JPH1121362A - Polyethylene resin porous film - Google Patents

Abstract

(57) [Problem] To provide a porous film having tough surface strength and high piercing elongation suitable for a battery separator and the like. A porous film composed of a high-molecular-weight polyethylene resin having a viscosity-average molecular weight of 300,000 or more and less than 1,000,000, having a thickness of 5 to 50 µm and an air permeability of 250 to 100
0 sec / 100cc, porosity 30-50%, pin puncture strength 4
A polyethylene resin porous film having characteristics of at least 00 gf / 25 μm and a pin pierceability of 2.0 mm or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a porous film useful as a separator for batteries, a separation membrane such as a microfiltration membrane, clothing such as a gas permeable jumper, a sanitary article such as a diaper, a sanitary article, and the like.

[0002]

2. Description of the Related Art Conventionally, porous films (including sheets)
Is widely used for various applications, and various proposals have been made on a method for producing such a porous film. For example, battery separators (battery separators)
As a method for producing a porous film for use as a resin, a method of melt-molding a film from a resin composition comprising polyethylene and a plasticizer, and then dissolving and removing the plasticizer contained in the film with an organic solvent (Japanese Patent Publication No. 60-231
No. 30), a method of forming a sheet from a solution in which polyethylene is dissolved in an organic solvent, adjusting the solvent content in the sheet, stretching, and then removing the residual solvent (Japanese Patent Publication No. 5-54495). Or a method in which a film is formed from a resin composition comprising polyethylene, an organic liquid and an inorganic fine powder, and the organic liquid and the inorganic fine powder contained in the film are extracted and removed (Japanese Patent Publication No. 60-23130). Etc. are known.

Also, the present applicant has been studying a method for producing a porous film for a battery separator, and has previously proposed a porous film having excellent surface strength, particularly excellent pin piercing strength ( JP-A-7-29563).

[0004]

When the above-mentioned surface strength is improved, when the porous film is mainly used as a separator for a wound type battery, the porous film comes into contact with the surfaces of the positive and negative electrode plates, and a pin is formed on the film. The generation of holes can be avoided. However, with the improvement of battery production technology in recent years, there is an increasing demand for higher quality porous films that can withstand higher battery production speeds.

[0005]

Means for Solving the Problems The present inventors have conducted intensive studies on the above problems, and found that not only the improvement of the surface strength but also the improvement of the piercing breaking elongation is effective for the pinhole resistance of the film. I found something. Then, the technical improvement was further performed to improve the piercing rupture elongation while maintaining a tough pin piercing strength as a battery separator or the like, thereby completing the novel polyethylene resin porous film of the present invention.

That is, the polyethylene resin porous film of the present invention is a porous film composed of a high molecular weight polyethylene resin having a viscosity average molecular weight of 300,000 or more and less than 1,000,000, and has a thickness of 5 to 50 μm and air permeability. 250-1000
Sec / 100cc, porosity 30-50%, pin sting strength 40
It is characterized by having characteristics of 0 gf / 25 μm or more and a pin piercing degree of 2.0 mm or more.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below in more detail. First, as a preferred production method for obtaining the porous film of the present invention, a resin composition containing a high-molecular-weight polyethylene resin and a plasticizer is melt-extruded into a film, and after cooling, the film is stretched. There is a method of performing heat treatment after removing a plasticizer contained in a stretched film.

As the polyethylene constituting the porous film of the present invention, a polyethylene resin having a viscosity average molecular weight of 300,000 or more and less than 1,000,000 is used. If the molecular weight is less than 300,000, it is difficult to obtain sufficient strength, and if the molecular weight is 1,000,000 or more, the orientation internal stress due to stretching tends to increase, and heat resistance tends to decrease, which is not preferable. The viscosity average molecular weight is measured according to ASTM D4020.

Further, the raw material polyethylene resin in the porous film of the present invention is not limited to a single polyethylene resin, and a blend of a plurality of grades of polyethylene can be used as long as the molecular weight after blending is within the range shown on the left. . In addition, as long as the physical properties specified in the present invention are not impaired, if necessary, polyethylene wax (molecular weight is usually 1,000 to 5,000), polybutene-1 (molecular weight is usually 4,000,000 or less), and polypropylene (molecular weight is usually 40,000 or less).
50% by weight or less based on polyethylene)
You may add in the following ranges.

As a plasticizer used in combination with polyethylene,
Good compatibility with the polyethylene, having a melting point lower than the melting point of the polyethylene and a boiling point higher than the melting temperature of the polyethylene, and a substance soluble in a polyethylene-insoluble organic solvent is preferably used, for example, Examples include higher aliphatic alcohols such as stearyl alcohol and ceryl alcohol, n-alkanes such as n-decane and n-dodecane, paraffin wax, liquid paraffin, and kerosene. Also, considering the simplicity of raw material handling at the time of extrusion molding, a plasticizer at room temperature is suitably used as the plasticizer.

The proportion of the polyethylene and the plasticizer depends on the porosity of the desired molded product, but the polyethylene is usually 5 to 60% by weight, preferably 10 to 50% by weight.
The plasticizer is usually 95 to 40% by weight, preferably 90 to 50% by weight.
% By weight. In addition to the polyethylene and the plasticizer, various known additives such as an antioxidant are usually added to the raw material composition containing polybutene-1 or the like in an amount of from 0.01 to 0.01%.
You may add in the range of about 5 weight%.

The above-mentioned raw material composition containing polyethylene and a plasticizer is usually 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 is usually carried out at a temperature of 140 to 240 ° C., and a raw film having a thickness of usually 10 μm to 1 mm is formed by a known method such as T-die or inflation molding. The film thickness at this time, considering the thickness change in the subsequent process and the thickness to finally obtain,
It can be set arbitrarily.

The obtained raw film is desirably subjected to uniaxial or biaxial stretching in order to improve its mechanical strength. 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.

After cooling, the stretched film is made porous by removing the plasticizer. As a method of removing the plasticizer, a so-called known organic solvent method is generally employed, in which the plasticizer in the film is dissolved in an organic solvent such as isopropanol, ethanol, or hexane, and is extracted and removed by solvent replacement.
The porous film obtained by removing the plasticizer as described above is subjected to a heat treatment in order to impart thermal dimensional stability. This heat 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. The heat treatment can be performed at any temperature lower than the melting point of polyethylene, but is preferably 1 in order to make the heat shrinkage in the width direction described later a suitable value.
10 ° C. or higher and lower than the melting point of polyethylene, more preferably 1
10 ° C. or more and 125 ° C. or less

In the heat treatment, a stretching treatment may be performed as long as the physical properties of the final porous film are not impaired. As the stretching method, not only the above-mentioned active stretching in the longitudinal direction and the transverse direction, but also the so-called passive stretching in which the stretching direction is imparted by restricting the flow direction and the width direction and inhibiting the heat shrinkage of the film. Can be used. Alternatively, a relaxation treatment for freely shrinking the film during the heat treatment can be performed. The stretching ratio during the heat treatment is appropriately kept within a range that does not impair the final physical properties of the porous film, and is usually 0.9 times or more and 1.5 times or less, preferably 0.9 times or more and 1.2 times or less. is there.

The porous film of the present invention produced as described above has a thickness of 5 to 50 μm, preferably 15 to 3 μm.
5 μm. The air permeability is 250 to 1000 seconds / 100 cc as measured according to JIS P8117,
Preferably it is 250-800 seconds / 100cc, more preferably 250-500 seconds / 100cc. For example, when the present porous film is used as a battery separator, if the value of the air permeability exceeds 1000 seconds / 100 cc, the resistance increases, and the desired performance as a battery may not be obtained.

The porosity of the porous film of the present invention is 30 to 50%, preferably 30 to 45%. In addition,
The porosity is obtained by punching out five places in the width direction of the film into a circle having a diameter of 4 cm, measuring the thickness and weight of the center part of the punched film, and calculating by the following formula.

Porosity (%) = (Vρ−W) / (Vρ) × 100 (where, V: volume of film (for 5 sheets) W: weight (for 5 sheets), ρ: density of material)

Further, the porous film of the present invention was measured according to Japanese Agricultural Standards Notification No. 1019 (pin diameter 1 mmφ,
The pin piercing strength at the tip 0.5R, the pin piercing speed 300 mm / min) is 400 gf / 25 μm or more, preferably 400 to 1000 gf / 25 μm, and particularly preferably 400 to 800 gf (25 μm). .

The porous film of the present invention is characterized by having not only the above-mentioned physical properties but also extremely high pin piercing elongation. The pin piercing elongation is the distance traveled by the pin from the point where the pin tip comes into contact with the film until the piercing break occurs in the above-described pin piercing strength measurement. By imparting sufficient pin piercing elongation to the porous film, for example, when the porous film is used as a separator for a wound type battery, a pinhole is generated in the film even when wound at a higher speed. It is possible to assemble the battery well without performing. The porous film of the present invention has a pin piercing elongation of 2.0 mm or more, preferably 2.3 mm or more, more preferably 2.6 mm or more. If the pin piercing elongation is less than 2.0 mm, there is a possibility that inconveniences such as lowering the winding speed may occur in order to produce a suitable battery.

Further, when a porous film is used as a battery separator, it is generally known that the heat shrinkage in the width direction is preferably low. The shrinkage is usually 5% or less, preferably 3% or less, and shows a sufficient heat shrinkage for practical use as a battery separator. The measurement of the heat shrinkage rate was 1
After leaving for 1 hour in a hot-air oven set at 05 ° C., the dimensions of the cooled porous film are measured, and calculated from the dimensional changes before and after heating.

When a porous film made of polyethylene resin is used as a battery separator, particularly as a separator for a lithium secondary battery, internal heat generation due to internal short-circuiting of the battery renders the separator non-porous and the battery is safely shut down. Is required. At this time, the separator should not only be made nonporous, but also have no defect such as a crack. One of the excellent points of the porous film of the present invention is that it has a good shutdown property at around 160 ° C.

[0023]

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. 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 two mixed compositions is about 800,000), and paraffin wax (average molecular weight of 389) Using a 40 mmφ twin screw extruder, 76 parts by weight of the mixture was extruded at an extrusion temperature of 1
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 6 times in the transverse direction at 110 ° C. using a tenter stretching machine. The obtained film is immersed in isopropanol at 60 ° C.
The paraffin wax was extracted and removed. The obtained film was heat-set at a temperature of 115 ° C. using a roll stretching machine. At the time of heat setting, the roll speed ratio was adjusted so that the longitudinal stretching ratio was 1.2 times. 25μ obtained
Table 1 shows the physical properties of the m porous film.

Further, the obtained porous film and a sandpaper (12
No. 0) was superimposed, heated to 160 ° C under a constant load, and when the film temperature reached 160 ° C, a sample was taken out and a shutdown test was performed to observe the nonporous state of the film and the presence of cracks. went. As a result, no cracks or the like were generated, and the film after the test was transparent and well nonporous, and the shutdown property was good.

Example 2 18 parts by weight of polyethylene having a viscosity-average molecular weight of 500,000, 6 parts by weight of polyethylene having a viscosity-average molecular weight of 1,000,000 (the viscosity-average molecular weight of a mixture of the two is about 600,000),
And a mixture of 76 parts by weight of paraffin wax (average molecular weight: 389) was extruded using a 40 mmφ twin screw extruder at an extrusion temperature of 170 ° C. and an extrusion rate of 10 kg / h, and a raw film was formed by a T-die method.

The obtained raw film was stretched 3 times in the machine direction and 8 times in the transverse direction at 110 ° C. using a biaxial stretching machine. The obtained film was immersed in ethanol at 60 ° C. to extract and remove paraffin wax. The obtained film was heat-set at a temperature of 115 ° C. using a roll stretching machine. At the time of heat setting, the roll speed ratio was adjusted so that the stretching ratio in the longitudinal direction was 1.1 times. 2 obtained
Table 1 shows the physical properties of the 5 μm porous film.

Comparative Example 1 A composition comprising 40 parts by weight of polyethylene having a viscosity average molecular weight of 700,000 and 60 parts by weight of stearyl alcohol was mixed with 40 m
Extrusion temperature 170 ° C, extrusion rate 10k using mφ twin screw extruder
g / h and extruded to form a film by the inflation method. The obtained film was immersed in ethanol at 60 ° C. to extract and remove stearyl alcohol. Next, the film was stretched 2.5 times in the machine direction and 5 times in the transverse direction at 120 ° C. using a biaxial stretching machine to obtain a 25 μm porous film. Table 1 shows the physical properties of the obtained porous film.

[0029]

[Table 1]

[0030]

The polyethylene resin porous film of the present invention has a tough surface strength and a high piercing elongation as a battery separator. Further, as a film for a battery separator, it is possible to cope with a higher battery production speed than ever before.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Yasushi Yasushi 3-10 Shiodori, Kurashiki-shi, Okayama Prefecture Mitsubishi Chemical Mizushima Plant (72) Inventor Kyoji Watanabe 3--10, Shiodori, Kurashiki-shi, Okayama Mitsubishi Chemical Corporation Mizushima Office

Claims (7)

[Claims] 1. A porous film comprising a high molecular weight polyethylene resin having a viscosity average molecular weight of 300,000 or more and less than 1,000,000, having a thickness of 5 to 50 μm and an air permeability of 250 to 100.
0 sec / 100cc, porosity 30-50%, pin puncture strength 4
A polyethylene resin porous film having characteristics of at least 00 gf / 25 μm and a pin pierceability of 2.0 mm or more. 2. The polyethylene resin porous film according to claim 1, wherein the pin puncture degree is 2.3 mm or more. 3. The polyethylene resin porous film according to claim 1, wherein the pin pierceability is 2.6 mm or more. 4. The polyethylene resin porous film according to claim 1, wherein the heat shrinkage in the width direction at 105 ° C. is 5% or less. 5. A resin composition containing a high molecular weight polyethylene resin having a viscosity average molecular weight of 300,000 or more and less than 1,000,000 and a plasticizer is melt-extruded into a film, and after cooling, the film is stretched. The method for producing a polyethylene resin porous film according to any one of claims 1 to 4, wherein the heat treatment is performed after removing the contained plasticizer. 6. The method for producing a polyethylene resin porous film according to claim 5, wherein the heat treatment temperature is 110 ° C. or higher and lower than the melting point of polyethylene. 7. The method for producing a polyethylene resin porous film according to claim 5, wherein the plasticizer is a solid at room temperature.