JP2001002813A - Porous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To impart excellent air permeability, liquid permeability, osmosis and good handling properties by using an olefin resin and specifying the meandering rate W and the firmness G. SOLUTION: This porous film has a meandering rate W of 0.10-0.30 and a firmness G of 20 mm or more. This porous film preferably has a permeation parameter, which shows a permeability of a liquid, P of 2.5 or more, a pore rate of 0.30-0.95, a peak pore diameter of 0.05-0.5 μm, and an air permeability of 10-1000 sea/100 cc and a thickness of 10-50 μm. Although no specific limitation is imposed on the method to obtain the porous film, it is preferred that a resin composition containing a high molecular weight polyolefin resin and a plasticizer is melt-extruded into a film, which is cooled, stretched to remove the plasticizer in the stretched film and then stretched again or subjected to a heat treatment. 10-50 wt.% of the polyolefin and 90-50 wt.% of the plasticizer are preferably mixed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous film having good air permeability and liquid permeability and good handling.

[0002]

2. Description of the Related Art Hitherto, porous films have been widely used in various applications such as battery separators, microfiltration membranes, electrolytic capacitors, and various filters, and various proposals have been made on methods for producing such porous films. For example,
A method in which a film is once produced by melt extrusion from a resin composition containing polyethylene and a plasticizer, and after stretching, the plasticizer contained in the film is dissolved and removed with an organic solvent (Japanese Patent Publication No. 6-21177). A method of preparing a polyethylene solution, adjusting the solvent content in the solution, removing the remaining solvent after stretching (Japanese Patent Publication No. 5-54495), forming a film from a composition comprising polyethylene, an organic liquid, and inorganic fine powder Thereafter, a method of extracting and removing the organic liquid and the inorganic fine powder from the film (Japanese Patent Publication No. 60-23130) is known.

[0003]

If the porosity is increased in order to improve the air permeability and liquid permeability of the porous film obtained by the above-mentioned known technique, the mechanical strength of the film decreases, The so-called stiffness of the film is eliminated, and its handling and running properties are significantly impaired. For example, when a porous film is used for a separator for a non-aqueous electrolyte battery, if the film is not stiff, wrinkles are likely to occur and the production yield is significantly reduced. In recent years, for the purpose of improving the performance and production efficiency of batteries, there has been an increasing demand for a porous film having an improved permeability of an electrolyte solution. Inadequate in balancing both.

[0004]

Means for Solving the Problems The inventors of the present invention have intensively studied the improvement of the performance of a porous film by paying attention to the above-mentioned points, and as a result, the tortuosity W representing the porous structure of a polyolefin-based porous film has its permeability. The present invention has been found to be closely related to properties such as gas permeability, liquid permeability, permeability, etc., and furthermore, has made technical improvements to improve various properties of the film in a well-balanced manner so that it can be used as a battery separator. Was completed. That is, the porous film of the present invention is a porous film composed of an olefin resin, the bending ratio W of which is 0.10 to 0.30, and whose stiffness G is 20 mm or more. A porous film and a battery separator comprising the same.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. The bend rate W defined in the porous film of the present invention represents an index of how much the bend is formed when the holes in the film are considered to be virtual tubes. This bending ratio W is calculated by the following equation.

[0006]

## EQU1 ## Bending rate W = air permeability Pa × porosity Pv × peak pore diameter dp / film thickness t

The bending ratio W of the porous film of the present invention
Is 0.10 to 0.30, preferably 0.13 to 0.27
It is. When the bending ratio W is smaller than 0.10, for example, when used as a battery separator, short-circuiting occurs with a high probability, which has a disadvantage of adversely affecting the productivity of the battery, and is larger than 0.30. For example, when used as a battery separator, there is a problem that the permeability of the electrolyte is poor, the productivity of the battery is deteriorated, and sufficient battery performance cannot be obtained.

[0008] The porosity Pv of the porous film of the present invention having such a bending ratio W is usually 0.40 to 0.95, preferably 0.45 to 0.70, and the peak pore diameter dp is usually 0. 05 to 0.5 μm, preferably 0.06 to 0.1
5 μm, and the air permeability Pa is usually 10 to 100
0 sec / 100 cc, preferably 30 to 500 sec / 100
cc, and the thickness of the film is usually set in the range of 5 to 100 μm, preferably 10 to 50 μm. Further, the thickness of the porous film is usually 10 to 50 μm, preferably 15 to 40 μm. If the thickness is smaller than 10 μm, it is not preferable because the positive electrode and the negative electrode may be separated from each other as a separator inside the battery to prevent short circuit.
On the other hand, if it is larger than 50 μm, the volume occupation ratio of the film inside the battery becomes too high, and the energy density per unit volume becomes small.

On the other hand, in the porous film of the present invention, the rigidity G representing the handling property of the film is 20 m.
It is also characterized by a range of m or more. The stiffness G is an index indicating the stiffness of the film measured according to JIS L1085, but is preferably 25 mm or more, more preferably 30 mm or more. If the softness G is less than 20 mm, the film has no stiffness, and handling becomes extremely difficult, for example, when the film is roll-passed. In addition, the film is likely to be wrinkled, and the production yield is significantly reduced. The porous film of the present invention has good mechanical strength and a tensile strength of usually 300 kg / cm ² or more, preferably 350 kg / cm ² or more.
/ Cm ² or more.

Further, in a preferred embodiment of the porous film of the present invention, the permeation parameter P indicating the permeability of the liquid to the film is 2.5 or more. Here, the permeation parameter P is an index representing the permeability of the liquid into the film, which can be considered from three viewpoints of the internal structure of the film, the surface structure of the film, and the compatibility between the material of the film and the liquid. The permeation parameter P is calculated using the porosity Pv, the contact angle θ and the pore diameter r obtained from the immersion heat of the test liquid and the film.

[0011]

## EQU2 ## Calculated as P = Pvcos θ / r. This θ needs to be calculated from immersion heat measurement in order to measure the contact angle between the porous body and the liquid without being affected by the roughness of the porous body. cosθ is determined according to the method of David A. Spagnolo et al. (J. Phys. Chem., vol. 100, No. 16, 6626 (1996)).

[0012]

## EQU3 ## It is determined by cos θ = (− 0.07T-hi) / γlv. Here, T is the absolute temperature (K), hi is the heat of immersion between the test liquid and the film (mJ / m ² · K), γlv
Is the surface tension of the liquid (mJ / m ² ). The heat of immersion per weight is measured by Isothe by Calorimetry Sciences Corporation.
Using a rmal Microcalorimeter (IMC), propylene carbonate is used as a test liquid, and the heat of immersion with the film is measured at 25 ° C. by a flow method. As test conditions, the film is 0.5 g, and the flow rate of the test liquid is 0.1 ml.
/ Min. The specific surface area of the film is determined by the BET method (nitrogen, one-point method), and the immersion heat per weight is converted into the heat per area. Further, the surface tension γlv of the test liquid is actually measured, and cosθ is calculated. Separately, the porosity Pv (no unit) is determined by a gravimetric method, the pore diameter r (unit: μm) is measured by a porometer, and the permeation parameter P is finally calculated.

The method for obtaining the porous film of the present invention having the above physical properties is not particularly limited, but preferably, a resin composition containing a high molecular weight polyolefin resin and a plasticizer is formed on the film. It is a method in which the film is stretched after melt extrusion molding, cooled, and after removing the plasticizer contained in the stretched film, the film is stretched again or heat-treated. In some cases, the stretching process before removing the plasticizer can be omitted. Then, in the method, in order to obtain a porous film having a specific range of the bending ratio W specified in the present invention, depending on the type and amount of the raw material polyolefin and the plasticizer, the film thickness, etc.
It is necessary to set optimal stretching conditions.

The polyolefin constituting the porous film of the present invention has a viscosity average molecular weight of usually 300,000 to 25.
It is preferable to use about 100,000 polyolefin resin, and particularly to use polyethylene resin. If the molecular weight is less than 300,000, it is difficult to obtain sufficient strength in applications requiring strength. In addition, such a viscosity average molecular weight is ASTM D402.
It is measured according to 0. The polyolefin resin may be a single type, or the polyolefin resin may have a molecular weight adjusted after blending within the above range. If necessary, a low molecular weight polyethylene wax having a molecular weight of about 1,000 to 5,000 may be added up to 50% by weight based on polyethylene.

On the other hand, as a plasticizer, considering the compatibility with the above-mentioned polyolefin, it has a melting point lower than the melting point of the polyolefin and a boiling point higher than the melting temperature of the polyolefin, and is suitable for a polyolefin-insoluble organic solvent. A soluble substance is preferably used. Taking into account the simplicity of raw material handling at the time of extrusion molding, a normal temperature solid plasticizer is suitably used as the plasticizer. For example, stearyl alcohol, higher aliphatic alcohols such as ceryl alcohol, n-decane, n-alkanes such as n-dodecane,
Examples include paraffin wax, liquid paraffin, and kerosene. The use ratio of the polyolefin and the plasticizer depends on the difference in the porous structure of the target molded product, but is usually 5 to 60% by weight of the polyolefin and 95 to 40% by weight of the plasticizer. The plasticizer is selected from the range of 90 to 50% by weight at 10 to 50% by weight.
Further, a stabilizer such as an antioxidant may be added up to a range of up to about 5% by weight.

Usually, such a raw material composition is uniformly kneaded with a known single-screw or twin-screw extruder and melt-extruded. As the extruder, a twin-screw extruder is preferably used in terms of the extrusion amount, extrusion stability, and kneading strength. Extrusion molding
It is usually carried out at a temperature of 140 to 300 ° C., and is usually 10 μm to 1
A raw film having a thickness of mm is formed. The thickness of the film can be arbitrarily set in consideration of a thickness change in a later step and a thickness to be finally obtained.

The obtained raw film is stretched uniaxially or biaxially. For stretching, any known stretching apparatus such as a roll stretching machine and a tenter can be used. Regarding uniaxial stretching, either longitudinal stretching or transverse stretching can be selected. As for the biaxial stretching, both sequential biaxial stretching and simultaneous biaxial stretching are possible. As a particularly preferred method, for example, when the area stretching ratio is 3 to 10 times in biaxial stretching and the stretching temperature is in the range from the crystal dispersion temperature of the polyolefin resin to the melting point + 10 ° C., the plasticizer is formed by removing the plasticizer in the subsequent step. This is desirable because the uniformity of the holes formed tends to be high. However, in some cases, such a stretching treatment step can be omitted by devising conditions of the stretching treatment after removing the plasticizer.

Next, after cooling, the stretched film is made porous by removing the plasticizer. As a method of removing the plasticizer, for example, the plasticizer in the film is isopropanol,
A so-called known organic solvent extraction method of dissolving in an organic solvent such as ethanol or hexane and extracting and removing the same by solvent replacement is exemplified.

It is desirable to carry out a stretching treatment on the porous film from which the plasticizer has been removed as described above. This not only provides thermal dimensional stability, but also allows the pore diameter and porosity to be adjusted within a large range, thereby further improving immersion properties and the like. The stretching treatment can be performed by any known method such as contact heating with a heating roll and heating in air in an oven. It is also possible to divert the above-mentioned stretching apparatus. In the stretching treatment, the stretching may be performed within a range that does not impair the physical properties of the final porous film. As a stretching method, not only the above-mentioned active stretching in the longitudinal direction and the transverse direction, but also the so-called heat treatment, in which the stretching effect is imparted by restricting the flow direction and the width direction and inhibiting the heat shrinkage of the film, that is, the so-called negative heat treatment. Stretching can also be used. In order not to impair the physical properties of the final porous film and to maintain the handleability of the film, the deformation operation at the time of heat treatment is performed at a temperature not lower than the melting point of the polyolefin resin −10 ° C. and not higher than the melting point, and the preferable area magnification is usually 2 It is suitably at least twice, preferably 5 to 10 times. If the deformation at the time of heat treatment exceeds 10 times, the bending ratio W of the film becomes large, and the desired permeability may not be obtained. If the ratio is less than twice, it is difficult to maintain the handling property of the film.

In order to obtain a porous film having a bending ratio W and a softness / softness G within the range of the present invention, it is necessary to optimize the stretching sequence, stretching ratio, stretching temperature and the like according to the raw material composition. However, in general, as the stretching conditions, it is preferable that the area stretching ratio is high to some extent and the stretching ratio in the vertical and horizontal directions is balanced to some extent. Usually, the total area stretching ratio is 2 to 30 times, and the vertical and horizontal stretching ratio is 0. A range of about 12 to 8 is desirable.

[0021]

EXAMPLES The present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited to the following examples unless it exceeds the gist of the present invention. In addition,
The test method in the examples is as follows. -The viscosity average molecular weight is based on ASTM D4020. -Air permeability Pa conforms to JIS P8117 (unit: sec / 100 cc). -Porosity Pv was measured by the gravimetric method. -The peak pore diameter dp is measured with a porometer manufactured by Coulter (unit: μm).・ The thickness t is measured with a Teklock film thickness meter (unit: μ
m)-Bending softness G is measured by the 45-degree cantilever method according to JIS L1085. -Tensile strength is the breaking strength in the MD direction (unit: kg / cm ² ) measured at a crosshead speed of 200 mm / min on a strip film having a length of 15 mm and a length of 40 mm.

Example 1 8 parts by weight of polyethylene having a viscosity-average molecular weight of 500,000, 16 parts by weight of polyethylene having a viscosity-average molecular weight of 1,000,000 (the viscosity-average molecular weight of the mixed composition of both becomes about 800,000), and paraffin wax ( The mixture having an average molecular weight of 389) and 76 parts by weight was subjected to an extrusion temperature of 1 using a 40 mmφ twin screw extruder.
The mixture was extruded at 70 ° C. at an extrusion rate of 10 kg / h, and a raw film was formed by an inflation method. The obtained raw film was stretched 2.5 times in the machine direction at 40 ° C. using a roll stretching machine, and then stretched 8 times in the transverse direction at 110 ° C. by a tenter stretching machine. The stretched film was immersed in isopropanol at 60 ° C. to extract and remove paraffin wax. Next, the film was processed using a roll stretching machine to obtain 115
Heat setting was performed at a temperature of ° C. At the time of heat setting, the roll speed ratio was adjusted so that the longitudinal stretching ratio was 2.5 times. Table 1 shows the results of evaluating the physical properties of the obtained porous film.

Next, a battery was assembled as follows using this porous film as a separator. The positive electrode is
The following substances were mixed to prepare a mixture for a positive electrode, applied to a 200 μm aluminum foil as a current collector with a thickness of 100 μm, dried, and then cut into a battery size. Raw material of positive electrode mixture LiCoO ₂ (specific surface area: 0.4 m ² / g) 85 parts by weight Carbon (specific surface area: 250 m ² / g) 10 parts by weight Polyvinylidene fluoride 5 parts by weight N-methyl-2-pyrrolidone 40 parts by weight The following materials are mixed to create a mixture for the negative electrode,
It was applied to a 10 μm copper foil as a current collector with a thickness of 100 μm, dried, and then cut into a battery size. Raw material of negative electrode mixture Carbon (specific surface area: 8 m ² / g) 85 parts by weight Polyvinylidene fluoride 15 parts by weight N-methyl-2-pyrrolidone 50 parts by weight

The non-aqueous electrolyte was prepared as follows. 1 mol of LiPF ₆ was added to a solvent obtained by mixing propylene carbonate and ethyl methyl carbonate at a volume ratio of 1: 1.
/ L dissolve. A separator was sandwiched between the obtained positive electrode and negative electrode, placed in a battery can, impregnated with the electrolyte, and then the lid was welded to complete the battery. After repeating charge and discharge for 10 cycles with this battery, the discharge capacity was measured under the conditions of 1C and 2C. The numerical value obtained by dividing the obtained numerical value by 2C / 1C is 2C / 1.
The discharge capacity ratio was C. Table 1 shows the obtained results.

Example 2 A raw film was formed from the same mixture as in Example 1 by a T-die method. The obtained raw film was immersed in ethanol at 60 ° C. to extract and remove paraffin wax. The film was rolled using a roll stretching machine at a temperature of 115 ° C. and
The film was stretched 8.0 times in a tenter at a temperature of 125 ° C. Table 1 shows the physical properties of the obtained porous film and the results of evaluating the film as a battery separator in the same manner as in Example 1.

Comparative Example 1 A raw film was formed in the same manner as in Example 2. The raw film was immersed in ethanol at 60 ° C. to extract and remove paraffin wax. The film was stretched 2.3 times at a temperature of 120 ° C. using a roll stretching machine and 5.4 times at a temperature of 127 ° C. using a tenter. Table 1 shows the physical properties of the obtained porous film and the results of evaluating the film as a battery separator in the same manner as in Example 1.

Comparative Example 2 An original film was formed in the same manner as in Example 2. The raw film was immersed in ethanol at 60 ° C. to extract and remove paraffin wax. The film was stretched 4.5 times at a temperature of 120 ° C. using a roll stretching machine and 7.0 times at a temperature of 120 ° C. using a tenter. Table 1 shows the results of evaluating the physical properties of the obtained porous film.
In addition, the film was evaluated as a battery separator in the same manner as in Example 1. However, this was stopped because the film could not be easily wound up and many wrinkles occurred.

[0028]

[Table 1]

[0029]

As described in detail above, the porous film of the present invention has excellent air permeability, liquid permeability or permeability, good handling properties, and excellent productivity. I have. Therefore, the porous film of the present invention,
It can be applied to various applications such as battery separators, microfiltration membranes, electrolytic capacitor membranes, and various filters.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Yasushi Yasushi 3-10 Ushidori, Kurashiki City, Okayama Prefecture Inside Mitsubishi Chemical Mizushima Plant (72) Inventor Kyosuke Watanabe 3-10, Shiodori, Kurashiki City, Okayama Prefecture Inside Mitsubishi Chemical's Mizushima Plant (72) Inventor Hideki Kiritani 3-chome Uchidori, Kurashiki-shi, Okayama Prefecture Mitsubishi Chemical Corporation Mizushima Plant (72) Inventor Miki Suzaki 3-chome, Shiodori, Kurashiki-shi, Okayama Prefecture Mitsubishi Chemical Mizushima Plant Co., Ltd.F-term (reference)

Claims (8)

[Claims] 1. A porous film composed of an olefin resin, having a bending ratio W of 0.10 to 0.30 and a softness G of 20 mm or more. Porous film. 2. A permeation parameter P representing the permeability of a liquid.
Is 2.5 or more. 3. The porous film according to claim 1, wherein the porosity is 0.30 to 0.95. 4. The porous film according to claim 1, which has a peak pore size of 0.05 to 0.5 μm. 5. An air permeability of 10 to 1000 seconds / 100 cc
The porous film according to any one of claims 1 to 4, wherein 6. The method according to claim 1, wherein the thickness is 10 to 50 μm.
5. The porous film according to any one of 5. 7. A battery separator comprising the porous film according to claim 1. 8. A battery comprising the battery separator according to claim 7.