JP4271750B2

JP4271750B2 - Microporous membrane and method for producing the same

Info

Publication number: JP4271750B2
Application number: JP22444298A
Authority: JP
Inventors: 成信松田; 藤治名古屋
Original assignee: Asahi Kasei Chemicals Corp
Current assignee: Asahi Kasei Chemicals Corp
Priority date: 1998-03-16
Filing date: 1998-08-07
Publication date: 2009-06-03
Anticipated expiration: 2018-08-07
Also published as: JPH11319522A

Description

【０００１】
【発明の属する技術分野】
本発明は、ウイルス除去フィルターを含む各種フィルター、精密濾過膜、限外濾過膜、電池用セパレータ、電解質コンデンサー用隔膜、固体電解質電池用電解質保持体等の用途に好適に使用される微多孔膜とその製造方法に関する。
【０００２】
【従来の技術】
微多孔膜は、ウイルス除去フィルターを含む各種フィルター、限外濾過膜、精密濾過膜、電池用セパレーター、電解コンデンサー用隔膜、固体電解質電池用電解質保持体等の各種用途に用いられている。これらの用途において重要な因子は、膜の孔径と構造の均質性、並びにこれらに依存する流体の透過特性及び流体から微粒子を分離する際の分離特性である。
【０００３】
また、フッ化ビニリデン系重合体からなる微多孔膜は、耐薬品性、耐熱性、機械的特性に優れた諸特性を持つ微多孔膜として期待される。
微多孔膜を分離膜として用いる場合、分離の対象となる物質のサイズによって膜の孔径を選択する必要がある。また、均質性すなわち孔径分布は膜の分離性能に著しく影響を及ぼす。さらに流体の透過性は分離効率に大きな影響を与える。一方、微多孔膜の製造的側面から見れば、上記の特性の制御の自由度が高く、製造条件の変動を吸収できる安定な方法が望まれる。
【０００４】
従来、フッ化ビニリデン系重合体微多孔膜の製造方法としては、（１）フッ化ビニリデン系重合体を溶媒に均一溶解した後、フッ化ビニリデン系重合体を溶解しない非溶媒中に浸すことによって固液または液々相分離させる湿式製膜法（例えば特開昭６０−９７００１号公報）、（２）フッ化ビニリデン系重合体と有機液状体と親水性無機微粉体を混合し溶融成形し、ついでその成型物から有機液状体と親水性無機微粉体を抽出して微多孔膜を得る方法（特開昭５８−９３７３４号公報）、（３）フッ化ビニリデン系重合体と有機液状体と疎水性無機微粉体を混合し溶融成形し、ついでその成型物から有機液状体及び疎水性無機微粉体を抽出して微多孔膜を得る方法（特開平３−２１５５３５号公報）などがある。
【０００５】
湿式製膜法では、スキン層を有する不均質な微多孔膜が殆どである。特開昭６０−９７００１号公報には網状組織を形成した微多孔膜を得る方法が開示されているが、湿式製膜法であるため機械的強度に問題がある。特開昭５８−９３７３４号公報に開示されている、親水性シリカを使用する方法で製造された微多孔膜には、マクロボイド（粗大孔）が多く存在し、破断伸度が小さく高温高圧での使用に耐えないという問題点がある。
【０００６】
また、フッ化ビニリデン系重合体と有機液状体及び疎水性または親水性シリカなどの無機微粉体を混合し、溶融成形する方法では、無機微粉体の分散状態が悪いとピンホール等の構造欠陥が生じやすく不都合である。また、性能面だけでなく製造的側面から見ても、構造欠陥が原因で生産収率が落ちたり、溶媒の抽出に加えて無機微粉体の抽出が加わり生産時間が長くなってしまう等の不都合がある。特開平３−２１５５３５号公報に開示されている疎水性シリカを使用する方法で製造された微多孔膜は、比較的均質な構造を持ち、破断強度・破断伸度も高いが、上記のシリカに由来する構造上の欠陥が存在する。
【０００７】
また、疎水性あるいは親水性シリカを抽出する際には、苛性ソーダ、苛性カリ等のアルカリ水溶液を用いる事が特開昭５８−９３７３４号公報や特開平３−２１５５３５号公報には開示されているが、アルカリ水溶液によりフッ化ビニリデン系重合体微多孔膜が薄茶色から褐色に着色される等の問題点も残る。また、シリカ抽出や脱色の際の力学的強度の低下が問題となる場合もある。
【０００８】
【発明が解決しようとする課題】
本発明は、上記の問題点が解決された、均質な構造をもち、流体の透過特性、流体から微粒子を分離する際の分離特性、機械的特性、及び耐薬品性が優れたフッ化ビニリデン系重合体微多孔膜及びその製造方法を提供することを目的としている。
【０００９】
【課題を解決するための手段】
本発明者等は、上記課題を達成するために、フッ化ビニリデン系重合体微多孔膜の構造を制御し得る種々の方法を検討し、その結果、重量平均分子量１×１０⁵以上のフッ化ビニリデン系重合体の使用、特定の溶媒と特定の温度でのフッ化ビニリデン系重合体の溶解、特定の冷却方法、必要な場合延伸残留歪みが１００％以下の延伸を組合せることにより、本発明に到達した。
【００１０】
即ち、本発明は、重量平均分子量１×１０⁵以上のフッ化ビニリデン系重合体を含む重合体相と、ハーフドライ法による平均孔径が０．００５〜５μｍであって、一方の面から他方の面に連通している空隙部とからなる微多孔膜であって、その内部構造がパーコレーション構造をとる微多孔膜である。
本発明において「ハーフドライ法による平均孔径」とは、後述の方法で測定した孔径である。
【００１１】
また、「パーコレーション構造」とは、重合体相は三次元的に任意の方向に分岐した等方性の網状構造を形成しており、空隙部は、網状構造の重合体相に囲まれて形成され、各空隙部は相互に連通している構造をいう。
上記の微多孔膜の中で、少なくとも一方の表面層の構造が内部構造と同一でなく、その内部構造と同一でない表面層の走査型電子顕微鏡法による平均孔径は内部構造と同じまたはそれ以上の微多孔膜、両表面層の構造もパーコレーション構造であり、両表面層の走査型電子顕微鏡法による平均孔径は、内部構造と同じまたはそれ以上である微多孔膜、及び少なくとも一方の表面層の走査型電子顕微鏡法による平均孔径は、内部構造よりも小さい微多孔膜が好ましい。
【００１２】
この微多孔膜は、前記のフッ化ビニリデン系重合体：パーコレーション構造をもった微多孔膜を形成可能な溶媒＝１０：９０〜６０：４０の重量比で、パーコレーション構造形成可能な温度Ｔs でフッ化ビニリデン系重合体を溶解した後、この溶液を押出装置で押出し、冷却して二相ゲルからなるゲル状成形体を形成した後、下記i）、ii）及びiii）から選ばれたいずれかの処理を行って製造される。
i）延伸を行うことなしに揮発性液体を用いて溶媒を除去する。
ii）溶媒を除去する前に延伸残留歪みが１００％以下になるように延伸を行った後に揮発性液体を用いて溶媒を除去する。
iii）揮発性液体を用いて溶媒を除去した後に延伸残留歪みが１００％以下になるように延伸を行う。
【００１３】
また、この微多孔膜は、前記のフッ化ビニリデン系重合体：パーコレーション構造をもった微多孔膜を形成可能な溶媒とそのフッ化ビニリデン系重合体と相溶する熱可塑性樹脂（以下、相溶性樹脂と言う。）の混合物＝１０：９０〜６０：４０の重量比で、しかもフッ化ビニリデン系重合体と相溶性樹脂の合計が６０重量％以下で、フッ化ビニリデン系重合体：相溶性樹脂＝４０：６０〜９０：１０の重量比の条件の下で、パーコレーション構造形成可能な温度Ｔs でフッ化ビニリデン系重合体及び相溶性樹脂を溶解した後、この溶液を押出装置で押出し、冷却して二相ゲルからなるゲル状成形体を形成した後、下記iv）、v）及びvi）から選ばれたいずれかの処理を行って製造される。
iv）延伸を行うことなしに、揮発性液体を用いて溶媒及び相溶性樹脂を除去する。
v）溶媒及び相溶性樹脂を除去する前に延伸残留歪みが１００％以下になるように延伸を行った後に揮発性液体を用いて溶媒を除去する。
vi）揮発性液体を用いて溶媒及び相溶性樹脂を除去した後に延伸残留歪みが１００％以下になるように延伸を行う。
【００１４】
本発明において、「パーコレション構造をもった微多孔膜を形成可能な溶媒」とは、フッ化ビニリデン系重合体１０〜６０重量％の範囲内の、任意の濃度のフッ化ビニリデン系重合体とその溶媒の溶液、または、任意の濃度のフッ化ビニリデン系重合体とその溶媒と相溶性樹脂の溶液について、横軸に溶解温度Ｔs 、縦軸に各溶解温度の溶液から製膜された膜の破断伸度ＴＬをＴs ＝１００℃を起点に５℃間隔でプロットした時に、−（ＴＬs + 5 −ＴＬs ）／｛（Ｔs ＋５℃）−Ｔs ｝（ＴＬs + 5 はＴs ＋５℃におけるＴＬ、ＴＬs はＴs におけるＴＬ）が最大となる溶解温度Ｔs に２．５℃をプラスした温度（Ｔs ＋２．５℃）をＴu とする。一方、横軸にＴs 、縦軸にその膜の空孔率Ｐを同様にプロットした時に（Ｐs + 5 −Ｐs ）／｛（Ｔs ＋５℃）−Ｔs ｝（Ｐs + 5 はＴs ＋５℃におけるＰ、Ｐs はＴs におけるＰ）が最大となる溶解温度Ｔs に２．５℃をプラスした温度（Ｔs ＋２．５℃）をＴl とする。上記の濃度範囲内の少なくとも一濃度の溶液がＴl 及びＴu の両者を保有し、（Ｔu −Ｔl ）＞０の場合、その溶媒はパーコレーション構造をもった微多孔膜を形成可能な溶媒である。
【００１５】
「パーコレーション構造形成可能な温度」とは、Ｔl ≦Ｔs ≦Ｔu を満たす溶解温度Ｔs をいう。
本発明の微多孔膜は、均質な構造を有し、流体の透過特性、流体から微粒子を分離する際の分離特性、機械的特性、及び耐薬品性に優れる。
本発明はまた、溶液を冷却して得られる二相ゲルからなるゲル状成形体に関し、これは後述するように溶媒を電解液で置換することにより、例えば、固体電解質電池用電解質保持体として好適に使用される。
【００１６】
以下に本発明を詳細に説明する。
微多孔膜の内部構造は、微多孔膜の任意の断面（多くの場合は垂直断面）を断面に垂直な方向より走査型顕微鏡等により検鏡する事により観察される構造である。微多孔膜の表面層の構造は、微多孔膜の表面を表面に垂直な方向より走査型電子顕微鏡等により検鏡する事により観察される構造である。
【００１７】
本発明の微多孔膜は、前記のフッ化ビニリデン系重合体とその溶媒の溶液、または、フッ化ビニリデン系重合体とその溶媒と相溶性樹脂の溶液を、冷却して二相ゲルを形成させることにより製造される。ここで言う二相ゲルとは、フッ化ビニリデン系重合体濃度の高い重合体濃厚相と重合体濃度の低い重合体希薄相から構成される。ここで、重合体濃厚相が重合体のみであり、重合体希薄相が溶媒のみである場合も含まれる。この場合の溶液は、目視では均一な溶液に見えるが、後述するように、溶液中に重合体の微結晶が散在している可能性がある。この様な二相ゲルができる場合は、均一な一相溶液の温度を、結晶化温度以上の任意の観察温度まで冷却して静置しても、重合体濃厚相と重合体希薄相の液々界面が目視で観察されることはない。代表的な液々界面は、例えば、”ＴＨＥＲＭＯＤＹＮＡＭＩＣＳＯＦＰＯＬＹＭＥＲＳＯＬＵＴＩＯＮＳ −ＰＨＡＳＥＥＱＵＩＬＩＢＲＩＡＡＮＤＣＲＩＴＩＣＡＬＰＨＥＮＯＭＥＮＡ−”（Ｋ．ＫＡＭＩＤＥ著、ＥＬＳＥＶＩＥＲ、１９９０）の図１．２に例示されている様に平面上であるが、重合体濃厚相と重合体希薄相が分離した状態が目視で観察される程度の場合もある。前記の二相ゲルができる場合は、溶液が全体的に白濁してゲル化する。ゲル化と液々相分離が拮抗しているという仮説が、液々界面が目視で観察されない理由として考えることができる。
【００１８】
本発明の微多孔膜のハーフドライ法による平均孔径は、０．００５〜５μｍの範囲であり、この範囲であれば、例えば、液体や気体の濾過のためのフィルターとして好適に使用できる。該平均孔径が５μｍより大きいとピンホールなどの構造欠陥の数が増加し、良好な分離特性を有する微多孔膜は得られない。液体や気体からの不純物の除去の分画特性の観点から言えば、微多孔膜のバブルポイント法による最大孔径と該平均孔径の比は２．０以下であることが好ましい。水処理用のフィルターとして使用される場合の平均孔径は、０．０５μｍ以上が好ましい。孔径が０．０５μｍ未満では、水処理の場合は、孔径が小さすぎて透過特性が低下してしまう。また、ウイルス除去フィルターとして使用される場合の平均孔径は、０．００５〜０．１μｍの範囲であることが好ましい。
【００１９】
重合体相の大部分が実質的に球状粒子と見なされる球状粒子網状構造（例えば、図６参照）は、本発明のパーコレーション構造とは異なる。球状粒子網状構造においては、球と球の接点で重合体相が繋がるために、力学物性が低下する。
空隙部の大部分が実質的に球状孔又は楕円体状孔と見なされる球状孔網状構造及び楕円体状孔網状構造も、本発明のパーコレーション構造とは異なる。球状孔網状構造又は楕円体状孔網状構造では、球と球又は楕円体と楕円体の接点で孔が繋がるために、液体の透過性が低下する。球状孔網状構造又は楕円体状孔網状構造は、球状または楕円体状の細胞が集合した構造に見えることから、細胞状構造と呼ぶ場合もある。
【００２０】
本発明の、二相ゲルによる三次元的に分岐した等方性の網状構造が、高伸度、高ウイルス除去性能、高透水量、高イオン伝導度や高充電効率などの作用効果に寄与しているのである。
本発明の微多孔膜の表面層の構造は、製造条件により、内部構造と同一の場合もあるし異なる場合もあるが、いずれの場合も表面の走査型電子顕微鏡法による平均孔径を内部構造と同じまたはそれ以上になるように工夫することができる。このことにより本発明の内部構造がパーコレーション構造であるフッ化ビニリデン系重合体微多孔膜は、本発明の目的とする微多孔膜の特性を発揮できている。また、表面の走査型電子顕微鏡法による平均孔径が内部構造以上の場合は、表面層がプレフィルターとしての効果を持つ様に工夫することもできる。
【００２１】
表面層の構造が内部構造と異なる場合、あるいは構造が同じであっても、表面層と内部構造の走査型電子顕微鏡法による平均孔径が異なる場合、表面層の厚みは、少なくとも０．１μｍ以上であり、通常３μｍ以内である。走査型電子顕微鏡法による平均孔径を求める際には、後述する様に画像処理装置を利用する。
微多孔膜の少なくとも一方の表面層の走査型電子顕微鏡法による平均孔径が、内部構造よりも小さい場合も本発明に含まれる。この場合は、内部構造よりも緻密な表面層が、液体や気体中の不純物が膜内へ侵入する事を防ぐ効果がある。この場合の表面層の厚みも、少なくとも０．１μｍ以上であり、通常３μｍ以内である。また、該緻密な表面層の平均孔径は、通常０．００１μｍ以上である。
【００２２】
これらの特徴を持つ微多孔膜は、均質な構造を有し、流体の透過特性、流体から微粒子を分離する際の分離特性、機械的特性、及び耐薬品性に優れる。液体の透過特性に優れるという事は、平均孔径が同一の膜と比べて液体の透過性に優れるということである。液体の透過性が優れた微多孔膜は、単位膜面積あたりの処理能力が高くなるので、膜モジュールをコンパクトにできる等の利点がある。
【００２３】
本発明において、フッ化ビニリデン系重合体の重量平均分子量は、１×１０⁵以上であり、該重量平均分子量が１×１０⁵未満では、溶液の粘度が低く、ゲル状多孔体を形成するのに不都合であり、得られた微多孔膜の機械的特性も劣悪なものとなる。上記のフッ化ビニリデン系重合体の重量平均分子量は、好ましくは３×１０⁵〜２×１０⁶であり、平均分子量の異なる複数のフッ化ビニリデン系重合体の混合物を使用してもよい。
【００２４】
本発明で使用されるフッ化ビニリデン系重合体の例としては、フッ化ビニリデンホモポリマー及びフッ化ビニリデン共重合体が挙げられ、フッ化ビニリデン共重合体としては、フッ化ビニリデンと四フッ化エチレン、六フッ化プロピレン、三フッ化塩化エチレン、およびエチレンから選ばれた１種類以上との共重合体であるものが用いられるが、特にフッ化ビニリデンホモポリマーが好ましい。また、これらの複数のフッ化ビニリデン系重合体の混合物でもよい。
フッ化ビニリデン系重合体には、必要に応じて、酸化防止剤、紫外線吸収剤、滑剤、アンチブロッキング剤などの各種添加剤を本発明の目的を損なわない範囲で添加することができる。
次に、本発明のフッ化ビニリデン系重合体微多孔膜の製造方法の一例を説明する。
【００２５】
本発明において、原料となるフッ化ビニリデン系重合体の溶液は、フッ化ビニリデン系重合体：パーコレーション構造をもった微多孔膜を形成可能な溶媒＝１０：９０〜６０：４０の重量比で、パーコレーション構造形成可能な温度で、フッ化ビニリデン系重合体を加熱溶解することによって調製する。
また、原料となるフッ化ビニリデン系重合体の溶液は、フッ化ビニリデン系重合体：パーコレーション構造をもった微多孔膜を形成可能な溶媒と相溶性樹脂の混合物（以下、溶媒／相溶性樹脂混合物と言う。）＝１０：９０〜６０：４０の重量比で、しかもフッ化ビニリデン系重合体と相溶性樹脂の合計が６０重量％以下で、フッ化ビニリデン系重合体：相溶性樹脂＝４０：６０〜９０：１０の重量比の条件を満たす組成で、フッ化ビニリデン系重合体及び相溶性樹脂を加熱溶解することによって調製することもできる。
【００２６】
図１にフッ化ビニリデンホモポリマー（重量平均分子量は３．６２×１０⁵）のフタル酸ジエチル（ＤＥＰ）溶液の結晶化温度Ｔc と溶解温度Ｔs の関係を示す。フッ化ビニリデン系重合体の重量分率は３０ｗｔ％（◇）、３５ｗｔ％（○）、４０ｗｔ％（△）である。いずれの重量分率においても、溶解温度Ｔs の増加に伴い結晶化温度Ｔc が減少し、溶解温度Ｔs が１７８℃付近以上では結晶化温度Ｔc はほぼ一定となる。この場合、Ｔs ＜１７８℃では、溶液中に重合体の微結晶が散在している可能性がある。また、Ｔs ＜１７８℃の範囲でＴs が低い程、単位体積中の微結晶の数が増えると考えることもできる。
【００２７】
図２は、フッ化ビニリデン系重合体とパーコレーション構造をもった微多孔膜を形成可能な溶媒の溶液の結晶化温度Ｔc と溶解温度Ｔs の関係を模式的に描いた図である。図２には、パーコレーション構造形成可能な温度範囲も書き加えた。
パーコレーション構造形成可能な温度の範囲内で、溶解温度が低い程、パーコレーション構造は緻密になる。パーコレーション構造形成可能な温度未満の温度で溶解した溶液からは無孔化した成形体しか得られず、空孔率が著しく低下する。更に溶解温度を下げると、均一溶液が得られなくなる。パーコレーション構造形成可能な温度を超える温度で溶解した溶液からは、内部構造が粗大化した成形体しか得られず、力学的強度及び伸度が著しく低下する。
【００２８】
図３にこの現象の一例を示す。図３、ａ）、ｂ）、ｃ）にそれぞれ製膜時の溶解温度Ｔs が異なるフッ化ビニリデン系重合体微多孔膜の空孔率、破断強度、破断伸度とその溶解温度Ｔs の関係を示す。Ｔs ＝１３５、１４０、１４５、１５０、１５５及び１６０℃の微多孔膜は、それぞれ比較例７、実施例９、実施例７、実施例８、比較例８及び比較例９に記載される方法によって調整された。図３ａ）より１３５℃以下では空孔率が著しく低下する。また、図３ｂ）及びｃ）より、１５５℃以上では破断強度及び破断伸度が著しく低下する。定義により、この場合は、Ｔl ＝１３７．５℃及びＴu ＝１５２．５℃であり、（Ｔu −Ｔl ）＞０となることから、ＤＥＰは（Ｂ）のパーコレーション構造をもった微多孔膜を形成可能な溶媒である。また、１３７．５℃≦Ｔs ≦１５２．５℃がパーコレーション構造形成可能な温度域である。
【００２９】
図４に示す、溶解温度が結晶化温度と曇り点曲線に及ぼす影響を示す模式図を用いてこの現象を定性的に説明する。
ここで曇り点曲線とは、曇り点温度を高分子の濃度に対してプロットした曲線である。曇り点温度が結晶化温度以上の場合には、溶液温度が曇り点温度を越せば、溶液は１相で均一に溶解した状態であり、逆に溶液温度が曇り点温度以下で結晶化温度以上の範囲にあれば、溶液は高分子濃度の高い高分子濃厚相と高分子濃度の低い高分子希薄相の２相に液々相分離する。溶液が結晶化温度以下に冷却されれば、高分子の結晶化が起こり、溶液は固化する。
【００３０】
図４の縦軸は温度、横軸はフッ化ビニリデン系重合体の濃度（例えば、重量分率）、二点鎖線は曇り点曲線である。ただし、この場合の曇り点曲線は結晶化曲線、即ち、固化曲線よりも低温側にあり、実際には観察されない。熱力学的な類推に基づき、この二点鎖線の位置に曇り点曲線が存在すると仮定した。ここで、液々相分離する系と同様に、曇り点曲線の低温側の領域を二相分離領域と考えることができる。
図４において、△はＴs ＞Ｔu の高温溶解、○はＴl ≦Ｔs ≦Ｔu を満たすパーコレーション構造形成可能な溶解温度Ｔs における溶解（図４には「中間」と記述）、◇はＴs ＜Ｔl の低温溶解を示している。一点鎖線、破線及び実線は、それぞれ高温溶解、パーコレーション構造形成可能な溶解温度における溶解及びに低温溶解の場合の結晶化曲線である。
【００３１】
既に述べた様に、Ｔl より低温溶解（◇）の場合は、無孔化し、空孔率の著しい低下が観察される。この場合は、図４から解るように、曇り点曲線が結晶化曲線よりも十分低温側にあり、二相分離よりも結晶化による均一なゲル化が支配的となって無孔化すると考えられる。また、Ｔu より高温溶解（△）の場合は、結晶化温度Ｔc が低くなり、構造が粗大化し、機械的特性の著しい低下が観察される。この場合は、図４に示したように、曇り点曲線が一部分結晶化曲線よりも高温側にあり、二相分離の影響が強くなるため、構造が粗大化すると考えられる。その中間の温度領域（○）では、（Ａ）のパーコレーション構造が形成される。その理由として、例えば、ゲル化と液々相分離が拮抗しているというメカニズムが考えられる。
【００３２】
パーコレーション構造形成可能な温度の範囲はフッ化ビニリデン系重合体と溶媒の組合せによって違う。また同一のフッ化ビニリデン系重合体と同一の溶媒であっても、それらの重量分率によって違ってくる。また、同じフッ化ビニリデン系重合体と溶媒の組合せで同一重量分率であっても、プレス製膜の様な比較的静的に微多孔膜を形成させる場合と、押出機などからの押出製膜の様な動的に微多孔膜を形成する場合では、動的に形成させる場合の方がパーコレーション構造形成可能な温度の範囲が低温側にシフトする傾向がある。即ち、パーコレーション構造形成可能な温度の範囲は、製膜方法によっても違ってくる。また、プレス製膜の際には、フッ化ビニリデン系重合体と溶媒を加熱混合した後、室温まで冷却したサンプルを熱プレス機を用いて、一定の溶解温度で再度溶解して平膜状等に成形する。この場合、熱プレス機で再度溶解する溶解温度が、パーコレーション構造形成するかどうかを支配する。言い換えれば、溶液は熱履歴を記憶せず、一番最後の溶解温度が膜の構造を支配する。
【００３３】
図３に示したように、高い強度及び伸度を保ちながら、高空孔率を保つ事ができる点からも、パーコレーション構造を形成する意義が確認できる。さらに、上述の溶媒は、溶融成形温度にて液体状態を保ち、かつ不活性であることが要求される。
パーコレーション構造をもった微多孔膜を形成可能な溶媒としては、フタル酸ジメチル、フタル酸ジエチル、フタル酸ジブチル、フタル酸ジオクチル、フタル酸ジイソデシル、フタル酸トリデシル等のフタル酸エステル、メチルベンゾエート、エチルベンゾエート等の安息香酸エステル、セバシン酸オクチル等のセバシン酸エステル、アジピン酸ジオクチル等のアジピン酸エステル、トリメリト酸トリオクチル等のトリメリト酸エステル、リン酸トリブチル、リン酸トリクレジル等のリン酸エステル、アセトフェノン等のケトンなどの単一溶媒あるいはこれら二種以上の混合溶媒が例示される。上記の溶媒中、アルキル基は各種異性体を含んでもよい。上記の単一溶媒または混合溶媒にアセトン、テトラヒドロフラン、メチルエチルケトン、ジメチルホルムアミド、ジメチルアセトアミド、ジメチルスルホキシド、Ｎ−メチルピロリドン等の良溶媒あるいは水等の非溶媒を混合し、パーコレーション構造をもった微多孔膜形成可能な溶媒になる程度に溶解性を調節した混合溶媒も本発明に使用しうる。
【００３４】
相溶性樹脂としては、メタクリル酸エステル樹脂やアクリル酸エステル樹脂やポリ（１、４−ブチレンアジペート）やポリ酢酸ビニルやポリビニルピロリドンなどを例示することができる。これらの中でメタクリル酸メチル樹脂やその共重合体が好ましく用いられる。メタクリル酸メチル樹脂の共重合体としては、アクリル酸メチルやスチレンやα−メチルスチレンやメタクリル酸や無水マレイン酸などのコモノマーとの共重合体等を挙げることができる。
【００３５】
加熱溶解は、上述したようにフッ化ビニリデン系重合体とパーコレーション構造をもった微多孔膜形成可能な溶媒とからなる混合物、または、フッ化ビニリデン系重合体とパーコレーション構造をもった微多孔膜形成可能な溶媒と相溶性樹脂とからなる混合物を、パーコレーション構造形成可能な温度で撹拌しながら行う。その温度は使用するフッ化ビニリデン系重合体及び溶媒及び相溶性樹脂の種類によってＴl ℃〜Ｔu ℃の範囲、より好ましくはＴl ＋２℃〜Ｔu −２℃の範囲で設定すればよい。
【００３６】
上述の混合物中におけるフッ化ビニリデン系重合体の濃度は、溶媒の溶解性によって溶解可能な濃度は変化するが、１０〜６０重量％、好ましくは１０〜４０重量％、より好ましくは１０〜３０重量％である。濃度が１０重量％未満では、溶液の粘度が低いために成形性が悪く、成形体の力学強度も弱い。一方、濃度が６０重量％を越えると、均一な溶液の調製が困難になるほか、パーコレーション構造が得られにくくなる。
【００３７】
また、加熱溶解に際し、フッ化ビニリデン系重合体とパーコレーション構造をもった微多孔膜を形成可能な溶媒と相溶性樹脂からなる混合物を選択する場合は、更にフッ化ビニリデン系重合体とそれと相溶性樹脂の合計が６０重量％以下で、フッ化ビニリデン系重合体：相溶性樹脂＝４０：６０〜９０：１０の重量比の条件が必要である。フッ化ビニリデン系重合体とそれと相溶性樹脂の合計濃度が６０重量％を越えると、均一な溶液の調製が困難になるほか、パーコレーション構造が得られにくくなる。また、フッ化ビニリデン系重合体と相溶性樹脂の合計量に対する相溶性樹脂の割合が６０重量％を越えると、フッ化ビニリデン系重合体の結晶性の低下が著しく、成形体の力学強度が弱くなる。逆に、フッ化ビニリデン系重合体と相溶性樹脂の合計量に対する相溶性樹脂の割合が１０重量％未満では、相溶性樹脂を添加する効果が期待できなくなる。
【００３８】
加熱溶解に際し、フッ化ビニリデン系重合体とパーコレーション構造をもった微多孔膜形成可能な溶媒と相溶性樹脂とからなる混合物を選択する場合は、ｖ）又はvi）に述べるような延伸によって成形体を製造すると透水性能の向上が著しい。相溶性樹脂により、フッ化ビニリデン系重合体の結晶性が適度に抑制され、構造欠陥が生じ易くなり、延伸の破壊によって貫通孔確率が向上しているものと考えられる。
【００３９】
次に、上述の混合物の加熱溶液をダイスから押し出して成形する。ダイスは、適宜選べば良いが、必要に応じて中空のダイスやＴダイや２重円筒のインフレーションダイス等を用いることができる。押出成形温度は、溶媒の種類によってＴl ℃〜Ｔu ℃の範囲内で適宜設定する。
ダイスから押し出された溶液は、冷却されて二相ゲルからなるゲル状成形体となる。冷却法としては、空気による冷却、ロールによる冷却または液体状の冷却媒体に直接接触させる方法等を用いることができる。
Ｔダイ等で押出し、平面上の膜を得る場合には、空気による冷却法やロールによる冷却法が多く用いられる。この場合は、微多孔膜の表面層の構造も内部構造と同じパーコレーション構造であり、通常は走査型電子顕微鏡法による平均孔径が内部構造と同じまたはそれ以上のフッ化ビニリデン系重合体微多孔膜が得られる。
【００４０】
また、中空のダイスから押出し中空状の膜を得る場合には、中空の断面形状及びそれらのサイズを安定させる目的で、液体状の冷却媒体に直接接触させる方法が有利である。空気冷却やロール冷却では、フッ化ビニリデン系重合体と溶媒の混合溶液の粘性が低いために、多くの場合中空糸の断面形状がつぶれてしまう。また、中空のダイス以外のダイス、例えばＴダイなどを用いる場合にも液体状の冷却媒体に直接接触させることができる。液体状の冷却媒体に直接接触させる場合には、パーコレーション構造をもった微多孔膜を形成可能な溶媒を、冷却媒体として使用することが好ましい。この場合は冷却媒体に接触する部分の表面層の構造は、内部構造と異なってくる。冷却温度はＴm −５０℃以下が好ましい。ここでＴm は、フッ化ビニリデン系重合体と溶媒の混合物中におけるフッ化ビニリデン系重合体の融点である。融点Ｔm はフッ化ビニリデン系重合体の濃度が下がる程、低温側へ移動する（融点降下現象）。フッ化ビニリデン系重合体と親和性の低い冷却媒体を使用すると、フッ化ビニリデン系重合体微多孔膜の表面層の構造がスキン状構造になったり、粒子状物集合構造になったりして、表面の開孔性が低くなる場合がある。
【００４１】
パーコレーション構造をもった微多孔膜形成可能な溶媒である冷却媒体として、フタル酸ジメチル、フタル酸ジエチル、フタル酸ジブチル、フタル酸ジオクチル、フタル酸ジイソデシル等のフタル酸エステル、メチルベンゾエート、エチルベンゾエート等の安息香酸エステル、セバシン酸オクチル等のセバシン酸エステル、アジピン酸ジオクチル等のアジピン酸エステル、トリメリト酸トリオクチル等のトリメリト酸エステル、リン酸トリブチル、リン酸トリクレジル等のリン酸エステル、アセトフェノン等のケトンなどの単一冷却媒体あるいは、これら二種以上の混合冷却媒体が例示される。上記の冷却媒体中、アルキル基は各種異性体を含んでも良い。また、上記の単一冷却媒体または混合冷却媒体にアセトン、テトラヒドロフラン、メチルエチルケトン、ジメチルホルムアミド、ジメチルアセトアミド、ジメチルスルホキシド、Ｎ−メチルピロリドン等の良溶媒をあるいは水等の非溶媒を混合し、パーコレーション構造をもった微多孔膜形成可能な溶媒となる程度に溶解性を調節した混合溶媒も冷却媒体として本発明に使用しうる。
【００４２】
得られたゲル状成形体は、溶媒と相溶性のある揮発性液体で洗浄し溶媒を除去する。洗浄用の揮発性液体としては、ペンタン、ヘキサン、ヘプタン等の炭化水素、塩化メチレン、四塩化炭素等の塩素化炭化水素、三フッ化エタン等のフッ化炭化水素、メチルエチルエーテル、ジエチルエーテル等のエーテル類、アセトン、メチルエチルケトン等のケトン類等を用いることができる。上述の揮発性液体は、使用した溶媒の種類によって適宜選択し、単独あるいは混合して用いられる。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、あるいはこれらの組合せによる方法等によって行うことができる。フッ化ビニリデン系重合体とパーコレーション構造をもった微多孔膜形成可能な溶媒と相溶性樹脂からなる混合物を選択した場合は、溶媒と相溶性樹脂を同時に洗浄して除去できる揮発性液体を使用する事が望ましい。
【００４３】
その後、微多孔膜を乾燥させる。微多孔膜の乾燥方法には、加熱乾燥、熱風による乾燥、加熱ロールに接触させる等の方法が挙げられる。
微多孔膜の表面開孔性を向上させる、即ち、表面層の走査型電子顕微鏡法による平均孔径を向上させたり、貫通孔確率を向上させたり、破断強度を向上させたりする目的で、ゲル状成形体、微多孔膜またはその両者に対して、上記の微多孔膜の構造的特徴を逸脱しない範囲の延伸倍率で延伸残留歪みが０〜１００％、好ましくは１０〜１００％の延伸を行なうことができる。ゲル状成形体または微多孔膜の延伸は、通常のテンター法、ロール法、圧延法等、もしくはこれらの方法の組合せによって所定の倍率で行う。延伸は一軸延伸または二軸延伸のどちらでもよい。また二軸延伸の場合、縦横同時延伸または逐次延伸のどちらでもよい。また、ここで言う延伸残留歪みとは、一軸延伸の場合、延伸前の試料長（初期長）に対する、延伸によって増加した試料長の割合である。また、二軸延伸の場合は、延伸前の膜面積（初期面積）に対する、延伸によって増加した膜面積の割合である。延伸残留歪みを１００％以下にするためには、条件によっても異なるが、一軸延伸の場合は延伸倍率３倍以下、二軸延伸の場合は面倍率で４倍以下である。ゲル状成形体または微多孔膜の延伸温度は、５０℃以下、好ましくは２５℃以下である。延伸温度が５０℃を越える場合は、延伸の効果が十分ではない。
【００４４】
ゲル状成形体を延伸した場合は、次に、前述の方法で溶媒を除去し、微多孔膜を乾燥させる。
得られた微多孔膜に対して、寸法の安定性などの目的で熱処理を施すことができる。熱処理温度は５０℃以上、フッ化ビニリデン樹脂の融点温度−２０℃以下の任意の温度に設定できる。
得られた微多孔膜は、必要に応じて、アルカリ処理、プラズマ照射、電子線照射、γ線照射、コロナ処理、界面活性剤含浸、表面グラフト、コーティング等で親水化処理することができる。
また、必要に応じてゲル状成形体または微多孔膜に対して、電子線照射やγ線照射等により架橋することもできる。
【００４５】
以上のようにして製造した微多孔膜は、好ましくは、空孔率が３０％以上、破断強度が５０Ｋｇｆ／ｃｍ²以上、より好ましくは７０〜５００Ｋｇｆ／ｃｍ²、破断伸度が１５０％以上、より好ましくは２００〜８００％、バブルポイントが１〜２０Ｋｇｆ／ｃｍ²、透水量が２００〜１００００リットル／ｍ²・ｈｒ・ａｔｍである。本発明の微多孔膜の厚さは、用途に応じて適宜選択しうるが、一般に２０〜１０００μｍ、好ましくは６０〜８００μｍである。
【００４６】
本発明の微多孔膜及びこの微多孔膜の製造過程において得られる二相ゲルからなるゲル状成形体は、微多孔膜の場合は電解質溶液を導入することにより、ゲル状成形体の場合は溶媒と電解質溶液を置換する事により得られる、固体電解質電池用電解質保持体の前駆体として使用できる。
本発明において使用される測定項目及び測定法は、以下の通りである。
（１）分子量及び分子量分布：ＧＰＣによりポリスチレン換算重量平均分子量Ｍｗを測定する。ＧＰＣ測定装置；東洋ソーダ製、カラム；ＧＭＨＸＬ、溶媒；ＤＭＦ、カラム温度；４０℃。
（２）微多孔膜の表面層の構造と内部構造の観察：走査型電子顕微鏡（ＳＥＭ）日立Ｓ−８００Ａを用いて微多孔膜の表面層の構造と内部構造を観察する。ここで内部構造とは、微多孔膜を凍結割断した断面の構造をいう。
（３）走査型電子顕微鏡法による平均孔径（μｍ）：微多孔膜の表面又は断面の走査型電子顕微鏡写真上に、画像処理装置（ＩＰ−１０００ＰＣ、旭化成工業（株）製）にて５０本の平行な直線を描き、直線が空隙部を通過する線分の長さの平均を平均孔径とする。任意の直線が少なくても１０個の空隙部を横切る様に、倍率及び領域の広さを選んだ。本発明では特に断らない限り、倍率６０００倍の電子顕微鏡写真の縦１６μｍ×横１６μｍの領域を利用する。
（４）微多孔膜の厚さ（μｍ）：ＳＥＭによって観察した微多孔膜の任意に選んだ断面の厚み５点以上の平均値を微多孔膜の厚みとする。
（５）平均孔径（μｍ）（ハーフドライ法）：ＡＳＴＭＦ３１６−８６に準拠してエタノールを用いて測定する。
（６）最大孔径（μｍ）（バブルポイント法）：ＡＳＴＭＦ３１６−８６及びＥ１２８−６１に準拠して、エタノールを用いて測定する。
（７）空孔率（％）：空孔率＝（空孔容積／微多孔膜容積）×１００。
【００４７】
（８）破断強度（Ｋｇｆ／ｃｍ²）、破断伸度（％）：中空糸状または幅１０ｍｍの短冊状試験片について、ＡＳＴＭＤ８８２に準拠して測定する。
（９）透水量（リットル／ｍ²・ｈｒ・ａｔｍ）：２５℃において、差圧１Ｋｇｆ／ｃｍ²にて、純水を用いて測定する。
（１０）延伸残留歪み（％）：
延伸残留歪み＝（（延伸後の試料長−初期長）／初期長）×１００。
（１１）融点Ｔm （℃）：フッ化ビニリデン系重合体と溶媒の混合物を密封式ＤＳＣ容器に封入し、セイコー電子製ＤＳＣ−２００を用いて測定した融解ピーク温度を融点とする（昇温速度５℃／ｍｉｎ）。
（１２）結晶化温度Ｔc （℃）：フッ化ビニリデン系重合体と溶媒の混合物を密封式ＤＳＣ容器に封入し、セイコー電子製ＤＳＣ２００を用いて、昇温速度５℃／ｍｉｎで溶解温度Ｔs まで昇温し、２０分間保持した後に、降温速度２℃／ｍｉｎで降温する過程で観察される結晶化ピーク温度を結晶化温度とする。
（１３）イオン伝導度（ｍＳ／ｃｍ）：シート状電解質保持体を金属電極（ステンレスシート）で挟み込み、ことで電気化学セルを構成し、電極間に交流を印加して抵抗成分を測定する交流インピーダンス法を採用して、ＥＧ＆Ｇ社製、３８９型インピーダンスメーターを用いてインピーダンスを測定する。イオン伝導度は、コール−コールプロットの実数インピーダンス切片から計算する。
【００４８】
【発明の実施の形態】
以下に本発明を実施例により具体的に説明する。
【００４９】
【実施例１】
重量平均分子量（Ｍｗ）が３．６２×１０⁵のフッ化ビニリデンホモポリマー４０重量部と、フタル酸ジエチル（ＤＥＰ）６０重量部を２軸混練機で１６０℃で加熱混合した後、室温まで冷却したサンプルを熱プレス機を用いて１５５℃で再度溶解して１００μｍの平膜状に成形した後、２０℃のプレス機で冷却してシート状ゲル状成形体を得た。成形されたゲル状成形体を塩化メチレン中に１時間浸漬する事によってＤＥＰを抽出し、室温で乾燥して微多孔膜を得た。この膜の平均孔径は０．１μｍであり、内部構造は図５ｂ）に示す様にパーコレーション構造であった。この膜に対して２０℃で延伸倍率１５０％の一軸延伸を施した後、２０℃で緩和させた。この際の延伸残留歪みは２０％であった。この延伸膜の内部構造はパーコレーション構造であった。
【００５０】
【実施例２】
熱プレス機で１６０℃で成形した以外は、実施例１に従った。この膜の平均孔径は０．１５μｍであり、内部構造は、図５ｃ）に示す様にパーコレーション構造であった。また、延伸膜の延伸残留歪みは３０％であり、内部構造はパーコレーション構造であった。
【００５１】
【比較例１】
熱プレス機で１５０℃で成形した以外は、実施例１に従った。この膜の内部構造は、図５ａ）に示す様に無孔化していた。
【００５２】
【実施例３】
ＤＥＰ６０重量部の代わりにアセトフェノン７０重量部を用いて、１４０℃で混練し、熱プレス機で１４０℃で成形した以外は実施例１に従った。この膜の平均孔径は０．１５μｍであり、内部構造はパーコレーション構造であった。
【００５３】
【実施例４】
ＤＥＰ６０重量部の代わりにフタル酸ジブチル（ＤＢＰ）７０重量部を用いて、１６５℃で混練し、熱プレス機で１６５℃で成形した以外は実施例１に従った。この膜の平均孔径は０．１５μｍであり、内部構造はパーコレーション構造であった。
【００５４】
【比較例２】
ＤＥＰ６０重量部の代わりにγ−ブチルラクトン（γ−ＢＬ）５５重量部を用いて、１２０℃で混練し、熱プレス機で１２０℃で成形した以外は実施例１に従った。この膜の内部構造は図６ａ）に示す様に球状粒子が連結した構造であった。
【００５５】
【比較例３】
γ−ＢＬの代わりにエチレンカーボネート（ＥＣ）を用いて１５０℃で混練し、熱プレス機で１５０℃で成形した以外は比較例２に従った。この膜の内部構造は図６ｂ）に示すように球状粒子が連結した構造であった。
【００５６】
【比較例４】
ＥＣの代わりにプロピレンカーボネート（ＰＣ）を用いた以外は比較例３に従った。この膜の内部構造は図６ｃ）に示すように球状粒子が連結した構造であった。
【００５７】
【実施例５】
ＤＥＰを７０重量部とした以外は実施例１に従って得られたゲル状成形体を、１６０℃のホットプレート上で再度溶解し、２０℃の空気により冷却し、引き続き塩化メチレン中に１時間浸漬する事によってＤＥＰを抽出し、室温で乾燥して微多孔膜を得た。この膜の平均孔径は０．１μｍであり、表面は図７ａ）に示すように多孔質であった。内部構造はパーコレーション構造であった。
【００５８】
【実施例６】
２０℃の冷却媒体中で冷却した以外は、実施例５に従った。冷却媒体としては、フタル酸ジメチル（ＤＭＰ）、ＤＥＰ、ＤＢＰ、フタル酸ジエチルヘキシル（ＤＯＰ）、フタル酸ジイソデシル（ＤＩＤＰ）、水、エチレングリコール（ＥＧ）をそれぞれ用いた。冷却媒体をＤＭＰ、ＤＥＰ、ＤＢＰ、ＤＯＰ、ＤＩＤＰ、水、ＥＧにした時の膜の表面は図７ｂ）、ｃ）、ｄ）、ｅ）、ｇ）、ｈ）にそれぞれ示すように多孔質であり、図７ｂ）、ｃ）、ｄ）、ｅ）の膜の平均孔径は０．１μｍ近傍であり、図７ｇ）、ｈ）の膜の平均孔径は０．０４μｍ近傍であった。いずれの膜も内部構造はパーコレーション構造であった。
【００５９】
【比較例５】
２０℃の冷却媒体中で冷却した以外は、実施例５に従った。冷却媒体としては、フタル酸トリデシル（ＤＴＤＰ）、デカリンをそれぞれ用いた。冷却媒体をＤＴＤＰ、デカリンとした時の膜の表面は図７ｆ）、ｉ）にそれぞれ示す様に無孔化していた。内部構造はパーコレーション構造であった。
【００６０】
【実施例７】
Ｍｗが３．６２×１０⁵のＰＶｄＦ４６．６重量部と、ＤＥＰ５３．４重量部の混合物を３５ｍｍφの２軸押出機で１４５℃で加熱混練し、内径０．９ｍｍφ、外径１．７ｍｍφの中空のダイスから押出し、中空糸状に成形した。この際、中空糸の径を安定させるために、糸の内部に１０ミリリットル／ｍｉｎの空気を流し、押し出した中空糸をＤＯＰからなる冷却媒体浴に浸漬する事により、冷却してゲル状成形体を得た。成形された中空糸状ゲルを塩化メチレン中に１時間浸漬してＤＥＰを抽出し、室温で乾燥して中空糸膜を得た。得られた中空糸膜は、内径０．８４ｍｍ、外径１．５９ｍｍ、空孔率５４．６％、平均孔径０．１４μｍ、最大孔径０．２１μｍでその比は１．５０であった。この中空糸膜の透水量は３００リットル／ｍ²・ｈｒ・ａｔｍで、破断強度は１３９Ｋｇｆ／ｃｍ²、破断伸度は３５３％であった。また、この膜の内部構造はパーコレーション構造であった。
【００６１】
【実施例８】
１５０℃で加熱混練した以外は実施例７に従った。得られた中空糸膜は、内径０．８８ｍｍ、外径１．６２ｍｍ、空孔率５４．３％、平均孔径０．１５μｍ、最大孔径０．２６μｍでその比は１．７３であった。この中空糸膜の透水量は３５０リットル／ｍ²・ｈｒ・ａｔｍで、破断強度は１２２Ｋｇｆ／ｃｍ²、破断伸度は２９０％であった。また、この膜の内部構造はパーコレーション構造であった。
【００６２】
【実施例９】
１４０℃で加熱混練した以外は実施例７に従った。得られた中空糸膜は、内径０．８６ｍｍ、外径１．６１ｍｍ、空孔率５４．０％、平均孔径０．１２μｍ、最大孔径０．１７μｍでその比は１．４２であった。この中空糸膜の透水量は２５０リットル／ｍ²・ｈｒ・ａｔｍで、破断強度は１５６ｇｆ／ｃｍ²、破断伸度は４００％であった。また、この膜の内部構造はパーコレーション構造であった。
【００６３】
【比較例７】
１３０℃で加熱混練した以外は実施例７に従った。得られた中空糸膜は、内径０．８５ｍｍ、外径１．６０ｍｍ、空孔率４２．０％と低く、全体的に収縮していた。また、空隙部は微細化し、ほとんどが独立孔となった内部構造をしており、透水量は零であった。この中空糸膜の破断強度は１５０Ｋｇｆ／ｃｍ²、破断伸度は３８０％であった。
【００６４】
【比較例８】
１５５℃で加熱混練した以外は実施例７に従った。得られた中空糸膜は、内径０．８１ｍｍ、外径１．５８ｍｍ、空孔率５２．９％、平均孔径０．１８μｍ、最大孔径０．５３μｍでその比は２．９４であった。この中空糸膜の透水量は５００リットル／ｍ²・ｈｒ・ａｔｍで、破断強度は９０Ｋｇｆ／ｃｍ²、破断伸度は９０％であった。この膜の内部構造は粗大化していた。
【００６５】
【比較例９】
１６０℃で加熱混練した以外は実施例７に従った。得られた中空糸膜は、内径０．８２ｍｍ、外径１．５８ｍｍ、空孔率５３．５％、平均孔径０．２０μｍ、最大孔径０．７９μｍでその比は３．９５であった。この中空糸膜の透水量は８１０リットル／ｍ２・ｈｒ・ａｔｍで、破断強度は８２Ｋｇｆ／ｃｍ²、破断伸度は７５％であった。またこの膜の内部構造は粗大化していた。
【００６６】
【実施例１０】
Ｍｗが５．４６×１０⁵のＰＶｄＦ３０重量部と、ＤＥＰ７０重量部の混合物を１４５℃で加熱混練した以外は実施例７に従った。得られた中空糸膜は、内径０．８５ｍｍ、外径１．６０ｍｍ、空孔率６９．１％、平均孔径０．１８μｍ、最大孔径０．２３μｍでその比は１．２７であった。この中空糸膜の透水量は２９００リットル／ｍ²・ｈｒ・ａｔｍで、破断強度は９３Ｋｇｆ／ｃｍ²、破断伸度は４３３％であった。またこの膜の内部構造はパーコレーション構造であった。
【００６７】
【実施例１１】
１５０℃で加熱混練した以外は実施例１０に従った。得られた中空糸膜は、内径０．８５ｍｍ、外径１．５８ｍｍ、空孔率６８．８％、平均孔径０．４４μｍ、最大孔径０．６７μｍでその比は１．５２であった。この中空糸膜の透水量は８２００リットル／ｍ²・ｈｒ・ａｔｍで、破断強度は８８Ｋｇｆ／ｃｍ²、破断伸度は４２５％であった。またこの膜の内部構造はパーコレーション構造であった。
【００６８】
【実施例１２】
ＤＥＰの代わりにＤＭＰを用いた以外は実施例９に従った。得られた中空糸膜は、内径０．８５ｍｍ、外径１．５３ｍｍ、空孔率６７．０％、平均孔径０．３４μｍ、最大孔径０．４９μｍでその比は１．４３であった。この中空糸膜の透水量は４３００リットル／ｍ²・ｈｒ・ａｔｍ、破断強度は９５Ｋｇｆ／ｃｍ²、破断伸度は２９２％であった。またこの膜の内部構造はパーコレーション構造であった。
【００６９】
【実施例１３】
ＤＭＰを６０重量部とし、熱プレス機で１５０℃で成形した以外は実施例１に従って、膜厚約１００μｍのシート状ゲル状成形体を得た。成形されたゲル状成形体をエーテル中に数時間浸漬する事によってＤＭＰを抽出し、室温で乾燥して微多孔膜を得た。この膜の平均孔径は０．１２μｍ、空孔率は５６％、膜厚は８７μｍ、破断強度は１２０Ｋｇｆ／ｃｍ²、破断伸度３００％であり、内部構造はパーコレーション構造であった。上記微多孔膜を、ＬｉＢＦ₄のＥＣ／ＰＣ＝１／１の組成の１ｍｏｌ／リットル溶液に室温で浸漬することにより、膜厚１００μｍのシート状電解質保持体を作製した。
【００７０】
上記シート状電解質保持体のインピーダンス測定を実施した結果、室温におけるイオン伝導度は０．８ｍＳ／ｃｍであった。
【００７１】
【実施例１４】
ＤＭＰを７０重量部とした以外は実施例１３に従って微多孔膜を得た。得られた微多孔膜の平均孔径は０．２５μｍ、空孔率は６３％、膜厚は６２μｍ、破断強度は１００Ｋｇｆ／ｃｍ²、破断伸度２７０％であり、内部構造はパーコレーション構造であった。実施例１３と同様の方法で膜厚８０μｍのシート状電解質保持体を作製した。このシート状電解質保持体の室温におけるイオン伝導度は１．１ｍＳ／ｃｍであった。
【００７２】
【比較例１０】
ＤＭＰをＥＣとした以外は実施例１３に従って微多孔膜を得た。得られた微多孔膜の空孔率は４３％、膜厚は７６μｍであり、内部構造は球状粒子が連結した構造であった。実施例１３と同様の方法で膜厚７０μｍのシート状電解質保持体を作製した。このシート状電解質保持体の室温におけるイオン伝導度は０．３ｍＳ／ｃｍであった。
【００７３】
【実施例１４】
Ｍｗが１．１８×１０⁶のＰＶｄＦ２５重量部と、ＤＭＰ７５重量部の混合物を１３５℃で加熱混練し、内径０．９ｍｍφ、外径１．４５ｍｍφの中空のダイスから押出し、ＤＯＰからなる冷却媒体浴の浴温を２０℃に制御し、ＤＭＰの抽出にメチルエチルケトンを用いた以外は実施例７に従った。得られた中空糸膜は、内径０．７５ｍｍφ、外径１．２５ｍｍφ、空孔率６９．８％、平均孔径０．１７μｍ、最大孔径０．２２μｍでその比は１．２２であった。この中空糸膜の透水量は２２００リットル／ｍ²・ｈｒ・ａｔｍで、破断強度は１１５Ｋｇｆ／ｃｍ²、破断伸度は３７１％であった。またこの膜の内部構造はパーコレーション構造であった。
【００７４】
【実施例１５】
実施例１４で得られた中空糸膜に、延伸伸度５０％の延伸を施した。延伸残留歪みは２８％、内径０．７２ｍｍφ、外径１．２２ｍｍφ、空孔率７３．０％、平均孔径０．１８μｍ、最大孔径０．２４μｍでその比は１．３３であった。この中空糸膜の透水量は２８００リットル／ｍ²・ｈｒ・ａｔｍで、破断強度は１０７Ｋｇｆ／ｃｍ²、破断伸度は３２１％であった。またこの膜の内部構造はパーコレーション構造であった。
【００７５】
【実施例１６】
Ｍｗが５．４６×１０⁵のＰＶｄＦ２４重量部と、アクリル樹脂（ＰＭＭＡ、デルペット８０Ｎ、旭化成工業（株）製）８重量部とＤＥＰ６８重量部の混合物を１４５℃で加熱混練し、ＤＢＰからなる冷却媒体浴を用いた以外は実施例７に従った。得られた乾燥膜に、延伸伸度５０％の延伸を施した。そうして得られた中空糸膜は、延伸残量歪み２９％、内径０．８５ｍｍ、外径１．６０ｍｍ、空孔率６９．１％、平均孔径０．１８μｍ、最大孔径０．２３μｍでその比は１．２７であった。この中空糸膜の透水量は３５００リットル／ｍ²・ｈｒ・ａｔｍで、破断強度は９３Ｋｇｆ／ｃｍ²、破断伸度は４３３％であった。またこの膜の内部構造はパーコレーション構造であった。
【００７６】
【実施例１７】
Ｍｗが１．１８×１０⁶のＰＶｄＦ２５重量部と、アクリル樹脂（ＰＭＭＡ、デルペット８０Ｎ、旭化成工業（株）製）５重量部とＤＭＰ７０重量部の混合物を１３７．５℃で加熱混練し、ＤＢＰからなる冷却媒体浴の浴温を２０℃に制御した以外は実施例１４に従った。得られた中空糸膜は、内径０．６９ｍｍφ、外径１．２５ｍｍφ、空孔率６９．３％、平均孔径０．１３μｍ、最大孔径０．１６μｍでその比は１．２３であった。この中空糸膜の透水量は１９００リットル／ｍ²・ｈｒ・ａｔｍで、破断強度は１０２Ｋｇｆ／ｃｍ²、破断伸度は４３９％であった。またこの膜の内部構造はパーコレーション構造であった。
【００７７】
【実施例１８】
実施例１７で得られた中空糸膜に、延伸伸度５０％の延伸を施した。延伸残留歪みは２６％、内径０．６８ｍｍφ、外径１．２３ｍｍφ、空孔率７２．０％、平均孔径０．１８μｍ、最大孔径０．２４μｍでその比は１．３３であった。この中空糸膜の透水量は２９００リットル／ｍ²・ｈｒ・ａｔｍで、破断強度は９９Ｋｇｆ／ｃｍ²、破断伸度は３７６％であった。またこの膜の内部構造はパーコレーション構造であった。
【００７８】
【発明の効果】
本発明の微多孔膜は、均質な構造をもち、流体の透過特性、流体から微粒子を分離する際の分離特性、機械的特性、及び耐薬品性が優れ、ウイルス除去フィルターを含む各種フィルター、精密濾過膜、限外濾過膜、電池用セパレータ、電解質コンデンサー用隔膜、固体電解質電池用電解質保持体等の用途に好適に使用される。
【図面の簡単な説明】
【図１】フッ化ビニリデンホモポリマー（重量平均分子量３．６２×１０⁵）のＤＥＰ溶液の結晶化温度Ｔc と溶解温度Ｔs の関係を示す図。
【図２】フッ化ビニリデン系重合体／溶媒系の溶液の結晶化温度Ｔc と溶解温度Ｔs の関係とパーコレーション構造形成可能な温度の範囲の関係を示す図。
【図３】中空糸状微多孔膜の、ａ）空孔率（％）、ｂ）破断強度（Ｋｇｆ／ｃｍ²）及びｃ）破断伸度（％）と溶解温度Ｔs の関係を示す図。
【図４】溶解温度によって位置を変えた結晶化曲線と曇り点曲線の関係を示す図。
【図５】ａ）、ｂ）、ｃ）は、各溶解温度Ｔs における微多孔膜断面の走査型電子顕微鏡写真。
【図６】ａ）、ｂ）、ｃ）は、種々の球状粒子網状構造をもった微多孔膜断面の走査型電子顕微鏡写真。
【図７】ａ）〜ｉ）は、各種の冷却媒体を使用したときの微多孔膜表面の走査型電子顕微鏡写真。[0001]
BACKGROUND OF THE INVENTION
The present invention is a microporous membrane suitably used for various filters including a virus removal filter, a microfiltration membrane, an ultrafiltration membrane, a battery separator, an electrolyte capacitor diaphragm, an electrolyte holder for a solid electrolyte battery, and the like. It relates to the manufacturing method.
[0002]
[Prior art]
Microporous membranes are used in various applications such as various filters including virus removal filters, ultrafiltration membranes, microfiltration membranes, battery separators, diaphragms for electrolytic capacitors, and electrolyte holders for solid electrolyte batteries. Important factors in these applications are the pore size and structural homogeneity of the membrane, as well as the permeation characteristics of the fluid and the separation characteristics when separating particulates from the fluid.
[0003]
A microporous film made of a vinylidene fluoride polymer is expected as a microporous film having various properties excellent in chemical resistance, heat resistance, and mechanical properties.
When using a microporous membrane as a separation membrane, it is necessary to select the pore size of the membrane according to the size of the substance to be separated. In addition, the homogeneity, that is, the pore size distribution, significantly affects the separation performance of the membrane. Furthermore, the permeability of the fluid has a great influence on the separation efficiency. On the other hand, from the manufacturing aspect of the microporous membrane, there is a demand for a stable method that has a high degree of freedom in controlling the above characteristics and can absorb fluctuations in manufacturing conditions.
[0004]
Conventionally, as a method for producing a vinylidene fluoride polymer microporous membrane, (1) by uniformly dissolving a vinylidene fluoride polymer in a solvent, and then immersing it in a non-solvent that does not dissolve the vinylidene fluoride polymer Wet film-forming method (for example, JP-A-60-97001) for solid-liquid or liquid-liquid phase separation, (2) Mixing and melt-molding vinylidene fluoride polymer, organic liquid and hydrophilic inorganic fine powder, Next, a method of obtaining a microporous film by extracting an organic liquid and hydrophilic inorganic fine powder from the molded product (Japanese Patent Laid-Open No. 58-93734), (3) a vinylidene fluoride polymer, an organic liquid and a hydrophobic There is a method (Japanese Patent Laid-Open No. 3-215535) for obtaining a microporous film by mixing an inorganic fine powder and melt-molding and then extracting an organic liquid and a hydrophobic inorganic fine powder from the molded product.
[0005]
In the wet film forming method, the heterogeneous microporous film having a skin layer is almost all. Japanese Patent Laid-Open No. 60-97001 discloses a method for obtaining a microporous film having a network structure, but there is a problem in mechanical strength because it is a wet film forming method. The microporous membrane manufactured by the method using hydrophilic silica disclosed in JP-A-58-93734 has many macrovoids (coarse pores), has a small elongation at break and is at high temperature and pressure. There is a problem that it can not stand the use of.
[0006]
Also, in the method of mixing vinylidene fluoride polymer, organic liquid, and inorganic fine powder such as hydrophobic or hydrophilic silica and melt molding, structural defects such as pinholes may occur if the inorganic fine powder is poorly dispersed. It is easy to occur and inconvenient. In addition, not only in terms of performance but also in terms of manufacturing, there are inconveniences such as a decrease in production yield due to structural defects and an increase in production time due to extraction of inorganic fine powder in addition to solvent extraction. There is. The microporous membrane produced by the method using hydrophobic silica disclosed in JP-A-3-215535 has a relatively homogeneous structure and high breaking strength / breaking elongation. There are structural defects derived from it.
[0007]
Further, when extracting hydrophobic or hydrophilic silica, it is disclosed in JP-A-58-93734 and JP-A-3-215535 that an alkaline aqueous solution such as caustic soda and caustic potash is used. There remains a problem that the microporous membrane of vinylidene fluoride polymer is colored from light brown to brown with an aqueous alkali solution. Moreover, the fall of the mechanical strength in the case of silica extraction or decoloring may be a problem.
[0008]
[Problems to be solved by the invention]
The present invention is a vinylidene fluoride system having a homogeneous structure in which the above-mentioned problems are solved, and having excellent fluid permeation characteristics, separation characteristics when separating fine particles from fluid, mechanical characteristics, and chemical resistance It aims at providing a polymer microporous membrane and its manufacturing method.
[0009]
[Means for Solving the Problems]
In order to achieve the above-mentioned problems, the present inventors have studied various methods capable of controlling the structure of the vinylidene fluoride polymer microporous membrane, and as a result, the weight average molecular weight was 1 × 10. ^Five Combining the use of the above-mentioned vinylidene fluoride polymers, the dissolution of vinylidene fluoride polymers at a specific solvent and a specific temperature, a specific cooling method, and a stretching with a residual strain of 100% or less if necessary. Thus, the present invention has been achieved.
[0010]
That is, the present invention has a weight average molecular weight of 1 × 10 ^Five A microporous structure comprising a polymer phase containing the above-mentioned vinylidene fluoride polymer and a void portion having an average pore diameter of 0.005 to 5 μm by a half dry method and communicating from one surface to the other surface. This is a microporous membrane whose inner structure has a percolation structure.
In the present invention, the “average pore diameter by the half-dry method” is a pore diameter measured by the method described later.
[0011]
In addition, the “percolation structure” means that the polymer phase forms an isotropic network structure branched in an arbitrary direction three-dimensionally, and the voids are formed surrounded by the polymer phase of the network structure. In addition, each of the gaps refers to a structure that communicates with each other.
Among the above microporous membranes, the structure of at least one surface layer is not the same as the internal structure, and the average pore diameter of the surface layer that is not the same as the internal structure is equal to or greater than that of the internal structure. The structure of the microporous membrane and both surface layers is also a percolation structure, and the average pore diameter of both surface layers by scanning electron microscopy is the same as or greater than the internal structure, and scanning of at least one surface layer A microporous membrane having an average pore diameter determined by scanning electron microscopy that is smaller than the internal structure is preferred.
[0012]
This microporous membrane is formed by the above-mentioned vinylidene fluoride polymer: solvent capable of forming a microporous membrane having a percolation structure = 10: 90 to 60:40 at a weight ratio of 10:90 to 60:40. After the vinylidene chloride polymer is dissolved, this solution is extruded with an extrusion device, cooled to form a gel-like molded product composed of a two-phase gel, and then selected from the following i), ii) and iii) It is manufactured by performing the process.
i) Remove the solvent with a volatile liquid without stretching.
ii) Before the solvent is removed, the solvent is removed using a volatile liquid after stretching so that the stretching residual strain is 100% or less.
iii) After removing the solvent using a volatile liquid, stretching is performed so that the stretching residual strain is 100% or less.
[0013]
The microporous film is composed of the above-mentioned vinylidene fluoride polymer: a solvent capable of forming a microporous film having a percolation structure and a thermoplastic resin compatible with the vinylidene fluoride polymer (hereinafter referred to as compatibility). A mixture of vinylidene fluoride polymer and compatible resin in a weight ratio of 10:90 to 60:40, and the total of the vinylidene fluoride polymer and the compatible resin is 60% by weight or less, and the vinylidene fluoride polymer: compatible resin. After dissolving the vinylidene fluoride polymer and the compatible resin at a temperature Ts at which a percolation structure can be formed under the condition of a weight ratio of 40:60 to 90:10, this solution is extruded with an extruder and cooled. After forming a gel-like molded body composed of a two-phase gel, it is produced by performing any of the treatments selected from the following iv), v) and vi).
iv) Remove solvent and compatible resin using volatile liquid without stretching.
v) Before removing the solvent and the compatible resin, stretching is performed so that the stretching residual strain is 100% or less, and then the solvent is removed using a volatile liquid.
vi) After removing the solvent and the compatible resin using a volatile liquid, stretching is performed so that the stretching residual strain is 100% or less.
[0014]
In the present invention, the “solvent capable of forming a microporous film having a percolation structure” means a vinylidene fluoride polymer having an arbitrary concentration within a range of 10 to 60% by weight of a vinylidene fluoride polymer. For the solution of the solvent, or the vinylidene fluoride polymer of any concentration and the solution of the solvent and the compatible resin, the horizontal axis represents the dissolution temperature Ts, and the vertical axis represents the film formed from the solution at each dissolution temperature. When the elongation at break TL is plotted at intervals of 5 ° C. starting from Ts = 100 ° C., − (TLs + 5−TLs) / {(Ts + 5 ° C.) − Ts} (TLs + 5 is TL at ts + 5 ° C., TLs Represents a temperature (Ts + 2.5 ° C.) obtained by adding 2.5 ° C. to the melting temperature Ts at which TL at Ts is maximized. On the other hand, when Ts is plotted on the horizontal axis and the porosity P of the membrane is plotted on the vertical axis, (Ps + 5-Ps) / {(Ts + 5 ° C) -Ts} (Ps + 5 is Ps at Ts + 5 ° C). , Ps is defined as a temperature obtained by adding 2.5 ° C. to the melting temperature Ts at which P) in Ts is maximum (Ts + 2.5 ° C.). When a solution having at least one concentration within the above concentration range has both Tl and Tu and (Tu-Tl)> 0, the solvent is a solvent capable of forming a microporous film having a percolation structure.
[0015]
“Temperature at which a percolation structure can be formed” refers to a melting temperature Ts that satisfies Tl ≦ Ts ≦ Tu.
The microporous membrane of the present invention has a homogeneous structure and is excellent in fluid permeation characteristics, separation characteristics when separating fine particles from fluid, mechanical characteristics, and chemical resistance.
The present invention also relates to a gel-like molded article comprising a two-phase gel obtained by cooling a solution, which is suitable as an electrolyte holder for a solid electrolyte battery, for example, by substituting a solvent with an electrolytic solution as described later. Used for.
[0016]
The present invention is described in detail below.
The internal structure of the microporous film is a structure observed by examining an arbitrary cross section (in many cases, a vertical cross section) of the microporous film with a scanning microscope or the like from a direction perpendicular to the cross section. The structure of the surface layer of the microporous film is a structure that is observed by examining the surface of the microporous film with a scanning electron microscope or the like from a direction perpendicular to the surface.
[0017]
The microporous membrane of the present invention forms a two-phase gel by cooling the vinylidene fluoride polymer and its solvent solution, or the vinylidene fluoride polymer and its solvent and compatible resin solution. It is manufactured by. The two-phase gel referred to here is composed of a polymer rich phase having a high vinylidene fluoride polymer concentration and a polymer dilute phase having a low polymer concentration. Here, the case where the polymer rich phase is only the polymer and the polymer dilute phase is only the solvent is also included. Although the solution in this case looks visually uniform, there is a possibility that polymer crystallites are scattered in the solution as described later. When such a two-phase gel can be formed, even if the temperature of the uniform one-phase solution is cooled to an arbitrary observation temperature higher than the crystallization temperature and allowed to stand, the liquid of the polymer concentrated phase and the polymer diluted phase The interface is not visually observed. A typical liquid-liquid interface is planar as illustrated in FIG. 1.2 of “THERMDYNAMICS OF POLYMER SOLUTIONS -PHASE EQUILIBRIA AND CRITICAL PHENOMENA-” (by K. Kamide, ELSEVIER, 1990), for example. In some cases, the state where the polymer rich phase and the polymer dilute phase are separated is visually observed. When the two-phase gel is formed, the solution becomes cloudy and gels. The hypothesis that gelation and liquid-liquid phase separation are antagonistic can be considered as the reason why the liquid-liquid interface is not visually observed.
[0018]
The average pore diameter of the microporous membrane of the present invention by the half dry method is in the range of 0.005 to 5 μm, and within this range, for example, it can be suitably used as a filter for liquid or gas filtration. If the average pore diameter is larger than 5 μm, the number of structural defects such as pinholes increases, and a microporous membrane having good separation characteristics cannot be obtained. From the viewpoint of the fractionation characteristics for removing impurities from liquids and gases, the ratio of the maximum pore size by the bubble point method of the microporous membrane to the average pore size is preferably 2.0 or less. The average pore diameter when used as a filter for water treatment is preferably 0.05 μm or more. When the pore diameter is less than 0.05 μm, in the case of water treatment, the pore diameter is too small and the transmission characteristics deteriorate. The average pore diameter when used as a virus removal filter is preferably in the range of 0.005 to 0.1 μm.
[0019]
The spherical particle network (see, eg, FIG. 6), where the majority of the polymer phase is considered substantially spherical particles, is different from the percolation structure of the present invention. In the spherical particle network structure, since the polymer phase is connected at the contact point between the spheres, the mechanical properties are deteriorated.
A spherical hole network structure and an elliptical hole network structure in which most of the voids are substantially regarded as spherical holes or ellipsoidal holes are also different from the percolation structure of the present invention. In the spherical hole network structure or the ellipsoidal hole network structure, since the holes are connected at the contact points of the sphere and the sphere or the ellipsoid and the ellipsoid, the liquid permeability is lowered. A spherical pore network structure or an ellipsoidal pore network structure is sometimes called a cellular structure because it looks like a structure in which spherical or ellipsoidal cells are gathered.
[0020]
The three-dimensionally branched isotropic network of the two-phase gel of the present invention contributes to effects such as high elongation, high virus removal performance, high water permeability, high ionic conductivity and high charging efficiency. -ing
The structure of the surface layer of the microporous membrane of the present invention may be the same as or different from the internal structure depending on the production conditions. In either case, the average pore diameter of the surface by scanning electron microscopy is determined as the internal structure. It can be devised to be the same or more. As a result, the vinylidene fluoride polymer microporous membrane having an internal structure of the present invention having a percolation structure can exhibit the characteristics of the microporous membrane targeted by the present invention. Further, when the average pore diameter by scanning electron microscopy on the surface is equal to or larger than the internal structure, the surface layer can be devised so as to have an effect as a prefilter.
[0021]
If the structure of the surface layer is different from the internal structure, or even if the structure is the same, the surface layer has a thickness of at least 0.1 μm or more when the average pore diameter of the surface layer and the internal structure is different by scanning electron microscopy. Yes, usually within 3 μm. When obtaining the average pore diameter by scanning electron microscopy, an image processing apparatus is used as described later.
The case where the average pore diameter of the at least one surface layer of the microporous membrane by the scanning electron microscopy is smaller than the internal structure is also included in the present invention. In this case, the surface layer denser than the internal structure has an effect of preventing impurities in the liquid or gas from entering the film. The thickness of the surface layer in this case is also at least 0.1 μm or more and is usually within 3 μm. Moreover, the average pore diameter of the dense surface layer is usually 0.001 μm or more.
[0022]
A microporous membrane having these characteristics has a homogeneous structure, and is excellent in fluid permeation characteristics, separation characteristics when separating fine particles from fluid, mechanical characteristics, and chemical resistance. The fact that the liquid permeability is excellent means that the liquid permeability is excellent compared to a membrane having the same average pore diameter. The microporous membrane having excellent liquid permeability has an advantage that the membrane module can be made compact because the processing capacity per unit membrane area is increased.
[0023]
In the present invention, the weight average molecular weight of the vinylidene fluoride polymer is 1 × 10 ^Five The weight average molecular weight is 1 × 10 ^Five Below, the viscosity of the solution is low, which is inconvenient for forming a gel-like porous body, and the mechanical properties of the obtained microporous film are also inferior. The vinylidene fluoride polymer preferably has a weight average molecular weight of 3 × 10 ^Five ~ 2x10 ⁶ A mixture of a plurality of vinylidene fluoride polymers having different average molecular weights may be used.
[0024]
Examples of the vinylidene fluoride polymer used in the present invention include a vinylidene fluoride homopolymer and a vinylidene fluoride copolymer, and the vinylidene fluoride copolymer includes vinylidene fluoride and tetrafluoroethylene. A copolymer of at least one selected from propylene hexafluoride, ethylene trifluoride chloride, and ethylene is used, and a vinylidene fluoride homopolymer is particularly preferable. Moreover, the mixture of these some vinylidene fluoride polymer may be sufficient.
Various additives such as an antioxidant, an ultraviolet absorber, a lubricant, and an antiblocking agent can be added to the vinylidene fluoride-based polymer as necessary, as long as the object of the present invention is not impaired.
Next, an example of the manufacturing method of the vinylidene fluoride polymer microporous film of the present invention will be described.
[0025]
In the present invention, the vinylidene fluoride polymer solution used as a raw material has a weight ratio of vinylidene fluoride polymer: solvent capable of forming a microporous film having a percolation structure = 10: 90 to 60:40, It is prepared by heating and dissolving a vinylidene fluoride polymer at a temperature at which a percolation structure can be formed.
The solution of the vinylidene fluoride polymer used as a raw material is a vinylidene fluoride polymer: a mixture of a solvent capable of forming a microporous film having a percolation structure and a compatible resin (hereinafter referred to as a solvent / compatible resin mixture). The weight ratio of 10:90 to 60:40, and the total of the vinylidene fluoride polymer and the compatible resin is 60% by weight or less, and the vinylidene fluoride polymer: compatible resin = 40: It can also be prepared by heating and dissolving a vinylidene fluoride polymer and a compatible resin with a composition that satisfies the weight ratio of 60 to 90:10.
[0026]
FIG. 1 shows a vinylidene fluoride homopolymer (weight average molecular weight of 3.62 × 10 6 ^Five ) Shows the relationship between the crystallization temperature Tc and the dissolution temperature Ts of diethyl phthalate (DEP) solution. The weight fractions of the vinylidene fluoride polymer are 30 wt% (◇), 35 wt% (◯), and 40 wt% (Δ). At any weight fraction, the crystallization temperature Tc decreases as the melting temperature Ts increases, and the crystallization temperature Tc becomes substantially constant when the melting temperature Ts is about 178 ° C. or higher. In this case, when Ts <178 ° C., polymer microcrystals may be scattered in the solution. It can also be considered that the lower the Ts in the range of Ts <178 ° C., the greater the number of microcrystals in the unit volume.
[0027]
FIG. 2 is a diagram schematically depicting the relationship between the crystallization temperature Tc and the dissolution temperature Ts of a solvent solution capable of forming a microporous film having a percolation structure with a vinylidene fluoride polymer. In FIG. 2, the temperature range in which the percolation structure can be formed is also added.
Within the temperature range in which the percolation structure can be formed, the lower the melting temperature, the denser the percolation structure. From a solution dissolved at a temperature lower than the temperature at which the percolation structure can be formed, only a non-porous shaped body can be obtained, and the porosity is remarkably lowered. If the dissolution temperature is further lowered, a uniform solution cannot be obtained. From a solution dissolved at a temperature exceeding the temperature at which the percolation structure can be formed, only a molded body having a coarse internal structure can be obtained, and the mechanical strength and elongation are significantly reduced.
[0028]
FIG. 3 shows an example of this phenomenon. Figures 3, a), b), and c) show the relationship between the porosity, breaking strength, breaking elongation, and melting temperature Ts of vinylidene fluoride polymer microporous membranes having different melting temperatures Ts during film formation. Show. The microporous membranes with Ts = 135, 140, 145, 150, 155 and 160 ° C. were obtained by the methods described in Comparative Example 7, Example 9, Example 7, Example 8, Comparative Example 8 and Comparative Example 9, respectively. Adjusted. From FIG. 3a), the porosity is remarkably lowered at 135 ° C. or lower. Also, from FIGS. 3b) and c), the breaking strength and breaking elongation are significantly reduced at 155 ° C. or higher. By definition, in this case, Tl = 137.5 ° C. and Tu = 152.5 ° C., and (Tu−Tl)> 0, so DEP is a microporous membrane having a percolation structure of (B). It is a solvent that can be formed. Further, 137.5 ° C. ≦ Ts ≦ 152.5 ° C. is a temperature range in which the percolation structure can be formed.
[0029]
This phenomenon will be qualitatively described with reference to the schematic diagram showing the influence of the dissolution temperature on the crystallization temperature and the cloud point curve shown in FIG.
Here, the cloud point curve is a curve in which the cloud point temperature is plotted against the polymer concentration. When the cloud point temperature is higher than the crystallization temperature, if the solution temperature exceeds the cloud point temperature, the solution is uniformly dissolved in one phase, and conversely, the solution temperature is lower than the cloud point temperature and higher than the crystallization temperature. In the range, the solution undergoes liquid-phase separation into two phases of a polymer rich phase having a high polymer concentration and a polymer dilute phase having a low polymer concentration. If the solution is cooled below the crystallization temperature, crystallization of the polymer occurs and the solution solidifies.
[0030]
In FIG. 4, the vertical axis represents temperature, the horizontal axis represents the concentration (for example, weight fraction) of the vinylidene fluoride polymer, and the two-dot chain line represents the cloud point curve. However, the cloud point curve in this case is on the lower temperature side than the crystallization curve, that is, the solidification curve, and is not actually observed. Based on the thermodynamic analogy, it was assumed that a cloud point curve exists at the position of the two-dot chain line. Here, as in the liquid-liquid phase separation system, the low temperature region of the cloud point curve can be considered as the two-phase separation region.
In FIG. 4, Δ is high-temperature dissolution with Ts> Tu, ○ is dissolution at a melting temperature Ts that can form a percolation structure satisfying Tl ≦ Ts ≦ Tu (described as “intermediate” in FIG. 4), and ◇ is Ts <Tl Shows low temperature dissolution. A one-dot chain line, a broken line, and a solid line are crystallization curves in the case of high-temperature melting, melting at a melting temperature at which a percolation structure can be formed, and low-temperature melting, respectively.
[0031]
As already mentioned, in the case of melting at a temperature lower than Tl (◇), it becomes non-porous and a significant decrease in porosity is observed. In this case, as can be seen from FIG. 4, the cloud point curve is sufficiently lower in temperature than the crystallization curve, and it is considered that uniform gelation by crystallization dominates and becomes nonporous rather than two-phase separation. . Further, in the case of melting at a temperature higher than Tu (Δ), the crystallization temperature Tc is lowered, the structure is coarsened, and a significant decrease in mechanical properties is observed. In this case, as shown in FIG. 4, the cloud point curve is partly on the higher temperature side than the crystallization curve, and the influence of two-phase separation becomes stronger, so the structure is thought to be coarse. In the intermediate temperature region (◯), the percolation structure of (A) is formed. As the reason, for example, a mechanism in which gelation and liquid-liquid phase separation are antagonized can be considered.
[0032]
The range of temperature at which the percolation structure can be formed varies depending on the combination of the vinylidene fluoride polymer and the solvent. Moreover, even if it is the same solvent as the same vinylidene fluoride type polymer, it changes with those weight fractions. In addition, even when the same vinylidene fluoride polymer and solvent are used in the same weight fraction, there is a case where a microporous film is formed relatively statically, such as press filming, In the case of dynamically forming a microporous film such as a film, the temperature range in which the percolation structure can be formed tends to shift to a lower temperature side when the film is dynamically formed. That is, the temperature range in which the percolation structure can be formed varies depending on the film forming method. In the case of press film formation, after heating and mixing the vinylidene fluoride polymer and the solvent, the sample cooled to room temperature is melted again at a constant melting temperature using a hot press machine, etc. To form. In this case, the melting temperature at which it is melted again by the hot press machine controls whether or not a percolation structure is formed. In other words, the solution does not store a thermal history, and the last melting temperature dominates the membrane structure.
[0033]
As shown in FIG. 3, the significance of forming a percolation structure can also be confirmed from the point that a high porosity can be maintained while maintaining high strength and elongation. Furthermore, the above-mentioned solvent is required to maintain a liquid state at the melt molding temperature and to be inert.
Solvents that can form a microporous membrane with a percolation structure include phthalates such as dimethyl phthalate, diethyl phthalate, dibutyl phthalate, dioctyl phthalate, diisodecyl phthalate, tridecyl phthalate, methyl benzoate, ethyl benzoate Benzoic acid esters such as octyl sebacate, adipic acid esters such as dioctyl adipate, trimellitic acid esters such as trioctyl trimellitic acid, phosphate esters such as tributyl phosphate and tricresyl phosphate, ketones such as acetophenone A single solvent such as or a mixed solvent of two or more of these is exemplified. In the above solvent, the alkyl group may contain various isomers. A microporous membrane having a percolation structure by mixing a good solvent such as acetone, tetrahydrofuran, methyl ethyl ketone, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone or a non-solvent such as water with the above single solvent or mixed solvent. A mixed solvent whose solubility is adjusted to such an extent that it can be formed can also be used in the present invention.
[0034]
Examples of compatible resins include methacrylic ester resins, acrylic ester resins, poly (1,4-butylene adipate), polyvinyl acetate, and polyvinylpyrrolidone. Among these, methyl methacrylate resin and its copolymer are preferably used. Examples of the copolymer of methyl methacrylate resin include a copolymer with a comonomer such as methyl acrylate, styrene, α-methylstyrene, methacrylic acid, and maleic anhydride.
[0035]
As described above, heat dissolution is performed by forming a mixture of a vinylidene fluoride polymer and a solvent capable of forming a microporous film having a percolation structure, or forming a microporous film having a vinylidene fluoride polymer and a percolation structure. A mixture of a possible solvent and a compatible resin is agitated at a temperature capable of forming a percolation structure. The temperature may be set in the range of Tl ° C. to Tu ° C., more preferably in the range of Tl + 2 ° C. to Tu −2 ° C., depending on the type of vinylidene fluoride polymer, solvent and compatible resin used.
[0036]
The concentration of the vinylidene fluoride polymer in the above-mentioned mixture varies depending on the solubility of the solvent, but is 10-60% by weight, preferably 10-40% by weight, more preferably 10-30% by weight. %. If the concentration is less than 10% by weight, the viscosity of the solution is low, so the moldability is poor and the mechanical strength of the molded article is also weak. On the other hand, when the concentration exceeds 60% by weight, it is difficult to prepare a uniform solution and it is difficult to obtain a percolation structure.
[0037]
In addition, in the case of selecting a mixture of a vinylidene fluoride polymer and a solvent capable of forming a microporous film having a percolation structure and a compatible resin, the vinylidene fluoride polymer is compatible with the polymer. The total amount of the resin is 60% by weight or less, and a condition of a weight ratio of vinylidene fluoride polymer: compatible resin = 40: 60 to 90:10 is necessary. If the total concentration of the vinylidene fluoride polymer and the compatible resin exceeds 60% by weight, it is difficult to prepare a uniform solution and a percolation structure is difficult to obtain. Further, if the ratio of the compatible resin to the total amount of the vinylidene fluoride polymer and the compatible resin exceeds 60% by weight, the crystallinity of the vinylidene fluoride polymer is remarkably lowered, and the mechanical strength of the molded product is weak. Become. On the contrary, when the ratio of the compatible resin to the total amount of the vinylidene fluoride polymer and the compatible resin is less than 10% by weight, the effect of adding the compatible resin cannot be expected.
[0038]
When a mixture of a vinylidene fluoride polymer, a solvent capable of forming a microporous film having a percolation structure, and a compatible resin is selected upon heating and melting, the molded article is stretched as described in v) or vi). When manufactured, water permeability performance is significantly improved. It is considered that the compatibility resin moderately suppresses the crystallinity of the vinylidene fluoride-based polymer, tends to cause structural defects, and improves the through-hole probability due to breakage of stretching.
[0039]
Next, the heated solution of the above mixture is extruded from a die and molded. A die may be selected as appropriate, but a hollow die, a T die, a double cylindrical inflation die, or the like can be used as necessary. The extrusion temperature is appropriately set within the range of Tl ° C to Tu ° C depending on the type of solvent.
The solution extruded from the die is cooled to become a gel-like molded body made of a two-phase gel. As the cooling method, cooling with air, cooling with a roll, direct contact with a liquid cooling medium, or the like can be used.
When extruding with a T-die or the like to obtain a flat film, an air cooling method or a roll cooling method is often used. In this case, the structure of the surface layer of the microporous membrane is the same percolation structure as the internal structure, and usually the vinylidene fluoride polymer microporous membrane having an average pore diameter equal to or greater than that of the internal structure by scanning electron microscopy Is obtained.
[0040]
Further, when obtaining a hollow membrane by extrusion from a hollow die, a method of directly contacting a liquid cooling medium is advantageous for the purpose of stabilizing the hollow cross-sectional shape and the size thereof. In air cooling and roll cooling, since the viscosity of the mixed solution of vinylidene fluoride polymer and solvent is low, the cross-sectional shape of the hollow fiber is often crushed. Further, when a die other than the hollow die, for example, a T die is used, it can be brought into direct contact with the liquid cooling medium. In the case of direct contact with a liquid cooling medium, a solvent capable of forming a microporous film having a percolation structure is preferably used as the cooling medium. In this case, the structure of the surface layer at the portion in contact with the cooling medium is different from the internal structure. The cooling temperature is preferably Tm-50 ° C or lower. Here, Tm is the melting point of the vinylidene fluoride polymer in the mixture of the vinylidene fluoride polymer and the solvent. The melting point Tm moves to a lower temperature side as the concentration of the vinylidene fluoride polymer decreases (melting point lowering phenomenon). When a cooling medium having a low affinity with the vinylidene fluoride polymer is used, the structure of the surface layer of the vinylidene fluoride polymer microporous film becomes a skin-like structure or a particulate matter aggregate structure, In some cases, the surface openness may be lowered.
[0041]
As a cooling medium that is a solvent capable of forming a microporous film having a percolation structure, phthalate such as dimethyl phthalate, diethyl phthalate, dibutyl phthalate, dioctyl phthalate, diisodecyl phthalate, methyl benzoate, ethyl benzoate, etc. Benzoic acid esters, sebacic acid esters such as octyl sebacate, adipic acid esters such as dioctyl adipate, trimellitic acid esters such as trioctyl trimellitic acid, phosphoric acid esters such as tributyl phosphate and tricresyl phosphate, and ketones such as acetophenone A single cooling medium or a mixed cooling medium of two or more of these is exemplified. In the above cooling medium, the alkyl group may contain various isomers. In addition, a good solvent such as acetone, tetrahydrofuran, methyl ethyl ketone, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone, or a non-solvent such as water is mixed with the above single cooling medium or mixed cooling medium to form a percolation structure. A mixed solvent whose solubility is adjusted to the extent that it becomes a solvent capable of forming a microporous film can also be used as a cooling medium in the present invention.
[0042]
The obtained gel-like molded product is washed with a volatile liquid compatible with the solvent to remove the solvent. Volatile liquids for cleaning include hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, fluorinated hydrocarbons such as ethane trifluoride, methyl ethyl ether, diethyl ether, etc. Ethers, ketones such as acetone and methyl ethyl ketone, and the like can be used. The above-mentioned volatile liquid is appropriately selected depending on the type of solvent used, and is 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, a method of a combination of these, or the like. When a mixture consisting of a vinylidene fluoride polymer and a solvent capable of forming a microporous film having a percolation structure and a compatible resin is selected, use a volatile liquid that can be removed by simultaneously washing the solvent and the compatible resin. Things are desirable.
[0043]
Thereafter, the microporous membrane is dried. Examples of the method for drying the microporous film include heat drying, drying with hot air, and contact with a heating roll.
In order to improve the surface openability of the microporous membrane, that is, to improve the average pore diameter of the surface layer by scanning electron microscopy, to improve the probability of through holes, and to improve the breaking strength, The molded body, the microporous film, or both of them are stretched with a stretching residual strain of 0 to 100%, preferably 10 to 100%, at a stretching ratio that does not depart from the structural characteristics of the microporous film. Can do. The gel-like formed body or the microporous film is stretched at a predetermined magnification by a usual tenter method, roll method, rolling method, or a combination of these methods. Stretching may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, either longitudinal or transverse simultaneous stretching or sequential stretching may be used. The stretch residual strain referred to here is the ratio of the sample length increased by stretching to the sample length before stretching (initial length) in the case of uniaxial stretching. In the case of biaxial stretching, it is the ratio of the film area increased by stretching to the film area (initial area) before stretching. In order to make the stretching residual strain 100% or less, although it depends on conditions, the stretching ratio is 3 times or less in the case of uniaxial stretching, and the surface magnification is 4 times or less in the case of biaxial stretching. The stretching temperature of the gel-like molded body or the microporous membrane is 50 ° C. or lower, preferably 25 ° C. or lower. When the stretching temperature exceeds 50 ° C., the stretching effect is not sufficient.
[0044]
When the gel-like molded body is stretched, next, the solvent is removed by the above-described method, and the microporous film is dried.
The obtained microporous membrane can be subjected to heat treatment for the purpose of dimensional stability. The heat treatment temperature can be set to an arbitrary temperature of 50 ° C. or higher and the melting point temperature of the vinylidene fluoride resin −20 ° C. or lower.
The obtained microporous membrane can be subjected to a hydrophilic treatment by alkali treatment, plasma irradiation, electron beam irradiation, γ-ray irradiation, corona treatment, surfactant impregnation, surface grafting, coating or the like, if necessary.
Moreover, it can also bridge | crosslink with an electron beam irradiation, a gamma ray irradiation, etc. with respect to a gel-like molded object or a microporous film as needed.
[0045]
The microporous membrane produced as described above preferably has a porosity of 30% or more and a breaking strength of 50 kgf / cm. ² Or more, more preferably 70 to 500 Kgf / cm ² The elongation at break is 150% or more, more preferably 200 to 800%, and the bubble point is 1 to 20 kgf / cm. ² The water permeability is 200-10000 liters / m ² · Hr · atm. The thickness of the microporous membrane of the present invention can be appropriately selected according to the use, but is generally 20 to 1000 μm, preferably 60 to 800 μm.
[0046]
The gel-like molded body comprising the microporous membrane of the present invention and the two-phase gel obtained in the production process of the microporous membrane is prepared by introducing an electrolyte solution in the case of a microporous membrane and a solvent in the case of a gel-like molded body. It can be used as a precursor for an electrolyte holder for a solid electrolyte battery obtained by replacing the electrolyte solution.
The measurement items and measurement methods used in the present invention are as follows.
(1) Molecular weight and molecular weight distribution: The polystyrene equivalent weight average molecular weight Mw is measured by GPC. GPC measuring apparatus: manufactured by Toyo Soda, column: GMHXL, solvent: DMF, column temperature: 40 ° C.
(2) Observation of the structure and internal structure of the surface layer of the microporous film: The structure and internal structure of the surface layer of the microporous film are observed using a scanning electron microscope (SEM) Hitachi S-800A. Here, the internal structure refers to a cross-sectional structure obtained by freezing and cutting a microporous membrane.
(3) Average pore diameter by scanning electron microscopy (μm): 50 on a scanning electron micrograph of the surface or cross section of a microporous membrane by an image processing apparatus (IP-1000PC, manufactured by Asahi Kasei Kogyo Co., Ltd.) The average straight line length is defined as the average length of the line segments that pass through the void. The magnification and the width of the region were selected so that at least 10 arbitrary voids crossed at least 10 voids. In the present invention, unless otherwise specified, a region of 16 μm length × 16 μm width of an electron micrograph having a magnification of 6000 times is used.
(4) Thickness of microporous film (μm): The average value of five or more thicknesses of arbitrarily selected cross-sections of the microporous film observed by SEM is defined as the thickness of the microporous film.
(5) Average pore diameter (μm) (half dry method): Measured using ethanol in accordance with ASTM F316-86.
(6) Maximum pore diameter (μm) (bubble point method): Measured using ethanol according to ASTM F316-86 and E128-61.
(7) Porosity (%): Porosity = (pore volume / microporous membrane volume) × 100.
[0047]
(8) Breaking strength (Kgf / cm ² ), Elongation at break (%): Measured according to ASTM D882 for a hollow fiber-shaped or strip-shaped test piece having a width of 10 mm.
(9) Water permeability (liter / m ² Hr.atm): Differential pressure of 1 kgf / cm at 25.degree. ² Then, measure with pure water.
(10) Stretch residual strain (%):
Stretch residual strain = ((sample length after stretching−initial length) / initial length) × 100.
(11) Melting point Tm (° C.): A mixture of a vinylidene fluoride polymer and a solvent is sealed in a sealed DSC vessel, and the melting peak temperature measured using a Seiko DSC-200 is taken as the melting point (heating rate) 5 ° C./min).
(12) Crystallization temperature Tc (° C.): A mixture of a vinylidene fluoride polymer and a solvent is sealed in a hermetically sealed DSC vessel, and the temperature rises to 5 ° C./min with a temperature rise rate of 5 ° C./min. After raising the temperature and holding for 20 minutes, the crystallization peak temperature observed in the process of lowering the temperature at a temperature lowering rate of 2 ° C./min is defined as the crystallization temperature.
(13) Ionic conductivity (mS / cm): sandwiching a sheet electrolyte holder between metal electrodes (stainless steel sheets) to form an electrochemical cell, and applying alternating current between the electrodes to measure the resistance component Employing the impedance method, the impedance is measured using a 389 type impedance meter manufactured by EG & G. The ionic conductivity is calculated from the real impedance intercept of the Cole-Cole plot.
[0048]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described specifically by way of examples.
[0049]
[Example 1]
Weight average molecular weight (Mw) is 3.62 × 10 ^Five 40 parts by weight of the vinylidene fluoride homopolymer and 60 parts by weight of diethyl phthalate (DEP) were heated and mixed at 160 ° C. with a twin-screw kneader, and the sample cooled to room temperature was again used at 155 ° C. with a hot press. After melting and forming into a 100 μm flat film, it was cooled with a press machine at 20 ° C. to obtain a sheet-like gel-like molded body. DEP was extracted by immersing the molded gel-like molded body in methylene chloride for 1 hour and dried at room temperature to obtain a microporous film. The average pore diameter of this membrane was 0.1 μm, and the internal structure was a percolation structure as shown in FIG. The film was uniaxially stretched at 20 ° C. and a draw ratio of 150%, and then relaxed at 20 ° C. At this time, the residual stretching strain was 20%. The inner structure of the stretched film was a percolation structure.
[0050]
[Example 2]
Example 1 was followed except that it was molded at 160 ° C. with a hot press. The average pore diameter of this membrane was 0.15 μm, and the internal structure was a percolation structure as shown in FIG. Further, the stretched residual strain of the stretched film was 30%, and the internal structure was a percolation structure.
[0051]
[Comparative Example 1]
Example 1 was followed except that it was molded at 150 ° C. with a hot press. The inner structure of this film was nonporous as shown in FIG. 5a).
[0052]
[Example 3]
Example 1 was followed except that 70 parts by weight of acetophenone was used instead of 60 parts by weight of DEP, kneaded at 140 ° C., and molded at 140 ° C. with a hot press. The average pore diameter of this membrane was 0.15 μm, and the internal structure was a percolation structure.
[0053]
[Example 4]
Example 1 was followed except that 70 parts by weight of dibutyl phthalate (DBP) was used instead of 60 parts by weight of DEP, kneaded at 165 ° C., and molded at 165 ° C. with a hot press. The average pore diameter of this membrane was 0.15 μm, and the internal structure was a percolation structure.
[0054]
[Comparative Example 2]
Example 1 was followed except that 55 parts by weight of γ-butyllactone (γ-BL) was used instead of 60 parts by weight of DEP, kneaded at 120 ° C., and molded at 120 ° C. with a hot press. The inner structure of this film was a structure in which spherical particles were connected as shown in FIG. 6a).
[0055]
[Comparative Example 3]
Comparative Example 2 was followed except that kneaded at 150 ° C. using ethylene carbonate (EC) instead of γ-BL and molded at 150 ° C. with a hot press. The inner structure of this film was a structure in which spherical particles were connected as shown in FIG. 6b).
[0056]
[Comparative Example 4]
Comparative Example 3 was followed except that propylene carbonate (PC) was used instead of EC. The internal structure of this film was a structure in which spherical particles were connected as shown in FIG. 6c).
[0057]
[Example 5]
Except for 70 parts by weight of DEP, the gel-like molded product obtained in accordance with Example 1 was dissolved again on a hot plate at 160 ° C., cooled with air at 20 ° C., and subsequently immersed in methylene chloride for 1 hour. As a result, DEP was extracted and dried at room temperature to obtain a microporous membrane. The average pore diameter of this membrane was 0.1 μm, and the surface was porous as shown in FIG. The internal structure was a percolation structure.
[0058]
[Example 6]
Example 5 was followed except that it was cooled in a cooling medium at 20 ° C. As the cooling medium, dimethyl phthalate (DMP), DEP, DBP, diethylhexyl phthalate (DOP), diisodecyl phthalate (DIDP), water, and ethylene glycol (EG) were used. When the cooling medium is DMP, DEP, DBP, DOP, DIDP, water, EG, the surface of the membrane is porous as shown in FIGS. 7b), c), d), e), g), h). Yes, the average pore size of the membranes of FIGS. 7b), c), d) and e) was around 0.1 μm, and the average pore size of the membranes of FIGS. 7g) and h) was around 0.04 μm. All the films had a percolation structure.
[0059]
[Comparative Example 5]
Example 5 was followed except that it was cooled in a cooling medium at 20 ° C. As the cooling medium, tridecyl phthalate (DTDP) and decalin were used. When the cooling medium was DTDP or decalin, the surface of the film was nonporous as shown in FIGS. 7f) and i). The internal structure was a percolation structure.
[0060]
[Example 7]
Mw is 3.62 × 10 ^Five A mixture of 46.6 parts by weight of PVdF and 53.4 parts by weight of DEP was heated and kneaded at 145 ° C. in a 35 mmφ twin screw extruder, extruded from a hollow die having an inner diameter of 0.9 mmφ and an outer diameter of 1.7 mmφ, and formed into a hollow fiber shape. Molded. At this time, in order to stabilize the diameter of the hollow fiber, the gel-like molded body is cooled by flowing 10 ml / min air inside the yarn and immersing the extruded hollow fiber in a cooling medium bath made of DOP. Got. The formed hollow fiber gel was immersed in methylene chloride for 1 hour to extract DEP and dried at room temperature to obtain a hollow fiber membrane. The obtained hollow fiber membrane had an inner diameter of 0.84 mm, an outer diameter of 1.59 mm, a porosity of 54.6%, an average pore diameter of 0.14 μm, a maximum pore diameter of 0.21 μm, and the ratio was 1.50. The water permeability of this hollow fiber membrane is 300 liters / m. ² ・ In hr ・ atm, the breaking strength is 139 kgf / cm ² The elongation at break was 353%. The internal structure of this film was a percolation structure.
[0061]
[Example 8]
Example 7 was followed except that the mixture was heated and kneaded at 150 ° C. The obtained hollow fiber membrane had an inner diameter of 0.88 mm, an outer diameter of 1.62 mm, a porosity of 54.3%, an average pore diameter of 0.15 μm, a maximum pore diameter of 0.26 μm and a ratio of 1.73. The water permeability of this hollow fiber membrane is 350 liters / m. ² ・ In hr ・ atm, the breaking strength is 122Kgf / cm ² The elongation at break was 290%. The internal structure of this film was a percolation structure.
[0062]
[Example 9]
Example 7 was followed except that the mixture was heated and kneaded at 140 ° C. The obtained hollow fiber membrane had an inner diameter of 0.86 mm, an outer diameter of 1.61 mm, a porosity of 54.0%, an average pore diameter of 0.12 μm, a maximum pore diameter of 0.17 μm and a ratio of 1.42. The water permeability of this hollow fiber membrane is 250 liters / m. ² ・ In hr ・ atm, the breaking strength is 156 gf / cm ² The elongation at break was 400%. The internal structure of this film was a percolation structure.
[0063]
[Comparative Example 7]
Example 7 was followed except that the mixture was heated and kneaded at 130 ° C. The obtained hollow fiber membrane had a low inner diameter of 0.85 mm, an outer diameter of 1.60 mm, and a porosity of 42.0%, and was entirely contracted. Moreover, the space | gap part refined | miniaturized and it had the internal structure which became the independent hole, and the water permeability was zero. The breaking strength of this hollow fiber membrane is 150 kgf / cm. ² The elongation at break was 380%.
[0064]
[Comparative Example 8]
Example 7 was followed except that the mixture was heated and kneaded at 155 ° C. The obtained hollow fiber membrane had an inner diameter of 0.81 mm, an outer diameter of 1.58 mm, a porosity of 52.9%, an average pore diameter of 0.18 μm, a maximum pore diameter of 0.53 μm and a ratio of 2.94. The water permeability of this hollow fiber membrane is 500 liters / m. ² ・ In hr ・ atm, breaking strength is 90Kgf / cm ² The elongation at break was 90%. The internal structure of this film was coarsened.
[0065]
[Comparative Example 9]
Example 7 was followed except that kneading was carried out at 160 ° C. The obtained hollow fiber membrane had an inner diameter of 0.82 mm, an outer diameter of 1.58 mm, a porosity of 53.5%, an average pore diameter of 0.20 μm, a maximum pore diameter of 0.79 μm and a ratio of 3.95. This hollow fiber membrane has a water permeability of 810 liters / m 2 · hr · atm and a breaking strength of 82 kgf / cm. ² The elongation at break was 75%. Moreover, the internal structure of this film was coarsened.
[0066]
[Example 10]
Mw is 5.46 × 10 ^Five Example 7 was followed except that a mixture of 30 parts by weight of PVdF and 70 parts by weight of DEP was heated and kneaded at 145 ° C. The obtained hollow fiber membrane had an inner diameter of 0.85 mm, an outer diameter of 1.60 mm, a porosity of 69.1%, an average pore diameter of 0.18 μm, a maximum pore diameter of 0.23 μm, and the ratio was 1.27. The water permeability of this hollow fiber membrane is 2900 liters / m ² ・ In hr ・ atm, the breaking strength is 93 kgf / cm ² The breaking elongation was 433%. The internal structure of this film was a percolation structure.
[0067]
Example 11
Example 10 was followed except that the mixture was heated and kneaded at 150 ° C. The obtained hollow fiber membrane had an inner diameter of 0.85 mm, an outer diameter of 1.58 mm, a porosity of 68.8%, an average pore diameter of 0.44 μm, a maximum pore diameter of 0.67 μm and a ratio of 1.52. The water permeability of this hollow fiber membrane is 8200 liters / m. ² ・ In hr ・ atm, breaking strength is 88Kgf / cm ² The elongation at break was 425%. The internal structure of this film was a percolation structure.
[0068]
Example 12
Example 9 was followed except that DMP was used instead of DEP. The obtained hollow fiber membrane had an inner diameter of 0.85 mm, an outer diameter of 1.53 mm, a porosity of 67.0%, an average pore diameter of 0.34 μm, a maximum pore diameter of 0.49 μm and a ratio of 1.43. The water permeability of this hollow fiber membrane is 4300 liters / m ² ・ Hr ・ atm, breaking strength is 95Kgf / cm ² The elongation at break was 292%. The internal structure of this film was a percolation structure.
[0069]
Example 13
A sheet-like gel-like molded body having a film thickness of about 100 μm was obtained according to Example 1 except that DMP was 60 parts by weight and molded at 150 ° C. with a hot press. DMP was extracted by immersing the molded gel-like molded body in ether for several hours and dried at room temperature to obtain a microporous film. The average pore diameter of this membrane is 0.12 μm, the porosity is 56%, the thickness is 87 μm, and the breaking strength is 120 kgf / cm. ² The elongation at break was 300%, and the internal structure was a percolation structure. The microporous membrane is made of LiBF _Four Was immersed in a 1 mol / liter solution having a composition of EC / PC = 1/1 at room temperature to prepare a sheet-like electrolyte support having a thickness of 100 μm.
[0070]
As a result of measuring the impedance of the sheet-like electrolyte support, the ionic conductivity at room temperature was 0.8 mS / cm.
[0071]
Example 14
A microporous membrane was obtained according to Example 13 except that DMP was changed to 70 parts by weight. The obtained microporous membrane had an average pore diameter of 0.25 μm, a porosity of 63%, a film thickness of 62 μm, and a breaking strength of 100 kgf / cm. ² The elongation at break was 270%, and the internal structure was a percolation structure. A sheet-like electrolyte holding body having a thickness of 80 μm was produced in the same manner as in Example 13. The ionic conductivity of this sheet-like electrolyte support at room temperature was 1.1 mS / cm.
[0072]
[Comparative Example 10]
A microporous membrane was obtained according to Example 13 except that DMP was EC. The obtained microporous film had a porosity of 43% and a film thickness of 76 μm, and the internal structure was a structure in which spherical particles were connected. A sheet-like electrolyte support with a film thickness of 70 μm was produced in the same manner as in Example 13. The ionic conductivity of this sheet-like electrolyte support at room temperature was 0.3 mS / cm.
[0073]
Example 14
Mw 1.18 × 10 ⁶ A mixture of 25 parts by weight of PVdF and 75 parts by weight of DMP was heated and kneaded at 135 ° C., extruded from a hollow die having an inner diameter of 0.9 mmφ and an outer diameter of 1.45 mmφ, and the bath temperature of the cooling medium bath composed of DOP was controlled at 20 ° C. Then, Example 7 was followed except that methyl ethyl ketone was used for extraction of DMP. The obtained hollow fiber membrane had an inner diameter of 0.75 mmφ, an outer diameter of 1.25 mmφ, a porosity of 69.8%, an average pore diameter of 0.17 μm, a maximum pore diameter of 0.22 μm and a ratio of 1.22. The water permeability of this hollow fiber membrane is 2200 liters / m. ² ・ In hr ・ atm, the breaking strength is 115Kgf / cm ² The elongation at break was 371%. The internal structure of this film was a percolation structure.
[0074]
Example 15
The hollow fiber membrane obtained in Example 14 was stretched with a stretch elongation of 50%. The stretching residual strain was 28%, the inner diameter was 0.72 mmφ, the outer diameter was 1.22 mmφ, the porosity was 73.0%, the average pore size was 0.18 μm, the maximum pore size was 0.24 μm, and the ratio was 1.33. The water permeability of this hollow fiber membrane is 2800 liters / m ² ・ In hr ・ atm, the breaking strength is 107Kgf / cm ² The elongation at break was 321%. The internal structure of this film was a percolation structure.
[0075]
Example 16
Mw is 5.46 × 10 ^Five A mixture of 24 parts by weight of PVdF, 8 parts by weight of acrylic resin (PMMA, Delpet 80N, manufactured by Asahi Kasei Kogyo Co., Ltd.) and 68 parts by weight of DEP is heated and kneaded at 145 ° C., and a cooling medium bath made of DBP is used. Example 7 was followed. The obtained dried film was stretched with a stretching elongation of 50%. The hollow fiber membrane thus obtained had an elongation remaining strain of 29%, an inner diameter of 0.85 mm, an outer diameter of 1.60 mm, a porosity of 69.1%, an average pore diameter of 0.18 μm, and a maximum pore diameter of 0.23 μm. The ratio was 1.27. The water permeability of this hollow fiber membrane is 3500 liters / m. ² ・ In hr ・ atm, the breaking strength is 93 kgf / cm ² The breaking elongation was 433%. The internal structure of this film was a percolation structure.
[0076]
[Example 17]
Mw 1.18 × 10 ⁶ A mixture of 25 parts by weight of PVdF, 5 parts by weight of an acrylic resin (PMMA, Delpet 80N, manufactured by Asahi Kasei Kogyo Co., Ltd.) and 70 parts by weight of DMP is heated and kneaded at 137.5 ° C., and the bath temperature of the cooling medium bath comprising DBP Example 14 was followed except that was controlled at 20 ° C. The obtained hollow fiber membrane had an inner diameter of 0.69 mmφ, an outer diameter of 1.25 mmφ, a porosity of 69.3%, an average pore diameter of 0.13 μm, and a maximum pore diameter of 0.16 μm, and the ratio was 1.23. The water permeability of this hollow fiber membrane is 1900 liters / m ² ・ In hr ・ atm, the breaking strength is 102Kgf / cm ² The breaking elongation was 439%. The internal structure of this film was a percolation structure.
[0077]
Example 18
The hollow fiber membrane obtained in Example 17 was stretched with a stretch elongation of 50%. The stretching residual strain was 26%, the inner diameter was 0.68 mmφ, the outer diameter was 1.23 mmφ, the porosity was 72.0%, the average pore size was 0.18 μm, the maximum pore size was 0.24 μm, and the ratio was 1.33. The water permeability of this hollow fiber membrane is 2900 liters / m ² ・ In hr ・ atm, the breaking strength is 99Kgf / cm ² The elongation at break was 376%. The internal structure of this film was a percolation structure.
[0078]
【The invention's effect】
The microporous membrane of the present invention has a homogeneous structure, excellent fluid permeation characteristics, separation characteristics when separating fine particles from fluid, mechanical characteristics, and chemical resistance, various filters including virus removal filters, precision It is suitably used for applications such as filtration membranes, ultrafiltration membranes, battery separators, diaphragms for electrolyte capacitors, and electrolyte holders for solid electrolyte batteries.
[Brief description of the drawings]
FIG. 1 Vinylidene fluoride homopolymer (weight average molecular weight 3.62 × 10 ^Five ) Is a graph showing the relationship between the crystallization temperature Tc and the dissolution temperature Ts of the DEP solution.
FIG. 2 is a graph showing a relationship between a crystallization temperature Tc and a dissolution temperature Ts of a vinylidene fluoride polymer / solvent solution and a temperature range in which a percolation structure can be formed.
FIG. 3 shows a) porosity (%) and b) breaking strength (Kgf / cm) of a hollow fiber-like microporous membrane. ² ) And c) A graph showing the relationship between the elongation at break (%) and the melting temperature Ts.
FIG. 4 is a diagram showing a relationship between a crystallization curve whose position is changed depending on a melting temperature and a cloud point curve.
FIGS. 5a, 5b, and 5c are scanning electron micrographs of a cross section of a microporous membrane at each melting temperature Ts.
FIGS. 6A and 6B are scanning electron micrographs of cross sections of microporous membranes having various spherical particle networks.
FIGS. 7a to 7i are scanning electron micrographs of the surface of a microporous membrane when various cooling media are used.

Claims

Vinylidene fluoride polymer having a weight average molecular weight of 1 × 10 ⁵ or more: at least selected from dimethyl phthalate, diethyl phthalate, dibutyl phthalate, and acetophenone capable of forming a microporous film having a percolation structure defined by (B) After dissolving the vinylidene fluoride polymer at a weight ratio of one solvent = 10: 90 to 60:40 and forming a percolation structure Ts defined in (C) at 135 ° C. to 165 ° C. The solution extruded by the extruder is cooled with at least one selected from a liquid cooling medium, air, and a roll to form a gel-like formed body composed of a two-layer gel, and then the following i), ii), and iii) A method for producing a microporous membrane comprising performing any of the treatments selected from the above.
i) Remove the solvent using a volatile liquid without stretching.
ii) Before the solvent is removed, the solvent is removed using a volatile liquid after stretching so that the stretching residual strain is 100% or less.
iii) After removing the solvent using a volatile liquid, stretching is performed so that the stretching residual strain is 100% or less.
(B) For a solution of vinylidene fluoride polymer having an arbitrary concentration within the range of 10 to 60% by weight of the vinylidene fluoride polymer, the horizontal axis represents the dissolution temperature Ts, and the vertical axis represents the solution at each dissolution temperature. When the breaking elongation TL of the film formed is plotted at intervals of 5 ° C. starting from Ts = 100 ° C., − (TLs + 5−TLs) / {(Ts + 5 ° C.) − Ts} (TLs + 5 is Ts + 5 The temperature (Ts + 2.5 ° C.) obtained by adding 2.5 ° C. to Ts at which TL and TLs at ° C. are the maximum TL at Ts) is defined as Tu. On the other hand, when Ts is plotted on the horizontal axis and the porosity P of the membrane is plotted on the vertical axis, (Ps + 5-Ps) / {(Ts + 5 ° C) -Ts} (Ps + 5 is Ps at Ts + 5 ° C). , Ps is a temperature obtained by adding 2.5 ° C. to Ts at which P) in Ts is maximum (Ts + 2.5 ° C.) as Tl. When a solution of at least one concentration within the above concentration range has both Tl and Tu, and (Tu-Tl)> 0, the solvent is a solvent capable of forming a microporous film having a percolation structure. (C) Melting temperature Ts satisfying Tl ≦ Ts ≦ Tu.

2. The liquid cooling medium is at least one selected from at least one solvent selected from dimethyl phthalate, diethyl phthalate, dibutyl phthalate, and acetophenone capable of forming a microporous film having a percolation structure. The manufacturing method of the microporous film as described in 2.

Vinylidene fluoride polymer having a weight average molecular weight of 1 × 10 ⁵ or more: at least selected from dimethyl phthalate, diethyl phthalate, dibutyl phthalate, and acetophenone capable of forming a microporous film having a percolation structure defined by (B) After dissolving the vinylidene fluoride polymer at a weight ratio of one solvent = 10: 90 to 60:40 at a temperature Ts capable of forming a percolation structure defined in (C) of 135 ° C. to 165 ° C. A gel-like molded body comprising a two-phase gel obtained by extruding with an extruder and cooling.
(B) For a solution of vinylidene fluoride polymer having an arbitrary concentration within the range of 10 to 60% by weight of the vinylidene fluoride polymer, the horizontal axis represents the dissolution temperature Ts, and the vertical axis represents the solution at each dissolution temperature. When the breaking elongation TL of the film formed is plotted at intervals of 5 ° C. starting from Ts = 100 ° C., − (TLs + 5−TLs) / {(Ts + 5 ° C.) − Ts} (TLs + 5 is Ts + 5 The temperature (Ts + 2.5 ° C.) obtained by adding 2.5 ° C. to Ts at which TL and TLs at ° C. are the maximum TL at Ts) is defined as Tu. On the other hand, when Ts is plotted on the horizontal axis and the porosity P of the membrane is plotted on the vertical axis, (Ps + 5-Ps) / {(Ts + 5 ° C) -Ts} (Ps + 5 is Ps at Ts + 5 ° C). , Ps is a temperature obtained by adding 2.5 ° C. to Ts at which P) in Ts is maximum (Ts + 2.5 ° C.) as Tl. When a solution of at least one concentration within the above concentration range has both Tl and Tu, and (Tu-Tl)> 0, the solvent is a solvent capable of forming a microporous film having a percolation structure. is there.
(C) Melting temperature Ts satisfying Tl ≦ Ts ≦ Tu.

A vinylidene fluoride polymer having a weight average molecular weight of 1 × 10 ⁵ or more: at least one solvent selected from dimethyl phthalate and diethyl phthalate capable of forming a microporous film having a percolation structure defined by (B) and the fluorine Mixture of vinylidene fluoride polymer and compatible thermoplastic resin = weight ratio of 10:90 to 60:40, and the total of the vinylidene fluoride polymer and the thermoplastic resin compatible therewith is 60% by weight or less. The temperature Ts at which the percolation structure defined in (C) can be formed at 135 ° C. under the weight ratio of the vinylidene fluoride polymer: thermoplastic resin compatible with the polymer = 40: 60 to 90:10. 165 was dissolved thermoplastic resin vinylidene fluoride polymer and compatible with it at ° C., extruded solution in an extrusion device, gels JoNaru consisting cooling and two-phase gel After forming the body, following iv), v) and method for producing a microporous film made by performing any one of the processes selected from vi).
iv) Using a volatile liquid without stretching, the thermoplastic resin compatible with the solvent and the vinylidene fluoride polymer is removed.
v) Before removing the thermoplastic resin compatible with the solvent and the vinylidene fluoride polymer, the solvent is removed using a volatile liquid after stretching so that the stretching residual strain is 100% or less.
vi) After removing the thermoplastic resin compatible with the solvent and the vinylidene fluoride polymer using a volatile liquid, stretching is performed so that the stretching residual strain is 100% or less.
(B) For a solution of vinylidene fluoride polymer having an arbitrary concentration within the range of 10 to 60% by weight of the vinylidene fluoride polymer, the horizontal axis represents the dissolution temperature Ts, and the vertical axis represents the solution at each dissolution temperature. When the breaking elongation TL of the film formed is plotted at intervals of 5 ° C. starting from Ts = 100 ° C., − (TLs + 5−TLs) / {(Ts + 5 ° C.) − Ts} (TLs + 5 is Ts + 5 The temperature (Ts + 2.5 ° C.) obtained by adding 2.5 ° C. to Ts at which TL and TLs at ° C. are the maximum TL at Ts) is defined as Tu. On the other hand, when Ts is plotted on the horizontal axis and the porosity P of the membrane is plotted on the vertical axis, (Ps + 5-Ps) / {(Ts + 5 ° C) -Ts} (Ps + 5 is Ps at Ts + 5 ° C). , Ps is a temperature obtained by adding 2.5 ° C. to Ts at which P) in Ts is maximum (Ts + 2.5 ° C.) as Tl. When a solution of at least one concentration within the above concentration range has both Tl and Tu, and (Tu-Tl)> 0, the solvent is a solvent capable of forming a microporous film having a percolation structure. is there.
(C) Melting temperature Ts satisfying Tl ≦ Ts ≦ Tu.