JP4098607B2 - Method for producing polyethylene microporous membrane - Google Patents

Description

【００３８】
【発明の効果】
本発明に係るポリエチレン微多孔膜の製造方法により、従来製法と同等な物性をもつポリエチレン微多孔膜が、従来よりも生産性に優れる方法で製造することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a polyethylene microporous membrane suitable for a battery separator.
[0002]
[Prior art]
Polyethylene microporous membranes are used in microfiltration membranes, battery separators, capacitor separators, and the like. Among these, it is suitably used as a battery separator, particularly as a lithium ion battery separator. The reason for this is that it has electrical insulation properties, has ion permeability while holding the electrolytic solution, is excellent in electrolytic solution resistance and oxidation resistance, and has a hole closing effect.
As a method for producing a polyethylene microporous membrane, for example, a method in which a raw material polyethylene and a plasticizer are melt-kneaded with an extruder or the like is formed into a sheet and the plasticizer is extracted after a process such as stretching, or a film made of the raw material polyethylene alone Conventionally, a method of removing the pore opening agent after the porous material is swollen once with a high-temperature pore opening agent and subjected to a porous treatment is known.
[0003]
Regarding the latter, Patent Document 1 discloses a polyethylene microporous membrane obtained by forming an ultrahigh molecular weight polyolefin inflation film, immersing it in a pore-opening agent, and then extracting the pore-opening agent, and then extracting the pore-opening agent. Yes. However, the porous treatment method in Patent Document 1 is a batch type using a metal frame, and a method for continuously performing the porous treatment is not disclosed.
Further, in Patent Document 2 by the same applicant, the same film as the above document is subjected to continuous porous treatment by bending and passing through the zigzag while restraining both ends of the film in a high-temperature liquid paraffin bath. A support method and a heat treatment apparatus are disclosed. Patent Document 3 discloses a method for producing a polyolefin microporous membrane using the heat treatment apparatus.
[0004]
If such a method is used, continuous microporous membrane production is possible, but film resistance in the film is large because of film transport in the liquid, for example, fluttering of the membrane occurs during high-speed production, In addition to the problem of breaking in some cases, there was a problem that a large amount of liquid paraffin was required to immerse the membrane. Furthermore, since the use of a submerged bearing is indispensable, there has been a problem that frequent maintenance for adhesion of dust and the like is required. Thus, as a production method for continuously swelling a polyethylene film with a pore-opening agent, a porous treatment method that is excellent in high-speed productivity, low in cost, and easy to maintain has been desired.
[0005]
[Patent Document 1]
JP-A-11-302436,
[Patent Document 2]
JP-A-10-278108,
[Patent Document 3]
JP-A-10-306168 [Problems to be solved by the invention]
An object of the present invention is to provide a method for producing a polyethylene microporous membrane by a porous treatment that is excellent in high-speed productivity, low cost, and easy to maintain.
[0006]
[Means for Solving the Problems]
As a result of intensive studies to solve the above-mentioned problems, it is possible to achieve the above-mentioned problems by applying a perforating agent and permeating it, instead of immersing the film in a perforating agent as in the prior art. The headline and the present invention were made.
That is, the present invention
[1] A method for producing a polyethylene microporous membrane having a polyethylene film preparation step and a porous treatment step, wherein the porous treatment step (1) applies a pore-opening agent to at least one surface of the polyethylene film. , (2) a step of permeating a pore-opening agent into the polyethylene film, and (3) a method for producing a polyethylene microporous membrane comprising a step of removing the pore-opening agent permeated into the polyethylene film,
[0007]
[2] The method for producing a polyethylene microporous membrane according to [1], wherein the polyethylene film is an oriented polyethylene film,
[3] The method for producing a polyethylene microporous membrane according to [1] or [2], wherein the viscosity of the pore-opening agent is 20 mPa · s or more and 9000 mPa · s or less,
[4] The method for producing a microporous polyethylene membrane according to [1] or [2], wherein the pore-opening agent has a viscosity of 100 mPa · s or more and 6000 mPa · s or less,
[ 5 ] The method for producing a polyethylene microporous membrane according to any one of [1] to [4], wherein the permeation method of (2) is a heat treatment,
[ 6 ] The method for producing a polyethylene microporous membrane according to any one of [1] to [ 5 ], wherein the steps (1) and (2) are performed in a state where the polyethylene film is constrained.
[0008]
[7] The process according to any one of [1] to [6], wherein the steps (1) and (2) are carried out using a lateral uniaxial stretching machine or a simultaneous biaxial stretching machine. production how of polyethylene microporous membrane,
It is.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The method for producing a polyethylene microporous membrane of the present invention is roughly divided into a polyethylene film preparation step and a porous treatment step. Further, the pore forming treatment step includes (1) a step of applying a pore-opening agent to at least one surface of the polyethylene film (hereinafter referred to as a pore-opening agent coating step), and (2) penetration of the pore-opening agent into the polyethylene film. A step (hereinafter referred to as a pore-opening agent permeation step), and (3) a step of removing the pore-opening agent that has penetrated the polyethylene film (hereinafter referred to as a pore-opening agent removal step).
Hereinafter, each process is demonstrated one by one.
[0010]
<Polyethylene film making process>
The polyethylene used in the present invention is preferably a high-density polyethylene which is a crystalline polymer mainly composed of ethylene or a copolymer of ethylene and an α-olefin, and further includes polypropylene, medium density polyethylene, linear low density. A blend of polyolefins such as polyethylene, low density polyethylene, and ethylene propylene rubber (EPR) at a ratio of 30 wt% or less can also be used.
The weight average molecular weight of the polyethylene is preferably 100,000 or more, more preferably 200,000 to 10,000,000. The weight average molecular weight of the polymer used may be adjusted to a preferred range by means such as blending or multistage polymerization.
[0011]
The method for producing the polyethylene film is not particularly limited, and examples thereof include a method for producing polyethylene by supplying polyethylene to an extruder and melt-kneading to produce a film, and a method for producing a film by compressing polyethylene powder. The polyethylene film is preferably an oriented polyethylene film. An oriented polyethylene film means that molecular chains are oriented in at least one axial direction. Of these, a biaxially oriented polyethylene film is preferred. The method for producing an oriented polyethylene film is not particularly limited, but in the case of a uniaxially oriented polyethylene film, a method by uniaxial stretching by a roll can be used, and in the case of a biaxially oriented polyethylene film, flat simultaneous biaxial stretching or flat A method by sequential biaxial stretching and tubular simultaneous biaxial stretching can be used.
[0012]
The stretching temperature is preferably 100 ° C. or higher from the viewpoint of stretchability of polyethylene, and preferably 250 ° C. or lower from the viewpoint of film strength. More preferably, it is 110 degreeC or more and 200 degrees C or less, More preferably, it is 120 degreeC or more and 180 degrees C or less, Most preferably, they are 130 degreeC or more and 170 degrees C or less. Stretching is also possible when the polyethylene contains a plasticizer, but stretching at 135 ° C. or lower is preferred. A plasticizer is an organic compound that can form a homogeneous solution at a temperature above the melting point of polyethylene. In the present invention, it is preferable not to include a plasticizer.
[0013]
The stretching ratio is preferably 4 times or more from the viewpoint of film strength, and preferably 400 times or less from the viewpoint of film breakage during stretching. More preferably, they are 10 times or more and 200 times or less, More preferably, they are 10 times or more and 100 times or less.
The stretching temperature and the stretching ratio are preferably determined under the optimum conditions in consideration of the molecular weight of the raw polyethylene and the presence or absence of a crosslinked structure.
The thickness of the film is preferably 1 μm or more from the viewpoint of mechanical strength, and preferably 200 μm or less from the use of the film. More preferably, they are 3 micrometers or more and 100 micrometers or less, More preferably, they are 5 micrometers or more and 50 micrometers or less.
[0014]
<Porosification treatment process>
(1) Opening agent coating step The pore opening agent used in the porous treatment step is a liquid that is permeable to the polyethylene film, or the liquid, the viscosity of the liquid, and the permeability to the polyethylene film. A mixture comprising a coatability improver, which can be added for the purpose of The coating property improving agent can be added to the liquid in one kind or two or more kinds. As used herein, “penetration” includes swelling of the polyethylene film as a result.
[0015]
Examples of liquids that are permeable to polyethylene film include hydrocarbons such as liquid paraffin, lower aliphatic alcohols, lower aliphatic ketones, nitrogen-containing organic compounds, ethers, glycols, lower aliphatic esters, and silicone oils. These can be used alone or in combination. Of these, liquid paraffin is preferred from the viewpoint of flash point.
The coatability improver is not particularly limited, but may be an oil-soluble liquid polymer such as polybutene, mineral oil such as gasoline oil, inorganic particles such as silica or alumina, organic particles such as starch, surfactants, etc. Water-soluble polymers emulsified and dispersed in oil, various surfactants, and the like can be used. Moreover, these can also be mixed and used. Among these, oil-soluble liquid polymer and mineral oil are preferable because they can be removed together with the pore opening agent in the pore opening agent removing step (3) described later in the pore forming treatment step.
[0016]
The content of the coatability improver is not particularly limited, but is preferably 0.001 wt% or more and 50 wt% or less from the viewpoint of the effect of the coatability improver. More preferably, it is 0.01 wt% or more and 20 wt% or less, More preferably, it is 0.05 wt% or more and 10 wt% or less, Especially preferably, it is 0.1 wt% or more and 5 wt% or less.
The viscosity of the pore-opening agent applied to the polyethylene film is not particularly limited, but is preferably 20 mPa · s or more from the viewpoint of uniform application, and preferably 9000 mPa · s or less from the viewpoint of fluidity during application. More preferably, it is 100 mPa * s or more and 6000 mPa * s or less, More preferably, it is 200 mPa * s or more and 4000 mPa * s or less, Especially preferably, they are 300 mPa * s or more and 2000 mPa * s or less. These viscosities can be suitably controlled, for example, by adding a coating property improving agent.
[0017]
The contact angle of the pore-opening agent with respect to the polyethylene film is not particularly limited, but the value measured by the liquid method at 25 ° C. and analyzed by the θ / 2 method is preferably 100 ° or less from the viewpoint of permeability to the film. More preferably, it is 80 degrees or less, More preferably, it is 60 degrees or less. These contact angles can be suitably controlled, for example, by adding a coating property improving agent.
Although it does not specifically limit as a coating method to a polyethylene film of a pore opening agent, A well-known coating technique can be used. In the case of batch processing, commercially available brushes for applying paint, doctor blades, and the like can be used. In the case of continuous processing, a reverse coater, a direct roll coater, a knife coater, a die coater and the like can be used.
[0018]
When applying the pore-opening agent, the polyethylene film is preferably constrained in at least one axial direction. In the case of batch processing, there are a method of constraining with a metal frame, for example, and a method of constraining with a batch-type biaxial stretching machine. The apparatus normally used in the continuous production by the flat type | mold stretching of a film, such as the method to perform, a horizontal uniaxial stretching machine, and a simultaneous biaxial stretching machine, can be used. The horizontal uniaxial stretching machine and the simultaneous biaxial stretching machine are, for example, those capable of continuous film stretching by providing a pair of endless link devices equipped with a film gripping device on both sides of the film stretching passage. Can be mentioned. Among these, the method of constraining with a horizontal uniaxial stretching machine or a simultaneous biaxial stretching machine is preferable because the porous treatment can be continuously performed and thermal shrinkage described later can also be suppressed in the lateral direction. .
[0019]
The pore-opening agent may be applied on one side or both sides, but is preferably applied on both sides from the viewpoint of air permeability of the polyethylene microporous membrane.
The coating amount of the pore-opening agent is not particularly limited, but preferably 1 g / m ² or more and 9000 g / m ² or less, more preferably 10 g / m ² or more and 1000 g / m ² , and still more preferably 50 g / m ² or more and 500 g / m ^2. It is as follows. Adjust the coating amount so that the pore penetration rate obtained by dividing the membrane mass after penetration of the pore opening agent by the membrane mass before penetration of the pore opening agent is 1.1 times to 10 times. Is preferred.
[0020]
The temperature of the pore opening agent during application is not particularly limited, but is preferably 25 ° C. or higher from the viewpoint of fluidity of the pore opening agent, and preferably 200 ° C. or lower from the viewpoint of the flash point of the pore opening agent. More preferably, it is 50 degreeC or more and 180 degrees C or less, More preferably, they are 80 degreeC or more and 160 degrees C or less, Most preferably, they are 100 degreeC or more and 140 degrees C or less.
The heat shrinkage rate when heat shrinkage is unavoidable when a pore-opening agent is applied is preferably 30% or less from the viewpoint of securing the porosity of the polyethylene microporous membrane. More preferably, it is 20% or less, More preferably, it is 10% or less.
[0021]
(2) Opening Agent Penetration Step The opening agent penetration step is a step of allowing the applied pore opening agent to penetrate into the polyethylene film. The method of permeation varies depending on the permeability of the pore-opening agent to polyethylene. For example, a pore-opening agent having low permeability can be suitably permeated by heating the membrane. Although it does not specifically limit as a heating method of a film | membrane, Well-known techniques, such as a hot air heating and a plate heater, can be used. At this time, in order to prevent thermal contraction of the film due to heating, it is preferable to restrain in at least one axial direction. Moreover, the timing which restrains may be any before heating and during heating, and is not specifically limited. The method for constraining the membrane can be the same as the method (1) above, and the porous treatment can be performed continuously, and furthermore, the thermal shrinkage can be suppressed in the lateral direction, so A method of restraining in the biaxial direction with a stretching machine or a simultaneous biaxial stretching machine is preferred.
[0022]
The heat shrinkage rate when heat shrinkage is unavoidable by heating is preferably 30% or less from the viewpoint of securing the porosity of the polyethylene microporous membrane. More preferably, it is 20% or less, More preferably, it is 10% or less.
The heating temperature is not particularly limited, but is preferably 30 ° C. or higher in order to improve the permeability, and is preferably 200 ° C. or lower from the viewpoint of the flash point of the pore opening agent. More preferably, it is 50 degreeC or more and 180 degrees C or less, More preferably, it is 100 degreeC or more and 160 degrees C or less, Most preferably, they are 120 degreeC or more and 140 degrees C or less.
The heating time is not particularly limited, but is preferably 1 second or longer from the viewpoint of the heating effect, and preferably 10 minutes or shorter from the viewpoint of maintaining the film strength. More preferably, they are 5 seconds or more and 5 minutes or less, More preferably, they are 10 seconds or more and 3 minutes or less.
[0023]
(3) Opening agent removal step The method for removing the opening agent is not particularly limited. However, when paraffin oil or dioctyl phthalate is used as the opening agent, washing is performed with an organic solvent such as methylene chloride or methyl ethyl ketone (MEK). Then, it can be removed by heating and drying at a temperature below the fuse temperature of the obtained microporous film. When a low boiling point compound such as decalin is used as the pore opening agent, the microporous membrane can be removed simply by heating and drying. In any case, it is preferable to restrain the film in at least one axial direction in order to prevent deterioration of physical properties due to film shrinkage.
[0024]
The polyethylene microporous film obtained by the above production method may be subjected to heat treatment (heat setting) as necessary in order to enhance dimensional stability.
Moreover, it is also possible to form a crosslinked structure in order to improve the heat resistance of the microporous membrane at least once in any of the film preparation step and the porous treatment step. The timing for forming the crosslinked structure may be before, during or after any of the above steps, but from the viewpoint of preventing thermal shrinkage of the polyethylene microporous membrane, it is preferable to form a crosslinked structure after the film preparation step.
[0025]
A method for forming a crosslinked structure is not particularly limited, and there are a method using ionizing radiation, a method of adding a crosslinking agent, and the like. Of these, the method using ionizing radiation is preferred.
Examples of the ionizing radiation include electron beams, γ rays, ultraviolet rays, etc. Among them, electron beams are preferable. The electron beam irradiation dose is preferably 1 Mrad or more in order to obtain a sufficient crosslinking density, and preferably 200 Mrad or less in order to prevent a decrease in mechanical strength due to excessive irradiation. More preferably, it is 2 Mrad or more and 100 Mrad or less, More preferably, it is 5 Mrad or more and 50 Mrad or less.
[0026]
In the case of a method using ionizing radiation, it is known that deterioration with time is caused by residual radicals. However, polyethylene is obtained by forming a cross-linked structure by ionizing radiation before (3) pore-opening agent removing step in the pore-forming treatment step, and further heat-treating at a temperature of 110 ° C. or higher during or after the pore-penetrating agent penetration step. Without impairing the permeability and mechanical strength of the microporous membrane, it is possible to produce a polyethylene microporous membrane that deactivates residual radicals and does not deteriorate with time.
Microporous polyethylene film can be created by the above processes has a porosity of 20% or more to 80% or less, 25 [mu] m in terms of air permeability 3000 sec /0.1dm ³ / 25μm or less, 25 [mu] m in terms of puncture strength is 2N / 25 [mu] m It has the above properties and can be suitably used as a battery separator.
[0027]
【Example】
Hereinafter, the present invention will be described in more detail based on embodiments. The test methods shown in the examples are as follows.
(1) Viscosity (mPa · s)
Using an E-type viscometer (VISCONIC ED type) manufactured by Tokimec Co., Ltd., measurement was performed at 20 ° C. using a standard cone (1 ° 34 ′) as a rotor and a rotor rotation speed of 10 rpm.
[0028]
(2) Pore Agent Penetration Ratio After the membrane infiltrated with the pore-opening agent was cut to a size of 10 cm square, the mass was measured, and the value divided by the 10 cm square membrane mass before the penetration was determined as the pore agent penetration rate. did.
(3) Film thickness (μm)
It was measured with a digital constant pressure thickness measuring instrument (manufactured by Toyo Seiki Seisakusho: Model B-1, probe diameter φ5 mm, measurement pressure 62.4 kPa).
[0029]
(4) Porosity (%)
A microporous membrane was cut into a 10 cm square, and the average density ρ (g / cm ³ ) of the membrane was calculated from its volume and mass. It calculated using the following formula from the obtained numerical value.
Porosity (%) = 100 × (1−ρ / 0.95)
(5) 25μm in terms of air permeability (sec /0.1dm ³ / 25μm)
Multiply the value obtained by JIS P-8117 compliant Gurley air permeability meter (manufactured by Toyo Seiki Seisakusho Co., Ltd .: Model G-B2C) by 25 (μm) / film thickness (μm) to obtain a 25 μm equivalent air permeability. It was.
[0030]
(6) Puncture strength in terms of 25 μm (N / 25 μm)
At a measurement temperature of 25 ° C., a piercing test is performed using a KES-G5 handy compression tester manufactured by Kato Tech under the conditions of a radius of curvature of the needle tip of 0.5 mm and a piercing speed of 2 mm / sec. ). The puncture strength was multiplied by 25 (μm) / film thickness (μm) to obtain a normal puncture strength of 25 μm.
[0031]
[Example 1]
100 parts by mass of high density polyethylene (density 0.95, viscosity average molecular weight 250,000) and 0.1 part by mass of triethylene glycol-bis [3- (3-t-butyl- 5-methyl-4-hydroxyphenyl) propionate was charged into a twin screw extruder having a diameter of 40 mm and L / D = 30, kneaded at 220 ° C. and 20 rpm, and cast on a 30 ° C. roll from a hanger coat die. Thus, a sheet having a thickness of 600 μm was prepared. The film was subjected to crosslinking treatment at an acceleration voltage of 600 keV and an irradiation dose of 300 kGy using a continuous electron beam irradiation apparatus (ELECTRONSHOWER: manufactured by Denki Kogyo Co., Ltd.), and then the crosslinked sheet was used as a simultaneous biaxial stretching machine. The film was stretched at a temperature of 145 ° C. and a stretching ratio of 7 × 7 to obtain a polyethylene film. Next, the film was constrained to a stainless steel metal frame of 15 cm square on the outer frame and 12 cm square on the inner frame, and the coating amount on one side was 100 g / m ² with a commercially available brush for paint on the 12 cm square of the film. Liquid paraffin was applied on both sides as a pore opening agent. The liquid paraffin used was 50 parts by weight of liquid paraffin with a viscosity of 791 mPa · s (manufactured by Matsumura Oil Research Co., Ltd.) and 50 parts by weight of liquid paraffin with a viscosity of 151 mPa · s (manufactured by Matsumura Oil Co., Ltd.) using a three-one motor. The viscosity was 403 mPa · s. Next, the coating film was put into a hot air oven adjusted to 130 ° C., left for 2 minutes after the surface temperature (measured with a contact-type thermocouple) reached the ambient temperature, taken out, and cooled to room temperature. Next, liquid paraffin was removed with methylene chloride and dried, followed by heat setting in an oven adjusted to 110 ° C. for 1 minute to obtain a polyethylene microporous membrane.
[0032]
[Comparative Example 1]
Instead of applying liquid paraffin to the polyethylene film, the polyethylene film was restrained with a 12 cm square metal frame and immersed in liquid paraffin with a viscosity of 151 mPa · s, temperature-controlled at 127 ° C. in an oil bath, for 2 minutes. The same procedure as in Example 1 was performed.
[0033]
[Example 2]
The same procedure as in Example 1 was performed except that polybutene-added liquid paraffin having a viscosity of 173 mPa · s (manufactured by Matsumura Oil Research Co., Ltd.) was used as the pore opening agent, and the set temperature of the hot air oven was set to 128 ° C.
[0034]
[Comparative Example 2]
Instead of applying liquid paraffin to polyethylene film, polybutene-added liquid paraffin with a viscosity of 173 mPa · s that is temperature-controlled at 128 ° C. in an oil bath (made by Matsumura Oil Research Co., Ltd.) This was performed in the same manner as in Example 1 except that the method was immersed in 2 minutes.
[0035]
[Example 3]
Ultra high molecular weight polyethylene powder (UH850: manufactured by Asahi Kasei Corporation, density 0.94, viscosity average molecular weight 2 million) is compression-molded for 5 minutes at a temperature of 240 ° C. and a hydraulic pressure of 200 kg / cm ² , 12 cm square, film thickness A 600 μm sheet was obtained. Next, the film was simultaneously biaxially stretched 8 × 8 times at a stretching temperature of 146 ° C. using a batch type biaxial stretching machine (manufactured by Toyo Seiki Seisakusho Co., Ltd.) to obtain a polyethylene film having a thickness of 10 μm. It was. Next, the polyethylene film is constrained to a stainless steel metal frame having an outer frame of 15 cm square and an inner frame of 12 cm square so that the coated amount on one side is 100 g / m ² with a commercially available paint brush for the 12 cm square of the film. Then, liquid paraffin of 151 mPa · s (manufactured by Matsumura Oil Co., Ltd.) was applied on both sides as a pore opening agent. Next, the coating film was put in a hot air oven adjusted to 132 ° C., left for 2 minutes after the surface temperature (measured with a contact thermocouple) reached the ambient temperature, taken out, and cooled to room temperature. Next, liquid paraffin was removed with methylene chloride and dried, and then heat-set in an oven adjusted to 120 ° C. for 1 minute to obtain a polyethylene microporous membrane.
[0036]
[Comparative Example 3]
Instead of applying liquid paraffin to the polyethylene film, the method was carried out except that the polyethylene film was restrained with a metal frame and immersed in liquid paraffin with a viscosity of 151 mPa · s adjusted to 130 ° C in an oil bath for 2 minutes. Performed as in Example 3.
Table 1 shows the porosity treatment conditions in the examples and the physical properties of the obtained polyethylene microporous membrane.
[0037]
[Table 1]

[0038]
【The invention's effect】
By the method for producing a polyethylene microporous membrane according to the present invention, a polyethylene microporous membrane having physical properties equivalent to those of a conventional production method can be produced by a method having higher productivity than the conventional one.

Claims (7)

  A method for producing a polyethylene microporous membrane comprising a polyethylene film preparation step and a porous treatment step, wherein the porous treatment step comprises (1) applying a pore-opening agent to at least one surface of the polyethylene film, (2 (1) A method for producing a microporous polyethylene membrane, comprising the step of permeating a pore-opening agent into the polyethylene film, and (3) a step of removing the pore-opening agent that has penetrated the polyethylene film.   2. The method for producing a polyethylene microporous membrane according to claim 1, wherein the polyethylene film is an oriented polyethylene film.   The method for producing a microporous polyethylene membrane according to claim 1 or 2, wherein the pore-opening agent has a viscosity of 20 mPa · s or more and 9000 mPa · s or less.   The method for producing a polyethylene microporous membrane according to claim 1 or 2, wherein the pore-opening agent has a viscosity of 100 mPa · s or more and 6000 mPa · s or less.   The method for producing a polyethylene microporous membrane according to any one of claims 1 to 4, wherein the permeation method of (2) is a heat treatment.   6. The method for producing a polyethylene microporous membrane according to any one of claims 1 to 5, wherein the steps (1) and (2) are carried out in a state where a polyethylene film is constrained.   The polyethylene microporous membrane according to any one of claims 1 to 6, wherein the steps (1) and (2) are carried out using a horizontal uniaxial stretching machine or a simultaneous biaxial stretching machine for the polyethylene film. Production method.