pumped to a tube-in-orifice spinneret at a rate of 4.1 mL/min. A precipitating solution comprised of 50 wt.% DMF and 50 wt.% glacial acetic acid was mixed, filtered and delivered to the spinneret at a rate of 4.5 mL/min. The polymeric dope solution was delivered through the outer, annular orifice of the spinneret having an outside dimension of about 0.029 inches (737 m) and an inside dimension of about 0.01 inches (about 254 m). The precipitating solution was delivered through a tube orifice within the annular orifice having an inside diameter of about 0.005 inches (about 127 m). Precipitated fiber was quenched in a reverse osmosis water bath and taken up at a rate of 49 m/min. Water Permeability All fibers produced in the Examples 1-7 above were evaluated for water permeability (flux) in the following manner. Water was passed through the lumens of potted test fibers with the filtering unit in a horizontal position. The ultrafiltrate port on the inlet side of the unit was plugged. Pressure monitors were placed at all inlet and outlet ports. With flow through the unit, backpressure was applied to the fiber outlet side of the unit to increase ultrafiltrate flow across the fibers. Three data points were taken at 10%, 50%, and 80-100% ultrafiltrate flow and transmembrane pressure (TMP) was calculated. Ultrafiltrate flow was plotted against TMP and the slope of this curve was used to determine flux or water permeability. As noted above, all of the above fibers for use as water filters, hemofilters and dialyzers had water permeabilities in excess of 500 mL/hr/mmHg/mê. Endotoxin Tests Filtering units prepared substantially in accordance with Examples 1-7 were tested with two liters of a bicarbonate solution containing a 15EU/ml endotoxin challenge at high flow rates. No endotoxin was passed even after repeated recirculations. The endotoxin solution was prepared by adding 0.25 ml of endotoxin (Control standard endotoxin, lot &num;47, 25 mcg/ml endotoxin, available from Associates of Cape Cod, MA) to a bicarbonate solution. The bicarbonate solution was made from an in-house preparation of bicarbonate concentrate powder by mixing the powder with sufficient reverse osmosis water to make 2 1/2 gallons. The limulus amebocyte lysate used for the assay had a sensitivity of 0.06 EU/ml. The bicarbonate solution tested negative for endotoxin. The solution with the added endotoxin tested positive at the ninth two-fold dilution tube (256X) giving an endotoxin concentration between 15.4 and 30.7. Test solution was recirculated from a two liter flask. The test solution was pumped through the filtering unit by a Sarns portable pump code 5M6002 serial &num;3397. Test Data &num;4 Endotoxin testing: Time Endotoxin Levels Observed After Filterina Before Filterina 1 minute none ( < 0.06 EUml) 15.4 5 minutes none 0.96 EU/ml 30 minutes none none 60 minutes none none Test Data &num;5 Endotoxin testing: Time Endotoxin Levels Observed After Filterina Before Filterina 1 minute none ( < 0.06 EU/ml) 5 minutes none 0.49 EU/ml 30 minutes none none 60 minutes none none 120 minutes none none Although the description of the preferred embodiment and best mode 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. An asymmetric, microporous, hollow fiber membrane comprising a polyimide polymer. 2. The hollow fiber membrane of Claim 1 wherein said fiber comprises 100 wt.% of said polyimide polymer. 3. The hollow fiber membrane of Claim 1 wherein said polyimide polymer comprises a polymer having the structure EMI33.1 wherein 10% to 90% of the R groups are EMI33.2 and the remaining R groups are EMI33.3 4. The hollow fiber membrane of Claim 3 wherein said fiber comprises from about 100 wt.% of said polyimide polymer. 5. The hollow fiber membrane of Claim 1 wherein said polyimide polymer comprises a polymer having the structure: EMI34.1 6. The hollow fiber membrane of Claim 5 wherein said polyimide polymer comprises from about 100 wt.% of said polyimide polymer. 7. The hollow fiber membrane of Claim 2 wherein said polyimide polymer comprises a polymer having the structure: EMI34.2 wherein 10% to 90% of the R groups are EMI34.3 and the remaining R groups are EMI35.1 8. The hollow fiber membrane of Claim 2 wherein said polyimide polymer comprises a polymer having the structure: EMI35.2 9. The hollow fiber membrane of Claim 1 wherein said membrane has a water permeability greater than 500 mL/hr/mmHg/mê. 10. The hollow fiber membrane of Claim 1 wherein said polyimide polymer has a molecular weight of about 40,000 daltons to about 105,000 daltons. 11. The hollow fiber membrane of Claim 1 wherein at a blood flow rate of 200mL/min, said membrane has a high rate of water permeability of about 17 mL/min/mmHg/mê; a high clearance rate of 175-190 mL/min for urea, 150-180 mL/min for creatinine and phosphorous, about 88 mL/min for Vitamin B12 and about 125165 mL/min for Cytochrome C for fibers having 1.25 m2 active surface area; and high sieving coefficients of 0.0 for albumin, 1.0 for myoglobin and 1.0 for inulin. 12. The hollow fiber membrane of Claim 1 wherein said membrane is uniformly porous throughout and wherein said membrane is characterized by the absence of voids. 13. The hollow fiber membrane of Claim 1 wherein said fiber is capable of being subjected to take-up rates in excess of 45 m/min. 14. The hollow fiber membrane of Claim 1 wherein said membrane rewets by maintaining a flux of at least 500 mL/hr/mmHg/m2 for about six sterilizations and wherein said fiber is further characterized by having 0.0% wetting agent additives selected from the group of hydrophilic polymers, nonionic, anionic, or amphoteric surfactants. 15. The hollow fiber membrane of Claim 14 wherein said hydrophilic polymers comprise polyvinylpyrrolidone. 16. The hollow fiber membrane of Claim 1 wherein said fiber membrane has a water permeability of at least 500 mL/hr/hhHg/mê. 17. A process for the manufacture of an asymmetric, microporous hollow fiber membrane comprising the steps of: (a) passing, through an outer annular orifice of a tube-in-orifice spinneret, a polymeric solution comprising about 15-25 wt.% of a highly polar polyimide polymer dissolved in a solvent with respect to the polymer, said solution having a viscosity of about 1500-5000 cps, 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 about (i) 99-65 wt.% of a solvent with respect to the polymer, and (ii) 1-35 wt.% of a non-solvent with respect to the polymer, 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 for about 0.5-2.0 meters; ; (d) quenching the polymer precipitate in a bath maintained at about 15-45 C to form a hollow fiber; and (e) taking up the fiber at a rate of about 40-70 meters/min. 18. The process of Claim 17 wherein said organic solvent is selected from the group consisting of dimethylformamide, dimethyylsulfoxide, dimethylacetamide, n-methylpyrrolidone and mixtures thereof. 19. The process of Claim 17 wherein said solvent with respect to the polymer comprises dimethylformamide. 20. The process of Claim 18 wherein said solvent comprises dimethylformamide. 21. The process of Claim 17 wherein said precipitating solution comprises: (i) 75-90 wt.% of a solvent with respect to the polymer; (ii) 20-10 wt.% water; and (iii) 5-0 wt.% additional non-solvent with respect to the polymer. 22. The process of Claim 21 wherein said solvent comprises dimethylformamide. 23. The process of Claim 21 wherein said non-solvent comprises isopropanol. 24. The process of Claim 17 wherein said polyimide polymer comprises a polymer having the structure: EMI38.1 wherein 10% to 90% of the R groups are EMI38.2 and the remaining R groups are EMI38.3 25. The process of Claim 17 wherein said polyimide polymer comprises a polymer having the structure: EMI39.1 26. The process of Claim 17 wherein said precipitating solution comprises about 81 wt.% dimethylformamide and 19 wt.% water. 27. The process of Claim 17 wherein said precipitating solution comprises about 85 wt.% dimethylformamide and 15 wt.% water. 28. The process of Claim 17 wherein said precipitating solution comprises about 85.5 wt.% dimethylformamide and 14.5 wt.% water. 29. The process of Claim 17 wherein said precipitating solution comprises 50 wt.% dimethylformamide and 50 wt.% water. 30. The process of Claim 17 wherein said polymeric solution comprises about 15 wt.% of a polyimide polymer, about 80 wt.% dimethylformamide, and 5.0 wt.% glacial acetic acid. 31. The product prepared in accordance with the process of Claim 17. 32. A process for the manufacture of an asymmetric, microporous hollow fiber membrane comprising the steps of: (a) passing, through an outer annular orifice of a tube-in-orifice spinneret, a polymeric solution comprising about 16-18 wt.% of a highly polar polyimide polymer dissolved in a solvent with respect to said polymer, said solution having a viscosity of about 1500-5000 cps, 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 about (i) about 80 wt.% of a solvent with respect to said polymer, and (ii) about 20 wt.% water, 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 for about 0.5-2.0 meters; ; (d) quenching the polymer precipitate in a bath maintained at about 15-4 50C to form a hollow fiber; and (e) taking up the fiber at a rate of about 40-70 meters/min; wherein said fiber membrane is uniformly porous, characterized by the absence of voids, and has a water permeability of at least 500 mL/hr/mmHg/mê 33. The process of Claim 32 wherein said polyimide polymer comprises a polymer having the structure: EMI41.1 wherein 10% to 90% of the R groups are EMI41.2 and the remaining R groups are EMI41.3 34. The process of Claim 32 wherein said polyimide polymer comprises a polymer having the structure: EMI42.1 35. The product prepared in accordance with the process of Claim 32.

Inventors: ANDRUS ROBERT G, COSENTINO LOUIS C, HALL ROBERT T II, WENTHOLD RANDAL M, ANDRUS, ROBERT, G., COSENTINO, LOUIS, C., HALL, ROBERT, T., II, WENTHOLD, RANDAL, M.
Applicants: MINNTECH CORP, MINNTECH CORPORATION
Application year: 1993
Main classification: B01D71
IPC codes: A61M1, B01D61, B01D69, B01D71, C08G73, D01D5, D01F6