JP5450944B2 - Polyolefin microporous membrane, battery separator and battery - Google Patents

Description

  The present invention relates to a polyolefin microporous membrane having a good balance of moderate permeability, puncture strength, smoothness and film formability, a battery separator comprising such a polyolefin microporous membrane, and a battery having such a separator.

  The polyolefin microporous membrane is useful as a separator for primary and secondary batteries. Such separators include lithium ion secondary batteries, lithium polymer secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, and the like. When a polyolefin microporous membrane is used as a separator for a lithium ion battery, the characteristics of the membrane greatly affect the performance of the battery. In particular, the permeability, mechanical properties, dimensional stability, shutdown properties, meltdown properties, etc. of polyolefin microporous membranes generally affect battery performance, productivity and safety.

  Although it is desired to have a relatively low shutdown temperature and a relatively high meltdown temperature for the battery, this generally results in batteries, particularly high capacity batteries that are exposed to high temperatures during storage and / or use. On the other hand, safety is improved. In order to obtain a high battery capacity, high separator permeability is desired. Since improved separator strength can result in improved battery assembly and manufacturing efficiency, separators with relatively high mechanical strength are desired.

  Generally, a polyolefin microporous membrane made only of polyethylene has a relatively low meltdown temperature of about 150 ° C. and a shutdown temperature of about 140 ° C., and a polyolefin microporous membrane made only of polypropylene has a relatively high shutdown temperature of about 155 ° C. And a meltdown temperature of about 165 ° C to about 170 ° C. Furthermore, a microporous membrane made of both polyethylene and polypropylene has been proposed. The polyolefin microporous film containing both polyethylene and polypropylene can be formed using a mixed resin of polyethylene and polypropylene.

  Japanese Patent Application Laid-Open No. 04-126352 discloses a battery separator having an appropriate shutdown characteristic and mechanical strength. This microporous separator is made of a mixture containing polyethylene having a viscosity average molecular weight of 300,000 or less, polyethylene having a viscosity average molecular weight of 1,000,000 or more, and polypropylene. This viscosity average molecular weight is defined by ASTM D 4020.

  Japanese Unexamined Patent Publication No. 7-268118 discloses a microporous biaxially stretched film having moderate permeability, mechanical strength, shutdown characteristics and meltdown characteristics. This biaxially stretched film is composed of 20 to 80% by weight of high molecular weight polyethylene having an intrinsic viscosity [η] of 10 dl / g or more, and 80 to 20 high molecular weight polypropylene having an intrinsic viscosity [η] of 3 to 15 dl / g. And a composition containing% by weight.

  Japanese Patent Application Laid-Open No. 2005-200578 discloses a polyolefin microporous membrane having appropriate permeability, mechanical strength, shutdown characteristics, heat resistance and oxidation resistance. This reference discloses a membrane that remains intact at high temperatures. This polyolefin microporous film is made of polyethylene and polypropylene, and polyethylene contains a component having a viscosity average molecular weight (Mv) of 50,000 to 300,000. The content of polypropylene is 7 wt% or more and less than 50 wt%.

  In order to improve the film formability and smoothness, it is desirable to optimize the properties of the polyolefin (especially polypropylene) of the polyolefin microporous membrane disclosed in the above three references. Polyolefin microporous membranes with poor film formability have the undesirable property of causing scratches on the surface of the membrane due to the polypropylene powder falling off when slitting. Polyolefin microporous membranes with poor smoothness are prone to short circuits when used as battery separators. Compression resistance and battery productivity are also adversely affected.

  JP 2002-194132 A is a polyolefin comprising a polyethylene having a weight average molecular weight / number average molecular weight of 8 to 100 and polypropylene having an MFR (Melt Flow Rate) of 2.0 or less, and a polypropylene content of 20% by weight or less. A polyolefin microporous membrane comprising the composition is disclosed. MFR is measured at 230 ° C. and 2.16 kg according to ASTM D 1238. This membrane has moderate smoothness and compression properties.

Reference literature JP 2004-196870 consists of polyethylene and polypropylene having a weight average molecular weight of 5 × 10 ⁵ or more and a heat of fusion (measured by a scanning differential calorimeter) of 90 J / g or more. A polyolefin microporous membrane having an amount of 20% by mass or less is disclosed. Reference literature JP 2004-196871 discloses a polyolefin microporous membrane made of polyethylene and polypropylene. This polypropylene has a weight average molecular weight of 5 × 10 ⁵ or more and a melting point of 163 ° C. or more (measured at a temperature increase rate of 3 to 20 ° C./min with a scanning differential calorimeter). The content of this polypropylene in the film is 20% by mass or less. The films disclosed in these references have the same heat resistance, permeability, shutdown temperature and meltdown temperature as before. However, all of the polyolefin microporous membranes disclosed in JP-A-2002-194132, JP-A-2004-196870, and JP-A-2004-196871 have desired permeability, puncture strength, smoothness and film formability. Does not have. WO2005 / 103127 discloses a polyolefin microporous membrane made only of polypropylene having a specific trouton ratio, but this membrane has a lower shutdown temperature than a polyolefin microporous membrane made of polyethylene.

  Therefore, a polyolefin microporous membrane for battery separators containing polyethylene and polypropylene and having a good balance of permeability, puncture strength, smoothness and film formability is desired.

  In one embodiment, the present invention relates to the discovery of polyolefin microporous membranes with improved permeability, puncture strength, smoothness and film formability.

In one embodiment, the present invention provides a polyolefin-based resin and a polyolefin-based resin having a Troughton ratio of at least about 15, or at least about 20, or at least about 30 at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ . The present invention relates to a method for producing a microporous membrane. This method provides a polyolefin microporous membrane having an excellent balance of appropriate permeability, puncture strength, smoothness and film formability. As used herein, the term “balanced” means that optimizing one of these properties does not result in a significant degradation of the other properties.

In another embodiment, the present invention relates to a polyolefin microporous membrane comprising polyethylene and a polypropylene having a trouton ratio of 15 or higher at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ .

This polypropylene may have, for example, an extensional viscosity of 50,000 Pa · sec or more at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ .

  In another embodiment, the present invention relates to a battery separator comprising the above-mentioned polyolefin microporous membrane.

  In yet another embodiment, the present invention relates to a battery comprising a negative electrode, a positive electrode, and the separator disposed therebetween.

  In yet another embodiment, the present invention relates to a method of using such a battery, for example as a source or sink for charging.

  The polyolefin microporous membrane of the present invention contains a polyethylene-based resin and polypropylene, and has an excellent balance of permeability, puncture strength, smoothness, and film formability. By using such a microporous polyolefin membrane as a separator, a battery excellent in capacity, cycle characteristics, discharge characteristics, heat resistance and productivity can be obtained.

[1] Composition of Material Used for Production of Polyolefin Microporous Membrane In one embodiment, the polyolefin microporous membrane is produced by extruding a polyolefin solution. The polyolefin solution comprises a polyolefin composition and at least one film forming solvent. The polyolefin composition is prepared by mixing polyethylene, polypropylene, and the like by a conventional method such as dry blending or melt kneading. The polypropylene in the polyolefin solution generally has a trouton ratio of at least about 15. The polyolefin solution may be prepared by mixing each resin with one or more kinds of film forming solvents without preparing the polyolefin composition in advance. In one embodiment, the polyolefin microporous membrane comprises a single microporous layer consisting of polyethylene and polypropylene having a trouton ratio of at least about 15. In one embodiment, the polyolefin multilayer microporous membrane consists of two layers. The first layer (for example, the upper layer) is made of a material for the first microporous layer, and the second layer (for example, the lower layer) is a second microporous layer made of polyethylene and polypropylene having a trouton ratio of at least about 15. Made of materials. The first material for the microporous layer is made of polyolefin, such as polyethylene and polypropylene. In yet another embodiment, the polyolefin multilayer microporous membrane consists of three or more layers. These layers include an outer layer (also referred to as a “surface” or “skin” layer) made of a first microporous layer material, and a second microporous layer made of polyethylene and polypropylene having a trouton ratio of at least about 15. It is at least one layer located in the middle (or inside) of the material for use. The inner layer of the polyolefin multilayer microporous membrane is located between both surface layers, and at least one inner layer is in planar contact with at least one surface layer. In another embodiment of the polyolefin multilayer microporous membrane comprising three or more layers, the surface layer is made of the second microporous layer material, and at least one intermediate layer is made of the first microporous layer material. When the polyolefin multilayer microporous membrane has three or more layers, it has at least one layer made of the first microporous layer material and at least one layer made of the second microporous layer material. As is obvious to those skilled in the art, the polyolefin multilayer microporous membrane can be produced by a conventional membrane production process such as lamination and coextrusion.

(1) Polyethylene resin The polyethylene used to prepare the polyolefin solution is generally formed of one or more polyethylene resins. In one embodiment, the polyethylene resin comprises (a) ultra high molecular weight polyethylene or (b) a second polyethylene having a lower molecular weight than the ultra high molecular weight polyethylene. The weight average molecular weight (Mw) of the polyethylene resin is not limited, but for example, about 1 × 10 ⁴ to about 1 × 10 ⁷ , about 1 × 10 ⁵ to about 5 × 10 ⁶ , about 2 × 10 ⁵ to about 3 × 10 ⁶ is sufficient. In one embodiment, the polyethylene resin consists mainly of polyethylene. In another embodiment, the polyethylene-based resin consists essentially of polyethylene. In yet another embodiment, the polyethylene-based resin consists of polyethylene (ie includes only polyethylene).

(a) Ultra High Molecular Weight Polyethylene In one embodiment, the ultra high molecular weight polyethylene has a Mw of about 1 × 10 ⁶ or greater. The ultrahigh molecular weight polyethylene may be not only an ethylene homopolymer but also an ethylene / α-olefin copolymer containing a small amount (up to 5 mol%) of a third α-olefin. As the third 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 Mw of ultra high molecular weight polyethylene is not limited, but may be about 1 × 10 ⁶ to about 15 × 10 ⁶ , about 1 × 10 ⁶ to about 5 × 10 ^6, or about 1 × 10 ⁶ to about 3 × 10 ⁶ . .

(b) Second polyethylene Mw of the second polyethylene is not limited. In one embodiment, the second polyethylene has a Mw of about 1 × 10 ⁴ to about 5 × 10 ⁵ . The second polyethylene can be, for example, high density polyethylene, medium density polyethylene, branched low density polyethylene and chained low density polyethylene, and mixtures thereof. For example, the second polyethylene may be high density polyethylene. The Mw of the high density polyethylene can be about 1 × 10 ⁵ to about 5 × 10 ^5, or about 2 × 10 ⁵ to about 4 × 10 ⁵ . The second polyethylene may be not only a homopolymer of ethylene but also a copolymer containing a small amount (up to 5 mol%) of the third α-olefin. Such a copolymer can be produced by a single site catalyst.

(c) Both ultra high molecular weight polyethylene and second polyethylene In one embodiment, the polyolefin solution comprises both ultra high molecular weight polyethylene and second polyethylene. The ultrahigh molecular weight polyethylene and the second polyethylene are as described in the above (a) and (b). When both ultra-high molecular weight polyethylene and second polyethylene are used, the molecular weight distribution (Mw / Mn) of the mixed polyethylene in the polyolefin solution is a conventional method, for example, a method of setting the quantitative ratio and Mw / Mn of each resin. Can be controlled. Mw / Mn is defined in section (d) below. In one embodiment, the second polyethylene is high density polyethylene. The amount ratio of the ultra high molecular weight polyethylene and the second polyethylene is not limited. For example, the content of the ultra high molecular weight polyethylene may be, for example, about 1% by mass or more, or about 1% by mass to about 60% by mass, based on the total of the ultra high molecular weight polyethylene and the second polyethylene.

(d) Molecular weight distribution Mw / Mn
Mw / Mn is a measure of molecular weight distribution. The larger this value, the wider the molecular weight distribution. The Mw / Mn of the polyethylene resin is not limited, but may be about 5 to about 300, about 5 to about 100, or about 5 to about 30. Although not limited, if Mw / Mn is less than about 5, extrusion may be difficult, and it may be difficult to produce a microporous polyolefin membrane having acceptable film thickness uniformity (ie, smoothness). On the other hand, when Mw / Mn exceeds about 300, it may be difficult to produce a polyolefin microporous membrane having sufficient strength. Mw / Mn of polyethylene (homopolymer or ethylene / α-olefin copolymer) can be appropriately adjusted by multistage polymerization. For example, 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 can be used. When both ultra high molecular weight polyethylene and second polyethylene are used, the greater the Mw / Mn, the greater the difference in Mw between the ultra high molecular weight polyethylene and the second polyethylene, and vice versa.

(2) Polypropylene resin The polyolefin solution also contains polypropylene. In the first stage, when a polyolefin composition is formed by mixing polypropylene with polyethylene, in the next stage, one or more film-forming solvents are mixed with the polyolefin composition to form a polyolefin solution. Instead, the polyolefin solution may be prepared by mixing one or more film-forming solvents with one or more resins without forming the polyolefin composition. The polypropylene-based resin is made of polypropylene having a trouton ratio of at least about 15 when measured at a temperature of about 230 ° C. and a strain rate of 25 sec ⁻¹ . When the Troughton ratio under the above measurement conditions is less than about 15, it may be difficult to produce a polyolefin microporous film having excellent smoothness and film formability. In one embodiment, the troopon ratio of polypropylene is at least about 25, or at least about 30. In general, when the polypropylene has (i) a Troughton ratio of at least about 15 and (ii) an elongational viscosity of at least about 50,000 Pa · sec at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ , the polyolefin microporous membrane is Has improved smoothness and film formability. For example, the extensional viscosity may be at least about 60,000 Pa · sec. In one embodiment, the polypropylene resin consists primarily of polypropylene. In another embodiment, the polypropylene-based resin consists essentially of polypropylene. In yet another embodiment, the polypropylene resin comprises polypropylene.

The Truton ratio and elongational viscosity are determined by, for example, the inflow pressure drop method, and the Cogswell theory ("Polymer Engineering and Science", 1972, Vol. 12, pp. 64-73). Can be measured according to. In that method, using a twin capillary rheometer (model number: RH-2200, manufactured by Rosand) with two dies of equal diameter and different length, the pressure loss of molten polypropylene at 230 ° C, shear rate Measure in the range of 10 to 1,800 sec ^-1 . In the low shear rate range (10 to 300 sec ^-1 ), a die with an inner diameter of 2.0 mm x length of 0.25 mm and a die with an inner diameter of 2.0 mm x length of 32 mm are used. In the high shear rate range (300 to 1,800 sec ^-1 ) A die having an inner diameter of 1.0 mm × a length of 0.25 mm and a die having an inner diameter of 1.0 mm × a length of 16 mm are used. The polypropylene sample is left in the holder and held at 230 ° C. for 3 minutes, and then the pressure loss of the molten polypropylene in each die is measured. For the shear rate, Rabinovitch correction [e.g., described in JIS K 7199 (1991), edited by the Japanese Society of Rheology, “Lectures / Rheology”, Polymer Publishing Association (1993), page 68, etc.] is performed.

The pressure loss p ₀ of molten polypropylene in both shear rate regions of a virtual die of length ₀ is expressed by the following formula (1):


(Where p ₁ is the pressure loss at the long die, p ₂ is the pressure loss at the short die, L ₁ is the length of the long die, and L ₂ is the length of the short die). calculate.

The elongational viscosity η _E is the following equation (2) according to Cogswell's theory in both shear rate regions:


[Where γ _a is the shear rate, η _S is the shear viscosity, n is defined by the formula: η _S = c γ _a ^n-1 (c is a constant), and p ₀ is the same as formula (1) It is. ]. The shear viscosity is determined using the twin capillary rheometer.

The strain rate ε is expressed by the following formula (3) in both shear rate regions:


(Where η _S, γ _a and n are the same as in equation (2) and p ₀ is the same as in equation (1)).

The relationship between the shear viscosity and the shear rate (10 to 1,800 sec ^-1 ) is expressed by the following equation (4) by the least square method:


And the relationship between the extensional viscosity and the strain rate (2 to 180 sec ^-1 ) is expressed by the following equation (5) by the least square method:


Approximate.

Using equation (3) and equation (4), the shear viscosity [η _S (25) (unit: Pa · sec)] at a strain rate of 25 sec ⁻¹ is obtained, and the strain rate is calculated using equation (5). The elongational viscosity [η _E (25) (unit: Pa · sec)] at 25 sec ⁻¹ is determined. From η _E (25) and η _S (25), the Troughton ratio [η _E (25) / η _S (25)] is obtained. The trouton ratio is measured five times, and the average value is defined as the trouton ratio of polypropylene at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ .

In one embodiment, the polypropylene can be any of a homopolymer of propylene, a copolymer of propylene and a fourth α-olefin, and a mixture thereof. Without limitation, the Mw of polypropylene in the first polypropylene-based resin can be from about 1 × 10 ⁴ to about 4 × 10 ^6, or from about 3 × 10 ⁵ to about 3 × 10 ⁶ . The Mw of polypropylene in the first polypropylene resin may be at least about 1.8 × 10 ⁶ . Without limitation, the molecular weight distribution (Mw / Mn) of polypropylene in the first polypropylene-based resin can be from about 1.01 to about 100, or from about 1.1 to about 50. If the crystallinity of polypropylene is low, it may be difficult to produce a membrane with a suitable porosity, so it is desirable that the crystallinity of polypropylene be relatively high. As the copolymer, any of a random copolymer and a block copolymer can be used. Examples of the fourth α-olefin include ethylene, butene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. Although not limited, the MFR (Melt Flow rate) of polypropylene measured according to JIS K 6758 may be, for example, about 0.01 to about 1.0, or about 0.05 to about 0.4. In one embodiment, the polypropylene has at least one of the following properties.
(i) Mw is at least about 6.5 × 10 ^5, or at least about 8 × 10 ⁵ .
(ii) The molecular weight distribution (Mw / Mn) is about 1 to about 100, does not exceed 5, does not exceed 4, or does not exceed 2.5.
(iii) The heat of fusion ΔHm is at least about 90 J / g, at least about 95 J / g, or at least about 100 J / g.
(iv) The elongational viscosity at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ is 50,000 Pa · sec or more.

In one embodiment, the polypropylene is an isotactic polypropylene having a relatively high melting point and long branched chains. Examples of commercially available polypropylene products include Basel's trade names Pro-fax PF611 ^™ and Pro-fax HS746S ^™ .

  In another embodiment, polypropylene can be produced by the method described in, for example, JP-A-62-121704, Japanese Patent No. 2869606, and the like. When the method described in JP-A-62-121704 is used, a Trundon ratio is obtained by irradiating a linear crystalline polypropylene in the presence of active oxygen and introducing a long-chain branched structure into the polypropylene molecule. At least about 15 polypropylene can be produced. Using the method described in Japanese Patent No. 2869606, 1 to 10 mmol of di-2-ethylhexyl peroxydicarbonate is reacted at 70 to 150 ° C. for 10 minutes to 3 hours with respect to 100 g of linear crystalline polypropylene. Then, it can be produced by melt-kneading.

  Commercially available polypropylenes such as the above include, for example, Prime Polypro E111G, E105GM (trade name, manufactured by Prime Polymer Co., Ltd.), Sun Allomer PS201A (trade name, manufactured by Sun Allomer Co., Ltd.), New Tren SB8000, SH9000 (above products) Name, manufactured by Nippon Polypro Co., Ltd.).

  Without limitation, the polypropylene content is generally from about 2% to about 98%, from about 3% to about 50%, or about 5% by weight, based on the weight of polyethylene and polypropylene. ~ 40% by weight. When the content of polypropylene is less than 2% by mass, it may be difficult to produce a polyolefin film having a sufficiently high meltdown temperature.

(3) Fifth polyolefin In addition to the polyethylene resin and the polypropylene resin, the polyolefin solution may contain a fifth polyolefin. The fifth polyolefin is composed of polybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate, polystyrene and ethylene / α-olefin copolymer. You may contain at least 1 type chosen from the group which consists of. These preferably have a Mw of about 1 × 10 ⁴ to about 4 × 10 ⁶ . The polyolefin solution may also include a polyethylene wax having a Mw of, for example, from about 1 × 10 ³ to about 1 × 10 ⁴ . The contents of the fifth polyolefin and the wax are not limited as long as the performance of the polyolefin microporous membrane is not significantly reduced. For example, the content of the polyethylene wax may be less than about 20% by mass based on 100% by mass of the polyolefin in the polyolefin solution containing the fifth polyolefin. In one embodiment, the polyethylene wax content is less than 10%. Polybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate and polystyrene contain not only homopolymers but also a fourth α-olefin It may be a copolymer.

[2] Method for Producing Polyolefin Microporous Membrane In one embodiment, a method for producing a polyolefin microporous membrane includes: (1) a polyethylene resin, a polypropylene resin, and at least one film forming solvent (both a process solvent or a diluent). To prepare a polyolefin solution. (2) Extrude the polyolefin solution from a die to form a gel-like molded product (also called an extrudate). (3) Generally form a sheet-like gel-like molded product. Cooled to form a gel-like sheet (also called cooled extrudate), (4) optionally stretched gel-like sheet to form a stretched gel-like sheet, (5) gel-like sheet or stretched The step of removing the film-forming solvent from the gel-like sheet to form a gel-like sheet from which the solvent has been removed, and (6) drying the solvent-free gel-like sheet to form a polyolefin microporous film. After step (6), any step, for example, step (7) for re-stretching the polyolefin microporous membrane, step (8) for heat treating the polyolefin microporous membrane, cross-linking step (9) by ionizing radiation, hydrophilization treatment step (10) etc. may be performed. The order of the arbitrary steps (6) to (10) is not limited. These steps will be described in detail below.

(1) A process of preparing a polyolefin solution by mixing a polyethylene resin, a polypropylene resin, and a film forming solvent. At least one process solvent (also called a diluent or a film forming solvent) is used as the polyethylene resin and the polypropylene resin. A polyolefin solution is prepared by mixing with, for example, melt mixing. The resin and solvent can be added sequentially, simultaneously or in combination. The polyolefin solution is prepared by first mixing at least a part of a resin to prepare a polyolefin composition, and then adding the polyolefin composition and at least one film-forming solvent (optionally with the remaining resin and / or additional resin). It may be prepared by mixing. One or more additives such as an antioxidant and finely divided silicic acid (pore forming agent) may be added to the polyolefin solution. The content of such an additive is not limited, but is set to such an extent that the film characteristics are not adversely affected. In general, the additive content should not exceed 1 wt.% In total, based on the weight of the polyolefin solution.

  By using a liquid film-forming solvent, stretching at a relatively high magnification becomes possible. Examples of the liquid solvent include aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin, and mineral oil fractions having boiling points corresponding to these, and dibutyl. Examples of the phthalate and dioctyl phthalate include liquid phthalates at room temperature. By using a non-volatile liquid solvent such as liquid paraffin, it becomes easy to obtain a gel-like molded body (gel-like sheet) in which the content of the liquid solvent is stable. In one embodiment, one or more solvents that are miscible with the polyolefin composition in the melt-kneaded state but are solid at room temperature may be mixed with the liquid solvent. Examples of such a solid solvent include stearyl alcohol, seryl alcohol, and paraffin wax. Although a solid solvent can be used without a liquid solvent, in this case, it may be difficult to uniformly stretch the gel-like sheet in the step (4).

  In one embodiment, the viscosity of the liquid solvent is from about 30 cSt to about 500 cSt, or from about 30 cSt to about 200 cSt when measured at 25 ° C. The viscosity is not particularly limited, but if the viscosity at 25 ° C. is less than about 30 cSt, foaming tends to occur and kneading is difficult. On the other hand, if it exceeds about 500 cSt, it may be difficult to remove the liquid solvent in the step (5).

  Although not limited, the polyethylene-based resin, the polypropylene-based resin, and the film-forming solvent are prepared by, for example, melt-kneading in a twin-screw extruder to prepare a polyolefin solution containing a relatively high concentration of polyethylene and polypropylene. Can be mixed. The film-forming solvent may be added before the start of kneading or during kneading. For example, the solvent can be added from the middle of the twin screw extruder during kneading. The resin may be dry mixed before melt kneading, and the solvent can be added before, during or after dry mixing.

  In one embodiment, the melt kneading temperature is about 80 ° C. above the polypropylene melting point (Tmp) to Tmp (denoted as Tmp + about 80 ° C.), about 160 ° C. to about 250 ° C., about 180 ° C. to about 230 ° C. It is. The melting point can be determined, for example, by differential scanning calorimetry (DSC) based on JIS K7121.

  The ratio (L / D) of the screw length (L) to the diameter (D) of the twin screw extruder is not limited. In one embodiment, L / D is about 20 to about 100, or about 35 to about 70. When L / D is less than about 20, melt kneading of the polyolefin solution may be difficult. If the L / D is greater than about 100, it may be difficult to prevent a decrease in molecular weight due to excessive shear due to excessive residence time of the polyolefin solution in the extruder. The cylinder inner diameter of the twin screw extruder is not limited. In one embodiment, the cylinder inner diameter is from about 40 mm to about 100 mm.

  The content of polyolefin (ultra high molecular weight polyethylene, second polyethylene, polypropylene, etc.) in the polyolefin solution is not limited. In one embodiment, the polyolefin solution is about 1 wt% to about 75 wt%, or about 20 wt% to about 70 wt%, based on 100 wt% polyolefin solution. If the polyolefin content in the polyolefin solution is less than about 1% by mass, it may be difficult to prevent swell and neck-in at the die outlet during the formation of the gel-like molded body. And self-supporting ability may deteriorate. On the other hand, when the content of the polyolefin in the polyolefin solution exceeds about 75% by mass, it may be difficult to mold the gel-like molded body.

(2) Extruding the polyolefin solution from the die to form a gel-like molded product In one embodiment, at least a part of the polyolefin solution is extruded from the die to form an extrudate. For example, the polyolefin solution may be extruded and led directly from the die to the first extruder. In another embodiment, a separately added extruder (second, third, etc.) may be used. Another extruder may be connected to the first extruder in series and / or in parallel. The product from the first extruder may be cooled and pelletized. The pellets can then be melt kneaded, for example, and extruded from a second extruder and die to form a gel shaped body or sheet. The shape of the die is not limited. For example, the die may be a sheet die having a rectangular base, a double cylindrical hollow die, a hollow die, an inflation die, or the like. The die gap is not critical. For sheet dies, the die gap is typically between about 0.1 mm and about 5 mm. The temperature of the polyolefin solution during extrusion (i.e., extrusion temperature) is not critical, but is generally from about 140 ° C to about 250 ° C. The extrusion rate is not critical but is generally from about 0.2 m / min to about 15 m / min.

(3) Step of cooling a sheet-like gel-like molded body In one embodiment, the extrudate (gel-like molded body) is cooled to form a gel-like sheet. Cooling is preferably performed at a rate of 50 ° C./min or more until the extrudate reaches at least the gelation temperature (that is, the temperature at which the extruded sheet starts to gel). In one embodiment, the extrudate is cooled to about 25 ° C. or less. While not wishing to be bound by any theory or model, it is believed that cooling forms a microphase consisting of polyolefin regions separated by a film-forming solvent. In general, when the cooling rate is low, the higher order structure of the gel sheet obtained becomes rough, and the pseudo cell unit forming the gel sheet is considered to be large. On the other hand, when the cooling rate is high, the cell unit becomes dense. If the cooling rate is less than 50 ° C./min, it may be difficult to stretch the gel-like sheet, possibly due to an increase in crystallinity. The cooling method is not limited. As a cooling method, for example, a method of directly contacting a cooling medium such as cold air or cooling water, a method of contacting a roll cooled by a refrigerant, or the like can be used.

(4) Step of stretching a gel-like sheet to form a stretched gel-like sheet In one embodiment, the obtained gel-like sheet is stretched in at least a uniaxial direction to form a stretched gel-like sheet. This step may be optional. Although not wishing to be bound by any theory or model, it is believed that the gel-like sheet can be uniformly stretched when it contains a film-forming solvent. Neither the stretching method nor the stretching ratio is particularly limited. For example, the stretching method may be any method capable of stretching the gel sheet at a predetermined magnification along the plane axis of the sheet (with or without optional heating). In one embodiment, stretching is performed by either tenter stretching, roll stretching, or inflation stretching (eg, using air). Without limitation, stretching can be done uniaxially or biaxially. Uniaxial stretching means stretching in the machine (ie, longitudinal) direction or the transverse direction. Biaxial stretching means sequential or simultaneous stretching in the machine direction and the transverse direction. Here, the machine direction is a direction oriented substantially along the moving direction during film formation on the film (in this case, a gel-like sheet), that is, the major axis direction of the film being manufactured. The transverse direction is a direction extending perpendicular to both the machine direction and a third axis substantially parallel to the thickness direction of the film on the film surface.

  In one embodiment, biaxial stretching is used. In the case of biaxial stretching (also referred to as biaxial orientation), any of simultaneous biaxial stretching, sequential stretching performed along one axis and another axis, and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used. In one embodiment, simultaneous biaxial stretching is used.

  The draw ratio is not limited. In one embodiment, in the case of uniaxial stretching, the linear stretching ratio may be about 2 times or more, or about 3 to about 30 times. In one embodiment, in the case of biaxial stretching, the linear stretching ratio is, for example, at least about 3 times in two planar directions, namely the machine direction and the transverse direction. The area magnification in biaxial stretching may be, for example, at least about 9 times, at least about 16 times, or at least about 25 times. Although not limited, when the area magnification is at least about 9, production of a polyolefin microporous membrane having a relatively high puncture strength is facilitated. If the area magnification is more than 400 times, it may be difficult to operate the stretching apparatus without tearing the gel-like sheet.

  Although not limited, the temperature of the gel-like sheet during stretching (stretching temperature) can be expressed as (Tme + 10 ° C), a temperature that exceeds the melting point of polyethylene used to prepare the polyolefin solution by about 10 ° C. It may be below the temperature. The stretching temperature may be in the range of more than Tcd to less than Tme. Tme and Tcd are the melting point and crystal dispersion temperature of polyethylene used to prepare the polyolefin solution. When the stretching temperature exceeds about Tme + 10 ° C., it may be difficult to orient the molecular chains of the polyolefin in the gel sheet during stretching. On the other hand, when the stretching temperature is lower than Tcd, it is difficult to stretch the gel sheet without tearing or tearing, and it may fail to achieve a desired stretching ratio. In one embodiment, the stretching temperature is from about 90 ° C to about 140 ° C, or from about 100 ° C to about 130 ° C.

  The Tme of the ultra high molecular weight polyethylene, second polyethylene, or polyethylene composition is generally about 130 ° C to about 140 ° C, and Tcd is about 90 ° C to about 100 ° C. Tcd can be determined from the temperature characteristics of dynamic viscoelasticity according to ASTM D4065.

  Although not wishing to be bound by any theory or model, the above stretching causes cleavage between polyethylene crystal lamellae and polypropylene crystal lamellae, and the polyethylene phase and the polypropylene phase are refined to form a large number of fibrils. It is thought. As a result, a polyolefin microporous film having a relatively large pore diameter and a relatively high mechanical strength can be easily produced by stretching. Such a polyolefin microporous membrane is considered to be particularly suitable for battery separator applications.

  The stretching may be performed under a condition in which a temperature distribution exists in the thickness direction (that is, a direction substantially perpendicular to the surface of the polyolefin microporous membrane). In this case, it becomes easy to produce a polyolefin microporous film having improved mechanical strength. Details of this method are described in Japanese Patent No. 3347854.

(5) A step of removing the solvent for film formation from the stretched gel-like sheet and forming a gel-like sheet from which the solvent has been removed In one embodiment, the gel-like sheet (when the step (4) is performed, the stretched gel-like sheet The film-forming solvent is removed (replaced) from the sheet to form a gel-like sheet from which the solvent has been removed. In order to remove (clean or replace) the film-forming solvent, a replacement (cleaning) solvent may be used. Although not wishing to be bound by any theory or model, the polyolefin phase formed in the gel-like sheet is separated from the solvent phase for film formation. It is considered that a porous film made of fibrils forming a network structure and having pores communicating irregularly in three dimensions can be obtained.

The cleaning solvent is not limited as long as the film forming solvent can be dissolved or replaced. 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 trifluoroethane 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 , C Examples include readily volatile solvents such as perfluoroethers such as ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F ₅ .

  The method for removing the film forming solvent is not limited, and any method can be used as long as most of the solvent can be removed, including a conventional solvent removing method. For example, the gel sheet after stretching can be washed by a method of immersing in a washing solvent, a method of showering a washing solvent, or a combination thereof. The amount of the cleaning solvent used is not limited, but generally depends on the method used to remove the film-forming solvent. For example, the washing solvent can be used in an amount of about 300 to about 30,000 parts by mass based on the weight of the gel sheet after stretching. Although it is generally desirable to do so, it is not always necessary to remove all film-forming solvents from the stretched gel-like sheet. If a considerable amount of the solvent remains in the stretched gel sheet after the solvent removing step, it may be difficult to produce a film having a desired porosity. Accordingly, in one embodiment, the gel after stretching until the amount of film-forming solvent remaining in the gel sheet after stretching is less than about 1% by weight, based on the weight of the gel sheet after stretching. The film-forming solvent is removed from the sheet.

(6) Step of drying the gel-like sheet after removing the solvent to form a polyolefin microporous membrane The gel-like sheet can be dried by any method capable of removing the washing solvent. For example, the film can be dried by a heat drying method or an air drying method. The temperature of the gel-like sheet during drying (that is, the drying temperature) is not limited. For example, the drying temperature may be equal to or lower than the crystal dispersion temperature Tcd. For example, the drying temperature may be 5 ° C. or more lower than Tcd. In one embodiment, the polyethylene has a crystal dispersion temperature in the range of about 90 ° C to about 100 ° C.

  Although not limited, drying can be performed until the residual washing solvent is about 5% by weight or less, based on the dried state, that is, based on the weight of the polyolefin microporous membrane after drying. In another embodiment, it may be carried out until about 3% by weight or less, based on after drying. Insufficient drying is generally considered to cause an undesirable decrease in the porosity of the microporous membrane. If drying is inadequate, the drying temperature and / or drying time should be increased. The polyolefin microporous membrane is formed by removing the washing solvent by drying or other methods.

(7) Re-stretching of polyolefin microporous membranes Although not essential, re-stretching of polyolefin microporous membranes after drying is at least uniaxial within a range that does not significantly reduce permeability, puncture strength, smoothness and film formability. May be.

  Although not essential, the polyolefin microporous membrane dried in step (6) may be stretched. The stretching may be performed uniaxially or biaxially, for example. The stretching method is not limited, and a conventional method (for example, dry stretching using a tenter clip) can be used. In order to distinguish the stretching in step (6) from the stretching in step (4), the stretching in step (6) is referred to as “re-stretching”, “dry stretching”, or “second stretching”. In step (4), the molecular orientation of the polyolefin in the polyolefin microporous membrane occurs by re-stretching, so the term “dry orientation” may be used instead of “dry stretching” or the like.

  Without limitation, stretching can be uniaxial or biaxial. In the case of biaxial stretching, stretching can be performed simultaneously in a direction substantially perpendicular to the plane (for example, the machine direction and the transverse direction), or sequentially in one direction on the plane and the other direction. In one embodiment, simultaneous biaxial stretching is used. The redraw ratio is not limited. In one embodiment, in the case of uniaxial restretching, the linear restretching ratio is generally about 1.1 times to about 2.5 times. In one embodiment, in the case of biaxial stretching, the linear stretching ratio is generally about 1.1 to about 2.5 times in at least two plane directions, such as the machine direction and the transverse direction. Dry stretching may be performed while heating the film (at a dry stretching temperature). The dry stretching temperature is not limited, but may be approximately the same or lower than the melting point Tme of polyethylene used for preparing the polyolefin solution. In one embodiment, the dry stretching temperature is from about Tcd to about Tme. When the dry stretching temperature is higher than Tme, it is difficult to produce a polyolefin microporous membrane having relatively high compression resistance and relatively uniform air permeability, particularly when the polyolefin microporous membrane after drying is stretched in the transverse direction. Sometimes. When the dry stretching temperature is lower than Tcd, it becomes difficult to soften the polyolefin in the microporous polyolefin membrane, and tearing occurs during dry stretching, or the magnification uniformity during dry stretching deteriorates. In one embodiment, the dry stretching temperature is from about 90 ° C to about 135 ° C, or from about 95 ° C to about 130 ° C.

  In one embodiment, the microporous membrane after drying may be heat treated. It is considered that the heat treatment stabilizes the polyolefin crystals in the polyolefin microporous membrane after drying, and a uniform lamella is formed. Conventional heat treatment such as heat setting treatment (HS) or heat relaxation treatment can be used. These may be only one or may be combined. In the case of HS, for example, a tenter clip or a roll can be used. The temperature of the membrane during the heat setting process (ie, “HS temperature”) may be from about Tcd to about Tme. In one embodiment, in the case of dry stretching, the HS temperature may be, for example, dry stretching temperature ± 5 ° C. or dry stretching temperature ± 3 ° C.

  Thermal relaxation treatment may be used instead of HS or in addition to HS. The thermal relaxation treatment is different from HS in that it is a heat treatment that does not apply stress (for example, tension) to the polyolefin microporous membrane. The method of the thermal relaxation treatment is not limited, but can be performed using, for example, a heating furnace having a belt conveyor or an air floating heating furnace. Further, after the heat setting treatment, the tenter clip may be loosened and the heat relaxation treatment may be performed as it is. The temperature of the polyolefin microporous membrane during the thermal relaxation treatment (that is, the thermal relaxation treatment temperature) is not limited. In one embodiment, the thermal relaxation treatment temperature is a temperature below about melting point Tme or about 60 ° C. to about (Tme-10 ° C.). It is considered that a polyolefin microporous film having relatively high permeability and strength can be easily produced by heat treatment such as HS and heat relaxation treatment.

(8) Crosslinking treatment In one embodiment, the polyolefin 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 about 0.1 to 100 Mrad may be used, and an acceleration voltage of about 100 to about 300 kV may be used. By the crosslinking treatment, a polyolefin microporous film having a relatively high meltdown temperature can be easily obtained.

(9) Hydrophilization treatment In one embodiment, the polyolefin microporous membrane may be subjected to a hydrophilic treatment, that is, a treatment capable of making the polyolefin microporous membrane more hydrophilic. The hydrophilization treatment can be performed by, for example, monomer grafting, surfactant treatment, corona discharge, or the like. Monomer grafting may be performed after the crosslinking treatment.

  In one embodiment, the polyolefin microporous membrane may be subjected to a hydrophilic treatment (that is, a treatment capable of making the polyolefin microporous membrane more hydrophilic). The hydrophilic treatment can be performed by monomer grafting, surfactant treatment, corona discharge or the like. Monomer grafting may be performed after the crosslinking treatment.

  In one embodiment, in the case of the surfactant treatment, at least one of a nonionic surfactant, a cationic surfactant, an anionic surfactant and an amphoteric surfactant can be used. In one embodiment, a nonionic surfactant is used. The method of using the surfactant is not limited. For example, the 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 microporous membrane by a doctor blade method.

[3] Physical properties and composition of polyolefin microporous membrane
(1) Physical Properties In one embodiment, the polyolefin microporous membrane has at least one of the following physical properties. For example, the polyolefin microporous membrane can have all of the following physical properties.

(a) Porosity of about 25 to 80% If the porosity of the polyolefin microporous membrane is less than about 25%, it may be difficult to produce a polyolefin microporous membrane having good air permeability. On the other hand, if it exceeds about 80%, it may be difficult to produce a battery having a sufficiently low electrical resistance and sufficiently long lasting.

(b) Air permeability of approximately 20 to 400 seconds / 100 cm ³ (converted to a film thickness of 20 μm)
When the air permeability of the polyolefin microporous membrane (measured by the Oken air permeability meter) is about 20 to 400 seconds / 100 cm ^3, it is easy to produce a battery having a large capacity and good cycle characteristics. If the air permeability is less than about 20 seconds / 100 cm ³ , it may be difficult to produce a battery having good shutdown characteristics. That is, the battery cannot support lithium ion conduction, for example, at excessive operating temperatures above 140 ° C.

(c) Puncture strength of at least about 2,000 mN The microporous membrane has a puncture strength (converted to a film thickness of 20 μm) of a 1 mm diameter needle with a spherical tip (curvature radius R: 0.5 mm) at a speed of 2 mm / sec. It is represented by the maximum load when piercing the multilayer microporous membrane. If the piercing strength is less than about 2,000 mN / 20 μm, it may be difficult to manufacture a battery having sufficient mechanical integrity to prevent an internal short circuit.

(d) Tensile rupture strength of at least about 80,000 kPa or more When the tensile rupture strength (measured by ASTM D882) of the microporous polyolefin membrane is at least about 80,000 kPa in both the longitudinal direction (MD) and the transverse direction (TD) It is easy to manufacture a battery that lasts long enough.

(e) Thermal shrinkage of 10% or less When the thermal shrinkage of the microporous polyolefin membrane (after holding the membrane at 105 ° C. for 8 hours) exceeds 10% in both the longitudinal and transverse directions, especially at the end of the separator It may be difficult to produce a battery that can withstand internal short circuits. In one embodiment, the thermal shrinkage is less than about 8% in both the longitudinal and transverse directions.

(f) Meltdown temperature of at least about 165 ° C or higher In one embodiment, the meltdown temperature of the polyolefin microporous membrane is from about 165 ° C to about 190 ° C. The meltdown temperature was increased by 50% for a test piece of 3 mm in the machine direction (also called the longitudinal direction) and 10 mm in the transverse direction, with the length of the test piece at room temperature (10 mm) being 100%. When the temperature. The specimen is heated at a rate of 5 ° C./minute from approximately ambient temperature (or room temperature) while pulling in the machine direction with a 2 g weight.

(g) Film thickness variation rate When the film thickness variation rate exceeds about 3 μm, it may be difficult to produce a battery that can sufficiently withstand an internal short circuit due to the relatively low resistance to short circuit compression of the battery. The film thickness fluctuation rate is obtained by measuring the thickness of the polyolefin microporous film with a contact thickness meter at intervals of 5 mm over a length of 30 cm in the TD direction.

(h) Polypropylene powder drop-off amount of about 2 g or less In order to obtain a film having a desired width, it is advantageous to cut the polyolefin microporous film in the machine direction. This operation is generally called slitting. During slitting, a small amount of polyolefin powder may adhere to the membrane. The amount of polypropylene powder falling off was the weight of the polypropylene powder adhering to the fixed bar when a 500 m long microporous film was slit in half at a speed of 50 m / min and each slit film was slid in contact with the fixed bar. It is. If the amount of polypropylene powder falling off is more than 20 g, the polyolefin microporous membrane should be examined for defects such as pinholes and black spots (impurities), which are indications of unacceptably high impurities. is there. The amount of polypropylene powder falling off can be improved by using a higher-purity polyolefin resin and a film-forming solvent, providing a step of filtering before extrusion, or combining them.

(i) Thickness In one embodiment, the thickness of the microporous membrane is from about 3 μm to about 200 μm, or from about 5 μm to about 50 μm.

(2) Composition of polyolefin microporous membrane
(a) Polyolefin The microporous membrane is generally made of a polyolefin used to form a polyolefin solution. Based on the weight of the polyolefin microporous membrane, it generally has a small amount of a washing solvent and / or a film-forming solvent of less than 1% by weight. During the manufacturing process, the molecular weight of a small amount of polyolefin may decrease, which is acceptable. In one embodiment, the difference between the Mw / Mn of the polyolefin and the Mw / Mn of the polyolefin solution in the membrane is less than about 10%, less than about 1%, Less than 0.1%. In one embodiment, the polyolefin microporous membrane comprises (i) polyethylene and (ii) polypropylene having a Trought ratio of 15 or more at 230 ° C. and a strain rate of 25 sec ⁻¹ .

(b) Polyethylene The polyethylene in the microporous polyolefin membrane comprises at least one of (a) ultrahigh molecular weight polyethylene or (b) a second polyethylene having a lower molecular weight than the ultrahigh molecular weight polyethylene. In one embodiment, the polyethylene has a weight average molecular weight (Mw) of about 1 × 10 ⁴ to about 1 × 10 ⁷ , about 1 × 10 ⁵ to about 5 × 10 ⁶ , about 2 × 10 ⁵ to about 3 ×. 10 is a ^6. Ultra-high molecular weight polyethylene and second polyethylene in the membrane are generally the same as those described for the preparation of the polyolefin solution, as any chemical or physical changes to the polyethylene during the manufacturing process are minor. Are the same. The amount of polyethylene in the microporous membrane is from about 2% to about 98%, from about 50% to about 97%, from about 60% to about 95% by weight, based on the weight of the polyolefin.

(c) Polypropylene The Troughton ratio of polypropylene in a polyolefin microporous membrane at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ is generally 15 or more. In one embodiment, the troopon ratio of polypropylene is at least about 25, or at least about 30. In one embodiment, the polypropylene has an elongational viscosity of 50,000 Pa · sec or more at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ . In another embodiment, the extensional viscosity is at least about 60,000 Pa sec. Even if changes in the physical and / or chemical properties of the polypropylene occur during the manufacturing process, the properties and composition of the polypropylene in the membrane are generally the same as described for the preparation of the polyolefin solution. is there. The amount of propylene in the microporous membrane is from about 2% to about 98%, from about 3% to about 50%, from about 5% to about 40% by weight, based on the weight of the polyolefin.

(3) Other polyolefins In addition to polyethylene and polypropylene, the polyolefin microporous membrane may optionally be polybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1, polyvinyl acetate, Any of polymethyl methacrylate, polystyrene, and ethylene / α-olefin copolymer may be included. These preferably have a Mw of about 1 × 10 ⁴ to about 4 × 10 ⁶ . The polyolefin microporous membrane may also comprise a polyethylene wax, preferably having a Mw of about 1 × 10 ³ to about 1 × 10 ⁴ . The amount of other polypropylene and wax is not limited, but should be an amount that does not cause a significant deterioration in the properties of the polyolefin microporous membrane. Polybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate, and polystyrene are homopolymers or copolymers containing a fourth α-olefin. It may be combined. The amount of other polyolefins in the microporous membrane may be less than about 20% by weight or less than about 10% by weight, based on the weight of the polyolefin.

[4] Battery separator The battery separator composed of the above-mentioned polyolefin microporous membrane can be appropriately selected depending on the type of the battery, but preferably has a film thickness of about 3 to about 200 μm, preferably about 5 to about 50 μm. It is more preferable to have a film thickness.

[5] Battery In one embodiment, the polyolefin microporous membrane of the present invention comprises a lithium ion battery, a lithium polymer secondary battery, a nickel-hydrogen secondary battery, a nickel-cadmium secondary battery, a nickel-zinc secondary battery, silver -It can be used as a separator for primary and secondary batteries such as zinc secondary batteries. The lithium ion secondary battery will be described below.

  The lithium ion secondary battery includes a positive electrode, a negative electrode, and a separator positioned between the negative electrode and the positive electrode. The separator generally contains an electrolytic solution (electrolyte). The structure of the electrode is not particularly limited and may be a conventional structure. For example, an electrode structure (coin type) in which disc-shaped positive and negative electrodes are arranged to face each other, and electrodes in which flat positive and negative electrodes are alternately stacked via at least one separator between them A structure (laminated type), an electrode structure in which laminated belt-like positive and negative electrodes are 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. In one embodiment, 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 usually 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. Solvents and / or lithium salts are not critical, but conventional solvents and lithium salts can be used. 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. The organic solvent may be an organic solvent having a relatively high boiling point (compared to the shutdown temperature of the battery) and a high dielectric constant. Suitable organic solvents include ethylene carbonate, propylene carbonate, ethyl methyl carbonate, γ-butyrolactone, low boiling point and low viscosity organic solvents such as tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane, dimethyl carbonate, diethyl carbonate, And the like including mixtures thereof. A high dielectric constant organic solvent usually has a high viscosity and vice versa, so a mixture of high and low viscosity solvents may be used.

  When assembling a battery, a separator is usually impregnated with an electrolytic solution. Thereby, ion permeability can be imparted to the separator (polyolefin microporous membrane). The impregnation method is not limited, and a conventional impregnation method can be used. For example, the impregnation treatment may be performed by immersing the microporous membrane in an electrolytic solution at room temperature.

  The battery assembly method is not limited, and a conventional battery assembly method can be used. For example, when assembling a cylindrical battery, for example, a positive electrode sheet, a separator made of a microporous film, and a negative electrode sheet are laminated in this order and wound to form a wound electrode assembly. A second separator may be necessary to prevent short-circuiting of the wound electrode assembly. The obtained wound electrode assembly is inserted into a battery can, impregnated with an electrolytic solution, and then a battery lid also serving as a positive electrode terminal provided with a safety valve is crimped through 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
The polyolefin resin was dry blended as follows. 2% by mass of ultra high molecular weight polyethylene (UHMWPE) with a weight average molecular weight (Mw) of 2.0 × 10 ⁶ , 88% by mass of high density polyethylene (HDPE) with an Mw of 3.5 × 10 ⁵ and a temperature of 230 ° C. and a strain rate of 25 sec. ^In 100 parts by mass of a polyolefin (PO) composition consisting of 10% by mass of a propylene homopolymer (PP) having a Trouton ratio of ^-1 of 36, an extensional viscosity of 67,000 Pa · sec, and an MFR (JIS K 6758) of 0.3, As an antioxidant, 1.0 part by mass of tetrakis [methylene-3- (3,5-ditertiarybutyl-4-hydroxyphenyl) -propionate] methane was dry blended. However, the melting point Tme measured for the polyethylene composition comprising UHMWPE and HDPE was 135 ° C., and the crystal dispersion temperature was 90 ° C.

Mw and Mw / Mn of UHMWPE and HDPE were determined by gel permeation chromatography (GPC) method under the following conditions.
・ 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 wt% (dissolution condition: 135 ° C./1 h)
・ Injection volume: 500μl
Detector: Waters Corporation differential refractometer (RI detector)
-Calibration curve: Prepared from a calibration curve obtained using a monodisperse polystyrene standard sample, using a predetermined conversion constant.

  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 conditions of 230 ° C. and 250 rpm to prepare a polyolefin solution. This polyolefin solution was extruded from a T die provided in a twin screw extruder. The extruded molded body was cooled while being drawn with a cooling roll adjusted to 10 ° C. to form a gel-like sheet.

  Using a tenter stretching machine, simultaneous biaxial stretching was performed at 115 ° C. in the longitudinal direction (the direction in which the film was conveyed) and the transverse direction at 5 times. The obtained stretched film was fixed to a 20 cm × 20 cm aluminum frame and immersed in methylene chloride (surface tension 27.3 mN / m (25 ° C.), boiling point 40.0 ° C.) adjusted to 25 ° C. Washing was performed while shaking at rpm for 3 minutes. The obtained membrane was air-dried at room temperature and then fixed on a tenter and heat-fixed at 125 ° C. for 10 minutes to produce a polyolefin microporous membrane.

Example 2
A polyolefin microporous membrane was prepared in the same manner as in Example 1 except that the composition of the PO composition was UHMWPE 2 mass%, HDPE 68 mass%, and PP 30 mass%.

Example 3
A polyolefin microporous membrane was prepared in the same manner as in Example 1 except that the composition of the PO composition was 90% by mass of HDPE and 10% by mass of PP.

Example 4
A polyolefin microporous membrane was prepared in the same manner as in Example 1 except that the composition of the PO composition was 70% by mass of HDPE and 30% by mass of PP.

Comparative Example 1
The composition of the PO composition is 90% by mass of HDPE, 10 masses of PP at a temperature of 230 ° C. and a trouton ratio of 10 at a strain rate of 25 sec ⁻¹ , an elongational viscosity of 19,000 Pa · sec, and an MFR (JIS K 6758) of 0.5. %, A polyolefin microporous membrane was prepared in the same manner as in Example 1 except that

Comparative Example 2
A polyolefin microporous membrane was produced in the same manner as in Example 1, except that the composition of the PO composition was 70% by mass of HDPE and 30% by mass of PP as in Comparative Example 1.

Comparative Example 3
A polyolefin microporous membrane was prepared in the same manner as in Example 1 except that the composition of the PO composition was UHMWPE 2% by mass, HDPE 88% by mass, and PP 10% by mass as in Comparative Example 1.

Comparative Example 4
A polyolefin microporous membrane was produced in the same manner as in Example 1 except that the composition of the PO composition was 5% by mass of UHMWPE, 90% by mass of HDPE, and 5% by mass of PP as in Comparative Example 1.

Comparative Example 5
The composition of the PO composition was UHMWPE 2 mass% and HDPE 98 mass%, and a polyolefin microporous membrane was prepared in the same manner as in Example 1 except that polypropylene was not added.

  The physical properties of the polyolefin microporous membranes obtained in Examples 1 to 4 and Comparative Examples 1 to 5 were measured by the following methods. The results are shown in Table 1.

(1) Average film thickness (μm)
The film thickness was measured by a contact thickness meter at an interval of 5 mm in the longitudinal direction over a width of 30 cm of the microporous membrane and obtained by averaging.

(2) Air permeability (sec / 100 cm ³ / 20μm)
The air permeability P ₁ measured according to JIS P8117 for a microporous film with a film thickness T ₁ is 20 μm when the film thickness is 20 μm according to the formula: P ₂ = (P ₁ × 20) / T ₁ It was converted to air permeability P _2.

(3) Porosity (%)
It was measured by a conventional mass method.

(4) 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 the piercing microporous film having a thickness T ₁ 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.

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

(6) Thermal shrinkage (%)
The thermal shrinkage of the polyolefin microporous membrane was obtained by holding the membrane at a temperature of 105 ° C. for 8 hours, and then measuring and averaging the shrinkage in the machine direction and the transverse direction three times each.

(7) Meltdown temperature (℃)
The meltdown temperature of the polyolefin microporous membrane is 150 ° C. or higher, preferably 150 to 190 ° C. The meltdown temperature was determined as follows. As shown in FIG. 1, test pieces TP having a size of 3 mm and 10 mm in the MD and TD stretching directions, respectively, were cut out from the polyolefin microporous membrane 1, and a thermomechanical analyzer (manufactured by Seiko Instruments Inc., Using the TMA / SS6000), hold the upper end 1a of the test piece TP with the holder 2, attach a 2g weight to the lower end 1b, and raise the temperature 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.

(8) Film thickness variation The thickness is measured with a contact thickness meter at intervals of 5 mm over a length of 30 cm in the TD direction of the microporous membrane, and the difference between the maximum and minimum values is obtained. ).

(9) Film forming property A polyolefin microporous film (length: 500 m) was slit in half at a speed of 50 m / min, and each slit film was brought into sliding contact with a fixed bar, and then wound on a reel. The powder adhering to the fixing bar was collected and its weight was measured. The film formability was evaluated based on the amount of adhering powder. Generally, the smaller the amount of powder attached, the better the film formability.

Table 1 (continued)

Table 1 (continued)

Notes: (1) Ratio of elongational viscosity to shear viscosity at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ .
(2) Measured at a temperature of 230 ° C and a strain rate of 25 sec- ¹ .

  As shown in Table 1, the polyolefin microporous membranes of Examples 1 to 4 are excellent in balance of permeability, puncture strength, smoothness and film formability, and further, tensile rupture strength, tensile rupture elongation, heat shrinkage resistance. In addition, the meltdown characteristics were excellent. On the other hand, since the polyolefin microporous membranes of Comparative Examples 1 to 4 all had a polypropylene trouton ratio of less than 15, the polyolefin microporous membranes of Examples 1 to 4 were inferior in film formability and smoothness. Moreover, since the polyolefin microporous film of the comparative example 5 did not contain polypropylene, the meltdown characteristic was inferior to the polyolefin microporous film of Examples 1-4.

Finally, embodiments of the present invention will be described.
[1] (1) (a) a polypropylene resin having a Troughton ratio of at least about 15 at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ ; and (b) (i) an ultra-high having at least 1 × 10 ⁶ Mw Polyolefin having a step of preparing a polyolefin solution by mixing a molecular weight polyethylene and / or (ii) a polyolefin resin composed of a polyethylene resin composed of a second polyethylene having a molecular weight lower than that of ultrahigh molecular weight polyethylene, and a film-forming solvent Article manufacturing method.

[2] The method according to [1], further comprising (2) extruding a polyolefin solution to form an extrudate, and (3) cooling the extrudate to form a cooled extrudate.

[3] The method according to [2], further comprising (4) stretching the cooled extrudate in at least one plane direction.

[4] In the method of [2] or [3], (5) a step of removing at least a part of the film-forming solvent from the cooled extrudate to form a polyolefin microporous membrane, and (6) a polyolefin microporous A method comprising the step of drying the membrane.

[5] In any one of the methods [1] to [4], the process solvent is aliphatic, cycloaliphatic or aromatic having a viscosity of 30 to 500 cSt when measured at a temperature of 25 ° C. One or more film-forming solvents and / or solid solvents selected from hydrocarbons, mineral oil fractions, and phthalates.

[6] The method according to [4], wherein the process solvent is liquid paraffin.

[7] A method that satisfies the following conditions (a) to (f) in the method of [6].
(a) The melt kneading in step (1) is performed in a twin screw extruder at a temperature in the range of melting point Tmp to Tmp + 80 ° C.
(b) The polyolefin content in the polyolefin solution is in the range of 1 to 75% by mass based on the mass of the polyolefin solution.
(c) A sheet die having a rectangular base and having a gap of about 0.1 mm to about 5 mm, heated to 140-250 ° C. during extrusion and having an extrusion rate of polyolefin solution of 0.2-15 m / min The polyolefin solution is directed to at least one die that is in the range of
(d) The cooling step (3) is performed at a cooling rate of at least 50 ° C./min.
(e) The stretching step (4) is biaxially performed at a magnification ranging from about 9 times to about 400 times.
(f) The process solvent is removed from the stretched molded article by a cleaning solvent comprising a volatile solvent selected from chlorinated hydrocarbons, esters, ketones, chain fluorocarbons, cyclic hydrofluorocarbons, perfluoroethers and mixtures thereof. Remove.

[8] The method according to [4] or [6], wherein at least one of the following steps is performed in an arbitrary order.
(7) (a) A step of redrawing the dried polyolefin microporous membrane at a temperature in the range of about 90 ° C to about 135 ° C.
(7) (b) A step of heat-treating the polyolefin microporous membrane after drying.
(8) A step of crosslinking treatment by irradiating the polyolefin microporous membrane after heat treatment with ionizing radiation.
(9) A step of hydrophilizing the polyolefin microporous membrane.

[9] A method for preparing a polyolefin solution using the polyolefin composition according to any one of [1] to [8]. The polyolefin composition comprises (a) a polyethylene resin having a weight average molecular weight in the range of 1 × 10 ⁴ to 1 × 10 ⁷ , and (b) a polypropylene content of about 2 based on the mass of the polyolefin composition. (C) polybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate, polystyrene and ethylene It contains at least one selected from the group consisting of α-olefin copolymers.

[10] A polyolefin microporous membrane produced by the process according to any one of [4], [6] and [8].

[11] A polyolefin containing polyethylene and a polypropylene having a Trouton ratio of at least about 15 at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ .

[12] The polyolefin according to [11], which is in the form of any one of (a) an extrudate, (b) a cooled extrudate, and (c) a polyolefin microporous membrane.

[13] A polyolefin that satisfies the following conditions (a) and / or (b) in the polyolefin of [11] or [12].
(a) polypropylene, has a temperature and strain rate 25 sec 15 more Trouton ratio at ^-1, and 50,000 Pa · sec or more elongational viscosity at a temperature and strain rate 25 sec ^-1 of 230 ° C. of 230 ° C..
(b) the polyethylene has a weight average molecular weight of about 1 × 10 ⁴ to about 1 × 10 ⁷ and (i) an ultra high molecular weight polyethylene, (ii) a second polyethylene having a lower molecular weight than the ultra high molecular weight polyethylene, or (iii) It consists of a polyethylene resin comprising a mixture of ultrahigh molecular weight polyethylene and second polyethylene.

[14] A film satisfying the following condition in the film of [13].
(a) The ultra high molecular weight polyethylene has a weight average molecular weight of at least about 1 × 10 ⁶ and comprises an ethylene homopolymer and / or an ethylene / α-olefin copolymer.
(b) the second polyethylene has a weight average molecular weight in the range of about 1 × 10 ⁵ to about 5 × 10 ⁵ , and is selected from high density polyethylene, medium density polyethylene, branched low density polyethylene, and chain low density polyethylene. Including at least one of
(c) The polyethylene is composed of ultra high molecular weight polyethylene having a Mw of 1 × 10 ⁶ or more and high density polyethylene, and the ultra high molecular weight polyethylene content in the polyethylene composition is about 100% by mass of polyethylene. It is in the range of 1% to about 60% by weight.

[15] The film according to [14], which satisfies one or both of the following conditions (a) and (b):
(a) The Mw / Mn of the polyethylene is in the range of about 5 to about 300.
(b) The polypropylene has at least one of the following characteristics (i) to (iv).
(i) The molecular weight (Mw) is at least about 6.5 × 10 ⁵ .
(ii) The molecular weight distribution (Mw / Mn) ranges from about 1 to about 100.
(iii) The heat of fusion ΔHm is at least about 90 J / g.
(iv) The elongational viscosity at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ is 50,000 Pa · sec or more.

[16] In the film of [14], the polypropylene is composed of any one of a propylene homopolymer, a copolymer of propylene and a second α-olefin, and a mixture thereof.

[17] A multilayer film comprising at least one layer composed of polyethylene and polypropylene having a trouton ratio of about 15 or more at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ .

[18] A polyolefin multilayer microporous membrane having a two-layer structure in the multilayer membrane according to [17].

[19] The multilayer multilayer microporous film according to [17], which is a polyolefin multilayer microporous film having a three-layer structure, and satisfies either or both of the following conditions (i) and (ii):
(i) At least one surface layer of the polyolefin multilayer microporous membrane is made of polyethylene and polypropylene having a Trought ratio of about 15 or more at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ .
(ii) At least one inner layer of the polyolefin multilayer microporous membrane is made of polyethylene and polypropylene having a Trought ratio of about 15 or more at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ .

[20] A battery separator comprising the film according to any one of [10] to [19].

[21] comprising an electrolyte, a negative electrode, a positive electrode, and a separator located between the negative electrode and the positive electrode, the separator comprising polyethylene and polypropylene having a Troughton ratio of 15 or more at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ . A battery in which the electrolyte is in contact with the negative electrode, the positive electrode and the separator.

[22] In the battery according to [21], 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, or a silver-zinc secondary battery Some batteries.

[23] The battery according to [21] or [22], wherein the positive electrode comprises a current collector and a positive electrode active material layer formed on the surface thereof and capable of occluding and releasing lithium ions.

[24] The battery according to any one of [21] to [23], wherein the electrolyte is a lithium salt dissolved in an organic solvent.

[25] A method of using the battery according to any one of [21] to [24], wherein the battery is charged or discharged.

It is the schematic which shows the measuring method of meltdown temperature.

Claims (5)

A polyethylene resin and a polypropylene having a Trouton ratio of 15 or more at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ are 15 to 50% by weight based on the weight of the polyethylene and the polypropylene. , and the result fibrils to form a three dimensional network structure, polyolefins characterized by having a three-dimensionally irregularly communicating pores microporous membrane. The polyolefin microporous membrane according to claim 1, wherein the polyethylene resin is 1 × 10 6. ⁶ Ultra high molecular weight polyethylene having a weight average molecular weight of 1 × 10 ^Four ~ 5 × 10 ^Five Wherein the content of the ultrahigh molecular weight polyethylene is 1 to 60% by mass based on the total of the ultrahigh molecular weight polyethylene and the second polyethylene. A polyolefin microporous membrane. 3. The polyolefin microporous membrane according to claim 1, wherein the polypropylene has an extensional viscosity of 50,000 Pa · sec or more at a temperature of 230 ° C. and a strain rate of 25 sec ⁻¹ . A battery separator comprising the polyolefin microporous film according to claim 1. A battery comprising a separator comprising the polyolefin microporous membrane according to claim 1.