JP2001190940A - Method of manufacturing polyethylene hollow fiber porous membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a polyethylene hollow fiber porous membrane simultaneously having dense micropores and water permeability, which is suited for the use in turbidity removal by the filtration. SOLUTION: In the method for obtaining a polyethylene hollow fiber porous membrane by fusing a polyethylene and an organic liquid at a high temperature, extruding the fused liquid into a liquid bath to solidify the fused liquid by cooling through the aie so to constitute hollow fibrous state and then extracting and removing said organic liquid, (1) the time period when said extrudate is running in the air between 0 to 1 second (not including 0), (2) the hollow part forming fluid is a liquid having a boiling point of a spinning nozzle temperature or more and (3) the hollow fibrous material is elongated so that the elongation retention becomes from 5% or more to 150% or less prior to or after the organic liquid after the solidifying by cooling procedure is extracted and removed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a polyethylene hollow fiber porous membrane having fine pores and high water permeability, which is suitable for filtration applications such as turbidity.

[0002]

2. Description of the Related Art Filtration operations using porous membranes such as microfiltration membranes and ultrafiltration membranes are carried out in the automobile industry (electrodeposition paint recovery and reuse system), the semiconductor industry (ultra pure water production), the pharmaceutical food industry (sterilization, It has been put to practical use in many fields such as enzyme purification. In particular, in recent years, it has been widely used as a method for producing drinking water and industrial water by turbidizing river water and the like. As the material of the membrane, various materials such as cellulose, polyacrylonitrile, and polyolefin are used. Among them, polyolefin-based polymers (polyethylene, polypropylene, polyvinylidene fluoride, etc.) are suitable for use as a material for aqueous filtration membranes because of their high water resistance due to their hydrophobicity, and are widely used. Among these polyolefin polymers, they do not contain halogen elements that pose a problem at the time of disposal,
Polyethylene, which is less prone to chemical deterioration during membrane cleaning due to less tertiary carbon with high chemical reactivity, is expected to have long-term use resistance, and is inexpensive, is considered to be particularly promising in the future.

As a polyethylene film, Japanese Patent Application Laid-Open No. 3-42
A membrane having a uniform three-dimensional porous structure (page 3, upper right column, lines 10-11) as disclosed in Japanese Patent Publication No. 025 is conventionally known. This uniform three-dimensional porous structure means a structure in which there is almost no change in the pore diameter in the membrane cross-sectional direction, and the pore diameters (and pore diameter distributions) at any two points in the membrane cross-section are almost equal. In such a film having a uniform structure in the film cross section direction, the transmission resistance of the entire film cross section is increased,
In order to obtain high water permeability, it was necessary to increase the pore diameter, and it was difficult to obtain a membrane having high water permeability while having fine pores (small pore diameter).

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a polyethylene hollow fiber porous membrane having dense pores and high water permeability, which is suitable for filtration applications such as turbidity.

[0005]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, the polyethylene membrane obtained by the following production method has high water permeability while having fine pores. To accomplish the present invention. That is, the present invention provides (1) a method in which polyethylene and an organic liquid are melted at a high temperature, and the melt is injected into a hollow portion through a spinning hole for forming a hollow fiber into a hollow portion while flowing the air into a hollow fiber shape. And extruding into a liquid bath to cool and solidify, and then extracting and removing the organic liquid to obtain a polyethylene hollow fiber porous membrane, wherein (a) the extruded material travels in the air for 0 to 1 second. (But not including 0), and (b) the hollow forming fluid is a liquid having a boiling point not lower than the spinning temperature, and (c) before or after extraction and removal of the organic liquid after cooling and solidification. , Residual elongation is 5
% Of a hollow fiber-like material, wherein the hollow fiber-like material is stretched so that the hollow fiber-like material becomes not less than 150% or less, and (2) the ability of the fluid forming the hollow part to separate liquid-liquid phase from the polyethylene at a high temperature. (1) The method for producing a polyethylene hollow fiber-like porous membrane according to the above (1), wherein the extrudate travels in the air for a period of 0 to 0.5 seconds (however, 0 is not included). Some of the above (1) and (2)
(4) The method for producing a polyethylene hollow fiber-like porous membrane according to the above (4).
The method for producing a polyethylene hollow fiber-like porous membrane according to (1) or (2), wherein the extruded material travels in the air for a time of from 0 to 0.25 seconds (excluding 0).
(5) The method for producing a polyethylene hollow fiber-like porous membrane according to the above (1) to (4), wherein the residual elongation by stretching before or after extraction and removal of the organic liquid after cooling and solidification is 10% or more and 100% or less. (6) The method according to (1) to (5), wherein the liquid bath is substantially composed of water.

[0006] The polyethylene hollow fiber porous membrane produced by the above-mentioned production method does not have a uniform three-dimensional structure with a uniform cross section, but has an anisotropic structure. The anisotropic structure refers to a structure in which the pore diameter changes rather than being uniform (uniform) in the membrane cross-sectional direction. In the anisotropic structure, the minimum pore size layer (the densest layer) that determines the separation function (blocking ability in filtration) of the membrane and causes a decrease in water permeability due to high permeation resistance
Occupies only one portion of the membrane cross section. In other words, the cross-sectional structure of the anisotropic structure membrane has a minimum pore diameter layer which has dense pores (pores of small pore diameter) that determine the separation function of the membrane and has a high permeation resistance but occupies only one part of the membrane cross-section, It consists of portions other than the minimum pore size layer which are not directly involved in the separation function and which are formed by communicating holes having a larger pore size than the minimum pore size layer and therefore do not contribute much to the increase in the permeation resistance of the membrane.

By forming such an anisotropic structure, it is possible to form a polyethylene porous membrane capable of satisfying both the existence of dense pores and the development of high water permeability. In particular, when the liquid bath is substantially composed of water, the outer surface portion (the outer surface itself and a cross-sectional portion from the outer surface to one-tenth of the film thickness) has a minimum pore size layer, and the pore size of the inner surface is A dense outer surface anisotropic structure larger than the pore diameter of the outer surface is formed. With this structure,
The pore diameter changes continuously from the outer surface to the inner surface,
The basic direction of the change is a direction in which the pore diameter increases from the outer surface to the inner surface.

Such an anisotropic structure having a dense outer surface is suitable as an external pressure filtration membrane for performing filtration from the outer surface side to the inner surface side. When performing turbidity or the like using a hollow fiber membrane, filtration from the outer surface side having a large membrane area (external pressure filtration) requires a load of contaminants (substances to be prevented such as turbid substances) per unit membrane area. This is advantageous because the amount can be reduced and efficient filtration can be performed. Note that Japanese Patent Application Laid-Open No. 3-4202
In the conventional uniform (non-anisotropic) three-dimensional porous structure as disclosed in Japanese Patent Application Publication No. 5 (1999), the entire membrane cross section is equivalent to the minimum pore diameter layer, and the existence of dense pores It is difficult to achieve a balance between high water permeability and high water permeability. Whether the film cross-sectional structure is anisotropic or uniform can be determined by observing the film cross section with an electron microscope.

The details of the manufacturing method of the present invention will be described below. Polyethylene is (1) inexpensive and has good mechanical strength properties, and (2) has a small amount of tertiary carbon, which is rich in chemical reactivity. It has the advantages of being able to expect long-term durability with little chemical deterioration due to chemical cleaning and the like, and (3) not containing a halogen element which is a problem at the time of disposal. Polyethylene includes high-density polyethylene and low-density polyethylene, and high-density polyethylene is preferable in view of the strength of the obtained film. Further, polyethylene has various molecular weights, and from the viewpoint of the strength of the obtained film, the viscosity average molecular weight is preferably 100,000 or more, preferably 200,000 or more. Further, from the viewpoint of moldability, the viscosity average molecular weight is preferably 1,000,000 or less. The viscosity average molecular weight (Mv) of polyethylene can be determined from the following equation by measuring the intrinsic viscosity ([η]) of a decalin solution at 135 ° C. ((J. Brandrup and EHI)
mergut (Editors), Polymer
Handbook (2nd Ed.), Page IV-7, J.
ohn & Sons, New York, 1975). [Η] = 6.8 × 10 ⁻⁴ × (Mv) ^{0.67 The} polyethylene may contain a small amount of a stabilizer such as an antioxidant or an ultraviolet absorber, if necessary.

The organic liquid used in the present invention, when mixed with polyethylene, is in a liquid-liquid phase separated state (polyethylene concentrated phase droplet /
The liquid is a liquid which can take a polyethylene dilute phase, that is, a two-phase coexistence state of an organic liquid concentrated phase droplet, and has a boiling point not lower than the upper limit temperature of the liquid-liquid phase separation temperature range. Instead of a single liquid, it may be a mixed liquid. When such an organic liquid and polyethylene are mixed in a concentration range in which liquid-liquid phase separation occurs, if the temperature is raised to a temperature higher than the upper limit temperature at which a liquid-liquid phase separation state is obtained in the mixed composition, polyethylene and the organic liquid are mixed. It is possible to obtain a uniformly dissolved molten phase. When the melt is cooled, a coexistence state (liquid-liquid phase separation state) of two liquid-liquid phases (polyethylene concentrated phase droplets and organic liquid concentrated phase droplets) appears, a pore structure is generated, and a temperature at which the polyethylene solidifies. The pore structure is fixed by cooling to (usually 100 to 150 ° C.), and a porous body is obtained by removing the organic liquid. At this time, the polyethylene rich phase portion at the time of liquid-liquid phase separation is cooled and solidified to form a porous structure (porous skeleton), and the polyethylene dilute phase (organic liquid rich phase)
The portion becomes the hole portion.

Examples of such organic liquids include dibutyl phthalate, diheptyl phthalate, dioctyl phthalate,
Phthalic acid esters such as di (2-ethylhexyl) phthalate, diisodecyl phthalate, ditridecyl phthalate,
Sebacic esters such as dibutyl sebacate, adipic esters such as dioctyl adipate, maleic esters such as dioctyl maleate, trimellitic esters such as trioctyl trimellitate, tributyl phosphate, trioctyl phosphate and the like Phosphoric esters,
Examples thereof include propylene glycol dicaprate, glycol esters such as propylene glycol dioleate, glycerin esters such as glycerin trioleate, and the like, alone or in combination of two or more.

Further, a liquid which is incompatible with polyethylene alone at high temperatures or a liquid such as liquid paraffin which is compatible with polyethylene at high temperatures alone, is too high in compatibility to change the phase separation state of two liquid-liquid phases. Definition of organic liquids that cannot be taken (liquids that can be in a liquid-liquid phase separation state at a certain temperature and polyethylene concentration range when mixed with polyethylene and have a boiling point higher than the upper limit of the liquid-liquid phase separation temperature range) A liquid mixture mixed with the above-mentioned organic liquid examples (phthalic acid esters and the like) within a range that does not deviate from the above can also be mentioned as examples of the organic liquid.

The polyethylene and the above-mentioned organic liquid are mixed and melt-mixed at a temperature higher than the upper limit temperature of the liquid-liquid phase separation temperature range at the predetermined mixing ratio by using, for example, a twin screw extruder. Can be. The mixing ratio of polyethylene and the organic liquid is disadvantageous because if the ratio of polyethylene is too small, the strength of the obtained membrane is too low, and conversely, if the ratio of polyethylene is too large, the water permeability of the obtained membrane is too low. Disadvantageous. The mixing ratio between polyethylene and organic liquid is 10/9 by weight of polyethylene / organic liquid.
0 to 40/60, preferably 15/85 to 30/7
0.

The melt is guided and extruded to a portion called the head at the tip of the extruder. By mounting a die for extruding the melt into a predetermined shape at the extrusion opening in the head, the melt can be formed into a predetermined shape and extruded. In the case of the present invention, a spinneret for forming into a hollow fiber shape (spinner for forming a hollow fiber) is attached to the extrusion opening of the head. The hollow fiber forming spinneret is provided with an annular hole for extruding the molten material into a hollow shape (annular shape) and an extruded hole for preventing the hollow portion of the extruded hollow material from closing and becoming a cylindrical shape. A spout nozzle having a hole (existing inside the above-mentioned annular hole and having a circular hole shape) for discharging a hollow part forming fluid to be injected into the hollow part of the hollow material formed on the extrusion side surface. is there. The melt of the polyethylene and the organic liquid is injected into the hollow portion through the hole inside the annular hole from the annular hole of the above-mentioned hollow fiber forming spinneret while the hollow portion forming fluid is injected into the hollow portion. Activated gas).

The hollow part forming fluid is, of course, non-reactive with the extrudate (polyethylene and organic liquid). It is necessary to maintain the roundness of the cross-sectional shape of the extruded hollow object. When the hollow part forming fluid is a gas (for example, nitrogen gas or air), it is difficult to maintain the roundness of the cross-sectional shape of the hollow object after being extruded from the spinneret. Since the hollow part forming fluid is discharged from the spinneret,
In order to be a liquid at the time of ejection, it is necessary to use a liquid having a boiling point equal to or higher than the spinning temperature as a hollow portion forming fluid.

As a characteristic of the fluid forming the hollow portion, in addition to the boiling point being higher than the spinning temperature, the use of a liquid capable of liquid-liquid phase separation with polyethylene at a high temperature improves the water permeability of the obtained membrane. It is preferable in that it is performed. However, in this case, the temperature of the hollow part forming fluid when discharged from the spinning nozzle for forming a hollow fiber is not necessarily required to be a temperature at which the liquid and liquid phase are separated from the polyethylene, but rather than a temperature range at which the liquid and liquid phase is separated. May be higher or lower. Examples of such a fluid for forming a hollow portion include the same examples as those of the above-described organic liquid.

The melt extruded into the air is guided to a liquid bath and cooled to a temperature at which the polyethylene in the extrudate solidifies. The melt extruded from the spinneret in this way
By cooling during passage from the spinneret outlet into the liquid bath, liquid-liquid phase separation occurs to generate a pore structure, which is then solidified to fix the pore structure. The composition of the liquid bath is not particularly limited as long as the liquid does not have reactivity with the extrudate (polyethylene and organic liquid), and may be the same as the organic liquid in the extrudate. However, the temperature must be equal to or lower than the solidification temperature of polyethylene in the extruded composition. Since the important function of the liquid bath is the function of cooling the extrudate, water having a high cooling capacity, that is, water, which has a large heat capacity, is preferable as the composition of the liquid bath.

The time required for the melt extruded from the spinneret into the air to enter the liquid bath, that is, the air travel time, is from zero to one.
Up to seconds (but not including zero). When the air travel time is zero, the extruded surface of the spinneret comes into contact with the liquid surface of the liquid bath. The spinning temperature is set to be higher than the compatibility temperature of polyethylene and the organic liquid, that is, the upper limit temperature of the liquid-liquid phase separation temperature range in the mixed composition, so that the liquid bath is necessarily set to be lower than the solidification temperature of polyethylene. High temperature. Therefore, when the air traveling time is zero, the spinneret is always cooled by the liquid bath, and the temperature control of the spinneret becomes unstable, which is not suitable. On the other hand, if the air traveling time is too long, the porosity of the outer surface decreases, and the water permeability of the obtained membrane decreases, which is not preferable. The air travel time is preferably between zero and 0.5 seconds (but not including zero), and more preferably between zero and 0.25 seconds (but not including zero). The air travel time is measured by measuring the winding speed and the air travel distance (the distance between the spout discharge surface and the liquid bath surface) when the hollow fiber is wound without tension at the liquid bath outlet.
From the following equation. Air travel time [sec] = (air travel distance [cm]) / (winding speed [cm / sec])

In the hollow fiber-like material coming out of the liquid bath, the polyethylene thick phase portion generated during liquid-liquid phase separation during cooling is solidified by cooling to form a porous structure (porous skeleton). The polyethylene dilute phase (organic liquid rich phase) portion is a pore portion filled with the organic liquid. A porous membrane can be obtained by removing the organic liquid clogging the hole. The removal of the organic liquid in the hollow material does not dissolve or degrade the polyethylene and removes the organic liquid to be removed by extraction with a volatile liquid that dissolves, and then, after drying, volatilizes and removes the remaining volatile liquid. Can be implemented. Examples of such volatile liquids for organic liquid extraction include hydrocarbons such as hexane and heptane, methylene chloride,
Chlorinated hydrocarbons such as carbon tetrachloride, methyl ethyl ketone and the like can be mentioned.

In the present invention, a stretching operation is performed such that the residual elongation becomes 5% or more and 150% or less before or after extraction and removal of the organic liquid from the hollow material after cooling and solidifying after leaving the liquid bath. I do. Residual elongation is determined by the following: when performing a stretching operation to release tension after stretching by applying tension in the axial direction of the hollow fiber-like material, the yarn length after tension release (after relaxation) and the yarn length before stretching. Defined by an expression. Residual elongation [%] = 100 {(yarn length after relaxation)-(yarn length before stretching)} / (yarn length before stretching) Since polyethylene has elasticity, the yarn length generally shrinks to some extent by release of tension. The residual elongation becomes smaller than the elongation under tension. However, when heat treatment or the like is performed under tension, the yarn length shrinks due to release of the tension is reduced. As described above, the hollow material after cooling and solidifying after leaving the liquid bath is subjected to a stretching operation before or after the extraction and removal of the organic liquid, whereby the water permeability of the obtained membrane can be improved. When the residual elongation is less than 5%, the effect of improving the water permeability is small, which is not preferable. Conversely, if the residual elongation exceeds 150%, the pore size of the obtained film increases, and the elongation also decreases, which is not preferable. Residual elongation is 10% or more and 100%
The following are particularly preferred.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below.
The present invention is not limited to this. The average pore size, pure water permeability, breaking strength and breaking elongation were determined by the following measuring methods. The stretching operation is performed at room temperature (25
C.), the film was stretched to a predetermined length by applying tension at a speed of about 1.4 cm / sec, held for 30 seconds, and then the tension was released. Thread length after tension release (thread length after relaxation)
Was used to determine the residual elongation by using the fiber length after the tension was released and the hollow fiber membrane was allowed to stand at room temperature for about one day to sufficiently relax.

Average pore diameter: measured according to the method described in ASTM: F316-86 (also known as half-dry method). Using ethanol as the liquid used, 25 ° C, pressure increase rate 0.01a
It was measured at tm / sec. The average pore diameter [μm] is determined by the following equation from the surface tension [dynes / cm] of the liquid used and the half dry air pressure [Pa]. Average pore diameter = 2860 (surface tension) / (half dry air pressure) The surface tension of ethanol at 25 ° C. is 21.97 dy.
nes / cm (edited by The Chemical Society of Japan, 3rd edition of Basic Handbook of Chemical Handbook, II-82, Maruzen Co., Ltd., 1984), the average pore diameter was calculated by the following equation.

Average pore diameter [μm] = 62834 / (half
(Dry air pressure [Pa]) The average pore size obtained by this half dry method is
This is the average pore size of the smallest pore size layer in the plane. Pure water permeability: After immersion in ethanol, immersion in pure water several times
Seal one end of the returned wet hollow fiber membrane of about 10 cm length,
Insert the injection needle into the hollow at the other end and inject at 25 ° C.
Pure water at 25 ° C at a pressure of 0.1 MPa from the shooting needle
Permeate volume of pure water injected into and permeating from the outer surface
[L] was measured, and the inner surface area [m ^Two]
Pure water permeability [L / m^Two/ H] was calculated. Pure water permeability = 60 (permeated water amount) / (membrane surface area) / (measurement
(Constant time [min]) In addition, surface area [m^Two] Was calculated from the following equation. Intramembrane surface area = π (membrane inner diameter [m]) x (membrane effective length [m]) Here, the effective membrane length excludes the part where the injection needle is inserted.
Refers to the net film length.

Breaking strength and breaking elongation: Using a tensile tester (Autograph AG-A type, manufactured by Shimadzu Corporation), the hollow fiber was pulled at a distance between chucks of 50 mm and a pulling speed of 200 mm.
The tensile strength at break and the elongation at break were determined from the load [kgf] and the displacement [mm] at break by the following formulas. Breaking strength [kgf / cm ^2] = (load at break) / (Makudan area [cm ^2]) elongation at break [%] = 100 (at break displacement) / 50 Here, Makudan area [cm ^2] is This is the area of the annular cross section of the membrane.

[0025]

Example 1 High-density polyethylene (manufactured by Mitsui Chemicals, Hyzex Million 030S, viscosity average molecular weight 450,000) 20
3 parts by weight (DIDP / DOP = 3/1) of a mixed organic liquid 8 by weight of diisodecyl phthalate (DIDP) and di (2-ethylhexyl) phthalate (DOP)
0 parts by weight and a twin screw extruder (TEM manufactured by Toshiba Machine Co., Ltd.)
-35B-10 / 1V) and melted by heating and kneading (23
0 ° C), an annular hole for extruding a melt having an outer diameter of 1.58 mm and an inner diameter of 0.83 mm on the discharge surface of the spinning hole for hollow fiber molding attached to the extrusion port in the head (230 ° C) at the tip of the extruder. The above melt is extruded from the above, and DOP is discharged as a hollow part forming fluid from the annular hole for discharging the hollow part forming fluid of 0.6 mmφ inside the annular hole for extruding the melt, and the hollow of the hollow fiber extrudate is discharged. It was injected into the part.

The hollow fiber extruded from the spinneret into the air is placed in a water bath at 25 ° C. through an air traveling distance of 0.5 cm, passed through about 2 m of water, solidified by cooling, and then formed into a hollow fiber. The film was wound out of the water bath at a speed of 16 m / min without applying tension. The air travel time at this time is 0.02 seconds based on the air travel distance and the winding speed. Next, the hollow fiber obtained was repeatedly immersed in methylene chloride at room temperature for 30 minutes five times to obtain DI in the hollow fiber.
The DP and DOP were extracted and removed, and then dried at 50 ° C. for half a day to volatilize and remove the remaining methylene chloride.

The polyethylene hollow fiber porous membrane thus obtained was stretched to a length of 40 cm by applying tension to the length of 20 cm, and then the tension was released to perform a stretching operation. The yarn length after releasing the tension was 27.6 cm, and the residual elongation was 38%.
Various physical properties (average pore size, pure water permeability, breaking strength, breaking elongation, yarn diameter) of the obtained membrane after the stretching operation are shown in Table 1, and an electron micrograph is shown in FIG. The cross-sectional structure of the obtained film was an external dense anisotropic structure having a minimum pore size layer on the outer surface and having a large pore diameter at the center and inner surface side of the cross section.

[0028]

Comparative Example 1 A polyethylene hollow fiber porous membrane was prepared according to Example 2 of JP-A-3-42025 (except that the volume ratio of polyethylene / silica / DOP was 27/14/59). I got The physical properties (average pore diameter, pure water permeability, breaking strength, breaking elongation, yarn diameter) of the obtained membrane are shown in Table 1, and an electron micrograph is shown in FIG. The cross-sectional structure of the obtained film was a uniform three-dimensional porous structure having no anisotropy in which the outer surface portion, the center portion of the cross section, and the inner surface portion had substantially the same pore diameter.

[0029]

Comparative Example 2 A polyethylene hollow fiber-like porous membrane was obtained in the same manner as in Example 1, except that the temperature of the water bath was set at 32 ° C. and the stretching operation was not performed. Various physical properties (average pore size,
Table 1 shows pure water permeability, breaking strength, breaking elongation, and yarn diameter). In addition, when the cross section of this film was observed with an electron microscope, it was found that the outer surface had a dense anisotropic structure similar to that of Example 1.

[0030]

Example 2 A tension was applied to a 20 cm length of the polyethylene hollow fiber porous membrane obtained in Comparative Example 2 to extend it to a length of 34 cm, then the tension was released and a stretching operation was performed. The yarn length after releasing the tension was 24.8 cm, and the residual elongation was 24%. Table 1 shows various physical properties (average pore diameter, pure water permeability, breaking strength, breaking elongation, and yarn diameter) of the obtained membrane after the stretching operation.
In addition, when the cross section of this film was observed with an electron microscope,
The outer surface had a dense anisotropic structure similar to that of Example 1.

[0031]

[Table 1]

[0032]

Example 3 A tension was applied to a 20 cm length of the polyethylene hollow fiber porous membrane obtained in Comparative Example 2 to extend it to a length of 50 cm, then the tension was released and a stretching operation was performed. The yarn length after releasing the tension was 33.6 cm, and the residual elongation was 68%. Table 1 shows various physical properties (average pore diameter, pure water permeability, breaking strength, breaking elongation, and yarn diameter) of the obtained membrane after the stretching operation.
In addition, when the cross section of this film was observed with an electron microscope,
The outer surface had a dense anisotropic structure similar to that of Example 1.

[0033]

According to the present invention, it has become possible to produce a polyethylene hollow fiber-like porous membrane having both fine pores and high water permeability, which is suitable for filtration applications such as turbidity.

[Brief description of the drawings]

FIG. 1 is an electron micrograph print of a film obtained in Example 1.

FIG. 2 is an electron micrograph print of the film obtained in Comparative Example 1.

Claims (1)

[Claims] 1. A mixed melt of polyethylene and an organic liquid is extruded into a liquid bath through a hollow fiber forming spout through air into a liquid bath while a hollow forming fluid is injected into the hollow, and solidified by cooling. Then, the organic liquid is extracted and removed to obtain a polyethylene hollow fiber-like porous membrane.
The extrudate travels in the air for a time of 0 to 1 second (not including 0), (2) the hollow forming fluid is a liquid having a boiling point not lower than the spinning temperature, and ( 3)
A method for producing a polyethylene hollow fiber-like porous membrane, characterized in that the hollow fiber is stretched so that the residual elongation is 5% or more and 150% or less before or after extraction and removal of the organic liquid after cooling and solidification.