JP2008255306A - Polyolefin multilayer microporous membrane, method for producing the same, separator for battery and battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a polyolefin multilayer microporous membrane having excellent balance of permeability, mechanical strength, meltdown properties, electrolyte absorption and electrolyte retention, to provide a method for producing the same and to obtain a separator for battery and a battery. <P>SOLUTION: The polyolefin multilayer microporous membrane has at least a first microporous layer for forming both surface layers and at least one second microporous layer arranged between both the surface layers. The first layer comprises a first polyethylene-based resin having the ratio of an ultra high molecular weight polyethylene having a weight-average molecular weight of ≥1×10<SP>6</SP>of ≥8 mass% and the second layer comprises a second polyethylene-based resin having the ratio of the ultra high molecular weight of ≤7 mass% and has a structure in which a pore diameter distribution curve obtained from a mercury porosimetry has at least two peaks. The total thickness of the first layer is 15-60% when the total thickness of the first and the second layers is 100%. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a polyolefin multi-layer microporous membrane having a suitable balance of permeability, mechanical strength, meltdown characteristics, electrolyte absorption and electrolyte retention, a method for producing the same, and a battery separator comprising such a multi-layer microporous membrane And a battery including the same.

  Polyolefin microporous membrane includes primary batteries, lithium ion secondary batteries, lithium polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver-zinc secondary batteries, etc. It is useful as a battery separator for use in secondary batteries. When the polyolefin microporous membrane is used particularly as a separator for a lithium ion secondary battery, the performance is deeply related to the characteristics, productivity and safety of the battery. For this reason, the polyolefin microporous membrane is required to have appropriate permeability, mechanical properties, dimensional stability, shutdown characteristics, meltdown characteristics, and the like. As is well known, it is required to have a relatively low shutdown temperature and a relatively high melt-down temperature in order to improve the safety of batteries, particularly those exposed to high temperatures during operation. High permeability is required to obtain a high battery capacity. In order to improve battery assembly and assembly, high mechanical strength is required.

In order to improve the physical properties of the polyolefin microporous membrane, it has been proposed to optimize the raw material composition, stretching conditions, heat treatment conditions, and the like. For example, Japanese Patent Laid-Open No. 6-240036 (Patent Document 1) discloses a polyolefin microporous membrane having a moderately large pore size and a sharp pore size distribution. This microporous membrane contains 1% by mass or more of ultrahigh molecular weight polyethylene having a weight average molecular weight (Mw) of 7 × 10 ⁵ or more, and a polyethylene resin having a molecular weight distribution (weight average molecular weight / number average molecular weight) of 10 to 300. The porosity is 35 to 95%, the average through hole diameter is 0.05 to 0.2 μm, the breaking strength (15 mm width) is 0.2 kg or more, and the hole diameter distribution (maximum hole diameter / average through hole diameter) The value is 1.5 or less.

WO 2000/20492 (Patent Document 2) discloses a polyolefin microporous membrane having improved permeability. The microporous membrane has fine fibrils made of polyethylene having a Mw of 5 × 10 ⁵ or more or a polyethylene composition containing the polyethylene. This microporous membrane has an average pore diameter of 0.05 to 5 μm, and the ratio of crystal lamellae having an angle θ with respect to the membrane surface of 80 to 100 degrees is 40% or more in the cross section in the longitudinal direction and the transverse direction.

  However, microporous membranes generally consisting essentially of polyethylene (ie, microporous membranes containing only polyethylene and no other important components) have a low meltdown temperature. Therefore, a microporous membrane made of a mixed resin of polyethylene and polypropylene and a multilayer microporous membrane made of a polyethylene layer and a polypropylene layer have been proposed.

  WO 2005/113657 (Patent Document 3) discloses a polyolefin microporous membrane excellent in moderate pore closing property, heat-resistant film breaking property, dimensional stability and high temperature strength. In this film, (a) the proportion of the component having a molecular weight of 10,000 or less is 8 to 60% by mass, the ratio Mw / Mn of Mw to the number average molecular weight (Mn) is 11 to 100, and the viscosity average molecular weight ( Mv) is formed of a polyolefin composition comprising a polyethylene resin having 100,000 to 1,000,000 and (b) polypropylene. This polyolefin microporous membrane has a porosity of 20 to 95%, and a thermal shrinkage at 100 ° C. of 10% or less. This polyolefin microporous membrane is obtained by extruding a melt-kneaded product of the above-mentioned polyolefin composition and a film-forming solvent from a die, stretching a gel-like sheet obtained by cooling, removing the film-forming solvent, and heat relaxation. Manufactured by processing.

  WO 2004/089627 (Patent Document 4) contains polyethylene and polypropylene as essential components, is composed of a laminated film of two or more layers, has a polypropylene content of at least 50% by mass and not more than 95% by mass on the surface layer on one side; A polyolefin microporous membrane having a polyethylene content of 50 to 95% by mass of the entire membrane is disclosed. This microporous membrane has improved permeability, membrane strength at high temperatures and safety, has a low shutdown temperature and a high short-circuit temperature.

  Japanese Patent Application Laid-Open No. 7-216118 (Patent Document 5) has two microporous layers having polyethylene and polypropylene as essential components and different proportions of polyethylene, and the proportion of polyethylene is 0 to 20 weight in one microporous layer. %, The other microporous layer is 21 to 60% by weight, and the whole film is 2 to 40% by weight. This battery separator has improved shutdown start temperature and mechanical strength.

  Recent separator characteristics include permeability, mechanical strength, dimensional stability, shutdown characteristics, and meltdown characteristics, as well as battery productivity characteristics such as electrolyte absorption, and battery cycle such as electrolyte retention. The characteristics related to the characteristics are also emphasized. In particular, the electrode of a lithium ion secondary battery expands and contracts with the ingress and desorption of lithium, but the expansion rate tends to increase with the recent increase in capacity of the battery. Since the separator is pressed when the electrode expands, the separator is required to have a small decrease in the amount of electrolyte retained by the pressing. However, when the pore diameter of the separator is increased in order to improve the electrolyte solution absorbability, there is a problem that the electrolyte solution retainability is lowered. In any of the battery separators of Patent Documents 1 to 5, either the electrolyte solution absorbability or the electrolyte solution retainability is insufficient. As described above, it is desirable that the microporous membrane for battery separators has improved permeability, mechanical strength, meltdown characteristics, electrolyte solution absorbability, and electrolyte solution retention, and has a good balance.

JP-A-6-240036 International Publication No. 00/20492 International Publication No. 05/113657 International Publication No. 04/089627 Japanese Patent Laid-Open No. 7-216118

  Accordingly, an object of the present invention is a polyolefin multilayer microporous membrane having an excellent balance of permeability, mechanical strength, meltdown characteristics, electrolyte absorption and electrolyte retention, a method for producing the same, and the multilayer microporous membrane. It is providing a battery separator and a battery including the same.

As a result of diligent research in view of the above object, the present inventors have (i) a first polyethylene-based resin in which the proportion of ultrahigh molecular weight polyethylene having a weight average molecular weight of 1 × 10 ⁶ or more is 8% by mass or more. And (ii) a second polyethylene resin in which the ratio of the ultrahigh molecular weight polyethylene is 7% by mass or less, and the pore size distribution curve obtained by mercury porosimetry has a structure having at least two peaks. A second microporous layer, the total thickness of the first and second microporous layers is 100%, and the thickness of the first microporous layer is 15 to 60%. The present inventors have found that a polyolefin multilayer microporous film having an excellent balance of mechanical strength, meltdown characteristics, electrolytic solution absorbability, and electrolytic solution retention can be obtained.

That is, the polyolefin multilayer microporous membrane of the present invention has at least a first microporous layer forming both surface layers and at least one second microporous layer provided between both surface layers, The one microporous layer includes a first polyethylene-based resin in which the ratio of the ultrahigh molecular weight polyethylene having a weight average molecular weight of 1 × 10 ⁶ or more is 8% by mass or more, and the second microporous layer includes the ultrahigh molecular weight. The first and second microporous layers include a second polyethylene resin having a molecular weight polyethylene ratio of 7% by mass or less, a pore size distribution curve obtained by a mercury intrusion method having at least two peaks, and The total thickness of the first microporous layer is 15 to 60%.

  The average pore diameter of the first microporous layer is preferably 0.005 to 0.1 μm, and the average pore diameter of the second microporous layer is preferably 0.02 to 0.5 μm. The ratio of the average pore diameter of the second microporous layer to the average pore diameter of the first microporous layer is preferably more than 1/1 and not more than 10/1.

  The second microporous layer has a dense region having a main peak in the range of 0.01 to 0.08 μm and a coarse region having at least one sub-peak in the range of more than 0.08 μm and 1.5 μm or less in the pore size distribution curve. Is preferred. The pore volume ratio between the dense region and the coarse region is preferably 0.5 to 49.

  The polyolefin multilayer microporous membrane preferably has a three-layer structure in which a pair of the first microporous layers are formed on both surfaces of one second microporous layer.

In the polyolefin multilayer microporous membrane according to a preferred embodiment of the present invention, the first microporous layer includes (i) the ultrahigh molecular weight polyethylene, (ii) the ultrahigh molecular weight polyethylene, and a weight average molecular weight of 1 × 10 ^4. A first polyethylene composition comprising high density polyethylene of ˜5 × 10 ⁵ (the proportion of the ultrahigh molecular weight polyethylene is 8% by mass or more), (iii) a mixture comprising the ultrahigh molecular weight polyethylene and polypropylene (the proportion of the polypropylene) Is formed by a mixture of the first polyethylene composition and polypropylene (the proportion of the polypropylene is 25% by mass or less), and the second microporous layer is ( i) the second polyethylene composition comprising the high density polyethylene, (ii) the ultrahigh molecular weight polyethylene and the high density polyethylene (the ultra high molecular weight polyethylene). The proportion of the low molecular weight polyethylene is 7% by mass or less), (iii) the mixture of the high-density polyethylene and polypropylene (the proportion of the polypropylene is 25% by mass or less), or (iv) the second polyethylene composition and the polypropylene. (The proportion of the polypropylene is 25% by mass or less).

The first method for producing the polyolefin multilayer microporous membrane of the present invention is as follows: (1) a first polyethylene resin having a weight average molecular weight of 1 × 10 ⁶ or more and a ratio of ultrahigh molecular weight polyethylene of 8% by mass or more; Preparing a first polyolefin solution containing a film solvent, and a second polyolefin solution containing a second polyethylene-based resin having a ratio of the ultra-high molecular weight polyethylene of 7% by mass or less and a film forming solvent; (2) The first and second polyolefin solutions are extruded simultaneously from one die so that the first polyolefin solution forms at least both surface layers and the second polyolefin solution forms at least one layer between both surface layers. (3) cooling the obtained extrusion-molded body, (4) stretching the obtained gel-like laminate sheet, (5) removing the film-forming solvent from the gel-like laminate sheet, and (6) obtaining Product Characterized by stretching the microporous membrane in at least one direction to 1.1 to 1.8 times.

The second method for producing the polyolefin multilayer microporous membrane of the present invention is as follows: (1) a first polyethylene-based resin having a weight average molecular weight of 1 × 10 ⁶ or more and a ratio of ultrahigh molecular weight polyethylene of 8% by mass or more; Preparing a first polyolefin solution containing a film solvent, and a second polyolefin solution containing a second polyethylene-based resin having a ratio of the ultra-high molecular weight polyethylene of 7% by mass or less and a film forming solvent; (2) The first and second polyolefin solutions were extruded from separate dies so that the first polyolefin solution formed at least both surface layers and the second polyolefin solution formed at least one layer between both surface layers. Immediately after lamination, (3) the obtained laminate is cooled, (4) the obtained gel laminate sheet is stretched, and (5) the film-forming solvent is removed from the gel laminate sheet, 6) Product obtained Characterized by stretching the microporous membrane in at least one direction to 1.1 to 1.8 times.

A third method for producing the polyolefin multilayer microporous membrane of the present invention is as follows: (1) a first polyethylene resin having a weight average molecular weight of 1 × 10 ⁶ or more and a ratio of ultrahigh molecular weight polyethylene of 8% by mass or more; Preparing a first polyolefin solution containing a film solvent, and a second polyolefin solution containing a second polyethylene-based resin having a ratio of the ultra-high molecular weight polyethylene of 7% by mass or less and a film forming solvent; (2) Extruding the first and second polyolefin solutions from separate dies, (3) cooling the obtained extruded products to form first and second gel sheets, (4) each gel sheet (5) the first and second gel-like sheets so that the first gel-like sheet forms at least both surface layers and the second gel-like sheet forms at least one layer between both surface layers. (6) The resulting laminated gel Removing the membrane-forming solvent from the sheet, characterized in that it stretched 1.1-1.8 times in at least one direction of the laminated microporous film was obtained (7).

The fourth method for producing the polyolefin multilayer microporous membrane of the present invention is as follows: (1) a first polyethylene resin and a synthetic resin having a weight average molecular weight of 1 × 10 ⁶ or more and a ratio of ultrahigh molecular weight polyethylene of 8% by mass or more; Preparing a first polyolefin solution containing a film solvent, and a second polyolefin solution containing a second polyethylene-based resin having a ratio of the ultra-high molecular weight polyethylene of 7% by mass or less and a film forming solvent; (2) Extruding the first and second polyolefin solutions from separate dies, (3) cooling the obtained extruded products to form first and second gel sheets, (4) each gel sheet (5) removing the film-forming solvent from each stretched gel sheet to form the first and second polyolefin microporous membranes, and (6) at least the second polyolefin microporous membrane at least uniaxially. Stretched 1.1 to 1.8 times in the direction (7) The first and second polyolefin microporous membranes so that the first polyolefin microporous membrane forms at least both surface layers, and the second polyolefin microporous membrane forms at least one layer between both surface layers. It is characterized by laminating films.

  In the first to fourth methods, the (laminated) microporous membrane is stretched after the gel-like (laminated) sheet is stretched, and can be referred to as “re-stretching”.

  The battery separator of the present invention is characterized by being formed of the above-mentioned polyolefin multilayer microporous membrane.

  The battery of the present invention is characterized by comprising a separator comprising the above-mentioned polyolefin multilayer microporous membrane.

  The polyolefin multilayer microporous membrane of the present invention has appropriate permeability, mechanical strength, meltdown characteristics, electrolyte absorption, and electrolyte retention. When the separator comprising the polyolefin multilayer microporous membrane of the present invention is used, a battery excellent in safety, heat resistance, storage characteristics and productivity can be obtained.

  The polyolefin multilayer microporous membrane of the present invention has at least a first microporous layer made of the first polyolefin and a second microporous layer made of the second polyolefin. When the polyolefin multilayer microporous membrane has a structure of three or more layers, it has a first microporous layer in both surface layers, and has at least one second microporous layer between both surface layers. The composition of both surface layers may be the same or different, but is preferably the same.

[1] Composition of polyolefin multilayer microporous membrane
(A) 1st polyolefin The 1st polyolefin which forms the 1st microporous layer (both surface layers in the case of the structure of three layers or more) of the polyolefin multilayer microporous membrane is (1) 1st polyethylene type Resin [(a) a first polyethylene composition comprising an ultra high molecular weight polyethylene having a weight average molecular weight (Mw) of 1 × 10 ⁶ or more, or (b) an ultra high molecular weight polyethylene + polyethylene other than ultra high molecular weight polyethylene (first The ratio of the ultrahigh molecular weight polyethylene is 8% by mass or more)], or (2) a mixture of the first polyethylene resin and 25% by mass or less of polypropylene. Hereinafter, the first polyolefin will be described in detail.

(1) The first polyethylene resin The first polyethylene resin is composed of (a) an ultra high molecular weight polyethylene having a weight average molecular weight (Mw) of 1 × 10 ⁶ or more, and (b) an ultra high molecular weight polyethylene and A first polyethylene composition comprising polyethylene other than ultrahigh molecular weight polyethylene (having a lower Mw than ultrahigh molecular weight polyethylene) (the first polyethylene composition is 100% by mass, and the proportion of ultrahigh molecular weight polyethylene is 8% by mass or more) ).

(a) Ultra high molecular weight polyethylene Ultra high molecular weight polyethylene has a Mw of 1 × 10 ⁶ or more. The ultra high molecular weight polyethylene may be not only an ethylene homopolymer but also an ethylene / α-olefin copolymer containing a small amount of other α-olefin. As the α-olefin other than ethylene, propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate and styrene are preferable. The content of α-olefin other than ethylene is preferably 5 mol% or less. Mw of the ultra-high-molecular-weight polyethylene is preferably ^{1 × 10 6 ~15 × 10 6} , more preferably ^{1 × 10 6 ~5 × 10 6} , 1 × 10 6 ~3 × 10 6 being particularly preferred.

(b) First polyethylene composition The polyethylene other than ultra high molecular weight polyethylene is preferably high density polyethylene, medium density polyethylene, branched low density polyethylene and chain low density polyethylene, and its Mw is 1 × 10 ^{4 to} 5 ×. 10 ⁵ is preferred. More preferable other polyethylene is high density polyethylene, and its Mw is preferably 1 × 10 ^{5 to} 5 × 10 ^5, more preferably 2 × 10 ⁵ to 4 × 10 ⁵ . The polyethylene other than the ultrahigh molecular weight polyethylene may be a copolymer containing a small amount of other α-olefins such as propylene, butene-1, and hexene-1 as well as an ethylene homopolymer. The content of α-olefin other than ethylene is preferably 5 mol% or less. Such a copolymer is preferably produced by a single site catalyst.

  The content of ultrahigh molecular weight polyethylene in the first polyethylene composition is 8% by mass or more. When the ultra high molecular weight polyethylene is less than 8% by mass, the strength of the multilayer microporous film is too low. The content of ultrahigh molecular weight polyethylene is preferably 20% by mass or more, and particularly preferably 25% by mass or more.

(2) Polypropylene When the first polyolefin is composed of the first polyethylene resin and polypropylene, the content of polypropylene is 25% by mass or less based on 100% by mass of the entire first polyolefin. When the proportion of polypropylene exceeds 25% by mass, the mechanical strength of the multilayer microporous membrane decreases. This ratio is preferably 15% by mass or less, and more preferably 10% by mass or less.

  Polypropylene may be either a homopolymer or a copolymer with another olefin, but a homopolymer is preferred. The copolymer may be either a random or block copolymer. Examples of olefins other than propylene include ethylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, α-olefins such as styrene, butadiene, And diolefins such as 5-hexadiene, 1,7-octadiene, and 1,9-decadiene. The proportion of other olefins in the propylene copolymer may be in a range that does not impair physical properties such as heat resistance, compression resistance, and heat shrinkage, and is preferably less than 10 mol%.

The weight average molecular weight (Mw) of polypropylene is preferably 1 × 10 ⁴ to 4 × 10 ^{6, and} more preferably 3 × 10 ^{5 to} 3 × 10 ⁶ . The molecular weight distribution (Mw / Mn) of polypropylene is preferably 1.01 to 100, and more preferably 1.1 to 50.

(B) Second polyolefin The second polyolefin consists of (a) the second polyethylene resin [other than ultra high molecular weight polyethylene with Mw of 1 × 10 ⁶ or more, or other than ultra high molecular weight polyethylene + ultra high molecular weight polyethylene A second polyethylene composition comprising a polyethylene (the proportion of ultrahigh molecular weight polyethylene is 7% by mass or less)], or (b) a mixture comprising a second polyethylene resin and 25% by mass or less polypropylene. Ultra high molecular weight polyethylene and other polyethylenes may be the same as described above. Ultra high molecular weight polyethylene and other polyethylenes may be different in the first and second polyethylene resins, but are preferably the same.

If the ratio of ultrahigh molecular weight polyethylene in the second polyethylene resin is more than 7% by mass, a hybrid structure described later is not formed. The ratio of the ultrahigh molecular weight polyethylene is preferably 5% by mass or less, and more preferably 3% by mass or less. Also Mw of the second polyethylene resin is preferably 1 × 10 ⁶ or less, more preferably ^{1 × 10 5 ~1 × 10 6} , 2 × 10 5 ~1 × 10 6 being particularly preferred. When the Mw exceeds 1 × 10 ⁶ , a hybrid structure is not formed. On the other hand, if it is less than 1 × 10 ⁵ , breakage tends to occur during stretching.

(2) Polypropylene When the second polyolefin is composed of the second polyethylene resin and polypropylene, the polypropylene content is 25% by mass or less, preferably 15% by mass or less, with the second polyolefin being 100% by mass. 10% by mass or less is more preferable. The polypropylene itself may be the same as in the first polyolefin.

(C) Molecular weight distribution Mw / Mn
Mw / Mn is a measure of molecular weight distribution. The larger this value, the wider the molecular weight distribution. In both the first and second polyolefins, the Mw / Mn of the ultra high molecular weight polyethylene and polyethylene other than the ultra high molecular weight polyethylene is preferably 5 to 300, more preferably 5 to 100, and most preferably 5 to 30. If Mw / Mn is less than 5, the high molecular weight component is too much and melt extrusion is difficult, and if Mw / Mn is more than 300, the low molecular weight component is too much and the strength of the multilayer microporous film is reduced. Mw / Mn of polyethylene (homopolymer or ethylene / α-olefin copolymer) can be appropriately adjusted by multistage polymerization. As the multi-stage polymerization method, a two-stage polymerization in which a high molecular weight polymer component is generated in the first stage and a low molecular weight polymer component is generated in the second stage is preferable. In the case of a polyethylene composition, the greater the Mw / Mn, the greater the difference in weight average molecular weight between ultrahigh molecular weight polyethylene and the other polyethylene, and vice versa. Mw / Mn of the polyethylene composition can be appropriately adjusted depending on the molecular weight and mixing ratio of each component.

(D) Other polyolefins The first and second polyolefins are components other than the above, as long as the performance of the multilayer microporous membrane is not impaired. (A) Polybutene having an Mw of 1 × 10 ⁴ to 4 × 10 ⁶ 1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate, polystyrene and ethylene / α-olefin copolymer, and (b) Mw is 1 × You may contain at least 1 type chosen from the group which consists of 10 < ³ > -1 * 10 < ⁴ > polyethylene wax. Polybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate and polystyrene contain not only homopolymers but also other α-olefins A copolymer may be used.

[2] Process for producing polyolefin multilayer microporous membrane
(A) First production method The first method for producing a polyolefin multilayer microporous membrane is as follows: (1) A first polyolefin solution is prepared by melt-kneading a first polyolefin and a film-forming solvent; 2) A second polyolefin solution is prepared by melt-kneading the second polyolefin and a film-forming solvent. (3) The first and second polyolefin solutions are simultaneously extruded from one die. The extruded product was cooled to form a gel-like laminate sheet, (5) the gel-like laminate sheet was stretched, (6) the film-forming solvent was removed from the gel-like laminate sheet, and (7) the resulting laminate And (8) stretching the resulting dried laminated microporous membrane. After step (8), heat treatment [step (9)], cross-linking treatment by ionizing radiation [step (10)], hydrophilization treatment [step (11)], and the like may be performed as necessary.

(1) Preparation of first polyolefin solution A film forming solvent is melt-kneaded with the first polyolefin to prepare a first polyolefin solution. Various additives such as an antioxidant, an ultraviolet absorber, an antiblocking agent, a pigment, a dye, an inorganic filler, and a pore forming agent (for example, finely divided silicic acid) are added to the polyolefin solution as necessary, so long as the effect of the present invention is not impaired Can be added.

  The film-forming solvent is preferably a liquid at room temperature. By using a liquid solvent, it is possible to stretch at a relatively high magnification. Liquid solvents include nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin and other aliphatic, cycloaliphatic or aromatic hydrocarbons, mineral oil fractions with boiling points corresponding to these, and dibutyl phthalate And phthalic acid esters which are liquid at room temperature such as dioctyl phthalate. In order to obtain a gel-like laminated sheet with a stable content of the liquid solvent, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. In the melt-kneaded state, it is miscible with polyethylene, but at room temperature, a solid solvent may be used in place of or in combination with the liquid solvent. Examples of such a solid solvent include stearyl alcohol, seryl alcohol, and paraffin wax. However, when only a solid solvent is used, there is a risk of uneven stretching.

  The viscosity of the liquid solvent is preferably 30 to 500 cSt at 25 ° C., more preferably 30 to 200 cSt. If the viscosity at 25 ° C. is less than 30 cSt, foaming tends to occur and kneading is difficult. On the other hand, if it exceeds 500 cSt, it is difficult to remove the liquid solvent.

  The uniform melt kneading of the first polyolefin solution is not particularly limited, but it is preferably performed in a twin-screw extruder to prepare a highly concentrated polyolefin solution. The film-forming solvent may be added before the start of kneading, or may be added from the middle of the twin-screw extruder during kneading, but the latter is preferred.

The melt kneading temperature of the first polyolefin solution is preferably set to the melting point Tm ₁ + 10 ° C. to Tm ₁ + 120 ° C. of the first polyethylene resin. The melting point is determined by differential scanning calorimetry (DSC) based on JIS K7121. Specifically, the ultra high molecular weight polyethylene, the polyethylene other than the ultra high molecular weight polyethylene, and the first polyethylene composition have a melting point of about 130 to 140 ° C., and therefore the melt kneading temperature is 140 to 250 ° C. Preferably, it is 170-240 degreeC.

  The ratio (L / D) of the screw length (L) to the diameter (D) of the twin screw extruder is preferably in the range of 20-100, more preferably in the range of 35-70. When L / D is less than 20, melt kneading becomes insufficient. When L / D exceeds 100, the residence time of the polyolefin solution increases too much. The inner diameter of the twin screw extruder is preferably 40 to 100 mm.

  The density | concentration of a 1st polyolefin solution is 1-75 mass%, Preferably it is 20-70 mass%. When the concentration is less than 1% by mass, not only the productivity is lowered, but also swell and neck-in are increased at the die outlet during extrusion, and the moldability and self-supporting property of the gel-like laminated molded product are lowered. On the other hand, if the concentration exceeds 75% by mass, the moldability of the gel-like laminated molded product is lowered.

(2) Preparation of second polyolefin solution A second polyolefin solution is prepared by melt-kneading a film-forming solvent in the second polyolefin. Except for the conditions described below, the description is omitted because it may be the same as in the case of preparing the first polyolefin solution. The film-forming solvent used for the second polyolefin solution may be the same as or different from the film-forming solvent used for the first polyolefin solution, but is preferably the same.

The melt kneading temperature of the second polyolefin solution is preferably set to the melting point Tm ₂ + 10 ° C. to Tm ₂ + 120 ° C. of the second polyethylene resin. Specifically, the ultra high molecular weight polyethylene, the polyethylene other than the ultra high molecular weight polyethylene, and the second polyethylene composition have a melting point of about 130 to 140 ° C., and therefore the melt kneading temperature is 140 to 250 ° C. Preferably, it is 170-240 degreeC.

  In order to obtain a good hybrid structure, the concentration of the second polyethylene resin in the second polyolefin solution is preferably 25 to 50% by mass, and more preferably 25 to 45% by mass.

(3) Extrusion The first and second polyolefin solutions are each fed from an extruder to one die, where both solutions are combined in layers and extruded into sheets. When producing a multilayer microporous membrane having a structure of three or more layers, preferably the first polyolefin solution forms at least both surface layers and the second polyolefin solution forms at least one layer between both surface layers (preferably The two solutions are combined in layers and extruded into a sheet (so that one or both of the surface layers are in contact).

  The extrusion method may be either a flat die method or an inflation method. In either method, the solution is supplied to separate manifolds and stacked in layers at the lip inlet of a multilayer die (multiple manifold method), or the solution is supplied to the die in a layered flow in advance (block method) Can be used. Since the multi-manifold method and the block method itself are known, a detailed description thereof will be omitted. The gap of the multi-layer flat die is preferably 0.1 to 5 mm. The extrusion temperature is preferably 140 to 250 ° C., and the extrusion speed is preferably 0.2 to 15 m / min. The film thickness ratio of the first and second microporous layers can be adjusted by adjusting the extrusion amounts of the first and second polyolefin solutions.

(4) Formation of gel-like laminated sheet A gel-like laminated sheet is formed by cooling the obtained laminated extruded product. Cooling is preferably performed at a rate of 50 ° C./min or more at least up to the gelation temperature. Cooling is preferably performed to 25 ° C. or lower. By cooling, the microphases of the first and second polyolefins separated by the film-forming solvent can be fixed. Generally, when the cooling rate is slowed down, the pseudo cell unit becomes large and the higher order structure of the resulting gel-like laminated sheet becomes rough. However, when the cooling rate is fastened, the cell unit becomes dense. When the cooling rate is less than 50 ° C./min, the degree of crystallinity increases and it is difficult to obtain a gel-like laminated sheet suitable for stretching. As a cooling method, a method of contacting with a refrigerant such as cold air or cooling water, a method of contacting with a cooling roll, or the like can be used.

(5) Stretching of the gel-like laminated sheet The obtained gel-like laminated sheet is stretched at least in the uniaxial direction. Since the gel-like laminated sheet contains a film-forming solvent, it can be uniformly stretched. The gel-like laminated sheet is preferably stretched at a predetermined ratio after heating by a tenter method, a roll method, an inflation method, or a combination thereof. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching, and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used, but simultaneous biaxial stretching is preferable.

  In the case of uniaxial stretching, the draw ratio is preferably 2 times or more, more preferably 3 to 30 times. In biaxial stretching, it is preferably at least 3 times in any direction (area magnification is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more). The puncture strength is improved by setting the area magnification to 9 times or more. When the area magnification exceeds 400 times, there are restrictions in terms of stretching apparatus, stretching operation, and the like.

In order to obtain a good hybrid structure, the stretching temperature is preferably within the range of the crystal dispersion temperature Tcd _{2 to} Tcd ₂ + 25 ° C. of the second polyethylene resin, and the range of Tcd ₂ + 10 ° C. to Tcd ₂ + 25 ° C. It is more preferable that the temperature be within the range of Tcd ₂ + 15 ° C. to Tcd ₂ + 25 ° C. When the stretching temperature is less than Tcd ₂ , the polyethylene resin is not sufficiently softened, and the film is easily broken by stretching, so that it cannot be stretched at a high magnification.

  The crystal dispersion temperature is determined by measuring the temperature characteristics of dynamic viscoelasticity using ASTM D4065. The ultrahigh molecular weight polyethylene, the polyethylene other than the ultrahigh molecular weight polyethylene, and the first and second polyethylene compositions have a crystal dispersion temperature of about 90-100 ° C., so that the stretching temperature is 90-125 ° C., preferably 100- 125 ° C., more preferably 105 to 125 ° C.

  By stretching as described above, cleavage occurs between polyethylene lamellae, the polyethylene phase becomes finer, and a large number of fibrils are formed. Fibrils form a three-dimensional irregularly connected network structure. Stretching improves the mechanical strength and enlarges the pores, which is suitable for battery separators.

  Depending on the desired physical properties, the film may be stretched by providing a temperature distribution in the film thickness direction, whereby a multilayer microporous film having further excellent mechanical strength can be obtained. Details of this method are described in Japanese Patent No. 3347854.

(6) Removal of film-forming solvent A cleaning solvent is used to remove (clean) the film-forming solvent. Since the first and second polyolefin phases are separated from the film forming solvent phase, the film forming solvent is made up of fibrils that form a fine three-dimensional network structure, and communicates irregularly in three dimensions. A porous film having pores (voids) to be obtained is obtained. Suitable washing solvents include, for example, saturated hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, ethers such as diethyl ether and dioxane, ketones such as methyl ethyl ketone, and trifluoride. Ethane, chain fluorocarbons such as C ₆ F ₁₄ and C ₇ F ₁₆ , cyclic hydrofluorocarbons such as C ₅ H ₃ F _7, hydrofluoroethers such as C ₄ F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H ₅ , Examples include readily volatile solvents such as perfluoroethers such as C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F ₅ .

  The gel laminated sheet can be washed by a method of immersing in a washing solvent, a method of showering the washing solvent, or a combination thereof. The washing solvent is preferably used in an amount of 300 to 30,000 parts by mass with respect to 100 parts by mass of the membrane. The washing temperature may be 15 to 30 ° C., and may be heated and washed as necessary. The temperature for the heat washing is preferably 80 ° C. or lower. The washing with the washing solvent is preferably performed until the residual amount of the liquid solvent becomes less than 1% by mass of the initial addition amount.

(7) Drying The laminated microporous film from which the film-forming solvent has been removed is dried by a heat drying method or an air drying method. The drying temperature is preferably at Tcd ₂ or less, preferably especially than Tcd ₂ 5 ° C. or more lower. Drying is preferably carried out until the residual washing solvent is 5% by mass or less, more preferably 3% by mass or less, assuming that the laminated microporous membrane is 100% by mass (dry weight). If the drying is insufficient, the porosity of the laminated microporous film is lowered when the stretching process and the heat treatment process of the latter laminated microporous film are performed, and the permeability is deteriorated.

(8) Stretching of the laminated microporous membrane The dried laminated microporous membrane is stretched (re-stretched) in at least a uniaxial direction. The laminated microporous membrane can be stretched by the tenter method or the like in the same manner as described above while heating. The stretching may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, any of simultaneous biaxial stretching and sequential stretching may be used, but simultaneous biaxial stretching is preferable. In addition, since re-stretching is usually performed on a long sheet-like laminated microporous membrane obtained from a stretched gel-like laminated sheet, MD direction and TD direction in re-stretching and MD direction and TD direction in stretching of the gel-like laminated sheet Matches. The same applies to other examples of the manufacturing method.

Stretching temperature is preferably in the second melting point Tm ₂ of polyethylene resin or less, and more preferably in the range of Tcd ₂ to Tm _2. When the stretching temperature exceeds Tm ₂ , the melt viscosity becomes low, the stretching cannot be performed well, and the permeability is deteriorated. On the other hand, when the stretching temperature is less than Tcd ₂ , the first and second polyethylene resins are not sufficiently softened, the film is easily broken during stretching, and the film cannot be stretched uniformly. Specifically, the stretching temperature is 90 to 135 ° C, preferably 95 to 130 ° C.

  The magnification of the laminated microporous membrane in the uniaxial direction is preferably 1.1 to 1.8 times. In the case of uniaxial stretching, the length is 1.1 to 1.8 times in the longitudinal direction or the transverse direction. In the case of biaxial stretching, the length is 1.1 to 1.8 times in the longitudinal direction and the transverse direction, and the longitudinal direction and the transverse direction may be the same or different from each other, but are preferably the same.

  By stretching the laminated microporous membrane 1.1 to 1.8 times, the layer made of the second polyethylene resin has a hybrid structure, and the average pore diameter is increased. The layer made of the first polyethylene-based resin has an ultra high molecular weight polyethylene ratio of 8% by mass or more, so even if it is stretched 1.1 to 1.8 times, the average is higher than the layer made of the second polyethylene-based resin. The pore size does not increase.

  When the draw ratio of the laminated microporous film is less than 1.1 times, a hybrid structure is not formed in the second microporous layer, and permeability, electrolyte solution absorbability, and electrolyte solution retention are reduced. On the other hand, when the draw ratio is more than 1.8 times, the fibrils become too thin, and the heat shrinkage resistance and the electrolyte solution holding ability deteriorate. The draw ratio is more preferably 1.2 to 1.6 times.

(9) Heat treatment It is preferable to heat-treat the laminated microporous membrane after drying. The crystal is stabilized by heat treatment, and the lamella is made uniform. As the heat treatment method, heat setting treatment and / or heat relaxation treatment can be used. The heat setting treatment is preferably performed by a tenter method or a roll method. The heat setting treatment temperature is preferably within the range of Tcd _{2 to} Tm ₂ , more preferably within the range of the stretching temperature ± 5 ° C. of the laminated microporous membrane, and within the range of ± 3 ° C. of the stretching (restretching) temperature of the laminated microporous membrane. Is particularly preferred.

The thermal relaxation process is a process of heating the laminated microporous film without applying stress, and can be performed by moving the inside of the heating furnace by, for example, a belt conveyor or an air floating method. Further, after the heat setting treatment, the tenter may be loosened and the heat relaxation treatment may be performed as it is. The thermal relaxation treatment is performed at a temperature of Tm ₂ or less, preferably within a temperature range of 60 ° C. to (Tm ₂ -5 ° C.). By the heat relaxation treatment as described above, a high-strength laminated microporous film having good permeability can be obtained.

(10) Crosslinking treatment The laminated microporous membrane may be subjected to a crosslinking treatment by irradiation with ionizing radiation such as α-rays, β-rays, γ-rays and electron beams. In the case of electron beam irradiation, an electron dose of 0.1 to 100 Mrad is preferable, and an acceleration voltage of 100 to 300 kV is preferable. The meltdown temperature of the laminated microporous membrane increases due to the crosslinking treatment.

(11) Hydrophilization treatment The laminated microporous membrane may be subjected to a hydrophilic treatment. The hydrophilic treatment can be performed by monomer grafting, surfactant treatment, corona discharge or the like. Monomer grafting is preferably performed after the crosslinking treatment.

  In the case of the surfactant treatment, any of nonionic surfactants, cationic surfactants, anionic surfactants and amphoteric surfactants can be used, but nonionic surfactants are preferred. The laminated microporous membrane is immersed in a solution obtained by dissolving a surfactant in water or a lower alcohol such as methanol, ethanol, isopropyl alcohol, or the solution is applied to the laminated microporous membrane by a doctor blade method.

(B) Second production method The second method for producing a polyolefin multilayer microporous membrane is as follows: (1) A first polyolefin solution is prepared by melt-kneading a first polyolefin and a film-forming solvent; 2) Melting and kneading the second polyolefin and the film-forming solvent to prepare a second polyolefin solution; (3) Laminating immediately after extruding the first and second polyolefin solutions from separate dies; 4) The obtained laminate is cooled to form a gel laminate sheet, (5) the gel laminate sheet is stretched, (6) the film-forming solvent is removed from the gel laminate sheet, and (7) is obtained. Drying the laminated microporous membrane, and (8) stretching the laminated microporous membrane. After the step (8), a heat treatment step (9), a crosslinking treatment step (10) by ionizing radiation, a hydrophilization treatment step (11) and the like may be performed as necessary.

  Since the second method is the same as the first method except for step (3), only step (3) will be described. The first and second polyolefin solutions are extruded into sheets from adjacent dies connected to each of a plurality of extruders, and immediately laminated while the temperature of each solution is high (for example, 100 ° C. or higher). Other conditions may be the same as in the first method.

(C) Third production method A third method for producing a polyolefin multilayer microporous membrane is as follows: (1) A first polyolefin solution is prepared by melting and kneading a first polyolefin and a film-forming solvent; 2) A second polyolefin solution is prepared by melting and kneading the second polyolefin and a film-forming solvent. (3) A first extruded product is prepared by extruding the first polyolefin solution from one die. (4) The second polyolefin solution is extruded from another die to prepare a second extrudate, and (5) the obtained first and second extrudates are cooled to obtain the first and second (6) The first and second gel sheets are each stretched, (7) The stretched first and second gel sheets are laminated, and (8) the gel obtained Removing the film-forming solvent from the laminated sheet, (9) drying the resulting laminated microporous film, and (10) lamination A step of stretching the microporous membrane. Between the steps (7) and (8), (11) a step of stretching the gel-like laminated sheet may be provided. Further, after the step (10), (12) a heat treatment step, (13) a crosslinking treatment step by ionizing radiation, (14) a hydrophilization treatment step and the like may be provided.

  Steps (1) and (2) may be the same as the first method, and steps (3) and (4) may be the same as the first method except that the first and second polyolefin solutions are extruded from separate dies. The step (5) may be the same as the first method except for forming an individual gel-like sheet, and the step (6) may be the same as the first method except for stretching the individual gel-like sheet. ) To (10) may be the same as in the first method. Steps (11) to (14) may be the same as described above.

In the stretching step (6), the crystal dispersion temperature Tcd ₁ to (Tcd ₁ + 25 ° C.) of the first polyethylene resin is preferable, (Tcd ₁ + 10 ° C.) to (Tcd ₁ + 25 ° C.) is more preferable, and (Tcd ₁ +15 ° C) to (Tcd ₁ + 25 ° C) is particularly preferred. The stretching temperature of the second gel-like sheet is preferably the crystal dispersion temperature Tcd ₂ to (Tcd ₂ + 25 ° C.) of the second polyethylene resin, more preferably (Tcd ₂ + 10 ° C.) to (Tcd ₂ + 25 ° C.), Particularly preferred is (Tcd ₂ + 15 ° C.) to (Tcd ₂ + 25 ° C.).

  Hereinafter, the step (7) of laminating the stretched first and second gel sheets will be described. When producing a multilayer microporous membrane having a structure of three or more layers, the stretched first gel sheet forms at least both surface layers, and the stretched second gel sheet forms at least one layer between both surface layers The stretched gel-like sheet is laminated as described above. The lamination method is not particularly limited, but the thermal lamination method is preferable. Examples of the heat lamination method include a heat sealing method, an impulse sealing method, an ultrasonic lamination method, and the like, but a heat sealing method is preferable. As the heat sealing method, a method using a heat roll is preferable. In the hot roll method, the first and second gel sheets are passed between a pair of heated rolls. The temperature and pressure at the time of heat sealing are not particularly limited as long as the gel-like sheet is sufficiently adhered and the properties of the obtained multilayer microporous film are not deteriorated. The heat sealing temperature is, for example, 90 to 135 ° C, preferably 90 to 115 ° C. The heat seal pressure is preferably 0.01 to 50 MPa. By adjusting the thickness of the first and second gel-like sheets, the thickness ratio of the first and second microporous layers can be adjusted. Moreover, you may extend | stretch, laminating | stacking by passing between multistage heating rolls.

(D) Fourth production methodA fourth method for producing a polyolefin multilayer microporous membrane is as follows: (1) A first polyolefin solution is prepared by melting and kneading a first polyolefin and a film-forming solvent; 2) Melt and knead the second polyolefin and the film-forming solvent to prepare a second polyolefin solution, (3) extrude the first polyolefin solution from one die, and (4) add the second polyolefin solution. Extruded from another die, (5) cooled each extruded molded body to form the first and second gel-like sheets, (6) respectively stretch the first and second gel-like sheets, (7) Remove the film-forming solvent from each stretched gel sheet, (8) dry the first and second polyolefin microporous membranes obtained, and (9) at least the second polyolefin microporous membrane. Stretching and (10) laminating a microporous film. If necessary, a heat treatment step (11) may be performed between steps (8) and (9). Further, after the step (10), a laminated microporous membrane stretching step (12), a heat treatment step (13), a crosslinking treatment step (14) by ionizing radiation, a hydrophilization treatment step (15), and the like may be performed.

  Steps (1) and (2) may be the same as the first method, and steps (3) and (4) may be the same as the first method except that the first and second polyolefin solutions are extruded from separate dies. Step (5) may be the same as the first method except for forming an individual gel-like sheet, step (6) may be the same as the third method, and step (7) is made up of individual gel-like sheets. The method may be the same as the first method except that the membrane solvent is removed, and the step (8) may be the same as the first method except that the individual microporous membrane is dried. Steps (12) to (15) may be the same as described above.

In step (9), at least the second polyolefin microporous membrane is stretched. The stretching temperature is preferably Tm ₂ or less, Tcd ₂ to Tm ₂ is more preferable. If necessary, the first polyolefin microporous membrane may also be stretched. The stretching temperature is preferable Tm ₁ or less, Tcd ₁ to Tm ₁ is more preferred. In either case of stretching the first and second polyolefin microporous membranes, the stretching ratio may be the same as in the first method except for stretching a non-laminated microporous membrane.

  The step (10) of laminating the stretched first polyolefin microporous membrane and the stretched second polyolefin microporous membrane will be described. When producing a multilayer microporous membrane having a structure of three or more layers, the first polyolefin microporous membrane forms at least both surface layers, and the second polyolefin microporous membrane forms at least one layer between both surface layers. Thus, both microporous membranes are laminated. The laminating method is not particularly limited, but the thermal laminating method is preferable as in the step of laminating the gel sheets by the third method, and the heat sealing method is particularly preferable. The heat sealing temperature is, for example, 90 to 135 ° C, preferably 90 to 115 ° C. The heat seal pressure is preferably 0.01 to 50 MPa. You may extend | stretch, laminating | stacking, and the method of letting it pass between multistage heating rolls at the said temperature and pressure is mentioned as the method.

In the heat treatment step (11), the heat setting temperature of the first microporous membrane is preferably Tcd _{1 to} Tm ₁ . The thermal relaxation temperature of the first microporous membrane is preferably Tm ₁ or less, more preferably 60 ° C. to (Tm ₁ −5 ° C.). The heat setting temperature of the second microporous membrane is preferably Tcd _{2 to} Tm _2, more preferably a stretching temperature ± 5 ° C., and particularly preferably a stretching temperature ± 3 ° C. The thermal relaxation temperature of the second microporous membrane is preferably Tm ₂ or less, more preferably 60 ° C. to (Tm ₂ −5 ° C.).

[3] Structure and physical properties of polyolefin multilayer microporous membrane
(A) First microporous layer
(1) Average pore diameter The average pore diameter of the first microporous layer is 0.005 to 0.1 µm, preferably 0.01 to 0.05 µm.

(2) Number of layers When the multilayer microporous membrane has a structure of three or more layers, the first microporous layer may be at least on both surface layers. When the multilayer microporous film has a structure of four or more layers, three or more first microporous layers may be provided as necessary. For example, you may provide the 1st microporous layer from which both surface layers differ in a component between both surface layers.

(3) Action of the first microporous layer When both surface layers are formed of the first microporous layer, a polyolefin multilayer microporous film exhibiting high mechanical strength, meltdown characteristics and electrolyte retention is obtained.

(B) Second microporous layer
(1) Average pore diameter The average pore diameter of the second microporous layer is 0.02 to 0.5 µm, preferably 0.02 to 0.1 µm.

(2) Structure As shown in FIG. 1, the second microporous layer has a hybrid structure in which the pore size distribution curve obtained by mercury porosimetry has at least two peaks (main peak and at least one sub peak). The main peak is preferably in the pore size range of 0.01 to 0.08 μm, and the sub-peak is preferably in the pore size range of more than 0.08 μm and not more than 1.5 μm. The main peak indicates a dense region, and the sub peak indicates a coarse region. The second microporous layer has a larger average pore diameter than the first microporous layer due to the presence of the coarse region. The hybrid structure is derived from the second polyethylene resin. When the ratio of ultra high molecular weight polyethylene is more than 7% by mass, a hybrid structure is not formed and the electrolyte absorbability is lowered.

  In a preferred example of the second microporous layer, the dense region has a main peak (first peak) in the range of about 0.04 to 0.07 μm, and the coarse region has a second peak in the range of at least about 0.1 to 0.11 μm, It has a third peak at about 0.7 μm and a fourth peak in the range of about 1-1.1 μm. However, the sub-peak does not necessarily have the third and fourth peaks. FIG. 1 shows an actual measurement example of a pore size distribution curve. In this example, there are first to fourth peaks at about 0.06 μm, about 0.1 μm, about 0.7 μm, and about 1.1 μm, respectively.

The ratio of the pore volume between the dense region and the coarse region is determined from S ₁ and S ₂ shown in FIG. Area S ₁ of the hatched portion of the smaller diameter side of the vertical line L ₁ passing through the first peak corresponds to the pore volume of the dense region, the area S ₂ of the hatched portion of the larger diameter side than the vertical line L ₂ passing through the second peak coarse Corresponds to the pore volume of the region. The pore volume ratio S ₁ / S ₂ between the dense region and the coarse region is preferably from 0.5 to 49, more preferably from 0.6 to 10, and particularly preferably from 0.7 to ₂ .

  Although not limited, the dense region and the coarse region in the second microporous layer are irregularly mixed in any of the cross sections in the longitudinal direction and the transverse direction to form a hybrid structure. The hybrid structure can be observed with a transmission electron microscope (TEM) or the like.

(3) Number of layers When the multilayer microporous membrane has a structure of three or more layers, it is sufficient that at least one second microporous layer is provided. When the multilayer microporous film has a structure of four or more layers, the second microporous layer may be multilayered as necessary. The plurality of second microporous layers may have different compositions.

(4) Action of the second microporous layer The second microporous layer has a larger average pore diameter than the first microporous layer. Therefore, if there is at least one second microporous layer between both surface layers, a polyolefin multilayer microporous membrane having good permeability and electrolyte solution absorbability can be obtained.

(C) Ratio of average pore diameter The ratio of the average pore diameter of the second microporous layer to the average pore diameter of the first microporous layer is preferably more than 1/1 and not more than 10/1, preferably 1.5 / 1. More preferably, it is ˜5 / 1.

(D) Arrangement and ratio of the first and second microporous layers The arrangement of the first and second microporous layers of the polyolefin multilayer microporous membrane is: (i) first microporous layer / second microporous In the case of a two-layer structure of layers, and (ii) a structure of three or more layers in which a first microporous layer is provided on both surface layers and at least one second microporous layer is provided between both surface layers It is roughly divided into As described above, in the case of a multilayer microporous membrane having a structure of three or more layers, as long as at least one second microporous layer is provided between both surface layers, the first and second microporous membranes are also provided. One or both of the porous layers may be provided. When a plurality of microporous layers are provided between both surface layers, the arrangement of the microporous layers between both surface layers is not particularly limited. Although not limited, the polyolefin multilayer microporous membrane preferably has a three-layer structure of first microporous layer / second microporous layer / first microporous layer.

  The ratio of the thickness of the first microporous layer (in the case of the structure of three or more layers, the total thickness of the first microporous layer) in both cases of the two-layer structure and the three-layer structure or more The total thickness of the first and second microporous layers is preferably 100% and 15-60%. If this ratio is less than 15%, the meltdown characteristics are low. On the other hand, if it exceeds 60%, the permeability, electrolyte absorption and electrolyte retention are low. This ratio is more preferably 15 to 50%. In the case of a three-layer structure, the layer thickness ratio of the first microporous layer / second microporous layer / first microporous layer is preferably 0.08 / 0.84 / 0.08 to 0.3 / 0.4 / 0.3, It is more preferable that it is 0.8 / 0.1-0.25 / 0.5 / 0.25.

(E) Physical Properties The polyolefin multilayer microporous membrane has the following physical properties.

(1) Air permeability of 20 to 400 seconds / 100 cm ³ (converted to a film thickness of 20 μm)
When the air permeability (air permeability) measured in accordance with JIS P8117 is 20 to 400 seconds / 100 cm ³ , the capacity of the battery increases when the multilayer microporous membrane is used as a battery separator. The cycle characteristics are also good. If the air permeability is less than 20 seconds / 100 cm ³ , shutdown may not be performed sufficiently when the temperature inside the battery rises. When the air permeability P ₁ measured according to JIS P8117 for a multilayer microporous film with a film thickness T ₁ is 20 μm according to the formula: P ₂ = (P ₁ × 20) / T ₁ in terms of the air permeability P _2.

(2) Porosity of 25 to 80% When the porosity is less than 25%, the multilayer microporous membrane does not have good air permeability. On the other hand, if it exceeds 80%, when the multilayer microporous membrane is used as a battery separator, the strength becomes insufficient, and the risk of a short circuit of the electrode increases.

(3) Puncture strength of 2,000 mN or more (converted to a film thickness of 20 μm)
Multilayer microporous membrane puncture strength (converted to a film thickness of 20 μm) is the maximum load when a 1 mm diameter needle with a spherical tip (radius of curvature R: 0.5 mm) is pierced into the multilayer microporous membrane at a speed of 2 mm / sec. It is represented by When the puncture strength is less than 2,000 mN / 20 μm, there is a possibility that a short circuit of the electrode may occur when the multilayer microporous membrane is incorporated in a battery as a battery separator.

(4) Tensile breaking strength of 49,000 kPa or more
When the tensile strength at break measured by ASTM D882 is 49,000 kPa or more in both the longitudinal direction (MD) and the transverse direction (TD), there is no concern about film breakage when used as a battery separator. The tensile strength at break is preferably 80,000 kPa or more.

(5) Tensile elongation at break of 100% or more
When the tensile elongation at break measured by ASTM D882 is 100% or more in both the longitudinal direction and the transverse direction, there is no fear of film breakage when used as a battery separator.

(6) Thermal shrinkage of 12% or less
When the thermal shrinkage rate when held at 105 ° C for 8 hours exceeds 12% in both the longitudinal and transverse directions, when the multilayer microporous membrane is used as a battery separator, the separator shrinks due to the heat generated by the battery, and its end This increases the possibility of short circuiting.

(7) Meltdown temperature of 150 ° C. or more The meltdown temperature of the polyolefin multilayer microporous membrane is 150 ° C. or more, preferably 150 to 190 ° C. The meltdown temperature is determined as follows. As shown in FIG. 2, a test piece TP having a size of 3 mm and 10 mm in the MD and TD stretching directions is cut out from the polyolefin multilayer microporous membrane 1, and the upper end 1a of the test piece TP is held by the holder 2. Then, a 2 g weight is attached to the lower end 1b and the temperature is raised from room temperature at a rate of 5 ° C./min. The temperature when the elongation of the test piece TP reaches 50% of the length (100%) at room temperature is defined as the meltdown temperature.

[4] Battery separator The battery separator composed of the above-mentioned polyolefin multilayer microporous membrane can be appropriately selected according to the type of battery, but preferably has a thickness of 3 to 200 μm, and a thickness of 5 to 50 μm. More preferably, it has a film thickness of 10 to 35 μm.

[5] Battery The polyolefin multilayer microporous membrane of the present invention comprises a lithium ion secondary battery, a lithium polymer secondary battery, a nickel-hydrogen secondary battery, a nickel-cadmium secondary battery, a nickel-zinc secondary battery, and a silver-zinc Although it is preferable for a separator for a secondary battery such as a secondary battery, it is particularly preferable for a separator for a lithium ion secondary battery. The lithium ion secondary battery will be described below. However, the present invention is not limited to the lithium ion secondary battery.

  In a lithium ion secondary battery, a positive electrode and a negative electrode are laminated via a separator, and the separator contains an electrolytic solution (electrolyte). The structure of the electrode is not particularly limited, and may be a known structure. For example, an electrode structure (coin type) arranged such that a disc-shaped positive electrode and a negative electrode face each other, an electrode structure (stacked type) in which flat plate-like positive electrodes and negative electrodes are alternately laminated, a laminated belt-like positive electrode, and An electrode structure in which the negative electrode is wound (winding type) can be used.

The positive electrode usually has a current collector and a layer that is formed on the surface thereof and includes a positive electrode active material that can occlude and release lithium ions. Examples of the positive electrode active material include transition metal oxides, composite oxides of lithium and transition metals (lithium composite oxides), and inorganic compounds such as transition metal sulfides. Transition metals include V, Mn, and Fe. , Co, Ni and the like. Preferable examples of the lithium composite oxide include lithium nickelate, lithium cobaltate, lithium manganate, and a layered lithium composite oxide based on an α-NaFeO ₂ type structure. The negative electrode has a current collector and a layer formed on the surface thereof and containing a negative electrode active material. Examples of the negative electrode active material include carbonaceous materials such as natural graphite, artificial graphite, cokes, and carbon black.

The electrolytic solution can be obtained by dissolving a lithium salt in an organic solvent. Lithium salts include LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , Li ₂ B ₁₀ Cl ₁₀ , Examples thereof include LiN (C ₂ F ₅ SO ₂ ) ₂ , LiPF ₄ (CF ₃ ) ₂ , LiPF ₃ (C ₂ F ₅ ) ₃ , a lower aliphatic carboxylic acid lithium salt, LiAlCl _{4 and the} like. These may be used alone or as a mixture of two or more. Examples of the organic solvent include organic solvents having a high boiling point and a high dielectric constant such as ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and γ-butyrolactone, and tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane, dimethyl carbonate, and diethyl carbonate. Examples include organic solvents having a low boiling point and a low viscosity. These may be used alone or as a mixture of two or more. Since a high dielectric constant organic solvent has a high viscosity and a low viscosity organic solvent has a low dielectric constant, it is preferable to use a mixture of the two.

  When assembling the battery, the separator is impregnated with the electrolytic solution. Thereby, ion permeability can be imparted to the separator (multilayer microporous membrane). The impregnation treatment is performed by immersing the multilayer microporous membrane in an electrolytic solution at room temperature. When assembling a cylindrical battery, for example, a positive electrode sheet, a separator made of a multilayer microporous membrane, and a negative electrode sheet are laminated in this order, wound, inserted into a battery can, impregnated with an electrolyte, and then a positive electrode terminal equipped with a safety valve Crimp the battery lid that also serves as a via a gasket.

  The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Example 1
(1) Preparation of first polyolefin solution 30% by mass of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight (Mw) of 2.0 × 10 ⁶ and a molecular weight distribution (Mw / Mn) of 8, and Mw of 3.5 × 10 6 ⁵ and 100 parts by mass of a first polyethylene composition comprising 70% by mass of high density polyethylene (HDPE) having an Mw / Mn of 13.5, tetrakis [methylene-3- (3,5-ditertiarybutyl- 4-Hydroxyphenyl) -propionate] 0.2 parts by mass of methane was dry blended. The melting point of the first polyethylene composition was 135 ° C., and the crystal dispersion temperature was 100 ° C. 35 parts by mass of the obtained mixture was put into a strong kneading type twin screw extruder (inner diameter 58 mm, L / D = 52.5), and liquid paraffin [50 cSt (40 ° C.)] 65 from the side feeder of the twin screw extruder. Mass parts were supplied and melt kneaded under the conditions of 230 ° C. and 250 rpm to prepare a first polyolefin solution.

(2) Preparation of second polyolefin solution Second polyethylene composition 100 comprising 2% by mass of the above UHMWPE, 98% by mass of HDPE having Mw of 3.0 × 10 ⁵ , Mw / Mn of 8.6, and Mw of 3.3 × 10 ⁵ 0.2 parts by mass of the antioxidant was dry blended with parts by mass. 35 parts by mass of the obtained mixture was charged into the same twin screw extruder as above, 65 parts by mass of liquid paraffin as described above was supplied from the side feeder of the twin screw extruder, and melt kneaded at 230 ° C. and 250 rpm. A second polyolefin solution was prepared.

Mw and Mw / Mn of UHMWPE and HDPE were determined by gel permeation chromatography (GPC) method under the following conditions (the same applies hereinafter).
・ Measurement device: GPC-150C made by Waters Corporation
・ Column: Shodex UT806M manufactured by Showa Denko KK
-Column temperature: 135 ° C
・ Solvent (mobile phase): o-dichlorobenzene ・ Solvent flow rate: 1.0 ml / min ・ Sample concentration: 0.1% by mass (dissolution condition: 135 ° C./1 h)
・ Injection volume: 500μl
-Detector: Differential refractometer manufactured by Waters Corporation-Calibration curve: Prepared from a calibration curve obtained using a monodisperse polystyrene standard sample using a predetermined conversion constant.

(3) Film formation First and second polyolefin solutions are fed from each twin-screw extruder to a three-layer T-die, and the layer thickness of the first polyolefin solution / second polyolefin solution / first polyolefin solution Extrusion was performed so that the ratio was 0.2 / 0.6 / 0.2. The extruded product was cooled while being drawn with a cooling roll adjusted to 15 ° C. to form a gel-like three-layer sheet. The gel-like three-layer sheet was simultaneously biaxially stretched 5 times in the longitudinal direction and the transverse direction at 117 ° C. by a tenter stretching machine. The stretched gel-like three-layer sheet is fixed to a 20 cm × 20 cm aluminum frame plate, immersed in a methylene chloride bath adjusted to 25 ° C., and the liquid paraffin is removed while shaking at 100 rpm for 3 minutes. Air dried. The dried film was redrawn 1.2 times in the transverse direction at 128 ° C. using a batch type drawing machine. With the re-stretched membrane fixed to a batch-type stretcher, a polyolefin three-layer microporous membrane was produced by heat setting at 128 ° C. for 10 minutes.

Example 2
(1) Preparation of the first polyolefin solution
A composition comprising 10% by weight of UHMWPE, 85% by weight of HDPE having Mw of 3.0 × 10 ⁵ and Mw / Mn of 8.6, and 5% by weight of propylene homopolymer (PP) having Mw of 5.3 × 10 ⁵ was used. A first polyolefin solution was prepared in the same manner as in Example 1 except that. The Mw of PP was determined by the GPC method in the same manner as described above (hereinafter the same).

(2) Preparation of second polyolefin solution A second polyolefin solution was prepared in the same manner as in Example 1.

(3) Film formation The layer thickness ratio of the first polyolefin solution / second polyolefin solution / first polyolefin solution is 0.1 / 0.8 / 0.1, and the stretching temperature and heat setting temperature of the laminated microporous film are 127.5 ° C. A polyolefin three-layer microporous membrane was prepared in the same manner as in Example 1 except that.

Example 3
(1) Preparation of first polyolefin solution A first polyolefin solution was prepared in the same manner as in Example 1.

(2) Preparation of the second polyolefin solution
A second polyolefin solution was prepared in the same manner as in Example 1 except that a composition comprising 95% by mass of HDPE and 5% by mass of PP having an Mw of 5.3 × 10 ⁵ was used.

(3) Film formation A polyolefin three-layer microporous membrane was prepared in the same manner as in Example 1 except that the stretching temperature of the gel-like laminated sheet was 116 ° C, and the stretching temperature and heat setting temperature of the laminated microporous membrane were 127 ° C. Produced.

Example 4
(1) Preparation of the first polyolefin solution
A first polyolefin solution was prepared in the same manner as in Example 1 except that only UHMWPE having Mw of 1.0 × 10 ⁶ and Mw / Mn of 8 was used.

(2) Preparation of the second polyolefin solution
A second polyolefin solution was prepared in the same manner as in Example 1 except that only HDPE having Mw of 3.0 × 10 ⁵ and Mw / Mn of 8.6 was used.

(3) Film formation The layer thickness ratio of the first polyolefin solution / second polyolefin solution / first polyolefin solution is 0.1 / 0.8 / 0.1, and the stretching temperature and heat setting temperature of the laminated microporous film are 127 ° C. A polyolefin three-layer microporous membrane was prepared in the same manner as in Example 1 except that.

Example 5
(1) Preparation of the first polyolefin solution
A first polyolefin solution was prepared in the same manner as in Example 2 except that a composition comprising 20% by mass of UHMWPE, 75% by mass of HDPE and 5% by mass of PP was used.

(2) Preparation of second polyolefin solution A second polyolefin solution was prepared in the same manner as in Example 3.

(3) Film formation A drying process was performed in the same manner as in Example 1 except that the layer thickness ratio of the first polyolefin solution / second polyolefin solution / first polyolefin solution was 0.1 / 0.8 / 0.1. The obtained film was re-stretched 1.2 times in the transverse direction at 127 ° C., heat-relaxed at 127 ° C. until it contracted to the size before re-stretching, and heat-fixed at the same temperature for 10 minutes. Thus, a three-layer polyolefin microporous membrane was produced.

Example 6
(1) Preparation of the first polyolefin solution
UHMWPE10 wt%, Mw in the 3.0 × 10 ^5, in the same manner as in Example 1 except for using HDPE70 mass% of Mw / Mn is 8.6, and the Mw is from PP20 mass% of 2.0 × 10 ⁶ composition, the One polyolefin solution was prepared.

(2) Preparation of second polyolefin solution A second polyolefin solution was prepared in the same manner as in Example 4.

(3) Film formation A drying process was performed in the same manner as in Example 1 except that the layer thickness ratio of the first polyolefin solution / second polyolefin solution / first polyolefin solution was 0.1 / 0.8 / 0.1. The obtained membrane was re-stretched 1.3 times in the transverse direction at 127 ° C., and a polyolefin three-layer microporous membrane was produced in the same manner as in Example 1 except that it was heat-set at the same temperature for 10 minutes.

Example 7
(1) Preparation of first polyolefin solution A first polyolefin solution was prepared in the same manner as in Example 1.

(2) Preparation of the second polyolefin solution
A second polyolefin solution was prepared in the same manner as in Example 1 except that a composition comprising 95% by mass of HDPE and 5% by mass of PP having an Mw of 2.0 × 10 ⁶ was used.

(3) Film formation A polyolefin three-layer microporous film was prepared in the same manner as in Example 1 except that the re-stretching temperature and the heat setting temperature were 127 ° C.

Comparative Example 1
Using only the first polyolefin solution having a concentration of 25% by mass, the gel sheet was stretched at 115 ° C., and heat fixed at 128 ° C. without stretching the microporous membrane, as in Example 1, A polyolefin microporous membrane was prepared.

Comparative Example 2
The first and second polyolefin solutions are extruded so that the layer thickness ratio of the first polyolefin solution / second polyolefin solution / first polyolefin solution is 0.4 / 0.2 / 0.4, and the stretching temperature of the gel-like laminated sheet Was set to 118 ° C., and a polyolefin three-layer microporous membrane was prepared in the same manner as in Example 1 except that the stretching temperature and heat setting temperature of the laminated microporous membrane were 126 ° C.

Comparative Example 3
The first and second polyolefin solutions were extruded so that the layer thickness ratio of the second polyolefin solution / first polyolefin solution / second polyolefin solution was 0.2 / 0.6 / 0.2, and the stretching temperature of the laminated microporous membrane A polyolefin three-layer microporous membrane was prepared in the same manner as in Example 1 except that the heat setting treatment temperature was 127 ° C.

Comparative Example 4
A polyethylene microporous membrane was prepared in the same manner as in Example 1 except that only the second polyolefin solution was used, the gel sheet was stretched at 118.5 ° C, and the microporous membrane was stretched and heat-set at 127 ° C.

Comparative Example 5
Example 1 except that the first polyolefin having 10% by mass of UHMWPE and 90% by mass of HDPE instead of the second polyolefin and having an Mw of 4.7 × 10 ⁵ was stretched and heat-set at 127 ° C., respectively. Similarly, a polyolefin three-layer microporous membrane was produced.

Comparative Example 6
A polyolefin three-layer microporous membrane was prepared in the same manner as in Example 3 except that the gel-like sheet was stretched at 115 ° C., the microporous membrane was not stretched, and the heat setting temperature of the microporous membrane was 126 ° C. .

Comparative Example 7
A polyolefin three-layer microporous membrane was produced in the same manner as in Example 1 except that the microporous membrane was stretched 1.9 times at 131 ° C and the heat setting temperature was 131 ° C.

  The physical properties of the polyolefin (three-layer) microporous membrane obtained in Examples 1 to 7 and Comparative Examples 1 to 7 were measured by the following methods. The results are shown in Table 1.

(1) Average film thickness (μm)
(Three layers) The film thickness was measured by a contact thickness meter over a 30 cm width of the microporous membrane at a longitudinal interval of 6 cm and obtained by averaging.

(2) Layer thickness ratio For the three films obtained by peeling the three-layer microporous film, the film thickness was measured at 10 mm longitudinal intervals over a width of 30 cm with a contact thickness meter. Asked. The layer thickness ratio was calculated from the average film thickness of each film.

(3) Air permeability (sec / 100 cm ³ / 20μm)
The thickness T ₁ of the (three-layer) air permeability P ₁ which was measured according to JIS P8117 with respect to the microporous membrane, wherein: P ₂ = a _{(P 1 × 20) / T} 1, 20μm thickness It was converted to air permeability P ₂ at the time of the.

(4) Porosity (%)
Measured by mass method.

(5) Puncture strength (mN / 20μm)
A spherical end surface (radius of curvature R: 0.5 mm) in diameter 1mm needle The maximum load was measured when pricked with a thickness of T ₁ to (three-layer) microporous membrane at a speed of 2 mm / sec. The measured value L ₁ of the maximum load was converted into the maximum load L ₂ when the film thickness was 20 μm by the formula: L ₂ = (L ₁ × 20) / T ₁ and used as the puncture strength.

(6) Tensile rupture strength and tensile rupture elongation Measured by ASTM D882 using a strip-shaped test piece having a width of 10 mm.

(7) Thermal shrinkage (%)
(Three layers) When the microporous membrane was held at 105 ° C. for 8 hours, the shrinkage in the longitudinal direction and the transverse direction were measured three times each and obtained by averaging.

(8) Meltdown temperature (℃)
Using a thermomechanical analyzer (manufactured by Seiko Instruments Inc., TMA / SS6000) and applying a load of 2 g to a 10 mm (TD) x 3 mm (MD) test piece TP by the method shown in FIG. The temperature was raised from room temperature at a rate of minutes. The temperature when the elongation of the test piece TP reached 50% of the length (100%) at room temperature was defined as the meltdown temperature.

(9) Average pore diameter of surface layer and inner layer The average pore diameter was determined by mercury porosimetry for the three membranes obtained by peeling the three-layer microporous membrane (equipment used: Porsizer Type 9320, manufactured by Micromeritic) . The average pore diameter of the surface layer is an average value of the average pore diameters of the two films constituting the surface layer.

(10) Inner layer pore size distribution The pore size distribution of the inner layer membrane obtained by peeling off the three-layer microporous membrane was measured by mercury porosimetry.

(11) Pore volume ratio of inner layer Obtained from S ₁ / S ₂ shown in FIG.

(12) Electrolyte absorption rate Electrolyte kept at 18 ° C using a dynamic surface tension measuring device (DCAT21 manufactured by Eihiro Seiki Co., Ltd., with precision electronic balance) [electrolyte: 1 mol / L LiPF ₆ , solvent: The microporous membrane is immersed in [ethylene carbonate / dimethyl carbonate = 3/7 (volume ratio)], and according to the formula [increase amount of microporous membrane mass (g) / microporous membrane mass before absorption (g)]. The absorption rate of the electrolytic solution was calculated. The absorption rate of the electrolytic solution in the microporous membrane of Comparative Example 1 is 1, and the absorption rate of the electrolytic solution is expressed as a relative value.

(13) Pressurized liquid retention ratio γ-butyrolactone was infiltrated into a microporous membrane sample having a width of 60 mm and a length of 100 mm until saturation, and the liquid retention amount A (g / g) per mass of the sample was measured. Filter paper and aluminum foil were stacked in this order on both surfaces of the liquid-absorbing sample. These were pressed for 5 minutes at 1.96 MPa (20 kgf / cm ² ) and 60 ° C. while being sandwiched between a pair of plate-like jigs. The liquid retention amount B (g / g) per sample mass after pressurization was measured in the same manner as described above. The liquid residual ratio (B / A) per sample mass was calculated and used as an index of liquid retention performance. The liquid residual ratio of the microporous membrane of Comparative Example 1 is expressed as a relative value, where 1.

Table 1 (continued)

Table 1 (continued)

Table 1 (continued)

Table 1 (continued)

Note: (1) Mw represents the weight average molecular weight.
(2) Weight average molecular weight / number average molecular weight (Mw / Mn).
(3) Concentration of first polyolefin solution / concentration of second polyolefin solution.
(4) Layer structure of surface layer / inner layer / surface layer.
(5) Layer thickness ratio of surface layer / inner layer / surface layer.
(6) (I) represents the first polyolefin solution, and (II) represents the second polyolefin solution.
(7) (Average pore diameter of second microporous layer) / (Average pore diameter of first microporous layer).
(8) The three layers of Comparative Example 5 are all composed of the first polyolefin, but one is shown in the column for the second polyolefin for convenience.

  As is clear from Table 1, the polyolefin three-layer microporous membranes of Examples 1 to 7 have a structure in which the second microporous layer has a larger average pore diameter than the first microporous layer. It was excellent in liquid absorption and electrolyte retention. Furthermore, it was excellent in permeability, puncture strength, tensile breaking strength, tensile breaking elongation, heat shrinkage resistance and meltdown properties.

  Since the non-laminated microporous membrane of Comparative Example 1 did not have the second microporous layer having a hybrid structure, the electrolyte solution absorbability was inferior to the polyolefin three-layer microporous membrane of Examples 1-7. . In the three-layer microporous membrane of Comparative Example 2, the total thickness of the first microporous layer is more than 60% with the total thickness of the first and second microporous layers being 100%. Electrolyte absorptivity was inferior compared with the polyolefin three-layer microporous membrane of ~ 7. Since the three-layer microporous membrane of Comparative Example 3 has a layer configuration of the second microporous layer / first microporous layer / second microporous layer, the polyolefin trilayer microporous membrane of Examples 1 to 7 Compared with the electrolyte solution absorbability and electrolyte solution retention were inferior. Since the non-laminated microporous membrane of Comparative Example 4 does not have the first microporous layer in which the ratio of ultrahigh molecular weight polyethylene is 8% by mass or more, compared to the polyolefin three-layer microporous membrane of Examples 1-7. Melt down characteristics and electrolyte retention were inferior. Since all the three layers of the microporous membrane of Comparative Example 5 were the first microporous layer, the electrolyte solution absorbability was inferior to the polyolefin trilayer microporous membrane of Examples 1-7. Since the three-layer microporous membrane of Comparative Example 6 did not stretch the microporous membrane, the electrolyte solution absorbability was inferior to the polyolefin three-layer microporous membrane of Examples 1-7. The microporous membrane of Comparative Example 7 was inferior in electrolyte solution retention compared to the polyolefin three-layer microporous membrane of Examples 1-7 because the draw ratio of the microporous membrane exceeded 1.8 times.

It is a graph which shows a typical pore size distribution curve. It is the schematic which shows the measuring method of meltdown temperature.

Claims (13)

A polyolefin multilayer microporous membrane having at least a first microporous layer forming both surface layers and at least one second microporous layer provided between both surface layers, wherein the first microporous layer Includes a first polyethylene resin having a weight average molecular weight of 1 × 10 ⁶ or more and a ratio of ultra high molecular weight polyethylene of 8% by mass or more, and the second microporous layer has a ratio of the ultra high molecular weight polyethylene of The pore size distribution curve obtained by mercury porosimetry includes 7% by mass or less of the second polyethylene resin, and has a structure having at least two peaks, and the total thickness of the first and second microporous layers is A polyolefin multilayer microporous membrane characterized in that the total thickness of the first microporous layer is 15 to 60% as 100%. 2. The polyolefin multilayer microporous membrane according to claim 1, wherein the first microporous layer has an average pore diameter of 0.005 to 0.1 μm, and the second microporous layer has an average pore diameter of 0.02 to 0.5 μm. A polyolefin multilayer microporous membrane characterized by the above. The polyolefin multilayer microporous membrane according to claim 1 or 2, wherein the ratio of the average pore diameter of the second microporous layer to the average pore diameter of the first microporous layer is more than 1/1 and less than 10/1. A polyolefin multilayer microporous membrane characterized by being. The polyolefin multilayer microporous membrane according to any one of claims 1 to 3, wherein the second microporous layer includes a dense region having a main peak in a range of 0.01 to 0.08 µm in the pore size distribution curve, and 0.08 µm. A polyolefin multi-layer microporous membrane having a coarse region having at least one sub-peak within a range of not more than 1.5 μm. 5. The polyolefin multilayer microporous membrane according to claim 4, wherein a pore volume ratio between the dense region and the coarse region is 0.5 to 49. The polyolefin multilayer microporous membrane according to any one of claims 1 to 5, which has a three-layer structure in which a pair of the first microporous layers are formed on both surfaces of one second microporous layer. A polyolefin multilayer microporous membrane. The polyolefin multilayer microporous membrane according to any one of claims 1 to 6, wherein the first microporous layer has (i) the ultrahigh molecular weight polyethylene, (ii) the ultrahigh molecular weight polyethylene, and a weight average molecular weight of 1; A first polyethylene composition comprising high-density polyethylene of × 10 ^{4 to} 5 × 10 ⁵ (the proportion of the ultrahigh molecular weight polyethylene is 8% by mass or more), (iii) a mixture comprising the ultrahigh molecular weight polyethylene and polypropylene (the above The proportion of polypropylene is 25% by mass or less), or (iv) a mixture comprising the first polyethylene composition and polypropylene (the proportion of polypropylene is 25% by mass or less), and the second microporous layer (Ii) a second polyethylene composition comprising the ultrahigh molecular weight polyethylene and (ii) the ultra high molecular weight polyethylene and the high density polyethylene (the ultra high molecular weight polyethylene). The proportion of the low molecular weight polyethylene is 7% by mass or less), (iii) the mixture of the high-density polyethylene and polypropylene (the proportion of the polypropylene is 25% by mass or less), or (iv) the second polyethylene composition and the polypropylene. A polyolefin multi-layer microporous membrane characterized in that it is formed by a mixture (the proportion of the polypropylene is 25% by mass or less). The method for producing a polyolefin multilayer microporous membrane according to any one of claims 1 to 7, wherein (1) the proportion of ultrahigh molecular weight polyethylene having a weight average molecular weight of 1 x 10 ⁶ or more is 8% by mass or more. A first polyolefin solution containing a polyethylene resin and a film forming solvent, and a second polyolefin solution containing a second polyethylene resin and a film forming solvent in which the ratio of the ultrahigh molecular weight polyethylene is 7% by mass or less. (2) The first and second polyolefin solutions are prepared so that the first polyolefin solution forms at least both surface layers and the second polyolefin solution forms at least one layer between both surface layers. Extruded simultaneously from one die, (3) The obtained extruded product is cooled, (4) The obtained gel-like laminated sheet is stretched, and (5) The film-forming solvent is removed from the gel-like laminated sheet. How to, characterized in that stretching in 1.1-1.8 times in at least one direction the laminated microporous film was obtained (6). The method for producing a polyolefin multilayer microporous membrane according to any one of claims 1 to 7, wherein (1) the proportion of ultrahigh molecular weight polyethylene having a weight average molecular weight of 1 x 10 ⁶ or more is 8% by mass or more. A first polyolefin solution containing a polyethylene resin and a film forming solvent, and a second polyolefin solution containing a second polyethylene resin and a film forming solvent in which the ratio of the ultrahigh molecular weight polyethylene is 7% by mass or less. Preparing (2) the first and second polyolefin solutions such that the first polyolefin solution forms at least both surface layers and the second polyolefin solution forms at least one layer between both surface layers. Laminated immediately after extrusion from a separate die, (3) the obtained laminate is cooled, (4) the obtained gel laminate sheet is stretched, and (5) from the gel laminate sheet for the film formation Remove solvent How to, characterized in that stretching in 1.1-1.8 times in at least one direction the laminated microporous film was obtained (6). The method for producing a polyolefin multilayer microporous membrane according to any one of claims 1 to 7, wherein (1) the proportion of ultrahigh molecular weight polyethylene having a weight average molecular weight of 1 x 10 ⁶ or more is 8% by mass or more. A first polyolefin solution containing a polyethylene resin and a film forming solvent, and a second polyolefin solution containing a second polyethylene resin and a film forming solvent in which the ratio of the ultrahigh molecular weight polyethylene is 7% by mass or less. And (2) extruding the first and second polyolefin solutions from individual dies, (3) cooling the obtained extruded products to form first and second gel sheets, 4) Stretch each gel sheet, and (5) the first gel sheet forms at least both surface layers, and the second gel sheet forms at least one layer between both surface layers. And a second gel-like sheet, and (6 ) Removing the film-forming solvent from the obtained laminated gel-like sheet, and (7) stretching the obtained laminated microporous film at least uniaxially 1.1 to 1.8 times. The method for producing a polyolefin multilayer microporous membrane according to any one of claims 1 to 7, wherein (1) the proportion of ultrahigh molecular weight polyethylene having a weight average molecular weight of 1 x 10 ⁶ or more is 8% by mass or more. A first polyolefin solution containing a polyethylene resin and a film forming solvent, and a second polyolefin solution containing a second polyethylene resin and a film forming solvent in which the ratio of the ultrahigh molecular weight polyethylene is 7% by mass or less. And (2) extruding the first and second polyolefin solutions from individual dies, (3) cooling the obtained extruded products to form first and second gel sheets, 4) Stretch each gel sheet, (5) Remove the film-forming solvent from each stretched gel sheet to form the first and second polyolefin microporous membranes, (6) at least the second Polyolefin microporous membrane at least uniaxial (7) so that the first polyolefin microporous membrane forms at least both surface layers, and the second polyolefin microporous membrane forms at least one layer between both surface layers, A method comprising laminating first and second polyolefin microporous membranes. A battery separator comprising the polyolefin multilayer microporous membrane according to claim 1. A battery comprising a separator comprising the polyolefin multilayer microporous membrane according to claim 1.

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