JP4177929B2 - Porous film and method for producing the same - Google Patents
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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
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Description
【０００１】
【発明の属する技術分野】
本発明は多孔質フィルム及びその製造方法に関する。さらに詳しくは、電池の正極負極間に配置されてこれらを隔離させる電池用セパレータ等として好適に用いられる多孔質フィルム及びその製造方法に関する。
【０００２】
【従来の技術】
近年、電子機器のコードレス化等に対応するために、電池として軽量で、高起電力、高エネルギーが得られ、しかも自己放電が少ないリチウム電池が注目を集めている。このリチウム電池の正極負極の間には、正極負極の短絡防止のためにセパレータが設けられており、このセパレータとしては正極負極間のイオンの透過性を確保するために多数の微孔を有する多孔質フィルムが使用されている。かかる多孔質フィルムの材料としては、高分子量ポリオレフィンを用いた多孔質フィルムが種々提案されている。
【０００３】
例えば、高温での耐短絡性を向上させる方法として、ポリオレフィン系樹脂に無機粉体及び／又は無機繊維とから構成された厚さ１０〜２００μｍの無機質多孔膜をセパレータとして用いることが、特開平１０−５０２８７号公報に開示されている。該公報では無機粉体として酸化チタン、酸化アルミニウム、チタン酸カリウム等が挙げられ、一般に球形ないし針状の無機物が挙げられている。
【０００４】
また、前記無機質多孔膜の延伸倍率は１〜１０倍程度とする旨が記載されているが、この程度の延伸倍率は低いため、得られる膜強度は十分でない。また、球形の無機粉体や針状の無機繊維を用いた多孔質フィルムをリチウム電池セパレータとして用いた場合、膜強度を向上させるために延伸倍率をさらに大きくすると、膜の通気度が過度に低下し、膜抵抗が低下する。この場合、例えば、外部短絡等が生じた際に一瞬で大電流が流れ、電池内部の温度が劇的に上昇するが、セパレータのシャットダウン機能も一瞬では作用できず、非常に危険である。
【０００５】
【発明が解決しようとする課題】
従って、本発明の目的は、微細孔構造及び適度な空孔率を有し、膜強度や通気度（ガーレ値）が高く、高温での正極負極間の短絡を防止する多孔質フィルム及び該多孔質フィルムの製造方法を提供することにある。
【０００６】
【課題を解決するための手段】
本発明者らは、前記課題を解決するために鋭意検討した結果、重量平均分子量が１×１０6 以上の超高分子量ポリオレフィンを含有するポリオレフィンと、平板構造を有する無機物を含有する無機粉体及び／又は無機繊維とからなる樹脂組成物を混練してシート状に成形し、延伸及び溶媒除去することにより得られる多孔質フィルムが、微細孔構造を有し、適度な空孔率を有し、膜強度や通気度が高く、高温においても正極負極間の短絡を防止することを見出し、本発明に到達した。
【０００７】
即ち、本発明の要旨は、
〔１〕 重量平均分子量が１×１０6 以上の超高分子量ポリオレフィンを含有するポリオレフィンと平板構造を有する無機物を含む無機粉体及び／又は無機繊維の総量５〜２５重量％並びに溶媒７５〜９５重量％からなる樹脂組成物を混練りし、シート状に成形し、延伸及び脱溶媒処理を行う工程を有する多孔質フィルムの製造方法、
〔２〕 平板構造を有する無機物がカオリナイト、ナクライト、ディッカイト等のカオリン族粘土、モンモリロナイト、ザウコナイト等のモンモリロナイト鉱物、リザルダイト等の蛇紋石、イライト、セリサイト、海緑石等のマイカ、バーミキュライトからなる群より選ばれる１種以上のものである前記〔１〕記載の製造方法、
〔３〕 多孔質フィルムが、重量平均分子量が１×１０ 6 以上の超高分子量ポリオレフィンをポリオレフィン中に３０重量％以上含有するポリオレフィン３０〜８５重量％と無機粉体及び／又は無機繊維１５〜７０重量％からなる、前記〔１〕又は〔２〕記載の製造方法に関する。
【０００８】
【発明の実施の形態】
本発明に用いることができるポリオレフィンは、重量平均分子量が１×１０6 以上の超高分子量ポリオレフィンを含有したものである。
【０００９】
超高分子量ポリオレフィンとしては、エチレン、プロピレン、１−ブテン、４−メチル−１−ペンテン、１−ヘキセン等のオレフィンの単独重合体、共重合体及びこれらのブレンド物等が挙げられる。これらの中では、多孔質フィルムの高強度化の観点から、機械的強度に優れる超高分子量ポリエチレンを用いることが好ましい。
【００１０】
超高分子量ポリオレフィンの重量平均分子量は１×１０6 以上であり、好ましくは１．５×１０6 以上である。また、超高分子量ポリオレフィンのポリオレフィン中における含有量は、高温での高い溶融粘度を得る観点から、３０重量％以上が好ましく、５０〜１００重量％がより好ましい。
【００１１】
ポリオレフィンの多孔質フィルム中における含有量は、適度な通気度及び優れた高温での耐短絡性を得る観点から、３０〜８５重量％であり、好ましくは４０〜８０重量％、より好ましくは５０〜８０重量％である。
【００１２】
本発明に用いることができる無機粉体及び無機繊維は、平板構造を有する無機物を含有するものである。即ち、平板構造を有する無機粉体及び無機繊維が好適に使用される。
【００１３】
本明細書において、平板構造とは、粒子や繊維の平面図において、その輪郭に接する最短間隔の二つの平行線間の距離（短径ｂ）と、水平面に平行で粒子や繊維表面に接する平面の高さ（厚みｔ）から以下の式により計算される偏平度が５以上のものを示す。本発明では、平板構造を有する無機物の中でも偏平度が７以上の無機物が特に好適である。短径ｂと厚みｔの測定は、例えばＳＥＭ観察により行うことができる。
【００１４】
偏平度＝短径ｂ／厚みｔ
【００１５】
また、本発明において無機粉体の平均径は０．１〜５μｍであり、無機繊維としては、平均繊維厚０．０１〜１μｍ、平均繊維長０．５〜１０μｍのものが好適である。本発明においては、平板構造を有する無機物を含有する無機粉体及び／又は無機繊維を用いることに一つの大きな特徴があり、かかる平板構造を有する無機物を用いることにより、高温での電極間の短絡を効率よく防止し、フィルム厚みを低減し、かつ通気度を高く保ち、安全性を向上させるという優れた効果が発現される。
【００１６】
このような平板構造を有する無機物を用いた場合に、延伸処理において延伸倍率を高くして多孔質フィルムを製造しても、微細孔構造や適度な空孔率を有し、膜強度や通気度が高くしかも高温での耐短絡性が高い多孔質フィルムが得られる理由については必ずしも明らかではないが、例えば、球状又は針状の無機物を用いた場合、延伸すると無機物を取り巻くような大きな空隙が生じると共にポリオレフィンとの接触が小さくなるのに対し、本発明においては無機物は平板構造を有しているため、延伸されてもポリオレフィンとの接触面積が大きく、連通孔の増大を抑制するためと考えられる。
【００１７】
前記無機物としては、平板構造を有する無機粉体や無機繊維であれば特に限定されないが、例えば、絶縁体であり、電池内部で不活性な無機材料が好ましい。特に限定されないが、その具体例としては、カオリナイト、ナクライト、ディッカイト等のカオリン族粘土、モンモリロナイト、ザウコナイト等のモンモリロナイト鉱物、リザルダイト等の蛇紋石、イライト、セリサイト、海緑石等のマイカ、バーミキュライト等が挙げられる。これらの中では、カオリン族粘土やマイカが特に好ましい。これらの無機物は、２種以上を混合して用いてもよい。
【００１８】
平板構造を有する無機物の前記無機粉体及び無機繊維中における含有量は、好ましくは５０重量％以上、より好ましくは６０〜１００重量％である。また、平板構造をもたない無機粉体及び／又は無機繊維を耐熱性と通気度を電池の特性にあわせ制御する目的で含有していてもよいが、無機粉体及び無機繊維中に５０重量％未満の量とするのが好ましい。平板構造をもたない無機粉体及び無機繊維としては、酸化チタン、チタン酸カリウム、酸化アルミニウム等の球形又は針状無機物が挙げられる。
【００１９】
無機粉体及び／又は無機繊維の多孔質フィルム中における含有量は、耐熱性向上とフィルム強度の観点から、１５〜７０重量％であり、好ましくは２０〜６０重量％、より好ましくは３０〜６０重量％である。
【００２０】
ポリオレフィンと無機粉体及び／又は無機繊維からなる本発明の多孔質フィルムの厚みは、電池の容量の増大を容易にし、また膜強度を確保する観点から、好ましくは１０〜１００μｍ、より好ましくは１５〜５０μｍである。
【００２１】
多孔質フィルムの空孔率は、好ましくは４０％以上、より好ましくは４０〜 ７０％である。特に、本発明の多孔質フィルムは、かかる範囲の空孔率に加えて、ポリオレフィンのみからなる多孔質フィルムと同様な微細孔構造を有するため、優れた電解液保持性を有するという優れた効果が発現される。
【００２２】
多孔質フィルムの通気度は、外部短絡時に一瞬で大電流が流れるのを防ぎ、電池の安全性を確保する観点及び膜抵抗を適度にし、急速充放電を効率よく行う観点から、好ましくは１００〜１５００ｓｅｃ／１００ｃｃ、より好ましくは１００〜１０００ｓｅｃ／１００ｃｃである。
【００２３】
多孔質フィルムの膜強度は電池組み立て時におけるフィルムの破膜を防止する観点から、突刺強度が７００ｇｆ／２５μｍ以上であることが好ましく、８００ｇｆ／２５μｍ以上であることがより好ましい。
【００２４】
多孔質フィルムの高温での耐短絡性としては、例えば、短絡を生じる温度が１８０℃以上であることが好ましく、１９０℃以上であることがより好ましい。
【００２５】
本発明の多孔質フィルムは、例えば、ポリオレフィンと無機粉体及び／又は無機繊維を溶媒と混合して樹脂組成物を調製し、これを混練し、シート状に成形し、延伸及び脱溶媒処理をすることにより製造することができる。
【００２６】
ポリオレフィンと無機粉体及び／又は無機繊維の使用量としては、その総量が樹脂組成物中において、５〜２５重量％であることが好ましく、１０〜２５重量％であることがより好ましい。樹脂組成物の調製におけるポリオレフィンと無機粉体及び／又は無機繊維の配合比率、無機粉体及び／又は無機繊維中における平板構造を有する無機物の配合比率は、得られる多孔質フィルムを構成する各成分比率が本発明で規定する所望の範囲となるように適宜選択される。
【００２７】
溶媒としては、ポリオレフィンを溶解するものであればよく、例えば、ノナン、デカン、ウンデカン、ドデカン、デカリン、流動パラフィン等の脂肪族又は環状の炭化水素、あるいは沸点がこれらに対応する鉱油留分が挙げられ、これらの中では、流動パラフィン等の不揮発性溶媒が好ましい。
【００２８】
溶媒の使用量としては、混練りトルク、圧延、延伸応力がそれぞれ低く、優れた生産性を得る観点及びシート化する際のネックインを小さくして生産性を上げる観点から、樹脂組成物中において、７５〜９５重量％であることが好ましい。
【００２９】
なお、樹脂組成物には、必要に応じて、酸化防止剤、紫外線吸収剤等の添加剤を、目的を損なわない範囲で添加することができる。
【００３０】
得られた樹脂組成物を混練りし、シート状に成形する工程は、通常用いられる公知の方法により行うことができる。例えば、樹脂組成物をバンバリーミキサー、ニーダー等を用いてバッチ式で混練りし、次いで、冷却された金属板に挟み込み急冷して急冷結晶化によりシート状成形物にしてもよく、Ｔダイ等を取り付けた押出機等を用いてシート状成形物を得てもよい。
【００３１】
樹脂組成物の混練りの際の温度は、ポリオレフィンを効率良く分散し、ポリオレフィンの分解を抑える観点から、該ポリオレフィンを該溶媒が溶解を開始させる温度（溶解開始温度）〜＋６０℃の範囲が好ましく、溶解開始温度＋２０℃〜＋５０℃の範囲がより好ましい。
【００３２】
シート状に成形するに際しては、押出機等から出てくるシート状成形物をさらに急冷してもよい。この時、過冷却度（ΔＴ）が２０℃以上になる条件で急冷することがより好ましい。
【００３３】
このようにして、樹脂組成物のシート状成形物を得ることができる。ここで、シート状成形物の厚みとしては、特に限定されないが、１〜２０ｍｍのものが好ましく、３〜１５ｍｍのものがより好ましい。
【００３４】
次に、シート状成形物の延伸及び脱溶媒処理を行う。延伸処理の方式は、特に限定されるものではなく、通常の圧延法（プレス法）、テンター法、ロール法、インフレーション法又はこれらの方法の組合せであってもよく、また、一軸延伸、二軸延伸等のいずれの方式も適用することができる。また、二軸延伸の場合は、縦横同時延伸又は逐次延伸のいずれでもよい。さらに、本発明では、延伸処理に先立ち、シート状成形物の圧延等の処理を行ってもよい。
【００３５】
延伸処理の温度は、延伸の均一性が良好で、充分な膜強度を得る観点から、ポリオレフィンの融点（Ｔｍ）＋５℃以下が好ましい。また、延伸倍率は、２５〜４００倍が好ましく、５０〜３００倍がより好ましい。その他の延伸処理条件は、通常用いられる公知の条件を採用することができる。
【００３６】
脱溶媒処理は、シート状成形物から溶媒を除去して多孔質構造を形成させる工程であり、例えば、シート状成形物を溶剤で洗浄して残留する溶媒を除去することにより行うことができる。溶剤としては、ペンタン、ヘキサン、ヘプタン、デカン等の炭化水素、塩化メチレン、四塩化炭素等の塩素炭化水素、三フッ化エタン等のフッ化炭化水素、ジエチルエーテル、ジオキサン等のエーテル類等の易揮発性溶剤が挙げられ、これらは単独で又は２種以上を混合して用いることができる。かかる溶剤を用いた洗浄方法は、特に限定されず、例えば、シート状成形物を溶剤中に浸漬して溶媒を抽出する方法、溶剤をシート状成形物にシャワーする方法等が挙げられる。
【００３７】
なお、本発明において、脱溶媒処理は、延伸前後に適宜行えばよい。例えば、前記シート状成形物を脱溶媒処理してから延伸処理に供してもよく、またシート状成形物をそのまま延伸処理してから脱溶媒処理を行ってもよい。あるいは、延伸処理前に脱溶媒処理を行い、延伸処理後に再度脱溶媒処理を行って残存溶媒を除去する態様であってもよい。
【００３８】
また、前記工程により得られた多孔質構造を有する成形物をヒートセット処理することができる。本発明において、ヒートセット処理は、フィルムの寸法変化を抑制して連続熱風炉へ通す等の公知の方法を用いることができる。ヒートセット処理の温度は、ポリオレフィンのＴｍ−２０℃以上、Ｔｍ＋５℃以下が好ましい。また、ヒートセット処理時間は、温度により異なり一概には限定できないが、例えば、３０秒〜１時間程度行うのが好ましい。
【００３９】
また、本発明においては、予め超高分子量ポリオレフィンの融点−２０℃以下の温度で予熱を行った後にヒートセット処理を行ってもよい。
【００４０】
このようにして得られる本発明の多孔質フィルムは、微細孔構造及び適度な空孔率を有し、膜強度や通気度が高く、高温での耐短絡性が優れているため、電池のセパレータとしての用途だけでなく、各種フィルター、電解コンデンサー用隔膜等にも好適に使用することができる。
【００４１】
【実施例】
以下、実施例及び比較例を挙げて本発明をさらに詳細に説明するが、本発明はかかる実施例により何ら限定されるものではない。なお、各種特性については、下記要領にて測定を行う。
【００４２】
（１）空孔率
測定対象の多孔質フィルムを直径６ｃｍの円状に切り抜き、その体積と重量を求め、得られる結果から次式を用いて計算する。
【００４３】
空孔率（％）＝１００×〔体積（ｃｍ3 ）−重量（ｇ）／平均密度（ｇ／ｃｍ3 ）〕／体積（ｃｍ3 ）
なお、式中の「平均密度」はポリオレフィン並びに無機粉体及び／又は無機繊維の平均密度である。
【００４４】
（２）通気度（ガーレ値）
ＪＩＳ Ｐ８１１７に準拠して測定する。
【００４５】
（３）耐熱性（高温での耐短絡性）
平均粒径１０μｍのＬｉＣｏＯ2 とカーボンブラック及びバインダーからなるペーストをステンレスメッシュに塗布した後乾燥固定した大きさφ１４ｍｍの正極と、平均粒径１０μｍのカーボンフレーク及びバインダーからなるペーストをステンレスメッシュに塗布した後乾燥固定した大きさφ１６ｍｍの負極でφ２０ｍｍ以上の大きさの多孔質フィルムを挟み、四フッ化エチレンシートを介してステンレス板に挟み、四隅のボルトをトルクレンチで１．０ｋｇｆ・ｃｍのトルクで締め、１０℃／ｍｉｎで昇温しながら両極の導通性をテスターにて調べて、短絡を生じた温度を測定する。
【００４６】
（４）膜強度（突刺強度）
突刺強度は、カトーテック（株）製の圧縮試験機「ＫＥＳ−Ｇ５」を使用して針突き刺し試験を行い、測定により得られた荷重変異曲線より最大荷重を読み取って突刺強度値とした。針は直径１．０ｍｍ、先端曲率半径０．５ｍｍを用い、突き刺しは２ｍｍ／秒の速度で行った。
【００４７】
実施例１
重量平均分子量が２×１０6 の超高分子量ポリエチレン（Ｔｍ：１３３℃、密度０．９４ｇ／ｃｍ3 ）１５重量部とマイカ（平均径２μｍ、密度２．６ｇ／ｃｍ3 、偏平度１５）１０重量部、流動パラフィン８５重量部をスラリー状に均一混合し、１６０℃の温度で小型ニーダーを用い、６０分間溶解混練りした。その後、これらの混練物を０℃に冷却された金属板に挟み込み、厚さ５ｍｍのシート状に急冷した。これらの急冷結晶化したシート状樹脂を、１２０℃でシート厚さが０．８ｍｍになるまでヒートプレス（延伸倍率６．１倍）し、１２０℃で同時に縦横３．５×３．５倍に二軸延伸し（総延伸倍率７５倍）、ヘプタンを用いて脱溶媒処理を行なった。次いで１３０℃で１０分間ヒートセット処理し、膜厚２７μｍ、空孔率５７％の微細孔構造を有する多孔質フィルムを得た。
【００４８】
比較例１
マイカを添加しない以外は実施例１と同様に製膜し、脱溶媒、ヒートセット処理を行ない、膜厚２５μｍ、空孔率５２％の多孔質フィルムを得た。
【００４９】
比較例２
マイカの代わりにチタン酸カリウム（繊維状、平均繊維厚０．１μｍ、平均繊維長５μｍ、密度３．３ｇ／ｃｍ3 、偏平度３）５重量部を添加した以外は実施例１と同様に製膜し、脱溶媒、ヒートセット処理を行ない、膜厚２８μｍ、空孔率６７％の多孔質フィルムを得た。
【００５０】
実施例１及び比較例１〜２で得られた多孔質フィルムの膜厚、空孔率、ガーレ値、短絡温度及び突刺強度を表１に示す。
【００５１】
【表１】
以上の結果より、実施例１で得られた多孔質フィルムは、比較例１〜２で得られた多孔質フィルムに比べ、適度な空孔率を有し、ガーレ値（通気度）や膜強度が高く、耐熱性（高温での耐短絡性）に優れたものであることがわかる。
【００５３】
【発明の効果】
本発明により、適度な空孔率を有し、膜強度や通気度が高く、高温での耐短絡性が高く、電解液保液性がよい多孔質フィルムを得ることができるという効果が奏される。また、本発明の多孔質フィルムは、電池セパレータとしての用途だけでなく、各種フィルター、電解コンデンサー用隔膜等に好適に用いることができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a porous film and a method for producing the same. More specifically, the present invention relates to a porous film that is preferably used as a battery separator or the like that is disposed between positive and negative electrodes of a battery to isolate them, and a method for manufacturing the same.
[0002]
[Prior art]
In recent years, lithium batteries, which are lightweight as batteries, have high electromotive force and high energy, and have few self-discharges, are attracting attention in order to cope with cordless electronic devices. A separator is provided between the positive electrode and the negative electrode of the lithium battery to prevent a short circuit between the positive electrode and the negative electrode. As the separator, a porous material having a large number of micropores to ensure the permeability of ions between the positive electrode and the negative electrode. Quality film is used. Various porous films using high molecular weight polyolefins have been proposed as materials for such porous films.
[0003]
For example, as a method for improving short-circuit resistance at high temperatures, it is possible to use, as a separator, an inorganic porous film having a thickness of 10 to 200 μm composed of a polyolefin resin and inorganic powder and / or inorganic fibers. -50287. In this publication, examples of the inorganic powder include titanium oxide, aluminum oxide, potassium titanate and the like, and generally spherical or needle-like inorganic substances are mentioned.
[0004]
Moreover, although it describes that the draw ratio of the said inorganic porous film shall be about 1-10 times, since the draw ratio of this grade is low, the film | membrane intensity | strength obtained is not enough. In addition, when a porous film using spherical inorganic powder or needle-like inorganic fibers is used as a lithium battery separator, if the stretch ratio is further increased in order to improve the film strength, the air permeability of the film is excessively decreased. And membrane resistance falls. In this case, for example, when an external short circuit or the like occurs, a large current flows instantaneously and the temperature inside the battery rises dramatically. However, the shutdown function of the separator cannot operate instantaneously, which is very dangerous.
[0005]
[Problems to be solved by the invention]
Accordingly, an object of the present invention is to provide a porous film having a fine pore structure and an appropriate porosity, high membrane strength and air permeability (Gurley value), and preventing a short circuit between the positive electrode and the negative electrode at a high temperature, and the porous film. It is in providing the manufacturing method of a quality film.
[0006]
[Means for Solving the Problems]
As a result of intensive studies to solve the above problems, the present inventors have found that a polyolefin containing an ultrahigh molecular weight polyolefin having a weight average molecular weight of 1 × 10 6 or more, an inorganic powder containing an inorganic substance having a flat plate structure, and The porous film obtained by kneading the resin composition comprising inorganic fiber and / or forming into a sheet, stretching and removing the solvent has a microporous structure and has an appropriate porosity, The present inventors have found that the film strength and air permeability are high and that a short circuit between the positive electrode and the negative electrode can be prevented even at high temperatures, and the present invention has been achieved.
[0007]
That is, the gist of the present invention is as follows.
[1] Total amount of inorganic powder and / or inorganic fiber including polyolefin containing ultra-high molecular weight polyolefin having a weight average molecular weight of 1 × 10 6 or more and an inorganic substance having a flat plate structure and 75 to 95% by weight of solvent %, A method for producing a porous film having a step of kneading a resin composition comprising, forming into a sheet shape, and performing stretching and desolvation treatment ,
[2] The inorganic material having a flat plate structure is composed of kaolinite clays such as kaolinite, nacrite, dickite, montmorillonite minerals such as montmorillonite and zauconite, serpentine such as lizardite, mica such as illite, sericite and sea green stone, and vermiculite. The production method according to the above [1], which is one or more selected from the group ,
[3] 30 to 85% by weight of a polyolefin containing 30% by weight or more of ultrahigh molecular weight polyolefin having a weight average molecular weight of 1 × 10 6 or more in the polyolefin and 15 to 70 inorganic powders and / or inorganic fibers The production method according to [1] or [2], which comprises% by weight .
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The polyolefin that can be used in the present invention contains an ultrahigh molecular weight polyolefin having a weight average molecular weight of 1 × 10 6 or more.
[0009]
Examples of the ultrahigh molecular weight polyolefin include homopolymers and copolymers of olefins such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, and 1-hexene, and blends thereof. Among these, from the viewpoint of increasing the strength of the porous film, it is preferable to use ultrahigh molecular weight polyethylene having excellent mechanical strength.
[0010]
The weight average molecular weight of the ultrahigh molecular weight polyolefin is 1 × 10 6 or more, preferably 1.5 × 10 6 or more. Further, the content of the ultrahigh molecular weight polyolefin in the polyolefin is preferably 30% by weight or more, more preferably 50 to 100% by weight, from the viewpoint of obtaining a high melt viscosity at a high temperature.
[0011]
The content of the polyolefin in the porous film is 30 to 85% by weight, preferably 40 to 80% by weight, more preferably 50 to 50% from the viewpoint of obtaining an appropriate air permeability and excellent short circuit resistance at high temperatures. 80% by weight.
[0012]
The inorganic powder and inorganic fiber that can be used in the present invention contain an inorganic substance having a flat plate structure. That is, inorganic powder and inorganic fiber having a flat plate structure are preferably used.
[0013]
In the present specification, the flat plate structure is a plane between particles and fibers in the plan view of the particles and fibers, the distance between the two parallel lines at the shortest distance in contact with the contours (minor axis b), and the plane parallel to the horizontal plane and in contact with the particles and fibers. The flatness calculated from the height (thickness t) by the following formula is 5 or more. In the present invention, among the inorganic materials having a flat plate structure, an inorganic material having a flatness of 7 or more is particularly suitable. The measurement of the minor axis b and the thickness t can be performed, for example, by SEM observation.
[0014]
Flatness = minor axis b / thickness t
[0015]
In the present invention, the average diameter of the inorganic powder is 0.1 to 5 μm, and the inorganic fiber having an average fiber thickness of 0.01 to 1 μm and an average fiber length of 0.5 to 10 μm is preferable. In the present invention, there is one major feature in using an inorganic powder and / or inorganic fiber containing an inorganic material having a flat plate structure. By using such an inorganic material having a flat plate structure, a short circuit between electrodes at a high temperature is achieved. Is effectively prevented, the film thickness is reduced, the air permeability is kept high, and the safety is improved.
[0016]
When an inorganic material having such a flat plate structure is used, even if a porous film is produced by increasing the draw ratio in the drawing process, it has a fine pore structure and an appropriate porosity, and has a membrane strength and air permeability. Although it is not always clear why a porous film having a high temperature resistance and a short circuit resistance at a high temperature is obtained, for example, when a spherical or acicular inorganic material is used, a large void that surrounds the inorganic material is generated when stretched. At the same time, the contact with the polyolefin is reduced, whereas in the present invention, the inorganic substance has a flat plate structure, so that even when stretched, the contact area with the polyolefin is large, which is considered to suppress the increase of the communication holes. .
[0017]
The inorganic material is not particularly limited as long as it is an inorganic powder or an inorganic fiber having a flat plate structure. For example, an inorganic material that is an insulator and is inert inside the battery is preferable. Specific examples include kaolinite clays such as kaolinite, nacrite and dickite, montmorillonite minerals such as montmorillonite and zauconite, serpentine such as lizardite, mica such as illite, sericite and sea green stone, vermiculite. Etc. Of these, kaolin clay and mica are particularly preferred. These inorganic materials may be used in combination of two or more.
[0018]
The content of the inorganic substance having a flat plate structure in the inorganic powder and the inorganic fiber is preferably 50% by weight or more, more preferably 60 to 100% by weight. Further, inorganic powder and / or inorganic fiber not having a flat plate structure may be contained for the purpose of controlling heat resistance and air permeability in accordance with the characteristics of the battery. The amount is preferably less than%. Examples of the inorganic powder and inorganic fiber having no flat plate structure include spherical or acicular inorganic substances such as titanium oxide, potassium titanate, and aluminum oxide.
[0019]
The content of the inorganic powder and / or inorganic fiber in the porous film is 15 to 70% by weight, preferably 20 to 60% by weight, more preferably 30 to 60% from the viewpoints of heat resistance improvement and film strength. % By weight.
[0020]
The thickness of the porous film of the present invention comprising a polyolefin and inorganic powder and / or inorganic fiber is preferably 10 to 100 μm, more preferably 15 from the viewpoint of facilitating increase in battery capacity and ensuring film strength. ~ 50 μm.
[0021]
The porosity of the porous film is preferably 40% or more, more preferably 40 to 70%. In particular, since the porous film of the present invention has a microporous structure similar to a porous film made of only polyolefin in addition to the porosity in such a range, it has an excellent effect of having excellent electrolytic solution retention. Expressed.
[0022]
The air permeability of the porous film is preferably 100 to from the viewpoint of preventing a large current from flowing instantaneously at the time of an external short circuit, ensuring the safety of the battery, making the membrane resistance appropriate, and performing quick charge / discharge efficiently. 1500 sec / 100 cc, more preferably 100 to 1000 sec / 100 cc.
[0023]
The membrane strength of the porous film is preferably 700 gf / 25 μm or more, more preferably 800 gf / 25 μm or more, from the viewpoint of preventing film breakage during battery assembly.
[0024]
As the short circuit resistance at a high temperature of the porous film, for example, the temperature at which a short circuit occurs is preferably 180 ° C. or higher, and more preferably 190 ° C. or higher.
[0025]
The porous film of the present invention is prepared, for example, by mixing a polyolefin and inorganic powder and / or inorganic fiber with a solvent to prepare a resin composition, kneading it, forming it into a sheet, and subjecting it to stretching and desolvation treatment. Can be manufactured.
[0026]
As the usage-amount of polyolefin, an inorganic powder, and / or an inorganic fiber, it is preferable that the total amount is 5-25 weight% in a resin composition, and it is more preferable that it is 10-25 weight%. The blending ratio of polyolefin and inorganic powder and / or inorganic fiber in the preparation of the resin composition, and the blending ratio of the inorganic substance having a flat plate structure in the inorganic powder and / or inorganic fiber are the components constituting the porous film to be obtained. The ratio is appropriately selected so as to be within a desired range defined in the present invention.
[0027]
Any solvent may be used as long as it dissolves polyolefin, and examples thereof include aliphatic or cyclic hydrocarbons such as nonane, decane, undecane, dodecane, decalin, and liquid paraffin, or mineral oil fractions having boiling points corresponding to these. Of these, non-volatile solvents such as liquid paraffin are preferred.
[0028]
As the amount of solvent used, the kneading torque, rolling and stretching stress are low, respectively, from the viewpoint of obtaining excellent productivity and from the viewpoint of increasing productivity by reducing neck-in when forming a sheet, in the resin composition 75 to 95% by weight is preferable.
[0029]
It should be noted that additives such as antioxidants and ultraviolet absorbers can be added to the resin composition as necessary within a range that does not impair the purpose.
[0030]
The step of kneading the obtained resin composition and molding it into a sheet can be performed by a commonly used known method. For example, the resin composition may be kneaded batch-wise using a Banbury mixer, a kneader, etc., then sandwiched between cooled metal plates and rapidly cooled to form a sheet-like molded product by rapid crystallization. A sheet-like molded product may be obtained using an attached extruder or the like.
[0031]
The temperature at which the resin composition is kneaded is preferably in the range of the temperature at which the solvent starts to dissolve the polyolefin (dissolution start temperature) to + 60 ° C. from the viewpoint of efficiently dispersing the polyolefin and suppressing the decomposition of the polyolefin. The range of dissolution start temperature + 20 ° C. to + 50 ° C. is more preferable.
[0032]
When forming into a sheet shape, the sheet-like molded product coming out of an extruder or the like may be further rapidly cooled. At this time, it is more preferable to quench rapidly under the condition that the degree of supercooling (ΔT) is 20 ° C. or higher.
[0033]
In this way, a sheet-like molded product of the resin composition can be obtained. Here, the thickness of the sheet-like molded product is not particularly limited, but is preferably 1 to 20 mm, and more preferably 3 to 15 mm.
[0034]
Next, the sheet-shaped molded product is stretched and desolvated. The stretching method is not particularly limited, and may be a normal rolling method (pressing method), a tenter method, a roll method, an inflation method, or a combination of these methods. Any method such as stretching can be applied. In the case of biaxial stretching, either longitudinal or transverse simultaneous stretching or sequential stretching may be used. Furthermore, in this invention, you may perform processes, such as rolling of a sheet-like molded object, before an extending | stretching process.
[0035]
The temperature of the stretching treatment is preferably not higher than the melting point (Tm) of the polyolefin + 5 ° C. from the viewpoint of good stretching uniformity and sufficient film strength. Moreover, the draw ratio is preferably 25 to 400 times, more preferably 50 to 300 times. The other well-known conditions used normally can be employ | adopted for other extending | stretching process conditions.
[0036]
The solvent removal treatment is a step of removing the solvent from the sheet-like molded product to form a porous structure, and can be performed, for example, by washing the sheet-like molded product with a solvent to remove the remaining solvent. Solvents include hydrocarbons such as pentane, hexane, heptane and decane, chlorine hydrocarbons such as methylene chloride and carbon tetrachloride, fluorinated hydrocarbons such as ethane trifluoride, ethers such as diethyl ether and dioxane, etc. A volatile solvent is mentioned, These can be used individually or in mixture of 2 or more types. The cleaning method using such a solvent is not particularly limited, and examples thereof include a method of extracting a solvent by immersing a sheet-like molded product in a solvent, and a method of showering the solvent on the sheet-like molded product.
[0037]
In the present invention, the solvent removal treatment may be appropriately performed before and after stretching. For example, the sheet-shaped molding may be subjected to a solvent removal treatment and then subjected to a stretching treatment, or the sheet-shaped molding may be subjected to a stretching treatment as it is, and then the solvent removal processing may be performed. Alternatively, a mode in which the solvent removal treatment is performed before the stretching treatment, and the solvent removal treatment is performed again after the stretching treatment to remove the residual solvent may be employed.
[0038]
Moreover, the molded object which has the porous structure obtained by the said process can be heat set-processed. In the present invention, a known method such as passing through a continuous hot stove while suppressing the dimensional change of the film can be used for the heat setting treatment. The temperature of the heat setting treatment is preferably Tm-20 ° C. or higher and Tm + 5 ° C. or lower of polyolefin. Further, the heat set treatment time varies depending on the temperature and cannot be generally limited, but for example, it is preferably performed for about 30 seconds to 1 hour.
[0039]
Moreover, in this invention, you may perform a heat set process, after preheating at the temperature below melting | fusing point-20 degreeC of ultrahigh molecular weight polyolefin beforehand.
[0040]
The porous film of the present invention thus obtained has a fine pore structure and an appropriate porosity, has high membrane strength and air permeability, and is excellent in short circuit resistance at high temperatures. It can be suitably used for various filters, diaphragms for electrolytic capacitors, and the like.
[0041]
【Example】
EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated further in detail, this invention is not limited at all by this Example. Various characteristics are measured as follows.
[0042]
(1) A porous film to be measured for porosity is cut out in a circular shape with a diameter of 6 cm, its volume and weight are determined, and the obtained results are used for calculation.
[0043]
Porosity (%) = 100 × [volume (cm 3 ) −weight (g) / average density (g / cm 3 )] / volume (cm 3 )
The “average density” in the formula is the average density of polyolefin and inorganic powder and / or inorganic fiber.
[0044]
(2) Air permeability (Gurley value)
Measured according to JIS P8117.
[0045]
(3) Heat resistance (short circuit resistance at high temperature)
A paste made of LiCoO 2 having an average particle size of 10 μm, carbon black and a binder was applied to a stainless steel mesh and then dried and fixed, and a paste made of carbon flakes having an average particle size of 10 μm and a binder was applied to the stainless steel mesh. A porous film with a diameter of φ20 mm or more is sandwiched between a negative electrode of φ16 mm after drying and fixing, sandwiched between stainless steel plates with a tetrafluoroethylene sheet, and bolts at four corners with a torque wrench at a torque of 1.0 kgf · cm. The temperature at 10 ° C./min is fastened, and the conductivity of both electrodes is examined with a tester to measure the temperature at which a short circuit occurs.
[0046]
(4) Film strength (puncture strength)
The puncture strength was determined by performing a needle puncture test using a compression tester “KES-G5” manufactured by Kato Tech Co., Ltd., and reading the maximum load from the load variation curve obtained by the measurement to obtain a puncture strength value. The needle had a diameter of 1.0 mm and a tip curvature radius of 0.5 mm, and the piercing was performed at a speed of 2 mm / second.
[0047]
Example 1
15 parts by weight of ultrahigh molecular weight polyethylene (Tm: 133 ° C., density 0.94 g / cm 3 ) having a weight average molecular weight of 2 × 10 6 and mica (average diameter 2 μm, density 2.6 g / cm 3 , flatness 15) 10 Part by weight and 85 parts by weight of liquid paraffin were uniformly mixed in a slurry state and dissolved and kneaded for 60 minutes at a temperature of 160 ° C. using a small kneader. Thereafter, these kneaded materials were sandwiched between metal plates cooled to 0 ° C. and rapidly cooled into a sheet having a thickness of 5 mm. These quenched and crystallized sheet-like resins are heat-pressed at 120 ° C. until the sheet thickness reaches 0.8 mm (stretching ratio: 6.1 times), and simultaneously at 120 ° C., the length and width are 3.5 × 3.5 times. The film was biaxially stretched (total stretch ratio: 75 times), and the solvent was removed using heptane. Next, heat treatment was performed at 130 ° C. for 10 minutes to obtain a porous film having a fine pore structure with a film thickness of 27 μm and a porosity of 57%.
[0048]
Comparative Example 1
A film was formed in the same manner as in Example 1 except that mica was not added, and solvent removal and heat setting treatment were performed to obtain a porous film having a film thickness of 25 μm and a porosity of 52%.
[0049]
Comparative Example 2
Made in the same manner as in Example 1 except that 5 parts by weight of potassium titanate (fibrous, average fiber thickness 0.1 μm, average fiber length 5 μm, density 3.3 g / cm 3 , flatness 3) was added instead of mica. Filming was performed, and solvent removal and heat setting were performed to obtain a porous film having a film thickness of 28 μm and a porosity of 67%.
[0050]
Table 1 shows the film thickness, porosity, Gurley value, short circuit temperature, and puncture strength of the porous films obtained in Example 1 and Comparative Examples 1 and 2.
[0051]
[Table 1]
From the above results, the porous film obtained in Example 1 has a moderate porosity as compared with the porous films obtained in Comparative Examples 1 and 2, and the Gurley value (air permeability) and membrane strength. It is understood that the heat resistance (short circuit resistance at high temperature) is excellent.
[0053]
【The invention's effect】
According to the present invention, it is possible to obtain a porous film having an appropriate porosity, high membrane strength and air permeability, high short circuit resistance at high temperature, and good electrolyte solution retention. The Moreover, the porous film of this invention can be used suitably not only for the use as a battery separator but for various filters, diaphragms for electrolytic capacitors, and the like.
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| JP2003026847A (en) * | 2001-07-16 | 2003-01-29 | Nitto Denko Corp | Porous film |
| JP4794098B2 (en) * | 2001-09-28 | 2011-10-12 | 東レ東燃機能膜合同会社 | Method for producing polyolefin microporous membrane |
| JP4659308B2 (en) * | 2001-09-28 | 2011-03-30 | 日東電工株式会社 | Porous film |
| JP4794099B2 (en) * | 2001-09-28 | 2011-10-12 | 東レ東燃機能膜合同会社 | Method for producing polyolefin microporous membrane |
| KR100599898B1 (en) * | 2002-08-28 | 2006-07-19 | 아사히 가세이 케미칼즈 가부시키가이샤 | Polyolefin Microporous Membrane and Method of Evaluating The Same |
| TWI305215B (en) | 2004-08-30 | 2009-01-11 | Asahi Kasei Chemicals Corp | Polyolefin microporous membrane and separator for battery |
| KR100758482B1 (en) * | 2004-12-07 | 2007-09-12 | 주식회사 엘지화학 | Surface-treated microporous membrane and electrochemical device prepared thereby |
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| KR100692972B1 (en) | 2005-10-20 | 2007-03-12 | 김희삼 | Manufacturing of mixture of polypropylene with illite |
| JP2007311367A (en) * | 2007-08-30 | 2007-11-29 | Matsushita Electric Ind Co Ltd | Battery |
| JP2009135076A (en) * | 2007-11-02 | 2009-06-18 | Panasonic Corp | Non-aqueous electrolyte secondary battery |
| JP5196969B2 (en) * | 2007-11-22 | 2013-05-15 | 旭化成イーマテリアルズ株式会社 | Multilayer porous membrane |
| CN101747549B (en) * | 2008-12-18 | 2013-06-19 | 比亚迪股份有限公司 | Polyolefin micro porous polyolefin membrane and method for preparing same |
| JP5629542B2 (en) * | 2010-10-04 | 2014-11-19 | 旭化成イーマテリアルズ株式会社 | Polyolefin microporous membrane |
| JP2011208162A (en) * | 2011-07-26 | 2011-10-20 | Sumitomo Chemical Co Ltd | Polyolefin resin film and composition for manufacturing polyolefin resin film |
| US20150099186A1 (en) * | 2012-03-02 | 2015-04-09 | Cornell University | Silicon nanocomposite nanofibers |
| WO2013130690A1 (en) | 2012-03-02 | 2013-09-06 | Cornell University | Lithium ion batteries comprising nanofibers |
| JP5316671B2 (en) * | 2012-04-20 | 2013-10-16 | 三菱樹脂株式会社 | Nonaqueous electrolyte secondary battery separator and nonaqueous electrolyte secondary battery using the same |
| JP5942134B2 (en) * | 2013-02-04 | 2016-06-29 | 旭化成株式会社 | Polyolefin microporous membrane |
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1999
- 1999-03-04 JP JP05743099A patent/JP4177929B2/en not_active Expired - Lifetime
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