JPH0912757A - Porous film, its production and separator for cell - Google Patents

Abstract

PURPOSE: To prepare a porous film which is excellent in mechanical strength and safety and useful as a battery separator by forming a crystalline polyethylene to a sheet, subjecting the sheet to annealing and drawing to form into a porous film and then annealing and finally subjecting to irradiation with radiation. CONSTITUTION: A composition containing a crystalline polyethylene as an essential component is formed to a sheet, the sheet is annealed and drawn to give a porous film. The porous film is annealed and irradiated with radiation to give the objective film having a heat-resistant temperature range over 20 deg.C, open pores of 0.05-2μm diameter and 30-70% porosity.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a porous film, a method for producing the same, a battery separator comprising the porous film and a battery having the battery separator.
The porous film has excellent mechanical strength and safety, and is particularly useful as a battery separator.

[0002]

2. Description of the Related Art Conventionally, various types of batteries have been put to practical use, and in recent years, as a battery for accommodating cordless electronic devices, etc., they have a high energy density and a high electromotive force, and self-discharge. Lithium batteries are attracting attention due to their low energy consumption.

The negative electrode of the lithium battery has, for example, metallic lithium, an alloy of lithium and a metal such as aluminum, the ability to adsorb lithium ions such as carbon and graphite, and the ability to occlude lithium ions by intercalation. For example, an organic material or a material formed of a conductive polymer doped with lithium ions is used.

Further, as the positive electrode of the lithium battery,
For example, fluorinated graphite generally represented by (CFx) n, M
nO ₂ , V ₂ O ₅ , CuO, Ag ₂ CrO ₄ , TiO ₂
Those formed by metal oxides such as CuS and sulfides such as CuS are used.

In the above-mentioned lithium battery, lithium metal and the like used as a negative electrode constituent material shows strong reactivity, and as an electrolyte, ethylene carbonate, propylene carbonate, acetonitrile, γ-butyl lactone, 1,2-dimethoxyethane. , Or an organic solvent such as tetrahydrofuran with LiPF ₆ , LiCF ₃ SO ₃ , Li
Since an organic solvent-based electrolytic solution in which ClO ₄ , LiBF ₄ or the like is dissolved as an electrolyte is used, when an abnormal current flows due to an external short circuit or incorrect connection, the battery temperature rises significantly. However, there is a problem that it causes thermal damage to the device incorporating it.

In the above lithium battery, a porous film is generally used as a battery separator, and the battery separator is interposed between a positive electrode and a negative electrode to prevent a short circuit between both electrodes during normal energization. At the same time, due to its porous structure, the electrical resistance between the two electrodes is kept low,
Battery voltage is maintained. On the other hand, when the internal temperature of the battery rises due to an abnormal current, when the internal temperature of the battery reaches a predetermined temperature, the porous structure of the porous film is transformed into a non-porous structure and its electric resistance is changed. It is trying to secure safety by increasing the temperature to cut off the battery reaction and preventing excessive temperature rise.

When the temperature rises due to an abnormal current,
The function to prevent battery overheating by keeping the battery reaction due to the increase in electrical resistance and to ensure battery safety is generally called the shutdown function (hereinafter abbreviated as SD function), which is essential for lithium battery separators. Is a function of.

In a porous film used as a separator for a lithium battery, the temperature at which the electrical resistance increases and reaches 200 Ω · cm ² (hereinafter referred to as “SD start temperature”) is low. If it is too high, the electrical resistance will increase with a slight temperature rise and the battery reaction will be interrupted, resulting in poor practicability. On the other hand, if the SD start temperature is too high, safety will be insufficiently secured. Therefore, it is recognized that the preferred SD onset temperature is about 110-140 ° C, and the more preferred SD onset temperature is 120-130 ° C.

Furthermore, it is desirable for the battery separator that the increased electrical resistance be maintained up to an appropriate temperature from the viewpoint of ensuring safety. The upper limit temperature at which the increased electrical resistance is maintained is referred to herein as "heat resistant temperature", and SD
The temperature range from the starting temperature to the heat resistant temperature is referred to as "heat resistant temperature range" in the present specification.

The above heat-resistant temperature can also be regarded as the film shape maintaining function of the battery separator. When the battery separator melts due to excessive temperature rise, the battery separator cannot maintain its shape and is broken, reducing the electrical resistance and destroying the SD function. Then, if the SD function is destroyed, there is a problem that the positive electrode and the negative electrode are short-circuited in the lithium battery due to contact with each other to cause a rapid rise in temperature, which causes thermal damage to a device incorporating the same. Therefore, from the viewpoint of ensuring safety, the battery separator will be approximately 110
It is desired that the SD starting temperature is within the range of 140 ° C, the heat resistant temperature is high, and the heat resistant temperature range is wide.

In addition to the characteristics such as the SD starting temperature described above, the basic characteristics of the battery separator are low electric resistance, high mechanical strength such as tensile strength, and uneven film thickness. It is required that the variation in characteristics such as electric resistance is small.

To meet the above requirements, Japanese Patent Laid-Open No. 60-23
Japanese Patent Application Laid-Open No. 954 and Japanese Patent Application Laid-Open No. 2-75151 disclose a battery separator made of a polyethylene (hereinafter abbreviated as PE) porous film or a polypropylene (hereinafter abbreviated as PP) porous film. Also,
In Japanese Patent Application Laid-Open No. 21559/1990, a porous film made of a mixture of PE having a normal molecular weight and high molecular weight PE is used as a battery separator. -30886
Japanese Patent No. 6 discloses that porous films made of different materials are stacked and used as a battery separator. Further, JP-A-5-331306 discloses that a porous sheet made of a resin composition containing PP and PE as essential components is used as a battery separator.

As a porous film used as a lithium battery separator, Japanese Patent Publication No. 46-401.
19, JP-B-55-32531, US Pat. No. 3,679,538 and US Pat. No. 380,140.
In No. 4, PP is extruded into a film at a high draft ratio (value obtained by dividing the film take-up speed during film forming by the resin extruding speed from the die).
A PP porous film obtained by heat-treating this and stretching it is a mixture of three components of PE having a specific molecular weight and molecular weight distribution, an inorganic fine powder and an organic liquid in JP-B-59-37292. A porous PE film obtained by extracting the inorganic fine powder and the organic liquid and then stretching the film is disclosed in JP-A-50-11.
Japanese Patent No. 1174 discloses a biaxially oriented porous film made of PP and PE. Further, in Japanese Patent Laid-Open No. 3-277640, ordinary high density PE (molecular weight less than 300,000) and ultra high molecular weight PE (molecular weight 8) are disclosed.
A porous film obtained by extracting an organic liquid from a porous film composed of an inorganic fine powder and an organic liquid and then subjecting the porous film to electron beam irradiation treatment is disclosed.

[0014]

[Problems to be solved by the invention]
40119, JP-B-55-32531, US Pat. No. 3,679,538 and US Pat. No. 380.
Since the battery starting separator made of the PP porous film disclosed in the specification 1404 and the like has a high SD starting temperature of 170 ° C. or higher, JP-A-60-23954 discloses the following.
JP-A-2-75151, JP-B-59-37292
The battery separator made of the PE porous film disclosed in Japanese Patent Laid-open Publication has a low heat resistance temperature of 140 ° C., which is insufficient in terms of ensuring safety.

Further, the porous film made of a mixture of a normal molecular weight PE and a high molecular weight PE disclosed in JP-A-2-21559 has a wide SD starting temperature range and a heat resistant temperature of about 145 ° C. Therefore, it is insufficient as a battery separator in terms of ensuring safety. Also,
The porous film disclosed in Japanese Unexamined Patent Publication No. 3-277640 discloses only the performance to prevent oxidative deterioration as a lead battery separator, and the SD characteristics necessary for the battery separator and related matters are completely disclosed. Not not.

Further, in the separators disclosed in JP-A-62-10857 and JP-A-63-308866, in which porous films made of different materials are laminated,
Although it seems that favorable results can be obtained in terms of SD characteristics, there is a problem in that the positions of the fine holes in each film deviate from each other due to the overlapping of the films, and the fine holes do not communicate from the front surface to the back surface, increasing the electrical resistance. . Further, when an adhesive is used for overlapping, some of the fine holes are blocked by the adhesive, which also increases the electric resistance. Further, as a matter of course, the thickness increases due to the stacking, which goes against the demands for downsizing the battery and increasing the energy density.

Further, the porous films made of PP and PE disclosed in JP-A-5-331306 and JP-A-50-111174 are manufactured at the time of production because of the difference in the characteristics of the respective resin compositions. However, it is difficult to control on an actual production scale.

Therefore, an object of the present invention is to provide excellent mechanical strength, and in particular, when used as a battery separator, has a low electric resistance and an appropriate predetermined SD onset temperature and a high heat resistance temperature, that is, It is an object of the present invention to provide a safe porous film having a wide heat resistant temperature range, a method for producing the porous film, a battery separator made of the porous film, and a battery having the battery separator.

[0019]

Means for Solving the Problems As a result of intensive studies, the present inventors have found that a porous film obtained by subjecting a specific porous film to radiation irradiation achieves the above object.

The present invention has been made on the basis of the above findings, and is a porous film composed of a composition containing crystalline polyethylene as an essential component, which has a heat resistance temperature range of 20 ° C. or more when irradiated with radiation. The present invention provides a porous film having:

The present invention also provides a preferred method for producing the above-mentioned porous film, in which a composition containing crystalline polyethylene as an essential component is molded into a sheet, the sheet is annealed, and then stretched to give a porous film. , Then
The present invention provides a method for producing a porous film, which comprises annealing the porous film and then subjecting it to radiation treatment.

Furthermore, the present invention provides a lithium battery separator comprising the above porous film, which is used by being interposed between a positive electrode and a negative electrode of a battery, and a lithium battery having the lithium battery separator. Has been achieved.

First, the porous film of the present invention will be described in detail below. Examples of the crystalline PE used in the present invention include high density PE, medium density PE, low density PE, linear low density PE, and a copolymer resin of ethylene and propylene. Among these, particularly, high-density PE having high crystallinity is preferably used, and particularly has a melt index of 0.05 to 5 (measured value at 190 ° C./2.16 kg).
Is preferably used, and a high-density PE having a melt index of 0.1 to 3 is more preferably used.

PP can be blended with the above crystalline PE. The blending ratio of PE and PP when PE is blended with PP varies depending on the melting point of PE used, but the PP content is preferably 70% by weight or less, and 50% by weight or less. Is more preferable. The melt index of PP to be blended (measured value at 230 ° C./2.16 kg) is 0.1 to 10.
Is more preferable, and it is more preferable that it is 0.2-5. In addition, the above crystalline PE may be used after being hydrophilized by a method of graft copolymerizing polar monomers such as acrylic acid, methacrylic acid and their methyl esters and vinyl acetate, or a method of impregnating a surfactant. Good.

The porous film of the present invention has the above crystalline P
The porous film before the irradiation treatment, which is composed of a composition containing E as an essential component, can be obtained by stretching a sheet molded from the raw material resin composition. The raw material resin composition contains the crystalline PE as an essential component, and CaC
O _3, may be used as the TiO _2, SiO ₂ or the like inorganic filler is blended. The average particle size of the inorganic filler is preferably 0.005 to 1 μm, more preferably 0.01 to 0.5.
μm.

As the porous film before the irradiation treatment of the present invention, a sheet is prepared by kneading a solvent-soluble organic liquid compound or solid compound with the raw material composition containing the crystalline PE as an essential component. It is also possible to use a product obtained by molding, extracting the organic liquid compound or solid compound from the sheet with a solvent, and removing the compound. Examples of the organic liquid compound or solid compound include liquid paraffin, paraffin wax, mineral oil such as process oil,
Examples thereof include ester compounds such as dioctyl phthalate, didecyl adipate, and tributyl phosphate. In the present invention, the above organic compounds or solid compounds may be used alone or in combination of two or more. The mixing ratio is preferably 1 to 2 with respect to 100 parts by weight of the crystalline PE.
00 parts by weight, more preferably 5 to 100 parts by weight.

In the composition containing the crystalline PE as an essential component, a surfactant, an antioxidant, a plasticizer, a flame retardant, a colorant, a compatibilizer, etc. are usually used for improving the physical properties of the resin, if desired. You may contain an appropriate amount of additives. The additive is preferably 0.05 to 100 parts by weight of the composition.
5 parts by weight can be used.

The thickness of the porous film of the present invention is generally preferably 10 to 50 μm, and 20 to 40 μm.
Is more preferable. The thickness of the porous film is 5
If it exceeds 0 μm, the volume occupied by the separator itself increases and the battery capacity (performance) per unit volume of the battery is deteriorated. Also, 1
If it is less than 0 μm, the SD performance at the time of abnormal temperature rise is insufficient, and the battery assembly processability is poor due to insufficient film strength.

In the porous film of the present invention, from the viewpoints of electric resistance when energized and performance of preventing permeation of fine particles, the pores of each pore are preferably permeation pores having a diameter of 0.05 to 2 μm, More preferable diameter is 0.2 to 0.8 μm
It is. The porous film of the present invention has a porosity of 3
It is 0 to 70%, preferably 35 to 55%. The porosity was calculated by the following formula from the volume of the entire membrane, the weight of the membrane, and the density measurement of the resin composition. Porosity = Void volume / Volume of entire membrane × 100 Pore volume = (Volume of entire membrane−Weight of membrane) / Density of resin composition

Thus, the porous film of the present invention is subjected to a radiation treatment of the above-mentioned porous film made of the composition containing the above crystalline PE as an essential component to obtain a heat-resistant temperature range of 20 ° C. or higher, preferably It has a heat resistant temperature range of 30 ° C. or higher. When the heat-resistant temperature range is less than 20 ° C., safety is insufficient when used as a battery separator. That is, SD is used for rapid temperature rise during abnormal conditions.
This is because even if it is carried out, the temperature rise continues due to the delay of heat conduction, and the temperature exceeds the heat resistant temperature to reheat. The radiation used for the radiation irradiation treatment will be described in detail in the description of the “manufacturing method” described later.

The porous film of the present invention has the S
The D onset temperature is preferably 110 to 140 ° C, more preferably 120 to 130 ° C. SD start temperature is 1
When the temperature is lower than 10 ° C., when used as a battery separator, a slight temperature rise inside the battery increases the electric resistance and the battery reaction is interrupted, which is not practical. Also, 14
If it exceeds 0 ° C., the safety as a battery separator is insufficient, so the above range is preferable.

In addition, the heat resistance temperature of the porous film of the present invention is preferably SD start temperature + 20 ° C. or more, more preferably SD start temperature + 30 ° C. so that the heat resistance temperature range is 20 ° C. or more. That is all. For example, SD
The heat resistant temperature of the porous film whose starting temperature is 130 ° C is preferably 150 ° C or higher, more preferably 160 ° C or higher.

The porous film of the present invention is suitable as a separator for batteries, and also used as a separation membrane, a breathable film for construction, a breathable film for medical use, a breathable film for clothes, a breathable film for foods and the like. You can

Next, a method for producing the porous film of the present invention, which is a preferable method for producing the porous film of the present invention, will be described. The manufacturing method of the porous film of the present invention,
Forming a composition containing crystalline PE as an essential component into a sheet,
After the sheet is annealed, it is stretched to give a porous film, and then the porous film is annealed and then irradiated with radiation.

In order to produce the porous film of the present invention in the method of producing the porous film of the present invention, first, a composition containing the above crystalline PE as an essential component is formed into a sheet. In order to mold the sheet, the composition containing the crystalline PE as an essential component can be carried out by an inflation method, a T-die method or the like which has heretofore been known as a thermoplastic resin sheet molding method. For example, the above composition is melt-kneaded by a twin-screw extruder, a kneader, a roll, a Banbury mixer, and then inflation molding, T
It can be melt-molded by die molding or the like to obtain a film-like material, but is not limited to such a method. Molding conditions, for example, the draft ratio is preferably 20 or more, more preferably 50 or more, and the film take-up speed is preferably 5 to 200 m / min, more preferably 10 to 1
00 m / min. The molding temperature is preferably 1
50-270 degreeC, More preferably, it is 180-240 degreeC. The draft ratio means a value represented by the following formula. Draft ratio = film take-up speed / linear velocity of resin extruded from die

Next, the formed sheet is annealed (heat-treated) (this annealing is hereinafter referred to as "first annealing"). By performing this first annealing, the formation of fine pores in the stretching step performed later is promoted, and a porous film having a high porosity can be obtained.

The first annealing can be carried out by any conventionally known method, for example, a method of bringing the sheet into contact with a heated roll or a metal plate, a method of heating the sheet in air or an inert gas. A method of winding a sheet or sheet on a core and heating this in a gas phase can be adopted.

The conditions of the first annealing (temperature / hour) differ depending on the type of crystalline PE used, but the temperature is preferably 80 to 140 ° C., and the time is preferably 10 seconds to 10 hours. The annealing conditions when using high-density PE as the crystalline PE are such that the temperature is preferably 100 to 140 ° C. and the time is preferably 10 seconds to 10 hours.

Next, the first annealed sheet is stretched to form a porous film. The stretching method is not particularly limited, conventionally known roll-type stretching method,
It can be carried out by a tenter type stretching method or the like. The stretching conditions (stretching temperature / stretching ratio) vary depending on the crystalline PE used, the target porosity and the like, but the stretching temperature is preferably 50 to 120 ° C., and the stretching ratio is preferably 30 to
It is 600%. Also, as crystalline PE, high-density PE
As for the stretching conditions when using, the stretching temperature is preferably 50 to 120 ° C., and the stretching ratio is preferably 30 to 6
00%.

Next, the porous film is further annealed (this annealing is hereinafter referred to as "second annealing"). The temperature of the second annealing is preferably 5 ° C. or more lower than the stretching temperature to the melting point of the crystalline PE used and 5 ° C. or more higher than the stretching temperature. Second after stretching
It is preferable to perform the annealing while controlling the tension so that the length of the porous film is reduced by 10 to 30% of the length of the porous film before the second annealing.

Next, the crystalline PE is crosslinked by subjecting the above porous film to radiation irradiation treatment. As the above-mentioned radiation, Yoshinori Sakamoto, "Electron beam processing for practitioners (New Polymer Bunko 27), published by Polymer Publishing Co., Ltd. (1989.
The radiation described in FIG. 1 on page 1 of 2.10) ”is mentioned, and among them, electron beams and gamma rays which are mainly used industrially are preferably used. In addition, the same document describes the handling of radiation in detail. The radiation dose is preferably 5 to 40 Mrad, more preferably 10 to 30 Mrad. When the irradiation amount exceeds 40 Mrad, the mechanical strength of the porous film becomes weak (brittle),
Battery assembly processability and impact resistance during use are poor. 5Mra
When it is less than d, the heat resistant temperature range does not reach 20 ° C., so that it is preferably within the above range. Further, when the irradiation amount of radiation exceeds 10 Mrad, it is preferable to perform irradiation in plural times. At this time, for example, 30 Mrad is
Irradiating with 10 Mrad of radiation 3 times. still,
In the present invention, the radiation irradiation treatment is performed on the entire surface of the porous film.

For example, a porous fill made of high-density PE
If the beam is irradiated with 20 Mrad electron beam,
The electrical resistance measured in an organic electrolyte is 5 Ω / sheet.
cm ^TwoAs low as below and at 130 to 170 ° C,
Its electrical resistance value is several tens of times higher than the initial resistance value at room temperature
-The electric resistance value per sheet suddenly rises to several hundred times or more and is 20
0 Ω · cm^TwoAbove, it is used as a battery separator
In this case, the SD characteristics will be excellent. In addition, the above
Porous fill consisting of a composition containing crystalline PE as an essential component
The electric resistance increased by irradiating the electron beam
Maintained at a temperature at least 20 ° C above the SD onset temperature
That is, it has been confirmed that it has a heat resistant temperature range of 20 ° C or more.
Was.

Next, the battery separator of the present invention will be described. The battery separator of the present invention comprises the porous film of the present invention and is used by being interposed between the positive electrode and the negative electrode of the battery. The battery separator has a porous structure in common with the battery separator made of the conventional PE porous film or PP porous film, but the SD function expression mechanism is different from these.

In the above-mentioned conventional battery separator, when the internal temperature of the battery reaches the softening point or melting point of the components constituting the battery separator, the porous structure changes to a non-porous structure due to the melting phenomenon, and the electric resistance The current is cut off by the increase. Therefore, the SD characteristic depends on the softening point or the melting point of the components constituting the battery separator. Therefore, the PE porous film battery separator has a low heat resistance temperature, while the PP porous film battery separator has a high SD start temperature and is close to the heat resistance temperature and has a narrow heat resistance temperature range. From this point as well, there was concern about safety.

However, in the battery separator made of the porous film of the present invention, when the internal temperature of the battery rises to reach the softening point or melting point of PE, the pores of the porous film forming the battery separator are closed. The electric resistance is increased and the electric current is cut off, but the film shape of the battery separator is maintained, so that a sufficient heat resistant temperature is exhibited.

That is, when a porous film made of a composition containing crystalline PE as an essential component is subjected to a radiation treatment, its melting point or softening temperature hardly changes and only the melt flow temperature rises. The battery separator made of a porous film is highly safe because it has a low SD initiation temperature that PE has and a high heat resistance temperature. In this way, the porous film made of one material has two functions in common so that it exhibits excellent practicality, which is completely different from the conventional product.

Next, the cylindrical battery which is the battery of the present invention will be described in detail with reference to FIG. Here, the cylindrical battery 10 of the present invention having the battery separator of the present invention will be described as an example, but in addition to this, the present invention can be applied to a prismatic battery, a disc-shaped (button-type) battery, or the like. it can.
The cylindrical battery 10 shown in FIG. 1 which is the battery of the present invention is
The battery separator 14 of the present invention is used by being interposed between a sheet-shaped positive electrode plate 12 and a sheet-shaped negative electrode plate 13, which are spirally wound to form a metal outer can 16
And has the structure in which the electrolytic solution is injected and the upper part is sealed by a sealing plate 18 having a safety valve attached.

The battery separator 14 separates the positive electrode plate 12 and the negative electrode plate 13 from each other. Further, since the battery separator 14 is the porous film of the present invention, it has a function of permeating electrolyte ions. ing. Further, since the porous film of the present invention has an appropriate SD initiation temperature, a high heat resistant temperature, and a wide heat resistant temperature range, the cylindrical battery 10
When the internal temperature of the battery reaches a certain temperature, the hole of the battery separator 14 is closed to interrupt the current, and safety is ensured at the time of internal short circuit, overfilling and the like.

As described above, the battery of the present invention is used with the battery separator of the present invention interposed between the positive electrode and the negative electrode of the battery. At this time, the materials for the positive electrode, the negative electrode, the battery case, the electrolytic solution, and the like, and the disposition structure of the components and the like of the battery of the present invention do not need to be special at all and are the same as those of the conventional battery.

[0050]

EXAMPLES The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to the following examples.

Example 1 A composition comprising 100 parts by weight of high-density PE having an apparent density of 0.960 and a melt index of 0.11 and 2 parts by weight of SiO _{2 having} an average particle diameter of 0.01 μm was prepared.
Sheets were molded using an inflation molding machine equipped with an inflation molding die having a die diameter of 200 mm and a slit gap of 1 mm. The molding temperature is 240
C., blow ratio is 1.0, and draft ratio is 140. While adjusting the air pressure in the bubble, air is blown from a position 5 cm above the die to cool, and the frost line is set to 3
The film was pulled out at 35 m / min while maintaining the length at 00 mm to form a sheet. The resulting sheet has a thickness of 2
It was 4 μm. The obtained sheet is 100 ° C. under no tension.
Annealing (heat treatment) for 30 minutes, and then applying the sheet to a roll stretching machine (preheating roll having a diameter of 500 mm,
Using a drawing roll having a diameter of 50 mm, an annealing roll having a diameter of 500 mm, and a cooling roll having a diameter of 700 mm and two sets of cooling rolls each having the same diameter, a preheating roll temperature of 80 ° C. and a drawing roll temperature of 70 ° C. Stretched to 3.0 times,
A porous film was obtained. The feeding speed of the unstretched sheet was 5 m / min. Then, the porous film was shrunk by 10% with an annealing roll heated to 110 ° C. to obtain a porous film having a final draw ratio of 2.7 times.

The surface of the obtained porous film was observed with a scanning electron microscope (SEM). The largest pore (elliptical shape) on the surface of the obtained porous film had a short diameter of 0.1 μm and a long diameter of 0.3 μm. The porosity of the obtained porous film was 41%,
The thickness was 20 μm, which was good without any unevenness in appearance. Furthermore, the tensile elastic modulus of the obtained porous film in the stretching direction was 4850 kg / cm ² , and the electric resistance at room temperature was 2.3 Ω · cm ² .

Next, the obtained porous film was treated with 10M.
The porous film of the present invention was obtained by irradiation treatment with an electron beam of rad. Electron beam irradiation processing is Nisshin High Voltage Co., Ltd.
Manufactured by CURETRON EBC-200-20-15, in nitrogen stream, applied voltage 150kv, current 7.6m.
A, the transportation speed was 10 m / min.

The obtained porous films were evaluated according to the following [Evaluation criteria for porous films]. The measurement results of the tensile elastic modulus and the softening temperature are shown in [Table 1] below, and the SD characteristic results are shown in [Fig. 2].
As is clear from [Table 1] and [FIG. 2], the obtained porous film had a high tensile elastic modulus and a heat resistant temperature range of 20 ° C. or higher.

[Evaluation Criteria for Porous Film] (1) Tensile Elastic Modulus: Measured in the stretching direction with a Tensilon tensile tester according to ASTM-D-882. (2) Softening temperature: A test film having a width of 2 cm and a length of 5 cm was prepared with the stretching direction as the length direction, and the two sides having shorter lengths were sandwiched by clamps having a weight of 3.2 g, respectively, and a gear oven was used. Suspended inside. The gear oven was heated at a rate of 10 ° C./minute, and the temperature of the oven when the film was melted and cut was defined as the softening temperature.

(3) SD characteristics; two sides were fixed so that the length of the porous film in the pulling direction was constant, and this was placed in an air thermostatic chamber kept at a predetermined temperature, left for 5 minutes, and then taken out. , Its electrical resistance is measured at room temperature, and the relationship between electrical resistance and temperature is shown in a graph. From this graph, SD
The characteristics were evaluated. The electrical resistance is measured according to JIS C 2
It carried out according to 313. That is, a solution prepared by mixing propylene carbonate and 1,2-dimethoxyethane in the same volume as an electrolytic solution and dissolving anhydrous lithium perchlorate as an electrolyte to a concentration of 1 mol / l was used. Co., Ltd. resistance meter, LCR meter K
AC resistance of 1 KHz was measured by C-532, and electric resistance R (Ω · cm ² ) of the porous film was calculated by the following formula. R = (R ₁ −R ₀ ) × S In the above formula, R ₀ , R ₁ and S represent the following. R ₀ : electric resistance of the electrolytic solution (Ω) R ₁ ; electric resistance measured with the porous film immersed in the electrolytic solution (Ω) S: cross-sectional area of the porous film (cm ² ) [Note] Since the electric resistance measuring cell used in the examples has a slight leakage current, even a completely non-porous film could measure only an electric resistance of about 600 Ω · cm ^{2 at} maximum.

[Embodiment 2] Irradiation with an electron beam is performed at 20 Mrad.
A porous film of the present invention was obtained in the same manner as in Example 1 except for the above. The 20 Mrad electron beam irradiation treatment was performed by repeating the electron beam irradiation treatment in Example 1 twice. The obtained porous film was evaluated in the same manner as in Example 1. The results are shown in the following [Table 1] and [Fig. 3]. As is clear from [Table 1] and [Fig. 3], the obtained porous film has a high tensile elastic modulus,
It had a heat resistant temperature range of 20 ° C. or higher.

[Embodiment 3] Irradiation with an electron beam is performed at 30 Mrad.
A porous film of the present invention was obtained in the same manner as in Example 1 except for the above. The 30 Mrad electron beam irradiation treatment was carried out by repeating the electron beam irradiation treatment in Example 1 three times. The obtained porous film was evaluated in the same manner as in Example 1. The results are shown in [Table 1] and [Fig. 4] below. As is clear from [Table 1] and [Fig. 4], the obtained porous film has a high tensile elastic modulus,
It had a heat resistant temperature range of 20 ° C. or higher.

Comparative Example 1 The porous film before the electron beam irradiation treatment of the porous film of Example 1 was evaluated in the same manner as in Example 1. The results are shown in the following [Table 1] and [Fig. 5].

[Comparative Example 2] A porous film was obtained in the same manner as in Example 1 except that the high density PE was an isotactic homopolymer of PP having a melt index of 0.4. The largest pores (oval shape) on the surface of the obtained porous film had a short diameter of 0.1 μm, a long diameter of 0.3 μm, and a porosity of 45%. The obtained porous film was evaluated in the same manner as in Example 1 without electron beam irradiation treatment. The results are shown in the following [Table 1] and [Fig. 6].

[0061]

[Table 1]

[0062]

EFFECT OF THE INVENTION The porous film of the present invention has excellent mechanical strength, and particularly when used as a battery separator,
It is a porous film that has a low electric resistance and an appropriate predetermined SD starting temperature, a high heat resistance temperature, and a wide heat resistance temperature range, and is therefore a safe porous film. Therefore, it is particularly useful as a battery separator. is there. And this porous film can be easily manufactured by the manufacturing method of the porous film of this invention.

[Brief description of the drawings]

FIG. 1 is a partially exploded perspective view showing a cylindrical battery, which is the battery of the present invention.

FIG. 2 is an SD of the porous film of Example 1.
It is a graph which shows a characteristic.

FIG. 3 is an SD of the porous film of Example 2.
It is a graph which shows a characteristic.

FIG. 4 is an SD of the porous film of Example 3.
It is a graph which shows a characteristic.

FIG. 5 is an SD of the porous film of Comparative Example 1.
It is a graph which shows a characteristic.

FIG. 6 is an SD of the porous film of Comparative Example 2.
It is a graph which shows a characteristic.

[Explanation of symbols]

 10 Cylindrical Battery 12 Positive Electrode Plate 13 Negative Electrode Plate 14 Battery Separator 16 Exterior Can 18 Sealing Plate

Claims (7)

[Claims] 1. A porous film comprising a composition containing crystalline polyethylene as an essential component, which is characterized by having a heat resistant temperature range of 20 ° C. or more when subjected to radiation treatment. the film. 2. The porous film according to claim 1, wherein the pores of each pore are permeation pores having a diameter of 0.05 to 2 μm and a porosity of 30 to 70%. 3. The porous film according to claim 1, wherein the radiation is an electron beam. 4. A composition comprising crystalline polyethylene as an essential component is formed into a sheet, the sheet is annealed, and then stretched to obtain a porous film, and then the porous film is annealed, followed by irradiation with radiation. A method for producing a porous film, which comprises treating. 5. The method for producing a porous film according to claim 4, wherein the radiation is an electron beam. 6. A separator for a lithium battery, which is made of the porous film according to claim 1 and is interposed between a positive electrode and a negative electrode of a battery. 7. A battery having the lithium battery separator according to claim 6.