JP2000044709A - Production of porous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a porous film having a high porosity, a small maximum pore diameter, and a narrow pore diameter distribution by dissolving under heating an ultrahigh-molecular-weight polyolefin resin in a mixed solvent comprising a good solvent and a poor solvent, kneading the solution at a specified temperature, cooling the solution to the solidifying point or below to form a gel-like sheet, rolling the sheet at a specified temperature, biaxially orienting the rolled sheet, and desolventing the oriented sheet. SOLUTION: Five to fifty wt.% ultrahigh-molecular-weight polyolefin resin having a weight-average molecular weight of 5×105 to 20×106 is dissolved under heating in a mixed solvent comprising 55-95 wt.% good solvent such as decane and 5-45 wt.% poor solvent such as tetraethylene glycol dimethyl ether and kneaded at 115-185 deg.C. Next, the mixture is molded into a gel-like sheet by cooling to the solidification temperature of the solvent. The sheet is crystallized, and then biaxially oriented at an areal draw ratio of 4-400 at a temperature in the range of (M+5) to (M-30) deg.C, and the oriented film is desolvented to obtain a film having a porosity of 50-80%, a maximum pore diameter of 0.030-0.045 μm, a pore diameter distribution of 1.5-2.5 and a thickness of 1-60 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a porous film containing an ultra-high molecular weight polyolefin resin,
More specifically, the present invention relates to a method for producing a porous film which has high strength, has fine pores and a narrow pore size distribution, and can be particularly preferably used as a battery separator.

[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 ensure the permeability of ions between the positive electrode and the negative electrode. In addition, various characteristics are required in relation to battery characteristics. Among them, high strength and high porosity are essential required characteristics.

[0005] The high strength of the film contributes to the improvement of the workability of the battery and the reduction of the internal short-circuit defect rate, and further to the improvement of the capacity by making the separator thinner. When the film has a high porosity, the ion permeability of the film is improved, and the charge and discharge characteristics, particularly, the charge and discharge characteristics at a high rate are improved.

As a method for producing such a porous film for a separator, conventionally, an ultra-high molecular weight polyolefin resin is heated and dissolved in a solvent, the solution is formed into a gel-like sheet, and the solvent is removed before and after stretching. A method of removing the residual solvent by performing stretching treatment is known (Japanese Patent Application Laid-Open No.
Nos. 0-242035, JP-A-61-495132, JP-A-61-195133, and JP-A-63-195133.
39602, JP-A-63-273651, etc.).

However, according to such a conventional method, each of the obtained porous films has a wide pore size distribution, and particularly has relatively large pore sizes. When used as a separator, a short circuit may occur when lithium dendrite is deposited.

[0008] For example, Japanese Patent Application Laid-Open No. Hei.
In the publication, the ultra-high molecular weight polyolefin resin is heated and dissolved in a mixed solvent of a good solvent and a poor solvent for the resin,
The solution was extruded from a die, cooled, and a gel-like sheet was prepared, stretched, and desolvated to obtain an ultra-high molecular weight polyolefin resin having an average pore size in the range of 0.05 to 0.2 μm. A method for producing a porous film is described.

However, with the recent development and development of lithium ion secondary batteries, films for separators are required to have more excellent electrolyte retention, ion permeability, high-speed charge / discharge characteristics and the like. There is a demand for a porous film having a pore structure with a small maximum pore size and a narrow pore size distribution.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above-mentioned problems in the porous film and to meet the above-mentioned demand, and has a porosity of 50 to 80% and a maximum pore size of 50 to 80%. It is an object of the present invention to provide a method for producing a porous film which has a small pore size and a narrow pore size distribution, and can be suitably used as a battery separator.

[0011]

According to the present invention, there is provided a method for producing a porous film, comprising the steps of: adding at least 30% by weight of an ultrahigh molecular weight polyolefin resin having a weight average molecular weight of 5 × 10 ⁵ or more;
The polyolefin resin containing 5 to 50% by weight is heated together with a solvent to dissolve the polyolefin resin in the solvent, and the resulting solution is kneaded at a temperature in the range of 115 to 185 ° C. While cooling to a temperature below the freezing point of the solvent using the kneaded material, it is molded into a gel-like sheet to crystallize the polyolefin resin, and then, when the melting point of the ultrahigh molecular weight polyolefin resin is M, the above-mentioned gel-like sheet Is rolled at a temperature in the range of (M + 5) ° C to (M-30) ° C, and further rolled from (M + 5) ° C to (M-30) ° C.
-30) Biaxial stretching at a temperature in the range of ° C, and then, in a method for producing a porous film, in which the obtained stretched film is subjected to a desolvation treatment, the solvent is 55 to 95% by weight based on the polyolefin resin. A poor solvent which is incompatible with the good solvent at the kneading temperature, selected from dialkyl ethers of (poly) alkylene glycol having 3 to 8 carbon atoms (alkyl group having 1 to 4 carbon atoms) or diglycidyl ether. 5 to 45% by weight, wherein the viscosity of the poor solvent is not more than the viscosity of the good solvent.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The material for the porous film used in the present invention is a polyolefin resin containing at least 30% by weight of an ultrahigh molecular weight polyolefin resin having a weight average molecular weight of 5 × 10 ⁵ or more.

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
Is 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.

In the present invention, the polyolefin resin is
A second polio is added together with the ultrahigh molecular weight polyolefin resin.
When a olefin resin is contained, the second polyolefin resin
Fat has a weight average molecular weight of 5 × 10^FiveLess than and preferred
In other words, the weight average molecular weight is 1 × 10^Four5 × 10 or more^FiveNot yet
Full range, preferably 1 × 10^Four~ 3 × 10 ^Five
Is in the range. Such polyolefin tree
As fats, for example, ethylene, propylene, 1-butene
, 4-methyl-1-pentene, 1-hexene and the like
Polymers, copolymers or mixtures thereof may be mentioned.
Wear. However, in the present invention,
As the olefin resin, high-density polyethylene resin is preferable.
It is used well.

In the present invention, the polyolefin resin is
At least 30% by weight of ultra high molecular weight polyolefin resin
It is necessary to include. When the proportion of the ultra-high molecular weight polyolefin resin in the polyolefin resin is less than 30% by weight, a porous film having the necessary strength as a battery separator cannot be obtained.

In the production of the porous film according to the present invention,
First, 5 to 50% by weight of the polyolefin resin is heated together with a solvent to dissolve the polyolefin resin in the solvent, and the resulting solution is kneaded at a temperature in the range of 115 to 185 ° C to obtain a kneaded material. Is prepared.

When the amount of the polyolefin resin is more than 50% by weight, it is difficult to uniformly dissolve the polyolefin resin in a solvent, so that a uniform gel-like sheet cannot be obtained and is finally obtained. Porous films also have low porosity. On the other hand, when the polyolefin resin is 5
When the amount is less than% by weight, the formability into a gel-like sheet is inferior, and in some cases, it cannot be formed into a gel-like sheet.

In the present invention, a mixed solvent consisting of 55 to 95% by weight of a good solvent and 5 to 45% by weight of a poor solvent is used as the solvent. The good solvent is a solvent that can well dissolve the polyolefin resin at a kneading temperature described below and can form a uniform solution, and is particularly limited as long as the solidification point is −10 ° C. or less. In particular, in the present invention, the freezing point is from −10 ° C. to −45 ° C.
Those in the range 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 them, a non-volatile solvent such as liquid paraffin is preferable, and in particular, the freezing point is −15 ° C. or less, and 40 ° C.
In this case, a non-volatile solvent having a kinematic viscosity of 65 cst or less is preferably used.

On the other hand, the poor solvent is a solvent that does not dissolve the polyolefin resin at a kneading temperature described later and cannot form a uniform solution. According to the present invention, a solvent selected from dialkyl ethers of a (poly) alkylene glycol having 3 to 8 carbon atoms (the alkyl group has 1 to 4 carbon atoms) or diglycidyl ethers, and the above-described good mixing temperature at the kneading temperature. A solvent that is incompatible with the solvent and has a viscosity at room temperature (30 ° C.) that is equal to or lower than the viscosity of the good solvent is used.

More preferably, according to the present invention, at the time of the kneading, the poor solvent is domainized in a solution of the polyolefin resin formed by the good solvent, that is, formed into fine droplets. Desirably, for this purpose, the poor solvent used has a solubility parameter SP value difference of at least 0.3, preferably at least 0.4, from the good solvent. Here, in the present invention, the solubility parameter is based on the Fedors calculation method.

Specific examples of the dialkyl ether of the (poly) alkylene glycol having 3 to 8 carbon atoms (the alkyl group has 1 to 4 carbon atoms) include tetraethylene glycol dimethyl ether.
Specific examples of the diglycidyl ether of a (poly) alkylene glycol having 3 to 8 carbon atoms include propylene glycol diglycidyl ether and butylene glycol diglycidyl ether.

In the mixed solvent, when the proportion of the poor solvent exceeds 45% by weight, a uniform solution of the polyolefin resin cannot be obtained during kneading, and when the proportion of the poor solvent is less than 5% by weight. Cannot obtain a porous film having the desired maximum pore size and pore size distribution.

In the present invention, when kneading at a temperature exceeding 185 ° C. when kneading a solution obtained by dissolving the polyolefin resin in the mixed solvent, the viscosity of the solution is too low, and When a sufficient shearing force cannot be applied, and when the kneading temperature is lower than 115 ° C., the polyolefin resin cannot be kneaded effectively. Thus, in the kneading of the polyolefin resin, the polyolefin resin is not kneaded. Entanglement of chains, especially
It is difficult to obtain sufficient entanglement of the polymer chains of the ultrahigh molecular weight polyolefin resin.

In the present invention, in order to obtain sufficient entanglement of the polymer chains of the ultrahigh molecular weight polyolefin resin, kneading is performed while applying a high shearing force to the solution mixture of the polyolefin resin and the solvent. Is preferred. When a sufficient shearing force cannot be exerted 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, for kneading a solution mixture of a polyolefin resin and a solvent, usually, a kneader or a twin-screw extruder capable of giving a strong shear force to the mixture is preferably used.

Next, according to the present invention, the temperature below the freezing point of the solvent, preferably −1, is obtained by using the solution-kneaded mixture of the polyolefin resin and the solvent thus obtained.
Temperature in the range of 0 ° C to -45 ° C, preferably -15 ° C
While cooling to a temperature in the range of −?
The polyolefin resin is crystallized by molding into a gel-like sheet having a thickness of 0 mm, preferably 2 to 10 mm.

As described above, the cooling to a temperature lower than the freezing point of the solvent (mixed solvent) using the kneaded material in the form of a solution of the polyolefin resin and the solvent is not particularly limited. The two metal plates may be cooled with dry ice, the kneaded material may be sandwiched between the 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 being cooled, the resin is finely crystallized not only to the surface layer of the obtained sheet but also to the center of the sheet, and finally to a high level. It is preferable to rapidly cool the kneaded material so that a porous film having a uniform, dense pore structure with high strength and high porosity can be obtained. Therefore, the cooling rate is preferably 50 ° C./min or more on average. .

Next, according to the present invention, when the melting point of the ultrahigh molecular weight polyolefin resin used is M, the above gel-like sheet is heated to a temperature in the range of (M + 5) ° C. to (M-30) ° C., preferably, At a temperature in the range of M ° C to (M-25) ° C, for example, using a heat press, the thickness is 0.1 to 3 m.
m, preferably 0.2-1 mm. The above melting point is a set temperature in DSC measurement. By this rolling, the gel-like sheet can be made into a rolled film having a uniform thickness, and thus a high-strength porous film can be finally obtained.

Then, according to the present invention, the rolled film thus obtained is heated from the above (M + 5) ° C. to (M-3).
0) a temperature in the range of ° C, preferably from M ° C to (M-2
5) Biaxially stretch at a temperature in the range of ° C. This biaxial stretching may be either sequential or simultaneous biaxial stretching,
Preferably, simultaneous biaxial stretching is performed. In the present invention, the stretch ratio of the rolled film is 2 times or more in one direction, the area stretch ratio is 10 times or more, and preferably 2 times or more in one direction.
And the area stretching ratio is in the range of 4 to 400 times.

Next, the stretched film thus obtained is washed with an appropriate solvent to remove the solvent remaining in the film to form a porous film, and, if necessary, to form a porous film. Heat and heat set (heat set) to prevent shrinkage.

Examples of the solvent used in the desolvation treatment include hydrocarbons such as pentane, hexane and heptane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; and volatile solvents such as ethers such as diethyl ether and dioxane. Is preferably used. These solvents are appropriately selected according to the solvent used for preparing the solution of the composition comprising the polyolefin resin and the saturated thermoplastic elastomer.
In order to remove the solvent remaining in the stretched film, for example, the stretched film may be immersed in the solvent.

The porous film according to the present invention thus obtained has a porosity of 50 to 80% and a thickness of 1 to 60.
μm, preferably in the range of 5 to 45 μm, the maximum pore size is in the range of 0.030 to 0.045 μm, the pore size distribution of the maximum pore size divided by the average pore size is in the range of 1.5 to 2.5, Preferably, it is in the range of 1.6 to 2.0.

[0033]

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 Gel permeation chromatograph (GPC- manufactured by Waters Inc.)
150C) using o-dichlorobenzene as a solvent and Shodex-80M (manufactured by Showa Denko KK) as a column at a temperature of 135 ° C. Data processing was performed using a data processing system manufactured by TRC. The molecular weight was calculated based on polystyrene.

Film Thickness: Measured from a 1 / 10,000 mm thickness gauge and a 10,000 × scanning electron micrograph of the cross section of the porous film.

The porosity was calculated from the weight W per unit area S of the film, the average thickness t and the density d by the following formula. Porosity (%) = (1− (10 ⁴ × W / S / t / d)) ×
100

Average pore diameter and maximum pore diameter of through-holes The distribution of pore diameters was measured by the BJH method using a specific surface area / pore distribution measuring device ASAP2010 manufactured by Shimadzu Corporation with a nitrogen adsorption / desorption method. The average pore size and the maximum pore size were determined. The value obtained by dividing the maximum pore size by the average pore size was defined as the pore size distribution.

Example 1 15 parts by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 2 × 10 ⁶ was mixed with liquid paraffin (viscosity of 8 at 30 ° C.).
2 cps) was added to a mixed solvent of 68 parts by weight of butylene glycol diglycidyl ether (viscosity at 30 ° C. 10 cps) of 17 parts by weight, and a slurry was prepared. The mixture was 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 gel-like sheet having a thickness of 5 mm while being rapidly cooled to -15 ° C. to crystallize the ultrahigh molecular weight polyethylene resin.

Next, the sheet is rolled by a heat press at a temperature of about 120 ° C. until the thickness becomes 0.5 to 0.6 mm. After stretching, the film was immersed in methylene chloride to remove the solvent. Thus, a porous film according to the present invention was obtained. The difference between the SP values of the two solvents used in this example was 1.3.
Met. Table 1 shows the thickness, porosity, maximum pore size, and pore size distribution of the obtained porous film.

Example 2 8 parts by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 2 × 10 ⁶ and 7 parts by weight of a high density polyethylene resin having a weight average molecular weight of 2 × 10 ⁵ were mixed with 68 parts by weight of liquid paraffin (viscosity at 30 ° C .: 82 cps). Part and butylene glycol diglycidyl ether (viscosity 10cps at 30 ° C) 1
7 parts by weight, a slurry was prepared, and the slurry was charged into a small kneader, heated at a temperature of 160 ° C. for about 50 minutes, dissolved, kneaded, and mixed with the ultrahigh molecular weight polyethylene resin and the solvent. A kneaded product was obtained. Thereafter, the kneaded product was formed into a gel-like sheet having a thickness of 5 mm while being rapidly cooled to -15 ° C. to crystallize the ultrahigh molecular weight polyethylene resin.

Next, the sheet is rolled by a heat press at a temperature of about 115 ° C. until the thickness becomes 0.5 to 0.6 mm. After stretching, the film was immersed in methylene chloride to remove the solvent. Thus, a porous film according to the present invention was obtained. The difference between the SP values of the two solvents used in this example was 1.3.
Met. Table 1 shows the thickness, porosity, maximum pore size, and pore size distribution of the obtained porous film.

Example 3 8 parts by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 2 × 10 ⁶ and 12 parts by weight of a high density polyethylene resin having a weight average molecular weight of 2 × 10 ⁵ were mixed with 56 parts by weight of liquid paraffin (viscosity 82 cps at 30 ° C.). Part and butylene glycol diglycidyl ether (viscosity 10cps at 30 ° C)
24 parts by weight of the mixed solvent,
This 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 gel-like sheet having a thickness of 5 mm while being rapidly cooled to -15 ° C. to crystallize the ultrahigh molecular weight polyethylene resin.

Next, this sheet is rolled by a heat press at a temperature of about 115 ° C. until the thickness becomes 0.5 to 0.6 mm. After stretching, the film was immersed in methylene chloride to remove the solvent. Thus, a porous film according to the present invention was obtained. The difference between the SP values of the two solvents used in this example was 1.3.
Met. Table 1 shows the thickness, porosity, maximum pore size, and pore size distribution of the obtained porous film.

Example 4 5 parts by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 2 × 10 ⁶ and 10 parts by weight of a high density polyethylene resin having a weight average molecular weight of 2 × 10 ⁶ were mixed with 68 parts by weight of liquid paraffin (viscosity at 30 ° C., 82 cps). Parts and propylene glycol diglycidyl ether (viscosity 20 cp at 30 ° C.)
s) A slurry was added to a mixed solvent consisting of 17 parts by weight, and the slurry was charged into a small kneader.
The mixture was heated for 0 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 gel-like sheet having a thickness of 5 mm while being rapidly cooled to -15 ° C. to crystallize the ultrahigh molecular weight polyethylene resin.

Next, the sheet is rolled by a heat press at a temperature of about 115 ° C. until the thickness becomes 0.5 to 0.6 mm, and then simultaneously biaxially 4 × 4 times at a temperature of about 115 ° C. After stretching, the film was immersed in methylene chloride to remove the solvent. Thus, a porous film according to the present invention was obtained. The difference between the SP values of the two solvents used in this example was 1.9.
Met. Table 1 shows the thickness, porosity, maximum pore size, and pore size distribution of the obtained porous film.

Example 5 10 parts by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 2 × 10 ⁶ was mixed with liquid paraffin (viscosity at 30 ° C. of 8
2 cps) 81 parts by weight and 9 parts by weight of tetraethylene glycol dimethyl ether (viscosity at 30 ° C. 10 cps) were added to form a slurry, which was charged into a small kneader and heated at a temperature of 160 ° C. for about 60 minutes. The mixture was 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 gel-like sheet having a thickness of 5 mm while being rapidly cooled to -15 ° C. to crystallize the ultrahigh molecular weight polyethylene resin.

Next, this sheet is rolled by a heat press at a temperature of about 120 ° C. until the thickness becomes 0.5 to 0.6 mm. After stretching, the film was immersed in methylene chloride to remove the solvent. Thus, a porous film according to the present invention was obtained. The difference between the SP values of the two solvents used in this example was 0.4.
Met. Table 1 shows the thickness, porosity, maximum pore size, and pore size distribution of the obtained porous film.

Example 6 8 parts by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 2 × 10 ⁶ and 7 parts by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 5 × 10 ⁵ were mixed with liquid paraffin (viscosity at 30 ° C. 11 cps) 81 Parts by weight and tetraethylene glycol dimethyl ether (viscosity 10 cp at 30 ° C.)
s) A slurry was added to a mixed solvent consisting of 17 parts by weight, and the slurry was charged into a small kneader.
The mixture was heated for 0 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 gel-like sheet having a thickness of 5 mm while being rapidly cooled to -15 ° C. to crystallize the ultrahigh molecular weight polyethylene resin.

Next, this sheet is rolled by a heat press at a temperature of about 115 ° C. until the thickness becomes 0.5 to 0.6 mm. After stretching, the film was immersed in methylene chloride to remove the solvent. Thus, a porous film according to the present invention was obtained. The difference between the SP values of the two solvents used in this example was 0.4.
Met. Table 1 shows the thickness, porosity, maximum pore size, and pore size distribution of the obtained porous film.

Comparative Example 1 15 parts by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 2 × 10 ⁶ was mixed with liquid paraffin (viscosity of 8 at 30 ° C.
2 cps) and a mixed solvent consisting of 68 parts by weight of diethylene glycol dibutyl ether (dynamic viscosity at 30 ° C., 10 cps) of 17 parts by weight to form a slurry, which is charged into a small kneader and heated at a temperature of 160 ° C. for about 60 minutes. The mixture was 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 gel-like sheet having a thickness of 5 mm while being rapidly cooled to -15 ° C. to crystallize the ultrahigh molecular weight polyethylene resin.

Next, the sheet is rolled by a heat press at a temperature of about 120 ° C. until the thickness becomes 0.5 to 0.6 mm. After stretching, the film was immersed in methylene chloride to remove the solvent, and a porous film was obtained. The difference between the SP values of the two solvents used in this comparative example was 0.2. Table 1 shows the thickness, porosity, maximum pore size, and pore size distribution of the obtained porous film.

Comparative Example 2 8 parts by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 2 × 10 ⁶ and 7 parts by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 5 × 10 ⁵ were mixed with liquid paraffin (viscosity at 30 ° C. 11 cps) 68 Parts by weight and a mixed solvent consisting of 17 parts by weight of dodecane (viscosity at 25 ° C., 1.4 cps), and a slurry was prepared.
The mixture was heated at a temperature of 60 ° C. for about 60 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 gel-like sheet having a thickness of 5 mm while being rapidly cooled to -15 ° C. to crystallize the ultrahigh molecular weight polyethylene resin.

Next, this sheet is rolled by a heat press at a temperature of about 120 ° C. until the thickness becomes 0.5 to 0.6 mm. After stretching, the film was immersed in methylene chloride to remove the solvent, and a porous film was obtained. The difference between the SP values of the two solvents used in this comparative example was 0.25. Table 1 shows the thickness, porosity, maximum pore size and pore size distribution of the obtained porous film.
Shown in

Comparative Example 3 5 parts by weight of an ultra high molecular weight polyethylene resin having a weight average molecular weight of 2 × 10 ⁶ and 10 parts by weight of a high density polyethylene resin having a weight average molecular weight of 2 × 10 ⁵ were mixed with 68 parts by weight of liquid paraffin (viscosity 11 cps at 30 ° C.). And 17 parts by weight of diglycidyl ether bisphenol A (Epicoat 828, manufactured by Yuka Shell Epoxy Co., Ltd., viscosity: 120 cps at 25 ° C.), and a slurry was prepared. The slurry was charged into a small kneader and heated at 160 ° C. For about 60 minutes, dissolving and kneading to obtain a kneaded product of an ultrahigh molecular weight polyethylene resin and a solvent. Thereafter, the kneaded product was formed into a gel-like sheet having a thickness of 5 mm while being rapidly cooled to -15 ° C. to crystallize the ultrahigh molecular weight polyethylene resin.

Next, this sheet is rolled by a heat press at a temperature of about 115 ° C. until the thickness becomes 0.5 to 0.6 mm. After stretching, the film was immersed in methylene chloride to remove the solvent, and a porous film was obtained. The difference between the SP values of the two solvents used in this comparative example was 2.8. Table 1 shows the thickness, porosity, maximum pore size, and pore size distribution of the obtained porous film.

[0057]

[Table 1]

[0058]

As described above, the porous film according to the present invention has a porosity of 50 to 70% and a maximum pore size of 0.0.
It has a pore structure with a pore size distribution of 5 μm or less and can be suitably used as a battery separator.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification FI FI Theme Court ゛ (Reference) B29L 7:00 C08L 23:04 (72) Inventor Mutsuko Yamaguchi 1-1-2 Shimohozumi, Ibaraki-shi, Osaka Inside Nitto Denko Corporation (72) Inventor Yutaka Kishi 1-1-2 Shimohozumi, Ibaraki City, Osaka Prefecture Inside Nitto Denko Corporation (72) Tomoaki Ichikawa 1-1-2 Shimohozumi, Ibaraki City, Osaka Nitto Denko (72) Inventor Shunsuke Nomi 1-2-1, Shimohozumi, Ibaraki-shi, Osaka Nitto Denko Corporation F-term (reference) 4F074 AA16 AA18 AA76 AA98 AB01 AD01 AD05 AH03 CA03 CA06 CB33 CC05X CC29Z CC32X CC32Y DA02 DA03 DA49 4F210 AA03A AA06 AE01 AG01 AG20 AH33 AR06 AR20 QA04 QC05 QD01 QD13 QD16 QG01 QG18 5H021 BB01 BB02 BB04 BB05 BB13 CC00 EE01 EE04 EE23 HH01 HH06 HH07 5H029 AJ11 CJ00 CJ02 CJ03 CJ06 CJ08 CJ12 DJ04 DJ13 EJ12 HJ00 HJ02 HJ10 HJ14

Claims (7)

[Claims] 1. A polyolefin resin containing at least 30% by weight of an ultrahigh molecular weight polyolefin resin having a weight average molecular weight of 5 × 10 ⁵ or more is heated together with a solvent to 5 to 50% by weight of the polyolefin resin to dissolve the polyolefin resin in the solvent. ,
The resulting solution is kneaded at a temperature in the range of 115 to 185 ° C., and then formed into a gel-like sheet while cooling to a temperature below the freezing point of the solvent using the obtained kneaded product,
After the polyolefin resin is crystallized, and then the melting point of the ultrahigh molecular weight polyolefin resin is M, the gel-like sheet is rolled at a temperature in the range of (M + 5) ° C to (M-30) ° C. , (M + 5) ° C to (M-3
0) biaxially stretching at a temperature in the range of 0 ° C., and then, in a method for producing a porous film in which the obtained stretched film is subjected to a desolvent treatment, the solvent is 55 to 95% by weight based on the polyolefin resin; 3 to 8 carbon atoms (poly)
Selected from dialkyl ethers of alkylene glycols (alkyl groups having 1 to 4 carbon atoms) or diglycidyl ethers, comprising 5 to 45% by weight of a poor solvent which is incompatible with the good solvent at the kneading temperature. A method for producing a porous film, wherein the viscosity of the poor solvent is not more than the viscosity of the good solvent. 2. The method for producing a porous film according to claim 1, wherein the polyolefin resin contains a second polyolefin resin having a weight average molecular weight of less than 5 × 10 ⁵ together with the ultrahigh molecular weight polyolefin resin. 3. The method for producing a 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 in the range of 1 × 10 ⁶ to 15 × 10 ⁶ . 4. The method for producing a porous film according to claim 2, wherein the second polyolefin resin is a high-density polyethylene resin having a weight-average molecular weight of 1 × 10 ^{4 to} 3 × 10 ⁵ . 5. The method for producing a porous film according to claim 1, wherein the stretch ratio from the gel sheet to the stretched film is in the range of 4 to 400 times in area ratio. 6. The method for producing a porous film according to claim 1, wherein the good solvent is liquid paraffin. 7. The method for producing a porous film according to claim 1, wherein the poor solvent is tetraethylene glycol dimethyl ether, propylene glycol diglycidyl ether or butylene glycol diglycidyl ether.