JP2010053245A - Polyolefin microporous membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polyolefin microporous membrane having both of excellent membrane strength and permeation performances. <P>SOLUTION: The polyolefin microporous membrane is composed of polyolefin fibrils and has micropores communicating in the thickness direction, wherein the average fibril diameter of the fibrils is from 40 to 80 nm and an average pore diameter of the micropores is from 15 to 50 nm. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a polyolefin microporous membrane, and particularly to a polyolefin microporous membrane useful for battery separators, filters and the like.

  Various polyolefin microporous membranes have been proposed as films suitable for applications such as battery separators and filters (see Patent Document 1). The technology of Patent Document 1 is a polyolefin having a surface structure composed of fine gaps divided by microfibrils and a network in which the microfibrils are uniformly dispersed, and having a specific average microfibril diameter and a specific average microfibril gap distance. It is a microporous membrane. However, this proposed polyolefin microporous membrane is insufficient in membrane strength and permeation performance as a separator for a lithium ion secondary battery with high energy density.

  A porous film having a specific average pore diameter, specific surface area, and porosity has been proposed as a technique for improving the membrane strength and permeation performance of a nonaqueous electrolyte battery separator (see Patent Document 2). However, even this proposed porous film is insufficient in membrane strength and permeation performance as a separator for a lithium ion secondary battery which has recently been increased in energy density and required to be thinned. In addition, when these proposed polyolefin microporous membranes and porous films are applied to filters, membrane strength and permeation performance are insufficient.

  As described above, conventional techniques are insufficient to satisfy the demands of battery separators with high energy density and thinning, and filters with high strength and thinning, and improvements are desired. .

International Publication No. 01/67536 JP 2002-367590 A

  As described above, practical separators and filters that have excellent membrane strength and permeation performance have not been obtained. Therefore, an object of the present invention is to provide a polyolefin microporous membrane having excellent membrane strength and permeability.

  The inventor has examined the porous structure of the polyolefin microporous membrane in detail, and is a polyolefin microporous membrane composed of polyolefin fibrils and having pores communicating in the thickness direction, and the average fibril diameter of the fibrils, The present inventors have found that the above-mentioned problems can be solved if the polyolefin microporous membrane has an average pore diameter within a specific range. That is, the gist of the present invention is as follows.

(1) A polyolefin microporous membrane composed of polyolefin fibrils and having pores communicating in the thickness direction, the fibrils having an average fibril diameter of 40 to 80 nm and the pores having an average pore diameter of 15 to 50 nm A polyolefin microporous membrane characterized by the above.
(2) The polyolefin microporous membrane according to (1), wherein the porosity of the polyolefin microporous membrane is 30 to 60%.
(3) The polyolefin microporous membrane according to (1) or (2), wherein the curvature of the pore is 1.2 to 1.8.
(4) The polyolefin microporous membrane according to any one of (1) to (3), wherein the piercing strength of the polyolefin microporous membrane is 300 g or more.
(5) The polyolefin microporous membrane according to any one of (1) to (4), wherein the polyolefin microporous membrane has a thickness of 5 to 25 μm.
(6) The polyolefin microporous membrane according to any one of (1) to (5), wherein the polyolefin is polyethylene.

  According to the present invention, a practical polyolefin microporous membrane having excellent membrane strength and permeability can be obtained. When such a microporous polyolefin membrane is used, for example, as a battery separator, an excellent separator that can sufficiently meet the demands for high energy density and thinning can be obtained. Moreover, even when such a polyolefin microporous membrane of the present invention is used in, for example, a filter, an excellent filter that can sufficiently meet the demands for high strength and thinning can be obtained.

  The polyolefin microporous membrane of the present invention has a polyolefin fibril network structure as a main structure and has pores communicating in the thickness direction. In the present invention, assuming that the entire structure of the polyolefin microporous membrane is a columnar fibril, the average fibril diameter is calculated from the measurement results of the volume and surface area of the polyolefin. Further, in the present invention, it is assumed that the pore structure is all cylindrical, and the average pore diameter is calculated from the measurement results of the pore volume and the surface area.

[Specific surface area]
In the present invention, the surface area of the polyolefin microporous membrane is used to calculate the average fibril diameter and the average pore diameter. This surface area is calculated from the measurement result of the specific surface area.

The specific surface area can be determined, for example from the N ₂ adsorption amount obtained by the gas adsorption method described below using the BET formula. (Method according to JIS Z 8830)
1 / [W · {(P ₀ / P) −1}] = {(C−1) / (Wm · C)} (P / P ₀ ) (1 / (Wm · C)... BET equation P is the gas pressure of the adsorbate in the adsorption equilibrium, P ₀ is the saturated vapor pressure of the adsorbate in the adsorption equilibrium, W is the adsorption amount at the adsorption equilibrium pressure P, Wm is the monomolecular adsorption amount, and C is the BET constant. Specific surface area measurement method by gas adsorption method (hereinafter sometimes referred to as “BET method”)

A plot (BET plot) is linear when the x-axis is relative pressure P ₀ / P and the y-axis is 1 / [W · {(P ₀ / P) −1}]. When the slope in this plot is A and the intercept is B, the monomolecular adsorption amount Wm is as follows.
Wm = 1 / (A + B)

Next, the specific surface area S _s is obtained by the following formula.
Ss = (Wm · N · Acs · M) / w
Here, N is the Avogadro number, M is the molecular weight, Acs is the adsorption cross section, and w is the sample weight. For example, in the case of N ₂ , the adsorption cross-sectional area is 0.16 nm ² .

[Average fibril diameter and average pore diameter]
The average fibril diameter in the present invention is as follows assuming that the entire structure of the polyolefin microporous membrane is a cylindrical fibril, and the average pore diameter in the present invention is as follows assuming that the pore structure is all cylindrical. Calculate as follows.

Let V _{s1 be} the total volume of fibril fiber, and V _{s2 be the} total pore volume. When the diameter of the fibril is R _s1 , the diameter of the cylindrical hole is R _s2 , the total length of the fibril is L _s1 , and the total length of the cylindrical hole is L _s2 , the following formulas (i) to (v) are established.
Ss · Ws = πR _s1 · L _s1 = πR _s2 · L _s2 (i)
V _s1 = π (R _s1 / 2) ² · L _s1 (ii)
V _s2 = π (R _s2 / 2) ² · L _s2 (iii)
V _s2 = ε · (V _s1 + V _s2 ) (iv)
V _s1 = Ws / ds … (V)
Here, Ss is a specific surface area (m ² / g), Ws is a basis weight (g / m ² ), ε is a porosity (%), and ds is a true density (g / cm ³ ).
R _s1 (fibril diameter) and R _s2 (pore diameter) can be determined from the above equations (i) to (v).

  In the polyolefin microporous membrane of the present invention, the average fibril diameter needs to be 40 to 80 nm. It is difficult to form an average fibril diameter smaller than 40 nm. Further, when the average fibril diameter is larger than 80 nm, the permeability is remarkably lowered and the film strength is also lowered.

  In the polyolefin microporous membrane of the present invention, the average pore size of the pores needs to be 15 to 50 nm. That is, high permeability can be realized by setting the average pore size to a very small pore size of 15 to 50 nm. This high permeability is realized by the aforementioned average fibril diameter being sufficiently small and in the range of 40 to 80 nm. When the average pore diameter is smaller than 15 nm, the permeability is inhibited and an excellent permeability cannot be obtained. On the other hand, when the average pore diameter exceeds 50 nm, the curvature increases, and when used as a battery separator, the membrane resistance increases, the load characteristics increase, and the battery performance may deteriorate. In addition, when the average pore diameter exceeds 50 nm as described above, the film strength tends to decrease, which is not preferable.

[Porosity]
In the polyolefin microporous membrane of the present invention, the porosity (ε) is calculated from the basis weight (g / m ² ), true density (g / cm ³ ), and film thickness (μm) of the polyolefin microporous membrane according to the following formula. .
ε = {1-Ws / (ds · t)} × 100
Here, Ws is a basis weight (g / m ² ), ds is a true density (g / cm ³ ), and t is a film thickness (μm).

  The porosity is preferably 30 to 60%, and more preferably 40% to 60%. If the porosity is less than 30%, the permeability is undesirably lowered. On the other hand, if the porosity is higher than 60%, the mechanical strength becomes insufficient and the handling property is lowered, which is not preferable.

[Curvature]
In the polyolefin microporous membrane of the present invention, the curvature is a value obtained by averaging the value obtained by dividing the channel length by the film thickness in the pores of an arbitrary polyolefin microporous membrane. This curvature can be calculated by impregnating a microporous polyolefin membrane with an electrolyte and measuring the membrane resistance. Specifically, it is calculated by the following formula.
τ = {(Rm · ε) / (ρ · t)} ^(1/2)
Here, τ is the curvature, Rm is the membrane resistance (ohm · cm ² ), ε is the porosity (%), ρ is the specific resistance of the electrolyte (ohm · cm), and t is the film thickness (μm). is there. In the present invention, 1M LiBF ₄ propylene carbonate / ethylene carbonate (1/1 weight ratio) is used at 20 ° C. for the electrolytic solution. In this case, ρ is 2.663 × 10 ⁻² ohm · cm. This value is an index of permeability, which is 1 or more from the definition.

  The curvature of the polyolefin microporous membrane of the present invention is preferably 1.2 to 1.8. The polyolefin microporous membrane of the present invention has a small value of 1.2 to 1.8 and excellent permeability even though the average pore diameter is small. This is because the average fibril diameter is small and the film has a uniform network structure.

[Puncture strength]
The polyolefin microporous membrane of the present invention also has good mechanical properties. Puncture strength is an index of mechanical properties of battery separators and filters. The polyolefin microporous membrane of the present invention has a puncture strength of 300 g or more despite having excellent permeability and high porosity, and has sufficient strength as a battery separator or filter. Because of such high strength, the polyolefin microporous membrane of the present invention has high handling properties when manufacturing and processing battery separators and filters.

[Thickness]
Since the polyolefin microporous membrane of the present invention has sufficient mechanical strength, it can be suitably used even at a thickness of 5 to 25 μm, and more preferably 5 to 15 μm.

[Polyolefin]
The polyolefin microporous membrane according to the present invention has a structure in which a polyolefin is a main component and has a large number of micropores inside, and these micropores are connected to each other. Is a membrane that can pass through. Examples of the polyolefin include polyethylene, polypropylene, polymethylpentene, and combinations thereof. Polyethylene is particularly preferable, and as this polyethylene, high-density polyethylene, a mixture of high-density polyethylene and ultrahigh molecular weight polyethylene, or the like is suitable.

[Production method of polyolefin microporous membrane]
The polyolefin microporous membrane of the present invention can be produced, for example, by the method shown below. That is, (I) a polyolefin composition is dissolved in a solvent such as paraffin, liquid paraffin, paraffin oil, mineral oil, castor oil, tetralin, ethylene glycol, glycerin, decalin, toluene, xylene, diethyltriamine, ethyldiamine, dimethylsulfoxide, hexane, etc. A step of preparing a solution, (II) a step of extruding the solution from a die at a temperature not lower than the melting point of the polyolefin composition and not higher than a melting point + 60 ° C., and cooling to form a gel-like composition, (III) the gel-like composition It is manufactured by a series of steps including a step of stretching a product, (IV) a step of heat-setting the stretched gel-like composition, (V) a step of removing the solvent, and (VI) a step of annealing. Here, the stretching step is preferably biaxial stretching, and any method of sequential biaxial stretching, longitudinal stretching, and transverse stretching simultaneously performing longitudinal stretching and transverse stretching separately can be suitably used. is there.

  The polyolefin microporous membrane having an average fibril diameter of 40 to 80 nm and an average pore diameter of 15 to 50 nm as in the present invention uses, for example, a mixed solvent composed of liquid paraffin and decalin as the solvent, and the concentration of the polyolefin composition is 15 to 35 wt. %, The draw ratio is 50 to 100 times (longitudinal draw ratio × lateral draw ratio), the heat setting temperature is 110 to 140 ° C., and the annealing temperature is a temperature not higher than the heat setting temperature. However, the present invention is not limited to these production conditions, and any conditions can be used as long as a polyolefin microporous membrane having an average fibril diameter of 40 to 80 nm and an average pore diameter of 15 to 50 nm can be obtained. Needless to say, it can be adopted.

  In addition, when the density | concentration of a polyolefin composition is made low or a draw ratio is enlarged, there exists a tendency for an average fibril diameter to become small or an average pore diameter to become large. Further, when the concentration of the polyolefin composition is increased or the draw ratio is decreased, the average fibril diameter tends to increase or the average pore diameter tends to decrease. Also, when the heat setting temperature is increased, the average pore diameter may be increased or the average fibril diameter may be increased. Conversely, when the heat setting temperature is decreased, the average pore diameter may be decreased or the average fibril diameter may be decreased. There is. If the annealing temperature is set higher than the heat setting temperature, or if the annealing temperature is greatly deformed during annealing, the average fibril diameter may increase or the average pore diameter may increase.

For the polyolefin microporous membrane having a porosity of 30 to 60%, for example, the concentration of the polyolefin composition is 15 to 35% by weight, the heat setting temperature is 140 ° C. or less, and the annealing is set to a temperature lower than the heat setting temperature. Can be obtained at If the concentration of the polyolefin composition is 35% by weight or more, the heat setting temperature is higher than 140 ° C., or annealing is performed at a temperature higher than the heat setting temperature, the porosity may be lower than 30%. Further, if there is a large deformation during annealing, the porosity may be lower than 30%. If the concentration of the polyolefin composition is lower than 15% by weight, the porosity may be higher than 60%.
Since the polyolefin microporous film having a curvature of 1.2 to 1.8 can be realized with the structure of the present invention, it can be produced by the method as described above.

[Usage]
Due to the above physical properties, the polyolefin microporous membrane of the present invention has excellent properties as a separator for non-aqueous secondary batteries. When applied to a separator for a non-aqueous secondary battery, polyethylene is appropriate as a polyolefin material from the viewpoint of shutdown characteristics.
The polyolefin microporous membrane of the present invention is excellent in permeation performance and membrane strength when used in a filter.

  Hereinafter, various measurement methods in this example will be described.

[Unit weight]
A sample is cut into 10 cm × 10 cm and the weight is measured. By dividing this weight by the area, the basis weight which is the weight per 1 m ² was obtained.

[Film thickness]
It was determined by measuring 20 points with a contact-type film thickness meter (manufactured by Mitutoyo Co., Ltd.) and averaging them. Here, the contact terminal used was a cylindrical one having a bottom surface of 0.5 cm in diameter.

[Porosity]
The porosity ε was calculated from the following formula.
ε = {1-Ws / (ds · t)} × 100
Here, Ws is a basis weight (g / m ² ), ds is a true density (g / cm ³ ), and t is a film thickness (μm).

[Specific surface area]
The measurement was performed according to JIS K 8830. Using NOVA-1200 (manufactured by Yuasa Ionics Co., Ltd.), the BET equation was used for analysis and determination by the nitrogen gas adsorption method. The sample weight during measurement was 0.1 to 0.15 g. The analysis was carried out by a three-point method, and the specific surface area Ss (m ² / g) was determined from the BET plot.

[Average fibril diameter and average pore diameter]
Let V _{s1 be} the total volume of fibril fiber, and V _{s2 be the} total pore volume. When the diameter of the fibril is R _s1 , the diameter of the cylindrical hole is R _s2 , the total length of the fibril is L _s1 , and the total length of the cylindrical hole is L _s2 , the following formulas (i) to (v) are established.
Ss · Ws = πR _s1 · L _s1 = πR _s2 · L _s2 (i)
V _s1 = π (R _s1 / 2) ² · L _s1 (ii)
V _s2 = π (R _s2 / 2) ² · L _s2 (iii)
V _s2 = ε · (V _s1 + V _s2 ) (iv)
V _s1 = Ws / ds … (V)
Here, Ss is a specific surface area (m ² / g), Ws is a basis weight (g / m ² ), ε is a porosity (%), and ds is a specific gravity (g / cm ³ ).
R _s1 (fibril diameter) and R _s2 (pore diameter) can be determined from the above equations (i) to (v).

[Membrane resistance]
Cut the sample to a size of 2.6 cm × 2.0 cm. The polyethylene microporous membrane substrate cut out in a methanol solution in which 3% by weight of a nonionic surfactant (manufactured by Kao Corporation; Emulgen 210P) is dissolved is immersed and air-dried. A 20 μm thick aluminum foil is cut into 2.0 cm × 1.4 cm and a lead tab is attached. Two aluminum foils are prepared, and a sample cut between the aluminum foils is sandwiched so that the aluminum foils are not short-circuited. The sample is impregnated with 1M LiBF ₄ propylene carbonate / ethylene carbonate (1/1 weight ratio) as an electrolyte. This is sealed under reduced pressure in an aluminum laminate pack so that the tab comes out of the aluminum pack. Such cells are prepared so that there are one, two, and three separators in the aluminum foil, respectively. The cell is placed in a constant temperature bath at 20 ° C., and the resistance of the cell is measured by an AC impedance method at an amplitude of 10 mV and a frequency of 100 kHz. The measured resistance value of the cell is plotted against the number of separators, and this plot is linearly approximated to obtain the slope. The film resistance (ohm · cm ² ) per separator was determined by multiplying the inclination by 2.0 cm × 1.4 cm which is the electrode area.

[Gurley value]
The Gurley value (second / 100 cc) was measured according to JIS P8117.

[Curvature]
The curvature τ was calculated from the following equation.
τ = {(Rm · ε) / (ρ · t)} ^(1/2)
Here, Rm is the membrane resistance (ohm · cm ² ), ε is the porosity (%), ρ is the specific resistance of the electrolyte (ohm · cm), and t is the film thickness (μm). In the present invention, 1M LiBF ₄ propylene carbonate / ethylene carbonate (1/1 weight ratio) is used at 20 ° C. for the electrolytic solution. In this case, ρ is 2.663 × 10 ⁻² ohm · cm.

[Puncture strength]
Using a KES-G5 handy compression tester manufactured by Kato Tech Co., Ltd., a piercing test was conducted under the conditions of a radius of curvature of the needle tip of 0.5 mm and a piercing speed of 2 mm / sec, and the maximum piercing load was defined as the piercing strength. Here, the sample was fixed by sandwiching a silicon rubber packing in a metal frame (sample holder) having a hole of Φ11.3 mm.

[Prototype of non-aqueous secondary battery]
89.5 parts by weight of lithium cobalt oxide (LiCoO ₂ ; manufactured by Nippon Chemical Industry Co., Ltd.), 4.5 parts by weight of acetylene black (manufactured by Denki Kagaku Co., Ltd .; trade name Denka Black), 6 polyvinylidene fluoride (manufactured by Kureha Chemical Co., Ltd.) 6 These were kneaded using an N-methyl-2pyrrolidone solvent so as to be parts by weight to prepare a slurry. The obtained slurry was applied onto an aluminum foil having a thickness of 20 μm, dried and pressed to obtain a positive electrode having a thickness of 100 μm.

  87 parts by weight of mesophase carbon microbeads (MCMB: Osaka Gas Chemical Co., Ltd.) powder, 3 parts by weight of acetylene black (manufactured by Denki Kagaku Kogyo; trade name Denka Black), 10 parts by weight of polyvinylidene fluoride (manufactured by Kureha Chemical Co., Ltd.) Thus, these were knead | mixed using the N-methyl-2 pyrrolidone solvent, and the slurry was produced. The obtained slurry was applied onto a copper foil having a thickness of 18 μm, dried and pressed to obtain a negative electrode having a thickness of 90 μm.

The positive electrode and the negative electrode were opposed to each other through a separator. This was impregnated with an electrolytic solution and sealed in an exterior made of an aluminum laminate film to produce a non-aqueous secondary battery. Here, 1M LiPF ₆ ethylene carbonate / ethyl methyl carbonate (3/7 weight ratio) (manufactured by Kishida Chemical Co., Ltd.) was used as the electrolytic solution.
Here, this prototype battery has a positive electrode area of 2 × 1.4 cm ² , a negative electrode area of 2.2 × 1.6 cm ² , and a set capacity of 8 mAh (in the range of 4.2V-2.75V).

[Load characteristic evaluation]
Dischargeability evaluation was implemented using the battery produced by the above methods. First, 10 charge / discharge cycles of 1.6 mA, 4.2 V for 8 hours constant current / constant voltage charge, 1.6 mA, 2.75 V for constant current discharge were performed, and the discharge capacity obtained in the 10th cycle was calculated. It was set as the discharge capacity of this battery. Next, constant current / constant voltage charging is performed at 1.6 mA and 4.2 V for 8 hours, and constant current discharging is performed at 16 mA and 2.75 V. The capacity obtained at this time was divided by the discharge capacity of the battery at the 10th cycle, and the obtained value was used as an index of load characteristics.

[Example 1]
As polyethylene powder, GUR2126 (weight average molecular weight 41.50 million, melting point 141 ° C.) and GRX143 (weight average molecular weight 560,000, melting point 135 ° C.) manufactured by Ticona were used. Mixing liquid paraffin (manufactured by Matsumura Oil Research Co., Ltd .; Smoyl P-350P; boiling point 480 ° C.) and decalin so that GUR2126 and GURX143 are 1: 9 (weight ratio) and the polyethylene concentration is 30% by weight. A polyethylene solution was prepared by dissolving in a solvent. The composition of the polyethylene solution is polyethylene: liquid paraffin: decalin = 30: 45: 25 (weight ratio).

  This polyethylene solution was extruded from a die at 148 ° C. and cooled in a water bath to prepare a gel tape (base tape). The base tape was dried at 60 ° C. for 8 minutes and at 95 ° C. for 15 minutes, and the base tape was stretched by biaxial stretching in which longitudinal stretching and lateral stretching were sequentially performed. Here, the longitudinal stretching was 5.5 times, the stretching temperature was 90 ° C., the transverse stretching was 11.0 times the stretching ratio, and the stretching temperature was 105 ° C. After transverse stretching, heat setting was performed at 125 ° C. Next, this was immersed in a methylene chloride bath to extract liquid paraffin and decalin. Then, it dried at 50 degreeC and obtained the polyethylene microporous film by annealing at 120 degreeC. The obtained polyethylene microporous membrane had a structure in which fibrillar polyolefin was entangled in a network and constituted pores.

  Table 1 shows the measurement results of the characteristics (weight per unit area, film thickness, specific surface area, average fibril diameter, average pore diameter, porosity, curvature, puncture strength, Gurley value, and membrane resistance) of the obtained polyethylene microporous film. . In addition, Table 1 shows the measurement results of load characteristics obtained by making a prototype of a non-aqueous secondary battery using the obtained polyethylene microporous membrane.

[Example 2]
As polyethylene powder, GUR2126 (weight average molecular weight 41.50 million, melting point 141 ° C.) and GRX143 (weight average molecular weight 560,000, melting point 135 ° C.) manufactured by Ticona were used. Mixing liquid paraffin (manufactured by Matsumura Oil Research Co., Ltd .; Smoyl P-350P; boiling point 480 ° C.) and decalin so that GUR2126 and GURX143 become 7: 3 (weight ratio) and the polyethylene concentration becomes 17% by weight. A polyethylene solution was prepared by dissolving in a solvent. The composition of the polyethylene solution is polyethylene: liquid paraffin: decalin = 17: 51: 32 (weight ratio).

  This polyethylene solution was extruded from a die at 148 ° C. and cooled in a water bath to prepare a gel tape (base tape). The base tape was dried at 60 ° C. for 8 minutes and at 95 ° C. for 15 minutes, and the base tape was stretched by biaxial stretching in which longitudinal stretching and lateral stretching were sequentially performed. Here, the longitudinal stretching was 5.5 times, the stretching temperature was 90 ° C., the transverse stretching was 11.0 times the stretching ratio, and the stretching temperature was 105 ° C. After transverse stretching, heat setting was performed at 120 ° C. Next, this was immersed in a methylene chloride bath to extract liquid paraffin and decalin. Then, it dried at 50 degreeC and obtained the polyethylene microporous film by annealing at 120 degreeC. The obtained polyethylene microporous membrane had a structure in which fibrillar polyolefin was entangled in a network and constituted pores.

  Table 1 shows the measurement results of the characteristics (weight per unit area, film thickness, specific surface area, average fibril diameter, average pore diameter, porosity, curvature, puncture strength, Gurley value, and membrane resistance) of the obtained polyethylene microporous film. . In addition, Table 1 shows the measurement results of load characteristics obtained by making a prototype of a non-aqueous secondary battery using the obtained polyethylene microporous membrane.

[Example 3]
As polyethylene powder, GUR2126 (weight average molecular weight 41.50 million, melting point 141 ° C.) and GRX143 (weight average molecular weight 560,000, melting point 135 ° C.) manufactured by Ticona were used. Mixing liquid paraffin (Matsumura Oil Research Co., Ltd .; Smoyl P-350P; boiling point 480 ° C.) and decalin so that GUR2126 and GURX143 become 3: 7 (weight ratio) and the polyethylene concentration becomes 30% by weight. A polyethylene solution was prepared by dissolving in a solvent. The composition of the polyethylene solution is polyethylene: liquid paraffin: decalin = 30: 45: 25 (weight ratio).

  This polyethylene solution was extruded from a die at 148 ° C. and cooled in a water bath to prepare a gel tape (base tape). The base tape was dried at 60 ° C. for 8 minutes and at 95 ° C. for 15 minutes, and the base tape was stretched by biaxial stretching in which longitudinal stretching and lateral stretching were sequentially performed. Here, the longitudinal stretching was 5.5 times, the stretching temperature was 90 ° C., the transverse stretching was 11.0 times the stretching ratio, and the stretching temperature was 105 ° C. After transverse stretching, heat setting was performed at 135 ° C. Next, this was immersed in a methylene chloride bath to extract liquid paraffin and decalin. Then, it dried at 50 degreeC and obtained the polyethylene microporous film by annealing at 120 degreeC. The obtained polyethylene microporous membrane had a structure in which fibrillar polyolefin was entangled in a network and constituted pores.

  Table 1 shows the measurement results of the characteristics (weight per unit area, film thickness, specific surface area, average fibril diameter, average pore diameter, porosity, curvature, puncture strength, Gurley value, and membrane resistance) of the obtained polyethylene microporous film. . In addition, Table 1 shows the measurement results of load characteristics obtained by making a prototype of a non-aqueous secondary battery using the obtained polyethylene microporous membrane.

[Comparative Example 1]
UH210 and HD6720 manufactured by Mitsui Chemicals, Inc. were used as polyethylene powder. Liquid paraffin (manufactured by Matsumura Oil Research Co., Ltd .; Sumoyle P-350P; Boiling point 480 ° C.) with UH210 and HD6720 at 2: 8 (weight ratio: average molecular weight 740,000) and polyethylene concentration of 20% by weight. ) And decalin in a mixed solvent to prepare a polyethylene solution. The composition of the polyethylene solution is polyethylene: liquid paraffin: decalin = 20: 50: 30 (weight ratio).

  This polyethylene solution was extruded from a die at 148 ° C. and cooled in a water bath to prepare a gel tape (base tape). The base tape was dried at 60 ° C. for 8 minutes and at 95 ° C. for 15 minutes, and the base tape was stretched by biaxial stretching in which longitudinal stretching and lateral stretching were sequentially performed. Here, the longitudinal stretching was 5.0 times, the stretching temperature was 105 ° C., the transverse stretching was 13.0 times the stretching ratio, and the stretching temperature was 115 ° C. After transverse stretching, heat setting was performed at 140 ° C. Next, this was immersed in a methylene chloride bath to extract liquid paraffin and decalin. Thereafter, the film was dried at 50 ° C. and annealed at 125 ° C. and a relaxation rate of 9% to obtain a polyethylene microporous film. The obtained polyethylene microporous membrane had a structure in which fibrillar polyolefin was entangled in a network and constituted pores.

  Table 1 shows the measurement results of the characteristics (weight per unit area, film thickness, specific surface area, average fibril diameter, average pore diameter, porosity, curvature, puncture strength, Gurley value, and membrane resistance) of the obtained polyethylene microporous film. . In addition, Table 1 shows the measurement results of load characteristics obtained by making a prototype of a non-aqueous secondary battery using the obtained polyethylene microporous membrane.

[Comparative Example 2]
As polyethylene powder, GUR2126 (weight average molecular weight 41.50 million, melting point 141 ° C.) and GRX143 (weight average molecular weight 560,000, melting point 135 ° C.) manufactured by Ticona were used. Mixing liquid paraffin (Matsumura Oil Research Co., Ltd .; Smoyl P-350P; boiling point 480 ° C.) and decalin so that GUR2126 and GURX143 become 3: 7 (weight ratio) and the polyethylene concentration becomes 30% by weight. A polyethylene solution was prepared by dissolving in a solvent. The composition of the polyethylene solution is polyethylene: liquid paraffin: decalin = 30: 45: 25 (weight ratio).

  A slurry was prepared by adding and dispersing 0.2 times the silica powder (manufactured by Tokuyama: Tokuseal) to the polyethylene solution. This polyethylene solution was extruded from a die at 148 ° C. and cooled in a water bath to prepare a gel tape (base tape). The base tape was dried at 60 ° C. for 8 minutes and at 95 ° C. for 15 minutes, and the base tape was stretched by biaxial stretching in which longitudinal stretching and lateral stretching were sequentially performed. Here, the longitudinal stretching was 5.5 times, the stretching temperature was 90 ° C., the transverse stretching was 13.0 times the stretching ratio, and the stretching temperature was 105 ° C. After transverse stretching, heat setting was performed at 138 ° C. Next, this was immersed in a methylene chloride bath to extract liquid paraffin and decalin. Then, it dried at 50 degreeC, annealed at 120 degreeC, and also removed the silica in acidic aqueous solution, and obtained the polyethylene microporous film. The obtained polyethylene microporous membrane had a structure in which fibrillar polyolefin was entangled in a network and constituted pores.

  Table 1 shows the measurement results of the characteristics (weight per unit area, film thickness, specific surface area, average fibril diameter, average pore diameter, porosity, curvature, puncture strength, Gurley value, and membrane resistance) of the obtained polyethylene microporous film. . In addition, Table 1 shows the measurement results of load characteristics obtained by making a prototype of a non-aqueous secondary battery using the obtained polyethylene microporous membrane.

[Comparative Example 3]
As polyethylene powder, GUR2126 (weight average molecular weight 41.50 million, melting point 141 ° C.) and GRX143 (weight average molecular weight 560,000, melting point 135 ° C.) manufactured by Ticona were used. Mixing liquid paraffin (manufactured by Matsumura Oil Research Co., Ltd .; Smoyl P-350P; boiling point 480 ° C.) and decalin so that GUR2126 and GURX143 become 4: 6 (weight ratio) and the polyethylene concentration becomes 20% by weight. A polyethylene solution was prepared by dissolving in a solvent. The composition of the polyethylene solution is polyethylene: liquid paraffin: decalin = 20: 40: 40 (weight ratio).

  This polyethylene solution was extruded from a die at 148 ° C. and cooled in a water bath to prepare a gel tape (base tape). The base tape was dried at 60 ° C. for 8 minutes and at 95 ° C. for 15 minutes, and the base tape was stretched by biaxial stretching in which longitudinal stretching and lateral stretching were sequentially performed. Here, the longitudinal stretching was 5.0 times, the stretching temperature was 105 ° C., the transverse stretching was 13.0 times the stretching ratio, and the stretching temperature was 115 ° C. After transverse stretching, heat setting was performed at 135 ° C. Next, this was immersed in a methylene chloride bath to extract liquid paraffin and decalin. Thereafter, the film was dried at 50 ° C., and annealed at 125 ° C. and a relaxation rate of 10% to obtain a polyethylene microporous film. The obtained polyethylene microporous membrane had a structure in which fibrillar polyolefin was entangled in a network and constituted pores.

  Table 1 shows the measurement results of the characteristics (weight per unit area, film thickness, specific surface area, average fibril diameter, average pore diameter, porosity, curvature, puncture strength, Gurley value, and membrane resistance) of the obtained polyethylene microporous film. . In addition, Table 1 shows the measurement results of load characteristics obtained by making a prototype of a non-aqueous secondary battery using the obtained polyethylene microporous membrane.

[Comparative Example 4]
As polyethylene powder, GUR2126 (weight average molecular weight 41.50 million, melting point 141 ° C.) and GRX143 (weight average molecular weight 560,000, melting point 135 ° C.) manufactured by Ticona were used. Mixing liquid paraffin (Matsumura Oil Research Co., Ltd .; Smoyl P-350P; boiling point 480 ° C.) and decalin so that GUR2126 and GURX143 become 3: 7 (weight ratio) and the polyethylene concentration becomes 20% by weight. A polyethylene solution was prepared by dissolving in a solvent. The composition of the polyethylene solution is polyethylene: liquid paraffin: decalin = 20: 60: 20 (weight ratio).

  This polyethylene solution was extruded from a die at 148 ° C. and cooled in a water bath to prepare a gel tape (base tape). The base tape was dried at 60 ° C. for 8 minutes and at 95 ° C. for 15 minutes, and the base tape was stretched by biaxial stretching in which longitudinal stretching and lateral stretching were sequentially performed. Here, the longitudinal stretching was 13.5 times, the stretching temperature was 90 ° C., the transverse stretching was 20.0 times the stretching ratio, and the stretching temperature was 105 ° C. After transverse stretching, heat setting was performed at 125 ° C. Next, this was immersed in a methylene chloride bath to extract liquid paraffin and decalin. Then, it dried at 50 degreeC and obtained the polyethylene microporous film by annealing at 120 degreeC. The obtained polyethylene microporous membrane had a structure in which fibrillar polyolefins crossed like a network to form pores.

However, some breakage and the like were observed in the stretching process, and the moldability was very poor.
Table 1 shows the measurement results of the characteristics (weight per unit area, film thickness, specific surface area, average fibril diameter, average pore diameter, porosity, curvature, puncture strength, Gurley value, and membrane resistance) of the obtained polyethylene microporous film. . In addition, Table 1 shows the measurement results of load characteristics obtained by making a prototype of a non-aqueous secondary battery using the obtained polyethylene microporous membrane.

  As is clear from the results shown in Table 1, the polyolefin microporous membranes of Examples 1, 2, and 3 in which the average fibril diameter of the fibrillated polyolefin and the average pore diameter of the pores in the polyolefin microporous membrane are within the scope of the present invention. Has a high porosity, a low curvature, and a high puncture strength, so that the permeation performance and the membrane strength when used in separators and filters were good. In addition, since the membrane resistance is small and the load characteristics are large, the battery performance when used as a battery separator is good.

  On the other hand, Comparative Examples 1 and 2 in which the average fibril diameter of the fibrillated polyolefin is outside the range of the present invention (over 80 nm) and the average pore diameter of the pores in the polyolefin microporous membrane is out of the range of the present invention (over 50 nm). The polyolefin microporous film had a large curvature and a low puncture strength, so that the permeation performance and the film strength when used in a separator or filter were inferior. Moreover, since the membrane resistance is large and the load characteristics are small, the battery performance when used as a battery separator is inferior.

  In addition, the polyolefin microporous membrane of Comparative Example 3 in which the average fibril diameter of the fibrillated polyolefin is outside the range of the present invention (over 80 nm) has a low porosity and inferior permeation performance when used in a separator or filter. Met. Moreover, since the membrane resistance is large and the load characteristics are small, the battery performance when used as a battery separator is inferior.

  In addition, the polyolefin microporous membrane of Comparative Example 4 in which both the average fibril diameter and the average pore diameter are outside the scope of the present invention (average fibril diameter less than 40 nm, average pore diameter less than 15 nm) was difficult to mold, Since the pore diameter was too small, it was difficult to impregnate the electrolyte solution. As a result, the membrane resistance was high, the curvature calculated from it was high, and the load characteristics were extremely poor.

  The polyolefin microporous membrane of the present invention is useful for separators and filters. In particular, it can be effectively utilized as a technique for improving the characteristics of non-aqueous secondary batteries.

Claims (6)

  A polyolefin microporous membrane composed of polyolefin fibrils and having pores communicating in the thickness direction, wherein the fibrils have an average fibril diameter of 40 to 80 nm, and the pores have an average pore diameter of 15 to 50 nm. A polyolefin microporous membrane.   The polyolefin microporous membrane according to claim 1, wherein the porosity of the microporous polyolefin membrane is 30 to 60%.   The polyolefin microporous membrane according to claim 1 or 2, wherein the pore has a curvature of 1.2 to 1.8.   The polyolefin microporous membrane according to any one of claims 1 to 3, wherein a puncture strength of the polyolefin microporous membrane is 300 g or more.   The polyolefin microporous membrane according to any one of claims 1 to 4, wherein the polyolefin microporous membrane has a thickness of 5 to 25 µm.   The polyolefin microporous membrane according to claim 1, wherein the polyolefin is polyethylene.