JPH10265596A - Composite porous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the subject porous film. comprising a porous substrate made of a resin and a coating film deposited on the surface thereof, having a specific thickness, a specified porosity, a specific air permeability and a specified thermal blocking temperature and suitable for a filtration film, a battery or a cell separator, a packaging material, a medical material, a clothes material, etc. SOLUTION: This film is a composite porous film, comprising a porous substrate made of a resin such as a polyolefin resin or a fluorine-containing resin and a coating film prepared by coating the surface of the substrate with a monomer polymerizable with energy rays or heat and then forming the film by irradiating thereof with energy rays and heating of the surface. The porous film has characteristics of 5-200 μm thickness, 20-80% porosity, 10-15,000 sec/100 cc Gurley type air permeability, 90-160 deg.C thermal blocking temperature and 180-300 deg.C thermal film breaking temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a porous membrane having gas permeability. Specifically, the present invention provides a porous film having improved heat resistance. Specifically, a porous membrane suitable for a filtration membrane, a battery separator, various packaging materials requiring air permeability, a medical material, a clothing material, and the like can be provided.
In particular, the present invention provides a porous membrane suitable for a separator for a lithium secondary battery.

[0002]

2. Description of the Related Art Porous membranes include filtration membranes, battery separators,
It is used in various industrial fields such as packaging, medical care, and clothing materials. In particular, when the porous film is used as a separator for a lithium secondary battery, it is necessary to have a property of retaining an electrolyte and having ion permeability while insulating the positive and negative electrodes so as not to directly contact them (insulating the electrodes). Is done.

[0003] In addition, with respect to a rapid temperature rise due to an abnormal reaction inside the battery or the like, the pores are closed to stop the permeation of ions (ion insulation) and safely stop the reaction inside the battery. Is also required. This is a property called heat blocking property, and it has been confirmed that a porous film using an olefin resin, for example, polyethylene, is blocked at 120 ° C. to 130 ° C. near its melting point.

[0004] For this purpose, for example, Japanese Patent Laid-Open No. 6-688
For example, a porous membrane as disclosed in JP-A-64 can be used. The patent uses ultra-high molecular weight polyethylene,
Clogging of the pores around 130 ° C. is observed. That is, the heat blocking property appears around 130 ° C., and the reaction inside the battery is 13 ° C.
It becomes possible to suppress the temperature around 0 ° C. However,
If the temperature inside the battery rises sharply, the temperature may be much higher than the melting point. In this case, the porous membrane will melt and the shape cannot be maintained. In such a case, a change such as a rupture state occurs, and the insulation property is impaired, and an abnormal reaction inside the battery may proceed again.

[0005] Maintaining the shape of the film against such a temperature rise is referred to as heat-resistant shape retention. In the case of the porous film made of ultra-high molecular weight polyethylene, the film breaks at around 150 to 160 ° C. That is, the heat-resistant shape maintaining property is impaired at a temperature higher than around 160 ° C. Furthermore, the polypropylene separator membrane has excellent shape retention at high temperatures, but when used as a lithium battery separator, the temperature at which self-occlusion is exhibited is about 175 ° C, which is close to the ignition temperature of lithium of 180 ° C. There is a problem that you are.

On the other hand, the separator membrane can be stretched to improve the strength. However, the stretched membrane has low heat resistance and heat characteristics.
It breaks at around 60 ° C and around 180 ° C with a polypropylene film. Therefore, as a means to obtain a porous film having high heat resistance,
For example, Japanese Patent Publication No. 6-55872 or Japanese Patent Publication No.
As can be seen in JP-A-62130, it is conceivable to use a high heat-resistant resin. However, in these methods, the temperature at which the plugging is basically stopped is high, and the property of stopping the abnormal reaction at an earlier time, that is, Thermal plugging at low temperatures is impaired.

Further, Japanese Patent Application Laid-Open No. 7-29563 describes a porous membrane made of ultra-high molecular weight polyethylene having improved heat-resistant shape maintaining properties.
Although it is a porous film having a heat blocking property falling within the range of 0 ° C. and maintaining a heat-resistant shape of 160 ° C. or more (200 ° C. or more depending on conditions), it is still insufficient.

[0008]

As described above, a battery separator, particularly a lithium secondary battery separator, has two characteristics, namely, heat blocking property against temperature change and heat resistant shape maintaining property. There is a strict demand, the former occurring at lower temperatures and the latter being required to be maintained at higher temperatures.

[0009]

Means for Solving the Problems Accordingly, the present inventors have:
We tried to form a deposited layer on the surface of the porous membrane to improve the heat blocking property against temperature change and the heat-resistant shape maintenance property more than before, and found that it could exhibit extremely high performance as a separator. Reached the present invention.

That is, according to the present invention, while maintaining the conventional characteristics of the insulating properties of the electrode, the shape can be maintained without melting or breaking the film at a high temperature, and the ion can be insulated at a higher temperature. The gist of the present invention is to provide a composite porous membrane comprising a resinous porous substrate and a coating deposited on the surface thereof, wherein the porous membrane comprises: The thickness of the porous substrate is 5 μm or more and less than 200 μm, (b) the porosity of the porous substrate is 20% or more and less than 80%, and (c) the Gurley-type air permeability of the composite porous membrane is 10 seconds / 100 cc or more and 1500 Seconds / 100cc or less,
(D) The heat blocking temperature of the composite porous membrane is 90 ° C. or higher and 160 ° C.
Hereinafter, (e) a composite porous membrane having a characteristic that the thermal rupture temperature of the composite porous membrane is 180 ° C. or more and 300 ° C. or less.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. The material of the resinous porous substrate used in the present invention is not particularly limited, but a polyolefin resin, a fluorine-based resin, or a kneaded product thereof is preferably used in view of the durability of the material. Among them, a porous ultrahigh molecular weight polyethylene membrane, a polytetrafluoroethylene porous membrane, a nonwoven fabric made of polyethylene or polypropylene, and the like are used. Most preferably, an ultra-high molecular weight polyethylene porous membrane that exhibits heat blocking properties at a lower temperature is used.

The physical properties of the porous substrate are as follows: the thickness is 5 μm or more and less than 200 μm, preferably 10 μm or more and 50 μm or less, and the porosity is 20% or more and 80% or less.
It is desirably less than 40%, preferably 40% or more. Such a porous substrate can be prepared by a known method. For example, in the case of an ultra-high molecular weight polyethylene porous membrane,
A swelling agent that is solid or liquid at room temperature and liquid under melt molding conditions is melt-kneaded with ultra-high molecular weight polyethylene, the composition is formed into a film by a molding method such as extrusion molding, and the swelling agent is extracted. Depending on the stretching, it is obtained by stretching before or after extraction.

The composite porous membrane of the present invention comprises the above-mentioned resin porous substrate and a film deposited on the surface thereof without substantially closing the pores of the substrate. The coating on the surface of the substrate is formed on one or both sides of the substrate, and in either case, the film is formed so as not to completely close the pores of the substrate.

As a method for forming a film by depositing a film on the surface of a porous substrate, (1) a method of applying a monomer polymerizable by energy rays or heat and then polymerizing the monomer; And (3) a method of forming a thin film by plasma polymerization.

Each of the above methods (1) to (3) can be specifically performed as follows. Examples of the monomer polymerizable by energy rays or heat in the method (1) include a compound having a vinyl group, for example, a hydrocarbon-based vinyl compound such as styrene and divinylbenzene, vinyl acetate, acrylonitrile, vinylpyridine, and N-vinylpyrrolidone. , Vinylformamide and the like, preferably a hydrocarbon-based vinyl compound, particularly preferably styrene. Further, a monomer having an acrylic group, for example,
Acrylic acid and its esters, such as acrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, polyethylene glycol acrylate, or acrylamide, acrylamides such as NN-dimethylacrylamide, or allyl groups such as allyl alcohol and allylamine And the like can also be used.

These monomers may be applied to a porous substrate as it is, or may be diluted with an appropriate solvent and then applied to the substrate. In addition, only one type of monomer may be used, or two or more types may be used in combination. After application, a film is formed on the substrate by irradiating ultraviolet light, electron beam, radiation, or the like, or by performing a heat treatment. At this time, an appropriate thermal dissociation type polymerization initiator such as a peroxide or an azo compound or a photopolymerization initiator may be used. In the above, it is possible to form a film on the surface of the substrate by closing the pores of the substrate by controlling the concentration of the monomer.

In the method (2) of performing graft polymerization on a substrate, a radical initiation point is formed on the substrate, and a radical polymerizable monomer is polymerized using the radical initiation point as a polymerization initiation point. The formation of the radical initiation point on the substrate can be achieved by treating the substrate with radiation, electron beam, corona discharge, plasma irradiation, fluorine, or the like. There is also a method of treating a substrate with an oxidizing agent such as potassium permanganate.

After the treatment for forming a radical starting point on the substrate as described above, application of a radical polymerizable monomer or immersion of the substrate in the radical polymerizable monomer, or contact with a monomer vapor To perform graft polymerization. At this time, the polymerizable monomer may be used alone, or may be used after being diluted with an appropriate solvent or the like. It is also possible to heat appropriately. Examples of the radical polymerizable monomer include the vinyl group-containing compound, acrylic acid, and esters thereof exemplified in the above-mentioned method (1), and the monomer is used alone or in combination of two or more. . Further, when a monomer having a plurality of polymerizable functional groups in one molecule such as divinylbenzene is used, the degree of cross-linking of the formed film is increased, and a tougher, heat-resistant film is obtained, which is one of preferred embodiments. . In the above, it is possible to form a film without closing the pores of the substrate by controlling the type of the monomer, the concentration of the monomer, and the like, and the surface energy of the film can be controlled by selecting the monomer.

As a plasma source used for forming a film on the surface of the base material by plasma polymerization in (3), a radio-frequency power source, a microwave power source, a direct-current power source, or the like is used. Also,
Plasma polymerization is usually performed in a suitable chamber,
The internal pressure is usually 0.1 Pa (about 1 mTorr) to atmospheric pressure, preferably 7 Pa (about 50 mTorr) to 400 P
a (approximately 3 Torr). Discharge is performed in the chamber to generate plasma, and appropriate monomer vapor is introduced into the plasma to perform polymerization.

At this time, any monomer having a vapor pressure under polymerization conditions can be polymerized, but benzene,
Aromatic hydrocarbons such as toluene, ethylbenzene, styrene, divinylbenzene and naphthalene, preferably those having a double bond, more preferably styrene, or nitrogen in the molecule of pyridine, triethylamine, piperidine, allylamine, etc. Organic compounds having atoms or alcohols such as ethanol, isopropyl alcohol, butanol, t-butyl alcohol, allyl alcohol, ethylene glycol, ethers such as tetrahydrofuran, diethyl ether, dioxane, ethyl acetate, methyl acrylate, propylene carbonate, etc. Esters, acetone, methyl ethyl ketone, organic compounds containing an oxygen atom in the molecule including ketones such as methyl isobutyl ketone, or hexamethyldisiloxane, Tiger ethoxy silane, trimethyl vinyl silane, octamethyltrisiloxane, can when combined organic compound containing a silicon atom such as hexamethyldisilazane to obtain more favorable results. These compounds can be used alone or in combination of two or more.

Further, the polymerization may be carried out in the presence of a gas such as argon, helium, oxygen, nitrogen, ammonia, carbon dioxide or the like simultaneously with the monomer. By controlling the polymerization conditions such as the amount of the monomer introduced and the chamber pressure plasma output, it is possible to form a film on the substrate surface without closing the pores of the substrate. Further, the film formed by the plasma polymerization method is generally a crosslinked body, and a film having excellent durability such as heat resistance and chemical resistance can be formed. Furthermore, the surface energy of the film can be controlled by appropriately selecting a monomer and controlling the polymerization conditions.

The thickness of the coating formed on the surface of the porous substrate is 0.5 μm or less, preferably 0.2 μm or less.
If it exceeds 0.5 μm, the pores of the base material are likely to be closed, which is not preferable. The lower limit of the thickness of the film is not particularly limited, and may be a monomolecular film as long as the film can be uniformly formed on the surface of the porous substrate. Usually, it is 0.01 μm or more.

As described above, the physical properties for specifying the composite porous membrane of the present invention comprising the resinous porous substrate and the film deposited on the surface thereof will be described.
It is less than 0 μm, preferably 10 μm to 50 μm, and the porosity is 20% to less than 80%, preferably 40% to 60%. The Gurley air permeability of the membrane is 10 seconds / 100 cc or more and 1500 seconds / 100 c.
c, preferably 10 seconds / 100 cc or more and 1000 seconds / 100 cc or less. The above film thickness and porosity are
It is preferable that the thickness and the porosity of the porous base material are substantially in the same range. If the thickness is out of the ranges, it is not preferable because the use as a battery separator becomes difficult.

Further, in the composite porous membrane of the present invention, the heat closing temperature is from 90 ° C. to 160 ° C., preferably from 90 ° C. to 150 ° C. On the other hand, the thermal rupture temperature, which is an index of the heat resistant shape characteristics, is 180 ° C. or more and 300 ° C. or less, preferably
It is 20 ° C. or more and 280 ° C. or less, and has extremely excellent characteristics as a battery separator, particularly as a lithium secondary battery separator.

In a lithium secondary battery using the composite porous membrane of the present invention as a separator, for example, a lithium metal, an alloy of lithium and aluminum, or a carbon electrode capable of absorbing and releasing lithium ions is used as a negative electrode. And a known electrode such as manganese dioxide is used as the positive electrode. As the shape of the battery, for example, a case in which the composite porous membrane of the present invention is wound between the positive electrode and the negative electrode, or a case in which each electrode is wrapped in a bag-shaped composite porous membrane together with an electrolytic solution, And a sealed battery. As the electrolyte, for example, a non-aqueous solution in which an electrolyte such as LiPF ₆ is dissolved in an aprotic polar solvent such as ethylene carbonate (EC) or dimethyl carbonate (DMC) is used.

[0026]

EXAMPLES The present invention will be described specifically with reference to Examples.
The present invention is not limited to the following examples unless it exceeds the gist. In addition, in the following examples, each measurement was based on the following method.

(1) Air permeability Measured according to JIS P8117. As a measuring device, a B-type Gurley type densometer (trade name) manufactured by Toyo Seiki Co., Ltd. was used. (2) Porosity From the volume and weight of the film and the density of the material, the volume of the void portion (space without material) occupying the inside of the film was expressed in percentage.

(3) Thermal obstruction test A sample having the same external dimensions was sandwiched from above and below by using two aluminum plates having a thickness of 80 mm square and having a square hole of 40 mm square in the center and having a thickness of 2 mm. Then, fix the outer periphery with a clip. In order to prevent the sample from directly contacting the aluminum plate, a Teflon film having the same outer dimensions as the aluminum plate and a thickness of 100 μm was sandwiched between both surfaces of the sample.
This is placed in an oven heated to 90 ° C. or 140 ° C., left to stand for 30 minutes, taken out, measured for air permeability, and measured for air permeability. Is less than 10 cc. In addition, the air permeability was displayed for those that were not determined to be clogged.

(4) Heat-resistant shape maintenance test Each of the test pieces was 100 mm in width and 50 mm in length, and had a thickness of 3 mm.
Glass plate / sample (porous membrane) / # 120 sandpaper /
Glass plates having a thickness of 3 mm "were stacked in this order, and fixed with clips so as not to shift. This was placed in an oven heated to a predetermined temperature, allowed to stand still for 30 minutes, taken out and the state of the porous membrane was observed, and a state in which a visible hole was opened in the membrane was determined to be a ruptured membrane. Further, the heat-resistant shape maintaining characteristics were evaluated at the temperature during the occurrence of the film breakage.

Comparative Example 1 80 parts by weight of stearyl alcohol and 0.5 part by weight of a phenolic stabilizer were powder-blended with 20 parts by weight of polyethylene having a viscosity average molecular weight of 1.5 million.
The polyethylene was left in a 70 ° C. oven for 30 minutes to wet the stearyl alcohol. This mixture was cooled to 10 ° C under the conditions of a jacket temperature of 170 ° C and a rotation speed of 100 rpm.
Kneaded for minutes. The resin temperature and torque became constant, and the mixture became transparent and uniform in the molten state.

Before the homogeneous mixture cools and solidifies,
At a temperature of 5 ° C. to obtain a pressed sheet having a thickness of 0.5 mm. The sheet was immersed in ethanol at 50 to 60 ° C. for 5 minutes to extract stearyl alcohol. This sheet was white due to porosity. The extracted sheet is simultaneously stretched at a temperature of 120 ° C. at 200% / sec.
X4 times stretching was performed. Table 1 shows the physical properties of the base film, and Table 2 shows the results of the heat resistance test.

Example 1 The ultrahigh molecular weight polyethylene porous membrane obtained by the method of Comparative Example 1 was placed in a chamber having a parallel plate type electrode inside, and the chamber was sufficiently evacuated. Was kept at 20 Pa. 13.56 MHz
Discharge by the radio wave power supply of
Minutes. Then, in addition to argon gas, styrene vapor was introduced into the chamber, and the chamber pressure was set to 300 m.
Torr. The discharge was performed again for 1 minute to form a plasma polymerized film of styrene on the substrate. Table 1 shows the physical properties of the obtained composite porous membrane. Table 2 shows the results of the heat resistance test of this composite porous membrane.

Example 2 A composite porous membrane was obtained in the same manner as in Example 1 except that pyridine was used instead of styrene. Table 1 shows the physical properties of the composite porous membrane. Table 2 shows the results of the heat resistance test.
Shown in

Example 3 A composite porous membrane was obtained in the same manner as in Example 1 except that trimethylvinylsilane was used instead of styrene. Table 1 shows the physical properties of the composite porous membrane. Table 2 shows the results of the heat resistance test.

[0035]

[Table 1] Note) In the above table, the unit of air permeability is “sec / 100cc”.

[0036]

[Table 2] Note) In the above table, the unit of air permeability is “sec / 100cc”.

Example 4 In the same manner as in Example 1, the ultrahigh molecular weight polyethylene porous membrane was treated with Alcon plasma for 1 minute. Thereafter, the inside of the chamber was leaked with nitrogen gas to return to atmospheric pressure, and immediately immersed in a degassed toluene solution containing 10% by weight of styrene and 1% by weight of divinylbenzene at 40 ° C. for 30 minutes to obtain a composite having a grafted polystyrene film. A porous membrane was obtained. Table 3 shows the physical properties of the composite porous membrane. Table 4 shows the results of the heat resistance test.

Comparative Example 2 The ultra-high molecular weight polyethylene porous membrane 1 used in each Example was
The procedure was the same as in Example 4, except that the polymer was not immersed in a toluene solution of styrene and 1 wt% of divinylbenzene by weight. Table 3 shows the physical properties of this film. Table 4 shows the results of the heat resistance test.

[0039]

[Table 3] Note) In the above table, the unit of air permeability is “sec / 100cc”.

[0040]

[Table 4] Note) In the above table, the unit of air permeability is “sec / 100cc”.

In the above Examples, it was confirmed that the heat-resistant shape maintaining property was improved as compared with the Comparative Example, and it was found that the heat-closing property was maintained. Comparative examples are
Although it tends to be closed by heating, it can also be seen that the degree of closing is smaller than in the examples.

[0042]

According to the present invention, the heat resistance, which cannot be achieved by the substrate alone, due to the coating formed on the porous substrate,
Chemical resistance and the like can be imparted. In particular, by introducing a crosslinked structure into the coating on the surface of the base material, the durability becomes more excellent. In addition, by controlling the material of the coating, it is possible to control the properties of the substrate surface as a result. As a result, the composite porous membrane of the present invention is useful as a battery separator, particularly as a lithium secondary battery separator.

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Claims (8)

[Claims] 1. A composite porous membrane comprising a resinous porous substrate and a coating deposited on the surface thereof, wherein (a) the composite porous membrane has a thickness of 5
μm or more and less than 200 μm, (b) porosity is 20% or more and 8
Less than 0%, (c) Gurley type air permeability is 10 seconds / 100cc
Not less than 15000 sec / 100 cc, (d) heat blocking temperature is 90 ° C. or more and 160 ° C. or less, and (e) thermal rupture temperature is 180 ° C.
A composite porous membrane having characteristics of not less than 300 ° C and not more than 300 ° C. 2. The composite porous membrane according to claim 1, wherein the thermal rupture temperature is 220 ° C. or more and 280 ° C. or less. 3. The coating deposited on the surface of the porous substrate,
The composite porous membrane according to claim 1, wherein the composite porous membrane is formed by applying a monomer polymerizable by an energy beam or heat onto a substrate, and then irradiating or heating the energy beam. 4. A film deposited on the surface of a porous substrate,
3. The composite porous membrane according to claim 1, wherein the composite porous membrane is formed by graft polymerization starting from a radical formed on a substrate surface. 5. A film deposited on the surface of a porous substrate,
3. The composite porous film according to claim 1, wherein the film is formed by a plasma polymerization method. 6. The composite porous membrane according to claim 1, wherein the porous substrate is a polyolefin resin or a fluorine-containing resin. 7. A battery separator comprising the composite porous membrane according to claim 1. 8. A lithium secondary battery incorporating the battery separator according to claim 7.