Multilayered microporous polyolefin film

Abstract

A polyolefin multilayer microporous membrane includes at least first microporous layers which form both surface layers of the membrane and at least a second microporous layer disposed between the both surface layers, wherein static friction coefficient of one of the surface layers of the polyolefin multilayer microporous membrane against another surface layer in a longitudinal direction (MD) is 1.1 or less, and wherein pore density calculated from an average pore radius measured by mercury porosimetry method and porosity, according to Formula (1) is 4 or more:
Pore density=(P/A.sup.3)×10.sup.4  (1)
wherein A represents the average pore radius (nm) measured by mercury porosimetry method and P represents the porosity (%).

Claims

1. A polyolefin multilayer microporous membrane comprising: at least first microporous layers which form both surface layers of the membrane and at least a second microporous layer disposed between said both surface layers, wherein static friction coefficient of one of said surface layers of said polyolefin multilayer microporous membrane against another surface layer in a longitudinal direction (MD) is 1.1 or less, and wherein pore density calculated from an average pore radius measured by mercury porosimetry method and porosity, according to Formula (1) is 4 or more:
pore density=(P/A.sup.3)×10.sup.4  (1) wherein A represents the average pore radius (nm) measured by mercury porosimetry method and P represents the porosity (%), the composition of the first microporous layer and the composition of the second microporous layer are different from each other, and the layer thickness ratio of the first microporous layer/the second microporous layer/the first microporous layer is from 0.05/0.9/0.05 to 0.3/0.4/0.3, said second microporous layer contains 35% by weight or more of ultra high molecular weight polyethylene having a Mw of 1×10.sup.6 or more, and said first microporous layers contain 5% to 30% by weight of polypropylene.

2. The polyolefin multilayer microporous membrane of claim 1, having a breakdown voltage of 1.4 kV/11.5 μm or more, and an air permeability of 250 seconds/100 cc or less.

3. The polyolefin multilayer microporous membrane of claim 2, having an electrochemical stability of 65 mAh or less.

4. The polyolefin multilayer microporous membrane of claim 2, wherein said first microporous layers contain 10% by weight or less of ultra high molecular weight polyethylene.

5. The polyolefin multilayer microporous membrane of claim 1, wherein said first microporous layers contain 10% by weight or less of ultra high molecular weight polyethylene.

6. The polyolefin multilayer microporous membrane of claim 1, wherein the first microporous layers contain 5% to 20% by weight of polypropylene.

Description

EXAMPLES

(1) The present invention will now be described in further detail with reference to the following Examples, but the present invention is not limited thereto. Methods for measuring the properties of the polyolefin microporous membranes in each of the Examples are as follows.

(2) (1) Average Membrane Thickness (μm)

(3) A sample piece of 100 mm square was cut out, and the membrane thickness of the four corners and the center of the sample were measured by a contact-type thickness gauge, and the measured values were averaged to obtain the average membrane thickness.

(4) (2) Air Permeability (sec/100 cm.sup.3)

(5) The air permeability was measured according to JIS P 8117.

(6) (3) Porosity (%)

(7) Weight per unit area W (g/cm.sup.2) was calculated from the mass of a 50 mm square sample. Then the porosity was calculated from the average density ρ (g/cm.sup.3) (the weighted average of the density of a single component) of the membrane components and the thickness d (cm), according to the formula below. For example, when the ratio of component 1 and component 2 are represented as y.sub.1 and y.sub.2 (y.sub.1+y.sub.2=1), respectively, and the density of each single component is represented as ρ1 and ρ2 respectively:
Average density ρ=y.sub.1×ρ.sub.1+y.sub.2×ρ.sub.2
Porosity P=(1−W/(d×ρ))×100(%)

(8) The porosity is preferably from 20 to 75%, more preferably from 25 to 60%.

(9) (4) Pin Puncture Strength (gf)

(10) A sample piece of 100 mm square was cut out, and the maximum load of the microporous membrane, when the four corners and the center of the membrane were punctured with a needle of 1 mm diameter with a sphere tip (curvature radius R: 0.5 mm) at a speed of 2 mm/second, was measured, and then the mean value was calculated.

(11) (5) Heat Shrinkage (%)

(12) The shrinkage rate was obtained by measuring the changes in lengths in the longitudinal direction (MD) and the transverse direction (TD) when the microporous membrane was exposed at 105° C. for 8 hours. The formula for the calculation is as follows.
Heat shrinkage (%)=(1−(length after heating/length before heating))×100
(6) Pore Density

(13) The pore density was obtained from the porosity P (%) as described in above (3) and the average pore radius A (nm) measured by mercury porosimetry method which will be described later, according to the formula below.
Pore density=(P/A.sup.3)×10.sup.4

(14) The average pore radius was calculated from Vp (cm.sup.3/g) which is the accumulated micropore volume measured by mercury porosimetry (apparatus employed: Poresizer Type 9320, manufactured by Micromeritic Corp.) and Sp (m.sup.2/g) which is the accumulated micropore specific surface area when assuming the micropores to be cylindrical, based on the micropore radius r obtained from Vp and each pressure, according to the formula below.
Average pore radius=2×Vp×1,000/Sp

(15) The average pore radius is preferably 57 nm or less, more preferably 53 nm or less.

(16) (7) Breakdown Voltage (kV)

(17) The breakdown voltage of the microporous membrane was evaluated according to the following method. A sample is cut into a width of 650 to 700 mm and a length of 600 mm or more, and spread on a copper plate electrode (650 mm×530 mm). The sample is then covered with a metal deposited film, and the surface of the film is stroked using a felt cloth to remove air and wrinkles A voltage is applied between the metal deposited film and the copper plate. In this case, after maintaining at 0.5 kV for 30 seconds, the voltage was elevated at a rate of 0.1 kV/10 seconds and maintained for 10 seconds at every 0.1 kV. This operation was repeated. During the operation, numbers of occurrences of short-circuit were counted at every 0.1 kV, and the measurement was terminated when the count exceeded 20. The voltage at that time point was determined as the breakdown voltage.

(18) This operation was repeated 5 times and the mean value of the measurements was converted to the value corresponding to 11.5 μm by proportionally distributing the thickness obtained in (1), to determine the breakdown voltage of the microporous membrane.

(19) (8) Electrochemical Stability (mAh)

(20) The electrochemical stability of the membrane in the present invention refers to the durability against oxidative degradation when used or stored under the conditions of high temperature (about 40 to 80° C.) in battery separator applications. The specific measurement method is as described below. LiCO2 was used as a cathode, graphite as an anode, and a solution in which LiPF6 was dissolved in a mixed solvent of ethylene carbonate and ethylmethyl carbonate (volume ratio: 4/6) to achieve a concentration of 1 mole/L was used as an electrolyte. Then the microporous membrane was immersed in the electrolyte and a battery was assembled. This battery was maintained under the charging conditions of 4.3 V at 60° C. for 21 days. The accumulated value of the charging current supplied to the battery during the charging was represented in the unit of mAh, wherein the voltage was maintained constant during charging. When this value is small, the battery capacity loss is small, and hence the membrane has an excellent electrochemical stability.

(21) (9) Weight Average Molecular Weight

(22) The molecular weights of the polyethylene and polypropylene are obtained by gel permeation chromatography (GPC). Measuring apparatus: PL-20 manufactured by Polymer Laboratories Co., Ltd. Column: Shodex UT806M manufactured by SHOWA DENKO K.K. Column temperature: 145° C. Solvent (mobile phase): o-dichlorobenzene Solvent flow rate: 1.0 mL/minute Sample preparation: To 10 mg of sample was added 5 mL of solvent for measurement, and the mixture was heated at 140 to 150° C. for approximately 20 minutes while stirring. Injection amount: 0.200 mL Detector: differential refractometer RI Standard sample: monodisperse polystyrene
(10) Static Friction Coefficient

(23) The static friction coefficient of one of the surface layers of the above mentioned polyolefin multilayer microporous membrane of the film against the other surface in MD was measured according to ASTM D1894. The measurement was performed at a test speed of 15 cm/min.

Example 1

(24) (1) Preparation of First Polyolefin Solution

(25) To one hundred parts by weight of a first polyolefin composition composed of 80% by weight of high density polyethylene (HDPE) having a weight average molecular weight (Mw) of 3.0×10.sup.5, and 20% by weight of polypropylene (PP) having a Mw of 1.2×10.sup.6, 0.2 parts by weight of tetrakis[methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate]methane as an antioxidant was dry blended. Into a strong kneading type twin-screw extruder (inner diameter 58 mm, L/D=52.5), 25% by weight of the resulting mixture was supplied, and 75% by weight of liquid paraffin [50 cSt (40° C.)] (resin concentration=25% by weight) was supplied via a side feeder of twin-screw extruder, followed by melt blending of the resultant under the conditions at 230° C., 250 rpm to obtain a first polyolefin solution.

(26) (2) Preparation of Second Polyolefin Solution

(27) To one hundred parts by weight of a second polyolefin composition composed of 40% by weight of ultra high molecular weight polyethylene (UHMWPE) having a Mw of 2.0×10.sup.6, and 60% by weight of the above described HDPE, 0.2 parts by weight of the antioxidant described above was dry-blended. Into the same twin-screw extruder as described above, 25% by weight of the resulting mixture was supplied, and 75% by weight of the same liquid paraffin as described above (resin concentration=25% by weight) was supplied via the side feeder of twin-screw extruder, followed by melt blending of the resultant conditions at 230° C., 250 rpm to obtain a second polyolefin solution.

(28) (3) Membrane Formation

(29) The first and the second polyolefin solutions were supplied to a T-die for trilayer from each of the twin-screw extruders, and extruded such that the layer thickness ratio of the first polyolefin solution/second polyolefin solution/first polyolefin solution achieved 0.1/0.8/0.1. The extruded molding was cooled while drawing with a chill roll controlled at a temperature of 30° C. to obtain a gel-like trilayer sheet. The resulting gel-like trilayer sheet was simultaneously biaxially stretched (first stretching) to 5 times the original length in both the longitudinal direction and traverse direction by a tenter stretching machine at 115° C., followed by heat setting at 100° C. Then the stretched gel-like molding sheet was immersed in a washing bath of methylene chloride, and washed to remove liquid paraffin. The washed membrane was air dried, and then stretched again (second stretching) to 1.4 times the original length in TD by the tenter stretching machine while heating at 125° C., followed by 86% relaxing in the same TD. The resultant was then subjected to heat setting treatment at 125° C. while maintaining in the tenter (the total amount of time of the second stretching, relaxing and heat setting treatment was 26 seconds) to obtain a polyolefin microporous membrane.

(30) The physical properties of the resulting microporous membrane are shown in Table 1.

Examples 2 to 9

(31) The same operation as in Example 1 was carried out except that the composition of the first polyolefin solution, the composition of the second polyolefin solution, the temperature of the first stretching, the temperature of the second stretching, and the relaxing ratio were changed, to obtain microporous membranes. The conditions and physical properties of these membranes are summarized in Table 1.

Example 10

(32) The same operation as in Example 1 was carried out except that 40% by weight of ultra high molecular weight polyethylene (UHMWPE) having a Mw of 2.0×10.sup.6 and 60% by weight of HDPE having a Mw of 3.5×10.sup.5 were used as the polyolefin composition for the second polyolefin solution, to obtain a microporous membrane.

Comparative Examples 1 to 4

(33) The same operation as in Example 1 was carried out except that the composition of the first polyolefin solution, the composition of the second polyolefin solution, the temperature of the first stretching, the temperature of the second stretching, and the relaxing ratio were changed, to obtain microporous membranes. The conditions and physical properties of these are summarized in Table 1.

(34) TABLE-US-00001 TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 First polyolefin solution UHMWPE 0 0 0 5 5 15 0 0 HDPE 80 80 60 95 90 78 80 100 PP 20 20 40 0 5 7 20 0 Resin 25 25 25 30 30 25 25 30 concentration Second polyolefin UHMWPE 40 40 40 40 40 40 40 40 solution HDPE (*1) 60 60 60 60 50 60 60 60 HDPE (*2) 0 0 0 0 0 0 0 0 PP 0 0 0 0 10 0 0 0 Resin 25 25 23 23 25 25 23 20 concentration Temperature of First stretching (° C.) 115 115 119 115 117 117 115 116 Temperature of Second stretching, 125 127 124 125 123 127 128 127 Relaxing and Heat setting treatment (° Second stretching magnification 1.4 1.4 1.4 1.4 1.1 1.4 1.6 1.4 in TD direction Relax ratio after second stretching (%) 86 86 96 96 0 97 0 0 Membrane thickness (μ) 11.5 11.5 11.0 11.5 11.0 11.5 11.5 11.0 Air permeability (sec/100 cc) 135 180 130 110 140 110 145 110 Porosity (%) 46 41 47 45 44 46 46 44 Pin puncture strength (gf) 330 350 300 330 330 330 370 270 Heat shrinkage ratio (%) MD 6.0 4.0 7.5 5.0 4.0 5.0 5.0 3.5 TD 2.0 1.0 8.5 4.5 3.5 4.5 5.0 5.0 Tensile strength MD 1,150 1,250 950 1,150 1,100 1,150 1,200 900 (kgf/cm.sup.2) TD 1,300 1,450 1,000 1,400 1,350 1,400 1,750 1,250 Static friction coefficient 0.90 0.90 0.85 1.00 0.95 0.95 1.00 0.95 Pore density 5.3 5.6 5.0 5.4 4.9 5.4 4.8 5.0 Breakdown Voltage (kV/11.5 μm) 1.7 1.8 1.5 1.6 1.6 1.6 1.6 1.5 Electrochemical stability (mAh) 50 55 35 85 65 60 50 95 Comparative Comparative Comparative Comparative Example 9 Example 10 Example 1 Example 2 Example 3 Example 4 First polyolefin solution UHMWPE 0 0 0 0 20 30 HDPE 100 100 80 100 78 70 PP 0 0 20 0 2 0 Resin 30 30 25 30 30 25 concentration Second polyolefin UHMWPE 40 40 30 30 40 40 solution HDPE (*1) 60 0 70 0 60 60 HDPE (*2) 0 60 0 70 0 0 PP 0 0 0 0 0 0 Resin 23 20 25 25 25 25 concentration Temperature of First stretching (° C.) 117 116 115 116 116 115 Temperature of Second stretching, 128 125 125 125 126 126 Relaxing and Heat setting treatment (° Second stretching magnification 1.4 1.4 1.4 1.4 1.4 1.4 in TD direction Relax ratio after second stretching (%) 96 86 86 86 96 96 Membrane thickness (μ) 11.5 11.5 11.5 11.5 11.5 11.5 Air permeability (sec/100 cc) 120 110 115 115 115 120 Porosity (%) 44 43 43 45 44 44 Pin puncture strength (gf) 320 240 320 270 320 330 Heat shrinkage ratio (%) MD 4.5 4.0 4.0 6.0 4.0 4.0 TD 4.0 1.0 4.0 1.5 4.0 4.0 Tensile strength MD 1,250 900 1,200 1,000 1,200 1,250 (kgf/cm.sup.2) TD 1,450 1,000 1,450 1,150 1,450 1.5 Static friction coefficient 1.00 1.00 0.80 1.00 1.30 1.50 Pore density 5.4 5.7 3.5 3.6 5.2 5.0 Breakdown Voltage (kV/11.5 μm) 1.5 1.6 1.2 1.2 1.5 1.5 Electrochemical stability (mAh) 100 95 55 100 80 75 (*1): Mw = 3.0 × 10.sup.5 (*2): Mw = 3.5 × 10.sup.5

(35) As shown in Table 1, the polyolefin multilayer microporous membrane of the present invention has good slip characteristics between membranes and has a dense microporous structure, and is suitable as a separator for batteries.

INDUSTRIAL APPLICABILITY

(36) The polyolefin multilayer microporous membrane of the present invention has good slip characteristics between membranes due to a static friction coefficient of not more than a specific value and a high durability due to a fine dense microporous structure. When the polyolefin multilayer microporous membrane of the present invention is used as a battery separator, a battery having an excellent handleability in battery production facilities, an improved quality, and an excellent safety and durability can be obtained.