JP2003138050A - Polyolefin microporous membrane - Google Patents

Abstract

(57) [Problem] To provide a polyolefin microporous membrane excellent in permeability and hardly deformed by compression. SOLUTION: The porosity is 10 to 80%, the bending ratio of the hole is more than 1.5 and less than 2.1, and the average hole diameter is 0.06 to
A polyolefin microporous membrane having a thickness of 0.3 μm.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyolefin microporous membrane suitable for a battery separator, and more particularly to a polyolefin microporous membrane suitable for use as a lithium ion secondary battery separator.

[0002]

2. Description of the Related Art Microporous membranes made of polyolefin have been used as separators in various batteries, and in particular, they are preferably used in lithium ion secondary batteries for which demand is rapidly increasing. The microporous polyolefin membrane has excellent electronic insulation, has ion permeability when impregnated with an electrolytic solution, has excellent resistance to electrolytic solution and oxidation, has a pore blocking effect, and has appropriate strength. And so on, and this is considered to be the reason why it is preferably used.

As a method for producing a microporous polyolefin membrane, a dry stretching perforation method is used in which only a polyolefin is formed into a membrane and then the pores are stretched by stretching, or a wet method in which a sheet is formed from a polyolefin and a plasticizer and then stretched and extracted. Various methods such as the method have been proposed. However, both methods involve a step of uniaxially or biaxially stretching in the plane direction. As a result, although the strength is improved in the machine direction and / or the direction orthogonal to the machine direction, only a film that is weak in the thickness direction and is easily deformed by compression has been conventionally obtained. For example, the present inventor has proposed a film having high puncture strength and high tensile rupture strength in Japanese Patent Laid-Open No. 2001-81221. There, although high strength was obtained in the plane direction (machine direction and its orthogonal direction), it was weak against compression and tended to be deformed.

The present inventor has filed Japanese Patent Application No. 2001-194405.
In this paper, we proposed a polyolefin microporous membrane that is not easily deformed by compression by satisfying certain specific conditions. However, the transmission performance is not always good,
Therefore, the use may be limited.

[0005]

DISCLOSURE OF THE INVENTION The present invention, as a battery separator, particularly a lithium ion secondary battery separator, is less likely to be deformed by compression and has a higher permeation performance than conventional polyolefin microporous membranes. An object is to provide an improved polyolefin microporous membrane.

[0006]

As a result of earnest studies, it was found that the above-mentioned problems can be solved by satisfying a certain specific condition, and the present invention has been completed. That is, the present invention provides (1) a porosity of 10 to 80%, a porosity of more than 1.5 and less than 2.1, and an average pore diameter of 0.06 to 0.3.
A microporous polyolefin membrane characterized by having a thickness of μm. (2) Porosity is 10 to 80%, bending ratio of pores is more than 1.5 and less than 2.1, and number of pores is 10 to 55 / μm.
^{2. A} microporous polyolefin film characterized by being ² . (3) Porosity is 10 to 80%, the bending ratio of the pores is more than 1.5 and less than 2.1, and the average pore diameter is 0.06 to 0.3.
A microporous film made of polyolefin, characterized in that it has a diameter of 10 μm and the number of pores is 10 to 55 holes / μm ² .

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below. The thickness of the polyolefin microporous film of the present invention is preferably 3 μm or more from the viewpoint of film strength, and 1 from the viewpoint of the effect of the present invention.
It is preferably 00 μm or less. More preferably 5 to 50 μm
And 5 to 30 μm is most preferable. The porosity of the polyolefin microporous membrane of the present invention is from 10 to 80%, preferably from 30 to 70%, from the viewpoint of electrolytic solution impregnation and the effect of the present invention. The air permeability of the polyolefin microporous membrane in the present invention is preferably 1 sec or more, and is preferably 1000 sec or less from the viewpoint of the effect of the present invention. It is more preferably 1 to 300 sec, and even more preferably 1 to 100 sec. The puncture strength of the polyolefin microporous film in the present invention is preferably 0.1 to 9.8 N / 25 μm, and more preferably 0.5 to 9.8 N / 25 μm from the viewpoint of productivity and the effect of the present invention.

The average pore diameter of the polyolefin microporous membrane in the present invention is preferably 0.06 to 0.3 μm from the viewpoint of withstand voltage performance and the effect of the present invention, and is 0.06.
More preferably, it is about 0.2 μm. The bending ratio of the pores of the polyolefin microporous film in the present invention needs to be more than 1.5 and less than 2.1 from the viewpoint of the effect of the present invention. The length of the hole can be obtained by multiplying the film thickness by the bending rate. The number of polyolefin microporous membrane of pores in the present invention further, withstand voltage performance, it is preferably 10 to 55 pieces / [mu] m ² from the viewpoint of the effect of the present invention, 10 to 50 pieces / [mu] m ² More preferably, 10 to 45 pieces / μm ² is most preferable.

Next, an example of the method for producing the microporous membrane of the present invention will be described. The microporous membrane of the present invention has, for example, the following (a)
It is obtained by a method including steps (e) to (e). (A) A step of melt-kneading a mixture of a polyolefin and a plasticizer. (B) A step of extruding the melt, forming it into a sheet, and cooling and solidifying. (C) A step of stretching at least uniaxially. (D) A step of extracting the plasticizer. (E) Heat setting step The order and number of times of these steps are not particularly limited, but the order of (a) step → (b) step → (c) step → (d) step → (e) step, ( a) step → (b) step → (c)
The order of step → (d) step → (c) step → (e) step or (a) step → (b) step → (d) step → (c) step → (e) step is preferable, and order of a) process → (b) process → (c) process → (d) process → (e) process,
(A) process → (b) process → (c) process → (d) process →
The order of step (c) → step (e) is more preferable.

The polyolefins used in the present invention are polyolefins alone and polyolefin compositions.
As the main component polyolefin, there is no particular limitation,
For example, polyethylene, polypropylene, poly-4-methyl-1-pentene and the like can be mentioned, and one or more kinds can be used, but polyethylene having excellent stretchability during film formation is used.
It is preferable to use one or more species. As for polyethylene,
A homopolymer polyethylene having a density of 0.940 g / cm ³ or more, or a copolymer polyethylene having an α-olefin comonomer content of 2 mol% or less is preferable, and a homopolymer polyethylene is more preferable. The type of α-olefin comonomer is not particularly limited. The viscosity average molecular weight of polyethylene is preferably 100,000 to 3,000,000.

The polyethylene polymerization catalyst is not particularly limited and may be any of Ziegler type catalyst, Phillips type catalyst, Kaminsky type catalyst and the like. Polyethylene can be polymerized by one-step polymerization, two-step polymerization, or more multi-step polymerization, and any method of polyethylene can be used, but polyethylene obtained by one-step polymerization is preferable.

As a polyolefin other than the main component,
Various polyolefins can be blended within a range that does not impair the film forming property and does not deviate from the requirements of the present invention.
For example, low melting point polyethylene having a high content of α-olefin comonomer for the purpose of improving the pore blocking property, polypropylene and poly-4-methyl-1-pentene for the purpose of improving the heat resistance can be blended. Further, also for polymer materials and other organic and inorganic materials other than polyolefin, without impairing the performance as a battery separator, without impairing the film-forming property, and may be blended within the range of not departing from the requirements of the present invention. it can.

The polyolefin used in the present invention includes:
If necessary, antioxidants such as phenol-based, phosphorus-based, and sulfur-based, metal soaps such as calcium stearate and zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, color pigments, etc. Known additives can be mixed and used. As the plasticizer used in the step (a), a non-volatile solvent capable of forming a uniform solution at a temperature equal to or higher than its melting point when mixed with the polyolefin to be used is used. For example, hydrocarbons such as liquid paraffin and paraffin wax, di-2-ethylhexyl phthalate, diisodecyl phthalate, and diheptyl phthalate are appropriately selected and used alone or in combination.

The extraction solvent used in the step (d) is a poor solvent for the polyolefin used and a good solvent for the plasticizer used, and its boiling point is higher than the melting point of the polyolefin used. Low one is desirable. As such an extraction solvent, for example, hydrocarbons such as n-hexane and cyclohexane, alcohols such as methanol, ethanol and isopropanol, acetone,
Examples thereof include ketones such as methyl ethyl ketone, ethers such as tetrahydrofuran, methylene chloride, organic solvents such as halogenated hydrocarbons such as 1,1,1-trichloroethane, and the like. Used.

As the method of melt kneading in the step (a), for example, after mixing with a Henschel mixer, a ribbon blender, a tumbler blender, etc., a screw extruder such as a single-screw extruder, a twin-screw extruder, a kneader, a Banbury mixer, etc. A method of melt-kneading can be mentioned. As a method for melt-kneading, an extruder capable of continuous operation is preferable, and a twin-screw extruder is more preferable because it has excellent kneading properties. The plasticizer may be mixed with the raw material polymer by the Henschel mixer or the like, or may be directly fed to the extruder at the time of melt kneading. The temperature at the time of melt-kneading is preferably 160 ° C. or higher and more preferably 300 ° C. or lower in order to obtain a uniform kneaded product. More preferably, it is 180 to 250 ° C.

Next, as the sheet forming method in the step (b), for example, a method of extruding the melt from an extruder equipped with a T-die and cooling it can be mentioned.
Examples of the cooling method include a method of directly contacting a cooling medium such as cold air and cooling water and a method of contacting with a roll cooled with a refrigerant, but a method of contacting with a roll cooled with a refrigerant is excellent in thickness control. preferable.

As the stretching method in the step (c), for example,
The stretching can be performed by a uniaxial stretching machine or a simultaneous biaxial stretching machine. (A) process → (b) process →
Stretching conditions for manufacturing in the order of (c) step → (d) step → (e) step, and (a) step → (b) step → (c)
In the case of manufacturing in the order of step → (d) step → (c) step → (e) step, as the stretching condition of the first stretching step, the stretching ratio is preferably 20 times or more in terms of area ratio, and the stretching temperature is 10.
0-135 degreeC is preferable and 110-135 degreeC is more preferable.

In the extraction step (d), the total plasticizer is extracted by immersing it in the extraction solvent described above, and then fully dried. It is preferable to reduce the residual amount of the plasticizer in the film to less than 1 wt% by extraction. In the step (e), heat shrinkage can be reduced by performing heat setting treatment. The heat setting can be performed by, for example, a TD tenter. The polyolefin microporous membrane of the present invention is performed in the order of (a) step → (b) step → (c) step → (d) step → (c) step → (e) step, and a stretching step after the extraction step. Was stretched at a high ratio and heat setting was performed near the melting point of the film.

Surface treatment such as electron beam irradiation, plasma irradiation, surface active agent coating, and chemical modification can be carried out, if necessary, within a range that does not impair the effects of the present invention. Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples, but these do not limit the scope of the present invention. Various physical properties used in the present invention were measured based on the following test methods. (1) Film thickness (μm) The film thickness (μm) was measured at a room temperature of 23 ° C. using a micro thickness meter (type KBM) manufactured by Toyo Seiki. (2) Porosity (%) Cut a 10 cm × 10 cm square sample from the microporous membrane,
The volume (cm ³ ) and the mass (g) were obtained, and the volume and the film density (g / cm ³ ) were calculated using the following formula. Porosity = (volume-mass / film density) / volume × 100 The film density was measured by the density gradient tube method (23 ° C.) according to ASTM-D1505. (3) Air Permeability (sec) Based on JIS P-8117, the air permeability was measured by a Gurley type air permeability meter (G-B2 type manufactured by Toyo Seiki Co., Ltd.).

(4) Puncture strength (N / 25 μm) A KES-G5 handy compression tester manufactured by Kato Tech was used and the radius of curvature of the needle tip was 0.5 m at room temperature (23 ° C.).
By performing the puncture test under the conditions of m and puncture speed of 2 mm / sec, the raw puncture strength (N) can be obtained as the maximum puncture load. By multiplying this by 25 (μm) / film thickness (μm), the 25 μm film thickness converted puncture strength (N / 25 μm) was calculated. (5) Viscosity average molecular weight Mv Determine the intrinsic viscosity [η] at 135 ° C. in a decalin solvent based on ASTM D4020. The Mv of polyethylene was calculated by the following formula. [Η] = 6.77 × 10 ⁻⁴ Mv ^0.67

(6) Average Pore Diameter (μm), Bending Rate, and Number of Holes (Pieces / μm ² ) The fluid inside the capillary is Knudsen flow when the mean free path of the fluid is larger than the pore diameter of the capillary.
It is known to follow Poiseuille's flow when it is small. Therefore, it is assumed that the air flow in the air permeability measurement of the microporous membrane follows the Knudsen flow, and the water flow in the water permeability measurement of the microporous membrane follows the Poiseuille flow.

In this case, the pore diameter d (μm) and the tortuosity τ (dimensionless) are determined by the air permeation rate constant R _{ga s} (m ³ / (m ² · s).
ec · Pa)), water transmission rate constant R _liq (m ³ / (m ²
・ Sec ・ Pa)), molecular velocity of air ν (m / se)
c), viscosity of water η (Pa · sec), standard pressure P _s (=
101325 Pa), porosity ε (%), film thickness L (μm)
Can be obtained from the following equation. d = 2ν × (R _liq / R _gas ) × (16η / 3Ps) × 1
0 ⁶ τ = (d × (ε / 100) × ν / (3L × P _s ×
R _gas )) ^1/2 Here, R _gas is calculated from the air permeability (sec) using the following equation. R _gas = 0.0001 / (air permeability × (6.424 × 10
^-4 ) x (0.01276 x 101325))

R _liq is the water permeability (cm ³ / (cm ² ·
sec · Pa)) is calculated using the following equation. R _liq = water permeability / 100 The water permeability is _{calculated as} follows. A microporous membrane preliminarily dipped in alcohol was set in a 41 mm diameter stainless steel permeable cell, the alcohol in the membrane was washed with water, and then water was permeated at a differential pressure of about 50,000 Pa for 120 seconds. From the water permeability (cm ³ ) at that time, the water permeability per unit time, unit pressure, and unit area was calculated, and this was taken as the water permeability.

Ν is a gas constant R (= 8.314),
Absolute temperature T (K), circular constant π, average molecular weight of air M (=
2.896 × 10 ⁻² kg / mol) is calculated using the following formula. ν = ((8R × T) / (π × M)) ^1/2 Further, the number of holes B (number / μm ² ) is calculated by the following equation. B = 4 × (ε / 100) / (π × d ² × τ)

(7) Deformation rate after compression (%) 100 circular samples having a diameter of 60 mm were cut out from the microporous membrane, and static electricity was removed by applying the air to a blower type static eliminator (KD-410 type manufactured by Kasuga Denki) for about 10 seconds. After doing, overlay,
The thickness T _b of the 100-ply sample was measured by the method (1). Next, this 100-sheet stack sample was pressed by a press molding machine. Press conditions are temperature 25 ℃, time 1.5
The compression load was set to 7 kN. After pressing, the plate was allowed to stand at room temperature of 23 ° C. for 100 minutes, and then the thickness T _a of the 100-ply sample after pressing was determined. From these values, the deformation rate (%) after compression of each sample was calculated by the following formula. After compression deformation ratio _{= (T b -T a) /} T b × 100

[Example 1] Pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant in 99.7 wt% of homopolymer polyethylene having Mv270,000. Of 0.3 wt% was added and dry blended using a tumbler blender to obtain a polymer mixture. The obtained mixture of polymer and the like was fed to the feed port of the twin-screw extruder by a feeder. Further, di-2-ethylhexyl phthalate was injected into the twin-screw extruder cylinder by a plunger pump.

The ratio of the amount of di-2-ethylhexyl phthalate in the total mixture melt-kneaded and extruded is 55 wt%.
The feeder and pump were adjusted so that Melt-kneading conditions are set temperature 200 ° C., screw rotation speed 35
It was performed at 0 rpm and a discharge rate of 15 kg / h. Subsequently, the melt-kneaded product is extruded through a T-die onto a cooling roll whose surface temperature is controlled to 25 ° C. and cast to give a thickness of 1
A 150 μm gel sheet was obtained.

Next, the film was guided to a simultaneous biaxial tenter drawing machine and biaxially drawn. The set stretching conditions are MD magnification 7.0 times,
The TD magnification is 4.0 times and the set temperature is 128 ° C. Next, the mixture was introduced into a methyl ethyl ketone tank and sufficiently immersed in methyl ethyl ketone to extract and remove di-2-ethylhexyl phthalate, and then the methyl ethyl ketone was removed by drying. Further, it was introduced into a TD tenter, and stretched and heat set.
The set draw ratio is 2.3 times and the set heat setting temperature is 135 ° C. Table 1 shows the physical properties of the obtained microporous membrane.

[Example 2] Pentaerythrityl-tetrakis- [3- (3,5-di-) was used as an antioxidant in 49.7 wt% of homopolymer polyethylene of Mv270,000 and 49.7 wt% of homopolymer polyethylene of Mv1,000,000. t
-Butyl-4-hydroxyphenyl) propionate]
Of 0.6 wt% was added and dry blended using a tumbler blender to obtain a polymer mixture. The obtained mixture of polymer and the like was fed to the feed port of the twin-screw extruder by a feeder. Further, di-2-ethylhexyl phthalate was injected into the twin-screw extruder cylinder by a plunger pump.

The ratio of the amount of di-2-ethylhexyl phthalate in the entire mixture melt-kneaded and extruded is 60 wt%.
The feeder and pump were adjusted so that Melt-kneading conditions are set temperature 250 ℃, screw rotation speed 20
It was performed at 0 rpm and a discharge rate of 15 kg / h. Subsequently, the melt-kneaded product is extruded through a T-die onto a cooling roll whose surface temperature is controlled to 25 ° C. and cast to give a thickness of 1
A 350 μm gel sheet was obtained.

Next, the film was guided to a simultaneous biaxial tenter drawing machine and biaxially drawn. The set stretching conditions are MD magnification 7.0 times,
The TD magnification is 4.0 times and the set temperature is 130 ° C. Next, the mixture was introduced into a methyl ethyl ketone tank and sufficiently immersed in methyl ethyl ketone to extract and remove di-2-ethylhexyl phthalate, and then the methyl ethyl ketone was removed by drying. Further, it was introduced into a TD tenter, and stretched and heat set.
The set draw ratio is 2.4 and the set heat setting temperature is 134 ° C. Table 1 shows the physical properties of the obtained microporous membrane.

[Comparative Example 1] A mixture of polymers and the like obtained in the same manner as in Example 2 was fed to the feed port of a twin-screw extruder by a feeder. In addition, liquid paraffin (37.
Kinematic viscosity at 78 ° C. of 7.59 × 10 ⁻⁵ m ² / s)
It was injected into a twin-screw extruder cylinder by a plunger pump. The feeder and pump were adjusted so that the ratio of the amount of liquid paraffin in the total mixture melt-kneaded and extruded was 55 wt%. Melt-kneading condition is set temperature 2
50 ° C, screw rotation speed 130 rpm, discharge amount 10 k
It was performed at g / h.

Subsequently, the melt-kneaded product was extruded through a T-die onto a cooling roll whose surface temperature was controlled at 30 ° C. and cast to obtain a gel sheet having a thickness of 1100 μm. Next, it was guided to a simultaneous biaxial tenter stretching machine and biaxially stretched. The set stretching conditions are an MD magnification of 7.0 times, a TD magnification of 7.0 times, and a set temperature of 119 ° C. Next, it was introduced into a methylethylketone tank and sufficiently immersed in methylethylketone to extract and remove liquid paraffin, and then dry and remove methylethylketone. Furthermore, lead to TD tenter,
Stretching and heat setting were performed. The set draw ratio is 1.8 times and the set heat setting temperature is 125 ° C. Table 1 shows the physical properties of the obtained microporous membrane.

[Comparative Example 2] The polymer mixture obtained in the same manner as in Example 1 was fed to the feed port of the twin-screw extruder by a feeder. In addition, liquid paraffin (37.
Kinematic viscosity at 78 ° C. of 7.59 × 10 ⁻⁵ m ² / s)
It was injected into a twin-screw extruder cylinder by a plunger pump. The feeder and pump were adjusted so that the ratio of the amount of liquid paraffin in the total mixture melt-kneaded and extruded was 55 wt%. Melt-kneading condition is set temperature 2
00 ℃, screw rotation speed 220rpm, discharge amount 15k
It was performed at g / h. Then, the melt-kneaded product was extruded through a T-die onto a cooling roll whose surface temperature was controlled at 30 ° C. and cast to obtain a gel sheet having a thickness of 1370 μm. Next, it was guided to a simultaneous biaxial tenter stretching machine and biaxially stretched. Set stretching conditions are MD magnification 7.0 times, TD
The magnification is 6.5 times and the set temperature is 125 ° C.

Next, the liquid paraffin was introduced into a methyl ethyl ketone tank and sufficiently immersed in methyl ethyl ketone to extract and remove the liquid paraffin, and then the methyl ethyl ketone was removed by drying.
Further, it was introduced into a TD tenter, and stretched and heat set. Set draw ratio is 2.0 times, set heat setting temperature is 127
℃. Table 1 shows the physical properties of the obtained microporous membrane.

[0036]

[Table 1]

[0037]

The polyolefin microporous membrane of the present invention is
Compared to conventional microporous membranes, it is characterized by superior permeation performance and being less likely to deform when compressed. Thereby,
For example, when used as a separator for lithium-ion secondary batteries, it has excellent charge and discharge characteristics, and retains the initial winding shape even after repeated charge and discharge, contributing to stability of performance during long-term use and reduction of defective rate. It is thought that.

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 4D006 GA41 MA22 MA24 MA31 MB03                       MB16 MB20 MC22 MC22X                       NA22 NA43 NA46 NA54 NA62                       NA66 PA01 PC80                 4F074 AA16 AA17 AD12 AG04 CA03                       CB03 CB16 CB28 CC02Y                       DA02 DA03 DA24 DA49                 5H021 CC00 EE04 HH01 HH02 HH03

Claims (3)

[Claims] 1. A porosity of 10 to 80%, a bending ratio of pores exceeding 1.5 and less than 2.1, and an average pore diameter of 0.06 to.
A microporous polyolefin film having a thickness of 0.3 μm. 2. A polyolefin, which has a porosity of 10 to 80%, a bending ratio of pores of more than 1.5 and less than 2.1, and a number of pores of 10 to 55 pores / μm ² . Microporous membrane. 3. The porosity is 10 to 80%, the bending ratio of the pores is more than 1.5 and less than 2.1, and the average pore diameter is 0.06 to.
A polyolefin microporous membrane having a diameter of 0.3 μm and a number of pores of 10 to 55 holes / μm ² .