JPH0676502B2 - Microporous flat membrane and method for producing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a microporous flat membrane and a method for producing the same, and more specifically, a polypropylene fine membrane having a large number of through fine pores obtained by a stretching method. The present invention relates to a porous flat membrane and a method for producing the same.

[Prior Art] A microporous flat membrane having a structure in which a large number of penetrating fine through holes are formed in a film made of a polymer material is a filtration membrane or separation membrane used for air cleaning, water treatment, etc., a battery or Separators used for electrolysis, gas exchange membranes or separation membranes used for artificial lungs or plasma separation, and as filter membranes or separation membranes for sterilization in the production of sake, draft beer, fresh juice, etc. and purification of various enzymes It is used in various fields.

As a method for producing a microporous flat membrane having a large number of through micropores, for example, after molding a polymer material in which an easily soluble substance is mixed and dispersed, the easily soluble substance is dissolved and removed by a solvent. A method of forming a large number of fine through holes in a film is known.

In recent years, a method in which a thermoplastic crystalline polymer material is formed into a film, which is then heat-treated and then stretched to generate pores in the film to form a porous body is also generally used. Has become.

It is known that various polymer substances can be used as the thermoplastic crystalline polymer material used for such a purpose. Especially, polyolefin, especially polypropylene, is inexpensive and has high strength and chemical resistance. Therefore, it is regarded as an excellent polymer material for producing a microporous flat membrane.

A method of producing a microporous flat membrane by stretching a crystalline polyolefin film to generate pores inside the film, for example, is disclosed in US Pat.No. 3,558,764, and according to this method, It has been obtained that the inside of the membrane has pores connected to each other and having an average pore diameter of 1000 to 2000Å. The method for producing the microporous flat membrane is the same as in Japanese Patent Publication No. 46-
No. 40119, Japanese Patent Publication No. 50-2170, Japanese Patent Publication No. 55-32531
It is also disclosed in the official gazette. According to these methods, the unstretched film is first heat-treated, and then stretched at a temperature near room temperature or at a temperature not lower than the second-order transition temperature of the resin used (for example, −40 ° C. or higher when polypropylene is used) to obtain pores. Is generated to form a porous body, and the pores formed are then subjected to heat treatment again to heat-fix the core of the method. However, the microporous flat membrane obtained by the above method has a wide pore size distribution and a low porosity.

[Problems to be Solved by the Invention] A conventionally known microporous flat membrane produced by a stretching method is
Voids are formed by the relatively thick part with uneven shape and size that forms a variety of large and small meandering meshes and the minute fibrils that run substantially parallel to the stretching direction of the film between these meshes. Not only has a non-uniform shape and size, but also has a low porosity and the winding part forms a continuous closed circuit of various sizes, so that it penetrates from one film surface to another. The through path of the through hole is long. Therefore, the obtained microporous flat membrane has a low permeation rate when it is used for plasma separation and is inferior in selective separation, and when it is used as a battery separator, it has a low porosity. Since the electrolyte cannot be sufficiently retained and the region in which ions can move is small, the internal resistance is high, and since the through path of the through hole is long, the substantial distance between the electrodes is large, and thus the internal resistance is high. The use of the battery as a separator may be limited, such as the performance being lowered.

[Means for Solving the Problems] Therefore, the inventors of the present invention continued to diligently study a polypropylene microporous flat membrane that overcomes the above-mentioned drawbacks and a method for producing the same, and as a result, a specific crystallinity and a small angle of a two-point figure are obtained. An unstretched polypropylene having an X-ray scattering diffraction image and a specific spread of the diffraction angle at half the maximum diffraction intensity in each figure (hereinafter referred to as the half width of the diffraction intensity) and a specific long period. By stretching the film under specific conditions, a microporous flat membrane with an extremely unique structure, that is, a pore size distribution that is sharp, a high porosity, and a penetration that spreads approximately two-dimensionally at right angles to the stretching direction The present invention has been achieved by succeeding in forming a microporous flat film having fine through holes.

That is, according to the present invention, there is provided a microporous flat film having a large number of penetrating fine through holes obtained by stretching an unstretched polypropylene film, the film being substantially predetermined at a right angle to the stretching direction of the film. A group of unstretched plate-like planes that run at intervals of and that are formed substantially parallel to a cross section perpendicular to the stretching direction of the film, and between the plate-like planes at substantially parallel and substantially predetermined intervals in the stretching direction of the film. A large number of fine transparent particles which are composed of a group of relatively thin fibrils running and connected between the plate-like planes and which have a substantially uniform shape in which the gaps between the thin fibrils connected between the plate-like planes spread in a substantially two-dimensional manner. A microporous flat film having pores is provided.

The method for producing the microporous flat film according to the present invention includes a crystallinity of 50 to 90%, a diffraction image of small-angle X-ray scattering is a two-point figure, and each figure is a substantially circular shape. The half-width of strength is 15 'or less and the long period is 120 Å or more.
It is carried out in a medium selected from the group consisting of oxygen, argon, carbon monoxide, methane and ethane, and the stretching temperature is -70.
A method (first method) in which the temperature is not higher than ℃ and is in the low temperature range from the freezing point of the medium to 50 ° C. higher than the boiling point of the medium (first method), and the stretching step of the unstretched polypropylene film is performed at room temperature in advance. And a method (second method) in which the stretching strain rate is less than 10% / min in a high temperature range of 110 to 155 ° C. without being provided.

Microporous polyolefin flat membrane obtained by a conventionally known method, for example, the membrane obtained by the method disclosed in JP-B-46-40119, a relatively thick portion is present in a meandering manner,
This forms a closed circuit of various sizes, similar to cells (cell-like), and microfibrils connect between them to form a porosity.

The present inventors have carefully examined various molding conditions, and formed this relatively thick portion only in a direction substantially perpendicular to the stretching direction of the film. In other words, the relatively thick portion has an unstretched plate shape. We have developed a special microporous flat membrane that is flat and has a large number of fine through holes that do not form in the film stretching direction.

To further describe this, in the microporous flat membrane of the present invention, the gap between the thin fibrils connected between the unstretched plate-like planes that become the through-holes has a substantially uniform shape in which it is two-dimensionally expanded. It can be said that it was presented.

Here, "substantially two-dimensionally spread" means that the through holes of the conventionally known microporous flat membrane penetrate from one membrane surface to the other membrane surface by meandering vertically and horizontally in the membrane. In contrast to the above, the pores of the microporous flat membrane of the present invention extend from one membrane surface to the other membrane surface substantially linearly or only slightly in the left-right direction between approximately two unstretched flat planes in the membrane. Means that it is winding and penetrating.

That is, the membrane obtained by the present invention overcomes the drawbacks of the membrane obtained by the conventionally known method, has strength and retains strength, and has increased porosity and pores of uniform shape and size. It is a microporous flat membrane having a short through path of pores.

The unstretched plate-shaped plane group referred to in the present invention does not form a closed circuit over three times the average length () of the microfibrils,
It is preferable that a closed circuit is not formed over 5 times or more, more preferably 10 times or more, of the average length () of the microfibrils. The average length of microfibrils () here is the average of the lengths of 20 points of any microfibrils around it including one point of any fibril that connects between the plate-like planes. Shall be represented.

In the present invention, the thickness (Δd) of the plate-shaped plane formed at right angles to the stretching direction of the film is preferably between 0.1 and 40 μm, and the thickness (Δd) of the plane and the fine fibrils. It is desirable that the relationship of 3Δl ≦ Δd ≦ 400Δl holds with the thickness (Δl). When Δd is less than 3Δl, the strength is insufficient, and when it is more than 400Δl, the porosity is lowered and the permeation rate is decreased. When the microporous membrane of the present invention is used, for example, in a plasma separation membrane, the ratio of the average length of fibrils () to the average interval between fibrils () is between 2 and 60, and is 0.02 to 2 μm. It is preferable to be located between.

The density of the fine fibrils may be any one of the cross-sections parallel to the stretching direction of the plate-shaped flat film on one surface or the other surface of the microporous flat film of the present invention.
When the cross section has the same length as the average length () of the fibrils starting from the point, it is preferable that the number of the fine fibrils is within the range of 3 or more and 30 or less.

Further, as the physical properties of the flat film according to the present invention, the porosity is 30
~ 85%, preferably 50-85% with an average pore size of 0.02-
2 μm, preferably 0.04 to 1.5 μm, with an average pore diameter (D
_A ) and the maximum pore size (D _M ) ratio D _A / D _M is in the range of 0.5 to 1.0, preferably in the range of 0.6 to 1.0, and the tensile modulus in the stretching direction is 5000 kg / cm ² or more, preferably 6000 kg / cm ² or more.

The polypropylene microporous flat membrane of the present invention having the above characteristics does not deteriorate permeation rate or separability over a long period of time when it is used as a filtration membrane, and when it is used for plasma separation, it is completely hemolytic. It has an interesting feature that it does not cause a phenomenon. When used as a battery separator, it is easy for ions to move, and various electrolytes can be used according to the type and shape of the battery, and the electrolyte can be cut into various shapes.

Next, the method for producing the microporous flat film according to the present invention will be described in detail.

The polypropylene used for producing the microporous flat film of the present invention is not particularly limited, and not only a homopolymer of propylene but also a random, block or graft copolymer of propylene and other monomer or oligomer. Also includes.

Also, the melt viscosity of the polypropylene used [melt flow index (MFI) or melt index (MFI
I)] is not particularly limited as long as the film can be formed, but in view of film formation or productivity, it is preferable to use one having an MFI of 0.5 to 40 g / 10 min.

In addition, polypropylene containing an additive (material) such as a plasticizer, a colorant, a flame retardant, and a filler can also be used.

In the present invention, first, the film is molded according to a known film manufacturing method to obtain an unstretched polypropylene film. Examples of usable film manufacturing methods include an inflation film molding method and a T-die film molding method. The molding conditions in such a molding method can be appropriately selected by a known technique. For example, the film forming temperature is a temperature in the range of not less than the temperature at which the polypropylene used can be discharged, and not more than the thermal decomposition temperature of the polyolefin, usually 170 to 300 ° C, preferably 190 to 270 ° C. is there.

Further, when the crystal orientation of the unstretched polypropylene film obtained by molding is extremely low, a microporous flat membrane having a unique pore structure in the present invention is obtained even when subjected to the stretching step of the present invention. Is difficult. Therefore, in addition to the discharge temperature described above, the draft ratio should also be considered in consideration of productivity.
The range from 6000 to 6000 is preferable, and the film must be formed by sufficiently considering the type of cooling medium, the time until the discharged resin comes into contact with the cooling medium, the distance, and the temperature of the resin at that time.

The obtained unstretched polypropylene film may be heat-treated before being subjected to the stretching step. By performing the heat treatment before the stretching, the crystallinity of the unstretched polypropylene film can be increased, so that the characteristics of the microporous membrane obtained by the stretching are further improved. The heat treatment is carried out by a method of heating the unstretched polypropylene film in air heated to a temperature 15 to 70 ° C. lower than the melting temperature of polypropylene for 3 seconds or more.

In the stretching step in the present invention, the crystallinity is 50 to 90%, preferably 60 to 90%, and the diffraction image of small angle X-ray scattering is 2%.
An unstretched polypropylene film having a dotted pattern in which each pattern is substantially circular and the half-value width of diffraction intensity in each pattern is 15 'or less, preferably 12' or less and the long period is 120 Å or more, preferably 150 Å or more is preferable. Be offered.

For the stretching step of the present invention, any one of the following methods is adopted.

The stretching step in a medium selected from the group consisting of nitrogen, oxygen, argon, carbon monoxide, methane and ethane, and the stretching temperature is -70 ℃ or less, the freezing point of the medium from the freezing point of the medium The stretching strain rate is less than 10% / min within a temperature range lower than the melting temperature of the polyolefin to be used without performing the stretching step at a room temperature in advance at a temperature lower than 50 ° C or higher. Done in.

Here, in the case of stretching by a method using polypropylene as the polyolefin, the temperature range is 110 to 155.
It is preferably set to ° C.

First, the method will be described.

In the cryogenic stretching step in the present invention, the above-mentioned media can be used alone or in combination.

Examples of preferred stretching temperature when using the above medium, when using nitrogen, the range of -209 ~ -146 ℃, when using oxygen, the range of -218 ~ -132 ℃, When argon is used, the range is -189 to -140 ° C, when carbon monoxide is used, the range is -205 to -141 ° C, and when methane is used, -182 to -111 ° C. In the range of −183 to −70 ° C. when ethane is used. Stretching temperature is -70
If the temperature is higher than 0 ° C, the rate of formation of effective through holes due to stretching becomes low. In the present invention, the temperature 50 ° C. or higher above the boiling point does not mean a temperature range accurately lower than the temperature 50 ° C. higher than the boiling point, but means a temperature 50 ° C. or higher higher than the boiling point.

At such an extremely low temperature, the medium exhibits a liquid state, a liquid / gas state, or a gas state, and the stretching step of the present invention can be carried out in any state of the medium described above.

It is presumed that the above-mentioned stretching according to the present invention occurs because the crazing action appears when the above-mentioned medium is stretched at an extremely low temperature. In ordinary media other than the above, the polypropylene film becomes a glass state at an extremely low temperature, and the polypropylene film is cut without showing elongation to cause a crazing action.

The cryogenic stretching temperature of the present invention is a temperature of −70 ° C. or lower, and from the freezing point of the medium to be used, it can be carried out in a range of a temperature 50 ° C. or higher higher than the boiling point. It is advantageous to carry out the reaction at a temperature near the boiling point of, in terms of production control, and for keeping the properties of the obtained polypropylene microporous flat membrane constant.

The stretching ratio in the above-mentioned cryogenic stretching step is generally a value in the range of 1 to 200% with respect to the unstretched polypropylene film. However, the preferred stretching ratio is a value in the range of 10 to 150%. With a draw ratio within these ranges, the number of pores tends to increase as the draw ratio increases, and this tendency is utilized to match the average pore diameter and porosity of the obtained polypropylene microporous flat membrane to the purpose. It is also possible to adjust.

The above-mentioned cryogenic stretching step can be repeated twice or more until a desired pore structure, average pore diameter, porosity and mechanical properties are obtained.

In the specific medium of the present invention, the polypropylene film is made porous by utilizing the drawing step under cooling at an extremely low temperature, unlike the conventional drawing step at around room temperature, the through holes are uniform, and the voids are An excellent polypropylene microporous flat membrane having a high rate can be obtained.

The polypropylene film which has been made porous through a drawing process at an extremely low temperature in the above-mentioned specific medium is preferably then subjected to a heat setting treatment. The main purpose of this heat setting treatment is heat setting for holding the formed fine through holes. In this heat-setting treatment, a polypropylene film which has been made porous while maintaining a stretched state at an extremely low temperature is heated for 3 seconds or more in air which is usually heated to a temperature of 110 to 165 ° C, preferably 130 to 155 ° C. It is carried out by a method or the like. If the heating temperature is significantly higher than the stated upper limit, the formed fine through holes may be closed, and if the temperature is significantly lower than the lower limit, or if the heating time is shorter than 3 seconds, heat fixation will occur. Insufficiency is likely to occur, the formed through-hole may be closed later, and thermal contraction is likely to occur due to temperature change during use. The above-mentioned cryogenic stretching and heat setting treatment can be repeatedly performed until the desired average pore diameter and porosity are obtained. That is, the temperature of the film can be returned to room temperature and repeatedly subjected to a step including a cryogenic stretching (and heat setting treatment). The number of through holes formed by repeating the cryogenic stretching can be increased, and the average through hole diameter can be increased.

The polypropylene microporous flat membrane prepared as described above has good characteristics with high porosity, but by further subjecting the polypropylene microporous flat membrane to the heat stretching step, the characteristics are further improved. .

The thermal stretching step of the polypropylene film which has been made porous through the above-mentioned ultra-low temperature stretching step at least once is carried out as follows. This hot drawing step is performed mainly for the purpose of expanding the diameter of the fine through holes formed at an extremely low temperature. This hot stretching step is carried out by stretching the porous polypropylene film in air heated to a temperature of 80 to 160 ° C, preferably 110 to 155 ° C. When the heating temperature is higher than the upper limit of the above temperature, the formed fine through holes may be closed, and when the temperature is lower than the lower limit, the expansion of the through hole diameter by stretching may be insufficient. There is.

The stretching ratio in this hot stretching step is usually 10 to 700%, preferably 50 to 550%, with respect to the film length (initial length) before being subjected to the cryogenic stretching step. If the stretching ratio is lower than 10%, the expansion of the through holes may be insufficient, and if it is higher than 700%, the film may be cut.

The hot stretching step is performed alternately with the cryogenic stretching step described above, or after at least one cryogenic stretching step is completed.

The film made porous by this stretching treatment is preferably subjected to a heat setting treatment between stretching steps. The main purpose of this heat setting treatment is to heat set the through holes formed through the heat drawing step.

This heat-setting treatment is usually carried out by heating the porous polypropylene film to a temperature of 3 seconds or more in the air while keeping the stretched state, usually 110 to 165 ° C, preferably 130 to 155 ° C. It

It is desirable that this heat setting treatment be similarly performed on the film which has undergone all the stretching steps.

If the heating temperature is higher than the above upper limit temperature, the formed pores may be closed, and if the temperature is lower than the above lower limit temperature or the heating time is shorter than 3 seconds, heat fixation tends to be insufficient. The through holes are closed, and thermal contraction easily occurs due to temperature change during use.

Next, the stretching method of the present invention will be described.

The stretching step in this case is usually 110 to 160 ° C., preferably 11
Stretching strain rate 10% / in the high temperature range of 0-150 ℃
Do in less than a minute.

When stretched at a temperature outside the above temperature range, when the temperature is low, only a small pore size can be obtained, or the film is cut at a small stretch ratio, and only a small porosity can be obtained. .

On the other hand, when the temperature is high, both the film thickness and the film width perpendicular to the stretching direction are reduced, polypropylene is melted or partially melted and no through holes are formed, or only small holes are obtained. Nothing happens.

If the stretching strain rate is 10% / min or more, only small holes may be obtained or no holes may be generated.

When the stretching strain rate is less than 10% / min, the average pore diameter of the through holes,
The porosity increases according to the draw ratio.

The draw ratio can be changed according to the average pore diameter of the through holes depending on the intended use of the intended polypropylene microporous flat membrane. The stretching ratio is 100 to 700%, preferably 150 to 600%, relative to the initial length of the unstretched polypropylene film. If the stretching ratio exceeds 700%, the film may be cut.

The polypropylene film that has been made porous through the stretching step is preferably heat-treated next. The main purpose of this heat treatment is heat fixation for holding the formed fine through holes. In this heat treatment, the polypropylene film made porous while maintaining the stretched state is kept in air for 3 seconds or longer, usually 110 to 165 ° C., preferably 130
It is carried out by a method of heating to a high temperature of ˜155 ° C.

If the heating temperature is higher than the above upper limit temperature, the formed pores may be closed, and if the temperature is lower than the above lower limit temperature or the heating time is shorter than 3 seconds, heat fixation tends to be insufficient. The through holes are closed, and thermal contraction easily occurs due to temperature change during use.

The polypropylene microporous flat film obtained by stretching the polypropylene film having the crystallinity, the small-angle X-ray scattering pattern, the full width at half maximum of the diffraction intensity and the long period by the above method and is a scanning electron microscope. When observed, between an unstretched plate-shaped plane group that runs at a substantially predetermined interval perpendicular to the stretching direction of the film and is formed substantially parallel to a cross section perpendicular to the stretching direction of the film, and between the plate-shaped planes. It is composed of a group of stretch-oriented relatively thin fibrils that run substantially parallel to each other in the stretching direction of the film and that run between the plate-like planes, and the gap between the thin fibrils that connect between the plate-like planes is approximately two-dimensional. A large number of fine through-holes having a substantially uniform shape are formed, and a membrane having such a unique structure is of course preferable for separation membrane applications such as water filtration and plasma separation. It is extremely preferable to use as the over data.

[Examples] Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

(Example 1) Polypropylene (UBE-PP-F109K, trade name: Ube Industries, Ltd., MFI = 9g / 10 minutes) was used with an inflation molding machine equipped with a die having an outer diameter of 150 mm, and a discharge temperature of 190 ° C. ,
An inflation film was formed at a take-up speed of 30 m / min. By heat-treating the obtained polypropylene film in a heated air bath at 145 ° C for 30 minutes, the crystallinity is 70%, the diffraction image of small-angle X-ray scattering is a two-point figure, and each figure is approximately circular and the diffraction intensity is Has a half width of 10 'and a long cycle of 21
An unstretched polypropylene film of 1Å was obtained.

This unstretched film was stretched in liquid nitrogen (-196 ° C) by 20% of the initial length, and the stretched state was maintained at 145 ° C.
The heat setting treatment was carried out for 2 minutes in the heated air bath.

This film was heat-stretched at 300% in an air atmosphere at 130 ° C., and then heat-set in a heated air tank at 145 ° C. for 30 minutes to produce a polypropylene microporous flat membrane.

Average pore size (D _A ) of the obtained polypropylene microporous flat membrane
And the maximum pore size (D _M ) are in accordance with the method specified in ASTM F316-80, the half dry method using ethanol (hereinafter,
The average pore size was 0.13 μm,
D _A / D _M was 0.85. The porosity is CARLOER
BA) (Italy) POROSIMETRO series (POROSI)
METRO SERIES) 1500 (hereinafter, the same) was measured by mercury porosimetry, and was 71.0%. The tensile elastic modulus in the stretching direction was measured according to the method specified in ASTM D882 (hereinafter the same), and was 7700 kg / cm ² .
The water permeation rate of the membrane hydrophilized with ethanol is 30.0 / m ² ·
It was min · kg / cm ² . The surface and cross section of this polypropylene porous flat membrane were observed with a scanning electron microscope (Hitachi: X-60
5, the same applies to the following), a large number of uniform flat plate groups formed substantially perpendicular to the stretching direction of the film and microfibril groups formed substantially parallel to the stretching direction of the film were observed. It was found that the two-dimensionally wide penetrating fine through holes were formed. Electron micrographs of the surface and cross section of the obtained polypropylene microporous flat film are shown in FIG. 1 (magnification: 12400 times) and FIG. 2 (magnification: 4500 times), respectively.

(Comparative Example 1) The same polypropylene as in Example 1 was used, and the crystallinity was 40.
%, The diffraction image of small-angle X-ray scattering is a two-point figure, but each figure is an ellipse, the half-width of the diffraction intensity in the minor axis direction of the ellipse is 20 ', and the long period is 165Å unstretched. A polypropylene film was obtained.

This unstretched film was stretched and heat set in the same manner as in Example 1 to produce a polypropylene microporous flat membrane.

The surface and cross section of this polypropylene microporous flat film were observed by a scanning electron microscope. As a result, it was found that the flat plate group and the size thereof, which were formed substantially at right angles to the stretching direction of the film, such as the film obtained in Example 1. No many through-holes with a uniform shape were observed, and the holes formed on the surface and cross section were not uniform in size and shape, and the part forming the surface between the fibrils existed in a meandering shape. A through-hole that formed a closed circuit and expanded two-dimensionally could not be observed.

(Example 2) A polypropylene film molded under the same conditions as in Example 1 was heat-treated in a heated air bath at 140 ° C for 20 minutes to give a crystallinity of 68% and a small-angle X-ray scattering diffraction image in a two-point pattern. An unstretched polypropylene film having a substantially circular shape, a half-value width of diffraction intensity of 11 'and a long period of 200Å was obtained.

This unstretched film, at a temperature of 145 ℃, strain rate 8.33% /
The film was stretched for 300 minutes with respect to the initial length, and heat-set in the heated air tank at 145 ° C. for 10 minutes while maintaining this stretched state to produce a polypropylene microporous flat membrane.

When the surface and cross section of this polypropylene microporous flat film were observed with a scanning electron microscope, a structure similar to that of the film obtained in Example 1 was recognized.

Comparative Example 2 The same unstretched polypropylene film used in Example 1 was stretched under the same conditions as in Example 1 except that the stretching in liquid nitrogen was changed to the stretching in air (25 ° C.). The
A polypropylene microporous flat membrane was produced.

Although the unstretched polypropylene film used was the same as that used in Example 1, the average pore diameter, porosity, D _A / D _M , elastic modulus and water permeation rate of the obtained microporous flat membrane were obtained. Both values were lower than those of the polypropylene microporous flat film obtained in Example 1. Furthermore, when the surface and cross section of the obtained polypropylene microporous flat film were observed by a scanning electron microscope, pores were formed on the film surface, but when the cross section of the film was observed, one surface to the other surface was observed. The number of through holes penetrating up to was small compared to the film obtained in Example 1.

(Example 3) Using the same unstretched polypropylene film as that used in Example 1, the same operation as in Example 1 was performed except that the stretching was performed in argon at -180 ° C.

The surface and cross section of the obtained polypropylene microporous flat film were observed with a scanning electron microscope.
A structure similar to that of the film obtained in 1. was observed.

Example 4 The same unstretched polypropylene film used in Example 1 was stretched in liquid nitrogen (−196 ° C.) by 20% with respect to the initial length, and the stretched state was maintained. The heat setting treatment was carried out for 2 minutes in a heated air bath at ℃, and the stretching and the subsequent heat setting treatment were repeated 5 times.

This film was heat-stretched and heat-set in the same manner as in Example 1 to produce a polypropylene microporous flat film.

When the surface and cross section of this polypropylene microporous flat film were observed with an operating electron microscope, a structure almost equivalent to that of the film obtained in Example 1 was observed.

Table 1 shows various characteristics of the films obtained in Examples 2 to 4 and Comparative Examples 1 and 2.

EFFECTS OF THE INVENTION The microporous flat membrane of the present invention has through-holes of uniform size and shape formed almost at right angles to the stretching direction of the membrane, and since it has a high porosity, It is a membrane that can satisfy both performances of speed and selective resolution. Therefore, it is possible to provide a membrane module with excellent filtration performance, and it can be applied not only as a separation membrane used for water treatment, air purification, etc., but also as a plasma separation module, especially as a module for donor pheresis. It was Further, the above-mentioned characteristics of the microporous flat membrane of the present invention have made it possible to apply this membrane extremely preferably as a battery separator.

[Brief description of drawings]

FIG. 1 is an electron micrograph showing the shape of minute fibrils (fiber shape) on the surface of the microporous flat membrane of the present invention,
FIG. 2 is an electron micrograph showing both the shape of fine fibrils (fiber shape) on the surface and cut surface of the microporous flat sheet.

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Claims (3)

[Claims] 1. A microporous flat membrane having a large number of fine through holes obtained by stretching a polypropylene film, the membrane running at a substantially predetermined interval at right angles to the stretching direction of the film, and An unstretched plate-shaped plane group formed substantially parallel to a cross section perpendicular to the stretching direction of the film, and a plate-shaped plane that runs between the plate-shaped planes in a direction substantially parallel to the stretching direction of the film. A plurality of fine through-holes having a substantially uniform shape that are formed by a group of stretch-oriented relatively thin fibrils connected to each other and the gaps between the thin fibrils connected between the plate-like planes spread in a substantially two-dimensional manner. A microporous flat membrane characterized by: 2. When producing a polypropylene microporous flat membrane having a large number of penetrating fine through-holes by stretching, the degree of crystallinity is 50 to 90%, and the diffraction image of small angle X-ray scattering is a two-point figure. Is a substantially circular shape and 1 / maximum of the maximum diffraction intensity in each figure
The stretching process of an unstretched film with a diffraction intensity spread (half width) of 15 'or less and a long period of 120 Å or more at the intensity of 2 is a group consisting of nitrogen, oxygen, argon, carbon monoxide, methane and ethane. In a more selected medium,
The stretching temperature is −70 ° C. or lower, and the stretching is performed in a low temperature range of 50 ° C. higher than the freezing point of the medium and 50 ° C. higher than the boiling point of the medium. A method for producing a microporous flat membrane. 3. A polypropylene microporous flat membrane having a large number of penetrating fine pores obtained by stretching a polypropylene film is produced, and a crystallinity of 50 to 90% and a diffraction image of small angle X-ray scattering are obtained. Each figure is a two-point figure, and each figure is approximately circular, and the spread (half-value width) of the diffraction intensity at half the maximum diffraction intensity in each figure is 15 'or less and the long period is 12
The stretching step of the unstretched film of 0 Å or more is performed at a stretching strain rate of less than 10% / min in a high temperature range of 110 to 155 ° C without performing it at room temperature in advance. The method for producing a microporous flat membrane described in 1.