JP4248093B2 - Porous film - Google Patents

Description

【００５４】
表１の結果より、実施例１〜２で得られた多孔質フィルムは、いずれも適度な通気度、突き刺し強度を有し、比較例１〜２で得られた多孔質フィルムに比べ、ＳＤ温度と熱破断温度のバランスに優れたものであることがわかる。
【００５５】
【発明の効果】
本発明の多孔質フィルムは、透過性能および機械的強度に優れると共に低温でのＳＤ機能と高温での耐熱性のバランスに優れたものであるため、電池用セパレータとして用いることで、電池の様々な大きさや用途に対してより安全性を向上させることが期待できるという効果が奏される。
【図面の簡単な説明】
【図１】図１は、第１のマトリックスポリマー中に第２のポリマー層が鱗片状に分散している多孔質フィルムの模式図である。
【図２】図２は、実施例１で得られた多孔質フィルムの断面の透過型電子顕微鏡写真（５０００倍）である。
【符号の説明】
１ 第１のマトリックスポリマー層
２ 第２のポリマー層[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a porous film. More specifically, the present invention relates to a porous film having a function of blocking permeability at a low temperature and an excellent function of a high melt fracture temperature, useful for battery separators and diaphragms for electrolytic capacitors.
[0002]
[Prior art]
Non-aqueous electrolyte batteries using light metals such as lithium as electrodes have a high energy density and low self-discharge, and thus have a wide range of applications against the background of high performance and miniaturization of electronic devices. As the electrode of such a non-aqueous electrolyte battery, a spiral wound body that secures a wide effective electrode area by stacking and winding a belt-like positive electrode, a negative electrode, and a separator is used. The separator basically prevents short-circuiting of both electrodes and allows the battery reaction by allowing ions to permeate due to its porous structure. However, when an abnormal current occurs due to misconnection or the like, the internal temperature of the battery A resin having a so-called shutdown function (SD function) in which the resin is thermally deformed to block the micropores and stop the cell reaction as the temperature rises is adopted from the viewpoint of improving safety.
[0003]
As a separator having such an SD function, for example, a polyethylene microporous film or a microporous film having a multilayer structure of polyethylene and polypropylene is known.
[0004]
However, due to recent advances in lithium ion secondary batteries and the like, not only the SD function described above, but also when the temperature further rises after shutdown, the separator itself melts down (melts down) or is plasticized and breaks If this happens, there is a risk of battery ignition and explosion, and therefore heat resistance is desired even at higher temperatures. In particular, if the capacity of the battery is increased or the internal resistance of the battery is reduced, the factors that increase the heat generation increase, which is more important.
[0005]
In view of the above problems, for example, in Japanese Patent Application Laid-Open No. 63-308866, a microporous film having high strength and excellent high-temperature characteristics is obtained by laminating a single film made of low melting point polyethylene and high melting point polypropylene. Although a method for obtaining a film has been disclosed, the internal resistance of the separator is increased due to the lamination, which is not suitable as a separator for a high-performance battery such as a high-power application. Japanese Patent Application Laid-Open No. 10-298325 discloses a method for obtaining a microporous film comprising a high molecular weight polyethylene composition containing low molecular weight polyethylene and polypropylene. However, when the temperature rises rapidly, polypropylene is used. In some cases, a heat resistance exceeding the heat resistance at a low temperature and a porous film having a good balance between the heat resistance at a high temperature are desired.
[0006]
[Problems to be solved by the invention]
In view of the above problems, an object of the present invention is to provide a porous film excellent in permeation performance and mechanical strength, and having an excellent balance between the SD function at a low temperature and the heat resistance at a high temperature.
[0007]
[Means for Solving the Problems]
As a result of diligent research to achieve the above object, the present inventors have found that the porous film has a low shutdown temperature by dispersing the heat-resistant layer spreading in the form of scales or flakes in the polymer layer having the SD function. The inventors have found that an excellent function having (SD temperature) and a high membrane breaking temperature can be obtained, and have reached the present invention.
[0008]
That is, the gist of the present invention is a porous film comprising a first matrix polymer and a second polymer dispersed in the form of scales or flakes in the first matrix polymer , wherein the first matrix polymer Is a polyolefin resin, and the second polymer is a reactive polymer capable of crosslinking reaction.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The first matrix polymer used in the present invention is desirably a polymer having an SD function at a low temperature as a battery separator, and a polyolefin resin is preferable. Examples of such a polyolefin resin include homopolymers, copolymers and blends thereof obtained by polymerizing ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and the like. Among these, polyolefin resins having a weight average molecular weight of 500,000 or less, particularly polyethylene having a low melting point are preferably used. These first matrix polymers may be used alone or in admixture of two or more. The content of the polyolefin resin having a weight average molecular weight of 500,000 or less is preferably 20 to 90% by weight, more preferably 30 to 80% by weight in the first matrix polymer.
[0010]
In order to increase the strength of the porous film, an ultrahigh molecular weight polyolefin resin having a weight average molecular weight exceeding 500,000, such as an ultrahigh molecular weight polyethylene having a weight average molecular weight exceeding 500,000, may be mixed as the first matrix polymer. . The content of the polyolefin resin is preferably 5 to 80% by weight, more preferably 8 to 70% by weight in the first matrix polymer.
[0011]
The content of the first matrix polymer is preferably 60 to 99% by weight and more preferably 60 to 97% by weight in the porous film. The lower limit of the content is preferably 60% by weight or more from the viewpoint of imparting sufficient shutdown temperature and film strength, and the upper limit is preferably 99% by weight or less from the viewpoint of maintaining heat resistance.
[0012]
Next, the second polymer used in the present invention is a reactive polymer that can undergo a crosslinking reaction after forming a porous film, a ring-opening polymer of an unsaturated condensed alicyclic compound, a diacryl phthalate resin, Examples thereof include an epoxy resin and an acrylic resin, and among them, a ring-opened polymer of an unsaturated condensed alicyclic compound is preferable. The ring-opened polymer of the unsaturated condensed alicyclic compound has an aliphatic ring derived from the monomer unit and a double bond in the main chain. The ring-opening polymer may be hydrogenated at a part of the double bond.
[0013]
The unsaturated condensed alicyclic compounds are roughly classified into the following three series. As the first series, among those classified as a condensed alicyclic compound in a narrow sense, an unsaturated compound having a double bond incorporated into a main chain after ring-opening polymerization is included in one of the rings. Moreover, it can use as an unsaturated condensed alicyclic compound also including the derivative | guide_body in which some of the hydrogen atoms of those unsaturated compounds replaced the other substituent. Specific examples thereof include bicyclo [3.2.0] hept-6-ene, bicyclo [4.2.0] oct-7-ene, and derivatives thereof.
[0014]
As the second series, among those classified as bridged ring compounds, unsaturated compounds having in one of the rings a double bond incorporated into the main chain after ring-opening polymerization can be mentioned. Moreover, it can use as an unsaturated condensed alicyclic compound also including the derivative | guide_body in which some of the hydrogen atoms of those unsaturated compounds replaced the other substituent. Specific examples thereof include bicyclo [2.2.1] hept-5-ene (also referred to herein as norbornene), bicyclo [2.2.1] hept-5-ene-2,3-dicarboxymethyl. Examples include norbornene derivatives such as esters, bicyclo [2.2.2] oct-2-ene, and derivatives thereof.
[0015]
The third series includes compounds having a bridged ring and a condensed alicyclic ring, and having an aliphatic ring and a double bond in the main chain after ring-opening polymerization. Specific examples thereof include tricyclo [5.2.1.0 ^2,6 ] deca-3,8-diene (dicyclopentadiene), tetracyclododecene, and derivatives thereof.
[0016]
Among these unsaturated condensed alicyclic compounds, norbornene and norbornene derivatives are preferable from the viewpoint of supplying raw materials. Further, these unsaturated condensed alicyclic compounds can be subjected to ring-opening polymerization alone or in admixture of two or more.
[0017]
As the ring-opening polymer of the unsaturated condensed alicyclic compound, polynorbornene or the like is preferably used. Among them, polynorbornene rubber having a high average molecular weight is more preferably used from the viewpoint of dispersibility.
[0018]
These 2nd polymers can be used individually or in mixture of 2 or more types.
[0019]
The content of the second polymer is preferably 1 to 40% by weight and more preferably 3 to 40% by weight in the porous film. The lower limit of the content is preferably 1% by weight or more from the viewpoint that a porous film having sufficient heat resistance can be obtained, and the upper limit is a viewpoint that the SD characteristic of the porous portion is easily maintained as a battery separator. Therefore, 40% by weight or less is preferable.
[0020]
The porous film of the present invention comprises the first matrix polymer and the second polymer dispersed in the form of scales or flakes in the first matrix polymer. Here, “scale-like” or “flaky” means that the thin polymer-like second polymer layer 2 is widely distributed in the first matrix polymer layer 1 as shown in FIG. Specifically, it generally has a thickness of 0.02 to 2 μm, a vertical diameter of 1 to 1000 μm, and a horizontal diameter of 1 to 1000 μm.
[0021]
The porous film of the present invention has one major feature in that it has the above structure. By having such a structure, when the porous film is subjected to a cross-linking reaction, the second polymer layers existing apart from each other in the film are connected to each other to increase the mechanical strength. Since the first matrix polymer layer that is basically separated has SD characteristics, it is considered that a porous film excellent in heat resistance at high temperature and SD function at low temperature can be obtained. It is done.
[0022]
When the first polymer and the second polymer are uniformly dispersed and exist in a minute domain form or a homogeneous single layer, the structure between the polymers is closely connected and fixed, so that the shutdown temperature Will be expensive. Therefore, as a preferable embodiment in the present invention, a porous film obtained by causing a crosslinking reaction of the second polymer in the porous film having the structure as described above is also included.
[0023]
Next, the manufacturing method of the porous film of this invention is demonstrated.
For the production of the porous film according to the present invention, known methods such as a dry film forming method and a wet film forming method can be used. For example, after the first matrix polymer is mixed with a solvent, kneaded, and heated and melted, the second polymer is added afterwards, and further kneaded and heated and melted. The kneaded product is formed into a sheet shape, and then rolled, stretched in a uniaxial direction or more, and the solvent can be extracted and removed.
[0024]
In the present invention, by using such a method, it is possible to obtain a form in which the second polymer is dispersed in the form of scales or flakes in the first matrix polymer.
[0025]
Examples of the solvent include aliphatic or cyclic hydrocarbons such as nonane, decane, undecane, dodecane, decalin, liquid paraffin, and mineral oil fractions having boiling points corresponding to these, and alicyclic such as liquid paraffin. Nonvolatile solvents rich in hydrocarbons are preferred. Further, the amount of the solvent used is preferably 60 to 95% by weight of the mixture of the first matrix polymer and the solvent. The step of kneading the mixture and forming it into a sheet can be performed by a known method, kneaded batch-wise using a Banbury mixer, kneader, etc., and then sandwiched in a cooled metal plate and rapidly cooled. Then, it may be formed into a sheet-like molded article by rapid crystallization, or a sheet-like molded article may be obtained using an extruder equipped with a T die or the like. The kneading conditions are appropriate in any case of kneading the first matrix polymer and the solvent (primary kneading) or kneading these kneaded product and the second polymer (secondary kneading). The temperature is not particularly limited as long as the temperature is appropriate, but the temperature is preferably 100 to 200 ° C. Also, the kneading time is not particularly limited, but it is preferable that the secondary kneading is longer than the primary kneading.
[0026]
Although it does not specifically limit as thickness of the sheet-like molding obtained in this way, 3-20 mm is preferable and you may make it thickness of 0.5-2 mm by rolling processes, such as a heat press. Moreover, as for the temperature of a rolling process, 100-140 degreeC is preferable.
[0027]
The method of stretching the sheet-like molded product is not particularly limited, and may be a normal denter method, roll method, inflation method, or a combination of these methods. Any method such as stretching can be applied. In the case of biaxial stretching, either longitudinal and transverse simultaneous stretching or sequential stretching may be used. It is preferable that the temperature of an extending | stretching process is 100-140 degreeC.
[0028]
The solvent removal treatment is a step of removing the solvent from the sheet-like molded product to form a microporous structure, and can be performed, for example, by washing the sheet-like molded product with a solvent to remove the remaining solvent. Solvents include hydrocarbons such as pentane, hexane, heptane and decane, chlorine hydrocarbons such as methylene chloride and carbon tetrachloride, fluorinated hydrocarbons such as ethane trifluoride, ethers such as diethyl ether and dioxane, etc. A volatile solvent is mentioned, These can be used individually or in mixture of 2 or more types. The cleaning method using such a solvent is not particularly limited, and examples thereof include a method of extracting a solvent by immersing a sheet-shaped molded product in a solvent, and a method of showering the solvent on the sheet-shaped molded product.
[0029]
After forming into a film by these known methods, the obtained porous film is subjected to a crosslinking treatment using one or more selected from the group consisting of heat, electron beam, ultraviolet light and visible light. It is preferable to eliminate all or part of the heavy bonds. Among these, a crosslinking treatment using heat is desirable from the viewpoint of the structural stability of the porous film. By performing such a crosslinking treatment, the heat resistance of the porous film is greatly improved.
[0030]
The reason for this improvement in heat resistance is not necessarily clear, but the polymer radical generated in each treatment is added to the double bond, and at that time, the second polymer, or the second polymer and other resin components, It is conceivable that a crosslinking reaction takes place between the two, and that the glass transition temperature of the second polymer chain itself is greatly increased by the disappearance of the double bond in the main chain. The ratio of losing the double bond is appropriately selected in consideration of the desired heat resistance, but an erasure rate of 80 to 100% (calculated based on the IR peak size) is preferable. And it is thought that heat resistance improves greatly by these.
[0031]
The method for the crosslinking treatment is not particularly limited. For example, in the case of using heat, even in the one-stage heat treatment method in which heat treatment is performed once, a multistage method in which heat treatment is first performed at low temperature and then heat treatment at higher temperature is performed. A heat treatment method or a temperature raising type heat treatment method in which heat treatment is performed while raising the temperature may be used, but it is desirable to perform the treatment without impairing the original characteristics of the porous film such as air permeability. In the case of one-stage heat treatment, although it depends on the composition of the porous film, it is preferably 40 ° C to 140 ° C. In addition, if heat treatment is started from low temperature and then the treatment temperature is raised, the heat resistance gradually improves with hardening of the porous film, so that heating does not impair the original characteristics such as air permeability. Be able to be exposed to high temperatures. Therefore, in order to complete the heat treatment in a short time without impairing various characteristics, a multistage type or a temperature rising type heat treatment method is preferable.
[0032]
The first heat treatment temperature of the multi-stage heat treatment method depends on the composition of the porous film, but preferably 40 to 90 ° C. The second heat treatment temperature depends on the composition of the porous film, but preferably Is 90-140 ° C. Moreover, you may perform the heat processing after the 3rd step | paragraph after a further short time at further high temperature as needed. The treatment time depends on the composition of the porous film, but is preferably about 3 to 48 hours for the first heat treatment and about 0.5 to 6 hours for the heat treatment at the second higher temperature. In the case of the temperature raising type heat treatment method, it may be performed under the conditions according to the multistage heat treatment method.
[0033]
When ultraviolet rays are used, for example, a porous film after film formation is impregnated with a methanol solution containing a polymerization initiator, and after the solvent is dried, the porous film is irradiated with a mercury lamp to perform a crosslinking treatment. Can do.
[0034]
When using an electron beam, for example, the porous film after film formation is performed by irradiating a radiation dose of 0.1 to 10 Mrad. The atmosphere during irradiation may be air, as in the heat treatment method, or may be an atmosphere of an inert gas such as nitrogen gas or argon gas in order to control the crosslinking state.
[0035]
Further, following the crosslinking treatment step, the porous film may generally be heat set (heat-fixed) to prevent thermal shrinkage. In particular, in the present invention, by performing the crosslinking treatment using heat as described above, heat setting is substantially possible depending on the processing conditions, but when the heat setting is insufficient, heat shrinkage is performed. In order to prevent it better, heat setting may be performed by further heating after the crosslinking treatment. What is necessary is just to perform the temperature at the time of this heat setting at about 110 to 140 degreeC for about 0.5 to 2 hours, for example.
[0036]
1-60 micrometers is preferable and, as for the thickness of the porous film obtained as mentioned above, 5-45 micrometers is more preferable. The porosity is preferably 20 to 80%, more preferably 25 to 75%. For example, the permeability according to JIS P8117 is preferably 100 to 1000 seconds / 100 cc, and more preferably 200 to 900 seconds / 100 cc. As the mechanical strength, for example, the piercing strength is preferably 200 gf / 25 μm or more, and more preferably 300 gf / 25 μm or more. In addition, as a measuring method of this piercing strength, the method as described in the below-mentioned Example is mentioned.
[0037]
As SD temperature of a porous film, 120-150 degreeC is preferable and 130-138 degreeC is more preferable. The thermal break temperature is preferably 150 ° C. or higher, more preferably 200 ° C. or higher.
[0038]
Since the porous film of the present invention thus obtained is excellent in low temperature SD effect and high temperature heat resistance, it can be used as a battery separator, which makes it more safe for various sizes and applications of batteries. It can be expected to improve.
[0039]
【Example】
EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples. In addition, the test method in an Example is as follows.
[0040]
(Film thickness)
The cross section of the 1/10000 thickness gauge and the porous film was measured with a scanning electron microscope.
[0041]
(Air permeability)
It measured based on JISP8117.
[0042]
(Porosity)
The porous film to be measured was cut into a circle with a diameter of 6 cm, the volume and weight were determined, and the calculation was performed using the following formula from the obtained results.
[0043]
Porosity (%) = 100 × [volume (cm ³ ) −weight (g) / average density of first matrix polymer and second polymer (g / cm ³ )] / volume (cm ³ )
[0044]
(Puncture strength)
A piercing test was conducted using a compression tester “KES-G5” manufactured by Kato Tech Co., Ltd. The maximum load was read from the obtained load displacement curve to determine the piercing strength. A needle having a diameter of 1 mm and a radius of curvature of the tip of 0.5 mm was used, and it was performed at a speed of 2 cm / second.
[0045]
(Shutdown temperature (SD temperature))
A stainless steel cell having a cylindrical test chamber of 25 mmφ, which can be sealed, was used, the lower electrode was 20 mmφ, and the upper electrode was a platinum plate (thickness 1.0 mm) of 10 mmφ. A measurement sample punched to 24 mmφ was immersed in an electrolytic solution, impregnated with the electrolytic solution, sandwiched between electrodes, and set in a cell. A certain surface pressure was applied to the electrode by a spring provided in the cell. The electrolytic solution used was a solution in which lithium borofluoride was dissolved to a concentration of 1.0 mol / l in a solvent in which propylene carbonate and dimethoxyethane were mixed at a volume ratio of 1: 1.
[0046]
A thermocouple thermometer and a resistance meter were connected to this cell so that the temperature and resistance could be measured, and the cell was put into a 180 ° C. thermostat and the temperature and resistance were measured. The average temperature increase rate from 100 to 150 ° C. was 10 ° C./min.
By this measurement, the temperature when the resistance reached 100 Ω · cm ² was defined as the SD temperature.
[0047]
(Thermal break temperature)
A strip-shaped sample having a width of 3 mm was attached with a chuck space of 10 mm, set in a thermal stress strain analyzer “TMA / SS100” manufactured by Seiko Electronics, and heated at a rate of temperature increase of 2 ° C. per minute. The temperature at the time of the temperature rise was evaluated, and the temperature at which the strip-shaped sample broke was defined as the thermal rupture temperature.
[0048]
Example 1
18 parts by weight of a polymer composition comprising 40% by weight of polyethylene having a weight average molecular weight of 300,000 (melting point 133 ° C., hereinafter the same) and 60% by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight of 3 million (melting point 144 ° C., the same shall apply hereinafter) 80 parts by weight of liquid paraffin (freezing point: −15 ° C., kinematic viscosity at 40 ° C .: 59 cst, hereinafter the same) was uniformly mixed in a slurry and melt kneaded at a temperature of 160 ° C. for about 15 minutes using a small kneader. In a kneader during kneading, 2 parts by weight of a ring-opening polymer powder of norbornene (manufactured by Nippon Zeon Co., Ltd., “Nosolex NB”, weight average molecular weight of 2 million or more, the same shall apply hereinafter) (10% by weight in the resin content) Was added and dissolved and kneaded for an additional 45 minutes. Thereafter, the kneaded product was sandwiched between metal plates cooled to 0 ° C. and rapidly cooled into a sheet shape. This quenched sheet-like resin is heat-pressed at a temperature of 115 ° C. until the sheet thickness becomes 0.4 to 0.6 mm, and biaxially stretched 3.5 × 3.5 times in length and width at the temperature of 115 ° C., The solvent removal treatment was performed using heptane. Then, the obtained porous film was heat-treated in air at 85 ° C. for 6 hours, and then heat-treated at 120 ° C. for 2 hours to obtain the porous film of the present invention.
Further, when the cross section of the obtained porous film was observed with a transmission electron microscope (5000 magnifications), as shown in FIG. 2, the layer 2 of the second polymer (norbornene ring-opening polymer) was In the matrix polymer (polyethylene) layer, it was dispersed in the form of scales.
[0049]
Example 2
15 parts by weight of a polymer composition consisting of 47% by weight of polyethylene having a weight average molecular weight of 300,000 and 53% by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight of 3 million and 80 parts by weight of liquid paraffin are uniformly mixed in a slurry state. It melt-kneaded for about 15 minutes using the small kneader at the temperature of degreeC. In a kneader during kneading, 5 parts by weight of a ring-opening polymer powder of norbornene (25% by weight in the resin content) was post-added and further dissolved and kneaded for 45 minutes. Thereafter, the kneaded product was sandwiched between metal plates cooled to 0 ° C. and rapidly cooled into a sheet shape. This quenched sheet-like resin is heat-pressed at a temperature of 115 ° C. until the sheet thickness becomes 0.4 to 0.6 mm, and biaxially stretched 3.5 × 3.5 times in length and width at the temperature of 115 ° C., The solvent removal treatment was performed using heptane. Then, the obtained porous film was heat-treated in air at 85 ° C. for 6 hours, and then heat-treated at 120 ° C. for 2 hours to obtain the porous film of the present invention. In the obtained porous film, the second polymer was dispersed in a scaly manner as in Example 1.
[0050]
Comparative Example 1
Norbornene ring-opening polymer powder 38.5% by weight (25% by weight in the resin content), 13 parts by weight of a polymer composition comprising 61.5% by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight of 3 million, and liquid paraffin 80 Part by weight was uniformly mixed in a slurry state, and dissolved and kneaded at a temperature of 160 ° C. using a small kneader for about 15 minutes. In a kneader during kneading, 7 parts by weight of polyethylene having a weight average molecular weight of 300,000 was post-added, and further dissolved and kneaded for 45 minutes. Thereafter, the kneaded product was sandwiched between metal plates cooled to 0 ° C. and rapidly cooled into a sheet shape. This quenched sheet-like resin is heat-pressed at a temperature of 115 ° C. until the sheet thickness becomes 0.4 to 0.6 mm, and biaxially stretched 3.5 × 3.5 times in length and width at the temperature of 115 ° C., The solvent removal treatment was performed using heptane. Thereafter, the obtained porous film was heat-treated in air at 85 ° C. for 6 hours, and then heat-treated at 120 ° C. for 2 hours to obtain a porous film. In the obtained porous film, the second polymer was dispersed in the form of fine domains.
[0051]
Comparative Example 2
15 parts by weight of a polymer composition consisting of 47% by weight of polyethylene having a weight average molecular weight of 300,000 and 53% by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight of 3 million and 85 parts by weight of liquid paraffin are uniformly mixed in a slurry state. It melt-kneaded for about 60 minutes using the small kneader at the temperature of (degreeC). Thereafter, the kneaded product was sandwiched between metal plates cooled to 0 ° C. and rapidly cooled into a sheet shape. This quenched sheet-like resin is heat-pressed at a temperature of 115 ° C. until the sheet thickness becomes 0.4 to 0.6 mm, and biaxially stretched 3.5 × 3.5 times in length and width at the temperature of 115 ° C., The solvent removal treatment was performed using heptane. Then, the obtained porous film was heat-processed at 120 degreeC for 2 hours, and the porous film was obtained. In the obtained porous film, the second polymer was uniformly integrated.
[0052]
Table 1 shows the characteristics of the porous films obtained in Examples 1-2 and Comparative Examples 1-2. A porous film having an SD temperature of 135 ° C. or lower and a thermal fracture temperature of 200 ° C. or higher is used as a well-balanced porous film.
[0053]
[Table 1]

[0054]
From the results in Table 1, the porous films obtained in Examples 1 and 2 all have appropriate air permeability and puncture strength, and the SD temperature is higher than that of the porous films obtained in Comparative Examples 1 and 2. It can be seen that it has an excellent balance between the thermal rupture temperature.
[0055]
【The invention's effect】
Since the porous film of the present invention is excellent in permeation performance and mechanical strength and has an excellent balance between the SD function at low temperature and the heat resistance at high temperature, it can be used as a battery separator. There is an effect that safety can be expected to be further improved with respect to size and use.
[Brief description of the drawings]
FIG. 1 is a schematic view of a porous film in which a second polymer layer is dispersed in a scaly manner in a first matrix polymer.
FIG. 2 is a transmission electron micrograph (magnified 5000 times) of a cross section of the porous film obtained in Example 1. FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 1st matrix polymer layer 2 2nd polymer layer

Claims (5)

A porous film comprising a first matrix polymer and a second polymer dispersed in a scale or flake form in the first matrix polymer , wherein the first matrix polymer is a polyolefin resin, 2. A porous film, wherein the polymer 2 is a reactive polymer capable of crosslinking reaction.   The porous film according to claim 1, wherein the second polymer is a ring-opening polymer of an unsaturated condensed alicyclic compound.   The porous film according to claim 2, wherein the ring-opening polymer of the unsaturated condensed alicyclic compound is a polynorbornene rubber. The porous film according to claim 1, wherein the polyolefin resin contains a polyolefin resin having a weight average molecular weight of 500,000 or less. The porous film according to any one of claims 1 to 4, wherein the porous film is obtained by crosslinking reaction of the second polymer.

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