JP2004335255A - Manufacturing method of polyolefine microporous membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stably provide a polyolefine microporous membrane with a basic property kept suitable as a separator for a lithium ion secondary cell, and having a small MD (machine direction) maximum shrinkage stress. <P>SOLUTION: When a heat solution of polyolefine in the middle of a cooling process is cast, a peripheral speed of a first roll is made faster than that of a second roll, and preferably, a film is formed without forming a bank at this time, then a lock means to retain a sheet to the roll is provided. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００４１】
上記表１に示すように、実施例で得られた本発明の微多孔膜は、膜厚分布が小さく、ＭＤ最大収縮応力も小さい。比較例ではこれらを共に満足させていないことが分かる。
【００４２】
【発明の効果】
本発明の方法によれば、熱可塑性樹脂シート作成時に第１ロールを第２ロールの周速度より大きくすることでＭＤ配向の小さい微多孔膜を安定して得ることができ、このときバンクを形成することなく成膜し、ロールにシートを係止する手段を持たせれば更にＭＤの収縮応力の小さい微多孔膜をより安定して得ることができる。
【図面の簡単な説明】
【図１】キャスト成型機の一例を示す模式図である。
【図２】ローレット目付きロールの一例を示す模式図である。
【符号の説明】
１ Ｔ−ダイ
２ 第１ロール
３ 第２ロール
４ 第３ロール
５ バンク（樹脂溜り）
６ 熱可塑性樹脂溶液
７ ローレット目加工
８ ロール[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a microporous polyolefin membrane, which is used as a separator for a secondary battery, and particularly suitably used as a separator for a lithium ion secondary battery.
[0002]
[Prior art]
BACKGROUND ART Microporous membranes made of polyolefin are used as separators in various batteries, and are particularly suitably used in lithium-ion secondary batteries, whose demand has been rapidly increasing in recent years. The microporous film made of polyolefin has, as its basic properties, excellent electronic insulation, has ion permeability by impregnation with an electrolyte, has excellent resistance to electrolyte and oxidation, has moderate strength, It has performances such as having a pore closing effect at about ° C, which seems to be the reason why these are preferably used.
[0003]
However, demands for polyolefin microporous membranes have become stricter with intensified performance competition for lithium ion secondary batteries. For example, thinning of a microporous film is indispensable for increasing the capacity and density of a battery, and the film forming technology accompanying this is becoming more advanced year by year.
Further, in order to prevent short circuit due to film shrinkage at the time of high temperature test of the battery, it is required to lower the heat shrinkage stress of the separator.
[0004]
Many proposals have been made so far as separators having a small shrinkage stress. For example, in the microporous membrane of Patent Document 1, in order to reduce the contraction stress in the TD (direction perpendicular to the machine direction), the contraction in the contraction stress relaxation step is 0.95 times or less in the TD direction to improve safety. It is disclosed that. However, this is a technique in which the contraction stress of the TD is reduced, and a sufficient effect has not been obtained in reducing the contraction stress in the MD (machine direction).
[0005]
When the shrinkage stress of the MD is large, especially when the separator is fixed for each turn part in a rectangular battery or the like, a short circuit may occur due to the MD shrinkage and the exposure of the electrode due to the film breakage during the high temperature test of the battery. . Further, the shrinkage stress of MD affects the shrinkage stress in the TD direction, which is not preferable.
Therefore, a method for reducing the MD orientation is required. For the purpose of suppressing the MD maximum shrinkage stress under the same stretching conditions, Patent Document 2 discloses an acrylic resin and Patent Document 3 describes a thermoplastic resin without rolling. Reports that the shrinkage stress of MD can be suppressed by molding.
However, this method alone was not sufficient to meet the current demand for polyolefin microporous membranes.
[0006]
That is, as a tendency of the recent method for producing a microporous membrane made of polyolefin, a higher molecular weight polyolefin is used in order to improve heat resistance and mechanical strength of the microporous membrane. Particularly, since ultrahigh molecular weight polyolefins have recently been used, the viscosity of the resin increases, and the orientation of MD during casting tends to be higher, and film formation tends to be difficult due to the high viscosity. As a countermeasure, for example, a film is formed so that there is no bank between rolls at the time of casting, or a film forming method using a sleeve belt is taken. However, film formation is unstable and film thickness distribution is deteriorated, I meandered on the roll.
[0007]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2001-81221 [Patent Document 2]
JP 2002-3620 A [Patent Document 3]
Japanese Patent Application Laid-Open No. Hei 9-164581
[Problems to be solved by the invention]
An object of the present invention is to stably provide a polyolefin microporous membrane having a small MD shrinkage stress while maintaining basic performance, which is suitable as a separator for a lithium ion secondary battery.
[0009]
[Means for Solving the Problems]
The present invention has solved the above problems. That is, the present invention is as follows.
(1) (A) A step of kneading a polyolefin and a plasticizer, extruding from a T-die as a hot solution, sandwiching between two rolls, and forming a gel-like sheet after cooling; A method for producing a microporous membrane, comprising a step of (C) extracting a plasticizer after stretching through a step of stretching a sheet, wherein in the step (A), the peripheral speed Av of the first roll and the peripheral speed of the second roll A method for producing a microporous membrane, wherein the speed Bv satisfies the following expression (1).
2 ≧ Av / Bv> 1 (1)
(2) The method for producing a microporous membrane according to (1), wherein no bank (resin pool) is generated between the first roll and the second roll in the step (A).
(3) The method for producing a microporous membrane according to (1) or (2), wherein in the step (A), a locking means for fixing the sheet to one or more rolls is provided.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention will be specifically described, particularly focusing on its preferred embodiments.
The feature of the present invention lies in the film forming method at the time of casting, that is, the speed ratio Av / Bv of the peripheral speed Av of the first roll and the peripheral speed Bv of the second roll at the time of casting is larger than 1. If the speed ratio Av / Bv exceeds 2, the film formation stability is affected. Also, although the reason is not clear, if the speed ratio Av / Bv exceeds 1, the orientation in the MD direction becomes small, and the film thickness further increases. The distribution tends to improve. Therefore, the speed ratio Av / Bv needs to be larger than 1 and equal to or smaller than 2, and more preferably, larger than 1 and equal to or smaller than 1.2.
[0011]
Here, the first roll is, of the two rolls that are first sandwiched after the hot solution is extruded from the T-die, a portion of the two rolls in contact with the gel-like sheet that is smaller than the second roll. The second roll refers to a roll having a portion in contact with the gel-like sheet larger than the first roll and serving to send the gel-like sheet to a subsequent process. Similarly, if necessary, it is possible to add third and fourth rolls from the side closer to the T-die. When the third roll is directly below the T-die, the roll with the higher peripheral speed is the first roll, and the roll with the lower peripheral speed is the second roll.
[0012]
In order to hold the gel-like sheet, it is necessary to rotate the first roll and the second roll in reverse, and the peripheral speeds of the respective rolls are opposite to each other. The peripheral speed of the second roll at this time depends on the extrusion amount of the polyolefin hot solution, the die shape, the film thickness, the film width, etc., but from the viewpoint of achieving both productivity and film formation controllability, the extrusion amount is 20 kg / h. It is preferably from 0.1 to 20 m / min in terms of conversion, more preferably from 0.3 to 15 m / min, even more preferably from 0.5 to 5 m / min.
[0013]
At the time of casting, a bank of the polyolefin hot solution may or may not be provided between the two rolls. However, without the bank, it is more preferable that the sheet is not easily rolled and the orientation in the MD direction is reduced.
The bank referred to in the present invention means a resin pool between two rolls as a polyolefin hot solution sandwiched between two rolls, and is generally obtained by the following two methods. And a state where there is no bank between the two rolls.
[0014]
(1) From a state in which there is a bank between two rolls under a constant extrusion amount and extrusion conditions, by increasing the distance between the two rolls, the polyolefin hot solution is prevented from being diffused between the rolls. I do.
(2) From a state where there is a bank between two rolls under a constant amount of extrusion and extrusion conditions, by increasing the peripheral speed of the first and second rolls and increasing the take-up speed of the polyolefin thermal solution, the polyolefin thermal solution is increased. Between rolls.
[0015]
Further, the locking means to be fixed in the present invention, the polyolefin hot solution or sheet is pulled by a take-up roll or the like, or does not slip on the roll even if the extrusion amount variation of the extruder occurs. Alternatively, it is a means mainly applied to reduce the slip of the seat.
The non-slip state here means that the peripheral speed of the roll contacting the sheet is the same as the traveling speed of the sheet, or the roll and the sheet contacting the sheet are discontinuous in the vertical and horizontal directions due to the difference in speed. It refers to a state in which no deviation occurs, and in the present invention, refers to a range in which the speed ratio between the roll contacting the sheet and the sheet is 0.9 or more and 1.1 or less. In addition, when each speed approaches even slightly from the sliding state, it can be said that the slip is reduced.If the degree of each speed approaches by introducing some locking means from the sliding state is large, the locking means The anti-slip effect is high.
[0016]
There are various types of such locking means, and there is no particular limitation on the use of the locking means, but among them, uneven processing is preferable because it has a high effect in reducing slip.
Here, the concavo-convex processing refers to processing that causes undulations on the surface of the material. Examples of the processing method include blast processing, gear shape processing, embossing, and knurling. Although there is no particular limitation on the method of processing the concavities and convexities applied to the roll, knurling is preferable because the processing is easy and the effect of reducing slip is high.
[0017]
The locking means for fixing according to the present invention may be provided on the entire surface of the roll or may be a part thereof. Although it is desirable to apply it to a part of both ends from the viewpoint that the post-press remains, there is no effect unless it is applied to the position related to the sheet. Depending on the application, it may be applied to the entire surface. The material of the roll to which the locking means is applied is not particularly limited, but a metal having high durability is preferable. The position and the number of the rolls to which the locking means are applied are not particularly limited. High effect and preferred.
[0018]
Further, the roll may be provided with a temperature control function, and the sheet may be gradually cooled or annealed. The temperature control may be dielectric heating or hot solvent heating, but hot solvent heating using a fluid such as water or oil having good heat conductivity is preferable, and oil temperature control that can be used up to high temperatures is more preferable.
Further, if the effect of preventing slippage on the sheet is exhibited, the sheet may or may not be pressure-bonded between the rolls. However, it is preferable to press-bond the sheet in order to securely fix it.
[0019]
The film thickness distribution in the TD direction of the stretched film in the invention is preferably 4 μm or less, more preferably 2 μm or less. If it is larger than 4 μm, the product yield of the target thickness is unfavorably low.
The piercing strength of the microporous membrane in the present invention is preferably from 1 N / 25 μm to 20 N / 25 μm, more preferably from 3 N / 25 μm to 10 N / 25 μm.
[0020]
Further, the MD maximum shrinkage stress of the microporous film in the present invention is preferably 0.1 MPa or more and 10 MPa or less, and when formed so that the piercing strength is 7 N / 25 μm, it is preferably 0.5 or more and 5 MPa or less. A range of 7 to 1.5 MPa is more preferable. If it is less than 0.5 MPa, the shrinkage stress in the TD direction inevitably increases, and a short circuit may occur due to film shrinkage during battery assembly or a short circuit may occur due to film shrinkage during a high temperature test of the battery. If the pressure is higher than 5 MPa, for example, when the separator is fixed for each turn portion in a rectangular battery or the like, a short circuit may occur due to the shrinkage of the MD and the exposure of the electrode due to the film breakage during the high temperature test of the battery.
[0021]
The microporous membrane of the present invention is obtained, for example, by a manufacturing method including the following steps (a) to (d).
(A) A mixture comprising a polyolefin and a plasticizer is melt-kneaded to form a hot solution.
(B) The hot solution is extruded, molded into a gel-like sheet, and cooled and solidified.
(C) The obtained gel-like sheet is stretched.
(D) After stretching, the plasticizer is extracted.
[0022]
The polyolefin used in the present invention is a homopolymer or a copolymer of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene. It can be a mixture. From the viewpoint of the performance of the porous membrane, polyethylene and a copolymer thereof are preferred.
The plasticizer as referred to herein is an organic compound capable of forming a uniform solution with a polyolefin at a temperature not higher than the boiling point.Specifically, decalin, xylene, dioctyl phthalate, dibutyl phthalate, stearyl alcohol, oleyl alcohol, decyl Examples include 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 is not particularly limited, but is preferably 20 wt% or more from the viewpoint of porosity of the obtained film, and is preferably 90 wt% or less from the viewpoint of viscosity. More preferably, the content is 50 wt% to 70 wt%.
[0023]
The extraction solvent used in the present invention is preferably a poor solvent for the polyolefin and a good solvent for the plasticizer, and has a boiling point lower than the melting point of the polyolefin. Examples of such an extraction solvent include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; alcohols such as ethanol and isopropanol; And ketones such as butanone. It is selected from these and used alone or in combination.
[0024]
The step of kneading the polyolefin composition and the plasticizer, extruding a hot solution into a gel-like sheet after cooling, for example, supplying the resin composition and the plasticizer to an extruder, kneading at about 200 ° C., A continuous sheet can be formed by casting from a hanger coat die onto a cooling roll adjusted to a crystallization temperature or lower.
Here, the temperature of the hot solution is preferably in the range of 150 to 300 ° C. The thickness of the gel-like sheet depends on the stretching ratio and the thickness of the microporous membrane in the final stage, but is preferably 0.1 to 3 mm. The temperature at which the hot solution is extruded and cooled is preferably from 10 to 130 ° C.
[0025]
The stretching step is performed at least once before extracting the plasticizer. The stretching is preferably simultaneous biaxial stretching by a tenter method. The stretching temperature is from room temperature to the melting point of the polyolefin, preferably 80 to 150 ° C, more preferably 100 to 140 ° C. The stretching ratio is preferably from 4 to 400 times, more preferably from 8 to 200 times, even more preferably from 16 to 100 times, in area ratio. The stretching ratio is preferably 4 times or more from the viewpoint of securing sufficient strength as a separator, and 400 times or less from the viewpoint of easiness of stretching and porosity.
[0026]
Next, in the extraction step (d), the plasticizer is extracted by immersing the stretched film obtained in (c) in the above-mentioned extraction solvent, and then sufficiently dried.
If necessary, a heat setting step can be added. In the heat setting step, the film after the extraction of the plasticizer is stretched in the MD or TD direction using a uniaxial stretching machine or a simultaneous biaxial stretching machine, and then the film is contracted to reduce the shrinkage stress. The polyolefin microporous film obtained by the above method can be subjected to surface treatment such as plasma irradiation, surfactant impregnation or application, surface grafting, etc., as necessary.
The polyolefin microporous membrane obtained by the above-described production method has a small MD orientation and a good film thickness distribution even under the same stretching conditions as before.
[0027]
Hereinafter, the present invention will be described specifically with reference to Examples and Comparative Examples, but these do not limit the scope of the present invention. Various physical properties used in the present invention were measured based on the following test methods.
(1) Film thickness was measured with a dial gauge (Ozaki Seisakusho: "PEACK No. 25" (trademark)).
(2) Film thickness distribution The film thickness of the obtained biaxially stretched film (film width 58 cm) was measured at intervals of 5 cm in the TD direction using a dial gauge, and this measurement was continuously performed in five rows in the MD direction. The difference between the value and the minimum was calculated.
[0028]
(3) Puncture strength A puncture test was performed using a “KES-G5 Handy Compression Tester” (trademark) manufactured by Kato Tech under the conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec, and a maximum puncture load ( N) was measured. The measured value was multiplied by 25 μm / film thickness to obtain a 25 μm converted puncture strength (N).
[0029]
(4) Maximum MD shrinkage stress (MPa)
The temperature of the obtained microporous membrane was measured using a thermomechanical analyzer (SHIMADZU TMA-50, trade name), and the shrinkage load (N) was measured. The measurement conditions are as follows: sample shape; width 3 mm × length 10 mm, initial load: 9.8 mN, temperature scanning range 30 to 200 ° C., heating rate: 10 ° C./min. The MD maximum shrinkage stress was calculated by substituting the maximum shrinkage load (N) in the obtained shrinkage load curve into the following equation.
MD maximum shrinkage stress (Pa) = maximum shrinkage load (N) / (A × T)
A: Sample width (m)
T: Sample thickness (m)
[0030]
【Example】
[Example 1]
0.3 parts by weight of an antioxidant was added to 30 parts by weight of HDPE having a Mv of 300,000, 25 parts by weight of UHDPE having a Mv of 2 million, 15 parts by weight of HDPE having a Mv of 700,000, and 30 parts by weight of an ethylene / propylene copolymer having a Mv of 120,000. This mixture was fed to a twin-screw extruder via a feeder so as to be 35 parts by weight. Further, liquid paraffin was injected into the cylinder of the extruder so as to be 65 parts by weight. Melt kneading was performed at a temperature of 200 ° C., a screw rotation speed of 280 rpm, and a discharge rate of 12 kg / h.
[0031]
Subsequently, the melt-kneaded polymer composition was extruded onto a roll through a T-die, and the peripheral speed of the first roll was 0.72 m / min and 0.6 m, respectively, such that the peripheral speed of the second roll was 1.2 times that of the second roll. / Min while casting to form a bank.
At this time, the speed ratio C / D between the speed C of the third roll and the speed D of the sheet on the third roll is 1.12, which is not in the range of 0.9 or more and 1.1 or less, and slippage is observed. Was done.
[0032]
Next, it was guided to a simultaneous biaxial tenter stretching machine to perform biaxial stretching. The stretching conditions are as follows: MD magnification 7.0 times, TD magnification 6.4 times, and temperature 120 ° C. Next, the mixture was led to a methyl ethyl ketone tank, and was sufficiently immersed in methyl ethyl ketone to extract and remove the plasticizer. Thereafter, methyl ethyl ketone was dried and removed to obtain a microporous membrane. The resulting microporous membrane was heat-set at 120 ° C.
[0033]
[Example 2]
Example 1 was repeated except that the peripheral speeds of the first roll and the second roll were set to 0.84 m / min and 0.7 m / min, respectively, and that there was no bank between the rolls.
[0034]
[Example 3]
The knurling (JIS B 0951-1962) was carried out in the same manner as in Example 1 except that 26% of the entire roll area was pulled off by third rolls provided on both ends of the roll. At this time, the speed ratio C / D between the speed C of the third roll and the speed D of the sheet on the third roll was 1.01, which was in the range of 0.9 or more and 1.1 or less. I didn't see it.
[0035]
[Example 4]
The knurling (JIS B 0951-1962) was taken up by third rolls each having 26% of the entire roll area applied to both ends of the rolls, and the peripheral speeds of the first roll and the second roll were each 0.84 m / min. The operation was performed in the same manner as in Example 1 except that the casting was performed so that there was no bank between the rolls at 0.7 m / min.
[0036]
[Comparative Example 1]
The same operation as in Example 1 was performed, except that the peripheral speed of the first roll and the second roll was 0.6 m / min, respectively, and molding was performed at a constant speed.
[0037]
[Comparative Example 2]
The same operation as in Example 2 was performed, except that the peripheral speed of the first roll and the second roll was 0.75 m / min, respectively, and molding was performed at a constant speed.
[0038]
[Comparative Example 3]
Example 3 was carried out in the same manner as in Example 3, except that the peripheral speed of the first roll and the second roll was 0.6 m / min, and molding was performed at a constant speed.
[0039]
[Comparative Example 4]
Example 4 was carried out in the same manner as in Example 4, except that the first roll and the second roll had a peripheral speed of 0.75 m / min, respectively, and were molded at a constant speed.
[0040]
[Table 1]

[0041]
As shown in Table 1, the microporous film of the present invention obtained in the examples has a small film thickness distribution and a small MD maximum shrinkage stress. It can be seen that both of these are not satisfied in the comparative example.
[0042]
【The invention's effect】
According to the method of the present invention, a microporous film having a small MD orientation can be stably obtained by making the first roll greater than the peripheral speed of the second roll during the preparation of the thermoplastic resin sheet. If the roll is provided with a means for locking the sheet, the microporous film having a smaller MD shrinkage stress can be obtained more stably.
[Brief description of the drawings]
FIG. 1 is a schematic view showing an example of a cast molding machine.
FIG. 2 is a schematic view illustrating an example of a knurled roll.
[Explanation of symbols]
1 T-die 2 First roll 3 Second roll 4 Third roll 5 Bank (resin pool)
6 Thermoplastic resin solution 7 Knurling 8 Roll

Claims (3)

(A) Kneading a polyolefin and a plasticizer, extruding from a T-die as a hot solution, sandwiching between two rolls, and after cooling into a gel-like sheet, (B) stretching the gel-like sheet And (C) extracting the plasticizer after stretching, wherein in the step (A), the peripheral speed Av of the first roll and the peripheral speed Bv of the second roll are increased. And a method for producing a microporous membrane characterized by satisfying the following formula (1).
2 ≧ Av / Bv> 1 (1) 2. The method for producing a microporous membrane according to claim 1, wherein in the step (A), no bank (resin pool) is generated between the first roll and the second roll. 3. The method for producing a microporous membrane according to claim 1, wherein in the step (A), a locking means for fixing the sheet to one or more rolls is provided.

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