WO2002092677A1 - Microporous polyolefin film - Google Patents

Abstract

A microporous polyolefin film made of a composition which comprises as essential ingredients (A) polyethylene having a viscosity-average molecular weight of 50,000 to 1,500,000 and (B) polypropylene having a viscosity-average molecular weight of 100,000 to 1,500,000 in such proportions that A+B is at least 80 wt.% based on the whole composition, A/(A+B) is from 51 to 90 wt.%, and B/(A+B) is from 10 to 49 wt.%, and which has a shutdown temperature lower than 140°C and has a continuous phase in which the polyethylene and polypropylene are intertwined with each other.

Description

 Description Polyolefin microporous membrane Technical field

 The present invention relates to a microporous polyolefin membrane, and more particularly, to a microporous polyolefin membrane suitable for a separator for a lithium battery.

Background art

 Conventionally, microporous polyethylene membranes have been used for lithium battery separators such as lithium batteries, lithium-ion batteries, and lithium polymer batteries.

 The main reason polyethylene is used is that polyethylene is used to melt the polymer at 130 ° C to 140 ° C to close the communication holes and to shut down the current, in order to ensure battery safety. It depends. When used as a battery separator, it is desirable that the shutdown temperature be as low as possible. Shutdown is a phenomenon in which the flow of lithium ions is interrupted by closing the pores of the porous membrane with the molten resin and increasing the electrical resistance of the membrane.

 Furthermore, as a function of the separator, it is necessary to maintain the shape even after the hole is closed, and to maintain insulation between the electrodes. However, polyethylene separators tend to decrease in strength after melting the polymer crystals. Therefore, how to raise the rupture-resistant temperature is an issue. For example, since it is necessary to guarantee battery safety at 150 ° C, the US Standard UL 1642 “Standard for Lithium Batteries” includes a battery safety rating of storing in an oven at 150 ° C for 10 minutes. Standards are set. To achieve this safety standard, the separator should be non-porous in shutdown and maintain its shape at 150 ° C or higher, preferably around 160 ° C, without rupture. There is a strong demand for a separator having excellent high-temperature strength. Also, if some holes are left unopened without shutting down completely when shut down, current leakage due to ion leakage or membrane rupture due to stress concentration is not desirable, so it is not desirable to shut down at shutdown temperature. There is a demand for a separator which is closed and nonporous.

Conventionally, as an attempt to raise the membrane rupture temperature of polyethylene separators, A number of attempts have been made to mix polypropylene with a high melting point with styrene or to laminate a microporous polyethylene membrane and a microporous polypropylene membrane.

 Japanese Patent Application Laid-Open Nos. 4-1126 / 52, 412/206/57, hei 5-313 / 306, hei 8-924 / 403, and 9-1224 / 1992 All of the publications disclose microporous membranes containing polyethylene and polypropylene. However, as is clear from the respective examples, all of the membranes disclosed in these examples are microporous membranes prepared by uniaxial stretching, and have strength and strength in the machine direction (MD) and the direction perpendicular to the machine direction (TD). The difference in shrinkage behavior tends to be large. In addition, since these are stretched and opened in the MD direction at a temperature lower than the melting point of polyethylene, they tend to contract in the MD direction near the shutdown temperature at which polyethylene melts.

 Japanese Patent Application Laid-Open Nos. 5-234578, Hei 6-96953, and Hei 6-222 002 describe separators made of specific polyethylene and polypropylene. Each of the disclosed methods is a method in which an organic solvent and an inorganic powder are mixed with a polymer mixture, extruded, and then the organic solvent and the inorganic powder are extracted and removed from the film. The temperature is also lower than the melting point of polyethylene. The microporous membranes disclosed in Japanese Patent Application Laid-Open Nos. 10-2893224 and 10-2983825 extrude a mixture of polyethylene, polypropylene and a solution, and then melt the polyethylene. It is prepared by stretching at a temperature of + 10 ° C or lower. These films are stretched biaxially and have isotropic physical properties, but the stretching temperature is not higher than the melting point of the constituent polyolefin + 10 ° C, preferably lower than the crystal melting point. Since the heat setting temperature is also below the melting point, the heat shrinkage during shutdown is large, and the challenge is how to achieve low shrinkage and low shrinkage stress near the shutdown temperature.

On the other hand, Japanese Patent Application Laid-Open No. 7-268118 discloses a microporous membrane having a mixed composition of ultra-high molecular weight polyethylene and ultra-high molecular weight polypyrene obtained by phase separation. And 130 which is usually found in polyethylene. Disclosed is a method for expressing a high-temperature strength by forming a crystal having an early temperature of C to 140 ° C and a crystal having a high melting temperature of 140 ° C to 152 ° C. ing. However, even with this method, although the film strength is developed up to 150 ° C, the strength is hardly developed at 160 ° C. In addition, the ratio of polypropylene increases, and polypropylene becomes a continuous phase. If it is formed, there are problems such as the shutdown temperature will be 140 ° C or more, and unblocked holes will remain even after shutdown, resulting in current leakage due to ion leakage. It is considered that this is because the microporous film disclosed in the present application is prepared by a so-called “phase separation method” using a mixture of a polyolefin and a plasticizer as a raw material. In the phase separation method, microporosity is formed in both the polyethylene phase and the polypropylene phase. If the ratio of polypropylene increases, it is considered that the unobstructed microporosity of the polypropylene phase increases even after the polyethylene is melted. Furthermore, since the molecular weight of the polymer described in JP-A-7-268118 is extremely high, uniform mixing with a plasticizer or extrusion tends to be difficult.

Disclosure of the invention

 The present invention relates to a polyolefin separator having a mixed composition of polyethylene and polypropylene as a main component, and a shutdown temperature equivalent to that of a separator composed of polyethylene alone, preferably less than 140 ° C, more preferably less than 138 ° C. As a separator for lithium batteries, it has a low shutdown temperature, a low shrinkage rate at the shutdown temperature, no pore leakage and no current leakage after shutdown, and high film strength at high temperatures. An object of the present invention is to provide a suitable microporous film.

 The present invention is as follows.

(1) Polyethylene (A) with a viscosity average molecular weight of 50,000 to 1,500,000 and polypropylene (B) with a viscosity average molecular weight of 100,000 to 1,500,000 are essential components, and A + B is 80 wt. ⁰ /. As described above, a polyolefin microporous membrane comprising a composition in which A / (A + B) is 51 to 90 wt% and / (A + B) is 10 to 49 wt%, has a shutdown temperature of 140 The polyolefin microporous membrane described above, wherein the polyethylene and polypropylene form a continuous phase entangled with each other at a temperature lower than ° C.

(2) Polyethylene (A) with a viscosity average molecular weight of 50,000 to 1,500,000 and polypropylene (B) with a viscosity average molecular weight of 100,000 to 1,500,000 are essential components, and A + B is 80 wt. % Or more, A / (A + B) is 51 to 90 wt%, B / (A + B) is 10 to 49 wt%, and the shutdown temperature is less than 140 ° C. If the piercing strength at the shut-down temperature is Fl and the piercing strength at 160 ° C is F2, Fl ^ lg / ni is? The microporous polyolefin membrane described above, wherein 2 / F 1≥0.2.

 (3) The polyolefin microporous membrane according to the above (1) or (2) having the following characteristics:

 1) Thermal shrinkage at shutdown temperature is 30% or less

 2) The thickness of the film is 5 m to 50 μπι,

 3) Porosity of 30% to 80%, and

 4) Room temperature piercing strength of 5 g / μm or more.

 (4) Polyethylene (A) with a viscosity average molecular weight of 50,000 to 1,500,000 and polypropylene (B) with a viscosity average molecular weight of 100,000 to 150,000 are essential components, and A + B is 80w in total. At least t%, a composition in which A / (A + B) is 51 to 90 wt% and B / (A + B) is 10 to 49 wt% is melt-kneaded by shearing, extruded, A precursor having a continuous phase in which polyethylene and polypropylene are entangled with each other is prepared, and the precursor is stretched to form a non-porous stretched film. Then, heat treatment at 110 ° C to 140 ° C is performed. A method for producing a microporous membrane, which comprises microporizing the film.

 (5) A lithium battery separator comprising the microporous membrane according to any one of (1) to (3).

BRIEF DESCRIPTION OF THE FIGURES

 FIG. 1 is a schematic diagram of a device for measuring a shutdown temperature and a short-circuit temperature. FIG. 2 is a partial view of an apparatus for measuring a shutdown temperature and a short-circuit temperature. FIG. 3 is a partial view of a device for measuring a shutdown temperature and a short-circuit temperature. BEST MODE FOR CARRYING OUT THE INVENTION

As described above, many prior arts of microporous membranes made by mixing polyethylene and polypropylene have been disclosed, but the shutdown temperature is low, the shrinkage at shutdown is small, and the current after shutdown is limited. There is no microporous membrane that does not leak and maintains high strength even at a high temperature of 160 ° C. This is thought to be due to the phase structure and the pore structure of polyethylene and polypropylene. That is, polyethylene and polypropylene are usually incompatible polymers Therefore, in the case of using the conventional manufacturing method, a polymer having a high ratio forms a continuous phase (sea) and a polymer having a low ratio forms a discontinuous phase (island), that is, a so-called sea-island structure. The formed microporous membrane cannot have low temperature shutdown, high temperature and high strength, and high temperature and low shrinkage at the same time.

 That is, when the ratio of polyethylene is high and polyethylene is a continuous phase, the film strength tends to decrease at a temperature higher than the melting point of polyethylene. This is probably because polypropylene with a high melting point is dispersed and does not contribute to strength development. On the other hand, when the proportion of polypropylene is high and polypropylene is a continuous phase, the film strength is increased even at a temperature higher than the melting point of polyethylene, but in this case, the shut down temperature tends to increase. This is considered to be due to the fact that the ratio of polypropylene increases, and the pore melting is greatly affected by the crystal melting temperature of polypropylene. In the present invention, a continuous phase in which polyethylene and polypropylene are entangled with each other is formed, high strength is exhibited even at a temperature higher than the melting point of polyethylene, and only the polyethylene phase is made microporous. In other words, it is possible to shut down by melting polyethylene.

 Hereinafter, the polyolefin-based microporous membrane of the present invention and the method for producing the same will be described in detail.

As the polyethylene used in the present invention, medium density and Z or high density polyethylene are preferable, and a homopolymer, a copolymer, or a graft polymer can also be used. Particularly, maleic anhydride-grafted polyethylene or the like is preferable when used for a separator because it improves wettability with an electrolytic solution. The molecular weight of the polyethylene used (polyethylene mixed when two or more kinds are mixed) is preferably 50,000 or more, more preferably 200,000 or more, from the viewpoint of strength. On the other hand, separately from the viscosity average molecular weight, the weight average molecular weight is preferably 50,000 or more. Further, the viscosity average molecular weight is preferably 50,000 or more and the weight average molecular weight is preferably 50,000 or more, and more preferably the viscosity average molecular weight is 200,000 or more and the weight average molecular weight is 50,000 or more. Polyethylene can be used alone or as a mixture of two or more. It is more preferable that ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 500,000 or more is used alone or as a mixture in terms of film properties. In addition, uniform melt-kneading is performed. In view of this, it is preferable that the viscosity average molecular weight is 150,000 or less and the weight average molecular weight is 200000 or less. The polyethylene used in the present invention having a high molecular weight within a range in which it can be uniformly kneaded is advantageous for forming a high melting point extended chain crystal which is effective for developing high-temperature strength.

 In other words, in Japanese Patent Application Laid-Open Nos. Hei 5—2 345 778, Hei 6-966 753, Hei 6—2 382 002, Hei 7—2 681 8 Therefore, even if ultra-high molecular weight polyethylene is used, it is difficult to form extended chain crystals that are effective for developing high-temperature strength, but the present invention does not use an organic solvent. Since it can be carried out both, it is easy to be super-stretched and has the advantage of easily forming an extended chain crystal. Incidentally, Japanese Unexamined Patent Publication (Kokai) No. 7-26881 / 18 discloses that if the intrinsic viscosity of polyethylene is not more than 10 d 1 / g (viscosity average molecular weight is about 1.6 million), high melting point extended chain crystals Although it is disclosed that no crystal is formed, in the microporous membrane according to the present invention, a chain crystal having a high melting point and an extended chain having a viscosity average molecular weight of about 200,000 is generated. In addition, it is preferable that the molecular weight is higher because the extended chain crystals are more easily formed. The continuous phase entangled with each other changes to a discontinuous phase with time when maintained at a high temperature in a non-shear field, but the transition time becomes longer as the viscosity of the system is increased using a high molecular weight polymer, and the continuous phase becomes longer. Easy to get a phase.

 The type of polypropylene used in the present invention is not limited, and examples thereof include isotactic polypropylene, atactic polypropylene, propylene'ethylene copolymer, and 1-butene-propylene copolymer. Examples include polypropylene having an isotactic index of 90% or more. Further, it is preferable that the viscosity average molecular weight of the polypropylene used (or the mixed polypropylene when two or more kinds are mixed) is 100,000 or more. On the other hand, independently of the viscosity average molecular weight, the weight average molecular weight is more preferably 100,000 or more. Further, the viscosity average molecular weight is preferably 150,000 or less from the viewpoint of uniform melt kneading. In the case of copolymerized polypropylene, the crystal melting temperature is 160. It is preferably C or more.

In the composition for forming the microporous membrane, the total of polyethylene and polypropylene must be at least 80 wt%. Other materials such as alumina and mon It is possible to add an appropriate amount of an inorganic substance such as morillonite, a polymer other than polyethylene or polypropylene, and an antioxidant.

 From the viewpoint of the porosity and high-temperature strength of the microporous membrane, it is essential that the ratio of polyethylene to polypropylene used is 51 to 9 wt% for polyethylene and 10 to 49 wt% for polypropylene.

 One of the features of the microporous membrane according to the present invention is that although polypropylene has a lower mixing ratio than polyethylene, it forms a continuous phase entangled with polyethylene. Since the polypropylene forms a continuous phase, sufficient piercing strength is exhibited even at a high temperature of 160 ° C.

 Whether polypropylene with a small mixing ratio forms a continuous phase is determined by measuring whether or not the strength of the film is developed in the temperature range between the crystalline melting temperature of polyethylene and the crystalline melting temperature of polypropylene. be able to. In addition, judgment can be made by using a transmission electron micrograph of ultrathin sections.

 The mixed system of polyethylene and polypropylene is usually incompatible, but since the phase separation temperature rises in the shearing field, a uniformly dissolved state can be obtained by selecting a specific temperature and shear stress ( For details, see Hiroshi Yui, Plastic Age, pl31-137, June, 2000). Melting and kneading in a shearing field can be performed by, for example, a twin-screw extruder having a large kneading effect. Melt kneading in the production method of the present application includes addition of an essential component or a third component from a feeder or the like in the middle of an extruder. The yarn-kneaded product melt-kneaded by a twin-screw extruder or the like is extruded using a sheet die such as a slit die or a T die, or a circular die such as a spiral die or a rotary die. It is said that when the shearing is stopped, the homogenous system separates with time and first reaches a discontinuous phase through a continuous phase that is entangled with each other. Is preferably fixed. After cooling and solidifying in the die, it is also possible to extrude solidified.

The precursor may be in any shape such as a sheet, a film, and a tube. The extruded precursor is stretched by flat stretching or tubular stretching to form a nonporous film. As flat stretching, there are uniaxial stretching and biaxial stretching, but biaxial stretching is preferred from the viewpoint of physical isotropy, sequential biaxial stretching, and simultaneous biaxial stretching. Any extension can be used. The stretching temperature is preferably from 100 ° C. to 160 ° C. from the viewpoint of the film strength, and more preferably from 130 ° C. to 150 ° C. Lowering the stretching temperature increases the strength of the film, but increases the shrinkage near the shutdown temperature. It is also possible to heat set at high temperature after drawing at low temperature. In order to achieve a low heat shrinkage at the shutdown temperature, it is preferable that the stretching or heat-setting step goes through a history of the crystalline melting temperature of polyethylene or higher.

 The stretched film is preferably made by heat treatment at 100 ° C. to 150 ° C., more preferably 110 ° C. to 140 ° C., and preferably 140 ° C. A microporous material is formed which falls down below C, more preferably below 138 ° C. The pores formed by the heat treatment are prepared by, for example, heat-treating the film in a liquid that selectively dissolves or melts the low melting point portion of polyethylene, and then washing the film with a detergent that is compatible with the liquid and does not dissolve polyethylene. It can be carried out by drying after removing the liquid. In order to prevent the film from shrinking, it is preferable to perform heat treatment under constraint. Micropores are formed in the polyethylene portion by performing the opening according to this method. Whether microporosity is formed only in the polyethylene part can be determined by heat-treating the microporous membrane at less than 140 ° C and shutting it down, and then measuring whether there is any leakage current or by measuring the surface. Also, it can be determined by observing with an electron microscope whether there are any unobstructed holes in the cross section. The liquid used for the heat treatment may be used alone or in combination with hydrocarbons such as paraffin oil, aliphatic alcohols, aliphatic ketones, nitrogen-containing organic compounds, ethers, glycols, aliphatic esters, and silicone oils. it can. The heat treatment time can be shortened if the treatment temperature is high. In order to maintain the strength of the resin after being made porous, the treatment time is preferably short. Examples of the solvent cleaning agent after the heat treatment include low-boiling hydrocarbons such as hexane, fluorine-based organic solvents such as hydrofluoroether and hydrofluorocarbon, chlorine-based solvents such as methylene chloride, and ketones such as methylethylketone. Can be used.

 Next, the present invention will be described in more detail with reference to examples.

The test method shown in the examples is as follows.

(1) Film thickness It was measured with a dialge (Ozaki Seisakusho: PEACOCK No. 25). (2) Porosity

A 20 cm square sample was taken and the volume and mass were calculated using the following formula. Porosity (%) = (Volume (cm ³ ) One mass (g) Density of Z polymer composition) / Volume

(cm) X 100

 (3) Room temperature piercing strength

 A piercing test was performed using a KES-G5 handy compression tester manufactured by Rikito Tech under the conditions of a radius of curvature of the needle tip of 0.5 mm and a piercing speed of 2 mmZsec, and the maximum piercing load (g) was measured. By multiplying the measured value by the 1Z film thickness (um), the piercing strength in terms of 1 zm (g) was obtained.

 (4) High temperature piercing strength

 The microporous membrane is sandwiched between two stainless steel washers with an inner diameter of 13 mm and an outer diameter of 25 mm, and the surrounding four points are clipped. The shutdown temperature of each membrane and a silicone oil of 160 ° C (Shin-Etsu) After immersion in KF-96-10CS), the piercing strength was measured after 30 seconds by the same method as in (3).

 (5) Heat shrinkage

 The microporous membrane was placed in an atmosphere at the shutdown temperature for one hour, and the values were determined from the changes in the length in the MD and TD directions. For the heat shrinkage of each sample, the larger value of MD and TD was used.

 (6) Shirt down temperature

 Figure 1 shows a schematic diagram of the shutdown temperature measurement device. 1 is a microporous membrane, 2A and 2B are nickel foils having a thickness of 10 / m, and 3A and 3B are glass plates. Reference numeral 4 denotes an electric resistance measuring device (LCR meter AG-431 1 manufactured by Ando Electric), which is connected to nickel foils 2A and 2B. Reference numeral 5 denotes a thermocouple connected to the thermometer 6. Reference numeral 7 denotes a data collector, which is connected to the electric resistance device 4 and the thermometer 6. 8 is an oven for heating the microporous membrane.

More specifically, as shown in FIG. 2, the microporous membrane 1 is stacked on the nickel foil 2A, and is fixed to the nickel foil 2A in the vertical direction with a Teflon tape. For the microporous membrane 1, 1 mo 1 Z liter of lithium borofluoride solution (solvent: pro Pyrene carbonate / ethylene carbonate / γ-butynoleratone = 1/1 /

2) is impregnated. As shown in Fig. 3, a piece of Teflon tape is stuck on the nickel foil 2mm, and masking is performed, leaving a 15mm x 10mm window at the center of the foil 2mm.

 The nickel foil 2A and the nickel foil 2B are overlapped so as to sandwich the microporous membrane 1, and two nickel foils are sandwiched between the two sides by the glass plates 3A and 3B. At this time, the window portion of the foil 2B and the porous film 1 are located at positions facing each other. The two glass plates are fixed by sandwiching them with a commercially available double clip. Thermocouple 5 is fixed to the glass plate with Teflon tape.

The temperature and electric resistance are continuously measured with such an apparatus. The temperature is raised from 25 ° C to 200 ° C at a rate of 2 ° C / min, and the electrical resistance is measured with an alternating current of 1 kHz. The shirt preparative down temperature defined as the temperature of Rutoki the electrical resistance of the microporous film reaches the 10 ³ Omega.

 (7) Viscosity average molecular weight

 The intrinsic viscosity [η] was measured with a 135 C decalin solution, and the viscosity average molecular weight (Μν) was calculated by the following equation. The intrinsic viscosity was determined by the viscosity method according to the “Polymer Analysis Handbook” (Ρ58).

[Η] = 6. 77X 10 one ^{4 Μν 0} ·. '

Example 1

 75 parts by weight of high-density polyethylene (density 0.95, viscosity average molecular weight 280,000), 25 parts by weight of polypropylene (density 0.90, viscosity average molecular weight 250,000), and as an antioxidant to the composition 0.3 parts by weight of tetrakis [methylene-3- (3 ', 5, di-butyne 4-hydroxypheninole) propionate] methane is supplied at 230 ° using a twin screw extruder with a diameter of 30 mm and LZD = 48. The mixture was kneaded under the conditions of C and 500 rpm, extruded from a T-die installed at the tip of the extruder, and immediately cooled and solidified with a cast roll cooled to 25 ° C to form a 0.5 mm thick sheet.

1 35 for this sheet. After stretching the film 7 × 7 times with a simultaneous biaxial stretching machine at C, this stretched film is fixed to a frame and immersed in paraffin oil at 130 ° C. for 2 minutes. The microporous membrane was obtained by washing with rutile ketone.

 Table 1 shows the physical properties of the obtained film. Sufficient high-temperature puncture strength was exhibited even at 160 ° C, confirming that the polypropylene phase had formed a continuous phase, and that the stretched film was observed with a transmission electron microscope using an ultrathin section of rutinium-stained stretched film. However, it was observed that the polyethylene phase and the polypropylene phase formed a continuous phase entangled with each other.

Example 2

 75 parts by weight of high-density polyethylene (density 0.94, viscosity-average molecular weight 1,000,000), 25 parts by weight of polypropylene (density 0.90, viscosity-average molecular weight 250,000), and 0.1 parts by weight of the composition as an antioxidant. 3 parts by weight of tetrakis [methylene-3- (3,, 5, di-t-butyl_4, -hydroxyphenyl) propionate] methane is heated to 240 ° C using a twin screw extruder with a diameter of 30 mm and LZD = 48. The mixture was kneaded under the conditions of 500 rpm, extruded from a T-die installed at the tip of the extruder, and immediately cooled and solidified by a cast roll cooled to 25 ° C. to form a sheet having a thickness of 0.5 mm.

 This sheet is stretched 7 x 7 times at 135 ° C with a simultaneous biaxial stretching machine, then this stretched film is immersed in paraffin oil at 130 ° C for 2 minutes, and then washed with methyl ethyl ketone Thus, a microporous membrane was obtained. Table 1 shows the physical properties of the obtained film.

Comparative Example 1

 75 parts by weight of high-density polyethylene (density 0.95, viscosity average molecular weight 280,000), 25 parts by weight of polypropylene (density 0.90, viscosity average molecular weight 250,000), and as an antioxidant to the composition 0.3 parts by weight of tetrakis [methylene-1-

(3,, 5'-g-t-butyl-1-4,1-hydroxyphenyl) propionate] Methane is kneaded at 250 ° C and 5 rpm in a batch kneader (Toyo Seiki Labo Plastomill) The obtained kneaded material was held in a hot press machine at 200 ° C. for 10 minutes to form a sheet having a thickness of 0.5 mm.

After stretching this sheet 7 x 7 times at 135 ° C with a simultaneous biaxial stretching machine, immerse the stretched film in paraffin oil at 130 ° C for 2 minutes, and then wash with methyl ethyl ketone. As a result, a microporous membrane was obtained. Table 1 shows the physical properties of the obtained film. When the stretched film was observed with a transmission electron microscope, a sea-island structure having propylene of 5 to 1θίπι as islands was observed.

Comparative Example 2

 75 parts by weight of high-density polyethylene (density 95, viscosity average molecular weight 280,000), 25 parts by weight of polypropylene (density 0.90, viscosity average molecular weight 250,000), and 0 as an antioxidant to the composition. Mix 3 parts by weight of tetrakis [methylene-1 3-(3,5,1-di-tert-butyl-4, -hydroxyphenyl) propionate] methane and feed it to a twin-screw extruder with a diameter of 30 mm and LZD = 48. Throw through one. Further, 100 parts by weight of liquid paraffin (kinematic viscosity at 37.78 ° C 75.9 cSt) is injected into the extruder by side feed, kneaded at 230 ° C and 500 rpm, and installed at the extruder tip. After being extruded from the T-die thus formed, it was immediately cooled and solidified with a cast roll cooled to 25 ° C. in the same manner as in Example 1 to form a sheet having a thickness of 1 mm.

 This sheet is stretched 7 × 7 times by a simultaneous biaxial stretching machine at 120 ° C, then this stretched film is immersed in methyl ethyl ketone to extract and remove the liquid paraffin, and then dried to remove microporous material. Obtained.

 Table 1 shows the physical properties of the obtained film. Observation of the stretched film with a transmission electron microscope revealed that the polypropylene phase was dispersed in the polyethylene continuous phase, and a sea-island structure was observed.

In the measurement of the shutdown temperature, not only was the shutdown temperature as high as 142 ° C, but the electrical resistance immediately after shutdown was unstable. When the microporous membrane immediately after shutdown was observed with a transmission electron microscope, an unobstructed pore structure was observed in the polypropylene phase. 1Example 1 例 Example 2 Comparative Example 1 Comparative Example 2 Kneading method Twin screw extruding Twin screw extruding 7 ° last mill Twin screw extruder Drilling machine Shear rate 髙 Shear High shear Low shear High shear Raw material composition * PE / PP PE / PPPE / P Ρ PE / PP

 / Solvent film thickness μ 20 1 8 22 20

: ^ 5 7 4 7 59 40 Room temperature piercing strength g 3 1 0 6 1 0 220 400 μm 俱 funeral hanging 1 5.5 3 3. 9 1 0 .0 20.0 溫 dish piercing strength & /

Every time

Shutta, -Pin

Stab at g 6 5 1 3 7 6 2 2 9 strength

 F 1 g / β ΐΆ 3. 3 7. 6 2. 8 1.45

G 23 3 1 3 at 160 ° C (puncture strength at 150 ° C rupture)

F 2 g / β m 1.2 1.70 0.1

F 2 / F 1 0. 3 6 0. 22 0.05 Shutdown "ON temperature ° C 1 3 1 1 33 1 3 1 1 42 Shutdown, ON temperature%

Heat shrinkage at 1 8 20 1 5 3 3

PE: polyethylene

 PP: Polypropylene

Industrial applicability

 The microporous membrane of the present invention has excellent high-temperature strength, low shirt down temperature and low shrinkage at shirt down temperature, and is particularly suitable for a lithium battery separator.

Claims

The scope of the claims 1. Polyethylene (A) having a viscosity average molecular weight of 50,000 to 1,500,000 and polypropylene (B) having a viscosity average molecular weight of 100,000 to 1,500,000 are essential components, and A + B is 80 wt% of the whole. As described above, a microporous polyolefin membrane made of a composition in which A / (A + B) is 51 to 90 wt% and B / (A + B) is 10 to 49 wt%, and has a shutdown temperature The polyolefin microporous membrane described above, wherein the temperature is lower than 140 ° C, and polyethylene and polypropylene form a continuous phase in which they are entangled with each other. 2. comprises a viscosity-average molecular weight of from 50,000 to 1 500,000 polyethylene (A) and the viscosity-average molecular weight of 1 00000-1 500,000 polypropylene (B) as essential components, A + B is of the total 80 w t ⁰ /. As described above, the composition is composed of AZ (A + B) of 51 to 90 wt% and B / (A + B) of 10 to 49 wt%, and the shutdown temperature is less than 140 ° C. a polyolefin microporous membrane, when the piercing strength at the shirt preparative down temperature piercing strength of at F l, 1 6 0 ° C and F 2, F in l≥ l gZ _J um, and F 2 / F 1 ≥0.2, the polyolefin microporous membrane described above.  3. The microporous polyolefin membrane according to claim 1 or claim 2 having the following characteristics:  1) Heat shrinkage at shutdown temperature is 30% or less,  2) When the film thickness is 5 μιη to 50 μηι,  3) Porosity of 30% to 80%, and  4) Room temperature piercing strength of 5 gZ / xm or more.  4. Polyethylene (A) with a viscosity average molecular weight of 50,000 to 1,500,000 and polypropylene (B) with a viscosity average molecular weight of 100,000 to 1,500,000 are essential components, and A + B is 80 wt% of the whole. As described above, a composition having AZ (A + B) of 51 to 90 wt% and B / (A + B) force S of 10 to 49 wt% is melt-kneaded by shearing, extruded, and polyethylene. To form a precursor having a continuous phase in which polypropylene and polypropylene are entangled with each other, stretch the precursor to form a non-porous stretched film, and then heat treat it at 110 ° C to 140 ° C to make it microporous. A method for producing a microporous membrane, comprising: 5. A lithium battery cell comprising the microporous membrane according to any one of claims 1 to 3. s ST  ZtLtO / ZOdT / lDd ..9Z60 / Z0 OAV