JPH06246139A - Heterogeneous hollow fiber membrane and its production - Google Patents

Abstract

PURPOSE:To produce a membrane passing gas at a higher rate as compared with a membrane having the same gas separation factor. CONSTITUTION:This heterogeneous hollow fiber membrane is made of a crystalline thermoplastic polymer, has a practically nonporous dense layer in the surface and contains open pores in the interior and this membrane has periodic ruggedness having 0.5-25mum pitch and 0.1-5mum depth at the dense layer side surface in the axial direction of the fibers. In order to produce this membrane, melt spinning, drawing and heat fixing at a temp. below (drawing temp.+10) deg.C under 15-40% shrinkage are successively carried out. First-stage heat fixing by holding in the temp. range of (Tm-50) deg.C to Tm deg.C for >=1 sec and second- stage heat fixing at a temp. below the first-stage heat fixing temp. under 10-40% shrinkage of the length of the hollow fibers after the first-stage heat fixing may be carried out in palce of the above heat fixing.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a separation membrane, an improved heterogeneous hollow fiber membrane, and a heterogeneous hollow fiber membrane having an improved gas permeation rate. The heterogeneous hollow fiber membrane of the present invention includes, for example, production of oxygen-enriched air, production of nitrogen-enriched air, gas separation such as concentration of carbon dioxide, and membrane oxygenator, for example, supply of diaphragm oxygen to a fish tank, Dissolution of gas into liquid such as absorption of carbon dioxide in a diaphragm, and removal of dissolved gas in liquid such as degassing of water, deoxidation of water, decarbonation of water, etc.
It is also used for pervaporation such as removal of organic solvent from water, defoaming and the like.

[0002]

2. Description of the Related Art Heterogeneous hollow fiber membranes, that is, hollow fiber type, which are composed of a crystalline thermoplastic polymer, have a dense layer of substantially non-porous surface, and have a continuous porous inside Of non-homogeneous membranes, in particular, having a dense layer that is substantially non-porous on the outer surface, the interior of the membrane is continuous porous, and the inner surface has a plurality of plate-like shapes extending in the direction perpendicular to the fiber axis. It is known that a heterogeneous hollow fiber membrane having open pores surrounded by crystals and a plurality of fibrils connecting the crystals can be produced by a melt molding method.

For example, a gas separation membrane and a method for producing the same are disclosed in JP-A-59-196706 and JP-A-59-22932.
No. 0, a diaphragm for gas-liquid contact is disclosed in JP-A-63-258605,
Diaphragms for artificial lung are disclosed in JP-A-1-104271, and membranes for all of them are disclosed in JP-A-4-210216. However, the surface of the heterogeneous hollow fiber membranes described in these prior arts on the side of the dense layer was smooth (for example, a photograph is disclosed in JP-A-1-104271).

A method for producing a heterogeneous hollow fiber membrane by melt molding is as follows. A crystalline thermoplastic polymer is melt-extruded into a hollow fiber shape, cooled and solidified under stress by pulling, heat-treated as required, and stretched. It is a method of heat fixing if necessary. In this manufacturing method, the surface dense layer and the porous support layer can be formed at the same time without coating, so the process is simple and the productivity is good. It has excellent properties such as no risk of peeling from the body.

The poly (4-methyl-1-pentene) -based polymer has a high oxygen / nitrogen separation coefficient and a high oxygen permeation coefficient as a material for a gas separation membrane, and has an oxygen-rich membrane and a nitrogen-rich membrane. It is a suitable material for the membrane.

[0006]

In the field of gas separation membranes and gas-liquid gas exchange membranes, gas permeation performance is improved, that is, gas permeation rate is improved without lowering the gas separation coefficient. That is an eternal task. The inventors of the present invention have studied a heterogeneous hollow fiber membrane having a dense layer on the outer surface, which is made of a crystalline thermoplastic polymer, as a result of studying the membrane structure for achieving a high gas permeation rate, It has been found that high gas permeability can be realized by providing an uneven structure on the outer surface of a heterogeneous hollow fiber membrane having a layer, and arrived at the present invention.

[0007]

The summary of the present invention is as follows.
Hollow fiber membrane made of a crystalline thermoplastic polymer, that is, a heterogeneous membrane type, that is, having a dense layer that is substantially non-porous on the surface, and the inside of the membrane is continuous and porous, especially the outer surface Has a substantially non-porous dense layer, the inside of the membrane is a continuous porous layer, and the inner surface has a plurality of plate-like crystals extending in the direction perpendicular to the fiber axis and a plurality of plate-like crystals connecting the crystals. A hollow fiber membrane having open pores surrounded by fibrils, the outer surface of which is 0.5 to 25 μm in the fiber axis direction and a depth of 0.1 to 5 μm.
A heterogeneous hollow fiber membrane characterized by having irregularities with a periodicity of μm.

The present invention will be described in more detail below. The heterogeneous membrane referred to in the present invention is a term generally used in the field of gas separation membranes, and a substantially non-porous dense layer formed on the surface of the membrane and a continuous porous support that supports the dense layer. A film composed of layers, which means that the dense layer and the supporting layer are composed of the same material, and the dense layer (or its precursor) and the supporting layer are substantially integrally molded.

The heterogeneous hollow fiber membrane of the present invention (hereinafter sometimes simply referred to as a membrane) is a hollow fiber type membrane having a substantially non-porous dense layer on the outer surface of the hollow fiber, The inside of the membrane (hollow fiber wall) is porous with pores communicating with each other, and the inner surface of the hollow fiber has a structure in which pores are opened.

No pores are substantially opened on the outer surface and it is non-porous, but it is acceptable that some pores are opened (hereinafter, the dense layer is penetrated. The pores are called pinholes). The amount of pinholes present on the outer surface can be observed by a scanning electron microscope (SEM), but it is difficult to quantify them. Therefore, the allowable amount of pinholes is defined by the gas separation coefficient. That is, if there are even a few pinholes in the dense layer, the gas separation coefficient, for example, the oxygen / nitrogen separation coefficient, drops sharply.
Although the nitrogen separation factor approaches 0.935 (for example, Tsutomu Nakagawa: High pressure gas, 18 (9), 471 (1981)), the heterogeneous membrane of the present invention has an oxygen / nitrogen separation factor of 0. .940 or more.

The upper limit of the oxygen / nitrogen separation coefficient of the membrane of the present invention, that is, the separation coefficient in the case where no pinholes are present in the dense layer, is determined by the type of the polymer and the additive, and the value thereof is different. Therefore, it is not necessary to set an upper limit in the present invention. The inside of the membrane, that is, the portion other than the outer surface of the hollow fiber wall is porous. The fact that the inside of the membrane is porous means that
It can be confirmed by observing a cross section obtained by cutting the hollow fiber vertically or obliquely with an SEM. When the longitudinal section and the transverse section of the heterogeneous hollow fiber membrane of the present invention were subjected to etching treatment as required and observed in more detail with SEM, the laminated plate-like crystals (the plate-like crystals were curved It can be seen that there are gaps between the plate crystals.

In some cases, microfibrils connecting plate crystals are observed. Pore diameter inside the membrane is 0.01-5μ
m. The pore diameter inside the membrane can be measured by SEM or a mercury porosimeter. On the inner surface of the film, pores surrounded by a plurality of plate crystals extending in a direction substantially perpendicular to the fiber axis and a plurality of fibrils connecting the crystals open. This state can be confirmed by SEM observation of the inner surface of the film. The opening shape of the pores may be circular, ellipse short in the fiber axis direction, ellipse long in the fiber axis direction, or slit-like long in the fiber axis direction.

In each case, the pore diameter (average of the longitudinal and lateral pores observed by SEM) is 0.01 to 5 μm. In addition, depending on the manufacturing conditions of the film, there are cases where a plurality of plate-shaped crystals extending in a direction substantially perpendicular to the fiber axis are not clear, but in such a case, an etching treatment such as ion etching is applied to form a laminated plate-like crystal. The presence of crystals is clearly observed. The film of the present invention achieves high strength by having such a structure.

The heterogeneous hollow fiber membrane of the present invention is characterized by having irregularities on the outer surface. This unevenness is wrinkled,
There are various cases such as scaly, wavy, and many hills, and paying attention to the convex portion, if it is a linear shape arranged almost perpendicular to the fiber axis, a meandering line arranged almost perpendicular to the fiber axis. It may be a shape, a mesh, an independent point, or the like. Further, in the case of independent dots, there may be cases where the points are arranged in a line in the fiber axis direction or randomly arranged.

The period of the irregularities in the fiber axis direction is 0.5 to 25 μm.
m, preferably 1 to 10 μm, and the depth of the unevenness is 0.
It is 1 to 5 μm, preferably 0.2 to 1.0 μm. However, this unevenness can be quite irregular, so
The period referred to here means the most frequent value, that is, the peak value in the Laplace-transformed spectrum of unevenness. of course,
It is also possible that the irregularities of a longer period or a shorter period exist so as to overlap with the irregularities of this period. Further, the depth of the unevenness means twice the amplitude of the unevenness of the most frequent cycle. The cycle and depth of the unevenness are determined by laser microscope, atomic force microscope (A
FM), scanning tunneling microscope (STM), etc.

The heterogeneous membrane of the present invention having irregularities on the outer surface exhibits a high gas permeation rate. However, there are many other factors that affect the gas permeation rate of the membrane. For example, when the film thickness (the thickness of the hollow fiber wall) is increased, a film having a low gas permeation rate tends to be obtained, and therefore, films having the same size should be compared. When compared with a film having the same size, the film of the present invention exhibits a higher permeation rate than the conventionally known film having no unevenness on the surface. In the present invention, when the film thickness is large, for example, 30 μm
It is particularly effective in the above cases.

Further, there are some pinholes in the dense layer, and the oxygen / nitrogen separation coefficient is 0.940 or more and less than αp (αp is the oxygen / nitrogen separation coefficient of the polymer itself. There are no pinholes in the dense layer. In some cases, the oxygen / nitrogen separation factor of the membrane is in agreement with this value) also requires careful comparison. When there are pinholes in the dense layer, the amount of gas that permeates the membrane is the sum of the amount of permeation in the dense layer and the amount of permeation of the pinholes. The gas permeation rate of the membrane largely depends on the density of pinholes because the rate of permeation through the part is high.

Therefore, the effect of the present invention cannot be judged by merely comparing the gas permeation rates. In such a case, the effect can be judged by comparing the gas permeation rates of the membranes having the same gas separation coefficient. Alternatively, it can be determined by, for example, the thickness of the dense layer calculated by the calculation formula described in JP-A-1-104271 described above becoming thin. The reason why such a heterogeneous hollow fiber membrane having irregularities on the outer surface exhibits an excellent gas permeation rate is unknown, but one possible reason is that the substantial surface area of the outer surface dense layer is increased. It is known that the gas permeation rate is proportional to the membrane surface area. Therefore, if the surface area is substantially increased due to the unevenness of the membrane surface, the gas permeation rate will increase even if the dense layer has the same thickness. .

Alternatively, it may be because manufacturing conditions can be adopted which can reduce the thickness of the dense layer. Of course, this is only a mechanism estimation and does not limit the present invention. The polymer constituting the membrane of the present invention is a crystalline thermoplastic polymer. The film of the present invention can be manufactured by a melt molding method. A crystalline thermoplastic polymer is required for production by the melt molding method. The crystalline polymer referred to in the present invention is one having an ultimate crystallinity of 30% or more.

Among the crystalline thermoplastic polymers, polyolefin-based polymers are preferable because they have high crystallinity and a high crystallization rate. Among them, poly (4-methyl-1-) is preferable.
The pentene) -based polymer has a high gas permeation rate and oxygen /
The high nitrogen separation coefficient makes it particularly suitable for producing oxygen-enriched air and membranes for producing nitrogen-enriched air. The poly (4-methyl-1-pentene) -based polymer used in the present invention is a homopolymer of 4-methyl-1-pentene or a copolymer or mixture containing 55% or more of 4-methyl-1-pentene. Is.

The size of the heterogeneous hollow fiber membrane of the present invention is not particularly limited, but the outer diameter is preferably 40 μm to 3 mm, more preferably 100 to 500 μm. The film thickness is preferably 5 μm to 1 mm, 10 to 50 μm
Is more preferable. In the production method of the present invention, first, a crystalline thermoplastic polymer is treated at Tm to (Tm + 100) ° C. (however,
Tm indicates the crystal melting temperature of the polymer), preferably (T
m + 20) to (Tm + 50) ° C., extruded from a hollow fiber nozzle, draft ratio (= take-off speed / discharge linear speed) 5
Melt spinning is carried out by cooling with a gas, preferably air, nitrogen or carbon dioxide while taking it in at 0 to 1500, preferably 200 to 1200.

The obtained spun yarn is heat-treated if necessary. The heat treatment may be omitted depending on the type of polymer, the size of the hollow fiber and the spinning cooling conditions, but it is preferably carried out to produce a high performance heterogeneous membrane. The heat treatment can be carried out by a method of exposing to a temperature of (Tm-50) to Tm ° C, more preferably (Tm-30) to Tm ° C. For the heat treatment, a method of staying in a high temperature gas or liquid atmosphere, a method of infrared heating, or the like can be adopted.

Next, the spun yarn or the heat treated spun yarn (referred to as undrawn yarn) is drawn. Since the inside of the membrane is made porous by the stretching, the hollow fiber which was transparent before the stretching is whitened by the stretching. In order to manufacture the membrane of the present invention, it is necessary to make the stretching ratio relatively large in order to allow shrinkage in the heat setting in the subsequent step. The required stretching ratio depends on the stretching temperature and heat setting conditions and is not constant, but it is 1.2 to 5, and preferably 1.3 to 3.5. The stretching temperature is Tg to (Tm-60) ° C (where Tg represents the glass transition temperature of the polymer), and Tg to (Tg + 1).
00) ° C. is preferred.

The stretching may be one-stage stretching or multi-stage stretching,
Multi-stage stretching, especially multi-stage stretching in which the temperature is sequentially increased, is preferable in that a required stretching ratio can be obtained without cutting and a film having excellent performance is obtained. In the case of multi-stage stretching, the necessary stretching ratio is the total stretching ratio in the stretching process. Stretched hollow fiber (referred to as stretched yarn)
Causes unevenness on the outer surface by shrinking and heat fixing at a relatively low temperature. The first of the heat setting methods in the production method of the present invention is, after stretching, (temperature of stretching step + 1
0) It is a method of heat-setting which shrinks by 15-40% at a temperature of 0 ° C or less.

The heat setting temperature is (the temperature of the drawing step +10) ° C.
If it is higher, even if it shrinks, sufficient unevenness does not occur on the outer surface. In this method, the stretching temperature and the shrinking temperature are preferably as high as possible in order to increase the dimensional stability of the obtained hollow fiber membrane. It is also possible to carry out heat setting in multiple stages. At this time, it is necessary that all the stages in the multi-stage heat setting are within the above temperature range. When the heat setting is multi-step heat setting, the shrinkage amount is the total.
After heat setting, aging may be performed under free length or limited shrinkage conditions, but aging in this case is also regarded as part of heat setting.

For the heat fixation, a method of retaining the hollow fibers in a high temperature gas or liquid atmosphere, a method of infrared heating or the like can be adopted. The second heat setting method of shrinking the drawn yarn to generate irregularities on the outer surface is a method of performing two-step heat setting under limited conditions. This two-stage heat setting method is preferable in order to increase the dimensional stability of the obtained hollow fiber membrane. The two-step heat-setting method of the present invention is performed by (Tm-50) to T
The first stage heat setting is carried out by keeping the temperature in the range of m ° C, preferably in the range of (Tm-30) to Tm ° C for 1 second or longer. The first-stage heat setting temperature is preferably 30 ° C. or more lower than the heat treatment temperature, and more preferably 15 ° C. or more lower.

The temperature of the first stage heat setting is preferably higher than the stretching temperature. The shrinkage amount of the hollow fibers in the first stage heat setting is preferably 0 to 30%. It is also possible to carry out the first stage heat setting in multiple stages. Not all stages in the multistage first stage heat setting need to be in the above temperature range, but heat setting in the above temperature range must be included. After the first stage heat setting, at a temperature lower than the first stage heat setting temperature, 10 to 40%, preferably 12% of the hollow fiber length after the first stage heat setting.
Second stage heat setting is performed while shrinking by ~ 35%, more preferably by 15 ~ 30%.

It is also possible to carry out the second stage heat setting in multiple stages. At this time, it is necessary that all the stages in the multistage second stage heat setting are within the above temperature range. When the second-stage heat setting is multi-stage heat setting, the shrinkage amount is the total thereof.
For the heat fixation, a method of retaining the hollow fibers in a high temperature gas or liquid atmosphere, a method of infrared heating or the like can be adopted. After heat setting, aging may be performed under free length or limited shrinkage conditions, but aging in this case is also regarded as part of the second stage heat setting. In this method, the irregularities on the outer surface of the film are formed in the second heat setting step.

[0029]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto.

Example 1 Melt Index (AST
M-D-1238, 260 ° C., 5 kg) 26 poly (4-methyl-1-pentene) (Tm = 240 ° C.) was melt extruded at 285 ° C. from a ring-shaped nozzle having a diameter of 6 mm at 0.2 m. After cooling with a cross wind of 1 / sec and passing through a spinning cylinder with a length of 4 m, the take-up speed is 12 at a position 5.5 m below the nozzle.
It was wound on a bobbin with a draft 350 at 0 m / min. This spun yarn is continuously heat-treated by passing it through a hot air oven having a temperature of 210 ° C. for a residence time of 5 seconds in a roller system, and 35 ° C.
Cold stretching at a draw ratio of 1.3, and hot stretching at 150 ° C. and a draw ratio of 1.4 (total draw ratio in the drawing step 1.
82), the first-stage heat-fixing in which the hot-air stove at 210 ° C. is shrunk by 5% and the residence time is 3 seconds, and the hot-air stove at 150 ° C. Sano 15
The second stage heat setting was carried out, in which the resin was allowed to pass with a residence time of 10 seconds while shrinking%.

The obtained hollow fiber membrane had an outer diameter of 304 μm, an inner diameter of 234 μm and a film thickness of 35 μm, and when observed by SEM, no pores were observed on the outer surface and scale-like irregularities were observed. On the inner surface, crystals that extend long in the fiber axis direction are stacked in the fiber direction, and fibrils are connected between the stacked crystals, and the part surrounded by the crystals and fibrils is long in the fiber axis direction. Observed as elliptical pores,
The average diameter of the pores was about 0.05 μm. In addition, a large number of pores having a pore diameter of about 0.1 μm were observed in the cross section obtained by obliquely cutting the hollow fiber membrane.

The outer surface of this hollow fiber membrane was measured by a scanning tunneling microscope (STM).
As shown in FIG. 1, the period is about 2.5 μm and the depth is about 0.
It was measured to be 3 μm. The gas permeation characteristics of the obtained heterogeneous hollow fiber membrane were such that the oxygen permeation rate was 2.2 × 10 ^−5.
[Cm ³ / cm ² · sec · cmHg] and the oxygen / nitrogen separation coefficient were 4.1 [−].

[Example 2] The draw ratio of heat drawing is 1.6, and the treatment time of the first stage heat setting is 5 seconds.
A heterogeneous hollow fiber membrane was produced in the same manner as in Example 1 except that the second stage heat setting conditions were free length, 70 ° C., and 6 hours. At this time, in the second stage heat setting, the shrinkage was 17% with respect to the hollow fiber length after the first stage heat setting.

When the obtained film was observed by SEM, a small number of pores having a pore size of about 0.05 μm were observed on the outer surface, and the size, outer surface shape, inner surface shape, and cross-sectional shape of Example 1 Was almost the same as. The gas permeation characteristics of this heterogeneous hollow fiber membrane are as follows: oxygen permeation rate of 1.8 × 10
^-5 [cm ³ / cm ² · sec · cmHg], and the oxygen / nitrogen separation coefficient was 4.1 [-].

[Example 3] The draw ratio of the heat drawing is 1.5, the heat setting is one stage under the conditions of 100 ° C and 28% shrinkage, and then the aging is carried out at 50 ° C for 24 hours. A heterogeneous hollow fiber membrane was produced in the same manner as in Example 1 except that the film was shrunk by 6%. On the outer surface of the obtained film, wrinkle-like irregularities and a small number of pores having a diameter of about 0.05 μm were observed. The inside and the inside surface of the film were the same as in Example 1. According to STM measurement, irregularities with a period of about 0.7 μm and a depth of about 0.2 μm, and a period of about 10 μm and a depth of about 1.2 μm
It had a structure in which irregularities of m were overlapped. The gas permeation characteristic of this heterogeneous hollow fiber membrane is that the oxygen permeation rate is 5.9 × 1.
It was 0 ⁻⁴ [cm ³ / cm ² · sec · cmHg] and the oxygen / nitrogen separation coefficient was 0.961 [−].

Example 4 The spinning temperature was 300 ° C., cold drawing was not performed, the draw ratio of hot drawing was 2.1, heat setting was 100 ° C., and 30% shrinkage condition was 1. A heterogeneous hollow fiber membrane was produced in the same manner as in Example 1 except that the step was performed and then the film was shrunk by 7% by free length aging at 70 ° C. for 1 hour. The outer surface of the obtained film has wrinkle-like irregularities and a small number of pores of about 0.5.
Micrometer pores were observed. The inside of the film was porous, and pores having a pore size of about 0.3 μm were opened on the inner surface.

The period of the irregularities on the outer surface of the film measured by STM was about 3.2 μm, and the depth of the irregularities was about 0.7 μm. The gas permeation characteristics of this heterogeneous hollow fiber membrane are that the oxygen permeation rate is 1.2 × 10 ⁻⁵ [cm ³ / cm ² · sec ·
cmHg], and the oxygen / nitrogen separation coefficient was 3.9 [-].

[Comparative Example] Heat setting was 210 ° C., 10 seconds, 1
A heterogeneous hollow fiber membrane was produced in the same manner as in Example 1 except that only one stage of the 5% shrinkage condition was used. This film is SEM
As a result, no irregularities were observed on the outer surface, and it was flat, and the cross section of the film and the inner surface of the film were the same as those of the film obtained in Example 1. In the measurement of unevenness by STM, no obvious period was observed and the depth was about 0.03 μm. The gas permeation characteristics of the obtained inhomogeneous hollow fiber membrane have an oxygen permeation rate of 1.3 × 10 ⁻⁵ [cm ³ / cm ² · sec · c].
mHg] and the oxygen / nitrogen separation coefficient were 4.2 [-].

[0039]

The present invention provides a membrane having a high gas permeation rate as compared with the same gas separation coefficient, and a method for producing the membrane.
That is, whether the pinch layer has no pinholes or some pinholes exist, the calculation results in a heterogeneous film having a thin pinch layer. As a result, the size and cost of the membrane module and device can be reduced.

[0040]

[Brief description of drawings]

FIG. 1 is a graph showing irregularities on the outer surface of a heterogeneous hollow fiber membrane produced in an example of the present invention, which was obtained by measurement with a scanning tunneling microscope. The horizontal axis is the distance in the fiber axis direction,
The vertical axis represents the distance in the depth direction, and the unit is μm.

Claims (7)

[Claims] 1. A heterogeneous hollow fiber membrane which is made of a crystalline thermoplastic polymer, has a substantially non-porous dense layer on its surface, and has a continuous porous inside the membrane, and A heterogeneous hollow fiber membrane, characterized in that it has irregularities of 0.5 to 25 [mu] m in the fiber axis direction and a depth of 0.1 to 5 [mu] m on the surface of the dense layer side. 2. The heterogeneous hollow fiber membrane has a substantially non-porous dense layer on the outer surface, the inside of the membrane is continuous porous, and the inner surface is in a direction substantially perpendicular to the fiber axis. The heterogeneous hollow fiber membrane according to claim 1, which is a heterogeneous hollow fiber membrane in which pores surrounded by a plurality of elongated plate crystals and a plurality of fibrils connecting the crystals are open. 3. The heterogeneous hollow fiber membrane according to claim 1, wherein the heterogeneous hollow fiber membrane is obtained by a melt spinning method. 4. The heterogeneous hollow fiber membrane according to claim 1, 2 or 3, wherein the crystalline thermoplastic polymer is a polyolefin polymer. 5. The polyolefin polymer is poly (4-
The heterogeneous hollow fiber membrane according to claim 4, which is a methyl-1-pentene) -based polymer. 6. A crystalline thermoplastic polymer is melt-spun,
Heat-treated as necessary, stretched by a stretch ratio of 1.2 to 5.0 in a temperature range of Tg to (Tm-60) ° C, and then shrunk by 15 to 40% at a temperature of (stretching temperature +10) ° C or lower. On the surface of the dense layer side, which is characterized by heat fixing while
A method for producing a heterogeneous hollow fiber membrane having periodic irregularities. 7. A crystalline thermoplastic polymer is melt-spun,
If necessary, after heat treatment and stretching at a draw ratio of 1.2 to 5.0 in the temperature range of Tg to (Tm-60) ° C, (T
m-50) to Tm ° C. for 1 second or longer to carry out the first stage heat setting, and then at a temperature lower than the first stage heat setting temperature, 10 to 40 of the hollow fiber length after the first stage heat setting. A method for producing a heterogeneous hollow fiber membrane having periodical irregularities on the surface of the dense layer, wherein the second stage heat setting is performed while shrinking by%.