JPH1017702A - Porous film made of polyethylene resin and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prepare the subject film having a suitable strength as a battery separator and high thermal property by including a specific aliphatic hydrocarbon. SOLUTION: This film is mainly composed of (A) a polyethylene resin having 30×10<4> -100×10<4> viscosity-average molecular weight and contains (B) 3-20wt.% of an aliphatic hydrocarbon (e.g. synthetic waxhaving <=3,000 weight-average molecular weight and 90-120 deg.C softening point. Preferably, the film has >=400g/25μm pinsticking strength, an excellent shut down property at 160 deg.C and <135 deg.C close-beginning temperature (a temperature at which a porous structure begins to vanish). The film is obtained by molding a composition containing the component A, the component B and (C) a plasticizer to a film state and removing the component C by extraction, then drawing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyethylene resin porous film useful as a battery separator. More specifically, the present invention relates to a polyethylene resin porous film having practically sufficient strength and excellent thermal properties.

[0002]

2. Description of the Related Art Conventionally, a porous film made of a polyethylene resin useful as a battery separator has been described.
Various proposals have been made. For example, JP-A-60-255
JP-A-107-24 discloses a method in which a polyethylene powder and a plasticizer powder are mixed, molded by an extruder, and a molded product obtained by removing the plasticizer after cooling is stretched.
No. 2035 proposes a method of dissolving, molding and stretching a polyethylene resin in a plasticizer and then removing the plasticizer.

[0003]

However, in either method, the porous film is stretched to give practically sufficient strength as a battery separator, and as a result, the oriented crystallization of the polymer constituting the film is performed. Progress, resulting in a decrease in heat resistance due to an increase in orientation distortion,
In particular, when applied to a lithium secondary battery or the like, there is a problem that the safety of the battery is poor.

An object of the present invention is to solve the above-mentioned conventional problems and to provide a polyethylene resin porous film having practically sufficient strength and excellent thermal characteristics.

[0005]

The present inventors have conducted intensive studies to obtain a polyethylene resin porous film having appropriate strength as a battery separator and having excellent thermal characteristics. Addition of group hydrocarbons
It has been found that the thermal properties of the porous molded article can be dramatically improved without impairing the physical properties thereof, and the present invention has been completed.

The polyethylene resin porous film of the present invention is characterized in that a polyethylene resin having a viscosity average molecular weight of 300,000 or more and less than 1,000,000 is obtained by adding an aliphatic hydrocarbon having a weight average molecular weight of 3,000 or less and a softening point of 90 to 120 ° C. -20% by weight.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. As the polyethylene constituting the 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 internal stress of orientation due to stretching tends to increase, and the heat resistance tends to decrease. Furthermore, not only single-grade polyethylene but also blends of multiple-grade polyethylene can be suitably used as long as the molecular weight after blending is within the range shown on the left. In addition, if necessary, polyethylene wax (molecular weight: 1,000 to 5,000), polybutene-1 (molecular weight: 4,000,000 or less), polypropylene (molecular weight: 4,000,000 or less), or the like may be used in an amount of 50% by weight based on polyethylene.
The following may be added. Here, in the case of polyethylene wax having a molecular weight of 3000 or less and a softening point of 90 to 120 ° C., it can also serve as an aliphatic hydrocarbon described later.

An aliphatic hydrocarbon having a weight average molecular weight of 3000 or less and a softening point of 90 to 120 ° C.
Even if the viscosity average molecular weight of a polyethylene resin composition containing 20% by weight is measured, the viscosity average molecular weight of the polyethylene resin containing no aliphatic hydrocarbon is not significantly different since the amount of the aliphatic hydrocarbon added is sufficiently small. Therefore, in the present invention, an aliphatic hydrocarbon is added to the polyethylene resin in an amount of 3 to 20.
The viscosity average molecular weight of the resin composition including the weight% can be regarded as the viscosity average molecular weight of the polyethylene resin. This means that the value obtained by measuring the viscosity average molecular weight of the obtained porous film can be regarded as the viscosity average molecular weight of the polyethylene resin.

The film of the present invention has a weight average molecular weight of 300
0 or less, preferably 2000 or less, more preferably 10 or less
The softening point measured by a method according to JIS K 2207 is 90 to 120 ° C, preferably 100 to 100 ° C.
It contains 3 to 20% by weight, preferably 3 to 10% by weight of an aliphatic hydrocarbon at 115 ° C. This content is 3% by weight
When the content is less than 20%, the effect of improving the thermal properties is not recognized, and when the content exceeds 20% by weight, a decrease in air permeability and the like are observed,
It is not suitable because the porosity of the molded article is impaired. When the weight average molecular weight of the aliphatic hydrocarbon exceeds 3,000, poor mixing with the polyethylene resin becomes apparent, and various physical properties are reduced. When the softening point is lower than 90 ° C. or higher than 120 ° C., the thermal properties of the obtained film are reduced.

The aliphatic hydrocarbon used in the present invention has a weight average molecular weight of 3000 or less and a softening point of 90 to 120.
Any temperature may be used as long as the temperature is ° C, for example, low molecular weight polyolefin such as polyethylene wax, petroleum wax such as paraffin wax, synthetic wax, or a mixture thereof. In particular, a synthetic wax obtained by a catalytic hydrogenation reaction of carbon monoxide is preferably used.

The film of the present invention is obtained by melt-extruding a mixture of polyethylene, an aliphatic hydrocarbon and a plasticizer,
Next, after extracting and removing the plasticizer, it can be manufactured by heat treatment such as heat setting or stretching. As the plasticizer, a substance that has good compatibility with the above polyethylene, has a melting point lower than the melting point of the polyethylene and a boiling point higher than the melting temperature of the polyethylene, and is soluble in an organic solvent insoluble in polyethylene. It is preferably used. 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, kerosene, and the like. Here, when an aliphatic hydrocarbon such as paraffin wax having a weight-average molecular weight of 3000 or less and a softening point of 90 to 120 ° C. is used as a plasticizer, the aliphatic hydrocarbon and the plasticizer can be combined. In this case, after the mixture of the polyethylene resin and the aliphatic hydrocarbon is melt-extruded, the extraction and removal treatment may be performed so that the aliphatic hydrocarbon remains at 3 to 20% by weight. However, in such a production method, it is difficult to control the conditions of the extraction and removal treatment. Therefore, it is preferable that the specific aliphatic hydrocarbon and the plasticizer be different substances.

The ratio of the polyethylene / aliphatic hydrocarbon mixture and the plasticizer to be used depends on the porosity of the target film, but usually the polyethylene / aliphatic hydrocarbon mixture is 5 to 60% by weight, preferably 10 to 10% by weight. ~ 50% by weight
The plasticizer is selected from the range of 95 to 40% by weight, preferably 90 to 50% by weight. In the present invention, known additives, such as an antioxidant, are added in an amount of about 0.01 to 5% by weight based on the raw material composition containing polyethylene, an aliphatic hydrocarbon, and a plasticizer in such a ratio. You may add in the range.

Extrusion molding is usually carried out at a temperature of 140 to 240 ° C., and is formed into a film or sheet having a thickness of 5 to 500 μm, particularly 10 to 300 μm, by a known method such as T-die or inflation molding. As a method for removing the plasticizer after molding, for example, a so-called known organic solvent method in which a plasticizer in a film or a sheet is dissolved in an organic solvent such as isopropyl alcohol, ethanol, and hexane and extracted and removed by solvent replacement, is used. No. In the extraction and removal, by appropriately selecting the extraction temperature and the extraction time, it is possible to remove only the plasticizer component and leave the aliphatic hydrocarbon without extraction.

[0014] The film or sheet obtained by removing the plasticizer and making it porous as described above,
Heat fixing may be performed by heating to about 100 to 180 ° C. In order to impart strength to the obtained porous film, it can be easily post-processed by uniaxial or biaxial stretching.

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. When a porous film made of polyethylene resin is used as a battery separator, particularly as a separator for a lithium secondary battery, measurement is performed according to Japanese Agricultural Standards Notification No. 1019 (pin diameter) in order to avoid pinholes in the film during battery assembly. 1mm
φ, tip 0.5R, pin piercing speed 300mm / min)
It is desirable that the pin puncture strength obtained is 400 g / 25 μm. The porous film of the present invention can have a pin piercing strength of 400 g / 25 μm by appropriately selecting the stretching processing conditions.
m or more and a polyethylene resin porous film.

The above polyethylene resin porous film has good shutdown properties at 160 ° C. at the same time. When a porous film made of polyethylene resin is used as a battery separator, particularly as a separator for a lithium secondary battery, it is required that the separator be non-porous due to internal heat generation due to an internal short circuit of the battery and the battery be safely shut down. At this time, the separator should not only be made nonporous, but also have no defect such as a crack.

The evaluation of the shutdown property can be performed by the following method. That is, a porous film made of a polyethylene resin and a sandpaper (No. 120) as a model of a lithium secondary battery electrode are overlaid and heated to 160 ° C. under a constant load. When the film temperature reaches 160 ° C., the sample is taken out and the state of nonporosity of the film and the presence of cracks are observed. The polyethylene resin porous film of the present invention has no cracks and the like in the above test, and the film after the test is transparent and satisfactorily nonporous.

Further, in order to enhance the safety of the battery, it is preferable that the temperature at which the porous structure starts to become nonporous (closing start temperature) is low. However, the polyethylene resin porous film has a closing start temperature of less than 135 ° C. It is suitable. The blockage start temperature can be evaluated based on the temperature at which the air permeability starts to increase, measured according to JIS P8117.

[0019]

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 viscosity average molecular weight was measured according to ASTM D4020. Example 1 A composition comprising 40 parts by weight of polyethylene having a viscosity-average molecular weight of 700,000, 5 parts by weight of a synthetic wax having a weight-average molecular weight of 670, a softening point of 105 ° C, and 55 parts by weight of stearyl alcohol was extruded using a 40 mmφ twin-screw extruder. The film was extruded at 170 ° C. at an extrusion rate of 10 kg / h, and a film was formed by an inflation method.

The resulting film was immersed in ethanol at 60 ° C. for 10 minutes to extract and remove stearyl alcohol. Next, the film was stretched 2.5 times in the MD direction and 5 times in the TD direction at 120 ° C. using a biaxial stretching machine to obtain a 25 μm porous film. Pin sting strength of the obtained film,
The air permeability, the shutdown property, and the onset temperature of clogging were measured. The results are shown in Table 1.

Comparative Example 1 A porous film was obtained in the same manner as in Example 1, except that the added amount of the synthetic wax was 20 parts by weight and the added amount of stearyl alcohol was 40 parts by weight. Table 1 shows the physical properties of the obtained film. Comparative Example 2 Weight average molecular weight 900, softening point 1 instead of synthetic wax
A porous film was obtained in the same manner as in Example 1 except that 10 parts by weight of polyethylene wax at 21 ° C. was added. Table 1 shows the physical properties of the obtained film.

Comparative Example 3 The porous film prepared in Example 1 was immersed in hexane at 80 ° C. for 60 minutes to extract the synthetic wax in the film. From the weight change before and after the hexane treatment, the content of the synthetic wax in the porous film after the treatment was 1% by weight or less. Table 1 shows the physical properties of the processed film.

[0023]

Table 1 Table 1 Pin piercing strength Air permeability Shutdown property Start of blockage (g / 25 μm) (sec / 100 cc) Non-porous state Crack etc. Temperature (° C) Example 1 450 500 Transparent None 131 Comparative Example 1 350 10000 Transparency None Unmeasurable Comparative Example 2 400 600 Cloudy Small Hole 136 Comparative Example 3 450 500 Cloudy Small Hole 135

As apparent from Table 1, the polyethylene resin has a weight average molecular weight of 1,000 or less and a softening point of 9 or less.
A polyethylene resin porous film containing an aliphatic hydrocarbon at 0 to 110 ° C and containing 3 to 20% by weight has good strength, good shutdown properties, and good closing temperature.

Example 2 20 parts by weight of polyethylene having a viscosity-average molecular weight of 500,000, 20 parts by weight of polyethylene having a viscosity-average molecular weight of 1,000,000 (the viscosity-average molecular weight of the above-mentioned mixture is about 800,000), and a weight-average molecular weight of 670 A mixture of 10 parts by weight of a synthetic wax having a softening point of 105 ° C. and 50 parts by weight of paraffin wax (average molecular weight: 389) is extruded using a 40 mmφ twin screw extruder at an extrusion temperature of 170 ° C. and an output of 10 kg / h, using a T-die method. To form a film.

The obtained film was immersed in hexane at 60 ° C. for 3 minutes to extract and remove paraffin wax.
At this time, a part of the synthetic wax was also extracted, and the residual wax component was about 5 parts by weight (about 11% by weight). From the solubility in hexane, it is considered that the residual wax is substantially only a synthetic wax. After the extraction treatment, the obtained film was stretched 4 times in the MD direction and 2.5 times in the TD direction at a temperature of 110 to 120 ° C. using a biaxial stretching machine. Table 2 shows the air permeability when the stretching temperature was changed.

Comparative Example 4 The procedure of Example 2 was repeated except that the amount of paraffin wax used was changed to 60 parts by weight and the stretching temperature was changed to 120 to 130 ° C. without using synthetic wax. Table 2 shows the results.

[0028]

[Table 2]

As is evident from Table 2, the polyethylene resin porous film of the present invention has low dependence on the stretching temperature conditions and has good quality stability.

[0030]

As described in detail above, the polyethylene resin porous film of the present invention has suitable strength as a battery separator and has high thermal characteristics.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kyosuke Watanabe 3-10 Ushidori, Kurashiki City, Okayama Prefecture Inside Mitsubishi Chemical R & D Co., Ltd. Mitsubishi Chemical Corporation Mizushima Development Laboratory Co., Ltd. (72) Inventor Taihei Nonobe 3-10 Ushidori, Kurashiki-shi, Okayama Prefecture Mitsubishi Chemical Corporation Mizushima Development Laboratory

Claims (5)

[Claims] (1) a viscosity average molecular weight of 300,000 or more and 10
A porous film made of a polyethylene resin having a weight average molecular weight of 3000 or less and a softening point of 90 to
A polyethylene resin porous film containing an aliphatic hydrocarbon at 120 ° C. in an amount of 3 to 20% by weight. 2. The porous film according to claim 1, wherein the pin puncture strength is 400 g / 25 μm or more, and 160
A porous film made of polyethylene resin, having a good shutdown property at a temperature of ° C. 3. The porous film made of polyethylene resin according to claim 1, wherein the closing temperature is less than 135 ° C. 4. A film-shaped composition comprising a polyethylene resin having a viscosity average molecular weight of 300,000 or more and less than 1,000,000, an aliphatic hydrocarbon having a weight average molecular weight of 3,000 or less and a softening point of 90 to 120 ° C., and a plasticizer. The method for producing a polyethylene resin porous film according to any one of claims 1 to 3, wherein the plasticizer is extracted and removed. 5. A film-shaped composition comprising a polyethylene resin having a viscosity average molecular weight of 300,000 or more and less than 1,000,000, an aliphatic hydrocarbon having a weight average molecular weight of 3,000 or less and a softening point of 90 to 120 ° C., and a plasticizer. The method for producing a polyethylene resin porous film according to any one of claims 1 to 3, wherein the plasticizer is extracted and removed, followed by stretching.