JPH10204197A - Porous membrane, battery separator made thereof and production of the membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous membrane having high air permeability and high mechanical strengths. SOLUTION: A nonporous film formed from a crystalline polymer is pre- stretched to a degree of reduction of thickness of 5-35% at a temperature from (Tm-50 deg.C) to Tm deg.C, while it is kept in the nonporous state. The film is then heat-treated and stretched to form a porous membrane. Preferably, the above stretching is carried out in two stages one of which consists of stretching at a temperature as low as -20 to 80 deg.C and the other of which consists of stretching at a temperature as high as (Tm-40) deg.C to Tm deg.C. It is desirable that the crystalline polymer is at least either of polyethylene and polypropylene. The porous membrane should have a porosity of 30-95% and have a tensile strength of 2,050kg/cm<2> or above in at least one direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a porous membrane, a battery separator using the same, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, various techniques have been known for producing a porous film by forming fine holes penetrating both front and back surfaces of a film (precursor film) formed from a crystalline polymer. For example, according to the technology described in Japanese Patent Publication No. 57-47017 or JP-A-7-216118, a porous film having high tensile strength can be produced.
Japanese Patent Application Laid-Open No. Hei 2-276833 describes a technique for improving the strength-to-width ratio of the strength of a precursor film before it is made porous.

[0003]

However, in various fields using a porous membrane, a porous membrane having higher mechanical strength such as tensile strength has been demanded.

[0004] For example, one of the uses of a porous membrane is a battery separator, and a battery separator used for a cylindrical battery is wound together with an electrode when a battery is manufactured. In this winding, if a high-strength separator is used, the winding speed can be increased, which leads to an improvement in productivity. In addition, if the mechanical strength of the battery separator is high, it is possible to prevent internal short circuit of the battery, and in terms of battery safety,
The mechanical strength of the porous membrane is important.

[0005] In addition, porous membranes are used for separation membranes, breathable films for clothing, and the like. In these applications, durability is important, and improvement in mechanical strength such as tensile strength is required. Required.

In response to such a demand, the above-mentioned Japanese Patent Publication No.
The techniques described in JP-A-47017 and JP-A-7-216118 cannot produce a porous membrane having sufficient mechanical strength. In addition, Japanese Patent Laid-Open No. 2-27
Japanese Patent No. 6833 does not describe in detail the problem of improving the mechanical strength of the porous film itself.

Accordingly, an object of the present invention is to provide a porous membrane having sufficiently high mechanical strength such as tensile strength, a battery separator using the same, and a method for producing the same.

[0008]

In order to achieve the above object, a porous membrane of the present invention is formed from a crystalline polymer, has a porosity in the range of 30 to 95%, and has a tensile strength in at least one direction. The strength is 2050 kg / cm ² or more.

That is, it is formed from a crystalline polymer,
When the porosity is in the range of 30 to 95% and the tensile strength in at least one direction is 2050 kg / cm ² or more, a porous film having high mechanical strength that can meet the requirements of various fields is obtained.

The battery separator of the present invention comprises the porous membrane of the present invention. As described above, since the porous membrane of the present invention has sufficient mechanical strength, if this is applied to a battery separator, for example, in the production of a cylindrical battery, the winding speed can be increased. And the safety of the resulting battery is improved.

Next, the method for producing a porous membrane of the present invention comprises:
When the melting point temperature of the crystalline polymer is Tm, the film formed from the crystalline polymer is (Tm-50)
° C) or more and Tm ° C or less but under a temperature condition of 5 to 35%.
%, Then pre-stretched, and then heat-treated and then stretched to make it porous.

According to this manufacturing method, the porosity is 30 to 9
5%, and the tensile strength in at least one direction is 20%.
The porous membrane of the present invention formed from a crystalline polymer of 50 kg / cm ² or more can be produced.

In the present invention, the melting point Tm of the crystalline polymer can be obtained as the crystalline melting point (° C.) of the crystalline polymer by, for example, measurement using a differential scanning calorimeter (DSC). When the film is formed of two or more types of crystalline polymers, the melting point having the lower melting point is defined as the melting point Tm of the crystalline polymer.

The porosity is the ratio of the volume of the pores to the volume of the porous film.

The thickness reduction rate (%) is a percentage of the film thickness (B) after the pre-stretching to the film thickness (A) before the pre-stretching, as represented by the following equation (Equation 1).

[0016]

## EQU1 ## Thickness reduction rate (%) = (A / B) × 100

In the method for producing a porous membrane according to the present invention, the pre-stretching is preferably performed while maintaining a non-porous structure of a non-porous film formed from the crystalline polymer.

In the method for producing a porous membrane according to the present invention, the crystalline polymer is preferably at least one of polyethylene and polypropylene. This is because these polymers are excellent in various properties such as chemical resistance, acid resistance and alkali resistance.

[0019]

Next, an embodiment of the present invention will be described. The porous membrane of the present invention can be produced, for example, by pre-stretching a non-porous film formed from a crystalline polymer under the above-mentioned predetermined conditions, then heat-treating the film, and then stretching the film to make it porous.

The crystalline polymer is not particularly restricted but includes, for example, polyolefins such as polyethylene, polypropylene, poly (4-methylpentene 1) and polybutene 1, and polyfluorinated olefins such as polyvinyl fluoride and polyvinylidene fluoride. And polyesters such as polyethylene terephthalate, polyethylene naphthalate and polybutylene terephthalate, polyphenylene sulfide, polyoxymethylene, polyamide and the like.

Among them, a resin having a high degree of attained crystallinity is preferable for efficiently forming micropores. Among the above, polyolefins, polyfluorinated olefins, polyamides, polyoxymethylene and the like are preferable. preferable. Particularly preferred are polyolefins having excellent properties such as chemical resistance, acid resistance, and alkali resistance, and most preferably, polyethylene and polypropylene, as described above. In addition,
These crystalline polymers may be used alone or in combination of two or more.

Next, the non-porous film formed from the crystalline polymer is pre-stretched. In addition, as a method of forming a nonporous film from the crystalline polymer, a film forming method used in a usual plastic forming method can be applied.

In the pre-stretching, the precursor film is preferably stretched while maintaining its non-porous structure. The stretching temperature is in a range from (Tm-50) ° C to Tm ° C. If the stretching temperature is lower than this, the film may be broken, and the film may be wrinkled, resulting in poor appearance. If the temperature exceeds the above range, it becomes difficult to make the film porous. A preferable range of the temperature condition is 100 to 125 ° C. This pre-stretching is usually
The stretching is uniaxial, and the method of preliminary stretching is not particularly limited. For example, a roll stretching method, a roll rolling method, a tenter stretching, and the like conventionally used in the field of film stretching can be used. The thickness reduction rate during pre-stretching is 5 to 3
It must be in the range of 5%. If the thickness reduction rate is smaller than this. When it becomes a porous membrane, the tensile strength does not increase,
On the other hand, if the thickness reduction ratio is larger than this, the film will break or it will be difficult to form a porous film. A preferable range of the thickness reduction rate is 7 to 25%.

Next, the pre-stretched film is heat-treated. The conditions for this heat treatment are not particularly limited, and are appropriately determined depending on the type of the crystalline polymer forming the film, and the like.
Usually, assuming that the crystal melting point of the film is Tm, the heat treatment temperature is (Tm-40) C or higher and (Tm-5) C or lower.
However, when there are two or more film constituent materials, the heat treatment temperature is (Tmb−40) ° C. to (Tma−), where Tma ° C. is the highest crystal melting point and Tmb ° C. the lowest.
5) It is ° C. The heat treatment time is usually about 2 seconds to
24 hours. By performing such a heat treatment,
The crystallinity of the film is increased, and the formation of fine pores by stretching performed later is facilitated, so that a porous film having a high porosity can be obtained.

Next, the heat-treated film is stretched to be porous. Usually, this stretching is preferably performed by a two-stage stretching process in which stretching is performed at a low temperature condition and then stretching is performed at a high temperature condition.

The stretching under the low temperature condition is usually performed at -20 to -20.
It is performed in a uniaxial direction in a low temperature region of 80 ° C. Hereinafter, the stretching at −20 to 80 ° C. is referred to as “low temperature stretching”. If the stretching temperature is lower than this range, the film may be broken during the operation, and if the stretching temperature is higher than this range, it may be difficult to form a porous film. A preferred range of the stretching temperature is 0 to 50 ° C. The stretching method is not particularly limited, and a roll stretching method, a tenter stretching method, and the like, which are conventionally used, can be applied. The stretching ratio in the low-temperature stretching is not particularly limited, but is usually about 20 to 400%, preferably about 40 to 300%. The stretching ratio (%) is obtained from the film size (L) before low-temperature stretching and the film size (LB) after low-temperature stretching by the following formula (Equation 2).

[0027]

## EQU2 ## Stretching ratio (%) = [(LB-L) / L] × 100

Next, in the case of a film stretched at a low temperature, the crystal melting point (° C.) of the film is defined as Tm.
The film is stretched in a high temperature range of not lower than Tm and not higher than Tm ° C. However, when the film is formed using two or more types of crystalline polymers, the melting point of the polymer having the lowest crystalline melting point is defined as T.
mb (° C.), the hot stretching temperature is (Tmb−40) ° C.
To Tmb ° C. Hereinafter, the stretching in this temperature range is referred to as “high-temperature stretching”. The reason for setting the temperature range of the high-temperature stretching to the above range is the same as the low-temperature stretching described above. If the stretching temperature is lower than the above range, the film may be broken during the operation, and the stretching temperature is higher than the above range. This is because the formation of a porous film may be difficult. The high temperature stretching is usually performed in the same direction as the stretching direction at the time of the low temperature stretching, but may be performed in another direction. The stretching method is not particularly limited, and the same method as the low-temperature stretching can be employed. The stretching ratio at the time of high temperature stretching is not particularly limited, but is usually about 10 to 500%, preferably about 1 to 500%.
00 to 300%. The draw ratios are as follows: the film size before low-temperature stretching (L), the film size after low-temperature stretching (before high-temperature stretching) (LB), and the film size after high-temperature stretching (L
H) is obtained from the following equation (Equation 3).

[0029]

## EQU00003 ## Stretching ratio (%) = [(LH-LB) / L] .times.100

Thus, the porous membrane of the present invention is obtained. In order to improve the dimensional stability, it is preferable to further perform at least one of the following shrinking treatment and heat setting, and particularly preferably Is to perform both processes.

In the shrinkage treatment, the stress acting during the low-temperature stretching and the high-temperature stretching remains in the porous film, and the residual stress causes the porous film to shrink in the stretching direction and easily cause a dimensional change. This is a process for improving the dimensional stability by contracting the dimension in the stretching direction after stretching. This shrinking treatment can be performed, for example, under heating conditions similar to the stretching temperature. Although the degree of shrinkage may be arbitrarily determined, it is usually set to such an extent that the film size after stretching is reduced by about 10 to 40%.

The heat setting is a treatment in which the dimension of the porous membrane in the stretching direction is controlled so as not to change, and heating is performed at a stretching temperature or higher. By this heat setting, the dimensional stability can be improved similarly to the shrinkage treatment.

The porous membrane of the present invention has a tensile strength in at least one direction of at least 2050 kg / cm ² and a porosity of 3
0 to 95%. If the porosity is lower than the above range, the air permeability decreases, and for example, when used for a battery separator, the discharge characteristics of the battery may be reduced. On the other hand, if the porosity is higher than the above range, the air permeability is improved, but the mechanical strength and durability of the porous film are reduced. A preferred range of the porosity is 35 to 60%.

The porous membrane of the present invention has a tensile strength in at least one direction of at least 2050 kg / cm ² and is excellent in mechanical strength. Such a high-strength porous membrane is required in various fields, and when actually used, leads to improvement in workability, productivity, and durability, and the porous membrane of the present invention has a high permeability. is there. Therefore, the porous membrane of the present invention,
It can be applied to various uses such as battery separators, separation membranes, architectural breathable films, and clothing films.

When the porous membrane of the present invention is used as a battery separator, it may be used as it is, or may be used in combination with other supporting materials.

[0036]

Next, examples will be described together with comparative examples. Example 1 Three layers made of polypropylene (PP) having a melt index (MI) of 0.5 and high density polyethylene (HDPE) having an MI of 1.3 and a density of 0.966 (composition: outer layer PP (14 μm), intermediate layer) Layer PP / HDPE = 2
/ 8 (10 μm)) to prepare a crystalline polymer precursor film having a total thickness of 38 μm. Note that 2/8 in the intermediate layer is a weight ratio.

The precursor film was rolled at a roll temperature of 120.
Preliminary stretching was carried out by roll stretching so that the temperature was reduced to 7 ° C and the thickness reduction rate became 7%. Next, the pre-stretched film was brought into contact with a metal roll having a surface temperature of 148 ° C. for 80 seconds and heat-treated.
Then, the film is stretched at 50 ° C. at a low temperature of 80%, then stretched at a temperature of 120 ° C. at a high temperature of 180%, and further shrinked at 120 ° C. by 20% based on the film length at the time of the maximum stretching. Obtained. This porous membrane has a porosity of 4
5% and a tensile strength of 2,070 kg / cm ² . In addition,
The tensile strength was measured using a tensile strength tester, and the porosity was determined by the following equation (Equation 4). In the following formula, the sample is the porous membrane used for the measurement.

[0038]

Porosity (%) = [1− (sample weight) / (sample area × thickness × resin density)] × 100

(Example 2) The thickness reduction rate in pre-stretching was 1
Except for 2%, the thickness was 24 μm in the same manner as in Example 1.
Was obtained. This porous membrane was measured in the same manner as in Example 1, and as a result, the porosity was 44% and the tensile strength was 2100 kg / cm ² .

Example 3 The rate of thickness reduction in pre-stretching was 2
Except for 1%, the thickness was 22 μm as in Example 1.
Was obtained. The porous membrane was measured in the same manner as in Example 1. As a result, the porosity was 42% and the tensile strength was 2200 kg / cm ² .

Example 4 The rate of thickness reduction in pre-stretching was 3
Except for 0%, the thickness was 21 μm in the same manner as in Example 1.
Was obtained. This porous membrane was measured in the same manner as in Example 1, and as a result, the porosity was 40% and the tensile strength was 2300 kg / cm ² .

(Comparative Example 1) Except that pre-stretching was not performed,
A porous film having a thickness of 29 μm was obtained in the same manner as in Example 1.
The porous membrane was measured in the same manner as in Example 1. As a result, the porosity was 44% and the tensile strength was 1600 kg / cm.
^{Was 2} .

(Comparative Example 2) The thickness reduction rate in the preliminary stretching was 3
Except that the thickness was 8%, the thickness was 14 μm in the same manner as in Example 1.
Was obtained. The porous membrane was measured in the same manner as in Example 1. As a result, the porosity was 26%, and the membrane had poor air permeability.

Comparative Example 3 The tensile strength was measured in the same manner as in Example 1 using a commercially available porous membrane film made of PP (thickness: 27 μm, porosity: 39%).
^{Was 2} .

[0045]

As described above, the porous membrane of the present invention has high air permeability, has high mechanical strength such as tensile strength as compared with conventional porous membranes, and sufficiently satisfies various fields. Things. Therefore, when the porous membrane of the present invention is used, for example, as a battery separator for a cylindrical battery, the winding speed can be increased, and the productivity of the battery can be improved. In addition, since the internal short circuit of the battery can be effectively prevented, improvement in the safety and reliability of the battery can be expected. Furthermore, when the porous membrane of the present invention is applied to, for example, a separation membrane, a building ventilation film, and a clothing ventilation film, a high-performance and highly durable separation membrane, a construction ventilation film, and a clothing ventilation film are obtained. .

Claims (5)

[Claims] 1. A porous membrane formed from a crystalline polymer, having a porosity in the range of 30 to 95% and a tensile strength in at least one direction of at least 2050 kg / cm ² . 2. A battery separator comprising the porous membrane according to claim 1. 3. When the melting point temperature of the crystalline polymer is Tm, the non-porous film formed from the crystalline polymer is reduced in thickness by 5% under a temperature condition of (Tm−50 ° C.) or more and Tm ° C. or less. A method for producing a porous film in which pre-stretching is performed so as to be 35% and then heat-treated and then stretched to make it porous. 4. The method for producing a porous membrane according to claim 3, wherein the pre-stretching is performed while maintaining the non-porous structure of the non-porous film formed from the crystalline polymer. 5. The method for producing a porous membrane according to claim 3, wherein the crystalline polymer is at least one polymer of polyethylene and polypropylene.