POROUS MEMBRANE AND PROCESS FOR ITS MANUFACTURE

Abstract: The invention relates to a porous membrane having excellent durability and being useful as an ultrafiltration membrane, which substantially comprises a phenylene sulfide copolymer represented by the formula (-Ph-S-) m (-Ph-X-) n and which has a nonuniform structure wherein a dense layer or layers substantially governing the separating function of the membrane are formed on one side or both sides of the membrane and a porous layer having a larger void volume is formed in contact with said dense layer. In said formula Ph represents a phenylene group, X represents -SO₂, - or-CO-, and m and n each represents a natural number.

Description: This invention relates to a porous membrane which is useful for ultrafiltration. Conventionally, applications of ultrafiltration membranes to uses in various fields such as production of pure water for the electronics industry, recovery of electroplating coatings, treatment of wastes of paper-producing or pulp factories, treatment of oil-containing wastes, recycling of the wastes from buildings, treatment of wastes in atomic power stations, clarification of fruit juices, production of live sake, concentration of whey, desalination, production of concentrated milk, concentration of egg white, treatment of bean proteins, recovery of enzymes, bioreactors, separation of biological products, removal of particles in the air and removal of particles in organic liquids have been proposed and practiced. In the fields where the ultrafiltration membranes are used, such as the fields mentioned above, the following properties concerning the durability of the membrane are usually required: i) Durability against washing with hot water or sterilization with steam (heat resistance); ii) Chemical Resistance (resistance to acids, bases, chlorine and so on); iii) Solvent Resistance; and iv) Low Fouling Properties (low protein-adsorbing properties). These properties are required because of the treatment conditions of the ultrafiltration membranes, sterilization, and of the washing for the recovery of the separation efficiency adversely affected by the concentrated solute layer formed on the surface of the membrane during processing. However, an ultrafiltration membrane which suffices all of these requirements has not yet been developed, so that ultrafiltration membranes with high durability are demanded. Cellulose acetate is often employed as a material for ultrafiltration membranes because it is easy to process. However, the ultrafiltration membranes made of cellulose acetate are not satisfactory in their durability such as heat resistance, alkali resistance and solvent resistance. Although ultrafiltration membranes made of a polyacrylonitrile and a polyvinylidene fluoride (Polymer, vol. 21, p.1047(1980)) have improved solvent resistance, they have poor heat resistance. Although those made of a polyamide are excellent in heat resistance, they cannot be sterilized with chlorine. Ultrafiltration membranes with which these problems are relatively well solved are those made of a polysulfone (JP-A-22508/75). However, they have problems associated with their solvent resistance and degradation of their permeation efficiency due to the adsorption of solutes. On the other hand, although the problem associated with solvent resistance was overcome by ultrafiltration membranes made of a polyimide (JP-A-90098/82), they have poor alkali resistance. As a porous membrane with excellent heat resistance, chemical resistance and solvent resistance, polyphenylenesulfide (PPS) membranes have been proposed. PPS membranes include two groups, that is, homogeneous porous moulded products prepared by moulding PPS into a film or hollow fiber by melt-extrusion and by stretching and heat setting the film or hollow fiber (JP-A-6733/83 and JP-B-44404/85), and heterogeneous porous moulded products prepared by dissolving PPS under severe conditions, moulding the resultant solution into a hollow fiber and coagulating the hollow fiber (JP-A-248202/85). With the former method, although they can exhibit high permeability as ultrafiltration membranes, it is difficult to control the pore size of the membrane and the membranes produced cannot have a heterogeneous structure. With the latter method, because of the high crystallinity of PPS, PPS can be dissolved in only very few solvents even under very severe conditions. Since porous membranes can only be prepared under these limited conditions, it is difficult to control the separation efficiency of the membranes, so that it is difficult to satisfy the demanded properties. On the other hand, it has been proposed to promote the durability of PPS porous membranes by oxidizing PPS so as to convert the sulfide bond into a sulfone bond (JP-A-225636/88). However, with this method, since the oxidization step is carried out after a porous membrane is prepared by the above-described method, it is difficult to control the separation efficiency. Further, in cases where partial oxidization of the sulfide bond is carried out, the polymer becomes a random copolymer, so that the reproducibility of the production process and the durability of the resulting membranes are not satisfactory. Another major problem in ultrafiltration membranes is the change in the separation efficiency due to the adsorption of the solute on the surface of the membrane. To reduce the adsorption, a method of making the membrane hydrophilic is now being investigated. For example, a method in which an ether bond is introduced into polyvinylidene fluoride (J. Memb. Sci., 36 , 257 (1988); Desalination, 70 , 207 (1988)), a method in which N-methylolamide and methylenebisamide are introduced into polyacrylonitrile (PCT/US88/00261), and a method in which a polysulfone membrane is treated with an aqueous nonionic polymer solution such as methyl or ethyl cellulose, polyethyleneglycol, and polyvinyl alcohol (ICOM'87) have been proposed. However, no membranes prepared by these method satisfy the above-mentioned durability. Thus, a hydrophilic membrane with excellent durability is strongly demanded. Accordingly, the object of the present invention is to provide an ultrafiltration membrane and a production process thereof, which ultrafiltration membrane has an excellent durability and excellent separation efficiency because of the ease of controlling its separation efficiency. We have conducted intensive studies to find that a porous membrane with heterogeneous structure made of a specific copolymer containing phenylenesulfide units has excellent durability and excellent separation efficiency and yet has high permeability. That is, the present invention provides a porous membrane having heterogeneous structure, which consists essentially of a phenylenesulfide-based copolymer of the following formula [I]: -(Ph-S-) m -(-Ph-X-) n - [I] , wherein Ph is a phenylene group, X is -SO₂- or -CO-, the total of m and n is 80-400 and [n/(m+n)] x 100% is not less than 50%, which membrane has a dense layer on one or both surfaces of the membrane, the dense layer controlling the separation performance of the membrane, and a porous layer next to the dense layer, which porous layer has a porosity higher than that of the dense layer. The porous membrane of the present invention has an excellent separation efficiency and yet has an excellent durability such as excellent solvent resistance, heat resistance, chemical resistance and low protein-adsorbing properties as well as a high permeability, so that it has excellent properties for ultrafiltration. Thus, the porous membrane of the present invention is excellent not only for ultrafiltration, but also as a support of a composite reverse osmosis membrane and of a gas separation membrane. Embodiments of the present invention will now be described in more detail with reference to the accompanying drawings in which Fig. 1 shows an infrared spectrum of a poly(phenylenesulfidesulfone) obtained in an example of the present invention; Fig. 2 is a scanning electromicrophotograph showing a cross section of a membrane of an example of the present invention; Fig. 3 is a field emission type scanning electromicrophotograph showing a cross section of a membrane of an example of the present invention; and Fig. 4 shows an NMR spectrum of a compound obtained by oxidizing a poly(phenylenesulfidesulfone) prepared in an example of the present invention. As mentioned above, the porous membrane of the present invention consists essentially of a phenylenesulfide-based copolymer of the above-described formula [I]. The formula [I] expresses a copolymer in which phenylenesulfide units of the formula -Ph-S- in the number of m and units of the formula -Ph-X- in the number of n are bonded in any order. Thus, the copolymer may be a random copolymer in which these units are randomly bonded, or may be an "alternate" copolymer of the following formula [II] in which these units are alternately bonded: -(-Ph-S-Ph-X-) q - [II] , wherein Ph and X are as defined above with reference to formula [I], and q is an integer of 80 - 400). Having regard to the reproducibility of the membrane performance and its durability, such alternate copolymers are preferred. That is, in order to stabilize the sulfide bond, it is preferred that an electron attractive group such as the sulfone or carbonyl group exists in the next unit. As the electron attractive group, the sulfone bond which has a larger electron attractive force and which makes the membrane hydrophilic is more preferred. It should be noted, however, if the copolymer has a regularity of about 50%, sufficient reproducibility and durability are obtained in many cases. The amount of the sulfone bond and/or carbonyl group is not less than 50 mol% in order to prevent the sulfide bonds from being adjacent to each other. In the formulae [I] and [II], Ph is a phenylene group. Although the phenylene group may be substituted with a group such as a hydroxyl, hydroxyethyl, sulfonic acid, carboxylic or hydroxymethyl group, a phenylene group with no substituent is preferred having regard to the properties of the membrane. Although the phenylene group may be o-, m- or p-phenylene group, p-phenylene group is preferred having regard to durability. X represents a sulfone group or carbonyl group and sulfone group which has a larger electron attractive force is more preferred. In the formula [I], m and n represent integers. If the molecular weight of the copolymer is too large, it is difficult to dissolve the copolymer in a solvent, and on the other hand, if the molecular weight is too small, the mechanical strength of the molded product may be poor. Thus, the copolymer preferably has a molecular weight of several tens thousand to about 100,000. Therefore, m + n is 80 - 200. In other words, the copolymer preferably has an intrinsic viscosity of about 0.1 to 1.0, more preferably 0.3 to 0.6. In order to make the molecular weight of the copolymer large or to stabilize the fusion of the copolymer, an inorganic salt may be added to the copolymer, by which the formation of the porous membrane is not particularly disturbed. Examples of the inorganic salt include zinc chloride, zinc fluoride, zinc bromide, zinc iodide, zinc acetate, calcium chloride, calcium fluoride, calcium bromide, calcium iodide and calcium acetate. The copolymer may be a blend of two or more copolymers. The porous membrane of the present invention may be in any form such as in the form of flat membrane, hollow fiber or tube. The flat membrane may be formed into an element of a spiral type, plate or frame type. The tube or hollow fiber may be bundled to form an element. Each of these elements has its own characteristics and an appropriate element may be selected for a particular use. The porous membrane of the present invention, either in the form of a flat membrane or a hollow fiber, has a heterogeneous structure having a dense layer substantially controlling the separation performance of the membrane on the surface of the membrane and a porous layer inside the dense layer, of which the porosity is higher than that of the dense layer. In the case of a flat membrane, the solute is separated on the surface of the membrane and after the fluid passes through the membrane surface, the porous layer is useful for increasing the flow rate of the fluid, so that a membrane with high performance may be attained. Although the efficiency of the separation process is higher when the dense layer is thinner, if the dense layer is too thin, the dense layer cannot withstand the pressure exerted upon the membrane when a fluid is treated. Thus, the dense layer preferably has a thickness of 0.1 µm to 10 µm. As for the fractionation performance, the membrane preferably has a solute rejection, as controlled by the dense layer, of not less than 10% when polyethyleneglycol with a molecular weight of 100,000, which is often employed in the evaluation of performance of ultrafiltration membranes, is used. In other words, the membrane preferably has a pore size of about 1 to 50 nm. This is because the substances to be separated by the ultrafiltration usually have a molecular weight of 10,000 to 200,000. It should be noted, however, that since a wide variety of substances may be separated by ultrafiltration, it is preferred that the separation performance be adjustable depending on the particular use of the membrane, so that the separation performance is not restricted to the above-mentioned range. The porous layer next to the dense layer has a larger porosity than the dense layer and it serves to support the dense layer. The porosity of the membrane may be gradually increased from the dense layer to the porous layer, or it may be sharply increased at a certain region. It is preferred that the membrane has a structure which has a required mechanical strength while the flow resistance of liquid is made small. In view of the pressure resistance, the thickness of the porous flat membrane of the present invention is preferably 10 to 300 µm, and taking the flow resistance of liquid into consideration, it is more preferably 30 to 300 µm. Although the overall porosity of the membrane is not restricted, it is preferably 40 to 90%, more preferably 50 to 80% having regard to the pressure resistance and liquid flow resistance. Although the porous flat membrane may be used by itself, the mechanical strength of the membrane may be promoted by forming the flat membrane on a support which is a fabric or a non-woven fabric made from polyester, polyamide, polyethylene, polypropylene and/or polyphenylenesulfide. The polyester may be employed for a wide variety of uses, the polyamide may be employed for the uses wherein a high heat resistance is required, the polyethylene and polypropylene may be employed for the uses wherein a high chemical resistance is required, and the polyphenylenesulfide may be employed for the uses wherein a high heat resistance and high chemical resistance are required. Although the weaving density of the support is selected depending on the solvent or concentration of the casting solution later described, it is preferred to select a weaving density with which the casting solution does not penetrate to the backside of the support but permeates to a certain depth of the support. As for the hollow fiber, the shape of the hollow fiber is primarily expressed by its outer diameter and inner diameter. Although the outer diameter is not restricted, it is usually 10 mm to 40 µm and may be appropriately selected depending on the particular use. Although the inner diameter is usually about half of the outer diameter, it may be appropriately selected depending on the particular use as long as the membrane thickness is 10 µm or more, so that it is not restricted to the above-described range. The hollow fiber membrane has dense layers at its surfaces and has a porous layer inside the membrane. The dense layers are formed on the inner and outer surfaces of the hollow fiber. In the case of an element applying pressure to outer surfaces of the hollow fibers, it is preferred that the dense layer on the outer surface has a greater density, and in the case of an element applying pressure to inner surfaces of the hollow fibers, it is preferred that the dense layer on the inner surface has a greater density, although the structure of the membrane is not restricted thereto. Although the porosity (excluding the hollow portion) of the fiber is not restricted, it is usually 10% to 80%, and preferably 20% to 50% having regard to the separation efficiency. From the practical viewpoint, the liquid permeability of the porous membrane of the present invention, irrespective of the form of the membrane, is preferably not less than 0.1 m³/m²·day when pure water is passed through the membrane at a pressure of 1 kg/cm² (0.1N/mm²) at 25°C, although the liquid permeability is preferably adjusted so as to attain an appropriate separation efficiency for a particular use. The polyphenylenesulfide-based copolymer of the formula [I] which constitutes the porous membrane of the present invention may be prepared by a known method such as described in, for example, JP-A-270736/88. That is, it can be prepared by polymerizing bis(4-chlorophenyl)sulfone and sodium hydrogen sulfide under heat in N-methylpyrrolidone as a solvent. The porous membrane of the present invention may be prepared by dissolving the above-described phenylenesulfide-based copolymer in a solvent, moulding the resulting solution and by casting the moulded solution into a non-solvent of the copolymer, which is freely miscible with the solvent of the copolymer solution so as to coagulate the copolymer. It should be noted, however, the membrane may be prepared by making the membrane porous after moulding the melted copolymer, or by moulding a dispersion of the copolymer, so that the production process of the membrane is not restricted to the wet process hereinbelow described. As a preferred solvent for dissolving the copolymer, although not restricted, aprotic polar solvents, acids and phenol compounds may be employed. Specific examples of the preferred solvent include dimethylsulfoxide (DMSO), N-methylpyrrolidone (NMP), N,N-dimethyl-2-imidazolidinone (DMI), hexamethylphosphoric triamide (HMPA), sulfuric acid, dichloroacetic acid, p-chlorophenol and m-cresol because these well dissolve the copolymer. As a characteristic common to these solvents, the solubility parameters (δ) of these solvents are about 11. The δh component contributes more largely to the solubility of the copolymer than δd and δp (wherein δh, δd and δp represent hydrogen bond force, dispersion force and polarity force components, respectively). In addition to the above-described solvents, ε-caprolactam, γ-butyrolactone, sulfolane, diphenylsulfide, diphenylamine, δ-valerolactam, dimethylformamide (DMF) and tetramethylurea may, for example, be employed. The solvent may also be a mixed solvent. That is, a mixture of two or more of the above-described solvents or a mixture of the above-described solvent and a solvent other than mentioned above may be employed. For example, in the case where NMP is used as the solvent, not only the aprotic polar solvents but also halogenated hydrocarbons, ketones, organic acids and amino compounds may be added to the solvent. By the addition of these solvents, the solubility of the copolymer may be promoted and the change in the structure of the membrane or the separation efficiency due to the change in the g value (coagulation value) may be controlled. To control the pore size of the membrane, an inorganic salt, acid, non-solvent of the copolymer and/or a macromolecular compound may be added to the phenylenesulfide-based copolymer solution. As the inorganic salt, those which are well dissolved in the aprotic solvent are preferred, of which examples include sodium chloride, sodium bromide, lithium chloride, sodium iodide, potassium thiocyanate, potassium iodide, iodine, lead acetate, lead chloride, lead nitrite and potassium permanganate. Preferred examples of the acid include acetic acid, trimellitic acid, lactic acid, propionic acid, valeric acid, citric acid, benzoic acid, sulfuric acid and dichloroacetic acid. Preferred examples of the macromolecular compound include polyvinyl alcohol, polyethyleneglycol and polyvinylpyrrolidone. The amount of these additives is usually, although not restricted, 0.1 - 40% by weight. As the amount of these additives is increased, the pore size of the membrane is enlarged. The dissolving of the copolymer is usually carried out at a temperature not higher than the boiling point of the solvent. In cases where the solubility of the copolymer is extremely low, a pressure may be applied to the solvent so as to elevate its boiling point, and the copolymer may be dissolved in the solvent at a temperature not higher than the elevated boiling point. To remove contaminants, it is preferred to filter the obtained copolymer solution. Although it is preferred to conduct the filtration using a membrane filter, in cases where the viscosity of the solution is high, a cotton filter may be employed, or the filtration may be carried out at a high temperature. The resulting filtrate is hereinafter referred to as a "casting" solution". Although the concentration of the casting solution may be selected arbitrarily within the range of the solubility of the copolymer in the solvent, if the concentration is too low, the formed membrane structure is weak, and if the concentration is too high, the solution is hard to permeate. In general, if the concentration is high, a membrane with high solute rejection and low permeation is obtained. Usually, the concentration of the copolymer in the casting solution is preferably 10 - 30% by weight. In the case of flat membranes, as a concentration giving an appropriate viscosity for obtaining practical membrane properties, a concentration of 15 - 25% by weight is preferred. Since the hollow fiber is not supported by a substrate, the concentration of the casting solution is preferably higher than that employed in the preparation of flat membranes. That is, 18 - 30% by weight is usually preferred. Having regard to the retention of the shape of the hollow fiber during the spinning step and of the separation performance, a viscosity of not less than 0.5 poise is preferred. When preparing a flat membrane, the casting solution is cast on a substrate in a prescribed thickness. When preparing a hollow fiber, the casting solution is extruded from a double spinneret and the spun fiber is introduced into a non-solvent of the copolymer, which is miscible with the solvent of the casting solution. When preparing a flat membrane, the thickness of the casting solution is preferably 30 - 600 µm because good membrane performance may be obtained. A hollow fiber may be prepared by extruding the copolymer solution from a double spinneret while spurting out a gas or a liquid from the inner spinneret so as to form a hollow fiber structure, and introducing the obtained fiber into a coagulating liquid. The separation performance of the membrane may be controlled by controlling the temperature at the spinneret, temperature of the coagulating liquid, composition of the liquid to be spurted out, the time from the extrusion to the coagulation and/or the draft value. The temperature of the casting solution, the time from the casting to the coagulation, the temperature and humidity of the environment from the casting to the coagulation, and the temperature of the coagulating liquid influence the separation performance of the membrane. In many cases, if the temperatures are low, a membrane with high solute rejection and low permeability is obtained. The coagulation liquid is a liquid which is freely miscible with the solvent of the casting solution and may be a single substance or a mixture of liquids. To control the coagulation speed of the casting solution, an inorganic salt and/or an organic compound may be added to the casting solution. Preferred examples of the inorganic salt include sodium chloride, sodium bromide, lithium chloride, sodium iodide, potassium thiocyanate, potassium iodide, potassium chloride, iodine, lead acetate, lead chloride, lead nitride and potassium permanganate. Preferred examples of the organic compound may include methanol, ethanol, isopropanol, butanol, polyethyleneglycol, polyvinylpyrrolidone, polyvinyl alcohol, N-methylpyrrolidone, N,N-dimethyl-2-imidazolidone, dimethylformamide, dimethylsulfoxide and hexamethylphosphoric triamide. In cases where it is desired to slow down the coagulation speed, a good solvent of the copolymer may be added to the coagulation liquid. In cases where an aprotic polar solvent and an acid are used as the casting solution, the coagulation liquid is preferably selected from water, methanol, ethanol, isopropanol and butanol. The temperature and composition of the coagulation liquid are important factors in controlling the separation performance of the membrane. Usually, the temperature of the coagulation liquid is preferably 0 - 50°C. By oxidizing the thus obtained porous membrane so as to convert a part of the sulfide bonds into sulfone bonds the heat resistance, chemical resistance and hydrophilicity of the membrane are further promoted. The oxidation may be carried out by using an oxidizing agent. Preferred examples of the oxidizing agent include hydrogen peroxide, an organic peroxide (e.g., peracetic acid, perbutyric acid, perbenzoic acid or chloroperbenzoic acid), alkali metals, alkaline earth metals, quaternary ammonium hypochlorite, chlorine, chromic acid, alkali metal permanganates and nitrates. Since the oxidation reaction is a solid-liquid heterogeneous reaction, the oxidizing agent per se and/or the solvent thereof should be one which can swell the copolymer without changing the microstructure of the membrane so much. From this viewpoint, organic peroxides and/or quaternary ammonium hypochlorite dissolved in an organic solvent are especially preferred. In particular, peracetic acid, chloroperbenzoic acid and tetrabutylammonium hypochlorite solution in ethyl acetate are especially preferred for efficiently promoting the durability of the copolymer. The oxidation may be carried out by immersing the porous membrane in a solution of the oxidizing agent. Although the temperature of the solution of the oxidizing agent is not restricted, usually 0 - 50°C is preferred. To carry out the reaction efficiently, the reaction may be conducted at a temperature higher than room temperature. In this case, however, care should be taken not to cause explosion if a peroxide is used. In cases where the reaction typically proceeds severely, it is desired to cool the reaction mixture at a temperature lower than room temperature. Although the time of the oxidation treatment is appropriately selected depending on the concentration of the oxidizing agent and the reaction temperature, it is usually 10 minutes to 5 hours. After the oxidation treatment, the porous membrane may be washed with water and/or alcohol to obtain a durable porous membrane of the present invention. The methods of measuring the characteristics concerning the present invention will now be summarized. i) Porosity A piece with a prescribed size is cut out from the porous membrane and an average thickness is measured to obtain an apparent volume. Then the weight of the porous membrane is measured to obtain the real volume. The porosity is defined by the following equation: ii) Solute Rejection When a liquid containing a solute is passed through the membrane, the solute rejection is defined by the following equation: (wherein A represents the concentration of the solute in the permeate and B represents the concentration of the solute in the supplied liquid before passing through the membrane) iii) Water Flux The weight of a liquid passing through the membrane is measured over a prescribed time at 25°C, and the volume of the liquid in terms of m³ which passed through 1 m² of the membrane in 24 hours is defined as the water flux. iv) Solubility Parameter (6) The δ value of low molecular weight compounds can be determined from the heat of vaporization and that of macromolecular compounds is determined from the degree of swelling or Small's equation. However, the δ values of many substances are given in the Polymer Handbook (published by Interscience Co., Ltd.). v) g value A 2% copolymer solution is prepared and water is added dropwise to the solution at 30°C. The amount of the added water at which the copolymer starts to precipitate is defined as the g value. Embodiments of the invention will now be described in more detail by way of examples thereof. Example 1 Poly(phenylenesulfidesulfone) (PPS) was synthesized by the method described in JP-A-270736/88. That is, into an about 1 litre autoclave equipped with means for measuring temperature and pressure, a stirrer and an outer heating means, 154.7 g of bis(4-chlorophenyl)sulfone, 56.5 g of sodium carbonate, 43.7 g of sodium acetate, 50.7 g of sodium hydrogen sulfide (supplied as an aqueous solution containing 59.0% by weight of NaSH), 211.7 g of N-methyl-2-pyrrolidone (NMP) and 14.4 g of deionized water were fed. The mixture was stirred while heating the mixture from 25°C to 200°C and was stirred for 3 hours at 200°C. Then a mixture of 160 ml of NMP and 26.7 ml of deionized water was added and the stirring was continued until the temperature of the mixture reached about 150°C. The reaction mixture was removed from the reactor as solid granules and the liquid was aspirated. The obtained solid was washed with hot deionized water (about 90°C, about 600 ml), filtered and once rinsed on the filter. These operations were repeated twice and the resultant solid was washed with cold deionized water to remove water-soluble impurities to complete the washing process. Into a one litre autoclave, 40 g of the purified copolymer obtained as above, 400 g of deionized water and 4.0 g of zinc acetate [Zn(C₂H₃O₂·2H₂O] were fed. The autoclave was equipped with stirrer, heating/cooling apparatus, a thermometer and a pressure gauge. While stirring the mixture of the copolymer/zinc acetate aqueous solution, the temperature of the mixture was raised to 185°C and was retained at this temperature for one hour. The mixture was then cooled to room temperature, and the recovered copolymer was washed once with hot water (about 90°C, 400 ml) while stirring the mixture. The recovered copolymer was dried at 160°C under reduced pressure. The weight average molecular weight of the thus obtained copolymer determined by gel permeation chromatography was 31600 (in terms of polystyrene). The infrared spectrum of the copolymer is shown in Fig. 1. The thus obtained copolymer is hereinafter referred to as "PPSS" (polyphenylenesulfidesulfone). The PPSS is expressed by the formula -(Ph-S-Ph-SO₂-) q - Fifteen grams of the thus obtained PPSS was added to 85 g of dry N-methylpyrrolidone, and the vessel containing the mixture was placed in a nitrogen atmosphere, followed by raising the temperature of the mixture to 180°C to dissolve the copolymer. The resulting solution was filtered through a membrane filter made of polytetrafluoroethylene having a pore size of 1 µm to remove the insoluble materials. This solution was cast on a glass plate to an average thickness of 150 µm at room temperature (20°C), and then the resultant film was immediately immersed in water (25°C) to obtain a porous membrane. The thus prepared porous membrane had a pure water permeation of 6.24 m³/m²·day under a pressure of 5 kg/cm² (0.5 N/mm²) at 25°C. As to the separation performance, the thus prepared porous membrane had a solute rejection of 98.0% and water flux of 1.22 m³/m²·day which were determined using 1000 ppm aqueous polyethylene glycol (molecular weight 100,000) at a pressure of 5 kg/cm² (0.5 N/mm²) at 25°C after three hours from the beginning of the filtration. Example 2 The same procedure as in Example 1 was repeated except that the casting solution was cast on a taffeta (both of the warp and weft were multifilaments of 150 denier (16.7 tex), weaving density of 90 warps/inch (35 warps/cm) and 67 wefts/inch (26 wefts/cm), 160 µm thickness) fixed on a glass plate. The pure water permeation was 12.85 m³/m²·day, and the solute rejection and water flux after 3 hours from the commencement of the filtration of the polyethyleneglycol solution was 95.0% and 2.27 m³/m²·day, respectively. A photograph taken with a scanning electronmicroscope, which shows the state of the cross section of the obtained membrane is shown in Fig. 2. A photograph taken with a field emission type scanning electronmicroscope, which shows the cross section of a porous membrane prepared by the same procedure as just mentioned above except that the solvent was dichloroacetic acid, is shown in Fig. 3. Examples 3 - 5 The same procedure as in Example 2 was repeated except that the PPSS concentration of the casting solution was changed. The results are shown in Table 1. Concentration of PPSS Casting Solution and Separation Performance PPSS Concentration (wt%) Solute Rejection (%) Water Flux (m³/m²·day) Example 3 16.0 91.8 1.27 Example 4 17.0 98.7 1.47 Example 5 18.0 98.0 1.44 Evaluation Conditions: Polyethyleneglycol 100,000 (1,000ppm) Pressure 5kg/cm², 25°C. Values after 3 hours' operation Examples 6 - 11 The same procedure as in Example 2 was repeated except the composition of the casting solution and that the molecular weight of the polyethyleneglycol used for the evaluation of the separation performance was 20,000. The results are shown in Table 2. When the solvent composition of Example 11 was employed, the elution of the fine particles from the membrane surface when the membrane was coagulated, which was observed in Examples 1 - 10, was reduced very much. Examples 12 - 41 The PPSS copolymer obtained in Example 1 was added to dry N,N-dimethyl-2-imidazolidinone (DMI) and the vessel containing the mixture was placed in a nitrogen atmosphere. After dissolving the copolymer by heating the mixture to 180 - 190°C, the insoluble materials were filtered off. The taffeta used in Example 2 was fixed on a glass plate, and the copolymer solution was cast on the polyester taffeta to an average thickness of 150 µm at room temperature (25°C). The resultant was then immediately immersed in water to obtain a porous membrane. In the process just mentioned above, the relationship between the separation performance of the membrane and the concentration of the casting solution, temperature of the coagulation liquid and/or the thickness of the casting solution is shown in Table 3. Example 42 To 75.6 ml or acetic acid, 22.8 ml of 30% aqueous hydrogen peroxide solution and 1.6 ml of concentrated sulfuric acid were added and the resulting mixture was left to stand for one day to obtain a peracetic acid solution. The porous membrane obtained in Example 1 was immersed in the thus obtained peracetic acid solution for three hours and the resulting membrane was washed with water. The separation performance of the membrane was evaluated using 1000 ppm aqueous polyethyleneglycol (molecular weight 100,000) solution under a pressure of 5 kg/cm² (0.5 N/mm²) and a temperature of 25°C. The solute rejection was 97.7% and the water flux was 1.23 m³/m²·day. In the above-mentioned treatment with peracetic acid, the change in the composition of the porous membrane with time was analyzed by elemental analysis, the results of which are shown in Table 4. Change in Composition of Porous Membrane Time (minute) Carbon (%) Hydrogen (%) Oxygen (%) 0 57.1 3.0 13.0 10 52.4 2.7 20.3 60 51.2 2.6 22.1 The morphology of the membrane was not changed after immersing the membrane in concentrated sulfuric acid for one hour at room temperature and no endothermic peak was observed up to 400°C in differential scanning calorimetry (DSC), so that the membrane was stable. Example 43 The porous membrane obtained in Example 2 was immersed in the peracetic acid solution prepared in Example 42 at room temperature for 3 hours and then the membrane was washed with water. The separation performance of the membrane was evaluated using 1000 ppm aqueous polyethyleneglycol (molecular weight 100,000) solution under a pressure of 5 kg/cm² (0.5 N/mm²) and a temperature of 25°C. The solute rejection was 91.9% and the water flux was 1.30 m³/m²·day. The contact angle of the interface between this membrane and water was 52°. Using 12.6 cm² of this membrane, ten-fold diluted bovine serum albumin (5 mg/ml) was two-fold concentrated under a pressure of 1.4 kg/cm² (0.14 N/mm²), and the loss of protein due to the adsorption by the membrane was measured by Bradfold's method. The protein loss was 4.0%. Examples 44 - 50 The same procedure as in Example 2 was repeated except that the support was a polyester non-woven fabric (MF-110 commercially available from Nippon Vilene) to obtain porous membranes. The obtained porous membranes were immersed in various respective liquids and their separation performance was evaluated. The results are shown in Table 5. Examples 51 - 58 The same procedure as in Example 43 was repeated except that the support was a polyester non-woven fabric to obtain porous membranes. The obtained porous membranes were immersed in various respective liquids and their separation performance was evaluated. The results are shown in Table 6. The CP/MAS¹³CNMR spectrum of the resulting oxidized membrane is shown in Fig. 4. Example 59 Two hundred fifty grams of the PPSS copolymer obtained in Example 1 was added to dry N-methylpyrrolidone and the vessel containing the mixture was placed in a nitrogen atmosphere. After dissolving the copolymer by heating the mixture to 180°C, the insoluble materials were filtered off. The obtained solution was extruded from a double spinneret (outer diameter 0.8 mm, inner diameter 0.6 mm) at an extrusion rate of 2.04 g/minute while injecting an injection solution (NMP/water = 60:40, injection rate of 0.75 g/minute). The extruded solution with a dry length of 5 mm was introduced into a water bath (25°C) and was drawn at a rate of 10 m/minute. After washing the resulting membrane twice (22 seconds each) with water at 30°C, the membrane was treated with hot water at 94°C for 36 seconds. The resulting hollow fiber membrane was cut into pieces with a length of 50 cm. Thirty pieces were bundled and the bundle was inserted into a glass tube with a diameter of 15 mm which had an outlet for the filtrate. The end of the glass tube was sealed with epoxy resin and both ends of the glass tube were cut off. The length of the hollow fibers in the resulting element was 26 cm. Distilled water with a pressure of 1.7 kg/cm² (0.17 N/mm²) was passed through the inner side of the hollow fiber. The amount of the permeated water was 5.86 m³/m²·day. Example 60 The same procedure as in Example 59 was repeated except that the hollow fiber was immersed in the peracetic acid solution prepared in Example 42 for 3 hours. The amount of the permeated water was 2.38 m³/m²·day. Further, water permeability was not degraded after the passage of methanol or tetrachloromethane for not less than 6 hours. Example 61 To 40 ml of aqueous sodium hypochlorite solution (free chlorine concentration of 4%), 40 ml of ethyl acetate and 1 g of tetrabutylammonium bromide were added and the mixture was vigorously stirred. The mixture was then left to stand until it separated into two phases and the ethyl acetate layer was recovered. The membrane obtained in Example 2 was immersed in the thus obtained solution at room temperature for 3 hours. By this treatment, the membrane became insoluble in methylene chloride or N-methylpyrrolidone. The separation performance of the membrane was evaluated using 1000 ppm aqueous polyethyleneglycol (molecular weight 100,000) solution under a pressure of 1 kg/cm² (0.1 N/mm²) and a temperature of 25°C. The solute rejection was 87% and the water flux was 1.9 m³/m²·day. The above exemplified porous membranes of the present invention are excellent in their durability such as heat resistance, chemical resistance and solvent resistance. Thus, the porous membrane of the present invention has a satisfactory durability even when it is used in the conditions which are severe for conventional ultrafiltration membranes, such as in the separation or recovery of solvents, treatment of vapor of organic compounds, treatment of hot liquids, thermal sterilization, sterilization with chlorine and washing with an alkali. Further, the porous membrane of the present invention can be used as a support of a composite reverse osmosis membrane or a support of a gas separation membrane. Thus, the porous membrane of the present invention may be used in various fields such as production of pure water for the electronics industry, recovery of electroplating coatings, treatment of wastes of paper-producing or pulp factories, treatment of oil-containing wastes, recycling of the wastes from buildings, treatment of wastes in atomic power stations, clarification of fruit juices, production of live sake, concentration of whey, desalination, production of concentrated milk, concentration of egg white, treatment of soybean proteins, recovery of enzymes and bioreactors.

Claims: A porous membrane having a heterogeneous structure, which consists essentially of a phenylenesulfide-based copolymer of the following formula [I]: -(Ph-S-) m -(-Ph-X-) n - [I] wherein Ph is a phenylene group, X is -SO₂- or -CO-, the total of m and n is 80-400 and [n/(m+n)] x 100% is not less than 50%, which membrane has a dense layer on one or both surfaces of the membrane, the dense layer controlling the separation performance of the membrane, and a porous layer next to the dense layer, which porous layer has a porosity higher than that of the dense layer. A porous membrane according to claim 1, wherein the phenylenesulfide-based copolymer is expressed by the following formula [II]: -(-Ph-S-Ph-X-) q - [II] wherein Ph and X are as defined in claim 1, and q is an integer of 80-400. A porous membrane according to claim 2, wherein X is -SO₂-. A porous membrane according to any preceding claim, wherein Ph is a p-phenylene group. A porous membrane according to any preceding claim, which has a thickness of 10-100µm. A porous membrane according to any preceding claim, wherein the porosity of the overall membrane is 20 to 80%. A porous membrane according to any preceding claim, having a pure water permeability, under a pressure of 1 kg/cm² (0.1N/mm²) at 25°C, of 0.1 m³ /m².day. A porous membrane according to any preceding claim, which is in the form of a flat membrane and is laminated on a woven or a non-woven fabric made of any of polyesters, polyamides, polyethylenes, polypropylenes and polyphenylenesulfides. A porous membrane according to any one of claims 1 to 7, which is in the form of a flat membrane, hollow fiber or a tube. A porous membrane according to any preceding claim, which is oxidized. A process of producing the porous membrane of Claim 1 comprising the steps of dissolving the phenylenesulfide-based copolymer represented by the formula [I] in at least one solvent selected from N-methylpyrrolidone, N,N-dimethyl-2-imidazolidone, hexamethylphosphoric triamide, sulfuric acid and dichloroacetic acid; moulding the resulting solution; and coagulating the moulded solution in a non-solvent of the phenylenesulfide-based copolymer, which is freely miscible with said solvent. A process according to claim 11, wherein the non-solvent of the phenylenesulfide-based copolymer is selected from at least one of water, methanol, ethanol and propanol. A process according to claim 11 or 12, wherein the temperature at which the copolymer is coagulated in the non-solvent of the phenylenesulfide-based copolymer is 0-50°C. A process according to claim 11, 12 or 13, wherein the concentration of the phenylenesulfide-based copolymer in said solution is 10-30% by weight. A process according to any of of claims 11 to 14, wherein the thickness of the moulded solution is 30-300µm. A process according to any one of claims 11 to 13, further comprising the step of adding to said solution of phenylenesulfide-based copolymer an inorganic salt, an acid, a non-solvent of the phenylene sulfide-based copolymer and/or a macromolecular compound. A process according to any one of claims 11 to 16, further comprising the step of oxidising the porous membrane. A process according to claim 17, wherein the oxidation is conducted using an oxidizing agent selected from peracetic acid, chloroperbenzoic acid and tetrabutylammonium hypochlorite. A process according to claim 17 or 18, wherein the temperature in the oxidation step is 0-50°C. A process according to any one of claims 17 to 19, wherein the oxidation is carried out in the presence of a solvent, at least one of the oxidizing agent and solvent being capable of swelling the phenylenesulfide-based copolymer.

Inventors: HIMESHIMA YOSHIO, KURIHARA MASARU, TANISUGI HIDEAKI, UEMURA TADAHIRO ROOM, WATANABE TETSUO, HIMESHIMA, YOSHIO, KURIHARA, MASARU, TANISUGI, HIDEAKI, UEMURA, TADAHIRO, ROOM 502, WATANABE, TETSUO
Applicants: TORAY INDUSTRIES, TORAY INDUSTRIES, INC.
Application year: 1989
Main classification: B01D71
IPC codes: B01D71