JPH09259858A - Polyethylene micro-porous film for separator, and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyethylene micro-porous film having excellent permeability performance and mechanical strength, and a low-temperature shutdown function. SOLUTION: For this porous film, a polyethylene micro-porous film for a separator comprising polyethylene of a weight-averaged molar weight of 5×10<5> or more, or a polyethylene composition of it by 70-90wt.%, and low molar weight polyethylene of a weight-averaged polar weight of 1000-4000 and a fusing point of 80-130 deg.C by 1-30wt.% is blended with solution of a composition comprising above polyethylene or its composition and above low molar weight polyethylene by 10-80wt.%, and solvent by 20-90wt.%. Obtained matter is cooled to form a gelatinized composition, and it is drawn at a temperature of the fusing point of the polyethylene composition plus 10 deg.C or less.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention is used in battery separators, electrolytic capacitor separators, and the like.
Particularly used as a separator corresponding to a non-aqueous solvent battery such as a lithium battery, and relates to a polyethylene microporous membrane for a separator having excellent permeability and mechanical strength and having a function of blocking permeability at low temperature, and a method for producing the same. Is.

[0002]

2. Description of the Related Art Microporous membranes are used in various separation membranes, battery separators, electrolytic capacitor separators and the like. Particularly in a lithium battery, since a lithium metal and lithium ion are used, an aprotic polar organic solvent is used as an electrolytic solution solvent, and a lithium salt is used as an electrolyte. Therefore, for the separator installed between the positive electrode and the negative electrode, a polyolefin-based material such as polyethylene or polypropylene, which is insoluble in an organic solvent and is stable to the electrolyte and the electrode active material, is processed into a microporous membrane or a non-woven fabric. The thing is used as a separator.

Recently, high-strength and high-elasticity microporous membranes have been developed using ultrahigh molecular weight polyolefins.
For example, a gel-like sheet is formed from a solution obtained by heating and dissolving an ultra-high molecular weight polyolefin having a weight average molecular weight of 7 × 10 ⁵ or more in a solvent, the amount of the solvent in the gel-like sheet is adjusted by desolvation treatment, and then heating is performed. A method for producing a microporous membrane by removing the residual solvent after stretching has been proposed (for example, JP-A-60-242035). Further, as a method for producing a polyolefin microporous film from a high-concentration solution of ultra-high molecular weight polyolefin, a method has been proposed in which the molecular weight distribution of the polyolefin composition containing the ultra-high molecular weight polyolefin is set to a specific value (JP-A-3-643).
34).

By the way, when the microporous polyolefin membrane is used in a battery such as a separator for a lithium battery, it is necessary to prevent an accident such as ignition when the temperature inside the battery rises due to a short circuit between electrodes. There is. For this reason, it is necessary for the separator to have a function of melting before lithium is ignited, clogging the holes, and shutting down the current. However, in each of the above microporous membranes, the permeability cutoff temperature due to microporous occlusion is not necessarily sufficiently low in terms of safety, and in order to further improve safety, the current must be shut down at a lower temperature. It is desirable to use a separator for raising. Therefore, it is considered that a battery separator having better high-temperature characteristics and higher safety can be obtained as it has a lower non-porous temperature and a larger difference between the non-porous temperature and the film breaking temperature. For example, in JP-A-63-308866 and JP-A-2-77108, high strength and excellent high temperature characteristics are obtained by laminating a single film made of low melting point polyethylene and high melting point polypropylene. Although a method for obtaining a microporous membrane has been disclosed, because of the lamination, the electric resistance of the separator becomes high, which makes it unsuitable as a separator for high-performance batteries. Further, Japanese Unexamined Patent Application Publication No.
In Japanese Patent Laid-Open Publication No. 77108, since the method of laminated extrusion is adopted, the productivity is inferior in terms of complication of the manufacturing process and manufacturing cost.

[0005]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a microporous membrane for a separator, which is excellent in permeation performance and mechanical strength and has a function of blocking permeation at low temperature, and a method for producing the same. Is.

[0006]

In order to solve the above-mentioned problems, the present invention provides a high molecular weight polyethylene or a composition thereof with a low-molecular weight polyethylene to provide excellent permeation performance and mechanical strength, and at low temperature. It was found that a microporous membrane that blocks permeability is obtained, and the present invention has been accomplished.

That is, the polyethylene microporous membrane for a separator of the present invention is a polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more or a polyethylene composition thereof 70 to 99.
It is characterized by being composed of 1 to 30% by weight of low molecular weight polyethylene having a weight percentage of 1000 to 4000 and a melting point of 80 to 130 ° C.

The method for producing a polyethylene microporous membrane for a separator of the present invention has a weight average molecular weight of 5 × 10 ^5.
Polyethylene or polyethylene composition 70 thereof
~ 99 wt% and weight average molecular weight of 1000-4000
And low molecular weight polyethylene having a melting point of 80 to 130 ° C.
10 to 80% by weight of a composition comprising 30% by weight, and a solvent 2
0 to 90% by weight of a solution is prepared, the solution is extruded from a die, cooled to form a gel composition, and the gel composition is stretched at a temperature not higher than the melting point of the polyethylene composition + 10 ° C. After that, the residual solvent is removed.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, polyethylene is
Weight average molecular weight is 5 × 10 ⁵ or more, preferably 1 × 10 5.
⁶ to 15 × 10 ⁶ . Weight average molecular weight is 5 ×
When it is less than 10 ⁵ , the maximum draw ratio is low in the drawing step during the production of the microporous film, and the desired microporous film cannot be obtained. On the other hand, the upper limit is not particularly limited, but if it exceeds 15 × 10 ⁶ , the moldability is poor in the formation of a gel-like molded product during the production of the microporous membrane.

Further, in the present invention, in order to increase the concentration of the polyolefin solution described later and to improve the strength of the microporous membrane, an ultrahigh molecular weight polyethylene having a weight average molecular weight of 1 × 10 ⁶ or more and a weight average molecular weight of 1 × are used. A composition with 10 ⁵ or more and less than 1 × 10 ⁶ polyethylene can be used. The content of ultra high molecular weight polyethylene in the polyethylene composition is preferably 1% by weight or more, more preferably 10 to 70% by weight, based on 100% by weight of the total polyethylene composition.
It is.

Further, the weight average molecular weight / number average molecular weight used as a measure of the molecular weight distribution of the polyethylene or the polyethylene composition thereof is 300 or less, preferably 5 to 50.

The low molecular weight polyethylene used in the present invention has a molecular weight of 1,000 to 4,000 and a melting point of 80 to 130.
C., an ethylene low polymer having a density of 0.92 to 0.9
A polyethylene wax of 7 g / cm ³ is preferred. Polyethylene wax is produced from ethylene by a low pressure direct polymerization process.

By adding this polyethylene wax to polyethylene or a polyethylene composition thereof, a polyethylene microporous membrane is used as a separator for a lithium battery or the like, and when the electrodes short-circuit and the temperature inside the battery rises, it shuts down at a low temperature. It is considered that the function of increasing the viscosity of the electrolytic solution is added at the same time as the application of. Therefore, if the molecular weight of the low-molecular-weight polyethylene is out of the range, the low-temperature shutdown effect cannot be obtained. The amount of low molecular weight polyethylene is 100% by weight of the total of polyethylene or its composition and low molecular weight polyethylene,
It is 1 to 30% by weight, preferably 5 to 25% by weight. 1
If the content is less than 5% by weight, the low temperature shutdown effect cannot be obtained and 3
When it is 0% by weight or more, the strength of the obtained polyethylene microporous membrane is significantly impaired.

The polyethylene microporous membrane of the present invention can be obtained by mixing an organic liquid or a solid with a resin component obtained by adding low molecular weight polyethylene to polyethylene, melt-kneading the mixture, and then extruding, stretching and extracting. Further, inorganic fine powder may be added to the mixture of the resin component and the organic liquid or solid without any problem. As a preferred method for obtaining the polyethylene microporous membrane of the present invention, a good solvent of polyethylene is supplied to the polyethylene composition to prepare a solution of the polyethylene composition, and the solution is extruded into a sheet form from a die of an extruder, This is a method in which the gel composition is cooled to form a gel composition, the gel composition is heated and stretched, and then the remaining solvent is removed.

In the present invention, a solution of a polyethylene composition as a raw material is prepared by heating and dissolving the above-mentioned polyethylene or polyethylene composition in a solvent.
The solvent is not particularly limited as long as it can sufficiently dissolve polyethylene. Examples thereof include nonane, decane, undecane, dodecane, liquid paraffin and other aliphatic or cyclic hydrocarbons, or mineral oil fractions having a boiling point corresponding to these, etc. In order to obtain it, a non-volatile solvent such as liquid paraffin is preferable. The heating and dissolution are carried out while vigorously stirring or kneading with an extruder at a temperature at which polyethylene is completely dissolved. The temperature is preferably in the range of 140 to 250 ° C, for example. The concentration of polyethylene solution is 10
-80% by weight, preferably 10-55% by weight. If the concentration is less than 10% by weight, not only is the amount of solvent used large and it is not economical, but also when forming into a sheet, swell and neck-in are large at the die outlet, making it difficult to form the sheet. In addition, at the time of heating and dissolving, it is preferable to add an antioxidant to prevent oxidation of polyethylene.

Next, the heated solution of polyethylene is preferably extruded from a die to be molded. As the die, a sheet die having a rectangular mouthpiece shape is usually used,
A double cylindrical inflation die or the like can also be used. When a sheet die is used, the die gap is usually 0.15 to 5 mm, and the extrusion molding temperature is 1
It is 40-250 degreeC. At this time, the extrusion speed is usually 2
It is 0 to 30 cm / min to 2 to 3 m / min.

The solution thus extruded from the die is cooled to form a gel composition.
Cooling is preferably performed at a rate of 50 ° C./min or more up to at least the gelation temperature. Generally, if the cooling rate is slow,
The higher-order structure of the obtained gel-like composition becomes coarse and the pseudo-cell units forming it become large, but when the cooling rate is fast, it becomes dense cell units. When the cooling rate is less than 50 ° C./minute, the crystallinity increases and it is difficult to obtain a gel composition suitable for stretching. As a cooling method, a method of directly contacting with cold air, cooling water, or other cooling medium, a method of contacting with a roll cooled with a refrigerant, or the like can be used. The solution extruded from the die may be taken up at a take-up ratio of preferably 1 to 10, more preferably 1 to 5, before or during cooling. When the take-up ratio is 10 or more, neck-in becomes large, and breakage easily occurs during stretching, which is not preferable.

Next, this gel-like molded product is stretched. The stretching is performed by heating the gel-like molded product and using a normal tenter method, roll method, inflation method, rolling method or a combination of these methods at a predetermined magnification. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. Further, in the case of biaxial stretching, either vertical and horizontal simultaneous stretching or sequential stretching may be used. Stretching temperature is the melting point of polyethylene +10
C. or less, preferably in the range from the crystal dispersion temperature of polyethylene to less than the crystal melting point. The stretching ratio varies depending on the thickness of the raw fabric, but in uniaxial stretching, it is preferably 2 times or more, more preferably 3 to 30 times. In biaxial stretching, the surface magnification is preferably 10 times or more, more preferably 15 to 400 times. If the surface magnification is less than 10 times, the stretching is insufficient and a highly elastic and high-strength microporous membrane cannot be obtained. On the other hand, the surface magnification is 4
If it exceeds 00 times, restrictions will occur in the stretching operation and the like.

The obtained stretch molded product is washed with a solvent to remove the remaining solvent. Examples of the cleaning solvent include hydrocarbons such as pentane, hexane, and heptane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; fluorinated hydrocarbons such as ethane trifluoride; and ethers such as diethyl ether and dioxane. Volatile ones can be used.
These solvents are appropriately selected according to the solvent used for dissolving the polyethylene composition, and used alone or as a mixture.
The washing method can be performed by a method of immersing in a solvent for extraction, a method of showering the solvent, a method of a combination thereof, or the like.

The above-mentioned washing is carried out until the residual solvent in the stretch-molded product is less than 1% by weight. Thereafter, the washing solvent is dried, and the washing solvent can be dried by a method such as heat drying or air drying. It is desirable that the dried stretch molded product is heat-set at a temperature in the range of the crystal dispersion temperature to the melting point.

The polyethylene microporous membrane produced as described above has a porosity of 35 to 95% and an average through-pore diameter of 0.
001-0.5μ, and breaking strength is 500 kg / cm
² or more. The thickness of the polyethylene microporous membrane of the present invention may be appropriately selected depending on the application, but is generally 0.1 to
It can be 100 μ, and preferably 2 to 50 μ.

The polyethylene microporous membrane obtained was
If necessary, further plasma irradiation, surfactant impregnation,
Surface modification such as hydrophilization treatment such as surface grafting can be performed.

[0023]

EXAMPLES The present invention will be described below in more detail with reference to examples, but the present invention is not particularly limited to the examples. In addition, the test method in an Example is as follows. (1) Film thickness: The cross section was measured with a scanning electron microscope. (2) Breaking strength: The breaking strength of a 15 mm wide strip test piece was measured according to ASTM D882. (3) Air permeability: Measured according to JIS P8117. (4) Shutdown temperature: A temperature at which the air permeability becomes 100,000 sec / 100 cc or more when heated to a predetermined temperature. (5) Meltdown temperature: The temperature at which the film melts and ruptures when heated to a predetermined temperature. Example 1 20% by weight of ultra high molecular weight polyethylene having a weight average molecular weight of 2 × 10 ⁶ and high density polyethylene 80 having a weight average molecular weight of 3.3 × 10 ⁵
A polyethylene composition was obtained in which 0.375 parts by weight of an antioxidant was added to 100 parts by weight of the polyethylene composition of wt%.
30 parts by weight of this polyethylene composition and 5 parts by weight of polyethylene wax (Mitsui High Wax-100P; melting point 115 ° C., molecular weight 1000; manufactured by Mitsui Petrochemical) are twin-screw extruder (58 mmφ, L / D = 42, strong kneading type). I put it in. Further, 70 parts by weight of liquid paraffin was supplied from the side feeder of this twin-screw extruder and melt-kneaded at 200 rpm to prepare a polyethylene solution in the extruder.

Subsequently, the T installed at the tip of the extruder
Extrude from the die at 190 ° C and collect with a cooling roll.
To form a gel-like sheet. Then this gel sheet
Was simultaneously biaxially stretched to 5 × 5 at 115 ° C. to obtain a stretched film.
Obtained. The stretched film obtained is washed with methylene chloride and left
After extruding and removing the liquid paraffin, dry and heat treatment
A polyethylene microporous membrane was obtained. This polyethylene fine
The results of physical property evaluation of the porous film are shown in Table 1.Example 2 In Example 1, 10 parts by weight of polyethylene wax
A microporous membrane was obtained in the same manner as in Example 1 except that it was used.
The microporous membrane obtained as described above has the physical properties shown in Table 1.
I wasExample 3 In Example 1, the melting point of polyethylene wax is 1
Mitsui High Wax 200P with a temperature of 00 ° C and a molecular weight of 2000
A microporous membrane was prepared in the same manner as in Example 1 except that 5 parts by weight was used.
Obtained. The microporous membrane obtained as described above has the physical properties shown in Table 1.
Had. Example 4 In Example 1, the melting point of polyethylene wax is 1
Mitsui High Wax 400P with a molecular weight of 4000 at 25 ° C
A microporous membrane was prepared in the same manner as in Example 1 except that 5 parts by weight was used.
Obtained. The microporous membrane obtained as described above has the physical properties shown in Table 1.
Had. Example 5 In Example 1, polyethylene has a weight average molecular weight of
1 × 10⁶ 30 parts by weight of ultra high molecular weight polyethylene
A microporous membrane was obtained in the same manner as in Example 1 except for the above. More than
The microporous membrane thus obtained has the physical properties shown in Table 1.
Was.Example 6 In Example 1, polyethylene has a weight average molecular weight of
5 × 10^Five 50 parts by weight of ultra high molecular weight polyethylene
A microporous membrane was obtained in the same manner as in Example 1 except for the above. More than
The microporous membrane thus obtained has the physical properties shown in Table 1.
Was.Comparative Example 1 In Example 1, the polyethylene wax was not added.
A microporous membrane was obtained in the same manner as in Example 1 except for the above. As above
The microporous membrane thus obtained had the physical properties shown in Table 2.Comparative Examples 2 and 3 In Example 1, the amount of polyethylene wax was 20
Same as Example 1 except that the amount is 0.1 part by weight or 0.1 part by weight.
A microporous membrane was obtained. Microporosity obtained as described above
The film had the physical properties shown in Table 2.Comparative Example 4 Example 1 has a melting point of 132 ° C. and a molecular weight of 12,000.
Example 1 except that 5 parts by weight of polyethylene wax was used.
A microporous membrane was obtained in the same manner. Obtained as above
The microporous membrane had the physical properties shown in Table 2.

[0025]

[Table 1]

[0026]

[Table 2] As is clear from Tables 1 and 2, the polyethylene microporous membrane of the present invention according to the methods of Examples 1 to 6 has high breaking strength and low shutdown temperature, and is used as a battery separator for lithium batteries. The safety of is high.

[0027]

Since the polyethylene microporous membrane of the present invention has high strength and low shutdown characteristics, it is highly reliable in terms of safety when used as a separator for lithium batteries.

Claims (2)

[Claims] 1. Polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more or 70 to 99% by weight of a polyethylene composition thereof, a weight average molecular weight of 1000 to 4000 and a melting point of 8
A polyethylene microporous membrane for a separator, which comprises 1 to 30% by weight of low molecular weight polyethylene at 0 to 130 ° C. 2. Polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more or 70 to 99% by weight of a polyethylene composition thereof, low molecular weight polyethylene 1 to 30 having a weight average molecular weight of 1000 to 4000 and a melting point of 80 to 130 ° C. weight%
A composition comprising 10 to 80% by weight of a composition comprising 20 to 90% by weight of a solvent is prepared, the solution is extruded through a die and cooled to form a gel composition, and the gel composition is formed into polyethylene. A method for producing a polyethylene microporous membrane for a separator, which comprises stretching at a temperature not higher than the melting point of the composition + 10 ° C., and then removing the residual solvent.