KR20090125007A - Polyolefin microporous membrane - Google Patents

Abstract

PURPOSE: A polyolefin microporous membrane and a manufacturing method thereof are provided to reduce failure rates of a slit process among a manufacturing line or a processing line of the polyolefin microporous membrane. CONSTITUTION: A manufacturing method of a polyolefin microporous membrane includes the following steps of: mixing at least polyolefin resin, plasticizers, and inorganic fine powder; mixing and melting a compound; molding a mixed compound to a sheet form; extracting the plasticizers and the inorganic fine powder from a molded product; drawing the molded compound; and alleviating a drawing process of an elongation sheet.

Description

Polyolefin microporous membrane {POLYOLEFIN MICROPOROUS MEMBRANE}

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

Polyolefin microporous membranes are widely used for separation of various materials, selective permeation membranes, and separators, and examples of their use include filling micropores, fuel cell separators, capacitor separators, and functional materials into holes to create new functions. The base material of the functional film for this, a battery separator, etc. are mentioned. Among these uses, it is especially preferably used as a separator for lithium ion batteries widely used for mobile devices, such as a notebook computer, a mobile telephone, and a digital camera. The reason for this is that the film has high mechanical strength or insulation performance.

In Patent Documents 1 and 2, polyethylene microporous membranes having both good permeability and high strength are disclosed. Patent Document 3 below discloses a polyolefin microporous membrane having suppressed heat shrinkage. Patent Document 4 discloses a polyolefin microporous membrane having a narrow distribution of pores and high strength.

[Patent Document 1] Japanese Unexamined Patent Publication No. 2002-194132

[Patent Document 2] Japanese Patent Application Laid-Open No. 10-258462

[Patent Document 3] Japanese Patent Application Laid-Open No. 9-012756

[Patent Document 4] International Publication No. 2005/061599

In recent years, as the capacity of a battery increases, the length of the wound electrode and the separator tends to be long. In addition, in terms of improving the productivity of the winding process, production at high speed is often performed.

Here, in terms of stably producing a polyolefin microporous membrane or a battery under high line speed conditions, the polyolefin microporous membrane has good slitability, and is not easily wound or wrinkled during battery winding. It is required to have

However, all of the polyolefin microporous membranes described in Patent Documents 1 to 4 still had room for improvement in terms of their slitability.

In view of the above circumstances, an object of the present invention is to provide a polyolefin microporous membrane capable of reducing a defective rate in a slit process in a production line, a processing line, or the like of a polyolefin microporous membrane.

MEANS TO SOLVE THE PROBLEM As a result of earnestly examining in order to solve the said subject, the polyolefin microporous membrane in which the ratio of the elasticity modulus of a longitudinal direction and the width direction, the maximum pore diameter, and 120 degreeC thermal contraction rate was adjusted to a specific range can solve the said subject. Discovered to complete the present invention.

That is, this invention is as follows.

[1] the ratio of the longitudinal modulus of elasticity / width modulus is 1.0 to 2.5,

The maximum pore size is 0.10 μm to 0.25 μm,

120 degreeC thermal contraction rate is 5% or less in both a longitudinal direction and the width direction

Polyolefin microporous membrane.

[2] The polyolefin microporous membrane according to [1], wherein the maximum shrinkage stress in the longitudinal direction is less than 0.1 N.

[3] The polyolefin microporous membrane according to [1] or [2], which contains 5 to 90% by mass of ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 700,000 or more.

[4] A separator for nonaqueous electrolyte secondary batteries, comprising the microporous membrane made of polyolefin according to any one of [1] to [3].

[5] A nonaqueous electrolyte secondary battery comprising the separator according to [4], a positive electrode, a negative electrode, and an electrolyte solution.

[6] A method for producing the polyolefin microporous membrane according to any one of [1] to [3], including each of the following steps (a) to (f).

(a) mixing at least a polyolefin resin, a plasticizer and an inorganic fine powder,

(b) melt kneading the mixture obtained in step (a),

(c) forming the kneaded product obtained in the step (b) into a sheet;

(d) extracting the plasticizer and the inorganic fine powder from the sheet-like molded article,

(e) biaxially stretching the sheet-like molded article,

(f) Process of extending | stretching relaxation of the stretched sheet obtained by the process (e) in the width direction,

The width direction draw rate in the said (e) process is 20-100% / sec, the width direction draw ratio is 1.1-4.0 times, and ratio of the longitudinal direction draw ratio / width direction draw ratio is 0.5-1.5,

The width direction relaxation rate of the stretched sheet in the said (f) process is 3% or more

The manufacturing method of a polyolefin microporous film.

According to this invention, the polyolefin microporous membrane which can reduce the defective rate in the process of performing a slit in the manufacturing line, processing line, etc. of a polyolefin microporous membrane is provided.

EMBODIMENT OF THE INVENTION Hereinafter, the form for implementing this invention (it abbreviates as "this embodiment" hereafter) is demonstrated in detail. In addition, this invention is not limited to the following embodiment, It can variously deform and implement within the range of the summary.

In the polyolefin microporous membrane of the present embodiment (hereinafter, simply abbreviated as "microporous membrane"), the ratio of the longitudinal elastic modulus / width elastic modulus is 1.0 to 2.5, the maximum pore diameter is 0.10 µm to 0.25 µm, and 120 ° C heat shrinkage is 5% or less in both the longitudinal direction and the width direction.

The polyolefin microporous membrane of this embodiment is formed from the polyolefin resin composition containing polyolefin resin and an inorganic fine powder, for example.

As a polyolefin resin used by this embodiment, the polymer obtained by superposing | polymerizing monomers, such as ethylene, propylene, 1-butene, 4-methyl-1- pentene, 1-hexene, and 1-octene (a single polymer, air, for example) And a multistage polymer). These polymers can be used individually by 1 type or in combination of 2 or more types. Among them, polyethylene resins are preferably used in terms of improving mechanical strength.

In addition, as the polyolefin resin, for example a density of 0.94 g / cm high-density polyethylene as described above ³ to a density of a density of in the range of 0.93 to 0.94 g / cm ³ of polyethylene, is lower than 0.93 g / cm ³ density Low density polyethylene, linear low density polyethylene, etc. are mentioned. Among them, from the viewpoint of increasing the film strength of the microporous membrane, high density polyethylene and medium density polyethylene are preferably used. These may be used alone or as a mixture. In addition, from the viewpoint of improving the permeability and mechanical strength of the microporous membrane, it is preferable to use polyethylene alone.

In terms of improving the slitability of the microporous membrane, the microporous membrane preferably has a viscosity average molecular weight of 700,000 or more ultra high molecular weight polyethylene, preferably 5 to 90 mass%, more preferably 10 to 90 mass%, even more preferably 20 to 80 It contains mass%.

In addition, from the viewpoint of improving the mechanical strength of the microporous membrane, the microporous membrane preferably contains 5 mass% or more of ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 700,000 or more.

In addition, from the viewpoint of improving the moldability of the microporous membrane, the microporous membrane preferably contains an ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 700,000 or more at 90 mass% or less.

On the other hand, in terms of improving the heat resistance of the microporous membrane, the polyolefin resin may include polypropylene.

As a specific example of such polypropylene, a propylene homopolymer, an ethylene propylene random copolymer, and an ethylene propylene block copolymer are mentioned, for example. Especially, a polypropylene homopolymer is used preferably.

In addition, when using a copolymer as polypropylene, it is preferable that content of ethylene which is a comonomer is 1.0 mass% or less from the point of not lowering the crystallinity degree of polypropylene and further reducing the permeability of a microporous membrane.

In addition, in all the polypropylenes used, it is preferable that the ethylene content which is a comonomer is 1 mol% or less, and it is preferable that all are propylene homopolymers.

In addition, from the viewpoint of improving the moldability to the microporous membrane, the intrinsic viscosity [η] of the polypropylene is preferably 1 to 25 dL / g, more preferably 2 to 7 dL / g. Here, [eta] is the value measured at the measurement temperature of 135 degreeC using decalin as a solvent based on ASTM-D4020.

Moreover, as a ratio which polypropylene occupies in the said polyolefin resin, Preferably it is 10 mass% or less, More preferably, it is 1-10 mass%, More preferably, it is 1-8 mass%, Especially preferably, it is 1-6 mass% to be. It is preferable to make this ratio 1 mass% or more from the point of improving the heat resistance of the polyolefin microporous membrane. On the other hand, it is preferable to make this ratio 10 mass% or less from the point of realizing a microporous film with a high puncture strength, while permeability is favorable.

Examples of the inorganic fine powder include silica, calcium silicate, aluminum silicate, alumina, calcium carbonate, magnesium carbonate, kaolin clay, talc, titanium oxide, carbon black, and diatomaceous earth. Especially, it is preferable to use a silica from a dispersibility and the ease of extraction.

The polyolefin resin composition may be, for example, an antioxidant such as phenol, phosphorus or sulfur; Metal soaps such as calcium stearate and zinc stearate; Various additives, such as a ultraviolet absorber, a light stabilizer, an antistatic agent, an antifogging agent, a coloring pigment, a lubricant, and an antiblocking agent, can also be mixed.

As a manufacturing method of the polyolefin microporous film in this embodiment, the manufacturing method including each process of following (a)-(f) can be used, for example.

(a) mixing at least a polyolefin resin, a plasticizer and an inorganic fine powder,

(b) melt kneading the mixture obtained in step (a),

(c) molding the kneaded product obtained in step (b) into a sheet;

(d) extracting the plasticizer and the inorganic fine powder from the sheet-like molded article,

(e) biaxially stretching the sheet-like molded article,

(f) Process of extending | stretching relaxation of the stretched sheet obtained by the process (e) in the width direction.

The step (a) is a step of mixing at least a polyolefin resin, a plasticizer, and an inorganic fine powder. The step (a) can be performed by a conventional mixing method using a blender such as a Henschel mixer, a V-blend mixer, a plowshare mixer, a ribbon blender, or the like. Moreover, it is also possible to mix and assemble.

As a plasticizer, when mixed with a polyolefin resin, it is preferable that it is a nonvolatile solvent which can form a uniform solution above melting | fusing point of a polyolefin resin. Moreover, it is preferable that it is a liquid at normal temperature.

Examples of the plasticizer include phthalic acid esters such as dioctyl phthalate, diheptyl phthalate and dibutyl phthalate, organic acid esters such as adipic acid esters and glycerin acid esters, phosphoric acid esters such as trioctyl phosphate, liquid paraffin, Solid wax, mineral oil and the like. Among the above, phthalic acid ester is preferable in view of compatibility with polyethylene, low air permeability, and low bubble point. These plasticizers may be used alone or as a mixture.

As a ratio which the said plasticizer occupies in the said mixture, Preferably it is 30 mass% or more, More preferably, it is 40 mass% or more, As an upper limit, Preferably it is 80 mass% or less, More preferably, it is 70 mass% or less. It is preferable to make the said ratio 80 mass% or less from the point of maintaining the melt tension at the time of melt molding high, and ensuring moldability. On the other hand, it is preferable to make the said ratio 30 mass% or more from the side which ensures moldability, and the aspect which efficiently extends lamellar crystal in the crystal region of a polyolefin. Here, the lamellae crystals are efficiently stretched out, which means that the polyolefin chains are efficiently stretched without breaking of the polyolefin chains, and the formation of uniform and fine pore structures or improvement of the strength and crystallinity of the polyolefin microporous membrane Can contribute.

Moreover, as for the mixing ratio of the polyolefin resin with respect to the total amount of the said polyolefin resin, a plasticizer, and an inorganic fine powder, 10-50 mass% is preferable, More preferably, it is 20-40 mass%. The proportion of the polyolefin resin is preferably 10% by mass or more from the viewpoint of improving the mechanical strength of the microporous membrane, and preferably 50% by mass or less from the viewpoint of improving the film forming property during extrusion molding and the permeability of the microporous membrane.

Moreover, as a ratio which the said inorganic fine powder occupies in the total amount of the said polyolefin resin, a plasticizer, and an inorganic fine powder, Preferably it is 3 mass% or more, More preferably, it is 5 mass% or more, As an upper limit, Preferably it is 60 mass% or less More preferably, it is 50 mass% or less. It is preferable to make the said ratio 3 mass% or more from the point which provides the outstanding permeability, and the aspect which stably forms a polyolefin microporous film. On the other hand, the ratio of 60% by mass or less means that the length direction of the polyolefin microporous membrane (the ejection direction of the resin during molding. Direction, which is sometimes abbreviated as "TD", in view of improving the heat shrinkage at 120 ° C. Moreover, it is also preferable at the point of improving the mechanical strength of a polyolefin microporous film.

The step (b) is a step of melt kneading the mixture obtained in the step (a). The step (b) can be performed by a screw extruder such as a single screw extruder, a twin screw extruder, a kneader, a Benbury mixer, or the like after mixing with a Henschel mixer, a ribbon blender, a tumbler blender, or the like. As a method of melt-kneading, the method of melt-kneading with the extruder which can operate continuously is preferable. In view of good kneading property, it is preferable that the extruder capable of continuously operating is a twin screw extruder.

Further, in the aspect of preventing deterioration of the resin due to heating during melt kneading, melt kneading can be performed in an inert gas atmosphere such as nitrogen.

(c) process is a process of shape | molding the kneaded material obtained at the process (b) in sheet form. The step (c) can be performed by, for example, a method of directly bringing the kneaded material into contact with a cooling medium such as cold air or cooling water, or by bringing the kneaded material into contact with a roll or press cooled with a refrigerant. Among them, a method of bringing the kneaded material into contact with a roll or press machine cooled with a refrigerant is preferable because of excellent thickness control of the microporous membrane.

The step (d) is a step of extracting the plasticizer and the inorganic fine powder from the sheet-like molded product. In the step (d), the plasticizer and the inorganic fine powder are extracted from the sheet-like molded product with a solvent. Examples of the solvent used for the extraction of the plasticizer include organic solvents such as methanol, ethanol, methyl ethyl ketone and acetone, ethers such as tetrahydrofuran, halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane, and the like. Can be used. In view of obtaining a microporous membrane having a high porosity and excellent adhesion to electrodes and excellent permeability, it is preferable to extract the inorganic fine powder after the plasticizer is extracted. As a solvent used for extraction of an inorganic fine powder, aqueous alkali solution, such as sodium hydroxide and potassium hydroxide, can be used. Moreover, when extracting an inorganic fine powder, you may leave a part of inorganic fine powder in a molding.

(e) A process is a process of biaxially stretching a sheet-like molded object. As TD drawing speed | rate in (e) process, Preferably it is 20-100% / second. In addition to improving the slit property, the TD drawing speed is preferably 20% or more in terms of improving the adhesion to the battery during hot pressing by making the surface roughness uniform, and 100% / in terms of improving the permeability. It is preferable to set it as second or less. On the other hand, MD extending | stretching speed becomes like this. Preferably it is 10-4000% / second, More preferably, it is 100-3000% / second. In addition to improving the slit property, from the viewpoint of improving the mechanical strength, the MD stretching speed is preferably at least 10% / second, and preferably at most 4000% / second in terms of improving the breaking resistance.

Moreover, as TD draw ratio in a process (e), Preferably it is 1.1 times or more, More preferably, it is 1.2 to 4.0 times. It is preferable to make polymer orientation to TD also by making TD draw ratio 1.1 times or more from the side which improves battery winding property, the slit improvement aspect, or the shrinkage property of the film width direction after a slit. In terms of improving safety at high temperatures, the TD magnification is preferably 4.0 times or less.

On the other hand, the MD draw ratio is preferably 1.5 to 8.0 times, more preferably 2.0 to 7.0 times, in terms of improving mechanical strength and breakability. It is preferable to make MD draw ratio 1.5 time or more from the viewpoint of improving mechanical strength, and it is preferable to make MD draw ratio 8.0 times or less from the viewpoint of improving the breakwater resistance at the time of hot press.

The ratio of MD draw ratio and TD draw ratio (MD draw ratio / TD draw ratio) is preferably 0.5 to 1.5, more preferably 0.5 to 1.3. It is preferable to make ratio of the said draw ratio into 0.5 or more from a viewpoint of improving mechanical strength and battery winding property, and it is preferable to make ratio of the said draw ratio into 1.5 or less from a viewpoint of improving the breaking resistance at the time of hot press. Do. Moreover, since the hole of the polyolefin microporous membrane becomes isotropic, it is also preferable from the viewpoint of stabilization of slit (reduction of winding shift). Moreover, when the shape of a hole is isotropic, ion permeability in a lithium battery improves and there exists a tendency for cycling characteristics to become favorable.

In addition, the said (e) process can be suitably implemented in any timing before and after the said (d) process.

(f) process is a process of extending | stretching relaxation of the stretched sheet obtained by the process (e) in the width direction. As TD relaxation rate in a process (f), Preferably it is 3 to 50%, More preferably, it is 5 to 40%. From the viewpoint of suppressing thermal contraction of the polyolefin microporous membrane and the stability at slit time, the TD relaxation rate is preferably 3% or more. In addition, in terms of reducing the occurrence of wrinkles during film formation, the TD relaxation rate is preferably 50% or less. The TD relaxation rate is calculated by (film width after stretching-film width after relaxation) ÷ (film width after stretching) x 100 = relaxation rate (%).

It is preferable to set the TD stretching speed, the TD stretching ratio, the ratio of the MD stretching ratio / TD stretching ratio, and the relaxation rate at the time of TD stretching in the above ranges from the viewpoint of improving the battery winding property, the slit property, the mechanical strength of the film, and the film forming property.

In terms of heat shrinkage and permeability of the polyolefin microporous membrane, the heat treatment temperature in the step (f) is preferably -5 to 15 ° C lower than the melting point of the film formed in the step (e), more preferably -3 to 13 ° C. low temperature, more preferably -2 ° C. to 10 ° C. low temperature.

The ratio (MD elastic modulus / TD elastic modulus) of MD elastic modulus and TD elastic modulus of a polyolefin microporous membrane is 1.0-2.5, Preferably it is 1.0-2.4. If the ratio of elastic modulus is 1.0 or more, MD deformation will not be caused easily at the time of winding. When the ratio of elasticity modulus is 2.5 or less, it becomes suppressed that only MD orientation becomes strong, and the winding shift at the time of slit is suppressed and slit property becomes favorable.

The maximum pore size of the polyolefin microporous membrane is 0.10 to 0.25 µm, preferably 0.12 to 0.23 µm, more preferably 0.12 to 0.21 µm, particularly preferably 0.12 to 0.20 µm. If the maximum pore size is larger than 0.10 mu m, the permeability is excellent. If the maximum pore size is smaller than 0.25 mu m, the insulation performance is increased. In addition, when hot-pressed, in order to maintain high insulation performance even at high temperature, the microporous membrane is required to have a high withstand voltage when compressed. Since the breakdown voltage and the maximum pore diameter are deeply related, if the pore size is too large, the breakdown voltage of the microporous membrane becomes low, and it becomes difficult to maintain sufficient insulation during compression. For this reason, it is necessary for the microporous membrane to have the pore size which makes high permeability and high insulation performance compatible with the slit property improvement side.

The 120 degreeC thermal contraction rate of the polyolefin microporous membrane is 5% or less in both a longitudinal direction and the width direction, More preferably, it is 4.5% or less in both a longitudinal direction and the width direction. It is preferable to set the thermal contraction rate at 120 ° C. in the above range in addition to the aspect of improving the slit property, and also in view of the safety of the battery at high temperatures.

The porosity of the polyolefin microporous membrane is preferably 40 to 70%, more preferably 40 to 65%, still more preferably 40 to 60%. When the porosity is 40% or more, the transmission performance tends to be excellent, and when it is 70% or less, the mechanical strength is excellent and the winding property at the time of slit tends to be good.

Since the air permeability of the polyolefin microporous membrane tends to improve ion permeability, it is preferably 10 to 220 seconds / 100 cc, more preferably 10 to 200 seconds / 100 cc, still more preferably 10 to 180. Second / 100 cc, still more preferably 10 to 150 seconds / 100 cc, particularly preferably 10 to 150 seconds / 100 cc. If the air permeability is 10 seconds / 100 cc or more, the insulation performance as a separator tends to be high, and if it is 220 seconds / 100 cc or less, the permeation performance at the time of hot press does not become low well, and battery life tends to be long.

The tensile strength in the longitudinal direction of the polyolefin microporous membrane is preferably 50 to 500 MPa, more preferably 80 to 400 MPa, still more preferably 100 to 300 MPa. If the tensile strength is 50 MPa or more, the battery winding property tends to be improved, and if it is 500 MPa or less, the slit property tends to be good.

The tensile strength of the polyolefin microporous membrane in the width direction is preferably 10 to 200 MPa, more preferably 15 to 150 MPa, still more preferably 20 to 100 MPa. If the tensile strength is 10 MPa or more, the slit property tends to be good, and if it is 200 MPa or less, the battery winding property tends to be good.

The maximum shrinkage stress in the longitudinal direction of the polyolefin microporous membrane is preferably less than 0.1 N, more preferably 0.09 N or less, and still more preferably 0.08 N or less. It is preferable to set the maximum shrinkage stress in the above range from the viewpoint that the battery is not easily twisted (easily molded) during battery production.

The polyolefin microporous membrane in the present embodiment is preferably used as a separation membrane for separation of substances, selective permeation and the like, and an insulating material for an electrochemical reaction apparatus such as a nonaqueous electrolyte secondary battery, a fuel cell, a capacitor, and the like. For example, it is used as a separator of a nonaqueous electrolyte secondary battery used together with a positive electrode, a negative electrode, and an electrolyte solution. Especially preferably, it is used as a separator for nonaqueous electrolyte-type square secondary batteries from the adhesive side of a separator and an electrode.

A nonaqueous electrolyte secondary battery is provided with the separator which consists of a polyolefin microporous film, a positive electrode, a negative electrode, and electrolyte solution. The nonaqueous electrolyte secondary battery may be provided with the same members as the known nonaqueous electrolyte secondary battery, and may have the same structure, except that the separator for the nonaqueous electrolyte secondary battery of the present embodiment is provided as a separator. It can be produced by the same method.

In addition, the various parameters mentioned above are measured according to the measuring method in the Example mentioned later unless there is particular notice.

<Example>

Next, although an Example and a comparative example are given and this embodiment is demonstrated more concretely, this embodiment is not limited to a following example, unless the summary is exceeded. In addition, the physical property in an Example was measured by the following method.

(1) Viscosity Average Molecular Weight (Mv)

First, [] was measured. [] was based on ASTM-D4020 and was measured at a measurement temperature of 135 ° C. using decalin as a solvent.

From the obtained [η], the viscosity average molecular weight was computed by following Formula.

[η] = 6.77 × 10 ^-4 Mv ^{0 .67} (Chiang's equation)

(2) film thickness (μm)

It measured at room temperature 23 degreeC using KBM (trademark) which is a micro thickness measuring instrument by Toyo Seiki.

(3) Porosity (%)

A sample of 10 cm × 10 cm edges is cut out of the microporous membrane, its volume (cm ³ ) and mass (g) are obtained, and the membrane density is 0.95 (g / cm ³ ) and calculated using the following equation. It was.

Porosity = (1-mass / volume / 0.95) × 100

(4) air permeability (seconds / 100 cc)

In accordance with JIS P-8117, it measured by G-B2 (trademark) which is a Gurley type air permeometer by Toyo Seiki Co., Ltd. product.

(5) puncture strength (N)

The microporous membrane was fixed with the sample holder of 11.3 mm in diameter of the opening using the handy compression tester KES-G5 (trademark) manufactured by Kato Tech. Next, the puncturing test was carried out in a 25 ° C. atmosphere at a curvature radius of 0.5 mm and a puncturing speed of 2 mm / sec at the tip of the fixed microporous membrane to obtain a net puncture strength N as the maximum puncture load.

(6) Maximum pore diameter (㎛)

Based on ASTM E-128-61, it calculated by the bubble point (BP) in ethanol.

(7) Tensile breaking strength (MPa), tensile elongation at break (%), modulus of elasticity (MPa) and modulus of elasticity of MD and TD

In accordance with JIS K 7127, it measured about MD and TD samples (shape; width 10mm x length 100mm) using the Shimadzu Corporation Corporation tension test machine and Autograph AG-A type | brand (trademark). In addition, the sample used the thing which attached the cellophane tape (Nitto Denko Co., Ltd. make, brand name: N.29) to one side of the both ends (25 mm each) of the chuck | zipper distance to 50 mm. In addition, in order to prevent sample slip during the test, a 1 mm thick fluororubber was adhered to the inside of the chuck of the tensile tester.

Tensile elongation at break (%) was calculated by dividing the amount of elongation (mm) to failure by multiplying the distance between the chucks by 50 mm. Tensile breaking strength (MPa) was determined by dividing the strength at break by the sample cross-sectional area before the test.

The MD / TD strength ratio was obtained by dividing the MD tensile breaking strength by the TD tensile breaking strength.

Tensile modulus was evaluated by the inclination of elongation between 1 and 4%. The MD / TD elastic modulus ratio was obtained by dividing the MD elastic modulus by the TD elastic modulus.

In addition, measurements include temperature; 23 ± 2 ° C., spinal pressure 0.30 MPa, tensile rate; 200 mm / min.

(8) Thermal contraction rate (%)

The polyolefin microporous membrane was cut out 100 mm square so that each edge | side may become parallel to MD and TD, and it left for 1 hour in the oven which adjusted the temperature to 120 degreeC, and MD and TD thermal contraction rate were measured.

(9) TMA (maximum shrinkage stress (N))

With a thermomechanical analyzer (Shimazu TMA50), measurement was carried out under the conditions of a sample length of 10 mm, a sample width of 3 mm, an initial load of 1.0 g, and an elevated temperature of 10 ° C / min. Maximum shrinkage load (N) was calculated | required in the shrinkage stress curve with respect to each direction of MD and TD.

(10) slitability

As an index for evaluating the slit, Nishimura Seisakusho Co., Ltd. slitter TH4C was used and evaluated by the degree of winding shift when rewinding at a traveling speed of 100 m / min. In addition, the thing which shift | deviated 0.3 mm or more in the slit state was made into the winding shift. Slitability was evaluated according to the following criteria.

(Double-circle): A winding shift | offset | difference generation is 0 windings or less in 10 windings.

(Circle): Winding shift | offset | difference generation is 1 winding or less of 10 windings.

X: Winding shift | offset | difference generation is more than 2 windings in 10 windings.

(11) battery winding properties

(11-1) Preparation of Electrode (Positive Electrode, Negative Electrode)

Preparation of positive electrode: 92.2 mass% of lithium cobalt composite oxide LiCoO ₂ as an active substance, 2.3 mass% of flaky graphite and acetylene black as a conductive agent, and 3.2 mass% of polyvinylidene fluoride (PVDF) as a binder, respectively, N-methylpi The slurry was prepared by dispersing in rollidone (NMP). This slurry was apply | coated to the one side | surface of the aluminum foil with a thickness of 20 micrometers used as a positive electrode collector by a die coater, and it dried at 130 degreeC for 3 minutes, and then compression-molded by the roll press. At this time, the active material application amount of the positive electrode was 250 g / m ² , and the active material bulk density was 3.00 g / cm ³ . This was cut to about 40 mm in width and 60 cm in length to form a band.

Preparation of negative electrode: 96.9 mass% of artificial graphite as an active material, 1.4 mass% of ammonium salts of carboxymethylcellulose as a binder, and 1.7 mass% of styrene-butadiene copolymer latex were dispersed in purified water to prepare a slurry. This slurry was apply | coated to one side of the copper foil of thickness 12micrometer used as a negative electrode electrical power collector with a die coater, and it dried at 120 degreeC for 3 minutes, and then compression-molded by the roll press. At this time, the active material coating amount of the negative electrode was 106 g / m ² , and the active material bulk density was 1.55 g / cm ³ to obtain a high filling density. This was cut to about 40 mm in width and 60 cm in length to form a band.

(11-2) Evaluation

Winding property was evaluated using the winding machine (KMW-2BY) of Kaido Seisakusho Co., Ltd. The length of the electrode was 60 cm, the length of the polyolefin microporous membrane was 50 cm, and the winding speed was 45 mm / sec. (The electrode has a positive electrode and a negative electrode, a polyolefin microporous membrane, a positive electrode, a polyolefin microporous membrane, 4 sheets in a negative order) In the wound state, it was evaluated whether or not wrinkles occurred in the microporous membrane. The criterion of wrinkle generation was evaluated as "wrinkling" in which at least 1 mm of wrinkles occurred in the winding direction.

(Double-circle): Wrinkle generation is 0 windings or less in 10 windings.

(Circle): Wrinkle generation is 1 winding or less in 10 windings.

X: Wrinkle generation is more than 2 windings in 10 windings.

(12) Battery evaluation (oven test, short circuit of wound body, cycle characteristic)

(12-1) Preparation of Battery

Preparation of Non-Aqueous Electrolyte: LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) so as to have a concentration of 1.0 mol / liter as a solute.

Battery Assembly: The electrode plate laminate was manufactured by superposing a polyolefin microporous membrane, a belt-shaped positive electrode, and a belt-shaped negative electrode in the order of a belt-shaped negative electrode, a microporous membrane, a belt-shaped positive electrode, and a microporous membrane and vortexing 12 times. This electrode plate laminated body was pressed in flat form at 2 MPa for 30 second on 70 degreeC temperature conditions, and the battery winding body was obtained.

The manufactured battery winding body accommodated the battery winding body in the container made of aluminum, and connected the lead made of aluminum derived from the positive electrode current collector to the container wall, and the lead made of nickel derived from the negative electrode current collector to the container lid terminal portion. The lithium ion battery thus produced was 6.3 mm long, 30 mm wide, and 48 mm high. In addition, the battery capacity of the lithium ion battery was 600 mAh.

(12-2) Paragraph of winding body

A short circuit test was conducted by applying voltages of 100 V and 120 V to the assembled battery. Evaluation was performed based on the following criteria, and the short circuited battery was disassembled to identify the cause.

◎: No short circuit at 120V

○: No short circuit at 100 V.

X: Shorted at 100V.

(12-3) Cycle characteristics

Cycle characteristics (500 cycles): Evaluated as capacity retention rate (%). As the first charge / discharge of the assembled battery, first charge a constant current to a voltage of 4.2V at a current value of 1 / 6C, start to narrow the current value to maintain a constant voltage of 4.2V, and then perform a first charge for a total of 8 hours. The discharge was performed to a terminal voltage of 2.5V with a current of 1 / 6C. Subsequently, as cycle charge and discharge, (i) current amount 0.5C, upper limit voltage 4.2V, constant current constant voltage charging for a total of 8 hours, (ii) 10 minute rest, (iii) constant current discharge with current amount 0.5C, final voltage 2.5V, (iv) A total of 50 charge / discharge cycles were performed under a cycle condition of 10 minutes of rest. All of the above charging / discharging treatments were performed in an atmosphere of 20 ° C and 45 ° C, respectively. Thereafter, the capacity retention rate (%) was determined by 100 times the ratio of the discharge capacity at the 500th cycle to the discharge capacity at the first charge.

◎: 90% or more capacity retention

○: 80% or more capacity retention

X: 80% or less of capacity retention rate

(12-4) oven test

In order to perform the oven test of the assembled battery, the charged battery was heated up at 5 degree-C / min from room temperature to 150 degreeC, and was left to stand at 150 degreeC and 170 degreeC for 30 minutes. Evaluation was performed based on the following criteria.

(Double-circle): It did not fire at 170 degreeC.

(Circle): It did not fire at 150 degreeC.

X: It fired at 150 degreeC.

The material used by the following example and the comparative example is as follows.

(1) polymer 1

Asahi Kasei Chemicals Co., Ltd. product name "UH650"

Ultra high molecular weight polyethylene with a viscosity average molecular weight of 1 million

(2) polymer 2

Asahi Kasei Chemicals Co., Ltd. product name "SH800"

High density polyethylene with a viscosity average molecular weight of 250,000

(3) inorganic fine powder

Doso Silica Co., Ltd. brand name "Nipsil LP"

Powder silica

(4) plasticizer

Dioctyl phthalate

[Examples 1 to 12 and Comparative Examples 1 to 8]

The raw materials were mixed and granulated by the compounding (mass%) shown in Table 1, followed by melt kneading with a twin screw extruder equipped with a T die at the tip, and then extruded and rolled into rolls heated from both sides to form a sheet having a thickness of 110 μm. . A plasticizer and an inorganic fine powder were extracted from the molding to prepare a microporous membrane. Then, the sheet was stretched under the stretching conditions shown in Table 1 to obtain a polyolefin microporous membrane. Table 2 shows the physical properties of the obtained microporous membrane. Here, "PC" in Table 1 shows the mixing ratio of the polyolefin resin with respect to the total amount of a polyolefin resin, a plasticizer, and an inorganic fine powder.

From the results of Table 2, the polyolefin microporous membrane of this embodiment was excellent in slitability without the occurrence of the winding shift when it winds again at the traveling speed of 100 m / min.

Moreover, the oven test, the short circuit of the wound body, and cycle characteristics were all favorable, and had the outstanding characteristic as a separator for nonaqueous electrolyte type secondary batteries.

<Industrial availability>

According to the present invention, there is provided a polyolefin microporous membrane capable of reducing a defective rate in a step of slitting in a production line of a polyolefin microporous membrane.

Claims (6)

The ratio of longitudinal elastic modulus / width elastic modulus is 1.0 to 2.5, The maximum pore size is 0.10 μm to 0.25 μm, 120 degreeC thermal contraction rate is 5% or less in both a longitudinal direction and the width direction Polyolefin microporous membrane. The polyolefin microporous membrane of Claim 1 whose largest shrinkage stress in a longitudinal direction is less than 0.1N. The polyolefin microporous membrane of Claim 1 or 2 containing 5-90 mass% of ultra-high molecular weight polyethylenes whose viscosity average molecular weights are 700,000 or more. The separator for nonaqueous electrolyte type secondary batteries which consists of the polyolefin microporous film of any one of Claims 1-3. A nonaqueous electrolyte secondary battery comprising the separator according to claim 4, a positive electrode, a negative electrode, and an electrolyte solution. It is a manufacturing method of the polyolefin microporous film of any one of Claims 1-3, Comprising each process of following (a)-(f) (a) mixing at least a polyolefin resin, a plasticizer and an inorganic fine powder, (b) melt kneading the mixture obtained in step (a), (c) forming the kneaded product obtained in the step (b) into a sheet; (d) extracting the plasticizer and the inorganic fine powder from the sheet-like molded article, (e) biaxially stretching the sheet-like molded article, (f) Process of extending | stretching relaxation of the stretched sheet obtained by the process (e) in the width direction, The width direction draw rate in the said (e) process is 20-100% / sec, the width direction draw ratio is 1.1-4.0 times, and ratio of the longitudinal direction draw ratio / width direction draw ratio is 0.5-1.5, The width direction relaxation rate of the stretched sheet in the said (f) process is 3% or more The manufacturing method of a polyolefin microporous film.