JP2015120835A - Polyolefin microporous film, separator for secondary battery, and secondary battery - Google Patents
Polyolefin microporous film, separator for secondary battery, and secondary battery Download PDFInfo
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- 229920000098 polyolefin Polymers 0.000 title claims abstract description 35
- 239000012982 microporous membrane Substances 0.000 claims description 61
- 230000035699 permeability Effects 0.000 claims description 34
- 239000012528 membrane Substances 0.000 claims description 10
- 239000004699 Ultra-high molecular weight polyethylene Substances 0.000 claims description 7
- 229920000785 ultra high molecular weight polyethylene Polymers 0.000 claims description 7
- 239000011248 coating agent Substances 0.000 description 14
- 238000000576 coating method Methods 0.000 description 14
- -1 polyethylene Polymers 0.000 description 13
- 239000004698 Polyethylene Substances 0.000 description 12
- 229920000573 polyethylene Polymers 0.000 description 12
- 238000005259 measurement Methods 0.000 description 11
- 238000000034 method Methods 0.000 description 10
- 230000000052 comparative effect Effects 0.000 description 5
- 238000009998 heat setting Methods 0.000 description 4
- 238000004519 manufacturing process Methods 0.000 description 4
- 230000000704 physical effect Effects 0.000 description 4
- 238000004804 winding Methods 0.000 description 4
- YMWUJEATGCHHMB-UHFFFAOYSA-N Dichloromethane Chemical compound ClCCl YMWUJEATGCHHMB-UHFFFAOYSA-N 0.000 description 3
- 238000001035 drying Methods 0.000 description 3
- 229920001903 high density polyethylene Polymers 0.000 description 3
- 239000004700 high-density polyethylene Substances 0.000 description 3
- 239000000203 mixture Substances 0.000 description 3
- 239000011148 porous material Substances 0.000 description 3
- 238000010586 diagram Methods 0.000 description 2
- 229940057995 liquid paraffin Drugs 0.000 description 2
- 239000000463 material Substances 0.000 description 2
- 239000011255 nonaqueous electrolyte Substances 0.000 description 2
- 229920013716 polyethylene resin Polymers 0.000 description 2
- 239000000243 solution Substances 0.000 description 2
- HBBGRARXTFLTSG-UHFFFAOYSA-N Lithium ion Chemical compound [Li+] HBBGRARXTFLTSG-UHFFFAOYSA-N 0.000 description 1
- 239000004743 Polypropylene Substances 0.000 description 1
- 229920010741 Ultra High Molecular Weight Polyethylene (UHMWPE) Polymers 0.000 description 1
- 230000008602 contraction Effects 0.000 description 1
- 238000001816 cooling Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 210000001787 dendrite Anatomy 0.000 description 1
- 230000005684 electric field Effects 0.000 description 1
- 239000003792 electrolyte Substances 0.000 description 1
- 239000008151 electrolyte solution Substances 0.000 description 1
- 238000001125 extrusion Methods 0.000 description 1
- 238000004898 kneading Methods 0.000 description 1
- 238000010030 laminating Methods 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 229910001416 lithium ion Inorganic materials 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 230000035515 penetration Effects 0.000 description 1
- 238000005191 phase separation Methods 0.000 description 1
- 229920000642 polymer Polymers 0.000 description 1
- 229920001155 polypropylene Polymers 0.000 description 1
- 239000011347 resin Substances 0.000 description 1
- 229920005989 resin Polymers 0.000 description 1
- 239000002904 solvent Substances 0.000 description 1
- 230000000007 visual effect Effects 0.000 description 1
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Abstract
Description
本発明は、ポリオレフィン微多孔膜、二次電池用セパレータおよび二次電池に関し、より詳しくは、ＭＤ方向の引張強度が高く透過性に優れたポリオレフィン微多孔膜、ならびにそれを用いてなる二次電池用セパレータおよび二次電池に関する。 The present invention relates to a polyolefin microporous membrane, a separator for a secondary battery, and a secondary battery, and more specifically, a polyolefin microporous membrane having a high tensile strength in the MD direction and excellent permeability, and a secondary battery using the same. The present invention relates to a separator and a secondary battery.
リチウムイオン電池などの非水電解液を用いた電池には、内部短絡を防止するため、所定の温度を超えた時点で反応をシャットダウンするシャットダウン機能を備えたセパレータが必須である。一般に電池用セパレータは微多孔膜からなり、温度が上昇すると、融点付近で収縮して微多孔が閉じ、電池反応をシャットダウンする。ここで、もしもセパレータの温度特性に局所的な偏りが存在すると、シャットダウンが不完全となり安全性の確保が困難となるおそれがあるため、セパレータは物性ムラの少ない均質な構造を有していることが望ましい。 For a battery using a non-aqueous electrolyte such as a lithium ion battery, a separator having a shutdown function for shutting down a reaction when a predetermined temperature is exceeded is essential to prevent an internal short circuit. In general, a battery separator is made of a microporous film. When the temperature rises, the battery shrinks in the vicinity of the melting point to close the micropore, and shuts down the battery reaction. Here, if there is a local bias in the temperature characteristics of the separator, shutdown may be incomplete and it may be difficult to ensure safety. Is desirable.
また、非水電解液系二次電池の普及および用途の拡大に伴い、セパレータに要求される品質水準も年々向上しており、安全性だけでなく、強度、透過性などの様々な特性が高いレベルで求められるようになっている。例えば、セパレータは張力をかけた状態で捲回されるので、捲回時の破膜防止のため、引張強度は高いことが好ましい。また、セパレータの捲回方向は通常ＭＤ方向であることから、ＭＤ方向の引張強度（以下、単に「ＭＤ引張強度」と記すこともある。）が、ＴＤ方向の引張強度（以下、単に「ＴＤ引張強度」と記すこともある。）に比べて適度に高いことが好ましい。 In addition, with the spread of non-aqueous electrolyte secondary batteries and the expansion of applications, the quality level required for separators has improved year by year, and not only safety but also various characteristics such as strength and permeability are high. It is required by level. For example, since the separator is wound in a tensioned state, it is preferable that the tensile strength is high in order to prevent film breakage during winding. Further, since the winding direction of the separator is usually the MD direction, the tensile strength in the MD direction (hereinafter sometimes simply referred to as “MD tensile strength”) is the tensile strength in the TD direction (hereinafter simply referred to as “TD”). It may be described as “tensile strength”).
電池の出力特性の観点からは、セパレータの透過性が高いことが好ましく、ＭＤ引張強度に優れていることが好ましい。透過性に優れた微多孔膜を用いることで電解質の通過が容易となり、高い電池特性を得ることができる。また、捲回時に高い張力をかけて密着させることで、電池の出力特性を高めることができる。 From the viewpoint of the output characteristics of the battery, it is preferable that the permeability of the separator is high, and it is preferable that the MD tensile strength is excellent. By using a microporous membrane having excellent permeability, the electrolyte can be easily passed and high battery characteristics can be obtained. Moreover, the output characteristic of a battery can be improved by making it adhere | attach with high tension | tensile_strength at the time of winding.
さらに、多様化する要求特性に応える方法として、近年では多孔質膜に機能材料を塗布して多機能化を実現するコーティング用セパレータの需要が高まりつつある。このようなコーティング用途においても、微多孔膜の強度は優れていることが好ましいが、コーティング用セパレータでは強度異方性の度合いが塗工時の張力制御に大きく影響することから、特定の配向方向のみ強度を高めるよりも、ＭＤ方向とＴＤ方向に適度な異方性を持たせつつ、塗工に適した強度バランスに調整することが好ましい。 Furthermore, as a method for meeting diversified demand characteristics, in recent years, there is an increasing demand for a coating separator that realizes multi-functionality by applying a functional material to a porous film. Even in such a coating application, it is preferable that the strength of the microporous film is excellent. However, in a coating separator, the degree of strength anisotropy greatly affects the tension control during coating, and therefore a specific orientation direction. It is preferable to adjust the strength balance suitable for coating while giving moderate anisotropy in the MD direction and the TD direction rather than increasing the strength only.
一方、自動車用電池等、二次電池の大型化に伴い、安全性の確保がますます重要になりつつある。そこで、電池内における異物や衝撃による破膜防止の観点からは、引張強度だけでなく突刺強度も高いことが好ましい。 On the other hand, with the increase in the size of secondary batteries such as automobile batteries, ensuring safety is becoming increasingly important. Therefore, from the viewpoint of preventing film breakage due to foreign matter or impact in the battery, it is preferable that not only the tensile strength but also the puncture strength is high.
微多孔膜の強度を向上させる方法としては、シートを延伸する方法や、多孔質シートを積層する方法などが広く知られている。しかしながら、従来技術では、強度を向上させると透過性が低下してしまうという問題点があり、強度と透過性のバランスに優れた微多孔膜を得ることは困難であった。 As a method for improving the strength of the microporous membrane, a method of stretching a sheet, a method of laminating a porous sheet, and the like are widely known. However, in the prior art, there is a problem that if the strength is improved, the permeability is lowered, and it has been difficult to obtain a microporous membrane having an excellent balance between strength and permeability.
例えば、特許文献１には、超高分子量ポリエチレンおよび高密度ポリエチレンを溶融混練して同時二軸延伸を行った後、所定の温度にて熱固定し、しかる後に溶媒を除去して微多孔膜を製造することが記載されている。しかしながら、この微多孔膜は高透過性で熱収縮率が低いものの、引張強度が必ずしも十分ではなかった。 For example, in Patent Document 1, after melt-kneading ultrahigh molecular weight polyethylene and high-density polyethylene and performing simultaneous biaxial stretching, heat fixing at a predetermined temperature, and then removing the solvent to form a microporous membrane. Manufacturing is described. However, although this microporous membrane has high permeability and low thermal shrinkage, the tensile strength is not always sufficient.
また、特許文献２には、ポリエチレンおよびポリプロピレンからなる微多孔膜Ａと、ポリエチレンからなる微多孔膜ＢをＡ／Ｂ／Ａの形態に３枚積層した積層微多孔膜が記載されている。しかしながら、このような積層微多孔膜は強度を確保しやすいという利点はあるものの、透過性が低く、また積層構造のため薄膜化が困難であった。
そこで本発明の課題は、ＭＤ引張強度が高く、ＭＤ引張強度とＴＤ引張強度のバランスに優れ、高い透過性を有し、二次電池用セパレータとして好適に用いることのできるポリオレフィン微多孔膜を提供することにある。 Therefore, an object of the present invention is to provide a polyolefin microporous membrane having high MD tensile strength, excellent balance between MD tensile strength and TD tensile strength, high permeability, and suitable for use as a separator for a secondary battery. There is to do.
本発明に係るポリオレフィン微多孔膜は、下記（１）〜（４）のいずれかの構成を有する。また、本発明に係る二次電池用セパレータは、下記（５）の構成を有する。さらに、本発明に係る二次電池は、下記（６）の構成を有する。
（１）１０μｍ四方の視野中に配向しているフィブリルの、機械方向（ＭＤ）に対し配向方向のなす角度が０〜３０°の範囲内にあるフィブリル部分をＭＤ配向したフィブリル部分とし、機械方向に垂直な方向（ＴＤ）に対し配向方向のなす角度が０〜３０°の範囲内にあるフィブリル部分をＴＤ配向したフィブリル部分とするとき、前記ＭＤ配向したフィブリル部分を径大順に２０％選択して算出した径大側平均フィブリル径ＤＭＤと、前記ＴＤ配向したフィブリル部分を径大順に２０％選択して算出した径大側平均フィブリル径ＤＴＤの比ＤＭＤ／ＤＴＤが１．２〜３．０であり、ＭＤ引張強度が１０００〜６０００ｋｇｆ／ｃｍ２であり、ＴＤ引張強度が１０００〜３０００ｋｇｆ／ｃｍ２であることを特徴とするポリオレフィン微多孔膜。
（２）前記ＤＭＤが８０〜２８０ｎｍであり、前記ＭＤ配向したフィブリル部分を径大順に２０％選択して算出した径大側フィブリル径標準偏差σＤＭＤがＤＭＤの２０％以下であり、前記ＤＴＤが２５〜２００ｎｍであり、前記ＴＤ配向したフィブリル部分を径大順に２０％選択して算出した径大側フィブリル径標準偏差σＤＴＤがＤＴＤの２０％以下である、請求項１に記載のポリオレフィン微多孔膜。
（３）微多孔膜の膜厚を２０μｍとしたときの透気抵抗度が６００ｓｅｃ／１００ｍＬ以下である、上記（１）または（２）のポリオレフィン微多孔膜。
（４）重量平均分子量が１．０×１０６〜５．０×１０６の超高分子量ポリエチレンを１〜５０重量％含有する、上記（１）〜（３）のいずれかのポリオレフィン微多孔膜。
（５）上記（１）〜（４）のいずれかのポリオレフィン微多孔膜を用いた二次電池用セパレータ。
（６）上記（５）のセパレータを含む二次電池。
The polyolefin microporous membrane according to the present invention has any one of the following constitutions (1) to (4). Moreover, the secondary battery separator according to the present invention has the following configuration (5). Furthermore, the secondary battery according to the present invention has the following configuration (6).
(1) The fibril portion oriented in the range of 0 to 30 ° with respect to the machine direction (MD) of the fibril oriented in the field of view of 10 μm square is defined as the MD-oriented fibril portion, and the machine direction When the fibril portion in which the angle formed by the alignment direction with respect to the direction perpendicular to the direction (TD) is in the range of 0 to 30 ° is a TD-aligned fibril portion, 20% of the MD-oriented fibril portions are selected in descending order of diameter. The ratio D MD / D TD between the large-diameter side average fibril diameter D MD and the large-diameter side average fibril diameter D TD calculated by selecting 20% of the TD-oriented fibril portions in descending order is 1.2 to is 3.0, MD tensile strength is 1000~6000kgf / cm 2, polyolefin TD tensile strength characterized in that it is a 1000~3000kgf / cm 2 fine Porous membrane.
(2) the D MD is 80～280Nm, the MD oriented large-diameter side fibril diameter standard deviation sigma DMD which fibrils portion was calculated by selecting 20% larger diameter order of 20% or less of D MD, the D TD is 25 to 200 nm, the large-diameter side fibril diameter standard deviation sigma DTD the TD oriented fibrils portion was calculated by selecting 20% larger diameter order of 20% or less of D TD, claim 1 Polyolefin microporous membrane.
(3) The polyolefin microporous membrane according to (1) or (2), wherein the air resistance when the thickness of the microporous membrane is 20 μm is 600 sec / 100 mL or less.
(4) The polyolefin microporous membrane according to any one of (1) to (3) above, containing 1 to 50% by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight of 1.0 × 10 6 to 5.0 × 10 6 .
(5) A separator for a secondary battery using the polyolefin microporous film according to any one of (1) to (4) above.
(6) A secondary battery including the separator of (5) above.
本発明のポリオレフィン微多孔膜は、ＭＤ配向したフィブリル部分のフィブリル径とＴＤ配向したフィブリル部分のフィブリル径が適切なバランスに保たれているので、引張強度および突刺強度に優れ、良好な透過性を発揮する。また、この微多孔膜は強度と透過性のバランスが優れているので、セパレータに使用した場合に良好なサイクル特性および出力特性を発揮でき、電池特性を向上させることが可能である。さらに、強度向上によって電池生産性や安全性も向上させることが可能となる。 The polyolefin microporous membrane of the present invention is excellent in tensile strength and puncture strength and has good permeability because the fibril diameter of the MD-oriented fibril portion and the fibril diameter of the TD-oriented fibril portion are kept in an appropriate balance. Demonstrate. Moreover, since this microporous membrane has an excellent balance between strength and permeability, it can exhibit good cycle characteristics and output characteristics when used in a separator, and can improve battery characteristics. Furthermore, battery productivity and safety can be improved by improving the strength.
また、本発明のポリオレフィン微多孔膜は、ＭＤ方向とＴＤ方向の異方性が適度な範囲内に調整されているので、コーティング用セパレータとしても好適に利用可能である。このような微多孔膜は張力制御が容易であり、塗りムラの少ない均一な塗工を実現することができる。また、高い張力に耐え得る優れた強度を有していることから、高速搬送による生産性の向上も可能となる。 The polyolefin microporous membrane of the present invention can be suitably used as a coating separator because the anisotropy in the MD and TD directions is adjusted within an appropriate range. Such a microporous membrane is easy to control the tension and can realize uniform coating with little coating unevenness. Further, since it has an excellent strength that can withstand high tension, productivity can be improved by high-speed conveyance.
以下に、発明の望ましい実施の形態について、図面を参照しながら説明する。 Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.
本発明のポリオレフィン微多孔膜は、フィブリル構造を有していることを一つの特徴とする。主としてフィブリルで構成された島状部や球状部の少ない構造を採用することにより、物性ムラや温度特性の局所的な偏りを未然に防止でき、シャットダウン特性に優れセパレータに適した微多孔膜を得ることができる。 One feature of the microporous polyolefin membrane of the present invention is that it has a fibril structure. By adopting a structure with few island-like parts and spherical parts mainly composed of fibrils, it is possible to prevent unevenness in physical properties and local bias in temperature characteristics, and to obtain a microporous membrane suitable for separators with excellent shutdown characteristics. be able to.
微多孔膜がこのようなフィブリル構造を有する場合、フィブリルがどのように配向しているかが、引張強度や強度異方性などの機械的特性に大きく影響すると考えられる。例えば、ＭＤ引張強度に優れた微多孔膜を製造する場合、フィブリルがＴＤ方向よりもＭＤ方向に多く配向していることが好ましい。また、塗工に適した強度異方性を実現するという観点からは、フィブリルの配向性が、ＭＤ方向、ＴＤ方向のいずれにも偏りすぎず、適度なバランスを保っていることが好ましい。 When the microporous membrane has such a fibril structure, it is considered that how the fibrils are oriented greatly affects mechanical properties such as tensile strength and strength anisotropy. For example, when producing a microporous film excellent in MD tensile strength, it is preferable that fibrils are oriented more in the MD direction than in the TD direction. Further, from the viewpoint of realizing strength anisotropy suitable for coating, it is preferable that the orientation of the fibrils is not excessively biased in either the MD direction or the TD direction, and an appropriate balance is maintained.
特に、ＭＤ引張強度に優れた微多孔膜を作るには、ＭＤ方向の延伸倍率をＴＤ方向の延伸倍率より高めながら同時に延伸し、かつ延伸後の熱固定温度の調整により、ＭＤ方向のフィブリルの成長を促し、最適なフィブリル径とすることが望ましい。 In particular, in order to make a microporous membrane having excellent MD tensile strength, the MD direction fibrils are stretched simultaneously by increasing the MD direction draw ratio higher than the TD direction draw ratio and adjusting the heat setting temperature after stretching. It is desirable to promote growth and achieve an optimum fibril diameter.
上述の観点から、本発明のポリオレフィン微多孔膜は、１０μｍ四方の視野中に配向しているフィブリルの、機械方向（ＭＤ）に対し配向方向のなす角度が０〜３０°の範囲内にあるフィブリル部分をＭＤ配向したフィブリル部分とし、機械方向に垂直な方向（ＴＤ）に対し配向方向のなす角度が０〜３０°の範囲内にあるフィブリル部分をＴＤ配向したフィブリル部分とするとき、前記ＭＤ配向したフィブリル部分を径大順に２０％選択して算出した径大側平均フィブリル径ＤＭＤと、前記ＴＤ配向したフィブリル部分を径大順に２０％選択して算出した径大側平均フィブリル径ＤＴＤの比ＤＭＤ／ＤＴＤが、所定の範囲内にあることを一つの特徴とする。 From the above viewpoint, the polyolefin microporous membrane of the present invention is a fibril in which the angle formed by the orientation direction with respect to the machine direction (MD) of the fibril oriented in a 10 μm square field is in the range of 0 to 30 °. When the portion is an MD-oriented fibril portion and the fibril portion having an angle of 0 to 30 ° with respect to the direction perpendicular to the machine direction (TD) is a TD-oriented fibril portion, the MD orientation was the fibrils portion large-diameter order 20% selected large diameter side average fibril diameter D MD calculated by the TD oriented fibrils portion of the large-diameter-side average fibril diameter D TD calculated by selecting 20% large diameter order One feature is that the ratio D MD / D TD is within a predetermined range.
図２は、走査型電子顕微鏡を用いてフィブリル表面を観察した結果の一例を示す模式図である。図２の視野１内には湾曲したフィブリル２が写っており、フィブリル２は、ＭＤ方向に対し０〜３０°の角度で配向しているフィブリル部分（区間Ａおよび区間Ｃ）と、ＴＤ方向に対し０〜３０°の角度で配向している部分（区間Ｂ）の両方を含んでいる。このような場合においては、各区間を独立して取り扱い、ＭＤ配向したフィブリル部分が２箇所（区間Ａおよび区間Ｃ）、ＴＤ配向したフィブリル部分が１箇所（区間Ｂ）存在するものとして、各フィブリル部分についてフィブリル径の測定を行う。また、区間内のフィブリル径が一定でない場合は、区間内の最大径を、当該区間におけるフィブリル径の測定値として採用する。なお、区間内のフィブリル部分の長さが５００ｎｍ以上あれば、当該区間をフィブリル径の測定に用いることができるものとする。
FIG. 2 is a schematic diagram showing an example of the result of observing the fibril surface using a scanning electron microscope. A
このようにしてフィブリル部分ごとにフィブリル径を測定した後、ＭＤ方向およびＴＤ方向のそれぞれに対し、所定の方向に配向したフィブリル部分を径大順に２０％選択して径大側平均フィブリル径を算出する。例えば、ＭＤ配向したフィブリル部分が視野内に１００箇所あった場合は、最大径が大きいものから順に２０箇所を選択して平均値を算出し、得られた値を径大側平均フィブリル径ＤＭＤ（ｎｍ）とする。ＴＤ方向の径大側平均フィブリル径ＤＴＤ（ｎｍ）についても同様である。 After measuring the fibril diameter for each fibril part in this way, the fibril part oriented in a predetermined direction is selected in the order of diameter in the order of 20% in the MD direction and the TD direction, and the large diameter side average fibril diameter is calculated. To do. For example, when there are 100 MD-oriented fibril portions in the field of view, the average value is calculated by selecting 20 locations in order from the largest maximum diameter, and the obtained value is used as the large-side average fibril diameter D MD. (Nm). The same applies to the larger diameter average fibril diameter D TD (nm) in the TD direction.
本発明のポリオレフィン微多孔膜において、上述のＤＭＤ／ＤＴＤは通常１．２〜３．０であり、好ましくは１．５〜２．９である。ＤＭＤ／ＤＴＤを１．２以上とすることで、ＭＤ引張強度に優れたポリオレフィン微多孔膜を得ることができる。また、ＤＭＤ／ＤＴＤを３．０以下とすることで、ＭＤ引張強度とＴＤ引張強度のバランスを適度な範囲に保つことができ、塗工時や搬送時の張力制御が容易となる。 In the polyolefin microporous membrane of the present invention, the above-mentioned D MD / D TD is usually 1.2 to 3.0, preferably 1.5 to 2.9. By setting D MD / D TD to 1.2 or more, a polyolefin microporous film excellent in MD tensile strength can be obtained. In addition, by setting 3.0 or less D MD / D TD, it is possible to maintain the balance of the MD tensile strength and TD tensile strength to an appropriate range, it is easy to tension control during or during transport coating.
引張強度と透気度とを両立させる観点から、上記ＤＭＤは８０〜２８０ｎｍであることが好ましく、１２０〜２６０ｎｍであることがより好ましく、１８０〜２４０ｎｍであることが特に好ましい。また、上記ＤＴＤは、２５〜２００ｎｍであることが好ましく、４０〜１８０ｎｍであることがより好ましく、６０〜１６０ｎｍであることが特に好ましい。一般に、フィブリル径が大きいほど透気抵抗度が高くなるため、フィブリル径が大き過ぎると所望の透気度を得ることが難しくなる。また、フィブリル径が小さ過ぎると、十分な引張強度を確保することが困難になる。 From the viewpoint of satisfying both tensile strength and air permeability, it is preferable that the D MD is 80～280Nm, more preferably 120～260Nm, particularly preferably 180～240Nm. Also, the D TD is preferably 25 to 200 nm, more preferably from 40～180Nm, particularly preferably 60～160Nm. In general, the larger the fibril diameter, the higher the air resistance. Therefore, if the fibril diameter is too large, it becomes difficult to obtain a desired air permeability. On the other hand, if the fibril diameter is too small, it is difficult to ensure sufficient tensile strength.
また、微多孔膜の孔径分布のばらつきを抑えるという観点からは、ＭＤ配向したフィブリル部分とＴＤ配向したフィブリル部分のいずれに関しても、フィブリル径の標準偏差は小さいことが好ましい。より具体的には、ＭＤ配向したフィブリル部分を径大順に２０％選択して算出した径大側フィブリル径標準偏差をσＤＭＤ（ｎｍ）とするとき、σＤＭＤはＤＭＤの２０％以下であることが好ましく、１８％以下であることがより好ましく、１６％以下であることが特に好ましい。同様に、ＴＤ配向したフィブリル部分を径大順に２０％選択して算出した径大側フィブリル径標準偏差をσＤＴＤ（ｎｍ）とするとき、σＤＴＤはＤＴＤの２０％以下であることが好ましく、１８％以下であることがより好ましく、１６％以下であることが特に好ましい。なお、σＤＭＤの下限はＤＭＤの１％以上であることが好ましく、σＤＴＤの下限はＤＴＤの１％以上であることが好ましい。このようにフィブリル径のばらつきを所定の範囲内に抑えることにより、孔径が均一で物性ムラの少ない微多孔膜を得ることができ、シャットダウン特性や塗工特性に優れたセパレータが製造可能となる。 Further, from the viewpoint of suppressing variation in the pore size distribution of the microporous membrane, it is preferable that the standard deviation of the fibril diameter is small for both the MD-oriented fibril portion and the TD-oriented fibril portion. More specifically, when the large diameter fibril diameter standard deviation calculated by selecting 20% of MD-oriented fibril parts in order of diameter is σ DMD (nm), σ DMD is 20% or less of D MD. It is preferably 18% or less, more preferably 16% or less. Similarly, when the large diameter fibril diameter standard deviation calculated by selecting 20% of TD-oriented fibril parts in order of diameter is σ DTD (nm), σ DTD is preferably 20% or less of D TD. 18% or less is more preferable, and 16% or less is particularly preferable. The lower limit of sigma DMD is preferably at least 1% of D MD, the lower limit of the sigma DTD is preferably 1% or more of D TD. Thus, by suppressing the variation in the fibril diameter within a predetermined range, a microporous film having a uniform pore diameter and little physical property unevenness can be obtained, and a separator having excellent shutdown characteristics and coating characteristics can be manufactured.
上述のようにフィブリル径のバランスを調整することで、ＭＤ引張強度が高く、ＭＤ引張強度とＴＤ引張強度のバランスに優れた微多孔膜を得ることができる。本発明の微多孔膜において、ＭＤ引張強度は、通常１０００〜６０００ｋｇｆ／ｃｍ２であり、好ましくは１５００〜４５００ｋｇｆ/ｃｍ２であり、より好ましくは２０００〜３５００ｋｇｆ/ｃｍ２である。また、ＴＤ引張強度は、通常１０００〜３０００ｋｇｆ／ｃｍ２であり、好ましくは１０００〜２０００ｋｇｆ/ｃｍ２であり、より好ましくは１０００〜１６５０ｋｇｆ/ｃｍ２である。このような微多孔膜は引張強度が優れているため、高い張力が掛かった場合も膜が破断しにくく、高い耐久性が要求される用途に好適に用いることができる。また、このような強度に優れた微多孔膜を電池用セパレータとして用いることで、電池作製時や使用時における短絡を防ぐことができるとともに、高い張力をかけてセパレータを巻回可能となり、電池の高容量化も図られる。なお、微多孔膜の引張強度（引張破断強度）は、例えばＡＳＴＭ Ｄ８８２に準拠した方法により測定することができる。 By adjusting the fibril diameter balance as described above, a microporous membrane having high MD tensile strength and excellent balance between MD tensile strength and TD tensile strength can be obtained. In the microporous membrane of the present invention, MD tensile strength is usually 1000~6000kgf / cm 2, preferably 1500~4500kgf / cm 2, more preferably 2000~3500kgf / cm 2. Further, TD tensile strength is usually 1000~3000kgf / cm 2, preferably 1000~2000kgf / cm 2, more preferably 1000~1650kgf / cm 2. Since such a microporous membrane has excellent tensile strength, the membrane is not easily broken even when high tension is applied, and can be suitably used for applications requiring high durability. In addition, by using such a microporous membrane with excellent strength as a battery separator, it is possible to prevent a short circuit during battery production or use, and it is possible to wind the separator by applying high tension. High capacity can also be achieved. In addition, the tensile strength (tensile breaking strength) of the microporous membrane can be measured by a method based on ASTM D882, for example.
本発明の微多孔膜は、膜厚を２０μｍとしたときの透気抵抗度が６００ｓｅｃ／１００ｍＬ以下であることが好ましい。ここで、膜厚を２０μｍとしたときの透気抵抗度とは、膜厚Ｔ１（μｍ）の微多孔膜において、ＪＩＳ Ｐ８１１７に準拠して測定した透気抵抗度をＰ１とするとき、式：Ｐ２＝（Ｐ１×２０）／Ｔ１によって算出される透気抵抗度Ｐ２のことを指す。なお、以下では、膜厚について特に記載がない限り、「透気抵抗度」を「膜厚を２０μｍとしたときの透気抵抗度」の意味で用いるものとする。 The microporous membrane of the present invention preferably has an air permeability resistance of 600 sec / 100 mL or less when the thickness is 20 μm. Here, the air resistance when the film thickness is 20 μm means that the air resistance measured in accordance with JIS P8117 is P 1 in a microporous film having a film thickness T 1 (μm). formula refers to the P 2 = (P 1 × 20 ) / T 1 air resistance P 2 calculated by. In the following description, “air permeability resistance” is used to mean “air resistance when the film thickness is 20 μm” unless otherwise specified.
微多孔膜を電池用セパレータとして用いる場合、透気抵抗度は低い値であることが好ましい。透気抵抗度が６００ｓｅｃ／１００ｍＬを超えると、電池の出力が小さくなることがある。なお、透気抵抗度の上限としては、４００ｓｅｃ／１００ｍＬ以下がより好ましく、３００ｓｅｃ／１００ｍＬ以下が特に好ましい。また、シャットダウン性能確保の観点から、透気抵抗度の下限は５０ｓｅｃ／１００ｍＬ以上であることが好ましく、１００ｓｅｃ／１００ｍＬ以上であることがより好ましい。 When the microporous membrane is used as a battery separator, the air permeability resistance is preferably a low value. When the air resistance exceeds 600 sec / 100 mL, the output of the battery may be small. In addition, as an upper limit of air permeability resistance, 400 sec / 100 mL or less is more preferable, and 300 sec / 100 mL or less is especially preferable. Further, from the viewpoint of ensuring the shutdown performance, the lower limit of the air permeability resistance is preferably 50 sec / 100 mL or more, and more preferably 100 sec / 100 mL or more.
上記ポリオレフィン微多孔膜において、１０５℃にて８時間保持したときのＭＤ方向の熱収縮率は、８％以下であることが好ましく、６％以下であることがより好ましい。また、１０５℃にて８時間保持したときのＴＤ方向の熱収縮率は、８％以下であることが好ましく、６％以下であることがより好ましい。熱収縮率が上記範囲内であると、セパレータを電極に捲回した後、乾燥工程にてセパレータが適度に収縮するため、セパレータと電極の密着性が向上し、従って電池組み立て時の不良率を低減できる。また、電池が異常作動し高温となった場合は、セパレータが徐々に縮んで少しずつショートしていくことが望ましく、セパレータが縮みすぎたり、逆にセパレータがほとんど縮まなかったりすると、十分な安全性を確保することが困難となるおそれがある。従って、安全性の観点からも、セパレータの熱収縮率は上記範囲内であることが好ましい。なお、熱収縮率が高過ぎると、コーティング時に乾燥工程などの熱によって基材が収縮し、乾燥皺や塗工膜のはがれを引き起こすおそれがある。そこで、コーティング時の不具合抑制の観点からは、ＭＤ方向の熱収縮率を６％以下とすることが好ましい。 In the above microporous polyolefin membrane, the thermal shrinkage in the MD direction when held at 105 ° C. for 8 hours is preferably 8% or less, and more preferably 6% or less. Further, the thermal shrinkage rate in the TD direction when held at 105 ° C. for 8 hours is preferably 8% or less, and more preferably 6% or less. If the thermal shrinkage rate is within the above range, the separator is appropriately shrunk in the drying process after the separator is wound on the electrode, so that the adhesion between the separator and the electrode is improved. Can be reduced. In addition, when the battery operates abnormally and becomes hot, it is desirable that the separator shrinks gradually and short-circuits gradually. If the separator shrinks too much, or conversely, the separator hardly shrinks, sufficient safety will be achieved. It may be difficult to ensure Therefore, from the viewpoint of safety, the thermal shrinkage rate of the separator is preferably within the above range. If the thermal shrinkage rate is too high, the base material shrinks due to heat during the drying process during coating, which may cause peeling of the drying flaws or the coating film. Therefore, from the viewpoint of suppressing defects during coating, it is preferable that the thermal shrinkage rate in the MD direction is 6% or less.
膜強度の観点から、空孔率の上限は、好ましくは７０％以下であり、より好ましくは６０％以下である。また、透過性能および電解液含有量の観点から、空孔率の下限は、好ましくは２０％以上であり、より好ましくは３０％以上である。空孔率を上記範囲内とすることにより、透過性、強度および電界液含有量のバランスが良くなり、電池反応の不均一性が解消され、デンドライト発生が抑制される。その結果、良好な安全性、強度、透過性が得られる。 From the viewpoint of film strength, the upper limit of the porosity is preferably 70% or less, and more preferably 60% or less. Further, from the viewpoints of permeation performance and electrolytic solution content, the lower limit of the porosity is preferably 20% or more, and more preferably 30% or more. By setting the porosity within the above range, the balance of permeability, strength and electric field liquid content is improved, the non-uniformity of the battery reaction is eliminated, and the generation of dendrites is suppressed. As a result, good safety, strength and permeability can be obtained.
ポリオレフィン微多孔膜を構成するポリオレフィンに関しては、ポリエチレンを用いることが好ましい。ポリエチレンの含有量は、ポリオレフィン全体を１００重量％としたとき、９０重量％以上であることが好ましく、９５重量％以上であることがより好ましく、９９重量％以上であることが特に好ましい。ポリエチレンの構成比率を上記範囲内とすることにより、ポリマー種の相分離がほとんどまたは全くない、均一性の高い微多孔膜を得ることができる。 Regarding the polyolefin constituting the polyolefin microporous membrane, it is preferable to use polyethylene. The polyethylene content is preferably 90% by weight or more, more preferably 95% by weight or more, and particularly preferably 99% by weight or more, based on 100% by weight of the entire polyolefin. By setting the composition ratio of polyethylene within the above range, a highly uniform microporous membrane with little or no phase separation of polymer species can be obtained.
また、物性ムラの少ない均質な微多孔膜を得るためには、フィブリル以外の構造を持つ領域、例えば、島状部や球状部、ラフト状部などができるだけ少ないことが望ましい。具体的には、微多孔膜の表面を観察したとき、フィブリル部分の面積の合計が、空孔部分を除いた樹脂部分全体の面積に対して９０％以上であることが好ましく、９５％以上であることがより好ましく、９９％以上であることが特に好ましい。フィブリル以外の構造を持つ領域をできるだけ少なくすることで、物性ムラや温度特性の偏りが少ない、シャットダウン特性に優れた微多孔膜を得ることができる。フィブリル部分の面積の算出にあたっては、微多孔膜の表面から無作為に選んだ複数の箇所で測定を実施し、得られた結果の平均値を求めることが好ましい。 Further, in order to obtain a homogeneous microporous film with little physical property unevenness, it is desirable that the number of regions having a structure other than fibrils, for example, an island-shaped portion, a spherical portion, and a raft-shaped portion is as small as possible. Specifically, when the surface of the microporous membrane is observed, the total area of the fibril parts is preferably 90% or more with respect to the entire area of the resin part excluding the pores, and is 95% or more. More preferably, it is 99% or more. By reducing the number of regions having structures other than fibrils as much as possible, it is possible to obtain a microporous film with excellent physical properties and uneven temperature characteristics and excellent shutdown characteristics. In calculating the area of the fibril portion, it is preferable to perform measurement at a plurality of locations randomly selected from the surface of the microporous membrane and obtain an average value of the obtained results.
強度向上の観点から、上記微多孔膜は、重量平均分子量が１．０×１０６〜５．０×１０６の超高分子量ポリエチレン（以下、単に「ＵＨＭＷＰＥ」とも記す。）を含有していることが好ましい。ＵＨＭＷＰＥの含有量は、ポリオレフィン全体の重量を１００重量％として、１〜５０重量％が好ましく、１〜４５重量％がより好ましく、１〜４０重量％が特に好ましい。ＵＨＭＷＰＥの含有量が５０重量％を超えると、ダイからのシート押出しが困難となるため、好ましくない。 From the viewpoint of improving the strength, the microporous membrane contains ultra high molecular weight polyethylene (hereinafter, also simply referred to as “UHMWPE”) having a weight average molecular weight of 1.0 × 10 6 to 5.0 × 10 6 . It is preferable. The content of UHMWPE is preferably 1 to 50% by weight, more preferably 1 to 45% by weight, and particularly preferably 1 to 40% by weight, based on 100% by weight of the entire polyolefin. If the content of UHMWPE exceeds 50% by weight, sheet extrusion from the die becomes difficult, which is not preferable.
強度と透過性を両立させるという観点から、上記ポリオレフィン微多孔膜の製造方法は、下記（１）、（２）の工程を含んでいることが好ましい。
（１）ダイから押し出されたシートを、延伸温度１００〜１３０℃、ＭＤ方向の延伸倍率２〜１０倍、ＴＤ方向の延伸倍率２〜１０倍にて同時二軸延伸する、湿式同時二軸延伸工程
（２）湿式同時二軸延伸工程によって延伸されたシートを洗浄し乾燥した後、乾燥したシートを、延伸温度が１１０〜１５０℃、ＭＤ方向の延伸倍率１．２〜３．０倍、ＴＤ方向の延伸倍率１．１〜２．０倍にてＭＤ方向歪速度１〜３０％／ｓｅｃ、ＴＤ方向歪速度１〜２０％／ｓｅｃで延伸し、その後、熱固定温度１１０〜１５０℃、保持時間３〜１５秒にて熱固定を行う、乾式同時二軸延伸工程
ここで、優れたＭＤ引張強度を確保する観点から、上記（２）の乾式同時二軸延伸工程において、ＭＤ方向の延伸倍率は、ＴＤ方向の延伸倍率よりも高いことが好ましい。製造時にこれらの工程を実施することにより、ＭＤ方向のフィブリルの成長を促し、最適なフィブリル径とすることができる。これにより、延伸倍率が同程度である従来のポリオレフィン微多孔膜と比較して、適切な径のフィブリルをＭＤ方向に異方性をもたせて存在させることができ、その結果、ＭＤ強度が向上し、捲回性に優れた微多孔膜を得ることができるため、電池特性の向上が望める。さらに、従来のポリオレフィン微多孔膜では、延伸倍率が高くなるとひずみが多く残り、透過性が悪化する傾向にあるが、上述の工程を実施することで、ひずみの発生を抑えて透気抵抗度を低く保つことができ、強度の向上と透過性の確保を高い水準で両立させることが可能となる。さらに、従来品と同程度の透過性を維持しつつより高い延伸倍率まで延伸が可能となるため、生産性も向上する。
From the viewpoint of achieving both strength and permeability, the method for producing a polyolefin microporous membrane preferably includes the following steps (1) and (2).
(1) Wet simultaneous biaxial stretching in which a sheet extruded from a die is simultaneously biaxially stretched at a stretching temperature of 100 to 130 ° C., a stretching ratio of 2 to 10 times in the MD direction, and a stretching ratio of 2 to 10 times in the TD direction. Step (2) After the sheet stretched by the wet simultaneous biaxial stretching process is washed and dried, the dried sheet is stretched at a stretching temperature of 110 to 150 ° C., a stretching ratio of 1.2 to 3.0 times in the MD direction, TD Stretched in the MD direction strain rate of 1 to 30% / sec and in the TD direction strain rate of 1 to 20% / sec at a direction stretching ratio of 1.1 to 2.0 times, and then maintained at a heat setting temperature of 110 to 150 ° C. The dry simultaneous biaxial stretching step in which heat setting is performed for 3 to 15 seconds. From the viewpoint of ensuring excellent MD tensile strength, in the dry simultaneous biaxial stretching step (2) above, the stretching ratio in the MD direction Is preferably higher than the draw ratio in the TD direction. Good. By carrying out these steps at the time of production, it is possible to promote the growth of fibrils in the MD direction and to obtain an optimum fibril diameter. As a result, fibrils having an appropriate diameter can be present with anisotropy in the MD direction as compared with the conventional polyolefin microporous membrane having the same draw ratio, and as a result, the MD strength is improved. Since a microporous film excellent in winding property can be obtained, improvement in battery characteristics can be expected. Furthermore, in the conventional polyolefin microporous membrane, when the draw ratio is increased, a large amount of strain remains and the permeability tends to deteriorate, but by performing the above-described process, the occurrence of strain is suppressed and the air resistance is reduced. It can be kept low, and it is possible to achieve both improvement in strength and securing of transparency at a high level. Furthermore, since it becomes possible to stretch to a higher stretching ratio while maintaining the same degree of permeability as the conventional product, productivity is also improved.
本発明はまた、上記ポリオレフィン微多孔膜を用いてなる二次電池用セパレータと、このような二次電池用セパレータを含む二次電池を提供する。強度に優れ透過性の高い本発明のポリオレフィン微多孔膜を用いることにより、安全性が高く優れた電池特性を有する二次電池が実現可能となる。 The present invention also provides a separator for a secondary battery using the polyolefin microporous membrane and a secondary battery including such a separator for a secondary battery. By using the polyolefin microporous membrane of the present invention having excellent strength and high permeability, a secondary battery having high safety and excellent battery characteristics can be realized.
以下に、本発明における具体例を、実施例を用いて説明するが、本発明はこれに限定されるものではない。 Specific examples of the present invention will be described below with reference to examples, but the present invention is not limited thereto.
１． フィブリル径
フィブリル径に関しては、以下の方法で測定を行った。まず、微多孔膜の表面のいずれかの場所を無作為に選んで走査型電子顕微鏡（ＳＥＭ）（ＪＥＯＬ製、型式：ＪＳＭ−６７０１Ｆ）で観察し、１０μｍ四方の広さの電子顕微鏡写真を視野として得た。この視野中に写っているフィブリルの、機械方向（ＭＤ）に対し配向方向のなす角度が０〜３０°の範囲内にあるフィブリル部分（ＭＤ配向したフィブリル部分）の全てについて、フィブリル径を測定した。なお、ＭＤ配向したフィブリル部分のフィブリル径が一定でない場合は、当該フィブリル部分における最大径を測定値として採用した。また、一つのフィブリルが複数のＭＤ配向したフィブリル部分を有している場合は、それぞれのＭＤ配向したフィブリル部分について測定を行った。得られた測定値のうち、径の大きさ順に上位２０％以内のものを選択し、それらの算術平均および標準偏差を、ＭＤ配向したフィブリル部分の径大側平均フィブリル径ＤＭＤおよび径大側フィブリル径標準偏差σＤＭＤとして求めた。ＴＤ方向についても同様の測定を行い、ＴＤ配向したフィブリル部分の径大側平均フィブリル径ＤＴＤおよび径大側フィブリル径標準偏差σＤＴＤを算出した。
1. Fibril diameter The fibril diameter was measured by the following method. First, any location on the surface of the microporous membrane was randomly selected and observed with a scanning electron microscope (SEM) (manufactured by JEOL, model: JSM-6701F). Got as. The fibril diameter was measured for all the fibril portions (MD-oriented fibril portions) in which the angle formed by the orientation direction with respect to the machine direction (MD) was within a range of 0 to 30 ° with respect to the machine direction (MD). . In addition, when the fibril diameter of the fibril part MD-oriented was not constant, the maximum diameter in the fibril part was adopted as a measurement value. In addition, when one fibril has a plurality of MD-oriented fibril parts, the measurement was performed on each MD-oriented fibril part. Among the obtained measured values, those within the top 20% are selected in the order of the diameter, and the arithmetic average and standard deviation thereof are calculated as the large diameter average fibril diameter DMD and large diameter side of the MD-oriented fibril portion. The fibril diameter standard deviation σ DMD was obtained. Perform similar measurement for the TD direction were calculated large diameter side average fibril diameter D TD and the large-diameter side fibril diameter standard deviation sigma DTD fibrils moieties TD orientation.
２．引張強度
ＭＤ引張強度およびＴＤ引張強度については、幅１０ｍｍの短冊状試験片を用いて、ＡＳＴＭ Ｄ８８２に準拠した方法により測定した。
2. Tensile strength About MD tensile strength and TD tensile strength, it measured by the method based on ASTMD882 using the strip-shaped test piece of width 10mm.
３．膜厚
微多孔膜の厚みは、接触式厚さ計を用いて、無作為に選択したＭＤ位置で測定した。測定は、膜のＴＤ（幅）に沿った点で、３０ｃｍの距離にわたって５ｍｍの間隔で行った。そして、上記ＴＤに沿った測定を５回行い、その算術平均を試料の厚さとした。
3. Film thickness The thickness of the microporous film was measured at a randomly selected MD position using a contact-type thickness meter. Measurements were taken at 5 mm intervals over a distance of 30 cm at points along the TD (width) of the membrane. And the measurement along said TD was performed 5 times and the arithmetic mean was made into the thickness of a sample.
４．透気抵抗度
膜厚Ｔ１の微多孔膜に対して透気度計（旭精工株式会社製、ＥＧＯ−１Ｔ）で透気抵抗度Ｐ１を測定し、式：Ｐ２＝（Ｐ１×２０）／Ｔ１により、膜厚を２０μｍとしたときの透気度Ｐ２に換算した。
4). Air permeability resistance The air resistance P 1 is measured with an air permeability meter (AGO-1T, manufactured by Asahi Seiko Co., Ltd.) with respect to a microporous film having a film thickness T 1 , and the formula: P 2 = (P 1 × 20) / T 1 , converted to air permeability P 2 when the film thickness was 20 μm.
５．突刺強度
先端に球面（曲率半径Ｒ：０．５ｍｍ）を有する直径１ｍｍの針を、平均膜厚Ｔ１（μｍ）の微多孔膜に２ｍｍ／秒の速度で突刺して最大荷重Ｌ１（貫通する直前の荷重、単位：ｇｆ）を測定し、Ｌ２＝（Ｌ１×２０）／Ｔ１の式により、膜厚を２０μｍとしたときの突刺強度Ｌ２（ｇｆ／２０μｍ）を算出した。
5. Puncture strength A needle with a diameter of 1 mm having a spherical surface (curvature radius R: 0.5 mm) at the tip is pierced into a microporous film with an average film thickness T 1 (μm) at a speed of 2 mm / sec, and the maximum load L 1 (penetration) The load immediately before the measurement, unit: gf) was measured, and the puncture strength L 2 (gf / 20 μm) when the film thickness was 20 μm was calculated by the formula L 2 = (L 1 × 20) / T 1 .
６．熱収縮率
微多孔膜を１０５℃にて８時間保持したときのＭＤ方向における収縮率を３回測定し、それらの平均値をＭＤ方向の熱収縮率とした。ＴＤ方向についても同様の測定を行い、ＴＤ方向の熱収縮率を算出した。
6). Thermal shrinkage The shrinkage in the MD direction when the microporous membrane was held at 105 ° C. for 8 hours was measured three times, and the average value thereof was taken as the thermal shrinkage in the MD direction. The same measurement was performed for the TD direction, and the thermal contraction rate in the TD direction was calculated.
７．空孔率
空孔率は、微多孔膜の質量ｗ１と、微多孔膜と同じポリエチレン組成物からなる同サイズの空孔のない膜の質量ｗ２から、空孔率（％）＝（ｗ２−ｗ１）／ｗ２×１００の式により算出した。
7). Porosity The porosity is calculated from the mass w1 of the microporous membrane and the mass w2 of the same size non-porous membrane made of the same polyethylene composition as the microporous membrane, and the porosity (%) = (w2-w1). ) / W2 × 100.
（実施例１）
（ａ）重量平均分子量Ｍｗ１が２．８９×１０６であり、分子量分布Ｍｗ１／Ｍｎ１（ここで、Ｍｎ１は数平均分子量である。）が５．２８である超高分子量ポリエチレン（ＵＨＭＷＰＥ）３０重量％ と、（ｂ）重量平均分子量Ｍｗ２が５．７２×１０５であり、分子量分布Ｍｗ２／Ｍｎ２（ここで、Ｍｎ２は数平均分子量である。）が４．８１である高密度ポリエチレン（ＨＤＰＥ）７０重量％とからなるポリエチレン組成物３０重量部を二軸押出機に投入し、この二軸押出機のサイドフィーダーから流動パラフィン７０重量部を供給し、２１０℃および２００ｒｐｍの条件で溶融混練して、ポリエチレン樹脂溶液を押出機中で調製した。続いて、このポリエチレン樹脂溶液を、押出機の先端に設置されたシート形成ダイから押し出し、得られたシート状押出物を２５℃の冷却ロールで引き取りながら、ゲル状成形物を形成した。得られたゲル状成形物に対して、１１５℃で５×５倍になるように湿式同時二軸延伸を施した後、延伸されたシートを２５℃の塩化メチレンに浸漬してリキッドパラフィンを除去し、室温で空気乾燥させた。そして、乾燥させたシートに対し、温度１３２℃、ＭＤ方向の延伸倍率１．６倍、ＴＤ方向の延伸倍率１．２倍、ラインスピード１ｍ／ｍｉｎ、熱固定温度１３２℃の運転条件で乾式同時二軸延伸を実施し、ポリエチレン微多孔膜を作製した。
得られた微多孔膜の膜特性を表１に示す。また、走査型電子顕微鏡による微多孔膜の表面観察図を図１（Ａ）に示す。この微多孔膜はＤＭＤがＤＴＤと比べて適度に高く、径の大きいフィブリルがＭＤ方向に多く配向する構造を有していた。また、σＤＭＤがＤＭＤと比べて小さく、σＤＭＤがＤＴＤと比べて小さいことから、フィブリル径のばらつきも小さかった。そして、延伸倍率が同程度である従来例（例えば、後述する比較例２など）と比較すると、この微多孔膜はＭＤ引張強度、突刺強度および透過性に優れており、二次電池用セパレータに適した特性を備えていた。
Example 1
(A) Ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight Mw1 of 2.89 × 10 6 and a molecular weight distribution Mw1 / Mn1 (where Mn1 is a number average molecular weight) of 5.28 And (b) high density polyethylene (HDPE) having a weight average molecular weight Mw2 of 5.72 × 10 5 and a molecular weight distribution Mw2 / Mn2 (where Mn2 is a number average molecular weight) of 4.81 30 parts by weight of a polyethylene composition comprising 70% by weight is charged into a twin screw extruder, 70 parts by weight of liquid paraffin is supplied from the side feeder of this twin screw extruder, and melt kneaded at 210 ° C. and 200 rpm. A polyethylene resin solution was prepared in an extruder. Subsequently, the polyethylene resin solution was extruded from a sheet forming die installed at the tip of the extruder, and a gel-like molded product was formed while taking the obtained sheet-shaped extrudate with a 25 ° C. cooling roll. The resulting gel-like molded product was wet biaxially stretched at 115 ° C to 5x5 times, and the liquid paraffin was removed by immersing the stretched sheet in methylene chloride at 25 ° C. And air dried at room temperature. The dried sheet was simultaneously dry-dried under the operating conditions of a temperature of 132 ° C., a draw ratio of 1.6 times in the MD direction, a draw ratio of 1.2 times in the TD direction, a line speed of 1 m / min, and a heat setting temperature of 132 ° C. Biaxial stretching was performed to produce a polyethylene microporous membrane.
Table 1 shows the film characteristics of the obtained microporous film. Further, FIG. 1A shows a surface observation view of the microporous film by a scanning electron microscope. The microporous membrane D MD moderately higher than the D TD, large fibril diameter had a structure oriented more in the MD direction. Furthermore, sigma DMD is smaller than the D MD, since sigma DMD is smaller than the D TD, was small variations in fibril diameter. And compared with the conventional example (for example, comparative example 2 etc. which are mentioned later) whose draw ratio is comparable, this microporous film is excellent in MD tensile strength, puncture strength, and permeability, and is used as a separator for secondary batteries. It had suitable characteristics.
（実施例２）
乾式同時二軸延伸において、ＭＤ方向への延伸倍率を２．０倍としたほかは、実施例１と同様にしてポリエチレン微多孔膜を得た。
得られた微多孔膜の膜特性を表１に示す。また、走査型電子顕微鏡による微多孔膜の表面観察図を図１（Ｂ）に示す。この微多孔膜は実施例１と同様にフィブリル径の測定結果が良好であった。また、この微多孔膜はＭＤ引張強度、突刺強度および透過性に優れており、二次電池用セパレータに適した特性を備えていた。
(Example 2)
In dry simultaneous biaxial stretching, a polyethylene microporous membrane was obtained in the same manner as in Example 1 except that the stretching ratio in the MD direction was 2.0 times.
Table 1 shows the film characteristics of the obtained microporous film. Further, FIG. 1B shows a surface observation view of the microporous film by a scanning electron microscope. This microporous membrane had good fibril diameter measurement results as in Example 1. Moreover, this microporous film was excellent in MD tensile strength, puncture strength and permeability, and had characteristics suitable for a secondary battery separator.
（実施例３）
乾式同時二軸延伸において、ＭＤ方向への延伸倍率を２．４倍としたほかは、実施例１と同様にしてポリエチレン微多孔膜を得た。
得られた微多孔膜の膜特性を表１に示す。また、走査型電子顕微鏡による微多孔膜の表面観察図を図１（Ｃ）に示す。この微多孔膜は実施例１と同様にフィブリル径の測定結果が良好であった。また、この微多孔膜はＭＤ引張強度、突刺強度および透過性に優れており、二次電池用セパレータに適した特性を備えていた。
(Example 3)
In dry simultaneous biaxial stretching, a polyethylene microporous membrane was obtained in the same manner as in Example 1 except that the stretching ratio in the MD direction was 2.4 times.
Table 1 shows the film characteristics of the obtained microporous film. Further, FIG. 1C shows a surface observation view of the microporous film by a scanning electron microscope. This microporous membrane had good fibril diameter measurement results as in Example 1. Moreover, this microporous film was excellent in MD tensile strength, puncture strength and permeability, and had characteristics suitable for a secondary battery separator.
（比較例１）
湿式同時二軸延伸後、乾式同時二軸延伸を行わなかったほかは、実施例１と同様にしてポリエチレン微多孔膜を得た。
得られた微多孔膜の膜特性を表１に示す。また、走査型電子顕微鏡による微多孔膜の表面観察図を図１（Ｄ）に示す。この微多孔膜は、フィブリル径のばらつきは比較的小さかったものの、径の大きいフィブリルはＭＤ方向とＴＤ方向にほぼ均等に配向しており、異方性が小さかった。また、ＭＤ引張強度や透気度が比較的低く、二次電池用セパレータとしての特性についても改善の余地が見られた。
(Comparative Example 1)
A polyethylene microporous membrane was obtained in the same manner as in Example 1 except that dry simultaneous biaxial stretching was not performed after wet simultaneous biaxial stretching.
Table 1 shows the film characteristics of the obtained microporous film. Further, FIG. 1D shows a surface observation view of the microporous film by a scanning electron microscope. Although the microporous membrane had a relatively small variation in the fibril diameter, the fibril with a large diameter was oriented almost uniformly in the MD direction and the TD direction, and the anisotropy was small. Moreover, MD tensile strength and air permeability were relatively low, and there was room for improvement in characteristics as a secondary battery separator.
（比較例２）
乾式同時二軸延伸において、ＭＤ方向への延伸倍率を１．０倍とし、ＴＤ方向への延伸倍率を２．０倍としたほかは、実施例１と同様にしてポリエチレン微多孔膜を得た。
得られた微多孔膜の膜特性を表１に示す。また、走査型電子顕微鏡による微多孔膜の表面観察図を図１（Ｅ）に示す。得られた微多孔膜の膜特性を表１に示す。また、走査型電子顕微鏡による微多孔膜の表面観察図を図１（Ｄ）に示す。この微多孔膜は、フィブリル径のばらつきは比較的小さかったものの、径の大きいフィブリルはＭＤ方向ではなくＴＤ方向に多く配向していた。また、ＭＤ引張強度や突刺強度、透気度などが比較的低く、二次電池用セパレータとしての特性についても改善の余地が見られた。
(Comparative Example 2)
In dry simultaneous biaxial stretching, a polyethylene microporous membrane was obtained in the same manner as in Example 1 except that the stretching ratio in the MD direction was 1.0 and the stretching ratio in the TD direction was 2.0. .
Table 1 shows the film characteristics of the obtained microporous film. Further, FIG. 1E shows a surface observation view of the microporous film by a scanning electron microscope. Table 1 shows the film characteristics of the obtained microporous film. Further, FIG. 1D shows a surface observation view of the microporous film by a scanning electron microscope. In this microporous membrane, although the variation in the fibril diameter was relatively small, the fibrils having a large diameter were largely oriented in the TD direction, not in the MD direction. Moreover, MD tensile strength, puncture strength, air permeability, etc. were relatively low, and there was room for improvement in characteristics as a secondary battery separator.
上述の実施例および比較例から示される通り、本発明により得られる微多孔膜は従来の微多孔膜に比べ延伸倍率が上がっているにもかかわらず、低い透気抵抗度と高い空孔率を維持しており、透過性を保ちつつ強度特性が改善されている点で優れている。 As shown from the above-mentioned Examples and Comparative Examples, the microporous membrane obtained by the present invention has low air resistance and high porosity even though the draw ratio is higher than that of the conventional microporous membrane. It is excellent in that the strength characteristics are improved while maintaining permeability.
なお、本出願では、数値範囲を示す「Ａ〜Ｂ」という表記を、下限値Ａおよび上限値Ｂを含めた範囲、すなわち「Ａ以上Ｂ以下」の意味で用いている。 In the present application, the notation “A to B” indicating a numerical range is used in a range including the lower limit value A and the upper limit value B, that is, “A or more and B or less”.
本発明に係るポリオレフィン微多孔膜は、特に二次電池用セパレータとして好適に使用することができる。 The polyolefin microporous membrane according to the present invention can be particularly suitably used as a separator for a secondary battery.
１ 視野
２ フィブリル
Ａ、Ｃ ＭＤ配向したフィブリル部分
Ｂ ＴＤ配向したフィブリル部分
1 Field of
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