JPH1121361A - Porous polyethylene resin film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous film suited for e.g. battery separators and having suitable facial strength and good air permeability. SOLUTION: This is a porous film made from a high-molecular weight- polyethylene resin having a viscosity-average molecular weight of 300,000 to below 1,000,000 and having a thickness of 5-50 μm, an air permeability of 100 to below 250 sec/100 cc, a porosity of 40-60%, a pin piercing strength of 350 gf/25 μm or above and a pin piercing elongation of 2.0 mm or above. The thermal shrinkage in the direction of width is 10% or below at 105 deg.C.

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.

[0003]

A major application of such a porous film is as a separator for non-aqueous electrolyte batteries. In recent years, there is an increasing demand for improving the air permeability of a porous film for the purpose of improving the battery. However, in the conventional technique, when the air permeability is improved, the pin piercing strength is also lowered, and the assembly speed of the battery is lowered, which makes it difficult to use the battery separator industrially as a battery separator.

[0004]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies on the above-mentioned problems, and have made technical improvements to improve the air permeability without impairing the pin piercing strength. A porous film was completed. 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, a thickness of 5 to 50 μm, and an air permeability of 100 seconds. / 100
cc or more and less than 250 seconds / 100 cc, porosity 40-6
0%, a pin piercing strength of 350 gf / 25 μm or more, and a pin piercing degree of 2.0 mm or more.

[0005]

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.

[0007] Further, the raw 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.

[0008] 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.

[0009] The proportion of polyethylene and plasticizer used depends on the porosity of the desired molded article, but 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 raw material composition containing the above polyethylene and plasticizer is usually kneaded uniformly by 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 the stretched film is cooled, the plasticizer is removed to make it porous. 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, it is preferable to perform a stretching treatment within a range that does not impair the physical properties of the final porous film. As the stretching method, the above-described longitudinal direction,
So-called passive stretching, in which a stretching effect is imparted by restricting the flow direction and width direction and inhibiting heat shrinkage of the film, as well as active stretching in the transverse direction, can be used.
Alternatively, a relaxation treatment for freely shrinking the film during the heat treatment can be performed. The stretching ratio during heat treatment is
It is appropriate to keep the final physical properties of the porous film within a range that does not impair the final properties, and it is usually 1.5 times or more, preferably 1.5 times or more and 2.5 times or less. When the ratio exceeds 2.5 times, the porosity of the film is impaired, and a desired air permeability may not be obtained.

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 seconds at 100 seconds / 100 cc or more as measured according to JIS P8117.
Seconds / 100 cc. Further, the porosity is 40-60.
%, Preferably 50 to 60%. The porosity is
Punched 5 places in the width direction of the film into a circle with a diameter of 4 cm, measured the thickness and weight of the center of the punched film,
It is calculated 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 is measured according to Japanese Agricultural Standards Notification No. 1019 (pin diameter 1 mmφ,
The pin piercing strength at a tip end of 0.5R and a pin piercing speed of 300 mm / min is 350 gf / 25 μm or more, preferably 350 to 1000 gf / 25 μm, particularly preferably 400 to 800 gf (25 μm). .

The porous film of the present invention is characterized by having not only the above 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 a sufficient pin piercing elongation to the porous film, for example, when the porous film is used as a separator for a wound type battery, even when wound at a higher speed, a pinhole is generated in the film. It is possible to assemble the battery satisfactorily without any trouble. The porous film of the present invention has a pin piercing elongation of 2.0 mm or more, preferably 2.3 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 lower. However, the heat resistance of the porous film of the present invention at 105 ° C. in the width direction is generally preferred. Shrinkage is usually 10% or less,
It is preferably at most 5%, and shows a sufficient heat shrinkage for practical use as a battery separator. In addition, the measurement of the heat shrinkage rate
After leaving for 1 hour in a hot-air oven set at 105 ° 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 makes 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.

[0020]

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 8 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 2.0 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 the two mixed compositions is about 600,000), and paraffin wax (average). The mixture having a 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 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 1.8 times. 2 obtained
Table 1 shows the physical properties of the 5 μm porous film.

Comparative Example 1 A composition comprising 25 parts by weight of polyethylene having a viscosity average molecular weight of 2,000,000 and 75 parts by weight of stearyl alcohol was mixed with 40 parts by weight of
Extrusion temperature 170 ° C, extrusion rate 10
The film was extruded at kg / h and a film was formed by an inflation method. The obtained film was immersed in ethanol at 60 ° C., and stearyl alcohol was extracted and removed.
Was heat set at a temperature of At the time of heat setting, the roll speed ratio was adjusted so that the longitudinal stretching ratio was 1.0 times. Table 2 shows the physical properties of the obtained 25 μm porous film.
It is shown in FIG.

[0026]

[Table 1]

[0027]

The polyethylene resin porous film of the present invention has a suitable surface strength and good air permeability as a battery separator.

Continued on the front page (72) Inventor Yasushi Yasumi 3-10 Ushidori, Kurashiki-shi, Okayama Pref. Mitsubishi Chemical Co., Ltd. Mizushima Office

Claims (6)

[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 100 seconds / 1.
More than 00cc and less than 250sec / 100cc, porosity 40
A porous film made of polyethylene resin, having characteristics of ６０60%, pin piercing strength of 350 gf / 25 μm or more, and pin piercing degree 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 thermal shrinkage in the width direction at 105 ° C. is 10%.
The polyethylene resin porous film according to claim 1 or 2, wherein: 4. 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. 4. Heat treatment after removing the plasticizer contained in the composition.
The method for producing a polyethylene resin porous film according to any one of the above. 5. The method for producing a polyethylene resin porous film according to claim 4, wherein the film is stretched by 1.5 times or more and less than 2.5 times during the heat treatment. 6. The method for producing a polyethylene resin porous film according to claim 5, wherein the plasticizer is a solid at room temperature.