JP3444712B2 - Microporous membrane suitable for battery separator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microporous membrane having a low pore blocking temperature, being less likely to be broken during membrane formation, and having a required strength. Furthermore, the microporous membrane of the present invention is suitable as a microporous membrane for organic solvent-based battery separators, and is particularly useful as a separator for lithium primary batteries, lithium ion secondary batteries and lithium secondary batteries.

[0002]

2. Description of the Related Art Polyethylene or polypropylene separators are used as organic solvent battery separators. In particular, polyethylene separators have a lower melting point than polypropylene separators, which may cause misuse of batteries. When an abnormal temperature rise occurs inside the battery, it melts at a low temperature and closes the micropores, so that the abnormal reaction can be stopped at an earlier stage and a further temperature rise can be suppressed. Therefore, it is useful in terms of battery safety. It can be said that the lower the melting temperature is, the more excellent the separator is in safety. As a technique therefor, Japanese Patent Application Laid-Open No. Hei 2
No. 21559 discloses a technique of mixing polyethylene having a viscosity average molecular weight of 300,000 or less and two types of polyethylene having a viscosity average molecular weight of 1 million or more. Further, JP-A-5-25305 discloses a technique of mixing low-density polyethylene having a low melting point with ultra-high-molecular-weight polyethylene and high-density polyethylene to obtain a low pore-closing temperature, and mixing low-density polyethylene with ultra-high-molecular-weight polyethylene. The technology is disclosed. Further, Japanese Patent Laid-Open No. 7-309965 discloses that holes are quickly closed in a narrower temperature range.
A technique is disclosed in which a high molecular weight ethylene / α-olefin copolymer alone is used instead of a blend.

JP-A-8-34873 discloses a high molecular weight polyethylene having an intrinsic viscosity [η] of 5 dl / g for the purpose of obtaining a separator excellent in tensile strength, dimensional stability at high temperature, and current blocking function. Density 0.915 to 0.93
Low density polyethylene of 0 g / cc and / or density of 0.
A technique for mixing with 950 to 0.970 g / cc of high density polyethylene is disclosed.

However, in JP-A-8-34873, the high molecular weight polyethylene is as high as 60% by weight or more, the pore closing temperature is not sufficiently lowered, and the moldability is poor. When two types of polyethylene are mixed as disclosed in JP-A-2-21559, when the ultra high molecular weight polyethylene and the high density polyethylene are mixed, the melting point is not sufficiently lowered. In order to improve it, if high-density polyethylene with a low molecular weight or high-density polyethylene with a wide molecular weight distribution is used, the film strength will decrease and the compatibility between polyethylenes will be poor. This causes a problem in thinning the film and increasing the strength. When ultra high molecular weight polyethylene and low density polyethylene are mixed, a low pore closing temperature can be obtained, but poor compatibility between polyethylenes poses a problem. As disclosed in Japanese Unexamined Patent Publication No. 25305/1993, in the case of mixing ultrahigh molecular weight polyethylene, high density polyethylene and low density polyethylene or ultrahigh molecular weight polyethylene and low density polyethylene, poor compatibility between polyethylenes still poses a problem. Becomes Japanese Unexamined Patent Publication No. 7-309965 discloses a technique in which a high molecular weight ethylene / α-olefin copolymer is used alone, but the decrease in the pore blocking temperature is insufficient due to the single composition of the high molecular weight product. .

[0005]

DISCLOSURE OF THE INVENTION The present invention provides a microporous membrane which has a low pore blocking temperature, is less likely to be broken during membrane formation, and has a required strength.

[0006]

[Means for Solving the Problems] That is, the present invention relates to ultrahigh molecular weight polyethylenes 10 to 5 having a viscosity average molecular weight of 1,000,000 or more.
0% by weight, α-olefin content is 0.05 to 2 mol
Polyethylene copolymer of less than 10% by weight,
And a polyethylene copolymer having an α-olefin content of 2 to 8 mol% and / or a low density polyethylene 3
The microporous membrane is characterized by comprising 5 to 70% by weight.

These microporous membranes are effective as organic solvent battery separators. The ultrahigh molecular weight polyethylene used in the present invention needs to have a viscosity average molecular weight of 1,000,000 or more, and preferably 2,000,000 to 3,500,000. The required film strength cannot be obtained unless polyethylene having a viscosity average molecular weight of 1,000,000 or more is added. Further, in consideration of compatibility between the film strength and the pore blocking temperature, the proportion of ultra high molecular weight polyethylene needs to be 10 to 50% by weight, preferably 20 to 40% by weight.

The α-olefin content used in the present invention is 0.05 to less than 2 mol%, preferably 0.2 to 2
When the polyethylene copolymer of less than mol% is added, the compatibility of the ultra-high molecular weight polyethylene and the polyethylene copolymer having an α-olefin content of 2 to 8 mol% and / or the low density polyethylene is improved and the moldability is improved. Therefore, breakage is unlikely to occur during film formation, and thinning and high strength can be achieved by high-stretching. Therefore, it became possible to manufacture a film having both a low pore blocking temperature and a required strength. The content of α-olefin is 0.05 mol%
If it is less, the content of α-olefin is 2 to 8 mol.
% Polyethylene copolymer or low density polyethylene has poor compatibility, and therefore does not function as a compatibilizer. On the other hand, if it is 2 mol% or more, the compatibility with the ultra-high molecular weight polyethylene deteriorates and the function as a compatibilizer is again lost. Examples of the α-olefin include propylene, butene-1, isobutene, hexene-1, octene-1, and the like, with propylene being preferred.

The α-olefin content is 0.05 to 2 mo.
The proportion of the polyethylene copolymer which is less than 1% is 10 to 5 in order to sufficiently exert the function as the compatibilizer.
It is 0% by weight, preferably 20 to 40% by weight.
In consideration of moldability, the viscosity average molecular weight of the polyethylene copolymer is preferably 5 to 1,000,000, more preferably 5 to 800,000, and further preferably 5 to 35.
In many cases.

The polyethylene copolymer and low density polyethylene having an α-olefin content of 2 to 8 mol% used in the present invention have a density of 0.930 g / cm 3.
^{It is 3} or less, and among them, a polyethylene copolymer having an α-olefin content of 2 to 8 mol%, and further 2 to 5 mol% is preferable. Also, considering the compatibility between the film strength and the pore blocking temperature, the α-olefin content is 2 to 8 mol%.
The ratio of the polyethylene copolymer and / or the low density polyethylene is 35 to 70% by weight, preferably 35 to 50% by weight. From the viewpoint of molding processability, the melt index is preferably 0.01 to 3 g / 10 min, more preferably 0.01 to 1 g / 10.
It is min.

The low density polyethylene in the present invention is so-called branched polyethylene produced by a high pressure method. The microporous membrane in the present invention,
It is preferable that the thickness is 1 to 50 μm, the porosity is 20 to 80%, the bubble point value is ² to 10 kg / cm ² , the pore closing temperature is 135 ° C. or less, and the puncture strength is 300 g or more, and further strengthening is achieved. Considering the above, the porosity is more preferably 20 to less than 50%. Also, the electric resistance value is 2Ω
· Cm ² or less, the vertical direction as the elastic modulus 5000 kg / cm ² or more, it is preferable as the rupture temperature is 0.99 ° C. or higher.

The microporous membrane in the present invention is produced, for example, by the following steps (a) to (e). (A) From ultra-high molecular weight polyethylene and a polyethylene copolymer having an α-olefin content of 0.05 to less than 2 mol% and a polyethylene copolymer having an α-olefin content of 2 to 8 mol% and / or low density polyethylene A step of granulating the following mixed polyethylene with an organic liquid, an inorganic filler and an additive. (B) A step of melt-kneading the mixture obtained in the step (a) in an extruder having a T-die attached to the tip thereof, and extruding from the T-die to form a sheet. (C) A step of extracting and removing the organic liquid material and the inorganic filler from the sheet-shaped molded product obtained in the step (b). (D) A step of uniaxially or biaxially stretching the molded product obtained by extracting and removing the organic liquid material and the inorganic filler, as it is or as several sheets are stacked.

The manufacturing process of the present invention will be described in more detail.
In the step (a), the ratio of the mixed polyethylene to the total weight of the mixed polyethylene, the organic liquid and the inorganic filler is 10 to 60% by weight, and the total ratio of the organic liquid and the inorganic filler is 40 to 90% by weight. When the proportion of the mixed polyethylene is less than 10% by weight, the strength is low, and when it exceeds 60% by weight, the fluidity at the time of extrusion molding is deteriorated and molding processability becomes difficult. Examples of the organic liquid include esters such as phthalic acid ester and sebacic acid ester, and liquid paraffin. These may be used alone or as a mixture. Examples of the inorganic filler include silica, mica and talc, and these may be used alone or as a mixture.

In addition to polyethylene, organic liquids, and inorganic fillers, various additives such as antioxidants, ultraviolet absorbers, lubricants, antiblocking agents, etc. may be added, if necessary, within a range that does not significantly impair the present invention. You can When biaxial stretching is performed in step (d), either sequential biaxial stretching or simultaneous biaxial stretching may be used. Further, heat treatment such as heat setting may be performed after the stretching treatment, if necessary.

[0015]

Various characteristics of the microporous membrane of the present invention were evaluated by the following test methods. 1. Viscosity average molecular weight: [η] was measured at a measurement temperature of 135 ° C. using a solvent (decalin), and the viscosity average molecular weight (Mv) was calculated by the equation (1). [Η] = 6.8 × 10 ⁻⁴ Mv ^0.67 (1) 2. Melt index: Measured according to JIS K6760. 3. Density: Measured according to ASTM D1238. 4. α-Olefin content: In a given spectrum obtained by ¹³ C-NMR measurement, the α-olefin content (mol%) was determined by the following formula.

Α-olefin content (mol%) =
(A) / [(A) + (B)] × 100 In the formula, (A) is the molar equivalent integrated value of the resonance intensity peak derived from the α-olefin component, and (B) is the resonance intensity peak derived from the ethylene unit. It is a molar conversion integrated value. For example, when the α-olefin is propylene, (A) = (I _{1 ′} + I _m + Iα / 2) / 3 (B) = (I ₁ + I ₂ + I ₃ + I _M + Iα / 2 + Iβ + I
γ) / 2 Here, the lower right subscript of the resonance intensity I represents the position of the carbon atom in the following formula.

[0017]

[Chemical 1]

5. Film thickness: Measured with a dial gauge (specified in JIS) having a minimum scale of 1 μm at a temperature of 23 ± 2 ° C. 6. Porosity: A sample of Xcm × Ycm was cut out and calculated by the equation (2). Porosity (%) = {1− (10 ⁴ × M) / (X × Y × T × ρ)} × 100 (2) In the formula, T: sample thickness μm, M: sample weight g,
ρ; Density / g / cm ^{3 The} density is defined by the equation (3).

1 / ρ = (1 / ρ ₁ ) X ₁ + (1 / ρ ₂ ) X ₂ + (1 / ρ ₃ ) X ₃ (3) In the formula, X ₁ + X ₂ + X ₃ = 1 X ₁ ; weight fraction of ultra-high molecular weight polyethylene X ₂ ; weight fraction of polyethylene copolymer having α-olefin content of 0.05 to less than ₂ mol% X ₃ ; α-olefin content of 2 to 8 mol% Weight fraction ρ _{1 of} polyethylene copolymer or low density polyethylene; Density ρ ₂ of ultra high molecular weight polyethylene; Density ρ ₃ of polyethylene copolymer whose α-olefin content is less than 0.05 to ₂ mol%; α-olefin content A mixed composition of a polyethylene copolymer having an amount of 2 to 8 mol% or a density polyethylene of low density polyethylene has a composition of “ultra high molecular weight polyethylene + polyethylene copolymer having an α-olefin content of 0.05 to less than 2 mol% + α-olefin content”. The amount is 2
~ 8mol% polyethylene copolymer + low-density polyethylene "is 1 / ρ = (1 / ρ ₁ ) X ₁ +
(1 / ρ ₂ ) X ₂ + (1 / ρ ₃ ) X ₃ + (1 / ρ ₄ ) X ₄ where X ₄ ; weight fraction ρ _{4 of} low-density polyethylene; density of low-density polyethylene 7. Bubble point value: The bubble point value in ethanol was measured according to ASTM E-128-61. 8. Electric resistance: LCR meter AG-43 manufactured by Ando Electric Co., Ltd.
11 and the cell shown in FIG. 1 were used to measure with an alternating current of 1 kHz, and the value was calculated by the equation (4).

Electric resistance value (Ω · cm ² ) = (resistance value when film exists−resistance value when film does not exist) × 0.785 (4) Electrolyte solution: propylene carbonate and dimethoxyethane The concentration of lithium perchlorate in the mixed solution (50/50% by volume) was 1 mol / liter. Electrode: platinum black electrode, electrode plate area: 0.785 cm ² , distance between electrodes: 3 mm 9. Puncture strength: A KES-G5 handy compression tester manufactured by Kato Tech Co., Ltd. was attached with a needle having a diameter of 1 mm and a curvature radius of 0.5 mm at a tip, a temperature of 23 ± 2 ° C., and a moving speed of the needle of 0.2 cm. A puncture test was performed at / sec. 10. Elastic modulus: Autograph AG-A manufactured by Shimadzu Corporation
A tensile test was performed on the mold. The elastic modulus was measured on the S-S curve with an inclination of elongation of 1 to 4%.

The measurement conditions are temperature: 23 ± 2 ° C., size of test piece; width 10 mm × length 10 mm, distance between chucks;
50 mm, pulling speed; 200 mm / min. 11. Pore blockage temperature and membrane rupture temperature: FIGS. 2A to 2C are schematic views of a device for measuring the hole blockage temperature and the rupture temperature. Figure 2
(A) is a block diagram of a measuring device. 1 is a microporous membrane, 2A and 2B are Ni foil with a thickness of 10 μm, 3A and 3
B is a glass plate. 4 is an electric resistance measuring device (LCR meter AG-4311 manufactured by Ando Electric Co., Ltd.)
A, 2B). A thermocouple 5 is connected to the thermometer 6. 7 is a data collector,
It is connected to an electric resistance measuring device (4) and a thermometer (6). 8 is an oven for heating the microporous membrane.

More specifically, the microporous membrane (1) is impregnated with a prescribed electrolytic solution, as shown in FIG.
As shown in (4), it is fixed on the Ni foil (2A) in a form fastened with Teflon tape only in the vertical direction (machine direction during film formation). The specified electrolytic solution is a 1 mol / l-lithium borofluoride / propylene carbonate solution. Ni foil (2
As shown in FIG. 2 (C), B) is masked with Teflon tape leaving a portion of 15 mm × 10 mm.
The Ni foil (2A) and the Ni foil (2B) are overlapped with each other so as to sandwich the microporous film (1), and two Ni foils are sandwiched by the glass plates (3A, 3B) from both sides thereof. Two
The glass plates are fixed by sandwiching them with a commercially available clip. The thermocouple 5 is fixed to the glass plate (3B) with Teflon tape.

Using the apparatus shown in FIG. 2A, the temperature and the electric resistance value are continuously measured. The temperature is raised from 25 ° C. to 200 ° C. at a rate of 2 ° C./min, and the electric resistance value is measured with an alternating current of 1 kHz. As shown in FIG. 3, the pore blocking temperature and the membrane rupture temperature are defined as the temperature at which the electric resistance value of the microporous membrane (1) reaches 10 ³ Ω.

[0024]

Example 1 Ultrahigh molecular weight polyethylene 12% by weight, polyethylene copolymer-A 12% by weight, polyethylene copolymer-D 16% by weight, dioctyl phthalate (DOP) 4
After 2.4% by weight and 17.6% by weight of finely divided silica were mixed and granulated, they were kneaded and extruded by a twin-screw extruder equipped with a T die to form a sheet having a thickness of 80 μm. DOP from the molded product
The finely divided silica was extracted and removed to give a microporous membrane. Two microporous membranes were stacked and heated at 118 ° C., and stretched 5 times in the longitudinal direction, and then stretched 1.5 times in the transverse direction. The characteristics of the polyethylene used are shown in Table 1, and the mixed composition of polyethylene is shown in Table 2.
The physical properties of the obtained microporous membrane are shown in Table 3 and FIG.

[0025]

Example 2 Ultrahigh molecular weight polyethylene 4% by weight, polyethylene copolymer-A 20% by weight, polyethylene copolymer-D 16% by weight, DOP 42.4% by weight, fine silica 1
Same as Example 1 except that 7.6 wt% was used. The characteristics of the polyethylene used are shown in Table 1, the mixed composition of polyethylene is shown in Table 2, and the physical properties of the obtained microporous membrane are shown in Table 3.
Shown in.

[0026]

Example 3 Ultrahigh molecular weight polyethylene 4% by weight, polyethylene copolymer-A 8% by weight, polyethylene copolymer-
D28% by weight, DOP42.4% by weight, finely divided silica 1
Same as Example 1 except that 7.6 wt% was used. The characteristics of the polyethylene used are shown in Table 1, the mixed composition of polyethylene is shown in Table 2, and the physical properties of the obtained microporous membrane are shown in Table 3.
3 and FIG. 3- (A).

[0027]

Example 4 Ultrahigh molecular weight polyethylene 12% by weight, polyethylene copolymer-B 12% by weight, polyethylene copolymer-D 16% by weight, dioctyl phthalate (DOP) 4
Same as Example 1 except that 2.4% by weight and 17.6% by weight of finely divided silica were used. The characteristics of the polyethylene used are shown in Table 1, the mixed composition of polyethylene is shown in Table 2, and the physical properties of the obtained microporous membrane are shown in Table 3.

[0028]

Example 5 Ultrahigh molecular weight polyethylene 4% by weight, polyethylene copolymer-C 20% by weight, polyethylene copolymer-D 16% by weight, dioctyl phthalate (DOP) 42.
Same as Example 1 except that 4% by weight and 17.6% by weight of finely divided silica were used. The characteristics of the polyethylene used are shown in Table 1, the mixed composition of polyethylene is shown in Table 2, and the physical properties of the obtained microporous membrane are shown in Table 3.

[0029]

Comparative Example 1 12% by weight of ultra high molecular weight polyethylene, 16% by weight of polyethylene copolymer-A, 12% by weight of polyethylene copolymer-D, 42.4% by weight of DOP, and 17.6% by weight of finely divided silica were used. This is the same as in the first embodiment. The characteristics of the polyethylene used are shown in Table 1, the mixed composition of polyethylene is shown in Table 2, and the physical properties of the obtained microporous membrane are shown in Table 3 and FIG.

[0030]

Comparative Example 2 Ultrahigh molecular weight polyethylene 4% by weight, polyethylene copolymer-A 4% by weight, polyethylene copolymer-
D32% by weight, DOP42.4% by weight, finely divided silica 1
Same as Example 1 except that 7.6 wt% was used. The characteristics of the polyethylene used are shown in Table 1, the mixed composition of polyethylene is shown in Table 2, and the physical properties of the obtained microporous membrane are shown in Table 3.
3 and FIG. 3- (B).

[0031]

Comparative Example 3 Ultrahigh molecular weight polyethylene 4% by weight, polyethylene copolymer-A 24% by weight, polyethylene copolymer-D 12% by weight, DOP 42.4% by weight, fine silica 1
Same as Example 1 except that 7.6 wt% was used. The characteristics of the polyethylene used are shown in Table 1, the mixed composition of polyethylene is shown in Table 2, and the physical properties of the obtained microporous membrane are shown in Table 3.
Shown in.

[0032]

[Comparative Example 4] Ultrahigh molecular weight polyethylene 2% by weight, polyethylene copolymer-A 22% by weight, polyethylene copolymer-D 16% by weight, DOP 42.4% by weight, fine silica 1
Same as Example 1 except using 7.6% by weight.
The characteristics of the polyethylene used are shown in Table 1, the mixed composition of polyethylene is shown in Table 2, and the physical properties of the obtained microporous membrane are shown in Table 3.

[0033]

Comparative Example 5 Ultrahigh molecular weight polyethylene 12% by weight, high density polyethylene 12% by weight, polyethylene copolymer-D
16% by weight, DOP 42.4% by weight, finely divided silica 17.
After 6 wt% was mixed and granulated, it was kneaded and extruded by a twin-screw extruder equipped with a T die to form a sheet having a thickness of 80 μm. DOP and finely divided silica were extracted and removed from the molded product to obtain a microporous membrane. Attempts were made to longitudinally stretch two microporous membranes under various conditions, but rupture occurred, and a membrane having a thickness of 25 μm as obtained in other examples could not be obtained. The characteristics of the polyethylene used are shown in Table 1, and the mixed composition of polyethylene is shown in Table 2.

[0034]

Comparative Example 6 12% by weight of ultra high molecular weight polyethylene, 28% by weight of polyethylene copolymer-D, 42.4% by weight of DOP, and 17.6% by weight of finely divided silica were mixed and granulated, and then twin-screw extrusion equipped with a T-die. Kneading and extruding with a machine 80 μm thick
Was formed into a sheet shape. DOP and finely divided silica were extracted and removed from the molded product to obtain a microporous membrane. Attempts were made to longitudinally stretch two microporous membranes under various conditions, but rupture occurred, and a membrane having a thickness of 25 μm as obtained in other examples could not be obtained. The characteristics of the polyethylene used are shown in Table 1, and the mixed composition of polyethylene is shown in Table 2.

[0035]

[Comparative Example 7] Polyethylene as polyethylene-C 40% by weight, DOP 42.4% by weight, finely divided silica 1
Same as Example 1 except using 7.6% by weight.
The characteristics of the polyethylene used are shown in Table 1, and the physical properties of the obtained microporous membrane are shown in Table 3 and FIG. 3 (B).

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

EFFECTS OF THE INVENTION The microporous membrane of the present invention has a low pore blocking temperature, is less likely to be broken during membrane formation, has required strength, and is suitable for use as an organic solvent battery separator.

[Brief description of drawings]

FIG. 1 is a schematic view of a cell for measuring the electric resistance of a microporous membrane of the present invention.

FIG. 2 (A) is a schematic view of an apparatus for measuring a pore blocking temperature and a membrane rupture temperature. FIG. 2B is a cross-sectional view of the Ni foil (2A) surface of FIG. 2A. FIG. 2C is the same as FIG.
It is sectional drawing in the Ni foil (2B) surface of (A).

FIG. 3A is a measurement diagram of pore blocking temperature and membrane rupture temperature of Examples 1 and 2. FIG. 3B is a measurement diagram of the pore blocking temperature and the membrane rupture temperature in Comparative Examples 1, 2, and 7.

[Explanation of symbols]

1: Microporous membrane 2A: Ni foil 2B: Ni foil 3A: glass plate 3B: Glass plate 4: Electric resistance measuring device 5: Thermocouple 6: Thermometer 7: Data collector 8: oven

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Claims (5)

(57) [Claims] 1. Ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 1,000,000 or more, 10 to 50% by weight, a polyethylene copolymer having an α-olefin content of 0.05 to less than 2 mol%, and an α-olefin content. A microporous membrane, characterized in that it comprises 35 to 70% by weight of polyethylene copolymer and / or low density polyethylene in an amount of 2 to 8 mol%. 2. An α-olefin content of 0.05 to 2 m
The microporous membrane according to claim 1, wherein the α-olefin in the polyethylene copolymer of less than ol% is propylene. 3. A film thickness of 1 to 50 μm and a porosity of 20 to 8
The microporous membrane according to claim 1 or 2, characterized in that the bubble point is 0%, the bubble point is ² to 10 kg / cm ² , the pore closing temperature is 135 ° C or lower, and the puncture strength is 300 g or higher. 4. The microporous membrane according to claim 3, which has a porosity of 20 to less than 50%. 5. The microporous membrane according to claim 1, which is used as an organic solvent battery separator.