DESCRIPTION

HOLLOW-FIBER MEMBRANE AND PROCESS FOR PRODUCING HOLLOW-FIBER MEMBRANE

TECHNICAL FIELD

The present invention relates to a hollow-fiber membrane suitable for use as a separation membrane of a water purifier and the like, and a process for producing a hollow-fiber membrane.

BACKGROUND ART

Hollow-fiber membrane modules for filtration, dialysis and the like of substances contained in a liquid to be treated have hitherto been used in many fields, for example, water treatment applications such as microfiltration and ultrafiltration, gas separation of nitrogen, oxygen, and hydrogen, chemicals, and biotechnology since they allow for a large effective membrane area per unit volume.

The hollow-fiber membranes originally used were mainly homogeneous membranes. Then, asymmetric membranes having a dense layer are becoming the mainstream as a result of a pursuit of permeability.

An asymmetric hollow-fiber membrane is formed by using of a double annular spinneret, where a liquid is injected into its center pipe to form the hollow shape. When the liquid is coagulable, a dense structure is formed on the inner surface side of the hollow-fiber membrane, whereas when the liquid is noncoagulable, a dense layer is formed on the outer surface side in a downstream coagulation bath.

A spinning draft ratio, which is an" important parameter in forming a hollow-fiber membrane, is defined as a ratio of an ejection speed of a membrane-forming stock solution from a spinneret to a drawing rate of a formed hollow-fiber membrane. The value varies greatly depending on the membrane forming methods.

For example, Patent Document 1 discloses a. process for producing a porous fiber having a spinning draft ratio of 100 or 185. However, this document relates to a melt spinning method, where membranes are stably formed even at a high spinning draft ratio. Also, even in a solution spinning method, membranes are stably formed at a high spinning draft ratio when the membranes are formed by gas injection. It is supposed that this is because a polymer of a membrane-forming composition is oriented along with the draft, and strength of the hollow-fiber membrane thus increases.

However, in forming a membrane by a liquid injection method, a polymer of a membrane-forming composition comes in contact with a non-solvent and thus coagulates to form a membrane. Therefore, membrane formation becomes unstable when the polymer is subjected to the draft at the same time. Specifically, for example. Patent Document 2 describes that the spinning draft ratio is normally set in the range of 2 to 5, since an extremely low or high spinning draft ratio makes the production unstable. Patent Document 3 also describes that the spinning draft ratio is set in the range of 10 to 300% {0.1 to 3) for a similar reason. Patent Document 4 points out that an inner surface of a hollow- fiber membrane has a torn structure when the spinning draft ratio exceeds only 2, which causes problems such that a target substance to be removed leaks easily. Moreover, when a balance between polymer coagulation and spinning draft is lost, there appears a star-shaped structure on the inner surface or a pleated structure on the outer surface in extreme cases and the fiber often has problems such as breakage.

Even in a liquid injection method, in the case of forming a homogenous hollow-fiber membrane, a membrane is more stably formed at a higher spinning draft ratio since the polymer component is oriented more steadily;. Based on this fact Patent Document 5 describes that the spinning draft ratio is 5 or more. Although not explicitly specified, it is considered that the membrane forming method employed therein is one by a thermally induced phase separation method since it is described that a homogeneous hollow-fiber membrane is obtained. Meanwhile, it is impossible to apply the same idea to a process of producing an asymmetric hollow-fiber membrane by a non-solvent induced phase separation method, since this process is not to coagulate a polymer component while making it oriented.

The thermally induced phase separation method and the non-solvent induced phase separation method are both membrane forming methods. The former is to induce phase separation by changing the temperature of a homogenous polymer solution, and is suitable for forming a homogenous membrane in terms of hollow-fiber membrane formation. The latter is to induce phase separation by adding a non- solvent composition to a homogenous polymer solution, and is suitable for forming an asymmetric membrane by controlling interfacial conditions such as those on inner and outer sides.

Meanwhile, development of thinner hollow-fiber membranes is in progress for improving turbidness removal performance by increasing the effective membrane area per unit volume of a hollow-fiber membrane module. However, as described above, it is considered that the spinning draft ratio needs to be kept 5 or less in forming a hollow-fiber membrane by a liquid injection method, since a high spinning draft ratio causes problems. Therefore, in terms of making a thinner hollow-fiber membrane, investigations have been made for a cycle where the diameter of the double annular spinneret is made smaller so as to achieve a thin hollow-fiber membrane while keeping the spinning draft ratio low.

However, making the diameter of it smaller means that spinneret should be exchanged for each of the desired hollow-fiber membranes, and this is not practical industrially. Moreover, the hollow-fiber membrane formed simply by a smaller double annular spinneret at; a low spinning draft ratio can improve turbidness removal performance only to the extent corresponding to- an increase in membrane area, when such fibers are filled into a module at the same filling rate as in a module filled with a conventional thick hollow-fiber membrane.

PRIOR ART DOCUMENTS
Patent Documents
Patent Document 1: JP-B-2550204
Patent Document 2: JP-B-31l7575
Patent Document 3: JP-B-2948856
Patent Document 4: JP-B-33l7876
Patent Document 5: JP-A-2008-l37004

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

It is an object of the present invention to provide a process for producing a hollow-fiber membrane which allows to increase the filled membrane area and improve turbidness removal performance of a hollow-fiber membrane module and is also applicable for industrial use.

Means for Solving the Problems

According to the present invention, it has been found that the above object can be achieved by either of the following constitutions.

(1) A hollow-fiber membrane including a dense layer on one of inner and outer sides, wherein pores formed on a membrane surface of a side having the dense layer have an aspect ratio of 3 or more and 5 or less.

(2) A hollow-fiber membrane including a dense layer on one of inner and outer sides, wherein a membrane surface of one side having the dense layer has an opening ratio of 8% or more and 14% or less, and a membrane surface of the other side has an opening ratio of 15% or more and 20% or less.

(3) A process for producing a hollow-fiber membrane, wherein an asymmetric hollow-fiber membrane is formed by a liquid injection method using a double annular spinneret at a spinning draft ratio of 6 or more and 13 or less.

(4) The process for producing a hollow-fiber membrane according to the above (3), wherein a membrane-forming stock solution in which a component of the hollow-fiber membrane is dissolved in an organic solvent has a viscosity of 1 Pa-s or more and 10 Pa-s or less.

(5) The process for producing a hollow-fiber membrane according to the above (3) or (4), wherein the component of the hollow-fiber membrane is a polysulfone system polymer.

(6) The process for producing a hollow-fiber membrane according to any one of the above (3) to (5), wherein an injection liquid used in the liquid injection method is noncoagulable.

(7) A water purifier including the hollow-fiber membrane obtained by the process according to any one of the above (3) to (6) or the hollow-fiber membrane according to the above (1) or (2).

In the present invention, the "aspect ratio of pores formed on a membrane surface" refers to a dimensional ratio of the formed pores in vertical/transverse directions, where the vertical direction refers to a longitudinal direction of a hollow-fiber membrane and the transverse direction refers to a direction along a surface of the hollow-fiber membrane perpendicular to the longitudinal direction of the hollow-fiber membrane.

EFFECTS OF THE INVENTION

The present invention is applicable to industrial use, since it makes it possible to produce a thin hollow-fiber membrane without having to exchange spinnerets. Moreover, when the' hollow-fiber membrane is filled into a; module at the same filling rate as in a module filled with a conventional thick hollow-fiber membrane, it can not only- increase the filled membrane area but also improve turbidness removal performance to a higher extent than that expected by the increase in membrane area. As a result, it becomes possible to lower exchange frequency of a cartridge, and thus to reduce energy burden caused by disposal of the cartridge, which makes the hollow-fiber membrane environmentally friendly.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1-a is a SEM micrograph of an outer surface of a hollow-fiber membrane obtained in Example 1.

Fig. 1-b is a SEM micrograph of an inner surface of the hollow-fiber membrane obtained in Example 1;.

Fig. 1-c is an example showing surface pores extracted from Fig. 1-a.

Fig. 2-a is a SEM micrograph of an outer surface of a hollow-fiber membrane obtained in Example' 2.

Fig. 2-b is a SEM micrograph of an inner surface of the hollow-fiber membrane obtained in Example 2.

Fig. 2-c is an example showing surface pores extracted from Fig. 2-a.

Fig. 3-a is a SEM micrograph of an outer surface of a hollow-fiber membrane obtained in Comparative Examples 1 and 2 .

Fig. 3-b is a SEM micrograph of an inner surface of the hollow-fiber membrane obtained in Comparative Examples 1 and 2.

Fig. 3-c is an example showing surface pores extracted from Fig. 3-a.

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in the following.

The present invention is a process for producing an asymmetric hollow-fiber membrane, for example, by a non- solvent induced phase separation method employing a liquid injection method using a double annular spinneret, wherein a membrane-forming stock solution and a noncoagulable liquid are injected respectively into an outer circumferential slit and a center pipe of the double annular spinneret, so as to form a hollow shape. The membrane-forming stock solution is, for example, ejected from the double annular spinneret together with the noncoagulable liquid or the like, runs for a predetermined interval, and then is led to a downstream coagulation bath. The hollow-fiber membrane coagulated to the hollow shape in the coagulation bath is washed with water, and then wound up into a reel.

In the membrane-forming stock solution, a hollow- fiber membrane component such as a polysulfone system polymer is dissolved. The polysulfone system polymer has a repeating unit of the following chemical formula (1) or (2), but is not limited thereto and a functional group may be added to part of the backbones.

[Chemical Formula 1]

[Chemical Formula 2]

(2)

Various solvents may be used as the solvent to dissolve the polymer, such as dimethylsulfoxide, dimethylacetoamide, dimethylformamide, N-methyl-2- pyrrolidone and dioxane. Particularly desirable are dimethylacetoamide, dimethylsulfoxide, dimethylformamide and N-methyl-2-pyrrolidone. The solvent may be properly
selected according to the viscosity of the membrane-forming stock solution and coagulability with the injection liquid.

The membrane-forming stock solution preferably has a viscosity of 1 Pa-s or more and 10 Pas or less: at the double annular spinneret. It is not preferable that the viscosity of the membrane-forming stock solution is too high, since the pressure at the spinneret becomes too high to keep the ejection state stable. On the contrary, it is not preferable that the viscosity of the membrane-forming stock solution is too low, since the fiber breaks due to decrease in spinnability before the membrane structure is formed. A more preferable viscosity range is 2 Pas or more and 8 Pa-s or less.

To adjust the viscosity of the membrane-forming stock solution, an additive may be added to the membrane-forming stock solution. For example, a hydrophilic polymer is suitably added in the case of a hollow-fiber membrane for a water purifier, since it provides hydrophilicity also to the hollow-fiber membrane itself. Considering affinity with a polysulfone system resin, polyvinylpyrrolidone and polyethylene glycol are the most preferable. When the hydrophilic polymer has a high molecular weight, it becomes possible to reduce the amount of the hydrophilic polymer to be added compared to the case of using a hydrophilic polymer having a low molecular weight. Thus, it is possible to adjust the viscosity of the membrane-forming stock solution by considering the molecular weight of the hydrophilic polymer.

The liquid injected into the center pipe may be properly selected from those coagulable or noncoagulable according to a desired form of the hollow-fiber membrane. The coagulation value is an indicator of .coagulability with the injection liquid. The coagulation value is the weight of the injection liquid added when it is gradually added to 50 g of a 1 weight% solution of a main polymer which constitutes the membrane until the system gets cloudy. A smaller coagulation value designates a higher coagulability with the injection liquid. It is judged according to the empirical rule that an injection liquid having a coagulation value of 40 or more (80% or more of; the original liquid amount) is noncoagulable,. since: such a liquid does not produce a particle structure of an coagulated polymer on a membrane surface on which a dense layer is formed.

When a coagulable liquid is used as the liquid, the coagulation starts from an inner surface and a dense layer is thus formed on the inner surface side of the hollow- fiber membrane. In contrast, when a noncoagulable liquid is used;, the coagulation starts from an outer surface by the downstream coagulation bath, and the dense layer is thus formed on the outer surface side of the hollow-fiber membrane. It is preferable that the liquid of the coagulation bath is water-based also because such a liquid is inexpensive. To adjust the coagulation speed of the polymer composition, a mixture of water and a solvent of the polymer composition is preferably used, and an additive such as a dispersant may be further added.

In the present invention, it is important to adjust the spinning draft ratio to 6 or more and 13 or less in the above process. The spinning draft ratio is the ratio of the ejection linear speed of the membrane-forming composition at the outer circumferential slit of the double annular spinneret to the wound-up speed of the hollow-fiber, or the value of the wound-up speed divided by the ejection linear speed of the membrane-forming composition. The ejection linear speed is a linear speed of the membrane- forming composition when it is ejected from the outer circumferential slit of the double annular spinneret, or the value of the ejection flow rate divided by the cross- sectional area of the outer circumferential slit.

When the spinning draft ratio is too low, it is expected that the turbidness removal performance is improved only to the extent corresponding to an increase in membrane area, even when the membrane is made thinner so as to increase the filled membrane area of a cartridge. A low spinning draft ratio means a narrow outer circumferential slit width. In such a case, pressure loss at the spinneret gets high at a high spinning rate, and the ejection cannot be kept stable and the membrane structure is disarranged, which causes problems of quality such as water permeability and fractional performance. Moreover, there occur problems such as high cost because of high difficulty in producing such a double annular spinneret having a narrow slit width.

On the contrary, when the spinning draft ratio is too high and the injection liquid is coagulable, the inner surface of the hollow-fiber membrane cannot keep its hollow shape by the influence of the spinning draft. As a result, the inner surface loses its smoothness and has a polygonal structure like a star, thus it is not preferablie. When the injection liquid is noncoagulable, the outer surface of the hollow-fiber loses its smoothness by the influence of the spinning draft. As a result, a pleated structure is formed, and thus it is not preferable.

Therefore, it is required in the present invention that the membrane is formed at a spinning draft; ratio of 6 or more and 13 or less, and preferably 6 or more and 10 or less.

The aspect ratio of pores formed on the surface can be given as a parameter for estimating the effect of the spinning draft ratio. The aspect ratio is defined as described above. Generally, the aspect ratio tends to be high when the draft ratio is high.

According to the production process of the present invention, the asymmetric membrane having the dense layer formed on one of the inner and outer sides is produced. By setting the spinning draft ratio as described above, the opening ratio becomes 8% or more and 14% or less on the surface of the side where the dense layer is formed. The opening ratio is more preferably 10% or more and 13% or less.

Alternatively, the aspect ratio of the pores formed on the membrane surface of the hollow-fiber membrane is 3 or more and 5 or less on the side where the dense layer is formed. The aspect ratio is more preferably in the range of 3 or more and 4 or less.

When the membrane of the present invention having the above opening ratio or aspect ratio is used in a water purifier, it becomes possible to improve turbidness removal performance to a higher extent than that expected by the increase in membrane area.

The turbidness removal performance is a numerical value until the filtration flow rate decreases to below a certain value due to deposition of turbidness, and is thought to generally proportional to the membrane area. However, it was found that use of the hollow-fiber membrane produced according to the present invention improves turbidness removal performance to a higher extent than that expected by the increase in membrane area.

A supposed mechanism of membrane pore formation is such that pores in form of micro nuclei formed by liquid- liquid phase separation grow gradually and are fixed structurally by polymer coagulation. Therefore, under the condition where there is no effect of the draft at the production of the membrane, the pores formed on the membrane are thought to be circular. Since the draft ratio is more than 1 in normal membrane production, pores have a longitudinally stretched structure in most cases.

The reason for the improvement of the turbidness removal performance to a higher extent than that expected by the increase in membrane area is supposed as; follows. When the formed pore is circular and 0.3 lam in size, water cannot flow through the pore anymore after the pore catches a spherical particle of 0.3 lim. In contrast, when the pore is affected by the draft, water can flow at both longitudinal ends of the pore even after the pore catches a particle of 0.3 jam in its center part. The draft has the largest effect on the outermost surface, but has a considerable effect also at the somewhat inner part. Some parts remain maintaining filtration performance, even after the particles are caught. The turbidness removal performance is thought to be improved by the effect of the steric pore structure formed around the outermost surface. Moreover, since the particles, which normally get into the membrane, are now caught on an outer layer of the membrane, it is expected that the filtration flow rate does not decrease easily and the turbidness removal performance is improved.

According to the hollow-fiber membrane of the present invention which is formed at the above draft ratio and thus has an opening ratio of 8% or more and 14% or less at the surface on the side of the dense layer or an aspect ratio of the pores on the surface of 3 or more and 5 or less, the above steric pore structure is formed by the influence of the draft at the time the dense layer is formed and the pores are opened on the membrane surface. Therefore, improvement in turbidness removal performance is expected for the; above reason. When the aspect ratio is 5 or more, the spinning draft ratio is thought to reach the above too high level and ejection of the membrane-forming stock solution becomes unstable, which is not preferable.

It is not necessary that the dense layer is located at the outermost surface of the membrane. It is preferable that the dense layer is formed at a slightly inner part from the outer surface by controlling conditions of the dry area, since it can make the dense layer thinner and improve permeability of the membrane.

It is necessary that the opening ratio of the membrane at the other side where the dense layer is not formed is higher than one at the side of the dense layer, since it does not affect permeation resistance of the membrane so much. However, when the opening ratio is made too high so as to improve water permeability, strength of the hollow-fiber membrane decreases. Therefore, the opening ratio is preferably 15% or more and 20% or less, more preferably 16% or more and 20% or less.

To keep strength of the hollow-fiber membrane, it is preferable that no macrovoid is formed on both surfaces. A macrovoid is a pore apparently larger than the neighboring pores, and is often regarded as a structural defect, because it seriously decreases pressure resistance.

The obtained hollow-fiber membrane is preferable since it can treat more water while maintaining; its removal performance on turbidness, iron rust and the like, when it is incorporated into a cartridge by a known method and used as a water purifier.

EXAMPLES

The present invention will be described in detail below by way of examples. The measuring method for the viscosity of the membrane-forming stock solution and the test method for turbidness removal performance of a water purifier cartridge are as follows.

(1) Viscosity Measurement

A B-type viscometer shown in JIS K7il7 (1999) was used for the measurement. An average value (n = 3) was taken as a measured value.

(2) Turbidness removal Performance Test of Water Purifier Cartridge

The clarification performance was evaluated according to the technique shown in JIS S3200 (1999).

(3) Measurement of Opening ratio on Hollow-fiber Membrane Surface

An outer surface of a hollow-fiber membrane was photographed by a field-emission scanning electron microscope (S-800, manufactured by Hitachi, Ltd.) at 3000- fold magnification. Since the hollow-fiber membrane was affected by the draft, it is suitable for mounting in the longitudinal direction for figuring out its structural character. The image size was set to 655 x 740 pixels. Matrox Inspector 2.2 (Matrox Electronic Systems, Ltd.) was used for image processing. A pore area was colored white and the area other than the pore area was' inverted to be black, and the number of pixels of the white area was counted. The threshold of the binarization was;set to an intermediate value between the brightest white part and the darkest black part. The opening ratio was a percentage obtained by dividing the sum of the pixels of the pores (total opening area) by the number of pixels of. the whole image.

Opening ratio (%) = (sum of pixels of pores)/(number of pixels of whole image) x 100

The resolution of the image was 0.04 694 8 μm/pixel, and the area S of the electron microscopic image was accordingly calculated to be 1,068.4 jam.

Regarding the average pore size, the number of pores displayed as white images was counted and the number of pixels of each pore was measured. Pores of 2 pixels or less were removed as noise. The pore area was calculated from the number of pixels of each pore by the following equation.

Pore area (lam) = (number of pixels of pore) x (resolution 0.140845)
The diameter of each pore was calculated from the above pore area, and the arithmetic average of diameter was obtained as the average pore size.

Since it was impossible to count the number of more than 3,000 pores, the analysis area was reduced to 465 x 525 pixels in such cases and the same processing was carried out.

The above processing was carried out on different 10 hollow membrane fibers, and the arithmetic average was obtained as the result.

(4) Measurement of Aspect Ratio of Pores on Membrane Surface

Using the image used in the measurement of opening ratio on a hollow-fiber membrane surface, 20 pores largest in the longitudinal length formed on the outermost layer were extracted from a visual field of 20 ym square (76 mm square as a printed image) , and the ratio of vertical/transverse dimensions (lengths) was each measured. In the visual field of 20 lam square, pores formed on the outermost layer have an elongated ellipsoidal shape, and the longitudinal direction (long axis of ellipse) of the pores was slightly different among the pores. In this case, the length along the longitudinal direction (long axis of ellipse) of each pore was defined as the length along the vertical direction (i.e. length of long axis of ellipse). That is, vertexes of the longitudinal length of; a pore were defined and the length between the vertexes was defined as a vertical length of the pore. In the visual field of 2 0 )im square, the transverse length was the length of the direction perpendicular to the vertical direction, and was measured at the longest part of the pore in the transverse direction. To measure the aspect ratio precisely, it is important to extract pores at the outermost surface which is thought to be greatly influenced, and to extract large pores (pores having a large sectional area) which are supposed to have been affected by the draft for a sufficient period after their formation. In some cases, a closed pore or a part of pore is further observed in a pore (having a closed ellipsoidal shape, the inside is slightly darker). However, these are not pores at the outermost layer but pores located inside. Therefore, these pores are ruled out as they do not relate to the measurement of the aspect ratio.

Polysulfone (15 parts by weight, hereinafter abbreviated to PSf, Ultrason S6010
manufactured- by BASF Corp.), 7 parts by weight of polyvinylpyrrolidone (hereinafter abbreviated to PVP, K-90 manufactured by ISP Corp., molecular weight: 1,200,000), 75 parts by weight of dimethylacetoamide (hereinafter abbreviated to DMAc) and 3 parts by weight of water were dissolved and stirred to prepare a membrane-forming stock solution. The;membrane- forming stock solution had a viscosity of- 3.4 Pa-s at 40°C. The membrane-forming stock solution was ejected from a double annular spinneret having an outer circumferential slit width of 0.15 mm and kept at 40°C, passed through a dry zone of a predetermined length, subjected to coagulation and water washing steps, and then wound-up at 3 6 tn/min. The ejection amount of the membrane-forming stock solution was adjusted so that a hollow-fiber membrane had a membrane thickness of 0.07 mm and an outer diameter of 0.36 mm. The ejection linear speed at the double annular spinneret, which was obtained by dividing the ejection amount from the double annular spinneret by the ejection sectional area, was 5.4 m/min and the spinning draft ratio was 6.7 under this condition.

The obtained hollow-fiber membrane had an asymmetric structure and a dense structure was formed on the outer surface side. Figs. 1-a and 1-b are SEM micrographs of the outer and inner surfaces of the membrane obtained in Example 1, respectively. The opening ratio of the membrane was measured by using them. The outer surface had an opening ratio of 13.4%, the inner surface had an opening ratio of 18.9% and no macrovoid was formed on the sides of the membrane.

In Fig. 1-a, the region surrounded by the white bold dotted lines designates an area of 2 0 pm square. The 20 largest pores in the longitudinal direction formed on the outermost surface on the outer surface side of the membrane were extracted from this area (Fig. 1-c), so aS; to measure the aspect ratio. The aspect ratio was 3.3 and the longitudinal dimension was 11.1 mm (2.9 μm in the actual dimension).

Then, 1,368 pieces of the above hollow-fiber membranes were bundled in a U-shaped, potted and then filled into a cartridge case. Activated carbon was further filled so as to produce a cartridge for a water; purifier. The cartridge had a membrane area of 0.147 m^ and turbidness removal performance of 3,300 L. Compared with Comparative Example 1, the turbidness removal performance improved by 3 8% relative to the increase in membrane area of 20%.

A hollow-fiber membrane was formed in the same manner as in Example 1, with adjusting the ejection amount of the membrane-forming stock solution so that the hollow-fiber membrane had a membrane thickness of 0.065 mm and an outer diameter of 0.35 mm. The spinning draft ratio was 7.0 under this condition. The obtained hollow-fiber membrane had an asymmetric structure and a dense structure was formed on the outer surface. Figs. 2-a and 2-b are SEM micrographs of the outer and inner surfaces of the membrane obtained in Example 2, respectively. The opening ratio of the membrane was measured by using them. The outer surface had an opening ratio of 13.7%, the inner surface had an opening ratio of 17.1% and no macrovoid was formed on the sides of the membrane.

Pores formed on the outer surface of the hollow-fiber membrane were extracted from the region surrounded by the white bold dotted lines in Fig. 2-a in the same manner as in Example 1 (Fig. 2-c) to measure the aspect ratio. The aspect ratio was 3.7 and the longitudinal dimension was 13.0 mm (3.4 ]am in the actual dimension) .

Then, 1,032 pieces of the above hollow-fiber membranes were bundled in a U-shaped, potted and then filled into a cartridge case. Activated carbon was further filled so as to produce a cartridge for a' water purifier. The cartridge had a membrane area of 0.086 m and clarification performance of 1,400 L. Compared with Comparative Example 2, the clarification performance improved by 82% relative to the increase in membrane area of 30%.

A hollow-fiber membrane was formed in the same manner as in Example 1, except the membrane thickness and outer diameter of the hollow-fiber membrane were 0.08: mm and 0.46 mm, respectively. The spinning draft ratio under this condition was 4.2. A dense structure was formed on the outer surface of the hollow-fiber membrane. According to the SEM micrographs of the outer and inner surfaces of the membrane obtained in Comparative Example 1, which are shown in Figs. 3-a and 3-b, respectively, the outer surface had an aperture ratio of 14.5% and the inner surface had an at aperture ratio of 19.2% and no macrovoid was formed the sides of the membrane.

Pores formed on the outer surface of the hollow-fiber membrane were extracted from the region surrounded by the white bold dotted lines in Fig. 3-a in the same manner as in Example 1 (Fig. 3-c) to measure the aspect ratio. The aspect ratio was 2.4 and the longitudinal dimension was 9.6 mm (2.5 jam in the actual dimension) .

Then, 888 pieces of the above hollow-fiber membranes were bundled to produce a cartridge having the same shape as in Example 1. The cartridge had a membrane area of 0.12 ml and clarification performance of 2,400 L.

The hollow-fiber membranes (600 pieces) of Comparative Example 1 were bundled to produce a; cartridge for a water purifier having the same shape as in Example 2. The cartridge had a membrane area of 0.066 m^ and clarification performance of 770 L.

An attempt was made to prepare a hollow-fiber membrane having a membrane thickness of 0.04 mm by taking- up the hollow-fiber membrane at a spinning draft ratio of 13.3. However, it was impossible to wound-up the fiber due to repeated breakage of the fiber.

The above results are summarized in Table 1.

The shape of the cartridge was same in Example 1 and Comparative Example 1. By filling the hollow-fiber membrane of the present invention, the turbidness removal performance improved by 38% while the membrane area increased by 20%. As such, an effect higher than that expected by the increase in membrane area was achieved.

The shape of the cartridge was same in Example 2 and Comparative Example 2. By filling the hollow-fiber membrane of the present invention, the turbidness removal performance improved by 82% while the membrane area increased by 30%. As such, an effect higher than that expected by the increase in membrane area was achieved.

[Table 1]

CLAIMS

1. A hollow-fiber membrane comprising a dense layer on one of inner and outer sides, wherein pores formed on a membrane surface of a side having the dense layer have an aspect ratio of 3 or more and 5 or less.

2. A hollow-fiber membrane comprising a dense layer on one of inner and outer sides, wherein a membrane surface of one side having the dense layer has an opening ratio of 8% or more and 14% or less, and a membrane surface of the other side has an opening ratio of 15% or more and 20% or less.

3. A process for producing a hollow-fiber membrane, wherein an asymmetric hollow-fiber membrane is formed by a liquid injection method using a double annular spinneret at a spinning draft ratio of 6 or more and 13 or less.

4. The process for producing a hollow-fiber membrane according to claim 3, wherein a membrane-forming stock solution in which a component of the hollow-fiber membrane is dissolved in an organic solvent has a viscosity of 1 Pa-s or more and 10 Pa-s or less.

5. The process for producing a hollow-fiber membrane according to claim 3 or 4, wherein the component of the hollow-fiber membrane is a polysulfone system polymer.

6. The process for producing a hollow-fiber membrane according to any one of claims 3 to 5, wherein an injection liquid used in the liquid injection method is noncoagulable.

7. A water purifier comprising the hollow-fiber membrane obtained by the process according to any one of claims 3 to 6 or the hollow-fiber membrane according to claim 1 or 2.