JPH1067871A - Production of microporous polyethylene film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce microporous polyethylene film having high heat resistance and strength at a high cross-linking efficiency by irradiating a stretched microporous polyethylene film with ionizing radiation in a specified oxygen atmosphere. SOLUTION: A hot soln. comprising polyethylene having a wt.-average mol.wt. of 100,000-4,000,000 and a plasticizer is cooled and solidified by the microphase separation method. After the resultant microporous sheet is stretched at least monoaxially, the plasticizer is extracted and removed from the sheet to give a stretched microporous polyethylene film. The film is irradiated with an ionizing radiation under an acceleration voltage of 500kV or lower and an absorbed dose of 1-200Mrad in an atmosphere with an oxygen concn. of 500ppm or lower, thus giving a microporous polyethylene film with a thickness of 1-500 μm, an air permeability of 3,000sec/10cc/25μm or lower, a porosity of 20-80%, a piercing strength of 300g/25μm or higher, a gel fraction of 20-98 %, and a breaking time in a silicone oil at 160 deg.C of 20sec or higher.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a separator used for battery materials such as various cylindrical batteries, prismatic batteries, thin batteries, button batteries, electrolytic capacitors and the like, and particularly as a separator for lithium batteries. The present invention relates to a method for producing a microporous polyethylene membrane suitable for use in the above.

[0002]

2. Description of the Related Art Microporous membranes have been used as materials for filter media for water purifiers and the like, for breathable clothing, separators for batteries and separators for electrolytic capacitors. In recent years, demand for lithium-ion secondary battery applications has been particularly growing, and as the energy density of batteries has increased, high performance has also been required for battery separators.

[0003] Lithium-ion secondary batteries use chemicals such as electrolytes and positive and negative electrode active materials. Therefore, polyolefin polymers are generally used as the material of the separator in consideration of chemical resistance. In particular, inexpensive polyethylene and polypropylene are used. The performance required for separators for non-aqueous electrolyte batteries such as lithium ion secondary batteries includes electrode short-circuit prevention function, ion permeability, assembly workability when winding the battery, battery safety, reliability, etc. Is raised.

The function of preventing an electrode short-circuit means that a separator functions as a partition wall interposed between the positive and negative electrodes to prevent an internal short-circuit. In order to prevent such an internal short circuit, it is necessary that the separator has a high strength, a small pore diameter and an appropriate film thickness. In a secondary battery, an internal electrode expands due to charge and discharge, and in some cases, a pressure of several tens of kg / cm ² may be applied to the separator. In addition, the electrode surface is not always smooth, and active material particles of various sizes may become projections. Even in such a case, the separator is required to have high strength so as not to be broken. With respect to the thickness of the separator, if it is too thin, current flows between the electrodes and does not function to prevent short circuit. Conversely, when the thickness of the separator is large, it is advantageous in terms of preventing short circuit, but since the volume occupied by the separator in the battery becomes large and the energy density becomes small, generally, in terms of battery design, An appropriate film thickness is set. Regarding the small pore diameter of the separator, the active material particles constituting the electrode pass through the pores of the separator and penetrate, and as a result, it is a necessary performance to prevent the internal short circuit and the ability to be reduced. . For the same reason, it is also important as a separator that there are no defects such as pinholes and thin portions.

[0005] The term "ion-permeable" means the ability of a separator to transmit only ions or an electrolytic solution without transmitting active material particles. Generally, ion permeability is high porosity,
It is evaluated for performance such as low air permeability and low electric resistance. In recent years, low-temperature discharge characteristics have become an important factor in consideration of the environment in which batteries are used. As to the assembly processability at the time of battery winding, when the separator is wound with a constant tension in the machine direction and wound with the electrode, it is required that the separator does not extend in the machine direction and does not change dimension in the width direction. . For this reason, the separator needs to have a high elastic modulus.

[0006] Battery safety means that when the battery heats up due to a problem such as an external short circuit or overcharge, the separator automatically cuts off the current and stops the heat generation, thereby suppressing runaway and explosion of the battery. It means a function. When the temperature inside the battery rises to near the melting point of the resin constituting the separator, the separator closes the pores due to heat flow, thermal deformation, or heat shrinkage, or the resin is absorbed on the electrode surface to form an insulating coating. By forming, a so-called shutdown function is exhibited. The temperature at which the pores are closed is generally called a fuse temperature, and the lower the temperature, the more desirable it is because it has the ability to cut off current at a low temperature and suppress heat generation. Further, the wider the temperature region in the shutdown state is, the longer the time during which the current is cut off is. Therefore, it is desirable to be able to withstand a temperature rise due to more intense heat generation. Furthermore, it is said that there is a temperature distribution inside a battery formed of a thick coil such as a cylindrical battery, but even if there is a locally high temperature portion, the temperature range in the shutdown state is wide. And is advantageous. The temperature at which the separator is broken at a high temperature and the electrode is short-circuited is generally called a short-circuit temperature. For the above-mentioned reason, the higher the temperature, the higher the heat resistance, which is desirable.

Therefore, it can be said that a separator having both a fuse function and heat resistance, realizing a wide shutdown area and having high strength is ideal in terms of battery safety and reliability. However, in reality,
There have been many difficulties in imparting reciprocal performance such as a fuse function and heat resistance to a battery separator. As a conventional technology of a battery separator, for example,
No. 73857 relates to a battery separator obtained by subjecting a sheet obtained by molding a composition comprising a polyolefin, a filler, and a plasticizer to a crosslinking treatment with ionizing radiation. However, the technology disclosed in this publication substantially relates to a technology for manufacturing a separator for a lead-acid battery, and does not require the extreme heat resistance required for a lithium ion secondary battery. Therefore, since there is no need to strictly control the environment such as the oxygen concentration in the ionizing radiation irradiation atmosphere, no detailed study on the oxygen concentration has been made.

As a separator for a non-aqueous electrolyte battery, Japanese Patent Application Laid-Open (JP-A) No. 63-205048 describes a microporous membrane of cross-linked polyethylene, but makes no mention of the production technique. As a technique for performing ionizing radiation treatment on a microporous film for the purpose of heat resistance for use in secondary batteries, Japanese Patent Application Laid-Open No. 3-274661 discloses a technique of irradiating ionizing radiation with a low absorption dose of 0.1 to 10 Mrad. Thus, attempts have been made to achieve both a fuse function and heat resistance. However,
Since the microporous membrane disclosed in this publication has a relatively large pore size, if the crosslinkage is too high and the melt viscosity of the resin is too high, the micropores will not be clogged, and the expression of the fuse function will be slow. It was difficult to impart high heat resistance to the film. For this reason, this publication discloses a technique for replacing the irradiation atmosphere with an inert gas such as nitrogen, but attempts to strictly control the oxygen concentration for the purpose of increasing the crosslinking efficiency have not been made. .

In Japanese Patent Application Laid-Open No. Hei 3-5997, a crosslinked film layer is made to have heat resistance by forming a radiation-crosslinked film and an uncrosslinked film into a microporous film.
It also discloses a technique in which an uncrosslinked film layer shares a fuse function. However, in this publication,
No detailed study has been made on the replacement of the inert gas in the irradiation atmosphere, the oxygen concentration, and the like.

[0010]

In the conventional microporous membrane manufacturing technology, when irradiating with ionizing radiation for the purpose of imparting heat resistance to the microporous membrane, the irradiation environment such as the atmospheric oxygen concentration is affected by the irradiation of the microporous membrane. Despite being a very important point in determining performance, it has not been clarified, so cross-linking efficiency cannot be sufficiently increased, and microporous membranes have problems such as insufficient heat resistance and strength. I was In the irradiation of ionizing radiation to the microporous film, the above-mentioned problem occurs because the microporous film has an extremely large surface area as compared with the non-porous film, so that oxygen and oxygen are irradiated during irradiation of ionizing radiation. This is because the contact probability is increased and the crosslinking reaction is inhibited. Therefore, it is necessary to reduce the oxygen concentration in the atmosphere in which the microporous film is irradiated with ionizing radiation as compared with the case where the oxygen concentration is applied to a normal nonporous film. Thus, an object of the present invention is to provide a method for producing a high-strength polyethylene microporous membrane having excellent heat resistance.

[0011]

Means for Solving the Problems The present inventor has conducted intensive studies to solve the above-mentioned problems. As a result of reducing the oxygen concentration in the irradiation atmosphere in the step of performing a cross-linking treatment with ionizing radiation, the present inventors have succeeded in reducing heat resistance. The present inventors have found a method for producing a polyethylene microporous membrane having excellent strength and high strength, and have accomplished the present invention.

That is, the present invention provides a method for producing a microporous polyethylene membrane comprising a step of subjecting a stretched microporous polyethylene film to a crosslinking treatment by irradiation with ionizing radiation. 50 concentration
The present invention relates to a method for producing a microporous polyethylene membrane, characterized in that the content is 0 ppm or less. Furthermore, the present invention relates to the above-mentioned method for producing a microporous polyethylene membrane, wherein the oxygen concentration in the ionizing radiation irradiation atmosphere is 100 ppm or less.

The polyethylene microporous membrane of the present invention refers to a porous sheet or film substantially composed of polyethylene, and is used, for example, as a battery material for a separator or the like. The form of the battery is not particularly limited, and is suitable for use in, for example, cylindrical batteries, square batteries, thin batteries, button batteries, electrolytic capacitors, and the like.

The polyethylene used for the polyethylene microporous membrane of the present invention is used for ordinary extrusion, injection, inflation, and blow molding, and has low density, medium density, high density, and linear low density. , And crystalline polymers having ethylene as a constitutional unit, such as ultrahigh molecular weight polyethylene. Further, as the polyethylene, if it is a polymer mainly composed of ethylene, a copolymer such as an ethylene-α-olefin copolymer or EPR may be blended. The average molecular weight of polyethylene is preferably 100,000 to 4,000,000, and more preferably 200,000 to 700,000. The average molecular weight of polyethylene refers to a weight average molecular weight obtained by GPC (gel permeation chromatography) measurement or the like. Generally, for polyethylene having an average molecular weight exceeding 1,000,000, accurate GPC Since measurement is not possible, the viscosity average molecular weight can be used instead. If the average molecular weight is less than 100,000, it is not preferable because stretchability is deteriorated or the strength becomes low, and if it is more than 4,000,000, it tends to be difficult to obtain a uniform composition.

In the present invention, as a method for obtaining a stretched microporous film before irradiation with ionizing radiation, there are, for example, the following two methods. The first method is
A heating solution comprising a polymer and a plasticizer is cooled and solidified by a microphase separation method to obtain a microporous sheet, and thereafter, a step of stretching at least uniaxially and a step of extracting and removing the plasticizer are performed. It is a method of obtaining a film. As the order of the step of stretching and the step of extracting and removing the plasticizer,
Stretching after extraction may be acceptable. However, stretching before extraction can effectively impart orientation to the microporous film and can suppress wasteful increase in porosity, so that high strength is preferred. Further, it is more preferable that the film is stretched before the extraction and further subjected to a stretching treatment in at least one axial direction after the plasticizer is extracted and removed, since the porosity and strength of the microporous membrane can be freely adjusted. Further, a filler, a heat stabilizer, a nucleating agent, and the like may be added to a heating solution composed of a polymer and a plasticizer.

The second method is a method in which a polymer is formed into an extruded sheet and a microporous film is obtained by a stretch opening method.
As the plasticizer, an organic compound capable of forming a homogeneous solution with polyethylene at a temperature equal to or lower than the boiling point of the plasticizer is used. And alcohols such as stearyl alcohol and oleyl alcohol.

As the solvent used for the purpose of extracting the plasticizer, an organic compound capable of mixing with the plasticizer at a temperature not higher than the boiling point of the solvent to form a uniform solution and extracting and removing the plasticizer from the microporous membrane. Is used. Examples of such extraction solvents include hydrocarbons such as n-hexane and n-heptane, alcohols such as ethanol and isopropanol,
Halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane, ketones such as acetone and 2-butanone, and ethers such as diethyl ether can be used.

The method for producing a microporous polyethylene membrane of the present invention is carried out by subjecting a stretched microporous polyethylene film to a crosslinking treatment by irradiation with ionizing radiation in an environment in which the oxygen concentration is sufficiently reduced. . Irradiation with ionizing radiation may be carried out at one time, but may be carried out several times in order to suppress the temperature rise due to self-heating of the microporous film. The irradiation direction of the microporous film is preferably reduced, for example, by irradiating from one side of the film to reduce equipment cost and production cost, and by irradiating from both sides of the film, uniformity in the thickness direction is improved.

In the present invention, the ionizing radiation is an electron beam,
α-rays, β-rays, γ-rays, X-rays, ultraviolet rays, and the like can be used. In particular, when an electron beam is used, the productivity is high because the facility is simple and the production speed can be increased. The absorbed dose of the ionizing radiation in the present invention is preferably 1 to 200 Mrad, more preferably 5 to 5 Mrad.
0 Mrad. Excessive irradiation such that the absorbed dose exceeds 200 Mrad is not preferable because the strength of the microporous film is reduced.

In the present invention, the accelerating voltage, particularly when the electron beam irradiation is performed, can be arbitrarily selected according to the thickness and the porosity of the microporous film.
V or lower, more preferably 100 to 300 kV.
The accelerating voltage is sufficient if the electron beam penetrates the microporous film. If the accelerating voltage is higher than 500 kV, equipment cost and production cost increase, which is not preferable. The magnitude of the acceleration voltage can be selected according to the purpose. That is, if a higher acceleration voltage is set, the electron beam penetrates the microporous film, so that a film uniformly irradiated in the film thickness direction is obtained.If a lower acceleration voltage is set, the electron beam penetrates. And an asymmetric film irradiated only on the surface is obtained.

In the method for producing a microporous polyethylene membrane according to the present invention, the method for reducing the oxygen concentration in the ionizing radiation irradiation atmosphere is such that an inert gas is fed into the irradiation atmosphere of an ionizing radiation irradiation apparatus. This is achieved by replacing the air and exhausting the atmosphere to the outside of the irradiation atmosphere, or maintaining the atmosphere replaced by the inert gas away from the outside. The inert gas to be used may be a normal temperature gas which does not deteriorate by irradiation with ionizing radiation and does not cause a chemical reaction with polyethylene. As such an inert gas, for example, helium, argon, nitrogen, or the like is preferable. Among them, the use of nitrogen is more preferable because the production cost can be reduced.

In the present invention, the oxygen concentration in the ionizing radiation irradiation atmosphere is 500 ppm or less, preferably 1 ppm.
00 ppm or less, and most preferably 50 ppm or less. The lower the oxygen concentration, the higher the gelation efficiency by cross-linking, which is preferable because the absorbed dose can be reduced. Oxygen concentration is 500
If the concentration is higher than ppm, the gelation efficiency of the resulting polyethylene microporous membrane due to crosslinking decreases and the heat resistance decreases, and if sufficient heat resistance is to be obtained, it is necessary to set a higher absorbed dose. This results in low strength, which is undesirable in any case. Such a method of measuring the oxygen concentration can be measured, for example, by installing a gas sampling tube of an oximeter in an irradiation atmosphere. As the oximeter, for example, a galvanic cell type oximeter or the like can be used.

The thickness of the polyethylene microporous membrane obtained by the production method of the present invention is preferably 1 to 500 μm.
m, more preferably 10 to 100 μm. When the film thickness is smaller than 1 μm, the mechanical strength becomes insufficient.
If it is larger than 00 μm, the volume occupied by the separator increases, which is disadvantageous in increasing the capacity of the battery, which is not preferable.

The air permeability of the microporous polyethylene membrane obtained by the production method of the present invention is preferably 3000 seconds /
100 cc / 25 μm or less, more preferably 1
000 sec / 100 cc / 25 μm or less. The air permeability is defined by the ratio between the air transmission time and the film thickness. If the air permeability is more than 3000 seconds / 100 cc / 25 μm, the ion permeability deteriorates or the pore size becomes extremely small, which is not preferable from the viewpoint of permeability.

The porosity of the polyethylene microporous membrane obtained by the production method of the present invention is preferably 20 to 80.
%, More preferably 30 to 60%. Porosity 2
If it is less than 0%, ion permeability typified by air permeability and electric resistance becomes insufficient, and if porosity is more than 80%, strength typified by piercing strength and tensile strength becomes insufficient.

The piercing strength of the microporous polyethylene membrane obtained by the production method of the present invention is 300 g / 25 μm.
m or more, and more preferably 400
g / 25 μm or more. The piercing strength is defined by the ratio between the maximum load and the film thickness in the piercing test. If the piercing strength is smaller than 300 g / 25 μm, defects such as short-circuit failure increase during battery winding, which is not preferable.

The gel fraction of the polyethylene microporous membrane obtained by the production method of the present invention is preferably 1% or more,
More preferably it is 20-98%, and most preferably 50-90%. If the gel fraction is less than 1%, sufficient heat resistance cannot be obtained. Here, the larger the gel fraction is, the higher the heat resistance of the microporous membrane becomes.
Irradiation with excessive ionizing radiation may cause side effects such as a decrease in intensity, so that excessive radiation irradiation is not preferred.

The breaking time of the microporous polyethylene membrane obtained by the production method of the present invention in silicone oil at 160 ° C. is preferably 20 seconds or more. The rupture time is correlated with the heat resistance of the battery separator evaluated in a battery mounting test such as an overcharge test, an external short-circuit test, and a heating test. The item is rejected, which is not preferable in terms of battery safety.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to examples. The test method shown in the examples is as follows. (1) Film thickness dial gauge (PEACOCK NO.
25). (2) Porosity A sample of 20 cm square was cut out from the microporous membrane, and the volume (cm)
³ ) and weight (g) were measured, and porosity (%) was calculated from the obtained result using the following equation. Porosity = 100 × (volume−weight / 0.95) / volume (3) Air permeability Air permeability time (sec / 100 cc) determined by measuring with a Gurley air permeability meter according to JIS P-8117. ) And the film thickness (μm) were converted into a film thickness according to the following formula to obtain an air permeability (sec / 100 cc / 25 μm). Air permeability = air permeability time × 25 / film thickness (4) Gel fraction Based on ASTM D2765, a sample cut to a certain size was subjected to a soluble component elution operation in boiling para-xylene for 12 hours. From the ratio of the sample weight (g) before the elution operation to the residual weight (g) after the elution operation, the gel fraction (%) was calculated according to the following equation. Gel fraction = 100 × remaining weight / sample weight (5) Puncture strength Using a compression tester (KES-G5 manufactured by Kato Tech),
A piercing test is performed 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 a maximum piercing load (g).
The film thickness was converted from the film thickness and the film thickness (μm) according to the following formula to obtain the piercing strength (g / 25 μm). Puncture strength = maximum pierce load × 25 / film thickness (6) Breaking time Two specimens each having a width of 10 mm were stacked and fixed between chucks with a spacing of 50 mm.
60 ° C silicone oil (KF-96- manufactured by Shin-Etsu Chemical Co., Ltd.)
The stress relaxation behavior when immersed in 10CS) and the time until the sample fractured were measured from visual observation. Here, when the rupture time was 10 minutes or more, it was evaluated as Δ. (7) Absorbed dose At the irradiation position in the electron beam irradiation device, a device dominant constant K was obtained with a film dosimeter (manufactured by FWT Inc.), and the current (mA) and the sample transfer speed (m / min) were obtained. ), The absorbed dose (Mrad) was calculated according to the following equation. Absorbed dose = K × current / transfer speed (8) Oxygen concentration A gas sampling tube of a galvanic cell type oxygen concentration meter (MKI-50 manufactured by Osaka Oxygen Industry Co., Ltd.) was installed in the irradiation atmosphere of the electron beam irradiation device, and the measurement range was 0. The oxygen concentration (ppm) was measured at ｐｐｍ2000 ppm.

[0030]

Example 1 High-density polyethylene (weight average molecular weight 25
38 parts by weight, linear copolymerized polyethylene (melt index 0.017, density 0.92)
9, 7 parts by weight of propylene content 1.6 mol%) liquid paraffin (kinematic viscosity at 37.78 ° C. 75.9 cS)
t) 55 parts by weight, and 0.3 based on the polyethylene
A cooling roll controlled at a surface temperature of 20 ° C. by kneading a composition consisting of 2,6-di-t-butyl-p-cresol in parts by weight at 200 ° C. with a 35 mm twin screw extruder through a coat hanger die. The sheet having a thickness of 1600 μm was obtained by extrusion casting. The obtained sheet is stretched with a simultaneous biaxial stretching machine to a stretching ratio of 7 × 7, followed by extracting and removing liquid paraffin using methylene chloride, and then drying and removing adhering methylene chloride. A porous film was obtained. Next, nitrogen was fed into the electron beam irradiation apparatus to replace the irradiation atmosphere gas with nitrogen, and the obtained film was irradiated with an electron beam under the conditions of an acceleration voltage of 150 kV and an absorbed dose of 5 Mrad. Before the electron beam irradiation, the oxygen concentration in the irradiation atmosphere of the apparatus was measured and found to be 30 p.
pm. Table 1 shows the physical properties of the obtained polyethylene microporous membrane.

[0031]

Example 2 An operation was performed in the same manner as in the production method described in Example 1, except that the flow rate of nitrogen fed into the electron beam irradiation apparatus was reduced and the electron beam was irradiated. When the oxygen concentration in the irradiation atmosphere of the apparatus was measured before the irradiation with the electron beam,
It was 200 ppm. Table 1 shows the physical properties of the obtained polyethylene microporous membrane.

[0032]

Comparative Example 1 Example 2 was repeated except that the electron beam was irradiated while the flow rate of nitrogen fed into the electron beam irradiation apparatus was further reduced.
The operation was carried out in the same manner as described in the above. Before the electron beam irradiation, the oxygen concentration in the irradiation atmosphere of the apparatus was measured and found to be 1000 ppm. Table 1 shows the physical properties of the obtained polyethylene microporous membrane.

[0033]

Comparative Example 2 The absorbed dose per irradiation was 5 Mrad, and the microporous film was alternately placed on the front and back four times in total (total 20 Mrad).
The same operation as in the production method described in Comparative Example 1 was performed except that irradiation with an electron beam (rad) was performed. Table 1 shows the physical properties of the obtained polyethylene microporous membrane.

[0034]

[Table 1]

[0035]

According to the method for producing a microporous polyethylene membrane of the present invention, in the step of performing a cross-linking treatment with ionizing radiation, the oxygen concentration in the irradiation atmosphere is reduced, thereby increasing the cross-linking efficiency and improving the heat resistance. It is possible to produce a polyethylene microporous membrane having excellent strength and high strength.

Claims (2)

[Claims] 1. A method for producing a polyethylene microporous membrane comprising a step of subjecting a stretched polyethylene microporous film to a crosslinking treatment by ionizing radiation irradiation, wherein the oxygen concentration in the ionizing radiation irradiation atmosphere is 500 ppm or less. A method for producing a microporous polyethylene membrane. 2. The method according to claim 1, wherein the oxygen concentration in the ionizing radiation irradiation atmosphere is 100 ppm or less.
3. The method for producing a microporous polyethylene membrane according to item 1.