JP4600970B2 - Method for producing polyolefin microporous membrane - Google Patents

Description

  The present invention relates to a production method excellent in productivity of a polyolefin microporous membrane effective as a battery separator.

Polyolefin microporous membranes are widely used as various battery separators. Polyolefin resins are often used particularly in lithium ion secondary batteries because they are excellent in resistance to organic solvents and in electronic insulation.
In recent years, multi-functionalization and weight reduction of portable devices are rapidly progressing, and batteries are required to have high capacity and high energy density, and in order to achieve these, thinning of separators is strongly desired. The thinned separator is required to have safety performance such as uniformity of thickness and low shrinkage at high temperature.

If the thickness of the microporous film is uneven, the lithium ion transmission resistance varies depending on the location, and the current distribution in the battery becomes non-uniform. Concentration of current at a specific portion may cause dendrite growth or heat generation. When the thermal shrinkage rate is large, when the internal temperature of the battery rises due to some abnormality, there is a possibility that the end of the electrode may come into contact with the separator due to the shrinkage, which may cause an internal short circuit. In the battery, the separator is wound in the MD direction, and it is particularly important to suppress shrinkage in the TD direction where shrinkage easily occurs.
As a main separator manufacturing method, a microporous membrane precursor is formed from a composition consisting of a polymer and a plasticizer by a phase separation process. There is a manufacturing method for forming a film. Among such known techniques, several proposals have been made for improving the thickness uniformity and shrinkage of the microporous membrane.

  In Patent Document 1, for the purpose of thickness uniformity, a sheet-like molded product made of polyolefin and a solvent is rolled by a belt press machine, and then stretched and desolvated to perform a microporous film having a uniform thickness. A manufacturing method is disclosed. Patent Document 2 discloses a method for producing a uniform microporous film by defining a pressing force and a pressing time in a rolling process using a belt press. However, when a sheet containing 70% or more of the solvent as described in Patent Document 1 or Patent Document 2 is heated and rolled, the amount of bleed out of the solvent from the sheet is large, and uniform rolling is continuously performed. It is difficult to maintain stable quality. Furthermore, a uniform and high-strength microporous film is obtained by using rolling and stretching in combination and making the rolling ratio larger than the stretching ratio, but high-magnification rolling with a belt press machine is difficult in a continuous production process. . Moreover, in the Example currently disclosed by patent document 1, the thickness dispersion | variation (difference of the maximum value and minimum value) is 2 micrometers-8 micrometers, and it cannot be said that it is necessarily excellent in thickness uniformity.

In Patent Document 2, a microporous film having a more uniform thickness is obtained by increasing the pressing time in rolling. However, in order to provide a long heating time as disclosed in the examples, the production line speed The productivity is low because it is necessary to reduce the energy consumption or to implement in large-scale equipment.
Further, Patent Document 3 discloses a method of rewinding and smoothing a product once wound up, but it adds a process and may cause uneven physical properties of the separator, resulting in low productivity.
For the purpose of low heat shrinkability, Patent Document 4 discloses a microporous film having a heat shrinkage rate of 20% or less in the MD direction and a heat shrinkage rate of 15% or less in the TD direction by heat treatment at a specific temperature and a manufacturing method. However, relaxing the orientation by heat treatment after the plasticizer extraction has a problem of reduced permeability and nonuniform thickness.

Patent Document 5 discloses a method for producing a polyolefin microporous membrane that has a high puncture strength and is difficult to tear by setting the ratio of the longitudinal draw ratio and the transverse draw ratio to 0.5 or more and 3 or less in the stretching step. . However, no attention has been paid to the relationship between the MD and TD stretch ratios in the initial stage of stretching, which affects the occurrence of stretching unevenness, that is, in the stretching process in which the area ratio is less than 4 times. In sequential stretching at a high MD ratio as described in Examples 3 and 4 of Patent Document 5, stretching unevenness may occur during TD stretching. Even in the simultaneous biaxial stretching described in Examples 1 and 2, stretching unevenness may occur depending on the relationship between the MD and TD stretching ratios in the stretching process in which the area ratio is less than 4 times. Absent.
JP 2000-230072 Japanese Patent Application Laid-Open No. 2002-292753 JP-A-8-39688 Japanese Patent Laid-Open No. 9-12756 Japanese Patent Laid-Open No. 9-3228

  An object of the present invention is to provide a method for producing a polyolefin microporous membrane that can be suitably used as a separator for a lithium ion secondary battery or the like, has both a uniform thickness distribution and low heat shrinkage, and is excellent in productivity. And

  As a result of intensive studies on the above problems, the inventors of the present invention have made (MD stretching ratio-1) / (TD stretching ratio-) in the process of stretching a polyolefin microporous membrane with an area ratio of less than 4 times. 1) It has been found that a microporous membrane excellent in thickness distribution and low heat shrinkability can be easily produced by satisfying the relationship of> 1 and having a final total area magnification of 4 times or more, thus achieving the present invention. It came to.

That is, the present invention is as follows.
(1) In a method for producing a polyolefin microporous membrane in which a polyolefin resin and a plasticizer are melt-kneaded and the resulting melt is formed into a sheet and then subjected to stretching and deplasticizing treatment, the area magnification is less than 4 times. The polyolefin microporous membrane characterized by satisfying the relationship of (MD draw ratio-1) / (TD draw ratio-1)> 1 and having a final total area ratio of 4 times or more. Production method.
(2) The method for producing a polyolefin microporous membrane according to (1), wherein the stretching method is simultaneous biaxial stretching.

  In the production method of the present invention, in the stretching process in which the area ratio is less than 4 times, the film is stretched under the conditions satisfying the relationship of (MD stretch ratio-1) / (TD stretch ratio-1)> 1, thereby obtaining a uniform thickness. It is possible to provide a method for producing a polyolefin microporous membrane having both distribution and low heat shrinkage and excellent productivity.

The method for producing a polyolefin microporous membrane of the present invention will be described in detail below, particularly focusing on its preferred form.
The polyolefin resin used in the present invention refers to a polyolefin resin used for normal extrusion, injection, inflation, and blow molding, and includes ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and Homopolymers and copolymers of 1-octene can be used. In addition, polyolefins selected from the group of these homopolymers and copolymers can be used alone or in combination. Representative examples of the polymer include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultrahigh molecular weight polyethylene, isotactic polypropylene, atactic polypropylene, polybutene, and ethylene propylene rubber. . When the microporous membrane obtained by the production method of the present invention is used as a battery separator, it is preferable to use a resin having a high melting point polyethylene as a main component because it is a low-melting-point resin and requires high strength. .

The viscosity average molecular weight of the polyolefin resin raw material used in the present invention and the microporous membrane of the present invention is preferably from 50,000 to less than 12 million, more preferably from 100,000 to less than 4 million, and most preferably from 200,000 to less than 2 million. is there. When the viscosity average molecular weight is less than 50,000, the melt tension at the time of melt molding becomes small and the moldability tends to be lowered, and it is difficult to impart sufficient entanglement and the strength tends to be low. When the viscosity average molecular weight exceeds 12 million, there is a tendency that uniform melt-kneading is difficult to obtain, and the sheet formability, particularly thickness stability, tends to be inferior.
In addition, the polyolefin composition used in the present invention includes phenolic, phosphorus-based, sulfur-based antioxidants, calcium stearate, zinc stearate, and the like, as necessary, as long as the advantages of the present invention are not impaired. Known additives such as metal soaps, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments can be mixed and used.

  The plasticizer used in the present invention may be any non-volatile solvent that can form a uniform solution above the melting point of the polyolefin resin when mixed with the polyolefin resin. Examples thereof include hydrocarbons such as liquid paraffin and paraffin wax, esters such as dioctyl phthalate and dibutyl phthalate, and higher alcohols such as oleyl alcohol and stearyl alcohol. In particular, when the polyolefin resin is polyethylene, liquid paraffin is preferable because it is highly compatible with polyethylene and is easy to perform uniform stretching because the resin and the plasticizer do not easily peel during stretching.

  The ratio of the polyolefin resin and the plasticizer used in the present invention is a ratio capable of uniform melt-kneading, is a ratio sufficient to form a sheet-like microporous membrane precursor, and has a high productivity. It is sufficient if it is not damaged. Specifically, the weight fraction of the polyolefin resin in the composition comprising the polyolefin resin and the plasticizer is preferably 10 to 70%, more preferably 20 to 60%. If the weight fraction of the polyolefin resin is smaller than 10%, the melt tension at the time of melt molding tends to be insufficient, and the moldability tends to be lowered. Moreover, the stretching stress applied during stretching may be reduced, and uniform stretching may be insufficient. On the other hand, when the weight fraction exceeds 70%, it is difficult to improve productivity due to an increase in extrusion load.

In the present invention, the polyolefin resin and the plasticizer are melt-kneaded by introducing the polyolefin resin into a resin kneading apparatus such as an extruder or kneader, introducing the plasticizer at an arbitrary ratio while heating and melting the resin, A method of obtaining a uniform solution by kneading a composition comprising a plasticizer is preferred. Moreover, what knead | mixed resin and a plasticizer previously may be thrown in. The polyolefin resin to be introduced may be in any form of powder, granules, and pellets. Moreover, when knead | mixing by such a method, it is preferable that the form of a plasticizer is a normal temperature liquid.
As an extruder, a single screw type extruder, a biaxial different direction screw type extruder, a biaxial same direction screw type extruder, etc. can be used. In particular, the twin screw extruder is preferable in that it can apply a shearing stress that sufficiently entangles molecular chains. The temperature at the time of melt kneading is preferably 140 ° C. or higher, and preferably 300 ° C. or lower in order to obtain a uniform kneaded product. More preferably, it is 160-250 degreeC.

In the present invention, a method for producing a sheet-like microporous membrane precursor by extruding a melt and cooling and solidifying is performed by extruding a uniform solution of a resin and a plasticizer into a sheet form via a T-die and the like and contacting the heat conductor. And cooling to a temperature sufficiently lower than the crystallization temperature of the resin. In the present invention, by extruding the melt under shear flow and then cooling and solidifying into a sheet shape, the obtained sheet is oriented more strongly in the MD direction than in the TD direction. Since the melt of the polyolefin and the plasticizer contains the plasticizer, the polyolefin molecules are easily stretched under shear, and a sheet arranged in the MD direction is obtained. The MD direction referred to here is the machine direction, that is, the winding direction during continuous film formation of the separator, and the TD direction is the width direction perpendicular to the MD direction. Polyolefin melts or polyolefin solutions are generally viscoelastic fluids and are non-Newtonian fluids whose shear viscosity varies with shear rate. Because of the viscoelastic fluid, the viscosity is constant when the shear rate is low, but the viscosity decreases when the shear rate is high. Shear rate ^-1 preferably 1000 seconds or less 1 sec ^-1 or more at the time of extrusion, more preferably 10 sec ^-1 to 800 sec ^-1. Since the shear rate is proportional to the extrusion amount and inversely proportional to the width of the T-die, when it is 1 second- ¹ or less, the extrusion amount is small and the product width is small, that is, the production speed and the production amount are reduced. When the shear rate is 1 second ⁻¹ or more, shear orientation in the inside of the T die or the like is large, and there is a tendency that MD orientation is high and the strength is high. Furthermore, since the molecular chains are melt-oriented, the entropy change in crystallization is reduced, and crystallization is more likely to occur, so that high strength can be expected. When the shear rate is 1000 seconds ⁻¹ or more, there is a case where the viscosity is greatly reduced and film formation is difficult. As the heat conductor used for cooling and solidification, metal, water, air, or the plasticizer itself can be used. In particular, a method of cooling by contacting with a metal roll has the highest heat conduction efficiency and is preferable. Also, when contacted with a metal roll, calendering or hot rolling such as sandwiching between rolls further increases the efficiency of heat conduction, and the sheet is oriented to increase the film strength. Since surface smoothness also improves, it is preferable.

  The present invention is characterized by the stretching process. In the stretching process, in the stretching process in which the area ratio is less than 4 times, the relationship of (MD stretching ratio-1) / (TD stretching ratio-1)> 1 is satisfied, and the final total area ratio is 4 times or more. Is essential to have both a uniform thickness distribution and low heat shrinkage. The area magnification is the product of the MD stretch ratio and the TD stretch ratio. The MD stretch ratio and the TD stretch ratio are the stretch ratios when the microporous membrane precursor is 1, and are defined as the ratio of the microporous membrane precursor to the original fabric even in the case of sequential stretching. The requirement of the present invention, “in the drawing process where the area magnification is less than 4 times, (MD draw ratio−1) / (TD draw ratio−1)> 1” is satisfied. , MD stretch ratio> TD stretch ratio, and MD stretch ratio <4 / TD.

  In the stretching process in which the area ratio is less than 4 times, (MD draw ratio-1) / (TD draw ratio-1) is preferably in the range of 1.2 to 20, more preferably in the range of 1.4 to 10. . A value of 1.2 or more is preferable because both thickness unevenness and TD shrinkage tend to be improved. If it is larger than 20, the stretching stress in the MD direction becomes too large, and there is a concern that the film may be detached from the chuck portion holding the film during stretching, or there is a concern of film breakage due to an increase in holding pressure, and stable stretching It can be difficult. The final total area magnification is preferably in the range of 9 to 100 times, more preferably in the range of 9 to 50 times. When the total area magnification is less than 9 times, sufficient strength may not be imparted to the film, and when it is 100 times or more, productivity may be reduced due to film breakage.

  The sheet-like microporous membrane precursor obtained by the film-forming method of the present invention comprises a polyolefin resin, a microcrystal consisting of a folded lamella or an extended chain, a spherulite with a large growth of microcrystals, a non-bonding crystal It is a heterogeneous structure that exists as crystal parts and the plasticizer exists between crystals, between spherulites, and inside amorphous parts. When these sheets are stretched, if the stretching stress is large, sufficient energy can be applied to stretch the folded lamella in the crystalline state, so that a uniform structure is easily obtained. When the stretching stress is small, sufficient energy cannot be applied to stretch the folded lamella, and the stress concentrates on weak parts such as tie molecules and thin fibrils between polyolefin crystals, which tends to cause uneven stretching. There is. When stretching unevenness occurs, there are a portion that has been highly stretched and a portion that has not been stretched, which causes uneven thickness, pore diameter, permeability, and the like. Briefly, stretching is performed uniformly when the stretching stress is large.

  The sheet-like microporous membrane precursor obtained by the film forming method of the present invention has a small stretching stress in the TD direction and a remarkably large stretching stress in the MD direction. This is due to the anisotropy of the orientation during film formation. Stretching in the MD direction with a large stretching stress can be performed uniformly, and stretching in the TD direction with a small stretching stress tends to be non-uniform stretching. In order to obtain a microporous film having a uniform thickness after stretching, it is necessary to preferentially perform stretching in the MD direction with a large stretching stress over stretching in the TD direction in a stretching process with an area ratio of less than 4 times. Specifically, in the stretching process in which the area ratio is less than 4 times, uniform stretching can be performed by controlling the stretching ratio in the relationship of (MD stretching ratio-1) / (TD stretching ratio-1)> 1. Is possible. After the area magnification reaches 4 times or more, uniform stretching is possible regardless of which of the MD direction and the TD direction is given priority. This is because when the area magnification reaches 4 times or more, the lamellae in the spherulites are stretched to form a network-like uniform fibril structure. As long as the present invention is satisfied, any method such as sequential stretching, multi-stage stretching, and multiple-time stretching may be used in combination with simultaneous biaxial stretching, but uniform stretching is that the stretching is simultaneous biaxial stretching. From the viewpoint of Here, the simultaneous biaxial stretching is a method in which stretching in the MD direction and stretching in the TD direction are simultaneously performed, and the deformation rate in each direction may be different. Sequential biaxial stretching is a technique in which stretching in the MD direction or TD direction is performed independently. When stretching is performed in the MD direction or TD direction, the other direction is in an unconstrained state or a constant length. It is in a fixed state.

Furthermore, although the reason is not clear, the area ratio is 4 times when the draw ratio is controlled in the relationship of (MD draw ratio-1) / (TD draw ratio-1)> 1 in the drawing process where the area ratio is less than 4 times. With the above microporous film, it is possible to obtain a microporous film having a similar puncture strength but small thermal shrinkage in the TD direction.
The stretching temperature is preferably a temperature in the range of 50 ° C. lower than the melting point Tm ° C. of the polyolefin microporous membrane to less than Tm ° C., more preferably 40 ° C. lower than the melting point Tm ° C. of the polyolefin microporous membrane to less than Tm ° C. The temperature in the range. If the stretching temperature is less than 50 ° C. lower than Tm ° C., the stretchability tends to decrease, and the strain component after stretching tends to remain, which tends to be insufficient in terms of shrinkage at high temperatures. When the stretching temperature is Tm ° C. or higher, the microporous membrane tends to melt and impair the permeation performance.

The method of extracting the plasticizer may be either a batch type or a continuous type. However, the plasticizer is extracted by immersing the microporous membrane in an extraction solvent and sufficiently dried, so that the plasticizer is substantially removed from the microporous membrane. It is necessary to remove. In order to suppress the shrinkage of the microporous membrane, it is preferable to constrain the end of the microporous membrane during a series of steps of immersion and drying. Further, the residual amount of plasticizer in the microporous membrane after extraction is preferably less than 1% by weight.
The extraction solvent used in the present invention is preferably a poor solvent for polyolefin and a good solvent for plasticizer, and its boiling point is preferably lower than the melting point of the polyolefin microporous membrane. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane, alcohols such as ethanol and isopropanol, diethyl ether, Examples include ethers such as tetrahydrofuran and ketones such as acetone and 2-butanone.

In the production method of the present invention, adding a heat treatment step such as heat fixation and thermal relaxation after each stretching step within a range not impairing the advantages of the present invention has the effect of further suppressing the shrinkage of the microporous membrane. is there.
Moreover, in the manufacturing method of this invention, you may post-process in the range which does not impair the advantage of this invention. Examples of the post-treatment include a hydrophilic treatment with a surfactant and the like, and a crosslinking treatment with ionizing radiation.
The final film thickness of the microporous membrane produced by the production method of the present invention is preferably 1 to 100 μm, and more preferably 5 to 50 μm. If the film thickness is smaller than 1 μm, the mechanical strength may be insufficient, and if it is larger than 100 μm, the occupied volume of the separator increases, which tends to be disadvantageous in terms of increasing the battery capacity.

  The porosity is preferably in the range of 25% to 60%, more preferably 30% to 55%. If the porosity is less than 25%, the permeability tends to decrease, while if it exceeds 60%, the mechanical strength tends to decrease. The puncture strength of the microporous membrane of the present invention is preferably 3.0N or more, and more preferably 4.0N or more. If it is less than 3.0N, when used as a battery separator, the separator is easily broken by the dropped active material or the like.

The invention will now be described in more detail by way of examples, which do not limit the scope of the invention. The test methods in the examples are as follows.
<Evaluation of microporous membrane>
(1) Film thickness Measured with a dial gauge (Ozaki Seisakusho: trademark, PEACOCK No. 25). A sample of MD 300 mm × TD 300 mm was cut out from the microporous membrane, and the film thickness at 16 points (4 points × 4 points) in a lattice shape was measured. The average value obtained was taken as the film thickness.
(2) Puncture strength Using a Kato-Tech, trademark, KES-G5 handy compression tester, a puncture test is performed under the conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec. (N).
(3) Thermal shrinkage A sample of MD 100 mm × TD 100 mm was cut out from the microporous membrane and placed horizontally in an oven at 120 ° C. and left for 1 hour. Then, it cooled with air and measured TD length (mm).
Shrinkage rate (%) = (1-TD length (mm) / 100) × 100
(4) Thickness unevenness A sample of MD 360 mm × TD 360 mm was cut out from the microporous film, and the film thickness was measured at 16 points (4 points × 4 points) in a lattice shape. The standard deviation of the obtained film thickness was defined as thickness unevenness.
Thickness unevenness (μm) = (Σ (thickness of each location - average film ^{thickness) 2/15) 1/2}

[Example 1]
As an antioxidant, pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] was added as an antioxidant to 60 parts by weight of HDPE having an Mv of 270,000 and 40 parts by weight of HDPE having an Mv of 950,000. Part by weight was added and dry blended using a tumbler blender to obtain a polymer mixture. The obtained mixture of polymers and the like was fed to a feed port of a twin screw co-directional screw type extruder having a diameter of 37 mm by a feeder. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected into the twin-screw extruder cylinder by a plunger pump.

The feeder and pump were adjusted so that the liquid paraffin content ratio in the total mixture (100 parts by weight) melted and kneaded and extruded was 65 parts by weight. The melt-kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 180 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded product was extruded through a T-die between cooling rolls controlled at a surface temperature of 25 ° C. to obtain a sheet-like polyolefin composition having a thickness of 1000 μm.
Next, the obtained sheet was cut into 80 mm square and stretched at a stretching temperature of 123 ° C. using a batch biaxial stretching machine (manufactured by Iwamoto Seisakusho). Stretching is constant so that (MD-1) / (TD-1) = 2.0 up to an area magnification of 6 times, specifically, the MD magnification is 3 times and the TD magnification is 2 times. Simultaneous biaxial stretching at speed. Thereafter, simultaneous biaxial stretching was performed so that the MD magnification was 7 times and the TD magnification was 7 times. Next, the four sides were fixed to a frame and immersed in methylene chloride to extract and remove liquid paraffin, followed by drying to obtain a microporous membrane.

  Table 1 shows the values of the film thickness, film thickness unevenness, thermal contraction rate, and puncture strength of the obtained microporous film. The values for the following examples and comparative examples are also shown in Table 1. The stretching ratio process is shown in FIG. The following examples and comparative examples are also shown in Table 1.

[Example 2]
Stretching is constant so that (MD-1) / (TD-1) = 4.0 up to an area magnification of 10 times, specifically, MD magnification is 5 times and TD magnification is 2 times. Simultaneous biaxial stretching at speed. Thereafter, a microporous membrane was obtained in the same manner as in Example 1 except that the simultaneous biaxial stretching was performed so that the MD magnification was 7 times and the TD magnification was 7 times.

[ Comparative Example 4 ]
Stretching was uniaxially stretched so that (MD-1) / (TD-1)> 8.3 when the area magnification was less than 4 times, specifically, the MD magnification was 3 times. Thereafter, a microporous membrane was obtained in the same manner as in Example 1 except that the simultaneous biaxial stretching was performed at a constant speed so that the MD magnification was 7 times and the TD magnification was 7 times.

[Example 3 ]
Specifically, the sheet-like polyolefin composition has a thickness of 600 μm and is stretched so that (MD-1) / (TD-1) = 4.0 until the area magnification is 10 times, specifically, the MD magnification is 5 times. The TD magnification was simultaneously biaxially stretched at a constant speed so as to be 2 times. Thereafter, a microporous membrane was obtained in the same manner as in Example 1 except that the simultaneous biaxial stretching was performed so that the MD magnification was 7 times and the TD magnification was 7 times.

[Example 4 ]
30 parts by weight of HDPE of Mv 270,000, 40 parts by weight of HDPE of Mv 950,000, 30 parts by weight of LDPE of Mv 350,000, pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) as an antioxidant ) Propionate] was added, and dry blended using a tumbler blender to obtain a polymer mixture. The obtained mixture of polymers and the like was fed to a feed port of a twin screw co-directional screw type extruder having a diameter of 37 mm by a feeder. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected into the twin-screw extruder cylinder by a plunger pump.

The feeder and pump were adjusted so that the liquid paraffin content ratio in the total mixture (100 parts by weight) melted and kneaded and extruded was 65 parts by weight. The melt-kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 180 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded product was extruded through a T-die between cooling rolls controlled at a surface temperature of 25 ° C. to obtain a sheet-like polyolefin composition having a thickness of 600 μm.
Next, the obtained sheet was cut into 80 mm square and stretched at a stretching temperature of 121 ° C. using a batch biaxial stretching machine (manufactured by Iwamoto Seisakusho). Stretching is constant so that (MD-1) / (TD-1) = 4.0 up to an area magnification of 10 times, specifically, MD magnification is 5 times and TD magnification is 2 times. Simultaneous biaxial stretching at speed. Thereafter, simultaneous biaxial stretching was performed so that the MD magnification was 7 times and the TD magnification was 7 times. Next, the four sides were fixed to a frame and immersed in methylene chloride to extract and remove liquid paraffin, followed by drying to obtain a microporous membrane.

[Example 5 ]
A sheet-like polyolefin composition having a discharge amount of 10 kg / h and a thickness of 270 μm was obtained. Next, the obtained sheet was cut into a 120 mm square and stretched at a stretching temperature of 121 ° C. using a batch biaxial stretching machine (manufactured by Iwamoto Seisakusho). Stretching is such that (MD-1) / (TD-1) = 1.3 until the area magnification is 10.8 times, specifically, the MD magnification is 3.6 times and the TD magnification is 3 times. Simultaneously biaxial stretching was performed at a constant speed. Thereafter, a microporous membrane was obtained in the same manner as in Example 1 except that simultaneous biaxial stretching was performed so that the MD magnification was 4 times and the TD magnification was 4 times.

[Example 6 ]
Specifically, the sheet-like polyolefin composition is continuously led to a simultaneous biaxial tenter, and (MD-1) / (TD-1) = 3.3 up to 2.6 times the area magnification, 1) The MD magnification is 2 times and the TD magnification is 1.3 times at a constant speed, and the area magnification is 2.6 times to 5.4 times (MD-1) / (TD-1)> 2. Specifically, (2) The MD magnification is 3 times and the TD magnification is 1.8 times at a constant speed, and the area magnification is 5.4 times to 10.4 times ( MD-1) / (TD-1)> 1.9, specifically, (3) Stretch ratio is changed stepwise so that MD magnification is 4 times and TD magnification is 2.6 times. Then, a microporous membrane was obtained in the same manner as in Example 1 except that the simultaneous biaxial stretching was performed smoothly so that the MD magnification was 7 times and the TD magnification was 7 times.

[Comparative Example 1]
Stretching is constant so that (MD-1) / (TD-1) = 0.5 until the area magnification is 6 times, specifically, the MD magnification is 2 times and the TD magnification is 3 times. Simultaneous biaxial stretching at speed. Thereafter, a microporous membrane was obtained in the same manner as in Example 1 except that the simultaneous biaxial stretching was performed so that the MD magnification was 7 times and the TD magnification was 7 times.

[Comparative Example 2]
Stretching is constant so that (MD-1) / (TD-1) = 0.25 until the area magnification is 10 times, specifically, the MD magnification is 2 times and the TD magnification is 5 times. Simultaneous biaxial stretching at speed. Thereafter, a microporous membrane was obtained in the same manner as in Example 1 except that the simultaneous biaxial stretching was performed so that the MD magnification was 7 times and the TD magnification was 7 times.

[Comparative Example 3]
Stretching at a constant speed so that (MD-1) / (TD-1) = 1 until the area magnification is 16 times, specifically, the MD magnification is 4 times and the TD magnification is 4 times. A microporous membrane was obtained in the same manner as in Example 6 except that simultaneous biaxial stretching was performed.

  As is apparent from Table 1, the microporous membrane produced by the production method of the present invention has both a uniform thickness distribution and low heat shrinkability as compared with Comparative Examples 1, 2, and 3.

  The polyolefin microporous membrane produced by the production method of the present invention can be suitably used particularly as a battery separator.

It is a graph which shows the change of MD magnification and TD magnification in an extending process in an Example and a comparative example.

Claims (1)

In the method for producing a polyolefin microporous membrane, which is obtained by melt-kneading a polyolefin resin and a plasticizer, molding the obtained melt into a sheet, and then performing stretching and deplasticizing treatment.
Said stretching in the stretching process of the area magnification is less than 4 times, be a (MD draw ratio -1) / met (TD stretching ratio -1)> 1 relationship, ultimately total area ratio is 4 times or more ,
The stretching is simultaneous biaxial stretching,
The method for producing a microporous polyolefin membrane is characterized in that the stretching profile is a polygonal line or a curve when the MD stretching ratio is the vertical axis and the TD stretching ratio is the horizontal axis .

Images