Microporous polyether ether ketone membranes and the preparation thereof

Abstract: Microporous polyetheretherketone (PEEK) membranes and hollow fibers are formed without sulfonating the PEEK polymer. A method of making membranes, fibers and articles from solutions of PEEK in non-sulfonating acid solvents is claimed. Solvents include methane sulfonic acid and trifluoromethane sulfonic acid. Sulfuric acid may be used as a diluent in non-sulfonating amounts. PEEK membranes are used as supports for composite ultrafiltration and reverse osmosis membranes.

Description: Microporous polymer membranes are staple products in the art of separating fluid fractions. Poly(oxy-1,4-phenyleneoxy-1,4-phenylene-carbonyl-1,4-phenylene), more commonly known as poly-(etheretherketone), or PEEK, displays advantageous chemical and physical properties for many such uses. The high melting point (about 367°C), high glass transition temperature (about 145°C), low solubility and high chemical resistance make PEEK a material of choice for separations applications for harsh environments. PEEK is not known to be affected by common organic solvents at room temperature. PEEK is also resistant to acids and bases with the exception of strong acids in high concentrations. The general insolubility of PEEK, while a useful and advantageous property of a membrane in place, complicates the formation of useful articles including microporous membranes from PEEK. Polymer film useful as ultrafiltration membranes and reverse osmosis membrane supports are traditionally formed by dissolving the polymer in a solvent, casting the polymer solution on a support as a thin film, followed by coagulation of the polymer by immersion of the support and polymer film into a bath of liquid in which the polymer solvent is miscible, but which is not a solvent for the polymer. PEEK solubility has been reported in concentrated sulfuric acid (German Patent 3,321,860). Membranes have been cast from PEEK dissolved in concentrated H₂SO₄. However, the membranes formed are no longer PEEK, but rather have been sulfonated. As a consequence of the change of chemical composition, the membranes no longer display the advantageous properties of PEEK, and are known to swell upon prolonged exposure to water. Bishop, M. T. et al., Macromolecules , 1985, 18, 86. It would be desirable to form PEEK membranes from solutions thereof that are not sulfonated and, thus, do not swell in water. An objective of the invention is the preparation of useful articles from PEEK, including porous solid fibers, membrane film, and hollow membrane fibers. The membranes of the invention have improved durability for ultrafiltration, microfiltration, and reverse osmosis. EP-A-297,744, which is a prior art in the meaning of Art. 54 (3) EPC, discloses a process of producing a structural network from a semicrystalline, melt crystallisable polyarylene ether or polyarylene thioether comprising providing a molten mixture of at least one semicrystalline polymer in the presence of a melt plasticiser for the polymer which melt contains at least some polymer entities having a crystalline form, the total polymer concentration being between 5 and 50% by weight of the mixture of polymer and melt plasticiser, and heating the mixture of polymer and melt plasticiser at a heating rate of between 1°C. hour and 1000° C. hr, Preferably between 10° C. hour and 100° C. hour until the mixture is at a temperature in the range 10° C below to 20° C above its clearing point and cooling the mixture until it solidifies whereby an open cell structure essentially free from granular superstructure is obtained. According to page 3, line 56 the open cell structure may be used as a filtration material. The product of Example 1 was in the form of a microporous product of high rigidity. The product of Example 4 is a microporous film of PEEK substantially free of diphenylsulphone. According to EP-A- 297 744, page 3, lines 11-12, the structure of the obtained product is essentially 3-dimensionally uniform. The present invention is directed to a method of making an article including porous solid fibers, membrane film and hollow membrane fibers of poly-(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene) known as a poly-(ether ether ketone) (PEEK) polymer, comprising: dissolving substantially uniformly 5-40% by weight of a PEEK polymer below a temperature that would cause degradation or sulfonation of the PEEK polymer in a strong but non-sulfonating acid solvent ( i.e. acids and mixtures of acids which dissolve PEEK in an amount of not less than five percent by weight but which do not react with the PEEK and change the chemical structure of the PEEK); forming the PEEK polymer solution into the shape of the article; and coagulating the article in a coagulation bath that is a non-solvent for the PEEK polymer but is miscible with the non-sulfonating acid solvent, said bath thereby leaching the non-sulfonating acid solvent from the article. Articles of PEEK, in accord with the present invention, including membranes, are prepared from PEER dissolved in a non-sulfonating acid or a non-sulfonating mixture of acids. A semi-permeable membrane suitable as an asymmetrical ultrafiltration (UF) membrane and as a support for a reverse osmosis (RO) membrane can now be prepared from a PEEK-type polymer by dissolving the PEEK polymer with a non-sulfonating acid solvent and forming the solution into an article by, for example, casting or extruding. The process then requires coagulating said article in a bath that is a non-solvent for PEEK polymers but is miscible with the non-sulfonating acid solvent whereby said bath leaches said solvent from the article. Microporous membranes in sheet form and in the form of hollow fibers may be prepared according to the method of the invention. Hollow fiber membranes may be formed according to the invention by spinning fibers from a PEEK-type polymer dissolved in a non-sulfonating acid. Enhanced flux PEEK membranes may be prepared from a PEEK-type polymer dissolved in non-sulfonating acids by adding small amounts of halogenated hydrocarbons. The invention is further related to a method of separating fluid components comprising filtering a fluid through a porous membrane containing a poly-(ether ether ketone) (PEEK) polymer obtainable according to the afore-mentioned method as well as to a method of separating ions from water comprising filtering water through a discriminating layer supported by a microporous poly-(ether ether ketone) (PEEK) membrane obtainable according to the afore-mentioned method. Figure 1 is a scanning electron photomicrograph of an end-view section of the hollow fiber microporous membrane prepared from a PEEK-type polymer in accordance with Example 8 of the invention. Figure 2 is a scanning electron photomicrograph of an end-view of a hollow fiber microporous membrane prepared from a PEEK-type polymer in accordance with Example 9 of the invention. Figure 3 is a scanning electron photomicrograph of a section of an end-view of a hollow fiber microporous membrane prepared from a PEEK-type polymer in accordance with Example 10 of the invention. Articles and microporous membranes of the present invention are conveniently formed from a solid particulate form of a PEEK-type polymer, such as e.g., a powder, crumbs, pellets, or other convenient or available PEEK subdivided form. PEEK, or a PEEK-type polymer, includes polymers prepared, for example, by a condensation polymerization reaction involving as the starting material 4,4′difluorobenzophenone, and hydroquinone to which is added anhydrous potassium carbonate in a nitrogen atmosphere at temperatures up to 320°C. Detailed instructions appear in U.S. Patent 4,320,224. Microporous membranes are prepared from solid PEEK dissolved in a strong but non-sulfonating acid. Non-sulfonating acid as used herein refers to acids and mixtures of acids which dissolve PEEK in an amount of not less than about five percent by weight but which do not react with the PEEK and change the chemical structure of the PEEK. Non-sulfonating acids include methane sulfonic acid (MSA), trichloromethane sulfonic acid, hydrofluoric acid, phosphoric acid, and trifluoromethane sulfonic acid (TFMSA). Sulfuric acid (H₂SO₄) may be effectively used as a diluent for a non-sulfonating acid, provided the proportion of sulfuric acid is limited so as not to increase the viscosity of the solution of PEEK which would impair the flow of the solution of PEEK in the membrane casting step, or result in sulfonating the PEEK. More than 60 percent H₂SO₄ in MSA at room temperature when used as a solvent for PEEK causes sulfonation of the PEEK. It has been noted that H₂SO₄ alone is not a suitable solvent to form PEEK membranes. In order to form uniform membranes according to this invention, the PEEK must be substantially uniformly dissolved in the acid or a mixture of acids. PEEK, suitable for forming microporous membranes of this invention, will be present in the casting solution in a range of 5 to 40 percent by weight preferably in a range of 8 to 20 percent by weight. The step of dissolving PEEK to form the membrane casting solution may be carried out readily at a temperature of room temperature to 40°C. Temperatures in excess of 40°C may be useful to reduce the time necessary to prepare a substantially uniform PEEK solution. However, temperatures should not be elevated so as to cause degradation or sulfonation of PEEK by the acid solvent. Temperatures lower than room temperature may be found advantageous to obtain particular membrane properties. Temperatures of 15° to 30°C are generally preferred for casting PEEK membranes in the method of this invention. The forms of PEEK polymer useful in the preparation of the membrane compositions include polymers with molecular weights from 7,000 to 100,000. The size and shape of subdivided solid PEEK polymer, crumbs, particles, or bead size may be varied, but is practically limited by the surface area available to the acid solvent for ease of dissolving and mixing. The casting of PEEK articles or membrane films is achieved in the traditional method of membrane manufacture. Typically, the article is formed by pouring a solution of PEEK into a mold of the article or, in the case of thin films, by spreading a solution of PEEK on an impervious surface. Uniform thickness of the resulting membrane is achieved by customary methods such as drawing a casting bar, glass rod or knife across a cast solution of PEEK deposited on a support medium. The casting solution of PEEK is permitted to remain on the support for a period of time which may range from the minimum time necessary to complete the casting step to minutes before the cast PEEK solution is coagulated in a fluid bath. In one preferred embodiment of the invention, the PEEK solution may be cast on a fibrous reinforcing medium that provides rigidity and strength such as a non-woven glass fiber mat or a non-woven polyester mat laid upon an impervious surface such as a glass plate. Alternatively, the PEEK membrane film may be cast without reinforcement on an impervious support surface such as a glass plate. After coagulation of PEEK in the coagulation bath, the PEEK membrane may be removed from the glass plate for further preparation or use. a further embodiment is a method wherein forming said membrane further comprises casting the solution of dissolved poly-(ether ether ketone)(PEEK) polymer upon a supporting substrate formed from woven or non-woven fibres of glass, polyester, natural fiber, or polyether sulfone. The coagulation bath is a solution in which the acid or mixture of acids is miscible, but which is not a solvent for PEEK. Water is a suitable coagulation fluid for both MSA and TFMSA. In the manufacture of an article from PEEK, the solution of PEEK cast in the shape of the article is contacted with a coagulation fluid bath, for example, by immersion. A coagulation bath causes coagulation of the dissolved PEEK by diluting the non-sulfonating acid solvent for PEEK. Shaped articles cast from PEEK dissolved in non-sulfonating acids generally maintain their integrity and form when immersed in a coagulation bath. Membranes cast from PEEK dissolved in non-sulfonating acids maintain their integrity as membranes when gently immersed in a coagulation bath. Membranes of PEEK demonstrate sufficient structural integrity after coagulation for use or further processing when carefully handled. After casting and coagulating, a PEEK membrane, is generally washed in water to remove residual acid solvent for approximately 30 to 60 minutes. Prior to use, membranes may optionally be further washed with dilute basic solutions to promote removal of the non-sulfonating acids. Solutions of PEEK described herein may also be formed into solid fibers and into hollow fibers by, for example, an air gap wet spinning process described in Encyclopedia of Polymer Science and Technology, 2d , "Fibers, Manufacture" Mark, H. F. et al., ed Wiley-Interscience, New York, 1986 and Encyclopedia of Chemical Technology, 3d , "Hollow Fiber Membranes", Mark, H. F., et al., ed Wiley-Interscience, New York, 1980. Preparation of PEEK solid or hollow fibers from solutions of PEEK is complicated by the fact that the room temperature solubility of PEEK in MSA is limited to about 8 percent on a weight-to-weight basis. This limited concentration results in a viscosity which approaches the lower limit for practical extrusion of polymer fibers, sometimes called "spinning" of a polymeric material. A feature of the present invention is that the solubility limit of PEEK in MSA may be increased by the addition of other non-sulfonating acids to form a solvent mixture for PEEK. The substitution of up to 100 percent TFMSA for MSA increases the solubility of PEEK to as high as about 40 percent by weight at room temperature. A mixture of 90 percent MSA/10 percent TFMSA is a convenient solvent for PEEK. PEEK fibers may be conveniently extruded from a PEEK solution in a non-sulfonating acid solvent in a PEEK concentration from 8 percent to 40 percent, preferably in a range from 16 percent to 10 percent. A further feature of the present invention is that the viscosity of the PEEK solution may be increased by the incorporation of H₂SO₄ into the non-sulfonating acid solvent for PEEK up to about 60 percent by weight sof solvent without the occurrence of adverse sulfonation of the PEEK. The amount of H₂SO₄ which may be incorporated as a diluent in the non-sulfonating acid solvent for PEEK is limited by the concentration at which the PEEK begins to be sulfonated by the H₂SO₄. The concentration of H₂SO₄ at which sulfonation begins will be lower as the temperature increases. In addition to the air-gap spinning process, the solution of dissolved PEEK may be extruded into fibers in the usual manner which is well known in the art. Hollow fiber membranes may be prepared by extrusion of the PEEK solution through an annular shaped orifice while a core fluid simultaneously flows through the center of the PEEK solution flow as it emerges from the annular orifice. The core fluid flow rate is empirically adjusted to a level compatible with the fiber flow rate. The fiber flow rate is also empirically adjusted to produce a uniformly shaped stream prior to immersion in a liquid coagulation bath. A further feature of the invention is controlling membrane surface properties of the membranes formed from PEEK dissolved in a non-sulfonating acid by variation of coagulation fluid. The use of various coagulation fluids for the preparation of PEEK membranes permits formation of a skin on one or both membrane surfaces, and the formation of an elongated honeycomb porous structure between the surfaces of the membrane. The porous structure between membrane surfaces advantageously provides structural support to the membrane surface, making microporous hollow fiber of PEEK-type polymer particularly suited for application as a support for high pressure uses such as a support for a composite membrane for reverse osmosis applications. A further advantage of the porous structure of the PEEK membranes is the formation of numerous flow channels between the membrane surfaces which permits unencumbered flow of the permeating fluid after the permeate passes through a skin layer or a composite membrane formed on a membrane surface. Membrane structure and properties may be varied by variation of the coagulation fluid, polymer molecular weight, or non-sulfonating acid solvent. By using a coagulation bath which has a high relative race of diffusion of the coagulation liquid into the PEEK polymer phase a microporous skin forms on the exterior surface of the hollow fiber. The rate of diffusion of the coagulation fluid into the PEEK polymer phase containing the non-sulfonating acid PEEK solvent may be slowed by the addition of an acid to the coagulation fluid, such as a coagulation bath of a non-sulfonating level of H₂SO₄, for instance 52 percent H₂SO₄ in water. Other methods of influencing the rate of diffusion of the coagulation fluid from the diffusion rate of water include the substitution of a polyglycol for water in the coagulation bath at a concentration up to 100 percent polyglycol. It has been observed that coagulation of PEEK polymer in fluids having a high relative rate of diffusion for the non-sulfonating acid solvent for PEEK polymer results in the formation of a thicker skin layer having smaller and fewer pores than does the coagulation of a solution of PEEK polymer in a coagulation fluid having a lower relative rate of diffusion in the PEEK polymer phase. Water is an example of a fluid with a high relative rate of diffusion for the non-sulfonating acid solvent for PEEK polymer. Examples of a fluid with a lower relative rate of diffusion than water are thickened water or polyethylene glycol. The viscosity of water is lower than the viscosity of thickened water or polyethylene glycol, consequently, the relative rate of diffusion of the non-sulfonating acid solvent in water is higher than the relative rate of diffusion in thickened water, or polyethylene glycol. It has also been observed that by coagulating PEEK polymer in coagulation fluids having a higher relative rate of diffusion in the non-sulfonating acid solvent for PEEK polymer, the structure between the surfaces of the membrane is more porous than the structure resulting from coagulation of PEEK polymer solution by a coagulation fluid having a lower relative rate of diffusion. The contrary observation of reduced porosity in the internal membrane structure results from use of coagulation bath fluids having lower relative rates of diffusion into the non-sulfonating acid solvent. As membrane flux is related to membrane skin thickness, porosity, polymer molecular weight, and pore size, as well as the porosity of the internal membrane structure, optimization of the membrane preparation parameters of non-sulfonation acid solvent, polymer molecular weight, and the relative rate of diffusion of the coagulation fluid will benefit each system in which a membrane separation of fluid fractions is attempted. One embodiment of a PEEK membrane prepared according to the present invention is characterized by a porous asymmetric structure. For example, the PEEK membrane fibers resulting from the hollow fiber extrusion process include an interior porous finger-like or ribbed structure between the opposing surfaces of the microporous membrane fibers with fingers or ribs extending generally radially between the outer surface or skin and the lumen of the fiber. The fingers or ribs surround vacuoles formed within the PEEK membrane. This radial structure is advantageous for hollow fiber membrane applications because it provides porous channels for flow between the discriminating microporous membrane skin and the lumen. The fingers or ribs in the asymmetric hollow membrane fiber provide additional structural support for the discriminating skin layer under elevated pressures employed in the use of the hollow fiber PEEK membrane as a reverse osmosis membrane support. If a core fluid which has a relatively low rate of diffusion for the PEEK solvent is injected into the bore of the hollow fiber membrane, skin formation in the bore may be reduced or altogether avoided. The physical characteristics of the hollow fiber PEEK membrane may be reversed by reversing characteristics of the bore fluid and the characteristics of the immersion bath. In this manner skin may be formed on the lumen surface by injecting in the bore of the hollow PEEK fiber at the extruder a fluid having a high affinity for the non-sulfonating acid PEEK solvent. If the immersion bath has a fluid which does not have a strong affinity for the non-sulfonating acid or acid mixture which dissolves PEEK, the finger or rib-like structure will result generally radially outward from the skin formed at the bore, or lumen, of the hollow fiber. In addition, by using a bore fluid and an immersion bath, both having a high affinity for the non-sulfonating acid PEEK solvent, a skin will result in both the bore and the outer surface of the PEEK hollow fiber membrane. The surface and internal properties of a fiber of PEEK extruded without a lumen may be similarly modified from the outer surface. The viscosity of the solution of PEEK in acid solvent at which PEEK hollow fiber membranes advantageously may be prepared will be empirically established by the practitioner having ordinary skill in the art and a working knowledge of the hollow fiber membrane spinning extruder and related machinery employed. Working viscosities are generally in the range from 2 to 12.10⁻³ Pa.s (2 centipoise to 12 centipoise). An additional feature of the invention i use of a halogenated hydrocarbon solvent of 3 or less carbon atoms in the non-sulfonating acid solvent for PEEK. The incorporation of a sufficient amount of the halogenated hydrocarbon solvent results in an enhanced level of fluid flux over membranes prepared without said solvent. A further advantage is that the enhanced flux rate does not occur at the expense of membrane selectivity. A suitable halogenated hydrocarbon for addition to the non-sulfonating acid PEEK solvent must be miscible with the membrane casting solution of dissolved PEEK in a non-sulfonating acid and not cause precipitation of PEEK from the solution. The halogenated hydrocarbon chosen should be immiscible with the coagulation fluid. The halogenated hydrocarbon chosen should be relatively volatile at room temperature or such other temperature at which the membrane preparation process is undertaken. Suitable halogenated hydrocarbons include, but are not limited to, chloroform, methylene chloride, carbon tetrachloride, 1,1,2-trichloroethane, 1,1,2-trichlorotrifluoroethane, or tetrachloroethane. The foregoing list of solvents is not intended to be exhaustive of all compounds included within the scope of the invention, or to be exclusive of other halogenated hydrocarbons possessing the characteristics necessary and described. While highly satisfactory and usable film, fiber and hollow fibers are made using the non-sulfonating acid or non-sulfonating mixture of acids, the enhanced flux referred to is noticeable when the membrane casting solution contains the halogenated hydrocarbon at a concentration up to 20 percent by weight, a range from 2 to 15 percent by weight is preferred, most preferably in a range from 4 to 10 percent by weight. A further feature of the present invention is that PEEK membranes prepared according to this invention are also useful as support in composite reverse osmosis membranes made up of thin-layer polymers, or coacervates, or ionic complexes. Typical of such composite membranes are those which may be prepared from interfacial condensation of polyfunctional secondary amines with polyfunctional acid halides as more fully disclosed in U.S. Patent 4,259,183, and composite membranes prepared from crosslinked, interfacially polymerized aromatic polyamides as more fully described in U.S. Patent 4,277,344. Composite reverse osmosis membranes may also be prepared according to most any other procedures known and practiced in the art of separation of fluid fractions by reverse osmosis membranes utilizing the PEEK membranes formed according to the present invention as a support for composite membranes. In a further embodiment of the invention, the microporous membrane film or hollow fiber of a poly-(ether ether ketone) (PEEK) polymer, obtainable according to the method according to the invention, comprises an asymmetric structure between membrane surfaces and has a skin on at least one surface, said film or fiber further comprises a discriminating layer of an interfacial condensation reaction product. Preferably, this microporous membrane or fiber has a pore size from 1 µm to 3 µm. Otherwise, the microporous membrane or fiber prepared according to the method of the invention may have a pore size from 0,1 µm to 3 µm. In addition, the above-mentioned asymmetric microporous membrane film or fiber may further comprise a thin layer of a crosslinked polymer. The invention described herein is illustrated by the following examples and comparisons. PEEK-type polymer used for the examples of the instant invention was secured from ICI Americas, Inc. The molecular weight provided by the manufacturer for grade 150G was 7,000, for grade 380G was 13,000, and for grade 450P was 18,000. A weight-average molecular weight determination is made from measurements of intrinsic viscosities of samples dissolved in sulfuric acid using a Mark-Houwink relation (K=0.000745, a=0.65) established from the data in Bishop, M. T. et al.; (Supra), and including a correction to account for the changes in molecular weights resulting from sulfonation of the PEEK samples in sulfuric acid. A weight-average molecular weight for a sample of grade 150G PEEK is 20,700; for grade 380G is 32,200 and for grade 450P is 39,200. Reference herein to molecular weight relates to the average molecular weights provided by the manufacturer. Example 1 4.0 Grams (g) of poly(etheretherketone) Victrex® PEEK grade 450P, a product of Imperial Chemical Industries, Wilmington, Delaware 19897, is weighed into a 50 ml single-neck ground glass round bottom flask. 17.6 g of methane sulfonic acid and 11.7 g of trifluoromethane sulfonic acid are added consecutively. The neck of the flask is connected with a ground glass bearing and a ground glass stir rod having a polytetrafluoroethylene blade. The mixture is stirred at approximately 30 to 60 rpm until a dark maroon solution results. Stirring time is approximately 12 hours at room temperature. A sheet of non-woven polyester fabric to be used as a membrane support is taped to a glass plate. A puddle of dissolved PEEK solution is poured onto the backing and spread across the non-woven fabric with a doctor blade shimmed to provide a gap of 0.152 mm above the glass plate. The glass plate is then immediately immersed in a water bath at room temperature for 30 minutes. A pink colored coagulated membrane results. Residual acid is removed from the membrane by soaking in 10 percent by weight sodium carbonate for 3 to 10 minutes followed by soaking in methanol for 3 to 10 minutes followed by a final rinsing with water. The membrane appears white in color and opaque to light transmission. Example 2 A series of membranes are prepared in the manner described in Example 1, except that various non-sulfonating acids are used as indicated. The membranes are tested to determine the flux and rejection of a solution of 0.1 percent dextran in water at a pressure of 345 kPa, said dextran having a molecular weight of 35,000 to 50,000. The flux rates for various PEEK loadings and acid solvent systems appear in Table I. Ultrafiltration Properties of PEEK Membranes Prepared According to Example 1 (Feed: 0.1 percent 35-50K Dextran at 345 kPa) Sample Solvent System Percent PEEK in Casting Solution Flux l/m 2. day Percent Dextran Rejection 2.1 60% MSA/40% TFMSA 10 1015 10.5 2.2 " 12 774 41.1 2.3 " 15 600 94.7 2.4 " 16 812 85.9 2.5 40% MSA/20% TFMSA/40% H₂SO₄ 9 5964 13.0 2.6 100% TFMSA 20 1104 38.7 Example 3 To use porous PEEK membranes as support for composite reverse osmosis membranes, PEEK membranes are prepared according to the method of Example 1 cast from a solution of 12 percent by weight PEEK in a solvent of a blend of 60 percent MSA/40 percent TFMSA. After the water rinse, the membranes are maintained in a wet condition until used as supports for thin film composite membranes. To prepare the composite reverse osmosis layer, sheet PEEK membrane samples are immersed in an aqueous solution of 2 percent m-phenylenediamine for about one minute. After removal of the PEEK membrane supports from the amine bath, they are placed on a glass plate with the side up of the PEEK membrane that was exposed to air at the time of casting and coagulation. Excess amine solution is gently squeezed from the PEEK-type membrane supports by gently rolling with a rubber roller. The PEEK-type supports are then placed flat in a solution of 0.1 weight/volume percent trimesoyl chloride in 1,1,2-trichlorotrifluoroethane for one minute, excess trimesoyl chloride solution is then drained from the resulting membrane. The resulting membranes are then air-dried at room temperature for at least two hours prior to testing for salt rejection at a working pressure of 1,380 kPa. Membranes prepared are tested for salt rejection using a solution of 0.2 percent NaCl at a working pressure of 1,380 kPa. Table II shows salt rejection. Samples PEEK Molecular Weight Flux (l/m 2. day) Percent NaCl Rejection 3.1 18,000 159 90.7 3.2 7,000 423 88.7 3.3 13,000 445 84.8 3.4 18,000 476 87.6 Example 4 PEEK membranes are cast in the method of Example 3 from a solution of PEEK-type polymer 12 percent by weight, dissolved in a solvent of 60 percent MSA/40 TFMSA. In the method of Example 3, the solution of m-phenylenediamine is replaced with an aqueous solution of 2 percent by weight piperazine, 2 percent by weight triethylamine and 0.2 percent by weight sodium lauryl sulfate. The membranes are tested for magnesium sulfate rejection using an aqueous solution of 0.2 weight percent of magnesium sulfate at an operating pressure of 1,380 kPa. Table III shows magnesium sulfate rejection. Samples PEEK Molecular Weight Flux (l/m².Day) Percent MgSO₄ Rejection 4.1 18,000 476 91.4 4.2 7,000 810 82.8 4.3 13,000 851 96.5 4.4 18,000 794 71.2 Example 5 0.51 Gram of PEEK crumbs available from Imperial Chemical Industries, under the registered trademark Victrex® PEEK 450P having an average molecular weight of 18,000 are mixed with 10.6 g of methane sulfonic acid in a 30 ml glass bottle. The mixture is stirred on a stirring plate at room temperature for 72 hours to form a clear homogeneous solution. The clear solution is cast on a glass plate and drawn to film membrane of a uniform thickness of 0.51 mm using a casting bar. After 15 seconds, the glass plate is carefully immersed into a 21°C water bath at an angle of 30° with the horizontal to coagulate the cast solution. The coagulated membrane sample is white and opaque. The coagulated PEEK sample lifts cleanly from the glass plate. The membrane is subsequently washed in water for 16 hours and air dried. The surface of the PEEK membrane exposed to the air at the time of casting appears to be shiny, while the surface of the membrane in contact with the glass support is dull in appearance. Scanning electron microscope examination discloses submicron pores dispersed uniformly in the textured surface of the matrix having a pore size of about 0.2 µm or less. The glass-contacting surface, however, exhibits larger pore diameters on the order of 3 µm or larger. The membrane is remarkably asymmetric. A water transmission rate of 10,824 l/m²/day at one atmosphere pressure is observed. The membrane retains a polystyrene latex having a particle size of 0.091 µm. Example 6 Samples of membranes prepared as described in Example 5 are cut and immersed in dimethylformamide for 21 days. Additional samples of membrane prepared as described in Example 5 are cut and immersed in dimethylacetamide for 21 days. Examination of the membrane samples after removal from the solvents discloses the membranes have retained their original integrity and remain undissolved and unsoftened in the respective organic solvents. The lack of solubility or softening suggests that the membranes are not sulfonated by the acid mixture solvent in the membrane casting process. Example 7 1.54 Grams of crumbs formed of PEEK having an average molecular weight of 39,200 are mixed with 20.62 g of methane sulfonic acid in a 30 cc glass bottle. The mixture is stirred on a stirring plate at room temperature for 72 hours to form a clear homogeneous solution. The solution is cast on a non-woven polyester fiber substrate of a thickness of 0.097 mm using a casting bar with a 0.254 mm clearance. The substrate with the cast solution thereon is immediately immersed in clear water maintained at 14°C to coagulate the membrane. The membrane is washed in running water for 30 minutes and air dried. The surface of the membrane is shiny and smooth. The flux rate and size retention characteristics of the membrane are tested with a water solution of 17 weight percent polyvinylpyrrolidone (PVP) having an average molecular weight of 40,000. The membrane exhibits an ultrafiltration flow rate of 146 liters/m² . day at a pressure of 345 kPa. The membrane retains 90 percent of the PVP. Example 8 Four grams of crumbs formed of PEEK having an average molecular weight of 39,200 are mixed with 50 g MSA in a glass bottle. The mixture is stirred for 72 hours to form a clear homogeneous solution. The prepared polymer solution is fed into the barrel of a ram extruder for hollow fiber membrane spinning. The extruder consists of a 15 mm diameter stainless steel barrel with a single hole spinnerette having a 0.343 mm center hole for a core fluid surrounded by a concentric annularly shaped material flow channel having an inner diameter ot about 1.6 mm and having an outer diameter of about 2.1 mm. These dimensions give an extrusion factor (nominal extrudate velocity at the die face, divided by the velocity of the plunger in the barrel) of 120. The prepared polymer solutions are fed into the barrel at room temperature. The plunger of the extruder moves in the barrel at a constant speed of 51 mm/minute. The PEEK solution emerges through the annular shaped flow channel. A fiber velocity at the die face of about 1.5 m/min results. The core fluid contains a mixture of 20 percent water and 80 percent polyglycol having an average molecular weight of 1,000. The core fluid stream is empirically adjusted with a needle valve to cause a uniform stream of shaped PEEK solution to develop before the shaped solution enters a water coagulation bath. The extruded hollow fiber exits the spinnerette flowing downward at right angles to the horizontal into a water coagulation bath. A 7.6 cm air gap exists between the spinnerette and the underlying water coagulation bath. After coagulation, the fiber is wound using a motor driven winder. The winding speed is adjusted to produce a PEEK fiber having a size of 1 mm outside diameter and 500 µm inside diameter. The collected fiber is leached and rinsed in water for from 15 to 60 minutes. The resulting fibers exhibit an asymmetric microporous structure. A scanning electron microscope photograph of an end-view of a section of a fiber appears as Figure 1 hereof. Filtration tests disclose a flux of 81 to 244 l/m² . day at 83.4 kPa. Molecular weight cut-off was 360,000 with 80 percent retention. Example 9 A polymer solution is prepared substantially as in Example 8. PEEK hollow fiber membranes are spun using the equipment and method of Example 8, except that the core liquid is water, and the coagulation bath is tetraethylene glycol. The spun fibers remain immersed in the coagulation bath until the fibers turn totally white and opaque, an indication of complete coagulation. Thereafter, the fibers are transferred to a water bath for rinsing. The fibers prepared in accordance with this example exhibit an asymmetric microporous structure with a skin formation at the lumen surface. A finger-like microporous structure with fingers extending radially from the skin of the lumen toward the outside surface and a layer of sponge-like porous structure with vacuoles therein is exhibited next to the outside surface. A scanning electron microscope photograph of an end-view of a section of the resulting membrane fiber wall appears as Figure 2. Scanning electron microscope examination reveals the outside surface to be porous having pore sizes in a range from 0.2 µm to 0.5 µm. Example 10 A polymer solution and membrane spinning methods are employed as in Example 8 to prepare PEEK hollow fiber membranes with the exception that both the core fluid and the coagulation medium are water. Membranes prepared exhibit both an inside and outside skin formation with a finger-like structure extending from the skin across the fiber thickness toward the opposing surface. A scanning electron microscope photograph of the fiber membranes prepared in the practice of this example appears as Figure 3. Example 11 Four grams of crumbs formed of PEEK having an average molecular weight of 39,200 are mixed with 50 g MSA in a glass bottle, stirring 72 hours to form a clear homogeneous solution. Liquid chloroform in varying amounts is then added to respective portions of PEEK solution according to the schedule of Table IV. The mixture is stirred for at least one additional hour. Membranes are cast from the solution on non-woven polyester substrates using a 0.25 mm clearance casting bar. The substrate and PEEK casting solution are then immediately immersed in water maintained at 15° to 18°C for a period of about twenty minutes to coagulate the solution to a membrane. The membranes are washed in running water for 1 hour and air dried. Flow rate and molecular size retention are tested using an Amicon Model 8050 filtration cell having a size of 50 ml. The filtration cell is available from the manufacturer at 182 Conant St., Danvers, Massachusetts, U.S.A. A 0.5 percent polyvinylpyrrolidone solution in water having a molecular weight of 40,000 is used to measure size retention. The permeate concentrations are determined by ultraviolet spectroscopy. Table IV shows membrane flux and PVP retention for chloroform loading. Sample Additional Percent CHCl₃ Flux l/m².day at 345 kPa Retention PVP 40,000 (percent) 11.1 0 50 >95 11.2 5 794 >90 11.3 10 186 >90 Membranes cast from solutions having 5 percent chloroform show a dramatic water flux improvement with no significant change in molecular weight cut-off. Example 12 Membrane solutions are prepared in accordance with Example 11 except a mixture of acids are used as solvent. MSA and TFMSA in a ratio of 90 percent MSA/10 percent TFMSA are used as solvents at room temperature. After PEEK is dissolved in the manner of Example 11, 5 percent chloroform is added to an aliquot and the solution stirred for an additional hour. The PEEK concentrations in the membrane casting solutions utilized and the results of testing the membranes are disclosed in Table V, the membranes cast being evaluated for flux and molecular size retention. Sample PEEK Loading Percent CHCl₃ Flux as l/m².day at 345 kPa Retention PVP, 40,000 (percent) 12.1 10 0 16 >95 12.2 10 5 percent 615 95 12.3 12 0 15 >95 12.4 12 5 percent 30 95 12.5 15 0 9 >95 12.6 15 5 percent 46 >95

Claims: A method of making an article including porous solid fibers, membrane film and hollow membrane fibers of poly-(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene) known as a poly-(ether ether ketone) (PEEK) polymer, comprising: dissolving substantially uniformly 5-40% by weight of a PEEK polymer below a temperature that would cause degradation or sulfonation of the PEEK polymer in a strong but non-sulfonating acid solvent ( i.e. acids and mixtures of acids which dissolve PEEK in an amount of not less than five percent by weight but which do not react with the PEEK and change the chemical structure of the PEEK); forming the PEEK polymer solution into the shape of the article; and coagulating the article in a coagulation bath that is a non-solvent for the PEEK polymer but is miscible with the non-sulfonating acid solvent, said bath thereby leaching the non-sulfonating acid solvent from the article. The method of Claim 1 wherein said article is a semi-permeable microporous membrane film of a poly-(ether ether ketone) (PEEK) polymer, further comprising forming said film by casting the solution of a PEEK polymer on an impervious surface, followed by coagulating the cast solution of a PEEK polymer in said coagulation bath thereby forming a microporous membrane of a PEEK polymer. The method of Claim 1 or 2 wherein the non-sulfonating acid solvent comprises methane sulfonic acid, trichloromethane sulfonic acid, hydrofluoric acid, phosphoric acid, trifluoromethane sulfonic acid, or a mixture thereof. The method of Claim 2 wherein forming said membrane further comprises casting the solution of dissolved poly-(ether ether ketone) (PEEK) polymer upon a supporting substrate formed from woven or non-woven fibers of glass, polyester, natural fiber, or polyether sulfone. The method of Claim 3 further comprising adding to said non-sulfonating acid solvent, a halogenated hydrocarbon having three or less carbon atoms such that the flux of the resulting membrane is enhanced. The method of Claim 3 wherein the non-sulfonating acid solvent contains up to 60 percent sulfuric acid. The method of Claim 3 wherein the coagulation bath has a high relative rate of diffusion into the polymer phase, i.e. one comparable to that of water. The method of Claim 3 wherein the coagulation bath has a low relative rate of diffusion into the polymer phase, i.e. one comparable to that of polyethylene glycol. The method of Claim 1 wherein said article comprises a poly-(ether ether ketone) (PEEK) fiber formed by extruding the solution of PEEK polymer through an orifice, and immersing the fiber in a coagulation bath. The method of Claims 3, 5, and 6, wherein said membrane is a hollow fiber, said method further comprising extruding a poly-(ether ether ketone) (PEEK) polymer solution through an annular shaped orifice, injecting at the orifice a core fluid in the lumen of the fiber thus formed, and immersing the fiber in a coagulation bath. A microporous membrane film or hollow fiber of a poly-(ether ether ketone) (PEEK) polymer obtainable according to the method according of any of the claims 1-10 comprising an asymmetric structure between membrane surfaces and having a skin on at least one surface. The microporous film or fiber of Claim 11 wherein said film or fiber further comprises a discriminating layer of an interfacial condensation reaction product. The microporous membrane or fiber as in Claim 11 having a pore size from 1 µm to 3µm. A microporous membrane or fiber prepared according to the method of Claim 3, 4, 5, or 6, having a pore size from 0,1 µm to 3µm. A method of separating fluid components comprising filtering a fluid through a porous membrane containing a poly-(ether ether ketone) (PEEK) polymer obtainable according to the method according to any of the claims 1-10. A method of separating ions from water comprising filtering water through a discriminating layer supported by a microporous poly-(ether ether ketone) (PEEK) membrane obtainable according to the method according to any of the claims 1-10. A microporous membrane film or fiber as in Claim 11 further comprising a thin layer of a crosslinked polymer.

Inventors: BISHOP MATTHEW T, CHAU CHIEH-CHUN, KOO JA-YOUNG, RACCHINI JOEL R, WESSLING RITCHIE A, BISHOP, MATTHEW T., CHAU, CHIEH-CHUN, KOO, JA-YOUNG, RACCHINI, JOEL R., WESSLING, RITCHIE A.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1989
Main classification: C08J9
IPC codes: B29C70, B29K105, B01D71, C08J9, C08J5, D01F6, B29C67, D01D5, B29K71, B29L31