Asymmetric polymer membrane and preparation thereof

Abstract: An asymmetric polymer membrane is disclosed characterized by being formed by irradiating a monomer and/or an oligomer poylmerizable with an energy ray and having a pore diameter distribution in the thickness direction of the membrane, a ratio of a permeability flux of oxygen to nitrogen of 0.9-1.1, and communicating pores.

Description: The present invention relates to a method of preparing an asymmetric polymer membrane according to the preamble of claim 1 or 3, and to an asymmetric polymer membrane as defined in claim 4. The membrane of the present invention is a porous membrane usable as an ultrafiltration membrane, a reverse osmosis membrane, a microfiltration membrane, etc. for the purpose of filtration and separation of proteins, colloids, bacteria, viruses, salts, etc., and in various separation processes in the food industry, pharmaceutical industry, and electronics industry, and in waste water treatment, artificial organs, purification of fresh water from sea water, etc. A method for preparing porous membranes utilizes a so-called wet process in which a polymer is dissolved in a solvent to form a sol which is transformed into a gel. In another method both a polymer compound and a material extractable by a nonsolvent for the polymer are dissolved in a solvent suitable for both compounds, and after a film has been made by evaporating the solvent, the film is extracted with the non-solvent. However, in these methods, not only are there problems in that the production speed is slow, but also in that it is necessary to prepare a polymer solution, and it is impossible to obtain a porous membrane having excellent strength, heat resistance and chemical resistance. In order to solve these problems, Japanese Patent Publications No. 34,329/1981 and No. 65,220/1988 describe a method for the preparation of a porous membrane with a cross-linked structure, in which a polymerizable monomer and/or oligomer are polymerized in the presence of a solvent which acts as a solvent for the monomer and/or oligomer, but does not swell a polymer made of these monomers. Furthermore, in Japanese Laid Open Patent No. 107,062/1974 an asymmetric membrane is described as consisting of a covalently cross-linked vinyl polymer. A porous membrane with excellent strength, heat resistance, and chemical resistance can be obtained with high production speed by the method of Japanese Patent Publications No. 34,329/1981 and No. 65,220/1988. However, since the porous membrane has a uniform pore diameter distribution in the cross-sectional direction of the membrane, in order to obtain a porous membrane with a desired filtering separability, it is necessary to select membrane forming conditions in very narrow ranges. In addition, when filtering separation is performed using the porous membrane, there are problems in that the permeation speed of the filtrate is extremely low and fouling of the membrane occurs easily. Furthermore, the method of Japanese Patent Publication No. 65,220/1988 is restricted to the preparation of a finely porous membrane with a fine pore dimension of 0.02-15 micrometers and without the capability of molecular weight cut-off. In addition, in the Proceedings of the Fourth Annual Membrane Technology/Planning Conference 231 (1986), there is a description of an asymmetric polymer membrane obtained by irradiating a homogeneous polymerizable solution of a monomer and/or an oligomer polymerizable by irradiation by an energy ray in a solvent which acts as a solvent for the monomer and/or the oligomer and which does not swell or dissolve a polymer produced of the monomer and/or oligomer cured by an energy ray. However, in this reference, only a description of a gas separation membrane with a ratio of permeation of oxygen to nitrogen of 2.9-4.2 is given; that is, a membrane having no pores which communicate with one another which could let a liquid pass through, and no description is given of a practical method for preparation thereof. In addition, in a method of Japanese Laid Open Patent Publication No. 107,062/1974 (corresponding to DE-A-2 301 553) an asymmetric membrane is prepared by exploiting the freezing of a solvent (hereinbelow described as a "freezing method"). This results in the thickness of a layer with a smaller pore diameter than that of the other parts in the thickness direction of the membrane (hereinbelow described as a dense layer) being 50 micrometers or greater. This results in the filtering rate being very low and the pore diameter of a layer with a larger pore diameter than that of other parts in the direction of thickness of the membrane (described below as a "porous supporting layer") being such that the membrane has insufficient strength. This is not practical. The document GB-A-2 124 973 (corresponding to the previously mentioned Japanese Patent Publication No. 65220/1988), in particular, describes a method of the kind mentioned initially for manufacturing a fluid-permeable microporous membrane by dissolving a radiation-polymerizable material selected from organic monomers and/or oligomers in a liquid vehicle, forming the resulting composition into a thin layer, exposing one or both sides of the layer to ultraviolet or electron beam irradiation to polymerize the material to a solid polymer which, being insoluble in the vehicle, segregates from the liquid vehicle to form a membrane, and then removing the liquid vehicle from the membrane by evaporation or washing. When ultraviolet radiation is used, a photoinitiator may be included in the solution. The document US-A-3 926 864 describes a polymer membrane which is a substantially insoluble, cross-linked, polymeric, non-laminated, one piece resin sheet suitable for the manufacture of ion-selective membranes by the introduction of ion-exchange groups. Both major surfaces of the sheet are of a rigid macroporous structure having pore diameters between about 0.05-0.15 µm, the remaining interior portion of the sheet substantially containing a gel type microporous structure having a pore diameter in the range of about 0.001-0.003 µm, and each macroporous surface or layer having a thickness between about 2.54 to 254 µm. The macroporous surface or layer is reported to lessen the tendency for membranes especially of the anion selective type to become fouled, i.e. clogged by material lodged or formed therein, when employed in the electrodialysis of solutions containing fouling constituents. Since the microporous portion of the membrane is of a gel type, crushing of the macroporous structure (which is called "compaction") occurs when pressure is applied to the membrane in order to filtrate, which lowers the flux extremely. Thus, such a membrane cannot be used in ultrafiltration, in which a membrane is normally used under a pressure difference ranging between 0.1 and 1 MPa. The resin sheet is prepared by polymerizing a mixture such as that of at least one monovinyl aromatic monomer with at least one polyvinyl aromatic cross-linking member in a mixed solvent containing at least one solvent that is totally miscible with the starting monomers and capable of acting as a swelling agent for the resulting copolymer, and at least one "poor" solvent that is miscible with the monomers but does not swell or become dissolved in the copolymer. The polymerization is performed under conditions preventing the escape of the poor solvent from the monomer mixture. The polymerization may be performed by irradiating the solution, for example with light or gamma rays. The document JP-A-63 69 845 describes a porous polymer material having pores of micrometer size, wherein the terminal groups of the polymer molecules are located on a surface of the material, and also a method for producing the porous polymer material by casting a solution of a curable prepolymer into final shape, drying the solution to provide a porous structure, and then curing the porous prepolymer by irradiating it with electromagnetic radiation that may consist of electron beams, alpha rays, gamma rays or ultraviolet rays. The porous structure is controlled by the selection of the type and/or composition of the solvent. It is the object of the present invention to provide a method for preparing an asymmetric polymer membrane having the features of the preamble of patent claims 1 and 3, the method allowing the membrane to be produced rapidly and with an arbitrarily choosable separating performance when used as a filter. It is also an object of the present invention to provide an asymmetric polymer membrane which exhibits excellent strength, heat resistance and chemical resistance, and with which, when used as a filter, a high or good filtering speed and, if desired, a high molecular weight cut-off capability can be attained, and which hardly becomes fouled. In accordance with the present invention these objects are achieved by the features of the characterizing portions of patent claims 1 or 3, and by the membrane of claim 4, respectively. The present inventors have performed extensive research to solve the above-described problems, and the present invention is the result of their efforts. That is, the present invention provides an asymmetric polymer membrane formed by irradiating a solution of a monomer and/or an oligomer which are polymerizable by irradiation with an energy ray; the membrane having an asymmetric pore diameter distribution in the thickness direction of the membrane and a ratio of permeation flux of oxygen/nitrogen (hereinbelow abbreviated as a "separation factor") of 0.9-1.1 and with pores which communicate with each other; and a method for preparation thereof. In an embodiment the thickness of a dense layer is 5 micrometers or less, and the pore diameter of the pores in the dense layer is 0.0005-0.015 micrometer, and the membrane has a molecular weight cut-off capability. In another embodiment the pore diameter of the dense layer is 0.02-20 micrometers and the membrane does not have a molecular weight cut-off capability. Preferably the pore diameter of a porous supporting layer is 0.1-20 micrometers. The monomer and/or oligomer can be used as a homogeneous polymerizable solution by mixing with a first solvent which acts as a solvent for the monomer and/or the oligomer, but does not swell or dissolve a polymer formed of the monomer and/or oligomer. In addition, it is possible to incorporate a second solvent which swells or dissolves a polymer formed of the monomer and/or oligomer in the homogeneous polymerizable solution. An asymmetric polymer membrane with a pore diameter distribution in the thickness direction of the membrane can be obtained by polymerizing the homogeneous polymerizable solution by irradiating it with an energy ray after a part of the first solvent and/or the second solvent is evaporated from the homogeneous polymerizable solution. In addition, an asymmetric polymer membrane with a pore diameter distribution in the cross-sectional direction of the thickness of the membrane can be obtained by polymerizing the homogeneous polymerizable solution by irradiating it with an energy ray under a condition in which a temperature gradient is provided in the thickness direction of the membrane. As an asymmetric polymer membrane of the present invention has a pore diameter distribution in the thickness direction of the membrane and communicating holes capable of allowing a liquid to pass through, an arbitrarily chosen filtering separator can be easily obtained in which the filtering speed is high and in which fouling is unlikely to occur. In addition, as it has a cross-linked structure, strength, heat resistance, and chemical resistance of the membrane are excellent. Furthermore, there exists an advantage of high production speed. In addition, in comparison with conventional wet methods and other methods for preparing asymmetric membranes, the method of preparation of the present invention is such that chemical modifications of the membrane are easy to perform; a cross-linked structure can be introduced into the membrane; the production speed is high as the phase separation is substantially and spontaneously completed; use of a high boiling point polar solvent such as dimethylformamide, the waste treatment of which is difficult, can be avoided; and, in comparison with methods for introducing a cross-linked structure, such as by cross-linking a film of a thermoplastic polymer by radiation, etc. , it has such features that the production speed is high; an apparatus on a large scale is unnecessary; the range of selection of polymers is wide; and a desired cross-linking density and cross-linking structure are easy to produce. As monomers used for the present invention, monofunctional monomers such as ethyl (meth)acrylate, hydroxyethyl (meth)acrylate, n-butyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, phenyl (meth)acrylate, phenylcellosolve (meth)acrylate (Cellosolve is a trademark of UCC), n-vinylpyrrolidone, isobornyl (meth)acrylate, dicyclopentenyl (meth)acrylate and dicyclopentenyloxyethyl (meth)acrylate, difunctional monomers such as diethylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,6-hexane diol di(meth)acrylate, polyethylene glycol di(meth)acrylate, 2,2-bis(4-(meth)acryloyloxypolyethylene-oxyphenyl)propane and 2,2-bis(4-(meth)acryloyloxypoly-propyleneoxyphenyl)propane, trifunctional monomers such as trimethylolpropane tri(meth)acrylate and trimethylolethane tri(meth)acrylate, tetrafunctional monomer such as pentaerythritol tetra(meth)acrylate, and hexafunctional monomers, such as dipentaerythritol hexa(meth)acrylate, can be cited. As examples of oligomers used in the present invention, those which are polymerizable by irradiation by an energy ray and have a weight average molecular weight of 500-50,000 can be cited; and in particular, an acrylate or methacrylate of an epoxy resin, an acrylate or methacrylate of a polyester resin, an acrylate or methacrylate of an polyether resin, an acrylate or methacrylate of a polybutadiene resin and a polyurethane resin having acrylic groups or methacrylic groups on its molecular terminals, can be cited. Selection of the monomer and/or the oligomer may be determined by the necessary heat resistance, strength, molecular weight cut-off capability, etc., of the polymer membrane. For example, to obtain a polymer membrane with excellent heat resistance, polyfunctional monomers and/or oligomers are selected. In contrast, when no heat resistance is required, only monofunctional monomers and/or oligomers may be selected. Furthermore, to produce such a polymer membrane which can filter substances having relatively small molecular weights, it is preferable that polyfunctional monomers and/or oligomers be selected, and that the molecular weight between cross-links be small. In contrast, to produce a polymer membrane which can filter substances with relatively large molecular weights, it is preferable that monomers and/or oligomers with low functionalities and large molecular weights be selected in order to make the molecular weight between cross-links large. As the first solvent used in the present invention, any solvent that can homogeneously dissolve monomers and/or oligomers used in the present invention, and which does not swell or dissolve the polymers formed of these monomers and/or oligomers, can be used. For example, when a polyurethane resin with acrylic groups on the ends of the molecule is used as an oligomer, alkyl esters such as methyl caprate, and dialkyl ketones such as diisobutyl ketone, etc., are preferably used as the first solvent. Solubility and boiling point of the first solvent can be suitably selected depending on the kind of the monomer and/or the oligomer, the necessary molecular weight cut-off capability, and the necessary degree of structural asymmetry. When a method of preparation is used in which a part of the first solvent is evaporated, there are correlations between the evaporation of the first solvent and the degree of the asymmetry of the structure; and the degree of the asymmetry of the structure correlates with the molecular weight cut-off capability and the amount of permeation of a filtrate. Therefore, selection of the boiling point of the first solvent is possibly one of the important factors determining the filtering characteristics of the obtained polymer membrane. One example related to the selection of boiling point of the first solvent is that when a part of the first solvent is evaporated at or below room temperature, and when the speed of the air flow blown on a polymerizable solution for evaporating a part of the solvent is low, or when the first solvent should be evaporated in an extremely short time, a substance with a boiling point of 80 °C or lower can be used as the first solvent. In addition, when heated air is blown on the polymerizable solution, or when the first solvent should be evaporated over a certain prolonged period, a substance with a boiling point of 60 °C or higher can be used as the first solvent. In addition, the first solvent may be a mixture of two or more. As the second solvent used in the present invention, any solvent that can homogeneously dissolve monomers and/or oligomers, and can swell or dissolve polymers formed of these monomers and/or oligomers, may be used. For example, when a polyurethane resin with acrylic groups on its molecular ends is used as the oligomer, acetone, methyl ethyl ketone, ethyl acetate, dimethylformamide, n-methyl-pyrrolidone, etc., can be suitably used as the second solvent. Solubility and boiling point of the second solvent can be appropriately selected, depending on the kinds of the monomers and/or the oligomers, the necessary molecular weight cut-off capability, and the necessary degree of the asymmetry in the structure. When a second solvent is incorporated in a polymerizable solution, the range of solubility control is widened, and accordingly, the range of selection of first solvents, polymerizable monomers and oligomers, and additives can be widened, and it thus becomes easier to improve membrane characteristics and to prepare a membrane in accordance with the desired use and purpose. In addition, it becomes easier to form a dense layer at an arbitrary position such as on the gas phase side, on the supporting body side or on the inner part of the membrane, by controlling the combination of boiling points of the first solvent and the second solvent. As in the case of the boiling point of the first solvent, selection of the boiling point of the second solvent is also possibly one of the important factors determining filtering characteristics of the obtained polymer membrane, and it is necessary to select it on the basis of the same considerations as in the case of the first solvent. In addition, in many cases, there exists a correlation between solubility of the second solvent and the molecular weight cut-off capability of the obtained polymer membrane. To give one example, in order to obtain a polymer membrane which can filter substances with relatively small molecular weights, a second solvent with a high solubility can be used. In addition, to obtain a polymer membrane which can filter substances with relatively large molecular weights, a second solvent with a low solubility may be used. The weight ratio of "a first solvent and a second solvent" to "a monomer and/or an oligomer" is preferably in the range of 0.1-4.0 per monomer and/or oligomer. If it is 0.1 or less, a sufficient amount of permeation is not obtained, and if it is 4.0 or larger, the strength of the membrane will be insufficient. As energy rays used in the present invention, electron beams, gamma rays, X-rays, UV rays, visible rays, etc., may be used. Among these rays, it is desirable to use UV rays because of the simplicity of the apparatus therefor and manipulation thereof. The intensity of the irradiating UV rays is preferably 10-500 mW/cm 2 , and the irradiation time is usually about 0.1-100 seconds. When UV rays and visible rays are used as energy rays, it is possible to incorporate an photopolymerization initiator in the polymerizable solution to accelerate the rate of polymerization. In addition, it is possible to further accelerate the polymerization by performing the irradiation with UV rays under an inert gas. An electron beam is another preferred energy ray which may be used for the present invention. When an electron beam is used, since there is no absorption of UV rays by the second solvent, the first solvent, and the other additives, the range of selection of these substances is widened and the rate of membrane formation is also improved. As UV polymerization initiators which can be mixed with a polymerizable solution in the present invention, acetophenones such as p-tert-butyltrichloroacetophenone, 2,2-diethoxyacetophenone and 2-hydroxy-2-methyl-1-phenylpropane-1-on, ketones such as benzophenone, 4,4-bisdimethylaminobenzophenone, 2-chlorothioxantone, 2-methylthioxantone, 2-ethylthioxantone and 2-isopropylthioxantone, benzoin ethers such as benzoin, benzoin methyl ether, benzoin isopropyl ether and benzoin isobutyl ether, benzyl ketals such as benzyl dimethyl ketal and hydroxycyclohexyl phenyl ketone, can be cited. For transforming a polymerizable solution into a thin membrane in the present invention, a method of coating a supporting body with a polymerizable solution by means of a roll coating method, a doctor blade method, a spin coating method, a spray method, etc., may be used. As the supporting body, a metal, a ceramic, a glass, a plastic, a fabric, a non-woven fabric, a paper, etc., can be used. When a belt-like material is used as the supporting body, a continuous thin film can be formed. The membrane can eventually be peeled off from the supporting body, and when the supporting body is porous, the product can be used integrated as it is. In addition, it is possible to use a method in which a thin membrane is formed without using a supporting body. In the specification of the present invention, the phrase "having a pore diameter distribution in the thickness direction of the membrane" means that a large number of pores of various diameters which vary continuously or discontinuously in the thickness direction of the membrane exist in the membrane. An example of the pore diameter distribution, in the case in which the pore diameters increase or decrease continuously from one surface of the membrane to the other surface thereof, is illustrated in FIG. 1. An example of the pore diameter distribution, in the case in which the pore diameters increase or decrease discontinuously from one surface of the membrane to the other surface thereof, is illustrated in FIG. 2. An example of the pore diameter distribution, in the case in which layers of pores having relatively small diameters exist near both surfaces of the membrane, and the pore diameters increase continuously or discontinuously from the surfaces of the membrane to the middle thereof, is illustrated in FIG. 3. The phrase "pores which communicate with one another" in the specification means that the pores are communicated to permit the passage of liquid from one surface of the membrane to the other surface. When the liquid being filtered is water, a hydrophobic membrane with pores which communicate with one another will not permit water to pass through unless great pressure is applied to one side of the membrane. However, such a membrane will permit the passage of water if the membrane is first dipped in an alcohol or an aqueous solution of a surfactant. The shape of the pores which communicate with one another in the asymmetric polymer membrane according to the present invention is not limited. For example, the pores which communicate with one another may consist of gaps between spherical domains of polymers in contact with one another; this situation is illustrated in FIG. 4. Another example, in which the polymers form a sponge-like structure, is illustrated in FIG. 5. An asymmetric polymer membrane of the present invention is a membrane that consists of a dense layer and a porous supporting layer. It is desirable that the thickness of the dense layer be 5 micrometers or less, and that the pore diameters in the dense layer then be 0.0005 to 0.015 micrometers. If the thickness is 5 micrometers or greater, the filtration speed is extremely low, and this is not practical. In addition, there is a practically attainable lower limit to the thickness of the membrane, but as there is no problem caused by the thinness of the membrane itself, there is no theoretical lower limit to the thickness of the membrane. In addition, it is preferable that in the porous supporting layer, the pore diameter of the pores be 0.1-20 micrometers. If the pore diameter is 0.1 micrometer or less, the filtration speed is extremely low, and this is not practical. In addition, if the pore diameter is 20 micrometers or greater, the strength of the membrane is extremely low, and this is again not practical. The dense layer and the porous supporting layer may be located on one surface of a membrane or on both surfaces thereof. Therefore, such a membrane having a dense layer inside and porous supporting layers on both surfaces, is to be included in the membranes of the present invention. The membrane of the present invention has pores which communicate with one another, has pore diameters of 0.0005-20 µm in the dense layer, and is capable of allowing a liquid to pass through. When the pore diameters of the holes of the dense layer of the membrane are 0.0005-0.015 µm, the membrane has a molecular weight cut-off capability and can separate polymer substances, low molecular weight substances, or ions dissolved in a solvent, from the solvent. Therefore, in this case, the membrane is suitable as an ultrafiltration membrane, a reverse osmosis membrane, etc. The assessment of whether the pore diameter of holes in the dense layer is not less than 0.0005 µm can be performed by noting whether the separation factor of oxygen and nitrogen is in the range of 0.9-1.1. The assessment whether the pore diameters of holes in the dense layer are not greater than 0.015 µm, can be performed by a filtration test using a substance with an apparent diameter, such as a protein or the like. When the pore diameters of holes of the dense layer of the membrane are 0.02-20 µm, the membrane does not have a molecular weight cut-off capability and allows polymer substances, low molecular weight substances, and ions to pass through. Therefore, in this case, the membrane is suitable as a microfiltration membrane, etc. The assessment of whether the pore diameters of holes in the dense layer are not less than 0.02 µm can be performed by a filtration test using a substance with an apparent diameter, such as microbes. The assessment of whether the pore diameter is not greater than 20 µm can be performed by electron microscopic observations. In addition, as the membrane of the present invention has a separation factor of 0.9 to 1.1 and the permeation rates of oxygen and nitrogen are at the same level, the membrane has no gas separation capability. If the pore diameters of the holes in the dense layer of the membrane are less than 0.0005 µm, a gas may pass through the membrane according to the dissolution-diffusion principle. The separation factor thereof depends on the material of the membrane; however, it is generally not less than 2. When the pore diameters of the pores which communicate with one another in the dense layer are not less than 0.0005 µm, a gas moves through the membrane according to the Knudsen flow or the Poiseuille flow. The separation factor thereof is 0.9 to 1.1 ("An Outline of Membrane Treatment Technique", Vol. 1, 1991, page 24, edited by Masayuki Nakagaki, Fuji Techno System). Furthermore, the ratio of the pore diameter in the dense layer to the mean pore diameter of the entire membrane is in a range of 10 -6 to 10 -1 in the membrane of the present invention. In addition, the density of the pores in the membrane of the present invention, that is, the void content, is preferably 10-70%. If the void content is 10% or less, the filtration speed is extremely low, and this is not practical. In addition, if it exceeds 70%, the strength of the membrane is extremely low, and this is again not practical. The present inventors found that there were various methods for polymerizing a polymerizable solution into a porous thin membrane having an pore diameter distribution in the thickness direction of the membrane by irradiation with an energy ray. A method can be cited in which, after a polymerizable solution is formed into a thin membrane and a part of a first solvent is evaporated, the thin membrane is irradiated with an energy ray; another method can be cited in which a second solvent is incorporated in a polymerizable solution and, after a part of this second solvent and/or a first solvent has been evaporated, this thin membrane is irradiated with an energy ray; and yet another method can be cited in which, after a polymerizable solution is formed into a thin membrane, this membrane is irradiated with an energy ray while a temperature gradient is provided in the thickness direction of the membrane. In order to evaporate a part of a solvent, methods may be chosen such as a method in which air, nitrogen, or an inert gas is blown on a polymerizable solution in a thin membrane-like condition; a method in which drying is performed for a specified time without blowing a gas over the membrane; and a method in which infrared rays are used. In order to evaporate selectively a portion of the solvent, the evaporation is performed by selecting substances, each having a different boiling point. When no temperature gradient exists and a first solvent, or both the first solvent and the second solvent, are nonvolatile, that is, under a condition in which the first solvent or the second solvent is not evaporated, an asymmetric structure is not formed. In addition, under a condition in which the first solvent or the second solvent is completely evaporated without any trace, only a non-porous homogeneous membrane or a symmetric porous membrane is obtained by energy ray curing. An asymmetric membrane consisting of a dense layer and a porous supporting layer cannot be obtained until a portion of a second solvent or a first solvent has been evaporated. In the present method, it is possible to easily introduce a cross-linked structure in a membrane (in comparison with other methods for preparing an asymmetric membrane such as wet methods and others), and to realize an extremely high production speed by evaporation in an extremely short time, and to produce phase separation substantially and spontaneously. In addition, in comparison with a freezing method, a high flux based on a thin dense layer, a high pressure-resistant strength resulting from a not-too-large pore diameter in a porous supporting layer, and a high production speed, can be realized. To provide a temperature gradient, a method in which the temperature of a supporting body and the atmospheric temperature or a gas flow temperature are made to differ from each other, and a method by means of infrared heating, may be used. The method of preparation in which an asymmetric structure is introduced by irradiating with an energy ray under a condition in which a temperature gradient is provided in a polymerizable solution exhibits a superior production speed and reproducibility in comparison with the method in which the irradiation with an energy ray is performed after a part of the solvent has been evaporated. The reason why and the mechanism by which a porous thin membrane with a pore diameter distribution in the thickness direction of the membrane is obtained in the present invention has not yet been discovered; however, it is hypothesized that a nonhomogeneous phase separation is produced in the thickness direction of the membrane by irradiating with an energy ray under a condition in which a concentration distribution of the monomer and/or the oligomer or a distribution of a concentration ratio of the first solvent to the second solvent is formed by evaporating a part of the solvent, and also under a condition in which a temperature gradient is formed in the thickness direction of the membrane. In addition, it is estimated that when a thin membrane is formed from a polymerizable solution on a supporting body, evaporation of the solvent from only one face of the polymerizable solution and differences in compatibility between each ingredient of the polymerizable solution and the supporting body contribute to generate the asymmetric pore diameter distribution. It is necessary that after polymerization is finished by irradiating with an energy ray, the first solvent and the second solvent be removed by evaporation and/or washing. For the washing, a washing agent which can dissolve thoroughly the first solvent, the second solvent, unreacted monomers and/or oligomers and a UV polymerization initiator, and which does not swell or dissolve the polymer can be used, and a cleaning agent with a low boiling point is preferable. In addition, the cleaning can be performed in a plurality of processes using a plurality of cleaning agents. As an asymmetric polymer membrane of the present invention exhibits high filtration rate, excellent strength, heat resistance, and chemical resistance, it is possible to apply it effectively as an ultrafiltration membrane, a reverse osmosis membrane, a precision filtration membrane, etc. FIGS. 1-3 are enlarged sectional views showing a membrane structure having a pore diameter distribution in the thickness direction of the membrane according to the invention. The inside of the circles in these FIGURES represent hole portions in the membrane. FIGS. 4 and 5 are drawings taken from electron micrographs showing Examples of the shape of pores which communicate with one another according to the present invention. Hereinbelow, the present invention will be explained in detail, but the range of the present invention is not restricted to these examples. "Parts" in the examples refer to parts by weight. [Example 1] (Preparation of a polymerizable solution) A polymerizable solution 1 was obtained by mixing 100 parts urethane acrylate oligomer with a number average molecular weight of 1,000 and three acrylic groups on average per molecule, 4 parts UV polymerization initiator Irgacure-651 (trademark; manufactured by Ciba-Geigy Co., Ltd.) and 70 parts diisobutyl ketone (a first solvent). (Preparation of an asymmetric polymer membrane) A glass plate was coated with the polymerizable solution 1 by means of a film applicator so as to obtain a thickness of 200 micrometers. After a part of the first solvent was evaporated by keeping the glass plate under a nitrogen flow for 2 minutes, the coating was irradiated with a UV ray having a wavelength of 360 nm and an irradiation intensity of 100 mW/cm 2 for 10 seconds by means of a metal halide lamp. It was observed that the coating film which was transparent before irradiation became opaque and white after irradiation. The obtained opaque white membrane was peeled off from the glass plate and the first solvent, unreacted monomer and oligomer, and the UV polymerization initiator were washed out by immersion in petroleum ether for 30 minutes. By thoroughly drying the membrane after cleaning, an asymmetric polymer membrane 1 with a luster on the glass plate side and no luster on the side brought into contact with the nitrogen flow was obtained. By observing the polymer membrane 1 by means of an electron microscope, it was found that the pore diameter on the glass plate side was about 0.01 micrometer, while the pore diameter on the side brought into contact with the nitrogen flow was about 2 micrometers. In addition, by observing a cross-section, it was found that a part with smaller pore diameters only existed in an extremely thin layer on the glass plate side, and the thickness was 1 micrometer or less. The separation factor of oxygen/nitrogen was 0.980. (Evaluation of filtration characteristics) A filtration experiment with a 0.3 % aqueous solution of polyethylene glycol having a molecular weight of 50,000 was performed by using an ultrafiltration apparatus SM-165-26 manufactured by Zaltrius Co., Ltd. Flux (transmittance) with the polyethylene glycol aqueous solution; rejection results of polyethylene glycol at a filtration temperature of 25 °C and under a filtration pressure of 3.94 bar (3 kg/cm 2 G) are shown in Table 1. The same evaluations were performed in the following Examples and Comparative Examples. [Example 2] (Preparation of a polymerizable solution) A polymerizable solution 2 was obtained by mixing 60 parts urethane acrylate oligomer with a number average molecular weight of 3,000 and three acrylic groups on average per molecule, 20 parts 1,6-hexane diol diacrylate, 4 parts Irgacure-651 (trademark; a UV polymerization initiator manufactured by Ciba-Geigy Co., Ltd.), 20 parts diisobutyl ketone (a first solvent), and 70 parts methyl caprate (a first solvent). (Preparation of an asymmetric polymer membrane) By performing the same procedures as in Example 1, an asymmetric polymer membrane 2 with a luster on the glass plate side and no luster on the side brought into contact with the nitrogen flow was obtained. Results of observations by means of an electron microscope were the same as those in Example 1. In addition, the separation factor of oxygen/nitrogen was 0.935. [Example 3] (Preparation of a polymerizable solution) A polymerizable solution 3 was obtained by mixing 50 parts urethane acrylate oligomer with a number average molecular weight of 1,000 and two acrylic groups on average per molecule, 30 parts acrylate of an epoxy resin with an epoxy equivalent of 190, 20 parts phenylcellosolve acrylate, 4 parts Irgacure-184 (trademark; a UV polymerization initiator manufactured by Ciba-Geigy Co., Ltd.), 70 parts methyl caprate (a first solvent), and 30 parts acetone (a second solvent). (Preparation of an asymmetric polymer membrane) A glass plate was coated with the polymerizable solution 3 by means of a film applicator so as to obtain a thickness of 200 micrometers. After a part of the solvent had been evaporated by keeping the glass plate under a nitrogen flow for 30 seconds, the coating was irradiated with a UV ray having a wavelength of 360 nm and an irradiation intensity of 100 mW/cm 2 for 10 seconds from a metal halide lamp. It was observed that the coating film which was transparent before irradiation became opaque and white after irradiation. The obtained opaque white membrane was peeled off from the glass plate and the first solvent, second solvent, unreacted monomer and oligomer, and the UV polymerization initiator were washed out by immersion in petroleum ether for 30 minutes. By thoroughly drying the membrane under a vacuum after cleaning, an asymmetric polymer membrane 3 with a luster on the glass plate side and no luster on the side brought into contact with the nitrogen flow was obtained. Results of observations by means of an electron microscope were the same as in Example 1. In addition, the separation factor of oxygen/nitrogen was 0.936. [Example 4] (Preparation of a polymerizable solution) A polymerizable solution 4 was obtained by mixing 60 parts urethane acrylate oligomer with a number average molecular weight of 3,000 and two acrylic groups on average per molecule, 20 parts trimethylolpropane triacrylate, 20 parts neopentyl glycol diacrylate, 4 parts Irgacure-651 (trademark; a UV polymerization initiator manufactured by Ciba-Geigy Co., Ltd.), 70 parts methyl laurate (a first solvent), and 30 parts methyl isobutyl ketone (a second solvent). (Preparation of an asymmetric polymer membrane) A polymer membrane 4 with a luster on the side brought into contact with the nitrogen flow and no luster on the glass plate side was obtained by the same method as in Example 3. By observing the polymer membrane 4 by means of an electron microscope, it was found that the pore diameter on the glass plate side was about 2 micrometers and the pore diameter on the side brought into contact with the nitrogen flow was about 0.01 micrometer. In addition, it was found by observation of the cross-section of the membrane that only an extremely thin layer on the side brought into contact with the nitrogen flow had pores of small diameter. In addition, the separation factor of oxygen/nitrogen was 1.02. [Example 5] (Preparation of a polymerizable solution) A polymerizable solution 5 was obtained by altering the amount of methyl caprate (a first solvent) in the polymerizable solution 3 of Example 3 to 80 parts. (Preparation of an asymmetric polymer membrane) An asymmetric polymer membrane 5 with a luster on the glass plate side and no luster on the side brought into contact with the nitrogen flow was obtained by the same method as in Example 3. Results of observations by means of an electron microscope were the same as those of Example 1. The separation factor of oxygen/nitrogen was 0.936. [Example 6] (Preparation of a polymerizable solution) A polymerizable solution 6 was obtained by mixing 60 parts urethane acrylate oligomer with a number average molecular weight of 3,000 and three acrylic groups on average per molecule, 20 parts 1,6-hexane diol diacrylate, 4 parts Irgacure-651 (trademark; a UV polymerization initiator manufactured by Ciba-Geigy Co., Ltd.), and 70 parts methyl caprate (a first solvent). (Preparation of an asymmetric polymer membrane) A glass plate was coated with the polymerizable solution 6 by means of a film applicator so as to obtain a thickness of 200 micrometers. After keeping the glass plate under a nitrogen flow at 80 °C for 3 seconds, the coating was irradiated with a UV ray having a wavelength of 360 nm and an irradiation intensity of 100 mW/cm 2 for 10 seconds by means of a metal halide lamp. It was observed that the coating film which was transparent before irradiation became opaque and white after irradiation. The obtained opaque white membrane was peeled off from the glass plate and the first solvent, second solvent, unreacted monomer and oligomer and the UV polymerization initiator were washed out by immersion in petroleum ether for 30 minutes. By thoroughly drying the membrane under a vacuum after cleaning, an asymmetric polymer membrane 6 with a luster on the side brought into contact with the nitrogen flow and no luster on the glass plate side was obtained. Results of observations by means of an electron microscope were the same as in Example 4. In addition, the separation factor of oxygen/nitrogen was 0.936. [Example 7] (Preparation of a polymerizable solution) A polymerizable solution 7 was obtained by mixing 80 parts urethane acrylate oligomer with a number average molecular weight of 1,000 and three acrylic groups on average per molecule, 20 parts dicyclopentenyl acrylate, 4 parts Irgacure-651 (trademark; a UV polymerization initiator manufactured by Ciba-Geigy Co., Ltd.), 200 parts methyl caprate (a first solvent), and 20 parts acetone (a second solvent). (Preparation of an asymmetric polymer membrane) A polymer membrane 7 with a luster on the glass plate side and no luster on the side brought into contact with the nitrogen flow was obtained by the same method as in Example 1. By observing the polymer membrane 7 by means of an electron microscope, it was found that the pore diameter on the glass plate side was about 0.2 micrometer and the pore diameter on the side brought into contact with the nitrogen flow was about 5 micrometers. In addition, it was found from an observation of the cross-section that only an extremely thin layer on the glass plate side had pores with a small diameter. In addition, the separation factor of oxygen/nitrogen was 0.934. [Comparative Example 1] The polymerizable solution 1 of Example 1 under a condition in which it was sandwiched between two glass plates with a spacer with a thickness of 200 micrometers was irradiated with a UV ray with a wavelength of 360 nm and an irradiation intensity of 100 mW/cm 2 for 10 seconds from a metal halide lamp. It was observed that the polymerizable solution which was transparent before irradiation became opaque and white after irradiation. The obtained opaque white membrane was peeled off from the glass plate and was immersed in petroleum ether for 30 minutes to wash out the first solvent, unreacted monomer and oligomer and the UV polymerization initiator. A polymer membrane 7 with no luster on both surfaces was obtained by thoroughly drying the membrane after cleaning under vacuum. By observing the polymer membrane 7 by means of an electron microscope, it was found that both faces had pore diameters of about 2 micrometers, and that the pore diameter was uniform in the thickness direction of the membrane, [Comparative Example 2] Using the polymerizable solution of Example 3, a polymer membrane 8 with no luster on both faces was obtained by the same method as in Comparative Example 1. Results of observations by means of an electron microscope were the same as those of Comparative Example 1. [Comparative Example 3] A glass plate was coated with the polymerizable solution 6 of Example 6 by means of a film applicator so as to obtain a thickness of 200 micrometers. The glass plate was irradiated with a UV ray with a wavelength of 360 nm and an irradiation intensity of 100 mW/cm 2 for 10 seconds by means of a metal halide lamp. It was observed that the coating film which was transparent before irradiation became opaque and white after irradiation. The obtained opaque white membrane was peeled off from the glass plate and was immersed in petroleum ether for 30 minutes to wash out the first solvent, unreacted monomer and oligomer and the UV polymerization initiator, resulting in a polymer membrane 9 with no luster on both faces. Results of observations by means of an electron microscope were the same as those of Comparative Example 1. [Comparative Example 4] (Preparation of a polymerizable solution) A polymerizable solution 7 was obtained by altering the fraction of the first solvent methyl caprate in the polymerizable solution 6 of Example 6 to 60 parts. (Preparation of a polymer membrane) A polymer membrane 10 with luster on both faces was obtained by the same method as in Comparative Example 3. By observing the polymer membrane 10 by means of an electron microscope, it was found that both upper and lower faces each had pore diameters of about 0.01 micrometer, and the pore diameter was uniform in the thickness direction of the membrane by observation of a cross-section. [Comparative Example 5] (Preparation of a polymerizable solution) A polymerizable solution 8 was obtained by altering the fraction of the first solvent methyl caprate in the polymerizable solution 6 of Example 6 to 80 parts. (Preparation of a polymer membrane) A polymer membrane 11 was obtained by the same method as that of Comparative Example 3. Results of observation by means of an electron microscope were the same as those for Comparative Example 1. [Comparative Example 6] (Preparation of a polymerizable solution) A polymerizable solution 9 was obtained by mixing 90 parts hydroxyethyl methacrylate, 10 parts ethylene glycol dimethacrylate, 4 parts Darocure-2959 (trademark; a UV polymerization initiator), 65 parts water, and 15 parts ethylene glycol. (Preparation of an asymmetric polymer membrane) Under a condition in which the polymerizable solution 9 was sandwiched between two glass plates having a spacer with a thickness of 400 micrometers, the lower glass plate was cooled to -40 °C. After the polymerizable solution 9 was frozen by cooling, a glass cell in which water at 20 °C was circulated was placed on the upper glass plate so as to melt the upper surface of the frozen polymerizable solution 9. Under this condition, the solution was irradiated with an ultraviolet ray with a wavelength of 360 nm and an irradiation intensity of 100 mW/cm 2 for 10 seconds by means of a metal halide lamp positioned above the upper glass plate. Then, the obtained membrane was unfrozen and was peeled off from the glass plate to obtain an opaque white polymer membrane 12. By observing the polymer membrane 12 by means of an electron microscope, it was found that the upper face of the membrane had a pore diameter of 0.01 micrometer or less and the lower face of the membrane had a pore diameter of about 100 micrometers. In addition, it was found from the result of observation of a cross-section that the thickness of the part in which the pore diameter of the upper face of the membrane was 0.01 micrometer was about 50 micrometers. (Evaluation of filtration characteristics) On the polymer membrane 12, evaluation of filtration characteristics was impossible as a polyethylene glycol aqueous solution did not pass through, even under a filtration pressure of 3.94 bar (3 kg/cm 2 G). In Examples 1 and 2, asymmetric polymer membranes were obtained by evaporation of diisobutyl ketone which was a first solvent. In Examples 3, 4, 5 and 7, asymmetric polymer membranes were obtained by evaporation of acetone and methyl isobutyl ketone which were the second solvents. In addition, in Example 6, phase separation on the surface of the membrane was not promoted by heating the surface of the membrane with a nitrogen flow at 80 °C to obtain an asymmetric polymer membrane. A polymer membrane with a pore diameter distribution in the cross-sectional direction of the membrane was not obtained in the Comparative Examples because in the Comparative Examples 1 and 2, evaporation of diisobutyl ketone did not occur as the polymerizable solution was sandwiched between glass plates and in Comparative Examples 3, 4 and 5, evaporation of a first solvent did not occur as the first solvent was methyl caprate with a high boiling point alone. It is clear from Table 1 that asymmetric polymer membranes in Examples 1-6 exhibited good filtration rates and molecular weight cut-off ability. In addition, the asymmetric polymer membrane in Example 7 could remove large substances with a diameter of 0.2 micrometer or larger and exhibited large filtration rates. The polymer membranes in Comparative Examples 1-5 which did not have a pore diameter distribution in the thickness direction of the membrane did not exhibit both excellent filtration rate and molecular weight cut-off capability at the same time. In addition, as is clearly shown in Comparative Examples 3, 4 and 5, even if the pore diameter of the membrane was controlled by changing the ratio of monomer and oligomer to a first solvent, it was impossible to obtain both excellent filtration rate and molecular weight cut-off capability at the same time. In Comparative Example 6, asymmetric polymer membranes were obtained; however, the dense layer was so thick that sufficient flux was not possible. Flux (l/m 2 ·h) Rejection (%) Example 1 25 80 Example 2 27 80 Example 3 30 89 Example 4 26 85 Example 5 35 80 Example 6 25 90 Example 7 30000 0 Comparative Example 1 28 35 Comparative Example 2 32 20 Comparative Example 3 33 50 Comparative Example 4 0 - Comparative Example 5 250 0

Claims: A method for preparing an asymmetric polymer membrane having a pore diameter distribution in the thickness direction of the membrane by preparing a homogeneous solution by mixing a radiation-polymerizable monomer and/or oligomer and a first solvent for the monomer and/or oligomer which does not swell or dissolve the formed polymer, forming the solution into a thin membrane, irradiating said thin membrane, and removing the solvent after the polymerization is finished by washing and/or evaporation, characterized in that a part of the solvent is evaporated from the solution in the form of the membrane before the solution is irradiated with an energy ray. A method according to claim 1, characterized in that a solvent which swells or dissolves the polymer is incorporated in the homogeneous solution as a second solvent. A method for preparing an asymmetric polymer membrane having a pore diameter distribution in the thickness direction of the membrane by preparing a homogeneous solution by mixing a radiation-polymerizable monomer and/or oligomer and a solvent for the monomer and/or oligomer which does not swell or dissolve the formed polymer, forming the solution into a thin membrane, irradiating said thin membrane, and removing the solvent after the polymerization is finished by washing and/or evaporation, characterized in that the homogeneous solution in the form of the membrane is irradiated with an energy ray while a temperature gradient is provided in the polymerizable solution along the direction of thickness of the membrane. An asymmetric polymer membrane with a dense layer and a porous supporting layer which may be located on one or both surfaces, communicating pores permitting the passage of a liquid from one surface of the membrane to the other, an asymmetric distribution of pore diameters along the direction of thickness of the membrane, and a ratio of the permeability to oxygen to that of nitrogen of 0.9 to 1.1, which membrane has been prepared by the method of claim 1, 2 or 3. An asymmetric polymer membrane according to claim 4, characterized in that the dense layer, having the smallest pore diameters, is at most 5 µm thick and the pore diameters in the dense layer are 0.0005 to 0.015 µm, the membrane having a molecular weight cut-off capability. An asymmetric polymer membrane according to claim 4, characterized in that the pore diameters in the dense layer, having the smallest pore diameters, are 0.02 to 20 µm, the membrane having no molecular weight cut-off capability. An asymmetric polymer membrane according to any one of claims 4 to 6, characterized in that in the porous supporting layer, having the largest pore diameters, the pore diameter is 0.1 to 20 µm.

Inventors: ANAZAWA TAKANORI, SAKAI KAZUNARI, ZHANG HUSHENG, ANAZAWA, TAKANORI, SAKAI, KAZUNARI, ZHANG, HUSHENG
Applicants: DAINIPPON INK & CHEMICALS, DAINIPPON INK AND CHEMICALS, INC.
Application year: 1991
Main classification: B01D67
IPC codes: C08F290, B01D67, B01D69