Aromatic polysulfone type resin hollow fiber membrane and process for producing the same
This invention relates to an aromatic polysulfone type resin hollow fiber membrane and a process for producing the same. More particularly, this invention is concerned with an aromatic polysulfone type resin hollow fiber membrane with a three-layer structure of inner and outer surface skin layers and a void layer disposed therebetween and connected thereto, which is extremely excellent in water permeabilty as well as in burst strength, and also is concerned with a process for producing the same. 
     There have hitherto been published many papers concerning aromatic polysulfone membranes and aromatic polyether sulfone membranes, but a limited number of them disclose the structure of such membranes. A membrane comprising a surface layer and a void layer as the support layer is disclosed in the specification of U.S. Pat. No. 3,615,024 assigned to Amicon Corporation as well as in Journal of Applied Polymer Science, 20, 2377-2394, ibid 21, 165-180 and 1883-1900 (1977) in which the reporter is Gulf South Research Institute. A hollow fiber membrane disclosed in the U.S. Pat. No. 3,615,024 specification has a dense surface layer on the inner surface thereof but has no such a layer on the outer surface thereof. The hollow fiber membrane of this prior art has a plurality of voids which opens at the outer surface of the membrane, said voids are being defined by portions where the polymer is deficient, and each having a size as large as 10 μm or more. The hollow fiber membrane with such a structure has such drawbacks that: 
     1 they are poor in mechanical strength; 
     2 they are impossible to back-wash; and 
     3 they are liable to be plugged or fouled. 
     The membranes disclosed in the latter were developed for use as supports for reverse osmosis membrane, and each of the membranes has, in both surface portions thereof, a number of pores of 250 Å to 0.44 μm in average diameter. The water permeability of such membranes is at most 1.3 m 3  /m 2  ·day·atm and, hence, of little practical use in ultrafiltration. 
     As described above, each of the conventional hollow fiber membranes is of little practical use because of its poor mechanical properties and low water permeability and, hence, there has been a strong demand in the art for development of a hollow fiber membrane having improved mechanical properties as well as a high water permeability. 
     As a result of the intensive studies made by us, it has been found that a hollow fiber membrane made of an aromatic polysulfone type resin and having a three-layer structure has improved mechanical properties, while exhibiting an extremely excellent water permeability. The present invention has been made based upon such a novel finding. 
     It is, therefore, an object of the present invention to provide a hollow fiber membrane which is extremely excellent in water permeability as well as in burst strength. 
     It is another object of the present invention to provide a process for producing a hollow fiber membrane of the kind mentioned above. 
    
    
     The foregoing and other objects, features and advantages of the present invention will be apparent to those who skilled in the art from the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a scanning electron photomicrograph (magnification: ×230) of a freeze-broken entire cross section of a hollow fiber membrane according to the present invention, said cross section being taken along the plane perpendicular to the fiber axis of said membrane; 
     FIG. 2 is a scanning electron photomicrograph (magnification: ×100) showing an outer surface of the hollow fiber membrane shown in FIG. 1; 
     FIG. 3 is a scanning electron photomicrograph (magnification: ×10000) showing an inner surface of the hollow fiber membrane shown in FIG. 1; 
     FIG. 4 is a scanning electron photomicrograph (magnification: ×10000) showing a portion of the cross section shown in FIG. 1, which portion includes part of the inner surface skin layer, i.e., the Ai layer and the void layer, i.e., the B layer; 
     FIG. 5 is a scanning electron photomicrograph (magnification: ×30000) showing an enlarged view of the portion shown in FIG. 4; 
     FIG. 6 is a scanning electron photomicrograph (magnification: ×10000) showing a portion of the cross section shown in FIG. 1, which portion includes part of the outer surface skin layer, i.e., the Ao layer and the void layer, i.e., the B layer; 
     FIG. 7 is a scanning electron photomicrograph (magnification: ×30000) showing an enlarged view of the portion shown in FIG. 6; 
     FIG. 8 is a scanning electron photomicrograph (magnification: ×1000) of a portion of a freeze-broken, oblique cross section showing the arrangement of voids of the hollow fiber membrane shown in FIGS. 1 through 7; 
     FIG. 9 is a graph showing the relationship between the membrane thickness and the water permeability of the hollow fiber membranes obtained in Examples 2 to 7 as will be given later, shown in comparison with that of hollow fiber membranes obtained in Comparative Examples 3 to 6 as will be given later and that of flat membranes obtained in Comparative Examples 8 to 13 as will be given later; 
     FIG. 10 is a graph showing the relationship between the membrane thickness and the burst strength of hollow fiber membranes obtained in Examples 2 to 7 as will be given later, shown in comparison with that of hollow fiber membranes obtained in Comparative Examples 3 to 6 as will be given later; 
     FIG. 11 is a scanning electron photomicrograph (magnification: ×670) showing a freeze-broken cross section of a hollow fiber membrane obtained in Example 1 as will be given later; 
     FIG. 12 is a scanning electron photomicrograph (magnification: ×3000) showing an enlarged view of the cross section shown in FIG. 11; 
     FIG. 13 is a scanning electron photomicrograph (magnification: ×150) of a freeze-broken entire cross section of a hollow fiber membrane obtained in Comparative Example 2 as will given later; 
     FIG. 14 is a scanning electron photomicrograph (magnification: ×125) of a freeze-broken entire cross section of a hollow fiber membrane with a double-layer structure obtained in Comparative Example 6 as will given later; and 
     FIG. 15 is a scanning electron photomicrograph (magnification: ×125) of a freeze-broken oblique cross section of the hollow fiber membrane shown in FIG. 14. 
    
    
     In one aspect of the present invention, there is provided an aromatic polysulfone type resin hollow fiber membrane having a thickness of less than 100 μm and a three-layer structure of inner and outer surface skin layers and a void layer disposed therebetween and connected thereto; said void layer having a thickness of at least 5 μm and comprising at least 30 voids each having a length of 5 μm or more in a radial direction of a cross section of said hollow fiber membrane, said inner and outer skin layers each having a maximum pore diameter of 0.80 μm or less and a thickness of 0.1 to 10 μm, provided that the average value of thicknesses of said inner and outer surface skin layers is 1/2 or less the thickness of said void layer. 
     The present invention will now be described in detail. 
     As the aromatic polysulfone type resin to be used for forming an aromatic polysulfone type resin hollow fiber membrane, there can be mentioned an aromatic polysulfone and an aromatic polyether sulfone respectively comprised of recurring units of the general formulae (I) and (II) ##STR1## (wherein X, X&#39;, X&#34;, X&#39;&#34;, X&#34;&#34; and X&#34;&#34;&#39; each independently stand for a non-dissociative substituent such as a lower alkyl group selected from the group consisting of methyl, ethyl, n-propyl and n-butyl or a halogen atom selected from the group consisting of F, Cl, I and Br, or a dissociative substituent such as --COOH, --SO 3  H, --NH 2  or the like, and l, m, n, o, p, q each independently stand for an integer of from 0 to 4). These aromatic polysulfone type resins each preferably have a number average molecular weight of 5,000 to 100,000 as determined according to the osmotic pressure method. These aromatic polysulfone type resins are capable of providing hollow fiber membranes which are excellent in heat resistance, acid resistance, alkali resistance, chemicals resistance and mechanical strength. 
     The hollow fiber membrane of the present invention which is made of the above-mentioned aromatic polysulfone or aromatic polyether sulfone has a three-layer structure of a dense inner surface skin layer (hereinafter often abbreviated as &#34;Ai layer&#34;), a void layer (hereinafter often abbreviated at &#34;B layer&#34;) and a dense outer surface skin layer (hereinafter often abbreviated as &#34;Ao layer&#34;). The larger the thickness of the hollow fiber membrane, the higher the burst strength but, needless to say, the lower the water permeability. The hollow fiber membrane of the present invention has a thickness as small as less than 100 μm, preferably 5 to 90 μm and has a water permeability as high as 9 m 3  /m 2  ·day·atm or more (in some cases, as high as 30 m 3  /m 2  ·day·atm). It is surprising that, in spite of such a small membrane thickness, the hollow fiber membrane of the present invention has a burst strength as high as 15 kg/cm 2  or more (in some cases, as high as 32  kg/cm 2 ). The outer diameter of the hollow fiber membrane of the present invention is small and usually is in the range of 100 to 700 μm. It is apparent from the foregoing, the hollow fiber membrane of the present invention has excellent mechanical properties, especially has an improved burst strength, in spite of its reduced membrane thickness and outer diameter. Accordingly, the hollow fiber membrane of the present invention can advantageously be employed in the applications where the use of a fine and thin hollow fiber is required. For example, a fine and thin hollow fiber membrane can advantageously be employed in the cases where: (1) high rate of low-viscosity liquid filtration is needed so that a large amount of the filtrate can be obtained; (2) the liquid to be subjected to filtration is valuable and, hence, the filtration is required to be performed with a narrow dead space so that no filtrate remains in the membrane after the filtration. 
     Further, the hollow fiber membrane of the present invention has a cut-off molecular weight of less than 100,000. A hollow fiber membrane having a cut-off molecular weight of less than 100,000 can be defined as a membrane being capable of cutting off: 25% or more of bovine serum albumin (molecular weight: 67,000); 90% or more of β-galactose DH (molecular weight: 95,000); and 15% or more of dextran T-500 (molecular weight: 500,000). 
     The basic structure of the hollow fiber membrane of the present invention will now be explained. 
     As mentioned above, the hollow fiber membrane of the present invention has a three-layer structure of an Ai layer, a B layer and an Ao layer. The thickness of each of the Ai layer and the Ao layer is in the range of 0.1 to 10 μm. It is required that the average value of the thicknesses of the Ai layer and the Ao layer is 1/2 or less the thickness of the B layer. If the average value of the thicknesses of the Ai layer and the Ao layer exceeds 1/2 the thickness of the B layer, the resulting hollow fiber membrane has a poor water permeability due to the presence of too thick skin layers which cause an increase of the permeation resistance of the membrane, or, has a descreased burst strength due to a uniform structure in which no voids are present. The Ai layer and the Ao layer are the layers in each of which pores selected from Cp pores, Up pores and Np pores (all of which will be mentioned later) are present. In the case where the surface skin layer possesses Cp pores or Up pores, the maximum pore diameter of such pores is in the range of 0.80 μm or less, preferably 0.50 μm or less, as measured by means of a scanning electron photomicrograph. When the maximum pore diameter of the pores present in the surface skin layer exceeds 0.80 μm, the burst strength and the compression strength of the resulting hollow fiber membrane is lowered. Further, if the pores present in the skin layers have a maximum pore diameter exceeding 0.80 μm, there is a danger of contamination of the filtrate, because the skin layer containing such pores allows bacteria to pass therethrough easily. The bacteria which intrude into the membrane propagate, bringing about accumulation of waste substances in the membrane. The hollow fiber membrane having such pores in its respective surface skin layers cannot be employed for preparation of the absolutely-purified water, because the water to be treated is easily contaminated by impurities, thereby causing lowering in specific resistance. Further, the hollow fiber membrane of this kind cannot be used for separation and purification of food stuffs or medical products, because, as is mentioned above, the filtrate is liable to be contaminated by bacteria. Especially, the use of such hollow fiber membrane in the medical applications involves a serious problem, because, due to the intrusion of bacteria into the membrane, the filtrate is liable to be contaminated by a large amount of pyrogenic substances which are very harmful to the human body. 
     For the reasons as set forth hereinabove, it is required that the pores present in the surface skin layers have a maximum pore diameter of 0.80 μm or less, preferably 0.50 μm or less. Further, it is preferred that the pores present in the surface skin layers have an average pore diameter of 0.50 μm or less. 
     As mentioned above, the Ai layer and Ao layer of the hollow fiber membrane of the present invention have pores selected from Cp pores, Up pores and Np pores. The Ai layer and Ao layer serve to permeate selectively molecules therethrough according to the size of molecules. Specifically, the Ai layer and Ao layer cut off large size molecules, while they allow molecules of small size to pass therethrough. The larger the thickness of any of the Ai and the Ao layers, the larger both the permeation resistance of the membrane and the mechanical properties of the membrane. Since the hollow fiber membrane of the present invention has two surface skin layers, i.e., the Ai layer and the Ao layer, leakage of molecules which must be cut off can be prevented even when one of these two surface skin layers accidentally becomes defective. Accordingly, by the use of a hollow fiber membrane of the present invention which possesses two surface skin layers, contrary to the case where a conventional hollow fiber having only one surface skin layer is employed, a safer filtration operation is ensured, and further, a cut-off molecular weight becomes sharp. 
     Between the Ai layer and the Ao layer are present at least 30 voids, which form a void layer, i.e., a B layer. In a cross section of the hollow fiber, each void has a length of 5 μm or more in a radial direction of the cross section of the hollow fiber. The thickness of the B layer is varied depending upon the membrane thickness, but usually at least 5 μm or more, preferably in the range of 5 to 90 μm. The voids are defined by portions where the polymer is deficient. 
     With respect to the shape of the void, there can be mentioned conic shape, spindle shape, cylindrical shape and spherical shape. 
     The observation of the entire cross section of the hollow fiber shows that the voids, as a whole, form an annulus having an almost uniform thickness and surrounded on its both sides by the Ai layer and the Ao layer, respectively. 
     Where 100 or more of voids are present in the B layer, the thickness of the B layer is determined as follows. The thickness of the B layer is obtained by using a scanning electron photomicrograph (magnification: ×100 to 500) showing an entire cross section of the hollow fiber, which cross section is taken along the plane perpendicular to the fiber axis. In such a photomicrograph, the thickness of the B layer is defined as a distance between a point at which a straight line drawn from the center of the cross section of the central bore of the hollow fiber in a radial direction thereof intersects the inner periphery of the annulus and another point at which the same straight line as mentioned just above intersects the outer periphery of said annulus. The inner and outer peripheries as mentioned above are determined as follows. First, determination of the outer periphery of the B layer is explained. Use is made of a scanning electron photomicrograph (magnification: ×100 to 500) of an entire cross section of a hollow fiber of the present invention. From the center of the cross section of the central bore of the hollow fiber, six lines are drawn, at angular intervals of 60°, in radial directions, thereby to form six fractions. Among the voids present in each of said six fractions, selected is a void extending most deeply into the exterior of the hollow fiber. On the periphery of the thus selected void, a point which is closest to the outer surface of the membrane is selected. Then, a distance between the above-mentioned point and the center of the cross section of the central bore of the hollow fiber is measured. As a result, six values in total with respect to the above-defined distance are obtained. Next, another set of six fractions is formed by shifting the phase of each fraction formed above by 20° in a clockwise direction. The same distance measurement as mentioned above is repeated, thereby to obtain another set of six values. Further, the phase of each of the six fractions obtained in the second attempt is shifted again by 20° in a clockwise direction, and six values are obtained in the same manner as mentioned hereinabove. In grand total, eighteen values are obtained. The arithmetical average of the thus obtained eighteen values is obtained. Then, a circle having a radius of a length corresponding to the above-obtained average value is drawn around the center of the cross section of the central bore of the hollow fiber shown in the electron photomicrograph. The outer periphery of the B layer is defined as the circle thus obtained. 
     The inner periphery of the B layer is determined in the same manner as mentioned above except that, from each fraction, a void extending most deeply into the interior of the membrane is selected; on the periphery of the thus selected void, a point which is closest to the inner surface of the membrane is selected; and a distance between the above-mentioned point and the center of the cross section of the central bore of the hollow fiber is measured. 
     On the other hand, where the member of voids present in the B layer is less than 100, the thickness of the B layer is determined as follows. Use is made of a scanning electron photomicrograph (magnification: ×100 to 500) of an entire cross section of the hollow fiber, which cross section is taken along the plane perpendicular to the fiber axis. From the voids shown in this photomicrograph, selected is a void of which the length is largest (the term &#34;length&#34; used herein means a length in a radial direction of the cross section). Taking the thus selected void as a standard, 30 voids are selected in order of length. The thickness of the B layer is defined as the arithmetical average of the lengths of the thus selected 30 voids. 
     The B layer functions as a support layer which contributes to the improvement in mechanical strength of the hollow fiber membrane. While, the B layer has such a function that it enables the liquid flowing into one surface skin layer of the present hollow fiber membrane to reach the other surface skin layer smoothly, without any resistance, thereby causing the water permeability of the hollow fiber membrane to be high. Accordingly, it can be said that the B layer much contribute to the improvement not only in mechanical strength such as burst strength but also in water permeability of the hollow fiber membrane. In the hollow fiber membrane of the present invention, it is preferred that the thickness of the void layer be 50 to 90% of the whole membrane thickness. 
     With respect to the thicknesses of the Ai layer and the Ao layer, they are determined as follows. Where the number of voids present in the B layer is 100 or more, the thicknesses of the Ai layer and the Ao layer are determined as follows. 
     (1) Thickness of the Ao layer; 
     Use is made of a scanning electron photomicrograph (magnification: ×1000 to 10000) of an entire cross section of a hollow fiber of the present invention. From the center of the cross section of the central bore of the hollow fiber, six lines are drawn, at angular intervals of 60°, in radial directions, thereby to form six fractions. Among the voids present in each of said six fractions, selected is a void extending most deeply into the exterior of the hollow fiber. On the periphery of the thus selected void, a point which is closest to the outer surface of the membrane is selected. Then, measured is a distance between the abovementioned point and a point at which a straight line drawn from the center of the cross section of the central bore of the hollow fiber in a radial direction thereof and passing said point of the selected void intersects the outer surface of the membrane. 
     As a result, six values in total with respect to the above-defined distance are obtained. Next, another set of six fractions is formed by shifting the phase of each fraction formed above by 20° in a clockwise direction. The same distance measurement as mentioned above is repeated, thereby to obtain another set of six values. Further, the phase of each of the six fractions obtained in the second attempt is shifted again by 20° in a clockwise direction, and six values are obtained in the same manner as mentioned hereinabove. In grand total, eighteen values are obtained. The arithmetical average of the thus obtained eighteen values is obtained. The thickness of the Ao layer is defined as the average value thus obtained. 
     (2) Thickness of the Ai layer: 
     The thickness of the Ai layer is determined in the same manner as mentioned in (1) above except that, from each fraction, a void extending most deeply into the interior of the membrane is selected; on the periphery of the void, a point closest to the inner surface of the membrane is selected; and a distance between the above-mentioned point and a point at which a straight line drawn from the center of the cross section of the central bore of the hollow fiber in a radial direction thereof and passing said point of the selected void intersects the inner surface of the membrane. 
     Where the number of voids present in the B layer is less than 100, the thicknesses of the Ai layer and the Ao layer are determined as follows. 
     (1) Thickness of the Ai layer: 
     Use is made of a scanning electron photomicrograph (magnification: 1000 to 10000) of an entire cross section of the hollow fiber. On the periphery of each void shown in the photomicrograph, a point which is closest to the inner surface of the membrane is selected. Then, with respect to all the voids shown in the photomicrograph, a distance between the above-mentioned point and a point at which a straight line drawn from the center of the cross section of the central bore of the hollow fiber in a radial direction thereof and passing said point of the void intersects the inner surface of the membrane is measured. As a result, corresponding number of distance values to the number of voids are obtained. Taking the smallest value as a standard, 30 distance values are selected in order of length. The thickness of the Ai layer is defined as the arithmetical average of the thus obtained 30 distance values. 
     (2) Thickness of the Ao layer: 
     Thickness of the Ao layer is determined in the same manner as mentioned in (1) above except that, on the periphery of each void shown in the photomicrograph, a point which is closest to the outer surface of the membrane is selected; and with respect to all of the voids shown the photomicrograph, a distance between the above-mentioned point and a point at which a straight line drawn from the center of the cross section of the central bore of the hollow fiber in a radial direction thereof and passing said point of the void intersects the outer surface of the membrane is measured. 
     As is apparent from the definition of the thickness of the Ai, B, Ao layers are mentioned above, the total of the thickness of each layer is not always identical with the whole membrane thickness of the hollow fiber. 
     The micro-structure of each layer will now be described in detail by reference to FIGS. 3 through 8. However, each of the micro-structures shown in FIGS. 3 through 8 is given as one example, and, the micro-structure of each layer of the present hollow fiber membrane is not limited to that shown in FIGS. 3 through 8. 
     From FIG. 3, it is observed that the inner surface of this hollow fiber membrane is very smooth. The presence of pores cannot be confirmed from FIG. 3. 
     With respect to a dense skin layer present on said inner surface of the membrane, i.e., the Ai layer, as is shown in FIGS. 4 and 5, it is comprised of globular polymer particles densely aggregated therein. Each polymer particle has a diameter as small as 0.5 μm or so. The pores present in the Ai layer are so small that they cannot be observed by the observation using a scanning electron microscope of 10,000 magnifications. However, by the measurement of the permeation resistance effected by passing aqueous solutions of dextran molecules or of various proteins, it is confirmed that there are formed micropores of which the diameter is in the range of approximately 10 to 100Å. The micropores of the kind as mentioned above are hereinafter referred to as &#34;Np pores&#34;. 
     With respect to the outer surface skin layer, i.e., the Ao layer, as is shown in FIGS. 6 and 7, the globular polymer particles are linked together, forming cylindrical aggregations each comprising a plurality of globular polymer particles. The Ao layer has a structure in which such cylindrical aggregations are densely present. The pores present in this layer are the so-called Up pores (as will be mentioned later). 
     The void layer disposed between the Ai layer and the Ao layer, i.e., the B layer, contains a plurality of voids. From a scanning electron photomicrograph given in FIG. 8, the arrangement of voids are clearly observed. As is apparent from FIG. 8, in the B layer, the voids are densely and regularly present in a thicknesswise direction of the membrane. As shown in FIG. 1, the voids have their respective longitudinal axes directed to the fiber axis, and each void has a length larger in a radial direction of the hollow fiber than in a direction perpendicular to said radial direction. 
     With respect to the Cp pores and the Up pores as mentioned in the foregoing, an explanation will be given below. 
     With respect to the Cp pores, as described in &#34;Kobunshi Ronbun Shu&#34;, vol. 34, No. 3, pp. 205-216 (1977), published by the Polymer Society of Japan, where in a water-in-soil type sphere as small as about 100 Å (small particle of emulsion) the oil is occupied by a polymer-rich phase and the water is occupied by a polymer-lean phase, there are formed structures called Cp pores. The Cp pore is characterized by having a circular shape in front and in cross section. On the other hand, a pore having a long and narrow shape is called a Up pore. 
     In another aspect of the present invention, there is provided a process for producing an aromatic polysulfone type resin hollow fiber membrane which comprises extruding a spinning solution of an aromatic polysulfone type resin in an organic polar solvent for said resin, said solution containing a glycol and having a resin concentration of 10 to 35% by weight, from an annular spinning nozzle which is provided with a resin-extruding annular orifice having an orifice width of 10 to 100 μm as measured on the resin-extruding side of the nozzle while simultaneously injecting as internal coagulating liquid a liquid miscible with said organic polar solvent but incapable of dissolving the polysulfone type resin into the annular spinning nozzle at an inside bore thereof, thereby to obtain an extrudate in the form of a hollow fiber, and introducing said extrudate into an external coagulating liquid comprising a liquid miscible with said organic polar solvent but incapable of dissolving the polysulfone type resin. 
     The important features of the process of the present invention are: adding a glycol as an additive into a spinning solution; controlling the resin concentration of a spinning solution to an appropriate value; and adjusting the membrane thickness of the hollow fiber to an appropriate value. 
     Any organic polar solvent can be used in the process of the present invention as far as it is capable of dissolving therein an aromatic polysulfone type resin. As preferred examples of such a solvent, there can be mentioned N-methylpyrrolidone, dimethylformamide, dimethylacetamide and diethylacetamide. 
     In the process of producing a hollow fiber membrane of the present invention, addition of a glycol into a spinning solution is of crucial importance. In the formation of a hollow semipermeable membrane, it is presumed that the polymer dissolved in the organic polar solvent precipitates around the glycol phases serving as nuclei when the coagulating liquids penetrate into the inside of the membrane through the inner and outer surfaces thereof, causing the respective boundary portions of the glycol phases to be converted to form micropores. The glycol phases are crucial as nuclei for the polymer precipitation which results in formation of micropores. Thus, it can be said that the addition of a glycol greatly contributes to the formation of a hollow fiber membrane with an Ai B Ao-composed three layer structure which is excellent in water permeability as well as in burst strength. 
     As preferred examples of a glycol to be used in the process of the present invention, there can be mentioned ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol (having a number average molecular weight of 200 to 6,000), propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol (having a number of average molecular weight of 200 to 6,000), glycerin, trimethylolpropane, polytetraethylene glycol, ethylene glycol methyl ether derivatives such as ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, diethylene glycol monomethyl ether, diethylene glycol dimethyl ether, triethylene glycol monomethyl ether and propylene glycol derivatives such as propylene glycol monomethyl ether or the like. These glycols as mentioned just above may be used alone or in combination. Of them, a glycol having a number average molecular weight similar to that of tetraethylene glycol is especially preferred. The proportion of the glycol to the resin solution is not critical in so far as the state of the resin solution in which the resin is uniformly dissolved can be maintained. Such a proportion may be varied depending upon, for example, the resin concentration of the solution and the kind of the polar solvent employed, but usually a glycol is incorporated in an amount of 0.5 to 30% by weight based on the total weight of the spinning solution. When the glycol content is less than 0.5% by weight, there can hardly be expected the desired effect of the glycol on the formation of a hollow fiber membrane. When the glycol content exceeds 30% by weight, the resin solution becomes instable and opaque and, in addition, the membrane-forming performance of the spinning solution is so lowered that there cannot be obtained a hollow fiber membrane with excellent properties. 
     The concentration of the resin in the spinning solution is in the range of from 10 to 35% by weight based on the total weight of the spinning solution, preferably from 15 to 30% by weight, more preferably 18 to 25% by weight. When the concentration of the resin exceeds 35% by weight, the resulting semipermeable membrane is so poor in water permeability that it cannot be put to practical use. In contrast, when the resin concentration is lower than 10% by weight, the resulting membrane is so poor in mechanical strength that it cannot be put to practical use. 
     In the process of the present invention, metal salts such as LiCl, LiNO 3 , CaCl 2  and MgCl 2  may be added to the spinning solution to increase the solubility of the polysulfone resin in the solvent for the resin. The metal salt may be employed in an amount of 5% by weight or less based on the amount of the spinning solution. 
     In preparing a hollow fiber, a coagulating liquid must be employed as the internal and external coagulating liquids so that coagulation of both of the outside and the inside of the extrudate in the form of a hollow fiber is performed. 
     As the coagulating liquid, water is most generally used. However, there may also be employed an organic solvent which is incapable of dissolving the resin. Examples of such an organic solvent include methanol, ethanol, dioxane, cyclohexanol, benzene, toluene, xylene, petroleum oil and the like. These non-solvents may be employed alone or in mixture. It is preferred that the internal coagulating liquid and the external coagulating liquid be the same. 
     The temperature of each of the spinning solution and the coagulating liquid is not critical, but generally, the temperature of the spinning solution is in the range of -100° C. to 140° C., preferably -20° to 70° C., and the temperature of the coagulating liquid is in the range of -100° to 100° C., preferably 0° to 80° C. 
     In practicing the process of the present invention, a spinning solution of an aromatic polysulfone type resin in an organic polar solvent for said resin is extruded from an annular spinning nozzle which is provided with a resin-extruding annular orifice while simultaneously injecting as internal coagulating liquid a liquid miscible with said organic polar solvent but incapable of dissolving the polysulfone type resin into the annular spinning nozzle at an inside bore thereof, thereby to obtain an extrudate in the form of a hollow fiber, and the extrudate thus obtained is introduced into an external coagulating liquid comprising a liquid miscible with said organic polar solvent but incapable of dissolving the polysulfone type resin. In the process of the present invention, the extrudate in the form of a hollow fiber may be directly introduced into the external coagulating liquid. However, in order to enhance the water permeability of the membrane without adversely affecting the burst strength, it is advantageous that the extrudate is, before being introduced into the external coagulating liquid, allowed to run 25 cm or less, preferably 15 cm or less, in an air space. The distance which the extrudate is allowed to run is defined as a distance between the tip of the spinning nozzle and the surface of the external coagulating liquid, and hereinafter will be abbreviated as the &#34;running-in-air distance&#34;. If the running-in-air distance exceeds 30 cm, the resulting hollow fiber membrane is caused to have a structure in which voids open to the outer surface of the membrane, or, have a structure in which the outer surface skin layer has an extremely small thickness. In case the running-in-air distance is 15 cm or less, it is desired that the resin concentration of the spinning solution is in the range of 10 to 35 wt%. On the other hand, when the running-in-air distance is 15 to 25 cm, it is desired that the resin concentration of the spinning solution is in the range of 15 to 35 wt%. 
     In order to obtain a hollow fiber membrane with a three-layer structure characteristic of the present invention, it is crucially important to control the membrane thickness. As mentioned in the foregoing, the hollow fiber membrane of the present invention has a membrane thickness of as small as less than 100 μm. A hollow fiber membrane having a thickness exceeding 100 μm is poor in water permeability, even if it has a three-layer structure, due to the presence of thick surface skin layers which causes an increase of the permeation resistance of the membrane. 
     According to the process of the present invention, the spinning solution is extruded from an annular spinning nozzle which is provided with a resin-extruding annular orifice having an orifice width of 10 to 100 μm as measured on the resin-extruding side of the nozzle. The thickness of the hollow fiber membrane is varied according to the orifice width of the annular spinning nozzle, and is not greatly affected by other spinning conditions. Usually, the membrane thickness is smaller than the orifice width of the nozzle. Accordingly, in order to obtain a hollow fiber membrane having a membrane thickness of as small as less than 100 μm, it is required to employ a spinning nozzle of which the orifice width is in the range of 10 to 110 μm. 
     As mentioned in the foregoing, the hollow fiber membrane of the present invention has a thickness as small as less than 100 μm and exhibits an extremely excellent water permeability. Further, the present hollow fiber membrane has, in spite of such a small membrane thickness, a high burst strength. Therefore, the hollow fiber membrane has a wide variety of prospective uses, and can advantageously be used especially in the applications where the use of a fine and thin hollow fiber having a high water permeability as well as a high burst strength is desired. 
     The present invention will now be illustrated in more detail by the following Examples that should not be construed as limiting the scope of the invention. 
     In Examples, the water permeability, the cut-off molecular weight, the burst strength and the average pore diameter and the maximum pore diameter of pores present in the surface skin layer of the aromatic polysulfone type resin hollow fiber membrane were determined by the following methods. 
     (1) Measurement of the water permeability 
     A predetermined number of hollow fibers whose outer and inner diameters have been measured in advance are bundled into a module and one end of said module is sealed. Water is injected from the other end of said module at which the hollow fibers are unified by means of an adhesive. The effective length of the module is 25 cm and the difference in pressure between the inside and the outside of the hollow fiber is 1 atm. Under such conditions, the permeability (m 3  /m 2 .day.atm) of the hollow fiber to a 25° C. distilled water is measured. 
     (2) Measurement of the cut-off molecular weight 
     One end of a hollow fiber of which the outer and inner diameters have been measured in advance is used as an inlet for water and the other end of said hollow fiber is used as an outlet for water. The effective length of the fiber is 25 cm, the pressure at the inlet portion is 1.2 kg/cm 2  or less, the pressure at the outlet portion is 0.8 kg/cm 2  or more, the average of the pressure in the inlet and outlet portions is 1.0 kg/cm 2  and the linear velocity of the water is 1.0 m/sec. Under such conditions, aqueous solutions of various molecules are, at a temperature of 25° C., introduced into said fiber from the inlet portion thereof. After 10 minutes, 0.5 ml of the filtrate is taken, and the proportions cut-off of various molecules are calculated from the amount of molecules contained therein. When the proportion cut-off of dextran molecules is determined, use is made of an aqueous 5.0 wt % solution thereof. On the other hand, when the proportion cut-off of a globular protein is determined, use is made of an aqueous 0.25 wt% solution thereof. In latter case, the hollow fiber should be, prior to injection of the solution, dipped into the said solution (5° C.) for 12 hours so that the hollow fiber is not adversery affected by adsorption of the protein. 
     (3) Measurement of the burst strength 
     A hollow fiber is bended in the form of a loop, and the both ends thereof are fixed. Pneumatic pressure is equally applied to the hollow fiber from each of the ends of the hollow fiber thus fixed, and thereafter, the pressure is gradually raised at an elevation rate of 10 kg/cm 2  /min. The burst strength is defined as a pressure at which the hollow fiber ruptures. 
     (4) Measurement of the average pore diameter and the maximum pore diameter of pores (Cp pores and Up pores) present in the surface skin layers 
     (i) Cp pores: In a scanning electron photomicrograph (magnification: ×10 3  to 10 4 ), 500 pores are arbitrarily selected from pores shown in said photomicrograph. The diameter of each of the thus selected pores is measured, thereby to obtain 500 values with respect to the pore diameter. The average pore diameter of Cp pores is defined as the arithmetical average of the thus obtained 500 values. 
     The maximum diameter of Cp pores is defined as a diameter of the pore of which the diameter is largest among at least 1,000 pores arbitrarily selected from the pores shown in the above-mentioned photomicrograph. 
     (ii) Up pores: A scanning electron photomicrograph showing an enlarged (×10 3  to 10 4 ) view of the surface of the membrane is prepared. On the thus prepared photomicrograph, 30 straight lines are arbitrarily drawn in such a manner that approximately 10 to 50 pores are crossed by each straight line. In this photomicrograph, with respect to each of 30 straight lines drawn thereon, measured are the following: 
     (1) number of pores (n) crossed by the straight line; and 
     (2) distance between two points on the periphery of a pore, at each of which the straight line intersects the periphery of the pore (the distance measurement should be made to the order of 10 -2  in terms of micron in real length). 
     The distance as defined above is measured with respect to all the pores crossed by the straight line, and the total (d) of the measured distance values is obtained with respect to each of 30 straight lines. 
     As a result, 30 values with respect to (n) and 30 values with respect to (d) are obtained. Then, 30 values for (n) and 30 values for (d) are separately added together, whereby the grand total of (n) and the grand total of (d) are respectively obtained, and referred to as (N) and (D), respectively. The average pore diameter of Up pores is defined as the value of (D/N). 
     The maximum pore diameter of Up pores is determined by the same process as used for the measurement of the maximum pore diameter of Cp pores. However, in place of the &#34;diameter&#34; used in the case of Cp pores, there is used, in the case of Up pores, the average value of the length of &#34;major axis&#34; and the length of &#34;minor axis&#34; of the pore. The terms &#34;major axis&#34; and &#34;minor axis&#34; of the Up pore are respectively defined as follows: 
     From a plurality of straight lines each passing across a pane and each having their respective end portions lying on the periphery of the pore, selected is a straight line of which the length is largest. The major axis of the Up pore is defined as the straight line having the largest length as mentioned above. The inner axis of the Up pore is defined as the straight line perpendicular to the major axis and passing the midpoint of the major axis, and having its end portions lying on the periphery of the pore. EXAMPLE 1 
     Dimethylacetamide (hereinafter often abbreviated as &#34;DMAc&#34;) as a solvent, tetraethylene glycol (hereinafter often abbreviated as &#34;TEG&#34;) as an additive and Udel polysulfone® (trademark of a polysulfone manufactured and sold by Union Carbide Corp., U.S.A.) comprising recurring units represented by the formula: ##STR2## as an aromatic polysulfone type resin were mixed in a weight ratio of 71:9:20 to obtain a homogeneous resin solution. The thus obtained resin solution was used as a spinning solution. The spinning solution of 30° C. was extruded from an annular hollow fiber spinning nozzle [provided with a resin-extruding annular orifice having an orifice width of 45 μm]. Purified water of 30° C. was used as internal and external coagulating liquids to effect coagulation of both the inside and outside of the resulting extrudate. As a result, there was obtained a porous membrane in the form of a hollow fiber having an inner diameter of 200 μm, an outer diameter of 276 μm and a membrane thickness of 38 μm. The distance between the tip of the spinning nozzle and the surface of the external coagulating liquid (hereinafter abbreviated as &#34;running-in-air distance&#34;) was 1.5 cm. 
     The thus obtained hollow fiber had the following properties: water permeability, 22 m 3  /m 2 .day.atm.water at 25° C.; burst strength, 20 kg/cm 2 . The proportion cut-off of ovalbumin which is a globular protein and has a molecular weight of 45,000 was 90% or more, and the proportion cut off of dextran T-500 (molecular weight: 500,000) was 53% or more. 
     The scanning electron photomicrograph (magnification: ×670) of a freeze-broken entire cross section of the resulting hollow fiber membrane is shown in FIG. 11. FIG. 12 is a scanning electron photomicrograph (magnification: ×3000) showing an enlarged view of a portion of the cross section shown in FIG. 11. From FIG. 11 and FIG. 12, it is seen that the hollow fiber membrane has a three-layer structure (Ao B Ai) characteristic of the present invention. Observation of the photomicrograph (magnification: ×1000 to 10000) of an entire cross section of the hollow fiber membrane showed that the thickness of the Ai layer was about 1.7 μm and that of the Ao layer was about 3.3 μm. 
     By the examination of both surfaces of the hollow fiber membrane using a scanning electron photomicrograph (magnification: ×10000), the pores present in the Ai layer were so small that they could not be observed, but it is confirmed that, in the Ao layer, there were formed Cp pores having an average pore diameter of 0.10 μm and a maximum pore diameter of 0.20 μm. 
     EXAMPLES 2 to 7 AND COMPARATIVE EXAMPLE 1 
     Hollow fiber membranes were prepared in substantially the same manner as in Example 1 except that the orifice width of the spinning nozzle was varied as indicated in Table 1. As a result, hollow fiber membranes differing in membrane thickness were obtained. The properties of the resulting hollow fiber membranes were examined and the results obtained are shown in Table 1. A hollow fiber membrane obtained in Comparative Example 1 did not have a three-layer structure but had a five-layer structure of an outer surface layer, an outer void layer, an intermediate layer, an inner void layer and an inner surface layer arranged in that sequence. In the hollow fiber membrane of five-layer structure, the thicknesses of the outer and inner void layers were largely different from each other, and the voids present in the outer void layer vary in length widely. 
     With respect to the hollow fiber membranes obtained in Examples 2 to 7, the relationship between the membrane thickness and the water permeability was examined and the results obtained are shown in FIG. 9. In FIG. 9, a curve -- -- obtained from the data on hollow fibers obtained in Examples 2 to 7, a curve --Δ--was obtained from the data on hollow fibers obtained in Comparative Examples 3 to 6 as will be given later and a curve -- -- was obtained from the data on flat membranes obtained in Comparative Examples 8 to 13 as will be given later. 
     The relationship between the membrane thickness and the burst strength of the hollow fiber membranes obtained in Examples 2 to 7 is shown in FIG. 10 in comparison with that of hollow fiber membranes obtained in Comparative Examples 3 to 6 as will be given later. In FIG. 10, a curve -- -- was obtained from the data on hollow fiber membranes obtained in Examples 2 to 7 and a curve --Δ13  was obtained from the data on hollow fiber membranes obtained in Comparative Examples 3 to 6. 
     
                                           TABLE 1                                 
__________________________________________________________________________
                  Mem-          A.sub.i layer                             
          Outer                                                           
              Inner                                                       
                  brane             pores*.sup.2                          
      Orifice                                                             
          diam-                                                           
              diam-                                                       
                  thick-                                                  
                      Water*.sup.1                                        
                           Burst                                          
                                Thick-   Average                          
                                               Maximum                    
Example                                                                   
      width,                                                              
          eter,                                                           
              eter,                                                       
                  ness,                                                   
                      perme-                                              
                           strength,                                      
                                ness,                                     
                                    Kind of                               
                                         pore diam-                       
                                               pore diam-                 
No.   μm                                                               
          μm                                                           
              μm                                                       
                  μm                                                   
                      ability                                             
                           kg/cm.sup.2                                    
                                μm                                     
                                    pores                                 
                                         eter, μm                      
                                               eter, μm                
__________________________________________________________________________
2     30  550 500 25  29   17   1.8 Np   --    --                         
3     45  580 500 40  27   20   1.7 Np   --    --                         
4     55  600 500 50  25   21   1.8 Np   --    --                         
5     80  640 500 70  23   25   1.8 Np   --    --                         
6     90  660 500 80  22   26   1.8 Np   --    --                         
7     105 690 500 95  21   28   1.8 Np   --    --                         
Compara-                                                                  
      180 800 500 150 2.5  32   1.8 Np   --    --                         
tive Ex-                                                                  
ample 1                                                                   
__________________________________________________________________________
                  A.sub.o layer                                           
                      pores*.sup.2                                        
                  Thick-   Average                                        
                                 Maximum                                  
                                       Cut-off molecular weight           
            Example                                                       
                  ness,                                                   
                      Kind of                                             
                           pore diam-                                     
                                 pore diam-                               
                                       Kind of pro-                       
                                               Molecular                  
            No.   μm                                                   
                      pores                                               
                           eter, μm                                    
                                 eter, μm                              
                                       tein employed                      
                                               Weight                     
__________________________________________________________________________
            2     1.8 Cp   0.10  0.25  Bovine serum                       
                                               67000                      
                                       albumin                            
            3     2.5 Cp   &#34;     &#34;     Bovine serum                       
                                               67000                      
                                       albumin                            
            4     3.0 Cp   &#34;     &#34;     Ovalbumin                          
                                               45000                      
            5     3.2 Cp   &#34;     &#34;     Trypsinogen                        
                                               24000                      
            6     3.5 Cp   &#34;     &#34;     Lysozyme                           
                                               14000                      
            7     4.0 Cp   &#34;     2.00  Cytochrome C                       
                                               13000                      
            Compara-                                                      
                  10.0                                                    
                      Np   --    --    Insulin  6000                      
            tive Ex-                                                      
            ample 1                                                       
__________________________________________________________________________
 Note:                                                                    
 *.sup.1 Water permeability: m.sup.3 /m.sup.2 · day · at
 · water at 25° C.                                        
 *.sup.2 Average pore diameter and maximum pore diameter were measured by 
 means of a scanning electron photomicrograph (magnification: × 1000
 to 10000)                                                                
 
    
     EXAMPLES 8 AND 9 AND COMPARATIVE EXAMPLE 2 
     In Example 8, the same resin solution as employed in Example 1 was extruded from an annular hollow fiber spinning nozzle having an orifice width of 90 μm. Purified water was used as internal and external coagulating liquids, thereby to effect coagulation of both the inside and outside of the resulting extrudate. The temperatures of the internal coagulating liquid and the external coagulating liquid were 30° C. and 50° C., respectively. The running-in-air distance was 0 cm. 
     In Example 9, a hollow fiber membrane was prepared in substantially the same manner as in Example 8 except that kerosene of 30° C. in temperature was used as the external coagulating liquid instead of the purified water and that the resulting extrudate was, after being contacted with the external coagulating liquid, washed in a 80° C. water bath. 
     Each of the hollow fiber membranes obtained in Examples 8 and 9 had a three-layer structure characteristic of the present invention. 
     A comparative hollow fiber membrane was prepared in substantially the same manner as in Example 9 except that the orifice width of the nozzle was changed to 150 μm. The resulting hollow fiber membrane had a three-layer structure, but the thickness of the membrane was 140 μm. The scanning electron photomicrograph of a freeze-broken cross section of this comparative hollow fiber membrane is given in FIG. 13. 
     The properties of each of the hollow fiber membranes obtained in Examples 8, 9 and Comparative Example 2 were examined and the results obtained are shown in Table 2. It is apparent from the results that the hollow fiber membrane (membrane thickness: 140 μm) obtained in Comparative Example 2 is extremely poor in water permeability, as compared with the hollow fiber membranes obtained in Examples 8 and 9. 
     
                                           TABLE 2                                 
__________________________________________________________________________
                            A.sub.i layer                                 
                                 pores*.sup.2                             
             Mem-                    Average                              
                                          Maximum                         
     Inner                                                                
         Outer                                                            
             brane                                                        
                  Water                                                   
                       Burst     kind                                     
                                     pore pore                            
Exam-                                                                     
     diame-                                                               
         diame-                                                           
             thick-                                                       
                  perme-                                                  
                       Strength,                                          
                            Thick-                                        
                                 of  diame-                               
                                          diame-                          
ple No.                                                                   
     ter, μm                                                           
         ter, μm                                                       
             ness, μm                                                  
                  ability*.sup.1                                          
                       kg/cm.sup.2                                        
                            ness, μm                                   
                                 pores                                    
                                     ter, μm                           
                                          ter, μm                      
__________________________________________________________________________
8    500 660 80   11.0 31   1.8  Np  --   --                              
9    500 660 80   10.0 30   1.8  Np  --   --                              
Compar-                                                                   
     420 700 140   2.0 35   1.8  Np  --   --                              
ative                                                                     
Exam-                                                                     
ple 2                                                                     
__________________________________________________________________________
                                    Cut-off                               
                                    molecular                             
                 A.sub.o layer      weight                                
                     pores*.sup.2                                         
                         Average                                          
                              Maximum                                     
                 thick-                                                   
                     kind                                                 
                         pore pore  Kind of                               
            Exam-                                                         
                 ness,                                                    
                     of  diame-                                           
                              diame-                                      
                                    protein                               
                                          Molecular                       
            ple No.                                                       
                 μm                                                    
                     pores                                                
                         ter, μm                                       
                              ter, μm                                  
                                    employed                              
                                          weight                          
__________________________________________________________________________
            8    4.0 Np  --   --    Insulin                               
                                           6000                           
            9    9.0 Cp  0.20 0.40  Lysozyme                              
                                          14000                           
            Compar-                                                       
                 65  Cp  0.20 0.50  Lysozyme                              
                                          14000                           
            ative                                                         
            Exam-                                                         
            ple 2                                                         
__________________________________________________________________________
 Note:                                                                    
 *.sup.1 Water permeability: m.sup.3 /m.sup.2 · day · at
 · water at 25° C.                                        
 *.sup.2 Average pore diameter and maximum pore diameter were measured by 
 means of a scanning electron photomicrograph (magnification: × 1000
 to 10000)                                                                
 
    
     COMPARATIVE EXAMPLES 3 TO 6 
     The same resin solution as employed in Example 1 was extruded from annular hollow fiber spinning nozzles differing in orifice width (orifice width: 50 μm, 100 μm, 150 μm and 200 μm), thereby to obtain hollow fiber membranes having various membrane thicknesses. Purified water was used as the internal coagulating liquid, but, the external coagulating liquid was not employed. 
     Each of the resulting hollow fiber membranes was 500 μm in inner diameter and had an Ai B-composed double-layer structure in which the voids present in the B layer open to the outer surface of the membrane. 
     With respect to the hollow fiber membranes obtained in Comparative Examples 3 to 6, the relationship between the membrane thickness and the water permeability is shown in FIG. 9 and indicated by a curve --Δ--, and the relationship between the membrane thickness and the burst strength is shown in FIG. 10 and indicated by a curve --Δ--. 
     COMPARATIVE EXAMPLE 7 
     The same resin solution as used in Example 1 was subjected to T-die extrusion to form a flat membrane, and the resulting membrane was dipped into a purified water to effect coagulation of the membrane. As a result, a flat membrane of 100 μm in thickness and having an Ai B Ao-composed three-layer structure was obtained. The resulting flat membrane had an extremely poor water permeability (0.01 m 3  /m 2  ·day·atm·water at 25° C.) and, hence, it was of little practical use. 
     EXAMPLE 10 
     Dimethylacetamide (DMAc) as the solvent, tetraethylene glycol (TEG) as the additive and VICTREX (trade name of a polyether sulfone manufactured and sold by ICI Company, England) comprising recurring units represented by the formula: ##STR3## as the aromatic polysulfone type resin were mixed in a weight ratio of 65:15:20 to obtain a homogeneous resin solution. The thus obtained resin solution was used as the spinning solution. The spinning solution of 30° C. was then extruded from an annular hollow fiber spinning nozzle having an orifice width of 55 μm. Purified water of 30° C. was used as the internal and external coagulating liquids to effect coagulation of both the inside and outside of the resulting extrudate. As a result, there was obtained a porous membrane in the form of a hollow fiber having an inner diameter of 500 μm, an outer diameter of 600 μm and a membrane thickness of 50 μm. The running-in-air distance, which is a distance between the tip of the spinning nozzle and the surface of the external coagulating liquid was 3.0 cm. 
     The thus obtained hollow fiber had following properties: water permeability, 30 m 3  /m 2  ·day·atm·water at 25° C.; burst strength, 23 kg/cm 2 . The proportion cut-off of insulin (molecular weight: 6000) was 90% or more. The resulting hollow fiber membrane had an Ai B Ao-composed three-layer structure characteristic of the present invention. By the examination of both surfaces of the hollow fiber membrane using a scanning electron photomicrograph (magnification: ×10,000), the pores present in the Ai layer were so small that they could not be observed, but it was confirmed that, in the Ao layer, there were formed Cp pores having an average pore diameter of 0.07 μm and a maximum pore diameter of 0.15 μm. 
     COMPARATIVE EXAMPLES 8 TO 13 
     The same resin solution as used in Example 1 was cast, at a temperature of 30° C., with doctor blades differing in thickness onto a glass plate to form membranes having various thicknesses. Each of the thus formed membranes was then dipped into a 30° C. purified water to effect coagulation of the membrane. As a result, there were obtained flat membranes having thicknesses of 10 μm, 20 μm, 50 μm, 70 μm, 80 μm and 90 μm, respectively. 
     With respect to the resulting flat membranes, the relationship between the membrane thickness and the water permeability is shown in FIG. 9 and indicated by a curve -- --. 
     COMPARATIVE EXAMPLE 14 
     The same polysulfone as employed in Example 1, DMAc as the solvent and TEG as the additive were mixed in a weight ratio of 16:75:9 to obtain a homogeneous resin solution. The thus obtained resin solution was cast, at a temperature of 30° C., with a doctor blade onto a glass plate to form a membrane, and the membrane was then dipped into a 30° C. purified water to effect coagulation of the membrane. The resulting flat membrane was 40 μm in thickness and had a three-layer structure. The water permeability of this flat membrane was as small as 0.2 m 3  /m 2  ·day·atm·water at 25° C. 
     EXAMPLES 11 TO 20 
     Hollow fibers were prepared in substantially the same manner as in Example 1 except that the kind of the additive was varied as indicated in Table 3. The additives employed and the properties of the resulting hollow fibers are shown in Table 3. Observation of a cross section of each of the thus obtained hollow fibers showed that each hollow fiber had a three-layer structure characteristic of the present invention. Each of the resulting hollow fibers was 500 μm in inner diameter, 600 μm in outer diameter and 50 μm in membrane thickness. 
     
                                           TABLE 3                                 
__________________________________________________________________________
                       A.sub.i layer       A.sub.o layer                  
                           Pores*.sup.3        Pores*.sup.3               
             Burst                                                        
                  Water*.sup.1                                            
                       Thick-                                             
                           Kind                                           
                               Average                                    
                                     Maximum                              
                                           Thick-                         
                                               Kind                       
                                                   Average                
                                                         Maximum          
Example      strength,                                                    
                  perme-                                                  
                       ness,                                              
                           of  pore diam-                                 
                                     pore diam-                           
                                           ness,                          
                                               of  pore                   
                                                         pore diam-       
No.  Additive                                                             
             kg/cm.sup.2                                                  
                  ability                                                 
                       μm                                              
                           pores                                          
                               eter, μm                                
                                     eter, μm                          
                                           μm                          
                                               pores                      
                                                   eter,                  
                                                         eter,            
__________________________________________________________________________
                                                         μm            
11   ethylene                                                             
             21   20   2.0 Np  --    --    5   Cp  0.05  0.10             
     glycol                                                               
12   diethylene                                                           
             21   21   2.0 &#34;   --    --    4   Cp  0.05  0.10             
     glycol                                                               
13   triethylene                                                          
             21   21   2.0 &#34;   --    --    5   Cp  0.08  0.15             
     glycol                                                               
14   ethylene                                                             
             21   26   1.8 &#34;   --    --    2   Cp  0.10  0.20             
     glycol/LiCl                                                          
15   polyethylene                                                         
             20   28   1.8 &#34;   --    --    5   Cp  0.10  0.30             
     glycol 200*.sup.2                                                    
16   polyethylene                                                         
             18   32   1.5 &#34;   --    --    8   Up  0.40  0.70             
     glycol 6000*.sup.2                                                   
17   propylene                                                            
             20   28   1.8 &#34;   --    --    2   Cp  0.10  0.20             
     glycol                                                               
18   dipropylene                                                          
             20   28   1.8 &#34;   --    --    2   Cp  0.10  0.30             
     glycol                                                               
19   polypropylene                                                        
             20   30   1.0 &#34;   --    --    6   Cp  0.30  0.50             
     glycol 1000*.sup.2                                                   
20   polypropylene                                                        
             17   33   1.0 &#34;   --    --    7   Up  0.40  0.70             
     glycol 3000*.sup.2                                                   
__________________________________________________________________________
                                      Cut-off molecular weight            
                                 Example                                  
                                      Kind of pro-                        
                                             Molecular                    
                                 No.  tein employed                       
                                             weight                       
                                                   Remarks                
__________________________________________________________________________
                                 11   Ovalbumin                           
                                             45000                        
                                 12     &#34;    45000                        
                                 13     &#34;    45000                        
                                 14   Trypsinogen                         
                                             24000                        
                                 15   Ovalbumin                           
                                             45000                        
                                 16   β-Galactose                    
                                             95000 Proportion cut-off of  
                                      DH           bovine serum albumin   
                                                   (molecular weight:     
                                                   67000) was 25% or      
                                                   more                   
                                 17   Ovalbumin                           
                                             45000                        
                                 18     &#34;    45000                        
                                 19   Bovine serum                        
                                             67000                        
                                      albumin                             
                                 20   β-Galactose                    
                                             95000 Proportion cut-off of  
                                      DH     bovine serum albumin         
                                                   (molecular weight:     
                                                   67000) was 25% or      
__________________________________________________________________________
                                                   more                   
 Note:                                                                    
 *.sup.1 Water permeability: m.sup.3 /m.sup.2 · day · at
 · water at 25° C.                                        
  *.sup.2 Molecular weight                                                
 *.sup.3 Average pore diameter and maximum pore diameter were measured by 
 means of a scanning electron photomicrograph (magnification: × 1000
 to 10000)                                                                
 
    
     EXAMPLES 21 TO 23 
     The same resin solution as used in Example 1 was extruded, at a temperature of 30° C., from annular hollow fiber spinning nozzles having orifice widths of 60 μm (Examples 21 and 22) and 55 μm (Example 23). Coagulation of the resulting extrudate was effected by using internal and external coagulating liquids as indicated in Table 4. The temperature of the coagulating liquid was 30° C. The running-in-air distance was 0 cm. Each of the hollow fiber membranes obtained in Example 21 and 22 was 500 μm in inner diameter, 610 μm in outer diameter and 55 μm in membrane thickness. The hollow fiber membrane obtained in Example 23 was 500 μm in inner diameter, 600 μm in outer diameter and 50 μm in membrane thickness. The properties of each of the resulting hollow fiber membranes were examined and the results obtained are shown in Table 4. 
     EXAMPLE  24 
     A hollow fiber membrane was prepared in the same manner as in Examples 21 to 22 except that a 50% aqueous DMAc solution of 30° C. was used as internal and external coagulating liquids and 10 cm of the running-in-air distance was employed. The properties of the resulting hollow fiber membrane were examined and the results obtained are shown in Table 4. 
     
                                           TABLE                                   
__________________________________________________________________________
                             A.sub.i layer                                
                                 Pores*.sup.3                             
     Internal                                                             
            External                                                      
                   Water*.sup.1                                           
                        Burst                                             
                             Thick-   Average                             
                                            Maximum                       
Example                                                                   
     coagulat-                                                            
            coagulat-                                                     
                   perme-                                                 
                        strength,                                         
                             ness,                                        
                                 Kind of                                  
                                      pore diam-                          
                                            pore diam-                    
No.  ing liquid                                                           
            ing liquid                                                    
                   ability                                                
                        kg/cm.sup.2                                       
                             μm                                        
                                 pores                                    
                                      eter, μm                         
                                            eter, μm                   
__________________________________________________________________________
21   H.sub.2 O                                                            
            50% aq.*.sup.2                                                
                   30   22   2.0 Np   --    --                            
            DMAc Soln.                                                    
22   50% aq.*.sup.2                                                       
            50% aq.*.sup.2                                                
                   32   17   2.3 Cp   0.10  0.20                          
     DMAc Soln.                                                           
            DMAc Soln.                                                    
23   50% aq.*.sup.2                                                       
            H.sub.2 O                                                     
                   30   20   2.0 Cp   0.10  0.20                          
     DMAc Soln.                                                           
24   50% aq.*.sup.2                                                       
            50% aq.*.sup.2                                                
                   33   17   2.0 Np   --    --                            
     DMAc Soln.                                                           
            DMAc Soln.                                                    
__________________________________________________________________________
A.sub.o layer                                                             
         Pores*.sup.3                                                     
Thick-        Average                                                     
                    Maximum                                               
                          Cut-off molecular weight                        
Example                                                                   
     ness,                                                                
         Kind of                                                          
              pore diam-                                                  
                    pore diam-                                            
                          Kind of pro-                                    
                                 Molecular                                
No.  μm                                                                
         pores                                                            
              eter, μm                                                 
                    eter, μm                                           
                          tein employed                                   
                                 weight                                   
                                       Remarks                            
__________________________________________________________________________
21   2.2 Cp   0.10  0.20  Ovalbumin                                       
                                 45000                                    
22   2.2 Cp   0.10  0.20  Bovine serum                                    
                                 67000                                    
                          albumin                                         
23   1.8 Np   --    --    Ovalbumin                                       
                                 45000                                    
24   8   Cp   0.30  0.50  β-Galactose                                
                                 95000 Proportion cut-off                 
                          DH           of bovine serum                    
                                       albumin (molecular                 
                                       weight: 67000) was                 
                                       25% or more                        
__________________________________________________________________________
 Note:                                                                    
 *.sup.1 Water permeability m.sup.3 /m.sup.2 · day · atm
 · water at 25° C.                                        
 *.sup.2 A 50% aqueous DMAc solution                                      
 *.sup.3 Average pore diameter and maximum pore diameter were measured by 
 means of a scanning electron photomicrograph (magnification: × 1000
 to 10000)                                                                
 
    
     EXAMPLES 25 TO 34 AND COMPARATIVE EXAMPLE 15 
     Hollow fiber membranes were prepared in substantially the same manner as in Example 1 except that the orifice width of the nozzle and the running-in-air distance were varied as indicated in Table 5. The inner diameter of each of the resulting hollow fiber membranes was adjusted to 500 μm. The properties of each of the resulting hollow fiber membranes were examined and the results obtained are shown in Table 5. 
     A hollow fiber membrane obtained in Comparative Example 5 (running-in-air distance: 30 cm) had a double-layer structure in which pores having an average pore diameter of 1.00 μm and a maximum pore diameter of 2.50 μm were present in the Ao layer. As shown in Table 5, the hollow fiber membrane obtained in Comparative Example 15 was poor in burst strength as compared with the hollow fiber membranes of the present invention. 
     
                                           TABLE 5                                 
__________________________________________________________________________
               Mem-          A.sub.i layer                                
Running-       brane             Pores*.sup.2                             
      in-air                                                              
           Orifice                                                        
               thick-                                                     
                   Burst                                                  
                        Water*.sup.1                                      
                             Thick-   Average                             
                                            Maximum                       
Example                                                                   
      distance,                                                           
           width,                                                         
               ness,                                                      
                   strength,                                              
                        perme-                                            
                             ness,                                        
                                 Kind of                                  
                                      pore diam-                          
                                            pore diam-                    
No.   cm   μm                                                          
               μm                                                      
                   kg/cm.sup.2                                            
                        ability                                           
                             μm                                        
                                 pores                                    
                                      eter, μm                         
                                            eter, μm                   
__________________________________________________________________________
25    20   110 75  15   20   2.3 Np   --    --                            
26    15   90  60  15   25   2.2 &#34;    --    --                            
27    10   70  55  15   25   2.0 &#34;    --    --                            
28    7    60  50  15   26   2.0 &#34;    --    --                            
29    5    60  50  16   25   2.2 &#34;    --    --                            
30    3    70  50  16   23   2.3 &#34;    --    --                            
31    2    70  60  18   22   2.3 &#34;    --    --                            
32    1.5  70  65  25   23   1.8 &#34;    --    --                            
33    1.0  70  60  26   20   1.8 &#34;    --    --                            
34    0    70  65  25   20   2.0 &#34;    --    --                            
Compara-                                                                  
      30   100 60   7   38   2.0 &#34;    --    --                            
tive Ex-                                                                  
ample 15                                                                  
__________________________________________________________________________
                A.sub.o layer                                             
                    Pores*.sup.2                                          
                Thick-   Average                                          
                               Maximum                                    
                                     Cut-off molecular weight             
          Example                                                         
                ness,                                                     
                    Kind of                                               
                         pore diam-                                       
                               pore diam-                                 
                                     Kind of pro-                         
                                            Molecular                     
          No.   μm                                                     
                    pores                                                 
                         eter, μm                                      
                               eter, μm                                
                                     tein employed                        
                                            weight                        
__________________________________________________________________________
          25    1.0 Up   0.50  0.80  Ovalbumin                            
                                            45000                         
          26    2.0 Up   0.40  0.70  Trypsinogen                          
                                            24000                         
          27    3.0 Up   0.40  0.60  &#34;      &#34;                             
          28    4.0 Cp   0.20  0.40  &#34;      &#34;                             
          29    3.0 Cp   0.18  0.35  &#34;      &#34;                             
          30    3.0 Cp   0.15  0.30  &#34;      &#34;                             
          31    5.0 Cp   0.15  0.30  &#34;      &#34;                             
          32    3.0 Cp   0.10  0.20  &#34;      &#34;                             
          33    2.5 Cp   0.05  0.10  &#34;      &#34;                             
          34    3.0 Np   --    --    &#34;      &#34;                             
          Compara-                                                        
                1.0 Up   1.00  2.50  Ovalbumin                            
                                            45000                         
          tive Ex-                                                        
          ample 15                                                        
__________________________________________________________________________
 Note:                                                                    
 *.sup.1 Water permeability m.sup.3 /m.sup.2 ·  day · at
 · water at 25° C.                                        
 *.sup.2 Average pore diameter and maximum pore diameter were measured by 
 means of a scanning electron photomicrograph (magnification: × 1000
 to 10000)                                                                
 
    
     400 Hollow fibers as prepared in Examples 34 were bundled into a module and unified by means of an adhesive at both end portions thereof (the resulting module is hereinafter referred to as &#34;module of the present invention&#34;). 400 hollow fibers as prepared in Comparative Example 15 were bundled into a module in the same manner as mentioned above (the resulting module is hereinafter referred to as &#34;comparative module&#34;). By using each of the thus obtained modules, filtration of a 30° C. water was carried out for a week, while back-washing of said module was effected (at a pressure of 0.5 kg/cm 2 ) for 3 minutes at a frequency of once an hour. The effective length of the fiber was 20 cm, the pressure at the inlet portion was 2 kg/cm 2  and the pressure at the outlet portion was 0 kg/cm 2 . 
     After the one-week operation, each module was diassembled and the hollow fiber bundle was taken out to examine the state of fibers. As a result of the examination, it was found that the hollow fiber bundle constituting the comparative module had been ruptured at several portions thereof. On the other hand, no damage was observed in the hollow fiber bundle constituting the module of the present invention. From the above, it will be understood that the hollow fiber obtained in Comparative Example 15 is so poor in mechanical strength that it cannot withstand a long-time continuous filtration operation. 
     Another module of the present invention and another comparative module were prepared in the same manner as mentioned above. Each of the modules was subjected to a continuous water filtration operation (30° C. water) effected for a week. In the operation, back-washing of the module was effected, at a pressure of 0.2 kg/cm 2 , for a 3 minutes at a frequency of once an hour. The pressure at the inlet portion was 0.2 kg/cm 2  and the pressure at the outlet portion was 0.1 kg/cm 2 . The resulting filtrate was subjected to a limurus test to examine the presence or the absence of pyrogen. As a result, pyrogen was not detected from the filtrate obtained using the module of the present invention but was detected from the filtrate obtained using the comparative module. The examination of each of the modules was effected using a scanning electron microscope after the continuous filtration operation. As a result, a large amount of dead or living bacteria were detected in the void layer and on the outer surface of the hollow fibers constituting the comparative module. By contrast, with respect to the module of the present invention, such bacteria were not detected. 
     Still another module of the present invention and still another comparative module were prepared in the same manner as described above. Each of the modules was subjected to a continuous filtration operation in the same manner as described just above. Thereafter, each of the modules was immersed in a 0.5% aqueous alkaline solution for a day and, thereafter, washed sufficiently with a pyrogen-free water. However, in the above step, pyrogen cannot be completely removed from the comparative module while pyrogen can be easily removed from the module of the present invention. Subsequently, the module of the present invention was subjected to a continuous water filtration operation for a week in the same manner as mentioned above. In the above operation, the obtained filtrate was free from pyrogen. 
     From the results as set forth above, it is understood that the hollow fiber membrane obtained in Comparative Example 15, in which pores having an average pore diameter of 1.00 μm and a maximum pore diameter of 2.50 μm were formed in the outer surface thereof, easily allows bacteria to enter the membrane. The bacteria present in the membrane propagate, causing generation of pyrogenic substances. Accordingly, if the hollow fiber membrane obtained in Comparative Example 15 is used for filtration of water, the resulting filtrate is liable to be contaminated by pyrogenic substances and, hence, a pyrogen-free water can hardly be obtained. 
     The term &#34;pyrogen&#34; as referred to herein means polyribosaccharide molecules present in the epidermis of the bacteria. 
     COMPARATIVE EXAMPLE 16 
     16 wt% of the same polysulfone as employed in Example 1, 55 wt% of DMAc as the solvent and, as the additive, 27 wt% of dimethylsulfoxide and 2 wt% of a 50% aqueous NaNo 3  solution were mixed, thereby to obtain a homogeneous resins solution. 
     The thus obtained resin solution was spun into a hollow fiber in substantially the same manner as in Example 1 except that 0 cm of the running-in-air distance was employed. The resulting hollow fiber membrane had a uniform structure in which no voids were present. The properties of the resulting hollow fiber membrane were examined and the results obtained are shown in Table 6. 
     EXAMPLE 35 
     The same polysulfone as employed in Example 1, DMAc as the solvent and TEG as the additive were mixed in a weight ratio of 16:74:10, thereby to obtain a homogeneous resin solution. 
     The thus obtained resin solution was spun into a hollow fiber in substantially the same manner as in Example 1 except that 0 cm of the running-in-air distance was employed. The properties of the resulting hollow fiber membrane were examined and the results obtained are shown in Table 6. 
     
                                           TABLE 6                                 
__________________________________________________________________________
                               Burst                                      
                                    Water*.sup.1                          
Example       Polysulfone,                                                
                     DMAc,                                                
                          Additive,                                       
                               Strength,                                  
                                    perme-                                
No.   Additive                                                            
              wt %   wt % wt % kg/cm.sup.2                                
                                    ability                               
__________________________________________________________________________
Compara-                                                                  
      Dimethyl                                                            
              16     55   27/2  8    4                                    
tive  sulfoxide/                                                          
Example                                                                   
      50% aq.                                                             
16    NaNo.sub.3 soln.*.sup.2                                             
Example                                                                   
      Tetraethylene                                                       
              16     74   10   18   20                                    
35    glycol                                                              
__________________________________________________________________________
 Note:                                                                    
 *.sup.1 Water permeabilty: m.sup.3 /m.sup.2 · day · atm
 · at 25° C.                                              
 *.sup.2 A 50% aqueous NaNo.sub.3 solution                                
 
    
     EXAMPLE 36 TO 38 
     Hollow fiber membranes were prepared in substantially the same manner as in Example 1 except that the polysulfone and DMAc were mixed in varied weight proportions so that spinning solutions differing in polysulfone concentration were prepared. The properties of the resulting hollow fiber membrane were examined and the results obtained are shown in Table 7. 
     
                                           TABLE 7                                 
__________________________________________________________________________
                            Ai layer                                      
                                pores*.sup.2                              
                                    Average                               
                                         Maximum                          
Exam-                                                                     
    Poly-         Burst                                                   
                       Water*.sup.1                                       
                            Thick-                                        
                                Kind                                      
                                    pore pore                             
ple sulfone,                                                              
         DMAc,                                                            
              TEG,                                                        
                  strength,                                               
                       perme-                                             
                            ness,                                         
                                of  diame-                                
                                         diame-                           
No. wt % wt % wt %                                                        
                  kg/cm.sup.2                                             
                       ability                                            
                            μm                                         
                                pores                                     
                                    ter, μm                            
                                         ter, μm                       
__________________________________________________________________________
36  16   74   10  15   20   2   Np  --   --                               
37  25   65   10  27   12   2   Np  --   --                               
38  27   60   10  30   9    2   Np  --   --                               
__________________________________________________________________________
                Ao layer                                                  
                    pores*.sup.2   Cut-off molecular                      
                        Average                                           
                             Maximum                                      
                                   weight                                 
            Exam-                                                         
                Thick-                                                    
                    kind                                                  
                        pore pore  kind of                                
            ple ness,                                                     
                    of  diam-                                             
                             diam- protein                                
                                         Molecular                        
            No. μm                                                     
                    pores                                                 
                        eter, μm                                       
                             eter, μm                                  
                                   employed                               
                                         weight                           
__________________________________________________________________________
            36  5   Cp  0.20 0.30  Ovalbumin                              
                                         45000                            
            37  4   Cp  0.05 0.10  &#34;     &#34;                                
            38  5   Cp  0.05 0.10  &#34;     &#34;                                
__________________________________________________________________________
 Note:                                                                    
 *.sup.1 Water permeability: m.sup.3 /m.sup.2 · day · at
 · water at 25° C.                                        
 *.sup.2 Average pore diameter and maximum pore diameter were measured by 
 means of a scanning electron photomicrograph (magnification: × 1000
 to 10000)                                                                
 
    
     EXAMPLE 39 AND 40 
     Hollow fiber membranes were prepared in substantially the same manner as in Example 1 except that the kind of the solvent was varied as indicated in Table 8. The properties of the resulting hollow fiber membranes were examined and the results obtained are shown in Table 8. 
     
                                           TABLE 8                                 
__________________________________________________________________________
                    A.sub.i layer                                         
                        Pores*.sup.2                                      
                            Average                                       
                                 Maximum                                  
Exam-     Burst                                                           
               Water*.sup.1                                               
                    Thick-                                                
                        Kind                                              
                            pore pore                                     
ple       strength,                                                       
               perme-                                                     
                    ness,                                                 
                        of  diam-                                         
                                 diam-                                    
No. Solvent                                                               
          kg/cm.sup.2                                                     
               ability                                                    
                    μm                                                 
                        pores                                             
                            eter, μm                                   
                                 eter, μm                              
__________________________________________________________________________
39  N--methyl                                                             
          15   18   1.8 Np  --   --                                       
    pyrrolidone                                                           
40  Dimethyl-                                                             
          14   19   1.8 Np  --   --                                       
    sulfoxide                                                             
__________________________________________________________________________
        A.sub.o layer                                                     
            Pores*.sup.2   Cut-off molecular                              
                Average                                                   
                     Maximum                                              
                           weight                                         
Exam-   Thick-                                                            
            Kind                                                          
                pore pore  Kind of                                        
ple     ness,                                                             
            of  diam-                                                     
                     diam- protein                                        
                                 Molecular                                
No.     μm                                                             
            pores                                                         
                eter, μm                                               
                     eter, μm                                          
                           employed                                       
                                 weight                                   
__________________________________________________________________________
39      4   Cp  0.10 0.20  Ovalbumin                                      
                                 45000                                    
40      4   Cp  0.10 0.20  Ovalbumin                                      
                                 45000                                    
__________________________________________________________________________
 Note:                                                                    
 *.sup.1 Water permeability: m.sup.3 /m.sup.2 · day · at
 · water at 25°  C.                                       
 *.sup.2 Average pore diameter amd maximum pore diameter were measured by 
 means of a scanning electron photomicrograph (magnification: × 1000
 to 10000)                                                                
 
    
     The conditions employed and the results obtained in Examples and Comparative Examples as described before will be summarized in Table 9. 
     
         TABLE 9
  Coagulating liquid    Internal External   Resin  Running- coagulating
 coagulating  Membrane Resin Solvent Additive soln. Orifice in-air liquid l
 iquid  Inner Outer Membrane  Example  Amount,  Amount,  Amount, temp.,
 width, distance,  Temp.,  Temp.,  diameter, diameter, thickness,
 Membrane No. Kind wt % Kind wt % Kind wt % °C. μm cm Kind
 °C. Kind °C. Type μm μm μm structure
    1 PS 20 DMAc 71 TEG 9 30 45 1.5 H.sub.2 O 30 H.sub.2 O 30 HF 200 276
 38 3 layers  2 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 30 &#34; &#34; &#34; &#34; &#34; &#34; 500 550 25 &#34;  3 &#34; &#34; &#34; &#34; &#34; &#34;
 &#34; 45 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 580 40 &#34;  4 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 55 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 600 50 &#34;
 5 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 80 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 640 70 &#34;  6 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 90 &#34; &#34; &#34; &#34; &#34;
 &#34; &#34; 660 80 &#34;  7 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 105  &#34; &#34; &#34; &#34; &#34; &#34; &#34; 690 95 &#34; Compara- &#34; &#34; &#34;
 &#34; &#34; &#34; &#34; 180  &#34; &#34; &#34; &#34; &#34;  &#34; &#34; 800 150  5 layers tive Ex- ample 1  8 &#34; &#34; &#34;
 &#34; &#34; &#34; &#34; 90 0 &#34; &#34; &#34; 50 &#34; &#34; 660 80 3 layers  9 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 90 &#34; &#34; &#34;
 Kerosene 30 &#34; &#34; &#34; &#34; &#34; Compara- &#34; &#34; &#34; &#34; &#34; &#34; &#34; 150  &#34; &#34; &#34; &#34; &#34; &#34; 420 700
 140  &#34; tive Ex- ample 2 Compara- &#34; &#34; &#34; &#34; &#34; &#34; &#34; 50 -- &#34; &#34; None -- &#34; 500
 540 20 2 layers tive Ex- ample 3 Compara- &#34; &#34; &#34; &#34; &#34; &#34; &#34; 100  -- &#34; &#34; &#34; --
 &#34; &#34; 600 50 &#34; tive Ex- ample 4 Compara- &#34; &#34;  &#34; &#34; &#34; &#34; &#34; 150  -- &#34; &#34; &#34; -- &#34;
 &#34; 660 80 &#34; tive Ex- ample 5 Compara- &#34; &#34; &#34; &#34; &#34; &#34; &#34; 200  -- &#34; &#34; &#34; -- &#34; &#34;
 690 95 &#34; tive Ex- ample 6 Compara- &#34; &#34; &#34; &#34; &#34; &#34; &#34; -- -- Water (30°
 C.) FH -- -- 100  3 layers tive Ex- ample 7 10 PES &#34; &#34; 65 &#34; 15  &#34; 55 3.0 H
 .sub.2 O 30 H.sub.2 O 30 HF 500 600 50 &#34; Compara- PS &#34; &#34; 71 &#34; 9 &#34; -- --  W
 ater (30° C.) FM -- -- 10 to 90 &#34; tive              See Examples
             FIG. 9 8 to 13 Compara- &#34; 16 &#34; 75 &#34; &#34; &#34; -- -- &#34; &#34; -- -- 40 &#34;
 tive Examples 8 to 14 11 &#34; 20 &#34; 71 EG 9 &#34; 55 1.5 H.sub.2 O 30 H.sub.2 O
 30 HF 500 600 50 &#34; 12 &#34; &#34; &#34; &#34; DEG &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 13 &#34; &#34; &#34; &#34;
 TriEG &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 14 &#34; &#34; &#34; &#34; EG/ 6/3 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34;
 &#34;      LiCl 15 &#34; &#34; &#34; &#34; PEG 9 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34;       200*.sup.1 16
 &#34; &#34; &#34; &#34; PEG &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34;      6000*.sup.1 17 &#34; &#34; &#34; &#34; PG &#34; &#34;
 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 18 &#34; &#34; &#34; &#34; DPG &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34;  &#34; &#34; 19 &#34; &#34; &#34;
 &#34; PEG &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34;      1000*.sup.1 20 &#34; &#34; &#34; &#34; PPG &#34; &#34; &#34; &#34;
 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34;      3000*.sup.1 21 &#34; &#34; &#34; &#34; TEG &#34; &#34; 60 0 &#34; &#34; 50% aq. &#34;
 &#34; &#34; 610 55 &#34;             DMAc             soln. 22 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 50%
 aq. &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34;           DMAc           soln. 23 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 55 &#34;
 &#34; &#34; H.sub.2 O &#34; &#34; &#34; 600 50 &#34; 24 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 60 10  &#34; &#34; 50% aq. &#34; &#34; &#34;
 610 55 &#34;             DMAc             soln. 25 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 110  20
 H.sub.2 O &#34; H.sub.2 O &#34; &#34; &#34; 650 75 &#34; 26 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 90 15  &#34; &#34; &#34; &#34; &#34; &#34;
 620 60 &#34; 27 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 70 10  &#34; &#34; &#34; &#34; &#34; &#34; 610 55 &#34; 28 &#34; &#34; &#34; &#34; &#34; &#34; &#34;
 60 7 &#34; &#34; &#34; &#34; &#34; &#34; 600 50 &#34; 29 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 5 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 30 &#34; &#34;
 &#34; &#34; &#34; &#34; &#34; 70 3 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 31 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 2 &#34; &#34; &#34; &#34; &#34; &#34; 620 60
 &#34; 32 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 1.5 &#34; &#34; &#34; &#34; &#34; &#34; 630 65 &#34; 33 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 1.0 &#34; &#34;
 &#34; &#34; &#34; &#34; 620 60 &#34; 34 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 0 &#34; &#34; &#34; &#34; &#34; &#34; 630 65 &#34; Compara- &#34; &#34;
 &#34; &#34; &#34; &#34; &#34; 110  30  &#34; &#34; &#34; &#34; &#34; &#34; 620 60 &#34; tive Ex- ample 15 Compara- &#34; 16
 &#34; 55 DMSO/ 27/2  &#34; 90 0 &#34; &#34; &#34; &#34; &#34; &#34; 650 75 A uniform tive Ex-     50%
 aq.             layer ample 16     NaNO.sub.3 35 &#34; &#34; &#34; 74 TEG 10  &#34; &#34; &#34;
 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 3 layers 36 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 1.5 &#34;  &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 37 &#34;
 25 &#34; 65 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 38 &#34; 27 &#34; 60 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34;
 &#34; &#34; 39 &#34; 20 NMP 71 &#34; 9 &#34; 60 &#34; &#34; &#34; &#34; &#34; &#34; &#34; 600 50 &#34; 40 &#34; &#34; DMF &#34; &#34; &#34; &#34; &#34;
 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34;
 Note:
 *.sup.1 Molecular weight
 *.sup.2 Water permeability: m.sup.3 /m.sup.2 · day · at
 · water at 25° C.
 *.sup.3 Average pore diameter and maximum pore diameter were measured by
 means of a scanning electron photomicrograph (magnification: × 1000
 to 10000)
 Abbreviations Used in Table 9
 PS: an aromatic polysulfone
 PES: an aromatic polyether sulfone
 DMAc: dimethylacetamide
 NMP: N--methylpyrrolidone
 DMF: dimethylformamide
 TEG: tetraethylene glycol
 EG: ethylene glycol
 DEG: diethylene glycol
 TriEG: triethylene glycol
 PEG: polyethylene glycol
 PG: propylene glycol
 DPG: dipropylene glycol
 PPG: polypropylene glycol
 DMSO: dimethyl sulfoxide
 HF: a hollow fiber membrane
 FM: a flat membrane
 
    
     
           Inner surface layer (Ai layer) Outer surface layer (Ao layer)  Propertie
 s  Pores*.sup.3  Pores*.sup.3 Void layer  Cut-off molecular  Average
 Maximum  Average Maximum (B layer)  weight  Thick- Kind pore pore Thick- K
 ind pore  pore Thick- Number Burst Water Kind of   ness, of diameter,
 diameter, ness, of diameter, diameter, ness, of strength perme- protein
 Molecular μm pores μm μm μm pores μm μm μm voids
 kg/cm.sup.2 ability*.sup.2 employed weight Remarks
   1.7 Np -- -- 3.3 Cp 0.10 0.20 31 More than 20 22 Ovalbumin 45000
     100 1.8 &#34; -- -- 1.8 &#34; &#34; 0.25 21 More than 17 29 Bovine serum 67000
        100   albumin &#34; &#34; -- -- 2.5 &#34; &#34; &#34; 36 More than 20 27 Bovine serum
 &#34;          100   albumin &#34; &#34; -- -- 3.0 &#34; &#34; &#34; 45 More than 21 25 Ovalbumin
  45000          100 &#34; &#34; -- -- 3.2 &#34; &#34; &#34; 65 More than 25 23 Trypsinogen
 24000          100 &#34; &#34; -- -- 3.5 &#34; &#34; &#34; 75 More than 26 22 Lysozyme 14000
          100 &#34; &#34; -- -- 4.0 &#34; &#34; 0.20 90 More than 28 21 Cytochrome 13000
         100   C &#34; &#34; -- -- 10.0  Np -- -- 138  More than 32 2.5 Insulin
 6000          100 &#34; &#34; -- -- 4.0 &#34; -- -- 74 More than 31 11 &#34; &#34;
 100 &#34; &#34; -- -- 9.0 Cp 0.20 0.40 70 More than 30 10 Lysozyme 14000
  100 &#34; &#34; -- -- 65 &#34; &#34; 0.50 75 More than 35 2.0 &#34; &#34;          100 4.0 &#34; --
 -- -- -- 5.00 10.00  26 More than  2  9 Ovalbumin 45000          100 &#34; &#34;
 -- -- -- -- 10.00  15.00  46 More than  5  3 Trypsinogen 24000  100 5.0
 &#34; -- -- -- -- 12.00  20.00  75 More than  8  2 &#34; &#34;          100 &#34; &#34; --
 -- -- -- 10.00  &#34; 90 About 100  9  2 Lysozyme 14000 4.0 &#34; -- -- 5.0 Np
 -- -- -- -- --  0.01 -- -- 2.0 &#34; -- -- 2.0 Cp 0.07 0.15 44 More than 23
 30 Insulin  6000          100 3.0 -- -- -- -- -- -- -- -- -- -- 0.1 to 1
 -- --            See            FIG. 9 2.8 -- -- -- -- -- -- -- -- -- --
 0.2 -- -- 2.0 Np -- -- 5.0 Cp 0.05 0.10 43 More than 21 20 Ovalbumin
 45000          100 &#34; &#34; -- -- 4.0 &#34; &#34; &#34; 44 More than &#34; 21 &#34; &#34;
 100 &#34; &#34; -- -- 5.0 &#34; 0.08 0.15 43 More than &#34; &#34; &#34; &#34;          100 1.8 &#34; --
 -- 2.0 &#34; 0.10 0.20 46 More than &#34; 26 Trypsinogen 24000          100 &#34; &#34;
 -- -- 5.0 &#34; &#34; 0.30 43 More than 20 28 Ovalbumin 45000          100 1.5 &#34;
 -- -- 8.0 Up 0.40 0.70 41 More than 18 32 β
 Galactose 95000 Proportion cut-off of          100   DH  bovine serum
 albumin               (molecular weight:               67000) was 25% or
 more. 1.8 &#34; -- -- 2.0 Cp 0.10 0.20 46 More than 20 28 Ovalbumin 45000
       100 &#34; &#34; -- -- &#34; &#34; &#34; 0.30 &#34; More than &#34; &#34; &#34; &#34;          100 1.0 &#34; --
 -- 6.0 &#34; 0.30 0.50 43 More than &#34; 30 Bovine serum 67000          100
 albumin &#34; &#34; -- -- 7.0 Up 0.40 0.70 42 More than 17 33 β
 Galactose 95000 Proportion cut-off of          100   DH  bovine serum
 albumin               molecular weight:               67000) was 25% or
 more. 2.0 &#34; -- -- 2.2 Cp 0.10 0.20 51 100 22 30 Ovalbumin 45000 2.3 Cp
 0.10 0.20 &#34; &#34; &#34; &#34; 50  70 17 32 Bovine serum 67000             albumin
 2.0 &#34; &#34; &#34; 1.8 Np -- -- 46 100 20 30 Ovalbumin 45000 &#34; Np -- -- 8.0 Cp
 0.30 0.50 45
  90 17 33 β                                            Galactose
 95000 Proportion cut-off of             DH  bovine serum albumin
       (molecular weight:               67000) was 25% or more. 2.3 &#34; --
 -- 1.0 Up 0.50 0.80 72 More than 15 20 Ovalbumin 45000          100 2.2
 &#34; -- -- 2.0 &#34; 0.40 0.70 56 More than &#34; 25 Trypsinogen 24000          100
 2.0 &#34; -- -- 3.0 &#34; &#34; 0.60 50 More than &#34; &#34; &#34; &#34;          100 &#34; &#34; -- -- 4.0
 Cp 0.20 0.40 44 More than &#34; 26 &#34; &#34;          100 2.2 &#34; -- -- 3.0 &#34; 0.18
 0.35 45 More than 16 25 &#34; &#34;          100 2.3 &#34; -- -- &#34; &#34; 0.15 0.30 &#34;
 More than &#34; 23 &#34; &#34;          100 &#34; &#34; -- -- 5.0 &#34; &#34; &#34; 53 More than 18 22 &#34;
 &#34;          100 1.8 &#34; -- -- 3.0 &#34; 0.10 0.20 62 More than 25 23 &#34; &#34;
   100 &#34; &#34; -- -- 2.5 &#34; 0.05 0.10 66 More than 26 20 &#34; &#34;          100 2.0
 &#34; -- -- 2.0 Np -- -- 61 More than 25 &#34; &#34; &#34;          100 &#34; &#34; -- -- 1.0 Up
 1.00 2.50 57 More than  7 38 Ovalbumin 45000          100 -- &#34; -- -- --
 Np -- -- --  0  8  4 &#34; &#34; 2.0 &#34; -- -- 2.0 &#34; -- -- 71 More than 18 20 &#34; &#34;
         100 &#34; &#34; -- -- 5.0 Cp 0.20 0.30 68 More than 15 &#34; &#34; &#34;
 100 &#34; &#34; -- -- 4.0 &#34; 0.05 0.10 69 More than 27 12 &#34; &#34;          100 &#34; &#34; --
 -- 5.0 &#34; &#34; &#34; 68 More than 30  9 &#34; &#34;          100 1.8 &#34; -- -- 4.0 &#34; 0.10
 0.20 44 More than 15 18 &#34; &#34;          100 &#34; &#34; -- -- &#34; &#34; &#34; &#34; &#34; More than
 14 19 &#34; &#34;          100
 
    
     In the meantime, the membrane thickness of the hollow fiber membrane of the present invention is defined as the minimum distance between the outer and inner peripheries of the hollow fiber membrane.