JP5572334B2 - Polyolefin microporous membrane - Google Patents

Description

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

  Polyolefin microporous membranes are widely used as separators for various substances, selective permeation separation membranes, and separators. Examples of applications include microfiltration membranes, separators for fuel cells, separators for capacitors, and functional materials. For example, a functional film base material for filling the pores into the holes and causing new functions to appear, battery separators, and the like. Among these uses, it is particularly suitably used as a separator for lithium ion batteries widely used in mobile devices such as notebook personal computers, mobile phones, and digital cameras. The reason is that the mechanical strength and insulation performance of the film are high.

  Patent Documents 1 and 2 disclose polyethylene microporous membranes having good permeability and high strength. Patent Document 3 discloses a polyolefin microporous membrane that suppresses thermal shrinkage. Patent Document 4 discloses a polyolefin microporous membrane having a narrow pore distribution and high strength.

JP 2002-194132 A Japanese Patent Laid-Open No. 10-258462 JP-A-9-012756 International Publication No. 2005/061599 Pamphlet

In recent years, with the increase in capacity of batteries, the lengths of winding electrodes and separators tend to be longer. In addition, high-speed production is often performed from the viewpoint of improving the productivity of the winding process.
Here, from the viewpoint of stably producing a polyolefin microporous membrane or a battery under a condition where the line speed is high, the polyolefin microporous membrane has a good slit property, It is desirable to have the characteristic that wrinkles do not easily occur.
However, all of the polyolefin microporous membranes described in Patent Documents 1 to 4 still have room for improvement from the viewpoint of slit property.

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

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have determined that the ratio of the elastic modulus in the length direction to the width direction, the maximum pore diameter, and the 120 ° C. heat shrinkage rate are adjusted to a specific range. The inventors have found that a porous membrane can solve the above-mentioned problems, and have completed the present invention.

That is, the present invention is as follows.
[1]
The ratio of the elastic modulus in the length direction / the elastic modulus in the width direction is 1.0 to 2.5,
The maximum pore size is 0.10 μm to 0.25 μm,
A polyolefin microporous membrane having a heat shrinkage of 120 ° C. of 5% or less in both the length direction and the width direction.
[2]
The polyolefin microporous membrane according to [1], wherein the maximum shrinkage stress in the length direction is less than 0.1 N.
[3]
The polyolefin microporous film according to [1] or [2], comprising 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 a non-aqueous electrolyte secondary battery comprising the polyolefin microporous membrane according to any one of [1] to [3].
[5]
[4] A nonaqueous electrolyte secondary battery comprising the separator according to [4], a positive electrode, a negative electrode, and an electrolytic solution.
[6]
[1] to a method for producing a polyolefin microporous membrane according to any one of [3], comprising the following steps (a) to (f):
(A) a step of mixing at least a polyolefin resin, a plasticizer, and an inorganic fine powder;
(B) a step of melt-kneading the mixture obtained in the step (a),
(C) a step of forming the kneaded material obtained in the step (b) into a sheet,
(D) a step of extracting the plasticizer and the inorganic fine powder from the sheet-like molded product,
(E) a step of biaxially stretching a sheet-like molded product,
(F) a step of stretching and relaxing the stretched sheet obtained in the step (e) in the width direction;
In the step (e), the width direction stretching speed is 20 to 100% / second, the width direction stretching ratio is 1.1 to 4.0 times, and the ratio of length direction stretching ratio / width direction stretching ratio is 0.5 to 1. .5,
A method for producing a polyolefin microporous membrane, wherein the stretched sheet in the step (f) has a widthwise relaxation rate of 3% or more.

  ADVANTAGE OF THE INVENTION According to this invention, the polyolefin microporous film which can reduce the defect rate in the process of performing a slit in the manufacturing line of a polyolefin microporous film, a processing line, etc. is provided.

  Hereinafter, a mode for carrying out the present invention (hereinafter abbreviated as “the present embodiment”) will be described in detail. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

  The polyolefin microporous membrane of the present embodiment (hereinafter sometimes simply referred to as “microporous membrane”) has a ratio of the elastic modulus in the length direction / the elastic modulus in the width direction of 1.0 to 2.5. The maximum pore diameter is 0.10 μm to 0.25 μm, and the 120 ° C. heat shrinkage ratio is 5% or less in both the length direction and the width direction.

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

  Examples of the polyolefin resin used in the present embodiment include polymers obtained by polymerizing monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene ( Homopolymers, copolymers and multistage polymers). These polymers can be used alone or in combination of two or more. Among these, a polyethylene resin is preferably used from the viewpoint of improving mechanical strength.

As examples of the polyolefin resin, for example, high density polyethylene such as density exceeds 0.94 g / cm ^3, density polyethylene in the range density of 0.93~0.94g / cm ^3, density of 0.93g Low density polyethylene lower than / cm ³ , linear low density polyethylene and the like. Among these, high-density polyethylene and medium-density polyethylene are preferably used from the viewpoint of increasing the film strength of the microporous film. They can be used alone or as a mixture. In addition, it is preferable to use polyethylene independently from a viewpoint of improving the permeability and mechanical strength of the microporous membrane.

From the viewpoint of improving the slit property of the microporous membrane, the microporous membrane is preferably an ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 700,000 or more (preferably 800,000 to 3,000,000), preferably 5 to 90% by mass, more preferably 10%. -90 mass%, More preferably, it contains 20-80 mass%.
Further, from the viewpoint of improving the mechanical strength of the microporous membrane, the microporous membrane preferably contains 5% by mass or more of ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 700,000 or more (preferably 800,000 to 3,000,000).
Furthermore, from the viewpoint of improving the moldability of the microporous membrane, the microporous membrane preferably contains 90% by mass or less of ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 700,000 or more (preferably 800,000 to 3,000,000).

On the other hand, from the viewpoint of improving the heat resistance of the microporous membrane, the polyolefin resin may include polypropylene.
Specific examples of such polypropylene include, for example, propylene homopolymer, ethylene-propylene random copolymer, and ethylene-propylene block copolymer. Among these, polypropylene homopolymer is preferably used.
In the case where a copolymer is used as polypropylene, the content of ethylene as a comonomer is 1.0% by mass or less from the viewpoint of not reducing the crystallinity of polypropylene and thus reducing the permeability of the microporous membrane. It is preferable.
Moreover, in all the polypropylenes used, the ethylene content as a comonomer is preferably 1 mol% or less, and all are preferably propylene homopolymers.
Furthermore, from the viewpoint of improving the moldability to a microporous membrane, the intrinsic viscosity [η] of polypropylene is preferably 1 to 25 dL / g, and more preferably 2 to 7 dL / g. Here, [η] is a value measured at a measurement temperature of 135 ° C. using decalin as a solvent based on ASTM-D4020.

  The proportion of polypropylene in the polyolefin resin is preferably 10% by mass or less, more preferably 1 to 10% by mass, still more preferably 1 to 8% by mass, and particularly preferably 1 to 6% by mass. . Setting the ratio to 1% by mass or more is preferable from the viewpoint of improving the heat resistance of the polyolefin microporous membrane. On the other hand, setting the ratio to 10% by mass or less is preferable from the viewpoint of realizing a microporous film having good permeability and high piercing strength.

  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. Among these, silica is preferably used from the viewpoint of dispersibility and ease of extraction.

  The polyolefin resin composition includes, as necessary, an antioxidant such as phenol, phosphorus, or sulfur; a metal soap such as calcium stearate or zinc stearate; an ultraviolet absorber, a light stabilizer, an antistatic agent, Various additives such as an antifogging agent, a coloring pigment, a lubricant, and an antiblocking agent may be mixed.

As a manufacturing method of the polyolefin microporous film in the present embodiment, for example, a manufacturing method including the following steps (a) to (f) can be used.
(A) a step of mixing at least a polyolefin resin, a plasticizer, and an inorganic fine powder;
(B) a step of melt-kneading the mixture obtained in the step (a),
(C) a step of forming the kneaded material obtained in the step (b) into a sheet,
(D) a step of extracting the plasticizer and the inorganic fine powder from the sheet-like molded product,
(E) a step of biaxially stretching a sheet-like molded product,
(F) A step of stretching and relaxing the stretched sheet obtained in the step (e) in the width direction.

  Step (a) is a step of mixing at least a polyolefin resin, a plasticizer, and inorganic fine powder. (A) A process can be performed by the normal mixing method using compounding machines, such as a Henschel mixer, a V-blender, a pro shear mixer, a ribbon blender, for example. It is also possible to perform granulation by mixing.

  The plasticizer is preferably a non-volatile solvent capable of forming a uniform solution above the melting point of the polyolefin resin when mixed with the 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 ester and glyceric acid ester, and phosphoric acid esters such as trioctyl phosphate. , Liquid paraffin, solid wax, mineral oil and the like. Among these, phthalic acid esters are preferable from the viewpoints of compatibility with polyethylene, low air permeability, and low bubble point. These plasticizers may be used alone or as a mixture.

  The proportion of the plasticizer in the mixture is preferably 30% by mass or more, more preferably 40% by mass or more, and the upper limit is preferably 80% by mass or less, more preferably 70% by mass or less. is there. Setting the ratio to 80% by mass or less is preferable from the viewpoint of maintaining high melt tension during melt molding and ensuring moldability. On the other hand, setting the ratio to 30% by mass or more is preferable from the viewpoint of securing moldability and efficiently extending the lamellar crystal in the polyolefin crystal region. Here, the fact that the lamellar crystal is efficiently stretched means that the polyolefin chain is efficiently stretched without causing the polyolefin chain to be broken, and the formation of a uniform and fine pore structure and the strength of the polyolefin microporous membrane And it can contribute to the improvement of crystallinity.

  Moreover, 10-50 mass% is preferable, and, as for the mixing ratio of the polyolefin resin with respect to the total amount of the said polyolefin resin, a plasticizer, and inorganic fine powder, 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 50% by mass from the viewpoint of improving the film forming property during extrusion molding and the permeability of the microporous membrane. % Or less is preferable.

  Furthermore, the proportion of the inorganic fine powder in the total amount of the polyolefin resin, the plasticizer, and the inorganic fine powder is preferably 3% by mass or more, more preferably 5% by mass or more, and the upper limit is preferably It is 60 mass% or less, More preferably, it is 50 mass% or less. Setting the ratio to 3% by mass or more is preferable from the viewpoint of imparting excellent permeability and stably forming a polyolefin microporous film. On the other hand, when the ratio is 60% by mass or less, the length direction of the polyolefin microporous membrane (the discharge direction of the resin during molding. Hereinafter, it may be abbreviated as “MD”) and the width direction (MD). In the direction substantially orthogonal to the following, which may be abbreviated as “TD” below). Moreover, it is preferable also from a viewpoint of improving the mechanical strength of the polyolefin microporous film.

The step (b) is a step of melt kneading the mixture obtained in the step (a). (B) A process can be performed with screw extruders, such as a single screw extruder and a twin screw extruder, a kneader, a Banbury mixer etc., for example after mixing with a Henschel mixer, a ribbon blender, a tumbler blender, etc. As a method of melt kneading, a method of melt kneading with an extruder capable of continuous operation is preferable. From the viewpoint of good kneadability, the extruder capable of continuous operation is preferably a twin screw extruder.
Furthermore, from the viewpoint of preventing deterioration of the resin due to heating during melt-kneading, the melt-kneading can be performed in an inert gas atmosphere such as nitrogen.

  (C) A process is a process of shape | molding the kneaded material obtained at the (b) process in a sheet form. The step (c) can be performed, for example, as a cooling method by bringing the kneaded material into direct 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 machine cooled with a refrigerant. Among these, the method in which the kneaded product is brought into contact with a roll or press machine cooled with a refrigerant is preferable in terms of excellent thickness control of the microporous membrane.

  (D) A process is a process of extracting a plasticizer and inorganic fine powder from a sheet-like molded product. In step (d), the plasticizer and the inorganic fine powder are extracted from the sheet-like molded product with a solvent. As the solvent used for the extraction of the plasticizer, 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, etc. are used. be able to. From the viewpoint of obtaining a microporous membrane having a high porosity, excellent adhesion to the electrode, and excellent permeability, it is preferable to extract the inorganic fine powder after extracting the plasticizer. As the solvent used for the extraction of the inorganic fine powder, an alkaline aqueous solution such as sodium hydroxide or potassium hydroxide can be used. In addition, when extracting 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 product. The TD stretching speed in the step (e) is preferably 20 to 100% / second. In addition to improving the slit property, from the viewpoint of improving the adhesion with the battery during hot pressing by making the surface roughness uniform, the TD stretching speed is preferably 20% / second or more, and the viewpoint of improving the permeability Therefore, it is preferable to set it to 100% / second or less. On the other hand, the MD stretching speed is preferably 10 to 4000% / second, more preferably 100 to 3000% / second. MD stretching speed is preferably 10% / second or more from the viewpoint of improving mechanical strength in addition to improving slit property, and 4000% / second or less from the viewpoint of improving film resistance. Is preferred.

  Moreover, as TD draw ratio in (e) process, Preferably it is 1.1 times or more, More preferably, it is 1.2 to 4.0 times. From the viewpoint of improving battery winding property, improving slit property, or reducing shrinkage in the film width direction after slitting, the TD stretch ratio may be 1.1 times or more and the polymer may be oriented to TD. preferable. From the viewpoint 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 times to 8.0 times, more preferably 2.0 to 7.0 times from the viewpoint of improving mechanical strength and film breakability. From the viewpoint of improving the mechanical strength, it is preferable to set the MD stretch ratio to 1.5 times or more, and from the viewpoint of improving the film resistance during hot pressing, the MD stretch ratio should be 8.0 times or less. Is preferred.

  The ratio of MD stretch ratio and TD stretch ratio (MD stretch ratio / TD stretch ratio) is preferably 0.5 to 1.5, more preferably 0.5 to 1.3. From the viewpoint of improving mechanical strength and battery winding property, the ratio of the draw ratio is preferably 0.5 or more, and from the viewpoint of improving film resistance during hot pressing, the ratio of the draw ratio is It is preferable to make it 1.5 or less. Moreover, since the pores of the polyolefin microporous membrane are isotropic, it is also preferable from the viewpoint of stabilizing the slit property (suppressing winding deviation). Further, the isotropic shape of the holes improves the ion permeability in the lithium battery and tends to improve the cycle characteristics.

  The step (e) can be appropriately performed at any timing before and after the step (d).

  Step (f) is a step of stretching and relaxing the stretched sheet obtained in step (e) in the width direction. The TD relaxation rate in the step (f) is preferably 3 to 50%, more preferably 5 to 40%. From the viewpoint of suppressing thermal shrinkage of the polyolefin microporous membrane and the stability at the time of slitting, the TD relaxation rate is preferably 3% or more. From the viewpoint of reducing wrinkling 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) × 100 = relaxation rate (%).

  Setting the TD stretching speed, the TD stretching ratio, the ratio of MD stretching ratio / TD stretching ratio, and the relaxation rate during TD stretching within the above ranges are battery winding properties, slitting properties, mechanical strength of the film, and film forming properties. It is preferable from the viewpoint of improvement.

  In addition, from the viewpoint of thermal 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 membrane formed in the step (e). The temperature is preferably −3 to 13 ° C., more preferably −2 to 10 ° C.

  The ratio of MD elastic modulus to TD elastic modulus (MD elastic modulus / TD elastic modulus) of the microporous membrane made of polyolefin is 1.0 to 2.5, preferably 1.0 to 2.4. When the elastic modulus ratio is 1.0 or more, MD deformation is difficult to occur during winding. When the elastic modulus ratio is 2.5 or less, only the MD orientation can be prevented from becoming strong, so that the winding deviation at the time of slitting is suppressed and the slit property is improved.

  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, and particularly preferably 0.12 to 0.20 μm. It is. When the maximum pore diameter is larger than 0.10 μm, the transmission performance is excellent, and when the maximum pore diameter is smaller than 0.25 μm, the insulation performance is improved. In order to maintain high insulation performance even at high temperatures when hot pressed, the microporous membrane is required to have a high withstand voltage when compressed. The withstand voltage and the maximum pore diameter are deeply related. If the pore diameter is too large, the withstand voltage of the microporous membrane is lowered, and it is difficult to maintain sufficient insulation during compression. Therefore, the microporous membrane needs to have a pore diameter that achieves both high permeability and high insulation performance, in addition to the improvement of slit property.

  The 120 ° C. heat shrinkage rate of the polyolefin microporous membrane is 5% or less in both the length direction and the width direction, and more preferably 4.5% or less in both the length direction and the width direction. Although it does not specifically limit as a minimum, Preferably it is 0.1% or more. Setting the 120 ° C. heat shrinkage rate within the above range is also suitable from the viewpoint of battery safety at high temperatures in addition to the improvement of slit property.

  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 the porosity is 70% or less, the mechanical strength is excellent and the winding property at the time of slitting 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, and even more preferably 10 to 10 seconds. 180 seconds / 100 cc, even more preferably 10-150 seconds / 100 cc, and particularly preferably 10-150 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 permeability performance during hot pressing tends to be low and the battery life tends to be long. It is in.

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

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

  The maximum shrinkage stress in the length direction of the polyolefin microporous membrane is preferably less than 0.1 N, more preferably 0.09 N or less, and even more preferably 0.08 N or less. Setting the maximum shrinkage stress within the above range is preferable from the viewpoint of making the battery difficult to scramble (easily formed).

  The polyolefin microporous membrane in the present embodiment is preferably used as a separation membrane for separation of substances, selective permeation, etc., and as a separator for electrochemical reaction devices such as non-aqueous electrolyte secondary batteries, fuel cells, capacitors, etc. More preferably, it is used as a separator of a non-aqueous electrolyte secondary battery used together with a positive electrode, a negative electrode, and an electrolytic solution. Particularly preferably, it is used as a separator for a non-aqueous electrolyte-based prismatic secondary battery from the viewpoint of adhesion between the separator and the electrode.

  The non-aqueous electrolyte secondary battery includes a separator made of a polyolefin microporous membrane, a positive electrode, a negative electrode, and an electrolytic solution. This non-aqueous electrolyte secondary battery includes the same members as the known non-aqueous electrolyte secondary battery, except that the separator for the non-aqueous electrolyte secondary battery of the present embodiment is used as a separator. As long as they have the same structure and can be manufactured by the same method.

  The various parameters described above are measured according to the measurement methods in the examples described later unless otherwise specified.

  Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist. In addition, the physical property in an Example was measured with the following method.

(1) Viscosity average molecular weight (Mv)
First, [η] was measured. [Η] was measured at a measurement temperature of 135 ° C. using decalin as a solvent based on ASTM-D4020.
The viscosity average molecular weight was calculated from the obtained [η] by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67 (Chiang's formula)

(2) Film thickness (μm)
The measurement was performed at a room temperature of 23 ° C. using a micro thickness measuring instrument manufactured by Toyo Seiki, KBM (trademark).

(3) Porosity (%)
A sample of 10 cm × 10 cm square was cut from the microporous membrane, its volume (cm ³ ) and mass (g) were determined, and the film density was calculated to be 0.95 (g / cm ³ ) using the following formula.
Porosity = (1−mass / volume / 0.95) × 100

(4) Air permeability (sec / 100cc)
Based on JIS P-8117, it measured with the Gurley type air permeability meter by the Toyo Seiki Co., Ltd. and G-B2 (trademark).

(5) Puncture strength (N)
Using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech, the microporous membrane was fixed with a sample holder having a diameter of 11.3 mm at the opening. Next, the center of the fixed microporous membrane is subjected to a piercing test in a 25 ° C. atmosphere at a needle radius of curvature of 0.5 mm and a piercing speed of 2 mm / sec. (N) was obtained.

(6) Maximum pore size (μm)
Based on ASTM E-128-61, it was calculated by bubble point (BP) in ethanol.

(7) Tensile breaking strength (MPa), tensile breaking elongation (%), modulus of elasticity (MPa), modulus of elasticity ratio of MD and TD Based on JIS K7127, Shimadzu Corporation tensile tester, Autograph AG-A type (Trademark) was used to measure MD and TD samples (shape; width 10 mm × length 100 mm). Moreover, the sample used the thing which stuck the cellophane tape (Nitto Denko Packaging System Co., Ltd. make, brand name: N.29) on the single side | surface of the both ends (25 mm each) of the sample for the distance between chuck | zippers to 50 mm. Furthermore, in order to prevent sample slippage during the test, a fluororubber having a thickness of 1 mm was attached to the inside of the chuck of the tensile tester.
The tensile elongation at break (%) was determined by dividing the amount of elongation (mm) up to fracture by the distance between chucks (50 mm) and multiplying by 100. The tensile strength at break (MPa) was determined by dividing the strength at break by the cross-sectional area of the sample before the test.
The MD / TD strength ratio was obtained by dividing the MD tensile strength by the TD tensile strength.
The tensile elastic modulus was evaluated with an inclination between 1 to 4% in elongation. The MD / TD elastic modulus ratio was obtained by dividing the MD elastic modulus by the TD elastic modulus.
The measurement was performed at a temperature of 23 ± 2 ° C., a chuck pressure of 0.30 MPa, and a tensile speed of 200 mm / min.

(8) Thermal shrinkage (%)
A polyolefin microporous membrane was cut into a 100 mm square so that each side was parallel to MD and TD, and allowed to stand in an oven adjusted to a temperature of 120 ° C. for 1 hour, and then the MD and TD thermal contraction rates were measured.

(9) TMA (maximum shrinkage stress (N))
Measured with a thermomechanical analyzer (Shimadzu TMA50) under the conditions of a sample length of 10 mm, a sample width of 3 mm, an initial load of 1.0 g, and a temperature rising temperature of 10 ° C./min. The maximum shrinkage load (N) was determined in the shrinkage stress curve for each direction of MD and TD.

(10) Slit property As an index for evaluating the slit property, a slitter TH4C manufactured by Nishimura Seisakusho Co., Ltd. was used, and the evaluation was made based on the degree of winding deviation when rewinding at a running speed of 100 m / min. In addition, the thing which 0.3 mm or more of deviation produced in the slit state was made into winding deviation. The slit property was evaluated according to the following criteria.
A: Winding deviation is 0 or less in 10 volumes.
○: Winding deviation is 1 volume or less in 10 volumes.
X: Winding generation | occurrence | production is 2 or more volumes in 10 volumes.

(11) Battery winding property (11-1) Production of electrode (positive electrode, negative electrode) Production of positive electrode: 92.2% by mass of lithium cobalt composite oxide LiCoO ₂ as active material, flake graphite and acetylene as conductive agent A slurry was prepared by dispersing 2.3% by mass of black in each case and 3.2% by mass of polyvinylidene fluoride (PVDF) as a binder in N-methylpyrrolidone (NMP). This slurry was applied to one side of a 20 μm thick aluminum foil serving as a positive electrode current collector with a die coater, dried at 130 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material coating 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 a width of about 40 mm and a length of 60 cm to form a strip.
Production of negative electrode: 96.9% by mass of artificial graphite as an active material, 1.4% by mass of ammonium salt of carboxymethylcellulose and 1.7% by mass of styrene-butadiene copolymer latex as a binder were dispersed in purified water to prepare a slurry. did. This slurry was applied to one side of a 12 μm thick copper foil serving as a negative electrode current collector with a die coater, dried at 120 ° C. for 3 minutes, and then compression molded with a 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, which was a high packing density. This was cut to a width of about 40 mm and a length of 60 cm to form a strip.

(11-2) Evaluation The winding property was evaluated using a winding machine (KMW-2BY) manufactured by Minato Manufacturing 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 / second. (The electrode has a positive electrode and a negative electrode. Four layers of a polyolefin microporous membrane, a positive electrode, a polyolefin microporous membrane, and a negative electrode were stacked and wound in this order). It was evaluated whether or not wrinkles were generated in the microporous membrane in the wound state. The standard for wrinkle generation was evaluated as “wrinkle generation” in which one or more wrinkles of 1 mm or more occurred in the winding direction.
A: Wrinkle generation is 0 or less of 10 volumes.
○: Wrinkle generation is 1 volume or less in 10 volumes.
X: Wrinkle generation is 2 or more of 10 volumes.

(12) Battery evaluation (oven test, winding short circuit, cycle characteristics)
(12-1) Preparation of battery Preparation of non-aqueous electrolyte: LiPF ₆ as a solute dissolved in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) to a concentration of 1.0 mol / liter Prepared.
Battery assembly: A microporous membrane made of polyolefin, a strip-like positive electrode, and a strip-like negative electrode were laminated in the order of the strip-like negative electrode, microporous membrane, strip-like positive electrode, and microporous membrane, and wound in a spiral shape 12 times to produce an electrode plate laminate. . This electrode plate laminate was pressed into a flat plate shape at 2 MPa for 30 seconds under a temperature condition of 70 ° C. to obtain a battery wound body.
The produced battery winding body is housed in an aluminum container, the aluminum lead led out from the positive electrode current collector is placed on the container wall, and the nickel lead led out from the negative electrode current collector is placed on the container lid terminal portion. Connected to. The lithium ion battery thus produced was 6.3 mm in length (thickness), 30 mm in width, and 48 mm in height. The battery capacity of the lithium ion battery was 600 mAh.

(12-2) Short-circuiting of wound body A short-circuit test was performed by applying voltages of 100 V and 120 V to the assembled battery. Evaluation was performed based on the following criteria, and the shorted battery was disassembled to confirm the cause.
(Double-circle): It did not short-circuit at 120V.
○: No short circuit occurred at 100V.
X: Short-circuited at 100V.

(12-3) Cycle characteristics Cycle characteristics (500 cycles): Evaluated as capacity retention rate (%). As the initial charge / discharge of the assembled battery, first the constant current is charged to a voltage of 4.2V with a current value of 1 / 6C, and then the current value is started to be reduced so that the constant voltage of 4.2V is maintained. Next, the battery was discharged with a current of 1 / 6C to a final voltage of 2.5V. Subsequently, as cycle charge / discharge, (i) current amount 0.5C, upper limit voltage 4.2V, constant current constant voltage charge for 8 hours in total, (ii) 10 minute rest, (iii) current amount 0.5C, termination Charging / discharging was carried out 50 times for convenience under a constant current discharge with a voltage of 2.5 V and (iv) a cycle condition of 10 minutes of rest. All the above charge / discharge treatments were carried out in an atmosphere of 20 ° C. and 45 ° C., respectively. Thereafter, the ratio of the discharge capacity at the 500th cycle to the discharge capacity at the initial charge was multiplied by 100 to obtain the capacity retention rate (%).
◎: Capacity maintenance ratio 90% or more ○: Capacity maintenance ratio 80% or more ×: Capacity maintenance ratio less than 80%

(12-4) Oven Test To perform an oven test on the assembled battery, the battery after charging was heated from room temperature to 150 ° C. or 160 ° C. at a rate of 5 ° C./min and left at 150 ° C. and 160 ° C. for 30 minutes. . Evaluation was based on the following criteria.
(Double-circle): It did not ignite at 160 degreeC.
○: No ignition at 150 ° C.
X: Fired at 150 ° C.

The materials used in the following examples and comparative examples are as follows.
(1) Polymer 1
Product name "Sunfine UH650" manufactured by Asahi Kasei Chemicals
Viscosity average molecular weight 1 million ultra high molecular weight polyethylene (2) polymer 2
Product name "Sunfine SH800" manufactured by Asahi Kasei Chemicals
High-density polyethylene with a viscosity average molecular weight of 250,000 (3) Inorganic fine powder Product name “Nipsil LP” manufactured by Tosoh Silica
Powdered silica (4) Plasticizer Dioctyl phthalate

[Examples 1-12, Comparative Examples 1-8]
After mixing and granulating the raw materials with the composition shown in Table 1 (% by mass), the mixture was melt-kneaded in a twin-screw extruder equipped with a T-die at the tip, extruded, rolled with rolls heated from both sides, Molded into a 110 μm sheet. A plasticizer and an inorganic fine powder were extracted from the molded product to prepare a microporous film. Thereafter, the sheet was stretched under the stretching conditions described 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 represents the mixing ratio of the polyolefin resin to the total amount of the polyolefin resin, the plasticizer, and the inorganic fine powder.

From the results shown in Table 2, the polyolefin microporous membrane of the present embodiment was excellent in slitting properties with no occurrence of winding deviation when rewinding at a running speed of 100 m / min.
In addition, the oven test, the short circuit of the wound body, and the cycle characteristics were all good, and they had excellent characteristics as a separator for a non-aqueous electrolyte secondary battery.

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

Claims (6)

The ratio of the elastic modulus in the length direction / the elastic modulus in the width direction is 1.0 to 2.5,
The maximum pore size is 0.10 μm to 0.25 μm,
A polyolefin microporous membrane having a heat shrinkage of 120 ° C. of 5% or less in both the length direction and the width direction.   2. The polyolefin microporous membrane according to claim 1, wherein the maximum shrinkage stress in the length direction is less than 0.1N.   The polyolefin microporous film according to claim 1 or 2, comprising 5 to 90 mass% of ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 700,000 or more.   A separator for a non-aqueous electrolyte secondary battery comprising the polyolefin microporous membrane according to any one of claims 1 to 3.   A non-aqueous electrolyte secondary battery comprising the separator according to claim 4, a positive electrode, a negative electrode, and an electrolytic solution. A method for producing a polyolefin microporous membrane according to any one of claims 1 to 3, comprising the following steps (a) to (f): (a) at least a polyolefin resin, a plasticizer, and an inorganic fine powder A process of mixing and granulating,
(B) a step of melt-kneading the mixture obtained in the step (a),
(C) a step of forming the kneaded material obtained in the step (b) into a sheet,
(D) a step of extracting the plasticizer and the inorganic fine powder from the sheet-like molded product,
(E) a step of biaxially stretching a sheet-like molded product,
(F) a step of stretching and relaxing the stretched sheet obtained in the step (e) in the width direction;
In the step (e), the width direction stretching speed is 20 to 100% / second, the width direction stretching ratio is 1.1 to 4.0 times, and the ratio of length direction stretching ratio / width direction stretching ratio is 0.5 to 1. .5,
A method for producing a polyolefin microporous membrane, wherein the stretched sheet in the step (f) has a widthwise relaxation rate of 3% or more.