JP2006045328A - Method for producing polyolefin fine porous film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing polyolefin fine porous film that has functions same as or more than those of the conventional porous film while using a plasticizer and facilitating the melt-kneading operation. <P>SOLUTION: Polyolefin is soaked in an excessive amount of plasticizer under heating so that at least a part of the polyolefin is allowed to swell with the plasticizer, then the excessive amount of plasticizer is separated by filtration. The resultant polyolefin including swelled polyolefin is melt-kneaded and extruded, then drawing and extraction of the plasticizer are carried out simultaneously to produce the polyolefin fine porous film. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  INDUSTRIAL APPLICABILITY The present invention is widely used as a separator for materials separation, permeation, and electrochemical reactors such as batteries and capacitors, and is particularly suitable for use as a separator for lithium ion batteries. Regarding the method.

  The microporous membrane made of polyolefin is widely used as a separator, selective permeation and separator for various substances, and examples thereof include microfiltration membranes, capacitor separators, battery separators and the like. Among them, it is particularly preferably used as a separator for a lithium ion battery mounted on a notebook personal computer, a mobile phone, a digital camera, or the like. As the reason, it is mentioned that it is excellent in mechanical strength, permeability, pore blockage, and heat resistance.

As a method for producing a polyolefin microporous membrane for these applications, in Patent Document 1, polyethylene is melt-extruded to form an extrusion precursor film, which is annealed at a temperature lower by 10 to 25 ° C. than the crystal melting point. Disclosed is a method in which uniaxial cold stretching is performed at 70 ° C. to a length of 120 to 160%, and then hot stretching is performed in the same uniaxial direction as cold stretching from 235 to 310% at a temperature 10 to 25 ° C. lower than the crystal melting point. Has been. This method does not require a special extruder to obtain a non-porous film, but it is difficult to use a high molecular weight polyolefin, and it flows easily and breaks at high temperatures above the film melting point. Further, since the nonporous film is locally torn and made porous, it is essentially difficult to make the film structure and film thickness uniform. Further, since the strength anisotropy of the film is large, it tends to tear easily.
In Patent Document 2, a gel sheet is formed from a solution obtained by heating and dissolving polyethylene in a solvent, the solvent amount in the gel sheet is removed to 10 to 80% by weight, and then heated and stretched, and then the residual solvent A method of removing is disclosed. This method requires a long time to prepare a homogeneous dilute solution of polyethylene, and it is necessary to remove excess solvent before stretching.

In Patent Document 3, a method of melting a polyolefin in an extruder, supplying a liquid solvent from the middle of the extruder, melt-kneading, extruding, forming a sheet, then heating and stretching, and then removing the residual solvent Is disclosed. This method does not require a long dilute solution preparation step, but requires equipment for supplying the solvent to the middle of the extruder. Moreover, in order to melt and knead the polyolefin and the solvent uniformly, a special extruder capable of kneading is required.
Patent Document 4 discloses a method in which a high-molecular-weight polyolefin blown film is formed, and the resulting polyolefin impermeable film is subjected to heat treatment to make it porous. This method requires a special inflation molding machine equipped with a special extruder to obtain a high molecular weight polyolefin film. Furthermore, since it is more porous than a nonporous film, it is essentially difficult to make the film structure and film thickness uniform.
Japanese Patent Publication No. 6-18915 Japanese Patent Publication No. 5-54495 Japanese Patent No. 3347835 Japanese Patent Laid-Open No. 10-258462

  An object of the present invention is to propose a production method capable of easily melt-kneading a microporous membrane having a performance equal to or higher than that of a conventional polyolefin microporous membrane using a plasticizer.

The present invention solves the above problems. That is, the present invention is as follows.
1. The polyolefin is immersed in an excess amount of a heated plasticizer, and at least a part of the polyolefin is swollen with the plasticizer, then the excess plasticizer is filtered, and the polyolefin containing the obtained swollen polyolefin is melt-kneaded and then extruded. A method for producing a microporous membrane made of polyolefin, comprising performing stretching and plasticizer extraction.
2. The polyolefin is immersed in an excess amount of a heated plasticizer, and at least a part of the polyolefin is swollen with the plasticizer, then the excess plasticizer is filtered, and the polyolefin containing the obtained swollen polyolefin is melt-kneaded and then extruded. A method for producing a polyolefin microporous membrane, characterized by cooling and solidifying, stretching at least in a uniaxial direction, and performing plasticizer extraction.

3. After immersing the polyolefin in an excessive amount of plasticizer heated, swell at least a part of the polyolefin with the plasticizer, filter the excess plasticizer, melt and knead the resulting polyolefin containing the swollen polyolefin and then extrude A method for producing a polyolefin microporous membrane, characterized in that a plasticizer is extracted by stretching in at least a uniaxial direction in a molten state.
4). The polyolefin to be swollen is in the form of pellets. ~ 3. The manufacturing method of the microporous membrane made from polyolefin in any one of these.

  In the present invention, since homogenization by melt-kneading is easy while using a plasticizer, a long-time solution adjustment step and a special extruder as in the prior art are not required. In addition, a polyolefin microporous membrane having performance equal to or higher than that of the conventional one can be obtained, whereby a highly safe and high performance secondary battery can be obtained.

Hereinafter, the present invention will be described in detail mainly focusing on preferred forms thereof.
Examples of the polyolefin used in the present invention include cyclic olefin copolymers such as polyethylene, polypropylene, methylpentene copolymer, ethylene / tetracyclododecene copolymer and ethylene / norbornene copolymer. From the viewpoint of stretchability, it is preferable to mainly use polyethylene. “Mainly used” means that 50% or more of the total polyolefin to be used is used, and one or more can be used.
The viscosity average molecular weight of polyethylene preferably used is preferably 100,000 or more, more preferably 200,000 or more from the viewpoints of film formation stability and film strength. Moreover, 2 million or less is preferable from a viewpoint of productivity, and 1 million or less is more preferable. Mainly preferably used polyethylene polymerization forms include single-stage polymerization, multi-stage polymerization, etc., but multi-stage polymerization is preferred from the viewpoint of membrane characteristics and productivity, and it is easy to obtain and stable in performance of polyethylene. From the viewpoint, two-stage polymerization is preferable.

In addition, homopolymers and copolymers can be used mainly as the polyethylene preferably used. However, when selecting the polyethylene of the copolymer, the comonomer content is preferably 2 mol% or less from the viewpoint of permeability. It is more preferably 1 mol% or less, and further preferably 0.6 mol% or less. Comonomers include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, And α-olefin comonomers such as 1-eicosene, cyclopentene, cyclohexene, cycloheptene, norbornene, 5-methyl-2-norbornene, tetracyclododecene, and 2-methyl-1.4,5.8-dimethano-1, It is represented by a cyclic olefin comonomer such as 2,3,4,4a, 5,8,8a-octahydronaphthalene or the general formula CH ₂ ═CHR (wherein R is an aryl group having 6 to 20 carbon atoms). Compounds such as styrene and vinylcyclohexane, 1,3-butadiene, 1,4-pentadiene, 1,5-hexadiene, 1,4-hexadiene, , 6-octadiene, 1,7-octadiene and cyclohexadiene, number 4-20 linear, although diene branched or cyclic and the like, preferably a α- olefin comonomer.
Examples of the polyethylene catalyst include Ziegler-Natta catalysts, Phillips catalysts, metallocene catalysts, and the like.

  In the present invention, it is essential to use a polyolefin swollen with a plasticizer (swelling polyolefin) as at least a part of the polyolefin used as a raw material for melt kneading. By using the swollen polyolefin, a long-time solution adjustment step as in the conventional method for producing a microporous membrane made of polyolefin using a plasticizer is unnecessary. Also, there is no need for equipment to supply the plasticizer in the middle of the extruder. Furthermore, since the plasticizer is already finely dispersed in the polyolefin by the swelling process, a twin screw extruder using special screw parts. There is no need for strong kneading, and for example, even uniform kneading with a full flight type single screw extruder can be made sufficiently uniform.

As the form of the polyolefin to be swollen, powder is also possible, but by using pellets, it is possible to prevent deterioration of the work environment such as dirt around the equipment due to powder scattering and suction into the body.
The swelling of the polyolefin is obtained by immersing the polyolefin in an excessive amount of heated plasticizer, and means a state in which the granular polyolefin such as powder or pellets retains the plasticizer while maintaining the granular shape. Usually, the particle size of polyolefin increases due to swelling. The excess amount means an amount that is twice or more the amount of plasticizer required for swelling.
As the plasticizer, a solvent having a high affinity with polyolefin and capable of forming a uniform solution at a temperature equal to or higher than the melting point of the polyolefin without being volatilized when melt-kneaded with the polyolefin is preferable. Specific examples include hydrocarbons such as liquid paraffin and paraffin wax, di-2-ethylhexyl phthalate (DOP), diisodecyl phthalate, diheptyl phthalate, etc. From the viewpoint of swelling, liquid paraffin, paraffin wax, etc. Hydrocarbons are preferred, and liquid paraffin is more preferred.

The temperature of the plasticizer to be heated is preferably 100 ° C. or higher, more preferably 110 ° C. or higher, further preferably 120 ° C. or higher, and most preferably 135 ° C. or higher from the viewpoint of swelling efficiency. Further, from the viewpoint of preventing the polyolefin from being welded, 200 ° C. or lower is preferable, 180 ° C. or lower is more preferable, and 160 ° C. or lower is further preferable.
The polyolefin charged into the heated plasticizer is preferably kept floating in the plasticizer by stirring or the like from the viewpoint of preventing the polyolefin from being welded.
The time for the swelling treatment is preferably 10 seconds or more, and more preferably 30 seconds or more in order to perform sufficient swelling. Further, from the viewpoint of processing efficiency, it is preferably 10 minutes or less, and more preferably 5 minutes or less.

Special care must be taken when swelling polyolefins that are easily dissolved in plasticizers, such as low melting point polyolefins, low molecular weight polyolefins, and powdered polyolefins. In that case, in order to prevent dissolution, the swelling treatment time may be shortened, and the preferred swelling treatment conditions may be deviated.
After the swelling treatment, the excess polyolefin is filtered to collect the swollen polyolefin. At this time, from the viewpoint of preventing the polyolefin from being welded, it is preferable to perform filtration after lowering the temperature in the swelling treatment system to be equal to or lower than the melting point of the polyolefin, and it is more preferable to perform filtration after the temperature is close to room temperature. You may remove the plasticizer adhering to the surface of swelling polyolefin as needed.

The plasticizer content in the swollen polyolefin is preferably 10 to 80 wt%, more preferably 20 to 70 wt%, and even more preferably 30 to 60 wt%, from the viewpoints of membrane permeability, film forming property, and swelling treatment efficiency.
In order to prevent deterioration during the swelling treatment, deterioration during melt kneading, and quality deterioration due to them, it is preferable that an antioxidant is blended. The concentration of the antioxidant is preferably 0.1 wt% or more, more preferably 0.2 wt% or more, based on the polyolefin weight. Moreover, from an economical viewpoint, 5.0 wt% or less is preferable and 3.0 wt% or less is more preferable.

  As the antioxidant, a phenolic antioxidant which is a primary antioxidant is preferable, and 2,6-di-t-butyl-4-methylphenol, pentaerythrityl-tetrakis- [3- (3,5- Di-t-butyl-4-hydroxyphenyl) propionate], octadecyl-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate, and the like. Secondary antioxidants can also be used in combination, such as tris (2,4-di-t-butylphenyl) phosphite, tetrakis (2,4-di-t-butylphenyl) -4,4- Examples thereof include phosphorus antioxidants such as biphenylene-diphosphonite and sulfur antioxidants such as dilauryl-thio-dipropionate.

Polymers other than polyolefin and other organic materials can also be blended without impairing the film forming property and within the range not impairing the effects of the present invention.
Furthermore, if necessary, known additives such as metal soaps such as calcium stearate and zinc stearate, UV absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments also impair film formation. It is possible to mix | blend and use in the range which does not impair the effect of this invention.

  Next, as a method of melt kneading and extruding, a polyolefin containing swollen polyolefin is introduced into a screw extruder such as a single screw extruder or twin screw extruder, a kneader, a mixer, etc., melt kneaded and homogenized, and a T-die Or extruded from a ring die. In this invention, the installation for adding a plasticizer is unnecessary. In addition, since the plasticizer is already finely dispersed in the polyolefin due to the swelling treatment, there is no need for toughening by a twin screw extruder using special screw parts, for example, weakness by a full flight type single screw extruder or the like. Even kneading can be made uniform enough. The temperature during melt kneading is preferably 140 ° C. or higher in order to obtain a uniform melt, and is preferably 300 ° C. or lower from the viewpoint of suppressing deterioration. Moreover, it is possible to suppress deterioration by performing melt-kneading in a nitrogen atmosphere.

The melted and kneaded melt may be passed through a screen to improve film quality.
Examples of a method for obtaining a stretched film from the extruded melt include a method in which the film is once cooled and solidified and then stretched in at least a uniaxial direction, and a method in which the stretched film is stretched in at least a uniaxial direction in a molten state.
As a method for stretching in at least a uniaxial direction after cooling and solidifying once, a method of forming a sheet and stretching with a stretching machine can be mentioned.
Examples of the method for forming a sheet include a method in which a plate-like melt extruded from a T-die is solidified by compression cooling. Examples of the cooling method include a method of directly contacting a cooling medium such as cold air and cooling water, a method of contacting a roll cooled by a refrigerant and a press, and a method of contacting a roll cooled by a refrigerant and a press. It is preferable in terms of excellent thickness control.

  As a method of stretching by a stretching machine, MD uniaxial stretching (MD means the machine direction) by a roll stretching machine using a roll, TD uniaxial stretching by a TD tenter (TD means a direction perpendicular to the machine direction), and simultaneous Examples thereof include simultaneous biaxial stretching with a biaxial tenter, and these can be performed in combination of one or more times. The stretching temperature is preferably 100 ° C. or higher, more preferably 110 ° C. or higher, from the viewpoint of stretchability. Further, from the viewpoint of film structure and film thickness uniformity, the melting point of the polyolefin mainly constituting the film is preferably not higher than the melting point (when polyethylene is mainly used, it is preferably 135 ° C. or lower), and more preferably 130 ° C. or lower. The draw ratio is a total surface magnification, and is preferably 8 times or more, more preferably 15 times or more, and further preferably 40 times or more from the viewpoint of film structure and film thickness uniformity. Further, from the viewpoint of production efficiency, 400 times or less is preferable, 200 times or less is more preferable, and 100 times or less is more preferable.

  Next, as a method for stretching at least in a uniaxial direction in a molten state, a plate-like melt extruded from a T-type die is directly subjected to MD uniaxial stretching by a roll stretching machine, or a cylinder extruded from an annular die. Examples of the method include simultaneous biaxial stretching by blowing air into the melt and expanding it. The temperature of the melt at the start of stretching is not lower than the melting point of the polyolefin mainly constituting the film, preferably 300 ° C. or lower, more preferably 250 ° C. or lower, and further preferably 200 ° C. or lower. This method is excellent in terms of thermal efficiency because it is drawn using heat during melt kneading. In addition, it can extend | stretch at the temperature below melting | fusing point of polyolefin which comprises a film | membrane from the middle of extending | stretching. The draw ratio is a total surface magnification, and is preferably 8 times or more, more preferably 15 times or more, and further preferably 40 times or more from the viewpoint of film structure and film thickness uniformity. Further, from the viewpoint of production efficiency, 400 times or less is preferable, and 200 times or less is more preferable.

The plasticizer extraction can be performed by immersing the stretched membrane in an extraction solvent. The extraction solvent is preferably a poor solvent for the polyolefin constituting the membrane and a good solvent for the plasticizer and having a boiling point lower than the melting point of the polyolefin constituting the membrane. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane, alcohols such as methanol, ethanol and isopropanol, ketones such as acetone and methyl ethyl ketone, ethers such as tetrahydrofuran, methylene chloride, 1,1, And organic solvents such as halogenated hydrocarbons such as 1-trichloroethane. It selects from these suitably, and is used individually or in mixture.
And a microporous film can be obtained by making it fully dry after plasticizer extraction.

Further, heat setting may be performed after the plasticizer is extracted and dried or / and stretched. The heat setting is an operation for reducing the shrinkage of the membrane by placing the microporous membrane at a high temperature for a predetermined time while performing dimension fixing or relaxation operation. The relaxation operation is an operation for reducing the film to MD and / or TD. The heat setting can be performed with a tenter or a roll stretching machine. When heat setting is performed after the stretching process, it can be performed with the same machine as the stretching process. The heat setting temperature is preferably 90 ° C. or higher, more preferably 100 ° C. or higher, and even more preferably 110 ° C. or higher from the viewpoint of the heat setting effect. Further, from the viewpoint of preventing deterioration of permeability, the melting point of the obtained microporous membrane is preferably not higher than the melting point, and in the case of a microporous membrane mainly composed of polyethylene, 135 ° C. or lower is preferable.
As long as the effects of the present invention are not impaired, surface treatment such as electron beam irradiation, plasma irradiation, surfactant coating, and chemical modification can be performed as necessary.

Next, preferred physical properties of the polyolefin microporous membrane obtained by the production method of the present invention will be described.
The viscosity average molecular weight of the microporous membrane obtained in the present invention is preferably 100,000 or more, more preferably 200,000 or more from the viewpoint of heat resistance. Moreover, 2 million or less is preferable from a viewpoint of productivity, and 1 million or less is more preferable.
The porosity is preferably 20% or more, more preferably 30% or more from the viewpoint of permeability. Further, from the viewpoint of film strength, 95% or less is preferable, 80% or less is more preferable, and 60% or less is more preferable.

The film thickness is preferably 3 μm or more, more preferably 5 μm or more from the viewpoint of film strength. Moreover, 100 micrometers or less are preferable from a viewpoint of an air permeability, and 50 micrometers is more preferable.
The air permeability is preferably 1 sec or more, and more preferably 50 sec or more. Moreover, 2000 sec or less is preferable from a viewpoint of the battery performance at the time of using for a battery, and 1000 sec or less is further more preferable.
The puncture strength is preferably 0.5 to 20.0 N / 20 μm, more preferably 1.0 to 20.0 N / 20 μm, from the viewpoint of battery productivity and battery safety when used for a battery.
Tensile strength is preferably 10 to 500 MPa, and more preferably 10 to 300 MPa for both MD and TD from the viewpoint of battery productivity and battery safety when used in a battery.
The hole closing temperature is preferably 140 ° C. or lower under a temperature rising condition of 2 ° C./min from the viewpoint of ensuring safety when the battery is heated. The thermal film breaking temperature is preferably 150 ° C. or higher under a temperature rising condition of 2 ° C./min from the viewpoint of ensuring safety during battery temperature rising.

Various physical properties used in the present invention were measured based on the following test methods.
(1) Viscosity average molecular weight Mv
Based on ASTM-D4020, the intrinsic viscosity [η] at 135 ° C. in a decalin solvent is determined. Mv of polyethylene was calculated by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67
(2) Melt index MI, MMI, HMI (g / 10 min)
Conforms to JIS K7210. MI was measured at a load of 2.16 kg, MMI at a load of 5 kg, and HMI at a load of 21.6 kg.
Polyethylene was measured at a temperature of 190 ° C, and polypropylene was measured at a temperature of 230 ° C.
(3) Density (g / cm ³ )
It is a value measured according to JIS K7112.

(4) Film thickness (μm)
The measurement was performed at a room temperature of 23 ° C. using a micro thickness measuring instrument manufactured by Toyo Seiki, KBM (trademark).
(5) Porosity (%)
A sample of 10 cm × 10 cm square was cut out from the microporous membrane, its volume (cm ³ ) and mass (g) were determined, and calculated from these and the film density (g / cm ³ ) using the following formula.
Porosity = (volume−mass / film density) / volume × 100
The film density was calculated at a constant 0.95.
(6) Air permeability (sec)
Based on JIS P-8117, it measured with the Gurley type air permeability meter by the Toyo Seiki Co., Ltd. and G-B2 (trademark).

(7) Puncture strength (N / 20μm)
Using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech, the maximum puncture is performed by performing a puncture test in a 25 ° C. atmosphere at a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec. Raw puncture strength (N) was obtained as a load. By multiplying this by 20 (μm) / film thickness (μm), the 20 μm film thickness equivalent puncture strength (N / 20 μm) was calculated.
(8) Tensile strength (MPa) of MD and TD
Measured using a tensile tester manufactured by Shimadzu Corporation, Autograph AG-A (trademark), and the tensile strength at break of MD and TD was determined by dividing the strength at break by the cross-sectional area of the sample before the test. . The measurement conditions are: temperature: 23 ± 2 ° C., sample shape: width 10 mm × length 100 mm, distance between chucks: 50 mm, tensile speed: 200 mm / min.

(9) Hole blockage temperature (° C) and thermal film breaking temperature (° C)
FIG. 1A shows a schematic diagram of a measuring apparatus. 1 is a microporous film, 2A and 2B are 10-micrometer-thick nickel foils, and 3A and 3B are glass plates. 4 is an electric resistance measuring device (LCR meter “AG-4411” (trademark) manufactured by Ando Electric Co., Ltd.), which is connected to the nickel foils 2A and 2B. A thermocouple 5 is connected to the thermometer 6. A data collector 7 is connected to the electric resistance device 4 and the thermometer 6. 8 is an oven that heats the microporous membrane.

More specifically, as shown in FIG. 1 (B), the microporous film 1 is overlaid on the nickel foil 2A, and the “Teflon (registered trademark)” tape (shaded portion in the figure) is vertically attached to the nickel foil 2A. Fix it. The microporous membrane 1 is impregnated with a 1 mol / liter lithium borofluoride solution (solvent: propylene carbonate / ethylene carbonate / γ-butyllactone = 1/1/2) as an electrolytic solution. As shown in FIG. 1 (C), “Teflon (registered trademark)” tape (shaded portion in the figure) is pasted on the nickel foil 2B, and masking is performed by leaving a window portion of 15 mm × 10 mm in the central portion of the foil 2B. It is.
The nickel foil 2A and the nickel foil 2B are overlapped so as to sandwich the microporous film 1, and two nickel foils are sandwiched by the glass plates 3A and 3B from both sides thereof. At this time, the window portion of the foil 2 </ b> B and the porous film 1 come to face each other.

Two glass plates are fixed by pinching with a commercially available double clip. The thermocouple 5 is fixed to the glass plate with “Teflon (registered trademark)” tape.
Temperature and electric resistance are continuously measured with such an apparatus. The temperature is raised from 25 ° C. to 200 ° C. at a rate of 2 ° C./min, and the electric resistance value is measured at an alternating current of 1 kHz. The pore closing temperature is defined as the temperature at which the electrical resistance value of the microporous membrane reaches 10 ³ Ω. The temperature at which the electrical resistance value again fell below 10 ³ Ω after closing the hole was taken as the thermal film breaking temperature.

The present invention will be described based on examples.
[Example 1]
In 100 parts by weight of liquid paraffin (kinematic viscosity 7.59 × 10 ⁻⁵ m ² / s at 37.78 ° C.) previously heated to 140 ° C. in an autoclave equipped with a stirrer, pentaerythrityl-tetrakis- [3- ( 3,5-di-tert-butyl-4-hydroxyphenyl) propionate], 5 parts by weight of pellet-shaped polyethylene having an MMI of 0.15, an HMI of 12, a viscosity average molecular weight of 300,000 and a density of 0.960 Charged and stirred for 3 minutes. Next, in order to make the temperature in the autoclave lower than the melting point of polyethylene (135 ° C.), 100 parts by weight of liquid paraffin at room temperature (kinetic viscosity of 7.59 × 10 ⁻⁵ m ² / s at 37.78 ° C.) was added. did. Next, the content was collected by filtration with a 30-mesh wire mesh, allowed to cool to room temperature, and then swollen from the excess liquid paraffin on the surface to collect swollen polyethylene (swelled polyethylene). From the change in weight of polyethylene before and after the swelling treatment, the swelling ratio (weight ratio of liquid paraffin in the swollen polyethylene) was calculated to be 40 wt%.

The obtained swollen polyethylene was supplied to a single screw extruder of 20 mmφ equipped with a full flight type screw. The set temperature was 200 ° C., the screw rotation speed was 80 rpm, and the discharge rate was 7 kg / h.
After the melt-kneaded material was extruded from a T-die, it was immediately cooled and solidified by a press molding machine cooled to 25 ° C., to obtain a 500 μm sheet.
Next, using a small biaxial stretching machine, the film was stretched at 115 ° C. to an MD magnification of 8.5 times and a TD magnification of 8.5 times.
Then, it immersed in methylene chloride, liquid paraffin was extracted and removed, and the methylene chloride was dried and removed to obtain a microporous membrane. The physical properties of the obtained microporous membrane are shown in Table 1.

[Example 2]
Swelled polyethylene was obtained in the same manner as in Example 1 except that liquid paraffin heated to 160 ° C. (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was used. From the weight change of the polyethylene before and after the swelling treatment, the swelling ratio was calculated to be 50 wt%.
96 wt% of the obtained swollen polyethylene and 4 wt% of pellet-like polypropylene having an MI of 0.5 and a density of 0.90 were blended in a blender. The obtained blend was melt-kneaded in the same manner as in Example 1, extruded from a T-type die, and immediately cooled and solidified by a press molding machine cooled to 25 ° C. to obtain a 1000 μm sheet.
Next, using a small biaxial stretching machine, the film was stretched at 120 ° C. to an MD magnification of 7.0 times and a TD magnification of 7.0 times.
Then, it immersed in methylene chloride, liquid paraffin was extracted and removed, and the methylene chloride was dried and removed to obtain a microporous membrane. The physical properties of the obtained microporous membrane are shown in Table 1.

[Example 3]
Containing 1 wt% of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate], MMI 0.05, HMI 5, viscosity average molecular weight 500,000, density 0.958 Swelled polyethylene was obtained in the same manner as in Example 2 except that pellet-shaped polyethylene was used. From the weight change of the polyethylene before and after the swelling treatment, the swelling ratio was calculated to be 50 wt%.
The obtained swollen polyethylene was melt-kneaded in the same manner as in Example 1 and extruded from an annular die (gap size 1350 μm). And expansion | swelling extending | stretching was performed by blowing air in a melt (temperature of 200 degreeC), and the 33-micrometer-thick stretched film was obtained.
Then, it immersed in methylene chloride, liquid paraffin was extracted and removed, and the methylene chloride was dried and removed to obtain a microporous membrane. The physical properties of the obtained microporous membrane are shown in Table 1.

[Comparative Example 1]
In an autoclave equipped with a stirrer, MMI 0.15, HMI12, viscosity average molecular weight 300,000, powdery polyethylene 60 wt% with a density of 0.960 and pentaerythrityl-tetrakis- [3- (3,5-di-t- (Butyl-4-hydroxyphenyl) propionate] 0.3 wt% and liquid paraffin (kinematic viscosity at 37.78 ° C. 7.59 × 10 ⁻⁵ m ^{2 / s)} 39.7 wt% were added and stirred at room temperature for 3 minutes. . During the work, the powdered polyethylene partly flew and scattered around, so cleaning was performed after the mixing work.
The obtained polyethylene slurry mixture was fed to the same extruder as in Example 1. The set temperature was 200 ° C., the screw rotation speed was 80 rpm, and the discharge rate was 7 kg / h.
After the melt-kneaded material was extruded from a T-die, it was immediately cooled and solidified by a press molding machine cooled to 25 ° C., to obtain a 500 μm sheet.
Next, an attempt was made to draw at a MD magnification of 8.5 times and a TD magnification of 8.5 times at 115 ° C. using a small biaxial stretching machine, but the film was broken because of insufficient homogenization by melt kneading.

  INDUSTRIAL APPLICABILITY The present invention is used as a method for producing a microporous membrane made of polyolefin, which is used for separation of substances, selective permeation, separators, and the like, and particularly suitably used as a separator for lithium ion batteries and the like.

It is a figure which shows the apparatus which measures a hole obstruction | occlusion temperature (degreeC) and a thermal membrane breakage temperature (degreeC), (A) is the schematic, (B) is a plane which shows the sample which affixed the microporous film on nickel foil FIG. 4C is a plan view showing the nickel foil masked leaving the window portion.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Microporous film 2A Nickel foil 2B Nickel foil 3A Glass plate 3B Glass plate 4 Electrical resistance measuring device 5 Thermocouple 6 Thermometer 7 Data collector 8 Oven

Claims (4)

  After immersing the polyolefin in an excessive amount of heated plasticizer, at least a portion of the polyolefin is swollen by the plasticizer, the excess plasticizer is filtered, and the polyolefin containing the obtained swollen polyolefin is melt-kneaded and then extruded. A method for producing a microporous membrane made of polyolefin, comprising performing stretching and plasticizer extraction.   After immersing the polyolefin in an excessive amount of heated plasticizer, at least a portion of the polyolefin is swollen by the plasticizer, the excess plasticizer is filtered, and the polyolefin containing the obtained swollen polyolefin is melt-kneaded and then extruded. A method for producing a microporous polyolefin membrane, characterized by cooling and solidifying, stretching at least in a uniaxial direction, and extracting a plasticizer.   After immersing the polyolefin in an excessive amount of heated plasticizer, at least a portion of the polyolefin is swollen by the plasticizer, the excess plasticizer is filtered, and the polyolefin containing the obtained swollen polyolefin is melt-kneaded and then extruded. A method for producing a polyolefin microporous membrane, comprising stretching in at least a uniaxial direction in a molten state and extracting a plasticizer.   The method for producing a polyolefin microporous membrane according to any one of claims 1 to 3, wherein the polyolefin to be swollen is in the form of pellets.

Images