JP2002088189A - Method for producing porous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a porous film with uniform structure having high mechanical strength, large specific surface area and large pore volume, also excellent in ion permeability and high-speed charge/discharge characteristics. SOLUTION: This method for producing a porous film comprises the following process: a resin composition comprising a polyolefin resin and a solvent is kneaded in a molten state, the resulting molten kneaded product is formed into a sheet, which, in turn, is subjected to rolling treatment followed by desolvation treatment; wherein the resin composition is characterized by having been agitated by an agitator in advance. Batteries each having the porous film obtained above are also provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a porous film. More specifically, the present invention relates to a method for producing a porous film which is preferably used as a battery separator or the like which is arranged between a positive electrode and a negative electrode of a battery to isolate them.

[0002]

2. Description of the Related Art Conventionally, various types of batteries have been put to practical use. In recent years, in order to cope with a cordless electronic device, a lightweight, high electromotive force and high energy can be obtained. In addition, lithium batteries with low self-discharge have attracted attention. For example, lithium secondary batteries are widely used for mobile phones and notebook personal computers, and are expected to be used as batteries for electric vehicles in the future.

As such a negative electrode material of a lithium battery, there can be mentioned an intercalation compound such as lithium metal, 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 usually 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. By stretching the sheet, performing a desolvation treatment before and after the stretching to remove the solvent remaining in the sheet,
Various methods for producing a porous film have been proposed.

For example, Japanese Patent Application Laid-Open No. 7-228718 discloses that 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, wherein the average diameter of fibril fibers is 0%. 0.01-0.2 μm, average diameter of through-holes 0.01-0.2
0.1 μm, porosity 35-95%, specific surface area 20-
A porous film having a curvature of 400 m ² / g and a ratio of a through path to a film thickness of 1.5 to 2.5 times the film thickness is described.

However, in order to use a porous film obtained using an ultrahigh molecular weight polyolefin resin practically as a separator for the above-mentioned applications, the film must have a higher strength, a higher specific surface area and a higher pore volume. There is a strong demand for excellent electrolyte retention properties, and further excellent ion permeability and high-speed charge / discharge characteristics. It is also a requirement that these characteristics be uniform over the entire area of the film.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a porous film having a high strength, a high specific surface area and a high pore volume, excellent ion permeability and high-speed charge / discharge characteristics, and a uniform film structure. It is to provide a method for producing a film.

[0009]

The gist of the present invention is to melt-knead a resin composition containing a polyolefin resin and a solvent, form the obtained melt-kneaded material into a sheet, and roll the sheet-like formed material. The present invention relates to a method for producing a porous film, comprising a step of performing a treatment and a desolvation treatment, wherein the method comprises subjecting a resin composition previously stirred by a stirrer to melt kneading.

[0010]

DETAILED DESCRIPTION OF THE INVENTION The polyolefin resin used in the present invention preferably contains an ultrahigh molecular weight polyolefin. Examples of the ultrahigh molecular weight polyolefin resin include homopolymers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, and 1-hexene, copolymers, and mixtures thereof. From the viewpoint of high strength of the resulting porous film, an ultrahigh molecular weight polyethylene resin is preferably used.

The ultrahigh molecular weight polyolefin has a weight average molecular weight of 5 × 10 ⁵ or more, preferably 5 × 10 ^{5 to} 20 ×
10 ⁶ , more preferably 1 × 10 ⁶ to 15 × 10 ⁶ is desirable.

The content of the ultrahigh molecular weight polyolefin resin is preferably 5 to 100% by weight, more preferably 8 to 100% by weight in the polyolefin resin.

The resin which may be contained in the polyolefin resin in addition to the ultrahigh molecular weight polyolefin resin includes homopolymers and copolymers of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and the like. And a mixture thereof. Among them, a high-density polyethylene resin is preferable from the viewpoint of increasing the strength of the obtained porous film. The weight average molecular weight of these resins is preferably 1 × 10 ⁴ or more and less than 5 × 10 ⁵ , more preferably 1 × 10 ^{4 to} 3 × 10 ⁵ .

The solvent that can be used in the present invention is not particularly limited as long as it is excellent in the solubility of the polyolefin resin, but those having a freezing point of -10 ° C. or lower are preferably used. Preferred specific examples of such a solvent include, for example, aliphatic or alicyclic hydrocarbons such as decane, decalin, and liquid paraffin, and mineral oil fractions whose boiling points correspond to these, among which, among others, liquid paraffin and the like. Non-volatile solvents are preferred, and have a freezing point of -45 to -10.
A non-volatile solvent having a kinematic viscosity at 65 ° C or lower at 40 ° C or lower is more preferable.

The mixing ratio of the polyolefin resin and the solvent varies depending on the type, solubility, kneading temperature and the like of the polyolefin resin, and cannot be unconditionally determined. However, the resulting slurry-like resin composition is melt-kneaded into a sheet. There is no particular limitation as long as it can be molded. For example, the polyolefin resin is preferably 5 to 30% by weight of the resin composition, and more preferably 8 to 20% by weight.
When the mixing ratio of the polyolefin resin is 5% by weight or more, the strength of the obtained porous film can be improved, and the mixing ratio of the polyolefin resin is 30% by weight.
When the content is below, the polyolefin resin can be sufficiently dissolved in the solvent and kneaded to near the stretched state, so that sufficient entanglement of the polymer chains can be obtained.

The resin composition may contain, if necessary, an antioxidant, an ultraviolet absorber, a dye, a nucleating agent, a pigment,
Additives such as antistatic agents can be added within a range that does not impair the purpose of the present invention.

The present invention has a great feature in that a resin composition previously stirred by a stirrer is subjected to melt-kneading.
It is preferable to subject the resin composition, which has been previously stirred with a stirrer, to melt-kneading, from the viewpoint of suppressing sedimentation of the polyolefin resin and improving the dispersibility of the resin composition.

In the present invention, the stirrer for stirring the resin composition is not particularly limited. However, a low-speed butterfly blade is used for suppressing the sedimentation of the polyolefin resin, and a high-speed turbine blade is used for loosening the lump at the initial stage of charging the resin composition. Can be suitably used. Here, the low speed is preferably a rotation speed of 10 to 500 rpm, and the high speed is preferably a rotation speed of 500 to 3000 rpm.

The temperature of the resin composition during stirring is 20 to 100.
C. is preferable, and 30 to 80 C. is more preferable. When the temperature of the resin composition is 20 ° C. or higher, unmelted gel of the polyolefin resin is less likely to be generated during kneading, and the amount of kneading can be increased. When the temperature is 100 ° C. or lower, the polyolefin resin does not dissolve during stirring and does not adhere to the stirrer tank, so that stable supply can be achieved.

In the present invention, it is preferable to supply a constant flow rate from the tank of the stirrer to the melt kneader via a pump. The method of the pump is not particularly limited as long as it is based on the composition of the resin composition, taking into account the flow rate stability, liquid leakage, and the like.

The melt-kneading of the resin composition is preferably performed by applying a sufficient shearing force to the resin composition in order to obtain sufficient entanglement of the polymer chains of the polyolefin resin.
Therefore, for melt-kneading the resin composition in the present invention, usually, a kneader or a twin-screw kneader capable of giving a strong shearing force to the mixture is preferably used.

The temperature at which the resin composition is melt-kneaded may be any suitable temperature condition, and is not particularly limited.
15-185 ° C is preferred. The temperature at the time of melt-kneading is preferably 115 ° C. or higher, in order to sufficiently knead the resin composition and obtain sufficient entanglement of the polymer chains of the polyolefin resin, at an appropriate viscosity, and with sufficient shearing for the resin composition. In order to exert a force, the temperature is preferably 185 ° C. or less.

Next, the obtained melt-kneaded material is formed into a sheet. The method of forming the melt-kneaded material into a sheet shape is not particularly limited, for example, may be sandwiched between cooled metal plates and rapidly cooled to form a sheet-shaped molded product by rapid crystallization.
After forming into a sheet shape using an extruder equipped with a fish tail die, a T die, or the like, it may be cooled and crystallized. For cooling the melt-kneaded product, a cooling roll or the like conventionally used can be used without particular limitation.In the present invention, not only the surface layer of the sheet-like molded product, but also the polyolefin resin up to the central portion. In order to crystallize finely, it is preferable to use a sizing die.

In the present invention, the polyolefin resin is finely crystallized not only to the surface layer of the obtained sheet-like molded product but also to the center, and is formed of fine and uniform fibrils having a large curvature ratio. In order to obtain a porous film having a membrane structure, the melt-kneaded product is preferably -15
It is desirable to rapidly cool to not more than ℃, more preferably not more than -20 ℃ to form a sheet. This is a solution state, that is, if the cooling rate at the time of forming a sheet from the melt-kneaded material is slow, it is stretched from the melt-kneading, the entangled fibrils return to hair spheres, forming thick fibers. That's why. However, since the gel-like sheet-shaped molded product generally does not have high thermal conductivity, the portion closer to the center than the surface layer is less likely to be cooled. However, when a cooled sizing die is used, due to the effect of heat conduction by the metal, it is possible to suppress the cooling unevenness of the molten mixture, and in addition to passing the space with high precision at a predetermined pressure, The shape stability of the obtained sheet-like molded product can be dramatically improved.

The thickness of the sheet-like molded product thus obtained is usually preferably 0.5 to 20 mm.

The sheet-like molded product thus obtained is stretched by melt-kneading, and the entangled fibril fibers return to a spherical shape to form a thick fiber, and a large through hole is formed in the sheet-like molded product. In order to prevent the polyolefin resin from being subjected to the rolling treatment described later, it is preferable to maintain the crystal structure of the polyolefin resin by keeping it at a temperature lower than the freezing point of the solvent used.

Next, a rolling process is performed. The method for performing the rolling process is not particularly limited, but it is preferable to perform the rolling process on the sheet-like molded product using a belt press. The belt press as used herein means one having a structure in which a sample is sandwiched between belts and rolled. In such a belt press machine, the belt can be moved at a constant speed by the driving drum, so that a continuous rolling process is possible.

The belt press used for the rolling process is as follows:
Although it is not particularly limited as long as it has the above structure, for example, a hydraulic double belt press using a press for pressurization, a pressure roll double belt press using a press roll, a belt gripping type belt press And a roto-cure, and among these, a pressure roll type double belt press is preferable from the flexibility of gap adjustment.

In the rolling process, it is preferable to perform heating rolling and cooling rolling continuously. The heating rolling and the cooling rolling may be performed using two belt presses by separating the heating belt press and the cooling belt press, and pressurization in contact with the sheet-like molded product in one belt press. The temperature of the contact portion of the means may be adjusted appropriately, but it is possible to use the heating belt press machine and the cooling belt press machine separately, since it is not affected by the temperature of each belt press machine. This is preferable because the rolling speed can be changed and the line speed can be increased. For example, in the case of a roll type, a pressure roll (heated roll) heated to a predetermined temperature
, And then cold-rolled by a pressure roll (cooling roll) cooled to a predetermined temperature.

Further, when two belt presses are used, it is possible to make a difference in the line speed between the heating belt press and the cooling belt press. By making a difference between the heating belt press line speed and the cooling belt press line speed, not only the stretching effect in the machine axial flow direction (MD direction) can be obtained, but also the rolling ratio in the MD direction can be controlled. This speed difference itself is the rolling ratio in the MD direction. Further, in order to suppress necking of the sheet-like molded product between the two belt presses, it is preferable to minimize the distance between the heating belt press and the cooling belt press.

Further, according to the present invention, it is possible to provide a feeding device in front of the heating belt press machine, and make a difference between the feeding speed and the heating belt press machine line speed. At this time, it is desirable that the ratio of the line speed / delivery speed is in the range of 2.5 to 3.5. When the ratio is 2.5 or more, the rolling ratio increases, and the orientation of the sample after rolling in the width direction (TD direction) is suppressed. Conversely, if it is less than 3.5, the machine axis direction (M
(D direction) is suppressed. By controlling so that anisotropy does not occur, the strength is further improved. Making the difference between the line speed of the heating belt press machine and the speed of the feeding machine can be expected to have an effect of suppressing meandering during sheet pressing, and can increase the yield.

The temperature at the time of hot rolling is preferably a temperature not lower than the melting point of the polyolefin resin −30 ° C. and not higher than the melting point of the polyolefin resin −10 ° C., more preferably a temperature not lower than the melting point of the polyolefin resin −20 ° C. The temperature is below -15 ° C. That is, in order to easily form a thin film by rolling, the melting point of the polyolefin resin is −3.
A temperature of 0 ° C. or higher is preferable, and in order to ensure strength and thickness uniformity when using the obtained porous film as a battery separator, the melting point of the polyolefin resin −
Temperatures below 10 ° C. are preferred. In addition, in this specification, the melting point of the polyolefin resin refers to an endothermic peak value temperature in a temperature rising process in DSC measurement.

The temperature during the cold rolling is preferably 40 ° C.
Hereinafter, the temperature is more preferably 10 to 20 ° C. That is, in order to maintain the rolling state, prevent the elastic recovery of the sheet-like molded product after the hot rolling, and make the sheet thickness uniform,
C. or less is preferred.

As a method of increasing the rolling ratio in the rolling process, there is a method of adjusting the gap between the pressure rolls. However, if the rolling ratio is set to be rapidly increased, the sheet-like molded product is belt-like. It slips between them and the biting becomes insufficient, so that rolling cannot be performed.

The number of pressure roll sets is not particularly limited, but is usually preferably about 10 to 30. Also, the bite angle of the pressure roll is not particularly limited,
0 to 1 ° is preferable, and 0 to 0.5 ° is more preferable. Here, the biting angle means an angle of a belt surface with respect to a traveling horizontal direction of the sheet-like molded product, and the belt surface indicates a region where the sheet-like molded product is bitten and rolled.

At the time of hot rolling, heat rolling is performed between belts having a biting angle in consideration of lubricating biting of the sheet-like molded product. It is preferable that the gap angle is 0 ° and the gap is constant.

Further, in order to increase the coefficient of friction between the belt surface and the sheet-like molded product and improve the biting, the surface roughness of the belt surface is controlled, or the sheet-like molded material such as paper is used. It is also possible to adopt a method in which the material is sandwiched and rolled.

Rolling by pressing is a kind of solid-phase processing. Since a resin composition is processed with high viscosity, a shear flow in which molecular friction occurs inside the resin causes brittle fracture, and uniform rolling is performed. Becomes difficult. In order to achieve ideal biaxial elongation, it is necessary to make the flow resistance as small as possible and to flow with a uniform plug flow (flag flow). For this purpose, a lubricant may be interposed between the resin composition and the belt interface. However, if the resin composition is composed of a polyolefin resin and a solvent as in the present invention, the solvent is applied between the composition and the belt surface during rolling. Oozes out and acts as a lubricant. From the viewpoint of expecting the behavior, the solvent is preferably 70% by weight or more in the resin composition of the polyolefin resin and the solvent.

A stretching process may be performed before and after the rolling process. The method of the stretching treatment is not particularly limited, and may be an ordinary tenter method, a roll method, an inflation method, or a combination of these methods. In addition, any method such as uniaxial stretching and biaxial stretching can be applied,
In the case of biaxial stretching, either vertical or horizontal simultaneous stretching or sequential stretching may be used, but vertical and horizontal simultaneous stretching is preferred. The temperature during the stretching treatment is preferably a temperature equal to or lower than the melting point of the polyolefin + 5 ° C. As other stretching treatment conditions, commonly used known conditions can be adopted.

Next, the sheet-like molded product is subjected to a desolvation treatment.

The desolvation treatment is a step of forming a porous structure by removing a solvent from a sheet-like molded product.
It can be carried out by washing the sheet-like molded product with a solvent to remove the solvent. The solvent can be appropriately selected according to the solvent used for the adjustment of the resin composition.Specifically, hydrocarbons such as pentane, hexane, heptane and decane, chlorination of methylene chloride, carbon tetrachloride and the like are used. Examples include volatile solvents such as hydrocarbons, ethers such as diethyl ether and dioxane, and alcohols, and these can be used alone or in combination of two or more. The method of desolvation treatment using such a solvent is not particularly limited, for example, a method of immersing a sheet-like molded product in a solvent to extract the solvent,
A method in which a solvent is showered on a sheet-like molded product is exemplified.

The solvent removal treatment may be performed before rolling. For example, the sheet-shaped molded product may be subjected to a desolvation treatment and then subjected to a rolling treatment, or may be subjected to a rolling treatment and then subjected to a desolvation treatment. Alternatively, the solvent may be removed before the rolling, and the solvent may be removed again after the rolling to remove the residual solvent.

In the present invention, the porous film thus obtained may be subjected to a heat setting treatment or the like for preventing heat shrinkage of the film, if necessary, to fix the shape.

The porous film thus obtained preferably has a thickness of 1 to 60 μm, preferably 5 to 45 μm, a BET specific surface area of 150 m ² / g or more and a pore volume of 0.5 cm ^3. / G, the average diameter of the through holes is preferably 0.03 μm or less, and the maximum diameter is preferably 0.1 μm or less. In addition, the pore volume and the pore diameter can be measured by the BJH method.

The porosity of the porous film is 35% or less.
Preferably, it is 75%, the air permeability is 100 to 800 seconds / 100 cc, and the needle penetration strength is 400 gf / 25 μm or more.

The porous film obtained according to the present invention comprises
It has high strength, high specific surface area and high pore volume, and is excellent in ion permeability and high-speed charge / discharge characteristics despite the long path of pores penetrating the membrane, that is, long penetration path.

In a glove box, a non-woven fabric (electrolyte-impregnated product) is formed by using the above-mentioned porous film in which a lithium cobalt oxide electrode is used as a positive electrode and a carbon electrode is used as a negative electrode in a glass, and an electrolyte is impregnated between the electrodes. ), The charge and discharge characteristics were examined. As a result, a high discharge efficiency was exhibited at a high current density, and a large output was possible in a short time.

Further, the porous film obtained by the present invention has good air permeability, but has a high specific surface area, fine fibrils are arranged at a high density, the average pore size is small, and the pore size distribution is uniform. Short circuit due to dendrite in the overcharge test hardly occurs. Therefore, it can be suitably used as a high-performance separator having excellent stability and durability in various batteries, particularly batteries for electric vehicles.

[0049]

The present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to these examples. In addition, various characteristics are measured in the following manner.

(Melting point) Using a differential scanning calorimeter “DSC-200” manufactured by Seiko Denshi Kogyo KK, the temperature was changed from room temperature to 200 ° C.
The temperature is raised at a rate of 10 ° C./min up to this point, and the endothermic peak value during this temperature raising process is defined as the melting point.

(Weight average molecular weight) A gel permeation chromatograph "GPC-150C" manufactured by Waters Co., Ltd. was used, o-dichlorobenzene was used as a solvent, and "Shodex-80M" manufactured by Showa Denko KK was used as a column. 13
Measure at 5 ° C. Data processing is performed using a data processing system manufactured by TRC. The molecular weight is calculated based on polystyrene.

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

(Porosity) Using a mercury porosimeter (Autoscan 33, Yuasa Ionics), the pore volume (ml / g) was determined, and the density of the polyolefin resin was determined to be 0.1%.
95 (g / ml) is calculated based on the following equation.

[0054]

(Equation 1)

(BET Specific Surface Area) A specific surface area / pore distribution measuring device “A” manufactured by Shimadzu Corporation using a nitrogen desorption method.
The BET specific surface area is measured using “SAP2010”.

(Average pore diameter and maximum pore diameter of through-holes)
The pore size distribution is measured by the BJH method using a specific surface area / pore distribution measuring device “ASAP2010” manufactured by Shimadzu Corporation using a nitrogen desorption method, and the average pore size and the maximum pore size are determined from this.

(Air permeability) Measured according to JIS P8117.

(Needle Penetration Strength) A handy compression tester “KES-G5” manufactured by Kato Tech Co., Ltd. is used. A needle having a diameter of 1.0 mm and a tip shape of 0.5 mm is measured at a holder diameter of 11.3 mm and a pushing speed of 2 mm / sec. The maximum load until the film breaks is defined as the needle penetration strength. All values are converted to 25 μm.

Example 1 15 parts by weight of an ultrahigh molecular weight polyethylene resin having a weight average molecular weight of 2 × 10 ⁶ (melting point 134 ° C.) and 85 parts by weight of liquid paraffin (solidification point −15 ° C., kinematic viscosity at 40 ° C. 59 cst) were mixed at a low speed. The mixture was stirred at a temperature of 20 ° C. for 30 minutes with a stirrer (manufactured by Inoue Seisakusho) having a butterfly blade (150 rpm) and a high-speed turbine blade (1500 rpm) to form a uniform slurry, and this was extruded into a twin-screw extruder (cylinder diameter: 40 mm, L / D = 42) and 16
The mixture was heated to a temperature of 0 ° C. and melt-kneaded to obtain a melt-kneaded product of an ultrahigh molecular weight polyethylene resin and a solvent. Then, immediately after extruding the kneaded material into a sheet at 160 ° C. using a fishtail die attached to the tip of a twin-screw extruder, quenched and crystallized through a sizing die cooled to −15 ° C. A shaped product was obtained.

Next, the sheet-like molded product (thickness: 1 m)
m) was heated and rolled at about 120 ° C. using a heating and pressing roll type double belt press machine in which the bite angle was set to 1 °, and the sheet-like molded product was rolled to a thickness of 100 μm, and then cooled and pressed. Using a roll type double belt press machine, 3
Cold rolling was performed at 0 ° C. (rolling magnification: 100 times). Next, it was immersed in heptane to remove the solvent, and the porous film thus obtained was further heat-set at 130 ° C. for 10 seconds to obtain a porous film having a thickness of 25 μm.

Example 2 A sheet-like molded product and a porous film having a thickness of 25 μm were obtained in the same manner as in Example 1 except that the stirring temperature was 60 ° C.

Example 3 A sheet-like molded product and a 25 μm-thick porous film were obtained in the same manner as in Example 1 except that the stirring temperature was changed to 100 ° C.

Comparative Example 1 A sheet-like molded product and a porous film having a thickness of 25 μm were obtained in the same manner as in Example 1 except that stirring was performed by a human hand without using a stirrer.

The unmelted polyolefin resin at the time of melt-kneading in the sheet-like molded products obtained in Examples and Comparative Examples was visually confirmed. In addition, the maximum throughput before the generation of unmelted material was confirmed at intervals of 5 kg / hr. Table 1 shows the results.

[0065]

[Table 1]

Table 2 shows the BET specific surface area, average pore size, maximum pore size, porosity, air permeability, and needle penetration strength of the porous films obtained in Examples and Comparative Examples.

[0067]

[Table 2]

From the above results, it can be seen that unmelted polyolefin resin is not confirmed in the sheet-shaped molded products of Examples 1 to 3. In addition, it was confirmed that the obtained porous film had high strength, high specific surface area and high pore volume, was excellent in ion permeability and high-speed charge / discharge characteristics, and had uniform characteristics and thickness. .

[0069]

According to the present invention, there is provided a method for producing a porous film having high strength, high specific surface area and high pore volume, excellent ion permeability and high-speed charge / discharge characteristics, and a uniform film structure. It became possible to do.

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Claims (4)

[Claims] 1. A process comprising melt-kneading a resin composition containing a polyolefin resin and a solvent, forming the obtained melt-kneaded product into a sheet, and performing a rolling treatment and a desolvation treatment of the sheet-like molded product. A method for producing a porous film, which comprises subjecting a resin composition previously stirred by a stirrer to melt kneading. 2. The method according to claim 1, wherein the temperature of the resin composition is adjusted to 20 to 100 ° C. and stirring is performed. 3. The method according to claim 1, wherein a stirrer having a low-speed butterfly blade and a high-speed turbine blade is used for stirring the resin composition. 4. A battery having a porous film obtained by the production method according to claim 1.