PROCESS FOR SPINNING HOLLOW FIBER MEMBRANES

Abstract: Process for producing a semipermeable hollow fiber, which comprises extruding a solution of polymer to form a hollow fiber extrudate, while simultaneously injecting a center medium into the bore of the hollow fiber extrudate, and coagulating the so formed extrudate. Gases and/or vapors are used as said center medium, for example air or components thereof. The polymer is preferably chosen among block copolymers comprising at least one hydrophilic polymer block and at least one hydrophobic polymer block. A polyethyleneoxide-polycarbonate block copolymer is particularly preferred.

Description: TECHNICAL FIELD This invention relates to a process for spinning a semipermeable hollow fiber membrane with improved properties. More specifically, the invention is concerned with a process for producing a hollow fiber membrane in that way that there occurs higher hydrophilicity of the hollow fiber inside which leads to higher performance and less fouling. Hollow fiber membranes are today extensively used in a large range of diffusive and/or convective separation processes, and this invention is particularly, though not exclusively, concerned with hollow fiber membranes for use in extracorporeal treatment of blood, such as hemofiltration, hemodialysis etc. BACKGROUND OF THE INVENTION It is a well known technique to produce hollow fiber membranes by coextrusion of a polymer solved in a suitable solvent or solvent system and a fluid that may be inert or may be a coagulation or precipitation fluid. The polymer solution is extruded through a circular orifice. The inert fluid or coagulating or precipitating fluid is extruded out of a bore in the center of the circular slit. This fluid is called center fluid or center medium. It is common practice to let the coextrudate run through atmosphere for short time, ranging from parts of seconds to some seconds and often it is claimed to influence the properties of the resulting hollow fiber membrane by the length of this air gap. Going on it is usual to draw the coextrudate through some fluids that may, but need not be identical with the center fluid. The center fluid and/or the consecutive treatment fluids cause a phase inversion and at last precipitation and solidification of the membrane. This process is decisive of the membrane properties as there are pore structure, pore size, pore size distribution, wettability, burst pressure, permeability and rejection properties. One property of a hollow fiber membrane that is of extreme importance especially for use of hollow fiber membranes in medical applications is the surface energy. The surface energy of a polymer depends on the nature of the groups that are present on the surface. In the case of blood contact it is well known that a high surface energy surface tends less to adsorb proteins or cellular components that may cause coagulation than surfaces with a low surface energy. The surface energy corresponds with the wettability. The higher the surface energy of a surface, the better is the wettability. Usually water is used as wetting agent. The lower the contact angle with water, the higher is the wettability and the surface energy. Highly wettable surfaces of membranes also lead to high diffusive permeabilities and therefore to higher clearances in dialysis processes. The object of this invention is therefore to provide a process for producing a hollow fiber membrane with an improved wettability of the fiber inside. It is a common technique to collect the hollow fiber membranes to bundles and to pot it into suitable housings. For potting with the mainly used potting material, two component polyurethane, it is essential to bring the hollow fiber membranes into a dry state. Therefore the hollow fiber membranes have to be stabilized against irreversible shrinkage and loss of filtration properties during drying. This commonly is done with glycerol especially with membranes for medical use as disclosed in for example European Patent Publication No 0 046 817. The common technique requires a lot of discontinuous production steps to come from the precipitated hollow fiber membrane to a bundle ready to be potted. The hollow fiber membranes have to be collected, made into bundles, the center fluid has to be removed, the hollow fiber membranes must be rinsed, treated with a stabilizing agent against shrinkage and loss of properties during drying, the excess stabilizing agent has to be removed and the bundle of hollow fiber membranes has to be dried to be ready for potting. It is obviously of high interest to find a production route that avoids expensive rinsing procedures and reduces discontinuous production steps. Another object of this invention is therefore to provide a process for producing a dry hollow fiber membrane bundle ready for potting on line and being made of a biocompatible material such as those described in DE-A-29 21 138, which discloses a process according to the preamble of claim 1. As regards the use of gases and/or vapors as center medium reference is being made to FR-A-2 435 276, EP-A-l00 285 and EP-A-77 098. DESCRIPTION OF THE INVENTION According to the invention is thus provided a process for producing a homogeneous semipermeable hollow fiber membrane with an inner fiber surface having improved wettability characteristics comprising extruding a solution of a block copolymer including at least one hydrophilic polymer and at least one hydrophobic polymer so as to produce an extrudate of said block copolymer in the form of a hollow fiber having an inner surface, simultaneously injecting a center medium into the center bore of said hollow fiber extrudate and coagulating said hollow fiber extrudate of said block copolymer, so as to form a solidified homogeneous membrane of said block copolymer by extruding said hollow fiber extrudate directly into a fluid capable of causing the coagulation of said hollow fiber extrudate, characterized in that said center medium comprises a non-polar center medium selected from the group consisting of gases, vapors and mixtures thereof, whereby said center medium causes an increase in the wettability of said inner surface of said hollow fiber. The polymer is chosen among block copolymers comprising at least one hydrophilic polymer block and at least one hydrophobic polymer block. Said block copolymers may be binary, tertiary or quartenary block copolymers. For example, said block copolymer may be chosen from a series of block copolymers with the schematic structure where A is a block of variable length of a hydrophilic or water soluble polymer and B is a block of variable length of a hydrophobic polymer, and where n is an integer from 1 to 1000. Suitable block copolymers may also be chosen from a series of block copolymers with the schematic structure in which A, B, C, D may, but need not, be identical, but at least one of which is a block of variable length of a hydrophilic or water soluble polymer and at least one of which is a block of variable length of a hydrophobic polymer, and in which n is an integer from 1 to 1000. Examples of said hydrophobic polymer block are polyvinylchloride, polyacrylonitrile, polyethylene, polypropylene, polytetrafluorethylene, polyvinylidenefluoride, polymethylmethacrylate, polycarbonate, polystyrene, polyphenylene, polysulfone, polyimide, polyetherimide and polysiloxane. Examples of said hydrophilic polymer block are polyvinylpyrrolidone, polyhydroxymethylmethacrylate, polymethacrylic acid, polyethyleneoxide, polyvinylalcohol, polysaccharide and polyimine. A copolymer of special interest is polyethyleneoxide-polycarbonate-copolymer. This material is useful for making membranes thereof for the use in diffusive and filtrative operation processes. Polymers that contain polar and apolar groups show a dependence of the surface energy on the process of their formation. For example it is reported by Mayrhofer and Sell (Angew. makromol. Chem. 20 (1971) 153) that different types of polyesters solidified against mica show contact angles with waters that are 8 to 16 degrees lower than with the similar polyester solidified against air. Generally there results a minimization of the interfacial energy. Therefore it was very surprising to find an increased wettability of the inner surface of a hollow fiber membrane made from polyethyleneoxide-polycarbonate-copolymer when, according to the invention, instead of the polar center fluid water the unpolar nitrogen gas was used. The process according to the invention also can be done with other gases or vapors of low boiling liquids or mixtures of different gases or mixtures of different vapors or mixtures of gases and vapors. The structure of the membrane was homogeneous without a skin on inside or outside. No change in structure and diffusive and filtrative properties could be seen with a hollow fiber membrane produced according to the invention with a constant wall thickness, when the diameter of the circular slit of the orifice was changed, or when the spinning speed was changed, or when the circular stretch of the membrane in the state of precipitation was changed, or when the temperature of the first coagulation bath was varied between 0 and 45°C. Besides the highly advantageous improved wettability, the process is characterized by an improved technique of preparation of a dry bundle of hollow fiber membranes ready to potting. It was found that the speed and the rate of the removal of solvent was increased. The uptake of stabilizing agent could be done on line and the drying too, without loss of filtrative and diffusive properties. The invention will be further illustrated by the following examples: Example 1 A solution of 14% by weight polyethyleneoxide-polycarbonate-copolymer with a ratio polyethyleneoxide: Polycarbonate of 20:80 and 86% by weight 1.3-Dioxolane with a viscosity of 19757 cP was prepared. The solution was extruded through a circular slit orifice. The center bore of the orifice was connected to a nitrogen pressure reducer with a constant pressure of 65 mm Hg. The orifice was immersed into water of 40°C. The depth was 770 mm. The fiber was drawn through some bathes to remove the dioxolane. The resultant hollow fiber membrane had an inner diameter of 250 µm and a constant wall thickness of 16 µm. After additional washing with distilled water for 16 hours, the advancing contact angle ϑ a of the inner surface against water was measured. This was done by observation of a very slowly moving air bubble in the water-filled, in water immersed hollow fiber membrane at 20°C. For comparison the advancing contact angles ϑ a of water against the inner surface of a conventional with water as center medium spun polyethyleneoxide-polycarbonate-copolymer hollow fiber membrane and of a cuprophane hollow fiber membrane also was measured. The following results were obtained: Example 2 Hollow fiber membranes were spun as described in example 1 and collected to bundles. The bundles were treated in a solution containing 40% by weight of glycerol and 60% of weight of water, dried and potted with PUR in dialyzer housings. The resulting polyethyleneoxide-polycarbonate-hollow fiber dialzyers had the properties listed in table 2 below: Example 3 Hollow fiber membranes were spun as described in example 1. Before the fibers were collected to bundles, they ran through a bath with a stabilizing fluid according to example 2. The treatment time was 12 sec at 22°C. The bundles were dried and potted with PUR into dialyzer housings. The resulting polyethyleneoxide-polycarbonate-hollow fiber dialyzers had the properties listed in table 3 below: Example 4 Hollow fiber membranes were spun according to example 3. Before collecting the hollow fiber membrane into bundles, the hollow fibers were dried in an air channel at 40°C. The drying time was 23 sec. The bundles were potted with PUR in dialyzer housings. The resulting polyethyleneoxide-polycarbonate dialyzers had the properties listed in table 4 below: Hollow fiber membranes out of a bundle were rinsed for 16 hours with distilled water and the advancing contact angle against water in the fibre inside was measured according to the method of example 1. The result was: ϑ a = 28.9 ± 3.1 (deg). INDUSTRIAL APPLICABILITY The process according to the invention may be used to produce a dry semipermeable hollow fiber membrane with improved wettability of the inside for any diffusive and/or convective separation process in an on-line process. The present process is, however, particularly, though not exclusively, useful in connection with semipermeable hollow fiber membranes for use in an extracorporeal treatment of blood, such as hemodialysis, hemodiafiltration, hemofiltration etc.

Claims: A process for producing a homogeneous semipermeable hollow fiber membrane with an inner fiber surface having improved wettability characteristics comprising extruding a solution of a block copolymer including at least one hydrophilic polymer and at least one hydrophobic polymer so as to produce an extrudate of said block copolymer in the form of a hollow fiber having an inner surface, simultaneously injecting a center medium into the center bore of said hollow fiber extrudate and coagulating said hollow fiber extrudate of said block copolymer, so as to form a solidified homogeneous membrane of said block copolymer by extruding said hollow fiber extrudate into a fluid capable of causing the coagulation of said hollow fiber extrudate, characterized in that said center medium comprises a non-polar center medium selected from the group consisting of gases, vapors and mixtures thereof, whereby said center medium causes an increase in the wettability of said inner surface of said hollow fiber. Process according to claim 1, characterized in that said center medium is air or components thereof. Process according to claim 2, characterized in that the hollow fiber is extruded directly into a coagulating fluid. Process according to claim 3, characterized in that the coagulated hollow fiber is dried. Process according to any of claims 1-4, characterized in that said hydrophilic polymer block is chosen from polyvinylpyrrolidone, polyhydroxymethyl-methacrylate, polymethacrylic acid, polyethyleneoxide, polyvinylalcohol, polysaccharide and polyimine. Process according to any of claims 1-5, characterized in that said hydrophobic polymer block is chosen from polyvinylchloride, polyacrylonitrile, polyethylene, polypropylene, polytetrafluorethylene, polyvinylidenefluoride, polymethylmethacrylate, polycarbonate, polystyrene, polyphenylene, polysurfone, polyimide, polyetherimide and polysiloxane. Process according to any of claims 1-6, characterized in that said polymer is a polyethyleneoxidepolycarbonate block copolymer.

Inventors: BUCK, REINHOLD JOHANN, GOEHL, HERMANN JOSEPH, HARTTIG, ERICH HERBERT, Göhl, Hermann Joseph
Applicants: GAMBRO DIALYSATOREN GMBH & CO. KG
Application year: 1986
Main classification: D01D5
IPC codes: D01D5, B01D69, D01F6, B01D71