JP2008013730A - Porous film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous film which, through the control of the crosslinked structure and the shrinkage force, has a high film strength at room temperature and even at a high temperature has excellent film rupture resistance, and provide its use. <P>SOLUTION: The porous film, formed when a resin composition containing a polyolefin is crosslinked, has a needle piercing strength of ≥2.5 [N/24 μm] and a peak value of the shrinkage force per unit cross-sectional area in the temperature range above the shut-down temperature. When the peak value is regarded as the shrinkage force of the porous film, the relationship between the gel fraction measured in boiling xylene and the above shrinkage force is ≥0.4 in terms of the value of the gel fraction [%]/the shrinkage force [N/cm<SP>2</SP>]. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a porous film obtained by crosslinking a resin composition containing polyolefins, and to its use. More specifically, the present invention relates to a porous film having high strength and strong tear resistance at high temperatures, and a battery using the porous film.

  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 spirally 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 a short circuit between the two electrodes and allows the battery to react by allowing ions to permeate through its microporous structure. However, if an abnormal current occurs due to an incorrect connection, 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. 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.

  However, due to recent advances in lithium ion secondary batteries and the like, not only the above-mentioned shutdown function, but also a heat-resistant element, that is, when the temperature further rises after shutdown, the separator itself melts or breaks down. In view of the possibility that a broken state may occur, it is desired to be able to cope with a higher temperature. 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.

  Furthermore, in recent years, the use of the non-aqueous electrolyte battery has been greatly expanded, and it has become necessary to design the battery assuming various dangerous situations. As an index for confirming the safety, for example, severe conditions are set such that, even if the battery is exposed to a high temperature state of 150 ° C., it does not immediately enter a non-steady state such as generating smoke. When such an abnormal temperature rise occurs, if the two internal electrodes are short-circuited, the stored energy is instantaneously released, which is extremely dangerous.

  In addition, the increase in the capacity of the battery does not stop, and the energy storage is larger. Therefore, it is very important for the separator to maintain its shape even when an abnormal temperature rise occurs and to keep the electrical insulation of the two electrodes of the battery.

  As a configuration of the nonaqueous electrolyte battery, there is a method of alternately stacking sheet-like electrodes and separators, but generally, from the viewpoint of manufacturing efficiency, as described above, the belt-like positive electrode, the negative electrode, and the separator are laminated and wound. Thus, a spiral wound body having a large effective electrode area is mainly used. For this reason, it can be said that the separator has a greater degree of freedom in deformation in the width direction (TD direction) than in the length direction (MD direction). Therefore, in order to maintain the shape of the separator configured in the battery, it is more important to maintain the shape in the TD direction.

  On the other hand, as such a separator, for example, in Patent Document 1 below, a porous 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. A method of obtaining is disclosed. However, since the porous film is a laminate, the internal resistance of the separator is high, and it is not suitable as a separator for high-performance batteries such as high-power applications.

  In addition, Patent Document 2 listed below specifies battery safety at or below the pore closure temperature by defining the maximum shrinkage stress in the TD direction and / or the maximum shrinkage rate below the pore closure temperature of the polyolefin porous film. A method for enhancing the performance is disclosed. However, this porous film does not have a crosslinked structure for improving heat resistance, and the safety of the battery at a temperature exceeding the pore closing temperature is not assumed. However, there have been reports of cases in which the battery temperature continues to rise due to a runaway reaction even after hole closure (shutdown) has occurred, and it is an urgent need to maintain safety above the SD temperature.

  On the other hand, in a polyolefin-based porous film, there is a case where a phenomenon called meltdown occurs at a temperature higher than the melting point of the constituent resin, and the phenomenon that the resin melts and the strength is lowered or flows to break the membrane or lose the electrical insulation function. For this reason, in order to improve the safety | security of the battery at high temperature, it is necessary to suppress the film breakage at high temperature reliably.

  Therefore, in Patent Document 3 below, a resin composition containing polyolefins and maleic anhydride-grafted polyethylene in a predetermined ratio for the purpose of improving durability at high temperatures (particularly, thickness maintenance at high temperatures). Is a porous film obtained by crosslinking, and has a needle stick strength at room temperature of 2 [N / 17 μm] or more.

  However, since this porous film has a small sheet thickness after heat pressing during production (for example, 0.4 mm in the examples), the resulting porous film has a large shrinkage force and a gel fraction. As a result, the balance deteriorated, and as a result, there was room for further improvement with respect to the film resistance when used as a separator at a high temperature.

  As described above, even when the film strength is strong and the battery is heated to a porous film used for a separator or the like, it is possible to simultaneously suppress a film breakage due to melting or an internal short circuit due to shrinkage. It is desired to make them compatible.

Japanese Patent Laid-Open No. 63-308866 Japanese Patent Laid-Open No. 11-322989 JP-A-2005-232289

  Therefore, an object of the present invention is to provide a porous film having a strong film strength at normal temperature and excellent in film-breaking resistance even at high temperatures by controlling the cross-linking structure and shrinkage force, and its use.

  As a result of intensive investigations to achieve the above object, the present inventors have determined that a porous film obtained by crosslinking a resin composition containing polyolefins has a needle stick strength of a certain level or more at normal temperature and shrinks. It has been found that by satisfying a certain balance between the force and the gel fraction, a porous film having a strong film strength at room temperature and an excellent film resistance at high temperatures can be provided, and the present invention has been completed. .

That is, the porous film of the present invention is a porous film obtained by crosslinking a resin composition containing polyolefins, and has a needle stick strength of 2.5 [N / 24 μm] or more and a temperature range of a shutdown temperature or more. The relationship between the gel fraction measured in boiling xylene and the shrinkage force when the peak value of the shrinkage force per unit cross-sectional area is the shrinkage force of the porous film is the gel content. The ratio [%] / shrink force [N / cm ² ] is 0.4 or more. The various physical property values in the present invention are values specifically measured by the measuring methods described in the examples.

According to the porous film of the present invention, since the film strength at room temperature is strong, the film-breaking resistance at room temperature is improved, so that the assembly workability of the battery can be improved and the defective rate can be reduced. Furthermore, by controlling the value of gel fraction [%] / contraction force [N / cm ² ], it becomes possible to achieve both high strength at normal temperature and film resistance at high temperature (160 ° C.). It is possible to improve safety by preventing internal short circuit at a high temperature of the used battery.

  In the above, the resin composition is 1 to 50% by weight of a polymer having at least a double bond, and at least one selected from the group consisting of polyolefins having a weight average molecular weight of 500,000 or less, thermoplastic elastomers, and graft copolymers It is preferable to contain 1-50 weight% of the resin component.

  Thus, it becomes easy to form a crosslinked structure by using a polymer having a double bond, and a shutdown function can be expressed at a relatively low temperature by using other components together. .

  Furthermore, it is preferable that the resin composition contains ultra high molecular weight polyethylene having a weight average molecular weight of 1,000,000 or more. By using ultra high molecular weight polyethylene, the strength is surely increased by the stretching effect at the time of porosity formation. In addition, it is advantageous for a crosslinking reaction, and a three-dimensional network of polyethylene polymer is developed by providing a crosslinked structure, and further high strength can be expected. In addition, it can contribute to the improvement of film resistance at high temperatures.

  The resin having a double bond is preferably at least one selected from the group consisting of polynorbornene, polybutadiene, polyisoprene, and EPDM. By these resin components. A crosslinking reaction for improving heat resistance can be suitably expressed.

In the present invention, in particular, by controlling the value of gel fraction [%] / contraction force [N / cm ² ], it is possible to achieve both high strength at room temperature and improved film resistance at high temperature. Therefore, a battery using the porous film as a battery insulation sustaining film (separator) can maintain electrical insulation and prevent short circuit, and can ensure high safety. The present invention is particularly effective when the battery is a non-aqueous electrolyte battery such as a lithium ion battery.

  The porous film of the present invention is obtained by crosslinking a resin composition containing polyolefins. Preferably, the resin composition comprises a polymer having at least a double bond, a polyolefin having a weight average molecular weight of 500,000 or less, a heat And at least one resin component selected from the group consisting of a plastic elastomer and a graft copolymer.

  A polymer having a double bond (hereinafter also referred to as a first resin component) has a double bond in the polymer main chain and / or side chain, and part of the double bond is hydrogen, halogen or the like. It may be eliminated by addition, or may be a derivative in which some hydrogen atoms of the double bond are replaced with other substituents. As the polymer, those in which a hydrogen atom is bonded to the α-position of the double bond are preferable. Specifically, for example, polynorbornene, polybutadiene, polyisoprene, natural rubber, acrylonitrile butadiene rubber, styrene butadiene rubber, EPDM (ethylene propylene diene terpolymer), polychloroprene, etc. are mentioned. As described above, some of these double bonds may be modified, or a mixture of two or more types may be used. Of these, polynorbornene, polybutadiene, and EPDM are more preferably used from the viewpoint of supplying raw materials and dispersibility.

  In EPDM, a type using ethylidene norbornene having excellent copolymerizability as a raw material is preferable. Among them, a higher molecular weight and a larger amount of residual double bonds are more preferable.

  In the porous film of the present invention, one or more resin components selected from the group consisting of polyolefins having a weight average molecular weight of 500,000 or less, thermoplastic elastomers, and graft copolymers (hereinafter also referred to as second resin components). Used. Examples of polyolefins having a weight average molecular weight of 500,000 or less include polyolefin resins such as polyethylene and polypropylene, and modified polyolefin resins such as ethylene-acrylic monomer copolymers and ethylene-vinyl acetate copolymers.

  Examples of the thermoplastic elastomer include thermoplastic elastomers such as polystyrene, polyolefin, polydiene, vinyl chloride and polyester. Examples of the graft copolymer include graft copolymers having a main chain of polyolefin and a side chain of a vinyl polymer having an incompatible group in the side chain. Polyacryls, polymethacrylates, polystyrene, polyacrylonitrile, polyoxyalkylenes Is preferred. Here, the incompatible group means a group incompatible with polyolefin, and examples thereof include a group derived from a vinyl polymer. These resins may be used alone or in combination of two or more.

  Among these, polyolefin resins having a weight average molecular weight of 500,000 or less, particularly low melting point polyethylene, polyolefin elastomers having crystallinity, and geraft copolymers having polymethacrylates having a low melting temperature in the side chain have low shutdown temperatures. It is preferable in terms of bringing about.

  In order to increase the strength of the porous film, it is preferable to further blend an ultrahigh molecular weight polyolefin resin such as ultrahigh molecular weight polyethylene having a weight average molecular weight of over 500,000, particularly a weight average molecular weight of 1,000,000 or more.

  That is, the porous film of the present invention comprises a resin composition containing the first resin and the second resin component and, if desired, an ultrahigh molecular weight polyolefin resin having a weight average molecular weight exceeding 500,000.

  In this invention, the compounding quantity of the polymer which has a double bond is the range of 1-50 weight% in a resin composition, for example, 1-40 weight% is preferable and 1-35 weight% is more preferable. The lower limit of the blending amount is 1% by weight or more from the viewpoint of obtaining a porous film having sufficient heat resistance by imparting a crosslinked structure, and the upper limit thereof maintains the characteristics of the porous film as a battery separator. From the viewpoint of achieving this, it is 50% by weight or less.

  Moreover, the compounding quantity of a 2nd resin component is the range of 1-50 weight% in a resin composition, for example, and 5-45 weight% is preferable. The lower limit of the blending amount is 1% by weight or more from the viewpoint of obtaining a sufficient SD temperature, and the upper limit has a sufficient porosity and maintains the characteristics of the porous film as a battery separator. From the viewpoint of achieving this, it is 50 wt% or less.

  Moreover, 5-98 weight% is preferable in a resin composition, and, as for the compounding quantity of the ultra high molecular weight polyolefin resin exceeding a weight average molecular weight 500,000, 10-90 weight% is more preferable.

  If necessary, additives such as antioxidants, ultraviolet absorbers, dyes, pigments, antistatic agents, and nucleating agents are added to the resin composition as long as the object of the present invention is not impaired. can do.

  Next, the manufacturing method of the porous film by this invention is demonstrated.

  For the production of the porous film according to the present invention, a known method relating to a wet film-forming method can be used. For example, it can be produced by mixing the resin composition with a solvent, forming into a sheet shape while kneading and heating and melting, rolling, stretching in a uniaxial direction or more, and removing the solvent by heating.

  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 rings such as liquid paraffin. Nonvolatile solvents rich in the formula hydrocarbon are preferred.

  Further, the amount of the solvent used cannot be generally determined, but the resin concentration is preferably 5 to 30% by weight. When the resin concentration is higher than this, kneading is insufficient and it becomes difficult to obtain sufficient entanglement of polymer chains.

  The step of kneading the mixture of the resin composition and the solvent and forming it into a sheet can be performed by a known method, kneaded batch-wise using a Banbury mixer, kneader, etc., and then cooled metal It may be sandwiched between plates and cooled to form a sheet-like molded product by rapid crystallization, or a sheet-like molded product may be obtained using an extruder equipped with a T die or the like.

  In addition, kneading | mixing should just be on suitable temperature conditions, Although it does not specifically limit, Preferably it is 100 to 200 degreeC.

  Although it does not specifically limit as thickness of the sheet-like molding obtained in this way, 2-20 mm is preferable, Furthermore, rolling processing, such as a heat press, may be performed, and you may be 0.5-3 mm in thickness. In that case, you may perform the cooling process for gelatinizing a sheet | seat before heat press. As for the temperature of a cooling process, -20-80 degreeC is preferable.

  Although it does not specifically limit as a heat press method, For example, the belt press machine of Unexamined-Japanese-Patent No. 2000-230072 can be used conveniently. Moreover, the temperature of the rolling process is preferably 100 to 140 ° C.

  The method for stretching the sheet-like molded product is not particularly limited, and may be a normal tenter method, roll method, inflation method, or a combination of these methods, and a known stretching method is also applied. can do. In the case of biaxial stretching, either longitudinal and transverse simultaneous stretching or sequential stretching may be used. From the viewpoint of film uniformity and strength, it is particularly preferable to form the film by simultaneous biaxial stretching. It is preferable that the temperature of an extending | stretching process is 100 to 140 degreeC.

The porous film of the present invention has a needle puncture strength of 2.5 [N / 24 μm] or more, has a peak value of contraction force per unit cross-sectional area in a temperature region above the shutdown temperature, and the peak value is porous. The relationship between the gel fraction measured in boiling xylene and the shrinkage force is 0.4 or more in terms of gel fraction [%] / shrinkage force [N / cm ² ] It is characterized by becoming.

  The generation of such a shrinkage force at a temperature higher than the crystal melting point is considered to be due to the release of the residual stress during stretching due to crystal melting. The residual stress at the time of stretching can be controlled in the process of forming the porous film, and can be controlled by each step or a combination of two or more steps.

  In order to increase the strength, it is indispensable to increase the draw ratio, but the residual stress at the time of stretching increases, and the shrinkage force at a high temperature above the crystal melting point also increases. That is, it is necessary to increase the strength while suppressing the contraction force. As a countermeasure, it is most preferable to increase the strength by providing a crosslinked structure without increasing the shrinkage force.

Specifically, the value of gel fraction [%] / contraction force [N / cm ² ] is 0.4 or more, preferably 0.5 or more, more preferably 0.6 or more, so that the contraction force is reduced. It is possible to increase the strength without increasing it. When the value of this gel fraction [%] / shrinkage force [N / cm ² ] is smaller than 0.4, the gel fraction is too low to suppress the shrinkage force, and the film resistance at high temperature I can not expect. The gel fraction can be controlled mainly by the amount of the polymer having a double bond, the ratio including the double bond, the molecular weight, the crosslinking conditions described later, and the like.

  On the other hand, in order to make the shrinkage force relatively small, it is preferable to perform the compression ratio of the heat press after cooling within a certain range in which the degree of compression is smaller than in the past, specifically to the sheet thickness before compression. On the other hand, it is preferable to compress to a thickness of 10 to 40%, and more preferable to compress to a thickness of 15 to 30%.

  The solvent removal treatment is a step of removing the solvent from the sheet-like molded product to form a porous structure, and the method is not particularly limited as long as the solvent can be removed. For example, the sheet-like molded product is washed with a solvent to remain. This can be done by removing the 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, methanol, Examples include readily volatile solvents such as alcohols such as ethanol and ketones such as acetone and methyl ethyl ketone, and these can be used alone or in admixture of two or more. 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.

  After forming the resin composition by these known methods to obtain a porous film, preferably, the porous film is made of at least one selected from the group consisting of heat, ultraviolet rays, electron beams and visible rays. By performing the crosslinking treatment used, all or part of double bonds such as polybutadiene are eliminated.

  Among these, crosslinking treatment using heat and ultraviolet rays is desirable from the viewpoint of the structural stability of the porous film. By carrying out these cross-linking treatments, the strength of the porous film is increased. In addition, the resistance to tearing at high temperatures is greatly improved. From the viewpoint of increasing the strength of the porous film and improving the tear resistance, the gel fraction by the crosslinking treatment is preferably 20% or more. When the gel fraction is 20% or less, a sufficient cross-linked structure cannot be obtained, and high strength or improved resistance to tearing cannot be expected. Adjustment of this gel fraction can be arbitrarily adjusted with the compounding quantity of the polymer which has a double bond, and crosslinking process conditions.

  The reason for improving the film resistance at high temperatures by the crosslinking treatment is not necessarily clear, but the polymer radical generated in each treatment is added to the double bond, and at that time, the polymers having the double bond, or the It is conceivable that a cross-linking reaction occurs between the polymer and other resin components, or that the glass transition temperature of the polymer chain itself will greatly increase due to the disappearance of the double bond in the main chain.

  When heat is used as the method for the crosslinking treatment, it may be a one-stage heat treatment method in which heat treatment is performed once, a multi-stage heat treatment method in which heat treatment is first performed at a low temperature and then heat treatment is performed at a higher temperature, or the temperature is increased. However, 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 a low temperature and then the treatment temperature is raised, the film resistance gradually increases with the cross-linking of the porous film, so the original properties such as air permeability are impaired by heating. Can be exposed to high temperatures without 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.

  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. Preferably it is 90-140 degreeC.

  When using ultraviolet rays, for example, by impregnating the porous film after film formation in the air as it is or by impregnating it with a methanol solution containing a polymerization initiator and drying the solvent, the porous film is irradiated with a mercury lamp, Crosslinking treatment can be performed. Moreover, you may perform ultraviolet irradiation in water for the heat control at the time of irradiation.

  When using an electron beam, for example, it is performed by irradiating a porous film after film formation with a radiation dose of 0.1 to 10 Mrad. The atmosphere at the time of irradiation may be air, as in the heat treatment method, or may be an inert gas atmosphere such as nitrogen gas or argon gas in order to control the crosslinking state.

  Further, following the cross-linking treatment step, the porous film may be fixed to a frame and heat set (heat-fixed) in order to prevent thermal shrinkage or to reduce the shrinkage force. In particular, in the present invention, by performing the crosslinking treatment using heat as described above, heat setting can be substantially performed depending on the processing conditions. In order to prevent 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.

  The thickness of the porous film of the present invention is 1 to 60 μm, preferably 5 to 50 μm. The air permeability is, for example, 100 to 1000 seconds / 100 cc, preferably 100 to 900 seconds / 100 cc by a method based on JIS P8117. The shutdown temperature is 150 ° C. or lower, preferably 140 ° C. or lower.

  Such a porous film according to the present invention, as a separator for non-aqueous electrolyte batteries having high strength and excellent tear resistance at high temperatures, can improve safety for various sizes and uses of batteries. I can expect.

  Like the conventional separator, the porous film of the present invention can be used in the state of being interposed between the positive electrode and the negative electrode to assemble a battery. In this case, the material of the positive electrode, the negative electrode, the battery case, the electrolytic solution, etc. and the arrangement structure of these components are not required to be special, and may be the same as the conventional one. May be street.

In addition, the porous film of the present invention can be used in the state of being interposed between a pair of electrodes in the same manner as a conventional separator to assemble a capacitor. In this case, the material of the electrode, electrolyte, case, etc. and the arrangement structure of these components are not required to be special, and may be the same as the conventional one. For example, in an electric double layer capacitor, an activated carbon electrode formed by using PTFE as a binder can be used as an electrode, and a solution obtained by adding 0.5 MEt ₄ PBF ₄ to propylene carbonate can be used as an electrolytic solution.

  The present invention will be described in more detail based on examples and comparative examples, but the present invention is not limited to only such examples. Various characteristics were measured as follows.

(Weight average molecular weight)
Using a gel permeation chromatograph [GPC-150C] manufactured by Waters, o-dichlorobenzene was used as a solvent, and [Shodex-80M] manufactured by Showa Denko KK was used as a column at 135 ° C. The data processing was performed using a data collection system manufactured by TRC. The molecular weight was calculated based on polystyrene.

(Film thickness)
It was measured with a 1/10000 thickness gauge.

(Porosity)
A porous film to be measured is cut out into a circle having a diameter of 6 cm, and its volume and weight are obtained, and calculation is performed using the following formula from the obtained results.
Porosity (volume%) = 100 × (volume (cm ³ ) −weight (g) / average density of resin (g / cm ³ )) / volume (cm ³ )
(Shutdown temperature)
A SUS cell having a cylindrical test chamber of 25 mmφ and capable of sealing the test chamber was used, a platinum electrode (thickness: 1.0 mm) of 10 mmφ was used for the lower electrode and the upper electrode was φ20 mm. A measurement sample punched to 24 mmφ was immersed in an electrolyte, impregnated with the electrolyte, 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 electrolyte used was a solution in which lithium borofluoride was dissolved at 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.

A thermocouple thermometer and a resistance meter were connected to the cell so that the temperature and resistance could be measured. The temperature and resistance were measured by placing the cell in a 180 ° C. thermostat. The average temperature increase rate from 100 to 150 ° C. was 10 ° C./min. By this measurement, the temperature when the resistance reached 100 Ωcm ² was defined as the shutdown temperature.

(Needle strength)
A needlestick test was performed at room temperature (25 ° C.) using a compression tester “KES-G5” manufactured by Kato Tech Co., Ltd. The maximum load was read from the obtained load displacement curve and used as the needle stick strength. A needle having a diameter of 1 mm and a radius of curvature of the tip of 0.5 mm was used, and piercing was performed at a speed of 2 cm / sec. The obtained value was multiplied by the thickness ratio and converted to the unit N / 24 μm.

(Shrink force)
A strip-shaped sample with a width of 10 mm is taken in the TD direction, attached between the chucks at 30 mm, and the lower chuck is placed in a windless electric heating and drying machine set at 170 ° C., and a digital force gauge (Nidec Sympo Co., Ltd.) The upper chuck was hung on FGC-02), the load at the time when the sample started to sag was set to 0 mN, and the temperature was raised. The average rate of temperature increase from 130 ° C. to 150 ° C. is 2 ° C./min. The peak value around 140 ° C. was read and used as the contractile force. This value was divided by the cross-sectional area of the sample, and the contractile force (unit N / cm ² ) per unit area was calculated. A graph showing the relationship between the temperature and the contractile force in Example 1 at that time is shown in FIG.

(Measurement of gel fraction)
A porous film cut into 4 cm square was sandwiched between 5 cm × 10 cm metal meshes to prepare a 5 cm × 5 cm square sample. The initial weight of this sample was measured, immersed in 100 mL of m-xylene (boiling point 139 ° C.), heated, boiled xylene for 5 hours, taken out, washed with ethyl acetate at 25 ° C., dried and dried. The gel fraction R was measured from the change. In addition, it can confirm that the crosslinked structure is formed with the result of a gel fraction.

R (%) = 100 × P1 / P0 (P0: initial weight, P1: weight after boiling xylene treatment)
(Heat-resistant film breakage test)
The porous film cut into a 14 cm square is fixed to a predetermined film-breaking test jig (outer frame 10 cm square, inner frame 7 cm square cut out) with an all-around clip, and then placed in a dryer at 160 ° C. The time [min] until the film breaks was measured.

Example 1
Polynorbornene resin (Norsolex NB manufactured by Nippon Zeon Co., Ltd.) 3% by weight, olefinic thermoplastic elastomer (TPE821 manufactured by Sumitomo Chemical Co., Ltd.) 16% by weight, ultrahigh molecular weight polyethylene having a weight average molecular weight of 1,000,000 (GUR4012, manufactured by Ticona, melting point 137) ° C) 17 parts by weight of a polymer composition comprising 81% by weight and 83 parts by weight of liquid paraffin were uniformly mixed in a slurry state, and melt-kneaded in a twin-screw extruder at a temperature of 160 ° C. Extrusion was carried out into a sheet. These kneaded materials are taken out under a constant tension, and once cooled and formed with a roll cooled with cooling water at 10 ° C., a 4 mm thick sheet is converted into a 1 mm sheet with a belt press machine at a set temperature of 120 ° C. Heat pressed. Thereafter, simultaneous biaxial stretching (stretching ratio: 4.5 × 4 times) was performed at a temperature of 123 ° C. to obtain a film. After removing the solvent from the stretched film using heptane, the end of the obtained porous film was fixed, heat-treated in air at 85 ° C. for 6 hours, and then heat-treated at 115 ° C. for 2 hours. A porous film according to the present invention was obtained.

Example 2
6% by weight of polynorbornene resin (Norsolex NB manufactured by Nippon Zeon Co., Ltd.), 16% by weight of olefin-based thermoplastic elastomer (TPE821 manufactured by Sumitomo Chemical Co., Ltd.), ultra high molecular weight polyethylene (GUR4012, manufactured by Ticona, melting point 137) C.) A porous film was obtained in the same manner as in Example 1 except that 17 parts by weight of a polymer composition consisting of 78% by weight and 83 parts by weight of liquid paraffin were mixed uniformly in a slurry state.

Example 3
Polynorbornene resin (Norsolex NB manufactured by Nippon Zeon Co., Ltd.) 3% by weight, high-density polyethylene (Mitsui Chemicals Co., Ltd., Hi-Zex 3000B, melting point 134 ° C.) 29% by weight, ultra-high molecular weight 1 million Example 1 except that 19 parts by weight of a polymer composition consisting of 68% by weight of a molecular weight polyethylene (GUR4012 manufactured by Ticona, melting point 137 ° C.) and 81 parts by weight of liquid paraffin were uniformly mixed and used in a slurry state. Thus, a porous film was obtained.

Example 4
EPDM resin (Esprene 553 manufactured by Sumitomo Chemical Co., Ltd.) 10% by weight, high-density polyethylene (Mitsui Chemicals, Hi-Zex 3000B, melting point 134 ° C.) 30% by weight, ultra-high molecular weight 1 million Example 1 except that 17 parts by weight of a polymer composition composed of 60% by weight of molecular weight polyethylene (GUR4012 manufactured by Ticona, melting point: 137 ° C.) and 83 parts by weight of liquid paraffin were uniformly mixed and used in a slurry state. Thus, a porous film was obtained.

Comparative Example 1
Without blending polynorbornene resin (Norex NB manufactured by Nippon Zeon Co., Ltd.), ultra-high molecular weight polyethylene (GUR4012, manufactured by Ticona) having 17% by weight of olefin-based thermoplastic elastomer (TPE821 manufactured by Sumitomo Chemical Co., Ltd.) A porous film was obtained in the same manner as in Example 1 except that 17 parts by weight of a polymer composition consisting of 83% by weight (melting point: 137 ° C.) and 83 parts by weight of liquid paraffin were mixed and used in a slurry state. .

Comparative Example 2
In Example 1, after melt-kneading with a twin screw extruder at a temperature of 160 ° C., extrusion into a sheet having a thickness of 10 mm, and cooling in the same manner, 0.8 mm with a belt press with a set temperature of 120 ° C. A porous film was obtained in the same manner as in Example 1 except that the sheet was molded (heat pressed).

  Table 1 shows the results of evaluating the porous films obtained in the above Examples and Comparative Examples by the above methods.

  As shown in the results of Table 1, the porous films of the examples had strong film strength at room temperature by controlling the cross-linking structure and shrinkage force, and were excellent in tear resistance even at high temperatures. On the other hand, in Comparative Example 1 having no cross-linked structure, the film breaking time at high temperature is extremely short, and even if it has a cross-linked structure, the amount of compression at the time of heat press is large, so the shrinkage force is large. In Comparative Example 2, the film breaking time at high temperature was remarkably short.

The graph which shows the relationship between the temperature in Example 1 and contraction force

Claims (7)

In a porous film formed by crosslinking a resin composition containing polyolefins, the needle stick strength is 2.5 [N / 24 μm] or more, and the peak contraction force per unit cross-sectional area in the temperature region above the shutdown temperature And the peak value is defined as the shrinkage force of the porous film, the relationship between the gel fraction measured in boiling xylene and the shrinkage force is gel fraction [%] / shrink force [N / A porous film having a value of cm ² ] of 0.4 or more.   The resin composition is at least one resin component selected from the group consisting of a polyolefin having at least 1 to 50% by weight of a polymer having a double bond and a weight average molecular weight of 500,000 or less, a thermoplastic elastomer, and a graft copolymer. 2. The porous film according to claim 1, comprising 1 to 50% by weight.   The porous film according to claim 1 or 2, wherein the resin composition contains ultrahigh molecular weight polyethylene having a weight average molecular weight of 1,000,000 or more.   The porous film according to claim 2, wherein the polymer having a double bond is at least one selected from the group consisting of polynorbornene, polybutadiene, polyisoprene, and EPDM.   The battery insulation maintenance film which consists of a porous film in any one of Claims 1-4.   The separator for nonaqueous electrolyte batteries which consists of a porous film in any one of Claims 1-4.   The battery which uses the porous film in any one of Claims 1-4 as a separator for nonaqueous electrolyte batteries.

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