JP5545928B2 - Method for producing polyolefin microporous membrane - Google Patents

Description

  The present invention relates to a method for producing a polyolefin microporous membrane.

  In recent years, power storage devices such as lithium ion secondary batteries and electric double layer capacitors (including those called lithium ion capacitors and non-aqueous lithium power storage elements) have been actively developed. In a power storage device, a microporous membrane (separator) is usually provided between the positive and negative electrodes. Such a separator has a function of preventing contact between positive and negative electrodes and transmitting ions.

  For example, in Patent Document 1, a raw material composition containing a polyolefin, a plasticizer, and a fine particle roughening agent is heated and melted to form a sheet, and the porous sheet containing the fine particle roughening agent is formed by removing the plasticizer. Thereafter, a production method is disclosed in which a porous sheet containing no particulate surface roughening agent is laminated and further heat-set by an appropriate technique. In Patent Document 2, a raw material composition containing a polyolefin, a plasticizer, and a filler is heated and melted to form a sheet. After removing the plasticizer to form a porous sheet, the porous sheet is at least in a single layer state. A method for producing a polyolefin microporous membrane that is stretched in one direction is disclosed.

JP-A-11-060792 Special table 2008-523211 gazette

Here, the separator is required to have a certain withstand voltage performance from the viewpoint of ensuring good safety of the electricity storage device. Moreover, low air permeability is calculated | required from a viewpoint of achieving the high output of an electrical storage device. Therefore, development of a method for producing a polyolefin microporous membrane having such functions is desired.
However, the polyolefin microporous membranes described in Patent Documents 1 and 2 still have room for improvement from the viewpoint of achieving both low air permeability and high withstand voltage performance.
This invention makes it a subject to provide the manufacturing method of the polyolefin microporous film which can make low air permeability and high withstand voltage performance compatible.

  As a result of intensive studies in view of the above circumstances, the present inventors have used a specific raw material and formed a microporous polyolefin film by using a specific technique, thereby achieving a low-permeability and high withstand voltage performance. The inventors have found that a porous film can be realized, and have completed the present invention.

That is, the present invention is as follows.
[1]
The following steps (A) and (B)
(A) a sheet forming step of kneading a raw material composition containing a polyolefin resin, a filler, and a plasticizer to form a sheet;
(B) After the step (A), the plasticizer is extracted from the sheet to form a filler-containing microporous sheet, and two or more filler-containing microporous sheets are stacked and stretched to TD.
A method for producing a polyolefin microporous membrane comprising:
[2]
[1] The production method according to [1], wherein the stretching step is a step of stretching the filler-containing microporous sheet one or more times in both MD and TD directions.
[3]
The manufacturing method according to [2], wherein the stretching step is a step of performing TD stretching after MD stretching.
[4]
The production method according to [2] or [3], wherein the stretching step is a step satisfying MD stretching ratio ≧ TD stretching ratio.
[5]
The production method according to any one of [1] to [4], wherein the polyolefin resin has a viscosity average molecular weight of 300,000 to 10,000,000.
[6]
The production method according to any one of [1] to [5], wherein the filler is silica powder.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the polyolefin microporous film which can make low air permeability and high withstand voltage performance compatible is provided.

  The best mode for carrying out the present invention (hereinafter abbreviated as “embodiment”) will be described in detail below. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

The manufacturing method of the present embodiment includes the following steps (A) and (B):
(A) a sheet forming step of kneading a raw material composition containing a polyolefin resin, a filler, and a plasticizer to form a sheet;
(B) After the step (A), the plasticizer is extracted from the sheet to form a filler-containing microporous sheet, and two or more filler-containing microporous sheets are stacked and stretched to TD.
including.

Here, in the present embodiment, the low-permeability and high withstand voltage performance of the obtained polyolefin microporous membrane can be achieved by going through the steps as described above.
The reason is not clear, but the filler contains a moderate interaction between the filler and the polyolefin resin (an interaction based on the affinity between the filler surface and the polyolefin resin, or an interaction between the polyolefin resin based on the filler shape). Not only within the same sheet but also between polyolefin resins forming other sheets via the interface, it is oriented in the resin discharge direction (in this specification, it may be abbreviated as “MD”). The polyolefin molecular chain is appropriately torn by stretching in the film width direction (sometimes abbreviated as “TD” in the present specification), realizing an appropriate porosity, pore size distribution, and strength expression. It is considered that low air permeability and high withstand voltage performance are achieved.

Examples of the polyolefin resin used in the step (A) include heavy polymers obtained by polymerizing monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Examples thereof include homopolymers (homopolymers, copolymers, multistage polymers, etc.). These polymers can be used alone or in combination of two or more.
Examples of the polyolefin resin include polyethylene (for example, low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultrahigh molecular weight polyethylene), polypropylene (for example, isotactic polypropylene, atactic polypropylene). , Polybutene, ethylene propylene rubber and the like.
Here, “low density polyethylene” means polyethylene having a density of 0.910 to 0.930 g / cm ³ , and “high density polyethylene” means polyethylene having a density of 0.942 g / cm ³ or more. Hereinafter, polyethylene may be abbreviated as “PE” and polypropylene may be abbreviated as “PP”.

The polyolefin resin preferably contains high-density polyethylene from the viewpoint of improving the voltage resistance of the polyolefin microporous membrane.
The proportion of the high density polyethylene in the polyolefin resin is preferably 10% by mass or more, more preferably 30% by mass or more, still more preferably 50% by mass or more, and may be 100% by mass.

  The viscosity average molecular weight of the polyolefin resin (when using a plurality of polyolefin resins, the total viscosity average molecular weight) is preferably 300,000 or more, more preferably 500,000 or more, and preferably 10 million or less. Preferably it is 3 million or less. Setting the viscosity average molecular weight to 300,000 or more is preferable from the viewpoint of maintaining high voltage resistance, mechanical strength, and melt tension during melt molding, and ensuring good moldability. On the other hand, setting the viscosity average molecular weight to 10 million or less is preferable from the viewpoint of achieving uniform melt-kneading and improving sheet formability, particularly thickness stability. Furthermore, it is preferable that the viscosity average molecular weight is 3 million or less from the viewpoint of further improving the moldability.

  The proportion of the polyolefin resin (the total amount when a plurality of polyolefin resins are used) in the raw material composition is preferably 10% by mass or more, more preferably 20% by mass or more, preferably as an upper limit. Is 50% by mass or less, more preferably 40% by mass or less.

As the filler used in the step (A), either organic fine particles or inorganic fine particles can be used.
Examples of the organic fine particles include modified polystyrene fine particles and modified acrylic resin particles.
Examples of the inorganic fine particles include alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, iron oxide and other oxide ceramics, silicon nitride, titanium nitride, boron nitride and the like. Nitride ceramics, silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amicite, bentonite, asbestos, Examples include zeolite, calcium silicate, magnesium silicate, diatomaceous earth, ceramics such as silica sand, and glass fiber.
These can be used alone or in combination of two or more. Among these, silica, alumina, and titania are more preferable from the viewpoint of electrochemical stability. Silica is particularly preferable.

The average particle size of the filler is preferably 1 nm or more, more preferably 6 nm or more, still more preferably 10 nm or more, and the upper limit is preferably 100 nm or less, more preferably 80 nm or less, still more preferably 60 nm or less.
When the average particle size is 100 nm or less, even when stretching or the like is performed, peeling between the polyolefin resin and the filler tends not to occur, which is preferable from the viewpoint of suppressing generation of voids. Here, it is difficult to cause peeling between the polyolefin resin and the filler from the viewpoint of increasing the hardness of the fibril itself constituting the microporous membrane, and the tendency to be excellent in compression resistance performance in the local region of the polyolefin microporous membrane, Or since the tendency which is excellent in heat resistance is observed, it is preferable. In addition, the close contact between the polyolefin resin and the filler improves the affinity of the electricity storage device separator with the non-aqueous electrolyte, from the viewpoint of realizing a separator excellent in output retention performance, cycle retention performance, etc. preferable.
On the other hand, setting the average particle size to 1 nm or more is preferable from the viewpoint of ensuring the dispersibility of the filler and improving the compression resistance in the local region.
In addition, the average particle diameter of the filler was obtained by magnifying and observing the sample with an electron microscope, randomly measuring 20 particle diameters, and taking the average value. The particle diameter was a diameter of a circle having the same area as the observed area.

  The proportion of the filler in the raw material composition is preferably 10% by mass or more, more preferably 15% by mass or more, preferably 70% by mass or less, more preferably 50% by mass or less, and still more preferably. 30% by mass or less. Setting the ratio to 10% by mass or more is preferable from the viewpoint of forming the polyolefin microporous film with a high porosity or improving the heat resistance. On the other hand, setting the ratio to 70% by mass or less is preferable from the viewpoint of improving the film formability at a high draw ratio and improving the puncture strength of the polyolefin microporous film.

Examples of the plasticizer used in the step (A) include hydrocarbons such as liquid paraffin and paraffin wax;
Esters such as diethyl hexyl phthalate and dibutyl phthalate;
Higher alcohols such as oleyl alcohol and stearyl alcohol;
Etc.
In particular, dioctyl phthalate is preferably used from the viewpoint of increasing the load during melt extrusion of the kneaded product and improving the dispersibility of silica.

  The proportion of the plasticizer in the raw material composition is preferably 30% by mass or more, more preferably 40% by mass or more, and the upper limit is preferably 80% by mass or less, preferably 60% by mass or less. . Setting the ratio to 80% by mass or less is preferable from the viewpoint of maintaining high melt tension during melt molding and ensuring moldability. On the other hand, setting the ratio to 30% by mass or more is preferable from the viewpoint of securing moldability and efficiently extending the lamellar crystals in the crystal region of the polyolefin resin. Here, the fact that the lamellar crystal is efficiently stretched means that the polyolefin chain is efficiently stretched without causing the polyolefin chain to be broken, and the formation of a uniform and fine pore structure and the mechanical strength of the polyolefin microporous membrane It can contribute to the improvement of crystallinity.

Various additives can be mixed and used in the raw material composition as necessary. Examples of such additives include phenol-based, phosphorus-based and sulfur-based antioxidants; metal soaps such as calcium stearate and zinc stearate; UV absorbers, light stabilizers, antistatic agents, antifogging agents, Examples thereof include coloring pigments.
In addition, as a ratio for which an additive accounts in the said raw material composition, Preferably it is 5 mass% or less.

In the step (A), examples of the method for kneading the polyolefin resin, filler, and plasticizer include the following methods (a) and (b).
(A) A method in which a polyolefin resin and a filler are put into a resin kneading apparatus such as an extruder or a kneader and kneaded, and further a plasticizer is introduced and kneaded while heating.
(B) A process of pre-kneading a polyolefin resin, a filler and a plasticizer in advance at a predetermined ratio using a Henschel mixer, etc., putting the kneaded product into an extruder and introducing and further kneading the plasticizer while heating and melting. how to.

  In the step (A), examples of the method for forming the sheet include a method in which the raw material composition is kneaded and then extruded into a sheet shape through a T-die or the like and brought into contact with a heat conductor to be cooled and solidified. . As the heat conductor, metal, water, air, plasticizer itself, or the like can be used. Moreover, it is preferable to cool and solidify by sandwiching between rolls from the viewpoint of increasing the film strength of the sheet-like molded body and improving the surface smoothness of the sheet-like molded body.

  On the other hand, in the step (B), examples of a method for forming a filler-containing microporous sheet by extracting a plasticizer include a method of immersing in methylene chloride. In addition, in such a plasticizer extraction process, it is preferable that a part of the filler is extracted in addition to the plasticizer, but the filler is not substantially extracted.

In the step (B), two or more filler-containing microporous sheets are stacked and stretched in the TD direction. Here, it is preferable that the filler-containing microporous sheets are directly stacked. The term “directly” as used herein means that there is substantially no other sheet layer interposed, but other layers (for example, filler-containing microporous materials) within the range not impairing the object of the present embodiment. A coating layer on the surface of the sheet, other thin film sheets, etc.) may be interposed.
In addition to stretching to TD, stretching to MD, especially performing TD stretching after MD stretching, achieves both low air permeability and high withstand voltage performance of the resulting microporous membrane at a higher level. From the viewpoint of making it.

Examples of the stretching method in the step (B) include simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple stretching. As the order of stretching, MD stretching can be performed after TD stretching.
The MD draw ratio is preferably 10 times or less, more preferably 5 times or less. The TD stretch ratio is preferably 8 times or less, more preferably 4 times or less.
Moreover, as a relationship between MD stretch ratio and TD stretch ratio, it is preferable that MD stretch ratio ≧ TD stretch ratio.
Further, the surface magnification is preferably 100 times or less, more preferably 50 times or less, still more preferably 16 times or less, and particularly preferably 10 times or less.
Performing such stretching is preferable from the viewpoint of suppressing generation of voids and reduction in mechanical strength.

  In addition, when performing stretching, a known stretching machine such as a roll or a tenter can be used. However, using a roll for a part of the stretching process can reduce the air permeability and the high resistance of the polyolefin microporous membrane. This is preferable from the viewpoint of better expressing the voltage performance.

  The MD stretching temperature is preferably a melting point temperature of −50 ° C. or higher, more preferably a melting point temperature of −30 ° C. or higher, and still more preferably a melting point temperature of −20 ° C. or higher, with the melting point temperature of the polyolefin resin constituting the film as a reference temperature. The upper limit is preferably the melting point temperature + 10 ° C. or lower. The stretching temperature of −50 ° C. or higher means that the interface between the polyolefin resin and the filler, or the interface between the polyolefin resin and the plasticizer can be satisfactorily adhered. It is preferable from the viewpoint of improving the compression performance and from the viewpoint of integrating the two films. For example, when high-density polyethylene is used as the polyolefin resin, the stretching temperature is preferably 115 ° C. or higher and 150 ° C. or lower. When a plurality of polyolefin resins are mixed and used, the melting point of the polyolefin resin having the larger heat of fusion can be used as a reference.

  The TD stretching temperature is preferably the melting point temperature + 40 ° C. or lower, more preferably the melting point temperature + 30 ° C. or lower, based on the melting point temperature of the polyolefin resin, and the lower limit is preferably the melting point temperature or higher. Setting the temperature to the melting point temperature or higher suppresses the occurrence of film breakage, etc., and reduces the thermal shrinkage rate of the polyolefin microporous film under the condition of 140 ° C., and integrates the two films. It is preferable from the viewpoint. On the other hand, setting the melting point temperature to 40 ° C. or lower is preferable from the viewpoint of suppressing the shrinkage of the polyolefin resin and reducing the thermal shrinkage rate of the polyolefin microporous membrane.

In addition, after the said (B) process, the process of performing heat processing and the process of performing post-processing may be included.
The temperature of the heat treatment is preferably the melting point temperature + 40 ° C. or lower, more preferably the melting point temperature + 30 ° C. or lower, based on the melting point temperature of the polyolefin resin, and the lower limit is preferably the melting point temperature or higher. Setting the temperature of the heat treatment to the melting point temperature or more is preferable from the viewpoint of suppressing the occurrence of film breakage and the like and reducing the thermal shrinkage rate of the polyolefin microporous film under the condition of 140 ° C. On the other hand, a melting point temperature of + 40 ° C. or lower is preferable from the viewpoint of suppressing the shrinkage of the polyolefin resin and reducing the thermal shrinkage rate of the polyolefin microporous membrane.

  Examples of the post-treatment include a hydrophilic treatment with a surfactant and the like, and a crosslinking treatment with ionizing radiation.

  The porosity of the microporous membrane obtained by the production method of the present embodiment is preferably 50% or more, more preferably 55% or more, preferably 90% or less, more preferably 85% or less. Setting the porosity to 50% or more is preferable from the viewpoint of securing the output of the battery. On the other hand, setting it to 90% or less is preferable from the viewpoint of securing a withstand voltage performance and mechanical strength.

  The final film thickness of the microporous membrane is preferably 2 μm or more, more preferably 5 μm or more, preferably 200 μm or less, more preferably 100 μm or less. Setting the film thickness to 2 μm or more is preferable from the viewpoint of improving the mechanical strength. On the other hand, a thickness of 200 μm or less is preferable because output characteristics are maintained and there is a tendency to be advantageous in terms of increasing the battery capacity.

  The air permeability of the microporous membrane is preferably 10 seconds or more, more preferably 50 seconds or more, preferably 1000 seconds or less, preferably 500 seconds or less, and more preferably 300 seconds or less. Setting the air permeability to 10 seconds or more is preferable from the viewpoint of suppressing self-discharge of the electricity storage device. On the other hand, setting it to 1000 seconds or less is preferable from the viewpoint of obtaining good charge / discharge characteristics.

  The withstand voltage of the microporous membrane is preferably 1.5 kV or more, more preferably 1.7 kV or more, in terms of a film thickness of 80 μm. Setting the withstand voltage to 1.5 kV or more is preferable from the viewpoint of ensuring the safety of the electricity storage device.

The porosity, film thickness, air permeability, and withstand voltage can be adjusted by adjusting the raw material composition, adjusting the stretching temperature and the stretching ratio, adjusting the temperature of the heat treatment step, and the like.
Moreover, the measured value of the various parameters mentioned above is measured according to the measuring method in the Example mentioned later.

The microporous membrane obtained by the production method of the present embodiment is particularly useful as a power storage device separator that uses a non-aqueous electrolyte. An electricity storage device generally uses a polyolefin microporous membrane as a separator, and includes a positive electrode, a negative electrode, and an electrolytic solution.
In the electricity storage device, for example, the microporous membrane is prepared as a vertically long separator having a width of 10 to 500 mm (preferably 80 to 500 mm) and a length of 200 to 4000 m (preferably 1000 to 4000 m). -Separator-negative electrode-separator or negative electrode-separator-positive electrode-separator are stacked in this order and wound into a circular or flat spiral shape to obtain a wound body, which is then housed in a battery can and further electrolyzed. It can be manufactured by injecting a liquid.
The electric storage device is manufactured through a process of laminating a flat plate in the order of positive electrode-separator-negative electrode-separator or negative electrode-separator-positive electrode-separator, laminating with a bag-like film, and injecting an electrolyte solution. You can also.

  An electricity storage device using a microporous membrane obtained by the manufacturing method of the present embodiment is excellent in high output performance (low air permeability) and reliability (high withstand voltage performance). Particularly useful for multilayer capacitors.

  Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist. In addition, the physical property in an Example was measured with the following method.

(1) Film thickness (μm)
The measurement was performed at a room temperature of 23 ° C. using a microthickness measuring instrument (type KBM manufactured by Toyo Seiki Co., Ltd.).

(2) Porosity (%)
A sample of 10 cm × 10 cm square was cut out from the microporous membrane, its volume (cm ³ ) and mass (g) were determined, and calculated from these and the film density (g / cm ³ ) using the following formula.
Porosity (%) = (volume-mass / density of mixed composition) / volume × 100
In addition, the value calculated | required by calculation from the density and mixing ratio of each used polyolefin resin and a filler was used for the density of the mixed composition.

(3) Air permeability (second)
It measured with the Gurley type air permeability meter (made by Toyo Seiki) based on JIS P-8117. It converted into the value per 80 micrometers thickness by the proportional calculation.

(4) Withstand voltage (kV)
A microporous membrane was sandwiched between aluminum electrodes having a diameter of 3 cm, a load of 15 g was applied, and this was connected to a withstand voltage measuring machine (TOS9201) manufactured by Kikusui Electronics Corporation to carry out the measurement. The measurement conditions were AC voltage (60 Hz) applied at a speed of 1.0 kV / sec, and the shorted voltage value was taken as the withstand voltage measurement value of the microporous membrane. It converted into the value per 80 micrometers thickness by the proportional calculation.
In addition, the measurement results were evaluated in the form of “(minimum value) to (maximum value)” using the minimum and maximum values of the values measured at 15 points on TD and 7 points on MD (total of 105 points). It was.

(5) Viscosity average molecular weight (Mv)
In order to prevent deterioration of the sample, 2,6-di-t-butyl-4-methylphenol is dissolved in decahydronaphthalene to a concentration of 0.1 w%, and this (hereinafter abbreviated as DHN) is used as the sample solvent. .
A sample solution is prepared by dissolving the sample in DHN at 150 ° C. to a concentration of 0.1 w%. 10 ml of the prepared sample solution is sampled, and the number of seconds passing through the marked line (t) at 135 ° C. is measured with a Canon Fenceke viscometer (SO100). Further, after heating the DHN to 0.99 ° C., and 10ml collected to measure the number of seconds passing between marked lines of the viscometer in the same manner _{(t B).} The intrinsic viscosity [η] was calculated by the following conversion formula using the obtained passing seconds t and tB.
[Η] = ((1.651 t / t _B −0.651) ^0.5 −1) /0.0834
Mv was calculated from the obtained [η] by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67

(6) Melting point It measured using DSC60 by Shimadzu Corporation. 3 mg of a sample was spread on an aluminum open sample pan having a diameter of 5 mm, and a clamping cover was placed thereon and fixed in the aluminum pan with a sample sealer. Under a nitrogen atmosphere, the temperature was increased from 30 ° C. to 200 ° C. at a rate of temperature increase of 10 ° C./min to obtain a melting endothermic curve. The peak top temperature of the obtained melting endotherm curve was defined as the melting point (° C.).

(7) Puncture strength Using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech, a microporous membrane was fixed with a sample holder having a diameter of 11.3 mm at the opening. Next, the puncture strength (N) is obtained as the maximum puncture load by conducting a puncture test on the fixed microporous membrane at a needle radius of curvature of 0.5 mm and a puncture speed of 2 mm / sec in a 25 ° C. atmosphere. )

(8) Tensile strength In accordance with JIS K7127, using a tensile tester manufactured by Shimadzu Corporation, Autograph AG-A (trademark), samples in the length direction (MD) and the width direction (TD) (shape; width 10 mm × length 100 mm). In addition, the sample had a chuck spacing of 50 mm. The tensile strength (MPa) was obtained by dividing the strength at break by the sample cross-sectional area before the test. The measurement was performed at a temperature of 23 ± 2 ° C., a chuck pressure of 0.30 MPa, and a tensile speed of 200 mm / min.

(9) Average pore diameter Based on ASTM F-316-86, it calculated by the half dry method.

(10) Maximum pore diameter Based on ASTM E-128-61, it was calculated by bubble point (BP) in ethanol.

[Example 1]
High-density polyethylene “SH800” with a viscosity average molecular weight (Mv) of 250,000 (manufactured by Asahi Kasei Chemicals Corporation, melting point 135.5 ° C.) 12.8 parts by mass, ultra high molecular weight polyethylene “UH650” with an Mv of 1,000,000 (Asahi Kasei Chemicals Corporation) ), Melting point 136.6 ° C.) 19.2 parts by mass, silica “LP” having an average primary particle size of 15 nm (manufactured by Tosoh Silica) 20 parts by mass, antioxidant 0.3 parts by mass, plastic As an agent, 48 parts by mass of dihexyl phthalate (DOP) was mixed with a Henschel mixer and granulated. Then, it knead | mixed and extruded at 200 degreeC with the twin-screw extruder equipped with T-die, and shape | molded in the sheet form of thickness 100 micrometers. DOP was extracted and removed from the molded product with methylene chloride to form a microporous membrane. Two of the microporous membranes were stacked and heated to 130 ° C. and stretched by MD and 5 times in a roll, and then heated to a maximum temperature of 140 ° C. and 1.7 times in TD and stretched by a tenter.
The obtained membrane had an MD tensile strength of 29 MPa, a TD tensile strength of 5 MPa, an average pore diameter of 0.20 μm, and a maximum pore diameter of 0.27 μm. Table 1 shows the film forming conditions and other film characteristics.

[Example 2]
The raw material composition of the polyolefin of Example 1 was changed to 21.3 parts by mass for SH800 with Mv250,000 and 20.7 parts by mass for UH650 with Mv1 million, and microporous as in Example 1 except for the conditions shown in Table 1. A membrane was obtained. The film forming conditions and film characteristics are shown in Table 1.

[Example 3]
The raw material composition of the polyolefin of Example 1 was changed to 29.9 parts by mass for SH800 with Mv 250,000 and 2.1 parts by mass with UH 650 for Mv 1 million, and microporous as in Example 1 except for the conditions shown in Table 1. A membrane was obtained. The film forming conditions and film characteristics are shown in Table 1.

[Example 4]
The raw material composition of the polyolefin of Example 1 was changed to 32 parts by mass of SH800 having an Mv of 250,000, and a microporous membrane was obtained in the same manner as in Example 1 except for the conditions shown in Table 1. The film forming conditions and film characteristics are shown in Table 1.

[Example 5]
Granulated by mixing 12.8 parts by weight of SH800 with Mv 250,000, 19.2 parts by weight of UH650 with Mv 1 million, 20 parts by weight of silica LP, 0.3 parts by weight of antioxidant, and 48 parts by weight of DOP with a Henschel mixer. did. Then, it knead | mixed and extruded at 200 degreeC with the twin-screw extruder equipped with T-die, and shape | molded in the sheet form of thickness 100 micrometers. DOP was extracted and removed from the molded product with methylene chloride to form a microporous membrane. The three microporous membranes were stacked and heated to 130 ° C. and stretched 5 times in MD and with a roll, and then heated to a maximum temperature of 140 ° C. and 1.7 times in TD and stretched with a tenter. The film forming conditions and film characteristics are shown in Table 1.

[Comparative Example 1]
Granulated by mixing 12.8 parts by weight of SH800 with Mv 250,000, 19.2 parts by weight of UH650 with Mv 1 million, 20 parts by weight of silica LP, 0.3 parts by weight of antioxidant, and 48 parts by weight of DOP with a Henschel mixer. did. Then, it knead | mixed and extruded at 200 degreeC with the twin-screw extruder equipped with T-die, and shape | molded in the sheet form of thickness 100 micrometers. DOP was extracted and removed from the molded product with methylene chloride to form a microporous membrane. One microporous film was heated to 130 ° C. and stretched by a roll 5 times to MD. The film forming conditions and film characteristics are shown in Table 1.

[Comparative Example 2]
Granulated by mixing 12.8 parts by weight of SH800 with Mv 250,000, 19.2 parts by weight of UH650 with Mv 1 million, 20 parts by weight of silica LP, 0.3 parts by weight of antioxidant, and 48 parts by weight of DOP with a Henschel mixer. did. Then, it knead | mixed and extruded at 200 degreeC with the twin-screw extruder equipped with T-die, and shape | molded in the sheet form of thickness 100 micrometers. DOP was extracted and removed from the molded product with methylene chloride to form a microporous membrane. One microporous membrane was heated to 130 ° C and stretched 5 times to MD, and then rolled to a maximum temperature of 140 ° C and 1.7 times to TD. The stretched product was not obtained because the film broke in the middle.

[Comparative Example 3]
Granulated by mixing 12.8 parts by weight of SH800 with Mv 250,000, 19.2 parts by weight of UH650 with Mv 1 million, 20 parts by weight of silica LP, 0.3 parts by weight of antioxidant, and 48 parts by weight of DOP with a Henschel mixer. did. Then, it knead | mixed and extruded at 200 degreeC with the twin-screw extruder equipped with T-die, and shape | molded in the sheet form of thickness 100 micrometers. DOP was extracted and removed from the molded product with methylene chloride to form a microporous membrane. Two of the microporous membranes were stacked and heated to 130 ° C. and stretched to MD by an 8.5 times roll. The film forming conditions and film characteristics are shown in Table 1.

[Comparative Example 4]
Granulated by mixing 12.8 parts by weight of SH800 with Mv 250,000, 19.2 parts by weight of UH650 with Mv 1 million, 20 parts by weight of silica LP, 0.3 parts by weight of antioxidant, and 48 parts by weight of DOP with a Henschel mixer. did. Then, it knead | mixed and extruded at 200 degreeC with the twin-screw extruder equipped with T-die, and shape | molded in the sheet form of thickness 250 micrometers. DOP was extracted and removed from the molded product with methylene chloride, and silica was extracted and removed with an aqueous sodium hydroxide solution to form a microporous membrane. The two microporous membranes were heated to 126 ° C. and stretched 6 times to MD with a roll. The film forming conditions and film characteristics are shown in Table 1.

[Comparative Example 5]
Granulated by mixing 12.8 parts by weight of SH800 with Mv 250,000, 19.2 parts by weight of UH650 with Mv 1 million, 20 parts by weight of silica LP, 0.3 parts by weight of antioxidant, and 48 parts by weight of DOP with a Henschel mixer. did. Then, it knead | mixed and extruded at 200 degreeC with the twin-screw extruder equipped with T-die, and shape | molded in the sheet form of thickness 400 micrometers. DOP was extracted and removed from the molded product with methylene chloride, and silica was extracted and removed with an aqueous sodium hydroxide solution to form a microporous membrane. The two microporous membranes were heated to 126 ° C. and stretched 6 times in MD to a roll, and then heated to a maximum temperature of 138 ° C. and stretched 2 times to TD in a tenter. The film forming conditions and film characteristics are shown in Table 1.

  From the results in Table 1, it was found that the polyolefin microporous membranes of the Examples achieved both low air permeability and high withstand voltage performance as compared with the polyolefin microporous membranes of the comparative examples. Moreover, the polyolefin microporous film of the Examples also had good stretchability. The polyolefin microporous membrane of the example was integrated, and a 400 m wound body could be obtained without peeling.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the polyolefin microporous film which can make low air permeability and high withstand voltage performance compatible is provided. The resulting polyolefin microporous membrane is particularly useful as a lithium ion battery separator.

Claims (6)

The following steps (A) and (B)
(A) a sheet forming step of kneading a raw material composition containing a polyolefin resin, a filler, and a plasticizer to form a sheet;
(B) After the step (A), the plasticizer is extracted from the sheet to form a filler-containing microporous sheet, and two or more filler-containing microporous sheets are stacked and stretched to TD.
A method for producing a polyolefin microporous membrane comprising:   The manufacturing method according to claim 1, wherein the stretching step is a step of stretching the filler-containing microporous sheet at least once in both directions of MD and TD.   The manufacturing method according to claim 2, wherein the stretching step is a step of performing TD stretching after MD stretching.   The manufacturing method according to claim 2, wherein the stretching step is a step satisfying MD stretching ratio ≧ TD stretching ratio.   The manufacturing method according to any one of claims 1 to 4, wherein the polyolefin resin has a viscosity average molecular weight of 300,000 to 10,000,000.   The manufacturing method according to claim 1, wherein the filler is silica powder.