JPH11144700A - Porous membrane, battery separator comprising porous membrane, and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous membrane comprising silane crosslinked polyethylene, high in obstacle to conductive particulates such as active materials, having low prermeation resistance of an electrolyte ion and characteristics tolerant to a tension upon manufacturing a battery, and further an excellent heat resistance in such a manner that even if an electrode surface is uneven, a projection does not pass through a separator and a short-circuit is not made; a separator for the battery comprising the porous membrane, and to provide a manufacturing method of them. SOLUTION: This comprises silane crosslinked polyethylene. A porosity ratio is 20 to 60%. The maximum pore size is equal to or less than 1 μm<2> . An air permeability (Gurley value) is equal to or less than 1000 sec. The number of projections having a height which is equal to or larger than twice the average thickness is equal to or less than 10 per area of 1 m<2> of the porous membrane surface. A raw material is a mixture resin comprising the silane denaturation polyethylene and organic low molecule indicative of a compatibility therewith. An extruding step, a crosslinking step, and an extracting step are included.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous film made of silane-crosslinked polyethylene having a specific porosity, a maximum pore size and air permeability, a battery separator made of the porous film, and a method for producing the same. More specifically, the present invention relates to a porous membrane having a good balance of barrier properties against conductive fine particles, permeability of electrolyte ions, mechanical strength, etc., and further excellent safety, a battery separator comprising the porous membrane, and a method for producing the same.

[0002]

2. Description of the Related Art Various types of polyethylene porous membranes have been developed. One of these uses is a battery separator, especially a separator for a non-aqueous solution high energy density secondary battery such as a lithium ion secondary battery. is there. Non-aqueous solution type separators for high energy density secondary batteries have the property that pores are automatically closed when a certain temperature is reached to form a substantially non-porous membrane (hereinafter referred to as "shutdown characteristics").
Further, a property of maintaining the film shape at a higher temperature (hereinafter, referred to as “heat resistance”) is required. The shutdown characteristics of the separator are such that when an excessive current is generated due to an external short circuit, the film material is melted or softened by Joule heat, the holes are closed and the current is cut off, thereby preventing further dangerous heat generation. I do. The heat resistance of the separator is such that if the temperature rise does not stop immediately after the holes are closed, or if it is heated from the outside for some reason, the insulation between the positive electrode and the negative electrode is maintained, causing an internal short circuit that causes dangerous heat generation. It works to prevent The present inventors have previously found that a porous film made of a crystalline resin and having a specific crystallinity and a specific gel fraction has shutdown characteristics and heat resistance. As an example of a porous film having the same, a silane cross-linked porous film has been proposed in Japanese Patent Application No. 8-150198.

The basic characteristics of a battery separator include a three-dimensionally meandering fine pore shape and a resistance to electrolyte ion permeation (hereinafter referred to as “electrolysis ion”) in order to prevent a short circuit caused by the movement of conductive fine particles such as an active material. (Referred to as "electrical resistance") and withstands tension during battery manufacturing.
Even if the electrode surface has irregularities, the mechanical strength is high so that the projecting portion does not penetrate the separator and cause a short circuit. As a method for making polyethylene porous, JP-A-51-74057 discloses a method in which a mixture of polyethylene, an inorganic fine powder and an organic liquid is extruded to extract at least one of the organic liquid and the inorganic fine powder. Have been. However, when inorganic fine powder is extracted, a pinhole having a large hole diameter may be formed and may become a moving path of the conductive fine particles. If the inorganic fine powder is not extracted, ionization may occur and battery performance may be reduced. It is not suitable for manufacturing battery separators.

Japanese Patent Application Laid-Open No. 2-67339 discloses that a polyolefin obtained by adding a polyethylene wax and / or a paraffin wax as an additive is silane-grafted with a silane compound, and then formed into a film to perform silane crosslinking. Next, a method for producing a porous polyolefin film, characterized by immersing such a molded product in a solvent to extract the additive, is described, which is suitable for a diaper cover or the like. However, the amount of paraffin wax added is 10% by weight.
Therefore, it was not suitable as a porous membrane for a battery separator. JP-A-55-60537 discloses a method in which a mixture of polyethylene and paraffin is extruded to obtain a sheet or film, and paraffin is extracted to make it porous. The publication further states that, when cooling and solidifying an extruded molten product, if the cooling is performed slowly, the phase separation between the resin and paraffin proceeds, resulting in a film having low mechanical strength. When cooled and solidified, the surface of the hand in contact with the rotating drum becomes a skin, resulting in a film with extremely poor air permeability.When it is introduced into water and quenched and solidified, a porous film with moderate air permeability and strength is obtained. It is described. As shown in these prior arts, there is a demand for the development of a porous membrane that satisfies the physical properties suitable for the application.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to comprise a silane-crosslinked polyethylene, which has a high barrier property against conductive fine particles such as an active material, has a low resistance to permeation of electrolyte ions, and withstands tension during battery production. Has the characteristics
Furthermore, even when the electrode surface has irregularities, the convex portion does not penetrate the separator and does not short-circuit, and a porous film having excellent heat resistance, a battery separator including the porous film, and a method for producing the same are provided. is there.

[0006]

Means for Solving the Problems The present inventors have studied a method of forming a film by melting and mixing an organic low-molecular substance such as polyethylene and paraffin wax, and extracting a porous organic film by extracting the organic low-molecular substance. In the process, the following points became clear. The first point is that when polyethylene having a large MFR, that is, polyethylene having a small molecular weight, is used, a phase of an organic low-molecular substance is formed between polyethylene spherulites, and the phase separation becomes coarse. However, if a porous membrane having low mechanical strength is used, on the other hand, if a polyethylene having a small MFR, that is, a polyethylene having a large molecular weight is used, a phase of an organic low-molecular substance is formed between polyethylene lamellas, and phase separation becomes fine. , High mechanical strength,
That is, it becomes a porous film having high electric resistance. Second, when polyethylene with low density, that is, polyethylene with low crystallinity, is used, fine phase separation is generally observed, but the maximum pore size is large, the electrical resistance is high, and the porous membrane has low mechanical strength. That is. The third point is that in conditions where the cooling temperature, the MFR and the density of polyethylene are optimized,
The purpose is to obtain a porous membrane in which basic characteristics as a battery separator are balanced.

As a result of further studies by the present inventors using silane-modified polyethylene, it has been found that an obstacle is caused by having a reactive functional group in the molecular chain. That is,
Since the crosslinking reaction occurs during the melt molding due to the slight moisture of several tens ppm level contained in the raw material, it is difficult to obtain a predetermined pore shape as if the molecular weight fluctuated during the molding. Further, when the melt molding temperature exceeds a certain temperature, a product such as an aggregate of crosslinked molecules is generated, and a projection is generated on the surface of the porous film of the final product. When manufacturing a secondary battery using a porous membrane, such protrusions may cause collapse of pores in the porous membrane,
In some cases, it has been found that this may cause breakage of the membrane and impair the performance of the porous membrane.

The present inventors have found that a porous membrane obtained by a specific production method has a specific range of porosity, maximum pore size and air permeability, and has a small number of projections per unit area. They have found that they can be solved and have completed the present invention.

That is, according to the first aspect of the present invention, the porosity is 20 to 60%, the maximum pore diameter is 1 μm or less, the air permeability (Gurley value) is 1000 seconds or less, and the average A porous membrane and a battery separator comprising the same are characterized in that the number of protrusions having a height of twice or more the thickness is 10 or less per square meter of the surface of the porous membrane. According to a second aspect of the present invention,
An extrusion process using a mixed resin of silane-modified polyethylene and an organic low-molecular substance showing compatibility with the raw material,
In the method for producing a silane cross-linked porous membrane including a cross-linking step and an extraction step, the MFR (melt flow rate) is 0.08 to 2 g / 10 min. The temperature of the mixed resin at the time of extrusion is in the range of from 180 ° C to 180 ° C, which is compatible with organic low-molecular substances, and the extruded melt is cooled and solidified at a temperature below the phase separation temperature. The present invention provides a porous membrane having the first property of the present invention, characterized by obtaining an intermediate molded body, and a method for producing a battery separator comprising the porous membrane.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
Made of silane cross-linked polyethylene, having a porosity of 20 to 60
%, Maximum pore size is 1 μm or less, air permeability (Gurley value) is 1
Less than 000 seconds means that the porous membrane has a uniform fine pore structure, the maximum pore size is sufficiently small to prevent the movement of the conductive fine particles, and the pores communicate so that the air permeability (Gurley value) is reduced. This means that a porous membrane having excellent electrolyte ion permeability can be obtained. Further, in a battery having a mainly wound structure such as a lithium ion secondary battery, if there is a protrusion on the separator, the sheet-like electrode in contact therewith is deformed and further presses another separator located on the back surface thereof. Since the pressed separator breaks the hole and increases the electric resistance, the battery performance is reduced. Further, depending on the degree of unevenness of the electrode, the separator may be broken and a short circuit may occur. That is, by reducing the number of protrusions having a height of twice or more the average thickness of the porous film, it is possible to contribute to the reduction of the battery performance, particularly to the prevention of short circuit. It is necessary that the number of protrusions having a height of at least twice the average thickness of the porous film is 10 or less per square meter of the surface of the porous film.

The method for producing a porous membrane according to the present invention is characterized in that, in the step of melt-extruding a mixed resin comprising silane-modified polyethylene and an organic low-molecular substance having compatibility with the same and cooling and solidifying to obtain an intermediate molded article, MF is a measure of
It is characterized in that silane-modified polyethylene having a specific range of R (melt flow rate) is used as a raw material and extruded in a specific mixed resin temperature range. The intermediate molded body obtained under such specific conditions undergoes phase separation between silane-modified polyethylene and an organic low-molecular substance that can be processed into a porous membrane suitable as a battery separator through a subsequent extraction step. Furthermore, under such specific conditions, the extruded molded body in a molten state is skinned even when cooled and solidified by, for example, a cooling drum, that is, the surface of the porous film is covered with a very thin resin film, and the surface of the porous film is removed from the surface. A state in which the holes are not penetrated to the back is unlikely to occur, and a porous film with less uneven thickness can be easily obtained.

The porosity of the porous membrane comprising the silane-crosslinked polyethylene of the present invention is in the range of 20 to 60%, preferably 30 to 50%. If the porosity is less than 20%, it becomes difficult to lower the air permeability (Gurley value), so that the electric resistance increases. If the porosity exceeds 60%, the mechanical strength becomes poor. The maximum pore diameter of the porous membrane of the present invention is 1 μm or less, preferably 0.05 to 0.90 μm, and more preferably 0.05 to 0.50 μm. Maximum pore size is 1μm
When the ratio exceeds the above, the barrier property against the conductive fine particles becomes inferior, for example, the chance that the conductive fine particles pass through the porous membrane to cause conduction increases. The air permeability of the porous membrane of the present invention is not more than 1000 seconds as a Gurley value, preferably not more than 800 seconds. A porous film having a Gurley value exceeding 1000 seconds is not preferable because an increase in electric resistance may be observed. In the silane-crosslinked polyethylene porous membrane of the present invention, the number of protrusions having a height of at least twice the average thickness of the porous membrane is 10 or less, preferably 5 or less per square meter of the surface of the porous membrane. Here, the height of the projection of the porous membrane is
It refers to the height from the surface of the porous membrane opposite to the surface from which the protrusions are projected to the top of the protrusion. In other words, it is the height up to the top of the protrusion including the thickness of the porous film. If there are many protrusions, the risk of a short circuit occurring due to damage of the porous membrane due to the protrusions during use of the battery increases. The susceptibility of the porous membrane to damage by the protrusions can be determined by the presence or absence of a short circuit in a battery winding test.

The silane-modified polyethylene used in the present invention includes polyethylene such as high-density polyethylene, low-density polyethylene, and linear low-density polyethylene, as well as polyethylene such as vinyltrimethoxysilane, vinyltriethoxysilane, and vinyltriacetoxysilane. It refers to a graft copolymer obtained by grafting a saturated silane compound, or an ethylene-ethylenically unsaturated silane compound copolymer.
The melting point is preferably in the range of 95 to 140 ° C. from the viewpoint of extrudability. The content of the unsaturated silane compound unit in the silane-modified polyethylene is preferably 0.001 to 5% by weight.

The MFR as a standard of the molecular weight of the silane-modified polyethylene used in the present invention is 0.08 to 2 g / 10 m
in, preferably in the range of 0.1 to 1.5 g / 10 min, more preferably in the range of 0.4 to 1.0 g / 10 min. If the MFR exceeds 2 g / 10 min, the phase separation between the silane-modified polyethylene and the low-molecular organic material becomes coarse, that is, the pores become large and non-uniform after extracting and removing the low-molecular organic material, and the mechanical strength of the porous membrane decreases. Easier to do. MF
When R is less than 0.08 g / 10 min, phase separation becomes too fine and electric resistance tends to increase. MFR 2g / 1
When the mixed resin temperature is 180
When melt-molded to exceed ℃, cross-linking proceeds during molding and acts as if the molecular weight increased, and phase separation becomes fine, but a product such as an aggregate of cross-linked molecules is formed, and the final product Protrusions are formed on the porous membrane. Also, if the MFR is 0.0
Even if a resin in the range of 8 to 2 g / 10 min is used, if the mixed resin temperature is melt-molded so as to exceed 180 ° C.,
It acts as if the molecular weight increased due to the progress of crosslinking during molding, and the phase separation became too fine and the electrical resistance easily increased.
Further, a product such as an aggregate of cross-linked molecules is formed, and a protrusion is generated on the porous film of the final product. Density of the silane modified polyethylene is preferably 0.945 g / cm ³ or more, and more preferably as long as 0.955 g / cm ³ or more. As for the upper limit of the density, it is possible to use a material having a high density as far as it can be polymerized. Normally, up to 0.97 g / cm ³ is easy to polymerize and is preferably used in the present invention. If the density is low, the maximum pore diameter increases, the electrical resistance increases, and the mechanical strength decreases. As a preferred example, a silane-modified polyethylene having a density of 0.958 g / cm ³ , a melting point of 130 ° C., and an MFR of 0.8 g / 10 min (manufactured by Mitsubishi Chemical Corporation; Rinklon HF-
700N), density 0.945 g / cm ³ , melting point 130 ° C.
Silane-modified polyethylene having an MFR of 0.4 g / 10 min (manufactured by Mitsubishi Chemical Corporation; Wrinklon HE-707), density of 0.946 g / cm ³ , melting point of 130 ° C., MFR of 0.45
g / 10 min. silane-modified polyethylene (manufactured by Sumitomo Bakelite Co., Ltd .; Moldex S241H).

The organic low molecular weight compound used in the present invention is preferably 80 to 1 when mixed with a silane-modified polyethylene.
It is an organic low-molecular compound showing compatibility at a temperature of 80 ° C, more preferably 110 to 180 ° C. If the temperature at which the components are compatible with each other is too low, phase separation is difficult to obtain as an intermediate molded product, which is not preferable. Further, the temperature at which the components are compatible with each other only needs to be equal to or lower than the resin temperature specified in the present invention. In the present invention, it is determined that compatibility is exhibited if the molten mixture is substantially transparent when visually observed. As the organic low-molecular substance exhibiting such compatibility, an organic low-molecular substance having at least one selected from the group consisting of paraffin wax, microcrystalline wax, synthetic wax, and liquid paraffin is preferable, and from the viewpoint of extractability. Paraffin wax and liquid paraffin are more preferred.

Hereinafter, a method for producing a porous membrane will be described. The mixing ratio of the silane-modified polyethylene and the organic low-molecular weight material is 40 to 60% by weight, and the organic low-molecular weight material is 60
A ratio of の 40% by weight is preferred. When the silane-modified polyethylene is less than 40% by weight, the porosity of the obtained porous membrane exceeds 60%, and when it exceeds 60% by weight, the porosity is 20%.
Below. The method of mixing the raw materials is not particularly limited, as long as the organic low-molecular substances are uniformly mixed. For example, there are a method of mixing and kneading with an extruder after mixing with a ribbon blender, a Henschel mixer, a fixed V-type blender, a tumbler, and the like, and a method of simultaneously charging and directly kneading both into an extruder.

As a method of melt-mixing a silane-modified polyethylene and an organic low-molecular substance to form a film or a sheet, a method in which both are supplied to an extruder and extruded from a slit die is used from the viewpoint of productivity. preferable. In the present invention, the silane-modified polyethylene having a specific MFR and the organic low-molecular weight compound are mixed at a temperature at which the mixed resin temperature at the time of melt-mixing shows compatibility to 180 ° C, preferably at a temperature at which the mixed resin temperature at the time of melt-mixing is compatible. Is formed in a temperature range of from 170 ° C to 170 ° C. If the temperature of the mixed resin is lower than the temperature at which the mixed resin is compatible, molding becomes difficult because the fluidity of the mixed resin is low. When the temperature of the mixed resin exceeds 180 ° C., a product such as an aggregate of crosslinked molecules is formed, and a projection may be formed on the surface of the porous film of the final product. The extruded intermediate molded product in a molten state is cooled and solidified at a temperature equal to or lower than the phase separation temperature of the silane-modified polyolefin and the organic low-molecular substance. Here, the phase separation temperature of the molten resin refers to a DSC curve when a mixed molten resin of a silane-modified polyolefin and an organic low molecular weight material is cooled at a cooling rate of 10 ° C. per minute using a DSC (differential scanning calorimeter). Of silane-modified polyethylene in Example 1. The phase separation temperature varies depending on the combination of the selected raw materials, the mixing ratio, and the like, but the approximate temperature is 20 to 130 ° C, preferably 50 to 120 ° C, and more preferably 50 to 115 ° C.
Range.

The cooling can be preferably carried out by introducing into a cooling drum or a coolant such as water. A method using a cooling drum is more preferable from the viewpoint of reducing uneven thickness. As the cooling temperature, for example, when paraffin wax is used as an organic low-molecular substance, it is preferably 110
° C or lower, more preferably 0 to 100 ° C, most preferably 20 to 90 ° C.

The low-molecular organic matter is extracted and made porous by a method such as immersing or exposing the cooled and solidified intermediate molded product in an extraction solvent. The extraction solvent may be a poor solvent or a non-solvent for the silane-modified polyethylene, and may be a good solvent for the organic low-molecular weight material. For example, if the organic low-molecular weight material is paraffin wax, pentane, hexane,
Hydrocarbons such as heptane, octane, isodecane, cyclohexane, ketones such as methylcyclohexanone, ethers such as ethyl ether, aromatics such as benzene, toluene and xylene, halogens such as carbon tetrachloride, perchloroethylene and chloroform Hydrocarbons and the like are preferably used. The extraction temperature and the extraction time may be appropriately determined in consideration of the boiling point and vapor pressure of the extraction solvent, the solubility of the low-molecular organic compound, the thickness of the intermediate molded product, and the like. After the extraction of the organic low-molecular substances and the completion of porosity, the obtained porous membrane is dried to remove the extraction solvent. Drying may be performed at a temperature of 50 ° C. to the melting point of polyethylene for 1 second to 1 hour.

The silane cross-linking may be performed at the stage of the intermediate molded body before the extraction step, at the stage of making porous after the extraction step, or at the stage of the extraction. Silane crosslinking can occur by exposing the intermediate compact or porous membrane to water. At this time, if a silanol condensation catalyst is used, the crosslinking reaction is promoted and the crosslinking can be completed in a short time. Exposure to water may be carried out by bringing the intermediate molded body or the porous membrane into contact with water at room temperature to 130 ° C., water vapor or moisture in the air for 0.1 second to 1 week, usually about 1 second to 1 hour. The silane cross-linked polyethylene used in the present invention refers to a polyethylene cross-linked by a silanol condensation reaction of a silane group previously introduced into a polyethylene molecular chain. Crystallinity of silane crosslinked polyethylene is 50-8
It is preferably 0% from the viewpoint of shutdown characteristics, and the gel fraction is preferably from 50 to 80% from the viewpoint of heat resistance. Silanol condensation catalysts are generally tin, zinc,
Carboxylates, organic bases, inorganic acids and organic acids of metals such as iron, lead and cobalt. As specific examples of the silanol condensation catalyst, dibutyltin dilaurate, dibutyltin diacetate, dibutyltin dioctoate, stannous acetate, stannous caprylate, lead naphthenate, zinc caprylate, cobalt naphthenate, ethylamine, dibutylamine, There are inorganic acids such as hexylamine, pyridine, sulfuric acid and hydrochloric acid, and organic acids such as toluenesulfonic acid, acetic acid, stearic acid and maleic acid.

The method of using the silanol condensation catalyst is as follows.
There are a method of forming a film by mixing a silanol condensation catalyst with the raw resin composition, and a method of applying a solution or dispersion of the silanol condensation catalyst to an intermediate molded article or a porous film. When the catalyst is blended in the raw resin composition, the amount of the catalyst may be appropriately determined depending on the degree of modification of the silane-modified polyethylene, the ratio of the organic low molecular weight substance, and the like. In the range of 0.0001 to 5% by weight. The concentration of the catalyst when the catalyst is applied is affected by the type of the solvent or the dispersion medium, the application method, and the like, but may be appropriately determined in the range of 0.001 to 30% by weight based on the solution or the dispersion. . The porous membrane finally obtained through the above steps is preferably obtained as a porous membrane having a thickness of 10 to 100 μm, more preferably 15 to 50 μm.

According to the above-mentioned production method, the porous membrane having the properties described in the first aspect of the present invention, ie, a silane-crosslinked polyethylene, has a porosity of 20 to 60%, a maximum pore diameter of 1 μm or less, and an air permeability (Gurley) Value) is 1000 seconds or less, and the height of the protrusion including the thickness of the porous film to the apex is twice or more the average thickness of the porous film. Porous membranes of 10 or less per square meter are obtained. This porous membrane has a high barrier property against conductive fine particles, has a low resistance to electrolyte ions, has few protrusions on the surface of the porous membrane, and is suitably used as a battery separator having excellent heat resistance.

[0023]

EXAMPLES The present invention will now be described specifically with reference to examples, but the present invention is not limited to these examples. The physical properties of the raw materials and the porous films described in the following examples were determined by the following measurement methods, and the physical properties of the raw materials and the porous films described in this specification were also based on the measurement results. I have. (MFR) Test temperature 1 according to JIS K6760
It measured at 90 degreeC and the test load of 2.16 kgf. (Density) Test temperature 23 according to JIS K6760
Measured at ° C. (Porosity) A sample porous membrane cut out into a square having a side of 40 mm was weighed (this weight is referred to as W1), and then mineral oil (Mineral oil, white, manufactured by Aldrich Chemical Company) was used.
light for 6 hours at room temperature, the sample porous membrane was taken out, the mineral oil on the surface was wiped off, weighed again (this weight is W2), and the porosity (%) was calculated from the following equation. Here, ρ1 represents the specific gravity (density) of the silane-modified polyethylene, and ρ2 is the specific gravity (density) of the mineral oil.
Represents Porosity (%) = [ρ1 (W2-W1) / (ρ2W1 + ρ1 (W2-W
1))] × 100 (Maximum pore diameter) Calculated from the bubble point in ethanol according to ASTM E-128-61. (Air permeability) It measured using the B type Gurley type densometer according to JISP8117.

(Electrical Resistance) The structure of the measuring cell is shown in FIG. Stainless steel can body of CR2332 type coin battery (1
a, 1b) to a stainless steel disk (2a, 2) having a diameter of 15 mm.
b) is attached as an electrode, a sample porous membrane (3) impregnated with an electrolyte is sandwiched between the two electrodes, the space is filled with the electrolyte, and the can body is placed through a Teflon packing (4). The measuring cell was assembled by swaging. Further, the packing portion was sealed with a sealant (trade name "Araldite") (5) to enhance the cell sealing property. Since the one electrode (2a) is attached to the can (1a) via the stainless steel spring (6), the electrode and the sample porous membrane come into contact without any gap. As the electrolyte, LIPASTE-EP3BLF7 manufactured by Toyama Pharmaceutical Co., Ltd. was used. This electrolytic solution is obtained by dissolving 7.4% by weight of LiBF ₄ as an electrolyte in a mixed solvent of 20.9% by weight of ethylene carbonate, 18.9% by weight of propylene carbonate, and 52.8% by weight of γ-butyrolactone (weight). % Are based on 100% by weight of the electrolyte). Using a 4274A MULTI-FREQUENCY LCR METER manufactured by Yokogawa Hewlett-Packard Company, an AC voltage having a frequency of 100 kHz and an applied voltage of 0.1 V was applied, and the AC impedance (resistance R) was measured.

(Puncture Strength) Using a Tensilon RTM-100 manufactured by Toyo Baldwin Co., Ltd., a yarn stop needle (R at the tip of the needle is 60 μm) as a pin and a crosshead speed of 20
The sample porous membrane was pierced at 0 mm / min, and the value obtained by dividing the maximum load by the thickness of the sample porous membrane was defined as the piercing strength. (Number of protrusions) A rectangular sample porous membrane having a length of 500 mm and a width of 50 mm was prepared, and a thickness gauge (Digital Micrometer M-30, manufactured by Sony Corporation) was used to measure 50 mm in the length direction.
Each time, every 10 mm in the width direction, it was visually confirmed that there were no protrusions, the thickness of a total of 50 points was measured, and the average value was taken as the average thickness of the porous membrane. Next, the protrusions on the surface of the porous film are visually confirmed, and the height thereof, that is, the height up to the top of the protrusions including the thickness of the porous film is measured by the thickness meter, and is at least twice the average thickness. The number of protrusions having a height of was counted. This measurement was performed on 40 sample porous membranes, and the total number of the counted protrusions was defined as the number of protrusions per square meter of area.

(Crystallinity) Using a TA-3000 thermal analysis system manufactured by Mettler, about 10 mg of a sample porous membrane was set in a measuring cell, and a temperature of 30 ° C. and a rate of 1 minute per minute were set in a nitrogen gas atmosphere.
The temperature was raised to 200 ° C. at 0 ° C., and the DSC curve was measured. From this DSC curve, the enthalpy of fusion ΔH (J / g) was determined from the area of the endothermic peak due to the melting of the crystal.
The crystallinity A (%) is expressed by the following equation: Crystallinity A (%) = (△ H /
ΔH ₀ ) × 100. Here, ΔH ₀ is the enthalpy of fusion of polyethylene perfect crystals, and is 288.8 J / g in this embodiment. (Gel fraction) Approximately 70 mg of sample porous membrane weighed in a test tube
(The weight and W ₀ mg) and placed in 1,2,4-trichlorobenzene about 10 ml, was heated 2 hours at a temperature 130 ° C.. This solution and undissolved components were filtered through a 100-mesh wire net, and the separated undissolved components were dried in a hot air oven at 50 ° C. for 12 hours. The dried undissolved components are weighed (this weight is defined as Wmg), and the following formula: gel fraction (%) =
The gel fraction (%) was calculated from (W / W ₀ ) × 100.

(Shutdown Characteristics and Heat Resistance) Using the cell shown in the figure where the electric resistance was measured, using the LCR meter, while applying an AC voltage having a frequency of 100 kHz and an applied voltage of 0.1 V, The temperature of the measurement cell was raised from room temperature at 15 ° C./minute, and the AC impedance (resistance R and reactance X) was measured. Shutdown (S
D) The temperature rises to a few hundred ohms with the resistance R rapidly increasing, and the reactance X overflows (OF; range over,
This is the temperature at which the measurement limit is 20 MΩ). When the resistance R indicates several hundred ohms and the reactance X overflows at this time, it indicates that the measuring cell is in the same state as a small-capacity capacitor, and the sample porous film is replaced with a non-porous insulating film. Indicates that it has changed. Note that the LCR meter does not guarantee the accuracy of the measurement of the resistance R when the reactance X overflows, and thus lacks quantitativeness with respect to the increase width of the impedance. The temperature at which the heat resistance disappears, that is, the rupture temperature, is several tens of ohms when the resistance R decreases again.
It is the temperature below. In addition, this measuring cell is 240 to
It bursts when heated to 300 ° C, and impedance cannot be measured at higher temperatures.

(Battery winding test) Length 500 mm, width 43
2 mm sample porous membrane, length 380 mm, width 38 m
m, a positive electrode sheet having a thickness of 0.14 mm, and a length of 400
mm, a width of 38 mm, and a thickness of 0.16 mm were prepared. The positive electrode active material is LiCoO ₂ which is a composite oxide of lithium and a transition metal (Cellseed C-0 manufactured by Nippon Chemical Industry Co., Ltd.)
5) As the negative electrode active material, Carbotron P manufactured by Kureha Chemical Industry Co., Ltd. capable of doping and undoping lithium was used. The sample porous membrane, the positive electrode sheet, the sample porous membrane, and the negative electrode sheet were stacked in this order, and this was wound using a battery electrode winding device at a winding speed of 2 seconds / 1 rotation and a tension load of 250 g,
Finally, the outer periphery was fixed with an adhesive tape to prepare a battery element having a diameter of 15 mm. Tester (E2373 manufactured by HEWLETTPACKARD)
Using A), the conduction between the positive electrode and the negative electrode of this battery element was examined. The case where the resistance value was in the overrange (30 MΩ or more) was evaluated as ○, and the case where the resistance value was measured was evaluated as ×. When the resistance value is measured, it indicates that a conduction point has occurred due to, for example, the electrode active material penetrating the porous film which is originally an insulator.

(Example 1) Silane-modified polyethylene (manufactured by Mitsubishi Chemical Corporation; Linklon HF-700N, MFR)
0.8 g / 10 min, density 0.958 g / cm ³ ) 45
% By weight and paraffin wax (manufactured by Nippon Seiwa Co., Ltd .;
Paraffin wax 135) 55% by weight, 30mmφ
A cooling drum supplied to a twin-screw extruder, melt-mixed at a resin temperature of 160 ° C. (the mixed resin was compatible), extruded from a T-die having a width of 350 mm, and maintained at a surface temperature of 80 ° C. in a molten state. The resulting mixture was cooled and solidified to obtain an intermediate molded body. Next, a 30% by weight aqueous dispersion of dibutyltin dilaurate was applied to both surfaces of the intermediate molded body, and then immersed in warm water maintained at a temperature of 85 ° C. for 1 hour to complete silane crosslinking.
Next, this intermediate molded body was immersed in cyclohexane at room temperature for 30 minutes to extract paraffin wax, and then dried in an oven at a temperature of 80 ° C. for 30 minutes to remove the extraction solvent, thereby obtaining a silane crosslinked porous membrane. . The production conditions are shown in Table 1, and the physical properties of the obtained porous membrane are shown in Table 2.

Example 2 A silane-modified polyethylene having an MFR of 0.4 g / 10 min and a density of 0.945 g / c
except for using those of m ³ was obtained a silane crosslinked porous membrane in the same manner as in Example 1. The production conditions are shown in Table 1, and the physical properties of the obtained porous membrane are shown in Table 2. The mixed resin was compatible at the resin temperature at the time of extrusion.

Example 3 A silane-crosslinked porous film was obtained in the same manner as in Example 1 except that liquid paraffin was used as the organic low-molecular substance and the surface temperature of the cooling drum was 40 ° C. The production conditions are shown in Table 1, and the physical properties of the obtained porous membrane are shown in Table 2. The mixed resin was compatible at the resin temperature at the time of extrusion.

Comparative Example 1 A silane-modified polyethylene having an MFR of 3 g / 10 min and a density of 0.957 g / cm ³
A silane-crosslinked porous membrane was obtained in the same manner as in Example 1, except that 50% by weight of paraffin wax and 50% by weight of the same paraffin wax as in Example 1 were used. The production conditions are shown in Table 1, and the physical properties of the obtained porous membrane are shown in Table 2.

Comparative Example 2 A silane-modified polyethylene having an MFR of 0.07 g / 10 min and a density of 0.945 g / min
A silane-crosslinked porous membrane was obtained in the same manner as in Example 1 except that the one having a diameter of cm ³ was used. The production conditions are shown in Table 1, and the physical properties of the obtained porous membrane are shown in Table 2.

Comparative Example 3 A silane-crosslinked porous film was obtained in the same manner as in Example 1, except that the resin was melted and mixed at a resin temperature of 200 ° C. The production conditions are shown in Table 1, and the physical properties of the obtained porous membrane are shown in Table 2.

Comparative Example 4 The silane-modified polyethylene used in Comparative Example 1 (MFR: 3 g /
A silane crosslinked porous membrane was obtained in the same manner as in Example 1 except that the mixture was melted and mixed at a resin temperature of 220 ° C. for 10 minutes at a density of 0.957 g / cm ³ ). The production conditions are shown in Table 1, and the physical properties of the obtained porous membrane are shown in Table 2.

Comparative Example 5 Polyethylene not modified with silane (MFR 0.3 g / 10 min, density 0.9
58 g / cm ³ ), and a porous membrane was obtained in the same manner as in Example 1 except that the resin temperature was 200 ° C. Table 1 shows manufacturing conditions
Table 2 shows the physical properties of the obtained porous membrane.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

According to the present invention, as shown in the examples, the porous film is made of a silane-crosslinked polyethylene porous film, has a high barrier property against conductive fine particles, a low permeation resistance of electrolyte ions, and a projection on the surface of the porous film. The present invention provides a porous membrane having a low heat resistance and excellent heat resistance, and a battery separator comprising the porous membrane. In addition, by using a silane-modified polyethylene having an MFR within a specific range, it has excellent properties with few surface protrusions, good film shape retention at high temperatures, and a balance between impedance and strength. A method for producing a porous membrane can be provided.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a measurement cell for measuring heat resistance.

[Explanation of symbols]

 1a: Can body (lid) 1b: Can body (bottom) 2a: Positive electrode (SUS) 2b: Negative electrode (SUS) 3: Porous membrane (separator) 4: Packing (Teflon) 5: Sealant 6: Spring ( SUS)

Claims (5)

[Claims] 1. A silane crosslinked polyethylene having a porosity of 20 to 60%, a maximum pore diameter of 1 μm or less, an air permeability (Gurley value) of 1000 seconds or less, and a height of at least twice the average thickness. The porous membrane, wherein the number of protrusions having the following is 10 or less per square meter of the surface of the porous membrane. 2. The battery according to claim 1, wherein the porous membrane is a battery separator.
The porous membrane as described in the above. 3. A method for producing a silane-crosslinked porous membrane comprising an extrusion step, a crosslinking step, and an extraction step using a mixed resin of silane-modified polyethylene and an organic low-molecular substance having compatibility with the same as a raw material. Melt flow rate) is 0.08 to 2 g / 10 min. The temperature of the mixed resin at the time of extrusion is in the range of from 180 ° C to 180 ° C, which is compatible with organic low-molecular substances, and the extruded melt is cooled and solidified at a temperature below the phase separation temperature. The method for producing a porous membrane according to claim 1, wherein an intermediate molded body is obtained by performing the method. 4. The silane-modified polyethylene has a density of 0.94.
4. The method for producing a porous membrane according to claim 3, wherein the amount is 5 g / cm ³ or more. 5. The battery according to claim 3, wherein the porous membrane is a battery separator.
Or the method for producing a porous membrane according to 4.