JP5453272B2 - Microporous membranes and methods of making and using such membranes - Google Patents

Description

  The present invention relates to a microporous membrane having moderate permeability, mechanical strength, heat shrinkage resistance, excellent electrolytic solution absorption characteristics, and compression resistance, and a method for producing and using such a microporous membrane. . The present invention also relates to a battery separator provided with such a microporous membrane and a battery using such a battery separator.

  Microporous membranes are used for primary batteries and secondary batteries such as lithium ion secondary batteries, lithium polymer secondary batteries, nickel metal hydride secondary batteries, nickel cadmium secondary batteries, nickel zinc secondary batteries, silver zinc secondary batteries, etc. It is useful as a battery separator. When a microporous membrane is used as a separator for a battery, particularly a lithium ion battery, the performance of the membrane greatly affects the characteristics, productivity and safety of the battery. Therefore, the microporous membrane should have appropriate permeability, mechanical properties, heat resistance, dimensional stability, shutdown properties, meltdown properties, and the like. Such a battery preferably has a relatively low shutdown temperature and a relatively high meltdown temperature for improving the safety characteristics of the battery, particularly for batteries that are exposed to high temperatures in operating conditions. The high permeability of the separator is preferred for high capacity batteries. A high mechanical strength separator is preferred for improved battery assembly and manufacture.

Optimization of material composition, stretching conditions, heat treatment conditions, etc. has been proposed, and the characteristics of microporous membranes used for battery separators have been improved. For example, JP-A-6-240036 discloses a polyolefin microporous membrane having an improved pore size and a sharp pore size distribution. The polyolefin 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 has a molecular weight distribution (weight average molecular weight / number average molecular weight) of 10 to 300. Made from polyethylene resin, porosity of 35 to 95%, average through hole diameter of 0.05 to 0.2 μm, breaking strength of 0.2 kg or more (15 mm width), pore diameter distribution of 1.5 or less (maximum pore diameter / Average through-hole diameter). This microporous membrane is obtained by die-extruding a melt blend of the above polyethylene resin and a film-forming solvent and cooling the gel sheet obtained at a temperature from the polyethylene resin crystal dispersion temperature (Tcd) to the melting point + 10 ° C. The film-forming solvent is removed from the gel-like sheet, and the obtained film is re-stretched at a surface magnification of 1.5 to 3 times at a melting point of the polyethylene resin of −10 ° C. or lower to obtain crystals of the polyethylene resin. It is manufactured by heat-setting at a temperature from the dispersion temperature to the melting point.

  International Publication No. 1999/48959 discloses a polyolefin microporous membrane having a uniform surface porous structure without local unevenness of permeability in addition to moderate strength and permeability. This membrane is made of a polyolefin resin such as high-density polyethylene having a Mw of 50,000 or more and less than 5,000,000 and a molecular weight distribution of 1 or more and less than 30, and formed by uniformly dispersed microfibrils. And has a network structure with fine voids, and has an average microfibril size of 20 to 100 nm and an average microfibril spacing of 40 to 400 nm. This microporous membrane is obtained by subjecting a gel-like sheet obtained by die-extruding a melt blend of the polyolefin resin and the film-forming solvent and cooling to a temperature above the melting point of the polyolefin resin to -50 ° C. or lower and below the melting point. The film-forming solvent is removed from the sheet, and the film is re-stretched 1.1 to 5 times at a temperature lower than the melting point above the melting point of the polyolefin resin −50 ° C. and heated at a temperature from the crystal dispersion temperature to the melting point of the polyolefin resin Manufactured by fixing.

International Publication No. 2000/20492 discloses a polyolefin microporous membrane with improved permeability, characterized by comprising polyethylene as a composition and polyethylene fine fibrils having a Mw of 5 × 10 ⁵ or more. The polyolefin microporous membrane has an average pore diameter of 0.05 to 5 μm, and the ratio of lamellae having an angle θ of 80 to 100 ° with respect to the membrane surface is 40% or more in the longitudinal and transverse cross sections. This polyethylene composition contains 1 to 69% by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight of 7 × 10 ⁵ or more, 98 to 1% by weight of high density polyethylene, and 1 to 30% by weight of low density polyethylene. This microporous membrane is obtained by die-extruding a melt blend of the above polyethylene composition and a film-forming solvent, stretching a gel-like sheet obtained by cooling, and from the crystal dispersion temperature of the polyethylene or its composition to the melting point + 30 ° C. It is manufactured by heat setting at a temperature and removing the film-forming solvent.

  WO2002 / 072248 discloses a microporous membrane with improved permeability, particulate blocking performance and strength. This microporous membrane is made using a polyethylene resin having a Mw of less than 380,000. The microporous membrane has a porosity of 50 to 95% and an average pore diameter of 0.01 to 1 μm. This microporous membrane is composed of a three-dimensional network skeleton formed by microfibrils having an average diameter of 0.2 to 1 μm connected to each other throughout the microporous membrane, and a size of 0.1 μm or more defined by the skeleton. It has openings with an average diameter of less than 3 μm. This microporous membrane is obtained by die extrusion of a melt blend of the above-mentioned polyethylene resin and a film-forming solvent, and removing the film-forming solvent from the gel-like sheet obtained by cooling. And a film stretched at a temperature of 80 to 140 ° C. is thermally fixed.

  WO 2005/113657 discloses a polyolefin microporous membrane having moderate shutdown properties, meltdown properties, dimensional stability and high temperature strength. This microporous membrane is (a) a polyethylene resin having a molecular weight of 10,000 or less and containing 8 to 60% by mass of a component having an Mw / Mn ratio of 11 to 100, where Mn is a polyethylene resin It is made using a polyolefin resin containing a polyethylene resin having a number average molecular weight and a viscosity average molecular weight (Mv) of 100,000 to 1,000,000, and (b) polypropylene. This microporous membrane has a porosity of 20 to 95% and a heat shrinkage rate of 10% or less at 100 ° C. This polyolefin microporous membrane is manufactured by die-extruding a melt blend of the above polyolefin and film-forming solvent, stretching the gel-like sheet obtained by cooling, removing the film-forming solvent, and annealing the sheet. The

  Regarding separator characteristics, not only permeability, mechanical strength, dimensional stability, shutdown characteristics, and meltdown characteristics, but also battery cycle characteristics such as battery productivity and electrolyte retention characteristics such as electrolyte absorption In recent years, the importance has increased. In particular, the lithium ion battery electrode expands and contracts as lithium is taken in and out, and an increase in battery capacity results in a larger expansion ratio. Since the separator is compressed when the electrode expands, it is preferred that the separator be affected as little as possible in terms of electrolyte retention when compressed.

  Further, improved microporous membranes are disclosed in JP-A-6-240036, International Publication No. 1999/48959, International Publication No. 2000/20492, International Publication No. 2002/072248, and International Publication No. 2005/113657. Even if disclosed, further improvements are needed, particularly in membrane permeability, mechanical strength, heat shrinkage resistance, compression resistance and electrolyte absorption properties. Thus, it is preferable to form a battery separator from a microporous film having improved permeability, mechanical strength, heat shrinkage resistance, compression resistance and electrolyte absorption characteristics.

The present invention relates to the discovery of a microporous membrane having good permeability, mechanical strength, heat shrinkage resistance, and improved electrolyte absorption and compression resistance. Embodiments of the present invention include micropores characterized by pore size distribution curves obtained by mercury intrusion porosimetry having at least two peaks (or pore diameters when the pores are substantially cylindrical). It relates to a porous membrane. Such membranes have been found to have good permeability, mechanical strength, heat shrink resistance, improved compression resistance and electrolyte absorption properties. A microporous membrane is a step of (1) mixing a polyolefin composition and at least one diluent, eg, a film-forming solvent, wherein the polyolefin composition is (a) eg about 2.5 × 10 ⁵ to about 5 × From about 74% to about 99% by weight of a first polyethylene resin having a weight average molecular weight of 5 × 10 ⁵ or less and a molecular weight distribution from about 5 to about 100, such as in the range of 10 ⁵ ; From about 1% to about 5% by weight of a second polyethylene having a weight average molecular weight greater than × 10 ⁵ , for example from about 5.1 × 10 ⁵ to about 1 × 10 ⁶ and a molecular weight distribution from about 5 to about 100; A resin, and (c) from about 0% to about 25 having a weight average molecular weight of about 3 × 10 ⁵ to about 1.5 × 10 ⁶ , a molecular weight distribution of about 1 to about 100, and a heat of fusion of 80 J / g or more. With weight percent polypropylene and this % Based on the weight of the polyolefin composition, (2) a step of die-extruding the mixed polyolefin composition and diluent to form an extruded product, and (3) a cooled extruded product by cooling the extruded product. (4) Stretching the cold-extruded product in at least one direction, for example about 9 to about 400 times, at a high stretch temperature of about Tcd to Tcd + about 30 ° C. of the mixed polyolefin of the cold-extruded product (5) forming a film from the stretched sheet by removing, for example, at least part of a diluent that is a film-forming solvent, and (6) forming the film in at least one direction from about 1.1 to about 1. It can be produced by a method comprising a step of stretching to a magnification in the range of 8 times to form a stretched membrane, and (7) heat-fixing the stretched membrane to form a final microporous membrane.

In the embodiment, the microporous membrane includes a dense region corresponding to a main peak in a range of 0.01 to 0.08 μm in a pore size (or pore size when the pore is substantially cylindrical), and a pore size ( Alternatively, when the hole is substantially cylindrical, it has a coarse area corresponding to at least one sub-peak in the range of 0.08 to 1.5 μm in the hole diameter distribution curve. In the embodiment, the pore volume ratio between the dense region and the coarse region is 0.5 to 49. In the embodiment, the microporous film has a surface roughness as the maximum value of the height difference between any two points on the film surface of 3 × 10 ² nm or more. In the embodiment, the upper limit of the surface roughness of the microporous film is 3 × 10 ³ nm. When the surface roughness is within this range, the microporous membrane has a large contact area with the electrolytic solution when used as a battery separator, and exhibits appropriate electrolytic solution absorption characteristics.

The resin used to form the polyolefin composition is (a) 5 × 10 ⁵ or less, such as from about 2.5 × 10 ⁵ to about 5 × 10 ⁵ , such as from about 2.5 × 10 ⁵ to about 4 × 10 ^5. A first polyethylene resin having a weight average molecular weight of about 5 to about 100, such as about 7 to about 50, and (b) greater than 5 × 10 ⁵ , for example about 5.1 × 10 ⁵ and weight average molecular weight of about 1 × 10 ⁶ from about 5 to about 100, for example, a second polyethylene resin having a molecular weight distribution of from about 5 to about 50, as (c) optionally from about 3 × 10 ⁵ A weight average molecular weight of about 1.5 × 10 ⁶ , such as about 6 × 10 ⁵ to about 1.5 × 10 ⁶ , a molecular weight distribution of about 1 to about 100, such as about 1.1 to about 50, and 80 J / G or more, for example 80 And a polypropylene resin having a heat of fusion et about 200 J / g. The microporous membrane suitably comprises 25% by weight polypropylene obtained from polypropylene resin and 75% by weight polyethylene obtained from polyethylene resin based on the weight of the polyolefin microporous membrane.

In an embodiment, the microporous membrane is
(1) A step of mixing a polyolefin composition and at least one film-forming solvent to form a polyolefin solution, wherein the polyolefin composition comprises (a) a weight average molecular weight of about 2.5 × 10 ⁵ to about 5 × 10 ⁵ And about 74% to about 99% by weight of a first polyethylene resin having a molecular weight distribution from about 5 to about 100, and (b) a weight average molecular weight of 5.1 × 10 ⁵ to 1.25 × 10 ⁶ From about 1% to about 5% by weight of a second polyethylene resin having a molecular weight distribution from about 5 to about 100; and (c) a weight average molecular weight of about 3 × 10 ⁵ to 1.5 × 10 ⁶ and about 1 From about 0% to about 25% polypropylene resin having a molecular weight distribution from about 100 to about 100 J / g and a heat of fusion of 80 J / g or more, wherein% is based on the weight of the polyolefin composition, Preferably a solution concentration of about 25% to about 50% by weight, for example about 30% to about 50% by weight, based on the weight of the reolefin solution, and (2) die extrusion of the polyolefin solution. A step of forming an extruded molded product, (3) a step of cooling the extruded molded product to form a cooled extruded molded product having a high polyolefin content, and (4) a mixing of the cooled extruded molded product with the cooled extruded molded product. Forming a stretched sheet by stretching in at least one direction at a high stretching temperature from about Tcd to about Tcd + 30 ° C., for example, at a magnification of about 9 to about 400 times, and (5) forming a film from the stretched sheet. Removing at least a portion of the solvent to form a film; and (6) a high magnification of about 1.1 to about 1.8 times, for example about 1.2 to about 1.6 times, in at least one direction. Stretching to and forming a stretched film is produced by a method comprising a step of forming a microporous film by heat (7) stretched film.

  In the above method, the stretching of the microporous membrane in the step (6) is performed after the stretching of the cooled extrusion-molded product in the step (4), and therefore is called “re-stretching”.

  In still another embodiment, the present invention relates to a microporous membrane including a polyolefin microporous membrane containing polyethylene and having an electrolyte solution absorption rate of 4 or more.

  The present invention relates to a microporous membrane (or film) having improved properties, particularly improved electrolyte injection and compression properties. As a first step in the method of making such a membrane, a particular polyethylene resin and optionally a particular polypropylene resin are mixed, for example, by melt blending, to form a polyolefin composition.

[1] Materials used to produce microporous membrane (1) Polyolefin composition Polyolefin composition is (a) 5 × 10 ⁵ or less, eg, weight in the range of about 2.5 × 10 ⁵ to about 5 × 10 ⁵ From about 74% to about 99% by weight of a first polyethylene resin having an average molecular weight and a molecular weight distribution of from about 5 to about 100, such as from about 75% to about 98% by weight, and (b) greater than 5 × 10 ⁵ From about 1% to about 5%, such as from about 2% to about 5% by weight of a second polyethylene resin having a weight average molecular weight in the range of, for example, 5.1 × 10 ⁵ to about 1 × 10 ⁶ ; (C) 0% to about 25% by weight polypropylene having a weight average molecular weight of about 3 × 10 ⁵ to about 1.5 × 10 ⁶ , a molecular weight distribution of about 1 to about 100, and a heat of fusion of 80 J / g or more. Resin and weight here Sento is based on the weight of the polyolefin composition, comprising a.

  In other embodiments, the polyolefin composition comprises 90% to 97% by weight of the first polyethylene resin, 2% to 5% by weight of the second polyethylene resin, and 5% based on the weight of the polyolefin composition. And 10% by weight polypropylene resin.

(A) Polyethylene resin (i) composition The first polyethylene resin, for example high density polyethylene (HDPE) resin, is about 2.5 × 10 ⁵ to 7 × 10 ⁵ , or 3 × 10 ⁵ to 6 × 10 ⁵ . It has a weight average molecular weight (Mw) and a molecular weight distribution (MWD) of about 5 to about 100. Non-limiting examples of the first polyethylene resin used herein are those having a Mw of about 2.5 × 10 ⁵ to about 4 × 10 ⁵ and a MWD of about 7 to about 50. The first polyethylene resin may be an ethylene homopolymer, or an ethylene-alpha olefin copolymer, such as one containing a small amount of a third alpha olefin, such as about 5 mole percent. The third alpha olefin is not ethylene, but it is propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, or styrene, or these The synthesis of Such copolymers are preferably produced using a single site catalyst.

The second polyethylene resin, eg ultra high molecular weight polyethylene (UHMWPE), has a larger Mw than the first polyethylene resin, for example in the range of 5.1 × 10 ⁵ to 1 × 10 ⁶ and an MWD of about 5 to about 100. Have. Non-limiting examples of the second polyethylene resin used herein are those having an MWD of about 5 to about 50. The second polyethylene may be an ethylene homopolymer or an ethylene-alpha olefin copolymer, such as one containing a small amount of a third alpha olefin, such as about 5 mol%. The third alpha olefin is not ethylene, but it is propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, or styrene, or these The synthesis of Such copolymers are preferably produced using a single site catalyst.

(Ii) Determination of Mw and MWD MWD is equal to the ratio of Mw to number average molecular weight (Mn). The MWD of the polymer used to produce the microporous membrane can be controlled by, for example, multistage polymerization. The MWD of the polyethylene composition can be controlled by the molecular weight and the mixing ratio of the polyethylene component.

The Mw and Mn of polyethylene are determined using high temperature size exclusion chromatography equipped with a differential refractive index detector (DRI) or “SEC” (GPC PL220, Polymer Laboratories). Use three commercially available PLgel Mixed-B columns from Polymer Laboratories. The nominal flow rate was 0.5 cm ³ / min and the nominal injection volume was 300 μl. The transfer line, column and DRI detector were placed in an oven maintained at 145 ° C. The measurement was performed according to the procedure disclosed in “Macromolecules”, Vol. 34, No. 19, (2001), pages 6812-6820.

  The GPC solvent used is filtered Aldrich reagent grade 1,2,4-trichlorobenzene (TCB) containing about 1000 ppm butylated hydroxytoluene (BHT). The TCB is degassed with an online degasser before being introduced into the SEC. A polymer solution was prepared by placing a dry polymer in a glass container, adding a predetermined amount of the above TCB solvent, and heating the mixture at 160 ° C. with continuous stirring for about 2 hours. The concentration of the UHMWPE solution was 0.25 to 0.75 mg / ml. The sample solution was filtered off-line using a SP260 Sample Prep Station model, commercially available from Polymer Laboratories, offline prior to injection into GPC.

  The separation efficiency of the column set was calibrated with a calibration curve generated using 17 individual polystyrene standards ranging from about 580 to about 10,000,000 Mp used to generate a calibration curve. . Polystyrene standards are commercially available from Polymer Laboratories (Amherst, Mass., USA). The calibration curve (log MP and holding capacity) is generated by recording the holding capacity of the peak in the DRI signal for each PS standard and applying the data to a quadratic equation. Samples are analyzed using IGOR Pro available from Wave Metrics.

(B) Polypropylene resin The polypropylene used here has a Mw of about 3 × 10 ⁵ to about 1.5 × 10 ⁶ , for example, about 6 × 10 ⁵ to about 1.5 × 10 ⁶ , and a non- As a limiting example, it has a heat of fusion (ΔHm) of about 80 to about 200 J / g, an MWD of about 1.0 to about 100, such as about 1.1 to about 50, and a propylene homopolymer or propylene Others, i.e. copolymers with a fourth olefin, may be used, although homopolymers are preferred. The copolymer may be a random copolymer or a block copolymer. The fourth olefin, which is an olefin other than propylene, is an alpha olefin such as ethylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, styrene, And diolefins such as butadiene, 1,5-hexadiene, 1,7-octadiene, 1,9-decadiene and the like. The percentage of the fourth olefin in the propylene copolymer is preferably in a range that does not degrade the properties of the microporous polyolefin membrane such as heat resistance, compression resistance, heat shrinkage resistance, etc., and is less than about 10 mol%, for example, From about 0 to less than about 10 mole percent is preferred.

  In embodiments, the MWD of polypropylene is from about 1.0 to about 100, for example from about 2 to about 50, or from about 2.5 to 6.

  The Mw, MWD and ΔHm of polypropylene are measured according to the method disclosed in US Patent Application Publication No. US 2008/0057389, the entire text of US Patent Application Publication No. US 2008/0057389 is incorporated herein by reference.

  The amount of polypropylene resin in the polyolefin composition is 25% by weight or less based on the weight of the polyolefin composition. If the percentage of polypropylene is higher than 25% by weight, the resulting microporous membrane will be relatively low in strength and difficult to form into a bimodal structure. The percentage of polypropylene resin can be, for example, from about 5% to about 20% by weight, and in another example from about 7% to about 15% by weight, based on the weight of the polyolefin composition. In embodiments, the propylene has a ΔHm in the range of about 110 J / g to 120 J / g.

(2) Other components In addition to the above components, the polyolefin composition comprises (a) an additional polyolefin and / or (b) a heat resistant polymer resin having a melting point or glass transition temperature (Tg) of about 170 ° C. or higher, It may be contained in an amount that does not deteriorate the properties of the microporous membrane, for example, 10% by weight or less based on the weight of the polyolefin composition.

(A) Additional polyolefins Additional polyolefins are: (a) polybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, each having a Mw of 1 × 10 ⁴ to 4 × 10 ⁶ , Polyoctene-1, polyvinyl acetate, polymethyl methacrylate, polystyrene and ethylene / alpha olefin copolymer, and (b) at least one polyethylene wax having a Mw of 1 × 10 ³ to 1 × 10 ⁴ . Polybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate and polystyrene are not limited to homopolymers, but also copolymers containing other α-olefins It may be.

(B) Heat-resistant resin The heat-resistant resin is (i) an amorphous resin having a melting point of about 170 ° C. or higher and partially having a crystal structure, and (ii) a completely amorphous resin having a Tg of about 170 ° C. or higher. A resin and a mixture thereof are preferable. The melting point and Tg are determined by differential scanning calorimetry (DSC) in accordance with the method of JIS K7121. Specific examples of the heat-resistant resin include polyesters such as polybutylene terephthalate (melting point: about 160 to 230 ° C.), polyethylene terephthalate (melting point: about 250 to 270 ° C.), fluororesin, and polyamide (melting point: 215 to 265 ° C.). , Polyarylene sulfide, polyimide (Tg: 280 ° C or higher), polyamideimide (Tg: 280 ° C), polyethersulfone (Tg: 223 ° C), polyetheretherketone (melting point: 334 ° C), polycarbonate (melting point: 220- 240 ° C.), cellulose acetate (melting point: 220 ° C.), cellulose triacetate (melting point: 300 ° C.), polysulfone (Tg: 190 ° C.), polyetherimide (melting point: 216 ° C.) and the like.

(C) Content The total amount of additional polyolefin and heat resistant resin is preferably 20% or less, for example from about 0% to about 20% by weight, based on the weight of the mixed diluent and polyolefin composition.

[2] Method for Producing Microporous Membrane The present invention relates to a method for producing a microporous membrane, which comprises (1) a specific polyolefin (generally in the form of a polyolefin resin) and at least one solvent or dilution. A step of mixing the agent to form a polyolefin solution, (2) a step of die-extrusion of the polyolefin solution to form an extrusion molded product, and (3) a step of cooling the extrusion molded product to form a cooled extrusion molded product. And (4) a step of stretching the cooled extruded product at a specific temperature to form a stretched sheet, and (5) a step of removing the solvent or diluent from the stretched sheet to form a solvent / diluent removal film. (6) A step of stretching the solvent / diluent removal film to a specific temperature, which is a specific temperature, to form a stretched film, and (7) a step of thermally fixing the stretched film to form a microporous film. . If necessary, (4i) heat setting treatment step, (4ii) heating roll treatment step, and / or (4iii) high temperature solvent treatment step may be performed between step (4) and step (5). . Between the step (5) and the step (6), the (5i) heat setting treatment step may be executed. If necessary, before step (6), following step (5i), (5ii) step of cross-linking by ionizing radiation, and after step (7) (7i) hydrophilization step and (7ii) surface coating Processing steps may be performed.

(1) Mixing of polymer and diluent A polyolefin composition comprising polyethylene and polypropylene is mixed with at least one diluent to form a mixture. If necessary, the mixture may contain various additives such as an antioxidant and fine particles of silicate (pore forming material) as long as the effects of the present invention are not reduced.

  The diluent is preferably a liquid at room temperature to allow for better stretching at relatively high magnification. If the diluent is a solvent for one or more of the polymers in the mixture, it is called a “solvent” or “film-forming solvent”. If the diluent is a solvent (or film-forming solvent), the mixture is called a “polyolefin solution”. Diluents include, for example, nonane, decane, decalin, p-xylene, undecane, dodecane, liquid paraffin, distillate of mineral oil having a boiling point equivalent to the boiling point of the above hydrocarbons, and dibutyl phthalate, dioctyl phthalate, etc. It may be an aliphatic, cycloaliphatic or aromatic hydrocarbon such as a phthalate which is liquid at room temperature. In order to most efficiently obtain an extruded product as a gel-like sheet having a stable solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. In embodiments, one or more solid solvents that are miscible with the polyolefin composition but are solid at room temperature during melt blending may be added to the liquid solvent. Such a solid solvent is preferably stearyl alcohol, seryl alcohol, paraffin or wax. In another embodiment, the solid solvent is used without a liquid solvent. However, when only a solid solvent is used, non-uniform stretching or the like can occur.

  The viscosity of the liquid solvent is preferably from about 30 to about 500 cSt, more preferably from about 30 to about 200 cSt, measured at a temperature of 25 ° C. If the viscosity at 25 ° C. is less than 30 cSt, the polyolefin solution is foamed and difficult to mix. On the other hand, if the viscosity is greater than 500 cSt, it is difficult to remove the liquid solvent.

  Although there is no particular limitation, it is preferable to prepare a high resin concentration polyolefin solution by melt blending the polyolefin solution with a twin screw extruder. The film-forming solvent may be added prior to initiating melt blending, or it may be fed to the twin screw extruder at the middle during blending, the latter being preferred.

  The melt blending temperature of the polyolefin solution is preferably in the range of the melting point (Tm) of the polyethylene resin + 10 ° C. to Tm + 120 ° C. The melting point is measured by differential scanning calorimetry (DSC) based on JIS K7121. In embodiments, the melt blending temperature is from about 140 to about 250 ° C, preferably from about 170 to about 240 ° C, particularly when the polyethylene resin has a melting point of from about 130 to about 140 ° C.

  In order to obtain a hybrid structure, the polyolefin composition concentration in the polyolefin solution is preferably about 25 wt% to about 50 wt%, for example, about 25 wt% to about 45 wt%, based on the weight of the polyolefin solution.

  The ratio L / D of the screw length L to the screw diameter D of the twin screw extruder is preferably in the range of about 20 to about 100, and more preferably in the range of about 35 to about 70. When L / D is less than 20, melt blending becomes insufficient. When L / D is larger than 100, the residence time of the polyolefin solution in the twin screw extruder becomes too long. In the latter case, the Mw of the membrane decreases as a result of excessive shearing and heating, which is not preferred. The cylinder of the twin screw extruder preferably has an inner diameter of about 40 to about 100 mm.

  In the twin screw extruder, the ratio Q / Ns of the amount Q (kg / h) of the filled polyolefin solution to the screw rotation speed Ns (rpm) is preferably about 0.1 to about 0.55 kg / h / rpm. If Q / Ns is less than 0.1 kg / h / rpm, the polyolefin is damaged by shear, resulting in a decrease in strength and meltdown temperature. When Q / Ns is larger than 0.55 kg / h / rpm, uniform blending cannot be performed. Q / Ns is more preferably about 0.2 to about 0.5 kg / h / rpm. The screw rotation speed Ns is preferably 180 rpm or more. Although there is no particular limitation, the upper limit of the screw rotation speed Ns is preferably about 500 rpm.

(2) Extrusion The polyolefin solution is melt blended in an extruder and die extruded. In another embodiment, the polyolefin is extruded and pelletized. In this latter embodiment, the pellets are melt blended and extruded in a second extrusion to produce a gel shaped article or sheet. In any of the embodiments, the die may be a sheet forming die having a rectangular opening, a double cylindrical hollow die, an inflation die, or the like. In the case of a sheet forming die, the die gap is preferably about 0.1 to about 5 mm. The extrusion temperature is preferably from about 140 to about 250 ° C., and the extrusion speed is preferably from about 0.2 to about 15 m / min. The direction of extrusion (i.e., the direction of movement of the extruded product between extrusion and downstream processes of extrusion) is referred to as the machine direction or "MD". The direction orthogonal to both film thickness and MD is called the transverse direction or “TD”.

(3) Cooling of Extruded Molded Products Extruded products manufactured from the die are cooled to form cooled extruded products, for example, high resin content gel shaped products or sheets. Cooling is performed by exposing the extruded product to a temperature below the gelation temperature of the extruded product at a cooling rate of about 50 ° C./min or more. Cooling is preferably performed to about 25 ° C. or less. By such cooling, the polyolefin microphase separated by the film-forming solvent is fixed. In general, a slow cooling rate provides a gel-like sheet with large pseudocell units, resulting in a coarser higher order structure. On the other hand, a high cooling rate is a dense cell unit. A cooling rate of less than 50 ° C./min results in a high degree of crystallinity, making it difficult to provide appropriate stretchability to the gel sheet. Usable cooling methods include bringing the extruded product into contact with a cooling medium such as cooling air and cooling water, and bringing the extruded product into contact with a cooling roller.

  In an embodiment, the relative amounts of polyolefin composition and diluent mixed for extrusion are selected so that the cooled extrudate has a high polyolefin content.

  Due to the high polyolefin content, the cooled extrudates can produce polyolefins derived from the polyolefin used to produce the polyolefin composition from at least about 25% by weight, for example from about 25% by weight, based on the weight of the cooled extrudate. It will contain about 50% by weight. If the polyolefin content of the cooled extruded product is less than about 25% by weight, it will be difficult to form a hybrid microporous membrane structure having both small and large pores. When the polyolefin content exceeds about 50% by weight, the viscosity becomes high and it becomes difficult to form a desired hybrid structure. The cooled extrudate preferably has a polyolefin content that is at least as high as the polyolefin solution.

(4) Stretching of the cooled extruded product The cooled extruded product is stretched in at least one direction. Without wishing to be bound by any theory or model, it is believed that chilled extrudates (eg, gel-like sheets) are uniformly stretched because the sheets contain a film-forming solvent. The gel-like sheet is preferably stretched at a predetermined ratio after being heated by, for example, a tenter method, a roll method, an inflation method, or a combination thereof. Stretching is performed uniaxially or biaxially, but biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching or multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used, but simultaneous biaxial stretching is preferred. When using biaxial stretching, the amount of stretching in each direction need not be the same.

  In the case of uniaxial stretching, the stretching ratio in the first stretching step is preferably 2 times or more, more preferably 3 to 30 times. In the case of biaxial stretching, the stretching ratio is preferably 3 times or more in any direction, that is, the surface magnification is preferably 9 times or more, more preferably 16 times or more, and most preferably 25 times or more. In one embodiment, the cooled extrusion is simultaneously stretched 3 to 7 times (eg 5 times) in MD and 3 to 7 times (eg 5 times) in TD. Examples of the first stretching step include stretching from about 9 times to about 400 times. In a further example, stretching is from about 16 times to about 49 times. The pin breaking strength of the microporous membrane is improved at a surface magnification of 9 times or more. When the surface magnification exceeds 400 times, the stretching device, stretching work, etc. include a large stretching device, which makes operation difficult.

  During stretching, the extrudate is obtained from a temperature in the range of about the crystal dispersion temperature (Tcd) of the polyethylene used to produce the extrudate to about Tcd + 30 ° C., eg, from the Tcd of the mixed polyethylene component of the cooled extrudate. Exposure to temperatures in the range of Tcd + 25 ° C., more preferably in the range of Tcd + 10 ° C. to Tcd + 25 ° C., most preferably in the range of Tcd + 15 ° C. to Tcd + 25 ° C. If the stretching temperature of the cooled extruded product is less than Tcd, the polyethylene resin is not sufficiently softened, so that the gel-like sheet is easily broken by stretching, and it is considered that high-strength stretching cannot be performed.

  The crystal dispersion temperature is determined by measuring the temperature characteristics of dynamic viscoelasticity according to ASTM D 4065. Since the polyethylene resin has a crystal dispersion temperature of about 90 to 100 ° C, the stretching temperature is about 90 ° C to 125 ° C, preferably about 100 ° C to 125 ° C, more preferably 105 ° C to 125 ° C.

  The above stretching creates cracks in the polyethylene lamella, making the polyethylene phase finer and forming a large number of fibrils. Fibrils form a three-dimensional network structure. Stretching is thought to improve the mechanical strength of the final microporous membrane, expanding the pores and making the microporous membrane suitable for use as a battery separator.

  Depending on the desired properties, the stretching may be performed with a temperature distribution in the film thickness direction to impart further improved mechanical strength to the final microporous membrane. A detailed description of this method is described in Japanese Patent No. 3347854.

(5) Diluent removal Any convenient method may be used to remove at least a portion of the diluent from the cooled extrusion. For example, it can be removed (eg, washed, transferred, or decomposed) using a cleaning solvent. Since the polyolefin composition phase is phase separated from the film-forming solvent phase, removal of the diluent provides a microporous membrane. Removal of the liquid solvent can be performed using one or more suitable cleaning solvents, i.e., those that can move the liquid solvent from the membrane. Examples of cleaning solvents include volatile solvents such as saturated hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, ethers such as diethyl ether and dioxane, and methyl ether ketone. Ketones, trifluoroethane, linear fluorocarbons such as C ₆ F ₁₄ , cyclic hydrofluorocarbons such as C ₅ H ₃ F _7, hydrofluoroethers such as C ₄ F ₉ OCH ₃ , C ₄ F ₉ OC ₂ H ₅ , C ₄ F ₉ OCF ₃ , C ₄ F ₉ OC ₂ F _{5 and} other perfluoroethers, and mixtures thereof.

  The stretched film can be washed by dipping in a washing solvent and / or applying a washing solvent. The cleaning solvent used is preferably about 300 parts by weight to about 30,000 parts by weight with respect to 100 parts by weight of the stretched membrane. The washing temperature is usually about 15 to about 30 ° C., and if necessary, heating may be performed during washing. The heating temperature during washing is preferably about 80 ° C. or less. Washing is preferably performed until the amount of residual liquid solvent is less than 1% by weight of the amount of liquid solvent present in the polyolefin solution prior to extrusion.

  The microporous film from which the film-forming solvent has been removed is dried by a heat drying method, an air drying method (for example, an air drying method using moving air) or the like. Any drying method that can remove large amounts of cleaning solvent may be used. Preferably, almost all of the cleaning solvent is removed during drying. During drying, the membrane may be exposed to a temperature that is preferably the same as or lower than Tcd, more preferably 5 ° C. or more lower than Tcd. Drying is performed until the residual cleaning solvent is preferably 5% by weight or less, more preferably 3% by weight or less, based on the weight (in a dry state) of the microporous membrane. Insufficient drying can be recognized because it causes an undesirable decrease in the porosity of the microporous membrane.

(6) Stretching of dry microporous membrane The dry microporous membrane is stretched (re-stretched) in the second stretching step at a high magnification at least uniaxially. The re-stretching of the microporous membrane can be performed during heating by the tenter method or the like, for example, as in the first stretching step. Re-stretching may be uniaxial or biaxial. In the case of biaxial stretching, both simultaneous biaxial stretching and sequential stretching can be used, but simultaneous biaxial stretching is preferred. Since re-stretching is usually carried out with a microporous membrane having a long sheet shape obtained from a stretched gel-like sheet, the MD and TD directions in re-stretching are the directions in stretching of a cooled extruded product. Usually it is the same. The high draw ratio in this step is about 1.1 to about 1.8 times, for example about 1.2 to about 1.6 times in at least one direction. The amount of stretching need not be the same in each stretching direction.

  The second stretching is preferably carried out while exposing the membrane to a second temperature (“second stretching temperature”), preferably Tm or less, more preferably in the range of Tcd to Tm. If the second stretching temperature is higher than Tm, it is considered that the melt viscosity is generally too low for stretching with a low permeability, resulting in low permeability. When the second stretching temperature is lower than Tcd, it is considered that the polyolefin is not sufficiently softened and the film is broken by stretching, that is, uniform stretching cannot be performed. In one embodiment, the second stretching temperature is typically about 90 to about 135 ° C, such as about 95 to about 130 ° C.

  As described above, the uniaxial second stretching ratio of the microporous membrane in this step is preferably about 1.1 to about 1.8 times. A magnification of 1.1 to 1.8 times usually makes the microporous membrane of the present invention a hybrid structure with a large average pore size. For uniaxial second stretching, the magnification may be 1.1 to 1.8 times in the longitudinal or transverse direction. In the case of biaxial second stretching, the microporous membrane can be stretched at the same or different magnification if the stretching ratio in both directions is 1.1 to 1.8 times, but at the same magnification Is preferred.

  If the second draw ratio of the microporous membrane is less than 1.1 times, it is difficult to produce a membrane having a hybrid structure and having good permeability, electrolyte absorption capability and compression resistance in the final membrane. It will be. If the second draw ratio is larger than 1.8 times, it is difficult to maintain the fiber structure, and it is considered that the heat shrinkage resistance and electrolyte absorption characteristics of the film are lowered. This second draw ratio may be, for example, 1.2 to 1.6.

  The stretching speed is preferably 3% / second or more in the stretching direction. In the case of uniaxial stretching, the stretching speed is 3% / second or more in MD or TD. In the case of biaxial stretching, the stretching speed is 3% / second or more for both MD and TD. A stretch rate of less than 3% / second can make it difficult to produce a membrane with good permeability without undue permeability variation over TD. The stretching speed is preferably 5% / second or more, more preferably 10% / second or more. Although there is no particular limitation, the upper limit of the stretching speed is, for example, 50% / second in order to avoid breakage of the film during stretching.

(7) Heat treatment The dried microporous membrane is thermally treated (heat-set) to stabilize crystals and create a uniform lamella in the membrane. The heat setting is performed by a tenter method or a roll method. Heat setting is performed while the membrane is exposed to a temperature in the range of the second stretching temperature ± 5 ° C, more preferably in the range of the second stretching temperature ± 3 ° C. If the heat setting temperature is too low, it becomes difficult to produce a film having the desired pin breaking strength, tensile breaking strength, tensile breaking elongation, and heat shrinkage resistance. If the heat setting temperature is too high, the permeability of the film is adversely affected. It is thought to give.

  The annealing process is performed after the heat setting step. Annealing is a heat treatment that does not apply a load to the microporous membrane, and is performed using, for example, a belt conveyor additional heat chamber or an air-floating heating chamber. Annealing may be performed continuously after loosening the tenter and heat setting. Annealing is performed by exposing the film to a temperature below Tm, more preferably in the range of about 60 ° C to about Tm-5 ° C. Annealing is thought to improve the permeability and strength of the microporous membrane. Optionally, the membrane may be annealed without prior heat setting.

(8) Heat setting treatment of stretched gel-like sheet The stretch-extruded product between step (4) and step (5) may be heat-set if necessary. The method of heat setting can be performed in the same manner as described above with respect to step (7).

(9) Heating roller treatment After step (4), subsequent to any of steps (4) to (7), at least one surface of the stretch-extruded product of step (4) is turned into one or more heating rollers. You may make it contact. The roller temperature is preferably in the range of Tcd + 10 ° C. to Tm. The contact time of the heated roll with the stretch extruded product is preferably from about 0.5 seconds to about 1 minute. The heated roll may have a flat surface or a rough surface. The heating roll may have a suction function for removing the solvent. Although there is no particular limitation, an example of a roller heating system holds heated oil that contacts the roller surface.

(10) High-temperature solvent treatment The stretch-extruded product may be brought into contact with a high-temperature solvent between step (4) and step (5). High temperature solvent treatment turns the fibrils formed by stretching into a fairly thick fiber backbone leaf shape, giving the microporous membrane a large pore size and moderate strength and permeability. The term “leaf vein shape” means that a fibril has a thick fiber backbone and thin fibers that spread from the backbone in a complex network structure. Details of the high temperature solvent treatment are described in International Publication No. WO2000 / 20493.

(11) Heat setting of microporous membrane containing cleaning solvent The microporous membrane containing at least a part of the cleaning solvent may be heat-set between step (5) and step (6) if necessary. The heat setting procedure is the same as described above in step (7).

(12) Crosslinking Bonding The heat-set microporous membrane can be crosslinked by ionizing radiation such as α rays, β rays, γ rays, electron beams and the like. When irradiating with an electron beam, the amount of electron beam is preferably about 0.1 to about 100 Mrad, and the acceleration voltage is preferably about 100 to about 300 kV. Crosslinking treatment is considered to increase the meltdown temperature of the microporous membrane.

(13) Hydrophilization treatment The heat-set microporous membrane is subjected to a hydrophilic treatment (a treatment for making the membrane more hydrophilic). The hydrophilic treatment may be a monomer graft treatment, a surfactant treatment, a corona discharge treatment, or the like. Optionally, a monomer grafting process is performed after the crosslinking treatment.

  In the case of hydrophilizing a heat-set microporous membrane by a surfactant treatment, any of a nonionic surfactant, a cationic surfactant, an anionic surfactant and an amphoteric surfactant may be used. Surfactants are preferred. The microporous membrane is immersed in a solution of a surfactant in water or a lower alcohol such as methanol, ethanol, isopropyl alcohol, or coated with the solution by a doctor blade method.

(14) Surface coating treatment As an option, the heat-set microporous membrane obtained from step (7) is made of porous polypropylene, porous fluororesin such as polyvinylidene fluoride or polytetrafluoroethylene, porous polyimide, porous To improve the meltdown characteristics when the membrane is used as a battery separator. The polypropylene used for coating preferably has a Mw of about 5,000 to about 500,000 and a solubility of about 0.5 g or more in 100 g of toluene at 25 ° C. Such polypropylene more preferably has a racemic diade with a fraction of about 0.12 to about 0.88, wherein the racemic dyad is a structural unit in which two adjacent monomer units are enantiomers of each other. That is. The surface coating layer has, for example, a structure in which a solution of the above coating resin in a good solvent is applied to a microporous film, and a part of the solvent is removed to increase the resin concentration, thereby separating the resin phase and the solvent phase. Is applied by removing the remaining solvent. Examples of good solvents for this purpose include aromatic compounds such as toluene and xylene.

[3] Structure, characteristics and composition of microporous membrane (1) Structure The polyolefin microporous membrane of the present invention has a hybrid structure derived from polyethylene resin, and pore size distribution (difference pore volume curve) obtained by mercury porosimetry. At least two peaks (a main peak and at least one sub-peak). The main peak and sub-peak correspond to the dense region and coarse region of the polyethylene phase, respectively.

  The microporous membrane produced by the method described above has a relatively wide pore size distribution when plotted as a differential pore volume curve. The pore size distribution can be measured by a conventional method such as mercury porosimetry.

として表される差分細孔容積を求め、ここで、Ｖｐは孔容積、ｒは円筒形孔と仮定した孔半径である。差分細孔容積は、孔径をｘ軸としてｙ軸にプロットすると、従来から「孔サイズ分布」と呼ばれる。ハイブリッド構造を示す膜では、差分細孔容積の少なくとも約２５％、または少なくとも約３０％、または少なくとも約４０％、または少なくとも約５０％、または少なくとも約６０％が、約１００ナノメートル以上の大きさ（直径）の孔に関連する。別の表現では、孔径に対する

の曲線では、約１００ナノメートルから約１，０００ナノメートルの孔径の曲線の下の面積の割合は、約１０ナノメートルから約１，０００ナノメートルの孔サイズ（または円筒形孔として孔径）の曲線の下の全面積の少なくとも約２５％、または少なくとも約３０％、または少なくとも約４０％、または少なくとも約５０％、または少なくとも約６０％である。一実施の形態では、約１００ナノメートルから約１，０００ナノメートルの孔径の曲線の下の面積は、約１０ナノメートルから約１，０００ナノメートルの孔径の曲線の下の全面積の約２５％から約６０％、または約３０％から約５５％、または約３５％から約５０％の範囲である。 When using mercury porosimetry to measure the pore size and pore volume distribution in the membrane, the pore size, pore volume and specific surface area of the membrane are conventionally measured. Measure

Where Vp is the pore volume and r is the hole radius assumed to be a cylindrical hole. The differential pore volume is conventionally called “pore size distribution” when the pore diameter is plotted on the y-axis with the x-axis being the x-axis. In membranes that exhibit a hybrid structure, at least about 25%, or at least about 30%, or at least about 40%, or at least about 50%, or at least about 60% of the differential pore volume is greater than about 100 nanometers. Related to (diameter) holes. In other words, for the pore size

In the curve, the percentage of the area under the pore diameter curve of about 100 nanometers to about 1,000 nanometers is about 10 nanometers to about 1,000 nanometers of pore size (or pore diameter as a cylindrical hole). At least about 25%, or at least about 30%, or at least about 40%, or at least about 50%, or at least about 60% of the total area under the curve. In one embodiment, the area under the pore diameter curve from about 100 nanometers to about 1,000 nanometers is about 25 of the total area under the pore diameter curve from about 10 nanometers to about 1,000 nanometers. % To about 60%, or about 30% to about 55%, or about 35% to about 50%.

  Although not limited, the dense region (relatively small pores) and the coarse region (relatively large pores) are entangled irregularly, and a hybrid structure can be formed in any cross section of the microporous polyolefin membrane viewed in the longitudinal and transverse directions. Form. The hybrid structure can be observed with a transmission electron microscope (TEM) or the like.

  Since the microporous membrane of the present invention has a relatively large internal space and opening due to the coarse region, it has an appropriate permeability and electrolyte solution that hardly changes in air permeability (air permeability) when compressed. Absorptive. The microporous membrane also has a relatively small interior space and opening, which affects the safety characteristics of the membrane when used as a battery separator, such as shutdown temperature and shutdown speed. Therefore, a lithium ion battery such as a lithium ion secondary battery including a separator formed of such a microporous membrane has moderate productivity and cycle characteristics while maintaining high safety performance.

The mercury intrusion porosimetry method used to determine the microporous membrane structure involves using a Pore Sizer 9320 (Micromeritics), a pressure range of 3.6 kPa to 207 MPa, and a cell volume of 15 cm ³ . In the measurement, a contact angle of 141.3 mercury and a surface tension of mercury of 484 dynes / cm were used. The parameters obtained in this way include pore volume, surface area ratio, maximum pore size, average pore size and porosity. References teaching this method include Raymond P. Mayer and Robert A. Stowe, J. Phys. Chem. 70, 12 (1966); LC Drake, Ind. Eng. Chem., 41, 780 (1949); HL Ritter and LC Drake, Ind. Eng. Chem. Anal, 17, 782 (1945) and EW Washburn, Proc. Nat. Acad. Sci., 7, 115 (1921).

すると、差分細孔容積の測定は以下のように行われる。第１に、孔に押し込められる水銀の容積Ｖは、圧力Ｐの関数として測定される。Ｐの測定値を用いて、上記で説明したように、孔半径ｒを計算する。Ｐを徐々に増大させ、水銀の容積をＰの各値で求める。この方法で、特定のｒの孔に関する孔容積を示す表が作成され、測定用に選ばれたＰの範囲により決まるｒの範囲にわたり表が作成される。ｒの表にされた値は、便利なようにＬｏｇ（ｒ）に変換される。孔容積Ｖｐは一般的に微多孔膜１ｇ当たりのｃｍ^３（ｃｍ^３／ｇ）で表わされる。

で表わされる差分細孔容積は、Ｖｐとｒの表の値から計算でき、ここで、ｄＶｐはＶｐの隣接する表の値間の差で近似され、ｄＬｏｇ（ｒ）は，Ｌｏｇ（ｒ）の隣接する表の値間の差で近似される。ポリオレフィン微多孔膜は、粗大領域のために相対的に大きな内部空間と開口部とを有するので、圧縮されたときにほとんど透気度に変化なく適度な透過性と電解液吸収性を有する。したがって、そのようなポリオレフィン微多孔膜から形成されたセパレータを備えるリチウムイオン二次電池のようなリチウムイオン電池は、適度な生産性とサイクル特性を有する。 The mercury intrusion porosimetry method is briefly summarized below. At a pressure P, pressurized mercury is applied to the flat surface of the microporous membrane. This pressure does work W1 on the mercury and pushes the mercury into the pores of the microporous membrane until equilibrium is reached. In an equilibrium state, the force F1 on the surface of mercury acting on the pressure P is balanced with a force F2 acting on the same magnitude as F1, but acting in the opposite direction. Accordance with conventional Washbum model, work load W1 = 2πrLγcosθ required to move the mercury surface in the pores, the balance with the work done by the opposing force represented by W2 = Pπr ² L. In these equations, the hole is cylindrical, L is the depth of the hole, and r is the radius of the hole. The contact angle of mercury is represented by θ. The surface tension of mercury is represented by γ, and is usually 0.48 Nm ⁻¹ . Therefore, according to the Washbum model, the hole radius r is expressed as a function of the pressure P according to the following equation.

Then, the differential pore volume is measured as follows. First, the volume V of mercury pushed into the hole is measured as a function of pressure P. Using the measured value of P, the hole radius r is calculated as described above. P is gradually increased, and the volume of mercury is determined for each value of P. In this way, a table showing the pore volume for a particular r hole is created and the table is created over a range of r determined by the range of P selected for measurement. The r tabulated values are converted to Log (r) for convenience. The pore volume Vp is generally expressed in cm ³ (cm ³ / g) per 1 g of the microporous membrane.

Can be calculated from the values of the Vp and r tables, where dVp is approximated by the difference between the values of the adjacent tables of Vp, and dLog (r) is the log (r) It is approximated by the difference between adjacent table values. Since the polyolefin microporous membrane has a relatively large internal space and opening due to the coarse region, the air permeability hardly changes when compressed and has appropriate permeability and electrolyte absorption. Therefore, a lithium ion battery such as a lithium ion secondary battery including a separator formed from such a polyolefin microporous film has moderate productivity and cycle characteristics.

  In an embodiment, the polyolefin microporous membrane is a single layer membrane and the membrane has a hybrid structure, for example, the microporous layer material has a differential pore volume curve under a pore size range of about 100 nm to about 1,000 nm. Is characterized by a differential pore volume curve, wherein the area is about 25% or more of the total area under the differential pore volume curve in the pore size range of about 10 nm to about 1,000 nm.

  If the percentage of the first polyethylene in the membrane exceeds 99% by weight, based on the total weight of polyolefin in the membrane, the hybrid structure becomes difficult to manufacture. Even if the percentage of polypropylene in the membrane exceeds 25% by weight based on the total weight of polyolefin in the membrane, the hybrid structure is difficult to manufacture. Even if the percentage of the second polyethylene in the membrane exceeds 5% by weight, the hybrid structure becomes difficult to manufacture. A membrane having a hybrid structure generally has better electrolyte absorption characteristics than a membrane not having a hybrid structure.

  In a preferred embodiment of the polyolefin microporous membrane, the main peak is in the pore size range of 0.01 μm to 0.08 μm and the at least one sub-peak is in the range of 0.08 μm to 1.5 μm. In other words, at least one sub-peak has a pore size greater than 0.08 μm and less than or equal to 1.5 μm. More preferably, the main peak is a first peak in the pore size range of about 0.04 μm to 0.07 μm and the sub-peak is a second peak in the pore size range of about 0.1 μm to 0.11 μm, about A third peak with a pore size of 0.7 μm and a fourth peak in the pore size range of about 1 μm to 1.1 μm. However, the sub-peak need not have the third and fourth peaks. For example, the pore size distribution curve may have first to fourth peaks of about 0.06 μm, about 0.1 μm, about 0.7 μm, and about 1.1 μm.

The pore volume ratio of the dense region to the coarse region of the microporous membrane of the present invention is standard using a transmission electron microscope (TEM), mercury intrusion porosimetry, etc., as disclosed in International Publication No. WO2008 / 016174. It is determined by the method and is incorporated herein by reference in its entirety in International Publication No. WO2008 / 016174. As can be seen from the above International Publication, the hatched area S ₁ of the vertical line L ₁ passing through the small diameter side and the first peak corresponds to the pore volume of the dense region, and the large diameter side and the second peak are shown. area S ₂ which gave a hatch vertical line L ₂ passing corresponds to the pore volume of the coarse domains. The pore volume ratio S ₁ / S ₂ of the dense region to the coarse region is preferably from about 0.5 to about 49, more preferably from about 0.6 to about 10, and most preferably from 0.7 to 2. When a clear peak is not recognized in the raw porosimetry data, an approximate position is obtained using, for example, conventional peak deconvolution analysis. For example, the change in the slope of the curve (for example, the inflection point) can be identified more clearly using the first derivative of the curve with the hole size on the x-axis and the number of holes on the y-axis.

  Although there is no particular limitation, the dense region and the coarse region are irregularly mixed and form a hybrid structure in any cross section of the polyolefin microporous film observed in the longitudinal direction and the transverse direction. The hybrid structure can be observed with a transmission electron microscope (TEM).

(2) Characteristics The final microporous film thickness is usually in the range of 3 μm to 200 μm. For example, the microporous membrane has a thickness in the range of about 5 μm to about 50 μm, such as in the range of about 15 μm to about 30 μm. The film thickness of the microporous film is, for example, measured by a contact thickness meter with a width of 10 cm and a distance of 1 cm in the longitudinal direction, and the film thickness is averaged. Thickness gauges such as Litematic available from Mitutoyo are suitable. For example, a non-contact thickness measurement method such as an optical thickness measurement method is also suitable.

(A) Air permeability of 500 seconds / 100 cm ³ or less (converted to a value of 20 μm film thickness)
The air permeability of the microporous membrane is measured according to JIS P8117. In one embodiment, the air permeability of the microporous membrane ranges from 20 to 400 seconds / 100 cm ³ . If necessary, the air permeability P ₁ measured for a microporous film having a film thickness T ₁ in accordance with JIS P8117 is the air permeability at a film thickness of 20 μm according to the formula P ₂ = (P ₁ × 20) / T _1. It is converted to degrees _{P 2.}

(B) Porosity of about 25 to about 80% The porosity of the microporous membrane is the same as the weight of the equivalent nonporous membrane of 100% polyethylene (with the same length, width and film thickness). It is measured in a conventional manner by comparing to the same). And the porosity is the formula: porosity% = 100 × (W2−W1) / W2, where W1 is the actual weight of the microporous membrane and W2 is equivalent to 100% polyethylene of the same size and thickness. Which is the weight of the non-porous membrane.

(C) Pin breaking strength of 2,000 mN or more (converted to the equivalent value of a 20 μm film thickness)
The pin breaking strength of the microporous membrane (converted to the value of a membrane having a thickness of 20 μm) is 2 mm / second with a needle having a diameter of 1 mm and a hemispherical end face (radius of curvature R: 0.5 mm). It is expressed as the maximum load measured when stabbed with. If the pin breaking strength is less than 2,000 mN / 20 μm, a short circuit may occur in a battery having a separator formed of a microporous film. In one embodiment, the pin break strength (converted to a 20 μm film thickness) of the microporous film is in the range of 4,000 to 5,000 mN.

Hemispherical end surface each with a diameter 1mm of T ₁ of the film thickness microporous membrane (the curvature radius R: 0.5 mm) Maximum load when stabbed at a speed of 2 mm / sec needle is measured. The measured maximum load L ₁ is converted into the maximum load L ₂ with a film thickness of 20 μm by the formula L ₂ = (L ₁ × 20) / T ₁ and is defined as the pin breaking strength.

(D) Tensile strength of 49,000 kPa or more The tensile strength of 49,000 kPa or more in both the longitudinal and transverse directions (measured using a test piece having a width of 10 mm in accordance with ASTM D-882) is particularly used as a battery separator. When used, it is characteristic of a microporous membrane with moderate durability. The tensile strength at break is preferably about 80,000 kPa or more.

(E) Tensile elongation greater than or equal to 100% Tensile elongation greater than or equal to 100% in both the longitudinal and transverse directions (measured in accordance with ASTM D-882) is particularly suitable when used as a battery separator. It is a characteristic of a certain microporous membrane.

(F) Thermal contraction rate of 12% or less in MD and TD The thermal contraction rate in the orthogonal plane direction (for example, the longitudinal direction or the transverse direction) at 105 ° C. is measured as follows. (I) measure the size of the microporous membrane specimen at room temperature in both the longitudinal and transverse directions; (ii) equilibrate the microporous membrane specimen at a temperature of 105 ° C. for 8 hours without applying a load. And (iii) measuring the size of the microporous membrane in both the longitudinal and transverse directions. Heating (or “heat”) shrinkage in the longitudinal or transverse direction is determined by dividing the result of measurement (i) by the result of measurement (iii) and expressing the percentage of the result in percent.

  In one embodiment, the microporous membrane has a TD heat shrinkage at 105 ° C. ranging from 3% to 10%, such as from 3.5% to 5%, and from 1% to 8%, such as from 1.5%. It has an MD heat shrinkage at 105 ° C. in the range of 3%.

(G) Film thickness change rate of 21% or less after heat compression (expressed in absolute value)
The rate of film thickness change after heating and compression at 90 ° C. for 5 minutes under a pressure of 2.2 MPa is generally 20% or less with respect to 100% of the film thickness before compression. A battery including a microporous membrane separator having a film thickness change rate of 20% or less has a moderately large capacity and good cycle characteristics. In one embodiment, the thickness change rate of the microporous membrane is in the range of 10% to 20%.

To measure the rate of change in film thickness after heat compression, a microporous membrane sample was placed between a pair of sufficiently flat plates and pressed at 90 ° C. for 5 minutes under a pressure of 2.2 MPa (22 kgf / cm ² ). The film is heated and compressed by a machine, and the average film thickness is determined by the same method as described above. The rate of change in film thickness is calculated by the equation: (average film thickness after heat compression−average film thickness before compression) / (average film thickness before compression) × 100.

(H) Air permeability after heat compression of 700 sec / 100 cm ³ or less When heated and compressed under the above conditions, the polyolefin microporous membrane usually has an air permeability of 700 sec / 100 cm ³ or less (Gurley value). Have A battery using such a microporous membrane has a reasonably large capacity and cycle characteristics. The air permeability is preferably 650 seconds / 100 cm ³ or less, for example in the range of 400 seconds / 100 cm ³ to 500 seconds / 100 cm ³ .

  The air permeability after heat compression is measured according to JIS P8117.

(I) Surface roughness of 3 × 10 ² nm or more The surface roughness of the microporous layer measured by the dynamic force mode of the atomic force microscope (AFM) is usually 3 × 10 ² nm or more (in the microporous film) Measured by average maximum height difference). The surface roughness of the microporous layer is preferably 3.5 × 10 ² nm or more, for example, in the range of 400 nm to 700 nm.

(J) Electrolytic solution absorption rate of 4 or more Using a dynamic surface tension measuring device (DCAT21 with a precision electronic balance commercially available from EKO), an electrolytic solution maintained at 18 ° C. (electrolyte: LiPF ₆ at 1 mol / L, solvent: The microporous membrane test piece was immersed in ethylene carbonate / dimethyl carbonate (volume ratio of 3/7) for 600 seconds, [weight of microporous membrane after immersion (g) / weight of microporous membrane before immersion (g)] The electrolytic solution absorption rate was determined by the following formula. The electrolytic solution absorption rate is represented by a relative value, where the electrolytic solution absorption rate of the microporous membrane of Comparative Example 1 is 1. In one embodiment, the electrolyte absorption rate of the microporous membrane is in the range of 4 to 10, for example 4.1 to 6.

(K) Pore size distribution The pore size distribution of the microporous membrane is determined by mercury porosimetry. A differential pore volume of at least about 25% is associated with pores larger than 100 nanometers (nm).

(L) Surface roughness The maximum surface height difference measured by dynamic force mode (DFM) AFM is used as the surface roughness.

(3) Composition of the microporous membrane The microporous membrane usually comprises the same polymer that produces the polymer composition and is usually of the same relative amount. A cleaning solvent and / or treatment agent (diluent) may be present, usually in an amount of less than 1% by weight, based on the weight of the microporous membrane. Slight polymer molecular weight degradation may occur during the process, but is acceptable. In one embodiment where the polymer is a polyolefin and the microporous membrane is produced by a wet process, the polymer MWD value in the microporous membrane does not exceed about 5%, even if there is molecular weight degradation during the process. The Mw / Mn of the polymer used to produce the microporous membrane in the range, in the range not exceeding about 1%, or in the range not exceeding about 0.1%.

(1) Polyolefins Embodiments of the microporous membrane of the present invention include (a) 2.5 × 10 ⁵ to 5 × 10 ⁵ , eg, about 2.5 × 10 ⁵ to about 4 × 10 ⁵ Mw and about 5 From about 74% to about 99% by weight of a first polyethylene having an MWD from about 100 to about 100, for example from about 7 to about 50, and (b) from 5.1 × 10 ⁵ to about 1 × 10 ⁶ , for example about 5. From about 1% to about 5% by weight of a second polyethylene having a Mw of 2 × 10 ⁵ to about 8 × 10 ⁵ and a MWD of about 5 to about 100, for example about 5 to about 50, and (c) as an option About 3 × 10 ⁵ to about 1.5 × 10 ⁶ , eg about 6 × 10 ⁵ to about 1.5 × 10 ⁶ Mw and about 1 to about 100, eg about 1.1 to about 50 MWD and 80 J About 0 weight with a heat of fusion of at least about 80 Jg / g, % To about 25% by weight polypropylene, where the weight percentage is based on the weight of the microporous membrane. In one embodiment, the heat of fusion of polypropylene ranges from 110 J / g to 120 J / g.

[4] Battery Separator In one embodiment, the battery separator formed from any of the above polyolefin microporous membranes of the present invention is about 3 to about 200 μm, or about 5 to about 50 μm, or about Although it has a film thickness of 7 to about 35 μm, the optimum film thickness is appropriately selected depending on the type of battery to be manufactured.

[5] Battery Although not particularly limited, the polyolefin microporous membrane of the present invention is used for primary batteries and secondary batteries, particularly lithium ion secondary batteries, lithium polymer secondary batteries, nickel hydrogen secondary batteries, nickel cadmium secondary batteries. It can be used as a separator for lithium ion secondary batteries, particularly for batteries, nickel zinc secondary batteries, silver zinc secondary batteries and the like.

  Lithium ion secondary batteries include an anode and a cathode stacked via separators, and a separator that typically includes an electrolyte in the form of an electrolyte (“electrolyte”). The structure of the electrode is not limited. Conventional structures are suitable. For example, the electrode structure is a coin type in which a disc-shaped positive electrode and a cathode are opposed to each other, a stacked type in which a flat positive electrode and a cathode are alternately stacked, a wound type in which a strip-shaped positive electrode and a cathode are wound, etc. It's okay.

The anode usually includes a current collector and an anode active material layer formed on the current collector and capable of absorbing and releasing lithium ions. The anode active material may be a transition metal oxide, a composite oxide of lithium and transition metal (lithium composite oxide), an inorganic compound such as a transition metal sulfide, or the like. The transition metal may be V, Mn, Fe, Co, Ni or 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 cathode includes a current collector and a cathode active material layer formed on the current collector. The cathode active material may be a carbonaceous material such as natural graphite, artificial graphite, coke, or carbon black.

The electrolytic solution may be a solution obtained by dissolving a lithium salt in an organic solvent. Lithium salt is 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 , 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 or the like may be used. These lithium salts may be used alone or as a mixture. Organic solvents are high boiling point and high dielectric constant organic solvents such as ethylene carbonate, propylene carbonate, ethyl methyl carbonate, γ-butyrolactone, and / or tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane, dimethyl carbonate, diethyl carbonate An organic solvent having a low boiling point and a low viscosity such as These organic solvents may be used alone or in combination. A high dielectric constant organic solvent usually has a high viscosity, and a low viscosity organic solvent usually has a low dielectric constant. Therefore, it is preferable to use a mixture of both.

  When assembling the battery, the separator is impregnated with an electrolytic solution to impart ion permeability to the separator (polyolefin microporous membrane). The impregnation treatment is usually performed by immersing the microporous membrane in an electrolytic solution at room temperature. When assembling a cylindrical battery, for example, an anode sheet, a microporous membrane separator, and a cathode sheet are laminated in this order, and the finished laminate is wound around a wound electrode assembly. The completed electrode assembly is loaded into a battery can, and then impregnated with the above electrolyte, and a battery lid that operates as an anode terminal equipped with a safety valve is caulked to the battery can via a gasket to manufacture a battery.

  The invention will now be described in more detail with reference to the following examples, without intending to limit the scope of the invention.

[Example 1]
(I) 2% by weight of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight (Mw) of 0.7 × 10 ⁶ and a molecular weight distribution (Mw / Mn) of 8 and (ii) of 3.0 × 10 ⁵ 100 parts by mass of polyethylene (PE) composition comprising 97.8% by mass of high density polyethylene (HDPE) having Mw and Mw / Mn of 8.6, and tetrakis [methylene-3- (3, 5-ditertiarybutyl-4-hydroxyphenyl) -propionate] 0.2 parts by weight of methane was dry blended to form a mixture. The polyethylene in the mixture has a melting point of 135 ° C and a crystal dispersion temperature of 100 ° C.

  40 parts by mass of the resulting mixture, that is, polyethylene solution, was packed into a strong blend twin screw extruder having an inner diameter of 58 mm and L / D = 42, and 60 parts by mass of liquid paraffin (50 cSt at 40 ° C.) was biaxially fed through a side feeder. Feeded to the extruder. A polyethylene solution was prepared by melt blending at 210 ° C. and 200 rpm. This polyethylene solution was extruded from a T die mounted on a twin screw extruder. While passing through a cooling roll adjusted to 40 ° C., the extruded product was cooled to form a cooled extruded product, that is, a gel-like sheet.

  Using a tenter stretching machine, the gel-like sheet was stretched 5 times biaxially at 118.5 ° C. in both the longitudinal and transverse directions. The stretched gel-like sheet is fixed to an aluminum frame plate of 20 cm × 20 cm, immersed in a methylene chloride bath adjusted to 25 ° C., liquid paraffin is removed by vibration at 100 rpm for 3 minutes, and dried by air flow at room temperature. did. The dried membrane was re-stretched at a magnification of 1.5 times in a transverse direction at 129 ° C. by a batch type stretching machine. The re-stretched film while being fixed to the batch-type stretching machine was heat-fixed at 129 ° C. for 30 seconds to produce a polyethylene microporous film.

[Example 2]
3% by mass of UHMWPE resin having a weight average molecular weight (Mw) of 0.54 × 10 ⁶ and a molecular weight distribution of 8 and an HDPE resin having a weight average molecular weight of 3 × 10 ⁵ and a molecular weight distribution of 8.6. Example 1 was repeated except that 97% by mass was provided.

[Example 3]
90% by mass of a polyolefin composition having a weight average molecular weight of 3 × 10 ⁵ and a molecular weight distribution of 8.6, and a PP resin having a weight average molecular weight of 6.6 × 10 ⁵ and a ΔHm of 83.3 J / g. Example 1 was repeated except that it was 8 mass%, the stretching temperature was 118 ° C., the re-stretching temperature was 129.5 ° C., and the heat setting was 129.5 ° C.

[Comparative Example 1]
HDPE resin having 20% by weight of UHMWPE resin having a weight average molecular weight (Mw) of 2 × 10 ⁶ and a molecular weight distribution of 8 and a weight average molecular weight of 3.5 × 10 ⁵ and Mw / Mn of 8.6. Example 1 was repeated except that 80% by mass was provided. Another difference from Example 1 of Comparative Example 1 was that 30 parts by mass of the finished polyolefin composition and 70 parts by mass of liquid paraffin (50 cSt at 40 ° C.) were packed in a biaxial stretching machine, and the stretching temperature was 115 ° C. And the heat setting temperature is 126.8 ° C.

  Table 1 shows the characteristics of the microporous membranes obtained in Examples and Comparative Examples.

[Comparative Example 2]
The polyolefin composition has 100% by mass of HDPE resin having a weight average molecular weight (Mw) of 3 × 10 ⁵ and a molecular weight distribution of 8.6, and is stretched at 118 ° C. and 1.4 times at 129 ° C. Example 1 was repeated except that it was re-stretched. In this comparative example, PP and UHMWPE were not included.

[Comparative Example 3]
3% by weight of UHMWPE resin having a weight average molecular weight of 2 × 10 ⁶ and a molecular weight distribution of 8, and 92% by weight of HDPE resin having a weight average molecular weight of 3 × 10 ⁵ and a molecular weight distribution of 8.6. A PP resin having a weight average molecular weight of 3 × 10 ⁵ and a PP resin having a ΔHm of only 77.2 J / g is stretched at 116 ° C. and restretched at 127 ° C. at a magnification of 1.4 times. And Example 1 was repeated, except that it was heat set at 127 ° C. Another difference from Example 1 in Comparative Example 3 is that 35 parts by mass of the finished polyolefin composition and 65 parts by mass of liquid paraffin (50 cSt at 40 ° C.) were packed in a twin screw extruder.

Table 1 shows the characteristics of the microporous membranes obtained in Examples and Comparative Examples.

As is apparent from Table 1, the microporous membrane of the present invention has a first peak in the pore size of 0.01 μm to 0.08 μm, and the second peak in the pore size of 0.08 μm to 1.5 μm. To 4 shows a pore size distribution curve obtained by mercury porosimetry with a fourth peak, and the surface roughness, which is the maximum height difference, was 3 × 10 ² nm or more. The microporous membrane of the present invention is improved in that the film thickness and air permeability hardly change after heat compression in addition to moderate air permeability, pin breaking strength, tensile breaking strength, tensile breaking elongation, and heat shrinkage resistance. It has electrolyte absorbability. On the other hand, the microporous membrane produced in the comparative example has low electrolyte solution absorbability, low compression properties, high heat shrinkability, and poor processability.

  The separator formed by the polyolefin microporous membrane of the present invention imparts appropriate safety, heat resistance, storage characteristics and productivity to the battery.

Claims (5)

A polyolefin microporous membrane,
74 wt% to 99 wt% of a first polyethylene having a Mw of 5 × 10 ⁵ or less, based on the weight of the polyolefin microporous membrane,
From 1% to 3% by weight of a second polyethylene having a Mw of 5.1 × 10 ⁵ to 1 × 10 ⁶ ;
The microporous membrane test piece was immersed for 600 seconds in an electrolytic solution maintained at 18 ° C., and obtained based on the formula [weight of microporous membrane after immersion (g) / weight of microporous membrane before immersion (g)]. The electrolyte absorption rate is 4 or more;
Polyolefin microporous membrane. The polyolefin microporous membrane has an electrolyte absorption rate in the range of 4.1 to 10;
The polyolefin microporous membrane according to claim 1 . The polyolefin microporous membrane has a pore size distribution curve having at least two peaks;
The polyolefin microporous membrane according to claim 1 or 2 . The polypropylene in the polyolefin microporous membrane is 25% by weight or less based on the weight of the polyolefin microporous membrane, and the polypropylene has a heat of fusion of 80 J / g or more and 3 × 10 ⁵ to 1.5 × 10 ^6. Having a Mw in the range of
The polyolefin microporous membrane according to any one of claims 1 to 3 . The polyolefin microporous membrane comprises 90% to 97% by weight of the first polyethylene and 5% to 10% by weight of the polypropylene, based on the weight of the polyolefin microporous membrane;
The polyolefin microporous membrane according to claim 4 .