JP2003103626A - Polyolefin microporous film and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a polyolefin microporous film excellent not only in the balance between voids, permeability and mechanical strength but also in dimensional stability. SOLUTION: A polyolefin containing polyethylene with a weight average molecular weight of 5×10<5> or more as an essential component and a liquid solvent are melted and kneaded to obtain a molten kneaded material which is, in turn, extruded from a die to be cooled to obtain a gel-like molded article. This gel-like molded article is biaxially stretched and the obtained stretched article is washed using a washing solvent of which the surface tension at 25 deg.C is not more than 24 mN/m to remove the liquid solvent. The stretched article after washing is again stretched at least in a uniaxial direction at a temperature below the crystal dispersing temperature of the polyolefin and subsequently thermally fixed at the temperature of the crystal dispersing temperature-melting point of the polyolefin so as not to generate a dimensional change in both of a machine direction (MD) and a lateral direction (TD).

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a microporous polyolefin membrane and a method for producing the same, and more particularly to a microporous polyolefin membrane having an excellent balance of porosity, air permeability and mechanical strength, and excellent dimensional stability. The manufacturing method is related.

[0002]

BACKGROUND OF THE INVENTION Polyolefin microporous membranes include separators for electrolytic capacitors, including separators for batteries used in lithium secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, polymer batteries, etc. It is widely used in various filters such as reverse osmosis filtration membranes, ultrafiltration membranes and microfiltration membranes, moisture-permeable waterproof clothing, and medical materials. When the polyolefin microporous membrane is used as a battery separator, particularly as a lithium ion battery separator, its performance is deeply related to battery characteristics, battery productivity and battery safety. Therefore, excellent air permeability, mechanical properties, dimensional stability, shutdown characteristics,
Meltdown characteristics are required.

With regard to battery characteristics, it is desired to improve discharge characteristics in a low temperature range, increase output, improve cycle characteristics, etc. Therefore, the pore diameter of a separator used in various battery systems,
Optimization of porosity and air permeability is required. Further, in terms of battery productivity, high mechanical strength is required because it is desired to improve the efficiency of the battery assembly process. Furthermore, regarding battery safety, it is desired to reduce defects such as voltage drop caused by pressure of impurities mixed on the electrodes and to improve high-temperature storability. Therefore, improvement of air permeability, mechanical strength and dimensional stability is required. It has been demanded.

For example, JP-A-60-242035, JP-A-60-255107 and JP-A-63-273651 disclose methods for increasing the strength of a microporous membrane. A method of manufacturing the membrane is proposed. These methods are methods in which an ultrahigh molecular weight polyolefin and various plasticizers or solvents are melt-kneaded, the resulting melt-kneaded product is extruded to form a gel-like sheet, and then stretched. However, since these methods use an ultra-high molecular weight polyolefin, a large amount of a plasticizer or solvent must be used in order to extrude a melt-kneaded product, and it takes time to remove the plasticizer or solvent, so productivity is high. However, the strength of the obtained microporous membrane was not sufficient.

On the other hand, JP-A-3-64334 proposes a method using a polyolefin composition containing an ultrahigh molecular weight polyolefin and having a value of (weight average molecular weight / number average molecular weight) within a specific range. ing. According to this method, it is possible to increase the concentration of the melt-kneaded product, that is, to reduce the amount of solvent used, and the microporous membrane obtained has both excellent strength and water permeability.

However, recently, the demands on the characteristics of batteries such as lithium ion batteries have become more severe, and it is required to further improve all the mechanical strength, permeability and dimensional stability of the microporous membrane. . However, in general, increasing the porosity to increase the permeability decreases the mechanical strength, and increasing the draw ratio to increase the mechanical strength decreases the dimensional stability. A microporous membrane that simultaneously satisfies the three physical properties of stability has not been obtained.

On the other hand, Japanese Patent Laid-Open No. 2000-248094 discloses that a kneaded product of ultrahigh molecular weight polyethylene and a solvent is formed into a gel-like sheet, rolled and stretched and subjected to solvent removal treatment, and then the melting point of the ultrahigh molecular weight polyolefin is measured. It proposes a manufacturing method in which heat-setting treatment is performed at a high temperature and a melting point + 10 ° C or lower.
However, the microporous membrane produced by this method has a puncture strength of 88.
89-10329 mN / 25μm, high but heat shrinkage is 9.8-
Since it is as large as 13.4%, when it is used as a lithium battery separator, there is a problem that an electrode short circuit easily occurs due to a temperature rise.

Further, JP-A-2001-081221 discloses that a kneaded product of a polyolefin having a viscosity average molecular weight of 150,000 to 1,000,000 and a plasticizer is formed into a gel-like sheet, which is biaxially stretched and then treated with a deplasticizer and then at least It proposes a manufacturing method in which stretching is performed in a uniaxial direction, and then TD relaxes the shrinking force. The microporous membrane obtained by this method has excellent puncture strength and tensile strength, but the improvement in heat shrinkage is only in the TD direction, and the air permeability is within a satisfactory range of about 800 seconds / 100 cc. Absent. Moreover, if the porosity is further increased by using this method, there is a problem that the heat shrinkage rate is further increased.

Therefore, an object of the present invention is to solve the above-mentioned drawbacks of the prior art, to produce a polyolefin microporous membrane having an excellent balance of porosity, air permeability and mechanical strength and excellent dimensional stability. Is to provide.

[0010]

As a result of earnest research in view of the above object, the inventors of the present invention melt-kneaded a polyolefin containing a polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and a liquid solvent, The resulting melt-kneaded product is extruded from a die and the gel-like molded product obtained by cooling is biaxially stretched. The stretched product has a surface tension of 2 at 25 ° C.
After removing the liquid solvent using a washing solvent (A) of 4 mN / m or less, the washed stretched product is stretched again in at least one axial direction at a temperature lower than the crystal dispersion temperature of the polyolefin,
Then, the above problems can be solved by heat setting treatment so that there is no dimensional change in both the machine direction (MD) and the transverse direction (TD: direction orthogonal to the machine direction) at a temperature of the crystal dispersion temperature to the melting point of the polyolefin. And found the present invention.

That is, the polyolefin microporous membrane of the present invention is composed of a polyolefin containing polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and having a porosity of 50.
~ 95%, machine direction (MD) after exposure at 105 ° C for 8 hours
And the transverse (TD) heat shrinkage rate is 5% or less, the puncture strength is 7840 mN / 25 μm or more, and the air permeability in terms of film thickness 20 μm is 20 to 500 seconds / 100 cc. And

The first method for producing the polyolefin microporous membrane of the present invention is to melt-knead a polyolefin containing polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and a liquid solvent, and obtain the melt-kneaded mixture. Extruding things from the die,
The gel-like molded product obtained by cooling was biaxially stretched, and the stretched product had a surface tension of 24 mN at 25 ° C.
After removing the liquid solvent using a cleaning solvent (A) of less than / m,
The washed stretched product is again stretched in at least uniaxial direction at a temperature lower than the crystal dispersion temperature of the polyolefin, and then in both the machine direction (MD) and the transverse direction (TD) at a temperature of the crystal dispersion temperature to the melting point of the polyolefin. Both are characterized by heat-setting treatment so that there is no dimensional change.

The second method for producing the polyolefin microporous membrane of the present invention is to melt-knead a polyolefin containing polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and a liquid solvent, and obtain the melt-kneaded mixture. Extruding things from the die,
The gel-like molded product obtained by cooling was biaxially stretched, and the stretched product had a surface tension of 24 mN at 25 ° C.
/ M or less washing solvent (A) to remove the liquid solvent to produce at least two microporous membrane (A), then at least one of the microporous membrane (A) of the polyolefin At least one of the microporous membrane (B) and at least one of the microporous membrane (A) is prepared by re-stretching in at least one axial direction at a temperature lower than the crystal dispersion temperature to produce a microporous membrane (B). The resulting laminated film (AB) is laminated by bonding with one sheet, and the dimensional change in both the machine direction (MD) and the transverse direction (TD) at the temperature of the crystal dispersion temperature to the melting point of the polyolefin. It is characterized in that it is heat-set so as not to exist.

The third method for producing the polyolefin microporous membrane of the present invention is to melt-knead a polyolefin containing polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and a liquid solvent, and obtain the melt-kneaded mixture. Extruding things from the die,
The gel-like molded product obtained by cooling was biaxially stretched, and the stretched product had a surface tension of 24 mN at 25 ° C.
At least two microporous membranes (A) are prepared by removing the liquid solvent with a washing solvent (A) of not more than 1 / m, and then at least two of the microporous membranes (A) are joined. By laminating, the resulting laminated film (A) is stretched again at least uniaxially at a temperature below the crystal dispersion temperature of the polyolefin, and then the machine direction (MD) at a temperature of the crystal dispersion temperature to the melting point of the polyolefin and It is characterized by heat setting so that there is no dimensional change in both the transverse direction (TD).

The resin raw material has a weight average molecular weight of 5 × 10 ⁵
Use of the above-mentioned polyolefin containing polyethylene as an essential component and use of a liquid solvent when preparing a melt-kneaded product are effective in improving puncture strength and dimensional stability. The piercing strength is improved by biaxially stretching (primarily stretching) the gel-like molded product obtained by extruding the melt-kneaded product through a die and cooling it. When the liquid solvent is removed, the washing solvent (A) having a surface tension of 24 mN / m or less at 25 ° C is used to shrink and densify the network structure by the washing solvent surface tension in the washing step and / or the drying step. Since this can be suppressed, the porosity and air permeability are improved. Further, by re-stretching (secondarily stretching) the stretched product after washing at a temperature lower than the crystal dispersion temperature of the polyolefin, a part of the fine fibrils is stretched, so that the pore size is expanded, and the porosity and the air permeability are increased. improves. After that, a heat setting treatment is performed at a temperature between the crystal dispersion temperature and the melting point to improve the dimensional stability without impairing the porosity and the air permeability. As a result, a microporous polyolefin membrane having an excellent balance of porosity, air permeability and puncture strength and excellent dimensional stability can be obtained. In the second method, the non-restretched membrane (microporous membrane (A)) and the restretched membrane (microporous membrane (B)) are laminated to form a porosity, an air permeability and a size. It is possible to balance stability.

The primary stretching is preferably simultaneous biaxial stretching, and the stretching magnification is preferably at least 3 times or more in any direction, and the area magnification is preferably 20 times or more. The primary stretching temperature is preferably not higher than the melting point of the polyolefin + 10 ° C., more preferably in the range from the crystal dispersion temperature to below the crystal melting point. On the other hand, the stretching temperature range of the secondary stretching is preferably 0 ° C or higher and lower than 90 ° C, more preferably 20 ° C or higher and lower than 85 ° C. By performing the secondary stretching at 85 ° C or lower, the effect of enlarging the pore size becomes large. The preferred temperature range for the heat setting treatment is 110 to 130 ° C. Further, it is preferable to perform a heat relaxation treatment (heat shrinkage treatment) before and / or after the heat setting treatment. This can further improve the heat shrinkage rate.

In order for the microporous polyolefin membrane to obtain more excellent properties, the polyolefin preferably satisfies the following conditions (1) to (7). (1) Weight average molecular weight contained in the above polyolefin 5 × 1
Polyethylene of 0 ⁵ or higher is ultra high molecular weight polyethylene. (2) The weight average molecular weight of the ultrahigh molecular weight polyethylene described in (1) above is 1 × 10 ⁶ to 15 × 10 ⁶ . (3) The weight average molecular weight of the ultra high molecular weight polyethylene described in (1) or (2) above is 1 × 10 ^{6 to} 5 × 10 ⁶ . (4) The above-mentioned polyolefin includes ultra-high molecular weight polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more and weight average molecular weight of 1 × 10 ⁴
Above is a composition with polyethylene of less than 5 × 10 ^{5 or} less. (5) The polyethylene having a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ^{5 in} the polyethylene composition described in (4) above is selected from high-density polyethylene, medium-density polyethylene, low-density polyethylene and linear low-density polyethylene. At least one selected from the group consisting of (6) The polyolefin composition as described in (4) or (5) above is an ultra high molecular weight polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more and a high density polyethylene having a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ^5. Consists of. (7) The polyethylene composition according to any one of (4) to (6) has an Mw / Mn of 5 to 300.

The removal of the liquid solvent is preferably carried out in two or more washing steps using a washing solvent, in which case the surface tension at 25 ° C. is at least 2 in the final washing step.
It is preferable to use a washing solvent (A) of 4 mN / m or less. This improves the cleaning effect and improves the porosity, air permeability, and dimensional stability of the polyolefin microporous film. The washing solvent (A) is preferably used within a temperature range where the surface tension is 24 mN / m or less.

In order for the microporous polyolefin membrane to obtain more excellent characteristics, the washing solvent (A) preferably satisfies the following conditions (8) to (14). (8) The surface tension becomes 20 mN / m or less at 25 ° C. (9) Hydrofluorocarbon, hydrofluoroether, cyclic hydrofluorocarbon, perfluorocarbon, perfluoroether, normal paraffin having 5 to 10 carbon atoms, isoparaffin having 6 to 10 carbon atoms, aliphatic ether having 6 or less carbon atoms, cyclopentane, etc. Cycloparaffin, 2-pentanone, methanol, ethanol, 1-propanol, 2-propanol, tertiary butanol, isobutanol, 2-pentanol, propyl acetate, tertiary butyl acetate, secondary butyl acetate, ethyl acetate, isopropyl acetate, It is at least one selected from the group consisting of isobutyl acetate, methyl formate, ethyl formate, isopropyl formate, isobutyl formate, and ethyl propionate. (10) C ₅ H ₂ F ₁₀ chain hydrofluorocarbon represented by the composition formula, C ₄ F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H ₅ hydrofluoroether represented by the composition formula, C ₅ H ₃ F Cyclic hydrofluorocarbon represented by the composition formula of ₇ , C ₆ F ₁₄ and perfluorocarbon represented by the composition formula of C ₇ F ₁₆ , and C ₄ F ₉ OCF ₃
And at least one fluorine-based compound selected from the group consisting of perfluoroethers represented by the composition formula C ₄ F ₉ OC ₂ F ₅ . (11) At least one normal paraffin selected from the group consisting of normal pentane, normal hexane, normal heptane, normal octane, normal nonane, and normal decane. (12) 2-methylpentane, 3-methylpentane, 2,2-dibutylbutane, 2,3-dibutylbutane, 2-methylhexane, 3-methylhexane, 3-ethylpentane, 2,2-dimethylpentane, 2 , 3-dimethylpentane, 2,4-dimethylpentane, 3,3-dimethylpentane, 2,2,3-trimethylbutane, 2-methylheptane, 3-methylheptane, 2,
2-dimethylhexane, 2,3-dimethylhexane, 2,5-
Dimethylhexane, 3,4-dimethylhexane, 2,2,3-trimethylpentane, 2,2,4-trimethylpentane, 2,3,3
-Trimethylpentane, 2,3,4-trimethylpentane, 2
-Methyloctane, 2,2,5-trimethylhexane, 2,3,5
-At least one isoparaffin selected from the group consisting of trimethylhexane, 2-methylnonane and 2,3,5-trimethylheptane. (13) At least one ether selected from the group consisting of diethyl ether, butyl ethyl ether, dipropyl ether and diisopropyl ether. (14) At least one selected from the group consisting of a mixture of water and an aliphatic alcohol having 3 or less carbon atoms, which is blended so that the surface tension at 25 ° C. is 24 mN / m or less.

The washing is preferably carried out by two or more washing steps, in which case a step of using a washing solvent (washing solvent (B)) other than the washing solvent (A) may be included. At this time, it is preferable to use the washing solvent (A) in the final washing step. As the washing solvent (B), it is preferable to use at least one selected from the group consisting of a readily volatile solvent and a non-aqueous solvent having a boiling point of 100 ° C. or higher and a flash point of 0 ° C. or higher.

In order for the microporous polyolefin membrane to obtain more excellent properties, the washing solvent (B) preferably satisfies the following conditions (15) to (27). (15) At least one selected from the group consisting of methylene chloride, carbon tetrachloride, ethane trifluoride, methyl ethyl ketone, pentane, hexane, heptane, diethyl ether and dioxane. (16) Normal paraffins having 8 or more carbon atoms, normal paraffins having 5 or more carbon atoms in which at least a part of hydrogen atoms are replaced by halogen atoms, isoparaffins having 8 or more carbon atoms, cycloparaffins having 7 or more carbon atoms, and hydrogen atoms Cycloparaffins having 5 or more carbon atoms, at least some of which are substituted with halogen atoms, aromatic hydrocarbons of 7 or more carbon atoms, aromatic hydrocarbons having 6 or more carbon atoms, in which at least some hydrogen atoms are substituted with halogen atoms , An alcohol having 5 to 10 carbon atoms in which a part of hydrogen atoms may be replaced with a halogen atom, an ester and an ether having 7 to 14 carbon atoms in which a part of hydrogen atoms may be replaced with a halogen atom, and It is at least one selected from the group consisting of ketones having 5 to 10 carbon atoms. (17) The normal paraffin having 8 or more carbon atoms has a carbon number of 8 to 12, and is preferably at least one selected from the group consisting of normal octane, normal nonane, normal decane, normal undecane, and normal dodecane. is there. (18) Normal paraffins having 5 or more carbon atoms in which at least a part of the hydrogen atoms are substituted with halogen atoms are 1-
Chloropentane, 1-chlorohexane, 1-chloroheptane, 1-chlorooctane, 1-bromopentane, 1-bromohexane, 1-bromoheptane, 1-bromooctane,
1,5-dichloropentane, 1,6-dichlorohexane and 1,
At least one selected from the group consisting of 7-dichloroheptane. (19) The isoparaffins having 8 or more carbon atoms are 2,3,4-trimethylpentane, 2,2,3-trimethylpentane, 2,2,5
-Trimethylhexane, 2,3,5-trimethylhexane,
It is at least one selected from the group consisting of 2,3,5-trimethylheptane and 2,5,6-trimethyloctane. (20) The cycloparaffins having 7 or more carbon atoms are cycloheptane, cyclooctane, methylcyclohexane, cis- and trans-1,2-dimethylcyclohexane, cis- and trans-1,3-dimethylcyclohexane and cis- and trans. It is at least one selected from the group consisting of -1,4-dimethylcyclohexane. (21) The above-mentioned cycloparaffin having 5 or more carbon atoms in which a part of hydrogen atoms are substituted with halogen atoms is at least one selected from the group consisting of chlorocyclopentane and chlorocyclohexane. (22) The aromatic hydrocarbon having 7 or more carbon atoms is at least one selected from the group consisting of toluene, orthoxylene, metaxylene and paraxylene. (23) Aromatic hydrocarbons having 6 or more carbon atoms in which some of the hydrogen atoms have been replaced by halogen atoms are chlorobenzene, 2
-Chlorotoluene, 3-chlorotoluene, 4-chlorotoluene, 3-chloroorthoxylene, 4-chloroorthoxylene, 2-chlorometaxylene, 4-chlorometaxylene, 5-chlorometaxylene and 2-chloroparaxylene At least one selected from the group consisting of (24) Alcohol having 5 to 10 carbon atoms in which a part of the hydrogen atoms may be replaced with a halogen atom is isopentyl alcohol, tertiary pentyl alcohol, cyclopentanol, cyclohexanol, 3-methoxy-1-
Butanol, 3-methoxy-3-methyl-1-butanol, propylene glycol normal butyl ether and
At least one selected from the group consisting of 5-chloro-1-pentanol. (25) The ester having 7 to 14 carbon atoms in which a part of the hydrogen atoms may be replaced with a halogen atom is diethyl carbonate, diethyl maleate, normal propyl acetate, normal butyl acetate, isopentyl acetate, 3-methoxy acetate. It is at least one selected from the group consisting of butyl, 3-methoxy-3-methylbutyl acetate, ethyl normal butyrate, ethyl normal valerate and 2-chloroethyl acetate. (26) The ether having 7 to 14 carbon atoms in which a part of the hydrogen atoms may be replaced with a halogen atom is at least one selected from the group consisting of normal butyl ether, diisobutyl ether and bischloroethyl ether. (27) The above-mentioned ketone having 5 to 10 carbon atoms is 2-pentanone, 3
-Pentanone, 2-hexanone, 3-hexanone, cyclopentanone and at least one selected from the group consisting of cyclohexanone.

In the washing, the washing solvent (A) alone or the washing solvent (A) and the washing solvent (B) may be subjected to a multi-step treatment of three or more steps. In this case, a washing step of three to five steps is performed. It is preferable to carry out.

As the optional component (C) in the washing solvent (B), for example, C ₅
A chain hydrofluorocarbon represented by a composition formula of H ₂ F ₁₀ , such as a hydrofluoroether represented by a composition formula of C ₄ F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H ₅ , such as a composition of C ₅ H ₃ F ₇ . A cyclic hydrofluorocarbon represented by the formula, for example, a perfluorocarbon represented by the composition formula of C ₆ F ₁₄ and C ₇ F _{16, and} a composition formula of the C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F ₅ , for example. A mixture of at least one solvent selected from the group consisting of perfluoroethers may be used.

Regarding the physical properties of the polyolefin microporous membrane produced by the method of the present invention, the air permeability is usually 20 to 500 seconds / 10.
0 cc (film thickness 30 μm conversion), porosity 50-95%, puncture strength 7840 mN / 25 μm or more, preferably
9800 mN / 25 μm or more, more preferably 11760 mN /
25 μm or more, heat shrinkage is 5 in both MD and TD directions
% Or less (105 ° C./8 hr).

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION [1] Polyolefin The polyolefin used for producing the polyolefin microporous membrane of the present invention contains polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component. Examples of such polyethylene include ultra high molecular weight polyethylene,
The weight average molecular weight thereof is preferably 1 × 10 ⁶ to 15 × 10 ⁶ , more preferably 1 × 10 ^{6 to} 5 × 10 ⁶ .

The polyethylene used may be a composition containing other polyolefin as an optional component as long as it contains polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more. Such other polyolefins, weight average molecular weight ^of 1 × 10 ⁴ or more to 5 × 10 ⁵ less than polyethylene, 1 × 10 ⁴ to 4 × 10 ⁶ polypropylene, the weight average molecular weight ^of 1 × 10 ⁴ to 4 × 10 ⁶ Polybutene-1, a polyethylene wax having a weight average molecular weight of 1 × 10 ³ or more and less than 1 × 10 ⁴ , and an ethylene / α-olefin copolymer having a weight average molecular weight of 1 × 10 ⁴ to 4 × 10 ^6. At least one selected from the above can be used. The amount of the other polyolefin added is 80 parts by weight or less based on 100 parts by weight of the entire polyolefin composition.

When a polyolefin composition is used, especially ultra high molecular weight polyethylene and a weight average molecular weight of 1 × 10 ⁴ or more
It is preferable to use a composition composed of polyethylene of less than 5 × 10 ⁵ because the molecular weight distribution (Mw / Mn) can be easily controlled depending on the application. The Mw / Mn of the polyolefin composition is not limited, but is preferably 5 to 300, more preferably 5 to 100. Weight average molecular weight is 1 x 10
^As the polyethylene of ⁴ or more and less than 5 × 10 ⁵ , any of high-density polyethylene, medium-density polyethylene or low-density polyethylene can be used, and not only ethylene homopolymer but also propylene, butene-1, hexene- 1
It may be a copolymer containing a small amount of other α-olefin such as. The polyolefin composition is not limited, but a composition composed of an ultra high molecular weight polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more and a high density polyethylene having a weight average molecular weight of 1 × 10 ⁴ or more to less than 5 × 10 ⁵ is used. It is suitable.

[2] Method for producing polyolefin microporous membrane The method for producing a polyolefin microporous membrane of the present invention comprises: (a) a step of adding a liquid solvent to the above polyolefin and melt-kneading to prepare a polyolefin solution; ) Extruding the polyolefin solution through a die, cooling to form a gel-like molded product, (c) a biaxial stretching step and a step of removing the liquid solvent, (d) a step of drying the obtained film, (e) Re-stretching process,
It includes a lamination step and a step of performing heat setting treatment. Furthermore (a)
After the steps (e) to (e), if necessary, (f) crosslinking treatment by ionizing radiation, (g) hydrophilic treatment, etc. may be performed. The method for producing the microporous polyolefin membrane of the present invention is divided into the first to third methods in the step (e) depending on the difference in the treatment method for the dried membrane.

(A) Process for preparing polyolefin solution First, a suitable liquid solvent is added to polyolefin and melt-kneaded to prepare a polyolefin solution. If necessary, various additives such as an antioxidant, an ultraviolet absorber, an antiblocking agent, a pigment, a dye and an inorganic filler can be added to the polyolefin solution within a range that does not impair the object of the present invention. For example, finely divided silicic acid can be added as a pore-forming agent.

It is essential to use a liquid solvent which is liquid at room temperature as a solvent for preparing the polyolefin solution. In the heating melt kneading state, it becomes a state of being mixed with the polyolefin, but at room temperature, when a solid solvent which is solid is used,
It becomes difficult to remove / recover the solvent, and thus the solvent cannot be sufficiently recycled. Liquid solvents include nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin and other aliphatic or cyclic hydrocarbons, and mineral oil fractions having boiling points corresponding to these, and room temperature such as dibutyl phthalate and dioctyl phthalate. Can use liquid phthalate ester. In order to obtain a gel-like molded product having a stable liquid solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin.

The viscosity of the liquid solvent is 30 to 500 at 25 ° C.
It is preferably cSt, more preferably 50 to 200 cSt. If the viscosity at 25 ° C is less than 30 cSt, foaming tends to occur, making it difficult to knead, and if it exceeds 500 cSt, removal of the liquid solvent becomes difficult.

The melt kneading method is not particularly limited, but it is usually carried out by uniformly kneading in an extruder. This method is suitable for preparing concentrated solutions of polyolefins. Melting temperature is the melting point of polyolefin +10 ℃ ~ +100
℃ is preferred, so 160-230 ℃ is preferred,
More preferably, it is 70 to 200 ° C. Here, the melting point is J
The value obtained by differential scanning calorimetry (DSC) based on IS K7121. The liquid solvent may be added before the start of kneading or may be added from the middle of the extruder during the kneading, but it is preferable to add the liquid solvent before the start of the kneading and form a solution in advance. Upon melt-kneading, it is preferable to add an antioxidant in order to prevent the oxidation of the polyolefin.

In the polyolefin solution, the blending ratio of the polyolefin and the liquid solvent is 1 to 50% by weight, preferably 20 to 4% by weight based on the total amount of both.
It is 0% by weight. When the amount of the polyolefin is less than 1% by weight, swell and neck-in increase at the die outlet when forming a gel-like molded product, and the moldability and self-supporting property of the gel-like molded product deteriorate. On the other hand, if it exceeds 50% by weight, the moldability of the gel-like molded article is deteriorated.

(B) Step of forming gel-like molded product The melt-kneaded polyolefin solution is extruded from the die directly or through another extruder, or once cooled and pelletized, and again through the extruder. As the die, a sheet die having a rectangular die shape is usually used, but a double-cylindrical hollow die, an inflation die, or the like can also be used. In the case of sheet dies, the die gap is usually 0.1 to 5 mm, and it is 140 to 250 ° C when extruded.
Heat to. The extrusion rate of the heated solution is preferably 0.2 to 15 m / min.

A gel-like molded article is formed by cooling the solution thus extruded from the die. Cooling is preferably performed at a rate of 50 ° C./min or more until at least the gelation temperature or less, and preferably 25 ° C. or less. In this way, the phase separation structure in which the polyolefin phase is microphase-separated by the solvent can be fixed. Generally, if the cooling rate is slow, the higher-order structure of the obtained gel-like molded article becomes coarse and the pseudo-cell units forming it become large, but if the cooling rate is fast, the cell units become dense. When the cooling rate is less than 50 ° C / min, the crystallinity increases,
It is difficult to obtain a gel-like molded product suitable for stretching. As a cooling method, a method of directly contacting with cold air, cooling water, or another cooling medium, a method of contacting with a roll cooled with a refrigerant, or the like can be used.

(C) Biaxial stretching step and liquid solvent removal step The gel-like molded product obtained by cooling the extruded solution is biaxially stretched (primary stretching). By performing biaxial stretching, the effect of improving the puncture strength by stretching is enhanced. The biaxially stretching method is carried out by heating a gel-like molded article and then carrying out a normal tenter method, a roll method, an inflation method, a rolling method or a combination of these methods at a predetermined magnification. The biaxial stretching may be either longitudinal / transverse simultaneous stretching or sequential stretching, but simultaneous biaxial stretching is particularly preferable.

The stretching temperature of the primary stretching is preferably not higher than the melting point of the raw material polyolefin + 10 ° C., more preferably in the range from the crystal dispersion temperature to below the crystal melting point. If the stretching temperature exceeds the melting point + 10 ° C, the resin melts and the molecular chains cannot be oriented by stretching. Further, if the stretching temperature is lower than the crystal dispersion temperature, the resin is not sufficiently softened, and the film is easily broken during stretching, so that high-stretching cannot be performed. In the present invention, the stretching temperature is usually 100 to 140 ° C, preferably 110 to 125 ° C.
Here, the crystal dispersion temperature means a value obtained by measuring the temperature characteristic of dynamic viscoelasticity based on ASTM D 4065.

The stretching ratio of the primary stretching varies depending on the thickness of the gel-like molded product, but it is preferably at least 3 times or more in any direction. The surface magnification is preferably 10 times or more, more preferably 20 times or more, and particularly preferably 20 to 400 times. If the surface magnification is less than 10 times, the stretching is insufficient and the puncture strength is not improved. On the other hand, if the surface magnification exceeds 400 times, a stretching device,
There are restrictions on the stretching operation and the like.

The liquid solvent is removed (washed) from the film obtained by the primary stretching. Since the polyolefin phase is microphase-separated by the solvent, a porous membrane can be obtained by removing the liquid solvent. For removing the liquid solvent, the washing solvent (A) having a surface tension at 25 ° C. of 24 mN / m or less, preferably 20 mN / m or less is used. The washing solvent (A) is preferably one that is not compatible with polyolefin. Such a washing solvent (A)
By using, it is possible to suppress shrinkage and densification of the network structure caused by the surface tension of the gas-liquid interface generated inside the micropores during drying after washing. As a result, the porosity and air permeability of the microporous membrane can be improved. When treated with a washing solvent whose surface tension at 25 ° C exceeds 24 mN / m, the network of the microporous membrane shrinks and becomes dense, and the air permeability is 500.
It exceeds the second / 100 cc. The surface tension of the washing solvent can be lowered as the use temperature is raised, but the usable temperature range is limited to the boiling point or lower. Further, in the present specification, the “surface tension” means the tension generated at the interface between gas and liquid, and is measured based on JIS K 3362.

As the washing solvent (A), fluorine compounds such as hydrofluorocarbon, hydrofluoroether, cyclic hydrofluorocarbon, perfluorocarbon and perfluoroether, normal paraffin having 5 to 10 carbon atoms, isoparaffin having 6 to 10 carbon atoms, Aliphatic ethers having 6 or less carbon atoms, cycloparaffins such as cyclopentane, aliphatic ketones such as 2-pentanone, methanol, ethanol, 1-propanol, 2-propanol,
Aliphatic alcohols such as tertiary butanol, isobutanol, 2-pentanol, propyl acetate, tertiary butyl acetate, secondary butyl acetate, ethyl acetate, isopropyl acetate, isobutyl acetate, methyl formate,
Examples thereof include ethyl formate, isopropyl formate, isobutyl formate, and aliphatic ester of ethyl propionate.

As the fluorine compound, a chain hydrofluorocarbon represented by a composition formula of C ₅ H ₂ F ₁₀ , C ₄ F ₉ OCH ₃
And C ₄ F ₉ OC ₂ hydrofluoroether represented by the composition formula H _5, C ₅ H ₃ cyclic hydrofluorocarbons represented by the composition formula of F _7, represented by a composition formula of C ₆ F ₁₄ and C ₇ F _{1 6} And at least one selected from the group consisting of perfluorocarbons represented by the composition formula of C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F ₅ . These fluorine compounds have a surface tension of 24 mN / m or less at 20 ° C,
The effect of suppressing shrinkage and densification of the network structure due to surface tension is high. Further, since the boiling point is 100 ° C or lower, it is easy to dry. Furthermore, because it has no ozone depletion, it can reduce the load on the environment, and its flash point is 40 ° C or higher (some compounds have no flash point), so the risk of flammable explosion during the drying process is low.

As the normal paraffin having 5 to 10 carbon atoms, normal pentane, normal hexane, normal heptane, normal octane, normal nonane and normal decane are preferable. These have a surface tension of 2 at 20 ° C.
It is 4 mN / m or less. Among them, normal pentane, normal hexane, and normal heptane, which have a boiling point of 100 ° C. or less and are easily dried, are more preferable.

2 as an isoparaffin having 6 to 10 carbon atoms
-Methylpentane, 3-methylpentane, 2,2-dibutylbutane, 2,3-dibutylbutane, 2-methylhexane, 3
-Methylhexane, 3-ethylpentane, 2,2-dimethylpentane, 2,3-dimethylpentane, 2,4-dimethylpentane, 3,3-dimethylpentane, 2,2,3-trimethylbutane, 2-methylheptane , 3-methylheptane, 2,2-dimethylhexane, 2,3-dimethylhexane, 2,5-dimethylhexane, 3,4-dimethylhexane, 2,2,3-trimethylpentane, 2,2,4-trimethyl Pentane, 2,3,3-trimethylpentane, 2,3,4-trimethylpentane, 2-methyloctane, 2,2,5-trimethylhexane, 2,3,5-trimethylhexane, 2-methylnonane and 2,3 5,5-Trimethylheptane is preferred. Among them, the surface tension is 24 mN / m or less at 20 ° C and the boiling point is 100 ° C or less. 2
-Methylpentane, 3-methylpentane, 2,2-dibutylbutane, 2,3-dibutylbutane, 2-methylhexane, 3
-Methylhexane, 3-ethylpentane, 2,2-dimethylpentane, 2,3-dimethylpentane, 2,4-dimethylpentane and 3,3-dimethylpentane are more preferred.

As the ether having 6 or less carbon atoms, diethyl ether, butyl ethyl ether, dipropyl ether and diisopropyl ether are preferable. They have a surface tension of 24 mN / m or less at 20 ° C and a boiling point of 100 ° C or less.

Cyclopentane, methanol, ethanol, 1-propanol and 2-propanol have a surface tension of 24 mN / m or less at 20 ° C. and a boiling point of 10
It is preferable that the temperature is 0 ° C. or less.

Among the above aliphatic esters, the surface tension is
Tertiary butyl acetate, secondary butyl acetate, isopropyl acetate, isobutyl acetate, ethyl formate, isopropyl formate and isobutyl formate which are 24 mN / m or less at 20 ° C. are preferable. Further, the boiling point is 100 ° C. or lower, but tertiary butyl acetate, ethyl formate and isopropyl formate are more preferable.

As the washing solvent (A), it is also possible to use a mixture of water and an aliphatic alcohol having 3 or less carbon atoms, which is blended so as to have a surface tension of 24 mN / m or less at 25 ° C.

The above-mentioned washing solvent (A) is mixed with another solvent.
It can be used as a thing. In this case, the mixing ratio
The surface tension to be 24 mN / m or less at 25 ℃
To For example C_FiveH₂F_TenChain hide represented by the composition formula
Rofluorocarbon, eg C _FourF₉OCH₃And C_FourF₉OC₂H_Fiveof
A hydrofluoroether represented by a composition formula, for example, C_Five
H₃F₇Cyclic hydrofluorocarbo represented by the composition formula
, Eg C₆F₁₄And C₇F_{1 6}Perf represented by the composition formula of
Luorocarbon, as well as eg C_FourF₉OCF₃And C_FourF₉OC₂F_Five
Is it a group consisting of perfluoroethers represented by the composition formula
At least one solvent selected from the above and carbonization of paraffin, etc.
Mixtures with hydrogen based solvents can be used.

The washing is preferably performed in two or more washing steps, and a step of using a washing solvent (washing solvent (B)) other than the washing solvent (A) may be included. In this case, the washing solvent (A) may be used in at least one step. Washing solvent
As (B), those not compatible with polyolefin are preferable, for example, pentane known as a washing solvent,
Hydrocarbons such as hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, fluorohydrocarbons such as trifluoroethane, ethers such as diethyl ether and dioxane, and volatile solvents such as methyl ethyl ketone can be used. . It is also possible to use a non-aqueous solvent having a boiling point of 100 ° C or higher and a flash point of 0 ° C or higher. Wash solvent as described above (B)
Is appropriately selected according to the liquid solvent used for dissolving the polyolefin composition, and is used alone or as a mixture.

By such a washing process of two or more stages,
Sufficient cleaning can be performed while suppressing shrinkage and densification of the network structure caused by the surface tension of the cleaning solvent. Preferably, a washing solvent is used at least in the final washing step.
Process with (A). As a result, when the washing solvent (B) is used, the washing solvent (B) can be removed (hereinafter referred to as “rinsing treatment”), and shrinkage and densification of the network that occurs during drying after washing can be prevented. As a result, the porosity and air permeability of the polyolefin microporous film are improved.

In the final washing step, the washing solvent (A)
When the treatment is performed with a washing solvent (A) having a boiling point of 100 ° C. or less, the efficiency of the drying step is improved. Furthermore, the above-mentioned C ₅ H ₂ F ₁₀
A chain hydrofluorocarbon represented by the composition formula of C ₄
Hydrofluoroether represented by the composition formula of F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H ₅ , cyclic hydrofluorocarbon represented by the composition formula of C ₅ H ₃ F ₇ , composition of C ₆ F ₁₄ and C ₇ F ₁₆ Perfluorocarbon represented by the formula, and C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F
_The use of a fluorine-based compound such as perfluoroether represented by the composition formula ₅ also has the effect of further reducing the environmental load in the manufacturing process as described above. Especially washing solvent
When using a solvent with a boiling point of 150 ° C or higher as (B), simply drying with hot air may take a long time to dry, and the porosity and air permeability may decrease in the subsequent heat treatment. The problem can be solved by using the washing solvent (A) at 100 ° C. or lower.

The non-aqueous solvent having a boiling point of 100 ° C. or higher and a flash point of 0 ° C. or higher which can be used as the washing solvent (B) is hardly volatile, has a low load on the environment, and has a risk of ignition and explosion in the drying process. Since it is low, it is safe to use. In addition, since it has a high boiling point, it is easy to condense, easy to collect, and easy to recycle. In the present specification, "boiling point" refers to a boiling point at 1.01 x 10 ⁵ Pa, and "flash point" refers to a value measured based on JIS K 2265.

As the non-aqueous solvent, for example, a boiling point of 100 ° C.
Examples thereof include paraffinic compounds having a flash point of 0 ° C. or higher, aromatics, alcohols, esters, ethers and ketones. The flash point thereof is preferably 5 ° C or higher, more preferably 40 ° C or higher. However, it is not preferable to make the non-aqueous solvent an aqueous solution because the liquid solvent cannot be sufficiently removed.

As the non-aqueous solvent, normal paraffins having 8 or more carbon atoms, normal paraffins having 5 or more carbon atoms in which at least a part of hydrogen atoms are substituted with halogen atoms,
Isoparaffins having 8 or more carbon atoms, cycloparaffins having 7 or more carbon atoms, cycloparaffins having 5 or more carbon atoms in which at least a part of hydrogen atoms are replaced with halogen atoms, aromatic hydrocarbons having 7 or more carbon atoms, and at least hydrogen atoms Aromatic hydrocarbons having 6 or more carbon atoms partially substituted with halogen atoms, alcohols having 5 to 10 carbon atoms in which some hydrogen atoms may be substituted with halogen atoms, and some hydrogen atoms being halogen atoms At least one selected from the group consisting of esters and ethers having 7 to 14 carbon atoms which may be substituted with and ketones having 5 to 10 carbon atoms is preferable.

As the normal paraffin having 8 or more carbon atoms, normal octane, normal nonane, normal decane, normal undecane and normal dodecane are preferable, and normal octane, normal nonane and normal decane are more preferable.

Normal paraffins having 5 or more carbon atoms in which at least a part of hydrogen atoms are substituted with halogen atoms include 1-chloropentane, 1-chlorohexane, 1-chloroheptane, 1-chlorooctane and 1-bromopentane. ,
1-Bromohexane, 1-bromoheptane, 1-bromooctane, 1,5-dichloropentane, 1,6-dichlorohexane and 1,7-dichloroheptane are preferable, 1-chloropentane, 1-chlorohexane, 1- Bromopentane and 1
-Bromohexane is more preferred.

2, as an isoparaffin having 8 or more carbon atoms,
3,4-trimethylpentane, 2,2,3-trimethylpentane, 2,2,5-trimethylhexane, 2,3,5-trimethylhexane, 2,3,5-trimethylheptane and 2,5,6-trimethyl Octane is preferred, and 2,3,4-trimethylpentane, 2,2,3-trimethylpentane, 2,2,5-trimethylhexane and 2,3,5-trimethylhexane are more preferred.

Cycloparaffins having 7 or more carbon atoms include cycloheptane, cyclooctane, methylcyclohexane, cis- and trans-1,2-dimethylcyclohexane, cis- and trans-1,3-dimethylcyclohexane and cis- and trans. -1,4-Dimethylcyclohexane is preferable, and cyclohexane is more preferable.

As the cycloparaffin having 5 or more carbon atoms in which at least a part of hydrogen atoms are substituted with halogen atoms, chlorocyclopentane and chlorocyclohexane are preferable, and chlorocyclopentane is more preferable.

As the aromatic hydrocarbon having 7 or more carbon atoms,
Toluene, ortho-xylene, meta-xylene and para-xylene are preferable, and toluene is more preferable.

Examples of the aromatic hydrocarbon having 6 or more carbon atoms in which at least a part of hydrogen atoms are replaced with halogen atoms are chlorobenzene, 2-chlorotoluene, 3-chlorotoluene, 4-chlorotoluene, 3-chloroorthoxylene, Four
-Chloroorthoxylene, 2-chlorometaxylene, 4-
Chlorometaxylene, 5-chlorometaxylene and 2-chloroparaxylene are preferred, and chlorobenzene, 2-chlorotoluene, 3-chlorotoluene and 4-chlorotoluene are more preferred.

Alcohols having 5 to 10 carbon atoms in which a part of hydrogen atoms may be replaced by halogen atoms include isopentyl alcohol, tert-pentyl alcohol, cyclopentanol, cyclohexanol and 3-methoxy-1- Butanol, 3-methoxy-3-methyl-1-
Butanol, propylene glycol normal butyl ether and 5-chloro-1-pentanol are preferred, and 3-
Methoxy-1-butanol, 3-methoxy-3-methyl-1
More preferred are butanol, propylene glycol normal butyl ether and 5-chloro-1-pentanol.

Examples of the ester having 7 to 14 carbon atoms in which a part of hydrogen atoms may be replaced with halogen atoms are diethyl carbonate, diethyl maleate, normal propyl acetate, normal butyl acetate, isopentyl acetate, 3-methoxybutyl acetate. , 3-methoxy-3-methylbutyl acetate, ethyl normal butyrate, ethyl normal valerate and 2-chloroethyl acetate are preferred, isopentyl acetate, 3-methoxybutyl acetate, 3-methoxy-3-methylbutyl acetate, ethyl normalbutyrate and acetic acid 2 -Chloroethyl is more preferred.

Diether glycol dimethyl ether, normal butyl ether, diisobutyl ether and bischloroethyl ether are preferable as the ether having 7 to 14 carbon atoms in which a part of hydrogen atoms may be replaced by a halogen atom, and dipropylene glycol dimethyl ether and Bischloroethyl ether is more preferred.

As the C5-10 ketone, 2-pentanone, 3-pentanone, 2-hexanone, 3-hexanone, cyclopentanone and cyclohexanone are preferable, and 2-pentanone and 3-pentanone are more preferable.

The washing solvent (B) as described above may be used as a mixture, but the washing solvent (B) may be used as an optional component (C).
For example, a chain hydrofluorocarbon represented by the composition formula of C ₅ H ₂ F ₁₀ mentioned as the cleaning solvent (A), for example, C ₄ F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H _{5 represented by} the composition formula hydrofluorocarbon. Ethers, such as cyclic hydrofluorocarbons having the composition formula of C ₅ H ₃ F ₇ , such as perfluorocarbons having the composition formula of C ₆ F ₁₄ and C ₇ F ₁₆ , such as C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC
_A mixture of at least one solvent selected from the group consisting of perfluoroethers represented by the composition formula of ₂ F ₅ may be used. In this case, the washing solvent (B) and optional component (C)
Has a surface tension of 24 at any temperature between 20 and 80 ° C.
It is preferable to mix at a ratio of mN / m or less. Specifically, 2 parts of the optional component (C) are mixed in 100 parts by weight of the mixed solvent.
To 98 parts by weight, preferably 5 to 50 parts by weight. By containing 2 to 98 parts by weight of the optional component (C), while suppressing shrinkage and densification of the network caused by the surface tension of the washing solvent,
Sufficient cleaning can be performed.

The washing solvent (B) has a surface tension of 20-80.
It is preferable to use a material having a temperature of 24 mN / m or less at any temperature of ° C. For example, normal pentane, hexane, heptane, ethane trifluoride, diethyl ether,
2-methylpentane, 3-methylpentane, cyclohexane, cyclopentane, acetone, methyl ethyl ketone and the like.

Washing solvent used in the first step of washing here
Preferred combinations of (B) and the washing solvent (A) used in the second step are shown below. However, as described below, the washing solvent (A)
Since the washing with the washing solvent (B) can be performed in three or more steps, these are not intended to be limited to two steps. For example, washing solvent (B) / washing solvent (A) =
Methylene chloride / C ₄ F ₉ OCH ₃ , methylene chloride / C ₆ F ₁₄ , methylene chloride / C ₇ F ₁₆ , methylene chloride / normal heptane,
Methylene chloride / normal hexane, methylene chloride / diethyl ether, ether / hydrofluoroether,
Ether / cyclic hydrofluorocarbon, ether /
Alcohol, mixture of ether / alcohol and water, normal paraffin / hydrofluoroether, normal paraffin / cyclic hydrofluorocarbon, normal paraffin / alcohol, mixture of normal paraffin / alcohol and water, isoparaffin / hydrofluoroether, isoparaffin / cyclic hydro Fluorocarbon, isoparaffin / alcohol, mixture of isoparaffin / alcohol and water, cycloparaffin / hydrofluoroether, cycloparaffin / cyclic hydrofluorocarbon, cycloparaffin / alcohol, mixture of cycloparaffin / alcohol and water, ketone / hydrofluoroether, Ketone / cyclic hydrofluorocarbon, ketone / alcohol, and ketone / alcohol with water Mixtures thereof. More preferably, the washing solvent (B) / the washing solvent (A) = methylene chloride / C ₄ F ₉
OCH ₃ , methylene chloride / C ₆ F ₁₄ , methylene chloride / C ₇ F ₁₆ , methylene chloride / normal heptane, methylene chloride / normal hexane, methylene chloride / diethyl ether, normal heptane / C ₄ F ₉ OCF ₃ , and normal heptane / C ₆ F ₁₄
Is. By using such a combination,
The porosity and air permeability of the microporous membrane can be improved while effectively removing the liquid solvent.

The washing solvent (A) alone or the washing solvent (A) and the washing solvent (B) may be washed in three or more steps. The one-step or two-step treatment is effective when the liquid solvent cannot be sufficiently removed and the physical properties of the obtained polyolefin microporous film are deteriorated. In this case, at least in the final washing step, it may be treated with the washing solvent (A), and the number of washings is not particularly limited, but usually 3 steps to
There are five stages, preferably three to four stages. In addition, even if treated with the same washing solvent in each stage, the manufacturing process is simply lengthened, the space of the polyolefin microporous membrane manufacturing facility is expanded, and the efficiency of liquid solvent removal is reduced. It is preferable to use a washing solvent. However, it is not limited to using different cleaning solvents. Therefore, for example, in the case of three-step treatment, it is possible to use the same washing solvent in the first step and the second step, and to use a washing solvent different from the first and second steps in the third step.

The washing method may be a method of immersing in the washing solvent (A) and / or the washing solvent (B) for extraction, a method of showering the washing solvent (A) and / or the washing solvent (B), or a combination thereof. It can be performed by a method or the like. Also wash solvent
The (A) and the washing solvent (B) are preferably used in an amount of 300 to 30,000 parts by weight per 100 parts by weight of the gel-like molded product. When the washing solvent (A) and the washing solvent (B) are used for washing in two or more steps, the amount of the washing solvent (A) used is
The amount of the washing solvent (B) used is preferably 100 parts by weight and 50 to 200 parts by weight. The washing with the washing solvent (A) alone or with the washing solvent (A) and the washing solvent (B) is preferably performed until the residual liquid solvent is less than 1% by weight based on the amount added.

The washing temperature with the washing solvent (B) is the washing solvent (B).
Depends on the boiling point of. When the boiling point of the washing solvent (B) is 150 ° C. or less, washing at room temperature is possible, and heating washing may be performed if necessary, and washing at 20 to 80 ° C. is generally preferable. Further, when the boiling point of the washing solvent (B) is 150 ° C. or higher, the penetrability into the inside of the membrane is poor at room temperature, so it is preferable to wash by heating.

The washing temperature and / or the rinsing temperature with the washing solvent (A) depend on the surface tension of the washing solvent (A). Specifically, it is preferable to carry out the washing and / or rinsing treatment at a temperature or higher at which the surface tension of the washing solvent (A) becomes 24 mN / m or less. Ambient temperature, surface tension of the cleaning solvent (A) is 24 mN / m
If it does not reach the temperature below, heat the surface tension to a temperature below 24 mN / m, if necessary. The washing solvent (A) has a surface tension of 24 mN at a temperature of at most 25 ° C.
/ M or less, so heating is not required in most cases,
Washing and / or rinsing can be performed at normal room temperature.

(D) Drying Step of Membrane The membrane obtained by stretching and removing the liquid solvent can be dried by a heating drying method or an air drying method. Here, the membrane dried by these drying methods is referred to as a microporous membrane (A). The drying temperature is preferably a temperature below the crystal dispersion temperature of the polyolefin, and particularly preferably a temperature lower than the crystal dispersion temperature by 5 ° C. or more.

By the drying treatment, the content of the washing solvent (B) remaining in the microporous polyolefin membrane is preferably 5% by weight or less (the weight of the dried membrane is 100% by weight),
More preferably, it is 3% by weight or less. When the drying is insufficient and the washing solvent (B) remains in a large amount in the film, the porosity is lowered by the subsequent heat treatment and the air permeability is deteriorated, which is not preferable.

(E) Re-stretching step, laminating step and heat-setting treatment The method for producing the polyolefin microporous membrane of the present invention is described below according to the difference in treatment method for the membrane after drying.
Divide into the third method. (e) -1. First method In the first method, the dried microporous membrane (A) is re-stretched at a temperature lower than the crystal dispersion temperature of the raw material polyolefin (secondary stretching).
To do. This improves porosity and permeability. It is presumed that this is because the re-stretching at a temperature lower than the crystal dispersion temperature stretches a part of the fine fibrils forming the polyolefin microporous film, thereby expanding the pore size. The secondary stretching may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, either longitudinal / transverse simultaneous stretching or sequential stretching may be used. As the stretching method, a tenter method, a roll method, a rolling method, or a combination thereof can be used.

The secondary stretching is performed at a temperature lower than the crystal dispersion temperature of the raw material polyolefin. For example, the crystal dispersion temperature of polyethylene is generally 90 ° C. The lower limit of the secondary stretching temperature is not particularly limited, but it is preferable to carry out at -20 ° C or higher, which is preferable. A more preferable range of the secondary stretching temperature is 0 ° C or higher to 90 ° C.
It is lower than 0 ° C, and a particularly preferable range is 20 ° C or higher and lower than 90 ° C. Stretching at a temperature higher than the crystal dispersion temperature cannot be expected to improve porosity. Although the reason for this is not clear, it is presumed that the effect of stretching a part of the fine fibrils forming the microporous polyolefin membrane is greater when stretched at a low temperature.

The draw ratio of the secondary drawing varies depending on the thickness of the raw material, but is 1 to 3 times in the uniaxial direction, preferably 1.1 to 2
1 times or more and less than 9 times in area magnification, preferably 1.2
~ 4 times. If it is 9 times or more, film rupture is likely to occur, which is not preferable. Here, the film obtained by secondarily stretching the microporous film (A) is referred to as a microporous film (B).

The microporous membrane (B) obtained by the secondary stretching is
Further, heat setting treatment is performed at a temperature of crystal dispersion temperature to melting point. The heat setting treatment is a heat treatment performed so that there is no dimensional change in both MD and TD. As a result, the thermal shrinkage can be improved without deteriorating the porosity and air permeability.
If the heat setting temperature is lower than the crystal dispersion temperature, the effect of improving the heat shrinkage is not sufficient, and if it exceeds the melting point, the air permeability deteriorates.
A preferable temperature range for the heat setting treatment is 110 to 130 ° C. The heat setting treatment is performed by either a tenter method, a roll method, or a rolling method. The treatment time is not particularly limited and is usually 1 second to 10 minutes, preferably 3 seconds to 2 minutes.

A heat relaxation treatment is preferably applied before and / or after the heat setting treatment. The thermal relaxation process is a process of causing thermal contraction. This can further improve the heat shrinkage rate. The heat relaxation treatment may be performed by a fixed method such as a tenter method, a roll method, a rolling method, or a free method using a belt conveyor method, a mesh drum (rotary drum) method, a floating method, or the like. The free method is preferable from the viewpoint of uniform physical properties in the width direction and lengthening of the winding length. The thermal relaxation treatment is carried out at a temperature from the crystal dispersion temperature of polyolefin to the melting point. The preferred temperature range for the thermal relaxation treatment is 110 to 130 ° C. The thermal relaxation treatment is preferably performed so as to maintain 80% or more of the length immediately before relaxation in both MD and TD, and relaxation in both directions or relaxation in only one direction may be performed.

(E) -2. Second Method In the second method, the above-mentioned microporous film (A) and microporous film (B) are joined to form a laminated film (AB). Since the microporous membrane (A) is not secondarily stretched, the heat shrinkage rate is smaller than that of the microporous membrane (B), while the microporous membrane (B) is excellent in puncture strength and permeability, so the microporous membrane (A ) And the microporous membrane (B) are joined together, the above three physical properties can be balanced. The microporous membrane (B) is preferably subjected to thermal relaxation treatment before being joined to the microporous membrane (A).

The lamination of the microporous membrane (A) and the microporous membrane (B) is usually carried out by thermally bonding the both. Thermal bonding can be performed by a known method, and an external heating method such as heat sealing or impulse sealing or an internal heating method such as ultrasonic bonding or ultrasonic bonding may be used. Usually done by heat sealing,
For this, a heat sealer equipped with a heat roll, a heating plate, an embossing roll, or the like, or a band sealer is used. The thermal bonding is performed at a temperature higher than the crystal dispersion temperature and lower than the melting point.

There is no limitation on the number of steps of the laminated structure.
It may be a two-tiered stack such as A / B or a three-tiered stack such as B / A / B. However, in the case of a laminated structure having three or more layers, it is preferable to form both surface layers by the microporous membrane (B) because the porosity and air permeability of the laminated microporous membrane are improved.

The laminated film (AB) thus obtained is heat-set in the same manner as in the first method. The heat setting treatment may be carried out at a temperature from the crystal dispersion temperature to the melting point by any of a tenter system, a roll system or a rolling system, and it is carried out for 1 second to 10 minutes, preferably 3 seconds to 2 minutes. If it is lower than the crystal dispersion temperature, the effect of improving the heat shrinkage is not sufficient, and if it exceeds the melting point, the air permeability is deteriorated. The preferred temperature range for heat setting is 110-130 ° C.
Is.

(E) -3. Third Method In the third method, at least two of the above-mentioned microporous membranes (A) are laminated by bonding to obtain a laminated membrane.
(A) is secondarily stretched. By stacking at least two microporous membranes (A), it is possible to prevent the membranes from becoming non-uniform or from breaking when stretched at a high magnification in the secondary stretching. For example, generally, if the thickness of the primary stretched raw material is 10 μm or less, it may not be possible to uniformly stretch in the secondary stretching, and when the primary stretching is performed at a high stretching ratio of more than 400 times, the film May be partially non-uniform, and if this is further stretched, the film may be broken in the worst case.

The lamination may be performed by thermal bonding using a heat sealer or the like as in the case of the second method. The thermal bonding is performed at a temperature higher than the crystal dispersion temperature and lower than the melting point. The secondary stretching may be uniaxial stretching or biaxial stretching as in the case of the first method. In the case of biaxial stretching, either longitudinal / transverse simultaneous stretching or sequential stretching may be used. The stretching method is a tenter method, a roll method,
Either rolling or a combination of these can be utilized.

Secondary stretching is carried out at a temperature below the crystal dispersion temperature of polyethylene. This improves the porosity and permeability as described above. The crystal dispersion temperature of polyethylene is generally 90 ° C. Although the lower limit of the secondary stretching temperature is not particularly limited, it is preferably −20 ° C. or higher. A more preferable range of the secondary stretching temperature is 0 ° C or higher and lower than 90 ° C, and a particularly preferable range is 20 ° C.
Above ~ 85 ℃ below. Stretching at a temperature higher than the crystal dispersion temperature cannot be expected to improve porosity.

The stretch ratio varies depending on the thickness of the original fabric,
The uniaxial direction is 1 to 3 times, preferably 1.1 to 2 times, and the area magnification is 1.01 times or more and less than 9 times, preferably 1.2 to 4 times. If it is 9 times or more, film rupture is likely to occur, which is not preferable.

The laminated film (A) after the secondary stretching is heat-set as in the case of the first method. The heat setting treatment may be carried out at a temperature from the crystal dispersion temperature to the melting point by any of a tenter system, a roll system or a rolling system, and it is carried out for 1 second to 10 minutes, preferably 3 seconds to 2 minutes. If it is lower than the crystal dispersion temperature, the effect of improving the heat shrinkage is not sufficient, and if it exceeds the melting point, the air permeability is deteriorated. The preferred temperature range for heat setting is 110-130
℃. Further, it is preferable to perform the above-mentioned thermal relaxation treatment before and / or after the heat setting treatment. The thermal relaxation treatment is carried out at a temperature from the crystal dispersion temperature of polyolefin to the melting point. The thermal relaxation treatment is preferably performed so as to maintain 80% or more of the length immediately before relaxation in both MD and TD, and relaxation in both directions or relaxation in only one direction may be performed.

(G) Crosslinking Treatment Step of Membrane It is preferable that the microporous membrane obtained by the first to third methods is subjected to a crosslinking treatment by ionizing radiation. Α-rays, β-rays, γ-rays and electron beams are used as ionizing radiation.
It can be performed at 0.1 to 100 Mrad and acceleration voltage of 100 to 300 kV. This can improve the meltdown temperature.

(H) Hydrophilization treatment step The microporous membrane obtained by the first to third methods can be used after being hydrophilized. As the hydrophilic treatment, a monomer graft, a surfactant treatment, a corona discharge treatment or the like is used. The hydrophilic treatment is preferably performed after ionizing radiation.

When the surfactant is used, any of nonionic surfactants, cationic surfactants, anionic surfactants and amphoteric surfactants can be used. Is preferred. In this case, the surfactant is made into an aqueous solution or a solution of a lower alcohol such as methanol, ethanol or isopropyl alcohol, and is hydrophilized by a method such as dipping and a doctor blade.

The obtained hydrophilized microporous membrane is dried. At this time, in order to improve air permeability, it is preferable to perform heat treatment while preventing or stretching shrinkage at a temperature equal to or lower than the melting point of the microporous polyolefin membrane.

[3] Polyolefin Microporous Membrane The polyolefin microporous membrane according to the preferred embodiment of the present invention has the following physical properties. (1) Air permeability is 20 to 500 seconds / 100 cc (film thickness 20 μm conversion). When it exceeds 500 seconds / 100 cc, the battery capacity becomes small when the polyolefin microporous membrane is used as a battery separator. On the other hand, if it is less than 20 seconds / 100 cc, the strength of the film will be significantly reduced. (2) Porosity is 50-95%. If it is less than 50%, sufficient permeability cannot be obtained. If it exceeds 95%, the balance between battery safety and impedance becomes unbalanced. (3) Puncture strength is 7840 mN / 25 μm or more, preferably
9800 mN / 25 μm or more, more preferably 11760 mN /
25 μm or more. If it is less than 7840 mN / 25 μm, pinholes are likely to occur when the microporous polyolefin membrane is incorporated into a battery as a battery separator. (4) Thermal shrinkage after exposure at 105 ° C for 8 hours is 5% or less in both the machine direction (MD) and the transverse direction (TD). When it exceeds 5%, when the polyolefin microporous film is used as a battery separator, a short circuit due to shrinkage is likely to occur. (5) The average curvature ratio is not limited, but is preferably 1.5-
2.2, and more preferably 1.5 to 2.0. When it exceeds 2.2, the ionic air permeability is significantly lowered, and when it is less than 1.5, the ionic air permeability is improved, but when the polyolefin microporous membrane is incorporated into a battery as a battery separator, a short circuit easily occurs.

The film thickness of the polyolefin microporous film can be appropriately selected depending on the application, but when it is used as a battery separator, for example, it is preferably 5 to 200 μm. Such a microporous polyolefin membrane can be obtained by the production method described in [2] above using the polyolefin described in [1] above.

As described above, the polyolefin microporous membrane obtained by the production method of the present invention has an excellent balance of porosity, air permeability and puncture strength, and excellent dimensional stability. And the like can be preferably used. In particular, when it is used as a battery separator, not only battery characteristics such as discharge characteristics and cycle characteristics in a low temperature region but also battery productivity and battery safety can be improved.

[0096]

EXAMPLES The present invention will be explained in more detail by the following examples, but the present invention is not limited to these examples.

Example 1 (First Method) 30% by weight of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 2.0 × 10 ⁶ and a weight average molecular weight of
A polyethylene composition consisting of 70% by weight of 3.0 × 10 ⁵ high-density polyethylene (HDPE) and having Mw / Mn = 16 (melting point: 13
Tetrakis [methylene-3- (3,5-ditertiarybutyl-4) as an antioxidant at 5 ° C and a crystal dispersion temperature of 90 ° C)
-Hydroxyphenyl) -propionate] methane was obtained by dry blending 0.25 parts by weight per 100 parts by weight of the polyethylene composition to obtain a polyethylene composition. 20 parts by weight of the obtained polyethylene composition was added to a twin-screw extruder (inner diameter 45 mm, L / D = 4
(2, strong kneading type), and supply 80 parts by weight of liquid paraffin from the side feeder of this twin-screw extruder.
Melt kneading was carried out under conditions of ° C and 200 rpm to prepare a polyethylene solution in an extruder. Then, this polyethylene solution was extruded from a T die installed at the tip of the extruder,
50 ℃ / min while taking it off with a cooling roll controlled to ℃
And cooled to form a gel-like sheet. The obtained gel-like sheet was simultaneously biaxially stretched (primary stretching) at 115 ° C. by a tenter stretching machine at 5 × 5 times to obtain a stretched film. The obtained membrane was fixed to a 20 cm × 20 cm aluminum frame, and ethyl perfluorobutyl ether [C ₄ F ₉ OC ₂ H ₅ , temperature-controlled at 25 ° C., Sumitomo 3M HFE-
7200, surface tension 13.6 mN / m (25 ° C), boiling point 76 ° C, no flash point (same below)] / n-heptane [surface tension 19.7 mN
/ M (25 ° C.), boiling point 98.4 ° C.] = 95/5 (wt / wt), and immersed in the first washing tank, and washed with shaking at 100 rpm for 3 minutes. Further, it was immersed in a second cleaning tank (rinse tank) containing HFE-7200 whose temperature was adjusted to 25 ° C. for 1 minute to perform a rinse treatment. The obtained membrane was dried with hot air at 70 ° C on a roll heated to 60 ° C to obtain a microporous membrane (A) (membrane
(A)) was produced. The obtained film (A) with a tenter stretching machine
A microporous membrane (B) (membrane (B)) was prepared by stretching the membrane in the TD direction only twice at 88 ° C. (secondary stretching). The resulting microporous membrane (B) was heat-set at 120 ° C for 10 minutes while fixing the membrane with a tenter stretching machine, and then heat-relaxed at 100 ° C for 10 seconds to give 5% MD and 5% TD. A microporous polyethylene film was produced by shrinking the film by%.

Example 2 (First Method) A membrane (A) was prepared in the same manner as in Example 1, and the film was simultaneously biaxially stretched in MD at a temperature of 85 ° C. by a tenter stretching machine at 1.5 times and at TD by 1.5 times. Then (secondary stretching), while holding the film in the tenter, heat relaxation treatment is performed at 95 ° C for 10 seconds, and MD is 7%.
A polyethylene microporous membrane was produced in the same manner as in Example 1 except that the membrane (B) was produced by shrinking TD and 8%.

Example 3 (First Method) 60% by weight of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 1.0 × 10 ⁶ and a weight average molecular weight of 60%
A polyethylene composition composed of 5.0 × 10 ⁵ high-density polyethylene (HDPE) 40% by weight and having Mw / Mn = 10 (melting point: 13
A polyethylene microporous membrane was produced in the same manner as in Example 1 except that 5 ° C and a crystal dispersion temperature of 90 ° C) were used.

Example 4 (First Method) Ultrahigh molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 8.0 × 10 ⁵ (melting point: 135 ° C., crystal dispersion temperature: 9)
A polyethylene microporous membrane was produced in the same manner as in Example 1 except that 0 ° C. and Mw / Mn = 7) were used.

Example 5 (Second Method) Two membranes (A) were prepared in the same manner as in Example 1,
Only one of them in the TD direction at 85 ℃ with a tenter stretching machine
The film (B) was produced by stretching 1.5 times (secondary stretching). Next, the obtained film (A) and film (B) were heat-bonded by a roll at 110 ° C, while holding the obtained laminated film (AB) in a tenter.
A polyethylene microporous membrane was prepared by heat setting treatment at 120 ° C. for 60 seconds.

Example 6 (Second Method) Three films (A) were prepared in the same manner as in Example 4,
A film (B) was prepared in the same manner as in Example 4 by using two of them. Then, obtain one film (A) and two films (B) as membrane B / membrane A /
By stacking so as to form a laminated structure of membrane B, heat-bonding with a roll at 110 ℃, and heat-treating the obtained laminated membrane (AB) at 120 ℃ for 60 seconds while holding the membrane in a tenter to make polyethylene. A microporous film of was prepared.

Comparative Example 1 A polyethylene microporous membrane was prepared in the same manner as in Example 1 except that the heat setting treatment was not performed.

Comparative Example 2 A microporous polyethylene membrane was produced in the same manner as in Example 1 except that paraffin wax (molecular weight: 400, melting point: 69 ° C.) was used instead of liquid paraffin.

Comparative Example 3 In the washing step and rinsing step, instead of the washing treatment with HFE-7200 / n-heptane and the rinsing treatment with HFE-7200, methylene chloride (surface tension 27.3
A polyethylene microporous membrane was produced in the same manner as in Example 1 except that the treatment was carried out by immersing for 1 minute in the first and second washing tanks containing mN / m (25 ° C) and boiling point 40.0 ° C).

The physical properties of the polyolefin microporous membranes obtained in Examples 1 to 6 and Comparative Examples 1 to 3 were measured by the following methods. -Film thickness: Measured with a contact thickness meter. -Air permeability: measured in accordance with JIS P8117 (film thickness 20 μm conversion). -Porosity: Measured by a gravimetric method.・ Puncture strength: 25 μm thick microporous membrane with a diameter of 1 mm (0.5 mm R)
The maximum load was measured when the needle was pierced at a speed of 2 mm / sec.・ Heat shrinkage ratio: When the microporous membrane was exposed at 105 ° C for 8 hours
The shrinkage rates of MD and TD were measured respectively.・ Average curvature ratio: Kozeny constant k of the Kozeny-Carman equation that can clarify the relationship between the fluid permeation velocity in a porous body and the porosity or specific surface area is approximated by the following equation when a three-dimensional orientation model of the fiber is introduced. Yes, it is usually 5.0. k≈ [4 ⁵ ε / (3π ² δ ₀ ² S _p ² )] × (L ₀ /
L) ² where ε is the porosity, δ ₀ is the fibril diameter, S _p is the surface area (specific surface area) of the porous solid only per unit volume,
L is the apparent transmission distance, that is, the film thickness, L ₀ is the true transmission distance, that is, the length of the penetrating path, and the curvature ratio defined by L ₀ / L is calculated.・ Cycle characteristics: Propylene carbonate (PC): Diethyl carbonate (DEC) = 1: 1 (weight ratio) was added to the solvent to prepare an electrolyte solution containing 1M LiClO ₄ (lithium perchlorate). A lithium battery using LiCoO ₂ as a positive electrode was prepared. A cycle test was performed in which the operation of charging this battery to 4.2V and then discharging it was repeated 700 times at 25 ° C, and the air permeability after the cycle test was examined.

[0107]

[Table 1]

As shown in Table 1, the polyethylene microporous membranes of Examples 1 to 6 produced by the method of the present invention have porosity,
It can be seen that it has an excellent balance of air permeability, puncture strength and heat shrinkage and has cycle characteristics that can withstand use. On the other hand, the microporous membrane of Comparative Example 1 was not subjected to heat setting treatment, Comparative Example 2 prepared a melt-kneaded product using a solid solvent, and Comparative Example 3 had a surface tension of 24 mN / 25 ° C.
Since the rinse treatment and the drying are performed using the cleaning solvent exceeding m, the heat shrinkage rate and the cycle characteristics are significantly inferior in Comparative Example 1 as compared with Examples 1 to 6, and the piercing strength in Comparative Example 2, The heat shrinkage is inferior, and the air permeability is inferior in Comparative Example 3.

[0109]

As described above in detail, the polyolefin microporous membrane of the present invention is obtained by melt-kneading a polyolefin having a weight average molecular weight of 5 × 10 ⁵ or more and polyethylene as an essential component and a liquid solvent. The melt-kneaded product is extruded from a die and the gel-like molded product obtained by cooling is biaxially stretched. The stretched product has a surface tension of 24 at 25 ° C.
After removing the liquid solvent by using a washing solvent (A) of mN / m or less, the stretched product after the washing is stretched again at least uniaxially at a temperature lower than the crystal dispersion temperature of the polyolefin, and then the polyolefin It is manufactured by heat-setting at a temperature from the crystal dispersion temperature to the melting point so that there is no dimensional change in both the machine direction (MD) and the transverse direction (TD), so the porosity, air permeability and puncture strength It is possible to produce a polyolefin microporous membrane that is excellent in balance and also in heat shrinkage. By using the obtained polyolefin microporous membrane as a battery separator, it is possible to improve not only battery characteristics such as discharge characteristics at low temperature range and cycle characteristics, but also battery productivity and battery safety. Become.

Front page continuation (51) Int.Cl. ⁷ Identification code FI theme code (reference) B29K 105: 04 B29K 105: 04 B29L 7:00 B29L 7:00 C08L 23:00 C08L 23:00 F term (reference) 4F074 AA16 AA17 AB01 CA03 CB34 CB37 CC02X CC02Z CC04Z CC05X CC29Y CC32Z DA49 4F210 AA03 AA04A AG01 AG20 AH33 QC12 QC15 QQ13 AQ33 A01 AQ03 AQ03 AQ03 AQ03 AQ11 AQ16 AA33 AQ17 AA17 A02 AM05 AM07 CJ02 CJ08 DJ04 EJ11 EJ12 HJ04 HJ09 HJ14

Claims (5)

[Claims] 1. A polyolefin microporous membrane comprising a polyolefin having a weight-average molecular weight of 5 × 10 ⁵ or more as an essential component of polyethylene, and having a porosity of 50 to 95% after exposure at 105 ° C. for 8 hours. Machine direction (MD) and transverse direction (TD) heat shrinkage is less than 5%, and piercing strength is 7840 mN
/ 25μm or more and the air permeability of 20μm is 20
A microporous polyolefin membrane characterized by ~ 500 sec / 100 cc. 2. A gel obtained by melt-kneading a polyolefin containing polyethylene as an essential component having a weight average molecular weight of 5 × 10 ⁵ or more and a liquid solvent, extruding the obtained melt-kneaded product through a die, and cooling. Biaxially stretching the shaped article,
The surface tension of the drawn product at 25 ℃ was 24 mN / m.
After removing the liquid solvent using the following washing solvent (A), the stretched product after washing is stretched again at least uniaxially at a temperature lower than the crystal dispersion temperature of the polyolefin, and then the crystal dispersion temperature of the polyolefin. A method for producing a microporous polyolefin membrane, which comprises heat-setting treatment so that there is no dimensional change in both the machine direction (MD) and the transverse direction (TD) at the temperature of the melting point. 3. A gel obtained by melt-kneading a polyolefin containing polyethylene as an essential component having a weight average molecular weight of 5 × 10 ⁵ or more and a liquid solvent, extruding the obtained melt-kneaded product through a die, and cooling. Biaxially stretching the shaped article,
At least two microporous membranes (A) were prepared from the obtained stretched product by removing the liquid solvent with a washing solvent (A) having a surface tension of 24 mN / m or less at 25 ° C. A microporous membrane (B) is prepared by stretching at least one of the membranes (A) again at least uniaxially at a temperature below the crystal dispersion temperature of the polyolefin, and then at least one of the microporous membranes (A). One piece and the microporous membrane (B)
The laminated film (AB) obtained by laminating at least one of the above is sized in both the machine direction (MD) and the transverse direction (TD) at the crystal dispersion temperature to the melting point of the polyolefin. A method for producing a polyolefin microporous membrane, which comprises heat-setting treatment so as not to change. 4. A gel obtained by melt-kneading a polyolefin having a polyethylene of 5 × 10 ⁵ or more in weight average molecular weight as an essential component and a liquid solvent, extruding the obtained melt-kneaded product through a die, and cooling. Biaxially stretching the shaped article,
At least two microporous membranes (A) were prepared by removing the liquid solvent from the obtained stretched product using a washing solvent (A) having a surface tension of 24 mN / m or less at 25 ° C. The porous film (A) is laminated by bonding at least two sheets, and the obtained laminated film (A) is stretched again at least uniaxially at a temperature lower than the crystal dispersion temperature of the polyolefin, and then the polyolefin. Crystal dispersion temperature ~
A method for producing a microporous polyolefin membrane, which comprises heat-setting at a melting point temperature so that there is no dimensional change in both the machine direction (MD) and the transverse direction (TD). 5. A battery separator using the polyolefin microporous membrane according to claim 1.