JP5373438B2 - Method for producing microporous film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a production method of a microporous film by which a microporous film having excellent air permeability can be obtained only by stretching a film in multiple stages without a step of extracting a plasticizer by using a solvent. <P>SOLUTION: The production method of a microporous film includes: a cold stretching step of cold stretching a film comprising a polyethylene resin composition containing a polyethylene resin and having a ratio &eta;<SB>e</SB>/&eta;<SB>s</SB>of the extensional viscosity &eta;<SB>e</SB>to the shear viscosity &eta;<SB>s</SB>ranging from 15 to 100, in at least one direction into 1.05 to 2.0 times of the dimension; and a hot stretch step of hot stretching the film subjected to cold stretching in the cold stretching step, in at least one direction into 1.05 to 5.0 times of the dimension. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a microporous film, a method for producing the same, and a battery separator.

  Microporous films, particularly polyolefin microporous films, are used in microfiltration membranes, battery separators, capacitor separators, fuel cell materials, and the like, and are particularly preferably used as lithium ion battery separators. . In recent years, lithium ion batteries have been used for small electronic devices such as mobile phones and notebook personal computers, while being applied to hybrid electric vehicles and the like.

  Here, a lithium ion battery used for a hybrid electric vehicle is required to have higher output characteristics for taking out a lot of energy in a short time. In addition, since lithium ion batteries used for hybrid electric vehicle applications are generally large and have high energy capacity, higher safety is required.

A battery separator used for a lithium ion battery is required to have a shutdown (hereinafter referred to as “SD”) function in order to ensure safety. The SD function is a function of stopping the battery reaction and preventing further temperature increase by rapidly increasing the electric resistance of the battery separator when the battery internal temperature rises excessively. As a mechanism for expressing the SD function, for example, in the case of a battery separator made of a microporous film, when the internal temperature of the battery rises to a predetermined temperature, the porous structure is lost to make it non-porous, and ion permeation occurs. Blocking.
For the purpose of providing a microporous film used as a separator that can cope with such circumstances, for example, Patent Document 1 and Patent Document 2 include a laminated microporous film composed of a polypropylene film and a polyethylene film. A manufacturing method has been proposed. On the other hand, in the microporous film proposed in Patent Document 3, the air permeability is greatly improved by extracting the plasticizer contained in the precursor of the microporous film with a solvent in the production process. .

Japanese Patent No. 3003830 Japanese Patent No. 338538 JP 2008-81513 A

  However, since the laminated microporous films described in Patent Documents 1 and 2 have insufficient air permeability, it is difficult to apply them to hybrid electric vehicles and the like that require high output characteristics. In addition, the microporous film disclosed in Patent Document 3 has a problem in that the production cost is high because the production process includes a step of extracting a plasticizer using a solvent.

  The present invention has been made in view of such circumstances, and without obtaining a plasticizer using a solvent, a microporous film having excellent air permeability can be obtained simply by stretching the film in multiple stages. An object of the present invention is to provide a method for producing a microporous film, a microporous film obtained by the production method, and a battery separator provided with the microporous film.

  As a result of intensive studies to achieve the above-mentioned object, the inventors of the present invention cold-stretched a polyethylene resin composition having a ratio of a specific elongation viscosity and a shear viscosity at a specific temperature, and then a higher specific temperature. It was found that a microporous film having air permeability suitable as a separator for a lithium ion secondary battery can be obtained by heat stretching at, and the present invention was completed.

That is, the present invention is as follows.
[1] A film made of a polyethylene resin composition containing a polyethylene resin and having a ratio η _e / η _s of the elongational viscosity η _e to the shear viscosity η _s of 15 to 100 is 1.05 to 2 in at least one direction. A microporous process comprising: a cold stretching process in which the film is cold-stretched by 0.0 times; and a hot stretching process in which the film that has been cold-stretched in the cold stretching process is heat-stretched in at least one direction by 1.05 to 5.0 For producing a conductive film.
[2] The method for producing a microporous film according to [1], wherein the temperature in the cold stretching step is −20 ° C. or higher and 70 ° C. or lower, and the temperature in the hot stretching step is 90 ° C. or higher and lower than 130 ° C.
[3] The method for producing a microporous film according to [1] or [2], wherein the number of terminal vinyl groups per 10,000 carbon atoms of the polyethylene resin is 3.0 or less .

  According to the present invention, a method for producing a microporous film capable of obtaining a microporous film having excellent air permeability only by stretching the film in multiple stages without extracting a plasticizer using a solvent, and A microporous film obtained by the production method and a battery separator provided with the microporous film can be provided.

  Hereinafter, a mode for carrying out the present invention (hereinafter simply referred to as “the present embodiment”) will be described in detail. The present invention is not limited to the following embodiment, and can be implemented with various modifications within the scope of the gist.

The method for producing a microporous film of the present embodiment includes a polyethylene resin composition (hereinafter referred to as “polyethylene resin”) having a polyethylene resin and a ratio η _e / η _s of the extensional viscosity η _e to the shear viscosity η _s of 15 to 100. A cold stretching step of cold-stretching a film (hereinafter referred to as “raw film Af”) comprising 1.05 to 2.0 times in at least one direction; A hot-stretching process in which the film that has been cold-stretched in the process is hot-stretched at least 1.05 times to 5.0 times in one direction. Here, in this specification, the “resin composition” is a concept that includes only one kind of resin (polymer material), and may be a mixture of two or more kinds of resins. An additive may be contained. The “polyethylene resin” refers to a polymer whose main component is ethylene. Here, the “main component” means a monomer occupying 50% by mass or more with respect to the total amount of the monomer of the polyethylene resin, preferably 80% by mass or more, more preferably 90% by mass or more, and still more preferably 95%. As mentioned above, it means a monomer showing preferably 98% or more, most preferably 100% by mass, that is, the total amount.

First, prior to the cold drawing step, a polyethylene resin composition Ac is prepared. Polyethylene resin composition Ac according to the present embodiment, the the ratio of the extensional viscosity eta _e to shear viscosity η _s η _e / η _s is 15 to 100, preferably 20 to 80, more preferably 30 to 50 It is. If η _e / η _s is 15 or more, the air permeability of the microporous film is good, and if it is 100 or less, it is difficult to break during film formation. In this specification, the units of the shear viscosity η _s and the extension viscosity η _e are both “Pa · s”.

The elongation viscosity η _e of the polyethylene resin composition Ac is a value measured under the conditions of a temperature of 180 ° C. and an elongation strain rate of 10 s ⁻¹ , and uses the inflow pressure loss method with a twin capillary rheometer to determine the Cogswell theory [Polymer Engineering]. Science, 12, 64 (1972)]. The shear viscosity η _s is a value measured under the conditions of a temperature of 180 ° C. and a shear strain rate of 100 s ⁻¹ , and the Cogswell theory using the inflow pressure loss method with a twin capillary rheometer as in the case of the extension viscosity η _e. According to the measurement.

The present inventors have found that the air permeability of the obtained microporous film is dramatically improved by using the polyethylene resin composition Ac having the ratio η _e / η _s within the above range. This factor is not completely elucidated, and the present inventors consider as one of them as follows. That is, by increasing the elongation viscosity to some extent with respect to the shear viscosity, the melted polyethylene resin in the polyethylene resin composition Ac is easily oriented in the extrusion direction during film forming, and as a result, the crystal orientation of the polyethylene resin. Is thought to improve. When the crystal orientation is improved, it is considered that in each stretching step described later, the inter-crystal opening is uniformly generated, and the air permeability of the obtained microporous film is improved.
Patent Document 1 and Patent Document 2 disclose a preferable melting point of the polyethylene resin composition, but no investigation is made on its extensional viscosity and melt viscosity. Further, when the ratios η _e / η _s of the polyethylene resin compositions used in Examples of Patent Document 1 and Patent Document 2 were measured, both were smaller than 15 and the air permeability of the microporous film was also low. It was insufficient.

  The MFR (melt flow rate) of the polyethylene resin composition Ac of the present embodiment is preferably 0.01 to 20 g / 10 minutes, more preferably 0.1 to 10 g / 10 minutes, and still more preferably 0. .5 to 5.0 g / 10 min. If the MFR is 0.01 g / 10 min or more, fish eye is hardly generated in the raw film Af, and if it is 20 g / 10 min or less, drawdown is difficult to occur and the film formability is improved. The MFR of the polyethylene resin composition Ac is measured according to the method described in the following examples.

  In general, fisheye is a small sphere formed in a film-like or sheet-like product, as defined in Polymer Editorial Board, Polymer Dictionary, Taiseisha, 2000, 6th edition, 337 pages, etc. It means a lump. The name was given because there are many things that show transparency like fish eyes. Fisheye is distinguished from its production factors, with unmelted lumps resulting from insufficient kneading of molding materials, lumps due to part of the raw material gelling, lumps due to partial deterioration of the material being molded, and foreign matter as the core. Various things, such as a thing, are mentioned. However, in this specification, the fish eye originates from the material used as the raw material of the microporous film, and excludes those having foreign matter as the core. In addition, the foreign material includes, for example, cellulose, dust, metal pieces, resin carbide, plastics of different types, lint, and pieces of paper.

  The portion of the fish eye in the raw film Af often does not become porous even when stretched. Therefore, if a large amount of fish eye exists, the air permeability of the microporous film decreases. Further, when the film is heat-stretched, the film may break from the fish eye, which is not preferable from the viewpoint of productivity of the microporous film. Therefore, in the raw film Af obtained from the polyethylene resin composition Ac, the number of fish eyes of 0.1 mm or more is preferably 100 or less per 10 g of the raw film Af, more preferably 50 or less. preferable. As a method of reducing the number of fish eyes of the raw film Af, a method of increasing the MFR of the polyethylene resin composition Ac to, for example, 0.01 g / 10 min or more, a molecular weight distribution of the polyethylene resin composition Ac, for example, 2 The method of adjusting to -30 is mentioned. In addition, the number of fish eyes of the raw fabric film Af is measured according to the method described in the following examples.

The density of the polyethylene resin composition Ac according to the present embodiment is preferably 945~970kg / m ^3, more preferably from 955~965kg / m ^3, more preferably from 960~965kg / m ^3. If the density is 945 kg / m ³ or more, a microporous film with better air permeability can be obtained, and if it is 970 kg / m ³ or less, the membrane is difficult to break during stretching. The density of the polyethylene resin composition Ac is measured according to the method described in the following examples.

  The molecular weight distribution of the resin in the polyethylene resin composition Ac according to this embodiment is a ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) (hereinafter referred to as “Mw / Mn”), and is 2.0 to 30. Is preferably 0.0, more preferably 6.0 to 10.0, and even more preferably 7.0 to 9.0. If Mw / Mn is 2.0 or more, heat generation when the polyethylene resin composition Ac is molded into the raw film Af is suppressed, and it is difficult for fish eyes due to resin degradation to occur, and if it is 30.0 or less. Fish eyes derived from high molecular weight components are reduced. Mw and Mn are obtained from gel permeation chromatography (hereinafter referred to as “GPC”) using polystyrene as a standard sample, and are measured in detail according to the methods described in the Examples below.

  In the polyethylene resin composition Ac according to this embodiment, the number of terminal vinyl groups per 10,000 carbon atoms of the polyethylene resin is preferably 5.0 or less, more preferably 3.0 or less, and 1.0. More preferably, it is as follows. If the number of terminal vinyl groups is 5.0 or less, the thermal stability at the time of molding the polyethylene resin composition Ac will be good, and fish eyes due to resin deterioration will not easily occur. The number of terminal vinyl groups per 10,000 carbon atoms of the polyethylene resin is measured according to the method described in the following examples.

The polyethylene resin composition Ac is not particularly limited as long as it contains a polyethylene resin and the ratio η _e / η _s is 15 to 100. For example, the polyethylene resin composition Ac preferably contains a polyethylene resin (hereinafter “Bp”) having a long branched chain produced by polymerizing ethylene using a chromium-based catalyst, and ethylene using a titanium-based catalyst. In addition, it is preferable to contain an ultra-high molecular weight polyethylene resin (hereinafter referred to as “Cp”) produced by other polymerization of other monomers as required, and it is more preferable to include both of these polyethylene resins.
For example, in the polyethylene resin composition Ac, a η _e / η _s produced using a chromium-based catalyst is larger than 100, and a η _e / η _s produced using a titanium-based catalyst is more than 15. You may contain a small polyethylene resin. Alternatively, the polyethylene resin composition Ac may contain a polyethylene resin obtained by melt-kneading a polyethylene resin having a ratio η _e / η _s smaller than 15 together with a crosslinking agent such as an organic peroxide and appropriately crosslinking the resin. Good. Such polyethylene resin composition Ac can keep ratio (eta) _e / (eta) _s in the range of 15-100.

  Next, a resin composition containing the above polyethylene resin (hereinafter referred to as “polyethylene resin Bp”) produced by polymerization of ethylene and, if necessary, other monomers using a chromium catalyst and having a long branched chain (Hereinafter referred to as “polyethylene resin composition Bc”).

  Examples of the chromium-based catalyst used for producing the polyethylene resin Bp include a solid catalyst in which a chromium compound is supported on an inorganic oxide carrier, or a catalyst obtained by mixing or reacting the solid catalyst and an organometallic compound. The catalyst may be a known catalyst. Specifically, JP-B Nos. 44-2996, 47-1365, 44-3827, 44-2337, 47-19985, 45-40902, 49 No. 38986, No. 56-18132, No. 59-5602, No. 59-50242, No. 59-5604, Japanese Patent Publication No. 1-1277, No. 1-112778, No. 1 -12781, JP-A-11-302465, JP-A-9-25312, JP-A-9-25313, JP-A-9-25314, JP 7-503739, US Pat. No. 5,104,841 No. 5,137,997 and the like are exemplified as the chromium-based catalyst.

  As polyethylene resin composition Bc which concerns on this embodiment, what contains polyethylene resin Bp manufactured by the method described in Unexamined-Japanese-Patent No. 2001-294613 etc. is preferable. That is, the polyethylene resin composition Bc is a mixture or reaction of a chromium oxide catalyst supported on a refractory compound and activated by heat treatment in a non-reducing atmosphere and an organoaluminum compound having both an alkoxy group and a hydrosiloxy group. Polyethylene resin Bp produced by polymerization of ethylene and other monomers as required in the presence of a polymerization catalyst obtained by mixing or reacting the solid catalyst component obtained and an organoaluminum compound having an alkoxy group. It is preferable to contain.

In order to obtain the polyethylene resin Bp of the present embodiment, a particularly preferable polymerization catalyst is represented by the following general formula (1) and a chromium oxide catalyst supported on a refractory compound and activated by heat treatment in a non-reducing atmosphere. Or a solid catalyst component obtained by mixing or reacting an organoaluminum compound having both an alkoxy group and a hydrosiloxy group, and an organoaluminum compound having an alkoxy group represented by the following general formula (2): It is a polymerization catalyst obtained by reaction.
_{^{AlR 1 p H q (OR 2}} ) x (OSiHR 3 R 4) y (1)
Here, in the formula (1), p, q, x, and y are p ≧ 1, 0 ≦ q ≦ 1, x ≧ 0.25, y ≧ 0.15, 0.5 ≦ x + y ≦ 1.5 and p + q + x + y = 3 is satisfied, and R ¹ , R ² , R ³ and R ⁴ represent a hydrocarbon group having 1 to 20 carbon atoms which may be the same or different from each other.
AlR ⁵ _3-n (OR ⁶ ) _n (2)
Here, in the formula (2), n satisfies 0 <n ≦ 1, and R ⁵ and R ⁶ represent a hydrocarbon group having 1 to 20 carbon atoms which may be the same or different from each other.

  The polyethylene resin Bp according to this embodiment is produced by a known polymerization method such as suspension polymerization, solution polymerization, or gas phase polymerization.

  The polyethylene resin composition Bc only needs to contain the polyethylene resin Bp obtained as described above, and may further contain various catalysts such as unreacted ethylene and a polymerization catalyst used for polymerization, and additives. Good.

The MFR of the polyethylene resin composition Bc is preferably 0.5 to 10 g / 10 minutes, more preferably 0.5 to 3 g / 10 minutes, and further preferably 0.8 to 2.0 g / 10 minutes. Yes, most preferably 1.0 to 1.6 g / 10 min. When the MFR is 0.5 g / 10 min or more, fish eyes are hardly generated on the raw film Af. When the MFR is 10 g / 10 min or less, drawdown hardly occurs and the film formability is improved.
The density of the polyethylene resin composition Bc is preferably 945 to 970 kg / m ³ . When the density is 945 kg / m ³ or more, the rigidity of the polyethylene composition of the present embodiment can be increased. The density of polyethylene (A) is more preferably 955 to 970 kg / m ³ , still more preferably 960 to 967 kg / m ³ , and most preferably 963 to 967 kg / m ³ . If the density is 945 kg / m ³ or more, a microporous film with better air permeability can be obtained, and if it is 970 kg / m ³ or less, the membrane is difficult to break during stretching.

  Next, a resin composition containing an ultrahigh molecular weight polyethylene resin (hereinafter referred to as “polyethylene resin Cp”) produced by other-stage polymerization of ethylene and, if necessary, other monomers using a titanium-based catalyst ( Hereinafter, it will be described as “polyethylene resin composition Cc”.

  Examples of the titanium-based catalyst used for producing the polyethylene resin Cp include a titanium compound on a carrier such as a porous polymer material, an inorganic solid oxide of an element belonging to Group 2, 3, 4, 13 or 14 of the periodic table. And a known catalyst may be used. In the above porous polymer material, examples of the matrix material include polyethylene, polypropylene, polystyrene, ethylene-propylene copolymer, ethylene-vinyl ester copolymer, styrene-divinylbenzene copolymer, ethylene-vinyl ester copolymer. Examples thereof include polyolefins such as coalesced parts or completely saponified products and modified products thereof, thermoplastic resins such as polyamide, polycarbonate, and polyester, and thermosetting resins such as phenol resin, epoxy resin, urea resin, and melamine resin. Examples of the inorganic solid oxide include silica, alumina, magnesia, magnesium chloride, zirconia, titania, boron oxide, calcium oxide, zinc oxide, barium oxide, vanadium pentoxide, chromium oxide, thorium oxide, and the like. A mixture or a composite oxide thereof may be mentioned.

  Specifically, Japanese Patent Publication Nos. 52-36788, 52-36790, 52-36791, 52-35792, 52-50070, 52-36794, 52 No. -36795, No. 52-36796, No. 52-36915, No. 52-36917, No. 53-6019, JP-A-50-21876, No. 50-31835, The catalyst described in 50-72044 gazette, 50-78619 gazette, 53-40696 gazette, and a pamphlet of WO99 / 28353 is illustrated as said titanium-type catalyst.

As a preferable titanium-based catalyst, for example, (a) a solid catalyst component obtained by reacting an organomagnesium compound represented by the following general formula (3) and (b) a titanium compound having at least one halogen atom; A polymerization catalyst obtained by mixing or reacting with an organometallic compound, or (a) comprising an organomagnesium compound, (b) a titanium compound, and (c) Al, B, Si, Ge, Sn, Te, and Sb. A polymerization catalyst obtained by mixing or reacting a solid catalyst component obtained by reacting a halide compound of an element selected from the group and an organometallic compound.
M _α Mg _β R ¹ _p R ² _q X _r Y _s (3)
Here, in the formula (3), α is 0 or a number greater than 0, p, q, r, and s are 0 or a number greater than 0, and the relationship of p + q + r + s = mα + 2β is satisfied. M represents a metal element belonging to Group 1, Group 2 or Group 3 of the periodic table, and m represents the valence of M. R ¹ and R ² each represent a hydrocarbon group having 1 to 20 carbon atoms which may be the same or different, and X and Y are monovalent groups which may be the same or different from each other, ^{-OR 3, -OSiR 4 R 5 R} 6, -NR 7 R 8, or an -SR ^{9. R 3, R 4, R 5} , R 6, R 7, R 8 represents a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms, R ⁹ represents a hydrocarbon group having 1 to 20 carbon atoms.

  The organometallic compound is preferably a compound having a metal element belonging to Group 1, Group 2 or Group 3 of the periodic table, and particularly preferably an organoaluminum compound or an organomagnesium compound complex containing an organoaluminum compound.

  The reaction between the solid catalyst component and the organometallic compound can be carried out by adding both components into the polymerization system and proceeding with the polymerization under the polymerization conditions, or may be carried out prior to the polymerization. Moreover, it is preferable that the reaction ratio of both components is the range of 1-3000 mmol of organometallic compounds with respect to 1g of solid catalyst components.

  The polyethylene resin Cp according to this embodiment is produced by a known polymerization method such as suspension polymerization, solution polymerization, or gas phase polymerization.

The polyethylene resin Cp is preferably a polymer composed of a low molecular weight polymer component and a high molecular weight polymer component, which are two-stage polymerized using a titanium-based catalyst. Here, the MFR of the low molecular weight polymer component is preferably 1 to 200 g / 10 minutes, more preferably 5 to 100 g / 10 minutes, and still more preferably 10 to 50 g / 10 minutes. The density of the low molecular weight polymer component is preferably 945 to 975 kg / m ³ , more preferably 955 to 975 kg / m ³ . The low molecular weight polymer component is obtained by homopolymerization of ethylene or copolymerization of ethylene and an α-olefin having 3 to 20 carbon atoms. When producing a low molecular weight polymer component by suspension polymerization, the low molecular weight polymer component is preferably produced in a solution containing 2.0 mol% or less of an α-olefin.

  In addition, since the high molecular weight polymer component obtained by two-stage polymerization is mixed with the low molecular weight polymer component in the polyethylene resin Cp, its MFR and density cannot be measured, but the polyethylene resin Cp or the polyethylene resin composition It is preferable that the substance Cc is obtained so as to have the following physical properties. The high molecular weight polymer component is obtained by adding ethylene and, if necessary, an α-olefin having 3 to 20 carbon atoms and polymerizing them in the presence of the low molecular weight polymer component obtained as described above. It is done. The content ratio of the high molecular weight polymer component to the total amount of the polymer composed of the low molecular weight polymer component and the high molecular weight polymer component is preferably 30 to 70% by mass.

The MFR of the polyethylene resin composition Cc is preferably 0.1 to 5.0 g / 10 minutes, more preferably 0.2 to 2.0 g / 10 minutes, still more preferably 0.4 to 1.6 g / 10. Min, most preferably 0.6 to 1.4 g / 10 min. If the MFR is 0.1 g / 10 min or more, fish eyes are hardly generated in the raw film Af, and if it is 5.0 g / 10 min or less, drawdown is difficult to occur and the film formability is improved. . The density of the polyethylene resin composition Cc is preferably 935 to 970 kg / m ³ , more preferably 940 to 965 kg / m ³ , and most preferably 950 to 963 kg / m ³ . If the density is 935 kg / m ³ or more, a microporous film with better air permeability can be obtained, and if it is 970 kg / m ³ or less, the membrane is difficult to break during stretching.

The polyethylene resin Cp is preferably obtained by homopolymerizing ethylene to produce a low molecular weight polymer component and then copolymerizing the low molecular weight polymer component, ethylene and an α-olefin having 3 to 20 carbon atoms. .
Here, the C3-C20 α-olefin is, for example, propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-decene, 1-dodecene, One or two or more compounds selected from the group consisting of 1-tetradecene, 1-hexadecene, 1-octadecene and 1-eicocene.

  The polyethylene resin composition Cc only needs to contain the polyethylene resin Cp obtained as described above, and may further contain various catalysts such as unreacted ethylene and a polymerization catalyst used for polymerization, and additives. Good.

When the polyethylene resin composition Ac contains the polyethylene resin composition Bc and the polyethylene resin composition Cc, the mass ratio (B / C) of the polyethylene resin composition Bc to the polyethylene resin composition Cc is B / C. C = 5/95 to 95/5 is preferable, and 30/70 to 70/30 is more preferable. When the mass ratio (B / C) is 5/95 or more, it becomes easy to obtain a polyethylene resin composition Ac with a ratio η _e / η _s of 15 to 100, and the resulting microporous film has good air permeability. Become. On the other hand, when the mass ratio is 95/5 or less, it becomes easy to obtain a polyethylene resin composition Ac having a terminal vinyl group number of 5.0 or less, and a raw film having good thermal stability and less fish eyes due to resin deterioration. Af can be obtained.

  There is no particular limitation on the mixing method when the polyethylene resin composition Ac is produced by mixing the polyethylene resin composition Bc and the polyethylene resin composition Cc, and a normal method of mixing in a powder state, a slurry state, a pellet state, or the like. Is used. When kneading them, they may be kneaded at a temperature of 150 to 300 ° C. using a uniaxial or biaxial extruder, a kneader or the like.

  Further, the polyethylene resin composition Ac according to the present embodiment includes, in addition to the above-described components, other additional components as necessary within a range that does not impair the features and effects of the present invention, such as olefin elastomers and antioxidants. , Metal deactivators, heat stabilizers, flame retardants (organic phosphate compounds, ammonium polyphosphate compounds, aromatic halogen flame retardants, silicone flame retardants, etc.), fluoropolymers, plasticizers (low molecular weight Polyethylene, epoxidized soybean oil, polyethylene glycol, fatty acid esters, etc.), flame retardant aids such as antimony trioxide, weather resistance (light) improvers, polyolefin nucleating agents, slip agents, inorganic or organic fillers, Reinforcing materials (polyacrylonitrile fiber, carbon black, titanium oxide, calcium carbonate, conductive metal fiber, conductive carbon black, etc.) Various coloring agents may also contain a releasing agent. The total content of these additional components is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less with respect to 100 parts by mass of the polyethylene resin composition Ac. is there.

  As a method for producing the raw film Af made of the polyethylene resin composition Ac according to the present embodiment, a sheet molding method such as T-die extrusion molding, inflation molding, calendar molding, and Skyf's method may be employed. Among these, T-die extrusion molding is preferable from the viewpoints of physical properties and applications required for the microporous film of the present embodiment.

  In the production method of the raw film Af, the draw ratio after extrusion, that is, the film winding speed (unit: m / min) is the extrusion speed of the polyethylene resin composition Ac (the line in the flow direction of the molten resin passing through the die lip). The value divided by the speed (unit: m / min) is preferably 10 to 500, more preferably 100 to 400, and still more preferably 150 to 350. Moreover, the winding speed of the film when winding the raw film Af is preferably about 2 to 400 m / min, more preferably 10 to 200 m / min. Setting the draw ratio in the above range is preferable from the viewpoint of improving the air permeability of the obtained microporous film.

  The raw film Af is preferably subjected to heat treatment (annealing) as necessary. As the annealing method, for example, a method of bringing the original film Af into contact with a heating roll, a method of exposing the original film Af to a heated gas phase before winding, a method of winding the original film Af on a core body, a heated gas phase or a heating liquid The method of exposing in a phase and the method of combining these are mentioned. The annealing conditions are preferably, for example, annealing at a heating temperature of 100 ° C. to 130 ° C. for 10 seconds to 100 hours. If the heating temperature is 100 ° C. or higher, the air permeability of the microporous film obtained later is further improved. If the heating temperature is 130 ° C. or lower, the films can be annealed even when the raw film Af is wound on the core. Is difficult to fuse. A more preferable heating temperature range is 115 ° C to 130 ° C.

  The elastic recovery rate after heat treatment of the raw film Af is preferably 50 to 80%, more preferably 60 to 75%. Setting the elastic recovery rate of the raw film Af within the above range is preferable from the viewpoint of obtaining a microporous film having a sufficient degree of porosity. The “elastic recovery rate after heat treatment” of the raw film Af is derived according to the method described in the following examples.

Next, the cold drawing process will be described.
In the cold drawing step, the raw film Af made of the polyethylene resin composition Ac is preferably cold drawn at 1.05 times to 2.0 times in at least one direction while being kept at -20 ° C or higher and 70 ° C or lower. .

  The stretching temperature for cold stretching in the cold stretching step is preferably -20 ° C or higher and 70 ° C or lower, more preferably 0 ° C or higher and 50 ° C or lower. When stretched at −20 ° C. or higher, the raw film Af is hardly broken, and when stretched at 70 ° C. or lower, the air permeability of the resulting microporous film becomes better. Here, the drawing temperature of cold drawing is the surface temperature of the film in the cold drawing process.

The draw ratio of cold drawing in the cold drawing step is 1.05 or more and 2.0 or less, preferably 1.2 or more and 1.7 or less. When the draw ratio is 1.05 times or more, a microporous film having good air permeability is obtained, and when it is 2.0 times or less, a microporous film having a uniform film thickness is obtained. The cold stretch of the raw film Af may be performed in at least one direction and may be performed in two directions. Preferably, the original film Af is uniaxially stretched only in the film extrusion direction (hereinafter referred to as “MD direction”).
In the present embodiment, in the cold stretching step, the raw film Af is preferably uniaxially stretched 1.1 to 2.0 times in the MD direction at a temperature of 0 ° C. or higher and 70 ° C. or lower.

Next, the heat stretching process will be described.
The method for producing a microporous film according to the present embodiment is 1.05 times or more and 5.0 times in at least one direction in a state in which the film that has been cold drawn in the cold drawing step is held at 90 ° C. or higher and 130 ° C. or lower. The following includes a heat stretching step for heat stretching.

  The stretching temperature for hot stretching only needs to be higher than the stretching temperature for cold stretching. Further, the stretching temperature of the heat stretching is preferably 90 ° C. or higher and 130 ° C. or lower, more preferably 100 ° C. or higher and 125 ° C. or lower. If it is hot-stretched at 90 ° C. or higher, the film is difficult to break, and if it is hot-stretched at 130 ° C. or lower, the air permeability of the resulting microporous film is improved. Here, the stretching temperature of the heat stretching is the surface temperature of the film in the heat stretching step.

The draw ratio of the hot drawing in the hot drawing step is 1.05 or more and 5.0 or less, preferably 1.1 or more and 5.0 or less, more preferably 2.0 or more and 4.0 or less. It is. When the draw ratio in the heat stretching step is 1.05 times or more, a microporous film having good air permeability is obtained, and when it is 5.0 times or less, a microporous film having a uniform film thickness is obtained. . The hot stretching may be performed in at least one direction and may be performed in two directions, but is preferably performed in the same direction as the cold stretching direction, more preferably uniaxial stretching only in the same direction as the cold stretching direction. Do.
In the present embodiment, in the hot stretching process, the film that has been cold-drawn through the cold-drawing process is uniaxially stretched 2.0 to 5.0 times in the MD direction at a temperature of 90 ° C. or higher and 130 ° C. or lower. Is preferred.

  The manufacturing method of the microporous film of the present embodiment includes a two-stage stretching process including a cold stretching process and a hot stretching process from the viewpoint of good air permeability and use required for the microporous film. When the manufacturing method of a microporous film is a method which performs an extending | stretching process in 1 step, the obtained microporous film becomes difficult to satisfy | fill the required favorable air permeability. In addition, the manufacturing method of the microporous film of this embodiment may include the further extending process in addition to each above-mentioned extending process.

  The method for producing a microporous film according to the present embodiment preferably includes a heat setting step in which heat setting is preferably performed at 100 ° C. or higher and 130 ° C. or lower with respect to the microporous film obtained through the heat stretching step. . This heat setting method includes a method of heat shrinking to such an extent that the length of the microporous film after heat setting is reduced by 3 to 50% with respect to the length of the microporous film before heat setting (hereinafter referred to as this method). The method is referred to as “relaxation”), and heat setting is performed so that the dimension in the stretching direction does not change.

  The heat setting temperature is preferably 100 ° C. or higher and 130 ° C. or lower, and more preferably 110 ° C. or higher and 125 ° C. or lower. Here, the heat setting temperature is the surface temperature of the microporous film in the heat setting process.

  In each step of the cold drawing step, the hot drawing step, other drawing steps and the heat setting step in the method for producing the microporous film of the present embodiment, the drawing or heat setting is performed in one step by a roll, a tenter, an autograph, or the like. Or it can carry out to a uniaxial direction and / or a biaxial direction in two steps or more. In particular, from the viewpoints of physical properties such as air permeability and porosity required for the obtained microporous film and applications, it is preferable to perform uniaxial stretching / fixing by two or more stages with a roll in at least one step.

  According to the method for producing a microporous film of the present embodiment, without including a plasticizer in the polyethylene resin composition Ac or the raw film Af, and without having a step of extracting the plasticizer using a solvent, A microporous film having excellent air permeability can be obtained.

  Next, the physical property of the microporous film obtained by the manufacturing method of the microporous film of this embodiment is demonstrated.

The porosity of the microporous film of this embodiment is preferably 20% to 80%, more preferably 40% to 70%, and still more preferably 50% to 70%. By setting the porosity to 20% or more, when a microporous film is used as a battery separator, sufficient ion permeability tends to be ensured. On the other hand, by setting the porosity to 80% or less, the microporous film tends to ensure sufficient mechanical strength.
The porosity of the microporous film can be adjusted to the above range by appropriately setting the composition of the polyethylene resin composition Ac, the stretching temperature in each stretching step, the stretching ratio, and the like. For example, in order to increase the porosity, the ratio η _e / η _s of the polyethylene resin composition Ac may be set to 15 to 100, or the draw ratio may be increased. Moreover, the porosity of a microporous film cuts out the sample of 10 cm x 10 cm square from the film, the volume V (cm < ³ >) and mass M (g) of the sample, and the density of resin composition Ac which comprises a film Calculated from d (g / cm ³ ) using the following formula.
Porosity (%) = {(VM−d / V) × 100

  The film thickness of the microporous film is preferably 5 to 40 μm, more preferably 10 to 30 μm.

The air permeability of the microporous film is preferably 10 seconds / 100 cc to 5000 seconds / 100 cc, more preferably 50 seconds / 100 cc to 1000 seconds / 100 cc, still more preferably 70 seconds / 100 cc to 150 seconds / 100 cc. is there. When the air permeability is 5000 seconds / 100 cc or less, the microporous film tends to ensure sufficient ion permeability. On the other hand, when the air permeability is 10 seconds / 100 cc or more, there is a tendency that a more uniform microporous film having no defect is obtained.
The air permeability of the microporous film can be adjusted to the above range by appropriately setting the composition of the polyethylene resin composition Ac, the stretching temperature in each stretching step, the stretching ratio, and the like. For example, in order to increase the air permeability, the stretch ratio may be increased or the relaxation ratio in heat setting may be decreased. The air permeability of the microporous film is measured using a Gurley type air permeability meter according to JIS P-8117.

  The microporous film produced by the production method of the present embodiment is suitably used as a battery separator, more specifically as a lithium secondary battery separator. The battery separator has a known configuration except that it includes the microporous film of the present embodiment as a microporous film, and may be produced by a known method. In addition, the microporous film of this embodiment is also used as various separation membranes.

  Next, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples unless it exceeds the gist. In addition, the raw material used and the evaluation method of various characteristics are as follows.

(1) MFR
MFR is synonymous with melt index, and the MFR of the polyethylene resin composition was measured under conditions of a temperature of 190 ° C. and a load of 2.16 kg in accordance with JIS K7210. The unit is g / 10 minutes.

(2) Density The density of the polyethylene resin composition was measured according to JIS K7112. The unit is kg / m ³ .

(3) Molecular weight distribution (Mw / Mn)
The molecular weight distribution of the resin in the polyethylene resin composition is a value of the ratio Mw / Mn of the weight average molecular weight (Mw) and the number average molecular weight (Mn) obtained from gel permeation chromatography (GPC). The GPC measurement was performed using a GPS device (trade name “HLC-8121GPC / HT”) manufactured by Tosoh Corporation. Using the trade name “TSKgel GMHHR-H (20)” (two) manufactured by Tosoh Corporation as the column, mobile phase o-dichlorobenzene (o-DCB), column temperature 155 ° C., flow rate 1.0 mL / min, sample concentration The test was performed under the conditions of 0.5 mg / mL (o-DCB), injection amount 500 μL, sample dissolution temperature 160 ° C., and sample dissolution time 3 hours. The molecular weight was calibrated with polystyrene, and Mw and Mn were obtained from the polystyrene-equivalent molecular weight to derive the molecular weight distribution.

(4) Elongation viscosity (η _e ), shear viscosity (η _s )
The elongation viscosity η _e and the shear viscosity η _s of the polyethylene resin composition were obtained by measuring according to Cogswell's theory [Polymer Engineering Science, 12, 64 (1972)] using the inflow pressure loss method. As a measuring apparatus, a twin capillary rheometer manufactured by Rosand was used, and the orifice was measured at 180 ° C. using the following long die and short die. A value at an elongation strain rate of 10 s ⁻¹ was used as the elongation viscosity η _e , and a value at a shear strain rate of 100 s ⁻¹ was used as the shear viscosity η _s .
Long die: length 16mm, diameter 1mm, inflow angle 180 °
Short die: Length 0.25mm, diameter 1mm, inflow angle 180 °

(5) Number of terminal vinyl groups The number of terminal vinyl groups per 10,000 carbon atoms of the polyethylene resin in the polyethylene resin composition was determined using a Fourier transform infrared spectrophotometer manufactured by JASCO Corporation. Specifically, using a sheet-like polyethylene resin composition that has been made non-porous by heating and pressing, the absorbance near the absorption wavelength of 909 cm ⁻¹ of the IR spectrum measured by the spectrophotometer was determined. This absorption wavelength is a wavelength resulting from the terminal vinyl group in the polyethylene resin. From the obtained absorbance, the number of terminal vinyl groups per 10,000 carbon atoms was determined by the following formula.
(Number of terminal vinyl groups per 10,000 carbon atoms) = 11.4 × absorbance / ((true sample density) × (sample thickness))
Here, the unit of the true density of the sample is g / cm ³ , and the unit of the thickness of the sample is mm.

(6) Film thickness (μm)
The film thickness of the microporous film was measured using a dial gauge (manufactured by Ozaki Seisakusho, trade name “PEACOCK No. 25”).

(7) Porosity (%)
The porosity of the microporous film was determined by cutting out a 10 cm × 10 cm square sample from the microporous film, the volume V (cm ³ ) and mass M (g) of the sample, and the density d of the resin composition constituting the film. It calculated using the following formula from (g / cm ³ ).
Porosity (%) = {(VM−d / V) × 100

(8) Air permeability (sec / 100cc)
The air permeability of the microporous film was measured with a Gurley type air permeability meter based on JIS P-8117. In addition, the value converted into the value when the film thickness of a microporous film was 20 micrometers was made into the air permeability of the microporous film.

(9) Number of fish eyes (pieces / 10g)
The number of fish eyes in the raw film was measured as follows. First, a 30 cm × 30 cm square sample was cut out from the raw film, and the sample was placed on a transparent plate irradiated with a fluorescent lamp from the lower side. It was visually confirmed from above the sample that the transmitted light from the fluorescent lamp passing through the transparent plate also penetrates the sample. At this time, since the degree of light transmission was different due to the difference in the thickness of the sample due to the fish eye, the difference was visually observed, and the portion having a diameter of 0.1 mm or more and a low degree of transmission was used as the fish eye. The number of the parts was measured. The arithmetic average value of the number of fish eyes in the three cut samples was converted into the number per 10 g of the original film, and the number of fish eyes in the original film was obtained.

(10) Elastic recovery rate The raw film was annealed in air at 130 ° C for 1 hour, and then cut into a strip shape having a width of 15 mm and a length of 2 inches (5.08 cm) to obtain a test piece. The test piece is set at a predetermined position of a tensile tester, and is up to 50% in the length direction at a speed of 2 inches / minute under a condition of 25 ° C. and 65% relative humidity, that is, a length of 3 inches. It extended to. Next, the test piece was held in its extended state for 1 minute, and then the test piece was relaxed at the same speed (2 inches / minute) to measure the length of the test piece when the load was zero. And based on following formula (4), "elastic recovery rate after heat processing" of original film Af was derived. In the formula, each length unit is the same unit.
Elastic recovery rate after heat treatment (%) = ((length of test piece at 50% extension) − (length of test piece when relaxed and load becomes zero)) / ((at 50% extension) Test piece length) − (length of test piece before stretching)) × 100 (4)

[Production Example 1]
(1) Synthesis of chromium oxide catalyst (I) 4 mol of chromium trioxide was dissolved in 80 liters of distilled water to obtain a solution, and 20 kg of silica (manufactured by WR Grace and Company, Grade 952) was immersed in this solution. And stirred for 1 hour at room temperature to obtain a slurry. Thereafter, the slurry was heated to distill off water, followed by drying under reduced pressure at 120 ° C. for 10 hours, and then firing by flowing dry air at 600 ° C. for 5 hours. Thus, a chromium oxide catalyst (I) containing 1.0% by mass of chromium was obtained.

(2) Synthesis of organoaluminum compound (II) 100 moles of triethylaluminum, 50 moles of methyl hydropolysiloxane (viscosity at 30 ° C .: 30 centistokes) (Si standard), and 150 liters of n-hexane were placed in a nitrogen atmosphere. An organoaluminum compound represented by the composition formula Al (C ₂ H ₅ ) _2.5 (OSi · H · CH ₃ · C ₂ H ₅ ) _0.5 by weighing in a pressure vessel and reacting at 50 ° C. for 24 hours with stirring. A hexane solution was prepared. Next, 100 moles (Al standard) of this hexane solution was transferred to a 600 liter reactor under a nitrogen atmosphere, and a mixed solution of 50 liters of ethanol and 50 liters of n-hexane was stirred while stirring the hexane solution in the reactor. Added at -10 ° C. After adding the mixed solution, the temperature in the reactor is raised to 50 ° C., and the reaction is carried out at this temperature for 1 hour, and the composition formula Al (C ₂ H ₅ ) _2.0 (OC ₂ H ₅ ) _0.5 (OSi · H · CH ₃ · C ₂ H ₅₎ represented by _0.5, to prepare a hexane solution of an organic aluminum compound having both alkoxy and Hidoroshirokishi group (II).

(3) Polymerization of polyethylene resin composition (Bc-1) In a single-stage polymerization process, the polyethylene resin composition (Bc-1) was polymerized in a polymerization vessel having a volume of 230 L. The polymerization temperature was set to 78 ° C., and the polymerization pressure was set to 0.98 MPa. Specifically, first, 50 g of the chromium oxide catalyst (I) was placed in the polymerization vessel, and then 5 mmol of the organoaluminum compound (II) (Al basis) was added and reacted at room temperature for 1 hour. A solid catalyst was obtained. Separately, an organoaluminum compound (III) having an alkoxy group was obtained by reacting ethanol and trihexylaluminum at a molar ratio of 0.98: 1 at a rate of 2 g / hour. The organoaluminum compound (III) was supplied to the polymerization vessel containing the solid catalyst so that its concentration (based on the hexane solution of the organoaluminum compound (II) in the reactor) was 0.08 mmol / liter. At the same time, purified n-hexane is fed to the polymerizer at a rate of 60 L / hr, and ethylene as a monomer is fed at a rate of 12 kg / hr and hydrogen is used as a molecular weight regulator so that the gas phase concentration becomes 20 mol%. At the same time, the polymerization was carried out by feeding the polymerization vessel. The polymer in the obtained polymerization vessel was dried to obtain a powdery polyethylene resin composition (Bc-1). The polyethylene resin composition (Bc-1) had an MFR of 1.8 g / 10 min and a density of 966 kg / m ³ .

[Production Example 2]
(1) Synthesis of Titanium Catalyst (IV) 3 liters of trichlorosilane as an n-heptane solution having a concentration of 2 mol / liter was charged into a 15 liter reactor sufficiently purged with nitrogen, and kept at 65 ° C. with stirring. There, a composition formula _{AlMg 6 (C 2 H 5)} 3 (n-C 4 H 9) 6.4 (On-C 4 H 9) the organomagnesium component represented by _5.6 n-heptane Solution 7 liters (magnesium terms 5 mol) was added over 1 hour, and the mixture was further reacted by stirring at 65 ° C. for 1 hour. After completion of the reaction, the supernatant was removed from the reaction product and washed 4 times with 7 liters of n-hexane to obtain a slurry containing solids. As a result of separating and drying the slurry containing this solid, 7.45 mmol of Mg was contained per gram of the solid.

  Among these, a slurry containing 500 g of solid was mixed with 0.93 liter of n-hexane solution of 1 mol / liter of n-butyl alcohol, and reacted at 50 ° C. for 1 hour with stirring. After completion of the reaction, the supernatant was removed from the reaction product and washed once with 7 liters of n-hexane. The slurry thus obtained was kept at 50 ° C., and 1.3 liters of n-hexane solution of 1 mol / liter of diethylaluminum chloride was added with stirring, and further reacted for 1 hour. After completion of the reaction, the supernatant was removed from the reaction product and washed twice with 7 liters of n-hexane. The obtained slurry was kept at 50 ° C., 0.2 liter of n-hexane solution of 1 mol / liter of diethylaluminum chloride and 0.2 liter of n-hexane solution of 1 mol / liter of titanium tetrachloride were added, The reaction was further continued for 2 hours. After completion of the reaction, the supernatant was removed from the reaction product, the solid catalyst was isolated, and the solid catalyst was washed with hexane until no free halogen was detected. The titanium catalyst (IV), which was a solid catalyst thus obtained, had 2.3% by mass of titanium.

(2) Polymerization of polyethylene resin composition (Cc-1) First, in the first stage polymerization, a stainless polymerizer 1 having a reaction volume of 300 liters was used for polymerization under the conditions of a polymerization temperature of 83 ° C and a polymerization pressure of 1 MPa. A molecular weight polymer component was obtained. As the catalyst, the titanium catalyst (IV) was supplied to the polymerizer 1 at a rate of 1.4 mmol / hour in terms of Ti atoms and triisobutylaluminum at a rate of 20 mmol / hour in terms of Al atoms. Further, n-hexane was introduced into the reactor 1 at a rate of 40 liters / hour. Polymerization was performed by using hydrogen as a molecular weight regulator and supplying ethylene and hydrogen as monomers so that the gas phase concentration of hydrogen was 38 mol%. The obtained polymer slurry solution containing the low molecular weight polymer component in the polymerization vessel 1 is led to a flash drum having a pressure of 0.1 MPa and a temperature of 75 ° C., and unreacted ethylene and hydrogen are separated, and then a polymerization with a reaction volume of 250 liters is performed. The pressure was introduced into the vessel 2 by a slurry pump. In the polymerization vessel 2, triisobutylaluminum was introduced at a rate of 7.5 mmol / hour and n-hexane was introduced at a rate of 40 liter / hour under the conditions of a temperature of 83 ° C. and a pressure of 0.5 MPa. Into this, ethylene as a monomer and hydrogen and butene-1 as molecular weight regulators were introduced so that the gas phase concentration of hydrogen was 6.8 mol% and the gas phase concentration of butene was 1.0 mol%. did. Thus, the mass ratio of the high molecular weight polymer component generated in the polymerization device 2 to the low molecular weight polymer component generated in the polymerization device 1 (high molecular weight polymer component) / (low molecular weight polymer component) is high so as to be 50/50. The molecular weight polymer component was polymerized and further dried to produce a powdery polyethylene resin composition (Cc-1) having an MFR of 1.3 g / 10 min and a density of 962 kg / m ³ . The MFR of the low molecular weight polymer component produced in the polymerization vessel 1 was 30 g / 10 minutes, and the density was 967 kg / m ³ .

[Example 1]
The polyethylene resin composition (Bc-1) and the polyethylene resin composition (Cc-1) powder produced as described above were, based on their mass, (Bc-1) / (Cc-1) = 50/50. Then, calcium stearate was added to this mixture to a concentration of 60 ppm, and the mixture was stirred and mixed with a mixer. The resulting mixture was kneaded using a twin-screw extruder having a cylinder diameter of 44 mm (trade name “TEX44HCT-49PW-7V” manufactured by Nippon Steel Works) under the conditions of a cylinder temperature of 180 ° C. and an extrusion rate of 45 kg / hour. Extrusion gave a polyethylene resin composition (Ac-1). About the obtained polyethylene resin composition (Ac-1), MFR, a density, (eta) _e / (eta) _s , Mw / Mn were measured. The results are shown in Table 1.

  The polyethylene resin composition (Ac-1) was introduced into a single screw extruder set at 180 ° C. having a diameter of 20 mm and L / D = 30 via a feeder, and a lip having a lip thickness of 5.0 mm installed at the tip of the extruder. Extruded from the die. The extruded polyethylene resin composition (Ac-1) after extrusion was immediately applied with cold air of 25 ° C., and then wound with a cast roll cooled to 95 ° C. under a draw ratio of 300 and a winding speed of 10 m / min. An original film (Af-1) was obtained. The number of fish eyes of the raw film (Af-1) was 72, and the elastic recovery after heat treatment was 70%.

After the obtained raw film (Af-1) was wound on the core, it was annealed at a temperature of 125 ° C. for 3 hours, cooled to 25 ° C., and 1.5 ° C. in the MD direction at a temperature of 25 ° C. Uniaxially stretching (cold stretching process) twice, followed by uniaxial stretching (thermal stretching process) 2.5 times in the MD direction at a temperature of 115 ° C., and further relaxed 0.9 times at a temperature of 125 ° C. Then, heat setting was performed to obtain a microporous film.
About the obtained microporous film, the film thickness, the porosity, and the air permeability were measured. The results are shown in Table 1.

[Example 2]
Example except that the mixing ratio of the polyethylene resin composition (Bc-1) and the polyethylene resin composition (Cc-1) was changed from (Bc-1) / (Cc-1) = 50/50 to 80/20 In the same manner as in Example 1, an original film (Af-2) was obtained. Next, a microporous film was obtained in the same manner as in Example 1 except that the original film (Af-2) was used instead of the original film (Af-1).
About the obtained original fabric film (Af-2), the number of fish eyes and the elastic recovery rate after heat processing were measured, and about the microporous film, the film thickness, the porosity, and the air permeability were measured. The results are shown in Table 1.

[Example 3]
Example except that the mixing ratio of the polyethylene resin composition (Bc-1) and the polyethylene resin composition (Cc-1) was changed from (Bc-1) / (Cc-1) = 50/50 to 10/90 In the same manner as in Example 1, a raw film (Af-3) was obtained. Next, a microporous film was obtained in the same manner as in Example 1 except that the original film (Af-3) was used instead of the original film (Af-1).
About the obtained original fabric film (Af-3), the number of fish eyes and the elastic recovery rate after heat processing were measured, and about the microporous film, the film thickness, the porosity, and the air permeability were measured. The results are shown in Table 1.

[Example 4]
A raw film (Af-4) was obtained in the same manner as in Example 1 except that a commercially available polyethylene resin composition (Ac-4) was used instead of the polyethylene resin composition (Ac-1). As the polyethylene resin composition (Ac-4), a polyethylene resin composition having an MFR of 0.3 g / 10 min, a density of 961 kg / m ³ , and η _e / η _s of 47 was used. Next, a microporous film was obtained in the same manner as in Example 1 except that the original film (Af-4) was used instead of the original film (Af-1).
About the obtained original fabric film (Af-4), the number of fish eyes and the elastic recovery rate after heat processing were measured, and about the microporous film, the film thickness, the porosity, and the air permeability were measured. The results are shown in Table 1.

[Example 5]
A raw film (Af-5) was obtained in the same manner as in Example 1 except that a commercially available polyethylene resin composition (Ac-5) was used instead of the polyethylene resin composition (Ac-1). In addition, as the polyethylene resin composition (Ac-5), a polyethylene resin composition having an MFR of 1.3 g / 10 min, a density of 964 kg / m ³ , and η _e / η _s of 37 was used. Next, a microporous film was obtained in the same manner as in Example 1 except that the original film (Af-5) was used instead of the original film (Af-1).
About the obtained original fabric film (Af-5), the number of fish eyes and the elastic recovery rate after heat processing were measured, and about the microporous film, the film thickness, the porosity, and the air permeability were measured. The results are shown in Table 1.

[Comparative Example 1]
Example except that the mixing ratio of the polyethylene resin composition (Bc-1) and the polyethylene resin composition (Cc-1) was changed from (Bc-1) / (Cc-1) = 50/50 to 0/100 In the same manner as in Example 1, a raw film (Af-6) was obtained. Next, a microporous film was obtained in the same manner as in Example 1 except that the original film (Af-6) was used instead of the original film (Af-1).
About the obtained original fabric film (Af-6), the number of fish eyes and the elastic recovery rate after heat processing were measured, and about the microporous film, the film thickness, the porosity, and the air permeability were measured. The results are shown in Table 1.

[Comparative Example 2]
Example except that the mixing ratio of the polyethylene resin composition (Bc-1) and the polyethylene resin composition (Cc-1) was changed from (Bc-1) / (Cc-1) = 50/50 to 100/0 In the same manner as in Example 1, an attempt was made to produce a microporous film. However, the molten resin was easily broken and the raw film could not be formed.

[Comparative Example 3]
A raw film (Af-8) was obtained in the same manner as in Example 1 except that a commercially available polyethylene resin composition (Ac-8) was used instead of the polyethylene resin composition (Ac-1). As the polyethylene resin composition (Ac-8), a polyethylene resin composition having an MFR of 1.0 g / 10 min, a density of 961 kg / m ³ , and η _e / η _s of 13 was used. Next, a microporous film was obtained in the same manner as in Example 1 except that the original film (Af-8) was used instead of the original film (Af-1).
About the obtained original fabric film (Af-8), the number of fish eyes and the elastic recovery rate after heat processing were measured, and about the microporous film, the film thickness, the porosity, and the air permeability were measured. The results are shown in Table 1.

[Comparative Example 4]
In the same manner as in Example 1, except that the polyethylene resin composition (manufactured by Prime Polymer, trade name “Hi-Zex 2208J”) exemplified in Patent Document 2 was used instead of the polyethylene resin composition (Ac-1), An anti-film (Af-9) was obtained. Next, a microporous film was obtained in the same manner as in Example 1 except that the original film (Af-9) was used instead of the original film (Af-1).
About the obtained original fabric film (Af-9), the number of fish eyes and the elastic recovery rate after heat processing were measured, and about the microporous film, the film thickness, the porosity, and the air permeability were measured. The results are shown in Table 1.

All of the microporous films of Examples 1 to 5 produced by the production method of the present invention exhibited extremely good air permeability (low air permeability).
In contrast, Comparative Example 1 and Comparative Example 3 using a polyethylene resin composition having a ratio η _e / η _s smaller than 15 had insufficient air permeability. Further, in Comparative Example 2 in which the ratio η _e / η _s exceeded 100, it was difficult to form a raw film, and a microporous film could not be obtained.

  The microporous film produced by the production method of the present invention has industrial applicability as a battery separator, more specifically as a lithium secondary battery separator.

Claims (3)

Containing polyethylene resin, the film the ratio of the extensional viscosity eta _e to shear viscosity η _s η _e / η _s is made of a polyethylene resin composition is 15 to 100, in at least one direction 1.05 to 2.0 times A cold drawing step of cold drawing,
A hot stretching process in which the cold-stretched film in the cold-stretching process is hot-stretched at least 1.05 times to 5.0 times in one direction;
The manufacturing method of the microporous film which has this.   The method for producing a microporous film according to claim 1, wherein the temperature in the cold stretching step is -20 ° C or higher and 70 ° C or lower, and the temperature in the hot stretching step is 90 ° C or higher and lower than 130 ° C.   The manufacturing method of the microporous film of Claim 1 or 2 whose number of terminal vinyl groups per 10000 carbon atoms of the said polyethylene resin is 3.0 or less.