JPH107831A - High heat resistant polyethylene microporous membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a microporous polyethylene membrane having excellent mechanical strength, permeability and productivity, and having extremely high heat resistance so as to ensure the safety of a battery even under severe conditions. SOLUTION: A polyethylene microporous membrane having a molecular weight between crosslinking points of 200,000 or less, a residual shrinkage of 15% or more, and a porosity of 20 to 80%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a microporous polyethylene membrane suitable for a battery separator.

[0002]

2. Description of the Related Art Microporous polyethylene membranes are used in microfiltration membranes, battery separators, condenser separators, and the like. Among these, when used as a battery separator, particularly a lithium ion battery separator, in addition to the general properties such as mechanical strength and permeability of the microporous membrane, the separator melts when the inside of the battery is overheated. A "fuse effect" is required, which is a film that covers the electrodes and that secures battery safety by interrupting current.

In the case of a polyethylene microporous membrane, the temperature at which the fuse effect is exhibited, that is, the fuse temperature is generally about 13 ° C.
It is known that the temperature is 0 to 150 ° C., and even if the inside of the battery is overheated for some reason, when the fuse temperature is reached, the microporous film melts and covers the electrode so that the current is interrupted. , The battery reaction stops. However, when the temperature rise is extremely rapid, the battery temperature further rises after the fuse, and as a result, the film may be broken and the current may be restored (short), resulting in a problem in safety. The development of a microporous polyethylene membrane having high heat resistance that can ensure the safety of the battery even under such severe conditions has been an issue.

For example, Japanese Patent Application Laid-Open No. 4-206257 discloses a method for improving heat resistance by blending polypropylene having a higher melting point than polyethylene with polyethylene. According to such a method, although the heat resistance of the microporous membrane is expected to be improved to some extent, although the polypropylene is blended, it easily flows after being melted due to overheating and breaks, resulting in a breakage of the battery. It was not an essential improvement in terms of security. Further, polypropylene has low compatibility with polyethylene, and has a disadvantage that strength is reduced because both are separated in the microporous membrane.

On the other hand, JP-A-3-1055851 discloses a method for improving mechanical strength by blending a specific amount of ultra-high molecular weight polyethylene with a high molecular weight polyethylene. Ultra-high molecular weight polyethylene has a considerable viscosity even after melting, that is, the polyethylene microporous membrane according to the method disclosed in the above-mentioned publication because it has shape retention properties, but it also becomes difficult for the membrane breakage after melting to occur as a secondary, but severe. Under the conditions, the membrane is also broken, and is not an essential solution like the invention disclosed in the above-mentioned publication.

Further, Japanese Patent Application Laid-Open No. 56-73857,
JP-A-63-205048, JP-A-3-2746
No. 61, JP-A-1-167344, JP-A-6
JP-A-329823 discloses a method for improving mechanical strength, oxidative strength, heat resistance and the like by crosslinking a polyolefin microporous membrane. According to these methods, it is possible to impart relatively high shape retention because the viscosity at the time of melting is increased by crosslinking.
As the performance of the battery has been improved, a highly heat-resistant polyethylene microporous membrane that can cope with severer conditions has been required.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, to provide an excellent mechanical strength, permeability, and productivity, and to provide an extremely high battery which can ensure the safety of a battery even under severe conditions. An object of the present invention is to provide a microporous polyethylene film having heat resistance.

[0008]

As a result of intensive studies to solve the above problems, it has been found that a microporous polyethylene membrane having a predetermined crosslinking density and a residual shrinkage ratio has extremely high heat resistance. I've reached the point. That is, the first aspect of the present invention is a microporous polyethylene membrane having a molecular weight between crosslink points of 200,000 or less, a residual shrinkage of 15% or more, and a porosity of 20 to 80%. A second aspect of the present invention is the microporous polyethylene membrane according to claim 1, wherein the gel fraction is 80% or more.

A third aspect of the present invention is the microporous polyethylene membrane according to claim 1 or 2, wherein the average pore diameter measured by a permeation method is 0.001 to 0.1 μm. A fourth aspect of the present invention is a battery separator using the microporous polyethylene membrane according to claims 1 to 3. further,
A fifth aspect of the present invention is a battery using the battery separator according to the fourth aspect. Hereinafter, the present invention will be described in detail.

When the polyethylene microporous film is heated in the battery and reaches a temperature near the fuse temperature, the strength of the microporous film decreases with melting of the crystal, and a strong shrinkage stress is generated by releasing the stretching orientation. The microporous polyethylene membrane is in a situation that is easily broken. At this time, it is considered that the microporous film is broken at a contact point with the active material on the electrode or the like, thereby causing a short circuit. That is, it is considered that the strength of the microporous film at the time of melting is one factor for improving the heat resistance, and such a property is quantitatively evaluated, for example, by the melt piercing strength as described below. Can do things.

<< Melting Puncture Strength >> The melt piercing strength was determined by melting a polyethylene microporous membrane constrained by a predetermined jig in silicon oil heated to a melting point or higher and pressing a predetermined metal needle against the molten film. It is determined from the load at the time of breaking. For example, since a general polyethylene microporous film has extremely low heat resistance, when immersed in silicon oil, it breaks due to shrinkage stress before the measurement of the melt piercing strength. On the other hand, a microporous polyethylene film blended with polypropylene or ultra-high molecular weight polyethylene shows a certain degree of melt piercing strength even after melting at a low temperature of about 150 ° C. due to these blending effects. The membrane breaks when immersed in silicone oil. That is, it is understood that the improvement of the heat resistance by the blend is limited to a relatively low temperature range.

On the other hand, a polyethylene microporous membrane whose melt strength has been improved by a cross-linking treatment such as electron beam irradiation has a certain degree of melt piercing strength even at a high temperature of about 200 ° C. regardless of the temperature, and is therefore higher than the blend film. It can exhibit heat resistance. However, in the prior art, the crosslinking density is relatively low in order to avoid various adverse effects (cross-linking, reduction of workability, film shrinkage, reduction of strength, increase of fuse temperature, slowdown of fuse effect, etc.) due to the crosslinking treatment, and When higher heat resistance is required, for example, when the microporous film undergoes large deformation due to an active material having a coarse particle size, the melt piercing strength is not sufficient.

On the other hand, the present inventors have established a technique for producing a microporous polyethylene membrane having an extremely high crosslink density while avoiding these adverse effects, and have produced an unprecedented microporous polyethylene membrane having a high crosslink density. . As a result, it has been found that when the crosslink density exceeds about one per polymer molecular chain, the melt-piercing strength of the microporous membrane is dramatically improved. The limit value of the crosslink density is in good agreement with the theoretical gelling conditions, and the molecular weight becomes apparently infinite with the completion of the three-dimensional crosslink, so that the fluidity during melting is substantially lost. It is considered that the strength was dramatically improved. The melt piercing strength in the present invention is 10 g or more, preferably 15 g or more, and more preferably 20 g or more. If the melt piercing strength is 10 g or less, sufficient heat resistance cannot be exhibited depending on the use and use conditions of the battery.

<< Molecular Weight Between Crosslinking Points >> The crosslink density can be evaluated by the molecular weight between crosslinking points obtained from a stress-strain curve when rubber elasticity theory is applied to a crosslinked polyethylene microporous membrane in a molten state. In the present invention, the condition for dramatically improving the melt piercing strength is that the crosslink density exceeds about one per polymer molecular chain, which means that the molecular weight between crosslink points is smaller than the molecular weight of the raw polyethylene. Corresponding to that. Here, considering that the average molecular weight of the raw material polyethylene used for the polyethylene microporous membrane is generally 100,000 or more or 200,000 or more, the molecular weight between crosslinking points is at least 200,000 or less,
If it is preferably 100,000 or less, the above conditions can be achieved irrespective of the molecular weight of the raw material polyethylene. On the other hand, when polyethylene having a relatively high molecular weight is used, the molecular weight between crosslinking points is equal to or less than the average molecular weight of the raw polyethylene (for example, if the polyethylene has an average molecular weight of 700,000, 7
It is needless to say that a dramatic improvement in the melt piercing strength can be achieved regardless of the range described above.

<Shrinkage Residual Ratio> Although the heat resistance of the separator is significantly improved by the crosslinking treatment, it is preferable that the shrinkage stress which causes short-circuit even if the heat resistance is improved is reduced as much as possible. For example, in the production method of the present invention, the timing of crosslinking is roughly divided into before and after stretching. Of these, when cross-linking after stretching, it is possible to suppress shrinkage of the microporous film at the time of fuse because molecules elongated by stretching are fixed at the crosslinking points,
For this reason, the heat resistance of the film can be further improved even with the same gel fraction as compared with the crosslinking before stretching.

On the other hand, if crosslinked before stretching, the fused microporous membrane tends to return to the shape at the time of crosslinking, and a large shrinkage stress is generated. Therefore, depending on the battery structure, compared with a microporous membrane crosslinked after stretching. It may be easy to short. As is clear from the above, the difficulty of shrinking the microporous film at the time of fuse is evaluated by the shrinkage residual ratio. The contraction residual ratio of the microporous membrane according to the present invention is 15% or more, preferably 20% or more, and more preferably 30% or more.

<< Gel Fraction >> The gel fraction, which is a measure of the crosslinked structure, can be evaluated by a measuring method in accordance with ASTM D2765, but the gel fraction required for a drastic improvement in the melt piercing strength is as follows. Like the molecular weight between cross-linking points, it depends on the average molecular weight of the raw material polyethylene, so that it is difficult to determine the range without limitation. For example, polyethylene having an average molecular weight of about 250,000 requires a gel fraction of about 80% or more, while polyethylene having an average molecular weight of about 140,000 can dramatically improve the penetration strength even at a gel fraction of about 50% or more. It is possible to achieve.

However, the strength at room temperature generally depends on the average molecular weight of the raw material polyethylene, so that when polyethylene having an average molecular weight of about 140,000 is used, the strength may be insufficient depending on the use of the battery. Therefore, assuming strength at room temperature, the average molecular weight is preferably 200,000 or more, and assuming the use of such polyethylene, the gel fraction is preferably 80% or more. Although the upper limit of the gel fraction is mainly limited by the production conditions, it is generally difficult to achieve a gel fraction of 99% or more, for example, in the case of crosslinking by electron beam irradiation. However, in the present invention, a gel fraction of 99% or more is not always necessary, and it is possible to impart a sufficient melt piercing strength even with irradiation of 99% or less.

<< Fuse Characteristics >> The fuse temperature of the microporous polyethylene film of the present invention can be determined from the temperature dependence of impedance in simple cell measurement. The fuse temperature of the microporous film according to the present invention is 100 ° C. to 160 ° C., preferably 110 ° C. to 140 ° C., more preferably 120 ° C.
C. to 135.degree. If the fuse temperature is higher than 160 ° C., when used as a battery separator, there is a concern about deterioration of the electrolytic solution and runaway reaction of the electrode. Also, considering that it is inevitable to use at high temperatures, such as inside an automobile, a fuse temperature of the microporous film of less than 100 ° C. is not preferable.

As described above, the polyethylene microporous membrane according to the present invention has high heat resistance, but other general physical properties also have an air permeability of 2,000 seconds or less in terms of 25 μ and a breaking strength of 50 or less.
It is 0 kg / cm ² or more, and has a performance exceeding that of a conventional microporous polyethylene membrane not only in heat resistance but also in mechanical strength and permeability. The polyethylene used in the present invention is preferably a high-density polyethylene or a copolymer of ethylene and an α-olefin, which is a crystalline polymer mainly composed of ethylene, and further includes polypropylene, medium-density polyethylene, and linear low-density polyethylene. And low-density polyethylene, polyolefin such as EPR blended at a ratio of 30% or less.

The weight average molecular weight of polyethylene is in the range of 100,000 to 4,000,000, preferably 200,000 to 1,000,000, and more preferably 200,000 to 700,000. Molecular weight 10
If it is smaller than 10,000, it is easy to break when the sheet is stretched.
If it is larger than 10,000, it becomes difficult to produce a hot solution for producing a sheet, and the fuse effect of the obtained microporous film becomes slow. Further, the weight average molecular weight of the polymer used may be adjusted to a preferred range by means such as blending or multi-stage polymerization.

The thickness of the microporous membrane is 1 to 500 μm, preferably 10 to 200 μm, more preferably 15 to 50 μm.
When the film thickness is smaller than 1 μm, the mechanical strength is not sufficient, and when the film thickness is larger than 500 μm, the reduction in size and weight of the battery is hindered. The porosity of the microporous membrane is 20-80
%, Preferably 30 to 60%. If the porosity is less than 20%, the permeability is not sufficient, and if it is more than 80%, sufficient mechanical strength cannot be obtained.

The average pore diameter of the micropores can be measured by a permeation method using a water-soluble polymer such as pullulan. The average pore diameter of the micropores of the microporous membrane according to the present invention is from 0.001 to 0.
1 μm, preferably 0.005 to 0.05 μm, more preferably 0.01 to 0.03 μm. If the average pore size is smaller than 0.001 μm, the permeability is not sufficient, and if the average pore size is larger than 0.1 μm. This is not preferable because the fuse effect becomes slow.

The piercing strength of the microporous membrane is 300 g or more,
Preferably it is 400 g or more, more preferably 450 g or more. If the piercing strength is less than 300 g, the separator may be short-circuited by the dropped active material or the like. Next, a method for producing the microporous polyethylene membrane of the present invention will be described. The method for producing a polyethylene microporous membrane of the present invention is basically based on three steps: a film formation step, a stretching step, and an extraction step.

<< Film Forming Step >> The polymer gel, which is an intermediate product of the present invention, is prepared by dissolving polyethylene in a plasticizer at a melting point or higher to form a hot solution, and cooling this to a crystallization temperature or lower. The plasticizer as referred to herein is an organic compound capable of forming a uniform solution with polyethylene at a temperature not higher than its boiling point. Specifically, decalin, xylene, dioctyl phthalate, dibutyl phthalate, stearyl alcohol, oleyl alcohol , Decyl alcohol, nonyl alcohol, diphenyl ether, n-decane, n
-Dodecane, paraffin oil and the like. Of these, paraffin oil and dioctyl phthalate are preferred. The ratio of the plasticizer in the polymer gel is not particularly limited.
To 90%, preferably 50% to 70%. 20
% Or less, it is difficult to obtain a microporous film having an appropriate porosity, and if it is 90% or more, the viscosity of the hot solution decreases, and continuous molding of the sheet becomes difficult.

Although there is no particular limitation on the film forming method,
For example, after supplying a powder and a plasticizer of high-density polyethylene to an extruder and melt-kneading at a temperature of about 200 ° C.,
By casting from a normal hanger coat die onto a cooling roll, a sheet having a thickness of several tens of μm to several mm can be continuously formed. In the present invention, since ultra-high molecular weight polyethylene is not an essential component, no special heating and melting equipment is required, and it is possible to prepare a homogeneous sheet extremely simply by simply adding polyethylene and a plasticizer to the extruder. It is.

<< Stretching Step >> Next, the obtained sheet is stretched in at least one axial direction to form a stretched film. The stretching method is not particularly limited, but a tenter method, a roll method, a rolling method, or the like can be used. Of these, simultaneous biaxial stretching by the tenter method is preferred. The stretching temperature is from room temperature to the melting point of the polymer gel, preferably 80 to 130C, more preferably 100 to 125C. The stretching ratio is 4 to 400 times, preferably 8 to 200 times, and more preferably 16 to 100 times in area ratio. If the stretching ratio is 4 times or less, the strength as a separator is insufficient. If the stretching ratio is 400 times or more, not only stretching is difficult, but also adverse effects such as a decrease in porosity of the obtained microporous membrane are liable to occur.

<< Extraction Step >> Next, a plasticizer is extracted and removed from the stretched film to obtain a microporous film. Although there is no particular limitation on the extraction method, when paraffin oil or dioctyl phthalate is used, it is removed by extracting with an organic solvent such as methylene chloride or MEK and then heating and drying the obtained microporous membrane at a fuse temperature or lower. can do. When a low boiling point compound such as decalin is used as the plasticizer, it can be removed only by heating and drying at a temperature lower than the fuse temperature of the microporous film. In either case, it is preferable to restrain the film in order to prevent a decrease in physical properties due to contraction of the film.

<< Crosslinking >> The timing of the crosslinking treatment is as follows.
Although it is possible to carry out any of the above three steps or before or after the step, it is generally difficult to stretch a sheet having a high crosslinking density. Therefore, it is preferable to carry out a crosslinking treatment after the stretching step, and to carry out a crosslinking treatment after the extraction step. It is more preferred to perform the treatment. Examples of the method of crosslinking include ionizing radiation typified by ultraviolet rays, electron beams, and gamma rays, and chemical crosslinking by addition of a crosslinking agent or a crosslinking assistant. Of these, the method of electron beam irradiation is preferable.

The dose for electron beam irradiation is 1 to 200
Mrad, preferably 2 Mrad to 100 Mrad, more preferably 5 Mrad to 50 Mrad. If the dose is too small, a sufficient cross-linking density cannot be obtained, and if the dose is too large, the microporous membrane may deteriorate and the mechanical strength may decrease. Since the efficiency of cross-linking by electron beam irradiation is generally strongly affected by the irradiation temperature, the cooling state of the sample, and the oxygen concentration, etc., it is possible to perform sufficient cross-linking treatment even at a low dose by optimizing these conditions. Becomes It is preferable to establish irradiation conditions in advance with reference to the molecular weight between crosslinking points and the gel fraction of the irradiated sample.

Although the acceleration voltage at the time of irradiation is not particularly limited,
For example, when irradiating a microporous membrane of about 30 μm,
The crosslinking treatment can be favorably performed at an acceleration voltage of about 200 kV. In addition, when a high dose is checked at once, polyethylene is heated by the energy of the electron beam, and disadvantages such as melting of the film are likely to occur. For this reason, when the dose is to be 10 Mrad or more, it is preferable to divide the dose into several times.
Also, in any of the production methods, by performing a heat treatment after crosslinking the microporous membrane, it is possible to increase the substantial crosslinking density (gel fraction) even at the same dose. The microporous polyethylene membrane obtained by the above-mentioned manufacturing method may be subjected to a heat treatment at a temperature lower than the fuse temperature, if necessary, in order to enhance dimensional stability. Further, a crosslinking treatment may be further performed after the heat treatment.

[0032]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail based on embodiments. The test method shown in the examples is as follows. (1) Film thickness dial gauge (Ozaki Seisakusho: PEACOCK No2
Measured in 5). (2) Porosity A 20 cm square sample was cut out from the microporous membrane, and its volume and weight were calculated. From the obtained results, calculation was made using the following equation. Porosity (%) = 100 × (volume (cm 3) −weight (g) / 0.
95) / volume (cm3)

(3) Average pore size SEM method: Measured with a scanning electron microscope. 2. Permeation method: When a 0.05% by weight aqueous solution of pullulan (manufactured by Showa Denko) was circulated under a differential pressure of 0.5 kg / cm ^2, the concentration of pullulan contained in the filtrate was determined by differential refractive index measurement. Then, the average pore diameter d (μm) was measured from the molecular weight M of pullulan at which the rejection became 50% and the intrinsic viscosity [η] of the aqueous solution using the following equation: [η] M = 2.1 × 10 ²¹ ((d / 2) ² ) ^3/2 (4) Gel fraction Extraction was performed based on the weight change of the sample of the microporous membrane cut into a certain size according to ASTM D2765 after extracting soluble matter in boiling paraxylene for 12 hours. The ratio of the residual mass after extraction to the mass of the previous sample was determined by the following equation. Gel fraction (%) = 100 × residual mass (g) / sample mass (g)

(5) Puncture Strength Using a KES-G5 handy compression tester manufactured by Kato Tech, the radius of curvature of the needle tip is 0.5 mm, and the piercing speed is 2 m.
A piercing test was performed under the conditions of m / sec, and the maximum piercing load was defined as the piercing strength (g). Also, by multiplying the piercing strength by 25 (μm) / film thickness (μm), 2
The puncture strength was calculated as 5 μ. (6) Melt piercing strength Silicone oil pre-heated to 160 ° C. after clipping the polyethylene microporous membrane between two SUS washers having an inner diameter of 13 mm and an outer diameter of 25 mm, and clipping the surroundings (Shin-Etsu Chemical: KF- 96-10CS) to melt the sample. Melt piercing strength (g) for the molten sample in this silicone oil in the same manner as (5)
Was measured.

(7) Molecular weight between cross-linking points A polyethylene microporous membrane is cut into a size of about 25 × 100 mm, and quickly immersed in silicon oil (Shin-Etsu Chemical: KF-96-10CS) preheated to 160 ° C. to be uniform. To obtain a non-oriented, non-porous sample. After washing this sample well with methylene chloride, width 5 mm,
A test piece having a length of 30 mm was cut out and the film thickness was measured. Using a tensile tester (TCM-500, manufactured by Minebea Co., Ltd.), the test specimen was subjected to a tensile test at a temperature of 160 ° C., a distance between chucks of 20 mm, and a speed of 100 mm / min.

The stress s (kg / cm ² ) at this time is represented by α−α ⁻²
When (α is plotted against the elongation ratio [α = L / L0]), a gentle S-shaped curve is obtained. This curve is
When α ^-2 is approximately 2 to 4, the gradient becomes the minimum. At this time, assuming that the test piece is in an ideal entropy elastic state, the molecular weight between crosslink points <Mc> becomes the minimum gradient A (kg / kg).
cm ² ) and the absolute temperature T can be obtained from the following equation. <Mc> = ρRT / A where ρ (g / cm ³ ) is the density of the test piece at the measurement temperature, and R is the gas constant.

(8) Residual shrinkage ratio A sample of a microporous membrane was sandwiched between two circular metal frames having an inner diameter of 54 mm, an outer diameter of 86 mm and a thickness of 2 mm via two fluororubbers, and the periphery was fixed with clips. . The film in this state is treated with 160 ° C silicone oil (Shin-Etsu Chemical: KF-96-10
Heat treatment was performed by immersion in CS) for 1 minute to remove the orientation of the uncrosslinked portion. Next, the sample was cut out along the inner diameter of the metal frame, immersed again in 160 ° C. silicone oil for 1 minute,
At this time, the shrinkage residual rate of the sample is calculated as the major axis a of the sample.
And the minor diameter b was calculated from the following equation. Residual shrinkage (%) = (ab / 54 ² ) × 100

(9) Fuse temperature As an electrolytic solution, a solution prepared by adding lithium borofluoride to a mixed solvent of propylene carbonate and butyrolactone (volume ratio = 1: 1) to a concentration of 1.0 M was used.
An electrolytic solution is impregnated into a polyethylene microporous membrane cut out to a diameter of 16 mm, and this membrane is sandwiched between ^two nickel electrodes at a pressure of 20 kg / cm ² , and is cooled from room temperature to 20 ° C.
The impedance change when the temperature rises at 1 / min is 1V,
It was measured under the condition of 1 kHz. In this measurement, the temperature at which the impedance reached 1000Ω was defined as the fuse temperature of the microporous film. (10) Absorbed dose The dose measured by the film dosimeter at the irradiation position in the electron beam irradiation device was taken as the absorbed dose of the sample to be irradiated.

(Comparative Examples 1 and 2) and (Examples 1 to 3) High-density polyethylene having a weight average molecular weight of 250,000 (density of 0.
956) 38.25 parts, melt index 0.017
6.75 parts of linear copolymerized polyethylene (density 0.929, propylene content 1.6 mol%) and 55 parts of paraffin oil (Matsumura Petroleum Institute: P350P) at 200 ° C. using a 35 mm twin screw extruder. To prepare a hot solution, and cast the hot solution from a hanger coat die with a lip of 1800 μm onto a cooling roll adjusted to 30 ° C. to form a 1800 μm thick polymer gel sheet. After stretching this sheet 7 × 7 times using a continuous simultaneous biaxial stretching machine, paraffin oil is extracted and removed with methylene chloride,
A polyethylene microporous membrane was prepared. The polyethylene microporous membrane was subjected to a crosslinking treatment under the conditions shown in the upper part of Table 1.
The acceleration voltage at this time was 150 kV. Table 1 shows the results
Shown below.

[0040]

[Table 1]

(Examples 4 to 6) 40 parts of high-density polyethylene having a weight average molecular weight of 140,000 (density 0.962) and 60 parts of paraffin oil (Matsumura Petroleum Institute: P350P) were mixed in a batch-type melt kneader (Toyo Seiki) : Labo Plastomill) at 200 ° C. and 50 rpm for 5 minutes. The obtained kneaded material was formed by a hot press at 200 ° C. and then cooled by a water-cooled press to form a sheet having a thickness of 1000 μm. The sheet was stretched 6 × 6 times using a batch-type simultaneous biaxial stretching machine (Toyo Seiki), and then the paraffin oil was extracted and removed with methylene chloride to form a microporous polyethylene membrane. The polyethylene microporous membrane was subjected to a crosslinking treatment under the conditions shown in the upper part of Table 2. The acceleration voltage at this time was 150 kV. The results are shown in the lower part of Table 2.

[0042]

[Table 2]

(Example 7) and (Comparative Examples 3 and 4) 28 parts of high-density polyethylene having a weight-average molecular weight of 250,000, linear copolymer polyethylene having a melt index of 0.017 (density 0.929, propylene content 1. 6 mol%) 1
2 parts, paraffin oil (Matsumura Oil Research Institute: P350P) 6
0 parts were kneaded at 200 ° C. using a 35 mm twin screw extruder to prepare a hot solution, and the hot solution was cast from a hanger coat die having a lip interval of 1400 μm onto a cooling roll adjusted to 30 ° C. A 1600 μm thick polymer gel sheet was prepared. The sheet was stretched 7 × 7 times using a continuous simultaneous biaxial stretching machine, and then paraffin oil was extracted and removed with methylene chloride to form a microporous polyethylene membrane. The polyethylene microporous membrane was subjected to a crosslinking treatment under the conditions shown in the upper part of Table 3. The acceleration voltage at this time is 150k
V. The results are shown in the lower part of Table 3.

[0044]

[Table 3]

[0045]

The microporous polyethylene membrane according to the present invention has both a sharp fuse effect and high heat resistance. Therefore, especially when used as a battery separator, the stability of the membrane under the fuse state is improved, and Preventing current recovery can further enhance the safety of the battery.

Claims (5)

[Claims] 1. A microporous polyethylene membrane having a molecular weight between crosslinking points of 200,000 or less, a residual shrinkage of 15% or more, and a porosity of 20 to 80%. 2. The polyethylene microporous membrane according to claim 1, wherein the gel fraction is 80% or more. 3. An average pore size determined by a permeation method of 0.001 to 0.001.
The polyethylene microporous membrane according to claim 1 or 2, wherein the thickness is 0.1 µm. 4. A battery separator using the microporous polyethylene membrane according to claim 1. 5. A battery using the battery separator according to claim 4.