JP3641319B2 - Method for producing polyolefin microporous membrane - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、ポリオレフィン微多孔膜及びその製造方法に関し、特に表面の粗度及び開孔度を向上させたポリオレフィン微多孔膜及びその製造方法に関する。
【０００２】
【従来の技術】
微多孔膜は、電池用セパレーター、電解コンデンサー用隔膜、各種フィルター、透湿防水衣料、逆浸透濾過膜、限外濾過膜、精密濾過膜等の各種用途に用いられている。特に、電池用セパレーター中でもリチウム二次電池等のセパレーターとしてその用途が期待されている。
【０００３】
リチウム二次電池は超電力が２．５〜４Ｖと高く、活性質の主成分をなすリチウムの分子量が小さいために、最もエネルギー密度の高い二次電池の一つとして期待されている。コイン型や単３サイズの小容量リチウム二次電池は、メモリーバックアップ用電池や携帯電話などで用いられている。しかし、電気自動車や小規模負荷調整には数１０ｋＷｈ程度の容量が必要とされており、大容量化と高出力化が課題である。リチウム二次電池は、超電力が高いなどのため水溶液系の電解液が利用できず、有機溶媒の電解液または高分子の固体電解質を用いるために、それらの導電率が低いので電流密度が小さい。このため大容量高出力の大形電池では、電極面積の増大が必要である。
【０００４】
これらの電池で用いられるセパレーターとしては、電極とセパレーターの接点によって電極の有効断面積を減少させない様にセパレーター表面が粗く孔径が大きいことが必要である。また安全性を考えた場合、両極間の距離を適切に取るために適度に厚い膜厚が必要である。さらに、電池特性である放電特性及びサイクル特性を良好とするため電解液保持量の大きい膜が必要である。
【０００５】
超高分子量ポリオレフィンを用いた高強度の微多孔膜の製造法が種々提案されている。例えば、特開昭６０−２４２０３５号、特開昭６１−４９５３１２号、特開昭６１−１９５１３３号、特開昭６３−３９６０２号、特開昭６３−２７３６５１号等には、超高分子量ポリオレフィンを含むポリオレフィン組成物を溶媒に加熱溶解した溶液からゲル状シートを成形し、前記ゲル状シートを加熱延伸、溶媒の抽出除去による微多孔膜が開発されている。これらのポリオレフィン微多孔膜は電池用セパレーターとして良好な物性を有しているが、前記の二次電池用のセパレーターとしてのより高容量化や大出力化には、必ずしも十分対応が出来ていない。
【０００６】
【発明が解決しようとする課題】
本発明の課題は、電池用セパレーターの構造、特に電極と接する膜の表面に凹凸をつけるようにした粗度及び開孔度をコントロールすることにより、電極の有効断面積の向上と電解液の保持量を改良させ、電解液リッチな層を形成せしめ、充放電時のリチウムイオンの電極間移動速度を向上させることによって電池特性を向上させることのできるポリオレフィン微多孔膜及びその製造方法を提供することである。
【０００７】
【課題を解決するための手段】
上記課題に鑑み鋭意研究の結果、本発明者らは、超高分子量ポリオレフィンを所定量以上含有したポリオレフィン組成物の溶液を押し出し、得られるゲル状シートを延伸、溶媒抽出によってポリオレフィン微多孔膜を得る際、特定の条件下で延伸をすることにより、微多孔膜表面を粗くし、開孔度を大きくすることができることを見いだし、本発明に想到した。
【０００８】
すなわち、本発明のポリオレフィン微多孔膜は、重量平均分子量が５×１０5 以上のポリオレフィンまたはそのポリオレフィン組成物からなり、表面の平滑度が５００００秒以下でかつ表面の開孔度が４０％以上あることを特徴とする。
【０００９】
また、上記にかかわる本発明のポリオレフィン微多孔膜の製造方法は、重量平均分子量が５×１０5 以上のポリオレフィンまたはそのポリオレフィン組成物を１０〜８０重量％と、溶媒９０〜２０重量％とからなる溶液を調製し、前記溶液をダイより押し出し、冷却してゲル状成形物を形成し、前記ゲル状成形物を降温条件下で加熱延伸し、しかる後残存する溶媒を除去することを特徴とする。
【００１０】
【発明の実施の形態】
本発明を以下に詳細に説明する。本発明においてポリエチレンは、重量平均分子量が５×１０5 以上、好ましくは１×１０6 〜１５×１０6 のものである。重量平均分子量が５×１０5 未満では、微多孔膜の製造時の延伸工程において最大延伸倍率が低く、目的の微多孔膜が得られない。一方、上限は特に限定的ではないが１５×１０6 を超えるものは、微多孔膜の製造時のゲル状成形物の形成において成形性に劣る。
【００１１】
また、本発明においては、後述のポリオレフィン溶液の高濃度化と微多孔膜の強度の向上を図るために、重量平均分子量１×１０6 以上の超高分子量ポリエチレンと重量平均分子量１×１０5 以上１×１０6 未満のポリエチレンとの組成物を用いることができる。超高分子量ポリエチレンのポリエチレン組成物中の含有量は、ポリエチレン組成物全体を１００重量％として１重量％以上が好ましく、より好ましくは１０〜７０重量％である。
【００１２】
さらに、前記ポリエチレンまたはそのポリエチレン組成物の分子量分布の尺度として用いられる重量平均分子量／数平均分子量は３００以下、好ましくは５〜６０である。分子量分布が３００をこえると、延伸時に低分子量成分の破断が起こり膜全体の強度が低下するため好ましくない。上記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−ペンテン−１、１−ヘキセンなどを重合した結晶性の単独重合体、２段重合体、または共重合体及びこれらのブレンド物等が挙げられる。これらのうちではポリプロピレン、ポリエチレン（特に高密度ポリエチレン）及びこれらの組成物等が好ましい。
【００１３】
上述したポリオレフィンまたはポリオレフィン組成物からなる本発明のポリオレフィン微多孔膜は、その表面の平滑度が５００００秒以下、好ましくは４００００秒以下で、かつ表面の開孔度が４０％以上である。平滑度が５００００秒を超えると、または開孔度が４０％未満では本発明のポリオレフィン微多孔膜をリチウム電池のセパレーターとして用いる場合、セパレーターの表面の凹凸が小さいために電極有効領域が小さくなり、電池特性が悪化する恐れがある。
【００１４】
ただし、本発明において、平滑度は平面板と膜の間に空気を流せば平滑度の悪いものは空気が流れ易いという原理にもとづき、有効面積１０ｃｍ2 の平面板を１ｋｇ／ｃｍ2 の圧力で膜に押しつけた時ほぼ３７０ｍｍＨｇの圧力差の下で１０ｍｌの空気が流れる秒数をもって平滑度としている。また、開孔度は膜表面で孔の占める割合を電子顕微鏡写真により測定した値である。
【００１５】
本発明の微多孔膜の製造方法は、上述のポリオレフィン組成物を溶媒に加熱溶解することにより、溶液を調製する。この溶媒としては、ノナン、デカン、デカリン、ｐ−キシレン、ウンデカン、ドデカン、流動パラフィンなどの脂肪族または環式の炭化水素、あるいは沸点がこれらに対応する鉱油留分などを用いることができる。またこの溶媒の粘度としては、２５℃における粘度が３０〜５００ｃＳｔ、特に５０〜２００ｃＳｔであるのが好ましい。２５℃における粘度が３０ｃＳｔ未満では、押出機で溶解する場合不均一吐出を生じ、混練が困難であり、一方５００ｃＳｔを超えると、後工程での脱溶媒が容易でなくなる。
【００１６】
加熱溶解は、ポリエチレン組成物を溶媒中で完全に溶解する温度で攪拌しながら行うか、または押出機中で均一混合して溶解する方法で行う。溶媒中で攪拌しながら溶解する場合は、温度は使用する重合体及び溶媒により異なるが、例えばポリエチレン組成物の場合には１４０〜２５０℃の範囲である。ポリオレフィン組成物の高濃度溶液から微多孔膜を製造する場合は、押出機中で溶解するのが好ましい。
【００１７】
押出機中で溶解する場合は、まず押出機に上述したポリオレフィンを供給し、溶融する。溶融温度は、使用するポリオレフィンの種類によって異なるが、ポリオレフィンの融点＋３０〜１００℃が好ましい。例えば、ポリエチレンの場合は１６０〜２３０℃、特に１７０〜２００℃であるのが好ましく、ポリプロピレンの場合は１９０〜２７０℃、特に１９０〜２５０℃であるのが好ましい。次に、この溶融状態のポリオレフィンに対して、液状の溶媒を押出機の途中から供給する。
【００１８】
ポリオレフィンと溶媒との配合割合は、ポリオレフィンと溶媒の合計を１００重量％として、ポリオレフィンが１０〜８０重量％、好ましくは１５〜７０重量％であり、溶媒が９０〜２０重量％、好ましくは８５〜３０重量％である。ポリオレフィンが１０重量％未満では（溶媒が９０重量％を超えると）、シート状に成形する際に、ダイス出口で、スウエルやネックインが大きくシートの成形が困難となる。一方、ポリオレフィンが８０重量％を超えると（溶媒が２０重量％未満では）、均一な溶液の調製が困難となる。
【００１９】
なお、上記溶媒は途中にサイドフィーダー等を有する押出機を用いて、押出機の途中から溶融状態のポリオレフィンに供給する必要がある。超高分子量ポリオレフィンを含むポリオレフィンと溶媒とを同時に供給すると、粘度差が大き過ぎるために混合ができず、ポリオレフィンと押出機のスクリューとが共回りを起こし溶液を調製できない。このようにして溶融状態のポリオレフィンに溶媒を添加し、押出機中で混練することにより、均一な濃度のポリオレフィンの高濃度溶液を短時間で調製することができる。
【００２０】
次に、このようにして溶融混練したポリオレフィンの加熱溶液を直接に、あるいはさらに別の押出機を介して、または一旦冷却してペレット化した後、再度押出機を介して、ダイス等から押し出して成形する。ダイスとしては、通常長方形の口金形状をしたシートダイスが用いられるが、２重円筒状のインフレーションダイスなども用いることができる。シートダイスを用いた場合のダイスギャップは通常０．１〜５ｍｍであり、押し出し成形温度は１４０〜２５０℃である。この際押し出し速度は、通常２０〜３０ｃｍ／分ないし２〜３ｍ／分である。
【００２１】
このようにしてダイスから押し出された溶液は、冷却することによりゲル状組成物に成形される。冷却は少なくともゲル化温度以下までは５０℃／分以上の速度で行うのが好ましい。一般に冷却速度が遅いと、得られるゲル状組成物の高次構造が粗くなり、それを形成する疑似細胞単位も大きなものとなるが、冷却速度が速いと、密な細胞単位となる。冷却速度が５０℃／分未満では、結晶化度が上昇し、延伸に適したゲル状組成物となりにくい。冷却方法としては、冷風、冷却水、その他の冷却媒体に置換接触させる方法、冷媒で冷却したロールに接触させる方法などを用いることができる。なお、ダイスから押し出された溶液は、冷却前あるいは冷却中に好ましくは１〜１０、より好ましくは１〜５の引き取り比で引き取ってもよい。引き取り比が１０以上になるとネックインが大きくなり、また延伸時に破断を起こしやすくなり好ましくない。
【００２２】
次に、このゲル状成形物に延伸を行う。延伸はゲル状成形物を加熱し、通常のテンター法、ロール法、インフレーション法、圧延法もしくはこれらの方法の組み合わせによって所定の倍率で行う。延伸は一軸延伸でも二軸延伸でもよいが、二軸延伸が好ましい。また、二軸延伸の場合は、縦横同時延伸または逐次延伸のいずれでもよい。
【００２３】
延伸温度はポリエチレンの融点＋１０℃以下、好ましくはポリエチレンの結晶分散温度から結晶融点未満の範囲で行う。例えば、ポリエチレン場合は、９０〜１４０℃、好ましくは１００〜１３０℃の範囲である。延伸温度が１４０℃以上になると、孔がつぶれて表面開孔度が低下する。また延伸温度が９０℃以下になると、孔が小さくなって有効な孔が減少して好ましくない。さらに本発明では、延伸を降温条件下で行う必要がある。降温条件は、シート表面温度で１〜２０℃／分、好ましくは５〜１５℃／分であり、延伸開始点の温度と延伸終了点の温度差が５℃以上であることが必要である。ポリエチレンの場合は、延伸開始点の温度は１１０〜１４０℃であり、延伸終了点の温度は９０〜１２０℃であるのが好ましい。降温条件下で延伸を行わない場合は、特に低温延伸条件では表面凹凸を形成することができるが、透過性が著しく損なわれた膜となる。また高温延伸条件下では表面開孔度の低い膜となる。従って、このようにシートを十分予熱した後、降温条件下でシート表層の延伸条件を変えることによって、容易に表面だけ凹凸があり開孔度が高く、透過性を維持した膜が製膜できる。
【００２４】
また延伸倍率は原反の厚さによって異なるが、一軸延伸では２倍以上が好ましく、より好ましくは３〜３０倍である。二軸延伸では面倍率で１０倍以上が好ましく、より好ましくは１５〜４００倍である。面倍率が１０倍未満では延伸が不十分で高弾性、高強度の微多孔膜が得られない。一方、面倍率が４００倍を超えると、延伸操作などで制約が生じる。
【００２５】
得られた延伸成形物は、溶剤で洗浄し残留する溶媒を除去する。洗浄溶剤としては、ペンタン、ヘキサン、ヘプタンなどの炭化水素、塩化メチレン、四塩炭素などの塩素化炭化水素、三フッ化エタンなどのフッ化炭化水素、ジエチルエーテル、ジオキサンなどのエーテル類などの易揮発性のものを用いることができる。これらの溶剤はポリエチレン組成物の溶解に用いた溶媒に応じて適宜選択し、単独もしくは混合して用いる。洗浄方法は、溶剤に浸漬し抽出する方法、溶剤をシャワーする方法、またはこれらの組合せによる方法などにより行うことができる。
【００２６】
上述のような洗浄は、延伸成形物中の残留溶媒が１重量％未満になるまで行う。その後洗浄溶剤を乾燥するが、洗浄溶剤の乾燥方法は加熱乾燥、風乾などの方法で行うことができる。乾燥した延伸成形物は、結晶分散温度〜融点の温度範囲で熱固定することが望ましい。
【００２７】
以上のようにして製造したポリエチレン微多孔膜は、空孔率が３５〜９５％で平均貫通孔径が０．００１〜０．５μで、かつ破断強度が５００ｋｇ／ｃｍ2 以上である。また本発明のポリエチレン微多孔膜の厚さは、用途に応じて適宜選択しうるが、一般に０．１〜１００μであり、好ましくは２〜５０μにすることができる。
【００２８】
なお、得られたポリエチレン微多孔膜は、必要に応じてさらに、プラズマ照射、界面活性剤含浸、表面グラフト等の親水化処理などの表面修飾を施すことができる。
【００２９】
【実施例】
以下に本発明について実施例を挙げてさらに詳細に説明するが、本発明は実施例に特に限定されるものではない。なお、実施例における試験方法は次の通りである。
（１）膜厚：断面を走査型電子顕微鏡により測定。
（２）透気度：ＪＩＳ Ｐ８１１７に準拠して測定した。
（３）破断強度：ＡＳＴＭ Ｄ８８２に準拠して測定した。
（４）平滑度：王研式測定機により測定。
（５）開孔度：電子顕微鏡写真により測定。
【００３０】
実施例１
重量平均分子量が２．５×１０6 の超高分子量ポリエチレン１００重量部に酸化防止剤０．３７５重量部を加えたポリエチレン組成物を得た。このポリエチレン組成物２０重量部を二軸押出機（５８ｍｍφ、Ｌ／Ｄ＝４２、強混練タイプ）に投入した。また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８０重量部を供給し、２００ｒｐｍで溶融混練して、押出機中にてポリエチレン溶液を調製した。
【００３１】
続いて、この押出機の先端に設置されたＴダイから１９０℃で押し出し、冷却ロールで引取りながら厚さ１．８ｍｍのゲル状シートを成形した。続いてこのゲル状シートを、降温速度を５℃／分で、延伸開始時は１１５℃、延伸終了時は１１０℃になるような温度条件下で、５×５に同時２軸延伸を行い、延伸膜を得た。得られた延伸膜を塩化メチレンで洗浄して残留する流動パラフィンを抽出除去した後、乾燥および熱処理を行い膜厚２５μｍのポリエチレン微多孔膜を得た。このポリエチレン微多孔膜の物性評価の結果を第１表に示す。
【００３２】
実施例２
実施例１において、重量平均分子量が１．０×１０6 の超高分子量ポリエチレンを用い、その３０重量部と流動パラフィン７０重量部を用い、ゲル状シートの厚さを１．１ｍｍにする以外は、実施例１と同様にしてポリエチレン微多孔膜を得た。このポリエチレン微多孔膜の物性評価の結果を第１表に示す。
【００３３】
実施例３
実施例２において、重量平均分子量が５．０×１０5 の高分子量ポリエチレンを用い、ゲル状シートの厚さを１．２ｍｍにする以外は、実施例１と同様にしてポリエチレン微多孔膜を得た。このポリエチレン微多孔膜の物性評価の結果を第１表に示す。
【００３４】
実施例４
実施例２において、ポリエチレン組成物として、重量平均分子量が２．５×１０6 の超高分子量ポリエチレン６重量部と重量平均分子量が３．５×１０5 の高密度ポリエチレン２４重量部を用いる以外は、実施例２と同様にしてポリエチレン微多孔膜を得た。このポリエチレン微多孔膜の物性評価の結果を第１表に示す。
【００３５】
実施例５
実施例４において、ゲル状シートの厚さを１．２ｍｍにし、延伸時の降温条件を表１に示す様にする以外は、実施例４と同様にしてポリエチレン微多孔膜を得た。このポリエチレン微多孔膜の物性評価の結果を第１表に示す。
【００３６】
実施例６
実施例４において、ゲル状シートの厚さを２．１ｍｍにし、延伸時の降温条件を表１に示す様にする以外は、実施例４と同様にしてポリエチレン微多孔膜を得た。このポリエチレン微多孔膜の物性評価の結果を第１表に示す。
【００３７】
実施例７
実施例２において、ゲル状シートの厚さを２．３ｍｍにし、延伸時の降温条件を表１に示す様にする以外は、実施例２と同様にしてポリエチレン微多孔膜を得た。このポリエチレン微多孔膜の物性評価の結果を第１表に示す。
【００３８】
比較例１
実施例１において、ポリエチレン組成物として、重量平均分子量が２．５×１０6 の超高分子量ポリエチレン３重量部と重量平均分子量が３．５×１０5 の高密度ポリエチレン１４重量部、流動パラフィン８３重量部を用い、ゲル状シートの厚さを１．６ｍｍにし、延伸条件を１１５℃の定温下で行う以外は実施例１と同様にして微多孔膜を得た。以上のようにして得られた微多孔膜は表２の物性を有していた。
【００３９】
比較例２
実施例１において、ポリエチレン組成物として、重量平均分子量が２．５×１０6 の超分子量ポリエチレン３重量部と重量平均分子量が３．５×１０5 の高密度ポリエチレン１４重量部、流動パラフィン８３重量部を用い、ゲル状シートの厚さを１．９ｍｍにし、延伸条件を１２０℃の定温下で行う以外は実施例１と同様にして微多孔膜を得た。以上のようにして得られた微多孔膜は表２の物性を有していた。
【００４０】
比較例３
実施例１において、ゲル状シートの厚さを２．９ｍｍにし、延伸条件を１２０℃の定温下で行う以外は実施例１と同様にして微多孔膜を得た。以上のようにして得られた微多孔膜は表２の物性を有していた。
【００４１】
【表１】
【表２】
【００４３】
【発明の効果】
以上詳述したように本発明の方法により得られるポリオレフィン微多孔膜は、適度な平滑度及び開孔度を有し、電池用セパレーターとして高容量化に十分対応出来るものである。特に、リチウム電池用セパレーターとして有用である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a polyolefin microporous membrane and a method for producing the same, and more particularly to a polyolefin microporous membrane having improved surface roughness and openness and a method for producing the same.
[0002]
[Prior art]
Microporous membranes are used in various applications such as battery separators, electrolytic capacitor membranes, various filters, moisture permeable and waterproof clothing, reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes. In particular, the battery separator is expected to be used as a separator for lithium secondary batteries.
[0003]
A lithium secondary battery is expected to be one of the secondary batteries with the highest energy density because the superpower is as high as 2.5 to 4 V and the molecular weight of lithium, which is the main component of the active material, is small. Coin-type and AA-size small-capacity lithium secondary batteries are used in memory backup batteries and mobile phones. However, the capacity of several tens of kWh is required for electric vehicles and small-scale load adjustment, and increasing capacity and increasing output are issues. Lithium secondary batteries cannot use aqueous electrolytes due to high superpower, etc., and use organic solvent electrolytes or polymer solid electrolytes, so their electrical conductivity is low, so the current density is low. . For this reason, it is necessary to increase the electrode area in a large-capacity large-power battery.
[0004]
The separator used in these batteries needs to have a rough separator surface and a large pore size so as not to reduce the effective sectional area of the electrode by the contact between the electrode and the separator. In consideration of safety, a moderately thick film thickness is necessary in order to obtain an appropriate distance between the two electrodes. Furthermore, in order to improve the discharge characteristics and cycle characteristics which are battery characteristics, a film having a large amount of electrolyte solution is required.
[0005]
Various methods for producing high-strength microporous membranes using ultrahigh molecular weight polyolefins have been proposed. For example, JP-A-60-242035, JP-A-61-495312, JP-A-61-195133, JP-A-63-39602, JP-A-63-273651, etc. include ultrahigh molecular weight polyolefins. A microporous membrane has been developed in which a gel sheet is formed from a solution obtained by heating and dissolving a polyolefin composition containing the solvent in a solvent, the gel sheet is stretched by heating, and the solvent is extracted and removed. Although these polyolefin microporous membranes have good physical properties as battery separators, they are not always sufficient for higher capacity and higher output as the secondary battery separator.
[0006]
[Problems to be solved by the invention]
The object of the present invention is to improve the effective cross-sectional area of the electrode and maintain the electrolyte by controlling the structure of the battery separator, in particular, the roughness and openness of the film in contact with the electrode. To provide a polyolefin microporous membrane capable of improving battery properties by improving the amount, forming an electrolyte-rich layer, and improving the inter-electrode transfer rate of lithium ions during charge and discharge, and a method for producing the same It is.
[0007]
[Means for Solving the Problems]
As a result of diligent research in view of the above problems, the present inventors obtained a polyolefin microporous membrane by extruding a solution of a polyolefin composition containing a predetermined amount or more of ultra-high molecular weight polyolefin, stretching the resulting gel sheet, and solvent extraction. At that time, the inventors have found that the surface of the microporous membrane can be roughened and the degree of openness can be increased by stretching under specific conditions, and the present invention has been conceived.
[0008]
That is, the polyolefin microporous membrane of the present invention comprises a polyolefin having a weight average molecular weight of 5 × 10 5 or more or a polyolefin composition thereof, and has a surface smoothness of 50000 seconds or less and a surface porosity of 40% or more. It is characterized by that.
[0009]
The method for producing a microporous polyolefin membrane of the present invention as described above comprises 10 to 80% by weight of a polyolefin having a weight average molecular weight of 5 × 10 5 or more or a polyolefin composition thereof and 90 to 20% by weight of a solvent. A solution is prepared, the solution is extruded from a die, cooled to form a gel-like molded product, the gel-like molded product is heated and stretched under a temperature lowering condition, and then the remaining solvent is removed. .
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below. In the present invention, the polyethylene has a weight average molecular weight of 5 × 10 5 or more, preferably 1 × 10 6 to 15 × 10 6 . When the weight average molecular weight is less than 5 × 10 5 , the maximum stretching ratio is low in the stretching step during the production of the microporous membrane, and the target microporous membrane cannot be obtained. On the other hand, although the upper limit is not particularly limited, those exceeding 15 × 10 6 are inferior in moldability in the formation of a gel-like molded product during production of the microporous membrane.
[0011]
In the present invention, in order to increase the concentration of the polyolefin solution described later and to improve the strength of the microporous membrane, the ultrahigh molecular weight polyethylene having a weight average molecular weight of 1 × 10 6 or more and the weight average molecular weight of 1 × 10 5 or more are used. Compositions with less than 1 × 10 6 polyethylene can be used. The content of the ultrahigh molecular weight polyethylene in the polyethylene composition is preferably 1% by weight or more, more preferably 10 to 70% by weight, based on 100% by weight of the entire polyethylene composition.
[0012]
Furthermore, the weight average molecular weight / number average molecular weight used as a measure of the molecular weight distribution of the polyethylene or polyethylene composition thereof is 300 or less, preferably 5 to 60. When the molecular weight distribution exceeds 300, the low molecular weight component breaks during stretching and the strength of the entire film is lowered, which is not preferable. Examples of the polyolefin include a crystalline homopolymer, a two-stage polymer, a copolymer and a blend thereof obtained by polymerizing ethylene, propylene, 1-butene, 4-methyl-pentene-1, 1-hexene, and the like. Is mentioned. Of these, polypropylene, polyethylene (particularly high-density polyethylene) and compositions thereof are preferred.
[0013]
The polyolefin microporous membrane of the present invention comprising the above-described polyolefin or polyolefin composition has a surface smoothness of 50000 seconds or less, preferably 40000 seconds or less, and a surface porosity of 40% or more. When the smoothness exceeds 50000 seconds, or the porosity is less than 40%, when the polyolefin microporous membrane of the present invention is used as a separator for a lithium battery, the surface area of the separator is small and the electrode effective area becomes small, Battery characteristics may be deteriorated.
[0014]
However, in the present invention, the smoothness is based on the principle that poor smoothness be allowed to flow air of air easily flows between the flat plate and the film, the flat plate having an effective area of 10 cm 2 at a pressure of 1 kg / cm 2 Smoothness is defined as the number of seconds in which 10 ml of air flows under a pressure difference of approximately 370 mmHg when pressed against the membrane. In addition, the degree of opening is a value obtained by measuring the ratio of holes on the surface of the film with an electron micrograph.
[0015]
In the method for producing a microporous membrane of the present invention, a solution is prepared by heating and dissolving the above-described polyolefin composition in a solvent. Examples of the solvent include aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, p-xylene, undecane, dodecane, and liquid paraffin, or mineral oil fractions having boiling points corresponding to these. The viscosity of this solvent is preferably 30 to 500 cSt, particularly 50 to 200 cSt at 25 ° C. If the viscosity at 25 ° C. is less than 30 cSt, non-uniform discharge occurs when it is melted by an extruder, and kneading is difficult. On the other hand, if it exceeds 500 cSt, solvent removal in the subsequent process becomes difficult.
[0016]
The dissolution by heating is carried out by stirring at a temperature at which the polyethylene composition is completely dissolved in a solvent, or by uniformly mixing and dissolving in an extruder. When dissolving with stirring in a solvent, the temperature varies depending on the polymer and solvent used, but in the case of a polyethylene composition, for example, it is in the range of 140-250 ° C. When producing a microporous membrane from a high concentration solution of a polyolefin composition, it is preferable to dissolve in a extruder.
[0017]
When melt | dissolving in an extruder, the polyolefin mentioned above is first supplied to an extruder, and is melted. Although melting temperature changes with kinds of polyolefin to be used, melting | fusing point of polyolefin + 30-100 degreeC is preferable. For example, in the case of polyethylene, it is preferably 160 to 230 ° C., particularly 170 to 200 ° C., and in the case of polypropylene, it is preferably 190 to 270 ° C., particularly preferably 190 to 250 ° C. Next, a liquid solvent is supplied to the molten polyolefin from the middle of the extruder.
[0018]
The blending ratio of the polyolefin and the solvent is 10 to 80% by weight, preferably 15 to 70% by weight, and 90 to 20% by weight, preferably 85 to 85% by weight, with the total of the polyolefin and the solvent being 100% by weight. 30% by weight. When the polyolefin is less than 10% by weight (when the solvent exceeds 90% by weight), when forming into a sheet, swell and neck-in are large at the die outlet, making it difficult to form the sheet. On the other hand, when the amount of polyolefin exceeds 80% by weight (when the solvent is less than 20% by weight), it is difficult to prepare a uniform solution.
[0019]
In addition, it is necessary to supply the said solvent to the molten polyolefin from the middle of an extruder using the extruder which has a side feeder etc. in the middle. If a polyolefin containing ultra-high molecular weight polyolefin and a solvent are supplied simultaneously, the difference in viscosity is too large to be mixed, and the polyolefin and the screw of the extruder co-rotate and a solution cannot be prepared. Thus, a high concentration solution of polyolefin having a uniform concentration can be prepared in a short time by adding a solvent to polyolefin in a molten state and kneading in an extruder.
[0020]
Next, the polyolefin melt solution thus kneaded and kneaded directly or through another extruder or once cooled and pelletized, and then extruded through a die or the like again through the extruder. Mold. As the die, a sheet die having a rectangular base shape is usually used, but a double cylindrical inflation die can also be used. When a sheet die is used, the die gap is usually 0.1 to 5 mm, and the extrusion molding temperature is 140 to 250 ° C. At this time, the extrusion speed is usually 20 to 30 cm / min to 2 to 3 m / min.
[0021]
The solution thus extruded from the die is formed into a gel composition by cooling. Cooling is preferably performed at a rate of 50 ° C./min or more at least up to the gelation temperature or less. In general, when the cooling rate is low, the higher order structure of the resulting gel-like composition becomes coarse, and the pseudo cell units that form the gel composition become large. However, when the cooling rate is high, the cells become dense cell units. When the cooling rate is less than 50 ° C./min, the degree of crystallinity increases and it is difficult to obtain a gel composition suitable for stretching. As a cooling method, it is possible to use a method of making contact with cold air, cooling water, or another cooling medium, a method of making contact with a roll cooled with a refrigerant, or the like. The solution pushed out from the die may be taken out at a take-up ratio of preferably 1 to 10, more preferably 1 to 5 before or during cooling. When the take-up ratio is 10 or more, the neck-in becomes large, and breakage tends to occur during stretching, which is not preferable.
[0022]
Next, the gel-like molded product is stretched. Stretching is performed at a predetermined magnification by heating the gel-like molded article and using a normal tenter method, roll method, inflation method, rolling method, or a combination of these methods. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, either longitudinal or transverse simultaneous stretching or sequential stretching may be used.
[0023]
The stretching temperature is the melting point of polyethylene + 10 ° C. or less, preferably in the range from the crystal dispersion temperature of polyethylene to less than the crystal melting point. For example, in the case of polyethylene, it is in the range of 90 to 140 ° C, preferably 100 to 130 ° C. When the stretching temperature is 140 ° C. or higher, the pores are crushed and the surface opening degree is lowered. On the other hand, when the stretching temperature is 90 ° C. or lower, the pores become small and the effective pores decrease, which is not preferable. Furthermore, in this invention, it is necessary to perform extending | stretching on temperature-fall conditions. The temperature lowering condition is 1 to 20 ° C./min, preferably 5 to 15 ° C./min in terms of the sheet surface temperature, and the temperature difference between the stretching start point and the stretching end point needs to be 5 ° C. or more. In the case of polyethylene, the temperature at the stretching start point is preferably 110 to 140 ° C, and the temperature at the stretching end point is preferably 90 to 120 ° C. When stretching is not performed under temperature-decreasing conditions, surface irregularities can be formed particularly under low-temperature stretching conditions, but the film has a significantly impaired permeability. Moreover, it becomes a film | membrane with a low surface opening degree under high temperature extending | stretching conditions. Therefore, after sufficiently preheating the sheet in this way, by changing the stretching condition of the sheet surface layer under the temperature-lowering condition, it is possible to easily form a film having irregularities only on the surface, high openness, and maintaining permeability.
[0024]
Moreover, although a draw ratio changes with thickness of an original fabric, in uniaxial stretching, 2 times or more are preferable, More preferably, it is 3 to 30 times. In biaxial stretching, the surface magnification is preferably 10 times or more, more preferably 15 to 400 times. If the surface magnification is less than 10 times, stretching is insufficient and a highly elastic, high-strength microporous film cannot be obtained. On the other hand, when the surface magnification exceeds 400 times, a restriction occurs in a stretching operation or the like.
[0025]
The obtained stretched molded product is washed with a solvent to remove the remaining solvent. Cleaning solvents include hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and tetrasalt carbon, fluorinated hydrocarbons such as ethane trifluoride, and ethers such as diethyl ether and dioxane. Volatile ones can be used. These solvents are appropriately selected according to the solvent used for dissolving the polyethylene composition, and used alone or in combination. The cleaning method can be performed by a method of immersing and extracting in a solvent, a method of showering a solvent, or a method using a combination thereof.
[0026]
Washing as described above is performed until the residual solvent in the stretched molded product is less than 1% by weight. Thereafter, the cleaning solvent is dried. The cleaning solvent can be dried by heat drying, air drying, or the like. The dried stretched molded product is preferably heat-set within the temperature range of the crystal dispersion temperature to the melting point.
[0027]
The polyethylene microporous membrane produced as described above has a porosity of 35 to 95%, an average through-hole diameter of 0.001 to 0.5 μm, and a breaking strength of 500 kg / cm 2 or more. The thickness of the polyethylene microporous membrane of the present invention can be appropriately selected according to the use, but is generally 0.1 to 100 μm, preferably 2 to 50 μm.
[0028]
The obtained polyethylene microporous membrane may be further subjected to surface modification such as plasma irradiation, surfactant impregnation, and hydrophilic treatment such as surface grafting, if necessary.
[0029]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not particularly limited to the examples. In addition, the test method in an Example is as follows.
(1) Film thickness: The cross section was measured with a scanning electron microscope.
(2) Air permeability: Measured according to JIS P8117.
(3) Breaking strength: measured in accordance with ASTM D882.
(4) Smoothness: measured with Oken type measuring machine.
(5) Opening degree: measured by electron micrograph.
[0030]
Example 1
A polyethylene composition in which 0.375 parts by weight of an antioxidant was added to 100 parts by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight of 2.5 × 10 6 was obtained. 20 parts by weight of this polyethylene composition was put into a twin screw extruder (58 mmφ, L / D = 42, strong kneading type). Further, 80 parts by weight of liquid paraffin was supplied from the side feeder of this twin-screw extruder and melt kneaded at 200 rpm to prepare a polyethylene solution in the extruder.
[0031]
Then, it extruded at 190 degreeC from T-die installed in the front-end | tip of this extruder, and formed the gel-like sheet | seat of thickness 1.8mm, drawing with a cooling roll. Subsequently, this gel-like sheet is simultaneously biaxially stretched 5 × 5 at a temperature drop rate of 5 ° C./min, 115 ° C. at the start of stretching, and 110 ° C. at the end of stretching, A stretched film was obtained. The obtained stretched membrane was washed with methylene chloride to extract and remove the remaining liquid paraffin, followed by drying and heat treatment to obtain a polyethylene microporous membrane having a thickness of 25 μm. The results of the physical property evaluation of this polyethylene microporous membrane are shown in Table 1.
[0032]
Example 2
In Example 1, except that ultra high molecular weight polyethylene having a weight average molecular weight of 1.0 × 10 6 is used, 30 parts by weight thereof and 70 parts by weight of liquid paraffin are used, and the thickness of the gel sheet is 1.1 mm. In the same manner as in Example 1, a polyethylene microporous membrane was obtained. The results of the physical property evaluation of this polyethylene microporous membrane are shown in Table 1.
[0033]
Example 3
In Example 2, a polyethylene microporous membrane was obtained in the same manner as in Example 1 except that high molecular weight polyethylene having a weight average molecular weight of 5.0 × 10 5 was used and the thickness of the gel sheet was 1.2 mm. It was. The results of the physical property evaluation of this polyethylene microporous membrane are shown in Table 1.
[0034]
Example 4
In Example 2, as the polyethylene composition, except that 6 parts by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight of 2.5 × 10 6 and 24 parts by weight of high density polyethylene having a weight average molecular weight of 3.5 × 10 5 are used. In the same manner as in Example 2, a polyethylene microporous membrane was obtained. The results of the physical property evaluation of this polyethylene microporous membrane are shown in Table 1.
[0035]
Example 5
In Example 4, a polyethylene microporous membrane was obtained in the same manner as in Example 4 except that the thickness of the gel sheet was 1.2 mm and the temperature lowering conditions during stretching were as shown in Table 1. The results of the physical property evaluation of this polyethylene microporous membrane are shown in Table 1.
[0036]
Example 6
In Example 4, a polyethylene microporous membrane was obtained in the same manner as in Example 4 except that the thickness of the gel sheet was 2.1 mm and the temperature drop conditions during stretching were as shown in Table 1. The results of the physical property evaluation of this polyethylene microporous membrane are shown in Table 1.
[0037]
Example 7
In Example 2, a polyethylene microporous membrane was obtained in the same manner as in Example 2 except that the thickness of the gel sheet was 2.3 mm and the temperature lowering conditions during stretching were as shown in Table 1. The results of the physical property evaluation of this polyethylene microporous membrane are shown in Table 1.
[0038]
Comparative Example 1
In Example 1, as a polyethylene composition, 3 parts by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight of 2.5 × 10 6 , 14 parts by weight of high density polyethylene having a weight average molecular weight of 3.5 × 10 5 , and liquid paraffin 83 A microporous membrane was obtained in the same manner as in Example 1 except that parts by weight were used, the thickness of the gel sheet was 1.6 mm, and the stretching conditions were performed at a constant temperature of 115 ° C. The microporous membrane obtained as described above had the physical properties shown in Table 2.
[0039]
Comparative Example 2
In Example 1, as a polyethylene composition, 3 parts by weight of supermolecular weight polyethylene having a weight average molecular weight of 2.5 × 10 6 , 14 parts by weight of high density polyethylene having a weight average molecular weight of 3.5 × 10 5 , and 83 parts by weight of liquid paraffin The microporous membrane was obtained in the same manner as in Example 1 except that the thickness of the gel sheet was 1.9 mm and the stretching conditions were performed at a constant temperature of 120 ° C. The microporous membrane obtained as described above had the physical properties shown in Table 2.
[0040]
Comparative Example 3
In Example 1, a microporous membrane was obtained in the same manner as in Example 1 except that the thickness of the gel sheet was 2.9 mm and the stretching conditions were performed at a constant temperature of 120 ° C. The microporous membrane obtained as described above had the physical properties shown in Table 2.
[0041]
[Table 1]
[Table 2]
[0043]
【The invention's effect】
As described in detail above, the polyolefin microporous membrane obtained by the method of the present invention has appropriate smoothness and openness, and can sufficiently cope with an increase in capacity as a battery separator. It is particularly useful as a lithium battery separator.
Claims (2)
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP13268496A JP3641319B2 (en) | 1996-04-30 | 1996-04-30 | Method for producing polyolefin microporous membrane |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP13268496A JP3641319B2 (en) | 1996-04-30 | 1996-04-30 | Method for producing polyolefin microporous membrane |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH09296060A JPH09296060A (en) | 1997-11-18 |
| JP3641319B2 true JP3641319B2 (en) | 2005-04-20 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP13268496A Expired - Fee Related JP3641319B2 (en) | 1996-04-30 | 1996-04-30 | Method for producing polyolefin microporous membrane |
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| Country | Link |
|---|---|
| JP (1) | JP3641319B2 (en) |
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1996
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| JPH09296060A (en) | 1997-11-18 |
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