JP2008226703A - Porous film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous film compatible with high thermal resistance and high shutdown responsiveness, a separator for cells using the porous film, and a nonaqueous electrolyte cells using the separator for cells. <P>SOLUTION: The porous film contains polyolephine having a weight-average molecular weight of 500,000 or larger, a polymer having a double bond in a molecule and polypropylene having a melting point of 140°C or lower, is crosslinked by UV irradiation or electron beam irradiation, and has a gel fraction of 10% or larger. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a porous film having both high heat resistance and high shutdown response, a battery separator using the porous film, and a nonaqueous electrolyte battery using the battery separator.

  Non-aqueous electrolyte batteries using light metals such as lithium as electrodes have 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 between the two electrodes and allows the battery reaction by allowing ions to permeate through its microporous structure. However, if an abnormal current occurs due to misconnection or the like, the internal temperature of the battery A polymer having a so-called shutdown function (SD function) in which the polymer 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 such a separator having the SD function, for example, a polyethylene microporous film, a microporous film having a multilayer structure of polyethylene and polypropylene, and the like are 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 become an unsteady 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 between the two electrodes of the battery.

  In view of the above problems, for example, Patent Document 1 discloses a method for obtaining a microporous porous film having high strength and excellent high-temperature characteristics by laminating a single film made of low melting point polyethylene and high melting point polypropylene. However, because of the lamination, 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.

  On the other hand, in the case of a polyolefin porous film, meltdown may occur above the melting point of the constituent polymer, and a phenomenon may occur in which the strength is reduced or the flow is lost, resulting in loss of the film breaking or 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.

  As a method for enhancing the safety of a polyolefin porous film, a method of laminating a nonwoven fabric of glass fiber or the like on a polyolefin-containing porous film (Patent Document 2), or a method of crosslinking by a technique such as polymerization after film formation Examples include a method using a molecular compound (Patent Documents 3 and 4), a method using a silane cross-linking agent (Patent Document 5), and a method using oxidation (Patent Document 6).

  However, the above-described method has problems such as limited applicable base materials, large-scale equipment, and a decrease in strength due to degradation of polyolefin that occurs simultaneously with crosslinking, and is difficult to put into practical use.

  Patent Document 7 discloses a method of irradiating ionizing radiation such as an electron beam. However, since the strength of polyolefin is lowered and the shutdown component is also cross-linked by ionizing radiation, sufficient heat resistance and sufficient There was a problem that the shutdown characteristics could not be compatible.

  The porous film used for separators, etc. has a strong film strength, and when the battery becomes hot, the separator pores close and the ion-permeability is lost quickly. It is necessary to suppress the film breakage due to the above.

Japanese Patent Laid-Open No. 63-308866 Japanese Patent Application Laid-Open No. 10-12111 JP-A-3-245457 Japanese Patent Laid-Open No. 3-193125 JP-A-9-216964 JP 2002-161165 A JP-A-10-7831

  Accordingly, the present invention provides a porous film that has both high heat resistance and high shutdown response, a battery separator using the porous film, and a nonaqueous electrolyte battery using the battery separator. With the goal.

  As a result of intensive studies to solve the above problems, the present inventors have found that the object can be achieved by using a porous film shown below, and have completed the present invention.

  The present invention contains a polyolefin having a weight average molecular weight of 500,000 or more, a polymer having a double bond in the molecule, and a polypropylene having a melting point of 140 ° C. or less, and is crosslinked by ultraviolet irradiation or electron beam irradiation, and has a gel fraction. The present invention relates to a porous film that is 10% or more.

  By using the porous film in the present invention, it is possible to obtain a porous film having high strength and excellent resistance to tearing at high temperatures.

  The present invention relates to a porous film having a shutdown index of 10 or less. When the shutdown index is 10 or less, the temperature rise is small and the film itself is not melted, so that it has high shutdown response and can improve the safety of the battery.

  The present invention relates to a battery separator using the porous film. Since the porous film is used, it is possible to provide a separator having high strength and excellent resistance to film breakage at high temperatures.

  The present invention relates to a non-aqueous electrolyte battery using the battery separator. Since the electrolyte battery has a high shutdown response, a highly safe battery can be provided.

  The porous film of the present invention comprises (first component) a polyolefin having a weight average molecular weight of 500,000 or more, (second component) a polymer having a double bond in the molecule, and (third component) a melting point of 140 ° C. or less. Of polypropylene.

  The porous film of the present invention contains an ultrahigh molecular weight polyolefin having a weight average molecular weight of 500,000 or more as the first component, preferably a weight average molecular weight of 500,000 to 3,000,000, more preferably 500 to 2,000,000. It is. In addition, when it is less than 500,000, the strength of the film cannot be obtained sufficiently. By containing the ultra-high molecular weight polyolefin, the strength is increased by the stretching effect at the time of porosity formation, and the heat resistance can be increased by constructing an advanced cross-linking network.

  In the present invention, the blending amount of the first component is preferably 20 to 90% by weight, more preferably 30 to 80% by weight, based on the porous film, from the viewpoint of high strength and high heat resistance. In addition, when it is less than 20% by weight, it becomes impossible to achieve high strength and high heat resistance of the porous film, and when it exceeds 90% by weight, it becomes difficult to obtain shutdown characteristics.

  Further, by containing the second component, the crosslinking reaction efficiently proceeds upon irradiation with ultraviolet rays, and the coating resistance at a high temperature is improved by the crosslinking. Furthermore, as a combined effect, a three-dimensional network is developed by crosslinking with ultra-high molecular weight polyolefin, which can contribute to further increase in strength and resistance to film breakage.

  The second component is a polymer containing a double bond in the polymer main chain and / or side chain, and part of the double bond may be lost by the addition of hydrogen, halogen or the like. A derivative in which a part of the hydrogen atoms of the bond is replaced with another substituent may be used. The polymer preferably has a hydrogen atom bonded to the α-position of the double bond, 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.

  The second component is more preferably at least one selected from the group consisting of polynorbornene, polybutadiene, polyisoprene, and EPDM. These polymers can suitably develop a crosslinking reaction for improving heat resistance. In particular, polynorbornene, polybutadiene, and EPDM are more preferable from the viewpoint of raw material supply and dispersibility.

  Regarding the EPDM, a type using ethylidene norbornene having excellent copolymerizability as a raw material is preferable, and among them, a higher molecular weight and a larger residual double bond amount are preferable.

  The blending amount of the second component is preferably 1 to 20% by weight in the porous film, more preferably 1 to 15% by weight, and still more preferably 1 to 10% by weight. When the blending amount is less than 1% by weight, sufficient heat resistance due to imparting a crosslinked structure cannot be obtained. On the other hand, if it exceeds 20% by weight, the characteristics of the porous film as a battery separator cannot be maintained.

  As a 3rd component of this invention, melting | fusing point contains the polypropylene of 140 degrees C or less. Examples of such polypropylene include a metallocene random copolymer having a very low melting point by a metallocene catalyst (manufactured by Nippon Polychem, trade name: Wintech, melting point: 125 ° C.). The copolymer has a low melting point, and can ensure shutdown characteristics at a temperature usually required for a separator for a lithium ion battery. Usually, low-density polyethylene or high-density polyethylene is generally used as a shutdown component. However, when the low-density polyethylene or high-density polyethylene is used, a cross-linking component is used upon irradiation with ultraviolet rays or electron beams. (For example, a polymer having a double bond in the molecule) undergoes a cross-linking reaction. As a result, fluidity due to melting does not appear even at a high temperature, and a sufficient shutdown function cannot be exhibited.

  When the third component of the present invention is used, there is almost no reaction with a component that causes a crosslinking reaction by ultraviolet irradiation or electron beam irradiation, so fluidity due to melting at high temperature is not impaired, and sufficient shutdown characteristics are ensured. be able to.

  The blending amount of the third component is preferably 5 to 79% by weight, more preferably 10 to 10% by weight in the porous film from the viewpoint of having sufficient porosity and maintaining the characteristics of the porous film as a battery separator. 50% by weight.

  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, a polymer component is mixed with a solvent, kneaded, formed into a sheet while being heated and melted, cooled to be gelled (solidified), stretched in a uniaxial direction or more by rolling or stretching under heating, It can be manufactured by removing by heating. In addition, it is preferable to form into a film by simultaneous biaxial stretching from a viewpoint of the uniformity and intensity | strength of a film | membrane. Moreover, it is preferable to use an antioxidant so that the oxidation reaction does not proceed during kneading and heating and melting.

  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.

  The blending amount of the polymer component and the solvent (mixture) used in the porous film of the present invention differs depending on the type of polymer component, solubility, kneading temperature, etc. There is no particular limitation as long as the mixture can be melt-kneaded and formed into a sheet. For example, the blending amount of the polymer component is preferably 5 to 30% by weight and more preferably 10 to 25% by weight with respect to the mixture.

  From the viewpoint of improving the strength of the resulting porous film, the amount of the solvent is preferably 70 to 95% by weight, more preferably 75 to 90% by weight, based on the mixture. In addition, if it is in the said range, a polymer component can fully be melt | dissolved in a solvent, and it can knead | mix to near the extended state, and sufficient entanglement of a polymer chain can be obtained.

  In addition, antioxidants, ultraviolet absorbers, dyes, nucleating agents, pigments, and antistatic additives can be added to the mixture as necessary, as long as the object of the present invention is not impaired. .

  The step of forming the melt-kneaded material into a sheet is not particularly limited and can be carried out by a known method, but it is kneaded batch-wise using a Banbury mixer, a kneader, etc., and then cooled onto a cooled metal plate. The sheet may be cooled by sandwiching and formed into a sheet-like molded product by rapid crystallization. Moreover, you may obtain a sheet-like molded object using the extruder etc. which attached T dies etc. 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 and you may make it thickness of 0.5-3 mm by rolling processes, such as a heat press. The heat press method is not particularly limited, but for example, a belt press machine described in JP-A-2000-230072 can be suitably used. Moreover, the temperature of the rolling process is preferably 100 to 140 ° C.

  In addition, you may perform the said rolling process before and after extending | stretching and a solvent removal process. For example, the sheet-like molded product may be subjected to a stretching treatment and a desolvation treatment (the order of stretching and desolvation may be any first) and then subjected to a rolling treatment. After that, stretching treatment and solvent removal treatment (the order of stretching and solvent removal may be any first) may be performed. Alternatively, a rolling process may be performed between the stretching process and the solvent removal process. For example, the solvent removal process is performed before the rolling process, and the stretching process and the solvent removal process are performed again after the rolling process. May be performed first) to remove the residual solvent.

  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. In view 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 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 used for the solvent removal treatment include hydrocarbons such as bentane, hexane, heptane and decane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, fluorinated hydrocarbons such as ethane trifluoride, diethyl ether, dioxane and the like. Examples include easily volatile solvents such as ethers, alcohols such as methanol and 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 the solvent by immersing the sheet-shaped molded product in a solvent, a method of showering the solvent on the sheet-shaped molded product, and the like.

  By these known methods, the polyolefin composition is formed into a porous film, and further subjected to a crosslinking treatment in which all or part of the double bonds of the crosslinking agent disappear by ultraviolet irradiation or electron beam irradiation. In addition, as a crosslinking method, simple ultraviolet irradiation is preferable.

  In the case of UV crosslinking, it is necessary to add a photoinitiator to the porous film before UV irradiation. The photoinitiator is not particularly limited, but benzophenone initiators such as benzophenone and methyl benzoylbenzoate, alkylphenone initiators such as benzoin ether, benzyldimethyl ketal and α-hydroxyalkylphenone, and acylphosphine oxide alone are used. Alternatively, it is preferably used in a blend, and a benzophenone-based initiator is particularly preferable because of its high activity.

  Examples of the method for adding the photoinitiator include a method of kneading with a raw material such as a polymer, and a method of applying or dipping as a solution. The photoinitiator concentration at this time is preferably 0.5 to 5%, more preferably 0.5 to 2%, based on the entire system (resin composition). If it is less than 0.5%, the photoreaction is insufficient and crosslinking is not sufficiently achieved. On the other hand, if it exceeds 5%, the photoreaction is not promoted even if a photoinitiator is added. Conversely, an excessive photoinitiator hinders the penetration of ultraviolet rays and inhibits the reaction, which is not preferable.

  The organic solvent used when preparing the solution containing the photoinitiator is not particularly limited and can be appropriately selected. Examples thereof include acetone, methyl ethyl ketone, toluene, methanol, ethanol, hexane, and the like. It is preferable to use an organic solvent.

  Ultraviolet irradiation is performed after making polyolefin into a porous film by stretching. When irradiated before stretching, it tends to break before a sufficient stretching ratio is achieved. Further, when a high-boiling solvent such as nonane, decane, undecane, dodecane, decalin, liquid paraffin or the like is used at the time of stretching, the irradiation is preferably performed after the solvent is replaced with a low-boiling solvent and then dried. When a large amount of a solvent is contained in the porous film, the photoinitiator is diluted with the solvent to reduce the reaction efficiency, and a photoreaction occurs with the solvent, which is not preferable.

  The degree of crosslinking of the porous film can be controlled by the amount of photoinitiator and the amount of ultraviolet irradiation. The degree of crosslinking can be adjusted by the gel fraction. In the present invention, the gel fraction is 10% or more, preferably 10 to 95%, and more preferably 30 to 80%. If the gel fraction is less than 10%, a sufficient cross-linked structure cannot be obtained, and an increase in strength or an improvement in film resistance cannot be expected.

  Furthermore, it is preferable to use ultraviolet rays having a wavelength distribution of 240 to 400 nm from the viewpoint of suppressing the decrease in the strength of the porous film and suppressing the deterioration of the porosity and air permeability due to the temperature rise of the porous film. By irradiating the ultraviolet rays, the porous film can be increased in strength and heat resistance, and the film resistance at high temperatures is greatly improved.

  The reason why the film-breaking resistance at high temperature is improved by the crosslinking treatment by ultraviolet irradiation is not necessarily clear, but the polymer radical generated by the ultraviolet treatment is added to the double bond, and the double bond having a double bond is formed at that time. A cross-linking reaction may occur between the polymers or between the polymer and other polymer components, or the glass transition temperature of the polymer chain itself will greatly increase due to the disappearance of the double bond in the main chain, It is conceivable that when polyolefin is kneaded, pseudo-crosslinking occurs due to complicated entanglement between very long polyolefin polymer chains or a cross-linking component and a linear polyolefin, thereby contributing to curing.

  Further, following the crosslinking treatment step, the porous film may generally be heat set (heat-fixed) to prevent thermal shrinkage. 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 600 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, for example, as disclosed in JP-A-63-205048. May be street.

  Next, the nonaqueous electrolyte battery of the present invention will be described. The non-aqueous electrolyte battery uses a battery separator made of the porous film as described above, and has a structure in which, for example, a wound electrode body obtained by laminating and winding a strip-like negative electrode, a positive electrode, and a separator is a battery. The battery is housed in a can, and an electrolytic solution is injected therein. Further, necessary members such as insulating plates above and below the battery are appropriately arranged according to a commercially available battery.

  As the electrolytic solution, for example, an electrolytic solution in which a lithium salt is used as an electrolytic solution and this is dissolved in an organic solvent is used. Examples of the organic solvent include, but are not limited to, esters such as propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate, diethyl carbonate, methyl propionate, and butyl acetate. Nitriles such as acetonitrile, ethers such as 1,2-dimethoxyethane, 1,2-dimethoxymethane, dimethoxypropane, 1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 4-methyl-1,3-dioxolane Furthermore, a single solvent such as sulfolane or a mixed solvent of two or more kinds can be used.

  Examples of positive electrode sheets include metal oxides such as lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, manganese dioxide, vanadium pentoxide, and chromium oxide, metal nitrides such as molybdenum disulfide, and lithium cobalt oxide. Is used as an active material, a mixture obtained by appropriately adding a conductive additive or a binder such as polytetrafluoroethylene to these positive electrode active materials, and a molded body using a current collecting material such as a stainless steel net as a core material The finished product is used.

  As the negative electrode sheet, one obtained by integrating an alkali metal or a compound containing an alkali metal with a current collecting material such as a stainless steel net is used. Examples of the alkali metal at that time include lithium, sodium, potassium and the like, and examples of the compound containing the alkali metal include alkali metals and alloys such as aluminum, lead, indium, potassium, cadmium, tin, magnesium, and further alkali. Examples thereof include a compound of a metal and a carbon material, a compound of a low potential alkali metal and a metal oxide, and a sulfide. When a carbon material is used for the negative electrode, the carbon material may be any material that can be doped and dedoped with lithium ions. For example, graphite, pyrolytic carbons, cokes, glassy carbons, and firing organic polymer compounds Bodies, mesocarbon microbeads, carbon fibers, activated carbon, and the like can be used.

  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)
A gel permeation chromatograph [GPC-150C] manufactured by Waters is used, and o-dichlorobenzene is used as a solvent, and [Shodex-80M] manufactured by Showa Denko KK is used as a column at 135 ° C. 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. In the present invention, a porous film of 30 μm was obtained in all of the examples and comparative examples.

(Porosity)
A porous film to be measured was cut out into a circle having a diameter of 6 cm, and its volume (cm ³ ) and weight (g) were determined, and the average density (g / cm ³ ) of the polymer components in the porous film was obtained. Was calculated using the following formula.
Porosity (volume%) = 100 × (volume−weight / average density) / volume

(Shutdown index)
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, soaked in 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 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. 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 at 100 to 150 ° C. was 10 ° C./min. This measurement temperature _{T 10} when the resistance is 10 [Omega · cm ^2, resistance from the difference between the temperature _{T 1000} when the 1000 [Omega] · cm ^2, was determined shutdown indicator.
Shutdown index = T ₁₀₀₀ −T ₁₀

(Measurement of gel fraction)
A porous film cut into a 4 cm square was sandwiched between 5 cm × 10 cm metal meshes to form a 5 cm square sample. The initial weight (P ₀ ) of this sample was measured, immersed in 100 ml of m-xylene (boiling point 139 ° C.) and boiled for 5 hours. Thereafter, the weight (P ₁ ) after the sample was taken out, washed and dried was measured, and the gel fraction (R) was calculated from the change in weight according to the following formula.
R (%) = 100 × (P ₁ / P ₀ )

(Heat-resistant film breakage test)
A 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 a clip, and then put into a dryer at 160 ° C. Measured the time (minutes) to break.

Example 1
Polynorbornene polymer (Norsolex NB manufactured by Nippon Zeon Co., Ltd.) 3% by weight, polypropylene (manufactured by Nippon Polychem, Wintech WFX4TA, melting point 125 ° C.) 19.4% by weight, ultra high molecular weight polyethylene (melting point 137 ° C.) having a weight average molecular weight of 1 million ) 17 parts by weight of a polymer composition consisting of 77.6% by weight and 83 parts by weight of liquid paraffin were uniformly mixed in a slurry state, melted and kneaded in a twin screw extruder at a temperature of 160 ° C. The sheet was extruded into a sheet.

  These kneaded materials were taken out under a constant tension, and once cooled and formed with a roll cooled with cooling water at 10 ° C., they were formed into 1 mm sheets with a belt press machine having a set temperature of 115 ° C.

  Then, simultaneous biaxial stretching was performed at a set temperature of 123 ° C., and the solvent removal treatment was further performed with heptane to obtain a porous film having a thickness of 30 μm. Thereafter, this porous film was immersed in a benzophenone / hexane solution having a concentration of 1% by weight for 1 minute and then air-dried.

  While this porous film is being conveyed by a compare-type high-pressure mercury irradiation device (UE021-203C made by Eye Graphics, emission length 250 nm, output 2 kW, 15 m / min), the front and back of the porous film are irradiated with ultraviolet rays (heat ray cut filter) By irradiating with UV light with a wavelength of 500 nm or more cut off.

  Immediately after UV irradiation, the porous film was heated and fixed at 126 ° C. for 2 hours to obtain a crosslinked porous film.

(Example 2)
EPDM polymer (Esbrene 5527F manufactured by Sumitomo Chemical Co., Ltd.) 10% by weight, polypropylene (manufactured by Nippon Polychem, Wintech WFX4TA, melting point 125 ° C.) 27% by weight, ultra high molecular weight polyethylene having a weight average molecular weight of 1 million (melting point 137 ° C.) A crosslinked porous film was obtained in the same manner as in Example 1 except that 17 parts by weight of the polymer composition consisting of 63% by weight and 83 parts by weight of liquid paraffin were uniformly mixed and used in the form of a slurry.

(Example 3)
EPDM polymer (Esbrene 5527F manufactured by Sumitomo Chemical Co., Ltd.) 10% by weight, polypropylene (manufactured by Nippon Polychem, Wintech WFX4TA, melting point 125 ° C.) 18% by weight, ultra high molecular weight polyethylene having a weight average molecular weight of 1 million (melting point 137 ° C.) A crosslinked porous film was obtained in the same manner as in Example 1 except that 17 parts by weight of a polymer composition comprising 72% by weight and 83 parts by weight of liquid paraffin were mixed and used in a slurry state.

Example 4
EPDM polymer (Esbrene 5527F manufactured by Sumitomo Chemical Co., Ltd.) 10% by weight, polypropylene (manufactured by Nippon Polychem, Wintech WFX4TA, melting point 125 ° C.) 9% by weight, ultra high molecular weight polyethylene (melting point 137 ° C.) having a weight average molecular weight of 1 million A crosslinked porous film was obtained in the same manner as in Example 1 except that 17 parts by weight of the polymer composition consisting of 81% by weight and 83 parts by weight of liquid paraffin were mixed and used in a slurry state.

(Comparative Example 1)
In Example 1, instead of polypropylene (manufactured by Nippon Polychem, Wintech WFX4TA, melting point 125 ° C.), high density polyethylene (Mitsui Chemicals Hi-Zex 3000B, density 0.96 g / cm ³ , melting point 137 ° C. ) Was used in the same manner as in Example 1 except that a porous film was obtained.

(Comparative Example 2)
In Example 1, except that the obtained porous film was not irradiated with ultraviolet rays. A porous film was obtained in the same manner as in Example 1.

  Table 1 shows the characteristics of the porous films obtained in the above Examples and Comparative Examples.

  From the results in Table 1, the porous films obtained in the examples of the present invention have a high gel fraction, a film breaking time at 160 ° C. of more than 60 minutes, and a small shutdown index. It was confirmed that a porous film having both responsiveness could be obtained.

On the other hand, Comparative Example 1 was confirmed to have a large shutdown index and poor shutdown response, and Comparative Example 2 was confirmed to have a low gel fraction, a very short film breaking time, and poor heat resistance.

Claims (4)

  Contains a polyolefin having a weight average molecular weight of 500,000 or more, a polymer having a double bond in the molecule, and a polypropylene having a melting point of 140 ° C. or less, and is crosslinked by ultraviolet irradiation or electron beam irradiation, and has a gel fraction of 10% or more. A porous film.   The porous film according to claim 1, wherein the shutdown index is 10 or less.   A battery separator comprising the porous film according to claim 1. A non-aqueous electrolyte battery using the battery separator according to claim 3.