Process for Preparing Isotropic Microporous Polysulfone Membranes
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Europa,schesP_ MM M M M MM Ml II II II II II I II J European Patent Office i-oo n * © Publication number: 0 362 588 B1 Office europeen* des.. brevets , © EUROPEAN PATENT SPECIFICATION © Date of publication of patent specification: 09.11.94 © Int. CI.5: B01 D 69/08, B01D 69/06, B01D 67/00, B01D 71/68 © Application number: 89117000.3 @ Date of filing: 14.09.89 © Process for preparing isotropic microporous polysulfone membranes. ® Priority: 23.09.88 US 248880 hollow fibers. II. Morphology" @ Date of publication of application: © Proprietor: W.R. GRACE & CO. 11.04.90 Bulletin 90/15 Grace Plaza 1114 Avenue of the Americas © Publication of the grant of the patent: New York New York 10036 (US) 09.11.94 Bulletin 94/45 @ Inventor: Ly, Ann L. © Designated Contracting States: 14 Sheridan Road DE FR GB IT Bedford MA 01730 (US) Inventor: Chu, Chaokang © References cited: 6 Wildwood Road EP-A- 0 086 365 Lexington MA 02173 (US) EP-A- 0 168 783 Inventor: Nguyen, Thanh D. DE-A- 3 342 824 28 Manning Street US-A- 4 051 300 Billerica MA 01821 (US) US-A- 4 612 119 INDUSTRIAL & ENGINEERING CHEMISTRY © Representative: UEXKULL & STOLBERG Paten- RESEARCH, vol. 26, no. 11, November 1987, tanwalte pages 2385-2389, American Chemical Soci- Beselerstrasse 4 ety,Washington, DC, US; L.Y. LAFRENIERE et D-22607 Hamburg (DE) al.:"Effect of polyvinylpyrrolidone additive on 00 the performance of polyethersulfone ultrafil- 00 tration membranes" 00 m Journal of applied polymer science, Vol.21, CM 1977, pp.1 65-80; Cabasso et al., "Polysulfone CO 00 Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid (Art. 99(1) European patent convention). Rank Xerox (UK) Business Services (3. 10/3.09/3.3.3) EP 0 362 588 B1 Description BACKGROUND OF THE INVENTION 5 This invention relates to the preparation of microporous polysulfone membranes. More specifically, the membranes prepared according to this invention are isotropic, or homogeneous, in that the porosity of the membrane is uniform over the cross section of the membrane. In addition, there is no skin present on either the inner or outer surface of the membranes of this invention. These characteristics are achieved by using a novel combination of casting solution, precipitation solution and spinning conditions. Liquid-liquid membrane io precipitation or coagulation is employed. Both hollow fiber and flat sheet membranes can be prepared according to this invention. Polysulfone membranes and hollow fiber membranes are known to the art and have been prepared to a variety of specifications. However, prior art hollow fibers typically are asymmetrical, or anisotropic. This type of fiber comprises a "skin" at the inner and/or outer surface and a microporous understructure. For 75 example, US-A-3,691 ,068 (Cross) discloses an anisotropic microporous polysulfone polymer membrane having a barrier layer at a surface thereof and a more porous support layer integral with the barrier layer. Similarly, US-A-4,029,582 (Ishii et al.) discloses poly(arylether-sulfone) semipermeable membranes having a thin dense layer and a porous supporting layer. Skinless polysulfone hollow fibers are known to the art. US-A-4,612,1 19 (Eguchi) discloses preparation 20 of a polysulfone hollow fiber filter medium having substantially no skin layer in the outer and inner surface areas. Eguchi uses a dry-wet spinning process, with carefully controlled temperatures. Cabasso et al., "Polysulfone Hollow Fibers II. Morphology," J. Applied Polymer Science, Vol. 21, pp. 165-80 (1977), which also utilizes a dry-wet spinning method, reports that control of the extrusion/coagulation procedure allows the formulation of skinned, porous skinned and non-skinned fibers. In a dry-wet spinning process, the 25 membrane casting solution is extruded into an air space and then is conducted into a liquid precipitation medium. DE-A-33 42 824 discloses a process for the preparation of hollow fiber polysulfone filter membranes and explicitly teaches a passage of the extruded material through air as an important feature resulting in an considerable improvement. It is apparent that this process uses an air space for production of a symmetric 30 and asymmetric cell structures. In all examples, the membrane was kept in air for 30 seconds to one minute. Additionally, temperature control is required in case of production of filter elements having symmetric cell structure and pore size. Furthermore, only a 1 or 2-component casting solution and a 1 or 2- component coagulating liquid are used. US-A-4 051 300 discloses a method of producing microporous hollow fibers by forming a solution of 35 fiber forming polymer in a suitable solvent adding to the solution a second polymer soluble in the solvent but with limited compatibility with the first polymer when the total concentration increases on coagulation, extruding the resulting solution through an orifice equipped for coaxial extrusion so that coagulation fluid within tube flow results, precipitating with liquid which is miscible with the solvent for the fiber forming material, is a non-solvent for the first polymer and a solvent for the second, contacting the extruded solution 40 with the precipitating liquid either coaxially through the extrusion device or by passing the extrudate through the precipitating liquid, and finally washing the resulting hollow fiber free of residual solvents and non- solvents, at air drying. Again there is a contact of the extruded material with air before entering the quench solution. Cited example 4 uses water-isopropanol-mixture as the center quench solution; the extrudate is then passed through a cold water outer quench solution. 45 EP-A-0 086 365 teaches preparation of a membrane having both inner and outer skin layers (AE and AO) and a void layer between them. These are anisotropic membranes. The invention relates to processes for preparation of microporous polysulfone membranes. The object to be solved is to provide processes which allows preparation of isotropic microporous polysulfone membranes in form of hollow fibers or flat sheets useful in industrial or pharmaceutical protein fractionation 50 which are skinless and homogeneous from surface to surface, having good tensile strength, a uniform, dense, spongy wall structure, more flexiblity with respect to application and quality control than would be true of the prior art anisotropic membranes, the processes offering the ability to control the porosity and membrane wall thickness by using appropriate spinning and setting conditions as well as casting and precipitation solutions. 55 This object is solved according to claim 1 by an ambient temperature process for forming an isotropic microporous polysulfone hollow fiber membrane, comprising: (a) preparing, in a solvent for polysulfone, a casting solution comprising a polysulfone polymer, a second polymeric component, that is one or more further polymers or prepolymers, and a pore forming 2 EP 0 362 588 B1 component, the polysulfone being present in an amount of 8.0 to 35.0 wt% and the second polymer or prepolymer component being present in an amount of up to 20 wt%, (b) preparing an outer precipitation solution comprising about 10.0 to 70.0 wt.% solvent for polysulfone, about 5.0 to 40.0 wt.% non-solvent for polysulfone and about 10.0 to 80.0 wt.% swelling agent, 5 (c) preparing a center precipitation solution comprising about 10.0 to 70.0 wt% solvent for polysulfone, about 5. to 40.0 wt% non-solvent for polysulfone and about 10.0 to 80.0 wt% swelling agent, the compositions of said outer precipitation solution and said center precipitation solution being adjusted to control the phase inversion rate such that an isotropic membrane is produced, (d) providing a precipitation bath containing said outer precipitation solution and having a hollow fiber- io forming spinnerette partially immersed therein, (e) extruding said casting solution and said center precipitation solution through said spinnerette directly into said precipitation bath to form an extruded hollow fiber membrane, (f) drawing said extruded hollow fiber membrane through said precipitation bath, and (g) drying said extruded hollow fiber membrane. is Alternatively, this object is solved according to claim 16 by an ambient temperature process for forming an isotropic microporous polysulfone flat sheet membrane, comprising: (a) preparing, in a solvent for polysulfone, a casting solution comprising a polysulfone polymer, a second polymeric component, that is one or more further polymers or prepolymers, and a pore forming component, the polysulfone being present in an amount of 8.0 to 35.0 wt% and the second polymer or 20 prepolymer component being present in an amount of up to 20 wt%, (b) preparing a precipitation bath comprising about 10.0 to 70.0 wt% solvent for polysulfone, about 5.0 to 40.0 wt% non-solvent for polysulfone and about 10.0 to 80.0 wt% swelling agent, (c) casting said casting solution directly onto a rigid non-porous support in said precipitation bath without exposure to air to form a flat sheet membrane, 25 (d) removing said flat sheet membrane from said rigid support, and (e) drying said flat sheet membrane. The process of this invention allows for the preparation of isotropic polysulfone microporous mem- branes having the above mentioned desired properties. The membranes obtained are skinless and are homogeneous from surface to surface. 30 For these membranes, the entire wall, rather than just the skin portion, performs the filtration. For that reason, it is expected that minor imperfections in isolated portions of the wall will not affect or impair the ability of the membrane to function.