JPH11322988A - Porous film, its preparation and use thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prepare a porous film preferably used in a separator of various batteries, particularly, batteries for an electric car, the film having high strength, a high specific surface, large pore volume and great path curvature, moreover, being excellent in ion permeability and further high speed charge and discharge characteristics. SOLUTION: This porous film comprises an ultra high mol.wt. polyolefin resin in a weight average molecular weight of 5×10<5> or more, or an ultra high mol.wt. polyolefin resin composition comprising at least 15 wt.% of the ultra high mol.wt. polyolefin resin and a polyolefin resin in a weight average molecular weight less than 5×10<5> , the porous film having a specific surface of 100 m<2> /g or more, pore volume of 0.5 cm<3> /g or more, the average diameter of through-holes of 0.03 μm or less and the maximum diameter of 0.1 μm or less, the average diameter of a fibril fiber constituting a three-dimensional network of 0.01-0.1 μm and the maximum diameter of 0.2 μm or less and the average path curvature of from above 2.5 magnifications to 10 magnifications or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous film made of an ultra-high molecular weight polyolefin resin, and more particularly, to a porous film having a high strength, a high specific surface area and a high pore volume.
Although the path of the hole through the membrane, that is, the through path, is long, it has excellent ion permeability and excellent high-speed charge / discharge characteristics. Therefore, the stability of various batteries, particularly batteries for electric vehicles, is high. The present invention relates to a porous film which can be suitably used as a high-performance separator having excellent durability and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, various batteries have been put to practical use, but recently, in order to cope with a cordless electronic device, a lightweight, high electromotive force and high energy can be obtained. Lithium batteries with low self-discharge are attracting attention. For example, a cylindrical lithium ion secondary battery is
It is widely used for mobile phones and notebook computers, and is expected to be used as a battery for electric vehicles in the future.

Examples of such a negative electrode material for a lithium battery include interlayer compounds such as metallic lithium, a lithium alloy and a carbon material capable of inserting and extracting lithium ions. On the other hand, examples of the positive electrode material include oxides of transition metals such as cobalt, nickel, manganese, and iron, and composite oxides of these transition metals and lithium.

Generally, in such a lithium battery, a separator is provided between the positive electrode and the negative electrode as described above in order to prevent a short circuit between the electrodes.
As such a separator, a porous film having a large number of micropores is generally used in order to secure the permeability of ions between the positive electrode and the negative electrode.

Conventionally, as such a battery separator, an ultra-high molecular weight polyolefin resin, together with other polyolefin resins, if necessary, is dissolved by heating in an appropriate solvent, and then formed into a gel-like sheet. Various methods have been proposed for producing a porous film by subjecting the sheet to a stretching treatment and performing a solvent removal treatment before and after the stretching treatment to remove a solvent remaining in the sheet.

For example, Japanese Patent Application Laid-Open No. 7-228718 discloses a polyolefin resin composition having at least 10% by weight of an ultrahigh molecular weight polyolefin resin having a weight average molecular weight of 1 × 10 ⁶ or more, and having an average fibril diameter of 0.01.
00.2 μm, the average diameter of the through holes is 0.01-0.1 μm, the porosity is 35-95%, the specific surface area is 20-400 m ² / g,
The curvature ratio, which is the ratio of the penetration path to the film thickness, is 1.5.
A porous film that is 22.5 times is described.

However, in order to use a porous film made of an ultra-high-molecular-weight polyolefin resin practically as a separator for an electric vehicle battery, the film must have higher strength, a higher specific surface area, a higher pore volume, There is a strong demand for excellent liquid retention properties, and further excellent ion permeability and high-speed charge / discharge characteristics.

[0008]

SUMMARY OF THE INVENTION The present inventors have made intensive studies to meet such a demand for a porous film made of an ultrahigh molecular weight polyolefin resin, and as a result, have found that fibrils constituting a three-dimensional network structure of the porous film are obtained. A porous film having a small diameter and a uniform distribution, and the through holes also have a maximum pore diameter of 0.1 μm or less, and a narrow distribution of the through holes can be obtained. The inventors have found that a porous film has all of the above-mentioned properties and, in particular, has excellent ion permeability despite having a very large average curvature, and has led to the present invention.

Therefore, the present invention provides a high strength, high specific surface area,
With high pore volume, large mean curvature,
In addition, it is an object of the present invention to provide a porous film having excellent ion permeability and excellent high-speed charge / discharge characteristics, and which can be suitably used as a separator for various batteries, particularly, batteries for electric vehicles, and a method for producing the same. Aim.

[0010]

According to the present invention, there is provided a porous film comprising an ultrahigh molecular weight polyolefin resin having a weight average molecular weight of 5 × 10 ⁵ or more, or at least 15% by weight of the ultrahigh molecular weight polyolefin resin and a weight average molecular weight of 5%. Is 5
An ultrahigh molecular weight polyolefin resin composition comprising a polyolefin resin having a specific surface area of less than 10 ⁵
0 m ² / g or more, pore volume 0.5 cm ³ / g or more, average diameter of through-holes 0.03 μm or less, maximum pore diameter 0.1 μm or less, average diameter of fibrils constituting a three-dimensional network structure Is 0.01 to 0.1 μm, the maximum diameter is 0.2 μm or less, and the average curvature is more than 2.5 times and 10 times or less.

The method for producing such a porous film according to the present invention is characterized in that the weight average molecular weight of the ultrahigh molecular weight polyolefin resin is not less than 5 × 10 ⁵ or at least 15% by weight. Is heated by heating a mixture comprising 5 to 30% by weight of an ultrahigh molecular weight polyolefin resin composition comprising a polyolefin resin having a solidification point of not more than 3 × 10 ⁵ and a solvent having a freezing point of not more than -10 ° C. The molecular weight polyolefin resin or its composition is dissolved in the above solvent, the mixture is kneaded at a temperature in the range of 115 to 185 ° C., and then cooled to a temperature below the freezing point of the solvent using the obtained kneaded product. While forming into a gel-like sheet, the ultra-high molecular weight polyolefin resin is crystallized, When the melting point of the fat is M, the gel sheet is biaxially stretched at a temperature in the range of (M + 5) ° C to (M-30) ° C, and then the obtained stretched film is subjected to a desolvation treatment. Features.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the production of a porous film according to the present invention will be described. The material for the porous film used in the present invention has a weight average molecular weight of 5 × 10
An ultrahigh molecular weight polyolefin resin composition comprising at least ⁵ ultrahigh molecular weight polyolefin resin or a polyolefin resin having at least 15% by weight of the ultrahigh molecular weight polyolefin resin and a weight average molecular weight of less than 5 × 10 ⁵ .

In the present invention, for the sake of simplicity, the ultrahigh molecular weight polyolefin resin and a composition containing the same are collectively referred to as an ultrahigh molecular weight polyolefin resin (composition).

In the present invention, the ultrahigh molecular weight polyolefin resin has a weight average molecular weight of 5 × 10 ^{5 to} 20 × 10 ^6.
And preferably 1 × 10 ⁶ to 15 × 10 ⁶
In the range. Examples of such an ultrahigh molecular weight polyolefin resin include homopolymers, copolymers, and mixtures thereof such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, and 1-hexene. . However, among them, in the present invention, an ultrahigh molecular weight polyethylene resin is preferably used.

The above ultra-high molecular weight polyolefin resin composition
Together with the ultrahigh molecular weight polyolefin resin
An outer polyolefin resin may be included. This super high
Polyolefin resins other than high molecular weight polyolefin resins
Means that the weight average molecular weight is 1 × 10 ^Four5 × 10 or more^FiveLess than
And preferably 1 × 10^Four~ 3 × 10^Fiveof
In range. As such a polyolefin resin,
For example, ethylene, propylene, 1-butene, 4-methyl
Homopolymers such as 1-pentene and 1-hexene, and copolymers
Coalescence or mixtures thereof. Only
In particular, in the present invention, high-density polyethylene
Resins are preferably used.

In the present invention, when an ultrahigh molecular weight polyolefin resin composition is used, the composition must contain at least 15% by weight of the ultrahigh molecular weight polyolefin resin.

In the production of the porous film according to the present invention,
First, the ultra-high molecular weight polyolefin resin (composition) 5
-30% by weight, preferably 8-20% by weight, and 95-70% by weight, preferably 92-80% by weight of a solvent having a freezing point of -10 ° C or less, are mixed in a uniform slurry,
This is heated and stirred to dissolve the ultrahigh molecular weight polyolefin resin (composition) in the solvent, and knead the resulting solution mixture to a temperature in the range of 115 to 185 ° C.
Prepare a kneaded product.

The solvent is not particularly limited as long as it dissolves the ultrahigh molecular weight polyolefin resin (composition) well and has a freezing point of -10 ° C. or lower. Has a freezing point of -10
Those having a temperature range of -45 ° C to -45 ° C are preferably used. Preferred specific examples of such a solvent include, for example, aliphatic or cyclic hydrocarbons such as decane, decalin, and liquid paraffin, and mineral oil fractions having a freezing point corresponding thereto. However, among these, a non-volatile solvent such as liquid paraffin is preferable, and a non-volatile solvent having a freezing point of -15 ° C or less and a kinematic viscosity at 40 ° C of 65 cst or less is particularly preferably used.

Ultra high molecular weight polyolefin resin (composition)
When the ultrahigh molecular weight polyolefin resin (composition) exceeds 30% by weight in the solution mixture of the solvent and the solvent, particularly, the solubility of the ultrahigh molecular weight polyolefin resin in the solvent is insufficient, and At times, the ultrahigh molecular weight polyolefin resin is not stretched and unraveled near the cut-off state, making it difficult to obtain sufficient entanglement of the polymer chains. Further, as described later, while cooling the kneaded material, the mixture is formed into a sheet, the resin is crystallized, and then the film is stretched to form a stretched film. It is difficult to obtain a porous film having characteristics regarding the fibril diameter, the through-hole diameter and the average curvature by the above method. On the other hand, when the ultrahigh molecular weight polyolefin resin (composition) is less than 5% by weight, it is obtained. The porous film has poor strength.

In the present invention, when kneading a solution obtained by dissolving the ultrahigh molecular weight polyolefin resin (composition) in a solvent at a temperature exceeding 185 ° C., the viscosity of the solution is too low. Therefore, a sufficient shearing force cannot be applied to the kneaded material, while the kneading temperature is 1
When the temperature is lower than 15 ° C., the mixture cannot be kneaded effectively. Thus, in kneading the mixture, it is possible to obtain sufficient entanglement of the polymer chains of the ultrahigh molecular weight polyolefin resin as described above. Have difficulty.

In the present invention, in order to obtain sufficient entanglement of the polymer chains of the ultrahigh molecular weight polyolefin resin, a high shear force is applied to the solution mixture of the ultrahigh molecular weight polyolefin resin (composition) and a solvent. It is preferable to knead while acting. If a sufficient shearing force cannot be applied during kneading, sufficient entanglement of the polymer chains of the ultrahigh molecular weight polyolefin resin may not be obtained. Therefore, according to the present invention, the kneading of the solution mixture of the ultra-high-molecular-weight polyolefin resin (composition) and the solvent usually requires a kneader or a twin-screw extruder capable of giving a strong shear force to the mixture. It is preferably used.

Next, according to the present invention, the temperature is lower than the freezing point of the solvent, preferably-, using the solution-kneaded mixture of the ultrahigh molecular weight polyolefin resin (composition) thus obtained and the solvent. A temperature in the range of 10 ° C to -45 ° C,
Preferably, the ultrahigh molecular weight polyolefin resin (and high-density polyethylene resin) is crystallized while cooling to a temperature in the range of −15 ° C. to −40 ° C. to form a gel-like sheet. Usually, the thickness of the gel-like sheet is suitably in the range of 0.1 to 5.0 mm.

As described above, the cooling to the temperature below the freezing point of the solvent using the kneaded product in the form of a solution of the ultrahigh molecular weight polyolefin resin (composition) and the solvent is not particularly limited, For example, two metal plates may be cooled in advance with dry ice, the kneaded material may be sandwiched between these metal plates, and the kneaded material may be pressed to form a sheet.

According to the present invention, when the kneaded material is formed into a sheet while cooling the kneaded material, not only the surface layer of the obtained sheet but also the center of the sheet can be finely crystallized with the kneaded material. Is preferably rapidly cooled. Therefore, the cooling rate is preferably 50 ° C./min or more on average.
In the solution state, that is, when the cooling rate at the time of molding from the kneaded material to the sheet is low, the kneading causes the fibrils to be stretched and entangled to return to the shape of a hair, forming a thick fiber, so that it is thin, and It is difficult to form a porous membrane structure having a large curvature ratio formed of uniform fibrils, and a porous membrane structure having large through holes is partially formed.

That is, in general, when a crystalline high molecular weight resin is crystallized, lamellar crystals are formed, and the thickness of the lamellar crystals largely depends on the crystallization temperature, and the larger the difference between the melting point and the crystallization temperature, the greater the difference. The thickness of the lamella crystal becomes smaller. According to the present invention, a kneaded product of an ultrahigh molecular weight polyolefin resin (composition) and a solvent is heated from a high temperature of 115 to 185 ° C to a temperature below the freezing point of the solvent used, that is, -10 ° C.
While being cooled to the following temperature, preferably, while being quenched, it is formed into a gel-like sheet, so that the thickness of the lamella crystal is very small, and therefore, when the gel-like sheet is stretched to obtain a stretched film, As a result of the crystal being divided into microcrystals, a porous film having a microfibril structure composed of fibrils having a small fiber diameter and being uniform and having a very large curvature ratio, which is a ratio of a penetration path to a film thickness, is obtained. Seems to be able to.

According to the present invention, even if the kneaded material is formed into a gel-like sheet while being rapidly cooled from a solution state and the resin is crystallized, the crystal structure of the resin in such a gel-like sheet is determined by the solvent used. When storing this gel-like sheet at a temperature equal to or higher than the freezing point, similar to the case where the cooling rate is slow, by kneading, it is stretched, the entangled fibrils return to the shape of a hair, forming thick fibers, It is difficult to form a fine and uniform porous structure, and large through holes are partially formed. Therefore, while rapidly cooling the kneaded product of the ultra-high-molecular-weight polyolefin resin composition and the solvent, it is formed into a gel-like sheet, and thus, the gel-like sheet having a crystal structure is immediately stretched or stored, It is desirable to store the obtained gel-like sheet at a temperature below the freezing point of the solvent.

As described above, when the cooling rate at the time of forming the kneaded material into a sheet is low, or when the obtained sheet is stored at a temperature higher than the freezing point of the solvent using the obtained sheet, the above-described phenomenon occurs. The resulting porous film lacks the fineness and uniformity of the pore structure and has relatively large pores, so that the strength, particularly the piercing strength, becomes poor.

According to the present invention, when the melting point of the ultrahigh molecular weight polyolefin resin is M, the gel-like sheet is heated at a temperature in the range of (M + 5) ° C. to (M-30) ° C., preferably M The film is biaxially stretched at a temperature in the range from ℃ to (M-25) ℃. This biaxial stretching may be either sequential or simultaneous biaxial stretching, but is preferably simultaneous biaxial stretching. In the present invention, the stretch ratio of the gel-like sheet is 3 to 32 times in one direction, and the area stretch ratio is 9 to 1 times.
A range of 024 times is appropriate, and preferably 3 times in one direction.
-20 times, and the area stretching ratio is in the range of 9-400 times.

Next, the biaxially stretched film thus obtained is washed with an appropriate solvent to remove the solvent remaining in the film to form a porous film. Heat and heat set (heat set) to prevent shrinkage. Examples of the solvent used in the desolvation treatment include volatile solvents such as hydrocarbons such as pentane, hexane, and heptane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; and ethers such as diethyl ether and dioxane. Is preferably used. These solvents are appropriately selected depending on the solvent used for preparing the solution of the ultrahigh molecular weight polyolefin resin (composition). In order to remove the solvent remaining in the sheet, for example, the sheet may be immersed in the solvent.

The porous film according to the present invention thus obtained has a thickness of 1 to 60 μm, preferably 5 to 60 μm.
45 μm, and the BET specific surface area is 100 m ² /
g, preferably in the range of 100 to 300 m ² / g, and the pore volume measured by the BJH method is 0.5 cm ³ / g or more, preferably in the range of 1 to 1.5 cm ³ / g, Further, the average diameter of the through holes measured by the BJH method is 0.03 μm.
Hereinafter, preferably, it is in the range of 0.01 to 0.03 μm,
Maximum pore diameter is 0.1 μm or less, preferably 0.1 to 0.03 μm
m, and the average diameter of the fibrils constituting the three-dimensional network structure is 0.01 to 0.1 μm, preferably 0.01 to 0.1 μm.
05 μm, the maximum diameter is 0.2 μm or less, preferably 0.02
The average curvature is in the range of more than 2.5 times to 10 times or less, preferably 3.0 to 6.0 times.

Further, the porous film according to the present invention has a porosity of 35 to 75%, preferably 50 to 70%, and an air permeability of 100 to 800 seconds / 100 cc, preferably 10 to 100 cc.
0 to 500 seconds / 100 cc.

The porous film according to the present invention has a high strength, a high specific surface area and a high pore volume.
Despite the long penetration path, it has excellent ion permeability and high-speed charge / discharge characteristics. In a glove box, a lithium cobaltate electrode is used as a positive electrode and a carbon electrode is used as a negative electrode in glass, and the porous film impregnated with an electrolyte is sandwiched between the glass and a nonwoven fabric (electrolyte impregnated product) serving as a cushioning material. When the charge / discharge characteristics were examined, high discharge efficiency was exhibited at a high current density, and large output was possible in a short time.

Further, the porous film according to the present invention has good air permeability, but has a high specific surface area, a high density of fine fibrils, and a small average pore size. It is hard to occur,
Thus, the battery separator has excellent stability and durability.

[0034]

EXAMPLES The present invention will be described below with reference to examples.
The present invention is not limited by these examples. Hereinafter, the melting point of the resin used and the properties of the obtained porous film were evaluated as follows.

(Melting Point) In the DSC (differential scanning calorimeter) measurement, the onset temperature was defined as the melting point. (Weight average molecular weight) Using a gel permeation chromatograph (GPC-150C, manufactured by Waters), o-dichlorobenzene was used as a solvent, and Shodex-8 was used as a column.
It was measured at a temperature of 135 ° C. using 0M (manufactured by Showa Denko KK). Data processing was performed using a data processing system manufactured by TRC. The molecular weight was calculated based on polystyrene. (Film thickness) The thickness was measured from a 1 / 10,000 mm thickness gauge and a 10,000 × scanning electron micrograph of the cross section of the porous film.

(BET specific surface area) A specific surface area / pore distribution measuring device AS manufactured by Shimadzu Corporation using a nitrogen adsorption / desorption method.
The BET specific surface area was measured using AP2010. (Pore volume) The pore volume was measured by the BJH method using a specific surface area / pore distribution measuring device ASAP2010 by a nitrogen adsorption / desorption method manufactured by Shimadzu Corporation. (Air permeability) Measured according to JIS P8117.

(Average and maximum pore diameters of through-holes)
The pore size distribution was measured by the BJH method using a specific surface area / pore distribution measuring device ASAP2010 manufactured by Shimadzu Corporation using a nitrogen adsorption / desorption method, and the average pore size and the maximum pore size were determined from this. (Average diameter and maximum diameter of fibrils) The surface and cross section of the porous film were read from a 50,000-fold scanning micrograph.

(5C Discharge Efficiency Ratio) The discharge capacity at the time of discharge at a discharge rate of 5 CmA was determined as a relative ratio with the average discharge capacity of a commercial product being 1. (Ion permeation coefficient ratio) Li of 1.4 mol / L concentration on the supply side
PF ₆ (EC / EMC), the EC / EMC solution on the permeate side is put into a glass cell via a porous film (separator), the permeation rate of LiPF ₆ is measured, the permeation coefficient is determined, and the permeation of a commercial product is determined. It was determined as a relative ratio with the coefficient being 1.

(Curve Ratio) The curvature ratio is obtained by the following Kozeny-Carman equation k ≒ [4 ⁵ ε / (3π ²⁾ relating to the relationship between the permeation rate of the fluid in the porous body and the porosity or specific surface area. δ ₀ ² · _Sp ² )] × (L ₀ / L), and is given by L ₀ / L. Here, epsilon is porosity, [delta] ₀ fibril diameter, S _p is the surface area per unit volume solids only of the porous body (specific surface area), L the transmission distance of the apparent (i.e., film thickness), L ₀ Is the true transmission distance (ie, the length of the penetration path), and the Kozeny constant k is usually set to 5.0.

Example 1 15% by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 2,000,000 (melting point: 134 ° C.) and 85% by weight of liquid paraffin (solidification point: −15 ° C., kinematic viscosity at 40 ° C .: 59 cst) were uniformly slurried. The mixture was charged in a small kneader, heated at a temperature of 160 ° C. for about 50 minutes, dissolved and kneaded to obtain a kneaded product of an ultrahigh molecular weight polyethylene resin and a solvent. Thereafter, the kneaded product was formed into a sheet having a thickness of 0.5 mm while being rapidly cooled to -15 ° C to crystallize the ultrahigh molecular weight polyethylene resin.

Then, the sheet was simultaneously biaxially stretched at a temperature of about 115 ° C. 4 × 4 times in length and width, and then immersed in methylene chloride to remove the solvent. The porous film thus obtained was further heat-set at 120 ° C. for 10 seconds to obtain a porous film having a thickness of 25 μm and a porosity of 52%.

Example 2 5% by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 2,000,000 (melting point 134 ° C.), 10% by weight of a high density polyethylene resin having a weight average molecular weight of 200,000 and liquid paraffin (solidification point −15 ° C., 40 ° C.) Kinematic viscosity at 59 cst) 85
% By weight and a slurry, and the mixture is charged into a small kneader and heated at a temperature of 160 ° C. for about 50 minutes.
The mixture was dissolved and kneaded to obtain a kneaded product of an ultrahigh molecular weight polyethylene resin, a high density polyethylene resin and a solvent. Then, while rapidly cooling the kneaded product to -15 ° C,
The sheet was molded into a 0.5 mm sheet, and the ultrahigh molecular weight polyethylene resin and the high density polyethylene resin were crystallized.

Next, this sheet was simultaneously biaxially stretched at a temperature of about 115 ° C. by 4 × 4 times in length and width, and then immersed in methylene chloride to remove the solvent. The porous film thus obtained was further heat-set at 120 ° C. for 10 seconds to obtain a porous film having a thickness of 30.6 μm and a porosity of 53%.

Example 3 20% by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 2,000,000 (melting point: 134 ° C.) and 80% by weight of liquid paraffin (freezing point: −15 ° C., kinematic viscosity at 40 ° C .: 59 cst) are uniformly slurried. And L / D = 42
Was heated at a temperature of 160 ° C. for about 5 minutes, dissolved and kneaded to obtain a kneaded product of an ultrahigh molecular weight polyethylene resin and a solvent. Thereafter, the kneaded product was formed into a sheet having a thickness of 0.5 mm while being rapidly cooled to -15 ° C to crystallize the ultrahigh molecular weight polyethylene resin.

Next, the sheet was simultaneously biaxially stretched at a temperature of about 115 ° C. 4 × 4 times vertically and horizontally, and then immersed in methylene chloride to remove the solvent. The porous film thus obtained was further heat-set at 120 ° C. for 10 seconds to obtain a porous film having a thickness of 20 μm and a porosity of 43%.

Example 4 15% by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 2,000,000 (melting point: 134 ° C.) and 85% by weight of liquid paraffin (solidification point: −15 ° C., kinematic viscosity at 40 ° C .: 59 cst) were uniformly formed into a slurry. The mixture was charged into a small kneader, heated at a temperature of 160 ° C. for about 50 minutes, dissolved and kneaded to obtain a kneaded product of an ultrahigh molecular weight polyethylene resin and a solvent. Thereafter, the kneaded material is cooled to -15 ° C.
While cooling rapidly, it is molded into a 0.5mm thick sheet,
The ultra high molecular weight polyethylene resin was crystallized.

Then, the sheet was simultaneously biaxially stretched 4 × 4 times at a temperature of about 115 ° C. and then immersed in methylene chloride to remove the solvent. The porous film thus obtained was further heat-set at 120 ° C. for 10 seconds to obtain a porous film having a thickness of 23.8 μm and a porosity of 54%.

Comparative Example 1 15% by weight of a high-density polyethylene resin having a weight average molecular weight of 200,000 and 85% by weight of liquid paraffin (freezing point: -15 ° C., kinematic viscosity at 40 ° C .: 59 cst) were uniformly mixed in a slurry form. The mixture was charged in a small kneader, heated at a temperature of 160 ° C. for about 50 minutes, dissolved, and kneaded to obtain a kneaded product of the high-density polyethylene resin and a solvent. Then, while rapidly cooling the kneaded product to -15 ° C,
It was formed into a 0.5 mm sheet to crystallize the high-density polyethylene resin.

Next, the sheet was simultaneously biaxially stretched at a temperature of about 115 ° C. 4 × 4 times in length and width, and then immersed in methylene chloride to remove the solvent. The porous film thus obtained was further heat-set at 120 ° C. for 10 seconds to obtain a porous film having a thickness of 16.2 μm and a porosity of 50%.

Comparative Example 2 5% by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 2,000,000 (melting point 134 ° C.), 10% by weight of a high density polyethylene resin having a weight average molecular weight of 200,000 and liquid paraffin (solidification point −15 ° C., 40 ° C.) Kinematic viscosity at 59 cst) 85
% By weight and uniformly mixed in a slurry, and the mixture was heated and stirred in a flask at a temperature of 200 to 230 ° C. to dissolve it.
The mixture was kneaded with a spatula for about 90 minutes to obtain a kneaded product of an ultrahigh molecular weight polyolefin resin, a high-density polyethylene resin and a solvent. Thereafter, the kneaded product was formed into a sheet having a thickness of 0.5 mm while being rapidly cooled to −15 ° C. to crystallize the ultrahigh molecular weight polyethylene resin and the high density polyethylene resin.

Next, this sheet was simultaneously biaxially stretched at a temperature of about 115 ° C. 4 × 4 times in length and width, and then immersed in methylene chloride to remove the solvent. The porous film thus obtained was further heat-set at 120 ° C. for 10 seconds to obtain a porous film having a thickness of 19 μm and a porosity of 28%.

Comparative Example 3 15% by weight of an ultra-high molecular weight polyethylene resin having a weight average molecular weight of 2,000,000 (melting point: 134 ° C.) and 85% by weight of liquid paraffin (solidification point: −15 ° C., kinematic viscosity at 40 ° C .: 59 cst) were uniformly slurried. The mixture was charged into a small kneader, heated at a temperature of 160 ° C. for about 50 minutes, dissolved, and kneaded to obtain a kneaded product of the ultrahigh molecular weight polyethylene resin and the solvent. Thereafter, the kneaded material is heated at 10 ° C.
The sheet was sandwiched between two metal plates kept under water cooling and pressed to form a sheet having a thickness of 0.5 mm to crystallize the ultrahigh molecular weight polyethylene resin.

Next, the sheet was simultaneously biaxially stretched 4 × 4 times at a temperature of about 115 ° C. and then immersed in methylene chloride to remove the solvent. The porous film thus obtained was further heat-set at 120 ° C. for 10 seconds to obtain a porous film having a thickness of 25 μm and a porosity of 52%.

Tables 1 and 2 show the characteristics of the porous films obtained in Examples 1 to 4 and Comparative Examples 1 to 3 and the commercially available porous films.

[0055]

[Table 1]

[0056]

[Table 2]

[0057]

As described above, the porous film according to the present invention has high strength and a BET specific surface area of 10
0 m ² / g or more, the pore volume measured by the BJH method is 0.5 c
m ³ / g or more, and the average pore size measured by the BJH method is 0.
Having a through-hole having a maximum pore diameter of not more than 0.3 μm, and having an average diameter of fibrils constituting a three-dimensional network structure.
0.01 to 0.1 μm, the maximum diameter is 0.2 μm or less, and the average curvature is more than 2.5 times and 10 times or less.

As described above, in the porous film according to the present invention, the fibrils forming the three-dimensional network structure are finely uniform, and the through holes are finely uniform. Since it has excellent ion permeability and high speed charge / discharge characteristics, it can be suitably used, for example, as a separator of a battery for an electric vehicle.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI C08L 23:04 (72) Inventor Shunsuke 1-2-1, Shimohozumi, Ibaraki-shi, Osaka Nitto Denko Corporation (72) Invention Person: Shigeru Fujita 1-2-1, Shimohozumi, Ibaraki-shi, Osaka Nitto Denko Corporation (72) Inventor Yutaka Kishi 1-2-1, Shimohozumi, Ibaraki-shi, Osaka Nitto Denko Corporation

Claims (15)

[Claims] 1. An ultrahigh molecular weight polyolefin resin having a weight average molecular weight of 5 × 10 ⁵ or more, or a polyolefin resin having at least 15% by weight of the ultrahigh molecular weight polyolefin resin and a weight average molecular weight of less than 5 × 10 ^5. It is composed of an ultra-high molecular weight polyolefin resin composition, has a specific surface area of 100 m ² / g or more, a pore volume of 0.5 cm ³ / g or more,
The average pore diameter of the through holes is 0.03 μm or less, and the maximum pore diameter is 0.1 μm
m, the average diameter of the fibrils constituting the three-dimensional network structure is 0.01 to 0.1 μm, the maximum diameter is 0.2 μm or less, and the average curvature is more than 2.5 times and 10 times or less. A porous film characterized by the following. 2. The porous film according to claim 1, wherein the ultrahigh molecular weight polyolefin resin is an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 1 × 10 ⁶ to 15 × 10 ⁶ . 3. The porous film according to claim 1, wherein the polyolefin resin having a weight average molecular weight of less than 5 × 10 ⁵ is a high density polyethylene resin having a weight average molecular weight of 3 × 10 ⁵ or less. 4. The fibril has an average diameter of 0.01 to 0.05 μm.
The porous film according to claim 1, wherein m and the maximum diameter are 0.1 µm or less. 5. The method according to claim 1, wherein the average curvature is 3.5 to 6.0 times.
The porous film according to the above. 6. A battery separator comprising the porous film according to claim 1. 7. An ultrahigh molecular weight polyolefin resin having a weight average molecular weight of 5 × 10 ⁵ or more, or a polyolefin resin having at least 15% by weight of the ultrahigh molecular weight polyolefin resin and a weight average molecular weight of 3 × 10 ⁵ or less. A mixture of 5 to 30% by weight of the ultra-high-molecular-weight polyolefin resin composition and 95 to 70% by weight of a solvent having a freezing point of -10 ° C or lower is heated, and the ultra-high-molecular-weight polyolefin resin or the composition is placed in the solvent. This mixture is kneaded at a temperature in the range of 115 to 185 ° C., and then, while cooling to a temperature below the freezing point of the solvent using the obtained kneaded product, the mixture is formed into a gel-like sheet, When the high-molecular-weight polyolefin resin is crystallized and the melting point of the ultra-high-molecular-weight polyolefin resin is M, the gel-like sheet is (M + ) Biaxially oriented from ° C. at (M-30) ° C. in the range of temperature, then the resulting production method of a porous film of claim 1 wherein the stretched film, characterized in that desolvation process. 8. The method according to claim 7, wherein the ultrahigh molecular weight polyolefin resin is an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 1 × 10 ⁶ to 15 × 10 ⁶ . 9. The polyolefin resin having a weight average molecular weight of 3 × 10 ⁵ or less is a high density polyethylene resin.
3. The method for producing a porous film according to item 1. 10. The method for producing a porous film according to claim 7, wherein the solvent is liquid paraffin. 11. The gel-like sheet is heated from M ° C. to (M-25) ° C.
The method for producing a porous film according to claim 7, wherein the film is stretched at a temperature in the range of: 12. The method for producing a porous film according to claim 7, wherein the stretch ratio from the gel sheet to the stretched film is in the range of 9 to 1024 times in area ratio. 13. The method for producing a porous film according to claim 7, wherein the stretched film is subjected to a desolvation treatment and then heat set. 14. The method for producing a porous film according to claim 7, wherein the solvent has a freezing point in a range of -10 ° C. to -45 ° C. 15. The method for producing a porous film according to claim 7, wherein the kneaded material is cooled to a temperature in a range of -10 ° C to -45 ° C.