JPH11106535A - Polyolefin porous membrane and its manufacture and battery separator therefrom - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a best polyolefin porous membrane for a battery separator having excellent ion permeability and mechanical strength at an ambient temperature and losing ion permeability at a lowest possible temperature in abnormal temperature range. SOLUTION: This membrane comprises an ultra high molecular polyolefin (component A) having not lower than 1×10<6> weight average mol.wt., a polyolefin wax (component B) having 700-10,000 weight average mol.wt., and a polyoleflin (component C) having less than 1×10<6> weight average mol.wt., and a polyolefin compsn. comprising not lower than 1 wt.% component A and not less than 20 wt.% component B is prepd. and 50-90 pts.wt. this compsn. and 10-50 pts.wt. solvent are mixed, then, this mixture is extruded from a dice into a sheet and, after cooling, it is extended into a porous sheet under a condition of a temperature not lower than a temperature 10 deg.C higher than the m.p. of the super high molecular polyolefin, then, the solvent is removed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a polyolefin porous membrane, a method for producing the same, and a battery separator using the same.

[0002]

2. Description of the Related Art Porous plastic membranes are used for various applications such as separators for electronic parts, various filters, moisture-permeable waterproof clothing, reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes. The porous membrane is used as a battery separator.

Conventionally, a porous film made of polyolefin has been prepared by, for example, mixing a pore-forming agent composed of fine powder of a different polymer or the like into a polyolefin, micro-dispersing the mixture, and extracting the pore-forming agent by a mixed extraction method. A phase separation method that forms a porous structure by microphase separation with a solvent, and interfacial fracture between heterogeneous solids is caused by giving strain such as stretching to a polyolefin molded body in which heterogeneous solids are micro-dispersed, and voids are generated. It was manufactured by a stretching method or the like for making it porous. However, conventionally, a polyolefin having a molecular weight of less than about 500,000 is usually used, so that there is a limit to a thin film and high strength by stretching.

To solve this problem, ultrahigh molecular weight polyolefins that can be formed into films having high strength and high tensile elasticity have been developed, and various methods for producing high-strength porous films using the same have been proposed.

For example, JP-A-58-5228 discloses that an ultrahigh molecular weight polyolefin is dissolved in a non-volatile solvent, a fibrous or film-like gel is formed from this solution, and the non-volatile solvent is removed from the gel. There is disclosed a technique of performing heat-drawing after performing an extraction treatment with a volatile solvent. However, this technique has various problems. First, a gel having porous fibers highly swollen with a non-volatile solvent was used as a gel.
Even if the film is stretched in the direction, the film cannot be stretched in a high orientation, is easily broken due to the expansion of the network structure accompanying the stretching, the strength of the obtained film is small, and the pore size to be formed is large. In addition, when the gel is dried after extracting the non-volatile solvent with the volatile solvent, the gel has a network structure that shrinks and densifies, but the unevenness of the volatile solvent causes warpage of the raw film. However, there is a problem that stretching at a high magnification cannot be performed due to shrinkage and densification.

In Japanese Patent Application Laid-Open No. 63-273652, a solution of an ultrahigh molecular weight polyolefin having a weight average molecular weight of 5 × 10 ⁵ or more is prepared, and this solution is extruded from a die while being rapidly cooled to a gelling temperature or lower. A technique is disclosed in which the content of the ultrahigh molecular weight polyolefin in the gel-like molded product is adjusted to 10 to 90% by weight, the film is stretched at a temperature not higher than the melting point of the ultrahigh molecular weight + 10 ° C, and then the residual solvent is removed. ing. However, in this technique, since the ultrahigh molecular weight polyolefin is biaxially stretched, it is necessary to prepare a dilute solution of the polyolefin, and it is difficult to form the dilute solution into a sheet, and furthermore, a solvent is contained in the sheet. Since it is contained in excess and the desired porous membrane cannot be obtained even if it is stretched as it is, it is necessary to remove the solvent and adjust the amount of solvent in the sheet before the stretching treatment. Had problems.

JP-A-3-64334 discloses a method for producing a polyolefin porous membrane containing an ultrahigh molecular weight polyolefin and having a value of (weight average molecular weight / number average molecular weight) in a specific range. . According to this method, the polyolefin solution can be made a high-concentration solution, and the stretchability of the sheet is good, so that the problem of the production efficiency is partially solved. However, in this production method, since the tensile modulus of the gel-like sheet is low, the thickness of the sheet tends to change during the production process, and the thickness of the obtained porous membrane may be non-uniform. Further, although the concentration of the polyolefin solution is high, 50% by weight or more of the solvent is present in the gel-like sheet, and the solvent extraction step cannot be omitted.

Therefore, the conventional manufacturing method has problems in terms of manufacturing efficiency and product quality.

Furthermore, when a polyolefin porous membrane is used as a battery separator, a function of preventing a rise in temperature due to a short circuit of the battery may be required. For example, when a polyolefin porous membrane is used as a battery separator for a lithium battery, at a temperature lower than the ignition temperature of lithium, the polyolefin is melted and pores of the membrane are closed, and it is necessary to lose ion permeability. . If the ion permeability is lost, the battery reaction can be stopped and no further temperature rise occurs. Such a function is usually called a shutdown function (SD function).
Since the SD function preferably occurs at a low temperature, it is desired to develop a battery separator that exhibits the SD function at a temperature lower than that of the conventional polyolefin porous membrane.

[0010]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a battery separator having excellent ion permeability and mechanical strength at ordinary temperatures and losing ion permeability at a temperature as low as possible in an abnormal temperature range. A first object is to provide an optimal polyolefin porous membrane, and a second object is a method for producing a polyolefin porous membrane having high production efficiency and uniform product quality.

[0011]

In order to achieve the above object, a polyolefin porous membrane of the present invention comprises an ultrahigh molecular weight polyolefin having a weight average molecular weight of 1 × 10 ⁶ or more (component A).
And a polyolefin wax having a weight average molecular weight of 700 to 10000 (component B), wherein the content of the component A is 1% by weight or more and the content of the component B is 20% or more.
Formed from a polyolefin composition that is at least
The thickness is 1 to 50 μm, the porosity is 10 to 90%, the Gurley type air permeability is 1 to 10000 sec / 100 cc, and the breaking strength is 1
00 kg / cm ² or more, ion permeation loss temperature of 135
It has a configuration that is not more than ° C.

As described above, in the polyolefin porous membrane of the present invention, the composition of the polyolefin composition is specified to the above-mentioned composition, and the physical properties are set to the above-mentioned specific ranges, so that the ordinary It is excellent in ion permeability and mechanical strength at temperature, and loses ion permeability at as low a temperature as possible in an abnormal temperature range, and is optimal as a battery separator. Therefore, the battery separator of the present invention comprises the polyolefin porous membrane of the present invention, and the battery of the present invention uses the battery separator of the present invention.

In the polyolefin porous membrane of the present invention, the ultrahigh molecular weight polyolefin (component A) is ultrahigh molecular weight polyethylene, the polyolefin wax (component B) is polyethylene wax, and the polyolefin composition comprises In addition to the component B, it is preferable to contain a polymer having a weight average molecular weight of 1 × 10 ⁴ or more and less than 1 × 10 ⁶ (component C), mainly composed of ethylene. Here, in the present invention, “mainly composed of ethylene” in the component C means that the proportion of ethylene units contained in the polymer is 50 mol% or more.

Next, the method for producing a polyolefin porous membrane of the present invention includes a first production method and a second production method as described below.

First, the first production method of the present invention comprises an ultrahigh molecular weight polyolefin having a weight average molecular weight of 1 × 10 ⁶ or more (component A), a polyolefin wax having a weight average molecular weight of 700 to 10,000 (component B), and a weight average molecular weight. Molecular weight 1
A polyolefin composition containing less than 10 ⁶ polyolefin (component C), wherein the content of the component A is 1% by weight or more and the content of the component B is 20% by weight or more, is prepared. 50 to 90 parts by weight of a product and a solvent 10
-50 parts by weight, extruding this mixture from a die to form a sheet, cooling, and then stretching the sheet under a temperature not higher than 10 ° C. higher than the melting point of the ultrahigh molecular weight polyolefin to form a porous membrane. Then, the solvent is removed.

The second production method of the present invention comprises the steps of: providing an ultrahigh molecular weight polyolefin having a weight average molecular weight of 1 × 10 ⁶ or more (component A), a polyolefin wax having a weight average molecular weight of 700 to 10,000 (component B), Molecular weight 1
A polyolefin composition containing less than × 10 ⁶ polyolefin (component C), wherein the content of the component A is 1% by weight or more and the content of the component B is 40% by weight or more, is prepared. A method of extruding the melt, extruding the melt from a die to form a sheet, cooling, and then stretching the sheet under a condition not higher than the melting point of the ultrahigh molecular weight polyolefin by 10 ° C. or less to form a porous film. It is.

The first and second production methods are excellent in sheet stretchability, good in sheet thickness stability, and do not require a solvent removal step before stretching. Therefore, according to the first production method and the second production method of the present invention, it is possible to efficiently produce the high-performance porous polyolefin membrane of the present invention with a uniform film thickness.

In the production method of the present invention, the polyolefin wax (component B) is a polyethylene wax,
The polyolefin (component C) is preferably a polymer mainly composed of ethylene and having a weight average molecular weight of 1 × 10 ⁴ or more and less than 1 × 10 ⁶ .

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION

The polyolefin porous membrane of the present invention contains the ultra high molecular weight polyolefin of the component A and the polyolefin wax of the component B, wherein the content of the component A is 1% by weight or more and the content of the component B is not less than 1% by weight. 20%
% Or more by weight of the polyolefin composition.

The ultrahigh molecular weight polyolefin of the component A must have a weight average molecular weight of 1 × 10 ⁶ or more, preferably 1 × 10 ⁶ to 15 × 10 ⁶ .
When the weight average molecular weight is less than 1 × 10 ⁶ ,
The maximum draw ratio is low, and the desired porous film cannot be obtained. On the other hand, the upper limit of the weight average molecular weight is not particularly limited, but those having a weight average molecular weight exceeding 15 × 10 ⁶ may be inferior in moldability in molding a sheet-like molded product in production.

Such an ultrahigh molecular weight polyolefin (A)
Component) as ethylene, propylene, 1-butene,
Crystalline homopolymers or copolymers obtained by polymerizing 4-methyl-1-pentene, 1-hexene and the like can be mentioned. Of these, ultrahigh molecular weight polyethylene is preferable, and particularly preferably high density ultrahigh molecular weight polyethylene.

The content ratio of the component A in the polyolefin composition must be 1% by weight or more based on the whole polyolefin composition. The content ratio of the component A is 1% by weight.
If it is less than 10, entanglement of the molecular chain of the ultra-high molecular weight polyolefin that contributes to improvement in stretchability is hardly formed at the time of production, and a high-strength porous membrane cannot be obtained.
On the other hand, the upper limit of the content ratio is not particularly limited, but if it exceeds 90% by weight, it may be difficult to achieve a high concentration of the polyolefin solution prepared at the time of production.

Next, the polyolefin wax as the component B must have a weight-average molecular weight of 700 to 10,000, and preferably in the range of 1,000 to 8,000. If the weight-average molecular weight is less than 700, the properties of the polyolefin composition at room temperature become waxy solids at the time of production, so that the tensile modulus of the extruded sheet does not improve, and upon extraction after stretching, The polyolefin wax is extracted together with the solvent, which leads to an increase in extraction waste, which is not economical. On the other hand, when the weight average molecular weight is 1
If it exceeds 0000, a problem arises in stretchability at a high magnification during production, uniform stretching cannot be performed, and loss of ion permeability of the obtained porous membrane at low temperature (135 ° C. or lower) cannot be achieved. . Also, when the addition amount of the polyolefin wax is less than 20% by weight, the uniformity of stretching is poor at the time of production, and sufficient loss of ion permeability in the obtained porous membrane cannot be achieved.

Examples of such a polyolefin wax include those of the same kind as the above-mentioned ultrahigh molecular weight polyolefin, and a polyethylene wax which is a polymer mainly composed of ethylene is preferred.

The polyolefin composition further comprises the above-mentioned A
In addition to the component and the component B, the weight average molecular weight is 1 × 10 ⁶
It is preferable to contain less than the polyolefin (component C). The lower limit of the molecular weight of the component C is preferably 1 × 10 ⁴ or more. This is because if a polyolefin having a weight average molecular weight of less than 1 × 10 ⁴ is used, there is a possibility that breakage is likely to occur during stretching during production. Note that this C
Particularly preferred weight average molecular weights of the components are from 1 × 10 ⁴ to 1 ×.
The range is 10 ⁵ . The compounding ratio of the component C is generally 0 to 80% by weight, preferably 20 to 70% by weight, particularly preferably 40 to 70% by weight, based on the whole polyolefin composition.
It is in the range of 60% by weight.

Examples of such a polyolefin include those of the same type as the above-mentioned ultrahigh molecular weight polyolefin, and high density polyethylene which is a polymer mainly composed of ethylene is particularly preferable.

The polyolefin composition according to the present invention may contain other components in addition to the components A, B and C. For example, if necessary, an antioxidant, an ultraviolet absorber, Various additives such as an anti-blocking agent, a pigment, a dye, and an inorganic filler can be added as long as the object of the present invention is not impaired.

Next, there are two types of manufacturing methods of the present invention, a first manufacturing method and a second manufacturing method. Examples of these manufacturing methods will be described below.

First, the component A, the component B and the component C are blended in the above-mentioned predetermined ratio (the above-mentioned various additives may be blended if necessary) to prepare a polyolefin composition.
Then, in the first production method, 50 to 90 parts by weight of the polyolefin composition and 50 to 10 parts by weight of the solvent are mixed and heated and melted to prepare a polyolefin composition solution. Further, in the second production method, the B component is blended by 40% by weight or more based on the whole polyolefin composition. Therefore, the polyolefin composition is heated and melted without using a solvent.

The solvent is not particularly limited as long as it can sufficiently dissolve the polyolefin composition. For example, nonane, decane, undecane, dodecane, aliphatic or cyclic hydrocarbons such as paraffin oil, or a mineral oil fraction having a boiling point corresponding thereto, and the like. To obtain, a non-volatile solvent such as paraffin oil is preferred.

The heat dissolution in the first production method is as follows:
The stirring is performed at a temperature at which the polyolefin composition is completely dissolved in the solvent. The temperature varies depending on the polymer and the solvent used. For example, in the case of a polyethylene composition, the temperature is usually in the range of 140 ° C to 250 ° C. The concentration of the polyolefin composition solution is 50 to 90% by weight, preferably 50 to 75% by weight. When the concentration is less than 50% by weight, not only is the amount of solvent used too large to be economical, but also the tensile elastic modulus of the molded sheet becomes low, and the thickness of the sheet changes during the course of conveyance, resulting in a porous film having good thickness accuracy. Can not get. On the other hand, if the concentration exceeds 90%, the amount of the solvent is too small, so that a problem occurs in dispersion of the solvent, and it becomes difficult to prepare a uniform solution. In addition,
Upon heating and dissolving, it is preferable to add an antioxidant for the purpose of preventing oxidation of the polyolefin.

The heating and dissolving in the second production method is preferably carried out with stirring at a temperature at which the polyolefin composition uniformly melts with the polyolefin wax. The temperature varies depending on the polymer used. For example, in the case of a polyethylene composition, the temperature is usually 140 ° C to 25 ° C.
It is in the range of 0 ° C. The amount of the polyolefin wax in the polyolefin composition is at least 40% by weight, preferably in the range of 40 to 70% by weight, based on the entire polyolefin composition. When the amount of the polyolefin wax is less than 40 parts by weight, the viscosity of the melt becomes high, so that sheet forming and uniform stretching become difficult. When heating and melting,
It is preferable to add an antioxidant to prevent oxidation of the polyolefin.

Next, a heated solution or melt of the polyolefin composition is extruded from a die and molded. As the die, a sheet die having a normal rectangular base shape is used, but a double cylindrical hollow die, an infection die, and the like can also be used, and there is no particular limitation.
The die gap when using a sheet die is usually
0.1 to 5 mm.
Heated to ~ 250 ° C. The extrusion speed at this time is usually 0.2 to 5 m / min.

The solution or melt extruded from the die is cooled and formed into a sheet. Cooling is performed until the sheet-shaped composition becomes lower than the softening point temperature of the polyolefin composition. 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 by a refrigerant, or the like can be used. The solution or the melt extruded from the die may be taken at a take-up ratio of 1 to 10, preferably 1 to 5, before or during cooling. When the take-up ratio is 10 or more, neck-in becomes large, and breakage tends to occur during stretching, which is not preferable.

Next, the sheet composition is stretched. Stretching can be performed at a predetermined magnification by heating a sheet-like molded product and using a usual tenter method, a roll method, an inflation method, a rolling method, or a combination thereof. The stretching is preferably biaxial stretching, and may be any of simultaneous longitudinal and transverse stretching or sequential stretching, and particularly preferably simultaneous biaxial stretching.

The stretching temperature is 10 ° C. or higher (Tm + 10 ° C.) higher than the melting point (Tm ° C.) of the ultrahigh molecular weight polyolefin.
Below), preferably in the range from the melting point to the softening point of the ultrahigh molecular weight polyolefin. For example, when the ultrahigh molecular weight polyolefin is ultrahigh molecular weight polyethylene, the stretching temperature is usually in the range of 90 ° C to 140 ° C, and preferably in the range of 100 ° C to 130 ° C. The stretching temperature is
When the temperature exceeds Tm + 10 ° C., the molecular chains cannot be oriented by stretching due to melting of the resin. When the stretching temperature is lower than the softening point temperature, the softening of the resin is insufficient, so that the sheet-like composition is liable to break in the stretching, and high-magnification stretching cannot be performed.

The stretching ratio varies depending on the thickness of the raw material (sheet-like composition), but is at least 2 in the uniaxial direction.
Times or more, preferably 3 to 20 times, 10 times or more in area magnification,
Preferably it is 20 to 400 times. If the areal magnification is less than 10 times, stretching may be insufficient and a high tensile elasticity and high strength porous membrane may not be obtained. On the other hand, when the area magnification exceeds 400 times, there is a possibility that restrictions may be caused in a stretching apparatus, a stretching operation, and the like. By this stretching operation, in the second manufacturing method,
The desired polyolefin porous membrane can be obtained.
In the production method described above, the solvent is further removed from the stretch-formed product.

As a method for removing the solvent, for example,
A method of washing the stretch molded product with a solvent can be used. As a solvent for washing, a solvent which dissolves the solvent without dissolving the polyolefin composition and which has high volatility is appropriately selected and used. As such a cleaning solvent, for example,
Hydrocarbons such as heptane and hexane, 1-butanol, 1
Alcohols such as propanol, methyl ethyl ketone,
Ketones such as methyl isobutyl ketone, dichloroethane,
Examples include chlorinated hydrocarbons such as carbon tetrachloride, and esters such as ethyl acetate and butyl acetate. These washing solvents may be used as a mixture of two or more as necessary. The washing method can be performed by a method of immersing in a solvent for extraction, a method of showering the solvent, a method of a combination thereof, or the like.

In this washing, the residual solvent in the stretch molded product is reduced to 1
It is preferable to carry out until the amount becomes less than% by weight. After this washing, the washing solvent is removed by drying. Examples of the drying method include heat drying, air drying, and vacuum drying. It is preferable that the dried stretch-formed product is heat-set at a temperature in the range from the softening point to the melting point. Thus, the target polyolefin porous membrane is obtained by the first production method.

The obtained polyolefin porous membrane is preferably subjected to a hydrophilization treatment by plasma irradiation, a surfactant impregnation treatment, a surface graft treatment or the like, if necessary.

The thus obtained polyolefin porous membrane of the present invention has a thickness of 1 to 50 μm and a porosity of 1
0 to 90%, Gurley type air permeability is 1 to 10000 sec / 10
0 cc, breaking strength of 100 kg / cm ² or more, and ion permeability loss temperature of 135 ° C. or less. In addition, the preferable range of the various physical property values is a thickness of 10 to 30 μm, a porosity of 30 to 70%, and a Gurley-type air permeability of 1 to 2000 sec / 10.
0 cc, breaking strength of 200 kg / cm ² or more, and ion permeability loss temperature of 90 to 130 ° C.

The polyolefin porous membrane of the present invention is most suitable for use as a battery separator. In addition, for example, a membrane for an electrolytic capacitor, various filters, a moisture-permeable waterproof clothing, a reverse osmosis filtration membrane, Applications include ultrafiltration membranes and microfiltration membranes.

[0044]

Next, examples will be described together with comparative examples. In addition, each test method in an Example and a comparative example is as showing below.

(1) Weight-average molecular weight A GPC apparatus manufactured by Waters Corporation was used, GMH-6 manufactured by Tosoh Corporation was used for the column, o-dichlorobenzene was used for the solvent, the temperature was 135 ° C., and the flow rate was 1.0 ml. / Min, gel permeation chromatography (GP
It measured by the C) method.

(2) Film thickness The thickness was measured with a 1/10000 linear gauge.

(3) Tensile modulus Measured according to ASTM D882.

(4) Tensile Rupture Strength A polyolefin porous membrane was formed into a 10 mm-wide strip-shaped test piece, and this fracture strength was measured in accordance with ASTM D882.

(5) Air permeability Measured by a Gurley-type air permeability meter.

(6) Porosity Measured by a mercury porosimeter.

(7) Membrane Resistance An electrolyte was prepared by dissolving LiClO _{4 in} a mixed solvent of propion carbonate and 1,2-dimethoxyethane at a weight ratio of 1: 1 to a concentration of 1 mol / l. Next, a separator (polyethylene porous membrane) was sufficiently immersed in the electrolytic solution. Then, a diameter of 2 on both sides of the separator
cm of a platinum electrode was contacted and the resistance was measured when a voltage was applied, and this value was defined as the film resistance.

(8) Temperature at which ion permeability is lost When a separator (porous polyethylene membrane) is brought into contact with an iron plate heated to a predetermined temperature for 0.5 seconds, and when the membrane resistance of (5) becomes ∞, This temperature was defined as the temperature at which the ion permeability was lost.

(Example 1) Weight average molecular weight was 2.0 × 1
0 and ultra high molecular weight polyethylene 3 parts by weight of ^6, a weight-average polyethylene 14 parts by weight and weight-average molecular weight having a molecular weight of 4.9 × 10 ⁵ is mixed with polyethylene wax 43 parts by weight of 1000, and this mixture 60 parts by weight And 40 parts by weight of liquid paraffin (weight average molecular weight: 450) to prepare a solution of the polyethylene composition. Next, tetrakis [methylene-3- (3,5-di-t-butyl-4-hydroxylphenyl) -propionate] methane (Irganox 1010, manufactured by Ciba Geigy) was added to 100 parts by weight of the solution of the polyethylene composition. 0.3 parts by weight as an antioxidant were added and mixed. Dissolve the mixed liquid while heating and stirring at a temperature of 200 ° C. with a heating and stirring dissolution apparatus,
A homogeneous solution was obtained.

This solution was extruded by an extruder having a diameter of 45 mm.
A sheet-like molded product was produced while extruding from a T-die and taking up with a cooling roll. The obtained sheet-like molded product was set in a biaxial stretching machine, and was simultaneously biaxially stretched 5 × 5 times at a temperature of 115 ° C. and a stretching speed of 5 mm / sec. The obtained stretched membrane was washed with ethyl acetate to extract and remove the remaining liquid paraffin, and then dried to obtain a 25 μm-thick polyethylene porous membrane. Various tests were performed on the porous membrane by the above-described method to examine the characteristics. The results are shown in Table 1 below.

(Example 2) Weight average molecular weight was 2.0 × 1
0 and ultra high molecular weight polyethylene 3 parts by weight of ^6, was mixed with polyethylene 14 parts by weight of the weight average molecular weight of 4.9 × 10 ^5, a polyethylene wax 63 parts by weight of the weight average molecular weight of 1,000, the mixture 80 parts by weight And 20 parts by weight of liquid paraffin were mixed to prepare a solution of the polyethylene composition. This solution was used, and a thickness of 2 mm was obtained under the same conditions and operation as in Example 1 except that the draw ratio was 6 × 6 times.
A 5 μm polyethylene porous membrane was obtained. Various tests were performed on the porous membrane by the above-described method to examine the characteristics. The results are shown in Table 1 below.

Example 3 The weight average molecular weight was 2.0 × 1
0 and ultra high molecular weight polyethylene 3 parts by weight of ^6, and a polyethylene 14 parts by weight of the weight average molecular weight of 4.9 × 10 ^5, were mixed and a polyethylene wax 83 parts by weight of the weight average molecular weight of 1,000 as a raw material. Next, tetrakis [methylene-3- (3,5-di-t-) was added to 100 parts by weight of this raw material.
Butyl-4-hydroxyphenyl) -propionate] methane (Irganox 1010, manufactured by Ciba Geigy) was added and mixed as an antioxidant. This mixture is heated at a temperature of 200 ° C.
The mixture was dissolved with stirring to obtain a uniform solution. This solution was extruded from a T-die by an extruder having a diameter of 45 mm, and a sheet-like molded product was formed while being taken up by a cooling roll. The obtained sheet-like molded product was set on a biaxial stretching machine, and simultaneously biaxially stretched 7 × 7 times at a temperature of 115 ° C. and a stretching speed of 5 mm / sec to obtain a 25 μm thick polyethylene porous membrane. Was. Various tests were performed on the porous membrane by the above-described method to examine the characteristics. The results are shown in Table 1 below.

Example 4 Weight Average Molecular Weight was 2.0 × 1
0 and ultra high molecular weight polyethylene 3 parts by weight of ^6, was mixed with polyethylene 14 parts by weight of the weight average molecular weight of 4.9 × 10 ^5, a polyethylene wax 43 parts by weight of the weight average molecular weight of 4000, the mixture 60 parts by weight And 40 parts by weight of liquid paraffin were mixed to obtain a polyethylene composition solution, and a 25-μm-thick polyethylene porous membrane was obtained under the same conditions and operations as in Example 1. Various tests were performed on the porous membrane by the above-described method to examine the characteristics.
The results are shown in Table 1 below.

(Example 5) The weight average molecular weight was 2.0 × 1
0 and ultra high molecular weight polyethylene 3 parts by weight of ^6, was mixed with polyethylene 14 parts by weight of the weight average molecular weight of 4.9 × 10 ^5, a polyethylene wax 43 parts by weight of the weight average molecular weight of 7000, the mixture 60 parts by weight And 40 parts by weight of liquid paraffin were mixed to obtain a polyethylene composition solution, and a 25-μm-thick polyethylene porous membrane was obtained under the same conditions and operations as in Example 1. Various tests were performed on the porous membrane by the above-described method to examine the characteristics. The results are shown in Table 1 below.

(Comparative Example 1) The weight average molecular weight was 2.0 × 1
0 and ultra high molecular weight polyethylene 3 parts by weight of ^6, was mixed with polyethylene 14 parts by weight of the weight average molecular weight of 4.9 × 10 ^5, and this mixture 17 parts by weight, the polyethylene composition was mixed with liquid paraffin 83 parts by weight 25 μm thick under the same conditions and operation as in Example 1 except that
Was obtained. Various tests were performed on the porous membrane by the above-described method to examine the characteristics. The results are shown in Table 1 below.

[0060]

[Table 1]

As shown in Table 1 above, the polyethylene porous membrane of the example produced from the polyolefin composition in which the prescribed materials were blended in the prescribed proportions showed the tensile elasticity, film thickness and breaking strength of the sheet-like molded product. , Air permeability, porosity, membrane resistance, and permeability loss temperature were all in the appropriate ranges.
On the other hand, in the polyethylene porous membrane of the comparative example in which the polyethylene wax was not used, the tensile elasticity, the air permeability, and the membrane resistance of the sheet-like molded product were poor, and the permeability loss temperature was high.

[0062]

As described above, the polyolefin porous membrane of the present invention has high strength, excellent handling and processability,
Furthermore, it has excellent air permeability and loses ion permeability at low temperatures. Therefore, the polyolefin porous membrane of the present invention is most suitable as a battery separator. Also,
In addition, the polyolefin porous membrane of the present invention is suitable for various uses such as a diaphragm for an electrolytic capacitor, an ultrafine filtration membrane, an ultrafiltration membrane, various filters, and a porous membrane for moisture-permeable waterproof clothing.

Further, the method for producing a polyolefin porous membrane of the present invention is a method which is excellent in production efficiency and has a small variation in the thickness of the obtained porous membrane. That is, in the production method of the present invention, a solvent is not used or a small amount is used even when a polyolefin composition as a raw material is molded, so that a polyolefin porous membrane can be efficiently obtained. Further, since the tensile modulus of the sheet-like material obtained at the time of manufacture is high, the accuracy of the thickness of the obtained porous film is also good, and the quality of the product is uniform.

Claims (7)

[Claims] An ultrahigh molecular weight polyolefin having a weight average molecular weight of 1 × 10 ⁶ or more (component A) and a polyolefin wax having a weight average molecular weight of 700 to 10,000 (component B), wherein the content of the component A is 1 % By weight, and the content of the component B is 20% by weight or more.
m, porosity is 10 to 90%, Gurley type air permeability is 1 to 10
000 sec / 100cc, breaking strength 100kg / cm ²
As described above, a polyolefin porous membrane having a loss of ion permeability of 135 ° C. or less. 2. The ultra-high-molecular-weight polyolefin (component A) is ultra-high-molecular-weight polyethylene, the polyolefin wax (component B) is a polyethylene wax, and the polyolefin composition is mainly composed of ethylene in addition to the components A and B. Weight average molecular weight 1 × 10 ⁴ or more 1 × 1
0 ⁶ less polymer polyolefin porous membrane according to claim 1, further comprising a component (C). 3. A battery separator comprising the polyolefin porous membrane according to claim 1. 4. A battery using the battery separator according to claim 3. 5. An ultrahigh molecular weight polyolefin having a weight average molecular weight of 1 × 10 ⁶ or more (component A), and a weight average molecular weight of 70
It contains a polyolefin wax (component B) of 0 to 10,000 and a polyolefin (component C) having a weight average molecular weight of less than 1 × 10 ^6, and the content of the component A is 1% by weight.
As described above, a polyolefin composition having a content ratio of the component B of 20% by weight or more is prepared.
90 parts by weight and 10 to 50 parts by weight of a solvent are mixed, and the mixture is extruded from a die to form a sheet. After cooling, the sheet is heated at a temperature not higher than 10 ° C. higher than the melting point of the ultrahigh molecular weight polyolefin. To form a porous membrane by stretching the polyolefin membrane, and then removing the solvent. ^6. An ultrahigh molecular weight polyolefin having a weight average molecular weight of 1 × 10 ⁶ or more (component A), and a weight average molecular weight of 70.
It contains a polyolefin wax (component B) of 0 to 10,000 and a polyolefin (component C) having a weight average molecular weight of less than 1 × 10 ^6, and the content of the component A is 1% by weight.
As described above, a polyolefin composition having a content ratio of the component B of 40% by weight or more is prepared, the composition is melted, and the melt is extruded from a die to form a sheet. A method for producing a porous film made of polyolefin, comprising stretching the sheet under a condition of not more than 10 ° C. higher than the melting point of the high molecular weight polyolefin to form a porous film. 7. The ultra-high-molecular-weight polyolefin (component A) is ultra-high-molecular-weight polyethylene, the polyolefin wax (component B) is a polyethylene wax, and the polyolefin (component C) has a weight-average molecular weight of 1 × 10 mainly composed of ethylene. ^The method for producing a polyolefin porous membrane according to claim 5 or 6, wherein the polymer is a polymer having ^{4 to} less than 1 x 10 ⁶ .