JP2001002812A - Preparation of porous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a film which has a uniform thickness, high strength, a high specific surface area and a high pore volume and which is excellent in ion permeability and rapid charging and discharging characteristics by melting and mixing a resin composition containing a polyolefin resin and a solvent, forming the mixture into a sheet, and subjecting the sheet to rolling by a belting press and deliquoring. SOLUTION: A polyolefin resin desirably contains 5-100 wt.% of an ultra high molecular weight polyolefin resin. Although no specific limitation is imposed on solvents, a solvent with a freezing point of -10 deg.C or lower is desired and, for example, liquid paraffin is desired. The polyolefin is incorporated in 5-30 wt.% of the resin composition. Heat rolling by a belting press is conducted at a temperature between the melting point of the polyolefin resin minus 30 deg.C and the melting point of the polyolefin resin minus 10 deg.C and cold rolling is conducted at 40 deg.C or lower. No specific limitation is imposed on the method of deliquoring and, for example, a sheet-like article may be immersed in a solvent to extract the solvent.

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.

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

Generally, in such a lithium battery, a separator for preventing a short circuit between the electrodes is provided between the positive electrode and the negative electrode as described above. 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 ultrahigh molecular weight polyolefin resin, together with other polyolefin resins, if necessary, is dissolved by heating in a solvent, and this is formed into a gel-like sheet. Various methods have been proposed for producing a porous film by subjecting a sheet to a stretching treatment and performing a solvent removal treatment before and after the stretching to remove a solvent remaining in the sheet.

For example, Japanese Patent Application Laid-Open No. 7-228718 discloses a polyolefin resin composition containing 10% by weight or more of an ultrahigh molecular weight polyolefin resin having a weight average molecular weight of 1 × 10 ⁶ or more. 0.
01 to 0.2 μm, the average diameter of the through holes is 0.01 to 0.1
μm, porosity 35-95%, specific surface area 20-400
m ² / g, the average tortuosity is the ratio of the through path for the film thickness is described porous film is 1.5 to 2.5 times the film thickness.

However, in order to use a porous film obtained by using an ultrahigh molecular weight polyolefin resin practically as a separator of a battery for an electric vehicle, the film has a higher strength, a higher specific surface area and a higher pore volume. At the same time, there is a strong demand for excellent electrolyte retention properties and further excellent ion permeability and high-speed charge / discharge characteristics.

[0008]

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

[0009]

The gist of the present invention is to melt-knead a resin composition containing a polyolefin resin and a solvent, form the resulting melt-kneaded product into a sheet, and obtain the obtained sheet-like molded product. The present invention relates to a method for producing a porous film, comprising a step of performing a rolling treatment and a desolvation treatment of a porous film, wherein the rolling treatment is performed using a belt press.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The polyolefin resin used in the present invention preferably contains an ultrahigh molecular weight polyolefin resin. Ultra high molecular weight polyolefin resins include ethylene, propylene, 1-butene, 4-methyl-
Examples thereof include homopolymers, copolymers, and mixtures thereof of olefins such as 1-pentene and 1-hexene. Among these, from the viewpoint of increasing the strength of the obtained porous film, an ultrahigh molecular weight polyethylene resin Is preferably used.

The ultrahigh molecular weight polyolefin resin has a weight average molecular weight of 5 × 10 ⁵ or more, preferably 5 × 10 ⁵ to 2
0 × 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.

[0013] In addition to the ultrahigh molecular weight polyolefin resin, the resin which may be contained in the polyolefin resin includes:
Ethylene, propylene, 1-butene, 4-methyl-1-
Pentene, homopolymer of olefin such as 1-hexene,
Copolymers and mixtures thereof, and the like. Among these, from the viewpoint of increasing the strength of the obtained porous film,
High density polyethylene resins are preferred. The weight average molecular weight of these resins is preferably not less than 1 × 10 ⁴ and 5 × 10 ^{4
It is} less than 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 liquid paraffin and the like. Is preferred, and has 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 in a range that does not impair the purpose of the present invention.

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 melted and kneaded may be any suitable temperature condition, and is not particularly limited.
15-185 ° C is preferred. In order to sufficiently knead the resin composition and obtain sufficient entanglement of the polymer chains of the polyolefin resin, the temperature at the time of melt-kneading is preferably 115 ° C. or higher, and at an appropriate viscosity, 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 for forming the melt-kneaded product into a sheet is not particularly limited. For example, the molten kneaded product may be sandwiched between cooled metal plates and rapidly cooled to form a sheet-like product by rapid crystallization.
After forming into a sheet using an extruder or the like to which a die or the like is attached, it may be cooled and crystallized. For cooling the melt-kneaded material, a conventionally used cooling roll or the like can be used without any particular limitation.In the present invention, not only the surface layer of the sheet-shaped molded product, but also the polyolefin resin up to the center 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-shaped 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 because in the solution state, that is, when the cooling rate at the time of forming the sheet from the melt-kneaded material is slow, the melt-kneading is stretched out, the entangled fibrils return to the spherical shape, and form a thick fiber. It is. 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 melt-kneaded material, and in combination with passing through a highly accurate space at a predetermined pressure. In addition, the shape stability of the obtained sheet-like molded product can be dramatically improved.

[0021] 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 the shape of a hair and form a thick fiber. In order to prevent the formation of the polyolefin resin, it is preferable that the polyolefin resin be immediately subjected to a rolling treatment described below or stored at a temperature below the freezing point of the solvent used to maintain the crystal structure of the polyolefin resin.

Next, the sheet-like molded product is subjected to a rolling treatment by a belt press. The belt press here means
It has 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 continuous rolling can be performed.

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-curer, among which a pressure roll type double belt press is preferable from the viewpoint of flexibility in gap adjustment.

In the rolling treatment, 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. When providing a difference in line speed, the heating belt press may be higher, or the cooling belt press may be higher. Further, in order to suppress necking of the sheet-like molded product between the two belt presses, it is preferable that the distance between the heating belt press and the cooling belt press be as small as possible.

It is also possible to provide a feeding device in front of the heating belt press machine, and to make a difference between the feeding speed and the heating belt press machine line speed. Making the difference between the line speed of the heating belt press machine and the feeding speed can also be expected to have an effect of suppressing meandering at the time of 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. It is a temperature of 15 ° C. or less. That is, in order to easily perform thinning by rolling, the melting point of the polyolefin resin is −30.
° C or higher is preferred, and the melting point of the polyolefin resin is -1 in order to ensure uniformity of strength and thickness when the obtained porous film is used as a battery separator.
Temperatures below 0 ° C. are preferred. In this specification,
The melting point of the polyolefin resin refers to an endothermic peak value temperature in a heating 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. It slips between them and the biting becomes insufficient and rolling is not possible.

The number of pressure rolls is not particularly limited, but is usually preferably about 10 to 30. The biting angle of the pressure roll is not particularly limited, but is preferably 0 to 1 °, more preferably 0 to 0.5 °. Here, the biting angle means an angle of a belt surface with respect to a horizontal direction of the sheet-like molded product, and the belt surface indicates an area where the sheet-shaped 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 friction coefficient 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 material such as paper is used to absorb oil. It is also possible to adopt a method in which the molded product is sandwiched and rolled.

Rolling by pressing is a kind of solid-phase processing. Since the resin composition is processed in a high-viscosity state, shear flow that causes molecular friction inside the resin causes brittle fracture, and uniform rolling is performed. It becomes difficult. In order to achieve ideal biaxial elongation, it is necessary to minimize the flow resistance and flow with a uniform plug 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. It exudes 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.

Next, the sheet-like molded product after the rolling treatment 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 residual solvent. The solvent can be appropriately selected according to the solvent used in the preparation of the resin composition.Specifically, pentane, hexane, heptane, hydrocarbons such as decane, methylene chloride, chlorination of carbon tetrachloride, etc. 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, and examples thereof include a method of immersing a sheet-like molded product in a solvent to extract the solvent, a method of showering the solvent to the sheet-like molded product, and the like. . The desolvation treatment may be performed before rolling. For example, the sheet-like composition may be subjected to a desolvation treatment and then subjected to a rolling treatment, or a desolvation treatment may be performed before the rolling treatment and the desolvation treatment may be performed again after the rolling treatment.

In the present invention, a stretching treatment may be further performed before and after the rolling and the desolvation treatment. For example, the sheet-shaped molded product may be subjected to a rolling treatment and a desolvation treatment (the order of rolling and desolvation may be performed first) and then subjected to a stretching treatment, or the sheet-shaped molded product may be subjected to a stretching treatment as it is. After that, the rolling treatment and the desolvation treatment (the order of the rolling and the desolvation may be any order). Alternatively, a stretching treatment may be performed between the rolling treatment and the desolvation treatment. For example, a desolvation treatment is performed before the stretching treatment, and a rolling treatment and a desolvation treatment are performed again after the stretching treatment (the order of the rolling and the desolvation treatment may be any. May be performed first) to remove the residual solvent.

The stretching method is not particularly limited, and may be a usual tenter method, roll method, 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 not higher than the melting point of the polyolefin resin + 5 ° C. As other stretching treatment conditions, commonly used known conditions can be adopted.

In the present invention, the shape of the porous film thus obtained may be further fixed, if necessary, by subjecting it to a heat setting treatment or the like to prevent the film from being thermally contracted.

The thickness of the porous film thus obtained is preferably 1 to 60 μm, more preferably 5 to 45 μm, and the BET specific surface area is 150 m ² / g or more.
It is preferable that the pore volume is 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 or less. Note that the pore volume and pore size are B
It can be measured by the JH method.

The porosity of the porous film is 35 to 7
It is preferable that the air permeability is 5%, 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.

A nonwoven fabric (electrolyte-impregnated product) in which a porous film in which a lithium cobaltate electrode is used as a positive electrode and a carbon electrode is used as a negative electrode in a glass in a glove box and an electrolyte is impregnated therebetween is used as a cushioning material. ), 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, although the porous film obtained by the present invention has good air permeability, it has a high specific surface area, fine fibrils are arranged at a high density, and the average pore size is small. Short-circuiting is unlikely to occur. Therefore, it can be suitably used as a high-performance separator having excellent stability and durability in various batteries, particularly batteries for electric vehicles.

[0046]

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 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.

[0051]

(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”.

(Air permeability) Measured according to JIS P8117.

(Average and maximum pore diameters 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.

(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.

(Thickness Variation) Using a 1/10000 thickness gauge, the thickness of the porous film was arbitrarily measured at 10 points from the edge to the center, and the difference between the maximum value and the minimum value was determined as the thickness variation. I do.

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 liquid paraffin (freezing point: −15 ° C., kinematic viscosity at 40 ° C .: 59 cst)
85 parts by weight in a slurry state, and the resulting mixture is subjected to a twin-screw extruder (cylinder diameter: 40 mm, L / L
D = 42) at a processing rate of 20 kg / hr,
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 melt-kneaded material at 160 ° C. into a sheet using a T-die attached to the tip of a twin-screw extruder, the material was rapidly cooled and crystallized through a sizing die cooled to −15 ° C.

Next, this sheet-like molded product (thickness: 10
mm) is heated and rolled at about 120 ° C. using a heating-pressing roll-type double belt press having a bite angle of 1 °, and the sheet-shaped product is 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 and a porosity of 45%.

Example 2 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 liquid paraffin (freezing point: −15 ° C., kinematic viscosity at 40 ° C .: 59 cst)
85 parts by weight in a slurry state, and the resulting mixture is subjected to a twin-screw extruder (cylinder diameter: 40 mm, L / L
D = 42) at a processing rate of 20 kg / hr,
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 melt-kneaded material at 160 ° C. into a sheet using a T-die attached to the tip of a twin-screw extruder, the material was rapidly cooled and crystallized through a sizing die cooled to −15 ° C.

Next, this sheet-like molded product (thickness: 20
mm) was heated and rolled at about 120 ° C. using a heating and pressing roll-type double belt press having a bite angle of 1 °, and the sheet was rolled to a thickness of 1.1 mm and then cooled. Using a pressure roll type double belt press machine, 3
Cold rolling was performed at 0 ° C. (rolling magnification: 18 times). Furthermore,
Simultaneously biaxially stretched 4.1 times vertically and horizontally at 120 ° C, thickness 6
6 μm (stretch ratio: 16.7 times, total stretch ratio: 300.
6 times), and then immersed in heptane to remove the solvent. The porous film thus obtained was further heat-set at 130 ° C. for 10 seconds to have a thickness of 17 μm and a porosity of 50 μm.
% Of a porous film was obtained.

Example 3 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 liquid paraffin (freezing point: −15 ° C., kinematic viscosity at 40 ° C .: 59 cst)
85 parts by weight in a slurry state, and the resulting mixture is subjected to a twin-screw extruder (cylinder diameter: 40 mm, L / L
D = 42) at a processing rate of 20 kg / hr,
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 melt-kneaded product into a sheet at 160 ° C. using a T-die attached to the tip of a twin-screw extruder, the crystal was rapidly cooled by passing through a cooling roll (roll diameter: 400 mm) cooled to 0 ° C. It was made.

Next, this sheet-like molded product (thickness: 10
mm) was heated and rolled at about 120 ° C. with a pressure roll type double belt press set at a bite angle of 1 °, and then 3
Cold rolling was performed at 0 ° C., and rolling was performed to a thickness of 100 μm (rolling magnification: 100 times). Subsequently, the porous film thus obtained 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 film having a thickness of 25 μm.
m, a porous film having a porosity of 45% was obtained.

Comparative 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 liquid paraffin (freezing point: −15 ° C., kinematic viscosity at 40 ° C .: 59 cst)
85 parts by weight in a slurry state, and the resulting mixture is subjected to a twin-screw extruder (cylinder diameter: 40 mm, L / L
D = 42) at a processing rate of 20 kg / hr,
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 melt-kneaded material at 160 ° C. into a sheet using a T-die attached to the tip of a twin-screw extruder, the material was rapidly cooled and crystallized through a sizing die cooled to −15 ° C.

Further, the sheet-like molded product (thickness: 10 m
m) was simultaneously biaxially stretched 10 times vertically and horizontally at 120 ° C. to obtain a thickness of 100 μm (stretching ratio: 100 times), and then immersed in heptane to remove the solvent. 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 and a porosity of 45%.

Table 1 shows the BET specific surface area, the average pore size, the maximum pore size, the air permeability, the needle penetration strength, and the variation in the thickness of the porous films obtained in the examples and comparative examples.

[0066]

[Table 1]

From the above results, it can be seen that the porous film of Comparative Example 1 manufactured without using a belt press had inferior needle penetration strength and a large variation in thickness.
It can be seen that the porous film has excellent values in any of the properties.

[0068]

According to the present invention, uniform thickness, high strength,
It has become possible to provide a method for producing a porous film having a high specific surface area and a high pore volume, and also having excellent ion permeability and high-speed charge / discharge characteristics.

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Claims (2)

[Claims] 1. A process in which a resin composition containing a polyolefin resin and a solvent is melt-kneaded, the obtained melt-kneaded product is formed into a sheet, and the obtained sheet-like formed product is subjected to a rolling treatment and a desolvation treatment. A method for producing a porous film, comprising: performing the rolling treatment using a belt press. 2. The method according to claim 1, wherein the rolling treatment is carried out at a temperature of not less than -30 ° C. and not more than -10 ° C. of the polyolefin resin, and then at not more than 40 ° C.